Now showing 1 - 10 of 21
  • 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
    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.
    Scopus© Citations 24  662
  • 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
    Variability in the three-dimensional geometry of segmented normal fault surfaces
    Normal faults are often complex three-dimensional structures comprising multiple sub-parallel segments separated by intact or breached relay zones. Relay zones are classified according to whether they step in the strike or dip direction and whether the relay zone-bounding fault segments are unconnected in 3D or bifurcate from a single surface. Complex fault surface geometry is described in terms of the relative numbers of different types of relay zones to allow comparison of fault geometry between different faults and different geological settings. A large database of fault surfaces compiled primarily from mapping 3D seismic reflection surveys and classified according to this scheme, reveals the diversity of 3D fault geometry. Analysis demonstrates that mapped fault geometries depend on geological controls, primarily the heterogeneity of the faulted sequence and the presence of a pre-existing structure, as well as on resolution limits and biases in fault mapping from seismic data. Where a significant number of relay zones are mapped on a single fault, a wide variety of relay zone geometries occurs, demonstrating that individual faults can comprise segments that are both bifurcating and unconnected in three dimensions.
    Scopus© Citations 19  32
  • 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.
    Scopus© Citations 5  417
  • Publication
    Fault displacement rates on a range of timescales
    Displacements on tectonic faults primarily accrue during earthquakes at rates that vary through time. To examine the processes that underlie the temporal changes in fault displacement rates we analyse displacements and displacement rates for time periods from the present to 5, 10, 20, 300, 500, 1 000 and 5 000 kyr for 261 active reverse or normal faults from a worldwide dataset. Displacement rates depart from million-year average rates by up to three orders of magnitude with the size of these departures inversely related to fault length and the duration of the sample period. Short-term (≤ 20 kyr) displacement rates generally span a greater range on small faults than large, a feature which suggests more variable growth on smaller faults. Simple earthquake-slip modeling shows that variations in displacement rates require changes in both recurrence interval and slip per event and do not support the Characteristic-slip earthquake model. As long as fault system strain rates are uniform, displacement rates generally become constant over time periods between 20 - 300 kyr, with the length of time required to reach stability being inversely related to the regional basin-wide strain rates. Stable long-term displacements rates and fluctuations in earthquake recurrence intervals and slip arise, in part, due to fault interactions.
    Scopus© Citations 96  814
  • Publication
    Examples of fault steps controlling event migration in seismic swarms
    (Society of Exploration Geophysicists, 2022) ; ;
    This study provides spatiotemporal constraints on seismicity within fault zones and identifies key links between fault step and event migration. We show that event distributions in seismic swarms can image stepping geometries reminiscent of relay zones commonly observed along fault zones. Earthquake migration can propagate across steps, indicating a transfer of deformation, but can be obstructed by others. Preliminary quantitative results show that whether a step transfers or blocks deformation depends on the separation between the bounding segments relative to the maximum magnitude of the events. These findings support the importance of understanding the role of internal fault geometry on seismicity and show that high accuracy event locations provide a critical understanding of seismicity.
  • Publication
    Bed-parallel slip associated with normal fault systems
    Stretching of the Earth's upper crust is commonly accommodated by normal faulting, fault-related folding and/or fracturing such as veins and joints. However, an increasing number of outcrop-scale studies highlight that extension is also accompanied by bed-parallel slip (BPS). The identification of BPS surfaces is, however, challenging due to their localised nature within bedded host rock sequences, the absence of suitable slip markers, and the scale and resolution of both outcrop and seismic reflection data. Here, we present examples of BPS identified within extensional fault systems in sedimentary sequences and outline the nature, magnitude, segmentation, and spatiotemporal distribution of BPS surfaces. These constraints provide a basis for defining the principal structural controls on BPS development and its geometric and kinematic relationship to normal faulting. We conclude that BPS is a common feature within multi-layered host rock sequences, irrespective of their lithological and mechanical properties, and is kinematically associated with a broad range of fault-related deformation, including bed rotations, flexural-slip folding, and both tectonic and gravity-driven sliding. The presence of BPS within normal fault systems can increase the complexity of the host rock volumes and fracture arrays with potential implications on subsurface fluid flow and seismicity.
    Scopus© Citations 6  140
  • 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.
    Scopus© Citations 241  2007
  • 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.
    Scopus© Citations 18  477