THERMOCHRONOLOGIC CONSTRAINTS ON THE EXHUMATION HISTORY OF THE SUSITNA GLACIER THRUST FAULT, EASTERN ALASKA RANGE

The Susitna Glacier Thrust Fault (SGTF), lies south of the right-lateral intracontinental Denali Fault, with these faults intersecting in the Susitna Glacier region. The SGTF was unknown until the 2002 M7.9 Denali Fault earthquake. Rupture initiated on this thrust, before propagating east along the Denali Fault and the Totschunda Fault. The SGTF is likely part of a flower structure forming part of the eastern Alaska Range, accommodating a portion of the normal component of transpression while strike-slip motion is largely taken up on the Denali Fault. Following the earthquake, the thrust was mapped extensively but the history and significance of the fault was poorly known. Low temperature thermochronology data collected over a significant elevation range, was applied to the footwall and hanging wall of the thrust to resolve the thermal history of the region, constrain the onset of faulting and better understand the role of the thrust in the formation of the eastern Alaska Range. U/Pb zircon ages constrain the timing of intrusion of granitic plutons sampled in the footwall (~57 Ma) and hanging wall (~98 Ma). ⁴⁰Ar/³⁹Ar biotite and feldspar age data constrain the onset of exhumation-related cooling in the Eocene. Together with AFT data and inverse thermal models, this data shows samples from both sides of the SGTF cooled as one coherent block during the Eocene and early Oligocene. However apatite (U-Th)/He ages from either side of the SGTF are different, ~28-24 Ma in the footwall and ~17-14 Ma from the hanging wall. This age difference, plus modeled T-t envelopes constrain it was during the Mid-Miocene when movement on the SGTF was initiated and the two sides of the fault were moving as independent blocks. The SGTF is active today, playing an important role in forming uplifted topography south of the Denali fault. It is a good example of strain partition in a transpressive regime.