DEFORMATION OF COOK INLET FOREARC BASIN, SOUTH-CENTRAL ALASKA: DEXTRAL-TRANSPRESSION WITHIN A CONVERGENT MARGIN

Late Neogene to Recent deformation in Cook Inlet forearc basin is caused by dextral-transpression in the plate margin of south-central Alaska. Collision and subduction of the Yakutat terrane along the plate margin at the northeastern end of the Aleutian subduction zone together with anticlockwise rotation of south-central Alaska south of the Denali fault is driving the accretionary complex of the Chugach and Kenai Mountains towards the Alaska Range on the opposite side of the basin. This deformation creates belts of fault-cored anticlines that are prolific sources of hydrocarbons and also potential sources for damaging earthquakes within the basin. The faults dip steeply into the basement beneath the Tertiary basin fill, and form conjugate flower structures at some localities. Comparing the geometry of the natural faults with physical analog models created in a sandbox deformation apparatus suggests that some of the faults accommodate significant dextral as well as reverse slip motion. The viability of the dextral-transpression model is supported by orientations of stresses inferred from focal mechanisms of inter-plate earthquakes that occur beneath Cook Inlet basin where the maximum horizontal compressive stress is oriented east to east-northeast with the least compressive stress nearly vertical. These stress orientations are compatible with those predicted by the dextral-transpression model, but significantly different than elsewhere along the plate boundary southwest of where the Yakutat terrane is colliding along the Alaska-Aleutian subduction zone.