Seismic images of crustal duplexing and continental subduction in the Brooks Range

Abstract

We have interpreted an integrated vertical incidence to wide‐angle seismic data set to develop a consistent migrated seismic reflection image and seismic velocity model of the Brooks Range fold and thrust belt in north central Alaska. The common midpoint (CMP) reflection data image the principal structures comprising the Brooks Range: the Endicott Mountains allochthon (EMA), the crustal scale Doonerak duplex, the master detachment, a 1.0–1.5 s thick zone of lower crustal reflectivity just above the crust‐mantle boundary, and a complex crustal root. The master detachment separates the crust into units which have been uplifted and deformed in the fold and thrust belt from those which have not. Least squares inversion of both reflection and refraction travel time data produced a velocity model consistent with the CMP image of the Brooks Range as well as with the Bouguer gravity data. The different layers comprising the seismic velocity model correlate well with the principal structural elements identified in the seismic reflection data, and seismic velocities in the model compare favorably to petrophysical data from Brooks Range rock samples. Maximum crustal thickness in the Brooks Range is 49 km, at an asymmetric root located under the EMA. At the root we observe an offset in the lower crustal reflectivity and two deep zones of reflections north of the root. We interpret these as a Moho offset of some 5 km near the range front, which in our favored interpretation resulted from subduction of the Brooks Range lower crust northward beneath the North Slope. A mantle reflective zone which we interpret as the subducted lower crust can be traced to depths as great as 65 km. Above this zone at considerably shallower depth is the original North Slope Moho. Proximity of the continental subduction zone to the crustal scale Doonerak duplex suggests that the development of the fold and thrust belt has been at least partially controlled by the lower crust/mantle subduction.