PARTITIONING OF IMBRICATE THRUSTING AND INVERSION IN THE CENTRAL BROOKS RANGE AND FOOTHILLS, NORTHERN ALASKA

MCDONOUGH, M.R., BALKWILL, H., BEVER, J., and LUKASIK, J., Petro-Canada Oil & Gas, 150 6th Ave SW, Calgary, AB T2P 3E3, Canada, [email protected]

Initial shortening in the Central Brooks Range is recorded by deposition of the Berriasian to Hauterivian Okpikruak and Barremian to Albian Fortress Mountain formations in the Colville Foreland Basin, which have an intervening angular unconformity. Seismic reflections from the Okpikruak display a north-tapering wedge geometry, which is interpreted as a foreland basin wedge. Many lines of evidence suggest Neocomian thrusting impacted the Central Brooks Range, prior to the Paleocene to Eocene principal phase of shortening. The frontal part of the Central Brooks Range is divided into domains of distinct structural style: 1) thin-skinned imbricate thrusting; and 2) deep seated thrust structures, some of which are clearly inversion structures. Imbricate thrust zones have an important detachment in Lisburne carbonates, while deep seated thrusts have an important detachment in Kayak shales, but ultimately root into Kekiktuk/Kanayut or deeper levels. Both structural styles are characterized by thrusts that are bounded by lateral ramps on the west, and terminate at thrust tips on their eastern ends. The hanging walls plunge steeply westward and terminate against lateral ramps, while thrust tips are associated with gentle easterly plunges. Prominent lateral ramp systems are controlled by Devono-Mississippian extensional faults that are at a high angle to the strike of the Brooks Range. In the Inner Foothills (south of Tuktu Escarpment), the Sukkak structure forms a large doubly plunging overturned anticline cored by Lisburne strata that was carried on a thrust with limited throw (<100m). Minimal throw and frontal/lateral ramp relations suggest that this is an inversion structure that was displaced by a footwall shortcut fault. Okpikruak in the footwall contains anomalous tectonic blocks of Lisburne carbonate (with the same CAI as the hanging wall) that were probably calved from the fault zone during displacement, indicating a Hauterivian time of inversion. The Inigok#1 well penetrated an inverted Kekiktuk depocenter beneath the Colville Basin, providing an analog for inversion of the Sukkak structure, and for Kanayut depocentres exposed in the Brooks Range. Balanced structure profiles developed from seismic and outcrop studies constrain shortening in the Foothills and Frontal Brooks Range to about 60-80 km.