Dilatation and shearing in tectono-volcanic systems from poro-elasto-plastic models set in the Southern Andes Volcanic Zone context, inferences on geofluid flow

Abstract

Geothermal fields near volcanic complexes and active crustal-scale fault zones require an understanding of the mechanical interactions that control variations in pore fluid pressure at a crustal scale. Crustal faults can trigger and modify fluid flow depending mostly on their geometry and mechanical properties. In turn, fluid flow reduces normal stresses causing either shearing or dilation through the rock mass, concomitant with hydraulic fracturing or seismic fault reactivation. The Southern Andes Volcanic Zone (SAVZ) documents widespread geofluid migration through the crust within a bulk regional transpressive regime. We address here the key role of dilatational domains potentially hosting geothermal fluids, in close relation to shear zones, by using elasto-plastic and poro-elasto-plastic models.First we define models considering Drucker-Prager elasto-plasticity, that account for either: 1) an inflating magmatic cavity or 2) a dextral slipping fault zone ca. 4 km apart, to assess the rheological conditions leading to brittle failure of the bedrock around the fault zone and the cavity, respectively. This setup is applied to the San-Pedro Tatara volcanic complex in the SAVZ. Parametric tests of Young’s moduli and frictional strength provide not only the conditions for macro-scale shear failure, but also shows the development of diffuse domains of dilatational strain in the intervening bedrock. Both void opening and/or volumetric cracking may lead to an increase in porosity and/or permeability, allowing over-pressurized geofluids to migrate within these domains. Our results (Ruz Ginouves et al., JVGR, 2021) show that generally, shallow magma chambers (~< 4 km) and fault zones must be close enough to trigger bedrock failure of the other counterpart (< 4 km), unless the magma chamber is deeper than 10 km, the magma overpressure is high or the regional strength is very low. We argue that alternating strike-slip faulting and magmatic overpressure promote a variety of stress fields that may explain observations of transient fluid pathways on seemingly independent timescales along the Andean margin.To gain further insights into these processes, we develop a numerical scheme to quantify stress and fluid flow with a coupled poro-mechanical approach implemented using Python’s Opensource FEM library FeniCS. Benchmarks are first presented to validate our poro-elasto-plastic approach. Then a synthetic setup shows how fluids get channelized around a fault zone several days after an imposed fault slip motion. Preliminary results are discussed in comparison to a high enthalpy geothermal system associated with another volcanic complex in the SAVZ.