Geophysical studies of Mono Basin -- Insights into the nature of a ‘volcano-tectonic depression’ in eastern California

Abstract:

Previously described as a volcano-tectonic depression, the Mono Basin is instead shown by gravity, magnetic, and magnetotelluric (MT) data to be bounded and intersected by structures that not only played a role in basin formation, but also controlled the location of a volcanic center in the middle of the basin. The Holocene volcano on Paoha Island in the middle of Mono Lake is the northernmost volcano in a series of north-trending, predominantly rhyolitic volcanic centers extending 40 km from the western margin of Long Valley caldera to Mono Lake. A product of regional transtensional deformation as well as host to an active and complex volcanic system, Mono Basin not only poses a volcanic hazard but may contain geothermal resources.

Although gravity measurements in Mono Lake are limited to Paoha and Negit Islands, the gravity-defined basin reveals a 30-mGal gravity low roughly centered on Paoha Island. The basin is approximately triangular shaped with steep, linear southwestern and northwestern margins, and a more irregular, less steep east-southeastern margin. Two-dimensional gravity models show a ~2-km deep, asymmetric basin with a steep southwestern margin, suggesting a normal range-front fault played a role in controlling basin shape. Recently collected ship-borne magnetic data shows a left-laterally offset magnetic anomaly along the northeastern basin margin suggesting strike-slip faulting characteristic of the nearby Mina deflection zone. Along the east-southeastern margin, the northeast trending basin edge bends abruptly to the north-northeast where a deep magnetic source protrudes into the basin, probably reflecting a pluton boundary.

Three-dimensional MT modeling reveals a conductive anomaly located ~10-km beneath Mono Lake, interpreted as a magma source that is on the order of 300 km³ and likely the source that feeds the Paoha Island volcano. A conductive connection from this deep source to the surface under the lake is offset to the west, where it appears to skirt around the deep magnetic source imaged in the basin, suggesting a magnetic pluton may be influencing the location of the volcano. A similar scenario is also observed further to the south where another pluton bounds the edge of the Mono craters.