Retrodeformable Cross
Sections and Oak Ridge Fault, Ventura
Basin, California
Robert S. Yeats, Gary F. Huftile
A retrodeformable (balanced) cross
section
is constructed such that
stratified rocks are restored to their undeformed state without loss or gain of
bed length or bed thickness. Ductile strata may be area-balanced if original
thickness is known. Near Ventura, folds in Pliocene-Pleistocene turbidites and
Miocene-early Pliocene shales (Rincon, Monterey, Sisquoc) overlie an unfolded
competent Paleogene sequence. The basal decollement of the foldbelt is in the
ductile Rincon Formation (lower Miocene). The overlying Sulphur Mountain,
Ventura Avenue, San Miguelito, and Rincon anticlines are fault-propagation folds
developing from south-dipping, largely late Quaternary frontal ramp thrusts
(Sisar-Big Canyon-Lion fault set, Barnard fault set, Padre Juan fault, and C-3
fault, respe tively) that rise from the decollement.
Cross
-
section
balancing
shows that the overlying fold-thrust belt has shortened 2.5-6 km more than
subjacent Paleogene competent strata. This excess bed length is taken up in the
Paleogene sequence on the Oak Ridge fault as a ramp from the brittle-plastic
transition zone through the upper crust. This implies that the basal decollement
is the frontal active thrust of the Oak Ridge fault. The decollement dies out
southeast of a line between Timber Canyon oil field and the west end of Oak
Ridge, possibly because of decreased ductility in the Miocene decollement
sequence due to appearance of sandstone interbeds. Farther southeast, late
Quaternary displacement concentrated on the Oak Ridge fault itself at rates
greater than 10 mm/year.
AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.