Style and age of late Oligocene-early Miocene deformation in the southern Stillwater Range, west central Nevada: Paleomagnetism, geochronology, and field relations

Paleomagnetic and geochronologic data combined with geologic mapping;tightl y restrict the timing and character of a late Oligocene to early Miocene ep isode of large magnitude extension in the southern Stillwater Range and adj acent regions of west central Nevada. The southern Stillwater Range was the site of an Oligocene to early Miocene volcanic center comprising (1) 28.3 to 24.3 Ma intracaldera ash flow tuffs, lava flows, and subjacent plutons a ssociated with three calderas, (2) 24.8 to 20.7 Ma postcaldera silicic dike s and domes, and (3) unconformably overlying 15.3 to 13.0 Ma dacite to basa lt lava flows, plugs, and dikes. The caldera-related tuffs, lava flows, and plutons were tilted 60 degrees-70 degrees either west or east during the i nitial period of Cenozoic deformation that accommodated over 100% extension . Directions of remanent magnetization obtained from these extrusive and in trusive, caldera-related rocks are strongly deflected from an expected Mioc ene direction in senses appropriate for their tilt. A mean direction for th ese rocks after tilt correction, however, suggests that they were also affe cted by a moderate (33.4 degrees +/- 11.8 degrees) component of countercloc kwise vertical axis rotation. Paleomagnetic data indicate that the episode of large tilting occurred during emplacement of 24.8 to 20.7 Ma postcaldera dikes and domes. In detail, an apparent decrease in rotation with decreasi ng age of individual, isotopically dated bodies of the postcaldera group in dicates that most tilting occurred between 24.4 and 24.2 Ma. The onset of t ilting immediately following after the final caldera eruptions suggests tha t the magmatism and deformation were linked. Deformation was not driven by magma buoyancy, however, because tilting equally affected the caldera syste ms of different ages, including their plutonic roots. It is more likely tha t regional extension was focused in the southern Stillwater Range due to ma gmatic warming and reduction of tensile strength of the brittle crust. Faul ts that accommodated deformation in the southern Stillwater Range initially dipped steeply and cut deeply to expose more than 9 km of crustal section. The exposed crustal sections are probably rotated blacks above an unexpose d basal detachment that lay near the early Miocene brittle-ductile transiti on.