Impact of host rock heterogeneity on failure around pressurized conduits: Implications for finger-shaped magmatic intrusions

Abstract

Rock failure accommodating the emplacement of magmatic intrusions is controlled by the local stress regime within the host rock. Most of the mechanical models addressing rock failure conditions (e.g., for a given tectonic regime or magma overpressure) simplify the stress calculation by assuming the homogeneity of the host rock properties. In this study, we highlight the importance of local heterogeneities in controlling the localization of the deformation and the failure mechanism around a magmatic intrusion. We numerically model the elastoplastic deformation of a heterogeneous host rock intruded by an overpressurized magmatic body of cylindrical or finger-shaped geometry. The plastic component of the deformation is considered with a mixed mode mechanism allowing dilatant and shear displacements to act simultaneously. Our simulations are performed for small strain amplitudes that reflect the pre-failure conditions of the host. We assess the subsequent failure mechanism of the system based on the development of the localized strain patterns. The heterogeneity in the model is introduced by a stochastic perturbation of the host rock cohesion with characteristic wavelength and amplitude. We show that a relatively small perturbation of ±10% of the cohesion field can efficiently localize the plastic deformation and control the subsequent emplacement mechanism. We further investigate a more realistic geological scenario in which the intrusion resides within sedimentary layers of contrasting strength, resulting in both heterogeneity and anisotropy of mechanical properties in the host rock. Our model reproduces similar deformation patterns as observed around finger-shaped magma intrusions within the Vaca Muerta shales in the Neuquén basin, Argentina. We conclude that heterogeneities within the host rock may locally “seed” dilatant shear faults around magmatic conduits and finger-shaped intrusions and result in the development of a process zone of length scale in order with the intrusion radius length scale.