CLEAR LAKE VOLCANIC FIELD IMAGED BY MAGNETOTELLURICS

The Clear Lake Volcanic Field (CLVF) in northern California is the youngest volcanic field associated with northward migration of the Mendocino Triple Junction and related magmatism created by a slab-free window. The CLVF has been active since ~2.1 Ma, with the most recent dated activity around 8.5 ka. Within the CLVF, the timing of eruption generally youngs towards the north and northeast. Included within the geographic bounds of the CLVF is The Geysers, the world’s largest energy producing geothermal field. To help understand the dynamics and interdependence of volcanic activity and the slab-free window, a regional scale magnetotelluric (MT) survey was collected between 2020 and 2023. MT measurement can be used to image subsurface variations in electrical resistivity and are sensitive to where fluids are or have been in the crust. The collected MT dataset was inverted to develop a 3D electrical resistivity model. Preliminary interpretations of the resistivity model suggest the presence of a large, resistive plutonic body underlying the well-constrained Geysers Plutonic Complex. This deeper body extends to the north below Cow Mountain and to the south through the Sonoma Volcanic Field. A conductive anomaly, which we interpret as delineating a possible crustal magma storage zone, extends either direction from The Geysers along a northeast trend beneath the southern edge of Clear Lake.