NEW INSIGHTS ON THE PLUVIAL HISTORY OF LAKE MANLY AND TECTONIC DEVELOPMENT OF SOUTHERN DEATH VALLEY RECORDED IN PLIOCENE-TO-RECENT LACUSTRINE STRATIGRAPHY

New mapping in Southern Death Valley (SDV) has defined four lacustrine sequences that record pluvial lake history, timing and style of progressive contractional deformation, and post-1.2 Ma right-lateral displacement across the main strands of the Southern Death Valley Fault Zone (SDVFZ). A middle-Pliocene sequence contains the ~3.4 Ma Mesquite Springs tephra. This sequence was tightly folded by earlier north-northeast directed shortening and subsequently refolded by northeast-directed shortening. An early-to-middle Pleistocene sequence includes Upper Glass Mountain (~1.2 to ~0.9 Ma) and Lava Creek B (0.64 ± 0.02 Ma) tephra and is moderately folded by northeast-directed shortening. A middle-to-late(?) Pleistocene sequence consists of shoreline sand and gravel, which is gently folded by northeast-directed shortening. This sequence has not been dated, but includes a prominent tephra layer that is currently being analyzed. We suspect that this sequence was deposited during the Oxygen Isotope Stage (OIS) 6 lacustrine phase of pluvial Lake Manly. The youngest sequence shows only minor deformation locally along strands of the SDVFZ. This unit includes fine-grained strata and shoreline gravel bars that reach a maximum elevation of ~46 m. Lacustrine shoreline deposits at an elevation of ~46 m in Northern Death Valley (NDV) and the northern part of SDV recently yielded tufa ages of 26.97 ± 0.62 and 24.38 ± 0.63 ¹⁴C kyrs B.P., respectively. We therefore correlate this sequence to the OIS 2 pluvial lacustrine phase of Lake Manly.

The main strands of the SDVFZ project through the lacustrine sequences in SDV. Post-1.2 Ma right-lateral offsets are on the order of 50 m. Headward eroded channels incised into the ~27 ka OIS 2 sequence consistently show ~5 m of right-lateral offset. These data yield average minimum right-lateral slip rates of <0.1 to 0.2 mm/yr, respectively. The low slip rates we estimate for the SDVFZ are consistent with its subdued geomorphic expression and stand in stark contrast to large right-lateral slip rates previously determined for the NDV fault zone (4-9 mm/yr over variable time frames), which accordingly exhibits marked geomorphic expression. Our preliminary studies in SDV indicate post-middle Pliocene deformation has been dominated by northeast-vergent contractional deformation.