MAGMATIC TECTONIC EVOLUTION OF A VOLCANIC RIFTED MARGIN

Many North Atlantic continental margins are underlain by huge volcanic edifices near the continent ocean boundary. A deep crustal hole drilled on the Voring Plateau (ODP Leg 104) has provided important constraints on the initial breakup history and the subsequent margin evolution by penetrating more than 900 m of igneous rocks and interbedded sediments below a post-early Eocene cover. The rocks constitute two distinct, different volcanic series. The upper series, comprising a seaward dipping reflector wedge, consists of transitional mid-oceanic tholeiitic lava flows and thin volcaniclastic sediments. Dacitic flows, some dikes and thicker interbedded sediments constitute the lower series. The margin evolved by Paleocene crustal extension accompanied by uplift and pervasive intrusion of the rift zone. Just prior to breakup, magma from shallow crustal melts produced the lower series. The upper series was constructed during an intense, rapidly waning, subaerial volcanic surge following breakup near the Paleocene-Eocene transition. It covers both the new oceanic crust and large areas of the continental crust. The dipping wedge was formed by subsidence due to loading and thermal contraction, possibly amplified by tectonism. When the surge abated, the injection center rapidly subsided and normal oceanic crust was formed. There is a direct temporal and compositional relationship between the onshore North Atlantic Volcanic Province and the margin volcanism. Compared to the transverse part of the province including the Iceland Hotspot, which has been active over 60 m.y., the volcanic margins reflect a 2500 km long transient segment related to continental breakup, lasting only about 3 m.y. The volcanic surge and uplift at breakup are related to a modest regional increased temperature at the base of the lithosphere. This mode of margin formation has had important paleoenvironmental implications.