ORIGIN OF THICK, HIGH-VELOCITY IGNEOUS CRUST ALONG THE US EAST-COAST MARGIN

Recent seismic results on the U.S. East Coast continental margin show that the zone between rifted continental and normal oceanic crust cons ists of thick (up to 25 km), high seismic velocity (nu(p) of 7.2-7.3 k m s(-1)) crust, interpreted as mafic igneous rocks emplaced during Tri assic/Jurassic continental rifting. The total volume of igneous rocks in this zone, which we call the East Coast Margin Igneous Province (EC MIP), may be as much as 2.7 x 10(6) km(3) placing the ECMIP among the world's large igneous provinces. We constrain the composition and orig in of the thick, igneous crust by using a compilation of laboratory me asurements to predict P wave velocities for rocks with the composition s of liquids produced by partial melting of mantle rocks. The high-vel ocity crust was produced from partial melting of mantle peridotite, wi th smaller melt fractions (< 10%) but at higher average pressures (gre ater than or equal to 2.0 GPa) than beneath normal mid-ocean ridges. T his requires higher than normal asthenospheric potential temperatures during rifting and a lid of lithosphere above upwelling asthenosphere to limit the minimum pressure of melting. Production of thick igneous crust at small melt fractions requires that the vertical flux of asthe nosphere during rifting exceeded the lateral flux of Lithosphere due t o extension; that is, mantle ''upwelling'' was more rapid than lithosp heric ''spreading.'' Thick igneous crust is strongly asymmetrical, ext ending up to 2000 km along the margin but only for about 80-100 lan se award. The rapid seaward transition to oceanic crust with normal thick ness and seismic velocity implies that the thermal anomaly and relativ ely rapid upwelling lasted for only 5-8 m.y. Moreover, there is no cru stal thickness anomaly in the Central Atlantic, in contrast to the Nor th Atlantic where the influence of the Iceland plume created thick cru st in a belt spanning the ocean from Greenland to the Faeroes Islands. These factors seem to preclude formation of thick igneous crust in re sponse to a deep-seated mantle plume. The ECMIP may have formed when h igh upper mantle temperatures induced asthenospheric upwelling. Magmat ism and seafloor spreading dissipated the thermal anomaly in the upper mantle, after which normal oceanic crust formed along the Mid-Atlanti c Ridge.