Abstract: Evolution of the Las Posas Anticline, Western Transverse Ranges, California

Robert A. Whitney, Eldon M. Gath

The Las Posas anticline is located in the Ventura Basin, in the central portions of the Western Transverse Range province of southern California, an area undergoing about 13 to 20 mm/yr of north south crustal shortening. The anticline resulted from folding and backthrusting during southerly propagation and emergence of the basal thrust, the Springville fault zone. The evolution of the anticline began with fault propagation folding and planation of the flat-lying sediments of the Oxnard flood plain (on-land portion of the Ventura Basin) accompanied by associated secondary and bending moment faulting. An inferred north-south tear fault separates the area being deformed from other parts of the Ventura Basin. The anticline became emergent about 120,000 yr ago. Folding was interrupted by i tercalation backthrusting at multiple depths and different time intervals, which created secondary anticlines on the south limb of the original fold.

Propagation of the basal thrust above the level of intercalation, terminated the backthrusting approximately 30,000 yr ago. Emergence of the Springville fault, also about 30,000 yr ago, rendered the anticlinal folds passive features, with deformation now occurring as folding north of the structure as the hanging wall is strained to conform to the geometry of the underlying thrust, and as overthrust displacement. Deformation in the hanging wall north of the anticline, as well as the geometry within the anticline, define the underlying thrust geometry. Soil chronologic and carbon-14 analyses indicate a strain rate of about 1.1 mm/yr for the Springville fault zone.

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