Evidence for paleoseismic slip on a continental low-angle normal fault: Tectonic pseudotachylyte from the West Salton detachment fault, CA, USA

Mitchell R. Prante, James P. Evans, & Susanne U. Janecke

Published February 1, 2014, SCEC Contribution #1829

The potential of continental low-angle normal faults (LANF) to nucleate large (Mw > 6.0) earthquakes at low angles remains unclear despite much focused research. We document evidence for ancient seismicity along a continental LANF (detachment fault) that formed and slipped at a low angle and produced tectonic pseudotachylyte. Thick and laterally persistent pseudotachylyte accumulations along the West Salton detachment fault (WSDF), Salton Trough, USA, preserve convincing evidence for a frictional melt origin including: spherulitic microlites, ductile-flow structures, preservation of high temperature phases as clasts, and injection veins. Pseudotachylyte thickness along the fault ranges from 0.1 to 1.5 m, and pseudotachylyte-cataclasite in the fault core and damage zone are exposed along ~2.6 km length of the fault. Reworked fragments of pseudotachylyte in cataclasites, and multiple generations of cataclasites provide evidence for the preservation of multiple earthquake cycles. The limited exposure (< 3% of the total exposed length), and unusually large volumes of pseudotachylyte along this section of the WSDF suggest special conditions for generation of frictional melt. Prior work, documenting the low dip of the WSDF throughout its history and abundant evidence for ancient seismicity presented here, shows that research must focus on discovering a mechanical explanation for seismic slip on LANFs. A new synthesis of pseudotachylyte along detachment faults from diverse tectonic settings provides convincing evidence that repeated ancient seismicity is common along detachment faults. These data constrain models for low-angle normal fault formation and strength

Citation
Prante, M. R., Evans, J. P., & Janecke, S. U. (2014). Evidence for paleoseismic slip on a continental low-angle normal fault: Tectonic pseudotachylyte from the West Salton detachment fault, CA, USA. Earth and Planetary Science Letters, 387(1), 170-183. doi: 10.1016/j.epsl.2013.10.048.