lateral velocity resolution from three dimensional seismic refraction data summary a method for estimating the lateral velocity resolution from 3 d seismic refraction traveltimes is presented the method is non linear in that synthetic data calculated from checkerboard models are inverted using an iterative tomographic approach incorporating smoothness constraints and updated ray paths at each iteration two applications are presented 1 real data from the faeroe basin experiment using the ray coverage and noise level of approximately 50 000 picks and 2 ideal data from the same experiment corresponding to 100 per cent data recovery equivalent to about three times as many ray paths the refraction data constrain the sedimentary and basement structure to approximately 12 km depth a comparison of the results from the two data sets provides a qualitative check on the resolution method illustrates the pitfalls of using ray coverage as an indicator of resolution and demonstrates the potential resolution of currently feasible 3 d refraction experiments 32 checkerboard models consisting of alternating positive and negative velocity anomalies superimposed on the preferred final model of the basin were tested each with a diverent cell size position or orientation the problem of ray bending associated with the 5 per cent checkerboard anomalies is shown to be small using models with four cell sizes allowed for resolution estimates from 3 to 15 km this range being imposed by the frequency content of the data and the areal dimensions of the study area 44 26 km an operator centred on each model node measures the local semblance between the known and recovered checkerboard models a determination of the smallest well resolved cell size at each model node is then made using a 0 7 semblance threshold to provide a spatially dependent lateral resolution estimate resolution for the real data set varies from an average of just over 3 km at 2 km depth to 10 km at 10 km depth where there is a local maximum in ray coverage due to a concentration of ray bottoming points for the ideal data set the average lateral resolution drops ov roughly linearly from better than 3 km at 2 km depth to 6 3 km at 10 km depth the largest diverence between the resolution associated with the ideal and real data occurs at 7 2 km depth where it averages 4 6 km for the former and 10 8 km for the latter due to the near absence of ray bottoming points between 7 and 9 km the results show that the final model for the faeroe basin obtained by regularized inversion with smoothness constraints is consistent with the resolution estimates in the sense that the model does not contain structure with a wavelength smaller than the resolution estimate at any point the results for the ideal data show that good quality 3 d refraction experiments can provide a lateral resolution of no worse than the receiver spacing shot spacing for land data throughout most of the sampled volume

venezuela basin crustal structure velocity depth profiles derived from six two ship expanding spread experiments in combination with other geophysical data define the characteristics of two distinct types of venezuela basin crust and the boundary between them each two ship common midpoint reflection refraction profile is automatically transformed into the p plane picked and interpreted to provide v z functions with appropriate confidence bounds the results together with gravity magnetic and near vertical incidence reflection data reveal a 50 000 km2 triangle of venezuela basin crust which resembles normal oceanic crust in a magnetic quiet zone north and west of this triangle lies the previously defined thick caribbean crust having two distinct layers above the m discontinuity acoustic basement there appears unusually smooth due to extensive basaltic sills and flows which were cored at deep sea drilling project sites 146 149 sills and 150 flows also depths to mantle are greater than normal interpretations of near vertical and wide angle reflection data show that the extra crustal thickness is due not only to the emplacement of the flows but also to the crust below being somewhat thicker than normal the boundary between the two crustal areas has a ne sw trend which parallels the dominant structural and magnetic lineations this boundary coincides in position though not in trend with the previously defined central venezuela basin fault zone further study is required to determine whether this boundary is of tectonic origin or if it represents a change in style of crustal production

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

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

finite difference calculation of traveltimes in three dimensions the traveltimes of first arriving seismic rays through most velocity structures can be computed rapidly on a three dimensional numerical grid by finite difference extrapolation head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 11 percent are found between the results of this technique and ray tracing for a complex but smooth model this scheme has proven useful for earthquake location and shows promise as an inexpensive well behaved substitute for ray tracing in forward modeling and kirchhoff inversion applications

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

the major features of the crustal structure in north eastern venezuela from deep wide angle seismic observations and gravity modelling abstract venezuela is located on the plate boundary zone between the south american continent and the caribbean plate a relative movement of 2 cm year is accommodated by a system of strike slip faults running from the andes to the gulf of paria the interior range a moderate height mountain range separates the oriental basin from the caribbean to the south predominantly precambrian rocks are outcropping in the guayana shield south of the orinoco river results of deep wide angle seismic measurements for the region were obtained during field campaigns in 1998 ecoguay for the guayana shield and in 2001 ecco for the oriental basin the total crustal thickness decreases from 45 km beneath the guayana shield to 39 km at the orinoco river and 36 km close to el tigre in the center of the oriental basin the average crustal velocity decreases in the same sense from 6 5 to 5 95 km s detailed information was obtained on the velocity distribution within the oriental basin velocities are as low as 2 2 km s for the uppermost 2 km 4 5 km s down to 4 km in depth and a maximum depth of 13 km was derived for material with seismic velocities up to 5 9 km s interpreted as the base of the sedimentary basin a gravimetric model confirms the structures derived from the seismic data discrete increases in sedimentary thickness along the basin may be associated to extension processes during the passive margin phase in the cretaceous or during earlier extension phases

the relationship between seismic velocity and density in the continental crust a useful constraint summary plots of seismic velocity and density of rock samples show that a range of densities is possible for rocks of each seismic velocity and vice versa although a single linear relationship is often assumed in crustal gravity calculations because of the scatter whenever rocks of known seismic velocity are converted to density using this relationship a reduction is made to the resolving power of the resulting gravity calculation if these rocks reach thicknesses of more than a few kilometres then the uncertainties become significant when compared with the size of commonly observed gravity anomalies examples are considered from the north sea mississippi and carolina trough it is concluded that the use of a seismic velocity measurement as the only indication of rock density does not provide a useful constraint when attempting to reproduce observed gravity variations an appropriate model for isostatic compensation is probably the most important factor for successful predictions of crustal structure on the basis of gravity data

first 40ar 39ar age constraints for the caledonian evolution of the trans european suture zone in ne germany from three rugen boreholes g14 rugen 5 loissin 1 across the caledonian deformation front four lower palaeozoic slate samples and three detrital muscovite concentrates from associated sandstones have been analysed by 40 ar 39 ar analysis microprobe analysis of the detrital muscovites show homogeneous but distinct chemical compositions for each concentrate particulate organic matter reflectance and illite crystallinity indicate that all the analysed whole rock samples have experienced anchizonal metamorphic conditions 40 ar 39 ar analysis of the samples from the g14 and loissin 1 wells give almost identical results muscovite concentrates define plateau ages of c 830 ma and show an isotopic rejuvenation of low temperature increments around 440 ma whole rock slate samples give plateau ages of c 440 ma and c 427 ma isotopic rejuvenation in the low temperature increments prior to 350 ma in the loissin 1 sample can be attributed to the permo carbonifereous volcanism which is known from this borehole in contrast samples from the borehole rugen 5 show a different isotope pattern a muscovite concentrate defines a plateau age of 609 ma two whole rock slate samples give internally discordant but identical 40 ar 39 ar spectra with consistent isotopic rejuvenation from high to low temperature increments from 510 ma to 420 ma while for the g14 and loissin 1 sandstone samples a baltica derived provenance can be assumed greywackes of rugen 5 indicate a cadomian influenced perigondwanan detrital source according to the results of whole rock slate samples the main caledonian tectonothermal event in ne germany occurred around the ordovician silurian boundary or in the early silurian 450 425 ma from these data the rugen palaeozoic appears as an allochthonous caledonian nappe complex which has been thrust much further onto the baltic shield than previously thought

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

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

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

origin of cariaco basin southern caribbean sea abstract the cariaco basin an anoxic basin in the central eastern venezuelan continental borderland is located in the area of overlap of two en echelon right lateral strike slip fault zones the moron and el pilar fault zones on the basis of 28 acoustic reflection profiles across it the basin is interpreted as a pull apart basin due to east west tension in the area of the right step between the two fault zones 125 km of overlap and 35 km separation the offset along the fault zones is less than 100 km and at least 25 km its age is most probably quaternary as suggested by the thickness of the sedimentary fill and the estimated rate of deposition and because strike slip displacement along the master faults probably began no earlier than the late tertiary the cariaco basin is the largest pull apart basin along the bocono moron el pilar fault system which is a part of the caribbean south america plate boundary

gravity inversion using seismically derived crustal density models and genetic algorithms an application to the caribbean south american plate boundary summary crustal density models derived from seismic velocity models by means of velocity density conversions typically reproduce the main features of the observed gravity anomaly over the area but often show significant misfits given the uncertainty in the relationship between velocity and density seismically derived density models should be regarded as an initial estimate of the true subsurface density structure in this paper we present a method for estimating the adjustments necessary to a seismically derived density model to improve the fit to gravity data the method combines the genetic algorithm paradigm with linear inversion as a way to approach the non linear and linear aspects of the problem the models are divided into three layers representing the sedimentary column the crystalline crust and the lithospheric mantle the depths of these layers are determined from the seismic velocity model each of the layers is divided into a number of provinces and a density adjustment value is found for each province so that the residual gravity difference between the observed gravity anomaly and the anomaly calculated for the seismically derived model is minimized while keeping between predefined bounds the preferred position of the province boundaries is found through the artificial evolution of a population of solutions given the stochastic nature of the algorithm and the non uniqueness of the problem different realizations can yield different solutions by performing multiple realizations we can analyse a set of solutions by taking their mean and standard deviation providing not only an estimate of the distribution in the subsurface but also an estimate of the associated uncertainty synthetic tests prove the ability of the algorithm to accurately recover the location of province boundaries and the values for a known model when using noise free synthetic data when noise is added to the data the algorithm broadly recovers the features that define the known model despite greater standard deviations of the solutions and the occurrence of artefacts in the mean solutions the algorithm was applied to four profiles across the caribbean south america plate boundary some general patterns in the distribution of were observed consistently in the profiles and are correlated with the interpretations of the velocity models positive values in the sedimentary layer negative values for island arc and extended island arc crust with an abrupt change to positive values in south american crust and positive values in the mantle under the continent and island arc with a transition to negative values under the caribbean oceanic crust

70 m y history 139 69 ma for the caribbean large igneous province it is commonly accepted that large igneous provinces are formed through catastrophic volcanic events occurring over vast areas within a few million years at the initiation of hotspots mantle plumes new 40ar 39ar ages 111 139 ma and geochemical results from the nicoya peninsula costa rica extend the age range of volcanism in the caribbean large igneous province to 70 m y 69 139 ma our results are not consistent with the formation of this vast igneous province through a single plume head at the initiation of a mantle plume such as the galapagos instead we propose that multiple oceanic intraplate igneous structures such as plateaus and hotspot tracks were accumulated through the subduction process the igneous structures could be remnants of the earlier history of the galapagos hotspot making it one of the oldest active hotspots on earth alternatively they could have been derived from several spatially distinct mantle melting events that sampled similar source material e g oceanic lithosphere of similar age

seismic structure of the crust and upper mantle in the peace river arch region canada four intersecting 300 km long reversed refraction lines within northwestern alberta and northeastern british columbia have been interpreted for crustal and upper mantle seismic velocity structure in the peace river arch pra region of the western canada sedimentary basin the data have been modeled using a two dimensional ray trace forward modeling approach based on asymptotic ray theory to match travel times and amplitudes of first and coherent later arrivals in addition an inversion of first arrival travel times along a fan shot profile has been performed to constrain crustal thickness northwest of the arch in a region not sampled by the reversed profiles the major features of the structural models are 1 weak to moderate lateral variations in crustal structure with no evidence of significant layering or thick low velocity zones within the crust 2 an average sub basement rms crustal velocity of 6 6 km s average upper mantle velocity of 8 25 km s and average crustal thickness of 40 km 3 regional variations in structure which appear related to the dominant n s trending cratonic structure including crustal thickness upper crustal and upper mantle velocities and pmp character and 4 subtle variations in structure that may be associated with the e w trending devonian axis of the pra including a shallowing of high lower crustal velocities thickening of the crust and an anisotropic pmp character beneath the arch and a weak trend in the dip of intracrustal reflectors away from the center of the pra region evidence from the refraction and other geophysical data for the presence of a local crustal expression of the pra is weak but suggests a thermal as opposed to flexural origin for its anomalous vertical movements

crustal structure of the caribbean northeastern south america arc continent collision zone 1 we present the results of a 568 km long regional wide angle seismic profile conducted in the southeastern caribbean that crosses an active island arc a remnant arc two basins possibly floored by oceanic crust an allochthonous terrane of forearc affinity and the passive margin of northern south america the velocity structures of the late cretaceous aves ridge remnant arc and miocene and younger lesser antilles arc are remarkably similar which implies that magmatic processes have remained moderately steady over time crustal thickness is 26 km at the aves ridge and 24 km at the lesser antilles arc in comparison to the izu bonin and aleutian arcs the lesser antilles arc is thinner and has no evidence for a lower crustal cumulate layer which is consistent with the estimated low magma production rates of the lesser antilles arc crustal thickness beneath the grenada and tobago basins is 4 10 km and the velocity structure suggests that these basins could be floored by oceanic crust a decrease of 1 km s in average seismic velocity of the upper crust is observed from nw to se across the north coast fault zone we argue that this marks the suture between the far traveled caribbean arc and the passive margin of the south american continent current strike slip motion between the caribbean and south american plates is located 30 km to the south and thus material originally deposited on the south american passive margin has now been transferred to the caribbean plate

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

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

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

high resolution mapping of bora winds in the northern adriatic sea using synthetic aperture radar 1 the adriatic sea is regularly subjected to strong bora wind events from the northeast during winter the events have a strong effect on the oceanography in the adriatic driving basin scale gyres that determine the transport of biogeochemical material and extracting large amounts of heat the bora is known to have multiple surface wind jets linked to the surrounding orography and have been the focus of many studies but it has not been possible to describe the detailed spatial structure of these jets by in situ observations using high resolution spaceborne radarsat 1 synthetic aperture radar sar images collected during an active bora period 23 january 16 february 2003 we created a series of high resolution 300 m maps of the wind field the obtained winds show reasonable agreement with several in situ wind observations with an rms wind speed error of 3 6 m s slightly higher than the 2 3 m s errors reported in previous studies these sar images reveal the spatial structure of the bora wind in unprecedented detail showing several new features in the senj region of croatia several images show rhythmic structure with wavelengths of 2 3 km that may reflect bora pulsation seen at fixed locations by previous investigators along the italian coast several images show a wide 20 30 km band of northwesterly winds that abruptly change to the northeasterly bora winds further offshore meteorological model results suggest that these northwesterly winds are consistent with those of a barrier jet forming along the italian apennine mountain chain

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

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

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

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

controls on the geometry of fold thrust belt salients salients convex to the foreland curves in the map traces of orogens typically involve fold thrust belts formed along the foreland margin of orogens an examination of the map view shape degree of asymmetry dimensions and structural trend line patterns i e the variation in fold trend and fault strike along the length of the belt of 20 fold thrust belt salients worldwide illustrates relationships between salient geometry and the geologic setting in which the salient formed in order to understand these relationships we used simple sandbox models to simulate the formation of three of the several types of salients specifically we examined indenter controlled salients the geometry of which reflects interaction of a fold thrust belt with a hinterland indenter basin controlled salients the geometry of which reflects lateral variations in the thickness of predeformational basin fill and detachment controlled salients the geometry of which reflects lateral variations in the strength of a detachment surface our models demonstrate that thrusts in these three types of salients initiate with a curved shape and that rocks within thrust sheets on the limbs of salients can undergo a vertical axis rotation in response to simple shear without requiring major rotation of the fault trace furthermore trend line patterns distinguish basin controlled from indenter controlled salients specifically structural trend lines in basin controlled salients converge at the end points of the curve while in indenter controlled salients they diverge at the end points of the curve the asymmetry of a salient correlates with the asymmetry of the predeformational basin and or of the indenter and with the convergence direction thus our results explain geometric characteristics of individual salients and address controversies concerning paleomagnetic data in salients they also may help prioritize petroleum exploration targets in salients foreland basin oil fields concentrate at the apex of basin controlled salients because the geologic factors that generate the salient also contribute to localizing oil fields