Vertical deformation along the Indio Hills, San Andreas Fault, California

Abstract:

Halfway between the Salton Sea and San Gorgonio Pass, the southernmost San Andreas Fault (SAF) bifurcates into the Mission Creek and Banning strands. These strands bound the Indio Hills (IH), and mark the first of a series of left-stepping branches that define the transpressional, southern Big Bend of the SAF. Between the fault strands, the Quaternary Ocotillo Formation is deformed with fold axis orientations consistent with dextral shear; structurally the IH are synclinal in the east, transitioning to a complex antiform with increased uplift suggested by exhumation of Tertiary units in the west. We report new long- and short-term erosion rates across the IH and uplift rates on the Banning strand, and we evaluate these measurements in terms of slip rates across the fault system and structural deformation within the IH. Two methods of catchment-averaged erosion rates provide minimum rates yield similar results, (0.08 to 0.34 mm/yr) across 6 catchments. The long-term rates are calculated from eroded volumes estimated from a 10-m DEM surface enveloping the Indio Hills and assume that all folding and uplift initiated ca. 500ka (the 750 ka Bishop ash is uplifted and warped within the IH). The short-term rates, determined from 10Be dating of alluvial sediments, increase gradually to the northwest. Similarity of the rates suggests steady state uplift over the history of the fold; ongoing structural analysis and dating needed to constrain the maximum rates will test this possibility. The new uplift rate for the Banning strand at the east end of the IH is determined from a 60 pts/m^2 DEM produced by structure from motion photogrammetry and U-series ages and cosmogenic dates that provide an age range of 20-76ka for a fan vertically offset by ~2.5 m. The resulting uplift rate on the fault (0.03-0.125 mm/yr) overlaps with the short-term catchment-averaged erosion rate for this location (0.08 mm/yr). Consequently, we interpret that vertical strain is partitioned onto both the Banning fault and in uplift and folding of the IH. The uplift rates increase westward along the IH, possibly indicating increased activity on the Banning strand to the west. This pattern will be considered in the context of paleoseismic and horizontal slip rate studies in the region, and implications for rupture directivity on this hazardous fault system.