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How Subduction Broke up Pangaea with Implications for the Supercontinent Cycle

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How Subduction Broke up Pangaea with Implications for the Supercontinent Cycle Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 How subduction broke up Pangaea with implications for the supercontinent cycle FRASER KEPPIE Department of Energy, Government of Nova Scotia, Joseph Howe Building, 12th Floor, 1690 Hollis Street, Halifax, Nova Scotia, B3J 3J9, Canada (e-mail: [email protected]) Abstract: Mechanisms that can explain the Mesozoic motion of Pangaea in a palaeomagne- tic mantle reference frame may also be able to explain its breakup. Calculations indicate that Pangaea moved along a non-rigid path in the mantle frame between the late Triassic and early Jurassic. The breakup of Pangaea may have happened as a response to this non-rigid motion. Tec- tonic forces applied to the margins of Pangaea as a consequence of subduction at its peripheries can explain both the motion and deformation of Pangaea with a single mechanism. In contrast, mantle forces applied to the base of Pangaea appear to be inconsistent with the kinematic con- straints and do not explain the change in supercontinent motion that accompanied the breakup event. Top-down plate tectonics are inferred to have caused the breakup of Pangaea. Strong coup- ling between the mantle and lithosphere may not have been the case during the Phanerozoic eon when the Pangaean supercontinent formed and subsequently dispersed. Gold Open Access: This article is published under the terms of the CC-BY 3.0 license. The mechanisms responsible for plate tectonic analysis builds on the previous work of D. F. change on Earth have been linked to the different Keppie (2015) in which only the relative plate stages of the supercontinent cycle (Worsley et al. motion data were evaluated. D. F. Keppie (2015) 1982; Gurnis 1988; Storey 1995; Tackley 2000; demonstrated that the Mesozoic opening of the Silver & Behn 2008; Li & Zhong 2009; Yoshida central Atlantic ocean was balanced by the closure & Santosh 2011; Buiter & Torsvik 2014; Nance of Palaeo-Tethys and Tethys oceans to the east and et al. 2014). Nance et al. (2014) provide a recent not by the closure of Panthalassa. D. F. Keppie review of the hypothesis that bottom-up or mantle- (2015) inferred that subduction-based processes based mechanisms are principally responsible for were most likely responsible for the breakup of the breakup of supercontinents, whereas top-down Pangaea because the geometry and timing of events or subduction-based mechanisms are inferred to identified in the linked Atlantic–Tethys compen- have governed the dispersal and (re-)amalgamation sation system appear most simple to explain if the of supercontinents (Fig. 1). In this hypothesis, the causal forces propagated from the Tethyan domain coupling of mantle convection, plumes or super- into the Atlantic domain and not vice versa. The plumes with the overlying lithosphere is thought to present study tests whether the motions of Pangaea have waxed and waned with the formation and dis- and its daughter plates in a palaeomagnetic refer- persal of supercontinents through time. Mantle pro- ence frame provide a further means to discrimi- cesses have been strongly implicated in the breakup nate between subduction-based and mantle-based mechanisms of past supercontinents including breakup mechanisms. Columbia, Rodinia, Pannotia and Pangaea (Nance et al. 2014, Fig. 1). The purpose of the present study is to re-evaluate Previous work the potential roles for both bottom-up and top-down mechanisms for the breakup of Pangaea (Figs 2 & As (Nance et al. 2014) have recently reviewed the 3). The present study focuses on the analysis of supercontinent cycle, only the main ideas pertinent plate motion data (Seton et al. 2012) in a palaeo- to Pangaea will be summarized here. Many studies magnetic mantle reference frame (Torsvik et al. have emphasized potential roles for mantle-based 2012). Boundary conditions calculated here are then mechanisms, such as convection cells, plumes or compared with basic predictions for mantle-driven superplumes, in the breakup of Pangaea (Doblas and subduction-driven breakup processes as sum- et al. 1998; Isozaki 2009; Santosh et al. 2009). marized from the results of analogue and numeri- Bottom-up plate tectonics are thought to have cal modelling studies (Zhong & Gurnis 1993; Li & been prominent during the breakup of superconti- Zhong 2009; Aitken et al. 2013). The present nents for the following reasons: (1) oceans close From:Li, Z. X., Evans,D.A.D.&Murphy, J. B. (eds) 2016. Supercontinent Cycles Through Earth History. Geological Society, London, Special Publications, 424, 265–288. First published online June 1, 2015, updated June 11, 2015, http://doi.org/10.1144/SP424.8 # 2016 The Author(s). 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 30, 2021 266 F. KEPPIE Maximum Columbia Rodinia Pannotia Pangea Amalgamation Critical Amalgamation palaeogeography Maximum Dispersal −2000 −1800 −1600 −1400 −1200 −1000 −800 −600 −400 −200 0 Age (Ma) top−down plate tectonics quiet plate tectonics? bottom−up plate tectonics (subduction−driven) (or minimally top−down) (mantle−driven) Fig. 1. Supercontinent cycles through time are plotted for the past 2 Ga during which time the supercontinents of Columbia, Rodinia, Pannotia and Pangaea are inferred to have formed and dispersed (Nance et al. 2014). The idea that the mechanisms of plate tectonics have alternated between top-down plate tectonics during the periods of supercontinent dispersal and (re-)amalgmation (blue) and bottom-up plate tectonics during the periods of supercontinent breakup (red) is shown (Nance et al. 2014). The possibility that subduction was minimal during supercontinent formation (Silver & Behn 2008) is portrayed (light blue), as is the hypothesis that the alternation of plate tectonic mechanism could reflect how the terrestrial supercontinents amalgamated continental lithosphere to a size larger than some critical size, after which mantle features capable of breaking supercontinents apart develop (Li & Zhong 2009). during the amalgamation of supercontinents and the 1984; Li et al. 1999; Dalziel et al. 2000; Ernst subduction zones previously accommodating the et al. 2005; Hou et al. 2008; Kouyate´ et al. 2013). closures are annihilated, thus subduction can The Central Atlantic Magmatic Province emplaced become minimal or absent (Silver & Behn 2008), during the breakup of Pangaea may be the largest or alternatively subduction may move to the periph- preserved Large Igneous Province in the geological ery of the supercontinent (Murphy & Nance 1991); record (Ruiz-Martı´nez et al. 2012). and (2) geodynamic modelling studies have shown Geodynamic models have shown that subducted that large amalgamations of continental lithosphere lithosphere can accumulate in the transition zone at can alter both the mechanical and thermal regimes the base of the upper mantle (Chen & Brud- in the underlying mantle. For example, the areal zinski 2001; Hamilton 2007), or at the base of the extent of a supercontinent can exceed a threshold lower mantle just above the core–mantle boundary size after which the insulation or thermal barrier (Kendall & Silver 1996; Spasojevic et al. 2010; supplied by the continental lithosphere is sufficient Sutherland et al. 2010; Tackley 2011). Accumu- to trigger changes in the underlying mantle flow lations of subducted lithosphere at the base of (Gurnis 1988; Zhong & Gurnis 1993; Zhong et al. the upper mantle can start to sink rapidly into the 2007; Li & Zhong 2009; Lenardic et al. 2011; deeper mantle in so-called slab avalanche events Yoshida 2013). In such cases, the underlying mantle (Condie 1998; Pysklywec et al. 2003; Capitanio may warm, expand, decrease in density, become et al. 2009). Slab avalanche events may trigger buoyant and exert upward and outward tractions superplumes in the mantle beneath superconti- on the base of the lithosphere. High volumes of nents, which may play roles in their amalgamation emplaced magmas called Large Igneous Provinces and breakup (Pysklywec et al. 2003; Nance et al. may indicate the influence of upwelling mantle 2014). Subducted lithosphere that has accumulated at these extension zones; the inferred melting of above the core–mantle boundary has been inferred mantle at elevated temperatures may support the from two low-velocity seismic zones interpreted hypothesis of mantle-driven breakup (Bond et al. from seismic tomography images of the lower Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 HOW SUBDUCTION BROKE UP PANGEA 267 mantle (Zhao 2004; Burke et al. 2008). These slab previous periods of supercontinent dispersal and graveyards presently underlie the African craton (re-)amalgamation (Nance et al. 2014). Neverthe- and the Pacific ocean (Zhao 2004), but one may less, plumes may yet be determined to have influ- have underlain the central Atlantic region during enced the motion and deformation of plates during the breakup of Pangaea (Morra et al. 2013). Slab the periods of supercontinent dispersal and amalga- graveyards can provide an origin of deep man- mation during the Proterozoic and Phanerozoic eons tle plumes as the subducted material is warmed (Hynes 1990; Burke & Cannon 2013; Conrad et al. across the core–mantle boundary (Hirose et al. 2013; Morra et al. 2013). 1999; Lay et al. 2004; Torsvik et al. 2006). Charac- teristic chemistry of emplaced magmas observed in the terrestrial rock record at the extension zones Methodology where supercontinents first began to fail may cor- respond to a source region that hosts substantial Data amounts of previously subducted oceanic litho- sphere (Senshu et al. 2009; Gonza´lez-Jime´nez et al. In this study, three 2012 datasets are used to con- 2013; Callegaro et al. 2014). strain and reconstruct the past evolution of continen- Thus, a variety of thermal and mechanical argu- tal lithosphere since c. 320 Ma: (1) a present-day ments have been assembled to explain why zones of continent or plate polygon model (Mu¨ller et al. upwelling mantle may have formed in the regions 2008; Seton et al.
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