Showing papers on "Gondwana published in 1988"

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.

Paleozoic paleogeography of North America, Gondwana, and intervening displaced terranes: Comparisons of paleomagnetism with paleoclimatology and biogeographical patterns

Abstract: New paleomagnetic data have become available in the past 5 yr that require modifications in previously published paleogeographic reconstructions for the Silurian and Devonian. In this paper, the new paleopoles are compared to published paleogeographic models based on paleoclimatologic and biogeographic data. The data from the three fields of paleomagnetism, paleoclimatology, and biogeography are generally in excellent agreement, and an internally consistent paleogeographic evolutionary picture of the interactions between North America, Gondwana, and intervening displaced terranes is emerging.During the interval of the Ordovician, Silurian, and Devonian, North America stayed in equatorial paleoposition, while rotating counter-clockwise. The northwest African part of Gondwana was in high southerly latitudes during the Late Ordovician and was fringed by peri-Gondwanide terranes, such as southern Europe (Armorica) and Avalonian basement blocks now found in eastern Newfoundland, Nova Scotia, the Boston Basin, the Appalachian Piedmont, and northern Florida. Subsequently, Gondwana and the peri-Gondwanide terranes displayed rapid drift with respect to the pole. This drift translates into the following pattern of movement for northwest Africa. During the latest Ordovician-Early Silurian, this area moved rapidly northward from polar to subtropical latitudes, followed by equally rapid southward motion from subtropical to intermediate (about 50°S) paleolatitudes during the Late Silurian-Middle Devonian. It is likely that significant east-to-west motion accompanied the latter shift in paleolatitudes, with the Caledonian-Acadian orogeny the result of Silurian to Early Devonian convergence and collision between Gondwana and North America. This collision sandwiched several of the intervening displaced terranes between Gondwana and North America. Subsequent to this collision, Gondwana was separated in the Late Devonian by a medium-width ocean from North America and the Avalonian and southern European blocks which were left behind adjacent to North America. This new ocean closed during the Carboniferous, and the resulting convergence and collision were the cause of the Hercynian-Alleghanian orogenic belt. Problems remaining for future research, besides the further gathering of reliable paleopoles, involve the uncertain pre-Devonian position of the southern British Isles in this scenario and the very rapid velocity with respect to the pole that results from the rapid Late Ordovician-Silurian apparent polar wander for Gondwana.

Isotopic mapping of age provinces in precambrian high-grade terrains: sri lanka'

Abstract: Nd model ages of amphibolite- and granulite-grade rocks in Sri Lanka form a simple regional pattern that broadly correlates with mappable geological units, and is in effect an isotopic map of the island's basement. The granulite-grade units of the Highland Group and Southwest Group have model ages of 2.2-3.0 Ga indicating derivation mainly from late Archean sources. They are bounded to the east and west by late Proterozoic gneisses of the Vijayan Complex with model ages of 1.1-2.0 Ga. The isotopic data identify three distinct crustal provinces and are not consistent with earlier suggestions that the Vijayan gneisses are retrograde equivalents of the Highland granulites. Sri Lanka is not a direct continuation of the Archean Dharwar Craton of southern India. Identification of "Vijayan-type" juvenile crustal terrains in other Gondwana fragments may play a key role in determining the precise attachment of southern India-Sri Lanka in eastern Gondwana.

Lower Palaeozoic facies and faunas around Gondwana

Abstract: Abstract A new base map for the Lower Palaeozoic of the Gondwanan continental plate incorporates various modifications to older reconstructions. An analysis is attempted of faunas at various times through the Lower Palaeozoic, in particular the Lower Ordovician (Arenig-Llanvirn), the middle and late Ordovician (Caradoc-Ashgill) and the early Silurian (Llandovery). Regressions and transgressions are identified and ‘western’ and ‘eastern’ Gondwanan platform faunas identified in contrast to the deeper-water peri-Gondwanan marginal faunas. The effects of climatic variation, both with time and between contemporary areas, are reviewed, and are seen to be the chief controlling factor on platform faunas, with a cline from Boreal to tropical faunas alongside the Gondwanan palaeocontinent. The biogeographical relationships of deeper-water faunas were more complex: certain distinctive taxa may be found distributed widely around the perimeters of the continent, and other faunas have taxa in common with marginal faunas around other contemporary palaeocontinents. Some older-described faunas, particularly brachiopods, trilobites and graptolites, are freshly reviewed, and new faunal data from Libya, Turkey, and Iran are presented and figured.

First Cretaceous mammal from India

Abstract: The record of Cretaceous mammals in particular and Mesozoic mammals in general is biased in favour of the Laurasian landmass1. There are relatively few reports of these mammals from the southern continents representing Gondwanaland, and most of these are confined to northwestern South America2,3. Here we report the discovery of a new eutherian mammal from the uppermost Maas-trichtian sediments intercalated in a volcanic sequence in Naskal, Andhra Pradesh, India. This first record of a south-Asian Cretaceous mammal indicates a wide distribution of these creatures and suggests that the Indian plate was not isolated from northern continents depsite the geological evidence. Our finds may also contribute to the understanding of the origin and evolution of therian mammals from mammal-like reptiles (therapsids) which are well known from the southern continents4.

Post-Hercynian plate reorganization in the Tethys and Arctic – North Atlantic domains

Abstract: During the late phases of the Hercynian orogeny convergence between Gondwana and Laurasia changed from a southeast – northwest direction to an essentially east – west one. Whilst during the Stephanian – Autunian crustal shortening persisted in the Appalachians the Variscan fold belt and its foreland became dissected by a system of wrench faults. These played an important role in the localization of rifts during the breakup of Pangea. The initial phase of the post-Hercynian plate reorganization, spanning some 90 Ma was governed by the Permo-Triassic southward propagation of the pre-existing Norwegian – Greenland Sea rift system and the westward propagation of the Tethys rift system. Gradual westward propagation of the Neo-Tethys sea-floor spreading axis during the Late Triassic – Early Jurassic preceded the Middle Jurassic crustal separation between Laurasia and Africa. This marked the onset of the second phase of the post-Hercynian plate reorganization which was governed by the northward and southward propagation of the Central Atlantic sea-floor spreading system. The Late Jurassic and Cretaceous opening of the Central Atlantic was coupled with a counterclockwise rotation of Africa relative to Laurasia, inducing the closure of the Dinaric – Hellenic ocean and the early Alpine orogenic phases.

Marine gravity of the southern ocean and Antarctic margin from Geosat

Abstract: Geosat altimeter data, collected from an orbit with a ground rack that repeated every 17 days and overlayed one of the 17-day Seasat ground tracks, were used to map the gravity field of the Southern Ocean and the continental margin of Antarctica. The combination of ascending an descending profiles produced a typical Geosat ground track spacing of 70 km at the equator, with the best coverage occurring between the latitudes of 60 and 72 deg in both the Northern and Southern hemispheres. The new data reveal many previously uncharted seamounts and fracture zones in the extreme Southern Ocean areas adjacent to Antarctica, showing the detailed gravity signatures of the passive and active continental margins of Antarctica. Seven large age-offset fracture zones apparent in the Geosat data record the early breakup of Gondwana.

Paleoclimatic Indicators and Inferred Devonian Paleolatitudes of Euramerica

Abstract: Paleolatitudinal inferences for Euramerica and adjacent continental blocks in the Devonian can be drawn from paleomagnetic, paleoclimatic, paleobiogeographic, and tectonic lines of evidence. The necessity for independent latitudinal tests in the Devonian is underscored by recent re-evaluation (Miller & Kent, 1986) of Catskill paleomagnetic data, heretofore the primary source of Late Devonian data for North America. Paleomagnetic latitudes shown on some published Devonian maps of Euramerica introduce notable climatic or biogeographic anomalies and need to be reconsidered. Zonal climatic belts (humid equatorial, arid tropical, and humid temperate) are modified by land masses in potentially predictable ways (orographic and monsoonal effects), providing constraints on paleolatitudes. Our approach attempts to: 1) confine climate-sensitive sedimentary indicators to respective climatic belts; and 2) establish a temporally consistent pattern of continental migration across these belts. Resulting patterns are checked against independent paleobiogeographic and paleomagnetic data. Devonian carbonate-evaporite facies occur over large areas of Euramerica and are constrained to a single arid belt at low latitudes. Two humid belts are delimited for the Middle and Late Devonian, separated by 35–40° of paleolatitude. The northern belt (Arctic Canada, Svalbard, and Urals) is interpreted as equatorial, where thick detrital sediments are associated in places with coal, bauxite, and thick local carbonates. The southern belt (eastern U.S.) is dominated by thick detrital sediments with minor carbonate and some coal; it probably was an orographic wet belt along the Acadians in warm temperate latitudes. Placement of Devonian Euramerica (Laurussia) in the southern hemisphere has advantages for global paleogeography. There is room to position the Devonian evaporites of Siberia north of Euramerica in an arid climatic belt (5–35°N), instead of their anomalous position in temperate latitudes (30–70°N) shown on paleomagnetic-based maps. A major seaway separating Euramerica and Gondwana on some Devonian maps creates serious paleobiogeographic anomalies, particularly for Appalachian Realm faunas. Southern Euramerica should occupy a position close to northwest Gondwana in order to keep North and South American parts of the Appalachian Realm together, and to keep both separated from the coeval Old World Realm.

Laurussia — The Old Red Continent

Abstract: Laurussia was formed during the latest Silurian by the welding of Laurentia-Greenland and Fennosarmatia along the Arctic-North Atlantic Caledonian mega-suture. During the Middle Devonian Acadian-Ligerian orogeny the Gondwana derived Avalonia, Aquitaine- Cantabrian and the ill defined Intra-Alpine Terrane were accreted to the southern margin of Laurussia. Latest Devonian initial contacts between Gondwana and Laurussia was followed, at the transition from Devonian to Carboniferous, by the Late Acadian-Bretonian orogenic pulse of the Appalchian- Variscan geosynclinal system. The late Visean onset of the Himalayan-type Main-Variscan orogenic cycle culuminated in the Permo-Carboniferous suturing of Gondwana and Laurussia. Devonian convergence and collision of Arctica with Laurussia governed evolution of the Inuitian fold belt, which became consolidated during Early Mississippian. On the western margin of Laurussia the Antler Orogen became uplifted during latest Devonian-Early Mississippian. The eastern, essentially passive margin of Laurussia was paralleled through Devonian time by the intra-oceanic Sakmanian-Magnitogorsk arc-trench system. The central landmass of Laurussia, the Old Red Continent, was characterised by a hot, seasonally dry climate; its size changed through time in response to eustatic sea level fluctuations, important intra-plate deformations and the development major delta plains in the Inuitian and Appalachian foredeep basins. The objective of this review is to retrace the latest Silurian to Early Carboniferous tectonic evolution of the Laurussian mega-continent.

Role of strike-slip faulting in the tectonic evolution of the Antarctic Peninsula

Abstract: The Antarctic Peninsula Mesozoic magmatic arc has had a long history of dextral, strike-slip deformation. The deformation was initially associated with the development of a wide accretionary complex, by the migration of fore-arc slivers, and the formation and inversion of a thick fore-arc basin succession. It also formed an important component within major shear zones in the arc, and may have controlled the formation of sedimentary basins in the back-arc region. Although some transcurrent motion within the fore-arc region was related to a component of oblique subduction, the main movement occurred during the breakup of Gondwanaland and the formation of a major transtensional rift system. A new reconstruction for this part of Gondwanaland is presented taking this transcurrent motion into consideration.

Gondwana, Tethys, and terrestrial vertebrates during the Mesozoic and Cainozoic

Abstract: Abstract During the Triassic, free dispersal of terrestrial vertebrates was possible in Pangaea; however provinciality appeared in Gondwana. Distribution of Triassic vertebrates may be explained even if the Earth diameter did not expand and if Pacifica did not exist. During the Jurassic, a free land communication persisted between Gondwana and Laurasia, and provinciality is not demonstrated. A Gondwanan pattern emerged during the Cretaceous, but since that time it is not possible to identify a global difference between Gondwanan faunas and Laurasian ones. An African/South American terrestrial community developed until the Aptian. Subsequently, the opening of South Atlantic hindered interchanges between these two areas, but did not end them; exchanges might have taken place then via trans-Atlantic filter, or sweepstakes, routes. For the Cretaceous/Eocene time interval, crossings of the Tethys occurred intermittently, the major event being the interchange that took place between North and South America by latest Cretaceous. By the Cretaceous/Palaeocene transition, a terrestrial route probably linked the still southern Indian Plate to Laurasia. It is uncertain whether this route reached Madagascar. After the Eocene/Oligocene transition, exchanges between Gondwanan continents vanished, whereas exchanges between Gondwanan areas and Laurasia sometimes occurred.

Paleomagnetism of the Silurian-Devonian Andreas redbeds: Evidence for an Early Devonian supercontinent?

Abstract: Two components of magnetization were isolated in the Silurian-Devonian Andreas redbeds of the central Appalachians of Pennsylvania (lat 40.75°N, long 75.78°W): a thermally distributed, synfolding B component, and a thermally discrete, pre-Alleghenian-age folding C component. The C component mean direction and associated pole position correspond to a Silurian-Devonian paleolatitude for the Andreas location of about 35°S, which, in conjunction with Early Devonian results from Gondwana, is consistent with an Early Devonian supercontinent configuration.

Limestone and chert in tectonic blocks from the Esk Head subterrane, South Island, New Zealand

Abstract: The Esk Head subterrane is a continuous belt, generally 10-20 km wide, of tectonic melange and broken formation on the South Island of New Zealand. This subterrane separates older and younger parts of the Torlesse terrane which is an extensive accretionary prism composed mostly of quartzo-feldspathic, submarine-fan deposits ranging from Permian to Early Cretaceous in age. The Torlesse is the most Pacific-ward of several Permian and Mesozoic accreted terranes in New Zealand that record tectonic amalgamation and ultimate accretion against the Pacific-facing Gondwana margin. The Esk Head subterrane of the Torlesse is especially informative because it includes within it conspicuous tectonic blocks of submarine basalt and a variety of basalt-associated seamount and sea-floor limestones and cherty rocks thought to be representative of the subducted plate. Limestones in tectonic blocks are of Late Triassic and probably Jurassic ages and include (1) submarine-cemented, pelagic-bivalve, geopetal packstone-grainstone; (2) brachiopod-bryozoan encrinite; and (3) radiolarian, pelagic lime mudstone. Most of the Triassic blocks have been dated using conodonts which have remarkably low color alteration index (CAI) values ( Paleogeographic inferences drawn from megafossils, bioclasts, and radiolarians, as well as from carbonate cements, indicate deposition of the oceanic sedimentary rocks at paleolatitudes somewhat lower than that of the New Zealand part of the Gondwana margin, but higher than paleoequatorial latitudes. These oceanic sediments and their basaltic substrates were evidently emplaced in the Torlesse accretionary prism following off-scraping from an extensive subducting oceanic plate, probably the Phoenix plate, which was obliquely convergent with the northwest-trending Gondwana margin during Late Jurassic and/or Early Cretaceous time.

Gondwana and Tethys

Abstract: Introduction. Gravity glide and plate tectonics lithospheric stress, deformation and tectonic cycles - the disruption of Pangaea and the closure of Tethys basement and cover rock history in western Tethys - HT/LP metamorphism associated with extensional rifting of Gondwana holocene serial folding in the Zagros Gondwana and the evolution of the Indian Ocean evolution of the southern margin of Tethys (north Australian region) from early Permian to late Cretaceous origin and assembly of south-east Asia continental terranes origin and assembly of the Tethyside orogenic collage at the expense of Gondwanaland lower Palaeozoic facies and faunas around Gondwana fossil plants as indicators of late Palaeozoic plate positions the nature, extent and subsequent dispersal of the Permian Tethys unexpected microfaunal communities within the Triassic Tethys structure and palaeogeography of the western Tethys during the Jurassic - tests based on ammonite palaeobiogeography middle Jurassic ammonite biogeography supports ambi-Tethyan origin of Tibet Mesozoic Turkey as part of Europe the trans-Pacific spread of equatorial shallow-marine benthos in the Cretaceous Gondwana, Tethys and terrestrial vertebrates during Mesozoic and Cainozoic tectonics from fossils? - analysis of reef-coral and sea-urchin distributions from late Cretaceous to recent, using a new method phytogeography of the eastern end of Tethys. Index.

Tectonic framework of the Himalaya, Karakoram and Tibet, and problems of their evolution

Abstract: The Himalaya, the Karakoram and Tibet were assembled by the successive accretion to Asia of continental and arc terranes during the Mesozoic and early Tertiary. The Jinsha and Banggong Sutures in Tibet join continental terranes separated from Gondwana. Ophiolites were obducted onto the shelf of southern Tibet in the Jurassic before the formation of the Banggong Suture. The Kohistan--Ladakh Terrane contains an island arc that was accreted in the late Cretaceous on the Shyok Suture and consequently evolved into an Andean-type batholith. Further east this Trans-Himalayan batholith developed on the southern active margin of Tibet without the prior development of an island arc. Ophiolites were obducted onto the shelf of India in the late Cretaceous to Lower Palaeocene before the closing of Tethys and the formation of the Indus--Yarlung Zangbo Suture at about 50 Ma. Post-collisional northward indentation of India at ca. 5 cm a$^{-1}$ since the Eocene has redeformed this accreted terrane collage; palaeomagnetic evidence suggests this indentation has given rise to some 2000 km of intracontinental shortening. Expressions of this shortening are the uplift of mid-crustal gneisses in the Karakoram on a late-Tertiary breakback thrust, folding of Palaeogene redbeds in Tibet, south-directed thrust imbrication of the foreland and shelf of the Indian Plate, north-directed back-thrusts along the Indus Suture Zone, post-Miocene spreading and uplift of thickened Tibet, giving rise to N--S extensional faults, and strike-slip faults, which allowed eastward escape of Tibetan fault blocks.

Late Palaeozoic Biogeography of East Asia and Palaeontological Constraints on Plate Tectonic Reconstructions

Abstract: Biogeographical patterns of late Palaeozoic rugose coral genera are analysed for the Lower Carboniferous (Visean), early Lower Permian (Asselian/Sakmarian), late Lower Permian (Qixian) and early Upper Permian (Maokoan) of East Asia. Boundaries to the biotic regions are defined to coincide with tectonically significant suture zones to test rival hypotheses about the plate tectonic reconstruction of that region. Three numerical techniques are employed to cluster areas on the basis of shared endemic taxa; parsimony analysis of endemism, principal coordinates analysis and single linkage cluster analysis. Geographical variation in overall diversity is also considered. These results are compared with empirically derived patterns based on other groups of organisms. Major conclusions from this work are as follows, (i) During the Carboniferous and early Permian, the Cathaysian region (North and South China Blocks, Tarim Terrane, Kunlun Terrane, Qiangtang Terrane) formed one cohesive biotic region lying tropically or subtropically; it did not start to fragment until the Upper Permian, (ii) This region was biotically isolated from Central Asia at least during the Carboniferous and Lower Permian, (iii) The southern boundary to the Cathaysian region does not coincide with a single suture zone through time, nor is it sharply defined. Instead there appears to be a gradual faunal impoverishment southwards across the Tibetan Plateau. This implies that faunal ranges are controlled only by the prevailing global climatic regime and not by a geographical barrier, (iv) The Lhasa and Himalaya Terranes shared a similar fauna until the mid-Permian, when a marked faunal disjunction developed coincident with the Zangbo Suture, (v) For terrestrial floras, the barrier to biotic exchange between the North China Block and Angaraland started to break down in the late Permian. It follows that no major oceanic break (‘Palaeotethys’) can be recognized within the Cathaysian region during the late Palaeozoic on palaeontological evidence. This region then formed an integral part of the Gondwanaland craton, extending up into broadly tropical latitudes, and did not become separated from it until the late Lower Permian. The Tienshan-Yinshan Suture is the most likely site of ‘Palaeotethys’, which appears to have occupied a broadly equatorial latitude. Combined with evidence on the ages of the various Asian sutures, this raises significant problems for those who demand a large ocean in their Carboniferous to early Permian palaeogeographical reconstructions of this region.

Permian age from radiolarites of the Hawasina nappes, Oman Mountains

Abstract: The Hawasina napper of the Oman Mountains yielded Permian radiolarians from cherts stratigraphically overlying a thick volcanic basement (Al Jil Formation) at the base of the Hamrat Duru Group. This fauna represents the first Permian radiolarians and radiolarites in the central and western Tethyan realm. A Permain age for pelagic sequences within the Hawasina Complex of Oman has major significance for regional paleogeographic reconstruction. A clear differentiation between platform (reefal sediments) and basin (radiolarites) from the base of the Late Permian (255 Ma) is implied. It suggests a flexure of the platform during Permian time; the present data implies that a zone of rifting was already developed adjacent to the northeast Gondwana platform margin during the Late Permian. The Hamrat Duru Basin corresponds to an opening intracontinental rift area (sphenochasm) between Arabia and northeast Gondwana, a reentrant of the paleo-Tethys.

Mammal teeth from the Cretaceous of Africa.

Abstract: We report here the discovery of two mammal teeth from the early Cretaceous of Cameroon. These, and some jaw fragments, all from Cameroon, are the only fossil evidence of mammalian evolution from Africa between late Jurassic and Paleocene, a span of at least 85 million years. A triangular upper tooth lacks the principal internal cusp of marsupials and placentals and is therefore of a similar evolutionary grade to most Jurassic and early Cretaceous therian mammals, but more primitive than the metatherian–eutherian grade. Early Cretaceous, or older, therian mammals are now known from all southern continents except Antarctica. The new find from Cameroon is consistent with the hypothesis that marsupials, the dominant living mammals of South America and Australia, were not present on any Gondwana continents until after the early Cretaceous separation of Africa by the opening of the South Atlantic.

Marine Ostracod Distributions during the Early Breakup of Southern Gondwanaland

Abstract: Plate tectonic and palaeo-oceanographical development of southern Gondwanaland, in particular the South Atlantic and Indian oceans, is documented in its marine Jurassic and Cretaceous ostracod faunas. During breakup, which commenced in mid-Jurassic times, seaways developed across the supercontinent via the Falkland Plateau and southern Africa, to Madagascar, India, and Australia. These were connected (probably via the SE Pacific) to the Neuquen Basin of west Argentina, but were isolated from the Equatorial South Atlantic by the Walvis-Rio barrier. The southern Gondwanaland oceans fostered a distinctive ostracod fauna that can be recognized in Bajocian to Cenomanian strata: South Gondwana Fauna (SGF). It contains numerous endemic forms, with the most characteristic species belonging to the Progonocytheridae. A major faunal change occurred between late Cenomanian and early Turonian times, when new taxa migrated from the Equatorial South Atlantic, via the Temperate South Atlantic, and ousted the extant SGF. Vigorous colonisation and rapid speciation was probably encouraged by radical oceanographical changes resulting from modifications in palaeogeography, especially the flooding of the Walvis-Rio barrier between the Equatorial and Temperate South Atlantic oceans. By Coniacian time at the latest, a cosmopolitan Pan Gondwana Fauna (PGF) was established, characterized by species belonging to the Trachyleberididae and Brachycytheridae. Subtle alterations in composition of the SGF and PGF may also be related to palaeogeographical changes.

On the Evolution of the Godavari Gondwana Graben, based on LANDSAT Imagery Interpretation

Abstract: Lineaments numbering about 110, show two major preferred directions, i.e., northwest-southeast and northeast-southwest. Majority of the lineaments are believed to be of Archaean ancestry. The Precambrian Pakhal, Sullavai, and the upper Palaeozoic to Mesozoic Gondwana sediments were deposited largely in a fault-controlled, successively developed troughs, broadly trending in a northwest-southeast direction. A five-stage tectonic evolution of the Godavari basin is envisaged. Absence of volcanism, and the narrow rift zones, place the Godavari Graben in the 'Crevice' type of platform rift zone of Milonovsky (1972).