Abstract

Geomorphic investigation of the San Andreas fault zone in the Indio Hills indicates many tectonically produced landforms, including beheaded streams, right-lateral deflected and offset streams, sags, shutter ridges, pressure ridges, and fault scarps. Near Biskra Palms, an alluvial fan-pediment complex has an apparent cumulative offset of about 0.7 km along the Mission Creek fault zone (north branch, San Andreas fault). Many of the tectonic landforms, as well as the fracture pattern that has developed during the Pleistocene, are explainable by simple shear or uplift associated with a small left bend in the main trace of the Mission Creek fault. The ratio of vertical to horizontal displacement in the vicinity of the bend is about 0.04.

Exposed in Pushawalla Canyon, 5 km northwest of the alluvial fan-pediment complex, are: (1) a sequence of stream terraces, (2) folded Plio-Pleistocene fanglomerates, and (3) an example of stream capture following a right lateral deflection or offset of several hundred metres. A left step of the Mission Creek fault in Pushawalla Canyon is a probably cause of folds and sporadic uplift that produced the stream terraces. Possible cause of recent stream capture are: (1) juxtaposition of Pushawalla Canyon with a relict canyon moving northwestward along the Mission Creek fault, or (2) right-lateral deflection or offset of Pushawalla Canyon along the fault, with simultaneous headward erosion of a shorter, steep stream flowing toward the Coachella valley.

Estimation of a slip rate and identification of paleoseismicity for the San Andreas fault in the Indio Hills is difficult. However, degree of topographic dissection, formation, and preservation of desert pavement, and relative soil profile development suggest that the age of the offset fan may be as old as 70,000 yr but that most likely it is on the order of 20,000 to 30,000 yr. These age estimates for the offset fan indicate a minimum slip rate for the San Andreas fault of 10 to 35 mm/yr, with about 23 to 35 mm/yr the most likely. The pattern of observed offset drainages is complex, but suggests that during the past few thousand years creep events or moderately large earthquakes have periodically produced several metres of right-lateral displacement.

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