CONSTRAINTS ON THE DISTRIBUTION AND MECHANISM OF ACTIVE SUBSIDENCE ON PAOHA ISLAND IN MONO LAKE, CALIFORNIA

Paoha Island at the center of Mono Lake is one of the youngest volcanic features in California. InSAR analysis (1992-present) has revealed that the northern third of the island is subsiding at 1 cm/yr. The driving mechanism for subsidence and whether it extends into the lake are unknown. Here we use a newly collected airborne LiDAR survey in conjunction with field mapping and InSAR analysis to show that active deformation is confined to the northern end of the island and does not extend into the lake. Specifically, the subsiding area is localized to a rhyodacite cryptodome and dacitic cones and lava flows. LiDAR reveals that sediments deposited on the volcanic features during the highest lake levels recorded on Paoha Island of 1959.3 m in the year 1919 have maintained their elevation and not subsided. Possible mechanisms for subsidence include island flank instability, compaction of lake sediments under a volcanic load, tectonic extension, cooling of a magma intrusion or hydrothermal system, and magma migration. Given that the subsidence is confined to the interior of the island, only cooling of a magma intrusion or hydrothermal system or magma migration can produce the observed spatial distribution of subsidence. Magnetotelluric studies have suggested the presence of a magma body at 2-5 km depth under Mono Lake, so magma migration is possible. Cooling of a magma intrusion would require an unrealistically large and shallow intrusion or high cooling rates to produce the magnitude of subsidence 300-350 years after intrusion. In contrast, cooling of a hydrothermal system with approximately 25-30% porosity at 0.5-1.5 °C/yr is a viable mechanism for producing subsidence of 1 cm/yr. However, there is no surface temperature anomaly above the subsiding zone on Paoha Island and heat flow measurements on the north, east, and west margin of the northern end of the island show no anomalies. It is possible that the dacite on the northern end of the island is hydraulically connected to a fumarole field within lava domes at Hot Springs Cove, 1.2 km to the southeast, and that the heat emerging there is the surface expression of the cooling hydrothermal system below the northern end of the island. This study provides constraints on both the distribution and mechanism of subsidence at Paoha Island and furthers our understanding of the active volcanic processes and hazards in the Mono Basin.