Date of Award

December 2018

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor(s)

Paul G. Fitzgerald

Keywords

Alaska, Apatite, Exhumation, Fission Track, Thermochronology, (U-Th)/He

Subject Categories

Physical Sciences and Mathematics

Abstract

The Denali Fault is an active intracontinental structure located ~500 km north of the southern Alaskan plate boundary. In 2002, a M 7.9 earthquake initiated on a previously unmapped thrust fault subsidiary to the Denali Fault (Susitna Glacier Thrust) and propagated ~227 km eastward along the east-central segment of the Denali Fault and the Totschunda Fault. The objective of this thesis is to evaluate how fault geometry and rheologic strength control spatial-temporal patterns of exhumation along large-scale transpressive structures. Apatite fission track (AFT) thermochronology and apatite (U-Th)/He (AHe) dating are used on a combination of bedrock and cobble samples to constrain cooling initiated by erosional exhumation, and make inferences about the vertical component of faulting activity for thrust faults subsidiary to the Denali Fault.

AFT analysis for bedrock samples collected south of the east-central Denali Fault reveals rapid Miocene exhumation (young AFT ages) west of the Gakona Glacier, and more complex cooling (old AFT ages) east of the Gakona Glacier. Using an estimated Holocene slip rate for the east-central Denali Fault, the location of the older eastern samples are constrained to the Mentasta releasing bend during a well-documented exhumation event in the Late Miocene. This suggests that fault geometry plays a vital role in controlling exhumation patterns.

Inverse thermal modeling of a combination of bedrock and cobble samples resolves episodes of cooling at 170-150, 100-80, 60-50, 35-25, 12-10 and ca. 6 Ma. These cooling trends correlate with changes in plate boundary processes at the North America-Pacific Plate boundary that control tectonic events in south-central Alaska. Miocene changes in plate motion of the Yakutat and Pacific plates correlate with the most dominant cooling episodes. AHe data further supports a subsequent Pleistocene acceleration of the cooling event that begins in the Late Miocene. The Pleistocene acceleration in cooling is interpreted to correspond with efficient glacial erosion during the Pleistocene glacial maximum.

Gaining a better understanding of how obliquely transpressive structures operate is important because the east-central Denali Fault is transected by the trans-Alaskan pipeline, and because the Denali Fault serves as an analog for similar structures with large population densities, such as the San Andreas and North Anatolian faults.

Access

Open Access

Available for download on Thursday, January 14, 2021

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