Abstract

A blind thrust zone and the overlying fold forming the Kettleman Hills South Dome were produced by late Cenozoic shortening normal to the San Andreas fault. West of Kettleman Hills, toward the San Andreas fault, the region of Pyramid Hills experienced structural growth as early as Late Cretaceous time, and growth accelerated in Pliocene to Holocene time. These structures were imaged to a depth of 7 km by reprocessing a seismic reflection record section. Restored structural cross sections indicate a three-stage growth history for Kettleman Hills South Dome: (1) late Miocene reverse faulting, (2) Pliocene fault-propagation-type folding, and (3) late Pliocene to Holocene tectonic wedge emplacement. Late Pliocene to Holocene structural growth at Pyramid Hills probably resulted from thrust emplacement of the Franciscan Complex. A forward model demonstrates that tectonic wedging could account for post-2.5 Ma deformation of Pyramid Hills and Kettleman Hills South Dome. Horizontal shortening across Pyramid Hills and Kettle-man Hills South Dome normal to the San Andreas fault since the Cretaceous period is 3.7 km. This figure does not include shortening within the basement; adding shortening of our tectonic wedge model yields 7.9 km of shortening since Cretaceous time. Post 2.5 Ma shortening determined by our tectonic wedge model is 6.3 km. The rate of post-2.5 Ma shortening (2.5 mm/yr), when added to that on the west side of the San Andreas fault, is consistent with the values derived from plate-motion models that describe the "San Andreas" discrepancy.

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