DIABASE DIKE AND STOCK INTRUSIONS COUPLED WITH HYPABYSSAL MAGMATIC STOPING AND SLUMP BLOCKS WITHIN THE BONEY MOUNTAIN FAULT ZONE IN THE TOPANGA FORMATION, WESTERN SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA

An exposure of the middle Miocene Topanga Formation intruded by the Conejo Volcanics in the western Santa Monica Mountains of southern California reveals evidence for Miocene extension ranging from 20 to 5 Ma based on recent research. Early Miocene extension (17 Ma), decompression, and melting during rotation of the western Transverse Range resulted in the extrusion of the Conejo Volcanics, which were then tilted northward during Pliocene-Quaternary deformation. At the Giant Sand Dune, near the mouth of Sycamore Canyon, the Boney Mountain Fault Zone with left-reverse motion dissects large outcrop exposures. Examination of this area shows that the fault zone contains a diabase intrusion, visible in a faulted-rock outcrop that is associated with two large slumps.

The eroded outcrop in this area provides a cross-sectional exposure of an igneous diabase intrusion into the Topanga Formation strata. Exposed are a swarm of feeder dikes and the upper portion of a stock containing sedimentary xenoliths. The main orientation of these dike swarms is N87°W, 88°NE. The walls of the feeder dikes are subparallel in pattern and they offset the intruded strata due to concurrent extensional fracturing, magmatic intrusion, and displacement of the dike walls as they settled into the still molten magma. Exposures of the diabase intrusions within the irregular, circular fault pattern show Topanga Formation strata that lie outside of, and against the upper middle Miocene Conejo Volcanics. The variable dip directions of the feeder dike swarm suggest forceful injection of the magma and crystallization in place. The irregular, circular fault pattern is interpreted as the bounding walls of a diabasic stock intrusion. The largest area represents magma that crystalized and was subsequently intruded by an underlying volcanic source, thereby allowing unmelted portions of Miocene rocks to settle into the magma chamber. Within the investigation area, up to one-third of the Miocene diabase rocks have replaced the Topanga Formation strata that consist of thick-bedded sandstone interbedded with clay-shale layers. These units may have formed by offshore marine turbidity currents. The dominant features are fine to- coarse-grained, often structureless sandstone beds, with parallel laminations, graded-bedding, rip-up clasts, and tool marks.