TERTIARY FRONTAL STRUCTURES OF THE BROOKIAN OROGEN BENEATH THE ARCTIC COASTAL PLAIN AND OFFSHORE NORTHERN ALASKA

The coastal plain on Alaska’s North Slope overlies a spatially and temporally varying array of largely-concealed Tertiary structures along the 700-km-long Tertiary frontal zone of the Mesozoic and Tertiary Brookian orogen. Although this thrust front has many characteristics that are common to numerous thrust belts worldwide, three aspects of it are unconventional: (1) the Tertiary frontal zone is superimposed on an orogen that appears to have been dominated by Jurassic and Cretaceous shortening; (2) in its eastern and youngest part, basement-involved structures have propagated northward nearly to the thrust front; and (3) locally in NE Alaska, thrusting has propagated across the modern Arctic continental hinge and has deformed the north-facing continental slope and rise.

We have combined a regional network of seismic reflection data with geologic field studies and fission-track analyses to establish a thorough understanding of the variations in structural style and timing of frontal Tertiary structures across the North Slope. The ages of frontal structures and foreland basin strata vary significantly from west to east (Paleogene structures deforming Cretaceous strata in the west; Paleogene to Neogene structures deforming Tertiary strata in the east). Timing of petroleum generation and maturation varies across the North Slope (mid-Cretaceous in the west, Paleogene in the east), but throughout the North Slope, formation of the youngest structural traps post-dates peak generation and migration. The frontal structures in the western North Slope include a broad area of incipient thin-skinned deformation that lies north of a fully developed passive-roof duplex, whereas the eastern North Slope is characterized by a fully developed passive-roof duplex upon which basement-involved thrusting and folding have been superimposed.