Vertical tectonism in oceanic fracture zones |
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| Authors: | Enrico Bonatti |
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| Institution: | Lamont-Doherty Geological Observatory of Columbia University, Palisades, N.Y. 10964 USA |
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| Abstract: | Anomalous topographic highs are found along many large oceanic fracture zones, frequently in the form of transverse ridges elongated parallel to the fractures both within and outside of the transform zone. These crustal highs are one or more kilometers shallower than the adjacent “normal” crust of equivalent age. Their elevation is frequently higher than the axial zone of lithospheric accretion. Geophysical and petrological data from the Vema, St. Paul, Romanche (Atlantic Ocean), Owen (Indian Ocean), Alula (Gulf of Aden), and other fracture zones suggest that the anomalous transverse ridges are not the result of excess volcanism, but rather of tectonic uplift of upper mantle and crustal blocks.Factors which may determine vertical tectonism along fracture zones include: (1) horizontal thermal conduction across a fracture zone from a lithospheric accreting segment; (2) viscodynamic forces operating in a fracture zone close to its intersection with an accreting segment; and (3) compressional and tensional horizontal stresses operating along a fracture zone, and created by several causes, the major one being small changes in the direction of spreading. Among these various factors, compressional and tensional horizontal stresses are probably the main cause of vertical tectonism in fracture zones. During the vertical motion of upper mantle/crustal blocks, ultramafic rocks, due to their physical properties, are uplifted preferentially.One of the implications of fracture zone vertical tectonism is that the age/depth of the crust relationship may not apply in regions with large fracture zones. In one well-documented case, a transverse crustal block at the Romanche fracture zone subsided during the last 5 m.y. at an average rate more than one order of magnitude faster than crust of equivalent age. |
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