COMPLEX SUBSURFACE NORMAL FAULT ARCHITECTURE RELATED TO MULTIPHASE EXTENSIONAL EVENTS AT THE BEAUFORT MARGIN, NORTHERN ALASKA

Numerous fault orientations at different stratigraphic levels, resulting from multiple extensional events since the Devonian, has created a structurally complex area on the Beaufort Margin of northern Alaska. This is an oil-producing region with numerous structurally-controlled active fields, such as Prudhoe Bay, Kuparuk, Milne Point, and Colville River units. While there have been many detailed studies of the North Slope of Alaska, published subsurface fault maps are scarce, and the tectonic history has not been fully elucidated. Our analysis of the Storms 3D seismic volume has resulted in the identification of three distinct normal fault orientations that affect different vertical ranges within the stratigraphy. Faulting extends from the Devonian up through to the Paleogene section, with different fault orientations likely caused by a rotation in stress fields associated with successive tectonic events (Ellesmerian, Beaufortian, and Brookian). Additionally, the effects of structural inheritance through reactivation of older faults during younger events is observed and enhances the subsurface complexity. Oblique reactivation of some faults resulted in en echelon segmentation during resultant upward fault propagation, with subsequent linkages across inferred relay ramps to form continuous faults. The evolution of the varying fault sets was facilitated by mechanical interactions between them, creating a complex structural history. Preliminary analysis shows that younger fault systems reactivated and utilized the inherited structure of a Devonian (Ellesmerian) rift system. These included Jurassic to Early Cretaceous rift faults (Beaufortian; 2 phases of faulting) related to the opening of the Canada Basin and Eocene flexure-induced normal faults (Brookian) related to northward resurgence of the eastern Brooks Range fold-and-thrust belt. Based on the changing extension directions through time, a new mechanism for the second phase Beaufortian rifting, which led to the opening of the Canada Basin, is proposed. This hypothesis involves a series of transform faults and oblique spreading along the seafloor spreading axis identified in gravity data.