Lithospheric Structure and Flat‐Slab Subduction in the Northern Appalachians: Evidence From Rayleigh Wave Tomography

Abstract

AbstractA billion years of tectonic history makes southeastern Canada and the northeastern United States an exciting area to investigate the evolution of continental lithosphere. Our study area comprises terranes with either Laurentian or Gondwanan provenance that accreted to eastern North America at different times. With the aim of resolving the isotropic velocity variations across the northern Phanerozoic Appalachians and the southeastern Proterozoic Grenville Province, we adopted a Rayleigh wave tomography technique that takes multipathing, scattering, and finite frequency effects into account. Our data sets include records of teleseismic earthquakes recorded by 71 broadband seismic stations over a 2‐year period. Our high‐resolution tomography models indicate significant (±3.5%) variations in shear wave velocity across different lithospheric domains, enabling us to discuss tectonic implications. In contrast to the Peri‐Laurentian zones, seismic signatures in the Peri‐Gondwanan domains are more complex and variable. Although systematic variations of seismic velocities across different tectonic zones are observed, we find no simple relation between the lithospheric thicknesses of different tectonic zones and their age. We interpret the lithosphere‐asthenosphere boundary in our study area, located at 70–120 km depth, with the thickest and fastest lithosphere beneath New Brunswick. We suggest that this relatively thicker lithosphere is due to a slab stacking process that occurred after flat subduction of a younger domain in the Late Silurian. Occurrence of flat subduction in the Late Silurian in the northern Appalachians is also supported by geochemical and paleomagnetic evidence.