The Toyuk thrust zone in the Endicott Mountains allochthon of the Brooks Range is a regional tectonic boundary. It has been interpreted previously as a large-displacement fault that emplaced the underlying Upper Devonian Hunt Fork Shale northward over the Upper Devonian to Mississippian (?) Kanayut Conglomerate. However, this University of Alaska Master's thesis study interprets the Toyuk thrust to be a duplex thrust zone that does not require major displacement.
This study addresses a segment of the Toyuk thrust zone along the West Fork of the Atigun River, just west of where it crosses the Dalton Highway north of Atigun Pass. In this area the thrust zone is defined by the intersection of the erosional surface with several imbricate thrust faults. These faults have cut detachment folds formed in the Kanayut between the incompetent underlying Hunt Fork Shale and the overlying Mississippian Kayak Shale. Within the area, each thrust fault places Kanayut over Kanayut or Kayak, except for the uppermost (southernmost) fault that places Hunt Fork over the Kanayut. However, Hunt, Fork is present in the hanging wall of these faults up-plunge to the east of the area. The structural geometry and the fact that each fault-bounded panel is limited to just these three units supports the interpretation that the imbricate thrust faults define a duplex.
A series of parallel cross sections across the area suggests two end-member models of duplex geometry to account for the exposed structural configuration. In both models, a floor thrust in Hunt Fork underlies a duplex in Kanayut. In model one, each linking thrust has roughly the same relatively small displacement and the roof thrust, in Kayak, is overlain by a normal stratigraphic succession. In model two, a much greater displacement on the uppermost (southern most) fault would have emplaced the Hunt Fork completely over all of the other fault-bounded panels in the area, thus forming the roof of the duplex. Discrimination between these models is precluded by subsequent erosion, which has removed the roof of the duplex.
Apatite fission-track dating of seven samples (by PaulO'Sullivan, La Trobe University, Australia) indicates cooling in the area during the Paleocene at approximately 60 Ma. Samples that cooled at this time overlie samples with older ages across the upper two faults in the area, which suggests that at least those two faults were active at that time. The data for the samples with older ages (79-69 Ma) are consistent with interpretation that these are "mixed ages" that reflect both the 60 Ma cooling and earlier cooling during the Cretaceous at approximately 100 Ma. Cooling events of these ages have been documented previously in the northern Brooks Range, but these samples are the first to record the 60 Ma event so far from the range front. This indicates that the formation and growth of the Toyuk duplex accommodated structural thickening within the orogenic wedge coeval with deformation at the range front and in the foothills.
Alaska Geological Society , 1998.