New Constraints on Along-Strike Variations in Synrift Magmatism on the Rifted Margin Offshore North Carolina and Virginia from Seismic Refraction Data

Although the Eastern North American Margin (ENAM) is one of the type-locales of a magma-rich passive rifted margin, our current knowledge about the volume and distribution of magma along and across the rifted margin, and the role of magmatism on rifting is limited. In particular, very few constraints are available here or in other rift systems on the 3D distribution of magmatic addition associated with rifting. Our study presents 2D and 3D tomography results from the Eastern North American Margin Community Seismic Experiment (ENAM CSE) offshore North Carolina and Virginia. Seventy-two short-period ocean-bottom seismometers (OBS) deployed along 4 main transects and thirty broadband OBS deployed across the margin recorded shots from a 3300- to 6600- cu. in. air gun array of the R/V Marcus G. Langseth at shot intervals of 37.5 to 225 m. We examine data from short-period and broadband OBS around the East Coast Magnetic Anomaly, which is thought to arise from synrift magmatic addition. Refractions from the sediments, crust and mantle (Ps, Pg and Pn, respectively) and reflections from the base of sediments and base of the crust (PsP and PmP) are observed on data from the majority of instruments examined. Arrivals from OBS lines shot with the larger volume array and larger shot spacing can be observed to offsets > 200 km. Reciprocity checks ensure consistent phase identifications between instruments. To obtain 2D models and a 3D model of P wave velocity structure based on first arrival travel time picks, we used the FAST (First Arrival Seismic Tomography) method (Zelt & Barton, 1998). We tested different grid spacing, initial models and parameterizations to examine their impact on the results. We also plan to use reflected and refracted arrivals for 3D reflection/refraction tomography. Preliminary velocity models show along-strike variations in the thickness of an interval with velocities of > 7 km/s, which we interpret to represent mafic magmatic addition. These results thus suggest along-strike variations in magmatism, consistent with results from previous 2D velocity models from this dataset. We will explore the possible causes for this variability and its consequences for the origin of magmatism and its role in extension and rifting.