ABSTRACT: Late Cenozoic Blind Thrusting and Transpressional Kinematics, Potrero Hills Region, Sacramento-San Joaquin Delta, California
The Potrero Hills is an east-west trending, asymmetric anticline along the northern margin of the Sacramento-San Joaquin delta region. Structural analysis using correlated electric log and dipmeter data indicates that the Potrero Hills is a fault-bend or fault-propagation fold underlain by a system of blind north-vergent thrusts. The thrusts ramp up to the north from an inferred detachment in Upper Cretaceous and/or Paleogene strata. Younger thrust imbricates have formed by ramping through the hanging walls of older imbricates. Most of the shortening has been accommodated by the Paleocene Martinez shale. Minimum structural relief is approximately 1030 m. Pliocene strata are folded on both limbs, indicating that at least some of the shortening has occurred during the late Cenozoic. Eas -west clusters of microseismicity in the Potrero Hills region suggest that deformation is actively occurring there.
Northwest-trending Quaternary folds and active seismicity indicate that the Western Great Valley is underlain by a system of northeast-vergent blind thrusts. The east-west strike of the Potrero Hills and northward vergence of thrusting are clearly anomalous compared to regional trends. Speculatively, the Potrero Hills may represent shortening at the northward termination of a system of north-trending strike-slip faults. The Potrero Hills may represent a restraining left step in the transfer of right-slip from the north-trending faults southeast of the delta to active strike-slip faults northwest of the delta.