Showing papers on "Gondwana published in 1995"

The early Paleozoic evolution of the Argentine Precordillera as a Laurentian rifted, drifted, and collided terrane: A geodynamic model

Abstract: Paleontologic and stratigraphic evidence points to the early Paleozoic Precordilleran terrane of western Argentina as being the conjugate rift pair of the Appalachians. Stratigraphic similarities of the Cambrian and early Arenig carbonate series and very strong affinities among trilobite, conodont, and brachiopod faunas show their close relationship. The most probable provenance areas are the Appalachian-Ouachita rifted margin and the Newfoundland Appalachians, although the former fits better with geometric and drifting paths suggested by faunal affinities. Increasing Celtic and Baltic brachiopod genera and divergent stratigraphy since the Arenig indicate the drifting of the Precordilleran terrane. Collisional foredeeps developed on collapsed former platform carbonates as flexural subsidence progressed. The collision of the Precordillera with western Gondwana occurred during the mid-Llanvirn to Llandeilo. A magmatic arc related to eastward subduction (present coordinates) was active in the Famatina Range east of the Precordillera. This region of Celtic affinity shows faunal exchange with the Precordilleran terrane since the late Arenig and may represent accreted intra-Iapetus volcanic island-arc complexes. The rifting and drifting stages are consistent with paleoclimatic and paleomagnetic data that show the migration of the Precordilleran terrane from periequatorial to peripolar latitudes between the Cambrian and latest Ordovician. The deep ocean to the west of the Precordillera started to close by the Late Ordovician with the eastward drift of the Chilenian terrane. Absence of volcanic or pyroclastic arc-derived rocks in the Precordillera indicate west-dipping subduction. As Chilenia approached the continental margin, a new forebulge was established on the former collided Precordilleran terrane, developing an erosional unconformity in central Precordillera (Talacasto-Tambolar arch). A Gondwanic signature was fully developed by the Middle Silurian when the Malvinokaffric Clarkeia Fauna flourished. Before then, the Late Ordovician glacial record and associated Hirnantia Fauna were the first clear tie to Gondwana. During the Silurian the marginal basin behaved as a foreland, with lithosphere rheology and eustasy governing the sequence stratigraphy. Wrench faulting along its eastern boundary displaced the Precordillera toward the south. Continued shortening during closure with the Chilenian terrane in the mid-Devonian produced thrust loading of the basin and generated a thick graywacke succession. Final accretion of Chilenia (Late Devonian) generated a regional angular unconformity between the lower and upper Paleozoic. New eastward subduction was initiated west of the accreted Chilenian terrane during the Late Permian–Triassic as indicated by the Choiyoi volcanic complex, which presently outcrops in the Frontal Cordillera.

Tectonic Evolution and Paleogeography of the Neuquén Basin, Argentina

Abstract: The tectonic evolution of the Neuquen basin spans about 220 m.y. of Mesozoic-Cenozoic subsidence. Initial rifting in the Triassic was driven by extensional collapse of the Permian-Triassic orogen. This period of extension was accommodated by inherited structural inhomogeneities and a southwest-oriented extensional stress field. From the Aalenian onward, fault-controlled subsidence was replaced by regional subsidence. Several episodes of structural inversion modified the shape of the depocenter and rejuvenated fringing sedimentary source areas. The most significant inversion occurred in the late Oxfordian-earliest Kimmeridgian when the Dorsal de Huincul was formed. This Late Jurassic diastrophism marks a fundamental reorganization of extensional stress fields related to fragmentation of southwestern Gondwana and the Atlantic opening. Late Jurassic-Cretaceous extension was northwest directed. This history of tectonic evolution is reflected in a complex structural framework, at least two major hydrocarbon source rock intervals, and numerous reservoir zones.

Neogene paleoenvironmental and paleoclimatic change in southern temperate ecosystems — a southern perspective

Abstract: Recently, a greatly increased number of macrofossil and pollen analytical records from Australasia and southern South America has permitted, for the first time, a comprehensive overview of past vegetation and climate change in southern temperate ecosystems. While the course of Neogene climatic change has been comparable to that of the temperate northern hemisphere, a distinctive southern hemisphere vegetation has evolved, not primarily because of its common Gondwana origin, but as a consequence of the minor amplitude of Quaternary change, absence of large ice sheets, and failure of full-glacial environments to persist through interglacials.

Oaxaquia, a Proterozoic microcontinent accreted to North America during the late Paleozoic

Abstract: Grenville-age granulite facies rocks in southern, central, and northeastern Mexico have distinctive geologic features that suggest a common tectonic evolution. The similarities include northwest-trending structural grain from Oaxaca to Tamaulipas, massif-type anorthosite-charnockite complexes, protoliths rich in sedimentary rocks of shallow-marine platform or continental rift-related facies devoid of calc-alkaline volcanic rocks, common metamorphism under granulite facies conditions, U-Pb zircon ages of about 1.0 Ga., and an apparently common history of uplift and cooling. Altogether, this evidence strongly suggests a coherent geologic history for this block. Paleontologic data from the overlying sedimentary sequences indicate that Oaxaquia was not part of Laurentia during most of the Paleozoic. This precludes emplacement of Oaxaquia in its present position by simple lateral displacement from the southern United States as well as a Taconic time of emplacement. Oaxaquia was probably emplaced to its present position during late Paleozoic time. The concept of a Mesoproterozoic “Oaxaquia” microcontinent extending for about 1000 km in Mexico needs to be considered in the reconstruction of the Grenville orogen as a whole and for the Paleozoic tectonic interactions between eastern Laurentia and western Gondwana.

Eocene arc-continent collision in New Caledonia and implications for regional southwest Pacific tectonic evolution

Abstract: New Caledonia preserves evidence that constrains models for the tectonic evolution of the southwest Pacific region. Onland geology reflects four main tectonic phases: (1) early Mesozoic development of subduction-related terranes and their accretion to the Gondwana margin; (2) Cretaceous passive margin development and sea-floor spreading during the Gondwana breakup; (3) foundering of an oceanic basin and the Eocene arrival of thinned Gondwana margin crust at a southwest-facing subduction zone, resulting in collisional orogenesis and obduction of an ophiolitic nappe from the northeast; and (4) detachment faulting during extensional collapse, resulting in unroofing of metamorphic core complexes. The last phase explains supposedly anomalous metamorphic gradients in the northeast of the island.

Pangea and the Paleogeography of the Permian

Abstract: In this chapter we review the paleogeographic and plate tectonic setting of the supercontinent of Pangea during the Early Permian (mid-Sakmarian) and the Late Permian (Kazanian). The paleogeographic reconstructions presented in Figs. 6 through 13 are based on the paleogeographIc maps assembled by the PALEOMAP Project (International Lithosphere Program) (Scotese and Golonka 1995) and have been revised to include information provided by authors of this Volume, as well as the recent paleogeographic syntheses published in the Decade of North American Geology and by Langford (1992), Yemane and Kelts (1990), Visser (1993), and other authors for the Gondwana continents.

Hypothesis for Cretaceous rifting of east Gondwana caused by subducted slab capture

Abstract: In the process of subducted slab capture, a spreading ridge approaches subparallel to a subduction zone following the trailing edge of a downgoing plate. Eventually the downgoing plate is too young and small to subduct, and spreading stops. The spreading ridge stalls many tens of kilometres outboard of the subduction zone. The subducted plate welds to the outboard plate across the dormant spreading center and is captured by it. The captured plate then acquires the motion of the plate it welded to. In the southwest Pacific the Pacific-Phoenix ridge approached the east Gondwana margin as the Phoenix plate subducted beneath New Zealand, the Chatham Rise and Campbell Plateau, the Lord Howe Rise (collectively, Zealandia), and Marie Byrd Land in Cretaceous time. Spreading and subduction shut down here between 110 and 105 Ma, and some sections of the Phoenix plate became welded to (captured by) the Pacific plate. Pacific plate northward motion began in Aptian time, pulling the captured subducted microplates with it. This movement exerted a basal traction on the overlying east Gondwana margin and resulted in extension of Zealandia and Marie Byrd Land. Continued Pacific northward motion rifted Zealandia from Marie Byrd Land at about 85 Ma.

On the Antarctic Peninsula batholith

Abstract: The plutonic rocks of the Antarctic Peninsula magmatic arc form one of the major batholiths of the circum-Pacific rim. The Antarctic Peninsula batholith is a 1350 km long by < 210 km wide structure which was emplaced over the period ˜240 to 10 Ma, with a Cretaceous peak of activity that started at 142 Ma and waned during the Late Cretaceous. Early Jurassic and Late Jurassic–Early Cretaceous gaps in intrusive activity probably correspond to episodes of arc compression. In a northern zone of the Antarctic Peninsula, the batholith intrudes Palaeozoic–Mesozoic low-grade meta-sedimentary rocks, and in a central zone it intrudes schists and ortho- and paragneisses which have Late Proterozoic Nd model ages and were deformed during Triassic to Early Jurassic compression. In a southern zone the oldest exposed rocks are Permian sedimentary rocks and deformed Jurassic volcanic and sedimentary rocks. All these pre-batholith rocks formed a belt of relatively immature crust along the Gondwana margin. With few exceptions, Jurassic plutons crop out only within the central zone: many are peraluminous, having ‘S-like’ mineralogies and relatively high 87sr/86sri. They are considered to consist largely of partial melts of upper crust schists and gneisses and components of mafic magmas that caused the partial fusion. By contrast, Early Cretaceous plutons crop out along the length of the batholith. Few magma compositions appear to have been affected by upper crust, the bulk being compositionally independent of the type of country rock they intrude. They are dominated by metaluminous, calcic, Si-oversaturated, 1-type granitoid rocks with relatively low 87sr/86sri intermediate-silicic compositions (< 5% MgO). We interpret these to represent partial melts of basic to intermediate, igneous, locally garnet-bearing, lower crust. Contemporaneous mafic magmas (e.g. syn-plutonic dykes) form a more alkaline, Si-saturated series having higher 143Nd/144Nd at the same87sr/86sr than the intermediate-silicic series, to which they are not petrogenetically related. The change from limited partial fusion of upper crust in Jurassic times to widespread partial fusion of lower crust in Early Cretaceous times is considered to be a result of an increasing volume of basaltic intrusion into the crust with time.

First record of the family Dromaeosauridae (Dinosauria: Theropoda) in the Cretaceous of Gondwana (Wadi Milk Formation, northern Sudan)

Abstract: The Cenomanian Wadi Milk Formation in Sudan yielded a rich continental vertebrate fauna. Apart from big theropod fragments, some disarticulated elements of a pes and a tooth indicate the presence of a dromaeosaurid theropod in the Late Cretaceous of Sudan. This is the first record of this theropod family from Gondwanian continents.

High‐pressure micro‐inclusions and development of an inverted metamorphic gradient in the Santiago Schists (Ordenes Complex, NW Iberian Massif, Spain): evidence of subduction and syncollisional decompression

Abstract: The Santiago Schists are located in the Basal Unit of the Ordenes Complex, one of the allochthonous complexes outcropping in the inner part of the Hercynian Belt in the north-west of the Iberian Peninsula. Their tectonothermal evolution is characterized by the development of an eo-Hercynian metamorphic episode (c. 374 Ma) of high-P, low- to intermediate-T. The mineral assemblage of the high-P episode is preserved as a very thin Si= S1 foliation included in albite porphyroblasts, being composed of: albite + garnet-I + white mica-1 + chlorite-1 + epidote + quartz + rutile ± ilmenite. The equilibrium conditions for this mineral assemblage have been estimated by means of different thermobarometers at 495 ± 10 °C and 14.7 ± 0.7 kbar (probably minimum pressure). The later evolution (syn-D2) of the schists defines a decompressive and slightly prograde P-T path which reached its thermal peak at c. 525 ± 10 °C and 7 kbar. Decompression of the unit occurred contemporaneously with an inversion of the metamorphic gradient, so that the zones of garnet-II, biotite (with an upper subzone with chloritoid) and staurolite developed from bottom to top of the formation. The estimated P-T path for the Santiago Schists suggests that the Basal Unit, probably a fragment of the Gondwana continental margin, was uplifted immediately after its subduction at the beginning of the Hercynian Orogeny. It also suggests that the greater part of the unroofing history of the unit took place in a context of ductile extension, probably related to the continued subduction of the Gondwana continental margin and the contemporaneous development of compensatory extension above it. The inverted metamorphic gradient seems related to conductive heat transferred from a zone of the mantle wedge above the subducted continental margin, when it came into contact with the upper parts of the schists along a detachment, probably of extensional character. The general metamorphic evolution of the Santiago Schists, with the development of high-P assemblages with garnet prior to decompressive and prograde parageneses with biotite, is unusual in the context of the European Hercynian Belt, and shows a close similarity to the tectonothermal evolution of several high-P, low- to intermediate-T circum-Pacific belts.

Structural and Tectonic Controls of Basin Evolution in Southwestern Gondwana During the Phanerozoic

Abstract: The continental lithosphere of southwestern Gondwana, comprising the southern part of South America and southern Africa, was largely assembled before the end of the Proterozoic. Geologic studies indicate that the basement anisotropy that controlled the development of Phanerozoic basins was established by Neoproterozoic-Early Cambrian tectonism. This tectonism reactivated the older terrane boundaries or cut across them. The backbone linking the system of Pan-African and Brasiliano basins was a system of northeast-trending structures. There were four areas of pronounced Neoproterozoic-Early Cambrian basin subsidence in the study area: the Chiquitanas trough in Bolivia, the Puncoviscana basin in Argentina, the Dom Feliciano-Ribeira basins in Brazil, and the Damara-Nama basin complex of southern Africa.

Late Carboniferous to Recent, Geodynamic Evolution of the West Gondwanian, Cratonic, Tethyan Margins

Abstract: The development of the Late Carboniferous-Liassic, Late Jurassic—Early Cretaceous, and Late Eocene—Early Miocene western Gondwanian rift systems and related magmatic provinces is related to the reactivation of preexisting Pan—African zones of lithospheric weakness. It resulted in the breakup of Pangea and Gondwana, and the opening of the Indian Ocean, western Tethys, and the South Atlantic Ocean. Changes in the intraplate stress regimes of Africa—Arabia and South America are indicated by alternating phases of crustal extension, sag-basin development and lithospheric compression. The compressive events can be correlated with changes in the rate and direction of the opening of the Central, South, and North Atlantic oceans. However, the repetition of rifting episodes, particularly within wide areas of Niger and Sudan, the persistence of some rift-independent magmatic provinces (e.g., Nigeria and Nubia) and geophysical data suggest the presence of large-scale, mantle upwelling below equatorial Africa, which could explain the specific geodynamic history of the African Plate.

Inverse relation between ice extent and the late Paleozoic glacial record of Gondwana

Abstract: The late Carboniferous–earliest Permian age estimate (300–280 Ma) for maximum ice volume during late Paleozoic glaciation of Gondwana is challenged. Past estimates assume a direct relation between extent of depositional glacial record and former ice cover; this assumption cannot be sustained, given that the glacial record is composed predominantly of glacially influenced marine strata that accumulated on the margins of ice-covered areas. We argue that Gondwana ice cover expanded in the Early Carboniferous in response to polar position, availability of moisture from a mediterranean sea, and epeirogenic uplift of the Gondwana interior. In Namurian time (ca. 325 Ma), Gondwana ice cover attained a maximum extent of about 21 × 10 6 km 2 , nearly the area of maximum Pleistocene ice cover. Despite extensive ice cover, the depositional record is meager; continental glacial deposits are poorly preserved on a regional unconformity. Thereafter, extensional subsidence of intracratonic basins promoted marine flooding, fragmentation of the ice cover, and the accumulation of thick glacially influenced marine deposits. In Stephanian-Asselian time (ca. 285 Ma), ice cover had decreased to about 15 × 10 6 km 2 , but glacial marine strata were being deposited and preserved across a very large area of the Gondwana supercontinent.

A new family of monotremes feom the Creataceous of Australia

Abstract: AUSTRALIA'S second Mesozoic mammal, Kollikodon ritchiei (Monotremata, Kollikodontidae, new family) has an extreme bunodont molar morphology. The existence of two distinctively different families of monotremes in the Early Cretaceous suggests that the order originated long before the Cretaceous and was very diverse in at least the Australian portion of eastern Gondwana. With four families now known from Australia, it is probable that monotremes originated and diversified in the Australian/Antarctic sector of Gondwana, followed by a single dispersal (ornithorhyn-chid) to the South American sector before or during the early Paleocene. It is probable that kollikodontids are the sister-group of a steropodontid/ornithorhynchid/tachyglossid clade. K. ritchiei and Steropodon galmani are among the largest Mesozoic mammals known.

A Late Ordovician high‐energy temperate‐water carbonate ramp, southern Quebec, Canada: implications for Late Ordovician oceanography*

Abstract: The Trenton Group (Late Ordovician), the youngest carbonate unit in the Taconic foreland basin of southern Quebec, is a tripartite unit with a distinctive coarse-grained middle part, the Deschambault Formation. Lithofacies of the Deschambault Formation are dominated by coarse-grained bioclastic/intraclastic limestones; finer-grained lithofacies are ubiquitous but subordinate. The complete spectrum of lithofacies indicates sedimentation ranging from above fairweather- to below storm-wave base. Skeletal components are indicative of the modern temperate-water bryomol association. Non-skeletal elements are represented by peloids and intraclasts. Accretion rates from areas of continuous sedimentation were low (<14 cm/103 years). From sedimentological and faunal evidence, it is proposed that the Late Ordovician Deschambault ramp was bathed by temperate waters. The model compares favourably with modern cool-water shelves rimming the southern edge of the Australian continent. Palaeomagnetic data locate southern Quebec in a low latitudinal setting during the Late Ordovician. Upper Ordovician facies distribution in eastern Canada and progressive disappearance of some faunal provinces through Late Ordovician time are used to conclude that the initiation of the Late Ordovician glaciation that covered most of Gondwana was instrumental in easing northward movement of cold oceanic currents. This resulted in the rapid contraction of the southern hemisphere warm-water tropical belt from a 30° latitudinal-wide zone in the early Caradoc to a 15° zone in the late Caradoc.

Milankovitch Sea-Level Changes, Cycles and Reservoirs on Carbonate Platforms in Greenhouse and Ice-House Worlds

Abstract: This short course is in three parts. Part 1 examines in general terms how carbonate cycles are generated on carbonate platforms, types of carbonate cycles developed, stacking patterns, margin geometries, degree of disconformity development, and briefly overview any characteristic diagenetic effects. Part 2 examines cycles and one- and two-dimensional stacking patterns, high resolution stratigraphy, and reservoir geometry on Later Permian platforms in the Permian Basin of West Texas. Part 3 examines reservoirs formed in an ice-house world during the major Carboniferous glaciation of Gondwana, using the Middle Pennsylvanian carbonates of the Giant Aneth oil field, Paradox Basin, Utah.

Arcs and backarc basins in the Early Paleozoic lapetus Ocean

Abstract: Understanding the evolution and destruction of past oceans not only leads to a better understanding of earth history, but permits comparison with extant ocean basins and tectonic processes. This paper reviews the history of the Early Paleozoic circum-Atlantic oceans by analogy with the Pacific Ocean and Mesozoic Tethys. Rifting and continental separation from 620 to 570 Ma led to the development of passive margins along parts of the northern margin of Gondwana (the western coast of South America); eastern Laurentia (eastern North America, NW Scotland and East Greenland), and western Baltica (western Scandinavia). Meagre paleomagnetic data suggest that western South America and eastern North America could have been joined together to form facing margins after breakup. Although western Baltica is an apparently obvious candidate for the margin facing NW Scotland and East Greenland, the paleomagnetic uncertainties are so large that other fragments could have been positioned there instead. The Iapetus Ocean off northeastern Gondwana was probably a relatively wide Pacific-type ocean with, during the late Precambrian to early Ordovician, the northern margin of Gondwana as a site of continentward-dipping subduction zone(s). The 650-500 Ma arc-related igneous activity here and the associated deformation gave rise to the Cadomian, ‘Grampian’, Penobscotian, and Famantinian igneous and orogenic events. By 490-470 Ma, marginal basins had formed along the eastern Laurentian margin as far as NE Scotland, along parts of the northern margin of Gondwana, and off western Baltica, but none are known from the East Greenland margin. These basins closed and parts were emplaced as ophiolites shortly after their formation by processes that, at least in some cases, closely resemble the emplacement of the late Cretaceous Semail ophiolite of Oman. This orogenic phase seems to have involved collision and attempted subduction of the continental margin of Laurentia, Gondwana and Baltica. In Baltica it gave rise to some eclogite facies metamorphism. Marginal basin development may have been preceded by arc formation as early as ca 510 Ma. A double arc system evolved outboard from the eastern Laurentian and western Baltica margins, analogous to some of the arc systems in the present-day western Pacific. At 480-470 Ma, there was a second phase of breakup of Gondwana, affecting the active Gondwanan margin. Eastern and Western Avalonia, the Carolina Slate Belt, Piedmont, and other North American exotic continental blocks rifted away from Gondwana. Farther east, Armorica, Aquitainia, Iberia and several European exotic continental blocks also rifted away, though it is unlikely that they all rifted at the same time. Between 460-430 Ma, peaking at ca 450 Ma, orogenic events involved continuing arc-continent collision(s). From 435-400 Ma the remaining parts of the Eastern Iapetus Ocean were destroyed and the collision of Baltica with Laurentia caused the 430-400 Ma Scandian orogeny, followed by suturing of these continents during the Siluro-Devonian Acadian orogeny or Late Caledonian orogeny to 380 Ma, leaving a smaller but new ocean south of the fragments that had collided with the Laurentian margin farther south. The Ligerian orogeny 390-370 Ma collision of Gondwana-derived Aquitaine-Cantabrian blocks with Eastern Avalonia-Baltica and removed the part of the Iapetus south of Baltica. Prior to any orogenic events, the Eastern Iapetus Ocean between Baltica and Laurentia may have resembled the present-day central Atlantic Ocean between Africa and North America. The ocean appears to have closed asymmetrically, with arcs forming first outboard of the western margin of Baltica while the East Greenland margin was unaffected. The Western Iapetus Ocean between Laurentia and Gondwana also closed asymmetrically with a dual arc system developing off Laurentia and an arc system forming off the northern margin of Gondwana. Like the Pacific Ocean today, the Eastern Iapetus Ocean had a longer and more complex history than the Western Iapetus Ocean: it was already in existence at 560 Ma, probably developed over at least 400 million years, by mid-Cambrian time was many thousands of kilometres wide at maximum extent, and was associated with a < 30 million year phase of marginal basin formation. In contrast, the Western Iapetus Ocean appears to have been much narrower, shorter lived (probably < 100 million years), and associated with the rifting to form two opposing passive carbonate margins, analogous to the Mesozoic Tethys or the present-day Mediterranean.