--> Abstract: New Constraints on Late Pleistocene Slip Across the San Andreas Fault Zone Near Indio, California, by N. Guzman and D. Yule; #90088 (2009)

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New Constraints on Late Pleistocene Slip Across the San Andreas Fault Zone Near Indio, California

N. Guzman and D. Yule
California State University, Northridge, CA, [email protected], [email protected]

A late Pleistocene alluvial fan surface near Indio, CA has been the focus of several slip-rate studies along the southern San Andreas fault system. Here, slip on the fault has cut the fan into three pieces (NE, middle, and SW) and the apparent right-lateral displacement of their NW margins has been used to constrain the displacement since the fan surface was abandoned at ~50 ka. However, some have interpreted that thrust faulting and/or landsliding has modified the NW edge of the SW fan, questioning its use as a valid piercing line.

For this study several trenches excavated across the NW edge of the SW fan expose a 1-m-thick, sub-horizontal, sheared boundary in support of the fault/landslide model. The shear zone may be an active structure as it emplaces Plio-Pleistocene Palm Spring Formation over the ~50 ka alluvial fan surface and colluvium/alluvium that is estimated to be late Holocene in age (based on its strong similarities to the modern deposits nearby). NE-SW folds in the hanging wall, and SE-trending slickenlines as well as SW-trending boudinage within the shear zone support SE-directed motion above a thrust fault. These features contradict the SW-directed sense of motion (from higher toward lower elevation) that one would expect if this feature were an active landslide.

The hanging wall of this apparent thrust includes alluvial terraces cut into the Palm Spring Formation, ~30-50 m above the fan surface in the footwall. The age of the hanging wall terraces is unknown, yet they strongly resemble the fan surface in the footwall. This tentative correlation yields an uplift rate of 0.6-1.0 mm/yr since fan abandonment. Previous slip estimates made from the offset NW fan margin therefore must be considered minimum values because they do not consider modification of the fan margin by displacement across the thrust.

AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009