THE NATURE OF BASINS IN THE TRANSITION ZONE FROM SIERRA NEVADA TO BASIN AND RANGE AS REVEALED BY GRAVITY, GEOLOGY, MAGNETOTELLURICS, AND HIGH-RESOLUTION AEROMAGNETIC DATA

High-resolution aeromagnetic data, combined with regional isostatic residual gravity, digital geology, and a magnetotelluric (MT) profile, reveal a wide range of structural styles for basins within a transect across the transition zone between the Sierra Nevada block near Bridgeport, California, and the Walker Lane near Mina, Nevada. In the southwest corner of the transect, the northern Mono Lake basin is underlain by a deep northeast-trending, fault-bounded graben, as revealed by a steep-sided gravity low and low magnetic intensities. Northeast-trending strike-slip faults dominate the surface structural style over much of the southern half of the transect. In contrast to northeast trends in the faulting and topography, the aeromagnetic data show strong east-west trends to the east of Mono Lake, suggesting that the northeast dominance at the surface represents thin-skinned deformation that is decoupled from an underlying east-trending structural grain. An MT profile crossing two east-west magnetic contacts reveals a sub-horizontal low-resistivity zone at several hundred meters depth that could represent hydrothermally altered volcanic rock along a decollement surface. This surface deepens at the northeast end of the MT profile, which is near the southwest end of the Huntoon Valley. This valley is bounded by two northeast-trending strike-slip faults, and represents a basin created entirely within the deformed upper plate. The valley is not seen in the gravity, and shows up in the aeromagnetic data only as an area of suppressed short-wavelength anomalies within a broad east-west belt of low magnetic intensity. To the north of Huntoon Valley, Whisky Flat is a half-graben bound on the west by a major Basin and Range fault. Low magnetic intensities over the topographic expression of the basin contrast with high intensities across the western bounding fault and across the southeastern basin margin. A gravity low occurs near the center of the basin, but there is no density contrast across the bounding fault or across the southern basin margin. On the eastern end of the transect, in the Walker Lane, the Rhodes Salt Marsh is underlain by a basin producing an elliptical gravity low. The aeromagnetic data reveal that the southern margin of this basin is controlled by an east-west strike slip fault that is exposed to the west.