MODERN TECTONICS OF SOUTHERN ALASKA AND IMPLICATIONS TO THE ACADIAN OROGENY

Results from numerical modeling of ongoing terrane accretion within southern Alaska have important implications for observed geological relationships within the northern Acadian aged rocks. A Paleocene to mid Eocene (66-50 Ma) aged volcanic arc formed within southern Alaska in response to subduction of the Kula plate. Over the last ~10 Ma, the Yakutat terrane, a relatively buoyant thickened oceanic plateau, has been shallowly subducting beneath North America at the eastern syntaxis of the Aleutian subduction zone. The buoyancy of the Yakutat terrane has major influences on the tectonics of southeastern Alaska, including: 1) advective offset of the hot mantle wedge to the north, resulting in tectonic refrigeration the lithosphere and a gap in arc magmatism, 2) creation of a broad seismogenic zone responsible for some of the largest strain releases on Earth, and 3) it drives deformation within the Alaska and Coastal Ranges. Numerical modeling of the coupled thermal-mechanical evolution of the southern Alaska orogen yields pressure-temperature paths for the downgoing Yakutat-Pacific terrane. The predicted paths do not cross the melting curve for wet basalt, suggesting a physical cause for the spatial and temporal gap in volcanism associated with the subducted extent of the Yakutat terrane. Arc volcanism within southern Alaska will resume with the transition back to the subduction of oceanic lithosphere. Paleozoic deformation and magmatism associated with subduction within western Maine and New Hampshire was perturbed by a decrease in the angle of the subduction driven by the accretion of a terrane. Following the terrane accretion event, subduction resumed with the creation of the younger coastal magmatic province. A spatial and temporal shift in deformation patterns and magmatism is consistent with tectonic reorganization following a terrane accretion event.