Ophiolite obduction and the Samail Ophiolite: the behaviour of the underlying margin

Abstract Samail ophiolite emplacement has become a type-example for ophiolite obduction. Despite this, problems and controversy remain with respect to the age of high- P metamorphism, the vergence of structures in deformed rocks beneath the ophiolite, the suprasubduction character of the ophiolite and the P-T conditions of the metamorphic sole. The presence of major, regional-scale NE-facing isoclinal folds and SW- and west-dipping shear zones with top-to-the-NE shear sense in Arabian margin rocks beneath the Samail Ophiolite nappe requires that the margin was not simply passively overridden during obduction of the ophiolite. The development of these folds at 72–76 Ma is at the time the ophiolite is emplaced finally onto the margin (80–70 Ma), accompanied by development of a major shear zone in Saih Hatat (the upper plate-lower plate discontinuity described by earlier workers) at c. 82–80 Ma. Structural scenarios that incorporate these folds and shear zones include lateral escape from a rising buoyant crustal slice along the former subduction interface, back-folding (‘retrocharriage’) associated with major oceanwards-directed underthrusting, or simple underthrusting of the margin by the oceanic realm. Previous models involving craton-directed overthrusting with domal culminations related to deep-seated footwall and lateral ramps are more applicable to the Tertiary structure and Tertiary evolution of the mountain range. Oman ophiolite obduction clearly involves ocean-vergent thrusting within the continental margin platform to slope facies sequences.