THE PAST AS A KEY TO THE PRESENT: ROLE OF INHERITED CRUSTAL STRENGTH IN THE DIFFUSE ALASKAN PLATE BOUNDARY ZONE

Alaska lies at the northern end of the Cordillera in a diffuse, wide, and seismically active plate boundary zone. What are the relative roles of inherited strength versus mantle dynamics throughout this zone? We explore what we know (and can infer) about crustal composition of Alaska from geologic and potential field data and compare this framework with earthquake locations and other evidence of tectonic strain localization. In some zones active deformation is localized along crustal scale compositional boundaries whereas other zones have deformation localized within what appears to be homogeneous crust, and other areas display diffuse deformation. Key questions relate to how relative strengths of crustal regions evolve through both igneous and tectonic processes.

The first-order assembly of the Alaskan lithosphere is the result of Paleozoic through present convergent margin processes. Different stages of subduction through collision are preserved in distinct crustal regions, and these ancient plate boundaries produce highly heterogeneous crustal fragments separated by narrow zones of potential weakness. In northern Alaska thick crust of the Arctic Alaska block preserves evidence of subduction, but to the south a large area of mafic to intermediate crust did not experience significant thickening or thinning since the Cretaceous. Although rheological contrasts are strong in this area, seismicity occurs both within crustal fragments and along their boundaries. Eastern interior Alaska, in contrast, has been extensively modified from its origins as a silicic Paleozoic arc and experienced several collisional events followed by extension that left the crust relatively thin; this may account for the very diffuse nature of seismicity here. In western interior Alaska simple collisional models do not explain the distribution of deformed and undeformed Paleozoic crust and we look forward to the Earthscope data providing insight. The southern Alaskan margin continues to be the site of ongoing subduction and collision, as it has over the last 200 m.y. Processes that have strongly modified the southern Alaska crust include subduction erosion and underplating, high heat flow associated with ridge subduction, and low-angle subduction which chills and strengthens the overlying crust and continues today.