RIGHT-STEPPING NORTH-SOUTH SUB-BASINS LINKING THE DIXIE VALLEY AND FAIRVIEW VALLEY STRUCTURAL BASINS, NEVADA, USA, IDENTIFIED FROM NEW LAND GRAVITY MEASUREMENTS

In 1954, a sequence of major earthquakes produced a widely-spaced, north-south trending, left-stepping surface rupture pattern distributed over the Transition Zone (TZ) between the Dixie Valley Structural Basin (DVSB) and Fairview Valley Structural Basin (FVSB). Although the seismological and neotectonic aspects of this event are well studied, particularly in the DVSB, the previous published exploration geophysics data (specifically gravity) across the TZ are too sparse and inconsistent to reveal the basement structural relief.

We present results and structural interpretations for a recently completed 300 station gravity survey covering 1,000 square km, which was designed to investigate the structural linkage between the DVSB and FVSB and the associated left-stepping 1954 Dixie Valley – Fair View Peak rupture pattern. The complete Bouguer gravity anomaly map shows what appears to be a remarkably clear picture of the basement structure in the TZ and adjoining DVSB and FVSB. Depths are interpreted assuming and Tertiary alluvium and volcanic basin fill with Mesozoic metasedimentary basement. Results show, within the study, the DVSB and FVSB are more than 2 km deep and have approximately northwestern trending terminations at their respective southern and northern extremes. These terminations effectively define the boundaries of the TZ, which measures 20 km north to south. Structurally projecting from the DVSB's southwest and FVSB's northeast termination quadrants are a roughly mirror image pair of north-south trending, right-stepping grabens (2 to 4 km wide and 500-700 m deep) which attenuate in relief toward a central overlap area. This overlap area is located 7 km east-northeast of where Elevenmile Canyon exits the Stillwater Range. The right-stepping grabens appear to represent a zone of maximum extension across the TZ, which also aligns with a southward projection of the maximum extension reported by other studies on the western structural boundary of the DVSB. The area defined by the graben pair's overlap and maximum depth attenuation aligns on a northwestern trend that we suggest may represent a crustal shear separating the two major basin systems. The relation of these grabens to the neotectonic surface faulting across the TZ is not clear at this time, but is under study.