Neogene crustal emersion and subsidence at the Romanche fracture zone,equatorial Atlantic |
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Authors: | E. Bonatti M. Sarnthein A. Boersma M. Gorini J. Honnorez |
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Affiliation: | 1. Lamont-Doherty Geological Observatory of Columbia University, Palisades, N.Y. 10964USA;2. Geology Institute, University of Kiel, Kiel Federal Republic of Germany;3. Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Fla. 33149USA |
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Abstract: | Partly phosphatized, oolitic-biogenic limestones were recovered 950–1300 m below sea level from two sites near the crest of a transverse ridge running parallel and adjacent to the Romanche fracture valley (equatorial Atlantic). Some of the biogenic contituents of the limestones (the benthonic foraminiferal genusAmphistegina; corals of the genusStylophora sp., scaphopods, etc.); their paleofacies assemblage (including echinoderms, gastropods, calcareous algae, etc.); and the presence of a well-developed oolitic facies, indicate that the limestones formed in very shallow water close to sea level. In addition, several features of the limestones (including the presence of stromatolite-like laminae, and dissolution features typical of subaerial diagenesis) suggest that the summit of the transverse ridge might have undergone episodes of emersion. The limestones were formed on a shallow carbonate bank at around the Miocene-Pliocene boundary, i.e., 5 ± 1 Myr ago, as determined by the age of fossil planktonic foraminifera and corals. The transverse ridge must have subsided since that time at an average rate of 0.2 mm/yr. It is unlikely that the vertical motions of the Romanche transverse crustal block were caused solely by accreting plate margin- or mantle plume-related volcanic and/or tectonic processes. It is suggested instead that such motions are related to vertical tectonism typical of large oceanic fracture zones. |
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