Abstract The status of Pannotia as an Ediacaran supercontinent, or even its mere existence as a coherent large landmass, is controversial. The effect of its hypothesized amalgamation is generally ignored in mantle convection models claiming the transition from Rodinia to Pangaea represents a single supercontinent cycle. We apply three geodynamic scenarios to Pannotia amalgamation that are tested using regional geology. Scenarios involving quasi-stationary mantle convection patterns are not supported by the geological record. A scenario involving feedback between the supercontinent cycle and global mantle convection patterns predicts upwellings beneath the Gondwanan portion of Pannotia and the arrival of plumes along the entire Gondwanan (but not Laurentian) margin beginning at c. 0.6 Ga. Such a scenario is compatible with regional geology, but the candidates for plume magmatism we propose require testing by detailed geochemical and isotopic studies. If verified, this scenario could provide geodynamic explanations for the origins of the late Neoproterozoic and Early Paleozoic Iapetus and Rheic oceans and the terranes that were repeatedly detached from their margins.

Abstract A supercontinent is generally considered to reflect the assembly of all, or most, of the Earth's continental lithosphere. Previous studies have used geological, atmospheric and biogenic ‘geomarkers’ to supplement supercontinent identification. However, there is no formal definition of how much continental material is required to be assembled, or indeed which geomarkers need to be present. Pannotia is a hypothesized landmass that existed in the interval c. 0.65–0.54 Ga and was comprised of Gondwana, Laurentia, Baltica and possibly Siberia. Although Pannotia was considerably smaller than Pangaea (and also fleeting in its existence), the presence of geomarkers in the geological record support its identification as a supercontinent. Using 3D mantle convection models, we simulate the evolution of the mantle in response to the convergence leading to amalgamation of Rodinia and Pangaea. We then compare this supercontinent ‘fingerprint’ to Pannotian activity. For the first time, we show that Pannotian continental convergence could have generated a mantle signature in keeping with that of a simulated supercontinent. As a result, we posit that any formal identification of a supercontinent must take into consideration the thermal evolution of the mantle associated with convergence leading to continental amalgamation, rather than simply the size of the connected continental landmass.

Abstract Pannotia is a hypothetical supercontinent that may have existed briefly during the Proterozoic–Cambrian transition. Various lines of evidence used to argue for its existence include global orogenesis in Ediacaran–Cambrian time, the development of Cambrian passive margins and some (but not all) tectonic reconstructions. Indirect measures used to infer Pannotia's veracity include patterns of biological diversity, palaeoclimate, sea level, magmatism and other palaeoenvironmental proxies. It is shown herein that neither the direct records nor the indirect proxies provide compelling support for Pannotia. If that ephemeral contiguous landmass existed at all, its effects on the broader Earth system are inextricably tied to the more fundamental processes of Gondwanaland assembly. This perspective emphasizes the remarkable consolidation of Gondwanaland as a semi-supercontinent within the early stages of the Pangaea cycle. Gondwanaland's size combined with its c. 300 myr longevity might have greater significance for mantle dynamics than the larger, but shorter-lived, Pangaea landmass.

Abstract Three supercontinents have been suggested to have existed in the last 1 Gyr. The supercontinent status of Pangaea and Rodinia is undisputed. In contrast, there is ongoing controversy on whether Pannotia existed at all. Here, we test the hypothesis of a Pannotian supercontinent. Using first-order tectonic constraints, we reconstruct the Paleozoic kinematics of major continents relative to the East European Craton. Back-rotation from Pangaea results in a supercontinent constellation in the early Paleozoic corroborating the existence of Pannotia. The presented model explains first-order constraints for both the break-up of Pannotia and the subsequent assembly of Pangaea. The break-up of Pannotia comprises (1) the early Paleozoic opening of Iapetus II and in turn the Rheic Ocean, concomitant with the subduction of the Neoproterozoic Iapetus I Ocean and (2) the coeval opening of the Palaeo-Arctic Ocean, which separated Siberia from the North American Craton. The subsequent convergence of the North American Craton, Avalonia, Gondwana and Siberia with the East European Craton resulted in Paleozoic collisional orogenies at different plate boundary zones. The existence of Rodinia, Pannotia and Pangaea as pari passu supercontinents implicates two complete supercontinent cycles from Rodinia to Pannotia and from Pannotia to Pangaea in the Neoproterozoic and the Paleozoic, respectively.

Abstract Lithospheric cusps occur where arcs are joined end to end. Where a subducting plate moves directly into a cusp, the slab experiences lateral constriction due to the cusp geometry. Buckled slabs of Cenozoic age occur at cusps (also known as ‘syntaxes’) in the Arabian, Indian, Pacific, Juan de Fuca and other plates. Here I report an Ediacaran example from the cusp of the Congo Craton where Pan-African collision zones meet at a right angle in NW Namibia. The craton was blanketed by syn- and post-rift Neoproterozoic marine carbonate, disconformably overlain by collision-related foredeep clastics. The disconformity has little stratigraphic relief in a 900 km-long fold belt rimming the craton, except within 60 km of the cusp apex where foredeep deposits bury a megakarst landscape floored by exhumed crystalline basement. Forebulge uplift, estimated from palaeokarst relief, was ≥1.85 km. This far exceeds characteristic forebulge heights of c. 0.5 km and matches the deepest part of the Grand Canyon of Arizona (USA). Coeval with megakarst development, map-scale mass slides moved coherently westwards and southwards towards the advancing accretionary prisms. Rapid burial by foredeep clastics preserved the megakarst palaeosurface and associated mass slides; folding them brought protection from complete destructive resurfacing for eons.

Abstract The Neoproterozoic tectonomagmatic evolution of West Avalonia comprises four major events. Tectonism started with the formation of a Tonian passive margin on a Baltica-derived ribbon dispersed into the Mirovoi Ocean. Obduction of an oceanic terrane onto the ribbon produced olistostromes, deformation and metamorphism before 750 Ma. Obduction was followed by a Tonian (750–730 Ma) arc on the created composite crust. A pause in magmatism between 730 and 700 Ma is the next event. Subsequently, a Cyrogenian (700–670 Ma) arc was formed, which may have collided with Baltica or another buoyant element nearby. Thereafter, a long-lasting (640–565 Ma) continental arc was erected which, combined with the late Ediacaran–Early Paleozoic sedimentary cover, represents the hallmark of West Avalonia. A Caribbean-style incursion of the Ediacaran arc into the widening Tornquist gap between Amazonia and Baltica led to a diachronous collision with the Ganderian arc. Strike-slip slivering produced a complex transfer of terranes to both: Carolinia and smaller terranes to Ganderia, and East Avalonia to West Avalonia. The Rheic Ocean opened diachronously at c. 500 Ma, following a plate reorganization and re-establishment of an oblique subduction zone beneath Amazonia. As a result, Avalonia and Ganderia became progressively separated and dispersed into the Iapetus Ocean.

Abstract The Variscan Orogen in Iberia and the Anti-Atlas Mountains in Morocco contains a set of ophiolites formed between Neoproterozoic and Devonian times, during the complex evolution of the NW African–Iberian margin of Gondwana. During this time interval, the margin evolved from an active margin (c. 750–500 Ma: the Reguibat–Avalonian–Cadomian arc) to the final collision with Laurussia in Devonian times to form Pangaea. In this context, one of the oldest recognized ophiolites is the Bou Azzer Ophiolite from the Anti-Atlas Mountains, dated at c. 697 Ma and containing two types of mafic rocks, the youngest of which has a boninitic composition. To the north, in the SW Iberian Massif, the Calzadilla Ophiolite contains mafic rocks also of boninitic composition dated at c. 598 Ma. Farther north, in the NW Iberian Massif, the Vila de Cruces Ophiolite is formed by a thick sequence of mafic rocks with an arc tholeiitic composition and minor alternations of tonalitic orthogneisses dated at c. 497 Ma. In the same region, the Bazar Ophiolite has a similar age of c. 495 Ma. Also in NW Iberia, there is a group of ophiolites with varied lithologies and dominant mafic rocks with arc tholeiitic composition (Careón, Purrido and Moeche ophiolites) dated at c. 395 Ma. The composition of all these peri-Gondwanan ophiolites is of supra-subduction zone type, showing no evidence of preserved mid-ocean ridge basalt type oceanic lithosphere. Consequently, these ophiolites were generated in the peri-Gondwanan realm during the opening of forearc or back-arc basins. Forearc oceanic lithosphere was promptly obducted or accreted to the volcanic arc, but the oceanic or transitional lithosphere generated in back-arc settings was preserved until the assembly of Pangaea. Based on the ages of the described ophiolites, the peri-Gondwanan realm has been a domain where the generation of oceanic or transitional lithosphere seems to have occurred at intervals of c. 100 myr. These regularly spaced time intervals may indicate cyclic events of mantle upwelling in the peri-Gondwanan mid-ocean ridges, with associated higher subduction rates at the peri-Gondwanan trenches and concomitant higher rates of partial melting in the mantle wedges involved. The origin of the apparent cyclicity for mantle upwelling in the peri-Gondwanan ocean ridges is unclear, but it could have possibly been related to episodic deep mantle convection. Cycles of more active deep mantle convection can explain episodic mantle upwelling, the transition from low- to fast-spreading type mid-ocean ridges and, finally, the dynamic context for the episodic generation of new supra-subduction zone type oceanic peri-Gondwanan lithosphere.

Abstract U–Pb zircon dating and geochemical investigations were applied to rocks from the Variscan deformed and metamorphosed Drosendorf Unit of the eastern Bohemian Massif, Austria. Data show that this unit contains remnants of a Mesoproterozoic granite (Dobra Gneiss Type A), early Neoproterozoic sediments (paragneiss, marble) and late Neoproterozoic volcanic-arc granitoids from the Avalonian–Cadomian peri-Gondwana Orogen (Dobra Gneiss Type B, Spitz Gneiss). A rock association of such old age is extraordinary in the Variscan Orogen. We interpret that these rocks were originally part of the Brunovistulian foreland plate (i.e. part of Avalonian Europe and the Devonian Old Red Continent, respectively) before being tectonically incorporated into the eastern flank of the Variscan Orogen. This interpretation is considered likely because the Dobra Gneiss Type B turned out to be extremely similar, geochemically and in age, to the Bittesch Gneiss in the Moravian Zone, which is generally accepted as a Brunovistulian (i.e. Avalonian) rock. Based on the zircon ages measured in ortho- and paragneiss samples, the Drosendorf Unit can be excellently correlated with West Amazonia. This supports the long-standing idea that Avalonian Europe contains terranes of Amazonian ancestry. A model is presented to show how West Amazonian rocks could have been transferred to Europe in the Early Paleozoic.

Abstract New U–Pb zircon ages from the Eastern Saghro massif in the Anti-Atlas of Morocco demonstrate a link between Pan-African transpressive collision at c. 600 Ma and transtension caused by the onset of Cadomian subduction and arc development from c. 570 Ma onwards. We present new U–Pb laser ablation inductively coupled plasma mass spectrometry ages of detrital and magmatic zircon from the Saghro, M'Gouna, and Ouarzazate Groups. The siliciclastic deposits of the Saghro Group were deposited in a back-arc setting developed on stretched continental crust of the West African margin. Collision with the Atlas–Meseta domain led to the closure of the back-arc basin before 600 Ma. Time of exhumation and surface exposure of the newly formed Pan-African basement is bracketed to c. 30 Ma owing to the maximum depositional age of 571 ± 4 Ma of the overlying M'Gouna Group. The U–Pb age of 567 ± 4 Ma for the lowermost ignimbrite of the Ouarzazate Group limits the time for the deposition of the M'Gouna Group to less than 4 Ma. The Pan-African orogeny was finished at c. 600 Ma whereas the onset of transtension related to Cadomian back-arc formation was very much younger from c. 570 Ma onwards.

Abstract Lithostratigraphic and structural data from the Early Paleozoic Hardangervidda Group in southern Norway indicate that SW Baltica was affected several hundred kilometres NE of the inferred collision zone with Avalonia (Thor Suture). The first sign of plate interaction was the deposition of a 50–60 m-thick quartz arenite (Holberg Formation) of Floian/Dapingian age in an otherwise mud- and carbonate-dominated shelf. An overlying 5–6 m-thick marble unit of Dapingian–Darriwilian age (‘Orthoceratite Limestone’) marks a change into greenish-grey, calcareous phyllite, locally with beds of impure marble and poorly sorted metasandstone (Solnut Formation). A series of décollement folds (D 2 structures) with axes orientated NW–SE (cross-folds) which superimposed a thin-skinned fold thrust with a NE–SW trend (D 1 structures) are interpreted as having developed during the progressive underthrusting of Baltica underneath Avalonia. Support for this model is seen in detrital zircon populations: with the Holberg and underlying formations having a Baltican signature, in contrast to the overlying Solnut Formation with a peri-Gondwana signature, including a distinct Late Neoproterozoic zircon population. It is further speculated that the c. 471–458 Ma Garborg eclogite and surrounding paragneisses in the Stavanger area are related to the suture zone between Baltica and Avalonia rather than being related to the Iapetus Ocean and Laurentia, as generally thought.

Abstract The late Neoproterozoic–Paleozoic Iapetus Ocean developed between Laurentia, Baltica, Siberia and Gondwana. Its Paleozoic closure history is recorded by volcano-sedimentary successions within the Caledonian orogen of Scandinavia, the British Isles and Newfoundland. We present new lithological, geochemical and geochronological data relevant for the Iapetan closure history from the hitherto poorly known Trollhøtta–Kinna Basin (central Norwegian Caledonides). This basin consists of alternating siliciclastic rocks, mid-ocean ridge basalts (MORBs), and felsic volcanic rocks highly enriched in, for example, Th, U and light REEs. Rhyolites from the stratigraphically upper part are dated by zircon U–Pb thermal ionization mass spectrometry to 473.3 ± 1.0 and 472.4 ± 0.7 Ma. Detrital zircon spectra indicate deposition after c. 480 Ma, with sediments derived from composite Cambro-Ordovician and Archean–Neoproterozoic landmass(es), possibly the Laurentian margin or a related microcontinent. The peculiar bimodal volcanic association is interpreted as an intermittent phase of marginal basin rifting, derived from a heterogeneous mantle source previously metasomatized by continental material. The tectonic mechanisms behind rifting could be slab retreat and/or break-off, or far-field tectonic forces within the Iapetan realm. Comparison of this basin with other Iapetus-related, similarly-aged volcano-sedimentary successions along the Caledonian–Appalachian orogen indicate that the bimodal MORBs and highly enriched rocks reflect a palaeotectonic setting hitherto unknown in the orogen.

Abstract The Scandinavian Caledonides consist of disparate nappes of Baltican and exotic heritage, thrust southeastwards onto Baltica during the Mid-Silurian Scandian continent–continent collision, with structurally higher nappes inferred to have originated at increasingly distal positions to Baltica. New U–Pb zircon geochronological and whole-rock geochemical and Sm–Nd isotopic data from the Rödingsfjället Nappe Complex reveal 623 Ma high-grade metamorphism followed by continental rifting and emplacement of the Umbukta gabbro at 578 Ma, followed by intermittent magmatic activity at 541, 510, 501, 484 and 465 Ma. Geochemical data from the 501 Ma Mofjellet Group is indicative of arc magmatism at this time. Syntectonic pegmatites document pre-Scandian thrusting at 515 and 475 Ma, and Scandian thrusting at 429 Ma. These results document a tectonic history that is compatible with correlation with peri-Laurentian and/or peri-Gondwanan terranes. The data allow correlation with nappes at higher and lower tectonostratigraphic levels, including at least parts of the Helgeland, Kalak and Seve nappe complexes, implying that they too may be exotic to Baltica. Neoproterozoic fragmentation of the hypothesized Rodinia supercontinent probably resulted in numerous coeval, active margins, producing a variety of peri-continental terranes that can only be distinguished through further combined geological, palaeomagnetic and palaeontological investigations.

Abstract Garnet Lu–Hf and Sm–Nd ages from the Shetland Caledonides provide evidence of a polyorogenic history as follows: (1) c. 1050 Ma Grenvillian reworking of Neoarchaean basement; (2) c. 910 Ma Renlandian metamorphism of the Westing Group; (3) c. 622–606 Ma metamorphism of the Walls Metamorphic Series but of uncertain significance because the eastern margin of Laurentia is thought to have been in extension at that time; (4) Grampian I ophiolite obduction at c. 491 Ma followed by crustal thickening and metamorphism between c. 485 and c. 466 Ma; (5) Grampian II metamorphism between c. 458 and c. 442 Ma that appears to have been focused in areas where pre-existing foliations were gently inclined and thus may have been relatively easily reworked; (6) Scandian metamorphism at c. 430 Ma, although the paucity of these ages suggests that much of Shetland did not attain temperatures for garnet growth. There is no significant difference in the timing of Caledonian orogenic events either side of the Walls Boundary Fault, although this need not preclude linkage with the Great Glen Fault. However, the incompatibility of Ediacaran events either side of the Walls Boundary Fault may indicate significant lateral displacement and requires further investigation.

Abstract The early Paleozoic rocks of eastern Ireland span the suture zone between the Laurentian and Ganderian continental margins of the Iapetus Ocean. The Grangegeeth Terrane comprises a Laurentian continental fragment and Ordovician volcanic arc that formed the southern margin of the late Ordovician Rathkenny Basin of Moffat Shale facies mudstone. Together, these were overstepped in Wenlock time by Laurentia-derived greywackes and became the southernmost tract of the Longford–Down Terrane accretionary prism as subduction brought them into the Laurentian-margin trench. South of the suture, Middle Ordovician failed rifting of a Ganderian volcanic arc terrane was followed by shortening during continuing subduction under the north-facing arc. In Newfoundland, a Ganderian volcanic arc migrated across the ocean in mid-Ordovician times to accrete to Laurentia. In Ireland, the late-accreted arc on the Laurentian margin was formed on a Laurentian microcontinent, and Ganderia had an active margin throughout Late Ordovician time. Silurian closure of Iapetus was between the leading edge of Ganderia and the Laurentian margin, unlike in the Canadian Appalachians, where separate Ordovician and Silurian sutures are recognized. Iapetus was narrow by Katian time but subduction-related magmatism continued into Wenlock times on both margins.