Lessons from Long Valley Caldera (LVC) for Considering the Unrest at Laguna del Maule (LdM)
Abstract:
Prehistoric LVC: 4 eruptive episodes of voluminous high-Si rhyolite (76–78% SiO2), poor in crystals, Sr, Eu, Ba, Zr, and Hf (interpreted as melt extracts from pluton-scale leucogranitic crystal mush), illustrate a variety of compositional sequences and patterns of ascent, assembly, and eruption. A 5th episode released 100 km3 of aphyric melt (74.3–75% SiO2) in ~40 batches that are all alike in major elements but range widely in those 5 trace elements.
Since 1980, seismicity in the adjacent Sierra Nevada has been accompanied inside the caldera by (1) uplift totaling ~83 cm, centered on the Pleistocene resurgent dome, and (2) many earthquake swarms along a 12-km-long segment of the ring-fault zone—contiguous with both the structural dome and the Sierran seismogenic domain. There is no evidence for new magma. Ongoing uplift and ring-fault seismicity may both be promoted by ascent of aqueous fluid released by second boiling of the leucogranitic pluton crystallizing at depths ≥10 km. There was no eruption on the dome after 500 ka, and since 230 ka volcanism is restricted to the west moat and areas west of the caldera. High-T hydrothermal activity in the central caldera waned after 300 ka, having cooled the caldera fill so that drilling on the resurgent dome to a depth of 3 km found T~100ºC. In contrast to extracaldera Mammoth Mtn, there is little seismicity beneath the dome at any depth, no emission of magmatic CO2, no elevated 3He/4He, and normal to below-normal heat flow. Most of the 75-km-long ring-fault zone is likewise aseismic, except only the 12-km segment. Since 1980, the nearby Sierran seismicity released 3.6 times more seismic energy than did the intracaldera earthquakes. Caldera seismicity is not related to stress associated with the uplift but, instead, driven by the regional tectonic stressfield. Sierran seismicity reactivated only the contiguous 12-km segment of the ring-fault zone. Upward-migrating swarms are thought to be triggered by ascending pulses of low-viscosity CO2-poor aqueous fluid, which provides the pressure source above the apex of the crystallizing caldera-wide pluton and escapes laterally to the newly reactivated segment of the ring-fault zone, where it mediates the ongoing seismicity along the caldera’s south moat.