Fluid-driven uplift at Long Valley Caldera, California: Geologic perspectives

Abstract

Since persistent seismicity began in the Sierra Nevada adjacent to Long Valley caldera in 1978–1980, intracaldera unrest has been marked by (1) episodes of uplift totaling ~83cm, centered on the middle Pleistocene resurgent dome, and (2) recurrent earthquake swarms along a 12-km-long segment of the caldera's ring-fault zone that is contiguous with both the dome and the Sierran seismogenic domain. Others have attributed the recent unrest to magmatic intrusion(s), but it is argued here that evidence for new magma is lacking and that ongoing uplift and ring-fault-zone seismicity are both promoted by ascent of aqueous fluid released by second boiling of the residue of the enormous Pleistocene rhyolitic reservoir terminally crystallizing at depths ≥10km. For 2Myr, eruptive vent clusters migrated southwestward from Glass Mountain to Mammoth Mountain. There has been no eruption on the resurgent dome since 500ka, and since 230ka volcanism has been restricted to the caldera's west moat and contiguous Sierran terrain, both outside the structural caldera. High-temperature hydrothermal activity in the central caldera waned after ~300ka, cooling the Pleistocene rhyolitic focus to the extent that drilling on the resurgent dome found mid-caldera temperature to be only 100°C and isothermal at depths of 2–3km. Beneath most of the resurgent dome, there is little seismicity at any depth, no emission of magmatic CO2 or other magmatic gases, no elevated 3He/4He ratios, and only normal to below-normal heat flow. Most of the 75-km-long ring-fault zone is likewise aseismic, excepting only the 12-km segment contiguous with the extracaldera seismogenic domain in the Sierra. Since 1980, the Sierran seismicity has released 3.6 times more cumulative seismic energy than have intracaldera earthquakes. The caldera seismicity is not driven by stresses associated with the adjacent uplift but, instead, by the extracaldera tectonic stressfield. Sierran seismicity activated the directly contiguous south-moat segment of the ring-fault zone, which had originated in the caldera-forming eruption at 767ka and everywhere else remains sealed. Hypocenter relocation studies of 1000s of earthquakes along the seismic segment have resolved recurrent upward-migrating swarms within networks of cryptic faults, apparently triggered by rapidly ascending pulses of high-pressure low-viscosity aqueous fluid. Entering the brittle crust at depths of 8–10km, such fluid is just what should be expected from second boiling of the late-stage CO2-poor rhyolitic residue. The fluid provides the pressure source above the apex of the crystallizing caldera-wide pluton and then escapes laterally to the newly reactivated southern segment of the ring-fault zone, its only available permeable pathway, where it mediates the ongoing south-moat seismicity.