LATE MIOCENE TO PLIOCENE LEFT-LATERAL REACTIVATION OF MIDDLE MIOCENE RIGHT-LATERAL FAULTS IN THE MINA DEFLECTION – A DETAILED STRATIGRAPHIC, STRUCTURAL, FAULT-KINEMATIC, AND GEOCHRONOLOGICAL INVESTIGATION IN THE COALDALE JUNCTION AREA, WALKER LANE, NEVADA

The Mina Deflection is a long-lived structural discontinuity in the western portion of the northern Basin and Range Province and the central Walker Lane belt that has controlled the structural grain of contractional and extensional deformation throughout much of the Phanerozoic. In middle Miocene times, the Mina Deflection appears to have acted as a right-lateral accommodation zone separating east-dipping normal fault systems in the north (e.g., Wassuk Range) from west-dipping normal fault systems in the south (e.g., White Mountains). During late Miocene and Pliocene Walker Lane transcurrent deformation, NE-trending normal faults and rhomboidal pull-apart basins (e.g., Columbus Salt Marsh) as well as left-lateral faults (e.g., Coaldale Fault) dominated the deformation field of the Mina Deflection. Detailed mapping and stratigraphic and structural analysis by the University of Kansas field camp reveals that the Coaldale Junction area, NE of the Silver Peak Range, exposes a thick (>2 km) east- to northeast-tilted Miocene stratigraphic succession resting unconformably on Oligo-Miocene welded rhyolitic ashflow tuffs. The Miocene succession consists of a lower portion composed of yellowish lacustrine siltstones and carbonaceous shales with abundant rhyolitic ashflow tuffs, gradually transitioning into an upper portion composed of greenish channelized fluvial sandstones and conglomerates and lahar deposits. Structurally, the tilted Miocene sedimentary succession is faulted and offset by major E-W trending faults that have previously been associated with the left-lateral Coaldale Fault. Detailed structural mapping, however, shows that the fault system is characterized by ~1.5 km of dextral displacement in the eastern portion of the mapping area, while fault kinematic and small-scale fault analysis in the western portion shows that these E-W trending right-lateral strike-slip fault systems have been reactivated as a left-lateral fault (Coaldale fault), offsetting 3 Ma basalts. The two main left-lateral fault strands are linked by a major right-stepping normal fault, omitting significant portions of the lower lacustrine stratigraphic section and clearly preserving a releasing-bend geometry consistent with the original right-lateral nature of the fault, prior to left-lateral reactivation.