Tectonic implications of out‐of‐sequence faults in an Accretionary Prism, Prince William Sound, Alaska

Abstract

The Chugach and Prince William terranes constitute a Mesozoic through Tertiary accretionary complex bordering the Gulf of Alaska. Faults of the Contact fault system juxtapose the two terranes. Across the previously mapped trace of the Contact fault in western Prince William Sound, we studied three belts of alternately higher and lower deformation intensity and metamorphism. Out‐of‐sequence reverse faults separate these belts. Until recently, researchers assumed that the Prince William terrane was accreted to the Chugach terrane along the Contact fault. However, our work supports an emerging view that the Chugach and Prince William terranes represent older and younger parts of a continuously evolved accretionary prism, best considered a single terrane. We found that we cannot distinguish the Chugach from the Prince William terrane on the basis of lithology, deformation, or metamorphism; therefore, we cannot precisely identify the local strand of the Contact fault system. Furthermore, the major faults in the study area are relatively late‐stage features, related not to initial accretion but rather to internal deformation of the prism. West‐northwest shortening characterizes the deformation along the Contact fault system in western Prince William Sound, whereas dextral‐slip faulting characterizes the deformation along the Contact fault system in eastern Prince William Sound. We ascribe this difference to development of the Alaska orocline coeval with changes in plate motions during Eocene time, which resulted in convergence in western Prince William Sound remaining relatively orthogonal while convergence in eastern Prince William Sound became increasingly oblique.