Tectonic Setting of the Big Pine Volcanic Field, Eastern California

The Big Pine Volcanic Field (BPVF) is situated in the northern part of the Owens Valley, a fault-controlled basin that has evolved between the Sierra Nevada and the White-Inyo Mountains during the last 7 m.y. The Owens Valley is part of the Eastern California Shear Zone (ECSZ), which is a broad trans-tensional zone of right-Iateral shear that defines an oblique rift zone in the wake of the NW-moving Sierra Nevada-Great Valley block. The BPVF occupies 400 square miles of the basin straddling the dextral Owens Valley Fault Zone and consists of scattered centers of volcanism dominated by cinder cones, lava flows, and domes associated with normal faults near the valley walls and with across-basin oblique fault systems. The lavas consist of moderately potassic olivine basalt, hawaiite and trachyandesite. One dome west of Poverty Hills consists of rhyolite. Most basaltic samples contain 1-4 mol% normative nepheline but a few have small amounts of normative hypersthene. All analysed samples have similar chondrite-normalized rare-earth element patterns with strong light rare-earth enrichment. La is up to 200 times chondrite and Lan/Smn ratios average 3.5. Two hawaiites from the Red Hill cone west of Bishop are distinct from the other samples. They have relatively high K2O (3.5 wt%), low Na2O (2.5 wt%), high FeOt (9.8 wt%) and very high Zr (350-400 ppm). The distinct geochemistry of the hawaiites suggest that they originated from a separate parental magma. Likewise, the geochemistry of the Poverty Hills rhyolite and the lack of intermediate samples between it and the basalts suggests that it too was derived from a separate parental magma. The Big Pine volcanic rocks are the youngest of the eastern Sierra Nevada potassic province. They have undergone relatively little fractionation and thus were probably erupted shortly after formation of the parental liquids. Development of the BPVF corresponds to the evolution of the Owens Valley Fault Zone (OVFZ) as a major transtensional dextral fault system during the Pleistocene-Holocene, as this fault system migrated westward within the basin in response to changes in local fault kinematics. We predict that volcanism in the BPVF became progressively younger both westward and northward as the OVFZ established itself as the most recently active fault system in this part of the ECSZ.