FOLDING IN THE UPPER PLATE OF THE BUCKSKIN-RAWHIDE DETACHMENT FAULT, SOUTHWESTERN ARTILLERY MOUNTAINS, WESTERN ARIZONA

The Artillery Mountains consist of 27-10 Ma strata deposited and tilted in a half-graben basin above the northeast-dipping Buckskin-Rawhide extensional detachment fault. The adjacent Rawhide Mountains to the southwest consist of variably mylonitic crystalline rocks in the footwall of the detachment fault, and abundant detachment-fault slivers of Paleozoic and Mesozoic metasedimentary rocks. The structure of the tapered end of the upper plate is revealed by new mapping of the Rawhide Wash and Artillery Peak 7.5' Quadrangles for the STATEMAP mapping program. Southwest-dipping strata in the half-graben basin are progressively less tilted up section. Stratigraphically highest tilted strata consist of red arkosic sandstone of the Chapin Wash Formation, the overlying, mylonite-clast-bearing Sandtrap Conglomerate, and the slightly tilted Manganese Mesa Basalt. Strata of the Chapin Wash Formation and Sandtrap Conglomerate, where exposed near the tapered edge of the upper plate, are folded at wavelengths of 100s to 1000s of meters, with shortening parallel to regional extension direction. Folds are truncated by the underlying detachment fault, indicating that folding pre-dated latest detachment-fault displacement. Such folding has been recognized previously at four other locations in the Rawhide, Buckskin, and Whipple Mountains. Folding is interpreted in the context of critical-taper theory, and is inferred to represent the shortening side of the critical-taper stability field wherein the surface slope is away from the trace of the detachment fault and wedge strength is low. Shortening occurs due to the weight of the tapered wedge on the underlying low-strength fault. Abundant conglomerate clasts of mylonite and chloritic breccia in folded strata were deposited in alluvial fans emanating from the lower plate of the detachment fault, and indicate that surface slope was to the northeast, consistent with an extensional-wedge setting conducive to wedge shortening.