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Magma Transport Pathways in Large Igneous Provinces: Lessons From 1 Magma Transport Pathways in Large Igneous Provinces: Lessons from 2 combining Field Observations and Seismic Reflection Data 3 4 1*Craig Magee ([email protected]), 2,3Richard E. Ernst 5 ([email protected]), 4James Muirhead 6 ([email protected]), 1Tom Phillips (Phillips, Thomas 7 ([email protected]), 1Christopher A-L Jackson ([email protected]) 8 9 *corresponding author 10 11 1Basins Research Group, Department of Earth Science and Engineering, Imperial College 12 London, Prince Consort Road, London, SW7 2BP, UK 13 2Deparment of Earth Sciences, Ottawa, ON, K1S 5B6, Canada 14 3Faculty of Geology and Geography, Tomsk State University, Tomsk 634050, Russia 15 4 Department of Earth Sciences, Syracuse University, Syracuse, New York 13210, USA 16 17 18 Abstract 19 Large Igneous Province (LIP) formation involves the generation, intrusion, and extrusion of 20 significant volumes (typically >1 Mkm3) of mainly mafic magma and is commonly 21 associated with episodes of mantle plume activity and major plate reconfiguration. Within 22 LIPs, magma transport through Earth’s crust over significant vertical (up to tens of 23 kilometres) and lateral (up to thousands of kilometres) distances is facilitated by dyke swarms 24 and sill-complexes. Unravelling how these dyke swarms and sill-complexes develop is 25 critical to: (i) evaluating the spatial and temporal distribution of contemporaneous volcanism 26 and hydrothermal venting, which can drive climate change; (ii) determining melt source 27 regions and volume estimates, which shed light on the mantle processes driving LIP 28 formation; and (iii) assessing the location and form of associated economic ore deposits. 29 Here, we review how seismic reflection data can be used to study the structure and 30 emplacement of sill-complexes and dyke swarms. We particularly show that seismic 31 reflection data can reveal: (i) the connectivity of and magma flow pathways within extensive 32 sill-complexes; (ii) how sill-complexes are spatially accommodated; (iii) changes in the 33 vertical structure of dyke swarms; and (iv) how dyke-induced normal faults and pit chain 34 craters can be used to locate sub-vertical dykes offshore. 35 36 Introduction 37 Large Igneous Provinces (LIPs) comprise large volume (>0.1 Mkm3 but frequently >1 38 Mkm3), predominantly mafic (-ultramafic) intrusion networks, extrusive sequences (e.g. 39 flood basalts), and crustal magmatic underplate emplaced either over a relatively rapid 40 timeframe (<5 Myr) or through multiple, short-lived pulses over a few 10’s of millions of 41 years (Ernst 2014; Ernst and Youbi 2017; Coffin and Eldholm 2005; Coffin and Eldholm 42 1994; Bryan and Ferrari 2013; Bryan and Ernst 2008). LIPs occur in intraplate continental or 43 oceanic settings, and many appear to result from the impingement of a mantle plume at the 44 base of the lithosphere (e.g. Ernst 2014; Ernst et al. 1995; Coffin and Eldholm 1992). 45 Transport of magma through Earth’s crust during LIP development is facilitated by sheet-like 46 conduits including dykes, sills, and inclined sheets. Dykes are traditionally considered to be 47 the most important component of LIP magma plumbing systems and typically form 48 impressive swarms that can extend laterally for >2000 km (Fig. 1) (e.g. Ernst 2014; Ernst and 49 Baragar 1992; Ernst et al. 2001; Ernst et al. 1995; Halls 1982). Giant circumferential (sub- 50 circular to elliptical) swarms up to 2000 km in diameter have also been recently recognized 51 (e.g. Buchan and Ernst, 2018a, b). However, field observations reveal that sill-complexes, i.e. 52 extensive interconnected networks of sills and inclined sheets (e.g. the Siberian Trap sill- 53 complex is exposed over >1.5 × 106 km2), form the primary volumetric elements of many LIP 54 plumbing systems (Fig. 1) (e.g. Svensen et al. 2012; Leat 2008; Elliot 1992; Elliot and 55 Fleming 2004; Burgess et al. 2017). Analysing and integrating our understanding of these 56 two key parts of LIP plumbing systems (dyke swarms and sill-complexes) is crucial to: (i) 57 estimating LIP melt volumes, which provide critical insight into mantle processes; (ii) 58 reconstructing palaeogeographies, particularly during the Precambrian (e.g. Ernst et al. 2013; 59 Bleeker and Ernst 2006); (iii) locating economic exploration targets associated with LIP 60 formation (e.g. Ernst and Jowitt 2013; Jowitt et al. 2014); (iv) quantifying LIP-related 61 greenhouse gas production, and its role in driving mass extinction (e.g. Ernst and Youbi 62 2017; Svensen et al. 2004); and (v) understanding the emplacement mechanics and surface 63 expression of sheet intrusions, which can allow similar features to be identified and 64 interpreted on other planetary bodies (e.g. Head and Coffin 1997; Ernst 2014). 65 To date, our understanding of magma transport through LIP plumbing systems has 66 primarily been driven by the mapping of dyke swarm and sill-complex extents and 67 geometries using traditional field approaches and geophysical techniques (e.g. aeromagnetic 68 data), coupled with a range of petrological, geochemical, and geochronological analyses (see 69 Ernst 2014 and references therein). Furthermore, application of different modelling 70 approaches has helped evaluate emplacement mechanics of dyke swarms and sill-complexes 71 (e.g. Townsend et al. 2017; Rivalta et al. 2015; Bunger et al. 2013; Malthe-Sørenssen et al. 72 2004; Galland 2012; Kavanagh et al. 2018; Kavanagh et al. 2015). Combining these 73 techniques has provided profound insights into LIP melt conditions, magma evolution and 74 distribution, emplacement mechanics, age and duration of emplacement, and the tectonic and 75 climatic impacts of these processes (e.g. Svensen et al. 2012; Svensen et al. 2004; Cooper et 76 al. 2012; Ernst 2014; Jerram and Bryan 2015; Bryan et al. 2010; Ernst et al. 2005; Burgess et 77 al. 2017; Ernst and Youbi 2017). Despite the importance of understanding LIP dyke swarms 78 and sill-complexes, we know relatively little about their 3D structure because of the 2D 79 nature of Earth’s surface. This limitation means that both the lateral and vertical extents of 80 dyke swarms or sill-complexes can seldom be examined simultaneously. Furthermore, it is 81 commonly challenging to resolve individual intrusions and determine their connectivity using 82 geophysical data that capture the subsurface expression of these LIP plumbing systems (e.g. 83 gravity and magnetics). Without being able to visualise the true 3D geometry of LIP dyke 84 swarms and sill-complexes, it is difficult answer a wide range of questions, including: How 85 does magma travel such large lateral distances (>100 km)? Can magma transport channels 86 within LIP plumbing systems remain open and facilitate sustained flow for protracted periods 87 of time? How do dyke swarms and/or sill-complexes feed flood basalts? How volumetrically 88 important are intrusive networks within LIPs compared to their extrusive counterparts? What 89 is their relationship to underlying, deep crustal magmatic underplate? How do sheet 90 intrusions relate to stress field conditions? 91 Reflection seismology is the only technique that can image intrusion networks in 3D, 92 at a relatively high-resolution (i.e. metres to tens of metres) over vast areas, allowing 93 individual magma bodies and associated structures to be mapped (e.g. Smallwood and 94 Maresh 2002; Thomson and Hutton 2004; Planke et al. 2005; Cartwright and Hansen 2006; 95 Schofield et al. 2017; Magee et al. 2016; Planke et al. 2015). For example, mapping of 96 magma flow indicators across regionally extensive sill-complexes imaged in seismic 97 reflection data reveal that these intrusion networks can transport melt over significant vertical 98 and lateral distances (e.g. Magee et al. 2016; Schofield et al. 2017; Reynolds et al. 2017). 99 These magma flow observations are further supported by field, magnetic, and geochemical 100 analyses of LIP sill-complexes (e.g. Airoldi et al. 2016; Leat 2008; Aspler et al. 2002; Marsh 101 2004). One challenge with reflection seismology, which relies on measuring arrival times of 102 acoustic energy reflected back from geological features, is that the technique favours imaging 103 of sub-horizontal to moderately inclined structures (e.g. sills). Dykes and dyke swarms are 104 thus rarely imaged in seismic reflection data (e.g. Thomson 2007), except when they have 105 been rotated post-emplacement to shallower dips (e.g. Phillips et al. 2017; Abdelmalak et al. 106 2015). Several studies have also shown, however, that dykes can be detected in seismic 107 reflection data through the recognition of dyke-related host rock structures and/or 108 geophysical artefacts (e.g. Wall et al. 2010; Malehmir and Bellefleur 2010; Zaleski et al. 109 1997; Bosworth et al. 2015; Kirton and Donato 1985; Ardakani et al. 2017). 110 In addition to imaging the 3D structure of dyke swarms and sill-complexes, seismic 111 reflection data also provide unprecedented insight into the host rock deformation mechanisms 112 that spatially accommodate magma emplacement and associated hydrothermal venting (e.g. 113 Hansen and Cartwright 2006b; Magee et al. 2014; Magee et al. 2013; Reeves et al. 2018; 114 Trude et al. 2003; Jackson et al. 2013; Svensen et al. 2004; Hansen 2006). Being able to 115 examine how intrusion induces deformation and the style of structures formed is critical 116 because it provides insight into the processes driving magmatism (e.g. Pollard et al. 1983; 117 Ernst et al. 2001; Magee et al. 2017; Galland 2012; van Wyk de Vries et al. 2014; Pollard and 118 Johnson 1973). Furthermore, understanding how subsurface magma movement and 119 accumulation translates into ground deformation, through 3D analysis of ancient intrusions 120 and associated deformation, informs how surficial intrusion-induced structures on Earth and 121 other planetary bodies relate to the underlying plumbing system (e.g.
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