New structures from the southern tip of the San Andreas fault zone near Durmid Hill

Susanne U. Janecke, & Daniel Markowski

In Preparation 2013, SCEC Contribution #7084

Field testing of a new kinematic model of the southern tip of the San Andreas fault (SAF) in the Durmid Hill area shows that contraction, rather than extension associated with the Extra fault zone to the southwest, dominate the area. Results from the first phase of a field study to remap structures between Bombay Beach and Salt Creek indicate a contractional stepover between the main SAF and an active but previously unmapped dextral fault zone near the northeast shore of the Salton Sea. This “Shoreline strand of the SAF system” may have a branch point with the main SAF at San Gorgonio Pass. There are many small to medium-displacement left-lateral and left-oblique faults and damage zones with east to NNW strikes between the main and Shoreline strands of the SAF. These cross faults roughly parallel fold axes and produced broad damage zones without a central slip surface in the mud-rich sediment of Durmid Hill. Damage zones contain fault blocks that resemble boudins but record brittle slip instead of ductile flow. ESE-trending folds are the other structural features between the main and Shoreline strands of the SAF. The main SAF zone is characterized by an extremely well developed damage zone that is up to 0.5 km wide and focused in mud- and gypsum-rich sediment southwest of the SAF. The intense shearing, subsidiary faulting, brecciation, and folding in the damage zone persists from north of Salt Creek to ~3 km NW of Bombay Beach. There, Pleistocene sediment cover the SAF, its entire damage zone, and both sides of the SAF. The ~0.78 Ma Bishop Tuff is near the base of the stratigraphic section that overlies the deactivated piece of the SAF. The ~1 Ma replacement of this piece of the SAF is probably another manifestation of the early Pleistocene reorganization described in Janecke et al 2010 GSASP along the San Jacinto fault zone. The “replacement piece” of the main SAF is probably located a few kilometers farther to the east, but it is so new that it is difficult to locate. The stratigraphic units in the Durmid Hill area consist of Ocotillo, Brawley, Borrego, Arroyo Diablo, and Olla formations, and possibly even Imperial Group rocks. The Extra fault zone does not appear to be an important structural element in the Durmid Hill area, except potentially in the poorly exposed area around Bombay Beach. Contractional strains across the strands of the SAF in the Durmid Hill area may inhibit the nucleation of large ruptures there.

Key Words
East Shoreline Strand San Andreas fault

Janecke, S. U., & Markowski, D. (2013). New structures from the southern tip of the San Andreas fault zone near Durmid Hill. Poster Presentation at 2013 Southern California Earthquake Center Annual Meeting.

Related Projects & Working Groups
Southern San Andreas Fault Evaluation (SoSAFE)