Abstract

Field mapping of a 7-mi-long part of the San Gabriel fault zone [in California] between Castaic and Gorman, 50 mi NW, of Los Angeles, provided information of possible displacement and directions of movement along the northeastern part of the fault zone, Within the area, the San Gabriel fault zone consists of 2 major NE.-dipping NW.-trending faults. The southwestern fault separates basement terrane from sedimentary rocks of the Ridge basin. A 5-mi-long fault sliver is outlined by the 2 faults, and contains sediments of middle and late Pliocene age - these have been tentatively correlated with the Ridge basin section NE. of the fault zone by matching stratigraphy, fossil horizons, and changes in clast composition in the Violin Breccia. The Violin Breccia is the most distinctive unit, and is a thin band of conglomerate and breccia representing talus from the fault zone. It ranges in age from middle to late Pliocene, and contains a distinctive diorite-anorthosite mass which probably represents a middle Pliocene landslide. The finer grained facies of the breccia consists of about 10,000 ft of alluvial and lacustrine sediments. The northeastern fault of the zone may have had a reverse dip-slip component of displacement of about 3500 ft as well as 3 1/2 mi of right lateral separation since early Pleistocene time. This produced a net slip of 4 1/2 mi and a net slip direction which plunges about 10 degrees to the NW.

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