Magma emplacement mechanism in visco-elasto-plastic crustal rock: an insight from quantitative 2D analogue experiments

Magma generated at depth ascends through the crust and erupts at the surface. In this process, the magma volume creates space by deforming the crustal rock. To study this process, researchers have employed analogue experiments for decades. Conventionally, most analogue models assume the crustal rock as a purely elastic, brittle (Coulomb), or viscous material. However, natural rocks exhibit more complex visco-elasto-plastic behaviours, with rheological properties transitioning from elastic-plastic to visco-elasto-plastic with increasing depth. Conversely, magma itself behaves as a Newtonian fluid, exhibiting a range of viscosities influenced by factors such as silica and volatile content. In this study, we conducted scaled 2D analogue magma emplacement experiments using Laponite^® RD (LRD), a complex visco-elasto-plastic rheological material, as a host rock analogue. The rheological properties of LRD were systematically varied by changing the curing time (t_c) from t_c = 30 min to t_c = 240 min to simulate a broad range of crustal rheological behaviours, corresponding to different depths from viscous to visco-elasto-plastic regimes. Food-coloured water and hydroxyethyl cellulose (HEC) aqueous solutions, with concentrations of 0.50 wt% and 0.75 wt%, are used to simulate magma with varying viscosities. Polyamide seeding particles (PSPs) of 60µm diameter were incorporated inside the LRD solution to quantify host-rock deformation during magma emplacement via particle image velocimetry (PIV). Our experimental results show that magma intrusions exhibit various shapes, ranging from straight-edged fractures in the case of low-viscosity water and viscoelastic LRD of high t_c to bulbous, rounded forms in high-viscosity HEC and viscous LRD of low t_c. Furthermore, PIV data enabled the identification of domains exhibiting distinct deformation types, thereby delineating changes in deformation regimes attributable to variations in the rheological properties of the host rock. Finally, by integrating geometric and PIV analyses, we established a relationship between the intrusion morphology and host rock rheology, and propose a mechanical model that elucidates the deformation mechanisms operative within the host during magma emplacement across different rheological combinations of host and magma materials.