Crustal and Upper-Mantle Anisotropy Related to Fossilized Transpression Fabric along the Denali Fault, Northern Canadian Cordillera

Research areas:
Year:
2014
Authors:
Journal:
BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA
Volume:
104
Number:
4
Pages:
1964-1975
Month:
August
ISSN:
0037-1106
Abstract:
We analyze crustal and upper-mantle structure near the Denali fault (northern Canadian Cordillera) using 11 broadband seismic stations. Receiver functions at five stations within 5-30 km of the fault trace display a strong P-to-S conversion within the midcrust (10-25 km depth) that systematically varies with back azimuth. Stacking and velocity inversion along complementary northwest-southeast and north-east- southwest back-azimuth ranges yield a strong anisotropy (> 10%) in the midcrust, with complex crustal anisotropic behavior (at least two layers) close to the Denali and Duke River faults junction. Anisotropy occurs in a low-velocity zone with the fast axis parallel to the Denali fault trend. Three additional stations close to the Denali fault (15-30 km), as well as two stations further away (> 100 km) show no significant crustal anisotropy. Shear-wave (SKS) splitting analysis indicates similar upper-mantle anisotropy, with a fast axis parallel to the trend of the Denali fault for all stations, except for station WHY located 125 km away. We relate this crustal and upper-mantle anisotropy to fossilized structural and mineral fabrics due to Eocene transpression on the Denali fault. These results suggest the main transpression phase (similar to 400 km displacement) was accommodated in a shear zone at least 50-60 km (less than 125 km) wide in the midcrust and upper mantle. Lack of clear anisotropy in the lowermost crust may relate to complex deformation within a detachment layer (orogenic float model).