Geology of the Semail ophiolite, northern Oman Mountains, is reviewed. The ophiolite is one of the largest in the world with an extension of 500 km along the Oman Mountains, which is a part of a series of nappes obducted onto the Arabian Continent during the closure of the Tetheys Sea at the end of Cretaceous. It preserves almost complete sections up to 14 km thick which are expected for the upper part of oceanic lithosphere formed at medium to fast spreading ridges: pillow and sheet flows with little intervening volcaniclastics, well developed sheeted dike swarm, large continuous plutonic bodies, and harzburgitic tectonite with foliations and lineations indicative of diapiritic upwelling, in descending order. These are superposed and modified by subsequent magmatic and tectonic episodes during the obduction, such as later lavas and plutonic bodies with island-arc affinities, deep erosion down to the sheeted dike swarm, steep low-T sheare zones in the mantle peridotite, and milonitic deformation at the base of the nappes.

Although the Semail ophiolite possesses many structural and volcanological similarities to the mid-ocean ridges, depleted but less primitive lava geochemistry cannot be reconciled by the origin as such, and still needs further research not only on ophiolites but also on the ocean floors.