Showing papers on "Gondwana published in 2005"

Long-lived glaciation in the Late Ordovician? Isotopic and sequence-stratigraphic evidence from western Laurentia

Abstract: The timing and causes of the transition to an icehouse climate in the Late Ordovician are controversial. Results of an integrated δ13C and sequence stratigraphic analysis in Nevada show that in the Late Ordovician Chatfieldian Stage (mid-Caradoc) a positive δ13C excursion in the upper part of the Copenhagen Formation was closely followed by a regressive event evidenced within the prominent Eureka Quartzite. The Chatfieldian δ13C excursion is known globally and interpreted to record enhanced organic carbon burial, which lowered atmospheric p CO2 to levels near the threshold for ice buildup in the Ordovician greenhouse climate. The subsequent regressive event in central Nevada, previously interpreted as part of a regional tectonic adjustment, is here attributed in part to sea-level drawdown from the initiation of continental glaciation on Gondwana. This drop in sea level—which may have contributed to further cooling through a reduction in poleward heat transport and a lowering of p CO2 by suppressing shelf-carbonate production—signals the transition to a Late Ordovician icehouse climate ∼10 m.y. before the widespread Hirnantian glacial maximum at the end of the Ordovician.

A review of the Mesoproterozoic to early Palaeozoic magmatic and tectonothermal history of south–central Africa: implications for Rodinia and Gondwana

Abstract: This paper provides a review of the tectonic evolution of central–southern Africa from Mesoproterozoic to earliest Palaeozoic times, using available geological information and a robust U–Pb zircon database. During the late Mesoproterozoic, the southern margin of the Congo–Tanzania–Bangweulu Craton was characterized by suprasubduction-zone magmatism and the accretion of arc and microcontinental fragments. Magmatism within the adjacent Irumide Belt formed by recycling of older continental crust. Ophiolite blocks, possibly part of an olistostromal melange, are present in a Neoproterozoic sequence overlying the Irumide Belt, and the occurrence of high-pressure/low-temperature subduction-zone metamorphism and protracted Neoproterozoic suprasubduction-zone magmatism demonstrates that there was an ocean to the south (present-day coordinates) of the Congo–Tanzania–Bangweulu Craton until the amalgamation of Gondwana at 550–520 Ma, indicating that the Congo–Tanzania–Bangweulu Craton was not part of Rodinia. On the basis of their different ages and styles of magmatism, the Mesoproterozoic Kibaran Belt, Choma–Kalomo Block and Irumide Belt are not components of the same orogen, therefore precluding a sub-Saharan-wide, linked ‘Kibaran’ ( sensu lato ) orogenic event. Evidence is presented to illustrate that the Congo–Tanzania–Bangweulu and Kalahari Cratons developed independently until their final collision during the Pan-African Orogeny along the Damara–Lufilian–Zambezi Orogen at c . 550–520 Ma.

Terrane processes at the margins of Gondwana

Abstract: The process of terrane accretion is vital to the understanding of the formation of continental crust. Accretionary orogens affect over half of the globe and have a distinctively different evolution to Wilson-type orogens. It is increasingly evident that accretionary orogenesis has played a significant role in the formation of the continents. The Pacific-margin of Gondwana preserves a major orogenic belt, termed here the ‘Australides’, which was an active site of terrane accretion from Neoproterozoic to Late Mesozoic times, and comparable in scale to the Rockies from Mexico to Alaska, or the Variscan-Appalachian orogeny. The New Zealand sector of this orogenic belt was one of the birthplaces of terrane theory and the Australide orogeny overall continues to be an important testing ground for terrane studies. This volume summarizes the history and principles of terrane theory and presents 16 new works that review and synthesize the current state of knowledge for the Gondwana margin, from Australia through New Zealand and Antarctica to South America, examining the evolution of the whole Gondwana margin through time.

Geochronology of Proterozoic basement inliers in the Colombian Andes: tectonic history of remnants of a fragmented Grenville belt

Abstract: Basement inliers of high-grade metamorphic rocks within the eastern Colombian Andes record a Grenvillian history. Among them, the Garzon Complex and the Dibulla, Bucaramanga and Jojoncito gneisses were studied using different geochronological methods to produce better correlations in the context of the reconstruction of the Grenville belt and of the supercontinent of Rodinia. The dynamic evolution of all of these units includes a final collisional event with exhumation of high-grade rocks. Such a tectonic history bears strong similarities with the Grenville Province in Canada and seems to confirm that these domains took part in the aggregation of Rodinia. Mesoproterozoic U-Pb zircon ages indicate heritage from magmatic protoliths, and the Sm-Nd model ages, as well as the e Nd values, suggest derivation from an evolved continental domain, such as the Amazonian craton, with some mixing with juvenile Neoproterozoic material. When these continental fragments are correlated with similar terrains in Mexico and the Central Andes, a large crustal fragment is implied; very probably it made up the southern portion of the Grenville belt within Rodinia, which was disrupted when Laurentia separated from Gondwana forming the Iapetus Ocean, leaving behind cratonic fragments that were later accreted to the South American Platform.

Devonian/Lower Carboniferous stratigraphy, facies patterns and palaeogeography of Iran Part II. Northern and central Iran

Abstract: Apart from a few small remnants of the Turan Plate in the north, Iran during the Palaeozoic was part of the northern margin of Gondwana On the basis of 65 sections, the majority of them covering the time-span from the Late Silurian into the Early Carboniferous, the stratigraphy and facies pattern of this area have been elucidated Biostratigraphical calibration and correlation of the sections, mostly by means of conodonts and brachiopods, show the evolution from a shallow carbonate-dominated shelf in the Silurian which, by a drop of sea-level, was transformed into a siliciclastic shelf during the Early Devonian Fully marine conditions were subsequently re-established in the Middle Devonian to early Frasnian and persisted into the early Late Carboniferous A widespread uplift in the latest Carboniferous turned the entire area into a continental regime before the onset of a new marine cycle during the late Early Permian With the exception of the northern zone (Talesh Range, Aghdarband), the Palaeozoic of Iran is characterized by continental to shallow marine deposits showing that enormous portions of the northern margin of this sector of Gondwana have been subducted during the convergence of the Turan and Iran Plates and the elimination of the Palaeotethys during the Late Triassic

Late Jurassic rifting along the Australian North West Shelf: margin geometry and spreading ridge configuration

Abstract: The Argo and Gascoyne Abyssal plains in the easternmost Indian Ocean document the last stages of eastern Tethys evolution before the breakup of eastern Gondwana. Thus they provide crucial information not only for modelling the evolution of the eastern Tethys and Proto-Indian Ocean, but also to understand the complex geodynamic history of the North West Shelf. We have revisited the marine magnetic anomaly record of the Argo and Gascoyne Abyssal Plains in combination with other geological and geophysical data from the North West Shelf and southeast Asia. Based on the combined data, we have created a revised plate-tectonic model and a set of palaeogeographic reconstructions for the evolution of the North West Shelf for the early stages after the breakup. The main difference between this model and previously published models is that we have interpreted a complete section of anomalies, M25A – M22A, in the Gascoyne Abyssal Plain, northwest of the Exmouth Plateau. The magnetic anomalies have the same trend as in...

Pacific subduction coeval with the Karoo mantle plume: the Early Jurasssic Subcordilleran belt of northwestern Patagonia

Abstract: The Early Mesozoic magmatism of southwestern Gondwana is reviewed in the light of new U-Pb SHRIMP zircon ages (181 ± 2 Ma, 181 ± 3 Ma, 185 ± 2 Ma, and 182 ± 2 Ma) that establish an Early Jurassic age for the granites of the Subcordilleran plutonic belt in northwestern Argentine Patagonia. New geochemical and isotopic data confirm that this belt represents an early subduction-related magmatic arc along the proto-Pacific margin of Gondwana. Thus, subduction was synchronous with the initial phase of Chon Aike rhyolite volcanism ascribed to the thermal effects of the Karoo mantle plume and heralding rifting of this part of the supercontinent. Overall, there is clear evidence that successive episodes of calc-alkaline arc magmatism from Late Triassic times until establishment of the Andean Patagonian batholith in the Late Jurassic involved westerly migration and clockwise rotation of the arc. This indicates a changing geodynamic regime during Gondwana break-up and suggests differential rollback of the subducted slab, with accretion of new crustal material and/or asymmetrical ‘scissor-like’ opening of back-arc basins. This almost certainly entailed dextral displacement of continental domains in Patagonia.

Stratigraphy and tectonics of Permo-Triassic basins in the Netherlands and surrounding areas

Abstract: This thesis addresses different aspects of the geological development during the Permian and Triassic (300 to 200 Ma) of the Netherlands and surrounding areas. The study area encompasses the Southern Permian Basin (SPB), a large intracratonic basin stretched out from the United Kingdom in the west to Lithuania in the east. This study revealed that, rather than one basin, the SPB actually comprised three basins, separated by subtle swells. The basins each originated differently with specific subsequent histories High-resolution stratigraphic correlations of the Permo-Triassic interval were carried out to (i) align the various stratigraphic schemes, and (ii) to define a single basin-wide stratigraphic framework for the SPB (Chapter 1). High resolution sequence stratigraphy as a correlation tool is presented in Chapter 2. Two new tectonic pulses in the Late Permian (Zechstein) are presented in Chapter 3. A detailed overview of the stratigraphy and tectonics of the Permian and Triassic in the Netherlands is presented in Chapter 4. The regional synthesis is presented in Chapter 5. It includes a series of new, high-resolution, paleogeographic maps illustrating the current insights in basin evolution. One of the recommendations of this study is a revision and redefinition of some units in the Permian and Triassic lithostratigraphy of the Netherlands (Appendix 1). Following the Variscan Orogeny, at the end of the Carboniferous (300 Ma), the Netherlands was situated in the interior part of the super-continent Pangea, north of the Alpine-style Variscan Mountain chain, which formed as a result of the accretion of the Gondwana to Laurussia plates. These mountains supplied the bulk of the sediments to the Netherlands until Mid Triassic times; further, they formed a barrier for humid air masses from the Tethys. Climatic conditions were arid to semi-arid for most of the Middle Permian to Middle Triassic, becoming slightly more humid during the Late Triassic. Marine incursions in the basin have been recorded in the Late Permian from the north, and from the late Early Triassic onwards from the Tethys. Late Carboniferous to Middle Permian rift/wrench tectonics triggered slab detachment and extrusive volcanism during a prolonged period (Early to Middle Permian). These processes resulted in the collapse of the Variscan Mountains. Initially molasse-type sediments were deposited, followed by eolian sediments and salts. Subsidence was essentially driven by thermal lithosphere contraction. Desert sediments (Rotliegend) were deposited during the Middle Permian, followed by cyclic evaporites of the Zechstein (Late Permian). The Triassic evolution is dominated by a complex interplay of climate, transgressions and short-lived pulses of rift tectonics. Cyclic sedimentation in the EarlyTriassic is attributed to climatic Milankovitch cyclicity. From the Middle Triassic onwards, rifting concentrated in the Central North Sea, Horn and Gluckstadt Grabens. During the pulses, up to 4,500 m of salt and fine-grained clastics accumulated here. Late Triassic compression, related to the closure of the Paleo-Tethys, resulted in the doming of Fennoscandia. Delta systems from the this high built out southward over the SPB during relatively humid periods. A major marine transgression flooded the area during the youngest part of the Triassic.

Paleozoic tectonic evolution of medio-Europa from the example of the French Massif Central and Massif Armoricain

Abstract: The French Massif Central and Massif Armoricain belong to three tectonic and paleogeographic domains of the Medio-Europa Variscan Orogen. The entire Massif Central and southern part of the Massif Armoricain belong to the North Gondwana margin, the Central Armorican Domain is a part of Armorica microcontinent and the Leon Domain is a piece of another microcontinent. The N. Gondwana margin and Leon Domain are made of a stack of metamorphic nappes, conversely, the Central Armorican Domain consists of a Proterozoic basement built up by the Neoproterozoic Cadomian orogeny and a Paleozoic sedimentary cover weakly deformed by upright folds related to wrenching. The architecture of the North Gondwana margin results of three main tectonic-metamorphic events that follow an early Late Silurian (ca 415 Ma) high-pressure metamorphism whose associated structures are poorly documented. The Early Devonian D1 event is responsible for top-to-the-SW nappes coeval with migmatization and exhumation of high-pressure rocks around 385-380 Ma. The Late Devonian-Early Carboniferous D2 event is a top-to-the-NW shearing coeval with an intermediate pressure-temperature metamorphism dated around 360-350 Ma. The Visean D3 event is a top-to-the-south shearing widespread in the south Massif Central whereas in north Massif Central, D3 corresponds to the onset of synorogenic extension. The Variscan Belt is also characterized by a widespread magmatism. The Early-Middle Devonian calc-alkaline magmatism is related to the southward subduction of the Rheic Ocean. The Carboniferous magmatic events are the crustal melting response of D2 and D3 tectonic events. Late Visean, Namurian and Westphalian magmatic stages are coeval with extensional tectonics controlled by NW-SE stretching. These structural, metamorphic and magmatic events are replaced in a geodynamic evolution model involving two cycles of microcontinent drifting, rewelding and continental collision.

New evidence on deinonychosaurian dinosaurs from the Late Cretaceous of Patagonia

Abstract: Deinonychosaurs, the theropod dinosaur group most closely related to birds, are known mostly from fossils from North America and Asia. A new example, from Patagonia, is the first to be found in the Southern Hemisphere, part of the Gondwana supercontinent in the Late Cretaceous. Its similarity to the northern forms suggests that there was less divergence between the southern and northern land masses than was thought. Most of what is known about the evolution of deinonychosaurs (that is, the group of theropods most closely related to birds) is based on discoveries from North America and Asia1. Except for Unenlagia comahuensis2,3 and some fragmentary remains from northern Africa4, no other evidence was available on deinonychosaurian diversity in Gondwana. Here we report a new, Late Cretaceous member of the clade, Neuquenraptor argentinus gen. et sp. nov., representing uncontroversial evidence of a deinonychosaurian theropod in the Southern Hemisphere. The new discovery demonstrates that Cretaceous theropod faunas from the southern continents shared greater similarity with those of the northern landmasses than previously thought. Available evidence suggests that deinonychosaurians were probably distributed worldwide at least by the beginning of the Cretaceous period. The phylogenetic position of the new deinonychosaur, as well as other Patagonian coelurosaurian theropods, is compatible with a vicariance model of diversification for some groups of Gondwanan and Laurasian dinosaurs.

Tectonic significance of the Lambert graben, East Antarctica: Reconstructing the Gondwanan rift

Abstract: Antarctica's Lambert graben, Australia's North West Shelf, and the eastern Indian Peninsula all host thick, fault-bounded Permian-Triassic successions. These terranes were adjacent to each other in Gondwana. The Lambert graben intersects the modern coastline, strikes oblique to shelf architecture, and has a geophysical signature that can be traced >1000 km inland. Vitrinite reflectance data from the graben margins record Permian-Triassic infill. Australia's North West Shelf is the relict of an intracontinental Carboniferous-Permian rift that was infilled during the Permian-Triassic then driven to oceanic completion during Jurassic-Cretaceous Gondwana breakup. This rift was compartmentalized over length scales of ∼650 km, corresponding to accommodation zones, margin-normal geophysical lineaments, and long-lived crustal weaknesses. In eastern India, similar compartmentalization is marked by extensive coal-bearing graben systems. Gondwana reconstructions indicate that the Lambert graben corresponds to the orientation and length scale of Carboniferous-Permian rift compartmentalization. The Lambert graben represents an accommodation zone of a wide intracontinental rift that extended from Australia's North West Shelf, between India and Antarctica, to southern Africa. This rift collected Gondwana's thick Permian-Triassic sedimentary blanket and rich alluvial coal deposits.

Hafnium Isotope and Trace Element Constraints on the Nature of Mantle Heterogeneity beneath the Central Southwest

Abstract: MORB appear to be most consistent with the hypothesis that this component represents fragments of subduction-modified lithospheric mantle beneath Proterozoic orogenic belts that foundered into the nascent Indian Ocean upper mantle during the Mesozoic breakup of Gondwana.

The accretionary history of southern South America from the latest Proterozoic to the Late Palaeozoic: some palaeomagnetic constraints

Abstract: It is now accepted that southern South America was formed from several terranes of diverse origin and evolution. However, a detailed history of the accretionary processes has not been unravelled yet. Palaeomagnetism can play an important role in such an endeavour. Palaeomagnetic constraints on the tectonic evolution of this region in the Proterozoic and Palaeozoic are reviewed and discussed. Data from the Rio de la Plata craton suggest that this block was already attached to most major Gondwana blocks by the end of the Proterozoic and may have formed a single continental mass with Congo-Sao Francisco, West Nile and Arabia throughout most of the Vendian. A large ocean separating these cratons from Amazonia and West Africa, prior to Gondwana assembly, is supported by available palaeomagnetic data. To the west of the Rio de la Plata craton is the Pampia terrane. Despite lack of palaeomagnetic data, geological evidence supports a model of Early Cambrian collision between these blocks. An Early Ordovician magmatic arc, the Famatina-Eastern Puna belt, which had developed on the western margin of the already accreted Pampia terrane, shows a systematic pattern of large clockwise rotation that has been interpreted as representative of the whole terrane. The favoured tectonic model portrays a continental magmatic arc with a back-arc basin to the east that was closed when the terrane rotated. There is little doubt of a Laurentian origin for the Cuyania (Precordillera) terrane, given the amount and diversity of evidence, including palaeomagnetism. The tectonic mechanism for accretion and its timing are still controversial. New palaeomagnetic data from Late Ordovician rocks of Cuyania support the 'Laurentian plateau' hypothesis, which suggests that Cuyania was still linked to Laurentia well into the Ordovician. Nevertheless, these new data do not rule out the more generally favoured 'microcontinent model'. To the west of Cuyania is the Chilenia terrane, separated by a belt of ophiolites of Late Ordovician age. Very little is known about this terrane, although some U-Pb ages and Nd model ages point to a Laurentian origin for its basement. Lack of palaeomagnetic data precludes determining its kinematic evolution. The Arequipa-Antofalla block may actually be a composite terrane. Palaeomagnetic data obtained so far come exclusively from the southern Antofalla block. Recently acquired data in the western Puna of Argentina confirm the originally proposed distribution of Early Palaeozoic palaeomagnetic poles, which, despite several uncertainties, delineate a pattern of significant counterclockwise rotations with a possible anomaly in palaeolatitude for the late Cambrian. The data suggest a major tectonic discontinuity between the Eastern and Western Puna of Argentina in the Early Palaeozoic. Four palaeomagnetic poles of Devonian to Permian age from the North Patagonian Massif are consistent in position and age with the Gondwana apparent polar wander path, suggesting that both continental masses have not experienced major relative displacement since the Devonian. The data do not, however, rule out a restricted separation of Patagonia orthogonal to its northern boundary in the Early or Middle Palaeozoic and subsequent collision in the Late Palaeozoic. © The Geological Society of London 2005.

Timing of subduction and exhumation along the Cambrian East Gondwana margin, and the formation of Paleozoic backarc basins

Abstract: The inversion of the Neoproterozoic-Cambrian passive margin of East Gondwana occurred during the early Paleozoic Delamerian-Ross orogeny. We present 4 0 Ar/ 3 9 Ar and structural data from deformed and metamorphosed Neoproterozoic clastic rocks beneath the Tasmanian ophiolite and the footwall of a high-pressure metamorphic complex in northern Tasmania. These data reveal the timing of accretionary deformation and the initiation of backarc extension along the Australian margin of Gondwana. 4 0 Ar/ 3 9 Ar analyses of muscovite from lower greenschist facies fault slices bounding the Forth metamorphic complex give plateau ages of 521.4 ′ 2.5 and 520.7 ′ 1.6 Ma. These data suggest that deformation within an accretionary prism off the margin of Tasmania, and possibly ocean arc collision, were under way by ca. 521 Ma. Muscovite from upper amphibolite and upper greenschist facies rocks in five locations of the Forth metamorphic complex, including retrograde shear zones, give 4 0 Ar/ 3 9 Ar cooling ages that average 508.1 ′ 2.6 Ma. Identical muscovite cooling ages from rocks originally at very different metamorphic P-T conditions suggest rapid cooling of the Forth complex at ca. 508 Ma, due to the juxtaposition of higher-grade against lower-grade rocks. Rapid cooling is also indicated by concordant 4 0 Ar/ 3 9 Ar ages of hornblende and muscovite in the high-grade core. Cooling was probably due to rapid exhumation along extensional shear zones within a regional extensional setting that also produced the Mount Read-Mount Stavely volcanic complexes (505-495 Ma) along with rift basins in Tasmania and southeast Australia. This continental rift magmatism and extension were caused by west-dipping subduction under the Australian margin of Gondwana after the collisional phase of the Delamerian-Ross orogeny. Rollback of subduction in the Australian sector of the margin between ca. 508 and 460 Ma produced a backarc basin >1000 km wide that became the basement for the Lachlan orogen turbidites. Similar amounts of subduction rollback seem not to have occurred in Antarctica at this time (unless the record is lost), suggesting significant along-strike differences in the early Paleozoic geodynamics of the Delamerian-Ross orogenic system.

East Java: Cenozoic Basins, Volcanoes and Ancient Basement

Abstract: East Java on land is divided here into four broadly EW zones: (1) the Southern Mountains Zone, an Eocene to Miocene volcanic arc, separated by (2) the present-day volcanic arc from (3) the Kendeng Zone which was the main Cenozoic depocentre in onshore East Java; and to the north (4) the Rembang Zone which represents the edge of the Sunda Shelf. Several synthems separated by unconformities can be identified and correlated between the different zones. There is a regional angular unconformity above Upper Cretaceous and older basement. The oldest rocks above the unconformity range from Mid Eocene to Lower Oligocene and record a gradual transgression and, in SE Java, an increase in volcanic material up-section. After an intra-Oligocene sea-level fall, volcanic material from the arc dominated in the Southern Mountains and Kendeng Zones while in the Rembang Zone carbonate deposition continued. In the Early Miocene, activity in the Southern Mountains Volcanic Arc culminated in a major eruptive phase at 20 Ma ± 1 Ma, similar in scale to the Pleistocene eruptions of Toba. To the north carbonate deposition was interrupted by clastic input containing reworked basement and Eocene material. The Mid Miocene was a period of reworking and carbonate sedimentation. In the Late Miocene volcanic activity recommenced at the position of the present-day arc and there was a series of deformation events throughout East Java. Volcanism has played an important role in the development of East Java, providing a source of material and contributing to subsidence by flexural loading. Provenance studies and dating of zircons * SE Asia Research Group, Geology Department, Royal Holloway University of London ** University of the West Indies *** Curtin University of Technology provide insight into the basement character and suggest that continental crust of Gondwana (possibly Western Australian) origin lies beneath part of the Southern Mountains Zone. It is suggested that continental Sundaland provided very little, if any, terrigenous material to East Java in the Cenozoic.

Tectonic evolution of the Andean Fold and Thrust Belt of the southern Neuquén Basin, Argentina

Abstract: The Andean Fold and Thrust belt between 368 and 398S can be divided in two sectors. The Eastern Sector corresponds to the Agrio Fold and Thrust Belt (FTB) characterized by a major exhumation during the Late Cretaceous, and minor deformation during the late Eocene and Late Miocene. The Western Sector corresponds to the main cordillera and is characterized by a complex evolution that involves periods of out-of-sequence thrusting with respect to the previously deformed outer sector, and pulses of relaxation of the compressive structure. Cretaceous uplift constituted an orogenic wedge that extended to the inner sectors of the Agrio FTB. Eocene compression was mainly concentrated within the Western Sector but may have reactivated the pre-existing structures of the Agrio FTB, such as the Cordillera del Viento. Late Miocene minor compressional deformation occurred in the retro-arc area and extended into the foreland area. This deformation event produced the closure of a short-lived intra-arc basin (Cura Mallin Basin, 25–15 Ma) at the innermost sector of the FTB. The Pliocene and Quaternary, between 378300and 398S, have been periods of relaxation of the inner part of the FTB and fossilization of the Agrio Fold and Thrust Belt. Localization of episodic late Oligocene–Early Miocene and Pliocene to the present extensional structures in the intraand inner retro-arc is controlled by pre-existing Jurassic half-grabens related to the formation of the Neuquen Basin. The Jurassic rift seems to be controlled by deep crustal–lithospheric discontinuities derived from a Proterozoic– Palaeozoic history of amalgamation in the area, now deeply buried under multiple episodes of Mesozoic–Tertiary synorogenic and synextensional sedimentation. Geophysical studies have revealed that the Andes mountain belt is extremely variable in crustal thickness and topography (Introcaso et al. 2000). The topography varies between broad amplitudes greater than 700 km measured from the trench, and narrow belts restricted to the inner sectors of the fold and thrust belt. These variations are mainly related to shortening within the Andes (Ramos et al. 2004). However, the causes of variable shortening and relief remain open to discussion and include several key factors: (1) shortening of the mantle lithosphere related to overthrusting of the Andes over old cratonic shields (Lyon-Caen et al. 1985; Lamb & Hoke 1997; Kley et al. 1999); (2) pre-existing anisotropies in the foreland of the orogenpre-dating theAndeanorogeny, which differentially deformed under compression (Allmendinger & Gubbels 1996); (3) changes in the lithospheric thermal structure and the consequent development of brittle–ductile transitions that become new detachments where the upper crust yields and is stacked over the foreland (James & Sacks 1999; Ramos et al. 2002); and (4) climate (Beaumont et al. 1992; Thomson 2002). At this latitude (378–398S), the Andean mountain belt deforms the Mesozoic Neuquen Basin (Fig. 1). The maximum topographic heights are restricted to a narrow band next to the volcanic arc, and the external zone represents a smooth surface where older deformations have taken place during the Late Cretaceous and Palaeogene (Fig. 1) (Zapata et al. 1999, 2002). The lack of amplitude and height of the orogenic system, in comparison with neighbour segments to the north, are in accordance with the minimum shortening computed from surface structures and crustal roots (Zapata et al. 1999; Ramos et al. 2004). The tectonic evolution of the Andean mountains in the southern portion of the Neuquen Basin reveals certain anomalies to the general From: VEIGA, G. D., SPALLETTI, L. A., HOWELL, J. A. & SCHWARZ, E. (eds) 2005. The Neuquen Basin, Argentina: A Case Study in Sequence Stratigraphy and Basin Dynamics. Geological Society, London, Special Publications, 252, 37–56. 0305-8719/05/$15.00 # The Geological Society of London 2005. picture of progressive foreland-propagating deformation in the Andes. These anomalies are characterized by a positive roll-back velocity, since the break-up of southern Gondwana (Ramos 1999b), and periods of foreland propagation of thrust sheets alternating with periods of tectonic relaxation, possibly originated from changes in the Wadati–Benioff geometry. However, are these real anomalies from the Andean orogeny point of view or do they exemplify a long-standing process in many segments along the Andean chain that elsewhere have been obscured by younger tectonic imprints? Other subduction-related orogens around the Fig. 1. Regional location map, where main morphostructural units of the Andes between 368 and 408S are displayed. (A) Arc and retro-arc morphostructural units. (B) Fore arc to retro-arc systems mentioned throughout the paper. (C) Thematic mapper scan of the area occupied by the Neuquen Embayment during the Mesozoic from the western to the eastern side of the present Andean belt. The square represents the area of Figure 5 and the black line indicates the position of the profile in Figure 10. T. ZAPATA & A. FOLGUERA 38

Review of Upper Paleozoic and Lower Mesozoic stratigraphy and depositional environments of central and west Mexico: Constraints on terrane analysis and paleogeography

Abstract: Reconstructing the geological evolution of central and western Mexico during the end of the Paleozoic and the beginning of the Mesozoic is very diffi cult because of a lack of exposures. The few outcrops available, and indirect information obtained from geophysical and geochemical data suggests that Central and Western Mexico are made up of a mosaic of pre-Jurassic terranes, and that previously defi ned terranes are mostly composites of basements of different origins. Most of those terranes are allochthonous with respect to North America, but some developed not far from their present position. It has been suggested that the Coahuila and Sierra Madre terranes (Oaxaquia block), part of Gondwana during Early Paleozoic, collided with North America by Late Paleozoic time. However, their Mississippian faunas of North American affi nity suggest that the collision might have occurred earlier. The nature of the basement of the Central terrane is unknown, but it is inferred to be allochthonous because there is an accretionary prism at its NE boundary. The basement of the Parral and Tahue terranes is formed by a deformed volcano-sedimentary complex of Early Paleozoic age, whose origin and paleogeographic evolution remains unknown. The Caborca and Cortes terranes are formed by Proterozoic metamorphic complexes and an accreted eugeoclinal Paleozoic sedimentary wedge. The basement of the Zihuatanejo terrane is made up of Triassic ocean-fl oor continental-rise assemblages accreted in Early Jurassic time. An overview of new stratigraphic and geochronologic data indicates that a number of tectonic events occurred during Late Paleozoic to Early Mesozoic time. A continental arc with a paleo-Pacifi c, east-dipping subduction zone evolved from Carboniferous to Early Permian time in eastern Mexico (Oaxaquia), and it was in part contemporaneous to deformation in the Ouachita belt. This was followed by a period of volcanic quiescence during middle Permian. A more felsic arc, with a different distribution of the volcanic axis, developed along all the paleo-Pacifi c margin in the Permo-Triassic. Terranes in northwestern Mexico show a completely different geological evolution during the Carboniferous and Permian time. They were characterized by passive margin sedimentation and by folding and thrusting of eugeoclinal rocks in the Mississippian and Late Permian. By Late Triassic, a passive or rifting Centeno-Garcia, E., 2005, Review of Upper Paleozoic and Lower Mesozoic stratigraphy and depositional environments of central and west Mexico: Constraints on terrane analysis and paleogeography, in Anderson, T.H., Nourse, J.A., McKee, J.W., and Steiner, M.B., eds., The Mojave-Sonora megashear hypothesis: Development, assessment, and alternatives: Geological Society of America Special Paper 393, p. 233–258. doi: 10.1130/2005.2393(08). For permission to copy, contact editing@geosociety.org. ©2005 Geological Society of America. 234 E. Centeno-Garcia spe393-08 3rd pages

New Zealand tectonostratigraphy and implications from conglomeratic rocks for the configuration of the SW Pacific margin of Gondwana

Abstract: Abstract The active margin of Gondwana is presently preserved in the southwest Pacific region in the formerly continuous Gondwana fragments of Australia, Antarctica and New Zealand. The Phanerozoic tectonic history of New Zealand is interpreted in terms of progressive Pacific-ward growth by accretion of arc-trench systems and the basement rocks are described in terms of a number of volcano-sedimentary accreted terranes, suites and batholiths that intrude the terranes. The age of these basement rocks ranges from Early Cambrian to late Early Cretaceous. The origin of the magmatic and sedimentary rocks and the time of accretion of the New Zealand terranes to the Gondwana margin are important for the understanding of Phanerozoic Pacific tectonics. Geochronological research over the last decade on igneous rocks and conglomeratic units shows that the Tutoko Complex/Amundsen Province plutons are major contributors of detritus to the Pahau depositional basin and that the Antarctic sector of the Panthalassan Gondwana margin has to be (re)considered as the likely source for the Permo-Triassic Rakaia sediments. Igneous clast data have greatly improved understanding of the evolution of the New Zealand microcontinent and have put tighter constraints on its Mesozoic tectonic setting within the southwest Pacific margin of Gondwana.