Rift-related Normal Faulting Guided by Structural Inheritance During Multiphase Extensional Events at the Beaufort Margin, Northern Alaska

A 3-D seismic analysis of Late Jurassic to Early Cretaceous normal faults in Arctic Alaska, associated with the Beaufortian rifting event that led to the subsequent opening of the Canada Basin, shows that fault growth was guided by the inherited structure of a Devonian to Mississippian extensional system (the Ellesmerian rifting). Later Eocene normal faulting associated with crustal flexure in response to thrust-sheet loading in the eastern Brooks Range was similarly guided by the inherited Beaufortian structure despite distinctly different fault orientations. The fault system architectures that ultimately developed have played an important role in defining the complex structure of producing oilfields in the North Slope of Alaska. Our fault mapping in the Storms 3-D seismic volume has revealed three distinct normal fault orientations spanning disparate stratigraphic ranges, reflecting three distinct ages of fault activity. Faulting extends throughout the imaged stratigraphy, with different fault orientations likely caused by a change/rotation in stress fields associated with successive tectonic events as well as the effects of structural inheritance evidenced by the reactivation of older faults during younger events. Locations, orientations, and throw patterns of Late Jurassic faults imply they formed in identical orientations to subjacent Ellesmerian faults, with both upward and downward fault growth from nucleation points in the Triassic section. Oblique reactivation of these Late Jurassic faults is manifested by en echelon segmentation during continued upward fault propagation in the Early Cretaceous, implying two phases of Beaufortian rifting, the second of which evolved to hyperextension and seafloor spreading in the Canada Basin. These faults were seemingly reactivated again in the Oligocene, resulting in upward growth through the Cretaceous and Paleogene sections that are host to recently discovered stratigraphic plays in the so-called Brookian section. The faults beneath the North Slope of Alaska thus provide a compelling example of the importance of structural inheritance in fault system development as well as creating challenges for oilfield characterization and development.