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1.
Benoît Dewandel Franoise Boudier Hartmut Kern Waris Warsi David Mainprice 《Tectonophysics》2003,370(1-4):77-94
Shallow seismic measurements in harzburgite from the Oman ophiolite performed in a zone where the maximum horizontal anisotropy is expected (vertical foliation and horizontal lineation) point to a dominant dependence of seismic properties on fracturing.
Optical microscopy studies show that microcracks are guided by the serpentine (lizardite) penetrative network oriented subparallel to the harzburgite foliation and subperpendicular to the mineral lineation, and that serpentine (lizardite) vein filling has a maximum concentration of (001) planes parallel to the veins walls. The calculated elastic properties of the oriented alteration veins filled with serpentine in an anisotropic matrix formed by oriented crystals of olivine and orthopyroxene are compared with seismic velocities measured on hand specimens.
Laboratory ultrasonic data indicate that open microcracks are closed at 100 MPa pressure, e.g. (J. Geophys. Res. 65, (1960) 1083) and (Proc. ODP Sci. Results Leg 118, (1990) 227). Above this pressure, laboratory measurements and modeling show that P-compressional and S-shear wave velocities are mainly controlled by the mineral preferred orientation. Veins sealed with serpentine are effective in slightly lowering P and S velocities and increasing anisotropy. The penetrative lizardite network does not affect directly the geometry of seismic anisotropy, but contributes indirectly in the fact that this network controls the microcrack orientations.
Comparison between seismic measurements of peridotite and gabbro in the same conditions suggest that P- and S-waves anisotropies are a possible discriminating factor between the two lithologies in the suboceanic lithosphere. 相似文献
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Marine Geophysical Research - 相似文献
3.
Boudier Françoise Godard Marguerite Armbruster Catherine 《Marine Geophysical Researches》2000,21(3-4):307-326
The eastern end of the Haylayn massif exposes a complex paleoridge structure interpreted as the tip of a northwestward propagating segment (Nicolas et al., this issue). The area, revisited from a petrostructural and geochemical viewpoint, offers the most documented exposures of the association of olivine gabbros and gabbronorites in Oman (Juteau et al., 1988). Gabbronorites were injected while the main gabbro unit was deforming in the magmatic state. Both units do not differ chemically, except for the SiO2 enrichment of the orthopyroxene-rich gabbros relative to olivine-gabbro. In addition, they display the same trace element signature, which implies the same parent magma for both units. The extension of the stability field of orthopyroxene is assigned to increase of oxygen fugacity due to hydration. The source of hydration is the ridge axis hydrothermal circulation, suggesting hydrothermal/magma interaction at temperatures above the gabbro solidus. The distribution of gabbronorites at the scale of the entire ophiolite suggests a relation with ridge tectonics where high-T conditions of hydrothermal-magmatic interaction are met. Such conditions are met when propagating segments rotate the structures of the dying magma chamber. 相似文献
4.
The Oman‐Emirates is the largest and best‐exposed ophiolite; consequently, it has attracted significant interest among scientists, together with serious conflicts. Most geologists regard this ophiolite as having formed in an intra‐oceanic subduction zone before being accreted to the Arabian continent. Here, we propose an alternative scenario, supported by detailed field observations and integrated geophysics. The smaller Emirates part of the ophiolite was forced into a nearby continent, in the pre‐collision stage of Tethyan closure. The contraction led to the exhumation of the mantle floor of segmented basins accreted in a rifted system similar to the present‐day Gulf of California. The implied high temperature–high pressure metamorphism and the range of geochemical signatures were introduced during the process of rifting, whereas the larger Oman ophiolite was emplaced by obduction onto and along the subducting continental shore. This Ridge–Trench–Transform system might call for a new process to obduct over continents in particular Tethyan ophiolites. 相似文献
5.
Françoise Boudier 《Contributions to Mineralogy and Petrology》1971,33(4):331-345
Complete physical and chemical data presented concern serpentine minerals choosen from epizonal to non metamorphic domains. It is tried to correlate well-studied serpentine minerals, generally coming from particular outcrops with the rock-forming ones known only through the microscope.It appears that ribbons filling serpentine veins in a serpentinized dunite, the whole mesh-structure of a non metamorphic serpentine, and bastite lamellae are made of lizardite. Antigorite, only serpentine mineral present in metamorphic domains, shows no significative difference between high-pressure facies (glaucophane schist) and low-pressure facies (green-schist). The data allow a better understanding of the serpentinization process. The geochemically antinomic behaviour of iron and magnesium is emphasized. 相似文献
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For the first time, the crystallized remnant of an oceanic ridge magma chamber is documented in the Oman ophiolite. It exists in the centre of a 40 km long monoclinal ridge (Jebel Dihm, Wadi Tayin massif), exposing a full crustal section perpendicular to the spreading direction. New detailed mapping supported by U‐Pb zircon geochronology suggests that the active, fast‐spreading ridge that died just prior to detachment of the ophiolite is preserved and largely intact. Our observations provide insights into the crystallizing mush zone of a magma chamber, before it crosses the external walls and solidifies as deformed gabbros. Our data provide new constraints on the shape and internal dynamics of a magma chamber, including gabbro subsidence from the floor of a perched melt lens and the limited contribution of sills to crustal accretion. By locating precisely the palaeo‐ridge axis, prior full spreading rate estimates can be increased to ~140 km Ma?1. 相似文献
8.
Structural contribution from the Oman ophiolite to processes of crustal accretion at the East Pacific Rise 总被引：1，自引：0，他引：1 
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下载免费PDF全文 A comprehensive model for the activity of the elementary accretion segment at fast‐spreading ridges relies on integration of structural data from the Oman ophiolite and geophysical results from the East Pacific Rise (EPR) around 9°N, which are of comparable size and spreading rates. The axial melt lens at shallow crustal level provides a link between Deval segmentation at the seafloor and a lower melt sill at Moho level, imaged at the EPR as a crustal melt zone (CMZ) and mapped in Oman as the Moho transition zone (MTZ). Both are attached to a mantle upwelling at the EPR, and to a frozen diapir in Oman. The physical link between diapiric mantle uprising at the Moho and Devals segmentation at the seafloor is the melt being injected from the mantle into the lower MTZ, ponding there, and then being released by powerful injections into the upper melt lens. The magma chamber covers the diapir at a distance of 5 km from the ridge axis. 相似文献
9.
In the Oman ophiolite, the large scale Makhibiyah shear zone, in Wadi Tayin massif was generated with no or little relative motion between the two adjacent blocks, in contrast with what is reported from otherwise similar shear zones in deep crust and upper mantle. This shear zone is asymmetrical with, along one margin an asthenospheric mantle (～1200 °C) and along the adjacent margin, a lithospheric mantle (～1000 °C). Within the hotter side and with increasing shear strain, horizontal flow lines smoothly swing towards the shear zone direction before abutting against the wall of the lithosphere side. Profuse mafic melts issued from the hotter mantle are frozen in the shear zone by cooling along this lithospheric wall. Tectonic and magmatic activities are entirely localized within the asthenospheric compartment. Mantle flow lines were rotated, during their channelling along this NW‐SE shear zone, in the NW and SE opposite directions. Depending on whether the flow lines are deviated NW or SE, dextral or sinistral shear sense is recorded in the shear band mylonitic peridotites. This demonstrates that the shear zone was not generated by strike‐slip motion, a conclusion supported by regional observations. 相似文献
10.
Françoise Boudier 《Contributions to Mineralogy and Petrology》1991,109(1):114-123
Experimental partial melting of plagioclase lherzolite at 0.7 MPa confining pressure has produced euhedral olivine crystals by corrosion and overgrowth during cooling. The particular conditions of this experiment allow observation of both processes recorded in the crystal shape: corrosion boundaries are rounded, or straight when parallel to [001] intersection of {110} planes; (010) and {110} facets are developed by fast overgrowth during quenching. These observations support the contention that phenocrysts in basaltic or picritic magmas are, in part, xenocrysts. The possible mantle origin of olivine crystals in two natural occurrences of ultramafic magmas; the picritic pillow lavas of the Troodos, and a wehrlitic intrusion of the oman ophiolite, is investigated. In both cases discriminant characteristics are deduced from detailed microstructural study. The mantle origin of olivine megacrysts in the investigated picrites raises the question about the existence of picritic magmas in the mantle. 相似文献