Constraints on the post-middle-Pleistocene tectonic development of the Confidence Hills, southern Death Valley, California

Our recent field mapping and tephrochronology offers new constraints on the style, timing, and rates of middle-Pleistocene-to-recent deformation in the Confidence Hills (CH), southern Death Valley. Movement on the active trace of the Southern Death Valley fault zone (SDVFZ) was preceded by earlier large-scale, northeast-vergent folding. This earlier folding involves several hundred meters of conformable late-Pliocene-to-middle-Pleistocene strata, which together form the common limb of a locally overturned fault-propagation fold pair. Geometric relations require that the blind thrust(s) responsible for earlier folding in the CH root well to the southwest of the active trace of the SDVFZ, which raises questions concerning previous flower structure models for earlier folding in the CH. Earlier folding began after deposition of Upper Glass Mountain tephra (1.1-0.9 Ma), which lies within the uppermost section of conformable, locally overturned Confidence Hills Formation(CHF), and ended prior to deposition of unconformably overlying fanglomerate, which contain tephra layers we tentatively correlate to the Bishop (0.76 Ma) and Lava Creek B (0.64 Ma) tephra. Earlier folding resulted in greater than 400 m structural relief and nearly 600 m of shortening in the span of 140-340 ka, yielding a middle-Pleistocene shortening rate of ~1.8-4.3 mm/yr. Dextral slip along the mappable traces of the SDVFZ began after earlier fault-propagation folding and also after deposition of the 0.76-0.64-Ma fanglomerates. Net right-lateral offset along the fault zone is well constrained ~4-km south of Shoreline Butte where a steeply-dipping contact marking the base of volcaniclastic conglomerate of the CHF is offset ~650 m. These relations yield a minimum post-middle-Pleistocene slip rate of ~1 mm/yr for the SDVFZ. Post-0.64-Ma shortening within the CH has been minor relative to earlier folding. The earlier fault-propagation folding in the CH appears to be related to a short-lived episode of northeast-directed motion of the Owlshead Mountains block, which was likely accommodated by left-lateral slip along the Wingate Wash fault.