Multi-scale along-margin variations in breakup volcanism at the Eastern North American Margin identified by magnetic anomaly modeling

To understand the magmatic processes that facilitated continental breakup and the transition to seafloor spreading at the Eastern North American Margin (ENAM), we investigate along-margin variations in the volume and location of volcanic emplacement, the surficial expression of the breakup magmatism, using three-dimensional magnetic forward modeling, a composite airborne and marine magnetic anomaly grid, and seismic reflection data. We focus on the East Coast Magnetic Anomaly (ECMA) extending along the ENAM, which has been attributed to subsided volcanic layers emplaced during continental breakup. We constrain the modeled magnetic source geometry using interpretations of seaward dipping reflectors, indicating the subsided volcanic layers, imaged by multichannel seismic data from both recent cruises and previous studies of the ENAM. We also create a three-dimensional model to investigate what contributions along-strike variations in magnetization could have on the ECMA character. Forward modeling results that produce a calculated magnetic anomaly character matching that of the ECMA show that the observed small-scale (~100 km) segmentation in ECMA amplitude can be explained by variations in volcanic source volume at a comparable scale. At a regional scale, the observed broad, double peak of the ECMA south of Cape Hatteras, North Carolina, can be attributed to a wider zone of volcanic emplacement, and the higher-amplitude, single peak of the ECMA to the north can be produced by a narrower, thicker volcanic package. Small-scale segmentation in the volume of volcanics suggest variations in the volume and distribution of magma production and/or the location of melt transport along the margin during breakup, which could be from factors including small-scale convection, melt channelization, and/or melt focusing. Regional-scale variations in the volcanism could be related to differences in mantle thermal conditions, lithospheric properties and preexisting structure, and/or the timing and progression of rifting towards breakup.