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Structures des ophiolites d'Oman : flux mantellaire sous un centre d'expansion d'expansion oceanique et charriage a la dorsale

Abstract : The Oman ophiolite is a piece of the tethysian oceanic lithosphere obducted on the arabian continental margin during late cretaceous times. Fortunately, it has not yet been crusnec by continental collision and has preserved the structures imprinted during its formation and emplacement. It crops out nearly continuously on a length of 475 km along the stike of the paleo-ridge axis and down to 9 km below the paleo-Moho. The structural data (Special Issue " Oman Ophiolite ", journal of Geophysical Research, 86, 84, 1981) favour the actualistic comparison of the Oman ophiolite with the present-day fast spreading centers. The mantle peridotites of Oman have recorded two successive plastic deformation events, the first one related to the accretion of the lithosphere (asthenospheric shear flow), the second one imprinted during the first step of its emplacement (intra-oceanic chrusting). The asthenospheric flow pattern and the composition of the mantle section show strong variations along the strike of the paleo-ridge axis. Small mantle diapirs spaced along the ridge axis by a few tens to more than one hundred kilometers are involved in the accretion processes. These diapirs display an exceptional magmatic activity : The modal composition of the peridotites is unusually depleted, chromite ore bodies and mafic dikes and impregnations features are particularly abundant. Most of the magma feeding the crustal magma chamber is probably channeled through them. Such a diapir, frozen and sampled during the oceanic thrusting, has been mapped in details (Maqsad area). The asthenospheric flow is vertical in a pipe 10 to 20 km in diameter and rotates abruptly into an horizontal attitude in a narrow transition zone lying at shallow depth below the paleo-Moho (a few hundred of meters at most). At the top and in the vicinity of the diapir, asthenospheric flow diverges in every directionsbut is preferentially channeled along the ridge axis. The zone of longitudinal flow extends at least 30 km away from the center of the diapir. The narrowness of the transition zone imolies a rheologic discontinuity at the top of the diapir attributed to an abrupt increase of the magma/rock ratio, consistent with field observations. Such a pattern is explained by a feed-back effect between the magma percolation, the viscosity drop of the asthenosphere and the steed rotation of the mantle flow below the Moho. Far from the diapirs, the asthenospheric flow can display a regular pattern at the scale of one hundred kilometers along strike. In such area, the flow plane is slightly oblique to the Moho and the flow lines are perpendicular to the ridge axis, suggesting steady-state accretion of the lithosphere on one flank of the ridge. The obliquity of about 10° between the Moho and the frozen flow plane reflects probably the slope of the base of the lithosphere equated with an isothermal surface in the zone of accretion. The size and the spacing of the mantle diapirs recognized in Oman are compatible with the structural, geophysical and petrological observations along present-day fast spreading ridges. During the intra-oceanic thrusting, the mantle peridotites were plastically deformed in high stress conditions in a thickness of a few hundred meters above the thrust plane. This deformation grades upward into mylonitic vertical shear zones. They can reach a thickness of a few kilometers and can be tracked on several tens of kilometers along strike. They are synchronous with important injections of hydrous magma thought to originate in the partial melting of the metamorphic sole. Major shear zones have recorded the same kinematics as the basal peridotites and the high-grade metamorphic sole. The early thrusting of the Oman ophiolite has involved considerable disolacement parallel to the ridge axis. These observations are best explained if the intra-oceanic thrusting was initiated along the ocean ridge itself. The thrusting at the ridge implies a rapid shift (less than 1 Ma duration) from expansion to compression as the thermal structure of the ridge has to be preserved. It could be the consequence of a short braking action in the tethysian subduction below Eurasia caused by collisions of microcontinents or island arcs with the eurasian active margin. Such collisions, contemporaneous with the oceanic thrusting of the south-tethysian ophiolites (Upper Albian times) occured along several segments of the north-tethysian active margin. Tectonic events recorded by the Oman ophiolite and by the associated metamorphic and sedimentary formations are usually well correlated with the kinematic and geological history of the tethysian area.
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Submitted on : Wednesday, February 1, 2012 - 9:55:48 AM
Last modification on : Wednesday, April 27, 2022 - 4:39:33 AM
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  • HAL Id : tel-00665031, version 1

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Georges Ceuleneer. Structures des ophiolites d'Oman : flux mantellaire sous un centre d'expansion d'expansion oceanique et charriage a la dorsale. Pétrographie. Université de Nantes, 1986. Français. ⟨NNT : ⟩. ⟨tel-00665031⟩

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