EVIDENCE FOR STRESS ROTATION NEAR THE WEST SALTON DETACHMENT FAULT (WSDF), SALTON TROUGH, CALIFORNIA

Low-angle normal faults share enigmatic mechanics with the San Andreas fault (SAF): both appear to slip at high angles (beta > 60°) to the regional maximum principal stress, S1. Hypotheses to explain this include 1) elevated pore-fluid pressure (Pf), 2) inherently weak fault-zone materials (i.e., clays), and 3) stress rotation in the fault zone (e.g., due to 1 or 2 causing weakening there). The brittle, top-E, late Miocene to middle Pleistocene WSDF forms the W margin of the Salton Trough, is folded by E-plunging folds, and was coeval with the S SAF. The latter implies regional stress rotation and/or partitioning (S1 subvertical near the WSDF but subhorizontal near the SAF).

We conclude that stress rotation also probably occurred within ~10 m of the WSDF. Faults with small separation (mostly 2-200 mm; few preserve striae) at three localities cut the upper ~10 m of the footwall chlorite breccia zone and lie below the ~3-30 cm-thick ultracataclasite layer subjacent to the WSDF. These include apparent conjugate sets, including R1 and R2 Reidel shears at two sites. Simple analysis (S1 bisecting acute conjugate angle) yields beta ranging from ~40 to ~70°, and varying over lateral distances of tens of meters and possibly over smaller vertical distances. In Nolina Canyon, steep conjugate sets yield beta of ~70°, and mutually crosscut a N-dipping, reverse/dextral set that may be related to folding of the WSDF. About 30 m away, R1 and R2 shears suggest beta of ~45°. Near Plum Canyon, Reidel shears occur in the upper 1-2 m of the footwall (beta of ~45°), yet slightly deeper (~2-10 m below the WSDF) faults have different, but consistent, orientations.

Small carbonate(?) veins (<2 mm thick) in the ultracataclasite layer at one site are subparallel to the WSDF, suggesting high Pf may have been important locally and/or intermittently. Granular flow in annular shear experiments can lead to beta of ~45°, so brecciation and cataclasis may be causative factors. The vertical fault-rock sequence suggests temporal fault-zone thinning, so vertical changes in fracture orientations may represent temporal and/or spatial changes in the stress field. Pseudotachylyte elsewhere along the WSDF implies seismogenic slip, so some stress changes may be due to temporal/spatial effects of the seismic cycle.