Structural relationships in the eastern syntaxis of the St. Elias orogen, Alaska

Abstract

The eastern syntaxis in the St. Elias orogen (Alaska, USA) is one of the most complex and least understood regions within the southern Alaska coastal mountain belt. The syntaxis contains many features unique to the orogen that are essential to understanding the structural architecture and tectonic history of the collision between North America and the allochthonous Yakutat microplate. The eastern syntaxis contains the transition from transpressional structures associated with the Queen Charlotte–Fairweather fault system in the east to the Yakataga fold-and-thrust belt (YFTB) to the west. Throughout the eastern syntaxis, a prominent unconformity at the base of the synorogenic Yakataga Formation records an erosional event related to the development of the YFTB. Strain accumulations in the eastern YFTB predate the deposition of the Yakataga Formation, extending estimates for the early development of the St. Elias orogen. Structural and stratigraphic relationships in the eastern syntaxis suggest that forethrusts associated with the transpressional system shut down and were overprinted by fold-and-thrust structures in the Early to latest Miocene. Basement in the eastern syntaxis consists of the Yakutat Group, part of the Chugach accretionary complex, which is carried by numerous low-angle thrust faults in the eastern syntaxis. Exposures of basement and fault patterns within the syntaxis have implications for tectonic reconstructions of the Yakutat microplate and the geodynamics of the orogen.