Heterogeneous Exhumation of Mid-crustal Rocks along the Hayes Restraining Bend of the Central Denali Fault
Abstract
The Denali fault has been continuously active as a dextral-oblique thrust fault since 25-27 Ma, which has resulted locally in rocks being exhumed from below the brittle-ductile transition. The youngest 40Ar/39Ar cooling ages (as young as 15 Ma for mica and 6 Ma for K-feldspar) occur in the eastern Alaska Range, within a narrow zone on the north side of the Hayes restraining bend. This pattern of maximum exhumation (> 11 km) 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 early Miocene, and the strain history should record exhumation in a dextral transpressive system. Vorticity analyses provide an opportunity to better understand the mechanisms for exhumation along this section of the Denali fault and were carried out on samples of both orthogneiss and quartz-rich metasedimentary rock from sites around the bend. The kinematic vorticity number (Wk) for each sample was determined using the Vorticity Diagram method and the EBSD-based Lattice Preferred Orientation method. Most of the rocks in this area display subvertical to steeply N-dipping foliations subparallel to the Denali fault with W to NW trending, moderately plunging mineral lineations. In contrast, the long axis of the strain ellipsoids determined for these samples trend overall to the NE. This suggests that lineations in this region are not indicative of the maximum stretching direction recorded by the strain ellipsoid. Preliminary vorticity results suggest that there are no systematic differences around the bend or between rock types and that deformation in this section of the fault is generally dominated by pure shear (Wk < 0.71). C-axis pole figures generated from EBSD yield predominantly single girdle patterns, which indicate that some component of simple shear is also preserved. Girdles from samples east of the bend are consistent with dextral motion on the Denali fault, but girdles from samples to the west appear to show either no or very slight sinistral slip sense. This may further support the idea that rocks in this area have not moved significantly with respect to the bend and do record primarily pure shear.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.T21D2851T
- Keywords:
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- 8012 High strain deformation zones;
- STRUCTURAL GEOLOGYDE: 8030 Microstructures;
- STRUCTURAL GEOLOGYDE: 8159 Rheology: crust and lithosphere;
- TECTONOPHYSICS