Hydrothermal history of the Long Valley Caldera, California: Life after collapse

Abstract

Drilling of the Long Valley Exploratory Well (LVEW) on the resurgent dome in the 760 ka Long Valley Caldera opened a window to view the geologic history of the central caldera. Stratigraphic relationships indicate piston/cylinder (Valles-type) collapse for this caldera, and a resurgent structure intimately linked to post-caldera-collapse rhyolitic intrusions. Samples recovered from this and other wells proximal to the resurgent dome were characterized through isotope microanalytical techniques, petrographic and microprobe study, and analysis of fluid inclusions within alteration minerals. This work revealed the complexity of primary magmatic and secondary hydrothermal activity involved in the formation of a resurgent dome. Measurements of the $\rm\delta\sp{18}O$ composition of silicate components forming the intracaldera lithologies display disequilibrium within samples as a result of variable exchange with hydrothermal fluids. A maximum calculated temperature of $350\sp\circ\rm C$ at 1800 m depth in LVEW indicates paleohydrothermal temperatures exceeded the known present-day hydrothermal conditions by more than $100\sp\circ\rm C.$ Contouring of $\rm\delta\sp{18}O$ values from wells on a line crossing the caldera define a pattern of convective flow with upwelling beneath the resurgent dome. Although surface volcanism at the LVEW site ended about 650 ka, laser probe $\rm\sp{40}Ar/\sp{39}Ar$ microanalysis of samples from sill-like intrusions into the intracaldera ignimbrite reveals intrusive events at ${\sim}650$ ka, ${\sim}450$ ka, and ${\sim}350$ ka. Sanidine phenocrysts from the Bishop Tuff at 1772 and 1792 m depths and whole rock samples of the Mesozoic metavolcanic basement rocks at 1957 m depth record times of disturbance by hydrothermal pulses at ${\sim}530$ ka and ${\sim}350$ ka. Repeated emplacement of intrusions into the centrally located caldera ignimbrite was a primary process of resurgence. In turn, the feeders for the intrusions and the intrusions themselves supplied heat for resurgent-dome-centered hydrothermal flow. After approximately 300 ka, all activity shut off in the central caldera only to resume at ${\sim}40$ ka in response to renewed Holocene volcanic activity in the West Moat. Geophysical evidence of recent intrusive activity beneath the resurgent dome indicates this shallow magma emplacement mechanism is not totally extinct in the central caldera. Most likely a new cycle of volcanism and hydrothermal circulation is underway as the caldera matures.