Fault Core Analysis of the Upper Seismic-Aseismic Transition Zone in the Western San Gabriel Fault, California

Geotechnical core acquired across the western San Gabriel Fault (wSGF), California provides insight into the composition, properties, and structure of the upper seismic-aseismic transition region of the fault zone. The wSGF accommodated >40 km of right-lateral slip between 10 and 5 million years ago and has since been exhumed ~2 km in the San Gabriel Mountains. The borehole Alt-B2 plunges ~65° south, forms a 30° angle with the 85° north dipping SGF, and provides a nearly continuous sampling of 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 contains two zones of well-developed shearing and alteration. On the northeast side of the fault, the damage zone consists of chloritically altered, fractured, and faulted Mendenhall gneiss; to the southwest the rocks consist of anorthosite and diorite. The interpreted fault core is at 251 m depth and consists of ~ 40 cm of foliated cataclasite. Fault slip surfaces several mm wide cut the foliated cataclasite in the damage zone. Fault-related rocks in the damage zone also consist of indurated breccias and cm-thick healed cataclasite zones. X-ray diffraction studies show the sheared fault-related rocks contain calcite, illite, analcime, clinochlore, chamosite, and laumontite. Calcite veins appear in the lower damage zone at ~370 m depth. These hydrous minerals 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 concomitant with fluids active within the fault zone.