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Description
The relationship between damage in fault zone rocks and rupture dynamics is still incompletely understood despite the importance to geophysical rupture models. Through detailed field observations, petrographic analysis, and quantitative chemical analysis, the following studies describe the damage zone properties along a portion of the Clark strand of the San Jacinto fault zone. This description is accomplished through two fundamentally different studies on the effects of dynamic rupture on fault zone rocks. Included within this thesis are two separate papers independently created for publication. Because of the independent nature of the works some sections will be contain repetitive material. The focus of Chapter l deals with a location within the valley floor of Horse Canyon, where the fault is exposed as a 42 cm wide ultracataclasite fault core embedded within an asymmetric damage zone comprised of fractured and pulverized rock and anastomosing cataclastic seams. The ultracataclasite fault core and cataclastic seams have undergone a loss in bulk mass, while bulk mass of the tonalitic rocks within the damage zone was increased. Utilizing the transport function and TiO2 as the framework element, a relative decrease of Na, Ca, Si, and Al in the fault core and cataclastic seams can be demonstrated, whereas the mass of these elements increases in the surrounding damaged rock. A-CN-K analysis suggests that the observed grain-size reduction is by comminution rather than weathering. A model explaining the above observations is selective pulverization of the quartz and feldspar grains with fluid accommodated mass loss in the fault core. In this model pressurized meteoric fluids enriched from the breakdown of mineralogical components within the core permeate out into the surrounding wall rocks, thereby transferring elemental mass from the core to the damage zone and increasing the bulk mass in the adjacent rock. Chapter 2 focuses on the patterns of damage throughout a double restraining bend of the Clark fault Detailed field mapping reveals an asymmetric pattern of damage with the most highly damaged rocks, or pulverized fault zone rocks (PFZR), located primarily southwest of the main fault strand. Field relationships reveal that the pulverization is not directly related to the restraining bend, but rather spatially related to the complex series of secondary faulting within the step-over region. Chemical analysis shows that the extreme grain size reduction of the PFZR is not due to chemical alteration. In addition to the damage asymmetry with respect to the fault, there is also a variation in damage degree with relation to lithology. The anisotropic rocks within the study area remain resistant to fault related grain size reduction while the isotropic rocks within the damage zone undergo a grain size reduction of several orders of magnitude. Quantitative analysis of the grain size reduction reveals a unimodal particle size distribution ranging from approximately 7-500 microns.
