Structural and stratigraphic evolution of the central and southern Outer California Continental Borderland

Newly-released multichannel seismic reflection data are used to map stratigraphic horizons in the Outer California Continental Borderland. This mapping reveals aspects of the geologic history of the region as it transitioned from a subduction margin to a transform margin. A regional unconformity, representing a paleo-sea-level reference datum, shows evidence for large vertical motions associated with the development of the transform plate boundary, including considerable post-lower-Miocene net subsidence. A down-cutting angular unconformity on the eastern side of the Outer Borderland suggests uplift of that area and subsequent erosion likely due to the oblique rifting of the Outer Borderland from the Peninsular Ranges of Baja California. Pinchout of pre-Miocene forearc basin rocks by this angular unconformity and an inferred change in lower crustal material data are found to define the Outer-Inner Borderland boundary. Data show that most of the current high bathymetric relief in the region is the result of post-lower-Pliocene transpression.