Browsing by Author "Mitchell, Thomas M."

Now showing 1 - 6 of 6
  • No Thumbnail Available
    Item
    Along-strike architectural variability of an exhumed crustal-scale seismogenic fault (Bolfin Fault Zone, Atacama Fault System, Chile)
    (2022) Masoch, Simone; Fondriest, Michele; Gomila, Rodrigo; Jensen, Erik; Mitchell, Thomas M.; Cembrano, Jose; Pennacchioni, Giorgio; Di Toro, Giulio
    Fault zone architecture and its internal structural variability play a pivotal role in earthquake mechanics, by controlling, for instance, the nucleation, propagation and arrest of individual seismic ruptures and the evolution in space and time of foreshock and aftershock seismic sequences. Nevertheless, the along-strike architectural variability of crustal-scale seismogenic sources over regional distances is still poorly investigated. Here, we describe the architectural variability of the >40-km-long exhumed, seismogenic Bolfin Fault Zone (BFZ) of the intra-arc Atacama Fault System (Northern Chile). The BFZ cuts through plutonic rocks of the Mesozoic Coastal Cordillera and was seismically active at 5-7 km depth and <= 300 degrees C in a fluid-rich environment. The BFZ in-cludes multiple altered fault core strands, consisting of chlorite-rich cataclasites-ultracataclasites and pseudo-tachylytes, surrounded by chlorite-rich protobreccias to protocataclasites over a zone up to 60-m-thick. These fault rocks are embedded within a low-strain damage zone, up to 150-m-thick, which includes strongly altered volumes of dilatational hydrothermal breccias and clusters of epidote-rich fault-vein networks at the linkage of the BFZ with subsidiary faults. The strong hydrothermal alteration of rocks along both the fault core and the damage zone attests to an extensive percolation of fluids across all the elements of the structural network during the activity of the entire fault zone. In particular, we interpret the epidote-rich fault-vein networks and associated breccias as an exhumed example of upper-crustal fluid-driven earthquake swarms, similar to the presently active intra-arc Liquin similar to e-Ofqui Fault System (Southern Andean Volcanic Zone, Chile).
  • No Thumbnail Available
    Item
    Evidence of hydrothermal fluid circulation driving elemental mass redistribution in an active fault zone
    (2021) Dorsey, Matthew T.; Rockwell, Thomas K.; Girty, Gary H.; Ostermeijer, Giles A.; Browning, John; Mitchell, Thomas M.; Fletcher, John M.
    Important fault zone processes can be discerned from the characterization of fracture damage and chemical transformations associated with active seismic sources. To characterize the 2010 M7.2 El Mayor-Cucapah rupture zone, continuous samples of fault core and 23 samples of damaged rock were collected perpendicular to strike of the Borrego fault. Samples were analyzed for clay mineralogy, bulk geochemistry, and bulk and grain density from which porosities and volumetric strains were derived. Prior to the Borrego fault forming, the tonalitic pmtolith, containing chlorite, epidote, and titanite, was subjected to temperatures of similar to 330-340 degrees C during deuteric alteration. Rocks within the damage zone are partially pulverized and contain abundant cataclastic seams. Porosity and volumetric strain peak in zones 1.5 m-10.5 m from the core. Within these zones, losses in Ca and P mass, increases in Mg and Na mass, along with the conservation of Fe and Si mass are consistent with oxidizing acidic conditions at < 200 degrees C. Gains in LOI are attributed to increases in clay content. The above data support a model of Mg- and Na-rich oxidizing fluid circulation within the damage zone of the Borrego fault.
  • 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.
  • Thumbnail Image
    Item
    Implications of longitudinal ridges for the mechanics of ice-free long runout landslides
    (2021) Magnarini, Giulia; Mitchell, Thomas M.; Goren, Liran; Grindrod, Peter M.; Browning, John
    The emplacement mechanisms of long runout landslides across the Solar System and the formation mechanisms of longitudinal ridges associated with their deposits remain subjects of debate. The similarity of longitudinal ridges in martian long runout landslides and terrestrial landslides emplaced on ice suggests that an icy surface could explain both the reduction of friction associated with the deposition of long runout landslides and the development of longitudinal ridges. However, laboratory experiments on rapid granular flows show that ice is not a necessary requirement for the development of longitudinal ridges, which instead may form from convective cells within high-speed flows. These experiments have shown that the wavelength (S) of the ridges is 2-3 times the thickness (T) of the flow, which has also been demonstrated at field scale on a tens-of-kilometre martian long runout landslide. Here, we present the case study of the 4-km-long, ice-free El Magnifico landslide in Northern Chile which exhibits clear longitudinal ridges, and show for the first time on a terrestrial landslide that the S/T ratio is in agreement with the scaling relationship found for both laboratory rapid granular flows and a previously measured martian long runout landslide. Several outcrops within the landslide allow us to study internal sections of the landslide deposit and their relationship with the longitudinal ridges in order to shed light on the emplacement mechanism. Our observations include interactions without chaotic mixing between different lithologies and the presence of meters-sized blocks that exhibit preserved original bedding discontinuities. We associate these observations with fluctuations in stress, as they are qualitatively similar to numerically modelled rapid granular slides, which were suggested, to some degree, to be associated with acoustic fluidization. Our results suggest that 1) the mechanism responsible for the formation of longitudinal ridges is scale- and environment-independent; 2) while the internal structures observed do not necessarily support a mechanism of convective-style motion, their interpretation could also point to a mechanism of internal deformation of the sliding mass derived from pattern-forming vibrations. Our novel observations and analysis provide important insights for the interpretation of similar features on Earth and Mars and for discerning the underlying mechanisms responsible for the emplacement of long run out landslides
  • Thumbnail Image
    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.