Recent seismic results on the U.S. East Coast continental margin show that the zone between rifted continental and normal oceanic crust consists of thick (up to 25 km), high seismic velocity (vp of 7.2--7.3 km s-1) crust, interpreted as mafic igneous rocks emplaced during Triassic/Jurassic continental rifting. The total volume of igneous rocks in this zone, which were call the East Coast Margin Igneous Province (ECMIP), may be as much as 2.7¿106 km3, placing the ECMIP among the world's large igneous provinces. We constrain the composition and origin of the thick, igneous crust by using a compilation of laboratory measurements to predict P wave velocities for rocks with the compositions of liquids produced by partial melting of mantle rocks. The high-velocity crust was produced from partial melting of mantle peridotite, with smaller melt fractions (<10%) but at higher average pressures (≥2.0 GPa) than beneath normal mid-ocean ridges. This requires higher than normal asthenospheric potential temperatures during rifting and a lid of lithosphere above upwelling astehosphere to limit the minimum pressure of melting.

Production of thick igneous crust at small melt fractions requires that the vertical flux of astenosphere during rifting exceeded the lateral flux of lithosphere due to extension; that is, mantle ''upwelling'' was more rapid than lithospheric ''spreading''. Thick igneous crust is strongly asymmetrical, extending up to 2000 km along the margin but only for about 80--100 km seaward. The rapid seaward transition to oceanic crust with normal thickness and seismic velocity implies that the thermal anomaly and relatively rapid upwelling lasted for only 5--8 m.y. Moreover, there is no crustal thickness anomaly in the Central Atlantic, in contrast to the North Atlantic where the influence of the Iceland plume created thick crust in a belt spanning the ocean from Greenland to the Faeroes Islands. There factors seem to preclude formation of thick igneous crust in response to a deep-seated mantle plume. The ECMIP may have formed when high upper mantle temperatures induced asthenospheric upwelling. Magmatism and seafloor spreading dissipated the thermal anomaly in the upper mantle, after which normal oceanic crust formed along the Mid-Atlantic Ridge. ¿ American Geophysical Union 1995