Lithosphere Structure and Seismic Anisotropy Offshore Eastern North America: Implications for Continental Breakup and Ultra-Slow Spreading Dynamics

The breakup of supercontinent Pangea occurred ∼200 Ma forming the Eastern North American Margin (ENAM). Yet, the precise timing and mechanics of breakup and onset of seafloor spreading remain poorly constrained. We investigate the relict lithosphere offshore eastern North America using ambient-noise Rayleigh-wave phase velocity (12–32 s) and azimuthal anisotropy (17–32 s) at the ENAM Community Seismic Experiment (CSE). Incorporating previous constraints on crustal structure, we construct a shear velocity model for the crust and upper ∼60 km of the mantle beneath the ENAM-CSE. A low-velocity lid (V_S of 4.4–4.55 km/s) is revealed in the upper 15–20 km of the mantle that extends ∼200 km from the margin, terminating at the Blake Spur Magnetic Anomaly (BSMA). East of the BSMA, velocities are fast (>4.6 km/s) and characteristic of typical oceanic mantle lithosphere. We interpret the low-velocity lid as stretched continental mantle lithosphere embedded with up to ∼15% retained gabbro. This implies that the BSMA marks successful breakup and onset of seafloor spreading ∼170 Ma, consistent with ENAM-CSE active-source studies that argue for breakup ∼25 Myr later than previously thought. We observe margin-parallel Rayleigh-wave azimuthal anisotropy (2%–4% peak-to-peak) in the lithosphere that approximately correlates with absolute plate motion (APM) at the time of spreading. We hypothesize that lithosphere formed during ultra-slow seafloor spreading records APM-modified olivine fabric rather than spreading-parallel fabric typical of higher spreading rates. This work highlights the importance of present-day passive margins for improving understanding of the fundamental rift-to-drift transition.