Continental Margin Of Gondwana Research Articles

The proto-Andean margin of Gondwana: an introduction

Abstract A basic background is presented for the discussion of the Early Palaeozoic geology of western Argentina covered by this book. This includes the definition and terminology of orogenic cycles on this part of the Gondwana margin, represented by the Eastern Sierras Pampeanas. The Pampean orogeny (Early Cambrian) relates to an intense but short-lived period of terrane collision predating the rifting of the Precordillera terrane from Laurentia. The Famatinian cycle is predominantly represented by intense subductionrelated magmatism of Early-Middle Ordovician age, developed on the continental margin of Gondwana during the rifting and drifting of the Precordillera terrane. The Grenvillian basement of the latter is further exemplified by a new Rb-Sr whole-rock isochron age of 1021 ± 12 Ma for orthogneisses from the Western Sierras Pampeanas. A mid-Ordovician granite in this area (dated at 481 ± 6 Ma by U-Pb ion microprobe data) may be related to rifting while the Precordillera terrane was still attached to Laurentia. A divergence of opinion is pointed out between some authors in this book who favour mid-Ordovician collision of the Precordillera with Gondwana, and others who place it much latter, in Silurian or Devonian times.

Geochemical and isotopic structure of the early Palaeozoic active margin of Gondwana in northern Victoria Land, Antarctica

The regional distribution of geochemical and isotopic compositions of granitoid rocks from a Gondwana continental margin is studied to highlight its structure and geodynamic evolution. The intrusive rocks emplaced during the early Palaeozoic Ross Orogeny in northern Victoria Land (Antarctica) constitute a high-K calc-alkaline association. The geographic patterns of isotope and geochemical data on granitoid rocks allow the distinction of two portions of the continental margin, separated by a sharp discontinuity. The portion towards the palaeo-Pacific Ocean (Oceanward Side) displays strongly regular inland increase of Sr- and decrease of Nd-isotope ratios, coupled with analogous variations in major and trace elements; on this basis we infer a NW-SE-trending margin affected by SW-directed subduction. The portion towards the East Antarctic Craton (Continentward Side) shows a similar regular variation only for Nd isotope compositions, consistent with a hypothesis of a N-S-striking margin with west-ward subduction. In the Oceanward Side, isotope and trace-element characteristics suggest that the granites were generated by extensive interaction of mantle-derived magmas with high-level crustal melts. The origin of Continentward Side intrusives is compatible with a process of interaction between mantle-derived melts and a mafic granulite lower crust. The granitoids of the two crustal sectors share the same Proterozoic Sm-Nd model ages, suggesting that they both belong to the same crustal province. We interpret this arrangement of crustal segments as due to the shift and rotation of a forearc sliver of the Gondwana margin. This movement was likely enhanced by oblique subduction under an irregular margin weakened by the presence of a magmatic arc.

Variscan accretionary complex of northwest Iberia: Terrane correlation and succession of tectonothermal events

The allochthonous terranes of northwest Iberia can be correlated with specific paleogeographic realms of the continental masses and intervening oceans involved in the Variscan collision. Assuming that the existing ophiolites represent the suture formed by the closure of the Rheic ocean, the units in the footwall to the suture correspond to the outer edge of the Gondwana continental margin, and the units in the hanging wall are interpreted as fragments of the conjugate margin, represented by the Meguma terrane. This correlation establishes a precise link between circum-Atlantic terranes, and makes it possible to draw a relatively simple scenario of the successive tectonothermal events recorded. Following the amalgamation of Avalon to Laurentia, the remaining outboard terranes underwent a progressive accretion to this continent that ended with the collision between Laurentia and Gondwana.

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.

Foraminiferal Stratigraphy and Paleoenvironments of Late Jurassic to Early Cretaceous Deposits in Thakkhola, Nepal

This is the first detailed account of Late Jurassic, deeper marine, southern hemisphere, agglutinated benthic faunas, and their close taxonomic affinity to coeval assemblages from northern high latitudes. Over 1100m of dark shale, siltstone and sandstone in Thakkhola (Nepal) are dated by means of ammonites, dinoflagellate cysts and foraminifera as Oxfordian through latest Albian. The succession was deposited along the northern Gondwana margin, bordering Tethys, while Thakkhola lay at mid-latitudes (30-41?S). A highly diversified agglutinated faunal record in the Oxfordian lower Nupra Formation, with 44 taxa, is segmented (in ascending order) into the Eomarssonella paraconica assemblage, Vemeuilinoides graciosus assemblage and Spiroplectammina suprajurassica assemblage. The assemblages are of restricted deeper shelf nature, in agreement with geochemical, sedimentary and ammonite-based depositional interpretation. The Tithonian upper Nupra Formation is attributed to a prodelta depositional setting, and contains a low-diversity, mainly agglutinated assemblage named after Trochammina annae. The Early Cretaceous Tangbe Formation shows a change to coarser terrigenous clastics deposited under shallow shelf conditions characterized by the Trochammina aff. schaimica assemblage of agglutinated taxa with extremely low diversity. The Late Jurassic assemblages of Nepal have a majority of taxa in common with those of Western Siberia, Svalbard and the Canadian Arctic Archipelago, which suggests that these contain many cosmopolitan taxa, suitable for global stratigraphic correlations. INTRODUCTION During 1988 and 1991, international field excursions were carried out to the Mesozoic strata of the Himalayan mountain range (Gradstein et al. 1989, 1992). These passive continental margin sediments are preserved immediately north of the central Himalayan range in the Thakkhola region of northern Nepal. The present report describes new paleontologic findings, with emphasis on biostratigraphic, paleoecologic and paleogeographic interpretations from studies of foraminifera. Field observations and sediment samples originate from the above-mentioned excursions. The field area is delineated by the villages Jomosom-TangbeChhukgaon-Muktinath, at approximately 28?N and 84?E. Here, relatively easily accessible Late Jurassic and Early Cretaceous strata, over 1100m thick, are exposed at altitudes of 2800 to 5000m. The sediments were deformed to varying degrees by folding and faulting during Himalayan collision in Cenozoic time. The geological literature refers to this area as Thakkhola and we follow this usage. The location of the field sections studied is shown in text-figure 1. Samples are designated according to field section, e.g. J02, followed by the sample number. The Late Jurassic through Early Cretaceous formations of Thakkhola contain exclusively or predominantly agglutinated assemblages. An exception is the topmost (Late Albian) part of the succession, which contains a calcareous assemblage mainly of planktonic species. Preliminary accounts of these assemblages were published previously in Gradstein et al. (1989, 1992). The Late Jurassic agglutinated assemblages are new to southern Asia, and show unexpected affinities to the so-called boreal faunas of the northern hemisphere, which may be more cosmopolitan than known so far. PALEOGEOGRAPHIC SETTING Paleomagnetic reconstructions indicate that Australia, Antarctica and Greater India, which included northern Nepal, formed part of the southern landmass of Gondwana in Jurassic time (Sengor 1985; Powell et al. 1988). The Thakkhola area lay on the continental margin of northern Gondwana, bordering the Tethys Ocean to the south, until at least the Early Cretaceous (text-fig. 2). Consequ ntly, the Jurassic and Early Cretaceous succession of Thakkhola shows marked similarities with the sequences drilled at ODP sites off northwestern Australia (Gradstein and von Rad 1991; Gibling et al. 1994). Northerly paleoflow directions measured in Early Cretaceous fluvio-deltaic deposits of Thakkhola are in accordance with deposition on the subsiding northern Gondwana continental margin. Paleomagnetic data confirm that the Thakkhola area lay at mid latitudes, 30-41?S, in Late Jurassic to Early Cretaceous times (Ogg in Gradstein et al. 1992). Early Jurassic carbonates (Jomosom and Bagung formations) reflect Thakkhola's position in the subtropical zone, and correspond to the most northerly paleolatitude recorded. In the Late Jurassic to Early Cretaceous the area drifted to higher latitudes, reflected by deposition of terrigenous clastics composing the Nupra, Chukh and Tangbe formations. A return to hemipelagic carbonate sedimentation in Late Albian time may reflect in part a reversal of drift (with movement northward), and in part a global rise in sea level, which drowned the shelves and restricted influx of terrigenous clastic sediments. STRATIGRAPHIC AND DEPOSITIONAL FRAMEWORK The Late Jurassic to Early Cretaceous succession of Thakkhola comprises deltaic, marine shelf and upper slope sediments. The micropaleontology, vol. 41, no. 2, pp. 143-170, text-figures 1-12, plates 1-5, tables 1-4, 1995 143 This content downloaded from 157.55.39.163 on Fri, 18 Nov 2016 04:19:11 UTC All use subject to http://about.jstor.org/terms Jeno Nagy et al: Foraminiferal stratigraphy and paleoenvironments of Late Jurassic to Early Cretaceous deposits in Thakkhola, Nepal

Cyclic development of sedimentary basins at convergent plate margins — 1. Structural and tectono-thermal evolution of some gondwana basins of eastern Australia

The Devono-Carboniferous Drummond Basin of eastern Australia formed by extensional tectonics, most probably in a back-arc setting, as an oceanic plate subducted westward under Gondwana's continental margin. Within this basin a syn-rift sequence and a relatively thick post-rift sequence are recognized, and the latter is separated from the overlying Galilee Basin by a mid-Carboniferous unconformity, which heralds a time of relatively mild compression, uplift and folding. The lower Galilee Basin formed as a foreland, secondary peripheral bulge, or mixed-style basin during the Late Carboniferous to Early Permian time interval. A mid-Permian unconformity separates it from the extensive Late Permian to Mid-Triassic upper Galilee Basin, here suggested to be a platform basin. Nest, the main, Mid- to Late Triassic compressional event led to reverse movement along previously normal faults, folding, uplift and erosion of up to 2 km of section from the Galilee and Bowen basins. During Jurassic and Cretaceous times, the Eromanga/Surat/Carpentaria Basin, a platform basin originally >1.7 million km 2 in extent, developed cratonward of the zone of continuing subduction. Subsequently, the culminating extensional event took place further east, where Lord Howe Rise rifted apart from the continental landmass and drifted eastward as oceanic seafloor spread in the Tasman Basin, and widespread uplift and erosion occurred over the eastern Australian coastal area. New data, re-interpretation of existing data and extensive literature support the interpretation of the Phanerozoic tectonic evolution of eastern Australia in a context of convergent plate margins. Within this context, overall eastward migration through time of the subduction zone and associated morphotectonic entities (e.g. the Warburton, Adavale, Drummond, Bowen and Tasman extensional basins) and vertical stacking of unconformity-bound extensional, foreland and platform basins (e.g. the Drummond, Galilee and Eromanga basins) occurred.

Late devonian accretion of the Gamilaroi Terrane to eastern Gondwana: Provenance linkage suggested by the first appearance of Lachlan Fold Belt‐derived quartzarenite

Timing of the accretion of an intra-oceanic island arc sequence (Gamilaroi Terrane) to the eastern margin of Gondwana is constrained by the appearance of Lachlan Fold Belt-derived quartzarenite clasts in the marine clastic sequence which unconformably overlies the island arc sequence. The oldest formation containing quartzarenite clasts is the Late Devonian (Famennian) Keepit Conglomerate. As a consequence of this accretion event the Gamilaroi Terrane became contiguous with the continental margin of Gondwana. This finding indicates that docking occurred some 30 Ma earlier than the Visean-Namurian event suggested by other researchers.

Reconstruction of the tectonic, volcanic, and sedimentary setting of strongly deformed Zn-Cu massive sulfide deposits at Benambra, Victoria

The Wilga and Currawong massive sulfide deposits at Benambra are hosted by a strongly deformed, Upper Silurian rhyolite-dacite-andesite-basalt-sediment succession, dominated by silicic volcanics and sedimentary rocks. This host succession developed in an ensialic back-arc or intra-arc basin of limited extensional origin on the active Gondwana continental margin. Strong deformation is related to compressional and transcurrent crustal movements during closing of the depositional basin at the end of the Silurian and to subsequent steep faulting.The massive sulfide deposits are essentially synvolcanic because (1) they have the same three generations of tectonic structures and the same intensity of deformation as their host rocks, suggesting that the ores predate the earliest compressional deformation, (2) the ores are constrained to a Late Silurian age similar to their host rocks because both the host rocks and this earliest deformation are Late Silurian in age, (3) there is a tight stratigraphic control on the location of the massive sulfides, and (4) the stratigraphy, depositional setting, and alteration are similar to many other less-deformed volcanic-associated massive sulfide districts.Deformation and alteration inhibit detailed stratigraphic correlation and modify rock textures, but do not destroy facies relationships, which allow sedimentological and volcanological interpretation. Sedimentary facies associations indicate that mineralization occurred in the basin center after fault-controlled subsidence from a mixed subaerial and subaqueous environment with active rhyolitic volcanism, through a marine shelf environment with mixed limestone-volcaniclastic sedimentation, to a moderate- to deep-water environment with mudstone and turbidite sedimentation and rhyolitic to basaltic volcanism.Juvenile elastic and nonelastic volcanic rocks are closely associated, subequal in volume, and comprise 50 to 70 percent of the volcano-sedimentary sequence. Evaluation of fragmentation and emplacement processes indicates that the juvenile volcaniclastics are mainly in situ, resedimented and intrusive hyaloclastites, rather than pyroelastics. Classification of primary contact relationships of the volcanics into passive, slightly disruptive and highly disruptive types, and the internal facies organization of the volcanic units, indicate that the volcanics form extrusive domes, extrusive tabular flows, partially emergent cryptodomes and sills, and entirely intrusive sills. Shallow sills and cryptodomes emplaced into wet subsea-floor sediments are texturally similar to, but more abundant than, lavas in the environment of mineralization. These shallow intrusions are distinguished by disruptive upper contacts including sediment-veined and sediment-matrix hyaloclastite. Their recognition is critical because the mineralized stratigraphic interval can only be correlated using volcanics and sediments emplaced on the sea floor. Based on extrusive units, the massive sulfides occur within 100 m above the last major extrusive rhyolites and within 50 m below the first major basalt lava.Integrating the tectonic, structural, sedimentological, and volcanological data shows that the massive sulfides formed in siltstone, in the proximal (near-vent) facies association of a low profile, mainly nonexplosive, moderate to deep-water submarine volcano composed of turbiditic sediments interleaved with numerous rhyolitic to basaltic sills, lavas, and associated hyaloclastites. The ores can be related in space and time to the advanced stage of extension and subsidence in a continental margin incipient rift, when relatively deep-marine conditions were attained, generation of voluminous rhyolitic crustal melts waned, and basaltic magma started to penetrate up deep extensional faults to the sea floor. The spatial association of the last rhyolite extrusions, the massive sulfides, and the first basalt extrusions, suggests that these rocks are genetically related and that all were fed via the same extensional fault system. This setting of silicic volcanic-associated massive sulfides at Benambra provides a model for the many deposits related to mainly nonexplosive volcanism and an alternative to the recently popular submarine pyroclastic caldera model.

Metamorphism and uplift of Alpine schist in the Franz Josef–Fox Glacier area of the Southern Alps, New Zealand

Geothermometry and geobarometry of 10 garnet–oligoclase zone schists in the Franz Josef–Fox Glacier area, Southern Alps, New Zealand, give temperatures ranging from 415 to 625°C and pressures from 5.2 to 9.2 kbar, indicating a T–P array of about 50°C/kbar and inferred peak temperature conditions over a c. 15‐km‐thick section at depths between c. 20 and 34 km. The present‐day distribution of the schist samples implies that only about one‐third of the original crustal section is now exposed.The garnet–oligoclase zone schists represent the deeper part of a metamorphosed and deformed accretionary complex that was associated with late Palaeozoic–early Mesozoic subduction along the Gondwana continental margin. Partial uplift (c. 0.2 m/Ma) and erosion of the complex during Jurassic–Cretaceous times (Rangitata uplift) was synchronous with D2 deformation and recrystallization, as recorded by the P–T array. Cenozoic (Kaikoura) uplift and exhumation of the schist since c. 30 Ma to form the Southern Alps was associated with oblique‐slip movement on the Alpine Fault. The present‐day position and steep eastward dip of isograds and D2 structures suggest considerable clockwise rotation during uplift associated with ductile attenuation and tectonic thinning by over two‐thirds of the original schist sequence, largely due to simple shear along schistosity planes. As the schist generally shows only incipient greenschist facies retrograde recrystallization, an apparently complete (although contracted) prograde mineral sequence has been preserved by rapid uplift (>5 km/Ma) of hot rock and the effects of limited shear heating near the Alpine Fault.

Carboniferous glaciation in Gondwana. Evidence for grounded marine ice and continental glaciation in southwestern Argentina

Upper Paleozoic sedimentary rocks in western Argentina accumulated in retro-arc foreland basins in the Gondwana continental margin. A glacial component in these rocks was recognized early in this century but reconstructions of the nature of the glaciation differ markedly. The dominant proposal sustains that glaciation in this region was of alpine-type, with glaciers descending from mountain ranges both inland and along the edge of the continent, and that the numerous intercalated diamictites were mostly deposited as subaquatic debris flows only indirectly connected with the presence of glacier ice. A stratigraphic and sedimentologic study of Middle Carboniferous glacigenic deposits of the Tepuel Group in central western Patagonia, southwestern Argentina, shows that two major diamictite facies are present: (a) massive diamictite, the more abundant, forms regionally extensive and thick (<60 m) pebbly bodies with boundaries transitional to non-glacial open shelf deposits; these are interpreted as rain-out till; and (b) chaotic diamictite, yielding thin (< 10 m) laterally discontinuous bouldery bodies showing erosive bases, strong substratal deformation, and upward transition to massive diamictite; these are interpreted as subglacial till. A third type of glacial deposit consists of interbedded current-deposited and wave-worked conglomerates and sandstones, in strata that downlap onto non-glacial shelf mudstones; these are interpreted as submarine outwash fan deposits. The position of subglacial till and outwash fans relative to the paleoshoreline indicate deep regional incursions of grounded ice onto the shelf. In addition, deepening of the shelf coincident with initial glacial marine sedimentation in the early Middle Carboniferous, at a time of global lowstand, argues for glacio-isostatic depression of the shelf and suggests a considerable ice thickness. Paleotopographic considerations indicate that the centres of ice dispersal were located on moderately high intracratonic highlands. A regional erosion surface across these highlands is attributed to subglacial scour under a Patagonian ice cap that expanded onto the adjacent shelf during glacial maxima. Published information concerning ice distribution in Middle Carboniferous Gondwana supports the existence of an ice cover similar in lateral extent to that in modern Antarctica. The Patagonian ice cap would have been peripheral to this continental-sized ice mass.

Events and sea-level changes on Gondwana's margin: The Meguma Zone (Cambrian to Devonian) of Nova Scotia, Canada

The Meguma Zone consists of thick, siliciclastic and volcaniclastic strata, and granitic plutons that underlie southern Nova Scotia and offshore areas. The strata were deposited from Late Cambrian through Early Devonian time on the continental margin of Gondwana. Grain size of the major stratigraphic units reflects the changing elevations of base level in the source region. Twelve major events in the zone can be related to changes in relative sea level and combined into four, second-order cycles. Each cycle consists of a fining-, then coarsening-upward sequence that is capped by subaerial volcaniclastic material. These subaerial events are assumed to represent significant hiati, and so are bounding unconformities. Two unconformities are at system boundaries; two, at sequence boundaries. Third-order cycles suggest idiosyncrasies of the source land. Ultimate cause of the Meguma cycles may be mantle activity that affected continental base level, relative sea level, and direction of continental drift. The Meguma curve is similar to that of northwest Africa.

Frontier Sedimentary Basins of the New Zealand Region

Petroleum-prospective basins of New Zealand began to form by mid-Cretaceous rifting of crustal elements previously assembled at the Gondwana continental margin. During the latest Cretaceous-early Cenozoic New Zealand separated from Australia and Antarctica by sea-floor spreading. An overall transgression in widely recorded in this post-rift phase, with decreasing clastic sediment supply as land area and relief were reduced. Mid-Cenozoic initiation of the modern plate boundary has resulted in uplift of mountain ranges, subsidence and filling of troughs, progradation of the shelf, and common reactivation or eversion of older structures. Petroleum potential of less explored basins can be compared to the productive Taranki basin. Source rocks are coal-rich deposits of the rift phase, also developed in Great South, Canterbury/Chatham, Western Southland, West Coast, and Northland basins. A different source contributes to oil and gas seeps on the East Coast, a continental margin during Late Cretaceous. The main reservoirs of Taranaki are early Cenozoic coastal and fluvial sands, also present in Great South, Canterbury, and West Coast and possibly other basins. Other Taranaki reservoirs include mid-Cenozoic limestone and Miocene turbidites, which are widespread in most other basins. Pliocene limestones have excellent reservoir potential on the East Coast. Late Cenozoic tectonics, essential tomore » trap development and significant for maturation in Taranaki, have created similar structures in basins near the plate boundary but are less significant in the development of Great South, eastern Canterbury/Chatham, and Northland basins.« less

Late paleozoic orogeny in the northwestern Gondwana continental margin, western Argentina and Chile

During Early Carboniferous-Early Permian time, the segment of the northwestern Gondwana continental margin (western Argentina and Chile) between present-day latitudes 27°S and 50°S was structurally organized into a marginal orogen and a foreland rising into cratonic highlands. Orogenesis and magmatism were governed by subduction of an ancestral Pacific plate under Gondwana. During the Late Devonian-Early Carboniferous, former slope-rise deposits were thickened to form the orogen and were thrust slightly onto the foreland; parts of the orogen emerged above sea level. Following this early diastrophic phase, tectonic uplift in the orogen was regionally checked by erosion until the late Early Permian when uplift rates overcame erosion. The development of this orogen is peculiar in that throughout much of this time: a) the orogenic bett was partly occupied by marine areas; b) there was no reversal of provenance, so that foreland basins were continuously fed from the interior highlands; and c) the orogenic belt did not advance further onto the foreland. Thus the orogen stabilized in position and relief. Magmatism was intense during the period of orogenic stability and was dominated by plutonism; there are no remnants of a well developed volcanic arc. Reconstructions involving Late Devonian-Early Carboniferous collision of crustal masses against Gondwana and a seaward jump of the subduction trench are not entirely substantiated by available data. An alternative suggests that from the Late Devonian to the Early Permian the trench maintained approximately the same position seaward of present-day coastal exposures.

Tectonic significance of Proterozoic ductile shortening and translation along the Antarctic margin of Gondwana

The central Transantarctic Mountains of Antarctica are divided into distinct lithotectonic packages of variable age, deformation style and metamorphic character. In the Miller Range (83°S, 155°E), the boundary between two such lithotectonic units is a low-angle, thrust-type ductile shear zone that carried high-grade gneisses of the East Antarctic craton to the southeast over a lower-grade metasedimentary sequence. Broad constraints limit the age of ductile deformation as late Early Proterozoic to Late Cambrian. In addition to a reverse component of movement, this zone also records along-strike motions in a direction subparallel to the present orientation of dominant Beardmore- and Ross-age orogenic trends. We postulate that major pre-Ordovician crustal shortening which occurred along this part of the Gondwana continental margin began in the Middle to Late Proterozoic as an early phase of long-lived convergence. Furthermore, orogen-parallel displacements indicate that oblique plate interactions may have played an important role in the early evolution of this active margin.

A non-cylindrical model for the northwest Iberian allochthonous terranes and their equivalents in the Hercynian belt of Western Europe

A case is made for a non-cylindrical development of the European Hercynian belt. Far-travelled allochthonous terranes in the belt consist of a mixture of units, including catazonal units, ophiolites and units derived from the continental margin of Gondwana. They often depict a metamorphic history of intermediate to high pressure type and Caledonian to Eo-Hercynian age, and are thought to have been put together in the early stages of the orogeny through a process partly obductive directed onto the continental margin of Gondwana. Considerations upon the disposition of the terranes, their relationships with the paleogeography of the Gondwana realm and the structures in them, suggest that their emplacement took place in an oblique fashion, at a small angle to the margin of the continent. The proposed allochthon could be related to the development of an island arc or a wedge-like subplate in the southern part of the Baltica plate. Later, convergence between Gondwana and Laurussia involving transcurrent, mostly dextral movements, combined with tectonic escape phenomena, would account for the present complexity of the belt and for the narrow strip the allochthonous terranes delineate in westernmost Europe.

Origin of the Chichibu Sea, Japan: Middle Paleozoic to Early Mesozoic Plate construction in the northern margin of the Gondwana continent

Pre‐Jurassic plate tectonic development of southwest Japan is discussed in the light of rifting tectonics using the symmetrical rock arrangement. In particular, a geodynamic model is presented of Middle Paleozoic to Early Mesozoic plate construction of the Chichibu Sea. The model involves five steps: (1) the deposition of Siluro‐Devonian rocks on the Gondwana continental margin; (2) incipient rifting into Hida + Hida Marginal continental block on the north and Kurosegawa continental block on the south, forming Chichibu Sea plate in Carboniferous time; (3) parts of the Chichibu Sea plate were subducted beneath the Kurosegawa continental block and newly formed Maizuru Island Arc in Permian time; (4) the rifting and subduction ceased in latest Permian time and the Chichibu Sea remained stable with the width of over 1000 km in Triassic time; and (5) the Kurosegawa continental block collided with the Hida + Hida Marginal continental block in Jurassic to earliest Cretaceous times. The geodynamic model is further discussed in the context of breakup and accretion tectonics.

The Paparoa Metamorphic Core Complex, New Zealand: Cretaceous extension associated with fragmeotation of the Pacific margin of Gondwana

In Westland‐Nelson provinces of New Zealand, high‐grade metamorphic and granitic basement rocks showing mylonitic ductile deformation are juxtaposed beneath low‐grade metasedimentary rocks and undeformed granites by uplift on low‐angle detachment faults. Several metamorphic core complexes analogous to those described from western North America are recognized. In the Paparoa Range, basement rocks include late Precambrian(?) paragneiss and granitic rocks of both Paleozoic and Cretaceous ages. Cover rocks include Ordovician turbidites, Paleozoic and Cretaceous granites, and mid‐Cretaceous breccia‐conglomerates. Brittle deformation and hydrothermal alteration (silica, chlorite, hematite, carbonate ± fluorite, uranium) characteristic of the detachment zone are also superimposed on uppermost lower‐plate mylonites. Kinematic indicators in the mylonitic rocks including composite S‐C febrics indicate that the detachment faults on the northeast and southwest sides of the Paparoa Core Complex had opposite senses of shear, with cover rocks on both sides moving away from the metamorphic core. Ductile deformation postdates several 114±18 Ma granitic plutons but by 108 Ma had ceased to affect at least some of the rocks currently exposed. Mylonitic rocks were uplifted to the surface and eroded into evolving half‐grabens by 105–100 Ma. Uplifted basement yields K‐Ar dates as young as 88 Ma, and tilting of the graben sediments indicates detachment continued well into the Late Cretaceous, when both cover and basement were intruded by alkali lamprophyre dykes. The Nelson‐Westland core complexes occur within an Early Cretaceous granitic province characterized by relatively radiogenic strontium. The boundary of this province, the NW trend to mid‐Cretaceous half‐grabens, the NNE trend of stretching lineations in mylonitic rocks, and the ESE trend of late Cretaceous lamprophyre dykes indicate that regional extension was maintained in a NNE direction for much of the Cretaceous. This regional extension may be part of an “extension corridor” which traversed the entire Gondwana continental margin from NE Queensland, Australia, to Marie Byrd Land, Antarctica. Extension preceded opening of the Tasman Sea between New Zealand and Australia at approximately 84 Ma and closely followed long‐lived compression on the Pacific convergent margin of Gondwana. The presence of core complexes in western New Zealand contrasts with the Australian margin to the Tasman Sea and lends support to simple shear models of continental rifting.

Overthrust terranes in the Lachlan fold belt, southeastern Australia

The Paleozoic history of the Lachlan fold belt involves terrane translation and accretion of discrete allochthonous fragments. Four major terranes developed adjacent to the Gondwana continental margin and were overthrust in the Middle Devonian to form a partly composite crust for southeastern Australia.

Magmatism and metamorphism in the Ladakh Himalayas (the Indus-Tsangpo suture zone)

Ladakh (India) provides a complete geological section through the northwestern part of the Himalayas from Kashmir to Tibet. Within this section the magmatic, metamorphic and geotectonic evolution of the northern Himalayan orogeny has been studied using petrographic, geochemical and isotope analytical techniques. The beginning of the Himalayan cycle was marked by large basaltic extrusions (Panjal Trap) of Permian to Lower Triassic age at the “northern” margin of the Gondwana continent (Indian Shield). These continental type tholeiitic basalts were followed by a more alkaline volcanism within the Triassic to Jurassic Lamayuru unit of the Gondwana continental margin. Lower Jurassic to Cretaceous oceanic crust and sediments (ophiolitic mélange s.s.) accompany the Triassic to Cretaceous flysch deposits within the Indus-Tsangpo suture zone, the major structural divide between the Indian Shield (High Himalaya) and the Tibetan Platform. So far, no relic of Paleozoic oceanic crust has been found. Subduction of the Tethyan oceanic crust during Upper Jurassic and Cretaceous time produced an island arc represented by tholeiitic and calc-alkaline volcanic rock series (Dras volcanics) and related intrusives accompanied by volcaniclastic flysch deposits towards the Tibetan continental margin. Subsequent to the subduction of oceanic crust, large volumes of calc-alkaline plutons (Trans-Himalayan or Kangdese plutons) intruded the Tibetan continental margin over a distance of 2000 km and partly the Dras island arc in the Ladakh region. The collision of the Indian Shield and Tibetan Platform started during the middle to upper Eocene and caused large-scale, still active intracrustal thrusting as well as the piling up of the Himalayan nappes. The tectonically highest of these nappes is built up of oceanic crust and huge slices of peridotitic oceanic mantle (Spongtang klippe). In the High Himalayas the tectonic activity was accompanied and outlasted by a Barrovian-type metamorphism that affected Triassic sediments of the Kashmir-Nun-Kun synclinorium up to kyanite/staurolite grade and the deeper-seated units up to sillimanite grade. Cooling ages of micas are around 20 m.y. (muscovite) and 13 m.y. (biotite). Towards the Indus-Tsangpo suture zone metamorphism decreases with no obvious discontinuity through greenschist, prehnite-pumpellyite to zeolite grade. Remnants of possibly an Eo-Himalayan blueschist metamorphism have been found within thrust zones accompanying ophiolitic mélange in the suture zone.

Foreword

The geodynamic origin of the Neoproterozoic ophiolites of the Arabian-Nubian Shield exposed in the Eastern Desert of Egypt remains controversial. In this study, we present new geochemical and field data from the Fawakhir ophiolite and from some mélange blocks along the Qift-Qusier Road in order to constraint the tectonic evolution of this part of the Central Eastern Desert. The Fawakhir ophiolite contains most lithological units of a Penrose-type ophiolite sequence, and includes ultramafic rocks in the west overlain by isotropic gabbro, sheeted dikes and pillow basalt in the east. These ophiolitic units are enriched in LREE (light rare earth elements) and LILE (large ion lithophile elements) but depleted in high field strength elements (La/Smcn = 0.40–1.22, Th/Nbpm = 1.7–10.9, La/Nbpm = 1.4–6.6). Their magmas appear to have been derived from a depleted (N-MORB-like) mantle source, and their geochemical characteristics are comparable to those of the Izu-Bonin-Mariana forearc oceanic crust formed during the initiation of an intra-oceanic subduction zone. Pillow lava blocks in the eastern mélange have geochemical signatures similar to those of oceanic crust generated in back-arc basins. The decrease in the magnitude of mantle depletion and the change of the geochemical signature along the Qift-Qusier Road from a forearc in the west to a back-arc in the east suggest the formation of the Fawakhir intra-oceanic arc system over an east-dipping subduction zone. With continued subduction and arc migration, this intra-oceanic arc system finally collided with the passive margin of the West Gondwana (the Saharan craton), resulting in the accretion of the Fawakhir arc–forearc units. Following its tectonic accretion onto the West Gondwana continental margin, the Fawakhir ophiolite was intruded by calc-alkaline dikes, whose magmas were derived partly from partial melting of the sub-continental lithospheric mantle. These dikes have geochemical characteristics similar to those of modern active continental margin (Andean-type) rocks (La/Smcn = 2.13–2.48, Gd/Ybcn = 2.04–4.25, Th/Nbpm = 3.2–5.8, La/Nbpm = 2.5–4.9), suggesting that the West Gondwana passive margin (Atlantic-type) was converted to an Andean-type margin subsequent to the arc-continent collision. The inferred conversion of the Atlantic-type margin to an Andean-type margin resulted from the collision-induced reversal of the subduction direction.