Browsing by Author "Meredith, Philip G."

Now showing 1 - 5 of 5
  • No Thumbnail Available
    Item
    A new anisotropic poroelasticity model to describe damage accumulation during cyclic triaxial loading of rock
    (2022) Lyakhovsky, Vladimir; Panteleev, Ivan; Shalev, Eyal; Browning, John; Mitchell, Thomas; Healy, David; Meredith, Philip G.
    Crustal rocks undergo repeated cycles of stress over time. In complex tectonic environments where stresses may evolve both spatially and temporally, such as volcanoes or active fault zones, these rocks may experience not only cyclic loading and unloading, but also rotation and/or reorientation of stresses. In such situations, any resulting crack distributions form sequentially and may therefore be highly anisotropic. Thus, the tectonic history of the crust as recorded in deformed rocks may include evidence for complex stress paths, encompassing different magnitudes and orientations. Despite this, the ways in which variations in principal stresses influence the evolution of anisotropic crack distributions remain poorly constrained. In this work, we build on the previous non-linear anisotropic damage rheology model by presenting a newly developed poroelastic rheological model which accounts for both coupled anisotropic damage and porosity evolution. The new model shares the main features of previously developed anisotropic damage and scalar poroelastic damage models, including the ability to simulate the entire yield curve through a single formulation. In the new model, the yield condition is defined in terms of invariants of the strain tensor, and so the new formulation operates with directional yield conditions (different values for each principal direction) depending on the damage tensor and triaxial loading conditions. This allows us to discern evolving yield conditions for each principal stress direction and fit the measured amounts of accumulated damage from previous loading cycles. Coupling between anisotropic damage and anisotropic compaction along with the damage-dependent yield condition produces a reasonable fit to the experimentally obtained stress–strain curves. Furthermore, the simulated time-dependent cumulative damage is well correlated with experimentally observed acoustic emissions during cyclic loading in different directions. As such, we are able to recreate many of the features of the experimentally observed directional 3-D Kaiser ‘damage memory’ effect.
  • No Thumbnail Available
    Item
    Conditions for fracture arrest in layered rock sequences
    (2020) Forbes Inskip, Nathaniel D.; Browning, John; Meredith, Philip G.; Gudmundsson, Agust
    Fracture arrest in layered rock sequences is important in many geodynamic processes, such as dyke-fed volcanic eruptions, earthquake ruptures, landslides, and the evolution of plate boundaries. Yet it remains poorly understood. For example, we do not fully understand the conditions for dyke arrest (preventing potential eruptions) or hydraulic-fracture arrest in gas shales (preventing potential aquifer pollution). Here we present new numerical results on the conditions for arrest of fluid-driven (mode-I) vertical fractures in layered rock sequences when the tips of the fractures approach the interface between two layers of contrasting mechanical properties. In particular, we explore the stress-field effects of variations in layer stiffness, proximity of fracture tip to layer interface, and layer thickness. When the layer hosting the fracture tip is stiffer, fracture arrest normally occurs at the interface with the more compliant layer. By contrast, when the layer above the interface is stiffer, fracture arrest may occur within the host layer well below the interface. These conclusions are supported by field observations of arrested fluid-driven joints and dykes and, therefore, provide a better understanding of the mechanical conditions for dyke-fed eruptions.
  • No Thumbnail Available
    Item
    Fracture growth and damage zone evolution in fault-vein systems determined through scaling relations in alteration halo-bearing hydrothermal veins
    (2024) Hofer Apostolidis, Karin Andrea; Cembrano Perasso, José Miguel; Browning, John; Pérez-Flores, Pamela; Mitchell, Thomas M.; Meredith, Philip G.; Rojas Guzmán, Flavia Jael; Tao Xu
    Understanding how fluids flow to form halo-bearing veins is essential to assess the fundamental processes involved in fracture propagation and the formation of hydrothermal ore deposits. Haloes may mimic damage zones during fracture propagation, contributing to the identification of scaling relations between halo width and fracture displacement. In this work, we examine geometry, kinematics and mineral composition of well-exposed halo-bearing fault-vein network field samples. We studied a total of 18 veins from Iron-Oxide Copper Gold (IOCG) deposits in the Chilean Atacama Desert and from the Chinese Cathaysia tectonic block. Vein length and width and halo width were measured directly at the outcrop and later under optical microscope. We established a scaling relation, over five orders of magnitude, between halo width (HW) and vein width (VW) of the form which suggests that the majority of analyzed haloes were formed as a result of crack tip process zone damage. Such ratios and scaling relationships, apart from elucidating the physical mechanisms driving halo/damage zone formation, have potential implications for a more reliable estimation of the nature and size of ore grade variations away from high-grade mineralized veins to the relatively lower grade surrounding wall rock volumes.
  • No Thumbnail Available
    Item
    Non-linear anisotropic damage rheology model: Theory and experimental verification
    (2021) Panteleev, Ivan; Lyakhovsky, Vladimir; Browning, John; Meredith, Philip G.; Healy, David; Mitchell, Thomas M.
    We extend the isotropic non-linear damage rheology model with a scalar damage parameter to a more complex formulation that accounts for anisotropic damage growth under true triaxial loading. The model takes account of both the anisotropy of elastic properties (associated with textural rock structure) and the stress- and damage-induced anisotropy (associated with loading). The scalar, isotropic model is modified by assuming orthotropic symmetry and introducing a second-order damage tensor, the principal values of which describe damage in three orthogonal directions associated with the orientations of the principal loading axes. Different damage components, accumulated under true triaxial loading conditions, allows us to reproduce both stress-strain curves and damage- and stress-induced seismic wave velocity anisotropy. The suggested model generalization includes a non-classical energy term similar to the isotropic non-linear scalar damage model, which allows accounting for the abrupt change in the effective elastic moduli upon stress reversal. For calibration and verification of the model parameters, we use experimental stress-strain curves from deformation of dry sandstone under both conventional and true triaxial stress conditions. Cubic samples were deformed in three orthogonal directions with independently controlled stress paths. To characterize crack damage, changes in ultrasonic P-wave velocities in the three principal directions were measured, together with the bulk acoustic emission output. The parameters of the developed model were constrained using the conventional triaxial test data, and provided good fits to the stress-strain curves and P-wave velocity variations in the three orthogonal directions. Numerical simulation of the true triaxial test data demonstrates that the anisotropic damage rheology model adequately describes both non-linear stress-strain behavior and P-wave velocity variations in the tested Darley Dale sandstone.
  • Thumbnail Image
    Item
    Textural evidence of fragmentation and densification processes in a fossilised shallow conduit on the flank of Nevados de Chillán Volcanic Complex
    (2024) Rojas, Flavia; Browning, John; Tuffen, Hugh; Cembrano, José; Espinosa-Leal, Javier; Unwin, Holly E.; Mitchell, Thomas M.; Hofer-Apostolidis, Karin; Meredith, Philip G.
    Eruptive style transitions are common in silicic volcanoes and an improved understanding of transitional controls is necessary for hazard forecasting. Examples of hybrid eruptions where both explosive and effusive eruptive behaviours occur simultaneously have led to a re-examination of models used to understand these complex and poorly understood processes. Exposed fossilised conduits record evidence of magmatic processes and provide the opportunity to examine structures and textures related to these transitions. Here we present a conceptual model of the evolution of a narrow (2.5 m wide) conduit located on the SW flank of the Nevados de Chillán Volcanic Complex, Chile. This conduit records evidence of fragmentation and densification processes through intercalated and juxtaposed banded, porous and dense domains. To understand how the products of each eruptive style relate and evolve during conduit formation, we combined qualitative textural analyses at different scales (outcrop, optical microscope and electron microscope), pore size and shape measurements using ImageJ, connected porosity measurements made using a helium pycnometer and total water content measurements using Fourier transform infrared spectroscopy. The results allow us to identify five principal phases of the conduit evolution: (I) an explosive phase where the conduit is filled with pyroclastic material, evidenced in the pyroclastic deposit preserved at the conduit wall, (II) a cyclic process of fragmentation and densification within the conduit that generates intercalation of the porous and dense domains, and leads to a hybrid explosive-effusive phase, (III) the formation of a dense magma plug that eventually seals the conduit and deforms vesicles and bands, (IV) the compaction of the pyroclastic domain due to the ascent of the plug, driving porosity reduction (to as little as 3% in the densest bands), with micro-folds and glassy fiamme, and (V) a final phase of post-sintering vesicle relaxation, yielding regular, mainly rounded, shapes. We compare our results with other exposed and examined conduits to propose a model of conduit evolution during small-volume, short-lived silicic eruptions.