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Thus they are of limited use to constrain under which conditions intrusions of contrasting shapes form.Here we present a series of 2D experiments where a viscous fluid (oil) was injected into a host matrix (laponite gel), the visco-elasto-plastic rheology of which is varied from dominantly viscous to dominantly elastic.This statement is corroborated by field observations of igneous sills emplaced in shale-carbonate rocks, which exhibit complex brittle-ductile deformation that accommodated the emplacement of the magma (Schofield et al., 2012; Duffield et al., 2016; Spacapan et al., 2017) (Figure 1D).Moreover, field observations in the host rock of thin laccoliths in the Henry Mountains, Utah, evidence significant plastic shear failure and ductile deformation of the overburden (Román-Berdiel et al., 1995; de Saint Blanquat et al., 2006; Wilson et al., 2016), in contrast to the elastic assumptions of the theoretical models (e.g., Pollard, 1973; Bunger and Cruden, 2011).
While dyke emplacement models assume tensile propagation in purely elastic host rock, earthquake swarms monitored during dyke propagation exhibit numerous, if not most dominant, double-couple focal mechanisms interpreted as shear faulting (e.g., Borandsdottir and Einarsson, 1979; Ukawa and Tsukahara, 1996; Battaglia et al., 2005; Roman and Cashman, 2006; White et al., 2011; Ágústsdóttir et al., 2016).In these models, magma intrusions are emplaced by pushing their host rock, which is displaced along fault planes (Figure 1B). Most models of emplacement of igneous sheet intrusions (i.e., dykes, sills, cone sheets, thin laccoliths) in the brittle crust address the host rock as an elastic solid (e.g., Rubin, 1995; Menand et al., 2010; Galland and Scheibert, 2013; Kavanagh et al., 2015; Rivalta et al., 2015).In these models, magma intrusions are addressed as idealized tensile hydraulic fractures, the thickening of which is accommodated by elastic bending of the host rock (Figure 1C). Diagram of idealized end-member cases of magma ascent.The mechanics of magma transport and emplacement in the Earth's crust generally corresponds to the flow of a viscous fluid into a solid, which deforms to accommodate the incoming volume of magma.In volcanic systems, however, the simplicity of this statement is challenged by the complexity of geological materials.