Post-obduction evolution of the Wadi Kabir syncline area and northern Saih Hatat, Oman

Abstract

The Saih Hatat tectonic window, situated in the eastern part of the Oman mountains, exposes HP/LT units that are associated with subduction of the continental margin or a microplate. The polarity and timing of this subduction is highly debated. The plural km-scale ENE – WSW trending Wadi Kabir fault (WKF) is situated along the northern edge of Saih Hatat, to the east of Muscat, where it juxtaposes the mantle sequence of the Samail ophiolite against >7 kbar metamorphosed Triassic shelf carbonates. Obduction of the Samail ophiolite and associated thrust sheets towards the SW onto the NE Oman margin commenced during the Turonian, contemporaneous with bulging and erosion of the autochthonous shelf carbonates. The Wadi Kabir fault acted as one of the major thrusts accommodating this obduction. The km-scale Wadi Kabir syncline (WKS) is overlying the mantle sequence harzburgites to the north of the Wadi Kabir fault, and consists of neo-autochthonous, post-obduction limestones and conglomerates, the age of which is at yet poorly constraint. The elevated peneplain to the south of the Wadi Kabir fault comprises neo-autochthonous, south-dipping limestones. They unconformably overlie the north-dipping Triassic shelf carbonates, from which they are separated by a tectonic melange. This research mainly deals with the evolution of the Wadi Kabir area after obduction of the ophiolite, which is subsequently mirrored against the evolution of adjacent neo-autochthonous basins and the northern Saih Hatat region. The basis for this research is formed by a detailed geological mapping exercise, which yielded stratigraphic columns, a geological map and structural profiles. Thin sections obtained at key stratigraphic intervals were analyzed for their fossil and mineral content. Movement vectors and sense from dragging features on shear bands, striations and slickenfibers on fault planes along the Wadi Kabir fault and on the tectonic melange overlying the elevated peneplain were obtained. They are analyzed using stress inversion software, in order to obtain the stress regimes active throughout the evolution of the area and their relative timing. During the exhumation stage the mantle sequence of the Samail ophiolite and associated thrust sheets was downfaulted towards the north, with the creation of ductile shear bands with top-to-N(E) extensional fabrics. This extensional regime continued into the Paleocene when the rocks entered the brittle domain, with motion over the WKF allowing hydrothermal alteration of the serpentinized mantle rocks to type I listwanites, and creation of a palaeo-topography. Sedimentation over the mantle sequence commenced during the Thanetian, during which the area was still subject to an (E)NE –(W)SW directed extensional regime as attested by NW – SE trending syn-sedimentary normal faults. These sediments are part of the Jafnayn and Rusayl formations, which were deposited in lagoonal to shallow-shelf environments. Extension ceased during the Ypresian, and was followed by thermal subsidence of Saih Hatat. Sedimentation reached the peneplain by latest Ypresian times with flooding of Saih Hatat, which was followed by deposition of shallow-shelf Seeb formation during the Lutetian – Bartonian. The area was affected by an ENE – WSW directed extensional regime during the Oligocene, with creation of WNW – ESE trending normal faults. An E – W directed compressive regime affected the area during the Miocene, resulting in creation of the km-scale WKS and order of magnitude smaller syncline on the peneplain. This also created synclinal structures in adjacent basins, and tightened Cretaceous folds in Saih Hatat. Strike-slip motion displaced the northern block towards the west, during which the type I listwanite was smeared over the fault surface of the WKF. The sediments on the peneplain were displaced relatively away from the WKF, which allowed tectonic wedging of type I listwanites over the basal conglomerates. A shift in stress regime to N – S compression during the Miocene – Pliocene resulted in uplift of the northern block containing the WKS with respect to the peneplain. This induced differential erosion cutting down to the Rusayl formation on the northern block, and down to the Seeb formation on the peneplain. Relieve of compression on the margin, possibly due a shift in the locus of deformation towards the south, resulted in relative normal motion and tilting of the northern block, emplacing the WKS in its current configuration. Simultaneous gravitational collapse of the sediments on the peneplain induced their tilting towards the south, during which the underlying formations were bulldozered towards the north. Additional low-temperature hydrothermal alteration locally created type II listwanite assemblages during this event.