NEW STRAND OF THE SAN JACINTO FAULT ZONE SW OF THE SALTON SEA AND A POSSIBLE CONTRACTIONAL STEP-OVER IN THE SAN FELIPE HILLS: A MODEL TO BE TESTED

Slip budgets across strands of the San Jacinto fault in the Salton Trough are poorly known. For example, the Clark strand separates a Cretaceous mylonite zone by ~15 km yet dies out into highly folded Plio-Quaternary deposits nearby. This slip discrepancy can be reconciled if the Clark fault is a major structure that transfers its slip across a left step to a blind NW-trending strike-slip fault ~5 km SW of the Salton Sea. Part of this poorly characterized fault was named the Palm Wash fault (PWF) by Petersen et al (1991), projects SE to the Imperial fault, and may reactivate a fracture zone produced by Plio-Quaternary spreading centers. Existing data sets and our preliminary analysis of folds in the western Salton Trough suggest that a 15-20 km wide left step in the Clark fault may transfer slip to the PWF. The steeply NE-dipping PWF was imaged on an seismic reflection profile (Severson, 1987), coincides with a major bedrock step, separates continental crust to the SW from mafic crust to the NE (Fuis et al, 1982), was the locus of right-lateral microseismicity (Petersen et al., 1991), coincides with a linear part of the shoreline of Lake Cahuilla, separates the emergent SW Salton Trough from the subsiding NE Trough, and bounds the NE edge of a >300 km2 area of distributed microseismicity and folding. Strongly folded Plio-Quaternary sediments and microseismicity are confined within a parallelogram between the PWF and San Felipe Hills fault. Within the parallelogram folds mostly trend E-W, yet near the blind PWF fault, they trend N-S (Dibblee, 1984 and several theses). Prior work interpreted the folding in the San Felipe Hills as deformation above a blind continuation of the Clark fault (Wells and Feragan, 1986), or as step-overs between more local strike-slip faults (Heitman, 2002) but the distribution and geometry of folding, and magnitude of shortening may conform better with a step-over model. Structural analysis of folds is underway to test our working hypothesis. We will also search for growth relationships to assess when E-W folding first deformed the basin fill of an older supradetachment basin. Analysis of aerial photographs shows no through-going angular unconformities, which might record the onset of folding in the contractional step-over. If confirmed, this would show very recent (<<1 Ma, post Brawley Formation) initiation of this part of the fault zone.