GeoScienceWorld
Volume

Ancient Orogens and Modern Analogues

Edited by J. B. Murphy, J. D. Keppie and A. J. Hynes

Abstract

Plate tectonics provide a unifying conceptual framework for the understanding of Phanerozoic orogens. More controversially, recent syntheses apply these principles as far back as the Early Archaean. Many ancient orogens are, however, poorly preserved and the processes responsible for them are not well understood. The effects of processes such as delamination, subduction of oceanic and aseismic ridges, overriding of plumes and subduction erosion are rarely identified in ancient orogens, although they have a profound effect on Cenozoic orogens. However, deeply eroded ancient orogens provide insights into the hidden roots of modern orogens. Recent advances in analytical techniques, as well as in fields such as geodynamics, have provided fresh insights into ancient orogenic belts, so that realistic modern analogies can now be applied. This Special Publication offers up-to-date reviews and models for some of the most important orogenic belts developed over the past 2.5 billion years of Earth history.

  1. Page 1
    Abstract
    Corresponding author (e-mail: bmurphy@stfx.ca)

    Plate tectonics provide a unifying conceptual framework for the understanding of Phanerozoic orogens. More controversially, recent syntheses apply these principles as far back as the Early Archaean. Many ancient orogens are, however, poorly preserved and the processes responsible for them are not well understood. The effects of processes such as delamination, subduction of oceanic and aseismic ridges, overriding of plumes and subduction erosion are rarely identified in ancient orogens, although they have a profound effect on Cenozoic orogens. However, deeply eroded ancient orogens provide insights into the hidden roots of modern orogens. Recent advances in analytical techniques, as well as in fields such as geodynamics, have provided fresh insights into ancient orogenic belts, so that realistic modern analogies can now be applied. This Special Publication offers up-to-date reviews and models for some of the most important orogenic belts developed over the past 2.5 billion years of Earth history.

    1. Page 9
      Abstract
      Corresponding author (e-mail: jim.cole@canterbury.ac.nz)

      The Taupo Volcanic Zone (TVZ) is an active continental volcanic arc/back-arc basin in central North Island, New Zealand. It is the youngest area of volcanic activity that extends southwards from the Coromandel Volcanic Zone (CVZ), where andesitic volcanism began c. 18 Ma and rhyolitic volcanism c. 10 Ma. It is an extensional basin (average c. 8 mm a−1) with numerous, predominantly normal (dip >60°) faults within the Taupo Rift, but with some strike-slip component. TVZ can be divided into three parts. In the north (Whakatane Graben – Bay of Plenty) and south (Tongariro volcanic centre) volcanism is predominantly andesitic, while in the central part it is predominantly rhyolitic. This central area comprises eight caldera centres; the oldest of which (Mangakino caldera; 1.62–0.91 Ma) may be transitional between CVZ and TVZ. Kapenga caldera (c. 700 ka) is completely buried by younger volcanics, but is probably a composite structure with most recent subsidence related to volcano-tectonic processes. Of the remaining five caldera centres, Rotorua, Ohakuri and Reporoa are all simple, sub-circular structures which collapsed c. 240 ka, and are each associated with one ignimbrirte outflow sheet (Mamaku, Ohakuri and Kaingaroa, respectively). Okataina and Taupo are caldera complexes with multiple ignimbrite eruptions and phases of collapse. The three simple calderas are extra-rift, occurring outside the main fault zone in the centre of the Taupo Rift system, while the two caldera complexes are both intra-rift. There is a close relationship between volcanism and structure in TVZ, and many of the structural caldera boundaries have rectangular geometry reflecting the fault pattern. Intrusion of high-alumina basalts as dykes, parallel to the fault trend, may have had a strong influence in causing rhyolitic eruptions in central TVZ.

    2. Page 31
      Abstract
      Corresponding author (e-mail: andes@gl.fcen.uba.ar)

      The analysis of magmatic distribution, basin formation, tectonic evolution and structural styles of different segments of the Andes shows that most of the Andes have experienced a stage of flat subduction. Evidence is presented here for a wide range of regions throughout the Andes, including the three present flat-slab segments (Pampean, Peruvian, Bucaramanga), three incipient flat-slab segments (‘Carnegie’, Guañacos, ‘Tehuantepec’), three older and no longer active Cenozoic flat-slab segments (Altiplano, Puna, Payenia), and an inferred Palaeozoic flat-slab segment (Early Permian ‘San Rafael’). Based on the present characteristics of the Pampean flat slab, combined with the Peruvian and Bucaramanga segments, a pattern of geological processes can be attributed to slab shallowing and steepening. This pattern permits recognition of other older Cenozoic subhorizontal subduction zones throughout the Andes. Based on crustal thickness, two different settings of slab steepening are proposed. Slab steepening under thick crust leads to delamination, basaltic underplating, lower crustal melting, extension and widespread rhyolitic volcanism, as seen in the caldera formation and huge ignimbritic fields of the Altiplano and Puna segments. On the other hand, when steepening affects thin crust, extension and extensive within-plate basaltic flows reach the surface, forming large volcanic provinces, such as Payenia in the southern Andes. This last case has very limited crustal melt along the axial part of the Andean roots, which shows incipient delamination. Based on these cases, a Palaeozoic flat slab is proposed with its subsequent steepening and widespread rhyolitic volcanism. The geological evolution of the Andes indicates that shallowing and steepening of the subduction zone are thus frequent processes which can be recognized throughout the entire system.

    3. Page 55
      Abstract
      Corresponding author (e-mail: keppie@glinx.com)

      The Middle Miocene, thin-skinned, Chiapas fold-and-thrust belt (Gulf of Mexico–southeastern Mexico–Belize) consists of WNW-trending folds and thrusts, and East–West sinistral transcurrent faults resulting from N60°E shortening. Balanced cross-sections indicate that shortening varies from 48% (SW) to c. 8% (NE) with a total shortening of 106 km, and that thrusts merge into a basal décollement in the Callovian salt horizon. The Middle Miocene age of the deformation is synchronous with collision of the Tehuantepec Transform/Ridge with the Middle America Trench off Chiapas. The presently exposed Tehuantepec Transform/Ridge varies from a transform fault across which the age of the oceanic crust changes producing a step (down to the east) to a ridge resulting from compression following a change in plate motion and a series of seamounts. On the other hand, the earthquake data show that the part of the Tehuantepec Transform/Ridge subducted during the past 5 Ma is a step with no accompanying ridge. Whereas collision of a ridge segment with the trench is inferred to be responsible for the 13–11 Ma deformation in the upper plate, its termination at 11 Ma suggests an along-strike transition to a step. Collision of the Tehuantepec Transform/Ridge also appears to have terminated arc magmatism along the Pacific coast of Chiapas. The similarity between the petroleum-producing, Cantarell structure in the Sonda de Campeche and the buried foldbelt in the Sierra de Chiapas suggests there is considerable further hydrocarbon potential.

    4. Page 71
      Abstract
      Corresponding author (e-mail: jdostal@smu.ca)

      The volcanic Triassic Takla Group constitutes a significant part of Stikinia and Quesnellia, two major terranes of the Canadian Cordillera that are separated by high-pressure rocks of the Cache Creek terrane containing Asian fauna. The geochemical and isotopic characteristics of the Takla Group in Quesnellia and Stikinia are similar, that is, tholeiitic basalts characterized by low abundances of strongly incompatible trace elements, negative Nb anomalies, +6 to +8 ɛNd values, the low initial Sr isotopic ratios, and relatively horizontal chondrite-normalized heavy REE patterns, all features typical of relatively primitive arcs with little or no continental crust involvement. These similarities have led to several geometric models: post-Middle Jurassic duplication by strike-slip faulting, and oroclinal or synformal folding. However, they are all inconsistent with either palaeomagnetic or faunal data, and the presence of a Triassic overstep sequence, which indicates amalgamation c. 50 ma before emplacement of the youngest oceanic rocks of the Cache Creek terrane. An alternative model is proposed: oblique eastward subduction of the Cache Creek accretionary prism and fore-arc producing high-pressure metamorphism, followed by extrusion into the arc and exhumation by the Middle Jurassic. This model implies that these high-pressure rocks, rather than marking an oceanic suture between disparate arc terranes, support a para-autochthonous origin.

    5. Page 89
      Abstract
      Corresponding author (e-mail: gerard.stampfli@unil.ch)

      New field data on the East Mediterranean domain suggest that this oceanic basin belonged to the larger Neotethyan oceanic system that opened in Permian times. A Greater Apulia domain existed in Mesozoic times, including the autochthonous units of Greece and SW Turkey. It also included a united Adria and Apulia microplate since Early Jurassic times. This key information implies that a new post-Variscan continental fit for the western Tethyan area is necessary, where the relationships between the Adriatic, Apulian and Iberian plates are defined with greater confidence. To construct a reliable palinspastic model of the Alpine realm, plate tectonic constraints must be taken into consideration in order to assess the magnitude of lateral displacements. For most of the plates and their different terranes, differential transport on the scale of thousands of kilometres can be demonstrated. This plate tectonic framework allows a better geodynamic scenario for the formation of the Alpine chain to be proposed, where the western and eastern transects have experienced contrasting geological evolutions. The eastern Alps–Carpathians domain evolved from the north-directed roll-back of the Maliac–Meliata slab and translation of the Meliata suture and Austroalpine domain into the Alpine domain. In the western Alps, the changing African plate boundary in space and time defined the interaction between the Iberian–Briançonnais plate and the Austroalpine accretionary wedge.

    6. Page 113
      Abstract
      Corresponding author (e-mail: stj@uvic.ca)

      Recent structural studies of the Apennines and the Calabrian orocline and a compilation of structural, stratigraphic, GPS and palaeomagnetic data from the central and western Mediterranean region show that beginning in the Late Miocene a N–S trending ribbon continent that had been previously deformed, and which we now recognize as the Apennine–Sicilian thrust belt, buckled eastward in response to northward movement of Africa relative to stable Europe. A simple geometric model is consistent with available data and shows how eastward buckling of an originally north–south continental beam explains: (1) opening of the Tyrrhenian Sea basin from 7–2 Ma, at which point sea-floor spreading ceases and the basin begins to shrink by southward subduction beneath Sicily; (2) the coeval development of an east-verging fold-and-thrust belt along the length of the Apennine–Sicilian belt in response to overthrusting of the autochthon to the east, followed by extension beginning at 1 Ma as the Apennine portion of the beam begins to retreat to the SW; and (3) subduction of continental and oceanic lithosphere east of the buckling beam into a trench that migrates eastward through time due to ‘push back’ by the buckling upper plate.

    7. Page 127
      Abstract
      Corresponding author (e-mail: dileky@muohio.edu)

      The modern Anatolian–African plate boundary is represented by a north-dipping subduction zone that has been part of a broad domain of regional convergence between Eurasia and Afro–Arabia since the latest Mesozoic. A series of collisions between Gondwana-derived ribbon continents and trench-roll-back systems in the Tethyan realm produced nearly East–West-trending, subparallel mountain belts with high elevation and thick orogenic crust in this region. Ophiolite emplacement, terrane stacking, high-P and Barrovian metamorphism, and crustal thickening occurred during the accretion of these microcontinents into the upper plates of Tethyan subduction roll-back systems during the Late Cretaceous–Early Eocene. Continued convergence and oceanic lithospheric subduction within the Tethyan realm were punctuated by slab breakoff events following the microcontinental accretion episodes. Slab breakoff resulted in asthenospheric upwelling and partial melting, which facilitated post-collisional magmatism along and across the suture zones. Resumed subduction and slab roll-back-induced upper plate extension triggered a tectonic collapse of the thermally weakened orogenic crust in Anatolia in the late Oligocene–Miocene. This extensional phase resulted in exhumation of high-P rocks and medium- to lower-crustal material leading to the formation of metamorphic core complexes in the hinterland of the young collision zones. The geochemical character of the attendant magmatism has progressed from initial shoshonitic and high-K calc-alkaline to calc-alkaline and alkaline affinities through time, as more asthenosphere-derived melts found their way to the surface with insignificant degrees of crustal contamination. The occurrence of discrete high-velocity bodies in the mantle beneath Anatolia, as deduced from lithospheric seismic velocity data, supports our Tethyan slab breakoff interpretations. Pn velocity and Sn attenuation tomography models indicate that the uppermost mantle is anomalously hot and thin, consistent with the existence of a shallow asthenosphere beneath the collapsing Anatolian orogenic belts and widespread volcanism in this region. The sharp, north-pointing cusp (Isparta Angle) between the Hellenic and Cyprus trenches along the modern Anatolian–African plate boundary corresponds to a subduction-transform edge propagator (STEP) fault, which is an artifact of a slab tear within the downgoing African lithosphere.

    1. Page 161
      Abstract

      The Uralian orogen is located along the western flank of a huge (>4000 km long) intracontinental Uralo-Mongolian mobile belt. The orogen developed mainly between the Late Devonian and the Late Permian, with a brief resumption of orogenic activity in the Lower Jurassic and Pliocene–Quaternary time. Although its evolution is commonly related to the Variscides of Western Europe, its very distinctive features argue against a simple geodynamic connection. To a first order, the evolution of Uralian orogen shows similarities with the ‘Wilson cycle’, beginning with epi-continental rifting (Late Cambrian–Lower Ordovician) followed by passive margin (since Middle Ordovician) development, onset of subduction and arc-related magmatism (Late Ordovician) followed by arc–continent collision (Late Devonian in the south and Early Carboniferous in the north) and continent–continent collision (beginning in the mid-Carboniferous). In detail, however, the Uralides preserve a number of rare features. Oceanic (Ordovician to Lower Devonian) and island–arc (Ordovician to Lower Carboniferous) complexes are particularly well preserved as is the foreland belt in the Southern Urals, which exhibits very limited shortening of deformed Mesoproterozoic to Permian sediments. Geophysical studies indicate the presence of ‘cold’, isostatically equilibrated root. Other characteristic features include a Silurian platinum-rich belt of subduction-related layered plutons, a simultaneous development of orogenic and rift-related magmatism, a succession of collisions that are both diachronous and oblique, and a single dominant stage of transpressive deformation after the Early Carboniferous. The end result is a pronounced bi-vergent structure. The Uralides are also characterized by Meso-Cenozoic post-orogenic stage and plume-related tectonics in Ordovician, Devonian and especially Triassic time. The evolution of the Uralides is consistent with the development and destruction of a Palaeouralian ocean to form part of a giant Uralo-Mongolian orogen, which involved an interaction of cratonic Baltica and Siberia with a young and rheologically weak Kazakhstanian continent. The Uralides are characterized by protracted and recurrent orogenesis, interrupted in the Triassic by tectonothermal activity associated with the Uralo-Siberian superplume.

    2. Page 197
      Abstract
      Corresponding author (e-mail: mandy.hofmann@senckenberg.de)

      The final pulse of the Variscan Orogeny in the northern Bohemian Massif (Saxo-Thuringian Zone) is related to the closure of the Rheic Ocean, which resulted in subduction-related D1-deformation followed by dextral strike-slip activity (D2-deformation, the Elbe Zone). Taken together, these deformation events reflect the amalgamation of Pangaea in central Europe. Lateral extrusion of high-grade metamorphosed rocks from an allochthonous domain (Saxonian Granulitgebirge) and the top–NW-directed transport of these domains (Erzgebirge nappe complex, Saxonian Granulitgebirge) are responsible for these dextral strike-slip movements. Geochronological data presented herein, together with published data, allow the timing of the final pulse of the Variscan Orogeny and related plutonic, volcano-sedimentary and tectonic processes. Marine sedimentation lasted at least until the Tournaisian (357 Ma). Onset of Variscan strike-slip along the Elbe Zone is assumed to be coeval with the beginning of the top–NW-directed lateral extrusion of the Saxonian Granulitgebirge at 342 Ma (D2-deformation). The sigmoidal shape of the Meissen Massif indicates that strike-slip activity was coexistent with intrusion of the pluton at c. 334 Ma into the schist belt of the Elbe Zone. In contrast, the intrusion of the Markersbach Granite provides a minimum age of c. 327 Ma for the termination of D2 strike-slip activity, because this undeformed pluton cross-cuts all strike-slip related tectonic structures. Geochronological data of an ash bed from the Permo-Carboniferous Döhlen Basin show clearly that post-orogenic sedimentation of Variscan molasse in that area was already active at 305 Ma. This pull-apart basin is a local example of regional Permo-Carboniferous extension within Pangaea. The uplift and denudation of the Variscan basement in the Saxo-Thuringian Zone occurred between c. 327–305 Ma.

    3. Page 215
      Abstract
      Corresponding author (e-mail: mpereira@uevora.pt)

      Following a Middle–Late Devonian (c. 390–360 Ma) phase of crustal shortening and mountain building, continental extension and onset of high-medium-grade metamorphic terrains occurred in the SW Iberian Massif during the Visean (c. 345–326 Ma). The Évora–Aracena–Lora del Rı́o metamorphic belt extends along the Ossa–Morena Zone southern margin from south Portugal through the south of Spain, a distance of 250 km. This major structural domain is characterized by local development of high-temperature–low-pressure metamorphism (c. 345–335 Ma) that reached high amphibolite to granulite facies. These high-medium-grade metamorphic terrains consist of strongly sheared Ediacaran and Cambrian–early Ordovician (c. 600–480 Ma) protoliths. The dominant structure is a widespread steeply-dipping foliation with a gently-plunging stretching lineation generally oriented parallel to the fold axes. Despite of the wrench nature of this collisional orogen, kinematic indicators of left-lateral shearing are locally compatible with an oblique component of extension. These extensional transcurrent movements associated with pervasive mylonitic foliation (c. 345–335 Ma) explain the exhumation of scarce occurrences of eclogites (c. 370 Ma). Mafic-intermediate plutonic and hypabyssal rocks (c. 355–320 Ma), mainly I-type high-K calc-alkaline diorites, tonalites, granodiorites, gabbros and peraluminous biotite granites, are associated with these metamorphic terrains. Volcanic rocks of the same chemical composition and age are preserved in Tournaisian–Visean (c. 350–335 Ma) marine basins dominated by detrital sequences with local development of syn-sedimentary gravitational collapse structures. This study, supported by new U–Pb zircon dating, demonstrates the importance of intra-orogenic transtension in the Gondwana margin during the Early Carboniferous when the Rheic ocean between Laurussia and Gondwana closed, forming the Appalachian and Variscan mountains.

    4. Page 239
      Abstract
      Corresponding author (e-mail: nance@ohio.edu)

      Detrital zircon age populations from Palaeozoic sedimentary and metasedimentary rocks in Mexico support palinspastic linkages to the northwestern margin of Gondwana (Amazonia) during the late Proterozoic–Palaeozoic. Age data from: (1) the latest Cambrian-Pennsylvanian cover of the c. 1 Ga Oaxacan Complex of southern Mexico; (2) the ?Cambro-Ordovician to Triassic Acatlán Complex of southern Mexico's Mixteca terrane; and (3) the ?Silurian Granjeno Schist of northeastern Mexico's Sierra Madre terrane, collectively suggest Precambrian provenances in: (1) the c. 500–650 Ma Brasiliano orogens and c. 600–950 Ma Goias magmatic arc of South America, the Pan-African Maya terrane of the Yucatan Peninsula, and/or the c. 550–600 Ma basement that potentially underlies parts of the Acatlán Complex; (2) the Oaxaquia terrane or other c. 1 Ga basement complexes of the northern Andes; and (3) c. 1.4–3.0 Ga cratonic provinces that most closely match those of Amazonia. Exhumation within the Acatlán Complex of c. 440–480 Ma granitoids prior to the Late Devonian–early Mississippian, and c. 290 Ma granitoids in the early Permian, likely provided additional sources in the Palaeozoic. The detrital age data support the broad correlation of Palaeozoic strata in the Mixteca and Sierra Madre terranes, and suggest that, rather than representing vestiges of Iapetus or earlier oceanic tracts as has previously been proposed, both were deposited along the southern, Gondwanan (Oaxaquia) margin of the Rheic Ocean and were accreted to Laurentia during the assembly of Pangaea in the late Palaeozoic.

    5. Page 271
      Abstract
      Corresponding author (e-mail: Cees.vanStaal@NRCan-RNCan.gc.ca)

      During the Early to Middle Palaeozoic, prior to formation of Pangaea, the Canadian and adjacent New England Appalachians evolved as an accretionary orogen. Episodic orogenesis mainly resulted from accretion of four microcontinents or crustal ribbons: Dashwoods, Ganderia, Avalonia and Meguma. Dashwoods is peri-Laurentian, whereas Ganderia, Avalonia and Meguma have Gondwanan provenance. Accretion led to a progressive eastwards (present co-ordinates) migration of the onset of collision-related deformation, metamorphism and magmatism. Voluminous, syn-collisional felsic granitoid-dominated pulses are explained as products of slab-breakoff rather than contemporaneous slab subduction. The four phases of orogenesis associated with accretion of these microcontinents are known as the Taconic, Salinic, Acadian and Neoacadian orogenies, respectively. The Ordovician Taconic orogeny was a composite event comprising three different phases, due to involvement of three peri-Laurentian oceanic and continental terranes. The Taconic orogeny was terminated with an arc–arc collision due to the docking of the active leading edge of Ganderia, the Popelogan–Victoria arc, to an active Laurentian margin (Red Indian Lake arc) during the Late Ordovician (460–450 Ma).

      The Salinic orogeny was due to Late Ordovician–Early Silurian (450–423 Ma) closure of the Tetagouche–Exploits backarc basin, which separated the active leading edge of Ganderia from its trailing passive edge, the Gander margin. Salinic closure was initiated following accretion of the active leading edge of Ganderia to Laurentia and stepping back of the west-directed subduction zone behind the accreted Popelogan–Victoria arc. The Salinic orogeny was immediately followed by Late Silurian–Early Devonian accretion of Avalonia (421–400 Ma) and Middle Devonian–Early Carboniferous accretion of Meguma (395–350 Ma), which led to the Acadian and Neoacadian orogenies, respectively. Each accretion took place after stepping-back of the west-dipping subduction zone behind an earlier accreted crustal ribbon, which led to progressive outboard growth of Laurentia. The Acadian orogeny was characterized by a flat-slab setting after the onset of collision, which coincided with rapid southerly palaeolatitudinal motion of Laurentia. Acadian orogenesis preferentially started in the hot and hence, weak backarc region. Subsequently it was characterized by a time-transgressive, hinterland migrating fold-and-thrust belt antithetic to the west-dipping A–subduction zone. The Acadian deformation front appears to have been closely tracked in space by migration of the Acadian magmatic front. Syn-orogenic, Acadian magmatism is interpreted to mainly represent partial melting of subducted fore-arc material and pockets of fluid-fluxed asthenosphere above the flat-slab, in areas where Ganderian's lithosphere was thinned by extension during Silurian subduction of the Acadian oceanic slab. Final Acadian magmatism from 395–c. 375 Ma is tentatively attributed to slab-breakoff.

      Neoacadian accretion of Meguma was accommodated by wedging of the leading edge of Laurentia, which at this time was represented by Avalonia. The Neoacadian was devoid of any accompanying arc magmatism, probably because it was characterized by a flat-slab setting throughout its history.

    6. Page 317
      Abstract
      Corresponding author (e-mail: arenas@geo.ucm.es)

      The ophiolites preserved in the Variscan suture of NW Iberia (Galicia) show a broad variability in lithology, geochemistry and chronology. This wide variety rules out the simplest plate tectonic scenario in which these ophiolites would have been exclusively related to the oceanic domain closed during the final Pangaea assembly, that is the Rheic Ocean. The ophiolitic units from Galicia also provide important data about the palaeogeography immediately preceding the opening of this ocean, and some information about pre-Gondwanan supercontinent cycles.

      Six different ophiolites can be distinguished in the allochthonous complexes of Galicia: the Purrido, Somozas, Bazar, Vila de Cruces, Moeche and Careón units. The Purrido Ophiolite is constituted by metagabbroic amphibolites with igneous protoliths dated at 1159±39 Ma (Mesoproterozoic), and geochemical affinities typical of island-arc tholeiites. These mafic rocks can be interpreted as one of the scarce members of the pre-Rodinian ophiolites, and they were probably generated in a back-arc setting in the periphery of the West African Craton. The Somozas Ophiolitic Mélange consists of a mixing of submarine volcanic rocks (pillow-lavas, submarine breccias, pillow-breccias, hyaloclastites), diabases, gabbros, microgabbros, diorites and granitoids, surrounded by a matrix of serpentinites or, less frequently, phyllites. Two granitic samples from this mélange yield U–Pb ages ranging between c. 527 and 503 Ma (Cambrian), which together with the characteristic arc signatures obtained in all the studied igneous rocks suggest that this ophiolite was generated in a peri-Gondwanan volcanic arc. The Bazar Ophiolite is formed by different tectonic slices with high temperature amphibolites, granulites, metagabbros and ultramafic rocks. The amphibolites are the most abundant rock type and show typical N–MORB compositions with igneous protoliths dated at 498±2 Ma (Cambrian). The high-temperature metamorphism affecting some parts of the unit has been dated at c. 480 Ma (lower Ordovician), and it is considered to be related to the development of an oceanic accretionary complex under the volcanic arc represented by the upper units of the allochthonous complexes of Galicia. Considering the most common palaeogeographic reconstructions for the Cambrian period, it is suggested that the oceanic lithosphere represented by the Bazar Ophiolite was formed into the peri-Gondwanan oceanic domain prior to the rifting of the Avalonian microcontinent, that is the Iapetus–Tornquist Ocean. According to current data about the Vila de Cruces Unit, it can be interpreted as a composite terrane, whose lithologies have U–Pb ages ranging from 1176–497 Ma, but constituted by metaigneous rocks with arc signatures. This dataset has been interpreted in relation to the development of a back-arc basin around the Cambrian–Ordovician limit, involving a Mesoproterozoic basement and the reactivation of a former suture. The opening of this back-arc basin can also be identified as the birth of the Rheic Ocean, and probably it would also include the lithological succession belonging to the Moeche Unit, although its basic rocks exhibit compositions with more oceanic character. Finally, the Careón Ophiolite includes remnants of an oceanic lithosphere generated in a supra-subduction zone setting at 395±2 Ma (middle Devonian). This ophiolite was formed in a contractive Rheic Ocean, shortly preceding the closure of this ocean. This is the only ophiolite in Galicia that can be related to mature stages of the Rheic Ocean, although as it is commonly observed in other regions the N–MORB crust is not preserved. This common oceanic crust has disappeared during subduction, probably in an intra-oceanic setting and during the generation of the igneous section preserved in the Careón Ophiolite.

    7. Page 343
      Abstract
      Corresponding author (e-mail: bmurphy@stfx.ca)

      The Rheic Ocean formed during the Late Cambrian–Early Ordovician when peri-Gondwanan terranes (e.g. Avalonia) drifted from the northern margin of Gondwana, and was consumed during the collision between Laurussia and Gondwana and the amalgamation of Pangaea. Several mafic complexes, from the Acatlán Complex in Mexico to the Bohemian Massif in eastern Europe, have been interpreted to represent vestiges of the Rheic Ocean. Most of these complexes are either Late Cambrian–Early Ordovician or Late Palaeozoic in age. Late Cambrian–Early Ordovician complexes are predominantly rift-related continental tholeiites, derived from an enriched c. 1.0 Ga subcontinental lithospheric mantle, and are associated with crustally-derived felsic volcanic rocks. These complexes are widespread and virtually coeval along the length of the Gondwanan margin. They reflect magmatism that accompanied the early stages of rifting and the formation of the Rheic Ocean, and they remained along the Gondwanan margin to form part of a passive margin succession as Avalonia and other peri-Gondwanan terranes drifted northward. True ophiolitic complexes of this age are rare, a notable exception occurring in NW Iberia where they display ensimatic arc geochemical affinities. These complexes were thrust over, or extruded into, the Gondwanan margin during the Late Devonian–Carboniferous collision between Gondwana and Laurussia (Variscan orogeny). The Late Palaeozoic mafic complexes (Devonian and Carboniferous) preserve many of the lithotectonic and/or chemical characteristics of ophiolites. They are characterized by derivation from an anomalous mantle which displays time-integrated depletion in Nd relative to Sm. Devonian ophiolites pre-date closure of the Rheic Ocean. Although their tectonic setting is controversial, there is a consensus that most of them reflect narrow tracts of oceanic crust that originated along the Laurussian margin, but were thrust over Gondwana during Variscan orogenesis. The relationship of the Carboniferous ophiolites to the Rheic Ocean sensu stricto is unclear, but some of them apparently formed in a strike-slip regimes within a collisional setting directly related to the final stages of the closure of the Rheic Ocean.

    1. Page 371
      Abstract

      Palaeomagnetic apparent polar wander (APW) paths from the world's cratons at 1300–700 Ma can constrain the palaeogeographic possibilities for a long-lived and all-inclusive Rodinia supercontinent. Laurentia's APW path is the most complete and forms the basis for superposition by other cratons' APW paths to identify possible durations of those cratons' inclusion in Rodinia, and also to generate reconstructions that are constrained both in latitude and longitude relative to Laurentia. Baltica reconstructs adjacent to the SE margin of Greenland, in a standard and geographically ‘upright’ position, between c. 1050 and 600 Ma. Australia reconstructs adjacent to the pre-Caspian margin of Baltica, geographically ‘inverted’ such that cratonic portions of Queensland are juxtaposed with that margin via collision at c. 1100 Ma. Arctic North America reconstructs opposite to the CONgo+São Francisco craton at its DAmaride–Lufilian margin (the ‘ANACONDA’ fit) throughout the interval 1235–755 Ma according to palaeomagnetic poles of those ages from both cratons, and the reconstruction was probably established during the c. 1600–1500 Ma collision. Kalahari lies adjacent to Mawsonland following collision at c. 1200 Ma; the Albany–Fraser orogen continues along-strike to the Sinclair-Kwando-Choma-Kaloma belt of south-central Africa. India, South China and Tarim are in proximity to Western Australia as previously proposed; some of these connections are as old as Palaeoproterozoic whereas others were established at c. 1000 Ma. Siberia contains a succession of mainly sedimentary-derived palaeomagnetic poles with poor age constraints; superposition with the Keweenawan track of the Laurentian APW path produces a position adjacent to western India that could have persisted from Palaeoproterozoic time, along with North China according to its even more poorly dated palaeomagnetic poles. The Amazonia, West Africa and Rio de la Plata cratons are not well constrained by palaeomagnetic data, but they are placed in proximity to western Laurentia. Rift successions of c. 700 Ma in the North American COrdillera and BRAsiliano-Pharuside orogens indicate breakup of these ‘COBRA’ connections that existed for more than one billion years, following Palaeoproterozoic accretionary assembly. The late Neoproterozoic transition from Rodinia to Gondwanaland involved rifting events that are recorded on many cratons through the interval c. 800–700 Ma and collisions from c. 650–500 Ma. The pattern of supercontinental transition involved large-scale dextral motion by West Africa and Amazonia, and sinistral motion plus rotation by Kalahari, Australia, India and South China, in a combination of introverted and extroverted styles of motion. The Rodinia model presented here is a marked departure from standard models, which have accommodated recent discordant palaeomagnetic data either by excluding cratons from Rodinia altogether, or by decreasing duration of the supercontinental assembly. I propose that the revised model herein is the only possible long-lived solution to an all-encompassing Rodinia that viably accords with existing palaeomagnetic data.

    2. Page 405
      Abstract

      The proposition that the Grenville Province is a remnant of a large hot long-duration collisional orogen is examined through a comparative study of its present orogenic front, the Grenville Front, and a former front, the Allochthon Boundary Thrust. Structural, metamorphic and geochronologic data for both boundaries and their hanging walls from the length of the Grenville Province are compared. Cumulative displacement across the Grenville Front was minor (10 s of km) whereas that across the Allochthon Boundary Thrust was major (100 s of km), consistent with the observation that the latter boundary separates rocks with a different age, and PT character, of metamorphism.

      On an orogen scale, Grenvillian metamorphism can be subdivided into two spatially and temporally distinct orogenic phases, a relatively high T Ottawan (c. 1090–1020 Ma) phase in the hanging wall of the Allochthon Boundary Thrust, and a relatively lower T Rigolet (c. 1000–980 Ma) phase in the hanging wall of the Grenville Front. It is argued that the structural setting and ≥50 My duration of Ottawan metamorphism are compatible with some form of channel flow beneath an orogenic plateau, with the Allochthon Boundary Thrust forming the base of the channel. Channel flow ceased at c. 1020 Ma when the Allochthon Boundary Thrust was reworked as part of a system of normal-sense shear zones, and following a hiatus of c. 20 My the short-lived Rigolet metamorphism took place in the former foreland and involved the development of a new orogenic front, the Grenville Front. Taken together, this suggests the Grenville Orogen developed as a large hot long-duration orogen during the Ottawan orogenic phase, but following gravitational collapse of the plateau the locus of thickening migrated into the foreland and active tectonism was restricted to a subjacent small cold short-duration orogen. The foreland-ward migration of the orogenic front from the Allochthon Boundary Thrust to the Grenville Front, the contrasting P–T–t character of the metamorphic rocks in their hanging walls, and the evidence for orogenic collapse followed by renewed growth, provide insights into the complex evolution of a long-duration collisional orogen.

    3. Page 445
      Abstract
      Corresponding author (e-mail: S.Reddy@curtin.edu.au)

      Argon isotopic data from mica from the southern Capricorn region of Western Australia record complex intra- and inter-grain systematics that reflect modification due to a range of processes. However, 40Ar/39Ar age distributions, though complex, generally show early Neoproterozoic ages in the west, increasing to Mesoproterozoic ages in the east. Palaeoproterozoic ages associated with cooling after the c. 1.8 Ga Capricorn Orogen or c. 1.6 Ga Mangaroon Orogen are not preserved. These data reflect cooling from a c. 300 °C thermal overprint that took place prior to 960 Ma that is related to the enigmatic Edmundian Orogeny. These data, combined with sediment provenance data from the Early Neoproterozoic Officer Basin and U–Pb age data from the nearby Pinjarra Orogen, indicate that the late Mesoproterozoic–Neoproterozoic Pinjarra and Edmundian events are dynamically linked and reflect tectonic activity on the western margin of the amalgamated West Australian Craton. The temporal framework for this event suggest a link to the evolving Rodinian supercontinent and reflect the oblique collision of either Greater India or Kalahari cratons with the West Australian Craton. These results illustrate that the temporal evolution of poorly preserved orogens can be constrained by low-temperature thermochronology in the adjacent cratons.

    4. Page 457
      Abstract
      Corresponding author (e-mail: dcorriga@NRCan.gc.ca)

      The Trans-Hudson Orogen (THO) of North America is one of the earliest orogens in Earth's history that evolved through a complete Wilson Cycle. It represents c. 150 Ma of opening of the Manikewan Ocean, from c. 2.07–1.92 Ga, followed by its demise in the interval 1.92–1.80 Ga, during the final phase of growth of the Supercontinent Columbia (Nuna). It is characterized by three lithotectonic divisions: (i) Churchill margin (or peri-Churchill); (ii) Reindeer Zone; and (iii) Superior margin (or peri-Superior). The peri-Churchill realm records progressive outward continental growth by accretion of Archaean to Palaeoproterozoic micro-continents (Hearne, Meta Incognita/Core Zone, Sugluk) and eventually arc terranes (La Ronge–Lynn Lake) to the Slave-Rae nuclei, with attendant development of orogenies and basin inversions related to the specific accretion events (1.92–1.89 Ga Snowbird; 1.88–1.865 Ga Foxe; 1.87–1.865 Ga Reindeer orogenies). The Reindeer Zone is characterized by primitive to evolved oceanic arcs, back-arc basins, oceanic crust and ocean plateaus that formed during closure of the Manikewan Ocean, and accretion of a micro-continent (Sask Craton) and smaller Archaean crustal fragments. The terminal phase of the Trans–Hudson orogeny represents collision between the Superior craton, the Reindeer Zone and the composite western Churchill Province during the interval 1.83–1.80 Ga.

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