Comparison of fault rocks formed paleoseismically and by paleocreep(?): Initial results from the West Salton detachment fault, southern California

Gary Axen, Katrina Soundy, & Virgil Leuth

Published August 15, 2017, SCEC Contribution #7724, 2017 SCEC Annual Meeting Poster #116

The West Salton detachment fault bounds the western Salton Trough (upper plate) above the Peninsular Ranges footwall. 10-15 km of top-E, low-angle normal slip from ~8 Ma to ~1 Ma (during San Andreas plate boundary activity) exhumed the footwall ~5-8 km. Studied fault rocks have intermediate plutonic protoliths. Typical footwall fault rocks formed in the upper seismogenic zone and were minimally reworked in the overlying aseismic zone, whereas typical upper-plate fault rocks formed at <3 km depth, generally lack pseudotachylyte and some contain clay gouge. Active dextral faults of the Peninsular Ranges have mostly similar protoliths, so should form similar fault rocks at depth. The detachment footwall offers plentiful, accessible fault-rock analogs.

The footwall typically displays a 2-part fault core: 10-40 cm of thinly layered (cm-scale) black to brown ultracataclasite above 1-3 m of cataclasite, both with mainly random microscopic fabrics and sparse mineralogical alteration (zeolite growth, chlorite locally replacing biotite). Pseudotachylyte veins record multiple paleoseismic events. In contrast, footwall fault rocks at Agua Caliente County Park, where hot springs flow from the detachment, lack pseudotachylyte, ultracataclasite is thin (<4 cm) and light grey, cataclasites are macroscopically foliated and lineated, and zeolites are more common. Foliations in brittle, low-temperature Agua Caliente fault rocks suggest that significant (late?) slip there was by creep, possibly assisted by chemically reactive fluids and zeolite growth.

We will compare grain shapes, grain-size distribution, fault-rock textures and mineralogy from Agua Caliente (paleocreeping?) versus paleoseismic sites. Based on experimental results, we hypothesize that fault-rock grains are more convex at paleoseismic sites than at Agua Caliente. XRD of footwall samples shows sparse, mainly calcium-bearing zeolites from both paleoseismic and paleocreeping(?) sites. In contrast, zeolites are abundant in Agua Caliente upper-plate fault rocks formed at shallow depth, so they may have crystallized late and/or above the paleoseismic zone. Textural analysis will constrain zeolite growth events and temperatures of fault-rock formation.

Key Words
fault rocks, cataclasis, West Salton detachment fault,

Citation
Axen, G., Soundy, K., & Leuth, V. (2017, 08). Comparison of fault rocks formed paleoseismically and by paleocreep(?): Initial results from the West Salton detachment fault, southern California. Poster Presentation at 2017 SCEC Annual Meeting.

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Earthquake Geology