FRONTAL DETACHMENT FOLDS ON ALASKA'S NORTH SLOPE

Tertiary detachment folds form a distinct structural element at the front of the Brooks Range orogen in northern Alaska. The Brooks Range is a dominantly north-vergent orogen that underwent hundreds of km of Late Jurassic to Aptian (dominantly Neocomian) shortening, followed by renewed shortening and uplift during the Tertiary. In the Brooks Range foothills of the central North Slope, frontal Neocomian structures are strongly overprinted by Tertiary faulting and folding. North of the Neocomian deformation front, a complex Tertiary passive-roof duplex (PRD) is developed primarily in Lower Cretaceous foreland basin strata above a regional detachment in the Jurassic to Lower Cretaceous Kingak Shale. North of this PRD, a frontal detachment-fold domain continues 100 km northward to the ultimate deformation front beneath the southern part of the Arctic coastal plain.

The frontal detachment folds are developed in the Cretaceous and Tertiary Brookian megasequence. In most cases (for example, Umiat anticline), the detachment folds are localized above local thrust ramps that splay from the regional Kingak Shale detachment upsection into Brookian strata. Thrust-ramp displacements are transferred up into the lower part of the Brookian section, where they are accommodated by local structural thickening of mudstone-rich turbidites, above which shallow marine to nonmarine upper Brookian sandstones are folded into prominent anticlines. These anticlines, each with 0.5–2.0 km of structural relief, are developed at spacings of 7–12 km, and each fold accommodates 4–6 km of shortening. North of these prominent folds, a higher detachment level is established within the Brookian section, carrying the last increment of northward displacement. This displacement feeds into one or more subtle, broad low-amplitude detachment folds, each of which has 100–300 m of structural relief and accommodates 0.3–2.5 km of shortening. The northernmost of these folds, (the “frontal tectonic welt”), is pinned at the deformation front.

The distinctive frontal detachment-fold domain is limited to the central and western parts of the North Slope, where the folding most likely occurred during Paleocene to Eocene time, based on local apatite-fission-track uplift ages and on regional deformational patterns.