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Precambrian supercontinents
Precambrian Research 244 (2014) 1–4
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Preface
Precambrian supercontinents
Reconstructions of continents during the Precambrian are critical in understanding the evolution of the Earth. They also provide a tectonic context for major ore deposits and in this way help in detecting metallogenic belts between blocks, and identifying new prospective regions for various kinds of mineral deposits. They are equally important for understanding sedimentary basins and their evolution. This Special Issue focuses on paleomagnetic, tectonic, crustal deformation, isotopic and paleogeographic studies of Precambrian continents and their building blocks to analyze the amalgamation, life span and break up of ancient supercontinents. The state of understanding pre-Pangea reconstructions of the continents or cratons and their specific paleogeographies is currently tentative at best. There is good evidence based, for instance, on the episodic nature of orogenic belts, and other kinds of evidence that there have been several supercontinents during the Precambrian such as the Late Archean Kenorland, the late Paleoproterozoic Nuna (a.k.a. Columbia, Hudsonland) and the Meso- to Neoproterozoic Rodinia (Pesonen et al., 2012a and references therein). It has also been suggested that during Archean times, the continental blocks were assembled into several supercratons, such as Superia, Sclavia and Vaalbara. Beyond these general concepts, the exact reconstructions are poorly constrained. In order to better characterize the nature and origin of the Precambrian paleogeography a special symposium “Precambrian Supercontinents” was held in Helsinki, Finland, September 25–28, 2012 (http://supercontinent2012.helsinki.fi/). The symposium was preceded by an excursion (September 22–25, 2012) to six geological landmark sites of the Baltic Shield. The aim of the symposium and excursion was to update concepts on various supercontinent topics and to discuss the geological and biological significance of supercontinents, and particularly, how many of the Precambrian reconstructions are supported by the increasing number of reliable paleomagnetic, radiometric age and isotope data. Various phenomena during the Precambrian, such as isotopic excursions, magmatic episodes like those involving the emplacement of major dyke swarms (e.g., Bleeker and Ernst, 2006), detrital zircon populations, occurrences of ophiolites, etc. have been identified in Precambrian data. An important aim of the symposium was to examine in which phase of the evolution of a supercontinent (amalgamation, life time or break up) their role becomes significant. Seventy researchers from all over the world attended the symposium and 74 abstracts were presented as oral talks or posters. The Abstract Volume and the Excursion Guidebook of the Symposium
http://dx.doi.org/10.1016/j.precamres.2014.02.014 0301-9268/© 2014 Elsevier B.V. All rights reserved.
are published by the Geological Survey of Finland (Mertanen et al., 2012; Pesonen et al., 2012b). The present Special Issue is the outgrowth of the achievements of the Symposium presentations in a form of 19 papers of which 17 are standard papers and 2 are reviews. The volume is divided under five topical sections. The first section (“Paleomagnetic pre-requisites for supercontinent reconstruction”) begins with an invited review paper by K.L. Buchan (2014), who defines the five requirements that a paleomagnetic pole must fulfill in order to become a key pole and to be an anchoring point in the construction of apparent polar wander paths for the cratons and in making reliable reconstructions. He was able to show that using a set of key poles it is plausible that Laurentia and Baltica formed a united landmass from 1590 to 1260 Ma, which forms the core of the Mesoproterozoic Nuna supercontinent. Another highlight of his analysis is the observation that the Slave and Superior cratons were separate cratons experiencing independent drift prior to ca. 1.9 Ga. Veikkolainen et al. (2014a) test the validity of the Geocentric Axial Dipole (GAD) hypothesis during Precambrian using a new Precambrian paleomagnetic database “PALEOMAGIA”. In the first analysis (Veikkolainen et al., 2014b) the authors used the classical inclination-frequency analysis of Evans (1976). Previously (e.g., Kent and Smethurst, 1998) this test has provided strong evidence for low inclination bias, which has been interpreted to represent a considerable non-dipole contamination in the Precambrian geomagnetic field and by this way has cast shadows over Precambrian reconstructions. The new analysis shows that the Precambrian field is actually close to GAD when only the highest quality data from igneous rocks are included and if binning is done cratonically and not geographically (see Veikkolainen et al., 2014c). The GAD model for Precambrian field is supported by the reversal asymmetryanalysis as shown by Veikkolainen et al. (2014d) in their second contribution to this volume. Considerable work has been done during the last twenty years to reconstruct the continents or cratonic blocks during the Precambrian. Although orogenic belts encircling large landmasses have been interpreted to represent ancient conjugate collisions, much less attention has been placed on what happens inside the crustal blocks when collisions take place. The second section of the volume (“Deformation of crust during supercontinent formation”) contains three papers. Henry Halls opens the discussion by addressing the possibility that significant crustal shortening has taken place in the Paleoproterozoic (Halls, 2014). Halls’ motivation comes from
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Preface / Precambrian Research 244 (2014) 1–4
the observations of Himalayan type collisions in Asia, which have lead to crustal shortening of ca. 2000 km between India and Siberia over the last 200 My. Using this as an analogue, Halls attempts to read whether there are paleomagnetic signatures of crustal shortening associated with the Paleoproterozoic Trans-Hudson orogen (THO) because this orogen bears close similarity both in size and deformation history to the India-Asia collision. The analysis shows that paleomagnetic data can accommodate crustal shortening of about 3000 ± 1000 km between the Superior and Slave cratons. The “crustal shortening” paper is followed by contribution of Fuck et al. (2014). They discuss of whether the cratons show continuity of the basement, or whether some cratons are exotic fragments accreted to the basement during Paleoproterozoic or Neoproterozoic events. They point out that in the generally accepted supercontinents, such as Nuna and Rodinia, the large central cratons like São Francisco and Amazonia are well described while the smaller crustal segments bordering these cratons have been often neglected. The key issue is to identify crustal units of various ages on the western part of the São Francisco craton of South America (Central Brazil) with successive magmatic events. The last paper of the crustal deformation section by Brito Neves and Fuck (2014) discusses the role of the three large cratons forming the basement of the South American platform. The key player is the Amazonian craton but the other cratons, and especially the role of the huge linear fault zones bounding these cratons, is emphasized. The various domains are discussed thoroughly in an attempt to find their “neighbouring” cratons during Archean and Proterozoic times. The third section (“Testing proposed supercontinents with new paleomagnetic data”) deals with paleomagnetic applications related to various phases of Precambrian supercontinents. The first paper of this section is by Salminen et al. (2014a) in which new paleomagnetic results from isotopically dated 2.4–2.3 Ga Taivalkoski diabase dykes, NE Finland are presented. This dyke swarm was also a key target in the pre-Symposium Excursion (Pesonen et al., 2012b). A reliable VGP is established from one E-W trending dyke showing a baddeleyite U–Pb age of 2339 ± 18 Ma and a positive baked contact test. Although preliminary, the result suggests that Baltica and Laurentia were close to each other but not attached at 2.4–2.3 Ga. Two paleomagnetic papers deal with Mesoproterozoic rocks in the Satakunta region, SW Finland. The new results allow testing of the NENA-configuration (Northern Europe-North America) of Baltica and Laurentia. In the first of them Klein et al. (2014) report paleomagnetic data from a red sandstone unit exposed in the Satakunta graben, SW Finland. Based on positive fold and reversal tests, the authors isolated the primary remanence direction of the sandstone, which is distinct from that of the cutting diabase with an age of 1.26 Ga. A tentative apparent polar wander (APW)-age for the sandstone is given ca. 1600 Ma, which is much older than the previously published K–Ar age of 1370 Ma. The relationship between Satakunta sandstone and nearby Eura and Laitila rapakivi massifs remains unsolved. Most likely the deposition of the sandstone started before the intrusion of the rapakivi granites as suggested by detrital zircon data, which does not reveal any hints of a rapakivi source. The other Satakunta paper by Salminen et al. (2014b) shows new primary Mesoproterozoic paleomagnetic data on the 1550 Ma diabase dykes occurring north of the Satakunta sandstone (see Klein et al., 2014). The isolated ChRM is of dual polarity and the primary origin is based on a positive contact test. One dyke is dated (U–Pb) with an age of 1564 Ma. The results conform to the previously proposed long-lived unity of Baltica-Laurentia from 1830 to 1260 Ma. One problem in constructing the Rodinia supercontinent by paleomagnetic data is the question of whether the Sveconorwegian and Grenvillian APW-loops (ca. 1020–830 Ma) can be matched as
would be the case if Laurentia and Baltica formed a drifting unity during Grenvillian times. To shed new light on this issue, Elming (2014) have studied the remanence of the 935–975 Ma diabase dykes exposed in Norrköping-Falun area, southern Sweden. The results provide a new key pole with an age of ca. 955 Ma for Baltica, and confirm the similarity of the Sveconorwegian and Grenvillian APW-loops, thus encompassing their joint drift within the Rodinia supercontinent during ca. 975–935 Ma. Previous paleomagnetically reconstructed PaleoMesoproterozoic supercontinents (see reviews by Pesonen et al., 2003, 2012a) do not include India, due to a lack of reliable data. This situation is now remedied due to a large amount of new, high-quality paleomagnetic results from various Indian cratons. Belica et al. (2014) make a brave attempt to review the drift history of Dharwar craton during Paleoproterozoic times (2.37–1.88 Ga). Most of the new poles from Dharwar are based on precise U–Pb dates, presence of dual polarities and in most cases on positive baked contact tests, thus providing a set of four new key poles for India (Dharwar). The authors discuss the position of India in the Columbia supercontinent, a topic also discussed in other papers of this volume (e.g., Pisarevsky et al., 2014). If India’s position in Paleo-Neoproterozoic supercontinents Columbia and Rodinia has been a problem, the same applies to Amazonia. Two papers provide new Proterozoic paleomagnetic data from the northern part of the Amazonia (Guiana Shield). In the first, Bispo-Santos et al. (2014a) present new dual polarity data of late Paleoproterozoic Surumu group and discuss their role in mapping the Columbia supercontinent. Comparison of the APWP’s of Amazonia and West Africa for the Paleoproterozoic interval suggest that these blocks were linked together at 1.98–1.96 Ga in a configuration whereby the known major shear zones become aligned. The results give support to the East Nuna landmass model and shed new light on the amalgamation time of Nuna. See also Johansson (2014) and Pisarevsky et al. (2014) in this volume. In the follow-up paper by Bispo-Santos et al. (2014b) new Mesoproterozoic paleomagnetic data are presented from the well-dated (1.79 Ga) Avanavero mafic sills, from the northern part of the Amazonian craton. The results constrain Amazonia’s position in the Columbia supercontinent, which is not very different from that of Johansson’s Mesoproterozoic Samba model (South America, Baltica and West Africa; Johansson, 2014), nor from the Columbia-model of Pisarevsky et al. (2014). However, as discussed in these papers, the slightly younger paleomagnetic data from Amazonia at ca. 1.42 Ga contrasts with the Samba model. The authors favour an interpretation that some kind of intra-cratonic movement is the cause for the problems at 1.42 Ga. The fourth section with a subtitle “Precambrian paleogeographies: from Nuna to Rodinia and Gondwana” contains four papers. The first paper dealing with Precambrian paleogeography is the contribution by Johansson (2014) who presents a revision of his previous SAMBA-model of Rodinia (Johansson, 2009), whereby Baltica, Amazonia and West Africa were attached to eastern Laurentia and East Antarctica, while Australia and India are docked with western Laurentia. In building his revised SAMBA-version of Rodinia, he assures us that it’s evolution from the preceding supercontinent NUNA is achieved in an acceptable kinematic way, i.e. not leading to anomalously high plate velocities. He looks the supercontinent cycle in terms of the intro-, extro- or orthoversion way (Mitchell et al., 2012) of breaking up a supercontinent, opening of oceans in between continents, and finally drifting of the continents into their new assembly. Notably, and unlike in his previous SAMBA model, he accepts that Baltica has been rotated by a considerable amount (ca. 75◦ clockwise) since its break up from NUNA and before final docking to Rodinia. Johansson points out the important role of E. Greenland in the Baltica-Laurentia landmass and addresses the question of whether E. Greenland
Preface / Precambrian Research 244 (2014) 1–4
shows Grenvillian type features (see also Pesonen et al., 2003). The answer is difficult since the existing evidence may lie below the ice or is obscured by tectonic reworking. Johansson also discusses other potential Rodinia continents like North China, South China, India and Australia and their positions around the Baltica-Laurentia landmass as discussed also in other papers of this volume (e.g., Pisarevsky et al., 2014). A drawback of Johansson’s model is that his images of SAMBA are cartoons rather than spherical projections, which makes it difficult to test using paleomagnetic data. The second paper in this section is by Pisarevsky et al. (2014) and is a test of Columbia during 1770–1270 Ma using updated paleomagnetic data, radiometric dating and geological evidence from major cratons. The authors divide the cratons into two or three stable landmasses such as West Nuna formed by Laurentia, BalticaSarmatia/Volgo Uralia and possibly India, East Nuna comprising North-, West- and South-Australia, North China and the Mawson craton. A possible third landmass consists of Siberia and Congo/Sao Francisco cratons. The life span of Nuna was short, perhaps from ca. 1650 Ma to its break up at ca. 1450–1380 Ma. The authors offer various plausible reconstructions of Columbia. For example, the SAMBA model (Johansson, 2009) of Columbia at ca. 1.79 Ga is paleomagnetically plausible but fails at 1.42 Ga. Various causes for the failure include intra-cratonic movements between various blocks as proposed also by Bispo-Santos et al. (2014b). The various Rodinia models include in most cases NorthEuropean cratons or fragments such as Baltica, Sarmatia, British Isles and Taimyr. However, often these models dismiss the European platform with its various Meso- to Neoproterozoic terranes. The Cadomian orogen crops out mainly in the Bohemian, Armorican and Iberian areas of the Variscides in Central and Western Europe. Linnemann et al. (2014) discuss the importance of detrital zircon as seen in rocks of the Schwarzburg Antiform in the Bohemian massif and other nearby terranes, which form the central part of the Cadomian orogen. One of the highlights of this paper is the result, using Hf - initial values, coupled with U–Pb TDM-model ages from both detrital and magmatic zircons showing that during the c. 180 Ma long Cadomian magmatic arc activity, juvenile magmas became contaminated by the recycling of Eburnean and Archean crust of Africa. The paper sheds light on the role of geotectonic processes, crustal growth, and crustal recycling of the Cadomian Orogen in Central and Western Europe. The fifth section (“The relevance of detrital zircons studies to supercontinent research”) begins with a paper by Andersen (2014) who offers a critical assessment of the use of detrital zircon data in analyzing the provenance of zircon grains in relation to supercontinents. Andersen’s main point is that the U–Pb and Lu–Hf data from detrital zircons in young, continental cover sequences suggest extensive reworking of older sediments, and therefore the path from “source to sink” is non-unique. He notes that cover sequences in Laurentia, Australia and southern Africa show detrital zircon U–Pb age and Hf isotope patterns that cannot be distinguished with confidence from bedrock sources in Fennoscandia. In the second paper of this section, Kuznetsov et al. (2014) uses detrital zircon to define the provenance in the South-Western Urals. The candidates for detrital zircon sources are either the East European Platform (proto-Baltica) or Australia. Solving this may help to establish the connection of Baltica to Australia in the Rodinia assembly, notably with Australia in the so called “up-side down” position in the Baltica-Australia connection. The last paper of this volume is by Geraldes et al. (2014) who use the LA-ICP-MS-technique to measure U–Pb ages in the Aquapei group sedimentary unit of SW Amazonia to discover its provenance. The interpretation is that Paleoproterozoic ages reflect a source rock some 1000–1200 km NE of the depositional basin. The Middle Mesoproterozoic ages probably reflect sources within the adjacent Lonas Maneches orogen in Bolivia. The somewhat younger zircon
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ages of ca. 1540 Ma may be related to the Cachoeirinha orogen or to magmatism of the Serra da Providencia orogen. The 1485–1411 Ma zircon populations may originate from the nearby Santa Helena orogen. Zircon data suggest that the maximum age of the deposition of the Aquapei sediments is 1.26 Ga.
Acknowledgements This special volume would not be possible without the valuable help of editorial officers of EES/Precambrian Research team: Jayakumar Balasaraswathi, Shalini Kumar, Randall Parrish, Guochun Zhao and Vaijanthi Sethuraman. The guest editors of this volume are much indebted to the reviewers, whose careful and timely reviews made the volume possible: Tom Andersen, Benjamin Brito Neves, Ken Buchan, Umberto Cordani, Roberto D’Àgnoll, Elton Dantas, Olav Eklund, Sten-Åke Elming, Richard Ernst, Reinhardt Fuck, Mauro Geraldes, Henry Halls, Eero Hanski, Doewe van Hinsbergen, Hannu Huhma, Åke Johansson, Feiko Kalsbeek, Dennis Kent, Alfred Kröner, Cris Lana, Laura Lauri, Joe Meert, Satu Mertanen, Brian Murphy, Heikki Nevanlinna, Vimal Pradhan, Sally Pehrsson, Lauri J. Pesonen, Sergei Pisarevsky, Augusto Rapalini, Tapani Rämö, Johanna Salminen, Phil Schmidt, Keith Sircombe, Peter Sorjonen-Ward, Neil Suttie, Nicholas Swanson-Hysell, John Tarduno, Ricardo Trindade and Rob van der Voo. We are also thankful for Dharmindar Maharaj and Toni Veikkolainen for their editorial help. The Supercontinent Symposium was organized by Lauri J. Pesonen and assisted by Lauri’s colleagues and students to commemorate his retirement and also in recognition of his service to Helsinki University, the Geological Survey of Finland and to the fields of supercontinent research, paleomagnetism and meteorite impact studies.
References Andersen, T., 2014. The detrital zircon record: supercontinents, parallel evolution – or coincidence? Precambrian Res. 244, 279–287. Belica, M.E., Piispa, E.J., Meert, J.G., Pesonen, L.J., Plado, J., Pandit, M.K., Kamenov, G.D., Celestino, M., 2014. Paleoproterozoic mafic dyke swarms from the Dharwar craton; paleomagnetic poles for India from 2.37 to 1.88 Ga and rethinking the Columbia supercontinent. Precambrian Res. 244, 100–122. Bispo-Santos, F., D’Agrella-Filho, M.S., Janikian, L., Reis, N.J., Trindade, R.I.F., Reis, M.A.A.A., 2014a. Towards Columbia: paleomagnetism of 1980–1960 Ma Surumu volcanic rocks, Northern Amazonian Craton. Precambrian Res. 244, 123–138. Bispo-Santos, F., DÀgrella-Filho, M.S., Trindade, R.I., Janikian, L., Reis, N.J., 2014b. Was there SAMBA in Columbia? Paleomagnetic evidence from 1790 Ma Avanavero mafic sills (Northern Amazonian Craton). Precambrian Res. 244, 139–155. Bleeker, W., Ernst, R., 2006. Short-lived mantle generated magmatic events and their dyke swarms: the key unlocking Earth’s paleogeographic record back to 2.6 Ga. In: Hanski, E., Mertanen, S., Rämö, T., Vuollo, J. (Eds.), Dyke Swarms – Time Markers of Crustal Evolution. Taylor and Francis/Balkema, London, pp. 3–26. Brito Neves, B.B., Fuck, R.A., 2014. The basement of the South American platform: Half Laurentian (N-NW) + half Gondwanan (E-SE) domains. Precambrian Res. 244, 75–86. Buchan, K.L., 2014. Key paleomagnetic poles and their use in Proterozoic continent and supercontinent reconstructions. Precambrian Res. 244, 5–22. Elming, S.-Å., 2014. A palaeomagnetic and 40 Ar/39 Ar study of mafic dykes in southern Sweden: a new Early Neoproterozoic key pole for the Baltic Shield and implications for Sveconorwegian and Grenville loops. Precambrian Res. 244, 192–206. Evans, M.E., 1976. Test of the dipolar nature of the geomagnetic field throughout Phanerozoic time. Nature 262, 676–677. Fuck, R.A., Dantas, E.L., Pimentel, M.M., Botelho, N.F., Laux, J.H., Junges, S.L., Soares, J.E., Praxedes, I.F., 2014. Paleoproterozoic crust-formation and reworking events in the Tocantins Province, Central Brazil: a contribution for Atlantica Supercontinent reconstruction. Precambrian Res. 244, 53–74. Geraldes, M.C., Nogueira, C., Vargas-Mattos, G., Matos, R., Teixeira, R., Valencia, V., Ruiz, J., 2014. U–Pb detrital zircon ages from the Aguapei group (Brazil): implications for the geological evolution of the SW border of the Amazonian Craton. Precambrian Res. 244, 306–316. Halls, H.C., 2014. Crustal shortening during the Paleoproterozoic: can it be accommodated by paleomagnetic data? Precambrian Res. 244, 42–52. Johansson, Å., 2009. Baltica, Amazonia and the SAMBA connection – 1000 million years of neighbourhood during the Proterozoic? Precambrian Res. 175, 221–234.
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Johansson, Å., 2014. From Rodinia to Gondwana with the ‘SAMBA’ model – a distant view from Baltica towards Amazonia and beyond. Precambrian Res. 244, 226–235. Kent, D.V., Smethurst, M.A., 1998. Shallow bias of paleomagnetic inclinations in the Paleozoic and Precambrian. Earth Planet. Sci. Lett. 160, 391–402. Klein, R., Pesonen, L.J., Salminen, J., Mertanen, S., 2014. Paleomagnetism of Mesoproterozoic Satakunta sandstone, Western Finland. Precambrian Res. 244, 156–169. Kuznetsov, N.B., Meert, J.G., Romanyuk, T., 2014. Ages of detrital zircons (U/Pb, LAICP-MS) from the Latest Neoproterozoic-Middle Cambrian(?) Asha Group and Early Devonian Takaty Formation, the Southwestern Urals: a test of an AustraliaBaltica connection within Rodinia. Precambrian Res. 244, 288–305. Linnemann, U., Gerdes, A., Hofmann, M., Marko, L., 2014. The Cadomian Orogen: neoproterozoic to early Cambrian crustal growth and orogenic zoning along the periphery of the West African Craton – Constraints from U–Pb zircon ages and Hf isotopes (Schwarzburg Antiform, Germany). Precambrian Res. 244, 236–278. Mertanen, S., Pesonen, L.J., Sangchan, P. (Eds.), 2012. Supercontinent Symposium 2012 – Programme and Abstracts. Geological Survey of Finland, Espoo, Finland, p. 159 pp. Mitchell, R.N., Kilian, T.M., Evans, D.A.D., 2012. Supercontinent cycles and the calculation of absolute paleolongitude in deep time. Nature 482, 208–212. Pesonen, L.J., Elming, S.-Å., Mertanen, S., Pisarevsky, S.A., D’Agrella-Filho, M.S., Meert, J.G., Schmidt, P.W., Abrahamsen, N., Bylund, G., 2003. Paleomagnetic configuration of continents during the Proterozoic. Tectonophysics 375, 289–324. Pesonen, L.J., Mertanen, S., Veikkolainen, T., 2012a. Paleo-Mesoproterozoic supercontinents – a paleomagnetic view. Geophysica 48 (1–2), 5–47. Pesonen, L.J., Mertanen, S., Sangchan, P., Koljonen, E. (Eds.), 2012b. Supercontinent Symposium 2012 – Pre-Symposium Excursion Guidebook. Geological Survey of Finland, Espoo, Finland, 100 pp. Pisarevsky, S.A., Elming, S.-Å., Pesonen, L.J., Li, Z.-X., 2014. Mesoproterozoic paleogeography: supercontinent and beyond. Precambrian Res. 244, 207–225. Salminen, J., Halls, H.C., Mertanen, S., Pesonen, L.J., Vuollo, J., Söderlund, U., 2014a. Paleomagnetic and geochronological studies on Paleoproterozoic diabase dykes of Karelia, East Finland – key for testing the Superia supercraton. Precambrian Res. 244, 87–99. Salminen, J., Mertanen, S., Evans, D.A.D., Wang, Z., 2014b. Paleomagnetic and geochemical studies of the Mesoproterozoic Satakunta dyke swarms, Finland, with
implications for a Northern Europe – North America (NENA) connection within Nuna supercontinent. Precambrian Res. 244, 170–191. Veikkolainen, T., Evans, D.A.D., Pesonen, L.J., 2014a. PALEOMAGIA – A PHP/MYSQL database of the Precambrian paleomagnetic data. Stud. Geophys. Geod. (submitted for publication). Veikkolainen, T., Evans, D.A.D., Korhonen, K., Pesonen, L.J., 2014b. On the lowinclination bias of the Precambrian geomagnetic field. Precambrian Res. 244, 23–32. Veikkolainen, T., Korhonen, K., Pesonen, L.J., 2014c. On the spatiotemporal averaging of paleomagnetic data. Geophysica (submitted for publication). Veikkolainen, T., Pesonen, L.J., Korhonen, K., 2014d. An analysis of geomagnetic field reversals supports the validity of the Geocentric Axial Dipole (GAD) hypothesis in the Precambrian. Precambrian Res. 244, 33–41.
Guest Editors Lauri J. Pesonen Department of Physics, Laboratory for Solid Earth Geophysics, University of Helsinki, FI-00014 Helsinki, Finland Henry C. Halls a,b Department of Chemical and Physical Sciences, University of Toronto Mississauga, Mississauga, Ontario L5 L IC6, Canada b Department of Earth Sciences, University of Toronto, 22 Russell Street, Toronto, Ontario M5S, 3B1, Canada a
Satu Mertanen Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland E-mail address: lauri.pesonen@helsinki.fi (L.J. Pesonen)
Contents lists available at ScienceDirect
Precambrian Research journal homepage: www.elsevier.com/locate/precamres
Preface
Precambrian supercontinents
Reconstructions of continents during the Precambrian are critical in understanding the evolution of the Earth. They also provide a tectonic context for major ore deposits and in this way help in detecting metallogenic belts between blocks, and identifying new prospective regions for various kinds of mineral deposits. They are equally important for understanding sedimentary basins and their evolution. This Special Issue focuses on paleomagnetic, tectonic, crustal deformation, isotopic and paleogeographic studies of Precambrian continents and their building blocks to analyze the amalgamation, life span and break up of ancient supercontinents. The state of understanding pre-Pangea reconstructions of the continents or cratons and their specific paleogeographies is currently tentative at best. There is good evidence based, for instance, on the episodic nature of orogenic belts, and other kinds of evidence that there have been several supercontinents during the Precambrian such as the Late Archean Kenorland, the late Paleoproterozoic Nuna (a.k.a. Columbia, Hudsonland) and the Meso- to Neoproterozoic Rodinia (Pesonen et al., 2012a and references therein). It has also been suggested that during Archean times, the continental blocks were assembled into several supercratons, such as Superia, Sclavia and Vaalbara. Beyond these general concepts, the exact reconstructions are poorly constrained. In order to better characterize the nature and origin of the Precambrian paleogeography a special symposium “Precambrian Supercontinents” was held in Helsinki, Finland, September 25–28, 2012 (http://supercontinent2012.helsinki.fi/). The symposium was preceded by an excursion (September 22–25, 2012) to six geological landmark sites of the Baltic Shield. The aim of the symposium and excursion was to update concepts on various supercontinent topics and to discuss the geological and biological significance of supercontinents, and particularly, how many of the Precambrian reconstructions are supported by the increasing number of reliable paleomagnetic, radiometric age and isotope data. Various phenomena during the Precambrian, such as isotopic excursions, magmatic episodes like those involving the emplacement of major dyke swarms (e.g., Bleeker and Ernst, 2006), detrital zircon populations, occurrences of ophiolites, etc. have been identified in Precambrian data. An important aim of the symposium was to examine in which phase of the evolution of a supercontinent (amalgamation, life time or break up) their role becomes significant. Seventy researchers from all over the world attended the symposium and 74 abstracts were presented as oral talks or posters. The Abstract Volume and the Excursion Guidebook of the Symposium
http://dx.doi.org/10.1016/j.precamres.2014.02.014 0301-9268/© 2014 Elsevier B.V. All rights reserved.
are published by the Geological Survey of Finland (Mertanen et al., 2012; Pesonen et al., 2012b). The present Special Issue is the outgrowth of the achievements of the Symposium presentations in a form of 19 papers of which 17 are standard papers and 2 are reviews. The volume is divided under five topical sections. The first section (“Paleomagnetic pre-requisites for supercontinent reconstruction”) begins with an invited review paper by K.L. Buchan (2014), who defines the five requirements that a paleomagnetic pole must fulfill in order to become a key pole and to be an anchoring point in the construction of apparent polar wander paths for the cratons and in making reliable reconstructions. He was able to show that using a set of key poles it is plausible that Laurentia and Baltica formed a united landmass from 1590 to 1260 Ma, which forms the core of the Mesoproterozoic Nuna supercontinent. Another highlight of his analysis is the observation that the Slave and Superior cratons were separate cratons experiencing independent drift prior to ca. 1.9 Ga. Veikkolainen et al. (2014a) test the validity of the Geocentric Axial Dipole (GAD) hypothesis during Precambrian using a new Precambrian paleomagnetic database “PALEOMAGIA”. In the first analysis (Veikkolainen et al., 2014b) the authors used the classical inclination-frequency analysis of Evans (1976). Previously (e.g., Kent and Smethurst, 1998) this test has provided strong evidence for low inclination bias, which has been interpreted to represent a considerable non-dipole contamination in the Precambrian geomagnetic field and by this way has cast shadows over Precambrian reconstructions. The new analysis shows that the Precambrian field is actually close to GAD when only the highest quality data from igneous rocks are included and if binning is done cratonically and not geographically (see Veikkolainen et al., 2014c). The GAD model for Precambrian field is supported by the reversal asymmetryanalysis as shown by Veikkolainen et al. (2014d) in their second contribution to this volume. Considerable work has been done during the last twenty years to reconstruct the continents or cratonic blocks during the Precambrian. Although orogenic belts encircling large landmasses have been interpreted to represent ancient conjugate collisions, much less attention has been placed on what happens inside the crustal blocks when collisions take place. The second section of the volume (“Deformation of crust during supercontinent formation”) contains three papers. Henry Halls opens the discussion by addressing the possibility that significant crustal shortening has taken place in the Paleoproterozoic (Halls, 2014). Halls’ motivation comes from
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the observations of Himalayan type collisions in Asia, which have lead to crustal shortening of ca. 2000 km between India and Siberia over the last 200 My. Using this as an analogue, Halls attempts to read whether there are paleomagnetic signatures of crustal shortening associated with the Paleoproterozoic Trans-Hudson orogen (THO) because this orogen bears close similarity both in size and deformation history to the India-Asia collision. The analysis shows that paleomagnetic data can accommodate crustal shortening of about 3000 ± 1000 km between the Superior and Slave cratons. The “crustal shortening” paper is followed by contribution of Fuck et al. (2014). They discuss of whether the cratons show continuity of the basement, or whether some cratons are exotic fragments accreted to the basement during Paleoproterozoic or Neoproterozoic events. They point out that in the generally accepted supercontinents, such as Nuna and Rodinia, the large central cratons like São Francisco and Amazonia are well described while the smaller crustal segments bordering these cratons have been often neglected. The key issue is to identify crustal units of various ages on the western part of the São Francisco craton of South America (Central Brazil) with successive magmatic events. The last paper of the crustal deformation section by Brito Neves and Fuck (2014) discusses the role of the three large cratons forming the basement of the South American platform. The key player is the Amazonian craton but the other cratons, and especially the role of the huge linear fault zones bounding these cratons, is emphasized. The various domains are discussed thoroughly in an attempt to find their “neighbouring” cratons during Archean and Proterozoic times. The third section (“Testing proposed supercontinents with new paleomagnetic data”) deals with paleomagnetic applications related to various phases of Precambrian supercontinents. The first paper of this section is by Salminen et al. (2014a) in which new paleomagnetic results from isotopically dated 2.4–2.3 Ga Taivalkoski diabase dykes, NE Finland are presented. This dyke swarm was also a key target in the pre-Symposium Excursion (Pesonen et al., 2012b). A reliable VGP is established from one E-W trending dyke showing a baddeleyite U–Pb age of 2339 ± 18 Ma and a positive baked contact test. Although preliminary, the result suggests that Baltica and Laurentia were close to each other but not attached at 2.4–2.3 Ga. Two paleomagnetic papers deal with Mesoproterozoic rocks in the Satakunta region, SW Finland. The new results allow testing of the NENA-configuration (Northern Europe-North America) of Baltica and Laurentia. In the first of them Klein et al. (2014) report paleomagnetic data from a red sandstone unit exposed in the Satakunta graben, SW Finland. Based on positive fold and reversal tests, the authors isolated the primary remanence direction of the sandstone, which is distinct from that of the cutting diabase with an age of 1.26 Ga. A tentative apparent polar wander (APW)-age for the sandstone is given ca. 1600 Ma, which is much older than the previously published K–Ar age of 1370 Ma. The relationship between Satakunta sandstone and nearby Eura and Laitila rapakivi massifs remains unsolved. Most likely the deposition of the sandstone started before the intrusion of the rapakivi granites as suggested by detrital zircon data, which does not reveal any hints of a rapakivi source. The other Satakunta paper by Salminen et al. (2014b) shows new primary Mesoproterozoic paleomagnetic data on the 1550 Ma diabase dykes occurring north of the Satakunta sandstone (see Klein et al., 2014). The isolated ChRM is of dual polarity and the primary origin is based on a positive contact test. One dyke is dated (U–Pb) with an age of 1564 Ma. The results conform to the previously proposed long-lived unity of Baltica-Laurentia from 1830 to 1260 Ma. One problem in constructing the Rodinia supercontinent by paleomagnetic data is the question of whether the Sveconorwegian and Grenvillian APW-loops (ca. 1020–830 Ma) can be matched as
would be the case if Laurentia and Baltica formed a drifting unity during Grenvillian times. To shed new light on this issue, Elming (2014) have studied the remanence of the 935–975 Ma diabase dykes exposed in Norrköping-Falun area, southern Sweden. The results provide a new key pole with an age of ca. 955 Ma for Baltica, and confirm the similarity of the Sveconorwegian and Grenvillian APW-loops, thus encompassing their joint drift within the Rodinia supercontinent during ca. 975–935 Ma. Previous paleomagnetically reconstructed PaleoMesoproterozoic supercontinents (see reviews by Pesonen et al., 2003, 2012a) do not include India, due to a lack of reliable data. This situation is now remedied due to a large amount of new, high-quality paleomagnetic results from various Indian cratons. Belica et al. (2014) make a brave attempt to review the drift history of Dharwar craton during Paleoproterozoic times (2.37–1.88 Ga). Most of the new poles from Dharwar are based on precise U–Pb dates, presence of dual polarities and in most cases on positive baked contact tests, thus providing a set of four new key poles for India (Dharwar). The authors discuss the position of India in the Columbia supercontinent, a topic also discussed in other papers of this volume (e.g., Pisarevsky et al., 2014). If India’s position in Paleo-Neoproterozoic supercontinents Columbia and Rodinia has been a problem, the same applies to Amazonia. Two papers provide new Proterozoic paleomagnetic data from the northern part of the Amazonia (Guiana Shield). In the first, Bispo-Santos et al. (2014a) present new dual polarity data of late Paleoproterozoic Surumu group and discuss their role in mapping the Columbia supercontinent. Comparison of the APWP’s of Amazonia and West Africa for the Paleoproterozoic interval suggest that these blocks were linked together at 1.98–1.96 Ga in a configuration whereby the known major shear zones become aligned. The results give support to the East Nuna landmass model and shed new light on the amalgamation time of Nuna. See also Johansson (2014) and Pisarevsky et al. (2014) in this volume. In the follow-up paper by Bispo-Santos et al. (2014b) new Mesoproterozoic paleomagnetic data are presented from the well-dated (1.79 Ga) Avanavero mafic sills, from the northern part of the Amazonian craton. The results constrain Amazonia’s position in the Columbia supercontinent, which is not very different from that of Johansson’s Mesoproterozoic Samba model (South America, Baltica and West Africa; Johansson, 2014), nor from the Columbia-model of Pisarevsky et al. (2014). However, as discussed in these papers, the slightly younger paleomagnetic data from Amazonia at ca. 1.42 Ga contrasts with the Samba model. The authors favour an interpretation that some kind of intra-cratonic movement is the cause for the problems at 1.42 Ga. The fourth section with a subtitle “Precambrian paleogeographies: from Nuna to Rodinia and Gondwana” contains four papers. The first paper dealing with Precambrian paleogeography is the contribution by Johansson (2014) who presents a revision of his previous SAMBA-model of Rodinia (Johansson, 2009), whereby Baltica, Amazonia and West Africa were attached to eastern Laurentia and East Antarctica, while Australia and India are docked with western Laurentia. In building his revised SAMBA-version of Rodinia, he assures us that it’s evolution from the preceding supercontinent NUNA is achieved in an acceptable kinematic way, i.e. not leading to anomalously high plate velocities. He looks the supercontinent cycle in terms of the intro-, extro- or orthoversion way (Mitchell et al., 2012) of breaking up a supercontinent, opening of oceans in between continents, and finally drifting of the continents into their new assembly. Notably, and unlike in his previous SAMBA model, he accepts that Baltica has been rotated by a considerable amount (ca. 75◦ clockwise) since its break up from NUNA and before final docking to Rodinia. Johansson points out the important role of E. Greenland in the Baltica-Laurentia landmass and addresses the question of whether E. Greenland
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shows Grenvillian type features (see also Pesonen et al., 2003). The answer is difficult since the existing evidence may lie below the ice or is obscured by tectonic reworking. Johansson also discusses other potential Rodinia continents like North China, South China, India and Australia and their positions around the Baltica-Laurentia landmass as discussed also in other papers of this volume (e.g., Pisarevsky et al., 2014). A drawback of Johansson’s model is that his images of SAMBA are cartoons rather than spherical projections, which makes it difficult to test using paleomagnetic data. The second paper in this section is by Pisarevsky et al. (2014) and is a test of Columbia during 1770–1270 Ma using updated paleomagnetic data, radiometric dating and geological evidence from major cratons. The authors divide the cratons into two or three stable landmasses such as West Nuna formed by Laurentia, BalticaSarmatia/Volgo Uralia and possibly India, East Nuna comprising North-, West- and South-Australia, North China and the Mawson craton. A possible third landmass consists of Siberia and Congo/Sao Francisco cratons. The life span of Nuna was short, perhaps from ca. 1650 Ma to its break up at ca. 1450–1380 Ma. The authors offer various plausible reconstructions of Columbia. For example, the SAMBA model (Johansson, 2009) of Columbia at ca. 1.79 Ga is paleomagnetically plausible but fails at 1.42 Ga. Various causes for the failure include intra-cratonic movements between various blocks as proposed also by Bispo-Santos et al. (2014b). The various Rodinia models include in most cases NorthEuropean cratons or fragments such as Baltica, Sarmatia, British Isles and Taimyr. However, often these models dismiss the European platform with its various Meso- to Neoproterozoic terranes. The Cadomian orogen crops out mainly in the Bohemian, Armorican and Iberian areas of the Variscides in Central and Western Europe. Linnemann et al. (2014) discuss the importance of detrital zircon as seen in rocks of the Schwarzburg Antiform in the Bohemian massif and other nearby terranes, which form the central part of the Cadomian orogen. One of the highlights of this paper is the result, using Hf - initial values, coupled with U–Pb TDM-model ages from both detrital and magmatic zircons showing that during the c. 180 Ma long Cadomian magmatic arc activity, juvenile magmas became contaminated by the recycling of Eburnean and Archean crust of Africa. The paper sheds light on the role of geotectonic processes, crustal growth, and crustal recycling of the Cadomian Orogen in Central and Western Europe. The fifth section (“The relevance of detrital zircons studies to supercontinent research”) begins with a paper by Andersen (2014) who offers a critical assessment of the use of detrital zircon data in analyzing the provenance of zircon grains in relation to supercontinents. Andersen’s main point is that the U–Pb and Lu–Hf data from detrital zircons in young, continental cover sequences suggest extensive reworking of older sediments, and therefore the path from “source to sink” is non-unique. He notes that cover sequences in Laurentia, Australia and southern Africa show detrital zircon U–Pb age and Hf isotope patterns that cannot be distinguished with confidence from bedrock sources in Fennoscandia. In the second paper of this section, Kuznetsov et al. (2014) uses detrital zircon to define the provenance in the South-Western Urals. The candidates for detrital zircon sources are either the East European Platform (proto-Baltica) or Australia. Solving this may help to establish the connection of Baltica to Australia in the Rodinia assembly, notably with Australia in the so called “up-side down” position in the Baltica-Australia connection. The last paper of this volume is by Geraldes et al. (2014) who use the LA-ICP-MS-technique to measure U–Pb ages in the Aquapei group sedimentary unit of SW Amazonia to discover its provenance. The interpretation is that Paleoproterozoic ages reflect a source rock some 1000–1200 km NE of the depositional basin. The Middle Mesoproterozoic ages probably reflect sources within the adjacent Lonas Maneches orogen in Bolivia. The somewhat younger zircon
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ages of ca. 1540 Ma may be related to the Cachoeirinha orogen or to magmatism of the Serra da Providencia orogen. The 1485–1411 Ma zircon populations may originate from the nearby Santa Helena orogen. Zircon data suggest that the maximum age of the deposition of the Aquapei sediments is 1.26 Ga.
Acknowledgements This special volume would not be possible without the valuable help of editorial officers of EES/Precambrian Research team: Jayakumar Balasaraswathi, Shalini Kumar, Randall Parrish, Guochun Zhao and Vaijanthi Sethuraman. The guest editors of this volume are much indebted to the reviewers, whose careful and timely reviews made the volume possible: Tom Andersen, Benjamin Brito Neves, Ken Buchan, Umberto Cordani, Roberto D’Àgnoll, Elton Dantas, Olav Eklund, Sten-Åke Elming, Richard Ernst, Reinhardt Fuck, Mauro Geraldes, Henry Halls, Eero Hanski, Doewe van Hinsbergen, Hannu Huhma, Åke Johansson, Feiko Kalsbeek, Dennis Kent, Alfred Kröner, Cris Lana, Laura Lauri, Joe Meert, Satu Mertanen, Brian Murphy, Heikki Nevanlinna, Vimal Pradhan, Sally Pehrsson, Lauri J. Pesonen, Sergei Pisarevsky, Augusto Rapalini, Tapani Rämö, Johanna Salminen, Phil Schmidt, Keith Sircombe, Peter Sorjonen-Ward, Neil Suttie, Nicholas Swanson-Hysell, John Tarduno, Ricardo Trindade and Rob van der Voo. We are also thankful for Dharmindar Maharaj and Toni Veikkolainen for their editorial help. The Supercontinent Symposium was organized by Lauri J. Pesonen and assisted by Lauri’s colleagues and students to commemorate his retirement and also in recognition of his service to Helsinki University, the Geological Survey of Finland and to the fields of supercontinent research, paleomagnetism and meteorite impact studies.
References Andersen, T., 2014. The detrital zircon record: supercontinents, parallel evolution – or coincidence? Precambrian Res. 244, 279–287. Belica, M.E., Piispa, E.J., Meert, J.G., Pesonen, L.J., Plado, J., Pandit, M.K., Kamenov, G.D., Celestino, M., 2014. Paleoproterozoic mafic dyke swarms from the Dharwar craton; paleomagnetic poles for India from 2.37 to 1.88 Ga and rethinking the Columbia supercontinent. Precambrian Res. 244, 100–122. Bispo-Santos, F., D’Agrella-Filho, M.S., Janikian, L., Reis, N.J., Trindade, R.I.F., Reis, M.A.A.A., 2014a. Towards Columbia: paleomagnetism of 1980–1960 Ma Surumu volcanic rocks, Northern Amazonian Craton. Precambrian Res. 244, 123–138. Bispo-Santos, F., DÀgrella-Filho, M.S., Trindade, R.I., Janikian, L., Reis, N.J., 2014b. Was there SAMBA in Columbia? Paleomagnetic evidence from 1790 Ma Avanavero mafic sills (Northern Amazonian Craton). Precambrian Res. 244, 139–155. Bleeker, W., Ernst, R., 2006. Short-lived mantle generated magmatic events and their dyke swarms: the key unlocking Earth’s paleogeographic record back to 2.6 Ga. In: Hanski, E., Mertanen, S., Rämö, T., Vuollo, J. (Eds.), Dyke Swarms – Time Markers of Crustal Evolution. Taylor and Francis/Balkema, London, pp. 3–26. Brito Neves, B.B., Fuck, R.A., 2014. The basement of the South American platform: Half Laurentian (N-NW) + half Gondwanan (E-SE) domains. Precambrian Res. 244, 75–86. Buchan, K.L., 2014. Key paleomagnetic poles and their use in Proterozoic continent and supercontinent reconstructions. Precambrian Res. 244, 5–22. Elming, S.-Å., 2014. A palaeomagnetic and 40 Ar/39 Ar study of mafic dykes in southern Sweden: a new Early Neoproterozoic key pole for the Baltic Shield and implications for Sveconorwegian and Grenville loops. Precambrian Res. 244, 192–206. Evans, M.E., 1976. Test of the dipolar nature of the geomagnetic field throughout Phanerozoic time. Nature 262, 676–677. Fuck, R.A., Dantas, E.L., Pimentel, M.M., Botelho, N.F., Laux, J.H., Junges, S.L., Soares, J.E., Praxedes, I.F., 2014. Paleoproterozoic crust-formation and reworking events in the Tocantins Province, Central Brazil: a contribution for Atlantica Supercontinent reconstruction. Precambrian Res. 244, 53–74. Geraldes, M.C., Nogueira, C., Vargas-Mattos, G., Matos, R., Teixeira, R., Valencia, V., Ruiz, J., 2014. U–Pb detrital zircon ages from the Aguapei group (Brazil): implications for the geological evolution of the SW border of the Amazonian Craton. Precambrian Res. 244, 306–316. Halls, H.C., 2014. Crustal shortening during the Paleoproterozoic: can it be accommodated by paleomagnetic data? Precambrian Res. 244, 42–52. Johansson, Å., 2009. Baltica, Amazonia and the SAMBA connection – 1000 million years of neighbourhood during the Proterozoic? Precambrian Res. 175, 221–234.
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Johansson, Å., 2014. From Rodinia to Gondwana with the ‘SAMBA’ model – a distant view from Baltica towards Amazonia and beyond. Precambrian Res. 244, 226–235. Kent, D.V., Smethurst, M.A., 1998. Shallow bias of paleomagnetic inclinations in the Paleozoic and Precambrian. Earth Planet. Sci. Lett. 160, 391–402. Klein, R., Pesonen, L.J., Salminen, J., Mertanen, S., 2014. Paleomagnetism of Mesoproterozoic Satakunta sandstone, Western Finland. Precambrian Res. 244, 156–169. Kuznetsov, N.B., Meert, J.G., Romanyuk, T., 2014. Ages of detrital zircons (U/Pb, LAICP-MS) from the Latest Neoproterozoic-Middle Cambrian(?) Asha Group and Early Devonian Takaty Formation, the Southwestern Urals: a test of an AustraliaBaltica connection within Rodinia. Precambrian Res. 244, 288–305. Linnemann, U., Gerdes, A., Hofmann, M., Marko, L., 2014. The Cadomian Orogen: neoproterozoic to early Cambrian crustal growth and orogenic zoning along the periphery of the West African Craton – Constraints from U–Pb zircon ages and Hf isotopes (Schwarzburg Antiform, Germany). Precambrian Res. 244, 236–278. Mertanen, S., Pesonen, L.J., Sangchan, P. (Eds.), 2012. Supercontinent Symposium 2012 – Programme and Abstracts. Geological Survey of Finland, Espoo, Finland, p. 159 pp. Mitchell, R.N., Kilian, T.M., Evans, D.A.D., 2012. Supercontinent cycles and the calculation of absolute paleolongitude in deep time. Nature 482, 208–212. Pesonen, L.J., Elming, S.-Å., Mertanen, S., Pisarevsky, S.A., D’Agrella-Filho, M.S., Meert, J.G., Schmidt, P.W., Abrahamsen, N., Bylund, G., 2003. Paleomagnetic configuration of continents during the Proterozoic. Tectonophysics 375, 289–324. Pesonen, L.J., Mertanen, S., Veikkolainen, T., 2012a. Paleo-Mesoproterozoic supercontinents – a paleomagnetic view. Geophysica 48 (1–2), 5–47. Pesonen, L.J., Mertanen, S., Sangchan, P., Koljonen, E. (Eds.), 2012b. Supercontinent Symposium 2012 – Pre-Symposium Excursion Guidebook. Geological Survey of Finland, Espoo, Finland, 100 pp. Pisarevsky, S.A., Elming, S.-Å., Pesonen, L.J., Li, Z.-X., 2014. Mesoproterozoic paleogeography: supercontinent and beyond. Precambrian Res. 244, 207–225. Salminen, J., Halls, H.C., Mertanen, S., Pesonen, L.J., Vuollo, J., Söderlund, U., 2014a. Paleomagnetic and geochronological studies on Paleoproterozoic diabase dykes of Karelia, East Finland – key for testing the Superia supercraton. Precambrian Res. 244, 87–99. Salminen, J., Mertanen, S., Evans, D.A.D., Wang, Z., 2014b. Paleomagnetic and geochemical studies of the Mesoproterozoic Satakunta dyke swarms, Finland, with
implications for a Northern Europe – North America (NENA) connection within Nuna supercontinent. Precambrian Res. 244, 170–191. Veikkolainen, T., Evans, D.A.D., Pesonen, L.J., 2014a. PALEOMAGIA – A PHP/MYSQL database of the Precambrian paleomagnetic data. Stud. Geophys. Geod. (submitted for publication). Veikkolainen, T., Evans, D.A.D., Korhonen, K., Pesonen, L.J., 2014b. On the lowinclination bias of the Precambrian geomagnetic field. Precambrian Res. 244, 23–32. Veikkolainen, T., Korhonen, K., Pesonen, L.J., 2014c. On the spatiotemporal averaging of paleomagnetic data. Geophysica (submitted for publication). Veikkolainen, T., Pesonen, L.J., Korhonen, K., 2014d. An analysis of geomagnetic field reversals supports the validity of the Geocentric Axial Dipole (GAD) hypothesis in the Precambrian. Precambrian Res. 244, 33–41.
Guest Editors Lauri J. Pesonen Department of Physics, Laboratory for Solid Earth Geophysics, University of Helsinki, FI-00014 Helsinki, Finland Henry C. Halls a,b Department of Chemical and Physical Sciences, University of Toronto Mississauga, Mississauga, Ontario L5 L IC6, Canada b Department of Earth Sciences, University of Toronto, 22 Russell Street, Toronto, Ontario M5S, 3B1, Canada a
Satu Mertanen Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland E-mail address: lauri.pesonen@helsinki.fi (L.J. Pesonen)