July 2014 LIP of the Month

Magma emplacement at Large Igneous Provinces

Nicolas Le Corvec

Lunar and Planetary Institute, USRA, 3600 Bay Area Blvd., Houston, TX, 77058, USA

(lecorvec@lpi.usra.edu)

Large Igneous Provinces (LIPs) are products of some of the largest magmatic (both volcanic and intrusive) events known on Earth [e.g., Coffin and Eldholm, 1994; Bryan and Ferrari, 2013; Ernst, 2014]. They have punctuated the Earth’s history and led to catastrophic mass extinctions through the injection of volatiles (CO₂ and SO₂) into the atmosphere during prolonged phases of LIP volcanic activity [e.g., Bond and Wignall, 2014]. LIPs are also associated with the dismembering of supercontinents through continental rifting and the formation of new ocean basins [Courtillot et al., 1999; Dalziel et al., 2000].

LIPs are commonly defined by having areal extents >0.1 Mkm² and are relatively short lived, with maximum lifespans of ~50 Myr, but typically restricted to one or more pulses of just a few millions years [Bryan and Ernst, 2008]. These intraplate voluminous magmatic events (>0.1 Mkm³) produce an extended range of material from high-level basaltic to silicic extrusive and intrusive materials (lava flow fields, sills, dikes including radiating dike swarms, Fig. 1) [Bryan et al., 2010; Ernst and Bleeker, 2010; Muirhead et al., 2012; Ernst, 2014].

Emplacement of magmatic systems

The formation of LIPs is believed to result from the upwelling of mushroom-shaped regions of solid mantle material, a.k.a. mantle plumes , but their existence is still an ongoing matter of debate [e.g., Anderson and Natland, 2005; Campbell and Kerr, 2007; Bryan and Ernst, 2008; Foulger, 2011; Ernst, 2014]. The rapid transport of magma toward the surface and formation of intrusive complexes are mainly controlled by the fracturing of the host rock through extensional fractures, also called hydrofracturing [Wadsworth, 1982]. The orientation of those magma-filled cracks is mainly controlled by the orientation of the principal stresses within the lithosphere [Rubin, 1995], thus the formation of dikes (i.e., discordant and generally subvertical, Fig. 1c) requires the minimum compressional stress (σ₃) to be horizontal, and the maximum compressional stress (σ₁) to be vertical or horizontal, and vice-versa for the formation of sills (i.e., bedding concordant, subhorizontal, Fig. 1b). Non-subhorizontal and non-subvertical intrusions are referred to sheets and require more complex stress orientations [Pasquarè and Tibaldi, 2007]. In addition, heterogeneities within the Earth have been recognized to play an important role in the behavior of magma propagation: e.g., numerical and analogue models have shown the influence of mechanical layering (Fig. 2 a, b, and c) [Kavanagh et al., 2006; Gudmundsson, 2011; Maccaferri et al., 2011; Barnett and Gudmundsson, 2014] or pre-existing fractures [Gaffney et al., 2007; Le Corvec et al., 2013] on direction of magma propagation.

Natural examples of giant sills can be observed in many LIP locations, e.g., the “Ferrar Dolerite” intrusions in Antarctica (Fig. 2d) [Airoldi et al., 2012; Muirhead et al., 2012]. Transition from feeding dikes to sills may occur at boundaries between two distinct layers. This reorientation mechanism is controlled by several parameters: e.g., the difference in stiffness between the two layers [Rivalta et al., 2005; Kavanagh et al., 2006; Maccaferri et al., 2011; Ritter et al., 2013; Barnett and Gudmundsson, 2014], stress barriers [Gretener, 1969; Gudmundsson, 1990; Barnett and Gudmundsson, 2014], the difference in fracture toughness [Maccaferri et al., 2011], the intrusion temperature [Chanceaux and Menand, 2014], and the rock interface strength (a.k.a., Cook-Gordon debonding) [Barnett and Gudmundsson, 2014; Kavanagh and Pavier, 2014].

Emplacement of large amounts of magma in the lithosphere requires space. Observation at the Ferrar Dolerite intrusions shows that in addition to the thickest sills (e.g., Finger Mountain Sill), other sill intrusions, although thinner and shorter in length, are found to ascend the stratigraphy in ‘step-wise’ fashion. These lead to a configuration of sills connected by sheets (Fig. 3a) [Muirhead et al., 2012]. As the dike rises toward the surface, it may change direction of propagation due to the potential mechanisms cited earlier. Depending of its size, a sill may bend its overburden which will influence the local stress and allow the formation of sheets. These new sheets may reach an interface along their path and transform into sills, which will lead to the formation of interconnected sills such as the ones observed by Muirhead and al. (2012) at Allan Hills and Terra Cotta Mountain at the Ferrar Dolerite intrusions. An alternative mechanism to the formation of interconnected sills is described by Barnett and Gudmundsson (2014) where a succession of stiff and soft layers acts as stress barriers (Fig. 3b).

Giant radiating dike swarms

Another peculiar type of magma intrusion in LIPs produces radiating dike swarms [Halls, 1982; Fahrig, 1987; Ernst and Baragar, 1992; Ernst et al., 1995; Hoek, 1995; Ernst et al., 2001]. Radiating dike swarm are seen on Earth, Mars and Venus [Grosfils and Head, 1994a; Grosfils and Head, 1994b; Mège and Masson, 1996; Ernst et al. 2003; Galgana et al., 2013] and have lengths ranging from tens up to 2200 km from the focal point of the swarm [Ernst et al., 2001]. Giant dike swarms, are defined to be longer than 300 km. Their formation may involve magma chambers at levels of neutral buoyancy [Grosfils and Head, 1995], or could be explained by a combination of tectonic loads applied to a homogeneous lithosphere [Galgana et al., 2013]. Despite the importance of the tectonic state of stress, heterogeneities within the lithosphere also have a strong influence on the behavior of magmatic systems as shown earlier. The presence of crustal low density rock above mantle higher density material may trap magmas of intermediate density near the level of neutral buoyancy, which favors the formation of magma chambers at the interface [e.g., Takada, 1989; Walker, 1989; Lister, 1991]. The stability of such chambers and their failure can vary depending on the tectonic environment and location of the chamber within the lithosphere. Magma chamber failure is controlled by the magma chamber wall in-plane tangential (σ_τ) and out-of-plane hoop (σ_θ) stress components in the context of an axisymmetric model [Galgana et al., 2011; Galgana et al., 2013]. Depending on the stress reaching the tensile failure of the host rock, different magmatic intrusions will be favored: σ_τ will favor vertical and circumferential dikes or lateral sill intrusions while σ_θ favors radial dike intrusions [Grosfils, 2007].

Using FEMs (finite element models), Le Corvec et al. [2014] have also studied the influence of upward flexure due to a rising mantle plume on the stability of magma chambers, the type of intrusion and their distance of propagation. Their result shows that failure of a magma reservoir in the upper part of the lithosphere is controlled by the hoop stress (σ_θ), which will lead to the formation of radial dikes (Fig. 4a and b). The failure of the magma chamber is also controlled by the thickness of the crust (i.e., the depth of the magma chamber within the lithosphere). The resulting radial dike can travel to distances up to ~300 km depending on the thickness of the lithosphere and the depth of the magma chamber (Fig. 4c). A cross-section of the lithosphere shows that the minimum deviatoric compressional stress is out of the plane favoring lateral dike propagation (Fig. 4d).

Conclusion

LIP eruptions lead to the emplacement of exceptionally large-scale magmatic intrusions and extrusions, during which large surfaces of the Earth or other planets are affected. The understanding of such phenomena lies in the understanding of magma propagation from source to surface. New evidence has shown the impact of crustal heterogeneities on the direction of magma propagation from dikes to sills to sheet to sills or dikes as well as the impact of the intrusive complex on the local state of stress, which can overcome the regional tectonic stress. The combination of fieldwork and analogue and numerical models has better constrained our understanding on how magma propagates within the lithosphere. More complex models will have to be developed in the future to constrain the thermal activity of these mechanisms as well as the temporal accommodation of the lithosphere to the intrusive and extrusive loads, since LIPs are related to attempted or successful continental breakup.

References