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Low-temperature thermochronological constraints on the timing and mechanisms behind the multi-stage exhumation history of the Alaska Range - Abstract
The Alaska Range is located ~500 km inland from the North American-Pacific plate boundary and is intimately related to the transpressive Denali fault system. Understanding the tectonics of southern Alaska and the Alaska Range offers insight into geologic processes inland of a subduction zone, such as the effects of plate motion changes, far-field effects, the partition of strain along a curved inland boundary, terrane accretion, and subduction of a buoyant slab (Yakutat microplate).
A previous study that combined apatite fission-track data from the central Alaska Range along with regional geological constraints indicated that the onset of uplift and denudation that resulted in the formation of the range was ~5–6 Ma and was related to a change in relative plate motion between the North American and Pacific plates (Fitzgerald, et al, 1995). By implication it was suggested that the formation of the entire Alaska Range was related to this plate motion change and thus occurred at about the same time.
This presentation summarizes past and recent preliminary low-temperature thermochronology (more work is planned) and paleoclimate results that point to a diverse exhumation pattern along the trend of the Alaska Range. Fission track data from the Tordillo Mountains in the western Alaska Range, suggests a period of rapid cooling at ~25 Ma with a latter period (~6 Ma) at about the same time as the central Alaska Range. In the eastern Alaska Range, 40Ar/39Ar multi-domain K-spar data from the Hayes Range show evidence of the exhumation of a proto eastern Alaska Range between 19 Ma and 9 Ma, whereas preliminary apatite fission track and (U-Th)/He data from the same region suggest rapid exhumation (~1.5–2 mm/yr) post ~1.4 Ma. These ages are consistent with regional changes in paleoclimate and depositional environments.
Recent passive seismic imaging of the buoyant Yakutat slab suggests that collision and subsequent underplating of the Yakutat slab likely contributed to the initiation of uplift within the central Alaska Range (Eberhardt-Phillips et al., 2006). The buoyant Yakutat slab has not been detected under most of the eastern Alaska Range (east of 148°W) (Rossi, et al, 2006). This, coupled with other results on: 1) Crustal thickness variations across major faults in Alaska 2) The nature and timing of the Yakutat microplate collision 3) Underplating of thicker crust 4) A re-evaluation of the timing of plate motion change (~8 Ma) based on global plate circuit reconstruction (Stock and Atwater, 1998) 5) The effects of climate change on mountain erosion and corresponding relief generating isostatic response, all point toward a combination of tectonic mechanisms and climate responsible for the variations in uplift and exhumation along the Alaska Range that is far more spatially, and temporally variable than previously thought.
Exhumation of the Tordillo Mountains (western Alaska Range) and the proto eastern Alaska Range may be related to the initial collision of the Yakutat microplate. Uplift and exhumation of the neo Tordillo Mountains and the central Alaska Range before the neo eastern Alaska Range may be related to the underplating of the Yakutat microplate, which has shifted northeast through time as a result of changes in plate motion leading to a pattern of “drifting exhumation.” Impingement of thick (Wrangellian) crust against thinner (Yukon-Tanana) crust north of the Hines Creek fault branch of the Denali Fault system, as the southern Alaska block moved NW as a result of the Yakutat microplate subduction may play a role in the development of the current high topography expressed in the eastern Alaska Range. Pliocene cooling in interior Alaska, independent of local tectonics that caused increased glacial erosion in valleys may have also have resulted in increased peak height in the Alaska Range due to the isostatic response.
Acknowledgments and Associated Footnotes
Copyright © 2014 by the Alaska Geological Society