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ABSTRACT: Magmatic-Tectonic Evolution of a Volcanic Rifted Margin

Olav Eldholm

Many North Atlantic margins are underlain by huge volcanic edifices near the continent-ocean boundary. A crustal hole drilled at the outer Voring Plateau during ODP (Ocean Drilling Project) Leg 104 has provided important constraints on the 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 recovered basement rocks constitute two 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 sediments constitute the Lower Series. The margin evolved by Paleocene crustal extension, uplift and pervasive intru ion in 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 surge following breakup in the earliest Eocene. The Upper Series covers both new oceanic crust and large areas of continental crust. The dipping wedge was formed by subsidence due to loading and thermal contraction, probably amplified by a tectonic force. When the surge had abated, the injection center subsided and a normal oceanic crust was formed. A direct temporal and compositional relationship exists between the onshore North Atlantic Volcanic Province and the volcanic margins. Whereas the central transverse part of the province, near the Iceland hotspot, has been active for 60 m.y., the volcanic margins reflect a 2000-km-long transient phenomenon lasting only 3 m.y. The breakup volcanism and lack of initial subsidence are related to a regional, about 50C°, increased temperature at the base of the lithosphere (hot carpet) combined with opening in previously extended crust. This mode of margin formation has profound implications for the Paleogene paleoceanography of the Norwegian-Greenland Sea.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990