bluuebunny/crossref_metadata_2025 huggingface dataset:
Oblique magnetic fabric in siderite-bearing pelitic rocks of the Upper Carboniferous Culm Basin, SW England: an indicator for palaeo-fluid migration?
Authors: De Wall Helga , Warr Laurence N.
Published: April 2008

A zone of siderite dominated magnetic fabrics is recognized within clastic argillaceous rocks of the southern part of the Upper Carboniferous Culm foreland basin of SW England. This zone was identified by measuring the anisotropy of magnetic susceptibility (AMS) before and after heat treatment of samples. A detailed investigation of a recumbent fold structure within this zone (at the well-known Crackington Haven locality) reveals the pre-folding nature of siderite formation. The restored
κ
max
axes of AMS-ellipsoids plot on a segment of a small circle, with a mean inclination of
c.
45° to the pole of the sedimentary bedding planes. This oblique magnetic fabric geometry is considered to reflect substrate-controlled siderite growth within a migrating fluid medium, which crystallized during diagenesis and the early stages of Variscan compression. The regional distribution of siderite growth, in combination with the directional information from the AMS, is discussed as an indicator for the palaeo-flow direction of diagenetic fluids within a foreland basin setting.
Three-dimensional geological models from outcrop data using digital data collection techniques: an example from the Tanqua Karoo depocentre, South Africa
Authors: Hodgetts D. , Drinkwater N. J. , Hodgson J. , Kavanagh J. , Flint S. S. , Keogh K. J. , Howell J. A.
Published: April 2008

Recent technological advances have made the collection of digital geological data from outcrops a realistic and efficient proposition. The world-class exposures of Permian basin-floor turbidite fans of the Tanqua depocentre, Karoo Basin, South Africa have been the focus of one such study. These outcrops are faulted at a subseismic scale (displacements of up to 40 m), with continuous exposures of up to 40 km in depositional dip and 20 km strike directions. Digital data collection has been undertaken using a variety of methods: differential global-positioning systems (DGPS) mapping, surveying using laser total station and laser rangefinders, ground- and helicopter-based digital photography and photogrammetry, and digital sedimentary outcrop logging as well as geophysical data from boreholes. These data have then been integrated into several 3-D geological models of the study area, built using a subsurface reservoir-modelling system. The integrated dataset provides insights into the stratigraphic evolution of a deep-water fan complex by allowing true 3-D analysis and interpretation of data collected in the field. The improved understanding of these deep-water fan systems will improve existing models of offshore analogues by enhancing understanding of geometries and trends not resolvable from existing offshore data and by identifying potential problematic areas for fluid flow. Initial results from the application of this approach have been successfully applied to the conditioning of stochastic geological models of a subsurface deep-water reservoir from the North Sea.
Building confidence in geological models
Authors: Bowden R. Andrew
Published: April 2008

Geological models are required because we do not have complete knowledge, in time or space, of the system of interest. Models are constructed in an attempt to represent the system and its behaviour based on interpretation of observations and measurements (samples) of the system, combined with informed judgement (expert opinion) and, generally, constrained for convenience by the limitations of the modelling medium. Geological models are inherently uncertain; broadly those uncertainties can be classified as epistemic (knowledge based) or aleatory (reflecting the variability of the system). Traditional, quantitative methods of uncertainty analysis address the aleatory uncertainties but do not recognize incompleteness and ignorance. Evidence-based uncertainty analysis provides a framework within which both types of uncertainty can be represented explicitly and the dependability of models tested through rigorous examination, not just of data and data quality, but also of the modelling process and the quality of that process. The inclusion of human judgement in the interpretation and modelling process means that there will be frequent differences of opinion and the possibility of alternative, inconsistent and/or conflicting interpretations. The analysis presented here uses evidence-based uncertainty analysis to formulate a complete expression of the geological model, including presentation of the supporting and the conflicting or refuting evidences, representation of the remaining uncertainties and an audit trail to the observations and measurements that underpin the currently preferred and the possible alternative hypotheses.
Beyond kriging: dealing with discontinuous spatial data fields using adaptive prior information and Bayesian partition modelling
Authors: Stephenson John , Gallagher K. , Holmes C. C.
Published: April 2008

The technique of kriging is widely known to be limited by its assumption of stationarity, and performs poorly when the data involve localized effects such as discontinuities or nonlinear trends. A Bayesian partition model (BPM) is compared with results from ordinary kriging for various synthetic discontinuous 1-D functions, as well as for 1986 precipitation data from Switzerland. This latter dataset has been analysed during a comparison of spatial interpolation techniques, and has been interpreted as a stationary distribution and one thus suited to kriging. The results demonstrate that the BPM outperformed kriging in all of the datasets compared (when tested for prediction accuracy at a number of validation points), with improvements by a factor of up to 6 for the synthetic functions.
Using prior subsidence data to infer basin evolution
Authors: White Nicky
Published: April 2008

Quantitative models, which predict the structural and thermal evolution of sedimentary basins and margins, can be used to extracted information from subsidence data derived from discrete and noisy stratigraphic records. Although many basin-modelling algorithms exist, most of them solve the forward problem and many assume that rifting is instantaneous. A 2-D optimization strategy, which calculates spatial and temporal variation of strain rate, is outlined. This general approach should help to elucidate the dynamical evolution of sedimentary basins but it also addresses three issues of interest to the hydrocarbon industry. First, the residual misfit between observed and predicted basin geometries allows competing structural and stratigraphic interpretations to be objectively tested. Second, the animated evolution of sedimentary basin and passive continental margins can be produced using the retrieved strain-rate tensor. Thirdly, spatial and temporal variations of strain rate control basal heat flow, which in turn constrains the temperature and maturation histories of the sedimentary pile. Here, a small selection of 2-D results are presented and the basis of a 3-D formulation is described.
Types of permeable fractures in granite
Authors: Sausse J. , Genter A.
Published: April 2008

This study presents a multidisciplinary approach to understanding and describing types of fracture permeability in the Soultz-sous-Forêts granite, Upper Rhine Graben. At Soultz, during the 1993 stimulation tests in the GPK1 well, it was shown that only a limited number of natural fractures contributed to flow, whereas there are thousands of fractures embedded within the massive granite. In order to understand the flow hierarchy, a detailed comparison between static (fracture apertures based on ARI raw curves) and dynamic data (hydraulic tests) was carried out. We propose that two scales of fracture networks are present: a highly connected network consisting of fractures with small apertures that may represent the far-field reservoir, and another network that contains isolated and wide permeable fractures (that produce an anisotropic permeability in the rock) and allows a hydraulic connection between the injection and production wells.
Natural fracturing and petrophysical properties of the Palisades dolerite sill
Authors: Goldberg D. , Burgdorff K.
Published: April 2008

This investigation of naturally occurring fractures in the mafic rocks of the Palisades dolerite sill characterizes the porosity of this crystalline rock sequence, and yields a method of determining the
in situ
porosity when complete down-hole information is not available. Two holes, 229 m and 305 m deep, were drilled 450 m apart through the sill and into the underlying Triassic sediments of the Newark Basin. Both holes were logged with geophysical tools, including the acoustic borehole televiewer (BHTV), to identify intervals of high porosity, fracturing, and potential zones of active fluid flow. Using the BHTV data, 96 and 203 fractures were digitally mapped within the sill in Well 2 and Well 3, respectively. Most fractures dip steeply (76–78°). There is a shift in fracture orientation between Well 2 and Well 3, although the lithology of the sill is continuous. The dolerite penetrated in both holes is fresh and unaltered, and intersects a 7-m thick olivine-rich layer about 15 m above the bottom of the sill. Several fractures identified in the sill have large apparent aperture (>6 cm) that correspond to high-porosity zones (6–14%), measured from both resistivity and neutron logs in Well 2. We use a relationship between porosity and apparent fracture aperture in Well 2 to infer the porosity in Well 3. This correlative method for estimating porosity may be applicable between holes in other crystalline rock environments where down-hole log data are incomplete. Changes in the temperature gradient log also indicate active fluid flow, although flow appears to be most active in fractured and high-porosity zones in the sediments.
Brittle fracture in two crystalline rocks under true triaxial compressive stresses
Authors: Haimson B. , Chang C.
Published: April 2008

We employed our new polyaxial cell to carry out true triaxial compression tests on dry (jacketed) rectangular prisms of two crystalline rocks, in which different magnitudes of the least and intermediate principal stresses σ
3
and σ
2
were maintained constant, and the maximum stress σ
1
was increased to its peak level in strain control. Both Westerly granite (Rhode Island, USA) and KTB amphibolite (Bohemian Massif, Germany) revealed similar mechanical behaviour, much of which is missed in conventional triaxial tests in which σ
2
= σ
3
. Compressive failure in both took the form of a main shear fracture, or fault, steeply dipping in the σ
3
direction. Compressive strength rose significantly with the magnitude of σ
2
, suggesting that the commonly used Mohr-type strength criteria, which ignore the σ
2
effect, predict only the lower limit of rock strength. The true triaxial strength criterion for each of the crystalline rocks can be expressed as the octahedral shear stress at failure as a function of the mean normal stress acting on the fault plane. We found that the onset of dilatancy increases considerably for higher σ
2
. Thus, σ
2
extends the elastic range for a given σ
3
and, hence, retards the onset of the failure process. The main fracture dip angle was found to increase as σ
2
rises, providing additional confirmation of the strengthening effect of σ
2
. SEM inspection of the micromechanics leading to specimen failure showed a multitude of stress-induced microcracks localized on both sides of the through-going fault. Here too the effect of σ
2
is noted, in that microcracks gradually align themselves with the σ
1
-σ
2
plane as the magnitude of σ
2
is raised.
Shear-wave anisotropy from dipole shear logs in oceanic crustal environments
Authors: Iturrino G. J. , Goldberg D. , Glassman H. , Patterson D. , Sun Y.-F. , Guerin G. , Haggas S.
Published: April 2008

The deployment of a down-hole dipole shear sonic tool in Hole 395A and Hole 735B marked the first two opportunities to measure high-resolution shear-wave velocity and

V
S

anisotropy profiles in oceanic crustal rocks. In Hole 395A near the Kane Fracture Zone, dipole sonic logs were recorded from 100–600 mbsf, and allow azimuthal anisotropy to be determined as a function of depth in the crust. The magnitude of

V
S

anisotropy varies with depth, from less than 3.2% in low-porosity flows at the bottom of the hole, to approximately 15.5% in highly fractured pillow basalts and breccias. The orientation of the fast

V
S

direction also varies over depth, with a mean value between 75°N and 80°E, and aligns with the strike of steeply dipping structures observed by down-hole electrical and acoustic images. This fast

V
S

angle orientation is locally oblique to the plate-spreading direction and to the Mid-Atlantic Ridge axis. In Hole 735B, drilled near the Atlantis Fracture Zone, dipole sonic logs from 23 to 596 mbsf indicate that

V
S

anisotropy varies with depth, with averages of 5.3% in the foliated and deformed gabbros recovered at the bottom of the hole; 4.5% in undeformed olivine and oxide-rich gabbros around 300 mbsf; and 6.8% in highly deformed mylonitic zones at shallow depths. The fast

V
S

angle also varies with depth, giving a mean orientation of approximately S45°E for well-resolved estimates in the upper interval of the hole. This direction aligns with the strike of steeply dipping fractures observed by down-hole imaging, and is locally oblique to the Southwest Indian ridge axis. Although the effects of regional stresses and local deformation of these holes may introduce anisotropy in the dipole sonic data, we conclude that crustal morphology in the vicinity of the holes contributes significantly to the magnitude and orientation of

V
S

anisotropy.
Electrical properties of slow-spreading ridge gabbros from ODP Hole 1105A, SW Indian Ridge
Authors: Einaudi F. , Pezard P. A. , Ildefonse B. , Glover P.
Published: April 2008

Physical properties of gabbroic samples from Ocean Drilling Program Hole 1105A were measured in the laboratory, with a particular emphasis on the analysis of electrical properties. This data-set includes the major lithologies sampled in ODP Hole 1105A: gabbros, olivine gabbros, oxide-rich gabbros, and, for all rock types, different ranges of alteration were sampled: from fresh to highly altered. All these lithologies correspond to the seismic Layer 3 layer of the oceanic crust, and large-scale geophysical data interpretation requires a complete understanding of the physical properties of rocks in this section. Electrical conductivities measured on brine-saturated gabbros reveal strong excess conductivity for samples rich in oxide minerals and, to a lesser extent, for altered samples. However, the classical models do not explain the excess conductivity reported in the oxide-rich samples when saturated with brine. The electrical conduction via electronic processes in metallic minerals has been taken into account in our analysis of the electrical properties. The oxide minerals’ contribution has been independently estimated through measuring dry electrical resistivity. These measurements allowed quantification of the electronic conduction, which can reach 80% of the full conductivity for the most oxide-rich gabbros.
Non-invasive characterization of fractured crystalline rocks using a combined multicomponent transient electromagnetic, resistivity and seismic approach
Authors: Meju M. A.
Published: April 2008

DC resistivity and seismic-refraction methods are well established in basement studies, and the multicomponent transient electromagnetic (TEM) method is also emerging as an effective tool for locating electrically conductive zones in crystalline rock underneath thick overburden. It can be expected that combining these three techniques will furnish a powerful non-invasive approach for characterizing fractured crystalline rocks. The aims of this paper are twofold, namely: (1) to establish the
in situ
geophysical signature of fracture-zones in weathered crystalline rock masses using the TEM profiling method, and (2) determine the resistivity and seismic-velocity relationship in the zones of fractured crystalline rock evinced by TEM profiling. Multicomponent TEM data from terrains with varying degrees of weathering are examined in this paper. The TEM response over a deeply weathered fracture zone in granite is found to be band-limited, with a consistent pattern of vertical voltage response (
V
z
) at early times enabling accurate location of the wet fracture zone which manifests as a steep conductive feature in 2D DC resistivity imaging. Depending on the thickness of the weathered mantle and the measurement bandwidth, the composite
V
z
signature consists of three parts: a high amplitude response with a single peak centred on the target fracture zone at very early times; a near invariant or slowly decreasing amplitude response across the fracture zone at some intermediate time; and a marked low-amplitude response at the centre of the fracture zone with higher amplitudes near the flanks (i.e. twin peaks) at later times. The across-strike voltage response (
V
x
) is diagnostic of conductive fracture zones and exhibits a marked sign reversal. The result of 2D inversion of resistivity, magnetotelluric and seismic-refraction recordings over an intensively fractured granodiorite suggests that resistivity (ρ) and compressional-wave velocity (
V
P
) can be related in the form log
10
ρ = m log
10
V
P
+
c
, where
m
and
c
are constants.
Non-linearity in multidirectional P-wave velocity: confining pressure behaviour based on real 3D laboratory measurements, and its mathematical approximation
Authors: Přikryl R. , Klíma Karel , Lokajíček T. , Pros Z.
Published: April 2008

Experimental laboratory measurements of P-wave velocity confirm the superposition of linearity over non-linearity by a progressive increase in confining pressure. The increase in confining pressure diminishes the influence of microcracks that are partly or totally closed. At a certain stress level, the trend of P-wave velocity with applied confining pressure approaches that of a solid without cracks, and a linear increase in elastic wave velocity occurs under high confinement.

Several studies have focused on the problem of mathematical approximation of this phenomenon (Carlson & Gangi 1985; Wepfer & Christensen 1991; Greenfield & Graham 1996; Meglis
et al.
1996). Although successful within a certain degree of error, they provide neither a multidirectional solution nor the comparison of results with rock fabric. In this study, an analytical relation was applied to describe the P-wave velocity-confining pressure behaviour of quasi-isotropic rocks (granites) and their anisotropic equivalents (orthogneisses). Two parameters of this relationship reflect the elastic properties of the rock matrix, and two others are related to the presence of microcracks, their density and genesis. The results of a mathematical approximation of the P-wave velocity-confining pressure behaviour show a favourable correlation to the measured data-sets. Comparison of individual fitted parameters with the rock fabric provides an improved understanding of the material’s mechanical behaviour.
The
14
C-polymethylmethacrylate (PMMA) impregnation method and image analysis as a tool for porosity characterization of rock-forming minerals
Authors: Oila E. , Sardini P. , Siitari-Kauppi M. , Hellmuth K.-H.
Published: April 2008

Accurate knowledge of porosity is essential for understanding the links between basic petrophysical parameters, such as diffusion coefficients, permeability and conductivity. Standard methods used to determine porosity quantify the bulk porosity and the distribution of pore sizes. Crystalline rocks are rarely monomineralic, and the porosity of polyphasic rocks is considered heterogeneous at the mineral grain scale. Calculation of bulk petrophysical parameters must take into account porosity and mineral-phase microstructures, as well as connectivity. The polymethylmethacrylate (PMMA) method uses radioactively (
14
C)-labelled methylmethacrylate (
14
C-MMA) liquid to impregnate the rock sample, which is then polymerized by irradiation, cut and autoradiographed. Porosity is quantified by digitizing the autoradiograph and subsequent densitometry. Staining of the same rock surface uses chemical agents that rapidly reveal the primary minerals of unaltered and altered crystalline rocks: mainly quartz, K-feldspar, plagioclase and dark minerals. The images of PMMA autoradiographs and stained rock surfaces are combined to quantify mineral-specific porosities.

The methodology has been applied here to a granite core from Palmottu (central Finland) representing coarse-grained granite adjacent to a potential water-conducting fracture. Imaging of the porosity relative to mineralogy is presented and complemented by mineral specific porosities.
Interactions between glaciers and permafrost: an introduction
Authors: Harris Charles , Murton Julian B.
Published: April 2008

A consideration of the interactions between glaciers and permafrost is essential to many environmental studies of cold regions. This paper reviews how concepts, field data and experimental studies from glaciology and geocryology can provide a basis for improved understanding of some of the key glacier-permafrost interactions at scales ranging from continental ice sheets to small proglacial streams. Glacitectonic processes are strongly influenced by water pressures beneath subglacial and proglacial permafrost, and by the amount of unfrozen water within the permafrost. Burial of glacier ice and growth of intrasedimental ice also occur within sub- and proglacial permafrost, and together they produce a complex assemblage of ground ice in glaciated frozen lowlands. In mountain regions, rock glaciers are associated with the presence of ground ice, and represent a landform that stradles the semantic fence separating glacial features from permafrost features. In proglacial and ice-marginal environments, geomorphological activity reflects the combined effects of glacially- and periglacially-conditioned processes operating synchronously in adjacent areas, or in succession; such activity includes the transport of sediment in glacierized catchments and the calving of glacial ice in ice-marginal lakes. Interaction between permafrost and glacial phenomena depends largely on their proximity: where permafrost occurs close to glaciers, the thermal regime of the active layer is influenced in part by the surface covering of adjacent glacial ice through its effect on albedo and ground heat flux. Glacier-permafrost interactions are particularly important in recently deglaciated terrain, where permafrost may be aggrading. This ‘paraglacial’ zone often shows rapid geomorphological change. Thus, glacier-permafrost interactions are complex and occur over a wide range of temporal and spatial scales.
The interaction of a surging glacier with a seasonally frozen foreland: Hagafellsjökull-Eystri, Iceland
Authors: Bennett Matthew R. , Huddart David , Waller Richard I.
Published: April 2008

This paper describes aspects of the landform-sediment assemblage produced by a recent surge of Hagafellsjökull-Eystri. This surge occurred during the winter and early spring of 1998/1999 and consequently advanced into a partially frozen foreland. Two aspects of this landform-sediment assemblage are considered. First the evidence for a frozen subglacial sediment layer beneath a lateral piedmont lobe formed during the surge is reviewed. This sediment layer consists of blocks of glacier ice set within a matrix of frozen sediment and was injected into basal crevasses to form a network of crevasse-squeeze ridges prior to freezing. The sediment layer appears to provide evidence of subfreezing deformation at the termination of the surge. Secondly the paper examines the detailed tectonic facies within a push-moraine formed along the eastern latero-frontal margin of the glacier during the surge. Architecturally this push moraine consists of a multi-layered slab of glaciofluvial sediments with a monocline structure that has been displaced laterally by the advancing ice margin. The sediment slabs within this monocline are characterized by both brittle and ductile styles of deformation. The authors argue that the observed variation in deformation style may be explained by spatial variation in the extent to which the glacial foreland was frozen or unfrozen at the time of displacement. Areas of frozen foreland would have behaved in a brittle fashion, while unfrozen areas deformed in a more ductile manor. Both these aspects of the landform-sediment assemblage examined in this paper appear to be the product of the seasonal timing of the surge. Not only do they add to our understanding of surge-type landsystems, but they also illustrate the potential of winter advances around the margins of some temperate glaciers to explore the coupling between glaciers and frozen proglacial sediments.
Glacier-permafrost hydrological interconnectivity: Stagnation Glacier, Bylot Island, Canada
Authors: Moorman Brian J.
Published: April 2008

The complex thermal structure in areas where polythermal glaciers and continuous permafrost are present increases the potential for direct linkages between subsurface water conduits within glaciers and permafrost. In this study, hydrologic features of a glacier and the surrounding ice-cored moraines were examined and the potential for englacial water to flow out of the glacier and into the moraine was investigated. Ground-penetrating radar investigations, dye trace tests and direct observations of hydrological features (e.g., moulins, springs and caves) on and around Stagnation Glacier on Bylot Island, Arctic Canada, were undertaken. Data reveal that englacial conduits extend from the glacier into the adjacent ice-cored moraine. Glacial meltwater may have experienced variable flow conditions over the last 10 years and the conduit closures have occurred over a much longer time period. The study illustrates the interconnectivity of the glacial and permafrost hydrological systems.
Cryological processes implied in Arctic proglacial stream sediment dynamics using principal components analysis and regression
Authors: Irvine-Fynn T. D. L. , Moorman B. J. , Sjogren D. B. , Walter F. S. A. , Willis I. C. , Hodson A. J. , Williams J. L. M. , Mumford P. N.
Published: April 2008

In high latitudes, recent research has demonstrated that both thermo-erosion and temperature dependence influence sediment release into fluvial systems. An analysis of proglacial suspended sediment concentration (SSC) dynamics is presented for three glacierized basins: cold-based Austre Brøggerbreen (Svalbard), polythermal Midre Lovénbreen (Svalbard) and polythermal Glacier B28 (Bylot Island). The temporal variation in processes dominating SSC patterns is assessed using stepwise multivariate regression following the subdivision of the time series. Partitioning of the time series is achieved through principal components and change point analyses. The regression models use discharge and surrogate predictor variables to model SSC, while improvements are made by using air temperature and radiation terms as independent variables. Comparisons are drawn between two sets of models with contrasting subseasonal division. By interpretation of the regression model characteristics, temporal changes in physical processes are implied over the course of the time periods. Numerical analyses suggest there is a trend for changes between fluvial, glacial and periglacial factors forcing responses in SSC. Therefore, it is conjectured that glaciofluvial sediment transfer at high latitudes is influenced by periglacial processes and conditions. This has implications for the predictions of fluvial sediment loads in a changing environment, and the use of sedimentary records for environmental reconstruction.
Late Holocene solifluction history reconstructed using tephrochronology
Authors: Kirkbride Martin P. , Dugmore Andrew J.
Published: April 2008

Phases of activity of four solifluction lobes at an altitude of 750–800 m are dated by tephrochronology at Snaefell, central eastern Iceland (64°48′ N 15°33′ E). The sample includes sorted lobes with tread gradients of 3–11° and unsorted (turf-banked terraces) in slope-foot locations. Trenches through lobe fronts reveal detailed internal structures picked out by multiple tephra layers. The tephras V1717, V1477, Ö1362, V870, Hekla-3 (2900 years
bp
and Hekla-4 (3800 years
bp
) provide isochronous surfaces of known age whose deformation and disturbance indicate mass movement and/or cryoturbation of the soil cover. Undisturbed soil including the Hekla-3 tephra indicates an absence of solifluction prior to 2900 years
bp
. Several centuries after Hekla-3, gravel-rich horizons mark wide-spread frost heave and solifluction of hillslopes. Later stabilization of these lobes allowed the accumulation of aprons of aeolian sediment below lee-side risers. These aprons contain
in situ
mediaeval tephras, dating the inception of solifluction to a considerable time prior to Norse settlement. The likely period of this first phase of solifluction is the Later Bog Period of the Subatlantic,
c.
2500-1000 years
bp
. The aprons are currently being overridden and deformed by solifluction lobes reactivated in the Little Ice Age.
Segmentation and interaction of normal faults within the Colfiorito fault system (central Italy)
Authors: Mirabella F. , Boccali V. , Barchi M. R.
Published: April 2008

Fault segments belonging to a fault population can link and interact, eventually forming a single larger fault, and thus affecting the estimation of the maximum expected earthquake. We present throw distribution data along the Quaternary normal faults of the Colfiorito fault system (central Italy), which consists of four main fault segments and where a seismic sequence occurred in 1997–1998. Throw values along the two central overlapping, en-échelon segments (8.5–9.5 km long) were measured on a good stratigraphic marker, by constructing a set of closely spaced geological cross-sections, perpendicular to the fault strike. As these faults are commonly retained active and border Quaternary basins, we compare morphological and geological throws in order to verify the faults neotectonic activity. Geological and morphological throw distributions show good correlation, testifying that recent faulting affected the topographic surface and suggesting that the observed offset completely accumulated during the Quaternary. The throw distribution along the fault segments is asymmetric and reaches maximum values (500–550 m) within the zone of fault overlap, suggesting mechanical interaction between the studied faults. Maximum length-throw correlation suggests that the studied faults grew according to a linear scaling relationship.
Textural controls on the brittle deformation of dolomite: variations in peak strength
Authors: Austin N. J. , Kennedy L. A.
Published: April 2008

Textural controls on the peak strength of dolomite are investigated through 23 triaxial deformation experiments, performed at confining pressures of 25, 50, and 100 MPa on texturally diverse dolomites. The mechanical data from these experiments are fit to an empirical failure criterion to elucidate the most significant parameters in dictating the peak strength of dolomite. Neither grain size nor porosity is required to quantitatively predict the peak strength of dolomite. Instead, only the effective Young’s modulus, and the empirically predicted uniaxial compressive strength, along with experimentally controlled confining pressure, are required in order to explain the peak strength to and
R
2
better than 0.89. A Hall-Petch relationship does not apply to this data set as a consequence of the variability in intragranular and grain boundary textures, which appear to overshadow the role of grain size. It is, therefore, essential that grain boundary textures and intragranular flaws be examined prior to making predictions regarding the relative peak strengths of chemically and mineralogically similar dolomites.
Textural controls on the brittle deformation of dolomite: the transition from brittle faulting to cataclastic flow
Authors: Austin N. J. , Kennedy L. A. , Logan J. M. , Rodway R.
Published: April 2008

To investigate the role of texture on the brittle deformation of dolomite, 23 triaxial deformation experiments were performed at confining pressures of 25, 50, and 100 MPa, dry, at room temperature, on dolomite from three texturally distinct sample suites. The variations in the mechanical response of these mineralogically and chemically similar dolomites, and the ensuing microstructures, indicate that grain boundary textures promote or inhibit the ability of grains to shear and rotate with respect to one another, whereas the presence of intragranular flaws, such as cleavage, that act as weaknesses, promote intragranular deformation. In samples with porosities greater than
c.
7%, inelastic pore collapse controls the transition from brittle faulting to extensive intragranular deformation and cataclastic flow. This porosity is much higher than has been observed at the onset of pore collapse in calcite, as a consequence of the inability of dolomite to deform by crystal plastic processes at room temperature. Combined, these textural features may dictate the transition from brittle faulting to cataclastic flow in brittle rocks in the upper crust.
Stress-strain rate history of a midcrustal shear zone and the onset of brittle deformation inferred from quartz recrystallized grain size
Authors: Gueydan F. , Mehl C. , Parra T.
Published: April 2008

The quantification of quartz shear stress and strain rate within a midcrustal shear zone provides a mechanical frame to describe the evolution from penetrative ductile deformation to localized deformation and the onset of brittle deformation. The quantification is based on the relationships between the quartz recrystallized grain size, the quartz shear stress (piezometric relation) and the strain rate (dislocation creep flow law). Increasing strain is accompanied by a general decrease of quartz recrystallized grain size and a decrease in grain size scattering. These are interpreted as a result of a complex loading history. The evolution from penetrative ductile deformation toward strain localization, marked by an increase of the strain rate by one order of magnitude, is inferred from grain size memory. Brittle deformation is triggered for quartz shear stress of the order of 70 MPa and strain rate close to 10
−12
s
−1
. This relative low value of the quartz shear stress necessary to trigger faulting implies a less important strength for the midcrust compared with strengths predicted by classical rheological envelopes.
A critical assessment of the tectono-thermal memory of rocks and definition of tectono-metamorphic units: evidence from fabric and degree of metamorphic transformations
Authors: Spalla M. Iole , Zucali Michele , Di Paola Silvia , Gosso Guido
Published: April 2008

A correlation procedure of scattered tectonic and metamorphic imprints in the reactivated crust is elaborated from recent analytical work in three Alpine metamorphic complexes. It consists of: interpretation of the time-sequence of tectonic fabrics and test of their kinematic coherence; determination of paragenetic compatibility among the mineralogical support of mesoscopic fabrics; cross-validation of mineral transformation over-prints; construction of
P-T-d-t
paths using a time-sequence of parageneses. The representation of structural and metamorphic information conveys the full tectono-metamorphic history on maps displaying combined tectonic and metamorphic effects. Shape and size definition of metamorphic units, now individuated mainly using their lithological homogeneity and dominant metamorphic imprint, is improved. The analysis of interaction between fabric and metamorphic imprint distributions, proposed in three Alpine examples, shows that the dominant metamorphic imprint does not coincide with
T
max
-
P

T
max

of each inferred
P-T-d-t
loop; the dominant metamorphic imprint is that given by the mineralogical support of the most pervasive fabric. Different metamorphic imprints may dominate in adjacent areas of a single tectono-metamorphic unit (TMU), or equivalent metamorphic imprints may dominate in different TMUs. Therefore, lithostratigraphic setting and dominant metamorphic imprint are inefficient to contour TMUs in terrains with polyphase deformation and metamorphism, without considering multiscale heterogeneity of superposed synmetamorphic fabrics.
Origin of the current stress field in the western/central Alps: role of gravitational re-equilibration constrained by numerical modelling
Authors: Delacou Bastien , Sue Christian , Champagnac Jean-Daniel , Burkhard Martin
Published: April 2008

We interpret the strain and stress fields of the western/central Alpine arc on the basis of 2.5D finite element modelling and a recent seismotectonic synthesis. Models have fixed boundary forces and different crustal geometries, so that they respond to buoyancy forces (variations in gravitational potential energies). The seismotectonic regime, characterized by orogen-perpendicular extension in the high topographic core of the belt and local orogenperpendicular compressional/transpressional deformation in the external zones, appears to be very close to the modelled gravitational regime. Rotation of Apulia has a minor effect on the current strain or stress fields of the Alpine realm. Nevertheless, it could help to explain the orogen-parallel dextral faulting that is observed all along external zones, from the northern Valais to the Argentera external crystalline massif. Our results highlight the consequences for the Alpine realm of ongoing convergence between the African and European plates. Our interpretation is that collision is no longer ongoing and that buoyancy-driven stresses dominate the present-day geodynamics of the western/central Alps.
Submarine slope systems: processes and products
Authors: Flint Stephen S. , Hodgson David M.
Published: April 2008

Understanding of the processes operating on submarine slopes, the preserved depositional products and post-depositional modifications to sediment body geometries through instability and remobilization requires integration of data from modern slopes with information derived from seismic and outcrop studies of ancient slope successions. Although local factors are important, key generic concepts that aid in predicting submarine slope processes and products include: (1) shelf accommodation/lateral sediment supply variations on sediment delivery; (2) the spatial/temporal distribution of characteristic styles/intensities of sediment instability and remobilization; and (3) a generalized model for the stratigraphic development of slope channel complexes, slope fans and the temporal relationships between these major components of slope stratigraphy.

Key remaining problems include bridging the gap between the timescales sampled by Recent to late Quaternary studies and those represented by ancient slope successions. Moreover, present-day highstand conditions may not provide a good analogue for lowstand slope settings. Future research efforts will also concentrate on better calibration of seismic facies to rock facies and linking of physical and numerical (process and forward) modelling techniques at different scales to observational datasets. The future lies in integration of these complementary research directions.
Deposition and stratigraphic architecture of an outcropping ancient slope system: Tres Pasos Formation, Magallanes Basin, southern Chile
Authors: Shultz M. R. , Fildani A. , Cope T. D. , Graham S. A.
Published: April 2008

The Tres Pasos Formation, Magallanes Basin, Chile, represents the deposit of a submarine slope depositional system. The formation is approximately 1500 m thick where exposed in the Ultima Esperanza district of southernmost Chile. It is characterized by a basal turbiditic sandstone unit up to 200 m thick that shows a north-to-south, proximal-to-distal facies evolution from turbidite channel-fill complexes to sheet-like sandstone units. This unit is interpreted as having been deposited at or near the base of slope. Overlying the basal sandstone unit is approximately 500 m of amalgamated mass transport complexes, fine-grained strata, and channelized and non-channelized turbidity current deposits, collectively comprising the middle part of the formation. Mass transport complexes exert a primary control on the character and grain size of turbidite sandstone bodies in the basal and middle part of the formation. In the southern part of the study area, a 300 m thick coarse-grained unit interpreted as a turbidite channel-fill complex partially replaces the middle part. The upper part of the formation is approximately 500 m thick and consists primarily of fine-grained strata. Failure scarps and thin turbidite channel-fill units are present in this upper part, interpreted as upper slope deposits.
Architecture and stratigraphic evolution of multiple, vertically-stacked slope channel complexes, Tanqua depocentre, Karoo Basin, South Africa
Authors: Wild Richard J. , Hodgson David M. , Flint Stephen S.
Published: April 2008

Early Permian deep-water deposits of the Tanqua depocentre, SW Karoo Basin, South Africa, include Unit 5, an extremely well-exposed 100 m-thick lower slope succession. Within the study area, Unit 5 comprises two partially-synchronous, vertically stacked, sub-parallel channel complex sets that lie 8 km apart along strike (the east-trending Klein Hangklip complex set and the NE-trending Groot Hangklip complex set). The detailed time-stratigraphic relationship between deposition in the interchannel areas and channel fill aggradation remains unresolved due to exposure limitations; however, it is suggested that most of the turbidite sheet deposits between the channels represent frontal lower slope splays from earlier slope feeder systems and are not genetically related to the channels. Gravitational instability in the sheet deposits drove a range of deformation processes from low velocity ‘slope creep’ to complete failure and slumping during times of maximum incision and bypass within the slope channels. Following the main phase of aggradation within the channels, periods of spill led to the formation of lateral splays and splay channels, which are distinct from the older frontal splay deposits. Each channel complex comprises two composite channel bodies and is interpreted to represent a fifth order sequence. In the absence of evidence of local (intraslope) tectonic controls, the vertical stacking of the channel complexes is interpreted to be due to fixed shelf edge entry points. Abrupt lateral facies changes along depositional strike, the ubiquity of instability features, the high proportion of sandstone preserved in the channel complexes and the absence of levees supports the interpretation that Unit 5 in the Hangklip area was deposited in a lower slope setting.
Quantitative textural analyses of TOBI sonar imagery along the Almería Canyon, Almería Margin, Alborán Sea, SE Spain
Authors: Sichi Olga Gómez , Blondel Philippe , Gràcia Eulàlia , Dañobeitia Juan José ,   The Hits-2001 Science Party
Published: April 2008

Hydrocarbon exploration and the ongoing assessment of potential seismic risks are the main drivers behind the high-resolution mapping of continental margins. The large volume of literature devoted to turbidite systems in particular shows the importance of detailed descriptions of their characteristics and overall geomorphological variability, primarily through the distribution of sediment facies. These descriptions rely mainly on acoustic measurements, which are notoriously difficult to interpret. Textural analyses quantify the second-order statistics of sonar imagery, detecting and quantifying details invisible to the human eye. We show the potential (and limitations) of this approach using high-resolution (6 m) towed ocean bottom instrument (TOBI) sidescan sonar imagery acquired in the Alborán Sea, south of Almería, Spain, during the high resolution imaging of Tsunamigenic structures (HITS) 2001 programme. The imagery is co-registered with EM-12 multibeam bathymetry, topographic parametric sonar (TOPAS) sub-bottom profiles, and localized ground truthing. Our study focuses on the Almería Canyon, a meandering channel system more than 57 km long and transporting large amounts of sediments from the coast down to the Alborán Trough,
c
. 1700 m deep. Textural analyses quantify the variations of sediment processes along the slope of the Almería Canyon. They agree with the results of previous studies, and they can be used to provide new insights in the dynamics and evolution of the canyon.
Mud prone entrenched deep-water slope channel complexes from the Eocene of eastern Turkey
Authors: Cronin Bryan T. , Çelik Hasan , Hurst Andrew , Turkmen Ibrahim
Published: April 2008

A series of deep-water channel deposits are exposed near Baskil, 50 km west of Elaziğ in eastern Turkey. The use of correlation panels, sedimentary logs, biostratigraphy and mapping revealed that all of the channel elements are found within much larger features. One series of channels from the southern margin of the basin lie, within an entrenched deep-water slope channel complex, over 3 km wide, called the Nohut Channel Complex. This channel complex has three main fill packages. The lower package is a highly disturbed, slumped interval that directly overlies the erosional base of the main channel complex. The slumped interval is interpreted as a series of mass transport complexes, derived from shallow marine or upper slope facies. The second package comprises a mixed carbonate-siliciclastic series, principally of interbedded calcarenites, marls and mudstones. Locally, outer shelf/upper slope offset-stacked channels with calcarenite fills occur. The upper package comprises isolated conglomerate and sandstone filled deep-water channels that are incised into laterally extensive depositional lobes. The coarse-grained fills are dominated by tabular beds. The lobes are comprised of fine-medium grained sandstones. Some of these channels have low aspect ratios (25:1), and are asymmetric in cross-section, suggesting sinuosity. The upper part of this fill package is either incised into by pro-fan delta conglomerate filled channels, or downlapped onto by shelfal calcarenites. A separate series of channels come from the northern margin of the basin, but are interpreted to have developed on an open, deep-water slope or ramp, rather than as an entrenched channel complex. These channels occur in two main sets, collectively referred to as the Aydinlar Channel Complexes. The lower set is characterized by syn-sedimentary foundering of the sand-filled channels within slumped enveloping finer-grained slope sediments. The upper set of channels, dominated by one large sand and gravel-filled fairway, is characterized by large bedforms at the base, passing upwards into stacked conglomerates, mud chip breccias and sandstones. The conglomerate facies is locally injected upwards into the fill or laterally into finer-grained marginal facies. The channel is capped by finer-grained facies, including graded sandstones and siltstones with complete Bouma sequences.