Heterogeneous Exhumation of Mid-crustal Rocks along the Hayes Restraining Bend of the Central Denali Fault

Laura Tait, University of California, Davis
Sarah Roeske, University of California, Davis
Matty Mookerjee, Sonoma State University

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. Both 40Ar/39Ar K-feldspar and mica cooling ages show greatest exhumation close to the fault on the north side of the Hayes restraining bend in the Eastern Alaska Range and indicate ongoing exhumation in this region since 25-27 Ma. Here, deformed orthogneisses and metasedimentary country rocks display generally subvertical to steeply N-dipping foliations with obliquely plunging lineations. The concentration of youngest 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. 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 (RXZ) to the orientation of λ1 with respect to foliation, and 2) the LPO method, which relates RXZ to the angle (β) between the flow plane (as determined by EBSD analysis) and the foliation. C-axis pole figures generated from EBSD data yield predominantly single girdle patterns, which are suitable for the LPO method, although several display unusual patterns that may require use of the a-axes to determine Wk. Preliminary results suggest that deformation in this section of the fault is dominated by pure shear (Wk < 0.71), especially in areas of greatest exhumation at the apex of the restraining bend.


Development of tectonic microstructures