Plate kinematics of the Denali fault system

Abstract

Two segments of the Denali fault system, the McKinley strand west of the Delta River and the Dalton–Shakwak fault east of the Delta River, have nearly perfect small circle geometries. This geometry permits interpretation of the right-lateral slip along these faults in terms of rigid plate tectonics. Their poles of rotation are in the Gulf of Alaska at 59.63°N, 147.38°W and 50.38°N, 154.02°W respectively. A model in which there has been simultaneous motion on both faults since 38 Ma ago predicts a third fault at their juncture which must act as a plate boundary with northwesterly thrust motion in this time interval. The Broxson Gulch thrust, which extends from near the Susitna River to its termination at the Delta River, meets these requirements. Paleozoic and Mesozoic volcanics, as well as Oligocene or younger strata, are thrust beneath sillimanite schists along this fault, and major pre-Tertiary fold structures are truncated by it. Given the direction of tectonic transport on all three faults and a displacement of 38 km on the McKinley strand, the minimum displacements on the Broxson Gulch and the Denali (Dalton–Shakwak) faults in the last 38 Ma are approximately 54 and 90 km respectively. The previously correlated Maclaren and Ruby Range metamorphic belts, however, indicate 300–400 km offset since about 55 Ma ago. Our results require that about 300 km of this be taken up west of the Maclaren belt, either on the McKinley strand or on thrust segments similar to the Broxson Gulch, or both. Our results further indicate that the arcuate shape of these segments of the Denali fault system are intrinsic properties of the faults themselves and that oroclinal bending need not be invoked to explain them.