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

cenozoic plate tectonic history of the northern venezuela trinidad area geological and geophysical data coupled with recent plate tectonic reconstructions suggest that the cenozoic geologic history of the northern venezuela trinidad area has been dominated by strike slip displacement of discrete crustal blocks allochthonous terranes within the area include metavolcanic rocks of the cretaceous villa de cura group and metamorphic rocks of the precambrian to cretaceous cordillera de la costa a relatively competent crustal block margarita block is defined by an outline around the metamorphic basement of margarita island the araya paria peninsula the northern range of trinidad and tobago island reconstruction of the margarita block to its original position requires at least partial closure of the falcon basin closure of the bonaire and cariaco basins and restoration of about 50 km of motion on both the oca and bocono faults post middle eocene eastward translation of the caribbean plate caused eastward motion of the margarita block a minor change in relative plate motion during the late oligocene or early miocene produced a right step in the moron fault forming the cariaco pull apart basin and el pilar fault zone maximum offset on el pilar fault is estimated to be no more than 125 km though displacement along the entire fault zone may have been greater transpressional stresses between the caribbean plate and northern south america caused folding of the serrania del interior of venezuela and the central range of trinidad eastward migration of transpressional stresses at the southeastern corner of the caribbean south american plate boundary is being accommodated by formation of oblique thrusts transpressive anticlines and downwarping of the crust bouguer gravity data suggest that jurassic aged atlantic oceanic crust is being depressed as the caribbean plate expands into the demerara plateau area this study suggests that the faults and transtensional transpressional compressional structures identified in this study are the result of stresses produced during the large eastward translation of the caribbean plate since the paleocene and are not the product of a shear couple

evolution of southern caribbean plate boundary vicinity of trinidad and tobago two processes oblique collision of an island arc and strike slip motion parallel to the south american coast have been suggested as dominant in the tectonic evolution of the southeastern corner of the caribbean field work in the el pilar fault zone of trinidad and interpretation of offshore seismic data lead us to conclude that the dominant process was strike slip motion on five or more major fault systems within a 250 km wide east west trending plate boundary zone extending north to south from grenada to the mouth of the orinoco river during the last 30 m y movements in this fault zone 1 diverted the orinoco river eastward 2 juxtaposed blocks with diverse stratigraphic characteristics and 3 generated secondary extensional compressional and strike slip str ctures including anticlines of northeast to north northeast trend of which the tobago arch the central range of trinidad and the deformed mud diapirs off the southeastern coast of trinidad are the most prominent

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

oceanic crust of the grenada basin in the southern lesser antilles arc platform seismic refraction data permit the southern lesser antilles arc and surrounding regions to be divided by the velocity of their basement we propose that high velocity basement of the arc platform beneath the grenadine islands and below a part of the tobago trough forearc basin is oceanic and continuous and was originally connected with oceanic crust of the grenada basin low velocity basements of the tobago terrane and the arc platform from st vincent north lie south and north respectively of the high velocity basement of the arc platform an oceanic origin of this high velocity crust in the grenadines is argued to be more plausible than an origin as unroofed lower arc crust the segment of probable oceanic crust in the arc platform was greatly uplifted during development of the present island arc mainly in late neogene time relative to the grenada basin and tobago trough accepting the proposition of shallow oceanic crust in the grenadines early middle eocene and possibly older pillow basalts of mayreau the oldest rock unit of the southern lesser antilles arc platform may be an exposure of such basement major and minor element compositions of mayreau basalt are indicative of a spreading rather than arc origin the stratigraphy of the pillow basalts indicates extrusion in an open marine environment distant or shielded from sources of arc or continental sediment followed by a period of pelagic sedimentation above the carbonate compensation depth the eocene basalt and pelagic cover formed a relatively deep floor of a marine basin in which arc derived turbidites and pelagic sediments accumulated over the succeeding 25 30 ma such basalts thus indicate a probable spreading origin of the grenada basin and an age of cessation of spreading in the region of mayreau in eocene time the configuration of the eocene basin and the direction of spreading however are unknowns regional structural relationships imply the spreading was probably backarc an origin also hinted at by ce zr ratios of mayreau basalt although the aves ridge probably was the remnant arc the existence and position of an early middle eocene frontal arc are unresolved the early paleogene tectonic architecture of the southeastern caribbean differed substantially from that of the neogene

water activity changes across an amphibolite granulite facies transition broken hill australia phase compositions in pelitic and mafic gneisses place tight constraints on pressure ranging from 3 up to 6 kb and to a lesser extent on temperature 500 up to 800 c during prograde regional metamorphism of the willyama complex broken hill se australia these limits allow an evaluation of water activity across the terrain using various equilibria in pelitic and mafic gneisses the stability of cummingtonite and biotite over much of the terrain places upper limits on temperature and the presence of syn metamorphic partial melts in the metasediments places lower limits on a h2o garnet biotitesillimanite k feldspar quartz relations combined with the partial melting data suggest a decrease in water activity from near 1 0 in the lower grade zones to 0 5 0 2 in the broken hill little broken hill part of the two pyroxene zone this result is compatible with less precise hornblende orthopyroxene clinopyroxene quartz relations

solar semidiurnal tide in the dusty atmosphere of mars abstract vertical coupling due to the solar semidiurnal tide in mars s atmosphere and effects on zonal mean temperature and wind structures are investigated using a numerical model the model provides self consistent solutions to the coupled zonal mean and tidal equations from the surface to 250 km breaking convective instability of the semidiurnal tide is parameterized using a linear saturation scheme with associated eddy diffusivities thermal forcing in the model gives rise to surface pressure perturbations and middle atmosphere zonal mean winds and temperatures that are consistent with available measurements and general circulation models results presented here primarily focus on globally elevated dust levels during southern hemisphere summer conditions similar to those experienced by the viking 1 and viking 2 landers during the 1977 global dust storms semidiurnal temperature and wind amplitudes maximize in the winter hemisphere and exceed 50 k and 100 m s 1 above 150 km and are typically 10 2

thermochronologic data from tobago west indies constraints on the cooling and accretion history of mesozoic oceanic arc rocks in the southern caribbean fission track thermochronologic studies on mid cretaceous dioritic and tonalitic rocks from tobago west indies provide important temporal constraints on post magmatic mesozoic cooling as well as the final phase of accretion of these oceanic arc plutonic rocks onto the continental margin of northern south america zircon fission track data indicate that the plutonic rocks of tobago cooled through the zircon closure isotherm 250 200 c at about 103 ma apatite ages and track length analysis suggest rapid cooling through the apatite annealing zone 120 60 c at about 45 ma furthermore the track lengths in apatites from these plutonic rocks are all greater than 14 microns which implies that these rocks have not been above 50 c since 45 ma if a geothermal gradient of 25 c km is inferred the analyzed samples have been at depths of less than 2 km since 45 ma tobago has occupied an upper crustal structural position since the mid cretaceous and significant final uplift of these mesozoic oceanic arc rocks occurred in the mid eocene the tectonothermal history of tobago since the mid eocene has not involved deep burial or a high temperature thermal flux this history apparently has chiefly involved translation of the tobago terrane along the caribbean south america plate boundary zone at upper crustal levels

tobago west indies a fragment of a mesozoic oceanic island arc petrochemical evidence the pre cenozoic rocks of tobago west indies represent an exceptional cross section through a portion of a composite mesozoic arc system other parts of which may be found in the villa de cura klippe the aruba blanquilla island chain and perhaps in the western cordillera of colombia and ecuador the geochemical data from tobago presented in this paper provide a basis from which a regional view of the petrologic character of the accreted mesozoic arc of the southern caribbean mountain system can be deduced the mesozoic rocks of tobago can be divided into four main groups the north coast schist the ultramafic to mafic plutonic complex the tobago volcanic group and late mafic dykes rocks of the north coast schist served as wallrock for the plutonic volcanic dyke complex which evolved during the mid cretaceous geochemical and nd and sr isotopic data presented demonstrate the oceanic island arc character of all the pre cenozoic rocks of the island all suites span the calc alkaline and tholeiitic regions on discrimination diagrams rare earth element ree patterns for the north coast schist tobago volcanic group and plutonic suite are essentially identical and are moderately lree enriched la yb 1 1 2 7 the dykes exhibit the most unevolved ree patterns and are interpreted as most closely representative of parental liquid compositions initial nd 6 6 to 9 4 and 87 sr 86 sr 0 70303 0 70364 for all samples except north coast schist and tobago volcanic group argillites are similar in value and overall range to other oceanic island arcs limited isotopic data from argillite samples from tobago suggest an important change in the continental derived sediment contribution during mesozoic arc growth nd isotopic data from north coast schist siliceous argillite indicate virtually no continental detritus in this neocomian sediment while nd isotopic data from the tobago volcanic group argillite suggest a definite continental contribution during albian arc growth

stable isotope profile across the orthoamphibole isograd in the southern marginal zone of the limpopo belt south africa abstract in the southern marginal zone of the limpopo belt the transition from granulite to amphibolite facies metamorphism is defined by a change from orthopyroxene to orthoamphibole bearing assemblages within metasedimentary rocks of the supracrustal bandelierkop formation oxygen isotope fractionations among constituent minerals within both amphibolite and granulite facies metasedimentary rocks are of similar magnitude implying a similar retrograde thermal history temperatures calculated from these fractionations can be arranged in the following order quartz garnet quartz orthoamphibole quartz pyroxene quartz plagioclase quartz biotite the observed temperature sequence is consistent with retrograde metamorphism under closed system conditions and oxygen closure of garnet orthoamphibole and orthopyroxene at or close to peak metamorphism while quartz plagioclase and biotite crystallized from and or re equilibrated via a melt fluid phase down to temperatures of 560 c small fluid rock ratios are implied by the simultaneous closure of quartz plagioclase and biotite given that quartz is the dominant oxygen bearing phase during retrogressive exchange and assuming garnet closure to oxygen diffusion at the time of formation then the quartz garnet fractionations may provide minimum peak temperature estimates of 736 52 c coarse grained concordant felsic veins within the metasediments texturally interpreted as anatectites are in isotopic equilibrium with the host rock the closed system defined here only on the basis of oxygen isotope systematics does allow for fluid melt extraction at high temperatures in contrast to the metasediments oxygen isotope systematics for mafic and ultramafic rocks of the bandelierkop formation suggest open system behaviour for at least some of the rocks such rocks are commonly cross cut by coarse grained felsic veins which are not in isotopic equilibrium with the host rock and contain quartz whose 18 o values are indistinguishable from those observed in the metasedimentary rocks the stable isotope data are consistent with a model involving anatexis of metasedimentary rocks at peak metamorphic temperatures partial extraction of the anatectic melts and their crystallization elsewhere in the sequence is suggested by the mineralogy and isotopic composition of large discordant pegmatites together with the isotopic disequilibrium and open system behaviour observed in some mafic and ultramafic rocks in the pelitic rocks the release of small amounts of fluid from residual melts and or somewhat larger amounts from partially collected melts during cooling may account for many of the retrograde mineralogic features observed for these rocks without perturbing the oxygen isotope systematics such retrograde features include the cordierite orthopyroxene symplectite after garnet and the incipient hydration of orthopyroxene and cordierite within the granulites and possibly the regional retrogression of granulites to form orthoamphibole gneisses retrograde kyanite within these rocks thus suggests an isobaric cooling path similar 13 c values of graphite separated from amphibolite and granulite facies metasediments together with significant oxygen and carbon isotope heterogeneity among charnockites metasediments banded iron formations and most mafic and ultramafic rocks are not consistent with a retrogressive influx of extraneous co 2 rich fluids as the cause of regional retrogression and orthoamphibole formation in the gneisses nor with charnockitization the data also argue against a pervasive co 2 infiltration as the cause of granulite facies metamorphism

oxygen isotope variations in granulite grade iron formations constraints on oxygen diffusion and retrograde isotopic exchange the oxygen isotope ratios of various minerals were measured in a granulite grade iron formation in the wind river range wyoming estimates of temperature and pressure for the terrane using well calibrated geothermometers and geobarometers are 730 50 c and 5 5 0 5 kbar the mineral constraints on fluid compositions in the iron formation during retrogression require either very co2 rich fluids or no fluid at all in the iron formation isotopic temperature estimates from quartz magnetite fractionations are controlled by the proximity to the enclosing granitic gneiss and range from 500 c qz mt 10 0 within 2 3 meters of the orthogneiss contact to 600 c qz mt 8 0 farther from the contact temperature estimates from other isotopic thermometers are in good agreement with those derived from the quartz magnetite fractionations

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

deep crystal response to continental collision the limpopo belt of southern africa the limpopo belt is an example of an archean 2700 ma continental collisional zone in which the thickened crust 70 km has virtually attained equilibrium with the adjacent non thickened plates by a process of upward movement and lateral spreading of the underthrust high grade metamorphosed plates

heavy mineral record of andean uplift and changing sediment sources across the ne margin of south america a case study from trinidad and barbados the heavy mineral compositions of sandstones in trinidad and barbados record the onset of andean related erosion and a reduction of craton derived sediments into ne south america the changing provenance was deduced by comparing heavy mineral assemblages interpreted from ancient sandstones with associations recognized in modern sands that can be reasonably correlated to existing tectonic domains the impact of the andean orogeny across the margin was to introduce a suite of minerals characteristic of low temperature metamorphism that today is prevalent adjacent to the caribbean mountain belt and differs from the zircon rich assemblage produced within cratonic plains twenty one paleocene late pliocene sandstone samples from trinidad revealed systematic changes in mineral diversity and maturity that recorded this provenance transition and suggests andean erosion during deposition of the late oligocene nariva formation similar to palaeogene sandstones of trinidad four eocene scotland formation samples from barbados support craton derivation but with additional evidence of minor andean input probably due to the proximity of the scotland formation delivery systems to an earlier uplift episode by the late miocene most of the sediments delivered into trinidad basins were supplied from the andean orogeny as suggested by the relative abundance of minerals of this affinity the heavy mineral records of trinidad and barbados are similar to that described across northern south america from both modern and ancient environments that collectively mark the uplift of the andean mountain belt with its strong influence on drainage patterns and reservoir provenance along this sector of the continental margin

a preliminary paleomagnetic pole for mid cretaceous rocks from tobago further evidence for large clockwise rotations in the caribbean south american plate boundary zone abstract the island of tobago occupies the eastern end of the central igneous belt of the caribbean south american plate boundary zone volcaniclastic sediment of the albian 100 ma tobago volcanic group and dikes of similar age within it were sampled in two homoclinal sections with different attitudes the mean of virtual geomagnetic poles for 12 sites 25 4 n 24 1 e a95 4 2 is well defined pre tilting and apparently reliable yet is far removed from a similar age reference pole for south america five other paleomagnetic studies of cretaceous rocks from widely separated localities farther west in the plate boundary zone yield pole positions that are remarkably similar to the tobago pole poles obtained from the guajira peninsula of colombia the islands of aruba and bonaire and the caribbean mountains of venezuela are among those that agree with the tobago result the paleolatitudes for study areas within the plate boundary zone are consistent with an origin on or near south america yet the poles throughout the zone are rotated roughly 90 dextral relative motion between the caribbean and south american plates was probably responsible for the rotation

hydrothermal graphite in new hampshire evidence of carbon mobility during regional metamorphism graphite precipitated from hydrothermal fluids pervades the sillimanite grade metasedimentary and plutonic rocks of new hampshire hydrothermal graphite occurs as microscopic veinlets halos of spherulites around shear zones and metre thick veins carbon isotope analyses of the graphite range from 25 to 9 13c pdb intermediate between the two biogenic crustal reservoirs of reduced organic matter and carbonate it is proposed that carbon was mobilized from sediments as co2 and ch4 during metamorphic devolatilization reactions the carbon bearing species were transported in aqueous fluids through hydraulic fractures graphite precipitated when aqueous fluids with different co2 ch4 ratios were mixed in fractures

diffusion effects on oxygen isotope temperatures of slowly cooled igneous and metamorphic rocks recently obtained data on oxygen diffusion in feldspars quartz and hornblende permit the prediction of the apparent18o16o temperatures that would be measured in a rock that consisted only of those three minerals and cooled slowly from high temperature the computed temperatures would be based on the differences in the18o16o ratios between coexisting pairs of minerals the present calculation takes into account the diffusion rates for oxygen as a function of temperature the cooling rate of the rock the mineral grain sizes and the mode of the rock for mineral grains 1 mm in radius and a cooling rate of 10 c m y the minimum difference in apparent temperature between quartz feldspar and feldspar hornblende pairs will be 115 c despite the assumption of a normal uneventful slow cooling history to room temperature further the apparent quartz hornblende temperature will range over 30 c 590 620 c depending on the mode of the rock for a cooling rate of 1000 c m y the apparent difference in temperature can be as much as 400 c consequently consistency in temperatures obtained by oxygen isotope analysis should not be expected in most high grade metamorphic rocks or igneous rocks which are cooled slowly departures from the pattern of temperatures obtained in this model would imply a very rapid quench from high temperature or a complex history for the rock for some minerals including hornblende the relation between temperature and the equilibrium fractionation of oxygen isotopes between coexisting phases has been derived from observed relations in natural specimens the choice of the specimens used for such calibrations needs to be re evaluated in light of these findings this may result in a change in the equilibrium equation constants an example from the literature the san jose tonalite baja california mexico was modelled and yields 18o concentrations in the minerals that correspond closely with the measured values this suggests that the model used is appropriate that the rock has had a simple thermal history and that it cooled at 100 200 c m y over the temperature range 800 500 c the set of paleotemperatures obtained for a rock will in general yield neither the mineral closure temperatures nor the formation or crystallization temperatures on the other hand the cooling rate of the rock may be derived from the data this in turn may have important tectonic implications with regard to denudation and uplift rates

four phase tectonostratigraphic development of the southern caribbean abstract although the stratigraphy of northern south america and the southern caribbean has been studied in detail in many areas a clear understanding of the sequence of tectonic events and their relation to regional stratigraphy has not been forthcoming since northern south america collided with southern north america in the late palaeozoic we identify four discrete tectonic phases from the stratigraphy of the area that is now the southern caribbean these phases derived from a synthesis of regional stratigraphy were 1 panama arc collistion with colombia in the late miocene early pliocene which drove a complex triangular wedge bounded by strike slip faults northward into the caribbean 2 right lateral strike slip faulting associated with post eocene eastern motion of the caribbean ocean floor into the gap between the separating americas 3 island arc development and collision associated with the entry of the present caribbean ocean floor an area of buoyant ocean crust into the gap between the separating americas and 4 continental rifting and passive margin development in the late jurassic early cretaceous associated with the separation of north and south america approximately along the palaeozoic suture zone overprinting of younger structures on inherited structures from previous phases is responsible for the overall complexity of the region