Tilted middle Tertiary ash-flow calderas and subjacent granitic plutons, southern Stillwater Range, Nevada: Cross sections of an Oligocene igneous center

Abstract

Steeply tilted late Oligocene caldera systems in the Stillwater caldera complex record a number of unusual features including extreme thickness of caldera-related deposits (>4–5.5 km), lack of conclusive evidence for structural doming of the calderas despite intrusion of cogenetic plutonic rocks, and preservation of vertical compositional zoning in the plutonic rocks. The Stillwater caldera complex comprises three partly overlapping ash-flow calderas and subjacent plutonic rocks that were steeply tilted during early Miocene extension. The calderas and cogenetic plutonic rocks are exposed in cross section over an unusually large depth range of ∼10 km. The Job Canyon caldera, the oldest (ca. 29–28 Ma) caldera, consists of two structural blocks. The north block consists of 0–1500 m of precollapse intermediate composition lava flows and breccias overlain by 2000 m of intracaldera rhyolite ash-flow tuff locally interbedded with thick sequences of caldera-collapse breccia, overlain in turn by 2500 m of intermediate lava flows and minor lacustrine and fluvial sedimentary rocks. The south block consists of thinner sequences (total thickness ≤2500 m) of intermediate lava flows and ash-flow tuff with local interbedded collapse breccia. The north part of the caldera is intruded by the cogenetic IXL pluton, which is vertically zoned downward from granodiorite to quartz monzodiorite. The 25 to 23 Ma Poco Canyon and Elevenmile Canyon calderas and underlying Freeman Creek pluton overlap in time and space with each other. Caldera-related deposits in the Poco Canyon caldera comprise two cooling units of crystal-rich rhyolite and high-silica rhyolite tuff (tuff of Poco Canyon) separated by a unit of crystal-poor high-silica rhyolite tuff and caldera-collapse breccia (megabreccia of Government Trail Canyon) and by a thick unit of crystal-rich rhyolite and trachydacite ash-flow tuff related to the Elevenmile Canyon caldera (tuff of Elevenmile Canyon). Total thickness of caldera-related deposits is locally >4500 m in the Poco Canyon caldera. The Elevenmile Canyon caldera is filled by >3000 m of ash-flow tuff (tuff of Elevenmile Canyon), locally overlain by a unit of water-laid rhyolite tuff and sedimentary rocks and by a locally thick unit of rhyolite ash-flow tuff (tuff of Lee Canyon). Total thickness of caldera-related deposits in the Elevenmile Canyon caldera is >4000 m. The composite Freeman Creek pluton intrudes the central and north parts of these calderas and consists of an older granodiorite porphyry phase probably related to the Elevenmile Canyon caldera and a younger granite phase probably related to the Poco Canyon caldera. A 7-km-long, texturally zoned rhyolite-porphyry to granite-porphyry dike intruded the north edges of these calderas and is probably a ring-fracture dike related to the Poco Canyon caldera. Caldera collapse occurred mostly along subvertical ring-fracture faults that penetrated to depths of >5 km and were repeatedly active during eruption of ash-flow tuffs. Subsidiary growth faults with relatively minor displacement are present in caldera-related deposits in the interior of the Job Canyon caldera. A fault separating the two structural blocks of the Job Canyon caldera later served as the north walls of both the Poco Canyon and the Elevenmile Canyon calderas. A second, long-active fault formed the south margin of the Job Canyon caldera, separated the Poco Canyon and Elevenmile Canyon calderas into blocks with greatly different amounts of caldera-related deposits, and later was reactivated during early Miocene extension. The calderas collapsed as large piston-like blocks, and there is no evidence for chaotic collapse. Preserved parts of caldera floors are relatively flat surfaces several kilometers across.