seismic stratigraphic analysis of the cenozoic sediments in the nw faroe shetland basin implications for inherited structural control of sediment distribution abstract the post basalt strata in the faroese area have been investigated based on interpretation of 2d and 3d reflection seismic data the post basalt package is divided into 5 units which have led to the constructions of 6 structural maps and 5 thickness maps within the 5 units 12 prograding sediment bodies have been identified based on the interpretation it is possible to obtain an overview during time of the location of depocentres and direction of prograding units within eocene time the depocentre was placed in the central part of the basin and the sediment influx was mostly from south and southwest during oligocene pliocene time the sediment influx was from north and northwest and the depocentre had moved in a westward direction closer to the faroe platform area emplacement of the cenozoic sediments in the faroese sector of the faroe shetland basin is controlled by thermal subsidence of the basin and local uplift of sediment source areas reactivation of older paleozoic and mesozoic structural elements seem to control the sediment path way and restrict the depositional areas various structural elements being re activated at different times caused considerable structural complexity understanding the older structural elements and their control on sedimentation is a potential tool for understanding deviations from normal thermal subsidence and for predicting the prospectivity in the post basalt succession in the faroe shetland basin

an analytical model of hanging wall and footwall deformation at ramps on normal and thrust faults abstract we determine stress strain and structural evolution for an analytical model of moderate angle 10 20 ramps developed on both normal faults normal ramps and thrust faults thrust ramps in the model a frictionless sliding surface with ramp configuration separates two incompressible isotropic viscous half spaces of different viscosity hanging wall and footwall ramps are planar but ramp flat corners are locally rounded deformation is due solely to translation across the ramp and regional extension or shortening is ignored sliding down a normal ramp causes vertical extension and horizontal shortening in both the hanging wall and the footwall this conforms with reverse faults seen in experimental models of normal ramps the stress in the footwall has the same magnitude as that in the hanging wall but the footwall strain rate is reduced in magnitude by the ratio both a hanging wall syncline and a footwall anticline form the normal ramp migrates in front of the rising footwall at a rate v ss v where v is the sliding velocity but the ramp does not change shape the continuous ramp motion in the model may correspond to the creation of an extensional duplex or chaos zone in brittle rocks at a normal ramp mean compressive stress is lowered and deviatoric stress is raised favoring local faulting and fracture sliding up a thrust ramp causes stresses strain rates and strains of the opposite sign horizontal extension and vertical shortening in both walls is consistent with normal faulting both a hanging wall anticline and a footwall syncline form passively although the deviatoric stress is raised at the ramp so is the mean compressive stress inhibiting fracturing however the mean compressive stress is lowered on the adjacent flats favoring fracturing there

3d crustal and lithospheric structures in the southeastern mediterranean and northeastern egypt crustal and lithospheric thicknesses of the southeastern mediterranean basin region were determined using 3d bouguer and elevation data analysis the model is based on the assumption of local isostatic equilibrium the calculated regional and residual bouguer anomaly maps were employed for highlighting both deep and shallow structures generally the regional field in the area under study is considered to be mainly influenced by the density contrast between the crust and upper mantle use of the gravity and topographic data with earthquake focal depths has improved both the geometry and the density distribution in the 3 d calculated profiles the oceanic continental boundary the basement relief moho depth and lithosphere asthenosphere boundary maps were estimated the results point to the occurrence of thick continental crust areas with a thickness of approximately 32 km in northern egypt below the coastal regions the thickness of crust decreases abruptly transition zone an inverse correlation between sediment and crustal thicknesses shows up from the study furthermore our density model reveals the existence of a continental crustal zone below the eratosthenes seamount block nevertheless the crustal type beneath the levantine basin is typically oceanic this is covered by sedimentary sequences more than 14 km thick the modeled moho map shows a depth of 28 30 km below cyprus and a depth of 26 28 km beneath the south florence rise in the northern west however the moho lies at a constant shallow depth of 22 24 km below the levantine basin which indicates thinning of the crust beneath this region the moho map reveals also a maximum depth of about 33 35 km beneath both the northern egypt and northern sinai both of which are of the continental crust the resulting mantle density anomalies suggest important variations of the lithosphere asthenosphere boundary lab topography indicating prominent lithospheric mantle thinning beneath south cyprus lab 90 km depth followed by thickening beneath the eratosthenes seamount florence rise levantine basin and reaching to maximum thickness below cyprian arc lab 115 120 km depth and further followed by thinning in the north african margin plate and north sinai subplate lab 90 95 km depth according to our density model profiles we find that almost all earthquakes in the study area occurred along the western and central segments of the cyprian arc while they almost disappear along the eastern segment the active subduction zone in the cyprian arc is associated with large negative anomalies due to its low velocity upper mantle zone which might be an indication of a serpentinized mantle this means that collision between cyprus and the eratosthenes seamount block is marked by seismic activity additionally this block is in the process of dynamically subsiding breaking up and being underthrusted beneath cyprus to the north and thrusted onto the levantine basin to the south

the role of mesozoic rifting in the opening of the ne atlantic evidence from deep seismic profiling across the faroe shetland trough the fast deep seismic reflection profile traverses the whole width of the faroe shetland trough the principal target of the profile was the structure of the crust beneath the faroe basalts in this region bright reflections are seen from between 7 and 9 km depth beneath the basalts dipping westwards in the opposite direction to the dip of the basalts and reflections within the basalts these sub basalt reflections are regarded as originating from near top basement the moho has not been imaged beneath the basalts possibly because of the absence of any impedance contrast at the base of the crust the profile shows that the basement of the faroe shetland trough thins to c 10 km beneath the centre of the trough thinning of the crystalline basement is probably the result of more than one phase of extension the most recent of which occurred in the mid to late cretaceous extension appears to have been concentrated on a series of east dipping normal faults cutting through the basement these faults may have originated during a precambrian rifting event it is suggested that opening of the ne atlantic occurred to the west of the faroe islands as mesozoic rifting in the faroe shetland trough had strengthened the lithosphere in this region

modelling hanging wall accommodation above rigid thrust ramps abstract experimental models are used to study the role of material rheology in hanging wall accommodation above rigid flat ramp flat thrust footwalls the deformation in the hanging wall was accomplished by forwards sliding along a rigid basal staircase trajectory with a variable ramp angle ranging from 15 to 60 we model different ramp angles to examine hanging wall accommodation styles above thrust ramps of overthrust faults ranging from 15 to 30 as well as above pre existing normal faults ranging from 45 to 60 for the hanging walls we used stratified frictional sand and viscous silicone putty materials in this paper we study three types of models type 1 models represent purely frictional hanging walls where accommodation above thrust ramps was by layer parallel thickening and by generating a series of back thrusts type 2 and 3 models represent stratified frictional viscous hanging walls in these models accommodation was by a complex association of reverse and normal faults mainly controlled by the rheological anisotropy as well as by the ramp inclination angle in type 2 models the silicone covered only the lower flat while in type 3 models it also covered the rigid ramp for 30 in type 2 models and 45 in type 3 models the viscous layer inhibited the development of back thrusts in the frictional hanging wall instead the silicone thickened to develop a ductile ramp for values higher than 30 in type 2 models and 45 in type 3 models back thrusts develop in response to the bulk compression the experiments simulate many structures observed above natural thrust ramps with 30 and pre existing normal faults with 45 the models emphasise the importance of a basal ductile layer which allows the hanging wall to step up over the rigid ramp by building up its own ductile ramp the models also emphasise that foreland directed normal faulting can develop at a thrust front in the case that the vertical stress due to gravity exceeds the horizontal stress due to end loading within a thrust wedge

limits of the seismogenic zone in the epicentral region of the 26 december 2004 great sumatra andaman earthquake results from seismic refraction and wide angle reflection surveys and thermal modeling the 26 december 2004 sumatra earthquake mw 9 1 initiated around 30 km depth and ruptured 1300 km of the indo australian sunda plate boundary during the sumatra obs ocean bottom seismometer survey a wide angle seismic profile was acquired across the epicentral region a seismic velocity model was obtained from combined travel time tomography and forward modeling together with reflection seismic data from the seacause ii cruise the deep structure of the source region of the great earthquake is revealed four to five kilometers of sediments overlie the oceanic crust at the trench and the subducting slab can be imaged down to a depth of 35 km we find a crystalline backstop 120 km from the trench axis below the fore arc basin a high velocity zone at the lower landward limit of the ray covered domain at 22 km depth marks a shallow continental moho 170 km from the trench the deep structure obtained from the seismic data was used to construct a thermal model of the fore arc in order to predict the limits of the seismogenic zone along the plate boundary fault assuming 100 150 c as its updip limit the seismogenic zone is predicted to begin 5 30 km from the trench the downdip limit of the 2004 rupture as inferred from aftershocks is within the 350 450 c temperature range but this limit is 210 250 km from the trench axis and is much deeper than the fore arc moho the deeper part of the rupture occurred along the contact between the mantle wedge and the downgoing plate

bending of the subducting oceanic plate and its implication for rupture propagation of large interplate earthquakes off miyagi japan in the japan trench subduction zone 1 we present the results of an onshore offshore wide angle refraction and reflection experiment off miyagi in the central japan trench forearc region there are two rupture zones of large interplate earthquakes here the landward rupture zone and the trenchward rupture zone to examine the influence of plate boundary geometry on the distributions of the rupture zones we determined reflector geometries from reflections the subducting oceanic plate increases its dip from about 5 to 13 around 143 2 e this bending point in the oceanic plate corresponds to the eastern edge of the trenchward rupture zone moreover another bending point may be present at approximately 142 3 e which corresponds to the eastern edge of the landward rupture zone the coincidence between bending points in the oceanic plate and the edges of rupture zones suggests that changes in the plate boundary geometry may affect rupture propagation of interplate earthquakes in the direction of plate convergence

optimal dip based on dissipation of back thrusts and hinges in fold and thrust belts 1 back thrusts and hinges are two types of transition between rigid sections of hanging walls observed in fold and thrust belts back thrusts are typical of frictional and homogeneous solids and hinges of creeping and layered materials our objective is to study the orientation of these transitions for the special case of a lower flat ramp transition in a fault bend fold with the following general two step methodology in the first step the forces acting on the transition are determined using equilibrium of each rigid section in the second step the optimal dip of the transition is obtained by minimizing the total dissipation of the structure the three sources of dissipation of comparable magnitude are at the transition on the flat and on the ramp for frictional material flows the back thrusts are velocity discontinuities with optimal dips always less than half the complementary ramp angle leading to hanging wall thickening the optimal dip agrees well with the results of physical analogue and numerical experiments for creeping and layered materials it is shown that a destabilizing deformation mechanism selected to be flexural slip is necessary for the strain to localize and the existence of hinges to be justified activation of flexural slip reduces dissipation at the transition and affects the optimal transition dip the two step methodology proposed here could be seen as a first attempt in producing mechanically balanced cross sections accounting for material rheology this approach should complement the now classical kinematic models of folding

imaging beneath an opaque basaltic layer using densely sampled wide angle obs data a combined reflection refraction wide angle seismic survey was conducted on the continental shelf north west of britain using a conventional streamer with an air gun source and static ocean bottom seismometers obs to record wide angle energy the shallow structure down to a basaltic layer was reasonably well imaged on the stacked reflection section the basalts however proved to be opaque to the conventional reflection method and prevented the imaging of deeper horizons where an important velocity inversion was anticipated this paper reports on the processing modeling and interpretation of the densely sampled wide angle obs data that were coincident with the reflection profile eleven obs instruments were deployed along a 75 km line and recorded signal from a powerful 149 liter 9100 in sup 3 air gun array fired every 50 m data processing was performed using a standard industrial reflection seismic software package prior to first arrival picking processing steps included geometry definition trace summation and display of the data using various scaling algorithms an initial model was constructed from 1d velocity time profiles digitized every 4 km along the stacked section first arrival travel time modeling rapidly converged to a detailed model of the structure of the top 5 km of the crust modeling revealedmore the existence of a buried low velocity mesozoic sedimentary basin of a prominent basement horst and of a normal fault penetrating to the basement less

the composition and structure of volcanic rifted continental margins in the north atlantic further insight from shear waves abstract imaging challenges caused by highly attenuative flood basalt sequences have resulted in the understanding of volcanic rifted continental margins lagging behind that of non volcanic rifted and convergent margins massive volcanism occurred during break up at 70 of the passive margins bordering the atlantic ocean the causes and dynamics of which are still debated this paper shows results from traveltime tomography of compressional and converted shear wave arrivals recorded on 170 four component ocean bottom seismometers along two north atlantic continental margin profiles this traveltime tomography was performed using two different approaches the first a flexible layer based parameterisation enables the quality control of traveltime picks and investigation of the crustal structure the second with a regularised grid based parameterisation requires correction of converted shear wave traveltimes to effective symmetric raypaths and allows exploration of the model space via monte carlo analyses the velocity models indicate high lower crustal velocities and sharp transitions in both velocity and vp vs ratios across the continent ocean transition the velocities are consistent with established mixing trends between felsic continental crust and high magnesium mafic rock on both margins interpretation of the high quality seismic reflection profile on the faroes margin confirms that this mixing is through crustal intrusion converted shear wave data also provide constraints on the sub basalt lithology on the faroes margin which is interpreted as a pre break up mesozoic to paleocene sedimentary system intruded by sills

heterogeneous structure around the rupture area of the 2003 tokachi oki earthquake mw 8 0 japan as revealed by aftershock observations using ocean bottom seismometers abstract large earthquakes have repeatedly occurred in the area off southeastern hokkaido island japan as the pacific plate subducts beneath the island which is on the north american plate the most recent large earthquake in this area the 2003 tokachi oki earthquake mw 8 0 occurred on september 26 2003 in order to investigate aftershock activity in the rupture area 47 ocean bottom seismometers obss were quickly deployed after the main shock in the present study we simultaneously estimate the hypocenters and 3 d seismic velocity models from the p and s wave arrivals of the aftershocks recorded by obss the subducting plate is clearly imaged as a northwest dipping zone in which vp is greater than 7 km s and the relocated hypocenters also show the subducting pacific plate the aftershock distribution reveals that the dip angle of the plate boundary increases abruptly around 90 km from the kuril trench the bending of the subducting plate corresponds to the southeastern edge of the rupture area the island arc crust on the overriding plate has p wave velocities of 6 7 km s and a vp vs of 1 73 a region of vp vs greater than 1 88 was found north of the epicenter of the main shock the depth of the high vp vs region extends about 10 km upward from the plate interface the plate boundary just below the high vp vs region has the largest slip at the main rupture a high vp anomaly 7 5 km s is found in the island arc crust in northeast part of the study area which we interpret as a structural boundary related to the arc arc collisional tectonics of the hokkaido region as the rupture of the main shock terminated at this high vp region we suggest that the plate interface geometry and the trench parallel velocity heterogeneity in the landward plate are principal factors in controlling the rupture area of the main shock

crustal structure of the faeroe shetland channel summary the deep structure of the faeroe shetland channel has been investigated as part of the north atlantic seismic project shot lines were fired along and across the axis of the channel with recording stations both at sea and on adjacent land areas at 61 n 1 7 km of tertiary sediments overlies a 3 9 4 5 km s 1 basement interpreted as the top of early tertiary volcanics a main 6 0 6 6 km s 1 crustal refractor interpreted as old oceanic crust occurs at about 9 km depth the moho 8 0 0 2 km s 1 is at about 15 17 km depth there is evidence that pn may be anisotropic beneath the faeroe shetland channel arrivals recorded at land stations show characteristics best explained by scattering at an intervening boundary which may be the continent ocean crustal contact or the edge of the volcanics the moho delay times at the shot points determined by time term analysis show considerable variation along the axis of the channel they correlate with the basement topography and the greatest delays occur over the buried extension of the faeroe ridge at about 60 15 n where they are nearly 1 s more than the delays at 61 n after correction for the sediments the large delays are attributed to thickening of the early tertiary volcanic layer with isostatic downsagging of the underlying crust and uppermost mantle in response to the load rather than to thickening of the main crustal ayer the new evidence is consistent with deeply buried oceanic crust beneath the faeroe shetland channel forming a northern extension of rockall trough the seabed morphology has been grossly modified by the thick and laterally variable pile of early tertiary volcanic rocks which swamped the region accounting for the anomalous shallow bathymetry the transverse ridges and the present narrowness of the channel

imaging of intrabasalt and subbasalt structure with full wavefield seismic tomography 1 the imaging of subsurface structure using conventional seismic reflection methods is problematic across areas where high velocity basaltic rocks are intercalated with sedimentary layers of low velocity in this study we use an approach that combines wavefield inversion of both normal incidence and wide aperture seismic data to constrain velocity structure within and beneath the basalt our field data are a two ship long offset profile acquired in the faeroe basin a conventional traveltime inversion of the wide angle data is performed to obtain a starting model for the wavefield inversion synthetic tests are used to explore resolution and to determine a strategy for inversion of the field data our final velocity model shows a high velocity basaltic layer underlain by an intermediate velocity layer that is presumably sedimentary the basalts appear highly heterogeneous in velocity and the degree of heterogeneity increases significantly with depth within the layer a prestack depth migration using the final velocity model reveals a number of new features despite the velocity heterogeneity within the basalts strong reflections are seen within and immediately beneath the basaltic section and these reflectors are more clearly imaged on the final depth migrated section than hitherto the transition from sediments to basalt does not appear to be sharp uniquely defined or locally subhorizontal the data suggest that velocity heterogeneity and the complexity of the transition from basalt to sediment may explain the difficulties of imaging beneath basalts west of britain wavefield tomography can resolve this complexity and hence improve both imaging and confidence of interpretation within and beneath basalts

structural factors controlling the coseismic rupture zone of the 1973 nemuro oki earthquake the southern kuril trench seismogenic zone 1 the kuril trench is an active seismogenic zone where historic great earthquakes have occurred repeatedly with temporal and spatial regularities this paper presents the results of a wide angle ocean bottom seismographic obs survey across the coseismic rupture zone of the 1973 nemuro oki earthquake mw 7 8 together with thermal modeling the main purpose of this study is to understand the crustal and thermal controls on the kuril seismogenic zone revealing structures that can be controlling factors of a megathrust earthquake helps in estimating earthquake hazards we focus specifically on the precise structure of the seismogenic interface our imaged subduction structure clearly indicates that the coseismic rupture of the 1973 event occurred between the subducting oceanic crust and the kuril island arc crust at 15 30 km depth a thermally determined updip limit of the seismic rupture 100 c corresponds to the seismically determined updip limit 15 km depth an analysis of amplitude variations of wide angle reflections demonstrated a highly reflective plate interface immediately trenchward of the coseismic rupture zone shallower than 15 km depth while a weakly reflective interface is observed at the coseismic rupture zone from the observed amplitude variation with offsets a thin low velocity layer 4 5 km s is observed at the top of the highly reflective interface this suggests the presence of fluid which would indicate weak coupling updip of the rupture zone we also deduced from these observations that a weakly reflective interface in the coseismic slip zone represents a strongly coupled zone pending the next megathrust earthquake

confirming sharp bending of the pacific plate in the northern japan trench subduction zone by applying a traveltime mapping method abstract many destructive earthquakes have occurred in the northern japan trench region owing to the subduction of the pacific plate two plate bending points where the dip of the subducting plate changes markedly have been found and correlations between the bending points and the updip limits of large earthquake rupture zones have been previously found in the regions off aomori 40 n and off miyagi 38 n in this study we investigate whether sharp plate bending is also present in the region off iwate 39 n located approximately halfway between the off aomori and off miyagi regions we analysed the existing off iwate wide angle seismic experimental data by applying a new method in which observed reflection phases are directly projected onto the depth distance domain from the time distance domain as a result we found a plate bending point in the region off iwate this is the first time such a point has been found there distances between the three bending points and the trench axis are almost uniform and sharp plate bending is a common feature in the northern japan trench subduction zone the bending axis which is defined by connecting bending points approximately coincides with the updip limit of the rupture zones of large interplate earthquakes seismic swarm activities have been observed inside the subducting oceanic crust along the bending axis and we infer that some of these activities are caused by bending of the subducting plate in addition p wave velocities of the upper plate vary across the bending axis implying that the position of the bending axis is closely related to physical properties of the upper plate and to seismic activities

centrifuge modelling of fold thrust structures in a tripartite stratigraphic succession abstract analog models measuring 127 76 mm in plan were deformed at 2500 4000 g in a centrifuge scaled stratigraphic sequences were constructed of anisotropic multilayers with individual layers of plasticine and silicone putty as thin as 40 m the plasticine silicone putty multilayers are analogs for interbedded competent carbonates and clastics and incompetent pelites given the model ratios of acceleration 2500 g length 5 10 6 specific gravity 0 6 time 10 10 modelling of fold thrust tectonics emphasizes the influence of stratigraphic succession on structural evolution the models are constructed with a tripartite stratigraphic succession comprising basal and upper well laminated and incompetent units and a middle somewhat more isotropic and competent unit the models deform by three mechanisms layer parallel shortening folding and thrust faulting they reproduce a number of fold thrust relationships that have been observed in nature folds are typically periclinal in en echelon arrays folds and thrusts are arcuate in plan reflecting differential shortening fold attitudes grade from upright at high levels to overturned at deeper levels within a structural panel reflecting drag against the basal decollement fold axial surfaces and thrust faults are listric while competent units may be offset by localized displacement on thrust faults the discrete faults may die out both upwards and downwards into regions of ductile strain in less competent units thrust faults appear to follow staircase trajectories through the strata transecting incompetent units at shallow angles to bedding and competent units at steeper angles however the apparent staircase pattern results from propagation of a fault along a relatively straight trajectory through previously folded strata foreland verging thrusts are more common than back thrusts the latter have steeper dips the models suggest a mechanism of thrust ramp nucleation following detachment folding long wavelength buckling of a competent unit can initiate localized strain folding and layer parallel shear in an underlying incompetent unit beneath the anticlines of the competent unit thrust faults propagate up section from these high strain zones through the foreland dipping limbs of buckle folds in the competent unit this mechanism may explain the commonly observed spatial periodicity of thrust ramps the model results bear similarities to natural fold thrust belts in which the stratigraphic succession consists of three mechanical units for example the asiak foreland and bear creek hills fold thrust belts of the slave province northwest territories canada

slip distribution of the 1952 tokachi oki earthquake m 8 1 along the kuril trench deduced from tsunami waveform inversion 1 we inverted 13 tsunami waveforms recorded in japan to estimate the slip distribution of the 1952 tokachi oki earthquake m 8 1 which occurred southeast off hokkaido along the southern kuril subduction zone the previously estimated source area determined from tsunami travel times hatori 1973 did not coincide with the observed aftershock distribution our results show that a large amount of slip occurred in the aftershock area east of hatori s tsunami source area suggesting that a portion of the interplate thrust near the trench was ruptured by the main shock we also found more than 5 m of slip along the deeper part of the seismogenic interface just below the central part of hatori s tsunami source area this region which also has the largest stress drop during the main shock had few aftershocks large tsunami heights on the eastern hokkaido coast are better explained by the heterogeneous slip model than previous uniform slip fault models the total seismic moment is estimated to be 1 87 1021 n m giving a moment magnitude of mw 8 1 the revised tsunami source area is estimated to be 25 2 103 km2 3 times larger than the previous tsunami source area out of four large earthquakes with m 7 that subsequently occurred in and around the rupture area of the 1952 event three were at the edges of regions with relatively small amount of slip we also found that a subducted seamount near the edge of the rupture area possibly impeded slip along the plate interface

calcite twin analysis in syntectonic calcite cape fold belt south africa implications for fold and cleavage formation within a shallow thrust front abstract lithologies of two formations in the frontal ranges of the cape fold belt south africa host synorogenic calcite precipitated at low metamorphic grade during the end permian deformation of this gondwana wide orogen the calcite crystals are mechanically twinned and this deformation is linked to both folding and the formation of an axial planar cleavage with associated extensional veins twinning strain analysis reveals a complex rotational synfolding history in the upper thin layered prince albert formation twinned calcite within the fold preserves two unique strain events the primary shortening strain fabric is layer parallel and transport parallel north south the shortening strain overprint 5 82 is layer normal and plunges steeply to the northeast these results are inconsistent with a flexural slip folding mechanism by contrast a homogeneous syn cleavage stress strain field is recorded in the underlying massive thick bedded dwyka tillite analyses of calcite twins in clast hosted syn cleavage fibrous calcite and rare limestone clasts define a sub horizontal n s shortening and sub horizontal e w extension the intermediate axis e 2 is vertical and preserves shortening 5 3 the extension axis e 3 is horizontal and parallel to the clast tension gash fracture plane these deformed syn cleavage calcite materials provide an independent constraint to the debate about contemporaneous stress strain fields associated with folding and formation of an axial planar cleavage

p and s wave velocities of consolidated sediments from a seafloor seismic survey in the north celtic sea basin offshore ireland a geophysical survey was conducted over a hydrocarbon prospect in the north celtic sea basin using a small array of ocean bottom seismographs obss the purpose of this study was to determine the ratio of p compressional to s shear wave velocity of consolidated sedimentary rocks in order to constrain possible subsurface variations in pore fluid content the ratio of vp and vs is known to be particularly sensitive to lithology porosity and pore fluid content making it a useful parameter for evaluating hydrocarbon prospects obss offer a relatively cheap and time effective means of acquiring multi component data compared with ocean bottom cables in this contribution we demonstrate the ability of an obs survey comprising three pairs of two obss spaced at 1 6 km to recover lateral variations in the vp vs ratio a key requirement of this type of study is that s waves will be generated by mode conversions in the subsurface since they cannot be generated in nor travel through fluids in this survey the contrast in physical properties of the hard seabed of the north celtic sea basin provided a means of generating converted s waves two dimensional ray tracing and forward modeling was used to create both vp and vs models along a profile crossing the blackrock prospect in the north celtic sea basin these models comprise four layers and extend to a maximum depth of 1 1 km the observed northward decrease in the vp vs ratio at depths of 500 1000 m below the seafloor in the study area is interpreted to represent lateral variation in the amount of gas present in the pore space of upper cretaceous chalks and shales overlying the prospective reservoir

moho and basement depth in the ne atlantic ocean based on seismic refraction data and receiver functions seismic refraction data and results from receiver functions were used to compile the depth to the basement and moho in the ne atlantic ocean for interpolation between the unevenly spaced data points the kriging technique was used free air gravity data were used as constraints in the kriging process for the basement that way structures with little or no seismic coverage are still presented on the basement map in particular the basins off east greenland the rift basins off nw europe are mapped as a continuous zone with basement depths of between 5 and 15 km maximum basement depths off ne greenland are 8 km but these are probably underestimated plate reconstructions for chron c24 c 54 ma suggest that the poorly known ammassalik basin off se greenland may correlate with the northern termination of the hatton basin at the conjugate margin the most prominent feature on the moho map is the greenland iceland faroe ridge with moho depths 28 km crustal thickness is compiled from the moho and basement depths the oceanic crust displays an increased thickness close to the volcanic margins affected by the iceland plume

crustal strain dependent serpentinisation in the porcupine basin offshore ireland abstract mantle hydration serpentinisation at magma poor rifted margins is thought to play a key role in controlling the kinematics of low angle faults and thus hyperextension and crustal breakup however because geophysical data principally provide observations of the final structure of a margin little is known about the evolution of serpentinisation and how this governs tectonics during hyperextension here we present new observational evidence on how crustal strain dependent serpentinisation influences hyperextension from rifting to possible crustal breakup along the axis of the porcupine basin offshore ireland we present three new p wave seismic velocity models that show the seismic structure of the uppermost lithosphere and the geometry of the moho across and along the basin axis we use neighbouring seismic reflection lines to our tomographic models to estimate crustal stretching c of 2 5 in the north at 52 5 n and 10 in the south at 51 7 n these values suggest that no crustal embrittlement occurred in the northernmost region and that rifting may have progressed to crustal breakup in the southern part of the study area we observed a decrease in mantle velocities across the basin axis from east to west these variations occur in a region where c is within the range at which crustal embrittlement and serpentinisation are possible c 3 4 across the basin axis the lowest seismic velocity in the mantle spatially coincides with the maximum amount of crustal faulting indicating fault controlled mantle hydration mantle velocities also suggest that the degree of serpentinisation together with the amount of crustal faulting increases southwards along the basin axis seismic reflection lines show a major detachment fault surface that grows southwards along the basin axis and is only visible where the inferred degree of serpentinisation is 15 this observation is consistent with laboratory measurements that show that at this degree of serpentinisation mantle rocks are sufficiently weak to allow low angle normal faulting based on these results we propose two alternative formation models for the porcupine basin the first involves a northward propagation of the hyperextension processes while the second model suggests higher extension rates in the centre of the basin than in the north both scenarios postulate that the amount of crustal strain determines the extent and degree of serpentinisation which eventually controls the development of detachments faults with advanced stretching

crustal structure of the ne rockall trough from wide angle seismic data modeling two wide angle seismic lines located in the northern rockall trough were acquired in may 2000 one line line e crosses the trough from the continental shelf off lewis to normal oceanic crust west of lousy bank in nw se direction the other line line d intersects with line e crosses the wyville thomson ridge in a sw ne direction and ends in the faeroe shetland basin sonobuoy data and expanding spread profiles acquired in the same area have been remodeled analysis of the seismic data using travel times and amplitudes reveals an up to 5 km thick sedimentary basin including an up to 1 5 km thick basaltic layer which is present in most of the trough further conclusions of this study are that the rockall trough is underlain by highly stretched continental crust of 13 km thickness the crust thickens to 24 km beneath lousy bank which is interpreted to be of continental nature beneath the hebrides continental shelf a three layer continental crust of 26 km is modeled an up to 12 km thick high velocity layer is observed underneath the ocean continent boundary and is interpreted as magmatic underplating resulting from excess volcanism during rifting no evidence for an underplate layer could be distinguished beneath the trough area modeling of the structure of the wyville thomson ridge revealed no existing igneous core of the ridge confirming existing theories that it is a compressional structure

plate interface properties in the northeast hikurangi subduction zone new zealand from converted seismic waves arrival time inversion of local earthquakes in the northeast hikurangi subduction zone has indicated high vp vs at the plate interface but the thickness of the anomalous zone is poorly constrained by the p and s travel times we investigate this plate interface zone further by modeling s to p converted phases using synthetic seismograms for upgoing rays of earthquakes in the subducting plate mantle the modeling indicates a low velocity layer at the plate interface which is generally 1 2 km thick and has vp of 5 0 5 35 km s and vp vs of 2 this is consistent with a subducting sediment channel with near lithostatic fluid the low rigidity indicates a weak zone which will have a strong influence on the distribution of deformation the modeling approach shows promise in defining rheological parameters at the plate interface and mapping their variation across the seismogenic zone of the subduction thrust

predicting fluid pressure in sedimentary basins from seismic tomography 10 pages 9 figures 2 tables geophysical journal international 2019 predicting fluid pressure in sedimentary basins from seismic tomography published by oxford university press on behalf of the royal astronomical society all rights reserved

sequences of mesostructures related to the development of alpine thrusts in the eastern pyrenees abstract the mesostructures formed during the development of an antiformal stack in the eastern pyrenees are studied the following structural evolution is proposed a pressure solution cleavage develops by layer parallel shortening at initial stages of the thrusting the cleavage is present in all units but mainly in the marls and marly limestones in the siliceous layers a weak cleavage and small scale thrust structures develop the cleavage and the bedding are deformed during the thrusting different structures form in various places of the antiformal stack where the bedding is flat lying southward directed layer parallel shears predominate in the hangingwall ramps kink bands disjunctive stylolitic cleavage and layer parallel shear opposite to the sense of thrust movement develop the described structures constitute a kinematical model for mesostructures developed during the evolution and ramping of thrusts

a quantitative study on the use of converted waves for sub basalt imaging the idea of imaging beneath a high velocity layer using converted waves has been popular since 1990 because these wave types have their maximum amplitudes at mid to far offsets the search for pure p waves at the highly multiple contaminated near offsets can be avoided for the atlantic margin with buried thin layered basalts our quantitative study shows that the initial single layered approach is not viable even in an unrealistic ideal geological setting the amplitude of the symmetrical psp mode is far too weak to be recognized on towed streamer data furthermore in the far offset window where locally converted waves have their strongest amplitudes there is a multitude of other reflections refractions and interbedded multiples which have similar moveouts and often higher amplitudes without the removal of these events a reliable image of the subsurface cannot be produced we show that even if this problem were solved it would be far easier to use the p wave reflection from beneath the basalt at near offsets our study shows that this wave type is by far the strongest response a borehole derived model using a thin layered basalt sequence reveals that the strongest locally converted wave has an asymmetrical path and is 10 times weaker all our results indicate that the pure p modes provide the best chance of imaging sub basalt sedimentary interfaces

a detailed subduction structure in the kuril trench deduced from ocean bottom seismographic refraction studies abstract in 1983 we conducted an ocean bottom seismograph obs experiment in the southernmost part of the kuril trench beneath which the pacific plate is subducting under hokkaido island japan the aim of this experiment was to determine crust and upper mantle structure from the oceanic basin to the continental slope by dense seismic refraction profiling using explosives and airguns the total length of the profiles was 560 km along which ten obs s were deployed the observed data were of good quality which enabled us to obtain a detailed velocity structure of the active margin down to 20 30 km we constructed a velocity structure model by ray tracing and amplitude modeling in the oceanic basin the crust has a typical oceanic structure characterized by three layers with p wave velocities of 1 8 3 8 6 5 and 6 5 7 0 km s the velocity gradient in layer 3 increases downward from 0 075 to 0 10 s 1 the moho discontinuity was well constrained by clearly observed p n and p m p phases the total thickness of the oceanic crust was determined to be 8 km and almost constant in the oceanic basin the p wave velocity is 7 9 km s beneath the moho discontinuity which increases downward with a rather small velocity gradient 0 015 0 03 s 1 beneath the continental slope we found relatively low velocity 2 5 to 5 5 5 8 km s material the oceanic moho discontinuity associated with the subducting plate was traced down to a depth of 25 km our seismic data strongly suggest that oceanic layer 2 is smoothly subducting and does not break up to form a wedge structure this result is in remarkable contrast with the velocity structure in the active margin of the ryukyu trench area about 1000 km south of the present experimental area where we found a 12 km thick prominent low velocity wedge is situated 50 150 km landward from the trench axis on the seaward side of the wedge the surface of the igneous basement undulates severely the wedge was probably formed by materials of oceanic origin such a difference in velocity structure suggests that the subduction mechanism in the trench area differs from region to region in the northwestern pacific

a mechanical analysis of the keystone muddy mountain thrust sheet in southeast nevada abstract the keystone muddy mountain thrust sheet was displaced for several tens of kilometres across the land s surface in cretaceous times its movement is related to the emplacement of the sierra nevada batholith which provided a source of compressive stress over a period of about 140 ma the mechanical analysis presented here examines the stresses operating throughout the whole thrust sheet i e the toe ramp and main thrust block which extends for approximately 200 km from front to rear frictional sliding can explain only the motion of the toe the ramp and perhaps part of the main thrust block a composite model involving elastic upper and viscous lower layers is used to account for the movement of most of the main thrust block

strains and folds within thrust sheets an analysis of the heilam sheet northwest scotland abstract the changes of shape and orientation of fossil worm burrows have been used to carry out a detailed strain analysis of the heilam thrust sheet in the moine thrust zone northwest scotland the strains have been factorised into components of layer parallel shear 1 layer normal shear 2 and layer parallel shortening or elongation variations in the 1 shear are largely lithologically controlled some strain is due to flexural processes as the thrusts and overlying beds change dip over thrust ramps or are tightened by later deformation while there may also be a component of drag which increases towards the overlying thrust this predating the shear due to flexure a contoured map of the values suggests that major folds within the thrust sheet developed as a result of stick or drag on the floor thrust prior to ramp climb this led to differential movement in the transport direction causing layer parallel shortening accompanied or followed by buckling of the layers a map showing layer normal shear strains 2 suggests that the heilam sheet forms a major surge zone the central part having moved further wnw than the northern and southern parts this has led to components of lateral shear along parts of the sheet especially in the north producing curvature of the imbricate faults

inversion of traveltime data under a statistical model for seismic velocities and layer interfaces we invert large aperture seismic reflection and refraction data from a geologically complex area on the northeast atlantic margin to jointly estimate seismic velocities and depths of major interfaces our approach combines this geophysical data information with prior information on seismic compressional velocities and the structural interpretation of seismic sections we constrain expected seismic velocities in the prior model using information from well logs from a nearby area the layered structure and prior positions of the interfaces follow information from the seismic section obtained by processing the short offsets instead of using a conventional regularization technique to smooth the interface velocity model we describe the spatial correlation of interfaces and velocities with a geostatistical model using a multivariate gaussian probability density function we impose a covariance function on the velocity field in each layer and on each interface in the model to control the smoothness of the solution the inversion is performed by minimizing an objective function with two terms one term measuring traveltime residuals and the other measuring the fit to the statistical model we calculate the posterior uncertainties and evaluate the relative influence of data and the prior model on estimated interface depths and seismic velocities the method results in the estimation of velocity and interface geometry beneath a basaltic sill system down to 7 km depth this method aims to enhance the interpretation process by combining multidisciplinary information in a quantitative model based approach

large explosive source wide recording aperture seismic profiling on the columbia plateau washington clear subsurface seismic images have been obtained at low cost on the columbia plateau washington the columbia plateau is perhaps the most notorious of all bad data areas because large impedance contrasts in surface flood basalts severely degrade the seismic wavefield this degradation was mitigated in this study via a large explosive source wide recording aperture shooting method the shooting method emphasizes the wide angle portion of the wavefield where fermat s principle guarantees reverberation will not interfere with the seismic manifestations of crucial geologic interfaces the basalt diving wave normally discarded in standard common midpoint cmp seismic profiling can be used to image basalt velocity structure via traveltime inversion maximum depth penetration of the diving wave tightly constrains basalt sediment interface depth an arrival observed only at shot receiver offsets greater than 15 km can be used to determine the velocity and geometry of basement via simultaneous inversion

three dimensional seismic refraction tomography a comparison of two methods applied to data from the faeroe basin this paper presents a comparison of two tomographic methods for determining three dimensional 3 d velocity structure from first arrival travel time data the first method is backprojection in which travel time residuals are distributed along their ray paths independently of all other rays the second method is regularized inversion in which a combination of data misfit and model roughness is minimized to provide the smoothest model appropriate for the data errors both methods are nonlinear in that a starting model is required and new ray paths are calculated at each iteration travel times are calculated using an efficient implementation of an existing method for solving the eikonal equation by finite differencing both inverse methods are applied to 3 d ocean bottom seismometer obs data collected in 1993 over the faeroe basin consisting of 53 479 travel times recorded at 29 obss this is one of the most densely spaced large scale 3 d seismic refraction experiments to date different starting models and values for the free parameters of each tomographic method are tested a new form of backprojection that converges more rapidly than similar methods compares favorably with regularized inversion but the latter method provides a simpler model for little additional computational expense when applied to the faeroe basin data bounds on two model features are assessed using regularized inversion with combined smoothness and flatness constraints an inversion of synthetic data corresponding to 100 data recovery from the real experiment shows a marked improvement in lateral resolution at deeper depths and demonstrates the potential of currently feasible 3 d refraction experiments to provide well resolved long wavelength velocity models the similarity of the final models derived from the two tomographic methods suggests that the results from the new form of backprojection can be relied on when limited computational resources rule out regularized inversion

crustal structure beneath the faroe islands and the faroe iceland ridge abstract we have mapped the transition from the continental faroe block the faroe islands and surrounding shelf to the thickened oceanic crust of the faroe iceland ridge in the north atlantic using the results of a detailed sea to land seismic profile with wide angle to normal incidence recordings of explosive and airgun shots fired at sea along the faroe iceland ridge interpretation of all available seismic and gravity data indicates that this aseismic ridge is composed of 30 3 km thick oceanic crust with a gradual transition to ancient continental crust from 100 to 40 km northwest of the faroe islands close to the shelf edge this confirms that the crust beneath the faroe islands which may be up to 46 km thick comprises continental material in agreement with previous seismic and geochemical results results suggest that the upper 5 2 0 7 km of the faroe crust consists of tertiary basalts generated during continental breakup overlying the continental crust beneath the lower crust where seismic constraint is poor may exhibit high seismic velocities 7 1 7 6 km s 1 which we attribute to underplating or intrusion by mafic melts during continental breakup in the early tertiary

upper and middle crustal deformation of an arc arc collision across hokkaido japan inferred from seismic refraction wide angle reflection experiments abstract the hidaka collision zone hcz central hokkaido japan is a good target for studies of crustal evolution and deformation processes associated with an arc arc collision the collision of the kuril arc ka with the northeast japan arc nja which started in the middle miocene is considered to be a controlling factor for the formation of the hidaka mountains the westward obduction of middle lower crustal rocks of the ka the hidaka metamorphic belt hmb and the development of the foreland fold and thrust belt on the nja side the hokkaido transect project undertaken from 1998 to 2000 was a multidisciplinary effort intended to reveal structural heterogeneity across this collision zone by integrated geophysical geological research including seismic refraction reflection surveys and earthquake observations an e w trending 227 km long refraction wide angle reflection profile found a complicated structural variation from the ka to the nja across the hcz in the east of the hcz the hinterland region is covered with 4 4 5 km thick highly undulated neogene sedimentary layers beneath which two eastward dipping reflectors were imaged in a depth range of 10 25 km probably representing the layer boundaries of the obducting middle lower crust of the ka the hmb crops out on the westward extension of these reflectors with relatively high vp 6 0 km s and vp vs 1 80 consistent with middle lower crustal rocks beneath these reflectors more flat and westward dipping reflector sequences are situated at the 25 27 km depth forming a wedge like geometry this distribution pattern indicates that the ka crust has been delaminated into more than two segments under our profile in the western part of the transect the structure of the fold and thrust belt is characterized by a very thick 5 8 km sedimentary package with a velocity of 2 5 4 8 km s this package exhibits one or two velocity reversals in paleogene sedimentary layers probably formed by imbrication associated with the collision process from the horizontal distribution of these velocity reversals and other geophysical geological data the rate of crustal shortening in this area is estimated to be greater than 3 4 mm year which corresponds to 40 50 of the total convergence rate between the nja and the eurasian plate this means that the fold and thrust belt west of the hcz is absorbing a large amount of crustal deformation associated with plate interaction across hokkaido island

the seismic velocity structure of the newfoundland appalachian orogen the deep structure of the newfoundland appalachian orogen is investigated by analyses of three intersecting seismic refraction wide angle reflection profiles which traverse the gander and dunnage zones or central mobile belt of newfoundland a simultaneous travel time inversion for velocity and interface was applied to the in line seismic refraction wide angle reflection data and constrained by synthetic amplitude models the results of the modeling procedure show a subhorizontally layered crust with upper crustal velocities ranging from 5 4 to 6 2 km s a midcrustal velocity of 6 25 6 35 km s and a lower crustal velocity of 6 7 0 2 km s the top of the lower crust is marked by a series of prominent reflections between 18 and 23 km depth which suggest a complex layered velocity interface strong laterally coherent moho reflections indicate a sharp crust mantle transition at 35 3 km the uppermost mantle has a velocity of 8 0 0 2 km s and a reflecting horizon at 55 km depth suggests an increase to velocities approaching 8 5 km s normal moveout corrections applied to fan profiles provide constraining evidence for the reflecting horizon at the top of the lower crust and laterally continuous moho reflections at 11 12 s two way travel time comparisons with a coincident deep seismic reflection profile show that the refraction and reflection mohos match to better than 2 3 km bulk poisson s ratios of 0 23 0 24 for the whole crust calculated from pmp sms travel times suggest a crust dominated by quartzofeldspathic lithologies and a notable absence of voluminous mafic additions to the lower crust the absence of a deep crustal root coupled with the bulk intermediate composition inferred for the lower crust from the seismic refraction wide angle data implies that the crust beneath central newfoundland has undergone multiple periods of reactivation and equilibration following successive orogenic episodes

crustal structure of the southeastern grenville province northern new york state and eastern ontario the grenville province exposes an oblique cross section through mid lower crustal lithologies that were pervasively deformed and subjected to regional thermal overprinting during the grenvillian orogeny 1 1 ga the southeastern grenville province is divided into two subterranes by the carthage colton mylonite zone a 110 km long lineament characterized by intense ductile shear and igneous intrusion which separates the amphibolite facies metasediments of the central metasedimentary belt to the west from the granulite facies metaplutonics of the central granulite terrane to the east the recognition of distinct lithotectonic domains separated by zones of intense ductile shear in the grenville province raises questions concerning the deep structure of these subterranes and in particular the means by which the mid lower crustal rocks exposed in the grenville province were emplaced seismic refraction wide angle reflection data were acquired to investigate the deep structural interrelationships within the southeastern grenville province a travel time inversion for velocity and interface depth was applied to the seismic data together with constraints from amplitude modeling to produce a seismic velocity model of the crust in the southeastern grenville province in the central metasedimentary belt the upper crust is characterized by velocities in the range 6 3 6 4 km s and a poisson s ratio of 0 26 0 01 which are attributed to quartzofeldspathic rocks farther east in the central granulite terrane upper crustal velocities of 6 55 km s and a poisson s ratio of 0 28 0 01 are associated with the marcy anorthosite the seismic homogeneity of the upper crust in the region of the carthage colton mylonite zone suggests that this boundary is a shallow feature limited to the upper 2 3 km of the crust the deep crustal structure of the southeastern grenville province is characterized by two discrete and laterally discontinuous seismic interfaces in the central metasedimentary belt the top of the lower crust is delineated by an eastward dipping interface at 24 28 km depth in the central granulite terrane prominent en echelon reflections referred to as the tahawus complex form a gently arched dome at 17 22 km depth interpretation of the tahawus complex as a zone of layered mafic cumulates is supported by its high velocity 7 1 km s and poisson s ratio 0 27 0 02 the lower crust is characterized by a velocity of 7 0 7 2 km s and an anomalously high poisson s ratio of 0 30 0 02 which are representative of pyroxene garnet granulites in contrast velocities of 6 8 7 0 km s are modeled beneath the central granulite terrane and appear to signify a lateral change in composition the moho lies at 44 45 km depth and is characterized by pronounced en echelon reflection segments suggesting compositional interlayering around the crust mantle boundary the velocity of the upper mantle is 8 0 8 2 km s an anomalous upper mantle layer with a reversed velocity of 8 6 km s dips eastward from 50 to 60 km depth beneath the southeastern grenville province our results indicate that remnants of magmatic intrusions that mobilized and thickened the crust during the grenvillian orogeny are preserved in the mid lower crust as a layered cumulate body tahawus complex and in the upper mantle as an eclogitic lens possibly delaminated from the overthickened crust during uplift of the southeastern grenville province

the crustal structure of the nw moroccan continental margin from wide angle and reflection seismic data summary the atlantic margin off morocco with its neighbouring jurassic oceanic crust is one of the oldest on earth it is conjugate to the nova scotia margin of north america the sismar marine seismic survey acquired deep reflection seismic data as well as wide angle seismic profiles in order to image the deep structure of the margin characterize the nature of the crust in the transitional domain and define the geometry of the synrift basins we present results from the combined interpretation of the reflection seismic wide angle seismic and gravity data along a 440 km long profile perpendicular to the margin at 33 34 n extending from nearly normal oceanic crust in the vicinity of coral patch seamount to the coast at el jadida and approximately 130 km inland the shallow structure is well imaged by the reflection seismic data and shows a thick sedimentary cover that is locally perturbed by salt tectonics and reverse faulting the sedimentary basin thickens from 1 5 km on the normal oceanic crust to a maximum thickness of 6 km at the base of the continental slope multichannel seismic mcs data image basement structures including a few tilted fault blocks and a transition zone to a thin crust a strong discontinuous reflection at 12 s two way travel time twt is interpreted as the moho discontinuity as a result of the good data quality the deep crustal structure depth and velocity field is well constrained through the wide angle seismic modelling the crust thins from 35 km underneath the continent to approximately 7 km at the western end of the profile the transitional region has a width of 150 km crustal velocities are lowest at the continental slope probably as a result of faulting and fracturing of the upper crust uppermantle velocities could be well defined from the ocean bottom seismometer obs and land station data throughout the model

reflection seismic evidence for a moho offset beneath the walls boundary strike slip fault birps has acquired two deep seismic reflection profiles across the walls boundary fault a major late caledonian strike slip fault that is probably the northern continuation of the great glen fault of scotland where crossed by the shet deep seismic survey north of the shetlands the walls boundary fault appears to be a narrow vertical structure which penetrates the entire crustal thickness and juxtaposes crusts of different thicknesses the seismic data show a moho offset in travel time of 1 0 1 5 seconds and two high amplitude diffractions originating at moho travel times these diffractions are as yet unique among seismic data across strike slip faults and can be used to argue that the moho is offset in depth by 2 3 km over a lateral distance of less than 6 km seismic modelling is used to constrain this interpretation the preservation of this moho topography suggests that the structure and rheology of the lower crust of the shetland platform was not significantly modified during post devonian extension of the north sea and atlantic margin

characterizing the mid faeroe ridge using seismic velocity measurements the character of the basement and overlying sedimentary sequences in the vicinity of the mid faeroe ridge is investigated through a synthesis of seismic velocity measurements a finite difference tomographic algorithm was applied to a wide aperture seismic profile acquired across the center of the faeroe basin to produce a velocity model free from a priori artifacts a series of parameterizations and smoothing constraints were run to assess the uniqueness of the final model uncertainties in the velocity structure were estimated at 5 comparison of the in situ velocity of the mid faeroe ridge with deterministic lithology velocity data extracted from nearby boreholes strongly implies that tertiary to cretaceous shale and sandstone sequences extend to depths of 4 5 km the occurrence of pre cretaceous sedimentary lithologies at 4 5 8 0 km depth is supported by in situ velocities of 4 0 4 5 km s which are best explained by well consolidated sediments and are incompatible with likely constituents of the basement evidence for a major structural discordance across the southeastern flank of the mid faeroe ridge is provided by a prominent fault whose location demarcates a lateral velocity variation of 0 5 km s a rapid increase of the in situ velocity at 8 km depth is strongly suggestive of a transition into the basement whose velocity 5 5 km s is comparable with high pressure laboratory measurements of basalts from the faeroe islands this basaltic basement in the center of the faeroe basin lies above a moho depth of 18 3 km which suggests cumulative stretching factors of about 3 0 and implies substantial igneous activity we conclude that in the region of the mid faeroe ridge the prerift lewisian basement has been highly attenuated pervasively intruded and possibly wholly replaced by basaltic material

numerical models for the control of inherited basin geometries on structures and emplacement of the klippen nappe swiss prealps cover nappes commonly deform above a shallow basal detachment surface located above rigid crystalline basement rocks inherited basin geometries are presumed to control the kinematic evolution and detachment of cover nappes in an accretionary wedge we compare results from two dimensional finite element modelling with the structural style of a natural cover nappe the klippen nappe which was detached from its basement along an evaporitic detachment horizon and thrust over a distance of roughly 100 km first order characteristics of the klippen nappe paleo basin include a complex distribution of weak detachment rocks and various changes in the sediment composition and layer thicknesses two types of structural style can be distinguished an imbricate fan at the rear end of the nappe prealpes medianes rigides and a fold dominated region at the front prealpes medianes plastiques the model results resemble the first order characteristics of the klippen nappe the model experiments suggest that the formation of imbricates and fault related folds are controlled by the discontinuity of detachment horizons discontinuities are locations where thrust ramps are triggered and layer heterogeneities control the locations for the initiation of detachment folds basement horsts may lead to the formation of recumbent folds which develop a melange zone on the highly sheared inverted limb the experiments also suggest that the thickness of the basal detachment horizon of the medianes plastiques decreases gradually from the foreland to the hinterland

a refraction seismic transect from the faroe islands to the hatton rockall basin 1 the crustal structure of the faroe rockall plateau was studied by a 790 km long refraction seismic transect consisting of two intersecting lines the air gun shot spacing was 200 m and the signals were recorded by 77 ocean bottom seismometers a p wave velocity model was developed from forward and inverse modeling of the wide angle seismic data and incorporation of coincident multichannel reflection seismic data continental crust with velocities ranging from 5 6 to 6 8 km s can be traced from the faroe islands across the banks to the sw of the faroes and into the hatton rockall basin the thickness of the subvolcanic crust is up to 25 km on the banks but is as little as 8 km in the channels between the banks the thinning in the channels may be related to nw trending shear zones extending from major lineaments in ne rockall trough basalt layers are found along the entire transect with a total thickness of up to 4 km two layers with velocities of 4 9 5 2 and 5 3 5 6 km s are thought to represent paleogene flood basalts that can be correlated from the faroe islands to george bligh bank close to george bligh bank an 80 km wide and up to 9 km thick body with velocities of 6 5 km s is interpreted as intrusion a 5 km thick high velocity lower crustal layer 7 3 km s extends from the area of the intrusion into the northern hatton rockall basin at the northern flank of lousy bank the transition zone to oceanic crust was encountered

viscous plastic finite element models of fault bend folds abstract in finite element models of fault bend folds viscous and plastic material properties are used to simulate pressure solution creep and cataclasis respectively plastic deformation occurs primarily above ramp hinges however a band of high plastic strain is created in the lower hanging wall as material moves over the ramp hinges viscous deformation dominates above the ramp and flats where plastic deformation is negligible hanging wall material undergoes layer parallel shortening on the lower flat and lower part of the ramp layerparallel extension on the upper part of the ramp and upper flat and layer parallel shortening farther along the upper flat the stress invariant j 2 is highest above the ramp hinges and below the ramp whereas stress invariant j 1 is highest above and below the ramp the high j 1 below the ramp suppresses plastic deformation so that footwall deformation is predominantly viscous in multilayer models strain is concentrated in the weak layers especially on the limbs of the hanging wall anticline in models with bedding parallel slip surfaces in the hanging wall the strain on fold limbs is accommodated by interlayer slip and layers between slip surfaces develop individual neutral surfaces the models indicate that the relative and absolute amounts of deformation in fault bend folds resulting from pressure solution creep and cataclasis vary as a function of structural position and history

exploration in the shetland faeroe basin using densely spaced arrays of ocean bottom seismometers recent exploration activity in the peripheral regions of the shetland faeroe basin offshore northwest scotland has led to the discovery of some of the largest oil reserves on the united kingdom uk continental shelf we present results from two ocean bottom seismometer profiles acquired by mobil north sea ltd across the center of the shetland faeroe basin these data provide a powerful tool for delineating long wavelength velocity variations and thus have potential for reducing the nonuniqueness associated with conventional seismic exploration methods analysis of the first arrival traveltime data using both forward and inverse ray based techniques produces a well constrained velocity depth model of the basin fill we estimate that the uncertainty in the velocity structure is 5 from a series of trial and error perturbations applied to the final models the velocity structure of the faeroe basin has three principal layers 1 a near surface layer with velocities in the range 1 6 to 2 2 km s 2 a 3

did expanded porous patches guide rupture propagation in 2003 tokachi oki earthquake 1 a time lapse seismic reflection survey before and after the 2003 tokachi oki interplate earthquake mw8 0 detected spatio temporal evolution in reflectivity near the source area from the evolution pattern and the aftershock observations we attribute the change in reflectivity to gradual fluid flow and estimate the subsequent permeability in the plate boundary shear zone to be 10 15 m2 the magnitude of the reflectivity change implies a 1 6 increase in porosity and a 2 2 5 6 mpa reduction in shear strength as a hypothesis to test we propose that expanded porous patches guided the seismic rupture propagation in the tokachi oki earthquake

kinematics and strain distribution of a thrust related fold system in the lewis thrust plate northwestern montana u s a abstract in order to understand interactions between motion along thrusts and the associated style of deformation and strain distribution in their hangingwalls geologic mapping and strain measurements were conducted in an excellently exposed thrust related fold system in the lewis thrust plate northwestern montana this system consists of 1 an e directed basal thrust the gunsight thrust that has a flat ramp geometry and a slip of about 3 6 km 2 an e verging asymmetric anticline with its nearly vertical forelimb truncated by the basal thrust from below 3 a 4 km wide fold belt the frontal fold complex that lies directly in front of the e verging anticline 4 a w directed bedding parallel fault the mount thompson fault that bounds the top of the frontal fold belt and separates it from the undeformed to broadly folded strata above and 5 regionally developed w dipping spaced cleavage although the overall geometry of the thrust related fold system differs from any previously documented fault related folds the e verging anticline itself resembles geometrically a rich type fault bend fold the observed initial cut off and fold interlimb angles of this anticline however cannot be explained by cross section balancing models for the development of either a fault bend fold or a fault propagation fold possible origins for the e verging anticline include 1 the fold initiated as an open fault bend fold and tightened only later during its emplacement along the basal thrust and 2 the fold started as either a fault bend or a fault propagation fold but simultaneous or subsequent volume change incompatible with any balanced cross section models altered its shape strain in the thrust related fold system was determined by the preferred orientation of mica and chlorite grains the direction and magnitude of the post compaction strain varies from place to place strains in the foreclimb of the hangingwall anticline imply bedding parallel thinning at some localities and thickening at others this inhomogeneity may be caused by the development of thrusts and folds strain in the backlimb of the hangingwall anticline implies bedding parallel stretching in the thrust transport direction this could be the effect of bending as the e verging anticline was tightened and transported across the basal thrust ramp strain measured next to the gunsight thrust again indicates locally varying shortening and extension in the transport direction perhaps in response to inhomogeneous friction on the fault or else to a history of alternating strain hardening and softening in the basal thrust zone

coal seismic surveying over near surface basalts experience from central queensland australia abstractseismic reflection surveying in basalt covered areas often fails to image underlying reflectors to gain insights into the nature of the problem and obtain potential solutions we have conducted experimental 2d seismic reflection and offset vsp surveys at two coal mines in the bowen basin of australia at the first mine the basalt is relatively deep 114 m and relatively thin 20 m conventional seismic acquisition and processing of a 2d seismic line provide poor results however upgoing reflections from layers below the basalt are clearly evident in the vsp survey and prestack depth migration is able to improve the continuity of the reflectors beneath the basalt at the second mine the 360 m wide basalt is at a depth of 40 m and has a thickness of about 40 m it is fresh and unweathered and consists of multiple flows which are interlayered with unconsolidated sediments long offset data acquisition combined with prestack depth migration was expected to produce satisfactory results but this i

seismological studies at parkfield i simultaneous inversion for velocity structure and hypocenters using cubic b splines parameterization a model for the 3d p and s wave velocity structure at parkfield has been developed by the systematic inversion of arrival times for microearthquakes and surface vibroseis sources recorded by nearby calnet stations and by the local borehole installed 10 station high resolution seismic network there a modification of the linear b splines model parameterization method of thurber 1983 to a cubic b splines formulation through tests inverting synthetic data sets for the parkfield configuration is found to produce smooth velocity distributions that still retain some small scale features application of the method to 953 p wave and 288 s wave arrivals from 111 microearthquakes and 8 vibroseis locations throughout the study area yields a velocity structure having variable resolution a low velocity zone appears to coincide with the creeping segment of the san andreas fault zone northwest of the 1966 parkfield earthquake epicenter an increase of about 15 per cent in seismic velocities is seen from the northeast to southwest across this segment of the fault zone the higher velocities to the southwest apparently correspond to the granitic basement the contrast across the fault is subdued to the southeast along the 1966 rupture zone a high vp vs ratio of 1 9 is seen in a 2 km wide volume near the location of the 1966 main shock hypocenter presumably the nucleation zone for the expected m 6 parkfield earthquake