[S-CG44] Evolution and movement of the crustal surface and application of geo- and thermochronology

[SCG44-02] Around the Bend: Constraining controls on Eocene cooling of the Yukon-Tanana upland, Interior Alaska

★Invited Papers

*Isabella Muller¹, Richard Ketcham¹, Robert Gillis², Evan Twelker² (1.University of Texas at Austin, 2.Alaska Division of Geological & Geophysical Surveys)

Keywords:Thermochronology, Fission Track, Alaska

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 bound to the north by the Tintina fault and to the south by the Denali fault, both regionally extensive dextral strike-slip faults. The Tintina fault was active in the Eocene (60-42 Ma) and estimates of displacement of the YTU along the Tintina fault range from 400-430 km. Additionally, the major strike-slip faults and terranes of southern and interior Alaska are bent 44 degrees to the southwest, and this bending, known as the Alaska orocline, is observed as far north as the Tintina fault. The Alaska orocline formed between 66-44 Ma, as constrained by existing paleomagnetic data. This bending occurred concurrently with dextral displacement of the YTU along the Tintina fault. 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. Here, we test the “luggage carousel” hypothesis (Glen, 2004) in the context of the YTU. As rocks are pushed around the Alaska orocline, crustal shortening is 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 and an overall younging of ages west to east, as terrain is pushed closer to the orocline. We collected a series of same-elevation paired samples along two major northeast striking faults in the YTU and two within-block elevation transects to substantiate this hypothesis. Preliminary AFT results from a set of paired samples collected on either side of the Shaw Creek Fault (central YTU) reveal differential ages (69.1 and 11.3 Ma, respectively), which suggest that these northeast striking faults did accommodate shortening and erosion during and after these major tectonic events.