Fracturing and Dome‐Shaped Surface Displacements Above Laccolith Intrusions: Insights From Discrete Element Method Modeling.

Inflation of viscous magma intrusions in Earth's shallow crust often induces strain and fracturing within heterogeneous host rocks and dome‐shaped ground deformation. Most geodetic models nevertheless consider homogeneous, isotropic, and linear‐elastic media wherein stress patterns indicate the potential for failure, but without simulating actual fracturing. We present a two‐dimensional Discrete Element Method (DEM) application to simulate magma recharge in a pre‐existing laccolith intrusion. In DEM models, fractures can propagate during simulations. We systematically investigate the effect of the host rock toughness (resistance to fracturing), stiffness (resistance to deformation) and the intrusion depth, on intrusion‐induced stress, strain, displacement and spatial fracture distribution. Our results show that the spatial fracture distribution varies between two end‐members: (a) for high stiffness or low toughness host rock or a shallow intrusion: extensive cracking, multiple vertical surface fractures propagating downward and two inward‐dipping highly cracked shear zones that connect the intrusion tip with the surface; and (b) for low stiffness or high toughness host rock or a deeper intrusion: limited cracking, one central vertical fracture initiated at the surface, and two inward‐dipping fractures at the intrusion tips. Abrupt increases in surface displacement magnitude occur in response to fracturing, even at constant magma injection rates. Our modeling application provides a novel approach to considering host rock mechanical strength and fracturing during viscous magma intrusion and associated dome‐shaped ground deformation, with important implications for interpreting geodetic signals at active volcanoes and the exploitation of geothermal reservoirs and mineral deposits. Plain Language Summary: Most viscous magmas are stored and solidified in the upper 1–2 km of Earth's crust and rarely reach the surface to produce explosive volcanic eruptions. At such shallow depths, these viscous magmas often form intrusions with a flat base and dome‐shaped roof, or "laccoliths." To accommodate the emplaced volume, the deformation of the host rock produces dome‐shaped ground uplift at the surface. Geological and geophysical observations show that laccolith emplacement often coincides with intense host rock fracturing. However, most of the numerical laccolith emplacement models ignore that fracturing and severely simplify the mechanical response of the host rock. To assess the effect of host rock fracturing on ground deformation, we designed a two‐dimensional Discrete Element Method (DEM) application, which can simulate the fracturing of the host rock during magma injection. We found that varying the strength of the host rock and/or the intrusion depth controls the spatial distribution of fracturing and magnitude of the ground uplift. Our results help to understand complex interplays between the depth of magma injection and the mechanical properties of host rocks, and to improve the modeling tools used to routinely monitor magmatic activity. Key Points: We present a two‐dimensional Discrete Element Method model that simulates host rock fracturing during the inflation of a magmatic laccolithThe host rock toughness and stiffness along with source depth control spatial fracture distribution between two end‐membersAt a constant injection rate, the vertical displacement maximum and lateral extent show sudden changes in response to host rock fracturing [ABSTRACT FROM AUTHOR]