LARGE IGNEOUS PROVINCE ON THE UNITED-STATES ATLANTIC MARGIN AND IMPLICATIONS FOR MAGMATISM DURING CONTINENTAL BREAKUP

RIFTED continental margins commonly include sections of igneous rock m ore than twice as thick as normal oceanic crust. Explanations for this voluminous magmatic accretion during rifting include plume models1-3, which require a deep-seated thermal or chemical anomaly in upwelling mantle, and non-plume models4-7, which call on broad, shallow thermal anomalies and/;or rapid upwelling of mantle through the melting zone. New seismic models from two transects across the continent ocean trans ition on the US Atlantic margin8-10 confirm the presence of a 20-25-km -thick igneous section. Here we argue that the similarity of the crust al structure on these and two previous transects, spanning 1,000 km of the margin, and the association of thick igneous crust with the East Coast magnetic anomaly11 imply that the thick igneous section extends along the entire margin and may have a volume of as much as 3.2 x 10(6 ) km3. The distribution of volcanic and plutonic rocks, details of the seismic structure, and lack of independent evidence for a hotspot are difficult to reconcile with plume models and suggest that non-plume p rocesses created the thick igneous crust.