Petrogenesis of Quaternary Potassic Volcanism (Big Pine Volcanic Field) Along the Owens Valley Fault Zone in the Eastern California Shear Zone

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 Quaternary BPVF occupies 400 square miles of the basin straddling the dextral Owens Valley Fault Zone (with long-term horizontal slip rate of 2-3 mm/yr) and consists of scattered volcanic centers dominated by cinder cones, lava flows, and domes spatially associated with normal faults bounding the Sierra Nevada and White-Inyo Mountains and with across-basin oblique-normal fault systems. The lavas, which are 1.18 m.y. and younger in age, 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 La_n/Sm_n ratios average 3.5. Two hawaiite samples 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 suggests 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 suggest that its felsic magmas too were derived from a separate parental melt. 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. 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. The plumbing system of the Big Pine basaltic volcanism is likely to have been controlled by lithospheric-scale extension developed in part as a result of displacement-transfer mechanism at the intersection of the NW-trending, dextral OVFZ and the NE-striking, NW-dipping Deep Springs normal fault.