Tethyan Oceanic Lithosphere Research Articles

Pre-collisional anastomosing shear zones in the Kohistan arc, NW Pakistan

Abstract Ductile strain localization commonly forms a pattern of shear zones anastomosing around lenses of less deformed rock. Initiation and development of anastomosing shear zones are studied through description of the structures and deformation history of plutonic rocks that form the lower crust of the Kohistan arc. Structures and textures developed in these rocks result from primary magmatic to solid state regional strain, overprinted by anastomosing shear zones. The primary strain was mainly acquired during magmatic emplacement at 100-90 Ma. Strain localization took place continuously from magmatic emplacement to solid state deformation during cooling of the plutons and formed three successive sets of shear zones. Set 1 is composed of associated discrete Riedel and thrust shear zones developed above solidus conditions during southwestward thrusting. Continuous deformation from solidus to amphibolite facies conditions between 100 and 83 Ma formed the second set of shear zones. The lower amphibolite facies set 3 shear zones are diffeentiated by larger strains recorded in the thicker mylonitic zones and enlargement of the spacing between shear zones during cooling. The anastomosing pattern of shear zones described here probably represents arc-related deformation during subduction of the Tethys oceanic lithosphere below the Kohistan arc.

PERIDOTITE BODIES IN THE W MEDITERRANEAN – TECTONIC SLICES IN AN ALPINE BELT FRAGMENTED BY LATE STAGE EXTENSION AND INHERENT SLAB ROLL-BACK

Integrating a wide range of multidisciplinary data, i.a., seismic tomography features, regional tectonic transport directions and uplift/cooling rates, and ages of subduction/collision, lithospheric extension and slab detachment, suggests a new model for the development of the Alpine belt in the W Mediterranean (Fig. 1; Zeck, 1996; 1997; 1999). The model implies that the backbone of the Alpine orogeny was formed by a composite SW-NE striking Betic-Ligurian subduction system, active until some time before 22 Ma (Fig. 1b). The system which consumed Mesozoic Tethyan oceanic lithosphere was W-ward dipping under the leading edge of Iberia which was drifting E-ward under the influence of the opening of the North Atlantic. Subduction activity was terminated by the formation of the Alpine orogenic belt through collision of Iberian and Betic-Ligurian continental lithospheres. A series of late stage extensional regimes, with local formation of Neogene oceanic lithosphere, in the Valencia Gulf, Provencal-Algerian basin and Tyrrhenian basin, and inherent slab roll-back subsequently have fragmented the original collisional belt (Fig. 1c). This has produced the seemingly erratic distribution of Alpine metamorphic core complexes in the western Mediterranean (Fig. 1a). Peridotite bodies are present in most of these Alpine core complexes: Betic-Rif, Calabria, Corsica and Liguria. The Ronda peridotites in the Betic-Rif belt are among the best known. These bodies outline the Arc of Gibraltar, and were shown to represent tectonic slices of mantle rocks within the major Alpujarride nappe complex. They reached their present position in three stages (Zeck, 1997). The first covers their travel from great mantle depths to crustal levels, in the order of 150-200 km (pseudomorphed diamonds). The second stage covers induction of peridotite slices in the crustal nappe pile of the Betic-Ligurian subduction zone system. Metamorphic conditions within the peridotite and its envelope at this stage are given by mineral parageneses formed at c. 11-15 kbar and 750-850oC (Tubia, 1994). Overprinting of these HP parageneses by mainly static LP parageneses (7-3 kbar and 750-850oC; Tubia, 1994) took place during the third and final stage of emplacement which started after cessation of subduction activity and slab break-off, shortly before 22 Ma as indicated by regional cooling rate studies (Zeck, 1996). Slabbreak- off caused lateral convective inflow of high-temperature and low-density asthenospheric material into the widening gap above the sinking lithospheric slab. This may have had a triggering effect on the uplift, but first of all created a HT metamorphic/anatectic regime within the overlying Betic lithosphere leading to thermal weakening in the crustal section promoting tectonic extrusion under the influence of compression between the rigid Iberian and African lithospheric plates. At higher levels, secondary extensional tectonic units, some of which contained peridotite slices, were individualized and displaced over distances of 50-200 km. Extremely high uplift/cooling rates (ca. 500oC/m.y.; Zeck, 1996) characterise this third stage of peridotite emplacement. Directions of extensional tectonic transport were variable – NW-ward for the Sa Bermeja/Alpujata peridotite body, W-ward for the body at Ceuta and SW-ward for that of Beni Bousera – and define a centrifugal pattern radiating from the central part of the Alboran Sea basin (Fig. 1c).

Tethyan sutures of northern Turkey

Abstract The two main Tethyan sutures of Turkey, the İzmir-Ankara-Erzincan and the Intra-Pontide sutures, are reviewed through several well-studied transects crossing the suture regions. Both sutures have formed during the Early Tertiary continental collisions following northward subduction of Tethyan oceanic lithosphere. The İzmir-Ankara-Erzincan suture is represented along most of its c. 2000 km length by Paleocene and younger thrust, which emplace the upper crustal rocks of the northern continent over that of the southern continent with an intervening tectonic layer of Cretaceous subduction-accretion complexes. These thrusts constitutes a profound stratigraphic, structural, magmatic and metamorphic break, of at least Carboniferous to Palaeocene age and form the main boundary between Laurasia and Gondwana in the Turkish transect. Voluminous subduction-accretion complexes of Triassic and Cretaceous ages occur respectively to the north and south of the suture giving the antithetic subduction polarities during these two periods. This, and evidence for a major accretionary orogeny of Late Triassic age north of the İzmir-Ankara-Erzincan suture suggest that two separate oceanic lithospheres, of Carboniferous to Triassic (Palaeo-Tethys) and of Triassic to Cretaceous ages (Neo-Tethys) respectively have been consumed along the suture. The final continental collision along the İzmir-Ankara-Erzincan suture was slightly diachronous and occurred in the earliest Palaeocene to the west and in the Late Palaeocene to the east. The c. 800 km long Intra-Pontide suture is younger in age and have formed during the Early Eocene and younger continental collisions linked to the opening of the Western Black Sea Basin as an oceanic back-arc basin. At present the North Anatolian Fault, which came into existence in the Late Miocene, follows the course of the older Intra-Pontide suture.

Structure and evolution of the northern Oman margin: gravity and seismic constraints over the Zagros-Makran-Oman collision zone

The obduction process in Oman during Late Cretaceous time, and continental-to-oceanic subduction along the Zagros-Makran region during the Tertiary are consequences of the Arabian-Eurasian collision, resulting in construction of complex structures composed of the Oman ophiolite belt, the Zagros continental mountain belt and the Makran subduction zone with its associated accretionary wedge. In this paper, we jointly interpret Bouguer anomaly and available petroleum seismic profiles in terms of crustal structures. We show that the gravity anomaly in northern Oman is characterized by a high-amplitude negative-positive couple. The negative anomaly is coincident with Late Cretaceous (Fiqa) and Tertiary (Pabdeh) foreland basins and with the Zagros-Oman mountain belts, whereas the positive anomaly is correlated to the ophiolite massifs. The Bouguer anomaly map indicates the presence of a post-Late Cretaceous sedimentary basin, the Sohar basin, centred north of the Batinah plain. We interpret the negative/positive couple in terms of loading of the elastic Arabian lithosphere. We estimate the different Cretaceous-to-Recent loads, including topography, ophiolite nappes, sedimentary fill and the accretionary prism of the Makran trench. A new method, using Mindlin's elastic plate theory, is proposed to model the 2D deflection of the heterogeneous elastic Arabian plate, taking into account boundary conditions at the ends of the subducted plate. We show that remnant ophiolites are isolated from Tethyan oceanic lithosphere in the Gulf of Oman by a continental basement ridge, a NW prolongation of the Saih-Hatat window. Loading the northward-limited ophiolite blocks explains the deflection of the Fiqa foredeep basin. West of the Musandam Peninsula, the Tertiary Pabdeh foredeep is probably related to the emplacement of a 8-km-thick tectonic prism located on the Musandam Peninsula and in the Strait of Hormuz. Final 2D density models along profiles through the Oman mountain belt and the Gulf of Oman are discussed in the framework of Late Cretaceous obduction of the Tethys and synchronous subduction and exhumation of the Oman margin.

Fabric development in a fragment of tethyan oceanic lithosphere from the piemonte ophiolite nappe of the western Alps, Valtournanche, Italy

South of the Matterhorn the Valtournanche cuts through Alpine serpentinites, metagabbros, meta-pillowbasalts and metasediments—dismembered remnants of the Jurassic Tethyan oceanic crust, reassembled in the Piemonte ophiolite nappe. This study deals with a serpentinized ultramafic to mafic layered complex stemming from a spreading ridge environment. Cumulus fabrics of various kinds can be read through antigorite pseudomorphs, still allowing the detailed reconstruction of deep oceanic crust. Relics of igneous and metamorphic olivine prove crustal conditions during deformation. Fracturing of cumulus olivine was succeeded by plastic flow that activated low-temperature slip systems. Concomitant recrystallization produced metaperidotite only along shear zones, which are ascribed to subduction of the oceanic crust. At the turning point from subduction to obduction a static metamorphic event resulted in recovery and grain growth of recrystallized olivine. Afterwards serpentinization of the complex took place still under static conditions. Deformation of the serpentinite led to a sequence of four phases, involving non-penetrative cleavage formation, stretching and folding. This deformation is structurally related to obduction of the complex although partly accompanied by subduction zone metamorphism. Final movements of the ophiolites were due to cataclastic thrusting forming subnappe boundaries.

Seismicity in the vicinity of the India-Burma border: Evidence for a sinking lithosphere

The vicinity of the India-Burma border region is among a few intracontinental regions in the world where intermediate-focus earthquakes occur. The recent installation of new seismic stations has improved the detection and location capabilities for earthquakes in this region. Three seismic stations belonging to this new array are located over the zone of intermediate-focus earthquakes. Analysis of recently acquired seismic data reveals a well-defined near-vertical zone of earthquake foci extending to 200 km beneath the Arakan-Yoma fold belt. On the basis of seismic, gravity and other geological data, it is suggested that this zone of earthquakes is associated with remnants of the already subducted, but not totally assimilated, Tethys oceanic lithosphere below the Burmese plate.

U [sbnd]Pb geochronology of Gangdese (Transhimalaya) plutonism in the Lhasa-Xigaze region, Tibet

A series of different plutons from the Lhasa-Xigaze segment of the Gangdese (Transhimalaya) belt has been studied by high-resolution U Pb analyses of zircon (using zircon fractions of 5–100 grains each, selected upon specific grain characteristics). For two diorites, located east of Xigaze (Dazhuka), the zircons yield concordant ages of 93.4 ± 1.0 and 94.2 ± 1.0 m.y., respectively. Also concordant ages of 41.1 ± 0.4 and 41.7 ± 0.4 m.y. have been obtained for two granodiorites, collected southwest of Lhasa (Qushui). The precision on the ages of two granites from the Xigaze and Lhasa area, is limited by two factors: the presence of inherited radiogenic lead and the occurrence of subsequent lead loss. However, some concordant zircons, detected in both granites, define approximate ages of about 67 and 53 m.y., respectively. The inherited lead components show that melting of Precambrian material was involved in magma genesis. The U Pb ages substantiate a magmatic activity lasting from mid-Cretceous (Cenomanian) to Eocene (Lutetian) time. Such a period of plutonism at the southern margin of Eurasia, as well as the occurrence of magma generation from continental crust, suggest that the Gangdese range results from the subduction of Tethys oceanic lithosphere (Indian plate) underneath Eurasia (Eurasian plate). If this model is true, the collision of India with Eurasia (along the Lhasa-Xigaze sector) postdates the emplacement of the 41 m.y. old Gangdese granodiorites, i.e. the collision occurred after Lutetian time.

Geologic section through the Samail Ophiolite and associated rocks along a Muscat‐Ibra Transect, southeastern Oman Mountains

Regional mapping at a 1 : 60,000 scale of a 30‐km strip from the Gulf of Oman (Muscat) across the Oman Mountains, 130 km to the south, provides the geologic setting for the (∼95 m.y.) Ibra section of the Samail ophiolite. Where best preserved, the Ibra ophiolite section is an ∼8 km‐thick section of oceanic consisting of ∼0.5 km of pillow lavas, 1.2–1.6 km of sheeted diabase dike complex, 0.2–1.0 km of high‐level noncumulate gabbro, and 3.0–5.0 km of cumulate gabbro that is underlain by tectonite peridotite 9–12 km thick. The Ibra section is found on the southward dipping limb of the Sayah Hatat antiform. The tectonite peridotite represents uniformly depleted harzburgite and dunite that have been deformed by high‐temperature, low‐stress asthenospheric flow. Discordant dunites within the tectonite peridotite appear to represent either flow crystallization products from primary picritic liquids or reaction products of these liquids with the harzburgite. The structural base of the tectonite peridotite is overprinted by a high‐stress, low‐temperature deformation that can be related to its oceanic detachment. The layered gabbros are predominantly olivine‐clinopyroxene‐plagioclase cumulates, and orthopyroxene does not occur as a cumulus phase. Occurrence of cumulate wehrlites and picrites at high stratigraphic levels within the layered gabbros is evidence that the gabbroic section crystallized predominantly from the bottom upward in a periodically replenished magma chamber. High‐level gabbro represents remnants of crystallization at the roof of the magma chamber and intrudes most overlying diabase dikes. Both the diabase dike complex and pillow lavas are hydrothermally altered, and alteration and metamorphism increase downward (zeolite (?) to epidote‐amphibolite facies). In spite of pervasive alteration, relict primary mineralogy and bulk chemistry suggest that the diabase dikes and pillow lavas are cogenetic with the underlying gabbros. The present‐day Samail thrust surface truncates ophiolite stratigraphy and puts the Samail ophiolite on top of unmetamorphosed Hawasina melange. The last motion on this surface was probably no older than Maestrichtian (70–65 m.y.). Garnet amphibolites exposed as remnants of earlier thrusting (∼90 m.y.) record initial ophiolite detachment at a 14–20‐km depth within the Tethyan oceanic lithosphere. The Hawasina Group, which underlays the Samail ophiolite, is a block melange where exposed near Muscat in the north, and it grades into an imbricated broken formation at the southern limit of the map area. The Hawasina Group is thrust over Permian to Late Cretaceous shelf carbonates that represent autochthonous Arabian continental shelf deposits. Recent (post‐Miocene) collapse and dome structures, such as the Ibra dome, have complicated ophiolite stratigraphy south of Jabal Dimh. Our geologic studies strongly indicate the Samail ophiolite represents a large, coherent slab of transported oceanic lithosphere formed at a Late Cretaceous spreading center in the Tethyan Sea.

Tertiary uplift history of the Troodos massif, Cyprus

The detailed uplift history of a portion of Late Cretaceous Tethyan oceanic lithosphere is documented by in situ sedimentary sequences around the Troodos massif of Cyprus. The pre-uplift tectonic setting first involved genesis of the Troodos massif at a spreading ocean ridge of Late Cretaceous age, followed by deformation in Maastrichtian time. Then, after a brief period of latest Cretaceous deep-water pelagic sedimentation, much of the area was blanketed in early Tertiary time by a wedge of pelagic carbonates derived from the northeast. Gradual uplift to the middle Miocene is documented by shallowing-upward carbonate sequences culminating in lagoons and localized reefs that were adjacent to emergent areas of vegetated and deeply weathered Troodos rocks. A pulse of vastly accelerated middle Miocene uplift is recorded by slumping, widespread erosion, and folding of the south Troodos sedimentary sequences. This uplift was associated with rapid deposition of both clastic and carbonate sediments in subsiding basins to the south. The Miocene uplift culminated in localized reef development and gypsum deposition, then pervasive peneplanation of the Troodos massif. In contrast, much of the Troodos massif remained low-lying and tectonically stable during Pliocene time, whereas to the north, sedimentary basins developed along high-angle faults initiated during the late Miocene emplacement of the adjacent Kyrenia Range. Major uplifts during the Pleistocene epoch were renewed, as shown by thick deposits of marine and continental fanglomerates. The uplift of Troodos oceanic crust is attributed to the progressive serpentinization of ultramafic rocks of the oceanic layer 4. These rocks, which were first deformed and partly serpentinized in Late Cretaceous time, were subsequently remobilized during periods of regional tectonic instability, especially during the middle Miocene and Pleistocene. The dominant driving force may have involved the liberation of water from a subduction zone dipping northward beneath Cyprus.