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Origin of culminations within the Southeast Oman Mountains at Jebel Ma-jhool and Ibra Dome

Department of Earth Sciences, Memorial University of Newfoundland, St. John’s, Nfld. Canada A1B 3X5

Structural culminations within the southeast Oman Mountains at Jebel Ma-jhool and the Ibra Dome developed through a process of multistage thrusting. This involved assembly and emplacement of the Semail and Hawasina thrust sheets over a footwall sequence of shelf carbonates and their pre-Permian basement, followed by progressive footwall collapse and breaching of the Hawasina thrust stack producing a hanging-wall anticline cored by shelf carbonates at Jebel Ma-jhool. At the Ibra Dome, foreland- and hinterland-dipping thrusts developed during the latter stages of thrusting forming a pop-up structure. Structural relations at Jebel Ma-jhool provide conclusive evidence that culminations within the Hawasina Complex are related to a decollement surface which lies at least as deep as the level of the shelf carbonates. Formation of the Ibra Dome and Jebel Ma-jhool structures was part of a widespread re-imbrication and culmination forming episode within the Oman Mountains. Major culminations, such as the Jebel Akhdar—Jebel Nakhl—Saih Hatat structures form significant morphological as well as structural highs. Slippage of the upper level Semail Ophiolite and Oman Melange sheets off these structures has resulted in truncation of the underlying Hawasina Complex thrust stack at Ibra Dome and Jebel Ma-jhool producing a break-back thrust sequence. The timing of initial thrusting, subsequent re-imbrication and culmination formation, and gravity-driven slippage of surficial thrust sheets is constrained to a 5 Ma period between the late Campanian and early Maastrichtian. This is based on the Coniacian to Campanian age of Aruma Group strata, the youngest rock unit affected by thrusting and the presence of the Maastrichtian Qahlah Formation unconformably overlying the flanks of both the Jebel Ma-jhool and Ibra Dome culminations. Broad scale folding and high-angle faulting of Maastrichtian and Tertiary strata in the southern Oman Mountains formed through regional mid-Tertiary compression and reactivation of Late Cretaceous culminations.

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