ABSTRACT: Geometry and Kinematics of an Active, Laterally Propagating Wedge-Thrust, Wheeler Ridge, California

Donald A. Medwedeff

Structural analysis of the lateral termination of a Quaternary fold-and-thrust system provides insight into the fault propagation process.

Located in the southern San Joaquin Valley, California, Wheeler Ridge anticline is underlain by a blind thrust system. Five quantitatively retrodeformable, cross sections based on more than 150 wells show that the anticline is a fault-bend fold forming at the tip of a north-verging wedge. A bend in the lower ramp and stacking of ramps at the wedge front make the fold asymmetric. Topographic expression of the fold is located only above the stacked ramps, where the uplift rate is greatest. Growth sediments bury the back limb and roll up the front limb. Section restoration, made by returning well segments to their predeformational locations, indicates that the wedge geometry of the thrust system is controlled by the depositional geometry of a Pliocene sand-rich unit.

Slip on the thrust decreases eastward along strike (from 660 m to zero in ^sim2,500 m), creating a bulk shear strain of 0.27, which is probably accommodated by large-scale cataclastic flow. The abrupt lateral termination of Wheeler Ridge at an escarpment suggests that lateral propagation occurs in discrete steps, creating tear faults at the fold edge and localizing antecedent streams which become entrenched to form water and wind gaps.

The well-constrained, three-dimensional, kinematic understanding of thrust development presented here provides a structural model for exploration of less well-constrained structures. The study also depicts the reconstruction of predeformational stratigraphy using quantitative restoration and true stratigraphic thickness correction techniques.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990