chronology of cenozoic tectonic events in western venezuela and the leeward antilles based on integration of offshore seismic reflection data and on land geology newly acquired deep penetration broadband onshore offshore lithospheric investigation of venezuela and the antilles arc region seismic reflection data from offshore western venezuela bonaire basin and around the leeward antilles are combined with existing geologic and geophysical data sets to examine the chronology of late cretaceous cenozoic tectonic events in this part of the caribbean south american plate boundary zone these tectonic events have controlled the maturation and structural trapping of known hydrocarbons in the offshore bonaire basin and the adjacent onland falcn basin we infer three tectonic phases that are constrained using these combined data sets 1 the late eocene early oligocene north south opening of the 3 6 km 1 8 3 7 mi thick falcn bonaire basin occurred along east west striking normal fault systems that have locally been inverted by later tectonic phases these paleogene normal faults rifted the upper cretaceous arc crust and paleogene marine depositional sequences within the offshore bonaire basin 2 northwest striking normal faults crosscut the older normal faults of the bonaire basin and leeward antilles and form deep submarine rifts that contain up to 4 km 2 5 mi of sedimentary fill and form deep water channels between islands of the leeward antilles offshore well data and age of onshore sediments in the falcn basin indicate that this second phase of rifting occurred mainly during the late oligocene to early miocene and remains active to the present 3 inversion of the subaerial falcn basin commenced during the middle miocene this inversion phase is reflected in the present day pattern of an east northeast trending fold thrust belt that can be traced over 200 km 124 mi along strike in the falcn basin a second offshore fold thrust belt la vela can be traced over a distance of 175 km 108 mi along strike and parallel to the northeast trending falcn basin coast restoration of imbricate thrusts seen on seismic lines perpendicular to the la vela fold thrust belt indicates a minimum of 7 km 4 3 mi of northeast southwest directed thin skinned shortening geochemical work indicates that source rocks for scattered occurrences of hydrocarbons in the falcn basin and its coastal zone are paleogene and miocene marine shale reservoir rocks are tertiary marine sandstone and shale deposited in paleogene rifts formed during the first tectonic phase in the late eocene to early oligocene structural traps were formed by thrusting during the second tectonic phase in the late oligocene to early miocene

from granulites to eclogites in the sesia zone italian western alps a record of the opening and closure of the piedmont ocean the sesia zone italian western alps offers one of the best preserved examples of pre alpine basement reactivated under eclogite facies conditions during the alpine orogenesis a detailed mineralogical study of eclogitized acid and basic granulites and related amphibolites is presented in these rare weak to undeformed rocks microstructural investigations allow three main metamorphic stages to be distinguished a a medium to low p granulite stage giving rise to the development of orthopyroxene garnet plagioclase brown amphibole ilmenite biotite in basic granulites and garnet k feldspar plagioclase cordierite sillimanite biotite ilmenite in acid granulites b a post granulite re equilibration associated with the development of shear zones producing discrete amphibolitization of the basic granulites and widespread development of biotite sillimanite cordierite spinel in the acid rocks c an eo alpine eclogite stage giving rise to the crystallization of high p and low t assemblages in an effort to quantify this evolution independent well calibrated thermobarometers were applied to basic and acid rocks for the granulite event p t estimates are 7 9 kbar and 700 800 c and for subsequent retrograde evolution p t was 4 5 kbar and 600 c for the eo alpine eclogite metamorphism pressure and temperature conditions were 14 16 kbar and 550 c the inferred p t path is consistent with an uplift of continental crust produced by crustal thinning prior to the subduction of the continental rocks in the light of the available geochronological constraints we propose to relate the pre alpine granulite and post granulite retrograde evolution to the permo jurassic extensional regime the complex granulite eclogite transition is thus regarded as a record of the opening and of the closure of the piedmont ocean

reaction enhanced ductility the role of solid solid univariant reactions in deformation of the crust and mantle abstract in collision belts and subduction zones penetrative deformation of thrust sheets may be catalyzed by metamorphic reactions if the reactions have a significant effect on the mechanical properties of rocks reaction enhanced ductility may result if a reaction produces a significant decrease in grain size thus allowing the rock to deform superplastically the product grain size of a reaction is a function of the ratio of nucleation and growth rates this ratio varies along any pressure temperature time ptt path and only attains a significant value at a certain distance from an equilibrium boundary a comparison of rates of change of p and t along typical segments of ptt paths for crustal rocks leads to the conclusion that a high nucleation rate a small grain size and resulting superplastic deformation are more likely to be achieved when reaction boundaries are crossed during underthrusting rather than during the thermal readjustments of a perturbed geotherm following crustal thickening or during uplift an example of reaction enhanced ductility occurs in eclogite facies metagranitoids in the sesia lanzo zone western alps where medium grained plagioclase reacted to fine grained jadeite quartz during early alpine underthrusting microstructural evidence from shear zones shows that the jadeite quartz aggregates were highly ductile and it is inferred that the small grain size allowed superplastic deformation further evidence suggests that this process operated during regional deformation of metagranitoids rubie 1982 the reaction of olivine to spinel or the beta phase by metastable overstepping of the two phase stability fields in a subducting plate ringwood 1975 may result in fine grained products and thus may be analogous to the albite jadeite quartz reaction the consequent decrease in effective viscosity is likely to have important consequences for plate dynamics at and below the depths of major phase transitions

deformation and metamorphism in the bard area of the sesia lanzo zone western alps during subduction and uplift the structure microstructure and petrology of a small area close to the village of bard in val d aosta italy has been studied in detail the area lies across the contact between the gneiss minuti gm and the eclogitic micaschist ems complexes of the lower element of the sesia portion of the sesia lanzo zone western alps both complexes have undergone high pressure metamorphism but the metamorphic assemblages indicate a sudden increase in pressure in going across the contact from the gm to the ems therefore we interpret the contact as a thrust dividing the lower element of the sesia into two sub elements this interpretation is supported by structural evidence the early alpine 90 70 ma metamorphic history is best preserved in the ems and is one of increasing pressure associated with thrusting the maximum p t recorded in the ems is 1500 mpa 15kbar and 550 c and in the gm is 1500 1300 mpa 15 13 kbar and 500 550 c we suggest that the rocks were probably in an active benioff zone during this time from then on the histories of the gm and ems are the same deformation continued and the thrust and thrust slices were folded during decreasing pressure we interpret the first postthrusting deformation in terms of uplift associated with continued shortening of the crust and underplating after the benioff zone had become inactive and a new benioff zone had developed further to the north west a still later deformation and the lepontine metamorphism 38 ma are related to continued uplift much of this deformation is characterized by structures indicative of vertical shortening and lateral spreading as the mountains rose above the general level of the surface

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

reaction pathways during the progressive deformation of a blueschist metabasite the role of chemical disequilibrium and restricted range equilibrium there are discrete masses of un deformed metabasite within the blueschist series of the island of syros greece around the margins of these masses are zonal sequences through rocks showing intracrystalline deformation but without a geometric fabric to rocks with discrete and anastomosing shear zones and finally to penetratively foliated rocks with isolated relics of the original undeformed texture textural relics suggest that this spatial sequence is at least qualitatively also a temporal sequence this progressive shear zone deformation took place concurrently with a glaucophane epidote to eclogite reaction the reaction pathways in the rocks that underwent the shear zone deformation can be compared with those in rocks of a similar composition that suffered a longer deformation history and show no relics of an undeformed parent although the final assemblages are in both cases the same the pathways are different these differences are in part related to reactions promoted by the change from local to bulk equilibrium on the onset of deformation in the rocks they are also related to the crystallization and later breakdown during the sequence of progressive equilibration of a metastable phase in this case an impure glaucophane

thickening history of the western alps a model of evolution for the western segment of the alpine chain is presented available data concerning the structural metamorphic and geochronological evolution in the majorunits involved in the thickening processes are takeninto account io constrain the model the successive stages of the collisional history are correlated with the evolution of thermal structure the african paleomargin was deformed at the onset of collision just after the abduction of the oceanic crust after these initial stages progressive stacking affected more and more external domains of the european plate lastly the deformation of the entire pile is linked io the thermal reequilibration of the considered thickened crust the alpine chain is a good example of a collision zone where the interaction of thermal and mechanical effects can be treated quantitatively

garnet orthopyroxene plagioclase quartz barometry refinement and application to the english river subprovince and the minnesota river valley recent experimental theoretical and thermodynamic studies permit better calibration of two reactions for geobarometry grossular pyrope quartz anorthite enstatite grossular almandine quartz anorthite ferrosilite if both reactions are applied using the same thermodynamic data and activity models they should yield the same pressure for a given garnet pyroxene plagioclase quartz assemblage application to a variety of high grade terrains generally yielded excellent results however poor results have been obtained for high fe rocks which can be traced to errors in activity models for garnet and or pyroxene either a two site ideal mixing model for orthopyroxene cf wood and banno 1973 underestimates enstatite activity for high fe orthopyroxenes or the ganguly and saxena 1984 model overestimates pyrope activity in low mg garnets

petrological arguments for high temperature extensional deformation in the pyrenean variscan crust saint barthelemy massif ariege france abstract the saint barthelemy massif is one of the variscan massifs of the north pyrenean zone thermobarometric analysis has been completed across a sequence comprising from bottom to top basal granulitic gneisses the main mylonite band and migmatites pt estimates are correlated with textures which represent progressive stages of gneissification and mylonitization the main foliation and the mylonitic textures were developed during the retrogressive evolution of the lp ht granulitic metamorphism retrogressive conditions are characterized by high heat flow associated with a pressure reduction the main penetrative deformation of the saint barthelemy massif is interpreted as resulting from a continuous extensional event

sequences of mesostructures related to the development of alpine thrusts in the eastern pyrenees abstract the mesostructures formed during the development of an antiformal stack in the eastern pyrenees are studied the following structural evolution is proposed a pressure solution cleavage develops by layer parallel shortening at initial stages of the thrusting the cleavage is present in all units but mainly in the marls and marly limestones in the siliceous layers a weak cleavage and small scale thrust structures develop the cleavage and the bedding are deformed during the thrusting different structures form in various places of the antiformal stack where the bedding is flat lying southward directed layer parallel shears predominate in the hangingwall ramps kink bands disjunctive stylolitic cleavage and layer parallel shear opposite to the sense of thrust movement develop the described structures constitute a kinematical model for mesostructures developed during the evolution and ramping of thrusts

an overview of the petroleum system of maracaibo basin the geologically complex maracaibo basin in northwestern venezuela is one of the most prolific hydrocarbon basins in the world having a basinal area of 50 000 km2 19 300 mi2 the basin has produced more than 30 billion bbl of oil with estimated recoverable oil reserves of more than 44 billion bbl the central elements of the petroleum system of the basin include 1 a world class source rock upper cretaceous la luna formation deposited on a shelf to slope environment under anoxic conditions and modified by intermittent oxygenated periods and tectonic events 2 high quality clastic reservoir rocks deposited in eocene and miocene fluviodeltaic settings 3 two main periods of rapid tectonic subsidence responsible for two pulses of voluminous hydrocarbon generation first during paleogene caribbean south american oblique plate collision and second during the neogene uplift of the sierra de perij mrida andes and 4 lateral and vertical migration of oil along strike slip normal and inverted faults as well as a regional unconformity of late eocene oligocene age the maturation migration and trapping of hydrocarbons were closely controlled by the tectonic evolution of the maracaibo basin during the paleogene the development of a foredeep along the northeastern margin of the basin and the strike slip reactivation of the rift related jurassic faults on the maracaibo platform controlled the early structural setting of the source and reservoir rocks hydrocarbons migrated updip from source rocks beneath the north northeastern margin of the basin along north south strike slip faults and into overlying eocene clastic reservoirs in the south central parts of the basin the second period of the maracaibo basin petroleum system developed during subaerial exposure of most of the maracaibo basin during oligocene miocene uplift of the adjacent sierra de perij and mrida andes uplift of mountain ranges surrounding the basin folded and depressed the interior of the basin to form the extensive maracaibo syncline because of the formation of the maracaibo syncline oil generation began in the central and southern parts of the synclinal basin and migrated northward hydrocarbons migrated up the flanks of the maracaibo syncline along reactivated strike slip faults and into miocene rocks adjacent to the uplifted mountain ranges escaping oil has formed numerous surface seeps along the edges of the maracaibo basin

statistical tests of aries data statistical tests are performed on project aries preliminary baseline measurements in the southern california triangle formed by the jet propulsion laboratory the owens valley radio observatory and the goldstone tracking complex during 1976 1980 in addition to conventional one dimensional tests a two dimensional test which allows for an arbitrary correlation between errors in individual components is formulated using the hotelling statistic on two out of three baselines the mean rate of change in baseline vector is statistically significant apparent motions on all three baselines are consistent with a pure shear with north south compression and east west expansion of 1 x 10 to the 7th year the aries measurements are consistent with the usgs geodolite networks in southern california and the safe laser satellite ranging experiment all three experiments are consistent with a 6 cm year motion between the pacific and north american plates and a band of diffuse shear 300 km wide except that corresponding rotation of the entire triangle is not found

coronitic reactions and ductile shear zones in eclogitised ophiolite metagabbro western alps north italy abstract the objective of this work is to study the most significant static and syn tectonic transformations which accompanied the eclogite metamorphism 450 50 c 12 15 kbar of an ophiolite metagabbro from the western alps in partly undeformed volumes abundant igneous mineral relics occur and the eclogite metamorphism produced coronitic or incomplete pseudomorphic reactions the disequilibrium or local equilibrium conditions implied by these features suggest that slow diffusion and interface kinetics were the rate limiting factors which prevented a large scale eclogite equilibration the undeformed volumes are bounded by syn eclogite ductile shear zones where the magmatic minerals and textures are obliterated and a new fabric foliation develops the enlargement of equilibrium domains observed in the shear zones indicates that a close approach to eclogite equilibration of these metagabbros was not attained without the catalytic effect of pervasive deformation the bulk rock chemistry also played an important role in determining the kind of eclogite assemblage in particular only sheared gabbros rich in fe can produce bimineralic eclogites

garnet chloritoid equilibria in eclogitic pelitic rocks from the sesia zone western alps their bearing on phase relations in high pressure metapelites abstract a detailed study of garnet chloritoid micaschists fom the sesia zone western alps is used to constrain phase relations in high pressure hp metapelitic rocks in addition to quartz phengite paragonite and rutile the micaschists display two distinct parageneses namely garnet chloritoid chlorite and garnet chloritoid kyanite talc has never been observed garnet and chloritoid are more magnesian when chlorite is present instead of kyanite the distinction of the two equilibria results from different bulk rock chemistries not from p t conditions or redox state estimated p t conditions for the eclogitic metamorphism are 550 600 c 15 18 kbar the presence of primary chlorite in association with garnet and chloritoid leads us to construct two possible afm topologies for the sesia metapelites the paper describes a kfmash multisystem for hp pelitic rocks which extends the grid of harte hudson 1979 towards higher pressures and adds the phase talc observed parageneses in hp metapelites are consistent with predicted phase relations critical associations are gt ctd chl and gt ctd ky at relatively low temperatures and gl chl ky and gt tc ky at relatively high temperatures

introduction to the special issue of marine and petroleum geology tectonics basinal framework and petroleum systems of eastern venezuela the leeward antilles trinidad and tobago and offshore areas

the stacking of thrust slices in collision zones and its thermal consequences the thermal consequences of stacking up thrust slices in collision zones are investigated using simple one dimensional thermal models the metamorphic evolution geotherms and p t time paths of a given tectonic unit belonging to a crustal stacking wedge made of more than two units is governed by three effects cooling and pressure decrease associated with erosion cooling by the lowest units screen effect heating by the upper units cover effect it is shown that the efficiency of these effects are dependent on the tectonic evolution of the collision zone the metamorphic evolutions are very sensitive to the following tectonic parameters the number of thrusted units involved in thickening the time delay between each thrust and finally the mode of stacking of the different units over and understacking it appears that for a given depth of burial the temperature increase during uplift is less important in a crust thickened by three or four units than in the case of two units the screen effect during understacking is more efficient for a short time delay 10 ma between each thrust overstacking leads to higher temperatures before uplift when compared to understacking it is also shown that the thermal perturbation induced in an intermediate unit of the pile is more efficiently recorded when its thickness is rather small 10 km it is shown that for an erosion controlled uplift the shape of the p t t path depends on the position of the rocks within a given unit and on the unit position within the pile finally the general metamorphic evolutions during crustal thickening hp lt metamorphism and subsequent overprint are discussed in terms of the previously mentioned parameters in the case of the western alps it appears that the more or less efficient greenschist overprint of the units involved in thickening can be explained by different time intervals between the thrust events

deep crustal extensional faulting in the ivrea zone of northern italy abstract the ivrea zone in northern italy consists of amphibolite and granulite facies metasedimentary metabasic and ultrabasic rocks and is thought to represent a section through the deep continental crust detailed mapping of the high grade part of this zone has revealed a series of high temperature shear zones which after the effects of later low temperature faulting and folding have been removed appears to accommodate extension of the sequence the shear zones formed at temperatures similar to those at which thermal equilibration occurred in the host rocks and locally show evidence for development under prograde conditions these observations fit well with thermal effects calculated from a simple heat flow model for extensional faulting in this model the effects of shear heating and displacement of cool hanging wall rocks against hotter footwall rocks are calculated these observations indicate that faulting as a mode of failure may be important during extension of the lower continental crust especially when basic rocks are the dominant components of the rock pile a restored section through the extended lower crust as observed in the ivrea zone is presented and the extent to which the features observed are able to explain seismic reflectivity of the lower crust is discussed

microstructural criteria for reliable thermometry in low temperature eclogites abstract two alpine eclogite bodies sampled in metaophiolitic complexes of the piemonte zone have been studied for geothermometry it is demonstrated that temperature estimates by both exchange thermometry and sizes of antiphase domains in omphacites give rise to erratic results when the defect structures of the minerals are not taken into account microstructural criteria by means of both optical and tem microscopy for reliable thermometry in low temperature eclogites are disscussed the dynamically recrystallized grains are the more suitable sites for the application of exchange thermometry while only low strained omphacites can be used for thermometry based on apd s

an evaluation of garnet clinopyroxene geothermometry in granulites four new formulations of the garnet clinopyroxene geothermometer ellis and green 1979 ganguly 1979 saxena 1979 dahl 1980 have been evaluated in the adirondacks and five other granulite terranes using results from 94 mineral pairs the saxena and ganguly formulations give temperatures that are generally 100 150 c above those constrained by phase equilibria and other independent thermometry while the empirical calibration of dahl gives widely scattered erratic results despite some scatter in the data the ellis and green calibration appears to be more accurate and precise than the others and is the most useful garnet clinopyroxene thermometer currently available for quantitative thermometry in granulites all four formulations are sensitive to large variations 70 80 c in temperature suggesting that problems with accuracy and precision can be improved with further refinement of model based aspects of the thermometers

pre oceanic rifting in the alps abstract a review of mesozoic mainly earliest jurassic to mid jurassic rifting in the western and central alps normal faults with throws up to a few kilometers are moderately inclined and involve rotation of crustal blocks the amount of stretching and thinning of the crust by jurassic rifting is discussed the rifting 200 165 m y pre dates the opening of the alpine oceans by spreading and transform movements 165 110 m y