Stratigraphic Record of Basin Formation, Deformation, and Destruction in the Past 2 Ma Along the Southern San Andreas Fault, Mecca Hills, California

Sedimentary rocks exposed in the Mecca Hills NE of Coachella Valley, southern California, record vertical crustal motions along the San Andreas (SAF) and associated strike-slip faults. A complex history of subsidence, transport, deposition, and uplift can be interpreted from mapping and measuring of sedimentary rocks, lithofacies analysis, and determination of provenance. Recent field studies suggest a poly-phase history of deformation and sedimentation for the stratigraphy in the Mecca Hills that is likely controlled by evolution of fault zone complexities. The Painted Canyon Fault (PCF), a reverse-oblique strike-slip fault sub-parallel to the SAF, has played a major role in basin formation and inversion over the past ca. 2 Myr. Deformation near the PCF coincides with deviations in fault strike relative to modern plate motion, and sedimentation appears to have been largely controlled by vertical motions accommodated by slip on the PCF. Preliminary magnetostratigraphic work provides tentative depositional age brackets. We infer that the Mecca Conglomerate, a locally derived cobble-boulder conglomerate with ~SSW directed paleoflow, was deposited near the PCF scarp during an early phase of SW-side down slip from ~1.8-1.5 Ma. Next, the Lower Palm Spring Formation, a regionally uniform unit of interbedded coarse fluvial sandstone and overbank siltstone with SSE directed paleoflow, appears to have been deposited during limited vertical motion on the PCF from ~1.5-1.2 Ma. We find that the base of the Upper Palm Spring Formation (~1.2 Ma) records the inversion of the PCF to a SW-side up oblique strike-slip fault based on the following: the contact between the Lower and Upper Palm Spring Formation is an angular unconformity throughout the Mecca Hills; large (km's) and small (10's m) scale syntectonic (growth) strata within the Upper Palm Spring; clasts of Lower Palm Spring reworked into the Upper Palm Spring Fm; and pronounced contrast between facies of the Upper Palm Spring Fm on the SW and NE sides of the PCF. Upper Palm Spring Fm on the SW side of the PCF makes up a >1300 m thick fining up sequence from coarse conglomerate to fluvial sandstone, overbank siltstone, and minor amounts of lacustrine mudstone. The NE side contains an ~800 m thick sequence that records NW-migrating trangression of lacustrine deposits over alluvial fan pebbly sandstone and nearshore sandstone and siltstone. The contrast across the PCF suggests basin partitioning at the beginning of Upper Palm Spring deposition along the PCF, initiating the modern configuration of uplift along the PCF. A nearly continuous 60-km long narrow strip on the NE side of Coachella Valley is undergoing recent (post-740 ka) inversion and uplift at a scale that greatly exceeds the length of local fault-zone complexities, suggesting an additional regional control on active uplift NE of the southern SAF. Pending magnetostratigraphic data will refine depositional ages and provide age correlations across the PCF. Higher-resolution ages will permit refined interpretation of the 4D deformation and sedimentation in the Mecca Hills over the past ~2 Myr.