bluuebunny/crossref_metadata_2025_split huggingface dataset:
Brittle deformation and graphitic cataclasites in the pilot research well KTB-VB (Oberpfalz, FRG)
Authors: Zulauf G. , Kleinschmidt G. , Oncken O. 
Published: April 2008

Abstract
Within the research well KTB-VB, late- to post-Variscan brittle deformation is widespread, indicating that the well is situated in a broad fault zone. Graphite-enriched foliated cataclasites within paragneisses are probably connected with late-Variscan NW trending reverse faults. These faults were succeeded by further, graphite-free, faults in post-Variscan time. During the formation of the graphitic cataclasites water-consuming retrograde metamorphism was active producing large amounts of phyllosilicates, especially at the expense of feldspar. The increase in phyllosilicates and graphite was associated with a switch of the dominant deformation mechanisms from brittle fracture to frictional sliding, crystal plasticity and diffusion-controlled processes. The cataclastic fabric suggests that the main part of the movement within the graphitic cataclasites was likely to have been aseismic rather than seismic.
Instability, softening and localization of deformation
Authors: Hobbs B. E. , Mühlhaus H.-B. , Ord A. 
Published: April 2008

Abstract

Material strain softening is commonly taken as a necessary and sufficient condition for localization in deforming rocks. However, there is a wide range of experimental and theoretical information which shows that localization can occur in sands, brittle rocks and ductile metals under strain-hardening conditions. This paper aims to bring these two contrasted views together. Three separate criteria are necessary in order to understand localization behaviour. The first involves the stability of the deforming system. The second determines whether a deforming system will undergo bifurcation so as to cease deforming in a homogeneous mode and instead deform in an inhomogeneous mode such as barrelling or localization. The stability and bifurcation criteria are independent of each other since barrelling is a stable mode whereas localization is unstable. The third criterion establishes if the unstable bifurcation mode is one of localization or of some other kind. Localization may arise from the presence of vertices on the yield surface (as in the case of pressure insensitive, rate dependent metals and in brittle rocks due to the development of preferred microfractures for slip) or from the constitutive relation being such that the plastic strain-rate vector is not normal to the yield surface (as in the cases of pressure sensitive, dilatant rocks, of materials deforming by crystal-plastic processes involving dislocation cross-slip and/or climb, and of visco-plastic materials in which voids are forming due to diffusive processes). It is important to distinguish between
material
and
system
softening (or hardening) behaviour. The theory for a kinematically unconstrained shortening experiment (that is, rigid, frictionless platens) indicates that localization can occur in strain-hardening materials but the system must strain-soften from then on; that is, localization occurs at peak stress for the system even though the material may continue to harden (or soften). However, the addition of kinematic constraints (such as friction at elastic platens, a constraint to deform in plane strain or at constant volume) means that localization may occur in a system that is monotonically strain hardening. Shear zones in naturally deformed rocks show ample evidence of dilatant behaviour in that evidence for the passage of large volumes of fluid during localization is common as is the development of dilatant vein systems. As such, since shear zones are strongly constrained by the elastic and (limited) plastic response of the relatively undeformed rocks surrounding the shear zones, strain-hardening behaviour of the system is to be expected as the norm, even if the rocks within the shear zones are undergoing material strain-softening.
Deformation of polycrystalline salt in compression and in shear at 250–350°C
Authors: Franssen R. C. M. W. , Spiers C. J. 
Published: April 2008

Abstract

Dry synthetic polycrystalline salt (NaCl) has been deformed in uniaxial compression and in shear in order to gain insight into the influence of deformation mode on the development of crystallographic preferred orientation, microstructural evolution and mechanical behaviour (flow strength). The experiments were carried out between 250 and 350°C, at strain rates between 10
−5
and 10
−7
s
−1
. Under these conditions halite deforms by climb-controlled dislocation creep. In our samples, the presence of deformed grains containing cellular networks of subgrains, and the development of texture are consistent with such mechanisms. The weaker {110} <110> slip plane appears to align parallel to the shear plane and perpendicular to the uniaxial compression direction. In the sheared samples, a <111> maximum is observed parallel to the shear direction. The textures obtained agree favourably with texture simulations. When the mechanical behaviour seen in uniaxial compression and shear is compared in terms of equivalent stress and strain, as defined in the von Mises theory of isotropic plasticity, the uniaxially deformed samples appear to be stronger. From a detailed consideration of the data, it is inferred that the difference in flow strength is largely due to anisotropy resulting from texture development. The results suggest that the assumptions underlying the von Mises theory of isotropic plasticity are not applicable in the present case, and that the associated flow rule does not offer an accurate method of generalizing creep laws for salt under the conditions investigated.
The SEM/ECP technique applied on twinned quartz crystals
Authors: Olesen Niels Ø. , Schmidt Niels-Henrik 
Published: April 2008

Abstract

The SEM/electron channelling technique, ECP, is useful in fabric studies, since a three-dimensional lattice orientation determination is performed with a precision of
c
. 0.5° from areas a few microns in diameter. Computer simulation of the ECP image (a line diffraction pattern) facilitates analysis, particularly in the case of low-symmetry crystals such as alpha-quartz. The most important types of twinning in quartz are after the Dauphiné and Brazil laws. The latter combines a left- and a right-handed crystal (enantiomorphism), and it is shown that it is
not
possible to discriminate between these two crystals, since the twin plane coincides with mirror planes in the ECP sphere resulting in identical ECPs for the individual Brazil twins. In contrast, Dauphiné (and other) twins are easily identified because their twin planes do not coincide with mirror planes. A specification is given of the point groups which permit a complete lattice orientation determination, and those which involve a loss of information (orientational solubility).
The undrained shear behaviour of fine-grained sediments
Authors: Yassir N. A. 
Published: April 2008

Abstract
A series of high pressure triaxial tests were conducted on three normally consolidated mud volcano silty clays to explore some of the effects on the undrained shear behaviour of argillaceous sediments. The tests followed two different consolidation paths before undrained shear: isotropic, with equal all-around stresses and anisotropic, at a horizontal to vertical effective stress ratio of 0.6. The tests showed that shear stresses produce overpressuring in the fine-grained sediments, and that the behaviour is sensitive to stress path, stress magnitude and grain size. Other important effects on the behaviour are also thought to include natural porosities and cementation.
Transition between seismic and aseismic deformation in the upper crust
Authors: Gratier J. P. , Gamond J. F. 
Published: April 2008

Abstract
Displacement on faults is often accommodated by a succession of two mechanical processes: aseismic sliding mass transfer and seismic cataclastic events. Pressure solution is attested both by dissolution markers of asperities which prevent sliding on and around the fault zone, and by the mechanism of growth of the mineral fibres by aseismic crack-seal in cavities opened by sliding. The cataclastic process is attested by observations of broken and kinked fibres, and by observations of euhedral crystals in the cavities opened by fault sliding. Natural examples are given to recognize and balance the mass transfers. Finally, a model is proposed to explain seismic/aseismic transitions on a given fault based on a gradual reduction in the cataclastic strength of the fault during sliding by successive breaking of asperities, and on the principle of minimum work consumed during the slip since the energy needed either for pressure solution sliding or for the cataclastic event may vary differently with the progressive sliding. Depending on the limiting processes for pressure solution slip, stable aseismic or unstable slip with seismic/aseismic transition is predicted.
Extensional veins and shear joint development in a thrust-fold zone (Northern Apennines, Italy)
Authors: Sani Federico 
Published: April 2008

Abstract
The Northern Apennines thrust system consists of clastic foredeep deposits that have been progressively involved in thrusting towards the northeast from the Oligocene to the Recent. The outcroping geological units are progressively younger towards the foreland implying that sedimentation and deformation developed together. The Umbrian Sequence is the most external unit and has been affected by Late Miocene to recent thrusting. The Marnoso-arenacea Formation, a marly arenaceous turbiditic sequence, belongs to this Unit. This formation provides good examples of pre-thrust and thrust-related structures. An extensional calcite vein system with two sub-vertical and orthogonal sets having constant orientation, developed before thrusting. A later conjugate shear joint system which cross-cuts the extensional veins, were developed in the early stages of thrusting. Folds related to the thrusting (i.e footwall synclines or hangingwall anticlines) spatially reoriented the previously formed extensional vein and shear-joint systems.
Rotliegend sediment distribution: a result of late Carboniferous movements
Authors: Glennie K. W. 
Published: April 2008

Abstract
The distribution of Rotliegend desert sandstones in Northwest Europe was mainly the outcome of four geological events that originated in the Carboniferous or earlier.

(1)
Westphalian culmination of collision between north-moving Laurussia and the faster moving Gondwana resulted in the E-W trending Variscan Mountains, which were to be the main source of Rotliegend fluvial sediment over successor basins to the former back-arc foreland.

(2)
During final convergence between Laurussia and Gondwana, the western Variscan orocline probably acted as a wedge that put Laurussia into transtension and, utilizing old lines of weakness, the Iapetus Suture between the Scottish-Greenland and Norwegian Caledonides and the Tornquist Line, initiated NW-SE and conjugate NE-SW systems of grabens.

(3)
Late orogenic relative movement between Laurussia and Gondwana also caused strike-slip faulting, related rapid collapse of the Variscan Mountains and the extrusion of Lower Rotliegend volcanics through parts of the Variscan foreland. The end of volcanism was followed by thermal subsidence of the Permian basins, many of them at rates that exceeded those of sedimentation.

(4)
The northward drift of Laurussia carried NW Europe from the equatorial zone of Carboniferous coal deposition to the latitude of a northern hemisphere trade-wind desert, where it lay in the rain shadow of the Variscan Mountains.
A review of Jurassic chronostratigraphy and age indicators for the UK
Authors: Cox B. M. 
Published: April 2008

Abstract
The basis of the chronostratigraphic subdivision of the Jurassic System is the sequence of ammonite faunas. At present, the 11 Jurassic stages (representing approximately 70 million years) can be divided into 145 ammonite-based zones or subzones. Methods of subdivision and correlation by various microfossil groups have also been developed for practical and economic reasons. Dating by fossils (biostratigraphy) underpins the broader-based approachs of event and sequence stratigraphy. It also supplies the primary stratigraphic control in the development of a standard magnetic polarity time-scale (magnetostratigraphy) of global applicability. At present, the Jurassic System of the UK area is divided into 38 units (biozones and subzones) on the basis of dinoflagellate cysts and 23 on the basis of calcareous nannofossils. Coverage is less comprehensive using the benthonic groups (foraminifera and ostracoda) which are more affected by environmental controls. The Lower Jurassic is divided into 16 units on the basis of foraminifera, but higher in the System only local divisions can be recognised in the Bathonian and informal divisions in the Callovian and oldest Oxfordian. There is fuller but still incomplete coverage using ostracoda which have proved particularly useful in the non-marine, brackish and marginal marine sequences of the Bathonian and Portlandian. The recovery of radiolaria from Jurassic sediments in the UK area is a new and exciting development which allows division of Kimmeridgian and Portlandian strata in graben areas in the North Sea Basin.
Mesozoic magmatic activity in the North Sea Basin: implications for stretching history
Authors: Latin David M. , Dixon John E. , White Nicky , Fitton J. Godfrey 
Published: April 2008

Abstract

Recent developments in the theory underlying magma generation allow the volumes and compositions of melt generated during extension of the lithosphere to be used to make quantitative statements about basin evolution. In the Mesozoic North Sea Basin there is a well defined relationship between magma chemistry and the degree of lithospheric attenuation. The largest extents of melting (<2%) resulted in the alkali basalts of the Forties province which occur in an area where β (the stretching factor) may be as high as 2, and which erupted within 30 million years of the onset of extension. In off-axis areas of the rift the degrees of melting are smaller and melt compositions are more undersaturated and extreme (nephelinites and ultrapotassic rocks). In the Forties province the observed melt compositions may be reconciled with melting on the dry peridotite solidus at a normal (1280°C) potential temperature (
T
p
) assuming a relatively thin mechanical boundary layer (MBL) of 70 km prior to rifting. Alternatively, a
T
p
of 1380°C will produce the alkali basalts from dry periodotite at the observed extension factors with an initial MBL thickness of 100 km. A
T
p
as high as 1480°C, which would give rise to regional pre- and syn-rift uplift, can be ruled out because the observed extent of melting is too small, the amounts of extension too large, and the rift episode too short. Off-axis magmatism may be explained by melting within a MBL with an initial thickness of ∼100 km or more at a
T
p
of 1280°C. Models for extension of the lithosphere by simple shear along an initially planar detachment fault fail to account for either the existence or the location of North Sea magmatism, with or without elevated potential temperatures if the dry solidus governs melting. This conclusion is independent of the initial dip of the detachment and also holds when the initial dip in the lithospheric mantle is double that in the crust. The timing, nature, and location of magmatic activity can be used to provide information about the onset of rifting, the rates of extension, the amount of strain, and the thermal history of a basin. When linked with studies of faulting and subsidence this approach may be of some importance for hydrocarbon exploration.
Timing, nature and sedimentary result of Jurassic tectonism in the Outer Moray Firth
Authors: Boldy S. A. R. , Brealey S. 
Published: April 2008

Abstract
Jurassic sedimentation in the Outer Moray Firth took place under a changing tectonic regime, accompanied by regional transgression in the late Jurassic. An early phase of tectonism is recognised, accompanying the collapse of the Central North Sea Dome. This Mid-Cimmerian phase of tectonism is characterized by North-South, Viking trend, faults and reactivated Northeast-Southwest Caledonian faults. These predominantly Bathonian age faults controlled subsidence and deposition of the Middle Jurassic Rattray Volcanics Formation. The Rattray Volcanics are unconformably overlain by the Sgiath and Piper Formations, of Oxfordian to early Kimmeridgian age, deposits of a northward prograding delta that continued to be affected by movement on Mid-Cimmerian faults.
A dramatic change in tectonic regime occurred during the Kimmeridgian, possibly concurrent with the major Eudoxus Zone transgression, that heralded the onset of Kimmeridge Clay deposition. During this Late Cimmerian rift-phase, sedimentation took place under conditions of active extension controlled by northwest-southeast, Witch Ground Graben trend, faults. The Kimmeridge Clay Formation is a typical syn-rift sequence with sands deposited on the downthrown sides of rotational fault blocks, whose crests were commonly eroded. Studies of ammonites recovered from cored sequences have refined the interpretation of timing of tectonic and transgressive events and have also highlighted anomalies in correlation between ammonite and dinocyst zonation schemes.
Fault traps in the Northern North Sea
Authors: Spencer A. M. , Larsen V. B. 
Published: April 2008

Abstract

The 250 hydrocarbon finds in the northern North Sea have total resources of 8000 × 10
6
Sm
3
oil-equivalent and 70% are in fault block traps, all with Jurassic or older reservoirs. They can be classified into plays by reservoir age: pre-rift is pre-Jurassic and Lower-Middle Jurassic; syn-rift is Upper Jurassic. 40 finds have sufficient published information to allow analysis of the geological relationships, structural and stratigraphic, which have given rise to their faulted hydrocarbon traps. All of the pre-rift and some of the syn-rift finds are traps in footwall blocks. They form a series with respect to the amount of conformable versus unconformable cap rock. In many the up-dip seal is due to stratigraphic truncation of the reservoir below an unconformable cap rock: the hydrocarbon pool does not extend to the bounding fault. The amount of erosion on the fault blocks and the footwall uplift which occurred seem to be related to the magnitude of fault throw. Most of the other syn-rift finds are hanging wall traps with entirely conformable cap rocks. The Brae-trend finds are classic syn-rift traps, located in the hanging wall of a major fault, movement on which was responsible for the supply of the reservoir clastics. The traps in the Central Graben are the most varied: all are different, probably due to complications produced by the underlying Permian salt.
Occurrence and depositional environment of the Lower Cretaceous sands in the southern Witch Ground Graben
Authors: Bisewski Hans 
Published: April 2008

Abstract
Lower Cretaceous sands in the southern Witch Ground Graben are of Aptian-Albian age and are part of the Sola Formation. In the Witch Ground Graben the Sola Formation is generally argillaceous. However, in the southern Witch Ground Graben, thick sands are developed and the argillaceous Sola Formation is replaced, almost in its entirety, by the sandy Kopervik Formation, also called the ‘Bosun Sands’. The increased clastic input in the upper part of the Lower Cretaceous is associated with the regression in the Central North Sea as a result of the Austrian tectonism. The sands of the Kopervik Formation are derived from sources on locally exposed and rotated fault blocks and emergent terraces and are deposited as submarine fan or turbiditic sands.
Early Palaeogene tectonics and sedimentation in the Central North Sea
Authors: Milton N. J. , Bertram G. T. , Vann I. R. 
Published: April 2008

Abstract
The early Palaeogene deposits of the Central North Sea have been divided by Stewart into ten depositional units, on the basis of seismic stratigraphy. These are interpreted as the products of variations in relative sea level. The units may be traced from the shelf into the basin using wireline log markers correlated within a biostratigraphic framework. These are interpreted as the signature of transgressive maxima when basinal clastic supply was at a minimum. The attitude of coal or lignite beds within the shelfal areas can be used to demonstrate a period of Palaeogene net uplift, followed by tilting and sinking of the shelf-edge and basinal areas during the later depositional episodes. Other than by a mechanism of load-induced differential compaction around buried or partially buried Mesozoic features no significiant rejuvenation of Mesozoic faults can be demonstrated. The area of uplift extended at least as far as the western limit of the Beauly Formation.
The products of Stewart’s depositional episodes vary depending on the tectonic activity. During the uplift phase, thick massive basinal fans were deposited and little or no shelfal deposits are preserved. During the tilting/sinking phase thick progradational wedges were deposited, and basinal deposition was largely confined to shales with relatively small local sand systems.
Cenozoic subsidence and uplift in the North Sea region: Implications for mechanisms of basin formation
Authors: Morgan Richard K. 
Published: April 2008

Abstract
The scale on which flexural subsidence has taken place in the North Sea region from the Late Palaeocene is very much greater than the scale on which fault-controlled subsidence and flexural subsidence occurred during the Mesozoic. In addition, uplift of the Scottish Highlands occurred during the Cenozoic on a scale comparable with that of the subsidence in the North Sea region during this time.
The Moray Firth, Mesozoic to Tertiary basin is divided into two distinct areas as a consequence of its Tertiary subsidence history, the Inner Moray Firth being uplifted and eroded and the Outer Moray Firth undergoing apparently continuous subsidence. The geometries of the pre-erosional sequences in the Moray Firth area indicate a Cenozoic history of both subsidence and uplift, providing a link between the subsiding North Sea region and the uplifted Scottish Highlands. Using seismic mapping and cross sections constructed therefrom, across the Moray Firth and North Central Graben areas, the relative subsidence and uplift is displayed as part of a unified process of crustal flexuring.
It is suggested that the flexural subsidence and uplift in the North Sea region and Scottish Highlands during the Cenozoic is associated with the onset of sea-floor spreading of the North Atlantic to the west. Furthermore it is proposed that the magnitude of Cenozoic subsidence in the North Sea region does not relate to the magnitude of Mesozoic extension which preceded it, the two processes being driven by different mechanisms.