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

a low velocity zone within the layer 3 region of 118 myr old oceanic crust in the western north atlantic summary during the north atlantic transect nat experiment 11 expanding spread profiles esps were acquired using a 3300 cubic inch 30 element tuned airgun array s v prospekta and a 48 channel seismic streamer r v moore extremely high density seismograms were obtained that have allowed us to transform the observed data to the domain of intercept time and ray parameter tau p with high precision velocity structures have been obtained by tau sum inversion ray tracing and synthetic wkbj and reflectivity seismogram methods both in tau p and x t domains a detailed analysis of esp 5 suggests the presence of a low velocity zone within the lower oceanic crust at the location of magnetic anomaly mo 118 ma it is apparently 2 86km thick has a p wave velocity of 6 5 km s 1 s wave velocity of 3 6 km s 1 and occupies much of the depth interval normally associated with oceanic layer 3 converted shear wave arrivals have been used to obtain the shear velocity structure we find that the vp vs ratio in the uppermost part of the crust is about 1 9 and much of the crustal section has a vp vsratio greater than 3 attenuation of p and s waves has been studied by amplitude analysis and we find that in most of the lower crust qs is equal to qp comparison with solutions from esps in younger and older parts of the basin suggests that esp 5 is unique in displaying a low velocity zone despite this the total thickness at esp 5 is not significantly different than at other locations we suggest that this provides strong evidence that the low velocity zone occurs within the lower crust and does not result from serpentinization of the upper mantle beneath thin oceanic crust

cenozoic collision of the lesser antilles arc and continental south america and the origin of the el pilar fault it is proposed that the cenozoic tectonic record of the southern lesser antilles arc and northeastern continental south america can be explained by ongoing right oblique collision between the arc and continent the collision has proceeded by the transport of the leading edge of the arc across the slope and outer shelf of a former north facing passive margin of the south american continent the overriding began in the study region near the gulf of cariaco in eastern venezuela in late eocene or oligocene time and has migrated with a generally se vector suturing has occurred between the arc and continent after the attainment of a critical distance of overlap today s point of suturing lies in the paria peninsula east of there overriding continues major tectonic elements engaged in or created by the collision are the southern lesser antilles magmatic arc forearc basin the araya tobago terrane a south american foreland thrust and fold belt and a foreland basin the araya tobago terrane is thought to consist of sediments of south american provenance that were accreted to the lesser antilles forearc during its transit of an ocean basin and the continental slope and outer shelf the emplacement of the magmatic arc and the araya tobago terrane caused tectonic imbrication of shelf strata to propagate ahead of the arc front as a foreland thrust and fold belt tectonic loading of the shelf also caused subsidence of a major foreland basin on the continentward side of the thrust belt it is proposed the el pilar fault exists between the gulf of cariaco and the paria peninsula as an active right slip fault but not east of paria it is not a throughgoing transform fault between the south american and caribbean plates the el pilar fault exists where the overlapping arc and the continent are sutured and takes up a suture parallel component of convergence between arc and continent the eastern tip of the fault propagates east with the point of suturing reconstructions of the cenozoic collision of the lesser antilles arc and the south american continent suggest that the arc lay somewhat north and west of its present position in the eocene this conclusion differs from that of plate reconstructions that assume that the arc was the leading edge of a caribbean plate that has moved east from pacific longitudes since the eocene

crustal structure of the sheared southwestern barents sea continental margin abstract eighteen expanding spread profiles from the southwestern barents sea continental margin were studied with a variety of seismic interpretation techniques and in conjunction with seismic reflection data the velocity depth profiles from the slope intercept herglotz wiechert and sum methods were compared and contrasted to the velocity depth curve calculated from the iterative forward modelling of the amplitudes the velocity depth profiles derived from the sum and forward modelling of the amplitudes were used to produce synthetics in both the x t and p domains to choose the technique that most accurately represented the data the results showed the oceanic crust at the senja fracture zone to be different from standard oceanic crust with a higher than average velocity for layer 2 of 5 7 6 4 km s 1 and slightly smaller crystalline crustal thickness the continent ocean transition at crustal depths was shown to be narrow in the range of 10 20 km our results were consistent with two different crustal provinces east of the senja fracture zone the tertiary marginal basin and the cretaceous basin region the velocity depth curves enabled us in a gross sense to suggest the depths to deep sedimentary interfaces and the top of the crystalline crust that were normally not observed in the reflection profiles depths down to 20 km were inferred for the base of the sedimentary section in the southwestern barents sea the crust thinned gently towards the continent ocean boundary at the senja fracture zone whereas the crustal thickness changed abruptly below the hornsund fault zone at the margin farther north we associated the change in crustal structure with cenozoic in the thick crust beneath the svalbard platform in the north the depths to mantle determined on the shelf were consistent with that observed on a deep multichannel seismic line that crosses the area

comment on chemical remagnetization of the upper carboniferous lower triassic pyeongan supergroup in the jeongseon area korea fluid migration through the ogcheon fold belt by y h park s j doh and d suk summary a recent paper published in geophysical journal international reports that the late palaeozoic to early mesozoic sedimentary rocks in the northeastern okcheon belt of korea were remagnetized during the late cretaceous to early tertiary and argues that the remagnetization was triggered by fluids travelling more than 800 km from the subducting slab of the kula pacific plates based on available geological data however we suggest that the remagnetization was caused by thermal effects of a late cretaceous pluton intruding the sedimentary rocks and or associated short range hydrothermal fluids rather than by long range fluids

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

holocene climate in the atlantic sector of the southern ocean controlled by insolation or oceanic circulation abstractthe holocene climate of the southern ocean is not well un derstood mainly because of the lack of high resolution reconstruc tions of ocean surface properties here we present a 12 500 yr long decadal scale record of holocene sea surface temperatures and sea ice presence from the polar front of the east atlantic southernocean the record shows gradual climate change with no abruptneoglacial cooling and an unprecedented late holocene warming the dominant forcing factor appears to be precessional insolation northern hemisphere summer insolation correlates to at least theearly to middle holocene climate trend spectral analysis revealscentennial scale cyclic climate changes with periods of 1220 1070 400 and 150 yr the record shows good correlation to east ant arctic ice cores and to climate records from south georgia andbunger oasis however the record shows out of phase behaviorwith regard to climate records from the western antarctic penin sula and the peru chile current such behavior hints at a climaticdivide through patagonia the drake passage and between westand east antarctica keywords southern ocean paleoclimate diatoms modern analogtechnique holocene insolation antarctic polar front introductionthe holocene climate variability has received less attention thanthe more dramatic changes during the last glacial period the holoceneis generally assumed to be a stable interglacial period when climatechanges in the north atlantic are transmitted to the rest of the worldthrough a strong thermohaline circulation imbrie et al 1992 thelarge uctuations of the last glacial period appear to continue into thisinterglacial although in a subdued manner bond et al 2001 the southern ocean has received even less attention than the highnorthern latitudes oceanic records are few and coarse they generallydisplay a warm early holocene followed by cooling labracherie etal 1989 pichon et al 1992 hodell et al 2001 similar to northatlantic climate e g koc and jansen 1994 high resolution recordsare restricted to antarctic ice cores which mostly display little climatevariability during the holocene jouzel et al 2001 an exception isthe taylor dome deuterium record in which the holocene appears tobe split by an abrupt cooling event ca 5 5 5 ka steig et al 1998 this cooling appears to t the late holocene global cooling trend por ter 2000 as a nonlinear response from the climate system to insolationchanges demenocal et al 2000 hodell et al 2001 high resolution holocene records available for southern oceanclimate reconstruction are rarely from the antarctic circumpolar cur rent proper e g domack et al 2001 stenni et al 2001 lamy et al 2002 with the exception of a multiproxy approach to holocene cli mate that includes a high resolution description of lithic fragments hodell et al 2001 we present data from site tn057 17 near the polar front in theeast atlantic southern ocean it is based on quantitative estimates ofsummer sea surface temperature ssst and sea ice presence sip utilizing the down core variability of diatom assemblages the recordshows good correlation to antarctic ice cores but a gradual change inphase relationships during the early holocene the record shows anin phase relationship with the ross sea region ice cores during thelate holocene the record shows an in phase relationship with the eastantarctic region ice cores antarctic polar frontthe antarctic polar front apf fig 1 is an important climateboundary in terms of air sea uxes and the heat and salt budgets ofthe oceans at the surface the apf is a steep sea surface temperaturegradient while the subsurface expression is the southern limit for sub duction of cold fresh antarctic surface water below warmer and moresaline subantarctic surface water orsi et al 1995 globally the apfcould be the southern limit of synchrony with northern hemisphereclimate broecker 1996 the apf is subject to large spring and sum mer blooms of diatoms fischer et al 2002 and achieves the largestbiogenic sedimentation rates of the diatom ooze belt of the southernocean burckle and cirilli 1987 it is these high sedimentation ratesof diatomaceous oozes and the sensitivity of diatoms especially to lightand temperature that make sites in the apf region suitable for pro ducing high resolution records of climate change

surface wave tomography of north america and the caribbean using global and regional broad band networks phase velocity maps and limitations of ray theory summary we present phase velocity maps of fundamental mode rayleigh waves across the north american and caribbean plates our data set consists of 1846 waveforms from 172 events recorded at 91 broad band stations operating in north america we compute phase velocity maps in four narrow period bands between 50 and 150 s using a non linear waveform inversion method that solves for phase velocity perturbations relative to a reference earth model prem our results show a strong velocity contrast between high velocities beneath the stable north american craton and lower velocities in the tectonically active western margin in agreement with other regional and global surface wave tomography studies we perform detailed comparisons with global model results which display good agreement between phase velocity maps in the location and amplitude of the anomalies however forward modelling shows that regional maps are more accurate for predicting waveforms in addition at long periods the amplitude of the velocity anomalies imaged in our regional phase velocity maps is three time larger than in global phase velocity models this amplitude factor is necessary to explain the data accurately showing that regional models provide a better image of velocity structures synthetic tests show that the raypath coverage used in this study enables one to resolve velocity features of the order of 800 1000 km however only larger length scale features are observed in the phase velocity maps the limitation in resolution of our maps can be attributed to the wave propagation theory used in the inversion ray theory does not account for off great circle ray propagation effects such as ray bending or scattering for wavelengths less than 1000 km scattering effects are significant and may need to be considered

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

migration velocity analysis based on common shot depth migration applied to the seismic data of the caribbean oceanic plateau summary in areas where the geology is characterized by strong vertical and lateral velocity gradients pre stack depth migration will produce the optimum subsurface image an accurate velocity model is required for an accurate pre stack depth migration we developed a velocity analysis based on recursive pre stack depth migration that gives better results than the traditional cmp velocity analysis when applied to data over the caribbean oceanic plateau first the shot records are migrated with a series of constant velocities for each identified geological layer the velocity that best flattens the event in common receiver gathers crg is picked a large number of velocity analyses are run along the seismic line in order to define the reflector geometry for each layer we obtain the depth and interval velocity in order to make a model down to that depth this model is then used as the input for the next layer velocity analysis finally when all the layers have been picked the entire line is depth migrated with the derived model the migration algorithm used is a common shot kirchhoff technique which uses the ray tracing method of vidale 1988 to compute the traveltimes needed to determine the operator shape we applied this migration velocity analysis technique to seismic profiles of the casis cruise to define the deep structure of the caribbean oceanic plateau we imaged a 2500 m deep reflection sub b beneath the top of the plateau b horizon which defines the top of a 4500 4700 m s 1 layer the images obtained with this method will help to define odp sites that will provide information about geochemical and petrological variations of the volcanic basement

basin structure on the norwegian margin from analysis of digitally recorded sonobuoys during r v robert d conrad cruise 21 a large number of sonobuoys were recorded in the voring more and adjacent basins at the margin off norway the digital sonobuoy data have been transformed into the domain of intercept time tau and ray parameter p and inverted by the tau sum method to obtain better definition of the near seafloor velocities using also the precritical energy a move out correction has been applied to the tau p data conventional analyses of the x t data were also performed in general the various types of analyses complement each other and should be used interactively together with forward modeling to obtain the optimal velocity depth function the sonobuoy results have outlined in a regional sense the pre cenozoic sedimentary basin at the margin this basin attains a maximum sediment thickness of 8 10 km of which only the upper half is defined by seismic reflection profiling at the outer margin the marginal escarpments form major discontinuities in the lateral velocity distribution the survey has proven that sonobuoy profiles recorded during standard seismic reflection profiling in major marginal basins may provide important new information particularly about the deeper parts of the basins that is not directly available from the reflection profiles

crustal structure in the western somali basin as part of integrated marine geophysical studies in the western somali basin we performed 118 sonobuoy experiments to define better the crustal structure of the margins and basin created by the separation of madagascar and africa after using t2 x2 conventional slope intercept methods and slant stacked t p techniques to analyse the data we combined our solutions with all previous velocity information for the area velocity functions were derived for the sediment coiumn and we detected a high velocity 4 58 0 29 km s 1 sediment layer overlying acoustic basement we confirmed that the crust is indeed seismically oceanic and that it may be considered either in terms of a layered model layers 2b 5 42 0 19 km s 1 2c 6 23 0 22 km s 1 3 7 03 0 25 km s 1 and mantle 7 85 0 32 km s 1 were identified or a more complex gradient model in which layer 2 is marked by a steeper velocity gradient than underlying layer 3 integrated igneous crustal thicknesses 1 62 0 22 s 5 22 0 64 km are significantly less than what is considered normal we present a revised seismic transect across the east african margin as well as total sediment thickness depth to basement and crustal thickness maps

the northern caribbean plate boundary in the jamaica passage structure and seismic stratigraphy multibeam bathymetry data and multichannel seismic reflection profiles have been collected at the end of 2012 along the enriquillo plantain garden fault zone epgfz in the jamaica passage between jamaica and hispaniola analysis of the data set reveals the tectonic evolution and the stratigraphic complexity of the northern caribbean boundary stratigraphic correlations with previous marine and on land studies are proposed to place the identified seismic sequences in their regional tectonic history two distinct crustal domains are interpreted typical stratigraphic sequences for the rifted blocks of the eastern cayman trough margin are identified in five basins of the jamaica passage highlighting the eastward limit of the cayman trough margin these inherited basins are deformed and folded during a first phase of compression that could correspond to the regional tectonic rearrangement recorded in the early miocene about 20 ma a distinct crustal domain that we propose to relate to the carib beds caribbean typical reflectors a b and v is identified in the southern part of the jamaica passage indicating that the caribbean large igneous province could extend up to the extreme northeast part of the lower nicaragua rise the left lateral epgfz currently cuts across two pre existing basins the morant and matley basins during the activity of the epgfz these basins are deformed and folded indicating a second phase of compression in contrast the navassa basin located in the middle of the jamaica passage results from the strike slip motion of the epgfz and is interpreted as an asymmetrical basin bordered by the epgfz only on its northern side

sub b layering in the southern caribbean the aruba gap and venezuela basin abstract multichannel seismic data in the aruba gap region near joides dsdp site 153 verify the presence of a deep sub b reflection one multichannel seismic line trends ne sw on and along the edge of beata ridge and passes within 1 km of site 153 and another line runs n s across the entire aruba gap with the drill site 4 km east from its northern end closely spaced velocity analyses indicate the presence of deep primary reflection events and enable calculation of interval velocities between the a b marker horizons deconvolved true amplitude normal incidence profiles sharply delineate the a b marker horizons and indicate the presence of the sub b reflection event on the ne sw line this deep reflector is best described as a diffuse discontinuous zone relatively horizontal lying about 0 8 seconds of two way travel time below the b reflector with an interval velocity of approximately 5 0 km s between horizon b and this reflection the n s line is more complex since the sub b reflection event is masked by a strong internal multiple from the a b interval in the central and western venezuela basin deep primary reflections beneath horizon b are also observed on the northern and western sides of what appears to be a major fault zone this fault zone separates the smooth b and sub b reflectors on the northern and western sides of this fault zone from what appears to be typical oceanic basement the widespread presence of sub b reflections yielding high interval velocities for the section between these events and horizon b suggest that this material is probably igneous in origin

morphostructure at the junction between the beata ridge and the greater antilles island arc offshore hispaniola southern slope abstract oblique convergence between the caribbean plate s interior and the inactive greater antilles island arc has resulted in the collision and impingement of the thickened crust of the beata ridge into southern hispaniola island deformation resulting from this convergence changes from a low angle southward verging thrust south of eastern hispaniola to collision and uplift in south central hispaniola and to left lateral transpression along the southern peninsula of haiti in western hispaniola using new swath bathymetry and a dense seismic reflection grid we mapped the morphological structural and sedimentological elements of offshore southern hispaniola we have identified four morphotectonic provinces the dominican sub basin the muertos margin the beata ridge and the haiti sub basin the lower slope of the muertos margin is occupied by the active muertos thrust belt which includes several active out of sequence thrust faults that were they to rupture along their entire length could generate large magnitude earthquakes the interaction of the thrust belt with the beata ridge yields a huge recess and the imbricate system disappears the upper slope of the muertos margin shows thick slope deposits where the extensional tectonics and slumping processes predominate the northern beata ridge consists of an asymmetrically uplifted and faulted block of oceanic crust our results suggest that the shallower structure and morphology of the northern beata ridge can be mainly explained by a mechanism of extensional unloading from the upper cretaceous onward that is still active residually along the summit of the ridge the tectonic models for the northern beata ridge involving active reverse strike slip faults and transpression caused by the oblique convergence between the beata ridge and the island arc are not supported by the structural interpretation the eastern bahoruco slope an old normal fault that acts as a passive tear fault accommodating the sharp along strike transition from low angle thrusting to collision and uplifting

late jurassic oceanic crust and upper cretaceous caribbean plateau picritic basalts exposed in the duarte igneous complex hispaniola abstract four distinct rock units have been recognized near el aguacate in the janico juncalito la vega area of the duarte complex dominican republic 1 serpentinites crosscut by numerous diabasic dikes 2 basalts interbedded with late jurassic ribbon cherts 3 picrites and ankaramites relatively enriched in incompatible trace elements and 4 amphibolites and gneissic amphibolites chemically similar to oceanic plateau basalts similar ar ar ages of late magmatic amphibole from a picrite and hornblende from an amphibolite documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty decl

venezuela basin of the caribbean sea stratigraphy and sediment distribution abstract the venezuela basin of the caribbean sea averages approximately 4800 m deep and is underlain by oceanic type crust it is bounded on the east and west by pre cenozoic ridges and on the north and south by late cenozoic deformed belts the age of the crust underlying the venezuela basin has not been firmly established but based upon drilling it is at least as old as late cretaceous the age lithology and origin of rock which underlies seismic reflecting horizon b the deepest horizon encountered by drilling is uncertain rocks which coincide with horizon b are tholeiitic basalt sills intruding pelagic carbonate sedimentary rock of late cretaceous age sedimentary strata above horizon b in the central venezuela basin are well known from deep sea drilling and by seismic delineation of their thickness the dominant sediment is pelagic carbonate seismic horizon a marks the onset of lithification pelagic chalk and marl oozes overlie limestones and marl sediments in the a b interval average approximately 500 m in thickness but vary from 0 to over 1000 m in thickness the thickest sections coincide with the venezuelan abyssal plain and with the base of the beata ridge in the northwest corner of the basin in the south central portion of the basin most of the a b section is absent the post a sediments average approximately 400 m in thickness with variations between 100 and 2600 m the principal thickening in this section is in the southeastern corner of the basin bounded by the curacao and aves ridges abyssal plain turbidites appear to be limited to the post a section

geologic and seismic velocity structure of the crust mantle transition in the bay of islands ophiolite complex geological investigations of the bay of islands ophiolite complex show that while a typical layered ophiolite suite is present the thickness of major lithologic units is extremely variable from place to place the composition and internal structure of map scale kilometers across lithologic units as well as the contacts that bound them are laterally variable inferred velocity depth functions of the crust mantle moho transition reconstructed for this terrane as oceanic lithosphere suggest an extremely complex internal seismic structure the crust mantle transition from mafic to ultramafic lithologies varies across the ophiolite from a sharp geologic and seismic velocity discontinuity to a complexly interlayered transition zone as much as 3 km thick the mafic ultramafic transition is characterized by laterally discontinuous lithologic units on the order of hundreds of meters to a few kilometers thick and up to several kilometers long these units megalenses may be composed of lithologies with higher or lower seismic velocities than those of the surrounding units solid state deformation has produced seismically anisotropic materials in the upper mantle and lower crustal units substantial relief and slopes of up to at least 12 over lateral distances of about 10 km occur on the top of the crust mantle transition zone as well as other geologic unit contacts although these types of geological features are striking to the field geologist working in ophiolites and are significant to the understanding of crustal accretion processes many of them occur on such a small scale that they might go undetected in different types of seismic experiments in contemporary oceanic lithosphere

shallower structure and geomorphology of the southern puerto rico offshore margin oblique convergence between the north american and caribbean plates along the eastern greater antilles island arc has yielded the asymmetric muertos thrust belt in the backarc region offshore south of puerto rico this thrust belt disappears and is replaced by a succession of neesw and eew trending deep basins and steep ridges that characterize the western anegada passage resulting in a complex deformation pattern using new high resolution multibeam bathymetry and seismic reflection profiles we studied the geomorphology and shallower structure of the southern puerto rico offshore margin we have identified four morphotectonic provinces the puerto rican sub basin and muertos trough the muertos margin the insular shelf and the western anegada passage the muertos margin province shows two distinct slope sub provinces the active muertos thrust belt e which includes lower and upper thrust belts with distinct deformational styles and lateral continuity e and the shelf slope highly incised by a dense canyon network this network is disrupted by the investigator fault zone consisting of a 130 km long eew trending band of active extensional deformation the investigator fault zone shows differential surface expression caused by along strike changes in the magnitude and distribution of the deformation though this deformation is driven by a nes oriented extension in the western anegada passage province the whiting basin and whiting and grappler ridges are formed by large dip slip normal faults driven by a nwese oriented extensional regime the western st croix rise shows a complex structure where the neesw trending nw dipping normal faults observed at the summit of the rise predate the eew bounding faults that could accommodate the extensional deformation at the present this study provides detailed observations on the active tectonic and sedimentary processes to help future studies assessing the natural resources and the seismic and tsunamigenic hazard in the puerto rico region

deep penetration seismic soundings across the northern margin of the south china sea twenty reversed two ship expanding spread profiles esps with maximum source receiver offsets of 100 km were collected in three transects across the rifted northern margin of the south china sea source receiver offset versus two way travel time x t data were mapped into the intercept time versus ray parameter p domain and velocity depth solutions were obtained by a combination of sum inversion in the p domain and ray tracing in both the p and x t domains arrivals from the moho were detected on 17 of the esps the depths to moho determined for individual esp interpretations have reproducibilities of 0 1 km to 3 km in most cases the moho depth has been determined to within 1 5 km moho depths determined in this investigation represent a significant improvement over previous estimates of moho along the margin from gravity data variations in present day crustal thickness measured from top of prerift basement to moho are one measure of the amount and nature of the crustal thinning associated with the rifting of continental crust preceding the formation of the adjacent south china sea basin the esp interpretations reveal that across the eastern portion of the south china margin the crust appears to thin more or less continuously toward the continent ocean boundary in the west esp interpretations also show a general trend of seaward crustal thinning but in addition indicate at least two instances of focused localized crustal thinning crustal velocities and the relative proportion of upper crust vp 6 4 km s are used to identify areas of the south china margin with similar and contrasting crustal structures variations in these properties are believed to result primarily from contrasting prerift crustal structure across the margin however magmatic underplating during rifting depth dependent extension and pleistocene igneous intrusions may also have contributed to the variations in present crustal structure reliable information about variations in crustal thickness and velocity structure across and along the south china margin is an important prerequisite to understanding better the nature of the spatially variable rifting processes which dominated the formation of this margin

seismic studies of crustal structure during the past four years seismology has continued its role as the key technique for determination of the structure of the continental and oceanic crust by and large however the field and analysis methods used bear little resemblance to those of ten years ago a continued push to increase the number of recording channels has led to the routine use of ocean bottom seismometers sonobuoys and multichannel streamers in oceanic studies and to the use of large sets of portable instruments and multichannel seismic reflection strings on land in studies using earthquakes as sources local and regional arrays provide a capability for obtaining geologically useful two and three dimensional information about the crust

transition from rifted continental to oceanic crust at the southeastern korean margin in the east sea japan sea 1 the southeastern korean margin documents the processes of continental rifting and seafloor spreading that eventually led to the opening of the southern part of the east sea japan sea two dimensional crustal structure of the southeastern korean margin was computed from ocean bottom seismometer data by tomographic inversion and iterative forward modeling the crustal structure shows the emplacement of high velocity 7 km s lower crust under the continental shelf and slope area associated with a rapid transition from rifted continental to oceanic crust the high velocity lower crust is interpreted as magmatic underplating formed by voluminous igneous activity during rifting magnetic modeling confirms its primary correlation with a prominent magnetic anomaly along the edge of the korean peninsula we suggest that the rifting and subsequent seafloor spreading at the korean margin was significantly controlled by the supply of magma in a region of hotter than normal mantle temperature

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

deformation of the caribbean region one plate or two new deep penetrating high resolution multichannel seismic reflection data collected in the eastern caribbean during r v ewing cruise ew9501 imaged both the crustal structure and overlying stratigraphic successions on the basis of this new multichannel seismic data we define the geologic development of the beata ridge and venezuelan basin the caribbean crust was formed by seafloor spreading in late jurassic early cretaceous time prior to the senonian widespread and rapid eruption of basaltic flows began in concert with extensional deformation of the caribbean crust thick volcanic wedges characterized by divergent reflectors are observed along the boundary that separates rough from smooth oceanic crust are coincident with an abrupt shallowing of the moho and appear to be bounded by a large northwest dipping fault system the locus of major extensional deformation migrated through time from the venezuelan basin to the western flank of the beata ridge extensional unloading of the beata ridge footwall caused uplift and rotation of the ridge sediment thicknesses and stratal geometry observed across the venezuelan basin and beata ridge suggest that the majority of the deformation in this region occurred during and soon after the emplacement of the volcanics minor fault reactivation in the neogene along the eastern flank of the beata ridge is associated with an accommodation zone i e tear fault that records a change in the deformation style from bending and subduction of the caribbean plate along the muertos trough south of puerto rico to compressional deformation and obduction of the caribbean plate south of hispaniola we propose that this difference in deformational style is in part a consequence of the thicker crust on the beata ridge which is more resistant to subduction

seismic stratigraphy and structure of the caribbean igneous province multichannel seismic reflection profiles reveal the tectonic and stratigraphic complexity of the caribbean province a smooth reflector b forms the top of widespread cretaceous volcanic flows drilled during dsdp leg 15 and odp leg 165 in addition we also show other seismic evidence of volcanism sills and dipping horizons the southern parts of the venezuela and colombia basins are underlain by rough basement that shows many characteristics of oceanic crust several thick volcanic plateaus separated by deep basins with thin crust comprise the caribbean volcanic province in the venezuela basin only normal oceanic crust is present but in other basins this crust is underlain by underplated volcanic material the crust of the venezuela basin is less than 5 km thick and was probably formed in the pacific during the jurassic refraction data indicate that a thin layer 2v is formed by original oceanic crust overlain by a thin volcanic layer an intra basement reflection sub b marks the top of original oceanic crust that is sandwiched between an upper volcanic layer and lower underplated material this underplated layer forms a very thick layer 3v beneath the beata and nicaragua volcanic plateaus the upper part of layer 3v is gabbroic and outcrops on the beata ridge the thickening occurs mainly in the high velocity lower part of layer 3v and is attributed to the presence of magnesian rocks picrites or ultramafic cumulates a highly reflective horizon r is located at the top of this layer we present a model where the galapagos plume is 2500 km wide but has narrow heads that formed the caribbean volcanic plateaus separated by deep basins the caribbean igneous province is compared to similar volcanic features in the indian ocean kerguelen plateau and in the western pacific ocean ontong java plateau nauru and pigafetta basins the thick plateaus of the caribbean province resemble the kerguelen and ontong java plateaus whereas the deep basins present some similarities with the nauru and pigafetta basins the caribbean igneous province is younger 90 75 ma than the indian and pacific plateaus 110 130 ma however several geological evidences from land and geophysical offshore observations indicate that the onset of volcanic activity in the caribbean province could be as old as 115 ma