GEOPHYSICAL INVESTIGATIONS OF THE MONO-INYO VOLCANIC CHAIN, MONO BASIN, CALIFORNIA

Potential-field models of data collected in Mono Basin, California suggest that a shallow basement high, possibly bounded by a fault, may have influenced the location of the Mono Domes. The Mono Domes are an arcuate chain of more than 30 overlapping domes and coulees located at the northernmost end of the Mono-Inyo Volcanic chain. During the summers of 2010 and 2011, new gravity, magnetic and audiomagnetotelluric (AMT) data were collected to investigate the subsurface structure of the Mono Basin. Over 500 new gravity stations and approximately 297 line-km of ground magnetic data were collected. Regional gravity data were collected throughout the basin and ground magnetic data were collected on major roads along numerous transects throughout the region, one of which is coincident with a seismic refraction line. Two AMT profiles were collected in Pumice Valley along the western margin of the Mono Domes. Gravity and magnetic data were compiled with pre-existing data from studies dating back to the 1960’s to produce a new regional isostatic gravity anomaly map and magnetic anomaly profiles throughout the basin.

The isostatic gravity anomaly map reveals the presence of gravity lows in Mono Lake and the Long Valley caldera in areas where low density volcanic sediments are prevalent. Gravity highs, observed to the east and west of Mono Lake and to the east of the Long Valley caldera, are due to relatively higher density plutonic rocks. Aeromagnetic data and ground magnetic data show an anomalous ring-shaped magnetic high centered over Aeolian Buttes, whose eastern margin follows the arcuate trend of the Mono Domes. This magnetic high is coincident with what Kistler (1966) inferred to be a ring-fracture. Two-dimensional (2D) forward modeling of potential field data along a profile across the basin suggest a basement high at approximately 600 m depth, the eastern edge of which lies beneath the Mono Domes. A fault may bound the eastern edge of the basement high that could have served as a conduit for the Mono Domes feeder dike. One-dimensional and 2D inverse models of the AMT profiles show a resistive body at roughly 600-800 m depth that coincides with the anomalous ring-like magnetic anomaly. The 2D resistivity model along the western edge of the Mono Domes may suggest the existence of a faulted basement block along this margin.