Browsing by Author "Ruz Ginouves, Javiera"

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    Dilatation and shearing in tectono-volcanic systems from poro-elasto-plastic models set in the Southern Andes Volcanic Zone context, inferences on geofluid flow
    (2022) Gerbault, Muriel; Saez, Felipe; Ruz Ginouves, Javiera; Cembrano, José; Iturrieta, Pablo; Hurtado, Daniel; Hassani, Riad; Browning, John
    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.
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    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
    (2021) Ruz Ginouves, Javiera; Gerbault, Muriel; Cembrano, Jose; Iturrieta, Pablo; Saez Leiva, Felipe; Novoa, Camila; Hassani, Riad
    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.