Crustal architecture and tectonic history of the Chukchi Borderland and Arctic Alaskan Brooks Range [electronic resource]
Keywords: Geological Sciences.

This thesis is composed of two independent projects, each of which consists of two distinct studies. The goal of the first project was to study and characterize a series of rocks collected from dredges from the Chukchi Borderland to understand the crustal architecture of this bathymetric high and potentially identify its pre-Cretaceous position prior to rifting and opening of the Amerasia Basin (Arctic Ocean). The second project sought to understand the exhumation history of high-pressure/low-temperature (HP/LT) metasedimentary rocks from the southern Brooks Range of northern Alaska. Better constraints on their time of exhumation was achieved by comparing the timing of deposition of sedimentary rocks in the synorogenic Yukon Koyukuk Basin deposited along the southern side of the Brooks Range to thermochronology results from the adjacent metamorphic core of the orogen. The Chukchi Borderland is a large bathymetric high that extends from the Alaskan Chukchi Shelf into the Amerasia Basin of the Arctic Ocean. Widely interpreted to be underlain by continental crust, the Chukchi Borderland has played a pivotal role in plate reconstructions of the Arctic, despite the fact that its geologic nature, origin and evolution are largely unknown. The first chapter uses microtextural, U-Pb, trace element and Lu-Hf analyses of metamorphic zircons to document a long-lived, Cambrian-Ordovician, granulite facies event that is recorded in gneisses dredged from the Chukchi Borderland. Based on the location and nature of dredged rocks, the second chapter suggests that the Chukchi Borderland comprises at least two distinct terranes, juxtaposed by the proposed Chukchi Borderland fault zone. Low-grade metasedimentary rocks recovered from Northwind Ridge yield a Laurentian detrital zircon signature that is statistically indistinguishable from that of Cambrian age units of the Franklinian passive margin exposed on Ellesmere Island in the Canadian Arctic Archipelago. In contrast, U-Pb and 40Ar/39Ar ages from high-grade gneisses and amphibolites dredged from the Chukchi Plateau suggest an affinity with rock units of the Pearya terrane of Arctic Canada. The new data suggest paleogeographic restoration of the Chukchi Borderland to a position adjacent to Ellesmere and Axel Heiberg islands prior to Amerasia Basin rifting. In northern Alaska, multiple tectonic models have been proposed to account for exhumation of HP/LT epidote-blueschist facies metasedimentary rocks of the southern Brooks Range. The third chapter attempts to constrain the timing of exhumation of the metamorphic core by determining the timing of basin formation and sedimentation in the Yukon Koyukuk Basin that flanks the Brooks Range to the south. Sediment composition, heavy mineral analyses, U-Pb geochronology and Lu-Hf analyses of detrital zircons suggests that the stratigraphy of the basin represents an unroofing sequence, reflecting the erosional striping of the neighboring orogenic highlands perhaps initiated as young as the latest Early Cretaceous (~107 Ma). The fourth chapter discusses the tectonic setting of HP/LT epidote-blueschist facies rocks in the Brooks Range and provides pressure-temperature-time constraints on their evolution using structural measurements and observations, Raman graphite thermometry, phengite barometry and 40Ar/39Ar thermochronology on high-P white mica obtained from two ~N-S transects through the orogen, along the John River and the North Fork of the Koyukuk River. Thermobarometry results indicate a southward increase in temperature and pressures from low-grade greenschist facies metamorphism (~360 -- 400 °C) recorded in rock units of the Doonerak window/Endicott Mountains allochthon to peak conditions (~515 °C and 10-12 kbar) epidote-blueschist facies conditions measured within rocks the Schist Belt. 40Ar/39Ar thermochronology of high-pressure white mica of the Schist Belt constrains the minimum timing for cooling below the partial retention zone for 40Ar in white mica to ~135-130 Ma, most likely during Brookian convergence. Latest Early Cretaceous regional extension along the southern flank of the Brooks Range resulted in exhumation of the HP/LT units from mid-crustal depths by ~107 Ma as recorded in the onset of sedimentation (and arrival of schist fragments and mineral assemblages) in the adjacent Yukon Koyukuk Basin. Crustal duplexing during latest Cretaceous -- Oligocene time led to ~15 km of uplift, development of the Doonerak antiform and regional southward tilting of the southern Brooks Range.