TECTONICS, VOL. 4 , NO. 7 , PAGES 739-750,
DECENEER 1955
QUATERNARY NORMAL AND REVERSE FAULTING AND
THE STATE OF STRESS I N THE CENTRAL ANDES
OF SOUTH PERU
Michel S é b r i e r , I J a c q u e s Louis Mercier
F r a n ç o i s Mégard , Gérard qaüiächer-$3
and Evelyne C a r e y - G a i l h a r d i s l
A b s t r a c t . F i e l d s t u d i e s i n t h e Andes
of s o u t h e r n P e r u show t h a t i n t h e High
Andes and P a c i f i c Lowlands , Quaternary and
Recent f a u l t s a r e normal. This e x t e n s i o n a l
t e c t o n i c s p o s t d a t e s compressional
deformations of P l i o c e n e - e a r l y Quaternary
age. I n t h e sub-Andes t h e observed
deformations are compres s i o n a l ; t h e y
a f f e c t e a r l y Quaternary d e p o s i t s . Some of
t h e f a u l t s s e p a r a t e Quaternary d e p o s i t s
from t h e bedrock and t h u s are c l e a r l y of
t e c t o n i c o r i g i n and n o t l a n d s l i d e e f f e c t s .
S t r i a t i o n s on t h e f a u l t p l a n e s i n d i c a t e
N-S t r e n d i n g e x t e n s i o n i n t h e High Andes
and P a c i f i c Lowlands. The t o t a l amount of
c r u s t a l s t r e t c h i n g i s s m a l l , probably of
t h e order of 1%d u r i n g t h e l a s t 1-2 m.y.
In t h e sub-Andes, f o l d s and f a u l t s
a f f e c t i n g Neogene and e a r l y Quaternary
d e p o s i t s r e s u l t from N-S s h o r t e n i n g .
~
4'
' L a b o r a t o i r e de Géologie Dynamique
I n t e r n e , U n i v e r s i t é P a r i s - S u d , Orsay,
2France.
Centre Géologique e t Géophysique,
U n i v e r s i t é d e s Sciences e t Techniques
gdu Languedoc, M o n t p e l l i e r , France.
O f f i c e de l a Recherche S c i e n t i f i q u e e t
Technique d'Outre-Mer , P a r i s , France.
Copyright 1985
by t h e American Geophysical Union.
Paper number 5T0550.
o278-7407I8sIOOST-OSS0 $10.00
N e v e r t h e l e s s , i t is supposed t h a t t h i s N-S
s h o r t e n i n g is of e a r l y Quaternary age. The
p r e s e n t - d a y compression probably s t r i k e s
E-W, judging from f o c a l mechanisms i n t h e
sub-Andes of c e n t r a l P e r u , s o u t h e r n
B o l i v i a , and n o r t h w e s t Argentina. Data
from s t r u c t u r a l a n a l y s i s of f a u l t s and
from earthquake f o c a l mechanism. a l l o w u s
t o surmise t h e s t a t e ,of stress i n t h e
Andes of s o u t h e r n Peru. The High Andes and
P a c i f i c Lowlands, s u b j e c t e d t o N-S
t r e n d i n g e x t e n s i o n , a r e bounded by two
zones of E-W t r e n d i n g compression: t h e
sub-Andes t o t h e east, and the c o n t a c t
between t h e convergent Nazca and South
b e r i c a p l a t e s t o t h e west. In our model
t h e maximum h o r i z o n t a l compressive stress
t r a j e c t o r y u Hmax i s roughly p a r a l l e l w i t h
t h e E-W convergence between t h e two
p l a t e s ; 6 Hmax corresponds t o 6 1, i n t h e
sub-Andes and t o d 2 i n t h e High Andes.
The l a t t e r s i t u a t i o n is caused by t h e
e l e v a t e d mass of t h e High Andes, where
d z z ( t h e v e r t i c a l s t r e s s ) is i n f e r r e d t o
b e d 1. Thus t h e t h i r d p r i n c i p a l s t r e s s
a x i s , being o r t h o g o n a l t o t h e o t h e r two
a x e s , i s o r i e n t e d N-S, a l l o w i n g e x t e n s i o n
t o occur i n t h a t d i r e c t i o n . On t h e o t h e r
h a n d , i n t h e sub-Andes 6 zz i s 6 3 , and
h o r i z o n t a l E-W s h o r t e n i n g occurs. The
s t a t e of s t r e s s i n t h e Andean c o n t i n e n t a l
c r u s t above t h e 30" d i p p i n g s l a b a p p e a r s
t o be d i f f e r e n t from t h a t i n t h e Andes o f
C e n t r a l P e r u s i t u a t e d above the f l a t
s u b d u c t i n g segment. In t h i s r e g i o n ,
compressional deformations a f f e c t a wider
p a r t of t h e C o r d i l l e r a .
ORSTOM Fonds Dochentaire
\
'*
7so
Sébrier e t al.:
INTRODUCTION
Two main types of s u b d u c t i o n zones have
been d i s t i n g u i s h e d by Uyeda and Kanamori
(1979): t h e Mariana t y p e w i t h e x t e n s i o n a l
t e c t o n i c s and back a r c s p r e a d i n g and t h e
C h i l e a n or Andean t y p e w i t h compressional
t e c t o n i c s and no back- a r c s p r e a d i n g .
However, i t i s a l r e a d y known t h a t normal
f a u l t s have been r e p o r t e d i n t h e C e n t r a l
Andes (Heim, 1949; S i l g a d o , 1951; Audebaud
e t a l . , 1973; Aubouin e t a l . , 1973). I n
t h e Andes of Peru and B o l i v i a , Quaternary
e x t e n s i o n a l n o m a l f a u l t s have been
a n a l y z e d (Dalmayrac, 1974; Lavenu, 1978;
S o u l a s , 1978; Lavenu and B a l l i v i a n , 1980;
Yonekura e t al., 1979; S k b r i e r e t al.,
1980b, 1982) and e x t e n s i o n has been
c o n s i d e r e d as the t y p i c a l s t a t e of s t r e s s
of t h e A l t i p l a n o d u r i n g Quaternary and
Recent times ( M e r c i e r , 1981). Some a u t h o r s
have considered t h i s e x t e n s i o n a l t e c t o n i c s
as a q u i t e n e g l i g i b l e , purely s u p e r f i c i a l ,
and l o c a l phenomenon (Allmendiger e t a l
1983; Jordan e t a l . , 1983). Nakamura and
Uyeda (1980) have suggested some p h y s i c a l
p r o c e s s e s which can b e r e s p o n s i b l e f o r
e x t e n s i o n a l t e c t o n i c s along subduction
z o n e s , such as landward d e c r e a s e of
o Hmax, g r a v i t a t i o n a l body f o r c e s , e t c .
R e c e n t l y , t h i s e x t e n s i o n a l t e c t o n i c s has
been i n t e r p r e t e d as an e f f e c t of t h e
h i g h l y compensated topography (Cross and
P i l g e r , 1982; Suarez e t a l . , 19831, and a
two-dimensional model h a s been p r e s e n t e d
(Dalmayrac and Molnar , 1981; Froidevaux
and Is acks , 1984).
In t h e Andes o f P e r u , B o l i v i a and
northernmost C h i l e , t w o r e c e n t and a c t i v e
t e c t o n i c domains have been s e p a r a t e d i n
r e l a t i o n t o t h e d i p of t h e subducted s l a b
( S t a u d e r , 1975; Barazangi and h a c k s ,
1976, 1979; Mkgard and P h i l i p , 19761,
t h e i r common boundary l y i n g a t about 14" S
(Hasegawa and Sacks, 1981; Grange, 1983;
Grange e t a l . , 198ba). North of t h i s
l a t i t u d e , t h e s l a b d i p s a t about 30" below
t h e t r e n c h i n n e r s l o p e , t h e s h e l f , and t h e
c o a s t . F a r t h e r e a s t , it f l a t t e n s and i s
a l m o s t h o r i z o n t a l under t h e Andes. In t h i s
r e g i o n , E-W Quaternary compression i s
observed i n a wide p a r t of t h e Andes
( S k b r i e r e t a l . , 1982; Blanc e t a l . , 1983;
Blanc, 19841, b u t e x t e n s i o n i s p r e s e n t
along t h e c o a s t ( S o u l a s , 1 9 7 8 ; S é b r i e r and
Macharé, 1980; Macharé,, 1981) and i n t h e
w e s t e r n p a r t of t h e High Andes,
p a r t i c u l a r l y i n t h e C o r d i l l e r a Blanca
(Dalmayrac, 1974; Yonekura e t a l . , 1979:
Dalmayrac and Molnar, 1981; S 6 b r i e r e t
.,
Quaternary Normal and Reverse F a u l t i n g
a l . , 1982). South of 1 4 " s l a t i t u d e , t h e
s l a b d i p s s t e e p l y , and Quaternary
e x t e n s i o n i s widespread. i n t h e Andes. T h i s
e x t e n s i o n a l t e c t o n i c s i s evidenced mainly
by E-W t r e n d i n g r e c e n t and a c t i v e normal
f a u l t s b o t h i n t h e P a c i f i c Lowlands and
t h e High Andes. I n t h e l a t t e r , e x t e n s i o n
i s a s s o c i a t e d w i t h Recent v o l c a n i c
a c t i v i t y . Compressional f a u l t i n g i s
r e s t r i c t e d t o t h e lowlands of t h e subAndes which bound t h e High Andes t o t h e
NE.
S t r u c t u r a l a n a l y s i s of t h e normal
f a u l t s i n d i c e t e s 9 N-S t r e n d i n g t e o s i o n a l
t e c t o n i c s which p o s t d a t e s e a r l y Quaternary
compressional movements. The few c r u s t a l
e a r t h q u a k e s analyzed by Grange (1983) i n
t h e Western C o r d i l l e r a a l s o . i n d i c a t e N-S
t e n s i o n . R e l a t i o n s between s e i s m i c i t y and
s u p e r f i c i a l deformations are more
complicated along t h e f a u l t e d boundary
between t h e c o a s t and t h e Western
C o r d i l l e r a . I n t h e p r e s e n t paper w e
d e s c r i b e a l l t h e Quaternary f a u l t s t h a t w e
have observed i n s o u t h Peru. S i n c e
s e i s m i c i t y i s g e n e r a l l y low, we f i r s t used
L a n d s a t imagery t o s e a r c h t h e most
conspicuous f a u l t zones of s o u t h Peru.
Then w e focused o u r f i e l d work on t h e
f a u l t e d Quaternary b a s i n s which a r e
a s s o c i a t e d w i t h t h e major Quaternary f a u l t
zones and o f f e r p o s s i b i l i t i e s t o d e c i p h e r
Recent f a u l t movements. F u r t h e r w e
surveyed s o u t h e r n Peru along t h e r o a d
network u s i n g a e r i a l photographs i n o r d e r
t o d i s c o v e r and s t u d y i n the f i e l d o t h e r
f a u l t zones n o t e a s i l y seen on Landsat
imagery. F i e l d s t u d i e s begin i n 1978.
Q u a t e r n a r y f a u l t s a r e of primary i n t e r e s t
because of t h e s c a r c i t y of f o c a l
mechanisms i n t h a t p a r t of t h e Andean
Cord i l 1 e r a .
EXTENSIONAL TECTOFICS I N THE H I G H ANDES
The Huambo-Cabana Conde A c t i v e F a u l t
------Western C o r d i l l e r a
Zone,
Geometry of t h e f a u l t s . I n t h e Western
C o r d i l l e r a . about 100 km n o r t h of Arequipa
( F i g u r e I ) , l a v a p l a t e a u s l i e a t a mean
e l e v a t i o n o f 4 km between t h e v i l l a g e s o f
Huambo and Cabana Conde ( F i g u r e 1, p o i n t
82. They a r e crossed by t h e major, n e a r l y
E-W t r e n d i n g T r i g a l and Solarpampa f a u l t s
and by some o t h e r minor f a u l t s n e a r l y
p a r a l l e l t o them ( F i g u r e s 2a and 2b). The
f a u l t e d a r e a i s about 28 km long i n t h e
E-W d i r e c t i o n and 10 km wide from n o r t h t o
south. This f a u l t e d zone extends t o t h e
S é b r i e r e t al.:
I n
Quaternary Normal and Reverse F a u l t i n g
w e s t i n t o t h e Uncapampa p l a t e a u l o c a t e d
7.5 kin n o r t h of Huambo. In t h e a r e a e a s t
o f Cabana Conde some s h o r t f a u l t s of t h e
same system c r o s s c u t t h e v o l c a n i c flows
vented from t h e Ampato v o l c a n i c c e n t e r
l o c a t e d 1 0 h f a r t h e r t o t h e s o u t h . West
of 72"W l o n g i t u d e , t h e mean t r e n d of t h e
f a u l t s i s N82"E, with extreme v a l u e s of
N77"E and N90"E. Toward t h e e a s t , t h e
f a u l t s c u r v e t o a N105"E-N12O0E d i r e c t i o n
i n t h e area of Cabana Conde. The T r i g a l
f a u l t i s about 1 0 kin long, and t h e
Solarpampa f a u l t i s 1 2 km long. The l e n g t h
of t h e minor f a u l t s range between 1 and 5
km. Some minor f a u l t s branch o u t from t h e
major o n e s ; g e n e r a l l y , throws d e c r e a s e
away from t h e main f a u l t . A l l t h e s e f a u l t s
a r e e a s i l y i d e n t i f i e d both i n t h e f i e l d
( F i g u r e 2 a ) and on a e r i a l photographs
( F i g u r e 2b) thanks t o well-developed
s c a r p s t h a t a r e u s a l l y 5-10 m h i g h b u t
r e a c h l o c a l l y 30 m f o r t h e major f a u l t s .
Most of t h e s c a r p s a r e s o u t h f a c i n g , t h i s
being o p p o s i t e t o t h e d r a i n a g e , c a u s i n g
sagponds t o form where t h e f a u l t s c r o s s
t h e few i n t e r m i t t e n t streams d r a i n i n g t h e
p l a t e a u s . Most of t h e s e sagponds a r e
f i l l e d w i t h sediments.
According t o s u r f a c e o b s e r v a t i o n s , most
of t h e f a u l t s d i p southward w i t h v a l u e s
r a n g i n g between 60" and 70", b u t t h e crude
columnar j o i n t i n g i n t h e l a v a s a t t h e
s u r f a c e makes p r e c i s e o b s e r v a t i o n s
d i f f i c u l t . Some s l i c k e n s i d e s w e r e measured
a l o n g t h e r o a d c u t on t h e r i g h t bank of
Quebrada T r i g a l ( F i g u r e s 3 and 4, s i t e 8).
A t i t s w e s t e r n end, t h i s f a u l t h a s a l o c a l
N114"E t r e n d and a 62" south d i p . . A t i t s
e a s t e r n end, v a l u e s of N108'E w i t h 74"
s o u t h d i p and N94"E w i t h 84" s o u t h d i p
were measured along two branches of t h e
f a u l t . Some n o r t h dipping f a u l t s have
been a l s o observed i n p a r t i c u l a r a
plurikilometric f a u l t crossing the
s o u t h e r n p a r t of t h e l a v a p l a t e a u 2 km
n o r t h of l a k e Mucurca has a n o r t h f a c i n g
s c a r p 5-10 m high. Along a l l t h e f a u l t s ,
d i p - s l i p movement always p r e v a i l s over
s t r i k e - s l i p , so t h a t no l a t e r a l
d i s p l a c e m e n t s of t h e streams have been
observed and a l l t h e s t r i a t i o n s t h a t w e
measured have p i t c h e s ranging between 48"
and 88" ( F i g u r e 4 , s i t e 8) with a c l e a r
maximum around 80: The g e n e r a l southward
d i p of t h e s e normal f a u l t s i s a s t r o n g
argument a g a i n s t a huge l a n d s l i d e f a u l t i n g
due t o t h e a t l e a s t 2000-m-deep i n c i s i o n
o f t h e Colca R i v e r l o c a t e d n o r t h of t h e
l a v a p l a t e a u s ( F i g u r e 2a).
Age o f f a u l t i n g . Norphological
741
arguments favor a very r e c e n t a g e f o r t h e
l a s t movements along t h e s e f a u l t s : (1) t h e
s c a r p s a r e f r e s h and c u t present-day
topography ( F i g u r e s 2b and 31, and (2) t h e
streams have been unable t o erod-e away t h e
r i d g e s t h a t dam t h e sagponds. A c l e a r
example of t h e l a t t e r i s exposed i n Cerro
T o c o a s i , 2 km SSW of Cabana Conde ( F i g u r e
2 a ) , where two small sagponds a r e
p r e s e r v e d i n a g u l l y , s l o p i n g a t about 20",
carved i n the c r e s t of t h e h i l l .
Other arguments may be drawn from t h e
space-time r e l a t i o n s of t h e f a u l t s with
t h e g e o l o g i c a l u n i t s t h a t they cut. ?hemost r e c e n t u n i t s a r e t h e somewhat
i n d u r a t e d s l o p e d e p o s i t s of p e r i g l a c i a l
o r i g i n f a u l t e d along t h e r i g h t bank of
Quebrada T r i g a l ( F i g u r e 3 , s e e d e t a i l e d
s t u d y i n t h e n e x t s e c t i o n ) . These r a t h e r
s t r a t i f i e d , ' angular, slope deposits ( c ,
F i g u r e 3 ) bury o l d e r s o i l s ( d , F i g u r e 3 )
and are covered only by t h e t h i n p r e s e n t day s o i l (a, F i g u r e 3 ) . We r e l a t e t h e s e
s l o p e d e p o s i t s ( c ) with t h e l a s t c o l d
epoch which is commonly considered t o b e
g r o s s l y c o e v a l w i t h t h e Wisconsinian
g l a c i a l epoch ( r o u g h l y 10,000 t o 50,000
y e a r s B.P.)
of North America ( C l a p p e r t o n ,
1972; Mercer and P a l a c i o s , 1977; S e r v a n t ,
1977). The youngest f a u l t e d a n d e s i t i c
flows (Mucurca flows V3, F i g u r e 2a)
o r i g i n a t e d from a v e n t l o c a t e d 2 km n o r t h
of l a k e Mucurca and flowed northward
a c r o s s t h e main l a v a p l a t e a u . They damed
l a k e Mucurca and f i l l e d i t s wide o u t l e t
v a l l e y . This small v e n t ( a l t i t u d e 4550 m)
and p a r t of t h e flows r e s t upon t h e d i s t a l
moraines t h a t cover t h e western s l o p e of
t h e huge eroded Lipayoc c a l d e r a . These
moraines are u n c o n s o l i d a t e d , have a f r e s h
morphology, and crop o u t down t o an
a l t i t u d e of 4400 m. These c r i t e r i a are
shared by many moraines r e l a t e d t o t h e
l a s t Andean g l a c i a l epoch i n s o u t h e r n
Peru. The Mucurca l a v a flow i s t h u s
p r o b a b l y younger t h a n t h e "Wisconsinian
epoch." Some v e n t s a r e l o c a t e d d i r e c t l y
a l o n g t h e f a u l t s : t h e Uncapahpa v o l c a n i c
v e n t , l o c a t e d 7 km n o r t h o f Huambo, l i e s
o v e r a N82OE t r e n d i n g f a u l t t h a t appears
t o be t h e western p r o l o n g a t i o n of t h e
Solarpampa f a u l t . .The neck ( a l t i t u d e 4210
m) s t a n d i n g 7 h t o t h e N22"E o f Huambo i s
a l s o l o c a t e d on t h e Solarpampa f a u l t .
A c l o s e r examination of the r e l a t i o n s
between t h e f a u l t s and t h e l a v a flows both
a t t h e Uncapampa v e n t and where t h e
Solarpampa f a u l t c r o s s e s t h e Mucurca (V3)
flow shows t h a t f a u l t s have been
r e a c t i v a t e d s e v e r a l times. T h e r e f o r e t h e i r
.
S6brier e t al. :
-,.N
Q u a t e r n a r y Normal and Reverse F a u l t i n g
-*
0-
-
-
1
\
-
I
b
S é b r i e r e t al.:
c
Q u a t e r n a r y Normal and Reverse F a u l t i n g
743
movements a r e i n p a r t contemporaneous with
l a v a e r u p t i o n s . Tne r e c e n t V3 Uncapampa
volcanoe vented f l u i d b a s i c a n d e s i t e s
toward t h e w e s t and NW ( F i g u r e 2 a ) . The
v e n t c o n s i s t s of two horseshoe-shaped
i n s e t sommas t h a t are widely opened
northwestward and a r e f i l l e d by a younger
c e n t r a l flow. W o minor E-W f a u l t s c u t
both sommas b u t do n o t c u t t h e c e n t r a l
flow ( F i g u r e 2a). A s t h e s t a t e of e r o s i o n ,
w e a t h e r i n g , and morphology of a l l t h e
f l o w s vented from Uncapampa c r a t e r i s t h e
same, it can be assumed t h a t they a r e
r e l a t e d t o an almost s i n g l e v e n t i n g
e p i s o d e i n p a r t coeval with t e n s i o n a l
t e c t o n i c s along t h e E-W t r e n d i n g normal
f a u l t s . Analysis of t h e r e l a t i o n s between
t h e d i f f e r e n t flow u n i t s of t h e Mucurca
flow and t h e Solarpampa f a u l t a l s o
d e m o n s t r a t e s t h a t t h i s f a u l t moved a t t h e
t i m e t h e l a v a s were flowing. In t h e o l d e r
v o l c a n i c s V2, t h e f a u l t throw i s l a r g e r
t h a n i n t h e Mucurca V3 f l o w ( F i g u r e 2a).
Upstream, i.e., s o u t h of t h e f a u l t , t h e
western p a r t of t h e V3 flow has b e e n
ponded a l o n g t h e a l r e a d y e x i s t i n g s c a r p ,
b u i l t i n t o V2 l a v a s , g i v i n g a p e c u l i a r
arrangement of t h e l a r g e pahoehoe waves on
t h e flow s u r f a c e . Once t h e pond had b e e n
f i l l e d by t h e o l d e r p a r t of V3 f l o w , t h e
younger one w a s a b l e t o flow f r e e l y
downstream and f i l l t h e v a l l e y f a r t h e r
n o r t h . A later normal movement a l o n g t h e
same Solarpampa f a u l t again d e p r e s s e d t h e
s o u t h e r n block. These d a t a i n d i c a t e t h a t
t h e Solarpampa f a u l t has moved a t l e a s t
twice d u r i n g i t s Recent h i s t o r y .
S i m i l a r l y , t h e throw of t h e T r i g a l f a u l t
a t t h e Pampa T r i g a l s i t e (see n e x t
s e c t i o n ) i s g r e a t e r t h a n 10 m i n t h e V i
v o l c a n i c s and only about 4 m i n t h e s l o p e
d e p o s i t s ( F i g u r e 3).
S t r u c t u r a l a n a l y s i s of T r i g a l s i t e .
The v o l c a n i c n a t u r e of t h e m a t e r i a l and
t h e l a c k of good o u t c r o p s make a d e t a i l e d
k i n e m a t i c a n a l y s i s of t h e Huambo-Cabana
Conde f a u l t system d i f f i c u l t . Only i n t h e
Quebrada T r i g a l , a f r e s h r o a d c u t a l o n g t h e
new Huambo-Cabana Conde road allowed us t o
s t u d y i n some d e t a i l t h e s t r i a t i o n s of a
f a u l t e d zone a s s o c i a t e d with t h e T r i g a l
f a u l t ( T r i g a l s i t e , F i g u r e s 2a and 3). The
throws of i n d i v i d u a l f a u l t s range between
1 dm and 1 m. The movements are v e r y
r e c e n t because t h e s e f a u l t s c u t
p e r i g l a c i a l s l o p e d e p o s i t s ( c , F i g u r e 3)
r e l a t e d t o t h e l a s t g l a c i a l epoch ( s e e
p r i o r section). In addition, the presentday s o i l ( a , F i g u r e 3) appears t o b e
s l i g h t l y f a u l t e d . These s l o p e d e p o s i t s are
U
c.ç'
Fig. 2a. G e o l o g i c a l map of t h e Huambo-Cabana Conde area (Western C o r d i l l e r a ) , 100 k m ' n o r t h of Arequipa ( s e e l o c a t i o n i n
F i g u r e 1, p o i n t 8). The l a v a p l a t e a u , ' a t a mean e l e v a t i o n of 4 km, i s c r o s s e d by t h e major normal T r i g a l and Solarpampa
f a u l t s ( t h i c k l i n e s h a t c h u r e d on t h e s i d e of t h e downthrown b l o c k ) . The f a u l t s o f f s e t t h e a n d e s i t i c Mucurca flow of p o s t
l a t e g l a c i a l epoch. Kinematics of t h e T r i g a l f a u l t analyzed i n d e t a i l a t T r i g a l s i t e ( p o i n t 8 ) i n d i c a t e a n e a r l y N-S
trending extension (Figure 4 , s i t e 8).
r.
m
Y
m
rt
Pl
i
,I!
km
5
Fig. 2b. A e r i a l photograph covering t h e c e n t r a l p a r t of t h e Huambo-Cabana Conde area seen on
a n d e s i t i c flows a r e d i r e c t e d northward as t h e Mucurca f l o w (M). The T r i g a l (TI and Solarpampa
most prominent s t r u c t u r e s seen on t h e a e r i a l photograph. The minor Uncapampa v e n t (U) and t h e
(small arrow heads) c u t flows produced by t h i s vent. The T r i g a l s i t e ( F i g u r e 3) i s shown by a
F i g u r e 2a. Most of t h e
(SI normal f a u l t s a r e t h e
minor E-W t r e n d i n g f a u l t s
small arrow marked t r .
S6brier e t al. :
746
Quaternary Normal and Reverse F a u l t i n g
fb
.
I
l
t
t o 42
to
Huambo
Cabana C o n d e
O
5m
Fig. 3. F i e l d view of t h e T r i g a l f a u l t a t T r i g a l s i t e ( F i g u r e s 2a and 2b,
p o i n t 8 ) a l o n g t h e Huambo-Cabana Conde road. Symbols a r e as f o l l o w s : a ,
p r e s e n t - d a y s o i l ; b y s l o p e b r e c c i a made of v o l c a n i c d e p o s i t s b e i n g a
l a t e r a l e q u i v a l e n t of t h e V l l a v a flows; c y s l o p e d e p o s i t s r e l a t e d t o t h e
l a s t g l a c i a l epoch; d , p a l e o s o i l between b and c u n i t s ; sf, f a u l t s c a r p ;
f p , f a u l t plane. S t r i a t i o n s have been measured on f a u l t p l a n e s w i t h i n t h e
same T r i g a l f a u l t zone ( F i g u r e 4 , s i t e 8).
somewhat c o n s o l i d a t e d because they have a
well-developed sandy-c layed m a t r i x which
c o n t a i n s only few s c a t t e r e d r o c k
fragments. A t t h e s c a l e of t h e o u t c r o p ,
m a t e r i a l has an homogeneous b e h a v i o r .
Moreover t h e r e i s no evidence of f a u l t
p l a n e r o t a t i o n , as t h e o r i g i n a l bedding
does n o t appear t o be warped.
Twelve s t r i a t e d normal s l i c k e n s i d e s
were measured. I t c l e a r l y appears t h a t t h e
k i n e m a t i c s of t h e s e f a u l t s ( F i g u r e 4 , s i t e
8 ) a g r e e b e t t e r w i t h a roughly N-S
lengthening. To d e f i n e more a c c u r a t e l y t h e
d i r e c t i o n s of Recent and a c t i v e e x t e n s i o n ,
we model b r i t t l e deformation c o n s i d e r i n g
t h e f a c t t h a t the rocks a r e highly
f r a c t u r e d and blocks a r e r i g i d . I t i s
c u r r e n t p r a c t i c e t o model deformation of
h i g h l y f r a c t u r e d bodies of rocks which
have been s u b j e c t e d t o a s t r e s s f i e l d by
u s i n g a simple mechanical s y s t e m (Carey
and B r u n i e r , 1974; Carey, 1976, 1 9 7 9 ) ( s e e
t h e appendix). This supposes t h a t t h e b u l k
change of t h e geometry of t h e rock body
occurs through t h e a d d i t i o n of independent
and small displacements of r i g i d blocks.
I f t h e s l i p s on t h e f a u l t s a r e
i n d e p e n d e n t , then t h e s l i p S g i v e n by t h e
s t r i a t i o n s on each f a u l t p l a n e has t o be
p a r a l l e l to the tangential s t r e s s t
r e s o l v e d on each f a u l t p l a n e . I n v e r s i o n o f
d a t a y i e l d s t h e azimuth of t h e p r i n c i p a l
s t r e s s axes and a r a t i o R of t h e p r i n c i p a l
s t r e s s d i f f e r e n c e s such as
R=(U 2-0 1 > / ( 6 3-6 1).
For the T r i g a l s i t e ( F i g u r e 4 , s i t e 81,
although the s t r i a t e d s l i c k e n s i d e s a r e
o n l y 12, they are r a t h e r w e l l s p a t i a l l y
d i s t r i b u t e d and t h u s e n a b l e a computation.
The o b t a i n e d s o l u t i o n ( s i t e 8 , T a b l e 1)
appears t o be good because the a n g l e s
( t , S ) between t h e p r e d i c t e d t and t h e
o b s e r v e d S s t r i a t i o n s a r e always l e s s t h a n
20" ( s e e histogram, F i g u r e 4 , s i t e 8 ) . The
minimum " t e n s i o n a l " p r i n c i p a l s t r e s s ( o 3)
d i r e c t i o n has an azimuth of N l O ' E , t h e
intermediate deviatoric principal s t r e s s
v a l u e (o 2 ) i s " t e n s i o n a l " b u t of much
smaller magnitude than t h e minimum
p r i n c i p a l s t r e s s , as shown by t h e r a t i o
R=0.77. Therefore t h e Recent and p r o b a b l y
a c t i v e s t a t e of s t r e s s of t h i s a r e a
i m p l i e s a n e a r l y N-S t r e n d i n g e x t e n s i o n .
A t l e a s t p a r t of t h e r e c e n t v o l c a n i c
S 6 b r i e r e t al.:
Quaternary Normal and Reverse F a u l t i n g
a c t i v i t y of t h e Western C o r d i l l e r a i s
a s s o c i a t e d w i t h t h i s N-S t r e n d i n g
tensional tectonics.
T h i s appears t o have a r e g i o n a l meaning
s i n c e i t i s i n very good agreement w i t h
t h e occurence of E-W t r e n d i n g a n d e s i t i c
dykes a t Achoma i n t h e Chivay b a s i n which
i s l o c a t e d 30 km f a r t h e r east of Cabana
Conde. Unpublished K-Ar d a t e s (G. Feraud,
w r i t t e n communication, 1984) from t h e s e
Chivay v o l c a n i c s and from t h e b a s a l p a r t
o f t h e Huambo-Cabana Conde r e c e n t
v o l c a n i c s (V2 i n F i g u r e 2a) confirm t h e
P l e i s t o c e n e age of t h e s e rocks (ages
between 0.2 and 1 m.y. B.P.) and t h e r e f o r e
o f t h e N-S e x t e n s i o n a l t e c t o n i c s .
Q u a t e r n a r y and A c t i v e Normal F a u l t s
Northeastern Altiplano
along t h e
--
Around Lake T i t i c a c a , Quaternary
l a c u s t r i n e t e r r a c e s and coeval e r o s i o n a l
s u r f a c e s are w e l l preserved. They do n o t
show c l e a r o f f s e t s due t o normal f a u l t i n g
.
e x c e p t along t h e n o r t h e a s t e r n b o r d e r of
t h e A l t i p l a n o (Lavenu, 1981; S é b r i e r e t
a l . , 1982; Lavenu e t al., 1984). f i e r e ,
Quaternary normal f a u l t s have been
d e s c r i b e d , and t h e i r k i n e m a t i c s are i n
agreement with r o u g h l y N-S s t r i k i n g
e x t e n s i o n (Lavenu, 1978; Bles e t al.,
1980).
From a g e n e r a l p o i n t of view,
e x t e n s i o n a l deformation appears t o b e
small on t h e A l t i p l a n o and a l o n g i t s
southwestern margin. However, normal
faulting is clear i n i t s northeastern part
and i n t h e t r a n s i t i o n a l zone with t h e
E a s t e r n C o r d i l l e r a . Here we d e s c r i b e t h e
normal f a u l t s t h a t we observed on t h e
P e r u v i a n A l t i p l a n o and i t s n o r t h w e s t e r n
c o n t i n u a t i o n as f a r as t h e Cuzco b a s i n
( F i g u r e 1).
The Cuzco b a s i n f a u l t s . The Cuzco
b a s i n i s a small intra-Andean b a s i n
l o c a t e d between t h e E a s t e r n C o r d i l l e r a and
t h e High P l a t e a u s ( F i g u r e s 1 and Sa). It
i s p a r t l y i n f i l l e d by Quaternary
continental deposits (Sébrier e t a l . ,
1982; Cabrera, 1984). To t h e n o r t h , t h i s
b a s i n i s l i m i t e d by t h e Tambomachay a c t i v e
f a u l t which is about 20 km long ( F i g u r e s
5a and 5b) and e x h i b i t s a 2-m-high s o u t h
f a c i n g morphological s c a r p l e t ( F i g u r e 6 ) .
This f a u l t has a n e a r l y N90" t o N120"E
t r e n d and a mean d i p of 60" t o t h e south.
A t i t s western end, a w e s t f a c i n g s c a r p l e t
a p p e a r s ; t h i s N-S s t r i k i n g recen; f a u l t
shows a l s o a normal downthrow. Thus a l l
t h e r e c e n t f a u l t s have a c l e a r normal
~
-
747
component (Figures 6 and 7 , s i t e 9 ) . The
a p p r o x i m a t e l y 300-m-high south f a c i n g
s c a r p which dominates t h e 2 m h i g h
s c a r p l e t (Figure 6 ) indicates t h a t the
Tambomachay f a u l t has moved s e v e r a l t i m e s
d u r i n g t h e P l e i s t o c e n e , producing a
cumulative normal throw t h a t may a t t a i n
300 m.
S t r i a t e d f a u l t planes have been
observed i n s e v e r a l p l a c e s (a, b y c y d ,
F i g u r e 5 a ) i n t h e v i c i n i t y of t h e major
f a u l t and n e a r San Sebastian. On t h e m a j o r
f a u l t zone, two o r t h r e e superimposed
f a m i l i e s of s t r i a t i o n s are c l e a r l y s e e n .
The f i r s t one shows a r e v e r s e s i n i s t r a l
s t r i k e - s l i p motion which agrees with an
E-W s h o r t e n i n g and i s r e s p o n s i b l e of t h e
l i n e a r trace of t h e Tambomachay f a u l t . The
l a s t one shows a normal s t r i k e - s l i p
movement which a g r e e s with a N-S
l e n t h e n i n g ( F i g u r e 7, s i t e 9). This i s t h e
l a s t kinematics which has probably
produced t h e normal s c a r p seen i n t h e
f i e l d ( F i g u r e 6). Near San S e b a s t i a n ,
f a u l t s have throws which range between
s e v e r a l decimeters and 3 m. They a r e
P l e i s t o c e n e t o very Recent i n age because
they o f f s e t Quaternary d e p o s i t s and
p o s t d a t e compressional deformation which
a f f e c t s e a r l y Pleistocene l a c u s t r i n e beds
(Figure 5a).
It i s e a s i l y seen ( F i g u r e 7, s i t e 9 )
t h a t normal f a u l t s having a NW-SE
d i r e c t i o n show a c l e a r s i n i s t r a l s t r i k e s l i p component so t h a t e x t e n s i o n does n o t
appear o r t h o g o n a l t o t h e Andean b e l t
s t r i k e . I n v e r s i o n of t h e d a t a f o r t h e two
s i t e s g i v e s minimum p r i n c i p a l stress (u 3 )
d i r e c t i o n s s t r i k i n g t o t h e N186"E ( T a b l e
1, s i t e 9d) and t o t h e N18'E ( T a b l e 1,
s i t e s 9 a , 9b, 9c). Grouping a l l t h e normal
s l i c k e n s i d e s measured i n t h e Cuzco b a s i n
( F i g u r e 7, s i t e 91, we o b t a i n a s o l u t i o n
o f good q u a l i t y ( s i t e 9 , Table 11, 1 6
s t r i a t i o n s having an a n g l e ( t , S ) l e s s e r
t h a n 20" (see histogram 9, F i g u r e 7 ) . Tne
stress f i e l d i n t h e Cuzco b a s i n i s enough
homogeneous t o g i v e a q u i t e r e l i a b l e
r e s u l t . The azimuth of t h e minimum
p r i n c i p a l s t r e s s is N8"E. Therefore t h e
Recent s t a t e of stress i n t h e Cuzco b a s i n
a p p e a r s t o be c h a r a c t e r i z e d by a n e a r l y
N-S t r e n d i n g e x t e n s i o n as i n t h e Western
C o r d i l l e r a (see above s e c t i o n ) and on t h e
northern Bolivian Altiplano.
The V i l c a n o t a r i v e r f a u l t system. The
r o u g h l y E-W t r e n d i n g Cuzco f a u l t zone i s
r e l a y e d t o t h e SE b; a NNW-SSE s t r i k i n g
f a u l t system which c o n t r o l s the d i r e c t i o n
o f t h e V i l c a n o t a r i v e r v a l l e y (VF on
Sébrier e t al.:
/”
Quaternary Normal and Reverse Faulting
I
*
i
r
r
S 6 b r i e r e t al.:
Quaternary Normal and Reveirse F a u l t i n g
743
c
F i g u r e 1). I t s l e n g t h i s a b o u t 9 0 km- from
Urcos t o Langui Lay0 lake ( F i g u r e 8 a ) and
i t s width v a r i e s between 1 0 and 20 kr. The
V i l c a n o t a r i v e r f a u l t . sys t e m s e p a r a t e s t h e
Meso-Cenozoic High P l a t e a u t e r r a n e s t o t h e
SW from t h e P a l e o z o i c f o r m a t i o n s of t h e
E a s t e r n C o r d i l l e r a t o t h e NE. It v a n i s h e s
t o t h e SE a t t h e n o r t h w e s t e r n t e r m i n a t i o n
of t h e A l t i p l a n o basin. F a u l t movements
h a v e produced an u p l i f t of t h e E a s t e r n
F i g . 4.
( o p p o s i t e ) Normal f a u l t d a t a from
P a c i f i c Lowlands and Western C o r d i l l e r a
used t o compute s o l u t i o n s of T a b l e 1
( l o c a t i o n s on F i g u r e 1). Numbers o u t s i d e
stereonets r e f e r t o data inside the
h i s t o g r a m s . Arrows a t t a c h e d t o f a u l t
t r a c e s correspond t o t h e measured s l i p
v e c t o r s s (WuIff s t e r e o n e t , lower
hemisphere). Thick segments on f a u l t
t r a c e s and histograms show d e v i a t i o n s
between measured S and p r e d i c t e d t s l i p
v e c t o r f o r each f a u l t plane. Large b l a c k
arrows g i v e azimuths of t h e minimum G 3
computed p r i n c i p a l ' s t r e s s d i r e c t i o n s . Note
t h a t t h e y s t r i k e n e a r l y N-S f o r a l l t h e
analyzed s i t e s . (A) Normal f a u l t s p r e a d
d a t a from P a c i f i c Lowlands and from High
Andes. Large numbers o u t s i d e t h e s t e r e o n e t
r e f e r t o l o c a t i o n of t h e s i t e s ( F i g u r e 11,
and s m a l l numbers r e f e r t o f a u l t d a t a .
F a u l t k i n e m a t i c s a r e i n good agreement
w i t h r o u g h l y N-S t r e n d i n g l e n g t h e n i n g . (BI
Compressional and e x t e n s i o n a l d i r e c t i o n s
deduced from f o c a l mechanisms of 11
e a r t h q u a k e s l o c a t e d under t h e t r a n s i t i o n a l
zone between P a c i f i c Lowlands and High
Andes (.data o b t a i n e d from a l o c a l s e i s m i c
network, depth f o c i between 1 2 and 40 km
Grange, 1983). E x t e n s i o n a l and
compressional q u a d r a n t s a r e superimposed
and t h u s d e f i n e common e x t e n s i o n a l and
compressional zones. A counting g r i d i s
b u i l t on t h e lower hemisphere w i t h p o i n t s
as c e n t e r s of c i r c l e s whose s u r f a c e s a r e
1%o f t h e hemispere s u r f a c e . Numbers on
s t e r e o n e t B r e p r e s e n t t h e number of t i m e s
such a d e f i n e d small c i r c l e i s common t o
t h e e x t e n s i o n a l zone d e f i n e d as above
( d e n s i t y diagram, Wulff s t e r e o n e t , lower
hemisphere). Ten f o c a l mechanisms d e f i n e
two s m a l l a r e a s (number 1 0 and shaded
a r e a s ) which r e s p e c t i v e l y c o n t a i n t h e
'common e x t e n s i o n a l d i r e c t i o n ( d i v e r g e n t
b l a c k arrows) and t h e common compressional
d i r e c t i o n s t r i k i n g roughly N105OE
(convergent b l a c k arrows). One f o c a l
mechanism (Grange's event 836) i s n o t i n
agreement with t h e o t h e r 1 0 s o l u t i o n s .
C o r d i l l e r a with r e s p e c t t o t h e High
P l a t e a u . S h o s h o n i t i c Quaternary volcanoes
( L e f e v r e , 1973) crop o u t along t h e
Vilcanota valley.
Evidences of t h e most r e c e n t f a u l t i n g
a r e l o c a t e d mainly 5-10 km west of t h e
V i l c a n o t a v a l l e y along a 70-km-long
l i n e a m e n t ( F i g u r e 8 a ) . This i s u n d e r l i n e d
by a s e r i e s of l a c u s t r i n e b a s i n s whose
a l t i t u d e s range between 3650 and 3950 m.
The most important of them a r e t h e
Pomacanchi l a k e t o t h e NW and t h e Langui
Layo l a k e t o t h e S E ( F i g u r e 8b). S e v e r a l
t i m e s , small h i s t o r i c a l e a r t h q u a k e s
( S i l g a d o , 1978) have d e s t r o y e d v i l l a g e s
a l o n g t h i s lineament ( c r o s s e s on F i g u r e
8b). Their f o c i were e s t i m a t e d t o be v e r y
shallow. NW o f t h e Pomacanchi l a k e , a 6km-long f a u l t t r a c e i s c l e a r l y seen i n t h e
f i e l d and on t h e a e r i a l photographs
(Suarez e t a l . , 1984 F i g u r e s 15 and 141,
and a l s o on Landsat imagery ( F i g y r e 8b).
T h i s Pomacanchi f a u l t has a N140 E s t r i k e
and a 60"-70" d i p t o t h e SW; i t e x i b i t s a
50-m normal .throw o f Quaternary age. It
c u t s a l l u v i a l conglomeratic f a n s r e l a t e d
t o t h e p e n u l t i m a t e g l a c i a l epoch ( n o t
a c c u r a t e l y d a t e d b u t more t h a n 0.1 m.y.1.
However , 'where streams c r o s s t h e f a u l t ,
t h e two l a s t a l l u v i a l t e r r a c e s do n o t show
any evidences of o f f s e t . Taking i n t o
a c c o u n t t h e accepted chronology of
a l l u v i a l formations i n s o u t h Peru ( S é b r i e r
e t al., 19821, t h e l a s t f a u l t movement i s
Recent, roughly between t h e l a s t and t h e
p e n u l t i m a t e g l a c i a l epochs, b u t does n o t
appear a c t i v e (i.e., l a s t movement o l d e r
t h a n 10,000 y e a r s B .P.
However, some
p a r t s of t h e V i l c a n o t a r i v e r f a u l t s y s t e m
must be a c t i v e as shown by t h e h i s t o r i c a l
s e i s m i c i t y and f r e s h s c a r p l e t a f f e c t i n g
t h e present-day morphology n o r t h of t h e
Langui v i l l a g e ( s t a r 11, F i g u r e 8 a ) .
Three s t r i a t e d f a u l t p l a n e s have been
measured on t h e s o u t h e a s t e r n p a r t of t h e
Pomacanchi f a u l t ( F i g u r e 1, p o i n t 10).
They show a normal s i n i s t r a l s t r i k e - s l i p
movement ( d a t a 10: 1, 2, 3, F i g u r e 4 , A).
These l i m i t e d d a t a do not p e r m i t
computation of t h e e x t e n s i o n a l d i r e c t i o n .
However, as o t h e r spread f a u l t d a t a of
s o u t h e r n P e r u , t h e y are i n agreement w i t h
a N-S t r e n d i n g e x t e n s i o n ( F i g u r e 4 , A).
The Mataro f a u l t s . Ten k i l o m e t e r s WNW
of t h e n o r t h w e s t e r n t e r m i n a t i o n of t h e
T i t i c a c a l a k e (Lago de Arapa), a t an
e l e v a t i o n of 3950 m ( F i g u r e 1, p o i n t 121,
t h e l a c u s t r i n e Mataro formation r e p r e s e n t s
d e p o s i t s of t h e lower P l e i s t o c e n e T i t i c a c a
l a k e (Lavenu e t al., 1984). %o small
>.
SQbrier e t al.:
7 50
TABLE 1.
Site
1
5
6-7
8
9
.
9a-b-c
9d
13
13b
13c
*
Quaternary Normal and Reverse F a u l t i n g
Parameters of t h e D e v i a t o r i c S t r e s s Tensors Computed From t h e Quaternary and
Recent Normal F a u l t s of t h e High Andes and P a c i f i c Lowlands
ND
Latitude
S
8'
16'
8'
12'
15'22'
16'11'
17'28'
15'381
13'30'
13'30'
13'301
1&'41'
14'411
14'411
19'
13'
6'
18'
6'
9'
Princioal Stress Directions
Longitude
a l
o 2
u 3
W
Azimuth Dip Azimuth Dip Azimuth Dip
75'091
72'02'
70'281
72'01'
71'56'
71'56'
71'56'
69'30'
69'301
69'301
115'
121'
115'
161'
324'
184'
321'
121'
170'
099'
84'
71'
75'
82'
8 O'
82'
74'
81'
80'
80'
282'
282'
266'
2 79'
o9 7'
287'
09 4'
265'
26 7'
267'
06'
18'
13'
04'
07'
02'
11'
07'
01'
10'
012'
014'
357'
009'
188'
018'
186'
356'
356'
357'
o1O
06'
O 7'
O 7'
O 7'
08'
11'
05'
10'
02'
R
0.79
O. 7 1
0.83
0.77
0.42
0-59
0.39
0.79
O , 77
0.76
These p a r a m e t e r s a r e computed from s u p e r f i c i a l Quaternary and Recent normal f a u l t s
o f P a c i f i c Lowlands ( s i t e s 1, 5 , and 6-71 and of High Andes ( s i t e s 8 , 9 , 9a-b-cy 9 d ,
1 3 , 13b, and 13c). S i t e l o c a l i z a t i o n s a r e shown o n F i g u r e 1. N D i s t h e number of
t r i a t e d f a u l t p l a n e s used t o compute t h e s o l u t i o n s . Azimuths a r e measured clockwise
from n o r t h ; d i p is toward the measured azimuth. R = a12-a'l/a'3-a11 i s t h e "stress
r a t i o " o r "shape f a c t o r 1 ' of t h e s t r e s s t e n s o r . I t s v a l u e v a r i e s between two extremes:
O as al2 = a l l and 1 a s d 2 = ~ 1 3 . Computed normal f a u l t s a r e on s t e r e o n e t s o f Figures
4 and 7.
f a u l t s have been observed on a 10-m-high
f l a n k of an i n t e r m i t t e n t stream, c u t i n t o
t h e Mataro formation. They have an E-W
s t r i k e and a 70" d i p t o t h e n o r t h w i t h a
10- t o 20-cm normal throw. They a r e
a n t i t h e t i c f a u l t s w i t h r e s p e c t t o major
f a u l t s bordering t h e l a k e T i t i c a c a t o t h e
NE. Although t h e y are s m a l l , t h e s e f a u l t s
may be i n d i c a t i v e of t h e Quaternary
deformation on t h e A l t i p l a n o because t h e y
a r e s i t u a t e d between t h e V i l c a n o t a r i v e r
f a u l t system and t h e B o l i v i a n A l t i p l a n o
Quaternary f a u l t s d e s c r i b e d by Lavenu
(1978). Moreover, t h e i r k i n e m a t i c s ( d a t a
1 2 : 1, 2 , F i g u r e 4 , A) a r e a l s o i n
agreement w i t h a N-S s t r i k i n g l e n t h e n i n g .
Quaternary Normal F a u l t i n g i n t h e
High Basins of t h e Eastern C o r d i l l e r a
----
North of Lake T i t i c a c a , s e v e r a l
Quaternary intermontane b a s i n s a r e
s i t u a t e d w i t h i n t h e c e n t r a l p a r t of t h e
E a s t e r n C o r d i l l e r a a t an e l e v a t i o n of
4000-5000 m ( F i g u r e 1, p o i n t s 1 3 , 1 4 , an
CA, CB, CR). Evidence of Quaternary normal
f a u l t i n g have been observed mainly in the
Ananea b a s i n ( F i g u r e 1, p o i n t 1 3 ) but a l s o
i n t h e Peruvian p a r t of t h e U l l a U l l a
b a s i n ( F i g u r e 1, p o i n t 1 4 ) .
The Ananea b a s i n f a u l t s . The WNW-ESE
t r e n d i n g Ananea b a s i n has a mean e l e v a t i o n
of 4600 m ( F i g u r e 1, p o i n t 1 3 ) . As f o r t h e
Altiplano, i t s general s t r u c t u r e r e s u l t s
mainly from Neogene compressional
deformations. The P l i o - P l e i s t o c e n e Ananea
d e p r e s s i o n i s superimposed on t h e ESE
t e r m i n a t i o n of a broad s y n c l i n o r i u m i n t h e
c e n t e r o f which Oligo-Miocene f o r m a t i o n s
c r o p o u t ; t h i s s y n c l i n o r i u m extends t o t h e
NW as f a r as t h e Crucero basin. This
Ananea b a s i n ( F i g u r e 9 ) i s i n f i l l e d by
P l i o c e n e p a l u s t r i n e sediments and by
s e v e r a l P l e i s t o c e n e moraines and outwash
d e p o s i t s ( F o r n a r i e t a l . , 1982).
I n t h i s b a s i n , w e measured Quaternary
normal f a u l t s a t t h r e e d i f f e r e n t p o i n t s
(13a, 13b, 13c on F i g u r e 9): t h r e e a t
Viscachani (13a), s i x a t Rinconada (13b) ,
and n i n e a t Pampa Blanca ( 1 3 ~ ) . Each of
t h e s e t h r e e p o i n t s i s less t h a n 5 km
d i s t a n t from each o t h e r . Normal f a u l t s
have been observed along d i s t a n c e s t h a t
r a n g e between s e v e r a l decameters t o
s e v e r a l hectometers. T h e i r throws v a r y
from s e v e r a l d e c i m e t e r s up t o 6 m. These
normal f a u l t s p o s t d a t e compressional
t e c t o n i c s which deformed P l i o c e n e and
e a r l y Q u a t e r n a r y d e p o s i t s . Moreover, some
o f them o f f s e t moraines which a r e r e l a t e d
t o the p e n u l t i m a t e g l a c i a l epoch ( o l d e r
The age of t h e
t h a n 100,000 y e a r s B.P.).
normal f a u l t i n g i s t h u s Quaternary. F a u l t s
having an E-W d i r e c t i o n a r e n e a r l y p u r e l y
72-00
,
Fig. 5a. S t r u c t u r a l s k e t c h of Cuzco b a s i n f a u l t s ( F i g u r e 1, p o i n t 9). 1, Recent Quaternary f o r m a t i o n s ; 2, e a r l y
P l e i s t o c e n e San S e b a s t i a n f o r m a t i o n ; 3 , Paleogene r e d b e d s ; 4, Cretaceous Yuncaypata formation. Roads: d o t t e d l i n e s .
Tambomachay a c t i v e normal f a u l t : t h i c k l i n e s (downthrown block h a t c h u r e d ) . F a u l t k i n e m a t i c s analyzed i n t h e v i c i n i t y of
t h e f a u l t ( s t a r s a , b , c ; s i t e s 9a-9c, F i g u r e 7) and n e a r San S e b a s t i a n (star d ; s i t e 9d, F i g u r e 7) show normal s l i p
w i t h a s m a l l . s i n i s t r a l component on t h e major f a u l t . North of San S e b a s t i a n , e a r l y P l e i s t o c e n e l a c u s t r i n e beds form
k i l o m e t r i c monoclines (arrows s t r i k e toward t h e h i g h e r d i p ) which r e s u l t from compressional deformation n o t p r e s e n t i n
Recent Q u a t e r n a r y d e p o s i t s .
1
S é b r i e r e t al.:
Q u a t e r n a r y Normal and Reverse F a u l t i n g
km
O
F i g . 5b. A e r i a l photograph c o v e r i n g c e n t r a l p a r t of Cuzco b a s i n ( s e e
F i g u r e 5 a ) . Most prominent s t r u c t u r e s are t h e two segments of t h e
Tambomachay a c t i v e f a u l t ( s m a l l b l a c k arrow h e a d s ) . Analyzed f a u l t s i t e s
a r e marked by s o l i d c i r c l e s a , b, d as o n F i g u r e 5a. TAM, Tambomachay; PU,
Puca P u c a r a Lncaic a r c h e o l o g i c a l s i t e s ; S. SEB, Cuzco suburb of San
S e b a s t i a n . NNE o f San S e b a s t i a n , e a r l y P l e i s t o c e n e formation appears as a
broad n e a r l y s y m e t r i c a n t i c l i n e .
normal; f a u l t s having a NE-SW d i r e c t i o n
a r e normal w i t h a d e x t r a l s t r i k e - s l i p
component, w h i l e f a u l t s s t r i k i n g NW-SE a r e
normal w i t h a s i n i s t r a l component ( F i g u r e
7, s i t e 13). These kinematics a g r e e with a
r o u g h l y N-S l e n g t h e n i n g . Computation of
the p r i n c i p a l stress d i r e c t i o n s confirms
t h i s f i r s t inference.
We f i r s t processed s e p a r a t e l y t h e d a t a
from Rinconada (13b) and Pampa Blanca
( 1 3 ~ )s i t e s ( F i g u r e 7). The numerical
r e s u l t s are good and comparable; t h e y ,
r e s p e c t i v e l y , show N4"W and N3"W t r e n d i n g
extensional principal s t r e s s d i r e c t i o n s
(13b and 1 3 c , T a b l e 1). Both c o m p u t a t i o n s
used few d a t a , and t h u s r e s u l t s a r e r a t h e r
p o o r l y c o n s t r a i n e d . We p r o c e s s e d
a l t o g e t h e r t h e 1 8 d a t a measured a t t h e
t h r e e s t u d i e d p o i n t s . This second
computation h a s been p o s s i b l e because a l l
t h e d a t a had been c o l l e c t e d i n a small
area and d i s p l a c e m e n t s a r e comparable i n
magnitude. The o b t a i n e d s o l u t i o n (13,
T a b l e 1) y i e l d s a N4"W t r e n d i n g e x t e n s i o n .
This s o l u t i o n i s of good q u a l i t y , t h e
( t , s ) a n g l e s b e i n g l e s s e r t h a n 15" f o r a l l
. -.
1-
S é b r i e r e t al.:
Q u a t e r n a r y Normal and Reverse F a u l t i n g
7 53
Fig. 6. F i e l d view of t h e Tambomachay f a u l t seen toward t h e n o r t h from
Puca P u c a r a ( l o c a t i o n : F i g u r e s 5a and 5b). A huge, south f a c i n g s c a r p
( a r r o w s ) dominates t h e 2-m-high s c a r p l e t ( s m a l l arrow heads). This
i n d i c a t e s t h a t t h e Tambomachay f a u l t has moved s e v e r a l t i m e s d u r i n g
P l e i s t o c e n e , producing a cumulative normal throw which i s about 300 m.
t h e f a u l t s (see h i s t o g r a m 13, F i g u r e 7).
I t i s s i m i l a r t o t h e 1 3 a and 13c s o l u t i o n s
( s e e Table 1). Thus i n t h e Ananea b a s i n
t h e Quaternary e x t e n s i o n appears t o have a
n e a r l y N-S d i r e c t i o n .
The Tuncuchi f a u l t s . The U l l a U l l a
b a s i n , s i t u a t e d a t a 4500-m e l e v a t i o n , h a s
a f l a t topography. However, n e a r Cerro
Tuncuchi ( F i g u r e 1, p o i n t 14) a meander of
t h e Suches r i v e r p e r m i t s a good
o b s e r v a t i o n of two f a u l t s on a 25-m-high
n a t u r a l s e c t i o n . These f a u l t s have a N-S
and N28"E s t r i k e , a s t e e p d i p t o t h e e a s t ,
and 0.5-lm
normal throws. They a f f e c t a
sandy conglomeratic a l l u v i a l t e r r a c e of
e a r l y Quaternary a g e and are covered by a
d i s t a l outwash t e r r a c e which i s n o t
o f f s e t . S t r i a t i o n s show normal d e x t r a l
s t r i k e - s l i p movements ( d a t a 14: 1, 2,
F i g u r e 4 , A). Although t h e s e f a u l t s a r e
seen on a small exposure, they seems t o b e
i n d i c a t i v e of t h e Quaternary normal
e x t e n s i o n i n t h e c e n t r a l p a r t of t h e
E a s t e r n C o r d i l l e r a s i n c e t h e y are i n good
agreement w i t h a N-S l e n t h e n i n g
demonstrated i n t h e nearby Ananea b a s i n .
Other examples o f Quaternary normal
f a u l t i n g i n t h e E a s t e r n C o r d i l l e r a may
i n c l u d e t h e N70°E t r e n d i n g f a u l t s t h a t c u t
moraine d e p o s i t s l o c a t e d t o t h e n o r t h of
t h e C o r d i l l e r a d e l Ausangate (Audebaud,
1973) and t h e NE edge of t h e N125'E
t r e n d i n g Ccatca b a s i n (CA and CB on F i g u r e
1). U n f o r t u n a t e l y , we have n o t been a b l e
t o c a r r y o u t f i e l d s t u d i e s on t h e s e
faults.
COMPRESSIONAL TECTONICS I N THE SUBANDES
From t h e snow-capped summits of t h e
n o r t h e a s t e r n edge of t h e High Andes,
toward t h e Amazonian f o o t h i l l s t h e
a l t i t u d e d e c r e a s e s markedly; more than 5
km of v e r t i c a l v a r i a t i o n i s observed a l o n g
a 40-km d i s t a n c e . The s t u n t e d v e g e t a t i o n
of t h e d r y High Andes i s r e p l a c e d by t h e
dense t r o p i c a l j u n g l e of t h e r a i n y subAndes. F i e l d c o n d i t i o n s become t h u s very
d i f f i c u l t , and u s e f u l o u t c r o p s e x i s t o n l y
a l o n g t h e t o r r e n t i a l r i v e r banks ( F i g u r e
7 54
S é b r i e r e t al.:
Quaternary Normal and Reverse F a u l t i n g
fib
N35 6'-10'
F i g . 7. Normal f a u l t d a t a o f t h e High Andes used t o compute s o l u t i o n s
of Table 1. See l o c a t i o n s on F i g u r e s 1, 5 a , and 9. Wulff s t e r e o n e t ,
lower hemisphere. Symbols as on F i g u r e 4 .
-
S é b r i e r e t al.:
755
Quaternary Normal and Reverse F a u l t i n g
72'
. i
71'
-
Cordillera
del
I4-
IL
-
'I
7;
7;
S t r u c t u r a l s k e t c h of t h e V i l c a n o t a r i v e r f a u l t system (High
F i g . 8a.
Andes, l o c a t i o n on f i g u r e 1, VF and p o i n t s 10 and 11). The r o u g h l y E-W
t r e n d i n g Anta b a s i n and Cuzco b a s i n f a u l t system i s r e l a y e d t o t h e SE by
t h e NNW-SSE s t r i k i n g V i l c a n o t a r i v e r f a u l t system. This f a u l t system
d i s a p p e a r s t o t h e SE a t t h e n o r t h w e s t e r n t e r m i n a t i o n of t h e A l t i p l a n o .
Thick s o l i d l i n e s a r e mapped normal f a u l t s ; dashed l i n e s are i n f e r r e d
Quaternary f a u l t s seen on L a n d s a t images. C r o s s e s , v i l l a g e s d e s t r o y e d by
s m a l l h i s t o r i c a l e a r t h q u a k e s ; shaded areas, snow-capped summits of t h e
E a s t e r n C o r d i l l e r a ; s t i p p l e d areas, P l i o - Q u a t e r n a r y b a s i n s ; s o l i d p a t c h e s ,
s h o s h o n i t i c Quaternary v o l c a n o e s ; a s t e r i s k , Quaternary v o l c a n i c cone.
O r i e n t a t i o n of t h e s k e t c h i s conform t o t h e Landsat image o r i e n t a t i o n of
F i g u r e 8b.
10). Geological change is a l s o
conspicuous, t h e E a s t e r n C o r d i l l e r a
c o n s i s t s mainly of Precambrian t o lower
P a l e o z o i c metamorphic and i n t r u s i v e r o c k s ,
whereas t h e sub-Andes a r e made o f Cenozoic
continental deposits (Figure 11).
The sub-Andean zone of s o u t h Peru i s
500 km long and about 50 km wide ( F i g u r e
1). It corresponds t o t h e n o r t h w e s t e r n
p a r t of t h e N120"E t r e n d i n g segment of t h e
sub-Andes which extends f o r 1200 km from
t h e lower Urubamba r i v e r (11" 3 0 ' s ) t o t h e
S a n t a Cruz ltelbow" (18"s). This narrow
b e l t has an a l t i t u d e which r a n g e s between
300 and 1000 m and can be subdivided i n
two p a r t s ( F i g u r e 1 1 ) : (1) The E a s t e r n
C o r d i l l e r a Piedmont b a s i n s ( P i l l c o p a t a ,
Quince M i l , Candamo) form a s t r i n g of
Neogene-Quaternary a l l u v i a l fan b a s i n s
l o c a t e d on a f a u l t zone l i m i t i n g t h e sub-
7 56
S6brier e t al.:
Q u a t e r n a r y Normal and Reverse F a u l t i n g
Fig. 8b. Landsat image MSS 7 (N. 8-2225- 14074-701) c o v e r i n g most of t h e
s t r u c t u r a l s k e t c h of t h e V i l c a n o t a r i v e r f a u l t system ( F i g u r e Ba). Snowcapped summits and Pomacanchi and Langui Layo l a k e s are r e a d i l y compared
w i t h t h e s t r u c t u r a l sketch. Cuzco b a s i n f a u l t s , V i l c a n o t a r i v e r f a u l t
system, and A l t i p l a n o n o r t h e r n b o r d e r f a u l t system a r e marked by small
arrow heads. S c a l e i s given on F i g u r e 8a.
Andes from t h e E a s t e r n C o r d i l l e r a . T h i s
f a u l t zone i s l o c a t e d a t t h e f o o t of t h e
High Andes, between 600 and 1000 m
a l t i t u d e . Deformation i s c h a r a c t e r i z e d by
r e v e r s e f a u l t i n g and t h r u s t i n g . ( 2 ) The
sub-Andean h i l l s ( S a l v a c i o n , Mazuko), a
30- t o 40-km-wide zone, a r e formed by
r i d g e s and v a l e s , t h e a l t i t u d e s of which
a r e between 300 and 800 m. Deformation i s
c h a r a c t e r i z e d by f o l d s and t h r u s t f a u l t s
a f f e c t i n g Neogene t o e a r l y Quaternary
sands and conglomerates. To t h e n o r t h , t h e
sub-Andean zone i s bounded by t h e Madre d e
Dios wide a l l u v i a l p l a i n which a p p e a r s t o
be t h e present-day a g g r a d a t i o n a l piedmont.
T h i s l i m i t between t h e sub-Andean h i l l s ,
where e r o s i o n p r e s e n t l y predominates, and
t h e a g g r a d a t i o n a l r i v e r p l a i n should mark
t h e a c t i v e sub-Andean t e c t o n i c f r o n t .
Owing t o u n f a v o r a b l e f i e l d c o n d i t i o n s ,
o n l y f o u r a r e a s (Quince M i l , Mazuko,
P i l l c o p a t a , and S a l v a c i o n ) have been
analyzed. Taking advantage of two r o a d s ,
w e have l e a c h e d t h e s o u t h e r n Peru sub-
Andes, and t h e n we have worked along t h e
t o r r e n t i a l r i v e r banks ( F i g u r e 1 0 ) .
E a s t e r n C o r d i l l e r a Piedmont b a s i n s :
Quince M i l and P i l l c o p a t a
---
The Quince M i l b a s i n i s s i t u a t e d a t
a l t i t u d e s r a n g i n g between 500 and 900 m ,
i s 40 km long and 15,km wide, and i s
i n f i l l e d by c o a l e s c i n g a l l u v i a l f a n s t h a t
l i e unconformably upon f o l d e d beds whose
ages a r e upper P a l e o z o i c , Cretaceous, and
Paleogene. These a l l u v i a l f a n s i n c l u d e two
s e r i e s : (1) an o l d e r one a t t r i b u t e d t o
P l i o c e n e - e a r l y Quaternary, and (2) a
younger one a t t r i b u t e d t o P l e i s t o c e n e and
Holocene (Laubacher e t a l . , 19821. The
o l d e r s e r i e s c o n s i s t s of some LO-m gray
carbonaceous sands p r e s e r v e d i n channels
and d i s c o r d a n t l y covered by about 250-m
t o r r e n t i a l conglomerates t h a t a r e f a i r l y
c o n s o l i d a t e d and weathered. The younger
s e r i e s c o n s i s t s of u n c o n s o l i d a t e d a l l u v i a l
f a n s and f l u v i a l t e r r a c e s . S e r i e s L h a s
Fig. 9. Geological s k e t c h of t h e e a s t e r n p a r t of t h e Ananea b a s i n ( E a s t e r n C o r d i l l e r a , p o i n t 1 3 , F i g u r e 1). 1,
Paleozoic f o r m a t i o n s ; 2 , P l i o c e n e Arcoaja f o r m a t i o n ; 3, e a r l y Quaternary moraines and outwash d e p o s i t s ; 4 , Recent and
upper Q u a t e r n a r y moraines; 5, Recent outwash d e p o s i t s ; 6, Recent a l l u v i a l f a n s . Quaternary normal f a u l t s have been
observed a t t h r e e d i f f e r e n t p l a c e s : a t Viscachani ( s t a r a ) , a t Rinconada (star b ) , and a t Pampa Blanca ( s t a r c ) .
7 58
Sébrier e t al.:
Quaternary Normal and Reverse F a u l t i n g
Fig. 10. E-W s t r i k i n g , s o u t h dipping r e v e r s e f a u l t along t h e Palcamayo
r i v e r , Q u i n c e M i l b a s i n ( s i t e 1 7 a., F i g- u r e 11). F a u l t ( t h i c k arrows) a f f e c t s
conglomerates of t h e Cancao formation, probably of e a r l y Q u a t e r n a r y age.
F a u l t kinematics a r e i n agreement with a N-S s h o r t e n i n g (17a, F i g u r e 1 2 ) .
been deformed by r e v e r s e f a u l t i n g , whereas
no deformations have been observed i n
s e r i e s 2. Thus t h e l a s t deformations t h a t
w e observed a r e n o t Recent and, as
d i s c u s s e d below, a r e probably of e a r l y
Quaternary age.
W
e measured r e v e r s e and s t r i k e - s l i p
f a u l t s a t two d i f f e r e n t p o i n t s : along t h e
Palcamayo r i v e r ( F i g u r e 1 2 , s i t e 17a) and
along t h e Huajiumbre r i v e r ( F i g u r e 1 2 ,
s i t e 17b). These two p o i n t s b e i n g 1.5 km
d i s t a n t . A t t h e Palcamayo s i t e , s t r i a t i o n s
a s s o c i a t e d w i t h r e v e r s e f a u l t s ( F i g u r e 10)
were measured on f l a t s i d e s of pebbles.
Owing t o t h e bad q u a l i t y o f Amazonian
f o r e s t o u t c r o p s , g e n e r a l l y it has n o t been
p o s s i b l e t o measure t h e throws of f a u l t s ,
b u t they can be e s t i m a t e d t o be of t h e
o r d e r of 1 m. A t both s i t e s , f a u l t s
s t r i k i n g E-W have a n e a r l y p u r e l y r e v e r s e
s l i p , f a u l t s s t r i k i n g NE-SW have a
s i n i s t r a l r e v e r s e s l i p , and f a u l t s
s t r i k i n g NW-SE have a d e x t r a l r e v e r s e s l i p
( F i g u r e 12, s i t e s L7a and 17b). These
kinematics a g r e e w i t h a roughly N-S
s h o r t e n i n g . , D e s p i t e t h e poor q u a l i t y of
t h e d a t a , computations of t h e p r i n c i p a l
s t r e s s d i r e c t i o n s (17a, 17b, Table 2) g i v e
good s o l u t i o n s ( s e e h i s t o g r a m s 1 7 a , 1 7 b ,
i
F i g u r e 1 2 ) and confirm t h i s i n f e r e n c e . The
maximum (compression) p r i n c i p a l d e v i a t o r i c
s t r e s s d i r e c t i o n s s t r i k e t o t h e N195"
( s i t e 17a) and N16" ( s i t e 17b). T h e r e f o r e
t h e l a s t f a u l t i n g observed i n t h e Quince
M i l b a s i n i s compressional and corresponds
t o a n e a r l y NNE t r e n d i n g s h o r t e n i n g .
These r e s u l t s a r e confirmed by
o b s e r v a t i o n s made i n t h e P i l l c o p a t a b a s i n ,
which i s l o c a t e d some 50 lun t o t h e WNW of
t h e Quince M i l b a s i n i n a s i m i l a r
s t r u c t u r a l s i t u a t i o n ( F i g u r e 11). It i s
s i t u a t e d a t an a l t i t u d e between 700 and
1100 m, and as t h e Quince M i l b a s i n , it i s
i n f i l l e d by c o a l e s c i n g a l l u v i a l f a n s . A t
Ubaldina, 2 lan south o f P i l l c o p a t a ( F i g u r e
11, s i t e 161, i n f l u v i a l conglomerates
s i m i l a r t o t h o s e of t h e o l d e r s e r i e s 1 o f
t h e Quince M i l b a s i n , we measured 12
s t r i a t i o n s a s s o c i a t e d with r e v e r s e f a u l t s
( F i g u r e 12, s i t e 1 6 ) . These s t r i a t i o n s
p o s t d a t e o l d e r ones which correspond t o
r o u g h l y E-W s h o r t e n i n g (M. S 6 b r i e r e t a l . ,
manuscript i n p r e p a r a t i o n , 1985). As i n
t h e Quince M i l b a s i n , t h e k i n e m a t i c s of
t h e l a s t f a u l t i n g t h a t w e observed a g r e e
w i t h a roughly N-S s h o r t e n i n g . Although
t h e s e d a t a are n o t e n t i r e l y i n agreement
w i t h t h e hypotheses of t h e numerical model
S é b r i e r e t al.:
759
Quaternary Normal and Reverse Fault'ing
. . . . . . . . . . . . . . . .
. . . . .
. . . . . . . .
. . . . . .
13
l.
.
o
1
1
4 - 6
-
,
,
,
-6
I
F i g . 11. S t r u c t u r a l s k e t c h showing s i t u a t i o n of t h e s t u d i e d Subandean
segment of s o u t h e r n Peru ( F i g u r e 1, p o i n t s 1 5 , 1 6 , 1 7 , 18). Two s t r u c t u r a l
s i t u a t i o n s are seen. The E a s t e r n C o r d i l l e r a Piedmont b a s i n s ( P i l l c o p a t a ,
Q u i n c e M i l , Candamo) a r e l o c a t e d on a r e v e r s e f a u l t zone ( t h i c k l i n e )
between 600 and 1000 m of a l t i t u d e . The Subandean h i l l s , between 300 and
800 m of a l t i t u d e ( S a l v a c i o n , Mazuko), a r e c h a r a c t e r i z e d by f o l d i n g and
high-angle t h r u s t i n g ( t h i c k lines w i t h a t t a c h e d s o l i d t r i a n g l e s ) . 1, Recent
Quaternary a l l u v i a l d e p o s i t s ; 2 , Neogene and e a r l y Quaternary f o r m a t i o n s ;
3, Paleogene red beds and upper Cretaceous s a n d s t o n e s ; 4 , Precambrian t o
lower Paleozoic metamorphic and i n t r u s i v e rocks of t h e E a s t e r n C o r d i l l e r a ;
5 , a n t i c l i n e ; 6 , s y n c l i n e . Numbered stars g i v e s i t u a t i o n of analyzed s i t e s
( F i g u r e 12). A l l deformations a r e compressional, r e s u l t i n g from N-S
trending shortening.
(see t h e appendix 1, t h e computed s o l u t i o n
( 1 6 , Table 2 ) i s f a i r l y good ( s e e
histogram 1 6 , F i g u r e 1 2 ) and g i v e s a N187"
s t r i k i n g maximum p r i n c i p a l s t r e s s (u 1).
Thus i n two piedmont b a s i n s of t h e E a s t e r n
C o r d i l l e r a t h e l a s t deformations t h a t w e
observed i n d i c a t e a roughly N-S t r e n d i n g
compression.
I
3
r'
The Sub-Andean H i l l s :
Mazuko and Salvacion
I
-_.
The E a s t e r n C o r d i l l e r a piedmont b a s i n s
are s e p a r a t e d from t h e sub-Andean h i l l s by
a small range of pre-Neogene rocks ( F i g u r e
11) t h a t i s e i t h e r a f a u l t e d a n t i c l i n o r i u m
(i.e., Puente Inambari a n t i c l i n e between
Quince M i l and Mazuko) or a complex
f a u l t e d and f o l d e d zone (i.e., Pongo de
Conec range between t h e P i l l c o p a t a b a s i n
and t h e S a l v a c i o n s y n c l i n e ) .
The Mazuko area i s l o c a t e d 2 0 km t o t h e
NE o f t h e Quince M i l b a s i n , along t h e
lower Inambari r i v e r , a t an a l t i t u d e t h a t
ranges between 300 and 650 m ( F i g u r e 11).
Upper Cretaceous t o e a r l y Quaternary
d e p o s i t s are s i m i l a r t o t h o s e of t h e
Quince M i l b a s i n excepted t h a t t h e Neogene
t o e a r l y Quaternary d e p o s i t s (Mazuko
formation) a r e t h i c k e r and do n o t r e s t
unconformably on t h e Paleogene r e d beds.
Thus t h e r e i s no c l e a r evidence f o r an
.
Sébrier e t al.:
TABLE 2.
P a r a m e t e r s of t h e D e v i a t o r i c S t r e s s Tensors Computed From Reverse S t r i k e - S l i p
Vector Gata of t h e South Peruvian Sub-Andes
Site
ND
Latitude
S
15
16
17a
21
12'50'
12' 55'
13'12'
17b
18a
18b
7 61
Q u a t e r n a r y Normal and Reverse F a u l t i n g
12
16
13
26
13
13'11'
13'05'
13'08'
Principal Stress Directions
Longitude
c1
a 2
a 3
W
Azimuth Dip Azimuth Dip Azimuth Dip
71'20'
71'23'
70'42'
70'42'
70'23'
70'23'
191'
187'
195'
016'
016'
003'
0.5'
02'
01'
08'
16'
08'
282'
277'
105'
284'
283'
094'
05'
04'
32'
10'
11'
05'
095'
071'
287'
145'
160'
215'
85'
86'
58'
77'
70'
80'
R
0.63
0.77
0.63
0.83
0.54
0.57
These p a r a m e t e r s are computed from t h e r e v e r s e s t r i k e - s l i p v e c t o r d a t a of t h e s o u t h
Peruvian sub-Andes. Same a b r e v i a t i o n s as on T a b l e 1. L o c a l i z a t i o n s of t h e s i t e s are
shown o n F i g u r e 1; d a t a are shown on s t e r e o n e t s of F i g u r e 12.
Oligo-Miocene compressional phase i n t h e
Mazuko area. The Neogene-early Quaternary
Mazuko f o r m a t i o n , a t l e a s t 3000 m t h i c k ,
i s f o l d e d . A s e t of f i v e o r s i x Quaternary
f l u v i a l t e r r r a c q s rests unconformably on
t h e l a t e r formation. We d i d n o t observe
any t r a c e of f a u l t i n g a f f e c t i n g t h e s e
t e r r a c e s . Thus, as i n Quince M i l b a s i n ,
t h e l a s t deformations are seen t o a f f e c t
d e p o s i t s which a r e a t t r i b u t e d t o e a r l y
Quaternary.
Dissymetric f o l d s a s s o c i a t e d w i t h
s t e e p l y dipping reverse f a u l t s s t r i k e
approximately N100" t o N 1 l O o E , and a f f e c t
t h e Mazuko conglomerates. Vergence is
toward the NE, i.e., toward t h e Amazonian
p l a i n . Two s i t e s have been a n a l y z e d : one
a l o n g Chiforongo r i v e r ( F i g u r e 11, s i t e
18b) and t h e o t h e r one a l o n g Dos d e Mayo
r i v e r ( F i g u r e 11, s i t e 1 8 a ) 2.5 km t o t h e
SSW and 3 lan t o t h e NNE o f Mazuko,
r e s p e c t i v e l y . A t Chiforongo, s t r i a t e d
r e v e r s e and s t r i k e - s l i p f a u l t s were
measured mainly i n t h e c l a y e d sands of t h e
Mazuko formation. A s i n o t h e r sub-Andean
l o c a l i t i e s , t h e bad q u a l i t y of o u t c r o p s
only p e r m i t s e s t i m a t i o n of t h e f a u l t
throws which a r e g e n e r a l l y of t h e o r d e r of
1 m. A t Dos d e Mayo, s t r i a t i o n s were
measured on f l a t s i d e s of cobbles w i t h i n a
20" dipping f o l d limb. They might o n l y
r e f l e c t l o c a l deformation w i t h i n t h i s f o l d
limb w i t h o u t any r e g i o n a l s i g n i f i c a n c e .
4
Fig. 12. ( o p p o s i t e ) Reverse s t r i k e - s l i p
v e c t o r . d a t a from sub-Andes of s o u t h Peru
used t o compute s o l u t i o n s of T a b l e 2 ( s i t e
s i t u a t i o n s on F i g u r e s 1 and 11). Wulff
s t e r e o n e t , lower hemisphere. Same symbols
as on F i g u r e 4.
However, a t both s i t e s , r e v e r s e and
s t r i k e - s l i p movements are such t h a t t h e y
a g r e e w i t h roughly N-S s h o r t e n i n g ( F i g u r e
1 2 , . s i t e s 18a and 18b). Despite t h e poor
q u a l i t y of t h e d a t a , w i t h r e s p e c t t o t h e
model hypotheses, computation g i v e s
s o l u t i o n s of f a i r l y good q u a l i t y (see
h i s t o g r a m s 18a and 1 8 b , F i g u r e 12) w i t h a
N3" t r e n d i n g compression a t Chiforongo
(18b, T a b l e 2) and a N16" t r e n d i n g
compression a t Dos de Mayo (18a, T a b l e 2).
Thus, i n t h e sub-Andean f o l d s t h e l a s t
d e f o r m a t i o n s observed a r e compressional
and r e s u l t from r o u g h l y NyS s h o r t e n i n g .
Another example of f a u l t i n g a s s o c i a t e d
w i t h sub-Andean f o l d s i s v i s i b l e i n t h e
NW-SE s t r i k i n g S a l v a c i o n s y n c l i n e ( F i g u r e
1 0 , p o i n t 15). It i s l o c a t e d some 100 km
t o t h e WNW of Mazuko and on t h e ' NE s i d e o f
t h e Pongo d e Conec f a u l t e d range t h a t
s e p a r a t e s it from t h e P i l l c c p a t a b a s i n .
T h i s Salvacion s y n c l i n e c o n s i s t s of L a t e
Cretaceous t o Paleogene r e d beds
conformably covered by gray-brown c l a y e d
s a n d s t h a t are i n t e r b e d d e d upward w i t h
conglomerates. These sands and
conglomerates are v e r y s i m i l a r t o t h e
Mazuko formation (Laubacher e t a l . , 1982)
and a r e probably a l s o of Neogene age. The
uppermost p a r t of t h e Salvacion s y n c l i n e
c o n s i s t s of f l u v i a l conglomerates t h a t
could be e q u i v a l e n t t o t h e upper
conglomerates of t h e Mazuko formation and
t h u s could be of e a r l y Quaternary age. No
unconformity h a s been observed i n t h e
Salvacion s y n c l i n e . N e v e r t h e l e s s , owing t o
t h e l a c k of good o u t c r o p s , t h e r e l a t i o n s
between t h e uppermost conglomerates and
t h e Neogene sands below have n o t been
c l e a r l y observed. Strong d i f f e r e n c e s o f
d i p of t h e two f o r m a t i o n s could i n d i c a t e
Sébrier e t al.:
7 62
Quaternary Normal and Reverse F a u l t i n g
70'30
1734
njo
I
?Ojd
Fig. 13. L o c a t i o n of t h e Chulibaya a c t i v e normal f a u l t ( F i g u r e 1, p o i n t
7). F a u l t belongs t o t h e Incapuquio f a u l t zone ( I F o n F i g u r e 11, s e p a r a t i n g
the High Andes from t h e P a c i f i c Lowlands. R e a c t i v a t i o n of t h e f a u l t c r e a t e d
a south f a c i n g s c a r p l e t ( t h i c k l i n e hatchured toward t h e downthrown block,
s e e F i g u r e 1 4 ) . Chulibava s i t e (6-7, F i g u r e 4 ) i s marked by a star. Arrow
head i n d i c a t e s l o c a t i o n ' o f F i g u r e 14.
an unconformity between them. Whatever t h e
case, t h e deformations of t h e s e uppermost
conglomerates are t h e youngest t h a t we
observed i n t h i s a r e a s i n c e no evidence of
deformation has been observed i n t h e
Quaternary f l u v i a l t e r r a c e s t h a t l i e
unconformably on t h e Salvacion s y n c l i n e .
Upstream from S a l v a c i o n v i l l a g e , along
t h e r i v e r , s t r i a t i o n s on f l a t s i d e s of
c o b b l e s of the uppermost conglomerates
have been measured w i t h i n a 20" d i p p i n g
f o l d limb ( F i g u r e 12, s i t e 15). A s i n t h e
o t h e r sub-Andean l o c a l i t i e s , t h e s e d a t a
a g r e e w i t h roughly N-S t r e n d i n g
s h o r t e n i n g . Bearing i n mind t h e same
r e s t r i c t i o n s as f o r t h e Dos de Mayo s i t e ,
we processed the d a t a with t h e numerical
method. The o b t a i n e d s o l u t i o n ( s i t e 1 5 ,
T a b l e 2) i s of good q u a l i t y ( s e e h i s t o g r a m
15, F i g u r e 1 2 ) and g i v e s a N191" t r e n d i n g
compression.
S l i p movements deduced from S t r i a t i o n s
have been measured i n s i x sub-Andean
l o c a l i t i e s situated i n various s t r u c t u r a l
s e t t i n g s : reverse faulting, t h r u s t i n g
a s s o c i a t e d w i t h f o l d i n g , and d i s c o n t i n u o u s
deformation of f o l d limbs. A l l evidence a
n e a r l y N-S t r e n d i n g compression.
U n f o r t u n a t e l y , the age of t h i s N-S
t r e n d i n g compression i s n o t v e r y p r e c i s e l y
c o n s t r a i n e d . According t o Laubacher e t a l .
(1982) it a f f e c t s e a r l y Quaternary
d e p o s i t s . The Quaternary a l l u v i a l t e r r a c e s
c o v e r i n g them unconformably do n o t show
any evidence of deformation. It i s not
known whether compressional deformation i s
s t i l l a c t i v e , s i n c e no f o c a l mechanisms
a r e a v a i l a b l e i n t h i s p a r t i c u l a r south
Peru segment of t h e sub-Andes. However, i t
i s l i k e l y t h a t t h i s N-S t r e n d i n g
compression i s of e a r l y Quaternary age, a s
i t i s w e l l known i n t h e whole High Andes
and P a c i f i c Lowlands o f Peru and B o l i v i a
( S é b r i e r e t a l . , 1982). This i s a l s o
s t r o n g l y supported by fo'cal mechanisms
which, evidence a present-day rdughly E-W
d i r e c t i o n of compression in t h e sub-Andes
of c e n t r a l P e r u , s o u t h B o l i v i a , and NW
Argentina.
EXTENSIONAL TECTONICS IN THE PACIFIC
LOWLANDS
F a u l t Zone S e p a r a t i n g
-From P a c i f i c Lowlands
High Andes
From t h e P a c i f i c Piedmont ( P a c i f i c
Lowlands) t o t h e Western C o r d i l l e r a , t h e
S é b r i e r e t al.:
.e
..-
Q u a t e r n a r y Normal and Reverse F a u l t i n g
7 63
F i g . 14. Western p a r t from the s o u t h f a c i n g s c a r p l e t (arrow) of t h e a c t i v e
Chulibaya normal f a u l t o f f s e t i n g up t o 2.5 m t h e topographic s l o p e p a r t l y
covered by s c r e e s of Holocene age ( l o c a t i o n on F i g u r e 13).
e l e v a t i o n g e n e r a l l y i n c r e a s e s from about
2000 m up t o more t h a n 4000 m along a 20km d i s t a n c e . This t r a n s i t i o n a l t o p o g r a p h i c
zone o f t e n c o i n c i d e s w i t h a f r a c t u r e d
b e l t , f a u l t s having g e n e r a l l y a high a n g l e
d i p . This f a u l t b e l t r e s u l t s from a long
t e c t o n i c h i s t o r y beginning i n t h e Upper
Cretaceous. T h r u s t s and r e v e r s e , s t r i k e s l i p , and normal f a u l t s are superimposed
( V i c e n t e e t al., 1979; S é b r i e r e t al.,
1979, 1982). Here, w e focused our s t u d y o n
t h e a c t i v e movements and on t h e f i n a l
kinematics of t h e f a u l t s observed
r e s p e c t i v e l y i n t h e r e g i o n of Chulibaya,
SE o f Arequipa ( F i g u r e 1, p o i n t s 6 and 7) ,
and of L l u c l l a , NW o f Arequipa ( F i g u r e 1,
p o i n t 5).
The Chulibaya a c t i v e normal f a u l t s .
The Chulibaya a c t i v e normal f a u l t ( F i g u r e
1, p o i n t 7 ) belongs t o t h e f a u l t b e l t ,
l o c a l l y known as t h e Incapuquio f a u l t zone
( I F on F i g u r e 1). It i s l o c a t e d upstream
from t h e Locumba r i v e r , about 2 km s o u t h
I:
o f t h e Chulibaya hamlet ( F i g u r e 13). The
f a u l t a f f e c t s t h e Paleocene Toquepala
v o l c a n i c formation. I t s t r i k e s N80°E t o
N120"E and d i p s 70" t o t h e s o u t h .
R e a c t i v a t i o n of t h e f a u l t c r e a t e d a
south f a c i n g s c a r p l e t ( F i g u r e 14) which
o f f s e t up t o 2 . 5 m t h e topographic s l o p e
p a r t l y covered by s c r e e s o f Holocene age.
S c a r p l e t and o p p o s i t e d i p p i n g s l o p e form a
narrow furrow. In some p l a c e s , it has been
f i l l e d , p r o b a b l y d u r i n g an e a r t h q u a k e , by
screes which do n o t have d e s e r t v a r n i s h
comparable w i t h t h e s u p e r f i c i a l s c r e e s . I n
o t h e r p l a c e s , t h i s furrow is u n d e r l i n e d by
puddles of d r y mud which has been s u p p l i e d
by t h e l a s t d e s e r t r a i n f a l l . I n t h e
P e r u v i a n d e s e r t t h e p e r i o d i c i t y of major
r a i n f a l l s i s about one o r two p e r c e n t u r y
( L e t t a u and L e t t a u , 1978). I n t h e R i o
Curibaya v a l l e y ( F i g u r e 131, i n h a b i t a n t s
a s s e r t t h a t l a s t heavy shower o c c u r r e d
approximately i n 1948. They do n o t
remember any major s e i s m i c event p r i o r t o
7 64
Sébrier e t al.:
t h i s d a t e , we assume t h a t t h e Chulibaya
normal f a u l t has n o t been r e a c t i v a t e d
s i n c e a t l e a s t one c e n t u r y .
Along t h e Chulibaya f a u l t w e have
observed only t h r e e s t r i a t e d s l i c k e n s i d e s
( d a t a 1, 2 , 3 o n s t e r e o n e t 6-7, F i g u r e 4 ) .
They confirm t h e n e a r l y pure normal
component on f a u l t s having a E-W
d i r e c t i o n . A t Cerro Garita ( F i g u r e 1,
p o i n t 6 ) nearby Toquepala, 30 km t o t h e
WNW of Chulibaya, we observed f i v e
a d d i t i o n a l normal f a u l t s t h e s t r i a t i o n s of
which correspond t o t h e l a s t movement
along t h e Incapuquio f a u l t zone ( d a t a 5-8
on s t e r e o n e t 6-7, F i g u r e 4 ) . Kinematics o f
most of t h e f a u l t s show a N-S lengthening.
The Chulibaya and Cerro G a r i t a f a u l t s are
r e g i o n a l f a u l t s of a comparable magnitude,
and we have assumed t h e r e f o r e t h a t t h e
deformation, i n t h e r e g i o n under
c o n s i d e r a t i o n , i s homogeneous enough t o
p r o c e s s t h e d a t a t o g e t h e r . The o b t a i n e d
s o l u t i o n (6-7, Table 1 ) g i v e s a N3'W
s t r i k i n g e x t e n s i o n u 3. This s o l u t i o n i s
of good q u a l i t y f o r t h e two s e t s of d a t a ,
t h e a n g l e s ( t , S ) being l e s s e r than 10" f o r
a l l t h e d a t a (see histogram 6-7, F i g u r e
4 ) . However, t h e obtained s o l u t i o n i s n o t
s t r o n g l y cons t r a i n e d due t o t h e
o r i e n t a t i o n s of t h e f a u l t p l a n e s which are
n o t w e l l d i s t r i b u t e d . The kinematics o f
t h e a c t i v e normal Chulibaya f a u l t i s i n
agreement
with a NS t r e n d i n g- extension.
The normal movement on t h e L l u c l l a
f a u l t . The L l u c l l a f a u l t ( F i g u r e 1, p o i n t
l o c a t e d i n t h e Rio Sihuas v a l l e y
_, . 60
km t o t h e WNW of t h e c i t y o f Arequipa. It
l i m i t s t h e Precambrian rocks of t h e
Western C o r d i l l e r a from t h e Oligo-Miocene
c o n t i n e n t a l d e p o s i t s of t h e P a c i f i c
Piedmont ( F i g u r e 1 5 ) . This f a u l t zone,
which occupies t h e same s t r u c t u r a l
s i t u a t i o n as t h a t of t h e Incapuquio f a u l t
zone ( F i g u r e 1, p o i n t s 6 and 7 1 , a l s o
e x h i b i t s a complex t e c t o n i c h i s t o r y . On
t h e major f a u l t p l a n e s t h e most important
movements r e s u l t from r e v e r s e s t r i k e - s l i p
or r e v e r s e s l i p . Normal and normal s t r i k e
s l i p s t r i a t i o n s ( F i g u r e 4, s i t e 5 )
p o s t d a t e t h e above compressional
deformations and appear t o r e s u l t from t h e
l a s t period of a c t i v i t y of t h e f a u l t zone.
Unfortunately, a t L l u c l l a t h i s e x t e n s i o n a l
t e c t o n i c s is n o t p r e c i s e l y dated. I t i s
younger than t h e compressional s t r u c t u r e s
which deformed t h e Oligo-Miocene d e p o s i t s
(Moquegua formation). On t h e o t h e r hand,
t h e l a c k of morphological s c a r p l e t seems
t o indicate t h a t it is p r i o r t o t h e
Holocene. Thus t h i s e x t e n s i o n a l t e c t o n i c s
I
...
-
Quaternary Normal and Reverse F a u l t i n g
could be o f P l i o c e n e - P l e i s t o c e n e age.
The 1 6 normal s t r i a t i o n s of t h e
u l t i m a t e p e r i o d of a c t i v i t y a r e i n
agreement w i t h a N-S l e n g t h e n i n g ( F i g u r e
4, s i t e 5). Computation has been performed
w i t h t h e s e data. The o b t a i n e d s o l u t i o n ( 5 ,
Table 1) g i v e s a N 1 4 " E s t r i k i n g
"extension" u 3 ; t h i s s o l u t i o n i s of good
q u a l i t y , a s 1 5 s t r i a t i o n s have ( t , S )
a n g l e lesser than 20" ( s e e histogram 5 ,
Figure 4).
Thus t h e youngest r e a c t i v a t i o n s of t h e
i i u c l i a f a u l t and of t h e Lncapuquio f a u l t
zone, both of which l i m i t t h e Western
C o r d i l l e r a from t h e P a c i f i c Lowlands,
appear t o r e s u l t from a N-S t r e n d i n g
e x t e n s i o n a l t e c t o n i c s . The N140 "E
t r e n d i n g , 70" s o u t h d i p p i n g Pampacolca
f a u l t , l o c a t e d 60 km t o t h e Nw of L l u c l l a
on t h e same f a u l t b e l t (PF on Figure 11,
has probably been r e a c t i v a t e d by t h i s N-S
e x t e n s i o n as it o f f s e t s e a r l y Quaternary
a l l u v i a l f a n s and shows a normal throw of
some 20 m. However, i t s k i n e m a t i c s has n o t
been s t u d i e d i n more d e t a i l due t o
unfavorable outcrops.
The Normal F a u l t s Along
-----
t h e P a c i f i c Coast
The c o a s t a l area of south Peru l i e s
above a zone of high s e i s m i c i t y which
o c c u r s e i t h e r a t t h e c o n t a c t between t h e
oceanic and c o n t i n e n t a l p l a t e s o r w i t h i n
t h e oceanic s l a b . I n c o n t r a s t , few seismic
f o c i a r e c l e a r l y located within t h e
c o n t i n e n t a l wedge of t h e o v e r r i d i n g p l a t e ,
and they have not provided f o c a l
mechanisms. The i n t e r p l a t e earthquakes a r e
c h a r a c t e r i z e d by t h r u s t-type f o c a l
mechanisms, whereas t h e f o c i l o c a t e d
w i t h i n t h e oceanic s l a b show normal
f a u l t i n g mechanisms ( S t a u d e r , 1973, 1975;
Isacks and Barazangi, 1977; Malgrange e t
al., 1981; Chinn and I s a c k s , 1983).
Microseismic d a t a o b t a i n e d from l o c a l
networks do n o t i n d i c a t e such a simp1.e
p a t t e r n (Grange e t a l . , 1984b), normal,
r e v e r s e , and s t r i k e - s l i p f a u l t s n o t being
c l e a r l y s e p a r a t e d i n space. Moreover, a
double-layered Benioff zone i s n o t
observed i n s o u t h e r n Peru (Bevis and
I s a c k s , 1984).
Many Q u a t e r n a r y f a u l t s with normal
throws have been r e p o r t e d a l l along t h e
P a c i f i c c o a s t (Lavenu and S o u l a s , 1976).
Our o b s e r v a t i o n s confirm t h a t t h e
Quaternary and Recent t e c t o n i c s i s
c h a r a c t e r i z e d by normal f a u l t s whose
extension i s of small magnitude, throws
being always l e s s t h a n few meters. We have
.
1
7200
F i g . 15. S t r u c t u r a l s k e t c h of t h e L l u c l l a f a u l t ( F i g u r e 1, p o i n t 5). Normal movements of t h e f a u l t ( h a t c h u r e d toward
t h e downthrown b l o c k ) , p o s t d a t i n g compressional d e f o r m a t i o n s , a r e n o t p r e c i s e l y dated b u t could be o f P l i o - P l e i s t o c e n e
age. 1, P l i o - P l e i s t o c e n e c o n t i n e n t a l d e p o s i t s ; 2, Oligo-Miocene d e p o s i t s (Moquegua f o r m a t i o n ) ; 3 , Precambrian rocks of
t h e Western C o r d i l l e r a i n t r u d e d by t h e c o a s t a l b a t h o l i t h . S o l i d s t a r (number 5) i n d i c a t e s t h e L l u c l l a s i t e .
U
m
VI
.
7 66
S6brier e t al.:
Q u a t e r n a r y Normal and Reverse F a u l t i n g
Fig. 16. Near San Juan de Marcona ( p o i n t l a , F i g u r e I), Quaternary marine
t e r r a c e (QI o v e r l i e s unconformably P l i o c e n e marine b e d s (Pl. Photograph
shows a N120"E s t r i k i n g normal f a u l t (F, normal movement i n d i c a t e d by t h i c k
arrows) w i t h a s m a l l 0.5-m downthrow (see datum 1 on s t e r e o n e t 1, F i g u r e
4 ) . The s t e e p , or s l i g h t l y o v e r t u r n e d , d i p of t h e curved f a u l t p l a n e seen
near t h e s u r f a c e i s r e l a t e d t o t h e upward d e c r e a s e of t h e v e r t i c a l load i n
p o o r l y c o n s o l i d a t e d sediments. T h i s g i v e s an apparent r e v e r s e f a u l t
geometry (F with t h i n arrows) on t h e s u r f a c e ( n o r t h r i g h t ) .
a l s o observed some r e v e r s e f a u l t s , b u t
t h e y a r e g e n e r a l l y due t o e a r l y Quaternary
compression. However, some small r e v e r s e
f a u l t s w i t h o f f s e t s l e s s t h a n few meters
a f f e c t middle Quaternary formations n e a r
Tacna and Chincha ( F i g u r e 1).
U n f o r t u n a t e l y , f i e l d evidence i s n o t
c l e a r l y demonstrative. I f they were of
t e c t o n i c o r i g i n , t h i s should mean t h a t a t
t h e s u r f a c e along t h e c o a s t t h e Recent
s t a t e of stress f l u c t u a t e s around a
n e u t r a l p o s i t i o n b u t with a p r e f e r e n t i a l
e x t e n s i o n a l regime of small magnitude.
Although Q u a t e r n a r y normal f a u l t s have
been r e p o r t e d i n many p l a c e s along t h e
P a c i f i c c o a s t (Lavenu and S o u l a s , 19761,
S é b r i e r e t al.:
s t r i a t e d f a u l t p l a n e s have been observed
o n l y a t f o u r l o c a l i t i e s : Marcona ( p o i n t
I), Chala ( p o i n t 21, C a l a v e r i t a s ( p o i n t
3 ) , and La Planchada ( p o i n t 4 , F i g u r e 1).
Normal f a u l t i n g a l o n g t h e P a c i f i c c o a s t
o p p o s i t e t h e Nazca r i d g e ( t h e Marcona
a r e a ) . The Marcona a r e a i s l o c a t e d on t h e
P a c i f i c c o a s t , o p p o s i t e t h e a s e i s m i c Nazca
r i d g e ( F i g u r e 1, p o i n t 1). A s e t o f
Quaternary stepped marine t e r r a c e s i s w e l l
devsloped i n t h i s area, t h e o l d e s t b e i n g
a t an a l t i t u d e of 700 m ( L e g a u l t , 1963).
They l i e unconformably on Pliocene marine
beds ( S é b r i e r and de Muizon, 1982). I n
t h i s a r e a t h e most r e c e n t f a u l t s a r e
normal w i t h throws ranging between s e v e r a l
decimeters and s e v e r a l meters. They
p o s t d a t e compressional deformations which
a f f e c t t h e P l i o c e n e marine beds. Moreover ,
some of them o f f s e t Quaternary marine
t e r r a c e s ( F i g u r e 16). Unfortunately, owing
t o t h e sandy n a t u r e of t h e m a t e r i a l , t h e
normal f a u l t p l a n e s do not e x h i b i t
s t r i a t i o n s t h a t could i n d i c a t e t h e s l i p
vectors.
I n t h e Marcona a r e a w e observed o n l y
e i g h t normal s t r i a t e d f a u l t planes. Two
were measured a t San Juan de Marcona
h a r b o r ( F i g u r e 1, p o i n t l a ) where a f r e s h
r o a d c u t shows normal f a u l t s o f f s e t i n g a
marine, upper P l e i s t o c e n e t e r r a c e . F i v e
o t h e r s l i c k e n s i d e s were measured a t A l t o
Grande ( F i g u r e 1, p o i n t l b ) , S O km t o t h e
ESE of San J u a n and one a t Aguada de Lomas
( F i g u r e 1, p o i n t I C ) , 7 km SE o f A l t o
Grande. They show c l e a r l y a n o r t h :o NE
s t r i k i n g l e n t h e n i n g ( F i g u r e 4 , s i t e 1).
T h i s i s i n agreement w i t h t h e occurrence
i n t h e Marcona a r e a of numerous
n o n s t r i a t e d Quaternary normal f a u l t s which
have a r o u g h l y E-W d i r e c t i o n (Lavenu and
Soulas , 1976).
A poorly c o n s t r a i n e d r e s u l t should b e
expected from computation of t h e s e e i g h t
measured s l i c k e n s i d e s s i n c e d a t a are few
and spread over a l a r g e area. However, we
have processed them assuming t h a t t h e
t e n s i o n a l deformation i s s u f f i c i e n t l y
homogeneous i n view of t h e s c a l e of t h e
s t u d y and because of t h e widespread E-W
normal f a u l t s which occur over t h e whole
a r e a . The o b t a i n e d s o l u t i o n (1, T a b l e 1
and F i g u r e 4 ) g i v e s a N12" s t r i k i n g
Ifextensiontf 6 3 . I n t h e Marcona area t h e
youngest Quaternary t e c t o n i c s i s
e x t e n s i o n a l and i s c h a r a c t e r i z e d , as i n
o t h e r s o u t h Peruvian l o c a l i t i e s , by a
roughly N-S t e n s i o n a l d i r e c t i o n .
Normal f a u l t i n g on t h e P a c i f i c c o a s t
s o u t h of Nazca area.
South of 1 5 " 4 0 ' S ,
'
7 67
Quaternary Normal and Reverse F a u l t i n g
t h e C o a s t a l C o r d i l l e r a 1000- t o 1300-mh i g h s c a r p f a c e s t h e P a c i f i c Ocean. This
south f a c i n g major s c a r p i s t h e r e s u l t o f
movements on a major f a u l t zone l i m i t i n g
t h e marine f o r e a r c b a s i n s from t h e P a c i f i c
Lowlands. This f a u l t zone does n o t o u t c r o p
c l e a r l y on land and seems t o be mainly
l o c a t e d o f f s h o r e . The c o a s t a l s c a r p i s
notched by marine t e r r a c e s of upper
Oligocene t o upper Quaternary age ( S é b r i e r
e t a l . , 1979; Huaman, 19851, and t h i s
k i l o m e t r i c s c a r p is mainly i n h e r i t e d from
t h e O1 ig o -Mio c ene t e c t o n i c s
U n f o r t u n a t e l y , T e r t i a r y , and moreover
Quaternary, d e p o s i t s a r e s c a r c e so t h a t
Q u a t e r n a r y movements on t h e abovementioned f a u l t zone are not p r e c i s e l y
known; it can only be s t r e s s e d t h a t a l l
t h e observed f a u l t s show small
displacements.
The Chala f a u l t s : Chala Bay ( F i g u r e 1,
p o i n t 2) i s l o c a t e d about 100 km ESE of
San Juan d e Marcona. A set o f Quaternary
stepped marine t e r r a c e s , t h e h i g h e s t
r e a c h i n g an a l t i t u d e of 250 m y o u t c r o p s
around- t h e bay ( L a h a r i e , 1970). They l i e
unconformably on T e r t i a r y marine beds. A s
i n t h e Marcona area, t h e youngest
Q u a t e r n a r y deformation t h a t we observed i s
c h a r a c t e r i z e d by small normal f a u l t s ,
whose p l a n e s g e n e r a l l y do n o t e x h i b i t
s t r i a t i o n s . We measured o n l y t h r e e
s t r i a t e d s l i c k e n s i d e s t h a t c u t upper
P l i o c e n e t o e a r l y Quaternary sandy c l a y
marine d e p o s i t s l o c a t e d on t h e western
p a r t of Chala Bay. They i n c l u d e a SW block
downthrown s e v e r a l decimeters and show a
normal s i n i s t r a l movement ( d a t a 2: 1, 2 ,
3, F i g u r e 4 , A). These t h r e e d a t a do n o t
permit computation of t h e e x t e n s i o n a l
d i r e c t i o n b u t a r e i n agreement w i t h a
roughly N-S lengthening.
The C a l a v e r i t a s and L a Planchada
f a u K : The upper P l i o c e n e marine beds of
t h e La Planchada formation outcrou on
b o t h s i d e s of t h e mouth of t h e Rio
Pescadores ( F i g u r e 1, p o i n t s 3 and 4 )
(Beaudet e t a l . , 1976). These formations
e x h i b i t s e v e r a l superimposed deformations:
(1) normal f a u l t s contemporaneous w i t h
Pliocene sedimentation, (2) compressional
d e f o r m a t i o n s , and ( 3 ) normal f a u l t s which
c u t deformations 1 and 2. A t C a l a v e r i t a s
( F i g u r e 1, p o i n t 3 ) and L a Planchada
( F i g u r e 1, p o i n t 4 ) we have measured two
s t r i a t e d normal f a u l t p l a n e s ( d a t a 3-1 and
4-1, F i g u r e 4 , A) whose NE block i s
downthrown s e v e r a l decimeters. Although
t h e s e are small, they show t h a t i n t h i s
a r e a , kinematics of t h e Quaternary normal
.
---
--
7 68
S 6 b r i e r e t al.:
Q u a t e r n a r y Normal and R e v e r s e F a u l t i n g
S é b r i e r e t al.:
Quaternary Normal and Reverse F a u l t i n g
7 69
f a u l t s i s i n agreement with t h e g e n e r a l
N-S lenghening ( F i g u r e 4 , A) v i s i b l e i n
t h e P a c i f i c Lowlands.
QUATERNARY AND RECENT STATE OF STRESS
I N THE ANDES OF SOUTHERN PERU
Q u a t e r n a r y F a u l t i n g , F o c a l Mechanisms, @
Andes of Southern
S t a t e of Stress i n
------
LI
I C
m
u
Within t h e Andes of s o u t h e r n P e r u ,
B o l i v i a , and n o r t h Argentina-Chile , which
are s i t u a t e d above a 30" e a s t d i p p i n g
s l a b , s e i s m i c i t y i s moderately a c t i v e
a l o n g t h e sub-Andes. No f o c a l mechanisms
have been p u b l i s h e d f o r t h e s o u t h P e r u v i a n
sub-Andes. Sub-Andes of s o u t h Peru are
c l e a r l y a f f e c t e d by compressional
t e c t o n i c s (Laubacher e t a l . , 1982).
S t r u c t u r a l a n a l y s i s of f o l d s and f a u l t s
a f f e c t i n g e a r l y Quaternary a l l u v i a l
d e p o s i t s shows a roughly N-S s h o r t e n i n g
( F i g u r e s 1 2 and 1 7 ) . Is t h i s N-S
s h o r t e n i n g t h e r e s u l t of t h e e a r l y
Q u a t e r n a r y compression as known i n many
p l a c e s of t h e Andes o f Peru ( S é b r i e r e t
al., 19821, o r i s i t t h e d i r e c t i o n o f
.
l a t e Quaternary-Recent s h o r t e n i n g , t h e
s t r i k e of which would have been induced by
t h e i n h e r i t e d N120"E o r i e n t a t i o n of t h e
sub-Andes of s o u t h Peru and n o r t h B o l i v i a ?
The poor q u a l i t y of o u t c r o p s i n t h e
Amazonian f o r e s t does n o t p e r m i t
s t r u c t u r a l a n a l y s i s i n t h e Recent
Q u a t e r n a r y d e p o s i t s . Comparison w i t h t h e
Recent s t a t e of s t r e s s i n t h e Andes o f
c e n t r a l Peru (M. S é b r i e r e t al.,,
manuscript i n p r e p a r a t i o n , 19851 argues i n
f a v o r of t h e f i r s t p r o p o s i t i o n . I n c e n t r a l
Peru t h e stress p a t t e r n i s similar t o t h a t
of s o u t h Peru ( F i g u r e 1 7 ) . N-S t r e n d i n g
e x t e n s i o n p r e v a i l s i n t h e High Andes, and
t h e l a s t compressional s t r u c t u r e s
a f f e c t i n g e a r l y Quaternary f o r m a t i o n s i n
t h e sub-Andes r e s u l t from N-S compression
as i n s o u t h Peru. However, f o c a l
mechanisms i n t h e c e n t r a l Peruvian subAndes ( S t a u d e r , 1975; Suarez e t al., 1983)
show t h a t t h e a c t i v e compression s t r i k e s
roughly E-W. I n a d d i t i o n , f o c a l mechanisms
i n sub-Andes of s o u t h B o l i v i a and NW
A r g e n t i n a (Chinn and I s a c k s , 1983; J o r d a n
e t a l . , 1983) a l s o show E-W s t r i k i n g
compression. T h i s s t r o n g l y s u p p o r t s t h e
f a c t t h a t i n s o u t h Peru as i n c e n t r a l P e r u
N-S compression i n sub-Andes i s of e a r l y
Q u a t e r n a r y age and t h a t a c t i v e compression
s t r i k e s E-W.
I n t h e High Andes and P a c i f i c Lowlands,
t h e youngest Quaternary deformations
S é b r i e r e t al.:
770
**
u-1
4%
Q u a t e r n a r y Normal and Reverse F a u l t i n g
A n a l y s i s of t h e exposed s t r i a t i o n s
p e r m i t s d a t e r m i n a t i o n of t h e s l i p v e c t o r s
o f t h e s e normal f a u l t s . Computations
performed o n 81 s t r i a t e d f a u l t p l a n e s from
s i x d i f f e r e n t areas show t h a t t h e r e c e n t
d e f o r m a t i o n s r e s u l t from a s t a t e of s t r e s s
( F i g u r e 181, such as t h e minimum p r i n c i p a l
s t r e s s (a 3) t h a t s t r i k e s r o u g h l y N-S, t h e
i n t e r m e d i a t e p r i n c i p a l s t r e s s (a 2 ) E-W,
whereas t h e maximum p r i n c i p a l s t r e s s (a 1 )
i s n e a r l y v e r t i c a l ( T a b l e 1 and F i g u r e s 1 7
and 18). The s t r e s s r a t i o
TABLE 3. Data Used t o
Estimate
the
Magnitude o f S t r e t c h i n g Produced by N-S
Trending E x t e n s i o n i n t h e High Andes and
P a c i f i c Lowlands a l o n g t h e 72" Meridian
Between Cuzco and Mollendo-Southern P e r u
F i g . 18, P r i n c i p a l s t r e s s d i r e c t i o n
computed from s l i p v e c t o r s o f Quaternary
and Recent normal f a u l t s of t h e High Andes
and P a c i f i c Lowlands of s o u t h Peru. Recent
Quaternary f a u l t i n g i n these regions
r e s u l t s from a s t a t e of s t r e s s such as CT 1
(compressional a x i s ) i s v e r t i c a l and Q 3
( " t e n s i o n a l " a x i s ) s t r i k e s between N5'W
and N18"E,
c l e a r l y r e s u l t from normal and normal
s t r i k e - s l i p f a u l t i n g . The major f a u l t s
which are p r e s e n t e d i n t h i s paper (CuzcoTambomachay, V i l c a n o t a , Huambo-Cabana
Conde, Chulibaya, etc., f a u l t s ) have
l e n g t h s r a n g i n g between 5 and 20 km.
Cumulative Quaternary normal throws a r e
g e n e r a l l y g r e a t e r t h a n 100 m. Moreover,
t h e s e f a u l t s a r e seen b o t h i n Quaternary
d e p o s i t s and i n bedrock of Precambrian t o
lower T e r t i a r y age. C l e a r l y , t h e i r
k i n e m a t i c s are n o t c o n t r o l l e d by l a n d s l i d e
e f f e c t s . Where Q u a t e r n a r y . f a u l t s a r e o f
smaller magnitudes a n d / o r are observed
o n l y i n p o o r l y c o n s o l i d a t e d Quaternary
sediments, t h e i r kinematics is i n
agreement w i t h t h e N-S e x t e n s i o n deduced
from a n a l y s i s of t h e major f a u l t s . I n t h e
Western C o r d i l l e r a , r e c e n t normal f a u l t i n g
i s contemporaneous w i t h v o l c a n i c a c t i v i t y .
Thus it a p p e a r s r e a s o n a b l e t h a t t h e normal
Quaternary f a u l t s described i n t h i s paper
a r e n o t t h e r e s u l t of s u r f a c e e f f e c t s s u c h
as l a n d s l i d e s b u t are r e p r e s e n t a t i v e of
t h e s t a t e of s t r e s s a t l e a s t i n t h e upper
p a r t of t h e c r u s t of t h e High Andes and
P a c i f i c Lowlands.
EC
CZ
WC
IF
PC
s,
D,
deg
deg
N-90
N 120
N 100
N 120
N 120
55
50
55
65
60
p,
deg
Tv,
m
85
200
400
400
200
500
75
80
70
75
-
x,
m
140
L
L
361
315
121
330
1267
O 3%
Middle t o Recent Q u a t e r n a r y d e p o s i t s
b e i n g u n t i l t e d , i t i s supposed t h a t n o
r o t a t i o n of t h e f a u l t planes has o c c u r r e d .
S t r e t c h i n g (XI may be c a l c u l a t e d f o r each
f a u l t u s i n g estimate v e r t i c a l throw t'hv),
mean d i p ( D I , mean f a u l t s t r i k e ( S I ,
and
t h e mean s t r i a t i o n p i t c h ( P l .
Subsequentl y , N-S components of s t r e t c h i n g f o r each
f a u l t h a v e t o b e added a l o n g t h e N-S
s e c t i o n : X = Tv! sinD(-cosS/tgP+sinS.cosD).
Data a r e t a k e n from t h e Ananea b a s i n ( 1 3 ) ,
E a s t e r n C o r d i l l e r a (EC); from Tambomachay
f a u l t (9), Cuzco b a s i n (CZ); from T r i g a l
t h e Western
and Solarpampa f a u l t s ( 8 ) ,
C o r d i l l e r a (WCl; from t h e Incapuquio f a u l t
( 6 1 , ( I F ) and from Marcona area ( 1 , F i g u r e
LI, P a c i f i c Coast (PC), t h e mean throw
b e i n g e s t i m a t e d from t h e maximum e l e v a t i o n
o f Q u a t e r n a r y marine t e r r a c e s . These d a t a
i n d i c a t e t h a t s t r e t c h i n g between Cuzco and
Mollendo a l o n g t h e N-S t r e n d i n g , 400-kml o n g s e c t i o n h a s a v a l u e of 0.3%. Study of
f a u l t s h a s been d i s c o n t i n u o u s a l o n g t h i s
s e c t i o n . We have supposed t h a t s t r e t c h i n g
due t o s m a l l unsurveyed f a u l t s might be of
t h e same o r d e r as s t r e t c h i n g due t o t h e
major f a u l t s . Thus t o t a l s t r e t c h i n g i s
p r o b a b l y of t h e o r d e r of 1% f o r t h e l a s t 1
o r 2 m.y.
L
S é b r i e r e t al.:
Quaternary Normal and Reverse F a u l t i n g
(R=u' 2 - ~ ~ 1 / ~ ~ 3 -h a~s ' v1a)l u e s r a n g i n g
between 0.4 and 0.8. This i m p l i e s t h a t t h e
minimum t tension al^^) p r i n c i p a l a x i s u 3
i s c l e a r l y d i f f e r e n t from t h e i n t e r m e d i a t e
one o 2 and t h u s w e l l determined w i t h
r e s p e c t t o t h e l a s t one. A s i m i l a r r e s u l t
i s o b t a i n e d from g r a p h i c a l a n a l y s i s of 1 2
a d d i t i o n a l s t r i a t e d f a u l t p l a n e s from s i x
o t h e r l o c a l i t i e s ( F i g u r e 4 , A). Thus 93
s t r i a t e d normal f a u l t p l a n e s demonstrate a
N-S t r e n d i n g t e n s i o n a l t e c t o n i c s of
Quaternary age.
A s mentioned above , t h e s o u t h Peruvian
Quaternary normal f a u l t s a r e e i t h e r s t i l l
a c t i v e , o f f s e t Quaternary d e p o s i t s , o r
p o s t d a t e compressional s t r u c t u r e s which
a f f e c t P l i o c e n e and e a r l y Quaternary
formations. The few r a d i o m e t r i c d a t e s o f
Peruvian- Quaternary u n i t s do n o t p e r m i t
p r e c i s e d e t e r m i n a t i o n of t h e t i m e d u r a t i o n
o f t h e N-S t r e n d i n g t e n s i o n a l t e c t o n i c s .
This depends mainly of t h e a c c u r a t e d a t i n g
of t h e e a r l y q u a t e r n a r y compressional
t e c t o n i c s . In t h e c e n t r a l Andes, two
compressional e p i s o d e s are observed
(Lavenu e t a l . , 1980; M e r c i e r , 1981;
Lavenu, 1981; S 6 b r i e r e t al., 1982). They
p r o b a b l y occurred between a b o u t 2 and 3
m.y. B.P.:
on one hand, t h e y are younger
than d a t e d l a t e P l i o c e n e beds and
conglomeratic deposits assigned t o the
e a r l y Quaternary ( S é b r i e r e t a l . , 1982) ;
o n t h e o t h e r hand, t h e y are o l d e r t h a n
s e v e r a l Quaternary u n i t s of assumed e a r l y
t o l a t e Quaternary ages. Hence a c c o r d i n g
to the presently available data the
Quaternary t e n s i o n a l t e c t o n i c s seems t o
have been a c t i v e d u r i n g t h e l a s t 1 o r 2
m.y. We estimate t h e h o r i z o n t a l s t r e t c h i n g
produced by Quaternary e x t e n s i o n along t h e
72"W m e r i d i a n l i n e . T h i s c a l c u l a t i o n ( s e e
T a b l e 3) is obviously a rough
approximation and g i v e s o n l y an o r d e r of
magnitude which appears t o be less than
1%.T h e r e f o r e t h e Quaternary N-S
s t r e t c h i n g i s very small and cannot b e
compared w i t h e x t e n s i o n r e p o r t e d e i t h e r i n
back a r c s p r e a d i n g s e t t i n g s o r i n t h e
i n t r a c o n t i n e n t a l Basin and Range province.
Within t h e Western and e a s t e r n
C o r d i l l e r a s , no t e l e s e i m i c f o c a l
mechanisms a r e a v a i l a b l e . I n t h e Western
C o r d i l l o r a , n e a r Chonta (CHO i n c i r c l e on
F i g u r e 171, a composite f o c a l mechanism o f
poor q u a l i t y according t o Grange (1983)
and Grange e t a l . (1984b) has been
determined from a l o c a l s e i s m i c network.
It i s i n agreement w i t h N-S t r e n d i n g
e x t e n s i o n . Beneath t h e High P l a t e a u of
s o u t h c e n t r a l Peru (Stauder , 1975; Suarez
i
77 1
e t a l . , 1983) one f o c a l mechanism ( 1 1 i n
c i r c l e on F i g u r e 17) shows s t r i k e - s l i p
f a u l t i n g w i t h a N-S s t r i k i n g T a x i s and a
E-W P a x i s . This earthquake i s l o c a t e d
w i t h i n t h e t r a n s i t i o n a l zone between t h e
30" d i p p i n g segment and t h e f l a t
s u b d u c t i n g segment of c e n t r a l Peru. The
Altiplano-Puna, above t h e 30" d i p p i n g
s l a b , i s almost a s e i s m i c , s u g g e s t i n g t h a t
t h e mechanical s t a t e i s n e a r l y n e u t r a l
( J o r d a n e t al., 1983). I n t h e s o u t h
P e r u v i a n High Andes, g e o l o g i c a l
o b s e r v a t i o n s c l e a r l y t e s t i f y an
e x t e n s i o n a l s t a t e of s t r e s s (U 1 v e r t i c a l ,
N-S u 3 and E-W CT 2 being h o r i z o n t a l ) ,
b u t t h e magnitude of e x t e n s i o n a l
d e f o r m a t i o n i s very small. A s i m i l a r s t a t e
of s t r e s s has been suggested on t h e
B o l i v i a n A l t i p l a n o (Lavenu, 1978).
Below t h e P a c i f i c Lowlands of s o u t h
P e r u , t h e f o c a l d e p t h s c a l c u l a t e d from
teleseismic d a t a are n o t a c c u r a t e l y
determined. Thus f o c a l mechanisms are n o t
c l e a r l y r e l a t e d t o t h e deformation of t h e
upper p l a t e , of t h e downgoing s l a b , o r of
t h e i n t e r f a c e between t h e two p l a t e s . From
a l o c a l s e i s m i c network, two groups of
f o c a l mechanisms have been determined
(Grange, 1983; Grange e t al., 1984b).
Beneath t h e c o a s t a t a depth s h a l l o w e r
t h a n 55 km, t h e y show a complex p a t t e r n of
normal, s t r i k e - s l i p , and r e v e r s e f a u l t i n g
which h a s n o t been e x p l a i n e d . Under t h e
t r a n s i t i o n a l zone between t h e P a c i f i c
Piedmont and t h e Western C o r d i l l e r a , a t
d e p t h s r a n g i n g between 1 2 and 40 km,
normal, r e v e r s e , and s t r i k e - s l i p f a u l t
s o l u t i o n s a r e more homogeneous. They a r e
i n agreement w i t h a N105"E s t r i k i n g
compression ( F i g u r e 4 , B y and 4B i n c i r c l e
on F i g u r e 1 7 ) . D i r e c t i o n of e x t e n s i o n i s
n o t s t r o n g l y c o n s t r a i n e d , being i n a p l a n e
normal t o t h e compressional d i r e c t i o n , and
p o s s i b l y d i p s g e n t l y toward t h e NNE
( F i g u r e 4 , B I . I f c o r r e c t , t h e s t a t e of
stress i s such t h a t o 2 i s v e r t i c a l , and
d 3 s t r i k i n g N20" t o 30"E and u 1 s t r i k i n g
N105 "E a r e h o r i z o n t a l . Whatever t h e c a s e ,
t h i s s t a t e of s t r e s s i s c l e a r l y d i f f e r e n t
from t h a t observed a t t h e s u r f a c e (see
F i g u r e 1 7 ) i n t h e whole P a c i f i c Lowlands
( v e r t i c a l d 1, u 3 s t r i k i n g N-S and u 2
s t r i k i n g E-W being h o r i z o n t a l ) . T h i s w i l l
be d i s c u s s e d i n t h e n e x t s e c t i o n .
A v a i l a b l e o f f s h o r e d a t a from t h e
Peruvian margin (Johnson and Ness, 1981)
seem t o demonstrate e x t e n s i o n a l t e c t o n i c s
b u t do not p r o v i d e any evidence of a N-S
e x t e n s i o n . Along t h e Peru-Chile t r e n c h ,
compressional s t r u c t u r e s have been
5.
Km O:
50;
100-
uZor3
Fig. 19. T e n t a t i v e model t o e x p l a i n t h e s t a t e of s t r e s s i n t h e Andes of s o u t h e r n Peru. P r l n c i p a l average l i t h o s p h e r i c
s t r e s s ( s e t s of t h i n s o l i d arrows) i n excess t o t h e r e f e r e n c e (sea l e v e l ) l i t h o s t a t i c s t r e s s a r e u x x ( s t r i k i n g E-W),
u yy ( s t r i k i n g N-S), and u zz ( v e r t i c a l ) ; u xx and u yy are c o n s i d e r e d a s f a i r l y c o n s t a n t ; u zz amount t o t h e weight of
t h e topography. Convergence between Nazca and South American p l a t e s ( t h i c k arrows) is roughly p a r a l l e l t o u xx. In t h e
sub-Andes, t e c t o n i c s is compressional (u xx>u yy>u z z ) , i.e., u 3 i s v e r t i c a l and u 1 i s h o r i z o n t a l . A s i m i l a r s t a t e of
s t r e s s i s i n f e r r e d from f o c a l mechanisms a t t h e c o n t a c t between t h e two p l a t e s . In t h e High Andes, u zz i n c r e a s e s , then
u zz>u xx>u yy; i.e., u 1 i s v e r t i c a l , O xx becomes u 2 , u 3 s t r i k e s N-S. I n t h e P a c i f i c Lowlands, according t o t h e
model, t e c t o n i c s should be compressional. However, a t t h e s u r f a c e , N-S e x t e n s i o n p r e v a i l s , i n d i c a t i n g a d e c r e a s e of u x x
v a l u e p o s s i b l y r e l a t e d t o t h e topographic e f f e c t of t h e deep oceanic t r e n c h . A t d e p t h , f o c a l mechanisms i n d i c a t e an E-W
s t r i k i n g u 1 p r i n c i p a l s t r e s s , t h e v e r t i c a l p r i n c i p a l s t r e s s being u 2 o r u 3.
3a
S é b r i e r e t al.:
d e s c r i b e d (Schweller e t a l . , 1981) and few
t h r u s t - t y p e f o c a l mechanisms from
e a r t h q u a k e s l o c a t e d a t t h e c o n t a c t between
t h e Nazca and South American p l a t e s show
a n E-W t o NE-SW d i r e c t i o n of P axes
( S t a u d e r , 1975; Chinn and I s a c k s , 1983)
( s e e c i r c l e d 8 , 9 , and 10 on F i g u r e 17).
The s t a t e of s t r e s s deduced from
s t r u c t u r a l a n a l y s i s of Recent-Quaternary
f a u l t s and from few f o c a l mechanisms i s
shown on F i g u r e 1 7 . The High Andes and
P a c i f i c Lowlands where t e n s i o n a l t e c t o n i c s
o c c u r s are bounded by two zones of
compressional deformation: t h e sub-Andes
t o t h e e a s t and t h e Peru-Chile t r e n c h t o
the w e s t .
The s t a t e of s t r e s s i n t h e Andes of
s o u t h e r n Peru and t h e e f f e c t of h i g h
topography.
toDonraDhv.
I n reg
ev i o n s where c r u s t a l
, thickening is present, extensional
t e c t o n i c s h a s been r e l a t e d t o t h e e f f e c t
of high topography (Tapponnier and Molnar ,
1976; Molnar and Tapponnier, 1978;
Dalmayrac and Molnar, 1981; Cross and
P i l g e r , 1982;. Suarez e t al., 1983;
Froidevaux and I s a c k s , 1984). Topographic
f e a t u r e s of l a r g e l a t e r a l e x t e n t are
compensated a t depth by mass d e f i c i e n c i e s
( i s o s t a s y ) . The l a t t e r , c r u s t a l r o o t
and/or hot asthenospheric m a t e r i a l , are
buoyant. The f i r s t - o r d e r e f f e c t of such a
l a r g e - s c a l e mass d i s t r i b u t i o n i s t o
perturb the vertical lithospheric stress.
The average l i t h o s p h e r i c v e r t i c a l stress a
z z i n excess t o t h e r e f e r e n c e (sea l e v e l )
l i t h o s t a t i c stress amounts t o t h e weight
o f t h e topography. The e x c e s s a v e r a g e
l i t h o s p h e r i c horizontal stress
a xx i s c o n s i d e r e d t o be f a i r l y c o n s t a n t ,
t h e s h e a r stress underneath t h e p l a t e
b e i n g n e g l e c t e d (Dalmayrac and Molnar,
1981; Froidevaux and I s a c k s , 1984). Then
i n lowlands, i f t e c t o n i c s is
compressional, u xxzo zz, s o t h a t a 1 i s
h o r i z o n t a l and o 3 v e r t i c a l . I n r e g i o n s of
h i g h topography, G zz i n c r e a s e s and may
exceed t
s xx, s o t h a t u 3 becomes
h o r i z o n t a l and u 1 v e r t i c a l ; t e c t o n i c s i s
e x t e n s i o n a l . Such a two-dimensional
a n a l y s i s could imply t h a t d i r e c t i o n o f
e x t e n s i o n i n r e g i o n s of h i g h topography
has t o be p a r a l l e l t o d i r e c t i o n of
compression i n lowlands. That i s supposed
i n Dalmayrac and Molnar s 1 a n a l y s i s of t h e
s t a t e of s t r e s s i n c e n t r a l P e r u ; e x t e n s i o n
i n C o r d i l l e r a Blanca i s presumed t o s t r i k e
r o u g h l y p a r a l l e l t o compression i n t h e
sub-Andes.
S t r u c t u r a l a n a l y s i s of r e c e n t
Quaternary f a u l t s of s o u t h Peru shows' t h a t
.
I
.
Quaternary Normal and Reverse F a u l t i n g
<
773
t h e "extensional" u 3 d i r e c t i o n s t r i k e s
between N5"W and N18'E ( F i g u r e 18). The
same N-S e x t e n s i o n h a s a l s o been
demonstrated i n C o r d i l l e r a Blanca (Mercier
and S é b r i e r , 1981; Bonnot, 1984). On t h e
o t h e r hand, as d i s c u s s e d above, where
compressional d i r e c t i o n h a s been
demonstrated i n t h e Andean c r u s t by f o c a l
mechanisms, it s t r i k e s roughly E-W.
T h e r e f o r e a three-dimensional approach
must be i n v e s t i g a t e d . Here ( F i g u r e 1 9 ) t h e
p r i n c i p a l average l i t h o s p h e r i c s t r e s s e s i n
e x c e s s t o t h e r e f e r e n c e (sea l e v e l )
l i t h o s t a t i c stress a r e named u x x
( h o r i z o n t a l , s t r i k i n g E-W), u yy
( h o r i z o n t a l , s t r i k i n g N-SI, and u z z
( v e r t i c a l ) . Focal mechanisms and
s t r u c t u r a l a n a l y s i s of r e c e n t Quaternary
f a u l t s i n b o t h s o u t h e r n Peru and c e n t r a l
Peru show t h a t o xx i s always t h e
h o r i z o n t a l maximum p r i n c i p a l s t r e s s
(a Hmax) and u yy t h e minimum one
(u Hmin). I n t h e sub-Andes, t e c t o n i c s i s
compressional. The s t a t e of stress i s
d e f i n e d by t h e r e l a t i o n : o x x r o y y x r zz;
t h u s d 3 i s v e r t i c a l and u 1 h o r i z o n t a l
and, as d i s c u s s e d above, probably s t r i k e s
E-W. I n t h e High Andes, o zz i n c r e a s e s
w i t h e l e v a t i o n of t h e topography. Two
s t a t e s of s t r e s s are p o s s i b l e ; t h e y a r e
d e f i n e d by r e l a t i o n s u x x m zz>u yy o r
a z z 7 c x x m yy. According t o o u r
o b s e r v a t i o n s t h e l a s t s t a t e of s t r e s s
p r e v a i l s i n t h e High Andes, i.e., (r 1 i s
v e r t i c a l ; whereas u 3 s t r i k i n g N-S and o 2
s t r i k i n g E-W are both h o r i z o n t a l ( F i g u r e
17). Whatever t h e c a s e , u 3 l i e s a l o n g t h e
y d i r e c t i o n and t h u s s t r i k e s N-S. I n t h e
Andes of s o u t h e r n P e r u , convergence
between t h e Nazca and South American
p l a t e s (Minster e t a l . , 1974; M i n s t e r and
J o r d a n , 1978) i s roughly p a r a l l e l t o u xx,
and t h u s e x t e n s i o n i n t h e South Andes i s
roughly o r t h o g o n a l t o t h e convergence
(Figure 19).
I n a g e n e r a l manner, i n high p l a t e a u s
which are submitted t o l a t e r a l
compression, i f e x t e n s i o n occurs due t o
t h e weight of t h e high topography,
e x t e n s i o n a l d i r e c t i o n must be r o u g h l y
o r t h o g o n a l t o s t r o n g e s t compressional
d i r e c t i o n s i n t h e a d j a c e n t lowlands. This
a p p e a r s c l e a r l y on t h e T i b e t a n p l a t e a u
(Molnar and Tapponnier, 1978; Tapponnier
e t a l . , 1981; Armijo e t a l . , 1984; Mercier
e t a l . , 1984). However, along a r c u a t e
s y s t e m s , compression a l o n g b o u n d a r i e s i s
n o t n e c e s s a r y p a r a l l e l t o convergence as
it i s t h e case i n t h e Andes. I t i s
noteworthy t h a t away from t h e convergence
.
,
Ï7A
Sébrier e t al.:
zone, a xx t r a j e c t o r i e s may be d e f l e c t e d
due t o p o s s i b i l i t i e s of l a t e r a l flowing o f
t h e c r u s t a l m a t e r i a l (Tapponnier and
Molnar, 1976). P r i n c i p a l d e v i a t o r i c s t r e s s
v a l u e s may v a r y along t h e same u xx
t r a j e c t o r y so t h a t s u c c e s s i v e u'l, u ' 2 ,
0 ' 3 d e v i a t o r i c v a l u e s may be p r e s e n t a l o n g
t h i s t r a j e c t o r y , a s demonstrated i n Asia
(Tapponnier and N o l n a r , 1976, 19791, i n
t h e Aegean a r c (Mercier e t a l . , 1979;
M e r c i e r , 19811, and i n t h e Japan a r c
(Nakamura and Uyeda, 1980). In t h e Andes
t h e s u c c e s s i v e al1 ( i n t h e sub-Andes1 and
al2 ( i n t h e High Andes) d e v i a t o r i c v a l u e s
e l c n g t h e u xx t r a j e c t o r y r e v e e l t h e
i n c r e a s e of u zz due t o t h e h i g h
topography i n comparison w i t h o Hmax.
Such a model does n o t e x p l a i n t h e s t a t e
of s t r e s s i n t h e P a c i f i c Lowlands, where
t h e s t a t e of s t r e s s i s n e a r l y n e u t r a l b u t
w i t h a p r e f e r e n t i a l tendency t o N-S
e x t e n s i o n . Many a u t h o r s (Sacks e t a l . ,
1978; Cross and P i l g e r , 1982; Froidevaux
and I s a c k s , 1984). have proposed t h a t
e x t e n s i o n a l deformation may be due t o
f l e x i o n of t h e c o n t i n e n t a l p l a t e along t h e
convergence boundary ( F i g u r e 1 9 ) . Above
t h e n e u t r a l s u r f a c e of t h e p l a t e , t r a c t i o n
should o c c u r and Q xx d e c r e a s e s . I f a xx
becomes s m a l l e r t h a n a z z b u t remains
h i g h e r t h a n CT yy, t h e n N-S e x t e n s i o n may
occur. However, bending might be roughly
p a r a l l e l t o t h e N120"E s t r i k i n g c o a s t , and
thus e x t e n s i o n above t h e n e u t r a l s u r f a c e
should b e o r i e n t a t e d roughly N30"E and n o t
along t h e u xx d i r e c t i o n (N80"E). D. Mc
Kenzie ( p e r s o n a l communication, 1985) h a s
suggested t h a t d e c r e a s i n g of cr xx a l o n g
t h e P a c i f i c c o a s t may be due t o a
topographic e f f e c t r e l a t e d t o t h e nearby
7000-m-deep t r e n c h .
F i n a l l y , on high p l a t e a u s w i t h c r u s t a l
t h i c k e n i n g , a s t h e Andes, e x t e n s i o n
roughly p a r a l l e l t o t h e convergence must
r e s u l t from a s t a t e of s t r e s s c l e a r l y
d i f f e r e n t from t h e present-day one. It i s
d e f i n e d by t h e r e l a t i o n d xx<a yy<o zz.
Such a s t a t e of stress i m p l i e s t h a t f o r c e s
a p p l i e d t o a convergence boundary
d r a s t i c a l l y decrease. Along s u b d u c t i o n
zones, s e v e r a l geodynamic p r o c e s s e s may be
p u t forward t o e x p l a i n such a s i t u a t i o n
(see Cross and P i l g e r , 19821, f o r
i n e t a n c e , seaward m i g r a t i o n of t h e s l a b ,
slow o r r e t r o g r a d e a b s o l u t e movement of
t h e upper p l a t e which a l l o w s seaward
m i g r a t i o n of t h e t r e n c h . Whatever t h e
s i t u a t i o n , t h e h i g h p l a t e a u i s no l o n g e r
l a t e r a l l y s u s t a i n e d and flows toward t h e
t r e n c h . This i s t h e Mariana-type a r c of
Quaternary Normal and Reverse F a u l t i n g
Uyeda and Kanamori (19791, and Uyeda
(1981, 1982).
E x t e n s i o n a l T e c t o n i c s i n Andean Convergent
Benioff Dip
-Zone Versus
There i s now a g e n e r a l agreement on t h e
c o n f i g u r a t i o n of t h e Andean Benioff zone.
I t d i p s w i t h an a n g l e of 30" t o t h e east
o r n o r t h e a s t beneath t h e A l t i p l a n o
(between l a t i t u d e 1 5 " s and 2 8 " s ) and
c e n t r a l C h i l e (south of l a t i t u d e 33 S I ,
where a c t i v e volcanism occurs. On t h e
c o n t r a r y , i t is r a t h e r f l a t b e n e a t h
c e n t r a l Peru ( n o r t h of l a t i t u d e 15"s) a d
n o r t h c e n t r a l C h i l e (between l a t i t u d e 28" S
and 33"S), where no a c t i v e volcanism i s
known ( S t a u d e r , 1973 ; Barazangi and
I s a c k s , 1976). The sub-Andean zone i s
wider and s e i s m i c a l l y more a c t i v e above
t h e f l a t subduction segments t h a n above
t h e s t e e p e r subduction o n e s (Jordan e t
a l . , 1983). Mégard and P h i l i p (1976)
proposed t h a t t h e r e c e n t and a c t i v e
deformations of t h e o v e r r i d i n g p l a t e
should be compressional over t h e f l a t
Benioff segments and t e n s i o n a l over t h e
s t e e p e r ones. However, Q u a t e r n a r y
e x t e n s i o n a l s o occurs i n c e n t r a l P e r u b u t
seems t o be r e s t r i c t e d t o t h e C o a s t a l
Lowlands and Western C o r d i l l e r a (e.g.,
C o r d i l l e r a Blanca) a r e a s , whereas
compression extends i n t h e whole E a s t e r n
C o r d i l l e r a and sub-Andes ( S é b r i e r e t a l . ,
19821, Thus t e n s i o n a l t e c t o n i c s a f f e c t s a
narrower zone i n c e n t r a l Peru than i n
s o u t h e r n Peru. T h e r e f o r e t h e r e seems t o
e x i s t a c o r r e l a t i o n between t h e s p a t i a l
o c c u r r e n c e of t e n s i o n a l / c o m p r e s s i o n a l
t e c t o n i c s and t h e geometry of t h e
u n d e r l y i n g subduction zone. A s commonly
accepted (Barazangi and I s a c k s , 1976;
Cross and P i l g e r , 1982) where t h e
subduction f l a t t e n s , t h e c o u p l i n g between
t h e Nazca and t h e South America p l a t e s
i n c r e a s e s . Consequently, u xx h a s a h i g h e r
magnitude, and compression p l a y s a more
important r o l e i n t h z o v e r r i d i n g
c o n t i n e n t a l l i t h o s p h e r e . On t h e c o n t r a r y ,
where t h e oceanic s l a b d i p s s t e e p e r , ' a n
a s t h e n o s p h e r i c wedge a p p e a r s , c a u s i n g
volcanism t o occur (Barazangi and I s a c k s ,
1976). I n a d d i t i o n , c o u p l i n g between t h e
two p l a t e s becomes weaker so t h a t
e x t e n s i o n a l domain a f f e c t i n g t h e High
Andes i s more e x t e n s i v e . This l a t t e r c a s e
i s i l l u s t r a t e d by t h e p r e s e n t - d a y s t a t e of
s t r e s s i n s o u t h e r n Peru. It i s noteworthy
t h a t d u r i n g Neogene and e a r l y Q u a t e r n a r y ,
compressional deformations have a f f e c t e d
S é b r i e r e t al.:
Q u a t e r n a r y Normal and Reverse F a u l t i n g
t h e whole c e n t r a l Andes, h i g h l a n d s and
lowlands (Dalmayrac and M a t t a u e r , 1980;
M a r t i n e z , 1980; S é b r i e r e t a l . , 1980b;
Mercier, 1981; Megard e t 'al., 1984). These
o b s e r v a t i o n s probably imply a s t r o n g
i n c r e a s e of CT xx v a l u e which i n d i c a t e s
t h a t geodynamic c o n d i t i o n s were d i f f e r e n t
from p r e s e n t ones.
CONCLUSIONS
F i e l d s t u d i e s i n s o u t h Peru c l e a r l y
show ( F i g u r e 1 7 ) t h e f o l l o w i n g :
1. I n t h e sub-Andes, d e f o r m a t i o n s are
compressional. They have been observed
a f f e c t i n g Neogene and e a r l y Q u a t e r n a r y
d e p o s i t s and r e f l e c t a r o u g h l y N-S
s t r i k i n g compression. U n f o r t u n a t e l y , the
poor q u a l i t y of o u t c r o p s i n t h e Amazonian
f o r e s t does n o t p e r m i t s t r u c t u r a l a n a l y s i s
of t h e Recent Quaternary d e p o s i t s .
However, as l a r g e l y d i s c u s s e d i n t h i s
p a p e r , t h i s N-S c o m p r e s s i m i s more l i k e l y
t h e e a r l y Quaternary d i r e c t i o r i of
compression r a t h e r than t h e p r e s e n t - d a y
one. This l a t t e r probably s t r i k e s E-W as
shown by f o c a l mechanisms i n t h e sub-Andes
of c e n t r a l P e r u , South B o l i v i a , and Nw
Argentina.
2. I n t h e High Andes, Recent and
a c t i v e deformations r e s u l t from normal
f a u l t i n g . S e v e r a l f a u l t s have l e n g t h s
r a n g i n g between 5 and 20 km and are s e e n
b o t h i n t h e bedrock and i n Q u a t e r n a r y
formations. Their k i n e m a t i c s i l l u s t r a t e a
N-S s t r i k i n g ex t e n s ion. However , magnitude
of s t r e t c h i n g i s small, of t h e o r d e r o f
1%.
I
3 . I n t h e P a c i f i c Lowlands, Quaternary
normal f a u l t s a l s o r e s u l t from N-S
t r e n d i n g e x t e n s i o n whose magnitude appears
t o be smaller than i n t h e High Andes.
S t r u c t u r a l a n a l y s e s of f a u l t s and some
f o c a l mechanisms enable us t o propose a
s k e t c h of t h e s t a t e of s t r e s s i n t h e Andes
of s o u t h Peru. High Andes and P a c i f i c
Lowlands a r e a f f e c t e d by N-S t r e n d i n g
e x t e n s i o n . They a r e bounded by two zones
o f roughly E-W s t r i k i n g compression: t h e
sub-Andes t o t h e NE and t h e c o n t a c t
between t h e Nazca and South American
p l a t e s t o t h e SW.
I n such a s k e t c h ( F i g u r e 1 9 1 , U Hmax
t r a j e c t o r i e s s t r i k e E-W and t h u s are
r o u g h l y p a r a l l e l t o t h e convergence
d i r e c t i o n ; D Hmax i s Q 1 i n t h e sub-Andes,
and a t t h e c o n t a c t between t h e two p l a t e s ,
u Hmax i s u 2 i n t h e High Andes. Thus
CT Hmin s t r i k e s N-S; i t i s CT 2 i n t h e subAndes and CI 3 i n t h e High Andes. This
,
77s
s t a t e of s t r e s s i s i n agreement w i t h an
e f f e c t of high topography such t h a t t h e
v e r t i c a l p r i n c i p a l stress CT z z , which i s
u 3 i n t h e lowlands, becomes u 1 i n t h e
High Andes. However, i n t h e P a c i f i c
Lowlands t h e s t a t e of s t r e s s a t t h e
s u r f a c e i s e x t e n s i o n a l and n o t
c o m p r e s s i o n a l , as could be expected. An
e f f e c t of warping of t h e c o n t i n e n t a l p l a t e
a l o n g t h e convergence boundary o r an
e f f e c t of topography due t o t h e nearby
deep t r e n c h may be p u t forward. T h e r e f o r e
i n h i g h p l a t e a u s such as t h e Andes o r
T i b e t , which are submitted t o l a t e r a l
compression, i f e x t e n s i o n i s due t o body
f o r c e s , i t should s t r i k e o r t h o g o n a l t o
compression i n t h e a d j a c e n t lowlands. I n
such a model, e x t e n s i o n a l d i r e c t i o n may
become p a r a l l e l t o t h e convergence
d i r e c t i o n only i f boundary f o r c e s d e c r e a s e
drastically.
The s t a t e f o r s t r e s s i n t h e Andean
c o n t i n e n t a l p l a t e s i t u a t e d above t h e 30"
d i p p i n g subduction appears t o be d i f f e r e n t
from t h a t of t h e Andes of c e n t r a l Peru
s i t u a t e d above t h e f l a t s u b d u c t i n g
segment. I n a subsequent paper w e s h a l l
r e p o r t a l l t h e Quaternary and a c t i v e
f a u l t s observed i n c e n t r a l Peru. These
f a u l t s w i t h a v a i l a b l e f o c a l mechanisms
w i l l p e r m i t us t o compare t h e d i f f e r e n t
s t a t e s of stress i n t h e c o n t i n e n t a l p l a t e
segments s i t u a t e d above s l a b s of d i f f e r e n t
d i p s . This may improve our u n d e r s t a n d i n g
c o n c e r n i n g t h e c o n t r o l of t h e s l a b
geometry and consequently of t h e
a s t h e n o s p h e r i c wedge on t h e d e f o r m a t i o n a l
regime i n t h e upper c o n t i n e n t a l p l a t e .
APPENDIX
Using t h e approach of B o t t (1959) and
P r i c e (19661, s e v e r a l a u t h o r s (e.g., Carey
and B r u n i e r , 1974; Carey, 1976, 1979;
Armijo and C i s t e r n a s , 1978; Angelier and
Goguel, 1979; Etchecopar e t a l . , 1981;
Armijo e t a l . , 1982; Angelier e t al.,
1982) have proposed q u a n t i t a t i v e computera i d e d methods t o i n t e r p r e t e k i n e m a t i c s of
f a u l t s i n a h i g h l y f r a c t u r e d bodies o f
rocks.
The b a s i c assumptions of a l l t h e s e
numerical methods are (1) f o r a p a r t i c u l a r
s i t e a given t e c t o n i c event i s
c h a r a c t e r i z e d by a s i n g l e homogeneous
stress t e n s o r , ( 2 ) f o r a g i v e n phase of
d e f o r m a t i o n , i f t h e material has a n
i s o t r o p i c and homogeneous b e h a v i o r , o n
each f a u l t p l a n e , s l i p ( r e s p o n s i b l e f o r
t h e s t r i a t i o n ) o c c u r s i n t h e d i r e c t i o n and
77 6
Sébrier e t al.:
Q u a t e r n a r y Normal and Reverse F a u l t i n g
of i on t h e f a u l t plane ( i ) ( s e e F i g u r e
20).
The-?bo_ve-mentzoned d e f i n i t i o n s imply
t h a t ( t i , S i ) = O ; t i i s a f u n c t i o n of t h e
f o u r parameters which d e f i n e t h e
d e v i a t o r i c s t r e s s t e n s o r : t h e t h r e e Euler
a n g l e s which g i v e t h e azimuthal d i r e c t i o n s
of t h e p r i n c i p a l s t r e s s axes and t h e
r e l a t i v e r a t i o ( R I of p r i n c i p a l s t r e s s e s
such as
R=!u'2-a'1
F i g . 20. S l i p movement on a f a u l t p l a n e
w i t h parameters used i n t h e Appendix ( s e e
text).
)!!a'3-o'l1
w i t h 0'1 0 ' 2 0'3; d l i s t h e
compressional d e v i a t o r i c s t r e s s , O' 2 i s
t h e i n t e r m e d i a t e d e v i a t o r i c s t r e s s , 0'3 i s
the tensional deviatoric stress.
Furthermore, compressional s t r e s s e s a r e
noted n e g a t i v e and t e n s i o n a l ones
positive.
The f o u r parametefs-values
must t h u s be
i n agreement w i t h (ti,Si)-O,
and t h e y
a r e t h e r e f o r e determined when t h e f u n c t i o n
N
s e n s e of t h e r e s o l v e d s h e a r s t r e s s a c t i n g
on t h i s f a u l t p l a n e , and ( 3 ) i f t h e r e i s
no continuous deformation w i t h i n t h e
b l o c k s s e p a r a t e d by f a u l t s , i f t h e r e i s no
f a u l t plane r o t a t i o n d u r i n g deformation,
and i f t h e s l i p s en t h e s l i c k e n s i d e s are
independent and s m a l l r e s p e c t i v e t o t h e
f a u l t l e n g t h , deformation o c c u r s as
r e l a t i v e displacements of r i g i d b l o c k s
along the faults.
A s t a t e of s t r e s s i s c h a r a c t e r i z e d by a
s t r e s s t e n s o r T which can be d i v i d e d i n t o
an i s o t r o p i c p r e s s u r e t e n s o r a1 and i n a
d e v i a t o r i c s t r e s s t e n s o r bD, t h i s l a t t e r
being r e s p o n s i b l e f o r t h e resolved s h e a r
stress
which induces of t h e movements
blocks. If t h e previous assumptions 1, 2 ,
and 3 a r e s a t i s f i e d , t h e n t h e d e v i a t o r i c
s t r e s s t e n s o r of a t e c t o n i c event can be
o b t a i n e d from, s e v e r a l independent
s t r i a t i o n s related to t h i s event, t o
w i t h i n a m u l t i p l i c a t i v e c o n s t a n t (Carey
and B r u n i e r , 1974; Carey, 1976).
For each measurred s t g a t e d s l i c k e n s i d e s
( i ) two v e c t o E n i and s i a r e d e f i n e d
( F i g u r e 2 0 ) ; n i i s t h e u n i t v e c t o r normal
t o t h e f a u l t _ p l a n e which has a downword
component; S i i s t h e u n i t v e c t o r which i s
o r i e n t e d p a r a l l e l t o t h e movement of t h e
f o o t w a l l block (FI w i t h r e s p e c t to-the
hangingwall block (H). The stress+ulL_
applied t o a f a u l t plane ( i > is a i = a n i + t i ;
o n i corresponds t o t h e s t r e s s component
normal t o t h e f a u l t p l s e ( i ) , t h e
r e s o l v e d shear s t r e s s t i i s t h e p r o j e c t i o n
-
i s minimum, w i t h N= n2mker of f a u l t p l a n e s
m e z s s e d , kf=-1 i f ( t i , S i ) a 9 0 ° , k i = l i f
( t i , S i ) < 90". More d e t a i l s on t h e
minimization process can be o b t a i n e d from
Carey (1976, 1979).
T h e o r e t i c a l l y , t h e minimum o f F might
b e equal t o -N. N e v e r t h e l e s s , w e c o n s i d e r
t h e minimizatAon-to be mathematically good
i f a l l t h e ( t i , S i ) a r e i n f e r i o r t o 20" or
i f t h i s minimum i s i n f e r i o r t o -90% o f N.
Anyway t h e minimum can be s u p e r i o r to -90%
of N and t h e res2lt-considered
as c o r r e c t
i f 80% of t h e ( t i , S i ) are i n f e r i o r t o 2 0 "
because t h e s o l u t i o n i s s t a b l e , i . e . ,
remains t h e same when only t h e 80% of t h e
N d a t a a r e used t o c a l c u l a t e t h e
d e v i a t o r i c s t r e s s t e n s o r . This l i m i t
p e r m i t s us t o t a k e i n t o account t h e e r r o r s
o f measurements and i s j u s t i f i e d by thef a c t t h a t t h e v a r i a t i o n of t h e c o s 2 ( t i , S i )
i s low, b e i n g up t o 20'. This simple
mechanical model was a p p l i e d t o many f i e l d
c a s e s and provided a good i n t e r p r e t a t i o n
i n terms of s t r e s s e s f o r t h e s t r i a t i o n s
observed on t h e f a u l t p l a n e s , e s p e c i a l l y
when t h e r e i s a s i n g l e phase of
deformation i n a h i g h l y f r a c t u r e d body of
r o c k s . More s o p h i s t i c a t e d c a l c u l a t i o n s
must be used t o s e p a r a t e d a t a
corresponding t o s e v e r a l t e c t o n i c phases
(Carey, 1979; Etchecopar e t al., 1981;
Armijo e t a l . , 1 9 8 2
>.
c
S é b r i e r e t al.:
.
Quaternary Normal and Reverse F a u l t i n g
Acknowledgments. F i e l d 'work h a s been
supported by I n s t i t u t o G e o f i s i c o d e l P e r u ,
ORSTOM L i m a , and ATP Geodynamique I I
( I n s t i t u t National d'Astronomie e t d e
Géophysique). We are g r a t e f u l t o t h e
S e r v i c i o A e r o f o t o g r a f i c o Nacional d e l Peru
which a u t h o r i z e d p u b l i c a t i o n of a e r i a l
photographs. The a u t h o r s thank C.
Froidevaux, D. H a t z f e l d and t h e two
r e v i e w e r s , B. I s a c k s and P. Tapponnier,
f o r h e l p f u l comments and B. P u r s e r f o r r e a d i n g t h e E n g l i s h manuscript.
. Allmendiger, R. W.,
V. A. Ramos, T. E.
J o r d a n , M. Palma, and B. L. I s a c k s ,
Paleogeography and Andean s t r u c t u r a l
geometry, n o r t h w e s t A r g e n t i n a ,
Tectonics,
1-16, 1983.
A n g e l i e r , J., and J. Goguel, .Sur une
méthode simple de d é t e r m i n a t i o n des
axes p r i n c i p a u x des c o n t r a i n t e s pour
une p o p u l a t i o n de f a i l l e s , C. R. Hebd.
Séances Acad. Sc.,
307-310, 1979.
A n g e l i e r , J., A. T a r a n t o l a , B. Valette,
and S. Manoussis, I n v e r s i o n of f i e l d
data i n f a u l t tectonics t o obtain the
r e g i o n a l s t r e s s , I, S i n g l e phase
p o p u l a t i o n : A new method of computing
t h e s t r e s s t e n s o r , Geophys. J. R.
Astron, Soc.,
607-621, 1982.
Armijo, R., and A. C i s t e r n a s , Un problème
i n v e r s e en m i c r o t e c t o n i q u e c a s s a n t e ,
R. Hebd. Séances Acad. Sc.,
595-
2,
c
h:
288,
69,
287,
C.
598, 1978.
Armijo, R., E. Carey, and A. C i s t e r n a s ,
The i n v e r s e problem i n m i c r b t e c t o n i c s
and t h e s e p a r a t i o n of t e c t o n i c p h a s e s ,
T e c t o n o p h y s i c s , 82, 145-160, 1982.
Armijo, R., P. Tapponnier, J. L. M e r c i e r ,
and Han T., Quaternary e x t e n s i o n of t h e
T i b e t a n p l a t e a u : F i e l d o b s e r v a t i o n s and
t e c t o n i c . i m p l i c a t i o n s , Paper p r e s e n t e d
at t h e I n t e r n a t i o n a l Symposium on
Geology of t h e Himalayas, Chengdu,
P e o p l e ' s Republic o f China, June 1984.
Aubouin, J., A. V. B o r r e l l o , G. C e c i o n i ,
R. C h a r r i e r , P. Chotin, J. F r u t o s , R.
T h i e l e , and J. C. V i c e n t e , E s q u i s s e
palgogéographique e t s t r u c t u r a l e d e s
Andes- m é r i d i o n a l e s , Rev. Géogr. Phys.
Géol. Dyn., g ( 1 - 2 1 , 11-72, 1973.
Audebaud, E., Geologia de l o s cuadrangulos
d e Ocongate y S i c u a n i , Bol. S e r . Geol.
Min., 25, 72 pp., 1973.
Audebaud, E. , R. Capdevila, B. Dalmayrac,
J. Debelmas, G. Laubacher, C. L e f è v r e ,
R. Marocco, C . M a r t i n e z , M. Mattauer,
2.
I
Y
Y
F. Mégard, J. P a r e d e s , and P. Tomasi,
Les t r a i t s géologiques e s s e n t i e l s d e s
Andes c e n t r a l e s (Pérou-Bolivie) , Rev.
Gbogr. Phys. Géol. D.yn., g ( 1 - 2 ) 73114, 1973.
Barazangi, M., and B. I s a c k s , S p a t i a l
d i s t r i b u t i o n of earthquakes and of t h e
Nazca p l a t e beneath South America,
Geology,
(11), 686-692, 1976.
B a r a z a n g i , M., and B. I s a c k s , Subduction
of t h e Nazca p l a t e b e n e a t h Peru:
Evidence f r o m ' s p a t i a l d i s t r i b u t i o n of
e a r t h q u a k e s , Geophys. J. R. A s tron.
SOC 57, 537-555, 1979
Beaudet, G., D. H e r m , R. L a h a r i e , and R.
Paskoff ,. S u r l ' e x i s t e n c e du P l i o c è n e
marin l e long de l a c ô t e du Pérou, C. R.
So".
Séances Soc. Géol. F r . ,
12113r
2,
-
REFERENCES
m
777
1,
1976.
B e v i s , M., and B. I s a c k s , Hypocentral
t r e n d s u r f a c e a n a l y s i s : Probing t h e
geometry of Benioff zones, J. Geophys.
Res
89,(B7), 6153-6170, 1984.
--y
Blanc, J. L., Etude n é o t e c t o n i q u e e t
s i s m o t e c t o n i q u e d e s Andes du Pérou
c e n t r a l dans l a r 6 g i o n de Huancayo,
t h e s i s , 158 pp., Univ. Paris-Sud,
Orsay, 1984.
B l a n c , J. L., J. Cabrera, and M. S é b r i e r ,
E s t u d i o m i c r o t e c t o n i c o de l a f a l l a
s i s m i c a d e H u a y t a p a l l a n a (Andes d e l
P e r u c e n t r a l ) ,-Rev. G e o f i s i c a I n s t .
Panam. Geogr. H i s t . , Mexico, 18/19, 524, 1983.
Bles, J. L., J. Goguel, A. Lavenu, P.
Masure, Néotectonique e t sismicité du
s i t e d e L a Paz ( B o l i v i e ) : Un exemple d e
f a i l l e s r é c e n t e s sans t r a c e de
s i s m i c I t 6 h i s t o r i q u e ; con séquences
p r a t i q u e s pour l e dévelotmement u r b a i n .
Bull. -Bur.-Rech. Géol. Min., Sect. 4 , '
-
..
2 , 109-117, 1980.
Bonnot, D. , Néotectonique e t t e c t o n i q u e
a c t i v e de l a C o r d i l l è r e Blanche e t du
C a l l e j o n d e Huaylas (Andes nordp é r u v i e n n e s ) , t h e s i s , 96 pp., Univ.
,
Paris-Sud , Orsay, 1984.
B o t t , M. H. P., The mechanics of o b l i q u e
s l i p f a u l t i n g , Geol. Mag., 9 7 , ( 2 ) , 109-
1 1 7 , 1959.
C a b r e r a , J., Etudes s u r l a n é o t e c t o n i q u e
d e l a r é g i o n de Cuzco: Zone Andine e t
zone Sub-Andine, s u d Pérou, r e p o r t
Diplom. Etud. Approf., 55 pp., Univ.
' Paris-Sud , O r s ay , 1984.
Carey, E., Analyse numérique d'un modèle
mécanique é l é m e n t a i r e a p p l i q u é à
l ' é t u d e d'une p o p u l a t i o n de f a i l l e s :
Calcul d'un t e n s e u r movem
,- d- -e -a
~
~~~
778
S é b r i e r e t al.:
contraintes
p a r t i r des s t r i e s de
g l i s s e m e n t , t h e s i s , 138 pp., Univ.
Paris-Sud, Orsay, 1976.
Carey E . , Recherche des d i r e c t i o n s
p r i n c i p a l e s de c o n t r a i n t e s a s s o c i é e s a u
j e u d'une p o p u l a t i o n de f a i l l e s , Rev.
Géogr. Phys. Géol. Dyn., E , ( l ) , 57-66,
1979.
Carey, E., and B. B r u n i e r , Analyse
t h é o r i q u e e t numérique d'un modèle
mécanique é l é m e n t a i r e appliqué à
1 ' 6 t u d e d'une p. o.p u l a t i o n de f a i l l e s , C.
R. Habd, Séances Acad, Stai 279, 891894, 1974.
Chinn, D . S.,
and B. L. I s a c k s , Accurate
s o u r c e depths and f o c a l mechanisms of
shallow earthquakes i n western South
America and i n t h e New Hebrides I s l a n d
Arc, T e c t o n i c s , 2 ( 6 ) , 52 9-56 3 , 1983.
C 1 a p p e r t o n , C., T h e P l e i s t o c e n e moraine
s t a g e s o f w e s t - c e n t r a l Peru, J.
G l a c i o l . , 12, 62, 255-263, 1 9 x .
Cross, T. A., a n d y . H. P i l g e r , C o n t r o l s
of subduction geometry, l o c a t i o n of
magmatic a r c s and t e c t o n i c s of a r c and
back-arc r e g i o n s , Geol. Soc. Am. Bull.,
93, 545-562, 1982.
D a l z y r a c , B., Un exemple de t e c t o n i q u e
v i v a n t e : Les f a i l l e s s u b a c t u e l l e s du
p i e d de la C b r d i l l k r e Blanche (Pérou),
Cah. ORSTOM, S6r. G o l . , a ( l ) , 19-27,
1974.
Dalmayrac, B., and M. M a t t a u e r , Subduction
e t phases de compression dans l a c h a î n e
des Andes, C. R. Hebd. Séances Acad.
SC., 209, 1345-1348, 1980.
Dalmayrac, B. and P. Molnar, P a r a l l e l
t h r u s t and normal f a u l t i n g i n Peru and
c o n s t r a i n t s on t h e s t a t e of s t r e s s .
Earth Planet. Sci. Lett.,
473-181,
-
-
55,
-I Y-t-l -l .
Etchecopar, A. , G . Vasseur, and M.
Daignères, An i n v e r s e problem i n
m i c r o t e c t o n i c s f o r the d e t e r m i n a t i o n o f
s t r e s s t e n s o r s from f a u l t s t r i a t i o n
a n a l y s i s , J. S t r u c t . Geol., 3(1),5165, 1981.
F o r n a r i , M., G . H e r a i l , and G. Laubacher,
E l oro en l a C o r d i l l e r a S u r o r i e n t a l d e l
Peru: E l p l a c e r f l u v i o g l a c i a l d e San
Antonio de P o t o (departamento de Puno)
y sus r e l a c i o n e s con l a m i n e r a l i z a c i o n
p r i m a r i a d e l a Rinconada. i n
Proceedings V Congreso Latin American
Geology-Argentina, vol. IV, pp. 369386, Buenos A i r e s , 1982.
F' r oidevaux, C.,
and B. L. Isacks, The
mechanical s t a t e of t h e Altiplano-Pun a
segment of t h e Andes, E a r t h P l a n e t .
Sci. L e t t . ,
305-314, 1984.
-
71,
Quaternary Normal and Reverse F a u l t i n g
Grange, F., Etude s i s m o t e c t o n i q u e
d é t a i l l é e de l a s u b d u c t i o n
l i t h o s p h é r i q u e au Sud Pérou, t h e s i s , 90
pp., Univ. Grenoble, Grenoble, France,
1983.
Grange, F., P. Cunningham, J. Gagnepain,
D. H a t z f e l d , P. Molnar, L. Ocola, A.
Rodriguez, S. W. Roecker, J. M. Stock,
and G. Suarez, The c o n f i g u r a t i o n of t h e
s e i s m i c zone and t h e downgoing s l a b i n
s o u t h e r n Peru, Geophys. Res. L e t t . ,
1 1 ( 1 ) , 38-41, 1984a.
Grange, F e j D. H a t z f e l d , P, Cunninghem, P i
Molnar, S. W. Roecker, G. S u a r e z , A.
Rodriguez, and L. Ocola, T e c t o n i c
i m p l i c a t i o n s of t h e microearthquake
s e i s m i c i t y and f a u l t plane s o l u t i o n s i n
s o u t h e r n P e r u , J. Geophys. Res.,
89(B7), 6139-6152, 1984b.
Hasegawa, A., and I. S. Sacks, Subduction
of t h e Nazca p l a t e b e n e a t h Peru a s
determined from s e i s m i c o b s e r v a t i o n s .
J. Geophys. Res., E ( B 6 1 , 4971-4980,
1981.
Human, D . , Evolution t e c t o n i q u e
cénozoique e t n é o t e c t o n i q u e du Piemont
P a c i f i q u e dans l a r é g i o n d'Arequipa
(Andes du sud P é r o u ) , t h e s i s , 220 p . ,
Univ. Paris-Sud, Orsay, 1985.
H e i m , A., Observaciones g e o l o g i c a s en l a
region d e l terremoto d e Ancash d e
Noviembre de 1946, Soc. Geol. Peru V.
2 ( 6 ) , 28 pp., 1949.
I s a c k s , B. L., and M. Barazanni.
Geometrv
- ,
o f Benioff zones : Lateral segmentation
and downward bending of t h e subducted
l i t h o s p h e r e , i n I s l a n d Arcs, Deep Sea
Trenches , and Back-Arc B a s i n s , Maurice
Ewing Ser., vol. 1, e d i t e d by M,
Talwani and W. C. Pitman I I I , pp. 991 1 4 , AGU, Washington, D. C., ,1977.
Johnson, S. H., and G. E. M s s , S h a l l a u
s t r u c t u r e s of t h e Peru margin,
Geol. Soc. Am.,
525-544, 1981.
Jordan, T. E., B.' L. I s a c k s . R. W.
A l i e n d i g e r , 3. A. Breve;, V. A. Wmos,
and C. J. Ando, Andean t e c t o n i c s
r e l a t e d t o geometry
of subducted Nazca
p l a t e , Geol. Soc. Am. Bull., 9 4 ( 3 ) ,
3 41-3 61, 1983.
L a h a r i e , R., Cronologia
d e l Cuaternario
Peruano, i n Proceedings I Congreso
L a t i n American Geology, v o l .
pp.
145-157, Soc. Geol. P e r u , L i m a . 1970.
Laubacher, G., G. Herail, M I F o r n a r i , and
M. S é b r i e r , Le P i h o n t Amazonien d e s
Andes Sud O r i e n t a l e s du Pérou
(Mar capa t a - Inambari 1, paper p r e s e n t e d
a t t h e C o l l . Piemonts, A s s . G601. S. O.
Toulouse, France, 1982.
-
-
Jub.,
154,
s.
6,
z
L
S é b r i e r e t al.:
Quaternary Normal and Reverse F a u l t i n g
Lavenu, A., Néotectonique des sédiments
p l i o - q u a t e r n a i r e s du Nord d e
1' 11tiDlano b o l i v i e n ( r é g i o n de La PazAyo A y b - h a l a ) , Cah. ORSTOM Sér. Géol.,
10(1>, 115-126,
1978.
Lavenu.. A., . Origine
e t évolution
n é o t e c t o n i q u e du l a c T i t i c q c a , Rev.
Hydrobiol. Trop., a ( & ) , 289-297, 1981.
Lavenu, A., and O. B a l l i v i a n , E s t u d i o s
n e o t e c t o n i c o s de las cuencas de l a s
r e g i o n e s de Cochabamba, S u c r e , T a r i j a ,
Cordillera Oriental, Bolivia,
Acad. Nac. Cienc. B o l i v i a , 2 ( 3 ) , 107-
e.
1 2 9 , 1980.
Lavenu, A., and J. P. S o u l a s , Observacion
de m i c r o f a l l a s P l i o - C u b t e r n a r i a s e n
d i s t e n s i o n a l o l a r g o de l a c o s t a s u r
d e l Peru, Bol. Soc.-Geol. Peru,
39-
2,
48. 1976.
Lavenu , A. , M. S é b r i e r , . and M. Servant ,
Néotectonique des Andes C e n t r a l e s :
3, pp. 56-58,
P é r o u , B o l i v i e , G.,
INQUA Neotectonic Comm. , Stockholm,
1980.
Lavenu, A. , M. F o r n a r i , and M. S é b r i e r ,
E x i s t e n c e de deux nouveaux épisodes
l a c u s t r e s q u a t e r n a i r e s dans l ' b l t i p l a n o
péruvo-bolivien, Cah. ORSTOM S 6 r
-Géol
*y
1 4 ( 1 ) , 103-114, 1984.
L e f è v r e , C . , Les c a r a c t è r e s magmatiques du
volcanisme p l i o - q u a t e r n a i r e d e s Andes
dans l e Sud du Pérou, - C o n t r i b . Mineral.
Petrol
41, 259-272, 1973.
L e g a u l t , R. Z. , P r e l i m i n a r y s t u d y o f
marine t e r r a c e s i n t h e Marcona-San Juan
a r e a of s o u t h e r n Peru, r e p o r t , Marcona
Mining Co , H i e r r o p e r u Marcona, 1963.
L e t t a u , H. H., and K. L e t t a u , Exploring
t h e w o r l d ' s d r i e s t c l i m a t e , Rep.
pp. 1-26, I n s t . f o r Environ. Stud.,
Univ. Wisc., Madison, 1978.
Macharé, J., Geologia d e l C u a t e r n a r i o e n
l a c o s t a d e l Peru c e n t r a l , t h e s i s , 197
pp., Univ. Nac. Ing., Lima, 1981.
Malgrange, M., A. Deschamps, and
R. Madariaga, Thrust and e x t e n s i o n a l
f a u t i n g under t h e Chilean coast: 1965,
1971 Aconcagua earthquakes , Geophys. J.
R. Astron. Soc.,
313-331, 1981.
M a r t i n e z , C., S t r u c t u r e e t é v o l u t i o n de l a
c h a î n e Hercynienne e t de l a c h a î n e Andine
dans l e nord de l a c o r d i l l h r e des Andes
d e B o l i v i e , Trav. Doc. ORSTOM,
352
pp., 1980.
Mégard, F., and H. P h i l i p , P l i o - Q u a t e r n a r y
tectono-magmatic z o n a t i o n and p l a t e
t e c t o n i c s i n t h e C e n t-r a -l __
Andes.
E
rth
- -- a~P l a n e t . S c i . L e t t . , 2, 231-238, 1976.
Mégard, F., D. C. Noble, E. McKee. and
-H. B e l l o n , M u l t i p l e p u l s e s of Neogene
.
-
-O'
-
101,
2,
119,
~
e
I
77 9
deformation i n t h e Ayacucho
i n t e r m o n t a n e b a s i n , Andes of c e n t r a l
Peru, Geol. Soc. Am. Bull., E ( 9 ) ,
1108-1117, 1984.
Mercer , J. H., and O. P a l a c i o s , Radioc a r b o n d a t i n g of t h e l a s t d a c i a t i o n i n
Peru, Geology, 5, 600-604, -1977.
M e r c i e r , J. L., Extensional-compressional
t e c t o n i c s a s s o c i a t e d w i t h t h e Aegean
A r c : bmparison w i t h t h e Andean
C o r d i l l e r a of s o u t h Peru-north B o l i v i a ,
P h i l o s . Trans. R. Soc. London, Ser. A,
300, 33 7-355 , 1981.
M e r x r , J. L., and M. & b r i e r , Informe
sobre e l e s t u d i o p r e l i m i n a r de l a s
f a l l a s r e c i e n t e s de la r e g i o n de l a
r e p r e s a d e R e c r e t a ( C o r d i l l e r a Blanca ,
Dep. Ancash), r e p o r t , 8 pp.,
E l e c t r o p e r u , Lima, 1981.
M e r c i e r , J. L., N. D e l i b a s s i s , A. Gaut h i e r , J. J. J a r r i g e , F. L e m e i l l e , H.
P h i l i p , M. S é b r i e r , and D. S o r e l , L a
n é o t e c t o n i q u e de l ' A r c Eggen, E.
Géogr. Phys. G6o1. Dyn., =(I),
67-92,
1979.
M e r c i e r , J. L., P. Tapponnier, R. Armijo,
E. C a r e y - G a i l h a r d i s , Han T., and Zhou
J., F o l d i n g , k i n e m a t i c s of f a u l t s and
e v o l u t i o n of t h e s t a t e of stress i n
s o u t h e r n T i b e t , paper p r e s e n t e d a t t h e
I n t e r n a t i o n a l Symposium on Geology of
t h e Himalayas, Chengdu, P e o p l e ' s
Republic o f China, June 1984.
M i n s t e r , J. B., and T. H. J o r d a n , P r e s e n t day p l a t e motions, J. Geophys. Res.,
83, 5331-5334, 1978.
M i n z e r , J. B., T. H. J o r d a n , P. Molnar,
and E. Haines, Numerical modelling of
i n s t a n t a n e o u s p l a t e t e c t o n i c s , Geophys.
J. R. Astron. Soc., 36, 541-576, 1974.
Molnar , P., and P. Tapponnier , A c t i v e
t e c t o n i c s of T i b e t , J. Geophys. Res.,
83, 5351-5375, 1978.
N a k G u r a , K., and S. Uyeda, Stress
g r a d i e n t i n arc-back a r c r e g i o n s and
p l a t e s u b d u c t i o n , J. Geophys. Res.,
SS(Bl1) , 6419-6428, 1980.
P r i c e , N. J., F a u l t and J o i n t Development
i n B r i t t l e and S e m i - B r i t t l e Rock,
Pergamon, New York, 1966.
Sacks, I. S., A. T. Linde, A. Rodriguez,
and J. A. Snoke, Shalow s e i s m i c i t y i n
subduction zones, Geophys. R e s . L e t t . ,
5 ( 1 1 ) , 901-903, 1978.
S c h w e l l e r , W. J., L. D. Kulm, and R. A.
P r i n c e , T e c t o n i c s , s t r u c t u r e and
sedimentary framework of t h e Peru-Chile
Trench, Mem. Geol. Soc. Am.,
323350. 1981.
S é b r i e r , M., and C. d e Muizon, Comporte-
-
I
154,
780
Sébrier e t al.:
ment P l i o - P l é i s t o c è n e de l a c ô t e
péruvienne au niveau d e l a d e f l e c t i o n
d e P i s c o (Andes c e n t r a l e s ) , I X Rdun.
Ann. Sc. T e r r e , p. 575, P a r i s , 1 9 8 2 .
S é b r i e r , M., and J. Machare, Observaciones
a c e r c a d e l C u a t e r n a r i o de l a Costa d e l
Peru c e n t r a l , Bull. I n s t . Fr. Etud.
5-22, 1 9 8 0 .
Andines, 9-(1-23,
S d b r i e r , M., R. Marocco, J. J. Gross,
S. Macedo, and M. Montoya, Evolucion
neogena d e l Piedemonte P a c i f i c o de l o s
Andes d e l S u r d e l P e r u , i n Proceeding
II Congreso Geology C h i l e , v o l .
p.
71-88, I n s t . I n v e s t . Geol., A r i c a ,
1979.
S é b r i e r , M., G . Laubacher, R. Marocco,
A. Lavenu, and M. S e r v a n t , Evolution
t e c t o n i q u e CénozoIque des Andes
C e n t r a l e s (Sud Pérou-Bolivie), paper
presented a t the 26th International
Geological Congress, Soc. Géol. Fr.,
P a r i s , 198Ca.
S é b r i e r , M., A. Lavenu, and M. S e r v a n t ,
Apuntes r e c i e n t e s s o b r e l a n e o t e c t o n i c a
en l o s Andes C e n t r a l e s (Peru-BoIivial,
Bull. I n s t . Fr. Etud. Andines, % 1 - 2 ) ;
1 - 3 , 1980b.
S é b r i e r , M., D. Huaman, J. L. Blanc,
J. Macharé, D. Eonnot, and J. Cabrera,
Observaciones a c e r c a de l a Neotectonica
d e l Peru, 109 pp., I n s t . G e o f i s i c o
P e r u , S e i s m i c i t y Seismic Risk Andin
Region, Lima, 1 9 8 2 .
S e r v a n t , M., L e cadre s t r a t i g r a p h i q u e du
P l i o - Q u a t e r n a i r e d e 1 ' A l t i p l a n o des
Andes t r o p i c a l e s de B o l i v i e , Rech. Fr.
TNQUA, suppl., Bull. A s s . Fr. Etud.
Quat., 1 _ ( 5 0 ) , 323.327, 1 9 7 7 .
S i l g- a d o ., E., The Ancash, Peru. earthquake
of November 10, 1 9 4 6 ; Bull: Seismol.
SOC. h.,41, 8 3 - 1 0 0 , 1 9 5 1 .
S i l g a d o , E., H i s t o r i a de l o s sismos mas
n o t a b l e s o c u r r i d o s en e l Peru (15131 9 7 4 ) , Bol. 3 , Ser. C , 1 3 0 pp., I n s t .
Geol. Min., L i m a , 1 9 7 8 .
S o u l a s , J. P., Tectonique q u a t e r n a i r e : La
c ô t e p a c i f i q u e e t l a chafne andine du
Pérou c e n t r a l , Rev. Géogr. Phys. G6o1.
Dyn., 0 ( 5 ) , 3 9 9 - 4 1 4 , 1 9 7 8 .
S t a u d e r , W., Mechanism and s p a t i a l d i s t r i b u t i o n of Chilean earthquakes w i t h
r e l a t i o n t o subduction of t h e oceanic
p l a t e , J. Geophys. Res., 2 ( 2 3 ) , 50335061. 1 9 7 3 .
S t a u d e r , W., Subduction of t h e Nazca p l a t e
under Peru as evidence by f o c a l
mechanisms and by s e i s m i c i t y , J.
Geophys. Res., g ( 8 ) , 1053-1064, 1 9 7 5 .
2,
-
Quaternary Normal and Reverse F a u l t i n g
Suarez, G . , P. Molnar, and B. C.
Burchfiel, Seismicity, f a u l t plane
s o l u t i o n s , depth of f a u l t i n g , and
a c t i v e t e c t o n i c s of t h e Andes of P e r u ,
Ecuador, and s o u t h e r n Colombia, J.
Geophys. Res., E ( B 1 2 ) , 10403-1Oz28,
1983 e
Tapponnier, P., and P. Molnar, S l i p - l i n e
f i e l d theory and l a r g e - s c a l e
continental tectonics, Nature,
319-324, 1976.
Tapponnier, P., and P. Molnar, A c t i v e
f a u l t i n g and Cenozoic t e c t o n i c s of t h e
Tien Shan, Mongolia,
and B a i k a l
r e g i o n s , J. Geolphys. Res., 84, 324 53459, 1979.
Tapponnier, P., J. L e M e r c i e r , R. Armijo,
Han T., and Zhou J., F i e l d evidence f o r
a c t i v e normal faultine: i n T i b e t .
9 -Nature
2 9 4 , 410-414,v1981.
Uyeda,. S., . Subduction zones and back a r c
b a s i n s : A review, Geol. Rundsch.,
7 0 ( 2 ) , 552-569, 1 9 8 1 .
U y e z ,. 'S., . Subduction zones: An i n t r o d u c t i o n t o comparative subductology,
Tectonophysics, 3, 1 3 3 - 1 5 9 , 1 9 8 2 .
Uyeda, S., and H. Kanamori, Back a r c
opening and t h e mode of s u b d u c t i o n , J.
Geophys. Res. , 84, 1 0 4 9 - 1 0 6 1 , 1 9 7 9 *
V i c e n t e , J. C., F. S e q u e i r o s , M. A.
V a l d i v i a , and J. Zavala, E l
cabalgamiento de Cincha-Pampacolca:
Elemento d e l a c i d e n t e mayor andino a l
NW d e Arequipa ( S u r d e l P e r u ) , Bol.
Soc, Geol. Peru, 61, 6 7 - 9 9 , 1 9 7 9 . ,
Yonekura, N., T. Matsuda, M. Nogami, a d
S. Kaizuka, An a c t i v e f a u l t along t h e
western f o o t of t h e C o r d i l l e r a Blanca.
Peru, Chigaku ZassRi, %(1),
1-19,
1979.
264,
-
E. Carey-Gailhardis, J. L. M e r c i e r , and
M. S é b r i e r , L a b o r a t o i r e d e Géologie
Dynamique I n t e r n e , U.A. 730 CNRS, b a t .
5 0 9 , U n i v e r s i t é Paris-Sud, 91405 Orsay
cedex, France.
G. Laubacher, ORSTOM, 2 4 r u e Bayard,
75008 P a r i s , France.
F. Mégard, Centre Géologique e t
Géophysique, U n i v e r s i t é des S c i e n c e s e t
Techniques du Languedoc, 34060
M o n t p e l l i e r , France.
(Received January 1 7 , 1 9 8 5 ;
r e v i s e d June 2 1 , 1 9 8 5 ;
accepted June 2 3 , 1985.)