Paper No. 1
Presentation Time: 9:00 AM-6:00 PM


BENOWITZ, Jeff1, BEMIS, Sean P.2, O'SULLIVAN, Paul B.3, LAYER, Paul W.4, FITZGERALD, Paul G.5 and PERRY, Stephanie5, (1)Geophysical Institute, University of Alaska Fairbanks, P.O. Box 755780, Fairbanks, AK 99775, (2)Earth & Environmental Sciences, University of Kentucky, Lexington, KY 40506, (3)Apatite to Zircon, Inc, 1075 Matson Rd, Viola, ID 83872-9709, (4)College of Natural Science and Mathematics, Univ of Alaska Fairbanks, PO 755780, Fairbanks, AK 99775, (5)Department of Earth Sciences, Syracuse University, 204 Heroy Geology Laboratory, Syracuse, NY 13244,

Geometric complexities (restraining/releasing bends, step-overs) are a ubiquitous feature of major strike-slip faults; however, controls on the formation and evolution of these features are rare. The Mount McKinley restraining bend of the Denali fault in Alaska exists as part of a strain-partitioned transpressional system and offers a natural laboratory for investigating how restraining bends may evolve. Positioned ~500 km inboard from the plate margin, Mount McKinley is south of the Denali fault on the inside of an abrupt ~17 degree bend. To the north, on the outside of the bend, the highest peak is Peter's Dome (3,221 m), and elevations rapidly decrease away from the Denali fault to a basin <15 km north. This topographic asymmetry suggests a strong structural control on local exhumation patterns and that this restraining bend has been a primary control on regional orogenic development for at least several million years.

We apply 40Ar/39Ar K-spar, AFT, and AHe dating and thermal modeling to samples collected on both sides of the Mount McKinley restraining bend combined with geomorphological observations to constrain both the restraining bend initiation and evolution. Preliminary overall thermochronologic results indicate prolonged rapid Neogene exhumation proximal to the Denali fault starting at ~28 Ma and continuing through to the present day. AFT cooling ages from samples on both sides of the Denali fault, range from ~8 Ma to ~3 Ma. Track length distributions and thermal models indicate all samples cooled quickly at the time of AFT closure. This preliminary data suggest a pattern of AFT ages younging to the west. One implication of this trend is that the Mount McKinley restraining bend may be migrating to the west at a long term rate of 2 km/Ma. This rate is less than half of the modern Denali fault slip rate. The available data does not yet have sufficient spatial coverage to discern when the restraining bend formed or if there was a change in strain accommodation in the region correlated with the previously documented ~6 Ma exhumation event at Mount McKinley. Integrating structural analysis of active faults and additional thermochronologic work will allow us to develop robust models for the evolution of the Mount McKinley restraining bend and to better define if the bend is indeed migrating to the west or is fixed to either side.