June 2016 LIP of the Mont

Lateral magma flow in mafic sill-complexes

Craig Magee¹, James D Muirhead², Alex Karvelas³, Simon P Holford⁴, Christopher A-L Jackson¹, Ian D. Bastow¹, Nick Schofield⁵, Carl TE Stevenson⁶, Charlotte McLean⁷, William McCarthy⁸, Olga Shtukert³

¹Basins Research Group, Department of Earth Science and Engineering, Imperial College, London, SW7 2BP, UK

²Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844, USA

³Schlumberger Multiclient, Schlumberger House, Buckingham Gate, West Sussex, RH6 0NZ, UK

⁴Australian School of Petroleum, University of Adelaide, Adelaide, SA 5005, Australia

⁵Geology & Petroleum Geology, School of Geosciences, University of Aberdeen, Aberdeen, AB24 3UE, UK

⁶School of Geography, Earth & Environmental Sciences, University of Birmingham, Birmingham, B15 2TT, UK

⁷School of Geographical and Earth Sciences, University of Glasgow, Glasgow, G12 8QQ, UK

⁸Department of Earth Sciences, University of St Andrews, St Andrews, KY16 9AL, UK

Email: c.magee@imperial.ac.uk

Based on: Magee C, Muirhead JD, Karvelas A, Jackson CAL, Bastow I, Holford SP, Schofield N, Stevenson CTE, McLean C, McCarthy W, Shtukert O, 2016, Lateral magma flow in sill-complexes, Geosphere, 12, 1-33, doi:10.1130/GES01256.1. http://geosphere.gsapubs.org/content/12/3/809.short

Introduction

Large Igneous Provinces (LIPs) (Ernst, 2014) preserve a record of major magma-tectonic events that have occurred throughout geological time. Constraining plumbing system structure is critical to unravelling the emplacement and melt history of these LIPs, which in turn provides insight into ancient plate configurations and tectonic processes (e.g., Ernst et al., 2005; Ernst et al., 2013). In particular, four key questions concerning plumbing system structure need to be addressed (Ernst et al., 2005):

1)      Where and how does magma enter the crust?

2)      How is magma distributed through the crust?

3)      What is the melt volume of the plumbing system?

4)      Over how long was the plumbing system emplaced (i.e. a proxy for melt production rate)?

To date, the vast majority of studies focus on the importance of mafic dike swarms in facilitating extensive magma transport within LIPs (e.g., Ernst et al., 2001; Ernst, 2014), whereas the role of sills is often overlooked. However, recent field- and seismic reflection-based research shows that magma transport in many LIP systems may occur through interconnected networks of mafic sills and inclined sheets (i.e. a sill-complex) (Thomson and Hutton, 2004; Cartwright and Hansen, 2006; Magee et al., 2014; Muirhead et al., 2014; Schofield et al., 2015; Magee et al., 2016). Mapping of flow indicators and magma geochemistry reveal that sill-complexes can enable magma transport over distances of up to 12 km vertically and 4100 km laterally (Cartwright and Hansen, 2006; Leat, 2008). It is the aim of this contribution, which is based on Magee et al., (2016), to bring together current research on sill-complex emplacement, by synthesizing new and published data from several case studies, providing a basis for future research focusing on sill-complexes within LIPs.

Summary of the use of seismic reflection data to understanding plumbing systems

High-quality, marine seismic reflection data, have arguably revolutionized our understanding of sill-complex emplacement because it allows sub-surface intrusion networks to be imaged in 3D at resolutions on the order of tens of meters (e.g., Fig. 1) (e.g., Smallwood and Maresh, 2002; Thomson and Hutton, 2004; Cartwright and Hansen, 2006; Magee et al., 2014; Magee et al., 2016). Before discussing specific case study examples, it is worthwhile to briefly consider seismic interpretation methodologies for recognizing and characterizing igneous intrusions. Sills are relatively well imaged in seismic data because igneous rocks commonly have greater densities and seismic velocities compared to the surrounding sedimentary strata. These physical differences at the intrusion-host rock contacts result in a high acoustic impedance contrast (Smallwood and Maresh, 2002), which reflects more seismic energy back to the surface than low impedance boundaries typically occurring between sedimentary rock sequence (Brown, 2004). In seismic data, igneous bodies are therefore commonly expressed as high-amplitude reflections (e.g., Fig. 1). One limitation of seismic data, however, is that sub-vertical dikes are rarely imaged because little acoustic energy  reflects back to the surface off the sub-vertical wall of a dike (Thomson, 2007).

An important technique in seismic interpretation of sill-complexes is the mapping of seismically resolvable magma flow indicators (e.g., intrusive steps, bridge structures, and magma fingers), which according to synthetic seismic forward modelling are manifest as minor vertical offsets in sill reflections (Magee et al., 2015). Field studies indicate that these structures form because sheet intrusions typically do not initially intrude as continuous bodies of magma but rather propagate as thin, laterally restricted magma segments that are vertically and/or laterally offset from each other (Fig. 2) (e.g., Rickwood, 1990; Hutton, 2009; Schofield et al., 2012a). With sustained magma input, these segments inflate and coalesce to form a continuous sheet intrusion, but the vertical offsets are preserved (Fig. 2) (Hutton, 2009; Schofield et al., 2012a). The long axes of these structures, which correlate to the primary magma flow axis (Fig. 2) (e.g., Schofield et al., 2012a), can be mapped to discern magma flow patterns and input zones (Thomson and Hutton, 2004; Magee et al., 2014).

Geologic examples of sill-complexes from field and seismic-based observations

Karoo-Ferrar LIP

Magmatism in the Karoo-Ferrar LIP began ~183 Ma (Encarnación et al., 1996), in response to plume and rift-related magmatism (Elliot and Fleming, 2000; Leat, 2008; Hasie et al. 2014; Hole, 2015). Initial activity involved rapid emplacement (~1 Myr) of significant magma volumes (>1 × 10⁶ km³) in the geographically separated sill-complexes of Karoo and Ferrar (Heimann et al., 1994; Elliot and Fleming, 2000; Leat, 2008; Svensen et al., 2012). The Ferrar sill-complex outcrops in Antarctica, SE Australia, and New Zealand, suggesting that the province extends for over 4100 km from the proposed site of a plume head source (Fig. 3B and C) (Mortimer et al. 1995; Elliot et al., 1999; Elliot and Fleming, 2000; Leat, 2008). Ferrar intrusive rocks of Antarctica occur almost exclusively within the 2.5 km thick Beacon Supergroup (e.g., Fig. 3C). The abundance of dikes is limited, and magma transport primarily occurred and was linked to the surface through interconnected sill-complexes (e.g., Figs 3A and B) ( Leat, 2008; Airoldi et al., 2012; Muirhead et al., 2012, 2014). The role of Ferrar sills in facilitating ~4100 km of lateral magma flow across the East Antarctic Margin is supported by: (1) observed sill-sill feeding relationships (Muirhead et al., 2012; Muirhead et al., 2014); (2) decreasing Mg# and MgO contents consistent with fractional crystallization during lateral magma flow as far as 4100 km from the Weddell Sea region (Elliot et al., 1999; Elliot and Fleming, 2000; Leat, 2008); and (3) the consistent orientation of broken bridge structure long axes and magnetic lineations within sills and interconnected sheets across Antarctica (e.g., in the Theron Mountains, Antarctica) (Dragoni et al., 1997; Hutton, 2009; Airoldi et al., 2012).

Sill-complexes in the North Atlantic Igneous Province

The North Atlantic Igneous Province formed ~62–53 Ma in response to the break-up of the North Atlantic and the impingement of a mantle plume at the base of the lithosphere (Saunders et al., 1997; Storey et al., 2007). Voluminous sill-complexes, dike swarms, volcanic centers, and extrusive igneous rocks, primarily of basaltic composition, were emplaced during these phases (~6.6 × 10⁶ km² in total) and are located along two volcanic rifted margins; the conjugate East Greenland–NW European and the West Greenland–Baffin Bay margins (Storey et al., 2007).

Irish Rockall Basin, North Atlantic

Magee et al. (2014) analyzed a sill-complex, located along the north-eastern margin of the Irish Rockall Basin, which consisted of 82 seismically resolved intrusions (Fig. 4). Individual intrusions predominantly displayed saucer-shaped and inclined sheet morphologies, which range in diameter from 0.3–7.1 km and transgress up to 1.79 km of strata (Magee et al., 2014). The sill-complex cross-cuts a ~2 km thick stratigraphic sequence of Cretaceous marine shale and Paleogene volcaniclastic sandstone (Fig. 4), and extends across an area of >748 km² (Magee et al., 2014). The main inclined portions of the intrusions primarily dip to the NW and the sills appear to be interconnected; i.e. inclined sheets and sill limbs apparently feed stratigraphically saucer-shaped sills (Fig. 4) (Magee et al., 2014). Connectivity between individual sills is supported by mapping of flow indicators, which commonly radiate outwards from inferred sill-sill feeder zones (Magee et al., 2014). Observations related to sill connectivity are consistent with a regional, south-easterly directed, lateral magma flow regime, perhaps indicative of a melt source associated with the Hebridean Terrace Igneous Complex located <10 km to the NW (Magee et al., 2014).

In addition to delimiting plumbing systems and flow patterns, it is also important to constrain the timeframe of magmatic activity. Above the sill-complex mapped by Magee et al. (2014), a series of dome-shaped folds are developed within the Upper Cretaceous to top Lower Eocene strata (Fig. 4). The outline of these folds corresponds to the lateral termination of underlying sills and, particularly along the Top Paleocene horizon, seismic reflections can be observed to onlap onto the folds (Fig. 4) (Magee et al., 2014). These seismic-stratigraphic relationships imply that fold growth was generated by and accommodated sill emplacement, resulting in the formation of bathymetric highs that were onlapped by syn-kinematic sediment (Trude et al., 2003; Hansen and Cartwright, 2006; Jackson et al., 2013; Magee et al., 2013e; Magee et al., 2014). Importantly, onlap relationships are observed throughout the folded succession, between the top Cretaceous and top Lower Eocene boundaries (Fig. 4), suggesting that sill emplacement and fold growth occurred incrementally over 15 Myr between ~65–54 Ma (Magee et al., 2014).

Faroe-Shetland Basin, North Atlantic

An extensive sill-complex covering >22,500 km² and emplaced primarily within Upper Cretaceous shale is imaged in seismic reflection data from the Faroe-Shetland Basin (Fig. 5A and B) (Passey and Hitchen, 2011; Schofield et al., 2015). Radiometric dating of sills penetrated by boreholes and relative ages of sills determined from seismic-stratigraphic relationships suggests that intrusion occurred between ~61–52 Ma (Passey and Hitchen, 2011; Schofield et al., 2015). Whilst >300 seismically resolved sills have been mapped in the basin (Fig. 5A), borehole data suggest that a significant proportion (up to ~80%) of intrusions have thicknesses below the limit of detectability of the seismic data and are not resolved (Schofield et al., 2015). The sills that are resolved display a wide variety of morphologies (e.g., saucer-shaped sills and inclined sheets) and have diameters up to tens of kilometers (e.g., Fig. 5) (Schofield et al., 2015). Individual intrusions may transgress up to 2 km of the sedimentary succession and appear to connect to overlying sills, inclined sheets, and lava flows (e.g., Fig. 5) (Schofield et al., 2012b; Schofield et al., 2015). Reconstruction of flow patterns within the Faroe-Shetland Basin sill-complex implies that magma was fed into the basin via basement-involved faults, from which sills propagated laterally for up to ~25 km into the hanging wall (Fig. 5) (Schofield et al., 2015).

Møre and Vøring basins, North Atlantic

Sill-complexes developed along the Norwegian margin (Fig. 6A) represent a continuation of the Late Cretaceous-to-Paleogene magmatic activity that generated networks of intrusions within the Rockall and Faroe-Shetland basins to the south-west (Skogseid et al., 1992; Planke et al., 2005; Cartwright and Hansen, 2006; Hansen, 2006). Sills were predominantly emplaced into Late Cretaceous and Early Paleocene shale, although some sills intruded older sedimentary strata and crystalline basement at ~55 Ma (Svensen et al., 2004; Planke et al., 2005; Hansen, 2006). Overall, the interconnected sill-complexes extend across >80,000 km² of the basin and, in places, transgresses up to 12 km of sedimentary strata and crystalline basement (e.g., Fig. 6) (Svensen et al., 2004; Planke et al., 2005; Cartwright and Hansen, 2006). Observations from the Møre and Vøring basins are consistent with those gathered from the Rockall and Faroe-Shetland basins; i.e. interconnected sill-complexes within sedimentary basins are laterally extensive and provide an efficient mechanism for magma ascent (e.g., Fig. 6B) (Cartwright and Hansen, 2006).

Northern Yilgarn Craton, Western Australia

The Youanmi Terrane in the Yilgarn Craton of Western Australia (Figs 7A and B) contains substantial greenstone belts separated by granite and granitic gneiss, which formed principally between ~3.05 and ~2.62 Ga (Cassidy and Australia, 2006). The interpretation of three deep seismic reflection profiles across the Youanmi Terrane has led to the identification of an interconnected network of highly reflective, generally saucer-shaped sills that extend for several hundreds of kilometers (e.g., Figs 7C and D) (Ivanic et al., 2013). Some sills appear to have been intruded along shear zones, whilst others cross-cut and truncate reflections associated with pre-existing shear zones and listric faults (Figs 7C and D) (Ivanic et al., 2013). These sills have been attributed to the ~1070 Ma Warakurna LIP (Ivanic et al., 2013) (Wingate et al., 2004). The sill that extends from Common Depth Point (CDP) 6500 to CDP 11000 on seismic profile 10GA-YU2 (Fig. 7D) is thought to be the Mount Homes Gabbro Sill, which crops out near the town of Sandstone and has a crystallization age of 1070 ± 18 Ma based on SHRIMP U-Pb dating of baddeleyite (Wingate et al., 2008). Aeromagnetic data suggests that the strike length of the sill exceeds 400 km (Wingate et al., 2008). The interconnected network of saucer-shaped sills interpreted from the Yilgarn Craton seismic data share many characteristics with those documented in the other case studies presented in this contribution. Importantly, the Yilgarn Craton data suggest that interconnected sill-complexes facilitating long-distance lateral magma transport can develop within crystalline continental crust.

Discussion

The seismic reflection data and field observations presented here demonstrate that sill-complexes can form integral components of LIP plumbing systems. In particular, it is clear that sill-complexes provide efficient magma flow pathways, transporting melt from source to surface over great vertical (10’s km) and lateral distances (>100’s km) (e.g., Cartwright and Hansen, 2006; Leat, 2008; Schofield et al., 2015; Magee et al., 2016). Here, we discuss how sill-complexes may fit into the answer of the four key questions concerning plumbing system structure that were highlighted at the start.

1)        Where and how does magma enter the crust?

Whilst the exact source of numerous sill-complexes remains poorly constrained, commonly due to a paucity of seismic and/or borehole data in relevant areas, mapping of magma flow patterns has suggested that some sill-complexes may: (1) originate from underlying melt zones (high-velocity bodies in seismic data) via dikes, fault-hosted inclined sheets, or transgressive sills (e.g., Figs 5A and B) (e.g., Cartwright and Hansen, 2006; Galerne et al., 2011; Rohrman, 2013; Schofield et al., 2015); (2) be fed by laterally propagating dike swarms that emanate from volcanic centers or mantle plume heads (e.g., Hyndman and Alt, 1987; Ernst et al., 1995); (3) propagate directly away from volcanic centers (Magee et al., 2014); or (4) be sourced directly from a plume head, with lateral flow promoted by emplacement into an area of topographic uplift above the plume head and the generation of a hydraulic gradient sufficient enough to allow sills (<10 m thick) to flow laterally for 1000’s of kilometers without freezing (Fialko and Rubin, 1999; Aspler et al., 2002; Leat, 2008).

2)        How is magma distributed through the crust?

Field- and seismic-based observations suggest that interconnected sill-complexes can transgress up to 12 km of strata and transport magma across lateral distances of up to 4100 km regardless of the rock types intruded (e.g., Figs 3-7) (e.g., Aspler et al., 2002; Cartwright and Hansen, 2006; Leat, 2008; Svensen et al., 2012; Magee et al., 2014; Muirhead et al., 2014; Schofield et al., 2015; Magee et al., 2016). An important constraint on the areal extent of sill-complexes appears to be the presence of fault arrays, or other pre-existing structures, which can limit lateral magma propagation by providing suitable magma ascent pathways to the Earth’s surface or to shallower stratigraphic levels (e.g., Fig. 5B) (Magee et al., 2016). Sill-complexes emplaced within relatively unstructured rocks (e.g., the Karoo Basin), however, cover areas that are orders of magnitude larger than those intruded into intensely faulted rocks; e.g., the Karoo sill-complex covers ~3 × 10⁶ km² (Fig. 3A), whereas sill-complexes across the Møre and Vøring rift basins only cover ~8 × 10⁴ km² (Fig. 6A) (cf. Planke et al., 2005; Leat, 2008; Svensen et al., 2012; Magee et al., 2014; Schofield et al., 2015).

From the case studies presented above  it is clear that inclined sheets transfer magma from deeper sills to shallower stratigraphic levels and are integral to facilitating magma ascent in sill-complexes (Thomson and Hutton, 2004; Cartwright and Hansen, 2006; Holford et al., 2012; Muirhead et al., 2012; Magee et al., 2013a; Magee et al., 2014). Several mechanisms have been recognized that may promote the development of inclined sheets: (1) numerical modelling has demonstrated that towards the lateral tips of a sill, during intrusion and/or inflation, σ₁ in the overburden is radially inclined inwards and can promote tensile failure and inclined sheet transgression (e.g., Gudmundsson, 2002; Malthe-Sørenssen et al., 2004; Barnett and Gudmundsson, 2014); (2) some sill-fed inclined sheet swarms are emplaced as mixed-mode fractures, whereby dilation normal to the inclined sheet plane and sub-vertical opening of the sill segments results in mixed-mode I-II fracturing (e.g., the Ferrar sill-complex, Antarctica, Muirhead et al., 2012, 2014); (3) emplacement of magma along reverse faults that nucleated during the growth of flat-topped forced folds (Thomson and Schofield, 2008; Bunger and Cruden, 2011); (4) intrusion along pre-existing faults (e.g., Fig. 5B) (Bedard et al., 2012; Magee et al., 2013a; McClay et al., 2013); or (5) the upward deflection, along a curved trajectory, of a propagating fracture beyond the lateral tip of a sill in response to the asymmetrical emplacement of magma above the initial level of the extension fracture (Hansen, 2015). Overall, magma ascent along inclined sheets in sill-complexes is likely facilitated by multiple processes and resolving the key controls on transgression is therefore challenging (Magee et al., 2016).

3)        What is the melt volume of the plumbing system?

An important consideration when examining sill-complexes using seismic reflection data concerns the role of dikes, which are rarely imaged, in facilitating magma transport between individual sills, inclined sheets, and the surface. Whilst field observations indicate that dikes may be volumetrically minor components of sill-complexes (e.g., Fig. 3) (e.g., the Karoo-Ferrar LIP; Leat, 2008; Hastie et al., 2014; Muirhead et al., 2014), it is likely that the volume of dikes seismically imaged in sill-complexes is underestimated. Furthermore, the majority of sills within a sill-complex likely have thicknesses below the limit of detectability in seismic data, implying that the volume of sills may too be underestimated (Schofield et al., 2015). Whilst seismic reflection data can thus provide unprecedented images of plumbing system structure in three-dimensions, extracting volumetric information remains challenging (Magee et al., 2016).

4)        Over how long was the plumbing system emplaced (i.e. a proxy for melt production rate)?

Sill-complexes represent tortuous, magma ascent pathways compared to swarms of dikes. It may therefore be expected that magma migration, from melt source to the Earth’s surface, will take longer if facilitated by sill, as opposed to dike, emplacement (Magee et al., 2016). The duration of sill-complex development may thus prolong magma residence times, affecting geochemical and petrological processes. For sill-complexes where absolute age data is available or relative dating techniques can be employed, studies suggest that magma injection occurs rapidly over a few million years (Skogseid et al., 1992; Archer et al., 2005; Planke et al., 2005; Cartwright and Hansen, 2006; Hansen and Cartwright, 2006; Hansen, 2006; Svensen et al., 2012). For example, radiometric dating suggests that the Karoo sill-complex were emplaced over 0.47 Myr at a rate of 0.78 km³/yr (Svensen et al., 2012). However, Magee et al. (2014) show multiple onlap relationships in forced folded successions above a sill-complex in the Rockall Basin, suggesting that fold growth, and thereby sill emplacement, occurred incrementally over ~15 Myr through the periodic injection of small magma batches (Fig. 4). Evidence for the prolonged development of a sill-complex in the Rockall Basin therefore implies that seismic-stratigraphic or radiometric dates obtained from only one or several intrusions in a sill-complex, which most previous studies rely upon, cannot reliably be applied to the entire magmatic network (Magee et al., 2014).

Numerous studies have documented that individual sills, as well as entire intrusion networks, may develop through the emplacement of distinct magma pulses (e.g., Glazner et al., 2004; Michel et al., 2008; Annen et al., 2015; Broderick et al., 2015). If some sill-complexes are indeed emplaced incrementally but over timeframes in which magma solidification should theoretically occur, how then does magma rheology vary and how do magma migration pathways remain operational within these systems? Consider a scenario whereby new distinct magma pulses may intrude along the contact, which likely represent a strong mechanical discontinuity, between fully or partially solidified intrusions and the host rock (i.e. under- or over-accretion; Fig. 8A) (see Annen et al., 2015 and references therein). This stacking and amalgamation of sills through repeated magma injection along sill-host rock contacts can influence its thermal and chemical evolution (see Annen et al., 2015 and references therein). If the duration between distinct pulses is too long, allowing the pre-existing intrusion to cool and potentially solidify, any new batch of magma is effectively intruded into a relatively low-temperature environment (Annen, 2011). The accretion of new magma above or below a pre-existing sill or stack of sills, within a relatively low-temperature environment will therefore simply serve to thicken the igneous body, potentially forming layered intrusions, without significant lateral propagation (e.g., Menand, 2008; Annen, 2011). If, however, the duration between the injection of distinct magma pulses is short and/or there is sufficient latent heat of crystallization, the maintenance of locally elevated temperatures may allow later magma pulses to remain hotter for longer, facilitating intrusion further up into the stratigraphic sequence (Fig. 8A) (e.g., Glazner et al., 2004; Annen, 2011; Annen et al., 2015; Magee et al., 2016).

An alternative explanation concerns the development and prolonged longevity of discrete magma flow pathways within individual intrusions (Fig. 8B). Numerous studies have shown that sills commonly form through the coalescence of magma segments or fingers and, on a larger scale, magma lobes (e.g., Fig. 2) (e.g., Pollard et al., 1975; Thomson and Hutton, 2004; Schofield et al., 2010; Schofield et al., 2012b; Schofield et al., 2012a). The channelization of magma into one or several of these zones can increase the segment thickness, allowing magma flow to be sustained whilst neighboring portions of the intrusion crystallize (Fig. 8B) (Holness and Humphreys, 2003). If these channelized segments remain partially molten, through the continued injection of small magma batches and/or insulation by adjacent lobes, subsequent magma pulses may preferentially reactivate the channels (Fig. 8B) (Currier and Marsh, 2015; Magee et al., 2016). Flow of magma through thin channels, which then bud off to form larger sills, is supported by mapped magma flow patterns which suggest that sills are intruded from a point-like source (Thomson and Hutton, 2004; Magee et al., 2013a; Magee et al., 2014; Schofield et al., 2015).

Conclusion

Sill-complexes are integral components of LIP plumbing systems. Mapping sill-complex structure is critical in defining the source area, volume, and production rate of melt within LIPs, as well as constraining how magma moves through the crust and where it is erupted. Seismic reflection data provides a powerful toolset for interpreting entire sill-complexes and mapping regional magma flow patterns. Integrating seismic- and field-based studies is therefore an exciting, new research avenue for LIP research.

Acknowledgements

The Department of Communications, Energy, and Natural Resources (Petroleum Affairs Division) in Ireland, Geoscience Australia, and PGS are also thanked for provision of seismic data. This work was completed as part of CM’s Junior Research Fellowship funded by Imperial College. JDM acknowledges support from Fulbright New Zealand and the Ministry of Science and Innovation.