Durham Research Online Deposited in DRO: 23 September 2020 Version of attached le: Published Version Peer-review status of attached le: Peer-reviewed Citation for published item: Heron, Philip J. and Murphy, J. Brendan and Nance, R. Damian and Pysklywec, R. N. (2020) 'Pannotia's mantle signature: the quest for supercontinent identication.', Geological Society, London, special publications. Further information on publisher's website: https://doi.org/10.1144/SP503-2020-7 Publisher's copyright statement: c 2020 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/). Published by The Geological Society of London. 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Pysklywec5 1Department of Earth Sciences, Durham University, Durham, DH1 3LE, UK 2Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia, B2G 2W5, Canada 3Department of Geological Sciences, Ohio University, Ohio, 45701, USA 4Department of Earth & Planetary Sciences, Yale University, Connecticut, 06511, USA 5Department of Earth Sciences, University of Toronto, Toronto, Ontario, M5S 3B1, Canada PJH, 0000-0002-4813-0504 *Correspondence: [email protected] Abstract: A supercontinent is generally considered to reflect the assembly of all, or most, of the Earth’s con- tinental lithosphere. Previous studies have used geological, atmospheric and biogenic ‘geomarkers’ to supple- ment 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 c. 0.65–0.54 Ga and was comprised of Gondwana, Laurentia, Baltica and possibly Siberia. Although Pannotia was considerably smaller than Pangaea (and also fleeting in its existence), the pres- ence of geomarkers in the geological record support its identification as a supercontinent. Using 3D mantle con- vection 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 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 (Fig. 1a) 2013), biogeochemical cycles (Nance et al. 1986), had a profound effect on the thermal evolution of and profound sea-level change (Worsley et al. the mantle. An increase in deep mantle upwelling 1984; Nance and Murphy 2013). Such geomarkers and plume formation is thought to follow supercon- are interpreted as signals of supercontinent conver- tinent formation (Gurnis 1988; Lowman and Jarvis gence leading to amalgamation and may be useful 1993; Zhong and Gurnis 1993; Zhong et al. 2007; proxies for identifying the existence of previous Li and Zhong 2009; Yoshida 2010; Heron and Low- supercontinents for which palaeocontinental recon- man 2010; Yoshida 2013; Gamal El Dien et al. structions may be less refined (Nance and Murphy 2019), with large igneous provinces (LIPs) being the 2019). surface manifestations of such plumes (e.g. Yale and Although their ages of amalgamation and Carpenter 1998; Courtillot et al. 1999; Ernst et al. break-up are still being refined, a number of pre- 2005; Ernst and Bleeker 2010; Sobolev et al. 2011). Pangaean supercontinents have been proposed Additional ‘geomarkers’ (see Nance et al. 1986) (based on the identification of geomarkers in the geo- have also been identified for supercontinent Pan- logical record). Rodinia (Fig. 1b), with Laurentia at gaea, including global-scale orogenesis (Nance et al. its centre, was formed by 1.1–0.9 Ga global-scale 1988; Santosh 2010a; Condie 2011; Müller et al. orogenesis (Dalziel 1991; Hoffman 1991; Moores 2013), crustal growth (Hawkesworth et al. 2010, 1991; Torsvik 2003; Li et al. 2004, 2008). It was 2016), the distribution of metamorphic belts fully assembled by c. 900 Ma and is believed to (Brown 2007) and mineral deposits (Goldfarb et al. have rifted apart in two separate episodes (c. 0.85– 2010), rapid climate swings (Hoffman et al. 1998; 0.70 Ga and c. 0.62–0.54 Ga) (Li et al. 2008, Strand 2012; Young 2012), major events in the evo- 2013; Li and Evans 2011). Earlier supercontinents lution of life and the atmosphere (Lindsay and Bras- include Nuna/Columbia (Hoffman 1997; Rogers ier 2002; Santosh 2010b ; Knoll 2013; Melezhik et al. and Santosh 2002) whose existence is most recently From: Murphy, J. B., Strachan, R. A. and Quesada, C. (eds) 2020. Pannotia to Pangaea: Neoproterozoic and Paleozoic Orogenic Cycles in the Circum-Atlantic Region. Geological Society, London, Special Publications, 503, https://doi.org/10.1144/SP503-2020-7 © 2020 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2020 P. J. Heron et al. (a) (b) (c) (d) (e) (f) Fig. 1. Continental configurations at (a) 320 Ma (Matthews et al. 2016) for Pangaea formation; (b) 900 Ma (Merdith et al. 2017) for Rodinia formation; (c) 700 Ma (Merdith et al. 2017) for Pannotia amalgamation ; (d) 650 Ma (Merdith et al. 2017) for Pannotia formation and change in subduction pattern; (e) 600 Ma (Merdith et al. 2017) showing Pannotia formation and region of CIMP; (f) 550 Ma (Merdith et al. 2017) for Pannotia dispersal. S, Siberia; L, Laurentia; B, Baltica; A, Amazonia; CIMP, Central Iapetus Magmatic Province. In (c–e) convergence zones leading to the amalgamation of the Gondwanan portion of Pannotia (blue lines) correspond with the approximate location of subduction zones. dated at c. 1.6–1.4 Ga (e.g. Pehrsson et al. 2015), Fig. 1c–f), is an example of the ongoing debate Kenorland (Williams et al. 1991) thought to have about what constitutes a supercontinent (see Mur- existed during the interval c. 2.7–2.5 Ga, and possi- phy et al. 2020; Evans, in press, this volume). bly Ur (c. 3.0 Ga), although the latter is better Given the uncertainties inherent in Neoproterozoic described as a supercraton (i.e. transient, late Archean palaeogeographical reconstructions, Nance and landmasses which broke up to form cratons, e.g. Murphy (2019) argue that Pannotia’s status as a Bleeker 2003). supercontinent is apparent from a number of Although there are geological markers for super- ‘supercontinent’ markers around the time of its pro- continent assembly (e.g. Nance et al. 2014), no for- posed formation (e.g. global-scale orogeny, rapid mal system exists for supercontinent identification, continental growth, profound changes in the chemis- as a result of which there is no formal definition of try of the oceans and atmosphere, rapid and dramatic a supercontinent (e.g. Meert 2012). Even a basic climate swings, as well as an explosion in biolog- understanding of how much contiguous landmass ical activity; Nance et al. 1986; Hoffman 1991; is required is open to debate. For example, Meert Hoffman et al. 1998; Maruyama and Santosh 2008; (2012) proposed a figure of 75% of available conti- Knoll 2013). However, there is general agree- nent crust but, as pointed out by Nance and Murphy ment that Pannotia’s size and duration were sig- (2019), this is both difficult to estimate and views nificantly less than that of Pangaea, drawing supercontinents as objects rather than the conse- controversy as to whether it can be classified as a quence of a geodynamic process. unique supercontinent or was simply part of Rodinia The status of Pannotia (c. 0.65–0.54 Ga; Gond- after initial break-up (e.g. Mitchell et al. 2012; Li wana–Laurentia–Baltica and possibly Siberia, et al. 2019). Downloaded from http://sp.lyellcollection.org/ by guest on September 23, 2020 Pannotia’s mantle signature An area of Pannotian dynamics that has not been mantle, describing the mass, momentum and energy fully explored is the response of the mantle to the balance (taking into account adiabatic heating, shear continental convergence leading to its amalgam- heating and radiogenic heat production), and the ation. Late Neoproterozoic continental convergence transport of chemical composition: and protracted subduction leading to the assembly of the Gondwanan portion of Pannotia is strongly −∇ · (2ηε˙) +∇p = ρg, (1) 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 ∇·(ρu) = 0, (2) effect of continental convergence and assembly on Late Neoproterozoic–Early Paleozoic (0.6–0.5 Ga) ∂T ρC + u ·∇T −∇·k∇T mantle convection patterns, could allow for super- p ∂t continent identification through its impact on mantle = ρ + ηε˙ ε˙ + α ·∇ρ dynamics (e.g.