Late Cretaceous exhumation of the Oman blueschists and eclogites: a two-stage extensional mechanism

The continental material of the Saih Hatat window has been affected by a Late Cretaceous, obduction-related, HP-LT metamorphism below the Oman ophiolite. A high-rate exhumation process is witnessed by the Maastrichtian-Palaeocene onlap onto the blueschist-facies rocks. Drastic metamorphic omissions are documented between the lowest, eclogitic units and the overlying, blueschist-facies ones. Widespread late-metamorphic shear structures point to a top-to-the-NNE detachment, in opposition to the sense of the Late Cretaceous obduction. The inversion of the shearing sense occurred under similar, low temperature conditions in both the blueschist and eclogite-facies units. Admitting that these HP-LT metamorphic rocks formed progressively at various depth in the subducting Arabian margin, a two-stage extensional mechanism of exhumation is suggested: (i) early uplift of the eclogitic rocks up to the blueschists depth by ductile thinning or squeezing of a 'blind extensional allochthon'; (ii) exhumation of the whole HP-LT metamorphic core complex by inversion of the obduction sole-thrust and isostatic rebound of the lower plate.