Surface textures of intrafault quartz grains as an indicator of fault movement

CATENA

Vol. 12, 271-279

Braunschweig 1985

SURFACE TEXTURES OF INTRAFAULT QUARTZ GRAINS AS AN INDICATOR OF FAULT MOVEMENT Y. Kanaori, Abiko SUMMARY Quartz grains obtained from gouges which accompany faults are observed by means of a scanning electron microscope. It is assumed that quartz grains in the gouge are formed from the breakage of quartz in the parent rock due to fault movement, with corrosion by groundwater occuring on grain surfaces after the movement. Therefore, surface textures or morphology of the grains can become a good indicator in examining the mode of fracture at the time of faulting, or the length of time elapsed since the fault moved. 1.

INTRODUCTION

Faults are generally found to be accompanied by fault gouge in basement rocks. It is of great importance to examine properties of the gouge in characterizing the fault (WU 1978, ANDERSON et al. 1980, 1983). Characteristics of faults, especially dimensions, sliding mode and activity, must be studied and assessed for the stability of powerplants, from the standpoint of earthquake design, when the plants must be constructed in potentially unstable areas. Recent experimental studies have been shown that it is important to elucidate mechanical properties of a fault with the presence of fault gouge, because the sliding mode of fault (stable sliding or stick-slip) is closely related to the mineralogy and the thickness of its gouge (SUMMERS & BYERLEE 1977, RUTI'ER & WHITE 1979, WANG et al. 1979). Quartz grains are commonly found in fault gouges. The grains collected from the gouges were indicated to be an important clue to determining the activity of fault (KANAORI et al. 1980, IKEYA et al. 1982, KANAORI 1983). Surface textures of quartz grains from fault gouges observed by means of a scanning electron microscope (SEM) were claimed to be related to the mode of fracture at the time of faulting and the length of time since the fault moved (KANAORI et al. 1980, KANAORI 1983). On the other hand, surface textures of quartz grains included in sedimentary rocks or deposits have been used for an effective indicator in estimating their sedimentary environments or the processes of transportation of deposits (e.g. KRINSLEY& DOORNKAMP 1973). This paper shows that surface textures of quartz grains from fault gouges observed using the SEM are a good indicator in examining the mode of fracture at the time of faulting or the length of time elapsed since the fault moved.

2.

METHOD

Quartz grains were mainly selected from fault gouges and prepared for SEM observation, according to the method established by KANAORI et al. (1980). At first, grains ISSN 0341 - 8162 (~ Copgright lg85 bg CATENA VERLAG. D- 3302 Cremlingen- Des|edt, W. 6ermany

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of 74/,tin to 250/.zm are selected from fault gouges by # 60 and # 200 Tyler sieves in supplying sufficient volume of water and are immersed in a solution of 10% HCI at room temperature until removing carbonate perfectly. Afther then, small weakly adhering particles are excluded from the surfaces in an ultrasonic bath of distilled water about 10 min. Then the grains are completely dried and magnetic minerals are excluded by a magnetic separator. Quartz and feldspar are included in remaining non-magnetic minerals. The following two methods of separating quartz from other mineral grains are employed; (1) checking the observed grains as to whether it is quartz or not by an energy dispersive X-ray detector (EDS) aller stuck on sample-stand, as mentioned in detail by KANAORI et al. (1980), and (2) picking carefully up quartz with a wetted single paint brush hair under a optical microscope before stuck on the sample-stand. The grains thus obtained are scattered over double-adhesive tape stuck to a copper sample-stand of the SEM and tire fixed so that no particles tire piled up oil each other. The grain surfaces are coated with gold. About 10 to 20 grains were randomly taken from each fault gouge sample and their surface textures were observed with a JEOL T-300 SEM tit magnifications of 50x to 10,000x. An accelerating voltage of 25 KeV is employed.

3.

SURFACETEXTURES OF QUARTZ GRAINS AND FAULT MOVEMENTS

Photo 1 shows an explanatory SEM photograph indicating an example of a quartz grain from a fault gouge. The gouge was obtained from the fault with the Median tectonic Line wllich distinguished Mesozoic sandstone from Plio-Pleistocene gravel beds in Kii peninsula, central Japan. The rounded grain with a slightly undulated surface is cut by a breakage surface (photo 1). A movement of the fault led to break rounded grains in the gravel bed, tbrming the grain shown in photo 1. The breakage surface on thegrain is considered to have been newly produced due to fault movement.

3.1.

LENGTH OF TIME AFTER FAULT MOVEMENT

I[ is assumed that quartz grains are formed from the breakage of quartz in tile parent rock duc to fault movement, with corrosion by groundwater occuring on grain surfaces after the movement. When grains originated from basement rocks due to fault movement, all their surl:,ices must be breakage surtaces. The degree of subsequent surface corrosion is, therefore, rehlted to the length ol'time since the fault moved (KANAORI 1983). The rate of corrosion of the surface must be ii]lluenced by chemical composition of groundwater hacontact with the grain, and the permeability and chemical composition of the fault gouge. Photos 3A and 3B indicate a breakage surface and an undulated surface, respectively. The latter surface must be a breakage surface as it was at the time oflbrmation. The sLtrl:ace has been modified by subsequent corrosion or precipitation ofsilica caused by groundwater. Although dilt"erences in chemical environments surrounding the grains should be coilsidereal, tile grain having the breakage surface (photo 3A) must have been formed at the time of later nlovement and/or have been in contact with groundwater during shorter length of tirnc than the grain with the undulated one (photo 3B). MARGOLIS (]968) reported that the surface texture of quartz grains from marine environment was a function of the length of time that tile grain had becn exposed to circulating water and groundwater and also environmental factors sucl] as cNmate and chemical

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composition of local groundwater should be taken into consideration. WHALLEY & KRINSLEY (1974) suggested that the surface alteration of grains from glacial environment was time-dependent in some way though as yet there was no absolute d.ating of the sequence. DOUGLAS & PLATI" (1977), DOUGLAS (1980) and WHITE (1981) observed, using the SEM surface textures of quartz grains in tills, soils or terrace deposits for which the depositional age was known and elucidated that the grain surface texture hac~ an intimate relationship with the age of their deposition. Although further works should be devoted to clarify the environmental factor such as chemical composition, amount and temperature of groundwater in contact with the surface of the grain in the natural fault, and the permeability and chemical composition of the gouge, the degree of alteration of grain surfaces must be related to the length of time since the grains were formed due to fault movement.

3.2.

MODE OF FRACTURE

Since quartz grains in faults were formed from the breakage of quartz w~thin the parent rock due to fault movement and after that were released into fault gouges, grains with breakage surfaces should have the same surface texture as when they were formed at the time of faulting. For example, a river pattern (cleavage step) (KITAGAWA & KOTERAZAWA 1977) is observed on the surface of the grain in photos 3Aand 5. This pattern is attributed to brittle fracture (cleavage fracture), which is categorized under transgranular fracture (KITAGAWA & KOTERAZAWA 1977). River patterns (cleavage steps), striations, granular fracture surfaces and dimple-like textures (KITAGAWA & KOTERAZAWA 1977) were observed to appear on less corroded surfaces of quartz grains from fault gouges and were considered to be related to the mode of fracture at the time of fault movement (KANAORI 1983). On the other hand, the cleavage fracture on surfaces of quartz grains from sedimentary rocks, or aeolian or glacial deposits has been already reported (e.g. KRINSLEY & DOORNKAMP 1973) and thought to be formed by the breakage of grains in sedimentational processes. In fracture tests on rocks the river pattern appeared on fractured surfaces generated by a point load test on quartzite (WlLLARD 1969) and the intergranular fracture appeared on surfaces resulting from torsion test on marble (ATKINSON 1979). Intergranular fracture has been also reported on surfaces of grains from mining-induced faults (GAY & ORTLEPP 1979). Fractographic analysis (KITAGAWA & KOTERAZAWA 1977) of quartz grain surfaces can be a clue to revealing the mode of fracture of the fault movement which released the grains into the gouge.

3.3.

BEHAVIOR OF GRAINS IN GOUGE DURING FAULT MOVEMENT

Wounded comers occasionally appear on quartz grains showing breakage surfaces from fault gouges. Photos 4A and 4B show fractures on the comer of a grain within a small fault in Mesozoic sandstone in northeastern Shikoku, southwestern Japan. Photos 4C and 4D show small scars crowding on the comer of a grain within the Atera fault cutting rhyolite in Gifu prefecture, central Japan. These fractures or scars are similar to abrasion pits observed on grains in aeolian and subaqueous deposits (BAKER 1970). WHALLEY(1978, 1979) also recognized scars similar to those on comers or edges of quartz grains in glacial deposits and ex-

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SURFACETEXTUREAS AN INDICATOROF FAULT

277

plained that they were formed by the abrasion of grains during transportation of the deposit, and reproduced these scars on the corner or edge of quartz grairls by experiments which simulated glacial grinding. Therefore, the fractures or scars presumably formed by the rotation ( E N G E L D E R 1974), or the collisionofgrains in fault gouges alter grains were formed due to ~ult movement. Fractures and small scars on the corner may be formed by violent collision and slow abrasion of grains, respectively. The fracture on the corner may be formed when the t~ault displaced at Paster rate than that including grains with'scars on their corner. This indicates that the rotation, collision or abrasion of grains occurs during the Pault movement, even in l~ault gouges.

Photo5: Spherical small particles adhering on a breakage surface of a quartz grain within a small Faultin Mesozoic alternating beds of sandstone and shale in Kii peninsula, central Japan.

Photo 4: SEM photographs of wounded comers of quartz grains. A and B: fractures on the corner of a quartz grain within a small fault in Mesozoic sandstone in northeastern Shikoku, southwestern Japan. C and D: small scars crowding on the corner of a quartz grain within the Atera faultcutting rhyolite in Gifu prefecture, central Japan. B and D are magnified views of A and C, respectively.

278 3.4.

KANAORI HEAT GENERATION AT THE TIME OF FAULT MOVEMENT

Spherical particles of 1/~m or less in diameter adher to the surface of a quartz grain within a fault gouge, even after the surface cleaned for 10 min. in the ultrasonic bath (photo 5). The particles are similar to those generated on the surface of quartz grains fractured by triaxial compression tests on granite at confining pressures of 6 to 20 MPa. They are also similar to spherical particles on surfaces of the Christianitos fault of southern California (SWAIN & JACKSON 1974) or well-rounded fragments on surfaces of grains in the gouge generated from sliding frictional tests at a confining pressure of 50 MPa (MOODY & HUNDLEY-GOFF 1980). Welded materials were recognized on the surface along a Himalaya thrust (SCOTT & DREVER 1953) and on the sliding surface by experiments at 24 ° to 410°C and up to 500 MPa (FRIEDMAN 1974). This indicates that spherical particles were probably generated by high heat generated on a fault surface at the time of its movement (MCKENZIE & BRUNE 1972). Thermal cracks were observed to propagate from negative crystal inclusions in sliding frictional tests at high temperature (MOORE & SIBSON 1978). Contrary to this, such cracks are not found to originate from a pore on the surface of the grain shown in photos 4C and 4D. Quartz grains existing in fault gouges may make it possible to know whether or not heat will generate on a fault surface at the time of fault movement.

4.

CONCLUSION

Quartz grains which are commonly included in fault gouges are considered to be formed from the breakage of quartz in the parent rock due to fault movement, with corrosion by groundwater occuring on grain surfaces after the movement. In other words, the history of fault movements is engraved on quartz grains in its gouge. Therefore, surface textures and morphology of quartz grains are strongly related to the mode of fracture at the time of faulting, or the length of time elapsed since the fault moved. As the changes of the surface textures or morphology are considered to be influenced in some degree by the chemical environment surrounding the grains in fault gouges, a further study would concentrate on clarifying the relation between quartz grains and gouge properties, such as clay mineralogy, chemical composition and texture, and on examining effects of groundwater in contact with the grains in the fault. ACKNOWLEDGEMENTS I am thankful to Prof. S. Mizutani at the Nagoya University and S. Ogata of Central Research Institute of Electric Power Industry. I would like to thank the staffs of the Institute, Messrs. Y. Satake, T. Kakuta, K. Miyakoshi and S. Inohara for discussion and some aspect of materials presented here.

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