Crustal anisotropy associated with fossilized transpression along the Denali Fault, northern Canadian Cordillera from receiver functions and ambient noise cross-correlations analysis

English Abstract:

The Denali Fault is a major, ~1200 km-long, continental strike-slip fault that participates to the plate boundary system of western North America since the Early Cenozoic. In southwest Yukon, it accommodated ~400 km of dextral displacement in a late Cretaceous–Eocene transpression phase during which allochthonous terranes were accreted to the North America margin. Smaller strike-slip and thrust faults mark a 50 – 100 km wide tectonic corridor between the St. Elias Mountains and the central Yukon plateau, with the Denali Fault along its eastern edge. To examine the crustal structure and strain localization, we analyze receiver functions (RF) combined with resulting ambient noise cross-correlations on a network of twelve broadband seismic stations deployed in the Denali Fault region.

RFs display a strong P-to-S conversion within the first 3 seconds, which systematically varies with back-azimuth. Stacking and velocity inversion of RF according the two complementary back-azimuth ranges show strong Vs anisotropy (> 10%) at mid crustal depths (15 – 20 km) for a subset of stations within 5 – 30 km of the Denali Fault and inside its tectonic corridor. Other stations, further away or in a different geological setting, show weaker (< 5%) anisotropy. In all cases, this Vs anisotropy occurs in a low-velocity zone with the slow velocity axis perpendicular to the Denali Fault trend. We analyze ambient noise cross-correlations for a period range of 4 – 15 s to extract average phase velocities between station couples. Preliminary analysis of phase velocities in the 4 – 10 s range (mid-crustal depth) suggests an anisotropy similar to the RF results, with fast directions parallel to the Denali Fault trend. Combining RFs and noise cross-correlation will better constrain the depth and width extensions of this high-anisotropy zone. These results suggest the ∼400 km-transpression phase was accommodated in a shear zone at least 50–60 km wide in the midcrust. Lack of clear anisotropy in the lowermost crust may relate to complex deformation within a detachment layer (orogenic float model).