Extracting tectonic histories from metamorphic rocks in mountain belts : insights from the snake range metamorphic core complex, Nevada, and the Brooks Range, Alaska
Keywords: Geological and Environmental Sciences.

The North American Cordillera is arguably the most well studied example of a long-lived accretionary orogen on Earth, and yet -- despite nearly a century-and-a-half of geologic study -- profoundly fundamental questions persist about its tectonic, metamorphic and magmatic evolution. These questions bear on the initiation and geodynamic drivers for deformation, the reasons for observed along-strike variations, the maximum thickness of the crust achieved during shortening events, the thermal evolution of thickened crust through time, and the timing and geodynamic drivers of the "collapse" of thick crust. Due to variable but often significant tectonic and metamorphic overprinting, studies to address the above questions require analytical techniques and tools that can peer through younger overprinting events. This thesis utilizes a combination of modern and classical geologic methods, including geologic mapping, structural analysis, U-Pb geochronology, metamorphic petrology, 40Ar/39Ar thermochronology, igneous petrology, isotopic geochemistry, and non-classical thermobarometry to peer through metamorphic overprints so as to refine our understanding of several important tectonic developments within the long-term evolution of the Cordillera orogen. This thesis characterizes and attempts to solve some outstanding questions and problems presented by metamorphic rocks exposed in the southern Brooks Range of Alaska and the Snake Range of Nevada which provide insights into the nature of lithospheric-scale processes that accompany continental orogenesis during both shortening and extension. A main focus of this thesis (Chapter 1) has been to reconcile field-based upper-crustal structural reconstructions with metamorphic and igneous processes occurring deeper in the crust in settings where these data sets are in conflict with one another. Specifically, in the northern Snake Range metamorphic core complex, Nevada, and in other similar settings, classical thermobarometry suggests that footwall rocks were buried twice as deep (> 8 kbar) as indicated by structural reconstructions based on geologic mapping (~4 kbar). Decades of disagreement have led to a variety of widely incongruent models for the burial and uplift of these rocks, most of which have since been applied globally. Chapter 1 evaluates the possibility of cryptic structures responsible for burial and exhumation from such depths and presents an independent test of the high pressure estimates with quartz-in-garnet piezothermobarometry and Ti in quartz thermobarometry. The discordance between paleo-depth determined from structural reconstructions and paleo-depth implied by > 7 kbar pressure estimates leads to the suggestion that Cordilleran metamorphic core complexes may have experienced super-lithostatic conditions due to shallow partial melting and increased deviatoric stresses in a region characterized by anomalously high geothermal gradients. A second focus of this thesis (Chapters 2-4) has been to better characterize the geology and tectonic history of the Arctic Alaska terrane, which has a history that links the Cordillera to the Arctic-North Atlantic region, where many terranes in the Canadian and western U.S. Cordillera are believed to have originated. Chapter 2 characterizes the growth of greenschist-facies metamorphic zircon within an extensive south-dipping extensional shear zone developed along the southern margin of the Brooks Range. This shear zone is interpreted as the result of a geodynamic switch in the Pacific subduction system that led to trench retreat / slab rollback just prior to the c. 115 Ma U-Pb ages of zircon in the normal sense shear zone. Chapter 3 utilizes detrital zircon "fingerprints" to better locate an important mid-Paleozoic suture within the Brooks Range of Arctic Alaska that provides an important geologic tie-point for paleogeographic reconstructions. Chapter 4 builds on this improved understanding of the crustal suture and paleogeography of the Arctic to show that establishment of subduction along the Cordilleran continental margin in the Devonian is the likely result of a major plate reorganization event during the latest stages of or post-dating the Caledonian orogeny. Together, these chapters support interpretations that the Cordilleran margin has been defined by cyclical advance and retreat of arc-barckarc systems since at least c. 400 Ma.