fluid overpressure estimates from the aspect ratios of mineral veins abstract several hundred calcite veins and mostly normal faults were studied in limestone and shale layers of a mesozoic sedimentary basin next to the village of kilve at the bristol channel sw england the veins strike mostly e w 239 measurements that is parallel with the associated normal faults the mean vein dip is 73 n 44 measurements field observations indicate that these faults transported the fluids up into the limestone layers the vein outcrop trace length 0 025 10 3 m and thickness 0 1 28 mm size distributions are log normal taking the thickness as the dependent variable and the outcrop length as the independent variable linear regression gives a coefficient of determination goodness of fit of r2 0 74 significant with 99 confidence but natural logarithmic transformation of the thickness length data increases the coefficient of determination to r2 0 98 indicating that nearly all the variation in thickness can be explained in terms of variation in trace length the geometric mean of the aspect length thickness ratio 451 gives the best representation of the data set with 95 confidence the true geometric mean of the aspect ratios of the veins lies in the interval 409 497 using elastic crack theory appropriate elastic properties of the host rock and the mean aspect ratio the fluid overpressure that is the total fluid pressure minus the normal stress on the fracture plane at the time of vein formation is estimated at around 18 mpa from these results and using the average host rock and water densities the depth to the sources of the fluids below the present exposures forming the veins is estimated at between around 300 m and 1200 m these results are in agreement to those obtained by independent isotopic studies and indicate that the fluids were of rather local origin probably injected from sill like sources water sills inside the sedimentary basin

modelling palaeostress magnitude and age in extensional basins a case study from the mesozoic bristol channel basin u k analysis of normal faults and extensional veins developed in the upper triassic and lower jurassic succession exposed along the bristol channel coast indicate a protracted period of rift related deformation from late triassic to early cretaceous during this period it is demonstrated that a consistently oriented stress system operated with 3 oriented ne sw but with stress ratios 2 1 3 varying from 0 9 0 1 two approaches are described to estimate rifting stress magnitudes the first involves data from synsedimentary faults and yields for late triassic rifting 1 0 714 mpa 2 0 169 mpa and 3 0 033 mpa the second is developed for faulting in the lower jurassic section where no direct evidence of age is available the method calculates stress magnitudes with the rifting stress ratio of 0 9 for varying increments of overburden load each increment represents the possible magnitude of tectonic stresses at the time indicated by the amount of burial by incrementally adding the estimated remaining lower jurassic lower cretaceous overburden load to the stress magnitudes of each case a plot of reducing stress ratio for each age of faulting is determined and that which best reproduces the range of stress ratios calculated represents the modelled estimate of stress magnitude and timing the results suggest an end early jurassic onset of faulting with principal stress magnitudes of 1 12 98 mpa 2 12 56 mpa 3 8 80 mpa several simplifying assumptions used in the analysis are discussed

a seismic study of deep geological structure in the bristol channel area sw britain summary results from eight seismic refraction lines 35 90 km long in the bristol channel area are presented the data mostly land recordings of marine shots have been interpreted mainly by ray tracing and time term modelling upper layer velocities through palaeozoic rocks usually fall within the range 4 8 5 2 km s 1 below the carboniferous limestone with a normal velocity of 5 1 5 2 kms 1 the old red sandstone with a velocity of 4 7 4 8 kms 1 acts as a low velocity layer as do parts of the underlying lower palaeozoic succession in the central south wales bristol channel area and the mendips a 5 4 5 5 km s 1 refractor is correlated with a horizon at or near the top of the lower palaeozoic succession under the whole area except for north devon a 6 0 6 2 km s 1 basal refractor has been located and is correlated with precambrian crystalline basement rocks in general this refractor deepens southwards from a series of basement highs which existed before the major movements of the variscan orogeny in south wales resulting in a southerly thickening of the pre upper carboniferous supra basement sequence in north devon a 6 2 km s 1 refractor at shallow depth interpreted as a horizon in the devonian or lower palaeozoic succession overlies a deep reflector that may represent the precambrian crystalline basement

structural geological factors in open pit coal mine design with special reference to thrusting case study from the ffyndaff sites in the south wales coalfield

estimating flow heterogeneity in natural fracture systems abstract examples of small to medium scale fault systems have been mapped in jurassic sedimentary rocks in north somerset england these examples include contractional and dilational strike slip oversteps as well as normal faults these maps form the basis of calculations performed to investigate heterogeneity in natural fracture systems with the aim of predicting fluid flow localisation in different fault styles as there is no way to measure fracture aperture directly we use vein thickness to represent an integrated flow path or palaeo aperture from which we derive a representation of the flow distribution three different methods are used to estimate flow heterogeneity based on 1 fracture density the ratio of fracture length to area 2 fracture aperture fracture porosity and 3 hydraulic conductance fracture permeability normalised to the pressure gradient and fluid properties our results show that fracture density and hydraulic conductance are poorly correlated and that fracture density does not fully represent the natural heterogeneity of fracture systems fracture aperture and hydraulic conductance indicate stronger degrees of flow localisation different types of structures also seem to display characteristic and predictable patterns of heterogeneity normal fault systems show the highest magnitude of localisation along the faults rather than in the relay ramps while contractional and dilational strike slip systems show very strong localisation in the faults and oversteps respectively in all cases the amount of damage in the oversteps can modify such patterns of heterogeneity

joint development in perturbed stress fields near faults abstract field evidence is presented for complex spatial and temporal perturbations of an otherwise systematic joint pattern around faults from well exposed faulted rock platforms joints propagating in perturbed stress fields will curve to follow the directions of the stress field trajectories a progressive change in joint direction is observed from unperturbed regions away from faults to strongly perturbed zones adjacent to faults this indicates that the joint pattern can reflect perturbations of the regional stress field around faults in the examples the stress field perturbations are probably due to points of high friction on the fault plane which concentrate stress and distort the stress field in the surrounding rock the corresponding joints converge at these points and are sub parallel to the fault along the remainder of the fault plane the possibility that a fault plane acts as a free surface contained within an elastic body is considered in this situation the fault plane induces a rotation of the principal stress axes to become either perpendicular or parallel to the fault the free surface model seems to explain the metre scale curvature of joints in the vicinity of existing joints but at the kilometre scale of a large fault plane the model becomes unrealistic unless the fault is open at the earth s surface two examples are investigated from the lias of great britain at nash point and robin hood s bay both comprise sub horizontal strata of relatively homogeneous lithology and bed thickness which provide striking examples of joints developed near faults

structural analysis and kinematic evolution of the inverted central south celtic sea basin abstract the central south celtic sea basin shows excellent examples of tectonics and sedimentation relationships as evidenced by detailed geological interpretation of a local grid of seismic profiles these relationships together with balanced restored geological cross sections and block diagrams have been used to unravel the post variscan evolution of this part of the celtic sea basin three sedimentary mega sequences separated by two erosional surfaces have been recognized overlying a deformed palaeozoic basement in this area 1 permo triassic jurassic syn rift mega sequence sedimentation of the lowest part of this succession is strongly controlled by extensional faults whereas the rest of the sequence indicates much less fault control lateral extent of these rocks is also controlled by the disposition of the palaeozoic basement which appears as an elevation in the pembrokeshire ridge area where no sedimentation occurs 2 cretaceous post rift mega sequence 3 tertiary post rift mega sequence the cretaceous and tertiary successions are more extensive than permo triassic jurassic sediments covering the central south celtic sea basin and the pembrokeshire ridge the structural framework displayed by the syn rift and post rift sequences is very different thus the general structure beneath the jurassic cretaceous late cimmerian unconformity consists of a wsw ene trending fault system and related folds this resulted from an extensional phase which led to a sub basin compartmentalization of the area and a subsequent inversional event which caused significant reverse movement along previous extensional structures early cretaceous erosion was responsible for an important and unequal denudation the timing of the switch between extension and inversion cannot be determined due to early cretaceous erosion overlying the jurassic cretaceous unconformity cretaceous sediments are slightly deformed by broad anticlines formed during early tertiary mainly as a result of tightening of previous folds the tertiary sequence shows a subhorizontal disposition responsible for the present day smooth bathymetric relief of the south celtic sea basin

palaeostress analysis a contribution to the understanding of basin tectonics and geodynamic evolution example of the permian cenozoic tectonics of great britain and geodynamic implications in western europe abstract microtectonic analysis in association with tectonic sedimentological observations enables us not only to define palaeostress tensors but also to date each of them such a study carried out in central and northeastern england and in southernmost wales has permitted us to point out several tectonic stages during the permian cenozoic period 1 inferred from syn sedimentary faulting in the late permian series the permian early triassic tectonic regime is characterised by a nnw sse oriented extension 2 during the late triassic to the early late jurassic an e w to ene wsw oriented extension was acting as suggested by syn sedimentary normal faulting in keuper lias and middle dogger sediments and post sedimentary normal faulting in oxfordian sediments 3 the following tectonic event probably started during the malm and was particularly active during the early cretaceous it was characterised by a n s to nne ssw oriented extension and was associated with an e w to wnw ese oriented strike slip tectonic regime transtension 4 a locally observed nw se oriented transpressional strike slip tectonic regime is thought to correspond to the laramide inversion phase of the middle paleocene 5 the later tectonic event is characterised by a n s oriented transpressional strike slip regime and affects a paleocene tholeiitic dyke cleveland dyke it is supposed to be synchronous with the so called eocene pyrenean to the early miocene helvetic compressional stages these paleostress tensors are compared with others defined in france the benelux countries and germany and are also collated to the tectonic evolution of oil basins assessed from geophysical analyses these comparisons raise discussions about the paleostress distribution within the geodynamic evolution of the west european shelf

the permian to cretaceous structural evolution of the western approaches basin melville sub basin uk summary the tectonic sedimentary evolution of the western part of the western approaches basin melville sub basin was initiated during early permian times when synrift clastic sedimentation and volcanism commenced within an ene trending half graben this sequence thickened to the south and onlapped a high to the north during the permian an early nnw sse basin extension was accommodated on an array of ene wsw trending normal faults linked by nnw sse orientated transfer faults this was followed by a period of ene wsw extension along the basin axis when the normal and transfer faults of the previous faulting episode swapped roles this was possibly related to early rifting in the bay of biscay both sets of faults appear to be controlled to some extent by variscan thrusts and strike slip faults within the basement during the upper triassic the ridge in the north subsided allowing a thick sequence of salt to be deposited where there had previously been a topographic high during jurassic times sedimentation extended across the permian triassic half graben into the french sector this was followed by an episode of rifting and wrench deformation and inversion during which the permian to jurassic basin was domed and uplifted the cretaceous to tertiary sedimentation which followed probably resulted from postrift thermal subsidence related to cooling of the continental platform adjacent to the opening atlantic ocean steep normal and or strike slip faults data from the earliest part of this postrifting stage

sequence architecture of ancient rocky shorelines and their response to sea level change an early jurassic example from south wales uk the sea cliffs of ogmore by sea in glamorgan south wales expose a succession of early jurassic nearshore carbonates that drape rocky palaeoplatforms a sequence stratigraphic interpretation allows the recognition of four retrogradationally stacked parasequences the internal facies architecture of each of which is controlled by the bathymetry and geometry of the underlying shore platforms preserved facies are interpreted as representative of foreshore shoreface and offshore environments supratidal facies are absent but representative storm terrace clast assemblages are preserved in all lithofacies as a result of the cannibalization of the backshore during periods of shoreline retreat driven by sea level rise biostratigraphical calibration allows the reconstruction of sea level history and the correlation of regionally important flooding surfaces in the rhaetian sinemurian of southern britain with the position of the contemporary shoreline this suggests that platform incision in south wales occurred several million years before the onset of major deposition superseding deposits above ancient rock shore platforms may therefore not be representative of the environment in which coastal incision took place as one of the best examples of its kind the rockshore at ogmore acts as a deposystem model for ancient rocky shores worldwide

hybrid veins from the southern margin of the bristol channel basin uk sinistral and dextral en echelon hybrid calcite veins are exposed on the southern margin of the bristol channel basin north somerset on a liassic carbonate platform these hybrid veins are transitional between pure extensional mode i and shear fractures mode ii mode iii and experienced both extensional and shear displacements during their formation in the study area they occur in conjugate sets with the conjugate angle ranging between 10 and 50 based on the kinematic analysis of these veins a new model for the opening of the bristol channel is proposed

mesozoic reactivation of variscan thrusting in the bristol channel area uk new seismic reflection data from the bristol channel area provide direct evidence of a major variscan thrust in the palaeozoic basement beneath the mesozoic bristol channel syncline the thrust has an average southerly dip of about 25 and it rises into the major fault the central bristol channel fault zone cbcfz at the northern margin of the bristol channel syncline the cbcfz is attributed to reactivation in extension of the variscan thrust probably during the early cretaceous there is no evidence for major synsedimentary movement on the cbcfz and the bristol channel syncline does not represent a rollover structure section restoration suggests that the cbcfz has cut up through a previously tilted sequence of triassic and jurassic strata pervasive nnw sse cross faults are interpreted as original variscan transfer faults that have been reactivated in the mesozoic and tertiary

late mesozoic reactivation of variscan faults in the north celtic sea basin ireland abstract three major ene trending reactivated fault systems downthrow to the north west in the north western part of the north celtic sea basin they control the location of three mesozoic half grabens the location of the faults is determined by the underlying basement structures these are variscan in age although they exploit older caledonide trends comparison of the variscan structures identified in the area with regional structures suggests a series of imbricate thrust features these structures reactivated during the mesozoic controlled both the location and depositional histories of the half graben basins in the region

strike slip relay ramps areas of reorientated bedding at contractional oversteps between strike slip faults are here called strike slip relay ramps metre scale examples are described from the jurassic sediments at east quantoxhead somerset u k larger strike slip relay ramps occur in the rio de peixe basin ne brazil along the newport inglewood trend california and in the bovey basin sw england although the geometry and development of strike slip relay ramps are similar to those of relay ramps in normal fault systems there are differences in the structures which accommodate the transfer of displacement between the overstepping faults whereas strike slip relay ramps are typically transpressional with pressure solution often occurring relay ramps in normal fault systems are dominated by extension or transtension care needs to be taken when interpreting areas of reorientated bedding between overstepping faults particularly when displacement directions are unknown for example when using seismic data this is because relay ramps can occur in both strike slip and normal fault zones

triassic jurassic development of the st george s channel basin offshore wales uk abstract isopach maps and backstripped tectonic subsidence plots from the triassic jurassic of the st george s channel basin reveal significant departures from current models of extensional basin development i rifting was highly protracted with discrete increments of crustal extension spanning ca 120 my ii substantial syn kinematic sediment on the footwalls of major faults records pure shear sub seismic scale deformation to have been important during regional triassic extension forward modelling of depth converted seismic interpretations is used to test the notion of combined pure and simple shear and to quantify the distribution of crustal extension between large and sub seismic scale faults the models indicate that triassic subsidence was generated by ca 2 5 km extension on major faults and 10 km extension by sub seismic scale faulting in fault bounded compartments jurassic extension was dominated by large scale faults whereas in the areas of overlap between major fault segments jurassic extension was taken up chiefly by sub seismic scale faulting comparison of measured vitrinite reflectance vr and vr modelled from heatflow history yielded by the forward models supports this scheme of episodic pure shear dominated extension this research highlights the inherent difficulty in using seismic interpretation in isolation from quantitative subsidence and or thermal history data to identify syn and post kinematic sedimentary sequences furthermore it provides an opportunity to more fully reconstruct mesozoic basin development in the west of uk area which in the cheshire central and east irish sea basins is largely precluded by erosion of post triassic strata

the moho in extensional tectonic settings insights from thermo mechanical models the lithospheric memory is key for the interplay of lithospheric stresses and rheological structure of the extending lithosphere and for its later tectonic reactivation other important factors are the temporal and spatial migration of extension and the interplay of rifting and surface processes the mode of extension and the duration of the rifting phase required to lead to continental break up are to a large extent controlled by the interaction of the extending plate with slab dynamics the finite strength of the lithosphere has an important effect on the formation of extensional basins this applies both to the geometry of the basin shape as well as to the record of vertical motions during and after rifting we demonstrate a strong connection between the bulk rheological properties of europe s lithosphere and the evolution of some of europe s main rifts and back arc systems the thermo mechanical structure of the lithosphere has a major impact on continental break up and associated basin migration processes with direct relationships between rift duration and extension velocities thermal evolution and the role of mantle plumes compressional reactivation has important consequences for post rift inversion borderland uplift and denudation as illustrated by poly phase deformation of extensional back arc basins in the black sea and the pannonian basin region

the undrained shear strength of soils containing large gas bubbles measurements of undrained shear strength are reported from a series of triaxial tests on reconstituted soil samples containing large bubbles of methane gas the experimental results indicate that the undrained strength can be either increased or reduced by the presence of large gas bubbles depending upon the values of the consolidation pressure and the initial pore water pressure a conceptual model for soils containing large gas bubbles developed in the companion paper is analysed to provide theoretical predictions for the undrained shear strength the analysis leads to theoretical upper and lower bounds for the undrained strength which show good agreement with the experimental data the experimental and theoretical developments have led to a better understanding of the strength of soils containing large gas bubbles this could be of considerable benefit to the offshore construction industry if platforms pipelines and other structures are to be built in areas of gas bearing sediments l article pre

selective reverse reactivation of normal faults and deformation around reverse reactivated faults in the mesozoic of the somerset coast abstract normal faults exposed in the triassic jurassic limestones and shales of the somerset coast were formed during the mesozoic development of the bristol channel basin reverse reactivation of some of these normal faults occurred during late cretaceous to early tertiary north south contraction the contraction is also evident from thrusts and conjugate strike slip faults preferential reactivation of the normal faults is attributed to 1 decreased fault plane friction 2 domino block rotation 3 displacement magnitude and 4 fault connectivity the geometries of overlapping and underlapping zones in reactivated fault zones are dependent on the existing structural geometry two distinctive styles of displacement accommodation occur between reverse reactivated normal faults 1 formation of a network of strike slip faults conjugate about nne ssw and 2 oblique steeply dipping reverse faults interaction between strike slip and an existing fault is dependent on whether the normal fault was reactivated the range of structures related to the north south contraction has been incorporated into a single deformation model controlled by the northwards movement of the hanging wall of the quantock s head fault pure dip slip movement occurred in the centre of its curved fault trace with a sinistral component at the western tip and a dextral component of displacement and strike slip block rotations occurred at the eastern tip shortening of these blocks was achieved through development of a strike slip fault network and nw striking thrusts in an underlap zone loading of the footwall by the hanging wall block modified the local stress system to allow formation of oblique steeply dipping reverse faults

the behavior of loose gassy sand methane gas found in loose deltaic deposits can contribute to the triggering of flow liquefaction of submarine slopes the behavior of loose gassy sand is studied in the laboratory using monotoni

crustal structure beneath southern england from deep seismic reflection profiles seismic reflection events from recently acquired profiles in southern england are interpreted in terms of stratigraphic and structural interfaces between rock units in particular evidence is brought forward that a series of reflection events dipping between 22 and 27 with azimuth averaging 160 represents 2 major thrust zones occurring in the variscan floor beneath mesozoic cover a geological model based upon borehole information and regional geological considerations is presented the tentative recognition of crystalline basement overlain and overstepped by late proterozoic rocks of variable thickness suggests that basement in this area was unstable from late precambrian times the mesozoic structural evolution can be related to tectonic re activation of major concealed thrust zones in the palaeozoic 9basement9

geometry and development of relay ramps in normal fault systems relay ramps occur between normal fault segments that overstep in map view the geometry and evolution of exposure scale relay ramps are described from the somerset coast england and are compared with larger scale ramps from elsewhere relay ramps can be classified into four groups based on the degree of interaction and linkage between the overstepping segments these groups are interpreted as being evolutionary stages in stage 1 the segments do not interact stage 2 involves the reorientation of bedding between two interacting faults to produce a relay ramp in stage 3 connecting fractures start to break the relay ramp stage 4 is when the relay ramp is destroyed to produce a single fault that has an along strike bend these evolutionary stages can develop through ti e but they can also be seen spatially a branch line between normal faults or an along strike bend may represent a stage 4 relay with progressively earlier stages occurring updip or downdip characteristic variability in displacement distance profiles for fault segments and linked faults accompanies the interaction and linkage processes displacement transfer by relay ramps is accompanied by steep displacement gradients along fault segments at oversteps relay ramps often contribute to a minimum in total fault displacement at a linkage point

deep seismic reflection survey across the variscan front of southern england in 1979 the institute of geological sciences commissioned two lines of seismic reflection profile a technique which has proved successful1 for studying crystalline basement the survey was sited to cross the concealed variscan front in southern england with recording time extended to 12 s two way time twt similar profiles in adjacent parts of europe have suggested that the variscan front is characterized by major thrust zones extending to considerable depths2 3 our profiles reported here show good primary reflections to 6 s twt and give new detail on the nature of variscan deformation in southern england zones of differing reflection character are evident a northern zone of low northerly apparent dips a central zone of poor reflector quality and a southern zone of steeper dips to the sse the southern zone is presumed to represent deformed variscan basement including several minor thrust planes and a proposed major thrust zone analogous to similar structures in europe impersistent reflections between 9 and 10 s twt could possibly relate to the moho

basin inversion by distributed deformation the southern margin of the bristol channel basin england several models of basin inversion described in the literature are tested in a study of triassic and early jurassic strata exposed along the southern margin of the bristol channel basin in somerset england that has been exhumed by 95 of them are still in net extension despite widespread kinematic evidence for reverse reactivation when coupled with the general absence of thin skinned thrusts and the widespread occurrence of regional contractional folds it appears that none of three main inversion models the fault reactivation model the thin skinned model and the buttress model are by themselves applicable we erect a new model of basin inversion the distributed deformation model which consists of three stages of basin inversion stage one involved early partial reactivation of large displacement steep normal faults stage two was dominated by folding wherein fault blocks underwent oblique non coaxial shortening by map scale folding accompanied by formation of outer arc normal faults minor cleavage and neoformed thrusts stage three involved reverse reactivation of outer arc normal faults and activation of oblique and strike slip faults that partitioned deformation into compartments

relationship between basement linked and gravity driven fault systems in the ukcs salt basins abstract extension of basement below significant detachment layers such as salt results in the formation of structures in the cover strata above the detachment layer in two ways thick skinned cover structures evolve to balance basement stretching these are linked to the basement faults via detachment in the salt layer or are directly linked to the basement structure depending on the amount of basement fault displacement relative to salt layer thickness basement extension also results in rotation of major basement fault blocks tilting the detachment layer and precipitating gravity driven thin skinned slip of the cover involving detached extensional faulting and folding detachment layers consisting specifically of salt play two roles in facilitating the development of thin skinned systems salt constitutes the detachment layer and also lends buoyancy to the cores of anticlinal folds reducing resistance to shortening the detachment layer may become tilted away from the basement fault scarp resulting in synthetic slip systems with the same polarity as the underlying basement fault or the detachment may be tilted toward the basement scarp giving antithetic slip systems of the opposite polarity in both cases the slides may terminate downdip in detachment folds and or reverse faults both types of thin skinned system will interact with those cover structures directly balancing basement extension and all of these components must be considered during section restoration tilt associated with 3d basement fault geometry e g relay zones may result in decoupled cover faults locally trending at high angles to the trend of underlying basement faults contrasting strain rates of basement faulting and related cover slip can lead to continuation of cover structural evolution after cessation of basement extension giving for example cover faults and folds of lower cretaceous age in the central north sea though basement extension had occurred in the upper jurassic synthetic systems may include footwall derived olistostromes emplaced above basement hangingwalls these commonly occur above basement relay ramps on segmented basin margins examples from the central graben and channel basin are discussed examples of antithetic systems from the irish sea central graben and a field example from the bristol channel basin are discussed

the sticklepath lustleigh fault zone tertiary sinistral reactivation of a variscan dextral strike slip fault dextral strike slip movement on the sticklepath lustleigh fault zone slfz is indicated by displacements of permian and older rocks previous authors have inferred that the main dextral movement which caused these displacements was post permian and noting the presence of small tertiary pull apart basins along the fault zone probably of tertiary age however the geometry of these early tertiary pull apart basins indicates sinistral rather than dextral strike slip movement we present an alternative model for the history of the sticklepath lustleigh fault zone summarized below 1 late variscan strike slip movement with a total displacement of up to 10 km produced the slfz and offset dextrally an earlier variscan thrust 2 extensional reactivation of this thrust led to rapid permo triassic subsidence in the crediton trough and a dextrally offset neighbour the hatherleigh outlier 3 approximately 6 km of early tertiary sinistral movement on the slfz produced small pull apart sedimentary basins and reduced the net dextral offset across the fault 4 in mid tertiary times minor dextral movements on the slfz may have produced reverse faulting on the margins of the tertiary basins

sedimentology of a devonian fault bounded braidplain and lacustrine fill in the lower part of the skrinkle sandstones dyfed wales the upper devonian to lower carboniferous skrinkle sandstones of the pembroke peninsula are predominantly continental deposits from the post caledonian synrift succession at the southern margin of the upper palaeozoic welsh landmass the lower part of the sandstones record deposition in the 30 10 km tenby angle fault block from which a 6 to 68 m thick interval is described and interpreted as a lacustrine deposit succeeded by a high energy sandy braidplain succession the lacustrine deposit is dominated by red mudstones and ripple cross laminated sandstones interbedded quartzose sandstones form a coarsening upward sequence from the red mudstones in the basin centre suggesting a deltaic origin and a smooth fronted braidplain delta model is proposed the sequence introduces the braidplain succession composed of groups of horizontal low angle laminated and trough cross bedded sandstones these are compared with recent ephemeral stream and sheetflood sediments and their characteristics used to depict a system of mutually erosive sheetflood and channel bodies the latter produced during the rising and falling stages of flood events which alternate to produce a thick multistorey sandstone palaeocurrent data indicate an axial drainage system from the north west running parallel to the main faults of the area this is supported by the maturity and sandy nature of the sediments basin closure towards the south and the postulated bristol channel landmass is inferred

thin skinned tectonics in the northern rhenish massif germany a network of profiles in the hercynian mountain system geo traverse rhenoherzynikum is needed to determine details of the geological structure and find the reasons for tectonic movements outstanding features within the mountain system such as the large fault zone along the southern boundary of the hunsruck1 and the left flank of the rhinegraben have already been investigated by reflection seismology2 3 we describe here a new reflection refraction experiment carried out in 1978 in the most northern part of the rhenish massif across the stavelot venn massif near aachen fig 1 while layers in the lower crust dip to the nnw a strong upper reflector at 3 4 km depth shows a slight dip to the sse and indicates a prominent fault zone near aachen known in belgium as the faille du midi here a comparable and apparently identical reflector was found some 100 and 175 km to the west of our profile we interpreted this reflector as a prominent and well lubricated thrust fault along which a huge horizontal nappe displacement took place during the last stages of the variscan orogeny

joints in the mesozoic sediments around the bristol channel basin abstract analysis has been carried out at four locations on the edges of the bristol channel basin to illustrate the later phases of deformation of a sedimentary basin and to illustrate the control on joint patterns of subtle changes in the stress system the characteristics of the joints are described and influences on joints are determined including the roles of faults folds and beds there is a low coefficient of correlation between joint spacing and bed thickness except in very thin limestone beds which have a high density of joints the lengths and spacings of earlier joint phases are usually greater than those of later phases later joints normally abut against earlier joints the joints abut the latest faults but are not displaced by them so the joints post date the main alpine contraction the joints formed in five main phases during reduction of the alpine stresses phase 1 joints are sub parallel to the regional compression direction 160 180 phase 2 joints are perturbed by faults often curving towards points of stress concentrations along the faults phase 3 joints are sub parallel to the earlier e w striking fold axes phase 4 joints are cross joints and phase 5 joints form polygonal patterns within joint bound blocks phases 2 and 3 do not occur in the absence of faults and folds and correspond with a reduction in horizontal compression and an increase in the importance of local factors phases 4 and 5 occur at all locations

variscan compressional structures within the main productive coal bearing strata of south wales the structure of the main productive coal measures within the anthracite zone in the western part of the south wales coalfield is re examined using data from recently operational opencast coal sites variscan thrusts and or folds and bed parallel shear structures are present within all of the sites studied at east pit unaffected by any large scale zone of disturbance bed parallel shear structures are developed along many of the gently dipping coal seams which form a series of simultaneously active detachments representing a process of easy slip thrusting a ne sw striking fold dominated disturbance at gilfach iago is interpreted to overlie a reactivated basement fault a more complex disturbance zone at ffos las combines both e w trending folds and more variably oriented thrusts which together shorten the coal bearing strata by up to 2 km and this is interpreted to be the major variscan frontal thrust ramp in the extreme west of the coalfield the variscan compressional structures within south wales are not consistent with a simple thin skinned thrust system the strain induced by deep seated variscan thrusts was distributed throughout much of the coal bearing sequence by easy slip deformation rather than being localized along a single regional detachment and additional thrusts present around the northern margin of the coalbasin appear to have formed as isolated autochthonous structures the inherent weakness of coal seams across south wales is attributed to the presence of greatly increased fluid pressures which would have been generated both by the maturing coal and by the influx of fluids along deep seated disturbance zones

a conceptual model for soils containing large gas bubbles offshore site investigations have indicated the presence of undissolved gas bubbles in the soils forming the sea bed at various locations throughout the world the gas bubbles are typically much larger than the normal void spaces so they cannot be considered as occluded bubbles within the pore water which simply change the compressibility of the pore fluid the effect of these large bubbles on the engineering properties of the soil must be understood if offshore construction is to take place in areas of gas bearing sediments this paper describes a conceptual model for soils containing large gas bubbles the model consists of a matrix of saturated soil surrounding isolated gas filled cavities the cavities forming the bubble sites are fixed within the soil so that gas drainage occurs only by the movement of dissolved gas the behaviour of the model is complicated by the processes of localized consolidation and bubble flooding des recherches conduites en mer ont indique qu il y a partout dans le monde d

new seismic evidence for a major geological boundary at shallow depth n devon recently acquired marine reflection data provide evidence for a major geological boundary at shallow depth under the southern bristol channel in the hangingwall block of the bristol channel thrust the boundary is recognized to equate with the top surface of a high velocity layer previously identified along refraction wide angle reflection seismic lines in the north devon area depth estimates from the reflection data and from a new interpretation of the earlier refraction data are combined to produce a contour map of the shallow boundary in the south central bristol channel and north devon areas the boundary has a wsw ene regional trend and lies at a depth of less than 2 km immediately north of the devon coast it is truncated to the north by the bristol channel thrust and deepens southwards across north devon and west somerset the boundary is interpreted as the top surface of a major litho stratigraphic unit older than the outcropping devonian sequence and in normal stratigraphic contact with it however in the absence of regional geological control the nature and age of the underlying high velocity layer remain uncertain

sediment damage caused by gas exsolution a key mechanism for mud volcano formation abstract gassy sediments are common in marine environments and are characterized by a specific mechanical behavior significantly different from that of water saturated sediments it is shown that gas causes damage and initiates fractures in sediments to define the controlling parameters dominating the damage process during gas exsolution and its consequences in terms of hydro mechanical behavior we developed a specific consolidation apparatus to test sediments collected from the active absheron mud volcano indeed mud volcano formation is initiated by gas exsolution and subsequent mud generation at depth from stratified sediments gas was generated in the samples by circulating carbonated water through the fine grained sediments then decreasing the total pressure particular attention was given to the impact of gas saturation and associated damage and fractures on p wave velocity sediment compressibility permeability and preconsolidation pressure results show that fracture geometry is mainly controlled by the degree of gas saturation and preconsolidation pressure the damage level increases with the degree of gas saturation while the elastic modulus of sediments is degraded experimental data show that sediments do not entirely recover their original mechanical behavior when gas dissipates finally the experimental data confirm that gas exsolution and expansion is a key mechanism for mud generation

relationship between polarity of extensional fault arrays and presence of detachments abstract extensional fault arrays are often dominated by a single fault vergence forming domino fault blocks the polarity of faults can be defined as synthetic or antithetic in relation to the overall shear sense across the fault system observations of the geometry of a number of field examples of extensional fault arrays vs the lithologies as a proxy for strength profile show that synthetic extensional arrays from the north sea and bristol channel basin detach on salt layers whereas examples of antithetic arrays from the north sea are found to pin out downwards and no basal detachment is present in the sacramento mountains core complex of the basin and range the major low angle detachment fault post dates antithetic shears that evolved at mid crustal level and are preserved in the granitic footwall these antithetic shears were cut by the main low angle detachment whose hanging wall disintegrated into an array of synthetic faults the kinematics of synthetic vs antithetic arrays dictate that faults within synthetic arrays must branch onto a basal detachment whereas faults within antithetic arrays may die out downwards therefore the results emphasise that fault polarity in domino arrays may be related to the boundary conditions of the fault blocks and therefore may be diagnostic of the strength profile of the faulted stratigraphy

the geometry of listric normal faults and deformation in their hangingwalls abstract in cross sections containing listric extensional faults area balancing techniques for depth to decollement are usually based on either bed length conservation or displacement conservation listric fault geometry may be constructed from a hangingwall roll over profile using the chevron construction this construction based on conservation of heave necessitates a reduction in fault displacement with decreasing fault dip a modification of this construction utilizing conservation of fault displacement predicts a listric fault that detaches at a shallower depth a new construction based on slip lines uses fault perpendicular displacement segments to generate listric fault shape fault propagation strain may be responsible for anomalous hangingwall geometries and these can be predicted by forward modelling using either a modified chevron construction or a slip line construction

linkage and evolution of conjugate strike slip fault zones in limestones of somerset and northumbria conjugate strike slip fault zones that cover metre scale areas at beadnell northumbria and kilve somerset were initiated as conjugate vein arrays early conjugate faults are linked by the propagation of one fault that eventually by passes the other fault a model for the development of strike slip faults is presented using fault and vein geometries and the position of damage zones with respect to the master faults as an indication of the propagation direction this model includes the evolution of networks from 1 the initial random development of vein arrays to 2 the isolated development of several unconnected conjugate fault segments that pass into vein arrays through 3 the intersection of a conjugate set of master faults and linkage with minor antithetic faults and the formation of new vein arrays with extensional geometries after a linked network of faults is established to 4 breaching of intersection points by dominant faults and finally 5 the propagation towards oversteps that are breached to form a through going fault the geometry of the active structures simplifies with time as strain is localised along the master fault but the complexities are preserved in the fault walls

the spatial distribution and geochemical variation of fault and fracture hosted calcite and gypsum cements in the eastern bristol channel basin abstract the spatial distribution and geochemical variation of fault and fracture hosted cements can be used to reconstruct the evolution and migration of pore fluids in sedimentary basins geochemical data from stable oxygen and carbon isotope and strontium isotope analyses of calcite and gypsum cements provide valuable insights into the source of pore fluids and their evolution in the eastern bristol channel basin overall at both an outcrop scale and a basin scale there were no observable relationships between the dip angle and direction of fractures with the oxygen and carbon isotopic composition of calcite cements indicating that the movement and distribution of fluids in fractures is not controlled by the orientation of fracture opening radiogenic strontium isotopic compositions of calcite and gypsum cements indicate at least two different mineralising fluid compositions that are strongly controlled by the lithologies fossiliferous limestones and shales and marls with gypsum nodules respectively in which they occur these fluids originated as connate pore fluids that were altered during the diagenesis of their host lithologies mineralising fluids within the lower jurassic lias group are characterised by enriched oxygen and strontium isotopic compositions and relatively depleted carbon isotopic compositions associated with the alteration of labile minerals and carbonaceous fossils mineralising fluids within the triassic mercia mudstone group have enriched strontium isotopic compositions as a result of the interaction of fluids with evaporitic intervals the mineralising fluids were redistributed through migration along faults and their associated damage zones this study highlights the importance of integrating a range of geochemical analyses when attempting to understand the source of fluids and their evolution through time the findings from this study can also be applied to nearby petroleum producing basins e g the wessex basin where equivalent rocks are economically significant source rocks and caprocks and have experienced similar structural histories

genesis of natural hydraulic fractures as an indicator of basin inversion abstract satin spar fibrous gypsum veins which occur in evaporite basins worldwide provide significant insights into host rock deformation and fluid flow although the genetic mechanism remains obscure satin spar veins in the red marls of the triassic mercia mudstone of the bristol channel basin were characterized in the context of regional and local setting the vein network in the keuper marl lower mercia exhibits a lack of systematic cross cutting between three distinct vein sets two sets of veins are observed in the overlying tea green marl with one set clearly crossing the other the gypsum veins commonly contain a blocky median zone of multiple thin bands of host rock inclusions and alabastrine gypsum crystals exhibiting crack seal patterns fibrous zones on either side of the median zone consist of pure parallel aligned gypsum fibres that are oblique to vein walls indicating a hybrid shear extensional mode of vein widening veins developed within reverse reactivated faults contain fibre lineations in the median zones and also on vein surfaces suggesting a minimum of two phases of fault slip the veins are interpreted to have formed as a result of overpressure in the low permeability mudstones by tectonic compression during basin inversion giving rise to the median zone growth of gypsum fibres commenced when the initial fractures were completely sealed the timing of vein formation is suggested to be miocene based on cross cutting relationships with folds formed during the latest phase of basin inversion because the fractures initiated in response to fluid overpressures they provide a useful analog to subsurface hydraulic fracture systems in low permeability rocks subjected to tectonic compression the fibrous widening of the veins post dates their initiation and led to continued propagation and increased connectivity

mechanical behaviour of gas charged marine plastic sediments understanding how free gas modifies the mechanical behaviour of marine plastic sediments remains a challenging issue special triaxial tests were carried out on natural sediments recovered from the gulf of guinea special attention was devoted to the laboratory preparation procedure involving saturation of natural marine sediment samples with carbonated water and generation of free gas following undrained unloading experimental data show that soil destructuration and damage generated by gas expansion and exsolution are at the origin of a the increase in the isotropic compressibility and decrease in the preconsolidation pressure and b the decrease in the peak undrained strength and abrupt increase of the pore pressure during undrained shearing the experimental data also show that flooding of gas bubbles and dissolution of free gas do not imply a complete strength recovery of natural sediments

possible variscan thrusting beneath the somerton anticline somerset vibroseis seismic data were acquired by goal petroleum plc during 1982 in the vicinity of the somerton anticline somerset these data show the simple form of the anticline but in addition show a southward dipping discontinuous seismic event beneath gravity modelling demonstrates that this event may be correlated with an e w trending gravity gradient which can be followed for approximately 150 km the dipping seismic event may represent a variscan thrust plane which dips to the ssw in contrast to similar planes seen to the east which dip to the sse the variation in dip seen between these postulated thrust planes may explain the curvature of structural trends seen within and to the east of the mendip hills the polden hills fault runs sub parallel to the proposed thrust plane beneath the somerton anticline and may be linked to a mesozoic reactivation of the postulated thrust

variscan structure of the inner bristol channel uk major variscan structures under the bristol channel are concealed beneath a mesozoic and quaternary cover mapping of reflectors on offshore seismic sections reveals two major wnw ese striking thrusts within the underlying palaeozoic sequence the more southerly with a moderate ssw dip has previously been identified as the bristol channel thrust another thrust the previously undescribed gravel margin thrust has been identified in the eastern part of the inner bristol channel and lies in the footwall of the bristol channel thrust it has a similar strike but steeper dip and extends to anticipated precambrian basement depths the bristol channel and gravel margin thrusts together juxtaposed the contrasting palaeozoic successions of south wales and sw england before mesozoic reactivation reflection packages identified on the offshore seismic sections are tentatively correlated with specific onshore palaeozoic sequences interpretation of the offshore seismic sections in conjunction with geological mapping along the north devon coast shows that the bristol channel and gravel margin thrusts have orientations and geometries typical of several mesoscale thrusts observed and measured at foreland point north devon along strike structural changes beneath the inner bristol channel seen on seismic sections provide evidence additional to earlier refraction and structural studies for the offshore continuation of the nw se trending watchet cothelstone hatch fault and its linkage with the previously described central bristol channel fault zone the bristol channel and gravel margin thrusts are interpreted to be offset from the cannington park thrust in north somerset by a 14 16 km variscan dextral strike slip displacement across the watchet cothelstone hatch fault the bristol channel gravel margin thrust and the cannington park thrust are interpreted as syngenetic structures linked by their lateral ramp the watchet cothelstone hatch fault using all the above information the overall structure of the variscides of the inner bristol channel is re evaluated and a new tectonic model is presented the structures discussed in this paper at a scale portrayed by commercial seismic data and published geological maps may represent merely the tips of linked thrusts that have previously been mapped down to mid crustal depths on birps seismic sections from the south west approaches traverse we show that enough data now exist to resolve outstanding problems of the concealed variscan structure in the inner bristol channel providing local structural detail that can be incorporated into the wider tectonic framework of nw europe

cyclic liquefaction of loose gassy sand gas can be found in many soils but none more common than in seabed soils gas charged sediments are known to be widely distributed throughout the world s oceans numerous research programs have studied saturated soil response to cyclic loading however little is known about the behavior of gassy soils subjected to cyclic loading the response of loose gassy specimens subjected to cyclic loading was studied in a laboratory program triaxial tests were performed on specimens of reconstituted ottawa sand prepared to various gas contents and densities the specimens were consolidated and then sheared under undrained cyclic loading gas content was measured at the beginning and end of each test using time domain reflectometry the response of the gassy specimens has been shown to be influenced by the soil state and grain characteristics and the intensity duration and frequency of the cyclic loading the propensity of a soil to liquefy is represented by a cyclic resistance ratio defined as the cyclic stres

on the undrained unloading behaviour of gassy sands soils that contain large amounts of dissolved gas within the pore fluid are called gassy soils gassy soils are common in marine environments and it is important to further our understanding of the unloading behaviour of gassy soils because of their potential to initiate and propagate submarine slope failures this paper focuses on the pore pressure responses and volumetric strains of loose gassy sands under different undrained unloading stress paths in laboratory specimens special attention was given to the constant deviatoric stress q constant undrained unloading stress path as it simulates the stress condition imposed by tidal drawdown one of the potential triggers of landslides in gaseous marine sediments gas exsolution was observed when the pore pressure was reduced below the liquid gas saturation pressure upon further decreases in total stress the resulting pore pressure change was much less than the total stress change hence effective stress decreased rapidly and at a certain point the sa

is the water still hot sustainability and the thermal springs at bath england the hot springs at bath are the largest natural thermal source in britain sustainable use of the waters for a spa requires maintenance of their temperature and flow rate together with smaller springs at hotwells bristol they form the outflow from a regional thermal aquifer that occurs where the carboniferous limestone is buried at depths 2 7 km in the bristol bath structural basin the aquifer is recharged via limestone outcrops forming the south and west portions of the basin rim current knowledge of the basin s structure is reviewed and important uncertainties identified concerning the hydrogeological role of thrust faults which may cut the limestone at depth a simple numerical model is used to determine the possible influence of thrusts upon groundwater flow within the thermal aquifer comparison of the modelled flow patterns with geochemical data and structure contours eliminates the hypothesis that thrusts completely disrupt the continuity of the aquifer the most successful model is used to simulate the possible impact of dewatering by large quarries at the limestone outcrops north and south of bath substantial reductions in modelled flow at bath result from proposed dewatering in the eastern mendips although the steady state approach adopted has severe limitations in that it does not take account of the incremental staging of actual dewatering nor allow for partial restitution of groundwater levels the geological uncertainties highlighted by the modelling could be addressed by future research into the effect of thrusts on the continuity of the carboniferous limestone more refined modelling to predict the timing of possible impacts of quarry dewatering will require measurements of the storativity of the thermal aquifer

the south wales coalfield low grade metamorphism in a foreland basin setting the south wales coalfield comprises namurian westphalian and stephanian sequences which developed in the external zone of the variscan orogenic belt sedimentation is thought to have been concentrated in a paralic basin related to crustal flexure in a foreland basin setting linked to variscan tectonic loading to the south it has long been recognized that coal rank varies across the coalfield from high volatile bituminous rank in the east and southeast to anthracite in the northwest this is confirmed here on the basis of comprehensive volatile matter and vitrinite reflectance data sets computer contoured to generate isovol and isoreflectance maps a steady increase in coalification with depth is also recorded clay mineral investigations show the presence of illite and kaolinite across the entire coalfield area while neither pyrophyllite nor mixed clays were detected illite crystallinity values from mudstones are in the range 1 20 0 44 2 vitrinite reflectance data indicate maturation temperatures in the range 123 290 c clay mineral and ic values suggest temperatures up to 230 c while previous descriptions of pyrophyllite from the coalfield would allow locally for even higher temperatures coalification appears to have occurred rapidly maximum 5 ma following deposition implying high heat flux during westphalian times this is also indicated by fluid inclusion evidence from quartz crystals in clay ironstone nodules the inference is of a high geothermal gradient c 50 c km 1 during westphalian times contrasting with an earlier gradient estimate of 20 37 c km 1 relatively high gradients have also been determined previously for the ruhr coalfield which occupies a similar tectonic setting in the external zone of the variscan orogenic belt such a gradient is at odds with the proposed foreland basin tectonic setting for the south wales coalfield in which low geothermal gradients would be anticipated

progressive accretion of antitaxial crystal fibres implications for the kinematics and dynamics of vein dilation abstract we made field and petrographic observations of antitaxial crystal fibres in the gypsum veins of the mercia mudstone sw england to examine the kinematic and dynamic implications of these fibres the gypsum fibres are predominantly oriented oblique to vein walls indicating that the veins were formed in a shear extension mode rather than pure tension mode fibres in single veins are parallel aligned regardless of the vein wall curvatures except those in the tapering tip zones field data demonstrate that there is no clear correlation between fibre plunges and vein dips fibres in many vein steps exhibit the same orientations as those in the main vein segments suggesting that these steps were formed as reactivated pre existing faults fractures instead of dilatational jogs vein tips that propagated progressively commonly curved towards the planes of neighbouring larger veins or interacted with the tips of adjacent sub parallel veins suggesting that vein propagation may have been dominated by the local stress field rather than remote stresses fibres in vein branches may not share the same orientations as those in the parent veins the host marls are commonly segmented into multiple parts by interconnected vein arrays that contain differently oriented fibres in each vein our study suggests that the gypsum veins lack evidence for crack seal increments and the crystal fibres in the veins had a continuous accretion towards vein walls the fibres are parallel to vein opening directions as indicated by the correlative vein wall irregularities and are of great potential for kinematic analysis and deformation restoration of the host rocks vein dilation could be driven by force of crystallization of veins exerted on the enclosing rocks which could have caused a growth related compaction and fracturing rather than uplifting the overlying strata that was accompanied with a bulk extension

stress dilatancy behaviour of bubbled fine grained sediments abstract although the strength and stiffness of fine grained gassy soil have been relatively well characterised its dilatancy remains poorly understood this study aimed to understand and formulate the stress dilatancy behaviour of fine grained gassy soil through a series of triaxial tests 19 in total conducted on fine grained gassy specimens where different initial gas volume fractions 0 and initial pore water pressures uw0 were considered compared to the stress dilatancy relation of saturated fine grained soil the addition of gas bubbles was found to result in more contractive or dilative soil behaviour depending on the combination of 0 and uw0 these distinct features cannot be captured by the existing stress dilatancy functions for fine grained saturated soils which solely depend on the stress ratio therefore a new stress dilatancy function is proposed within the framework of critical state soil mechanics to quantify the distinct modification effects of bubbles which either suppress or enhance dilatancy on the dilatancy of gassy soils with different 0 and uw0 values in a unified manner this new function has been validated and can be readily implemented in critical state based constitutive models for predicting the shear behaviour of fine grained gassy soil and therefore the response of various offshore structures i e foundations and pipelines founded on a gas bearing seabed

quantifying vein attributes in massive mudstones triassic sw england implications for progressive evolution of opening mode fracture networks abstract attributes of gypsum vein arrays in the triassic massive mudstones of sw england were analyzed to infer the process of vein network evolution field data demonstrate that there is no clear correlation between vein orientation and thickness however sub horizontal veins have a higher average thickness than the inclined veins vein thickness distribution is best characterized by a log normal distribution the fractal dimensions d of vein networks analyzed range from 1 44 to 1 71 vein networks which contain abundant bridging veins between neighbouring sub horizontal veins to form an anastomosing pattern exhibit a higher d value the fraction of vein volume derived from image analysis ranges from 6 75 to 17 94 a positive correlation is revealed by plotting the fractal dimensions of the vein networks versus the number of vein intersection points suggesting that an anti clustering distribution of veins could contribute to more interactions and also intersections of the gypsum veins the vein attribute data presented indicate that mineralization in the massive mudstones is mainly localized to some of the largest veins as a result of mechanical interaction of neighbouring veins during vein propagation hindering the nucleation of new veins as a consequence accommodation of the bulk rock strain could be facilitated mainly by expansion of the large backbone veins which follow a positive feedback mechanism of growth this study reveals a strong link between the geometric parameters of opening mode fractures with the history of their formation propagation and intersection

effect of initial pore pressure on undrained shear behaviour of fine grained gassy soil many of the world reserves of fossil fuels are located at various water depths in fine grained sediment under the seabed the fine grained sediment contains relatively large biogas bubbles which has been posing challenges to the stability of offshore foundations supporting oil and gas platforms although fine grained gassy soil was found to exhibit different undrained shear strengths cu by altering the initial pore pressure ui relevant to water depth systematic studies concerning the effect of ui on undrained shear behaviours of the soil are still lacking this study reports a series of undrained triaxial tests aiming to compare and investigate the responses of reconstituted fine grained gassy soil with the same consolidation pressure p c but at a wide range of varying ui 0 1000 kpa the shearing induced excess pore pressure u in the gassy specimens highly depends on ui it can be either smaller than that of the saturated specimen with the same p c due to partial dissipation of u int

upper carboniferous source rocks north and south of the variscan front nw and central europe abstract some topical aspects of upper carboniferous source rocks within the three main tectonic settings in nw and central europe are described these are likely to offer an effective tool for reduction of risk in future exploration in the foreland the main geological factors responsible for the nitrogen content of gas accumulations and for the isotopic composition of methane are outlined it will probably be possible to predict the molecular and isotopic composition of gas accumulations in the future in addition the application of the dynamics of diffusion processes to the loss of gas accumulations over geological time offers the possibility of predicting hydrocarbon fill in some areas of the basin in the variscan fold belt the model of thin skin tectonics suggests that the lowermost parautochthonous low rank thrust sheets are more likely to be source rocks than are allochthonous high rank thrust sheets additionally young subsidence is required in order to meet the conditions of gas dynamics the possible source rock potential of the permo carboniferous fills of the intramontane grabens south of the variscan front are reported here for the first time some promising discoveries below the swiss and german molasse basin as well as in the eastern paris basin mean that the intramontane grabens are now an interesting exploration objective