The Triassic of the Thakkhola (Nepal). II: Paleolatitudes and comparison with other Eastern Tethyan Margins of Gondwana

During the Triassic, the Thakkhola region of the Nepal Himalaya was part of the broad continental shelf of Gondwana facing a wide Eastern Tethys ocean. This margin was continuous from Arabia to Northwest Australia and spanned tropical and temperate latitudes.A compilation of Permian, Triassic and early Jurassic paleomagnetic data from the reconstructed Gondwana blocks indicates that the margin was progressively shifting northward into more tropical latitudes. The Thakkhola region was approximately 55° S during Late Permian, 40° S during Early Triassic, 30° S during Middle Triassic and 25° S during Late Triassic. This paleolatitude change produced a general increase in the relative importance of carbonate deposition through the Triassic on the Himalaya and Australian margins. Regional tectonics were important in governing local subsidence rates and influx of terrigenous clastics to these Gondwana margins; but eustatic sea-level changes provide a regional and global correlation of major marine transgressions, prograding margin deposits and shallowing-upward successions. A general mega-cycle characterizes the Triassic beginning with a major transgression at the base of the Triassic, followed by a general shallowing-upward of facies during Middle and Late Triassic, and climaxing with a regression in the latest Triassic.     

The Triassic of the Thakkhola (Nepal). I: stratigraphy and paleoenvironment of a north-east Gondwanan rifted margin

The Mesozoic sediments of Thakkhola (central Nepal) were deposited on a broad eastern north Gondwanan passive margin at mid-latitudes (28–41 °S) facing the Southern Tethys ocean to the north. The facies is strikingly similar over a distance of several thousand kilometres from Ladakh in the west to Tibet and to the paleogeographically adjacent north-west Australian margin (Exmouth Plateau, ODP Legs 122/123) and Timor in the east. Late Paleozoic rifting led to the opening of the Neo-Tethys ocean in Early Triassic times. An almost uninterrupted about 2 km thick sequence of syn-rift sediments was deposited on a slowly subsiding shelf and slope from Early Triassic to late Valanginian times when break-up between Gondwana (north-west Australia) and Greater India formed the proto-Indian Ocean. The sedimentation is controlled by (1) global events (eustasy; climatic/oceanographic changes due to latitudinal drift; plate reorganization leading to rift-type block-faulting) and (2) local factors, such as varying fluvio-deltaic sediment input, especially during Permian and late Norian times. Sea level was extremely low in Permian, high in Carnian and low again during Rhaeto-Liassic times. Third-order sea-level cycles may have occurred in the Early Triassic and late Norian to Rhaeto-Liassic. During the Permian pure quartz sand and gravel were deposited as shallowing upward series of submarine channel or barrier island sands. The high compositional maturity is typical of a stable craton-type hinterland, uplifted during a major rifting episode. During the early Triassic a 20–30 m thick condensed sequence of nodular ‘ammonitico rosso’-type marlstone with a ‘pelagic’ fauna was deposited (Tamba Kurkur Formation). This indicates tectonic subsidence and sediment starvation during the transgression of the Neo-Tethys ocean. During Carnian times a 400 m thick sequence of fining upward, filament-rich wackestone/shale cycles was deposited in a bathyal environment (Mukut Formation). This is overlain by about 300 m of sandy shale and siltstone intercalated with quartz-rich bioclastic grain- to rudstone (Tarap Shale Formation, late Carnian-Norian). The upper Norian to (?lower) Rhaetian Quartzite Formation consists of (sub)arkosic sandstones and pure quartz arenites, indicating different sediment sources. The fluvio-deltaic sandstones are intercalated with silty shale, coal and bioclastic limestone, as well as mixed siliciclastic-bioclastic rocks. The depositional environment was marginal marine to shallow subtidal. The fluvio-deltaic influence decreased towards the overlying carbonates of Rhaeto-Liassic (?) age (Jomosom Formation correlative with the Kioto Limestone), when the region entered tropical paleolatitudes resulting in platform carbonates.     

Comparison of the Jurassic and Cretaceous sedimentary cycles of Somalia and Madagascar: implications for the Gondwana breakup

The Jurassic and Cretaceous sedimentary history of northern Somalia and the Morondava Basin of south-western Madagascar have been studied. Both regions display an independent facial development; however, a comparison of the sequential evolution of the Mesozoic sedimentary successions in these two presently widely separated areas reveals a surprisingly high level of similarity, which probably reflects major events during the disintegration of Eastern Gondwana during the Jurassic and Cretaceous. Although in Jurassic times the onset of transgressions and regressions in both areas compares well with eustatic development, major deviations in combination with the tectonic activities of different degrees are observed in the Early and Late Cretaceous synchronously in both regions. Transgressions are observed in Toarcian, Bajocian (not dated in northern Somalia), Callovian, Valanginian (Madagascar only), Aptian and Campanian times. Tectonism is noted before the Aptian and Campanian transgressions in northern Somalia and the Morondava Basin of south-western Madagascar.     

Geology of the Grove Mountains in East Antarctica

Grove Mountains consists mainly of a series of high-grade (upper amphibolite to granulite facies) metamorphic rocks, including felsic granulite, granitic gneiss, mafic granulite lenses and charnockite, intruded by late tectonic gneissic granite and post-tectonic granodioritic veins. Geochemical analysis demonstrates that the charnockite, granitic gneiss and granite belonged to aluminous A type plutonic rocks, whereas the felsic and mafic granulite were from supracrustal materials as island-arc, oceanic island and middle oceanic ridge basalt. A few high-strained shear zones disperse in regional stable sub-horizontal foliated metamorphic rocks. Three generations of ductile deformation were identified, in which D₁ is related to the event before Pan-African age, D₂ corresponds to the regional granulite peak metamorphism, whereas D₃ reflects ductile extension in late Pan-African orogenic period. The metamorphic reactions from granitic gneiss indicate a single granulite facies event, but 3 steps from mafic granulite, with P-T condition of M1 800°C, 9.3×10⁵ Pa; M₂ 800–810°C, 6.4 × 10⁵ Pa; and M₃ 650°C have been recognized. The U-Pb age data from representative granitic gneiss indicate (529±14) Ma of peak metamorphism, (534±5) Ma of granite emplacement, and (501±7) Ma of post-tectonic granodioritic veins. All these evidences suggest that a huge Pan-African aged mobile belt exists in the East Antarctic Shield extending from Prydz Bay via Grove Mountains to the southern Prince Charles Mountains. This orogenic belt could be the final suture during the Gondwana Land assemblage.     

The Evolution of the Geothermal Gradient from Lambert Graben and Mahanadi Basin – A Contribution to the Indo-Antarctic Rift Debate

The East Antarctic Lambert Graben and the Indian Mahanadi Basin are considered to represent segments of an intra-Gondwanan rift structure that was active at least since the Paleozoic. Fission track analyses of apatites from a comprehensive data set across the shoulders of both grabens were used to compare their low temperature history, and to estimate the paleo-geothermal gradients before the onset of the last denudation/rifting stage (Late Jurassic). The paleo-geothermal gradients of both juxtaposed Gondwana margins similarly increase from the basement towards the respective rift shoulder from 15–20°Ckm⁻¹ to 25–30°Ckm⁻¹. This trend of increasing paleo-geothermal gradients, together with a denudation episode commencing in the Early Cretaceous and coeval igneous activity, indicates a common rifting stage accompanying the breakup of Gondwana in the India-Antarctica sector.     

The Phanerozoic Reconstitution of Indian Shield as the Aftermath of Break-up of the Gondwanaland

The Indian crust, generally regarded as a stable continental lithosphere, experienced significant tectono-thermal reconstitution during the Phanerozoic. The earliest Phanerozoic tectonic process, which grossly changed the geological and geophysical character of the Precambrian crust, was during the Jurassic when this crustal block broke up from the Gondwana Supercontinent. There were two earlier abortive attempts to fragment the supercontinent in the Palaeozoic. Different types of geological processes were associated with these aborted events. The first was the intrusion of anorogenic alkali granites during the Early Palaeozoic (at 500±50 Ma), while the second was linked with formation of the Gondwana rift basins during Late Palaeozoic. The tectonic history of the Indian Shield subsequent to its separation from the Gondwanaland at around 165 Ma is a complex account of its northward journey, which was culminated with its collision with the northern continental blocks producing the mighty Himalayas in the process. Considerable reconstitution of the Indian Shield took place due to magma underplating when this lithospheric block passed over the four mantle plumes. While the underplating events grossly changed the geophysical character of the Indian Shield in isolated patches, the propagation of the underplated materials was assisted by the deep crustal fractures (geomorphologically expressed as lineaments), which formed during the break-up of the Gondwanaland. Several of these deep fractures evolved through the reactivation of the pre-existing (Precambrian) tectonic grains, while some others developed as new fractures in response to either the extensional stresses generated during the supercontinental break-up or the plume-lithosphere reactions. Significant geomorphological changes occurred in peninsular India subsequent to the continental collision. Most of these changes were brought about by the movements along the lineaments, which fragmented the Indian Shield into a number of rigid crustal blocks. The present day seismic behaviour of the Indian Shield is a reflection of movements of the rigid crustal blocks relative to each other. An interesting feature of the Phanerozoic geological history of the Indian Shield is the evolution of a number of sedimentary basins under different tectono-thermal regimes.     

Structure and Evolution of the East Antarctic Lithosphere: Tectonic Implications for the Development and Dispersal of Gondwana

Lithospheric evolution of the Antarctic shield is one of the keystones for understanding continental growth during the Earth's evolution. Architecture of the East Antarctic craton is characterized by comparison with deep structures of the other Precambrian terrains. In this paper, we review the subsurface structure of the Lower Paleozoic metamorphic complex around the Lützow-Holm area (LHC), East Antarctica, where high-grade metamorphism occurred during the Pan-African orogenic event. LHC is considered to be one of the collision zones in the last stage of the formation of Gondwana. A geoscience program named ‘Structure and Evolution of the East Antarctic Lithosphere (SEAL)’ was carried out since 1996-1997 austral summer season as part of the Japanese Antarctic Research Expedition (JARE). Several geological and geophysical surveys were conducted including a deep seismic refraction/wide-angle reflection survey in the LHC. The main target of the SEAL seismic transect was to obtain lithospheric structure over several geological terrains from the western adjacent Achaean Napier Complex to the eastern Lower Paleozoic Yamato-Belgica Complex. The SEAL program is part of a larger deep seismic profile, LEGENDS (Lithospheric Evolution of Gondwana East iNterdisciplinary Deep Surveys) that will extend across the Pan-African belt in neighboring fragments of Gondwana.     

青藏高原羌塘北部托和平错一带二叠系展金组火山岩的特征及构造环境

西藏改则县托和平错地区展金组中发育一套早-中二叠世以玄武岩类为主、玄武安山岩次之、安山岩少量的海相火山岩组合.该火山岩中玄武岩、玄武安山岩的SiO2含量为44.43%～54.87%,TiO2含量多在4%以上,K2O+Na2O平均为4.57%,玄武岩类富K,玄武安山岩富Na.区内火山岩富集K、Ba、Th、Ta、Ce、P、Zr、Sm、Ti、Cr,亏损Rb、Nb、Hf、Sc,Y/Nb在0.322～0.499之间.综合常量元素和微量元素特征,可将该火山岩归入碱性玄武岩系列.区内火山岩从基性到中性稀土元素总量基本没有变化,REE平均为402.31×10-6,LREE/HREE为8.17～12.69,为轻稀土强烈富集型,δEu为0.93～1.04,平均为0.99,无Eu异常,玄武岩的Th/Ta值多数大于1.6.火山岩的地质特征和地球化学特征表明,本区火山岩产出的大地构造环境为冈瓦纳大陆北缘裂谷环境.