Key paleomagnetic poles and their use in Proterozoic continent and supercontinent reconstructions: A review

Precambrian Research 238 (2013) 93–110

Contents lists available at ScienceDirect

Precambrian Research journal homepage: www.elsevier.com/locate/precamres

Review

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Key paleomagnetic poles and their use in Proterozoic continent and supercontinent reconstructions: A review

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Kenneth L. Buchan ∗ Geological Survey of Canada, 601 Booth Street, Ottawa, Canada K1A 0E8

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Article history: Received 7 March 2013 Received in revised form 19 September 2013 Accepted 23 September 2013 Available online 8 October 2013

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Keywords: Key paleopole Supercontinent reconstruction Apparent polar wander path Proterozoic

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Contents

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Key paleomagnetic poles are poles that are well defined and precisely dated. The rock unit from which the pole is derived must have a precise (usually U–Pb) age and the pole itself must be demonstrated primary with a rigorous field test. The use of key poles is essential in defining reliable apparent polar wander paths (APWPs) and establishing continental reconstructions. Many hundreds of Proterozoic paleopoles have been published from around the globe, but only ∼45 are from large craton interiors and pass the key pole criteria. Most key poles are from mafic dykes and sills in the Superior craton (pre-1.83 Ga) or Laurentia (post-1.80 Ga) or Baltica. As a result, with occasional exceptions, it is difficult to define or compare reliable APWP segments in order to test Proterozoic continental reconstructions. However, there are now sufficient age matches or approximate age matches for pairs of key poles from a number of cratons to help constrain their relative locations. In this analysis, Proterozoic key poles are identified and their use in constructing APWPs and testing continent and supercontinent reconstructions is discussed. This key pole database establishes a well constrained Superior craton-Laurentia APWP for much of the Proterozoic that can be used as a reference track against which a growing number of individual key poles from other cratons can be compared. There is now a robust Baltica–Laurentia reconstruction for ∼330 m.y. between 1.59 and 1.26 Ga using this approach and potentially for ∼570 m.y. between 1.83 and 1.26 Ga if additional key and non-key poles from well-dated units are considered. Key pole comparisons for several other cratons yield preliminary constraints on the relative movement of cratons (e.g., Slave and Superior cratons in the Paleoproterozoic) or on specific elements of continental reconstructions (e.g., Amazonia and Baltica in the Mesoproterozoic, South China craton and Australia in the Neoproterozoic, or Baltica and Laurentia also in the Neoproterozoic). Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Key paleomagnetic pole criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Field tests for primary remanence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Baked contact test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Baked contact profile test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Intraformational conglomerate test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Polarity correlation test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Secular variation correlation test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6. Remanence direction correlation test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7. Regional consistency test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Field tests which do not establish a remanence as primary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. “Partial” baked contact test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Antipodal reversals (reversal test) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Reversals in a single stratigraphic section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Fold test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Conglomerate test on conglomerate in younger sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Tel.: +1 613 995 4386. E-mail address: [email protected] 0301-9268/$ – see front matter Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.precamres.2013.09.018

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Methods for utilizing key paleopoles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Coeval APWP segment method (Method 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Coeval key paleopole method (Method 2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Coeval great circle method (Method 3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Proterozoic key paleopole database . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Late Archean-Proterozoic continents and supercontinents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Key paleopole analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1. Pre-1.85 Ga . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2. 1.85–1.20 Ga . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3. 1.20–0.544 Ga . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction

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Key paleomagnetic poles are poles that are well defined and precisely dated (Buchan et al., 2000; Buchan, 2007b). In the Precambrian, key poles are a prerequisite for establishing reliable apparent polar wander paths (APWPs) and testing continent and supercontinent reconstructions. Many if not most non-key poles are so poorly constrained, especially in age, that they cannot be reliably sequenced along APWPs. Only about a dozen years ago, most key Precambrian poles were derived from Laurentia (post-1.80 Ga) and the Superior craton (pre-1.83 Ga) or Baltica. There were few key pole age matches making it difficult to attempt paleocontinental reconstructions. Since then the database of key poles has expanded to include poles from a significant number of the large cratons around the world, as well as a significant number of key pole age matches. Although the importance of key paleopoles is acknowledged in many recent publications, most attempts to use paleomagnetic data to reconstruct Precambrian continents rely heavily or even almost entirely on non-key poles. As a result of the large uncertainties in many of these poles, especially in their ages, numerous quite discordant reconstructions have been proposed. Only occasionally have reviews of Precambrian reconstructions focused largely on key paleopoles (Buchan et al., 2000, 2001; Evans and Pisarevsky, 2008). In this review the current database of Proterozoic key poles is catalogued and analyzed to determine what constraints can be placed on APWPs and continent/supercontinent reconstructions. To aid in the discussion several non-key poles are considered in order to clarify or support reconstructions based on key poles. They are from well-dated, unmetamorphosed rock units but lack a primary field test, or are precisely dated virtual geomagnetic poles (VGPs) that may not average out secular variation.

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2. Key paleomagnetic pole criteria

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There are two basic criteria for a key paleopole (Buchan, 2007b): (1) The age of the paleopole is precisely determined. In particular, the pole must be demonstrated primary with a field test (see Section 3) and the rock unit precisely dated. Only U–Pb or occasionally Ar–Ar ages are sufficiently precise. Most key paleopoles catalogued in this review have ages that have been determined within ±10 m.y., although a maximum uncertainty of ±20 m.y. is permitted. As noted by Buchan (2007b), it should in future be possible to significantly tighten this latter uncertainty. (2) The paleopole is of good quality. In particular, the primary remanence must be properly isolated using paleomagnetic cleaning techniques such as stepwise alternating field or thermal demagnetization, and secular variation largely averaged

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out. In addition, it is important that the remanence is corrected for the effects of structural rotation. As a result, it is usually best to avoid cratonic margins where complicated deformation may be difficult to resolve. In sedimentary rocks, the remanence may need to be corrected if inclination shallowing has occurred either during deposition or later compaction (Kodama, 2012). Inclination shallowing is greatest (up to 15 or 20◦ ) for sediments deposited in intermediate latitudes and decreases progressively to zero for deposition at the geomagnetic equator and pole. The presence of inclination shallowing can be determined by comparing data from coeval sedimentary and igneous rocks, by comparing fine-grained and coarse-grained sediments which experience different degrees of shallowing, or with more sophisticated rock magnetic experiments summarized in Kodama (2012).

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It should be noted that the key pole age criteria are more stringent than criteria used in other quality tests. For example, the most widely cited pole quality ranking in use today is the Q index of Van der Voo (1990) which does not require that the remanence is demonstrated primary or that the age of the rock unit is precisely determined. 3. Field tests for primary remanence Field tests are critical to determining if a remanence is primary. They are often poorly understood and frequently misused. Tests which indicate that a remanence is primary are summarized below, and those which do not establish a remanence as primary are outlined in Section 4. 3.1. Baked contact test A classic baked contact test (Everitt and Clegg, 1962; Buchan, 2007a) involves sampling an igneous unit and sampling both the baked and unbaked zones of nearby host rocks. The test is positive and the remanence of the igneous unit is considered to be primary if the igneous rock and its baked host carry similar stable remanence directions, whereas the unbaked host a short distance away carries a significantly different stable direction. Diabase dykes that crosscut older diabase dykes, sills or mafic volcanics often yield excellent tests. On the other hand, felsic host rocks frequently carry unstable remanences which are unsuitable for baked contact tests. There is the possibility of a “false positive” if, for example, chemical alterations occurred along the contact of the igneous unit long after its emplacement and locally reset the remanence direction on either side of the contact, while not affecting the remanence farther away. Such chemical alterations can usually be detected by comparing the magnetic characteristics of the host rock in the baked and unbaked zones, or by comparing the magnetic directions of the margin and

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interior of the igneous unit (see further discussion in Schwarz and Buchan, 1989; Halls, 2008). The baked contact test is often misused as described in Section 4.

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3.2. Baked contact profile test

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A baked contact profile test (Buchan, 2007a) is more demanding and more powerful than the classic baked contact test, and to date has only been carried out in a few cases, usually associated with depth of burial studies following the method of Schwarz (1977). Samples are collected in an igneous unit as well as along a detailed profile into the host rock. The aim is to identify the zone of hybrid magnetization in the host rocks where host and baked overprint remanence components are superimposed, in addition to identifying the remanence zones of the classic test. If the maximum unblocking temperature of the overprint component decreases systematically through the hybrid zone with increasing distance from the igneous contact, as expected from heat conduction theory (Jaeger, 1964), it can be concluded that the overprint component is a thermoremanent magnetization acquired at the time of emplacement of the igneous unit.

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3.3. Intraformational conglomerate test

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The intraformational conglomerate test (Graham, 1949) is another powerful tool to demonstrate a primary remanence. If conglomerate clasts carry stable but randomly oriented remanence directions then the clasts have not been magnetically overprinted since the conglomerate was formed. If the clasts were derived from volcanic or sedimentary rocks of the same formation, then those units must also carry a primary remanence. Care must be taken to ensure that the conglomerate is truly intraformational and not marking a disconformity.

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3.4. Polarity correlation test

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The “polarity correlation” test requires detecting magnetic polarity reversals which are correlated with rocks that are of different age but emplaced over a relatively short time interval (e.g., Buchan and Halls, 1990). For example, if sampling sites along individual dykes in a single swarm consistently carry either reversed or normal remanence directions, then the different polarities undoubtedly reflect changes in the polarity of the earth’s magnetic field as the swarm was being emplaced. This is because even wide dykes (e.g., 200 m) cool through the blocking temperatures of their stable remanence much too quickly to record a magnetic field reversal. If the polarities are not consistent along individual dykes, the remanence is an overprint. The test can also be carried out in sedimentary (or volcanic) stratigraphy (e.g., Evans et al., 2000). In this case, to demonstrate that the remanence is primary it is necessary to demonstrate that the polarity pattern can be traced laterally. In other words, a given sedimentary bed (or volcanic flow) should carry a consistent polarity over some distance (e.g., >100 m). This can be accomplished by sampling two or more separate stratigraphic profiles between which individual beds are correlated. In the case of volcanic flows, it should be noted that the correlation may be complicated because a given flow may overprint the flow, or at least the upper portion of the flow, below. As discussed in Section 4, the presence of reversals in a single stratigraphic section does not yield a conclusive test of primary remanence.

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3.5. Secular variation correlation test

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In analogue to the polarity reversal test, the “secular variation correlation” test requires detecting secular variation between rock units that have been emplaced over a short time interval (e.g., Halls,

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1986; Buchan and Halls, 1990). For example, if sampling sites along one dyke in a swarm consistently carry slightly different directions from sites along a second dyke, then it is likely that the difference reflects secular variation that occurred as the dykes were emplaced, because even wide dykes cool too quickly through the blocking temperatures of their stable remanence to average out secular variation. The more dykes that carry distinct directions the stronger the test. It should be noted that neighbouring dykes may also exhibit distinctly different geochemistry consistent with their injection at slightly different times in the evolution of the parent magma chamber. 3.6. Remanence direction correlation test If two rock units of distinctly different age with similar magnetic characteristics (e.g., two diabase dyke swarms whose magnetizations are carried by low-titanium titanomagnetite with similar unblocking temperatures) occur in the same area and have stable but significantly different remanence directions, it is likely that the remanence of the younger unit is primary. Otherwise, one might expect that overprinting would have affected both swarms and their remanence directions would be similar. For this “remanence direction correlation” test to be valuable, the units in question should be sampled over a significant geographical area to ensure that there is a systematic difference in remanence direction between the units. 3.7. Regional consistency test If a single rock unit emplaced over a short time interval (e.g., dyke swarm) yields consistent paleopoles over a large geographical area (e.g., spanning hundreds of kilometres) in the interior of a craton with no evidence of subsequent regional metamorphism, it is likely that the remanence is primary. This also applies to widely scattered but discrete units of similar age (Buchan et al., 2000). 4. Field tests which do not establish a remanence as primary There are also several field tests that do not establish that a remanence is primary, although they are assumed to do so in some published studies. 4.1. “Partial” baked contact test Although less common today than in the past, there are still a significant number of cases where paleomagnetic remanences are described as primary based on so-called “partial” baked contact tests. In a typical example, stable and consistent remanence directions are observed in an igneous unit and its baked host, but unstable or scattered remanence directions are obtained from unbaked host rocks. Occasionally, no samples are actually collected from the unbaked host rocks. As regional overprinting will usually reset the remanence direction of both an igneous unit and its host, “partial” baked contact tests yield no information whatever about the primary or secondary nature of the remanence and should be termed “inconclusive” or “incomplete” baked contact tests (Buchan et al., 2001). 4.2. Antipodal reversals (reversal test) The presence of reversals that are antipodal is frequently used to suggest that a remanence is primary. However, reversals occur during overprinting, and indeed are almost inevitable when the overprint is acquired during a long-term metamorphic event, except during a polarity superchron. Therefore this test does not

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establish that a remanence is primary, except in the specific circumstances described for the polarity correlation test in Section 3. It does suggest that there is no unresolved partial overprint. But complete overprinting may have occurred. It should also be noted that there are a number of instances of primary remanences from a single magmatic event where reversals are not antipodal because emplacement extends over a significant time interval during which the continent was in motion. For example, the 2.13–2.10 Ga Marathon dyke swarm of North America was emplaced over ∼25 m.y. and shows polarity asymmetry in its primary remanence (Halls et al., 2008). Polarity asymmetry has also been reported in 1.11–1.08 Ga Kewenawan volcanics of the MidContinent Rift of North America, although analysis of a very detailed section at Mamainse Point indicates that the data actually record three symmetric reversals superimposed on rapid apparent polar wander (Swanson-Hysell et al., 2009).

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4.3. Reversals in a single stratigraphic section

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The presence of magnetic reversals within a single sedimentary or volcanic stratigraphic section has also been used to suggest that the remanence is primary. However, reversals will likely be observed even if the remanence is an overprint. As noted in the discussion of the polarity correlation test of Section 3, to establish that the remanence is primary, it is necessary to demonstrate lateral consistency in the polarity pattern.

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4.4. Fold test

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The fold test (Graham, 1949; McElhinny, 1964; McFadden, 1990) establishes that the age of remanence is pre-, syn- or post-fold, but not usually that it is primary. A positive fold test can only indicate that the remanence is primary (or nearly primary) if it is demonstrated that folding occurred during deposition or emplacement of the rock unit under study.

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4.5. Conglomerate test on conglomerate in younger sediments

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As noted in Section 3, a positive intraformational conglomerate test can be used to demonstrate that the remanence carried by sediments or volcanics is primary, provided that the conglomerate cobbles are derived from the formation being studied or have the same magnetic characteristics as the formation under study. On the other hand, if the conglomerate occurs stratigraphically above the sediments or volcanics that are being studied, a positive test only establishes that the remanences in the conglomerate cobbles and in the sediments or volcanics pre-date deposition of the conglomerate, even if the cobbles are known to be derived from the formation in question. In other words, if the time between the formation of the conglomerate and the sediments below is sufficiently long the sediments below could have been remagnetized prior to conglomerate formation.

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5. Methods for utilizing key paleopoles

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APWPs will have similar lengths and shape (e.g., see discussion in Evans and Pisarevsky, 2008), and superimposing these APWPs will yield a unique reconstruction. In most studies a wide variety of nonkey poles are used to construct APWP segments, because so few key poles are yet available from most cratonic blocks. However, it should be emphasized that key poles must form the basis of any reliable APWP. When non-key poles are used, their sheer number often swamps the tiny number of key poles and can result in very misleading paths (e.g., Buchan et al., 1994). Therefore, at present, this method is seldom applicable in the Proterozoic because there are insufficient reliable poles to construct APWP segments. 5.2. Coeval key paleopole method (Method 2) This method involves a direct comparison of individual coeval key paleopoles from different cratons and is important when there is an insufficient number of key poles with which to construct APWP segments (e.g., Buchan et al., 2000). At present matching key pole ages is still the only viable technique for most cratons and most time intervals in the Proterozoic. The locations of two cratons are compared by superimposing their coeval key poles. However, the reconstruction is not unique because a single pole does not constrain paleolongitude and its polarity is ambiguous. These uncertainties can be addressed as more key pole matches become available. In particular, if the two cratons are attached and rotating, or separated but rotating on the same tectonic plate, then two (or more) key pole matches will establish a unique reconstruction (see discussion on p. 186 of Buchan et al., 2000). 5.3. Coeval great circle method (Method 3) Regardless of whether the two cratons are rotating, if two key pole matches are available, then a simple comparison of the lengths of the great circle arcs for the pole pairs from each craton yields information on whether the cratons were in relative motion or on the same tectonic plate through the time interval in question (Graham et al., 1964; Evans and Pisarevsky, 2008). It is important that the poles are from stable (rigid and undeformed) cratonic interiors, as cratonic margins may have suffered substantial deformation. Significantly different arc lengths indicate that the two cratons were not on the same plate throughout the interval. Similar arc lengths are a necessary condition for the two cratons to have been on the same plate. However, it does not unequivocally establish that this is the case, as the similarity could be coincidental. Assuming the match in length is not coincidental, then superimposing the matching poles places the cratons in their unique relative position during the time interval. At present this method can only be applied in a few cases. For example, Evans and Pisarevsky (2008) found only three instances of pairs of coeval poles with similarlength great circle arcs in the entire pre-0.8 Ga paleomagnetic record. 6. Proterozoic key paleopole database

291

There are a number of methods in which key paleomagnetic poles can be used to establish or test continental reconstructions in the Proterozoic.

292

5.1. Coeval APWP segment method (Method 1)

289 290

293 294 295 296 297

Comparing coeval APWP segments for two cratonic blocks (e.g., Graham et al., 1964), provided they are based on key paleopoles, is the most powerful method for comparing the locations of the cratons and determining if they were moving on the same tectonic plate. During the period when cratons were moving in unison their

There are many hundreds of published Proterozoic paleopoles. Of these ∼50 can be considered as key poles. Approximately 45 are from the interior of relatively large cratons (Table 1 and Fig. 1), and are the subject of this analysis. Poles from cratonic margins, which may have suffered significant deformation, and from microcontinents are not included. Most key data in Table 1 are from mafic dykes and sills which typically carry strong, stable magnetizations and can be precisely dated with the U–Pb method on baddeleyite or occasionally with the Ar–Ar technique. Other methods (e.g., K–Ar, Rb–Sr, Sm–Nd) do not generally yield ages that are sufficiently precise or accurate for key poles. Baked contact tests are available for many of these units

298 299 300 301 302 303 304 305 306 307 308

309

310 311 312 313 314 315 316 317 318 319 320 321 322 323

324

325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344

345

346 347 348 349 350 351 352 353 354 355 356 357

K.L. Buchan / Precambrian Research 238 (2013) 93–110

97

Table 1 Key paleopoles from large cratons. N

Plat (◦ N) and Plon (◦ E)

A95 (◦ )

Pol

Test

Paleomagnetic references

Age (Ma)

Age references

41

−51, 257

2

Da

x

GM: Evans and Halls (2010)

Ubz 2473–2446

68

−43, 239

2

Da

b

10

−56, 255

5

D

b

19

−53, 243

3

Da

x

GM: Evans and Halls (2010) T: Buchan et al. (1990) GM: Evans and Halls (2010) T: Halls (1991) GM: Evans and Halls (2010)

Ubz 2473–2446

a

6u

−17, 272

10

D

b

Ub 2217 ± 4

8d

−16, 281

6

D

s

Ub 2216 + 8/−4

Buchan et al. (1993, 1996)

6d

17, 233

10

S

d, x

GM: Buchan et al. (2000) T: Buchan (1991) GM: hereinb T: Buchan et al. (1993) Halls and Davis (2004)

Heaman (1997) and Halls et al. (2005) Heaman (1997) and Halls et al. (2005) Heaman (1997) and Halls et al. (2005) Heaman (1997) and Halls et al. (2005) Noble and Lightfoot (1992)

Ub 2172–2168

Halls and Davis (2004)

6d

28, 223

11

S

b(p)

Ubz 2167 ± 2

Buchan et al. (1993)

16s

45, 198

8

D

d

Buchan et al. (1993) T: Buchan and Schwarz (1981) GM: Evans and Halls (2010)

U 2126–2121

13s

55, 182

8

D

b

U 2106–2101

8d

53, 180

9

S

b

GM: Evans and Halls (2010) T: Buchan et al. (1996) Halls and Heaman (2000)

Ubz 2091 ± 2

Buchan et al. (1996) and Halls et al. (2008) Hamilton et al. (2002) and Halls et al. (2008) Halls and Heaman (2000)

13d

43, 184

6

S

s

Halls (1986)

Ubz 2076 + 5/−4

Buchan et al. (1996)

8d

62, 169

7

s, d

Buchan et al. (2007a)

Ub 2069 ± 1

Buchan et al. (2007a)

9s 34s

38, 174 29, 218

10 4

D

b b

Buchan et al. (1998) GM: Evans and Halls (2010) T: Zhai et al. (1994)

Uzb 1998 ± 2 Uz 1884–1877

Buchan et al. (1998) Halls and Heaman (2000)

12, 268

7

S

b

Buchan et al. (2009)

Ub 2027–2023

Buchan et al. (2009)

17d 35s

19, 277 09, 245

6 6

S S

2b s, d

Irving et al. (2004) Irving et al. (1972a)

Irving et al. (2004) Hamilton and Buchan (2010)

18s

−13, 219

7

S

ef, xc

Meert and Stuckey (2002)

Ub 1740 + 5/−4 Ub 1592 ± 3, 1590 ± 4 1476 ± 16

12s

−02, 218

5

S

bc

Emslie et al. (1976)

Uz ∼1460

24s 37s

02, 206 −07, 215

4 4

D D

xc xc

Irving et al. (1977) Harlan et al. (1994)

Uz ∼1450 Uz ∼1434d

7s

−01, 201

8

D

xc

Fahrig and Jones (1976)

Uz ∼1420

Mackenzie dykes

14s

04, 190

5

S

b

Ub 1267 ± 2

Sudbury dykes

52s

−03, 192

3

S

b(p)

Abitibi dykes

5d

43, 208

14

S

b

Logan sills

5st

49, 220

4

S

be

Upper Osler lavas R Portage Lake lavas

25s 4st

43, 195 27, 181

6 2

S S

eff s

30s

22, 181

5

S

b

GM: Buchan and Halls (1990) T: Irving et al. (1972b) and Buchan et al. (2009) Palmer et al. (1977) T: Schwarz and Buchan (1982) Ernst and Buchan (1993) T: Ernst (1989) GM: Halls and Pesonen (1982) T: Pesonen (1979) Halls (1974) GM: Halls and Pesonen (1982) T: Books (1972) Diehl and Haig (1994) T: Palmer et al. (1981) GMh : Harlan et al. (2008)

Unit

Superior craton Matachewan R dykes (W of KSZ) Matachewan R dykes (E of KSZ) Matachewan N dykes (W of KSZ) Matachewan N dykes (E of KSZ) Nipissing N1 sills (E of KSZ) Senneterre dykes (E of KSZ) Biscotasing dykes (W of KSZ) Biscotasing dykes (E of KSZ) Marathon N dykes (W of KSZ) Marathon R dykes (W of KSZ) Cauchon dykes (West of KSZ) Fort Frances dykes (W of KSZ) Lac Esprit dykes (E of KSZ) Minto dykes Molson (B + C2) dykes Slave craton Lac de Gras dykes Laurentia Cleaver dykes Western Channel Diabase St. Francois Mountains volcanics and intrusives Michikamau anorthosite pluton Harp Lake complex Laramie complex and Sherman Granite Mistastin complex

Lake Shore traps g

Gunbarrel intrusions

6u

09, 139

9

g

x

Ubz 2473–2446 Ubz 2473–2446

Complied by Meert and Stuckey (2002) from data in Van Schmus et al. (1993) Krogh and Davis (1973) Krogh and Davis (1973) Scoates and Chamberlain (1995) Roddick in Emslie and Stirling (1973) LeCheminant and Heaman (1989)

Ub 1235 + 7–3

Dudas et al. (1994)

Ub 1141 ± 1

Krogh et al. (1987)

Uz 1108 ± 1

Davis and Green (1997)

Uz 1105 ± 2 Ub 1095 ± 3

Davis and Green (1997) Davis and Paces (1990)

Ub 1087 ± 2

Davis and Paces (1990)

Ub 780 ± 2

Harlan et al. (2003)

98

K.L. Buchan / Precambrian Research 238 (2013) 93–110

Table 1 (Continued) Unit

N

Plat (◦ N) and Plon (◦ E)

A95 (◦ )

Pol

Test

Paleomagnetic references

Age (Ma)

Age references

Franklin magmatic event

26s

08, 163

4

D

xi

GMi : Buchan et al. (2000)

Ub 723–712

Long Range dykes

5d

19, 355

18

D

b

Hodych et al. (2004) (recalculated from Murthy et al., 1992); also see McCausland et al. (2009)

Uzb 615 ± 2, 614 + 6/−4

Heaman et al. (1992), Pehrsson and Buchan (1999), and Denyszyn et al. (2009) Kamo et al. (1989) and Kamo and Gower (1994)

10

D

b

GM: hereinj T: Abrahamsen and Van der Voo (1987)

Ub 1382 ± 2

Upton et al. (2005)

GM: Buchan et al. (2000) T: Mertanen and Pesonen (1995) Pesonen and Neuvonen (1981) T: L.J. Pesonen (in Buchan et al., 2000) Pesonen and Neuvonen (1981) T: L.J. Pesonen (in Buchan et al., 2000) Lubnina et al. (2010b)

Uz 1630 ± 9; 1617 ± 2

Neuvonen (1986) and Törnroos (1984)

Uz 1576 ± 13; 1571 ± 20; 1571 ± 9

Suominen (1991)

Uzb 1577 ± 12; 1540 ± 12

Suominen (1991)

Ub 1452 ± 12

Lubnina et al. (2010b)

Northeastern Greenland (Laurentia) 10s Midsommersø sills, 11, 239 dykes and related 9s volcanics 19s Baltica Subjotnian quartz porphyry dykes

14s

29, 177

6

S

s, xk , dk

Åland quartz porphyry dykes

7s

12, 182

7

S

b, xk

Åland dolerite dykes

6s

28, 188

9

S

b, xk

Lake Ladoga mafic rocks Jotnian dolerite intrusions (CSDG)l

4u

15, 177

6

D

b

41s

04, 158

4

S

b, x

Egersund dykes

12s

31, 044m

16

S

2b

Yilgarn craton Widgiemooltha dykes

9d

−10, 159

8

D

14d

46, 135

4

Kaapvaal Black Hills dykes

8

09, 352

Kalahari Umkondo dolerites

10u

GM: Buchan et al. (2000) T: Neuvonen (1973); L.J. Pesonen (in Buchan et al., 2000) Walderhaug et al. (2007)

Ub 1268–1256l

Suominen (1991) and Söderlund et al. (2006)

Ub 616 ± 3

Bingen et al. (1998)

b

Smirnov et al. (2013) and Evans (1968)

Ub 2418 ± 3, 2411 ± 2, 2410 ± 2

Nemchin and Pidgeon (1998), Doehler and Heaman (1998) and French et al. (2002)

S

b

Wingate and Giddings (2000)

Uzb 755 ± 3

Wingate and Giddings (2000)

5

D

b, c

Lubnina et al. (2010a)

Ub ca. 1880; 1865

Olsson in Söderlund et al. (2010) and Olsson in Lubnina et al. (2010a)

64, 039

4

S

b

Gose et al. (2006) T: Jones and McElhinny (1966) and Jones (1968)

Ubz 1112 ± 1; 1108 ± 1

Hanson et al. (2004)

7s

31, 279

4

S

c

Didenko et al. (2009)

Uz 1878 ± 4

Didenko et al. (2009)

Dharwar Bangalore dykes

25d

16, 057

6

S

b

Halls et al. (2007) and references therein

Ub 2368–65

French and Heaman (2010) and Halls et al. (2007)

India Mahoba dykes VGPn

4d

−39, 050

12

S

do

Pradhan et al. (2012)

Uz 1113 ± 7

Pradhan et al. (2012)

2st

04, 161

13

D

p

GM: Evans et al. (2000)

SHRIMP Uz 748 ± 12

Ma et al. (1984)

Australia Mundine Well and related dykes

Siberia Lower Akitkan sediments

S. China Basal Liantuo Fm sediments

K.L. Buchan / Precambrian Research 238 (2013) 93–110

99

Table 1 (Continued) Unit

N

Plat (◦ N) and Plon (◦ E)

A95 (◦ )

Pol

Test

Paleomagnetic references

Age (Ma)

Age references

N. China Taihang dykes

19d

36, 247

3

S

b

Halls et al. (2000)

Uz 1769 ± 3

Halls et al. (2000)

Amazonia Nova Guarita intrusives

19s

−48, 246

7

B

b

Bispo-Santos et al. (2012)

Abi 1419 ± 4

Bispo-Santos et al. (2012)

Notes: ‘Unit’ is the rock unit studied. For Superior craton 2069 Ma and older units are identified as either east of the Kapuskasing Structural Zone (KSZ) or west of the KSZ (see text). ‘N’ is the number of sites (s), dykes (d), units (u) and studies (st) utilized in calculating mean remanence directions. ‘Plat’ and ‘Plon’ are latitude and longitude of the mean paleopole. ‘A95 ’ is radius of the circle of confidence about the mean pole and is usually calculated with Fisher statistics on site mean poles. When site mean poles are not available A95 is estimated as the square root of dp × dm, where dp and dm are the semi-axes of the 95% ellipse of confidence about the mean pole (Khramov, 1987, p. 97). ‘Pol’ is remanence polarity (S is single polarity and D is dual polarity). ‘Test’ is the positive field test that establishes that the remanence is primary (b is baked contact test; b(p) is baked contact profile test; c is intraformational conglomerate test; p is polarity correlation test; s is secular variation correlation test; d is remanence direction correlation test; ‘ef’ is fold test roughly contemporaneous with emplacement; x is regional consistency test. See description of tests in text. ‘Paleomagnetic references’: GM indicates a grand mean was calculated in this study or elsewhere; T indicates a reference for the field test, unless it is described in the general paleomagnetic reference. ‘Age’ is age of rock unit and magnetization. U is U–Pb, A is Ar–Ar, b is baddeleyite, z is zircon, bi is biotite. a Dual polarity for Matachewan dykes refers to the overall swarm, although normal (N) and reversed (R) polarity results are listed as separate entries. b Grand mean for Senneterre dykes calculated from data of Buchan et al. (1993, 2007a). c These units from northeastern, central and western North America all have rather similar paleopoles and ages, although field tests are only available for the Michikamau (baked contact test) and St. Francois Mountains (positive fold test where folding is thought to be contemporaneous with emplacement). d Harlan et al. (1994) propose that, based on Ar–Ar data, the Laramie anorthosite and Sherman granite magnetizations were blocked at ca. 1415 Ma during slow cooling of the plutons. e Baked contact test of Pesonen (1979) is for an undated dyke that it is thought to feed the Logan sills. f Remanence is interpreted as primary because Keweenawan polarity reversal coincides exactly with an unconformity in the Osler volcanic sequence, and scatter in paleomagnetic directions decreases following correction for syn-volcanic tilting (Halls, 1974). g Gunbarrel sills, dykes and sheets from three widely separated regions of western North America (Park et al., 1995; Harlan et al., 2003, 2008). h Grand mean for Gunbarrel intrusions calculated for data from northern Cordillera, northern Canadian Shield and Wyoming craton of Montana (Harlan et al., 2008). i The Franklin grand mean is based on data from a very wide geographic area of mainland Canada, Baffin Island and Victoria Island which is interpreted as a coherent block. A similar Franklin grand mean paleopole at 8◦ N, 164◦ E (A95 = 3) has also been calculated by Denyszyn et al. (2009) based on data from this block as well as from northwest Greenland (corrected for the Cretaceous-Early Tertiary opening of Baffin Bay) and Ellesmere and Devon Islands (corrected for probable Cretaceous-Early Tertiary rotation relative to North America). j Grand mean calculated for Midsommersø dolerites, Zig-Zag Dal basalts (Marcussen and Abrahamsen, 1983) and related dykes (Abrahamsen and Van der Voo, 1987). k Subjotnian quartz porphyry dykes, Åland quartz porphyry dykes and Åland dolerite dykes all have broadly consistent poles over a wide region in southern Finland whereas older units from the same area as the Subjotnian quartz porphyry dykes have different poles. l Ages for intrusions of the Central Scandinavian Dolerite Group (CSDG) span 1271–1247 Ma (Söderlund et al., 2006), although paleomagnetic sites are likely confined to those dated in the range 1268–1256 Ma. m Egersund paleomagnetic directions show a small amount of streaking towards the present Earth’s magnetic field, suggesting that some sites might carry a small unresolved present field component. n Mahoba pole is considered a VGP (Pradhan et al., 2012) because it is based on 4 dykes and hence is unlikely to fully average out secular variation. o Mahoba dykes are located in the same region where older (1.98 Ga) dykes carry distinctly different remanence directions.

358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386

to demonstrate that their remanence is primary. In a few cases other field tests have been reported. There is an increasing awareness of the importance of collaborative paleomagnetic-geochronological studies, especially on dyke swarms. Such studies are critical because it is common to find subparallel dykes of different ages in the same geographical area. Two of the key poles in Table 1 are derived from sedimentary rocks whose remanence can be susceptible to inclination shallowing. The data in these studies, however, suggest that shallowing is not present or had little effect on the paleopoles. In the case of the Basal Liantuo Fm sediments of South China, Evans et al. (2000) compared the inclinations of sandstones and mudstones, found that they were indistinguishable, and therefore concluded that there was no measurable inclination shallowing due to compaction (this would also suggest no depositional inclination error). In the case of the Lower Akitkan sediments of Siberia, Didenko et al. (2009) did not describe a test, but the remanence inclination is very low (mean inclination of 8◦ ), and hence shallowing is undoubtedly very small if present. For example, for a flattening factor, f = 0.5, the original remanence inclination of the Siberia sediments would be 16◦ . This would indicate an 8◦ shallowing which translated to a change in the paleopole (or paleolatitude) of only 4◦ . Furthermore the Siberia sediments, described as fine-grained sandstones and siltstone, have a single remanence direction grouping. A dozen years ago, most key data were derived from Laurentia and one of its constituent cratons, the Superior craton. There were three rather large gaps in the Superior-Laurentia data set, between 2.45 and 2.22 Ga, between 1.88 and 1.46 Ga, and between 1.087 and 0.78 Ga. Few key poles were available from the

remaining cratons around the globe. Age matches or approximate matches were only available in three instances – for Baltica – Laurentia at ca. 1.26 Ga, Laurentia–Kalahari at 1.11 Ga and Laurentia–Australia at ca. 0.78–0.72 Ga (Buchan et al., 2000, 2001). Today, the key paleopole database is significantly improved. There are now 28 key poles from the Superior craton and Laurentia (assuming that the four Matchewan poles of Table 1 are considered as one, as are the two Biscotasing poles). The 1.88–1.46 Ga gap in the Superior-Laurentia dataset has been closed, and other smaller gaps filled. In addition, a primary pole for the Midsommersø sills and dykes and Zig-Zag Dal basalts (Marcussen and Abrahamsen, 1983; Abrahamsen and Van der Voo, 1987) of northeastern Greenland, generally thought to have been part of Laurentia, has now been precisely dated at 1.38 Ga (Upton et al., 2005). Northeastern Greenland is generally assumed to have been part of Laurentia prior to the separation of Greenland from North America in the CretaceousEarly Tertiary, although the presence of the Greenland ice sheet prevents a definitive understanding of the linkage. As a result of the improved Superior-Laurentia data set, the Superior-Laurentia APWP can now be used as a reliable reference path against which poles from other cratonic blocks can be compared through the 2.22–1.087 Ga period. The number of key paleopoles from various other cratons around the world has risen sharply – from seven to 17. Several of these can be matched in age with those from Superior-Laurentia or with those from other cratons. In total, precise or approximate age matches are now available in eight time intervals (Fig. 1). Perhaps surprisingly, given the amount of study that has focused on Rodinia in recent years, there are still no key poles, even from Laurentia,

387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415

Amazonia

N. China

S. China

India

Siberia

Kalahari

600

800

700

900

1100

1000

1300

Australia

Rodinia

1200

1500

1400

1700

1600

1800

1900

2100

Baltica

Columbia/Nuna

2000

2300

2200

2500

600

Kenorland or supercratons

2400

Ma

K.L. Buchan / Precambrian Research 238 (2013) 93–110

Laurentia

100

Ma

Fig. 2. Approximate age range of the most widely discussed supercontinents or supercratons in the latest Archean and Proterozoic. Prolonged periods of assembly and breakup are depicted by the lighter shading.

700 800 900 1000 1100

VGP

1200 1300 NE Greenland

1400 1500 1600 1700 1800 1900 2000 2100 2200

used geological evidence and paleomagnetic data (key and non-key poles with age uncertainties up to ±200 m.y.) to suggest a longlived supercontinent Protopangaea. In the late Paleoproterozoic-early Mezoproterozoic various versions of a supercontinent called Columbia or Nuna have been advanced (see summary in Meert, 2012), based largely on geological data (e.g., Rogers and Santosh, 2002), a combination of geological and paleomagnetic information (e.g., Zhao et al., 2004) or a combination of key and non-key paleomagnetic data (e.g., Zhang et al., 2012). Rodinia is the most often discussed supercontinent in the latest Mesoproterozoic-Neoproterozoic. Widely divergent reconstructions have been proposed based on geological and/or paleomagnetic analysis (e.g., McMenamin and McMenamin, 1990; Dalziel, 1997; Pisarevsky et al., 2003; Li et al., 2008; Evans, 2009). The number and configuration of cratons that may have been involved in proposed supercontinents in the Proterozoic are hotly debated, as is the timing of their assembly and breakup. The wide variety of proposals reflects, in part, the lack of reliable paleomagnetic data from most cratons.

434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453

2300

Dharwar

In this section the key paleopoles catalogued in Table 1 and Fig. 1 are considered in relationship to three time intervals: pre-1.85, 1.85–1.20, 1.20–0.544 Ga. Several non-key poles from well-dated rock units, which are used to help clarify the discussion or which support reconstructions based on key poles are listed in Table 2. When paleopoles from two cratons are compared in the following discussion, it should be noted that there are several uncertainties that need to be considered.

Kaapvaal

2500

Yilgarn

8. Key paleopole analysis

Superior Slave

2400

Fig. 1. Age of key Proterozoic paleopoles from individual cratons. Key poles for craton margins and micro-continents are not included (see text). Precise and approximate age matches are indicated by grey shading. The entry for India at 1.11 Ga may not average out secular variation and hence is identified as a VGP.

416 417 418 419 420 421 422

423 424

425 426 427 428 429 430 431 432 433

within the period 1.087–0.78 Ga which is though to include the period in which the supercontinent was fully assembled. The potential for establishing many more key paleopoles in the near future appears great. Precise U–Pb baddelyite dating of mafic magmatic events around the world is proceeding at a rapid pace (Ernst et al., 2013). A determined effort will be required to obtain primary paleopoles from the newly dated rock units. 7. Late Archean-Proterozoic continents and supercontinents Numerous continent or supercontinent reconstructions have been proposed for various time intervals in the late Archean and Proterozoic based on geological or paleomagnetic data or a combination of the two. The most widely discussed supercontinents fall into three time intervals (Fig. 2). In the late Archean-early Paleoproterozoic supercontinents such as Kenorland (e.g., Williams et al., 1991) or smaller supercratons such as Superia, Vaalbara, and Sclavia (e.g., Bleeker, 2003) have been proposed based mainly on geological considerations. Piper (2010)

(a) The quoted uncertainty for a paleomagnetic pole (typically A95 = 5–10◦ ). Note that this uncertainty will be an underestimate of the actual uncertainty if secular variation has not been fully averaged out. (b) The mismatch between the mean ages for the two poles (typically 0–30 m.y. in this analysis). (c) The quoted uncertainty on the mean age of a rock unit. Most key poles are derived from fast cooling dyke swarms (or sills) that may be emplaced over several million years. If the U–Pb age is from a single dyke, its uncertainty may underestimate the uncertainty in the age of the paleopole, which is derived from a number of dykes. Dating more than one dyke helps to eliminate this problem. Occasionally, key poles are from large plutons whose magnetization may have been blocked during slow cooling later than the age recorded by the U–Pb age (e.g., Harlan et al., 1994). In plotting reconstructions it is common practice to simply show the cratonic blocks in a single reconstruction–usually the one which brings the blocks closest together within the constraints of the geological and/or paleomagnetic data. In this section, however, an attempt is made to illustrate the range of reconstructions that is

454

455 456 457 458 459 460 461 462

463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478

479 480 481 482 483

101

Huhma (1981) Nilsson and Wikman (1997) Pisarevsky and Sokolov (2001) Lubnina et al. (2012)

permitted by the key paleomagnetic data, given the uncertainty in relative paleolongitude and often in relative magnetic polarity. 8.1. Pre-1.85 Ga

Note: See notes for Table 1. sa is number of samples. a Malley dyke magnetization is likely significantly older than the 2.19 Ga emplacement of Dogrib dykes (see Buchan et al., 2012). b The 17 Dogrib dyke sites are from two prominent dykes, and unlikely to average out secular variation. c Dogrib dyke magnetization is interpreted to significantly predate the 2.11 Ga emplacement of Indin dykes and is likely primary (see Buchan et al., 2012). d Data are from a single large sill, and unlikely to average out secular variation.

Uz 1840–1837 Uz 1780 ± 3; 1776 + 8/−7 Ca. 1790–1751 Ub 1751 ± 3 Neuvonen et al. (1981) Pisarevsky and Bylund (2010) Pisarevsky and Sokolov (2001) GM: Pisarevsky and Bylund (2010) – – – – S D D 3 8 4 7 48, 225 46, 183 40, 221 39, 217

Halls et al. (2011) Ub 1635–1622 Halls et al. (2011) – D 9 5, 274 9s

12s 11s 36 sa 2std Baltica Haukivesi lamprophyre dykes Småland intrusions Shoksha sandstones Ropruchey sill VGP

Western Greenland (Laurentia) Melville Bugt dykes

Bostock and van Breemen (1992) Ub 1827 ± 4 McGlynn et al. (1974) – D 8 12, 291 10s Churchill Province Sparrow dykes

Buchan et al. (2012) LeCheminant et al. (1997) Ub 2231 ± 2a Ub ca. 2190c Buchan et al. (2012) McGlynn and Irving (1975) – – S S 7 4 −51, 310 −35, 310 9s 17sb Slave craton Malley dykes Dogrib dykes VGP

N Unit

Table 2 Selected non-key paleopoles utilized in the figures.

Plat (◦ N) and Plon (◦ E)

A95 (◦ )

Pol

Test

Paleomagnetic references

Age of rock unit (Ma)

Age references

K.L. Buchan / Precambrian Research 238 (2013) 93–110

The configuration of postulated late Archean-early Paleoproterozoic supercontinents or alternatively several small supercratons is poorly understood. Timing of breakup is also uncertain but likely occurred sometime between 2.5 and 2.0 Ga (e.g. Bleeker, 2003), perhaps occurring progressively over an extended period, based on evidence from rifted margins and large igneous provinces. Until recently there were only a few pre-1.85 Ga Proterozoic key paleopole, all from a single craton – the Superior craton of North America. However, the Superior database has now been improved, and key poles have been obtained from the Slave craton of North America, the Siberia craton, the Kaapvaal craton of southern Africa, the Yilgarn craton of Australia and the Dharwar craton of India (Fig. 1). The Superior key paleopoles (Table 1), derived from mafic dykes and sills, now define a reasonably robust APWP (Fig. 3a) that, except for a long gap between 2.45 and 2.22 Ga, can be used as a reference track against which individual key poles from other cratons can be compared. The long Superior track establishes that the craton was drifting throughout the period and suggests that plate tectonic processes operated in the early Paleoproterozoic. The Superior craton key pole data set is also of sufficient detail to identify a 10–20◦ relative rotation of the eastern and western portions of the craton since 2.07 Ga. The analysis is based on comparing coeval key poles at ca. 2.47, 2.17 and 2.08–2.07 Ga from either side of the Kapuskasing Structural Zone, which marks the boundary between the eastern and western Superior (Halls and Davis, 2004; Buchan et al., 2007a; Evans and Halls, 2010). A series of on-going collaborative studies of paleomagnetism and U–Pb geochronology in the Slave craton of North America are yielding paleopoles for several precisely dated dyke swarms (Buchan et al., 2007b, 2009, 2010, 2012; Mitchell et al., 2009) and have the potential to produce an APWP that rivals that for the Superior craton. A key paleopole has been reported for the 2.025 Ga Lac de Gras swarm (Buchan et al., 2009) and is plotted in Fig. 3a for comparison with the Superior APWP. Although an exact age match is not available from the Superior craton data, the Lac de Gras pole falls far from the older 2.07 Ga Lac Esprit and slightly younger 2.00 Ga Minto key poles of the Superior track. This demonstrates that the Superior and Slave cratons were not in their current relative positions and orientations (Buchan et al., 2009). This, in turn, indicates that supercontinent reconstructions (e.g., Piper, 2010) that incorporate Laurentia (i.e. with Superior and Slave cratons in their present relative position) at ca. 2.025 Ga or earlier are invalid. Two older Slave craton poles, a paleopole from the precisely dated 2.23 Ga Malley swarm (Buchan et al., 2012) and a VGP for the precisely dated 2.19 Ga Dogrib swarm (McGlynn and Irving, 1975; LeCheminant et al., 1997), are also plotted in Fig. 3a. Buchan et al. (2012) conclude that the Dogrib pole significantly pre-dates 2.11 Ga and is likely primary, and that the Malley pole is significantly older than 2.19 Ga, likely acquired near the time of emplacement. However, as definitive field tests are not available, these poles are not considered key poles. If the Malley and Dogrib poles are primary, then a comparison of the Slave and Superior paleopoles of Fig. 3a between 2.23 and 2.025 Ga indicates that the two cratons did not drift adjacent to one another throughout this period (more detailed analysis and discussion in Buchan et al., 2012), in agreement with geological evidence that they were not together in the early Paleoproterozoic (e.g., Bleeker, 2003). Fig. 3b illustrates the drift of Superior craton (based on key poles) and Slave craton (based on key and non-key poles). Buchan et al.

484 485

486

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Fig. 4. Ca. 1.88 Ga reconstruction of Superior, Siberia and Kaapvaal cratons based on key poles. The Superior craton is fixed at an arbitrary longitude and in one hemisphere. The Siberia and Kaapvaal cratons are permitted to vary in longitude and hemisphere to indicate the range of possible reconstructions. For clarity, presentday north (N) is indicated for Siberia and Kaapvaal cratons.

Fig. 3. (a) Key paleopoles and probable primary poles from precisely dated units on the Superior (squares) and Slave (circles) cratons of North America (modified from Buchan et al., 2012). Pole ages are in Ga. Ellipses of 95% confidence are shown. All poles are key poles except for the Malley paleopole and Dogrib VGP which are from precisely dated dyke swarms and known to significantly pre-date 2.11 Ga, but which do not have a primary test (see text). (b) Drift plot for Superior and Slave cratons (modified after Buchan et al., 2012) based on 2.23–2.00 Ga poles in (a). The Superior craton is shown in one hemisphere, whereas the Slave craton is shown in both hemispheres. Longitude is not constrained. The arrow labelled N on the Slave craton indicates present north. EQ is equator. The inset locates Superior and Slave cratons within Laurentia.

548 549 550 551 552 553 554 555 556 557 558 559 560 561 562

(2010) report that key poles have also been obtained for the 2.11 Ga Indin and 1.88 Ga Ghost dyke swarms in the Slave craton and that these data demonstrate independent drift of the Slave and Superior cratons. This interpretation is consistent with geological evidence that final cratonization of the Superior within Laurentia likely did not occur until ca. 1.80 Ga (e.g., Corrigan et al., 2009; St Onge et al., 2009). Ca. 1.88–1.86 Ga key poles have been reported for the lower Akitkan sedimentary rocks of the Siberian craton (Didenko et al., 2009) and the Black Hills dykes of the Kaapvaal craton (Lubnina et al., 2010a) (Table 1). They provide a direct age match (Fig. 1) with the coeval 1.88 Ga Molson dyke pole from the Superior craton. The locations of the three cratons at 1.88 Ga are compared in Fig. 4. The paleolatitude and azimuthal orientation of each block is fixed by its paleomagnetic data. Paleolongitude and polarity are not

defined. The Superior craton is placed at an arbitrary longitude and in one hemisphere. The Siberia and Kaapvaal cratons are permitted to occupy any longitude in either hemisphere to account for the longitudinal and polarity ambiguity. This yields the various possible reconstructions of the three blocks. Siberia straddles the equator, whereas the Superior and Kaapvaal cratons are both at intermediate latitudes. This suggests that Siberia was likely not a nearest neighour of either the Superior or Kaapvaal craton at 1.88 Ga. On the other hand, Kaapvaal is permitted to be adjacent to Superior (positions C or D in Fig. 4), although Bleeker (2003) has suggested that a Superior-Kaapvaal connection is unlikely based on geological considerations. More widely separated locations of the two cratons (e.g., A, B, E, F or G–L) are equally possible from the paleomagnetic data. Further key pole age matches are required to establish which reconstruction is correct and whether any of these cratons were part of a single supercontinent in the early Paleoproterozoic. In addition to the 2.47–2.45 Ga Matachewan dyke key pole for the Superior craton, very early Paleoproterozoic key poles have been published for the 2.41 Ga Widgiemooltha dykes of the Yilgarn craton of southwestern Australia (Smirnov et al., 2013) and the 2.37 Ga Bangalore dykes of the Dharwar craton of India (Halls et al., 2007). Unfortunately, the ages of these three poles span 80–100 m.y., so that reconstructions are of only a very tentative nature. For example, Pesonen et al. (2012) utilized poles from all three blocks, plus a non-key pole from Baltica and a comparison of dyke trends. Halls et al. (2007) compared the Dharwar and Yilgarn positions and proposed that the Bangalore and Widgiemooltha dykes were part of a giant radiating swarm associated with a long-lived plume. Smirnov et al. (2013) compared the key 2.41 Ga Yilgarn pole with a non-key pole derived from a 2.41 Ga dyke in the Zimbabwe craton, and suggested that the two cratons could have been nearest neighbours with their coeval dykes aligned parallel to each other.

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8.2. 1.85–1.20 Ga The 1.85–1.20 Ga interval is thought to correspond to the final assembly, drift and progressive breakup of the proposed supercontinent of Columbia/Nuna (Fig. 2; e.g., Meert, 2012). There are a number of key pole age matches in this period (Fig. 1). Three of these age matches involve Baltica and Laurentia at ca. 1.59, 1.45 and 1.26 Ga. Using the key pole match for the Mackenzie dykes of Laurentia and the Central Scandinavian Dolerite Group of Baltica at ca. 1.26 Ga, Buchan et al. (2000) proposed a reconstruction of Baltica and Laurentia with northern Norway adjacent to northeastern Greenland (Position B in Fig. 5a), a reconstruction that is similar to the Nena (Northern Europe-North America) fit of Gower et al. (1990). Only one polarity option for Baltica is shown in this reconstruction because only a single polarity is observed in the Baltica data and only a single polarity is observed in the Laurentia data. Therefore, given that the ages of the poles from the two cratons overlap, their polarity is undoubtedly the same, and there is no relative polarity ambiguity. Because of the paleolongitude ambiguity when dealing with a single key pole match the reconstruction of position B was not considered unique and other positions for Baltica (e.g., A and C of Fig. 5a) are also possible. However, a very much older comparison at 1.83 Ga between a pair of non-key poles from precisely dated units in Baltica and Laurentia appeared to give a rather similar fit despite a large overall rotation of the two blocks. Therefore, Buchan et al. (2000) concluded that if the 1.83 Ga data is primary the combined 1.83 and 1.26 Ga data would define a unique reconstruction (position B) as described in Method 2 of Section 5.2. Since that time there has been a dramatic increase in key data from both Laurentia and Baltica that bear on their relative positions in the early Mesoproterozoic. In addition to the 1.26 Ga result, key poles can now be compared at 1.46–1.45 and 1.59–1.57 Ga. In Fig. 5b, data for the ca. 1.45 Ga Harp Lake Complex of Laurentia (Irving et al., 1977; Krogh and Davis, 1973) are compared with those for the 1.45 Ga Lake Ladoga mafic rocks of Baltica (Lubnina et al., 2010b; see also Salminen and Pesonen, 2007). Utilizing the ca. 1.46 Ga Michikamau anothosite pole for Laurentia (Emslie et al., 1976) would yield a roughly similar fit. In Fig. 5c, results for the 1.59 Ga Western Channel Diabase of Laurentia (Irving et al., 1972a; Hamilton and Buchan, 2010) are compared with those for the slightly younger 1.58–1.57 Ga Åland dykes of Baltica (Pesonen and Neuvonen, 1981; Suominen, 1991). The slightly older 1.63–1.62 Ga Subjotnian quartz porphyry dykes (Mertanen and Pesonen, 1995; Neuvonen, 1986; Törnroos, 1984) is shown for comparison. In each of Fig. 5a–c, the position labelled B places northern Norway adjacent to northeastern Greenland within the uncertainty of the paleomagnetic and age data. Because the cratons are rotating through the 1.59–1.26 Ga period, position B defines a unique reconstruction of Baltica and Laurentia (see Method 2 of Section 5.2) and indicates that the two cratons were moving together during this ∼330 m.y. period (Hamilton and Buchan, 2010; Lubnina et al., 2010b). There are now also age matches that involve key and/or nonkey poles from well-dated units at three older times, ca. 1.63, ca. 1.79–1.74 and ca. 1.84–1.83 Ga. They are discussed here to indicate that the reconstruction described for key poles may be valid as early as 1.83 Ga. In Fig. 5d, non-key data for the Melville Bugt dykes (Halls et al., 2011) of western Greenland portion of Laurentia, with Greenland corrected for the opening of the Labrador Sea, are compared with the key data for the ca. 1.63–1.62 Ga Subjotnian quartz porphyry dykes of Baltica (Mertanen and Pesonen, 1995; Neuvonen, 1986; Törnroos, 1984). They yield a reconstruction similar to that of Pesonen et al. (2012). In Fig. 5e, key results from the 1.74 Ga Cleaver dyke swarm of Laurentia (Irving et al., 2004) are compared with non-key data for the ca. 1.78–1.77 Ga Småland

103

intrusions (Pisarevsky and Bylund, 2010; Nilsson and Wikman, 1997) and ca. 1.79–1.75 Ga Shoksha sediments (Pisarevsky and Sokolov, 2001) of Fennoscandia. Using the non-key VGP for the precisely dated Ropruchey sill of Fennoscandia (Pisarevsky and Bylund, 2010; Lubnina et al., 2012) yields a similar fit to that of the Shoksha sediments. In Fig. 5f, non-key results are compared for the 1.83 Ga Sparrow dykes of the Churchill Province of North America (McGlynn et al., 1974; Bostock and van Breemen, 1992) and the 1.84–1.83 Ga Haukivesi lamprophyres of Fennoscandia (Neuvonen et al., 1981; Huhma, 1981). Once again, in each of the reconstructions of Fig. 5d–f, position B places northern Norway against northeastern Greenland within the uncertainty of the data even as the two blocks continue to rotate. These non-key comparisons suggest that the 1.59–1.26 Ga reconstruction based on key poles may be valid through the entire 570 m.y. period between 1.83 Ga and 1.26 Ga. It should be noted that Fig. 5e–f do not require that Baltica and Laurentia be attached at ca. 1.83–1.74 Ga. They only indicate that at 1.83 Ga the Churchill Province and Fennoscandia from which the paleopoles were derived were together as shown, and that at 1.78–1.74 Ga Fennoscandia and Laurentia were attached. As noted earlier, geological evidence suggests that the final cratonization of the Superior craton within Laurentia likely did not occur until ca. 1.80 Ga (Corrigan et al., 2009; St Onge et al., 2009). In addition, the Volgo-Sarmatia portion of Baltica was likely not fully amalgamated with Fennoscandia until ca. 1.75 Ga (e.g., Bogdanova et al., 2012). Q3 It should also be noted that, unlike Fig. 5a where there is no relative polarity ambiguity, in Fig. 5b–f there is a relative polarity ambiguity and Baltica could be placed in either hemisphere. The polarity option that is not shown would not yield a unique reconstruction of the two blocks, and is not included in order to keep the diagram reasonably simple. In Fig. 6, rudimentary APWP segments based on the 1.83–1.26 Ga paleopoles for Laurentia and Baltica utilized in Fig. 5 are compared (see Method 1 of Section 5.1). In present day coordinates (Fig. 6a) the data from the two cratons form discordant paths. Using a rotation of +55◦ about an Euler pole at 47.5◦ N, 001.5◦ E the two data sets can be matched within the uncertainty of the poles and their ages as illustrated in Fig. 6b (cf. Fig. 5 of Evans and Pisarevsky (2008) based on a more limited data set available at that time and a slightly smaller rotation angle of +49◦ ). This rotation also brings the present-day northern coast of Norway and the Kola Peninsula of Russia against the northeastern coast of Greenland (Fig. 6b). In Fig. 6c the individual poles from the two cratonic blocks are grouped by age to show the systematic age progression from east to west. The Laurentia–Baltica reconstruction of Fig. 6b, with northern Norway against northeastern Greenland, comprised a rather large landmass and, therefore, likely formed a major portion of any supercontinent (e.g., Columbia/Nuna) that may have existed in the latest Paleoproterozoic or early Mesoproterozoic. It will be assumed in further discussions of other cratons that this Baltica–Laurentia (or Fennoscandia-Churchill Province) reconstruction is valid during the 1.84–1.26 Ga time interval. It should be emphasized that the key pole comparison permits only reconstructions in which northern Norway and the Kola Peninsula face northeastern Greenland (e.g., Gower et al., 1990; Evans and Mitchell, 2011; Bispo-Santos et al., 2012; Zhang et al., 2012; Pesonen et al., 2012). However, many other published reconstructions for this period place the two blocks adjacent to one another, but in quite different configurations, such as western Norway facing northeastern Greenland (e.g., Rogers and Santosh, 2002; Johansson, 2009), northwestern Norway facing southeastern Greenland (Hoffman, 1997; Zhao et al., 2004) or southwestern Norway facing southeastern Greenland (Halls et al., 2011). Such reconstructions are not permitted by the well-constrained paleomagnetic data of Figs. 5 and 6.

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Fig. 5. Ca. 1.26–1.83 Ga reconstructions of Laurentia and Baltica or their predecessor cratons. Reconstructions in (a)–(c) are based on key poles. Those in (d)–(f) include non-key poles usually from well-dated rock units. Labels for key poles are in bold; those for non-key poles are in italics. (a) Reconstruction at ca. 1.27–1.25 Ga (modified after Buchan et al., 2000). Only one polarity option is shown for Baltica because only a single polarity is observed in the Laurentia data and only a single polarity in the Baltica data. Reconstructions at (b) ca. 1.46–1.45 Ga, (c) ca. 1.59–1.57 Ga, (d) ca. 1.64–1.62 Ga, (e) ca. 1.79–1.74 Ga, and (f) ca. 1.84–1.83 Ga. In (e) the reconstruction is between Fennoscandia and Laurentia. The outline of Baltica is shown for comparison. In (f) the reconstruction is between the Churchill Province (assumed to extend across Greenland) and Fennoscandia. Outlines of Baltica and Laurentia are shown for comparison. To keep the figure relatively simple the second polarity option in (b)–(f) has not been shown as discussed in the text. 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752

In addition to the key (and non-key) paleopoles that establish the Baltica–Laurentia fit, there are key poles from two other cratons in the 1.85–1.20 Ga period. Bispo-Santos et al. (2012) have reported a key pole for the 1.42 Ga Nova Guarita intrusives of Amazonia and compared it with roughly coeval poles for Baltica and Laurentia. They propose a reconstruction of Amazonia and Baltica which, within the uncertainty of the paleomagnetic data and age control, is similar to reconstructions that have been proposed based on geological considerations, such as the SAMBA (South America-Baltica) fit of Johansson (2009). The various possible reconstructions of Amazonia and Baltica are illustrated in Fig. 7. Laurentia is located according to the Baltic-Laurentia fit of Fig. 6b. Position C is closest to the SAMBA reconstructions of Amazonia and Baltica (position C’) shown by Johansson (2009) and Bispo-Santos et al. (2012). It corresponds to a +95◦ rotation of Amazonia to Baltica (in present coordinates) about an Euler pole at 59◦ N, 254◦ E. Note that the apparent overlap of Amazonia and Baltica and their relative rotation with respect to the SAMBA fit can be accounted for by the combined uncertainty in the paleomagnetic data and the age uncertainties and age mismatch of ∼30 m.y. Although details of the data have not yet been published, Reis et al. (2012) have indicated that a primary pole has also been obtained from 1.79 Ga Avanavero sills and dykes of northern Amazonia, and that when compared to non-key poles from precisely dated units in Baltica, supports the SAMBA fit at that time. If the 1.79 Ga data is confirmed as primary the approximate SAMBA reconstructions at 1.79 and 1.45 Ga would

indicate that this fit may have lasted at least 450 m.y. It should be noted that a non-key pole from the precisely dated 1.79 Ga Colider Suite volcanics of southwestern Amazonia (Bispo-Santos et al., 2008) is distinctly different from the Avanavero pole, and does not yield the SAMBA fit. Reis et al. (2012) reinterpret this Colider pole as a probable secondary overprint. Based on a key pole for the 1.77 Ga Taihang dykes (Halls et al., 2000), the location of the North China craton can be compared with the position of Laurentia–Baltica at 1.74 Ga (Fig. 8). In Fig. 8, Laurentia is located on the basis of the 1.74 Ga Cleaver dyke pole, and Fennoscandia is then positioned with respect to Laurentia following the reconstruction of Fig. 6b. Baltica is thought to have been in the final assembly phase at this time (Bogdanova et al., 2013), so is shown as a dashed outline. Both polarity options place North China at the equator. Fennoscandia lies within 20◦ of the equator (Baltica within 10◦ ) and Laurentia between latitudes of 25◦ and 80◦ . Therefore, it appears unlikely that North China was adjacent to Laurentia (e.g., compare Fig. 20a of Halls et al., 2000), although a fit close to the present northern Laurentia margin cannot be entirely ruled out given the uncertainties in the paleomagnetic data and the 30 m.y. mismatch in ages. A position close to Fennoscandia (or Baltica) is possible (e.g., positions b or C), as well as more distance positions (A, B, D, E or a, c, d). Incorporating data from non-key poles, Zhang et al. (2012) and Chen et al. (2013) have proposed distinctly different reconstructions of continents/supercontinents in which North China is located away from Laurentia–Fennoscandia. Further key

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Fig. 6. Comparison of Laurentia and Baltica paleopoles (with circles of 95% confidence) between 1.26 and 1.84 Ga. Pole ages are in Ga. (a) Baltica and Laurentia are shown in their present locations, except that Greenland (and its poles) have been corrected for opening of the Labrador Sea using an Euler pole of 67.5◦ , 241.3◦ E, and rotation angle of −13.8◦ (Roest and Srivastava, 1989). Broad arrows with solid outlines indicate the overall trend of poles for Laurentia and Baltica, and can be considered a rudimentary APWP segment. The 1.38 Ga pole for northeastern Greenland, which has no age match in the Baltica data, appears discordant. It may represent a loop (narrow dashed arrow) in the Laurentia path (e.g., Marcussen and Abrahamsen, 1983), or perhaps relative rotation of northeastern Greenland and Laurentia as the geological relationship them is obscured by the Greenland ice sheet and rotations of Ellesmere and Devon Island with respect to Laurentia have been documented (Denyszyn et al., 2009). (b) Baltica and its paleopoles are rotated +55◦ about an Euler pole at 47.5◦ N, 001.5◦ E. This rotation yields a close fit of Baltica (Fennoscandia pre-1.75 Ga) and Laurentia (Churchill Province pre-1.80 Ga) as well as an overlap of the poles from the two blocks. For simplicity the 1.38 Ga pole from northeastern Greenland is not shown. (c) Poles are grouped by age interval with circle of confidence removed for clarity.

781

pole comparisions are required to better constrain reconstructions of North China and Laurentia–Fennoscandia (Baltica) in the late Paleoproterozoic.

782

8.3. 1.20–0.544 Ga

779 780

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During the 1.20–0.544 Ga interval the supercontinent Rodinia is believed to have assembled, drifted and broken apart (Fig. 2). As noted in Section 7, there have been a large number of proposed reconstructions of Rodinia or portions of the supercontinent over the past few years based on geological, paleomagnetic or a combination of geological and paleomagnetic constraints. Buchan et al. (2001) examined the paleomagnetic record for the Rodinia period and concluded that there were insufficient key poles to confirm the existence of a supercontinent or to identify significant components of a supercontinent reconstruction. Meert and Torsvik (2003) also analyzed paleomagnetic data and found major discrepancies with published Rodinia reconstructions. Unfortunately, the growing number of paleopoles (mostly non-key poles) since those two reviews were carried out has not helped to resolve most of the major discrepancies as evident from the radically different Rodinia reconstructions that have been recently proposed based heavily or almost entirely on paleomagnetic data (e.g., Evans, 2009; Li et al., 2008). The lack of key poles in the Rodinia period is clear from an examination of Fig. 1. They are only available during the proposed assembly and breakup of the supercontinent, and are discussed briefly in this section.

The key paleopole data that are currently available during the time of Rodinia assembly yield a precise age match at 1.11 Ga (Fig. 1) for the Logan sills of the Keweenawan large igneous province (LIP) of Laurentia (Halls and Pesonen, 1982; Davis and Green, 1997) and the Umkondo LIP of the Kalahari craton (Gose et al., 2006; Hanson et al., 2004). The data suggest that the two blocks (and their coeval LIPs) were not together at that time (Gose et al., 2006). As illustrated in Fig. 9 the Kalahari craton is on the equator, whereas Laurentia lies between latitude 40◦ and 75◦ . Primary VGPs from precisely data units of similar age in Baltica (1.12 Ga Salla dyke; Salminen et al., 2009; Lauerma, 1995) and India (1.11 Ga Mahoba dykes; Pradhan et al., 2012) have also been reported. The Salla dyke VGP is based on only one dyke (13 sites) and hence is quite uncertain. The Mahoba dykes VGP, on the other hand, is based on 4 dykes and so is likely to have averaged out much of the secular variation. It is used to roughly locate India with respect to Laurentia and Kalahari craton in Fig. 9. Given the uncertainty in the paleopoles the data would permit India to be located close to either Laurentia or Kalahari, although positions far from these blocks are equally possible. During the early phase of Rodinia breakup key 0.76–0.75 Ga paleopoles have been reported for the Mundine Well dykes of Australia (Wingate and Giddings, 2000) and the basal Liantuo Formation sedimentary rocks of the South China craton (Evans et al., 2000; Ma et al., 1984). Although not precisely matched in age with key poles from Laurentia, they do fall in age between the key 0.78 Ga Gunbarrel intrusions pole (Harlan et al., 2003, 2008) and the key 0.72–0.71 Ga Franklin dykes pole (grand mean by Buchan et al., 2000; ages summarized in Denyszyn et al., 2009) from Laurentia

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60 0

60 0

30

0

E

F

30

G

Laurentia

Laurentia

0

Amazonia N

EQ

Baltica N

30

N

Kalahari

1.42 Ga

B A

2 polarity options

EQ

1.45 Ga

C

N

D

Amazonia

0

30

C’

0

India

SAMBA fit

60 0

ca. 1.11 Ga

ca. 1.45-1.42 Ga

60 0

Fig. 7. Ca 1.45–1.42 Ga reconstruction of Laurentia–Baltica and Amazonia based on key paleopoles. Baltica, based on its 1.45 Ga key pole, is placed at an arbitrary longitude and in one hemisphere. Laurentia is located with respect to Baltica based on the reconstruction of Fig. 6b. The 1.42 Ga position of Amazonia is permitted to vary in longitude and hemisphere to indicate the range of possible reconstructions. For clarity, present-day north (N) is indicated for Amazonia. Amazonia position C is similar to the SAMBA fit of Johansson (2009), which is shown here as C’ (modified after Bispo-Santos et al., 2012). The apparent overlap of Amazonia (position C) and Baltica can be accounted for by the combined uncertainty of the paleomagnetic data and the age mismatch.

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Fig. 8. Ca. 1.77–1.74 Ga reconstruction of Laurentia–Fennoscandia and North China based on key paleopoles. Laurentia is located at an arbitrary longitude and in one hemisphere using its key 1.74 Ga pole and Fennoscandia is then positioned according to the reconstruction of Fig. 6b. The outline of Baltica is also shown as it was in the final stage of assembly at this time (Bogdanova et al., 2013). The 1.77 Ga position of North China is permitted to vary in longitude and in either polarity option to indicate the range of possible reconstructions. For clarity, present-day north (N) is indicated for the North China craton.

Fig. 9. Ca. 1.11 Ga reconstruction of Laurentia and Kalahari craton based on key paleopoles, and India based on a well-dated VGP. Laurentia is fixed at an arbitrary longitude in one hemisphere. Kalahari and India are permitted to vary in longitude and in either polarity option to indicate the range of possible reconstructions. For clarity, present day north (N) is indicated for the Kalahari craton.

(Fig. 1). The various possible reconstructions of the three cratons are shown in Fig. 10. Laurentia is located near or at the equator, with Australia and South China at progressively higher latitudes. In many Rodinia reconstructions, Australia and South China are located west of Laurentia (in present-day coordinates). For example, Li et al. (2008) position South China against the western margin of Laurentia, with Australia further outboard (the so-called “missing link” model). In this scenario, it is assumed that by ca. 0.75 Ga the supercontinent was beginning to fragment with South China, Australia and Laurentia rifting apart (Evans et al., 2000; Li et al., 2008) (see positions B and b in Fig. 10). However, various other configurations of the three blocks are permitted by the key pole data as illustrated in the figure. More key pole data are required to clarify the relationship between these blocks. A key pole age match is also available at 0.615 Ga (Fig. 1) for the Egersund dykes of Baltica (Walderhaug et al., 2007; Bingen et al., 1998) and the Long Range dykes of Laurentia (Murthy et al., 1992; Hodych et al., 2004; Kamo and Gower, 1994). These data permit a reconstruction with present-day western Norway near southeastern Greenland (position B in Fig. 11). It corresponds to a −46◦ rotation of Baltica to Laurentia (in present coordinates) about an Euler pole at 67◦ N, 243◦ E. This or roughly similar fits are often used in recent Rodinia reconstructions (e.g., Li et al., 2008; Pisarevsky et al., 2008; Nance et al., 2008; Evans, 2009; Pesonen et al., 2012). It has the advantage of placing the precisely coeval Egersund and Long Range dykes in relatively close proximity to one another, as might be expected if they are parts of the same LIP event. Amazonia is usually inserted into the right-angle gap along the eastern margin of Laurentia and southern margin of Baltica (present coordinates), although there are no key poles to test this fit in the late Neoproterozoic, and radically different locations for Amazonia are sometimes proposed in the Rodinia period. For example, Tohver et al. (2006) place Amazonia adjacent to northeastern Laurentia, whereas Santos et al. (2008) place it against southwestern Laurentia. Based only on the key paleomagnetic data, reconstructions of Baltica and Laurentia other than the western Norway-southeastern

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Karlstrom et al., 2001). As discussed in Section 8.2, the 0.615 Ga western Norway-southeastern Greenland fit is sometimes shown (incorrectly) in reconstructions of the pre-1.26 Ga period. In conclusion, the configuration of most cratons in Rodinia remains difficult to assess based on key paleomagnetic data.

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9. Conclusions

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ca. 0.78-0.71 Ga 60 0 Fig. 10. Ca. 0.78–0.71 Ga reconstruction of Laurentia, Australia, and South China. Laurentia at 0.78 and 0.72–0.71 Ga is fixed at an arbitrary longitude (any longitudinal drift is not shown) and a single hemisphere. Australia and South China at 0.75 Ga are permitted to vary in longitude and hemisphere. For clarity, present day north (N) is indicated for the South China craton. Positions b for Australia and B for South China are those related to the “missing link” Rodinia model (cf. Evans et al., 2000; Li et al., 2008) in which South China is originally between Australia and the presentday western margin of Laurentia, but the three blocks have begun to separate by 0.75 Ga as indicated by the spreading ridges between the blocks.

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Greenland fit (position B in Fig. 11) are possible (e.g., positions A, C, D–F). It should be noted that the fit of position B differs radically from the pre-1.26 Ga Baltica–Laurentia reconstruction of Fig. 6b, indicating that the two blocks separated and rotated ∼90 relative to one another between 1.26 and 0.615 Ga (e.g., Fig. 1 of

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(1) Key paleopoles are essential for establishing reliable APWPs and continental reconstructions in the Proterozoic. (2) There have been several important advances related to key paleopoles over the past decade: (a) The Proterozoic reference APWP for the Superior craton and Laurentia is now more secure, with few very long time gaps. (b) A significant number of key poles from outside Superior/Laurentia are now available. (c) A robust Baltica–Laurentia reconstruction has been established for ∼330 m.y. (1.59–1.26 Ga) based on key pole matches and potentially for ∼570 m.y. (1.83–1.26 Ga) based on additional key and non-key poles. (d) Key pole age matches for several other cratons and times yield constraints on specific continental reconstructions. (3) Despite these substantial gains few key paleopoles have been obtained from the Rodinia period especially between 1.05–0.80 Ma, so that it is not possible to test most elements of the numerous and widely divergent Rodinia reconstructions using key poles. (4) There is great potential for many more key Proterozoic paleopoles given the rapid progress that has occurred recently in precise dating of mafic magmatic units around the globe (e.g., Ernst et al., 2013). Acknowledgements I thank Mike Hamilton, Richard Ernst, Tony LeCheminant, Wouter Bleeker, Otto van Breemen, Sally Pehrsson and Joe Hodych for many helpful discussion concerning key paleopoles and precise dating of magmatic events. Sally Pehrsson and two anonymous reviewers provide helpful comments on the manuscript. Reconstructions in the figures were drawn with the assistance of the computer program GMAP32 (Torsvik and Smethurst, 1997).

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Fig. 11. The 0.615 Ga reconstruction of Laurentia and Baltica, with Laurentia fixed at an arbitrary longitude and in a single hemisphere and Baltica permitted to vary in longitude and hemisphere. Position B brings the coeval Egersund and Long Range dyke swarms to their closest approach, and is roughly the fit used in many published reconstructions of Rodinia.

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