TRANSPRESSION AND EXHUMATION OF MID-CRUSTAL ROCKS ALONG THE DENALI FAULT SYSTEM

The dextral transpressive Denali fault system provides a rare opportunity to discern the mechanism for exhumation from mid-crustal levels along an active continental strike-slip fault. The deepest exhumation is closest to the fault along the N-side of the broad Mount Hayes restraining bend in the eastern Alaska Range. Here the rocks have a variably developed, locally strong subvertical to steeply N-dipping foliation with low to moderate plunging lineations recording dextral and oblique-thrust slip. The fabrics exist in orthogneisses with young protoliths (~38 Ma), as well as in older country rocks and ~70-50 Ma protolith-age orthogneisses. Muscovite ⁴⁰Ar/³⁹Ar cooling ages from rocks along the N-side of the fault within 1.5 km of the active fault trace have the youngest ages near the apex of the bend, with ages as young as ~ 15 Ma both E and W of the bend. The “bull’s-eye” pattern of youngest cooling ages indicates that the rocks along the N-side have remained relatively fixed with respect to the bend since exhumation via dextral oblique convergence began around 25-27 Ma (based on multi-mineral ⁴⁰Ar/³⁹Ar ages).

In order to more fully understand the mechanisms for deep exhumation along this fault system, we performed a vorticity analysis on samples of orthogneiss as well as the metasedimentary country rock. Vorticity number (Wk) estimates were calculated via two independent methods, 1) the “Vorticity Diagram Method”: relating the length of the maximum (λ₁) and minimum (λ₃) principal strain axes ratio (R_xz) to the orientation of λ₁ with respect to the foliation, and 2) the “LPO Method”: where R_xz is related to the angle (β) between flow plane (determined via EBSD analysis) and the penetrative foliation. The orthogneisses produced far more consistent results with respect to these two methods. The samples yielded a mean vorticity number of 0.51, which suggests that the deformation is pure shear dominated (66%). The pure shear component of the deformation resulted in shortening of the deforming zone perpendicular to the zone boundaries and near-vertical extrusion. From the strain magnitudes and vorticity numbers, we estimate approximately 18% shortening across the zone. The parallelism of the ductile fabrics with the active fault trace supports the crystallographic evidence that exhumation occurred by near-vertical uplift.