Discontinuous Igneous Addition Along the Eastern North American Margin Beneath the East Coast Magnetic Anomaly

Detailed models of crustal structure at volcanic passive margins offer insight into the role of magmatism and the distribution of igneous addition during continental rifting. The Eastern North American Margin (ENAM) is a volcanic passive margin that formed during the breakup of Pangea similar to 200 Myr ago. The offshore, margin-parallel East Coast Magnetic Anomaly (ECMA) is thought to mark the locus of syn-rift magmatism. Previous widely spaced margin-perpendicular studies seismically imaged igneous addition as seaward dipping reflectors (SDRs) and high velocity lower crust (HVLC; >7.2 km/s) beneath the ECMA. Along-strike imaging is necessary to more accurately determine the distribution and volume of igneous addition during continental breakup. We use wide-angle, marine active-source seismic data from the 2014-2015 ENAM Community Seismic Experiment to determine crustal structure beneath a similar to 370-km-long section of the ECMA. P-wave velocity models based on data from short-period ocean bottom seismometers reveal a similar to 21-km-thick crust with laterally variable lower crust velocities ranging from 6.9 to 7.5 km/s. Sections with HVLC (>7.2 km/s) alternate with two similar to 30-km-wide areas where the average velocities are less than 7.0 km/s. This variable structure indicates that HVLC is discontinuous along the margin, reflecting variable amounts of intrusion along-strike. Our results suggest that magmatism during rifting was segmented. The HVLC discontinuities roughly align with locations of Mid-Atlantic Ridge fracture zones, which may suggest that rift segmentation influenced later segmentation of the Mid-Atlantic Ridge. Plain Language Summary The East Coast of the United States is a passive margin that formed during continental breakup of Pangea, the most recent supercontinent. Although passive margins are generally not locales of active faulting and magmatism, by investigating their current structure, we gain insight into processes during past rifting. We know that extensive volcanism and magmatic addition to the Earth's crust occurred during the breakup of Pangea, but we do not fully understand how the distribution changes from north to south along the margin. To study these rocks, which are buried 10-20 km beneath the seafloor, we use ocean bottom seismometers to determine the sound speed of rocks beneath the surface, which can tell us about composition and rock type. We found that the thickness and extent of magma-derived rocks are variable along the margin, with gaps up to 30 km wide. This variability is likely related to processes happening in the continental rift which may have influenced the structure of the Mid-Atlantic Ridge that formed after rifting ended.