a mechanism for magmatic accretion under spreading centres the most active volcanic zone on earth occurs along the global mid ocean ridge system where the lithosphere is pulled apart and new oceanic crust is created at rates between 10 and 200 km myr 1 this volcanism is segmented and is concentrated along the axial valleys and highs of slow and fast spreading ridges respectively with transform fault zones separating and generally offsetting each active ridge segment volcanic activity may be nearly absent at the ridge transform intersection where a slow spreading ridge is offset and greatly reduced where a fast spreading ridge is offset implying thinner crust in these regions1 the source of magmas erupted along ocean ridges is the underlying mantle which undergoes decompression melting when rising to fill the gap between the spreading plates the resulting magma aggregates in the upper mantle and ascends to the crust due to its low density compared with the parent mantle rock the melt is believed to pool in crustal magma chambers whence it periodically erupts to the surface typically the formation ascent and aggregation of magmas along the mid ocean ridges is regarded two dimensionally and modelled in a section through the crust and mantle across the strike of the ridges see ref 2 here however we consider the problem with a three dimensional model for which we provide supporting geological evidence in which the magmas rise as a result of a gravitational instability modified rayleigh taylor in the underlying partial melt zone

axial topographic relief associated with ridge transform intersections the axial depths of mid ocean ridges are consistently observed to increase along strike from the central part of a ridge segment to its intersection with a transform fault variations in the magnitude of this axial relief probably reflect variations in the magmatic and tectonic processes that characterize plate accretion and seafloor spreading at each individual segment constrained by observations from 41 ridge transform intersections we investigate the relative effect on axial depths of transform offset segment length regional depth and spreading rate we first solve for a model of ridge segment relief as a function of the age offset of the transform and the segment length both scaled by spreading rate through a series of regression analyses covering a range of spreading rate scalings we determine a model which predicts segment relief at both fast and slow spreading centers to within 240 m standard deviation we find that half segment scale deepening is more than a direct thermal effect of the juxtaposition of a cold lithospheric edge and the end of a ridge at a transform fault a second inversion problem is solved to determine the parameters that contribute to the absolute depth of the ridge axis we can show that the absolute depth of the shallowest section of a ridge segment is strongly dependent on regional depth and that this dependence is more complex at slow spreading ridges reflecting the common but not ubiquitous existence of a median valley there these results in combination with the segment relief analysis clearly indicate that the axial depth at a ridge transform intersection is a function of both long wavelength processes such as may control hot spot distribution and local tectonic factors including but not limited to transform offset

mechanisms and depths of atlantic transform earthquakes we have studied 40 earthquakes on seven major transforms along the mid atlantic ridge the majority show the expected transform parallel strike slip motion on steeply dipping fault planes some anomalous mechanisms suggesting either unusually shallow dip or a component of dip slip motion occur near ridge transform intersections the vertical tectonism suggested by transform morphology may thus be restricted to intersections where thermal contrast across the transform is greatest and stress orientations can be oblique to both ridge and transform centroid depths determined by waveform inversion are limited to a shallow zone corresponding approximately to the area above the 400 c isotherm of the thermal structure expected from lithospheric cooling the depth extent of faulting predicted by estimating fault area from seismic moments extends approximately to the predicted 600 c isotherm assuming that the areas above the 400 c or 600 c isotherms fail seismically moment release for six of the transforms accounts for varying fractions of the slip predicted by plate motion suggesting that some aseismic slip occurs if the time period sampled is representative in contrast on the vema transform the inferred seismic slip is greater than predicted unless faulting extends to the 650 c isotherm alternatively the vema may have an unusually long recurrence time or be mechanically anomalous due to its complex history in contrast to transform events centroids for oceanic intraplate earthquakes extend to approximately the 750 c isotherm this result is intriguing if depths reflect lithospheric strength as a function of depth transforms have much higher strain rates than intraplate areas so if both had the same rheology the limiting isotherm for transforms should be higher alternatively if a weaker rheology were appropriate the higher strain rate should cancel the weakening and yield a limiting isotherm similar to that for intra plate regions transforms may thus be either weaker and or have higher temperatures than expected in the latter model the required high temperatures perhaps due to intrusion at depth predict measurable heat flow anomalies the second possibility unusual rheological weakness is reminiscent of the unexplained weakness of the san andreas fault where heat flow data exclude anomalously high temperatures

thermal model of oceanic transform faults a thennal model of the oceanic transform fault which includes effects of the lateral heat conduction across the fault and the shear heating along the fault has been developed the results show that shear heating along the transform fault significantly raises the temperature along the fault the depths to isotherms of the thermal model were compared to the depths of earthquakes along transform faults by including the shear heating the limiting isotherm of the focal depths of atlantic transform earthquakes is 700 c 600 c if not including the shear heating which is close to the limiting temperature of the intraplate oceanic earthquakes if the transform earthquakes are limited by the 750 c isotherm then the transform earthquakes should be shallow events as predicted by our thermal model the model predicts the maximum depth of the 750 c isotherm increases with the length of the transform fault and decreases with the half slip rate both the effects of the lateral heat conduction across the transform fault and the shear heating along the fault generate topographic and geoid anomalies in a 30 to 60 km wide zone at the fault with magnitudes of the order of a few hundred meters and tens of centimeters respectively given the thermal structure of the transform fault the shear traction along the fault can be estimated the average shear traction per unit length of the transform fault is of the order of 1012 n m which is comparable to the magnitude of the major plate driving forces

contrasts in morphology and deformation offshore montserrat new insights from the sea calipso marine cruise data 1 during the december 2007 sea calipso experiment we collected seismic reflection profiles offshore of montserrat off the east coast we imaged deep fans of volcaniclastic debris from three volcanoes progressively active from 2 ma to present near shelf sedimentation rates of 8 9 cm ka are approximated following cessation of local volcanic activity the fans were deposited on sediments with apparent dips towards the ese trending montserrat havers fault system mhfs in southern montserrat the mhfs encloses the soufriere hills volcano has elevated crustal blocks at roche s bluff st georges hill and garibaldi hill and extends off the west coast off the west coast the n dip of two faults supports a n dip interpretation for a major component of mhfs the belham valley fault we propose that local deformation is affected by stress redistributions consistent with a right stepping sinistral en echelon fault system but the interplay of transtension and magmatism has resulted in complex and evolving stress regimes

new insights on the tectonics along the new hebrides subduction zone based on gps results 1 at the new hebrides nh subduction zone ridges born by the subducting australia plate enter the trench and collide with the overriding margin results from gps surveys conducted on both sides of the trench and new bathymetry maps of the nh archipelago bring new light on the complex tectonics of this area convergence vectors present large variations that are not explained by australia pacific a p poles and that define four segments vectors remain mostly perpendicular to the trench and parallel to the earthquake slip vectors slow convergence i e 30 40 mm yr is found at the central segment facing the d entrecasteaux ridge the southern segment moves faster than a p motion predicts 89 to 124 mm yr relatively to a western north fiji basin wnfb reference the northern and southern segments rotate in opposite directions consistently with the extension observed in the troughs east of both segments both rotations combine in central vanuatu into an eastward translation that bulldozes the central segment into the wnfb at 55 mm yr that model suggests that the motion of the central segment forced by the subduction collision of the d entrecasteaux ridge influences the motion of the adjoining segments the new caledonia archipelago is motionless with respect to the rest of the australia plate despite the incipient interaction between the loyalty ridge and the nh margin southeast of the interaction area convergence is partitioned into a 50 mm yr trench normal component accommodated at the trench and a 90 mm yr trench parallel component close to the a p convergence and presumably accommodated by a transform boundary at the rear of the nh arc

an alternative interpretation of the cayman trough evolution from a reidentification of magnetic anomalies magnetic data were collected during the wilkes 1973 and seacarib ii 1987 cruises to the cayman trough a new interpretation of magnetic data is carried out an isochron pattern is drawn up from our anomaly identifications an early eocene age 49 ma ypresian for cayman trough opening is proposed instead of the late oligocene or middle eocene ages suggested by previous studies our plate tectonic reconstruction is simpler and fits the on land geology jamaica and cuba and the tectonics our reconstruction shows a southward propagation of the spreading centre between magnetic anomalies 8 and 6 26 and 20 ma the trough width increases by 30 km in this period the southward propagation of the cayman spreading centre from the middle oligocene to the early miocene induced the development of the restraining bend of the swan islands the formation of a 1 km high scarp on the eastern trace of the cayman trough transform fault walton fault and the formation of a pull apart basin hendrix pull apart magnetic anomalies and magnetization maps give information about the deformation and the rocks the proposed evolutionary model of the cayman trough from the inception of seafloor spreading to the present configuration is presented in relation to the tectonic escape of the northern boundary of the caribbean plate from the maastrichtian to the present

donnees sur la structure de l arc insulaire des petites antilles entre ste lucie et anguilla abstract geological and geophysical data were collected during a marine geological cruise arcante 1 carried out in 1977 1978 between st lucia and anguilla corresponding to about two thirds of the length of the lesser antilles archipelago they allow of a reconsideration of the structural framework of this island arc considered as a whole the lesser antilles ridge is characterized by a pattern contrasting sharply between its northern and southern halves the major discontinuity axis runs to the south of the guadeloupe archipelago the basement of the northern half is supposed to be mainly mesozoic and is connected with the evolution of the greater antilles whilst the southern half was generated only since the eocene the ne flanks of the lesser antilles ridge are steep and dissected by large submarine valleys such as the desirade sea trough their recent sedimentary cover post lower miocene is lacking or very poor enabling the outcropping of older formations volcanic mesozoic basement around la desirade upper cretaceous eocene and oligocene sediments conversely the nw flanks are gently sloping and are draped by a rather thick overburden of plio pleistocene deposits to the south the steep sw flanks are covered with a moderate thickness of recent sediments and display characteristics contrasting with those of the se flanks facing the barbados accretionary prism from the south up to martinique there is only one volcanic ridge of eocene to present age to the north of martinique this ridge splits into two axes the submarine landmarks of these two arcs have been disclosed during the present survey the eastern one or outer arc shows a row of seamounts and islands often marked by strong magnetic dien bien phu bank marie galante agouti bank or heavy gravimetric marie galante grande terre anomalies this outer arc was superimposed on the pre eocene basement and is expressed in the eocene and oligocene volcanic terrains cropping out in the islands of antigua st bartholomew st martin and anguilla which rise from the wide northern insular shelves a major ca 70 km interruption in the continuity of this axis takes place between dien bien phu bank and marie galante la desirade and barbuda islands although pertaining to the limestone caribbees are not included in the outer arc the western or inner arc made up of upper miocene to present volcanics begins with dominica island and terminates at luymes bank but an interruption of about 50 km is to be noted between basse terre and montserrat the outer and inner arcs are separated by the narrow ca 50 km and elongated ca 250 km kallinago depression this intra arc trough is generally filled by less than 1 km of recent sediments and is subdivided into three smaller basins it was not initiated by extensional rifting related to back arc spreading as was previously assumed it is possible that from martinique to guadeloupe an intermediate arc was in activity during the miocene data from exploratory oil surveys provide some new insights into the geology and the structure of the wide saba bank located to the west of the inner arc and near the northern end of the archipelago its volcanic substratum is of upper cretaceous paleocene age and is part of the pre eocene basement of the northern lesser antilles the general characteristics of the ne flanks of the lesser antilles ridge may indicate some tilting toward the west of the northern part of the island arc this may be ascribed to the arrival of atlantic fracture zone ridges barracuda and tiburon in the subduction zone during the miocene this event could explain the jump of the volcanic line from the outer to the inner arc

pliocene to pleistocene vertical movements in the forearc of the lesser antilles subduction insights from chronostratigraphy of shallow water carbonate platforms guadeloupe archipelago an integrated stratigraphic study was conducted on the shallow water carbonate platforms of the guadeloupe archipelago to refine the tectonic evolution of the lesser antilles forearc the carbonate platforms are now dated to the zanclean calabrian interval and their demise occurred between 1 5 and 1 07 ma the precise chronostratigraphy allows dating of the main extensional tectonic events since the late miocene an initial episode occurred during the late miocene related to the reactivation of inherited n130 e trending shear zones and led to the emergence of most parts of the forearc subsequently zanclean to early piacenzian carbonate platforms developed in association with a general subsidence of the forearc during the late piacenzan a second extensional episode occurred at this time la desirade underwent major uplift and emergence whereas most of the forearc remained submerged prior to 1 07 ma a third north south extensional episode occurred and led to the final demise of the carbonate platforms thus the forearc was characterized by general subsidence since the early pliocene interrupted by three main extensional episodes and related differential uplifts this suggests that the lesser antilles subduction is probably erosive north of latitude 15 n since c 5 ma related to aseismic ridge subduction supplementary data planktonic foraminiferal assemblages calcareous nannofossil taxa associations 40 ar 39 ar geochronological data and palaeomagnetic data are available at www geolsoc org uk sup18724

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

paleomagnetic evidence for tertiary anticlockwise rotation in southwest puerto rico late cretaceous and eocene paleomagnetic results from calcareous mudstone limestone and volcaniclastic rocks in southwest puerto rico yield a significant anticlockwise discordance in magnetization direction relative to north american reference data progressive thermal demagnetization has revealed stable characteristic high blocking temperature magnetization components that display an intrasite bipolarity the direction of this stable magnetization in tilt corrected coordinates is consistently either northwest and down or southeast and up the pole position 39 4 n lat 209 6 e long dp 9 3 dm 16 7 n 10 suggests post eocene anticlockwise rotation of the sampling region relative to north america by 45 about a vertical axis a rotating block domain model in which north south compression is superimposed on wnw ese strike slip faults in sw puerto rico can explain the inferred rotation

anomalous opening of the equatorial atlantic due to an equatorial mantle thermal minimum abstract the geology of the equatorial atlantic is dominated by a broad east west megashear belt where a cluster of large fracture zones offsets anomalously deep segments of the mid atlantic ridge mar the origin and evolution of this megashear region may lie ultimately in an equatorial mantle thermal minimum the notion of a mantle thermal minimum in the equatorial atlantic is supported by an equatorial minimum of zero age topography a maximum in mantle shear waves seismic velocity and a minimum in the degree of melting indicated by the chemistry of mar basalts and peridotites this thermal minimum has probably been a stable feature since before the cretaceous separation of africa from south america it caused a pre opening equatorial continental lithosphere thicker and colder than normal the cretaceous benue trough in western africa and the amazon depression in south america are interpreted as morphostructural depressions created or rejuvenated by strike slip transpressional and transtensional tectonics ducing extension of the cold thick equatorial lithosphere the oceanic rift propagating northward from the south atlantic impinged against the equatorial thicker colder and therefore stronger than normal continental lithosphere that consequently acted as a locked zone this and a low magmatic budget due to the cold upper mantle caused a lower than normal rate of propagation of the oceanic rift into the equatorial belt with diffuse deformation during mostly amagmatic extension the thick cold lithosphere prevented major cretaceous igneous activity from the st helena plume eventually initial weak isolated nuclei oceanic lithosphere were emplaced separated by e w continent continent transforms opening occurred largely by strike slip motion along these initial transforms the consequences were that the equatorial atlantic opened prevalently along an e w direction in contrast to the n s opening of the north and south atlantic and that sheared continental margins are particularly well developed in the equatorial atlantic after further continental separation the cold equatorial mantle caused a low degree of melting with na rich morb and alkali basalt rather than normal morb and with undepleted mantle peridotities thin crust depressed ridge segments and a prevalence of amagmatic extension similar conditions still exist today long transforms offsetting short ridge segments kept sea floor spreading unstable and dominated by transform tectonics with transform migration transpression and transtension causing strong vertical motion emersion and subsidence of lithospheric blocks development of deep pull apart basins and preservation of relict slivers of old lithosphere occasionally even of continental lithosphere within younger crust the equatorial transforms are caused ultimately by a long lived thermal minimum in the upper mantle and not vice versa however they then create second order rebound thermal effects that help maintain the thermal minimum in the upper mantle it can be speculated that mantle thermal minima at the earth s equator might be related to true polar wander triggered by subduction of dense masses into the mantle

the spacing of rift axis highs dependence upon diapiric processes in the underlying asthenosphere abstract detailed bathymetric mapping has revealed a near harmonic distribution of intermediate wavelength morphologic highs on slow to fast spreading centers one model suggests that the highs may result from the thermal instabilities generated by the interaction of cold fracture zone walls with the axial asthenosphere this may force a deepening of the isotherms near the fracture zone and an uplift of the isotherms midway between the fracture zones a preferred model based on newly accumulated data suggests that the highs may be a result of upwelling within the asthenosphere underlying the ridge axis where a linear region of instability develops at the base of a partial melting zone less dense material at depths of 40 80 km may penetrate the overlying material at discrete and ideally evenly spaced points harmonically spaced upwellings should generate excess magma at discrete points on the ridge axis creating volcanic highs the spacing between highs may depend upon the continuity and thickness of the deep axial instability layer the temperature and viscosity ratios within the magma and the distance between major transform faults instability of the diapirs may stimulate the migration of the highs through time

actively evolving microplate formation by oblique collision and sideways motion along strike slip faults an example from the northeastern caribbean plate margin abstract the pattern of folding faulting and late quaternary coral reef uplift rates in western and central hispaniola haiti and dominican republic suggest that the elongate gonave microplate a 190 000 km2 area of the northeastern caribbean plate is in the process of shearing off the caribbean plate and accreting to the north american plate late cenozoic transpression between the southeastern bahama platform and the caribbean plate in hispaniola has inhibited the eastward motion of the northeastern corner of the plate transpression is manifested in western and central hispaniola by the formation of regional scale folds that correspond to present day anticlinal topographic mountain chains continuous with offshore anticlinal ridges areas of most rapid quaternary uplift determined from onland coral reefs 125 ka and younger coincide with the axial traces of these folds offshore data suggest recent folding and faulting of the seafloor onshore reef data do not conclusively require late quaternary folding but demonstrate that tectonic uplift rates of the axial areas of the anticlines decrease from the northwest peninsula of haiti 0 37 mm yr to to the central part of the coast of western haiti 0 19 mm yr to the south central part of western haiti 0 mm yr formation of the 1200 km long enriquillo plantain garden walton fault zone as a bypass strike slip fault has isolated the southern edge of the gonave microplate and is allowing continued unimpeded eastward motion of a smaller caribbean plate past the zone of late neogene convergence and quaternary uplift of coral reefs in hispaniola offshore seismic reflection data from the jamaica passage the marine strait separating jamaica and haiti show that the enriquillo plantain garden fault zone forms a narrow but deep active fault bounded trough beneath the passage the active fault is continuous with active faults mapped onshore in western haiti and eastern jamaica the bathymetric deep is present because the jamaica passage fault segment represents a 50 km wide transtensional left step of the fault trace between haiti and jamaica onshore satellite imagery and field observations suggest that the enriquillo plantain garden fault forms a continuous trace extending from central hispaniola east of lake enriquillo dominican republic to the westernmost end of the southern peninsula of haiti the regional lineament corresponds to a recent fault scarp in quaternary alluvium of the clonard pull apart basin in the central part of the southern peninsula of haiti and suggests that at least this part of the lineament has undergone recent slip calmus 1983 has suggested a total offset of 30 50 km of the enriquillo plantain garden fault zone in the southern peninsula of haiti using the apparent offset of lithologic units seismic reflection data from lake enriquillo document recent deformation of quaternary lake sediments where the lineament crosses the lake leveling of the crest of a late holocene coral reef and associated algal tufa around lake enriquillo demonstrate late holocene vertical movement and tilting in a 1500 m wide zone parallel to the fault trend lateral offset is difficult to show in the enriquillo valley area because of rapid recent sedimentation into the valley the pattern of inactive strike slip faults and fold belts of cenozoic age in cuba and the yucatan basin suggest that two elongate microplates were sheared off the proto caribbean plate and accreted to the north american plate by a similar process in paleocene and eocene times age of terminal deformation in western central and eastern cuba is consistent with southeastward younging and migration of arc collision the similarity of the size and sequence of events in cuba suggests that the process of oblique collision and sideways motion of the plate along a new strike slip fault towards a free face may be an important process of microplate formation and interplate transfer in other areas

collisional model for rapid fore arc block rotations arc curvature and episodic back arc rifting in subduction settings 1 we review geophysical and geological data from 23 active and ancient plate boundaries to document a compelling spatial and temporal relationship between collision plate boundary curvature rapid tectonic rotations and the occurrence of back arc rifting our observations support a conceptual model where the change from subduction to collision causes rapid fore arc rotation when viewed in a reference frame relative to the bounding tectonic plates leading to marked plate boundary curvature our global compilation reveals that most active back arc rifts are associated with rapidly rotating fore arcs and nearby collisions thus we propose that collision and rapid fore arc rotations play a major role in the evolution and kinematics of back arc basins we conduct numerical modeling to better understand the physical processes behind these observed rapid tectonic rotations at convergent margins our results suggest that the presence of an indentor or choke point in the subduction system e g collision can generate rapid rotation of the fore arc about a nearby pole and lead to back arc rifting the rate of fore arc rotation and back arc rifting depends on the incoming indentor velocity and can be greatly enhanced by slab rollback and the presence of a low viscosity back arc where viscosity of the back arc is low fore arc rotation dominates where back arc viscosity is high the formation of strike slip faults and tectonic escape dominates our observational and model derived results illustrate that shallow crustal forces produced by the entry of buoyant features into subduction zones are a fundamental mechanism for the generation of fore arc block rotations plate boundary curvature back arc rift evolution and tectonic escape in subduction systems rollback of the subducting slab within the mantle will certainly enhance the processes we describe but it is not the only driving force

the tertiary geodynamical evolution of the aegean arc a paleomagnetic reconstruction abstract the paleomagnetic results obtained in the last 5 6 years from tertiary formations in the aegean domain indicate that the lower miocene arc was almost rectilinear with an e w trend and that its curvature has been acquired tectonically in two major phases during the middle miocene a first phase of deformation is characterized by rotations occurring at the two terminations of the arc clockwise in the west epirus anticlockwise in the east southeastern anatolia a second phase of rotation occurring in the last 5 ma about a pole situated in the southern adriatic sea has affected only the northwestern part the data also indicate that the antalya basin has not been involved in the geodynamical evolution of the arc so that the bey daglari represent its easternmost unit a more complex rotational pattern is observed in the internal zones the large rotations measured in evia 48 and skyros 26 probably occurred in the last 5 ma and result from rotation of fault bounded blocks in a shear zone connecting the north aegean trough to the external arc in the izmir region in the karaburun peninsula of western anatolia and in the island of lesbos significant counterclockwise clockwise or null rotations of coherent blocks have taken place during the neotectonic extensional regime furthermore the values of the inclination obtained from both sedimentary and volcanic oligo miocene formations are systematically shallower than expected on the basis of a geocentered dipole field this suggests that the entire studied area has undergone a northward drift of over 1000 km since the middle miocene

the east pacific rise near 21 n 13 n and 20 s inferences for along strike variability of axial processes of the mid ocean ridge abstract data obtained along various segments of the mid ocean ridge mor are used to construct an idealized model for crustal accretion the model seeks to predict the topographic volcanic tectonic and hydrothermal characteristics of any given spreading segment of the mor as a function of distance away from the bounding transform faults this model is based on a series of detailed mapping efforts carried out on segments of the mor having a broad range of spreading rates between 2 and 16 cm yr this paper includes the results of the french searise program carried out during the summer of 1980 aboard the n o jean charcot and two american cruises conducted in 1981 aboard the r v melville

caribbean intra plate deformation paleomagnetic evidence from st barthelemy island for post oligocene rotation in the lesser antilles forearc abstract as subduction zones and their related processes are often studied in 2d or cylindrical 3d sections the dynamic effects of trench curvature and its evolution through time remain under explored whereas temporal variations in trench trend may be estimated through restoring upper plate deformation we investigate the forearc deformation history of the strongly curved northern lesser antilles trench connecting the near orthogonal lesser antilles subduction zone with the motagua cayman transform plate boundary our new paleomagnetic dataset consists of 310 cores from eo oligocene magmatic rocks and limestones from st barthelemy island the limestones yielded a post folding magnetization containing a similar magnetic direction to those stored in magmatic rocks that intrude the folded carbonates both indicating a post oligocene 15 and perhaps up to 25 counterclockwise rotation of the island our results highlight that the present day trench curvature formed progressively during the cenozoic allowing us to discuss different tectonic scenarios explaining ne caribbean plate deformation and to identify key targets for future research on tectonic architecture and the potential present day activity of intra plate deformation that may pose seismic hazards

three dimensional flow beneath a slow spreading ridge axis a dynamic contribution to the deepening of the median valley toward fracture zones the axial valley along a slow spreading mid ocean ridge may be explained by a vertical pressure gradient due to viscous flow of asthenosphere upwelling in a relatively narrow conduit beneath the ridge axis along a ridge segment the axial valley floor deepens by as much as 2 3 km over distances of several tens of kilometers approaching a ridge transform intersection this deepening may be explained in part by a pressure gradient associated with horizontal flow in the ridge axis conduit vanishing vertical velocity on a conduit end wall at a ridge transform intersection results in reduced vertical flow within about one conduit width of the intersection horizontal flow along the conduit toward the intersection must occur to form lithosphere at a uniform rate along the ridge axis a simple model with a conduit of uniform width that terminates with vertical planar endwalls at ridge transform intersections is considered the model is based on an analytical solution for flow in a narrow conduit and boundary layer approximations for the flow structure near the vertical end walls this model indicates that the induced horizontal flow and pressure gradient extend for a distance along the ridge axis of several lithosphere thicknesses from an intersection for reasonable values of the conduit width and asthenosphere viscosity the model predicts that induced horizontal flow can contribute significantly to the deepening of an axial valley approaching an intersection

solution to the volume problem in serpentinization kernel pattern consists of a rim of completely serpentinized peridotite surrounding a core of partly serpentinized or unserpentinized peridotite it is found adjacent to shear zones in some serpentinites kernels are commonly rectangular with length width ratios of 1 to 3 the boundary between the completely serpentinized rim and the unserpentinized core is commonly cut at a high angle by cross fractures the cross fractures which contain either picrolite or chrysotile asbestos taper out in the core and widen in the rim they are almost always perpendicular to the core rim boundary and point into the core of the kernel in this sense they are radially distributed in the rim the kernel pattern can be observed in thin section in outcrop and on the megascopic scale of tens of metres cross fractures represent expansion fractures in serpentinite in the rim of the kernel caused by an increase in volume acompanying serpentinization of the peridotite in the core of the kernel as the serpentinization front penetrates into the kernel the serpentinized peridotite expands however the serpentinite in the rim cannot expand and must fracture to accommodate the increase in volume as serpentinization proceeds more expansion occurs extending the fractures farther into the core and requiring the fractures in the existing serpentinite to widen these fractures are subsequently filled by serpentine minerals the resulting pattern consists of a rim of serpentinite cut by serpentine veins that surrounds a core of peridotite measured densities of cross fractures in the rims of kernels are consistent with those expected from the magnitude of the volume change accompanying serpentinization of peridotite

a test of alternative caribbean plate relative motion models two contrasting models have been proposed to describe the present day motions between the caribbean plate and neighboring plates one model based on both north america caribbean and cocos caribbean data assumes that north america caribbean motion is reflected by the spreading rate approximately 2 cm yr inferred from magnetic anomalies at the cayman spreading center and the azimuths of nearby transforms jordan 1975 this geometry was used by minster and jordan 1978 in deriving global plate motion model rm2 the other model based only on north america caribbean data uses rates and azimuths inferred from the geometry of the lesser antilles wadati benioff zone sykes et al 1982 the cayman spreading center data were discounted assuming that they underestimate the full north america caribbean motion owing to the complex tectonics of the greater antilles the latter model assumes plate convergence at the lesser antilles at a rate about twice as fast as and an azimuth differing by 25 from the jordan model the two models also differ in their predictions of south america caribbean motion the jordan model predicts oblique convergence whereas sykes et al predict oblique divergence we use the nuvel 1 global relative motion data set which incorporates recent data not used in earlier studies to discriminate between the alternative models we find that the jordan geometry provides a better fit to the direction of north america caribbean motion moreover we find that models based on this geometry better fit the direction of cocos caribbean motion observed in earthquake slip vectors along the middle america trench since the prediction of this direction depends strongly on the rate of north america caribbean motion we conclude that the jordan model better describes caribbean plate motions the discrepancy between the two models results from the different methods of estimating plate motions sykes et al 1982 report that they determined a caribbean north america convergence rate of approximately 4 cm yr from the length of the lesser antilles wadati benioff zone we employ a similar procedure and obtain a rate closer to 2 cm yr we also find that the method they report using to determine the convergence direction is unable to distinguish between the alternative azimuths we thus conclude that estimates of the rate and direction of plate motion from wadati benioff zone configurations are insufficiently robust and that such estimates are better derived using rates from magnetic anomalies and azimuths from transform faults and slip vectors we further conclude that successful estimates of relative motion require internal consistency between the euler vectors of all relevant plates as demonstrated by the powerful cocos caribbean constraint not incorporated in the sykes et al model

plate kinematics of the south atlantic chron c34 to present a high resolution seafloor spreading history of the south atlantic since chron c34 is constrained by a combination of seasat altimeter data and underway marine geophysical data a set of 45 finite rotation poles defines the relative position of africa and south america at roughly 2 m y intervals a set of 12 stage poles constrain the relative motion of these two plates at 5 to 10 m y intervals the position of the stage poles continuously migrates reflecting the continuously changing azimuths of the fracture zones major changes in spreading direction are observed in the late cretaceous and early cenozoic as the fracture zones sweep out broad s shaped curves similar to the pattern seen on the kane fracture zone in the central atlantic small offset fracture zones were found to be the most accurate recorders of changes in plate motion large offset fracture zones such as the agulhas falkland fracture zone were the least reliable recorders at 30 s spreading rates decrease throughout the late cretaceous from a high of 75 mm yr at the end of chron c34 to a low of 30 mm yr around chron c27 a period of slow spreading between chron c30 and chron c20 corresponds to a zone of fracture zone proliferation an increase in the amplitude of geoid anomalies over fracture zones greater relief on topographic profiles and locally evidence of intraplate crustal deformation spreading rates increase at chron c20 to about 50 mm yr and then gradually decrease during the late paleogene and neogene a comparison of synthetic fracture zones based on the south atlantic stage poles to the observed trends of fracture zones in the equatorial atlantic indicates that the vema and marathon fracture zones were generated by south atlantic spreading as opposed to central atlantic spreading at least as far back as 35 m y ago and possibly 50 m y ago

segmentation and kinematics of the north america caribbean plate boundary offshore hispaniola we explored the submarine portions of the enriquillo plantain garden fault zone epgfz and the septentrional oriente fault zone sofz along the northern caribbean plate boundary using high resolution multi beam echo sounding and shallow seismic reflection the bathymetric data shed light on poorly documented or previously unknown submarine fault zones running over 200 km between haiti and jamaica epgfz and 300 km between the dominican republic and cuba sofz the primary plate boundary structures are a series of strike slip fault segments associated with pressure ridges restraining bends step overs and dogleg offsets indicating very active tectonics several distinct segments 50 to 100 km long cut across pre existing structures inherited from former tectonic regimes or bypass recent morphologies formed under the current strike slip regime along the most recent trace of the sofz we measured a strike slip offset of 16 5 km which indicates steady activity for the past 1 8 ma if its current gps derived motion of 9 8 2 mm yr has remained stable during the entire quaternary

upper mantle heterogeneity below the mid atlantic ridge 0 15 n small scale variations in composition of mantle derived peridotites have been investigated in the 0 15 n portion of the mid atlantic ridge mar thanks to a relatively close spaced peridotite sample coverage achieved by combining samples collected by russian and u s expeditions areal variations in the composition of mantle equilibrated minerals olivine orthopyroxene clinopyroxene and spinel have been interpreted as due primarily to regional variations in the initial composition degree of partial melting and thermal structure of the upper mantle mantle rocks from the eastern part of the romanche transform frequently contain a trapped fraction of basaltic melt while undepleted mantle prevails in the western part of the romanche suggesting a cold upper mantle thermal regime in this region which prevented significant melting immediately to the north the st paul fracture zone fz upper mantle shows intermediate degrees of melting except for st peter paul island which exposes metasomatized mantle rocks chemically and isotopically different from other oceanic peridotites between st paul fz and 4 n strakhov fz we have an area of strongly depleted upper mantle farther north the doldrums fz area 8 n appears to be underlain by moderately depleted upper mantle with some melt entrapment the vema fz 11 n is underlain by relatively homogenous upper mantle which has undergone a rather low degree of melting the mercurius and marathon transforms between 12 and 13 n expose moderately depleted peridotites finally the 15 20 fz area shows relatively undepleted upper mantle on the northern side of the transform and at sites distant from the mar axis and strongly depleted mantle south of the transform the strongly depleted mantle from the 2 3 n and 14 15 n regions is associated spatially with light rare earth element enriched mid ocean ridge basalt showing a hot spot type geochemical signature the areal association of refractory peridotites with enriched basalt and with zero age topographic highs in the 2 3 n and 14 15 n regions can be explained either by the influence of mantle thermal plumes or by the presence in the mantle of metasomatized h2o rich domains which would cause enhanced melting and provide a source for basalt enrichment these mantle domains might be relicts of an originally subcontinental mantle

muertos trough subduction microplate tectonics in the northern caribbean the relative plate motion between the north american and caribbean plates has been commonly assumed to take place in the form of oblique subduction along the puerto rico trench1 west of hispaniola this plate boundary becomes pure strike slip nevertheless it has been suggested that this boundary is more diffuse with part of the plate motion taken up by subduction along the muertos trough mt south of hispaniola2 5 fig 1 but there has been no direct seismological evidence of subduction to confirm this here we present the results of a study of the 24 june 1984 earthquake beneath mt the focal mechanism shows thrusting at a depth of 32 km which together with reports of a great earthquake in 1751 confirms that mt is an active plate boundary this implies that plate motion is accommodated by several tectonic boundaries delineating microplates caught between the caribbean and north american plates fig 4

tectonic evolution and mantle structure of the caribbean we investigate whether predictions of mantle structure from tectonic reconstructions are in agreement with a detailed tomographic image of seismic p wave velocity structure under the caribbean region in the upper mantle positive seismic anomalies are imaged under the lesser antilles and puerto rico these anomalies are interpreted as remnants of atlantic lithosphere subduction and confirm tectonic reconstructions that suggest at least 1100 km of convergence at the lesser antilles island arc during the past 45 myr the imaged lesser antilles slab consists of a northern and southern anomaly separated by a low velocity anomaly across most of the upper mantle which we interpret as the subducted north america south america plate boundary the southern edge of the imaged lesser antilles slab agrees with vertical tearing of south america lithosphere the northern lesser antilles slab is continuous with the puerto rico slab along the northeastern plate boundary this results in an amphitheater shaped slab and it is interpreted as westward subducting north america lithosphere that remained attached to the surface along the northeastern boundary of the caribbean plate at the muertos trough however material is imaged until a depth of only 100 km suggesting a small amount of subduction the location and length of the imaged south caribbean slab agrees with proposed subduction of caribbean lithosphere under the northern south america plate an anomaly related to proposed oligocene subduction at the nicaragua rise is absent in the tomographic model beneath panama a subduction window exists across the upper mantle which is related to the cessation of subduction of the nazca plate under panama since 9 5 ma and possibly the preceding subduction of the extinct cocos nazca spreading center in the lower mantle two large anomaly patterns are imaged the westernmost anomaly agrees with the subduction of farallon lithosphere the second lower mantle anomaly is found east of the farallon anomaly and is interpreted as a remnant of the late mesozoic subduction of north and south america oceanic lithosphere at the greater antilles aves ridge and leeward antilles the imaged mantle structure does not allow us to discriminate between an intra americas origin and a pacific origin of the caribbean plate key points tectonic reconstructions for the caribbean validated by seismic tomography cenozoic lesser antilles and mesozoic greater antilles subduction confirmed tomography gives insight into geodynamics of lesser antilles

tectonics of the virgin islands basin north eastern caribbean the tectonics of the virgin islands basin are controlled by the plate boundary between the puerto rico virgin islands microplate and the stable part of the caribbean plate several contradicting theories about the formation and development of this basin have previously been proposed as part of the danish galathea 3 expedition extensive marine geological investigations of the basin were carried out in march 2007 including sediment coring and acquisition of multi beam and two dimensional seismic data this paper represents a summary of the key observations from the multi beam and the seismic data set the interpretation of these observations leads to the proposition of a tectonic model for the virgin islands basin the model consists of n s to nw se directed extension combined with e w trending sinistral strike slip and the new structural evidence from the virgin islands basin is entirely consistent with the most recently published gps data

dust accumulation in the new zealand region since the last glacial maximum abstract loess is widespread in new zealand deposits 1 m thick cover 10 of the land area it has mainly been derived from dust deflated from river floodplains during the last glacial maximum lgm dust accumulation continues today downwind of major river floodplains most loess is quartzofeldspathic having its origins in mesozoic and neogene rocks of the axial ranges and hill country in the central north island there are deposits of volcanic loess derived from aeolian reworking of tephras loess morphology and properties vary greatly due to diverse parent materials post depositional climates and drainage conditions the widespread 26 170 cal yr kawakawa tephra provides a datum for calculating mass accumulation rates mars rates are mostly within the range 70 150 g m 2 yr 1 but enhanced deposition at one site gave a rate of 360 g m 2 yr 1 contemporary mars of 40 100 g m 2 yr 1 were determined for distances of 1 75 0 4 km downwind of the rakaia river lgm mars of quartz for two marine cores p69 q858 drilled 100 300 km east of new zealand are 40 70 g m 2 yr 1 the mar of the aeolian component of p69 is estimated to be ca 15 g m 2 yr 1

mapping of a segment of the romanche fracture zone a morphostructural analysis of a major transform fault of the equatorial atlantic ocean one square nautical degree within the active part of the romanche fracture zone was surveyed at about long 17 w during the 1988 equamarge ii cruise of the r v jean charcot to the equatorial atlantic seabeam mapping and seismic reflection profiles of the trench and adjacent northern and southern transverse ridges emphasize the contrast between the apparent simplicity of the major morphostructural lineaments and the complexity of the second order features the vertical displacement associated with the main strike slip movement of major oceanic transform faults is clearly illustrated by positive topographic anomalies of the transverse ridges the bathymetric map and samples collected confirm that the flat topped northern transverse ridge actually emerged at a time when peridesertic climate shaped basaltic basement into ventifact pebbles coated with desert varnish

tectonic and sedimentary architecture of the karukera spur a record of the lesser antilles fore arc deformations since the neogene the karukera spur is an easternmost submerged rise of the lesser antilles fore arc in the guadeloupe archipelago culminating about 4000 m above the fore arc basin 150 km west of the deformation front the analysis of 3500 km 2d high resolution multichannel seismic reflection and multibeam bathymetric data and the study of 14 core samples allow for the first time to reconstruct its sedimentary anatomy and tectonic evolution seven seismic units us1 to us7 are evidenced organised into four major depositional sequences separated by erosional surfaces units us1 to us4 thicken to the south southeast on the spur the sedimentary deposits rest upon a deeply eroded metamorphic basement of the caribbean plate as in la desirade island sequence 1 units us1 and us2 remains unknown in its lower part because it does not outcrop its upper part comprises early miocene shallow water carbonates sequence s2 unit us3 only comprises an upper miocene transgressive systems tract deposited into basinal environment it is bounded by major erosional surfaces sb2 and sb3 sequence 3 us4 to us6 is zanclean to pleistocene it is dominantly composed of shallow water reefal deposits sequence 4 corresponds to recent deposits the karukera spur experienced three main extensional episodes with alternations between uplift and subsidence periods uplifts are responsible for emersions and erosions subsidence severely drowned shallow water deposits the first extensional episode led to the formation of horsts and grabens in the basement prior to early miocene deposits the second extensional episode occurred during upper miocene in accordance with data in surrounding islands the third tectonic episode developed since calabrian noticeable is the constant reactivation of n130 n150 n150 n180 and n40 trending faults since early miocene and the neoformation of e w trending faults since calabrian our results suggest that at the latitude of guadeloupean archipelago the lesser antilles subduction zone is an erosional margin because of the successive sweepings of the santa lucia then tiburon ridges

palaeomagnetism of the central cuban cretaceous arc sequences and geodynamic implications abstract a detailed palaeomagnetic study of cretaceous age volcanic and sedimentary arc rocks from central cuba has been carried out samples from 32 sites 12 localities were subjected to detailed demagnetisation experiments nineteen sites from the los paso matagua provincial and cabaiguan formations yielded high unblocking temperature dual polarity directions of magnetisation which pass the fold tests with confidence levels of 95 or more and are considered to be primary in origin the palaeomagnetic inclinations are equivalent to palaeolatitudes of 9 n for the aptian 18 n for the albian a synfolding remanence identified in 5 sites from the younger hilario formation indicates a late cretaceous remagnetisation at a palaeolatitude of 16 n our results are in good agreement with previous palaeogeographic models and provide the first high quality palaeomagnetic data demonstrating the gradual northward movement of the cretaceous volcanic arc throughout the cretaceous the declination values obtained all indicate significant and similar amounts of anticlockwise rotation from the oldest sequences studied through to the late cretaceous remagnetisation this rotation is most likely related to collision of the arc with the north american plate and transpressional strike slip movement along the northern margin of the caribbean plate as it progressed eastwards into the large proto caribbean basin

seismicity and seismic gap in the lesser antilles arc and earthquake hazard in guadeloupe summary a seismic study of the lesser antilles arc has been carried out first for the period 1950 1978 for which we can use local seismic networks to draw maps of instrumental seismicity then for the period 1530 1950 for which we have catalogues of felt earthquakes the striking feature of the spatial distribution of foci is the cluster of epicentres in the northern half of the arc all large earthquakes m 7 5 are located north of 14 latitude seismicity cross sections through the arc show a variable dipping subduction zone along the arc the deep seismic zone is steeper in the centre of the arc than on the extremity the time space diagram for historical seismicity and the evidence of a seismic gap at the east of guadeloupe lead us to consider the northern half arc as a likely site for a large earthquake in the near future the seismic slip rate calculated from all major earthquakes since 1530 is of much greater value than that obtained from recent plate tectonic models suggesting that the recurrence rate of earthquakes is more than many hundreds of years with a possible aseismic creep

transpressional rupture of an unmapped fault during the 2010 haiti earthquake the enriquillo plantain garden strike slip fault accommodates the relative motion between the north american and caribbean plates and was thought to have ruptured during the 2010 haiti earthquake satellite data instead indicate that a blind thrust fault possibly related to the haitian fold thrust belt was responsible and caused some contractional deformation on 12 january 2010 a mw 7 0 earthquake struck the port au prince region of haiti the disaster killed more than 200 000 people and caused an estimated 8 billion in damages about 100 of the country s gross domestic product1 the earthquake was initially thought to have ruptured the enriquillo plantain garden fault of the southern peninsula of haiti which is one of two main strike slip faults inferred to accommodate the 2 cm yr 1 relative motion between the caribbean and north american plates2 3 here we use global positioning system and radar interferometry measurements of ground motion to show that the earthquake involved a combination of horizontal and contractional slip causing transpressional motion this result is consistent with the long term pattern of strain accumulation in hispaniola the unexpected contractional deformation caused by the earthquake and by the pattern of strain accumulation indicates present activity on faults other than the enriquillo plantain garden fault we show that the earthquake instead ruptured an unmapped north dipping fault called the leog ne fault the leog ne fault lies subparallel to but is different from the enriquillo plantain garden fault we suggest that the 2010 earthquake may have activated the southernmost front of the haitian fold and thrust belt4 as it abuts against the enriquillo plantain garden fault as the enriquillo plantain garden fault did not release any significant accumulated elastic strain it remains a significant seismic threat for haiti and for port au prince in particular

incipient continental collision and plate boundary curvature late pliocene holocene transtensional hellenic forearc crete greece u ring t c brachert write in their recent paper ten veen kleinspehn 2003 proposed that the western messara graben of central crete underwent transtensional deformation since about 3 4 ma transtension is supposed to be dominantly resolved along 070 striking sinistral faults and that the 070 striking faults have strike slip normal slip ratios of 10 100 1 ten veen kleinspehn 2003 then use their results from the western messara graben to propose that sinistral transtension should represent tectonic escape of central and eastern crete during incipient collision of the libyan passive margin the interpretation of the kinematic data on which ten veen kleinspehn 2003 base their tectonic model is in contrast to earlier studies which have attributed the young i e pliocene to recent kinematics of crete largely to radial extension angelier et al 1982 fassoulas 2001 ring et al 2001 ten veen kleinspehn 2003 p 167 argued that onshore structural assemblages is critical to deciphering forearc kinematics and therefore we present our kinematic data from onshore faults in the western and central messara graben and compare our data together with the fault slip data of fassoulas 2001 from the heraklion graben and microseismicity data hatzfeld et al 1993 delibasis et al 1999 with the fault slip data of ten veen kleinspehn 2003 fig 9 for discussing the kinematics of the young faults we believe that fault slip data directly reflect the kinematics of the faults from which they were collected and therefore allow testing the proposition often veen kleinspehn that sinistral transtension is supposed to be mainly resolved at the 070 striking faults the latter of which are known to be an integral part of the youngest set of faults on crete fassoulas 2001 and are in part seismically active delibasis et al 1999 in general

rapid microplate rotations and backarc rifting at the transition between collision and subduction using global positioning system velocities from convergent plate boundaries in papua new guinea new zealand tonga vanuatu and the marianas we note a spatial correlation between rapid tectonic block rotations and the transition from subduction to collision we present a mechanism for the block rotations in which the change from collision of a buoyant indentor to normal subduction exerts a torque on the upper plate microplate this work improves our understanding of the causes of rapid vertical axis rotations often observed in paleomagnetic studies we also show how collision induced rotations may lead to backarc rifting

crustal structure of the continent ocean transition off the cote d ivoire ghana transform margin implications for thermal exchanges across the palaeotransform boundary summary crustal structure of the continent ocean transition off the cote d ivoire ghana transform margin is determined by 2 d modelling of wide angle seismic data and gravity data the resulting models together with geological data recently collected by drilling on the margin are compared with available thermomechanical models of transform margins in particular we test the predicted effects on the transform margin crust of continental heating by the adjacent oceanic lithosphere these effects include i ductile flow of the lower continental crust either parallel to the transform direction or perpendicular to it from the continent towards the ocean and ii thermal uplift of the transform margin in the region of the cote d ivoire ghana transform margin the seismic and gravity data indicate that the continental crust remains uniform in thickness as the continent ocean boundary is approached the interpreted models lack a transitional domain between continental and oceanic crusts and abnormally thin oceanic crust is found along the margin these results exclude large scale ductile flow of lower continental crust during the margin formation the geological data are moreover not consistent with thermal uplift of the margin during the period of contact between the transform margin and the spreading centre our geophysical and geological data thus infer only moderate heating of the transform continental margin by the adjacent oceanic lithosphere moderate heating of the cote d ivoire ghana transform margin during margin formation implies that the thermal exchange between the transform margin and the adjacent spreading centre were lower than predicted by available thermal models two main factors may have contributed to reduce thermal exchanges across the continent ocean boundary the first factor is that the oceanic edge may have been colder than assumed by thermal models an abnormally cold spreading axis along the transform margin when the oceanic crust was emplaced is supported by the atypical structure of the oceanic crust along the cote d ivoire ghana transform margin the oceanic crustal structure is similar to that found along oceanic fracture zones where oceanic crust is also emplaced near a ridge transform intersection in both cases abnormally low thermal gradients at the spreading centre along the transform margin may be explained by the cumulative effect of i a cold edge effect of the adjacent continental lithosphere ii accretion at the end of a ridge segment away from the main magma supply point along the spreading axis and iii an efficient upward cooling of the oceanic lithosphere owing to deep hydrothermal circulation within the faulted lithosphere the second factor causing moderate thermal exchanges is the possibility of large amounts of water in oceanic lithospheric faults which reduces the lateral thermal conduction in the lithosphere since water is much less conductive than rock during the margin formation the oceanic edge thus seems to have acted as a thermal buffer between the cold continental margin and the hot spreading centre we propose that this thermal buffer hypothesis may be generalized to most transform margins implying a lower amount of continental heating than assumed by the available thermomechanical models

diffuse impact of the mid atlantic ridge with the romanche transform an ultracold ridge transform intersection the romanche is a long offset 950 km slow slip 1 7 cm yr transform thus a hot ridge axis should meet a 50 m y old thick and cold lithosphere at the ridge transform intersection rti a strong thermal topographic transform cold edge effect is therefore predicted a morphobathymetric seismic reflection and petrologic study of the eastern romanche rti shows that as the mid atlantic ridge approaches the transform a well formed axial rift valley disappears about 80 km from the rti and is substituted by short en echelon poorly developed axial ridge segments they too disappear about 30 km from the edge of the transform valley the predicted gradual deepening of the ridge axis toward the transform was not observed an active nodal deep and an inside corner high are also absent these observations and the distribution of earthquake epicenters suggest a poorly developed diffuse rti an inactive rift valley 80 km west of the present rti suggests ridge jumping within the last 4 m y the present poorly developed rti may reflect the attempts of an embryonic spreading axis to become established and to propagate toward the transform we infer from bottom rock sampling that the basaltic crust is patchy or absent and mantle derived serpentinized peridotites outcrop ubiquitously on the seafloor starting 30 km from the edge of the transform valley the unusually deep 4 km below sea level axial ridge segments the lack of crust and the chemistry of the peridotites suggest a prevalently amagmatic regime due to an ultracold upper mantle in this region absence of basaltic crust would favor massive serpentinization of a several kilometers thick peridotite column mass balance modeling suggests that the decrease of density and volume expansion resulting from serpentinization could explain the absence of the predicted deepening of the seafloor as it approaches the transform these results suggest that the topographic effect of the transform edge thermal contrast may disappear at ultracold rtis and that ultracold rtis are magma starved short lived and unstable in time and space

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

neotectonics of the caribbean a compilation of earthquake focal mechanism solutions together with the interpretation of inclined intermediate depth seismic zones neogene strike slip faults fault controlled basins and volcanism is used to test three existing models for the neotectonic i e neogene or post oligocene development of the caribbean the current models although based on different hypotheses using different data sets are similar and suggest varying degrees of convergence between the caribbean and either north or south american plates or convergence between the caribbean plate and continental fragments of the northwestern corner of south america using model 1 previous workers have interpreted the disposition of fracture zones and magnetic anomalies in the central and south atlantic as indicating that the caribbean plate is being compressed by north south convergence between the americas with the result that it is being driven eastward relative to the americas along strike slip faults while producing east west trending subductionlike convergent zones along northern and southern plate margins using model 2 previous workers have interpreted the azimuths of active strike slip faults and focal mechanism solutions and closure of three plate velocity circuits as indicating that the south american plate converges in a west northwesterly direction on the southern margin of the caribbean plate and has produced compressional features while strike slip motion dominates along the northern boundary using model 3 previous workers have interpreted focal mechanism solutions and the pattern of strike faulting and basin development in northwestern south america to suggest right lateral displacement of the northwestern corner of south america relative to the craton with convergence and compression in the south central caribbean our integration of geologic information on faults basins and volcanism considerably extends the limited record of plate interactions known from historical reports of the past few hundred years and recorded earthquakes and suggests that 1 relative motion between the north american and caribbean plates is dominantly east west strike slip motion although areas of complex internal deformation associated with restraining bend or compressional strike slip fault segments have been identified in jamaica hispaniola and central america 2 relative motion between the south american and caribbean plates is dominantly east west strike slip motion although an area of restraining bend deformation has been identified in the araya peninsula of venezuela 3 relative motion between a triangular continental fragment of northwestern south america the maracaibo block ranges from northwest to northeast along the south central margin of the caribbean and appears to be driven mostly by the late miocene pliocene collision of the panama arc although additional driving forces may include the collision of the carnegie ridge and or oblique subduction to the northeast in ecuador and 4 relative motion internal deformation may be occurring at a very slow rate between fault bounded blocks within the caribbean plate as a result of the convergence of the maracaibo block and or eastward motion of the caribbean plate relative to a continental promontory of north america in northern central america because the caribbean margins are composed mainly of anisotropic easily deformed continental and arc lithosphere plate boundaries are zones of diffuse deformation and it is difficult to assess the degree of convergence between the americas although data from the caribbean itself suggest that it may be smaller than the amount inferred by some workers using model 1 fracture zone and magnetic anomalies in the atlantic some of the compressional features cited as evidence for the relative convergence of the americas may be instead related to the convergence between the maracaibo block relative to the southern margin of the caribbean many aspects of caribbean neotectonics remain less than completely understood but more integration of seismic and geologic studies particularly exploratory fault trenching of onshore faults and seismic profiling of offshore faults should lead to rapid advances

serpentinization faults and their role in the tectonics of slow spreading ridges two major families of faults dominate the tectonics of slow spreading ridges the transform faults paralleling the spreading direction and the gloria faults observed by large scale side scan sonar normal to it the observation of microearthquakes to 8 km depth beneath the mid atlantic ridge axis implies that not only the transform faults but the gloria faults as well penetrate into the mantle since large horizontal temperature gradients are often associated with them both types of fault provide the means of circulating large volumes of sea water through the oceanic mantle under these conditions serpentinization of upper mantle rocks will occur in a predictable way its influence on the tectonic development of slow spreading ridges is discussed

is there a northern lesser antilles forearc block 1 a systematic discrepancy exists between slip vectors of thrust fault earthquakes at the lesser antilles trench lat and the predicted direction of north american caribbean convergence a possibility has been that the discrepancy resulted because neither was well constrained estimating caribbean motion has been challenging owing to the limited data along the plate s complex boundaries similarly earlier studies had few slip vectors because interplate thrust events are infrequent to address these difficulties we estimate a new caribbean north america euler vector using recently available gps data from sites in the presumably stable interior of the caribbean and compare the predicted velocities to a larger set of slip vectors the discrepancy persists suggesting the northern lesser antilles forearc nlaf moves as a distinct entity from both the caribbean and north america for simplicity we treat its motion as a coherent block but because gps sites are not within the nlaf distributed deformation is also possible although there is no geologic evidence for the boundaries of the presumed nlaf block gps data show that the motions of martinique barbados and trinidad are similar to that of the caribbean suggesting that none are on the nlaf block and the southern lat is weakly coupled

morphotectonics of the central muertos thrust belt and muertos trough northeastern caribbean multibeam bathymetry data acquired during the 2005 spanish r v hesperides cruise and reprocessed multichannel seismic profiles provide the basis for the analysis of the morphology and deformation in the central muertos trough and muertos thrust belt the muertos trough is an elongated basin developed where the venezuelan basin crust is thrusted under the muertos fold and thrust belt structural variations along the muertos trough are suggested to be a consequence of the overburden of the asymmetrical thrust belt and by the variable nature of the venezuelan basin crust along the margin the insular slope can be divided into three east west trending slope provinces with high lateral variability which correspond to different accretion stages 1 the lower slope is composed of an active sequence of imbricate thrust slices and closed fold axes which form short and narrow accretionary ridges and elongated slope basins 2 the middle slope shows a less active imbricate structure resulting in lower superficial deformation and bigger slope basins 3 the upper slope comprises the talus region and extended terraces burying an island arc basement and an inactive imbricate structure the talus region is characterized by a dense drainage network that transports turbidite flows from the islands and their surrounding carbonate platform areas to the slope basins and sometimes to the trough in the survey area the accommodation of the ongoing east west differential motion between the hispaniola and the puerto rico virgin islands blocks takes place by means of diffuse deformation the asymmetrical development of the thrust belt is not related to the geological conditions in the foreland but rather may be caused by variations in the geometry and movement of the backstop the map view curves of the thrust belt and the symmetry of the recesses suggest a main north south convergence along the muertos margin the western end of the investigator fault zone comprises a broad band of active normal faults which result in high instability of the upper insular slope

tectonics and history of the andaman sea region abstract the andaman sea is an active backarc basin lying above and behind the sunda subduction zone where convergence between the overriding southeast asian plate and the subducting australian plate is highly oblique the effect of the oblique convergence has been formation of a sliver plate between the subduction zone and a complex right lateral fault system the late paleocene collision of greater india and asia with approximately normal convergence started clockwise rotation and bending of the northern and western sunda arc the initial sliver fault which probably started in the eocene extended through the outer arc ridge offshore from sumatra through the present region of the andaman sea into the sagaing fault with more oblique convergence due to the rotation the rate of strike slip motion increased and a series of extensional basins opened obliquely by the combination of backarc extension and the strike slip motion these basins in sequence are the mergui basin starting at 32 ma the conjoined alcock and sewell rises starting at 23 ma east basin separating the rises from the foot of the continental slope starting at 15 ma and finally at 4 ma the present plate edge was formed alcock and sewell rises were separated by formation of the central andaman basin and the faulting moved onshore from the mentawai fault to the sumatra fault system bisecting sumatra

paleomagnetic evidence for late miocene counterclockwise rotation of north coast carbonate sequence puerto rico a paleomagnetic study of the essentially undeformed middle tertiary carbonate sequence along the north coast of puerto rico reveals statistically significant pre pliocene discordance of characteristic component directions against those expected from cratonic north america for much of the section despite generally weak to moderately weak magnetic intensities confirmation of the magnetization as primary in origin comes from the presence of two distinct components of magnetization intrasite bipolarity and or the reproducibility of measurements the mean geographic direction for the upper oligocene to middle miocene strata is 335 2 degree 32 9 degree and the corrected mean paleomagnetic pole is 207 6 degree 66 5 degree n 3 alpha 95 4 3 degree this suggests a counter clockwise ccw block rotation of puerto rico and its microplate of 24 5 degrees plus minus 5 8 degrees during the late miocene using a width of 250 km for the northern caribbean plate boundary zone ncpbz between the north american plate and caribbean plate the mean left lateral displacement implied is 1 8 to 2 4 cm yr which agrees fairly well with published relative motion rates for the two plates average rotation rate for 50 ma to 20 ma was 0 7 degree my but perhaps as great as 4 degree my in the miocene resolution of mean paleolatitude indicates northward motion of a degree or lessmore during the period of rotation causes of this short lived rotation may include 1 tectonic escape from the inhibiting presence of the bahama banks and beata ridge during eastward motion of puerto rico along the sinistral transpressive puerto rico trench and muertos trough fault systems or 2 changes in relative plate motions of the caribbean and north american plate during the late miocene less

extension and transtension in the plate boundary zone of the northeastern caribbean the authors propose that the caribbean ca north american na plate boundary zone pbz from the puerto rico trench to the venezuelan basin from mona canyon east has been in left transtension over the last 15 20 ma a boundary normal component of extension occurs throughout the pbz and is a principal cause of the puerto rico trench such extension is due to wnw velocity of na ca and the northward pullaway of na from its s dipping slab which is below puerto rico strike slip motion may be taken up among terranes in the pbz by rigid ccw rotation and by oblique slip at their boundaries rotation of the largest terrane puerto rico virgin islands prvi has caused such major structures as the muertos thrust and anegada passage the model implies na ca velocity estimated from cayman transforms is more accurate than that from slip vectors from seisms in the na slab

transform migration and vertical tectonics at the romanche fracture zone equatorial atlantic the romanch transform offsets the mid atlantic ridge mar axis by about 950 km in the equatorial atlantic multibeam and high resolution multichannel seismic reflection surveys as well as rock sampling were carried out on the eastern part of the transform with the riv akademik strakhov as part of the russian italian mid atlantic ridge project primar morphobathymetric data show the existence on the northern side of the transform of a major 800 kin long aseismic valley oriented 10 o to 15 o from the active valley it disappears about 150 km from the western mar segment the aseismic valley marks probably the former location of the romanche transform paleoromanche that was active up to roughly 8 10 ma when the transform boundary migrated to its present position a temporary microplate developed during the migration and reorientation of the transform this microplate changed its sense of motion as it was transferred from the south american to the african plate a prominent transverse ridge extends for several hundred kilometers parallel to the transform on its northern side reaching its shallowest part shallower by over 4 km than the predicted thermal contraction depth in a zone opposite the eastern mar axis transform intersection rti flat top peaks on the summit of the transverse ridge are capped by acoustically transparent weakly stratified shallow water platform lagunal reef limestones this limestone unit is a few hundred meters thick and overlies igneous basement evaluation of the seismic reflection data as well as study of samples of carbonates ventifact basaltic pebbles and gabbroic peridofitic and basaltic rocks recovered at different sites on the transverse ridge suggest that 1 the summit of the transverse ridge was above sea level at and before about 5 ma 2 the transverse ridge subsided since then at an average rate 1 order of magnitude faster than the predicted thermal contraction rate its summit was flattened by erosion at sea level during subsidence 3 the transverse ridge is an uplifted sliver of lithosphere and not a volcanic construcfional feature and 4 transtensional and transpressional tectonics have affected the transverse ridge hypotheses on the origin of the romanche transverse ridge include 1 lateral heat conduction across the rti 2 shear heating 3 lithospheric flexure due to thermal stresses in the cooling lithosphere 4 viscoelastic deformation of the lithosphere 5 hydration dehydration of mantle peridotites and 6 longitudinal flow of melt and igneous activity across the rti these processes cannot by themselves explain the transverse ridge although some of them could contribute to its formation to a small extent vertical tectonics due to transpressional and transtensional events related to a nonstraight transform boundary and to regional changes in ridge transform geometry is probably the primary process that gave rise to the uplift of the transverse ridge and to its recent subsidence uplift may have been caused primarily by thrust faulting induced by transpression related to the oblique impact of the lithospheric plate against the former paleoromanche and the younger transform boundaries before and during the transition to the present boundary after migration of the transform boundary to its present position transpression was replaced by transtension and by subsidence of the transverse ridge an aseismic axial rift valley impacting against the transform valley about 80 km west of the present rti suggests eastward ridge jumping that probably followed transform migration localized transtension or transpression due to bends in the orientation of the transform may have caused intense although localized vertical movements such as those that formed an ultradeep 7800 m pull apart basin along the transform valley

late amagmatic extension along the central and eastern segments of the west philippine basin fossil spreading axis the spreading processes within the west philippine basin wpb remain partly unknown this study presents an analysis of the tectono magmatic processes that happened along its spreading axis during the conclusion of the last spreading phase at 33 30 ma we demonstrate that the late episode of n s opening from an e w trending spreading system has been followed by a late tectonic event occurring in the central and eastern parts of the basin this event was responsible for transtensional strain accommodated along the nw se faults cutting through the former e w rift valley in the central part of the basin in its eastern part the same event occurred at a larger extent and led to the creation of a new nw se axis obliquely cutting the older e w spreading segments and their associated spreading fabrics at this location several tens of kilometers of slightly oblique amagmatic extension occurred following a v60 direction we propose that this late event is associated with the onset of e w opening of the parece vela basin located along the eastern border of the wpb at 30 ma extensive stresses within this basin were probably transmitted to the hot and easily deformable rift zone of the wpb the newly created nw se axis most likely propagated from east to west being responsible for scissors opening within the wpb ne sw extension ceased when well organized spreading started at 26 ma in parece vela basin accommodating entirely the global extensive stress pattern 2002 elsevier science b v all rights reserved

transtensional deformation of montserrat revealed by shear wave splitting here we investigate seismic anisotropy of the upper crust in the vicinity of soufriere hills volcano using shear wave splitting sws analysis from volcano tectonic vt events soufriere hills which is located on the island of montserrat in the lesser antilles became active in 1995 and has been erupting ever since with five major phases of extrusive activity we use data recorded on a network of seismometers between 1996 and 2007 partially spanning three extrusive phases shear wave splitting in the crust is often assumed to be controlled either by structural features or by stress aligned cracks in such a case the polarization of the fast shear wave would align parallel to the strike of the structure or to the maximum compressive stress direction previous studies analyzing sws in the region using regional earthquakes observed temporal variations in which were interpreted as being caused by stress perturbations associated with pressurization of a dyke our analysis which uses much shallower sources and thus only samples the anisotropy of the upper few kilometres of the crust shows no clear temporal variation however temporal effects cannot be ruled out as large fluctuations in the rate of vt events over the course of the study period as well as changes in the seismic network configuration make it difficult to assess average delay times of approximately 0 2 s similar in magnitude to those reported for much deeper slab events suggest that the bulk of the anisotropy is in the shallow crust we observe clear spatial variations in anisotropy which we believe are consistent with structurally controlled anisotropy resulting from a left lateral transtensional array of faults which crosses the volcanic complex

in situ study of the eastern ridge transform intersection of the vema fracture zone abstract fourteen dives of the submersible nautile have been carried out at the eastern intersection of the vema fracture zone with the mid atlantic ridge direct observations allow us to locate precisely the surficial expression of the transform fault motion in the southern trough of the fracture zone valley the present day displacement zone corresponding to an extremely narrow less than 300 m transform fault zone is confined to a belt of disturbed terrain situated at the foot of the southern wall and characterized by transform parallel 30 100 m high ridges of basaltic fragments and pelagic sediments active splays of the transform fault zone lie generally between these ridges creating steep v shaped furrows despite a rather sinuous trend one of the fault strands appears to be laterally continuous and other minor active splays branch out from it or run parallel to it the surface manifestations of the tectonic activity along the present day displacement zone include r riedel shears low angle shears p shears en echelon structures open fissures small pull apart structures and funnel alignments in the sedimentary cover the transform fault motion reactivates large normal faults on the adjacent flanks of the southern wall and the median ridge the transform parallel topography defines a 10 km wide zone with dip slip motion the ridge transform intersection area is entirely floored by fresh basalts contrasting with other intersections of major fracture zones such as the kane and oceanographer fracture zones recent volcanic activity is not restricted to the central volcanic ridge but is widespread across the whole nodal basin area the present day displacement zone cross cuts the neovolcanic zone and the disconnected northernmost extension of the volcanic ridge is attached to the median ridge and so belongs to the african plate the extensive volcanism is suggestive of a magmatic production phase in the intersection area extremely fresh pillowed lava forms have been observed at the rti inside corner high indicating possible off axis volcanism

rate of serpentinization in seafloor environments the permeability of partially serpentinized peridotites has been measured near room temperature and found to be in the range of 10 11 to 10 12 darcys or 10 7 to 10 8 cm2 sec 1 comparable to shaley materials serpentinites act as semipermeable membranes to salt ions in ocean crust and deep groundwater environments serpentinization must be a continuous process controlled only by water access to ultramafic rocks through their cover given this it will generate high local strains and stresses with episodic cracking the serpentinization process may also contribute to heat flow and ocean floor topography

implications of paleomagnetic data on the collision related bending of island arcs paleomagnetic studies from the marianas central philippines ryukyus sulawesi fiji new hebrides all show major differences in the declination values observed from individual islands within the same arc comparable differences are not observed in the inclination values a simple plate tectonic model would predict that each island arc should experience the same rotation along its entire length as the plate moves around its rotation axis bathymetry from each of these arcs show that the differentially rotated segments of the upper plate occur in areas where the downgoing plate carries a feature such as a seamount chain rifted continental fragment or an island arc the coincidence of the bathymetric feature and the discordant paleomagnetic declination data support an earlier hypothesis of vogt 1973 that these features are buoyant zones on the downgoing plate which interact with the margin of the upper plate as a result of this interaction the upper plate boundary is locally deformed in addition to the paleomagnetic evidence other geologic and geophysical data from these areas suggest such interactions for example there appears to be a good correlation between the deflections in major structures around the collision zone and the paleomagnetic directions that are reported as this deformation proceeds eventually the stresses resulting from the collision may result in any of the following tectonic processes 1 slip line style strike slip faults allowing for the sideward extrusion of portions of the upper plate tapponnier et al 1983 2 changes in the polarity of the subduction zone 3 development of strike slip faults around the margin of the indenter or 4 reorganization of the entire plate margin in addition data from the marianas southern end of the ryukyu arc and the lau basin suggest that back arc basin development may result from the collision related deformation

three dimensional temperature structure of a ridge transform ridge system abstract we have solved the problem of three dimensional heat transfer by conduction and material transport within a ridge transform ridge system the boundary conditions are those of a finite plate with temperatures fixed at the top and bottom we do not solve for the pattern of flow so must arbitrarily describe the material transport within triangular regions beneath the spreading centers flow is assumed to be vertical and at constant velocity with the volume of upwelling matching the volume of horizontal transport by the plates the steady state solution is approached by repeated finite steps in time each involving two stages first to represent material transport temperatures in the grid of points are shifted horizontally and or vertically to the point at which the associated particle would arrive given the time period and assumed velocity field second a three dimensional fourier transform technique is used to calculate the cooling which would occur in the static case during the same period we find that the geometry of flow in the upwelling region here parameterized by velocity of upwelling or width of the upwelling zone is more important than spreading rate in determining the temperature structure the results indicate that conductive cooling cannot alone account for deepening of the median valley towards the fracture zone for a 10 m y offset on the transform fault conductive cooling across the fracture zone to the older lithosphere affects the upwelling material only to a distance of about 10 km from the fault in contrast to the deepening of the median valley over several tens of kilometers observed on the mid atlantic ridge

seismic crustal structure and morphotectonic features associated with the chain fracture zone and their role in the evolution of the equatorial atlantic region oceanic transform faults and fracture zones fzs represent major bathymetric features that keep the records of past and present strike slip motion along conservative plate boundaries although they play an important role in ridge segmentation and evolution of the lithosphere their structural characteristics and their variation in space and time are poorly understood to address some of the unknowns we conducted interdisciplinary geophysical studies in the equatorial atlantic ocean the region where some of the most prominent transform discontinuities have been developing here we present the results of the data analysis in the vicinity of the chain fz on the south american plate the crustal structure across the chain fz at the contact between 10 and 24 ma oceanic lithosphere is sampled along seismic reflection and refraction profiles we observe that the crustal thickness within and across the chain fz ranges from 4 6 5 9 km which compares with the observations reported for slow slipping transform discontinuities globally we attribute this presence of close to normal oceanic crustal thickness within fzs to the mechanism of lateral dike propagation previously considered to be valid only in fast slipping environments furthermore the combination of our results with other data sets enabled us to extend the observations to morphotectonic characteristics on a regional scale our broader view suggests that the formation of the transverse ridge is closely associated with a global plate reorientation that was also responsible for the propagation and for shaping lower order mid atlantic ridge segmentation around the equator

analysis of nrm directions from the rossello composite implications for tectonic rotations of the caltanissetta basin sicily abstract the paleomagnetic results of the pliocene marine marls from the rossello composite section 30 in the caltanissetta basin of sicily have been analysed to determine the amount and timing of tectonic rotations the analysis shows that there is a consistent inclination error for the nrm component which is removed between 500 and 600 c which may be caused by compaction of the sediment differences in inclination error throughout the composite section are related to differing carbonate content a higher carbonate content implying less compaction and smaller inclination error furthermore consistent differences are found between directions of normal and reversed polarities as well as between directions of the lower and upper part of the composite section these differences are to a large extent related to differences in carbonate content porosity and influence of weathering the caltanissetta basin shows a total clockwise rotation of 34 occurring since the early pliocene the magnetostratigraphy of the rossello composite section shows that from the miocene pliocene boundary 5 2 ma up to the gilbert gauss boundary 3 6 ma there were no tectonic rotations of the caltanissetta basin further we speculate that 10 of this total clockwise rotation occurred during approximately kaena times ca 3 05 ma and that at least 24 of this clockwise rotation is younger than the gauss matuyama boundary 2 6 ma

kinematic reconstruction of the caribbean region since the early jurassic abstract the caribbean oceanic crust was formed west of the north and south american continents probably from late jurassic through early cretaceous time its subsequent evolution has resulted from a complex tectonic history governed by the interplay of the north american south american and paleo pacific plates during its entire tectonic evolution the caribbean plate was largely surrounded by subduction and transform boundaries and the oceanic crust has been overlain by the caribbean large igneous province clip since 90 ma the consequent absence of passive margins and measurable marine magnetic anomalies hampers a quantitative integration into the global circuit of plate motions here we present an updated quantitatively described kinematic reconstruction of the caribbean region back to 200 ma integrated into the global plate circuit and implemented with gplates free software our reconstruction includes description of the tectonic units in terms of euler poles and finite rotation angles our analysis of caribbean tectonic evolution incorporates an extensive literature review to constrain the caribbean plate motion between the american continents we use a novel approach that takes structural geological observations rather than marine magnetic anomalies as prime input and uses regionally extensive metamorphic and magmatic phenomena such as the great arc of the caribbean the clip and the caribbean high pressure belt as correlation markers the resulting model restores the caribbean plate back along the cayman trough and major strike slip faults in guatemala offshore nicaragua offshore belize and along the northern andes towards its position of origin west of the north and south american continents in early cretaceous time we provide the paleomagnetic reference frame for the caribbean region by rotating the global apparent polar wander path into coordinates of the caribbean plate interior cuba and the chortis block we conclude that formation of the caribbean plate west of the north and south americas as a result of panthalassa pacific spreading leads to a much simpler plate kinematic scenario than proto caribbean atlantic spreading placing our reconstruction in the most recent mantle reference frames shows that the clip originated 2000 3000 km east of the modern galapagos hotspot and may not have been derived from the corresponding mantle plume finally our reconstruction suggests that most if not all modern subduction zones surrounding the caribbean plate initiated at transform faults two of these along the southern mexican and nw south american margins evolved diachronously as a result of migrating trench trench transform triple junctions

stress interaction between subduction earthquakes and forearc strike slip faults modeling and application to the northern caribbean plate boundary 1 strike slip faults in the forearc region of a subduction zone often present significant seismic hazard because of their proximity to population centers we explore the interaction between thrust events on the subduction interface and strike slip faults within the forearc region using three dimensional models of static coulomb stress change model results reveal that subduction earthquakes with slip vectors subparallel to the trench axis enhance the coulomb stress on strike slip faults adjacent to the trench but reduce the stress on faults farther back in the forearc region in contrast subduction events with slip vectors perpendicular to the trench axis enhance the coulomb stress on strike slip faults farther back in the forearc while reducing the stress adjacent to the trench a significant contribution to coulomb stress increase on strike slip faults in the back region of the forearc comes from unclamping of the fault i e reduction in normal stress due to thrust motion on the subduction interface we argue that although coulomb stress changes from individual subduction earthquakes are ephemeral their cumulative effects on the pattern of lithosphere deformation in the forearc region are significant we use the coulomb stress models to explain the contrasting deformation pattern between two adjacent segments of the caribbean subduction zone subduction earthquakes with slip vectors nearly perpendicular to the caribbean trench axis is dominant in the hispaniola segment where the strike slip faults are more than 60 km inland from the trench in contrast subduction slip motion is nearly parallel to the caribbean trench axis along the puerto rico segment where the strike slip fault is less than 15 km from the trench this observed jump from a strike slip fault close to the trench axis in the puerto rico segment to the inland faults in hispaniola is explained by different distributions of coulomb stress in the forearc region of the two segments as a result of the change from the nearly trench parallel slip on the puerto rico subduction interface to the more perpendicular subduction slip beneath hispaniola the observations and modeling suggest that subduction induced strike slip seismic hazard to puerto rico may be smaller than previously assumed but the hazard to hispaniola remains high

geomagnetic secular variation and the statistics of palaeomagnetic directions in this study we examine the role of palaeosecular variation psv in the use of statistics for palaeomagnetic studies and we provide new reliability criteria for palaeomagnetic poles or directions we first conclude that fisher statistics should not be applied to average palaeomagnetic directions but to virtual geomagnetic pole vgp distributions instead secondly we strongly advocate that typical properties of geomagnetic field behaviour are taken into account in the assessment of palaeomagnetic data sets the latitude dependent properties e s k provide useful guidelines for the reliability of a palaeomagnetic data set a reliable assessment of these properties depends on the sufficient number of palaeomagnetic samples being taken therefore as an additional instrument of assessing data sets we provide a n dependent a95 envelope bounded by an upper limit a95max and a lower limit a95min that helps to ascertain whether or not a distribution has sufficiently well sampled psv and therefore geomagnetic field behaviour applying these criteria is indispensable for studies of geomagnetic behaviour or for studies aiming at using tk03 gad for inclination error correction through the elongation inclination e i method for palaeomagnetic studies aimed at geological reconstructions they form helpful guidelines and increase the confidence in the rocks having faithfully recorded the field an analysis of published eastern mediterranean data shows that the vast majority of studies do not conform to the van der voo criteria in particular with respect to n and a95 we have provided criteria that are on the one hand more lenient lower n may still provide relevant information and on the other hand more strict for high n the criterion of a95 16 should be adapted to a requirement of lower a95 e g a95 80

reconnaissance de structures tectoniques et volcaniques sous marines de l arc recent des petites antilles kick em jenny qualibou montagne pelee nordouest de la guadeloupe abstract a multibeam bathymetric survey seacarib 1 was carried out in april 1985 on board the r v conrad lamont doherty geological observatory off the western coasts of several islands of the lesser antilles complementing a previous survey of the same type arcante 2 thermosite 1980 devoted to adjoining areas in guadeloupe martinique and st lucia it allowed us to precisely map submarine volcanoes in a marginal setting behind the recent arc the main volcanic axis of the archipelago emplaced during the neogene the kick em jenny volcano south of the grenadines is the only active submarine volcano of the lesser antilles it is a small and very young cone probably only tens or hundreds of years old which was hydrothermally active during the survey the directeur volcano guadeloupe is the largest submerged volcanic centre thus far detected in the lesser antilles having been formed between 3 3 and 2 9 ma during the 1980 survey two other large volcanoes were discovered off southern basse terre guadeloupe vieux fort volcano 2 5 2 0 ma and in front of fort de france bay martinique schoelcher volcano older than 4 2 ma several other much smaller volcanoes have been mapped off the same coasts particularly the bouillante seapeaks guadeloupe moreover large tectonic structures cut the recent arc transversally and obliquely the well known qualibou depression on the island of st lucia was interpreted in various ways neither the hypothesis of a large caldera the western side of which collapsed offshore nor the suggestion of a huge rotational gravity slide are confirmed the depression seems more likely to have been controlled by tectonic factors shaping a complex graben structure the seaward extension appears to be restricted to a northern channel the soufriere channel the st pierre fault in martinique is a large transverse feature extending seaward down to some 2000 m depth to the south it bounds the mount pelee active volcanic system and the escarpment of the fault forms a dam against subaqueous pyroclastic flows running down from the volcano these flows are responsible for the unusually smooth morphology of the submarine flanks the zombi fault of northwest guadeloupe is part of a first order feature extending along at least 130 km from montserrat in the north to marie galante in the east this curved fracture cuts across the northern central part of the lesser antilles passing progressively from a subparallel to a transverse orientation with respect to the axis of the island arc these types of surveys are important because they provide better understanding of the structure and recent geological evolution of the recent volcanic arc they are also a useful complementary tool for geothermal and seismological studies

what drives microplate motion and deformation in the northeastern caribbean plate boundary region the north caribbean plate boundary zone is a broad deformation zone with several fault systems and tectonic blocks that move with different velocities the indentation by the bahamas platform the bahamas collision is generally invoked as a cause of this fragmentation we propose that a second driver of deformation is the western edge of the south dipping puerto rico slab moving sideways with the north america plate the westward motion of the slab edge results in a push on the caribbean plate farther west we refer to this second mechanism for deformation as slab edge push the motion of the north america plate relative to the caribbean plate causes both drivers to migrate from east to west the bahamas collision and slab edge push have been operating simultaneously since the miocene the question is the relative importance of the two mechanisms we use mechanical finite element models that represent the two mechanisms from the late oligocene 30 ma to the present for the present both models successfully reproduce observed deformation implying that both models are viable back in time the slab edge push mechanism better reproduces observations neither mechanism successfully reproduces the observed miocene counterclockwise rotation of puerto rico we use this rotation to tune a final model that includes fractional contributions of both mechanisms we find that the slab edge push was the dominant driver of deformation in the north caribbean plate boundary zone since 30 ma

arc parallel extension and localization of volcanic complexes in guadeloupe lesser antilles 1 subduction of atlantic seafloor under the caribbean plate causes shallow earthquakes within the lesser antilles volcanic arc such earthquakes above the subduction interface show strike slip or normal fault plane solutions the latter with e w striking nodal planes to better assess seismic hazard and the coupling between volcanism and tectonics we investigated faulting related to overriding plate deformation in the guadeloupe archipelago using aerial photographs satellite spot images and topographic maps 1 25000 scale we mapped active and middle to late pleistocene fissures and normal fault systems that cut the uplifted coral platforms grande terre and marie galante and the volcanic rocks of basse terre the available marine geophysical data show that the faults extend offshore to bound submarine rifts the e w striking 1500 m deep v shaped marie galante rift separates the two islands of marie galante and grande terre normal faults in the north of grande terre appear to mark the similarly v shaped western termination of the 5000 m deep n 50e to n130 e striking desirade graben three shallow m 5 5 earthquakes 6 may 1851 29 april 1897 3 august 1992 appear to have ruptured segments of the marie galante rift boundary faults the young la grande decouverte volcanic complex of basse terre including the 1440 a d soufriere dome lies within the western termination of the marie galante rift the ancient volcanic shoulders of the rift buttress the active dome to the north and south which may explain why major prehistoric sector collapses and pyroclastic avalanches have been directed southwestward into the caribbean sea or southeastward into the atlantic ocean the marie galante rift is typical of other troughs transverse to the northeastern edge of the caribbean plate we interpret such troughs which are roughly orthogonal to the arc to result from slip partitioning and extension perpendicular to plate convergence that they disappear southward implies that they result from interaction between the caribbean and north american plates

recent investigations of the 0 5 ma geomagnetic field recorded by lava flows we present a synthesis of 0 5 ma paleomagnetic directional data collected from 17 different locations under the collaborative time averaged geomagnetic field initiative tafi when combined with regional compilations from the northwest united states the southwest united states japan new zealand hawaii mexico south pacific and the indian ocean a data set of over 2000 sites with high quality stable polarity and declination and inclination measurements is obtained this is a more than sevenfold increase over similar quality data in the existing paleosecular variation of recent lavas psvrl data set and has greatly improved spatial sampling the new data set spans 78 s to 53 n and has sufficient temporal and spatial sampling to allow characterization of latitudinal variations in the time averaged field taf and paleosecular variation psv for the brunhes and matuyama chrons and for the 0 5 ma interval combined the brunhes and matuyama chrons exhibit different taf geometries notably smaller departures from a geocentric axial dipole field during the brunhes consistent with higher dipole strength observed from paleointensity data geographical variations in psv are also different for the brunhes and matuyama given the high quality of our data set polarity asymmetries in psv and the taf cannot be attributed to viscous overprints but suggest different underlying field behavior perhaps related to the influence of long lived core mantle boundary conditions on core flow psv as measured by dispersion of virtual geomagnetic poles shows less latitudinal variation than predicted by current statistical psv models or by previous data sets in particular the brunhes data reported here are compatible with a wide range of models from those that predict constant dispersion as a function of latitude to those that predict an increase in dispersion with latitude discriminating among such models could be helped by increased numbers of low latitude data and new high northern latitude sites tests with other data sets and with simulations indicate that some of the latitudinal signature previously observed in vgp dispersion can be attributed to the inclusion of low quality insufficiently cleaned data with too few samples per site our matuyama data show a stronger dependence of dispersion on latitude than the brunhes data the taf is examined using the variation of inclination anomaly with latitude best fit two parameter models have axial quadrupole contributions of 2 4 of the axial dipole term and axial octupole contributions of 1 5 approximately 2 of the octupole signature is likely the result of bias incurred by averaging unit vectors