MULTIPLE EPISODES OF EXTENSION ACROSS THE EAST RANGE, NEVADA, A TERTIARY EXTENSIONAL TRANSITION ZONE IN THE NORTHERN BASIN AND RANGE PROVINCE

The East Range in north-central Nevada is situated between two fairly well documented Miocene Basin-and-Range extensional domains, a central Nevada domain characterized by large magnitude extension centered around 17-15 Ma and a northwestern Nevada domain characterized by later extension (<12 Ma) of considerably smaller total magnitude. In the southern East Range, Oligocene plutons yield new zircon U-Pb SHRIMP ages of 31.4 ± 0.4 and 33.0 ± 0.3 Ma. New apatite fission-track and (U-Th)/He data from these plutons record rapid uplift along range-bounding normal faults between 16 - 15 Ma. Extension continued at slower rates until ~ 10 Ma. Cumulative slip along these brittle structures has exhumed more than 6 km of crust, suggesting mean slip rates of at least ~ 0.6-0.9 km/my over ~ 6 my. Quaternary fault scarps and geothermal and seismic activity indicate deformation has continued to the present-day. Geologic mapping of uplifted Tertiary lacustrine deposits and volcanic flows atop the East Range and in the nearby Sou Hills record multiple phases of uplift and basin formation closely timed with widespread bimodal volcanism. Tuffaceous siltstone, sandstone, and volcaniclastic conglomerate (in depositional contact with pre-Cenozoic footwall rocks) dip east between 35-45° and are unconformably overlain by moderately east-dipping (10-15°) capping basalt flows. Existing K-Ar ages of nearby basalt flows suggest volcanism between 17-6 Ma; the basalt flows in the East Range likely fall within this age-range. New ⁴⁰Ar/³⁹Ar dating (in progress) of these flows and interbedded tuffs will further constrain the timing of faulting and volcanism.

These new data complement our knowledge of highly variable and heterogeneous extension; faulting began in the East Range area concurrently with other ranges in the central Nevada domain but apparently continued ~ 5 - 6 my after much extension in central Nevada ceased. The younger faulting history in the East Range coincides with the onset of extension in the northwestern domain. Considered in a regional context, the East Range defines a substantial transition in the geophysical character of the crust, most notably, changes in crustal thickness and heatflow, suggesting that it represents a fundamental crustal-scale boundary in the northern Basin and Range crust.