GEOLOGY AND GEOMORPHOLOGY OF THE INTERSECTING MISSION CREEK AND BANNING FAULT STRANDS, SAN ANDREAS FAULT

The Mission Creek and Banning strands of the San Andreas fault zone merge at Biskra Palms, near Indio, CA. Here faulting appears to have displaced the northern margin of an ~50 ka alluvial fan by ~600 to 700 m in a right-lateral sense. However, some geologic maps show a thrust fault that merges with the downstream edge of the fan and raises the possibility that the fan edge has been structurally modified since abandonment. We excavated a series of cuts across the fan edge to test this hypothesis. These cuts reveal features that support the thrust fault model as they show a 1-m-thick, sub-horizontal shear zone that contains Plio-Pleistocene Palm Spring Formation in its hanging wall and late Pleistocene alluvium and Holocene(?) colluvium and alluvium in its footwall. The excavations clearly show that the shear zone overrides paleo-topography of the ~50 ka fan. In places, colluvium and alluvium filled low-lying areas ahead of the hanging wall block that was subsequently overridden by motion on the thrust. Therefore, faulting post-dates the youngest footwall unit. Though no dateable material has been obtained, the unconsolidated nature and lack of soil development suggest that the colluvium and alluvium found in the footwall is quite young. Several features support a SE-vergence of the fault including map-scale E-W trending open folds in the hanging wall, and SE-trending slickenlines and SW-trending boudinage within the shear zone. In addition, the hanging wall block contains alluvial strath terraces that are cut into the Palm Spring Formation, ~30-50 m above the footwall. Though their age is unknown, the soil and desert varnish on terrace boulders resembles the ~50 ka fan surface in the footwall. This possible correlation yields an uplift rate of 0.5-1.0 mm/yr, and slip rate of 1-2 mm/yr assuming a 30-degree dipping thrust and a San Andreas-parallel slip vector. The margin of the ~50 ka fan is therefore substantially modified by thrust faulting, and using the fan edge to constrain motion would underestimate the total amount of right-lateral slip across the San Andreas fault here. Our preferred model interprets the thrust as the surface trace of the Banning strand where it overrides the Biskra Palms fan and merges with Mission Creek strand.