USING POTENTIAL FIELD INVERSIONS TO IMAGE THE MAGMATIC PLUMBING OF NORTHERN CALIFORNIA'S CLEAR LAKE VOLCANIC FIELD

Located ~120 km north of the San Francisco Bay Area, the Clear Lake Volcanic Field (CLVF) is the youngest in a series of northwards-younging volcanic fields in California’s Coast Range mountains. Situated within the broad San Andreas Fault System, intermittent volcanic activity has persisted in the CLVF for the past ~2 Ma with eruptions occurring as recently as ~8.5 Ka. Heat beneath the CLVF sustains the world’s largest producing geothermal field, The Geysers.

Our geophysical research aims to image the magmatic plumbing of the CLVF and determine if one or more crustal partial melt zones persist from previous volcanic or intrusive episodes. Understanding the heat source underlying the volcanic and geothermal fields will inform updated volcanic hazard assessments and help engineers sustainably manage The Geysers.

We compiled a dataset of over 3,000 existing ground-based gravity measurements and completed the first 3-D gravity inversions of the CLVF region. Several regularization styles and a series of synthetic model inversion tests were used to characterize the resolution and non-uniqueness of the recovered models. The inversion results indicate that the pronounced gravity low centered around Mount Hannah, a ~0.8 Ma dacitic edifice near the center of the CLVF, is best explained by a mid-crustal (6 to 13 km deep) partial melt zone with a density of 2.37 to 2.52 g/cm³. Thermodynamic modeling with rhyolite-MELTS suggests that this low-density zone could contain 10-30% partial melt given temperatures of ~700°C at 8 km depth (210 MPa). Our inversion results and a reanalysis of borehole cuttings refute previous assertions that the Mount Hannah gravity low can be predominantly explained by a 5-7 km thick lens of Great Valley Sequence argillite. The proposed partial melt zone is further supported by the ongoing occurrence of deep long-period earthquakes, regionally high heat flow, and ³He enrichment of hydrothermal fluids. These observations provide additional evidence of the enduring mantle-sourced recharge of one or more melt-bearing bodies beneath the CLVF.

Our ongoing research focuses on reanalyzing regional aero-magnetic data and the collection of a large magnetotelluric (MT) dataset. Jointly inverting gravity, magnetic, and MT data from the area will provide a more comprehensive image of CLVF’s magmatic plumbing.