Thermotectonic evolution of the Alaska Range: Low-temperature thermochronologic constraints.

摘要

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.;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. 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.;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).;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. (Abstract shortened by UMI.).