Analysis of Quaternary faults and associated deformation of sedimentary basin fill: Inner continental borderland of southern California

The San Andreas fault system is distributed across strike hundreds of kilometers in southern California. This transform system includes offshore faults along the shelf, slope and adjacent basin, comprising part of the Inner California Continental Borderland. Previously, offshore faults have been interpreted as being discontinuous, or certain faults have been interpreted as thrusts between Long Beach and San Diego. Our work, based on ∼3000 kilometers of deep-penetration industry multi-channel seismic reflection data (MCS) as well as high resolution U.S. Geological Survey reflection profiles indicate that many of the offshore faults are more geometrically continuous than previously reported including Newport-Inglewood(NI)-San Mateo-Carlsbad(SMC), and Coronado Bank Detachment(CBD)-Descanso faults. We interpret a ∼18 km wide right step over from the NI-SMC positive flower structure in the north to the CBD-Descanso negative flower structure in the south adjacent to San Diego. These digital fault and stratigraphic interpretations were gridded and depth converted for modeling displacement and its direction on the San Mateo-Carlsbad fault.;Stratigraphic interpretations of reflection profiles included the ca. 1.8 Ma top Lower Pico (TLP), which was correlated from wells located offshore Long Beach. Four younger Quaternary unconformities (Q1,Q2,Q3,Q4) are interpreted through much of the study area. We correlate the Q horizons to corehole data in Los Angeles harbor and constrain their ages: Q1 is 160-300; ka Q2 is 300 ka; Q3 300-450 ka; and Q4 ∼600 ka. These ages are several times older than the stratigraphic age model published by Covault and Romans (2009) and an order of magnitude older for the top Lower Pico horizon. Fault slip rates modeled using our new ages would be correspondingly slower than would be the case using the previous ages.;We estimate an average right-lateral slip rate of ∼0.44mm/yr since 1.8 Ma on the San Mateo-Carlsbad, which had been published as a thrust. Our modeling also indicates that the SMC fault is kinematically continuous for at least 60km through a major bend that is part of the right step over. This change in SMC strike marks a boundary between transpression in the north and transtension in the south and is regionally significant to understanding earthquake hazard and globally significant to understanding distributed fault systems that reactivate preexisting structure.