POST-EOCENE EXTENSION IN THE NORTHERN SHOSHONE RANGE, NEVADA

Tertiary rocks in the northern Shoshone Range locally overlie and intrude highly deformed Lower Paleozoic rocks of the Roberts Mountains allochthon. The larger Tertiary exposures were previously interpreted as subvertical breccia pipes (intrusions), but our new field data indicate that these “pipes” consist of stratified ash-flow tuff and sedimentary rock. They crop out in a WNW-trending belt of exposures at Mt. Lewis-Maysville Summit, Horse Canyon, and Rocky Canyon, and consist of a 35.6 Ma densely welded dacitic ash flow (informally called the tuff of Mt. Lewis) interbedded with sandstones and coarse volcaniclastic deposits (probably debris flows). These units strike N-S and dip 30-70º E—the steeply-dipping compaction foliation in the tuffs was interpreted as flow-foliation in breccia pipes. South of Mill Creek Canyon, another WNW trending series of outcrops previously mapped as undivided welded tuff includes the tuff of Cove Mine (34.2 Ma), Caetano Tuff (33.8 Ma), unit C of the Bates Mountain Tuff (28.8 Ma), and sparse conglomerate. These units also dip 30-50º east, suggesting that their west-dipping contacts with underlying Paleozoic rocks (previously mapped as depositional) are normal faults. Tertiary rocks at Mill Creek and in the Mt. Lewis area were deposited unconformably on Paleozoic basement (none appear to be breccia pipes), and we infer that their present east tilt is due to extension on west-dipping normal faults. Some of these faults may be the northern strands of middle Miocene (ca. 16 Ma) faults that cut and tilted the Caetano caldera ~40° east in the central Shoshone Range (<5 km south of Mill Creek Canyon), but further mapping is necessary to trace the faults through the highly deformed Paleozoic rocks that surround the isolated Tertiary outcrops. The tilted Tertiary rocks described here are all west of the Mt. Lewis - Maysville Summit area and it is unclear if the zone of tilting extends farther east. Nevertheless, significant post-Eocene extensional faulting in the northern Shoshone Range has profound implications both for the structure of the Roberts Mountains allochthon and the exposure of potentially mineralized rocks in its lower plate, both of which were likely east-tilted and repeated by west-dipping faults together with overlying Tertiary rocks.