TECTONIC INVERSION OF EXTENSIONAL STRUCTURES IN THE CALIFORNIA CONTINENTAL BORDERLAND ALONG THE SOUTH BOUNDARY OF THE WESTERN TRANSVERSE RANGES PROVINCE

Major east-west faults within the Western Transverse Ranges Province (WTRP) strike at high angles across the northwest-trending faults, basins and ridges of the California Continental Borderland. Recently released multichannel seismic-reflection (MCS) data (donated by WesternGeco, Inc.) illustrate the structural style where the East Santa Cruz Basin fault zone and the San Clemente fault zone of the Continental Borderland terminate to the northwest against the WTRP. These fault zones are interpreted to form the western limit of intense middle Miocene oblique extension that accompanied rotation of the WTRP. East of these fault zones, basement is mainly exhumed Catalina Schist, interpreted as a regional metamorphic core complex, whereas west of the fault zones, the basement is sedimentary and igneous rocks of a late Mesozoic and early Cenozoic subduction complex. We use MCS, gravity and magnetic data to investigate the tectonic interaction between the ridges and the south boundary of the WTRP.

The transition between extended and intact crust is coextensive with the northwest-trending Santa Cruz-Catalina and Santa Rosa-Cortes Ridges. Near the WRTP boundary, rocks making up the Santa Rosa-Cortez Ridge are little deformed. In contrast, northwestward along the Santa Cruz-Catalina Ridge toward the WRTP, thrust faulting is increasingly more intense. Adjacent to the boundary, the thrust-faulted rocks completely override Miocene extensional structures, and the thrust faults are currently active. The difference in deformation of the two ridges could result from a combination of: 1) an eastward crustal thinning and consequent weakening that developed during the Miocene extension; 2) a difference in horizontal strain across the right-slip San Clemente fault near its termination at the WRTP boundary; 3) strain partitioning along this boundary; and 4) a contrast in bulk rheological properties of the ridges. One explanation for the geometry of the thrust faults that override extensional structures is that the faults result from tectonic inversion of Miocene low-angle extensional faults. Comparison between seismic-reflection data and analog models of graben inversion suggests that an extensional fault with a ramp-flat geometry and downthrown on the east was compressed.