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1

Rifting and closure of two branches of the Tethys; from Neotethys to Alpine Tethys

Fodor Laszlo

Geotourism / Geoturystyka

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2023

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nr 1-2 (72-73)

19--20

EN

The western termination of the Neotethys is marked by a complex interaction of several small oceanic basins which were formed and closed progressively. The western end of the Neotethys was opened from Permian to Middle Triassic; spreading started from Anisian. The rifting was associated with acidic, sometimes basic magmatism; Permian intrusions are widespear in certain zones (Eastern Alps), and together with Middle Triassic volcanites, played a role in weakening of the extending continental lithosphere. The rifting process was interacting with evaporite tectonism in regions where Late Permian evaporites were formed potentially as a post-rift or intra-rift stage. Due to loading of the ovelying Early Triassic clastic-carbonate ramp sequence, and the still ongoing extensional deformation, and/ or gravitational sliding of shelf domains toward deepening extended continental margin, salt tectonics probably started in latest Early Triassic. The uprising salt walls strongly influenced shelf and eventually slope deposition; the minibasins between salt walls often hosted carbonate ramp or platform development while collapsing salt structures could turn to deep “intra-platform” basins. The salt tectonics controlled the continuing facies differentiation during the Late Triassic. The development of salt-cored normal faults are not characteristic for a typical post-rift passive margin, but due to their relation to underlying salt, facies differentiation was maintained. The earliest sign of rifting of the Alpine Tethys can be seen in the Late Triassic deep grabens (Southern Alps, southern Transdanubian Range). This is the reason that separation of salt-related deformation, and crustal extension is not evident in some zones. The closure of the Neotethys started with intra-oceanic subduction, probably with a double polarity, and the formation of a supra-subductional new oceanic lithosphere (the Vardar zone in some interpretations). The age of this process is somewhat controversial in different models. Isotopic ages of metamorphic sole of the Vardar ophiolites suggest 175–170 Ma while neutral to acidic differentiates in the eastern Vardar testify ongoing Late Jurassic oceanic magmatism (~155–155 Ma). A complex system of melange was formed under and in front of the emplacing upper plate Vardar ophiolite. While sub-ophiolitic melange with serpentinitic matrix formed below the overlying hot oceanic lithosphere, the sediment-hosted melange contains blocks from different zones of the passive margin and partly the overlying ophiolite. Stratigraphic ages indicate that this processes happend during the Middle and Late Jurassic. The obduction happened in latest Jurassic (Tithonian) indicated by reef limestone on top of ophiolites. This was followed by the imbrication of the underlying passive margine Adriatic continental lithospere during the entire Cretaceous and Cenozoic. Clastic foreland basins were formed within this lower plate supplied partly by the passive upper plate ophiolite. The Alpine Tethys went on intensive rifting which ended with break-up in late Middle or in the Late Jurassic on its southern Piemont-Ligurian branch. The onset of subduction is not exactly clear but could happen in the Late Cretaceous resulted in high-pressure metamorphism of the oceanic domains in the Eocene (Tauern window). The Transdanubian Range of Hungary was situated between the two oceanic domains during the whole Mesozoic. While this unit has not been buried and only deformed modestly, the sedimentary events reflect the complex evolution. Middle Triassic rifting resulted in disruption of Early Triassic mixed siliciclastic-carbonate ramp into platform and somewhat deeper grabens. Small-scale synsedimentary faults and neptunian dykes testify this phase. Away from the break-up zone, the area underwent important post-rift subsidence compensated by platform carbonate sedimentation through the Late Triassic. However, the trace of initial Late Triassic rifting is present in forms of synsedimentary faults in the western side, closer to the future Neotethys. Following the earliest Jurassic decline of platform biota, the ongoing Alpine rifting disintegrated the entire TR carbonate platform into shallower, sediment free ridges and somewhat deeper grabens. This rifting and subsidence resulted in deposition of pelagic red nodular limestone in the Aalenian-Bajocian. After cherty sedimentation in the Callovian–Oxfordian, very modest extension appeared in the latest Jurassic. Although this phase could be considered as the final extension of the Alpine Tethys rifting far to the west, it is more probable that in fact this is due to slight downbending of the TR below the distal ophiolite emplacement to the east. The Neotethyan influence prevaild in the eastern TR during the Early Cretaceous. A clastic foreland basin was supplied by ophiolite and supra-ophiolite detritus of the obducted Neotethyan Vardar unit. Structural cituation changed in the late Early Cretacoues, around 115 Ma (Albian). The entire TR underwent shortening. The unit, formerly the lower plate of the Neotethyan system, was emplaced, as the highest nappe, on to the other continental units of the Austroalpine system. Within the Eastern Alps, this was associated with intracontinental subduction initiated in zone of Permian magmatism having thermally weakened the lithosphere. The relationship of this subduction, and associated high to ultrahigh pressure metamorphism is not clear, but eventually could have connected to large-scale displacement of the Neotethyan subduction zone at its northernmost termination zone. The complete change of the TR, from lowermost position to upper plate, is the reflection of complex 3D geometry of overlapping oceanic domains and could happen in other Tethyan areas

2

Cretaceous magmatic evolution in the Deylaman igneous complex, Alborz zone, Iran : change from extensional to compressional regime

Akmali Sheida, Asiabanha Abbas, Haghnazar Shahrooz

Geological Quarterly

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2019

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Vol. 63, No. 4

757--770

EN

The Deylaman igneous complex, as a part of the Late Cretaceous rock unit that lies behind the Paleogene Alborz magmatic arc, in the northern Alborz zone, is composed of basaltic sheet lavas alternating with the pelagic calcareous sediments, basaltic pillow lavas, felsic lavas and gabbroic-monzodioritic intrusions. The pelagic calcareous deposits contain microfossils representing the Santonian-Maastrichtian ages. Furthermore, petrographic textures such as the hyalomicrolitic texture and swallow-tail plagioclase crystals in the pillow lavas, and also segregation vesicles in the basaltic sheet lavas, imply high external (hydrostatic) pressures as the magma was extruded in a deep-water environment. The rock samples show both compositional bimodality and characteristic trends in the variation diagrams. Also, some geochemical characteristics imply that the basaltic lavas originated from the partial melting of an undepleted deep mantle source containing spinel lherzolite: the enrichment patterns of LREE/HREE ratios of the samples [(La/Yb)n = 3.93-4.16 for basaltic lavas and 10.92 for felsic lavas] lying between those characteristic of OIBs [(La/Yb)n = 12.92] and EMORBs [(La/Yb)n = 1.91]; similarities between the patterns of multi-element spider-diagrams; LILE bulges in the basaltic samples compared with those of OIBs. Moreover, the samples show influence from two geotectonic environments: supra-subduction zone (SSZ) settings and plume-type within-plate magmas. Therefore, because of the deep submarine environment inferred for the effusive volcanic eruptions in Santonian-Maastrichtian time, it seems that the Deylaman igneous complex evolved through two stages: first, a tensional regime in a supra-subduction zone (farther from the Mesozoic magmatic arc) and formation of an embryonic rift-related oceanic basin in the Late Jurassic-Early Cretaceous; secondly, a compressive regime in the Late Cretaceous-Early Paleocene and inland migration of the magmatic arc. Consequently, the Cretaceous magmatism can be interpreted as a prelude to the Eocene magmatic flare-up in the magmatic arcs of Iran.

3

Geochemistry of siliciclastic rocks from the Shemshak Group (Upper Triassic-Middle Jurassic), northeastern Alborz, northern Iran : implications for palaeoweathering, provenance, and tectonic setting

Taheri A., Jafarzadeh M., Armstrong-Altrin J. S., Mirbagheri S. R.

Geological Quarterly

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2018

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Vol. 62, No. 3

522--535

EN

Combined petrographic and geochemical data of the siliciclastic sedimentary rocks from the Shemshak Group in the northeastern Alborz Mountains, north of Iran are described, together with their implications for palaeoweathering, their provenance, and tectonic setting. Based on field observations and modal composition, the sandstones are classified as litharenites. The chemical index of alteration (CIA) indicated that the source terrains underwent a moderate intensity of chemical weathering. The index of chemical variation (ICV) values indicated that the Shemshak Group rocks were immature and related to a source area with an active tectonic regime. Major, trace and rare earth element (REE) data suggested the domination of mixed sedimentary (recycled) and igneous rocks in the source area of the Shemshak Group. Petrographic and geochemical characteristics of Shemshak Group rocks suggest an active continental margin (ACM), which corresponds to the collision of the Iran plate with the Turan plate.

4

Geochemistry and tectonic setting of the Chah-Bazargan sub-volcanic mafic dykes, south Sanandaj–Sirjan Zone (SSZ), Iran

Fazlnia A.

Geological Quarterly

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2018

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Vol. 62, No. 2

447--458

EN

The Chah-Bazargan sub-volcanic mafic dykes (trachybasalt and basaltic trachyandesite) are located in the south of the Sanandaj–Sirjan Zone (SSZ), Iran. The dyke mineralogy mostly comprises amphibole, clinopyroxene, olivine, orthopyroxene, and plagioclase as phenocrysts and fine-grained plagioclase and some ferromagnesian minerals in the matrix. The rocks are alkaline and shoshonitic in composition. The mafic melts relate to Neotethys subduction activity beneath the southern SSZ in the ~Eocene–Miocene interval. Markedly positive Ba, U, K, Pb, and Sr and negative HFSE (high field strength elements: Nb, Ta, Zr, Hf, P, and Ti) anomalies demonstrate this subduction. The sub-volcanic mafic dykes were produced from a metasomatized upper lithospheric mantle wedge at a depth consistent with the stability field of phlogopite-spinel (or -spinel/garnet) lherzolite. Geochemical studies on the basis of the rare earth elements (REE) and HFSE, and large ion lithophile elements (LILE) display that the mantle wedge underwent degrees of partial melting averaging between 5 and 15% to form the Chah-Bazargan sub-volcanic mafic dykes. It is possible that the chemical composition of the rocks was changed due to fractional crystallization and crustal contamination during emplacement.

5

Late Ladinian radiolarians from the Tahtalidag Nappe of the Antalya nappes, SW Turkey : remarks on the late Middle and Late Triassic evolution of the Tahtalidag Nappe

Tekin U., Sonmez I.

Acta Geologica Polonica

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2010

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Vol. 60, no. 2

199-217

EN

The late Ladinian to Late Triassic succession of the Tahtalidag (upper) Nappe of the Antalya nappes was studied in the Egregindere section, north of the city of Antalya, SW Turkey. The chert bands in the central part of the section have yielded poorly to moderately preserved radiolarians documenting the Late Ladinian Muelleritortis firma and Muelleritortis cochleata radiolarian zones. Based on the Egregindere succession, a major deepening event, evidenced by radiolarian cherts, took place between the middle and late Late Ladinian. The Late Triassic thick-bedded neritic limestones represent a shallowing-upward sequence, which formed as a result of the horst-like rising of the Tahtalidag Nappe during the Late Triassic block faulting. Fifty-nine radiolarian taxa have been determined from the Upper Ladinian of the Egregindere section. One species (Muelleritortis elegans) and two subspecies (Muelleritortis firma equispinosa and Muelleritortis firma globosa) are described as new.