Imaging the magmatic system of Mono Basin, California, with magnetotellurics in three dimensions

Abstract

A three‐dimensional (3‐D) electrical resistivity model of Mono Basin in eastern California, unveils a complex subsurface filled with zones of partial melt, fluid‐filled fracture networks, cold plutons, and regional faults. In 2013, 62 broadband magnetotelluric stations were collected in an array around southeastern Mono Basin from which a 3‐D electrical resistivity model was created with a resolvable depth of 35 km. Multiple robust electrical resistivity features were found that correlate with existing geophysical observations. The most robust features are two 300 ± 50 km3 near‐vertical conductive bodies (3–10 Ω m) that underlie the southeast and northeastern margin of Mono Craters below 10 km depth. These features are interpreted as magmatic crystal‐melt mush zones of 15 ± 5% interstitial melt surrounded by hydrothermal fluids and are likely sources for Holocene eruptions. Two conductive east dipping structures appear to connect each magma source region to the surface. A conductive arc‐like structure (< 0.9 Ω m) links the northernmost mush column at 10 km depth to just below vents near Panum Crater, where the high conductivity suggests the presence of hydrothermal fluids. The connection from the southernmost mush column at 10 km depth to below South Coulée is less obvious with higher resistivity (200 Ω m) suggestive of a cooled connection. A third, less constrained conductive feature (4–10 Ω m) 15 km deep, extending to 35 km is located west of Mono Craters near the eastern front of the Sierra Nevada escarpment and is coincident with a zone of sporadic, long‐period earthquakes that are characteristic of a fluid‐filled (magmatic or metamorphic) fracture network. A resistive feature (103–105 Ω m) located under Aeolian Buttes contains a deep root down to 25 km. The eastern edge of this resistor appears to structurally control the arcuate shape of Mono Craters. These observations have been combined to form a new conceptual model of the magmatic system beneath Mono Craters to a depth of 30 km.

Geophysical characterization of a Proterozoic REE terrane at Mountain Pass, eastern Mojave Desert, California, USA

“…Magnetotellurics (MT) is an electromagnetic method that is typically employed for investigating deep (10 km or more) subsurface resistivities and is especially sensitive to fluids or hydrothermal mineralization in geologic structures (e.g., Peacock et al, 2015). Electrical conductivity is commonly attributed to enhanced porosity and permeability that creates high fluid connectivity.…”

Three‐dimensional electrical resistivity model of the hydrothermal system in Long Valley Caldera, California, from magnetotellurics

“…Newly collected MT data, combined with data from Wannamaker et al [1991] and Peacock et al [2015], were inverted to produce a preferred 3-D electrical resistivity model, where two inversion programs were used, WS [Siripunvaraporn et al, 2005] that is based on an Occam-type inversion, and ModEM [Egbert and Kelbert, 2012] that utilizes a nonlinear conjugate gradient method. Topography was not used in the model, and the first layer is average elevation above sea level (2300 m).…”

Constraining the Early Eruptive History of the Mono Craters Rhyolites, California, Based on 238U‐230Th Isochron Dating of Their Explosive and Effusive Products

“…These Holocene Mono-Inyo eruptions are inferred to have resulted from intrusion and venting of an 8-to 10-km-long, north striking dike, which accommodates regional east-west extension (Bursik & Sieh, 1989;Riley et al, 2012). Beneath the Mono Craters chain, geophysical data suggest the presence of as much as 20% silicic melt (Achauer et al, 1986;Dawson et al, 1990;Peacock et al, 2015), which is inferred as part of a granitic reservoir that has fed the eruptions of high-silica rhyolite at Mono Craters (Hildreth, 2004). The area with the youngest silicic volcanism (dacites to low-silica rhyolites) is centered in Mono Lake where intrusive, effusive, and explosive events formed the islands of Negit and Paoha between about 2000 and 350 years ago, respectively (Figure 2a; Colman et al, 2014;Stine, 1987).…”