THREE-DIMENSIONAL FLOW AND LOCALIZED RAPID EXHUMATION ASSOCIATED WITH THE HAYES RESTRAINING BEND OF THE EASTERN DENALI FAULT

The Denali Fault is a dextral-oblique strike slip fault system that has been active since ca. 50 Ma. Both ⁴⁰Ar/³⁹Ar K-feldspar and mica cooling ages show greatest exhumation and exhumation rates within 1.5 km of the active fault on the north side of the gentle Hayes restraining bend in the eastern Alaska Range. Rapid exhumation in this region has been ongoing since ca. 30 Ma. This pattern of maximum exhumation near the apex of the bend supports the idea that rocks to the north of the fault have been relatively fixed with respect to the bend since the Oligocene with ages as young as 40 Ma for hornblende, 15 Ma for mica, and 6 Ma for K-feldspar.

Three-dimensional kinematic data will illustrate the mechanism for rapid exhumation occurring at the apex of the Hayes restraining bend. Material flow within this region needs to have a significant vertical component in order to explain the observed thermal history and macrostructures. Vorticity analyses provide an opportunity to better understand the kinematics of exhumation along this section of the Denali Fault by disentangling the relative proportions of pure and simple shear. Elevated pure shear percentages can accommodate the vertical flow needed to achieve the observed rapid exhumation.

Our first analyses were on orthogneiss and quartz-rich metasedimentary rock from sites around the bend. Rocks from the most deeply exhumed areas display subvertical to steeply N-dipping foliations subparallel to the Denali Fault with W to NW trending, moderately plunging mineral lineations. Whenever possible, we estimated the kinematic vorticity number (Wk) using two independent methods: the grain shape-based Vorticity Diagram method and the EBSD-based Lattice Preferred Orientation (LPO) method. The results of the vorticity analyses indicates that deformation in this section of the fault is generally dominated by pure shear (Wk < 0.71), but the results varied depending on protolith and location in the bend. Our current study focuses on just the latest Cretaceous – Eocene feldspar- and quartz-rich orthogneisses in the area of maximum exhumation. These plutonic rocks record only the youngest deformation. The LPOs within the orthogneisses indicate a wide range of deformation temperatures and their vorticity analyses will help to further elucidate the mechanism of exhumation along the Denali Fault.