RUBY MOUNTAINS CORE COMPLEX AS A GUIDE TO EOCENE DEEP CRUSTAL STRUCTURE IN THE ELKO-CARLIN REGION, NEVADA

The deep-crustal rocks exposed in the Ruby-East Humboldt metamorphic core complex, northeastern Nevada, provide a guide for reconstructing Eocene crustal structure ~50 km to the west near the Carlin trend of Eocene gold deposits. The deep-crustal rocks, in the footwall of a west-dipping normal-sense shear system, may have underlain the Piñon and Adobe ranges ~50 km west before Tertiary extension. Eocene lakes formed on the hanging wall during an early phase of the extensional fault system. The magnitude and timing of Paleogene extension remain indistinct, but dikes and tilt axes in the upper crust indicate that spreading was east-west to northwest-southeast, perpendicular to a Paleozoic and Mesozoic orogen which the spreading overprinted. High geothermal gradients associated with Eocene or older crustal thinning may have contributed to hydrothermal circulation in the upper crust. Late Eocene eruptions, upper-crustal dike intrusion, and gold mineralization approximately coincided temporally with deep intrusion of Eocene sills of granite and quartz diorite and shallower intrusion of the Harrison Pass pluton into the core-complex rocks. Stacked Mesozoic nappes of metamorphosed Paleozoic and Precambrian rocks in the core complex lay at least 13–20 km deep in Eocene time based on published geobarometry. Only in the northern part of the complex had the presently exposed rocks been buried even deeper in the late Mesozoic, to >30 km depths, before losing part of their cover by Eocene time. Nappes in the core plunge northward beneath the originally thicker Mesozoic tectonic cover in the north part of the core complex. Mesozoic nappes and tectonic wedging likely occupied the thickened mid-level crustal section between the deep-crustal core-complex intrusions and nappes and the overlying upper crust. These structures as well as the subsequent large-displacement Cenozoic extensional faulting and flow in the deep crust would be expected to blur the expression of any ancient regional structures that could correlate with mineral belts.