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Conjugate Volcanic Margins |
Department of Geology, University of Oslo, P.O. Box 1047 Blindern, N-0316 Oslo, Norway
Volcanic margins in the Atlantic Ocean reveal a series of common crustal units and structural features developed during continental extension and break-up. We suggest that four main crustal zones can be recognized on volcanic margins. This tectono-magmatic zonation implies a history of development where tectonic and magmatic styles and dimensions depend on the interaction of lithospheric and asthenospheric properties and dynamics. The amount of excess igneous activity depends on the temperature and fluid content of the asthenosphere along the incipient plate boundary and the dynamic history of the lithosphere during the rift phase. An adequate understanding of the margin history requires studies of the entire rift, i.e. the conjugate margins. We also note that the spectacular wedges of seaward-dipping reflectors observed along many rifted margins are only one of many igneous features originating during the process of break-up and initial seafloor spreading. Probably, most passive rifted margins represent intermediate cases relative to the volcanic and non-volcanic end-members. A mantle plume impinging on lithosphere already under extension emplacing Large Igneous Province-type initial oceanic crust, including an extensive extrusive cover, is considered the most likely explanation for volcanic margins. Hydrocarbon resource evaluations of volcanic margins have to include their characteristic tectono-magmatic features and their consequences for vertical motion, erosion, sedimentation, thermal and burial histories, and maturation.
