Discontinuous High Velocity Lower Crust and Early Margin Segmentation along the Eastern North American Margin's Rift to Drift Transition

We used wide-angle marine active source seismic data from the 2014-2015 GeoPRISMS Eastern North American Margin (ENAM) Community Seismic Experiment (CSE) to investigate along-strike changes in the crustal structure beneath the East Coast Magnetic Anomaly (ECMA). The ENAM formed during the breakup of Pangea and the transition to seafloor spreading ~200 myr ago. Crustal structure of a passive margin records extension and magmatism during rifting, as well as the rift to drift transition where seafloor spreading becomes dominant. Previous studies imaged igneous addition to the margin as seaward dipping reflectors and high velocity lower crust (HVLC) but along-strike variability was interpolated between widely spaced profiles. The ECMA and regional gravity anomalies imply along-strike variability on much smaller wavelengths. Along-strike changes of the ENAM can provide insight into small scale processes that may be critical for a successful rift to drift transition and that influence margin/mid-ocean ridge segmentation.

Tomographic modeling from ocean bottom seismometer data on lines 4A and 4B of the ENAM CSE shows a P-wave velocity structure resembling extremely thick oceanic-type crust, indicating that the ECMA marks the rift to drift transition. We observe minimal variation in crustal thickness, but substantial variation in the extent of HVLC. Within the HVLC, velocities at the base of the crust reach 7.5 km/s but two ~30 km wide HVLC gaps are present where velocities only reach 7.2 km/s. The HVLC variability does not correlate with ECMA variability but the gaps are coincident to isostatic gravity lows and align with offsets within the Inner Magnetic Quiet Zone/Mid-Atlantic Ridge fracture zones. The HVLC gaps resemble imaged velocity reductions across mid-ocean ridge transforms, but there are no reductions along the Blake Spur Magnetic Anomaly further seaward. Further, the existence of developed transforms at the rift to drift transition is unlikely. More likely, the regions of >7.5 km/s HVLC represent localized enhanced upwelling that emplaced more mafic material in some locations and less mafic material in others. The ECMA's crust likely formed from diffuse upwelling through continental mantle lithosphere resulting in along-strike HVLC variability that influenced segmentation of the Mid-Atlantic Ridge.