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Long-lived continental magmatic arcs may migrate large (hundreds of kilometers) trench perpendicular distances as convergent margin configurations and slab geometry vary over time; however, many arc-magmatic belts are spatially localized over tens of millions of years.We document, by compiling published crystallization geochronology data for southern Alaska (6,485 total bedrock and single-grain detrital ages combined), that since ca. 100 Ma, arc magmatism has been localized along the Alaska Range suture zone (in places within a 10km × 5km swath) and at times over 500km inboard. However, since ca. 100 Ma, incoming subducting slab characteristics and convergent margin configurations varied greatly and include both normal oceanic plate and oceanic plateau subduction, plate vector changes, oroclinal bending and reconfiguration of trench shape, terrane accretion, long-distance terrane translation, and a Paleocene slab break off/slab window event. Therefore, it is inferred that inherited upper-plate lithospheric thickness and thermal variations must control in part the geometry of the subducting slab below a mobile southern Alaskan margin through hydrodynamic mantle wedge “suction” forces. Additionally, crustal thickness heterogeneity may focus magma ascent through melt ponding along Moho offsets, and upper-plate lithospheric-scale strike-slip faults may be acting as passive and active conduits for arc magmatism. 

Southern Alaska is a collage of accreted terranes. The deformation history of accreted terranes and the geometric history of their bounding faults reflect both inherited features and associated convergent margin events. We employ S-to-P receiver functions on multiple dense (<20km spacing) arrays of broadband seismometers across southern Alaska to investigate signals of dynamic tectonic activity. An inboard-dipping (∼15∘) boundary is imaged aligning with the trace of the Border Ranges Fault, which is interpreted as an unrotated inboard-dipping paleo-subduction (Mesozoic) interface. This observation, along with previous seismic imaging of the Border Ranges Fault and the next outboard terrane-bounding fault, the Contact Fault, buttresses a known history of convergent tectonics that varies along the margin. Large (>10 km) crustal thickness offsets imaged across both the Denali Fault system and the Eureka Creek Fault support a Mesozoic-to-Present inboard-dipping (east and northward) subduction polarity in the region. Additionally, our imaging reveals a significant velocity increase with depth at ∼25km beneath the Copper River Basin, which we interpret as the top of a region of active underplating and/or intrusion of basaltic magmatism. This feature may be related to the generation of a newWrangell Volcanic Field volcano, resulting from the underlying tear in the subducting slab.