Géométrie et dynamique de l'accrétion dans les ophiolites téthysiennes : Himalaya du Ladakh, Oman et Turquie

Ophiolites are of wide interest, for the geodynamic reconstructions of mountain chains on one hand, for detailed studies of the oceanic accretionary processus on the other hand. This volume is mainly based on the study of three regions belonging to the Téthyan domain and presenting contrasting characters : - the Semai! ophiolite in Oman is one of the best preserved ophiolites of the world; its size allows to evidence along-strike variations of the accretional processus at a scale comparable to the scale of oceanic observations; - the ophiolites in Ladakh, preserved as small klippen thrust onto the Indean plate, like the Spongtang ophiolite, or in the form of small slices along the suture zone and as basement of the arc represent important geodynamic tracers and indicate conditions of accretion and and history of obduction quite opposite to the ones deduced from the Oman ophiolite; - the tauric ophiolites represent intermediate characteristics in ail these aspects. The results of petro-structural studies within these ophiolites and their associated terraines are presented here, together with their implication for the accretionary setting, their intra-oceanic tectonic history and the mode of obduction. The Oman ophiolites comprise commonly of outcrops of complete sections from mantle harzburgite to pillow lavas. Their composition and internai structures indicate an environment of a fast spreading ridge. Transverse structures are rare and none interupts the crustal sequence. The alongstrike variations of the different facies suggest segmentation of the ridge at the scale of about 30 km that varied with time. At several places (Fizh, Haylayn) overlapping spreading centers or propagating rifts have been evidenced. The plutonic sequence is constituted to nearly equal parts by gabbros and wehrlites which are commonly interlayered and could represent mutually intrusive sill assemblages, and/or two coexisting but immiscible liquids. The internai structures of the plutonic sequence commonly show variations and intersections that suggest the existance of numerous succesive magma chambers of reduced size. Leur formation can be explained by periodic impregnation cycles of the transition zone : due to increasing melt contents this zone is supposed to become periodically unstable and to intrude into the crustal section. These primary structures are commonly transposed by a viscous flow, coupled with the plastic flow of the peridotites weil visible in the basal part of the gabbro sequence where it is in direct contact with the harzburgites. The other studied ophiolites present quite different characteristics, rarely complete sequences, and peridotites that are commonly less depleted, and more abundantly affected by lithospheric shear zones. The Spongtang Klippe is essentially composed of peridotites , in which the asthenospheric structures, are overprinted by shear zones of two directional families : one of near to . vertical foliations associated with near to horizontallineations indicating a transcurrent mouvement, the other with near to horizontal structures. Rare gabbros and wehrlites are cross-cut by diabase dike swarms orientated perpendicular with respect to the transverse shear zones. These structures suggest that the Spongtang ophiolite was accreted at the intersection of a ridge with a transform fault. The facies and their petrology have the characteristics of an ophiolite accreted at a slow spreading ridge. Structural and petrological characteristics, as weil as its Lower Cretaceous age are comparable with the Tibetan ophiolites, suggesting their accretion in a common oceanic domain. The obduction of the Spongtang Klippe is very late, coeval with collision and the result of a gravitary gliding of the small oceanic slice, special by its relief or rigidity, into a melange basin developed at the foot of the passive margin and active as late as Lower Eocene, while both units continue onto the Indean plate. This development iscompletely different from the one suggested for the Oman ophiolite. The Owen-Chaman fracture zone constitutes the limit between the two domains. The structures of the tauric ophiolites suggests equally their accetion in an oceanic bassin rich in transform faults of comparable age as the one where the Himalayan ophiolites were accreted. At least locally weil developed cumulates indicate a slightly higher accretion rate. Intra-oceanic tectonics are witnessed by near to horizontal shear zones at the base of the mantle sequence, in the amphibolitic sole, and also by surpra-ophiolitic breccias.