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Maps are presented of the fractures and faults that developed in originally intact cylinders of granite during triaxial experiments at room temperature. Additional deformation features are described graphically. With increasing axial compression prior to sample failure, the samples show increases in the numbers of microfractures, kinked micas, and flame-perthite-bearing feldspars; the latter feature is an exsolution/replacement texture in K-feldspar whose development is commonly attributed to the application of a differential stress. The overall pattern of fracturing following sample failure varies from sample to sample, but the orientation of the main fault in a given sample can be correlated with the confining pressure of the experiment. Interpretive maps of the paths of concentrated shear show that shearing is much more localized during stick-slip motion than during stable sliding. Localization of shear accompanying stick slip has also been observed following friction experiments in which the samples contained a layer of artificial or natural gouge placed along a sawcut. These similar results suggest that the deformation textures reflect the processes controlling the sliding behavior. INTRODUCTION In order to study the initiation and development of fault zones and the relationships between fault structures and sliding behavior, we have run a series of triaxial friction experiments using originally intact cylinders of granite. To aid in the investigation, maps were prepared of the fracture patterns and the zones of concentrated shear that were visible in thin sections of the samples. Preliminary results of this study are presented in Moore et al. (1990). Because of space limitations, only selected fault maps could be included in that paper. The main purpose of this report is to present all of the deformation maps, to serve as a data base for Moore et al. (1990) and for any future papers on this topic. Measurements made of some additional deformation features observable in thin section, namely kink bands and flame perthite, are included at the end of this report. STARTING MATERIALS The experiments were run using intact cylinders of Westerly and Barre granite, both of which rock types have been extensively employed in geophysical studies. They are typically referred to as granites in the rock mechanics literature and will be called granites in this paper as well. However, both rocks classify as granodiorites based on their mineral proportions (Moore et al., 1987); principally, this means that Barre and Westerly contain considerably less K-feldspar and more plagioclase than a true granite. The two rock types have very similar primary and secondary mineral assemblages, but they differ somewhat in their textures, as illustrated in Figure 1. The mineral assemblages and mineral compositions of Westerly and Figure 1. Photomicrographs of a) Westerly and b) Barre starting materials. Westerly is medium-grained, relatively uniform-looking and essentially undeformed. Barre is considerably coarser grained, particularly with respect to quartz and K-feldspar, and also somewhat deformed, as indicated in b) by the strained quartz phenocryst and elsewhere by kinked micas and aligned fractures. Note the long, intragranular fracture in the quartz phenocryst in b). ~ Barre are described in Moore et al. (1987); of principal interest to this study are their textural characteristics, which are described briefly below. The Westerly samples are medium-grained and haye a uniform appearance (Fig. 1a). They consist of subhedra! plagioclase crystals up to 2.2 mm long and somewhat smaller (about 1 mm diameter) anhedral quartz and anhedra! to subhedral K-feldspar crystals. Mafic minerals, including biotite which averages 0.35 mm in its longest dimension with a maximum of 1.5 mm, are evenly distributed throughout the rock. The Westerly samples are not foliated, and neither the quartz nor the feldspar crystals are highly strained. Some of the crystals, particularly quartz, do contain intragranular and grain-boundary fractures; an example of such fractures that are filled with fine-grained muscovite is shown in Figure 9 of Moore et al. (1987, p. 20). The Barre samples (Fig. 1b) are coarser grained than the Westerly ones (Fig. 1a), and they are dominated by anhedral and highly strained quartz and K-feldspar crystals, up to 3.5 mm long, which wrap around smaller (0.8 to 1 mm diameter) crystals of plagioclase, quartz, and mafic minerals. The biotites in the Barre samples are alsp coarser grained than their counterparts in Westerly, averaging 0.6 mm in their longest dimension with a maximum of about 2 mm. The Barre starting material contains a more,extensive array of fractures than Westerly; the fractures are principally intragranular ones within quartz and, to a lesser extent, feldspar. Because of the larger maximum grain size of Barre, its intragranular fractures also tend to be longer than those found in the Westerly samples. Many of the fractures in Barre have approximately the same trend, which imparts a mild fabric to the rock when viewed in thin section. Some of these fractures are filled with calcite, muscovite, or other secondary minerals. EXPERIMENTS CONDUCTED The experiments can conveniently be divided into 3 groups that were conducted at different times. The experimental conditions for the 3 groups of samples are summarized in Table 1. All of the experiments were run at room temperature; Groups I and III use Westerly cylinders and Group II uses Barre. Group I is derived from an older set of experiments whose strength data are reported in Summers and Byerlee (1977); the deformation textures developed in these samples have not previously been described. The granite cylinders were approximately 25 mm in diameter and 65 mm in length. All of the Westerly experiments were conducted on room-dry samples, whereas all but two of the Barre experiments used cylinders that had been pre-saturated with deionized water (Table 1). No externally controlled pore pressure was applied during any of the experiments. The Westerly experiments of Group I are separable into two sets that were run at different rates of axial loading; the experiments of Groups II and III were run at the slower of the two Group I strain rates. The main variable in the experiments was the confining pressure; because the sliding behavior is a function of the confining pressure, a wide range of pressures was tested to provide several examples each of stick-slip and stable motion for textural comparison. All of the Group I experiments were run until the jackets failed; with a single exception, this involved several millimeters of shear along the generated fault surface. Two of the Group II and four of the Group III experiments were stopped just after the failure point of the sample; Table 1. EXPERIMENTS CONDUCTED Group I Westerly Wla W2a W3a RSW1 R90 R91 W2b W3b R120 R122 R123 R125 Group n Barre RSG1B RSG2B RSG3B RSG4B RSG5B RSG6B RSG7B RSG8B RSG9B RSG10B RSG11B RSG12B Group HI Westerly RSG501 RSG502 RSG503 RSG504 RSG505 RSG506 RSG507 RSG508 Pc(MPa) 41 83 120 277 621 313 78 118 389 486 485 585 485 485 50 100 200 300 400 500 100 500 500 500 100 100 100 100 500 500 500 500 Axial loading rate (mm/sec) .00635 .00635 .00635 .00635 .00635 .00635 .000635 .000635 .000635 .000635 .000635 .000635 .000635 .000635 .000635 .000635 .000635 .000635 .000635 .000635 .000635 .000635 .000635 .000635 .000635 .000635 .000635 .000635 .000635 .000635 .000635 .000635 5 Dry/Fluid saturated Dry Dry Dry Dry Dry Dry Dry Dry Dry Dry Dry Dry Dry Dry Sat. Sat. Sat. Sat. Sat. Sat. Sat. Sat. Sat. Sat. Dry Dry Dry Dry Dry Dry Dry Dry Comments Stable slip Stable slip Stable slip Stick slip Stick slip Stick slip Stable slip Stable slip Stick slip To failure Stick slip Stick slip To 90% failure To 67% failure Stable slip Stable slip Mostly stick slip Stick slip Stick slip Stick slip To failure To failure To 90% failure To 67% failure To failure To failure To 99% failure To 90% failure To failure To failure To 99% failure To 90% failure these experiments are useful for separating the textural effects of the initial break from those superimposed during shear. Four experiments each of Groups II and III were halted prior to sample failure, at some percentage of the failure stress as determined from previous runs that went beyond the failure point. These latter experiments were conducted to study the development of deformation features such as microfracturing and kink bands in the cylinders prior to sample failure. STRENGTH AND SLIDING! BEHAVIOR Plots of differential stress versus axial compression for Groups I and II of Table 1 are presented in Figures 2-5; only those experiments are shown for which sliding was continued after the initial break. All of the experiments of Group III were terminated either immediately before or after the initial failure, and the strength plots for report. The results for Westerly granite in Figures 2 and 3 hat group are not included in this are taken from Summers and Byerlee (1977); their report also contains additional strength data from intact-rock experiments on Westerly. With increasing confining pressure there is a corresponding increase in the stresses supported both at the initial failure point and during the subsequent frictional sliding of the samples. The sliding behavior also changes from stable slip at low confining pressures to stick-slip motion at high confining pressures. The Barre sample at 200 MPa (Fig. 4) displays transitional sliding behavior, with an early period of stable slip followed almost completely by stick-slip motion. No Group I Westerly experiment (Figs. 2 and 3) was conducted at a confining pressure near 200 MPa; however, transitional behavior was observed in several other intact-rock experiments on Westerly granite that were run at confining pressures between 150 and 200 MPa (Summers and Byerlee, 1977). Otherwise comparable Westerly experiments run at different r

Faults, fractures, and other deformation features produced during loading of granite in triaxial equipment