Trond H. Torsvik

Earth geography from 500 to 400 million years ago: a faunal and palaeomagnetic review

Very different palaeogeographical reconstructions have been produced by a combination of palaeomagnetic and faunal data, which are re-evaluated on a global basis for the period from 500 to 400 Ma, and are presented with appropriate confidence (or lack of it) on six maps at 20 Ma intervals. The palaeomagnetic results are the most reliable for establishing the changing palaeolatitudes of Baltica, Laurentia and Siberia. However, global palaeomagnetic reliability dwindles over the 100 Ma, and more evidence for relative continental positioning can be gleaned from study of the distribution of the faunas in the later parts of the interval. The new maps were generated initially from palaeomagnetic data when available, but sometimes modified, and terranes were positioned in longitude to take account of key faunal data derived from the occurrences of selected trilobites, brachiopods and fish. Kinematic continuity over the long period is maintained. The many terranes without reliable palaeomagnetic data are placed according to the affinities of their contained fauna. The changing positions of the vast palaeocontinent of Gondwana (which has hitherto been poorly constrained) as it drifted over the South Pole during the interval have been revised and are now more confidently shown following analysis of both faunal and palaeomagnetic data in combination, as well as by the glacial and periglacial sediments in the latest Ordovician. In contrast, the peri-Gondwanan and other terranes of the Middle and Far East, Central Asia and Central America are poorly constrained.

The making and unmaking of a supercontinent: Rodinia revisited

Abstract During the Neoproterozoic, a supercontinent commonly referred to as Rodinia, supposedly formed at ca. 1100 Ma and broke apart at around 800–700 Ma. However, continental fits (e.g., Laurentia vs. Australia–Antarctica, Greater India vs. Australia–Antarctica, Amazonian craton [AC] vs. Laurentia, etc.) and the timing of break-up as postulated in a number of influential papers in the early–mid-1990s are at odds with palaeomagnetic data. The new data necessitate an entirely different fit of East Gondwana elements and western Gondwana and call into question the validity of SWEAT, AUSWUS models and other variants. At the same time, the geologic record indicates that Neoproterozoic and early Paleozoic rift margins surrounded Laurentia, while similar-aged collisional belts dissected Gondwana. Collectively, these geologic observations indicate the breakup of one supercontinent followed rapidly by the assembly of another smaller supercontinent (Gondwana). At issue, and what we outline in this paper, is the difficulty in determining the exact geometry of the earlier supercontinent. We discuss the various models that have been proposed and highlight key areas of contention. These include the relationships between the various ‘external’ Rodinian cratons to Laurentia (e.g., Baltica, Siberia and Amazonia), the notion of true polar wander (TPW), the lack of reliable paleomagnetic data and the enigmatic interpretations of the geologic data. Thus, we acknowledge the existence of a Rodinia supercontinent, but we can place only loose constraints on its exact disposition at any point in time.

Rodinia refined or obscured: palaeomagnetism of the Malani igneous suite (NW India)

Abstract New palaeomagnetic data from the Neoproterozoic felsic volcanic rocks of the Malani igneous suite (MIS) in NW India, combined with data from an earlier study, yield a palaeomagnetic pole with latitude=74.5°N, longitude=71.2°E (dp/dm=7.4/9.7°). A statistically positive fold test and remanences carried by typical high-temperature oxidation (deuteric) minerals support a primary magnetic signature. U/Pb ages from MIS (771–751 Ma) overlap with those for granitoids and dolerite dykes from the Seychelles microcontinent (mainly 748–755 Ma), and palaeomagnetic data for both entities can be matched with a tight reconstruction fit (Seychelles→India: Euler latitude=25.8°N, longitude=330°E, rotation angle=28°). In this Neoproterozoic time interval, MIS and the Seychelles must have been located at intermediate northerly latitudes along the western margin of Rodinia, with magmatism that probably originated in a continental arc. The most reliable, dated palaeomagnetic data (±756 Ma) from MIS, Seychelles and Australia require a crucial reappraisal of the timing and plate dynamics of Rodinia break-up and Gondwana assemblage. These new data necessitate an entirely different fit of East Gondwana elements than previously proposed, and also call to question the validity of the Southwest US–East Antarctic and Australia–Southwest US models. The palaeomagnetic data mandate that Greater India was located west of Australia rather than forming a conjugate margin with East Antarctica in the Mid-Neoptroterozoic. Break-up of Rodinia along western Laurentia may therefore have taken place along two major Neoproterozoic rifts; one leading to separation of Laurentia and Australia–East Antarctica, and the second between Australia and India.

Reconstructions of the continents around the North Atlantic at about the 60th parallel

Abstract Late Carboniferous–Early Tertiary apparent polar wander (APW) paths (300–40 Ma) for North America and Europe have been tested in various reconstructions. These paths demonstrate that the 500 fathom Bullard et al. fit is excellent from Late Carboniferous to Late Triassic times, but the continental configuration in northern Pangea changed systematically between the Late Triassic (ca. 214 Ma) and the Mid-Jurassic (ca. 170 Ma) due to pre-drift extension. Best fit North Atlantic reconstructions minimize differences in the Late Carboniferous–Early Jurassic and Late Cretaceous–Tertiary segments of the APW paths, but an enigmatic difference exists in the paths for most of the Jurassic, whereas for the Early Cretaceous the data from Europe are nearly non-existent. Greenland’s position is problematic in a Bullard et al. fit, because of a Late Triassic–Early Jurassic regime of compression (>300 km) that would be inherently required for the Norwegian Shelf and the Barents Sea, but which is geologically not defensible. We suggest a radically new fit for Greenland in between Europe and North America in the Early Mesozoic. This fit keeps Greenland ‘locked’ to Europe for the Late Paleozoic–Early Mesozoic and maintains a reconstruction that better complies with the offshore geological history of the Norwegian Shelf and the Barents Sea. Pre-drift (A24) extension amounted to approximately 450 km on the Mid-Norwegian Shelf but with peak extension in the Late Cretaceous.

The Neoproterozoic and Palaeozoic palaeomagnetic data for the Siberian Platform: From Rodinia to Pangea

Abstract Using the most reliable palaeomagnetic data from the Siberian Platform we have constructed an apparent polar wander (APW) path extending between 1100 Ma and 250 Ma. From this we derive the palaeo-latitudinal drift history and orientation change of Siberia through the Neoproterozoic and Palaeozoic. Comparison of selected palaeomagnetic data from Siberia north and south of the Viljuy basin confirms a mid-Palaeozoic anticlockwise rotation of northern Siberia relative to southern Siberia. The rotation of approximately 20 degrees was first proposed by Gurevich in 1984. The Viljuy basin runs approximately east–west along latitude 64°N. APW paths based on data compilations including, for example, Ordovician data from both the Lena river section (south) and Moyero river section (north) will be adversely affected by this relative rotation. The palaeomagnetic data indicate an inverted orientation for Siberia in `Rodinia times (ca. 750 Ma) in a palaeo-latitudinal belt between 15°S and 20°N. This is inconsistent with a palaeo-position on the northern margin of Rodinia if the rest of Rodinia is located according to palaeomagnetic data from Laurentia, Baltica and East Gondwana. The final convergence between Siberia and Baltica is poorly constrained by palaeomagnetic data. At 360 Ma Siberia was in an inverted position in mid-northerly latitudes, separated from Baltica (to the south) by an east–west oceanic tract approximately 1500 km wide. The next palaeomagnetic constraint on the position of Siberia is at 250 Ma which puts Siberia and Baltica together at the northern end of Pangea. The convergence of the two is characterised by the northerly drift of Baltica and clockwise rotation of Siberia. Although the APW paths for Siberia, Baltica and Laurentia differ, they imply broadly similar palaeo-latitudinal drift trends for the three continents. During the time-period studied all three continents start in southerly/equatorial palaeo-latitudes, drift south, then drift north, changing drift sense at approximately the same time. The smaller scale differences in palaeo-latitude change reflect the opening and closing of intervening oceans. The overall pattern of movements may reflect a large scale (temporal and spatial) geodynamic system which survived the construction and destruction of supercontinents. If we hold to the concept that true polar wander is not significant, we conclude that large continents, although intermittently separated by oceanic tracts, may be driven across the globe in a weak union for periods of 800 Ma or more.

North Atlantic sea-floor spreading rates: implications for the Tertiary development of inversion structures of the Norwegian–Greenland Sea

The Tertiary development of the Norwegian continental margin was dominated by the opening of the Arctic–North Atlantic Ocean. The correct identification of magnetic anomalies and their ages and the analysis of spreading rates during the formation of this ocean are important in understanding the development of the region and specifically the history of its passive margins. Three ocean domains, the Ægir, Reykjanes and Mohns regions, were investigated in an effort to understand the lateral changes in structural development of the passive margin after continental break-up. Spreading rates generally slowed down from 2 cm a−1 after Early Eocene initiation of sea-floor spreading, to values around 0.5 cm a−1 in Oligocene time. An increase in spreading rates to around 1 cm a−1 coincided with the positioning of the Iceland hotspot under the North Atlantic mid-ocean ridge. At the same time, the European plate changed its absolute plate motion from a north-directed drift to a motion more towards the east. The location of inversion structures in the Vøring and Faeroes Basin rather than in the Møre Basin is related to differences in spreading rates. The Mohns and the Reykjanes Ridges produced more ocean floor than the Ægir–Kolbeinsey Ridges. Asymmetric ocean-floor formation in the Ægir Ridge led to differential stress at the base of the lithosphere, which probably explains the absence of inversion features in the Møre Basin (less mantle drag). Furthermore, upper plate margins such as the Vøring Basin and possibly the Faeroe Basin have a lower compressional strength than lower plate margins such as the Møre Basin, and therefore preferentially developed inversion structures. Along the transform boundaries separating the domains, additional stress probably built up along extension of the transform zones into the extended continental crust. This additional stress probably also assisted initiation of the inversion structures in the Vøring Basin and the Faeroes area. The amplification of the inversion structures in the Vøring Basin and the Faeroes Basin was subsequently caused by a variety of processes related to sedimentation and uplift–erosion.

Baltica upside down: A new plate tectonic model for Rodinia and the Iapetus Ocean

We propose that Baltica was geographically inverted throughout the Neoproterozoic and therefore suggest reassessment of the classic Wilson Cycle paleoreconstruction depicting an Atlantic-type early Paleozoic (Iapetus) ocean between western Norway and East Greenland. Our new reconstruction dismisses the need for 180° rotation of Baltica after breakup of Iapetus and presents more plausible geologic correlations between Baltica, Laurentia, and Gondwana than those accompanying previous fits. In our reconstruction, the breakup that led to the formation of the Iapetus Ocean was initiated at a junction between a rift (Laurentia-Gondwana), a right-lateral fault (Laurentia-Baltica), and a trench (inverted Baltica-Gondwana), thereby linking the late Precambrian Iapetus opening to the Timanian and Avalonian orogenies between Gondwana and an inverted Baltica.

U–Pb geochronology of Seychelles granitoids: a Neoproterozoic continental arc fragment

Abstract New high-precision U–Pb zircon ages for 14 granitoid rocks of the Seychelles, including samples from Mahe, Praslin, La Digue, Ste. Anne, Marianne, Fregate and Recifs Islands, yield dates between 748.4±1.2 Ma and 808.8±1.9 Ma, interpreted as representing magmatic crystallization ages. U–Pb zircon ages as young as 703 Ma have been reported by other workers, and the time span of Seychelles magmatic activity, therefore, is ∼100 Myr. The vast majority of ages, however, fall in the period 748–755 Ma, suggesting a major period of granite plutonism at this time. At least some Seychelles dolerite dikes have equivalent ages, indicating the contemporaneity of granitic and basaltic magmas, and supporting field and chemical evidence for the production of minor quartz dioritic rocks by hybridization and magma mingling. Possible correlatives of the late Neoproterozoic (∼700–800 Ma) granitoids and dolerites of the Seychelles include volcanic and plutonic rocks in Madagascar and northwestern India (Malani Igneous Suite, Rajasthan), which may have formed in a continuous continental (Andean-type) arc located at the western margin of the Rodinia supercontinent. This idea is more consistent with the time span of magmatism, petrologic character of the igneous rocks, and paleomagnetically determined reconstructions, than the commonly held view of an intra-plate extensional setting for the Seychelles and Malani Igneous Suite.

The Tornquist Sea and Baltica–Avalonia docking

Early Ordovician (Late Arenig) limestones from the SW margin of Baltica (Scania-Bomholm) have multicomponent magnetic signatures, but high unblocking components predating folding, and the corresponding palaeomagnetic pole (latitude = 19degreesN, longitude = 051degreesE) compares well with Arenig reference poles from Baltica. Collectively, the Arenig poles demonstrate a midsoutherly latitudinal position for Baltica, then separated from Avalonia by the Tomquist Sea. Tornquist Sea closure and the Baltica-Avalonia convergence history are evidenced from faunal mixing and increased resemblance in palaeomagnetically determined palaeolatitudes for Avalonia and Baltica during the Mid-Late Ordovician. By the Caradoc, Avalonia had drifted to palaeolatitudes compatible with those of SW Baltica, and subduction beneath Eastern Avalonia was taking place. We propose that explosive vents associated with this subduction and related to Andean-type magmatism in Avalonia were the source for the gigantic Mid-Caradoc (c. 455 Ma) ash fall in Baltica (i.e. the Kinnekulle bentonite). Avalonia was located south of the subtropical high during most of the Ordovician, and this would have provided an optimum palaeoposition to supply Baltica with large ash falls governed by westerly winds. In Scama, we observe a persistent palaeomagnetic overprint of Late Ordovician (Ashgill) age (pole: latitude-4degreesS, longitude= 012degreesE). The remagnetisation was probably spurred by tectonic-derived fluids since burial alone is inadequate to explain this remagnetisation event. This is the first record of a Late Ordovician event in Scania, but it is comparable with the Shelveian event in Avalonia, low-grade metamorphism in the North Sea basement of NE Germany (440-450 Ma), and sheds new light on the Baltica-Avalonia docking

Plate tectonic modelling: virtual reality with GMAP

Abstract Palaeogeographic reconstructions have been an integral part of global tectonic research since the advent of the plate tectonic paradigm, and GMAP is a state of the art computer program which performs all processing and plotting tasks associated with the generation of palaeogeographic reconstructions and plate tectonic modelling. GMAP is menu-driven and easy to use; the user is never far removed from the basic data from which palaeogeographic reconstructions are derived, and therefore has a sense of total control over the programs performance. GMAP can generate reconstructions based on known Euler rotation data poles or palaeomagnetic poles. The user is also free simply to move continents around on the screen, according to less tangible constraints. GMAP is supplied with a full range of continental outlines. It is also possible to import new continents via simple ASCII files. GMAP is in use at leading institutions world-wide, and has been the work-horse of the EUROPEAN GEOTRAVERSE and EUROPROBE projects.