Cimmerian Blocks Research Articles

Lower Cisuralian brachiopod faunas from the Lhasa and South Qiangtang blocks in Tibet and their biostratigraphical and palaeobiogeographical implications

The opening time of the Bangong-Nujiang Ocean (BNO, also known as the Meso-Tethys Ocean) remains controversial. It was documented to have opened during the Permian, Early Palaeozoic or Mesozoic times. The Permian faunas and their palaeobiogeographical affinities of the Lhasa and South Qiangtang blocks bordered by the Bangong-Nujiang Suture Zone are crucial for reconstructing the tectonic evolution of both blocks and determining the opening time of the BNO. This paper describes two new lower Cisuralian brachiopod faunas from the uppermost Yunzhug Formation of the Lhasa Block and the Zhanjin Formation of the South Qiangtang Block. These two faunas exhibit a high similarity in composition, both containing species of Bandoproductus, Spirelytha and Sulciplica thailandica. Comparable brachiopod faunas are found from the glaciomarine diamictites in the blocks of the eastern Cimmerian belt including the Baoshan, Irrawaddy and Sibuma, as well as the Tethys Himalaya and the Gondwanaland. They are herein named the Bandoproductus-Spirelytha association, which exhibits apparent Gondwanan affinities. Cluster analysis and principal components analysis are employed to conduct a palaeobiogeographical reconstruction of the Asselian–Sakmarian brachiopod faunas primarily from the aforementioned tectonic units. A single palaeobiogeographical unit, the Indoralian Province, is recognised and represented by the unified brachiopod association. This suggests that most of the Cimmerian blocks, including the Lhasa and South Qiangtang blocks, were attached to the northern Gondwanan margin during the early Cisuralian. This is in contrast to our previous results revealed by multiple quantitative analyses for the Kungurian–Roadian brachiopod faunas in the Tethys, which show distinct faunal differences between the Lhasa and South Qiangtang blocks. This palaeobiogeographical discrepancy suggests that the two blocks probably had varying northward drifting tempos after they detached from the Gondwanaland, that is, the South Qiangtang Block drifted faster and incorporated into the temperate-warm region of the Northern Cimmerian Subprovince; whereas the Lhasa Block drifted relatively slowly and retained to be close to the northern Gondwanan margin and belonged to the Southern Cimmerian Subprovince during the late Cisuralian. The faunal differences provided critical implications that the BNO probably had opened in the Artinskian and reached a certain width with distinct faunal differences during the Kungurian.

A MODEL OF THE LATE MESOZOIC AND CENOZOIC THERMOTECTONIC EVOLUTION OF THE PRE-MESOZOIC BASEMENT ROCKS IN SOUTH TUVA

Thermotectonic modeling was performed for the crystalline rocks of South Tuva using the apatite fission-track analysis. Thermotectonic modeling made it possible to visualize the Late Mesozoic and Cenozoic cooling history of the Pre-Mesozoic basement rocks, and to reconstruct the chronology and scale of the denudational processes over the last 125 myr and the evolution of paleorelief of South Tuva over the last 100 myr. The modeling results depicted several Mesozoic-Cenozoic episodes of cooling due to differential denudation and exhumation of the Pre-Mesozoic basement rocks. A differential denudation is related to an asynchronous activation of fault structures controlling the tectonic evolution of South Tuva. It is shown that the Early Cretaceous (~125–100 Ma) activation of the Agar-Dag-Oka thrust fault zone could result from the post-collisional processes after the collision between Siberia and Amuria and/or consecutive collision between the Cimmerian blocks. An intense activation of the Agar-Dag-Oka fault zone in the Late Cretaceous (~100–75 Ma), accompanied by significant basement rock exhumation in the eastern South Tuva to absolute heights of 1200 m, could be caused by the Karakoram-Pamir collision in the south of Eurasia. The Late Cenozoic (25–0 Ma) activation of the main fault zones of South Tuva represents a far-field effect of the Indo-European collision on the southern Eurasian continent. At the same time, there were the maximum basement uplift in the junction zone between the South Tannuola and Ubsunur-Bii-Khem fault zones and the transformation of relief of South Tuva from moderately dissected, with absolute heights of 500 to 1400 m, to modern, with absolute heights of 800 to 2600 m.

The Aptian orbitolinid records of the Lut Block (Central Iran): Biostratigraphic and palaeobiogeographic implications

The biostratigraphic and palaeogeographic aspects of the Lower Cretaceous “Orbitolina limestone” of the eastern Cimmerian block in Central Iran were analyzed, providing the first high resolution dating of this unit in the Lut Block. The fossil assemblages are dominated primarily by orbitolinid larger benthic foraminifera. The identified orbitolinids of the studied section, including Palorbitolina lenticularis (Blumenbach), P. ultima Cherchi and Schroeder, Palorbitolinoides pileus (Fossa-Mancini), Praeorbitolina cormyi Schroeder, and P. wienandsi Schroeder, were observed in pack-/grainstone facies. Based on the stratigraphic value of the index orbitolinids, an age of late early Aptian is constrained for the orbitolinid bearing unit of the study area. The results from Central Iran, palaeobiogeographically located at the northern margin of the Neo-Tethys, suggest that the previous inferred restriction of the Praeorbitolina/Palorbitolinoides Association to the southern margin of the Neo-Tethys during the Lower Cretaceous has to be corrected.

Cimmerian block detachment from Gondwana: A slab pull origin?

The Cimmerian blocks are ribbons-like micro-continents that detached from the northern margin of Gondwana during the closure of the Paleotethys ocean at the end of the Permian. While it has been hypothesized that slab pull forces induced by subduction of the Paleotethys beneath Eurasia could have driven extension in the lower plate after the Paleotethys mid-oceanic ridge subduction, so far, numerical modeling studies have concluded that these forces alone are not sufficient to drive continental breakup. However, the previous modeling studies neglect olivine phase transitions in the mantle and likely underestimated the slab pull. Here, we present numerical simulations that consider the evolution of the density of the slab associated with metamorphic reactions and analyze carefully when, where, and how the slab pull can lead to the formation of micro-continents. Our results show that the marked increase in slab density related to olivine phase transitions at 410 km discontinuity, in addition to gravitational potential energy, can cause continental rifting of the lower plate. Nonetheless, continental breakup is only achieved in absence of horizontal shear flow at the base of the continental lithosphere. In these successful experiments, the deformation localizes 100-200 km on shore due to bending stress pointing out that this process probably always leads to the formation of small continental ribbons. Our results confirm that the tensional force induced by the Paleotethys subduction dynamics may have caused the detachment of the Cimmerian blocks. Since continental breakup of the lower plate is promoted in the absence of basal shear below Gondwana's lithosphere (with respect to the underlying mantle), we argue that the formation of the Cimmerian blocks provides information on the relative motion between the tectonic plates and the underlying mantle. Our simulations demonstrate that there is a significant time-lag between ridge subduction and continental breakup (i.e. the time needed for the slab to reach the 410 km discontinuity), which is rarely accounted by paleogeographic reconstructions.

Quantitative palaeobiogeography of the Kungurian–Roadian brachiopod faunas in the Tethys: Implications of allometric drifting of Cimmerian blocks and opening of the Meso-Tethys Ocean

Benthic faunas and their palaeobiogeographic affinities related to climate gradient can provide insightful information to unravel the tectonic histories of their hosted blocks if they moved across different latitude zones. In this study, three different quantitative analyses (network, cluster and non-metric multi-dimensional scaling analyses) are employed to carry out a palaeobiogeographic study of the Kungurian–Roadian brachiopod faunas of the Cimmerian blocks. A spin-glass algorithm that enables network community detection is here used for the first time and its six groups-partition is considered as the optimal solution. Two new palaeobiogeographic subprovinces, the Southern and Northern Cimmerian subprovinces in the Southern Transitional Zone, are first recognised besides the previously perceived Cathaysian, Transhimalayan, Westralian and Austrazean provinces. Based on distinct palaeobiogeographic discrepancies, it is suggested that the Lhasa, Tengchong and Irrawaddy blocks, constituting the main parts of the Southern Cimmerian Subprovince, were probably situated near the peri-Gondwanan region; whereas the South Qiangtang, Baoshan and Sibuma blocks of the Northern Cimmerian Subprovince probably were located in the temperate region of the southern hemisphere during the Cisuralian to Guadalupian transition. Such discrepancies in palaeobiogeographic affinitites between two continental slices imply that the Meso-Tethys Ocean had existed before the late Cisuralian and its present oceanic remnants are represented by the Bangong-Nujiang Suture Zone in central Tibet, the Gaoligongshan shear zone in western Yunnan and the Medial-Myanmar Suture Zone in southern Myanmar and Peninsula Thailand. Moreover, the Neo-Tethys Ocean had likely co-existed with the Meso-Tethys Ocean during that time. Given that all these blocks were attached to the northern peri-Gondwanan margin during the early Cisuralian, the Kungurian-Roadian faunal discrepancies among different continental slices with a distinct latitude gradient can be interpreted by the allometric northward drifting of the Cimmerian blocks, which led to the formation of the Meso-Tethys and Neo-Tethys oceans.

Geophysical and Geochemical Evidence for a New Mafic Magmatic Province Within the Northwest Shelf of Australia

AbstractThe formation of mafic magmatic provinces are significant geological events that can drive mass extinctions and continental rifting and can influence basin evolution, petroleum prospectivity and mineralization. Buried magmatic provinces, however, are rarely identified and difficult to define. The Northwest Shelf of Australia contains large volumes of potentially interconnected mafic igneous material across several sedimentary basins. However, limited study and a lack of surface exposure have prevented detailed description and classification of these rocks. In this study, the distribution and composition of these mafic igneous rocks are described using an integrated geophysical and geochemical approach, which included over 10,000 km line length of 2D seismic data, well log data and chemical analysis of samples from 14 wells across the Browse, Roebuck, Canning and North Carnarvon basins. Using this combined data set, we demonstrate interconnectivity of buried mafic igneous rocks across the Northwest Shelf and calculate a total surface area exceeding 280,000 km2 and a cumulative minimum volume of ∼140,000 km3. Petrology and geochemistry of samples indicate they are basaltic and doleritic with alkaline and sub‐alkaline compositions and formed in a continental rift setting. Collectively, the igneous rocks meet the criteria for classification as a mafic magmatic province (MMP) and closely match the criteria required for classification as a large igneous province (LIP). Emplacement of the newly defined Northwest Shelf MMP may represent hotspot magmatism that could have initiated rifting of the Cimmerian Block from NW Australia during the Permian and could have potential for future, large scale CO2 sequestration and storage.

Fusulinid-bearing oolites from the Tengchong Block in western Yunnan, SW China: Early Permian warming signal in the eastern peri-Gondwana

Discovery of fusulinid-bearing oolites from the Tengchong Block in western Yunnan, China, provides new insights into Permian paleoclimatic history of the eastern peri-Gondwana. In the northern Tengchong Block, new carbonate samples from the lower part of the Dadongchang Formation yield a monotonous fusulinid assemblage, comprising Eoparafusulina tschernyschewi tschernyschewi, E. tschernyschewi oblonga, E. tibetica, E. aff. tschernyschewi tschernyschewi and E. aff. laudoni. This assemblage correlates well to late Sakmarian–Artinskian fusulinids widespread among the Cimmerian blocks in peri-Gondwana, which were characterized by low diversity and dominance of cosmopolitan taxa. Such fusulinids have been interpreted to indicate temperate-water environment in the aftermath of late Paleozoic glaciation. Moreover, the eastern Cimmerian region has been suggested, by both paleontological and sedimentological data, to be even cooler than the western region especially during the Early Permian. However, the fusulinids of this study unexpectedly occur within oolites, as ooids diagnostic for warm-water settings, have never been recorded from Early Permian strata of any eastern Cimmerian block. This new finding in the northern Tengchong Block indicate that the shallow marine there was warm enough to accommodate not only poorly diversified fusulinids but also oolites, even immediately posterior to the late Paleozoic glaciation. The appearance of these oolites is more likely ascribed to a warming pulse of sea-surface temperature in the context of post-glacial climatic amelioration than to the northward drift of the Tengchong Block.

Triassic evolution of the western Neotethys: constraints from microfabrics and U–Pb detrital zircon ages of the Plattenkalk Unit (External Hellenides, Greece)

The Plattenkalk Unit forms the structurally deepest nappe of the External Hellenides and plays a key role for the reconstruction of Triassic paleogeography in the western Tethyan realm. To shed light on the provenance of the Plattenkalk Unit, we present microfabrics and U–Pb detrital zircon data obtained from quartzite and metaconglomerate of its basal part (Kastania and Transitional Beds exposed on the southern Peloponnesus). A Minoan-type age spectrum of detrital zircons, together with inherited deformation microfabrics of detrital components, points to a very low- to low-grade metamorphic basement as source rock that was situated along the northern passive margin of Apulia facing the Neotethys in the north. The siliciclastic rocks of the Kastania Beds reflect the late Permian/early Triassic opening of the Neotethys, whereas the Transitional Beds indicate the late Triassic facies change toward the Pantokrator-type dolomite, which is a characteristic rock of late Triassic Tethyan carbonate platforms. Dolomite and evaporites of the Trypali Unit, on the other hand, were deposited vis-a-vis along the southern passive margin of Cimmeria. This Triassic configuration of Tethyan basins fits well with the S/SW-directed transport and the recent vertical succession of the Hellenic nappes. The proposed Triassic position of the Plattenkalk basin implies that the Apulian microplate was not part of the Cimmerian block since Permian times. The separation of the Apulian microplate from Gondwana is attributed to the Jurassic breakup of Pangea resulting in the opening of the Mesogean ocean, the relics of which are undergoing subduction beneath the Aegean microplate still today.

Evidence from detrital chrome spinel chemistry for a Paleo-Tethyan intra-oceanic island-arc provenance recorded in Triassic sandstones of the Nakhlak Group, Central Iran

Detrital chrome spinel (Cr-spinel) is for the first time described from the Nakhlak Group, which is a distinctive Triassic (Late Olenekian‒?Early Carnian) sedimentary succession in Central Iran. Previously published data from the Nakhlak Group suggest deposition along an active margin during the Triassic and Eurasian affinity. Presently, the Nakhlak Group is located in the northwestern region of the Central-East Iranian Microcontinent, a major segment of the Cimmerian block. The Nakhlak region is believed to have been dislocated from its prior position at the Turan Plate from the southern Eurasian active margin after deposition in Triassic time. Almost all of the analyzed detrital Cr-spinels have high Cr-numbers (0.6–0.85), variable Mg-numbers (0.33–0.78), and low Fe3+ (<0.25), Al2O3 (<20 wt%) and TiO2 (<0.4 wt%) contents, suggesting a magmatic source for the Cr-spinels formed both within oceanic (mainly harzburgitic) mantle and in a supra-subduction zone (SSZ) tectonic setting. The data suggest that mafic‒ultramafic rock assemblages with SSZ signatures were generated in the Paleo-Tethyan realm before their obduction as an ophiolite. By comparing data from the present study with results from previous work on the Triassic in Central Iran it is evident that Paleo-Tethyan ophiolites of intra-oceanic island-arc (IOIA) origin supplied detrital material to the Nakhlak Group at the southern margin of Eurasia during pre-to syn-Eocimmerian tectonics in the Triassic.

Global Permian brachiopod biostratigraphy: an overview

Abstract Establishing a Permian brachiopod biochronological scheme for global correlation is difficult because of strong provincialism during the Permian. In this paper, a brief overview of brachiopod successions in five major palaeobiogeographical realms/zones is provided. For Gondwanaland and peri-Gondwanan regions including Cimmerian blocks, Bandoproductus and Punctocyrtella (or Cyrtella ) are characteristic of the lower Cisuralian, as is Cimmeriella for the middle Cisuralian. As the Cimmerian blocks continued drifting north during the late Kungurian, accompanied by climate amelioration, contemporaneous brachiopods inhabiting these blocks showed a distinct shift from cold-water to mixed or warm-water affinities. However, coeval brachiopods in the Northern Transitional Zone (NTZ) are characterized by warm-water faunas and are associated with fusulinids in the lower Cisuralian. The Guadalupian brachiopods of the NTZ were clearly mixed between the Boreal and palaeoequatorial affinities. The end-Guadalupian is marked by the disappearance of a few characteristic genera, such as Vediproductus , Neoplicatifera and Urushtenoidea , in the Palaeotethyan region. The onset of the end-Permian mass extinction in the latest Changhsingian is clearly exhibited by the occurrence of the dwarfed and thin-shelled brachiopods commonly containing Paracrurithyris .

About this title ‐ Geological Evolution of Central Asian Basins and the Western Tien Shan Range

The geological evolution of Central Asia commenced with the formation of a complex Precambrian–Palaeozoic orogen. Cimmerian blocks were then accreted to the southern margin in the Mesozoic, leading to tectonic reactivation of older structures and discrete episodes of basin formation. The Indian and Arabian blocks collided with Asia in the Cenozoic, leading to renewed structural reactivation, intracontinental deformation and basin development. This complex evolution resulted in the present-day setting of an elongated Tien Shan range flanked by large Mesozoic–Cenozoic sedimentary basins with smaller intramontane basins distributed within the range. This volume presents multidisciplinary results and reviews from research groups in Europe and Central Asia that focus on the western part of the Tien Shan and some of the adjacent large sedimentary basins. These works elucidate the Late Palaeozoic–Cenozoic tectono-sedimentary evolution of the area. Emphasis is given to the collision of terranes and continents and the ensuing fault reactivations. The impact of climatic changes on sedimentation is also examined.

Geological evolution of Central Asian Basins and the western Tien Shan Range

Abstract The geological evolution of Central Asia commenced with the evolution of a complex Precambrian–Palaeozoic orogen. Cimmerian blocks were then accreted to the southern margin during the Mesozoic, leading to tectonic reactivation of older structures and discrete episodes of basin formation. The Indian and Arabian blocks collided with Asia during the Cenozoic, leading to renewed structural reactivation, intracontinental deformation and basin development. This complex evolution resulted in the present-day setting of an elongated Tien Shan range flanked by large Mesozoic–Cenozoic sedimentary basins with smaller intramontane basins distributed within the range. The aim of this volume is to present multidisciplinary results and reviews from research groups in Europe and Central Asia that focus on the western part of the Tien Shan and some of the large sedimentary basins in that area. These works elucidate the Late Palaeozoic–Cenozoic tectono-sedimentary evolution of the area. Emphasis is placed on the collision of terranes and/or continents and the ensuing fault reactivation; the impact of changes in climate on the sedimentation is also examined.

An upper Kungurian/lower Guadalupian (Permian) brachiopod fauna from the South Qiangtang Block in Tibet and its palaeobiogeographical implications

A brachiopod fauna including 15 species belonging to 14 genera is described from a thin carbonate succession of the Lugu Formation at the Anmu section in the northern part of the South Qiangtang Block, which is about 15km south to the Longmu Co-Shuanghu Suture. The brachiopod fauna is late Kungurian or early Guadalupian in age in terms of the presence of the brachiopod species Vediproductus punctatiformis, “Cryptospirifer” omeishanensis, and Paraplicatifera regularis. This is also generally consistent with the age indicated by the fusulines (e.g., Cancellina, Chusenella, Monodiexodina, Nankinella, Neofusulinella, Pseudofusulina, and Sphaerulina) from this formation at the nearby Mari and Duoma A (DMA) sections. Palaeobiogeographically, the brachiopod fauna exhibits a typical Cathaysian affinity and all the species are reported commonly from the equivalents in South China and Cimmerian blocks, which strongly suggest that the South Qiangtang Block had drifted to a position in the warm-water or temperate regime close to South China.

Provenance of Permian–Triassic Gondwana Sequence units accreted to the Banda Arc in the Timor region: Constraints from zircon U–Pb and Hf isotopes

Analysis of zircons from Australian affinity Permian–Triassic units of the Timor region yield age distributions with large age peaks at 230–400Ma and 1750–1900Ma, which are similar to zircon age spectra found in rocks from NE Australia and crustal fragments now found in Tibet and SE Asia. It is likely that these terranes, which are now widely separated, were once part of the northern edge of Gondwana near what is now the northern margin of Australia. The Cimmerian Block rifted from Gondwana in the Early Permian during the initial formation of the Neo-Tethys Ocean. The zircon age spectra of the Gondwana Sequence of NE Australia and in the Timor region are most similar to the terranes of northern Tibet and Malaysia, further substantiating a similar tectonic affinity. A large 1750–1900Ma zircon peak is also very common in other terranes in SE Asia.Hf analysis of zircon from the Aileu Complex in Timor and Kisar Islands shows a bimodal distribution (both radiogenically enriched and depleted) in the Gondwana Sequence at ~300Ma. The magmatic event from which these zircons were derived was likely bimodal (i.e. mafic and felsic). This is substantiated by the presence of Permian mafic and felsic rocks interlayered with the sandstone used in this study. Similar rock types and isotopic signatures are also found in Permian–Triassic igneous units throughout the Cimmerian continental block.The Permian–Triassic rocks of the Timor region fill syn-rift intra-cratonic basins that successfully rifted in the Jurassic to form the NW margin of Australia. This passive continental margin first entered the Sunda Trench in the Timor region at around 7–8Ma causing the Permo-Triassic rocks to accrete to the edge of the Asian Plate and emerge as a series of mountainous islands in the young Banda collision zone. Eventually, the Australian continental margin will collide with the southern edge of the Asian plate and these Gondwanan terranes will rejoin.

Paleomagnetic investigation of the Early Permian Panjal Traps of NW India; regional tectonic implications

The ∼289Ma Panjal Traps of NW India (Kashmir) are part of a series of rift-related mafic suites (Abor, Sikkim etc.) that were erupted onto northern India (present-day coordinates) around the same time as separation of the Cimmerian blocks of Qiangtang and Sibumasu. We report new data from only the second paleomagnetic investigation of this unit. Standard alternating field and thermal demagnetization methods were used to isolate characteristic magnetizations from seven outcrops at three locations within the Kashmir Valley, NW India. Analysis of four sections (14 individual cooling units) from close to Srinagar, that together form a tectonically coherent sequence spanning ∼3km of stratigraphy, yield a single-component, primary magnetization with a mean direction of Dec: 134.8°, Inc: 55.3° (α95=8.9°, k=21.0). An inclination-only mean of 52.5° (α95=8.9°, k=47.2) gives a paleolatitude of ∼33°S (±5°). A paleopole of 110.5°E 8.4°S (A95=10.7) is also calculated. Assuming the magnetization records a portion of the reverse polarity Kiaman Superchron, the new result indicates extrusion of the Panjal Traps basalts at mid-latitudes in the southern hemisphere. By inference this constrains the location of central Gondwana, and informs debates related to Cimmeria’s detachment from Gondwana.

Mesozoic tectonic and topographic evolution of Central Asia and Tibet: a preliminary synthesis

Abstract During the Late Palaeozoic–Mesozoic, Central Asia and Tibet were affected by several geodynamic episodes that induced either large-scale compression or widespread extension. The Late Palaeozoic final amalgamation of the Central Asian Orogenic Belt, the accretion of the Cimmerian blocks, the closure of the Mongol–Okhotsk Ocean and the accretion of the Neocimmerian blocks set the structural pattern of the continent. This Mesozoic tectonic heritage plays a first-order role in the localization and evolution of the Tertiary deformation of the continent. Similarly, large-scale Mesozoic topographic features are still preserved in Central Asia, where they form a non-negligible part of the present-day topography. This work aims at providing an overview of the major tectonic events that affected Central Asia and Tibet during the Late Palaeozoic and Mesozoic periods. The general topographic evolution of the continent is also described together with the accompanying climatic changes through time.

Rugose corals at the Tournaisian–Viséan transition in the Central Taurides (S Turkey) – Palaeobiogeography and palaeoceanography of the Asian Gondwana margin

This paper gives the first taxonomic description of the Upper Tournaisian–Lower Viséan rugose coral fauna of the Yaricak Formation (Aladag Unit, Central Taurides, South Turkey). Fifteen species belonging to twelve genera were identified, one species is newly described: Eokoninckocarinia gemmina. The corals are stratigraphically distributed in four assemblages. The two typical assemblages of the Upper Tournaisian are composed of widely distributed taxa (Uralinia, Caninia, Proheterelasma, Zaphrentites). The assemblage crossing the Tournaisian–Viséan boundary is characterized by Eurasian and cosmopolitan and widely distributed taxa (Calmiussiphyllum, Siphonophyllia, Bifossularia Amygdalophyllum, Caninophyllum, Keyserlingophyllum) and Asian taxa (Kueichouphyllum). The youngest assemblage, dominated by Eokoninckocarinia gemmina sp. nov., has yielded foraminifers Moliniacian (Lower Viséan) in age. These assemblages form a low diversity level-bottom community which is typical of the South Palaeotethys ‘Kueichouphyllum Zone’ extending along the Asian margin of Gondwana (Cimmerian Terrane) during Lower Carboniferous times. As in the other Cimmerian blocks, all the corals are solitary and colonial taxa are virtually absent. This absence is tentatively explained by the high palaeolatitude (c. 50°S) position of the Cimmerian Terrane in the southern part of the Palaeotethys Ocean for this time slice. A cold-water palaeo-current running eastward along the Gondwana margin might also be considered as it possibly could explain the wide distribution of the Kueichouphyllum fauna, restricted east of Africa in the southern coast of the Palaeotethys. Palaeoceanography, palaeoclimate and facies issues are discussed as possible causes of the diversity gradient observed between the eastern (Australia, Malaya) and the western (North Africa) margin of the Gondwana.

Further paleomagnetic results for lower Permian basalts of the Baoshan Terrane, southwestern China, and paleogeographic implications

The Baoshan Terrane of southwestern China is considered to have been part of the Cimmerian block during the late Paleozoic; consequently, knowledge of its paleoposition and geological evolution can provide constraints on the Permian breakup of northern East Gondwana. Therefore, we conducted paleomagnetic and rockmagnetic studies on lower Permian basalts from four localities in the Baoshan Terrane. The basalts hold a stable characteristic remanent magnetization (ChRM) at high temperatures (300–680°C) that is carried by magnetite, maghemite, and hematite with both pseudo-single and multiple domains. To test the reliability of data from these volcanic rocks, we analyzed the geomagnetic secular variation (GSV) and reliability of both the present data and previous paleomagnetic data. The results from 23 sites yield a single reversed polarity directed downwards to the southwest, giving a site-mean direction of Dg/Ig=156.7°/56.6° (kg=8.0, α95=11.4°) before tilt correction, and Ds/Is=218.3°/60.1° (ks=14.1, α95=8.4°) after tilt correction. The result passed the fold test, but the GSV was able to be averaged out in only two sections. All available data were examined section-by-section using the angular dispersion (SB) of virtual geomagnetic poles (VGPs) to ensure that the GSV was completely averaged out. Because the dispersion in declinations is likely to have been affectedby subsequent tectonic deformation, the paleosecular variation (PSV) could not be evaluated from all the data amassed from different sections, and the PSV was able to be removed from only four (combined) sections. A small-circle fit of these VGPs gives an averaged paleocolatitude of 51.9°±3.7° (N=31 sites) centered on 24°N, 99°E. The result indicates that the sampled area of the Baoshan Terrane was located at a latitude of 38°S±3.7° during the late early Permian. A comparison of this result with early Permian data from Gondwanan blocks suggests that the Baoshan Terrane was situated near the junction of northern India and northwestern Australia, and that it broke away from western Australia after the early Permian.

Paleomagnetic data support Early Permian age for the Abor Volcanics in the lower Siang Valley, NE India: Significance for Gondwana-related break-up models

Confusion exists as to the age of the Abor Volcanics of NE India. Some consider the unit to have been emplaced in the Early Permian, others the Early Eocene, a difference of ∼230 million years. The divergence in opinion is significant because fundamentally different models explaining the geotectonic evolution of India depend on the age designation of the unit. Paleomagnetic data reported here from several exposures in the type locality of the formation in the lower Siang Valley indicate that steep dipping primary magnetizations (mean=72.7±6.2°, equating to a paleo-latitude of 58.1°) are recorded in the formation. These are only consistent with the unit being of Permian age, possibly Artinskian based on a magnetostratigraphic argument. Plate tectonic models for this time consistently show the NE corner of the sub-continent >50°S; in the Early Eocene it was just north of the equator, which would have resulted in the unit recording shallow directions. The mean declination is counter-clockwise rotated by ∼94°, around half of which can be related to the motion of the Indian block; the remainder is likely due local Himalayan-age thrusting in the Eastern Syntaxis. Several workers have correlated the Abor Volcanics with broadly coeval mafic volcanic suites in Oman, NE Pakistan–NW India and southern Tibet–Nepal, which developed in response to the Cimmerian block peeling-off eastern Gondwana in the Early-Middle Permian, but we believe there are problems with this model. Instead, we suggest that the Abor basalts relate to India–Antarctica/India–Australia extension that was happening at about the same time. Such an explanation best accommodates the relevant stratigraphical and structural data (present-day position within the Himalayan thrust stack), as well as the plate tectonic model for Permian eastern Gondwana.

Pangea Megasequences of Tethyan Gondwana-margin reflect global changes of climate and tectonism in Late Palaeozoic and Early Triassic times—A review

The maximum concentration of continental crust at the Pangea stage is characterized by a specific depositional sequence generally referred to as the Pangea Megasequence. Extending in time from the Late Carboniferous to the middle of the Triassic, the succession exhibits similar trends across the whole of Gondwana. Invariably, the sequence was initiated by Late Carboniferous to Early Permian glacial and periglacial deposits. Deglaciation occurred in Early Sakmarian time, evidenced by a typical, commonly transgressive facies. The succeeding formations comprise, in ascending order, coal measures, redbeds, some more coal measures and again redbeds with an intercalation of fluviatile sands in the Early Triassic. After deglaciation the basic depositional theme was modified, depending on postglacial adjustments of climate and on the type of regional tectonic regimes. Extension of the tropical climatic belt after deglaciation was one factor that governed the resulting sediment facies. Coal deposition that prevailed in central Gondwana in the Early Permian gave way to dominance of redbeds in the Middle and Late Permian and, in more distal positions, evaporitic deposits were laid down, following deglaciation. Within marine realms, coralline limestones were formed. Within Gondwana the depositional period of the Pangea Megasequence was governed by three distinctive tectonic regimes: collision dominated the Panthalassa margin, transpressional sag controlled the interior basins, and extension and rifting was experienced along the entire Tethyan margin. In the Early Permian, large and complex graben structures commenced to develop between Africa and India (Malagasy Trough) and between India and Australia (West Australian Trough), giving access to Tethyan waters during deglaciation, commencing in the late Early Sakmarian. Rifting along the Tethyan margin commenced in the Early Permian and was associated with active volcanism between Cashmere and Yunnan and in north-western Australia. Spreading of Neo-Tethys and the formation of oceanic crust, leading to the separation of the Cimmerian Blocks from Gondwana, commenced in the late Early Permian and continued into the Triassic. Thus two facies realms developed, an intracratonic rift facies comprising the Cashmere, Lhasa and Baoshan blocks and a facies controlled by detachment, comprising more distal blocks, such as Tengchong, Malay and Sumatra. The present distribution of individual blocks was governed by fold movements of the Himalayan Orogeny, complicated by transpression along the eastern Himalayan Syntaxis.