--> Miocene Basin Configuration and Tectonic Evolution of the Inner Borderland Rift

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Miocene Basin Configuration and Tectonic Evolution of the Inner Borderland Rift


The California Continental Borderland underwent a transformation from subduction to rifting and crustal block rotation during the organization of the Pacific-North America transform plate boundary. In this paper we will attempt to unravel the rifting / rotation phase of the tectonic evolution of the borderland. Rifting was initiated in early Miocene time as oblique transtension between triple junctions migrating north and south along the continental margin. Miocene basins are preserved along the flanks of the Inner Borderland Rift offshore Southern California from the Northern Channel Islands along the transpressional Santa Cruz-Catalina Ridge and the southern California coast and southward into the San Clemente Basin and coastal basins of northern Baja California. High-resolution bathymetry, seismic and other geophysical data are used to define basin margins and identify important segmentation of the rift. Major rift segments are defined by trapezoidal basins floored by Catalina Schist with northwest-trending dextral oblique(?) transform fault boundaries. Mapping of the basin geometry for middle and late Miocene basin sedimentary sequences provides evidence regarding the initial evolution of the Inner Borderland Rift. Correlation of distinct basins and fault boundaries on conjugate margins of the rift enable reconstruction of the Miocene plate boundary configuration and tectonic evolution. Abundant wells near the coast, and sparse, but widespread outcrop samples across the Borderland provide stratigraphic data to constrain timing of major tectonic events. Our working model for oblique rifting in the Borderland resembles the Gulf of California, where right-stepping echelon transform faults link left-stepping extensional basins. Neogene dextral shear and block rotations that disrupted the initial rift geometry may be extracted from the basin evolution.: We recognize a change from ductile extension while the rift formed above attenuated crust, heated from mantle upwelling near the former spreading center, to brittle deformation including block faulting and tilting as the crust cooled and thickened. The complex spatial and temporal transgressive deformation style of the Pacific-North America continental margin transform plate boundary evolution may be determined with increased resolution.