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

다국어 초록 (Multilingual Abstract)

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 (VS 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.
The Eastern North American Margin (ENAM) formed during the breakup of supercontinent Pangea, marking the opening of the Atlantic Ocean. However, details of the timing and mechanics of the breakup are not well understood. The ENAM region provides a fossilized record of this transition from continental rifting to seafloor spreading, informing understanding of the fundamental “rift‐to‐drift” plate‐tectonic process. Using Rayleigh‐waves, we solve for the 3‐D shear velocity structure of the lithosphere offshore North Carolina, revealing a 15–20 km thick low‐velocity “lid” that extends ∼200 km from the margin, terminating at the Blake Spur Magnetic Anomaly. We interpret this as stretched continental lithosphere, which implies that complete breakup of Pangea did not occur directly at the margin but rather ∼200 km seaward. This corresponds to a breakup age of ∼170 Ma, ∼25 Myr later than previously thought. We also observe a directional dependence of Rayleigh‐wave velocities, where waves traveling parallel to the margin propagate 2%–4% faster than waves traveling perpendicular, opposite of what is expected for oceanic lithosphere. This provides evidence for margin‐parallel deformation in the mantle during breakup ∼170–200 Ma. We propose that relative motion of the overriding plate drives mantle deformation in ultra‐slow seafloor spreading environments.



A 15–20 km thick low‐velocity lid extends ∼200 km from the margin and is interpreted as stretched continental mantle lithosphere

Complete continental breakup and onset of normal seafloor spreading occurred ∼170 Ma, ∼25 Ma later than previously thought

Observed margin‐parallel lithospheric anisotropy resulted from plate‐motion induced shear during ultra‐slow spreading 170–200 Ma


A 15–20 km thick low‐velocity lid extends ∼200 km from the margin and is interpreted as stretched continental mantle lithosphere
Complete continental breakup and onset of normal seafloor spreading occurred ∼170 Ma, ∼25 Ma later than previously thought
Observed margin‐parallel lithospheric anisotropy resulted from plate‐motion induced shear during ultra‐slow spreading 170–200 Ma