60 MILLION YEARS OF MAGMATISM AND DEEP-CRUSTAL FLOW: DOES THE RUBY-EAST HUMBOLDT CORE COMPLEX PRESERVE THE “ROOF-ZONE” OF A DEEP-CRUSTAL CHANNEL BENEATH THE NEVADAPLANO?

Accumulating evidence supports the concept that the Great Basin of the U.S. formed an orogenic high plateau, the ‘Nevadaplano,’ that persisted from Late Cretaceous well into Cenozoic time. In addition, a variety of geophysical observations and geodynamic models indicate that analogous modern orogenic plateaus such as the Andean Altiplano overlie deep-crustal channels of rheologically weakened, partially melted rock at depths of 20-40 km, with lateral flow serving to modulate both crustal thickness and surface topography. I propose that the Ruby Mountains-East Humboldt Range metamorphic core complex (RM-EHR) of northeastern Nevada represents the exhumed, frozen-in roof zone of a deep-crustal channel active beneath the Late Cretaceous to Paleogene Nevadaplano. Centrally located in the Nevadaplano region, the RM-EHR was exhumed through polyphase Cretaceous to Late Cenozoic rock uplift from paleodepths of 15 - 38 km (400-1000 MPa), increasing northward. Deeper structural levels throughout the RM-EHR are permeated by leucogranites and migmatites ranging in age from Late Cretaceous to Oligocene, with a series of km-scale fold-nappes coinciding with the roof zone of the migmatite complex. These fold-nappes record large-scale lateral deep-crustal flow and dramatic vertical thinning of the pre-deformational stratigraphy before, during, and after fold emplacement. Interpretation of these folds in the context of channel flow is illustrated by the Winchell Lake fold-nappe in the northern East Humboldt Range. The Neoarchean to Neoproterozoic Angel Lake allochthon was first thrust over an upright Neoproterozoic to Mississippian stratigraphy during deep tectonic burial to >35 km associated with kyanite zone metamorphism. Then, dramatic plastic thinning of the underthrust stratigraphy accompanied intracrustal heating, sillimanite zone metamorphism and large-scale partial melting. Large-scale lateral flow from beneath the over-thickened region resulted in lateral extrusion of the fold-nappe with continued plastic attenuation in a general shear regime. Finally, rejuvenation of the channel flow system during Eocene to Oligocene mantle-derived magmatism and renewed crustal melting accompanied the early history of core complex exhumation before its ultimate capture by Miocene detachment faulting.