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Textures of sheared halite and their implications for the seismogenic slip of deep faults
Tectonophysm. Elsevier
69
144 (1987) 69-86
Science
Publishers
B.V., Amsterdam
- Printed
in The Netherlands
Textures of sheared halite and their implications for the seismogenic slip of deep faults HIROYUKI
HIRAGA
* and TOSHIHIKO
SHIMAMOTO
Institute of Geolo~ and Mineralogy. Facult~v of Science, Hiroshima Utwersiy, (Received
December
5, 1985; revised version accepted
Huoshma
730 (Jupun)
May 8. 1986)
Abstract Hiraga.
H. and Shimamoto,
T.. 1987. Textures
faults.
In: R.L. Wesson
Halite
exhibits
(Editor),
a wide spectrum
even at room temperature,
precut
reports
Shimamoto
and
well-developed unstable
Logan
discrete
unstable surfaces.
potentially associated
fault
motion
At pressures
unstable
deformation,
homogeneous
plastic
fault
with seismogenic
occurs
above
motion
and mylonites
shearing
deformation
attention
address:
Danbara-yamazaki.
0040.1951/87,‘$03.50
shears.
strain
complete shearing slip thus
localization. takes
place
of a
and that (2) the potentially
although
of this heterogeneous slip withln no direct
of halite.
along
zones become extends
results by
by the formation
some kind of brittle
recrystallization
1985c, 1986).
the earlier
mode of internal
Thus,
and at confining
well into
narrow. wider
More nearly
the shear
deformation evidence planar
and irregular
the semiductile
of
importantly. and
and the regime
with the view that at least some of them could
of
well be
fault motion.
mation ranges from brittle deformation near the surface to fully ductile deformation at depth. Such the mechanical behavior of and plate boundaries, including
Danbara Minami-ku,
Junior
High
Hiroshima
School.
4-42
732 (Japan)
ii) 1987 Elsevier Science Publishers
a complete spectrum of shearing deformation has not yet been investigated on geologically significant silicates in laboratory experiments, primarily due to technical difficulties. Shimamoto (1985a,b,c, 1986) and Shimamoto and Logan (1986) thus used simulated halite shear zones as an analog for faults and plate boundaries, which consist mainly of silicates, and observed a complete transition between frictional slip and ductile shearing flow at
the generation of large earthquakes, requires knowledge of the mechanical properties of rocks under extreme shear. The relevant shearing defor-
* Present
that the primary
the discontinuous
Seismogenic
receive renewed
shear strain
on
halite along a 35 o
(Shimamoto,
characterized
flow
of large-scale
were performed
or natural
Our new data reconfirm
after a critical
after
internal
100 MPa.
disappears.
should
experiments
previously
shearing
properties
within the shear zones. The nature
to the nearly
only when
about
field to fully ductile
from 3 to 300 pm/s,
deformation,
with R, Riedel
Introduction Understanding large-scale faults
specimens.
and it is shown
owing
ranging
slip of deep
144: 69-86.
mechanical
triaxial
with those reported
shearing
interconnected
can be found,
Thirty-nine
slip rate along the precut
with this strain localization
the predominantly
for the seismogenic
Tectonophyws.
slip in the brittle
apparatus.
are consistent
closely in this paper, shears
Faulting.
with a 0.5 mm thick layer of synthetic
into heterogeneous
is associated
deformation
potentially
data
(1) nearly
Y Riedel
overlaps
cataclastic
available
on these, and some previous
changes
is examined
is slip along
possibly
that
foliation,
fault motion
deformation
Mechanical
observations
from frictional
sandstone
with a constant
up to 200 MPa.
This paper
zones
of Tennessee
halite and their implications
of Earthquake
halite shear zone is ideal for studying
with the currently
specimens
at room temperature,
pressures
of behavior,
and the simulated
faults and plate boundaries dry cylindrical
of sheared
The Mechanics
B.V
70
room temperature, data in the brittle
thereby
fundamental
to the earthquake
potentially
seismogenic
of earthquakes,
1982; Sibson. and
depths 1983).
slip
faults
0.5 mm thick
of
initiate (Meissner
Shimamoto
or
plate
important to the ini-
axis,
along
the
near the base of
pm/s,
Strehlau, 1983; Das
(1985a,c)
re-
mogeneous deformation changes into heterogeneous deformation beyond some critical shear strain which increases with increasing normal stress, and that the onset of this localized deformation coincides with a shift in mechanical behavior from stable to potentially unstable slip with negative velocity dependence (i.e., lower friction at a faster slip rate). Thus, potentially unstable fault motion is associated with the localization of deformation even in this semiductile regime. The principal objective of this paper is to examine the nature of this heterogeneous deformation with the aim of gaining a better understandinitiation sults are mylonites sent work Shimamoto
related
procedures
Velocity-stepping ducted by Dieterich (1986). were made
and
layer
of compacted
at an angle
at room precut
sandstone
the confining
et al. (1972).
placements
from
were corrected
along
The
pressure
a
to the cylinslip
rate
300 down
apparatus
Measured
with a
halite
of 35”
temperature.
ranged
200 MPa, using a triaxial Handin
at room humidity
of Tennessee
ranged
to 3 up to
described
stresses
for apparatus
in
and disdistor-
tion, contact area change and the strength of the polyolefin jackets (see Shimamoto, 1977; Shimamoto et al., 1980). Details of the experimental procedures are described in Shimamoto and Logan (1981). Reagent grade pure NaCl (about 0.1 mm in grain size) and iodized cooking salt (0.2-0.3 mm in grain size) were used as the halite layer. This pure NaCl is the same as that used previously (Knapp, 1983; Shimamoto and Logan. 1984,1986; Shimamoto, 1985a,b,c, 1986) but the cooking salt is not. The halite layer was precompacted by leaving the specimen for 15-30 min under a confining pressure of 200 MPa and a differential stress of about 100 MPa;-not adequate to initiate the slip along the precut. The average thickness of the halite zone was estimated from its weight assuming zero porosity. Mechanical
data
to the rupture
of large earthquakes. Observational realso discussed briefly with respect to and metamorphic tectonites. The preis reported in an abstract by Hiraga and (1985).
Experimental
precut
drical
and
textures.
cm in length and 4.8 cm in diameter)
of
the majority
pressure-insensitive ductile flow and yet the shear resistance still depends on the normal stress across the shear zones. Shimamoto and Logan (1986) further recognized that the apparently ductile, ho-
processes
ground
were conducted specimens
(9-11
cognized that potentially unstable fault motion extends well into the semiductile field where deformation textures are nearly identical to those of
ing of physical
on cylindrical
is how deep the
1982; Chen and Molnar,
Scholz,.
with
problem
because
large or great earthquakes the seismogenic
Experiments
questions
boundaries extends. This is particularly in understanding the processes leading tiation
the effect of slip rate on the resulting
the mechanical
and ductile domains.
One of the most regard
linking
tests, similar to those con(1979) and Tullis and Weeks in previous shearing experi-
ments on halite (Shimamoto, 1985a,b,c, 1986; Shimamoto and Logan, 1986). In the experiments reported herein, the slip rate along simulated halite shear zones was kept constant in order to isolate
Ten and 29 experiments were performed on natural and synthetic halite. respectively. Mechanical results from our constant slip rate experiments are similar to those from previous velocitystepping tests (Shimamoto, 1985c, 1986), so they are described only briefly here (see Hiraga (1986) for a complete description). Figure 1 shows the shear stress and normal stress on the precut at steady state or nearly steady state friction from 31 tests on natural and synthetic halite. The shear stress that can be sustained by halite is slightly smaller than that for many rocks that were compiled by Byerlee (1978) at confining pressures below about 50 MPa, but it deviates markedly from his relationship and
STEADY-STATE
FRICTION ZSOMW
200
100
t I
: SYNTHETIC
.
x
:
I[
HALITE
NATURAL HALITE
300
STRESS
NORMAL
.
(Mpa) \qo
Fig. 1. Shear stress plotted at steady
state or nearly
Tennessee
sandstone
(squares)
or synthetic
two categories along
intercept
is selected
When
normal
layers
Data
intermediate
stick-slip
for specimens
thick
halite.
occurs.
are grouped ranges
gives the confining
the frictional
strength
each group
pressure.
for many
---.
--~__-
into
stick-slip
compiled
line by
Byerlce (1978).
,
-3
-2
-1
log(SLIP
Fig. 2. Steady synthetic logarithm
much
less sensitive
to the normal
stress
of slip rate along
slip and stable
slip, as shown
of normal stress at confining pressures above about 250 MPa at fast slip rates (30-300 pm/s), above 150-200 MPa at intermediate slip rates (0.3-3 pm/s)
and
above
about
100 MPa
at slow slip
rates (0.003-0.03 pm/s). The relationship between steady state or nearly steady state friction and the slip rate along the precut is summarized as follows (Shimamoto, 1985c, 1986) (Fig. 2). (1) In the pressure-insensitive flow regime at a confining pressure of 250 MPa, shear resistance increases monotonically with increasing slip rate. (2) A regime of negative velocity dependence (frictional regime) is clearly present at intermediate and slow slip rates and at pressures below about 100 MPa, and stick-slip was observed only in this regime. (3) The negative velocity dependence changes into a positive dependence with increasing slip rate (high-speed
bar(s).
The
Shimamoto
(SLIP
IN
3
pm/s)
state shear
resistance
of
against
the
symbols
are
indicates
episodic
left hand
corner.
the confining
stick-slip
occurs.
pres-
a regular
and the shear stresses at the onset
of abrupt
experimental
Different
in the upper
test. When
is selected
(1985c,
2 RATE
halite and for stick-slip,
next to each symbol
portion
, 1
the precut.
and synthetic
stick-slip
$5
0
steady
used for natural
and at the cessation
/lOMPa 15 5
---
halite (0.5 mm thick) plotted
sure in MPa for each
at pressures above about 100 MPa. The data in Fig. 1 agree quantitatively with the previous results (Shimamoto, 1985c, fig. 8) which revealed that shear resistance becomes nearly independent
RATE)
state or nearly
and natural
The number
becomes
15
------+I0
0
The
of data
The dashed
rocks
of
of slip rate
a regular
_.30-' 30
of natural
and the peak in the stress is plotted. line through
+30 o\30r+x30
stress on the precut
state sliding
0.5 mm
(circles)
of the fine broken
with the abscissa indicates
steady
with
for fast and
the precut.
portion
against
slip events are tied with vertical results
for
1986) are also shown
solid lines with the confining
pressure
synthetic
halite
for comparison
by by
given on each curve.
frictional regime) or with decreasing slip rate (flow regime). (4) The frictional regime diminishes and the high-speed
frictional
regime
expands
with in-
creasing confining pressure, and the friction velocity relationship degenerates into the relationship for the pressure-insensitive flow. Our experimental results are far more incomplete than previous results (Shimamoto, 1985a.b.c. 1986) but are consistent with them as compared in Fig. 2. Microscopic
observations
Microscopic observations were polished specimens with a polarizing under reflected light, described in Shimamoto
made on microscope
following the method and Logan (1986). Speci-
72
mens
were polished
standard and
procedures
the final
paste
with Carborundum using benzene
polish
was made
(1 pm in grain
by use of
size). Grain
lipsoid)
a diamond
boundaries
on
the polished surface were etched with a small amount of saliva. The polished surface was then vacuum-coated
with platinum-palladium
by use of the same procedures of SEM specimens, reflectivity ducing
boundaries reported
of the polished
sharp
contrasts
under
reflected
by Shimamoto
surfaces
between
increased thereby
grains
(i.e., the major
pro-
and grain
light. Some specimens
and Logan (1986) are also
from
ment and the thickness
of the maximum
axis of the strain the measured
el-
displace-
of the shear zone assuming
homogeneous simple shear. When the foliation is uniformly and well-developed as in Fig. 4. the between
5a). These
the
the orientation
calculated
agreement
(Pt-Pd)
for the preparation
and this greatly
and
elongation
as a lubricant
with
boundary
formed halite
/& and 13,.,,is excellent (Fig. indicate that the foliation is
results
parallel grains
preserved
to the maximum
and
that
despite
its original
elongation
of
orientation
is
likely postdeformational
recrys-
tallization. The foliation
defined
by straight
and continu-
included in our description. The thickness of the halite shear zones often increases slightly toward
ous grain boundaries
the lower leading
in those composed of synthetic halite (Shimamoto and Logan, 1986, fig. 8) or crushed natural halite (Knapp, 1983). Shimamoto and Logan (1986) pro-
edge of the precut
specimen,
so
that the average shear strain, y, at the location of a photomicrograph is given in most figures below.
has been recognized
the shear zones consisting
of cooking
only in
salt, but not
Foliation und its origin
posed that the cooking salt grains contain impurities such as various nutriments near their surfaces (upper diagram in Fig. 5b), so that the grain
The starting material, precompacted only by a confining pressure of 200 MPa and a differential
surfaces extreme
stress of 100 MPa, is a structureless
and
aggregate
of
become nearly straight and planar after elongation (lower diagram in Fig. 5b).
that
the
movement
the
original
grain
surfaces dynamic
surfaces (e.g., A, B and C in Fig. 3b) seem to have recrystallized. Fine striations in many other grains such as D in Fig. 3b are perhaps mostly { 100) cleavage fractures. The most conspicuous texture in halite shear zones is well-developed foliation, defined by
foliation. If this hypothesis is correct. halite grains must have elongated more or less uniformly parallel to the direction of maximum elongation to form the foliation, and hence the predominant mechanism of deformation is most likely to have been in-
straight and very continuous grain boundaries (Fig. 4). Almost all of the halite grains in the shear zones are recrystallized grains by dynamic and/or postdeformational recrystallization, as described
tracrystalline deformation
and discussed fully by Knapp (1983) and Shimamoto and Logan (1986). Thus, the textures in halite shear zones do not necessarily directly reflect the processes of deformation. However, the foliation does appear to convincingly reflect the deformation processes (Shimamoto and Logan. 1986). The measured angle, I!$,,~,between the foliation and the shear zone boundary in Fig. 4 is 4.4” f 0.5 O, the error being one standard deviation of ten measurements. This angle agrees very closely with the angle. e,.,, (4.44’), between the shear zone
is hindered or static
of
halite clasts and granulated material (Fig. 3 and fig. 11 of Knapp, 1983). Some grains with clear
by the impurities even during recrystallization to form the
gliding. However, the mechanism of cannot be identified in our specimens
since the halite within entirely recrystallized. observed confining
the shear zones is almost Such a foliation was
in the halite shear zones deformed pressures at and above 30 MPa.
Strain localization
at
and unstable fault motion
Shimamoto and Logan (1986) have recognized that the homogeneous shearing deformation such as that shown in Fig. 4 changes into heterogeneous deformation (as demonstrated by discontinuous and distorted foliation), after a critical shear strain, y,,, which increases with increasing confining pressure. Of particular interest in their
73
UNDEFORMED ONLY
; PRECOMPACTION
(PC = ZOOMPa;
cd =~OOM~a)
100
0 Fig. 3. Photomicrographs differential surfaces reflected
findings
of starting
material
stress of 100 MPa for about
are recrystallized
(synthetic
halite) which was compacted
30 min. (b) shows a close-up
grains. All photomicrographs
pm
1
1
of the upper
in this and subsequent
under
central
a confining
portion
pressure
of (a). Euhedral
figures were taken with a polarizing
of 200 MPa and a grains
with clear
microscope
under
light.
are that the onset
of this heterogeneous
deformation nearly coincides with the shift from stable shearing flow-type slip to potentially unstable friction-type slip and, therefore, that the potentially unstable fault motion is associated with the localization of deformation within the shear zone. These results are reexamined below, based
on our new observational
results (Fig. 6).
The same mechanical data as those in Shimamoto and Logan (1986) are plotted in Fig. 6. Briefly, the mechanical behavior of simulated faults at intermediate slip rates is divided into flow-type and friction-type as schematically shown in Fig. 6b, and they are plotted in the confining
w I-
i I”
1 ---
0
\ pC
=15() MPa, T = 12.8, C&,=4.44",
Fig. 4. Photomicrograph deformed between maximum
under
at a confining
reflected
pressure
light showing
of 150 MPa (specimen
uniformly
8obs=4a40t o-5"
and well-developed
HO2 described
by Shimamoto
foliation
and the shear zone boundaries
agrees quite closely with the calculated
elongation
and the shear zone boundaries
assuming
calculated
from the measured
axis to the minor axis of the strain
ellipsoid
displacement corresponding
homogeneous
along the precut
simple shear.
and the thickness
to this shear strain
across
and Logan,
the foliation
this location.
400 pm
I
angle,
hahte
shear
1986). The observed
a natural
angle,
/3,,,, between
y is the average
the direction
shear strain
zone BohS, of the
of halite at
of the shear zone. The ratio of the major
is 166.
pressure versus shear strain space in Fig. 6a. Here, the shear strain is averaged over the entire shear zone. The friction-type behavior is characterized
roughly classified into (1) homogeneous deformation, (2) heterogeneous deformation and (3) very heterogeneous deformation, and the results from
(1) by an instantaneous viscous-like response lowed by a gradual decay in friction toward
Shimamoto and Logan (1986) and from the present study are all plotted in Fig. 6a, using different
folthe
steady state value upon a step increase in the slip rate and (2) by a negative velocity dependence of steady state friction (Dieterich, 1979, 1981; Ruina, 1983). On the other hand, the flow-type behavior exhibits (1) a gradual and monotonic change in the shear resistance toward the steady state value upon a step change in the slip rate and (2) a positive velocity dependence of the steady state shear resistance. As can be seen from the results at 150 MPa pressure in Fig. 6a, the flow-type behavior changes into friction-type behavior with increasing shear strain. Deformation
within
halite
shear
zones
was
circular symbols formation. Here,
for the different types of dethe shear strain is the average
shear strain at the location of the polished men, not of the entire shear zone. Homogeneous the development
speci-
deformation is characterized by of a uniform foliation with a
good agreement between cobs and f?,,, (see Figs. 4 and 5). Deformation classified as homogeneous includes local and incipient heterogeneous deformation, but the cross-sectional area showing this heterogeneous deformation is less than 10% of the area of the entire shear zone. The incipient heterogeneous deformation was recognized at the
NATURAL
, <,’
HALITE
,’
[ ORIGIN
,’ , ,’I’
OF
FOLIATION
/
NONDEFORMED
8(?p Halite
grains
SUt‘aCe
bwth impurltles
HOMOGENEOUSLY
HOMOGENEOUS
1
SIMPLE
SHEAR
\ 1 %
1
2
3
Fig. 5. a. The measured correspondence between
between
19~~.and
l?,,,. b. Schematic
y and R denote
NATURAL (Cooking
0
-
,’
,’ /’ 0
of the maximum
elongation
and the ratio of the major
of the formation
hne indicates of foliation.
with the standard
axis to the minor
0 ,’ ,‘O /’
_
Boha is the angle deviation
axis of the strain
shown and the
ellipsoid,
F&AT
SLOW
Flow type (-_) Friction type (mm)
t
,’
!
the
TYPE]
/’
? m
against
the one to one
(the major axis of the strain ellipsoid)
[FLOW
0
,’
,’
the process
The dashed
over eight to twenty measurements
Homown. def (0)
,’
:200-
within halite shear zones plotted
shear.
Heterogen. def (0) Very heterogen.def.(o)
Salt)
!k
showing
foliation
simple
of (b).
HALITE
ml
diagram
the direction
developed
homogeneous
averaged
the shear strain
See text for the explanation
_I
assuming
and the shear zone boundaries,
zone boundaries.
300
and homogeneously
elongation
bars and SC,, is the angle between
respectively.
R = 32
(b)
(degrees)
of uniformly
of the maximum
the foliation
by vertical shear
orientation
orientation
T = 5.5.
4
6Cdl
(4
predicted
SHEARED
00
DISPLACEMENT
-
DISPLACEMENT
----,
0
,’ ,’
%
,’
ZlOO-
0
,‘-
ii
l
-I
g v
marrm 0.0 ,
OO
20
(a) Fig. 6. a. Strain pressure
shown indicate of strain
localization
a,
L
SHEAR within
STF%N
pm/s)
The nature
is divided circular
is roughly
symbols
the onset of strain localization localization
and friction-type
(b:
60
r-
from Shimamoto
into flow and friction
of deformation
by the different
,
halite shear zones and a change
versus shear strain space (modified
rates (0.3-3 symbols.
l .
,+
types as shown
classified
as determined
from textural
of simulated
1986). b. Mechanical
schematically
into homogeneous,
in (a). The horizontal
behavior.
in the slip behavior
and Logan,
evidence
faults
shown
during
(b), and these are plotted
heterogeneous
bars with short
behavior
vertical
m the confining
the intermediate
and very heterogeneous
deformations,
lines in the lower left hand
(see text). The dashed
slip
m (a) with different
line in (a) roughly
corner
as in (a)
gives the onsets
76
central
parts of the shear zones (Hiraga,
22) and the strain the central
part
localization
tends to initiate
due perhaps
to the stress
centration there (see Paterson, heterogeneous deformation ranges
from
(e.g.,
slightly
Shimamoto
heterogeneous able
distortion
as to alter
pletely.
Examples
tion are described
at
but
its original
to such
arrangement
of very heterogeneous below.
7 shows a natural halite shear zone at a confining pressure of 5 MPa, which
appear
textural nature
to be recrystallized
evidence
has
of deformation
been
grains. found
within
HALITE
Fig. 7. Photomicrograph
under
rate of 3 pm/s.
evidence
rock-halite procedures
No clear
interface. when
saliva
Planar
reflected
as very heterogeneous
de-
halite
shear
MPa.
Foliation
showing
extending
off the polished
a halite
the
surface.
9 shows
another
deformation deformed
within
is spread
of
a natural
at a pressure
in this specimen
and is discontinuous
example
of 70
is locally
wavy
across many surfaces, over the entire
shear
behavior
coincides
with those of het-
erogeneous and very heterogeneous deformations. At a confining pressure of 30 MPa, deformation appears to become concentrated at one of the rockkhalite interfaces soon after the onset of heterogeneous deformation. Perhaps because of this, foliation remains relatively undistorted and its orientation is nearly uniform throughout the foliated portions (e.g., Fig. 8). Thus, the angle between the foliation and the shear zone boundary
shear
&25
zone
can be seen in the shear from
zone
deformation
: P, = 5MPa,
of deformation
surfaces
was wiped
light
and
this, too, was classified
friction-type
the
the shear zone. The
1981).
NATURAL
con-
Figure
eventually
The result in Fig. 6 substantiates Shimamoto and Logan’s (1986) conclusion that the domain of
neous deformation. Such a concentration of slip at one of the rock-gouge interfaces is characteristic of the brittle deformation of intrafault materials (Engelder et al., 1975; Byerlee et al., 1978; and Logan,
became
shear zone boundary
and
slip is evidently concentrated at the upper rockkhalite interface and hence the deformation of this specimen was classified as very heteroge-
Shimamoto
Deformation
part of
zone.
No clear
to infer
areas near the upper
at the upper
shaped
consists of a structureless aggregate of halite grains. More than half of the grains have clean surfaces and
in the dotted
very heterogeneous
com-
deforma-
across a planar
absent
formation.
a
is discontinuous
(e.g., the thick solid line in Fig. 8b) and is
the sketch.
consider-
at and
in Fig. 8. In this speci-
surface
centrated
fig. 11) to
not
begins to form at pressures
30 MPa as shown
men, the foliation
con-
deformation
1986,
displaying
of foliation,
degree
Figure deformed
Logan,
deformation
above
1978, fig. 13). The mentioned above
heterogeneous
and
The foliation
1986, fig.
upper
left
deformed zone,
hand
and
to the
at a confining
pressure
the slip is concentrated lower
center
were
of 5 MPa
and
entirely
along
created
during
with
a slip
the upper the
etching
:
^ ,>
.z-
of a R, Riedel shear (cf. Fig. 10). Thin ‘vertical striations
the dotted
_
_
‘6 =
26
-
_
during
portions
,
rate of 3 pm/s.
&I=
,,,
the etching
.-
process
_
-<
,
and are not microfractures.
-
0
__-
a clear discontinuity
a. Photomicrograph
2.2”
in b. The heavy solid line in b indicates
of 30 MPa and at a shortening were created
in or across
pressure
PC = 30 MPa ,
of halite at a confining
HALITE
or discontinuous
deformation
shown by solid lines, is absent
Fig. 8. Heterogeneous
b)
(a>
NATURAL
.-
______
Foliation, with the orientation
light. b. Sketch. the foliation
reflected cutting
under
200pm
=:
J)‘Z
=
1-0
’ 82=5
+dWOL=
‘d :
snoauaiioralaH'6 %.J
pallop ay1 ssone 10 "1 s"onuyIox~p
Jo uoyxuo~ap
31IlVH1WfUVN
19
should strain zontal
correspond
bars with short
left hand corner strain,
approximately
at the time of strain
vertical
shear strain
shear
The hori-
lines in the lower
as such from
shown with the dashed
shear
two specimens.
for the strain
localization,
100 MPa, but it markedly
,*I’PC=lOO’iiPa
pressures increases
: ..--- ..
(b) I/
/’
The nature of heterogeneous
‘, 30”‘o
‘&, , *
,o~,~
below about with increas-
at higher pressures.
The orientations planar discontinuities
. -
(4
line in Fig. 6a, seems to be
nearly the same at confining ing pressure
the
of Fig. 6a gives the critical
determined
The critical
to
localization.
pc
=
T&pa
_____ _---------..
deformation
of continuous and nearly of foliation were analyzed
(cl
/I/pc q 5()&pal
,_ _- -.-.--_.
,o%
“.ios
/’
statistically based on detailed sketches of sheared halite in order to examine the nature of the heterogeneous deformation associated with unstable fault motion. The most predominant discontinui-
“‘,
30%
8’
-
(d)
ties in the halite shear zones deformed at a pressure of 30 MPa are R, Riedel shears (Fig. lOa, e). (See Bartlett
et al. (1981) for a discussion
complete set of Riedel shears.) an example of such R, shears
of the
Figure 11 displays along which folia-
-
(e) Fig. 10. a. A complete
set of Riedel shears and their notations.
tion has been dragged, showing the correct sense of R, shearing. The predominant discontinuities
b-e.
showing
are Y and R, shears at confining 50-100 MPa (Fig. lob-d). That
deformed
at a confining
Preferred
orientation
pressures Y shears
of are
poorly developed at 30 MPa is due to the fact that the slip tends to become concentrated at a rock-halite interface at that pressure. On the other hand, slip is more diffuse and occurs along Y shears
developed
higher shown
pressures. The discontinuities of foliation, with the dotted portions in Fig. 12b, be-
throughout
come less sharp, less planar, wider at a confining pressure
the
shear
zone
Rose
straight
diagrams
discontinuities
frequency
pressure
orientation within
as specified
of the discontinuities
in percent.
were made,
the
of foliation
respectively,
230, 498,
taken horizontally
with the right-lateral
zones
in each diagram. by the
130 measurements
one and
mens in (b), (c), (d) and (e), respectively.
relatively
shear
is shown
145 and
for one, three,
of
halite
three speci-
The shear
zone is
sense of shear.
at
less continuous and of 150 MPa than at
lower pressures. Although some of those discontinuities seem to be Y and R, shears, their orientations could not be identified in many cases and were not examined statistically. The patterns of these discontinuities of foliation thus resemble those of Riedel shears, and this strongly suggests that some kind of brittle deformation has overlapped the apparently ductile deformation associated with the formation of foliation. However, these discontinuous regions consist of an aggregate of relatively equigranular grains
(e.g.,
Figs.
microfractures.
8 and
12) and
Unfortunately,
are virtually
free of
the mechanism
of
deformation along these discontinuities cannot be inferred from textural observations as a consequence of postdeformational recrystallization. Heterogeneous deformation within halite shear zones is complex and variable, and its exact nature is not yet elucidated completely through our observations. The following are just a few examples showing simple but essential processes during the heterogeneous shearing deformation in the semiductile field. The development of R, shears is commonly accompanied by anticlockwise rotation of foliation under right-lateral shearing (e.g., EF in Fig. lib), and this rotation appears to be
80
NATURAL
P,=lOOMPa,
HALITE:
0
Fig. 11. Sharp Shimamoto discontinuity, rotation
discontinuity
and Logan,
relative
of foliation
in a halite
1986). a. Photomicrograph
AB, showing
the correct
to the general
shear
under
sense of R, Riedel
trend of foliation
shearing.
in the shear zone.
at a confining
light. b. Sketch. Foliation
O,,I=l-lo
200 pm
I
zone deformed
reflected
r=50,
Foliation
in the central
pressure
of 100 MPa (specimen
(thin solid lines) is dragged part (e.g.. EF)
exhibits
along
H19, the
anticlockwise
light. b. Sketch.
in the dotted
areas in (b).
pressure
: PC =150MPa,
within a halite shear zone at a confining
HALITE
shown with solid lines, is absent
deformation
Foliation,
Fig. 72. Heterogeneous
---
NATURAL
of 150 MPa (specimen
-
r=45,
---_
and Logan,
ql-3”
H05, Shimamoto
&I
1986).
a. Photomicrograph
under
reflected
82
E Fig.
13. A schematic
disptacement
diagram
in a shear the effect of slip along a R, Riedel shear on the deformation right hand side of “R,” was drawn assuming a smalf amount of rotation of the R,shear
illustrating
field in the upper
and the shear strain distribution only shows one of the simplest
associated
as shown on the topmost kinematic
models
line. The diagram
to account
with slip along R, shears. The effect of
local slip along the RI shears is to produce a strain component of shortening parallel to the shear zone as schematically illustrated in Fig. 13. Such slip would no doubt reduce the shear strain in the neighboring areas. Figure 13 shows a simple
NATURAL
HALITE
is not based on a mechanical
for the observed
heterogeneous
case in which triangular shearing rotation
the shear
shear
The
(GE-WE)
of deformation;
it
deformation.
strain
area is arbitrarily
the average
analysis
zone.
strain
in the upper
taken
as one-fifth
in the shear
zone.
left of Such
deformation would cause slight clockwise of the R, shear (GE-HE in Fig. 13) and
considerable
shortening
: PC = 100 MPa , 7 =
parallel
60 ,eca, =
to the shear zone
1.0’
200pm
0 L
Fig. 14. Folds of foliation Logan,
within
1986). a. Photomicrograph
of a R, Riedel shear,
a halite shear zone deformed under reflected
AB (in b). The average
at a confining
pressure
of 100 MPa (specimen
light. b. Sketch. Folds seem to have developed
shear strain
at this location
is 60.
in association
H19. Shimamoto
and
with the development
x3
NATURAL
HALITE
: PC = 100 MPa I 7 =
79,
ecal = 0.7’
(a) .‘,.‘.
.
\ .
b)
‘_‘.
.
~ .,
-0
200pm I
Fig. 15. Distortion Photomicrograph location
of foliation under
reflected
along
a P Riedel
light. b. Sketch.
shear,
AB, in a halite
R, and Y Riedel shears
shear
zone (the same specimen
also have been formed.
as shown
The average
in Fig. 14). a.
shear strain
at this
is 79.
in the central part (e.g. C-D in Fig. 13). This shortening must have produced not only the anticlockwise rotation of the foliation (Fig. 11) but also well-developed folds of foliation that are frequently observed in shear zones (Fig. 14). Devel-
opment of other Riedel shears induces additional complexity in the mode of deformation. Foliation has become folded in association with the development of a P Riedel shear in an example shown in Fig. 15.
84
Discussion
and conclusions
HETEROGENEOUS IN
Because extends
the potentially
well into the semiductile
tion characterized oped initiate
fault
(e.g.,
and
Fig.
because
from deeper and Scholz,
4
most
of well-devel-
and
Shimamoto,
large
earthquakes
the mechanical
of faults
in the semiductile
field emerges
primary
area
with
processes
leading
of
interest
to the initiation
zone
behavior
regard
as the to
the
of large earth-
quakes. Unstable fault motion is associated with the strain localization within the shear zones as shown
by Shimamoto
and
ZONES
motion
parts of the seismogenic
1983)
SHEAR
field of deforma-
by the formation
foliation
1985a.b,c), (Das
unstable
DEFORMATION
HALITE
Logan
(1986)
and
is
j,.
15,OMPa
,_&:,-.-z_ . ..i .t --_._
reconfirmed by our more extensive data (Fig. 6). This paper has been intended as a clarification of
.- e., . _~~
.. 7
the nature of this heterogeneous deformation. Our observational results are shown schematically Slip
in Fig. 16 and are summarized as follows. becomes concentrated at one of the
rock-halite 5-10 MPa.
interfaces Stick-slip
at confining pressures of associated with such don-
centrated deformation is typical of the brittle behavior of simulated faults (Shimamoto, 1977; Byerlee et al., 1978). Plastic deformation associated with the formation of foliation begins to occur at pressures above about 30 MPa. The most predominant discontinuities of foliation that develop within the shear zones after the strain localization are Y and R, shears (Figs. 10 and 16), and it appears that a typical mode of internal slip is the
slip
along
Y shears
interconnected
by
R,
shears (lowermost diagram in Fig. 16). Slip often occurs at the other rock-halite interface in different areas of the shear zone at a pressure of 30 MPa, and in such cases, a continuous R, shear is developed across the shear zone (Fig. 16). At a pressure of 70 MPa, Y shears tend to develop throughout the halite shear zone and hence the internal slip mode is distributed over more of the shear zone. Despite considerable variation in the mode of internal slip, the discontinuities developed at pressures of 30-70 MPa are generally narrow, planar and continuous. Of particular significance is that stick-slip or potentially unstable slip with negative velocity dependence does seem to occur only when
Fig. 16. Summary
diagram
neous deformation
within halite shear zones at various
ing pressures, indicate
shown
as demonstrated absent pressure
of heteroge-
sketch.
confin-
Solid
lines
in the shear zones. Plastic deformation,
by the development
pressures in the
the nature
to the left of each
discontinuities
confining
showing
above
dotted
of 150 MPa.
about
areas Large
in the and
of foliation,
30 MPa. sketch
small
occurs
at
The
foliation
is
for
a confining
arrows
indicate
the
senses of shear along the shear zones and along the discontinuities within agram
the shear
schematically
Riedel shears connected
zones, shows
respectively. the
The lowermost
displacement
by the displacement
along
two
diY
along a R, Riedel
shear. See text for explanation.
the slip occurs along such discrete surfaces, although it is still uncertain whether deformation along the discontinuities is predominantly brittle or not. At higher pressures, the discontinuous zones of foliation become wider and less straight (Fig. 16) and the potentially unstable behavior disappears. Stick-slip disappears at fast slip rates, even at pressures below 70 MPa (high-speed frictional regime). However, we were unable to recognize any textural differences between the frictional and high-speed frictional regimes (cf. Shimamoto, 1986). Halite recrystallizes so easily, even at room
85
temperature,
under
extreme
that its texture cannot differences
in the deformation
servations primary mation
within
factor
processes.
Our ob-
pressure
of the mode
is the
of defor-
the halite shear zones.
results
(Shimamoto
and
as
well
Logan,
of the significance
morphic
deformation
to reveal subtle
only show that confining controlling
Present praisal
shearing
be expected
1986)
tectonites.
Mylonites of ductile
with stable
son, 1977, 1983). However, in the halite
shear
zones
suggest
a reap-
have been motion
considand to
(e.g., Sib-
the foliation during
ones
and meta-
deformation
fault
developed
unstable
slip is
similar to planar structures in mylonites, and hence the mylonite could well be the product of semiductile or semibrittle deformation and could be associated with seismogenic fault motion (Shimamoto, in prep.). Large or great thrust-type earthquakes along subducting plate boundaries initiate from depths of 30-50 km, immediately below the estimated depth for high-pressure type metamorphism, so that metamorphic tectonites, too, could well be the products of semiductile deformation (Shimamoto.
Part
1985~). There is a possibility
that the
deformation processes near the focal depth of the large or great earthquakes can be inferred from the studies of mylonites and metamorphic
Byerlee,
J.D..
Wrench
faults
Byerlee,
1978. Friction
J.D.,
1978.
Mjachkin,
Structures
Chen,
W.-P.
of rocks.
thank
J.M. Logan,
M. Friedman,
I. Hara and M. Ohnaka for their useful suggestions, J.N. Magouirk and A. Minami for their help in the experimentation
and observa-
tions and J.M. Logan and an anonymous reviewer for their careful review of the manuscript, and the U.S. Geological Survey (grant 14-08-0001-21181) and the U.S. Department of Energy (grant DEFG05-84ER13228) for partially supporting this work. The experiments reported here were done in Center for Tectonophysics, Texas sity, during the summer of 1984.
A&M
Univer-
cations
Geophys.,
R. and Voevoda,
in fault
gouge
Tectonophysics,
Molnar,
P..
for the thermal
Focal
of
Dieterich,
at shallow
in-
and their impliproperties
of the
Res., 88: 4183-4214.
Das, S. and Scholz, C.H., 1983. Why larg’e earthquakes nucleate
0.. stable
depths
earthquakes
and mechanical
J. Geophys.
during
44: 161-171.
1983.
and intraplate
lithosphere.
depths.
Nature,
J.H.. 1979. Modeling
do not
305: 621-623.
of rock friction,
tal results and constitutive
equations.
1. Experimen-
J. Geophys.
Res.. 84:
2161-2168. Dieterich,
J.H..
simulated
1981. Constitutive
gouge. Geophys.
properties
Monogr.,
of faults
Am. Geophys.
with
Union,
24: 103-120. Engelder,
J.T.,
sliding
Logan,
J.M.
characteristics
H.S.,
confining
pressure:
H.,
Hiraga,
M., Logan. buckling
1986. Textures
H. and
shear J.,
Limits
of stresses
T., 1985a.
Tectonics. Rock
and state variable
study.
The friction
Ph.D.
semibrittle
23rd. Tokyo.
prop-
thesis. Texas
Tex., 198 pp.
Velocity-dependent
T.. 1985b. Friction-velocity
friction relationship
and ductile
of simu-
Meet., 1: 221.
regimes.
for halite Gen. As-
2: 387 pp. (abstr.).
T.. 1985~. The origin of large or great thrust-type
earthquakes
ductile
1: 73-89.
Deformation:
lated halite gouge. Seismol. Sot. Jpn.. Annu.
sem. IASPEI.
in
Res., 88: 10359-10370. an experimental
gouge in brittle.
Texas
New York, N.Y.. 254 pp.
A&M Univ., College Station,
Shimamoto.
of halite
thesis.
T.. 1977. Effect of fault gouge on frictional
Shimamoto.
Seis-
to the depth-frequency
earthquakes.
1983. Slip instability
erties of rocks:
Shimamoto,
of sheared
instability.
M.Sc.
1982.
1978. Experimental
Brittle Field. Springer,
Shimamoto.
thesis.
Tex.. 78 pp.
of shallow
Shimamoto,
Textures
zones.
distribution
A.L.,
MSc.
flow and recrystallization
Strehlau,
laws. J. Geophys.
Geo-
halite.
to the plastic
crust and their relation
Ruina,
under
beams.
16: l-28.
T., 1985.
continental M.S.,
L.J. and
of rocks
Meet., 2: 117.
in experimental R. and
The
72pp.
reference
ST.. 1983. Gliding
gouge
Pattison.
folding
of sheared
Shimamoto,
mol. Sot. Jpn., Annu. Knapp,
J.M.,
Union,
Hiroshima
halite with special
1975.
fault gouge.
of single-layer
Am. Geophys.
Univ. Hiroshima,
J.W.,
on quartz
1972. Experimental
phys. Monogr., Hiraga,
Handin.
113: 69-86.
J.W.. Friedman,
Swolfs.
and
of sandstone
Pure Appl. Geophys., Handin,
physics,
References
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Pure Appl.
V., Summers,
developed
and
tracontinental
Paterson.
We sincerely
limestone
116: 615-626.
Meissner,
Acknowledgements
in
19: 255-211.
A&M Univ.. College Station,
tectonites.
technical
IX,
Tectonophysics.
sliding and stick-slip.
previous
of mylonites
ered to be products be associated
as
sure.
along
subducting
plate
boundaries.
Tectono-
119: 37-65. T.. 1986. Transition flow for halite
shear
between zones
frictional
at room
slip and
temperature.
Science, 231: 711-714. Bartlett, mental
W.L.. Friedman. folding
M. and Logan,
and faulting
J.M.,
of rocks under
1981. Expericonfining
pres-
Shimamoto. J. Struct.
T.. in prep. A new fault model. To be submitted Geol.
to
86
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T. and
Logan,
J.M.,
1981. Effects
fault gouge on the sliding behavior non-clay
gouges.
Shimamoto,
Logan,
and
natural
experiments long-term Shimamoto, behaviors silicates.
J. Geophys.
T. and
of simulated Geophys.
1984.
Laboratory
earthquakes:
Logan,
sandstone:
Res., 86: 2902-2914.
J.M.,
tests. Tectonophysics, T. and
of simulated
of Tennessee
J.M., halite
Monogr.,
an
Sibson, friction
argument
for
Sibson,
United
shear
zones: Geophys.
an analog
for
Union,
37:
Sibson,
two-degree-of-freedom
67: 175-205.
of earthquakes
States. Bull. Seismol.
R.H.,
earthquake
1983. Continental
in the continental
men-apparatus
J.W. and
interaction
Logan.
during
J.M..
fault
structure
source. J. Geol. Sot. London,
stick-slip
1980. Speci-
stability
in a triaxial
phys.. 124: 3833414.
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sliding
crust of the
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of granite.
and
shallow
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Tullis, T.E. and Weeks, J.D., 1986. Constitutive T., Handin,
J. Geol.
133: 191-213.
R.H.. 1982. Fault zone models, heat flow and the depth
1986. Am.
a decoupled
R.H., 1977. Fault rocks and fault mechanisms.
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Sot.. London,
109: 165-175.
49-63. Shimamoto.
compression
model. Tectonophysics,
behavior
Pure Appl.
and Geo-
69
144 (1987) 69-86
Science
Publishers
B.V., Amsterdam
- Printed
in The Netherlands
Textures of sheared halite and their implications for the seismogenic slip of deep faults HIROYUKI
HIRAGA
* and TOSHIHIKO
SHIMAMOTO
Institute of Geolo~ and Mineralogy. Facult~v of Science, Hiroshima Utwersiy, (Received
December
5, 1985; revised version accepted
Huoshma
730 (Jupun)
May 8. 1986)
Abstract Hiraga.
H. and Shimamoto,
T.. 1987. Textures
faults.
In: R.L. Wesson
Halite
exhibits
(Editor),
a wide spectrum
even at room temperature,
precut
reports
Shimamoto
and
well-developed unstable
Logan
discrete
unstable surfaces.
potentially associated
fault
motion
At pressures
unstable
deformation,
homogeneous
plastic
fault
with seismogenic
occurs
above
motion
and mylonites
shearing
deformation
attention
address:
Danbara-yamazaki.
0040.1951/87,‘$03.50
shears.
strain
complete shearing slip thus
localization. takes
place
of a
and that (2) the potentially
although
of this heterogeneous slip withln no direct
of halite.
along
zones become extends
results by
by the formation
some kind of brittle
recrystallization
1985c, 1986).
the earlier
mode of internal
Thus,
and at confining
well into
narrow. wider
More nearly
the shear
deformation evidence planar
and irregular
the semiductile
of
importantly. and
and the regime
with the view that at least some of them could
of
well be
fault motion.
mation ranges from brittle deformation near the surface to fully ductile deformation at depth. Such the mechanical behavior of and plate boundaries, including
Danbara Minami-ku,
Junior
High
Hiroshima
School.
4-42
732 (Japan)
ii) 1987 Elsevier Science Publishers
a complete spectrum of shearing deformation has not yet been investigated on geologically significant silicates in laboratory experiments, primarily due to technical difficulties. Shimamoto (1985a,b,c, 1986) and Shimamoto and Logan (1986) thus used simulated halite shear zones as an analog for faults and plate boundaries, which consist mainly of silicates, and observed a complete transition between frictional slip and ductile shearing flow at
the generation of large earthquakes, requires knowledge of the mechanical properties of rocks under extreme shear. The relevant shearing defor-
* Present
that the primary
the discontinuous
Seismogenic
receive renewed
shear strain
on
halite along a 35 o
(Shimamoto,
characterized
flow
of large-scale
were performed
or natural
Our new data reconfirm
after a critical
after
internal
100 MPa.
disappears.
should
experiments
previously
shearing
properties
within the shear zones. The nature
to the nearly
only when
about
field to fully ductile
from 3 to 300 pm/s,
deformation,
with R, Riedel
Introduction Understanding large-scale faults
specimens.
and it is shown
owing
ranging
slip of deep
144: 69-86.
mechanical
triaxial
with those reported
shearing
interconnected
can be found,
Thirty-nine
slip rate along the precut
with this strain localization
the predominantly
for the seismogenic
Tectonophyws.
slip in the brittle
apparatus.
are consistent
closely in this paper, shears
Faulting.
with a 0.5 mm thick layer of synthetic
into heterogeneous
is associated
deformation
potentially
data
(1) nearly
Y Riedel
overlaps
cataclastic
available
on these, and some previous
changes
is examined
is slip along
possibly
that
foliation,
fault motion
deformation
Mechanical
observations
from frictional
sandstone
with a constant
up to 200 MPa.
This paper
zones
of Tennessee
halite and their implications
of Earthquake
halite shear zone is ideal for studying
with the currently
specimens
at room temperature,
pressures
of behavior,
and the simulated
faults and plate boundaries dry cylindrical
of sheared
The Mechanics
B.V
70
room temperature, data in the brittle
thereby
fundamental
to the earthquake
potentially
seismogenic
of earthquakes,
1982; Sibson. and
depths 1983).
slip
faults
0.5 mm thick
of
initiate (Meissner
Shimamoto
or
plate
important to the ini-
axis,
along
the
near the base of
pm/s,
Strehlau, 1983; Das
(1985a,c)
re-
mogeneous deformation changes into heterogeneous deformation beyond some critical shear strain which increases with increasing normal stress, and that the onset of this localized deformation coincides with a shift in mechanical behavior from stable to potentially unstable slip with negative velocity dependence (i.e., lower friction at a faster slip rate). Thus, potentially unstable fault motion is associated with the localization of deformation even in this semiductile regime. The principal objective of this paper is to examine the nature of this heterogeneous deformation with the aim of gaining a better understandinitiation sults are mylonites sent work Shimamoto
related
procedures
Velocity-stepping ducted by Dieterich (1986). were made
and
layer
of compacted
at an angle
at room precut
sandstone
the confining
et al. (1972).
placements
from
were corrected
along
The
pressure
a
to the cylinslip
rate
300 down
apparatus
Measured
with a
halite
of 35”
temperature.
ranged
200 MPa, using a triaxial Handin
at room humidity
of Tennessee
ranged
to 3 up to
described
stresses
for apparatus
in
and disdistor-
tion, contact area change and the strength of the polyolefin jackets (see Shimamoto, 1977; Shimamoto et al., 1980). Details of the experimental procedures are described in Shimamoto and Logan (1981). Reagent grade pure NaCl (about 0.1 mm in grain size) and iodized cooking salt (0.2-0.3 mm in grain size) were used as the halite layer. This pure NaCl is the same as that used previously (Knapp, 1983; Shimamoto and Logan. 1984,1986; Shimamoto, 1985a,b,c, 1986) but the cooking salt is not. The halite layer was precompacted by leaving the specimen for 15-30 min under a confining pressure of 200 MPa and a differential stress of about 100 MPa;-not adequate to initiate the slip along the precut. The average thickness of the halite zone was estimated from its weight assuming zero porosity. Mechanical
data
to the rupture
of large earthquakes. Observational realso discussed briefly with respect to and metamorphic tectonites. The preis reported in an abstract by Hiraga and (1985).
Experimental
precut
drical
and
textures.
cm in length and 4.8 cm in diameter)
of
the majority
pressure-insensitive ductile flow and yet the shear resistance still depends on the normal stress across the shear zones. Shimamoto and Logan (1986) further recognized that the apparently ductile, ho-
processes
ground
were conducted specimens
(9-11
cognized that potentially unstable fault motion extends well into the semiductile field where deformation textures are nearly identical to those of
ing of physical
on cylindrical
is how deep the
1982; Chen and Molnar,
Scholz,.
with
problem
because
large or great earthquakes the seismogenic
Experiments
questions
boundaries extends. This is particularly in understanding the processes leading tiation
the effect of slip rate on the resulting
the mechanical
and ductile domains.
One of the most regard
linking
tests, similar to those con(1979) and Tullis and Weeks in previous shearing experi-
ments on halite (Shimamoto, 1985a,b,c, 1986; Shimamoto and Logan, 1986). In the experiments reported herein, the slip rate along simulated halite shear zones was kept constant in order to isolate
Ten and 29 experiments were performed on natural and synthetic halite. respectively. Mechanical results from our constant slip rate experiments are similar to those from previous velocitystepping tests (Shimamoto, 1985c, 1986), so they are described only briefly here (see Hiraga (1986) for a complete description). Figure 1 shows the shear stress and normal stress on the precut at steady state or nearly steady state friction from 31 tests on natural and synthetic halite. The shear stress that can be sustained by halite is slightly smaller than that for many rocks that were compiled by Byerlee (1978) at confining pressures below about 50 MPa, but it deviates markedly from his relationship and
STEADY-STATE
FRICTION ZSOMW
200
100
t I
: SYNTHETIC
.
x
:
I[
HALITE
NATURAL HALITE
300
STRESS
NORMAL
.
(Mpa) \qo
Fig. 1. Shear stress plotted at steady
state or nearly
Tennessee
sandstone
(squares)
or synthetic
two categories along
intercept
is selected
When
normal
layers
Data
intermediate
stick-slip
for specimens
thick
halite.
occurs.
are grouped ranges
gives the confining
the frictional
strength
each group
pressure.
for many
---.
--~__-
into
stick-slip
compiled
line by
Byerlce (1978).
,
-3
-2
-1
log(SLIP
Fig. 2. Steady synthetic logarithm
much
less sensitive
to the normal
stress
of slip rate along
slip and stable
slip, as shown
of normal stress at confining pressures above about 250 MPa at fast slip rates (30-300 pm/s), above 150-200 MPa at intermediate slip rates (0.3-3 pm/s)
and
above
about
100 MPa
at slow slip
rates (0.003-0.03 pm/s). The relationship between steady state or nearly steady state friction and the slip rate along the precut is summarized as follows (Shimamoto, 1985c, 1986) (Fig. 2). (1) In the pressure-insensitive flow regime at a confining pressure of 250 MPa, shear resistance increases monotonically with increasing slip rate. (2) A regime of negative velocity dependence (frictional regime) is clearly present at intermediate and slow slip rates and at pressures below about 100 MPa, and stick-slip was observed only in this regime. (3) The negative velocity dependence changes into a positive dependence with increasing slip rate (high-speed
bar(s).
The
Shimamoto
(SLIP
IN
3
pm/s)
state shear
resistance
of
against
the
symbols
are
indicates
episodic
left hand
corner.
the confining
stick-slip
occurs.
pres-
a regular
and the shear stresses at the onset
of abrupt
experimental
Different
in the upper
test. When
is selected
(1985c,
2 RATE
halite and for stick-slip,
next to each symbol
portion
, 1
the precut.
and synthetic
stick-slip
$5
0
steady
used for natural
and at the cessation
/lOMPa 15 5
---
halite (0.5 mm thick) plotted
sure in MPa for each
at pressures above about 100 MPa. The data in Fig. 1 agree quantitatively with the previous results (Shimamoto, 1985c, fig. 8) which revealed that shear resistance becomes nearly independent
RATE)
state or nearly
and natural
The number
becomes
15
------+I0
0
The
of data
The dashed
rocks
of
of slip rate
a regular
_.30-' 30
of natural
and the peak in the stress is plotted. line through
+30 o\30r+x30
stress on the precut
state sliding
0.5 mm
(circles)
of the fine broken
with the abscissa indicates
steady
with
for fast and
the precut.
portion
against
slip events are tied with vertical results
for
1986) are also shown
solid lines with the confining
pressure
synthetic
halite
for comparison
by by
given on each curve.
frictional regime) or with decreasing slip rate (flow regime). (4) The frictional regime diminishes and the high-speed
frictional
regime
expands
with in-
creasing confining pressure, and the friction velocity relationship degenerates into the relationship for the pressure-insensitive flow. Our experimental results are far more incomplete than previous results (Shimamoto, 1985a.b.c. 1986) but are consistent with them as compared in Fig. 2. Microscopic
observations
Microscopic observations were polished specimens with a polarizing under reflected light, described in Shimamoto
made on microscope
following the method and Logan (1986). Speci-
72
mens
were polished
standard and
procedures
the final
paste
with Carborundum using benzene
polish
was made
(1 pm in grain
by use of
size). Grain
lipsoid)
a diamond
boundaries
on
the polished surface were etched with a small amount of saliva. The polished surface was then vacuum-coated
with platinum-palladium
by use of the same procedures of SEM specimens, reflectivity ducing
boundaries reported
of the polished
sharp
contrasts
under
reflected
by Shimamoto
surfaces
between
increased thereby
grains
(i.e., the major
pro-
and grain
light. Some specimens
and Logan (1986) are also
from
ment and the thickness
of the maximum
axis of the strain the measured
el-
displace-
of the shear zone assuming
homogeneous simple shear. When the foliation is uniformly and well-developed as in Fig. 4. the between
5a). These
the
the orientation
calculated
agreement
(Pt-Pd)
for the preparation
and this greatly
and
elongation
as a lubricant
with
boundary
formed halite
/& and 13,.,,is excellent (Fig. indicate that the foliation is
results
parallel grains
preserved
to the maximum
and
that
despite
its original
elongation
of
orientation
is
likely postdeformational
recrys-
tallization. The foliation
defined
by straight
and continu-
included in our description. The thickness of the halite shear zones often increases slightly toward
ous grain boundaries
the lower leading
in those composed of synthetic halite (Shimamoto and Logan, 1986, fig. 8) or crushed natural halite (Knapp, 1983). Shimamoto and Logan (1986) pro-
edge of the precut
specimen,
so
that the average shear strain, y, at the location of a photomicrograph is given in most figures below.
has been recognized
the shear zones consisting
of cooking
only in
salt, but not
Foliation und its origin
posed that the cooking salt grains contain impurities such as various nutriments near their surfaces (upper diagram in Fig. 5b), so that the grain
The starting material, precompacted only by a confining pressure of 200 MPa and a differential
surfaces extreme
stress of 100 MPa, is a structureless
and
aggregate
of
become nearly straight and planar after elongation (lower diagram in Fig. 5b).
that
the
movement
the
original
grain
surfaces dynamic
surfaces (e.g., A, B and C in Fig. 3b) seem to have recrystallized. Fine striations in many other grains such as D in Fig. 3b are perhaps mostly { 100) cleavage fractures. The most conspicuous texture in halite shear zones is well-developed foliation, defined by
foliation. If this hypothesis is correct. halite grains must have elongated more or less uniformly parallel to the direction of maximum elongation to form the foliation, and hence the predominant mechanism of deformation is most likely to have been in-
straight and very continuous grain boundaries (Fig. 4). Almost all of the halite grains in the shear zones are recrystallized grains by dynamic and/or postdeformational recrystallization, as described
tracrystalline deformation
and discussed fully by Knapp (1983) and Shimamoto and Logan (1986). Thus, the textures in halite shear zones do not necessarily directly reflect the processes of deformation. However, the foliation does appear to convincingly reflect the deformation processes (Shimamoto and Logan. 1986). The measured angle, I!$,,~,between the foliation and the shear zone boundary in Fig. 4 is 4.4” f 0.5 O, the error being one standard deviation of ten measurements. This angle agrees very closely with the angle. e,.,, (4.44’), between the shear zone
is hindered or static
of
halite clasts and granulated material (Fig. 3 and fig. 11 of Knapp, 1983). Some grains with clear
by the impurities even during recrystallization to form the
gliding. However, the mechanism of cannot be identified in our specimens
since the halite within entirely recrystallized. observed confining
the shear zones is almost Such a foliation was
in the halite shear zones deformed pressures at and above 30 MPa.
Strain localization
at
and unstable fault motion
Shimamoto and Logan (1986) have recognized that the homogeneous shearing deformation such as that shown in Fig. 4 changes into heterogeneous deformation (as demonstrated by discontinuous and distorted foliation), after a critical shear strain, y,,, which increases with increasing confining pressure. Of particular interest in their
73
UNDEFORMED ONLY
; PRECOMPACTION
(PC = ZOOMPa;
cd =~OOM~a)
100
0 Fig. 3. Photomicrographs differential surfaces reflected
findings
of starting
material
stress of 100 MPa for about
are recrystallized
(synthetic
halite) which was compacted
30 min. (b) shows a close-up
grains. All photomicrographs
pm
1
1
of the upper
in this and subsequent
under
central
a confining
portion
pressure
of (a). Euhedral
figures were taken with a polarizing
of 200 MPa and a grains
with clear
microscope
under
light.
are that the onset
of this heterogeneous
deformation nearly coincides with the shift from stable shearing flow-type slip to potentially unstable friction-type slip and, therefore, that the potentially unstable fault motion is associated with the localization of deformation within the shear zone. These results are reexamined below, based
on our new observational
results (Fig. 6).
The same mechanical data as those in Shimamoto and Logan (1986) are plotted in Fig. 6. Briefly, the mechanical behavior of simulated faults at intermediate slip rates is divided into flow-type and friction-type as schematically shown in Fig. 6b, and they are plotted in the confining
w I-
i I”
1 ---
0
\ pC
=15() MPa, T = 12.8, C&,=4.44",
Fig. 4. Photomicrograph deformed between maximum
under
at a confining
reflected
pressure
light showing
of 150 MPa (specimen
uniformly
8obs=4a40t o-5"
and well-developed
HO2 described
by Shimamoto
foliation
and the shear zone boundaries
agrees quite closely with the calculated
elongation
and the shear zone boundaries
assuming
calculated
from the measured
axis to the minor axis of the strain
ellipsoid
displacement corresponding
homogeneous
along the precut
simple shear.
and the thickness
to this shear strain
across
and Logan,
the foliation
this location.
400 pm
I
angle,
hahte
shear
1986). The observed
a natural
angle,
/3,,,, between
y is the average
the direction
shear strain
zone BohS, of the
of halite at
of the shear zone. The ratio of the major
is 166.
pressure versus shear strain space in Fig. 6a. Here, the shear strain is averaged over the entire shear zone. The friction-type behavior is characterized
roughly classified into (1) homogeneous deformation, (2) heterogeneous deformation and (3) very heterogeneous deformation, and the results from
(1) by an instantaneous viscous-like response lowed by a gradual decay in friction toward
Shimamoto and Logan (1986) and from the present study are all plotted in Fig. 6a, using different
folthe
steady state value upon a step increase in the slip rate and (2) by a negative velocity dependence of steady state friction (Dieterich, 1979, 1981; Ruina, 1983). On the other hand, the flow-type behavior exhibits (1) a gradual and monotonic change in the shear resistance toward the steady state value upon a step change in the slip rate and (2) a positive velocity dependence of the steady state shear resistance. As can be seen from the results at 150 MPa pressure in Fig. 6a, the flow-type behavior changes into friction-type behavior with increasing shear strain. Deformation
within
halite
shear
zones
was
circular symbols formation. Here,
for the different types of dethe shear strain is the average
shear strain at the location of the polished men, not of the entire shear zone. Homogeneous the development
speci-
deformation is characterized by of a uniform foliation with a
good agreement between cobs and f?,,, (see Figs. 4 and 5). Deformation classified as homogeneous includes local and incipient heterogeneous deformation, but the cross-sectional area showing this heterogeneous deformation is less than 10% of the area of the entire shear zone. The incipient heterogeneous deformation was recognized at the
NATURAL
, <,’
HALITE
,’
[ ORIGIN
,’ , ,’I’
OF
FOLIATION
/
NONDEFORMED
8(?p Halite
grains
SUt‘aCe
bwth impurltles
HOMOGENEOUSLY
HOMOGENEOUS
1
SIMPLE
SHEAR
\ 1 %
1
2
3
Fig. 5. a. The measured correspondence between
between
19~~.and
l?,,,. b. Schematic
y and R denote
NATURAL (Cooking
0
-
,’
,’ /’ 0
of the maximum
elongation
and the ratio of the major
of the formation
hne indicates of foliation.
with the standard
axis to the minor
0 ,’ ,‘O /’
_
Boha is the angle deviation
axis of the strain
shown and the
ellipsoid,
F&AT
SLOW
Flow type (-_) Friction type (mm)
t
,’
!
the
TYPE]
/’
? m
against
the one to one
(the major axis of the strain ellipsoid)
[FLOW
0
,’
,’
the process
The dashed
over eight to twenty measurements
Homown. def (0)
,’
:200-
within halite shear zones plotted
shear.
Heterogen. def (0) Very heterogen.def.(o)
Salt)
!k
showing
foliation
simple
of (b).
HALITE
ml
diagram
the direction
developed
homogeneous
averaged
the shear strain
See text for the explanation
_I
assuming
and the shear zone boundaries,
zone boundaries.
300
and homogeneously
elongation
bars and SC,, is the angle between
respectively.
R = 32
(b)
(degrees)
of uniformly
of the maximum
the foliation
by vertical shear
orientation
orientation
T = 5.5.
4
6Cdl
(4
predicted
SHEARED
00
DISPLACEMENT
-
DISPLACEMENT
----,
0
,’ ,’
%
,’
ZlOO-
0
,‘-
ii
l
-I
g v
marrm 0.0 ,
OO
20
(a) Fig. 6. a. Strain pressure
shown indicate of strain
localization
a,
L
SHEAR within
STF%N
pm/s)
The nature
is divided circular
is roughly
symbols
the onset of strain localization localization
and friction-type
(b:
60
r-
from Shimamoto
into flow and friction
of deformation
by the different
,
halite shear zones and a change
versus shear strain space (modified
rates (0.3-3 symbols.
l .
,+
types as shown
classified
as determined
from textural
of simulated
1986). b. Mechanical
schematically
into homogeneous,
in (a). The horizontal
behavior.
in the slip behavior
and Logan,
evidence
faults
shown
during
(b), and these are plotted
heterogeneous
bars with short
behavior
vertical
m the confining
the intermediate
and very heterogeneous
deformations,
lines in the lower left hand
(see text). The dashed
slip
m (a) with different
line in (a) roughly
corner
as in (a)
gives the onsets
76
central
parts of the shear zones (Hiraga,
22) and the strain the central
part
localization
tends to initiate
due perhaps
to the stress
centration there (see Paterson, heterogeneous deformation ranges
from
(e.g.,
slightly
Shimamoto
heterogeneous able
distortion
as to alter
pletely.
Examples
tion are described
at
but
its original
to such
arrangement
of very heterogeneous below.
7 shows a natural halite shear zone at a confining pressure of 5 MPa, which
appear
textural nature
to be recrystallized
evidence
has
of deformation
been
grains. found
within
HALITE
Fig. 7. Photomicrograph
under
rate of 3 pm/s.
evidence
rock-halite procedures
No clear
interface. when
saliva
Planar
reflected
as very heterogeneous
de-
halite
shear
MPa.
Foliation
showing
extending
off the polished
a halite
the
surface.
9 shows
another
deformation deformed
within
is spread
of
a natural
at a pressure
in this specimen
and is discontinuous
example
of 70
is locally
wavy
across many surfaces, over the entire
shear
behavior
coincides
with those of het-
erogeneous and very heterogeneous deformations. At a confining pressure of 30 MPa, deformation appears to become concentrated at one of the rockkhalite interfaces soon after the onset of heterogeneous deformation. Perhaps because of this, foliation remains relatively undistorted and its orientation is nearly uniform throughout the foliated portions (e.g., Fig. 8). Thus, the angle between the foliation and the shear zone boundary
shear
&25
zone
can be seen in the shear from
zone
deformation
: P, = 5MPa,
of deformation
surfaces
was wiped
light
and
this, too, was classified
friction-type
the
the shear zone. The
1981).
NATURAL
con-
Figure
eventually
The result in Fig. 6 substantiates Shimamoto and Logan’s (1986) conclusion that the domain of
neous deformation. Such a concentration of slip at one of the rock-gouge interfaces is characteristic of the brittle deformation of intrafault materials (Engelder et al., 1975; Byerlee et al., 1978; and Logan,
became
shear zone boundary
and
slip is evidently concentrated at the upper rockkhalite interface and hence the deformation of this specimen was classified as very heteroge-
Shimamoto
Deformation
part of
zone.
No clear
to infer
areas near the upper
at the upper
shaped
consists of a structureless aggregate of halite grains. More than half of the grains have clean surfaces and
in the dotted
very heterogeneous
com-
deforma-
across a planar
absent
formation.
a
is discontinuous
(e.g., the thick solid line in Fig. 8b) and is
the sketch.
consider-
at and
in Fig. 8. In this speci-
surface
centrated
fig. 11) to
not
begins to form at pressures
30 MPa as shown
men, the foliation
con-
deformation
1986,
displaying
of foliation,
degree
Figure deformed
Logan,
deformation
above
1978, fig. 13). The mentioned above
heterogeneous
and
The foliation
1986, fig.
upper
left
deformed zone,
hand
and
to the
at a confining
pressure
the slip is concentrated lower
center
were
of 5 MPa
and
entirely
along
created
during
with
a slip
the upper the
etching
:
^ ,>
.z-
of a R, Riedel shear (cf. Fig. 10). Thin ‘vertical striations
the dotted
_
_
‘6 =
26
-
_
during
portions
,
rate of 3 pm/s.
&I=
,,,
the etching
.-
process
_
-<
,
and are not microfractures.
-
0
__-
a clear discontinuity
a. Photomicrograph
2.2”
in b. The heavy solid line in b indicates
of 30 MPa and at a shortening were created
in or across
pressure
PC = 30 MPa ,
of halite at a confining
HALITE
or discontinuous
deformation
shown by solid lines, is absent
Fig. 8. Heterogeneous
b)
(a>
NATURAL
.-
______
Foliation, with the orientation
light. b. Sketch. the foliation
reflected cutting
under
200pm
=:
J)‘Z
=
1-0
’ 82=5
+dWOL=
‘d :
snoauaiioralaH'6 %.J
pallop ay1 ssone 10 "1 s"onuyIox~p
Jo uoyxuo~ap
31IlVH1WfUVN
19
should strain zontal
correspond
bars with short
left hand corner strain,
approximately
at the time of strain
vertical
shear strain
shear
The hori-
lines in the lower
as such from
shown with the dashed
shear
two specimens.
for the strain
localization,
100 MPa, but it markedly
,*I’PC=lOO’iiPa
pressures increases
: ..--- ..
(b) I/
/’
The nature of heterogeneous
‘, 30”‘o
‘&, , *
,o~,~
below about with increas-
at higher pressures.
The orientations planar discontinuities
. -
(4
line in Fig. 6a, seems to be
nearly the same at confining ing pressure
the
of Fig. 6a gives the critical
determined
The critical
to
localization.
pc
=
T&pa
_____ _---------..
deformation
of continuous and nearly of foliation were analyzed
(cl
/I/pc q 5()&pal
,_ _- -.-.--_.
,o%
“.ios
/’
statistically based on detailed sketches of sheared halite in order to examine the nature of the heterogeneous deformation associated with unstable fault motion. The most predominant discontinui-
“‘,
30%
8’
-
(d)
ties in the halite shear zones deformed at a pressure of 30 MPa are R, Riedel shears (Fig. lOa, e). (See Bartlett
et al. (1981) for a discussion
complete set of Riedel shears.) an example of such R, shears
of the
Figure 11 displays along which folia-
-
(e) Fig. 10. a. A complete
set of Riedel shears and their notations.
tion has been dragged, showing the correct sense of R, shearing. The predominant discontinuities
b-e.
showing
are Y and R, shears at confining 50-100 MPa (Fig. lob-d). That
deformed
at a confining
Preferred
orientation
pressures Y shears
of are
poorly developed at 30 MPa is due to the fact that the slip tends to become concentrated at a rock-halite interface at that pressure. On the other hand, slip is more diffuse and occurs along Y shears
developed
higher shown
pressures. The discontinuities of foliation, with the dotted portions in Fig. 12b, be-
throughout
come less sharp, less planar, wider at a confining pressure
the
shear
zone
Rose
straight
diagrams
discontinuities
frequency
pressure
orientation within
as specified
of the discontinuities
in percent.
were made,
the
of foliation
respectively,
230, 498,
taken horizontally
with the right-lateral
zones
in each diagram. by the
130 measurements
one and
mens in (b), (c), (d) and (e), respectively.
relatively
shear
is shown
145 and
for one, three,
of
halite
three speci-
The shear
zone is
sense of shear.
at
less continuous and of 150 MPa than at
lower pressures. Although some of those discontinuities seem to be Y and R, shears, their orientations could not be identified in many cases and were not examined statistically. The patterns of these discontinuities of foliation thus resemble those of Riedel shears, and this strongly suggests that some kind of brittle deformation has overlapped the apparently ductile deformation associated with the formation of foliation. However, these discontinuous regions consist of an aggregate of relatively equigranular grains
(e.g.,
Figs.
microfractures.
8 and
12) and
Unfortunately,
are virtually
free of
the mechanism
of
deformation along these discontinuities cannot be inferred from textural observations as a consequence of postdeformational recrystallization. Heterogeneous deformation within halite shear zones is complex and variable, and its exact nature is not yet elucidated completely through our observations. The following are just a few examples showing simple but essential processes during the heterogeneous shearing deformation in the semiductile field. The development of R, shears is commonly accompanied by anticlockwise rotation of foliation under right-lateral shearing (e.g., EF in Fig. lib), and this rotation appears to be
80
NATURAL
P,=lOOMPa,
HALITE:
0
Fig. 11. Sharp Shimamoto discontinuity, rotation
discontinuity
and Logan,
relative
of foliation
in a halite
1986). a. Photomicrograph
AB, showing
the correct
to the general
shear
under
sense of R, Riedel
trend of foliation
shearing.
in the shear zone.
at a confining
light. b. Sketch. Foliation
O,,I=l-lo
200 pm
I
zone deformed
reflected
r=50,
Foliation
in the central
pressure
of 100 MPa (specimen
(thin solid lines) is dragged part (e.g.. EF)
exhibits
along
H19, the
anticlockwise
light. b. Sketch.
in the dotted
areas in (b).
pressure
: PC =150MPa,
within a halite shear zone at a confining
HALITE
shown with solid lines, is absent
deformation
Foliation,
Fig. 72. Heterogeneous
---
NATURAL
of 150 MPa (specimen
-
r=45,
---_
and Logan,
ql-3”
H05, Shimamoto
&I
1986).
a. Photomicrograph
under
reflected
82
E Fig.
13. A schematic
disptacement
diagram
in a shear the effect of slip along a R, Riedel shear on the deformation right hand side of “R,” was drawn assuming a smalf amount of rotation of the R,shear
illustrating
field in the upper
and the shear strain distribution only shows one of the simplest
associated
as shown on the topmost kinematic
models
line. The diagram
to account
with slip along R, shears. The effect of
local slip along the RI shears is to produce a strain component of shortening parallel to the shear zone as schematically illustrated in Fig. 13. Such slip would no doubt reduce the shear strain in the neighboring areas. Figure 13 shows a simple
NATURAL
HALITE
is not based on a mechanical
for the observed
heterogeneous
case in which triangular shearing rotation
the shear
shear
The
(GE-WE)
of deformation;
it
deformation.
strain
area is arbitrarily
the average
analysis
zone.
strain
in the upper
taken
as one-fifth
in the shear
zone.
left of Such
deformation would cause slight clockwise of the R, shear (GE-HE in Fig. 13) and
considerable
shortening
: PC = 100 MPa , 7 =
parallel
60 ,eca, =
to the shear zone
1.0’
200pm
0 L
Fig. 14. Folds of foliation Logan,
within
1986). a. Photomicrograph
of a R, Riedel shear,
a halite shear zone deformed under reflected
AB (in b). The average
at a confining
pressure
of 100 MPa (specimen
light. b. Sketch. Folds seem to have developed
shear strain
at this location
is 60.
in association
H19. Shimamoto
and
with the development
x3
NATURAL
HALITE
: PC = 100 MPa I 7 =
79,
ecal = 0.7’
(a) .‘,.‘.
.
\ .
b)
‘_‘.
.
~ .,
-0
200pm I
Fig. 15. Distortion Photomicrograph location
of foliation under
reflected
along
a P Riedel
light. b. Sketch.
shear,
AB, in a halite
R, and Y Riedel shears
shear
zone (the same specimen
also have been formed.
as shown
The average
in Fig. 14). a.
shear strain
at this
is 79.
in the central part (e.g. C-D in Fig. 13). This shortening must have produced not only the anticlockwise rotation of the foliation (Fig. 11) but also well-developed folds of foliation that are frequently observed in shear zones (Fig. 14). Devel-
opment of other Riedel shears induces additional complexity in the mode of deformation. Foliation has become folded in association with the development of a P Riedel shear in an example shown in Fig. 15.
84
Discussion
and conclusions
HETEROGENEOUS IN
Because extends
the potentially
well into the semiductile
tion characterized oped initiate
fault
(e.g.,
and
Fig.
because
from deeper and Scholz,
4
most
of well-devel-
and
Shimamoto,
large
earthquakes
the mechanical
of faults
in the semiductile
field emerges
primary
area
with
processes
leading
of
interest
to the initiation
zone
behavior
regard
as the to
the
of large earth-
quakes. Unstable fault motion is associated with the strain localization within the shear zones as shown
by Shimamoto
and
ZONES
motion
parts of the seismogenic
1983)
SHEAR
field of deforma-
by the formation
foliation
1985a.b,c), (Das
unstable
DEFORMATION
HALITE
Logan
(1986)
and
is
j,.
15,OMPa
,_&:,-.-z_ . ..i .t --_._
reconfirmed by our more extensive data (Fig. 6). This paper has been intended as a clarification of
.- e., . _~~
.. 7
the nature of this heterogeneous deformation. Our observational results are shown schematically Slip
in Fig. 16 and are summarized as follows. becomes concentrated at one of the
rock-halite 5-10 MPa.
interfaces Stick-slip
at confining pressures of associated with such don-
centrated deformation is typical of the brittle behavior of simulated faults (Shimamoto, 1977; Byerlee et al., 1978). Plastic deformation associated with the formation of foliation begins to occur at pressures above about 30 MPa. The most predominant discontinuities of foliation that develop within the shear zones after the strain localization are Y and R, shears (Figs. 10 and 16), and it appears that a typical mode of internal slip is the
slip
along
Y shears
interconnected
by
R,
shears (lowermost diagram in Fig. 16). Slip often occurs at the other rock-halite interface in different areas of the shear zone at a pressure of 30 MPa, and in such cases, a continuous R, shear is developed across the shear zone (Fig. 16). At a pressure of 70 MPa, Y shears tend to develop throughout the halite shear zone and hence the internal slip mode is distributed over more of the shear zone. Despite considerable variation in the mode of internal slip, the discontinuities developed at pressures of 30-70 MPa are generally narrow, planar and continuous. Of particular significance is that stick-slip or potentially unstable slip with negative velocity dependence does seem to occur only when
Fig. 16. Summary
diagram
neous deformation
within halite shear zones at various
ing pressures, indicate
shown
as demonstrated absent pressure
of heteroge-
sketch.
confin-
Solid
lines
in the shear zones. Plastic deformation,
by the development
pressures in the
the nature
to the left of each
discontinuities
confining
showing
above
dotted
of 150 MPa.
about
areas Large
in the and
of foliation,
30 MPa. sketch
small
occurs
at
The
foliation
is
for
a confining
arrows
indicate
the
senses of shear along the shear zones and along the discontinuities within agram
the shear
schematically
Riedel shears connected
zones, shows
respectively. the
The lowermost
displacement
by the displacement
along
two
diY
along a R, Riedel
shear. See text for explanation.
the slip occurs along such discrete surfaces, although it is still uncertain whether deformation along the discontinuities is predominantly brittle or not. At higher pressures, the discontinuous zones of foliation become wider and less straight (Fig. 16) and the potentially unstable behavior disappears. Stick-slip disappears at fast slip rates, even at pressures below 70 MPa (high-speed frictional regime). However, we were unable to recognize any textural differences between the frictional and high-speed frictional regimes (cf. Shimamoto, 1986). Halite recrystallizes so easily, even at room
85
temperature,
under
extreme
that its texture cannot differences
in the deformation
servations primary mation
within
factor
processes.
Our ob-
pressure
of the mode
is the
of defor-
the halite shear zones.
results
(Shimamoto
and
as
well
Logan,
of the significance
morphic
deformation
to reveal subtle
only show that confining controlling
Present praisal
shearing
be expected
1986)
tectonites.
Mylonites of ductile
with stable
son, 1977, 1983). However, in the halite
shear
zones
suggest
a reap-
have been motion
considand to
(e.g., Sib-
the foliation during
ones
and meta-
deformation
fault
developed
unstable
slip is
similar to planar structures in mylonites, and hence the mylonite could well be the product of semiductile or semibrittle deformation and could be associated with seismogenic fault motion (Shimamoto, in prep.). Large or great thrust-type earthquakes along subducting plate boundaries initiate from depths of 30-50 km, immediately below the estimated depth for high-pressure type metamorphism, so that metamorphic tectonites, too, could well be the products of semiductile deformation (Shimamoto.
Part
1985~). There is a possibility
that the
deformation processes near the focal depth of the large or great earthquakes can be inferred from the studies of mylonites and metamorphic
Byerlee,
J.D..
Wrench
faults
Byerlee,
1978. Friction
J.D.,
1978.
Mjachkin,
Structures
Chen,
W.-P.
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thank
J.M. Logan,
M. Friedman,
I. Hara and M. Ohnaka for their useful suggestions, J.N. Magouirk and A. Minami for their help in the experimentation
and observa-
tions and J.M. Logan and an anonymous reviewer for their careful review of the manuscript, and the U.S. Geological Survey (grant 14-08-0001-21181) and the U.S. Department of Energy (grant DEFG05-84ER13228) for partially supporting this work. The experiments reported here were done in Center for Tectonophysics, Texas sity, during the summer of 1984.
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