Constrictional extensional tectonics in the northern Oman mountains, its role in culmination development and the exhumation of the subducted Arabian continental margin

The NE margin of the Arabian continent was overthrust by 'exotic' sheets of oceanic and continental margin units (the Semail Ophiolite allochthon) in the Late Cretaceous. Although parts of this margin (Saih Hatat Massif) were deeply buried, through subduction, to depths suitable for eclogite-facies metamorphism, other parts are unmetamorphosed (Jebel Akhdar Massif). Hence an almost continuous metamorphic gradient is preserved. This forms an ideal setting within which to relate shallow and deeper-seated tectonic processes within an orogen. Structural data are presented from the Jebal Akhdar Massif, a composite antiformal structure that contains a network of structures that post-date allochthon emplacement. These include down-to-the-NNE layer-extensional shears and steeper faults. Layer-extensional shears contain open to close folds with hinge lines parallel to regional elongation directions. Larger-scale NNE-trending folds include the regional Jebel Nakhl Antiform. The same kinematic style can be traced into the exhumed high-pressure metamorphic terrane of Saih Hatat. Coeval orthogonal layer contraction and layer-thinning and elongation describes bulk constrictional 3D strain. Although this might be indicative of regional transtension, large-scale strike-slip faults, active during the extension, as predicted by general transtensional models are not evident. Consequently, it is inferred that constriction was the result of laterally varying crustal extension whereby top-to-the-NNE extension was locally combined with left-lateral shearing. Exhumation of the metamorphic series occurred under a carapace of extending allochthons, defining an elongate 'pip' of material returning to shallow crustal levels. There is, however, an imbalance between net extension and possible contraction within the Arabian continent that requires deformation within a volume of net-divergent tectonics. Thus crustal extension continued after the end of convergent tectonics in the region.