A microstructural study of S-C mylonites of part of the Tanque
Verde Mountains, Tucson, Arizona
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Martins, Verónica E. de Sousa Carvalho
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A MICROSTRUCTURAL STUDY OF S-C MYLONITES
OF PART OF THE TANQUE VERDE MOUNTAINS,
TUCSON, ARIZONA
by
Veronica E. de Sousa Carvalho Martins
A Thesis Submitted to the Faculty of the
DEPARTMENT OF GEOSCIENCES
In Partial Fulfillment of the Requirements
For the Degree of
MASTER OF SCIENCE
In the Graduate College
THE UNIVERSITY OF ARIZONA
19
8 4
5? £ 2 o
STATEMENT BY AUTHOR
This thesis has been submitted in partial fulfillment
of requirements for an advanced degree at The University
of Arizona and is deposited in the University Library to
be made available to borrowers under the rules of the Library.
Brief quotations from this thesis are allowable
w i t h o u t s p e c i a l p e r m i s s i o n , p r o v i d e d that a c c u r a t e
acknowledgement of source is made.
Requests for permission
for extended quotation from or reproduction of this manuscript
in whole or in part may be granted by the head of the major
department or the Dean of the G r a duate College when in
his or her judgment the proposed use of the material is
in the interests of scholarship.
In all other instances,
however, permission must be obtained from the author.
SIGNED:
6 ir Soosa
r /V<UJA
LttO
Aev? a ):>
APPROVAL BY THESIS DIRECTOR
This thesis has been approved on the date shown below:
W
f
r
i
f
.
l
l
-
GEORGE H. DAVIS, Ph.D.
Professor of Geosciences
Date
ACKNOWLEDGEMENTS .
I am very grateful to Dr. G. H. Davis for suggesting
this project to me and for guiding me throughout its execution.
His
generosity
of
financial
resources,
his
enthusiasm,
and his encouragement are deeply appreciated.
I would
Dr.
J. G a n g u l y
also
for
like
serving
to
thank
Dr.
as m e m b e r s
P. J.
of
the
Coney
and
committee,
and Dr. J. F. Schreiber, Jr. for his photographic help.
This
research
was
supported,
in
p a r t , by
Geotectonics Laboratory, The University of Arizona.
the
Additional
funding by the Graduate College of The University of Arizona,
through a Graduate Student Program Development Grant helped
to m a k e
this
study
possible.
Personal
financial
support
was partially provided by the African-American Institute.
Finally I would like to thank my parents,
and
lido
given me.
Carvalho
for
the moral
Carolina
support they have always
TABLE OF CONTENTS
Page
LIST OF I L L U S T R A T I O N S .................................
LIST OF T A B L E S .......................................
A B S T R A C T .............................................
INTRODUCTION
.........................................
Scope and P u r p o s e ............................
Location and Methods
........................
Previous W o r k .................................
GEOLOGY OF THE STUDY A R E A ............................
Geologic Setting
............................
Rock T y p e s ...................................
Mesoscopic Description
......................
Mylonitic G n e i s s ............... .. .
Microbreccia
........................
Petrographic Description
....................
Mylonitic Gneiss
....................
........................
Microbreccia
OUTCROP EXPRESSION
v
vii
viii
1
1
3
6
8
8
10
11
11
13
14
14
17
......................................
21
Exfoliation Parting .............................
F o l i a t i o n ........................................
L i n e a t i o n ........................................
F r a c t u r e s ..............................
21
21
26
MICROSTRUCTURES
26
........................................
32
S-C Foliations
.............................
Asymmetric porphyroclasts ......................
Quartz Fabrics
. ................................
Microbreccia Fabrics
...........................
40
41
42
INTERPRETATION
..........................................
R E F E R E N C E S ...........................................
iv
32
43
49
LIST OF ILLUSTRATIONS
Figure
Page
1.
Northeast view of the F l a t r o c k s ..............
2
2.
Photomicrograph of spectacularly developed
S-C foliations in the mylonitic gneiss
indicating dextral shear ......................
4
Location map and structural geologic map
of the study a r e a .............................
5
Schematic cross section of metamorphic core
c o m p l e x e s ................. ....................
9
5.
Rock slabs of typical S-C mylonitic gneiss . .
12
6.
Photomicrographs of mylonitic gneiss .........
15
7.
Photomicrographs of microbreccia ...........
.
18
8.
Outcrop expression of the mylonitic gneiss . .
22
9.
Detached block of the mylonitic gneiss . . . .
23
10.
Outcrop expression of mylonitic gneiss and
"exfoliation parting" looking NW ..............
24
Lower-hemisphere equal area projection of
(a) poles to mylonitic foliation and (b)
lineation as measured in outcrop ..............
25
12.
Lineation in the mylonitic g n e i s s ...........
27
13.
Two views of the outcrop appearance of the
vein-like s e a m s ...............................
29
14.
Map of the fracture and vein pattern . . . . .
30
15.
Lower-hemisphere equal area projection of
fractures and s t r i a t i o n s .................
31
A series of photographs of the "S-C"
mylonite in thin s e c t i o n ......................
33
3.
4.
11.
16.
v
vi
LIST OF ILLUSTRATIONS— Continued
Figure
Page
17.
Evolution of S-C
angular relationship
18.
Failure envelope
diagram
. . . .
..................
39
47
LIST OF TABLES
Table
1.
Page
S-C angular r e l a t i o n s ......... ..
vii
. . . . . . .
38
ABSTRACT
The
Verde
exquisitely
Mountain,
developed
S-C
exposed
Tucson,
surfaces,
L-S mylonites
of Tanque
Arizona,
exhibiting spectacularly
are
record
the
of simple
shear
deformation in a thick regional shear zone of core complex
affinity.
This
study,
prima r i l y a microstructural one,
also
consisted of geologic mapping, including systematic measurement
and description of foliation,
lineation, joints, and veins.
The major lithologic unit is a quartzo-feldspathic mylonite
derived
from
a guartz-monzonitic
strikes N49°W and dips 16°SW.
across
Foliation
Lineation lies in the plane
of foliation and plunges 14° S61°W.
cut
protolith.
Joints and vein structures
the mylonite orthogonally to the lineation and
microscopically are seen to be veins of quartzo-feldspathic
microbreccia clearly derived from the tectonite,
The
most
conspicuous
of
two
sets
of
foliations
Present in the gneiss, the C-surfaces, are defined by trails
of m i c a c e o u s minerals
and
are
parallel to the shear zone walls.
interpreted
to be aligned
The second set of foliations,
the S-surfaces, is marked by elongate quartz grains, aligned
unequidimensional feldspars and quartz ribbons.
The S—surfaces
are
the plane
interpreted
to be aligned
viii
parallel
to
of
ix
flattening.
C-surfaces
the
sense
The
angu l a r
constitutes
of
normal-slip.
evaluation
shear,
relationship between the S- and
a useful
which
for
criterion
the
e ntire
in determining
study
area
is
Such a result is also supported by the kinematic
of
microfractures.
asymmetric
porphyroclasts
and
tensi o n a l
INTRODUCTION
Scope and Purpose
In
of
recent years
interest
among
interpretation
shear
zones
there
str u c t u r a l
has
a dramatic
geologists
of m i c r o s t r u c t u r e s
(Ramsay
been
and G r a h a m
in the study and
in m y l o n i t i c
1970;
surge
Watts
rocks of
and Williams
1979? Brunei 1980; Ramsay 1980a? Burg, Iglesias, and Ribeiro
1981;
The
Brown
and
importance
sense
of
Mu r p h y
Simpson
of m i c r o s t r u c t u r e s
tectonic
deformation,
1982?
and
transport,
the
lies
and
of
1983) .
in evaluating
the physical
nat u r e
Schmid
the
environment of
progressive
deformation
through time.
Within very wide mylonitic
significance,
the shear
zone
the
as
difficult
a whole.
absence
of
zones of regional
zone boundaries may not be clearly
exposed or may no longer be preserved.
it b e c o m e s
shear
to
interpret sense of shear
However,
expo s e d
Under such circumstances
certain microfabrics,
shear
for the
even in
zone boundaries can be used
for kinematic interpretation.
In
part
is
of
the
study
Tanque Verde
extr a o r d i n a r y .
area,
lo c a l l y
Mountain,
Large
k nown
as
Flatrocks,
the exposure of mylonites
expanses
display
100%
of beautifully developed L-S tectonites (Figure 1).
1
exposure
The
2
Figure 1.
Northeast view of the Flatrocks.
3
tectonites,
when examined microscopically,
reveal pervasive
S-C fabrics (Figure 2) which can of course be used to determine
sense
The
of
shear
mylonites
h ave
formed
(Berthe,
w ere
Choukroune,
interpreted
within
a thick
by
and
Jegouzo
Davis
regional
(1980,
structures
of
Ta n q u e
Verde
1983)
to
shear zone which has
accommodated regional extension in the Tertiary.
and
1979).
Mountain
are
The rocks
regarded as
one of the metamorphic core complexes of the western Cordillera
(Crittenden, Coney, and Davis 1980).
The
area
and
to
in
exposure
evaluate
detail
sense
the
document
also
the
shear,
of
to
the
as the study
describe
fabrics,
carefully
in p a r t i c u l a r
and to speculate on the origin of veins
It is hoped that this research will further
view
provide
of
nature
the S-C structures,
of microbreccia.
at Flatrocks was chosen
a
of
a southwest sense of shear and will
better
understanding
of
the
kinematic
significance of thick zones of mylonitic rocks.
Location and Methods
The
of
the
Flatrocks
southeastern
approximately
An
a rea
mapping
1:4800.
study
30
km
of about 0.5
of
the
More
flank
east
site
of
is
located
Tanque
at
Verde
base
Mountain
of
Tucson,
Arizona
km2 was
studied
in detail.
entire
the
(Figure 3).
Geologic
area was carried out at a scale of
detailed
mapping
at
a scale
of
1:1200
was carried out to capture details of the fracture and vein
Figure 2.
N e g a t i v e p r i n t of a thin section of exquisite
S-C my l o n i t i c gneiss indicating dextral shear.
Note the perfect diamond-shaped sphenes (scale:
1 cm = 1.4 mm) .
Figure 2
Figure 3.
(a) Location map.
(b) Structural geologic map of the study area.
A rizo n a
Tucson
^
Explanation
y, strike & dip of foliation
Ql trend & plunge of lineation
/
/
trace of fracture, dashed
where covered
□
mylonitic gneiss
Figure 3
b
6
system.
Approximately
gneiss
w ere
40
collected,
oriented
and
from
s a mples
these,
of mylonitic
30
oriented thin
sections were prepared for detailed petrographic examination.
Previous Work
A number
of
studies
have ,been
made
of
the
and structures of the Tanque Verde Mountain area.
with the exception of Di Tul l i o ’s work
rocks
However,
(1983), these studies
noticeably lack analysis of microstructural fabrics.
Pashley
(1966)
focused on the structures surrounding
the Tucson basin and was the first to recognize the relationship
betw e e n
Verde
the
topography
Ridge.
Leger
and
(1967)
the
structure
studied
of
f o lding
the
Tanque
and jointing
in the gneiss and like Pashley (1966) attributed the structures
to lateral compression from the southeast.
Drewes
(1974,
1977)
of the Rincon Mountains
fault
as
a thr u s t
east-northeast
mapped
in detail
and interpreted
fault,
attributing
Laramide compression.
the geology
the Santa Catalina
the
deformation
He further
to
suggested
some reactivation of older thrust faults by low-angle normal
faulting.
New
ideas
eme r g e d
in
the
eighties,
particularly
among researchers of The University of Arizona
(Davis 1980,
1983? Davis, Gardulski, and Anderson, 1981; Di Tullio 1983).
Davis
the
(1983)
Rincon
proposed
Mountains
that
are the
the
rocks
and
structures
of
record of normal-slip simple
7
shear within a thick regional shear zone.
this
point
of view,
Di
Tullio's
Further documenting
study of the fault
rocks
of the Tanque Verde Ridge indicates a large degree of brittle
deformation that suggests simple shear on a microstructural
scale.
GEOLOGY OF THE STUDY AREA
Geologic Setting
The Tanque Verde Mountains are one of several plunging
antiforms that comprise the Santa Catalina-Rincon metamorphic
core
complex.
s t r uctural
(Davis
The
complex
packages:
and
Coney
typically
tectonites,
1979;
Davis
consists
of
microbreccias,
1980;
Davis
et
three
and cover
al.
1981)
(Figure 4).
The tectonites derived from Precambrian and Tertiary
granitic protolith are characterized by low-dipping foliation
and pervasive lineation, and they constitute the predominant
lithologic
1977)
unit.
They
have
been mapped by Drewes
(1974,
as the Continental Granodiorite and the Wrong Mountain
Quartz Monzonite.
Overlying the mylonites of the core is the decollement
zone
and
characterized
by
intense
hematite-epidote-chlorite
the m y l o n i t i c
overprinted,
fabric
sometimes
of
the
brecciation,
alteration.
catac l a s i s ,
In
this
underlying gneissic
completely destroyed,
zone
rocks
is
by grain-size
reduction and intense alteration.
Most of the gneissic complex
rocks,
but
at
unmetamorphosed
the
base
of
the
sedimentary
mou n t a i n s ,
rocks
8
is denuded of younger
of
the
deformed
cover
rest
but
in
9
Ledge of
m icrobreccia
Detachment
Pdcollem ent
De'collement
zone
Tectonite
gneiss
M ic ro hrecciate
mylpnitic
news
Precam brtan
q u a r tz monzonite
protolith
Figure 4.
T e rtia ry
quartz, monzonite
p ro to lith
S c h e m a t i c c r o s s s e c t i o n s h owing s t r u c t u r a l
e l e m e n t s of m e t a m o r p h i c core complexes (Davis
et al. 1981).
10
fault
contact
Catalina
and
on
fault
conforms
the
gneiss.
(Pashley
with
the
This contact,
1966;
Drewes
general
known as the
1977),
orientation
dips gently
of
foliation
in the mylonitic rocks.
Recent
have
workers
proposed
earlier
figures
in the
a kinematic model
interpretations.
in
T a n q u e V e r d e Mountain area
these
Davis
later
that differs
(1983),
studies,
shear
deformation
by n o r m a l - s l i p
to
crustal
along
a thick
displacement
extension
in
the
suggested
that
the
are products of simple
shear
that
from
one of the leading
has
rocks and structures of the complex
greatly
took
zone characterized
place
Tertiary.
The
in
response
fault
rocks
represent progressive deformation through time which reflects
a passage
to
higher
and
higher
level
conditions
causing
the overprinting of the tectonites by microbrecciation.
Rock Types
The entire study area is dominated by a medium-grained,
light-colored
L-S m y l o n i t i c
rocks of the Catalina fault.
this
unit
as
gneiss,
part of the footwall
Although Drewes
(1977)
Precambrian Wrong Mountain Quartz
mapped
Monzonite,
apparently correlative rocks in the Santa Catalina Mountain
are now
believed
to
be
of
Tertiary
age
(44
to
47
m . y .)
(Shakel, Silver, and Damon 1977; Keith et al. 1980).
Minor
include
small
lithologic
pegmatitic
u nits
exposed
intrusions
in the study area
and
mic r o b r e c c i a t e d
11
material
which
that
crosscut
lineation.
to
be
occupies
the m y l o n i t i c
At
quartz,
narrow
first
but
the v e i n - f i l l i n g
glance
vein-like
gneiss perpendicular to the
these vein-like
microscopic
is
indiscrete seams
intensely
examination
seams appear
reveals
shattered and crushed
that
rocks
comprising microbreccia (see Figure 7).
Two different kinds of microbreccia were identified;
a fine-grained,
greenish white,
a medium-grained,
quartz-rich breccia; and
brownish-white, altered and essentially
quartz-feldspathic breccia.
Mesoscopic Description
Mylonitic Gneiss
In hand
medium-grained,
foliated,
sample,
almost equigranular,
and p e n e t r a t i v e l y
of very large size,
unit
the mylonitic gneiss
exposed
in
is typically
moderately to strongly
lineated
(Figure
5) .
Augen
so characteristic of the other gneissic
the
Tanque Verde
Mountains
and
derived
from 1.4 b.y. quartz monzonite, are absent and porphyroclasts
of feldspars are seldom larger than 5 mm.
T his
unit
is
greenish
gray
it weathers to a brownish gray color.
in
and
composition
apatite.
with
quartz
Feldspar
mostly
fresh surfaces
forms
and
It is quartz-feldspathic
smaller amounts of biotite,
porphyroclasts
2 mm to as much as 10 mm long.
but
on
range
chlorite,
in size from
Augen can also be of quartz,
st r i n g s
that
can
be
up
to 3 cm
Figure 5.
P o l i s h e d s u r f a c e s of typical S-C m y l o n i t i c
gneiss.
(a) View is parallel to lineation and perpendicular
to foliation (scale:
1 cm = 2.5 m m ) .
(b) v i e w is p e r p e n d i c u l a r to l i n e a t i o n and
foliation (scale:
1 cm = 3.9 mm).
12
Figure 5
13
long.
Typically the quartz strings are seen to bend around
the porphyroclasts.
The alignment of micas and elongate
minerals on the visible foliation surface defines a mineral
lineation that is pervasive.
Microbreccia
The light-colored microbreccia is a greenish white,
and very
fine-grained
material.
gray,
It w e a t h e r s
black,
almost
rock
to
and white.
entirely
of
with
long
po c k e t s
of coarser
a variety of colors:
It is highly silicified,
m i c r o c r y s t a l l i n e quartz.
brownish
composed
The pockets
of coarser grained material are made up of quartz, feldspar,
and
clay.
Here the quartz
seen to be randomly oriented.
grains
are discernible and are
Some of the clasts are angular
and have perfect square shapes that are up to 2 mm in length.
The
darker
variety
of microbreccia present
in the
area and occupying the extensional fractures is dark greenish
gray and it weathers
the
light-colored
m u c h greater.
up m ost
of
a v e r aging
to brownish gray.
one,
and
the p e r c e n t a g e
The dominant mineralogy
the
about
matrix,
3 mm
and clasts
in
It is coarser than
size.
of
c lasts
is chlorite,
of quartz
Opaques
are
is
making
and feldspar
abundant and
commonly associated with the fragments of quartz and feldspar.
Thin qu a r t z
this
unit
in
veins
of approximately 1 mm
a ran d o m
fashion.
in width crosscut
Alteration
is dominantly
chloritic with minor amounts of epidote and hematite.
14
Petrographic Description
Mylonitic Gneiss
The
from
tectonite
quartz
within
monzonitic
porphyroclasts
of
the
plutonic
feldspar
in
various accessory minerals.
minerals
Two
biotite
size
sections:
zircon,
group s
is
it
derived
contains
matrix
(chlorite),
of
and
Its mineralogical composition,
26% potassium feldspar,
9% biotite and chlorite,
(sphene,
distinct
thin
r o c k , and
consists of 46% quartz,
15% plagioclase,
area
a fine-grained
q u a r t z , feld s p a r , plagioclase,
on average,
study
apatite,
were
and 3%
opaques,
r e c ognized
porphyroclastic
grains
accessory
and epidote) .
in most of the
of
feldspars
and
more rarely quartz and sphene; fine-grained matrix (48-220 n)
consisting predominantly of quartz and feldspar (Figure 6).
Augen
are
xenoblastic
and
display
a wide variety
of shapes.
Although circular to ellipsoidal augen predominate,
i r r e gular
to
tab u l a r
augen
are
also
present.
Feldspar
porphyroclasts are generally rounded and possess an asymmetry
which
They
is
defined
exh i b i t
by
traces
the
of
asymmetrically
fractures
rotated
tails.
that are preferentially
oriented at a high angle to the dominant foliation as seen
in thin sections and in many cases the fractures are filled
by
quartz
and
chlorite.
in pl a c e s
are
clustered
shaped patches.
Porphyroclasts
into
occur
singly and
crude bands and irregularly
Secondary alteration of feldspars to sericite
Figure 6.
P h o t o m i c r o g r a p h s of
(a) and p o l a r i z e d
ribbons, boudin-like
augen (scales
1 cm =
mylonitic gneiss in plane
light (b) showing quartz
structures, and asymmetric
0.7 mm).
15
Figure 6
16
is common.
Many augen are poikiloblastic with inclusions
of quartz, chlorite, apatite, and zircon.
Quartz may occur as porphyroclasts, but it is largely
part of the fine-grained matrix.
of new grains with
grain
that
boundaries,
can
be m o r e
Recrystallized into aggregates
fairly constant grain size and serrated
it is distributed
than
10
mm
in
in ribbons and lenses
length,
and w h i c h
occur
in parallel to subparallel groups causing a strong lineation.
Elongation of quartz grains,
best developed in the ribbons,
is prevalent and elongate grains show boudin-like structures.
I r r egular
masses
typically
interstitial
grains
show
of qua r t z
typical
to
strain
are
also
a ugen
or
found
and
they
feldspar groups.
te x t u r e s
such
as
are
The
undulatory
extinction.
Biotite is distributed both as disseminated plates
displaying a consistent alignment, and as irregular clusters.
Most grains are tabular with a shredded appearance.
biotite is associated with chlorite,
Generally
the latter being a
replacement product of the former.
The
matrix.
remaining
Of
inte r e s t i n g
forms
perfect
It
a very
is
minerals
form
diamond-shaped
common
are m o s t l y present
is
sphene
crystals
ac c essory mineral
which
(see
in the
typically
Figure
2).
and is distributed
as scattered single grains and less commonly in small clusters.
17
A large percentage of the grains are fractured.
The crystals
have lengths of .9 mm to 1.5 mm.
Zircon
fine-grained
augen.
and
apatite
matrix
and
They both have
o ccur
as
as
constituents
idioblastic
of
inclusions
the
in the
the same size range and are usually
less than 1 mm in length.
Opaque
minerals
all thin sections analyzed.
grains
associated
with
occur
as
minor
constituents
in
They are distributed as individual
clusters
and
bands
of
biotite.
They also occur as inclusions in porphyroclasts.
Microbreccia
Viewed microscopically the fine-grained, light-colored
microbreccia
finely
less
crushed,
than
quartz
is m a d e
.05
up primarily of quartz.
with
mm
mineral
to
.1 mm
grains
ranging
(Figure 7a).
It is very
in size from
Larger
clasts of
and feldspar float in the matrix and can be as large
as .7 m m in length.
Both quartz and feldspar exhibit undulose
extinction and possess
alteration
products
fractures
such as
that are filled with dark
iron oxides.
Kinked twinning
of plagioclase is another common deformational feature.
Biotite is present in small amounts and is commonly
replaced
dark
by
clots.
chlorite.
Other
Epidote
accessory
zircon, and opaque minerals.
occurs
minerals
as
l arge
include
tabular
apatite,
Figure 7.
Photomicrographs of (a) fine- and (b) coarse-grained
m i c r o b r e c c i a in p l a n e l ight (scale:
1 cm =
0.7 mm) .
18
Figure 7
19
In thin section the microbreccia is seen to display
a network
of
crosscutting
veins,
the
older
ones showing
more signs of strain such as very strong undulose extinction.
Some
large
fr a g m e n t s
are
seen
to be
displaced when
cut
by the veins and generally display normal-slip displacement.
The
and,
dark-colored
microbreccia
is
similar to the other microbreccia,
joints filled with quartz
medium-grained
is cut by numerous
(see Figure 7b) .
The percentage
of large clasts is much higher and most clasts are of feldspars
and quartz.
by
Domains
f r a ctures
are
to the m y l o n i t i c
in the microbreccia which are bounded
compositionally
gneiss.
In
and
these
texturally
domains
similar
the foliated
fabric is preserved and quartz ribbons are still common.
The
mineralogical
microbreccia
replacing
and
sphene
comprises
biotite.
composition
quartz,
Opaques,
of
this variety
feldspars,
apatite,
are minor constituents.
and
chlorite
epidote,
In general
of
zircon,
the grains
are subangular to subrounded.
Quartz
and
feldspars
typically
show deformational
features such as undulose extinction and pervasive fractures.
Commonly they are poikiloblastic with
and
apatite.
Plagioclase
clasts
inclusions of zircon
are
both
fractured
and
kinked.
Chlorite
is d i s t r i b u t e d
in
is
ra n d o m l y
ir r e g u l a r
disseminated
clusters.
and
S phene
in
cases
occurs
as
20
broken pieces of the once perfect diamond-shaped crystals
and
is
usu a l l y
found
fabric is preserved.
in the
do m a i n s
where
the mylonitic
OUTCROP EXPRESSION
Exfoliation Parting
"Exfoliation
very
clos e l y
parting"
spaced,
is
here
used
foliation-parallel
to
describe
fractures
which
impart to the mylonite outcrops their characteristic expression
(Figure 8).
The planes
correspond
to
the
mylonitic gneiss,
are
shear
surfaces,
the
well
t hat p a r a l l e l
developed
the
these partings
C-surfaces
to be described shortly.
through-going,
faults
of weakness marking
of the
In places there
low-angle
detachment
exfoliation parting
(Figure 9).
Along such detachments no brecciated material was recognized,
but the mylonite is intensely altered and the fabric obliterated
by alteration.
Foliation
In
outcrop
measureable
elongate
(Figure
grains
Foliation
southwest.
a single
and/or
generally
The
10),
dip
foliation
defined
conspicuous
r i bbons
aggregates of quartz
strikes
of
by
is
the
N49°W,
and
of quartz,
and feldspar.
dipping gently to the
foliation
rarely
exceeds 20°
(Figure 11 and see Figure 3 b ) .
Locally in outcrop a second foliation can be observed.
Fashioned by the planar preferred orientation of micas
(biotite and chlorite) and aligned rotated tails of asymmetric
21
22
Figure 8.
View of the outcrop expression of the mylonitic
gneiss in relation to the overall disposition
of t h e r o c k s in t h e T a n q u e V e r d e Ridge
(background).
23
Figure 9.
Detached block of mylonitic gneiss
24
Figure 10.
O u t c r o p e x p r e s s i o n of m y l o n i t i c g n e i s s
and "exfoliation parting" looking NW.
Figure 11.
L o w e r - h e m i s p h e r e e q u a l area p r o j e c t i o n of
(a) poles to mylonitic foliation and (b) lineation
as measured in outcrop.
25
foliation
yi points
Figure 11
26
porphyroclasts,
up to
this
foliation
30°
to
the most
foliation
is
b est
is
conspicuous
viewed
on
inclined
at angles
foliation.
surfaces
This
which
of
second
are parallel
to lineation and perpendicular to foliation.
Lineation
Lineation
trending
3b) .
S61°W
+ 8°
Lineation
mafic minerals
quartz
is p e n e t r a t i v e
and
plunging
is d e f i n e d
(biotite,
aggregates
and parallel
by
14°
the
chlorite)
(Figure 12).
in outcrop,
(see Figures 11 and
alignment
of
inequant
and smeared and elongate
Lineation lies
in a plane
corresponding to the most prominent foliation.
The attitude of the lineation
by
the
across
wa s
large
it.
number
Though
observed
spaced
that
fractures
of
no
in
the
fractures
seems
that
to be affected
orthogonally
cut
displacement can be recognized,
certain
"panels"
lineation
it
bounded by closely
displays
a slight
shift
in trend from neighboring panels.
Fractures
One
area
cut
are
of
the
across
the
many
most
striking
steeply
di p p i n g
fe a t u r e s
of
vein-like
the m y Ionite perpendicular
the
seams
to lineation.
study
that
They
extend for long distances (more than 30 m) without interruption,
bending,
to
splaying,
adjacent
and/or
fractures
in
transferring
en
echelon
their displacement
or
stepped
fas h io n
27
Figure 12.
Lineation
the NE.
in
outcrop,
as
viewed
toward
28
(Figures
13
and 14) .
The thickness of these veins
from less than 0.5 cm to more than 4 cm.
ranges
Some show a crude
pinch-and-swell structure.
These
co m m o n
fil l e d
qu a r t z
fractures
veins,
but
are
closer
easily mistaken to be
examination
reveals
that
they are narrow zones containing microbreccia.
The
they
are
average
strike
essentially
of the fractures
vertical
(Figure 15).
is N30°W,
and
The surfaces
are typically striated but the exposed areas are so miniscule
(100
of
c m 2 ) it becomes difficult
displac e m e n t .
down-dip
is
The steep plunge of striations
movement.
a major
to assume any great amount
obstacle
The
ab s e n c e
of
in determining
indicate
i d e n t i f i a b l e markers
displacement.
two localities was separation observable,
At just
and both revealed
normal displacement.
The
is
fact
orthogonal
that
the
trend
of
the
fracture
traces
to the lineation and their vein— like nature
suggest they are extensional features.
Figure 13.
Two views of the
v e i n - l i k e seams.
stepped nature.
o u t c r o p a p p e a r a n c e of the
Note t heir i n d i s c r e t e and
29
Figure 13
Figure 14.
Map of the fracture and vein pattern.
Figure 15.
L o w e r - h e m i s p h e r e e q u a l area p r o j e c t i o n of
(a) pol e s to minor f r a c t u r e s (crosses) and
p e r v a s i v e fractures (dots) and (b) striations
on pervasive fracture surfaces.
31
Figure 15
MICROSTRUCTURES
S-C Foliations
When
viewed microscopically the mylonites
are seen
to possess two foliations which conform in physical appearance
and
geometry
(Berthe
et
developed
be
for
to
characteristics
a l . 1979) .
in
thin
successfully
the
the
S-C
r e l ations
secti o n s
used
Flatrocks
of S- and C-surfaces
of
are
spectacularly
the my l o n i t e s .
in d e t e r m i n i n g
the
locality as a whole,
sense
They
of
following
of Nicolas, Bouchez, and Boudier (1972), Berthe et al.
Watts
and W i l l i a m s
(1980) , B o u c h e z
and
(1979) , Ponce
Pecher
de
(1976),
Leon
Burg
can
shear
the work
(1979),
and Choukroune
et
al.
(1981),
and Brown and Murphy (1982).
The S-C surfaces are best defined
cut p a r a l l e l
to
lineation and perpendicular to foliation.
Of these two foliations,
the
foliation
so
in thin sections
it is the S-surfaces that express
conspicuous
in outcrop.
Ironically,
the
C-surfaces are more prominent than the S-surfaces in microscopic
view;
but
they are only rarely discernible
at the outcrop
scale within the Flatrocks locality.
The C-surfaces are defined by the preferred orientation
of biotite and chlorite aligned parallel with rotated tails
of the porphyroclasts (Figure 16.)
32
Spacing of the C-surfaces
Figure 16. A series of photographs of thin sections showing
spectacularly developed S-C foliations indicating
dextral sense of shear (scale in (a) :
1 cm
= 1.7 mm; in (b) and (c) : 1 cm = 1.5 mm; and
in (d) ; 1 cm = 2 mm) .
33
Figure 16
Figure 16 —
Continued
#
Figure 16
Continued
c
w
cn
Figure 16
Continued
d
w
C T>
37
ranges
from
0.2
mm
to
2.5
mm,
and
is controlled in part
by the size of the asymmetric porphyroclasts.
These surfaces
are
are
con s i d e r e d
inter p r e t e d
boundaries
to be p l a n e s
of
shear
and
to form and remain parallel
generally
to the shear
zone
(Berthe et al. 1979? Simpson and Schmid 1983).
The S-surfaces are defined by elongate quartz grains,
quartz ribbons, and the aligned longest dimensions of asymmetric
porphyroclasts
a n astomosing,
(see
Figure
bending
16).
ar o u n d
the
The
augen
S-surfaces
and
are
deflected by
the C-surfaces.
The
f r o m 19°
angle
to
23°
between
on
the
average
S-
and
(Table
C-surfaces
1).
and C-angular relationships, Berthe et al.
four
stages
in
the
development
of
S-C myl o n i t e s :
mylonite
develo p m e n t ,
C-surfaces;
ultramylonitic
(Figure
relationship
in
second
angular
the
stage
the S-C surfaces.
of m y l o n i t e s
in
stage,
17) .
the
S-surfaces
at the third stage,
parallelism
in the
the
on
the S-
(1979)
recognized
ca u s i n g
initial
the
stage
of
f o r m at 45° to the
the average value of the
it is 15°;
the
two
area
and ultimately
surfaces
Therefore,
Flatrocks
Based
deformation
at the second stage,
angle is 25°?
in th e
progressive
ranges
the
places
of B e r t h e ' s progressive
come into
S-C
angu l a r
the mylonites
evolution of
Besides reflecting the stage of development
and therefore the degree of deformation,
relationships provide a means of determining
S-C
sense
38
TABLE 1.
S-C ANGULAR RELATIONSHIPS
SAMPLE
ANGLE (DEGREES)
FRVM 1
22
23
17
15
16
21
19
PRVM 2
25
23
29
35
22
21
24
FRVM 4
22
27
24
27
26
24
24
FR 8
26
21
30
23
16
17
21
FRVM 10
26
23
22
23
24
27
21
FRVM 14
20
26
22
18
18
23
18
FRVM 17
24
26
26
27
31
34
29
FRVM' 17
25
25
21
26
21
15
23
FRVM 18
23
19
25
18
24
21
24
FRVM 22
21
24
21
22
25
19
24
Range:
15o-30°
Mean:
23°
Standard Deviation:
4°
39
Figure 17. E v o l u t i o n of S - C a n g u l a r
progressive simple shear.
relationship
with
40
of
shear,
in
that
the
intersection
of
the
two
surfaces
is oriented so that the apices of the acute angles between
t h e m p oint
and 17) .
in
the
S-C
correct
angular
sense of shear
relationships
of
(see Figures 16
the
mylonites
in
the Flatrocks area call for a normal-slip simple shear.
Asymmetric Porphvroclasts
As
in m a n y
the m y l o n i t i c
q u a r t z - f e l d s p a t h i c mylonitic
gneis s e s
microscopically
of
re s i s t a n t
the
terranes,
Flatrocks locality display
retort-shaped
porphyroclasts
of feldspar that lie within a more ductile and fine-grained
matrix.
The
retort shape
distribution
of
Figure
Each
by
16) .
is accentuated by the asymmetric
foliation
feldspar
a non-recrystallized
a round
porphyroclast
core
fine recrystallized grains.
augen
are
composed
of
the porphyroclasts
surrounded
(see
is characterized
by
a mosaic
of
The tails of the retort-shaped
f iner
recrystallized
material
and
they extend in the direction of the trace of the C-surfaces.
During
tails
progressive
is m o r e
and so they
deformation the weaker material
easily
rotated
record the last
and Schmid 1983).
This
of the
than the large porphyroclast
increment of rotation
(Simpson
asymmetric fabric element has been
widely and successfully used to infer the sense of displacement
(Eisbacher
1970;
and Schmid 1983).
Etchecopar
1977;
Jegouzo
1980;
Simpson
41
The
interior
retort-shaped
spaced
portions
of
porphyroclasts
are
(0.05 mm), penetrating,
along
which
alteration
systematically
oblique
has
to
many
of
marked
parallel,
the
by
a ugen-
very
to
closely
hairline fractures
occurred.
Microfractures
are
C-surfaces
(see Figure 16)
and
in some cases are filled with quartz and/or chlorite microveins.
Quartz Fabrics
Quartz
in t h i n . s e c t i o n s
of
the m y l o n i t i c
gneiss
expresses conspicuous evidence for intracrystalline plastic
deformation.
of
new
are
in
Original quartz grain5are replaced by aggregates
recrystallized
turn
elongate
grains,
and
parallel
to
the
the
quartz
aggregates
s-surfaces.
Grain
size reduction is prevalent in the fine quartz-rich matrix.
Ind i v i d u a l
grains
are m a r k e d
by
serrated
boundaries
and
show strong undulatory extinction.
Quartz
strain,
"ribbons,"
are wavy,
expressions
curved around
rigid feldspar porphyroclasts
in m e c h a n i c a l
behavior
of
extreme plastic
and draping
(see Figure 6).
between
quartz
over the more
The contrast
and feldspar
gives
rise to an inosculating texture so characteristic of mylonites.
Tiny
boudin-like
quartz
of
ribbons.
a variation
structures
are typically associated with
Such boudin-like structures are the result
in d u c t i l i t y
with
foliation (Platt and Vissers 1980).
o r i e ntation due to the
42
Microbreccia Fabrics
The
microbreccias
which
occupy
the
extensional
fractures in outcrop are seen in thin section to be composed
of randomly oriented quartz and feldspar clasts in a random
fabric
with
matrix.
serrated
about
0.2
Clasts
grain
very
an
average
They
and can be up to 2.4 mm
small,
and tabular
range
in the light-colored
coarser-grained microbreccia.
of
subangular
boundaries.
m m to 0.5 mm
microbreccia,
have
subangular,
forms
in size from
finer-grained
in the dark-colored
The matrix is made up primarily
equant
interlocking
grains with
size of 0.01 mm and 0.05 mm for the light and
dark microbreccias,
respectively.
The grain size reduction
is due to comminution rather than neomineralization accompanying
plastic
deformation.
In
places
the
matrix
is
seve r e l y
strained so mineral grains are hardly discernible.
The
late-stage
branch
microbreccias
are
crosscut
by
a ne t w o r k
of
intersecting microfractures and microveins which
out,
pinch and swell,
and cut across
large clasts.
However, they have no detectable displacement.
In some parts,
the foliated fabric of the mylonitic
gneiss is preserved in small pieces of the tectonite floating
in the fine-grained cataclastic matrix.
INTERPRETATION
The
majority
of
the
microstructures
present
in
the mylonitic gneiss in the Flatrocks study area constitute
strong
evidence
progressive
for
si m p l e
plastic
shear
deformation brought about by
within
a
thick
r e g ional
shear
zone.
The parallelism of the fold axes
in adjacent areas
with the mineral lineation, and the perpendicularity between
lineation and the axes of boudin and pinch-and-swell structures
(Davis 1980)
suggest that the orientation of the lineation
is the direction of maximum elongation.
is
interpreted to be equivalent
Thus the lineation
to the X direction of the
finite strain ellipsoid.
The C-surf aces represent planes of shear, and throughout
the p r o g r e s s i v e
zone boundaries.
deformation
remain parallel
to the shear
The S-surfaces correspond to the XY plane
of the strain, thus defining the plane of flattening (Ramsay
and Graham 1970).
initially
shearing
at
they
45°
The S-surfaces are thought to have developed
to
come
the
into
C-surfaces,
and with progressive
parallelism.
An
average
val u e
of 23° for the intersection of the S- and C-surfaces places
the mylonitic gneisses
of the Flatrocks
stage of mylonitization.
43
in an intermediate
44
Using
inclination
revealed
the
of
orientation
the
that
asymmetry
of
the
resp e c t
the
the
lineation
f o l i a t i o n s , a sense
requires
southwesterly with
the
two
of
upper
to
rocks
lower
porphyroclasts
of
to
ones.
and
and
shear
have
the
is
moved
Furthermore,
the
orientation
of the microfractures in the augen in relation to the planes
of
shear
also
supp o r t
a s ense
of
shear
compatible with
that documented by the S-C relationships.
From
the
mesoscopic
and microscopic
observations,
it is then c o n d u c e d that the mylonitic gneisses were developed
in
a shear
of
the
zone
upper
characterized by a southwest displacement
r o c k s , a conclusion
consistent with that of
Davis (1983) who based his inference on fold analysis.
While
the
intracrystalline
is
a product
the m y l o n i t i c
clasts
of
mylonitic
plastic
of b r i t t l e
gn e i s s
is
deformation,
the
a
result
of
microbreccia
deformation clearly derived from
as
microbreccia
gneiss
evidenced by
are
composed
the fact that small
of
relic
mylonitic
fabric.
Few studies have focused on cohesive,
random fabric
fault rocks, and in the Flatrocks study site, the generation
of m i c r o b r e c c i a
is p r o b l e m a t i c .
Cataclasites of similar
composition and internal geometry to those of the microbreccias
of
the
study
Flatrocks are described by Di Tullio
of
the
fault
rocks
of
(1983)
in her
the Tanque Verde decollement
45
zone.
Based
on
their
(1983)
suggested
intrusive-like contacts,
these
cataclasites
in a state of fluidized flow.
are
Di Tullio
injected
Interestingly,
rocks
similar seams
of microbreccia in the Newport fault zone are briefly mentioned
by
Harms
for
(1982)
them.
w ho
suggests
Several
a hydraulic-fracture origin
mechanisms
account for microbrecciation:
superplasticity
pressure
been
and G r a y
and
rigid
upon
to
structural
due to high fluid
1982 ? P h i l l i p s
Osborne, and Palmer 1983).
c alled
cataclastic flow,
and hydraulic fracturing
(House
fracturing
have
1982;
Anderson,
Cataclastic flow involves repeated
body
rotation.
Although
the
lack
of cementing material does not completely support the hypothesis
of
cataclastic
pervasive
in
p art
is not
p lace
the
presence
of
microfractures
in d i c a t e
cataclastic
a mechanism
a viable
at
mineral
be
of
assemblage
the
brecciation.
mechanism
temperatures
microbreccia.
to
flow,
The
in this
higher
and
rotated
flow
was
Superplastic
flow
case because it takes
than those
deformational
presence
clasts and
indicated by the
fe a t u r e s
of
of high fluid pressure
the
seems
controlling parameter and a mechanism analogous
to Ramsay's
(1980b)
crack seal mechanism is very plausible.
The driving force of this mechanism is a high fluid pressure.
Hubbert
and
role
fluid
an
of
Rubey
environment
(1959)
pressure
in
and
Secor
jointing
characterized
by
(1967)
examined
and concluded
that
the
in
high hydraulic pressure.
46
jointing will occur regardless of the depths.
The existence
of high fluid pressure in a rock helps attain the stress
conditions required for brittle fracture, in that a constant
increase
in the fluid pressure systematically causes a
decrease in the differential stress and therefore drives
the Mohr
stress
circle against the envelope of failure
(Figure 18).
In
the
Flatrocks
study
area
it
is not
difficult
to imagine such a hydraulic-fracture mechanism, since intense
fra c t u r i n g
necessary
and
alteration point
to b ring
brittle fracture.
stress
about
to great amounts of water
stress
conditions
required for
An influx of water reduces the differential
and failure takes place,
injected material which
followed by
infilling with
is later cracked when ideal
stress
conditions are again reached.
Based
on
detailed
petrographic
in the Santa Catalina Mountains,
that mylonitic gneisses,
study on gneisses
Sherwonit
(1974)
concluded
like those in the Tanque Mountain
area, are of amphibolite facies and were formed at temperatures
of 650° to 730° C. and pressures of 4kb to 6kb
(12-18 k m).
Sibson (1977) proposed temperatures above greenschist facies
conditions
and
deformation
depths
behavior.
of
10-15
On
the
km
for
o ther
his
hand,
quasi-plastic
the
brittle
deformational fractures of the microbreccia and a diagnostic
47
N orm al
stress
Figure 18. F a i l u r e e n v e l o p e d i a g r a m s h owing the effect
of an incremental fluid pressure on the position
of the Mohr stress circle (Secor 1965).
48
mineral
at
the
lower
assemblage
rich
in
chlorite
pressure-temperature
development
suggest
conditions.
they formed
In conclusion,
of mylonites on one hand and microbreccia
on the other suggest the initiation of a shear zone charact­
erized by normal-slip simple shear in relatively deep crustal
conditions followed by late-stage development of microbreccia
at the expense of mylonites by brittle deformation at shallow
levels
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