The Long Valley/Mono Basin Volcanic Complex: A preliminary magnetotelluric and magnetic variation interpretation

Abstract

The Long Valley/Mono Basin volcanic complex in eastern California is one of the few major silicic caldera systems in western North America which have exhibited volcanic activity so recently that they may still be potentially active. Whereas, in principle, magnetotelluric measurements offer a significant opportunity to study such systems through mapping subsurface electrical features associated with hydrothermal and magmatic processes in the crust, our studies in this area indicate that such techniques are profoundly affected by the highly three‐dimensional structures associated with these complicated volcanic terranes. We argue that while simple plane‐layered (one‐dimensional) interpretations are likely to be misleading in such a complicated three‐dimensional environment, magnetotelluric observations can delineate important physical features associated with caldera structures; in particular, the method can closely determine the location and magnitude of major boundary faults. Because of the high resistivity contrast between basin fill and crystalline basement, these techniques are very useful for characterizing the subsurface hydrologie regime as well. In addition, sites have been occupied in the southwest moat of Long Valley caldera, an area that is currently exhibiting a variety of tectonic activity. Both the telluric field and magnetic induction arrows imply the presence of a structurally controlled east‐west electric current system at relatively shallow depth in the crust. This elongated east‐west zone is aligned along the belt of recent seismic activity in the southwest moat, a zone of seismic shear wave attenuation, and a zone of known hydrothermal alteration at the surface. Finally, we summarize observations from the vicinity of Pumice Valley (adjacent to Mono Craters) which has been thought to be an active incipient caldera. Since our present data do not indicate an observable decrease in resistivity at shallow depths beneath the valley, we are led to conclude that the parent magma body feeding Mono Craters (and perhaps Inyo domes) is either too thin or too deep to be resolved or is significantly displaced from a position beneath the center of the inferred ring fractures.