This study outlines the structure and tectonic history of the De Long Mountains, the western end of the Brooks Range and the Lisburne Hills, an area extending from the Chukchi Sea to long 156° W., 250 mi eastward. Seven distinct, but roughly coeval, stratigraphic sequences from Upper Devonian to Lower Cretaceous, can be recognized. The Brooks Range and Ivotuk Hills Sequences characterize the Brooks Range and its foothills, although the Brooks Range Sequence is also present in the De Long Mountains. The De Long, Nuka Ridge, and Ipnavik Sequences are confined to the De Long Mountains. The Lisburne Hills Sequence is characteristic of and confined to the Lisburne Hills.

The Brooks Range and De Long Mountains constitute two distinct tectonic provinces; compressional tectonics identify the former, and gravity-slide tectonics, the latter. The two styles of deformation are considered complementary and were developed during the orogeny. Each component tectonic unit, that is, thrust-sheet or slide-sheet, consists of only one stratigraphic sequence, which has enabled the relative allochthonicity of the sheets to be ascertained. The gravity slide-sheets have been dislocated at least 60 to 70 mi in the De Long Mountains.

The orogeny was initiated in the Jurassic Period; the early phase was marked by the emplacement of an ophiolitic suite marginal to the geanticline. Flysch trough subsidence had commenced by earliest Cretaceous time and flanked a geanticline centered on the present Baird Mountains area. Subsidence continued throughout the Cretaceous Period with successive maxima accompanied by migration northward of the axes of subsidence. Compressional overthrusting of the Brooks Range and Ivotuk Hills Sequences developed in association with these sequences that were downwarped with the subsiding troughs. Where downwarping was most prolonged, or most strongly developed, gravity sliding took place, specifically in the region of the present De Long Mountains and Lisburne Hills. These gravity slides form the bulge of the De Long Mountains and are postulated to override the overthrust Brooks Range “spine.” These movements reached a climax in the Late Cretaceous with the final uplift and isostatic adjustments occurring in the Late Cretaceous, early Tertiary. Accompanying this final orogenic phase was the development of large-scale transcurrent faulting and secondary thrusting.

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