STRAIN PATH AND THERMAL HISTORY OF QUARTZITE IN THE DEEP CRUST OF ANDEAN-STYLE OROGENIC PLATEAUS: A CASE STUDY FROM THE WOOD HILLS, NEVADA

Analysis of microstructure and quartz crystallographic preferred orientations (CPOs) in quartzite from the Wood Hills of Elko County, Nevada elucidates the strain path and thermal history of part of the Ruby Mountains-East Humboldt Range-Wood Hills metamorphic core complex. This area is thought to represent the deeper crust of an early Cenozoic orogenic plateau — “the Nevadaplano” — and so by analogy it may shed light on tectonic processes in the middle-crust of modern day orogenic plateaus. The quartz CPOs resulted from a polyphase deformational history including Late Cretaceous to Paleocene shortening locally overprinted by Eocene to Oligocene extension. The earlier deformation is characterized by a south to north increase in grain size accompanying a transition in the dominant recrystallization mechanism from subgrain rotation to grain boundary migration; preliminary analysis of crystallographic misorientations across subgrain boundaries indicate that these changes are also associated with a transition in the most active quartz slip systems from predominantly prism <a> in the southeast to an increase in activity on the rhomb <a> systems to the northwest. Taken together, these relationships indicate an increase in metamorphic grade and structural depth toward the north. Quartz c-axis pole figures also transition from symmetric crossed girdles in the southern Wood Hills to top-SE asymmetric crossed and single girdles in the north. The increasing contribution from simple shear with depth suggests the possibility that the overlying thrust system broadens into a more diffuse shear zone as it penetrates deeper structural levels. Finally, overprinting the older deformation in the northwestern corner of the Wood Hills is an intense top-NW mylonitize zone associated with core complex exhumation that shows significant grain size reduction and top-NW shear inferred from both microstructure shear sense indicators and asymmetric quartz CPOs.