a newly recognized type of slickenside striation abstract this note summarizes current views on the origin of slickenside striations and points to the existence of a type of striation not readily explained by established genetic ideas such striations have been seen on fault surfaces in naturally deformed shale and in experimentally deformed paraffin wax they are defined by arrays of shallow slip parallel ridges and grooves of millimetric width and below on any given slip surface the ridges are as common as the grooves and display similar dimensions and profile shapes across any pair of slip surfaces in contact at the end of faulting ridges protruding from one surface fit into or nest with grooves on the other surface the origin of these features is not yet understood

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

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

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

the geometry of echelon fractures in rock implications from laboratory and numerical experiments abstract the traces of echelon joints veins and dikes in rock range from curving to straight theoretical analyses using boundary element numerical methods have concluded that straight open fractures imply a significant remote differential stress whereas curving traces imply a more nearly isotropic stress we present a series of laboratory experiments which investigate the two dimensional propagation paths of echelon fractures in pmma plates as a function of the applied biaxial loading and the initial geometry of a simple fracture array the experimental results support the theoretical conclusions and verify the accuracy of the numerical method at the grain scale rock does not rigorously conform to the assumptions of isotropy homogeneity and linear elasticity demanded by the numerical method nevertheless results from crack path stability theory suggest that small scale deviations of a fracture from the ideal path generally do not affect its large scale behavior fracture paths which become kinked or curved by local heterogeneities are found to self correct to a path governed by competition between the fracture tip and remote stress fields provided the remote differential stress is non zero this enhances our confidence that the numerical method can be used to produce and explain realistic fracture geometries in rock an unexpected experimental result is the non perpendicular intersection of fractures grown in the laboratory free surface boundary conditions require that the walls of an open fracture be free of shear stress the principal stresses are therefore aligned with the crack wall and we might consequently expect an approaching opening mode fracture either to intersect the free surface at a right angle or to turn away and follow an asymptotic nonintersecting path experimental and numerical modeling however shows how the near tip stresses generated by an approaching fracture dominate the local stress field and allow it to propagate along an oblique path very close to intersection with the adjacent free surface

extension tectonics in the wessex basin southern england the permian to cretaceous tectonic evolution of the wessex basin was controlled by horizontal tensional and vertical isostatic forces within the lithosphere the gross morphologies of its constituent structures were governed by the location of variscan thrust and wrench faults in the upper and middle crust which suffered extensional reactivation in tensional stress fields oriented approximately nw se several episodes of crustal extension can be resolved in early permian early triassic early jurassic and late jurassic early cretaceous times these were characterized by the rapid subsidence of fault bounded basins and commonly by erosion of adjacent upfaulted blocks superimposed upon the fault controlled subsidence dominant during periods of fault quiescence and becoming increasingly important with time a component of regional subsidence is considered to have a thermal origin this suggests that crustal extension was accompanied by some form of not necessarily uniform lithospheric thinning subsidence analyses assuming local airey isostasy give cumulative crustal extension factors of 20 28 beneath the grabens a more reasonable assumption of regional airey compensation indicates basinwide crustal extension of 13 17 which is consistent with birps offshore deep seismic reflection data

the structure and evolution of the wessex basin southern england an example of inversion tectonics abstract the wessex basin of southern england consists of four main sub basins each sub basin is thought to be fundamentally controlled by the normal reactivation of hercynian basement thrust and wrench or transfer faults post hercynian polyphase reactivation of both east west thrust and northwest southeast transfer faults tended to compartmentalise the basement thereby leading to the initiation of discrete permian cretaceous depocentres while rifting was confined to the western portions of the area during the permo triassic easterly migration in the mesozoic allowed the initiation of the weald and channel basins the sub basins represent therefore simple sinistral pull apart basins opening along northwest southeast faults this same fault direction is postulated to have also played a major role in controlling and or modifying the development of european basins in general major pre aptian uplift and erosion is largely attributed to the thermal effects and hence the isostatic uplift of the basin associated with continental rifting of the bay of biscay and the north atlantic margins major inversion and hence basin destruction began in the late cretaceous early tertiary with the uplift of the channel weald and vale of pewsey basins and culminated in the oligocene miocene synchronous with the helvetic phase of alpine deformation wessex basin inversion was facilitated by the thrust reactivation of variscan faults above an intracrustal detachment surface the same surface which allowed upper crustal extension and hence basin initiation in the permo triassic the structural development of the wessex basin both during extension basin formation phase and compression inversion phase was ultimately controlled by the plate motion of africa relative to europe such plate motions successfully predict the observed nw se sinistral motion from the stephanian to the aptian e w sinistral motion during the aptian cenomanian and largely nw se dextral motion from the cenomanian to the present day

vein growth mechanisms and fluid sources revealed by oxygen isotope laser microprobe in situ laser microprobe analyses of millimeter wide quartz veins has revealed the largest 18o gradients 9 mm yet documented in terrestrial samples the systematic isotopic variations indicate a crack seal growth mechanism that variably interleaved 18 o enriched quartzite porphyroclasts with newly precipitated 18 o depleted vein quartz the isotopically light quartz requires open system infiltration of meteoric fluids into the quartzite during its incorporation into the ruby gap duplex central australia documentation of the fluid source in these small veins would not be possible without submillimetre isotopic sampling capabilities application of this laser based technology to other geologic problems should result in similar documentation of large isotopic gradients and provide new insight into fluid rock interactions

earthquake faulting as a structural process abstract structural geology is concerned with the history of movement in the earth s crust and the processes by which displacements occur in the upper one third to one half of deforming continental crust displacement is accommodated largely by seismic slip increments on existing faults it follows that earthquakes and related processes are an integral part of structural geology traditionally structural geologists have been preoccupied with the complexity of the finite deformation within fault zones and with the stress states prevailing at the initiation of faults in intact crust future structural work should be directed more towards understanding the dynamic character of fault reactivation during incremental slip and related effects questions of interest include rheological and geometrical controls on the initiation perturbation and termination of ruptures directivity effects associated with rupture propagation the recognition of structures resulting from repeated stress cycling within seismogenic crust and identification of structural features diagnostic of shear stress levels during faulting structures arising from the inter relationships between slip episodes and induced fluid flow are of special importance because these dynamic fault processes appear influential in the development of much fault hosted mineralization mesothermal gold quartz lodes hosted in high angle reverse shear zones of mixed brittle ductile character form illustrative examples of structures that arguably can only be interpreted by seismo structural analysis embodying the concepts listed above

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

possible paleostress tensor configurations derived from fault slip data in eastern vermont and western new hampshire the configuration of six possible paleostress tensors have been derived from 152 faults measured in eastern vermont and western new hampshire populations of potentially genetically related faults were separated using two techniques tensor configurations for each population were derived using a linear least squares inversion method based in part on the work of reches 1987 and a grid search inversion method which tests over 100 000 possible tensors for compatibility with all or portions of the data faults belonging to the oldest population set r n 24 occur primarily in high grade rocks this set is composed of semiductile northeast trending reverse and west northwest trending left lateral faults fault fabrics of quartz rods and thin mylonite layers suggest that the host rocks were at crustal levels of 8 10 km during faulting the derived tensor indicates a roughly east west 1 and near vertical 3 set r faults are offset by normal faults sets n1 and n2 and are interpreted to be pre mesozoic in age perhaps related to late paleozoic alleghanian compression normal and normal oblique faults mineralized with chlorite calcite and strained quartz n 73 have been separated into two populations sets n1 n 49 and n2 n 24 even though these faults are likely of similar age host rocks were probably at moderate crustal depths of perhaps 5 km during faulting faults of both sets are most likely related to mesozoic rifting tensor configurations indicate that 1 plunges steeply northwest n1 and southeast n2 and 3 plunges gently roughly east west these faults cut presumably mesozoic age dikes and are themselves offset by normal oblique faults set t and right lateral faults set rl strongly deviatoric near vertical 1 suggests a thermally driven tectonic regime during the development of normal faulting in new england set t n 25 is composed of normal oblique slip faults mineralized similarly as sets n1 and n2 the plunge of 1 is 50 n and 3 plunges gently east field relations indicate set t faults document the transition from normal sets n1 and n2 to strike slip faulting regimes set rl a compatible sequence of mesozoic stress fields is suggested de boer et al 1988 north northeast trending right lateral faults of set rl n 7 are mineralized with quartz limonite calcite chlorite calcite twins are bent and fractured and quartz grains show minor internal strain suggesting that these faults developed while host rocks were at relatively shallow crustal levels the plunge of 1 is gently northeast and 3 plunges gently east southeast seven distinctly mineralized feldspar siderite quartzoalcite 1imonite chlorite gently northwest dipping thrust faults of set f define the sixth tensor these faults are cut by essentially vertical east side up faults and are assumed to be mesozoic in age by analogy with similar faults in western massachusetts

from orientation to magnitudes in paleostress determinations using fault slip data abstract determinations of reduced stress tensors using fault slip data yield the orientation of principal stress axes and the ratio of the differences between principal stress magnitudes the use of rupture and friction laws allows determination of the two remaining unknowns that is the reconstruction of the complete stress tensor taking into account the depth of overburden brings an additional constraint the method is applied and discussed in the case of the hoover dam site western u s a where large data sets and rock mechanics information are available differences between intact sample and rock mass properties account for apparent disagreements between paleostress levels determined in similar tectonic environments pore pressure plays an important role where information about pore pressure is absent zero and hydrostatic pore pressure cases should be considered as limits

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

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

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

size of snow particles in a powder snow avalanche little quantitative information is available concerning the size of ice particles in the turbulent clouds of powder snow avalanches to quantify particle size distributions we have developed an experimental device that collects particles in real scale powder avalanches the device was placed on the concrete bunker of the swiss vallee de la sionne avalanche dynamics test site on 31 january 2003 a large powder snow avalanche struck the bunker and we were able to collect particle samples the collected particles have been photographed and the pictures digitized an image analysis tool allows us to determine an equivalent particle radius the captured particles have a geometric mean of 0 16 mm the largest particles were 0 8 mm in size and the smallest particles 0 03 mm

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

extensive underthrusting of undeformed sediment beneath the accretionary complex of the lesser antilles subduction zone along the eastern margin of the caribbean plate which overrides the atlantic oceanic lithosphere at 20 km myr 1 in an easterly direction1 east of the lesser antilles island arc lies a wide accretionary complex barbados ridge complex 260 km wide at the latitude of barbados and 20 km thick beneath barbados3 4 it increases in width and thickness from north to south2 fig 1 the style of deformation varies considerably along the eastern margin of the complex2 3 5 11 and is related to the thickness and type of sediment on the atlantic ocean floor a common feature is that a proportion of undeformed sediment from the ocean floor is brought beneath the deformed sediments of the accretionary complex and in some areas a clearly defined seismic reflector separates the deformed and undeformed sediments which has been interpreted as a decollement surface5 12 13 a seismic section from an investigation of the accretionary complex conducted from rss discovery in 1980 using 12 channel seismic reflection and long range side scan sonar gloria shows undeformed sedimentary rocks carried horizontally 74 km beneath deformed rocks forming the accretionary wedge we report here that the low shear stresses required on the decollement between the deformed and undeformed rocks are made possible by porewater at pressures close to that of the lithostatic load

short paper evidence of negative structural inversion beneath central england from new seismic reflection data a major seismic reflection transect acquired by the british geological survey between 1984 and 1986 has elucidated the deep geology and structure of some well known surface features of central england the worcester basin a mesozoic extensional graben system is bounded to west and east by major sub planar normal faults which formed by the extensional reactivation of much older structures respectively the malvern axis and a similar though concealed structure forming its mirror image to the east within the worcester basin a thick permo triassic sequence rests directly upon precambrian basement rocks to the west and east permo triassic rocks are thin or absent but relatively thick palaeozoic sequences are preserved above the precambrian these strikingly different successions together with the fault geometry suggest that the worcester basin lies within a major inverted oblique slip fault system or flower structure in palaeozoic times transpressivt stresses led to reverse faulting along the malvern axis and its complementary structure to the east with consequent regional uplift of the intervening tract and the formation of a positive flower structure in permian and early mesozoic times transtensional stresses caused collapse and negative inversion of the flower structure and consequent subsidence of the worcester basin

geological evidence for intra jurassic faulting in the wessex basin and its margins geological observations on jurassic outcrops close to major faults in the wessex basin mendip area reveal the local presence of ammonite and brachiopod bearing sediments penetrating underlying strata toarcian and bajocian neptunian dykes and particularly sills are associated with the eypemouth fault and bajocian sills with the bride fault and mere fault in the mendip area numerous neptunian dykes of hettangian sinemurian pliensbachian and bajocian ages cross cutting carboniferous limestone are recorded typically also associated with major basement faults e g cranmore and leighton faults these periods of assumed sediment injection are taken as indicating times of displacement along the faults in question variations in facies hettangian sinemurian toarcian bajocian uppermost oxfordian kimmeridgian spatially linked to faults are documented from some areas and boreholes reveal considerable fault controlled thickness changes in hettangian sinemurian bajocian and kimmeridgian sediments the timing of jurassic faulting in the wessex basin mendip area thus polarizes into two intervals hettangian bajocian and latest oxfordian onwards correlating with the early rifting phases of the central and north atlantic respectively

detailed structure and possible fluid paths at the toe of the barbados accretionary wedge odp leg 110 area during the summer of 1987 a bottom navigated heat flow and deep tow side scan sonar survey was carried out over the toe of the barbados accretionary wedge in the ocean drilling program odp leg 110 drilling area surface traces of the leg 110 drill holes were recognized on the sonar images precise relocation of the odp holes relative to a reference multichannel seismic line led to a new interpretation of some of the structures previously described by the leg 110 scientific party for example one thrust formerly interpreted as a forward propagating major thrust is interpreted here as a near surface backthrust developed above a probable shallow decollement located at the base of the pleistocene sequence we use these new structural data as surface guidelines in building a balanced cross section of and defining a fluid flow pattern within the toe of the wedge using new closely spaced heat flow data we discuss briefly the fluid flow along the major fractures through the wedge

stress variability and brittle tectonic structures the problem of stress variability both in space and time during brittle deformation of rocks is faced in this note several possible causes of the temporal and spatial variations of the stress tensor are reviewed and discussed a genetic partitioning of the stress tensor is proposed showing that any stress field within a crustal volume can be seen as the sum of relatively simple stress tensors associated to specific genetic components the genetic components discussed in this note are the gravitational the tectonic the fluid the thermal and the diagenetic one each genetic component is a function of several parameters but for all of them the possible important role played by the variable time is emphasised according to numerous researches based on theoretical and experimental grounds brittle deformation in a rock mass occurs when some critical conditions are reached starting from initial equilibrium conditions and leading to even different stress conditions as a consequence of failure field examples are presented and data from literature are discussed to exhibit evidences from complex tectonic structures and to argue about small and large scale spatial variations as well as about short and long term temporal variations of the stress trajectories and of the stress magnitudes that continuously occur during a brittle deformational event the relationships between local and temporal stress variations and what is commonly defined the regional stress field are discussed adding recommendations for future researches

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

the south celtic sea bristol channel basin origin deformation and inversion history abstract the south celtic sea bristol channel basin is a relatively narrow and elongated sedimentary trough located within the northern part of the hercynian fold belt and filled with permo triassic redbeds to jurassic marine sediments its present configuration is the result of a three fold sequence of tectonic phases initiated during the late palaeozoic the origin of the basin appears to be related to localised extensional reactivation of hercynian thrust zones during the permian and early triassic followed by regional subsidence during the late triassic and jurassic uplift of the basin margins together with dextral strike slip movements in an east west sense and large scale erosion during the late jurassic to early cretaceous resulted in the present elongated configuration of the lower mesozoic basin regional subsidence from aptian time onwards resulted in widespread deposition of a relatively thin section of wealden elastics and upper cretaceous chalk unconformably overlying folded pre cretaceous strata sedimentation was interrupted during the early tertiary by a series of tectonic events that included emplacement of the lundy igneous mass during the early eocene and mild basin inversion during the oligo miocene inversion was accompanied by dextral reactivation of the nw se oriented sticklepath fault zone the total amount of uplift probably not exceeding 350 m reflects the rather stable configuration of the underlying lithospheric crust as indicated by seismic reflection and refraction data

the thermal and mechanical development of the wessex basin southern england summary the wessex basin of southern england is an e w trending late palaeozoic to tertiary extensional basin formed on early palaeozoic variscan crust extension occurred over a protracted period from late carboniferous to cretaceous times resulting in the evolution of four main depocentres or sub basins the channel basin winterborne kingston trough vale of pewsey and weald basins each sub basin was controlled fundamentally by the normal reactivation of basement thrust faults and wrench or transfer faults the basement faults represent primary structures associated with the fold and thrust terrain of the devonian carboniferous variscan orogeny subsequent but synchronous reactivation of both thrust and transfer faults tended to compartmentalize the basement leading to discrete depocentres and the production of general rhomboidal shaped sub basins characteristic of pull apart basins in strike slip terrains to investigate the thermo mechanical properties of the lithosphere during basin formation 50 boreholes within the wessex basin were back stripped to isolate the driving tectonic subsidence responsible for basin formation the resultant driving subsidence is characterized by a gentle sometimes negative exponential form with superimposed rapid discrete subsidence events as the geological development of the wessex basin was characterized by polyphase extension and the utilization of crustal detachments the lithospheric model used to determine crustal extension above the detachment and stretching below the detachment included the effects of depth dependent stretching and finite re rifting from the back stripping analysis the basin wide averaged upper crustal extension and lower crustal sub crustal lithospheric stretching were 11 and 5 respectively the distribution of is strongly influenced by the location of growth faults and hence basement thrust fault reactivation suggesting that is reflecting the brittle failure and collapse of hanging wall blocks repeated slip and block collapse resulted in the polyphase extension responsible for renewed basin subsidence in contrast is uniformly distributed across the basin suggestive of a ductile failure of the lower crust and responsible for the low magnitude thermal subsidence strain compatibility between the upper crust and lower crust sub crustal lithosphere is maintained partly within the basin 5 and partly external to the basin as the variscan basal thrust was the fundamental relay in the sense of wernicke responsible for the wessex basin formation we postulate that strain balancing induced a cumulative essentially thermal isostatic uplift of 60 m under the northern margin of the armorican massif

uranium lead isochron age and preliminary sulfur isotope systematics of the felder uranium deposit south texas uranium lead isotope systematics of roll front ores in miocene sandstone at the felder and mclean uranium deposits south texas coastal plain give a well defined 207 pb 204 pb 235 u 204 pb isochron age of 5 07 or 0 15 m y the relatively slight degree of scatter of the points defining the isochron is probably due to initial pb isotope inhomogeneity and the resulting inferred persistence of closed system behavior for u and pb is probably the result of the long term presence of u and pb immobilizing h 2 s 206 pb 238 u systematics are badly scattered owing to long term migration of radioactive daughters of 238 u beta and gamma activity systematics of the ores consistently identify those with grossly anomalous pb isotope systematics however and proved highly useful in identifying ores that have gained gross amounts of uranium daughters fes 2 minerals in the altered tongue of the host sandstone are characterized by abundant postore pyrite and heavy delta 34 s values whereas fes 2 minerals in mineralized and unaltered barren rock are characterized by abundant ore stage marcasite and by light delta 34 s values the delta 34 s values of fes 2 minerals in the altered tongue are similar to those defined for sour gas from the underlying edwards limestone of cretaceous age indicating the probable source for the sulfur of an inferred resulfidization event the 5 07 m y isochron age probably reflects the end of roll front development we believe that the end was due to the resulfidization and does not preclude significant earlier periods of mineralization modern ground water in the area is also sulfide bearing thus helping preserve the deposit but with delta 34 s values distinct from those of sulfides in the resulfidized zone of the host rock

the interplay between fluids folds and thrusts during the deformation of a sedimentary succession abstract in this paper field evidence is presented to support the idea that fluid pressures in excess of lithostatic pressure were operating during the initiation and reactivation of large thrusts and folds in the wenlock shales of llangollen north wales it is also argued that as major thrusts are reactivated and as major folds amplify into finite structures they in turn exert an influence on the subsequent migration and concentration of fluids within the deforming sedimentary succession

how facies and diagenesis affect fracturing of limestone beds and reservoir permeability in limestone marl alternations abstract facies and diagenetic heterogeneities in carbonate reservoir rocks affect both fracture distribution and fracture permeability many studies focussed on fracture patterns in limestone marl alternations as e g fluid flow models are based on laterally continuous beds here we examine 4010 fractures in multiple layers of limestone marl alternations using a modified scan line method the studied successions belong to the blue lias formation hettangian sinemurian exposed on the coast of the bristol channel united kingdom we combine methods of sedimentology and structural geology with rock physics to gain a better understanding of the role of facies diagenesis and petrophysical properties tensile and compressive strength hardness porosity on the distribution of fractures fracture orientation density spacing and height fracture distribution varies significantly despite similar bed thicknesses indicating that planar bedding planes i e well bedded limestones wbl and beds with bedding plane irregularities i e semi nodular limestones snl must be distinguished snl show higher percentages of non stratabound fractures 67 while they are more stratabound in wbl 57 additionally beds with variable bed thicknesses in scale of 15 m long beds exhibit a wide range of fracture spacing whereas fractures in beds with more continuous bed thicknesses are more regularly spaced considering all lithologies the percentage of non stratabound fractures increases proportionally with caco 3 content three subsections studied in detail reveal different main sedimentological and diagenetic features from early lithified over differentially compacted to physically compacted all of them are characterised by dissimilar percentages of stratabound and non stratabound fractures in limestone beds and marl interbeds our findings demonstrate that the distribution of fractures in individual well bedded limestones is not necessarily representative for successions of limestone marl alternations multiple layers should therefore be studied in outcrop analogues as basis for fluid flow models of reservoirs composed of such lithologies

extension and rifting the zeit region gulf of suez abstract a field analysis of faults and fractures in the ras gharib ras gemsa region of the gulf of suez shows that the main late cenozoic extension occurred perpendicular to the rift axis three main types of dip slip normal faults successively developed as the tilt of blocks bounded by antithetic normal faults increased determinations of the amount of extension from structural data are compatible with estimates made using subsidence data through a simplified model of lithospheric stretching the uplift of rift shoulders is related in chronology and volume to the subsidence of the rift the geometry of fault patterns and directions of extension suggests that the late cenozoic total movement corresponds to a counterclockwise rotation of 4 5 of sinai relative to africa with a pole close to cairo