Mantle flow-induced crustal thinning in the area between the easternmost part of the Anatolian plate and the Arabian Foreland (E Turkey) deduced from the geological and geophysical data

Eastern Anatolia consisting of an amalgamation of fragments of oceanic and continental lithosphere is a current active intercontinental contractional zone that is still being squeezed and shortened between the Arabian and Eurasian plates. This collisional and contractional zone is being accompanied by the tectonic escape of most of the Anatolian plate to the west by major strike-slip faulting on the right-lateral North Anatolian Transform Fault Zone (NATFZ) and left-lateral East Anatolian Transform Fault Zone (EATFZ) which meet at Karlıova forming an east-pointing cusp. The present-day crust in the area between the easternmost part of the Anatolian plate and the Arabian Foreland gets thinner from north (ca 44 km) to south (ca 36 km) relative to its eastern (EAHP) and western sides (central Anatolian region). This thinner crustal area is characterized by shallow CPD (12–16 km), very low Pn velocities (< 7.8 km/s) and high Sn attenuation which indicate partially molten to eroded mantle lid or occurrence of asthenospheric mantle beneath the crust. Northernmost margin of the Arabian Foreland in the south of the Bitlis–Pötürge metamorphic gap area is represented by moderate CPD (16–18 km) relative to its eastern and western sides, and low Pn velocities (8 km/s). We infer from the geophysical data that the lithospheric mantle gets thinner towards the Bitlis–Pötürge metamorphic gap area in the northern margin of the Arabian Foreland which has been most probably caused by mechanical removal of the lithospheric mantle during mantle invasion to the north following the slab breakoff beneath the Bitlis–Pötürge Suture Zone. Mantle flow-driven rapid extrusion and counterclockwise rotation of the Anatolian plate gave rise to stretching and hence crustal thinning in the area between the easternmost part of the Anatolian plate and the Arabian Foreland which is currently dominated by wrench tectonics.     

Counterclockwise P-T-t trajectory from the metamorphic sole of a Neo-Tethyan ophiolite (Turkey)

The Kiziltepe ophiolitic thrust sheet in the Bolkar Mountains of Turkey occurs between two subparallel ophiolite belts bounding the Tauride carbonate platform and represents a remnant of the Cretaceous Neo-Tethyan oceanic lithosphere. It is underlain by foliated amphibolite that represents a metamorphic sole developed at the inception of an intra-oceanic subduction zone in the Neo-Tethys 92-90 Ma. Blueschist-facies overprinting of the amphibolite indicates that the metamorphic sole was dragged deeper into the subduction zone where it experienced increasing P/T with cooling. Regional tectonic constraints suggest a Maastrichtian age for the timing of this blueschist-facies metamorphism. Sodic amphibole-rich veins and crossite/Mg-riebeckite rims on hornblende suggest that growth of blueschist-facies minerals was facilitated by infiltration of fluid along fractures and grain boundaries. We infer a counterclockwise P-T-t trajectory during which metamorphism was accompanied/succeeded by rapid uplift along the northern edge of the Tauride belt in Late Cretaceous-early Tertiary time.     

Reactivated Tibetan block in a Tethyan context

The Tibetan block originated during the late Palaeozoic-early Mesozoic by separation from Gondwanic India and the opening of Neo-Tethys. The event, which was responsible for reactivating the Tibetan basement, closed Palaeo-Tethys, lying to the north of it, and created the Kun Lun fold system as a consequence of the collision of the north-moving Tibetan block with the Tarim—Tsaidam block.During the late Cretaceous, South Tibet developed into an Andean-type foldbelt (Nyenchen Thangla) by subduction of Neo-Tethyan lithosphere beneath Tibet which had begun in the late Triassic. Mesozoic sedimentary sequences in Tibet are the response to an extensional regime behind an active margin. A Neo-Tethys island arc system at this margin was crushed during the early Eocene with the development of the Transhimalayan ophiolite and plutonic belts. Final collision during mid-Eocene times between the Himalayan microcontinent, a fragment of India and reworked Tibet in the Mid-Eocene produced the Indus—Tsangpo suture zone. A large part of the microcontinent, with an unconsumed segment of Neo-Tethys oceanic crust attached to it, presumably underthrust Tibet prior to the Himalayan orogeny. This feature is responsible for the double thickness of Tibetan crust and its young volcanism.     

Comment on “207Pb–206Pb single-zircon evaporation ages of some granitoid rocks reveal continent-oceanic island arc collision during the Cretaceous geodynamic evolution of the Central Anatolian crust, Turkey” – Boztug, D., Tichomirowa, M. & Bombach, K., 2007, JAES 31, 71–86

A continent-oceanic island arc collision model was proposed as a new geodynamic scenario for the evolution of the Cretaceous Central Anatolian granitoids in the Central Anatolian crystalline complex (CACC) by Boztug et al. (2007b) [Boztug, D., Tichomirowa, M., Bombach, K., 2007b. 207Pb–206Pb single-zircon evaporation ages of some granitoid rocks reveal continent-oceanic island arc collision during the Cretaceous geodynamic evolution of the central Anatolian crust, Turkey. Journal of Asian Earth Sciences 31, 71–86]. The key aspects of this model include an intra-oceanic subduction in the Neotethyan Izmir-Ankara Ocean, formation of an island arc and its subsequent collision with the northern margin of the Tauride–Anatolide Platform. The identical scenario was initially proposed by Göncüoglu et al. (1992) [Göncüoglu, M.C., Erler, A., Toprak, V., Yalınız, K., Olgun, E., Rojay, B., 1992. Geology of the western Central Anatolian Massif, Part II: Central Areas. TPAO Report No: 3155, 76 p] . Moreover, the weighted mean values of the reported 207Pb–206Pb single-zircon evaporation ages by Boztug et al. (2007b) [Boztug, D., Tichomirowa, M., Bombach, K., 2007b. 207Pb–206Pb single-zircon evaporation ages of some granitoid rocks reveal continent-oceanic island arc collision during the Cretaceous geodynamic evolution of the central Anatolian crust: Turkey. Journal of Asian Earth Sciences 31, 71–86] from A-type granitoids in the CACC seem to be miscalculated and contrast with the field data.     

A new tectonic scenario for the Sanandaj–Sirjan Zone (Iran)

Recent geochemical studies of volcanic rocks forming part of the ophiolites within the Zagros and Naien-Baft orogen indicate that most of them were developed as supra-subduction ophiolites in intra-oceanic island arc environments. Intra-oceanic island arcs and ophiolites now forming the Naien-Baft zone were emplaced southwestward onto the northeastern margin of the South Sanandaj–Sirjan Zone, while those now in the High Zagros were emplaced southwestward onto the northern margin of Arabia. Thereafter, subduction continued on opposite sides of the remnant oceans. The floor of Neo-Tethys Ocean was subducted at a low angle beneath the entire Sanandaj–Sirjan Zone, and the floor of the Naien-Baft Ocean was subducted beneath the Central Iranian Micro-continent. The Naien-Baft Ocean extended into North-West Iran only temporarily. This failed ocean arm (between the Urumieh-Dokhtar Magmatic Assemblage and the main Zagros Thrust) was filled by thick Upper Triassic–Upper Jurassic sediments. The Naien-Baft Ocean finally closed in the Paleocene and Neo-Tethys closed in the Early to Middle Eocene. After Arabia was sutured to Iran, the Urumieh-Dokhtar Magmatic Assemblage recorded slab break-off in the Middle Eocene.     

Subduction,ophiolite genesis and collision history of Tethys adjacent to the Eurasian continental margin: new evidence from the Eastern Pontides,Turkey

This paper presents several types of new information including U–Pb radiometric dating of ophiolitic rocks and an intrusive granite, micropalaeontological dating of siliceous and calcareous sedimentary rocks, together with sedimentological, petrographic and structural data. The new information is synthesised with existing results from the study area and adjacent regions (Central Pontides and Lesser Caucasus) to produce a new tectonic model for the Mesozoic–Cenozoic tectonic development of this key Tethyan suture zone.

The Tethyan suture zone in NE Turkey (Ankara–Erzincan–Kars suture zone) exemplifies stages in the subduction, suturing and post-collisional deformation of a Mesozoic ocean basin that existed between the Eurasian (Pontide) and Gondwanan (Tauride) continents. Ophiolitic rocks, both as intact and as dismembered sequences, together with an intrusive granite (tonalite), formed during the Early Jurassic in a supra-subduction zone (SSZ) setting within the ?zmir–Ankara–Erzincan ocean. Basalts also occur as blocks and dismembered thrust sheets within Cretaceous accretionary melange. During the Early Jurassic, these basalts erupted in both a SSZ-type setting and in an intra-plate (seamount-type) setting. The volcanic-sedimentary melange accreted in an open-ocean setting in response to Cretaceous northward subduction beneath a backstop made up of Early Jurassic forearc ophiolitic crust. The Early Jurassic SSZ basalts in the melange were later detached from the overriding Early Jurassic ophiolitic crust.

Sedimentary melange (debris-flow deposits) locally includes ophiolitic extrusive rocks of boninitic composition that were metamorphosed under high-pressure low-temperature conditions. Slices of mainly Cretaceous clastic sedimentary rocks within the suture zone are interpreted as a deformed forearc basin that bordered the Eurasian active margin. The basin received a copious supply of sediments derived from Late Cretaceous arc volcanism together with input of ophiolitic detritus from accreted oceanic crust.

Accretionary melange was emplaced southwards onto the leading edge of the Tauride continent (Munzur Massif) during latest Cretaceous time. Accretionary melange was also emplaced northwards over the collapsed southern edge of the Eurasian continental margin (continental backstop) during the latest Cretaceous. Sedimentation persisted into the Early Eocene in more northerly areas of the Eurasian margin.

Collision of the Tauride and Eurasian continents took place progressively during latest Late Palaeocene–Early Eocene. The Jurassic SSZ ophiolites and the Cretaceous accretionary melange finally docked with the Eurasian margin. Coarse clastic sediments were shed from the uplifted Eurasian margin and infilled a narrow peripheral basin. Gravity flows accumulated in thrust-top piggyback basins above accretionary melange and dismembered ophiolites and also in a post-collisional peripheral basin above Eurasian crust. Thickening of the accretionary wedge triggered large-scale out-of-sequence thrusting and re-thrusting of continental margin and ophiolitic units. Collision culminated in detachment and northward thrusting on a regional scale.

Collisional deformation of the suture zone ended prior to the Mid-Eocene (~45?Ma) when the Eurasian margin was transgressed by non-marine and/or shallow-marine sediments. The foreland became volcanically active and subsided strongly during Mid-Eocene, possibly related to post-collisional slab rollback and/or delamination. The present structure and morphology of the suture zone was strongly influenced by several phases of mostly S-directed suture zone tightening (Late Eocene; pre-Pliocene), possible slab break-off and right-lateral strike-slip along the North Anatolian Transform Fault.

In the wider regional context, a double subduction zone model is preferred, in which northward subduction was active during the Jurassic and Cretaceous, both within the Tethyan ocean and bordering the Eurasian continental margin.     

Relicts of an intra-oceanic arc in the Sapi-Shergol mélange zone (Ladakh, NW Himalaya, India): implications for the closure of the Neo-Tethys Ocean

In the Ladakh–Zanskar area, relicts of both ophiolites and paleo-accretionary prism have been preserved in the Sapi-Shergol mélange zone. The paleo-accretionary prism, related to the northward subduction of the northern Neo-Tethys beneath the Ladakh Asian margin, mainly consists of tectonic intercalations of sedimentary and blueschist facies rocks. Whole rock chemical composition data provide new constraints on the origin of both the ophiolitic and the blueschist facies rocks. The ophiolitic rocks are interpreted as relicts of the south Ladakh intra-oceanic arc that were incorporated in the accretionary prism during imbrication of the arc. The blueschist facies rocks were previously interpreted as oceanic island basalts (OIB), but our new data suggest that the protolith of some of the blueschists is a calc-alkaline igneous rock that formed in an arc environment. These blueschists most likely originated from the south Ladakh intra-oceanic arc. This arc was accreted to the southern margin of Asia during the Late Cretaceous and the buried portion was metamorphosed under blueschist facies conditions. Following oceanic subduction, the external part of the arc was obducted to form the south Ladakh ophiolites or was incorporated into the Sapi-Shergol mélange zone. The incorporation of the south Ladakh arc into the accretionary prism implies that the complete closure of the Neo-Tethys likely occurred by Eocene time.     

Early Triassic potassic volcanism in the Afyon Zone of the Anatolides/Turkey: implications for the rifting of the Neo-Tethys

Afyon Zone, which was derived from the Anatolide–Tauride platform during closure of the Neo-Tethys, is made up of pre-Mesozoic basement and unconformably overlying Triassic–Early Tertiary cover series. The Afyon Zone contains widespread metavolcanic rocks, which are dominated by rhyolite, dacite, and trachyandesite. They form a distinct volcanic succession, which is separated from the underlying Silurian–Lower Carboniferous metacarbonates and meta-siliciclastics by a regional unconformity. Trachyandesitic metavolcanics are made up of massive lava flows, pyroclastics and epiclastics, less frequently, domes and dikes, which were developed on a deeply eroded subaerial landmass. U/Pb and Pb/Pb zircon geochronology yielded Lower Triassic (~250 Ma) ages, which are interpreted as extrusion age of trachyandesitic volcanics. Based on the stratigraphic, geochronological, and geochemical data, we suggest that these Lower Triassic magmatic rocks represent an extensional tectonic setting on the northern active margin of the Gondwana, which led to the development of the northern branch of the Neo-Tethys.     

Submarine fan and slope-apron deposition in a Cretaceous Forearc Basin: The Gürsökü Formation (Kavak–Samsun, N. Turkey)

The Late Cretaceous Gürsökü Formation represents the proximal fill of the Sinop–Samsun Forearc Basin that was probably initiated by extension during the Early Cretaceous. The succession records sedimentation in two contrasting depositional systems: a slope-apron flanking a faulted basin margin and coarse-grained submarine fans. The slope-apron deposits consist of thinly bedded turbiditic sandstones and mudstones, interbedded with non-channelized chaotic boulder beds and intraformational slump sheets representing a spectrum of processes ranging from debris flow to submarine slides. The submarine fan sediments are represented by conglomerates and sandstones interpreted as deposited from high density turbidity currents and non-cohesive debris flows. The occurrence of both slope apron and submarine fan depositional systems in the Gürsökü Formation may indicates that the region was a tectonically active basin margin during the Late Cretaceous.     

The Misis–Andırın Complex: a Mid-Tertiary melange related to late-stage subduction of the Southern Neotethys in S Turkey

The Mid-Tertiary (Mid-Eocene to earliest Miocene) Misis–Andırın Complex documents tectonic-sedimentary processes affecting the northerly, active margin of the South Tethys (Neotethys) in the easternmost Mediterranean region. Each of three orogenic segments, Misis (in the SW), Andırın (central) and Engizek (in the NE) represent parts of an originally continuous active continental margin. A structurally lower Volcanic-Sedimentary Unit includes Late Cretaceous arc-related extrusives and their Lower Tertiary pelagic cover. This unit is interpreted as an Early Tertiary remnant of the Mesozoic South Tethys. The overlying melange unit is dominated by tectonically brecciated blocks (>100 m across) of Mesozoic neritic limestone that were derived from the Tauride carbonate platform to the north, together with accreted ophiolitic material. The melange matrix comprises polymict debris flows, high- to low-density turbidites and minor hemipelagic sediments.The Misis–Andırın Complex is interpreted as an accretionary prism related to the latest stages of northward subduction of the South Tethys and diachronous continental collision of the Tauride (Eurasian) and Arabian (African) plates during Mid-Eocene to earliest Miocene time. Slivers of Upper Cretaceous oceanic crust and its Early Tertiary pelagic cover were accreted, while blocks of Mesozoic platform carbonates slid from the overriding plate. Tectonic mixing and sedimentary recycling took place within a trench. Subduction culminated in large-scale collapse of the overriding (northern) margin and foundering of vast blocks of neritic carbonate into the trench. A possible cause was rapid roll back of dense downgoing Mesozoic oceanic crust, such that the accretionary wedge taper was extended leading to gravity collapse. Melange formation was terminated by underthrusting of the Arabian plate from the south during earliest Miocene time.Collision was diachronous. In the east (Engizek Range and SE Anatolia) collision generated a Lower Miocene flexural basin infilled with turbidites and a flexural bulge to the south. Miocene turbiditic sediments also covered the former accretionary prism. Further west (Misis Range) the easternmost Mediterranean remained in a pre-collisional setting with northward underthrusting (incipient subduction) along the Cyprus arc. The Lower Miocene basins to the north (Misis and Adana) indicate an extensional (to transtensional) setting. The NE–SW linking segment (Andırın) probably originated as a Mesozoic palaeogeographic offset of the Tauride margin. This was reactivated by strike-slip (and transtension) during Later Tertiary diachronous collision. Related to on-going plate convergence the former accretionary wedge (upper plate) was thrust over the Lower Miocene turbiditic basins in Mid–Late Miocene time. The Plio-Quaternary was dominated by left-lateral strike-slip along the East Anatolian transform fault and also along fault strands cutting the Misis–Andırın Complex.     

Geological Evolution of Northeastern Anatolia

The paleogeography of the Eastern Pontides was defined by the Paleotethys ocean to the north and a continental assemblage to the south, prior to Carboniferous time. The S-dipping subduction of Paleotethys beneath Gondwana caused the development of arc magmatism, mostly active in the Early Carboniferous. Cessation of magmatism during Late Carboniferous-Early Permian time was accompanied by the deposition of platform carbonates, which were rifted to open a back-arc oceanic basin (Karakaya ocean) during the Triassic.

Accompanying closure of this Triassic basin, the Ladinian-Late Triassic products of Neotethys, opening in the south, transgressively overlay the basement in the Keban continent to the south. However, transgression reached the northern region (Pontide continent) during Liassic time, because of a topographic high created by southward subduction of the Paleotethys ocean as well as closure of the Karakaya ocean.

During the late Cenomanian/Turonian to Eocene, an island arc evolved as a result of N-dipping subduction of Neotethys. The ophiolite-melange association was obducted onto the Pontide continent as a retrocharriage process in the Turonian-Maastrichtian, Paleocene, and the end of the early Eocene, and onto the Keban continent in Campanian-Maastrichtian and pre-late Lutetian time.

A continental-lacustrine environment developed, and partial melting of the thickened crust initiated the development of volcanic units in the Miocene. The region was affected by right-lateral strike-slip faulting (the North Anatolian fault) and a NE-SW-trending left-lateral strike-slip fault system (the Northeast Anatolian fault).     

http://www.sciencedirect.com/science/article/pii/S1674987112001569

The southeastern Anatolia comprises numbers of tectono-magmatic/stratigraphic units such as the metamorphic massifs,the ophiolites,the volcanic arc units and the granitoid rocks.All of them play important role for the late Cretaceous evolution of the southern Neotethys.The spatial and temporal relations of these units suggest the progressive development of coeval magmatism and thrusting during the late Cretaceous northward subduction/accretion.Our new U-Pb zircon data from the rhyolitic rocks of the wide-spread volcanic arc unit show ages of(83.1±2.2)-(74.6±4.4) Ma. Comparison of the ophiolites,the volcanic arc units and the granitoids suggest following late Cretaceous geological evolution.The ophiolites formed in a suprasubduction zone(SSZ) setting as a result of northward intra-oceanic subduction.A wide-spread island-arc tholeiitic volcanic unit developed on the top of the SSZ-type crust during 83-75 Ma.Related to regional plate convergence, northward under-thrusting of SSZ-type ophiolites and volcanic arc units was initiated beneath the Tauride platform(Malatya-Keban) and followed by the intrusion of l-type calc-alkaline volcanic arc granitoids during 84-82 Ma.New U-Pb ages from the arc-related volcanic-sedimentary unit and granitoids indicate that under-thrusting of ophiolites together with the arc-related units beneath the Malatya-Keban platform took place soon after the initiation of the volcanic arc on the top of the SSZtype crust.Then the arc-related volcanic-sedimentary unit continued its development and lasted at～75 Ma until the deposition of the late Campanian—Maastrichtian shallow marine limestone.The subduction trench eventually collided with the Bitlis-Ptrge massif giving rise to HP-IT metamorphism of the Bitlis massif.Although the development of the volcanic arc units and the granitoids were coeval at the initial stage of the subduction/accretion both tectono-magmatic units were genetically different from each other.     

Upper Palaeozoic subduction/accretion processes in the closure of Palaeotethys: Evidence from the Chios Melange (E Greece), the Karaburun Melange (W Turkey) and the Teke Dere Unit (SW Turkey)

During Late Palaeozoic time a wide ocean, known as Palaeotethys, separated the future Eurasian and African continents. This ocean closed in Europe in the west during the Variscan orogeny, whereas in Asia further east it remained open and evolved into the Mesozoic Tethys, only finally closing during Late Cretaceous–Early Cenozoic.Three Upper Palaeozoic lithological assemblages, the Chios Melange (on the Aegean Greek island), the Karaburun Melange (westernmost Aegean Turkey) and the Teke Dere Unit (Lycian Nappes, SW Turkey) provide critical information concerning sedimentary and tectonic processes during closure of Palaeotethys. The Chios and Karaburun melanges in the west are mainly terrigenous turbidites with blocks and dismembered sheets of Silurian–Upper Carboniferous platform carbonate rocks (shallow-water and slope facies) and poorly dated volcanic rocks. The Teke Dere Unit to the southeast begins with alkaline, within-plate-type volcanics, depositionally overlain by Upper Carboniferous shallow-water carbonates. This intact succession is overlain by a tectonic slice complex comprising sandstone turbidites that are intersliced with shallow-water, slope and deep-sea sediments (locally dated as Early Carboniferous). Sandstone petrography and published detrital mineral dating imply derivation from units affected by the Panafrican (Cadomian) and Variscan orogenies.All three units are interpreted as parts of subduction complexes in which pervasive shear zones separate component parts. Silurian–Lower Carboniferous black cherts (lydites) and slope carbonates accreted in a subduction trench where sandstone turbidites accumulated. Some blocks retain primary depositional contacts, showing that gravitational processes contributed to formation of the melange. Detached blocks of Upper Palaeozoic shallow-water carbonates (e.g. Chios) are commonly mantled by conglomerates, which include water-worn clasts of black chert. The carbonate blocks are restored as one, or several, carbonate platforms that collided with an active margin, fragmenting into elongate blocks that slid into a subduction trench. This material was tectonically accreted at shallow levels within a subduction complex, resulting in layer-parallel extension, shearing and slicing. The accretion mainly took place during Late Carboniferous time.Alternative sedimentary-tectonic models are considered in which the timing and extent of closure of Palaeotethys differ, and in which subduction was either northwards towards Eurasia, or southwards towards Gondwana (or both). Terrane displacement is also an option. A similar (but metamorphosed) accretionary unit, the Konya Complex, occurs hundreds of kilometres further east. All of these units appear to have been assembled along the northern margin of Gondwana by Permian time, followed by deposition of overlying Tauride-type carbonate platforms. Northward subduction of Palaeotethys beneath Eurasia is commonly proposed. However, the accretionary units studied here are more easily explained by southward subduction towards Gondwana. Palaeotethys was possibly consumed by long-lived (Late Palaeozoic) northward subduction beneath Eurasia, coupled with more short-lived (Late Carboniferous) southward subduction near Gondwana, during or soon after closure of Palaeotethys in the Balkan region to the west.     

Identification and origin of bitumen in Neolithic artefacts from Demirköy Höyük (8100 BC): Comparison with oil seeps and crude oils from southeastern Turkey

Two ring-like artefacts from the aceramic Neolithic site of Demirköy Höyük in southeastern Turkey were analysed using geochemical techniques in order to determine whether they were prepared using a bitumen amalgam or not. The artefacts, dated 8100 BC, are early evidence of the innovative use of a petroleum-based material to prepare pieces of ornaments (beads, rings, etc.) for the elite of a Neolithic settlement. In order to trace the source of the presumed bitumen, two oil seeps, Boğazköy and Yeşilli, were sampled. To complete the genetic references, geochemical data on crude oils from the main oil fields from the area were compiled.Basic geochemical data show that bitumen is present in the artefacts. Sterane and terpane patterns, as well as carbon isotopic data on C₁₅₊ saturated and C₁₅₊ aromatic hydrocarbons, allowed us to conclude that the Demirköy Höyük bitumen and the Boğazköy oil seep were generated from a Silurian source rock. The detailed geochemical characteristics show, however, that the Demirköy Höyük bitumen does not correlate perfectly with the Boğazköy oil. This discrepancy suggests several explanations: the real bitumen source may be elsewhere in the vicinity and has not been discovered or was at the Boğazköy oil seep location but with slightly different properties in Neolithic times, or has disappeared. Another possibility is that the slight molecular differences are due to weathering effects, which affected the pristine bitumen within the archaeological sample.     

Petrology and geochemistry of post-collisional Middle Eocene volcanic units in North-Central Turkey: Evidence for magma generation by slab breakoff following the closure of the Northern Neotethys Ocean

The Eocene volcano-sedimentary units of Northern Anatolia are confined into a narrow zone trending parallel to the Intra Pontide and İzmir–Ankara–Erzincan sutures, along which the northern branch of the Neotethys Ocean was closed during a period between Late Maastrichtian and Paleocene. The Middle Eocene formations overlie both the imbricated and highly deformed units of the suture zone, which are Paleocene or older in age, as well as the formations of adjacent continental blocks with a regional disconformity. Therefore, they can be regarded to be post-collisional. These units are composed of subaerial to shallow marine sedimentary beds (i.e. the Örencik formation) at the base and a subaerial volcanic unit (i.e. the Hamamözü formation) in the middle and at the top. This sudden facies change from marine to subaerial environment in the Middle Eocene is a common phenomenon across northern Turkey, implying that a regional uplift event occurred possibly across the suture zone before the initiation of the volcanism during Lutetian. The Middle Eocene lavas span the whole compositional range from basalts to rhyolites and display a calc-alkaline character except for alkaline to mildly-alkaline lavas from the top of the sequence. All lavas display a distinct subduction signature. Our geochemical data indicate that calc-alkaline lavas were derived from a subduction-modified source, whereas alkaline to mildly-alkaline lavas of the late stage were possibly sourced by an enriched mantle domain. Magmas evolved in magma chambers emplaced possibly at two different crustal levels. Magmas in deeper (> 13 km) and possibly larger chambers fractionated hydrous mafic minerals (e.g. amphibole and biotite), two pyroxenes and plagioclase and assimilated a significant amount of crustal material. Intermediate to acid calc-alkaline lavas and pyroclastics were derived from these chambers. Magmas in the shallower chambers, on the other hand (~ < 12 km), crystallized anhydrous mineral assemblages, assimilated little or no crustal material and fed basic to intermediate lavas in the region. Both deep and shallow chambers were periodically replenished by mafic magmas. We argue that a slab breakoff model explains better than any alternative model (i) why the volcanism during the Middle Eocene was confined into a rather narrow belt along the suture zone, (ii) why it initiated almost contemporaneous with a regional uplift after the continental collision event, (iii) why it postdated arc volcanism along the Pontides in the north by 15–20 My, (iv) why it assimilated significant amount of crustal material, and (v) why alkalinity of lavas increased in time.     

Tectonic Setting and Evolution of the Sivas Basin,Central Anatolia,Turkey

The Sivas Basin is one of several Central Anatolian basins. It developed mainly after the closure of the northern branch of Neotethys. Its location between the Kirsehir Massif and the Taurides implies that it should not be confused with the Inner Tauride ocean located south of the Eastern Taurides. The basement of the Sivas Basin consists of ophiolitic nappes and melanges that were thrust toward the margins of the continental blocks present in this area—the Pontide belt to the north and the Anatolide-Tauride platform to the south. The basin was initiated by tectonic subsidence at the end of the Cretaceous, and it can be compared to a foreland basin during Paleocene and early to middle Eocene time. It was emergent during late Eocene and Oligocene time, although it continued to subside. A transgression in some parts of the basin occurred during the Oligocene and early Miocene (maximum flooding). During the Pliocene, it was affected by regional compression directed toward the NNW, which resulted from convergence of the Arabian and Eurasian plates. This basin may have developed as an intracontinental basin within the Tauride platform and probably never had an oceanic basement. As a result of this work, the general paleogeographic organization of Central Anatolia and Northern Tethys during the Mesozoic should to be revised.     

Festigkeit, verformbarkeit und gefügeregelung von anhydrit — experimentelle stauchverformung unter manteldrucken bis 5 kbar bei temperaturen bis 300°C

The dependence of strength, ductility, and preferred orientation of polycrystalline anhydrite upon confining pressure (up to 5 kbar), temperature (up to 300° C), and strain (up to 30%) has been evaluated by compression tests.Strength and ductility increase at room temperature with increasing mantle pressure. Up to 1 kbar mantle pressure anhydrite is brittle and failure occurs by tension and shear fractures. Homogeneous flow between 1 and 3 kbar mantle pressure is mostly due to intercrystalline slip which is sensitive to pressure. Beyond the elastic limit the stress—strain curves are nearly horizontal. No preferred orientation develops. Between 3 and 4 kbar mantle pressure the intracrystalline mechanisms become noticeable. The stress—strain curves show weak strain hardening. The (210)-planes reveal a weak preferred orientation perpendicular to the axis of compression.With increasing temperature the strength decreases at low strains (< 5%). Intracrystalline mechanisms become more dominant, because the critical resolved shear stresses are lower with increasing temperature. At high strains (> 15%) both strength and ductility increase at higher temperatures. At even higher strains, strain hardening ceases once again and the stress—strain curves become nearly horizontal. From that point on preferred orientation is no longer increased. The stress—strain curves differ with the orientation of the specimen axis to the original fabric.

Zusammenfassung

In Stauchversuchen an polykristallinem Anhydrit wurde die Abhängigkeit der Festigkeit, der Verformbarkeit und der Gefügeregelung vom Manteldruck (bis 5 kbar), von der Temperatur (bis 300°C) und vom Verformungsgrad (bis 30%) untersucht.Festigkeit und Verformbarkeit nehmen bei Raumtemperatur mit steigenden Mantel-drucken zu. Bis etwa l kbar Manteldruck verhält sich der Anhydrit spröde und es treten Trenn und Verschiebungsbrüche auf. Die gleichmässige Fliessverformung bei Manteldrucken über 1 kbar erfolgt durch überwiegend interkristalline, manteldruckempfindliche Verformungsmechanismen. Die Spannungs—Verformungskurven verlaufen nach Überschreiten der elastischen Verformung nahezu horizontal. Eine Regelung tritt nicht ein. Ab 3 kbar bis 4 kbar Manteldruck beginnen sich intrakristalline Mechanismen bemerkbar zu machen. Die Spannungs—Verformungskurven weisen eine schwache Verfestigung auf. Durch die eintretende schwache Regelung stellt sich eine (210)-Ebene bevorzugt senkrecht zur Stauchachse ein.Mit steigender Temperatur bei sonst gleichen Bedingungen nimmt die Festigkeit bei geringen Verformungsgraden (< 5%) ab. Die Verformung erfolgt überwiegend durch intrakristalline Mechanismen, deren kritische Schubspannungen mit zunehmender Temperatur geringer werden. Die Spannungs—Verformungskurven steigen nach Überschreiten des elastischen Bereichs stärker an als zuvor. Es tritt mit zunehmender Temperatur eine stärkere Verfestigung und eine intensivere Einregelung von (210) senkrecht zur Stauchachse ein. Im Bereich hoher Verformungsgrade (> 15%) liegen die Festigkeiten bei höheren Temperaturen höher als bei niedrigen Temperaturen. Bei sehr hohen Verformungsgraden wird die Verfestigung so gross, dass interkristalline Mechanismen die intrakristallinen wieder ablösen. Die Spannungs—Verformungskurven werden wieder flacher und die Zunahme in der Regelung hört auf.Je nach der Orientierung der Stauchachse zum Ausgangsgefüge verlaufen die Spannungs—Verformungskurven unterschiedlich.Die Abhängigkeit der Verformungsmechanismen vom Manteldruck, Verformungsgrad und der Temperatur wird in einem dreidimensionalen Modell vorgestellt.     

西藏西南部达巴-休古嘎布蛇绿岩带的形成与演化

:该蛇绿岩带的岩体由地幔橄榄岩组成,主要岩石类型是方辉橄榄岩和纯橄榄岩,缺少典型蛇绿岩剖面中的洋壳单元.微量元素和稀土元素特征显示蛇绿岩形成于类似洋中脊的构造环境.笔者提出该区蛇绿岩来源于印度大陆北缘洋盆的洋壳碎片,这个陆缘洋盆与新特提斯洋主体的形成和演化准同步.洋盆的演化模式是:早三叠世,随着印度(冈瓦纳)大陆向南漂移,其北部边缘因引张裂解产生裂谷,于晚三叠世向东开口与新特提斯洋主体连通,洋盆初具洋壳性质,北侧形成阿依拉-仲巴微陆块.侏罗-白垩纪为洋盆洋壳演化期,处于类似洋中脊的构造环境.晚白垩世末洋盆开始闭合.在新特提斯洋板块向北俯冲消减过程中,阿依拉-仲巴微陆块、陆缘洋盆和印度大陆一起随着向北漂移,在印度大陆向北挤压作用下洋盆逐渐收缩以致最终闭合.     

Upper Carboniferous retroarc volcanism with submarine and subaerial facies at the western Gondwana margin of Argentina

During Late Carboniferous times a continental magmatic arc developed at the western margin of Gondwana in South America, as several marine sedimentary basins were formed at the same time in the retroarc region. North of 33°S, at Cordón Agua del Jagüel, Precordillera of Mendoza, Argentina, a volcanic sequence crops out which was emplaced in a submarine environment with some subaerial exposures, and it is intercalated in marine sediments of Agua del Jagüel Formation, which fills of one of these retroarc basins. This paper presents, for the first time, a facies analyses together with geochemical and isotopic data of this volcanic suite, suggesting its deposition in an ensialic retroarc marine basin. The volcanic succession comprises debris flows with either sedimentary or volcanic fragments, base surge, resedimented massive and laminated dacitic–andesitic hyaloclastite, pillow lava, basic hyaloclastite and dacitic–andesitic lavas and hyaloclastite facies. Its composition is bimodal, either basaltic or dacitic–andesitic. The geochemistry data indicate a subalkaline, low K calk-alkaline and metaluminous affinity. The geochemistry of the basalts points to an origin of the magmas from a depleted mantle source with some crustal contamination. Conversely, the geochemistry of the dacites–andesites shows an important participation of both crustal components and subduction related fluids. A different magmatic source for the basalts than for the dacites–andesites is also supported by Sr and Nd isotopic initial ratios and Nd model ages. The characteristics of this magmatic suite suggest its emplacement in an extensional setting probably associated with the presence of a steepened subduction zone at this latitude during Upper Carboniferous times.