clastic domains of sandstones in central eastern venezuela trinidad and barbados heavy mineral and tectonic constraints on provenance and palaeogeography current models for the tectonic evolution of northeastern south america invoke a palaeogene phase of inter american convergence followed by diachronous dextral oblique collision with the caribbean plate becoming strongly transcurrent in the late miocene heavy mineral analysis of cretaceous to pleistocene rocks from eastern venezuela barbados and trinidad allow us to define six primary clastic domains refine our palaeogeographic maps and relate them to distinct stages of tectonic development 1 cretaceous passive margin of northern south america 2 palaeogene clastics related to the dynamics of the proto caribbean inversion zone before collision with the caribbean plate 3 late eocene oligocene southward transgressive clastic sediments fringing the caribbean foredeep during initial collision 4 oligocene middle miocene axial fill of the caribbean foredeep 5 late eocene middle miocene northern proximal sedimentary fringe of the caribbean thrustfront and 6 late miocene recent deltaic sediments flowing parallel to the orogen during its post collisional mainly transcurrent stage domain 1 3 sediments are highly mature comprising primary guayana shield derived sediment or recycled sediment of shield origin eroded from regional palaeogene unconformities in trinidad palinspastic restoration of neogene deformation indicates that facies changes once interpreted as north to south are in fact west to east reflecting progradation from the maturin basin into central trinidad across the nw se trending bohordal marginal offset distorted by about 70 km of dextral shear through trinidad there is no mineralogical indication of a northern or northwestern erosional sediment source until oligocene onset of domain 4 sedimentation paleocene middle eocene rocks of the scotland formation sandstones in barbados do show an immature orogenic signature in contrast to venezuela trinidad domain 2 sediments this requires 1 at least a bathymetric difference if not a tectonic barrier between them and 2 that the barbados deep water depocentre was within turbidite transport distance of the early palaeogene orogenic source areas of western venezuela and or colombia domains 4 6 from late oligocene show a strong direct or recycled influence of caribbean orogen igneous and metamorphic terranes in addition to substantial input from the shield areas to the south the delay in the appearance of common caribbean detritus in the east relative to the paleocene and eocene appearance of caribbean influenced sands in the west reflects the diachronous eastward migration of caribbean foredeep subsidence and sedimentation as a response to eastward younging collision of the caribbean plate and the south american margin

first cycle quartz arenites in the orinoco river basin venezuela and colombia modern first cycle quartz arenites are forming in the orinoco drainage basin by at least two distinct mechanisms common to both mechanisms is an environment of intense chemical weathering and extended time over which weathering can occur in the llanos andean foreland basin extended time is provided by temporary storage of orogenically derived sediments on extensive alluvial plains in lowland portions of the guayana shield long soil mineral residence times are produced by very low erosion and transport rates resulting from low relief and tectonic quiescence both weathering intensity and the duration of weathering are important in the modification of sand composition first cycle quartz arenites are produced in diverse tectonic settings and cannot be used by themselves to infer paleotectonic settings of ancient sequences furthermore although tectonic setting is of great importance in determining sand composition modifications during transport and deposition can overprint the effects of the tecto

mineralogically mature sandstones in accretionary prisms abstract plate tectonic interpretations of sandstone composition are made in the hope that detrital framework modes of sandstone suites provide information about the tectonic setting of basins of deposition and associated provenances dickinson et al 1983 the underlying assumption that provenance tectonics are directly associated with tectonics of depositional and post depositional sedimentary settings is not however universally true as sandstones from two contemporary accretionary prism complexes nias island indonesia and barbados west indies illustrate mineralogically immature sandstones rich in feldspar and volcanic lithic fragments are expected in such tectonic settings but both nias island and barbadian sandstones are mineralogically very mature dominated by quartz in b th cases this discrepancy between inferred and actual tectonic setting has been explained by postulating that sands from a provenance petrogenetically unrelated to the arc trench system e g continental or recycled orogen sands have been introduced into the accretionary prism by sedimentary or tectonic processes sandstones which are transported sedimentologically or tectonically across the boundaries of sedimentary tectonic provinces and which therefore occur in tectonic settings petrogenetically unrelated to the tectonics of the sediment source area are important components of at least some active accretionary complexes the possibility of trans boundary sedimentary and or tectonic transport must be allowed for in making plate tectonic interpretations based on sandstone petrog aphy especially in reconstructions of ancient orogenic belts

modern sands of south america composition provenance and global significance standard petrographic methods were used to study 604 modern sands from south america of which 351 came from rivers and 253 from beaches in spite of the wide geomorphic contrasts these sands belong to only three great families 1 an immature andean family of lithic arenites rich in volcanic and metamorphic grains that covers about 30 of south america 2 a cratonic association rich in quartz that covers about 62 of the continent and 3 a transitional molasse association which occurs between these two and covers only about 8 of the continent the grand arithmetic average q f rf ratio for the entire continent is 60 11 29 and the area weighted carbonate free average qa fa rfa is 68 10 22 a superior continental estimate and the first ever for an entire continent

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

petrology chemistry and texture of modern river sands amazon river system key idea following the spirit of p d krynine the sands of the world s largest tropical river system are studied to reveal much about the general origin of sand and sandstone thin section and chemical study of 95 sand samples from the amazon river system in brazil peru ecuador and colombia a river system with great contrasts in climate and rock types reveals that sands derived from the andes are lithic arenites whereas sands derived from precambrian terrains are much richer in quartz as are sands in rivers draining tertiary molasse sand becomes more mature downstream along the main stem of the amazon solimoes maranon so that a sample analyzed at the mouth does not accurately reflect headwater composition dilution by quartz rich tributaries and quartz rich tertiary outcrops possible differential elimination of rock fragments and even possible weathering on the modern flood plain of the amazon contribute to enhanced maturity downstream we speculate that although data on modern large subarti

weathering processes and the composition of inorganic material transported through the orinoco river system venezuela and colombia abstract the composition of river borne material in the orinoco river system is related primarily to erosion regime which in turn is related to tectonic setting especially notable is the contrast between material derived from tectonically active mountain belts and that from stable cratonic regions for a particular morpho tectonic region the compositional suites of suspended sediment bed material overback deposits and dissolved phases are fairly uniform are are typically distinct from whose of other regions for each region a consistent set of chemical weathering reactions can be formulated to explain the composition of dissolved and solid loads in developing these formulations erosion on slopes and storage of solids in soils and alluvial sediments are important considerations compositionally verymature sediment is derived from areas of thick soils where erosion is transport limited and from areas where sediments are stored for extended periods of time in alluvial deposits compositionally immature sediments are derived from tectonically active mountain belts where erosion is weathering limited weathering limited erosion also is important in the elevated parts of the guayana shield within areas of sleep topography compared to the mountain belts sediments derived from elevated parts of the shield are more mature a greater degree of chemical weathering seems to be needed to erode the rock types typical of the shield the major element chemistry and mineral composition of sediment delivered by the orinoco river to the ocean are controlled by rivers that have their headwaters in mountain belts and cross the llanos a region of alluvial plains within the foreland basin the composition of sediments in rivers that drain the shield seems to be established primarily at the site of soil formation whereas for rivers that drain the mountain belts additional weathering occurs during s episodes of storage on alluvial plains as sediments are transported across the llanos to the main stem of the orinoco after mixing into the main stem there seems to be little subsequent alteration of sediment

regional controls on geomorphology hydrology and ecosystem integrity in the orinoco delta venezuela interacting river discharge tidal oscillation and tropical rainfall across the 22 000 km2 orinoco delta plain support diverse fresh and brackish water ecosystems to develop environmental baseline information for this largely unpopulated region we evaluate major coastal plain shallow marine and river systems of northeastern south america which serves to identify principal sources and controls of water and sediment flow into through and out of the orinoco delta the regional analysis includes a summary of the geology hydrodynamics sediment dynamics and geomorphic characteristics of the orinoco drainage basin river and delta system because the amazon river is a major source of sediment deposited along the orinoco coast we summarize amazon water and sediment input to the northeastern south american littoral zone we investigate sediment dynamics and geomorphology of the guiana coast where marine processes and holocene history are similar to the orinoco coast major factors controlling orinoco delta water and sediment dynamics include the pronounced annual flood discharge the uneven distribution of water and sediment discharge across the delta plain discharge of large volumes of water with low sediment concentrations through the rio grande and araguao distributaries water and sediment dynamics associated with the guayana littoral current along the northeastern south american coast inflow of large volumes of amazon sediment to the orinoco coast development of a fresh water plume seaward of boca grande disruption of the guayana current by trinidad boca de serpientes and gulf of paria and the constriction at boca de serpientes

extensional collapse in the neoproterozoic aracuai orogen eastern brazil a setting for reactivation of asymmetric crenulation cleavage the aracuao orogen of eastern brazil is one of many brasiliano pan african orogens formed during the neoproterozoic assembly of gondwana its western edge bordering the sao francisco craton is the serra do espinhaco fold thrust belt in which top up to the west reverse sense faults west verging folds f1 and east dipping spaced to phyllitic cleavage s1 developed we have found that the kinematics of deformation changes markedly at the hinterland margin of this fold thrust belt here beneath a plateau known as the chapada acaua metadiamictite and fine grained pelitic schist comprise an east dipping belt that contains an assemblage of structures indicative of top down to the east normal sense shear this assemblage includes a cascade of f2 folds that refold f1 folds and verge down the dip of the belt s enveloping surfaces vertical tension gashes and top down to the east rotated clasts based on the presence of these structures we propose that the plateau exposes a regional scale normal sense shear zone here called the chapada acauashear zone casz because f2 folds refold f1 folds normal sense shear in the casz occurred subsequent to initial west verging thrusting considering this timing of motion in the casz we suggest that the zone accommodated displacement of the internal zone of the aracuao orogen down relative to its foreland fold thrust belt and thus played a role in extensional collapse of the orogen the casz trends parallel to preserved thrusts to the west and thus may represent an inverted thrust fault notably throughout the casz s1 schistosity has been overprinted by a pervasive west dipping asymmetric crenulation cleavage s2 the sigmoid shape of s1 surfaces in s2 microlithons require that slip on each s2 surface was top down to the west s2 cleavage is axial planar to the down dip verging f2 folds based on its geometry we suggest that s2 cleavage initiated either as an antithetic extensional crenulation cleavage during reverse sense shear or as a near vertical asymmetric crenulation cleavage formed during east west shortening of a preexisting east dipping schistosity subsequent normal sense shear in the casz reactivated this cleavage causing clockwise rotation of s2 domains as viewed looking along strike to the north in a manner similar to that of rotational bookshelf faults such movement could have accommodated concomitant vertical flattening of the casz during extensional collapse

cretaceous collision and indentation in the west carpathians view based on structural analysis and numerical modeling 1 a model of indentation of a rigid promontory into weak metasedimentary rocks during cretaceous convergence is suggested for tectonic evolution of southern part of west carpathians an early arcuate cleavage fan has developed in front of northward moving southern rigid basement block the interaction of the moving indenter with western stationary basement promontory is responsible for development of the boundary parallel shear zone along which the main southern indenter is shifted to the east this results in development of a new steep transpressional cleavage overprinting the early fabric eastward displacement of the southern indenter causes the development of a thrust zone parallel to the margin of the eastern stationary promontory a proposed numerical model of the deformation of a thin viscous sheet in front of oval rigid indenter reliably simulates the development of the observed deformation pattern modeled discrete partitioning between the western promontory and the indenting block fully agrees with the observed secondary cleavage associated with the transpressional shear zone our numerical model interconnects this complex kinematic frame with finite strain pattern which was to date possible only for simple boundary conditions in addition the model explains the polyphase cleavage patterns in terms of complex shapes of promontories and changes in movements of indenting blocks

the interaction of a two layer isolated mesoscale eddy with bottom topography abstract the propagation of an isolated mesoscale eddy onto a western bounding topographic slope is examined through the use of a two layer primitive equation numerical model variable parameters in the study are sense of rotation cyclonic anticyclonic vertical structure baroclinic barotropic and layer thickness ratio h1 h2 eddy size and strength rotational velocity and frictional parameterization are fixed dispersion in the isolated eddy is induced by planetary and topographic beta effects giving the eddy an asymmetric distribution this asymmetric distribution allows for nonlinear self advective propagation tendencies these nonlinear tendencies play a key role in the direction of propagation of the eddy specifically in a quiescient background an anticyclone can have eastward propagation tendencies which can overcome planetary and topographic beta effects conversely cyclones can have westward propagation velocities which are augmented greater than maximum rossby wave phase speed b

apparent shear band geometry resulting from oblique fold sections abstract small scale shear zones inclined at intermediate angles to an earlier anisotropy are often observed in deformed rocks they are traditionally described as shear bands c bands extensional crenulation cleavage or normal kink bands formed as a result of extension along the anisotropy their asymmetries are widely used to describe the large scale kinematics of deformation and the deformational history of a given area we demonstrate that when various three dimensional fold structures are observed on two dimensional outcrop surfaces or in thin section they can appear geometrically identical we have developed a simple technique that allows the geometrical evaluation of any section across a cylindrical fold of arbitrary geometry the ranges of planar sections on which a fold exhibits shear band like geometry are presented on a stereographic projection in order to simplify the determination of critical orientations we demonstrate that for any fold geometry there are two distinct groups of sections showing shear band like geometry with opposite senses of shear criteria systematically arranged around the axial plane and which are inclined at a high angle to the major anisotropy we provide a field example from western carpathians where kinematic analysis mainly based on apparent extensional shear bands led to overemphasis of the role of post orogenic extension on the final structural pattern of the belt

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

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

paleogeographic and tectonic implications of quartzose sandstones of barbados the provenance of paleogene sandstones on the island of barbados is relevant to various models of the tectonic evolution of the southern caribbean area modal point counts of 26 sandstones from barbados show that the constituent grains most likely were derived from a composite craton interior and orogenic margin abundant detrital polycrystalline quartz grains have affinities to a spectrum of low to high rank metamorphic plutonic crystalline source rocks microprobe data suggest that feldspars were derived from metamorphic or plutonic source terrances the complex heavy mineral suite of the terrigenous sandstones including glaucophane chloritoid titanite zircon and aluminosilicates supports derivation from orogenic and cratonal terranes due to the complex history of the south american caribbean plate boundary and poor age constraints on timing of deposition widely contrasting models have been proposed concerning the provenance of the sandstones convergence between the aruba orchila arc and continental south america during the paleogene resulted in the formation of an e w trending foreland fold thrust belt and foredeep dominant sediment dispersal was to the east northeast along tectonic strike the present study shows that the terrigenous sandstones on barbados probably were deposited in deep sea fans to the north of present day araya peninsula during and shortly after the late middle eocene probable source terranes include the guayana shield the caribbean mountain system interior foreland fold thrust belt and possibly to a lesser extent the lesser antilles magmatic arc or predecessor arc

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

two contrasting pressure temperature time paths in the villa de cura blueschist belt venezuela possible evidence for late cretaceous initiation of subduction in the caribbean the villa de cura blueschist belt is one of several east west trending allochthonous belts comprising the caribbean mountain system of northern venezuela this blueschist belt consists of four structurally coherent subbelts that also trend east west from north to south these are characterized by 1 pumpellyite actinolite 2 glaucophane lawsonite 3 glaucophane epidote and 4 barroisite the retrograde pressure temperature p t path of the northern three subbelts generally parallels their prograde path such p t paths are typical for franciscan style subduction settings and are characterized by relatively low geothermal gradients indicative of refrigeration during subduction zone parallel ascent and exhumation of these rocks the barroisite subbelt formed at high pressures similar to those of the glaucophane epidote subbelt but at substantially higher temperatures and followed a counterclockwise p t path new 40 ar 39 ar ages record peak metamorphism at 96 3 0 4 ma for the barroisite subbelt and 79 8 0 4 ma for the northern three subbelts the caribbean plate is thought to have been a fragment of the farallon plate which together with the great arc of the caribbean greater antilles aves ridge lesser antilles leeward antilles migrated northeastward after a subduction polarity reversal and overrode the young proto caribbean lithosphere that had formed by spreading between north america and south america the more silicic barroisite subbelt may have been part of the arc that was subducted immediately after polarity reversal whereas the other three belts formed much later when the geothermal gradient had decreased substantially the villa de cura belt was exhumed in two stages first by late cretaceous arc parallel extension and second by miocene southward thrusting onto the south american continent

tectonic versus chemical weathering controls on the composition of fluvial sands in tropical environments the compositions of fluvial sands collected in mountainous regions along the volcanic front in central america and along the andean front in venezuela and colombia correlate with the tectonic setting of their source terrains the sands derived from the central american volcanic arc are compositionally distinct from those derived from the andean terrains and minor variations in tectonic style can also be discriminated subtle differences in tectonic setting and source rock composition are recognizable only when modification by chemical weathering is slight they are strongest in sands from low order streams draining mountainous areas with modest alluvial plain development sands that have been exposed to weathering for extended periods during alluvial storage are progressively enriched in chemically stable phases with increased weathering sands from continental terrains converge toward a supermature quartz arenite end member and the imprint left by source rock composition and tectonic setting is obscured in intraoceanic terrains a similar but much less pronounced enrichment in stable phases relative to less stable phases is observed similarly overprinting the tectonic signal although detailed tectonic information is easily lost sands derived from intraoceanic terrains are distinguishable from those derived from continental terrains even after long exposure to intense chemical weathering conditions

secondary cleavages in ductile shear zones abstract secondary cleavages developed at late stages in ductile shear zones show several features that are inconsistent with progressive simple shear in the zone these are the orientation of a single secondary cleavage oblique to the shear zone boundaries conjugate sets with opposite senses of shear and multiple sets with the same sense of shear these features can be explained if the bulk flow is partitioned into slip along discrete failure planes parallel to the primary foliation s coaxial stretching along the foliation and spin

the detrital record of cretaceous to pliocene sandstones across the ne south american margin the cretaceous to pliocene erosional history of the ne south american margin depicts the increasing influence of the uplifting caribbean mountain belt petrographic analyses of 59 cretaceous to late pliocene sandstone samples derived from across the islands of trinidad and barbados demonstrate systematic mineralogical changes whereby early cretaceous arkosic arenites gave way to mature quartz arenites by the late cretaceous and caribbean mountain derived lithic rich arenites during the neogene changes in the framework compositions are accompanied by changes in textural attributes with caribbean mountain detritus being notably finer grained than cratonic input silica cements predominate in quartzose sandstones and sericitic and clay cements within litharenites the changing mineralogical compositions record the gradual deformation of the northern south american passive margin and initiation of the andean caribbean mountain foreland basin system with incipient orogenic detritus evident from the latest eocene and unambiguous orogenic input by the late oligocene the preponderance of finer grain sizes within deltaic pliocene sandstones records the later fill of an established foredeep with sediments sourced primarily from erosion of the andean hinterland these interpretations agree with heavy mineral compositional changes and the timing of uplift from geo and thermo chronological data across the margin supplementary material qualitative descriptions sample coordinates and raw point counts are available at https doi org 10 6084 m9 figshare c 3659381

cretaceous evolution of a metamorphic core complex the veporic unit western carpathians slovakia p t conditions and in situ40ar 39ar uv laser probe dating of metapelites alpine metamorphism related to the development of a metamorphic core complex during cretaceous orogenic events has been recognized in the veporic unit western carpathians slovakia three metamorphic zones have been distinguished in the metapelites 1 chloritoid chlorite garnet 2 garnet staurolite chlorite 3 staurolite biotite kyanite the isograds separating the metamorphic zones have been modelled by discontinuous reactions in the system k2o feo mgo al2o3 sio2 h2o kfmash the isograds are roughly parallel to the north east dipping foliation related to extensional updoming along low angle normal faults thermobarometric data document increasing p t conditions from c 500 c and 7 8 kbar to c 620 c and 9 10 kbar reflecting a coherent metamorphic field gradient from greenschist to middle amphibolite facies 40ar 39ar data obtained by high spatial resolution in situ ultraviolet uv laser ablation of white micas from the rock slabs constrain the timing of cooling and exhumation in the late cretaceous mean dates are between 77 and 72 ma however individual white mica grains record a range of apparent 40ar 39ar ages indicating that cooling below the blocking temperature for argon diffusion was not instantaneous the reconstructed metamorphic p t t path is clockwise reflecting post burial decompression and cooling during a single alpine orogenic cycle the presented data suggest that the veporic unit evolved as a metamorphic core complex during the cretaceous growth of the western carpathian orogenic wedge metamorphism was related to collisional crustal shortening and stacking following closure of the meliata ocean exhumation was accomplished by synorogenic orogen parallel extension and unroofing in an overall compressive regime

topographic generation of an eddy at the edge of the east greenland current wadhams and squire 1983 describe an ice water eddy at the edge of the east greenland current located at about 79 and 1 e the eddy has been sighted earlier vinje 1977 and appears to occur in that general location with some regularity wadhams and squire explain the eddy in terms of baroclinic instability the criteria of griffiths and linden 1982 and phillips 1954 suggest that the east greenland current is baroclinically unstable and phillips theory indicates that the most unstable wavelength of a sinusoidal velocity perturbation is 50 km in agreement with the approximate size of the observed eddy

feedback between ice flow barotropic flow and baroclinic flow in the presence of bottom topography coupling between externally driven barotropic flow and locally driven baroclinic flow in the presence of an ice cover and topography is studied the topography is a necessary ingredient in this coupling this study shows that the observed mesoscale activity of the ice edge seen in satellite imagery does not necessarily reflect the mesoscale baroclinic activity in the ocean besides oceanic eddies the ice cover can trace the topographic changes via the coupling to the barotropic flow a two layer ocean model coupled to an ice model is constructed to simulate an ice ocean system with a varying bottom topography in the absence of wind forcing the ice cover reflects the externally driven barotropic ocean response especially topographically forced taylor columns by forming ice streamers or meanders some of these features propagate advected with the ocean currents as a whole downstream creating an image of eddy propagation even though there is no baroclinic structure underneath when downwelling favorable winds ice on the left from the wind direction are turned on in addition to this background barotropic flow the ekman flow in the ocean toward the open ocean will enhance the meandering of the ice edge due to the barotropic flow during upwelling favorable winds the ice edge stays rather compact including the case when a strong baroclinic cyclonic vortex is developing beneath an ice meander supported by the taylor column in the ocean

the fluvial geochemistry and denudation rate of the guayana shield in venezuela colombia and brazil abstract the guayana shield is composed of early to mid precambrian igneous and metamorphic basement rocks with a quartzitic platform cover the complete absence of limestones and evaporites allows a clear chemical expression in the stream data of the primary weathering of the basement in a humid tropical environment total erosion rates are extremely slow 10 m m y with equal contributions from the dissolved and suspended loads however the former is largely silica with ratios of si to total cation equivalents si tz ranging to in excess of three weathering is complete to kaolinite and gibbsite i e the environment is one of active laterisation with a penetration rate of the weathering front into the fresh substrate about twice the denudation rate in basins of relatively homogeneous lithology rb sr isochrons constructed from the river data agree with the whole rock ages from the drainages thus all the common refractory rb containing minerals k feldspar mica are completely dissolved the thick lateritic regolith that is accumulating as a result of this intense weathering is a common relict feature on other southern hemisphere shields in the absence of active tectonics or greatly accelerated mechanical erosion the weathering rates of these basement rocks must be quite insensitive to environmental change

an ice water vortex at the edge of the east greenland current during the y mer 80 cruise in august 1980 we observed an ice feature of 60 km diameter having the shape of a breaking wave it lay at the edge of the east greenland drift ice in fram strait with its center at 79 15 n 00 38 e and the wave was breaking upstream relative to the east greenland current two conductivity temperature and depth sections across the feature in n s and e w directions revealed warm up to 4 3 c water some 60 km inside the polar front in lenses centered at 40 m depth but with an effect to beyond 600 m the temperature anomalies were accompanied by salinity anomalies so that there was little net effect on the density profile the form of the sections together with the ice distribution observed by vertical photography from a helicopter and surface motion observed by tracking four radar transponders all suggest that the feature is a vortex produced by an instability in the polar front it has similar characteristics to vortices investigated experimentally and theoretically by griffiths and linden 1981a b 1982 and observationally by wadhams et al 1979

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

restoration of early variscan structures exposed along the tepla shear zone in the bohemian massif constraints from kinematic modelling perpendicular or high angle relationship of lineations is common in shear zones where synchronous fabric transposition and shearing occur this paper documents a polyphase early variscan structural record exposing the transition from orogenic superstructure to infrastructure along a major detachment shear zone at the western margin of the tepla barrandian domain bohemian massif on the scale of tens of kilometers this region shows major but continuous reorientation of s2 s3 intersection lineation from its nne ssw trends in the superstructure to the ese wnw trends in the infrastructure a simple kinematic numerical model which allows deformation parameters such as fabric attractor deformation symmetry and proportion of simple shear to be varied was formulated to explain the rotation of fabrics observed in the field in line with the results of our kinematic modelling these rotations are interpreted as a result of progressive transposition of the originally subvertical s2 foliation into the gently se dipping s3 marked by stretching at high angle to the incipient s2 s3 intersection the mineral fabric relations suggest that the s2 was associated with crustal thickening and is well preserved in the superstructure while s3 was associated with exhumation of deeper parts of the infrastructure the kinematic model confirmed the operation of major detachment shear zone and allowed a set of the most probable deformation parameters leading to successful simulation of the fabric geometries to be identified in addition the successful model parameters were used to restore the vertical structure of the studied region prior to exhumation on the basis of available pressure temperature data we believe that unlike the commonly used balanced cross section restoration our restoration technique can be successfully applied in orogen interiors if the mineral fabric relations are well constrained and high quality pressure temperature estimates exist

shear band formation and strain localization on a regional scale evidence from anisotropic rocks below a major detachment betic cordilleras spain regional scale deformation taking place in a strongly anisotropic yet homogeneous metapelitic protolith during an apparently single tectonic event was systematically investigated as a function of the distance to the main tectonic contact i e the filabres shear zone a major detachment in the betic cordilleras spain the density of c3 shear bands or extensional crenulation cleavage reworking the earlier s2 schistosity increases exponentially towards the contact in parallel with the decrease in the size of the shear domains systematic variations in angles and shape ratios are also reported deformation and age patterns however suggest that this spectacular trend at least partly results from a progressive localization of the deformation through time this fossilized shear strain gradient was thus produced somewhat diachronously such shear strain patterns nevertheless provide a mean to constrain the rheological properties for such weak lithologies and a mean to better understand crustal deformation

exceptions to the relationship between plate tectonics and sandstone composition abstract routine use of qfl qplsmlvm and lmlslv plate tectonic provenance diagrams has led to the recognition of error populations for these sandstones the interpretation of tectonic setting based on data exclusive of petrography does not coincide with the interpretation of tectonic setting based on their location on compositional diagrams the anomalous sandstones can be separated into at least four categories i sandstones deposited during the transition between tectonic regimes may be derived in part from relict source rocks ii the relative enrichment of sandstones in detrital quartz by weathering and or depositional reworking may lead to an inaccurate interpretation of tectonic setting from compositional data iii sandstones deposited in tectonic settings as yet unrepresented on he provenance diagrams may plot between the provenance fields or overlap existing fields iv the role of detrital carbonate rock fragments directly affects the location of data points on provenance diagrams and is an unresolved problem the first two categories represent permanent problems in the use of sandstone composition as an indicator of plate tectonic setting whereas the last two may only be temporary problems that can be solved by more data and a consensus on determining detrital modes recognition of anomalous sandstones in the rock record will reduce the probability of erroneous interpretations of tectonic setting and enhance the use of sandstone petrology as an indicator of plate tectonic setting

the tectonic fabric of the barbados ridge accretionary complex abstract a map showing near surface structure and other surficial geological features on the barbados ridge accretionary complex has been constructed from gloria long range side scan sonographs detailed bathymetric surveys using seabeam and single and multichannel seismic reflection sections the map shows major variations in the size orientation and style of structures across the accretionary complex that are principally related to the increasing thickness of the complex and the sediment drape upon it and variations along the complex that derive from the northward thinning of sediment accreted from the ocean floor and the effects of ridges in the oceanic basement the development of crossfaulting is mainly associated with basement ridges mud diapirism is abundant in the southern part of the complex and is characteristic of that part of the complex into which turbidites from the orinoco submarine fan are accreted slumping on a variety of scales is another feature of the complex the map is presented at a scale of 1 1063415 4 inches per degree of longitude and also shows bathymetry at 200 m contour intervals

the changing course of the orinoco river during the neogene a review abstract recent studies have revealed that the rich fresh water vertebrate fossil fauna of the urumaco formation in northwestern falcon of late miocene age belongs biogeographically to the orinoco river system this finding together with other evidence can be used to chart the changing course of the orinoco river during the neogene and thus date the uplift of various mountain ranges that affected the change in this course presence of the misoa delta in the middle eocene of the maracaibo basin has been suggested as evidence for a large river running in a south north direction and draining the central cordillera of colombia and the guayana highlands the late middle eocene uplifting of western venezuela changed the paleogeographic setting and a new delta building shifted to the south represented by the extensive carbonera formation of late eocene to oligocene age in the llanos basin of colombia and southwestern venezuela in the earliest miocene the falcon basin situated to the east of the maracaibo basin was primarily the site of marine shale sedimentation stratigraphic and microfaunal evidence suggests the possibility of the presence of the proto orinoco river in the western margin of the basin in any case from the mid early miocene until the end of the middle miocene a fluvio deltaic sequence was deposited in the northwestern falcon with a total thickness of 4 7 km the urumaco formation overlies this sequence and its vertebrate fauna indicates that the river which built the prior delta was definitely the proto orinoco the deformation and uplift of the eastern cordillera of colombia and of the southwestern end of the merida andes caused the distal course of the river to be deflected to a west east direction a late middle miocene age for this change of course is precisely dated on marine microfaunal evidence the late miocene sedimentary record of the orinoco river delta appears east of maturin in the eastern venezuela basin and pliocene and pleistocene deltaic sediments are especially evident in trinidad and the columbus basin

a precautionary note on shear bands as kinematic indicators the orientation of shear bands relative to foliations defined by elongated mineral aggregates is often used to determine the large scale sense of displacement in ductile faults data from contact strain zones at the bases of large overthrust complexes in the eastern alps and the betic cordilleras demonstrate that there is not always a simple geometrical relation between shear band orientation and sense of vorticity in bulk non coaxial flow in addition to single shear band sets that show displacements synthetic with the large scale displacement there are single sets with antithetic displacements and conjugate sets the last two observations are at variance with published data and interpretations and cast doubts on the general applicability of shear bands as indicators of large scale flow kinematics

caribbean tectonics and relative plate motions during the last century three different ways of interpreting the tectonic evolution of the gulf of mexico and the caribbean have been proposed taking into account the bailey willis school of a permanent pre jurassic deep sea basin the edward suess school of a subsided continental terrain and the alfred wegener school of continental separation the present investigation is concerned with an outline of an interpretation which follows that of pindell and dewey 1982 an attempt is made to point out ways in which the advanced hypotheses can be tested the fit of africa north america and south america is considered along with aspects of relative motion between north and south america since the early jurasic attention is given to a framework for reconstructing caribbean plate evolution the evolution of the caribbean the plate boundary zones of the northern and southern caribbean and the active deformation of the caribbean plate

the mineralogy and metamorphic geology of low grade metasediments northern range trinidad forty one low grade metasediments from the northern range of trinidad and from the neighbouring island of tobago west indies were investigated by microscopic x ray powder diffraction guinier camera technique and electron microprobe methods the following minerals were encountered in order of decreasing abundance quartz muscovite chlorite paragonite muscovite mixed layer paragonite organic material calcite pyrophyllite dolomite albite hematite chloritoid pyrite rutile titanite rectorite stilpnomelane the chemical composition of muscovite is dependent on the mineral assemblage with si 6 1 6 2 per formula unit in two high alumina assemblages versus si 6 6 in a low alumina assemblage microprobe analyses indicate that muscovite paragonite and pyrophyllite are sometimes intergrown on a micrometre scale chlorite composition estimated from x ray data shows good correspondence with microprobe data for two al poor samples but is less satisfactory for an al rich sample observed mineral assemblages in akna are indicative of the pyrophyllite paragonite albite fades for the first time chemical data for coexisting pyrophyllite chlorite m mgo mgo feo 0 377 and chloritoid m 0 110 are given this assemblage is indicative of the low or medium pressure range of the lower greenschist facies pressure conditions during metamorphism are not well constrained but a minimum pressure of 2 kbar corresponding to 5 6 km of overburden can be estimated from the na m4 content of actinolite temperatures derived from the calcite dolomite geothermometer range between 300 and 350 c and are consistent with the presence of the assemblage pyrophyllite calcite quartz

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

petrology of fluvial sands from the amazonian foreland basin peru and bolivia modal sand compositions in nine rivers draining the proximal part of the amazonian retroarc foreland basin in peru and bolivia provide support for the recycled orogen provenance model of dickinson and suczek average compositions of 52 sand samples from the rio madre de dios and its tributaries are qfl 74 5 21 qmflt 60 5 35 qplvmlsm 44 10 46 lmlvls 69 18 13 and qmpk 91 4 5 these compositions accurately reflect the predominance of sedimentary and metasedimentary rocks in the cordillera oriental of the andean orogen the ultimate source terrane for the sands of the modern foreland basin rivers of the madre de dios basin can be classified in one of three ways 1 rivers that head in the subandean fold thrust belt 2 rivers that are actively incising a subtle intraforeland uplift overlain by quaternary and upper tertiary foreland basin fill and 3 trunk rivers that collect both type 1 and type 2 rivers the rivers that drain the fold thrust belt carry sands that are slightly more lithic than those carried by rivers that drain the intraforeland uplift the intraforeland derived sands are relatively enriched in volcanic rock fragments lmlvls 70 22 8 whereas sands derived from the fold thrust belt are relatively enriched in sedimentary rock fragments lmlvls 66 9 24 the overriding control on sand composition is the relative area of bedrock type exposed in the fold thrust belt sands reworked from tertiary and quaternary foreland basin fill are relatively enriched in quartz possibly owing to intense weathering of sediments in the soil profile the preservation of predictable and systematic mixing patterns in sand compositions from different rivers suggests that sandstone petrologic data may be augmented by paleocurrent isopach and textural data to facilitate detailed reconstruction of drainage patterns in ancient nonmarine foreland basins

quartzose turbidites of the accretionary complex of barbados i chalky mount succession abstract quartzose sandstone and mudstone turbidites of cenozoic and possibly older age constitute a large proportion of strata in the accretionary complex of the lesser antilles arc exposed at its structural high on barbados the chalky mount succession is the thickest 628 m and best exposed of structurally continuous turbidite sequences on the island four classes of layer types are distinguished among chalky mount beds single sandstones between 0 2 and 12 m thick amalgamated sandstones multilayers of sandstone 20 cm thick and mudstone and thick mudstone units layer types are divided among 65 types based on bouma sequences grading within sandy divisions and sand to mud gradation of these less than a dozen types occur frequently although many layer types cannot be in erpreted uniquely as turbidites the occurrence of classic turbidites throughout the section indicates the succession is of subsea sediment gravity flow origin the vertical succession of layer types can generally be interpreted as reflecting lateral facies migration of a radial fan and includes from bottom to top basin plain distal outer fan proximal outer fan midfan inner fan midfan abandoned fan proximal outer fan the sequence indicates initial progradation followed by retrogradation and abandonment then rejuvenation of the former fan paleocurrent directions at several horizons in the section indicate transport mainly between north and east relative to present coordinates the applicability of the radial fan model to the chalky mount and other turbidite successions of barbados suggests that a base of slope configuration of the depositional sites was likely