Crustal Extension in the Southern Snake Range and Vicinity, Nevada-Utah: An Integrated Geological and Geophysical Study.

This study of late Cenozoic crustal extension in the southern Snake Range metamorphic core complex, the southern Snack and the Hamlin Valleys utilizes industry-acquired seismic data, two petroleum exploration boreholes, gravity data and surface geology of the adjacent ranges. Isotopic age determinations from outcropping igneous rocks were used to date the structural and tectonic events. The dominant structural elements of the southern Snake and the Hamlin Valleys are relatively young ($\u3c$17 Ma), and are north-south striking, planar normal faults and associated half-grabens. The valleys are separated by an east-west striking accommodation zone and evolved independently by temporal and spatial interference and superposition of half-grabens. Progressive thickening of basin-fill sequences towards the bounding normal faults within each half-graben suggest semi-continuous slip along the bounding faults; interruptions are represented by angular unconformities which define three seismic sequences above the volcanic unit. K-Ar age determinations of 34 and 26 Ma from volcanic rocks in the Needles Range are correlated with the bottom and top of the volcanic unit; this volcanism was followed by inception of basin formation recorded by the oldest valley-fill sequence. The geometry of the oldest valley-fill sequence indicates that it continued to the west but is now eroded as the Snake Range core complex was uplifted. The intermediate valley-fill sequence records a major stage of tectonism synchronous with displacement along the Snake Range Decollement in the southern Snake Range between 17-18 Ma. Deposition of the youngest valley-fill sequence began at 10 Ma synchronously with a 45$\sp\circ$ change in the regional extension direction. A late-stage (Late Cenozoic) uplift was recognized in the south-east corner of the Snake Range core complex as a top lap in the seismic data and as a previously recognized unconformity in the outcrop separating angular, older Tertiary conglomerates from flat-lying younger conglomerates on top. Late-stage, local uplifts, as results of disruption of a previously bent elastic plate by normal faults and of differential deformation of various components of a heterogeneous crust in response to bending stress, superpose broad, regional uplifts and explain the total uplift in the Snake Range metamorphic core complex