VARIATIONS IN TECTONIC STYLE AND SETTING ALONG THE SOUTHERN ALASKA MARGIN SINCE THE JURASSIC (Invited Presentation)

Prior to the middle Jurassic, much of modern Alaska and major components of the northern Cordillera were dispersed along the western North American margin as loosely connected arc and pericratonic terranes. Progressive accretion of these terranes to one another and, eventually, to the continental margin marked a major transition from offshore arc systems to a long-lived continental arc. That arc, remnants of which are exposed in the northern Aleutian Range and western Alaska Ranges, preceded the modern Aleutian system and preserves evolving tectonic styles from the middle Cretaceous to Eocene. Overlapping magmatic belts record episodic igneous activity that varied in volume and spatial extent, geochemistry, and isotopic character. Some magmatic episodes host significant mineralization and others do not, but spatiotemporal patterns of key deposit types are poorly understood. Regional deformation was associated with Late Cretaceous magmatism and Eocene spreading ridge subduction, and deformation style and kinematics were influenced by terrane architecture, accretion, and translation along the margin. These complex tectonic patterns represent a long-lived convergent margin that exhibited considerable along-strike variation because of terrane interactions and changing plate dynamics from the Jurassic to Eocene. Similar variations in tectonic style and associated features are observed along the Oligocene–Recent convergent margin of southern Alaska, which extends ~3600 km from the western end of the Aleutian arc to a prominent syntaxis formed above the subducting Yakutat microplate. These variations are generally controlled by subduction-related parameters (convergence rate, obliquity, and slab dip) and the nature (age, composition, and architecture) of crust within the upper and lower plates. Evaluating the relative influence of these factors on Alaska’s geologic framework since the Eocene is aided by detailed knowledge of the modern geodynamic setting. Interpretation and reconstruction of the pre-existing architecture and evolution of the southern Alaska margin provides critical geologic insights despite the challenges and uncertainties caused by incomplete bedrock exposure and access, now-subducted oceanic features, and complex, highly overprinted geological reorganization.