EVOLUTION OF CENOZOIC EXTENSIONAL NORMAL FAULTING IN THE TOBIN RANGE, NEVADA; AND IMPLICATIONS FOR THE BASIN AND RANGE PROVINCE

The Tobin Range of north-central Nevada lies near the transition between more extended terrane (>50%) to the south and east, and less extended terrane to the north and west. Geologic mapping, Ar/Ar dating and rock geochemistry establish the Cenozoic volcanic stratigraphy, geometry and timing of normal faults, and magnitude of extension near Golconda Canyon.

Three phases of normal faulting are identified. The earliest consists of N- and NE-striking and west-dipping faults coeval with ~33 Ma andesitic volcanism that produced <20ºE tilting. The second phase consisted of a major NW-striking fault and associated NE-striking faults that dip west. These faults moved between late Oligocene and middle Miocene (25 to 14 Ma) and produced syntectonic basin-fill landslide mega-breccia and sedimentary rocks. The last phase extends from the middle Miocene to present. North-striking, west-dipping (and a few east-dipping) faults include the active Tobin Range fault and accommodated ~25º – 30ºE tilting on the east side of the range. These three phases of normal faulting resulted in ~25º – 30ºE Cenozoic tilt. The fault timing varies, with the western part of the range being dominated by early Oligocene to early Miocene tectonism, and the eastern part dominated by middle Miocene and younger tectonism. Palinspastic restorations of cross-sections suggest the southern Tobin Range has undergone ~50% east-west crustal extension since 34 Ma. Normal faulting began in northeastern Nevada at ~38 to 35 Ma, synchronous with dacitic magmatism and Carlin-type gold mineralization. Extension was non-uniform in NE Nevada, with areas of >100% extension alternating with little-extended zones. The Tobin Range lies immediately west of this zone, and was not affected by pre-34 Ma normal faulting. Similar to areas farther west (Yerington) Oligocene ignimbrites derived from central Nevada filled paleocanyons >1 km deep. We suggest canyons result from central Nevada magmatism and extension that uplifted western Nevada in the Oligocene, similar to the modern Sierra Nevada-Great Basin boundary. Late Oligocene to middle Miocene normal faulting and extension are moderate near and west of the Tobin Range. The Miocene (~15 Ma) zones of >100% extension are localized in the Walker Lane belt of strike-slip faulting and andesitic magmatism south of 40°N latitude.