SHORTENING IN THE UPPER PLATE OF THE BUCKSKIN-RAWHIDE EXTENSIONAL DETACHMENT FAULT, SOUTHWESTERN USA, AND IMPLICATIONS FOR STRESS STATE DURING EXTENSION

Detailed geologic mapping in the Buckskin, Rawhide, and Artillery Mountains in western Arizona identified numerous folds in Oligocene-Miocene strata above the Buckskin-Rawhide extensional detachment fault. The folds are above or adjacent to the Harcuvar metamorphic core complex which was uplifted and exposed by highly effective tectonic exhumation at ~27-9 Ma. Folds affect strata deposited during extension but the folds are truncated by the detachment fault, demonstrating that folding occurred during the period of detachment faulting. Fold axes are approximately perpendicular to regional extension direction. In two of the four areas of folding, fanglomerates derived partially from lower plate mylonitic rocks are the stratigraphically highest folded strata. Folding could have occurred above low-angle normal faults with curved or ramp-flat geometries, but problems with such origins, and the abundance of folds, lead us to consider the possibility that at least some folds reflect shortening in the same direction as regional extension. Application of critical-taper theory to an extensional wedge with very low basal friction indicates that wedge shortening would be expected if the wedge developed a sufficient surface slope that was uphill toward the wedge tip. We suggest that, in at least two of the study areas, folding was triggered by development of alluvial fans shed off of the core complex with associated surface slope that was uphill toward the rising core complex. Alternatively, folding was triggered by footwall uplift sufficient to produce such a slope, without alluvial fans. In either case, potential energy due to the surface slope would be available for wedge shortening, just as it would be available for wedge extension if surface slope in the wedge is downhill toward the wedge tip. Wedge shortening would be expected to promote effective core-complex denudation.