151-5 Apatite Fission Track Evidence for Exhumation of the Yukon-Tanana Upland during Eocene Oroclinal Bending, Interior Alaska

Session: Chronology of Orogenesis: Unlocking the Timelines of Mountain Building (Posters)

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Abstract:

The late-stage tectonic evolution of the Yukon-Tanana upland (YTU) in interior Alaska is complex and has received limited attention, due in large part to the inaccessibility of the area. The overlap of major tectonic events in the Eocene, including the development of regional strike-slip faults (Tintina and Denali) and oroclinal bending, complicates interpretations of geological datasets from this time. The YTU is bounded by the Tintina fault to the north and the Denali fault to south, both regionally extensive dextral strike-slip faults. The Tintina fault accommodated at least 400 km of displacement between 60-42 Ma. Concurrently, the Alaska orocline, a 44 ˚ ± 11˚ southwest bend in the faults and terranes of Alaska, formed between ~66-44 Ma, as constrained by paleomagnetic data. This bending extends as far north as the Tintina fault and likely coincided with dextral translation of the YTU. An open question remains as to what the upper crustal structure looked like in interior Alaska during this time. For this study, we use the low-temperature cooling history of the YTU to better understand the Eocene tectonic configuration of this terrane in the interior. Existing apatite fission track (AFT) data in the YTU is limited but broadly suggests a tectonic control on cooling. We combine these data with new AFT analyses to test the “luggage carousel” hypothesis in the context of the YTU. As rocks are moved through the Alaska orocline, crustal shortening was accommodated by a system of northeast striking faults. In this model, we expect to see differential AFT ages across faults where shortening and subsequent erosion occurred with progressively younger ages from west to east as rocks were transported in and around the orocline. To test this hypothesis, we collected a series of paired same-elevation samples along two major northeast striking faults in the YTU and two within-block elevation transects. AFT ages along Mt. Harper, a Cretaceous pluton and the highest peak in the YTU, record early Eocene exhumation, while paired samples from opposite sides of the northeast faults yield markedly different ages, suggesting a complex fault geometry and exhumation between ~60-40 Ma. Together, these data support a model in which fault-controlled exhumation occurred during regional tectonic reorganization of interior Alaska.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10353

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