NEOGENE POLYPHASE EXTENSION IN THE NORTHERN BASIN AND RANGE, STILLWATER RANGE, NEVADA

New geologic mapping data from the northern Stillwater Range recognizes a previously unknown low-angle fault separating Oligocene to early Miocene welded tuffs and from the footwall Jurassic and Triassic metamorphic rocks. This fault has low dips of 5 to 25 degrees and fault striae and kinematics indicating top-to-the-west displacement. The fault itself is a 1-2 meter thick zone of scaly clay gouge derived from the footwall and hanging wall rocks. Beds of middle Miocene clastic sedimentary rocks derived from the welded tuffs were deposited angular unconformably over the welded tuffs. Numerous mafic dikes and sills intrude the welded tuff and clastic sedimentary rocks and this section is capped by a 200 meter thick section of basaltic lava flows. The welded tuffs correlate with regional tuffs with ages of 31 to 25 Ma and the capping basalts correlate with middle Miocene basalts. The low-angle fault cuts the welded tuffs, clastic sedimentary rocks and mafic sills. The basalt flows are not in contact with the low-angle fault, but appear to be younger than this fault based on their more consistent and lower dips, whereas the welded tuffs and sediments have highly variable, generally steeper dips. Later sets of faults cut all of the Oligocene and Miocene rocks: northerly-striking high-angle strike-slip faults and ENE-striking normal faults. The ENE-striking faults accommodate a similar direction of extension as the active range front faults on both sides of the Stillwater Range. These observations add to the growing regional dataset of significant middle Miocene extension overprinted by younger extension in north-central Nevada.