Geological and seismic evidence for the tectonic volution of the NE Oman continental margin and Gulf of Oman

Late Cretaceous obduction of the Semail ophiolite and underlying thrust sheets of Neo-Tethyan oceanic sediments onto the submerged continental margin of Oman involved thin-skinned SW-vergent thrusting above a thick Guadalupian–Cenomanian shelf-carbonate sequence. A flexural foreland basin (Muti and Aruma Basin) developed due to the thrust loading. Newly available seismic reflection data, tied to wells in the Gulf of Oman, suggest indirectly that the trailing edge of the Semail Ophiolite is not rooted in the Gulf of Oman crust but is truncated by an ENE-dipping extensional fault parallel to the coastline. This fault is inferred to separate the Semail ophiolite to the SW from in situ oceanic Gulf of Oman crust to the NE. It forms the basin margin to a “hinterland” basin formed atop the Gulf of Oman crust, in which 5 km of Late Cretaceous deep-water mudstones accumulated together with 4 km of Miocene and younger deep-water mudstones and sandstones. Syndepositional folding included Paleocene–Eocene folds on N-S axes, and Paleocene to Oligocene growth faults with roll-over anticlines, along the basin flank. Pliocene compression formed, or tightened, box folds whose axes parallel the modern coast with local south-vergent thrusts and reversal of the growth faults. This Pliocene compression resulted in large-scale buckling of the Cenozoic section, truncated above by an intra-Pliocene unconformity. A spectacular 60-km-long, Eocene(?) to Recent, low-angle, extensional, gravitational fault, down-throws the upper basin fill to the north. The inferred basement of the hinterland basin is in situ Late Cretaceous oceanic lithosphere that is subducting northwards beneath the Makran accretionary prism.

Shallow-marine serpentinization-derived fluid seepage in the Upper Cretaceous Qahlah Formation, United Arab Emirates

Serpentinization of ultramafic rocks in the sea and on land leads to the generation of alkaline fluids rich in molecular hydrogen (H2) and methane (CH4) that favour the formation of carbonate mineralization, such as veins in the sub-seafloor, seafloor carbonate chimneys and terrestrial hyperalkaline spring deposits. Examples of this type of seawater–rock interaction and the formation of serpentinization-derived carbonates in a shallow-marine environment are scarce, and almost entirely lacking in the geological record. Here we present evidence for serpentinization-induced fluid seepage in shallow-marine sedimentary rocks from the Upper Cretaceous (upper Campanian to lower Maastrichtian) Qahlah Formation at Jebel Huwayyah, United Arab Emirates. The research object is a metre-scale structure (the Jebel Huwayyah Mound) formed of calcite-cemented sand grains, which formed a positive seafloor feature. The Jebel Huwayyah Mound contains numerous vertically orientated fluid conduits containing two main phases of calcite cement. We use C and O stable isotopes and elemental composition to reconstruct the fluids from which these cements precipitated and infer that the fluids consisted of variable mixtures of seawater and fluids derived from serpentinization of the underlying Semail Ophiolite. Based on their negative δ13C values, hardgrounds in the same section as the Jebel Huwayyah Mound may also have had a similar origin. The Jebel Huwayyah Mound shows that serpentinization of the Semail Ophiolite by seawater occurred very soon after obduction and marine transgression, a process that continued through to the Miocene, and, with interaction of meteoric water, up to the present day.

Architecture of the crust and lithosphere beneath the Semail Ophiolite from ambient noise tomography and receiver functions: insights on the tectonic evolution of eastern Arabia

<p>The Semail Ophiolite is the world<span>‘</span>s largest and best exposed oceanic lithosphere on land and a primary reference site for studies of creation of oceanic lithosphere, initiation of subduction, geodynamic models of obduction, subduction and exhumation of continental rocks during obduction. Five decades of geological mapping, structural, petrological and geochronological research provide a robust understanding of the geodynamic evolution of the shallow continental crust in northern Oman and how the late Cretaceous obduction process largely shaped the present-day landscape. Yet, prior to obduction, other first-order tectonic processes have left their imprint in the lithosphere, in particular the Neoproterozoic accretion of Arabia and Permian breakup of Pangea. Due to the scarcity of deep structure imaging below the ophiolite, the presence and significance of inherited structures for the obduction process remain unclear.</p><p>We discuss a new 3-D anisotropic shear wave velocity model of the crust below northern Oman derived from ambient noise tomography and Receiver Function analysis which allows to <span>resolve</span> some key unknowns in geodynamics of eastern Arabia: (1) <span>Several NE-trending structural boundaries in the middle and lower crust are attributed to the Pan-African orogeny and align with first-order lateral changes in surface geology and topography.</span> (2) The well-known Semail Gap Fault Zone is an upper crustal feature whereas two other deep crustal faults are newly identified. (3) Permian rifting occurred on both eastern and northern margins but large-scale mafic intrusions and/or underplating occurred only in the east. (4) While obduction is inherently lithospheric by nature, its effects <span>are mostly observed at shallow crustal depths, and lateral variations in its geometry and dynamics can be explained by effects on pre-existing Pan-African and Permian structures. (5) Continental subduction and exhumation during late Cretaceous obduction may be the cause for crustal thickening below today‘s topography.</span> (6) Thinning of the continental lithosphere below northern Oman in late Eocene times – possibly related to thermal effects of the incipient Afar mantle plume - provides a plausible mechanism for the broad emergence of the Oman Mountains and in particular the Jabal Akhdar Dome. Uplift might thus be unrelated to compressional tectonics during Arabia-Eurasia convergence as previously believed.</p>

Chapter 6 Conclusions, differences between the Jabal Akhdar and Saih Hatat domes and unanswered questions

This chapter provides the conclusions/outlines of the tectonics, affecting the Southeastern Oman Mountains, including the Jabal Akhdar and Saih Hatat domes. The main tectonic events include amongst others (1) Neoproterozoic rifting, (2) two distinct early Paleozoic compressive events, (3) large-scale open ‘Hercynian’ folding and formation of a pronounced unconformity during the late Paleozoic, (4) rifting preceding the opening of the Neo-Tethys Ocean during the late Paleozoic, (5) late Cretaceous obduction of the Semail Ophiolite and the response of the Arabian lithosphere as well as (6) post-obductional tectonics. Also of major geological significance are the three major glaciations (Sturtian, Marinoan and Late Paleozoic Gondwana glaciation) which have been recorded in the rocks of northern Oman. Moreover, major lithological, structural and metamorphic differences exist between the Jabal Akhdar and Saih Hatat domes. It appears likely that a major fault, striking parallel to the eastern margin of the Jabal Akhdar Dome, probably originating during Neoproterozoic terrain accretion, acted as a divide between both domes until present. This fault was multiple times reactivated and could explain the differences between the two domes. A catalogue of unanswered questions is included in chronological order to express that many geological aspects need further investigation and future research projects.

Chapter 5 Tectonic evolution of the Oman Mountains

The tectonic evolution of the Oman Mountains as of the Neoproterozoic begins with a major extensional event, the Neoproterozoic Abu Mahara rifting. It was followed by the compressional Nabitah event, still during the Neoproterozoic, in Oman but possibly not in the study area. During the earliest Cambrian, the Jabal Akhdar area was affected by the Cadomian Orogeny, marked by NE--SW shortening. It is unclear, whether the Saih Hatat area was exposed to the Cadomian deformation, too. Still during the lower Cambrian, the Angudan Orogeny followed, characterized by NW--SE shortening. An episode of rifting affected the Saih Hatat area during the mid-Ordovician. During the mid-Carboniferous, both dome areas were deformed by tilting and large-scale open folding in the course of the ‘Hercynian’ event. As a consequence, a major unconformity formed. As another Late Paleozoic event, the Permian break-up of Pangaea and subsequent formation of the Hawasina ocean basin, are recorded in the Southeastern Oman Mountains. As a result, a passive margin formed which existed until the mid-Cretaceous, characterized by deposition of mostly shelfal carbonates. This interval of general tectonic quiescence was interrupted during the early Jurassic by uplift and tilting of the Arabian Platform. The platform collapsed during the late Cretaceous, related to the arrival of the obducted allochthonous nappes including the Semail Ophiolite, transforming the passive margin to an active margin.The Semail Ophiolite formed most likely above a subduction zone within the Neo-Tethys Ocean during the Cenomanian while parts of the Arabian Plate were subducted to the NE. Formation of oceanic lithosphere and SW-thrusting was broadly coeval, resulting in ophiolite obduction onto the Hawasina Basin. The Semail Ophiolite and the Hawasina rocks combined were thrust further onto the Arabian Plate. Their load created a foreland basin and forebulge within the Arabian Platform. Once the continental lithosphere of the Arabian Platform was forced into the subduction zone, a tear between the dense oceanic lithosphere and the buoyant continental lithosphere developed. This led to rapid uplift and exhumation of subducted continental lithosphere of the Saih Hatat area, while obduction was still going on, causing in multiple and intense folding/thrusting within the eastern Saih Hatat Dome. Exhumation of the Saih Hatat Dome was massive. The emplacement of the ophiolite was completed during the Campanian/Maastrichtian. For completeness, we also present alternative models for the developmental history of the Semail Ophiolite.Immediately after emplacement, the Arabian lithosphere underwent intense top-to-the-NE extensional shearing. Most of the Saih Hatat Dome was exhumed during the latest Cretaceous to Early Eocene, associated with major extensional shearing at its flanks. Further convergence during the late Eocene to Miocene resulted in exhumation of the Jabal Akhdar Dome and some gentle exhumation of the Saih Hatat Dome, shaping the present-day Southeastern Oman Mountains. In the coastal area, east and SE of the Saih Hatat Dome, some late Cretaceous to present-day uplift is evident by, e.g., uplifted marine terraces. The entire Oman Mountains are uplifting today, which is evident by the massive wadi incision into various rock units, including wadi deposits which may form overhangs.