Fault-Core Microtextures And Slip Rate Of The West Salton Detachment Fault, Southern California

Katrina L. Soundy

In Preparation December 4, 2018, SCEC Contribution #8964

The West Salton Detachment Fault (WSDF) is a low-angle normal fault that bounds the western Salton Trough and "desert ranges" (upper plate) from the Peninsular Ranges (footwall) in Southern California. Footwall and hanging wall fault rocks have quartzo-feldspathic plutonic protoliths in our study sites. We focus on footwall fault rocks that formed mainly in the upper seismogenic zone and were exhumed mainly by WSDF slip. Footwall fault rocks show little overprint due to transiting the shallower aseismic zone: hanging wall fault rocks formed at <2-3 km paleodepth, lack a well-developed ultracataclasite layer, and show clay, zeolite and/or potassic alteration, which are minimal, post tectonic and/or absent in the footwall. (U-Th)/He dating of apatite and zircon from the hanging wall and footwall of the WSDF record 2.3-4 to 8 km of footwall exhumation and >8-10 km of top-east slip accumulated on the WSDF mainly after ~12 Ma. Syntectonic upper-plate sediments suggest that most WSDF slip accumulated after ~5 Ma, possibly beginning ~8 Ma. WSDF slip ended at ~1.1 Ma when the detachment was cross-cut and deactivated by dextral strike-slip faults of the southern San Andreas fault system.

We focus on footwall fault rocks that were unroofed from the upper seismogenic zone by WSDF slip. Two study sites along the WSDF were selected to test for contrast in fault-rock microstructure due to different slip rates, creep versus seismic: Agua Caliente (AC) and Powder Dump (PD). The PD footwall displays a two-part fault core (ultracatclasite above cataclasite), as seen along most of the WSDF, and has pseudotachylite fault and injection veins. This and other observations nearby indicate that the WSDF at the PD site experienced frequent seismic slip events. In contrast, at AC the footwall fault-core rocks lack pseudotachylite, ultracataclasite is thin (a few cm), and subjacent cataclasites are macroscopically foliated and lineated with normal-sense S-C-C' fabrics. Active hotsprings issue from the WSDF and/or the nearby Elsinore fault at AC and strong upper-plate alteration at AC suggests this has been the case over significant time. The observed foliation in otherwise brittle, low-temperature Agua Caliente footwall fault rock suggests that significant slip accumulated by creep, perhaps assisted by fluid-related processes.

We conclude that size-dependent grain-shape variations (elongation, concavity, circularity, and roundness) differ between the two sites. AC has more convex, more round, and more equant grains within 1 meter of the detachment, while PD has more concave, less round, and more elongate grains within 1 meter of the detachment. There is a quantifiable difference in size-dependent elongation measurements between AC and PD. AC has relatively similar elongation averages for grains above 25 microns, but below 25 microns the elongation average increases. At PD, elongation decreases with grain size for plagioclase, while quartz does not seem to have a clear relationship between grain size and elongation. There are also differences in concavity between mineral phases between the two sites. At AC, feldspar is generally more concave than quartz, while the opposite is true at PD. We infer that all of these observations are due to different fracturing processes at AC versus PD: AC grains fragmented by chipping and rolling, while PD grains fragmented by tensile cracking. These different fracturing processes affect the grain shapes of mineral phases differently depending on their fracture properties.

Grain size distribution at both WSDF slip sites had a D-value of 2.8-3.0, which shows the grains fragmented by constrained comminution and were overprinted by subsequent shear. Additionally, the grains at AC are later rounded and chipped, so the GSD at AC was mostly preserved from initial tensile cracking and then additionally overprinted by grain shapes that occur from unconstrained comminution.

Finally, pulverized rock was identified only at PD, within the fractured damage zone, where paleo-seismic slip events occurred. This supports previous arguments for pulverized rock being an indicator of seismic slip.

Key Words
West Salton Detachment Fault, low-angle normal fault, microstructures, fault slip rate, fault mechanics, grain shape analysis, grain size distribution, particle size distribution, S-C foliation, cataclastic foliation, pulverization texture, pseudotachylite

Soundy, K. L. (2018). Fault-Core Microtextures And Slip Rate Of The West Salton Detachment Fault, Southern California (New Mexico Institute of Mining and Technology, Socorro).