FAULT CORE ANALYSIS OF THE SEISMIC-ASEISMIC TRANSITION ZONE IN THE SAN GABRIEL FAULT, CA

Geotechnical core acquired across the San Gabriel Fault (SGF), California provides insight into the composition, properties, and structure of the upper seismic-aseismic transition region of the fault zone. The SGF accommodated ~20 km of right-lateral slip between 13 and 5 million years ago and has since been uplifted ~2 km in the San Gabriel Mountains. The borehole B2 plunges ~65° south, forms a 30° angle with the 85° north dipping SGF, and samples almost 500 m of protolith, fault damage zone, and fault core with little interference from near-surface alteration. We examine rock core acquired to a maximum vertical depth of 472 m, across the ~170 m wide fault zone. The fault damage zone consists of chloritically altered, fractured, and faulted gneiss. The fault core samples consist of highly sheared fault-related rocks including zones of foliated cataclasite and clay-rich gouge developed within granodiorite and granitoid gneiss. Preliminary X-ray diffraction studies show the sheared fault-related rocks contain calcite, illite, analcime, clinochlore, chamosite, and laumontite. These hydrous mineral phases may be products of syntectonic fluid-rock interactions, likely at elevated temperatures associated with the fault. The presence of these minerals within the foliated cataclasites suggest that the upper seismic-aseismic transition in some faults may be the result of the formation of frictionally weak minerals within the fault core. Quantifying the values of fault-rock properties is needed to provide fundamental constraints on the dimensions and elastic properties of faults, which are used to model seismic energy dissipation, and are essential in earthquake hazards planning, particularly along the active San Andreas Fault.