THE RELATIONSHIP BETWEEN SEDIMENTATION AND ACTIVE TECTONICS IN A 100-M-WIDE TRANSTENSIONAL STEPOVER BASIN ALONG THE SAN ANDREAS FAULT SYSTEM NEAR BANNING, CALIFORNIA

Deformation of sediment layers observed in a 100-m-long trench across the entire width of a transtensional stepover basin located along the San Bernardino strand of the San Andreas fault system at Burro Flats near Banning, California provides evidence of multiple episodes of faulting. In this type of structural setting, folding is the primary style of deformation and faulting is secondary. The area is located on the toe of an alluvial fan, where Holocene sedimentation is dominated by debris flows. A trench log was constructed by measuring detailed stratigraphic sections at 2-m intervals and correlating units between columns. Ground-rupture-related deformation formed topographic lows on the depositional surface, which could be maintained over long intervals in this semi-arid region. Nearest the fault, offset of aquifer layers caused ponding of water and the development of marshes in the lows, where thick deposits of peat formed. Dry depressions were either filled by single debris flows or sand-silt-mud packages derived from local ponding and/or stream overbanking. Importantly, the magnitude of deformation (fold amplitude) increases with depth in the sediment layers (from ~0.5 to 2 m), indicating multiple episodes of faulting. At least four earthquake events in the past 500 years are recorded by deformation of the sediment layers in the stepover basin.