Description
The kinematic behavior displayed in the California Continental Borderland appears to be associated with a penetratively developed tectonic regime that affected the crust prior to the onset of major wrench-fault deformation. Several authors of the Borderland literature have insightfully described this subsea province as having a "basin-and-range topography." Extensional disruption of the Mesoroic accretionary wedge-subduction complex has fundamentally altered the original rock distribution and set the stage for later strike-slip faulting. The extensional deformation may be far more important than previously recognized, giving strike-slip deformation a less pervasive role in the kinematic history of southern California. A complex interplay of extension and transform faulting seems to characterize the mid-Tertiary strain path within different segments of the borderland. Mid-Tertiary extensional deformation, which is well documented in the adjoining Colorado River, Mojave, and Salton Trough regions, is now considered to be an intricate component in the structural evolution of the Borderland. The genetic tie between these geographic regions is inferred to be the interaction of the lithosphere and asthenosphere during the mid-Tertiary slab disintegration and formation of the slab gap as described by Severinghaus and Atwater. Santa Cruz Island is a manifestation of this mid-Tertiary extension in the Borderland region. The southern portion of the island is a tilted crustal block, which has exposed the SCI Schist and overlying crystalline and Tertiary units. In the extensional model proposed for the island, the tilt block was originally bounded by normal faults. One of these formed an ancestral SCI fault, along which the SCI Volcanics and underlying San Onofre Breccia on the north side of the island were down-dropped from the footwall (southern portion of the island). Another, larger normal fault is inferred to have been active just south of the island, where it has displaced Santa Cruz Island from a footwall of Catalina Schist, now represented by the submerged Kelez Ridge. The Kelez Ridge is considered to be the source for the synorogenic San Onofre Breccia which occurs along the southern margin of Santa Cruz Island. Tilting and anticlinal folding within the southern portion of the island appear to be directly related to the geometry of the detachment system. Mid-Tertiary disruption of the Mesozoic accretionary wedge-subduction complex has exhumed and juxtaposed multiple structural levels, much as seen in the highly extended corridor of the Colorado River region. The juxtaposition of terranes within the Borderlands may be largely a result of crustal disruption along a linked system of normal faults. The contribution of lateral transport along strike-slip faults may still be important but not nearly as dramatic as many have suggested in the past.
