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Received: 10 December 2020 | Revised: 15 December 2020 | Accepted: 19 December 2020 DOI: 10.1111/bre.12537 EAGE RESEARCH ARTICLE Salt– magma interactions influence intrusion distribution and salt tectonics in the Santos Basin, offshore Brazil Craig Magee1 | Leonardo M. Pichel2 | Amber L. Madden- Nadeau3 | Christopher A.- L. Jackson2 | Webster Mohriak4 1School of Earth and Environment, University of Leeds, Leeds, UK Abstract 2Basins Research Group, Department of Many sedimentary basins host thick evaporite (salt) deposits. Some of these ba- Earth and Environment, Imperial College sins also host extensive igneous intrusion networks. It thus seems inevitable that, London, London, UK in some locations, magma will interact with salt. Yet how interaction between these 3Department of Earth Sciences, University materials may influence salt tectonics or magma emplacement, particularly at the of Oxford, Oxford, UK 4Faculty of Geology, Universidade do basin- scale, remains poorly understood. We use 3D seismic reflection data from the Estado do Rio De Janeiro, Rio de Janeiro, Santos Basin, offshore Brazil to image 38 igneous intrusions spatially related to thick Brazil Aptian salt. Based on identified seismic– stratigraphic relationships, we suggest sill Correspondence emplacement likely occurred during the late Albian-to- Santonian. We show intra- Craig Magee, School of Earth and salt sills are geometrically similar to but laterally offset from supra- salt sills. We Environment, University of Leeds, Leeds, suggest ascending magma was arrested by the salt in some areas, but not others, UK. Email: [email protected] perhaps due to differences in evaporite lithology. Our mapping also reveals most sills occur within and above the presalt Merluza Graben, an area characterized by Funding information Albian- to- Neogene, salt- detached extension. In adjacent areas, where there are few Imperial College London intrusions, salt deformation was driven by post- Santonian diapir rise. We suggest emplacement of hot magma within evaporites above the Merluza Graben enhanced Albian- to- Santonian salt movement, but that crystallization of the intrusion network restricted post- Santonian diapirism. Our work indicates salt– magma interaction can influence salt tectonics, as well as the distribution of magma plumbing systems, and thus could impact basin evolution. KEYWORDS basin, Brazil, magma, salt, sill, tectonics 1 | INTRODUCTION plumbing systems, which in places may have intruded ear- lier salt- formed structures (North Sea, offshore Netherlands, Thick evaporite deposits are common in many sedimentary Blažić & Moreau, 2017; e.g. Danakil Depression, Ethiopia, basins, where they can flow to form a range of salt structures Schofield et al., 2014). On their own, both salt tectonics and (e.g. diapirs) (e.g. Hudec & Jackson, 2007; Jackson, 1995; magmatism can impact the evolution of sedimentary basins Rowan, 2014; Rowan et al., 2004; Warren, 2006; and continental margins, including the accumulation of natu- Warren, 2010). Many salt basins also contain magma ral resources (e.g. Bedard et al., 2012; Holford et al., 2012; This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2021 The Authors. Basin Research published by International Association of Sedimentologists and European Association of Geoscientists and Engineers and John Wiley & Sons Ltd. Basin Research. 2021;00:1–24. wileyonlinelibrary.com/journal/bre | 1 2 MAGEE ET AL. | EAGE Jackson & Hudec, 2017; Peron-Pinvidic et al., 2019; Rohrman, 2007; Schofield et al., 2017; Skogseid et al., 2000; Highlights Tari et al., 2003). Yet despite the common occurrence of ig- neous intrusions in many salt basins, and their importance to • Magma inevitably intrudes salt in some basins, but basin evolution and natural resource development, we have what are the consequences of such interaction? a poor understanding of how magma may interact physically • Use seismic reflection images of sills below, and chemically with salt, or the potential consequences of within and above salt in the Santos Basin, Brazil. such interactions (e.g. Heimdal et al., 2019; Li et al., 2009; • We show the salt- trapped ascending magma in López- García et al., 2020; Schofield et al., 2014). places, whereas other areas were bypassed by Few structurally oriented studies have explored poten- dykes. tial salt– magma interactions (López-García et al., 2020; Schofield et al., 2014; Underhill, 2009). For example, • Intrusion of hot magma enhanced salt flow, but Schofield et al. (2014) showed mechanical variations within crystallized sills inhibited later salt movement. the sub- horizontally layered, Werra salt complex, Germany, • Salt rheology and structure impacts magma locally controlled magma emplacement mechanics and intru- plumbing systems, which in turn impacts salt sion architecture at a metre- scale. In the Werra salt complex, tectonics. relatively strong halite layers were shown to host vertical dykes emplaced via brittle processes, whereas syn- intrusion heating, dehydration and fluidization of hydrous salt (e.g. carnallite) layers promoted sill emplacement (Schofield seismic datasets will arguably be critical to addressing the et al., 2014). However, it remains unclear whether and shortcomings associated with each data type. how these salt–magma interactions occur at the basin scale Here, we use 3D seismic reflection data from the Santos (Schofield et al., 2014). For example, can salt bodies provide Basin, offshore Brazil, to image the structure of 38 igneous preferential flow pathways for magma, or do they arrest intru- intrusions emplaced below, within and above an Aptian evap- sions and restrict magma distribution (Schofield et al., 2014; orite layer that flowed to form a range of kilometre-scale salt Underhill, 2009)? Furthermore, we do not fully know: (a) structures. No boreholes in the study area penetrate the salt whether emplacement of hot magma could induce salt flow structures or sills, meaning we cannot directly comment on (López- García et al., 2020; Underhill, 2009); or (b) if the small- scale structural and chemical salt– magma interactions. intrusion and crystallization of magma within hydrous salts However, with our data, we aim to establish the basin-scale like carnallite, which can lubricate and facilitate salt move- structural consequences of salt– magma interactions, allow- ment (e.g. Jackson & Hudec, 2017; Urai et al., 1986; Van ing us to test whether: (a) the behaviour and rheological prop- Keken et al., 1993), could locally increase the mechanical erties of salt during emplacement can influence the geometry strength of the salt and create a rigid framework that restricts of individual intrusions and intrusion networks; (b) salt layers salt flow and limits diapirism (e.g. Schofield et al., 2014). can facilitate or inhibit magma ascent and (c) the presence There is also a lack of information regarding how the heating of magma or crystallized intrusions impacts large- scale salt and potential melting of salt (e.g. halite melts at ca. 800°C) tectonics. By evaluating the consequences of magma intru- during intrusion affects the chemistry, rheology and evolution sion within salt, we anticipate our research will provide a of a magma and/or the formation of associated ore deposits framework for future, integrative studies probing physical (e.g. Barton & Johnson, 1996; Heimdal et al., 2019; Iacono- and chemical salt– magma interactions at a variety of scales. Marziano et al., 2017; Li et al., 2009; Mohriak et al., 2009; Pang et al., 2013; Ripley et al., 2003). There are two key problems that currently limit our abil- 2 | GEOLOGICAL SETTING ity to answer questions regarding salt–magma interactions. First, field or mine exposures of intrusions within salt allow The Santos Basin located offshore south- eastern Brazil and chemical and small- scale (e.g. metre- scale) structural anal- formed in the Early Cretaceous (ca. 130– 125 Ma; Barremian- ysis of salt–magma interactions, but provide little insight to- Early Aptian) due to the opening of the South Atlantic into how the whole system may have behaved in 3D at sub- Ocean (Figure 1a,b) (e.g. Davison et al., 2012; Mohriak stantially larger scales (e.g. Schofield et al., 2014). Second, et al., 1995; Quirk et al., 2012). During the post-rift, in the seismic reflection images reveal the 3D architecture of entire latest Aptian to possibly earliest Albian (ca. 116– 111 Ma), intrusions, salt structures and halokinetic sequences, but pro- a ca. 2- to 3.5- km- thick evaporite- dominated layer was de- vide little insight into the chemical or small- scale structural posited across the basin, covering a sag basin sedimentary processes associated with salt– magma interaction (cf. Blažić succession (Figure 1a,b) (e.g. Davison et al., 2012; Karner & & Moreau, 2017; Underhill, 2009). Integrating field and Gambôa, 2007; Pichel & Jackson, 2020; Rowan, 2014). 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