Structure and petrology of the San Gabriel fault zone near the wildwood picnic area, San Gabriel Mountains, California

Brittle fault zones are lithologically heterogeneous and structurally anisotropic discontinuities in the upper crust. Evaluating the macroscopic and microscopic structure of fault zones is important for a variety of applications, including earthquake nucleation and propagation, rheological and mechanical behavior of faults, hydrogeology, hydrocarbon migration, and toxic and non-toxic waste isolation. Field and laboratory investigations of uplifted and eroded fault zones provide an insight to fault behavior at depth. An analogy can be established to constrain behavior of recent faults at the similar depths. The complex appearance of the San Gabriel fault at my study area has evolved through time resulting in multiple fault zones. At least four different generations of faults can be documented, and each fault generation has its own distinctive structure and a well developed cataclastic zone. This is different from how the San Gabriel fault appears at Pacoima Canyon. At Pacoima canyon the fault zone is narrow, and one main strand of faulting is documented. This difference in fault structure and appearance between the Pacoima canyon exposure and my study area must be due a fundamental differences in how the fault initiated, evolved over time, and reached its maturity. My study suggests that in order to improve our understanding of the San Gabriel fault dynamics, modelers should incorporate the heterogeneous nature of the brittle San Gabriel fault into their models. Hence, sampling should be sensitive to different fault zones and the hierarchical arrangement of faults. A true and correct model must be established on firm awareness of the geology and petrology of the study area. Hence, this study provides a detailed geological and petrologic context map, which is necessary for future development of a successful chemical and physical model.