MICROSTRUCTURAL AND GEOCHEMICAL STUDY OF THE BORREGO FAULT ZONE, NORTHERN BAJA CALFIORNIA, MX

Understanding the development and evolution of fault cores and damage zones is vital to understanding the seismic cycle and how fluids behave and interact with the various fault zone architectural components. Unfortunately, a complete understanding of these relationships is not yet possible, as documentation of the different physical properties associated with damage zones and fault cores in a wide variety of settings has not yet been completed. Damage zones consist of densely fractured parent material which lowers the yield strength and increases the ease of rupture (Caine et al., 1996). The formation of fractures also leads to a higher permeability and porosity than in the unaltered parent material (Morton et al. 2012, Rempe et al., 2013). This increase in permeability and porosity thus provides pathways for fluid migration. In addition to acting as a migration pathway for fluids, some faults can also act as seals (Knipe, 1992). The fault core is composed primarily of very fine-grained clay gouge which can act as an impermeable boundary upon healing. This study will utilize geochemical studies including X-ray diffraction and X-ray fluorescence as well as microstructural analyses, to evaluate the sealing versus open character of the various fault zone architectural components associated with the Borrego fault, which ruptured in the 2010 El Mayor-Cucapah earthquake in the Sierra de Los Cucapah in northern Baja California, Mexico.

The comprehensive sampling performed in this study provides a new method for continuous sampling through a fault core and into damage zone. High-quality epoxy was poured across the fault core extending into the damage zone and the epoxied section was subsequently removed from the ground as one continuous section taken perpendicular to the fault. This study focuses on damage (i.e. fracture density) and geochemical trends at varying distances to the principal slip surface but the implications for this method of sampling are widely applicable.