Now showing 1 - 10 of 16
  • Publication
    Definition of a fault permeability predictor from outcrop studies of a faulted turbidite sequence, Taranaki, New Zealand
    Post-depositional normal faults within the turbidite sequence of the Late Miocene Mount Messenger Formation of the Taranaki basin, New Zealand are characterised by granulation and cataclasis of sands and by the smearing of clay beds. Clay smears maintain continuity for high ratios of fault throw to clay source bed thickness (c. 8), but are highly variable in thickness, and gaps occur at any point between the clay source bed cutoffs at higher ratios. Although cataclastic fault rock permeabilities may be significantly lower (c. 2 orders of magnitude) than host rock sandstone permeabilities, the occurrence of continuous clay smears, combined with low clay permeabilities (10's to 100's nD) means that the primary control on fault rock permeability is clay smear continuity. A new permeability predictor, the Probabilistic Shale Smear Factor (PSSF), is developed which incorporates the main characteristics of clay smearing from the Taranaki Basin. The PSSF method calculates fault permeabilities from a simple model of multiple clay smears within fault zones, predicting a more heterogeneous and realistic fault rock structure than other approaches (e.g. Shale Gouge Ratio, SGR). Nevertheless, its averaging effects at higher ratios of fault throw to bed thickness provide a rationale for the application of other fault rock mixing models, e.g. SGR, at appropriate scales.
  • Publication
    Localisation of normal faults in multilayer sequences
    Existing conceptual growth models for faults in layered sequences suggest that faults first localise in strong, and brittle, layers and are later linked in weak, and ductile, layers. We use the Discrete Element Method (DEM) for modelling the growth of a normal fault in a brittle/ductile multilayer sequence. The modelling reveals that faults in brittle/ductile sequences at low confining pressure and high strength contrast localise first as Mode I fractures in the brittle layers. Low amplitude monoclinal folding prior to failure is accommodated by ductile flow in the weak layers. The initially vertically segmented fault arrays are later linked via shallow dipping faults in the weak layers. Faults localise, therefore, as geometrically and kinematically coherent arrays of fault segments in which abandoned fault tips or splays are a product of the strain localisation process and do not necessarily indicate linkage of initially isolated faults. The modelling suggests that fault tip lines in layered sequences are more advanced in the strong layers compared to weak layers, where the difference in propagation distance is most likely related to strength and/or ductility contrast. Layer dependent variations in fault propagation rates generate fringed rather than smooth fault tip lines in multilayers.
      744Scopus© Citations 126
  • Publication
    Controls on Metal Distributions at the Lisheen and Silvermines Deposits: Insights into Fluid Flow Pathways in Irish-Type Zn-Pb Deposits
    The world-class Irish Zn-Pb(-Ag) deposits occur within one of the world’s major metallogenic provinces. While it has been well documented that these orebodies are structurally controlled, exactly how fluids migrated from source to trap is still poorly understood. Using 3-D modeling techniques, the current study investigates metal distribution patterns at the Silvermines and Lisheen deposits to gain insights into fluid pathways and structural controls on mineralization. Distinct points along segmented normal faults are identified as the feeders to individual orebodies, allowing hot, hydrothermal, metal-bearing fluids to enter host rocks and form orebodies. These points are characterized by highly localized and elevated Ag, Cu, Co, Ni, and As concentrations as well as low Zn/Pb ratios, which increase away from the feeders. Metal distributions are initially controlled by major and minor normal faults and subsequently affected by later oblique-slip dextral and strike-slip faults. High-tonnage areas without typical feeder signals are interpreted to be structural trap sites, which are distal to fault-controlled feeder points. This study highlights both the importance of a well-connected plumbing system for metal-bearing fluids to reach their basinal traps and the control that an evolving structural framework has on spatial distribution of metals.
      389Scopus© Citations 12
  • Publication
    The origin and nature of hydraulic fractures and veins within the Burren, County Clare, Ireland
    Carboniferous (Mississippian) limestones of the Burren are cross-cut by sub-vertical veins, from 1µm up to 50cm thick, defining a strongly clustered and scale-indepen-dent system in which predominantly N-S veins are transected by longer NNE-trending veins. Vein infills mainly comprise of calcite, but with subordinate amounts of quartz, sulphide (mainly galena and sphalerite) and fluorite also occurring, particularly in the south-central part of the area. Thinner and shorter veins are planar and discontinuous in plan view, sometimes forming en-echelon arrays, with thicker veins forming better connected and more complex structures which extend for several kilometres across the Burren region. Veins with ‘exotic’ infills are generally both longer and thicker, and they appear to be spatially associated with, or up to 5km to the north of, a 5km wide zone of ENE-trending Variscan monoclinal folding. Individual veins are vertically persistent, and the same structures are seen throughout the exposed ca 1200m thick Carboniferous sequence, from Tournaisian limestones through to Serpukhovian-Bashkirian siliciclastics. The veins are mainly extensional, sometimes with a component of sinistral displacement particularly on NNE-trending veins, displaying fibrous growth through to hydraulic fracturing and brecciation. Their formation is attributed to the valving of overpressured fluids within Mississippian basins during N-S Variscan compression. Pb isotope analysis supports a model in which sulphide infills are scavenged from underlying basement rocks or hydrothermal Zn-Pb mineralisation during the tectonic inversion of post-rift sequences overlying Lower Carboniferous normal faults.
      283Scopus© Citations 5
  • Publication
    Earthquake histories and Holocene acceleration of fault displacement rates
    (Geological Society of America, 2009-10) ; ; ;
    Displacement rates for normal and reverse faults (N = 57) are generally higher when averaged for the Holocene (~10 ka) than for the late Quaternary (~300 ka) and longer time scales. Holocene acceleration of displacement rates could be attributed to geological processes that produce increases of tectonic tempo. We propose an alternative model in which the observed rate changes arise from variability in earthquake slip and/or recurrence coupled with a sampling bias toward those faults that are best represented at the Earth’s surface and accrued displacement fastest during the Holocene. This model is consistent with displacement rates measured over time intervals of up to ~300 k.y. for 129 faults from the Taupo Rift, New Zealand. Departures of earthquake parameters and associated displacement rates from their long-term (>300 k.y.) averages are attributed to fault interactions and occur on time intervals inversely related to these long-term displacement rates and to regional strain rates.
  • Publication
    3-D Modeling of the Lisheen and Silvermines Deposits, County Tipperary, Ireland: Insights into Structural Controls on the Formation of Irish Zn-Pb Deposits
    (Society of Economic Geologists, 2019-02-01) ; ; ; ;
    Faults are important structures in the formation of many mineral deposits, often acting as conduits for ore-forming fluids and sometimes providing, or generating, the bounding structures to associated mineralizing sites. Using 3-D analysis and modeling of the Lisheen and Silvermines deposits within the Irish ore field, we investigate the geometry of normal fault systems and their implications on the origin and nature of associated deposits. These Irish-type deposits are carbonate hosted and developed within the hanging walls of normal faults arising from an Early Carboniferous episode of north-south rifting, with relatively limited amounts of later deformation. Structural analysis of high-quality mine datasets indicates that fault segmentation is ubiquitous with left-stepping segments arising from north-south stretching developed above generally ENE-NE-trending fault arrays, which are subparallel to older Caledonian penetrative fabrics and structure within underlying Silurian and Ordovician rocks. Fault segments occur on different scales and have a profound impact on structural evolution, with larger scale segments and intervening relay ramps defining distinct orebodies within deposits and smaller scale segments and relays potentially providing paths for upfault fluid flow. The difference in behavior is attributed to the integrity of associated relay ramps where intact ramps represent orebody-bounding structures, and smaller breached ramps provide enhanced associated hydraulic properties and act as vertical conduits. Hanging-wall deformation along the rheological boundary between host-rock limestones and underlying shales has an important control on the localization of earlier dolomitization and/or brecciation and later mineralization adjacent to this contact, and on the migration pathways for basinal brines and mineralizing fluids.
      535Scopus© Citations 16
  • Publication
    2D distinct element modeling of the structure and growth of normal faults in multilayer sequences : 1. Model calibration, boundary conditions, and selected results
    (American Geophysical Union, 2007) ; ;
    The distinct element method is used for modeling the growth of normal faults in layered sequences. The models consist of circular particles that can be bonded together with breakable cement. Size effects of the model mechanical properties were studied for a constant average particle size and various sample widths. The study revealed that the bulk strength of the model material decreases with increasing sample size. Consequently, numerical lab tests and the associated construction of failure envelopes were performed for the specific layer width to particle diameter ratios used in the multilayer models. The normal faulting models are composed of strong layers (bonded particles) and weak layers (nonbonded particles) that are deformed in response to movement on a predefined fault at the base of the sequence. The modeling reproduces many of the geometries observed in natural faults, including (1) changes in fault dip due to different modes of failure in the strong and weak layers, (2) fault bifurcation (splaying), (3) the flexure of strong layers and the rotation of associated blocks to form normal drag, and (4) the progressive linkage of fault segments. The model fault zone geometries and their growth are compared to natural faults from Kilve foreshore (Somerset, United Kingdom). Both the model and natural faults provide support for the well-known general trend that fault zone width increases with increasing displacement.
      822Scopus© Citations 62
  • Publication
    Geometrical analysis of the refraction and segmentation of normal faults in periodically layered sequences
    Normal faults contained in multilayers are often characterised by dip refraction which is generally attributed to differences in the mechanical properties of the layers, sometimes leading to different modes of fracture. Because existing theoretical and numerical schemes are not yet capable of predicting the 3D geometries of normal faults through inclined multilayer sequences, a simple geometric model is developed which predicts that such faults should show either strike refraction or fault segmentation or both. From a purely geometrical point of view a continuous refracting normal fault will exhibit strike (i.e. map view) refraction in different lithologies if the intersection lineation of fault and bedding is inclined. An alternative outcome of dip refraction in inclined multilayers is the formation of segmented faults exhibiting en échelon geometry. The degree of fault segmentation should increase with increasing dip of bedding, and a higher degree of segmentation is expected in less abundant lithologies. Strike changes and associated fault segmentation predicted by our geometrical model are tested using experimental analogue modelling. The modelling reveals that normal faults refracting from pure dip-slip predefined faults into an overlying (sand) cover will, as predicted, exhibit systematically stepping segments if the base of the cover is inclined.
      479Scopus© Citations 40
  • Publication
    2D distinct element modeling of the structure and growth of normal faults in multilayer sequences : 2. Impact of confining pressure and strength contrast on fault zone growth and geometry
    (American Geophysical Union, 2007) ; ;
    The growth of normal faults in periodically layered sequences with varying strength contrast and at varying confining pressure is modeled using the Distinct Element Method. The normal faulting models are comprised of strong layers (bonded particles) and weak layers (non-bonded particles) that are deformed using a predefined fault at the base of the sequence. The model results suggest that faults in sequences with high strength contrast at low confining pressure are highly segmented due to different types of failure (extension vs. shear failure) in the different layers. The degree of segmentation decreases as the strength contrast decreases and confining pressure increases. Faults at low confining pressure localize as extension (Mode I) fractures within the strong layers and are later linked via shallow dipping faults in the weak ones. This leads to initial staircase geometries that, with increasing displacement, cause space problems that are later resolved by splaying and segmentation. As confining pressure increases the modeled faults show a transition from extension to hybrid and to shear fracture and an associated decrease in fault refraction, with a consequent decrease in fault surface irregularities. Therefore the mode of fracture, which is active in the strong layers of a mechanical multilayer at a particular confining pressure, exerts an important control on the final fault geometry.
      424Scopus© Citations 50
  • Publication
    The impact of porosity and crack density on the elasticity, strength and friction of cohesive granular materials : insights from DEM modelling
    Empirical rock properties and continuum mechanics provide a basis for defining relationships between a variety of mechanical properties, such as strength, friction angle, Young’s modulus, Poisson’s ratio, on the one hand and both porosity and crack density, on the other. This study uses the Discrete Element Method (DEM), in which rock is represented by bonded, spherical particles, to investigate the dependence of elasticity, strength and friction angle on porosity and crack density. A series of confined triaxial extension and compression tests was performed on samples that were generated with different particle packing methods, characterised by differing particle size distributions and porosities, and with different proportions of pre-existing cracks, or uncemented grain contacts, modelled as non-bonded contacts. The 3D DEM model results demonstrate that the friction angle decreases (almost) linearly with increasing porosity and is independent of particle size distribution. Young’s modulus, strength and the ratio of unconfined compressive strength to tensile strength (UCS/T) also decrease with increasing porosity, whereas Poisson’s ratio is (almost) porosity independent. The pre-eminent control on UCS/T is however the proportion of bonded contacts, suggesting that UCS/T increases with increasing crack density. Young’s modulus and strength decrease, while Poisson’s ratio increases with increasing crack density. The modelling results replicate a wide range of empirical relationships observed in rocks and underpin improved methods for the calibration of DEM model materials.
      1790Scopus© Citations 230