Structural evolution of underthrusted sediments, Kodiak Islands, Alaska

Abstract

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 (D1) in the coherent terranes is marked by normal faults, mineralized layer‐perpendicular veins in sandstones, and microfaults in shales. These features suggest that σ1 was vertical and that σ3 was horizontal and NW trending during D1. We interpret D1 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 D1 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 D1 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.

Fluid-rock interaction recorded in black fault rocks in the Kodiak accretionary complex, Alaska

“…The Ghost Rocks Formation, consisting of the latest Cretaceous-earliest Paleocene ridge-flank deposits, is distributed over the southeastern part of Kodiak Island (Fisher and Byrne 1987). This formation is overthrust by the Late Cretaceous Kodiak Formation and is underthrust by Eocene marine sediments (Moore 1969).…”

Earthquake faulting in subduction zones: insights from fault rocks in accretionary prisms

“…In the early Tertiary accretionary complex of Kodiak Island, Alaska, the argillaceous mélange continuously develops along strike and has been interpreted as representing a paleo-subduction thrust that was subducted at depths of 12 to 14 km and temperatures of 230°C to 260°C (Byrne 1984;Fisher and Byrne 1987;Vrolijk et al 1988). The approximately 0.3-m-thick black and aphanitic fault rocks have developed in the 15-to 20-m-thick cataclasite that is derived from the argillaceous mélange (Rowe et al 2005;Meneghini et al 2010).…”

Tectono-metamorphic history of the ophiolitic Lento unit (northern Corsica): evidences for the complexity of accretion-exhumation processes in a fossil subduction system

“…The depth of the accretion is provided by T and P conditions of the metamorphic climax, indicative of a deformation at 25-30 km. The structural features of the D1 phase, consisting of rootless isoclinal folds, bedding-parallel foliation, mineral-stretching lineations and HP metamorphism, are coherent with deformation achieved in the subduction channel during the accretion process [63,64,68,69,70,71]. In this tectonic setting, as result of the high strain values of non-coaxial deformation, the mineral-stretching lineations are assumed to be aligned along the shear direction [71 and quoted references].…”