EVOLUTION OF THE NORTHERN SNAKE RANGE METAMORPHIC CORE COMPLEX, NEVADA: INTEGRATING FIELD, STRUCTURE, GEOCHRONOLOGIC, AND THERMOCHRONOLOGIC DATA SETS

After more than 40 years of research, disagreement continues about the mechanism by which metamorphic core complexes (Mccs) within the western U.S. formed; the northern Snake Range metamorphic core complex (NSR), Nevada has been central to this debate. In models proposed for the formation of Mccs, such as those based on the geometry & kinematics of extensional deformation, and two-dimensional (2D) thermal mechanical, three-dimensional (3D) analogue, and energy constrained models, there is a close temporal and spatial link between upper crustal normal faulting, lower crustal ductile extension and flow, and detachment faulting. To test these models in the NSR we completed new U-Pb geochronology on zircon from deformed and undeformed rhyolite dike swarms emplaced into ductilely thinned and horizontally stretched lower plate rocks. The new geochronology provides tight bounds on the timing of ductile extension at between 38.2 ± 0.3 Ma and 22.50 ± 0.36 Ma. The older age bracket is from the northern dike swarm, which was emplaced in the northwestern part of the range pretectonic to syntectonic with ductile extension. The younger age bracket is from the Silver Creek dike swarm, which was emplaced in the southern part of the range after ductile extensional deformation. These ages, with associated field observations and petrography, combined with published data on the geology, kinematics, and finite strain of extension, moderate and low temperature thermochronology on lower plate rocks, and age and faulting histories of Cenozoic sedimentary basins adjacent to the NSR, are interpreted as recording an episode of localized upper crustal brittle extension during the Eocene that drove upward ductile extensional flow of hot middle crustal rocks from beneath the NSR detachment soon after, or simultaneous with, emplacement of the oldest dike swarm. Exhumation of the lower plate continued in a rolling hinge/isostatic rebound style; the western part of the lower plate was exhumed first and the eastern part extended ductilely either continuously or episodically until the early Miocene when the post-tectonic youngest dike swarm was emplaced. Major brittle slip along the eastern part of the NSR detachment and along high angle normal faults exhumed the lower plate during middle Miocene.