Duplex accretion and underplating in an ancient accretionary complex, Kodiak Islands, Alaska

Abstract

The landward belt of the Kodiak Formation represents a sequence of lower Maastrichtian turbidites that was underplated to the Kodiak accretionary complex by duplex accretion. Evidence for duplexes exists on the mesoscopic and map scale. At the microscopic scale, incremental strain histories are consistent with early shear during underthrusting abrupt landward rotation of individual thrust slices over fault ramps, and locking of thrust packets in steeper orientations. Duplex geometries at the mesoscopic scale are represented by high-angle reverse faults bounded either above or below by low-angle, southeast-verging detachments. At the map scale, different structural belts can be related to a low-angle detachment zone (central belt) flooring a more landward duplex (landward belt). Duplex underplating can result in the uplift and thickening of accretionary prisms without necessarily deforming the overlying, previously accreted material and slope sediments.