Detailed models of crustal structure at volcanic passive margins offer insight into the role of magmatism and 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 ~200 Myr ago. The offshore, margin-parallel East Coast Magnetic Anomaly (ECMA) is thought to mark the locus of synrift 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 ~370-km-long section of the ECMA. P-wave velocity models based on data from short-period ocean bottom seismometers reveal a ~21-km-thick crust with laterally variable lower crust velocities ranging from 6.9 to 7.5 km/s. Sections with HVLC alternate with two ~30-km-wide areas where the velocities do not exceed 7.0 km/s. This variable structure indicates that HVLC is discontinuous along the margin. Velocity-thickness analysis indicates that the HVLC discontinuity is the result of variable intrusion along-strike. Our results suggest that magmatism during early rifting was segmented and was lower in volume than previously thought. The HVLC discontinuities roughly align with locations of Mid-Atlantic Ridge fracture zones, which may indicate that early rift segmentation influenced later segmentation of the Mid-Atlantic Ridge.

The Patagonian slab window has been proposed to enhance the solid Earth response to ice mass load changes in the overlying Northern and Southern Patagonian Icefields (NPI and SPI, respectively). Here we present the first regional seismic velocity model covering the entire north-south extent of the slab window. A slow velocity anomaly in the uppermost mantle indicates warm mantle temperature, low viscosity, and possibly partial melt. Low velocities just below the Moho suggest that the lithospheric mantle has been thermally eroded over the youngest part of the slab window. The slowest part of the anomaly is north of 49°S, implying that the NPI and the northern SPI overlie lower viscosity mantle than the southern SPI. This comprehensive seismic mapping of the slab window provides key evidence supporting the previously hypothesized connection between post-Little Ice Age anthropogenic ice mass loss and rapid geodetically observed glacial isostatic uplift (≥ 4 cm/yr).