HETEROGENEOUS EXHUMATION OF MID-CRUSTAL ROCKS ALONG THE HAYES RESTRAINING BEND OF THE CENTRAL DENALI FAULT

The right-lateral transpressive Denali fault has been a principal feature in Alaskan tectonics for over 60 million years and offers the chance to study exhumation mechanisms of rocks from below the brittle-ductile boundary. Areas of exhumation along the length of the fault are heterogeneously distributed, with at least 11 km of exhumation recorded along the north side of the Hayes restraining bend in the eastern Alaska Range since 25-27 Ma. The concentration of youngest ⁴⁰Ar/³⁹Ar cooling ages (as young as 15 Ma for mica and 6 Ma for K-feldspar) at the apex of the bend appears to support the idea that rocks to the north of the fault have been relatively fixed with respect to the bend since the Miocene and have experienced significant exhumation over that time. 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. The kinematic vorticity number (Wk) for each sample was determined using two different methods: 1) the Vorticity Diagram Method, which relates the length of the maximum (λ₁) and minimum (λ₃) principal strain axes ratio (R_XZ) to the orientation of λ₁ with respect to foliation, and 2) the LPO method, which relates R_XZ to the angle (β) between the flow plane (as determined by EBSD analysis) and the foliation.

In general, rocks in this area display subvertical to steeply N-dipping foliations with obliquely plunging mineral lineations. Most lineations, however, trend to the W or NW, while the orientation of strain ellipsoids determined for these samples shows the long strain axis trending 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 are consistent with strike-slip motion on the Denali fault and indicate that some component of simple shear is also preserved.