Mesozoic rifting along the eastern seaboard of North America : insights from the seismic velocity structure of the Newfoundland margin and the northern Gulf of Mexico
Eddy, Drew Richard
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Passive margins along the eastern seaboard of North America formed during early Mesozoic continental rifting and seafloor spreading, tectonic processes that are not fully understood. Seismic refraction and reflection data at the northeastern and north-central Gulf of Mexico and the Grand Banks of Newfoundland, Canada, are used to interpret the deep seismic velocity structure of sediments, crust, and mantle. These interpretations allow for a better understanding of continental rifting, mantle upwelling, magmatism, and seafloor spreading. Magma-poor rifting of the Newfoundland-Iberian margin developed a wide continent-ocean transition zone (COT). I present an analysis of 2-D marine seismic refraction and reflection data from the SCREECH project, including a shear velocity model to constrain the composition of the Newfoundland COT. Comparing SCREECH Line 2 Vp/Vs ratios with depth to potential lithologies supports a COT comprised of hyperextended continental crust and serpentinized mantle. Reconstructions of the opening of the Gulf of Mexico basin are impeded by a lack of seafloor magnetic anomalies and an abundance of sediments that obscure acquisition of seismic refraction datasets. Accordingly, the roles of mantle upwelling, magmatism, and lithospheric extension in this small ocean basin are poorly known. I present new 2-D marine seismic refraction data from the U.S. Gulf of Mexico collected during the 2010 GUMBO project. Rifting in the eastern Gulf of Mexico developed above a zone of anomalously high mantle potential temperatures that led to abundant magmatism. Syn-rift basins in continental crust, high velocity lower crust, a narrow zone of crustal thinning, and seaward-dipping reflectors support this interpretation. Oceanic crust here is thick despite slow seafloor-spreading rates, implying continuation of a thermal anomaly after rifting. In the north-central Gulf of Mexico, transitional crust is consistently thin (~10 km) across a wide zone. Velocity-depth comparisons, asymmetry of the north-central Gulf with the Yucatán margin, and dating of onshore xenoliths support either stretched and magmatically intruded continental crust or a multi-stage episode of seafloor spreading with ridge jumps. I contend that although tectonic inheritance may ultimately influence the location of a passive margin, the rifting process is largely controlled by mantle potential temperature and upwelling rate.