MANTLE AND DEEP CRUSTAL DYNAMICS AND OCEANIC SPREADING CENTERS AS DEDUCED FROM OPHIOLITES

Structural mapping of mantle and crustal structures in ophiolites, mainly the Oman ophiolite, reveals a contrasted behavior between center and tips of ridge segments. Large deformation ascribed to homogeneous HT flow beneath the ridge of origin is always recorded in segments. It corresponds either to signature of mantle upwelling or mantle drifting away from the upwelling areas. In ophiolites thought to represent fast spreading situations as Oman, the large plastic deformation in the horizontally drifting mantle peridotites is coupled with a similarly large magmatic deformation in the overlying an flat laying layered gabbros. In contrast, tips of propagating segments are intensely deformed, displaying vertical mohos underlined by shear zones in the mantle and low crust. This crust is also injected by mafic dikes and brecciated on a large scale. HT thrusts affecting the mantle section about 1Ma after accretion and associated with fast rotations (30°/Ma) deduced from paleomagnetic studies are related to microplate activity.