Structural evolution of Jabal Qumayrah: A salt-intruded culmination in the northern Oman Mountains

Abstract

The Jabal Qumayrah area of the northern Oman Mountains records the evolution and subsequent destruction of a Mesozoic passive continental margin in the Oman segment of the Neo-Tethys Ocean, followed by the re-establishment of a passive margin, punctuated by phases of Tertiary compression. Almost uniquely along the Oman Mountains, it also contains intrusions of salt. Detachment of oceanic sediments and volcanics during the early phases of NE-directed subduction beneath the nascent Semail Ophiolite created an in-sequence stack of imbricated thrust units comprising distal trench units (Haybi Complex), and deep-ocean and continental rise sediments derived from the Mesozoic Oman margin (the Hawasina Complex). These were emplaced onto the depressed margin beneath and ahead of the ophiolite during its obduction in the Cenomanian– Coniacian. The Mesozoic continental slope sediments of the Sumeini Group had already been largely over-ridden by the more distal thrust sheets when the Hawasina sole thrust propagated into those sediments. This detached a Sumeini Group thrust sheet, which was transported westward for at least 7 km, carrying with it the overlying Hawasina thrust stack. Structurally lower parts of the Hawasina thrust stack (Hamrat Duru Group) also extended ahead of the Sumeini Group thrust sheet, but they were not restacked with it, indicating motion continued along this part of the Hawasina sole thrust. Further footwall collapse detached at least one more imbricate within the Sumeini Group and the combined thrust stack was then folded along a N-S axis, possibly above a frontal ramp. This was associated with complex out-of-sequence forward and back-thrusting at the lower structural levels. A right-lateral scissors fault developed at right angles to the direction of nappe transport, associated with normal faulting down-to-south. Late-stage culmination within the nappe pile created an asymmetrical west-facing dome, around which the structurally overlying Hawasina thrust sheets are folded. Passive margin sedimentation was re-established in the Campanian–Maastrichtian following subsidence of the locally emergent nappe pile and was dominated by carbonate sedimentation with little clastic input from the ophiolite or Hawasina sediments. Stable sedimentation persisted until Oligocene–Miocene compression, synchronous with the Zagros compressional event in Iran, resulted in west-facing folding along the western side of the northern Oman Mountains and their subsequent uplift. The Jabal Qumayrah massif preserves a salt intrusion composed of gypsum and anhydrite, the top of which is now exposed in the centre of the culmination. The origin of the salt remains unclear and investigations continue. Possible sources include the extension of the major regional salt basins found in the foreland, in particular those at the Ediacaran/Cambrian boundary (Ara Group), beneath the Hawasina Nappes and Semail Ophiolite. Alternatively, evaporitic basins may have developed locally along the edge of the proto Neo-Tethyan margin during the earliest rifting phase, beneath what became the continental slope deposits, although there is little evidence for these elsewhere in the autochthonous shelf succession.