THERMOTECTONIC EVOLUTION OF THE ALASKA RANGE: LOW-TEMPERATURE THERMOCHRONOLOGIC CONSTRAINTS

The research presented here seeks to constrain the multiple episodes of uplift and denudation contributing to the formation of the Alaska Range. The approach will be to determine the thermal history (~30 Ma to the present) of the central, and eastern Alaska Range both temporally and spatially. Mechanisms of deformation (resulting in periodic episodes of rock uplift) along an intracontinental strike-slip margin may be constrained by investigating and understanding patterns of exhumation through time throughout the central and eastern Alaska Range. The southern Alaska continental margin is a complex boundary defined by active subduction and accretion of numerous terranes including the Yakutat microplate, the formation of a distinct volcanic arc system, and the development of major intracontinental strike-slip fault systems. The Alaska Range is located ~500 km inland of the active plate margin, along a major continental strike-slip fault system, the Denali Fault system (DFS). The high topography of the central Alaska Range, south of the DFS, occurs where its McKinley strand changes from a mainly east-west orientation to a southwest-northeast orientation. The high topography of the eastern Alaska Range lies to the northwest of the junction between the Hines Creek strand of the DFS and the main DFS strand to the south. I propose two approaches to resolve the thermotectonic history of the mountain range: 1) Direct sampling and quantification of the thermal evolution of the range itself including the central and eastern segments through low-temperature thermochronologic study of basement rocks; and 2) thermochronologic study of sediments of the Nenana Gravel and Usibelli Group in adjacent basins where deposition is related to the erosional unroofing of the Alaska Range. CHAPTER ONE The goal is to constrain the lower temperature, and hence more recent cooling history of the region, and to determine whether there was variation (episodicity) in the exhumation rate since the Late Miocene, as well as to examine the exhumation record across the DFS. The chapter revisits the same sampling set used in the original Fitzgerald et al., (1995), study applying (U-Th)/He apatite (AHe) thermochronology and HeFTy thermal modeling to evaluate the newer techniques against the old. AHe thermochronology was applied to samples in the broader McKinley area to better constrain regional exhumation patterns in relation to the master strand of the Denali Fault and to look for a predicted increase in regional exhumation rates associated with a trend in Late Cenozoic cooling. The general results of the AHe dating and the thermal modeling support the initiation of rapid cooling at ~6 Ma conclusion of the original AFT study. Another goal of this chapter is to better constrain the timing of onset and rate of exhumation of the northern foothills (i.e., terrain north of the DFS), as well as the central Alaska Range. There is also evidence of an earlier, but less significant, period of cooling at ~10 Ma. Denali Fault proximal cooling age trends suggest northward propagation of deformation that may be accommodated along previously unmapped thrust faults. Contrary to previous thermochronology work in Alaska, there is no evidence of an increase in exhumation rates after the introduction of glaciers to the region (~3 Ma) in the current thermochronology data set. The over-all new data supports the earlier conclusion of initiation of rapid exhumation related to a change in oblique convergence of the Southern Alaska Block. HeFTy modeling combined with the vertical transect approach provided additional constraints on the regions thermal history. CHAPTER TWO The southern Alaskan continental margin has been tectonically active since at least the Cretaceous (~65 Ma), including ridge subduction (~65-50 Ma), Yakutat microplate translation (beginning ~30-25 Ma), Yakutat collision (~10 Ma-present) with a decrease in subduction angle to flat slab (~beginning at 10 Ma), microplate rotation (Southern Alaska Block) and Pacific plate motion changes relative to the North American plate at ~25-20, 12-10 and 5-6 Ma. Tectonic events along this southern margin resulted in intracontinental deformation and the development of the Alaska Range along the curved right-lateral strike-slip Denali fault system (DFS). In addition to higher-temperature thermochronologic methods applied to constrain the earlier thermal history of the eastern Alaska Range (EAR), low-temperature thermochronology on basement rocks of the EAR is being applied to constrain the more recent thermal history and formation of the modern day range. Apatite fission-track (AFT) ages from north and south of the DFS located at, and around Nenana Mountain range from ~43-0.9 Ma. A distinct break in slope (the base of an exhumed partial annealing zone) in the Nenana Mountain vertical profile at ~6 Ma indicates the onset of rapid cooling, with an average cooling rate of ~20-30°C/m.y., and exhumation rate \u3e1 km/m.y since that time. Apatite (U-Th)/He (AHe) ages from multi-grain aliquots range from ~40-1 Ma for samples north and south of the DFS. In general, all but three AHe ages are consistent with rapid cooling due to exhumation from ~6-1 Ma. The EAR low-temperature thermochronology data has similarities with that from the western and central Alaska Ranges, namely the onset of an episode of rapid cooling at ~6 Ma; interpreted as rapid exhumation and uplift to form the modern day Alaska Range. We have yet to differentiate changes in cooling rates for AHe and/or AFT ages \u3c6 Ma and hence better constrain the younger exhumation history of the range. Within the chapter I relate the near synchronicity between the onset of rapid cooling and exhumation in the central and eastern Alaska Range (~6 Ma) along the Denali fault, with plate motion change between the Pacific and North American plates. Plate motion change and an increase in relative convergence at ~6 Ma led to a greater-normal component of collision of the Yakutat microplate at the same time, as its more buoyant southern half of continental affinity entered the southern Alaskan subduction zone. This relationship either initiated or enhanced counterclockwise rotation of the Southern Alaska block (lying north of the plate boundary and south of the DFS) changing the partition of strain along the DFS, as well as causing thrusting along faults that splay off the DFS (e.g., Susitna Glacier thrust fault). All of these tectonic components contribute to the transference of stress from the active subduction zone to the DFS, along which the Alaska Range was formed, and where uplift, exhumation and deformation continues today. CHAPTER THREE The aim of this chapter is to constrain multiple periods and patterns of denudation along an intra-continental strike-slip fault system that may then be related to specific tectonic periods of compression accommodated by the northward propagation of basin growth and development (Ruiz et al., 2004; Reiners and Brandon, 2006; Ridgway et al., 2007). The location and subsidence of interior basins located from northwestern Canada to eastern Alaska have been inferred to be controlled by the geometry of the neighboring major convex faults and the state of stress along these faults (i.e. Denali, Tintina and Border Ranges fault systems (Shultz and Aydin, 1990). By evaluating the distribution of locally reduced mean stresses through the use of a boundary element model, the development and subsidence of Tertiary interior basins can be explained by a combination of forces. The two main controlling factors on these basins include the geometry along major convex-shaped right-lateral faults and the state of stress and the change in that stress though time along these faults during the Tertiary based on plate motion vectors. Of note is the conclusion that rotation due to oroclinal bending did not prove a sufficient mechanism by itself to induce right-lateral slip and therefore result in enough mean stress to induce basin subsidence according to the model. Trop, et al. (2007), examined the development of the Wrangell volcanic field (WVF) intra-arc basin during the Miocene. Based on analysis of the Frederika Formation (FF) and FF/WVF lavas, it was concluded that WVF intra-arc basin development overlapped with the cessation of strike-slip basin development and magmatism in the eastern WVF (~10 Ma) and is consistent with a diachronous collision of the Yakutat terrane against the northern Pacific margin. As a result of the uplift of the Alaska Range, sediment shed off the system was deposited to the north of the range within the Neogene Tanana and Cantwell basins. Defined periods of compression and northward propagation of thrust-top basin development can be related to tectonic driving mechanisms operating along the margin (Ridgway et al., 2007). These periods of unroofing should be defined by the decomposition of apatite fission-track cooling ages of samples collected throughout the Tanana basin stratigraphy. The goal of this chapter is to interpret links between depositional and deformational processes (i.e. exhumation cyclicity and/or episodic events) as related to the tectonic evolution of the collisional southern Alaska convergent margin. Deciphering the evolution of mountain belts and plate movements can be accomplished by combining basin analysis with exhumation research