Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 Accepted Manuscript Geological Society, London, Special Publications Pannotia's mantle signature: the quest for supercontinent identification Philip J. Heron, J. Brendan Murphy, R. Damian Nance & R. N. Pysklywec DOI: https://doi.org/10.1144/SP503-2020-7 To access the most recent version of this article, please click the DOI URL in the line above Received 10 January 2020 Revised 2 July 2020 Accepted 6 July 2020 © 2020 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0/). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics When citing this article please include the DOI provided above. Manuscript version: Accepted Manuscript This is a PDF of an unedited manuscript that has been accepted for publication. The manuscript will undergo copyediting, typesetting and correction before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the book series pertain. Although reasonable efforts have been made to obtain all necessary permissions from third parties to include their copyrighted content within this article, their full citation and copyright line may not be present in this Accepted Manuscript version. Before using any content from this article, please refer to the Version of Record once published for full citation and copyright details, as permissions may be required. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 Pannotia's mantle signature: the quest for supercontinent identification Philip J. Heron1*, J. Brendan Murphy2, R. Damian Nance3, 4, R. N. Pysklywec5. 1Department of Earth Sciences, Durham University, Durham, England. 2Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia, Canada. 3Department of Geological Sciences, Ohio University, Ohio, USA. 4Department of Earth & Planetary Sciences, Yale University, Connecticut, USA. 5Department of Earth Sciences, University of Toronto, Toronto, Ontario, Canada. * Corresponding author (email: [email protected]). Abstract: A supercontinent is generally considered to reflect the assembly of all, or most, of the Earth’s continental lithosphere. Previous studies have used geological, atmospheric, and biogenic ‘geomarkers’ to supplement supercontinent identification. However, there is no formal definition of how much continental material is required to be assembled, or indeed which geomarkers need to be present. Pannotia is a hypothesized landmass that existed in the interval ~0.65-0.54 Ga and was comprised of Gondwana, Laurentia, Baltica, and possibly Siberia.ACCEPTED Although Pannotia was considerably MANUSCRIPT smaller than Pangaea (and also fleeting in its existence), the presence of geomarkers in the geological record support its identification as a supercontinent. Using 3-D mantle convection models, we simulate the evolution of the mantle in response to the convergence leading to amalgamation of Rodinia and Pangaea. We then compare this supercontinent ‘fingerprint’ to Pannotian activity. For the first time, we Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 show that Pannotian continental convergence could have generated a mantle signature in keeping with that of a simulated supercontinent. As a result, we posit that any formal identification of a supercontinent must take into consideration the thermal evolution of the mantle associated with convergence leading to continental amalgamation, rather than simply the size of the connected continental landmass. The formation of supercontinent Pangaea (Figure 1a) had a profound effect on the thermal evolution of the mantle. An increase in deep mantle upwelling and plume formation is thought to follow supercontinent formation (Gurnis 1988; Zhong & Gurnis 1993; Lowman & Jarvis 1993; Zhong et al. 2007; Li & Zhong 2009; Yoshida 2010; Heron & Lowman 2010; Yoshida 2013; Gamal el Dien, 2019), with large igneous provinces (LIPs) being the surface manifestations of such plumes (e.g. Yale & Carpenter 1998; Courtillot et al. 1999; Ernst et al. 2005; Ernst & Bleeker 2010; Sobolev et al. 2011). Additional “geomarkers” (see Nance et al., 1986) have also been identified for supercontinent Pangaea, including global-scale orogenesis (Nance et al., 1988; Santosh, 2010a; Condie, 2011; Müller et al. 2013), crustal growth (Hawkesworth et al. 2010, 2016), the distribution of metamorphic belts (Brown, 2007) and mineral deposits (Goldfarb et al. 2010), rapid climate swings (Hoffman et al. 1998; Strand 2012; Young 2012), major events in the evolution of life and the atmosphere (Lindsay & Brasier 2002; Santosh 2010b; Knoll, 2013; Melezhik et al. 2013), biogeochemical cycles (NanceACCEPTED et al., 1986), and profound sea-level MANUSCRIPT change (Worsley et al. 1984; Nance and Murphy, 2013). Such geomarkers are interpreted as signals of supercontinent convergence leading to amalgamation and may be useful proxies for identifying the existence of previous supercontinents for which palaeocontintental reconstructions may be less refined (Nance and Murphy, 2019). Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 Although their ages of amalgamation and breakup are still being refined, based on the identification of these markers in the geological record, a number of pre-Pangaean supercontinents have been proposed. Rodinia (Figure 1b), with Laurentia at its centre, was formed by 1.1-0.9 Ga global-scale orogenesis (Hoffman 1991; Dalziel 1991; Moores 1991; Torsvik 2003; Li et al. 2004, 2008). It was fully assembled by approximately 900 Ma and is believed to have rifted apart in two separate episodes (~0.85-0.70 Ga and ~0.62-0.54 Ga) (Li et al. 2008, 2013; Li & Evans 2011). Earlier supercontinents include Nuna/Columbia (Hoffman, 1997; Rogers and Santosh, 2002) whose existence is most recently dated at ~1.6- 1.4 Ga (e.g., Pehrsson et al., 2015), Kenorland (Williams et al., 1991) thought to have existed during the interval ~2.7-2.5 Ga, and possibly Ur (~3.0 Ga), although the latter is better described as a supercraton (i.e. transient, late Archean landmasses which broke up to form cratons, e.g. Bleeker 2003). Although there are geological markers for supercontinent assembly (e.g., Nance et al., 2014), no formal system exists for supercontinent identification, as a result of which there is no formal definition of a supercontinent (e.g., Meert 2012). Even a basic understanding of how much contiguous landmass is required is open to debate. For example, Meert (2012) proposed a figure of 75% of available continent crust but, as pointed out by Nance and Murphy (2019), this is both difficult to estimate and views supercontinents as objects rather than the consequence of a geodynamic process. The statusACCEPTED of Pannotia (~0.65-0.54 Ga; Gondwana-Laurentia-Baltica MANUSCRIPT and possibly Siberia, Figure 1c-f), is an example of the ongoing debate about what constitutes a supercontinent (see Murphy et al., this volume; Evans, this volume). Given the uncertainties inherent in Neoproterozoic palaeogeographic reconstructions, Nance and Murphy (2019) argue that Pannotia’s status as a supercontinent is apparent from a number of ‘supercontinent’ markers around the time of its proposed formation (e.g., global-scale orogeny, rapid continental Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 growth, profound changes in the chemistry of the oceans and atmosphere, rapid and dramatic climate swings, as well as an explosion in biological activity; Nance et al., 1986; Hoffman 1991; Hoffman et al., 1998; Maruyama and Santosh, 2008; Knoll, 2013). However, there is general agreement that Pannotia’s size and duration were significantly less than that of Pangaea, drawing controversy as to whether it can be classified as a unique supercontinent or was simply part of Rodinia after initial breakup (e.g., Mitchell et al., 2012; Li et al., 2019). An area of Pannotian dynamics that has not been fully explored is the response of the mantle to the continental convergence leading to its amalgamation. Late Neoproterozoic continental convergence and protracted subduction leading to the assembly of the Gondwanan portion of Pannotia is strongly indicated by widespread “Pan-African” collisional orogenic activity between 0.62 and 0.53 Ga (e.g. Li et al., 2008; Merdith et al., 2017). Investigating the effect of continental convergence and assembly on Late Neoproterozoic-Early Paleozoic (0.6-0.5 Ga) mantle convection patterns, could allow for supercontinent identification through its impact on mantle dynamics (e.g., Heron, 2019). Furthermore, as the legacy of such an impact would affect Palaeozoic mantle convection patterns, it would also need to be factored into models purported to explain the amalgamation of Pangaea. The Central Iapetus Magmatic Province (CIMP, Figure 1e-f) is one potential manifestation of changes in deep mantle circulation that may have been an outcome of continental convergence (in a similar way to the Central Atlantic Magmatic Province and the convergenceACCEPTED of Pangaea). CIMP comprises MANUSCRIPTseveral large igneous provinces (LIPs) and predominantly consists of mafic dyke swarms, basalts, bimodal basalt-rhyolite complexes, lamprophyres and carbonatites (Kamo et al., 1995; Higgins