APATITE (U-TH)/HE DATING OF THE SUSITNA GLACIER THRUST FAULT REGION, EASTERN ALASKA RANGE, AK

The Denali Fault (DF) is a major intracontinental strike-slip fault system, along which the Alaska Range formed. We focus in the eastern Alaska Range (EAR) on the Susitna Glacier Thrust Fault (SGTF), a NE-trending thrust located south of the DF. The SGTF was first mapped as a result of the 2002 M7.9 Denali Fault Earthquake which initiated on the thrust, then transferred onto the DF travelling east over 300 km. It has been suggested that the SGTF is part of a positive flower structure with the DF facilitating uplift and accommodating NW-directed oblique convergence along the DF.

The objective of this study is to use low temperature thermochronology techniques; apatite (U-Th)/He dating (AHe) and apatite fission track (AFT) thermochronology to determine cooling and exhumation patterns associated with the SGTF. We present AHe results from samples collected in the Susitna Glacier region.

A total of 74 apatite (U-Th)/He single grain and 3 multi-grain analysis were done on 18 samples collected in two vertical profiles and one transect across the region. Samples north of the DF have ages that range from 1.6-2.9 Ma. The SGTF footwall has ages that range from 22-31 Ma, clustering around 24-28 Ma. The ages do not vary with elevation, suggesting rapid cooling. The AHe ages in the SGTF hanging wall are younger, as expected, but also have greater variation ranging from 6.4-27 Ma, strongly clustering around 14-17 Ma.

The ages north of the DF are consistent with other AHe ages nearby and known greater, more recent exhumation. The ages in the SGTF hanging wall are a result of cooling due to erosion after thrusting and suggest fault activity since the mid-Miocene (~16 Ma). In the SGTF footwall, ages may reflect regional cooling due to rock uplift and are consistent with the suggested initial timing of the Yakutat collision (~25 Ma). The SGTF likely formed as a result of the NW-directed oblique convergence from the far-field deformation caused by the Yakutat collision, but the 2002 earthquake is evidence that the fault is still active. Whether fault motion was continual or episodic is undetermined, but the pattern of ages suggests the SGTF was a local structural control on topographic development of the region during the Neogene. AFT data will provide greater constraints on the cooling history of the region and the role of the SGTF in the formation of the EAR.