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\begin{document}

\title{Lithosphere structure and seismic anisotropy offshore eastern North
America: Implications for continental breakup and ultra-slow spreading
dynamics}



\author[1]{Joshua B. Russell}%
\author[2]{James B. Gaherty}%
\affil[1]{Brown University}%
\affil[2]{Northern Arizona University}%


\vspace{-1em}



  
  \date{May 25, 2024}


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\begin{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 relic 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 ( 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.%
\end{abstract}%



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