Northern Margin Of Gondwanaland Research Articles

Detrital zircon U–Pb isotopes and whole‐rock geochemistry of early Palaeozoic sediments of the Baoshan and Lancang Blocks, SW China: Implications for Proto‐Tethys evolution and Gondwana reconstruction

The palaeogeographic reconstruction of the microcontinents on the northern margin of Gondwanaland and the tectonic evolution model of the Proto‐Tethys in the Early Palaeozoic remain controversial. The Baoshan and Lancang Blocks in SW Yunnan, China, were located on the northern margin of the Gondwana continent during the Early Palaeozoic. In this study, we present new zircon U–Pb ages and whole‐rock geochemistry data for the sedimentary rocks of the Lancang Group in the Lancang Block and the Mengtong Group in the Baoshan Block to reveal the nature of sedimentary provenance with a further tectonic evolution model of the Proto‐Tethys. Our new detrital zircon U–Pb results, in conjunction with the published age of the metamorphic volcanic rock interlayers, suggest that the Mengtong and Lancang groups were probably deposited during the Early Palaeozoic. The Mengtong and Lancang groups have similar detrital zircon age spectra and display three distinctive age peaks at 1.2–1.1 Ga, ~0.95 Ga, and 0.6–0.5 Ga. This implies that the Baoshan‐Lancang Block was situated in the transitional zone between India and Australia on the Gondwana margin and received detritus from both cratons. Combined with abundant volcanic rocks in the Lancang Group and SSZ‐type ophiolites in the Changning‐Menglian belt, the Lancang Block should be reconstructed as a continental island arc of the Baoshan Block in the Early Palaeozoic. The geochemical analysis results show that the sediments of the two strata were primarily sourced from felsic rocks in the upper crust, and they were deposited in the active continental margin. Combined with previous research, we propose a new tectonic evolution model for the Proto‐Tethys Ocean in the Early Palaeozoic, from bisubduction to collisional orogeny.

Upper Neoproterozoic garnet-bearing granites in the Zeber-Kuh region from east central Iran micro plate: Implications for the magmatic evolution in the northern margin of Gondwanaland

Upper Neoproterozoic garnet-bearing granites in the Zeber-Kuh region from east central Iran micro plate: Implications for the magmatic evolution in the northern margin of Gondwanaland

First evidence for the subduction initiation and boninitic magmatism from the Armutlu Peninsula (NW Turkey): geodynamic significance for the Cadomian magmatic arc system of the Gondwanan margin

ABSTRACT The Cadomian Orogeny was active along the northern margin of Gondwanaland during the Late Neoproterozoic–Early Cambrian period. Remains of this orogenic activity are traced from Western Europe to the Himalayas, including some metamorphic basement rocks of Turkey. In this study, we provide new whole-rock and mineral chemistry, and zircon U–Pb age data for the metagabbro and metagranites in the Armutlu Peninsula, north of the Intra Pontide suture zone (IPSZ), which are the equivalent of the Cadomian basement of the İstanbul Zone (İZ). Geochemically, the metagabbros show an extreme depletion in Nb, Zr, Th, and Hf; low TiO2 contents; and high Mg# values exhibiting boninitic affinity, which are commonly associated with fore-arc settings. The metagranites are calc-alkaline and high-K calc-alkaline in character. They are slightly enriched in LREE and have similar features to shear-type plagiogranites. The yielded zircon U-Pb crystallization ages are 564 ± 4.4 Ma for metagabbros and 561 ± 9.5 Ma (Late Ediacaran) for metagranites. In this study, we provide additional information for the Cadomian basement of the İZ and the geodynamic evolution of Cadomian magmatism by discussing the tectonic setting for two geochemically contrast but almost coeval magmatic products.

Late Neoproterozoic to early Paleozoic paleogeographic position of the Yangtze block and the change of tectonic setting in its northwestern margin: Evidence from detrital zircon U-Pb ages and Hf isotopes of sedimentary rocks

AbstractThe crustal evolution of the Yangtze block and its tectonic affinity to other continents of Rodinia and subsequent Gondwana have not been well constrained. Here, we present new U-Pb ages and Hf isotopes of detrital zircons from the late Neoproterozoic to early Paleozoic sedimentary rocks in the northwestern margin of the Yangtze block to provide critical constraints on their provenance and tectonic settings. The detrital zircons of two late Neoproterozoic samples have a small range of ages (0.87–0.67 Ga) with a dominant age peak at 0.73 Ga, which were likely derived from the Hannan-Micangshan arc in the northwestern margin of the Yangtze block. In addition, the cumulative distribution curves from the difference between the depositional age and the crystalline age (CA–DA) together with the mostly positive εHf(t) values of these zircon crystals (−6.8 to +10.7, ~90% zircon grains with εHf[t] > 0) suggest these samples were deposited in a convergent setting during the late Neoproterozoic. In contrast, the Cambrian–Silurian sediments share a similar detrital zircon age spectrum that is dominated by Grenvillian ages (1.11–0.72 Ga), with minor late Paleoproterozoic (ca. 2.31–1.71 Ga), Mesoarchean to Neoarchean (3.16–2.69 Ga), and latest Archean to early Paleoproterozoic (2.57–2.38 Ga) populations, suggesting a significant change in the sedimentary provenance and tectonic setting from a convergent setting after the breakup of Rodinia to an extensional setting during the assembly of Gondwana. However, the presence of abundant Grenvillian and Neoarchean ages, along with their moderately to highly rounded shapes, indicates a possible sedimentary provenance from exotic continental terrane(s). Considering the potential source areas around the Yangtze block when it was a part of Rodinia or Gondwana, we suggest that the source of these early Paleozoic sediments had typical Gondwana affinities, such as the Himalaya, north India, and Tarim, which is also supported by their stratigraphic similarity, newly published paleomagnetic data, and tectono-thermal events in the northern fragments of Gondwana. This implies that after prolonged subduction in the Neoproterozoic, the northwestern margin of the Yangtze block began to be incorporated into the assembly of Gondwana and then accept sediments from the northern margin of Gondwanaland in a passive continental margin setting.

Petrology of the shoshonitic Çambaşı pluton in NE Turkey and implications for the closure of the Neo-Tethys Ocean: Insights from geochemistry, geochronology and Sr–Nd isotopes

The Eastern Pontide Orogenic Belt (EPOB) is one of the well-preserved continental magmatic arcs in the Alpine–Himalayan Belt and consists of Paleozoic to Miocene plutonic bodies. This paper presents new geological, bulk-rock major and trace element analyses, biotite Ar–Ar ages and Sr–Nd isotope data on representative samples from the Çambaşı granitic body exposed in the northwestern part of the EPOB. Although the majority of the plutons in the EPOB show calc-alkaline affinity and I-type character, the Çambaşı pluton has a shoshonitic affinity and an I-type, metaluminous signature. The pluton is composed of mainly monzonite, quartz monzonite and granite bodies including monzo-dioritic mafic microgranular enclaves (MMEs). Excluding the MMEs, these rocks have 57.7–70.2wt.% SiO2, 13.1–17.5wt.% Al2O3, 3.06–3.64wt.% Na2O, 3.66–5.92wt.% K2O, 0.33–3.3wt.% MgO and Mg# <30, indicating intermediate to evolved compositions. They are characterized by light rare earth element (LREE)-enrichment, relatively flat heavy rare earth element (HREE) patterns with small negative Eu anomalies and moderately fractioned REE patterns [average (La/Yb)N=11.32]. Decreasing Fe2O3T, MgO, CaO, TiO2, Ba, Eu, and Sr with increasing SiO2 contents are consistent with fractional crystallization and can be related to fractionation of plagioclase, clinopyroxene, hornblende, apatite and Fe–Ti oxides. Initial 87Sr/86Sr ratios vary between 0.70470 and 0.70478, while those of 143Nd/144Nd lie between 0.51262 and 0.51263. Major, trace element and initial Sr–Nd homogeneity show that fractional crystallization played a key role in the evolution of the pluton. 40Ar/39Ar dating of four biotite samples from the pluton gave plateau ages of between 44.62±0.1 Ma and 44.10±0.0 Ma, which are interpreted as cooling ages of the plutonic rocks. We suggest that the Çambaşı pluton formed in a post-collisional extensional setting where partial melting of chemically enriched lithospheric mantle was controlled by lithospheric thinning and upwelling of asthenosphere that followed the closure of the northern branch of Neo-Tethys Ocean in Lutetian time on the northern margin of Gondwanaland.

Continental weathering in the Early Triassic in Himalayan Tethys, central Nepal: Implications for abrupt environmental change on the northern margin of Gondwanaland

The geochemistry of Triassic mudstones in the Himalayan Tethys sequence, central Nepal, was studied with respect to changes in sedimentary facies, grain size, and source rocks. The Triassic sedimentary facies of mudstone and carbonates show deposition in offshore to hemiplegic environments. The rare earth element (REE) pattern of the Permian and Triassic mudstones suggests uniformity correlatable to average shale. The major element geochemistry of the Early Triassic Griesbachian-early Smithian mudstones indicates a sediment supply from strongly weathered sources with the chemical index of alteration (CIA) values of 76–81. However, the mudstones in the late Smithian show weakly weathered sources with CIA values of 68–74. The lower part of the Middle Triassic Anisian mudstones return to Early Triassic paleoweathering levels. There are no significant relationships among lithofacies, the grain size of the sediments, and CIA values. Thus, the abrupt change of the degree of paleoweathering in the Early Triassic, late Smithian time, suggests a dramatic decrease in continental weathering, which is related to a predominantly arid climate in the northern marginal area of Gondwana.

Identification of Early Carboniferous Granitoids from Southern Tibet and Implications for Terrane Assembly Related to the Paleo-Tethyan Evolution

AbstractThis article presents zircon U-Pb and Hf isotope data, together with the whole-rock major- and trace-element composition, of Early Carboniferous granitoids newly identified from the Jiacha and Langxian areas in the southern Lhasa terrane, southern Tibet. The Jiacha rocks are monzogranites that yield zircon U-Pb ages of 347–345 Ma and eHf(t) values from −5.4 to −4.9. The Langxian rocks are granodiorites with slightly older zircon U-Pb ages of 355–352 Ma and lower eHf(t) values from −6.8 to −6.5. Our data suggest that these granitoids were generated largely by reworking of Paleoproterozoic ( Ga) basement materials. In conjunction with literature data, it is further argued that the southern and central parts of the Lhasa terrane, separated by the Sumdo eclogite belt, should have been an integrated block before the late Paleozoic. Our study supports the notion that the Lhasa terrane was derived from the northern margin of Gondwanaland, in association with formation of at least two stages of Tethyan Oc...

Late Palaeozoic corals of Tibet (Xizang) and West Yunnan, Southwest China: successions and palaeobiogeography

A dynamic pattern of coral faunal provincialism in the Carboniferous to Permian sequence is preserved in Tibet–West Yunnan. During the Early Carboniferous, an undifferentiated Eurasian province was present, containing the Kueichouphyllum, Keyserlingophyllum- Siphonophyllia, and Cyathaxonia faunas, that reflect major environmental differences relative to previous interpretations. During the Late Carboniferous–Early Permian, the Indoralian province and the Cathaysian province can be distinguished. The former is recognised by an absence of Late Carboniferous–Asselian corals and by the presence of the Sakmarian–Artinskian Cyathaxonia fauna. The latter contains the Late Carboniferous and Early Permian compound corals Nephelophyllum and Kepingophyllum. As many blocks drifted northward beginning in the late Early Permian, the Indoralian province had evolved into two discrete provinces: the Himalayan and Cimmerian provinces. The Himalayan province as a relic of the Indoralian province was in the northern margin of Gondwanaland. The Cimmerian province between the Himalayan and the Cathaysian provinces consists of the present tectonic blocks: Lhasa, Qiangtang, Tengchong, and Baoshan in Tibet and West Yunnan. It is characterised by Roadian non-dissepimental solitary corals and Wordian–Capitanian compound Waagenophyllidae, as well as some endemic Cimmerian taxa such as Thomasiphyllum and Wentzellophyllum persicum. The Cathaysian province is dominated by Szechuanophyllum and Ipciphyllum. During the Late Permian, the Himalayan province and the Cathaysian province can be recognised. The former contains only small solitary corals, referred to as the Lytvolasma fauna, and the latter is identified by Liangshanophyllum, a fasciculate waagenophyllid.

SHRIMP U–Pb in zircon geochronology of the Chor granitoid: evidence for Neoproterozoic magmatism in the Lesser Himalayan granite belt of NW India

Porphyritic and peraluminous granitoids of the Lesser Himalayan granite belt fringe the High Himalaya from Nepal to Pakistan. Magmatic zircons in the Chor granitoid in Himchal Pradesh define a SHRIMP U–Pb age of 823±5 Ma. This Neoproterozoic age contrasts with Cambro-Ordovician ages for other dated Lesser Himalayan granites, but is coeval with granite magmatism in the South China craton. Neoproterozoic magmatism within the northern margin of Gondwanaland may provide a more local source for detrital zircons in High Himalayan metasediments during the Neoproterozoic and early Paleozoic than the recently postulated East African orogen.

Artinskian conodonts from the Dingjiazhai Formation of the Baoshan Block, west Yunnan, southwest China

Permian conodonts were recovered for the first time from the Dingjiazhai Formation, a well-known diamictite-bearing stratigraphic unit in the Gondwana-derived Baoshan Block in West Yunnan, Southwest China. The conodont fauna occurs in limestone units within the upper part of the formation and consists ofSweetognathus bucaramangus(Rabe),S. whitei(Rhodes),Mesogondolella bisselli(Clark and Behnken), and an unidentified ramiform element. Based on the known stratigraphic distribution of 5.bucaramangus(Rabe), the fauna is referable to the upperSweetognathus whitei-Mesogondolella bisselliZone, and thus is dated as middle Artinskian according to the current definition of the stage. The Dingjiazhai Formation is overlain paraconformably by the Woniusi Formation, which is represented mostly by basalts and basaltic volcaniclastics related to rifting volcanism during the separation of the Baoshan Block from Gondwanaland. The present discovery of conodonts from the upper part of the Dingjiazhai Formation reveals that the glaciogene diamictites in the Dingjiazhai Formation are older than middle Artinskian, and the inception of rifting volcanism of the Baoshan Block is later than middle Artinskian.Occurrence of an essentially warm water element,Sweetognathus bucaramangus(Rabe), in the Dingjiazhai conodont assemblage notwithstanding, the entire fossil faunas including brachiopods and fusulinoideans from the limestone units of the formation can be best interpreted as a middle latitudinal, non-tropical, and still substantially Gondwana-influenced assemblage developed at the northern margin of Gondwanaland just after deglaciation in the southern hemisphere during Early Permian time. This time could be regarded as the beginning of the Cimmerian Region, which had mixed or transitional paleobiogeographic characteristics between the Paleoequatorial Tethyan and cool/cold Gondwanan realms, and which became well developed during Middle Permian time.

The origin of the Woyla Terranes in Sumatra and the Late Mesozoic evolution of the Sundaland margin

The Jurassic–Cretaceous Woyla Group of northern Sumatra includes fragments of volcanic arcs and an imbricated oceanic assemblage. The arc rocks are intruded by a granitic batholith and are separated from the original continental margin of Sundaland by the oceanic assemblage. Rocks of the arc assemblage are considered to be underlain by a continental basement because of the occurrence of the intrusive granite and of tin anomalies identified in stream sediments. Quartzose sediments associated with the granite have been correlated with units in the Palaeozoic basement of Sumatra. From these relationships a model has been proposed in which a continental sliver was separated from the margin of Sundaland in the Late Jurassic to Early Cretaceous in an extensional strike-slip faulting regime, producing a short-lived marginal basin. The separated continental fragments have been designated the Sikuleh and Natal microcontinents. In the mid-Cretaceous the extensional regime was succeeded by compression, crushing the continental fragments back against the Sundaland margin, with the destruction of the marginal basin, now represented only by the imbricated oceanic assemblage. Modifications of this scenario are required by subsequent studies. Age-dating of the volcanic assemblage and intrusive granites in the Natal area showed that they formed part of an Eocene–Oligocene volcanic arc and are not relevant to the model. Thick-bedded radiolarian chert and palaeontological studies in the oceanic Woyla Group rocks of the Natal and Padang areas showed that they formed part of a more extensive and long-lived ocean basin which lasted from at least Triassic until mid-Cretaceous. This raised the possibility that the Sikuleh microcontinent might be allochthonous to Sumatra and encouraged plate tectonic reconstructions in which the Sikuleh microcontinent originated on the northern margin of Gondwanaland and migrated northwards across Tethys before colliding with Sundaland. Since these models were proposed, the whole of Sumatra has been mapped and units correlated with the Woyla Group have been recognised throughout western Sumatra. These units are reviewed and the validity of their correlation with the Woyla Group of northern Sumatra is assessed. From this review a revised synthesis for the Late Mesozoic tectonic evolution of the southwestern margin of Sundaland is proposed.

Wuchiapingian (early Lopingian, Permian) global brachiopod palaeobiogeography: a quantitative approach

A global presence/absence database of 212 Wuchiapingian (early Lopingian, Permian) brachiopod genera from 30 stations is analysed by cluster analysis, nonmetric multidimensional scaling and minimum spanning tree to document the global palaeobiogeographical patterns. Five core groups are revealed by the quantitative analysis and interpreted as representing five marine biotic provinces. They are the Cathaysian (tropical), Western Tethyan (tropical), Himalayan (warm temperate), Austrazean (cold temperate) and Greenland–Svalbard Provinces (cold temperate). The Cathaysian Province is composed of many isolated or semi-isolated islands situated in the Palaeotethys, whereas the other four provinces occurred mainly on the continental shelves of Pangea: the Western Tethyan Province along the western coast of the Palaeotethys, the Himalayan Province on the northern margin of Gondwanaland, the Austrazean Province along the southeastern margin of Gondwanaland, and the Greenland–Svalbard Province on the northern margin of Pangea. In addition, nonmetric multidimensional scaling helped to identify key biogeographic determinants: latitude-related thermal gradient appears to have accounted for most of the variance in the data; geographic distance and ocean circulation may have also played a major, but subordinate, role in the delineation and/or enhancement of some of the provinces. Comparison with Early and Middle Permian global marine provincialism indicates that marine biotic provinces had significantly reduced during the Lopingian (Late Permian) in the lead up to the end-Permian mass extinction.

Formation and Enrichment of the Yangla Copper Deposit and its Implications for the Evolution of the Jinshajiang Suture in the Northern Margin of Gondwanaland

Formation and Enrichment of the Yangla Copper Deposit and its Implications for the Evolution of the Jinshajiang Suture in the Northern Margin of Gondwanaland

Gondwanaland dispersion, Asian accretion and evolution of eastern Tethys∗

Most east and southeast Asian continental blocks, comprising North and South China, Indochina (including the Qamdo‐Simao Block), Tarim (including the Kunlun and Ala Shan Terranes), Qaidam, Sibumasu, Qiangtang, Lhasa, Kurosegawa, Northwest and Southeast Hainan, West Burma and Woyla Terranes, had their origins on the northern margin of Gondwanaland. Phanerozoic evolution of eastern Gondwanaland and Tethys involved the successive rifting and separation of three continental slivers (now recognised as collages of terranes) from northern Gondwanaland, their northwards drift, and amalgamation/accretion to form east and southeast Asia. These continental slivers separated from the margin of Gondwanaland in the Late Devonian (North China, South China, Indochina, Qaidam, Tarim and Hainan Island Terranes), Early‐mid‐Permian (the Cimmerian continent including the Sibumasu and Qiangtang Terranes), and Late Triassic‐Late Jurassic (Lhasa, West Burma and Woyla Terranes). The northwards drift of these terranes was accompan...

Pre-Cretaceous evolution of SE Asian terranes

Abstract During the last decade, a wide range of geological and geophysical data has led to the recognition of various continental terranes in SE Asia which, on tectonostratigraphic, palaeobiogeographic and palaeomagnetic grounds are ‘suspect’ or allochthonous in nature. Some of the recognized terranes may be composite and there is some disagreement with regard to the number of terranes and their boundaries. The continental terranes are bounded by sutures (representing former oceans), by narrow mobile belts or major fault zones. Comparative studies of the stratigraphy, palaeontology, and palaeomagnetism of the various pre-Cretaceous continental terranes of East and SE Asia suggest that they were all derived directly or indirectly from Gondwanaland. Asian continental terranes that are placed on the India-Australian margin of Gondwanaland in the Early Palaeozoic include Tarim (here regarded to include the Kunlun and Ala Shan terranes), Qaidam, Indochina/East Malaya (which includes the Qamdo-Simao block of western China), North &amp; South China, Sibumasu, Qiangtang, Lhasa, Kurosegawa, NW and SE Hainan, West Burma and the Woyla terranes. The evolution of Gondwanaland and Tethys during the Palaeozoic and Mesozoic involved the rifting of continental slivers/fragments from northern Gondwanaland, and the northwards drift and amalgamation/accretion of these to form proto-East and SE Asia. Three continental slivers were rifted from the northern margin of Gondwanaland in the Early to Late Devonian (North China, South China, Indochina/East Malaya/Qamdo-Simao, Qaidam and Tarim terranes); Early-Middle Permian (the Cimmerian continent including the Sibumasu and Qiangtang terranes and possibly NW and SE Hainan); and Late Triassic to Late Jurassic (Lhasa, West Burma and Woyla terranes). The northwards drift of these terranes was accompanied by the opening and closing of three successive oceans, the Palaeo-Tethys, Meso-Tethys and Ceno-Tethys. Assembly of Gondwanaland-derived Asian terranes began with the amalgamation of South China and Indochina/East Malaya along the Song Ma/Song Da zone during the Late Devonian/Early Carboniferous to form ‘Cathaysialand’. Palaeomagnetic, climatic and biogeographic data indicate that Cathaysialand and North China were located within the Palaeo-Tethys at low northern/equatorial latitudes during the Late Carboniferous and Permian. Palaeomagnetically determined palaeolatitudes are consistent with the development of tropical Cathaysian floras on these terranes. The Tarim, Kunlun, Qaidam and Ala Shan terranes accreted to Kazakstan/Siberia in the Permian. A major episode of rifting occurred on the northern margin of Gondwanaland in the Late Carboniferous-Early Permian and the Cimmerian continent separated in the late Early Permian resulting in the opening of the Meso-Tethys. Suturing of Sibumasu and Qiangtang to Cathaysialand occurred in the Late Permian-Triassic, closing a major branch of the Palaeo-Tethys. South and North China amalgamated and then accreted to Laurasia by Late Triassic-Early Jurassic times. The highly disrupted Kurosegawa terrane of Japan, possibly derived from Australian Gondwanaland, accreted to Japanese Eurasia, also in the Late Jurassic. The Lhasa, West Burma and Woyla terranes rifted from NW Australian Gondwanaland in the Late Triassic to Late Jurassic and drifted northwards during the Jurassic and Early Cretaceous as the Ceno-Tethys opened and the Meso-Tethys was destroyed by subduction beneath Eurasia. Accretion of these terranes to proto-SE Asia occurred in the Cretaceous. The SW Borneo and Semitau terranes were derived from the South China/Indochina margin by the opening of a marginal basin in the Cretaceous which was subsequently destroyed by southwards subduction during the rifting of the Reed Bank-Dangerous Grounds terrane from South China when the South China Sea opened. The NW and SE Hainan terranes, which formed part of Early Palaeozoic and possibly Late Palaeozoic Gondwanaland, reached their current positions, relative to South China, sometime in the Jurassic-Cretaceous.

Phanerozoic evolution of the basins of Northern Egypt and adjacent areas

The results of integrating geological well data and geophysical information from the subsurface of northern Egypt are presented in terms of basin dynamics. Recent biostratigraphic data from wells and scarce outcrops are shown to be critical to an understanding of syndepositional tectonics. Six tectonostratigraphic phases of basin evolution are recognized to span the Phanerozoic. These phases initially record the development of intracratonic subsidence, controlled by deep crustal strike-slip tectonics, as Nubian continental and Tethyan marine influences competed across the northern margin of Gondwanaland. Evidence is also presented for the formation of the present day continental margin to northern Egypt. After a phase of crustal stretching, oceanic rifting focused on the western margin of the Arabian Platform, propagating progressively westwards during the Early-Mid-Jurassic. Thereafter, the effects of passive subsidence on the continental margin were disturbed by discrete phases of intracratonic strike-slip, associated ‘Syrian Arc’ folding, and the formation of deep basins in the opening Red Sea and Gulf of Suez. Three structural fabrics persisted throughout Phanerozoic basin evolution, the result of repeated extensional reactivation and inheritance from Pan-African basement.

Paleozoic Stratigraphy in the Qiangtang Region of Tibet: Relations of the Gondwana and Yangtze Continents and Ocean Closure Near the End of the Carboniferous

Paleozoic sequences are defined for the northern and southern Qiangtang by linking field observations in the Gaize Province to the known Paleozoic record in neighboring areas. The pre-Devonian Gemuri Group of earlier authors is divided and the use of that term is no longer recommended. The upper Paleozoic rock assemblages, geological structures, and biota of the southern Qiangtang are of Gondwana glacial aspect, but those of the northern Qiangtang show more resemblance to those of the ancient Tethys. The two assemblages are interpreted as representing, respectively, the northern margin of Gondwanaland and the southwestern margin of the Yangtze continent. The two continents are suggested to have begun to separate during the Early Carboniferous and to have rejoined in the earliest Permian. Throughout this cycle of events, the northern Qiangtang occupied a passive margin while the southern Qiangtang rocks show evidence of the development of an active margin. Basaltic rocks from the southern Qiangtang are ind...

Origin and assembly of the Tethyside orogenic collage at the expense of Gondwana Land

Abstract The Tethysides are a superorogenic complex flanking the Eurasian continent to the south and consisting of the Cimmerides and Alpides , products of Palaeo- and Neo-Tethys respectively. We here review their evolution, mainly on the basis of new maps showing the distribution of sutures, magmatic rocks, certain palaeobiogeographically and palaeoclimatologically significant taxa and facies, and fragments of Pan-African (900–450 Ma) orogenic system forming the basement of many Tethyside blocks. These are supplemented by palaeomagnetic data reported in the literature. A fundamental tenet of this paper is that major sutures which contain ophiolite fragments, represent tectonic sections between continental blocks where oceanic crust has been subducted. Palaeo-Tethys came into existence largely in late Carboniferous time. Coevally, it began to be consumed by both internal and peripheral subduction zones, which continued into the Permian; some of these had been inherited from pre-Tethyan times. In the later Permian, rifting subparallel with the northern margin of Gondwana Land began between the Zagros and Malaysia, separating a Cimmerian continent from N. Gondwana Land, and thus heralding the opening of Neo-Tethys and other smaller oceans that were back-arc basins of Palaeo-Tethys. This rifting possibly also extended farther west into Crete and mainland Greece. However, the North China block, Yangtze block, Huanan block, the eastern moity of the Qangtang block (North Tibet), and Annamia, all originally pieces of the end-Proterozoic-early Palaeozoic Gondwana Land, had already separated from it in pre-late Carboniferous times, possibly during the Devonian. All of these blocks, and the Cimmerian continent, were characterized by Cathaysian floral elements in late Palaeozoic time. Palaeomagnetic and palaeontological data showing the original Gondwana Land affinity of these continental blocks are supplemented by correlating late Proterozoic-early Palaeozoic Pan-African sutures, orogenic belts, and sedimentary basin fragments across Tethyside sutures. Late Permian foraminiferal provinces are related to this palaeogeographical interpretation. By Triassic times, most Cimmeride subduction zones were already in existence. The Cimmerian Continent accelerated its separation from Gondwana Land and—locally in the late Permian—began disintegrating internally along the Waser/Rushan-Pshart/Banggong Co-Nu Jiang/Mandalay ocean. By late Triassic time all of the Chinese blocks—except Lhasa-and Annamia had collided with each other and with Laurasia. The resulting enormous orogenic collage had a ‘soft cushion’ between itself and Laurasia, in the form of the enormous accretionary complex of the Songpan-Ganzi. This connection enabled Laurasian land vertebrates to reach south-east Asia by late Triassic time. In late Triassic to middle Jurassic times, most major Cimmeride collisions were completed. Widespread aridity in Central Asia occurred in late Jurassic time, probably in the rain shadow of the newly formed Cimmeride mountain wall. Neo-Tethyan subduction systems formed along the S. margin of the Cimmerides or within Neo-Tethyan oceanic lithosphere during the Jurassic. Most, if not all, were north- or east-dipping. They continued the northerly migration of the Tethyside blocks. Evolution of the Tethysides influenced the distribution of marine and terrestrial organisms, and affected sea-level changes and patterns of atmospheric circulation during much of the Mesozoic and Cainozoic. It is likely to have reflected the surface expression of a persistent trend in the large-scale convective circulation in the mantle, that continuously transported material northward into the Tethyan domain.