Abstract: Geology of the San Rafael Mountains Melange, California

WAHL, A. D., Ventura College, Ventura, CA

The origin and evolution of Franciscan melanges are commonly associated with Farallon-North American plate interaction. Detailed mapping (1:10,000), combined with general lithologic and structural observations, reveals that the melange of this study formed during Late Cretaceous time as a contractional, vertical, brittle-ductile shear zone. This information is combined with investigation of the nearby Coast Range fault (Hidden Potrero fault), the local coherent rocks belt of the Franciscan Complex, and the Oso syncline to reveal Cenozoic deformation of this melange. The results of this study indicate that this melange was thrust beneath the Great Valley sequence during Paleocene time, and redeformed by late Eocene or early Oligocene to Quaternary tectonism that reflects transpression.

The post-Paleocene transpressional structures of this melange are related to the Littie Pine fault and to the movement of local crustal blocks. The timing and tectonic style of the Late Eocene or early Oligocene diastrophism indicate that the Little Pine Mountain block, which includes the San Rafael Mountains melange, formed a buttress against the northward translation of the San Marcos block (onshore Santa Maria province) by about 35 Ma. This diastrophism is related to the establishment of the known transform boundary that formed later, during Miocene time, and the local, oblique-convergent geometry this plate boundary must have had as the Santa Ynez Mountains block (western Transverse Ranges province) subsequently rotated around this buttress.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California