Magma emplacement at Large Igneous Provinces
Nicolas Le Corvec
Lunar and Planetary Institute, USRA, 3600 Bay Area Blvd., Houston, TX, 77058, USA
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].
Figure 1: Types of extrusive and intrusive materials in LIPs. a. Deccan lavas of the Wai Sub-group exposed in the Western Ghats of India near Mahabaleshwar. These lavas are part of the sheet-lobe dominated Ambenali (lower part of cliffs) and Mahabaleshwar Formations (upper part). Scarp face seen here is approximately 450 m high (http://www.largeigneousprovinces.org/12may); b. A quarry in southeastern Botswana exposes a weathering profile into a Mesoproterozoic sill of the Umkondo Large Igneous Province (http://www.swanson-hysell.org/field-work-photos/); c. A 20-km-long dike in the western coastal region of the Deccan flood basalt province, where most dike swarms have N-S to NNW-SSE trends. The dike forms a dam across the Surya River near the coastal town of Dahanu (http://www.geology.sdsu.edu/how_volcanoes_work/Fissure.html); and d. 2500-2450 Matachewan and Mistassini radiating swarms, Superior Province, Canada (http://www.mantleplumes.org/GiantRadDykeSwarms.html).
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.
Figure 2: a. Example of analogue modeling: A vertically propagating dyke has turned to intrude as a sill along the interface separating upper rigid layer from lower layer. Note the protrusions from the sill periphery into the lower less rigid layer (from Kavanagh et al. 2006), b. Example of analogue modeling: Formation of a dyke–sill hybrid. The dyke has reached the interface and has intruded horizontally as a sill and into the upper rigid layer (from Kavanagh et al. 2006), c. Example of numerical modeling: Successive “snapshots” of the propagating dike reaching a mechanical boundary. The Coulomb failure function variation induced on horizontal planes is shaded in background (blue : negative values; red : positive values), the dashed line represents the energetically preferred path and the shape of the dike is drawn exaggerated by a factor 1400 (from Maccaferri and al. 2011), and d. Network of Ferrar Dolerite sills south of Terra Cotta Mountain, Transantarctic Mountains, Antarctica (http://www.otago.ac.nz/geology/people/students/airoldi/)
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).
Figure 3: a. Distribution of the basement sill in South Victoria Land (from Muirhead et al., 2012), b. Finite element model (FEM) of a layered crustal segment where both dyke and sill follow the maximum compressive principal stress (σ1) trajectories represented by the red ticks. The black line shows the paths which the dyke and later the sill would follow as they reach different layers with varying stiffness (from Barnett and Gudmundsson 2014).
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 (στ) and out-of-plane hoop (σθ) 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: στ will favor vertical and circumferential dikes or lateral sill intrusions while σθ favors radial dike intrusions [Grosfils, 2007].
Using FEMs (finite element models), Le Corvec et al.  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 (σθ), 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).
Figure 4: a. Amount of overpressure at failure and type and location of failure affecting the magma chamber for the upward flexure environment. The dotted, dash and dots and dashed lines represents FEMs with a lithosphere thickness of 20, 30 and 40 km, respectively. The symbols in green and orange represent a failure mechanism controlled by the tangential stress (στ) and hoop stress (σθ), respectively. The diamond and triangle symbols represent the failure location at the center and bottom of the magma chamber’s wall, respectively. Open and filled symbols represent stable and unstable magma chambers, respectively. Magma chambers that fail at their bottom are considered unviable. The grey area represents the appropriate conditions for the formation of radial dikes with radial distances > 200 km. b. Overpressure and stresses controlling the failure of the magma chamber for three different lithosphere thicknesses (20, 30 and 40 km). For the upward flexure environment, the left side of each figure represents the stresses (στ and σθ) along the lower half of the magma chamber. In some cases, no overpressure is needed to reach failure as the magma chamber is unstable. c. Distance favorable to the propagation of radial dikes within lithospheres subjected to upward flexure. Each symbol represents FEMs with their magma chamber failure controlled by the hoop stress (i.e., favoring radial dike formation). The symbol surrounded by a circle is the example of the deviatoric stress orientation for a lithosphere subjected to upward flexure shown in d. for a lithosphere subjected to downward flexure. The deformation in d. is increased by a factor of 5.
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.
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