TWO PERIODS OF MIOCENE TO CONTEMPORARY FLATTENING STRAIN DURING DISPLACEMENT ON A LOW-ANGLE DETACHMENT AND ON SUPERPOSED CURVED HIGH-ANGLE FAULTS, VOLCANIC HILLS, SOUTHWEST NEVADA

The Eastern California Shear Zone (ECSZ) and central Walker Lane (CWL) form part of a tectonic boundary zone separating the Sierra Nevada and the central Great Basin. The ECSZ and CWL are misaligned and were kinematically linked by the Sliver Peak-Lone Mountain (SPLM) detachment from 12 to 4 Ma and subsequently by the active curved-array of east-northeast and north-northwest striking faults of the Mina deflection. Pliocene to contemporary transtension (constrictional strain) dominates within the ECSZ and CWL but in the Mina deflection, the geometry of individual curved fault systems results in areas of finite flattening. South of the Mina deflection in northern Fish Lake Valley, faults in the Volcanic Hills formed in the hanging wall of the curved Emigrant Peak fault zone, which truncates the SPLM detachment exposed in the footwall in the Silver Peak Range to the east. Volcanic and sedimentary rocks in the Volcanic Hills were deposited in localized basins active during displacement on the SLPM detachment. Geologic mapping, kinematic analysis, and fault-slip inversion reveal that the high-angle faults have a progressive history involving two simultaneous extension directions (flattening). A total of 392 slip lineations with shear-sense determinations were collected on all orientations of high-angle faults in the array. Slip superposition documents four extension directions separated into two successive periods, each characterized by mutually cross-cutting slip relations indicating two directions of simultaneous extension (flattening strain). The youngest system of flattening strain has a primary extension direction of west-northwest and a secondary direction of north-northeast, consistent with contemporary earthquake focal mechanisms and strain-gauge results. The older history shows simultaneous north-northwest (primary) and east-northeast (secondary) extension and is interpreted to reflect the state of strain in the upper-plate of the SLPM extensional complex.