REINTERPRETATION OF THE TACT SEISMIC PROFILE AND THE CRUSTAL ARCHITECTURE OF THE BROOKS RANGE

Recent investigations indicate that the Brooks Range was formed as a consequence of four distinct orogenic events. These include: (1) north-directed arc-continent collision in the Jurassic and Neocomian that produced thin-skinned deformation, emplacement of far-traveled allochthons, and high-pressure metamorphism; (2) regional extension and down-to-the-south normal faulting in the hinterland dated at 103-96 Ma and coeval deposition of voluminous clastic strata in the foreland and hinterland; (3) thin- to thick-skinned deformation due to retro-arc thrusting emanating from southern Alaska in the Paleocene and Eocene, and (4) thrusting in the northeastern Brooks Range as a consequence of terrane accretion in southern Alaska in the Oligocene to Recent. The effects of this deformational history can be distinguished in the seismic reflection and refraction data collected by the north-south Trans-Alaska Crustal Transect (TACT) in 1989-1990. Jurassic to Neocomian structures are interpreted to form the hummocky reflections at shallow levels (<5 km) under the Brooks Range north of Mt. Doonerak, probably due to antiformal stacks of the Lisburne Group carbonate rocks in the Endicott Mountains allochthon and in the underlying autochthon. Mid-Cretaceous extensional fabrics in the southern Brooks Range are associated with a southward thickening wedge-like region of flat to gently dipping reflections in the middle and upper crust south of Mt. Doonerak. Below and north of this wedge is a 20-km-thick south-dipping panel of interlacing reflections whose base climbs from a depth of at least 30 km in the southern Brooks Range to about 10 km in the northern foothills. This panel cores the 70- and 25-Ma-age Mt. Doonerak antiform and pinches out in mid-Cretaceous foreland basin strata, suggesting it is a crustal-scale duplex formed in the early Tertiary and reactivated in the Oligocene. The Moho attains its deepest level in this area (nearly 50 km), descending southward from the North Slope and northward from the metamorphic hinterland (both 32 km). This configuration is interpreted to be the result of the mid-Cretaceous ductile thinning of the North Slope autochthon, increasing southward beneath the hinterland, and of Tertiary downbowing of the North Slope autochthon beneath the load of the Mt. Doonerak crustal-scale duplex.