476
Fourth fSAG. Goettin gen (G ernuuty], 04-(}6IfOlf999
QUATERNARY BRITTLE DEFORMATION IN THE
CALDERA AREA, NORTHERN CHILE (27°S)
Car los MARQUARDT( 1) and Alain LAVENU(2)
( 1) SERNAGEO MIN Chile, Av. Santa Mar ia 0 104. Santiago (cmarquar@sernageo min.cl)
(2) [RD. casilla 53 390. correo central. Santiago 1.Chile ([email protected])
KEY WORDS : Neotectonics. exrensional deformation. uplift. marine terrace.
INTRODUCTION
Even though evidence of young brirtle deformation is well preserved in severa] localiries of the
coastal belt south of Caldera (2]0-27° 15'S). the only reference available regarding faulting in the area is
found in Mortimer ( 1969). who describes a Neogene reverse fault followed by normal reacti vation SE of
Morro Copiap ô (figure 1). Deformation is characterised by variously oriented high angle normal faults.
T hey show centirneter ta rneter long displacernents in sedimentary layers assoc iated to Quaternary marine
terraces and. occasionaly, originale scarps that may be weil distinguished l'rom marine cliffs .
We thus infer the normal faults present in the southern part of the area based on the fact that they
eut old beach-rid ges and ether shorelines, specially those more oblique to the fault traces. A second
criterium we have applied considers the fact thar, in contrast to marine c liffs, the base of fault scarps
shows a variable elevation. refered to sea-Ievel.
Two case studies have been selected in order to better describe the geornetry, kinematics and age
of these faults : the Calderilla and Bahia Inglesa localities.
CALDERILLA
One NI OOE and three N600E trending normal faults outcrop in the area between Caldera and
Bahia Inglesa. Fault planes are weil exposed only in the major one l'rom the second set. whose base scar p
varies l'rom sea level up to 45 m above il. In figure 2 we describe the following three profiles. located in
figure 1. along this fault :
Fourth /SAG, Goettingen (Germany), 04-06//0//999
Profile A.-
two
477
normal faults are exposed at a elevation of 9 m, one km inland northeast l'rom
Bahia Inglesa. They offset both the hase of the sedimentary coyer and erosional unconformities within
littoral and continental unconsol idated sediments with an age range l'rom Neogene to Quaternary. These
N72 °E/66°N and N81° E/64° N trending faults suggest an hemigraben related to NNW-SSE extension. The
small western antithetic normal fault in the hanging wall may reflect minor rigid block rotation.
Deformation took place after deposition of the sedimentary coyer , whose age is given by that of
the marine terrace (Qm), formed at 125 kyr (Marquardt et al ., 1999).
Profile 8 .- a branched normal fault that crops out at 13 m a.s.\. displaces at least 3 m the base of
the Quaternary shallow marine sediments (outside l'rom figure 2B). The fault seems to be sealed by eolian
and aluvial deposits, yet the development of a superficial scarp favours their involvement in the
deformation .
The NE-SW trend of this Iault suggests a NW-SE extensional deformation . As shown in the
sarne figure, 10 m southeast of this structure a set of four small reverse faults may be related to footwall
rigid rotation .
Profile
c.- 2.5 km northeast of the previous profile and 10 m southeast from the main fault trace
(figure 2C); we recognise two secondary SE dipping normal faults. A srnall NW dipping reverse fault is
associated to one of them. Because no scarp is preserved in the loose overlying sands, faulting seems to
have displaced only the marine deposits, assigned here to the 220 kyr (l .S. 7) marine high stand. Their
offset is less than a meter and their trend once more suggests a NW-SE exten sional deformation.
Age of deformation may here be sornewhat older than in profile 2A, considering the age
assigned to the only markers clearly displaced.
478
FOItrlh /5AG. Goettingen (German)'), 04-06//0//999
70 '55'
N
t
27 '0 5'
Bahia Inglesa
[=:::J
[=:::J
Shore sed iment (Quaternary)
~
Plutons (Jurassic)
........
Beach-ridge (Quat ernary)
-
Marine sediment ( Neogene)
tcu:
Marine di ff ( Q uarer nar y )
...t.....:
Normal Fault (Quaternary)
~
......,
Reverse Fault (Neoge ne)
......... Fault in ferred/cov ered
Ca ldera harbour
~ Profile localitie s, figure 2
Figure 1.- Geologie sketch map show ing the main structure of the coastal south Caldera harbour.
Fourtli /SAG. Goettingen (Germanyï. 04-06//01/999
479
A
1 NW·
SE
?
C)
1:: ::::::1Qe
~
1
Eolian (Quaternary)
Qal Aluvial(Quaternary)
L" 1 Qm Fossils (Qu aternary )
~I Qm Marin e (Q uaternary)
INW - SE
~ Ne Marine (Neogene)
Pluton (lurassic)
Figure 2.- Profile along the same normal fault scarp, inland Calderilla.
480
Fourth /5AG, Goettingen (Ge rmany), 04-06//0/1999
BAHIA INGLESA
A NNE-SSW trending normal fault scarp, subparallel to a marine cliff whose base lies at 30 m
a.s.l., develops 2 km east of Bahia Inglesa.
The fault scarp faces westward in its northern half and eastward in the southern one, showing
that the structure corresponds to a scissor fault (figure 3). A small graben is developed westward of the
northern section. Vertical offsets range from 4 m down to half a meter in the vertica l dipping central
zone. A WNW-ESE extensional deformation, younger than 125 kyr (age of the displaced marine
sediments) may be infered for this structure.
WNW
ESE
~
(No rthe rn most pro file)
+22
-===::
'\-
m
J
N25"WJ80.W
\
D
Quaternary shore sediment
associated with the 125 kyr
CIS 5) carver marine terrace
D
Quaternary shore sediment
associated with the 220 kyr
(lS 7) carver marine terrace
r:------
-=::::=::::::::======1" Fau 1t scarp
(
F"
~
• ) Marine cliff
+30 m
Altitud in m a.s.I.
1
(
1
+30 m
=
_+2o--.Js1T
o
1
5 m
C1-=-C1
1
:x=
\
(Southern mos t pro file)
.: =-
~
+30~
:==::=::::;'
1
N280 E/80o E
Ir
Figure 3.- NS advanced profiles
alon g the fault inland of Bahfa
Inglesa area.
Fourth ISAG, Goettingen (Germany) , 04-0611011999
481
CONCLUSIONS
In the coastal belt between Tl " and 28°S evidence of neotectonic activity is recorded by :
•
Quaternary marine terraces uplifted at various heights under 200 m a.s.1.
•
brittle deformation found in the terrace sediments close to Caldera (27°00' to 27° 15' S).
The brittle deformation is recorded by probably coseismic normal faults that may be related to
reactivation of preexistent structures . Based on their trend and the age of the sediments involved in the
deformation, we propose that extensional deformation took place after 220 kyr
(most like1y after 125 kyr), with a WNW-ESE to NNW -SSE trend. Extension may have resulted from
uplift and trenchward migration of the coast ,
The "major" faults show a maximum vertical displacement of 4 m. Normal and reverse cm scale
faults, associated to rigid block rotation, are recognized in both their foot and hanging walls.
ACKNOWLEDGEMENTS
This study was supported by the chilean Servicio Nacional de Geologîa y Minerfa, as a part of its
"Geological Mapping of the Atacama Fault System and Coastal Belt between 26° and 28°S", a project
carried out with Estanislao Godoy, to whom we acknowledge his help in both field work and discussion
of the text.
REFERENCES
Marquardt, C; Ortlieb, L., Guzmân, N. and Lavenu, A., 1999. Recent Vertical Motions and Quaternary Marine
Terraces in the Caldera Area. (This Simposium).
Mortimer, C; 1969. The GeomorphologicaJ Evolution of the Southern Atacama Desert, Chi le. A thesis submitted
the University of London for the Degree of Philosophy. Departament of Geology, University College London.
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