Seismic profiling constraints on the evolution of the Brooks Range, Arctic Alaska from an integrated reflection/refraction survey
Wissinger, Eugene Scott
Doctor of Philosophy
An integrated vertical incidence to wide-angle seismic data set has been used to develop a consistent migrated seismic reflection image and seismic velocity model of the Brooks Range fold and thrust belt in north central Alaska. 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 sec 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 reflected and refracted travel time data produced a velocity model consistent with CMP images of the Brooks Range, Bouguer gravity data, and seismic velocities determined from petrophysical data. Maximum crustal thickness in the range is 49 km, in an asymmetric root located under the EMA. At the root, an offset in lower crustal reflectivity is observed along with two deep zones of reflections north of the root. These reflections are interpreted as a Moho offset of some 5 km, resulting from subduction of the Brooks Range lower crust northward beneath the North Slope. The seismic reflection data, velocity data, and surface geologic constraints are used to identify the boundaries of major structural assemblages in the Brooks Range and restore 3 interpretations of the range to their pre-Jurassic configurations. Minimum shortening estimates derived above the basal decollement for the 3 models approximate 500-600 km of Mesozoic-Recent shortening. The amount of sub-decollement shortening may be as little as that now observed, 50-65 km, or may be comparable to the 500-600 km observed for upper-intermediate crustal rocks. 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.