INITIATION OF THE SOUTHERN CALIFORNIA BIGHT DURING MID-MIOCENE TRANSROTATION

The Southern California Bight is the result of complex tectonic and geologic processes spanning more than 100 million years. From Cretaceous into Oligocene time, subduction of the Farallon plate beneath North America produced a trench, above which a subduction complex and a forearc basin formed. The forearc basin widened by westward outbuilding of the trench-slope break and eastward transgression as the associated magmatic arc migrated eastward. This simple pattern was disrupted when the Pacific plate began to interact with North America soon after 30 Ma. Early to Middle Miocene (~17-13 Ma) crustal extension of the Los Angeles and eastern Ventura basin areas occurred during vertical-axis rotation (transrotation) of the western Transverse Ranges above the captured Monterey microplate. Detachment faulting, high-angle faulting, upper-plate block rotation around horizontal axes and isostatic uplift of the lower plate produced accommodation space for diverse depositional systems. High-angle and low-angle normal faults, as well as through-going strike-slip and transfer zones defined sub-basin margins. Palinspastic reconstruction of Topanga Formation sub-basins and related offshore features suggest a network of coeval normal and transfer zones that define multiple deforming blocks that moved along complex arc paths during clockwise transrotation. Transrotation led to extension and basin development both along the primary breakaway zone (what is now near the southern edge of the western Transverse Ranges) and areas to the southeast toward the unrotated upper plate of the Peninsular Ranges. Basin subsidence and filling were especially rapid near the hinge point in the area that would later become the Los Angeles basin and its margins. These complex transrotational processes have been overprinted by northwest-trending dextral faults, northeast-trending sinistral faults and regional north-south shortening to form the modern Southern California Bight.