Active Margin Of Gondwana Research Articles

Potential sources of Ediacaran strata of Iberia: a review

Advances in stratigraphy, geochemistry and U-Pb detrital zircon geochronology from Ediacaran strata of Iberia allow for the improved characterisation of crustal growth in the North Gondwana active margin. The formation of Cadomian magmatic arcs and associated back-arc basins that took place in the North Gondwana active margin was a long-term process. Iberia has been placed in the Cadomian belt in currently accepted palaeogeography reconstructions at c. 570–560 Ma, based on the characteristics of Ediacaran strata. The Ediacaran strata of Iberia with outstanding geochemical homogeneity are distributed across three zones of Iberia: (1) Narcea Slates in the Cantabrian and West Asturian Leonese zones (maximum depositional age of c. 600 and 553 Ma); (2) Schist–Greywacke Complex (Lower Series) in the Central Iberian Zone (maximum depositional age of c. 578 to 550 Ma); and (3) Série Negra in the Ossa-Morena Zone (maximum depositional age of c. 590–545 Ma). Pre-Cryogenian detrital zircons found in the Ediacaran strata of Iberia seem to be related to distal sources distributed across three main areas of North Gondwana inland. The oldest detrital zircons probably derive from distal sources such as the West African craton, the Trans-Saharan belt and the Arabian–Nubian Shield, in view of the increase in distance from sedimentary basins. The West African craton is the most likely source for Archean and Palaeoproterozoic detrital zircons, while the Trans-Saharan belt and the Arabian–Nubian shield could provide a source for Tonian and Mesoproterozoic grains. The youngest zircon ages (c. 630–545 Ma), which make up the dominant population in the Ediacaran strata of Iberia, are probably derived from proximal sources as would be the Cadomian magmatic arc system, not excluding the contribution of the Pan-African orogen.

Cambrian ultrapotassic rhyolites from the Lhasa terrane, south Tibet: Evidence for Andean-type magmatism along the northern active margin of Gondwana

The petrogenesis and tectonic significance of early Paleozoic magmatic rocks in south Tibet is debated. In this paper, we report Cambrian ultrapotassic felsic volcanics from the Lhasa terrane. Petrographic and whole-rock geochemical studies indicate that these rocks are rhyolite and rhyolitic ignimbrite, and have varying SiO2 (69.56–82.09wt.%), Al2O3 (9.52–16.61wt.%) and Fe2O3t (1.0–7.9wt.%). The rocks are ultrapotassic (high K2O 4.19–6.90wt.%, and very low Na2O 0.01–0.06wt.%) and peraluminous (A/CNK=1.41–2.22). They are enriched in Rb, Th, U, Pb, Zr and Y, and showed negative Ba, Sr, P, Nb, Ta and Ti anomalies, and slightly fractionated REE patterns with moderately negative Eu anomalies, typical of A-type granitoids. Zircons from six samples yielded crystallization ages of ca. 512Ma. These zircons show εHf(t) values in the range of −4.7 to +1.3, and two-stage Hf model ages of 1734–1392Ma. Combining with previous results, we propose that these ultrapotassic rhyolites were derived from the partial melting of middle Proterozoic crustal rocks in the lower crust under high-temperature and water-absent conditions in an extensional environment of active magmatic arc. Thus, this study provides new insights into early Paleozoic Andean-type orogeny along the northern margin of Gondwana supercontinent.

Cadomian (Ediacaran–Cambrian) arc magmatism in the ChahJam–Biarjmand metamorphic complex (Iran): Magmatism along the northern active margin of Gondwana

The ChahJam–Biarjmand complex (CJBC), flanked by the Alborz Mountains in the north and the Lut–Tabas block to the south, is part of Central Iranian block, where the oldest continental crust of Iran is found. This complex contains granitic to tonalitic orthogneissic rocks (old plutons) and associated metasediments, amphibolites and paragneisses. Metamorphosed granitic and granodioritic dikes intrude orthogneisses as well as metasediments and are abundant close to the plutons (orthogneissic rocks). Based on the results of bulk rock trace and rare earth elements, the orthogneissic rocks are inferred to have crystallized from subduction-related melts. Amphibolites also have subduction-related signatures and are inferred to have formed both as metamorphosed volcanoclastic sediments and as attenuated basic dikes. The presence of para-amphibolites associated with paragneisses and metasediments (mica schists) could represent a sedimentary basin filled with magmatic arc erosional products. U–Pb zircon dating of the ChahJam–Biarjmand rocks yielded 238U/206Pb crystallization ages of ca. 550 to 530Ma (Ediacaran–early Cambrian). Sr–Nd isotope systematics on whole rocks (εNd(t)=−2.2 to −5.5) and zircon Hf isotope results indicate that CJBC Cadomian granitic rocks contain older, possible Mesoproterozoic, continental crust in their source. The ChahJam–Biarjmand granitic–tonalitic gneissic rocks are coeval with other similar-aged metagranites and gneisses within Iranian basement exposed in Central Iran, the Sanandaj–Sirjan Zone and the Alborz Mountains, as well as in the Tauride–Anatolide platform in western Anatolia and in NW Turkey. All these dispersed Cadomian basement rocks are interpreted to show fragments of Neoproterozoic–early Cambrian continental arcs bordering the northern active margin of Gondwana.

Seismic stratigraphic record of transition from Mesozoic subduction to continental breakup in the Zealandia sector of eastern Gondwana

The southwest Pacific between Australia, New Zealand and New Caledonia is a block of continental crust, Zealandia, that moved away from Australia and Antarctica after a long period of subduction beneath eastern Gondwana. We use >100,000line-km of seismic-reflection profiles to identify intra-continental basins related to the Gondwana active margin, overlain with erosional unconformity by retrogradational strata. We interpret this regional-scale first-order unconformity, the Eastern Gondwana Composite Surface, and seismic-stratigraphic megasequence pattern to represent the transition from subduction to continental breakup and separation of eastern Gondwana. Rocks that make up the lower seismic-stratigraphic megasequence (Zealandia-3, Permian to Early Cretaceous) have been drilled near New Zealand and can be correlated with Murihiku Supergroup rocks. Farther north on the Lord Howe Rise, we correlate this megasequence with Triassic to Jurassic rocks of the Clarence-Moreton Basin of eastern Australia. We reinterpret the Fairway–Aotea Basin to be underlain by a wedge of Zealandia-3 deposits thrust in a retro-arc-like foreland basin that was active immediately prior to breakup. The lower rift and retrogradational megasequence (Zealandia-2, Late Cretaceous to Eocene) we correlate with the Pakawau and Kapuni Groups of Taranaki Basin, New Zealand, and the “Formation à charbon” and “phtanites” of New Caledonia. The boundary with the shallowest megasequence (Zealandia-1, Late Eocene to present) is the “Eocene–Oligocene Unconformity”, which is primarily overlain by pelagic carbonate rocks. This regional unconformity likely represents the onset of the modern Tonga–Kermadec subduction system.

Anatectic reworking and differentiation of continental crust along the active margin of Gondwana: a zircon Hf–O perspective from West Antarctica

Abstract The Fosdick migmatite–granite complex of West Antarctica preserves evidence of two crustal differentiation events along a segment of the former active margin of Gondwana, one in the Devonian–Carboniferous and another in the Cretaceous. The Hf–O isotope composition of zircons from Devonian–Carboniferous granites is explained by mixing of material from two crustal sources represented by the high-grade metamorphosed equivalents of a Lower Palaeozoic turbidite sequence and a Devonian calc-alkaline plutonic suite, consistent with an interpretation that the Devonian–Carboniferous granites record crustal reworking without input from a more juvenile source. The Hf–O isotope composition of zircons from Cretaceous granites reflects those same two sources, together with a contribution from a more juvenile source that is most evident in the detachment-hosted, youngest granites. The relatively non-radiogenic εHf isotope characteristics of zircons from the Fosdick complex granites are similar those from the Permo-Triassic granites from the Antarctic Peninsula. However, the Fosdick complex granites contrast with coeval granites in other localities along and across the former active margin of Gondwana, including the Tasmanides of Australia and the Western Province of New Zealand, where the wider range of more radiogenic εHf values of zircon suggests that crustal growth through the addition of juvenile material plays a larger role in granite genesis. These new results highlight prominent arc-parallel and arc-normal variations in the mechanisms and timing of crustal reworking v. crustal growth along the former active margin of Gondwana. Supplementary material: Figs S1 and S2 are available at www.geolsoc.org.uk/SUP18625

Reconstruction of the early Mesozoic plate margin of Gondwana by U–Pb ages of detrital zircons from northern Victoria Land, Antarctica

Abstract Detrital zircons of eight sandstone samples from the Triassic–Early Jurassic Section Peak Formation (Victoria Group, Beacon Supergroup) in northern Victoria Land, Antarctica, were investigated by U–Pb LA–ICPMS dating. The basin was flanked by the East Antarctic craton, and by a magmatic arc at the palaeo-Pacific margin of Gondwana. It accommodated sandstones ranging from quartzo-feldspathic to volcaniclastic in composition. The detrital zircon age spectra yield pronounced concentrations at c. 190–250 Ma, 500–700 Ma and 800–1200 Ma. The proportion of Triassic–Early Jurassic zircons increases from base to top of the formation, and correlates positively with the abundance of detrital volcanic rock fragments. The youngest zircon ages are close to the stratigraphic age of each sample, indicating contemporaneous magmatic activity along the active margin of Gondwana. Igneous rocks that formed during the Ross Orogeny ( c. 470–545 Ma) were a minor source only, suggesting that the Ross Orogen became progressively covered by sediments as the basin expanded. Pan-African ( c. 500–700 Ma) and Grenville ( c. 800–1200 Ma) age zircons may have been derived from crustal sources currently covered beneath the polar ice sheet, although recycling from Cambro-Ordovician units provides an alternative explanation. Supplementary material: The exact results of the age analyses of all samples are presented in Tables A1 to A8 at www.geolsoc.org.uk/SUP18624

Antarctica and supercontinent evolution: historical perspectives, recent advances and unresolved issues

Abstract The Antarctic rock record spans some 3.5 billion years of history, and has made important contributions to our understanding of how Earth's continents assemble and disperse through time. Correlations between Antarctica and other southern continents were critical to the concept of Gondwana, the Palaeozoic supercontinent used to support early arguments for continental drift, while evidence for Proterozoic connections between Antarctica and North America led to the ‘SWEAT’ configuration (linking SW USA to East Antarctica) for an early Neoproterozoic supercontinent known as Rodinia. Antarctica also contains relicts of an older Palaeo- to Mesoproterozoic supercontinent known as Nuna, along with several Archaean fragments that belonged to one or more ‘supercratons’ in Neoarchaean times. It thus seems likely that Antarctica contains remnants of most, if not all, of Earth's supercontinents, and Antarctic research continues to provide insights into their palaeogeography and geological evolution. One area of research is the latest Neoproterozoic–Mesozoic active margin of Gondwana, preserved in Antarctica as the Ross Orogen and a number of outboard terranes that now form West Antarctica. Major episodes of magmatism, deformation and metamorphism along this palaeo-Pacific margin at 590–500 and 300–230 Ma can be linked to reduced convergence along the internal collisional orogens that formed Gondwana and Pangaea, respectively; indicating that accretionary systems are sensitive to changes in the global plate tectonic budget. Other research has focused on Grenville-age ( c. 1.0 Ga) and Pan-African ( c. 0.5 Ga) metamorphism in the East Antarctic Craton. These global-scale events record the amalgamation of Rodinia and Gondwana, respectively. Three coastal segments of Grenville-age metamorphism in the Indian Ocean sector of Antarctica are each linked to the c. 1.0 Ga collision between older cratons but are separated by two regions of pervasive Pan-African metamorphism ascribed to Neoproterozoic ocean closure. The tectonic setting of these events is poorly constrained given the sparse exposure, deep erosion level and likelihood that younger metamorphic events have reactivated older structures. The projection of these orogens under the ice is also controversial, but it is likely that at least one of the Pan-African orogens links up with the Shackleton Range on the palaeo-Pacific margin of the craton. Sedimentary detritus and glacial erratics at the edge of the ice sheet provide evidence for the c. 1.0 and 0.5 Ga orogenesis in the continental interior, while geophysical data reveal prominent geological boundaries under the ice, but there are insufficient data to trace these features to exposed structures of known age. Until we can resolve the subglacial geometry and tectonic setting of the c. 0.5 and 1.0 Ga metamorphism, there will be no consensus on the configuration of Rodinia, or the size and shape of the continents that existed immediately before and after this supercontinent. Given this uncertainty, it is premature to speculate on the role of Antarctica in earlier supercontinents, but it is likely that Antarctica will continue to provide important constraints when our attention shifts to these earlier events.

Neoproterozoic to early Cambrian Franciscan-type mélanges in the Teplá–Barrandian unit, Bohemian Massif: Evidence of modern-style accretionary processes along the Cadomian active margin of Gondwana?

Mélanges have been reported from a variety of modern tectonic settings and are a hallmark of the Franciscan-type subduction–accretion complexes. In the Precambrian, however, well documented examples of mélanges become extremely scarce. This paper describes in detail Neoproterozoic to early Cambrian mélanges recently recognized in the Cadomian basement of the Teplá–Barrandian unit, Bohemian Massif. The mélanges are of sedimentary and tectonic origin and include voluminous successions of graywackes interpreted to represent mass-transport deposits presumably along the upper slope of an accretionary wedge. The clast compositions indicate multiple episodes of mixing of terrigenous arc-derived and deep-water (?pelagic) material. Moreover, abundant bodies of (meta-)basalts, interpreted as dismembered seamounts or pillow volcanoes, were emplaced tectonically into various structural levels of this sedimentary mélange by two different mechanisms. At an upper level, the irregular to elongated bodies of basalts were offscraped, only weakly strained, and incorporated in the sedimentary matrix that exhibits steep cleavage indicating horizontal pure shear shortening and oblate strain. In contrast, at a lower level, disc-shaped and closely spaced basalt bodies are ductilely sheared and reworked into a flat-lying prolate to plane-strain fabric suggesting horizontal ductile flow, presumably along the base of the accretionary wedge during flat-slab oceanic subduction. Our new U–Pb ages also reveal that the mélange formation continued until the early Cambrian (<527Ma). We conclude that these sedimentary–tectonic mélanges and the inferred mélange-forming processes are identical to those that operate along modern active plate margins and, in general, reveal their potential as excellent markers for establishing modern-style subduction in Precambrian settings.

First U–Pb SHRIMP zircon and 40Ar/39Ar ages of metarhyolites from the Afyon–Bolkardag Zone, SW Turkey: Implications for the rifting and closure of the Neo-Tethys

The Afyon–Bolkardag Zone (ABZ) is one of the most important metamorphic zones in Turkey. This zone consists of metasedimentary and metavolcanic rocks, which have undergone regional greenschist facies metamorphism. The metavolcanic rocks are dominated by rhyolite, dacite, and trachyandesite. In the Ilgin (Konya) area, the low grade metavolcanics were originally rhyolitic lavas and tuffs. We present new precise Sensitive High-Resolution Ion MicroProbe (SHRIMP) 206Pb–238U age analyses of zircons and 40Ar/39Ar age data of phengite from metarhyolites that enable a more precise reconstruction of the ABZ. SHRIMP analyses of euhedral or sub-euhedral magmatic zircons from two metarhyolite samples yield weighted means 230±2Ma and 229±2Ma. These ages are interpreted as formation age of rhyolitic volcanics, implying they were formed in the Late Triassic time (Carnian). We suggest that these Upper Triassic magmatic rocks were formed in an extensional tectonic setting on the northern active margin of Gondwana, which led to the development of the northern branch of the Neo-Tethys. 40Ar/39Ar dating of two phengite samples from metarhyolites gave plateau ages of between 63.7±0.1Ma and 62.6±0.1Ma. These dates are interpreted as the metamorphic age of these volcanic rocks, and as the age of metamorphism that affected the ABZ. Consequently, these earliest Tertiary ages also constrain the timing of the closure of the Neo-Tethys.

To date or not to date - Does luminescence dating work in the European Alps and their foreland?

The Squantum ‘Tillite’ (c. 593–570 Ma) consists of thick (up to 215 m) massive and crudely-stratified diamictites conformably interbedded with subaqueously-deposited conglomerates and sandstones within a thick (~ 7 km) Boston Basin fill which is dominated by argillite turbidites. The Squantum Tillite was first interpreted as being glacigenic in origin in 1914 because of the presence of diamictites; argillites were interpreted as glaciolacustrine ‘varves’ with rare ice-rafted debris, and conglomerates as glaciofluvial outwash. More recently these have been shown to be the product of deep marine mass flow processes with no glacial influence, yet because of its age equivalence with the deep marine, glacially-influenced Gaskiers Formation, the Squantum Tillite is still seen by some as supporting evidence for a widespread ‘Snowball Earth’ event at c. 580 Ma. New sedimentological work confirms that conglomerate and sandstone facies are deep marine sediment gravity flows genetically related to massive (homogeneous) and crudely-stratified (heterogeneous) diamictites produced subaqueously by downslope mixing of gravel and cobbles with muddy facies. Rare horizons of ‘ice rafted debris’ in thin-bedded and laminated turbidite facies interbedded with thick debrites show a weak but positive correlation of lamina thickness with grain size, suggesting these facies are non-glacial co-genetic ‘debrite–turbidite’ couplets. A significant volcanic influence on sedimentation is identified from reworked lapilli tuff beds and reworked ash in turbidites. The depositional setting of the Squantum ‘Tillite’ appears to be that of a submarine slope/fan setting in an open marine volcanic arc basin receiving large volumes of poorly-sorted sediment on the mid-latitude active margin of Gondwana. No direct glacial influence is apparent.

Fractionation and incipient self-granulitization during deep-crust emplacement of Lower Ordovician Valle Fértil batholith at the Gondwana active margin of South America

Large granite batholiths were emplaced at the Gondwana active margin during Lower Ordovician in South America. These have contributed to crustal growth by net addition of silicic rocks to the continental crust. New U–Pb SHRIMP zircon age determinations, together with thermobarometric and geochemical data, yield that batholith magma intrusion is the responsible of heating and self-granulitization of early gabbro pulses. Partially molten granulitic gabbros, which appear as either early intruded into the metasedimentary host or as large inclusions within the batholith-forming Qtz-diorites, contain Opx-bearing, trondhjemitic leucosomes surrounding Hbl+Opx+Pl mafic mesosomes forming typical agmatitic structures. Hornblende–Plagioclase equilibria, applied to mineral pairs of granoblastic aggregates in textural equilibrium of metagabbro mesosomes, yield temperatures in the range 850–910°C for core-to-core pairs and in the range 1000–1075°C for rim-to rim pairs, at pressures of about 0.7GPa. SHRIMP zircon age revealed that the whole batholith was emplaced over a narrow time interval of 20Ma from 465 to 485Ma, with most ages clustered at about 470Ma. The age of metagabbros is 473±7Ma for older zircons and 454±4 for younger zircons. These ages are almost coincident within error with the age of host migmatites (477±5Ma) and those of batholith intrusion of 476±9Ma and 475±3Ma for Qtz-diorites and 475±5Ma for granites. Zircon overgrowths of these intrusive rocks yield sages clustered around 450Ma, revealing a protracted thermal history, more complex than previously believed. The geochemical study reveals that Qtz-diorites, tonalites and granodiorites form a continuous trend produced by magmatic fractionation from a parental dioritic magma. A weak adakitic tendency, with Sr/Y>15 in several samples, implies the presence of Grt in the source or magma chamber at a minimum pressure of 1.0GPa, higher than the depth of emplacement at 0.7GPa. The high temperature of magma emplacement, which induced the incipient self-granulitization of early magmatic pulses, together with the cotectic-like fractionation linking coeval Qtz-diorites, tonalites and granodiorites, is compatible with fractionation at the lower crust of a deep-generated, infracrustal, (sublithospheric?) intermediate magma.

The Basement of the Andean Frontal Cordillera in the Cordón del Plata (Mendoza, Argentina): Geodynamic Evolution

The Andean Paleozoic basement of the Cordon del Plata (Argentina) consists of two sets of rocks showing different stratigraphy, structure and metamorphism. The lower one is represented by the pre-Carboniferous (Devonian?) Vallecitos beds. These rocks have been affected by folds and associated cleavage, developed under low-grade metamor- phic conditions and related to the Chanic orogenic event of the Famatinian Orogenic Cycle (Upper Devonian-Lower Carboniferous). The Vallecitos beds is pre-orogenic to the Chanic deformation event and must have been deposited in a fore-arc basin located on the active margin of Chilenia, before its collision with the passive margin of Gondwana. The upper set unconformably rests on the Vallecitos beds and consists of the Late Carboniferous El Plata Formation, and the Rio Blanco Conglomerates, probably Late Carboniferous-Permian in age. These rocks have been affected by east- directed thrusts and associated folds formed under very low-grade to non-metamorphic conditions. This deformation can be related to the San Rafael phase (Gondwanan Orogen). The El Plata Formation was deposited in an extensional back-arc basin while the Rio Blanco conglomerates must have been deposited in a retro-arc basin, both of them on the active margin of Gondwana. The Choiyoi Group, essentially volcanic and Permo-Triasic in age, rests unconformably on the previously described successions and was deposited in an extensional setting (pre-orogenic stage) associated with the beginning of the Andean Cycle. This cycle lead to the uplift of the Frontal Cordillera during the Cenozoic and the deposition of thick continental units at the base of the Cordon del Plata in Neogene-Quaternary times.

The provenance of Late Ediacaran and Early Ordovician siliciclastic rocks in the Southwest Central Iberian Zone: Constraints from detrital zircon data on northern Gondwana margin evolution during the late Neoproterozoic

U–Pb geochronology of detrital zircon from Late Ediacaran (Beiras Group greywackes) and Early Ordovician (Sarnelhas arkosic quartzites and Armorican quartzites of Penacova) sedimentary rocks of the southwest Central Iberian Zone (SW CIZ) constrain the evolution of northern Gondwana active-passive margin transition. The LA-ICP-MS U–Pb data set (375 detrital zircons with 90–110% concordant ages) is dominated by Neoproterozoic ages (75% for the greywakes and 60% for the quartzites), among which the main age cluster (more significant for Beiras Group greywackes) is Cryogenian (c.840–750 Ma), while a few Mesoproterozoic and Tonian ages are also present (percentages <8%). These two features, and the predominance of Cryogenian ages over Ediacaran ages, distinguish the Beiras Group greywackes (SW CIZ) from the time-equivalent Serie Negra (Ossa-Morena Zone – OMZ), with which they are in inferred contact. The age spectra of the Beiras Group greywackes also reveal three major episodes of zircon crystallisation in the source area during the Neoproterozoic that are probably associated with a long-lived system of magmatism that developed either along or in the vicinity of the northern Gondwana margin at: (1) c. 850–700 Ma – Pan-African suture (not well represented in OMZ); (2) c. 700–635 Ma – early Cadomian arc; and (3) c. 635–545 Ma – late Cadomian arc. Comparison of Neoproterozoic ages and those of the Paleoproterozoic (c. 2–1.8 Ga) and Archean (mainly Neoarchean – 2.8–2.6 Ga, but also older) in the Beiras Group greywackes with U–Pb ages of Cadomian correlatives shows that: (1) SW CIZ, OMZ, Saxo-Thuringian Zone, North Armorican Cadomian Belt and Anti-Atlas) evolved together during the formation of back-arc basins on the northern Gondwana active margin and (2) all recorded synorogenic basins that were filled during the Ediacaran by detritus resulting from erosion of the West African craton, the Pan-African suture and a long-lived Cadomian magmatic arc. Differences in detrital zircon age populations in the greywackes of the Beiras Group (SW CIZ Cadomian basement) and the Serie Negra (OMZ Cadomian basement) are also observed in their respective overlying Early Ordovician quartzites. Since both these SW Iberia Cadomian basements evolved together along the active margin of Gondwana (but sufficiently separated to account for the differences in their detrital zircon content), this continuation of differing zircon populations into the Early Ordovician suggests that the inferred contact presently juxtaposing the Beiras Group and the Serie Negra is not pre-Early Ordovician and so is unlikely to demonstrate a Cadomian suture.

Long‐lived orogenic construction along the paleo‐Pacific margin of Gondwana (Deep Freeze Range, North Victoria Land, Antarctica)

The paleo‐Pacific margin of Gondwana records a prolonged history of convergence during the Cambrian–Early Ordovician Ross‐Delamerian and the Middle Ordovician–Early Silurian Lachlan orogenies. This study describes structure, petrology, and geochronology of a set of NW‐SE striking ductile shear zones that crosscut the Ross age, Early Cambrian granitoid rocks of the Wilson Terrane in the Deep Freeze Range (North Victoria Land, Antarctica). The shear zones developed under amphibolite facies metamorphic conditions (650–700°C and 0.5–0.7 GPa) and show a systematic top‐to‐the‐NE sense of shear. The shear zone activity interferes with emplacement of late, subhorizontal leucocratic dikes and combined U‐(Th)‐Pb (zircon and monazite) and 40Ar‐39Ar (biotite and phengite) geochronology constrains the shearing event at ∼470 Ma, with the sheared granite yielding U‐Pb zircon crystallization ages of ∼508 Ma. The reconstructed P‐T path followed by the granite protoliths indicates an anticlockwise trajectory, suggesting the synshearing amphibolite metamorphism was associated with the burial of an early formed, Ross continental crustal section. These new findings are interpreted as evidence of a renewed, Ordovician episode of orogenic construction at the paleo‐Pacific margin of Gondwana that predated the onset of the Lachlan orogeny in the region. A polycyclic reactivation of the Ross age Wilson Terrane of North Victoria Land is suggested, which is used to propose a unitary framework for the space‐time transition from the Ross‐Delamerian to the Lachlan orogeny along the proto‐Pacific active margin of Gondwana.

Plate-driven extension and convergence along the East Gondwana active margin: Late Silurian–Middle Devonian tectonics of the Lachlan Fold Belt, southeastern Australia

Plate-driven extension and convergence along the East Gondwana active margin: Late Silurian–Middle Devonian tectonics of the Lachlan Fold Belt, southeastern Australia

Plate-driven extension and convergence along the East Gondwana active margin: Late Silurian–Middle Devonian tectonics of the Lachlan Fold Belt, southeastern Australia

Plate-driven extension and convergence along the East Gondwana active margin: Late Silurian–Middle Devonian tectonics of the Lachlan Fold Belt, southeastern Australia

Structure and stratigraphy of the Teplá–Barrandian Neoproterozoic, Bohemian Massif: A new plate-tectonic reinterpretation

The Teplá–Barrandian unit (TBU) of the Bohemian Massif exposes a section across the once extensive Avalonian–Cadomian belt, which bordered the northern active margin of Gondwana during late Neoproterozoic. This paper synthesizes the state-of-the-art knowledge on the Cadomian basement of the TBU to redefine its principal component units, to revise an outdated stratigraphic scheme, and to interpret this scheme in terms of a recent plate-tectonic model for the Cadomian orogeny in the Bohemian Massif. The main emphasis of this paper is on an area between two newly defined fronts of the Variscan pervasive deformation to the NW and SE of the Barrandian Lower Paleozoic overlap successions. This area has escaped the pervasive Variscan (late Devonian to early Carboniferous) ductile reworking and a section through the Cadomian orogen is here superbly preserved. The NW segment of the TBU consists of three juxtaposed allochthonous belts of unknown stratigraphic relation (the Kralovice–Rakovník, Radnice–Kralupy, and Zbiroh–Šárka belts), differing in lithology, complex internal strain patterns, and containing sedimentary and tectonic mélanges with blocks of diverse ocean floor (meta-)basalts. We summarize these three belts under a new term the Blovice complex, which we believe represents a part of an accretionary wedge of the Cadomian orogen. The SE segment of the TBU exposes the narrow Pičín belt, which is probably a continuation of the Blovice complex from beneath the Barrandian Lower Paleozoic, and a volcanic arc sequence (the Davle Group). Their stratigraphic relation is unknown. Flysch units (the Štěchovice Group and Svrchnice Formation) overlay the arc volcanics, and both units contain material derived from volcanic arc. The former was also sourced from the NW segment, whereas the latter contains an increased amount of passive margin continental material. In contrast to the Blovice complex, the flysch experienced only weak Cadomian deformation. The new lithotectonic zonation fits the following tectonic scenario for the Cadomian evolution of the TBU well. The S- to SE-directed Cadomian subduction beneath the TBU led to the involvement of turbidites, chaotic deposits, and 605 ± 39 Ma ocean floor in the accretionary wedge represented by the Blovice complex. The accretionary wedge formation mostly overlapped temporally with the growth of the volcanic arc (the Davle Group) at ~ 620–560 Ma. Upon cessation of the arc igneous activity, the rear of the wedge and some elevated portions of the arc were eroded to supply the deep-water flysch sequences of the Štěchovice Group, whereas the comparable Svrchnice Formation (~ 560 to < 544 Ma) was deposited in a southeasterly remnant basin close to the continental margin. The Cadomian orogeny in the TBU was terminated at ~ 550–540 Ma by slab breakoff, by final attachment of the most outboard ~ 540 Ma oceanic crust, and by intrusion of ~ 544–524 Ma boninite dikes marking the transition from the destructive to transform margin during the early/middle Cambrian.

Arc accretion to the early Paleozoic Antarctic margin of Gondwana in Victoria Land

The Antarctic Ross Orogen was built up during the early Paleozoic in the framework of the convergence between the Paleo-Pacific oceanic plate and the Gondwana continental margin. Models for the Ross Orogen in northern Victoria Land are based on terranes having a variable provenance with respect to the margin. However, recent studies provide evidence for the occurrence of different pieces of the lithospheric puzzle: (i) the Wilson continental magmatic arc, representing the main part of the active Gondwana margin, (ii) the Bowers arc–backarc system, (iii) the Admiralty crustal ribbon including continental material of the Wilson forearc, and (iv) the newly discovered, Cambrian oceanic magmatic Tiger arc, along the Ross Sea coast. An updated model is presented in which, after the Early Cambrian magmatic activity of the Wilson arc, a retreat of the subduction zone in the Early–Middle Cambrian gave way to boudinage of the Wilson forearc, trenchward arc migration, opening of the Bowers backarc basin and inception of the outboard Tiger subduction zone. Renewed convergence resulted in the development of the Middle Cambrian Bowers arc, closure of the backarc and deep underthrusting of portions of it at the Middle–Late Cambrian. Finally, in the latest Cambrian to earliest Ordovician, fast exhumation was coupled in the north with erosion and sediment shed to the northeast, and with extension and potassic magmatism in central and southern Victoria Land.

Plate-driven extension and convergence along the East Gondwana active margin: Late Silurian–Middle Devonian tectonics of the Lachlan Fold Belt, southeastern Australia

The Lachlan Fold Belt of southeastern Australia developed along the Panthalassan margin of East Gondwana. Major silicic igneous activity and active tectonics with extensional, strike-slip and contractional deformation have been related to a continental backarc setting with a convergent margin to the east. In the Early Silurian (Benambran Orogeny), tectonic development was controlled by one or more subduction zones involved in collision and accretion of the Ordovician Macquarie Arc. Thermal instability in the Late Silurian to Middle Devonian interval was promoted by the presence of one or more shallow subducted slabs in the upper mantle and resulted in widespread silicic igneous activity. Extension dominated the Late Silurian in New South Wales and parts of eastern Victoria and led to formation of several sedimentary basins. Alternating episodes of contraction and extension, along with dispersed strike-slip faulting particularly in eastern Victoria, occurred in the Early Devonian culminating in the Middle Devonian contractional Tabberabberan Orogeny. Contractional deformation in modern systems, such as the central Andes, is driven by advance of the overriding plate, with highest strain developed at locations distant from plate edges. In the Ordovician to Early Devonian, it is inferred that East Gondwana was advancing towards Panthalassa. Extensional activity in the Lachlan backarc, although minor in comparison with backarc basins in the western Pacific Ocean, was driven by limited but continuous rollback of the subduction hinge. Alternation of contraction and extension reflects the delicate balance between plate motions with rollback being overtaken by advance of the upper plate intermittently in the Early to Middle Devonian resulting in contractional deformation in an otherwise dominantly extensional regime. A modern system that shows comparable behaviour is East Asia where rollback is considered responsible for widespread sedimentary basin development and basin inversion reflects advance of blocks driven by compression related to the Indian collision.

Subduction-driven shortening and differential exhumation in a Cadomian accretionary wedge: The Teplá–Barrandian unit, Bohemian Massif

The Teplá–Barrandian unit (TBU) of Central Europe's Bohemian Massif exposes perhaps the best preserved fragment of an accretionary wedge in the Avalonian–Cadomian belt, which developed along the northern active margin of Gondwana during Late Neoproterozoic. In the central TBU, three NE–SW-trending lithotectonic units (Domains 1–3) separated by antithetic brittle faults differ in lithology, style and intensity of deformation, magnetic fabric (AMS), and degree of Cadomian regional metamorphism. The flysch-like Domain 1 to the NW is the most outboard (trenchward) unit which has never been significantly buried and experienced only weak deformation and folding. The central, mélange-like Domain 2 is characterized by heterogenous intense deformation developed under lower greenschist facies conditions, and was thrust NW over Domain 1 along a SE-dipping fault. To the SE, the most inboard (arcward) Domain 3 is lithologically monotonous (dominated by graywackes and slates), was buried to depths corresponding up to the lower greenschist facies conditions, where it was overprinted by a pervasive SE-dipping cleavage and then was exhumed along a major NW-dipping normal fault. We interpret these domains to represent allochtonous tectonic slices that were differentially buried and then exhumed from various depths within the accretionary wedge during Cadomian subduction. The NW-directed thrusting of Domain 2 over Domain 1 may have been caused by accretion at the wedge front, whereas the SE-dipping cleavage and SE-side-up exhumation of Domain 3 may record inclined pervasive shortening during tectonic underplating and subsequent horizontal extension of the rear of the wedge. The boundary faults were later reactivated during Cambro–Ordovician extension and Variscan compression. Compared to related terranes of the Cadomian belt, the TBU lacks exposed continental basement, evidence for regional strike-slip shearing, and extensive backarc magmatism and LP–HT metamorphism, which could be interpreted to reflect flat-slab Cadomian subduction. This, in turn, suggests that Cadomian accretionary wedges developed in a manner identical to those of modern settings, elevating the TBU to a key position for understanding the style, kinematics, and timing of accretionary processes along the Avalonian–Cadomian belt.