Structural evolution of underthrusted sediments, Kodiak Islands, Alaska

The late Cretaceous to early Tertiary sediments of the Kodiak accretionary complex preserve the earliest deformation associated with underthrusting along a thickly sedimented convergent margin. Because of the thick trench fill, this early deformation resulted in the formation of two distinct structural terranes: (1) coherent terranes of layered turbidites and (2) melange terranes. The earliest deformation (D 1 ) in the coherent terranes is marked by normal faults, mineralized layer‐perpendicular veins in sandstones, and microfaults in shales. These features suggest that σ 1 was vertical and that σ 3 was horizontal and NW trending during D 1 . We interpret D 1 structures as hydrofractures that formed in flat‐lying sediments with high fluid pressures caused by rapid tectonic loading. In contrast, the melanges appear to record a more protracted and complex history that culminated in structures similar to the D 1 structures above. We have recognized three successive, transitional stages in the melanges: (I) breakup of sand beds along cataclastic shear zones with dispersal of sand inclusions within a relatively ductile shale matrix, resulting in the blocks‐in‐matrix character of the melange, (II) development of ductile shear bands that crosscut the melange fabric at a low angle, and (III) hydrofracturing with the formation of calcite and quartz‐filled veins in sandstone inclusions and microfaults in the shales. The sequence of microstructures associated with stages I–III records increased cohesion of the sediments during deformation dominated by layer‐parallel shear. Intense mineralization and microfaulting during stage III suggest that the melanges are zones of concentrated fluid flow with a fracture dominated permeability. Stage III structures are texturally indistinguishable and contemporaneous with the less pervasive D 1 structures in the coherent terranes. Based on these observations, we believe that the melanges are major shear zones that formed at the top of the subducting pile and immediately below a major decollement. The coherent terranes represent the structurally and stratigraphically lowest sediments, and they bypassed the stage I and II deformations of the melanges because they were the greatest distance from the decollement when they were deformed. Moreover, both the melange and coherent terranes appear to have bypassed the shallow zone of intense horizontal shortening that dominates the toe of the overriding accretionary prism.