Tectonic Controls on the Development of Unconformities - the North Slope, Alaska

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The geology of northern Alaska is defined by two complementary, synchronous tectonic events. Opening of the Canada basin breached the south facing Ellesmerian passive margin in the Late Jurassic, cutting off northerly derived sediment and initiating thermal subsidence of the north facing Beaufort passive margin. Uplift of the Brooks Range on thrust faults loaded the lithosphere, creating the Colville trough, a foreland basin more than 10 km deep, and providing the bulk of the sediments that filled it. The northerly advance of the Brooks Range caused the two independent loads to flexurally interfere. The Beaufort passive margin is separated from the foreland by the Barrow Arch, which is, in part, the flexural bulge due to the interaction of these two loading events [Nunn et al., 1987]. A record of this interaction is preserved in the sediments north and south of the Barrow Arch. We have used this stratigraphy, coupled with simple basin modeling, to consider the flexural and stratigraphic effects of the evolving thrust wedge and basin filling. Using a simple sedimentation model and loading on an elastic lithosphere, we have produced two unconformity-bounded sequences without sea level change. The unconformities are produced as a consequence of sedimentation and two increments in the thrust load that cause the flexural bulge to translate laterally, rise, and fall. The generation of unconformities over a flexural bulge by mass redistribution on a purely elastic lithosphere calls into question the use of these unconformities in other yoked systems to document viscoelastic lithospheric rheologies. Even if the lithospheric strength decays significantly on geologic time scales, we may not observe this decay in stratigraphy if the basin filling time is a significant fraction of the relaxation time. The Beaufort passive margin and the thrust belt strike at an oblique angle to each other, causing the independent bulges to constructively interfere in the west and destructively interfere in the east. The advance of the thrust-related bulge beyond the eastern Beaufort shelf has, by flattening the basal decollement, exerted significant control on the thrust wedge of the Brooks Range, allowing the thrust wedge tip to advance beyond the coastline.


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