shear wave splitting in northeast venezuela trinidad and the eastern caribbean abstract we present new measurements of splitting of core shear wave phases sks skks pks recorded at seven stations in the eastern caribbean region six stations in northeastern venezuela and trinidad constituted the secasa92 temporary broadband array the seventh station sjg is installed on puerto rico we also attempted to measure source side splitting of s waves from earthquakes in the lesser antilles subducted slab but with little success mean splitting parameters fast polarization direction and delay time t at the stations are consistently east west trending and over 1 s respectively delay times at the secasa92 array are amongst the highest yet measured reaching 2 1 s and generally over 1 5 s we note an approximately linear decrease in delay times here from maxima at the northernmost stations to a minimum at the southernmost the secasa92 array is situated in the wide south america caribbean sa ca plate boundary zone and sjg is within the north america caribbean na ca plate boundary zone in both cases trends are parallel to the plate boundary zones locally we interpret these results to indicate that the sa ca plate boundary deformation extends to depths of at least 200 km and that south american crust lithospheric mantle and asthenosphere are coherently deformed the extreme delay times we measure are probably the result of transpression in the sa ca plate boundary and overriding of the lesser antilles slab by south america compressional flattening results in extremes of foliation development and dextral shearing yields a strong lineation in upper mantle olivine by contrast the delay time at sjg 1 2 s is near the global average and reflects the fact that the na ca plate boundary zone is purely sinistral or includes minor transtension we note that recent counterclockwise rotation of puerto rico does not apparently affect upper mantle deformation beneath the island we suggest that this rotation is crustal driven by far field sinistral shear and partly decoupled from the mantle shear our results are consistent with the trench parallel mantle flow hypothesis russo and silver 1994 science 263 1105 1111 and indicate that asthenospheric mantle flow beneath the caribbean plate is eastward and probably drives the plate s eastward motion

constraints on present day plate motions south of 30 s from satellite altimetry abstract gridded free air gravity anomalies derived from geosat and seasat altimetric observations are used to locate all mid ocean ridge transform faults south of 30 s and wherever possible assign azimuths and uncertainties useful for plate kinematic analysis a total of 131 ridge offsets longer than 30 km were located of which 114 are clearly transform faults with well defined valleys azimuthal uncertainties range from 0 6 for the longest transform faults to nearly 10 for the transform faults with lengths close to 30 km to assess whether the altimetrically derived azimuths are representative of present day plate slip directions azimuths of transform faults mapped with side scan sonar and side looking or conventional bathymetric systems were compared to their altimetrically derived counterparts comparison of these two independently estimated sets of azimuths using the x2 test shows no significant difference indicating that the altimetrically derived azimuths can be used where bathymetrically derived azimuths are not available we also find no evidence that large age offset transform faults are less representative of present day slip directions than short age offset transforms models constructed using the altimetrically derived azimuths fit the nuvel 1 spreading rates transform azimuths and earthquake slip vectors nearly as well as nuvel 1 as well as decreasing the model uncertainties thus the altimetrically derived azimuths offer improved accuracy and decreased uncertainties in estimates of global plate velocities without increasing non closures of plate circuits beyond the level already present in nuvel 1 in practical terms plate velocities predicted by models that substitute the altimetrically derived azimuths for all nuvel 1 transform azimuths from below 30 s never differ from velocities predicted by nuvel 1 by more than 0 3 mm yr 1 and 4 however the several degree azimuthal misfits to altimetrically derived azimuths from the africa antarctic and pacific antarctic plate boundaries are statistically significant and suggest there are several degree inaccuracies in the predictions of the nuvel 1 angular velocities the new catalog of southern hemisphere transform fault azimuths should prove useful for at least the following 1 improvement of models of deformation of the australian plate west of the macquarie ridge complex 2 improvement of estimates of motion between the australian and pacific plates 3 revision of upward bounds on the amount of seafloor deformation south of africa related to rifting of the african plate and 4 studies of a possible deforming zone within the south american plate east of the south sandwich trench

spectral analysis of gravity anomalies and the architecture of tectonic wedging ne venezuela and trinidad we have analyzed the spectral content of free air gravity anomalies in the caribbean south american plate boundary zone in order to determine better the near surface 0 120 km distribution of crustal and upper mantle elements which give rise to the unusual gravity field of this region the plate boundary zone in northeastern venezuela and trinidad is the site of the world s sea level continental minimum of bouguer gravity anomalies yet the region is also one of mild topography mean value 43 m maximum 1200 m we find the mean depths to interfaces of significant density contrast at a variety of depths for portions of the plate boundary zone we interpret interfaces at 30 35 km and 32 km beneath the guyana shield and the aves ridge respectively to be the moho other shallow interfaces 5 14 km are most likely sediment cover basement contacts in the maturin foreland basin and southern grenada basin deeper interfaces 54 63 km we associate with loaded and downwarped continental and oceanic south american lithosphere the deepest boundaries at depths of 89 120 km may be related to detached or detaching oceanic lithosphere overridden by continental south america we use our results to test the tectonic wedging model of the plate boundary zone recently published by russo and speed 1992 we find that the tectonic wedging model adequately describes many of the structural boundaries inferable from our analysis of gravity anomalies but that the model must be modified to include a thinner guyana shield crust

ferrobasalts from the spiess ridge segment of the southwest indian ridge highly vesicular microporphyritic basaltic rocks have been dredged from the slow spreading spiess ridge segment of the southwest indian ridge all the samples recovered are hyalocrystalline with plagioclase clinopyroxene and olivine as phenocryst and microphenocryst phases titanomagnetite occurs as euhedral microphenocrysts in some of the more evolved samples in terms of bulk rock and quench glass chemistry the lavas are characterised by highly evolved compositions e g feo 10 3 14 2 tio2 2 0 3 4 k2o 0 50 1 1 mgo 6 0 3 5 zr 160 274ppm nb 14 32ppm and can be classified as ferrobasalts isotopic and incompatible element ratios of the lavas e g 87sr 86sr 0 70325 0 70333 zr nb 8 4 11 3 y nb 2 3 1 4 indicate their strongly enriched nature see also dickey et al 6 quantitative major and trace element modelling indicates that most of the compositional variations observed can be attributed to low pressure fractional crystallisation of plagioclase clinopyroxene and minor olivine and titanomagnetite the range in composition can be accounted for by up to 65 fractional crystallisation we suggest that the extreme differentiation of the spiess ridge lavas is related not to spreading rate but to rate of magma supply the basaltic melts appear to have evolved in a newly established zone of magmatic activity associated with the most recent northward jump of the bouvet triple junction where they were effectively isolated from significant admixture of primitive magmas

the 1946 hispaniola earthquakes and the tectonics of the north america caribbean plate boundary zone northeastern hispaniola we have determined focal mechanisms for the largest earthquake ms 7 8 recorded instrumentally in the caribbean basin the august 4 1946 hispaniola earthquake and three of its large magnitude ms 6 1 aftershocks we also relocated 63 aftershocks and one foreshock of the event series the aftershock series is elongate trends wnw and is centered on the samana peninsula of northeast hispaniola shallow aftershocks are in a 75 km wide linear zone and intermediate depth 70 to 130 km aftershocks apparently delineate a moderately south or ssw dipping slab it is not clear however whether these events indicate active subduction of north american atlantic ocean lithosphere or are strike slip events on the interface between subducted but no longer sinking slab and caribbean mantle we constrained focal mechanisms of the main shock and three aftershocks by combining observed p and s polarities and amplitude ratios and also by waveform modeling the two methods yield consistent results the mechanisms include strike slip and thrust displacements on nw striking nodal planes fault dip is variable ne or sw the nw striking fault planes parallel mapped terrane boundaries and faults in the north america na caribbean ca plate boundary zone and are also parallel to the aftershock series trend we interpret the events to be motions on a wnw trending restraining bend segment of the na ca plate boundary in eastern hispaniola we have calculated magnitudes for eight of the earthquakes in the series for the three events including the main shock for which data are available our magnitudes are systematically less than the previously published magnitude estimates given the high magnitude and large aftershock area of the august 4 1946 event these earthquakes probably represent the true long term interplate motions between north america and the terranes in this portion of the plate boundary zone

easter microplate evolution geochemical evidence confirms that there is current active spreading on both the east and west rift zones of the easter microplate and northwards propagation of the east rift with the active tip at 25 s the east rift is influenced by lateral flow of plume derived material from a hotspot located beneath either sala y gomez or easter island spreading rate variation does not seem to be influential in controlling the composition of erupted basalts

aftershock pattern of the july 9 1997 mw 6 9 cariaco earthquake in northeastern venezuela abstract the mw 6 9 cariaco earthquake of july 9 1997 was the strongest earthquake that occurred on the el pilar fault between cumana and the gulf of paria in the 20th century the el pilar fault is the most prominent strike slip fault in the transition zone between the caribbean plate and the south american plate in this region we installed 18 seismic and 10 strong motion stations for recording aftershocks we calculated the hypocenters of 1306 aftershocks with magnitudes between 0 5 and 4 1 most aftershocks concentrated near the documented trace of the el pilar fault the spatial distribution of the aftershocks allowed us to separate the aftershock region on the el pilar fault into six segments with different dip angles we computed and classified fault plane solutions of 222 aftershocks the majority of fault plane solutions are of strike slip type some segments show differences in the dominating type of focal mechanisms from the aftershock distribution and spatial variation of b values we inferred a rupture length of the cariaco earthquake of 60 km and thus an average displacement of 1 3 m during the main shock is estimated two out of four stepovers which were found on this length were seismically activated during the monitoring time interval the b value distribution on the fault plane appears to define two areas of major stress relaxation beneath cariaco and half way between casanay and el pilar close to the surface

evolution of abyssal lavas along propagating segments of the galapagos spreading center abstract the unusual petrological diversity of abyssal lavas erupted along some segments of the galapagos spreading center is a direct consequence of the propagation elongation of these segments into older oceanic crust with increasing distance behind propagating rift tips relatively unfractionated morb erupted close to the tips are joined first by feti basalts bimodal assemblage and then by a wide range of basaltic and siliceous lavas further behind propagating rift tips this broad range diminishes again approaching the narrow compositional range of adjacent normal ridge segments these compositional variations reflect the evolution of the subaxial magmatic system beneath the newly forming spreading center as it propagates through a pre existing plate we envisage this evolution as proceeding from small isolated ephemeral magma chambers through increasing numbers of larger increasingly interconnected chambers to the steady state buffered system of a normal ridge throughout this evolution magma supply rates gradually increase and cooling rates of crustal magma bodies decrease high degrees of crystal fractionation are favored only when a delicate balance between cooling rate and resupply rate of primitive magma is achieved at other propagating and non propagating ridge transform intersections the degree to which the balance is achieved and the length of ridge over which it evolves control the distribution of fractionated lavas these effects may be evaluated provided a number of tectonic variables including transform length spreading and propagation rates are taken into account

global systematics of unaveraged mid ocean ridge basalt compositions comment on global correlations of ocean ridge basalt chemistry with axial depth and crustal thickness by e m klein and c h langmuir

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

historical seismicity of the southeastern caribbean and tectonic implications we have relocated the twenty eight largest magnitude 4 3 ms 7 3 historical 1922 1963 earthquakes of the southeastern caribbean we also present new focal mechanisms for seven of these events the relocations are based on reported issp ands arrival times that we analyzed using generalized linear inversion techniques the new focal mechanisms were constrained by first motionp polarities as reported by the iss and as picked by us where records were available and by the polarities and ratios ofsh andssh andsv andssv arrivals that we determined from seismograms the results of the relocations are commensurate with the distribution of seismicity observed in the recent era hypocenters are shallow and intermediate in depth 0 200 km and the events occur almost exclusively in areas known to be currently seismic the frequent seismic activity in the vicinity of the paria peninsula venezuela is clearly a persistent feature of the regional earthquake pattern intermediate depth earthquakes indicative of subduction beneath the caribbean plate occur here and along the lesser antilles arc the grenadines seismic gap is confirmed as an area of low seismic moment release throughout the historical era trinidad and the eastern gulf of paria were also largely quiescent

p wave velocity tomography of the venezuelan region from local arrival times arrival times from the local seismological network of venezuela were used to estimate a three dimensional p wave velocity model for the region between longitude 60 74 w and latitude 6 14 n to a depth of more than 80 km the inversion was carried out by damped least squares describing the media by homogeneous velocity blocks the resolved lateral velocity variations in the first layer 0 30 km depth showed a correlation with the main stratigraphic features of the area while second layer 30 50 km depth showed the influence of moho depth variations through the region generating a pattern well correlated to the bouguer anomaly map lithospheric seismic velocities below the moho appear to be influenced by the major crustal fault systems an important low velocity zone is present below the triple junction of the fault systems of oca bocono and moron in northwestern venezuela farther south a similar low velocity zone is present below the junction of the bocono and the santa marta fault systems those are the two continental corners of the triangular maracaibo block below 80 km depth the fourth layer the low velocity zones show a connected pattern that follows or is adjacent to the crustal fault zones the presence of subducted atlantic lithosphere below the eastern venezuelan basin could explain the high velocity zone at this location a similar interpretation emerges from the tectonic wedging model previously proposed to explain the pronounced minimum of the gravity anomaly

bouvet triple junction in the south atlantic geology and evolution the south american african and antarctic lithospheric plates meet in the bouvet triple junction btj located in the south atlantic near the island of bouvet multibeam magnetic gravimetric and seismic reflection data have been used to understand the evolution of the three accretionary transform boundaries that converge in the btj the easternmost segments of the american antarctic ridge aar have a spreading full rate of 19 5 mm yr for the last 8 m y they are oriented n s except for some ne sw segments probably created by magma poor extension the southernmost portion of mid atlantic ridge mar spreading full rate of 30 5 mm yr for the last 9 m y has elevated topographic anomalies it is segmented by transform and overlapping discontinuities and shows evidence of axial propagation the mar axial valley bifurcates at its southern tip in two branches oriented 115 o and 180 o that are or have been up to recently loci of crustal accretion the bifurcation represents a former ridge ridge ridge triple junction the westernmost segments of the southwest indian ridge swir are anomalously high the segment adjacent to the island of bouvet spreading rate 14 5 mm yr is shallower than normal by almost 1 km due to the influence of the bouvet hot spot the westernmost swir segment spiess ridge consists of a swollen volcanic ridge that reaches 320 rn below sea level and has a deep caldera on its summit spiess ridge narrows and deepens to the nw v shaped topographic and magnetic lineations suggest that it propagates nw at a rate of 40 to 50 mm yr the spiess magmatic event started at roughly 1 ma when it caused deactivation of the 115 o spreading branch therefore the antarctic south american and african plates meet presently not in a triple point but in a broad zone of diffuse deformation an area of extensional deformation observed east of spiess ridge may be caused by excess crustal formation at spiess ridge that cannot be accommodated by motion of rigid plates the evolution of the btj since 10 ma involves stages of rrr rff and rrf configurations with highly variable geometry of the accretionary transform boundaries topographic anomalies anomalously thick crust and excess volcanism suggest that the upper mantle below this region is affected by widespread strong thermal anomalies that have influenced the configuration of the btj and determined indirectly intraplate deformation in wide areas of the btj region the thermal anomaly that gave rise to the swir spiess excess magmatism is the prime cause of the recent disruption of a former rrr configuration and of the imminent establishment of a new rrr triple junction to the north

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

petrologic consequences of rift propagation on oceanic spreading ridges abstract the production of anomalously differentiated lava compositions at several mid ocean spreading centers can be attributed to magmatic processes associated with propagating rifts the degree of differentiation attained by magmas beneath oceanic spreading ridges depends mainly on the balance between cooling rate and the supply rate of new magma to shallow chambers low supply rates and moderate cooling rates allow advanced degrees of closed system fractionation to occur high supply rates result in open systems in which magma compositions are buffered by frequent replenishment with new hot magma propagating rift tips are a special class of ridge transform intersection in which the balance between cooling and supply rates is conducive to the development of advanced degrees of differentiation over an expanded length of ridge this balance is affected by the spreading rate the propagation rate of the rift the length of the bounding transform and proximity to hotspots maximum compositional variability and maximum degree of differentiation occur within 50 km of propagating rift tips and subsequently diminish with increasing distance rifts that propagate through plates in directions approximating their absolute motion relative to the lower mantle are characterized by the presence of anomalously differentiated lavas over longer ridge segments than are rifts that propagate against their absolute motion geochemical anomalies may persist though changing in degree and extent for several million years on ridge segments that stop propagating the concept of magnetic telechemistry is generally supported by our study but in the vicinity of hotspots magnetic anomaly amplitude may be controlled more by bathymetric and or thickened magnetic layer effects than by geochemistry

mantle discontinuity structure beneath the colorado rocky mountains and high plains analysis of mantle discontinuity structure using converted p to s pas phases beneath colorado from the program for array seismic studies of the continental lithosphere passcal rocky mountain front rmf experiment reveals significant topography at the 410 and 660 km depth discontinuities and corresponding transition zone thickness variations a stack of all radial receiver functions resolves the 410 and 660 km discontinuities at average depths of 419 and 677 km respectively imaging of lateral variations in mantle discontinuity structure is accomplished by geographically binning the pds conversion points and then stacking the receiver functions in each bin to form spatial images analogous to common depth point stacking corrections for lateral velocity heterogeneity are calculated using the local s wave tomographic model of lee and grand 1996 and a constant lnvs lnvp scaling of 1 3 this scaling value is determined from the relative scaling between teleseismic p and s wave travel time residuals measured from the rocky mountain front deployment mantle discontinuity images using 150 km square bins show 20 km of 410 km discontinuity topography 30 km of 660 km discontinuity topography and up to 40 km of transition zone thickness variation features of the discontinuity structure include a 20 km depression of the 660 km discontinuity beneath western colorado and a gradual 10 km dip of the 410 km discontinuity beneath the high plains the thickening of the transition zone beneath southwest colorado is consistent with the presence of the subducted farallon slab in this region as imaged by van der lee and nolet 1997 in general our results show that the transition zone discontinuity structure is more complex than that predicted by the simple model of olivine phase boundaries modulated by vertically coherent thermal anomalies

structure and deformation history of the northern range of trinidad and adjacent areas conflicting models have been proposed for both the evolution of northern south america and the neotectonics of the south caribbean plate boundary zone the trinidadian portion of the margin is particularly controversial but surprisingly it has been little studied we present a structural analysis of trinidad s northern range pertinent updates of the island s stratigraphy and sedimentology and new zircon fission track age determinations and use them to constrain trinidad s geologic history and to better understand the controlling tectonic processes in our interpretation trinidad s three e ene striking ranges which are separated by late neogene recent depocenters expose 1 the northern range group generally greenschist metamorphosed upper jurassic to cretaceous north facing continental slope sediments of the northern range deposited on the northern south american passive margin 200 400 km to the wnw and 2 the trinidad group cretaceous paleogene shelf slope sediments of the central and southern trinidad deposited less than 100 km wnw of their present location a small allochthon composing the sans souci group cretaceous tholeiitic volcaniclastic basaltic and gabbroic rocks sans souci formation and sediments toco formation now in the northeastern northern range has been transported hundreds of kilometers from the west with the caribbean plate despite earlier references to cretaceous orogenesis all deformation in trinidad is of cenozoic age the first deformation in the northern range d1 formed north vergent nappes and induced greenschist metamorphism probably in the late eocene or oligocene the nappes developed either by the underthrusting of the proto caribbean crust beneath south america due to convergence between north and south america or as gravity slides caused by oversteepening induced by this convergence and or the passage of the caribbean plate s peripheral bulge and arrival of its foredeep northern range d2 deformation is south vergent and represents the incorporation of northern range metasediments into the caribbean accretionary prism the transition to d3 brittle transpressive right lateral strike slip faulting is interpreted to be due to the uplift and east southeastward transpressive emplacement of northern range caribbean prism rocks onto the south american stepped shelf this emplacement formed the miocene transpressive thrust belts and foreland basin in central and southern trinidad in the final phase of northern range deformation d4 e w normal faults and shear zones and conjugate nnw sse and ne sw normal faults developed and displacement on preexisting e w right lateral strike slip faults continued the 11 ma northern range zircon fission track ages suggest rapid uplift from the late miocene to recent late miocene subsidence of the tobago platform immediately to the north of the northern range and greater than 3 km of normal down to the north displacement indicated for the north coast fault zone separating the northern range and tobago platform leads us to postulate that the rapid uplift of the northern range was in response to the northward detachment of the tobago platform from above the northern range along the north dipping transtensional north coast fault zone this late miocene change in deformation style can be explained by a change from caribbean south american right lateral transpression to right lateral strike slip generally striking 080 this has generally induced a component of extension on pre existing faults striking at greater than 080 and a component of compression on faults striking at less than 080

probabilistic seismic hazard assessment at the eastern caribbean islands a probabilistic seismic hazard analysis has been performed to compute probabilistic seismic hazard maps for the eastern caribbean region 10 n 19 n 59 w 64 w which includes in the north the leeward islands from anguilla to dominica and in the south the windward islands from martinique to grenada barbados trinidad and tobago the analysis has been conducted using a standard logic tree approach that allowed systematically taking into account the model based i e epistemic uncertainty and its influence on the computed ground motion param eters hazard computations have been performed using a grid of sites with a space resolution of 0 025 degrees covering the territory of the considered islands two different computation methodologies have been adopted the standard cornell mcguire approach cornell 1968 mcguire 1976 based on the definition of appropriate seismogenic zones sz and the zone free approach developed by woo 1996 which overcomes the ambiguities related with the definition of seis mic sources the interplay and complexities between shallow crustal intraplate and interface subduction seismicity of the caribbean region have been thoroughly investigated by merging all available databases a comprehensive and updated earthquake catalog for the region has been compiled also a thorough investigation has been undertaken to identify the most suitable ground motion prediction equa tions to be used in the analyses uniform hazard spectra have been calculated for the horizontal component of ground motion rock and level site conditions 4 return periods rp 95 475 975 and 2475 yr and 22 spectral accelerations sa with structural periods ranging from 0 to 3 s sas at 0 2 and 1 0 s for 2475 yr rp have been calculated to allow the definition of seismic hazard in the region of study according to the international building code ibc international code council icc 2009

spiess ridge an axial high on the slow spreading southwest indian ridge we report recent mapping of spiess ridge with the hawaii mr1 sidescan sonar spiess ridge is an unusual elongate 90 by 50 km volcanic feature on the slow spreading southwest indian ridge near the bouvet hotspot the northwest half of the ridge has a narrow 7 km neovolcanic zone in mr1 sonar images and a simple magnetic anomaly sequence including possible anomalies c1n and c2n in contrast the southeast half shows extrusive volcanism over 40 km with distributed eruption sites a broadened central magnetic anomaly with no anomaly c2n and a volcanic ridge radiating from the summit of spiess ridge oblique to the spreading orthogonal trend the images show no evidence for large offset normal faults or an axial rift valley typical of slow spreading ridges overall spiess ridge has an appearance very unlike that of either fast or slow spreading ridges and more transitional between a spreading ridge and a seamount we compare the morphology of spiess ridge to other large volcanic structures in oceanic rifts spiess ridge and bouvet island represent localized zones of excess melting along the southwest indian ridge and we discuss their origin in the light of current ideas on ridge hotspot interaction

petrography and geochemistry of basaltic rocks from the conrad fracture zone on the america antarctica ridge intrusive and extrusive basaltic rocks have been dredged from the conrad fracture zone transecting the slow spreading america antarctica ridge the majority of rocks recovered are holocrystalline with the dominant mineral assemblage being plagioclase plus clinopyroxene with or without minor fe ti oxides olivine occurs in only three samples and many of the samples show evidence of extensive alteration secondary minerals include chlorite actinolite k and na feldspar analcite and epidote in terms of bulk chemistry the rocks are characterized by their generally evolved and highly variable compositions e g mg 0 65 0 35 tio2 0 7 3 6 zr 31 374ppm nb 3 21ppm y 17 96ppm ni 100 9ppm but with respect to the immobile incompatible element ratios e g zr nb y nb la sm are similar to normal or depleted mid oceanic ridge basalts quantitative major and trace element modelling indicate that most of the variation observed can be attributed to low pressure fractional crystallization of plagioclase plus clinopyroxene in approximately equal proportions with or without minor fe ti oxides the range in composition can be accounted for by up to 76 fractional crystallization although ferrobasalts have not frequently been associated with slow spreading ridges the extreme differentiation observed in the conrad fracture zone basalts implies some additional constraint other than spreading rate on the formation of ferrobasalt and reaffirms the importance of extensive crustal differentiation during the production of this basalt type

evolution of the grenada and tobago basins and implications for arc migration abstract the tectonic mechanisms controlling how volcanic arcs migrate through space and geologic time within dynamic subduction environments is a fundamental tectonic process that remains poorly understood this paper presents an integrated stratigraphic and tectonic evolution of late cretaceous to recent volcanic arcs and associated basins in the southeastern caribbean sea using seismic reflection data wide angle seismic refraction data well data and onland geologic data we propose a new tectonic model for the opening of the grenada and tobago basins and the 50 250 km eastward jump of arc volcanism from the late cretaceous aves ridge to the miocene to recent lesser antilles arc in the southeast caribbean based on the mapping of three seismic megasequences the striking similarity of the half graben structure of the grenada and tobago basins that flank the lesser antilles arc their similar smooth basement character their similar deep marine seismic facies and their similar paleogene sediment thickness mapped on a regional grid of seismic data suggest that the two basins formed as a single saucer shaped oceanic crust paleogene forearc basin adjacent to the now dormant aves ridge this single forearc basin continued to extend and widen through flexural subsidence during the early to middle eocene probably because of slow rollback of the subducting atlantic oceanic slab rollback may have been accelerated by oblique collision of the southern aves ridge and southern lesser antilles arc with the south american continent uplift and growth of the southern lesser antilles arc divided the grenada and tobago basins by early to middle miocene time inversion of normal faults and uplift effects along both edges of the lesser antilles arc are most pronounced in its southern zone of arc collision with the south american continent the late miocene to recent depositional histories of the grenada and tobago basins are distinct because of isolation of the grenada basin by growth and uplift of the neogene lesser antilles volcanic ridge

magmatic and hydrothermal processes in the bouvet triple junction region south atlantic results of electron microprobe and microthermometric studies of samples collected from the bouvet triple junction region btjr during a joint russian italian geological expedition on the r v academician nikolaj strakhov 1994 have revealed new data on the composition of basaltic magmas and oceanic hydrothermal fluids connected with magmatic processes detailed analysis of basaltic glasses shows that the modem mid atlantic ridge mar rift valley is composed of normal mid ocean ridge basalts with low concentrations of k2 o and tioz n morb while its flanks are more enriched with these components approaching e morb a marked influence of the bouvet hot spot volcanism on magma generation on the south west indian ridge swir near bouvet island is observed basaltic melts in this area belong to alkalic and transitional series and have maximum contents of k2o tio2 h2o microthermometric analyses of fluid inclusions in the samples from the btjr have revealed major differences in the oceanic hydrothermal fluid systems on the mar and near swir which depends on the peculiarities of magma in the area of the mar with dry melts only h2o solution inclusions in quartz were found thus seawater is probably the only primary source of hydrothermal fluids nacl mgcl2 h2o t 170 200 c in the swir area with the high content of water in melts syngenetic liquid co2 and h2o solution inclusions in quartz indicate the influence of the magmatic fluid component on the ore forming water carbon dioxide solutions nacl cac12 h2o co2 t 200 310 c p 900 1700 bar

low s velocities under the tornquist teisseyre zone evidence for water injection into the transition zone by subduction a strong low s velocity anomaly at 300 500 km depth coincides with the western boundary of the russian platform the anomaly is too large to be explained by a simple temperature anomaly or by compositional variations nor is it an artifact induced by seismic anisotropy we present a model that explains the anomaly through the injection of water at the time of closure of the tomquist ocean that separated the continents of avalonia and baltica in the early paleozoic dense hydrous magnesium phases are the most likely agents for transporting water to the transition zone but an important role may also be played by nominally anhydrous clinopyroxene when these minerals are brought out of their stability field water is released it may accumulate in spinel or k amphibole and may be released when the mantle warms up after subduction halts the effect of this water is to induce weakening of the shear modulus or even creation of a heavy melt by a conservative estimate a subduction episode lasting 85 m y would inject enough water to saturate a large volume of mantle rock with 0 3 h2o low velocities or low q anomalies have also been observed in the transition zones near currently active slabs

source processes of large m 6 5 earthquakes of the southeastern caribbean 1926 1960 we have conducted body waveform modeling studies of 13 historic earthquakes to provide a better understanding of the long term spatial and temporal pattern of seismicity and deformation within a region extending from barbuda lesser antilles to cumana venezuela our results suggest that shallow earthquakes 50 km deep along the south american caribbean plate margin reflect right lateral and extensional deformation intermediate depth events 100 km show left lateral strike slip motion beneath the paria peninsula of venezuela in the lesser antilles the 1960 barbuda and 1946 martinique earthquakes appear to be interplate thrust events however the greatest moment release in the region has occurred at intermediate depths as a mixture of normal and strike slip faulting generally along trends oblique to the arc the deformation rate estimated from the seismic moment release between 1926 and 1960 is only 1 to 10 of the estimated plate convergence rate for the region

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

tectonic inversion in the caribbean south american plate boundary gps geodesy seismology and tectonics of the mw 6 7 22 april 1997 tobago earthquake on 22 april 1997 the largest earthquake recorded in the trinidad tobago segment of the caribbean south american plate boundary zone mw 6 7 ruptured a shallow 9 km ene striking 250 azimuth shallowly dipping 28 dextral normal fault 10 km south of tobago in this study we describe this earthquake and related foreshock and aftershock seismicity derive coseismic offsets using gps data and model the fault plane and magnitude of slip for this earthquake coseismic slip estimated at our episodic gps sites indicates movement of tobago 135 6 to 68 6 mm nne and subsidence of 7 9 to 0 mm this earthquake was anomalous and is of interest because 1 its large component of normal slip and ene strike are unexpected given the active e w dextral shearing across the caribbean south american plate boundary zone 2 it ruptured a normal fault plane with a low 28 dip angle and 3 it reactivated and inverted the preexisting tobago terrrane south america ocean continent thrust boundary that formed during early tertiary oblique plate convergence

oceanic versus emplacement age serpentinization in the josephine ophiolite implications for the nature of the moho at intermediate and slow spreading ridges we present field petrographie and geochemical evidence for oceanic serpentinization in the 162 ma josephine ophiolite of nw california and sw oregon undeformed and unrodingitized dikes that intruded into serpentinized shear zones provide time markers for serpentinization and deformation the dikes intruding serpentinites are of two types 1 fe ti enriched dikes with normal mid ocean ridge basalt nmorb magmatic affinity which are geochemically linked to the uppermost lavas and a late fe ti dike within the crustal sequence of the ophiolite and 2 hornblende bearing calc alkaline dikes intruded during ophiolite emplacement between 150 and 146 ma based on crosscutting relationships between dikes and serpentinites serpentinization of upper mantle peridotites took place at or near the ridge axis during periods of amagmatic structural extension in the absence of a magma chamber lizardite bearing seipentinites in the northern peridotite are constrained to be oceanic and indicate temperatures of 350 c within the oceanic upper mantle oceanic lizardite bearing serpentinized shear zones from the ultramafic cumulate section to the basal sole indicate that the entire josephine peridotite may have been transected by serpentinized shear zones prior to ophiolite emplacement the ultramafic cumulate sequence was completely serpentinized prior to ophiolite emplacement and we suggest that the paleo moho in the josephine ophiolite is a serpentinization boundary the basal sole is interpreted to be a reactivated oceanic fault along which antigorite mylonites formed from a preexisting serpentinite during ophiolite emplacement oceanic serpentinization in the josephine ophiolite took place beneath a 2 3 km crustal sequence and suggests that serpentinization may be an important feature at intermediate spreading rate mid ocean ridges as well as at slower spreading ridges

estimation of the intrinsic absorption and scattering attenuation in northeastern venezuela southeastern caribbean using coda waves northeastern venezuela has been studied in terms of coda wave attenuation using seismograms from local earthquakes recorded by a temporary short period seismic network the studied area has been separated into two subregions in order to investigate lateral variations in the attenuation parameters coda q 1 q c 1 has been obtained using the single scattering theory the contribution of the intrinsic absorption q i 1 and scattering q s 1 to total attenuation q t 1 has been estimated by means of a multiple lapse time window method based on the hypothesis of multiple isotropic scattering with uniform distribution of scatterers results show significant spatial variations of attenuation the estimates for intermediate depth events and for shallow events present major differences this fact may be related to different tectonic characteristics that may be due to the presence of the lesser antilles subduction zone because the intermediate depth seismic zone may be coincident with the southern continuation of the subducting slab under the arc

compositional and kinetic controls on liquid immiscibility in ferrobasalt rhyolite volcanic and plutonic series we present major element compositions of basalts and their differentiation products for some major tholeiitic series the dry low pressure liquid lines of descent are shown to approach or intersect the experimentally defined compositional space of silicate liquid immiscibility ferrobasalt rhyolite unmixing along tholeiitic trends in both volcanic and plutonic environments is supported by worldwide occurrence of immiscible globules in the mesostasis of erupted basalts unmixed melt inclusions in cumulus phases of major layered intrusions such as skaergaard and sept iles and oxide rich ferrogabbros closely associated with plagiogranites in the lower oceanic crust liquid immiscibility is promoted by low pressure anhydrous fractional crystallization that drives the low al2o3 high feo liquids into the two liquid field kinetic controls can be important in the development of two liquid separation the undercooling that occurs at the slow cooling rates of plutonic environments promotes early development of liquid immiscibility at higher temperature in contrast rapid cooling in erupted lavas leads to large undercoolings and liquid immiscibility develops at significantly lower temperatures unmixing leads to the development of a compositional gap characterized by the absence of intermediate compositions a feature of many tholeiitic provinces the compositions of experimental unmixed silica rich melts coincide with those of natural rhyolites and plagiogranites with high feotot and low al2o3 suggesting the potential role of large scale separation of immiscible si rich liquid in the petrogenesis of late stage residual melts no trace of the paired ferrobasaltic melt is found in volcanic environments because of its uneruptable characteristics instead fe ti p rich gabbros are the cumulate products of immiscible fe rich melts in plutonic settings the immiscibility process may be difficult to identify because both melts crystallize the same phases with the same compositions the two liquids might form incompletely segregated emulsions so that both liquids continue to exchange as they crystallize and remain in equilibrium even if segregated both melts evolve on the binodal surface and exsolve continuously with decreasing temperature the two liquids do not differentiate independently and keep crystallizing the same phases with differentiation further evolution by fractional crystallization potentially drives the bulk liquid out of the twoliquid field so that very late stage liquids could evolve into the single melt phase stability field

chronostratigraphy and tectonostratigraphy of the columbus basin eastern offshore trinidad the columbus basin forming the easternmost part of the eastern venezuela basin is situated along the obliquely converging margins of the caribbean and south american plates the two primary structural elements that characterize the basin are 1 transpressional northeast southwest trending anticlines and 2 northwest southeast oriented down to the northeast extension normal faults the basin was filled throughout the pliocene and pleistocene by more than 40 000 ft 12 200 m of clastic sediment supplied primarily by the paleo orinoco delta system the delta prograded eastward over a storm influenced and current influenced shelf during the pliocene pleistocene depositing marine and terrestrial clastic megasequences as a series of prograding wedges atop a lower pliocene to pre pliocene mobile shale facies biostratigraphic and well log data from 41 wells were integrated with thousands of kilometers of interpreted two dimensional and three dimensional seismic data to construct a chronostratigraphic framework for the basin as a result several observations were made regarding the basin s geology that have a bearing on exploration risk and success 1 megasequences wedge bidirectionally 2 consideration of hydrocarbon system risk across any area requires looking at these sequences as complete paleofeatures 3 reservoir location is influenced by structural elements in the basin 4 the lower limit of a good quality reservoir in any megasequence deepens the closer it comes to the normal fault bounding the wedge in a proximal location 5 reservoir quality of deep marine strata is strongly influenced by both the type of shelf system developed bypass or aggradational and the location of both subaerial and submarine highs and 6 submarine surfaces of erosion partition the megasequences and influence hydrostatic pressure migration and trapping of hydrocarbons and the distribution of hydrocarbon type begin page 1906

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

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

global scale analysis of the mantle pds phases a global scale analysis of travel times of the mantle pds phases converted from p to s from the discontinuity at a depth d beneath the receiver is presented most of the data are related to p 410 s and p 660 s recorded by the broadband stations of geoscope incorporated research institutions for seismology iris and other networks in order to provide a good global scale coverage published measurements at 37 stations are complemented by our measurements at 45 stations lateral resolution of our data is many times higher than that of ss and its precursors previously used for similar purposes but we have fewer data in the oceans lateral variations of travel times of the major pds phases are strongly correlated with the lateral p and s velocity variations in the uppermost mantle as revealed by tomographic studies the scaling coefficient between the teleseismic s and p residuals as found from the variations of travel times of the major pds phases is around 3 7 for north america and around 3 3 for the rest of the world almost all the differential travel times t p 660 s t p 410 s are in the range of 1 s relative to the average value of 24 0 s if the lateral variations of the velocities within the mantle transition zone mtz can be neglected the corresponding variations of thickness of the mtz are in the range of 10 km relative to the standard depth of 250 km and do not show significant correlation with either velocities in the upper mantle above the mtz or with the variations of thickness of the mtz as found in some recent studies of precursors to ss there is a weak but marginally significant positive correlation between the variations of thickness of the mtz inferred from our data and the s velocity variations in the mtz as given by su et al 1994 this correlation suggests that both variables contain temperature dependent components lateral variability of the mtz is also manifested by extreme weakness of p 410 s at several locations this may indicate either anomalous topography on 410 km discontinuity or strong broadening of the otherwise sharp discontinuity at some stations there are arrivals that can be interpreted as p 520 s this interpretation implies that the strengh and or sharpness of 520 km discontinuity as well as its depth are laterally variable in a broad range

regional structure and tectonic history of the obliquely colliding columbus foreland basin offshore trinidad and venezuela abstract cenozoic eastward migration of the caribbean plate relative to the south american plate is recorded by an 1100 km long venezuela trinidad foreland basin which is oldest in western venezuela 65 55 ma of intermediate age in eastern venezuela 34 20 ma and youngest beneath the shelf and slope area of eastern offshore trinidad submarine columbus basin 15 0 ma recent in this study of the regional structure fault families and chronology of faulting and tectonic events affecting the hydrocarbon rich columbus foreland basin of eastern offshore trinidad we have integrated approximately 775 km of deep penetration 2d seismic lines acquired by the 2004 broadband ocean land investigations of venezuela and the antilles arc region bolivar survey 325 km of vintage gulfrex seismic data collected by gulf oil company in 1974 and published industry well data that can be tied to some of the seismic reflection lines top cretaceous depth structure maps in the columbus basin made from integration of all available seismic and well data define for the first time the elongate subsurface geometry of the 11 15 km thick and highly asymmetrical middle miocene recent depocenter of the columbus basin the main depocenter located 150 200 km east of trinidad and now the object of deepwater hydrocarbon exploration is completely filled by shelf and deepwater sediments derived mainly from the orinoco delta the submarine darien ridge exhibits moderate 20 140 m seafloor relief forms the steep 12 24 northern structural boundary of the columbus basin and is known from industry wells to be composed of 0 5 4 5 km thick folded and thrust imbricated hydrocarbon bearing section of cretaceous and early tertiary limestones and clastic rocks the eastern and southern boundaries of the basin are formed by the gently 1 7 4 5 northward dipping cretaceous paleogene passive margin of south america that is in turn underlain by precambrian rocks of the guyana shield interpretation of seismic sections tied to wells reveals the following fault chronology 1 middle miocene thrusting along the darien ridge related to highly oblique convergence between the caribbean plate and the passive margin of northern south america continuing thrusting and transpression in an oblique foreland basin setting through the early pleistocene 2 early pliocene recent low angle normal faults along the top of the cretaceous passive margin these faults were triggered by oversteepening related to formation of the downdip structurally and bathymetrically deeper and more seaward columbus basin large transfer faults with dominantly strike slip displacements connect gravity driven normal faults that cluster near the modern shelf slope break and trend in the downslope direction to the south no normal faults are present because the top cretaceous horizon has not been oversteepened as it is adjacent to the foreland basin 3 early pliocene recent strike slip faults parallel the trend of the darien ridge and accommodate present day plate motions

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

constraints on upper mantle discontinuities from observations of long period reflected and converted phases stacked images combining over 5 years of long period global digital seismograph network data reveal many phases associated with reflections and conversions from upper mantle discontinuities these images show travel time and amplitude relative to a reference phase which is aligned and normalized on all seismograms prior to stacking results obtained for p s ss and pp reference arrivals resolve numerous phases from discontinuities near 410 and 660 km while some of the stacks also show evidence for a weaker discontinuity near 520 km phases of particular interest include p and sh multiples resulting from topside reflections precursors to ss from underside reflections and p to sv converted phases these phases can be clearly seen both in the waveform stacks and in cross correlation analysis of individual seismograms travel times for these arrivals are converted to discontinuity depths relative to velocities in the preliminary reference earth model and empirical corrections are applied for the effects of lateral velocity variations in the upper mantle average apparent depths to the discontinuities for the different phases agree to within 3 km for the 410 km discontinuity to within 4 km for the 520 km discontinuity and to within 8 km for the 660 km discontinuity the best global averages are obtained from the ss precursor data which indicate discontinuities at 415 519 and 659 km discontinuity depths obtained from the p to sv converted phases at over 35 individual seismic stations exhibit variations of less than 20 km apparent depths to the 660 km discontinuity consistently show greater variability than depths to the 410 km discontinuity supporting recent laboratory results which indicate that the clapeyron slope for the 660 km discontinuity is significantly larger in magnitude than the slope for the 410 km discontinuity precursors to ss seen between 110 and 180 are particularly useful for mapping possible lateral variations in discontinuity depths since each arrival can be associated with a single underside reflection point apparent discontinuity depths computed from ss precursors for different tectonic regions agree to within about 5 km the ss precursors have especially good coverage near the subducting slabs in the northwest pacific analysis of apparent discontinuity depths in this area suggests the presence of a broad 1000 to 1500 km wide region near the slab in which the 660 km discontinuity is depressed by about 20 km measuring absolute amplitudes for these phases is difficult due to the large corrections required to compensate for the effects of incoherent stacking relative amplitude analysis suggests that the p and s wave impedance changes at 410 km are about 0 8 1 1 times the size of the changes at 660 km and that the contrasts at 520 km are between 0 3 and 0 6 of the changes at 410 km

the use of velocity spectrum for stacking receiver functions and imaging upper mantle discontinuities summary in order to improve the signal to noise ratio of receiver function data it is typical to stack receiver functions calculated from events at similar distances and back azimuths we have adapted the velocity spectrum stacking vss technique used extensively in reflection seismology to the receiver function method in order to stack data with different ray parameters thereby improving further the signal to noise ratio perhaps more importantly by producing the velocity spectrum stacks we take advantage of the differences in the shapes of the moveout curves of converted phases and reverberations to identify and separate the various phases and to infer velocity structure by comparison of velocity spectrum stacks produced from the observed data at the iris ida broad band station at obninsk russia obn with those produced from prem synthetics we have identified ps phases from the 400 and 670 km discontinuities the p to s ps converted phase from the 400 km discontinuity observed at obn is much larger compared with that of the 670 km discontinuity than is predicted by prem this suggests a higher velocity contrast at 400 km than in prem by adapting a bootstrap method to assess the depth and velocity estimates determined by vss we find that the ps phase from the 670 km discontinuity is best imaged with higher than prem upper mantle p and s velocities and at a depth of 663 3 km we find no evidence of a 220 km discontinuity beneath obn in these data

evidence for local variations in the depth to the 410 km discontinuity beneath albuquerque new mexico we stack data from the three component broadband seismic stations at pinon flat california pfo ontario canada rson and albuquerque new mexico anmo using the velocity spectrum stacking method by doing so we estimate the transition zone thickness tzt beneath southern california and ontario to be 236 and 263 km respectively beneath anmo we find a bimodal distribution in our estimates of the tzt the transition zone west of anmo appears to be similar in thickness to that beneath pfo 232 km the transition zone east of anmo is 253 km similar to that estimated for rson we conclude that surface tectonics across the rio grande rift are closely related to the transition zone structure beneath it and that the rift which separates the region of active tectonism from the stable high plains of eastern new mexico is also associated with a boundary in transition zone structure most of this 20 km change in tzt appears to be due to topography on the 410 km discontinuity this implies that beneath new mexico lateral variations in temperature do not cross the transition zone and that the 410 km discontinuity reflects surface tectonics more closely than does the 660 km discontinuity

death and transfiguration of a triple junction in the south atlantic three major lithospheric plates antarctic south american and african meet in the south atlantic near bouvet island where the mid atlantic ridge mar the southwest indian ridge swir and the american antarctic ridge converge toward a fast evolving triple junction a major magmatic pulse has recently built a new swollen segment of the swir spiess ridge that is propagating toward the mar at a rate of 4 to 5 centimeters per year disrupting a former ridge ridge ridge rrr triple junction a new triple junction will be established about 70 kilometers to the north when the propagating swir spiess segment will impact with the mar probably within the next 1 million years the american antarctic ridge will take advantage of the mar swir duel by capturing an approximately 70 kilometer stretch of mar whereas the antarctic plate will increase its size

thickness estimates of the upper mantle transition zone from bootstrapped velocity spectrum stacks of receiver functions summary we modify the receiver functions stacking technique known as velocity spectrum stacking vss so as to estimate combinations of velocity model v p and v s and depth that stack the ps conversion from upper mantle discontinuities most coherently we find that by estimating the diverences in the depths to the 660 and 410 km discontinuities using velocities that maximize the stacked amplitudes of p410s and p660s phases we can estimate the thickness of the transition zone more accurately than the depths to either of these discontinuities we present two examples indicating that the transition zone beneath obninsk russia is 252 6 km thick and that beneath pasadena california is only 220 6 km thick

semi periodic sequences and extraneous events in earthquake forecasting ii application forecasts for japan and venezuela in order to analyze observed seismicity in central japan and venezuela we applied a new method to identify semi periodic sequences in the occurrence times of large earthquakes which allows for the presence of multiple periodic sequences and or events not belonging to any sequence in the time series we also explored a scheme for diminishing the effects of a sharp cutoff magnitude threshold in selecting the events to analyze a main four event sequence with probability p c 0 991 of not having occurred by chance was identified for earthquakes with m 8 0 in central japan venezuela is divided from west to east into four regions for each of these the magnitude ranges and identified sequences are as follows region 1 m 6 0 a six event sequence with p c 0 923 and a four event sequence with p c 0 706 region 2 m 5 6 a five event sequence with p c 0 942 region 3 m 5 6 a four event sequence with p c 0 882 region 4 m 6 0 a five event sequence with p c 0 891 forecasts are made and evaluated for all identified sequences having four or more events and probabilities 0 5 the last event of all these sequences was satisfactorily aftcast by previous events whether the identified sequences do in fact correspond to physical processes resulting in semi periodic seismicity is of course an open question but the forecasts properly used may be useful as a factor in seismic hazard estimation

upper mantle seismic discontinuities and the thermal structure of subduction zones the precise depths at which seismic velocities change abruptly in the upper mantle are revealed by the analysis of data from hundreds of seismometers across the western united states the boundary near 410km depth is locally elevated that near 660 km depressed the depths of these boundaries which mark phase transitions provide an in situ thermometer in subduction zones the observed temperature contrasts require at least moderate thickening of the subducting slab near 660 km depth in addition a reflector near 210 km depth may mark the bottom of the aesthenosphere

minor and trace element geochemistry of volcanic rocks dredged from the galapagos spreading center role of crystal fractionation and mantle heterogeneity a wide range of rock types abyssal tholeiite fe ti rich basalt andesite and rhyodacite were dredged from near 95 w and 85 w on the galapagos spreading center computer modeling of major element compositions has shown that these rocks could be derived from common parental magmas by successive degrees of fractional crystallization however the p2o5 k2o ratio averages 0 83 at 95 w and 1 66 at 85 w and implies distinct mantle source compositions for the two areas these source regions also have different rare earth element ree abundance patterns with la sm ef 0 67 at 95 w and 0 46 at 85 w the sequence of fractionated lavas differs for the two areas and indicates earlier fractionation of apatite and titanomagnetite in the lavas from 95 w the mantle source regions for these two areas are interpreted to be depleted in incompatible and volatile elements although the source region beneath 95 w is less severely depleted in la and k incompatible trace element abundances in 26 samples are used to infer that the range of fe ti rich basalt from 85 w represents 19 to 35 residual liquid following crystal fractionation of a mineral assemblage of plagioclase clinopyroxene and lesser olivine the most highly differentiated samples have also had less than 1 titanomagnetite removed most samples from 85 w can be related to a common parental magma that contained approximately 9 wt feo 1 wt tio2 and had an mg number mg 100 mg mg fe2 of about 65 although the samples from 95 w cannot all be derived from a common parental magma the inferred parental magmas may have been derived by varying degrees of partial melting of a common source the fractionation sequence consists of two parts an initial iron enrichment trend followed by a silica enrichment trend we interpret the trace element data to indicate that the most iron rich lavas represent about 32 residual liquid derived by crystal fractionation of plagioclase clinopyroxene and lesser olivine from a parental magma with an mg number of about 66 the silica enrichment trend results from crystallization of titanomagnetite and some apatite fractionation of pigeonite which is a minor phase in the major element models cannot be distinguished from clinopyroxene fractionation by using trace elements

constraints on crustal and mantle structure of the oceanic plate south of iceland from ocean bottom recorded rayleigh waves abstract from april to july 2002 we carried out a deployment of 6 ocean bottom seismometers and 4 ocean bottom hydrophones in the north atlantic south of iceland during the deployment period we recorded clear rayleigh waves from 2 regional and 14 teleseismic earthquakes this corresponds to a rayleigh wave detection rate of nearly 92 for events with mw 6 06 0 and epicentral distance less than 110 close to detection rate estimates based on noise level variability we measured rayleigh wave event station group dispersion and inter station phase dispersion for one mid atlantic ridge event the group dispersion curve is sensitive to the structure of the north east atlantic with an average age of 39 myr the phase dispersion curve is sensitive to the structure just south of iceland average plate age 33 myr both dispersion curves indicate faster velocities than previously postulated for oceanic plate generated at the reykjanes ridge a grid search approach was used to constrain the range of models fitting the data the high velocity seismic lid just south of iceland in the model for the phase dispersion path is slower or thinner than in the group dispersion model which averages over a larger area and a somewhat older plate age but the velocities in the low velocity half space are similar we further consider the residual bathymetry in the experimental area the residual anomaly decreases by 300 400 m from the reykjanes ridge to the 30 myr old plate south of iceland this decrease can be explained by the disappearance of a mantle thermal anomaly associated with the iceland plume both the residual bathymetry and the surface wave data are thus consistent with the notion that the southward spreading of the icelandic plume is channelised underneath the reykjanes ridge and does not spread far outside this channel based on the experience from the pilot experiment we estimate that a minimum recording time of 13 15 months in favourable weather conditions april september is required to record enough data to map the spreading plume with surface waves and to produce a tomographic image to a depth of 1000 km using body waves this can be achieved by a continuous deployment of at least 20 months or by two or three deployments during the spring and summer of consecutive years

constraining crustal and uppermost mantle structure beneath station tbz trabzon turkey by receiver function and dispersion analyses summary we jointly invert teleseismic radial component receiver functions and regional rayleigh and love surface wave group velocities for 1 d shear wave velocity structure beneath station tbz located on the northern side of the eastern pontides an influence factor is employed to control the relative influence of receiver function and surface wave dispersion on the resultant velocity depth profile radial and transverse component receiver functions at station tbz exhibit an azimuthal amplitude and polarity pattern consistent with 2 d receiver structure that has a general dip direction towards approximately south the radial component receiver functions are least affected by the dipping structures along the strike direction and thereby we prefer teleseismic events sampling along strike structures to alleviate the deflecting effect of dipping interfaces on the 1 d solution the 1 d inversion effectively reveals the two layer nature of the crust which is perturbed by high and low velocity layers and serves as a provisional model for the 2 d forward modelling minor to moderate changes to the 1 d model such as changing depth to and velocity contrast across an interface are needed to achieve the results with the 2 d modelling dipping interfaces and seismic anisotropy are included in the 2 d modelling to fit both radial and transverse component receiver functions the upper crust is characterized by a shear velocity of 3 5 km s 1 and cut through by a 4 km thick high velocity i e 3 8 km s 1 layer overlying the upper crust the sedimentary cover i e the top 5 km has velocities within the range 2 0 3 5 km s 1 a mid crustal velocity discontinuity between the upper granitic crust and the lower basaltic crust is identified at 16 km depth this boundary is analogous to the mid crustal discontinuity found under the black sea basin across which the shear velocity jumps from 3 5 to 4 1 km s 1 a relatively thick i e 12 km low velocity layer in the lower crust with a velocity reversal from 4 1 to 3 7 km s 1 is needed to better explain reverberations off this depth range we infer a 2 d moho discontinuity placed at 35 km depth beneath the station the proposed dip angle for the moho is rather steep i e 25 although coincident with regional gravity studies the associated sn velocity i e 4 4 km s 1 is rather low indicating disturbed upper mantle structure beneath the region initial amplitudes of transverse component receiver functions are rather energetic for which we propose substantial p and s velocity anisotropy 12 per cent for the topmost depths 5 km

petrogenesis of oceanic plagiogranites by partial melting of gabbros an experimental study we performed hydrous partial melting experiments at shallow pressures 0 2 gpa under slightly oxidizing conditions nno oxygen buffer on oceanic cumulate gabbros drilled by odp ocean drilling program cruises to evaluate whether the partial melting of oceanic gabbro can generate sio2 rich melts with compositions typical of oceanic plagiogranites the experimental melts of the low temperature runs broadly overlap those of natural plagiogranites at 940 c the normalized sio2 contents of the experimental melts of all systems range between 60 and 61 wt and at 900 c between 63 and 68 wt these liquids are characterized by low tio2 and feotot contents similar to those of natural plagiogranites from the plutonic section of the oceanic crust but in contrast to fe and ti rich low temperature experimental melts obtained in morb systems at 950 c the 1 500 m long drilled gabbroic section of odp hole 735b legs 118 and 176 at the southwest indian ridge contains numerous small plagiogranitic veins often associated with zones which are characterized by high temperature shearing the compositions of the experimental melts obtained at low temperatures match those of the natural plagiogranitic veins while the compositions of the crystals of low temperature runs correspond to those of minerals from high temperature microscopic veins occurring in the gabbroic section of the hole 735b this suggests that the observed plagiogranitic veins are products of a partial melting process triggered by a water rich fluid phase if the temperature estimations for high temperature shear zones are correct up to 1 000 c and a water rich fluid phase is present the formation of plagiogranites by partial melting of gabbros is probably a widespread phenomenon in the genesis of the ocean crust

relocated aftershocks of the march 10 1988 trinidad earthquake normal faulting slab detachment and extension at upper mantle depths abstract we relocated aftershocks of the march 10 1988 earthquake off the east coast of trinidad in order to determine the fault plane for the main shock and to understand this anomalous earthquake series the march 10 main shock m w 6 6 was the first significant instrumentally recorded earthquake to occur in the epicentral region east of trinidad the reported depth of the main shock 53 km by the international seismological centre isc is unusual but is consistent with observations of pp p times the main shock harvard centroid moment tensor cmt solution nearly pure normal faulting on nodal planes striking ene is also unusual given the depths of the seisms and the rarity of extensional faulting at these depths using p arrival times tabulated by the isc we relocated the aftershocks using the joint hypocentral determination method to find new locations relative to the main shock in map view the relocated epicenters form a tight elliptical cluster with major axis trending wnw ese and minor axis nne ssw virtually all the seisms in the earthquake series occurred at depths ranging from 30 to 60 km deeper than normal crustal earthquakes in an area previously not considered a locus of subducting lithosphere the aftershocks define a distinct plane dipping 34 nw this dip is consistent with the harvard cmt focal mechanism of the main shock one cmt nodal plane dips 38 n and strikes ene very close to our estimate of the active fault plane thus the march 10 aftershocks are consistent with slip on the north dipping plane of the main shock because the earthquake series occurred 100 km se of oceanic south american slab subducting beneath the caribbean plate at a depth generally inconsistent with shallow normal faulting outside of slabs we interpret the main shock as having occurred deep 50 km in south american lithosphere currently moving nw into the subduction zone the occurrence of this earthquake indicates an extensional stress applied to the lithosphere of the south american plate likely related to a combination of slab pull to the nw as the slab subducts beneath the caribbean plate and to detachment of the subducting lithosphere from buoyant continental south america

calcite and quartz microstructural geothermometry of low grade metasedimentary rocks northern range trinidad abstract the northern range of trinidad is a mountainous exposure of deformed metasedimentary rocks with mesozoic depositional ages and tertiary metamorphic ages located in the caribbean south american plate boundary zone these rocks lack precise mineralogical indicators of metamorphic grade using temperature sensitive quartz and calcite microstructures and fission track data we identify describe quantify and map a nearly continuous spectrum of relatively low l50 c to relatively high 250 400 c deformation temperatures from east to west across the range average estimated rock exhumation rates also increase systematically along this trend the analysis illuminates a sharp east west trending major thermal discontinuity i e a fault along the southern boundary of the range kinematic analysis of faults and shear bands adjacent to this boundary and modeling indicate that the fault probably dips southward at 80 85 and accommodated normal dip slip at rates that decreased from west to east earlier experimental studies at high strain rates illustrated a strong strain rate dependency for quartz microstructural transitions our work indicates a possible strong temperature control for quartz microstructural transitions at natural strain rates

petrogenesis and tectonic implications of the iron rich tholeiitic basalts in the hutuo group of the wutai mountains central trans north china orogen abstract the hutuo group located in the wutai mountains area is characteristic of paleoproterozoic strata in the north china craton and plays an important role in establishing evolutionary processes in the trans north china orogen we present petrological observations along with geochemical and nd isotopic data for basalts in the qingshicun and hebiancun formations of the hutuo group these basalts are enriched in tfeo 11 97 18 01 wt consistent with occurrences of iron rich basalt world wide they also display relatively high mgo 4 95 12 25 wt ni 79 121 ppm and cr 37 101 ppm and low in sio 2 41 37 51 95 wt therefore the iron rich character of the hutuo group basalts originated with their parental magma rather than via shallow crustal fractionation at low oxygen fugacity as inferred for many high iron basalts the hutuo group basalts exhibit weak to moderate differentiation of light ree over heavy ree la lu cn 2 61 6 79 with minor or slight eu anomalies eu eu 0 77 1 41 and also show the nb and ti troughs without obvious zr and hf anomalies the nd t values of the basalts range from 1 23 to 0 81 with t dm of 2489 2664 ma the basalts most plausibly originated from the fertile subcontinental lithosphere mantle and in the case of the qingshicun formation basalts also underwent the crustal contamination basalts in the hutuo group also have the high zr hf ratios and present high ti v values of 24 55 and zr y ratios ranging from 4 6 to 13 6 suggesting that they were emplaced in a within plate setting considering the sedimentary associations and detrital fragments in the hutuo and gantaohe groups and bimodal magmatism along the trans north china orogen we propose that 2 2 2 0 ga geological events within the orogen were rift related but did not result from subduction or island arc processes

oblique collision and tectonic wedging of the south american continent and caribbean terranes our studies of the surface structures seismicity and gravity anomalies of the caribbean south american plate boundary zone of northeast venezuela and trinidad show that the crustal and upper lithospheric structure and kinematics of this zone are complex and differ markedly along strike we propose a model of the zone9s lateral transition from near trinidad west to caracas emphasizing three concepts 1 the steepening detachment and then sinking away of atlantic oceanic lithosphere that is was attached to the northern edge of continental south america 2 the wedging seaward and imbricate thickening of the northern margin of the continent in the region of the detaching atlantic slab the continental wedge overrides the slab and is overridden by terranes attached to the caribbean plate and 3 that the structural and kinematic transitions are a consequence of the progressive oblique collision between continental south america and the overriding caribbean terranes

geochemistry of lau basin volcanic rocks influence of ridge segmentation and arc proximity side scan sonar imaging of the central lau basin sw pacific has revealed a central lau spreading centre clsc propagating southwards at the expense of an eastern lau spreading centre elsc with a small intermediate lau spreading centre ilsc forming a relay between the two volcanic rocks sampled along these spreading centres and from two adjacent seamounts are glassy to fine grained pillow lavas and sheet flows of basalts ferrobasalts and andesites the evolved rocks are mostly confined to the propagating tip of the clsc and can be explained by a high rate of cooling relative to magma supply as invoked for magma genesis at propagating ridges elsewhere compared with equivalent rocks from the eastern pacific the most evolved members of the clsc suite require similarly high degrees 90 of fractional crystallization from their basaltic parents fractional crystallisation cannot however account for the compositional differences between clsc and elsc lavas whereas the composition of the clsc lavas lies just within the compositional spectrum of typical n morb the elsc lavas are distinctly enriched in alkali and alkaline earth elements reach oxide and apatite saturation at lower fe ti and p concentrations and generally show greater vesicularity despite slightly greater depths of eruption all indicative of a water rich subduction component they also have lower contents of ni sc na and fe and higher contents of si at a given mgo concentration that indicate a more depleted and more hydrous mantle beneath the elsc compared with the clsc these results provide further evidence that beneath the central lau basin the source composition changes progressively from morb type to island arc tholeiite type as the subduction zone is approached both eastwards from the clsc to elsc and southwards along the elsc to the valu fa ridge they also indicate that the composition of the subduction component may vary systematically away from the arc with th lree ba rb and h as h2o all present close to the arc only ba rb and h2o present at intermediate distances and just h2o perceptible at the furthest distances

imaging crustal structure along refraction profiles using multicomponent recordings of first arrival coda in a way similar to receiver function imaging coda of the first arrivals can be used to constrain crustal structure along controlled source refraction profiles with multicomponent recording stacked cross correlations of the radial and vertical components of recordings from three peaceful nuclear explosions of the 3850 km long profile quartz russia exhibit good correlation with the depth to the basement and provide a horizontal resolution level close to recording station spacing 10 15 km the results also suggest high 0 35 0 4 average poisson s ratios within the sediments when applied to other multicomponent long range refraction profiles this approach could provide a simple and inexpensive way to constrain the structure of the upper crust that is required for interpretation of the deeper structures and that cannot be constrained by other means most importantly reverberations within the sedimentary column appear to account for much of the observed complexity of the first arrival waveforms and therefore such reverberations should be taken into account in the interpretations of seismic scattering from within the mantle manuscript received 24 january 2002

crustal scattering and some artifacts in receiver function images the existing 2d and 3d depth imaging approaches using multichannel teleseismic receiver functions rfs can result in significant artifacts caused by misinterpretation of signal generated noise resulting from broadside scattering within the crust a synthetic example using acquisition geometry of the 1993 cascadia experiment shows that scattering of the primary p wave arrivals from a sedimentary accretionary wedge could produce rf events close to those observed in real data quasi linear crustal structures with significant velocity contrasts near moho depths such as subduction zones should create particularly strong and coherent noise arrivals conventional interpretations of these arrivals based on assumptions of their origin in mode conversions could lead to images of spurious dipping structures of landward dips steeper than 10 that may be very difficult to distinguish from the true structures detailed analysis of prestack premigration rf data is required for correct identification of the signal generated noise and validation of the resulting depth images identification of scattered events in rf records could also help constrain major crustal structures such as crustal sutures and subduction fault zones and particularly those with strong moho expressions manuscript received 10 june 2003

gneiss domes metamorphic core complexes and wrench zones thermal and structural evolution of the nigde massif central anatolia 1 the nigde massif a migmatite cored structural dome in south central turkey was metamorphosed deformed intruded and exhumed in an oblique tectonic regime involving a switch from transpression prograde metamorphism to transtension exhumation an 80 ma history of transpression and transtension is recorded in metamorphic rocks of the massif and the last 50 ma are recorded in both basement and overlying basin deposits nigde metamorphic rocks record a high temperature top to north syntranspression ductile fabric that was overprinted by lower grade top to south syntranstension ductile shear zones the 40ar 39ar data for hornblende muscovite and biotite document rapid late cretaceous cooling with the oldest ar hornblende ages 88 79 ma overlapping u pb ages in monazite and zircon 92 78 ma muscovite ar ages are dominantly 81 79 ma suggesting a short time between crystallization of high temperature metamorphic rocks and their cooling biotite ar ages are 79 74 ma with the exception of disturbed samples from the eastern margin of the massif and from eocene granitic stocks in the dome core the eastern margin adjacent to the sinistral central anatolian fault zone records another cycle of burial and exhumation that overprinted earlier fabrics and disturbed isotopic systems these two cycles of burial exhumation took place repeatedly along the same zone this may be characteristic of highly oblique tectonic regimes in the nigde massif burial and exhumation became more localized and of lower amplitude over time suggesting stabilization of the crust

seismicity and tectonics of the southeastern caribbean we present 33 new focal mechanisms for se caribbean earthquakes 1963 1988 we use these mechanisms in conjunction with 28 previously available mechanisms to distinguish between two models of plate boundary zone interaction in the se caribbean the trench trench transform and hinge faulting model and the right oblique collision model shallow 0 70 km and intermediate 70 200 km depth earthquakes occur in the study region we focus on the tectonic causes of these events and the motions they delineate the shallow earthquakes are in a broad linear zone which trends ne from the paria peninsula of venezuela towards barbados intermediate depth earthquakes cluster beneath and nw of the peninsula and deepen to the nw perpendicular to the ne trending shallow events the vertical distribution of the earthquakes suggests a slab with steep nw dip shallow dextral strike slip on e striking faults is restricted to a 60 km wide linear zone between the gulf of cariaco and the western margin of the gulf of paria dextral strike slip is active only as far east as the gulf of paria and not within or east of trinidad shallow thrust events with ene striking planes distributed between the araya peninsula and the gulf of paria indicate collision at crustal levels between south america and caribbean and that folding and thrusting are still active over a 60 km interval south of the araya paria isthmus active thrusting in venezuela corroborates predictions of transpression between caribbean and south america and discounts transtensional motions between the two plates in the se caribbean the conjunction of shallow thrust strike slip and normal earthquakes in the gulf of paria at around 62 3 may be the expression of unpartitioned oblique compressive deformation in the plate boundary zone intermediate 165 km h 70 km depth thrust and dip slip events within the nw dipping slab indicate that oceanic lithosphere probably originally attached to south america subducts to the nw beneath the caribbean plate shallow normal faulting events e and ne of trinidad are expressions of plate bending about near horizontal axes parallel to the lesser antilles subduction zone we conclude that the earthquake mechanisms provide strong support for the right oblique collision model of caribbean south american plate interaction

the indian ocean nodule field petrotectonic evolution and ferromanganese deposits abstract the indian ocean nodule field ionf is significant from several points of view roughly bordered by 10 s to 16 30 s and 72 e to 80 e and located within the central indian ocean basin ciob the field hosts the world s second largest and second high grade manganese nodule deposit after the equatorial nodule belt in the north pacific ocean moreover the crust underlying this field is characterised by unique morphotectonic signatures owing to its formation between 60 and 49 ma under three variable spreading conditions fast intermediate and slow from the indian ocean ridge system iors the nodule field has been surveyed both extensively more than 0 4 million km2 area and intensively comprising of a large geophysical data set and geological sample inventory during the last two decades several morphotectonic features such as seamounts hills ridge normal lineaments and ridge parallel lineations have disturbed the apparently smooth topographic gradient 1 7000 of the seafloor here variations in the rate of spreading and formation of new oceanic crust along the ridge crest during more than one episode of india eurasia collision are imprinted in the ionf based on the nature of the ridge parallel lineations which are related to the rate of crustal accretion the field is divided in to four sectors a b c and d from north to south sectors a and c were formed at a fast rate of spreading 90 95 mm year half rate and sectors b and d were formed at an intermediate 55 mm year and slow 26 mm year rates respectively the predominance of tensional stress in sectors a and c caused asymmetrical flexuring of the seafloor resulting in widely spaced faults and folds with low amplitude and large wavelength in contrast the seafloor flexuring in sector d are closely spaced long symmetrical and of high amplitude the timing and intensity of the collision of india with eurasia is constrained by the variable intensity of these flexures suggesting probably a soft touch at 58 ma and the hard collision at about 51 ma the nodule field hosts several seamounts both as isolated entities and in linear chains which are arranged parallel to the flow lines along the direction of absolute motion of the indian plate the distribution morphology and growth patterns of a majority of these seamounts are related to spreading rate suggesting their formation at the ridge crest however many of the seamounts show more than one stage of growth with local intraplate volcanism contributing to the enlargement of the larger ancient seamounts varieties of volcanics such as tholeiitic basalts spilites ferrobasalts and pumice occur within the ionf the alteration of some of these volcanics has resulted in palagonitisation of the glass and formation of zeolites subsequently during its journey away from the ridge crest to the abyssal areas the crust underlying the nodule field witnessed intraplate volcanism this is evident from the addition of younger rocks at the base of the ancient seamounts inconsistent growth of volcanoes eruption of ferrobasalt corresponding to the areas of high amplitude magnetic zones and presence of volcanogenic hydrothermal materials vhm of 10 ka age these findings collectively hint that the ionf is geodynamically unstable and may have been volcanically active in the recent past during the last 8 ma growth of authigenic ferromanganese deposits in the form of manganese nodules and crusts has occurred in the nodule field the deposits occur at an average water depth of 5000 m the basinal geomorphology intraplate tectonic deformations and volcanic eruptions considerably influence the formation development morphology mineralogy and composition of these deposits the data show that the large seamounts reverse faults and fracture zones fzs supply rock fragments as seeds for the nodule formation the hydrogenous precipitation from the overlying water column is the primary source of metal accumulation in the nodules the secondary relatively weak intraplate eruptions along the base of ancient seamounts or lineaments subsurface igneous activity and diagenetic remobilisation of metals have also played significant role in the growth and enrichment of the deposit based on a large data set we estimate the contribution of various physico chemical parameters and model the probable conditions of formation of the ferromanganese deposits in the ionf the model also hints at the location where the resource exploitation should be rewarding

triangulation to gps and gps to gps geodesy in trinidad west indies neotectonics seismic risk and geologic implications we estimated horizontal velocities at 25 sites first surveyed in a 1901 1903 british ordnance survey triangulation and then resurveyed with gps in 1994 1995 to identify trinidad s principal active on land faults quantify fault slip rates and test for elastic locking our best fit single fault elastic dislocation model put 12 3 mm yr of dextral strike slip on the central range fault 1 2 km locking depth an apparently aseismic active fault the estimated motions also showed statistically insignificant horizontal motion 2 2 1 8 mm yr of right slip 2 7 2 0 mm yr of n s shortening on the eastward on strike extension of the el pilar fault known to be the active transform fault in venezuela repeat gps measurements made between 1994 and 2005 at two sites spanning the island north to south showed a1 4 3 mm yr eastward plate motion parallel dextral velocity differential consistent with our best fit historic 1901 1995 fault slip rate paleoseismology trenching demonstrates that the central range fault cuts 5000 year old sediment and is capped by w550 year old sediment suggesting that it may be locked and may have ruptured at least once during this time interval about w5 mm yr of slip could be taken up on the los bajos fault and additional faults in the offshore south of trinidad the existing 1901 1995 and 1994 2005 geodetic data alone cannot resolve whether the central range fault is

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

early paleozoic tectonic evolution of the chinese south tianshan orogen constraints from shrimp zircon u pb geochronology and geochemistry of basaltic and dioritic rocks from xiate nw china traditionally the chinese south tianshan has been regarded as a late paleozoic orogenic belt however little is known about the early paleozoic tectonic architecture of the region this paper presents the first evidence of cambrian ordovician morb type basalts and adakitic diorites on the southern margin of the yili plate in china basalts from xiate in southwestern tianshan show a typical transitional t morb and ferrobasalt composition which indicate a formation at a propagating spreading ridge the basalts give a weighted mean 206pb 238u crystallization age of 516 3 7 4 ma by shrimp u pb zircon dating and have experienced contact metamorphism due to the intrusion of a dioritic pluton the dioritic pluton has a weighted mean 206pb 238u crystallization age of 470 12 ma and geochemical characteristics resembling that of adakitic rocks the pluton is considered to have been formed by partial melting of garnet amphibolites from thickened lower crust in arc or continental collision settings the basalts and diorites are considered to outline the eastern extension of the early paleozoic suture zone the nikolaev line which stretches east west for hundreds of kilometers between the northern tianshan and central tianshan terranes of kyrgyzstan our findings substantiate that the yili and central tianshan plates were separated by the early paleozoic terskey ocean the terskey ocean probably closed during the early stage of the late ordovician lomize et al in geotectonics 31 6 463 482 1997 resulting in the final amalgamation of the yili and central tianshan plates consequently an early paleozoic suture zone is documented in the chinese tianshan region which is most likely represented by the north nalati fault

tectonic wedges geometry and kinematic interpretation abstract the geometry of several tectonic wedge combinations was studied in an outcrop in the basque basin western pyrenees three types of tectonic wedges were distinguished simple double and triple wedge on the surface the geometry of the observed tectonic wedges is always highly elemental emergent thrust horse or klippe the final stage in the evolution of a tectonic wedge is its delamination this can occur by the spreading of the thrusts which conform the wedge or by the development of back thrusts associated to them the relative age of the back thrust with respect to the thrust indicates whether the original structure was a simple thrust or a tectonic wedge if this criterion is applied to the pyrenean orogen this range corresponds to a tectonic wedge

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

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

surface rupture of the cariaco july 09 1997 earthquake on the el pilar fault northeastern venezuela abstract this paper discusses the surface rupture of the cariaco july 09 1997 ms 6 8 earthquake in northeastern venezuela located at 10 545 n and 63 515 w and about 10 km deep the field reconnaissance of the ground breaks confirms that this event took place on the ene wsw trending onshore portion of the dextral el pilar fault between the gulfs of cariaco and paria which is part of the major wrenching system within the caribbean south america plate boundary zone dextral slip along this fault was further supported by the structural style of this rupture en echelon right lateral r shears connected by mole tracks at restraining stepovers and by larger geometric complexities pop ups at las manoas and guarapiche as well as by the focal mechanism solutions determined for the event by several authors this 1997 surface ruptre comprised two distinct sections from west to east a a main very conspicuous continuous 30 km long rather straight 075 n trending alignment of en echelon surface breaks with a rather constant purely dextral coseismic slip of about 25 cm but reaching a maximum value of 40 cm slightly northwest of pantono and b a secondary discontinuous 10 km long boomerang shaped rupture with a maximum coseismic slip of 20 cm at guarapiche the onshore extent of the surface rupture totalled 36 km but may continue westward underwater as suggested by the very shallow aftershock seismicity this aftershock activity also clearly defined the steep north dip of the fault plane along the western rupture suggesting tectonic inheritance on this major fault from many locals accounts the rupture seems to have propagated from pantono to the west highly asymmetric bidirectionality this suggests that earthquake nucleation happened at or near the casanay guarapiche restraining bend and rupture quickly propagated westward allowing only a small fraction to progress eastwards beyond the bend additionally the large fraction of after slip or creep released is to be related to such restraining bend which seems to have partly locked slip during rupture