Observation of fault gouge development in laboratory see-through experiments

Tectonophysics, 204 (1992) 123- 136 Elsevier

Science

Publishers

123

B.V., Amsterdam

Observation of fault gouge development see-through experiments

in laboratory

Kerry Haggert a, Simon J.D. Cox b,c and Mark W. Jesse11 ’ (

’ Geology Department, UniLlersityof Melbourne, Parkdle, Vie. 3052, Australia ’ CSIRO D&ion of Geomechanics, PO Box 54, MI Wacerley, Vie. 3149, Australia VIEPS, Department of Earth Sciences, Monash University, Clayton, Vie. 3168, Australia (Received

November

1, 1990; revised version

accepted

August

5, 1991)

ABSTRACT

Haggert, K., Cox, S.J.D. and Jessell, M.W., experiments. Tectonophysics, 204: 123-136.

1992. Observation

of fault

gouge

development

in laboratory

see-through

We describe the results of a set of brittle sliding and wear experiments on thin sheets of polycrystalline sodium nitrate. We have built a new brittle deformation apparatus that allowed us to simultaneously observe the microstructural evolution under the microscope, and measure the mechanical response of the material. The microstructural history was recorded using a video microscopy technique and selected frames were then digitized and analysed. Fault gouge in these experiments was the direct product of frictional wear and was produced dominantly by intense intergranular fracturing and accompanying grain surface erosion. Although an increase in displacement was accompanied by an increase in gouge zone width, the development of a gouge zone is not a constant process either with respect to displacement, or with respect to the spatial distribution along the fault zone.

Introduction

Fault gouge or breccia is non-cohesive cataelastic rock produced by shear-stress-induced brittle failure within fault zones at shallow depths Gibson, 1977). It is present in most natural faults and is partly related to the interaction of the rough surfaces generated during the formation of the fault. It has been suggested that the mechanics of fault development may be reflected in morphological details of natural gouges, particularly in terms of the gouge zone thickness (Robertson, 1983; Scholz, 1987) and the particle size distribution (Rutter et al., 1986; Sammis et al., 1987). However, the micromechanical details of the processes governing gouge evolution have been dis-

Correspondence Sciences,

Monash

0040-1951/92/$05.00

to: S.J.D. Cox, VIEPS, University,

Clayton,

Department

of Earth

Vie. 3168, Australia.

0 1992 - Elsevier

Science

Publishers

cussed in fairly idealized terms (Scholz, 1987; Sammis et al., 1987). Most previous laboratory-based studies of the development of fault gouge structures have concentrated on the evolution of existing gouge zones of various thicknesses containing loose granular material (e.g., Engelder, 1974). These have shown that the deformation within a fault gouge layer tends to localize onto surfaces with characteristic orientations (e.g., Logan et al., 19791, and the degree of this microstructural development is dependent on the composition of the gouge, the pressure and temperature applied, the relative sliding velocity of the surfaces in contact, and the total slip (Byerlee et al., 1978; Moore et al., 1986, 1988; Power et al., 1988). Experimental procedures, which start with clean surfaces and achieve sufficient displacements to generate reasonable amounts of gouge, have only recently been developed for rock using rotary apparatuses (Blanpied

B.V. All rights reserved

K. HAGCEK

124

et al., 1987; Wang and Scholz, 1990). The results from these, however, appear to largely follow the earlier experiments. It has also been found that the presence of gouge within a fault zone increases the stability of sliding and reduces the magnitude of the stress drops associated with stick-slip (Byerlee and Summers, 1976; Byerlee et al., 1978; Engefder, 1978). Clearly, it is of major interest to understand the processes invoIved in the initiaI development of gouge and the subsequent microstructural modification of the gouge layer because of this significance for fault mechanics. A major limitation in the previous studies is that observations are restricted to after-the-event sectioning, so the deformation sequence can only be inferred, or involve interrupting the run with the danger of disturbing the geometry. It is desirable to make sequential observations of an evolving gouge zone in situ. However, shearing experiments on real rocks pose several problems stemming from the difficulty in imposing the high strains, pressures and temperatures which exist in the Earth’s crust. Accordingly, analogue materials, which deform similarly to the rocks they model, may be used for studies at physical conditions which are more easily attained in the laboratory (Means, 1989). Using these materials, essentially two-dimensional experimental techniques have been developed enabling the processes of deformation to be directly viewed through an optical microscope. In such experiments, the use of analogue materials allow experiments to be conducted in apparatus small and light enough to operate on a conventional microscope stage. In the ductile field, this has revolutionised our understanding of the processes involved in dynamic recrystallisation (Urai et al., 1980; Means and Xia, 1981; Tungatt and Humphreys, 1981; Wilson, 1986; Urai et al., 1986), and their effects on fabric development in deforming rocks (Jesse& 1986). In this study we have used an experimental configuration similar to those previously applied in. the ductile field of deformation and have further developed the method to allow the observation of brittle sliding. The technique permits the processes involved in the initial production of wear material at a microscopic level to be moni-

L’ ET Al,.

tored in real time. By correlating direct obsefvations of the development of fault gouge with changes in the mechanical parameters of the experimental system, we can obtain a further understanding of the development of fault gouge and the mechanics of fault movement. Experimental methods Material used nnd gamble preparation

Sodium nitrate, NaNO,, has the same crystal structure, cleavage and twinning symmetry and similar mechanical behaviour as Calcite (Tungatt and Humphreys, 1981). This makes it a close analogue of an actual rock material. It is transparent, has a low melting point (307”C), and is chemically stable up to the melting point. We used granular, 99.5% pure reagent-grade sodium nitrate for these experiments. Thin, poIyc~stalline aggregates were prepared by hot-pressing sodium nitrate between glass slides coated with a releasing agent (Dow-Corning) at pressures of 8 MPa. The temperature was maintained at 305°C to allow the grains to anneal without melting. The resulting sample had dimensions roughly 40 x 20 X 0.2 mm and a grain size in the order of 0.5 mm in diameter, so most grains extended through the material to both free surfaces. The sampfe was removed from the slides and dissected using a razor blade. The long edge of each sample was then ground to produce a macroscopically smooth surface. The sample preparation processes also caused some fracturing of the samples. On the microscope in transmitted light these appeared as opaque zones when the fractures propagated obliquely through the sample, or when some dilation occurs. The majority of these developed along grain boundaries, and displayed no systematic orientation.

We have developed the micro-friction rig (Fig. 1) by extension of the principles used in the shear rigs described by Means and Xia (1981) and Urai et al. (1980).

fiA1Ji.i’

GOU(ili

l~i:Vti.OPMEN’l’

IN l.AROKA-WRY

ShbTWROUGH

EXPERlMEN

I’S

------+ shear

force Fig. 1. The micro-friction

rig. The mounting

blocks and sam-

ples are placed within this outer casing.

Two foils of the analogue material are prepared with long straight edges. The foils are each glued over part of their area with a cyanoacrylate adhesive to a thick glass mounting block, leaving the prepared edges unattached. The blocks are placed together so that the half-samples lie side by side with only the long edges in contact, which comprises the initial fault trace (Fig. 2a). The samples are kept within the plane between the mounting blocks by a confining force applied by two leaf springs on the upper glass block. The rig design partitions the free motion of the two glass blocks such that the bottom block is constrained to transverse motion, while the top block is constrained to forward motion, and in this way we can separately measure the loads applied in these two directions. A motor and gear system provides a shear force between 2 and 10 N which pushes the upper glass block and sample longitudinally over the lower glass block at a constant velocity (0.008 mm/s in the experiments reported here) for a maximum distance of 30 mm. The lower block remains stationary resulting in relative shearing between the half-samples along the fault trace. The two half-samples are pushed laterally together by a spring-loaded device, producing a normal force between 10 and 20 N. The normal stress varies during an experiment because the area of contact along the sliding interface changes as the samples are offset. In this sequence of experiments the sample thickness was not recorded systematically, but converted to stresses the applied loads would fall in the range 0.25-20 MPa.

0 Fig.

2.

(shaded),

(a)

Relationship

the thick

between

the

glass mounting

forces in micro-friction

rig. When

thin

blocks,

half-samples

and the applied

the upper glass block slides

over the lower block, the two half-samples

lie side by side and

grind along their long edges. Microstructural

changes result-

ing from the shearing are viewed from above as indicated. Cross section through indicate

the sample

the sliding interfaces:

the simulated

assembly. The

the short vertical

fault, while the longer horizontal low-friction

sample-glass

thick

(h) lines

segment

is

segments are

interfaces.

Roller bearings are installed between the glass blocks and the rig’s casing to ensure that the only major frictional surfaces in contact during an experiment are at the fault trace, and the interface between the smooth, flat, unbonded surfaces of the half-samples and the glass blocks against which they slide (Fig. 2b). We measured the background friction in the system in a run in which the half-samples were not in contact at the fault trace, and the mechanical data presented have been corrected with respect to this background level. This was, however, only 1530% of the measured signal.

The rig was placed on the stage of an optical at room temperature so that the de-

micrownpe

126

K liAC;(;ERT

El‘ AL

r.Alit.r

GOUfiE

I~~VEI,OPMtNI’

Fig. 3. Sequence of view

of images from

is 2.2 mm wide.

preparation.

The opaque

shear. Grain

boundary

(b) Microstructures

experiment

(a) Initial

state

zone through

A, grain

seen after

are seen (e.g., in grain half-sample

IN i.ABORAl’OKY

Na#32

the centre

5 and a complex

margin

half-sample

Extensive

arc all oriented

in the expected

samples

the

during

contributed their

mutual movement

tensile

grain boundary

boundaries.

to the dilation. creating

is accommodated

Some

structure

are as follows: in the lower

moving

of twins and fractures

fractures

and twin

The opaque

Cd) After A series

half-sarn~le~

half-sample

rotations

voids (e.g., at L and M).

These

which

formation occurring during a sliding experiment could be viewed in transmitted light. A video camera attached to the microscope altowed the microstructural data to be recorded onto video tape. These were replayed through a microcomputer (Macintosh II) and selected images were digitized using a frame-grabber and stored for later comparison. The maximum size of the images is 768 X 512 pixels. In the analysis of the experiments sequential images were digitally enhanced using the software package Image (W. Rashband, NIH, Washington D.C., U.S.A.). The images in Figures 3-S were taken under planepolarized light and magnified so the fields shown in the figures correspond to an area of only 2.2 X 1.8 mm of the sample. In each image shown the upper sample is stationary with respect to the microscope stage, which we will refer to as the hanging wall, while the lower sample, the foot-

fractures

develop

orientated

The field

during

sample

have a dextral

sense of

images for reference.

but some conjugate

are observed

at D and r?:

into the gouge zone. Cc) After

This

image large

shows

grain

and many grains

arrangements

sets

is best seen in the upper

new fractures

and as the samples

in en-whelon

will

development

of the

in width

dilate

which

B and J; the new fractures

6.9 mm of displacement. of interacting

the substrate.

in this and the other

of material

in grains

gouge zone has increased

along these free surfaces

fault,

fracture

in size by erosion

is seen in the upper

orientation.

F are labeled

within

sets have developed

twins are not consistently

out of the field of view-two

Q has been reduced

last stages of the experiment.

particularly

the development

of the image is the trace of the simulated

is constantly

twin development

EXPERIMENIS

half-samples.

of the image

a void in the gouge zone is seen at 7’; grain of displacement.

showing

of the two

4.1 mm of displacement

at the left

since the lower

SEE-.~HROCIGH

5.1 mm

at G, N and !

disaygregation

N with

grains

of the 0

and

have lost cohesion

continue through

to disaggregate

P

along slip

both half-samples.

wall, is moving toward the right. This allows direct comparison of the structure at successive stages through the experiment within the hanging wall. The forces applied during the experiments were measured using load-ceils installed between the screw-feed and the upper block for the shear force, and between the spring and the lower block for the normal force. These each consisted of a stiff cantilever with a full strain-gage bridge bonded to the arms. The signals were recorded digitally at 5-s intervals.

The processes observed in britrle sliding We have identified four main deformation processes, which progress in a series of aperiodic,

128

K HACiCiERT

Ll- Al

FACiLT

GOUGE

DEVEL~3OPMENT

IN LABORATORY

SEk-THROUGH

episodic events which thus appears to be a fundamental characteristic of brittle deformation in these experiments. Deformation is not limited to the areas immediately adjacent to the fault zone, but considerable fracturing and twinning occurs in the wall-rock material at distances away from the fault trace up to twice the gouge zone width. Sequential images from experiments Na#32, Na#36 and Na#30 (Figs. 3, 4 and 5) are presented to illustrate the development of many of the following features. Twirming

Twins were found within many grains prior to the initiation of sliding, probably in response to the forces imposed during sample preparation and during the application of the initial normal load. The composition planes were randomly orientated with respect to the experimental geometry. As sliding progresses new twins developed. These often formed conjugate sets so that the resultant finite shear strain was in the direction parallel to the sample shear direction (Fig. 31, The opticaf definition of pre-existing twins was often enhanced, possibly indicating fracturing.

Most new fractures propagated so that the acute angle they made with the sliding surface faced away from the direction of movement, especially intragranular fractures. These directions are at a high angle to the orientation of the most tensile stress component, and thus supports previous suggestions that tensile cracking plays an important roIe even in shear deformation (e.g., Brace and Bombolakis, 1963; Evans and Wang: 198.5;Cox and Scholz, 1988). Fractures inclined in the reverse orientation, however, were occasionally seen in the early stages of sliding. In Figure 3 we indicate typical fracture orientations at successive stages through an experiment. Throughout the duration of sliding, much of the fracturing was concentrated along pre-exist-

Fig. 4. Sequence

of images

from experiment

wide, and the images correspond

Na#36

showing

129

EXPERIMENTS

ing lines of weakness, especially along grain boundaries, while intragranular fracturing was only common during the later stages of an experiment. Both types of fractures opened in a direction parallel to the predicted maximum tensile stress, though in several experiments they accommodated some of the &ding movement just prior to the samples undergoing complete disaggregation (Fig. 3d). Although the initial sliding surfaces in some of our experiments were quite rough even macroscopically, we only observed fracturing through an asperity at the onset of sliding in one experiment (Fig. 5). The dislodged grain shown was further eroded to form a fine-grained, opaque powder which infilled irregularities along the sliding interface but did not accommodate any shear movement. At the normal stresses used in these experiments, gouge interactions quickly become dominant over the deformation of asperities on the substrate. Grain surface erosion

As the grain boundary fractures propagated, the edges of the grains became damaged in a process we term grain surface erosion. A shattered zone progressed inward towards the grain centre, creating a halo of wear materiai around a less deformed region of the grain. This continually reduced the size of the grain until only small, angular particles remained (Fig. 4), ranging in size from about 0.01 mm to the original grain size of about 0.5 mm in diameter. In some cases the cohesion at the grain boundaries was sufficiently decreased for the whole grain to become dislodged from the substrate and included into the gouge zone. These loose grains also underwent grain surface erosion as part of the processes of comminution operating within the gouge zone. The damaged areas appeared opaque since particles smaller than the thickness of the sample were stacked normal to the plane of the image and the fine fractures through grains were very

the process

of grain boundary

to (a) 0.5 mm, (b) 1.7 mm and Cc) 4.8 mm of displacement.

of grain A in the centre,

accompanied

by the creation

of opaque

erosion. showing

The field of view is 2.2 mm a gradual

gouge material.

reduction

in the size

130

K. HAGGERl’

ET Al

FAU1.T

GC)IJC;k

I~EVtzl.OPM~N’I

Fig. 5. Sequence irregularities still

smaller

tensile particles

are being

from

in the substrate

concentrated

expected

of images

IN

rolled

along

experiment

are often

a rigid

orientation

I.ARORA~l’ORY

are then

around

within

Na#30

not filled

intert‘ace.

The

in a grain where

which

Slit-‘I-HROUGH

incorporated

tXPEKIMLN7S

showing

the process

by gouge material. field

of view

is 2.2 mm wide.

the stress is initially

(a) Displacement

concentrated.

This dislodges

The new gouge appeared quite cohesive and did not move independently between the two samples. Previous studies have reported the development of subsidiary shear planes within unconsolidated gouge zones (e.g., Logan et al., 1979; Moore et al., 1986, 1988; Marone and Scholz, 1989). In our experiments slip remains localized near the original sample interface and we did not observe the development of any other localized shear planes. Our observations were restricted by the opaque appearance of the gouge in transmitted fight, but may also have been limited by the small total displacements reached, together with relatively low normal and confining pressures used. Increased normal forces tended to decrease sliding stability in this apparatus which resulted in complete sample failure by disaggregation at an early stage of fault movement, so the upper range of normal force was limited to the values indicated.

opaque,

We also see how voids

0.03 mm.

created

by

that the slip movement

is

Fracturing

two large particles

0.2 mm. (c) Displacement

in bize by the process of grain

5.6 mm. All that is left is fine-grained,

the transmission

plucking.

these holes it is evident

into the gouge zone. (h) Displacement

the gouge zone and decreasing

closely spaced, both preventing of light.

of asperity

and through

boundary

erosion.

is seen in the and a number

I .4 mm.

of

The grains

Cd) Displacement

gouge material.

Mechanical changes such as slip weakening (discussed below) within a gouge zone of constant thickness would require some microstructural modification within the fault zone, but these processes were not resolvable at the scale of our observations due to the opaque nature of the gouge. Voids within the gouge zone were created by bridging between asperities on the sample surfaces. These usually changed in morphology as sliding occurred but they did not always act as a sink for gouge particles. They remained supported along the sliding interface for most of the experiments’ duration, and through them we saw that the fault surface remained close to its initial position (Fig. Sd). Derelopnzent

qf the gouge zone

Although the experiments began with a gouge-free interface, there was an opaque rim along the sample edges due to damage introduced during sample preparation. This opaque zone cannot be differentiated from gouge mate-

K. HAGGEKT

132

Na#53

.

,

0.01 0

10

5

slip displacement

Fig. 6. Relationship and slip displacement rate

of 0.3 mm/mm

approximate

a steady

are approximate

between

the average

for experiment decreases state

(mm)

gouge

gouge

Na#S3.

with

slip displacement

zone width.

fits to the gouge zone growth

the large reduction

zone width

The initial wear to

The two lines law, indicating

in wear rate after longer displacements.

rial under the microscope, so we take it to represent the initial width of the gouge zone; around 0.16 mm in all experiments. In our subsequent measurements of the gouge zone thickness, areas of the sample adjacent to the interface which contained opaque, fine-grained material were included, but not fractured grains where no grain comminution to this level had occurred. The thickness of the gouge material generated in our experiments was not a simple function of slip, however, portions of the width/displacement curves could be approxinlated to linear functions. In the early stages of sliding the amount of gouge, and hence the thickness of the gouge layer, increased with slip displacement at rates ranging up to 0.41 mm/mm, and following this, the rate of gouge development decreased substantially (Fig. 6). The displacements attained before this decrease ranged from 2.4 to 6.0 mm. The variation between experiments was probabIy due to the different initial strengths and thicknesses of the sampIes.

proportionali~ is called the coefficient of friction. Figure 7 shows that these experiments were generally consistent with this. A slight increase in the coefficient of friction ‘and then a decay to a constant value was usually observed to follow a pause in sliding, such as when the normal force was increased. Such transient effects have been described previously, and it has been suggested that for a particular surface these may be characterised by a constant displacement for the development of an equilibrium value for any particular sliding velocity (e.g., Dieterich, 1979; Blanpied et al., 1987; Cox, 1990). The fact that we observe transient effects with the expected sense and a reasonable magnitude supports the validity of the dynamic observations made using the micro-friction rig for the small displacements monitored. During experiments changes in the shear and normal forces could be correlated with stages of the development of the gouge zone and the nature of the fault movement as observed under the microscope. Figures 6 and 8 show the average gouge zone width and the shear force, with respect to the slip displacement, for one experiment. The first 3.5 mm of sliding were characterized by rapid growth of the gouge zone at a rate of > 0.3 mm/mm. The friction coefficient was roughly constant through this period, though

0.6 _ 2 .I 2 B 8 5 ._ 6 ._ +

0.4 -

0.2 -

N-l 1 SN

,

0.0

Mechanical

observations

To a first approximation, shear resistance due to friction follows Amontons’ law with shear stress proportional to normal stress. The constant of

trl’ AL.

.

.

N=13..5N

.

0

’

I .

.

.

N=15.5N .

sIip displacement

of shear

force

experiment

Na#37.

constant

between

to normal

and

coefficient

(the ratio

slip displacement

coefficient

step changes

15

(mm)

the friction force)

The friction

through

I

10

5

Fig. 7. Relationship

_I ..,.

is approximately

in normal

force.

for

MAUI

‘I’ GOUCil:

III~V~I,Of’M~N

I’ IN

l,AI3OKA’I‘ORY

Strk- IHKOUGH

EXPkKIMtN

0.6 2 .z lz t e

0.4

x ‘Z .i! t 0.2

10

5

slip displacement Fig.

8. Relationship

friction

stages of sliding spond

the sudden events,

sliding

is in progress. with

experiment material

(Fig.

through Comparing

During

in the shear

during stable

that

force

mechanism, cunie

the microscope the

and the the early corre-

the later stages of the

changes

in the gouge zone width

h), it appears

the shear resistance

displacement

Na#S3.

friction-displacement

changes

approaches

slip

drops

while

is by a more

observations

coefficient

the

experiment

by the smoother

the direct ment

for

to stick-slip

experiment cated

between

coefficient

(mm)

as indias well

as

as the experiin the

friction

for the same

as the amount

a constant,

steady-state

volume

of the fault

zone is dramatically

of gouge or width, reduced.

many stick-slip events occurred, seen as sharp decreases in the friction in Figure 8. The thickness of the gouge zone approached a constant value of 1.2 mm during the later stages of the experiment (Fig. 61, accompanied by more stable sliding. There was also a weakening of the sliding interface with displacement as indicated by the gradual decrease in the shear force necessary to maintain a constant sliding velocity. Although the gouge zone appeared to reach a steady-state width, micr~)structural development below the resolution of our images must have continued which caused this weakening. Discussion

Mechanisms of gouge formation These experiments have illustrated some mechanisms of gouge development and substrate deformation in addition to those which have been previously recognised.

1’S

I.13

(1) Deformation is generally episodic, but this does not always produce stick-slip events in the mechanical data. We could only recognise this through the real-time observations possible in such two-dimensional experiments. The sudden deformation episodes appear to be the dominant contribution to gouge accumulation in the case described here. (2) Deformation is not limited to the areas immediately adjacent to the fault zone, but considerable fracturing and twinning occurs in the wall-rock material for a distance approximately twice the gouge zone width. (31 Fracturing is often concentrated around the entire perimeter of grains in the substrate, and damage progresses inward creating a halo of wear material around a less defornled region of the grain. This may eventually result in complete destruction of the grain, creating additional gouge material in the process, or may substantially decrease the cohesion associated with the grain boundaries such that the whole grain becomes dislodged from the substrate and included into the gouge zone. Sammis et al. (1987) have proposed that splitting of grains due to impingement of similar-sized particles provides a geometrical explanation of particIe size distributions observed in fault gouges. In these experiments. however, transgranular fracture of grains loosened from the wall rims is clearly less important than grain surface erosion described above. Grain surface erosion tends to lead locally to a bimodal particle size distribution, but because many grains will be at different stages of size reduction, the overall particle size distribution coufd still be similar to that suggested by Sammis et al. (1987).

The dimensions of the micro-friction rig allows an absolute displacement of 30 mm, which, although large compared with many previous experiments conducted in triaxial apparatuses, still appears to be insufficient for an equilibrium microstructure to develop. The gouge particles arc highly angular, and the gouge zone does not attain a steady state width in many of the experi-

134

ments analyzed. Localized slip zones within the developing gouge zones, as might be expected in a mature gouge (Logan et al., 1979), have not been observed. The initial microstructure is the main control on the pattern of deformation within the sodium nitrate samples. The physical characteristics of each sample (i.e., the initial size and number of asperities on the sIiding surfaces, the thickness and length of the samples, and the strength and cohesion of the individual grains) are variable. Thus, while the overall class of mechanisms is quite consistent, the distribution of the processes of deformation within the sample are different for each experiment. Gouge zone thickness and wear models In a model of gouge development through the interaction of initially rough surfaces, both the thickness and grain size of the gouge zone will be determined by displacement and the initial surface roughness until sufficient sliding has occurred to produce equilibrium conditions and a decreases in the rate of gouge development (Yoshioka, 1986; Sammis et al., 1987; Power et al., 1988). Although artificially prepared surfaces may initially be regarded as “macroscopically smooth”, a11surfaces are rough at some scale. An asperity can be defined as “. . . any roughness, bump or other projection on a sliding surface” (Engelder, 1978). The static frictional force on a fault zone is due to the strength of interlocked asperities, and only when these are overcome can sliding occur (Bowden and Tabor, 1950; Archard, 1953; Byerlee, 1967). In overcoming asperities, Engelder and Scholz (1976) recognized that a hard asperity ploughing into a soft substrate will eventually fracture, and a soft asperity may adhere to a hard substrate. Material dislodged along sliding surfaces in this way has been considered as the classic mechanism of gouge generation. In these experiments, however, the grain surface erosion process was the principal means for the creation of gouge material, and the intersections of grain boundaries and the shearing surface were the loci of greatest gouge development.

K. HAGGEK’I

L’l; Al..

In previous studies on naturai faults, approximately linear relationships have been found to characterize the relationship between fault zone thickness W and shear displacement D. These include:

D = 83W’.“’ and D = 6OW’.“’ (Otsuki, 1978); D = cW; c = 10-1000 ( Robertson. 1983) ; D =cW; c = 1000 (Wallace and Morris, 1986) ; D = 309W ‘B (Clout, 1989, Monash Univ. PhD). Previous studies of experimentally produced fault gouge, however, indicate that there is a non-linear relationship between thickness and displacement with the wear rate decreasing with slip displacement (Yoshioka, 1986; Power et al., 1988: Wang and Scholz, 1990). We have measured the slope of portions of the wear curves obtained for the experiments discussed in this study (Fig. 6), and find that the coefficient c ranges from - 3 to - 160. While the total shear displacements achieved in these experiments were limited, the larger values of c found in the latter stages are within the range expected since any approximately linear portions in the early stages of these wear curves will show growth rates much greater than the long-term average, which must be represented in the natural cases. However, we may also interpret the non-linear growth characteristic of experimental faults with reference to a wear model which depends on brittle interactions between surface irregularities on the fault walls (e.g., Byerlee, 1967). An explanation for the change in behaviour for the artificial surfaces may be drawn from work by Power et al. (1988) and Wang and Scholz (19901, based on ideas of the scale dependence of surface roughness. Wang and Scholz found that the gouge formation process in their experiments may be divided into two phases, with a decrease in wear rate between them. On the basis of some detailed measurements of natural faults, however, Power et al. (1988) suggested that there was a major distinction between the topography of natural and artificial surfaces. They found that on natural

FAtJill- GOUGE

DEVELOPMENT

IN LABORATORY

SEE-THROUGH

surfaces roughness continued to increase up to the longest scale lengths measured, and suggested that damage and gouge creation, through intersurface interaction, would continue indefinitely. In contrast, because of the way in which they are usually prepared from an initially flat surface, artificial faults have a long wavelength cut-off, above which the amplitude of surface features does not increase, so this will lead to a reduction in the wear rate as sliding progresses.

135

EXPERIMENTS

We would like to thank A. White for assistance with the construction of the prototype rig, C. Marone for discussion, and W. Power for a review of the manuscript. Financial assistance came from a Monash University/CSIRO collaborative research grant.

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