Showing papers on "Permian published in 2006"

Contrasting Late Carboniferous and Late Permian–Middle Triassic intrusive suites from the northern margin of the North China craton: Geochronology, petrogenesis, and tectonic implications

Abstract: Two contrasting intrusive suites have been identified from the northern margin of the North China craton: a Late Carboniferous dioritegranodiorite suite mainly made up of quartz diorite, diorite, granodiorite, tonalite, and hornblende gabbro, and a Late Permian–Middle Triassic suite of granitoid intrusions consisting of monzogranite, syenogranite, and quartz monzonite. Plutons from the Late Carboniferous suite exhibit variable SiO2 contents and calc-alkaline or high-K calc-alkaline, metaluminous geochemical features. Most have low negative whole-rock ϵNd(T) values (where T is the crystallization age) of −17.1 to −11.5 and zircon ϵHf(T) values of −38.3 to −11.2, indicating that they were derived mainly from anatectic melting of the ancient lower crust with some involvement of mantle materials. However, an older pluton in the suite exhibits higher ϵNd(T) values of −11.5 to −9.9, Nd model ages of 1.82–1.64 Ga, lower initial 87Sr/86Sr ratios of 0.7046–0.7048, and it contains some zircon grains that are characterized by high negative to positive zircon ϵHf(T) values of −8.7 to 1.2, indicating strong involvement of juvenile materials derived from the lithospheric mantle. The Late Carboniferous plutons are interpreted as subduction-related and to have been emplaced in an Andean-style continental-margin arc during the southward subduction of the paleo–Asian oceanic plate beneath the North China craton. Rocks from the Late Permian–Middle Triassic intrusive suite display geochemical signatures ranging from highly fractionated I-type to A-type. They exhibit higher zircon ϵHf(T) values of −14.9 to −6.7, whole-rock ϵNd(T) values of −10.6 to −8.8, and younger Hf and Nd model ages than most of the Late Carboniferous plutons, indicating that they could have been produced by extreme fractional crystallization of hybrid magmas resulted from mixing of coeval mantle- and crust-derived melts. They are interpreted as postcollisional/postorogenic granitoids linked to lithospheric extension and asthenosphere upwelling due to slab break-off and subsequent sinking after final collision and suturing of the Mongolian arc terranes with the North China craton. These two contrasting intrusive suites suggest that the final closure of the paleo–Asian Ocean and collision between the Mongolian arc terranes and the North China craton occurred during the Late Permian, and these events were followed by postcollisional/postorogenic extension, large-volume magmatism, and significant continental growth. No significant syncollisional crustal thickening, high-pressure metamorphism, or S-type granitoid magmatism occurred during the collision process.

The diversification of Paleozoic fire systems and fluctuations in atmospheric oxygen concentration.

Abstract: By comparing Silurian through end Permian [≈250 million years (Myr)] charcoal abundance with contemporaneous macroecological changes in vegetation and climate we aim to demonstrate that long-term variations in fire occurrence and fire system diversification are related to fluctuations in Late Paleozoic atmospheric oxygen concentration. Charcoal, a proxy for fire, occurs in the fossil record from the Late Silurian (≈420 Myr) to the present. Its presence at any interval in the fossil record is already taken to constrain atmospheric oxygen within the range of 13% to 35% (the “fire window”). Herein, we observe that, as predicted, atmospheric oxygen levels rise from ≈13% in the Late Devonian to ≈30% in the Late Permian so, too, fires progressively occur in an increasing diversity of ecosystems. Sequentially, data of note include: the occurrence of charcoal in the Late Silurian/Early Devonian, indicating the burning of a diminutive, dominantly rhyniophytoid vegetation; an apparent paucity of charcoal in the Middle to Late Devonian that coincides with a predicted atmospheric oxygen low; and the subsequent diversification of fire systems throughout the remainder of the Late Paleozoic. First, fires become widespread during the Early Mississippian, they then become commonplace in mire systems in the Middle Mississippian; in the Pennsylvanian they are first recorded in upland settings and finally, based on coal petrology, become extremely important in many Permian mire settings. These trends conform well to changes in atmospheric oxygen concentration, as predicted by modeling, and indicate oxygen levels are a significant control on long-term fire occurrence.

Initiation of the Indosinian Orogeny in South China : Evidence for a Permian Magmatic Arc on Hainan Island

Abstract: It has been widely accepted that an active continental margin existed along the coast of Southeast China during the Mesozoic time that produced extensive magmatism in the region. However, there is little constraint as to when this active margin was first initiated. Here we present new SHRIMP U‐Pb zircon ages and geochemical and Sr‐Nd isotopic data for syntectonic granites on Hainan Island. Our data demonstrate that these rocks, dated at 267–262 Ma, are typical of calc‐alkaline I‐type granites formed in continental arc environments. The age of this magmatic arc coincides with a sudden change in sedimentary environments in South China during the Permian time, suggesting that the South China Indosinian Orogeny was likely contemporaneous with the onset of continental arc magmatism.

Europe from the Variscan to the Alpine cycles

Abstract: The time span between the Variscan and Alpine cycles is not devoid of any major tectonic activity, and corresponds to the Cimmerian cycle. Between the Early Permian and Late Triassic, the Eocimmerian cycle was marked by the closure of Palaeotethys and opening of Neotethys and of an array of south Eurasian back-arc basins. This was followed by the break-up of Pangaea and the Early Jurassic opening of the central Atlantic and Alpine Tethys. However, in the area of the Eocimmerian collision, the geodynamic evolution is relatively uninfluenced by this event, and a new cycle of Cimmerian deformation affected the Hellenides, Dinarides, Balkans and Pontides in Jurassic-Early Cretaceous times. The anti-clockwise rotation of Africa during the Late Cretaceous heralded the onset of Alpine orogenic processes, characterized first by major east-west shortening, and opening and closure of younger oceanic basins of back-arc type.

The Variscan Orogen in Poland

Abstract: The structure and evolution of the Polish part of the Variscan Orogenic Belt is reviewed, based on published data and interpretations. The Sudetic segment of the Variscides, together with adjacent areas, experienced multi-stage accretion during successive collisional events that followed the closure of different segments of the Rheic Ocean. In SW Poland, Variscan tectono-stratigraphic units are tectonically juxtaposed and often bear record of contrasting exhumation/cooling paths, constrained by palaeontological and geochronological data. This points to the collage-type tectonics of this area. A three-partite subdivision of the Sudetes is proposed that reflects timing differences in deformation and exhumation of the respective segments. The Central, West and East Sudetes were deformed and amalgamated during the Middle/Late Devonian, at the turn from the Devonian to Carboniferous and during Early Carboniferous times, respectively. Problems in extending the classical tectono-stratigraphic zonation of the Variscides into the Sudetes are discussed and attributed to activity along Late Palaeozoic strike-slip faults and shear zones, disrupting and dispersing the initially more simply distributed tectono-stratigraphic units into the present-day structural mosaic. Relationships between the Variscan Externides and the foreland basin are explored. Sediments of the foreland basin locally onlap the external fold-and-thrust belt that had undergone an earliest Carboniferous partial tectono-thermal overprint. During the Late Carboniferous, the SW part of the foreland basin was heavily affected by thrusting and folding and incorporated into the Externides. During Westphalian C to Early Permian times, localized folding and thrusting affected the distal parts of the foreland basin, probably in response to dextral transpressional movements along NW-SE trending basement faults.

Permo-Carboniferous climate: Early Pennsylvanian to Late Permian climate development of central Europe in a regional and global context

Abstract: A well-justified stratigraphical correlation of continental successions and new palaeogeographic reconstruction of Pangaea reveal new insights into the northern Pangaean climate development influenced by palaeogeography, palaeotopography, glacio-eustatic sealevel changes and ocean currents. The overall Permo-Carboniferous aridization trend was interrupted by five wet phases. These are linked to the Gondwana icecap. The aridization and weakening of wet phases over time were not only caused by the drift of northern Pangaea to the arid climatic belt, but also by the successive closure of the Rheic Ocean, which caused the expansion of arid/semi-arid environments in the Lower/Middle Permian. The end of the Gondwana glaciation rearranged ocean circulation, leading to a cold, coast-parallel ocean current west of northern Pangaea, blocking moisture coming with westerly winds. The maximum of aridity was reached during the Roadian/Wordian. The Trans-Pangaean Mountain Belt was non-existent. Its single diachronous parts never exceeded an average elevation of 2000 m. The maximum elevation shifted during time from east to west. The Hercynian orogen never acted as an orographic east–west barrier, and the Inter-Tropical Convergence Zone was widely displaced, causing four seasons (dry summer/winter, wet spring/autumn) at the equator and a strong monsoon system. The climate history of the European realm during the Late Carboniferous (Pennsylvanian) and Permian is stored in many solitary basins (Fig. 1) within the Hercynian orogen and the foreland basin. The story of Westphalian climate is well known because of the numerous investigations of the coal-bearing Variscan foredeep. The younger Westphalian is characterized by a slight aridization (Abbink & van Kronijnenburgvan Cittert 2003; Oplustil 2004), which was accompanied by an increase of seasonality. Nevertheless, the environment was strongly influenced by the ocean and epi-continental seas with multiple transgression events. The last extensive marine ingression was the Aegir/Mansfield Band (Westphalian B/C). The post-Westphalian climate development is more differentiated and

Late Paleozoic vertical growth of continental crust around the Junggar Basin,Xinjiang,China(PartI):Timing of post-collisionai plutonism

Abstract: As an important part of the Paleozoic orogenic belt in northern Xinjiang,the Junggar is characterized by intense and extensive development of Late Paleozoic granitoid plutons together with minor mafic plutons,and it is one of the representative regions for the Phanerozoic continental crustal growth in the Central Asian Orogenic Belt.On the basis of new SHRIMP zircon U-Pb dating on the plutons in the East and West Junggar,in association with the zircon U-Pb ages from the literature,an attempt is made in this study to establish the timing of the late Paleozoic post-collisional plutonism in the region.It is concluded that the Late Paleozoic post- collisional plutonism occurred from middle-late Visean of Carboniferous to the end of Early Permian in accordance to the Geological Time Scale(Gradstein et al.,2004).Temporally,the plutons in the East and West Junggar formed in a period of 65 Ma,from 330 to 265Ma and from 340 to 275Ma,respectively.However,the plutonism in the East Junggar showed two peaks of 330 to 310Ma and 305 to 280Ma,whereas most plutons in the West Junggar were emplaced from 310 to 295Ma.Spatially,the plutonism occurred in all tectonic units separated by ophiolite belts and even some granitoid plutons intruded in ophiolites.Furthermore,the post-collisional plutons can be found not only in the Junggar,but also in the Altai orogenic belt to the north and in the Tianshan orogrnic belt to the south.

Middle-Late Permian mass extinction on land

Abstract: The end-Permian mass extinction has been envisaged as the nadir of biodiversity decline due to increasing volcanic gas emissions over some 9 million years. We propose a different tempo and mechanism of extinction because we recognize two separate but geologically abrupt mass extinctions on land, one terminating the Middle Permian (Guadalupian) at 260.4 Ma and a later one ending the Permian Period at 251 Ma. Our evidence comes from new paleobotanical, paleopedological, and carbon isotopic studies of Portal Mountain, Antarctica, and comparable studies in the Karoo Basin, South Africa. Extinctions have long been apparent among marine invertebrates at both the end of the Guadalupian and end of the Permian, which were also times of warm-wet greenhouse climatic transients, marked soil erosion, transition from high- to low-sinuosity and braided streams, soil stagnation in wetlands, and profound negative carbon isotope anomalies. Both mass extinctions may have resulted from catastrophic methane outbursts to the atmosphere from coal intruded by feeder dikes to flood basalts, such as the end-Guadalupian Emeishan Basalt and end-Permian Siberian Traps.

Evolution and importance of wetlands in earth history

Abstract: The fossil record of wetlands documents unique and long-persistent fl oras and faunas with wetland habitats spawning or at least preserving novel evolutionary characteristics and, at other times, acting as refugia. In addition, there has been an evolution of wetland types since their appearance in the Paleozoic. The fi rst land plants, beginning in the Late Ordovician or Early Silurian, were obligate dwellers of wet substrates. As land plants evolved and diversifi ed, different wetland types began to appear. The fi rst marshes developed in the mid-Devonian, and forest swamps originated in the Late Devonian. Adaptations to low-oxygen, low-nutrient conditions allowed for the evolution of fens (peat marshes) and forest mires (peat forests) in the Late Devonian. The differentiation of wetland habitats created varied niches that infl uenced the terrestrialization of arthropods in the Silurian and the terrestrialization of tetrapods in the Devonian (and later), and dramatically altered the way sedimentological, hydrological, and various biogeochemical cycles operated globally. Widespread peatlands evolved in the Carboniferous, with the earliest ombrotrophic tropical mires arising by the early Late Carboniferous. Carboniferous wetlandplant communities were complex, and although the taxonomic composition of these wetlands was vastly different from those of the Mesozoic and Cenozoic, these communities were essentially structurally, and probably dynamically, modern. By the Late Permian, the spread of the Glossopteris fl ora and its adaptations to more temperate or cooler climates allowed the development of mires at higher latitudes, where peats are most common today. Although widespread at the end of the Paleozoic, peat-forming wetlands virtually disappeared following the end-Permian extinction. The initial associations of crocodylomorphs, mammals, and birds with wetlands are well recorded in the Mesozoic. The radiation of Isoetales in the Early Triassic may have included a submerged lifestyle and hence, the expansion of aquatic wetlands. The evolution of heterosporous ferns introduced a fl oating vascular habit to aquatic wetlands. The evolution of angiosperms in the Cretaceous led to further expansion of aquatic species and the fi rst true mangroves. Increasing diversifi cation of angiosperms in the Tertiary led to increased fl oral partitioning in wetlands and a wide Greb, S.F., DiMichele, W.A., and Gastaldo, R.A., 2006, Evolution and importance of wetlands in earth history, in Greb, S.F., and DiMichele, W.A., Wetlands through time: Geological Society of America Special Paper 399, p. 1–40, doi: 10.1130/2006.2399(01). For permission to copy, contact editing@geosociety.org. ©2006 Geological Society of America. All rights reserved. 2 S.F. Greb, W.A. DiMichele, and R.A. Gastaldo

Pre-Alpide Palaeozoic and Mesozoic orogenic events in the Eastern Mediterranean region

Abstract: We review the Palaeozoic-Early Mesozoic evolution of the Eastern Mediterranean-Balkan region with special reference to Anatolia, and provide new isotopic data on the Palaeozoic magmatic and metamorphic rocks. The pre-Alpide evolution of the region involves episodic growth of Laurussia by accretion of oceanic terranes and Gondwana-derived microcontinents. Terrane accretion, associated with deformation, magmatism and regional metamorphism, took place in the Late Ordovician-Early Silurian, Carboniferous, Late Triassic-Early Jurassic and Mid-Jurassic. The Late Ordovician-Early Silurian accretion is inferred from strati-graphic and faunal records in the Pontides; other evidence for it is buried under young cover on the northern margin of the Black Sea. The Carboniferous orogeny is related to southward subduction and continental collision on the southern margin of Laurussia. It is marked in the Pontides by high-grade regional metamorphism, north-vergent deformation and post-orogenic latest Carboniferous-Early Permian plutonism. The latest Triassic-Early Jurassic Cimmeride orogeny involved the collision and amalgamation of an oceanic plateau to the southern margin of Laurasia. It is represented by voluminous accretionary complexes with Late Triassic blues-chists and eclogites. Late Jurassic regional metamorphism and deformation is confined to the Balkans, and is the result of continental collision between the Rhodope-Serbo-Macedonian and Strandja blocks in the Late Jurassic. The Palaeozoic geological history of the Balkans and the Pontides resembles that of Central Europe, although the similarities end with the Mesozoic, as a consequence of the formation of Pangaea.

Unravelling the multi-stage burial history of the Swiss Molasse Basin: integration of apatite fission track, vitrinite reflectance and biomarker isomerisation analysis

Abstract: A complex basin evolution was studied using various methods, including thermal constraints based on apatite fission-track (AFT) analysis, vitrinite reflectance (VR) and biomarker isomerisation, in addition to a detailed analysis of the regional stratigraphic record and of the lithological properties. The study indicates that (1) given the substantial amount of data, the distinction and characterisation of successive stages of heating and burial in the same area are feasible, and (2) the three thermal indicators (AFT, VR and biomarkers) yield internally consistent thermal histories, which supports the validity of the underlying kinetic algorithms and their applicability to natural basins. All data pertaining to burial and thermal evolution were integrated in a basin model, which provides constraints on the thickness of eroded sections and on heat flow over geologic time. Three stages of basin evolution occurred in northern Switzerland. The Permo-Carboniferous strike-slip basin was characterised by high geothermal gradients (80-100 degrees C km(-1)) and maximum temperature up to 160 degrees C. After the erosion of a few hundreds of metres in the Permian, the post-orogenic, epicontinental Mesozoic basin developed in Central Europe, with subsidence triggered by several stages of rifting. Geothermal gradients in northern Switzerland during Cretaceous burial were relatively high (35-40 degrees C km(-1)), and maximum temperature typically reached 75 degrees C (top middle Jurassic) to 100 degrees C (base Mesozoic). At least in the early Cretaceous, a stage of increased heat flow is needed to explain the observed maturity level. After erosion of 600-700 m of Cretaceous and late Jurassic strata during the Paleocene, the wedge-shaped Molasse Foreland Basin developed. Geothermal gradients were low at this time ( 45 km). Thus, maximum temperature as well as maximum burial depth ever reached in Mesozoic strata occurred at different times in different regions. Since the Miocene, 750-1050 m were eroded, a process that still continues in the proximal parts of the basin. Current average geothermal gradients in the uppermost 2500 m are elevated (32-47 degrees C km(-1)). They are due to a Quaternary increase of heat flow, most probably triggered by limited advective heat transport along Paleozoic faults in the crystalline basement.

The Pattern and Timing of Biotic Recovery from the End-Permian Extinction on the Great Bank of Guizhou, Guizhou Province, China

Abstract: Microfacies analysis and point counts of thin sections from 608 hand samples were used to track changes in the abundance and diversity of fossil grains through the extended recovery interval following end-Permian mass extinction on the Great Bank of Guizhou (GBG)—an isolated Late Permian to Late Triassic carbonate platform in south China. Exposure of a two-dimensional cross-section of the platform permits the comparison of faunal patterns along an environmental gradient from shallow to deep water. The diverse Late Permian biota was dominated by calcareous sponges, crinoids, articulate brachiopods, foraminifera, and calcareous algae. In contrast, Early Triassic communities were dominated by mollusks, with increasing abundance of crinoids beginning in the Spathian. Increase in the diversity and abundance of fossils on the GBG was confined to a brief interval near the Spathian–Anisian boundary and concentrated along the platform margin. Later Middle Triassic diversification, the return of calcareous...

From wetlands to wet spots; environmental tracking and the fate of Carboniferous elements in Early Permian tropical floras

Abstract: Diverse wetland vegetation flourished at the margins of the Midland Basin in north-central Texas during the Pennsylvanian Period. Extensive coastal swamps and an ever-wet, tropical climate supported lush growth of pteridosperm, marattialean fern, lycopsid, and calamite trees, and a wide array of ground cover and vines. As the Pennsylvanian passed into the Permian, the climate of the area became drier and more seasonal, the great swamps disappeared regionally, and aridity spread. The climatic inferences are based on changes in sedimentary patterns and paleosols as well as the general paleobotanical trends. The lithological patterns include a change from a diverse array of paleosols, including Histosols (ever-wet waterlogged soils), in the late Pennsylvanian to greatly diminished paleosol diversity with poorly developed Vertisols by the Early-Middle Permian transition. In addition, coal seams were present with wide areal distribution in the late Pennsylvanian whereas beds of evaporates were common by the end of the Early Permian. During this climatic transition, wetland plants were confined to shrinking "wet spots" found along permanent streams where the vegetation they constituted remained distinct if increasingly depauperate in terms of species richness. By Leonardian (late Early Permian) time, most of the landscape was dominated by plants adapted to seasonal drought and a deep water table. Wetland elements were reduced to scattered pockets, dominated primarily by weedy forms and riparian specialists tolerant of flooding and burial. By the Middle Permian, even these small wetland pockets had disappeared from the region.

Global Permian tetrapod biostratigraphy and biochronology

Abstract: Abstract The most extensive Permian tetrapod (amphibian and reptile) fossil records from the western United States (New Mexico-Texas) and South Africa provide the basis for definition of 10 land-vertebrate faunachrons that encompass Permian time. These are (in ascending order): the Coyotean, Seymouran, Mitchellcreekian, Redtankian, Littlecrotonian, Kapteinskraalian, Gamkan, Hoedemakeran, Steilkransian and Platbergian. These faunachrons provide a biochronological framework with which to determine and discuss the age relationships of Permian tetrapod faunas. Their correlation to the marine time scale and its numerical calibrations indicate that the Coyotean is a relatively long time interval of about 20 Ma, whereas most of the other faunachrons are much shorter, about 1–2 Ma long each. The Platbergian may also be relatively long, 14 Ma, although this is not certain. This suggests slow rates of terrestrial tetrapod faunal turnover during most of the Early Permian and late Middle to Late Permian, but more rapid rates of turnover during the latest Early and most of the Middle Permian, especially during the explosive initial diversification of therapsids.

The Carboniferous system: use of the new official names for the subsystems, series, and stages

Abstract: As a result of votes by the Subcommission on Carboniferous Stratigraphy [SCCS] that were ratified by the International Commission on Stratigraphy [ICS] and the International Union of Geological Sciences [IUGS] over the period 1999-2004, the official subdivision of the Carboniferous System has been substantially modified. For subsystems, the terms Mississippian and Pennsylvanian should be used in all regions of the world to replace the more ambiguous and more awkward terms Lower and Upper Carboniferous. Regional geographic names for series and stages may continue to be used in those regions in which they developed, specifically in Western Europe, the USA, and China. However, their global equivalents should be denoted equally, particularly as they become better correlated, in order to facilitate global correlation in future work. The SCCS also voted to standardize the scale of all regional units termed stages at rough equivalency with the global stages now recognized in the Carboniferous (which are similar in scale to those in the adjacent Devonian and Permian Systems). Therefore, the up to 26 subdivisions of the Tournaisian, Visean, Namurian, Westphalian and Stephanian of the regional western European classification should now be ranked and termed only as substages.