The Hawasina Basin: A fragment of a starved passive continental margin, thrust over the Arabian Platform during obduction of the Sumail Nappe

Abstract A complete revision of the lithostratigraphy of the Hawasina Nappes in the Eastern and Central Oman Mountains, as compared to that of the previous stratigraphy ∗ , leads to a new palaeogeographical reconstruction of the basin. Five very distinct zones have been determined, representing five principal structural units that succeed one another from the external to the internal zones as follows: 1. (1) the Hamrat Duru Basin, 2. (2) the Baid Platform, 3. (3) the Al Avidh Trough, 4. (4) the Misfah Platform, and 5. (5) the Umar Basin. The history of the Hawasina Basin began in the Late Permian with the formation on the northeastern edge of Gondwana of a vast intracontinental basin, the Hamrat Duru Basin. However, this basin became a passive continental margin only during the Middle-Late Triassic. Formation of the Neo-Tethys by crustal spreading and accretion began at this time, in the internal part of the basin and in continuity with the Umar Basin, the Misfah Platform and the Al Aridh Trough which formed at the same time within the Hawasina Basin. From this period onwards, the main geomorphological features of the basin and of the margin were fixed and persisted up to the Late Cretaceous. The phase of extension by which the basin was subsequently affected during the Late Tithonian-Berriasian was mainly manifested by generalized subsidence. These main stages in the formation and evolution of the Hawasina Basin are marked by extensive development of in general alkaline, but in places transitional and locally tholeiitic MORB magmatism. The sediments of the Hamrat Duru Basin are dominated by turbidites composed of material mainly derived from the Arabian Platform, whereas the Al Aridh Trough and Umar Basin are characterized by pelagic sediments. However, the Al Aridh and Umar grabens also contain layers of breccia of variable extent, containing clasts and blocks of Late Permian and Late Triassic shallow-water carbonates derived from the adjacent Baid and Misfah platforms. Sedimentation in the Hawasina Basin thus began locally in the Late Permian and by the Triassic it was taking place in the remaining parts of the basin, continuing up to the Late Turonian-Early Senonian. When the basin closed and overthrusting during obduction of the Sumail Ophiolite occurred, sedimentation ended. The overthrusting took place in three stages: 1. (1) a southerly directed, compressive, pre/early obduction phase during which the Hawasina Basin was detached from its substratum to form an obduction prism, 2. (2) an obduction phase sensu stricto which, by gravitational mechanisms, induced major displacement of the principal Hawasina units and their piling up on the Arabian Platform (with the outermost units at the base and the innermost at the top), and 3. (3) a late obduction phase which caused local north-northeastward backfolding, possibly associated with the development of a fracture cleavage which, in the basal structural levels, may pass into true schistosity.