Pacific Section AAPG, SPE and SEPM Joint Technical Conference

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Tectonic Evolution Of Borderland Transpressional Systems, Southern California

Abstract

Strike-slip basins created along the southern California continental margin record the complex Pacific-North America transform plate boundary evolution. A model based on changing relative plate motions and boundary geometry describes the transpression and restraining bend development along the margin. Geological structure inherited from Neogene oblique rifting of the Inner Borderland was reactivated by post-Miocene oblique convergence when the locus of plate boundary deformation jumped eastward to the southern San Andreas and Gulf of California transform fault systems. Crustal thinning during oblique rifting was followed by basin inversion and uplift of lower crustal structure. Three major transpressional fault systems are investigated to map the complex faulting and related deformation: 1) San Clemente fault system (Santa Cruz-Catalina Ridge); 2) Palos Verdes fault system (PV, San Pedro Bay); 3) Newport-Inglewood-Rose Canyon fault system (NIRC, South Coast). Basin overlap onto rifted and rotated crustal blocks that flanked the breakaway provide important tectono-stratigraphic sequences that define the early rift configuration. Uplifted basin sediments record the evolution and character of the deformation. Widespread volcanism provided intrusions, flows, and volcaniclastics for age control of basin sequences. In Phase 1, right-stepping en echelon transform faults form to link extensional ‘pull-apart’ basins during oblique-rifting. In Phase 2, northward rotation of the relative plate motion vector and the eastward jump in the plate boundary create transpression on NW-trending transform faults and inversion of Miocene basins. Parts of the former low-angle normal fault system active during initial rifting and dismembered by transform faulting may be reactivated as blind thrusts during subsequent transpression. The THUMS-Huntington Beach fault may represent a final stage where a former vertical transform fault was abandoned and pushed over as new faults by-pass the restraining bend and convergence increases. The PV and NIRC fault zones provide straighter paths for right-slip, but still sustain local basin inversion during Quaternary transpression. The same process occurred at the Santa Catalina restraining bend and is active now at the San Bernardino Mountains segment of the San Andreas fault. Repeated structural style reveals the active process of transpressional fault system evolution within the PAC-NAM plate boundary in southern California.