Accretion,structure and hydrology of intermediate spreading-rate oceanic crust from drillhole experiments and seafloor observations |
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| Authors: | Philippe A Pezard Roger N Anderson William B F Ryan Keir Becker Jeffrey C Alt Pascal Gente |
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| Institution: | (1) Département de Génie Océanique, Institut Méditerranéen de Technologie, 13451 Marseille, France;(2) Borehole Research Group of the Lamont-Doherty Geological Observatory, and Department of Geological Sciences, Columbia University, 10964 Palisades, New York, USA;(3) Marine Geology and Geophysics of the Lamont-Doherty Geological Observatory, and Department of Geological Sciences, Columbia University, 10964 Palisades, New York, USA;(4) Rosentiel School of Marine and Atmospheric Sciences, Division of Marine Geology and Geophysics, 4600 Rickenbacker Causeway, 33149 Miami, Florida, USA;(5) Department of Geological Sciences, University of Michigan, 48109 Ann Arbor, Michigan, USA;(6) Université de Bretagne Occidentale, 6 Avenue Le Gorgeu, 29283 Brest, France |
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| Abstract: | Downhole measurements recorded in the context of the Ocean Drilling Program in Hole 504B, the deepest hole drilled yet into the oceanic crust, are analyzed in terms of accretion processes of the upper oceanic crust at intermediate spreading-rate. The upper part of the crust is found to support the non steady-state models of crustal accretion developed from seafloor observations (Kappel and Ryan, 1986; Gente, 1987). The continuous and vertical nature of borehole measurements provides stratigraphic and structural data that cannot be obtained solely from seafloor studies and, in turn, these models define a framework to analyze the structural, hydrological, and mineralogical observations made in the hole over the past decade.Due to the observed zonation with depth of alteration processes, and its relation to lava morphologies, the 650-m-thick effusive section penetrated in Hole 504B is postulated to be emplaced as the result of two main volcanic sequences. Massive lava flows are interpreted as corresponding to the onset of these sequences emplaced on the floor of the axial graben. The underlying lava made of structures with large porosity values and numerous cm-scale fractures is thus necessarily accreted at the end of the previous volcanic episode. On top of such high heterogeneous and porous intervals, the thick lava flows constitute crustal permeability barriers, thereby constraining the circulation of hydrothermal fluids.Accreted in the near vicinity of the magma chamber, the lower section is that exposed to the most intense hydrothermal circulation (such as black smokers activity). Once capped by a massive flow at the onset of the second volcanic phase, the lower interval is hydrologically separated from ocean-waters. A reducing environment develops then below it resulting, for example, in the precipitation of sulfides. Today, whereas the interval corresponding to the first volcanic episode is sealed by alteration minerals, the second-one is still open to fluid circulation in its upper section. Thus, upper part of the volcanic edifice is potentially never exposed to fluids reaching deep into the crust, while the lower one is near the ridge axis.Considering that most of the extrusives are emplaced within a narrow volcanic zone, the first unit extruded for a given vertical cross-section is necessarily emplaced at the ridge-axis. In Hole 504B, the 250-m-thickTransition Zone from dikes to extrusives is interpreted as the relict massive unit flooding the axial graben at the onset of the first volcanic sequence, and later ruptured by numerous dikes. Further from the axis, the same massive unit constitutes a potential permeability cap for vertical crustal sections accreted earlier. Also, the upper 50 meters of the basement might be considered as the far-end expression of massive outpours extruded near the ridge-axis. |
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| Keywords: | Accretion model oceanic crust downhole measurements hydrothermalism mid-ocean ridge |
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