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

The breakup of supercontinent Pangea occurred similar to 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 similar to 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 similar to 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 similar to 15% retained gabbro. This implies that the BSMA marks successful breakup and onset of seafloor spreading similar to 170 Ma, consistent with ENAM-CSE active-source studies that argue for breakup similar to 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.

Automated summary

Researchers investigated the relict lithosphere offshore eastern North America, revealing features such as a low-velocity lid and typical oceanic mantle lithosphere characteristics. They propose that the Blake Spur Magnetic Anomaly marks the successful breakup and onset of seafloor spreading, occurring approximately 25 million years later than previously thought. Additionally, they observe margin-parallel Rayleigh-wave azimuthal anisotropy in the lithosphere that correlates with absolute plate motion during spreading.