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.

A document by Sarah Jaye Oliva. Click on the document to view its contents.

We conduct a high-resolution teleseismic receiver function investigation of the subducting plate interface within the Alaskan forearc beneath Kodiak Island using data collected as part of the Alaska Amphibious Community Seismic Experiment in 2019. The Kodiak node array consisted of 398 nodal geophones deployed at ~200-300 m spacing on northeastern Kodiak Island within the southern asperity of the 1964 Mw9.2 Great Alaska earthquake. Receiver function images at frequencies of 1.2 and 2.4 Hz show a coherent, slightly dipping velocity increase at ~30-40 km depth consistent with the expected slab Moho. In contrast to studies within the northern asperity of the 1964 rupture, we find no evidence for a prominent low-velocity layer above the slab Moho thick enough to be resolved by upgoing P-to-S conversions. These results support evidence from seismicity and geodetic strain suggesting that the 1964 rupture connected northern (Kenai) and southern (Kodiak) asperities with different plate interface properties.