Microstructures of pulverized granitoids from the San Andreas Fault near Little Rock, CA

Abstract:

Pulverized fault zone rocks have been observed in the damage structure of several large bimaterial faults. They appear to have been shattered in situ, are associated with significant grain size reduction, and may be produced in response to dynamic reduction of normal stress during earthquake ruptures. Here we present results of a detailed microstructural analysis of pulverized granitoids from samples of ~40 m deep borehole near the San Andreas Fault (Wechsler et al., 2011). The study makes extensive use of optical and electron microscopy (SEM and TEM) and associated techniques. The host rock is a coarse-grained biotite granite slightly metasomatised collected about 1 km from the San Andreas fault. The pulverized rocks have fine to very fine grain sizes, heterogeneously distributed in thin-sections, with some grains as small as <5 µm. Nevertheless the mineralogical composition is essentially the same as the host granodiorite. All minerals, regardless of their composition, are fractured, and in most cases do not show evidence of rotation/translation. Shattered grains are highly angular and still preserve the high-temperature microstructures of the undeformed granodiorite, such as subgrains in quartz and feldspars. While quartz crystals have more anhedral shapes with few recognizable faces, plagioclase and K-feldspar fragments are more euhedral, suggesting that the pulverization process in feldspars is initiated and facilitated along their mechanical anisotropies. In some samples feldspar are mildly sericitized. Biotite grains might show intense kink-band development, typical of low-temperature deformation, and might be partially transformed to chlorite. When present, calcite grains have a very high twin density, suggesting relatively high stress conditions for their development. Further results (still in acquisition process) at µm-to-nm scale will be presented at the conference.