Abstract: Late Quaternary Structure and Convergence Rates in the Coastal Ventura Basin

Gary J. Huftile, Robert S. Yeats

Present-day shortening across the coastal Ventura Basin is dominated by the north-dipping Red Mountain and San Cayetano faults and the south-dipping Oak Ridge fault. The Oak Ridge fault has surface expression to the west end of Oak Ridge, west of which active displacement shifts to a decollement in the Miocene Rincon Formation and ramps upward as the Ventura Avenue-San Miguelito-Rincon north-verging fault propagation folds. The Ventura Avenue anticline accounts for 2 to 2.5 km shortening since the end of Saugus deposition, similar to post-Saugus displacement on the Oak Ridge fault from South Mountain eastward. A section along Ventura River crosses no major S-verging reverse fault, but the Red Mountain fault to the west and the San Cayetano fault to the east have late Quaternary displa ement up to several km. Shortening from Red Mountain to Upper Ojai is relatively constant because of the Sulphur Mountain homocline underlain by the Lion backthrust. We calculated convergence rates > 20 mm/yr assuming that the reverse faults flattened to a decollement at the base of brittle crust and that the top of the Saugus was 250 ka based on amino-acid racemization age estimates. However, 4.6 yr of GPS measurements show convergence of only 7 to 10 mm/yr, causing us to reevaluate our assumptions. Suppose that the top Saugus is 500 ka based on paleomagnetic data and that the reverse faults continue into lower crust as ductile shear zones instead of flattening out at the base of brittle crust. This produces corrected convergence rates of 7 ± 3 mm/yr at Red Mountain, 9 ± 0. mm/yr at Ventura River, and 8 ± 2 mm/yr at upper Ojai-Sulphur Mountain, in agreement with geodetic data.

AAPG Search and Discovery Article #90981©1994 AAPG Pacific Section Meeting, Ventura, California, April 27-29, 1994