Structural evolution of the Jebel Akhdar culmination and its implications for exhumation processes in the northern Oman Mountains

Many orogenic belts show evidence for coeval extension and compressional tectonics. Here I present new evidence for such activity during the tectonic evolution of the Oman Mountains, a Late Cretaceous orogenic system on the NE margin of the Arabian continent. The tectonic significance and implications of this discovery are discussed within the context of convergent plate boundary processes. During the Late Cretaceous the NE margin of the Arabian continent was overthrust by "exotic" sheets of oceanic and continental margin units (the Semail ophiolite allochthon). Some parts of this margin (Saih Hatat Massif) were deeply buried through subduction, to depths suitable for eclogite facies metamorphism, while other parts are unmetamorphosed (Jebel Akhdar Massif). Consequently an almost continuous metamorphic gradient is preserved, that creates an ideal setting with which to relate both shallow and deep-seated tectonic processes to orogenic development Current mapping and structural studies reveal that the extensional deformation that followed the obduction of the allochthons and concomitant partial subduction of the continental margin is very much more extensive than previously recognized and was synchronous with the folding of the culminations of the Oman orogen. The extensional deformation appears to be partitioned into two orthogonal structural styles: NNE-directed shearing and vertical thinning, together with a local NW -SE contraction manifested by lineation-parallel folds. In the lebel Akhdar massif the NNE-shear deformation is extensively developed, forming steep faults and bedding-parallel detachments that extend stratigraphy with a top-to-the-NNE sense of shearing. No evidence of ongoing NW-SE contraction is seen, and hence deformation is apparently plane strain. In contrast, lebel Nakhal is arked with widespread deformation of both NNE-shearing and the concomitant NW -SE contraction, indicating non-plane strain. In Saih Ratat NNE-shearing is penetrative, with a component of orthogonal contractjon. Coeval orthogonal layer contraction, layer-thinning and elongation describe the bulk constrictional 3D strain. While 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 the constriction was the result of laterally-varying crustal extension, where the top NNE extension was locally combined with left-lateral shearing. For the Nakhal anticline, where the direction of the finite elongation axis is N030E, the orientation of sinistral transtension deformation is NNE-SSW. Exhumation of the metamorphic series ccurred beneath a carapace of extending allochthons which defined an elongate pip of material that returned to shallow crustal levels. There is an imbalance however, between the net extension and the possible contraction within the Arabian continent which requires deformation within a volume of net-divergent tectonics. Thus crustal extension continued after the end of convergent tectonics in the region.