TEPHROCHRONOLOGY OF THE LAST CHANCE RANGE, EASTERN CALIFORNIA: DOCUMENTATION OF THE SALINE RANGE AS A SIGNIFICANT PLIOCENE TEPHRA SOURCE (Invited Presentation)

Tephrochronology is an effective tool for stratigraphic correlation throughout the structurally complex western Great Basin, however, tephrochronologists are confounded by Neogene volcanism that produced rhyolite tephra with very similar glass shard composition over the last18 Ma. For example, the Death Valley area has multiple 4-3 Ma tephra layers with major- and minor-element shard composition similar to Long Valley caldera eruptions, but Long Valley caldera is a Pleistocene feature. In addition, questions often arise as to whether multiple tephra layers result from individual eruptions, are duplicated by faulting, or are fluvially reworked.

To resolve some of these issues, we collected chemical and 40Ar/39Ar data from 14 tephra layers in the Last Chance Range (LCR), which separates Death Valley from Eureka and Saline Valleys. In this study, we informally name the 3.57 Ma tuff of Dry Mountain and ~3.5 Ma tuff of Crater Road. Trace element data from the glass shards of all three tephra layers of the ~3.34-3.1 Ma Mesquite Spring family, present in the LCR, show the glass shard compositions of these three tephra layers are too similar to reliably distinguish them by shard composition alone; however, the Mesquite Spring family are distinguishable from Long Valley tephra layers. In addition, glass shard trace element data identify the Curry canyon tephra family, named herein, representing multiple eruptions between 3.58 and 3.3 Ma. Thick beds (1-6 m) and coarse lapilli indicate that many of these Pliocene tephra layers were deposited close to their eruptive sources. We propose that the tephra erupted from the Saline Range volcanic center, which borders southern Eureka Valley. This volcanic center was one in a series of rhyolite-producing Neogene volcanic centers situated between the Silver Creek caldera, western AZ, and Mono Lake.

We use the interbedded sediments and tephra at Hanging Rock Canyon to document basinward stepping of the normal-oblique Eureka Valley fault zone (EVFZ) bounding the LCR during both the Pliocene and Pleistocene and constrain the timing of exposure of the Last Chance thrust by slip on the EVFZ to the late Pliocene. The EVFZ stepped basinward during the late Pliocene uplifting alluvial fan deposits containing a 1.9-1.7 Ma lower Glass Mountain tephra layer. The youngest trace of the EVFZ offsets late Pleistocene (ca. 30 ka) alluvial fans.