The transition from a passive margin to an Upper Cretaceous foreland basin related to ophiolite emplacement in the Oman Mountains
The Upper Cretaceous Muti Formation in the Oman Mountains exemplifies the transition from a passive continental margin to a foreland basin related to thrusting and ophiolite emplacement. Field data largely substantiate theoretical models that depict a flexural forebulge moving continentward of an advancing thrust-load, followed by flexural downwarping and collapse as nappes override a continental margin. Five units of the Upper Cretaceous (? late Cenomanian–late Campanian) Muti Formation depositionally, or structurally, overlie Mesozoic passive-margin paleoenvironments ranging from abyssal plain to continental rise, slope, and carbonate platform. The Arabian continental margin and Tethys ocean rifted in Permian and Triassic time, giving rise to a mature passive margin in the Jurassic and Lower Cretaceous. At the end of Lower Cretaceous time, northeastward subduction apparently began, and the Semail ophiolite formed, probably in a marginal basin setting. As subduction continued, the trench migrated toward the Arabian continent preceded by a flexural fore-bulge that soon reached the adjacent Arabian continent (Cenomanian-Turonian; ca. 90−88.5 Ma), halting deposition regionally (“Wasia-Aruma break”). Flexural uplift was greatest near the shelf edge, with removal of as much as 600 m of the platform edge succession, and redeposition onto the base of slope as slump sheets, debris flows, and lithoclastic turbidites. As the advancing thrust-load impinged on the edge of the Tethys ocean, the crust was downflexed (late Coniacian–Campanian time; 88.5−73 Ma), creating a foredeep that partly filled with terrigenous clastics, olistostromes, detached blocks of rift-related carbonate build-ups, seamount volcanics and continental basement rocks. The basin depocenter migrated inboard with time, collapsing, in turn, Mesozoic abyssal plain, base of slope, and slope settings to establish a foreland basin on the foundered shelf. Terrigenous sediments accumulated in deep water below the carbonate compensation depth (CCD), together with lithoclastic limestones and detached blocks shed from submarine fault scarps. Successive foreland basins were overridden by the advancing nappes. During the later stages of thrusting onto the continent, Mesozoic sediment decollement nappes were bulldozed ahead of the Semail nappe, ploughing into the existing foreland basin. In conjunction with the final emplacement of the Semail ophiolite, the foreland basin migrated to its most continentward location and filled with clastics derived by subaerial erosion of the Semail nappe and other units. The nappes were submerged again in late Maastrichtian time, possibly in response to flexural relaxation, followed by re-establishment of a carbonate platform. Further compression and thrusting in the Paleocene-Eocene gave rise to a successor foreland basin adjacent to the northern Oman Mountains front.