Browsing by Author "Mitchell, Thomas"

Now showing 1 - 4 of 4
  • Thumbnail Image
    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
    Fault intersection-related stress rotation controls magma emplacement at the Nevados de Chill´ an Volcanic Complex
    (2025) Espinosa Leal, Javier; Browning, John; Cembrano, José; Mitchell, Thomas; Rojas, Flavia; Moorkamp, Max; Griffith, W. Ashley; Meredith, Philip
    It has been suggested that fracture and fault intersections promote enhanced transport of fluids in the brittle crust by forming zones of increased permeability. However, the underlying mechanisms that control the emplacement of magma at fault intersections remain poorly understood. To better understand the relation between magma emplacement, volcano development and fault zone intersections, we examine the Nevados de Chillán Volcanic Complex (NChVC, 36.8°S) in the Southern Andean Volcanic Zone. The complex is thought to be located atop the intersection between two sets of NE-right lateral strike-slip faults and a seismically active regional scale NW-oriented inherited structure, also interpreted as a regional fault zone. We collected data on the orientation and frequency of tens of dykes and thousands of fractures, at the volcano scale, from representative outcrops using three-dimensional digital image correlation techniques, with images taken from Unmanned Aerial Vehicles (UAVs). We use these data to generate a conceptual model of the response of the different fracture sets to regional loads and the potential consequence in terms of magma emplacement. In our conceptual model, N-S to NW-SE striking fractures become reactivated by fault intersection-related local stress field rotations. This, in turn, favors NW-SE aligned magma emplacement, and the evolution of NW-SE aligned volcanoes. Our findings provide a mechanical explanation for rotated magma emplacement pathways, which do not necessarily require a transient stress state imposed by unlocking the megathrust.
  • Thumbnail Image
    Item
    Fault-fluid interaction in porphyry copper hydrothermal systems : faulted veins in radomiro Tomic, northern Chile
    (2019) Jensen, Erik; González, Gabriel; Faulkner, Daniel R.; Cembrano, José; Mitchell, Thomas
  • No Thumbnail Available
    Item
    Structural Evolution of a Crustal-Scale Seismogenic Fault in a Magmatic Arc: The Bolfin Fault Zone (Atacama Fault System)
    (2021) Masoch, Simone; Gomila, Rodrigo; Fondriest, Michele; Jensen, Erik; Mitchell, Thomas; Pennacchioni, Giorgio; Cembrano, Jose; Di Toro, Giulio
    How major crustal-scale seismogenic faults nucleate and evolve in crystalline basements represents a long-standing, but poorly understood, issue in structural geology and fault mechanics. Here, we address the spatio-temporal evolution of the Bolfin Fault Zone (BFZ), a >40-km-long exhumed seismogenic splay fault of the 1000-km-long strike-slip Atacama Fault System. The BFZ has a sinuous fault trace across the Mesozoic magmatic arc of the Coastal Cordillera (Northern Chile) and formed during the oblique subduction of the Aluk plate beneath the South American plate. Seismic faulting occurred at 5-7 km depth and <= 300 degrees C in a fluid-rich environment as recorded by extensive propylitic alteration and epidote-chlorite veining. Ancient (125-118 Ma) seismicity is attested by the widespread occurrence of pseudotachylytes. Field geologic surveys indicate nucleation of the BFZ on precursory geometrical anisotropies represented by magmatic foliation of plutons (northern and central segments) and andesitic dyke swarms (southern segment) within the heterogeneous crystalline basement. Seismic faulting exploited the segments of precursory anisotropies that were optimal to favorably oriented with respect to the long-term far-stress field associated with the oblique ancient subduction. The large-scale sinuous geometry of the BFZ resulted from the hard linkage of these anisotropy-pinned segments during fault growth.