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Description
The Sierra Madre-Cucamonga fault zone (SMCFZ) is a 100-km long emergent thrust system responsible for uplift of the San Gabriel Mountains, which form the central Transverse Ranges of southern California. Tectonic geomorphology and pedochronology were used to quantify the late Quaternary activity along the fault zone at three localities. Study areas were selected on the basis of historical or clear late Quaternary neotectonic deformation, alluvial terrace formation, and degree (lack) of urbanization. Ages of deformed alluvial surfaces were established using a least squares regression model that compares soil indices to chronosequence soils. On the western SMCFZ, recurrent uplift of the banging-wall block during the late Pleistocene has produced well defined flights of strath terraces along Pacoirna Wash, Little Tujunga Wash and Big Tujunga Wash, in the vicinity of the 1971 (M6.7) San Fernando earthquake. Ratios of scarp height or perched terrace elevation to estimated surface age yield a vertical separation rate of 1.1 +1.4/-0.4 mm/yr for the San Fernando fault of the western SMCFZ. Based on the vertical separation of a late Pleistocene terrace in the Arroyo Seco area, the active strand of the central SMCFZ has an average vertical uplift rate of 0.5 +0.8/-0.1 mm/yr. Well-expressed scarps across late Pleistocene to Holocene alluvial surfaces in the Day Canyon area delineate multiple strands of the eastern SMCFZ. Soil development and radiocarbon analysis of an alluvial surface deformed by strand C and strand A/B suggest an average Holocene vertical separation rate of 3.2 +0.7/-0.5 mm/yr. Thus, the vertical uplift rate decreases from about 3.2 mm/yr along the Cucamonga fault to less than 1 mm/yr for the central SMCFZ in the Altadena area and greater than 1 mm/yr for the western SMCFZ along the San Fernando fault. The dramatic decrease in uplift rate and fault expression from the Cucamonga fault to the eastern Sierra Madre fault is coincident with left-lateral strike-slip faulting on the San Jose fault suggesting that left-lateral faults such as the San Jose and Raymond faults partition strain away from the Sierra Madre-Cucamonga fault zone, perhaps in part by block rotation.
