Juxtaposition Of Terranes Research Articles

Ganderia–Laurentia collision in the Caledonides of Great Britain and Ireland

During terrane convergence, an influx of clastic sediment from an upper plate onto a lower plate is an early indication of terrane juxtaposition. In the Caledonides of Great Britain and Ireland, units accreted to Laurentia during the early Palaeozoic Era include peri-Gondwanan terrane assemblages that earlier separated from West Gondwana. However, the Southern Uplands Terrane contains detrital zircon populations apparently derived entirely from Laurentia, characterized by a large, asymmetric Mesoproterozoic peak and a scarcity of zircon at 600 Ma and 2.1 Ga. In contrast, Cambrian and Ordovician rocks from the Lake District and the Leinster Massif of Ireland show abundant grains with these ages, together with a range of Mesoproterozoic zircon. These characteristics are shared with the Monian terrane of Anglesey and with Ganderia in the Appalachians, indicating probable derivation from Amazonia in West Gondwana. Silurian sandstones from the Lake District show an influx of Laurentia-derived zircon, and lack the peri-Gondwanan signal. This indicates that in the Caledonides, Ganderia was not accreted to the Laurentian margin until c . 430 Ma, in contrast to the Ordovician accretion of Ganderian fragments recorded in the Appalachians, suggesting that the configuration of the closing Iapetus Ocean varied significantly along the strike of the orogen. Supplementary material: Details of sample coordinates, analytical procedure and U/Pb detrital zircon analytical data are available at www.geolsoc.org.uk/SUP18739 .

Anomalous electrical structure in the northwestern Paraná Basin, Brazil, observed with broadband magnetotellurics

Abstract The deep structure of the northern part of the Parana Basin has been investigated through the analysis of a SW–NE magnetotelluric (MT) profile composed of 24 broadband stations with periods from 0.001 to ∼1000 s that are regularly spaced ∼10 km apart. The SW portion of the profile crosses a negative Bouguer anomaly with amplitudes of ∼20 mGal. Our analyses show that there are two lithospheric domains with different gravity and electric conductivity properties. In the regularized two-dimensional MT inversion model the mid-to-lower crust in the SW part of the profile is almost two orders in magnitude more conductive compared to the NE portion. This difference in conductivity extends to the uppermost mantle depths, suggesting the existence of a major lithospheric discontinuity in the north-central Parana Basin, locally coincident with the position of the Apore River. We interpret this discontinuity as a juxtaposition of terranes with distinct natures and ages, possibly as result of tectonic accretion during the formation of Western Gondwana.

The Mecubúri and Alto Benfica Groups, NE Mozambique: Aids to unravelling ca. 1 and 0.5 Ga events in the East African Orogen

We present new field and geochronological data for two newly identified sequences of Neoproterozoic metasedimentary rocks, the Mecubúri and Alto Benfica Groups, interfolded with the Nampula Complex, NE Mozambique. Both groups comprise clastic sequences of meta-psammites and meta-conglomerates, which were strongly deformed, metamorphosed to sillimanite grade and locally migmatised. The relatively unsheared contacts with the surrounding Mesoproterozoic gneisses suggest that both groups are largely autochthonous. The depositional environment of the sequences is interpreted as a proximal, continental-fluvial environment in small depositional, possibly inter-montane, fault-controlled basins. Evidence of post-Mesoproterozoic exposure to the surface is provided by secondary nodular sillimanite growth in the Mecubúri Group and the underlying high-grade basement, pointing to weathering and/or epigenetic hydrothermal alteration to clays prior to burial, deformation and metamorphism. U–Pb analyses of detrital zircons constrain maximum depositional ages of 530 ± 18 and 938 ± 32 Ma for the Mecubúri and Alto Benfica Groups, respectively. The two groups have different provenances as shown by their detrital zircon age-frequency distributions. The Mecubúri Group was derived from the adjacent eastern Nampula Complex basement (ca. 1100 Ma), with minor early and mid-Neoproterozoic (950–700 Ma) and abundant late Neoproterozoic components (650–550 Ma). The latter were probably derived from Neoproterozoic terranes north of the Lurio belt, such as the Cabo Delgado Nappe Complex (CDNC). Hf isotope data indicate a juvenile origin to the Nampula Complex basement (ɛHf ca. +4 to 8) which was then re-worked as the source for the late Neoproterozoic melts (ɛHf ca. 0 to +5). Juvenile magmatic input also continued in the late Neoproterozoic (ɛHf ca. +11). In addition to the dominant ca. 1100 Ma local source rocks, the Alto Benfica Group, in the western Nampula Complex, records input from older Mesoproterozoic (1200–1400 Ma), Palaeoproterozoic (1600–2000 Ma) and rare Archaean sources, with scant evidence of Neoproterozoic provenance. These sources themselves reflect re-working of older pre-existing crustal components with crustal separation ages up to 3000 Ma. Zircon rims to detrital grains and metamorphic monazite from both groups date high-grade metamorphism at 500 ± 10 Ma, within uncertainty of the age for late to post-tectonic granitoids of the Nampula Complex and the Lurio belt. This suggests an intimate genetic relationship between the metamorphism and migmatisation of the two metasedimentary sequences and granite emplacement at that time. The detrital zircon data from the Alto Benfica Group suggest that the Nampula Complex was juxtaposed with the adjacent terranes of northern Mozambique and SE Africa, which were uplifted, exposed to sub-aerial weathering and locally lateritised at the time of deposition. The terrane juxtaposition event can be equated with the main, NW–SE-directed collision phase of the East African Orogeny at ca. 550 Ma. The resulting mountain belt was rapidly eroding and providing detritus for the two clastic sequences at ca. 530 Ma, probably during the early phases of extensional orogenic collapse. Later in the extensional phase, local SW–NE oriented compression deformed the Mecubúri and Alto Benfica Groups, along with the basement, into tight upright folds, with associated axial planar foliation. Sillimanite-grade metamorphism and migmatisation, associated with the emplacement of late-tectonic granites took place at ca. 500 Ma. It follows that none of the regional fabrics and large-scale structures in the Nampula Complex are Mesoproterozoic in age; the present distribution of lithological units and structures are almost entirely the result of the East African collisional orogeny at ca. 550 Ma and its subsequent extension and collapse.

Paired metamorphic belts revisited

The modern plate tectonics regime is characterized by a duality of thermal environments, one representing the subduction zone and the other representing the arc–backarc or orogenic hinterland. This duality is the hallmark of one-sided (asymmetric) subduction, and the characteristic imprint of one-sided subduction in the geological record is predicted to be the broadly contemporaneous occurrence of two contrasting types of metamorphic belt, one of high d T/d P type and the other low d T/d P type. The broadly contemporaneous occurrence of granulite and ultrahigh-temperature metamorphism with eclogite–high-pressure granulite metamorphism in the geological record since the Neoarchean Era is evidence of dual thermal environments and indicates that subduction has operated on Earth since that time. Classic ‘paired’ metamorphic belts in which an inboard high d T/d P metamorphic belt is juxtaposed against an outboard low d T/d P metamorphic belt along a tectonic contact—such as the Ryoke and Sanbagawa belts in Japan—are found in Phanerozoic accretionary orogens of the circum-Pacific. Generally, they appear to result from juxtaposition of terranes with different metamorphic facies series that may or may not be exactly contemporaneous and that may or may not be far-traveled. This is a consequence of the difference between globally-continuous subduction, generating a low-to-intermediate d T/d P environment in the subduction zone and a high d T/d P environment in the arc–backarc system, and metamorphic imprints in the geological record that represent discrete ‘events’ due to changes in plate kinematics or subduction boundary dynamics, or as a result of collision of ridges, arcs or continents with the upper plate at the trench. The concept of ‘paired’ metamorphic belts may be generalized and extended more widely than in the original proposition to subduction-to-collision orogenic systems in addition to accretionary orogenic systems. In this wider application, the term “paired metamorphic belts” may be used for “penecontemporaneous belts of contrasting type of metamorphism that record different apparent thermal gradients, one warmer and the other colder, juxtaposed by plate tectonics processes” (Brown, 2009). This extends the original concept of Miyashiro (1961) beyond the simple pairing of high d T/d P and low d T/d P metamorphic belts in circum-Pacific accretionary orogens, and makes it more useful in the context of our better understanding of the relationship between thermal regimes and tectonic settings. This is particularly useful in subduction-to-collision orogenic systems, where the suture and lower plate materials will register the imprint of low-to-intermediate d T/d P and the upper plate will register penecontemporaneous high d T/d P metamorphism commonly manifested at shallow crustal levels by the occurrence of granites in the rock record.

The ‘microcontinent’ Perunica: status and story 15 years after conception

Abstract Central Europe consists of a complex mosaic of more or less independent terranes with varying tectonometamorphic histories, usually also of different lithological compositions and protolith, and thus it is reasonable to suppose that the majority of these blocks have experienced somewhat different palaeogeographical evolution. The present terrane juxtaposition has been interpreted in general as a result of the Variscan collision of peri-Gondwanan and peri-Baltic derived terranes, with Gondwana on one side and Baltica and/or Laurentia on the other side. However, reconstruction of the pre-Variscan development and mutual palaeogeographical relationships remains a major challenge of interpretation.

Adjacent terranes with ca. 2715 and 2650 Ma high-pressure metamorphic assemblages in the Nuuk region of the North Atlantic Craton, southern West Greenland: Complexities of Neoarchaean collisional orogeny

In the gneiss complex of the Nuuk region of the North Atlantic Craton in southern West Greenland, terrane juxtaposition was followed by Neoarchaean folding under amphibolite facies conditions, with widespread low-pressure recrystallisation (5 kbar and 550–700 °C). The complex metamorphic overprinting requires that the P– T history related to actual terrane assembly has to be extracted from very small relicts of older metamorphic assemblages combined with the U/Pb dating, petrology and geochemistry of metamorphic zircons. In the south of the region, the Færingehavn terrane (Eoarchaean orthogneisses) is tectonically overlain by a supracrustal package of amphibolites and paragneisses (ca. 2840 Ma felsic volcano-sedimentary protoliths). This package is juxtaposed against a higher tectonic level represented by the Tre Brødre terrane (2825 Ma orthogneisses) and the Tasiusarsuaq terrane (2920–2810 Ma orthogneisses). The terranes were assembled by 2710–2720 Ma, as shown by dating of granitic sheets intruded along the terrane boundary mylonites. In the Færingehavn terrane and in the overlying 2840 Ma supracrustal package, relict early high-pressure assemblages (12–8 kbar, 700–750 °C) are clinopyroxene + garnet + plagioclase + quartz ± hornblende in mafic rocks and garnet + kyanite + rutile bearing assemblages in paragneisses. These are commonly replaced by lower pressure assemblages (7–5 kbar) such as cordierite ± sillimanite ± garnet in paragneisses and hornblende + plagioclase + quartz ± garnet in mafic rocks. In situ partial melting took place during both low- and high-pressure regimes. Metamorphic zircon in the high- and low-pressure assemblages yields dates of ca. 2715 Ma, mostly with errors of <±5 Ma, thereby demonstrating rapid decompression at high temperatures. Zircons in the overlying Tre Brødre and Tasiusarsuaq terranes show little response to the ca. 2715 Ma event supporting structural interpretations that they were at a higher structural level at ca. 2715 Ma. High temperature recrystallisation continued after ca. 2715 Ma, as demonstrated by intergrowth of sillimanite with 2680 Ma metamorphic zircon. The Kapisilik terrane (3050–2960 Ma orthogneisses) and another supracrustal assemblage of amphibolites and ca. 2800 Ma quartzo-feldspathic metasedimentary rocks are exposed north of the Færingehavn terrane and within fold cores along its western margin, are bounded by folded Neoarchaean mylonites. The Kapisilik terrane and this supracrustal assemblage include high-pressure metamorphic remnants in amphibolites and metasediments (metamorphic segregations with garnet + clinopyroxene and kyanite, respectively) that formed at ca. 2650 Ma. These remnants are overprinted by high temperature, but lower pressure metamorphic events at ca. 2630, 2610 and 2580 Ma. Thus remnants of early metamorphism reveal mutually exclusive ca. 2715 and 2650 Ma high-pressure events in adjacent tectonically juxtaposed terranes. Repeated high-pressure metamorphism in adjacent terranes is observed in younger complex collisional orogens.

The Ross orogeny of the transantarctic mountains: a northern Victoria Land perspective

Northern Victoria Land (Antarctica) is made up of three terranes of Cambrian–Ordovician rocks: the Wilson (WT), Bowers (BT) and Robertson Bay terranes (RBT). The WT comprises a low- to high-grade metasedimentary sequence intruded by calc-alkaline plutons with magmatic arc affinity; the BT is composed of low-grade metavolcanic and metasedimentary rocks usually interpreted as an intra-oceanic arc; the RBT is a very low-grade flysch-like sequence. Terrane juxtaposition has traditionally been attributed to accretion during the Cambro-ordovician Ross orogeny. We propose a new model in which the WT, BT and RBT are interpreted as an arc/back-arc/trench system, developed in the context of a SW-dipping subduction zone. The subducting plate carried a continent originally located outboard of the turbidite fan of the RBT. Collision between this continent and the East Antarctic craton caused partial subduction of the intervening back-arc basin and, ultimately, the end of Ross-orogenic subduction. The turbidite fan of the RBT originally sedimented above the trench and on the subducting oceanic plate; due to collision it was thrusted on the continent, that constitutes, at least in part, the present basement of the RBT turbidite. The eastern portions of this continental mass were later dissected by the tensile tectonics related to the opening of the of the Southern Ocean.

The40Ar‐39Ar laser analysis of K‐feldspar: Constraints on the uplift history of the Grenville Province in Ontario and New York

A comprehensive geochronologic database describes a period of late extension across the Metasedimentary Belt (MB) of the Grenville Province in northeastern North America. Because extension continued through much of the young unroofing history of the region, these data do not bracket the timing of final extension across all shear zones and do not constrain the timing of final juxtaposition of terranes. Analysis of K‐feldspar in the MB by the40Ar‐39Ar method can give information on the very latest perturbations in the area since K‐feldspars have multiple, low closure temperatures, ranging from about 150° to 300°C. These multiple diffusion domains require temperature‐time modeling to fully interpret argon release spectra. This modeling requires precise knowledge of the temperature of degassing of each step, which usually requires the use of a resistance furnace for sample analysis. We establish a first‐order relationship between laser power and temperature, which can be used for multidiffusion domain modeling of K‐feldspars. Comparison of samples analyzed by both methods reveals virtually no difference between the systems, supporting the validity of K‐feldspar analyses by laser step heating. Our data suggest that using the laser for K‐feldspars can give results that are geologically reasonable, precise, and easier to collect. In this case, these data are then applied to the cooling history of the North American Grenville Province. Our K‐feldspar analyses show that the latest extensional motion along shear zones in the MB is after 900 Ma, and the region is uplifting as a uniform block by 780 Ma.

Assessing trilobite biodiversity change in the Ordovician of the British Isles

AbstractThe Ordovician Period saw the most sustained, steep rise in marine biodiversity in the history of life on Earth and set the ecological pattern for the rest of the Palaeozoic Era. The long history of research, wide variety of depositional settings and juxtaposition of terranes with very different palaeobiogeographical histories makes the Ordovician of the British Isles an excellent laboratory in which to study this change. A database approach to analysis of trilobite biodiversity change is described using a simple compilation of stratigraphical ranges of 663 species of Avalonian trilobites, and a much more sophisticated information‐rich relational database based on the fundamental information of the fossil record–species at localities. The latter includes 2001 occurrence records of 617 trilobite species at 508 localities in the Welsh Basin. The first ever species‐level diversity curves for the whole Ordovician of an entire region are presented. Both databases reveal an overall increase in species‐ and genus‐level diversity through the Ordovician. Random resampling tests and environmental information are used to remove sampling effects from the Welsh Basin diversity curves. The main features of trilobite biodiversity change in Avalonia are the following: a late Arenig–early Llanvirn increase, during which time the highest species‐to‐genus ratios occur, is contemporaneous with a rise in global diversity and rifting of Avalonia from Gondwana; a late Abereiddian dip is followed by recovery during the Llandeilian–early Caradoc; decline during the late Caradoc–early Ashgill is at least partly attributable to lack of preserved rock and restriction of most of the preserved faunas to deep‐water environments. The greatest diversity occurs in the palaeoenvironmentally differentiated Rawtheyan and is followed by the diversity crash seen in the Hirnantian throughout the world. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Rheological controls on Grenvillian intrusive suites: implications for tectonic analysis

Studies of plutonic suites in western Grenville, along with the recent advances in understanding magma ascent and emplacement, and interactions between magma and their country rock, provide insights on the use of intrusive rocks to infer the accretionary history of deeply-exposed Precambrian orogens. In the Grenvillian Central Metasedimentary Belt of Québec, terrane juxtaposition was inferred to be late based on the distribution of 1.08 Ga potassic alkaline plutons. This view was questioned, however, upon recognition that magma ascent by dyke propagation can be stalled to enable the formation of plutons upon intersecting rheologically softer units, for example Grenvillian marble. In this context, preferential ponding of magmas in marble rather than tectonics can control regional distribution of plutons. The spatial association of plutons with marble and their contact relationships are, at the local scale, obscured by transformation of marble to skarn and by mechanical excision of marble from the paragneiss wall-rock sequence. In contrast, plutons and dykes of an older, 1.17 Ga, less alkaline magma association are shown by field and remote-sensing studies to be evenly distributed across the various lithotectonic domains of the belt. Their sheet-like emplacement along the belt boundary constrains Grenvillian tectonic assembly in the region to be early. Contrasting loci, type and degree of deformation of intrusive bodies illustrate that host-rock rheological differences influence the final characteristics of an intrusive suite. Consequently, the nature of the host is an important variable in inferring timing relationships and tectonic scenarios from intrusive suites in high-grade terranes.

Extending a thickened crustal bulge: toward a new geodynamic evolution model of the paleozoic NW Bohemian Massif, German Continental Deep Drilling site (SE Germany)

Fault-bounded (tectonic) metamorphic complexes assembling the NW Bohemian Massif around the German Continental Deep Drilling (KTB) site are seen to be extremely heterogeneous in tectonic and metamorphic histories. In current models, the different complexes were supposed to reflect a puzzle of small pre-Devonian microplates, and the related collision events supposedly lasted until the Carboniferous. Opposed to these models, it will be shown that all the boundaries among the complexes were formed by detachment, late in a prolonged overall geodynamic history of a thickened crustal bulge, during extensional tectonics and associated thermal events that outlasted the onset of collision in the Silurian/Lower Devonian by about 70–80 Ma. (Micro-)structures, petrological and geochronological data of individual complexes predominantly preserve the late stages rather than the unbroken record of their tectonometamorphic histories. Such partial histories strongly different among individual complexes, depict diverse snapshots taken at different places in the evolving thickened crustal bulge and at different instants in its overall evolution, and do not define different precollisional microplates. Predominantly P– T and deformation episodes after terrane juxtaposition are preserved. This article presents an integrated view of the structural geology, microscopic fabrics, P– T data and geochronology of such diverse metamorphic complexes. This integrated view provides a new understanding of (1) the tectonic evolution during Upper Silurian/Devonian collision of the Gondwana-derived Central European lithosphere with Laurussia, (2) the postaccretionary events that lasted through the Upper Carboniferous and (3), the earlier (Lower Ordovician) metamorphic and magmatic history, which is only locally recorded. Metamorphic complexes occupying the structurally highest position (upper tectonic complexes) record Devonian and earlier tectonometamorphic and magmatic events. After the Mid-Devonian, such complexes did not experience any metamorphism. The recorded Devonian events consist of subduction and exhumation of HP-rocks and their exhumation involved thrusting and extensional tectonics. Upper tectonic complexes comprise fragments of both the plate that was subducted in this period and the overriding plate. In the footwall, Carboniferous extension has brought upper tectonic complexes against metamorphic complexes that essentially record Lower and Upper Carboniferous tectonometamorphic and magmatic events (lower tectonic complexes). In the lower tectonic complexes, such events are (1) consecutive extensional stages that created at least three sets of ductile normal detachment systems intersecting each other, and various associated thermal pulses, as well as (2) predetachment events that are only recorded as petrologic and structural relics transposed during extension. Inferred as predetachment events are crustal subduction as well as stacking that outlasted thrusting and exhumation of the upper tectonic complexes. In the deeper portion of the thickened crustal bulge represented by the lower tectonic complexes, spatial variations of P– T– t– d histories during decompression occurred as a result of continued differential exhumation on the three sets of detachment systems, and also from various thermal pulses that have affected different parts of this section during progressive shearing and exhumation to various degrees. Whereas the lower tectonic complexes of the Erzgebirge Gneiss Dome preserve the record of a Lower Carboniferous history of HP-metamorphism and crustal thickening followed by extension, in those of the Oberpfalz region (KTB site), a major Upper Carboniferous thermal pulse mostly erased the pre-Upper Carboniferous strain fabrics and metamorphic record. In the Oberpfalz region, ongoing extension emplaced mid-crustal rocks that pervasively equilibrated in the Upper Carboniferous at high temperatures and low pressures (low P/T ratio) against rocks not exposed to high temperatures at that time. In summary, a prolonged postaccretionary history concealed large-scale structural relationships linked to crustal subduction and thickening associated with former juxtaposition of Gondwana lithosphere to Laurussia. Polyphase crustal extension and strike slip created the variety of different metamorphic tectonic complexes observed in the NW Bohemian Massif.

Terrane amalgamation in the Clew Bay region, west of Ireland

AbstractThe Caledonides of the west of Ireland provide a well-exposed and well-mapped example of an oblique collision zone. The east-northeast trending Deer Park and Achill Beg Fault system is a crustal scale ductile sinistral strike-slip duplex of late Ordovician age, imbricating late Precambrian granulite facies lower crustal rocks, near eclogite facies supracrustal rocks, up to amphibolite facies Dalradian metasedimentary rocks and greenschist facies Cambro-Ordovician rocks. This fault system is correlated with a pre-Devonian component of the Highland Boundary Fault system in southern Scotland. In the Clew Bay area, the high pressure-low temperature facies metamorphic rocks, in tectonic contact with greenschist facies Cambro-Ordovician rocks, are together interpreted as an accretionary prism complex related to northwestward directed subduction. Both of these are allocthonous terrains with respect to the Dalradian terrane to the north (North West Mayo). To the south, the Cambro-Ordovician rocks docked with a probable Dalradian block containing ultramafic intrusives (Deer Park Complex) during the late Ordovician. The Deer Park Complex and South Mayo Trough linked earlier, during the Arenig.Silurian and Lower-Middle Devonian redbed successions sit unconformably on the metamorphic rocks. Deposition and deformation of these cover rocks was controlled by oblique strike-slip movements on the Leek Fault whose strike swings from west-northwest to north-northeast, following earlier basement trends, as it is traced eastwards from Clew Bay. The Leek Fault System may be correlated with the Leannan Fault of northwest Donegal, a splay of the Great Glen Fault system of central Scotland. East of Clew Bay, this sinistral shear generated local dilation on the more northerly trending bend of the Leek Fault. Lower and Middle Old Red Sandstone redbeds were developed here. The west-northwest trend of the Leek Fault in Clew Bay acted as a compressional bend during these sinistral movements and transpressional southwest directed thrusting developed in Silurian rocks. Post-Middle Old Red Sandstone pre-late Tournaisian dextral displacement on the Leek Fault reversed this pattern with transtension in Clew Bay allowing intrusion of small carbonated peridotite bodies into Silurian rocks and easterly directed thrusting of Middle Old Red Sandstone rocks east of the Bay on the transpressional north-south bend.A tectonic model for the region is presented here. This model involves a northwestward directed subduction system, 150 to 750 km of Arenig sinistral strike slip movement, and eastwards insertion of the Connemara block with formation of the Ordovician South Mayo Trough as a pull-apart basin. Subsequently, a further 130 to 650 km eastward displacement of rocks took place south of the Deer Park Fault in later Ordovician times. The magnitudes of these estimates are directly proportional to an assumed maximum wavelength of 1500 km for promontories on the original Laurentian margin, and using the current juxtaposition of terranes, a minimum wavelength of 300 km is inferred.

Application of cladistics to terrane history—parsimony analysis of qualitative geological data

Hypotheses of terrane dispersal or accretion can be represented graphically as branching diagrams (cladograms), but an assessment of competing hypotheses of terrane history requires a method of analysis of supporting evidence which resolves the most parsimonious explanation of all available data. Cladistics is a rigorous analytical method first developed for phylogeny reconstruction (i.e. biological history), but applicable to any hierarchical data set. Given appropriate definitions, the various types of geological, geophysical and biological data used to support hypotheses of fragmentation or fusion history for geological regions (terranes) assumed to have had independent geological histories can be organized hierarchically. Terrane fragmentation is equivalent to phylogenetic splitting of biological taxa, and standard algorithms for parsimony analysis may be directly applied. Terrane accretion may be represented as a coalescing area cladogram, and the supporting evidence also forms a hierarchical data set, but with two main differences. The less general attributes historically precede the more general (the reverse applies in phylogeny reconstruction), and the branching points (nodes on the cladogram), unlike hypothetical common ancestors in phylogeny reconstruction, represent defined geographic areas, with a geological structure which can be investigated. In cladistic reconstruction of evolutionary history the common ancestors are hypothetical, and their attributes can only be inferred from the distribution of attributes amongst the terminals (known biological taxa); in contrast, the end product of terrane accretion is a composite structure (geological province) within which juxtaposition of terranes may eliminate some of the possible historical sequences which led to its formation.

Geochemical, oxygen, and neodymium isotope compositions of metasediments from the Abitibi greenstone belt and Pontiac Subprovince, Canada: Evidence for ancient crust and Archean terrane juxtaposition

The Abitibi greenstone belt (AGB) and Pontiac Subprovince (PS) in the southwestern Superior Province are adjacent greenstone-plutonic and metasedimentary-dominated terranes, respectively, separated by a major fault zone. Metasediments from these two contrasting terranes are compared in terms of major- and trace-element and O- and Nd-isotope compositions, and detrital zircon ages. The following two compositional populations of metasediments are present in the low-grade Abitibi southern volcanic zone: 1. (1) a mafic-element-enriched population (MEP) characterized by flat, depleted REE patterns; enhanced Mg, Cr, Co, Ni, and Sc; low-incompatible-element contents; and minor or absent normalized negative troughs at Nb, Ta, and Ti; and 2. (2) a low-mafic-element population (LMEP) featuring LREE-enriched patterns; enhanced Rb, Cs, Ba, Th, and U contents; and pronounced normalized negative troughs at Nb, Ta, and Ti. These geochemical features are interpreted to indicate that the MEP sediments were derived from an ultramafic- and mafic-dominated oceanic provenance, whereas the LMEP sediments represent mixtures of mafic and felsic arc source rocks. The PS metasediments are essentially indistinguishable from Abitibi LMEP on the basis of majorelement and transition metal abundances, suggesting comparable types of source rocks and degrees of maturity, but are distinct in terms of some trace elements and O-isotope compositions. The Pontiac metasediments are depleted in 18O and enriched in Cs, Ba, Pb, Th, U, Nb, Ta, Hf, Zr, and total REE and also have higher ratios of Rb K , Cs Rb , Ba Rb , Ta Nb , Th La , and Ba La relative to the Abitibi LMEP. Two subtypes of REE patterns have been identified in PS metasediments. Both have moderately fractionated distributions, with La Yb n = 3.9–17 and 7.8–13.6, respectively; but the first has minor Eu anomalies ( Eu Eu ∗ = 0.89–1.16 ), whereas the second features pronounced negative anomalies ( Eu Eu ∗ = 0.52–0.81 ) coupled with high concentrations of incompatible elements such as K, Rb, Th, U, Nb, and Ta. The first subtype is interpreted to be derived from provenances of mixed mafic and felsic volcanic rocks, whereas the Eu-depleted type has geochemical features that are typical of post-Archean sediments or Archean K-rich granites and volcanic equivalents. Abitibi metasediments have ϵ Nd (2.7 Ga) values of −0.71 to +1.83, corresponding to model T CHUR = 2.82−2.55 Ga, with the majority around 2.65 Ga, close to the depositional age of ~2.7 Ga. In contrast, PS metasediments are characterized by ϵ Nd (2.7 Ga) = +0.95 to −3.51, corresponding to T dhur = 2.7 to ~3.1 Ga, consistent with a detrital zircon age spectrum of ~2.69 Ga to ~3.1 Ga. Pontiac Subprovince and Abitibi metasediments have contrasting provenances, suggesting their separate, initial geodynamic settings and later tectonic juxtaposition. Major and trace elements, O- and Nd-isotopic compositions, and detrital zircon ages collectively suggest that some of the PS metasediments were derived mainly from juvenile rocks; whereas others require various degrees of contribution from older (3.0–3.5 Ga) evolved crust mixed with juvenile (~2.7 Ga) rocks. Older components were derived either directly from erosion of an ancient continental arc or from contemporaneous volcanic rocks which may contain recycled older components. If the PS represents in part an accretionary prism to the Abitibi southern volcanic zone, then oblique subduction may have been involved, which allowed the Pontiac Subprovince to juxtapose or sample exotic source rocks.

Early Permian transform margin development of the southern New England Orogen, eastern Australia (eastern Gondwana)

The southern New England orogen, part of the eastern margin of Gondwana, evolved from a zone of high‐angle plate convergence during the Carboniferous, into either a transform margin or a highly oblique‐convergent margin by the Early Permian. Fault‐bounded, narrow, elongate, rapidly filled sedimentary basins that developed along the Peel‐Manning fault system in the Early Permian provide support for this hypothesis. They are similar to sedimentary basins developed along strike‐slip fault zones worldwide. Strike‐slip dislocations provide the best explanation for juxtaposition of terranes with distinctly different geological histories and discontinuities in regional geological trends across these basins. A close spatial relationship also exists between occurrences of serpentinite‐matrix ophiolitic melange, the Peel‐Manning fault system, and related secondary faults. Early Permian transtension may have facilitated the diapiric rise of serpentinites, which originated from an older underlying ultramafic precursor, up these fault zones. Rise and Early Permian emplacement of the Bundarra Plutonic Suite, an S‐type batholith across which regional geological trends are offset, is also best accommodated in a model that involves a transtensional regime.

Tectonostratigraphic terranes within Archaean gneiss complexes: examples from Western Australia and southern West Greenland

New field work and isotopic data show that the Godthabsfjord region of West Greenland consists of a collage of tectonostratigraphic terranes, which evolved separately prior to tectonic juxtaposition in the late Archaean. In Western Australia the Narryer Gneiss Complex, which lies on the northwestern margin of the Yilgarn Craton, is, unlike the Godthabsfjord region, very poorly exposed (less than 1 % ). In consequence it is impossible to follow geological boundaries in this complex, and instead the complex has been studied by a very extensive use of within-grain zircon U-Pb geochronology on the ion microprobe SHRIMP. The zircon geochronology suggests that the Narryer Gneiss Complex also consists of several discrete terranes of early to mid Archaean gneisses. In both the Godthabsfjord region and the Narryer Gneiss Complex, late Archaean juxtaposition of terranes was accompanied by intrusion of crustally­derived granites, deformation, and amphibolite facies metamorphism. Thus some Archaean high grade gneiss complexes consist of terranes that underwent independent evolution until they were brought together at a later time. In this respect their anatomy resembles post-Archaean orogenic belts that formed as a consequence of plate tectonic processes.

Middle Paleocene terrane juxtaposition along the Median Tectonic Line, southwest Japan: Evidence from 40Ar/ 39Ar mineral ages

The Median Tectonic Line (MTL) is a regional transcurrent fault separating anadalusite-sillimanite-type metamorphic rocks and granitic rocks within the Ryoke terrane from glaucophanic-type metamorphic rocks within the Sambagawa terrane in southwest Japan. The late Cretaceous Izumi Group unconformably overlies Ryoke metamorphic rocks and is tectonically separated from the Sambagawa terrane by the MTL. Recent 40Ar/ 39Ar mineral ages reported for the Sambagawa terrane in this area suggest extensive Late Cretaceous uplift; however, no Sambagawa erosional debris has been described from the Izumi Group. 40Ar/ 39Ar ages have been determined for hornblende and biotite separated from samples systematically collected along a traverse from the Ryoke terrane into the MTL (Kashio area, Chubu district). In addition whole-rock samples of protomylonite within the MTL have been analyzed. Similar 68–70 Ma isotope correlation ages are recorded by hornblende within massive (Ikuta) and foliated (Minakata) granite and protomylonite within the MTL. Biotite from these samples records ages ranging between 66 and 68 Ma. Together the mineral ages suggest relatively rapid post-magmatic cooling through appropriate argon closure temperatures. These also indicate that temperatures maintained during protomylonite formation within the MTL were less than those required to effect even partial rejuvenation of intracrystalline argon systems within hornblende. Protomylonite samples yielded well-defined whole-rock plateau ages of 62–63 Ma which are interpreted to closely date the last phase of ductile flow within the MTL. These results suggest that a significant ductile phase of movement occurred within the MTL in the Middle Paleocene. This may have been associated with significant sinistral displacement and resultant tectonic juxtaposition of previously separated portions of the Sambagawa and Ryoke belts (with unconformably overlying Izumi Group).

Subduction tectonic erosion leading to fore-arc (imbricate) collapse: A major process with implications for terrane juxtaposition

Subduction tectonic erosion leading to fore-arc (imbricate) collapse: A major process with implications for terrane juxtaposition

Thermal neutron dosimetry for fission track dating with metal activation monitors: A reinvestigation

The modern plate tectonics regime is characterized by a duality of thermal environments, one representing the subduction zone and the other representing the arc–backarc or orogenic hinterland. This duality is the hallmark of one-sided (asymmetric) subduction, and the characteristic imprint of one-sided subduction in the geological record is predicted to be the broadly contemporaneous occurrence of two contrasting types of metamorphic belt, one of high dT/dP type and the other low dT/dP type. The broadly contemporaneous occurrence of granulite and ultrahigh-temperature metamorphism with eclogite–high-pressure granulite metamorphism in the geological record since the Neoarchean Era is evidence of dual thermal environments and indicates that subduction has operated on Earth since that time. Classic ‘paired’ metamorphic belts in which an inboard high dT/dP metamorphic belt is juxtaposed against an outboard low dT/dP metamorphic belt along a tectonic contact—such as the Ryoke and Sanbagawa belts in Japan—are found in Phanerozoic accretionary orogens of the circum-Pacific. Generally, they appear to result from juxtaposition of terranes with different metamorphic facies series that may or may not be exactly contemporaneous and that may or may not be far-traveled. This is a consequence of the difference between globally-continuous subduction, generating a low-to-intermediate dT/dP environment in the subduction zone and a high dT/dP environment in the arc–backarc system, and metamorphic imprints in the geological record that represent discrete ‘events’ due to changes in plate kinematics or subduction boundary dynamics, or as a result of collision of ridges, arcs or continents with the upper plate at the trench. The concept of ‘paired’ metamorphic belts may be generalized and extended more widely than in the original proposition to subduction-to-collision orogenic systems in addition to accretionary orogenic systems. In this wider application, the term “paired metamorphic belts” may be used for “penecontemporaneous belts of contrasting type of metamorphism that record different apparent thermal gradients, one warmer and the other colder, juxtaposed by plate tectonics processes” (Brown, 2009). This extends the original concept of Miyashiro (1961) beyond the simple pairing of high dT/dP and low dT/dP metamorphic belts in circum-Pacific accretionary orogens, and makes it more useful in the context of our better understanding of the relationship between thermal regimes and tectonic settings. This is particularly useful in subduction-to-collision orogenic systems, where the suture and lower plate materials will register the imprint of low-to-intermediate dT/dP and the upper plate will register penecontemporaneous high dT/dP metamorphism commonly manifested at shallow crustal levels by the occurrence of granites in the rock record.

Sr and Nd isotope systematics of polymetamorphic Archean gneisses from southern West Greenland and northern Labrador

Sr and Nd isotopic data for middle to late Archean polymetamorphic felsic gneisses from localities in the Nuuk area, West Greenland, are compared and contrasted with new isotopic results for early Archean Amîtsoq gneisses and with data for isotopically reworked Kiyuktok gneisses from the Saglek area, Labrador. Sr isotopic data for individual suites of felsic gneisses record the time-integrated effect of variable Rb–Sr fractionation during prograde and retrograde events as well as the effect of source inhomogeneity.Contrasting petrologic and Sr–Nd isotopic characteristics are the result of differences in level of exposure, caused partially by juxtaposition of terranes of different metamorphic character by movement on ductile shear zones and post-shearing folding deformation. Sm–Nd systematics of felsic gneisses from Nordafar, Ikerasakitsup akornga, Tinissaq, and Kangimut sammisoq – Qasigianguit define a geologically meaningless ca. 3280 Ma Nd "isochron", which is the result of mixing of samples from unrelated suites and the effect of open-system behaviour. Gneisses lying on this pseudoisochron were variably affected by ca. 2800–2900 Ma prograde granulite-facies metamorphism and structurally controlled retrogression under amphibolite- to greenschist-facies conditions.The study shows that Sr–Nd isotope systematics of geologically identifiable units may be modified by open-system behaviour during prograde and retrograde metamorphism. Isotopic data from gneiss complexes metamorphosed under granulite-facies conditions may therefore yield equivocal information concerning isochron interpretation, significance of model ages, and estimates of crustal residence time.