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The Role of Megacontinents in the Supercontinent Cycle Chong Wang1,2, Ross N

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The Role of Megacontinents in the Supercontinent Cycle Chong Wang1,2, Ross N https://doi.org/10.1130/G47988.1 Manuscript received 6 June 2020 Revised manuscript received 1 October 2020 Manuscript accepted 5 October 2020 © 2020 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 25 November 2020 The role of megacontinents in the supercontinent cycle Chong Wang1,2, Ross N. Mitchell1*, J. Brendan Murphy3, Peng Peng1 and Christopher J. Spencer4 1 State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China 2 Deparment of Geosciences and Geography, University of Helsinki, Helsinki 00014, Finland 3 Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada 4 Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario K7L 2N8, Canada ABSTRACT Baltica (and subsequently outboard additions) Supercontinent Pangea was preceded by the formation of Gondwana, a “megacontinent” (Wan et al., 2019). Assembly of Eurasia started about half the size of Pangea. There is much debate, however, over what role the assembly with the central Asian orogenic belt that weld- of the precursor megacontinent played in the Pangean supercontinent cycle. Here we dem- ed Baltica and Siberia at ca. 250 Ma. This as- onstrate that the past three cycles of supercontinent amalgamation were each preceded by sembly overlaps with the tenure and breakup of ∼200 m.y. by the assembly of a megacontinent akin to Gondwana, and that the building of a Pangea and represents an early assembly phase megacontinent is a geodynamically important precursor to supercontinent amalgamation. of the proposed future supercontinent Amasia The recent assembly of Eurasia is considered as a fourth megacontinent associated with (Mitchell et al., 2012). Following Siberia’s future supercontinent Amasia. We use constraints from seismology of the deep mantle for assimilation into Pangea, accretion along the Eurasia and paleogeography for Gondwana to develop a geodynamic model for megacontinent eastern margin of Siberia of continental blocks assembly and subsequent supercontinent amalgamation. As a supercontinent breaks up, a and terranes occurred between 200 and 100 Ma megacontinent assembles along the subduction girdle that encircled it, at a specific location (Torsvik et al., 2012; Wan et al., 2019). Much where the downwelling is most intense. The megacontinent then migrates along the girdle of Eurasia represents the reassembly of rifted where it collides with other continents to form a supercontinent. The geometry of this model fragments of Gondwana since the Devonian. is consistent with the kinematic transitions from Rodinia to Gondwana to Pangea. The sense of plate motion has been meridional, translating rifted pieces of Gondwana from the INTRODUCTION percontinent formed during the breakup phase, Southern Hemisphere to assemble in Eurasia in The supercontinent cycle of continental as- a megacontinent is viewed as an assembly of the Northern Hemisphere. With the presently sembly and breakup has been linked to global- multiple continents geodynamically linked to rapid northward migration of Australia and its scale orogenesis, mantle convection patterns, the incipient amalgamation phase of the next imminent collision with Eurasia, assembly of and the evolution of climate, the environment, supercontinent. The megacontinent becomes a the megacontinent is likely ongoing. and life (Nance et al., 2014). Three superconti- large subset of the next supercontinent, which nents are proposed to have formed since 2 Ga: results from the amalgamation of a majority Gondwana in Pangea Pangea, Rodinia, and Columbia (Evans, 2013). of continents into one contiguous, long-lived Gondwana assembled in two sectors: West Today, Earth is in between supercontinent con- landmass (Evans, 2013). The new term fits in Gondwana, with the Brasiliano and Pan- figurations (Pangea in the past, Amasia in the fu- the continental hierarchy: supercontinent (e.g., African orogens; and East Gondwana, with ture) (Mitchell et al., 2012), with supercontinent Pangea) > megacontinent (e.g., Gondwana) > the East African and Kuunga orogens. The assembly beginning (e.g., Eurasia) while super- continent (e.g., Africa) > microcontinent (e.g., amalgamation of Gondwana was initiated as continent breakup is ongoing (e.g., East Africa). Japan). We provide evidence for the assembly of early as ca. 750 Ma and was fully complete Gondwana was a major and early-forming part megacontinents as precursors to every supercon- by ca. 520 Ma (Collins and Pisarevsky, 2005). of supercontinent Pangea (Fig. 1), but its role in tinent through time. We also offer a conceptual Gondwana formed the southern portion of su- the supercontinent cycle remains controversial. framework for the origin of megacontinents that percontinent Pangea (Fig. 1). The collision of Some researchers emphasize the importance of refines models of the supercontinent cycle. Gondwana and Laurussia (Laurentia-Baltica- large Gondwana, even referring to it as a super- Avalonia) to finally form Pangea at ca. 350 Ma continent (Spencer et al., 2013), whereas others MEGACONTINENTS OF EARTH (Torsvik et al., 2012) thus implies that the as- interpret Gondwana as only part of the larger HISTORY sembly of megacontinent Gondwana predates Pangea (Doucet et al., 2019). Eurasia in Amasia amalgamation of supercontinent Pangea by We present the concept of a “megacontinent” Eurasia is presently Earth’s largest landmass ∼170 m.y. (Fig. 2). as a geodynamic precursor of supercontinent (Figs. 2 and 3A), containing Siberia, the North formation. If continents are rifted pieces of a su- China craton, the South China craton, the Tarim Umkondia in Rodinia craton, India, and many small blocks bounded The Grenville orogen between eastern Lau- *E-mail: [email protected] by multiple orogenic systems that collided with rentia and, most likely, Amazonia and other CITATION: Wang, C., et al., 2021, The role of megacontinents in the supercontinent cycle: Geology, v. 49, p. 402–406, https://doi.org/10.1130/G47988.1 402 www.gsapubs.org | Volume 49 | Number 4 | GEOLOGY | Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/4/402/5252882/402.pdf by guest on 30 September 2021 Figure 1. Paleogeo- graphic reconstruction of Pangea before breakup, showing Gondwana megacontinent (orange) in Pangea supercontinent (blue)(Mitchell et al., 2012; Mitchell et al., 2020). continents did not occur until ca. 1 Ga (Spen- an megacontinent is based on paleomagnetism, ontinent as Nuna because (1) “Columbia” was cer et al., 2013), and Neoproterozoic supercon- geochemistry, and geochronology of a coeval the name used for the first attempts at global- tinent Rodinia was not finally assembled until ca. 1.1 Ga large igneous province (Choudhary scale supercontinent reconstruction (Zhao et al., ca. 900 Ma (Merdith et al., 2017). However, et al., 2019). 2002), and (2) Hoffman (1997) used the term ∼100 m.y. before the Grenville collision and “Nuna” (an Inuit word for the lands bordering ∼200 m.y. before Rodinia amalgamation, sev- Nuna in Columbia the northern oceans and seas of North Ameri- eral continents, including Amazonia, were part The existence of a Paleoproterozoic– ca) to refer to the larger continent of Laurentia, of a megacontinent “Umkondia”, including the Mesoproterozoic supercontinent Columbia which was suspected to be contiguous with Bal- Congo–São Francisco, India, Kalahari, and West (a.k.a. Nuna) has been proposed (Zhao et al., tica. Peripheral constituents of supercontinent Africa cratons, that assembled before 1.1 Ga 2002; Kirscher et al., 2020). There is debate Columbia such as Australia were sutured by (Spencer et al., 2017; Choudhary et al., 2019). over its name, and legitimate cases for prece- ca. 1.6 Ga, heralding final supercontinent amal- Thus, assembly of the Umkondia megacontinent dence can be made for either option (Meert, gamation (Kirscher et al., 2020). The internal began while the breakup of the previous super- 2012; Evans, 2013). In a possible resolution to orogens of Laurentia indicate the assembly of continent was ongoing (Evans and Mitchell, this semantic standoff, we refer to the supercon- the Nuna megacontinent by ca. 1.8 Ga via the 2011). The case for Umkondia as a pre-Rodini- tinent as Columbia and to its precursor megac- Trans-Hudson orogen (Hoffman, 1997) and, Figure 2. Megacontinents through time. (Top) Timeline of megacontinents and associated supercontinents. (Bottom) Megacontinent reconstructions at age of final assembly (Eurasia at present day). EQ—equator. Geological Society of America | GEOLOGY | Volume 49 | Number 4 | www.gsapubs.org 403 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/4/402/5252882/402.pdf by guest on 30 September 2021 its supercontinent by ∼200 m.y. (Fig. 2). During A this time interval, a large ocean closed, imply- ing a large initial spatial separation between the megacontinent and the supercontinent. Megac- ontinent Gondwana, for example, assembled at high southerly latitudes as evidenced by glacial deposits and paleomagnetic data (Caputo and Crowell, 1985), whereas the amalgamation of supercontinent Pangea straddled the equator af- ter the closure of the Rheic and Iapetus Oceans (Torsvik et al., 2012). We calculated megacon- tinent sizes using the areas of continent and/or craton geographic shape files (Pisarevsky et al., 2014; Torsvik et al., 2014) with GPlates
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