BISHOP ASH AMS FABRICS ADJACENT TO THE SOUTHERNMOST SEGMENT OF THE SAN ANDREAS FAULT, DURMID HILL, SALTON TROUGH, CALIFORNIA

Recent studies of the southeastern segment of the San Andreas Fault (SAF) reveal its complex structure and confirm its potential to nucleate major earthquakes. Durmid Hill, one suggested point of origin for such a quake, is the surface expression of a doubly-plunging anticline just east of the Salton Sea where the SAF intersects the Brawley seismic zone. Folds and faults within the anticline indicate that both dextral wrenching and convergence across the SAF contribute to local uplift, as the local trend of the SAF is oblique to the plate motion vector. The Bishop Ash, a prominent marker horizon at Durmid Hill, is tightly folded, boudinaged parallel to fold hinges, and cut by thrust and sinistral strike-slip faults. We examined the magnetic traits of the Bishop Ash in a region 1-3 km from the SAF to assess deformation at hand-sample scale.

The Ash consists mainly of rhyolitic glass shards, with rare biotite and feldspar grains. The primary magnetic mineral carrier in the ash is single-domain magnetite contained within the volcanic glass, observed through a prominent Verwey transition and higher magnetization at cooler temperatures in field cooled versus zero-field cooled experiments. Previous studies of the Ash’s anisotropy of magnetic susceptibility (AMS) close (≤1 km) to the SAF revealed a magnetic fabric strongly influenced by tectonic deformation, while the AMS fabric farther (~30 km) from the SAF is more consistent with distal ash-fall deposition. Additionally, the characteristic remanent magnetization (ChRM) of these far-field samples aligns with the orientation of the field at the time of eruption. In this study we analyzed 26 samples collected at distances 1 to 3 km from the SAF. The AMS and remanent magnetization of our samples do not agree with the results of these previous studies. Instead, our samples exhibit universally low anisotropies, with magnetic fabrics showing no consistent orientation. ChRMs cluster loosely in situ but become more scattered when corrected for bedding, indicating that the ChRM is not primary; rather, it was acquired during or after deformation. In short, the area from which our samples were collected seems to be close enough to the SAF to have disrupted the original detrital remanence and AMS fabric without generating a directionally coherent tectonic overprint.