URANIUM-SERIES DATING OF CALCITE FILLED FRACTURES FROM ALONG THE BENEDICT CANYON FAULT, SANTA MONICA MOUNTAINS, CALIFORNIA

The Benedict Canyon Fault is located within the eastern Santa Monica Mountains of Los Angeles County. In general, this fault exists as a northeasterly trending splay off the Santa Monica-Raymond Hill Fault zone. The Benedict Canyon Fault trends from Kenter Canyon, northeastward to the Cahuenga Pass and Universal City. This fault consists of a near-vertical main trace with several sub-parallel breaks, that offset rocks of Jurassic (Santa Monica Slate) to Upper Miocene (Modelo Formation). The southern-most exposure of the fault terminates at the Pleistocene and younger terraces that flank the southern Santa Monica Mountains. The northern-most portion of the fault bifurcates and the various branches of which then terminate at the recent alluvium of the San Fernando Valley. Motion on the fault appears to be left-oblique, similar to that of the nearby Santa Monica-Raymond Hill Fault zone. A characteristic of the sedimentary rocks proximal to the Benedict Canyon Fault is a wide zone of intense fracturing and jointing, often misinterpreted as evidence of recent landsliding. In many areas, these fractures have been recemented, both completely and partially, with calcite. Previous work by the author has shown that the orientations of the facture and joint sets suggest that the mode of deformation is consistent with movement along the Benedict Canyon Fault. In an attempt to determine the last episode of movement on the fault, calcite was collected from filled rock fractures. Uranium-series disequilibrium-method dating utilizing an isochron scheme for the calcite cement has shown that the most recent period of movement, fracturing and cementation is pre-Pleistocene.