wave diffraction by multiple arbitrary shaped cracks in an infinitely extended ice sheet of finite water depth flexural gravity wave interactions with multiple cracks in an ice sheet of infinite extent are considered based on the linearized velocity potential theory for fluid flow and thin elastic plate model for an ice sheet both the shape and location of the cracks can be arbitrary while an individual crack can be either open or closed free edge conditions are imposed at the crack for open cracks zero corner force conditions are further applied at the crack tips the solution procedure starts from series expansion in the vertical direction based on separation of variables which decomposes the three dimensional problem into an infinite number of coupled two dimensional problems in the horizontal plane for each two dimensional problem an integral equation is derived along the cracks with the jumps of displacement and slope of the ice sheet as unknowns in the integrand by extending the crack in the vertical direction into the fluid domain an artificial vertical surface is formed on which an orthogonal inner product is adopted for the vertical modes through this the edge conditions at the cracks are satisfied together with continuous conditions of pressure and velocity on the vertical surface the integral differential equations are solved numerically through the boundary element method together with the finite difference scheme for the derivatives along the crack extensive results are provided and analysed for cracks with various shapes and locations including the jumps of displacement and slope diffraction wave coefficient and the scattered cross section

alaska landfast sea ice links with bathymetry and atmospheric circulation 1 using radarsat synthetic aperture radar sar imagery of northern alaska and northwestern canada we calculated a mean climatology of the annual landfast ice cycle for the period 1996 2004 we also present the monthly minimum mean and maximum landfast ice extents throughout the study area these data reveal where and when the landfast is most stable and which sections of the coast are susceptible to midseason breakout events stabilization of landfast ice is strongly related to the advance of the seaward landfast ice edge slie into waters around 18 m deep isobaths near this depth are a good approximation for midseason landfast ice extent comparison with work from the 1970s suggests a reduced presence of landfast ice in this region of the arctic due to later formation and earlier breakup this will likely lead to an increase in coastal erosion and may also have profound effects upon subsistence activities which are intimately linked to the timing of marine mammal migration patterns interannually landfast ice formation correlates with the incursion of pack ice into coastal waters suggesting that the later mean date of formation in recent years may be related to the increasingly northward location of the perennial sea ice edge the timing of breakup correlates well with onset of thawing air temperatures analysis of regional data shows a multidecadal trend toward earlier thaw onset which suggests that the observed change in breakup dates may be part of a longer term trend

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

the monagas fold thrust belt of eastern venezuela part i structural and thermal modeling abstract the eastern venezuelan basin evb contains one of the largest hydrocarbon accumulations in the world main petroleum targets are buried structures of the monagas fold thrust belt mftb which forms the northeastern edge of the evb the objective of this study is to integrate the seismic and well data that has been acquired over the last 10 years across the mftb and evb to create an updated structural model three regional cross sections 60 75 km long are presented across an area of 4000 km2 five structural domains are described amarilis furrial jusepin cotoperi and pirital they are characterized by thrusts and high angle reverse faults structural style changes along strike are related to variations in depth of detachment levels and to the strike slip component of the deformation we have estimated a shortening between 43 and 59 km that increases eastward over a distance of 40 km the evolution of the mftb is divided in four episodes based on stratigraphic structural and thermal maturity evidences 1 oligocene early miocene initial movement of pirital thrust 2 early miocene simultaneous movement on pirital furrial and cotoperi thrusts 3 middle miocene increases in velocity and change in geometry of pirital thrust during an out of sequence period of thrusting 4 late miocene to holocene minor thrust activity this evolution is consistent with the oblique convergence between the caribbean and south american plates and the convergence between north and south america that affected eastern venezuela during the cenozoic by analyzing the along strike variations in structural style new exploratory opportunities have been identified under the orocual and santa barbara fields two untested duplex structures are proposed they were developed during the middle miocene other prospective hydrocarbon traps are associated to oblique transpressive faults that create anticline structures

oblique collision and accretion of the netherlands leeward antilles to south america 1 accretion of the netherlands leeward antilles volcanic island arc to northern south america offshore venezuela was accompanied by deformation and clockwise block rotation structural analysis of aruba curacao and bonaire indicates that they were deformed during at least one ductile event d1 and three brittle events f1 f3 the first deformation phase d1 and f1 began in the early cretaceous at approximately 110 ma deformation features are consistent with displacement partitioning at the obliquely convergent caribbean atlantic proto caribbean plate boundary this deformation was accompanied by an approximate 90 clockwise rotation of the forearc terrane the onset of the second phase of deformation f2 occurred in the eocene 55 ma also accompanied by a small amount of rotation and structures are consistent with wrench tectonics in a pull apart regime this coincided with the collision of the caribbean plate with the great bahama bank and the initiation of convergence between north and south america the youngest deformation structures and rotation f3 are also associated with wrench faulting likely motion along the oca fault and the caribbean plate transform fault initiated a major restraining bend in the late paleogene beginning 12 ma the northeastward escape of the maracaibo block further increased the complexity of tectonism at the caribbean south american plate boundary deformation structures are consistent with a total of 135 clockwise rotation of the leeward antilles islands and regional diachronous deformation from west to east since the early cretaceous

geometry and kinematics of the north monagas thrust belt venezuela abstract the complex geometry and structural evolution of the north monagas thrust belt were investigated and new exploration trends in this part of eastern venezuela considered the pirital thrust and related structures of the maturin basin are used to discuss the sequence of thrusting and to illustrate the constant balance between erosion and thrust reactivation in a recent fold and thrust belt a careful analysis of the chronostratigraphy and the internal geometry of the foredeep and piggy back syntectonic deposits has been used to constrain balanced cross sections and to establish the kinematics of each individual structure basement involvement is postulated for the pirital thrust which remobilizes previously emplaced cover structures such as the el furrial or orocual structural units it thus probably relates to the pinch out of a shallow decollement level coal measures of the barranquin formation older late jurassic to lower cretaceous evaporites or other mesozoic plastic horizons and to the reactivation of a deeper basement fault the westward emergence of the pirital and el furrial thrusts in a complex transfer area the urica fault is also analysed according to transverse profiles the surficial urica fault does not connect directly to a basement fault but instead acts as a lateral ramp for the el furrial cover unit and as a tear fault for the pirital thrust reactivation of ancient thrust faults also occurs at the rear of the pirital thrust in the west in areas where the topographic profile of the eastern venezuelan ranges has recently been modified by the pirital thrust emplacement these out of sequence thrusts resulting from fault reactivation in the inner parts of the prism are discussed in terms of the preservation of the critical taper they effectively relate directly to erosional processes which significantly decrease the load applied on this portion of the wedge the influence of the sequence of thrusting on the potential plays is considered

opening of the grenada back arc basin and evolution of the caribbean plate during the mesozoic and early paleogene abstract geological and geophysical data indicate that the grenada basin was presumably created during the paleocene by sundering of a proto eastern caribbean arc into a remnant arc to the west aves swell and an active arc to the east lesser antilles ridge grenada basin spreading is thought to have been penecontemporaneous with the creation of the yucatan basin located at the opposite side of the caribbean sea i suggest that a continuous mesozoic caribbean arc m c a including the greater antilles the aves lesser antilles system and the aruba blanquilla chain netherland venezuelan antilles was initiated in the pacific probably about 130 120 ma ago its arrival in front of and its subsequent motion inside the central atlantic tethyan seaway caused the opening of both yucatan and grenada basins which occurred at the two initial points of contact with the north and south american cratons in contrast to the style of many other island arcs this back arc spreading event occurred only once in the long history of the m c a the lesser antilles appear to be the oldest currently active intra oceanic island arc

wave attenuation and wave drift in the marginal ice zone abstract surface gravity waves in a viscous rotating ocean are studied theoretically when they penetrate an area covered by highly concentrated brashlike ice the motion is described by a lagrangian formulation and the brash is modeled by a viscous newtonian fluid results for wave attenuation and wave drift are obtained in the asymptotic limit of a thin very viscous upper layer the derived damping rate compares favorably with field data from the marginal ice zone miz the drift velocity in the ocean exhibits a marked maximum in the viscous boundary layer near the ice ocean interface at the outer edge of the boundary layer it exceeds the inviscid stokes drift by a factor of 7 4 computed values for the mean viscous drag on the ice induced by the wave motion show that this effect may compete with the frictional effected of the wind in packing the ice finally it is demonstrated that the integrated horizontal mass transports in the open ocean and under the ice do not match which leads to upwelling in

the effect of the marginal ice zone on the directional wave spectrum of the ocean abstract during the mizex 84 experiment in the greenland sea in june july 1984 a cooperative program was carried out between the scott polar research institute spri and the institute of oceanographic sciences ios to measure the change in the directional character of the ocean wave spectrum in the immediate vicinity of the ice edge the aim was to extend one dimensional spectral measurements made hitherto so as to study in full the processes of reflection and refraction directional spectrum analysis of these records shows that i significant reflection of wave energy occurs at the ice edge detected using long hasselmann analysis ii within the ice the directional spectrum at high frequencies where attenuation is rapid broadens to become almost isotropic whereas iii the directional spectrum at swell frequencies where the attenuation is slower becomes initially narrower before broadening more slowly than the high frequency energy an explanation of these effects is offered in terms of scatte

propagation of a lithospheric tear fault step through the western boundary of the calabrian accretionary wedge offshore eastern sicily southern italy the rollback of a segmented slab of oceanic lithosphere is typically accompanied by vertical lithospheric tear fault s along the lateral slab edge s and by strike slip movement in the upper plate defined as a step fault subduction tear edge propagator the neogene evolution of the central mediterranean is dominated by the interaction between the slow africa eurasia convergence and the se ward rollback of the ionian slab that leads to the back arc opening of the tyrrhenian sea here we present post stack time migrated and pre stack depth migrated archimede 1997 multichannel seismic lines that were acquired offshore eastern sicily at the foot of the malta escarpment first we identify the recent deformation along the lateral ramp of the calabrian accretionary wedge towards the east the calabrian wedge is formed by the accretion of the post evaporitic sediments above a decollement at the base of the messinian evaporites at the latitude of syracuse 50 km east of the malta escarpment a major n150 degrees e trending crustal scale and vertical fault slices through the entire accretionary wedge this fault cuts by several kilometers through the pre evaporitic messinian sediments and into the basement the vertical offset along this vertical fault decreases from north to south and the fault is no longer observed on the seismic lines 50 km se of the alfeo seamount a previously published moho depth isocontour map offshore sicily and the recent gps data combined with the presence of strike slip movements ne onshore sicily allow us to identify this 200 km long crustal scale fault as the surface expression of a step fault the presence of syntectonic pleistocene sediments on top this crustal scale fault suggests a recent lithospheric vertical movement of the step fault in response to the rollback of the ionian slab and to the se ward advance of the calabria peloritan block

finite floe wave reflection and transmission coefficients from a semi infinite model a model to describe the reflection and transmission of ocean waves by a single ice floe is developed from the semi infinite model of fox and squire 1990 1991 this is done by considering the coefficients for the transition from ice to water in the semi infinite case in terms of those from water to ice finite floe reflection and transmission coefficients r and t respectively are then found as the solution of a set of four simple simultaneous equations the properties of r and t are investigated and examples of their absolute values are given for several geometries r compares well with the predictions of a precise model in the case of deep water these results suggest that the analytical model described has applications to defining the sea state within marginal ice zones given the floe size and ice thickness distributions and the incoming sea wave spectrum

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

observations and analyses of an intense waves in ice event in the sea of okhotsk 1 ice draft ice velocity ice concentration and current profile data gathered at an array of eight continental shelf monitoring sites east of sakhalin island were analyzed in conjunction with regional meteorological data to document and explain intense wave occurrences several hundred kilometers inside the sea of okhotsk ice pack the studied event was associated with the 19 21 march 1998 passage of an intense cyclone which produced waves with amplitudes in excess of 1 m at the most offshore monitoring location the relatively monochromatic character of the waves allowed extraction of wave intensity time series from ice draft time series data spatial and temporal variations in these data were used to establish directions and speeds of wave energy propagation for comparisons with an earlier interpretation liu and mollo christensen 1988 of an antarctic intense waves in ice event it was concluded that although both events are compatible with a two stage process in which initially slowly advancing wave activity increases subsequent ice cover wave transmissivity the first stage of the sea of okhotsk event was not explicable in terms of the static stress induced changes in the waves in ice dispersion relationship proposed by liu and mollo christensen an alternative explanation is offered that eschews the linkage between wave group velocities and the observed slow rates of wave energy propagation and attributes the subsequent transition to more normal wave propagation behavior to ice pack divergence

the response of ice floes to ocean waves a precise linear mathematical theory is reported to model the response of a solitary ice floe in ocean waves allowing the floe to bend with the passing wave both infinite and finite water depths are considered and the model is also extended to include a pair of separated floes of different length the case of n floes then being a natural and straightforward development for a single ice floe perfect transmission is achieved whenever the wavelength beneath the ice couples perfectly to the length of the ice floe then the strains induced in the bending ice floe reach a maximum and because multiple cycle tuning can occur in floes which are long compared to the wavelength strain response amplitude operators raos are complicated by considering the case of infinite stiffness heave and roll raos are also found for typical ocean wave periods and these agree well with two dimensional rigid body models finally ice floes of different diameters but constant 1 m thickness are subjected to spectral forcing and the strain spectral density for each is found spectral density envelopes increase gradually with floe diameter achieving a maximum value for a floe of about 102m thereafter strains never decrease below those for the 80 m curve owing to multiple cycle tuning this may explain the presence of zones within an ice field where floe size never exceeds some prescribed value results obtained from the complete theory for two adjacent floes do not differ significantly from those found by applying the single floe model serially except when the separation is very small

crustal history of margarita island venezuela in detail constraint on the caribbean plate tectonic scenario the pressure temperature time deformation evolution for the crust of margarita island venezuela has been established to allow comparison with current plate tectonic models for the caribbean region on margarita the 12 recognizable stages of development can be summarized in terms of the following evolving tectonic settings protolith evolution as aptian albian or older oceanic crust as well as continental crust with paleozoic basement stages 1 and 2 accretion and high pressure metamorphism 500 600 c 10 14 kbar as the margarita complex in the deep level of a fore arc at 100 90 ma stage 3 ascent cooling and emplacement into the intermediate crustal level of a volcanic arc at 90 80 ma stage 4 transform plate margin setting at a comparable level at 80 50 ma stage 5 second episode of rapid uplift and cooling stages 6 and 7 and shallow crustal level close to transform plate margin from 50 ma to present stages 8 to 12 this complex sequence is in excellent agreement with plate tectonic scenarios that require a pacific origin for the caribbean plate and eastward migration of the margarita complex and its correlatives along northern south america since the cretaceous

a theoretical laboratory and field study of ice coupled waves field and laboratory data of ice coupled waves propagating beneath shore fast ice are presented and compared with a theoretical model for sea ice which allows temperature variation although the laboratory data set is essentially qualitative because of scaling problems the measured wave characteristics compare favorably with those seen in the field the theory fits the field data set well both for the dispersive behavior and for the energy decay as the ice coupled waves penetrate the ice sheet moreover a maximum displacement suggested by theory is seen in field data for the first time it is proposed that this peak may be associated with the breakup of shore fast ice in exposed regions the theoretical treatment presented assumes sea ice to be thermorheologically simple an untested hypothesis which will require later experimental verification however the theory is quite general and can be used to solve a variety of sea ice mechanical problems where creep is considered important but where the loads and displacements are small enough that linearity prevails

age and spreading history of the cayman trough as determined from depth heat flow and magnetic anomalies the opening of the cayman trough records aspects of relative motion between the north american and caribbean plates essential to accurate reconstruction of caribbean evolution assessment of opening rates based upon basement subsidence analysis as well as magnetic anomaly interpretation shows that the trough may have opened at one third to one half previously proposed rates depths to basement show that the crust has subsided away from the present axis in two stages a younger stage characterized by substantial and asymmetric greater to the east subsidence preceded by an older stage of slight increases of depth with distance depth to age relations calculated from subsidence curves which accommodate lateral as well as vertical cooling of the accreting slab indicate that the trough has opened at an overall rate of less than 15 mm yr since 25 30 ma and at 20 30 mm yr prior to 30 ma marine magnetic anomalies show lineations perpendicular to the trough including a distinct central anomaly but the pattern is neither well defined nor symmetrical about the present axis however partial anomaly sequences can be identified on both sides of the axis and the distribution of these sequences indicates that spreading may have involved frequent short ridge jumps mostly eastward the overall rate of opening as indicated by this interpretation of the magnetics is about 15 and 30 mm yr since and prior to 26 ma both subsidence and magnetics indicate that the trough opened by at least 45 50 ma

identification and tectonic implications of a tear in the south american plate at the southern end of the lesser antilles 1 in the southeast corner of the caribbean westward subduction of atlantic oceanic south america beneath the lesser antilles transitions to east west transform motion between continental south america and the caribbean plate this geometry requires negatively buoyant subducting oceanic south american lithosphere to progressively detach from positively buoyant continental south american lithosphere the most widely accepted model is slab break off with oblique arc continent collision and northwest dipping continental subduction precipitating narrow rifting in the subducting slab in contrast the subduction transform edge propagator step model conceptualizes progressive detachment along a vertical dip slip tear through the lithosphere with stress focused at the edge of the propagating transform boundary we present four types of seismic data to resolve the ongoing lithospheric detachment local seismicity receiver functions wide angle seismic velocity inversion and a regional balanced cross section constrained by petroleum industry data these four data sets image a near vertical tear extending through the entire lithosphere revealing a key mechanism for the structural evolution of venezuela

observations of wind generated waves in antarctic fast ice abstract observations of naturally induced propagating waves in the fast ice cover of mcmurdo sound antarctica were made during the austral winter of 1986 with a rosette of three wire strainmeters the data collected over a seven month period and including a wide range of ice conditions and wind velocities constitute a winter wave climatology and show for the first time the nature and magnitude of flexural gravity waves induced by the direct action of turbulent air flow over the ice surface it is observed that below a threshold wind velocity corresponding roughly to the minimum group velocity for wave propagation in the ice 10 m s 1 no resolveable waves are produced at wind speeds above the threshold wave energy increases significantly with increasing wind velocity analyses of the wave energy density spectra show the observed wave characteristics to be in general agreement with existing theory although considerable scatter in the data prevents detailed comparisons the absolute magnitudes of

ice coupled wave propagation across an abrupt change in ice rigidity density or thickness the model of fox and squire 1990 1991 1994 which discusses the oblique propagation of surface gravity waves from the open sea into an ice sheet of constant thickness and properties is augmented to include propagation across an abrupt transition of properties within a continuous ice sheet or across two dissimilar ice sheets that abut one another but are free to move independently rigidity thickness and or density may change across the transition allowing for example the modeling of ice coupled waves into across and out of refrozen leads and polynyas across cracks and through coherent pressure ridges reflection and transmission behavior is reported for various changes in properties under both types of transition conditions

the caribbean plate pulled pushed or dragged 1 mechanical coupling between the lithosphere and the asthenosphere remains a controversial topic in the geosciences beneath the caribbean plate shear wave splitting measurements indicate ew strain in the asthenosphere which can be interpreted as mantle flow driving or resisting motion of the overlying lithosphere here we constrain the average shear traction on the base of the caribbean plate by balancing all torques these torques result from body forces that act on the caribbean slab pull ridge push lateral density variations from plate boundary friction and from basal shear tractions we obtain a range of physically realistic torque solutions which we examine further by computing the corresponding stresses and rotations within the caribbean plate for comparison with observations the deformation field for the caribbean is particularly sensitive to the amount of friction on intraplate faults representative models have a good fit with observations and are characterized by 1 a near zero basal shear traction 0 3 mpa 2 lithosphere averaged plate boundary friction 10 mpa 3 local forces due to indenters and trench pull 4 a net pull by the caribbean slab and 5 intraplate fault shear stresses on the order of tens of megapascals we conclude that the mechanical coupling of the caribbean plate to the underlying asthenosphere is small

transient quaternary erosion and tectonic inversion of the northern range trinidad abstract cosmogenic 10be measured in sediments from northern range catchments on the island of trinidad reveals low millennial scale rates of erosion average 40 mm ka that increase roughly eightfold from 11 to 92 mm ka eastward across the mountain range these results in conjunction with an analysis of mountain morphometrics are consistent with quaternary east side up tilting of the northern range which has occurred recently 100 ka the highest millennial scale erosion rates coincide spatially with quaternary east side surface uplift albeit not in magnitude high modern rainfall rates low topographic relief and convex longitudinal stream profiles indicating transient quaternary erosion we interpret that hillslope adjustment and erosion of the northern range is buffered from climatic and tectonic forcings by dense tropical vegetation cover thick saprolite and porous regolith compared with longer term thermochronology exhumation rates we document that an order of magnitude deceleration of and reversal in the locus of erosion and exhumation has occurred during the pliocene to the holocene we suggest that these combined data sets highlight distinct syn and post collisional phases of northern range development that are related to a major change in relative caribbean south american plate motion from oblique convergence to transform motion oblique collision during the mid miocene likely caused relatively higher rates of and asymmetric exhumation of the northern range post collisional mountain scale tilting is likely caused by a combination of crustal extension in the gulf of paria and by deep subsurface lithospheric detachment processes that drives dynamic topography

reflection and transmission characteristics at the edge of shore fast sea ice the reflection and transmission of ocean waves at a sea ice boundary is reconsidered the sea ice is modelled as a continuous thin elastic plate of uniform thickness floating on water of arbitrary constant depth unlike earlier solutions matching of potentials between the free surface domain and the ice covered domain is done at all depths previous solutions were incompletely matched as the potentials in each domain were deficient in the present solution a number of the infinity of evanescent modes which were hitherto ignored are included in the solution and allow matching to be carried out by minimization of an integrated error term from surface to seafloor reflection and transmission are found to be markedly influenced by the inclusion of these modes suggesting that conclusions based on the incomplete potentials may be substantially in error

the attenuation rates of ocean waves in the marginal ice zone during field operations in the greenland and bering seas in 1978 1979 and 1983 a number of experiments were carried out in which wave energy was measured along a line of stations running from the open sea deep into an icefield wave buoys in the water and accelerometer packages on floes were the instruments employed with airborne vertical photography to supply information on floe size distribution it was found that the decay of waves is exponential with a decay coefficient which generally increases with frequency except for a roll over at the highest frequencies the observations can be fitted reasonably well to a theory of one dimensional scattering

negligible convergence and lithospheric tearing along the caribbean south american plate boundary at 64 w 1 prior studies of the caribbean south american plate boundary have suffered from poor constraint on the structure of the crust and uppermost mantle we use a recent wide angle velocity model from the broadband ocean land investigation of venezuela and the antilles arc region project to constrain new seismic reflection data and previously published line drawing interpretations of the caribbean south american plate boundary at 64 w though commonly characterized as obliquely convergent we determine that convergence is negligible in our study area previous estimates of miocene to present north south shortening onshore eastern venezuela have commonly been 115 km or higher but we constrain shortening to 35 km onshore with an additional 30 km offshore with such minor convergence we conclude that uplift and basin subsidence in eastern venezuela does not derive from typical collisional orogeny instead the largely vertical tectonics likely result from mantle dynamics associated with an eastward propagating near vertical tear in the lithosphere along the former passive margin

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

spectral modeling of ice induced wave decay abstractthree dissipative two viscoelastic and one viscous ice models are implemented in the spectral wave model wavewatch iii to estimate the ice induced wave attenuation rate these models are

ocean excited plate waves in the ross and pine island glacier ice shelves ice shelves play an important role in buttressing land ice from reaching the sea thus restraining the rate of grounded ice loss long period gravity wave impacts excite vibrations in ice shelves that can expand pre existing fractures and trigger iceberg calving to investigate the spatial amplitude variability and propagation characteristics of these vibrations a 34 station broadband seismic array was deployed on the ross ice shelf ris from november 2014 to november 2016 two types of ice shelf plate waves were identified with beamforming flexural gravity waves and extensional lamb waves below 20 mhz flexural gravity waves dominate coherent signals across the array and propagate landward from the ice front at close to shallow water gravity wave speeds 70 m s 1 in the 20 100 mhz band extensional lamb waves dominate and propagate at phase speeds 3 km s 1 flexural gravity and extensional lamb waves were also observed by a 5 station broadband seismic array deployed on the pine island glacier pig ice shelf from january 2012 to december 2013 with flexural wave energy also detected at the pig in the 20 100 mhz band considering the ubiquitous presence of storm activity in the southern ocean and the similar observations at both the ris and the pig ice shelves it is likely that most if not all west antarctic ice shelves are subjected to similar gravity wave excitation

lithosphere tearing at step faults response to edges of subduction zones abstract slab edges are a relatively common feature in plate tectonics two prominent examples are the northern end of the tonga subduction zone and the southern end of the new hebrides subduction zone near such horizontal terminations of subduction trenches ongoing tearing of oceanic lithosphere is a geometric consequence we refer to such kinks in the plate boundary as a subduction transform edge propagator or step other steps are the north and south ends of the lesser antilles trench the north end of the south sandwich trench the south end of the vrancea trench and both ends of the calabria trench volcanism near steps is distinct from typical arc volcanism in some cases slab edges appear to coincide with mantle plumes using 3d mechanical models we establish that step faults are stable plate tectonic features in most circumstances in the probably rare cases that the resistance to fault propagation is high slab break off will occur relative motion along the transform segment of the plate boundary often is non uniform and the step is not a transform plate boundary in the rigid plate tectonics sense of the phrase step propagation may result in substantial deformation rotation topography and sedimentary basins with a very specific time space evolution surface velocities are substantially affected by nearby steps

neogene quaternary tectonic evolution of the leeward antilles islands aruba bonaire curacao from fault kinematic analysis aruba bonaire and curacao are islands aligned along the crest of a 200 km long segment of the east west trending leeward antilles ridge within the broad caribbean south america plate boundary zone presently characterized by east west right lateral strike slip motion the crust of the leeward antilles ridge represents the western segment of the cretaceous early cenozoic great arc of the caribbean which obliquely collided with the continental margin of northern south america in early cenozoic time following the collision the ridge was affected by folding and was segmented by oblique northwest striking normal faults that have produced steep sided northwest trending elongate islands and narrow shelves separated by deepwater sediment filled and fault controlled basins in this paper we present the first fault slip observations on the neogene carbonate rocks that cover large areas of all three islands our main objective is to quantify the timing and nature of neogene to quaternary phases of faulting and folding that have affected the structure and topography of this area including offshore sedimentary basins that are being explored for their petroleum potential these data constrain three fault phases that have affected aruba bonaire and curacao and likely the adjacent offshore areas 1 nw se directed late paleogene compression 2 middle miocene syndepositional nnw sse to nne ssw extension that produced deep rift basins transverse to the east west trending leeward antilles ridge and 3 pliocene quaternary nne trending compression that produced nw se trending anticlines present on aruba curacao and bonaire islands our new observations that include detailed relationships between striated fault planes paleostress tensors and bedding planes show that prominent bedding dips of neogene limestone on aruba bonaire and curacao were produced by regional tectonic shortening across the entire leeward antilles ridge rather than by localized syndepositional effects as proposed by previous workers we interpret pliocene quaternary nne directed shortening effects on the leeward antilles ridge as the result of northeastward extrusion or tectonic escape of continental areas of western venezuela combined with southeastward shallow subduction of the caribbean plate beneath the ridge

a method for restoring sedimentary sequence original structural profiles a case study of miocene strata from the northern continental slope of the south china sea abstract most sequence stratigraphy models have been derived from the observation of seismic profiles which still retain the original form of the sedimentary stage for folded and fractured formations there is no way to establish a full view of the basin wide continuous stratal configuration this paper presents an approach to restore the sedimentary sequence original structural profile through a process of segmented line drawing of seismic events reassembly and modification we use a case study from the pearl river mouth basin in the south china sea to demonstrate the principle and method of this technology after restoration we obtained a basin wide sedimentary stage form and continuous sequence stratigraphic structure profiles and thus further investigated the sedimentary landforms palaeo water depth sedimentary systems stacking pattern stratal configuration parasequence sets and systems tract which may be ambiguous in conventional seismic profiles this approach may be applied to the study of basin and sequence stratigraphy and basin sedimentary process

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

effect of shot interval on ocean bottom seismograph and hydrophone data data collected by 18 ocean bottom receivers for a seismic line shot at both 50 m 24 s shot interval and 125 m 58 s shot interval shot spacing provide a direct field comparison of the effect of shot interval on marine wide angle seismic data our results indicate that both shot spacings produce high quality refraction data in shallow water 1000 m on hydrophone and vertical channel data in deeper water the data quality of the 50 m line is adequate for the vertical channel but it is often poor at large offsets for the hydrophone channel in comparison to the 125 m shot spacing data a theoretical model to explain these observations provides further information useful for designing an experiment using ocean bottom receivers

transpression displacement partitioning and exhumation in the eastern caribbean south american plate boundary zone the caribbean south american plate boundary zone in northeastern venezuela is a transpressive orogenic belt consisting from north to south of a nascent subduction zone south caribbean deformed belt a volcanic arc leeward antilles arc a hinterland with high pressure p low temperature t metamorphic rocks cordillera de la costa belt and a southern nonmetamorphic foreland fold and thrust belt serrania del interior the geometry style and orientation of mid cretaceous to tertiary synmetamorphic deformation structures d1 in the hinterland are compatible with formation in a right oblique subduction or collision zone in which displacement partitioning has occurred late oligocene to recent right oblique convergence resulted in the emplacement of the arc and hinterland on the passive south american margin and the formation of the foreland fold and thrust belt d2 the displacements between the caribbean and south american plates are partitioned as well both d1 and d2 deformations are diachronous they are older in the west and younger in the east and related to the eastward passage of the caribbean plate with respect to south america the ascent decompression and exhumation of the high p low t metamorphic rocks occurred in two stages the first in the cretaceous by arc parallel extension d1 and the second in neogene time by thrusting d2 and subsequent erosion

a mechanism for the formation of ice edge bands we propose a mechanism for the formation of multiple bands at an ice edge that is subjected to an off ice wind the wind moves the outermost floes seaward faster than the interior ice dilating the marginal ice zone and opening up random polynyas in the largest of the polynyas the off ice wind generates a significant fetch limited wave spectrum which produces a wave radiation pressure on floes at the downwind end of the polynya increasing their velocity they are herded against their downwind neighbors creating a band of high ice concentration the band is stabilized by an internal compacting stress due to the exponential decay of the wind wave spectrum across the band and to a slower decay of swell incident from seaward the band s growth terminates when it encounters a second downwind polynya of sufficiently large size we show how an array of immature bands generated in this way matures by selective mergers into larger regularly spaced bands of approximately equal width

floe pair interaction event rates in the marginal ice zone data from an accelerometer package are used to estimate the rates at which interaction events occur between adjacent ice floes in the marginal ice zone observations suggest that the events are principally the result of a modified collision process where brash ice partially filling the gap between two floes acts as an intermediary in the transfer of momentum from one to the other a model is presented from which the rate at which these interactions occur can be predicted comparison of the rates estimated from the data with those predicted by the model shows there to be some agreement although there is considerable room for improvement the predicted event rate depends strongly on the ratio of the root mean square wave height to the amount of open water between pairs of floes the form of this dependence offers an explanation for grouping of events a feature that may be readily observed in many extracts of data

late cretaceous miocene diachronous onset of back thrusting along the south caribbean deformed belt and its importance for understanding processes of arc collision and crustal growth 1 the venezuelan basin is obliquely subducted to the east southeast beneath the continental south american plate at the east west trending south caribbean deformed belt scdb a 50 100 km wide wedge of accreted sedimentary rocks derived both from offscraping of the 0 5 to 1 km thick sedimentary cover of the subducting venezuelan basin and from incorporation of clastic sediments derived from the continental margin of south america in this paper we describe the structure and sequence stratigraphy of five late cretaceous to recent sedimentary sequences overlying a 230 000 km2 area of the southern venezuelan basin and beata ridge the data set includes 5900 km of 2d seismic reflection data that are tied to lithologic and age data taken from dsdp sites 146 149 and 150 in the central part of the venezuelan basin we use observed patterns of wedging in mapped and dated sedimentary sequences adjacent to the scdb to determine the age of the diachronous west to east initiation of thrusting and subduction at the scdb the onset of subduction in the scdb corresponds with the cessation of thrusting from west to east along the zone of collision between the intraoceanic caribbean arc and the south american continent this correlation in the age of deformation indicates that collision between the caribbean arc and the south american continent led to subduction polarity reversal a process that is commonly observed at various stages of tectonic evolution in other areas of active arc continent collision including japan and sunda

geological integration and evaluation of northern monagas eastern venezuelan basin abstract over the last 60 years the eastern venezuelan basin has been heavily explored with giant oil accumulations being developed in such areas as oficina anaco quiriquire and the orinoco oil belt in the states of anzoategui and monagas since 1986 interdisciplinary studies and seismic interpretations have led to discovery of a trend of deep structures on the northern flank of the basin some of them beneath old early tertiary producing fields the first exploratory well ful 1 tested 12 000 bopd of 28 api crude the new reservoirs consist of massive sandstones of the eocene caratas formation and oligocene naricual and los jabillos formations between depths of 13 000 and 17 000 feet the main structures placed in a thrust fold belt as a series of anticlines extending over 100 km across northern monagas currently four wells are being drilled and six more exploratory wells are planned to be drilled in the new future to date over 1900 mmbbls of reserves have been discovered and possible reserves for the area are close to 8 6 billion bbls

tectonic evolution of the caribbean the development of the caribbean is discussed in terms of modern tectonic theory the nature of the site on which the caribbean formed is examined and the development of the rifted margins of the caribbean is described constraints on caribbean evolution from the relative motions of north and south america are briefly examined and the caribbean oceanic plateau is discussed the great island arc system of the caribbean is addressed in detail emphasizing the way the great arc of the caribbean was segmented the role of central america in caribbean history is briefly considered

the natural oscillations of an ice covered channel les modes d oscillations naturelles d un chenal infiniment long recouvert de glace sont envisages en utilisant l approximation hydrostatique et en supposant que la glace ait un comportement elastique la relation de dispersion le mode de forme et les forces associees sur les bords du chenal sont obtenus pour les trois modes inferieurs une attention particuliere est portee aux limitations dues aux approximations hydrostatiques et elastiques

geophysical and oceanographic information in the marginal ice zone from ocean wave measurements consequent to recent experiments in marginal ice zones miz advances in remote sensing and theoretical progress studies have been published which invoke wave ice interactions to explain or predict certain geophysical phenomena while admirable and desirable in principle these works are based on a refutable surface boundary condition intended for a sheet or plate of solid ice rather than a medium made up of individual ice floes a case is argued for prudence as derived results may originate entirely from the application of an inappropriate miz model

formation of the maturin foreland basin eastern venezuela thrust sheet loading or subduction dynamic topography 1 the maturin basin in eastern venezuela is considered a good example of a peripheral foreland basin earthquake and tomographic data indicate that eastern venezuela is affected by the oblique subduction of the south american plate underneath the caribbean plate new forward flexural isostatic modeling of eastern venezuela has been carried out in order to determine whether the maturin basin was generated purely by thrust sheet loading from the serrania and monagas foreland thrust belts a sequence of forward models from middle miocene to present was generated for 3 profiles across the serrania del interior thrust belt the monagas foreland thrust belt and the maturin foreland basin the predictions of these models are constrained using seismic reflection and well data the flexural isostatic modeling shows that thrust sheet loading associated with the serrania del interior and monagas foreland thrust belts is insufficient to generate the observed subsidence within the maturin basin dynamic fluid flow modeling of subduction related dynamic topography of eastern venezuela has been used to investigate the influence of south american plate subduction on the generation of the accommodation space observed in the maturin basin fluid flow modeling of subduction related dynamic topography suggests that the subduction of the south american lithospheric mantle caused downward deflection of the south american crust affecting the maturin basin and the serrania thrust belt this modeling suggests that the maturin basin subsidence has two components 55 related to thrust sheet loading and 45 driven by continental subduction

wave effects on ocean ice interaction in the marginal ice zone the effects of wave train on ice ocean interaction in the marginal ice zone are studied through numerical modeling a coupled two dimensional ice ocean model has been developed to include wave effects and wind stress for the predictions of ice edge dynamics the sea ice model is coupled to the reduced gravity ocean model through interfacial stresses the main dynamic balance in the ice momentum is between water ice stress wind stress and wave radiation stresses by considering the exchange of momentum between waves and ice pack through radiation stress for decaying waves a parametric study of the effects of wave stress and wind stress on ice edge dynamics has been performed the numerical results show significant effects from wave action the ice edge is sharper and ice edge meanders form in the marginal ice zone owing to forcing by wave action and refraction of swell system after a couple of days upwelling at the ice edge and eddy formation can be enhanced by the nonlinear effects of wave action wave action sharpens the ice edge and can produce ice meandering which enhances local ekman pumping and pycnocline anomalies the resulting ice concentration pycnocline changes and flow velocity field are shown to be consistent with previous observations

a plate kinematic framework for models of caribbean evolution abstract we define the former relative positions and motions of the plates whose motions have controlled the geological evolution of the caribbean region newly determined poles of rotation defining the approximate spreading histories of the central north and the south atlantic oceans are given for the late jurassic early cretaceous anomaly sequence of the central north atlantic we have used previously published definitions of fracture zone traces and magnetic anomaly picks redetermining the pole positions and angular rotations for various isochrons on an evans and sutherland interactive graphics system for magnetic anomalies younger than the cretaceous quiet period in both oceans we 1 used seasat altimeter data to help define fracture zone traces and 2 identified and used marine magnetic anomalies to determine the positions of spreading isochrons along the flowlines indicated by the fracture zones by the finite difference method the relative paleopositions and the relative motion history between north and south america were computed this analysis defines the size and shape and the rate at which the size and shape changed of the interplate region between north and south america since the middle jurassic thus a plate kinematic framework is provided for the larger plates pertaining to the caribbean region in which can be derived more detailed scenarios for gulf of mexico and caribbean evolution north and south america diverged to approximately their present relative positions from late triassic to early campanian about 84 m y ago time this is the period during which the gulf of mexico and a proto caribbean seaway were formed since the campanian only minor relative motion has occurred from early campanian through to middle eocene times south america diverged only another 200 km and since the middle eocene minor n s convergence has occurred these very minor post early campanian motions have probably been accommodated by imperfect shear and compression along the atlantic fracture zones to the east of the lesser antilles and along the northern and southern borders of the caribbean plate accordingly it is suggested that from campanian time to the present the relative motions between the north and south american plates have had only minor effects on the structural development of the caribbean region primarily using the data of engebretson et al the convergence history of pacific plates with north america was calculated for two points near the western caribbean by completing finite difference solutions the convergence history of the pacific plates with the caribbean and south american plates can be approximated the direction and rate of convergence of the pacific plates with the americas may have controlled the style of subduction and possible microplate migration along the north american south american and western caribbean boundaries that define the eastern pacific plate margin

observations of flexural waves on the erebus ice tongue mcmurdo sound antarctica and nearby sea ice abstract new strain data relating to lexural oscillations of the erebus glacier tongue egt mcmurdo sound antarctica are presented and are analysed in the frequency domain the data were collected during november 1989 just 3 months prior to the most recent calving of the ice tongue which occurred in march 1990 a broad band oscillation centred on 50 s is found in both the strain measurements collected on the egt and those collected on the sea ice nearby the oscillation is shown at least in part to be propagating with a phase velocity of approximately 65 m s j in a direction away from the snout towards the grounding line rather than being wholly due to a standing wave pattern in the egt a coupling model between the sea ice and the egt is proposed and is shown to compare reasonably well with the data introduction the erebus glacier tongue egt flows from the western side of mount erebus into mcmurdo sound about 20 km north of new zealand s scott base and the nearby united states base mcmurdo fig 1 its close proximity to excellent logistical support has meant that the egt has been the focus of a number of experimental and theoretical studies to map its geometry and structure holdsworth 1982 and to understand its dynamics holdsworth 1969 1974 goodman and holdsworth 1978 ho1dsworth and ho1dsworth 1978 ho1dsworth and glynn 1981 vinogradov and holdsworth 1985 gui and squire 1989 the egt is known to have calved at least three times since the turn of the cen tury in march 1911 members of scott s british antarctic