Lithospheric structure and seismic anisotropy of the northern Appalachian region

Over the course of more than 30 years, the seismic structure of the Appalachian Orogen of northern New England and Canada has been studied using several active- and passive-source methods. The resulting models shed light on present-day crust and upper-mantle wave speed structure, seismic anisotropy and their relationship to the complex geological history of the region. Notable contributions were made by the Lithoprobe East project in the 1980s/1990s, and by the EarthScope USArray in the 2010s. Crustal studies revealed significant heterogeneity across the northern Appalachians. Variations in upper- and mid-crustal seismic wave speeds correlate with metamorphic grade and composition across the Appalachian domains, and the interaction between the Laurentian margin and accreted terranes varies along strike. Crustal thicknesses typically range from ∼30 to 46 km. Lithospheric thicknesses vary from ∼70 to 120 km beneath Maine and the Canadian Maritimes, preserving orogenic structures including a potential signature of flat-slab subduction. Anisotropic orientations are oblique to orogenic trends in New England, but sub-parallel in the Canadian Appalachians, with spatial variations that suggest a significant contribution from lithospheric deformational fabric associated with Paleozoic orogenesis and later rifting processes.