The Oman Mountains formed by late Cretaceous obduction of the Tethys-derived Semail Ophiolite. This study concerns the postobductional extension on the northern flank of the mountain belt. Nine sites at the northern margins of the Jabal Akhdar/Nakhl and Saih Hatat domes of the Eastern Oman (“Hajar”) Mountains were investigated. The northern margins are marked by a system of major interconnected extensional faults, the “Frontal Range Fault”. While the vertical displacements along the Saih Hatat and westerly located Jabal Nakhl domes measure 2.25–6.25 km, 0.5–4.5 km and 4–7 km, respectively, it amounts to 1–5 km along the Jabal Akhdar Dome. Extension had started during the late Cretaceous, towards the end of ophiolite emplacement. Two stages of extension can be ascertained (late Cretaceous to early Eocene and probably Oligocene) at the eastern part of the Frontal Range Fault System (Wadi Kabir and Fanja Graben faults of similar strike). Along the intervening and differently striking fault segments at Sad and Sunub the same two stages of deformation are deduced. The first stage is characterized again by extension. The second stage is marked by dextral motion, including local transtension. Probable Oligocene extension affected the Batinah Coast Fault while it also affected the Wadi Kabir Fault and the Fanja Graben. It is unclear whether the western portion of the Frontal Range Fault also went through two stages of deformation. Bedding-parallel ductile and brittle deformation is a common phenomenon. Hot springs and listwaenite are associated with dextral releasing bends within the fault system, as well as a basalt intrusion of probable Oligocene age. A structural transect through the Frontal Range along the superbly exposed Wadi Bani Kharous (Jabal Akhdar Dome) revealed that extension affected the Frontal Range at least 2.5 km south of the Frontal Range Fault. Also here, bedding-parallel shearing is important, but not exclusive. A late Cretaceous thrust was extensionally reactivated by a branch fault of the Frontal Range Fault. Extension may be ductile (limestone mylonites), ductile and brittle (ooid deformation, boudinaged belemnite rostra, shear bands) or brittle. Extension is heterogeneously distributed within the Frontal Range. Extension is mainly related to orogenic/gravitational collapse of the Oman Mountains. Collapse may have been associated with isostatic rebound and rise of the two domes. In the western part of the study area, the Frontal Range Fault has a listric morphology. It is probably horizontal at a depth of 15 km below the Batinah coastal area. The fault seems to use the clay- and tuff-bearing Aruma Group as shear horizon. The depth of 15 km may coincide with the brittle-ductile transition of quartz- and feldspar-rich rocks. Close to this depth, the listric Batinah Coast Fault curves into the Frontal Range Fault. Extension along the Frontal Range and Batinah Coast faults probably reactivated preexisting late Cretaceous thrust faults during post-late Eocene time. The latter fault is likely mechanically related to the Wadi Kabir Fault via the Fanja Graben Fault and the Sunub fault segment. Listwaenite and serpentinite cluster preferably around the extensional faults. The Semail Gap probably functioned as a sinistral transform fault or fault zone during the Permian.
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