PLIO-PLEISTOCENE TECTONIC DEVELOPMENT OF THE NOBLE HILLS, SOUTHERN DEATH VALLEY, CALIFORNIA

Previous estimates of long-term, dextral offsets and average slip rates are comparable for both the Northern Death Valley fault zone (NDVFZ) and the Southern Death Valley fault zone (NDVFZ). However, recent studies in the northern part of Southern Death Valley (SDV) suggest that large offsets reported in the literature for the SDVFZ overestimate long-term slip by an order of magnitude. To resolve this apparent discrepancy, we reexamined the stratigraphic and structural relations in the Noble Hills (NH). New mapping shows that strata exposed along the extent of the NH represent a conformable package of Pliocene basin deposits, herein referred to as the Noble Hills Formation (NH Fm), that record the Pliocene-to-recent tectonic history of SDV, and provide limiting constraints on timing and magnitude of slip on the SDVFZ. The NH Fm is a nearly one kilometer thick conformable package of ephemeral fluvial-lacustrine strata, upward coarsening alluvial deposits derived from both the north and south, and proximally-derived rock-avalanche deposits. Deposition of these strata continued through at least 3.34 Ma based on the presence Mesquite Springs tephra which we identified in the uppermost NH Fm. After 3.4 Ma, SDV experienced a major change in tectonic regime resulting in formation of NE-vergent contraction along the length of the NH. Large-scale fault propagation folds that locally involve nearly the entire stratigraphic thickness of the NH Fm exhibit up to 800 m of both structural relief and horizontal shortening in the northern NH. Structural shortening decreases toward the southeast. However, contractional deformation is markedly more intensive and complex at the south end of the NH. A subsequent episode of deformation is represented by north-striking dextral faults that cross-cut NE-vergent structures. Previous studies attribute contraction in the NH to right-slip on the SDVFZ, though map relations show movement on the SDVFZ initiated well after NE-vergent contraction. Field relations show that the SDVFZ is limited to a single 13 km long trace along the NE margin of the NH. At the south end of the NH the fault strikes oblique to earlier-formed contractional structures, displacing contacts within the NH Fm ~200 m.