Orientation of underthrusting during latest Cretaceous and earliest Tertiary time, Kodiak Islands, Alaska

The Uganik thrust and correlative faults constitute a major structural boundary between a mélange and a deformed turbidite sequence exposed along the continental margin of southwestern Alaska. During under-thrusting between 60 and 70 m.y. B.P., the lower plate of this thrust was completely disrupted as much as 1 km or more below the fault, whereas the upper plate was overprinted by a cleavage inclined to the thrust surface. A new technique utilizing the geometry of the cleavage-thrust intersection provides the basis for determining 37 slip vectors that indicate that the lower plate of the fault was underthrust along a vector that now trends 334° ± 7° and plunges 42°NW. The movement of the Uganik thrust probably was caused by underthrusting of the sea floor. At the 95% confidence level the direction of movement is similar to slip vectors determined from adjacent accreted terranes and confirms the subduction polarity inferred from regional geology.