K-FELDSPAR THERMOCHRONOLOGIC ANALYSIS OF THE TORDRILLOS AND MOUNT MCKINLEY REGIONS OF THE ALASKA RANGE: EOCENE EXHUMATION

We apply K-feldspar thermochronology (Tc of ~350-150 C) to bedrock samples from the Tordrillo Mountains (western Alaska Range) and the Mt. McKinley area (central Alaska Range) to investigate exhumation history and to examine temporal and structural interpretations of thrust faulting in the Tordrillo Mountains. Previous apatite fission track (AFT; Tc ~120 C) studies from the Tordrillos (Haeussler et al., 2008) implied a complex age to elevation relationship that led to an interpretation of two unmapped thrust faults, and proposed cooling events at ~23 Ma and ~6 Ma. We subsequently performed K-feldspar thermochronologic analysis on the same sample set, and our data constrain the same fault blocks that were previously proposed by the AFT work. Thus, the high-temperature thermochronometric approach provided an independent confirmation of the structural interpretation based on the AFT work.

K-feldspar thermochronology also shows evidence of widespread exhumation-related cooling between ~50 Ma and ~40 Ma. Eocene deposits from Cook Inlet (West Foreland), Matanuska Valley (Wishbone) and the Tanana Basin (Upper Cantwell) all provide evidence of Eocene high-energy depositional environments. These spatially scattered (~600 km apart) formations consist, in part, of conglomerates and sandstones and imply region-wide deformation and topographic development. Our cooling ages are not only coeval with the depositional interval of these formations, but are also the same as an exhumation event on Mt. Logan in the Wrangell-St. Elias Mountains (O’Sullivan and Currie, 1996). We infer these cooling events and clastic deposition are related to the same far-field tectonic processes. A consideration of the regional tectonic history indicates the most likely cause is the re-establishment of a normal convergent margin thermal regime after passage of a slab window in Paleocene-Eocene time. However, how stresses are transferred inboard is unclear, but changes in relative plate motions may have been an important factor and strike-slip faulting during this time.

References:

Haeussler, P.J., O’Sullivan, P., Berger, A.L., and Spotila, J.A., 2008, in Active Tectonics and Seismic Potential of Alaska, 269-285

O’Sullivan, P., and Currie, L., 1996, Earth Planet. Sci. Lett., v. 144, 251-261