bluuebunny/crossref_metadata_2025 huggingface dataset:
Phanerozoic strike-slip faulting in the continental interior platform of the United States: examples from the Laramide Orogen, Midcontinent, and Ancestral Rocky Mountains
Authors: Marshak S. , Nelson W. J. , McBride J. H.
Published: April 2008

The continental interior platform of the United States is that part of the North American craton where a thin veneer of Phanerozoic strata covers Precambrian crystalline basement. N- to NE-trending and W- to NW-trending fault zones, formed initially by Proterozoic/Cambrian rifting, break the crust of the platform into rectilinear blocks. These zones were reactivated during the Phanerozoic, most notably in the late Palaeozoic Ancestral Rockies event and the Mesozoic-Cenozoic Laramide orogeny — some remain active today. Dip-slip reactivation can be readily recognized in cross section by offset stratigraphic horizons and monoclinal fault-propagation folds. Strike-slip displacement is hard to document because of poor exposure. Though offset palaeochannels, horizontal slip lineations, and strain at fault bends locally demonstrate strike-slip offset, most reports of strike-slip movements for interior-platform faults are based on occurrence of map-view belts of en echelon faults and anticlines. Each belt overlies a basement-penetrating master fault, which typically splays upwards into a flower structure. In general, both strike-slip and dip-slip components of displacement occur in the same fault zone, so some belts of en echelon structures occur on the flanks of monoclinal folds. Thus, strike-slip displacement represents the lateral component of oblique fault reactivation; dip-slip and strike-slip components are the same order of magnitude (tens of metres to tens of kilometres). Effectively, faults with strike-slip components of displacement act as transfers accommodating jostling of rectilinear crustal blocks. In this context, the sense of slip on an individual strike-slip fault depends on block geometry, not necessarily on the trajectory of regional σ
1
. Strike-slip faulting in the North American interior differs markedly from that of southern and central Eurasia, possibly because of a contrast in lithosphere strength. Weak Eurasia strained significantly during the Alpine-Himalayan collision, forcing crustal blocks to undergo significant lateral escape. The strong North American craton strained relatively little during collisional-convergent orogeny, so crustal blocks underwent relatively small displacements.
Transform zones in the South Atlantic rifted continental margins
Authors: Mohriak W. U. , Rosendahl B. R.
Published: April 2008

Integration of seismic, potential field, and borehole data from conjugate basins along the South Atlantic continental margin, particularly the northeastern Brazilian and northwestern African segments, indicates that the rift architecture is controlled by fracture zones that extend from the oceanic crust and penetrate through the continental crust, locally corresponding to Precambrian structures in cratonic regions. The fracture zones may divide the continental margin into several compartments with independent sedimentary depocentres, separate crustal domains along oceanic transforms, and affect the rift architecture by shearing. Oceanic transform zones may leak igneous rocks originated from the mantle.

This work discusses conjugate sedimentary basins in the South Atlantic salt basins, particularly from Jacuípe to Sergipe-Alagoas on the Brazilian side, and from Gabon to Rio Muni on the African side. The following aspects are emphasized: (1) rift depocentres are controlled by border faults subparallel to the margin and by transverse faults that may continue as transform fractures in the oceanic crust; (2) the southernmost segment of the South Atlantic continental margins is characterized by Early Cretaceous volcanic rocks that underlie continental lacustrine Neocomian to Barremian syn-rift sediments; (3) the pre-rift sequences (Mesozoic and Palaeozoic sediments) that underlie the syn-rift depocentres in Gabon and Sergipe/Alagoas are mainly devoid of volcanics; (4) there is seismic evidence of magmatic underplating in the deeper portions of the continental crust, which are expressed by antiformal features locally aligned with transform fractures; (5) basement-involved extensional faults and volcanic activity along leaking transform faults are imaged along several conjugate segments of the margin, particularly along the equatorial margin (Romanche fracture zone); (6) in some segments of the divergent margin, the transition from outer rift blocks to oceanic crust is characterized by wedges of seaward-dipping reflectors with a possible origin associated with emplacement of oceanic ridges; (7) locally, the outermost rift blocks near the continental-oceanic crust boundary seem to be highly eroded by post-rift uplift caused by transform fault shearing or by magmatic underplating; (8) tectonomagmatic episodes climaxed in the Late Cretaceous/Early Tertiary in northeastern Brazil and extended to the Late Tertiary on the West African margin, forming large volcanic complexes along transverse lineaments that affect both oceanic and continental crust.
Tectonic stress in the Earth’s crust: advances in the World Stress Map project
Authors: Sperner B. , Müller B. , Heidbach O. , Delvaux D. , Reinecker J. , Fuchs K.
Published: April 2008

Tectonic stress is one of the fundamental data sets in Earth sciences comparable with topography, gravity, heat flow and others. The importance of stress observations for both academic research (e.g. geodynamics, plate tectonics) and applied sciences (e.g. hydrocarbon production, civil engineering) proves the necessity of a project like the World Stress Map for compiling and making available stress data on a global scale. The World Stress Map project offers not only free access to this global database via the Internet, but also continues in its effort to expand and improve the database, to develop new quality criteria, and to initiate topical research projects. In this paper we present (a) the new release of the World Stress Map, (b) expanded quality ranking schemes for borehole breakouts and geological indicators, (c) new stress indicators (drilling-induced fractures, borehole slotter data) and their quality ranking schemes, and (d) examples for the application of tectonic stress data.
3D analogue models of variable displacement extensional faults: applications to the Revfallet Fault system, offshore mid-Norway
Authors: Dooley T. , McClay K. R. , Pascoe R.
Published: April 2008

The Revfallet Fault system, Heidrun area, offshore mid-Norway, illustrates considerable variation in structural style along strike which is attributed to variation in coupling between basement and cover (separated by Triassic salt), due to the variable displacement along the fault system and to distinct phases of shallow level as well as basement-involved extension. Deformation in the southern part of the fault is partitioned over a number of cover-graben and associated with the development of a monoclinal flexure above the basement fault system. With an increase in displacement northwards, cover-graben become grounded on the pre-salt basement resulting in the rotation and breaching of the extensional drape fold and, eventually, partial and full coupling between basement and cover fault systems.

Analogue models were designed to simulate the 3D evolution of cover deformation above a variable displacement extensional fault separated from the cover by a ductile basal unit and to validate the structural interpretations and evolutionary models for the Revfallet Fault system. Analogue model results reveal the changing geometry along strike from a gentle extensional drape fold with minimal brittle deformation at low extension, to a breached drape fold system with significant rotation and collapse of the cover across the basement fault as displacement increases. Models conducted with a two-phase history — shallow extension (orthogonal and oblique) followed by basement-involved extension illustrate variable reactivation of cover-graben along strike during the basement-involved event, and collapse and rotation of the extensional footwall to the basement fault as displacement increases to a maximum. Coupling between cover faults and the basement fault only occurred where an oblique, precursor cover-graben cuts across the basement fault from hanging wall to footwall. Closer examination of the structural and stratigraphic relationships reveals major discrepancies between throw on cover and basement fault segments. The analogue model results compare favourably to seismic data from the Revfallet Fault system and validate the structural model for this fault system.
3D evolution of a pop-up structure above a double basement strike-slip fault: some insights from analogue modelling
Authors: Schellart W. P. , Nieuwland D. A.
Published: April 2008

In analogue modelling, pop-up structures have previously only been described in models of basement-involved strike-slip deformation with a stepover or restraining bend configuration. We describe the results of an analogue experiment with an alternative model configuration to investigate the structural development of pop-up structures. This model consists of a cover sequence of sand overlying two parallel running basement faults, which experience the same sense and the same amount of slip during deformation. The ratio of basement fault separation (1 cm) to overburden thickness (4 cm) is small. The experiment has been recorded in an X-ray tomograph, by which it was possible to investigate the 3D structural evolution of the model. During deformation, an elongated rhombic pop-up structure developed centrally above the basement faults. The faults bounding the pop-up structure displayed convex-upward as well as straight to concave-upward shapes. During deformation the topography above the basement faults increased to a maximum of
c.
4.8 mm, compared to a maximum subsidence of
c.
1.5 mm on both sides of the basement faults. During progressive deformation, local maximum horizontal principal stresses displayed a considerable rotation, from 45° in the initial stages, to (sub)parallel in a more advanced stage and finally back to
c.
45° to the strike of the basement faults.
Hanging wall accommodation styles in ramp-flat thrust models
Authors: Mulugeta G. , Sokoutis D.
Published: April 2008

In this paper we study the dynamic and rheologic control of hanging wall accommodation in ramp-flat thrust models. In particular we vary the dimensionless ratio of shear strength to gravity stress to model hanging wall accommodation styles in different materials. In all models we require that the flat-ramp-flat footwall provides a surface of low frictional resistance. In viscous materials hanging wall accommodation progresses by wedge flow. In Bingham materials, wedge flow is also the preferred mode in cases where the gravity stress exceeds the yield limit of materials. Such models simulate the flow of salt or snow glaciers above ramp obstructions. At high ratios of shear strength to gravity stress the hanging wall blocks translate forward without bending and unbending to the form of the rigid footwall. In elastic-plastic strain-hardening materials ramp-flat accommodation progresses by fault-bend folding in case there is a near balance between the yield stress and gravity stress. In frictional materials hanging wall accommodation progresses by shear or kink-band nucleation above fault-bends. The shear or kink-bands which initially nucleate at the lower fault-bend change shape and reactivate by normal faulting or tensile failure at the upper fault-bend, depending on the ratio of shear strength to gravity stress. In nature, hanging wall accommodation by thrust nucleation above ramps and their subsequent reactivation may be anticipated in frictional sediments at upper crustal levels, where temperatures and pressures are low.
3D discrete kinematic modelling applied to extensional and compressional tectonics
Authors: Cornu T. , Schneider F. , Gratier J. -P.
Published: April 2008

The 3D simulation of coupled backward and forward deformation of geological layers is a new step in basin modelling. Although this problem could be addressed with either mechanical or kinematic approaches, the mechanical approach remains too complex to be addressed properly. The kinematic model described here allows a geologically valid path, which takes into account an incremental evolution in time. To obtain a better description of 3D geometries, the model uses a full hexaedric discretization and the discrete neutral surface of each layer is used when performing the flexural slip deformation. An application to a synthetic geological case is then proposed, to study the behaviour of the structure in compressional and extensional contexts.
Experimental constraints on volatile abundances in arc magmas and their implications for degassing processes
Authors: Scaillet B. , Pichavant M.
Published: April 2008

Recent phase equilibrium studies, combined with analytical and petrological data, provide rigorous constraints on the pre-eruptive
P-T-f
H
2
O-
f
O
2
-
f
S
2
-
f
CO
2
conditions of silicic to mafic arc magmas. Pre-eruptive melts show a broad negative correlation between temperature and melt H
2
O contents. Pre-eruptive melt S contents cluster around 100 ppm in residual rhyolitic liquids of silicic to andesitic magmas, and range up to 5000 ppm in more mafic ones. For the entire compositional spectrum, melt sulphur contents are almost independent of prevailing
f
O
2
. In contrast, they are positively correlated to
f
S
2
, in agreement with experimental observations. Using these intensive constraints, the composition of coexisting fluid phases has been modelled through a MRK equation of state. Pre-eruptive fluids in silicic to andesitic magmas have
X
H
2
O (mole fraction of H
2
O) in the range 0.65–0.95.
X
H
2
O decreases as pressure increases, whereas
X
CO
2
increases up to 0.2–0.3. Pre-eruptive fluids in hydrous mafic arc magmas, such as high-alumina basalts, generally have similar mole fractions of H
2
O and CO
2
at mid-crustal levels, with
X
H
2
O increasing only for magmas stored at shallow levels in the crust (<1 kbar). The sulphur content of the fluid phase ranges from 0.12 up to 6.4 wt% in both mafic and silicic magmas. For silicic magmas coexisting with 1–5 wt% fluid, this implies that more than 90% of the melt+fluid mass of sulphur is stored in the fluid. Calculated partition coefficients of S between fluid and melt range from 17 up to 467 in silicic to andesitic magmas, tending to be lower at low
f
O
2
, although exceptions to this trend exist. For mafic compositions, the sulphur partition coefficient is constant at around 20. The composition of both melt and coexisting fluid phases under pre-eruptive conditions shows marked differences. For all compositions, pre-eruptive fluids have higher C/S and lower H/C atomic ratios than coexisting melts. Comparison between volcanic gas and pre-eruptive fluid compositions shows good agreement in the high temperature range. However, to reproduce faithfully the compositional field delineated by volcanic gases, silicic to andesitic arc magmas must be fluid-saturated under pre-eruptive conditions, with fluid amounts of at least 1 wt%, whereas mafic compositions require lower amounts of fluid, in the range 0.1–1 wt%. Nevertheless, volcanic gases colder than 700 °C are generally too H
2
O-rich and S-poor to have been in equilibrium with silicic to andesitic magmas under pre-eruptive conditions, which suggests that such gases probably contain a substantial contribution from meteoric or hydrothermal water.
Sulphur release from flood lava eruptions in the Veidivötn, Grímsvötn and Katla volcanic systems, Iceland
Authors: Thordarson T. , Self S. , Miller D. J. , Larsen G. , Vilmundardóttir E. G.
Published: April 2008

Emissions of SO
2
by volcanic eruptions have been shown to be important for short-term environmental and climate changes. Stratospheric sulphur mass-loading by explosive silicic eruption is commonly considered to be the principal forcing factor for these changes. The SO
2
emissions from basaltic flood lava eruptions have not featured strongly in the discussions on volcano-climate interactions, notwithstanding the fact that basaltic magma is typically richer in sulphur (by a factor of two to four), than silicic magmas, as well as the evidence of widespread atmospheric impact associated with historical flood lava eruption.

Fourteen Holocene flood lava eruptions are known from the Veidivötn, Grímsvötn, and Katla volcanic systems of the Eastern Volcanic Zone in South Iceland, which include the three largest of its kind in Iceland; the 1783–1784 Laki, 934–40 Eldgjá, and
c.
8600 years
BP
Thjórsá events. We present new data on the sulphur content in melt inclusions from the Veidivötn system and use this information, along with existing inclusion data from the Grímsvötn and Katla volcanic systems, to establish an empirical method for estimating the sulphur mass release from these basaltic flood lava eruptions. The results show that these eruptions released a total of
c.
700 Mt SO
2
into the atmosphere in four 600- to 850-year-long eruption periods. During each period, between 98 and 328 Mt SO
2
were emitted into the atmosphere, and the mass loadings from individual eruptions ranged from 5 to 210 Mt SO
2
. These flood lava eruptions are likely to have resulted in widespread atmospheric perturbations and, by analogy with the 1783–1784 Laki eruption, the effects of the largest eruptions may have been felt on a hemispheric scale.
Optical sensing of volcanic gas and aerosol emissions
Authors: McGonigle A. J. S. , Oppenheimer C.
Published: April 2008

Volcanic gas and aerosol surveillance yield important insights into magmatic, hydrothermal, and atmospheric processes. A range of optical sensing and sampling techniques has been applied to measurements of the composition and fluxes of volcanic emissions. In particular, the 30-year worldwide volcanological service record of the Correlation Spectrometer (COSPEC) illustrates the point that robust, reliable, straightforward optical techniques are of tremendous interest to the volcano observatory and research community. This chapter reviews the field, in particular the newer and more versatile instruments capable of augmenting or superseding COSPEC, with the aim of stimulating their rapid adoption by the volcanological community. It focuses on sensors that can be operated from the ground, since they generally offer the most flexibility and sensitivity. The success of COSPEC underlines the point, however, that such devices should be comparatively cheap, and easy to use and maintain, if they are to be widely used.
Fumarole migration and fluid geochemistry at Poás Volcano (Costa Rica) from 1998 to 2001
Authors: Vaselli O. , Tassi F. , Minissale A. , Montegrossi G. , Duarte E. , Fernández E. , Bergamaschi F.
Published: April 2008

We report the results of a geochemical survey of fumaroles, thermal springs, and gas discharges from areas in and around the active crater lake of Poás volcano (Costa Rica) from February 1998 to February 2001. The springs are highly acidic-sulphate waters with temperatures approaching boiling point, whereas gas chemistry is characterized by typical magmatic species, such as SO
2
, HF, HCl, H
2
, and CO. From February 1998 new fumarolic fields formed inside the southern part of the crater. They moved anticlockwise from the S to the NE inner walls of the crater, while those located in the southern part of the crater and close to the pyroclastic cone south of the crater lake diminished or disappeared altogether, during 1999 and 2000. This shift was also characterized by chemical variation of the magmatic gas species. In spite of the chemical changes of fumaroles, the composition of the lake changed little during this time. This fact, together with the chemical profile with depth of the lake, suggests that the lake is a very efficient condenser of magmatic fluids. An apparent chemical stratification of the lake suggests that dilution with meteoric water is not complete, due to the presence of liquid sulphur at the lake bottom and/or due to the continuous influx of new magmatic components.
Degassing processes of Popocatépetl and Volcán de Colima, Mexico
Authors: Varley N. R. , Taran Y.
Published: April 2008

To understand the behaviour of a volcano and try to predict future eruptive activity, it is imperative to investigate its degassing processes. Areas of study include direct degassing of the magma through the conduit, the interaction between the hydrothermal system and volcanic gases and fluids, and diffuse degassing through the volcanic edifice. Various aspects of the degassing processes of Popocatépetl and Volcán de Colima in Mexico have been studied. The results are summarized in this chapter, and new data on diffuse degassing at both volcanoes and analyses of fumarolic gases from Volcán de Colima are presented.

Variations in the flux of SO
2
at both volcanoes have led to models of the degassing processes. At Colima, isotopic analyses of condensates suggest a shallow degassing magma body. Volcán de Colima showed geochemical precursors prior to its 1998/9 eruption and variations in certain parameters continue to relate to the behavior of the volcano during its on-going eruption. Both volcanoes yield interesting data on their diffuse degassing of CO
2
and radon: at Popocatépetl, the large plume degassing contrasts with a small diffuse contribution. Volcán de Colima has anomalous concentrations of diffuse CO
2
associated with faults. The challenge is how to interpret the geochemical data to provide a fresh insight into the mechanics of the volcanic system.
Hydrocarbon occurrence and exploration in and around igneous rocks
Authors: Schutter Stephen R.
Published: April 2008

Hydrocarbons can occur within and around igneous rocks, sometimes in commercially significant quantities. Igneous or closely associated rocks can be hydrocarbon sources in the conventional sense (biotic) as well as possibly through abiotic processes. Maturation is extremely variable, depending on the extrusive/intrusive nature of the activity and the relative importance of a deep heat source. Igneous volatiles and hydrothermal fluids may also be important in mobilizing and moving hydrocarbons. Igneous rocks can have good reservoir qualities, and they can produce their own trapping structures as well as being part of a larger feature. Many exploration methods are individually unreliable in and around igneous rocks, and an integrated approach is most effective. Seismic, magnetotelluric, gravity and magnetic surveys may all provide helpful information. Geological mapping, geochemistry and remote imagery may also be helpful. Evaluation of potentially commercial hydrocarbon accumulations requires interpretation of well logs, which may have unusual characteristics. Drill stem and production tests may also be needed for evaluation before exploration ends and development begins.
Volcanic reservoir rocks of northwestern Honshu island, Japan
Authors: Magara Kinji
Published: April 2008

The main Japanese oil producing region lies on the Japan Sea side of northern Honshu island. Although the total reserve is small and production supplies only threetenths of a percent of total Japanese oil consumption, it has two distinguishing features: (1) the main reservoir rocks are volcanic, pyroclastic, or tuffaceous, and (2) primary oil and gas migration seems to have taken place downward from the overlying source rocks. Marine volcanic activity since 15 Ma formed the main reservoir sections together with significant secondary porosity development. Thick and continuous deposition of organic-rich shales and mudstones followed and lower parts of these fine-grained rocks became the main source rocks. The principal direction of primary hydrocarbon migration occurred vertically downward from them. These fine-grained rocks seem to have acted as pressure seals as well as capillary seals over the oil/gas saturated zones below.
Controls on primary porosity and permeability development in igneous rocks
Authors: Petford Nick
Published: April 2008

Some of the more important processes leading to the development of primary igneous porosity due to the cooling and crystallization of magma are reviewed. A distinction is made between volcanic and plutonic rocks, and crystalline and granular volcanic material. Porosity in each rock type is classified according to a proposed effective length scale and geometry into diffusive (Class D) and macroscopic flow (Class F) features. Estimated ranges in values of porosity and permeability are given for a wide selection of igneous rock types, and comparison is made with permeability variations (Δ
k
) derived for both the continental and oceanic crust. While fracture porosity is dominant in most crystalline materials, primary porosity development may play an important role in the final (total) porosity in igneous basement. Some types of primary porosity and permeability in igneous rocks will be strongly time- and scale-dependent due to thermal effects associated with the emplacement and cooling of magmas and volcanic material. Tectonic reworking of the primary petrophysical properties of basement-forming igneous rocks may be significant in the development of regions of anisotropy and enhanced porosity.
Characterization of rough-walled fractures in crystalline rocks
Authors: Ogilvie S. R. , Isakov E. , Taylor C. W. , Glover P. W. J.
Published: April 2008

The full characterization of the rough surfaces of fractures and their resulting apertures is an important step in the drive toward an improved understanding of the factors which control fluid flow through rocks. This is crucial in igneous and metamorphic rocks, since fractures in these rocks may form the only significant pathways for fluid migration. Here we describe a three-pronged approach for the full characterization of rough fracture surfaces in a selection of crystalline rocks using a suite of software developed in house. Firstly, profiling is carried out using an optical method, which converts images of epoxy fracture surfaces covered with dyed water into topographies using the Lambert-Beer Law. Many hardware and software (OptiProf™) developments give this method the upper hand over previous attempts at spectrophotometric analysis. It is not possible to profile every fracture surface, therefore numerical modelling of fluid flow is carried out using synthetic fractures with rough fracture surfaces that are representative of the natural rock fractures. ParaFrac™ allows the analysis and parametrization of fracture surfaces and apertures. SynFrac™ enables the numerical synthesis of fracture surfaces and apertures with prescribed basic parameters. Both procedures take full account of the complex matching properties of the fracture surfaces as a function of wavelength, as well as anisotropy within the properties defining the fracture surfaces and their resulting aperture. They have been rigorously tested on a large suite of synthetic fractures as well as real rock fractures. These tests have allowed relationships between the standard deviation of surface asperity heights, the fractal dimension and the matching parameters to be related to the resulting aperture of the fractures.
Thermally induced primary fracture development in tabular granitic plutons: a preliminary analysis
Authors: Koenders M. A. , Petford Nick
Published: April 2008

We present an analytical model that predicts some of the mechanical effects associated with the intrusion and subsequent cooling of a rectangular intrusion emplaced at a uniform temperature into elastic continental crust. Assuming an idealized geometry and initial conditions, we recover the temperature field and subsequent strain field as a function of both position and time. The strain field is particularly relevant as it provides information on the
primary
(cooling-related) fracture formation pattern and direction within and immediately surrounding the pluton. We find a large strain jump across the pluton-country rock contact, implying that fracture formation should be maximized at the edges and corners of the intrusion. The direction of the fractures is predominantly vertical within the pluton centre, but becomes progressively more inclined towards the pluton margin and into the adjacent country rock. Fracture orientation may depend critically on the geometry of the intrusion, in particular the ratio of the longest to shortest dimension
L
1
/
L
2
.
Applications of X-ray computed tomography in the geosciences
Authors: Mees F. , Swennen R. , Geet M. Van , Jacobs P.
Published: April 2008

X-ray computed tomography (CT) is a non-destructive technique with wide applications in various geological disciplines. It reveals the internal structure of objects, determined by variations in density and atomic composition. Large numbers of parallel 2D sections can be obtained, which allows 3D imaging of selected features. Important applications are the study of porosity and fluid flow, applied to investigations in the fields of petroleum geology, rock mechanics and soil science. Expected future developments include the combined use of CT systems with different resolutions, the wider use of related X-ray techniques and the integration of CT data with results of compatible non-destructive techniques.
Porosity and fluid flow characterization of granite by capillary wetting using X-ray computed tomography
Authors: Géraud Y. , Surma F. , Mazerolle F.
Published: April 2008

The porosity and transfer properties of a very low porosity material (granite) are measured. A new procedure is defined using a capillary test and X-ray computed tomography (CT) scanning. Injected volumes are very low, i.e. a few cm
3
for a sample volume of 1 dm
3
, using a fluid/rock ratio lower than 0.1%. This technique allows monitoring of the anisotropy of fluid flow during the test. Flow along the injection direction is higher than along the perpendicular direction. Saturation depends on the specific saturation of each mineral zone. Multiscale analysis allows defining the flow conditions as being controlled at both the mineral and the sample scale. Results indicate the specific role for various constituting parts of the material. High speed flow occurs in the crack network of K-feldspar, while the storage function is localized in the reaction zone forms by quartz and muscovite.
Characterization by X-ray computed tomography of water absorption in a limestone used as building stone in the Oviedo Cathedral (Spain)
Authors: Ruiz de Argandoña V. G. , Rodriguez-Rey A. , Celorio C. , Calleja L. , Suárez del Rio L. M.
Published: April 2008

Water plays a fundamental role in rock weathering processes. Its penetration and movement inside rocks greatly influences the nature and intensity of damage affecting building stones. X-ray computed tomography (CT) is a useful technique for non-destructive mapping of water penetration. For the reported study, CT was used to investigate the internal structure and water penetration patterns for the Piedramuelle Stone, a Cretaceous limestone used as building material in the Oviedo Cathedral (North of Spain).

X-ray CT provides good images of the internal structure of the samples: the sedimentary layering due to differences in composition and porosity is clearly seen, as well as other textural features. The movement and penetration rate of water was monitored during standard free absorption water tests. The images that were obtained show a clear difference between dry and wet zones in the interior of the sample. Water movement is related to the petrographic characteristics of the rock, mainly to layering which controls the direction of water penetration. Hounsfield Unit numbers provide a quantitative approach for assessing the penetration rate of water.
Estimation of porosity and hydraulic conductivity from X-ray CT-measured solute breakthrough
Authors: Anderson S. H. , Wang H. , Peyton R. L. , Gantzer C. J.
Published: April 2008

Heterogeneities are common in natural porous media and are present on different scales. Use of X-ray computed tomography (CT) may provide a tool for quantifying smallscale heterogeneities in porosity and hydraulic conductivity in porous media. Porosity and saturated hydraulic conductivity distributions were estimated using CT for a series of undisturbed soil core samples taken from a field site. CT measurements were collected during breakthrough experiments using an iodide tracer. Techniques were developed to estimate porosity and hydraulic conductivity from solute breakthrough data. Results were compared with bulk sample measurements. CT-measured porosity compared well with laboratory-measured porosity. Hydraulic conductivity estimated from CT methods slightly overestimated laboratory-measured values. These techniques provide a method to quantify the spatially variable porosity and hydraulic conductivity on a millimetre scale rather than on a core-averaged scale. Chemical transport through the soil was predicted using a finite element method for each core using the CT-measured soil properties. Comparisons between measured and predicted chemical transport suggest that small-scale heterogeneities cause departures between measured and simulated solute breakthrough curves, and that a smaller grid size may be needed to improve the simulation.
Industrial X-ray computed tomography studies of lake sediment drill cores
Authors: Flisch A. , Becker A.
Published: April 2008

An industrial X-ray computed tomography (CT) system was used for the study of sedimentary and deformation structures in weakly consolidated late Pleistocene and Holocene lake sediments. CT analysis revealed details of structures that could not be detected by X-ray radiography or visual core logging. Examples include sand dykes, dropstones and plant remains, which are potentially important in palaeoseismological and palaeoenvironmental research. The CT images also help to discriminate between drill core artefacts and natural structures. X-ray tomography was also used for the determination of physical properties, particularly for bulk density measurements based on calibrated grey-scale values.
Analysis of analogue models by helical X-ray computed tomography
Authors: Schreurs G. , Hänni R. , Panien M. , Vock P.
Published: April 2008

The aim of analogue model experiments in geology is to simulate structures in nature under specific imposed boundary conditions using materials whose rheological properties are similar to those of rocks in nature. In the late 1980s, X-ray computed tomography (CT) was first applied to the analysis of such models. In early studies only a limited number of cross-sectional slices could be recorded because of the time involved in CT data acquisition, the long cooling periods for the X-ray source and computational capacity. Technological improvements presently allow an almost unlimited number of closely spaced serial cross-sections to be acquired and calculated. Computer visualization software allows a full 3D analysis of every recorded stage. Such analyses are especially valuable when trying to understand complex geological structures, commonly with lateral changes in 3D geometry. Periodic acquisition of volumetric data sets in the course of the experiment makes it possible to carry out a 4D analysis of the model, i.e. 3D analysis through time. Examples are shown of 4D analysis of analogue models that tested the influence of lateral rheological changes on the structures obtained in contractional and extensional settings.

Supplementary material:
The nineteen movies referred to in the article are available at
https://doi.org/10.6084/m9.figshare.c.4788519
Tubular structures of northern Wairarapa (New Zealand) as possible examples of ancient fluid expulsion in an accretionary prism: evidence from field and petrographical observations
Authors: Ledésert Béatrice , Buret Christophe , Chanier Frank , Ferrière Jacky , Recourt Philippe
Published: April 2008

The Cape Turnagain area is located on the inboard portion of the Hikurangi subduction margin, on the northern Wairarapa coast of the North Island of New Zealand. A 4.5 km long coastal section of sea cliffs of Mio-Pliocene sediments contains numerous tubular carbonate-rich concretions. Their morphology and petrographical observations suggest they were possibly formed by fluid flows of carbonate-rich water through a silty sediment. These tubular concretions could be fossil fluid expulsion structures similar to dewatering chimneys described offshore in New Zealand. The external diameter of the concretions observed
in situ
reaches 60 cm and internal canal up to 4 cm. Up to four canals are encountered in a single concretion. A positive relationship is observed between the chimney size and the number of canals or cumulative diameter of canals, suggesting that the size of the concretion is a function of the fluid which flowed through the plumbing network. The increased number of tubular concretions in upper Miocene siltstones compared to overlying Pliocene strata could be linked to a compressive event that caused overpressuring and the expulsion of fluids through the sediment pile.
Fluidization structures produced by upward injection of sand through a sealing lithology
Authors: Hurst Andrew , Cartwright Joe , Duranti Davide
Published: April 2008

Subsurface and outcrop data are used to describe sand injectites, a group of genetically related features that includes sandstone dykes and sills, but also structures within depositional sand bodies. Fluidization is identified as the process by which sand is injected but we draw attention to the lack of constraints regarding fluidization velocity and fluid viscosity. Injectites are shown to develop between <10 m and 500 m below the seafloor. No relationship between depth of generation and injection geometry is found. Liquefaction of sand may produce sufficient excess pore fluid to create small sand injections during shallow burial. Large injectite bodies are identified on seismic data that may exceed 4 × 10
7
m
3
are unlikely to be related to sand liquefaction. The general validity of hydraulic fracture as the mechanism for seal failure and propagation of injections is questioned. The association between the formation of polygonal faults and sand injection provides one of several alternative mechanisms for seal failure. Multi-phase intrusion is proposed as a likely mechanism for the formation of large sand intrusions, both because of the cyclical nature of most of the process invoked in their formation, and the author’s own observations. Many of the processes of sand injection remain poorly constrained.
The geometry, growth and linkage of faults within a polygonal fault system from South Australia
Authors: Nicol A. , Walsh J.J. , Watterson J. , Nell P.A.R. , Bretan P.
Published: April 2008

Quantitative analysis of faults within a South Australian polygonal fault system, interpreted from a 3-D onshore seismic survey, provides a basis for establishing their growth and linkage histories. The geometric characteristics of faults are consistent with an origin arising from the gravitational instability of an underlying low-density, overpressured, mobile layer. Fault size populations have scale-bound, non-power-law properties reflecting the thicknesses of the faulted and mobile layers and the strongly connected nature of the system. The spatial distributions of faults reflect the localization of conjugate faults at the top of the mobile layer and the scale of fault-bounded polygons. Displacement variations on faults show marked decreases at or adjacent to the top of the mobile layer and attest to its active role in faulting. The wide range of fault strike directions provides numerous fault intersections with high intersection angles (≥ 60°) forming triple-junctions at which fault linkage and capture occurred. Fault linkage and capture is attributed to a simple model in which continued movement on faults which share a mutual footwall is favoured and hanging wall faults are deactivated. The model involves thickening of the mobile layer within the footwalls of faults and thinning and eventual grounding of the overlying sequence, within their hanging walls.
Polygonal faults in the Ormen Lange Field, Møre Basin, offshore Mid Norway
Authors: Stuevold Liv M. , Faerseth Roald B. , Arnesen Linn , Cartwright Joe , Möller Nicola
Published: April 2008

3D seismic and well data from the Ormen Lange Field, Mid Norway have been used to analyse the development of a system of polygonal faults affecting the Late Cretaceous-early Paleocene reservoir. These faults have the typical properties of polygonal fault systems recognized elsewhere in mainly fine-grained successions. They grew by upward propagation from the thick, shale-prone interval of the Late Cretaceous in the Møre Basin and were reactivated during the deposition of the Balder Formation. They have throws ranging from a few metres to 80 m, are typically 1–3 km in length and have highly irregular throw distributions along strike, mainly as a result of complex fault intersection geometries. The Ormen Lange Field is the first described example of polygonal faults that completely transect a major sandstone reservoir interval. The presence of these faults has important implications for the likely production behaviour of the field. Fault seal analysis shows that they are unlikely to form juxtaposition seals, except locally, but that they may have a significant risk for clay smear seals, particularly in the lower reservoir unit.
Normal faulting in chalk: tectonic stresses vs. compaction-related polygonal faulting
Authors: Hibsch C. , Cartwright J. , Hansen D.M. , Gaviglio P. , André G. , Cushing M. , Bracq P. , Juignet P. , Benoit P. , Allouc J.
Published: April 2008

This paper documents normal fault sets observed in chalks exposed in widely separated localities in the UK and France. These faults are characterized by having a wide range of strikes at any one locality, are developed entirely within the chalk succession and do not seem to interconnect to deeper or shallower structures. These structures may result from two different mechanisms: (1) complex polyphase deformational histories involving contrasting stress states; or (2) a single deformational phase in which the faults develop to accommodate compactional strains. Evidence is presented from microstructural and petrographic data to support the latter interpretation. In particular, the association of calcite and marcasite mineralizations with fracture surfaces and fault zones and textural observations relating flint occurrence to early fault formation point towards fault propagation at a very early stage of burial and compaction of the chalky sediments. An analogy is drawn between these outcrop-scale structures and polygonal fault systems at a larger scale recognised from seismic observations of chalk sequences deposited at passive continental margins. The origin of these structures may be related to syneresis at an early stage of deformation followed by pressure solution phenomena that may reactivate this early-inherited polygonal fault pattern until the present day.
The origin and development of joints in the Boom Clay Formation (Rupelian) in Belgium
Authors: Mertens J. , Vandenberghe N. , Wouters L. , Sintubin M.
Published: April 2008

A system of natural, vertical and mutual perpendicular joints is described in the clay pits of the Rupelian Boom Clay in the Antwerp area of North Belgium. Joints are the dominant discontinuity surfaces in the clay outcrops. Neither the stress evolution of the clay during burial and uplift nor the regional tectonic history can explain the tensional jointing. It is suggested that the negative horizontal stresses required for the joint formation in clays are caused by shrinkage of the clay when the formation was located near the surface. The origin of the loss of pore fluid in a several tens of metres thick clay layers remains unknown. The joints in the Boom Clay are a scarce field example of the possibility of lateral contraction of a clay layer, without involving tectonics or burial/uplift as an origin. When such a jointed clay layer is buried again, the presence of the vertical joints might offer pathways for fluid migration through a relatively impermeable layer. A general relationship between a maximal depth beneath which no tensional joints can occur and the cohesion of the clay has been derived. In the case of the Boom Clay this limiting depth is around 40–50 m. The time of the joint formation in the outcrop area is most probably late Oligocene/early Miocene. The burial history of the clay at a particular location can be used as a predictive tool for the presence or absence of tensional joints.
Pore pressure/stress coupling in Brunei Darussalam — implications for shale injection
Authors: Tingay Mark R. P. , Hillis Richard R. , Morley Christopher K. , Swarbrick Richard E. , Okpere Eugene C.
Published: April 2008

Shale dykes, diapirs and mud volcanoes are common in the onshore and offshore regions of Brunei Darussalam. Outcrop examples show that shale has intruded along both faults and tensile fractures. Conventional models of overpressure-induced brittle failure assume that pore pressure and total stresses are independent of one another. However, data worldwide and from Brunei show that changes in pore pressure are coupled with changes in total minimum horizontal stress. The pore pressure/stress-coupling ratio (Δσ
h
/ΔP
p
) describes the rate of change of minimum horizontal stress magnitude with changing pore pressure. Minimum horizontal stress measurements for a major offshore field where undepleted pore pressures range from normal to highly overpressured show a pore pressure/stress-coupling ratio of 0.59. As a consequence of pore pressure/stress coupling, rocks can sustain a greater increase in pore pressure prior to failure than predicted by the prevailing values of pore pressure and stress. Pore pressure/stress-coupling may favour the formation of tensile fractures with increasing pore pressure rather than reactivation of pre-existing faults. Anthropogenically-induced tensile fracturing in offshore Brunei supports this hypothesis.
Re-evaluation of mobile shale occurrences on seismic sections of the Champion and Baram deltas, offshore Brunei
Authors: Van Rensbergen Pieter , Morley Christopher K.
Published: April 2008

3D seismic data in the Baram and Champion delta provinces offshore Brunei show that regions thought to be occupied entirely by chaotic seismic data and conventionally interpreted as shale diapirs, are regions of dimmed, but coherent reflectivity. Such data indicate shale diapir masses are not present, instead dimming can be attributed to sediment intrusive complexes, overpressured fluids and gas clouds, or processing artefacts. In this way significant delta structures are masked on 2D seismic data, which are important to interpret delta tectonic evolution. The Middle Miocene-Recent Champion and Baram deltaic provinces are characterized by typical gravity tectonics-related structures. However, being situated on an active margin they are also affected by episodic development of contractional structures, which are located on older reactive shale bulges and result in inversion of motion on some growth faults. The emplacement of shale pipes, gas clouds and intrusive complexes is generally relatively late (Pliocene) in comparison with the underlying reactive diapirs (Late Miocene) and their emplacement events may be separated in time by several million years. Late overpressured systems may be related to phases of pore fluid pressure increase during or following periods of inversion tectonics, which resulted in phases of enhanced fluid migration in the basin, where fluids were either expelled laterally oceanwards, or vertically.