New Mexico Geological Society Annual Spring Meeting — Abstracts

The primary rift structure in the western Salton trough is the low angle West Salton Detachment Fault (WSDF). The WSDF is a low-angle normal fault that bounds the western Salton Trough (upper plate) from the Peninsular Ranges (footwall) in Southern California. The detachment was active from ~5-8 to ~1 Ma. Slip along the detachment ended when the dextral San Jacinto, San Felipe, and Elsinor systems cut across it ~1.1-1.2 Ma and dominated local deformation. (U-Th)/He dating of apatite and zircon of the hanging wall and footwall of the WSDF indicate at least 2.3-4 to 8 km of exhumation and >8-10 km of eastward horizontal extension starting 5 Ma.

Footwall and hanging wall fault rocks have intermediate plutonic protoliths. Footwall fault rocks were formed mainly in the upper seismogenic zone and were minimally reworked while transiting to the aseismic zone. Hanging wall fault rocks formed at <2-3 km paleodepth, lack a well-developed ultracataclasite layer, and show clay alteration.

Along most of the WSDF, the top of the footwall displays a 2-part fault core. Immediately adjacent to the principal fault plane is a thin 10-40 cm layer of black-brown ultacataclasite. The ultracatacaslite is above 1-3 meters of cataclasite. Both of these layers have several random microscopic fabrics. Pseudotachylyte veins injected into both the hanging wall and footwall are observed in multiple locations throughout the center of WSDF. Significant hydrothermal alteration is seen in the southern section of the WSDF, where the ultracataclasite is thinner, the cataclasite is macroscopically foliated, and there is no pseudotacylyte.

Two study sites were selected to observe any contrast in fault microstructures: Agua Caliente and Powder Dump. Powder Dump displays the typical two-part fault core seen through most of the WSDF and has psedutachylyte injection veins. This indicates that Powder Dump slipped seismically at some times. In contrast, Agua Caliente displays abundant hotspring activity from the detachment. The fault core rocks at Agua Caliente are different: pseudotachylyte is absent, ultracataclasite is thinner (a few cm), and cataclasites are macroscopically foliated and lineated with normal-sense S-C fabrics. The observed foliation in otherwise brittle, low-temperature Agua Caliente fault rock suggests that significant slip accumulated by creep.

Our study aims to compare the fault-rock textures (grain size distribution, grain shapes, micro - and macroscopic fabrics) and mineralogy from Agua Caliente (paleocreeping) and Powder Dump (paleoseismic). Some lab work (Keulen, et al. 2007) suggests that grain shapes are more convex when formed at high slip rates, but this has not been shown in natural examples (to our knowledge). We hypothesize that we should be able to replicate the observation of the correlation from samples in the field between the increasing rate of slip and the increasing convexity of fault grains.

References:

1. Keulen, N., Heilbronner, R., Stünitz, H., Boullier, A.-M., & Ito, H. (2007). Grain size distributions of fault rocks: A comparison between experimentally and naturally deformed granitoids. Journal of Structural Geology, 29(8), 1282–1300. https://doi.org/10.1016/j.jsg.2007.04.003