Coupled Poro-elasto-plastic models of transient fluid flow in response to a crustal strike-slip fault : insight from a geothermal setting in the South Andean volcanic zone

Abstract

Geothermal systems are recognized as key energy resources as well as locations where hydrothermally enhanced chemical reactions can favour mineralizations of economic interest. While fluid-fault interactions in the upper crust have received a wealth of investigations using observational, experimental and modelling approaches, the multi-parametric processes at play are still poorly constrained. While faults can alter fluid flow in their surroundings, potentially acting as barriers or conduits for fluids, magmatic and hydrothermal fluids can also modify pore pressure and alter faults resistance to slip motion. The Planchon-Peteroa geothermal system of the South Andean Volcanic Zone (Chile), illustrates at tectonic crustal scale, how strike-slip faults appear closely involved in the localization of hydrothermal fluid flow. Here, we carry a preliminary modelling approach to be considered as a proof of concept, to show how within such a tectonic setting, a strike slip fault influences fluid flow out from a geothermal reservoir. We developed an original poro-elasto-plastic Finite Element Method (FEM) based on the FEniCS library, and in which the poro-elastic and the elasto-plastic constitutive equations are implicitly coupled. Once this implementation is benchmarked, we assess the development of fluid flow due to a slipping vertical strike-slip left-lateral fault set at 5 km depth. The development of dilational and contractional domains in the fault’ surroundings lead to mean stresses and volumetric strains that range between ±1 MPa and ±10−4, respectively. The appearance of negative and positive fluid pressure in these domains lead to a time-dependent focused fluid flow, which resembles the suction-pump mechanism proposed ca. 30 years ago. We investigate the spatial and temporal evolution of this fluid flow when varying fault permeability, shear modulus, fluid viscosity, and rock frictional strength. We report a maximum fluid flux reaching 8 to 70 times the initial stationary flux. Pressure-driven fluid diffusion returns to stationary state between weeks to months after fault slip. We also show how a plasticity criterion as simple as the von Mises criterion already enhances fluid flow, locally. This transient process highlights the importance of addressing such solid-fluid coupling in studies aiming at constraining volcanic eruption triggers as well as seismic fault destabilization, and the means and pros of geothermal system development.