PEDOGENIC CONSTRAINTS ON SLIP ALONG THE SAN ANDREAS FAULT, SOUTHERN CALIFORNIA

The Mission Creek drainage at the southern end of the San Gorgonio Pass in southern California provides a unique opportunity to examine the history of late Quaternary slip on the San Andreas Fault (SAF). There, a nested sequence of alluvial-fan fills derived from catchment areas north of the Mission Creek and Mill Creek strands of the SAF lies south of both strands, and records a history of alluviation and erosion that we are investigating using detailed geologic mapping, morphometric and stratigraphic analysis, geochronology of fills (IRSL) and surfaces (cosmogenic-nuclides), and pedogenic analysis. Surfaces are correlated by morphometric analysis, including longitudinal profiles, surface texture and drainage density. The degree of soil development, compared to independently-dated surfaces regionally, provides temporal boundaries for the tectonic evolution. Pedogenic properties utilized in age determination include tiron-extractable pedogenic silica, soil-development indices and iron oxide composition and content. IRSL dates of 106 and 95 Ka from (respectively) 5 and 4 m beneath the youngest pedon are consistent with age estimates based on soil-profile development. We use the data to reconstruct a preliminary movement history for the SAF that includes the following: (1) the fill associated with the youngest pedon is positioned directly downstream from the canyon mouth of Mission Creek, suggesting that the Mill Creek and Mission Creek strands of SAF along this reach have not generated significant slip in approximately the last 100 Ka; (2) The deposits associated with the two older pedons have been displaced by the SAF no farther northwest than about 8-10 km. This displacement is similar to slip estimates for the Mill Creek strand of the SAF north of San Gorgonio Pass (Matti and Morton, 1993); (3) All alluvial fills in the Mission Creek area are within 10 km of the source drainage, suggesting that the Mission Creek strand of the SAF has not generated significant dextral slip in at least the last 400 Ka years, and probably longer.