Arabian Margin Research Articles

Red Sea related history of extension and magmatism in the Jizan area (Southwest Saudi Arabia): indication for simple-shear during early Red Sea rifting

The tectonic and magmatic history of the Jizan coastal plain of the Red Sea (Tihama Asir) in Southwest Saudi Arabia is dominated by SW-NE extension. This extension manifests itself by a basic dike swarm and several generations of normal faults that trend roughly NW-SE. The oldest synrift deposits are non-marine clastics and bimodal volcanics of the Jizan Group, which were intruded by basic dikes, gabbros and granophyres of the Tihama Asir Magmatic Complex (TAMC). Radiometric ages for the TAMC-rocks range from 26 to 18 Ma. Rifting in the southern part of the Red Sea System therefore is certainly older than 26 Ma. Between 26 and 20 Ma the southwestern Arabian margin was affected by a monoclinal downwarp (flexure). The oldest extensional features in the Jizan area are N-S to NNW-SSE striking dikes and normal faults which were overprinted by other NW-SE striking extension structures. An array of en-echelon extensionzones is linked by complex patterns of transfer faults, mainly WNW-ESE striking dextral strike slip or oblique-slip faults. The observed superposition of dikes and normal faults of different orientations was not necessarily caused by a counterclockwise rotation of the regional stress-field, but more likely reflects the clockwise rotation of individual faultbounded blocks between the two phases of extension. Uplift of the graben shoulder east of the Jizan coastal plain is probably younger than 14 Ma and its development coincides with a major rearrangement of the kinematics along the Red Sea. This new kinematic pattern probably led to the onset of strike-slip movement along the Aqaba-Levant transform. At the same time extension and volcanism ceased in the Jizan area or shifted further to the west. After a long hiatus of about 10 Ma renewed extension facilitated extrusion of alkali-olivine-basalts since 2 Ma ago.

Tectonic Evolution of the Foothills Belt and Foreland Areas of the Taurus Overthrust Belt, Southeast Turkey: ABSTRACT

In the Foothills belt of the Taurus overthrust belt in southeast Turkey, a number of Late Cretaceous to late Miocene southward-migrating flexural foredeeps were generated in response to contractional and crustal loading events occurring along their internal margins. From a study of the disposition and deformation of the foreland basin sediments, the relative importance of the following events can be demonstrated: (1) A thin-skinned shortening of the northward-facing Arabian passive margin, which occurred during closure of Neotethys and ophiolitic obduction during the Late Cretaceous. Molasse-type marine and nonmarine sediments generated during this event are restricted to a narrow east-west-trending, southward-migrating flexural foredeep of Campanian to Paleocene age. (2) A late Miocene contractional event related to continental collision between the Arabian and Eurasian plates. The molasse sediments generated by this event are still being deposited and are volumetrically similar to those generated by the Late Cretaceous event. The importance of this late Miocene contractional event is emphasized, and its structural expression differs throughout the North Arabian platform. In the Foothills belt domain, the structures are largely thin skinned and the Late Cretaceous thrust front has been rejuvenated. In the foreland area to the south, the late Miocene shortening has a muchmore » deeper structural expression, reactivating old extensional faults and resulting in the inversion of Paleozoic- and Mesozoic-age half-graben basins up to 400 km south of the Taurus thrust front.« less

Upper Cretaceous Muti Formation: transition of a Mesozoic nate platform to a foreland basin in the Oman Mountains

ABSTRACTThe Upper Cretaceous (Turonian‐Campanian) Muti Formation (Sayja Member) documents the transition from a passive continental margin to a foreland basin, related to overthrusting of continental margin and ophiolitic nappes derived from the Tethys ocean. Upper Cretaceous northeastward subduction culminated in collision of a trench with the Arabian margin. As the trench docked with the margin the lithosphere was flexed, forming a peripheral bulge that migrated cratonward with time. The platform edge was initially uplifted (Turonian) and deeply eroded, creating the ‘Wasia‐Aruma break’. After passage of the peripheral bulge subsidence began, with accumulation first of ferruginous crusts on hardgrounds. Lime‐muds were then deposited on a deepening unstable sea‐floor, along with phosphatic nodules and crusts (Turonian‐Coniacian). Passage of the overthrust load over the Arabian continental‐margin edge downflexed the lithosphere (Santonian‐Campanian), resulting in drastic foundering of the old shelf edge to form a foredeep. Upper platform horizons collapsed as slump‐sheets and debris‐flows. Limestone blocks and lithoclastic debris‐flows were shed by mass‐wasting of the already deeply eroded old platform edge. Mud and silt were derived from the uplifted Arabian continent and deposited by mainly gravitational processes in a foredeep below the C.C.D. Subsidence of the Arabian platform edge allowed the Semail ophiolite nappe finally to override the Muti basin (late Campanian) with little internal deformation. Submarine emplacement is suggested by the absence of ophiolitic detritus in the Muti Formation. The stratigraphic evolution of the Muti Formation is in good general agreement with a model of the transition of an old, thermally mature, passive continental margin to a foreland basin, where the emplaced load is submerged.

The role of fracture zones during early Red Sea rifting: structural analysis using Space Shuttle radar and LANDSAT imagery

The structural pattern of the Red Sea region, an area where continental lithosphere is cut by a young oceanic rift, was studied by means of satellite and space shuttle radar imagery. The Space Shuttle Mission 2 made two passes over the Red Sea bracketing a region transitional between the southern Red Sea, where true seafloor spreading started about 5 Ma BP, and the northern Red Sea where crust is probably continental, though thinned and stretched. Structures on these passes define bands of shearing in the African and Arabian margins parallel to the Dead Sea Fracture Zone (000–020°). Combinations of these data with LANDSAT information reveal four shear bands along the margins of the Red Sea and one oblique to the Gulf of Suez. We have named the northern three Red Sea shear zones detected on the SIR-A data, the Brothers–Dead Sea Fracture Zone, the Zabargad Fracture Zone and the Shagara Fracture Zone. Two sets of normal faults were also detected, trending 330–340° (parallel to the northern Red Sea coastline) and 310–320° (parallel to the axial deeps in the Red Sea). A map of the 310–320° faults bordering the Red Sea resembles a V-shaped wedge narrowing to the north near the Gulf of Suez. These faults may have formed as a result of the most recent push of seafloor spreading. If this is so, then there may be a propagating tectonic wedge moving from south to north which thins the continental crust up to distances of 300 km from the axis of spreading. Following continental deformation, oceanic crust may be emplaced in 'hot points' along the rift axis. These 'punctiform' oceanic spreading centres should propagate in the same direction as continental rifting. During this episode palaeo-shear zones probably act as barriers ('locked zones') to the axial propagation. One such case is at the Zabargad Fracture Zone where the oblique impact of the propagating rift against the palaeo-shear may have placed the Red Sea's western side under compression and uplift while the eastern side may have been subject to oblique extension with the formation of pull-apart basins. The Space Shuttle imagery allows integration of some of the northern Red Sea morphotectonic features with structures on the continent.

Geochemical evidence for the origin of Late Triassic melange units in the Oman Mountains as a small ocean basin formed by continental rifting

A useful tool to elucidate past tectonic environments is the geochemistry of volcanic and sedimentary rocks when used together. The regional structural setting of the Oman Mountains indicates that deep-water sediments and volcanic rocks formed adjacent to the rifted Arabian margin in the Late Triassic near the axis of a narrow ocean basin of Red Sea-type. Tholeiitic to trachytic extrusives formed seamounts associated with Late Triassic reefal build-ups. “Immobile” trace element compositions point to a within-plate origin. The interbedded and overlying Late Triassic deep-sea sedimentary cover comprises ribbon radiolarites and both distal siliclastic and calcareous turbidites that accumulated on an abyssal plain at least ca. 180 km northeast of the Arabian continent. Associated ferromanganiferous oxide-sediments are interpreted as chemical precipitates derived from high-temperature vents in the spreading axis of the young ocean basin. Pervasive regional subsidence took place during end Triassic/Early Jurassic time. Later, in the Cretaceous, oceanic crust was consumed in a northeast-dipping subduction zone. MORB-type crust was subducted while Late Triassic volcanic edifices and sedimentary cover were accreted. During eventual trench-margin collision the Semail ophiolite split into blocks allowing sub-ophiolite melange rocks to be expelled upwards through corridors, creating the Batinah Melange. As the ophiolite nappe ploughed inboard over already thrust-assembled abyssal plain sediments (Hawasina Complex), some duplexes were uplifted, oversteepened, overturned and then slid backwards onto the ophiolite to form the Batinah Sheets.

Ophiolites: Figments of Oceanic Lithosphere?

Summary To resolve the problem of what a particular ophiolite represents it is necessary to look both at the ophiolite and at the rocks surrounding it: those immediately adjacent to the ophiolite and those of the region. Detailed geochemical and petrological studies are unlikely, on their own, to contribute to an understanding of the tectonic setting of ophiolite formation, nor of emplacement. Evidence from ophiolite complexes of the Mesozoic Arabian margin is inconsistent with the view that they originated at the spreading centre of a major ocean, or in back-arc basins. Instead, it is argued that these ophiolites represent discontinuous ‘oceanic’ rifts formed within the Arabian passive margin. Metamorphism of rocks beneath the ophiolite is considered to mark extension of the margin associated with the formation of these oceanic rifts in the late Cretaceous. These ophiolites do not mark a major suture and the true suture lies to the north of the ophiolite zone.

Genesis of the Batinah mélange above the Semail ophiolite, Oman

The Batinah mélange which overlies the late Cretaceous Semail ophiolite in the northern Oman Mountains comprises mostly sedimentary rocks of deep-water facies, alkalic lavas and intrusives, all of continental margin affinities, together with smaller volumes of Semail ophiolitic and metamorphic rocks. Four intergradational textural types of mélange can be recognized. Sheet mélange has large (>1 km) intact sheets either with little intervening matrix or set in other mélange types, and with an organised sheet orientation fabric. Slab mélange is finer textured (>100 m) and more disrupted. Block mélange has smaller (> m) blocks with some matrix and a weak to random block fabric. Clast mélange is matrix-supported rudite with a weak depositional clast fabric. Structural relationships, particularly the absence of tectonic fabrics, the decreasing strength of fragment fabrics with increasing fragmentation, and the abundance of brittle fragmentation, suggest that these mélange types formed by either gravity-driven sedimentary processes or superficial sliding or thrusting of individual rock slabs. In the slab mélange, long sequences can be pieced together, passing up from Upper Triassic mafic sub-marine extrusives and sediments into radiolarian cherts, hemipelagic and redeposited limestones, and terminating in non-calcareous radiolarities with Mn-deposits of early Cretaceous age. Mafic sills are numerous. These sequences can be matched with sub-ophiolite rocks now exposed in fault corridors through the Semail. These sequences become progressively disrupted upwards in the corridors and can be traced continuously into overlying mélange, which then thins away from the corridors. We argue that, during late Cretaceous emplacement over the Arabian margin, active fault corridors split the Semail slab and acted as conduits up which sub-ophiolite rocks were supplied to the ophiolite surface. There the rocks were redisributed by superficial processes.

K—Ar ages of alkaline igneous rocks in the northern Oman mountains, NE Arabia, and their relations to rifting, passive margin development and destruction of the Oman Tethys

SummaryK–Ar ages of biotites from a variety of alkaline volcanics and minor intrusive rocks in the nothern Oman mountains allochthon give a range of ages from Triassic (230 Ma) to mid Cretaceous (92 Ma) and represent igneous activity on the Oman continental margin throughout the Mesozoic. This was a passive margin destroyed by the emplacement across it of a pile of nappes in the late Cretaceous, including a largely intact thrust sheet of Upper Cretaceous oceanic lithosphere (the Semail ophiolite). Biotite ankaramite dykes, cutting compositionally similar volcanics, in the thrust complex immediately beneath the ophiolite, give Triassic ages and are related to the rifting and break-up of the northeast Arabian margin at the beginning of formation of the Oman Tethys. Mid Cretaceous (Cenomanian–Turonian) ages are mostly recorded from the northern part of the mountains where there are alkaline tuffs in a sedimentary melange. They are approximately the same age as the ophiolite and may be related to tectonic instability of the Oman margin immediately prior to ophiolite emplacement. Alkaline sills, intrusive into a variety of rocks, including Triassic volcanics, give Jurassic and Cretaceous ages and are interpreted as periodic alkaline activity on the Oman margin throughout passive margin development.

The upper Batinah Complex, Oman: allochthonous sediment sheets above the Semail ophiolite

The upper Batinah Complex consists of allochthonous sedimentary sheets that structurally overlie both the Semail ophiolite and its cover of mélange (lower Batinah Complex). Sequences represent a former sedimentary transition across the outer part of a passive Mesozoic continental margin. From proximal to distal, units include redeposited quartzitic sandstones and shallow-water limestones, large carbonate buildups floored by Triassic mafic lavas, and pelagic radiolarites or hemi-pelagic calcilutites also underlain by Triassic mafic lavas. Sheets of these upper Batinah Complex sequences were emplaced onto the upper surface of the Semail ophiolite in Late Cretaceous time, during or soon after the advance of the Semail Nappe onto the Arabian margin. Emplacement was along subhorizontal fault zones, with sheets of distal facies followed by successively more proximal sheets. Structural style suggests a low overburden during deformation, with ductile flow of argillaceous material but more brittle behavior of other lithologies. Large recumbent folds are present. Fold orientations imply sheet transport from between northeast and southeast.We conclude that the Batinah sheets represent part of the passive margin to a continental mass originally northeast of the main Arabian margin, but which is no longer present. We favour high-level gravity emplacement of Batinah sheets, and argue against trench accretion as an emplacement mechanism for either the sheets or the Semail Nappe itself.

Stratigraphy of the Mesozoic rocks above the Semail Ophiolite, Oman

SummaryA revised stratigraphy is presented for the rocks which lie between the Upper Cretaceous Semail ophiolite and its post-emplacement sediment cover of uppermost Cretaceous to Tertiary age. Above the pelagic sediments of the ophiolite (Suhaylah Formation, Cenomanian–Turonian) occur ophiolite-derived clastic sediments (Zabyat Formation, ?Turonian–?Campanian) related to early stages of ophiolite emplacement. These formations are structurally overlain by the Batinah Complex, comprising Triassic to Cretaceous lithologies, all previously equated with the Hawasina rocks that now lie tectonically beneath the ophiolite. We recognize a lower melange unit to the complex, which demonstrably includes Hawasina lithologies. The melange formed by superficial rather than deep processes, but the mechanism by which Hawasina material was supplied to the ophiolite surface is unproven. The upper unit of the Batinah Complex shows a coherent stratigraphy which contrasts with that of the Hawasina. It comprises three main allochthonous tectonic sheets, within which we recognize three formations and one other informal unit. The upper Batinah Complex may have been tectonically derived not from the now subjacent Arabian margin but from another margin with a passive Mesozoic history.

A geophysical survey of the westernmost Gulf of Aden

The Gulf of Aden has the features of a miniature Atlantic Ocean, namely a central rough zone, main trough and continental margins. It has probably evolved within the last 45 Ma, i.e. it is approximately one third the age of the Atlantic. Being youthful, it is a good place for studying the early stages of continental drift, sea floor spreading and evolution of continental margins. Sixteen precision depth, gravity and total intensity magnetic profiles have been obtained in the westernmost Gulf of Aden along the direction N 32/212°, estimated to be the direction of sea floor spreading from the computer fit of Arabia and Somalia. In addition, a continuous seismic reflexion profile was obtained over the northern part of one of the profiles from the axial rift zone to the Arabian continental margin. The reflexion profile reveals that the basement (surface of oceanic layer 2) has at least three distinct slopes. Changes in the characters of the gravity and magnetic anomalies are noticed corresponding to the changes in slopes of the basement. In accord with recent ideas on the formation and cooling of oceanic lithosphere, it seems unlikely that the Gulf of Aden has evolved by continuous sea floor spreading and more likely it has evolved in at least three distinct phases. The earliest of these is difficult to date from the magnetic anomalies and three possible models are presented. The most likely indicates sea floor spreading from 0 to 4.5 Ma (Plio-Pleistocene), 16 to 23.5 Ma (latest Oligocene to lower Miocene) and 35.5 to 43 Ma (upper Eocene to lower Oligocene). The most surprising result is that the seismic reflexion and gravity data require the ocean-continent boundary to be between the 100 fathom contour and the coast. This implies that the continental margins are underlain by early oceanic crust and should more accurately be called oceanic margins rather than continental margins. Other discoveries include three previously unmapped transform faults and a jump in the spreading axis in the eastern part of the survey area. These, together with the locations of the recent spreading axes and a possible triple junction, are shown on a new tectonic map of the area.