sar image statistics related to atmospheric drag over sea ice the possibility of using synthetic aperture radar sar data to distinguish sea ice regions with different atmospheric drag is explored both the amplitude of the radar return and statistics derived from sar image data are examined roughness statistics data from several pack ice areas are used in a backscatter model to predict the return from surfaces with measured drag coefficients the results suggest that the scattering coefficient for typical radar wavelengths is insensitive to the roughness elements responsible for the observed drag coefficient variations over pack ice free of major ridges for marginal ice zones where ice concentration and floe deformation contribute to atmospheric drag a simple model for the atmospheric boundary layer is used to provide qualitative relationships between drag coefficient and regional ice properties ice concentration floe size distribution floe edge density derivable from sar data simple algorithms to produce maps of ice concentration and edge density are outlined and applied to 23 5 cm sar digital image data

dispersion of sea ice in the bering sea the dispersion of sea ice in the bering sea is analyzed in terms of the changes in separation between pairs of ice floes the statistics of these changes depend on the separation itself and on the time interval over which the changes are measured rather than being constant as in the case of molecular or fickian diffusion the diffusivity increases according to the 1 8 power of the separation for short times the diffusivity is proportional to time for times greater than 3 days the diffusivity appears to approach a constant value for any particular separation comparison with other data shows that the ice floe diffusivity is about an order of magnitude smaller than the diffusivity at the surface of the temperate ocean and about an order greater than the diffusivity of sea ice in the beaufort sea

the structural development of the wessex basin the mesozoic and cenozoic structures of southern england form part of a system of disturbances which extends across the channel into northern france they are reviewed in the light of published data from south dorset basin development started with permo triassic rifting and is believed to have continued through the jurassic and early cretaceous with intermittent growth on deep seated listric normal faults probably associated with roll over anticlines this regime had ceased by the aptian and the basin passed through a period of relative stability in the late cretaceous followed by tectonic inversion and in the miocene by northwards compressive movement along the former normal faults this partially restored the earlier separative displacement at depth and probably accentuated pre existing roll over anticlines it was expressed in the surficial upper cretaceous palaeogene sequence as monoclinal flexuring the early rifting and the change from overall n s crustal stretching to compression may reflect plate tectonic events in the north atlantic region

an assessment of the reliability of sea ice motion and deformation retrieval using sar images the local mass balance of sea ice is dependent on the advection of ice into and out of an area and on the deformation processes in that area sea ice motion can be observed from space by synthetic aperture radar sar and quantified by drift detection algorithms due to the scarcity of field observations it remains a challenging task to validate the resulting motion fields we analyse the quality of sea ice motion fields derived from sar data using an example dataset from the weddell sea region we apply a quality indicator for sea ice motion fields which is independent of field data and evaluate it with reference data obtained from visual analysis of the sar images together with the motion field sea ice deformation can also be retrieved from sar data similarly to ice motion it is very difficult to obtain field data to evaluate the quality of the results based on a manually derived reference dataset we introduce a method to validate the retrieved deformation rates this procedure requires no additional field data our analysis shows that deformation rates derived from sar data are consistent with results obtained from buoy analysis by previous studies

upper mantle structure along a profile from oslo noress to helsinki to leningrad based on explosion seismology abstract waveforms from the noress array were analyzed for 147 industrial explosions during the 1985 to 1988 period along a profile running east from oslo noress to helsinki to leningrad ohl profile the events were 250 to 1300 km from noress and had local magnitude in the range 2 0 to 3 5 event locations and origin times constrained by the university of helsinki9s regional seismic network provide a reliable basis for travel time estimation at noress we also used data recorded by norsar in 1979 for three shots on the fennolora north south long range seismic profile which were near the ohl profile analysis of mantle p wave signals from the explosions showed that first arrivals could be traced continuously to a distance of 750 to 800 km where there is a cutoff and shift of approximately 2 0 to 2 5 sec in the travel time curve and an increase in average apparent velocity interpretation of the observed travel times and waveforms for this profile suggests a low velocity zone from approximately 105 to 135 km depth combined analysis of the seismic data with a bouguer gravity map indicates the presence in the upper mantle of a high velocity high density body of linear extent approximately from 200 to 300 to 500 to 600 km east of the noress array it is postulated that this body may represent the root of an ancient volcanic system in which lighter silicic constituents were depleted from the upper mantle during the eruptive phase

strike slip orogens an explanation for the hercynides the nature of oblique plate interactions is not yet well understood the oblique interaction of pacific and n american plates has resulted in large scale migration rotation and accretion of microplates intervening basins are ephemeral individually filled and deformed oblique motion results in zones of both compression and extension the latter may be sufficient to generate new oceanic crust and the former to obduct it both subduction and collision are restricted in time and space the resulting orogen is extraordinarily complex in detail the far more complicated hercynides are the result of dextral interaction between continental plates europe and africa the lateral variability of the orogen the major and long lived strike slip faults the bimodal volcanism and related ore deposits of the european trough the lizard ophiolite and its melanges the nw spanish ophiolites and the iberian pyrite belt are all explained using an oblique collision model in particular the generation deformation and obduction of ophiolites in oblique slip mobile zones is discussed

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

observation of cyclone induced inertial sea ice oscillation in fram strait 1 in march 2007 the field campaign framzy took place in the region between spitsbergen and greenland one goal of the experiment was the observation of cyclone impact on ice drift by means of autonomous meteorological buoys deployed on sea ice on 23 march an intense cyclone of about 500 km diameter passed with about 20 m s from south to north over the ice covered part of the fram strait and induced strong inertial oscillations of the sea ice ice drift velocity fluctuations abruptly increased by a factor of three to values of more than 1 m s this was caused by a resonant wind forcing for almost three quarters of the inertial oscillation depending on the local ice conditions the oscillations lasted over a two to five days period the inertial oscillations caused ice drift divergence fluctuations of 6 10 6s 1 and a reduction of ice concentration by up to 11

the role of plastic ice interaction in marginal ice zone dynamics under appropriate conditions the nonlinear nature of plastic ice interaction together with a nonlinear coupling between ice thickness characteristics and ice rheology can substantially modify the character of marginal ice zone dynamics this paper examines the steady state ramifications of these nonlinearities by using a one dimensional simplification of hibler s 1979 two level viscous plastic sea ice model a series of idealized small scale simulations 4 km resolution are carried out with the model formulated in a moving lagrangian grid in order to remove diffusion effects analytic solutions for the equilibrium plastic adjustment case are also constructed the results show that if the ice thickness distribution is allowed to equilibrate in response to a constant wind field the thickness strength coupling will yield a sharp ice edge with the compactness dropping rapidly to zero near the ice margin despite this compactness falloff the steady adjusted ice velocity is nearly constant across the marginal ice zone however the adjustment to a steady state constant velocity can be preceded by a period of shearing which will abate as the ice strengthens by ridging with an increase in forcing near the edge the ice velocity scales with the wind velocity but with a speed less than free drift in the absence of water drag and wind stress the model exhibits a very slow outward creep but yields no ice edge jet effects the ramifications of plastic interaction on upwelling and downwelling are also examined with the same drag coefficients over ice and ocean and for a north south ice edge the model yields in the northern hemisphere upwelling for northward winds downwelling for southward winds and no effect for off ice winds

mesoscale sea ice deformation in the east greenland marginal ice zone in this paper mesoscale 10 km ice kinematics data obtained during the drift phase of the 1983 marginal ice zone experiment are analyzed the measurements were made with a microwave transponder system accurate to better than 1 m from the point of view of granular media theory the ice pack was close to ideal over the scale of the array the pack was quite regular with floes of relatively uniform size closely packed together the main external driving force for the ice was the ocean current simultaneous current measurements were made at three of the strain array sites the ice behaved in a relatively rigid manner with more shear than dilatation occurring least squares fits of the strain rate tensor showed the deformation field to be quite homogeneous superimposed on the rigid motion were smaller fluctuations with a spectrum falling off proportional to frequency to the power of 3 2 to 2 close examination of individual strain lines showed rather discontinuous distance changes more representative of plastic slip rather than floe bumping although a substantial signal at the inertial period was present in the absolute drift no clear peaks at this period occurred in the spectra of the strain rate tensor invariants analysis of the spatial variation of the underlying ocean currents revealed quite a different picture from that of the ice kinematics in particular the current field exhibited a much greater spatial variability than the ice motion with considerable variance at the inertial period coherence between the ice and ocean differential velocity was small for all frequencies overall the rigid interactive character of the compact ice cover prevented most of the differential ocean currents from being transferred to the differential ice motion

anomalous dispersion of sea ice in the fram strait region the single particle dispersion of sea ice in the fram strait region is investigated using ice drift buoys deployed from 2002 to 2009 within the fram strait cyclones and the arctic climate system study campaigns a new method to estimate the direction of the mean flow based on a satellite drift product is introduced as a result the bias in the dispersion introduced by the mean flow is eliminated considering only the displacements of the buoys in the cross stream direction results show an absolute dispersion growing quadratically in time for the first 3 days and an anomalous dispersion regime exhibiting a strongly self similar scaling following a 5 4 power law for time scales larger than 6 days persisting over the whole time series of length 32 days the non gaussian distribution of the velocity fluctuations with a skewness of 0 15 and a kurtosis of 7 33 as well as the slope of the lagrangian frequency spectrum between 2 and 1 are in agreement with the anomalous diffusion regime comparison with data from the international arctic buoy program yields similar results with an anomalous dispersion starting after 10 days and persisting over the whole time series of length 50 days the results suggest the presence of deformation and shear acting on the sea ice dispersion the high correlation between the cross stream displacements and the cross stream wind velocities shows the important role of the wind as a source for the anomalous dispersion

a dynamic and thermodynamic sea ice model for subpolar regions a mesoscale dynamic thermodynamic sea ice model is presented the model differs from the large scale sea ice model hibler 1979 parkinson and washington 1979 in both the dynamic and thermodynamic parts in the dynamic calculation a constitutive relation based on ice floe collision is used for the determination of the internal ice force in the marginal ice zone miz while in the thermodynamic calculation a three level approach for the ice thickness distribution is adopted instead of the common two level approach hibler 1979 the simulation results from the mesoscale sea ice model show that the three level approach is more sensitive to geophysical forcing than the two level approach thus it is demonstrated that the mesoscale sea ice model has the potential of a more accurate description of the dynamics and the thermodynamics than the previous models in the simulation of sea ice processes in subpolar regions giving the exact initial conditions and the atmospheric and oceanic forcings this model can be used as a sea ice forecasting model in subpolar regions

marginal ice zone rheology comparison of results from continuum plastic models and discrete particle simulations computer simulations of ice behaviour in the marginal ice zone miz are described two cases were studied a constant velocity wind blowing at certain angle with respect to the coast in what can be considered a simplified miz problem and a vortex wind force field the viscous plastic approach of hibler has been used to model the ice rheology we study the effects of adopting two different rheologies cavitating fluid and mohr coulomb the introduction of shear strength effects through a mohr coulomb yield criterion plays an important role in determining ice drift velocities in the miz the effects that ice floe collisions can have in determining the ice rheological behavior have been tested by using discrete particle simulations in order to do so simulations for the miz were performed using different mean floe sizes as well as different miz widths and wind conditions the predictions of the discrete particle simulations are in general agreement with the results obtained by using the mohr coulomb rheology in the viscous plastic model the continuum models were solved by using a fully lagrangian method known as smoothed particle hydrodynamics sph the method has the advantage of the lagrangian techniques in avoiding problems of numerical diffusion an additional advantage of sph in comparison with classical lagrangian approaches is that sph does not need grids and this makes it well suited to handle problems of large deformations

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

the variscan fold thrust belt of southeast ireland and its control on early mesozoic extension and deposition a method to predict the sherwood sandstone

root zone of the late proterozoic salma caldera northeastern arabian shield kingdom of saudi arabia the eroded root of the late proterozoic salma caldera crops out in a striking roughly elliptical feature about 27 km long and 22 km wide near the northeastern edge of the arabian shield the caldera is genetically part of an elongate alkalic granitic massif jabal salma that extends 35 km from the caldera to the southwest comenditic ash flow tuff and lava of the caldera fill probably more than 1 km thick are the oldest recognized rocks of the caldera complex these rocks were erupted during caldera collapse associated with the rapid evacuation of the upper mildly peralkalic part of a zoned magma reservoir within the caldera fill a massive lithic rich intracaldera rhyolite probably a lava in excess of 1 km thick is overlain by a layered ash flow sequence numerous megabreccia blocks probably derived from the caldera wall occur in the massive rhyolite open folds in the layered volcanic rocks may be due to high temperature slumping of the rocks toward the center of the caldera following collapse later peralkalic granite that intruded the caldera ring fracture zone occurs in an arcuate pattern outside the area of exposed caldera fill after caldera collapse metaluminous to peraluminous magma rose beneath the caldera at approximately 580 ma and solidified as biotite alkali granite rim syenogranite and late high level granophyre rare earth element abundances indicate that the layered rhyolite tuff peralkalic granite and granophyre are chemically more evolved than the biotite alkali granite and rim syenogranite the granophyre intruded the caldera fill as a dome shaped body composed of numerous sheetlike masses granophyric texture resulted from rapid pressure release and quenching accompanying the intrusion of each sheet maximum penetration of the granophyre into overlying rocks occurred in the central region and along the west side of the caldera where the caldera fill volcanic rocks have been removed by erosion no apparent structural doming of the exposed volcanic rocks along the east side of the caldera took place the layered ash flows commonly dip steeply toward the center of the caldera postemplacement deformation and metamorphism of the caldera are minimal small displacement strike slip faults cut the complex which is tilted to the northeast by no more than about 2

modeling linear kinematic features in sea ice abstract sea ice deformation is localized in narrow zones of high strain rate that extend hundreds of kilometers for example across the arctic basin this paper demonstrates that these failure zones may be modeled with a viscous plastic sea ice model using an isotropic rheology if the ice is assumed to be heterogeneous at the grid scale and allowed to weaken in time intersecting failure zones propagate across the region the direction of failure propagation depends upon the stress applied to the ice wind stress and boundary conditions and the rheological model describing plastic failure of the ice the spacing between failure zones is controlled by the magnitude of the wind stress and the distribution describing spatial variability of ice strength sea ice motion and deformation oscillate at close to a 12 h period throughout the arctic and antarctic pack ice this oscillation is found at all spatial scales from hundreds of kilometers to the lead scale it is shown that with an inertial embedded mo

welded tuffs deformed into megarheomorphic folds during collapse of the mcdermitt caldera nevada oregon the montana mountains contain part of a resurgent dome complex within the southwestern sector of the mcdermitt caldera exposed intracaldera units consist of welded tuffs lavas domes and epiclastic deposits the majority of volcanic units have a comenditic composition but a few units are nonperalkaline and biotite bearing a deformed series of comenditic welded tuffs occur in a strongly folded rheomorphic zone more than 500 m thick the geometry of vitrophyres flow bands foliations and lineations causes the rheomorphic zone to superficially resemble a series of intrusive domes however the contacts between units are conformable the vitrophyres are continuous from one fold to the next and the units display pumice shards and other features common in welded tuffs the deformed tuff sequence lies on top of biotite tuffs and epiclastic deposits that are tilted and faulted but not folded and beneath relatively undeformed tuffs and lake deposits this relationship suggests that their large scale folds in the deformed sequence developed shortly after caldera collapse

the development of a regional scale intraplate strike slip fault system alpine deformation in the north of ireland abstract a paucity of displacement markers can make constraining the kinematics and interaction of regional scale strike slip faults difficult in this study a high resolution aeromagnetic survey allows quantitative analysis of kilometre scale paleogene faulting in the north of ireland mapping offset dykes and igneous centres reveals the presence of two orientations of cenozoic strike slip faults attributed to broadly n s alpine shortening arising from the convergence of africa and europe reactivated ne sw striking faults accommodated up to 2 3 km of sinistral displacement mainly during the paleocene following tectonic quiescence up to 2 5 km of oligocene aged dextral movement was accommodated on newly formed nnw sse striking faults displacement analysis suggests that the faults were kinematically linked and experienced a switch in polarity of the predominantly active fault orientation during the paleogene forming an apparent conjugate system with asymmetric displacement distributions the switch in fault polarity is attributed to a rotation in the regional maximum horizontal stress orientation and the locus of deformation moving from pre existing reactivated structures to newly formed faults the analysis also illustrates that despite low strain rates the interaction of the principal components of the fault system controlled the formation of sedimentary basins and defined the present day geography of the region far from an active plate boundary

the post variscan development of the british isles within a regional transfer zone influenced by orogenesis abstract the break up of pangaea after the variscan orogeny included rifting extending southwards from the barents sea via the norwegian greenland rift and into the north sea and northwards from the central atlantic these two major rift systems interacted to form an approximately 1200 km wide transfer zone across the british isles where a complex network of basins developed during the mesozoic fault patterns were commonly controlled by reactivation of precambrian caledonian and variscan structures the two main rift systems were unable to breach this regional transfer zone where the crust had been thickened by the caledonian and variscan orogenies until the eocene breaching did not occur down the north sea and through the english channel because of alpine contraction in nw europe instead breaching occurred around the west of ireland and nw scotland so the british isles remained connected to europe rather than to the north american plate

integrating 3 d seismic data field analogs and mechanical models in the analysis of segmented normal faults in the wytch farm oil field southern england united kingdom we propose a methodology for the analysis of normal fault geometries in three dimensional 3 d seismic data sets to provide insights into the evolution of segmented normal fault systems and to improve recovery efforts in fault controlled oil fields limited seismic resolution can obscure subtle fault characteristics such as segmentation and gaps in fault continuity that are significant for oil migration and thus accurate reservoir characterization detailed seismic data analyses that incorporate principles of normal fault mechanics however can reveal evidence of fault segmentation we integrate seismic attribute analyses outcrop analog observations and numerical models of fault slip and displacement fields to augment the use of 3 d seismic data for fault interpretation we applied these techniques to the wytch farm oil field in southern england resulting in the recognition of significant lateral and to a lesser extent vertical segmentation of reservoir scale faults slip maxima on fault surfaces indicate two unambiguous segment nucleation depths controlled by the lithological heterogeneity of the faulted section faults initiated preferentially in brittle sandstone and limestone units subsequent growth and linkage of segments predominantly in the lateral direction resulted in composite fault surfaces that have long lateral dimensions and multiple slip maxima reservoir compartmentalization is greatest at the level of prevalent segment linkages which corresponds at wytch farm with the predominant hydrocarbon producing unit the sherwood sandstone at relatively shallower depths fault segments are younger and less evolved resulting in a greater degree of segmentation with intact relay zones

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 role of floe collisions in sea ice rheology a collisional rheology for an idealized two dimensional flow of a fragmented ice field is derived this fragmented ice field is modeled as an assembly of identical smooth disks collisions between neighboring disks are caused by the mean deformation field these collisions transfer momentum which produces the internal stresses in the deforming ice field by equating the collisional energy losses to the deformational energy a relationship between the stress and strain rate is quantified to demonstrate the essential idea an analytical derivation is first given under quite restricted assumptions a monte carlo simulation is then developed to provide a more general approach for the analysis it is found that the collisional stresses are proportional to the square of disk diameter and the square of the deformation rate the magnitude of stresses is also found to increase rapidly as the collisional restitution of disks increases the collisional rheology yields zero tensile strength the associated normal flow rule commonly used in the plastic rheology is not valid in the collisional rheology it is found that the collisional stresses are very small consequently the resulting stress divergence is estimated to be much lower than the air stress typically encountered in the marginal ice zone however these collisional stresses become singular as the maximum compactness is reached indicating that a different mechanism may exist in that extreme

on the role of ice interaction due to floe collisions in marginal ice zone dynamics a constitutive relation based on floe collisions as introduced by shen et al 1986 is used in a sea ice model for calculating the internal ice force the formulation is generalized to collisions of discs of two sizes and of multiple sizes and a comparison with existing constitutive models is presented a coupling of the floe collision model with a viscous plastic model hibler 1977 in the simulation of mesoscale sea ice processes in subpolar regions is suggested in order to account for the marginal ice zone dynamics

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

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

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