The Mechanical Interaction and Potential Mutual Trigger of a Fault Zone and a Volcanic Reservoir from 3D Elasto-Plastic Models Based on a Field Case from the Andean Southern Volcanic Zone

Abstract

Active and fossil magmatic-geothermal systems have been spatially associated with crustal faults, and the feedback relationships between them has been widely identified through structural geology methods, laboratory studies, and more recently through numerical modeling. However, although recognized, the mechanical constraints driving their interaction are yet to be better understood. Here, we aim at understanding the local mechanical interaction between a crustal magmatic reservoir and a strike-slip dextral fault zone set 4 km apart, inspired by a specific field case in the Southern Andes. To do so, we performed a series of 3d elasto-plastic models and examined how shear stress, volumetric strain, and plastic strain develop in the rock volume between the fault and reservoir. We tested the potential mutual trigger by imposing either (1) a strike-slip displacement along the fault zone, or a (2) magmatic overpressure along the reservoir walls, and through parametric tests of Young’s modulus and frictional strength. Our results show that both accumulated fault displacement and magmatic loading consistently create dilational strain to develop fluid flow pathways and fluid accumulation within the crust. Moreover, parametric tests indicate that the bedrock’s Young’s modulus is one of the most determinant parameter for failure of the reservoir when examining the perturbation of a fault. In this case, the stiffer the bedrock, the easier it is to induce reservoir failure and localized shear failure, whereas compliant bedrock and fault domains promote higher diffuse and distributed volumetric strain. The latter could be significant for fluid flow and increase in rock permeability. On the other hand, bedrock and fault friction are key to determine whether a magmatic loading can induce fault failure. In this case, the friction angle of the fault zone and the bedrock are required to be sufficiently low for failure to occur. We conclude that a fault zone and a geofluid reservoir can significantly interact at a kilometric scale. Further work should aim at combining this local interaction with the active regional stress field (e.g. transpressional stress field of the Southern Andes).