http://hdl.handle.net/10197/2750 2013-05-21T12:28:57Z http://hdl.handle.net/10197/3869 Zn, Fe and S isotope fractionation in a large hydrothermal system Gagnevin, Damien; Boyce, Adrian; Barrie, Craig; Menuge, Julian; Blakeman, Robert The genesis of hydrothermal ore deposits is of crucial economic importance. This study investigates the extent, causes and consequences of zinc and iron isotope fractionation in a large hydrothermal system at the world-class Navan Zn–Pb orebody, Ireland. Large variations in Zn, Fe and S isotope compositions have been measured in microdrilled sphalerite (ZnS) at the millimetre scale. d66Zn and d56Fe display a well-defined positive correlation and both also correlate with d34S. These relationships represent the combined effects of kinetic Zn and Fe isotope fractionation during sphalerite precipitation, and S isotope variation through mixing of hot, metal-rich hydrothermal fluids and cool, bacteriogenic sulfide-bearing brines. Combined with S isotope data, d56Fe and d66Zn data on mine concentrates confirm that hydrothermal sulfide is a minor component of the overall deposit signature. Our data suggest that incoming pulses of metal-rich hydrothermal fluid triggered sulfide mineralisation, and that rapid precipitation of sphalerite from hydrothermal fluids will lead to strong kinetic fractionation of Zn and Fe isotopes at very short time and length scales, thereby limiting the use of Fe and Zn isotopes as exploration tools within deposits, but revealing the possibility of detecting new deposits from isotopically heavy Zn–Fe geochemical halos. 2012-07-01T00:00:00Z http://hdl.handle.net/10197/3787 Large-scale, linked drainage systems in the NW European Triassic: insights from the Pb isotopic composition of detrital K-feldspar Tyrrell, Shane; Souders, A. Kate; Daly, J. Stephen; Shannon, Patrick M.; Haughton, P. D. W. Pb isotopic data from K-feldspars in Middle Triassic (Anisian) sandstones in the Wessex Basin, onshore southwest UK, and the East Irish Sea Basin, some to the north, show that the same grain populations are present. This indicates that the drainage system (the “Budleighensis” River) feeding these basins originated from the same source/s, most probably the remnant Variscan Uplands to the south. Fluvial and aeolian sandstones have the same provenance, suggesting that if water- and wind-driven sands were originally derived from different sources, this has been obscured through reworking prior to final deposition. Significant recycling of feldspar from arkosic sandstones in earlier sedimentary basins can be ruled out. The provenance data agree with previous depositional models, indicating transport distances in excess of , with a drainage pattern that linked separate basins. This supports the idea that the regional fluvial system was driven by topography and episodic flooding events of sufficient magnitude to overcome evaporation and infiltration over hundred’s of kilometres. Importantly, this drainage system appears to have been isolated and independent from those operating contemporaneously to the northwest of the Irish and Scottish massifs, where the remnant Variscan Uplands apparently exerted no influence on drainage or sand supply. 2012-05-01T00:00:00Z http://hdl.handle.net/10197/3706 A late-Holocene climate record in stalagmites from Modrič Cave (Croatia) Rudzka, Dominika; McDermott, Frank; Suric, Masa Few terrestrial Holocene climate records exist from Southeastern Europe despite its important geographic position as a transitional climatic zone between the Mediterranean and mainland continental Europe. In this study we present new petrographic and stable isotope data for two Holocene speleothems from Modrič Cave, Croatia (44o15’N, 15o32’E), a coastal Adriatic site (120 metres inland). Modern meteorological and cave conditions have been monitored for two years to understand the links between the climate variability and the stable isotope time-series records in speleothems. Typical of a Mediterranean-type climate, a negative water balance exists between April and September, so that recharge of the aquifer is restricted to the winter months. The weighted mean δ18O of the rainfall is -5.96‰ (2σ =2.83), and the weighted mean D/H rainfall value is -36.83‰ (2σ = 19.95), slightly above the Global Meteoric Water Line (GMWL), but well below the Mediterranean Meteoric Water Line (MMWL). Modern calcite from the tops of each stalagmite exhibits δ18O values that are close to isotopic equilibrium with their respective drip water values. Unfortunately, the relatively young ages and low uranium contents (c. 50 ppb) of both stalagmites hamper the use of U-series dating. Radiocarbon dates have been used instead to constrain their chronology using a dead carbon correction. Aside from some Isotope Stage 3 material (c. 55 ka), both stalagmites were deposited during the late Holocene. Climatic conditions during the late Holocene are inferred to have been sufficiently wet to maintain stalagmite growth and any hiatuses appear to be relatively short lived. Inferred changes in the stalagmite diameters during deposition are linked to δ13C and δ18O variations, indicating alternating periods of drier and wetter conditions. Drier conditions are inferred for the late Roman Ages warm period and the mid-Medieval Warm Period (MWP). Wetter conditions are associated with the Little Ice Age period. 2012-01-01T00:00:00Z http://hdl.handle.net/10197/3705 The coupled δ13C-radiocarbon systematics of three late Glacial/early Holocene speleothems; insights into soil and cave processes at climatic transitions Rudzka, Dominika; McDermott, Frank; Baldini, Lisa M.; Fleitmann, Dominik; Moreno, Ana; Stoll, Heather The coupled δ13C-radiocarbon systematics of three European stalagmites deposited during the Late Glacial and early Holocene were investigated to understand better how the carbon isotope systematics of speleothems respond to climate transitions. The emphasis is on understanding how speleothems may record climate-driven changes in the proportions of biogenic (soil carbon) and limestone bedrock derived carbon. At two of the three sites, the combined δ13C and 14C data argue against greater inputs of limestone carbon as the sole cause of the observed shift to higher d13C during the cold Younger Dryas. In these stalagmites (GAR-01 from La Garma cave, N. Spain and So-1 from Sofular cave, Turkey), the combined changes in δ13C and initial 14C activities suggest enhanced decomposition of old stored, more recalcitrant, soil carbon at the onset of the warmer early Holocene. Alternative explanations involving gradual temporal changes between open- and closed-system behaviour during the Late Glacial are difficult to reconcile with observed changes in speleothem δ13C and the growth rates. In contrast, a stalagmite from Pindal cave (N. Spain) indicates an abrupt change in carbon inputs linked to local hydrological and disequilibrium isotope fractionation effects, rather than climate change. For the first time, it is shown that while the initial 14C activities of all three stalagmites broadly follow the contemporaneous atmospheric 14C trends (the Younger Dryas atmospheric 14C anomaly can be clearly discerned), subtle changes in speleothem initial 14C activities are linked to climate-driven changes in soil carbon turnover at a climate transition. 2011-08-01T00:00:00Z http://hdl.handle.net/10197/3698 Source separation on seismic data : application in a geophysical setting Moni, Aishwarya; Bean, Christopher J.; Lokmer, Ivan; Rickard, Scott This article gives a brief description of the Degenerate Unmixing Estimation Technique (DUET) and applies it in a geophysical setting. Source separation has not been fully addressed by geophysicists and is a crucial first step to locating simultaneous sources, which in turn helps with understanding the dynamics of the sources and their source mechanisms. DUET is applied to synthetic seismic signals. The source separation method works successfully to separate two contemporary explosive sources, and two simultaneous oblique tensile cracks in a 3D structural model of Mt Etna. The method is also applied to field recordings on Mt Etna from 2008. The method separates Long Period events from tremor, Long Period events from Volcano Tectonic events and different sources of tremor from each other. 2012-05-01T00:00:00Z http://hdl.handle.net/10197/3697 A first evaluation of the spatial gradients in δ18Orecorded by European Holocene speleothems McDermott, Frank; Atkinson, Tim; Fairchild, Ian J.; Baldini, Lisa M.; Mattey, David P. Oxygen isotope data for well dated Holocene speleothems from Europe have been compiled for the first time. The data were analysed at 1 ka time slices through the Holocene by taking averages of 50 year duration. After filtering the data to exclude high altitude, high latitude and sites proximal to the Mediterranean Sea, the data exhibit surprisingly tight linear correlations between speleothem O isotope values and longitude. The slope of the data on δ18O vs. longitude plots changes systematically from the early to the late Holocene, exhibiting a much steeper zonal gradient in the early Holocene. Changes in the isotope gradient through the course of the Holocene reflect both a gradual increase in δ18O in speleothems from the western margin of the transect and a simultaneous decrease in speleothem δ18O on the eastern end of the transect. These changes follow summer insolation trends through most of the Holocene, but show marked deviations from c. 4 ka to the present day. Steeper early Holocene zonal isotope gradients are attributed primarily to a combination of early Holocene warming in the west and intense convective rainfall over the European continent in summer time driven by high early Holocene summer insolation. Although the absolute δ18O values preserved in speleothems do not precisely reflect the equilibrium values with respect to the waters from which they are precipitated, the tight isotope-longitude correlations indicate that speleothems are reliable recorders of combined rainfall O isotope signals and air temperature. 2011-12-01T00:00:00Z http://hdl.handle.net/10197/3080 Earthquake histories and Holocene acceleration of fault displacement rates Nicol, Andrew; Walsh, John J.; Mouslopoulou, Vasiliki; Villamor, Pilar 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. 2009-10-01T00:00:00Z http://hdl.handle.net/10197/3058 Drainage reorganization during breakup of Pangea revealed by in-situ Pb isotopic analysis of detrital K-feldspar Tyrrell, Shane; Haughton, P. D. W.; Daly, J. Stephen The Pb isotopic composition of detrital K-feldspar grains can be rapidly measured using laser ablation MC-ICPMS. The feldspar Pb signal can survive weathering, transport and diagenesis, and careful targeting avoids problems with inclusions and alteration. As common Pb isotopic compositions show broad (100s km scale) variation across the continents, the method provides a powerful provenance tracer for feldspathic sandstones. Here we combine a new Pb domain map for the circum-North Atlantic with detrital feldspar Pb isotopic data for Triassic and Jurassic sandstones from basins on the Irish Atlantic margin. The Pb compositions reveal otherwise cryptic feldspar populations that constrain the evolving drainage pattern. Triassic sandstones were sourced from distant Archean and Paleoproterozoic rocks, probably in Greenland, Labrador and Rockall Bank to the NW, implying long (>500 km) transport across a nascent rift system. Later Jurassic sandstones had a composite Paleo- and Mesoproterozoic source in more proximal sources to the north (<150 km away). Little or no feldspar was recycled from Triassic into Jurassic sandstones, and the change in provenance is consistent with distributed, low relief Triassic extension in a wide rift, followed by narrower Jurassic rifting with more localised fault-controlled sediment sources and sinks. 2007-11-01T00:00:00Z http://hdl.handle.net/10197/3057 Sedimentology, sandstone provenance and palaeodrainage on the eastern Rockall Basin margin : evidence from the Pb isotopic composition of detrital K-feldspar Tyrrell, Shane; Souders, A. Kate; Haughton, P. D. W.; Daly, J. Stephen; Shannon, Patrick M. The Rockall Basin, west of Ireland, is a frontier area for hydrocarbon exploration but currently the age and location of sand fairways through the basin are poorly known. A recently developed provenance approach based on in-situ Pb isotopic analysis of single K-feldspar grains by laser ablation multi-collector inductively-coupled mass spectrometry (LA-MC-ICPMS) offers advantages over other provenance techniques, particularly when applied to regional palaeodrainage issues. K-feldspar is a relatively common, usually first-cycle framework mineral in sandstones and its origin is typically linked to that of the quartz grains in arkosic and sub-arkosic rocks. Consequently, in contrast to other techniques, the Pb-in-K-feldspar tool characterises a significant proportion of the framework grains. New Pb isotopic data from K-feldspars in putative Permo-Triassic and Middle Jurassic sandstones in Well 12/2-1z (the Dooish gas condensate discovery) on the eastern margin of the Irish Rockall Basin are reported. These data suggest that three isotopically distinct basement sources supplied the bulk of the K-feldspar in the reservoir sandstones and that the relative contribution of these sources varied through time. Archaean and early Proterozoic rocks (including elements of the Lewisian Complex and its offshore equivalents), to the immediate east, north-east and north of the eastern Rockall Margin, are the likely sources. More distal sourcelands to the north-west cannot be ruled out but there was no significant input from southern sources, such as the Irish Massif. These data, together with previously published regional Pb isotopic data, highlight the important role played by old, near and far-field Archaean – Proterozoic basement highs in contributing sediment to NE Atlantic margin basins. The Irish Massif appears to have acted as a significant, but inert, drainage divide from the Permo-Triassic to the Late Jurassic and hence younger, Avalonian and Variscan, sand sources appear to have been less important on the Irish Atlantic Margin. 2010-01-01T00:00:00Z http://hdl.handle.net/10197/3056 K-feldspar sand-grain provenance in the Triassic, west of Shetland : distinguishing first-cycle and recycled sediment sources? Tyrrell, Shane; Leleu, Sophie; Souders, A. Kate; Haughton, P. D. W.; Daly, J. Stephen Sandstone provenance studies can help constrain palaeogeographic reconstructions and ancient drainage system scales and pathways. However, these insights can be obscured by difficulties in geochemically distinguishing or adequately characterising potential sourcelands, or by failure to identify sedimentary recycling. Triassic basins west of Shetland accumulated ~2.5 km of sand-rich sediment. The Middle-Upper Triassic Foula Formation represents fluvial, aeolian and sabkha facies deposited in the northern interior of the Pangean supercontinent. Published U-Pb zircon geochronology and heavy mineral analysis suggest that these sandstones were derived from East Greenland. They contain significant fresh K-feldspar which is likely to be first-cycle and derived directly from its source. Pb isotopic analyses of individual K-feldspar sand-grains show a single, unradiogenic Pb population, consistent with the provenance indicated by U-Pb zircon geochronology. Archaean and Palaeo-Mesoproterozoic rocks – the Nagssugtoqidian Mobile Belt, the Lewisian Complex or equivalents - are the likely source, with terranes south of the Moine Thrust (Grampian, Caledonian and Variscan) ruled out by both the Pb and U-Pb data. However, it is not possible to distinguish between rift flank sources to the east and west, as both areas have similar crustal affinity and/or share the same tectonic history. It is possible that the sediment was derived from the West Shetland Platform and not from Greenland. The comparison of provenance signals from robust and less stable mineral phases provides a means of recognising sedimentary recycling. Robust zircon populations and less stable feldspar in Foula Formation sandstones concur in indicating the same source, suggesting that they are likely to be first-cycle. The Triassic sand supply can be contrasted with that in Upper Carboniferous (Namurian) basins in the north of England where a significant zircon population has no corresponding K-feldspar component. This zircon population is likely to have been recycled from Lower Palaeozoic greywackes from the Southern Uplands Belt or its along strike extension. 2009-11-01T00:00:00Z http://hdl.handle.net/10197/3034 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 Schöpfer, Martin P. J.; Childs, Conrad; Walsh, John J. 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. 2007-01-01T00:00:00Z http://hdl.handle.net/10197/3033 2D distinct element modeling of the structure and growth of normal faults in multilayer sequences : 1. Model calibration, boundary conditions, and selected results Schöpfer, Martin P. J.; Childs, Conrad; Walsh, John J. 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. 2007-01-01T00:00:00Z http://hdl.handle.net/10197/3032 Geometrical analysis of the refraction and segmentation of normal faults in periodically layered sequences Schöpfer, Martin P. J.; Childs, Conrad; Walsh, John J.; Manzocchi, Tom; Koyi, Hemin A. 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. 2007-02-01T00:00:00Z http://hdl.handle.net/10197/3031 Definition of a fault permeability predictor from outcrop studies of a faulted turbidite sequence, Taranaki, New Zealand Childs, Conrad; Walsh, John J.; Manzocchi, Tom; Strand, Julian; Nicol, Andrew; Tomasso, Mark; Schöpfer, Martin P. J.; Aplin, Andrew C. 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. 2007-01-01T00:00:00Z http://hdl.handle.net/10197/3030 Static and dynamic connectivity in bed-scale models of faulted and unfaulted turbidites Manzocchi, Tom; Walsh, John J.; Tomasso, Mark; Strand, Julian; Childs, Conrad; Haughton, P. D. W. A range of unfaulted and faulted bed-scale models with sheet-like bed geometries have been built and analysed in terms of static bed connectivity and fractional permeability assuming permeable sands and impermeable shales. The models are built using a new method which allows amalgamation ratio to be included explicitly as model input and this property, rather than net:gross ratio, is found to be the dominant control on inter-bed connectivity. The connectivity of faulted sequences is much more complex and is dominated by interactions of variables. A comprehensive modelling suite illustrates these results and highlights the extremely rare combinations of circumstances in which faulted sequences have lower connectivities than their unfaulted sedimentological equivalents, irrespective of whether fault rock properties are included or not. In general, models containing stochastically placed shale smears associated with each faulted shale horizon are better connected than if deterministic Shale Gouge Ratio cut-offs are applied. Despite the complex interactions between geological input and bed-scale connectivity, the flow properties of a system are controlled by only three geometrical, rather than geological, variables describing connectivity, anisotropy and resolution. If two different faulted or unfaulted systems have identical values of these three variables they will have the same flow properties. 2007-01-01T00:00:00Z http://hdl.handle.net/10197/3029 Fault displacement rates on a range of timescales Mouslopoulou, Vasiliki; Walsh, John J.; Nicol, Andrew 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. 2009-02-25T00:00:00Z http://hdl.handle.net/10197/3028 Calibrating fault seal using a hydrocarbon migration model of the Oseberg Syd area, Viking Graben Childs, Conrad; Sylta, Oyvind; Moriya, S.; Morewood, Nigel; Manzocchi, Tom; Walsh, John J.; Hermanssen, D. It is widely acknowledged that fault rock capillary properties are important in controlling the distribution of hydrocarbons in sedimentary basins, and methods exist for predicting the capillary seal capacity of prospect bounding faults. However, fault seal capacity is rarely incorporated into models of hydrocarbon migration. This paper presents the results of migration modelling of the Oseberg Syd area of the Viking Graben incorporating fault rock capillary properties. Seal capacity is calculated in the model as a function of Shale Gouge Ratio (SGR), i.e. the percentage shale in the sequence moved past a point on a fault. Over 3 000 model realisations were run for different SGR to fault seal capacity relationships and the calculated hydrocarbon distributions were compared with known distributions. Realisations were ranked according to the closeness of fit between model and actual oil-water contacts for 7 traps. The best-fit to all 7 traps was provided by realisations with significant seal capacity at SGR values greater than ca. 0.2; a value which is in agreement with an independently derived fault-by-fault calibration between SGR and seal capacity. The level of fill calculated for an individual trap is extremely sensitive to minor changes in the seal capacity relationship because it is controlled not only by the seal capacities of the faults that bound the trap, but also by the pattern of fill-spill of upstream traps. This sensitivity to minor changes in seal capacity introduces large uncertainties when fault seal capacity relationships are used in a predictive mode and emphasises the requirement for migration modelling in fault seal prospect evaluation. 2009-06-01T00:00:00Z http://hdl.handle.net/10197/3027 The impact of porosity and crack density on the elasticity, strength and friction of cohesive granular materials : insights from DEM modelling Schöpfer, Martin P. J.; Abe, Steffen; Childs, Conrad; Walsh, John J. 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. 2009-02-01T00:00:00Z http://hdl.handle.net/10197/3026 Reconciliation of contrasting theories for fracture spacing in layered rocks Schöpfer, Martin P. J.; Arslan, Arzu; Walsh, John J.; Childs, Conrad Natural and man-made brittle layers embedded in a weaker matrix and subjected to layer-parallel extension typically develop an array of opening-mode fractures with a remarkably regular spacing. This spacing often scales with layer thickness, and it decreases as extension increases until fracture saturation is reached. Existing analytical one-dimensional (1-D) 'full-slip' models, which assume that interfacial slip occurs over the entire length of the fracture-bound blocks, predict that the ratio of fracture spacing to layer thickness at saturation is proportional to the ratio of layer tensile to interface shear strength (T/s). Using 2-D discontinuum mechanical models run for conditions appropriate to layered rocks, we show that fracture spacing at saturation decreases linearly with decreasing T/s ratio, as predicted by 1-D models. At low T/s ratios (ca. <3.0), however, interfacial slip is suppressed and the heterogeneous 2-D stress distribution within fracture-bound blocks controls further fracture nucleation, as predicted by an existing 2-D 'fracture infill criterion'. The applicability of the two theories is hence T/s ratio dependent. Our models illustrate that fracture spacing in systems permitting interfacial slip is not necessarily an indicator of fracture system maturity. Fracture spacing is expected to decrease with increasing overburden pressure and decreasing layer tensile strength. 2011-04-01T00:00:00Z http://hdl.handle.net/10197/3025 Localisation of normal faults in multilayer sequences Schöpfer, Martin P. J.; Childs, Conrad; Walsh, John J. 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. 2006-05-01T00:00:00Z http://hdl.handle.net/10197/3024 Geometric and kinematic controls on the internal structure of a large normal fault in massive limestones : the Maghlaq Fault, Malta Bonson, Chris G.; Childs, Conrad; Walsh, John J.; Schöpfer, Martin P. J.; Carboni, Vincent The Maghlaq Fault is a large, left-stepping normal fault (displacement >210 m) cutting the Oligo-Miocene pre- to syn-rift carbonates of SW Malta. Two principal slip zones separate the deformed rocks of the fault zone from the undeformed wall rocks. Fault rocks derived from fully lithified, pre- to early syn-rift sediments comprise relatively continuous fine-grained veneers of cataclasite and localised fault-bound lenses of wall rock, occurring over a range of scales, which are commonly brecciated. The lenses result from the linkage of slip surfaces, the inclusion of asperities and the formation of Riedel shears within the fault zone. In contrast, fault rock incorporated from unlithified syn-rift sediments comprise relatively continuous veils of rock that deformed in a ductile manner. Anomalously thick parts of the fault zone with highly complex structure and content are associated with breached relay zones, branch-lines and bends; these structures represent progressive stages of fault segment linkage. The progressive evolution and bypassing of fault zone complexities to form a smoother and more continuous active fault surface, results in complex fault rock distributions within the fault zone. Segment linkage structures have high fracture densities which combined with their significant vertical extents suggest they are potentially important up-fault fluid flow conduits. 2007-02-01T00:00:00Z