A NEW MODEL FOR CONTINENTAL BREAKUP AND INCIPIENT SEAFLOOR SPREADING ALONG THE EASTERN NORTH AMERICAN MARGIN

The mechanisms that enable continental breakup are widely debated. In particular, the role of magmatism and its proposed efficiency in weakening the lithosphere causing rapid failure remains unclear. Mantle melts are thought to be a significant factor influencing continental rift processes, and thus rift systems are often classified as magma-poor or magma-rich.

To investigate the feedbacks between magmatism and deformation during the late stages of rifting, we use active-source seismic data from the Eastern North American Margin, which records the initial breakup of Pangea in the Jurassic. With a high resolution 3D seismic velocity volume, we analyze azimuthal traveltime variations of mantle refractions to constrain the orientation and strength of frozen seismic anisotropy across the rift-to-drift transition.

We find evidence of an isotropic upper mantle lid beneath a proto-oceanic domain. The base of the ~10-12 km lid marks a sharp transition to a strongly anisotropic layer below. These results support recent interpretations of the overlying proto-oceanic crustal structure and composition, suggesting that a subcontinental mantle lithospheric lid resisted breakup during late stages of rifting. Beneath the relict lid, anomalously strong anisotropy of ~8-15% in the margin-parallel direction is best explained by an organized olivine fabric that was assisted by large melt volumes during deformation near the fossil lithosphere-asthenosphere boundary. The strength of this fabric increases seaward and culminates at the Blake Spur Magnetic Anomaly, suggesting that melts and deformation became focused here and facilitated complete lithospheric failure. In younger oceanic lithosphere farther seaward, we find evidence for a weaker ~6% fabric rotated ~90° in the margin-perpendicular direction, indicating that seafloor spreading in the Central Atlantic was underway at this time.

We argue that prolonged and arduous rifting was influenced by ultraslow extension rates and a thick inherited Pangea orogenic lithosphere, which resisted breakup despite an abundance of magmatism. The rifting sequence preserved offshore eastern North America is unique in the global spectrum of rift systems, and these characteristics might represent the style of rifting associated with the initial breakup of supercontinents.