Structural mapping in the Oman ophiolites: Mantle diapirism along an oceanic ridge

Abstract

Structures induced in peridotites by high-temperature plastic flow at the oceanic spreading centre source have been mapped in the Oman ophiolite. They can be reset to the geometry of the spreading system by rotating the boundary between these peridotites and the layered gabbros (the Moho) to the horizontal and by equating the trend of the sheeted dyke complex with that of the ridge. A number of typical asthenospheric flow patterns thus emerge which are more specifically described in a companion paper ∗ ∗ Ceuleneer et al. (this issue). . The most spectacular is a structure characterized by divergence of flow planes and flow lines from the vertical to the horizontal above and around mantle diapirs 10 × 15 km in diameter. Such diapirs are also recognized by traces of an intense magmatic activity (magmatic impregnation, and an abundance of gabbro dykes and intrusions and chromite pods). The general structure of the 500 km long Oman palaeo-ridge is dominated by these diapirs which are spaced approximately every 50 km along strike. In the preliminary picture which is revealed by this study, no other mantle structure typical of a ridge segmentation is documented with the possible exception of a transform structure within the Wadi Tayin massif. Evidence converges to indicate that Oman ophiolites correspond to a fast-spreading ridge.