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

Abstract

Though shallow flow of hydrothermal fluids in Long Valley Caldera, California, has been well studied, neither the hydrothermal source reservoir nor heat source has been well characterized. Here a grid of magnetotelluric data were collected around the Long Valley volcanic system and modeled in 3‐D. The preferred electrical resistivity model suggests that the source reservoir is a narrow east‐west elongated body 4 km below the west moat. The heat source could be a zone of 2–5% partial melt 8 km below Deer Mountain. Additionally, a collection of hypersaline fluids, not connected to the shallow hydrothermal system, is found 3 km below the medial graben, which could originate from a zone of 5–10% partial melt 8 km below the south moat. Below Mammoth Mountain is a 3 km thick isolated body containing fluids and gases originating from an 8 km deep zone of 5–10% basaltic partial melt.

Seismic evidence for significant melt beneath the Long Valley Caldera, California, USA

“…2E) attributed to 30-60% melt (Seccia et al, 2011). While hydrothermal alteration contributes to reduced velocity at shallow depths (<7 km) (Peacock et al, 2016), it does not likely have significant effects at greater depths (Barnes, 1997). Similarly, the velocity is too low to be explained by heat remaining from a crystallized magma reservoir, and implies the presence of residual melt.…”

“…Despite this limitation, these studies have found evidence for a low V P and V S or high S-wave attenuating zone near the base of their models (Romero et al, 1993;Foulger et al, 2003;Seccia et al, 2011;Lin, 2015). However, given the proximity to the relatively low-temperature base (100 °C) of the Long Valley Exploration Well (Sorey et al, 2000), low V P /V S , and low resistivity, many suggest these anomalies reflect magmatically derived fluids and/or hydrothermal alteration (Romero et al, 1993;Lin, 2015;Peacock et al, 2016). The best evidence to date for a deep magma reservoir comes from limited lower-resolution teleseismic studies, which show an 8-30% reduction in V P between 7 km and 20 km depth (Dawson et al, 1990;Steck and Prothero, 1994;Weiland et al, 1995).…”

Subsurface Structure of Long Valley Caldera Imaged With Seismic Scattering and Intrinsic Attenuation

“…Sanders () and Ponko and Sanders () image an area of high P ‐wave attenuation at 4–5 km beneath the east side of Mammoth Mountain, which they interpreted as a region of fractured rock with supercritical hydrothermal fluids. More recently, Peacock et al () also inferred a shallow isolated hydrothermal system in their magnetotelluric (MT) images, which extends to around 3‐km depth below the northeastern part of Mammoth Mountain (C4 in Figure 3 in Peacock et al, ). This anomaly is also coincident with a lower P ‐wave velocity (Foulger et al, ; Lin, ; Seccia et al, ) and average V p / V s ratio (Dawson et al, ; Foulger et al, ; Lin, , ).…”

“…The role of active magmatism beneath Mammoth Mountain is more clear given the combination of large gas emissions (Lewicki et al, ) and both shallow and deep seismicity (Shelly & Hill, ; Shelly et al, ). At Mammoth, electrical resistivity images also identify anomalies that may be produced by active hydrothermal systems (Hill, ; Peacock et al, ).…”

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“…Seismologic and magnetotelluric studies (Achauer et al, 1986;Peacock et al, 2015Peacock et al, , 2016 suggest that numerous, separate, mid-crustal, potentially active magmatic sources (partial melt zones) lie in an irregular, but N-S elongated zone extending from Mono Lake to Mammoth Mountain.…”