TWO-STAGE MODEL FOR EXHUMATION OF THE RUBY MOUNTAINS - EAST HUMBOLDT METAMORPHIC CORE COMPLEX, NORTHEASTERN NEVADA

The Ruby Mountains – East Humboldt metamorphic core complex exposes igneous and metasedimentary rocks unroofed from Mesozoic depths >30 km, but studies of these deeply-buried crystalline rocks and of the surrounding Tertiary volcanic and sedimentary rocks each yield very different pictures of the role played by crustal extension in exhumation of the core complex. Geochronologic and geobarometric data from the metamorphic rocks indicate that exhumation began as early as the Cretaceous, with final exhumation taking place during Eocene to Miocene (dominantly late Oligocene) slip on the west-dipping normal fault that bounds the core complex to the west. In contrast, studies the of age, composition, and distribution of Eocene ash-flow tuffs and Miocene basin fill suggest minor regional extension in the Eocene, no Oligocene extension, and rapid slip on the west-dipping fault beginning ca. 17 Ma and over by 10-12 Ma. These contradictory findings can be reconciled if the core complex was exhumed in two distinct phases. In the Eocene to early Oligocene phase, intense magmatism was accompanied by diapiric rise, partial melting, and intense local attenuation of deeply-buried sedimentary rocks to depths as shallow as 12-15 km. Exhumation of these rocks was accompanied by only minor deformation of the brittle crust and no crustal thinning, requiring that it be compensated in the middle and lower crust—perhaps by downward flow on the margins of the “dome,” although there is no direct evidence for this. After a period of tectonic quiescence from the late Oligocene to the early Miocene, the igneous and metamorphic rocks were rapidly exhumed to the surface during middle Miocene slip on the west-dipping fault. Slip on this fault was accompanied by formation of a deep hanging-wall basin and minor bimodal volcanism. In this two-stage scenario, magmatism and extension both played key roles in formation of the core complex, but these processes were separated widely in time and had different causes—magmatism occurred during the mid-Tertiary “ignimbrite flareup,” and extension followed ~20 m.y. later during regional Miocene Basin and Range extension.