Dispersal Of Terranes Research Articles

The southern Ribeira Belt in Western Gondwana: a record of a long-lived continental margin and terrane collage

The Ribeira Belt is an ENE-trending orogen in southeastern South America associated with the Neoproterozoic and Early Cambrian assembly of Western Gondwana. Its final configuration results from the amalgamation of several Statherian to Neoproterozoic continental margin tectonostratigraphic terranes. This contribution provides a revision of the southern Ribeira Belt, encompassing the São Roque, Apiaí, Embu, Costeiro, Curitiba, and Paranaguá terranes, bounded by branches of long-lived Tonian–Ediacaran shear zone systems. An initial Statherian rifting (1750 Ma) associated with anorogenic magmatism and sedimentation is followed by Calymmian to Ectasian (1600–1200 Ma) large carbonate platforms as well turbiditic volcanosedimentary successions, including both passive and active continental margins, and possible island arc collision, making up the Apiaí and São Roque composite terranes. The Tonian is represented by the Embu Terrane accretion and orogenesis at 850–760 Ma. The Brasiliano orogeny in the terranes north of the Lancinha–Cubatão shear zone is mainly represented by 630–600 Ma granitic magmatism associated with a Barrovian metamorphism, related to a tectonic scenario evolving from subduction to oblique collisional and final post-collisional stabilization. Terranes south of the Lancinha-Cubatão shear zone display evidence of an early Ediacaran suture zone, including a high-grade complex, ultramafic suites, a paired low-P and high-P belt corresponding to a back-arc basin, and an accretionary wedge, with a metamorphic collisional peak between 590 and 585 Ma. Terrane dispersion along strike-slip shear zones occurred after 590 Ma, resulting in the final configuration of the southern Ribeira Belt, associated with anorogenic A-type granitic magmatism and pull-apart continental sedimentary and volcanic basins. The deformation in this final period may reflect a far-field stress related to continental collisions that proceed south and southeast as the docking of the Costeiro (Oriental) and Paranaguá terranes, and later the Cabo Frio Terrane collision, as well the southernmost Dom Feliciano and Kaoko final collisional process, until the upper Cambrian (500–490 Ma).

A detective duo of apatite and zircon geochronology for East Avalonia, Johnston Complex, Wales

The Johnston Complex represents a rare inlier of the Neoproterozoic basement of southern Britain and offers a window into the tectonomagmatic regime of East Avalonia during the assembly of Gondwana. This work presents in situ zircon (U–Pb, Lu–Hf), apatite (U–Pb), and trace element chemistry for both minerals from the complex. Zircon and apatite yield a coeval crystallization age of 570 ± 3 Ma, and a minor antecrystic zircon core component is identified at 615 ± 11 Ma. Zircon Hf data imply a broadly chondritic source, comparable to Nd data from East Avalonia, and T DM 2 model ages of c. 1.5 Ga indicate source extraction during the Mesoproterozoic. Zircon trace element chemistry is consistent with an ensialic calc-alkaline continental arc setting and demonstrates that magmatism was ongoing prior to terrane dispersal at 570 Ma. Apatite trace element chemistry implies a sedimentary component within the melt consistent with voluminous S-type granite production during the formation of Gondwana. The similarity of the ɛHf and geochemistry between both zircon age populations suggest derivation from a uniform source that did not undergo significant modification between 615–570 Ma. Time-constrained apatite–zircon chemistry addresses complexities in dating S-type granitoids (zircon inheritance) and permits inferences on post-magmatic thermal histories. Supplementary material: Zircon U–Pb, Lu–Hf and trace element data, and apatite U–Pb and trace element data are available at https://doi.org/10.6084/m9.figshare.c.6484464

DEFORMATION HISTORY AND CORRELATION OF PAIKON AND TZENA TERRANES (AXIOS ZONE, CENTRAL MACEDONIA, GREECE)

Paikon and Tzena terranes are situated in the centre part of Axios zone, between Almopia and Paionia ophiolitic belts. Tectonostratigraphic data reveal that both have been affected by the same polyphase deformation and metamorphism, as well that they have the same lithostratigraphic column. The first deformation phase took place during the Middle to Late Jurassic and is associated with ophiolite obduction, nappe- stacking, terrane accretion and crustal thickening (D1). Metamorphism does not exceed greenschist facies (M1). Relict HP-LT metamorphic assemblages predating M1 metamorphism are possibly developed during subduction processes and overloading of the obducted ophiolites on the continental margin, characterized the initial stages of deformation. Compressional tectonics and intense thrusting with the same kinematics continued in Lower Cretaceous time, affected all pre-Upper Cretaceous units and the obducted ophiolites (D2). This phase is associated with low-greenschist metamorphism (M2). The first main extensional event occurs in the Late Cretaceous, related to basin formation and sedimentation (D3). During Paleocene to Eocene, D4 intense imbrication of all tectonic units towards mainly SW takes place again. Nappes collapse and finally crustal exhumation taken place during Oligocene to Miocene, associated with low - angle normal faults, with a main top to the SW sense of movement (D5). In Miocene to recent times, high - angle normal and strike-slip faults are formed in an extensional to transtensional strain regime (D6), associated with Neogene to Quaternary basin formation and terrane dispersion. The basement rocks of both terranes are of Pelagonian origin, exhumed as a multiple tectonic window.

Ediacaran high-pressure collision metamorphism and tectonics of the southern Ribeira Belt (SE Brazil): Evidence for terrane accretion and dispersion during Gondwana assembly

The Curitiba Terrane represents a major segment of the southern Ribeira Belt (SE Brazil), which was derived from the collision between the São Francisco, Congo, Paranapanema and Luís Alves Cratons during the Neoproterozoic (Brasiliano/Pan-African Orogeny). The tectonic setting and the metamorphic records of two major juxtaposed units from the Curitiba Terrane, a Neoproterozoic shallow continental-shelf metasedimentary assemblage (Turvo-Cajati Formation) and an Archaean to Paleoproterozoic TTG-type orthogneiss assemblage (Atuba Complex), were investigated. Migmatitic paragneisses from the Turvo-Cajati Formation underwent a deep collision metamorphism. Conventional geothermobarometry and petrological modelling in the system NCKFMASHTi indicate peak metamorphic conditions between 670 and 810 °C at 9.5–12 kbar. Metamorphic paths calculated from zoned garnet and plagioclase using the Gibbs method of differential thermodynamics indicate distinct evolution for two major groups of migmatites from the Turvo-Cajati Formation: (i) kyanite migmatites evolved from low-temperature eclogite to high-pressure granulite facies conditions following near isobaric heating; (ii) sillimanite migmatites underwent near isothermal decompression and apparently evolved from high-temperature eclogite facies conditions. Chemical dating of monazite indicates that the peak metamorphism of the Turvo-Cajati Formation occurred at 589 ± 12 Ma, followed by a greenschist facies metamorphic overprint at 579 ± 8 Ma related with late transcurrent shear zones. 40Ar– 39Ar biotite ages indicate that the Turvo-Cajati Formation cooled below 250–300 °C at 555 ± 4 Ma. P– T data and petrological evidence of rocks from the Atuba Complex suggest a retrograde metamorphic path with cooling from 730 to 630–650 °C at 6–7 kbar. Available K–Ar and 40Ar– 39Ar data indicate that the Atuba Complex had cooled to below 300–500 °C between ca. 590 and 580 Ma. Geochronological data indicate that the main metamorphism of the Turvo-Cajati Formation and the Atuba Complex are coeval, but very contrasting metamorphic signatures reflect formation in different parts of a collisional suture. The integration of structural and petrological data indicates that the structural pattern of the Curitiba Terrane is related to Ediacaran westward directioned nappes during the late- to postmetamorphic period. This is concomitant with a main, crustal-scale, strike-slip regime, dominant throughout the Ribeira Belt. The nappe stack was later deformed by cylindrical folds with E–W trending sub-horizontal axes parallel to the synthrusting stretching lineation and was dismembered and dispersed by late sinistral strike-slip shear zones. The late tectonic assembly of the Ribeira Belt was controlled by significant postcollision terrane dispersion along major strike-slip shear zones.

Role of the Ollo de Sapo massive felsic volcanism of NW Iberia in the Early Ordovician dynamics of northern Gondwana

The Ollo de Sapo Fm. in the Sanabria region of NW Iberia consists of subvolcanic, volcanic and volcano-sedimentary rocks underlying and partly intercalated with Early Ordovician, platform-facies sediments. Granitic plutons of the same age, presently transformed into orthogneisses, are genetically related. The metavolcanic rocks are dacites and rhyolites with potassic and peraluminous character, and volcanic arc signature, possibly inherited from the melting of calc-alkaline crustal material. In the study area, the Ollo de Sapo Fm. is made up of two large superposed volcanic domes separated by a thin screen of contemporaneous metapelites and sandstones. The domes underwent endogenous growth with subvolcanic, phenocryst-rich magma intruding and cooling under a volcanic and volcaniclastic shield. The western dome tapers to the east and underlies the younger eastern dome, suggesting that the whole formation consists of several volcanic edifices replacing each other along the 570 km outcrop belt of the Ollo de Sapo Fm. Two new U–Pb protolith ages of 488 ± 6 Ma and 472 ± 12 Ma match, within error, the known age span for the Ollo de Sapo Fm. in Iberia (495–483 Ma). The younger age dates an augen gneiss at the core of the eastern volcanic dome, whereas the older date comes from the overlying volcanic sequence, suggesting intrusion of the augen gneiss protolith into the overlying volcanic rocks. An incipient rift involving necking of crustal-scale boudinage a few hundred km landward of the edge, but still within the wide passive margin of northern Gondwana may explain the linear distribution of the Ollo de Sapo Fm. and the limited time span of massive volcanism. The heat for crustal melting was probably supplied by mafic magmas intruding or underplatting the lower crust. The Early Ordovician felsic magmatism of NW and Central Iberia seems to form part of a large siliceous province linked to break-up and terrane dispersion of the northern Gondwana realm.

Ordovician–earliest Silurian rift tholeiites in the Acatlán Complex, southern Mexico: Evidence of rifting on the southern margin of the Rheic Ocean

The Acatlán Complex of southern Mexico is a vestige of a Paleozoic Ocean inferred to be either the Cambro-Ordovician Iapetus and/or the Ordovician-Carboniferous Rheic oceans. Ordovician granitoids in the complex have been interpreted as either the products of dehydration melting, arc or rift magmatism, however, the geochemistry of felsic rocks is inconclusive. The geochemistry of a recently discovered, major, Ordovician–earliest Silurian mafic igneous suite associated with these granitoids is critical to defining the tectonic setting of the igneous event, and to paleogeographic reconstructions. Such data from three areas in the Acatlán Complex (Xayacatlán, Patlanoaya, and Cuaulote) document tholeiitic, within-plate characteristics with a source in either primitive mantle or mantle previously modified by subduction-related magmatism possibly combined with crustal contamination. This, combined with their occurrence as a dike swarm intruding rift-passive margin clastic sedimentary rocks, indicates rifting of a continental margin. Mafic dikes at Xayacatlán yielded a concordant U–Pb TIMS zircon age of 442 ± 1 Ma and a 40Ar/ 39Ar hornblende plateau age of 434 ± 3 Ma. The age of mafic magmatism at other localities is defined by the ages of associated granitoids intruded at ca. 461 Ma and by the age of the youngest detrital zircon in the host rocks: 496 ± 25 Ma at Patlanoaya. Previously published age data suggest that this igneous event may have started earlier at 478 ± 5 Ma (Early Ordovician). Although the life spans of the Iapetus and Rheic oceans overlap during the Ordovician, subduction and collision characterize the former, whereas the latter was in a rift–drift mode. Thus, this Ordovician–earliest Silurian magmatism is more consistent with rifting along the southern margin of the Rheic Ocean. Initiation of rifting at ca. 480 Ma is contemporaneous with separation of peri-Gondwanan terranes, such as Avalonia and Carolinia, from Amazonia–Oaxaquia. Subsequent rifting may be analogous to Mesozoic–Cenozoic western North America where transtensional deformation is related to dispersal of terranes (e.g. Baja California and Cretaceous Baja British Columbia) leading to a gradual collapse of the inboard continental margin.

Zircon Inheritance Reveals Exceptionally Fast Crustal Magma Generation Processes in Central Iberia during the Cambro-Ordovician

The Variscan basement of the Central Iberian Zone contains abundant Cambro-Ordovician calc-alkaline to peraluminous metagranites and metavolcanic rocks with two notable features: first, they were apparently produced with no connection to any major tectonic or metamorphic event; second, they have an unusually high zircon inheritance. U–Pb dating combined with cathodoluminescence imaging reveals that about 70–80%, in some samples nearer 100%, of the zircon grains contain inherited pre-magmatic cores, despite the temperature reached by the magmas (about 900°C, calculated using the Ti-in-zircon thermometer) being high enough to dissolve all the available zircon (from the rock's zircon saturation temperature, 770–860°C). The fact that the dissolution of zircon was so incomplete can only be attributed to the kinetics of heat transfer to and from the magmas. Three-dimensional modeling of zircon dissolution behavior in melts with a composition similar to the Iberian Cambro-Ordovician magmas indicates that the survival of zircons from the suggested late Pan-African protolith would be possible only if melt production was rapid, specifically less than 104 years, and probably about 2 × 103 years, from the beginning of melting (700°C) to the thermal peak (900°C). Melt production was followed by fast magma transfer to upper crustal levels resulting either in surface eruption or in the emplacement of small (< 400 m thick) sills or laccoliths. We suggest that these elevated rates of crustal melting could only have been caused by intrusion of mantle-derived mafic magmas, most probably at the base of the crust. This scenario is consistent with a rifting regime in which crust and mantle were mechanically decoupled; this would explain the scarcity of contemporaneous crustal deformation. Furthermore, fast melting rates in the lower crust followed by fast melt transportation to the upper crust could also explain the lack of contemporaneous metamorphism. The speed of the partial melting process resulted in the production of felsic magmas that inherited the geochemical characteristics of their granitoid crustal protolith. This explains the apparent contradiction between the calc-alkaline to peraluminous geochemical characteristics of the magmas and the inferred extensional (i.e. rift-related) tectonic setting. Our model is compatible with the hypothesis of fragmentation and dispersal of terranes from the northern margin of Gondwana that led to the opening of the Rheic and Galicia–South Brittany oceans and, ultimately, caused the detachment of the Iberian microplate from Armorica and Gondwana during the early Paleozoic.

Tectonic correlations of pre-Mesozoic crust from the northern termination of the Colombian Andes, Caribbean region

Reconnaissance zircon U/Pb SHRIMP, Ar–Ar, and Sm–Nd geochronology, petrological, and geochemical data were obtained from selected localities of two pre-Mesozoic metamorphic belts from the northern termination of the Colombian Andes in the Caribbean region. The older Proterozoic belt, with protoliths formed in a rift- or backarc-related environment, was metamorphosed at 6–8 kb and 760–810 °C during Late Mesoproterozoic times. This belt correlates with other high-grade metamorphic domains of the Andean realm that formed a Grenvillian-related collisional belt linked to the formation of Rodinia. The younger belt was formed over a continental arc at <530–450 Ma in a Gondwanide position and metamorphosed at 5–8 kb and 500–550 °C, probably during the Late Paleozoic–Triassic, as part of the terranes that docked with northwestern South America during the formation of Pangea. A Mesozoic Ar–Ar tectonothermal evolution can be related to regional magmatic events, whereas Late Cretaceous–Paleocene structural trends are related to the accretion of the allocthonous Caribbean subduction metamorphic belts. Lithotectonic correlations with other circum-Caribbean and southern North American pre-Jurassic domains show the existence of different terrane dispersal patterns that can be related to Pangea’s breakup and Caribbean tectonics.

Early Palaeozoic rift‐related magmatism in Variscan Europe: fragmentation of the Armorican Terrane Assemblage

Early Palaeozoic bimodal rift‐related magmatism is widespread throughout much of the Variscides of Europe. It is traceable from the Polish Sudetes to NW Iberia. Granitic plutonism generally predates Cambro–Ordovician bimodal magmatism. In the N Bohemian Massif this early Palaeozoic granitic plutonism was generated by partial melting of Cadomian basement, whereas contemporaneous alkali granites with a mantle component are typical of the NW Iberian Massif. Silurian‐Devonian mafic magmatism in the N Bohemian Massif, Massif Central and NW Iberian Massif is partly preserved as obducted ophiolites. Compositional diversity displayed by Cambro‐Ordovician mafic magmatism can be accounted for by interaction between a spreading centre and an upwelling mantle plume. This indicates that combined tensional forces and mantle plume convection assisted the early Palaeozoic dispersal of terranes from the N Gondwana margin. Continued fragmentation resulted in development of an archipelago of related terranes separated by a network of seaways and formation of oceanic crust.

Fault reactivation within Avalonia: plate margin to continental interior deformation

Lithotectonic terranes commonly have faults with movement histories that reflect their original tectonic setting and their subsequent re-activation during terrane accretion and post-accretionary dispersal. Since later movements tend to overprint evidence of earlier motions, documenting fault reactivation can be a difficult task. Avalonia, the largest terrane within the Canadian Appalachians preserves evidence for repeated episodes of movement along NE-trending fault zones in a variety of tectonic settings between the late Neoproterozoic and late Paleozoic. Evidence of Neoproterozoic motion is preserved in pre-final crystallization deformation fabrics in arc-related igneous complexes which intrude shear zones. These record strike-slip motion related to oblique subduction along the continental margin of Gondwana. In the Paleozoic, Avalonia migrated from its original peri-Gondwanan setting. Mid-Ordovician to earliest Silurian deformation and magmatism is attributed to the sinistral accretion of Avalonia to Laurentia. This reactivated Neoproterozoic shear zones and resulted in basin inversion in mainland Nova Scotia and the formation of mylonites and injection of dikes into brittle fractures in southern New Brunswick. Sinistral motion was accompanied by terrane dispersal and was followed in the Late Silurian and Early Devonian by further reactivation in the form of dextral strike-slip that reflects convergence between Laurentia and Gondwana. Following the accretion and dispersal of Avalonia, the fault zones became stranded within the continental interior. Subsequently, the fault zones accommodated local stresses which were far-field responses to collisional tectonics associated with the mid-Late Devonian Acadian and the Late Carboniferous Alleghanian orogenies. Hence, the generation of these fault systems along plate margins in the late Neoproterozoic and early Paleozoic profoundly influenced the tectonic evolution of late Paleozoic intra-continental deformation. Late Carboniferous deformation is recorded in (a) the mylonitic and cataclastic fabrics of the shear zones themselves, (b) the orientations of spatially related fold structures, (c) local controls on basin formation and sedimentary facies and (d) offsets in stratigraphy. This motion was predominantly dextral and records continued convergence between Laurentia and Gondwana during the amalgamation of Pangea.

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.

Early vertebrates andpaleogeographic models

Studies on the distribution of early vertebrates indicate high endemism in the Early Devonian, and continental connection between West Gondwana and Euramerica in the Frasnian, and between East Gondwana and Asia in the late Famennian. The absence of a northern equivalent to the Siluro-Devonian Malvinokaffric Province is consistent with plate tectonics, but inconsistent with the Expanding Earth hypothesis. A detailed palaeogeography of land and sea areas based on analysis of sealevel, climate, tectonism, and other data provided by the rock record, is necessary for biogeography. Cladistic methods used in the phylogenetic analysis of early vertebrates can be adapted to accommodate various qualitative data sets ( e.g., paleobiogeographic, paleoclimatic, paleogeographic information) relevant to the history of continental or terrane dispersion and accretion, permitting true data integration and parsimony analysis of competing palaeogeographic hypotheses.

Structural and 40Ar/39Ar mineral age constraints for the tectonothermal evolution of the Green Head Group and Brookville Gneiss, southern New Brunswick, Canada: Implications for the configuration of the Avalon composite terrane

AbstractIn the late Precambrian Avalon composite terrane of the Canadian Appalachians, the local juxtaposition of Avalonian successions against gneiss complex–platformal metasedimentary rock associations of uncertain relationship to the Avalonian overstep sequence has raised important questions about the configuration of the composite terrane. Typical of this relationship is the juxtapostion of Avalonian arc‐related packages (Caledonia assemblage) with the migmatitic Brookville Gneiss and metacarbonate–quartzite Green Head Group (Brookville assemblage) along the Caledonia Fault in southern New Brunswick. Polyphase deformation of the predominantly greenschist facies Green Head Group accompanied development of a regional ductile shear zone that separates the group from the amphibolite facies Brookville Gneiss. Heterogeneous ductile flow in carbonate rocks and the development of a regional foliation was followed by NW‐directed shortening and the local development of a penetrative axial planar fabric that intensifies towards the shear zone. Associated structural elements suggest regional dextral transpression, consistent with the metamorphic contrast across the shear zone. Steeply plunging east–west folds may record younger, sinistral movement on associated NE–SW faults. Deformation coincident with metamorphic culmination in the Brookville Gneiss produced a gneissic foliation that was later deformed to produce widespread minor folds of sheath‐like geometry. These folds are best developed proximal to the shear zone where they locally document dextral shear, and probably include several generations that overlap early phases of deformation of the Green Head Group. Kinematic indicators within the gneiss are predominantly dextral.36Ar/40Ar versus 39Ar/40Ar isotope‐correlation ages recorded by metamorphic hornblende suggest that regional cooling of the Brookville Gneiss through ca. 500°C occurred at ca. 540 Ma, providing a minimum age for initial deformation and concomitant metamorphic culmination in the gneiss. 40Ar/39Ar plateau ages for metamorphic muscovite suggest cooling through ca. 375°C at ca. 500–520 Ma, providing a minimum age for progressive deformation in both lithotectonic sequences. Low temperature age discordance in the muscovite spectra suggest partial rejuvenation in the mid‐ and late Palaeozoic.Protracted Cambrian tectonothermal activity in the Brookville assemblage contrasts with the Avalonian tectonostratigraphic record of the Caledonia assemblage in which late Precambrian arc‐related packages are overstepped by Cambrian–Ordovician shallow marine strata. Significant Cambrian separation between the two assemblages is therefore suggested, despite Precambrian similarities in their tectonothermal evolution. Separation as a consequence of terrane dispersal is suggested, and may imply a significant rearrangement of the Avalon composite terrane at this time. Final juxtaposition of the two assemblages pre‐dates their shared late Palaeozoic rejuvenation, and may correspond to an earlier, mid‐Palaeozoic thermal overprint correlated with tectonothermal activity accompanying accretion of the Avalon and outboard Meguma terranes to more inboard tectonic elements of the northern Appalachians.

Lower-crustal deformation during terrane dispersion along strike-slip faults

After accretion, terranes caught between obliquely converging plates are often displaced several hundreds of kilometers. How this motion is accommodated at depth is not well known. Spurred by our Trans-Alaskan Crustal Transect (TACT) seismic studies of the Yukon-Tanana terrane, Alaska, I propose that the lower crust responds ductilely to accommodate differential movement between upper crust and upper mantle during margin-parallel translation. The locations of these ductile regions are dependent on temperature, composition and strain rate. For geotherms based on moderate surface heat flow (60–90 mWm −2) and reasonable geological strain rates (100% in 3 Ma), typical lower-crustal rocks can deform ductilely. The resultant subhorizontal layered fabric should be detectable with seismic techniques. Two terranes that have experienced hundreds of kilometers of margin-parallel translation, the Yukon-Tanana terrane sampled by our seismic refraction/wide-angle reflection profiles and the Salinian block (California) sampled by both seismic refraction and reflection, have reflective lower crust. Both of these terranes have moderate to high surface heat flows, have relatively thin crust, and are interpreted seismically to be underlain by a quartzo-feldspathic lower crust; conditions favorable for ductile deformation in the lower crust. Interpretations of lower-crustal layering for these two terranes is based on an increase in lower-crustal reflectivity. This layering is attributed to tectonic banding consistent with ductile deformation.

Contrasting metamorphic terranes in northwest Wales

Abstract The oldest rocks exposed in northwest Wales crop out as a series of fault-bounded slices. Whereas some of these fault-bounded units are clearly part of the same stratigraphic package (e.g. those belonging to the Monian Supergroup), others show little or no affinity with contiguous areas. Three suspect terranes are identified on Anglesey: (1) the Monian Supergroup; (2) the gneisses and granitic rocks of central Anglesey; (3) the Monian blueschists. The metamorphic histories of these terranes are markedly different and PT conditions vary from anchizonal/lower greenschist (1) to sillimanite grade (2) to epidote- or lawsonite-bearing blueschists (3). Each terrane has been juxtaposed by faults, the earliest movements along which (where not obscured by brittle reactivation) can often be shown to have been ductile and accompanied by dynamic recrystallization. The most prominent of these ductile fault boundaries occur between terranes 1 and 3 in Anglesey (Berw Shear Zone), and between terrane 1 and the Sarn Complex (terrane 4) in the Llŷn peninsula (Llŷn Shear Zone). A similar ductile mylonite zone exists between the Cullenstown Group (terrane 1?) and the Rosslare gneisses (terrane 2?) in SE Eire. All these shear zones are steep, and kinematic indicators indicate oblique sinistral shear. Radiometric, lithostratigraphic and palaeontological data constrain the ductile fault movement to c. 550–540 Ma. This latter event is interpreted as recording an important phase of terrane dispersion and subsequent assembly that occurred along the Avalonian margin immediately prior to marine transgression over southern Britain. It is likely that this transcurrent event is recorded inboard from the Monian area by shear zones within the late Precambrian (Avalonian/Cadomian) basement in southern Britain and northwest France.

Metallogeny and tectonic development of the Tasman Fold Belt System in New South Wales

The northwestern corner of New South Wales consists of the paratectonic Late Proterozoic to Early Cambrian Adelaide Fold Belt and older rocks, which represent basement inliers in this fold belt. The rest of the state is built by the composite Late Proterozoic to Triassic Tasman Fold Belt System or Tasmanides. In New South Wales the Tasman Fold Belt System includes three fold belts: (1) the Late Proterozoic to Early Palaeozoic Kanmantoo Fold Belt; (2) the Early to Middle Palaeozoic Lachlan Fold Belt; and (3) the Early Palaeozoic to Triassic New England Fold Belt. The Late Palaeozoic to Triassic Sydney—Bowen Basin represents the foredeep of the New England Fold Belt. The Tasmanides developed in an active plate margin setting through the interaction of East Gondwanaland with the Ur-(Precambrian) and Palaeo-Pacific plates. The Tasmanides are characterized by a polyphase terrane accretion history: during the Late Proterozoic to Triassic the Tasmanides experienced three major episodes of terrane dispersal (Late Proterozoic—Cambrian, Silurian—Devonian, and Late Carboniferous—Permian) and six terrane accretionary events (Cambrian—Ordovician, Late Ordovician—Early Silurian, Middle Devonian, Carboniferous, Middle-Late Permian, and Triassic). The individual fold belts resulted from one or more accretionary events. The Kanmantoo Fold Belt has a very restricted range of mineralization and is characterized by stratabound copper deposits, whereas the Lachlan and New England Fold Belts have a great variety of metallogenic environments associated with both accretionary and dispersive tectonic episodes. The earliest deposits in the Lachlan Fold Belt are stratabound Cu and Mn deposits of Cambro-Ordovician age. In the Ordovician Cu deposits were formed in a volcanic are. In the Silurian porphyry CuAu deposits were formed during the late stages of development of the same volcanic are. Post-accretionary porphyry CuAu deposits were emplaced in the Early Devonian on the sites of the accreted volcanic arc. In the Middle to Late Silurian and Early Devonian a large number of base metal deposits originated as a result of rifting and felsic volcanism. In the Silurian and Early Devonian numerous SnW, Mo and base metal—Au granitoid related deposits were formed. A younger group of MoW and Sn deposits resulted from Early—Middle Carboniferous granitic plutonism in the eastern part of the Lachlan Fold Belt. In the Middle Devonian epithermal Au was associated with rifting and bimodal volcanism in the extreme eastern part of the Lachlan Fold Belt. In the New England Fold Belt pre-accretionary deposits comprise stratabound Cu and Mn deposits (pre-Early Devonian): stratabound Cu and Mn and ?exhalite Au deposits (Late Devonian to Early Carboniferous); and stratabound Cu, exhalite Au, and quartz—magnetite (?Late Carboniferous). S-type magmatism in the Late Carboniferous—Early Permian was responsible for vein Sn and possibly AuAsAgSb deposits. Volcanogenic base metals, when compared with the Lachlan Fold Belt, are only poorly represented, and were formed in the Early Permian. The metallogenesis of the New England Fold Belt is dominated by granitoid-related mineralization of Middle Permian to Triassic age, including SnW, MoW, and AuAgAs Sb deposits. Also in the Middle Permian epithermal AuAg mineralization was developed. During the above period of post-orogenic magmatism sizeable metahydrothermal SbAu(W) and Au deposits were emplaced in major fracture and shear zones in central and eastern New England. The occurrence of antimony provides an additional distinguishing factor between the New England and Lachlan Fold Belts. In the New England Fold Belt antimony deposits are abundant whereas they are rare in the Lachlan Fold Belt. This may suggest fundamental crustal differences.

Tectonostratigraphic Terranes of the Frontier Circum-Pacific Region: ABSTRACT

Many major exploration frontiers around the Pacific are in regions where complex geologic relations reflect plate-tectonic processes, crustal mobility, and accretion of exotic terranes. A preliminary map at a scale of 1:20,000,000 portrays the location and character of major terranes, as well as the position of suture zones and ophiolitic belts. The destruction of the proto-Pacific ocean (Panthalassa) involved accretion of terranes to cratonal regions such as Gondwana and Laurasia. In eastern Australia, accretion occurred in the Lachlan foldbelt during the early Paleozoic, followed by accretion within the New England foldbelt from late Paleozoic to early Mesozoic time. Terranes in southwestern New Zealand and eastern Antarctica were also probably accreted during the Paleozoic. The southern margin of Siberia, extending into China, underwent a protracted period of accretion from the late Precambrian through the early Mesozoic. Mid-Paleozoic accretion is reflected in the Innuitian foldbelt of the Arctic Ocean, the Black Clastic unit of the northern Rocky Mountains, and the Antler orogeny of the western U.S. cordillera. The Mesozoic breakup of Pangaea and the acceleration of subduction aided in the rifting and dispersal of terranes from equatorial paleolatitudes. Fragments of these terranes now compose much of the continental margins of the Pacific basin, including New Zealand, Indochina, southern China, southeast Siberia, the North American cordillera, and South America. Combined paleomagnetic, paleobiogeographic, and lithologic data substantiate that some terranes have been displaced thousands of kilometers, but adequate data of these kinds are still lacking for many terranes. Some terranes are presently being further fragmented by post-accretionary dispersion processes such as strike-slip faulting in western North America and Japan. Although the character and distribution of terranes in the western U.S. are fairly well documented, details are needed for other terranes around the Pacific basin. Interpretation of structure and stratigraphy at depth will be aided by more data on the timing of End_Page 485------------------------------ accretion and the nature of deformation associated with accretion and dispersion. Such data are needed for further define specific exploration targets in the circum-Pacific region. End_of_Article - Last_Page 486------------