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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 (ν_p 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 we 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 …