drained and undrained anisotropic elastic stiffness parameters equations that map drained to undrained elastic stiffness parameters in a cross anisotropic material are presented graphs of various anisotropic stiffness ratios are used to illustrate the important trends a new 2 d graph showing the elastic bound enables measured elastic parameters to be plotted which all lie well within it the 2 d graph enables uncoupled and incompressible materials to be shown plus materials with particular initial effective stress path slopes in an undrained triaxial test bounds more restrictive than the elastic one are also discussed and the likely ranges of various anisotropic stiffness ratios are presented in general undrained anisotropy is much less than drained that is closer to isotropic in undrained plane strain conditions vertical and horizontal moduli are found to be the same in drained plane strain conditions the ratios of the moduli are almost the same as the young s moduli

chronology of cenozoic tectonic events in western venezuela and the leeward antilles based on integration of offshore seismic reflection data and on land geology newly acquired deep penetration broadband onshore offshore lithospheric investigation of venezuela and the antilles arc region seismic reflection data from offshore western venezuela bonaire basin and around the leeward antilles are combined with existing geologic and geophysical data sets to examine the chronology of late cretaceous cenozoic tectonic events in this part of the caribbean south american plate boundary zone these tectonic events have controlled the maturation and structural trapping of known hydrocarbons in the offshore bonaire basin and the adjacent onland falcn basin we infer three tectonic phases that are constrained using these combined data sets 1 the late eocene early oligocene north south opening of the 3 6 km 1 8 3 7 mi thick falcn bonaire basin occurred along east west striking normal fault systems that have locally been inverted by later tectonic phases these paleogene normal faults rifted the upper cretaceous arc crust and paleogene marine depositional sequences within the offshore bonaire basin 2 northwest striking normal faults crosscut the older normal faults of the bonaire basin and leeward antilles and form deep submarine rifts that contain up to 4 km 2 5 mi of sedimentary fill and form deep water channels between islands of the leeward antilles offshore well data and age of onshore sediments in the falcn basin indicate that this second phase of rifting occurred mainly during the late oligocene to early miocene and remains active to the present 3 inversion of the subaerial falcn basin commenced during the middle miocene this inversion phase is reflected in the present day pattern of an east northeast trending fold thrust belt that can be traced over 200 km 124 mi along strike in the falcn basin a second offshore fold thrust belt la vela can be traced over a distance of 175 km 108 mi along strike and parallel to the northeast trending falcn basin coast restoration of imbricate thrusts seen on seismic lines perpendicular to the la vela fold thrust belt indicates a minimum of 7 km 4 3 mi of northeast southwest directed thin skinned shortening geochemical work indicates that source rocks for scattered occurrences of hydrocarbons in the falcn basin and its coastal zone are paleogene and miocene marine shale reservoir rocks are tertiary marine sandstone and shale deposited in paleogene rifts formed during the first tectonic phase in the late eocene to early oligocene structural traps were formed by thrusting during the second tectonic phase in the late oligocene to early miocene

seismic evidence for blind thrusting of the northwestern flank of the venezuelan andes surface geology and seismic and well data from the northwestern flank of the venezuelan andes indicate overthrusting of andean basement rocks toward the adjacent maracaibo basin along a blind thrust fault the frontal monocline is interpreted as the forelimb of a northwestward verging fault related fold deformed over a crustal scale ramp the andean block has been thrust 20 km to the northwest and uplifted 10 km on a ramp that dips about 20o 30 o southeastward the thrust fault ramps up through crystalline basement rocks to a decollement horizon within the shaly units of the cretaceous colon mito juan formations backthrusts in the monocline produce a wedge geometry and reduce the amount of blind slip required on the decollement northwest of the andes the rigid andean uplift was caused by northwest southeast compressive tectonic forces related to the convergence of the caribbean plate the panama volcanic arc and northwestern south america the thick up to 6 km molasse deposits accumulated in the foredeep basin indicate that the venezuelan andes started to rise as early as the early miocene however a late miocene intramolasse unconformity marks the beginning of the formation of the monocline and the greatest uplift the crustal scale fault related fold model may explain structural features seen in other areas of basement involved foreland deformation the regional geologic setting of the area can be described as a mountain front adjacent to a foredeep basin the mountain front represents a precambrian paleozoic igneous metamorphic complex partially covered with sediments of mesozoic and cenozoic ages the foredeep is an asymmetric structural basin that contains up to 12 km of sedimentary section the early miocene to pleistocene molasse deposits up to 6 km in thickness dominate the sedimentary section the oldest sedimentary rocks that crop out in the mountain front are jurassic red beds which are unconformably overlain by a cretaceous sequence up to 2 km in thickness from early cretaceous to middle eocene sediments were deposited in an extensive passive continental margin setting figure 3 coarse clastic sediments of the rio negro formation bioclastic carbonates of the apon formation and siliciclastic sediments of the aguardiente formation were deposited first in the initial stage of the cretaceous transgressive event limestones calcareous shales and black shales of the capacho and la luna formations represent anoxic facies deposited during the maximum marine transgressiolof late cretaceous cenomanian campanian age regression began in the latest cretaceous maestrichtian with marine deposition of the shaly colon formation and the sandier mito juan formation this

undrained stiffness anisotropy from hollow cylinder experiments on four eocene to jurassic uk stiff clays the paper describes the anisotropic undrained stiffness behaviour of four medium plasticity heavily overconsolidated uk stiff marine clays as revealed through hollow cylinder testing the experimen

nature of the lithosphere across the variscan orogen of sw iberia dense wide angle seismic reflection data 1 two wide angle seismic transects have been acquired across the sw iberian massif they crossed three major geological zones south portuguese zone ossa morena zone and central iberian zone with their tectonic contacts and the pyrite belt being of greatest interest a total of 690 digital seismic recording instruments 650 texans and 40 reftek 3 component units from the iris passcal instrument pool were used the transects a and b are each approximately 300 km long and consist of 3 and 6 shot points respectively with an approximately 60 km shot point interval the charge sizes range from 1000 kg at the edges to 500 kg at the center these recently acquired experiments were designed to provide velocity constraints on the lithosphere and to complement the previously acquired normal incidence seismic profile iberseis both data sets are part of the sw iberia project which was developed within the europrobe program and designed to address fundamental questions about the nature and dynamics of the variscan lithosphere the acquisition parameters provide closely spaced wide angle seismic images of the lithosphere beneath sw iberia in transect a the station spacing was on average 400 m while along transect b the receiver spacing was approximately 150 m because of this close trace spacing the lateral continuity of the seismic arrivals is greatly improved frequency analysis revealed that the recorded events feature relatively low frequencies 6 25 hz after processing the shot records show high amplitude and well defined arrivals the interpreted pmp arrival located at approximately 11 s normal incidence traveltime is characterized by high amplitude and relatively low frequency 6 12 hz a well defined pn arrival appears at offsets beyond 120 km at far offsets greater than 180 km an upper mantle reflection is observed furthermore within the upper crust the shots records feature a relatively high velocity arrival located at 4 5 s normal incidence traveltime the analysis of this arrival indicates that it probably corresponds to the top of the iberian reflective body identified in the iberseis deep seismic profile the velocity models obtained by forward modeling show a complex crust especially in the middle crust the velocity models are the most detailed ones that have been produced in the area and contain a large amount of new features that are relevant to the understanding of the composition of the crust and upper mantle beneath the zone the velocity depth functions derived from the velocity models have higher middle crustal velocities than the average in other continental areas a comparison between laboratory seismic velocity measurements and the velocities of the models was carried out in order to estimate the crustal and the upper mantle composition results indicate that the high middle crust velocities correspond to rocks of a mafic composition the combined data set reveals new aspects related to the lithospheric evolution of this transpressive orogen and allows us to attempt an interpretative cross section of the upper lithosphere in sw iberia

permeability of aged grout around tunnels in london prediction of the long term settlement of clay soils over tunnels requires a knowledge of the permeability of the soil and of the tunnel lining however determination of the lining permeability in the field is difficult an important contributor to this problem is the lack of knowledge concerning the permeability of the grout between the lining and the soil this paper presents the results of tests to characterise the properties of grout samples from london underground tunnels investigating permeability porosity microstructure and composition the tests revealed that the newer grout was impermeable relative to the surrounding clay however the older samples showed much greater permeabilities and an altered grout composition suggesting that degradation had taken place exposure to groundwater appeared to have caused carbonation and sulfate reaction the combination of chemical reaction and leaching of cementitious and degradation products appears to have made these grouts more permeable so that the

tectonic controls of the right lateral burro negro tear fault on paleogene structure and stratigraphy northeastern maracaibo basin the northeastern maracaibo basin in western venezuela was deformed by paleogene thrusting and an associated tear fault burro negro right lateral strike slip fault zone related to paleogene oblique collision between the caribbean and south american plates two different tectonic models have been previously proposed for the thick paleogene depocenter located along the northeastern margin of the maracaibo basin the first model proposes that the depocenter is a foreland basin controlled by southwestward directed overthrusting during a late paleocene middle eocene oblique collision between the caribbean and south american plates the second model supported by data presented in this study proposes that the asymmetric paleogene maracaibo sedimentary wedge was controlled by strike slip displacement along a right lateral tear fault separating southeast directed thrust sheets to the east lara nappes from a more stable platform area to the west maracaibo basin regional seismic lines recorded to 5 s two way traveltime reveal the structure of the asymmetric paleogene depocenter in the northeastern part of the maracaibo basin the approximately 100 km 62 mi long burro negro fault is a right lateral strike slip fault separating less deformed inner to outer shelf rocks of the western maracaibo basin from highly deformed deep marine rocks of the eastern maracaibo basin seismic lines northeast of the burro negro fault zone show elongate subsurface basins bounded by partially inverted reverse and strike slip faults filled with about 3 km 1 8 mi of oligocene and miocene clastic marine sedimentary rocks structural highs of eocene rocks are characterized at depth on seismic reflection lines by chaotic seismic reflections that underlie the more coherently stratified oligocene and miocene subbasins we interpret these structural highs as steeply dipping fault zones and shale diapirs activated during eocene oligocene oblique plate convergence the geology and overall structural configuration of the northeastern maracaibo basin and the burro negro fault zone support its origin as a right lateral tear fault in our model the burro negro fault zone accommodated southeastward migration of the thrust front in the deep water area east of the fault in the present day falcn region the paleogene clastic wedge of the maracaibo basin exhibits many common features of a classic foreland basin including onlap onto an arch or forebulge located near the center of the present day lake maracaibo this paleogeographic coincidence of the burro negro fault zone and the maracaibo shelf edge suggests that the paleotrend of the south american passive margin prior to collision was serrated in map view west to east migration of the caribbean south america oblique collision formed progressively younger parallel tear faults to the east of the maracaibo basin that may have formed by the same tectonic process as the burro negro fault zone

gps estimate of relative motion between the caribbean and south american plates and geologic implications for trinidad and venezuela global positioning system gps data from eight sites on the caribbean plate and five sites on the south american plate were inverted to derive an angular velocity vector describing present day relative plate motion both the caribbean and south american velocity data fit rigid plate models to within 1 2 mm yr the gps velocity uncertainty the caribbean plate moves approximately due east relative to south america at a rate of 20 mm yr along most of the plate boundary significantly faster than the nuvel 1a model prediction but with similar azimuth pure wrenching is concentrated along the approximately east striking seismic el pilar fault in venezuela in contrast transpression occurs along the 068 trending central range warm springs fault in trinidad which is aseismic possibly locked and oblique to local plate motion

current block motions and strain accumulation on active faults in the caribbean the caribbean plate and its boundaries with north and south america marked by subduction and large intra arc strike slip faults are a natural laboratory for the study of strain partitioning and interseismic plate coupling in relation to large earthquakes here we use most of the available campaign and continuous gps measurements in the caribbean to derive a regional velocity field expressed in a consistent reference frame we use this velocity field as input to a kinematic model where surface velocities results from the rotation of rigid blocks bounded by locked faults accumulating interseismic strain while allowing for partial locking along the lesser antilles puerto rico and hispaniola subduction we test various block geometries guided by previous regional kinematic models and geological information on active faults our findings refine a number of previously established results in particular slip rates on the strike slip faults systems bounding the caribbean plate to the north and south and the kinematics of the gonave microplate our much improved gps velocity field in the lesser antilles compared to previous studies does not require the existence of a distinct northern lesser antilles block and excludes more than 3 mm yr of strain accumulation on the lesser antilles puerto rico subduction plate interface which appears essentially uncoupled the transition from a coupled to an uncoupled subduction coincides with a transition in the long term geological behavior of the caribbean plate margin from compressional hispaniola to extensional puerto rico and lesser antilles a characteristics shared with several other subduction systems

shear strength anisotropy of natural london clay the paper reports the shear strength anisotropy of the natural highly overconsolidated london clay from heathrow terminal 5 as established by comprehensive hollow cylinder apparatus hca testing

tectonic inheritance crustal architecture and contrasting structural styles in the venezuela andes surface and subsurface data image contrasting structural styles for the two flanks of the venezuela andes in the north a flexural basin developed in neogene times between the andes and the lake maracaibo north verging thrusts are mainly detached in the pre cretaceous substratum and form a deeply buried antiformal stack secondary decollement levels occur both in the upper cretaceous and tertiary strata accounting for the passive roof thrust of a conventional frontal triangle zone in the south the barinas basin hardly compares with a flexural basin it is largely dominated by both north and south verging basement involving structures paleogene normal faults are locally inverted and caribbean nappes are frequently reactivated or refolded by younger oblique neogene andean structures seismic profiles in this area also attest to the strong neogene structural inversion of upper jurassic lower cretaceous grabens hercynian or more likely early paleozoic structures account for local preexisting crustal heterogeneities reactivated during both the tethyan rifting and the andean deformations two trans andean regional profiles have been balanced and constrained by an inversion of the gravimetric data they imply a progressive deepening of the northern moho and a south dipping subduction of the infracontinental lithospheric mantle of the maracaibo block the shortening for both sections averages 60 km palinspastic restorations assume a relative cylindricity for the deep crustal architecture of the andes and minimize the possible effects of a progressive right lateral escape of the maracaibo block with respect to stable south america along the bocono fault strain partitioning during neogene oblique convergence induced surficial thrust fronts parallel to the plate boundary strike slip motion in the allochthon along the bocono fault and an asymmetric subduction wedging at depth

velocity field across the southern caribbean plate boundary and estimates of caribbean south american plate motion using gps geodesy 1994 2000 global positioning system gps observations between 1994 and 2000 at twenty two sites in the lesser antilles and northern south america indicate that the caribbean plate along its southern boundary slips at a rate of 20 5 2 mm a with an azimuth of n 84 2 e at 65 w relative to the south american plate east of 68 w 80 of the dextral slip is contained within a 80 km wide shear zone centered on the el pilar san sebastian fault system west of 68 w the plate boundary broadens to more than 300 km with dextral shear shared between the northeast trending bocono fault 9 11 mm a in western venezuelan and an offshore system near the northern coast

crustal thickness variations in venezuela from deep seismic observations abstract the caribbean south america plate boundary zone is a complex zone of plate interactions forming thrust belts and foreland basins in northern venezuela within the framework of the bolivar and geodinos projects the geodynamics of plate interactions is being investigated using interdisciplinary geological and geophysical methods here we focus on the results of the land based active seismic observations done in 2004 along four deep seismic wide angle profiles acquired perpendicular to the caribbean south america plate boundary in northern venezuela between longitudes 63 w and 70 w and ranging from about latitudes 12 n to about 9 n the mostly unreversed profiles provide information on the crustal structure from the oceanic transitional crust on the southern border of the caribbean plate to the continental crust of the caribbean mountain system and their associated foreland basins which are bordered to the south by the guayana shield which corresponds to stable south america plate the derived crustal thickness oscillates around 35 km along the coastline corresponding to the caribbean mountain system and decreases only slightly towards the leeward antilles to the south in the area of the eastern venezuela basin crustal thickness reaches 40 km increasing towards the guayana shield to 45 km nevertheless there are two regions of anomalous crustal thickness proven by arrivals from the lower crust and the moho discontinuity in the eastern part of the eastern venezuela basin crustal thickness reaches up to 50 km with high velocity anomalies within the lower crust which are interpreted as reworked lower crustal and upper mantle material associated to the plate interactions of the south american and the caribbean plates the second anomalous zone is a remarkable crustal thinning from 35 km to 27 km in the falcon basin in western venezuela which extends eastwards into the bonaire basin as documented by pmp reflections derived from land shots and observations of the air gun blasts on the stations of the venezuelan seismological network

characteristics of the london clay from the terminal 5 site at heathrow airport the innovative engineering approach adopted for the new terminal 5 at heathrow airport called for an advanced investigation of the london clay strata including detailed in situ profiling and stress path laboratory testing on high quality rotary cored samples although the scope of the investigations exceeded that normally specified for conventional design questions relating to the structure and anisotropy of stiffness and strength of the clay remained that could not be answered further research was required and the deep excavations at the site provided the opportunity for a team from imperial college to take multiple block samples from three depths supplemented by two additional dedicated rotary cored boreholes intensive research was performed at imperial college on these samples as described in three companion papers by gasparre et al and nishimura et al this overview paper integrates the findings from the recent research with those from the commercial investigation and with earlier studies to e

tectonic implications of tomographic images of subducted lithosphere beneath northwestern south america we used seismic tomography to investigate the complex structure of the upper mantle below northwestern south america images of slab structure not delineated by previous seismicity studies help us to refine existing tectonic models of subducted caribbean pacific lithosphere beneath the study area beneath western venezuela and colombia we distinguish two slabs a maracaibo and a redefined bucaramanga slab the maracaibo slab coinciding with most of the bucaramanga slab previously defined by w d pennington dips in a direction of 150 at an angle of 17 to a depth of 275 km and correlates to the subducted late cretaceous oceanic plateau of the caribbean plate farther south a second slab dips at an angle of 50 in a direction of 125 to a depth of at least 500 km and correlates to the subducted oceanic crust of the nazca plate and the downdip extension of the panama island arc we refer to this slab as the redefined bucaramanga slab because it is different from the bucaramanga slab segment defined by pennington the area of the south american plate overriding both slabs is characterized by the absence of an active volcanic arc an anomalously wide topographically uplifted and tectonically active area and the northward escape of the maracaibo block along active strike slip faults in support of earlier studies we attribute this to the underthrusting of the caribbean oceanic plateau our shallowly dipping maracaibo slab along the base of the south american lithosphere and to the recent collision of the panama island arc rafted in on more steeply dipping crust of the nazca plate our redefined bucaramanga slab

tectonic implications of cretaceous pliocene fission track ages from rocks of the circum maracaiho basin region of western venezuela and eastern colombia

flexural and gravity modelling of the merida andes and barinas apure basin western venezuela abstract the kinematic evolution of the barinas apure basin and the southern merida andes from lower miocene to the present is numerically modelled using flexural isostatic theory and geophysical and geological data two published regional transects are used to build up a reference section which is then used to constrain important parameters e g shortenings and sedimentary thicknesses for the flexural modelling to control the location of the main fault system in the flexural model earthquake information is also used the estimated flexural elastic thickness of the south american lithosphere beneath the barinas apure basin and the merida andes range is 25 km the value for the final total shortening is 60 km the flexural isostatic model shows that the andean uplift has caused the south american lithosphere subsidence and the development of the barinas apure basin in addition gravity modelling was used to understand deep crustal features that could not be predicted by flexural theory consequently the best fit flexural model is used to build a gravity model across the merida andes and the barinas apure basin preserving the best controlled structural features from the flexural modelling e g basin wavelength and depth and slightly changing the main bodies density values and deep crustal structures the final gravity model is intended to be representative of the major features affecting the gravity field in the study area the predicted morphology in the lower crustal level of the final gravity model favours the hypothesis of a present delamination or megathrust of the maracaibo crust over the south american shield this process would use the conrad discontinuity as a main detachment surface within an incipient nw dipping continental subduction

aseismic continuation of the lesser antilles slab beneath continental south america 1 we present results of travel time inversions of teleseismic p and s waves recorded at the secasa92 southeast caribbean south america 1992 temporary broadband array in northeastern venezuela and trinidad the inversions reveal the unusual structure of the southern termination of the lesser antilles subduction zone a minimum 2 relatively high velocity anomaly trends wsw from the seismically defined lesser antilles slab beneath and nw of the paria peninsula to a point below the venezuelan serrania del interior well south of the caribbean coast resolution tests utilizing actual ray geometries and densities of the source data indicate that the regional scale structure beneath the study area is reasonably well resolved thus a detached and detaching subducted south american slab appears to lie beneath continental south america we infer that oceanic south american lithosphere has been overridden to a significant degree by continental south america the detached slab now lying beneath continental south america was driven into its current position after detaching from the former eastward striking mesozoic ocean continent passive margin as this margin entered the subduction zone because oceanic and continental south america are still attached without apparent relative motion between them along the atlantic passive margin southeast of our study region the slab must be the actively moving element during continental overriding thus the slab and its surrounding mantle both caribbean and south america beneath northeastern south america are mobile and have moved ese relative to the stable guyana shield craton

the post yield behaviour of four eocene to jurassic uk stiff clays a detailed study is described of the post yield behaviour of four medium plasticity heavily overconsolidated uk stiff clays sub layers of the stiff to hard gault kimmeridge and oxford clays were identified sampled and tested these along with facies investigated in an earlier london clay study had broadly similar depositional histories the intention in considering a spread of similar sediments from the jurassic to the eocene was to allow any strong effects of geological age or burial depth to be identified a strongly fissured meso structure was present in three of the four clays which had a controlling influence on their effective shear strengths considering that the representative element volume is of paramount importance in measuring the strengths of such soils all four soils were brittle in shear and when sheared to sufficient displacements developed low residual shear strengths the stiff clays were investigated further through comparisons between natural and reconstituted behaviour usi

measurements of the permeability of london clay using a self boring permeameter the coefficient of permeability of the london clay at bradwell essex has been measured using various methods including a self boring permeameter the results are compared with in situ permeability measurements made using conventional borehole piezometer techniques and with laboratory permeability determinations the self boring permeameter can be successfully used in stiff clays and the self boring technique has potential advantages over the conventional in situ method that uses borehole piezometers laboratory samples show that the ratio of horizontal to vertical permeability of the london clay at the site is approximately 2 the in situ horizontal permeability measured with the self boring permeameter is about four times that measured on 51 mm dia laboratory samples le coefficient de permeabilite de l argile de londres a bradwell essex fut mesure par plusieurs methodes y compris l emploi d un permeametre autoforeur l article compare les resultats avec des mesures de permeabilite effectuees a

plate motions in the north andean region repeated geodetic measurements with the global positioning system gps provide direct measurements of displacements due to plate motions and active crustal deformation in central america and northern south america an area of complex interaction of the nazca cocos caribbean and south american plates the displacement rates for the period 1988 1991 obtained from the results of the first three central and south america casa gps campaigns are in general agreement with the predictions of the nuvel 1 plate motion model but there are differences in detail between the observations and the model the nazca north andes convergence rate vector measured by gps is different from the nuvel 1 vector at 95 confidence the difference implies that the north andes are moving northward relative to south america the measured convergence between the caribbean plate and the north andes suggests that the southern margin of the caribbean plate is located in the south caribbean deformed belt the april 1991 costa rica earthquake and the cocos caribbean convergence rate determined by gps suggest the possibility of significant ongoing deformation between central america and the stable interior of the caribbean plate our gps results are consistent with deformation of the overriding plates at the convergent margins of central and south america and confirm that active convergence is occurring around much of the southern margin of the caribbean plate from colombia west to costa rica costa rica and panama are not part of the stable caribbean plate instead the south caribbean deformed belt and the north panama fold belt probably represent the southern margin of the caribbean plate

subduction of the caribbean plate and basement uplifts in the overriding south american plate the new tectonic interpretations presented in this paper are based on geologic field mapping and gravity data supplemented by well logs seismic profiles and radiometric and earthquake data the present caribbean south american plate boundary is the south caribbean marginal fault where subduction is indicated by folding and thrusting in the deformed belt and a seismic zone that dips 30 to the southeast and terminates 200 km below the maracaibo basin the caribbean south american convergence rate is estimated as 1 9 0 3 cm yr on the basis of the 390 km length of the seismic zone and a thermal equilibration time of 10 m y the caribbean south american convergence has produced a northwest southeast maximum principal stress direction 1 in the overriding south american plate the mean 1 direction for the maracaibo santa marta block is 310 10 based on earthquake focal mechanism determinations and structural and gravity data on the overriding south american plate basement blocks have been uplifted 7 12 km in the last 10 m y to form the venezuelan andes sierra de perija and the colombian santa marta massif crystalline basement of the venezuelan andes has been thrust to the northwest over tertiary sediments on a fault dipping about 25 and extending to the mantle in the sierra de perija mesozoic sediments have been thrust 16 26 km to the northwest over tertiary sandstones along the cerrejon fault a thrust fault dipping 15 10 to the southeast is consistent with field mapping and gravity and density data the santa marta massif has been uplifted 12 km in the last 10 m y by northwest thrusting over sediments the basement block overthrusts of the perijas venezuelan andes and the santa marta massif are pliocene pleistocene analogs for laramide orogenic structures in the middle and southern rocky mountains of the united states the nonmagmatic basement block uplifts along low angle thrust faults reveal horizontal compression in the overriding plate over 500 km from the convergent margin present day east northeast west southwest 080 compression is indicated by earthquake focal mechanisms and strike slip motion on the bocono fault these earthquakes are intraplate deformation associated with east west 080 nazca south american convergence

electrical conductivity in the earth s mantle inferred from champ satellite measurements i data processing and 1 d inversion summary a novel time domain approach to the global electromagnetic induction problem is applied to vector magnetometer data observed by the champ satellite data recorded during 11 storm events in 2001 2003 are processed track by track yielding time series of spherical harmonic coefficients the data are then interpreted in terms of 1 d layered electrical conductivity models the inversion is performed by full search of model parametric space which yields sensitivity of misfit with respect to conductivities of layers and positions of interfaces in the upper 50 km the inversion solidly recovers a conductive layer corresponding to averaged surface conductance the conductivity of the lower mantle is established at 6 s m 1 assuming the upper lower mantle interface is fixed at the seismic based 670 km boundary however the satellite data favour the models with a large jump around 1000 km to unrealistic conductivity values exceeding 10 3 sm 1 the resolution of the method in the resistive upper mantle sandwiched between conductive crust and lower mantle is poor nevertheless an upper bound of 0 01 s m 1 is suggested by the data a conductivity increase in the transition zone is not

the influence of structure on the behaviour of london clay an intensive investigation is described into the london clay units from heathrow terminal 5 intrinsic properties and composition were established and relating the behaviour of intact and reconsti

the undrained shear strength anisotropy of four jurassic to eocene stiff clays the shear strength of heavily overconsolidated stiff to hard plastic clays is crucial to their stability and also influential on the ground movements they develop in many geotechnical engineering applications this paper considers the shear strength anisotropy of the london gault kimmeridge and oxford clays through advanced hollow cylinder experiments on multiple high quality samples taken at similar depths from inland sites where the geotechnical profiles have been established by comprehensive laboratory and in situ testing suites of undrained tests are reported which loaded specimens from their in situ stress states to reach ultimate failure at pre defined final major principal stress axis orientations defined in the vertical plane while also controlling or monitoring the intermediate principal stress ratio b both stress path and simple shear tests were undertaken with the hollow cylinder apparatus which offers key advantages over conventional simple shear equipment the interpretation reveals

analysis of tunnel excavation in london clay incorporating soil structure recent studies on london clay have identified a number of different units in the geological profile and have highlighted the role of soil structure in mechanical behaviour in fact structure is the dominant factor determining the differences in the mechanical response of different units in the paper numerical analyses simulating the undrained excavation of a tunnel in st james s park are presented london clay behaviour is characterised by a kinematic hardening structured soil model incorporating structure and stiffness degradation the parameters and initial conditions are based on a careful calibration that takes into account the presence of different units within the london clay formation and the different degrees of soil structure the analyses performed result in a very satisfactory reproduction of the magnitude and patterns of short term surface and subsurface displacements as well as pore pressures the paper concludes with a discussion of the results in the context of other analyses performed

tearing and breaking off of subducted slabs as the result of collision of the panama arc indenter with northwestern south america we present two regional lithospheric cross sections that illustrate eastward and southeastward dipping subducted slabs to depths of 315 km beneath the surface of colombia in northwestern south america these cross sectional interpretations are based on relocated earthquake hypocentral solutions models supported on gravity and magnetic regional data and coda q q c tomography the method of tomographic imaging based on spatial inversion of the coda wave has advantages of providing information on the lateral variations of the anelastic properties and thermal structure of the lithospheric system mapping of earthquake defined benioff zones combined with tomographic imaging reveals the presence of an 240 km long east west striking slab tear named here the caldas tear the proposed caldas tear separates a zone of shallow 20 30 dipping southeastward subduction in the area of colombia adjacent to panama and the caribbean sea which is not associated with subduction related volcanism from an area of steeper 30 40 dipping slab adjacent to the eastern pacific ocean that is associated with an active north south chain of active arc volcanoes we propose that the caldas slab tear separating these two distinct subducted slabs originally formed as the southern boundary of the panama indenter an extinct island arc that began subducting beneath northwestern south america about 12 ma the area south of the panama indenter is miocene oceanic crust of the nazca plate which subducts eastward beneath northwestern south america at normal angles and melts to form a north south trending active volcanic arc in addition to the formation of the caldas tear we propose that impedance of the thicker crustal area of the panama arc indenter over the past 12 ma may have led to down dip break off of previously subducted oceanic crust that is marked by an extremely concentrated and active earthquake swarm of intermediate depth earthquakes beneath east central colombia

compaction driven evolution of porosity and permeability in natural mudstones an experimental study this paper describes a series of experiments designed to investigate the influence of lithology on the compactional loss of porosity and permeability in mudstones two intact samples of london clay with clay fractions of 40 and 67 were compacted to 33 mpa effective stress clay fraction permeability porosity pore size distribution and specific surface area were measured and their evolution was monitored throughout the compaction process electron microscopy was combined with mercury porosimetry to trace the collapse of the pore structure with increasing effective stress in both cases porosity loss occurred primarily by the collapse of large pores this process is more obvious in the coarser grained sample because throughout the compaction process it has a much broader range of pore radii and a much greater mean pore radius consistent with the pore size distributions the permeability of the coarser sample ranges from 10 10 m s 1 to 10 12 m s 1 while that of the finer grained sample ranges from 4 10 12 m s 1 to 5 10 14 m s 1 during progressive compaction from 2 to 33 mpa the compressibility of the finer grained sample is greater than that of the coarser grained sample 0 15 as opposed to 0 07 however in both cases the compressibility is much lower than that inferred for lithologically similar samples compacted over geological timescales the demonstration that both porosity and lithology clay fraction influence the permeability of mudstones should allow the development of more realistic porosity permeability relationships which take into account lithological variations exhibited by mudstones

segmentation of the lesser antilles subduction zone abstract precise hypocentral locations of earthquakes recorded on local seismic networks from 1978 to 1984 have established the configuration of the benioff zone beneath the lesser antilles island arc there are two distinct segments to the zone one to the north of martinique which trends nnw and one to the south of st lucia which trends nne the northern zone dips at 60 50 the southern zone has a dip of 50 45 in the north but is vertical in the south pleistocene volcanoes are aligned in distinct segments which closely mirror the kink in the benioff zone mid miocene swarms of dykes in martinique and st lucia have nw and ne azimuthal modes respectively we interpret this to mean that this kink in the subduction zone existed as long ago as the miocene the kink may represent the deformation of a single subducting plate or two separate north and south american plates subducting beneath the caribbean plate with a triple junction in the region of martinique st lucia

deep structure of the merida andes and sierra de perija mountain fronts maracaibo basin venezuela the maracaibo basin is a 50 000 km2 19 305 mi2 topographic depression bounded to the east and south by the mrida andes of venezuela and to the west by the sierra de perij of venezuela and colombia both uplifted mountain blocks expose paleozoic basement rocks and mesozoic cenozoic carbonate and clastic rocks that were mainly folded and thrusted by regional shortening in the paleogene and late neogene using geologic maps seismic reflection data and wells from the steep mountain front areas we test different structural models of how late paleogene neogene convergence is accommodated along subsurface faults and folds at the mountain fronts seismic imaging of the deep 5 km 3 1 mi structure of both maracaibo mountain fronts shows basinward dipping monoclines with stratal dips ranging from 20 to overturned and an almost complete lack of faulting in the basin edge monocline or in adjacent horizontally bedded strata of the maracaibo basin seismic data reveal the presence of one or more triangle zones at depth along both the mrida andes and sierra de perij that exhibit characteristic thrust wedge geometries the lower part of the wedge is defined by imbricate thrust faults dipping beneath the mountain block and involving the paleozoic basement the upper part of the wedge is defined by a basinward dipping thrust associated with fault propagation folds at the surface and an overlain basin edge monocline the creation of the steep to overturned dips of the monocline is attributed to the effects of 6 10 km 3 7 6 2 mi of shortening along the lower zone of imbricated thrust faults this 6 10 km 3 7 6 2 mi of shortening calculated from triangle zones of both the sierra de prija and mrida andes is significantly less than regional estimates from 12 to 60 km 7 4 to 37 mi of shortening inferred by previous workers from regional balanced cross sections that assume low angle thrust type geometries we propose that a pop up style of deformation related to the inversion of jurassic rift features may be a more realistic interpretation of the convergent fault systems that have controlled uplift of the mrida andes and sierra de perij inversion of relatively steep basement normal faults at the edges of and within both ranges may explain lesser amounts of observed shortening

determination of permeability of overconsolidated clay from pressuremeter pressure hold tests a method was developed to interpret the horizontal permeability kh from pressuremeter pressure hold tests phts of approximately 3 min duration the method relies on a regression analysis of the

tectonic shear zones in the london clay formation subhorizontal shear zones observed in the london clay formation are described the physical properties of the shear zones are examined and it is demonstrated both that they are of tectonic origin the result of bedding plane slip during minor folding and that they were probably formed before there had been any significant erosion of the london clay or later overlying formations the occurrence of the shear zones is reviewed and it is shown that similar features have been recorded in other areas to the west of london and it seems likely that they will be encountered elsewhere in the london basin wherever minor folding occurs the shape of the london clay formation reading formation interface appears to give an indication of the degree of folding and hence of the possible occurrence of shear zones the displacements of the shear zones examined are believed to be small perhaps only a few centimetres and consequently the strength along the shear zones is probably slightly above the true large displace

tectonics basin subsidence mechanisms and paleogeography of the caribbean south american plate boundary zone abstract using a mega regional dataset that includes over 20 000 km of on and offshore 2d seismic lines and 12 wells we illustrate three different stages of fault formation and basin evolution in the caribbean arc south american continent collisional zone transpressional deformation associated with oblique collision of the caribbean arc migrates diachronously over a distance of 1500 km from western venezuela in paleogene time 57 ma to a zone of active deformation in the eastern offshore trinidad area each diachronous stage of pre syn and post collisional basin formation is accompanied by distinct patterns of fault families we use subsidence histories from wells to link patterns of long term basinal subsidence to periods of activity of the fault families stage one of arc continent collision initial collision is characterized by overthrusting of the south and southeastward facing caribbean arc and forearc terranes onto the northward subducting mesozoic passive margin of northern south america northward flexure of the south american craton produces a foreland basin between the thrust front and the downward flexed continental crust that is initially filled by clastic sediments shed both from the colliding arc and cratonic areas to the south as the collision extends eastward towards trinidad this same process continues with progressively younger foreland basins formed to the east on the overthrusting caribbean arc and forearc terranes north south rifting adjacent to the collision zone initiates and is controlled by forward momentum of southward thrusting arc terranes combined with slab pull of the underlying and subducting north dipping south american slab uplift of fold thrust belts arc continent suture induces rerouting of large continental drainages parallel to the collisional zone and to the axis of the foreland basins stage two this late stage of arc continent collision is characterized by termination of deformation in one segment of the fold thrust belt as convergent deformation shifts eastward rebound of the collisional belt is produced as the north dipping subducted oceanic crust breaks off from the passive margin inducing inversion of preexisting normal faults as arc continent convergence reaches a maximum strain partitioning also begins to play an important role as oblique convergence continues accommodating deformation by the formation of parallel strike slip fault zones and backthrusting southward subduction of the caribbean plate beneath the south caribbean deformed belt as subsidence slows in the foreland basins sedimentation transitions from a marine underfilled basin to an overfilled continental basin offshore sedimentation is mostly marine sourced by the collided caribbean terranes localized islands and carbonate deposition stage three this final stage of arc continent collision is characterized by 1 complete slab breakoff of the northward dipping south american slab 2 east west extension of the caribbean arc as it elongates parallel to its strike forming oblique normal faults that produce deep rift and half grabens 3 continued strain partitioning strike slip faulting and folding the subsidence pattern in the caribbean basins is more complex than interpreted before showing a succession of extensional and inversion events the three tectonic stages closely control the structural styles and traps source rock distribution and stratigraphic traps for the abundant hydrocarbon resources of the on and offshore areas of venezuela and trinidad

a biostratigraphy for the london clay in london biostratigraphy enables specific layers of strata to be given a unique stratigraphic address that can be used to identify the horizons in which in situ tests are conducted and from which samples for testing are collected this is particularly valuable when working in fine grained deposits where stratigraphic position is not easily recognised by lithology alone and permits any data bank of geotechnical values and performance to be used without fear of confusing one stratigraphic layer with another construction in the london clay formation and its supporting research has necessitated an accurate and speedy method for identifying with a precision of centimetres the stratigraphic layer from which geotechnical data have been obtained the principles that enable biostratigraphy to be used for this purpose are explained and illustrated with reference to the london clay formation in london this approach can be used in most sedimentary strata an example of biostratigraphic data used in london is presented

in situ lateral stress coefficient k0 from shear wave velocity measurements in soils abstractthe at rest lateral stress coefficient k0 is an important soil parameter in geotechnical design problems and yet it is quite elusive in our ability to assess its value either by laboratory or in situ tests in this study an innovative geophysics approach toward the evaluation of the in situ k0 profile with depth is obtained by using small strain stiffness anisotropy ratio g0 hh g0 vh in soils the newly proposed k0 equation is derived from a database compiled from 12 test sites which have direct k0 measurements as procured from field tests either self boring pressuremeters sbp or total stress cells tsc and or laboratory triaxial tests instrumented consolidometers and or suction measurements the data show a strong relationship between the overconsolidation difference ocd p vo and stiffness anisotropy ratio thus enabling a k0 assessment in clays silts and sands

a 3 d lithospheric model of the caribbean south american plate boundary a 3 d structural model of the caribbean south american plate boundary was constructed by gravity modeling the model was constrained by four wide angle seismic refraction sections moho depth estimations from receiver functions and additionally seismological hypocenters surface geology and geodynamic information density values were calculated from empirical velocity density functions and mineralogical chemical composition considering specific p t conditions we tested different structural models for western and eastern venezuela in the final model the fit of the measured and modeled gravity fields for a long caribbean slab in western venezuela was better than the fit obtained for a short one this interpretation is consistent with the constraining data the slab is interpreted to extend further to the south beneath northern colombia and culminates in the area of the seismic cluster of the bucaramanga nest the modeling estimates a slab dip angle under maracaibo and merida andes of 15 which increases to 32 below 100 km depth the dip direction of approx n150 e 5 increases lightly eastward in eastern venezuela considering its short wavelength lineaments analyzed from gravity data by curvature methods and euler deconvolution seem to be related to shallow structures and density contrast in the serrania del interior and not from a deep detached slab beneath the continental crust it is deduced from modeling results that this slab configuration has a very small influence on the gravity field the slab was modeled according to the subduction transform propagation model with purely westward subduction and a slab break off along a vertical dip slip tear through the lithosphere

on the reactivation of a large landslide induced by rainfall in highly fissured clays abstract the paper discusses the field investigations geotechnical characterization and time evolution of horizontal displacements in a wide landslide the latter triggered by an earthquake occurring in september 2002 is located in sicily and involves a thick body of stiff and highly fissured clays belonging to a varicoloured clay formation in order to characterise the reactivation of landslide mechanisms induced by rainfall a three year monitoring programme 2008 2011 was implemented to measure rain pore water pressures and deep and superficial displacements the monitoring data made it possible to recognize three distinct landslides which evolve at variable rates in different directions within the overall landslide area the landslides were reactivated by the increase in pore water pressures in the period autumn 2009 spring 2010 the data clearly show the mechanical role of pore water pressures in the stability of the slope and point out the dependence of the time evolution of the horizontal displacement on the pore water pressure variation induced by rainfall back analysis carried out on the single landslides confirmed that they are reactivated ones since the residual shear strength is mobilised on the pre existing sliding surfaces the pore water pressure variation and the landslide body displacement rates can be effectively related to 5 month cumulative rainfall by simple models characterised by three parameters these models calibrated on the period october 2009 september 2010 made it possible to predict the horizontal displacement rates in the ensuing time period the displacements forecasted by the model in the period september 2010 may 2011 agree very well with the measured ones

bayesian identification of soil strata in london clay identification of soil strata in the london clay formation lcf is of practical significance as it allows geotechnical property data within the same soil strata to be compared or used effectively the soil strata in lcf are generally determined based on their lithological characters and fossil contents with which geotechnical engineers are less familiar to assist geotechnical engineers in identifying the soil strata in lcf this paper aims to develop bayesian approaches to identification of soil strata in lcf using water content data equations are derived for the proposed bayesian approaches and the approaches are illustrated using a water content profile at st james s park london the proposed bayesian approaches are shown to correctly identify the soil strata in lcf in addition a sensitivity study is performed to explore the effect of data quantity i e both the measurement interval and number of measurements at the same depth in a water content profile and provide guidance on the water conten

quaternary tectonics and present stress tensor of the inverted northern falcon basin northwestern venezuela abstract the tertiary falcon basin in northwestern venezuela has a privileged position in the geodynamic puzzle of northwestern south america occurring in a region where several major plates caribbean south america and nazca and minor lithospheric blocks maracaibo bonaire and western colombia are interacting a combination of good exposures due to aridity and a near continuous sedimentary record in a now inverted basin helps to unravel the neogene and quaternary geodynamic evolution of this region a neotectonic and microtectonic investigation of the plio quaternary sedimentary rocks of the northern falcon basin reveals that this region is subject to a compressive to transpressive regime at present this regime is characterized by a nnw sse oriented maximum horizontal stress and a ene wsw trending intermediate or minimum horizontal stress as is confirmed by focal mechanism solutions this stress field is in agreement both with the nne directed extrusion of the maracaibo and bonaire blocks in western venezuela where the falcon basin is located and present day transpression along the caribbean south america plate boundary zone

structure of the merida andes venezuela relations with the south america caribbean geodynamic interaction for over 50 years several models based on diverse geologic concepts and variable quality of data have been proposed to explain the major structure and history of the merida andes ma in western venezuela lately this chain growth and associated flexural basins deepening have been related to incipient type a subductions of either polarity accounting for the across chain asymmetry however these recent models have not well integrated the present tectonically active setting driven by neighboring major plate interactions at present this chain exhibits ongoing strain partitioning where cumulative right lateral slip along chain axis is as much as half of or about the same as the transverse shortening since late miocene thus implying that the nne directed maracaibo block extrusion with respect to the south america sa plate is not a secondary feature consequently this paper discusses some limitations exhibited by the se directed continental subduction models maracaibo crust underthrusting the merida andes in the light of available geological and geophysical data besides it is herein proposed that the merida andes structuration is related to a nw directed gently dipping incipient type a subduction where chain growth and evolution are similar to those of a sedimentary accretionary wedge i e barbados but at crustal scale and with ongoing strain partitioning this continental subduction is the se portion of a major orogenic float that also comprises the perija range and the santa marta block d 2002 elsevier science b v all rights reserved

the stiffness of natural london clay an investigation of natural london clay is reported involving advanced triaxial hollow cylinder apparatus hca and dynamic testing techniques significant anisotropy was revealed at all scales of deformation and the framework of cross anisotropic elasticity was found to apply broadly to the initial elastic behaviour the stiffness parameters obtained by independent techniques generally exhibited good agreement with the greatest deviation being seen in the poisson s ratios which fell far from the values usually assumed in conventional foundation analysis probing tests established the limits to the elastic domain over a range of depths showing that these scaled in proportion to the mean effective stress level as did those of a second kinematic surface that surrounded the elastic domain once engaged this second surface signified a new pattern of strain increment directions faster elastic plastic stiffness decay with strain and also a greater dependence of behaviour on recent stress history howev

orogenic float of the venezuelan andes abstract the venezuelan or merida andes are a ne trending intracontinental orogen that started to rise from the middle miocene due to the e w far field convergence between the maracaibo block to the northwest and the guyana shield to the southeast oblique convergence is responsible for strain partitioning with thrusting along both foreland basins and right lateral strike slip faulting along the ne sw bocono fault cutting the venezuelan andes along strike the central part of the belt is also cut by the n s left lateral strike slip valera fault that connects the bocono fault both faults bounding the trujillo block that escapes towards the nne even though the regional geology of belt is well known its structure at depth remains a matter of debate our work based on the integration of geological and geophysical data aims to better constrain the deep geometry of faults and the tectonic evolution of the mountain belt we used the orogenic float model to construct two nw se trans andean crustal scale balanced sections the late neogene quaternary shortening varies from 40 km in the south to 30 km in the north across the trujillo block indicating that a quarter of the deformation seems to be absorbed by the tectonic escape process more importantly a major reorganization in the crust took place in the early pliocene it is characterized by the imbrication of the maracaibo crust into the guyana crust this resulted in the subduction of the guyana lower crust and the formation of a nw vergent basement thrust propagating upwards and surfacing along the las virtudes thrust rapid uplift of the northern flank of the belt subsequently occurred together with massive deposition of the plio quaternary coarse grained betijoque formation in the northwestern foreland basin

the crustal structure of the guayana shield venezuela from seismic refraction and gravity data abstract we present results from a seismic refraction experiment on the northern margin of the guayana shield performed during june 1998 along nine profiles of up to 320 km length using the daily blasts of the cerro bolivar mines as energy source as well as from gravimetric measurements clear moho arrivals can be observed on the main e w profile on the shield whereas the profiles entering the oriental basin to the north are more noisy the crustal thickness of the shield is unusually high with up to 46 km on the archean segment in the west and 43 km on the proterozoic segment in the east a 20 km thick upper crust with p wave velocities between 6 0 and 6 3 km s can be separated from a lower crust with velocities ranging from 6 5 to 7 2 km s a lower crustal low velocity zone with a velocity reduction to 6 3 km s is observed between 25 and 25 km depth the average crustal velocity is 6 5 km s the changes in the bouguer anomaly positive 30 mgal in the west and negative 20 mgal in the east cannot be explained by the observed seismic crustal features alone lateral variations in the crust or in the upper mantle must be responsible for these observations

receiver function study of the crustal structure of the southeastern caribbean plate boundary and venezuela 1 we have investigated crustal thickness and composition across the southeastern caribbean plate boundary with the receiver function technique we used teleseismic data recorded by a temporary broadband array deployed under the bolivar project and the permanent national seismic network of venezuela we used the primary p to s conversion and crustal reverberations to estimate crustal thickness and average crustal vp vs ratio over the region we observe large variations in crustal thickness and poisson s ratio estimated moho depth ranges from 16 km beneath the southeastern caribbean sea to 52 km beneath northeastern venezuela and the venezuelan andes there is a good correlation between crustal structure and tectonic terranes data from the precambrian guayana shield suggest that the underlying crustal structure is relatively uniform with a moderate thickness 37 km and an intermediate composition a thick crust is found below the foreland basins the two mountain systems in northern venezuela the serrania del interior and the serrania del falcon have a thin crust with arc composition and are likely dynamically supported by elastic rebound or underthrusting of the oceanic plateau that characterizes the southern caribbean on the other hand the venezuelan andes and perija range on the western side of the country are probably isostatically balanced by thick crustal roots

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

the caribbean south american plate boundary at 65 w results from wide angle seismic data 1 we present the results of the analysis of new wide angle seismic data across the caribbean south american plate boundary in eastern venezuela at about 65 w the 500 km long profile crosses the boundary in one of the few regions dominated by extensional structures as most of the southeastern caribbean margin is characterized by the presence of fold and thrust belts a combination of first arrival traveltime inversion and simultaneous inversion of pmp and pn arrivals was used to develop a p wave velocity model of the crust and the uppermost mantle at the main strike slip fault system we image the cariaco trough a major pull apart basin along the plate boundary the crust under the southern caribbean deformed belt exhibits a thickness of 15 km suggesting that the caribbean large igneous province extends to this part of the caribbean plate the velocity structures of basement highs and offshore sedimentary basins imaged by the profile are comparable to those of features found in other parts of the margin suggesting similarities in their tectonic history we do not image an abrupt change in moho depth or velocity structure across the main strike slip system as has been observed elsewhere along the margin it is possible that a terrane of caribbean island arc origin was accreted to south america at this site and was subsequently bisected by the strike slip fault system the crust under the continental portion of the profile is thinner than observed elsewhere along the margin possibly as a result of thinning during jurassic rifting

earth s middle age this research was funded through natural environment research council grant ne j021822 1 and the apc was paid through the rcuk oa block grant

the shear stiffness characteristics of four eocene to jurassic uk stiff clays a large proportion of the southern uk is underlain by stiff clays improving their geotechnical characterisation is important for many current and future infrastructure projects this paper presents an integrated study of the complex stiffness behaviour of four key medium plasticity highly overconsolidated strata the gault kimmeridge oxford and london clays the latter were deposited between the jurassic and the eocene under broadly similar marine conditions coordinated programmes of advanced static and dynamic laboratory measurements have been undertaken on high quality samples concentrating on samples taken from similar depths at inland sites and including triaxial and hollow cylinder stress path experiments employing high resolution local strain multi axial bender element and resonant column techniques a new approach was employed to interpret the hollow cylinder experiments and the laboratory measurements are examined in combination with independent field shear wave data the clays stiffness c

an assessment of the earth pressure coefficient in overconsolidated clays the determination of the earth pressure coefficient k0 in a natural clay deposit is a problem of considerable significance in geotechnical engineering while the methods for evaluation of k0 are reliable for normally consolidated soils significant difficulties still exist in evaluating k0 in overconsolidated clays given that it is influenced by the stress history of the material together with the age structure mineralogical composition and depositional environment indeed some of these factors are responsible for the soil becoming anisotropic the existing framework for prediction of k0 in overconsolidated soils does not account for any influences caused by anisotropy the work reported in this paper evaluates the validity of a revised relationship between k0oc and ocr overconsolidation ratio using data obtained from laboratory investigations the tests were performed on reconstituted and undisturbed samples of belfast upper boulder clay london clay and gault clay tests were also performed on re

mantle flow beneath northwestern venezuela seismic evidence for a deep origin of the merida andes abstract we measured shear wave splitting from sks data recorded by the national seismic network of venezuela and a linear broadband passcal rice seismic array across the merida andes the linear array was installed in the second phase of the passive seismic component of the bolivar project to better understand the complicated regional tectonics in western venezuela using the method proposed by wolfe and silver 1998 sks waveforms from 2 to 36 earthquakes mostly from the tonga subduction zone were selected for each of the 23 stations in the region in order to do splitting analysis the fast polarization directions can be divided into 3 zones all in agreement with local gps data the first zone comprises the stations north of the dextral strike slip oca ancon fault these stations show the largest split times 1 6 3 2 s oriented in a roughly ew direction and are similar to splitting observations made further to the east along the strike slip plate boundary growdon et al 2009 we attribute this to trench parallel mantle flow that passes around the northwest corner of the subducting caribbean plate and along the northern edge of south america as proposed by russo and silver 1994 forming an eastward flow beneath the southern caribbean plate zone two is the merida andes with the right lateral bocono fault in the center where split orientations are at n45 e suggesting that the observed seismic anisotropy is likely caused by lithospheric deformation parallel to the bocono fault zone three is east of the bocono fault inside the barinas apure basin where the measured split times are smaller 0 8 s with an ew fast direction that is consistent with those observed at the guarico basin maturin basin and the guayana shield in the east and are interpreted as orientation with the motion of the continent

on the interaction of the north andes plate with the caribbean and south american plates in northwestern south america from gps geodesy and seismic data

palaeozoic to early jurassic history of the northwestern corner of gondwana and implications for the evolution of the iapetus rheic and pacific oceans abstract the palaeozoic to mesozoic igneous and metamorphic basement rocks exposed in the merida andes of venezuela and the santander massif of colombia are generally considered to define allochthonous terranes that accreted to the margin of gondwana during the ordovician and the carboniferous however terrane sutures have not been identified and there are no published isotopic data that support the existence of separate crustal domains a general paucity of geochronological data led to published tectonic reconstructions for the evolution of the northwestern corner of gondwana that do not account for the magmatic and metamorphic histories of the basement rocks of the merida andes and the santander massif we present new zircon u pb icp ms data from 52 igneous and metamorphic rocks which we combine with whole rock geochemical and pb isotopic data to constrain the tectonic history of the precambrian to mesozoic basement of the merida andes and the santander massif these data show that the basement rocks of these massifs are autochthonous to gondwana and share a similar tectono magmatic history with the gondwanan margin of peru chile and argentina which evolved during the subduction of oceanic lithosphere of the iapetus ocean the oldest palaeozoic arc magmatism is recorded at 500 ma and was followed shortly by barrovian metamorphism peak metamorphic conditions at upper amphibolite facies are recorded by anatexis at 477 ma and the intrusion of synkinematic granitoids until 472 ma subsequent retrogression resulted from localised back arc or intra arc extension at 453 ma when volcanic tuffs and interfingered sedimentary rocks were deposited over the amphibolite facies basement continental arc magmatism dwindled after 430 ma and terminated at 415 ma coevally with most of the western margin of gondwana after pangaea amalgamation in the late carboniferous to early permian a magmatic arc developed on its western margin at 294 ma as a result of subduction of oceanic crust of the palaeo pacific ocean intermittent arc magmatism recorded between 294 and 225 ma was followed by the onset of the andean subduction cycle at 213 ma in an extensional regime extension was accompanied by slab roll back which led to the migration of the arc axis into the central cordillera of colombia in the early jurassic

predicting the stand up time of temporary london clay slopes at terminal 5 heathrow airport the paper describes the results of finite element analyses of the temporary slope geometries in london clay at london heathrow airport s terminal 5 the aims of the analyses were to examine the times before failures developed and to identify the failure mechanisms involved the brittle behaviour of the london clay was modelled and the effects of progressive failure were taken into account the possible presence of tectonic shears with their strength close to residual was considered by comparing analyses with and without a tectonic shear zone the predicted time to failure of the slopes and the form of the failure whether shallow or deep seated were determined by a combination of the assumed permeability profile and whether or not allowances were made for increases in permeability as the clay swelled the average surface suction the in situ k0 profile the depth of excavation and whether or not a low strength tectonic shear surface was present in the slope the analyses fall into the category of clas

characterizing the caribbean south american plate boundary at 64 w using wide angle seismic data 1 we present wide angle velocity modeling results from profile 64 w of the broadband ocean land investigation of venezuela and the antilles arc region bolivar project line 64w is a 460 km long approximately north south onshore offshore reflection refraction transect located approximately at 64 w longitude the profile extends across the transform plate boundary between the southeastern caribbean car and south american sa plates east of the profile the plate boundary bends to the north and sa subducts beneath car we utilize first arrival tomography to resolve a smooth velocity field for the sediments and upper middle crust and then use a layered approach to resolve a sharp velocity contrast for the moho simultaneously inverting refracted pn and reflected pmp arrivals we image crustal and upper mantle structure across the plate boundary zone we interpret that the strike slip system that accommodates relative motion between car and sa extends near vertically through the entire crust and offsets the moho we see no evidence supporting a major component of convergence and rather than a wide boundary zone of overlapping lithospheric plates we interpret the plate boundary to be confined to the 33 km wide near vertical strike slip system previously interpreted thrust faults flanking the strike slip system appear to be confined to the upper middle crust and may be related to the detachment of subducting south american lithosphere at the southern terminus of the lesser antilles subduction zone east of 64 w

stiff sedimentary clays geological origins and engineering properties this keynote paper summarises and extends earlier work on the characterisation of clays with particular reference to stiff sedimentary clays examples of clay characterisation are given that include the various main geological events from sedimentation to erosion and weathering and other phenomena such as diagenesis and brecciation the examples demonstrate the value of plotting basic data such as the current stress state and undrained strength in terms of the void index these three parameters permit an immediate simple categorisation in terms of geological history of sedimentary clays further examples illustrate the engineering consequences of weathering of stiff clays and the associated phenomenon of brecciation produced by the processes of cambering and valley bulging it is emphasised that at all stages in a clay s geological history it is the bond strength between the soil particles not the geological preconsolidation stress that controls the mechanical behaviour of sedimentary clays

subduction driven recycling of continental margin lithosphere seismic images of the subducted atlantic slab under northeastern south america and the alboran slab beneath the gibraltar arc region show that subducting oceanic plates viscously entrain and remove the bottom of the continental thermal boundary layer from adjacent continental margins driving surface tectonics and pre conditioning the margins for further deformation alan levander and co authors present seismic images that show larger than expected volumes of high seismic velocity mantle associated with the subducted atlantic slab under northeastern south america and the alboran slab beneath the gibraltar arc region the anomalous regions are under and aligned with the continental margins at depths greater than 200 km with the lithospheric mantle under the continental margins significantly thinner than expected the authors conclude that subducting oceanic plates viscously entrain and remove the bottom of the continental thermal boundary layer from adjacent continental margins driving surface tectonics and preconditioning the margins for further deformation by creating topography along the lithosphere asthenosphere boundary whereas subduction recycling of oceanic lithosphere is one of the central themes of plate tectonics the recycling of continental lithosphere appears to be far more complicated and less well understood1 delamination and convective downwelling are two widely recognized processes invoked to explain the removal of lithospheric mantle under or adjacent to orogenic belts2 3 4 5 here we relate oceanic plate subduction to removal of adjacent continental lithosphere in certain plate tectonic settings we have developed teleseismic body wave images from dense broadband seismic experiments that show higher than expected volumes of anomalously fast mantle associated with the subducted atlantic slab under northeastern south america and the alboran slab beneath the gibraltar arc region6 7 the anomalies are under and are aligned with the continental margins at depths greater than 200 kilometres rayleigh wave analysis8 9 finds that the lithospheric mantle under the continental margins is significantly thinner than expected and that thin lithosphere extends from the orogens adjacent to the subduction zones inland to the edges of nearby cratonic cores taking these data together here we describe a process that can lead to the loss of continental lithosphere adjacent to a subduction zone subducting oceanic plates can viscously entrain and remove the bottom of the continental thermal boundary layer lithosphere from adjacent continental margins this drives surface tectonics and pre conditions the margins for further deformation by creating topography along the lithosphere asthenosphere boundary this can lead to development of secondary downwellings under the continental interior probably under both south america and the gibraltar arc8 10 and to delamination of the entire lithospheric mantle as around the gibraltar arc11 this process reconciles numerous sometimes mutually exclusive geodynamic models proposed to explain the complex oceanic continental tectonics of these subduction zones12 13 14 15 16 17

deformation characteristics of a clayey interbed during fluid injection surface deformation due to fluid withdrawal has long been observed at the surface above aquifers and oil reservoirs uplift associated with fluid injection has also been observed although the processes of subsidence and uplift are reversible in a poroelastic setting the presence of clayey interbeds can result in ground deformation behavior non reversible because of their low permeability and nonlinear behavior in this investigation a cam clay model is used in conjunction with a poroelastic model to simulate the presence of a laterally extensive lens shaped clayey interbed in a sandstone aquifer during fluid injection spatial and temporal changes associated with this interbed during aquifer pressurization are captured and can be clearly differentiated from the deformation of the surrounding poroelastic aquifer formation results for pore pressures and ground surface deformation patterns can be categorized into three distinct time intervals an early time interval where the aquifer is pressurized but not the interbed leading to a lower region above the interbed an intermediate time interval in which the interbed pressures slowly increase approaching the pressure of the adjacent aquifer leading to a potentially large surface uplift above the interbed and a late time interval in which pressure equilibration is achieved between the interbed and aquifer and a highly non symmetrical irregular surface deformation pattern results which may be higher or lower than the background depending on the interbed characteristics reservoir configuration is found to be an important factor influencing ground deformation behavior with more obvious deformation always occurring in a laterally confined aquifer as opposed to a laterally infinite aquifer

the use of kinematic hardening models for predicting tunnelling induced ground movements in london clay the use of a kinematic hardening soil model for predicting short and long term ground movements due to tunnelling in london clay is investigated the model is calibrated against oedometer and triaxial tests on intact samples from different units of the london clay the calibrated model is then used in finite element analysis to simulate the field response at st james s park during excavation of the jubilee line extension tunnels the finite element predictions compare well with the available field monitoring data the importance of using consistent initial conditions for this complex boundary value problem in conjunction with the model parameters selected is highlighted the stiffness response of different regions of the finite element mesh indicates that the rate at which the stiffness degrades and the stiffness response further away from the tunnel boundary affect the short term predictions significantly the long term predictions confirm that the compression characteristics of the soil control the mag

asynchronous miocene pliocene exhumation of the central venezuelan andes constraining the uplift and exhumation history of the venezuelan andes is critical for understanding the complex geodynamic evolution of northern south america we present new apatite fission track aft data from the sierra nevada de merida in the central venezuelan andes and compare the results with previously published aft data for the sierra la culata el carmen block to the north the aft data are combined with three dimensional thermal modeling to constrain the exhumation and relief history of both blocks which show significant differences the sierra nevada indicates exhumation rates of 1 7 km m y from ca 10 ma until 4 ma decreasing to 0 4 km m y from 4 ma to present the el carmen block in contrast underwent much more rapid recent exhumation with rates of 1 4 km m y since 4 ma the data suggest 4 km of relative rock uplift of the sierra la culata with respect to the sierra nevada since 4 ma and support significant uplift of basement blocks along major preexisting discontinuities during transpression in the northern andes linked to accretion of the panama arc and indentation of the maracaibo block rapid middle late miocene exhumation in the sierra nevada resulted from surface uplift that caused the late miocene deflection of the orinoco river inferred from the neogene sedimentary record

geodynamics of the northern andes subductions and intracontinental deformation colombia new regional seismological data acquired in colombia during 1993 to 1996 and tectonic field data from the eastern cordillera ec permit a reexamination of the complex geodynamics of northwestern south america the effect of the accretion of the baudo panama oceanic arc which began 12 myr ago is highlighted in connection with mountain building in the ec the istmina and ibague faults in the south and the santa marta bucaramanga fault to the northeast limit an e se moving continental wedge progressive indentation of the wedge is absorbed along reverse faults located in the foothills of the cordilleras northward of 5 n and transpressive deformation in the santander massif crustal seismicity in colombia is accurately correlated with active faults showing neotectonic morphological evidences intermediate seismicity allows to identify a n ne trending subduction segment beneath the ec which plunges toward the e se this subduction is interpreted as a remnant of the paleo caribbean plateau pcp as suggested by geological and tomographic profiles the pcp shows a low angle subduction northward of 5 2 n and is limited southward by a major e w transpressive shear zone normal oceanic subduction of the nazca plate np ends abruptly at the southern limit of the baudo range northward the np subducts beneath the choco block overlapping the southern part of the pcp cenozoic shortening in the ec estimated from a balanced section is 120 km stress analysis of fault slip data in the ec northward of 4 n indicates an e se orientation of 1 in agreement with the pcp subduction direction northward near bucaramanga two stress solutions were observed 1 a late andean n80 e compression and 2 an early andean nw se compression

crustal density structure in northwestern south america derived from analysis and 3 d modeling of gravity and seismicity data abstract this paper presents a three dimensional 3 d interpretation of new gravity and seismicity datasets for northern south america a 3 d forward density model was constructed on the basis of deep wide angle seismic refraction sections moho depth from receiver functions and surface geology density values were estimated from published borehole data for sediments by using empirical velocity density functions and considering mineralogical chemical composition variations under typical pressure temperature conditions for upper and lower crustal rocks the modeled 3 d density structure was kept as simple as possible the continental and oceanic plates were formed by two sedimentary bodies one crustal body and one mantle lithosphere body overlying a sub lithospheric mantle the caribbean plate was modeled with an atypical crustal thickness of 18 km including sediments the geometry of the caribbean plate was modeled using a combination of gravity modeling and analyses of the seismicity and focal mechanism solutions intermediate seismicity and the orientation of the t axes appeared aligned along the predicted position of the slab as a result the estimated slab dip angle under maracaibo and the merida andes was 15 and increases up to 20 after 100 km depth the model shows two orientations in the slab strike n150 e 5 in western colombia and southward underneath the maracaibo block the modeling results suggest that the northern south american upper and lower crusts are relatively light and the density of the caribbean crust is typical for an oceanic crust

geodetic exploration of strain along the el pilar fault in northeastern venezuela we use global navigation satellite systems observations in northeastern venezuela to constrain the el pilar fault epf kinematics and to explore the effects of the variable elastic properties of the surrounding medium and of the fault geometry on inferred slip rates and locking depth the velocity field exhibits an asymmetric velocity gradient on either side of the epf we use five different approaches to explore possible models to explain this asymmetry first we infer a 1 6 km locking depth using a classic elastic half space dislocation model second we infer a 1 5 km locking depth and a 0 33 asymmetry coefficient using a heterogeneous asymmetric model including contrasting material properties on either side of a vertical fault suggesting that the igneous metamorphic terranes on the northern side are 2 times more rigid than the sedimentary southern side third we use a three dimensional elastostatic model to evaluate the presence of a compliant zone suggesting a 30 reduction of rigidity in the upper 3 km at the depth of a 1 to 5 km wide fault zone fourth we evaluate the distribution of fault slip revealing a widespread partial creep pattern in the eastern upper segment while the upper western segment exhibits a partially locked area which coincides with the rupture surface of the 1797 and 1929 earthquakes to supplement these models we upgrade the previously published displacement simulation method using nonvertical dislocations with data acquired between 2003 and 2013 the localized aseismic displacement pattern associated with creeping or partially creeping fault segments could explain the low level of historic seismicity

formation of the rocky mountains western united states a continuum computer model one hypothesis for the information of the rocky mountain structures in late cretaceous through eocene time is that plate of oceanic lithosphere was underthrust horizontally along the base of the north american lithosphere the horizontal components of the motion of this plate are known from paleomagnetism and the edge of the region of flat slab can estimated from reconstructed patterns of volcanism new techniques of finite element modeling allow prediction of the thermal and mechanical effects of horizontal subduction on the north american plate a model that has a realistic temperature dependent rheology and a simple plane layered initial condition is used to compute the consequences of horizontal underthrusting in the time interval 75 million to 30 million years before present successful prediction of this model include i the location amount and direction of horizontal shortening that has been inferred from laramide structures ii massive transport of lower crust from southwest to northeast iii the location and timing of the subsequent extension in metamorphic core complexes and the rio grande rift and iv the total area eventually involved in basin and range style extension in a broad sense this model has predicted the belt of laramide structures the transport of crust from the coastal region to the continental interior the subsequent extension in metamorphic core complexes and the rio grande rift and the geographic region of late tertiary basin and range extension its principal defects are that i many events are predicted about 5 million to 10 million years too late and ii the wave of crustal thickening does not travel far enough to the east reasonable modifications to the oceanic plate kinematics and rheologies that were assumed may correct these defects the correspondence of model predictions to actual geology is already sufficiently close to show that the hypothesis that horizontal subduction caused the laramide orogeny is probably correct the rocky mountain thrust and reverse faults formed in an environment of east west to northeast southwest compressive stress that was caused by the viscous coupling between the oceanic plate and the base of the north american crust nonuniform crustal thickening by simple shear transport also caused relative uplifts therefore this model is consistent with both of the range forming mechanisms that have been inferred 1 a new proposal that arises from this simulation is that horizontal subduction also caused the subsequent extensional basin and range taphrogeny by stripping away the mantle lithosphere so that the crust was exposed to hot asthenosphere after the oceanic slab dropped away

subduction in the southern caribbean images from finite frequency p wave tomography 1 the eastern boundary of the caribbean plate is marked by subduction of the atlantic under the lesser antilles the southeastern plate boundary is characterized by a strike slip margin while different configurations of subduction of the southwest caribbean under south america have been proposed we investigate the slab geometry in the upper mantle using multiple frequency teleseismic p wave tomography waveforms from p and pkpdf phases from 285 mb 5 0 events occurring at epicentral distances from 30 to 90 and greater than 150 were bandpass filtered and cross correlated to obtain up to three sets of delay times for each event the delay times were inverted using approximate first fresnel zone sensitivity kernels our results show the subducting atlantic slab as well as a second slab in the west of the study area that we interpret as a subducting fragment of the caribbean plate both slabs have steep dips where imaged and can be traced to depths greater than 600 km these results are consistent with transition zone boundary topography as determined by receiver function analysis the atlantic slab extends continent ward south of the plate bounding strike slip margin we interpret this extension as continental margin lithospheric mantle that is detaching from beneath south america and subducting along with the oceanic atlantic slab the steep subduction of the caribbean occurs 500 km landward from the trench implying an initial stage of shallow subduction as far to the east as the lake maracaibo merida andes region as has been inferred from intermediate depth seismicity

modeling wide angle seismic data for crustal structure southeastern grenville province a modeling methodology for obtaining two dimensional 2 d crustal structure from wide angle seismic data is applied to data from the southeastern grenville province pre modeling steps include 1 assignment of arrival pick uncertainties for appropriate data fitting and weighting using an empirical relationship based on signal to noise ratio 2 using a modified form of travel time reciprocity to avoid unreasonable levels of model heterogeneity and 3 identifying data unsuitable for 2 d modeling the goal of the travel time inversion amplitude modeling approach is to obtain a minimum structure and minimum parameter model that takes into account both horizontal and vertical variations in the resolution of typical wide angle data each step of a layer stripping procedure involves a series of inversions in which a one dimensional or simple starting model is improved with additional velocity and or interface nodes until a satisfactory trade off between travel time fit parameter resolution and complete ray coverage of all source receiver pairs is achieved using zero vertical velocity gradient layers and head waves during preliminary first arrival inversion can 1 decrease the number of intermediate models 2 allow greater lateral heterogeneity to be imaged and 3 simplify incorporation of amplitude modeling constraints into the final model using amplitude distance curves allows quantitative modeling of the relative amplitude and offset variations of phases discrepancies between observed and calculated reflection amplitudes are used to infer fundamental non step like velocity changes at layer boundaries later arrivals due to unresolved velocity anomalies are modeled using reflecting segments that float within the model without an associated velocity structure these reflectors provide a spatial image like that obtained from vertical incidence reflection data as opposed to a velocity image the model of grenville crustal structure is more detailed than a model obtained from a previous interpretation of the data and includes elements analogous to those imaged in nearby deep reflection data a crustal scale zone of wide angle reflectors with an average easterly apparent dip of 13 defines a major grenvillian terrane boundary

quaternary fault kinematics and stress tensors along the southern caribbean from fault slip data and focal mechanism solutions deformation along the southern caribbean coast as confirmed by the compilation of stress tensors derived from fault plane kinematic indicators microtectonics and further supported by focal mechanism solutions herein presented results from a compressive strike slip transpressional senso lato regime characterized by a nnw sse maximum horizontal stress 1h 11 and or an ene wsw minimum 1h 13 or 12 horizontal stress which is responsible for present activity and kinematics of six sets of brittle features east west right lateral faults nw se right lateral faults synthetic riedel shears ene wsw to east west dextral faults p shears nnw sse normal faults almost north south left lateral faults antithetic riedel shears and ene wsw reverse faults sub parallel to fold axes and mostly in the subsurface the latter ones being associated to ene wsw trending folding in this particular region the brittle deformation obeys the simple shear model although not all the deformation can be accounted for it since partitioning is also taking place regional folding and thrusting is essentially due to the normal to structure component of the partitioned maximum horizontal stress conversely the maximum horizontal stress on