STRUCTURAL GEOLOGY AND EVOLUTION OF A DUPLEX SOUTHWEST OF GALBRAITH LAKE, NORTH-CENTRAL BROOKS RANGE, ALASKA

The fold-and-thrust belt of the north-central Brooks Range of Alaska is included within the Endicott Mountains allochthon (EMA), a part of the ancient passive continental margin that was thrust northward a minimum of 88 km over the North Slope parautochthon (NSP). Thrust-related folds located along the leading edge of the EMA are interpreted to be detachment folds that formed in the competent Lisburne Limestone over a regional decollement in the incompetent Kayak Shale. North vergent imbricate thrusts ramped up from a floor thrust in the Kayak Shale, truncated the pre-existing detachment folds in the Lisburne Limestone and connected with a roof thrust in the overlying incompetent Siksikpuk Formation thereby creating a duplex informally termed the Lisburne duplex. Current models of duplex structures typically assume that folds within the thrust-bound horses form synchronous with or subsequent to thrust faulting and thus limit the fold style to be either fault-propagation or fault-bend folds. This study indicates that fold styles within duplexes can also include folds that pre-date truncation by thrust faults, such as detachment folds.

A second significant result is that the exposed and reconstructed structural geometries of the Lisburne duplex reveal a floor and a roof thrust that slope northward toward the foreland. Typically, foreland sloping duplexes are attributed to twice as much displacement on the imbricate thrusts as the initial spacing between the thrusts. However, displacement on imbricate thrusts in the study area is less than half the distance of the initial spacing between the thrusts and therefore does not account for the foreland slope of the roof thrust. Instead, analysis of the sectional geometry of the duplex indicates that the slope can be attributed to the entire duplex being tilted toward the foreland. This interpretation is consistent with previous studies that focused on duplex geometry within the EMA. Two explanations of the foreland slope of the Lisburne duplex are: A) the underlying NSP is imbricated thus tilting the overlying EMA or B) a wedge of competent Kanayut Conglomerate is emplaced under the Lisburne duplex in the EMA.