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Older and Deeper Seismic Deformation in the San Gabriel Crystalline Basement Complex Along the South Branch of the Punchbowl Fault

Matthew T. Dorsey, Judith S. Chester, & Frederick M. Chester

Published August 15, 2019, SCEC Contribution #9839, 2019 SCEC Annual Meeting Poster #117

The main trace of the Punchbowl fault (PF), part of an ancient, exhumed fault zone of the San Andreas system, has been investigated repeatedly to understand the structure and deformation processes of seismic faults at depth. The previous studies focus on the arkosic sandstone of the Punchbowl Formation and the ultracataclasite layer at the contact with the San Gabriel crystalline basement complex (CBC). In contrast, the South Branch of the Punchbowl fault (SBPF), as well as the distributed brittle deformation within the CBC, remains largely uninvestigated even though they likely provide a record of deformation within crystalline rock at earlier stages and greater depths of faulting relative to the PF in this system. Here, we present observations of three representative exposures of deformation in the CBC and SBPF, 1) a thick, preserved section of sheared CBC adjacent to the ultracataclasite layer of the PF, 2) the CBC in the damage zone between the PF and SBPF that displays cross-cutting relations for three phases of subsidiary faulting, and 3) a well-exposed section across the SBPF in the CBC. Outcrop grid maps, fabric element orientations, and survivor clast sizes were recorded to document fault structure at the mesoscopic scale, and to record position and context of samples collected for geochemical, petrologic and microscale structural studies. Exposure 1 is a 6 m wide zone of foliated white and green cataclastic basement rock with brown and red ultracataclasite bands that is bounded to the NE by the ultracataclasite layer of the PF. The zone is bounded to the SW by a second distinct, 0.6 m-thick ultracataclasite layer containing large, fractured, elongate clasts of aplite, and is interpreted as a remnant of faulting in the CBC prior to juxtaposition with the Punchbowl Formation. Exposure 2 displays multiple discrete faults up to ~10 cm-thick that we interpret as sites of seismic slip. The faults cut diorite and aplite, contain cataclasite, and record fluidization, cementation and alteration processes. At exposure 3, the SBPF is a zone of distributed shear, ~10 m thick, that contains multiple cm to dm thick layers of cataclasite and ultracataclasite, and that is bordered by broad zones of intense fracturing and subsidiary faulting. The cataclasite and ultracataclasite layers display fluidized and flow structures as well as surfaces of localized slip, consistent with fluid-assisted and seismic faulting.

Key Words
Punchbowl fault, structural mapping, seismic deformation, fluidization

Dorsey, M. T., Chester, J. S., & Chester, F. M. (2019, 08). Older and Deeper Seismic Deformation in the San Gabriel Crystalline Basement Complex Along the South Branch of the Punchbowl Fault. Poster Presentation at 2019 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology