The interplay of a fault zone and a volcanic reservoir from 3D elasto-plastic models: Rheological conditions for mutual trigger based on a field case from the Andean Southern Volcanic Zone

The Southern Andes margin hosts active and fossil volcanic, geothermal, and mineralized systems documenting intense geofluid migration through the crust. Fluid flow is also spatially associated with crustal faults that accommodate the bulk deformation arising from oblique plate convergence. Although recognized, the precise local mechanical interaction between faults and crustal reservoirs is yet to be better understood. Here we present 3D numerical models of a magmatic reservoir and a fault zone set about 4 km apart, inspired by the Tatara-San Pedro volcanic complex in the Southern Volcanic Zone (similar to 36 degrees S), which displays a geothermal field and a margin-parallel dextral active fault zone constrained by published magnetotelluric profiles and crustal seismicity respectively. We investigate elasto-plastic deformation and stress patterns in the intermediate bedrock space between the reservoir and the fault zone and test how shear stress, volumetric strain, and plastic strain develop. We also test the potential of enabling brittle failure of their counterpart by imposing either (1) a strike-slip displacement along the fault zone, or (2) a magmatic overpressure at the cavity walls. Parametric tests of Young's modulus and frictional strength provide the conditions for macro-scale brittle failure and show the development of diffuse domains of dilational strain of the order of 10(-5) -10(-3) in the intervening bedrock. This dilation is a proxy to the opening of voids or volumetric cracking in the bedrock, which tends to increase porosity and permeability allowing over-pressurized geoflu ids to migrate within these domains. Our results show that a minimum of 60 m of fault displacement is required to trigger brittle failure of an upper crustal cavity if the bedrock is stiff, whereas, for a more compliant bedrock, more than 100 m of localized slip motion is required. This implies that it is rather the accumulated effect of repeated crustal fault displacement that potentially favors fluid pathways upwards, rather than a single seismic event. On the other hand, a minimum of 7.5 MPa of fluid overpressure is required for a mid-crustal cavity (15 km depth) to trigger brittle failure of the fault zone. This threshold overpressure increases up to 50 MPa when the cavity is shallower (6 km depth). Our results show that in general, shallow reservoirs must be very dose to fault zones (less than 1-2 km apart) to reactivate them. The models show that localized strike-slip tectonics and magma intrusions build a dilational stress field at the scale of several kilometers, that promotes fluid pathways to the surface. Further combining this interaction with the regional transpressional stress field may explain observations of transient fluid pathways on seemingly independent timescales along the Andean margin. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The Southern Andes margin contains active and fossil volcanic, geothermal, and mineralized systems that demonstrate intense geofluid migration through the crust, associated with crustal faults that accommodate bulk deformation from oblique plate convergence. 3D numerical models of a magmatic reservoir and a fault zone were presented to investigate elastic-plastic deformation and stress patterns in the intermediate bedrock space, showing potential for brittle failure triggered by strike-slip displacement or magmatic overpressure. The results suggest that repeated crustal fault displacement may favor fluid pathways upwards, rather than a single seismic event, and that shallow reservoirs must be close to fault zones to reactivate them and promote fluid pathways to the surface.