bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of fractal flaw distributions on rock deformation in the brittle field
Authors: Main Ian G. , Meredith Philip G. , Sammonds Peter R. , Jones Colin
Published: April 2008

The geometrical distribution of flaws plays a crucial role in the physical behaviour of geological materials under stress. Flaws are present in the earth on all scales, from microcracks to plate-rupturing faults. They may be distributed on one characteristic length scale (e.g. joints, ‘characteristic’ earthquakes), or more commonly exhibit scale-invariance over a specified range of sizes. Scale-invariance implies that the discrete length distribution in a finite range is a power law of negative exponent
D
, where 1 ≤
D
< 3. Fault systems where motion is concentrated on a dominant fault (e.g. San Andreas) have
D
≈ 1, but more diffuse fault systems have
D
near 2.
D
is one of the fractal dimensions of the fracture system. The length distribution of faults or microcracks may be inferred from the slope
b
of the log-linear frequency—magnitude distribution of earthquakes, or laboratory-scale acoustic emissions, since it can be shown that
D
= 3
b/c
. The scaling factor
c
depends on the relative time constants of the seismic event and the recording instrument, and is usually equal to 3/2.
b
is found experimentally to be negatively correlated with the stress intensity on the dominant flaw, which depends in turn on the applied stress and the flaw length. Thus a fracture mechanics model of rock failure which includes a range of flaw sizes can be tested by seismic monitoring.

We describe a fracture mechanics model of rock failure for a variety of styles of deformation, ranging from elastic failure to quasi-static cataclastic flow, and predict the time-dependence of
D
and the seismic
b
-value at different times up to and including failure. Critical coalescence of microcracks during dynamic failure (e.g. earthquake foreshocks) occurs when
D
= 1 (
b
= 0.5); random processes (e.g. cataclastic flow, background seismicity) are associated with
D
= 2 (
b
= 1); positive feedback in the concentration of stress on the dominant flaw (e.g. during strain softening and shear localisation) occurs when
D
< 2 (
b
< 1); negative feedback in stress concentration (e.g. during the early stages of dilatancy), and where a highly diffuse fracture system is produced, occurs at low stress intensities and is associated with
D
> 2 (
b
> 1).

It has long been a goal of structural geologists to measure stress on rocks, since most geometrical signatures of deformation are strain-related. We show that stress is not usually as significant in rock fracture as stress intensity, and furthermore that the geometric signature of the length distribution of microcracks is well-correlated with the stress intensity.
Mechanical compaction and the brittle—ductile transition in porous sandstones
Authors: Wong Teng-Fong
Published: April 2008

Grain scale brittle fracture is an important mechanism for porosity reduction and tectonic deformation in rocks under diagenetic and low grade metamorphic conditions. Microstructural observations and acoustic emission measurements were used to elucidate the micromechanical processes involved (1) in pressure-induced grain crushing, and (2) in the brittle-ductile transition in porous sandstones. Experimental observations show that the critical effective pressure for the onset of grain crushing decreases as a function of increasing grain size and of increasing porosity. The correlation among grain crushing pressure, porosity and grain size can be interpreted using a Hertzian fracture mechanics model. Experimental observations show that both porosity and effective pressure have profound influence on the inelasticity and failure mode. Based on such observations, a brittle-ductile transition map for porous rocks can be constructed in the effective pressure-porosity space. The theoretical predictions of a fracture mechanics model of a pore-emanated crack are in qualitative agreement with experimental observations on the effect of pressure and porosity on inelastic behaviour and failure mode. However, such a model overestimates the crack stabilization effect due to confining stresses since it neglects grain rotation and pore collapse which are important deformation mechanisms in porous rocks. Implications of the experimental and theoretical results on depth — porosity relation, aseismicity at the shallow region of subduction zones, and faulting in sandstones are discussed.
Shear bands in a plastic layer at yield under combined shortening and shear: a model for the fault array in a duplex
Authors: Davies Russell K. , Fletcher Raymond C.
Published: April 2008

To model the regular spacing of the thrust faults in duplexes we subject a single layer embedded in an overlying viscous fluid, and an underlying rigid medium, to combined layer-parallel shortening and shear. The layer is a rigid-plastic solid at yield. A cusp introduced as a small perturbation on the upper layer interface perturbs the flow. The lower layer interface is weak and remains flat throughout the deformation, whereas the upper interface is deformable and can assimilate both weak and strong conditions. If the upper interface is strong it folds into a broad open fold. If the interface is weak the flow localizes into narrow shear bands that cut into the layer from the cusp, and reflect from the layer interfaces at a constant spacing. Material is displaced across the shear bands in a fault-like manner. If the layer is subjected solely to layer-parallel shortening these shear bands are symmetric. Superposing a shear component on the layer interface reorients the shear bands. The shear band dipping opposite to the sense of shear is more strongly developed and intersects the interface at a lower angle than the other. This is in agreement with the observations of duplex configurations.
The failure mechanism for deep-focus earthquakes
Authors: Green H. W. , Burnley P. C.
Published: April 2008

Experimental deformation of Mg
2
GeO
4
olivine at pressures between 1 and 2 GPa in the spinel stability field has led to discovery of a faulting instability that develops at the kinetically-controlled threshold of transformation. Very fine-grained olivine and spinel are found in fault zones. Deformation at lower temperatures is ductile; transformation is inhibited and specimens are very strong. Deformation at higher temperatures also is ductile but transformation is rapid and specimens are much weaker. Detailed examination of the microstructures of specimens deformed in the faulting regime lead to an anticrack theory of faulting that explains the experimental data and provides a fundamentally new mechanism for deep-focus earthquakes. The new mechanism is analogous to the Griffith theory of fracture; nucleation and growth of spinel under stress produces spinel-filled microanticracks normal to the maximum compressive stress that link up to produce faulting. The friction paradox for deep earthquakes is resolved because this faulting process provides a fine-grained, superplastic, ‘lubricant’ for faults. The temperature distribution within subducting slabs of lithosphere requires that the conditions of instability are reached as a natural consequence of subduction; metastable olivine in the interior of deep slabs warms to a critical temperature where faulting ensues in the presence of a shear stress.
Mechanical controls on dilatant shear zones
Authors: Ord A.
Published: April 2008

The finite difference code FLAC is used to examine the distribution of regions of high and low mean normal stress (or pressure) and of maximum dilation around deforming, periodic shear zones. The assumption is made here that the fluid pressure is equal to the mean normal stress. Fluid flow is favoured by large pressure gradients, and is enabled by regions of dilatancy. It is commonly assumed that regions of high dilation are necessarily associated with regions of low pressure. However, it is shown here that this need not be the situation. Cases in which maximum dilation is associated with the maximum pressure may be useful for understanding the presence of periodic melt segregations whereas cases in which maximum dilation is associated with minimum pressure may be useful for understanding metamorphic differentiation during crenulation cleavage development.
Experimental determination of constitutive parameters governing creep of rocksalt by pressure solution
Authors: Spiers C. J. , Schutjens P. M. T. M. , Brzesowsky R. H. , Peach C. J. , Liezenberg J. L. , Zwart H. J.
Published: April 2008

Theoretical models for compaction creep of porous aggregates, and for conventional creep of dense aggregates, by grain boundary diffusion controlled pressure solution are examined. In both models, the absolute rate of creep is determined by the phenomenological coefficient
Z
* =
Z
0
exp (−Δ
H/RT
), a thermally activated term representing effective diffusivity along grain boundaries. With the aim of determining
Z
0
, Δ
H
and hence
Z
* for pressure solution creep in rocksalt, compaction creep experiments have been performed on wet granular salt. Compaction experiments were chosen since theory indicates that pressure solution creep is accelerated in this mode. The tests were performed on brine-saturated NaCl powder (grainsize 100–275 μm) at temperatures of 20–90°C and applied stresses of 0.5–2.2 MPa. The mechanical data obtained show excellent agreement with the theoretical equation for compaction creep. In addition, all samples exhibited well-developed indentation, truncation and overgrowth microstructures. We infer that compaction did indeed occur by diffusion controlled pressure solution, and best fitting of our data to the theoretical equation yields
Z
0
= (2.79 ± 1.40) × 10
−15
m
3
s
−1
, Δ
H
= 24.53 kJ mol
−1
. Insertion of these values into the theoretical model for conventional creep by pressure solution leads to a preliminary constitutive law for pressure solution in dense salt. Incorporation of this creep law into a deformation map suggests that flow of rocksalt in nature will tend to occur in the transition between the dislocation-dominated and pressure solution fields.
Phenomenological superplasticity in rocks
Authors: Gilotti Jane A. , Hull Joseph M.
Published: April 2008

Superplastic deformation of rocks can be defined as: homogeneous deformation without loss of continuity to very large strains. This phenomenological definition differs from current geological usage that equates superplasticity with either grain boundary sliding, a deformation mechanism, or with a specific set of material parameters. Two examples of natural, phenomenological superplasticity in rocks are presented. A diabase dyke deformed continuously in progressive simple shear to shear strains of at least γ = 100, despite the low grade of metamorphism and relatively coarse grain size of the resultant tectonite. Very coarse-grained pegmatite dykes show continuous deformation to high strains under predominantly pure flattening; intracrystalline plasticity was an important micromechanism of flow. A much wider range of material and mechanical parameters and physical conditions give rise to phenomenological superplasticity in rocks than predicted by experiments on metals. By inference and example, more deformation mechanisms will be associated with superplasticity in nature. The profound differences between the metallurgical experiments and the geological environment prohibit simple rheological interpretations of natural superplastic behaviour.
Experimental study of grain-size sensitive flow of synthetic, hot-pressed calcite rocks
Authors: Walker A. N. , Rutter E. H. , Brodie K. H.
Published: April 2008

Synthetic calcite rocks of controlled grain-size were prepared by crushing large, clear calcite crystals, centrifuged to separate restricted grain-size fractions, cold-pressed and then hot-pressed to produce low-porosity polycrystals of mean grain-size covering the range 2–40 µm. These were deformed dry over the range 400–700°C, mainly at 200 MPa confining pressure, to investigate the onset with decreasing grain-size of grain-size sensitive flow.

The deformation of the fine-grained material can be described by a flow law of the form

ė
=
A
exp (−
H/RT
) σ

n

d

m

in which at low stresses (<25 MPa differential stress)
A
= 10
4.9
,
H
= 190 kJ mol
−1
,
n
= 1.7 and
m
= −1.9, when strain-rate,
ė
, is in s
−1
, stress, σ, is in MPa and grain-size,
d
is in µm. At higher stresses (25–250 MPa)
A
= 100,
H
= 190 kJ mol
−1
,
n
= 3.3 and
m
= −1.3. In the low stress regime, grains remain equidimensional during flow, a pre-existing preferred crystallographic orientation produced during cold-pressing tends to weaken and grain-boundary sliding is important. At higher stresses, a grain flattening fabric develops, the pre-existing preferred crystallographic orientation pattern is preserved, and recovery-accommodated intracrystalline plastic flow is inferred to be the dominant deformation mechanism. Grain-size sensitivity of the flow stress persists even into the intracrystalline plastic flow regime. Grain-size insensitive flow only develops at grain-sizes greater than 40 µm, as demonstrated by reference to mechanical data for the flow of Carrara and Taiwan marbles.

The flow laws which describe most of the experimental data do not extrapolate well through the lowest strain-rate data at the coarser grain-sizes. This indicates that a complete description of the flow, which can be used reliably to extrapolate to geological conditions, requires one or more of the material parameters
A, n
and
m
to be a function of one or more of stress, strain-rate and grain-size. This implies variable contributions from different deformation mechanisms as the deformation conditions change.
Estimates of the rates of microstructural changes in mylonites
Authors: Prior David J. , Knipe Robert J. , Handy Mark R.
Published: April 2008

Microstructures developed during dislocation creep such as the dynamically recrystallized grain-size and the subgrain-size are modified in response to increasing differential stresses and shear-strain rates adjacent to rigid minerals in mylonites. In two specimens variations in quartz subgrain-size around rigid minerals has been quantified from orientation contrast SEM images. In a deformed granite from the Pogallo ductile fault zone, quartz two-dimensional subgrain-size is reduced from background values of about 30 μm to 8 μm and then 4 μm in progressively narrower channels between rigid feldspars. In pelitic mylonites from the Alpine fault zone, New Zealand, quartz bands are wrapped around rigid garnets and the subgrain-size is reduced from background values of about 20 μm to 14 μm within 2000 μm of the garnet to 11 μm adjacent to the garnet. In both specimens examined the microstructure responds to both increases and decreases in differential stresses and coupled strain-rates. The analysis of the distribution of subgrain sizes provides information for the assessment of microstructural stability and the estimation of the rates of microstructural changes. The New Zealand specimens have good regional shear strain-rate controls and minimum subgrain boundary migration rates (during reduction in differential stresses and strain-rates) of 1.2 × 10
−9
μm s
−1
to 1.2 × 10
−11
μm s
−1
are estimated, for bulk shear strain rates of 10
−10
s
−1
and 10
12
s
−1
respectively. Palaeopiezometry coupled with rheological considerations suggests that these microstructural changes correspond to minimum differential stress-rates of 5 × 10
−9
MPa s
−1
to 5 × 10
−11
MPa s
−1
. Similar or slower regional stress-rates during fault zone evolution, will allow continuous re-equilibration of the microstructure. Thus the time at which any given microstructure is frozen in will be critically dependent upon fault zone deformation history. In the Alpine fault zone rapid uplift and high near surface temperatures combine to generate rapid stress drops as mylonitic deformation gives way to cataclastic faulting during uplift. Mylonite microstructure is frozen in during this rapid stress drop and represents the shallowest level of major crystal-plastic deformation.
Crystallographic fabrics: a selective review of their applications to research in structural geology
Authors: Law R.D.
Published: April 2008

In this brief introductory review the potential geological use of crystallographic fabrics is illustrated by considering selected geological problems which, given appropriate conditions, may be investigated in plastically deformed rocks using fabric analysis. Quartz and calcite are taken as the main illustrative examples. Numerical fabric simulations indicate that the imposed strain path (strain symmetry, vorticity etc.) is reflected in the relationship between the fabric pattern, kinematic framework and finite strain axes. Although these fabric patterns are sensitive to the numerical model and combination of crystallographic slip systems chosen, many of the major fabric types have been observed in experimentally and naturally deformed rocks. The fabric pattern itself may contain important information on strain symmetry, orientation of fields of extension and contraction and operative slip systems. Similarly, the angular relationship between the fabric pattern and finite strain features (foliation and lineation) may provide information on shear sense and vorticity of deformation. Spatial transitions with decreasing grain size in naturally deformed rocks, from strongly defined fabrics to a complete lack of crystallographic preferred orientation, have been interpreted as indicating a switch to deformation mechanisms involving grain boundary sliding. Potential problems associated with using the absence of a fabric as an indication of grain boundary sliding (and by inference superplastic flow) are discussed. Experimental studies indicate that geometrical relationships between intracrystalline strain features and the crystal lattice of individual grains may be used to deduce palaeo-stress directions. Results of palaeo-stress analysis techniques based on such relationships are compared.
A simulation of the temperature dependence of quartz fabrics
Authors: Jessell M. W. , Lister G. S.
Published: April 2008

This paper presents the predictions of a hybrid model of fabric development in quartzites that combines aspects of previous models based on lattice rotations and dynamic recrystallization. The input parameters for the model have been systematically varied in order to investigate changes in deformation fabrics due to increasing temperature, and predictions of crystallographic preferred orientations and grain shape fabrics are made. The resulting fabrics reflect the combination of both deformation processes over the range of the parameters used. The fabrics evolve continuously with progressive deformation, and produce crossed girdles, single girdles and point maxima for a single deformation geometry. The results suggest that the coupling of dynamic recrystallization with lattice rotations can actually produce a larger variation in crystallographic preferred orientations with temperature than is predicted by using a model that only considers lattice rotations. Using this model point maxima develop even though a multiple slip model has been assumed.
Practical application of entropy optimization in quantitative texture analysis
Authors: Schaeben H. , Siemes H. , Höfler S. , Will G.
Published: April 2008

Finite series expansion methods are applied to the texture problem of recovering an orientation density function (odf) from its pole density functions (pdf) measured in X-ray or neutron diffraction experiments, the mathematical model of entropy maximization is applied to pole figure data and numerical results are presented as well as an interpretation of their geological significance.
Deformation in an accretionary melange, Alexander Island, Antarctica
Authors: Nell P.A.R.
Published: April 2008

Alexander Island contains several belts of melange in a wide accretionary complex. One melange belt in the northwest of the island incorporates both oceanic and trench-fill material. It evolved by many different deformation mechanisms: dispersed independent particulate flow (IPF) and limited cataclasis at shallow levels; and diffusion mass transfer (DMT) and limited crystal plastic processes at deeper levels. Fluid pressures may have risen due to the subduction of young hot oceanic crust, which probably affected the structural evolution of the region by controlling the strength of the decollement and hence the taper of the accretionary prism.
Centrifuge modelling of thrust faulting: strain partitioning and sequence of thrusting in duplex structures
Authors: Liu Shumin , Dixon John M.
Published: April 2008

The kinematic evolution of fold-thrust duplex structures has been investigated by analog scale modelling using the centrifuge technique. Plasticine and silicone putty simulate rocks such as limestone and shale, respectively. Model stratigraphic units consist of microlaminations (55–250 μm thick) of these two materials, different ratios of thickness of the constituent laminae simulating different limestone/shale ratios (bulk competence). The models involve six stratigraphic units totalling 4 mm in thickness, with alternating bulk competence (low at the base, high at the surface). They were subjected to layer-parallel compression from one end, and kinematic evolution was monitored by cutting profile sections at six deformation stages.

Structural style varies with depth. The top competent unit develops upright buckle-folds which propagate from ‘hinterland’ to ‘foreland’. The middle competent unit also buckles, but the folds have pronounced foreland vergence. The bottom competent unit first develops gentle folds that propagate forwards, but the foreland-dipping limbs of these folds are soon cut by thrust faults. As shortening continues, this unit is telescoped into a classic blind duplex structure: thrust faults root in the underlying incompetent unit, ramp through the competent unit, and merge into a roof thrust in the overlying incompetent unit. The ‘wavelength’ of the duplex structure is inherited from the initial buckling instability. The duplex develops by serial nucleation of faults from hinterland to foreland as in natural thrust belts. However, all the faults remain active while the stratigraphic pile deforms as a tapered wedge.

Palinspastic reconstruction by restoration of buckle folds and fault slip shows that layer-parallel shortening accounts for approximately half of the total shortening at all structural levels. Folding and faulting account for the remainder, with folding dominating at high levels and faulting at depth. The transfer is accommodated by decollement in the incompetent units.

The models bear striking resemblance to fold-thrust structures of the Appalachian Valley and Ridge Province of West Virginia and Virginia, USA.
Vein distribution in a thrust zone: a case history from the Northern Apennines, Italy
Authors: Coli Massimo , Sani Federico
Published: April 2008

In the Northern Apennines the Cervarola Thrust superposes the Cervarola Unit (Early-Middle Miocene sandstones and shales) on the Marnoso Arenacea Unit (Middle-Late Miocene marly sandstones) and has the geometry of a leading imbricate fan. Each thrust sheet consists of a thick turbiditic sandstones sequence (Mt Cervarola Fm.), which grades downward into shales (Scisti Varicolore Fm.). During thrusting, the Scisti Varicolore Fm. was intensely deformed and widely affected by shear veins, filled with calcite. A regional pattern has been detected in the vein distribution: veins are concentrated some ten metres above the thrust fault, in a band a few metres thick, and can be grouped into two systems. The principal one presents two sets of veins striking in the same direction as the thrust: one set dips at a high angle to the thrust fault (High Angle Set, HAS), whereas the other one lies sub-parallel to the thrust fault (Low Angle Set, LAS). Both of these sets of veins have dip-slip slickenfibres. The secondary system comprises two other sets of veins, which lie obliquely to the thrust fault. Slickenfibres are strike-slip and trend nearly parallel to the thrusting direction and to the intersection lineation between the two sets. Vein development can be explained in the light of the fluid-dynamic studies recently developed for present-day accretionary prisms.
Tectono-stratigraphic development and hydrocarbon habitat of the Carboniferous in northern England
Authors: Fraser A. J. , Gawthorpe R. L.
Published: April 2008

Over 70 years of exploration in the Carboniferous of northern England has resulted in the discovery of rather modest recoverable reserves totalling 75 million barrels of oil and 27 billion cubic feet of gas. Nevertheless during this time the petroleum industry has amassed a substantial quantity of borehole and seismic information. Integrating this essentially subsurface database with information derived from outcrop studies has permitted a hitherto unachievable understanding of the Carboniferous in terms of its tectonostratigraphic development and hydrocarbon habitat.

The strong NW-SE and NE-SE structural trends developed in the northern England Carboniferous were inherited from the late Palaeozoic Caledonian orogeny. These fault trends were consistently reactivated throughout the Carboniferous in both an extensional and compressional sense. The main influence on Carboniferous basin evolution in northern England was the Variscan collision-type orogeny. The Variscan plate cycle controlled the development of syn-rift, post-rift and inversion megasequences from late Devonian to early Permian times. Sequences developed within these Carboniferous megasequences are primarily controlled by episodic rifting and periodic fault reactivation with eustatic sea-level changes providing only minor control at the subsequence level.

The late Carboniferous-early Permian culmination of the Variscan orogeny is seen to be the main trap forming event. All hydrocarbon discoveries to date display some element of Variscan inversion in their geometry. Variscan tectonics have also exerted a subtle but important control on play fairway evolution. The main source rocks (pro-delta shales) are confined to isolated, syn-rift depocentres. Syn-rift siliciclastic reservoirs are also restricted to the rifted half graben. Carbonate grainstone reservoirs rim the margins of the deeper half graben where terrigenous input has been limited. Delta top channel and mouth bar reservoirs are best developed where they axially infill remnant syn-rift bathymetry.

Mesozoic burial, ensuring hydrocarbon generation post Variscan trap formation, is, however, the main control on the present day distribution of hydrocarbons in the Carboniferous of northern England. Several key areas where Carboniferous source rocks have generated significant hydrocarbons during the Mesozoic have been identified; the East Midlands being the most significant in terms of produced hydrocarbons and perceived future potential.
The structural controls on Upper Jurassic and Lower Cretaceous reservoir sandstones in the Witch Ground Graben, UK North Sea
Authors: O’Driscoll D. , Hindle A. D. , Long D. C.
Published: April 2008

The Witch Ground Graben is a product of Mid- and Late Cimmerian extensional tectonism. The principal orientation of normal faulting was NNE-SSW during the Mid-Cimmerian (Bajocian to Early Kimmeridgian) and a combination of NW-SE and E-W during the Late Cimmerian (Late Kimmeridgian to Barremian). Block rotation on the dominant Late Cimmerian trends, combined with reactivation of older structural grains formed the fault block traps in the area.

Two major depositional systems form the reservoirs of the late Jurassic and early Cretaceous of the Witch Ground Graben, namely, the deltaic and shallow marine systems of the Sgiath and Piper Formations and the deeper water turbiditic sands of the Kimmeridge Clay and Valhall Formations. The transition between the systems is highly diachronous. The turbiditic sandstones are often enclosed within or onlap sealing strata, thus providing a stratigraphic element to many of the traps in the area.

A chronostratigraphic framework was established and used as the basis for understanding firstly, the timing of the tectonic episodes responsible for moulding the Witch Ground Graben and secondly, the importance of these events in determining reservoir sandstone distribution and trap development.
Tertiary structures and hydrocarbon entrapment in the Weald Basin of southern England
Authors: Butler Malcolm , Pullan Christopher P.
Published: April 2008

The Weald Basin of southeast England was formed by rapid subsidence associated with thermal relaxation following early Mesozoic extensional block faulting. The basin appears initially to have taken the form of an easterly extension of the Wessex Basin but became the major depocentre during the Upper Jurassic and Lower Cretaceous, with associated active faulting. These movements appear to have ceased prior to Albian times and a full Upper Cretaceous cover is believed to have been deposited in a gentle downwarp which extended far beyond the confines of the Weald and Wessex Basins. Major inversion of the Weald Basin took place in the Tertiary, with both gentle regional uplift, which in the eastern part of the basin is estimated to have exceeded 5000 feet (1525 metres), and intense local uplift along pre-existing zones of weakness, which led to the formation of compressional features such as tight folds and reverse faults. Zones of Tertiary deformation appear to have been strongly influenced by underlying, particularly Hercynian, structural trends.

Lower Jurassic source rocks reached maturity in the early Cretaceous and initial migration occurred at this time, often over long distances, into traps closed by pre-Aptian faults. Tertiary tilting and uplift led to the breaching of many of these pre-existing traps and the formation of large folded closures. A second phase of hydrocarbon migration, particularly of gas, took place at this time, with significant vertical migration along fault zones. Major reservoirs located to date occur in Middle Jurassic carbonates and Upper Jurassic sandstones, but deep burial in the basin has caused considerable destruction of primary reservoir characteristics; changes in the temperature and pressure regimes and the mobilization of fluids within the basin resulting from the Tertiary uplift caused further diagenetic changes, particularly in the carbonate reservoirs.

Exploration of the Weald Basin remains at a very early stage, with a low drilling density to date. The more recent drilling has focussed on earlier structures, but traps formed or modified during the Tertiary movements represent important exploration objectives, although general deterioration in reservoir quality towards the centre and east of the basin makes large fold closures in these areas less attractive.
Active normal faulting in central Greece: an overview
Authors: Roberts Steven , Jackson James
Published: April 2008

Rapid extension in central Greece is accommodated by large active normal faults that largely control the geomorphology. The faults bound graben whose asymmetry is evident in the topography, bathymetry, vertical movements of the coastline and tilting of Neogene sediments adjacent to them. Graben up to 20 km in width appear to be simple half-graben, whereas the hangingwalls in wider graben are generally broken by faults antithetic to the major bounding faults. The graben form basins up to 100 km long, but the faults bounding these basins are segmented, with individual segments no longer than about 20 km. The fault segments step en echelon or change polarity along strike of the large basins, but are not linked by simple strike-slip (‘transfer’) faults: the regions between the main fault segments appear to be pervasively deformed by numerous small faults. Drainage in central Greece is controlled by fault segmentation and footwall lithology. Where footwalls consist of Mesozoic limestone the drainage is often axial, breaking through the fault system into the main basin by flowing round the ends of fault segments or through gaps where the fault segments step. In such places, drainage directly off the fault scarps is relatively minor, and the fault segmentation controls the spacing of major fans along the basin margin. Where footwalls consist of Neogene sediments, drainage may flow directly across the fault scarp, though in such cases the catchment areas are relatively small and linear. The origin of the fault segmentation is unclear, but may be related to rotations of the fault-bounded blocks about horizontal and vertical axes.
The geometry of normal faults in a sector of the offshore Nile Delta, Egypt
Authors: Beach A. , Trayner P.
Published: April 2008

This paper presents an interpretation of the geometry of normal faults in part of the offshore Nile Delta. The structure is dominated by north-dipping faults that are clearly imaged on seismic reflection profiles, and affect sediments of Pliocene-Pleistocene age, together with the underlying Miocene sequences that contain potential hydrocarbon reservoir rocks. The geometrical analysis is carried out in several stages. Firstly, time slice maps of the fault pattern are produced in order to establish the overall fault geometry of the area. Secondly, for each indivdiual fault identified, structure contours on the fault surface itself are mapped. This procedure serves to highlight any complexities in the geometry of the faults, and allows potential inconsistencies in the interpretation to be identified. Thirdly, horizon interpretations and correlations are made across the faults, and these are checked for consistency by constructing maps of the magnitude of the dip slip component of displacement within the fault surface. The complete geometric analysis makes it possible to extend the results from the good data areas in the upper parts of seismic sections to the poorer quality areas lower down. In this way it is possible to derive an interpretation of the structural geometry of the Miocene reservoir sequences. The seismic reflections from the Miocene are poor and irregular, and structural interpretation and modelling reduces the uncertainty associated with the reservoir geometry. The structural interpretation is presented as a model of tilted and fault bounded blocks, with considerable horizontal separation. This provides a much clearer and more consistent picture of the reservoir geometry, and contrasts with previous interpretations that have shown a more or less continuous reservoir sequence with little structure. The geometrical interpretation should form the basis for a new hydrocarbon play in the area because it predicts the locations of footwall crests, as well as areas where reservoir is missing because of separation across faults.
Geometric and kinematic coherence and scale effects in normal fault systems
Authors: Walsh John J. , Watterson Juan
Published: April 2008

Fault displacements derived from a seismic reflection survey of an offshore oilfield are projected onto a vertical plane parallel to the fault strike and the displacement values contoured. In addition to those for single faults, displacement diagrams are also constructed for fault arrays by aggregating the displacement values on selected faults. Displacement contours form regular and systematic patterns, even when there is no continuity between the fault surfaces in the array. A system in which linkage between the elements of a fault array is achieved by ductile strain of the intervening rock is referred to as soft-linked and is regarded as the general case. Geometrical coherence, with regular and systematic displacement patterns, exists at all stages in the growth of a fault array, implying a kinematic coherence requiring a high degree of synchronous movement, as opposed to sequential development, on individual elements in the array.

Conventional maps and diagrams allow representation of only two orders of magnitude of fault displacement rather than the five or more which may occur. A soft-domino model is presented in which the role of ductile strains is acknowledged both in accommodating varying displacements on fault surfaces and in extension on structures too small to be represented individually, and in which the rotation of rigid fault blocks is of reduced importance when compared with rigid-domino models.
Numerical and analogue modelling of normal fault geometry
Authors: Dresen G. , Gwildis U. , Kluegel Th.
Published: April 2008

In an attempt to constrain the influence of boundary conditions on fault nucleation and propagation processes, basement-controlled normal faulting has been simulated using analogue clay models and plane-strain finite element models. Two opposing basement configurations were investigated and the results of the analogue and numerical models were compared.

In the first series of clay models subsidence of a basement block along an inclined normal fault and along a vertical fault was simulated. An upwards-propagating fault initiated at the base of the clay model. The final fault geometry was consistent with the geometry of the stress distribution calculated in the finite element model. No listric fault developed.

In the second experiment horizontal basement extension was simulated and, as expected from the numerical calculations, these boundary conditions resulted in the formation of listric faults. The results of the analogue and numerical experiments show that initial fault geometry can be predicted successfully from the stress distribution at the elastic limit.
Calculating normal fault geometries at depth: theory and examples
Authors: White Nicky , Yielding Graham
Published: April 2008

In recent years, a large number of methods relating the shape of a normal fault at depth to the shape of a sedimentary horizon in its hangingwall have been described. Such methods are best used in areas where it can be assumed that the footwall remains relatively undeformed during extension. This assumption may not always be valid, especially in the case of large-scale, basement-extending normal faults. Conservation of area (or solid area when compaction occurs) must be the fundamental constraint. The advantages and disadvantages of the various methods are briefly outlined. A general model which assumes that hangingwall deforms by arbitrarily inclined simple shear and differential compaction is discussed. Fault geometry, the inclination of simple shear, and compaction parameters may all be determined from
N
beds using a simple inversion scheme based on this general method. The algorithm has previously been tested on synthetic data using a range of fault geometries. Such testing indicates that all unknown parameters including fault geometry can, in general, be uniquely determined provided that the geometries of two or more beds within the hangingwall are known. It is important that hangingwall and footwall stratigraphies are accurately known, though uncertainty in either could be included as a bounded variable within a formal inversion scheme. In this paper, the method is applied to laboratory-modelled normal faulting and to depth-converted seismic reflection data. Results suggest that hangingwalls deform by bulk, antithetically inclined, simple shear. Differential compaction of hangingwall sediments is often important and should be taken into account.
Quartz grain types in Holocene deposits from the Spanish Central System: some problems in provenance analysis
Authors: Tortosa Amparo , Palomares Marta , Arribas José
Published: April 2008

Holocene sands of the Spanish Central System were exclusively derived from plutonic, middle-upper grade and low-grade metamorphic rocks. Modal composition of studied sands is mostly controlled by grain size and source area lithology. Thus, sands derived from slates and schists plot near the QR edge on the QFR diagram for all grain size fractions. Sands derived from granitic or gneissic rocks have a wide dispersion on the QFR diagram, from the R pole to the QF edge, depending on sand grain size.

Percentages of quartz types in granitic-derived sands are Qnu
42
, Qu
40
, Qp2–3
14
and Qp > 3
4
. Sands of gneissic origin have Qnu
51
, Qu
15
, Qp2–3
23
and Qp > 3
11
. Sands derived from slates and schists have Qnu
20
, Qu
12
, Qp2–3
5
and Qp > 3
63
. Quartz types easily discriminate sands of low-grade origin, but distinction of sands derived from plutonic rocks from those derived from middle-upper grade metamorphic rocks is difficult because of the highly variable Qu content of plutonic rocks related to strain history and crystallization conditions. Thus, quartz types must be used with caution in source discrimination if plutonic rocks are present in the source area.
The role of fission track dating in discrimination of provenance
Authors: Hurford Anthony J. , Carter Andrew
Published: April 2008

Determination of single crystal ages by fission track dating analysis of zircons (and subordinately apatites) within a detrital heavy mineral assemblage, together with frequency distribution and radial plot analysis of the measured ages, permits identification of different age modes. Such modes may characterize the provenance of a sediment and assist in defining source areas. A limitation of the application is the thermal stability of tracks within a mineral above which tracks are lost and ages partially reset: in broad terms over geological time 200–250°C for zircon and <100°C for apatite. Synopses are presented of published fission track provenance studies in north central Victoria, northeastern Tasminia, Barbados, the Rockall Trough, the English Wealden, Mexico, Utah and the Central Alps.
A local source for the Ordovician Derryveeny Formation, western Ireland: implications for the Connemara Dalradian
Authors: Graham John R. , Wrafter John P. , Daly Stephen , Menuge Julian F.
Published: April 2008

Facies and palaeocurrent data indicate that the conglomeratic Ordovician Derryveeny Formation is the deposit of an alluvial fan whose apex was located a few kilometres east of the present outcrop. Clasts in the conglomerates include migmatite, schist, gneiss, granite, acid porphyry, spilite and vein quartz. Sillimanite bearing migmatite clasts closely resemble rocks of the neighbouring Lough Kilbride Formation, a unit of the Connemara Dalradian (Lower Proterozoic) basement from which they are presumed to be derived. This local source for the conglomerates is also suggested by the similarity of the Nd-model ages (
c.
2.15 Ga) from the metamorphic clasts and from the Lough Kilbride Formation. Moreover peraluminous granite clasts give the same model ages, suggesting the granites are intracrustal melts from the same source. Interpretation of the Connemara Dalradian as a suspect terrane requires that it docked before Upper Ordovician times. Differences between the clast assemblage in the Derryveeny Formation and the presently exposed Lough Kilbride Formation are related to stripping of the original cover, emphasizing the value of clastic detritus in the study of the uplift history of metamorphic basement.
Controls on the petrographic evolution of an active margin sedimentary sequence: the Larsen Basin, Antarctica
Authors: Pirrie Duncan
Published: April 2008

The sedimentary fill of the Cretaceous-Tertiary Larsen Basin, located at the northern tip of the Antarctic Peninsula, records the evolution of the Antarctic Peninsula source terrain. Uplift, and the possible renewal of arc volcanism within this area, is documented by the wide petrographic spread in the lower Gustav Group (Barremian-Coniacian). The Hidden Lake Formation (Coniacian-Santonian) records a major pulse in proximal calc-alkaline arc volcanism. The overlying Marambio Group shows a change from lithic-volcanic sandstones to quartz-feldspar-rich sandstones, reflecting a change in source terrain. An abrupt change in sandstone composition in the upper Santa Marta Formation (Campanian) may reflect both a switch in volcanism and plutonism from the east to the west coast of the Antarctic Peninsula (i.e. away from the basin margin), and a change to more mafic volcanism. The Lopez de Bertodano Formation (Campanian-Palaeocene) dominantly reflects a change to a quartzo-feldspathic source terrain (the Trinity Peninsula Group). This is also related to both a decrease in the intensity of arc volcanism and an increase in distance from the location of concurrent volcanism in relation to the site of deposition. Controls on petrography other than simple arc-unroofing (e.g. location and nature of arc volcanism and depositional setting) play an important role in sandstone petrography. These controls on sandstone composition should always be considered in interpreting sequences from active plate margin settings.
An integrated approach to provenance studies: a case example from the Upper Jurassic of the Central Graben, North Sea
Authors: Humphreys Bernard , Morton Andrew C. , Hallsworth Claire R. , Gatliff Robert W. , Riding James B.
Published: April 2008

The provenance of Upper Jurassic sandstones in a small half graben in the southern Central Graben of the North Sea has been investigated by studying a range of parameters: the composition of feldspars, the nature of rock fragments and their cathodoluminescence properties, heavy mineral assemblages, clay mineral assemblages, whole rock geochemistry and the occurrence of reworked palynomorphs. The combined result of these investigations is that rocks of Triassic, Permian and Carboniferous age are believed to have supplied detritus to the Upper Jurassic basin and to have diluted the contemporaneous bioclastic deposits. However, each individual provenance indicator tended to emphasize the importance of one particular source. Only an integrated study provided a complete picture of the interplay of different sources during deposition. Such integrated studies also identify the limitations of individual provenance indicators, in particular the adverse role of burial diagenesis in the removal of diagnostic grains such as volcanic feldspars and certain heavy mineral grains.
Petrographic evidence of different provenance in two alluvial fan systems (Palaeogene of the northern Tajo Basin, Spain)
Authors: Arribas José , Arribas M. Eugenia
Published: April 2008

Palaeogene detrital deposits of the northern Tajo Basin are coalescent alluvial fan systems interfingering distally with lacustrine carbonates. Non-carbonate extrabasinal clasts increase to the east while carbonate extrabasinal clasts decrease. Rock fragments increase to the west, while the feldspar/quartz ratio remains constant. Rock fragments define two sedimentary domains: the Iberian, in the east, was derived from Mesozoic rocks of the Iberian Range, and the Central System, to the west, was derived from Cretaceous cover and Palaeozoic metamorphic basement. Evolution of sandstone composition is related to erosion of the source areas and is different in the two domains. The tectonic setting is apparently ‘recycled orogen’, providing calcareous rock fragments are included in the total lithic clasts.