quaternary glaciations in the northern andes venezuela colombia and ecuador abstract the nature and distribution of glacial drift in the high andes of venezuela colombia and ecuador suggest that deposits of only the last two quaternary glaciations isotope stages 6 and 4 2 are present the penultimate glaciation in ecuador may be represented by deeply weathered drift that is beyond the range of radiocarbon dating in colombia and venezuela subdued moraines covered with loess may be of equivalent age in places this drift extends 1000 m lower in altitude than the limits of the last glaciation which comonly reach 3400 3600 m radiocarbon dated organic deposits 33 43 ka bp lying between tills composing superposed terminal moraines suggest that andean glaciers may have been as extensive during isotope stage 4 as during isotope stage 2 a relatively long interstadial from 43 33 ka bp preceded the glacier advance that culminated during the interval ca 28 15 ka bp two late glacial stadials are marked by moraine complexes at 3900 4200 m altitude they are partly constrained by radiocarbon ages of 12 9 ka bp and 12 10 ka bp moraines apparently deposited during neoglacial advances of the last ca 5 ka lie in front of most of the glaciers remaining in the northern andes

cambrian ordovician acritarchs in the meguma terrane nova scotia canada resolution of early paleozoic stratigraphy and implications for paleogeography organic walled microfossils acritarchs provide age constraints for the previously poorly dated cambrian and lower ordovician turbiditic goldenville and halifax groups in the northern appalachian meguma terrane of nova scotia and address controversies about stratigraphy and provenance the oldest exposed formation of the 8 km thick goldenville group contains the trace fossil oldhamia which suggests an age of late early cambrian whereas the coticule bearing uppermost formation yielded an acritarch species consistent with middle cambrian epoch 3 age the conformably overlying 5 km thick halifax group includes basal pyritiferous units that yielded a late cambrian furongian assemblage of acritarch species providing further confirmation that the underlying manganese bearing formations are of cambrian epoch 3 age and not ordovician as recently claimed overlying nonpyritiferous formations contain the early ordovician graptolite rhabdinopora flabelliformis flabelliformis and acritarchs of similar age samples collected up section from the graptolite occurrence yielded acritarch species that are indicative of the later tremadocian and floian the new fossil data confirm a gap in age of 30 m y between the halifax group and the overlying silurian to lower devonian rockville notch group the new ages are consistent with stratigraphic units defined in regional mapping and support recent interpretation of fundamental differences in depositional and tectonic environments between the northwestern and southeastern parts of the meguma terrane they also provide new constraints on paleogeography of the terrane by confirming age similarities to stratigraphy in north wales with which correlation has been proposed previously based mainly on lithological similarities

gps estimate of relative motion between the caribbean and south american plates and geologic implications for trinidad and venezuela global positioning system gps data from eight sites on the caribbean plate and five sites on the south american plate were inverted to derive an angular velocity vector describing present day relative plate motion both the caribbean and south american velocity data fit rigid plate models to within 1 2 mm yr the gps velocity uncertainty the caribbean plate moves approximately due east relative to south america at a rate of 20 mm yr along most of the plate boundary significantly faster than the nuvel 1a model prediction but with similar azimuth pure wrenching is concentrated along the approximately east striking seismic el pilar fault in venezuela in contrast transpression occurs along the 068 trending central range warm springs fault in trinidad which is aseismic possibly locked and oblique to local plate motion

crystallization and vesiculation of the 1984 eruption of mauna loa calculations based on existing thermodynamic models for silicate melts are used to model the physical and chemical evolution of the 1984 mauna loa magma specifically the calculations simulate the vesiculation induced crystallization of the ascending magma the calculations provide a physiochemical model to explain the increase in crystallization under isothermal conditions the evolution of the mauna loa magma is modeled in three separate stages corresponding to 1 the subsurface crystallization of the phenocryst assemblage 2 the crystallization and vesiculation of the magma during its ascent from depth and 3 the crystallization and vesiculation of the lavas at the surface the preeruptive crystallization of olivine and orthopyroxene phenocrysts began at 1155 c and 0 2 gpa olivine and plagioclase alone crystallized during the ascent of the magma the bulk of the crystallization and vesiculation of the mauna loa magma occurred at pressures less than 20 mpa under lithostatic pressures this corresponds to depths as shallow as 600 700 m ancillary calculations have established the net heat effect of the prescribed ascent path and demonstrate that the heat associated with vesiculation easily compensates for the heats of crystallization the calculations suggest that the near surface processes can be quite endothermic depending on the initial h2o content of the magma for a magma with 1 wt dissolved h2o the heat balance demands approximately 5 10 c cooling further calculations predict the liquid line of descent for this ascent path and document the corresponding variations in the physical properties of the melt phase

petrology age and tectonic setting of the white rock formation meguma terrane nova scotia evidence for silurian continental rifting the white rock formation in the yarmouth area of the meguma terrane of southern nova scotia consists mainly of mafic tuffaceous rocks with less abundant mafic flows epiclastic and clastic sedimentary rocks and minor intermediate and felsic crystal tuff it is divided into seven map units that appear to young from west to east inconsistent with a previously assumed synclinal structure the white rock formation is flanked on both northwest and southeast by mainly the cambrian to lower ordovician halifax formation the western contact is interpreted to be a sheared disconformity whereas the eastern contact appears to be a major brittle fault and shear zone that juxtaposes different crustal levels the granitic brenton pluton forms a faulted lens within the eastern shear zone a felsic tuff from the upper part of the white rock formation yielded a u pb zircon age of 438 3 2 ma identical within error to published ages for the brenton pluton and felsic volcanic rocks near the base of the white rock formati

stress field and seismotectonics of northern south america we have integrated in situ stress neotectonic and global positioning system data to investigate the complex interactions among the south american caribbean and nazca plates and the costa rica panama microplate and to examine different seismotectonic models that have been proposed for the region the resulting data set was used to generate an integrated stress map of the region that shows that the stress field in northern south america varies systematically in both orientation and relative magnitude in the southwestern part of the study area the ecuadorian andes stress province exhibits strong e w compression resulting from the subduction of the nazca plate beneath the south american plate in the north andes stress province the observed nw se compression may result from the convergence between the caribbean and the south american plates and or the negative buoyancy of the already subducted caribbean plate beneath northwestern south america possible convergence between the costa rica panama microplate with respect to northwestern south america may also be a source of compression in this region in north and northeastern venezuela normal and strike slip faulting with a ne sw direction of extension characterizes the san sebastian el pilar stress province

age and duration of volcanic activity at diamond craters southeastern oregon abstract diamond craters one of several young basalt lava fields dotted across southeastern oregon was active in the time between about 7320 and 7790 years ago calibrated ages this timing is determined by a two step analysis first new radiocarbon ages from samples that underlie the lava field and existing ages from overlying deposits constrain the volcanic activity to between 6130 and 7790 years ago second paleomagnetic directions specifically a steep inclination of about 71 further constrain the age of emplacement to the older part of that interval the time before 7320 years ago paleomagnetic directions for lava emplaced early and late in the eruptive activity are essentially the same the duration of diamond craters eruptive activity must have been at least 1 2 decades in order to account for the extrusion of 1 6 km 3 of dense rock equivalent magma at rates typical of long lived basaltic eruptions whether the duration was only that brief or persisted to a century or more is irresolvable from paleomagnetic directions although the coincidence of directions from beginning to end is evidence that the eruption certainly was not protracted over many centuries

saddle reef auriferous veins in a conical fold termination oldham anticline meguma terrane nova scotia canada reconciliation of structural and age data many auriferous veins dated at 370 ma in the meguma terrane of nova scotia are concentrated in tight domes and conical fold hinges that are transected by 378 ma low pressure and high temperature metamorphic isograds and 380 370 ma granitoid plutons published experiments suggest that such folds are diachronous initiating at inhomogeneities and propagating both in amplitude and along their length thus fold terminations are typically conical in geometry and record only the youngest increment of folding the northeastern termination of the oldham anticline is characterized by 1 a conical fold 2 saddle reef auriferous veins 3 divergent and convergent cleavage fans 4 a downdip lineation on the slaty cleavage defined by biotitic mineral aggregates inferred to have developed during fold growth the age of the biotitic mineral aggregates is bracketed between peak metamorphism at 600 c defined by a nearly concordant 378 ma u pb monazite age from a xenolith in lamprophyre dyke and 366 ma 40 ar 39 ar muscovite biotite ages recording cooling through 400 300 c this suggests that the conical termination of the oldham anticline grew between 378 and 366 ma an observation that reconciles the empirical structural control of the saddle reef auriferous veins with the 370 ma age dating of vein minerals application of this conclusion to saddle reef auriferous veins in domes suggests that mineralization was related to the youngest increment of fold amplification

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

velocity field across the southern caribbean plate boundary and estimates of caribbean south american plate motion using gps geodesy 1994 2000 global positioning system gps observations between 1994 and 2000 at twenty two sites in the lesser antilles and northern south america indicate that the caribbean plate along its southern boundary slips at a rate of 20 5 2 mm a with an azimuth of n 84 2 e at 65 w relative to the south american plate east of 68 w 80 of the dextral slip is contained within a 80 km wide shear zone centered on the el pilar san sebastian fault system west of 68 w the plate boundary broadens to more than 300 km with dextral shear shared between the northeast trending bocono fault 9 11 mm a in western venezuelan and an offshore system near the northern coast

the history of a continent from upb ages of zircons from orinoco river sand and smnd isotopes in orinoco basin river sediments abstract we report shrimp u pb ages of 49 zircons from a sand sample from the lower orinoco river venezuela and nd model ages of the fine sediment load from the main river and tributaries the u pb ages reflect individual magmatic or metamorphic events the sm nd model ages reflect average crustal residence ages of the sediment sources together they allow delineation of the crust formation history of the basement precursors of the sediments the u pb ages range from 2 83 to 0 15 ga and most are concordant or nearly so discrete age groupings occur at 2 8 2 1 and 1 1 ga the oldest group contains only three samples but is isolated from its closest neighbors by a 600 ma age gap larger age groupings at 2 1 and 1 1 ga make up about a third and a quarter of the total number of analyses respectively the remaining analyses scatter along concordia and most are younger than 1 6 ga the 2 8 and 2 1 ga ages correspond to periods of crust formation of the imataca and trans amazonian provinces of the guyana shield respectively and record intervals of short but intensive continental growth these ages coincide with 2 9 and 2 1 ga nd model ages of sediments from tributaries draining the archean and proterozoic provinces of the guyana shield respectively indicating that the u pb ages record the geological history of the crystalline basement of the orinoco basin zircons with ages corresponding to the major orogenies of the north atlantic continents the superior at 2 7 ga and hudsonian at 1 7 1 9 ga were not found in the orinoco sample the age distribution may indicate that south and north america were separated throughout their history nd model ages of sediments from the lower orinoco river and andean tributaries are 1 9 ga broadly within the range displayed by major rivers and dusts this age does not coincide with known thermal events in the region and reflects mixing of sources with different crust formation ages the igneous and metamorphic history of these sources as recorded by the detrital zircons is that of the orinoco basin basement this implies that despite evidence of fast sedimentary recycling global similarities in nd crustal residence ages and the probability of cross continent mixing through continental drift the sedimentary material carried by individual rivers is mainly derived from the crystalline basement in the basin the global semblance in nd isotope ratios in major river sediments and atmospheric dusts results from the averaging effect of large scale sampling of the continents which are heterogeneous in age on smaller regional scales a large portion of the continental crust in the orinoco basin formed during the trans amazonian orogeny at 2 0 2 1 ga and smaller portions formed both earlier at 2 8 ga and later after 1 6 ga these observations which are consistent with the relative sizes of crustal age provinces in the orinoco basin indicate that sediments from the lower orinoco and andean tributaries contain 25 35 of material added to the crust since trans amazonian times nd model ages of these sediments underestimate the average crust formation age of the basement of the orinoco basin by only about 10 if this relationship holds in other river basins then nd model ages of major rivers and wind blown particulates indicate that the mean age of the continental crust is 1 9 2 ga

early crystallization history of alkali olivine basalts diamond craters oregon abstract basaltic lavas form the isolated quaternary volcanic centers of diamond craters and occur at cow lakes in s e oregon the exposed rocks are olivine and plagioclase phyric alkali olivine basalts the holocrystalline groundmasses contain plagioclase titanaugite olivine fe ti oxides and minor apatite and occur in the high alumina greater than 17 petrographic province of the northwestern great basin the whole rock and phenocryst compositions provide enough information to define the equilibrium state of the magmas the solutions to the thermodynamic equations are valid for systems of known bulk composition and can be used to evaluate whether rock compositions are close to magma compositions two of the lavas from diamond craters have whole rock and phenocryst compositions consistent with their being natural melts rock compositions that reflect the effects of crystal separation open systems are recognized by significant differences in predicted versus observed phenocryst modes the p t conditions of phenocryst saturation and the crystallization history of the magmas can be inferred from petrologic data combined with thermodynamic modelling and mass balance constraints the initial melts are inferred to have equilibrated at 0 8 1 0 gpa and 1210 1240 c the majority of the lavas from diamond craters have compositions that reflect olivine and plagioclase crystal separation these lavas can be related to two possible parent rock compositions by small amounts 2 8 by weight of olivine and plagioclase separation

flexural and gravity modelling of the merida andes and barinas apure basin western venezuela abstract the kinematic evolution of the barinas apure basin and the southern merida andes from lower miocene to the present is numerically modelled using flexural isostatic theory and geophysical and geological data two published regional transects are used to build up a reference section which is then used to constrain important parameters e g shortenings and sedimentary thicknesses for the flexural modelling to control the location of the main fault system in the flexural model earthquake information is also used the estimated flexural elastic thickness of the south american lithosphere beneath the barinas apure basin and the merida andes range is 25 km the value for the final total shortening is 60 km the flexural isostatic model shows that the andean uplift has caused the south american lithosphere subsidence and the development of the barinas apure basin in addition gravity modelling was used to understand deep crustal features that could not be predicted by flexural theory consequently the best fit flexural model is used to build a gravity model across the merida andes and the barinas apure basin preserving the best controlled structural features from the flexural modelling e g basin wavelength and depth and slightly changing the main bodies density values and deep crustal structures the final gravity model is intended to be representative of the major features affecting the gravity field in the study area the predicted morphology in the lower crustal level of the final gravity model favours the hypothesis of a present delamination or megathrust of the maracaibo crust over the south american shield this process would use the conrad discontinuity as a main detachment surface within an incipient nw dipping continental subduction

application of la icp ms analysis of arsenopyrite to gold metallogeny of the meguma terrane nova scotia canada abstract investigation of gold metallogeny in the paleozoic meguma terrane canada is conducted through la icp ms analysis of arsenopyrite collected from eight slate belt style vein gold deposits using a novel approach integrating elemental distribution maps and their derived elemental paragenesis with multi element binary plots the data reveal two distinct gold events 1 an early event characterized by a co ni mo sb se elemental association related to initial growth of arsenopyrite that reflects the presence of invisible gold 10 ppm and 2 a second event spatially associated with late fracture sets that is characterized by an al ti v mn element association and reflects either remobilization or upgrading of primary invisible gold and is manifest as visible gold the results of this study indicate a complex and protracted history of gold mineralization which has important ramifications for the meguma gold deposits as well as other orogenic gold districts globally in the case of the meguma terrane it involves an initial gold event that is followed by element mobilization and in the case of precious metals au ag an upgrading through a zone refining process in addition the variable coupling and decoupling of elements is only revealed using in situ derived la icp ms data

the kingston terrane southern new brunswick canada evidence for an early silurian volcanic arc the fault bounded kingston terrane of southern new brunswick consists of felsic and less abundant mafic volcaniclastic rocks and flows and minor interbedded sedimentary rocks of the kingston group intruded by mainly granitic plutons and abundant younger mafic sheets the granitic plutons form about half of the terrane and are characterized by fine grain size and typically granophyric and porphyritic textures indicative of high level emplacement mafic sheets were emplaced in the granitic plutons and parallel to primary layering in the kingston group which is typically steeply dipping and trends northeast to north northeast hence in contrast to some earlier interpretations the kingston terrane does not consist of a bimodal dike complex the kingston group granitic rocks and mafic sheets contain mineral assemblages consistent with metamorphism to upper greenschist lower amphibolite facies new u pb zircon ages of 442 6 ma from a dacitic tuff unit and 437 10 ma from a granitic pluton corroborate previous u pb dates that had suggested that the volcanic and granitic rocks in the kingston terrane are early silurian rather than precambrian as had been earlier assumed the felsic volcanic and granitic rocks are interpreted to be comagmatic on the basis of their field relationships and similar u pb ages and chemical character compositions of both the felsic rocks and less abundant mafic and intermediate and mafic rocks are consistent with calc alkalic affinity and emplacement in a continental margin volcanic arc herein termed the kingston arc a u pb date of 435 5 ma was also obtained from compositionally similar granite that forms a dike in an adjacent fault bounded amphibolite facies metasedimentary unit the similarity in age and composition of the granite dike suggests a link between these metasedimentary rocks and the kingston terrane possibly as part of an accretionary complex if so its location southeast of the kingston terrane indicates that subduction was to the present northwest mafic sheets in the kingston terrane are tholeiitic but have some arc like characteristics consistent with their origin in a former arc setting but the sheets were likely emplaced in a subsequent extensional setting the compositional similarity of the mafic sheets to late silurian mafic volcanic rocks and plutons of the coastal maine magmatic province suggests that they may be related a possible model for evolution of the kingston terrane is proposed wherein the kingston group and related granite formed in an early silurian arc on the margin of ganderia as represented by the new river terrane of late precambrian and early paleozoic age northwestward subduction culminated in collision between the kingston arc and a continental block now represented by the brookville terrane to the southeast subsequent transpressive mo tions in the late silurian may have been responsible for the crustal scale extensional environment in which the coastal maine magmatic province was formed including emplacement of mafic dikes into the deformed former kingston arc transpressive motions related to juxtaposition of the avalon and meguma terranes with the previously accreted more inboard terranes continued through the devonian and into the carboniferous

440 ma igneous activity in the meguma terrane nova scotia canada part of the appalachian overstep sequence abraded zircons from the basal rhyolitic tuff member of the white rock formation which disconformably overlies the cambrian early ordovician meguma group have yielded a nearly concordant u pb age of 442 or 4 ma interpreted as the age of extrusion this age straddles the approximately 443 ma ordovician silurian boundary abraded zircons from the brenton granite lie on a chord with an upper intercept age of 439 4 3 ma which is interpreted to be the age of intrusion thus supporting its inferred subvolcanic nature on the other hand monazite from the brenton pluton yielded a nearly concordant analysis with a 207 pb 206 pb age of 380 or 3 ma which is interpreted to be the time of the low pressure high temperature metamorphism using these new data the following observations suggest that the meguma and avalon terranes were neighbors during the silurian early devonian 1 both have latest ordovician earliest silurian bimodal subaerial alkalic tholeiitic rift related volcanic rocks with nd signatures indicating a similar continental basement source 2 both show a similar progression of depositional environments subaerial in the earliest silurian progressively deeper water marine strata in the llandovery and wenlock switching to gradually shallowing marine environments in the ludlow and reverting to subaerial in the pragian and 3 both contain rhenish bohemian early devonian fauna furthermore the southeast to northwest transition from an offshore sandbar to a beach sand in the silurian white rock formation suggests the presence of land to the north now recognized as avalonia these conclusions support published suggestions that the siluro devonian successions in the meguma and avalon terranes form part of the overstep sequence that extends across most of the northern appalachians published data indicate that avalonia was adjacent to gondwana in the neoproterozoic that it separated from gondwana in the early ordovician and was accreted to eastern laurentia in the late ordovician early silurian on the basis of the data and correlations presented here we suggest that the meguma terrane travelled with avalonia this is consistent with the absence of a phase of deformation between the cambro ordovician meguma group and the silurian white rock formation

combined slope ratio analysis and linear subtraction an extension of the pearce ratio method abstract a new technique called combined slope ratio analysis has been developed by extending the pearce element ratio or conserved denominator method pearce 1968 to its logical conclusions if two stoichiometric substances are mixed and certain chemical components are uniquely contained in either one of the two mixing substances then by treating these unique components as conserved the composition of the substance not containing the relevant component can be accurately calculated within the limits allowed by analytical and geological error the calculated composition can then be subjected to rigorous statistical testing using the linear subtraction method recently advanced by woronow 1994 application of combined slope ratio analysis to the rocks of the uwekahuna laccolith hawaii usa and the lavas of the 1959 summit eruption of kilauea volcano hawaii usa yields results that are consistent with field observations

origins of the 1954 1960 lavas kilauea volcano hawaii major element constraints on shallow reservoir magmatic processes in the 1954 1960 period kilauea volcano hawaii erupted four times summit eruptions occurred in 1954 halemaumau and 1959 kilauea iki and flank eruptions occurred in the lower east rift zone in 1955 and 1960 our analysis demonstrates that the chemical compositions of lavas from each eruption are related solely through fractionation and sorting of the observed phenocryst assemblages we have used pearce element ratio diagrams to make the following observations and inferences 1 the conserved element ratio p k for each eruption has less variance than is expected from analytical uncertainty thus we cannot reject the hypothesis each set of lavas represents a comagmatic suite of compositions 2 where each of the four groups of lava analyses is plotted on a pearce element ratio diagram with axes of si k and 0 5 mg fe 1 5ca 2 75na 0 25al k the data fit a slope of 1 0 therefore we cannot reject the hypothesis the chemical variation in each data set is consistent with sorting of ol pl au 3 each of the 4 groups of lavas has the same mean p k ratio as a result we cannot reject the hypothesis the 1954 1960 lavas originated from a single magma batch or several magmas with identical p k values finally using the most magnesian glass analysis in the kilauea data suite 1959 sg 05 we have modelled the differentiation path with thermodynamic based calculations these calculations predict a liquid line of descent that is consistent with the path inferred from rock and glass analyses therefore the simulations provide an additional test of our hypotheses and corroborate the postulated differentiation model additionally these calculations suggest that differentiation took place at less than 0 3 gpa and extended over a temperature range of between 100 c and 150 c

glacial effects limiting mountain height the height of mountain ranges reflects the balance between tectonic rock uplift crustal strength and surface denudation tectonic deformation and surface denudation are interdependent however and feedback mechanisms in particular the potential link to climate are subjects of intense debate spatial variations in fluvial denudation rate caused by precipitation gradients are known to provide first order controls on mountain range width crustal deformation rates and rock uplift moreover limits to crustal strength are thought to constrain the maximum elevation of large continental plateaus such as those in tibet and the central andes there are indications that the general height of mountain ranges is also directly influenced by the extent of glaciation through an efficient denudation mechanism known as the glacial buzzsaw here we use a global analysis of topography and show that variations in maximum mountain height correlate closely with climate controlled gradients in snowline altitude for many high mountain ranges across orogenic ages and tectonic styles with the aid of a numerical model we further demonstrate how a combination of erosional destruction of topography above the snowline by glacier sliding and commensurate isostatic landscape uplift caused by erosional unloading can explain observations of maximum mountain height by driving elevations towards an altitude window just below the snowline the model thereby self consistently produces the hypsometric signature of the glacial buzzsaw and suggests that differences in the height of mountain ranges mainly reflect variations in local climate rather than tectonic forces

systematics of modal phenocryst assemblages in submarine basalts petrologic implications phenocryst assemblages in ocean ridge basalts generally show an increasing proportion of plagioclase as the total amount of phenocrysts increases the variations in phase assemblages as well as most crystal liquid kd s are similar to variations equimodal trends predicted by low pressure laboratory experiments suggesting that many of these basalts have experienced varying degrees of low pressure cyrstallization prior to quenching with little sorting of crystals and liquid important exceptions include moderately to highly phyric basalts enriched either in plagioclase or olivine which lie well off the experimental trends in these basalts megacrysts and xenocrysts usually cited as evidence for magma mixing commonly represent a small proportion of the total crystalline phase assemblage however phase proportions for many of these basalts lie well outside the range that could be produced by simple mixing selective gravitative sorting either prior or subsequent to mixing appears to be the likely explanation for these phyric basalts a relation between spreading rate and phase proportions is neither supported nor refuted by the data which as yet do not adequately represent fast spreading ridges

electron beam analytical instruments and the determination of modes spatial variations of minerals and textural features of rocks in polished section point counts based on x ray intensities collected with an automated electron beam analytical instrument can be used to estimate the accurate modes required by modern physical chemical modelling of rock forming processes in addition the spatial distributions of mineral species and mineral compositions can be extracted from the coordinates of the point counting grid statistical tests of whether the mineral and porosity distributions are uniform can be performed by comparing the results from subareas of the polished section the separate and composite contributions of the mineral phases to the texture and structure of the rock can be visually represented with mineral distribution maps qualitative differences in grain size can be recognized because the nonzero finite electron beam diameter can detect grain boundaries spots on the minerals are identified by comparing the ranking of x ray intensities with the ranking of intensities on a set of standards for example this method identified more than 88 of the spots on the surface of a polished section of an alkali olivine basalt a small additional number of spots approximately 1 were identified because their spectra were dominated by the x ray intensities of two characteristic elements this method was particularly efficient in identifying phases with a small grain size and simple chemistry e g apatite the modal amounts of an alkali olivine basalt obtained with a microprobe are within approximately one standard deviation of the results obtained by point counting the same areas of the polished section under reflected and transmitted light if the minerals are major constituents and occur only in the groundmass accessory and phenocryst phases agree to within approximately three standard deviations

birth and demise of the rheic ocean magmatic arc s combined u pb and hf isotope analyses in detrital zircon from sw iberia siliciclastic strata paleozoic continental reconstructions indicate that subduction of rheic oceanic lithosphere led to collision between laurussia and gondwana which was a major event in the formation of the ouachita appalachian variscan orogenic belt and the amalgamation of pangea however arc systems which record rheic ocean subduction are poorly preserved the preservation of devonian detrital zircon in late devonian early carboniferous siliciclastic rocks of sw iberia rather than arc related igneous rocks indicates that direct evidence of the arc system may have been largely destroyed by erosion here we report in situ detrital zircon u pb isotopic analyses of late devonian early carboniferous siliciclastic rocks from the pulo do lobo zone which is a reworked late paleozoic suture zone located between laurussia and gondwana detrital zircon age spectra from the pulo do lobo zone frasnian formations show striking similarities revealing a wide range of ages dominated by neoproterozoic and paleoproterozoic grains sourced from rocks typical of peri gondwanan terranes such as avalonia the meguma terrane and the ossa morena zone pulo do lobo rocks also include representative populations of mesoproterozoic and early silurian zircons that are typical of avalonia and the meguma terrane which are absent in the ossa morena zone the famennian tournaisian formations from the pulo do lobo zone however contain more abundant middle late devonian zircon indicating the contribution from a previously unrecognized source probably related to the rheic ocean magmatic arc s the middle late devonian to early carboniferous zircon ages from the siliciclastic rocks of sw iberia south portuguese pulo do lobo and ossa morena zones have a wide range in ehft values 8 2 to 8 3 indicating the likely crystallization from magmas formed in a convergent setting the missing rheic ocean arc was probably built on a meguma avalonia type basement we propose for the pulo do lobo zone that the frasnian sedimentation occurred through the opening of a back arc basin formed along the laurussian active margin during rheic ocean subduction as has been recently proposed for the rhenohercynian zone in central europe detrital zircon ages in the frasnian siliciclastic rocks indicate provenance in the meguma terrane avalonia and devonian rheic ocean arc s as a result of back arc basin inversion the frasnian formations underwent deformation metamorphism and denudation and were unconformably overlain by famennian to visean siliciclastic strata including the phyllite quartzite formation of the south portuguese zone the latest devonian early carboniferous detritus were probably shed to the pulo do lobo zone represa and santa iria formations by recycling of devonian siliciclastic rocks from the south portuguese zone meguma terrane and from a new distinct source with baltica laurentia derivation preserved in the horta da torre formation and alajar melange

simultaneous calculation of metamorphic fluid fluxes reaction rates and fluid rock interaction timescales using a novel inverse modeling framework abstract to study metamorphic carbonation at greenschist facies conditions in the sw scottish highlands a novel inverse modeling framework which combines solutions of the transport equation with a global optimization method of differential evolution was developed using this framework we calculated simultaneously time integrated and time averaged metamorphic fluid fluxes of 83 4 35 4 m 3 m 2 and 10 10 1 0 5 m 3 m 2 s 1 respectively a time averaged reaction rate constant of 10 12 7 10 2 m s 1 and comparable timescales for fluid flow and fluid driven reaction of 10 4 3 0 5 yr and 10 2 7 5 2 yr respectively these parameters were calculated using an empirical estimate of the coefficient of molecular diffusion and a calculated value for metamorphic porosity our estimates are 1 consistent with single pass flow of fluid released by metamorphic devolatilization 2 within the range where heat is transported by conduction and matter is transported by advection 3 in agreement with an emerging consensus that metamorphic events are relatively short lived and 4 supportive of applying laboratory based estimates of kinetic parameters to metamorphic systems based on a sensitivity analysis we show that 1 selecting the diffusion coefficient rather than fluid velocity reaction rate or flow duration as an input parameter yields more robust estimates of metamorphic fluid flow parameters and 2 ignoring reaction dependent porosity and reaction rates can result in an order of magnitude uncertainty in best fit flow parameters evaluated from concentration profiles finally similarity between our calculated time averaged metamorphic fluid fluxes which were obtained numerically and those which were obtained analytically confirms the validity of using the quasi stationary state assumption to quantify metamorphic fluid flow parameters

analysis of petrologic hypotheses with pearce element ratios pearce element ratios can test whether the members of a rock suite are comagmatic and can illustrate the causes of chemical diversity in comagmatic suites comagmatic rocks have constant ratios for elements conserved in the system during changes that led to the chemical diversity in basaltic systems the incompatible elements ti k and p are often conserved the slope of the trend on a pearce element ratio diagram is sensitive to the stoichiometry of the crystallizing and segregating phases a judicious choice of ratios as axes for the diagram provides a signature for the phases involved and estimates of their compositions in basaltic rocks diagrams with ti k vs p k can provide a test of the comagmatic hypothesis diagrams with 0 5 mg fe k vs si k have trends that are distinct for each comagmatic suite and different mineral assemblage different suites are distinguished by the intercepts in diagrams whereas mineral assemblages are recognized by the slopes of the trends for example if olivine is the sole crystallizing and segregating phase the trend will have a slope of 1 diagrams with 2ca na k vs al k distinguish plagioclase from augite assemblages and in conjunction with 0 5 mg fe k diagrams unravel the crystallization sequences of suites that have suffered three phase crystallization and segregation analyses from the uwekahuna laccolith kilauea the 1955 and 1967 68 eruptions of kilauea diamond craters volcanic field oregon and experimental data on morb glasses provide illustrations of the interpretations that can be obtained from pearce element ratios

40ar 39ar white mica ages reveal neoproterozoic paleozoic provenance and an alleghanian overprint in coeval upper ordovician lower devonian rocks of meguma and avalonia abstract 40ar 39ar analyses ir single grain total fusion of white micas from upper ordovician lower devonian sedimentary strata in the meguma annapolis section and avalon arisaig section terranes of the canadian appalachians complement recent detrital zircon studies by providing constraints on local source areas and identifying the age of low temperature a remarkable outcome of the study is that many muscovite ages are significantly younger than the depositional age of the strata they do not record the middle late devonian age of acadian folding or adjacent intrusions but instead preserve a precise age of 322 6 2 1 ma 16 analyses although the sections sampled display no discernable evidence for carboniferous tectonism these ages are coeval with documented alleghanian deformation in both avalonian and meguma terrane rocks and we interpret them to reflect distributed fluid flow coeval with dextral shear along the avalon meguma terrane boundary

petrologic hypothesis testing with pearce element ratio diagrams derivation of diagram axes in petrology pearce element ratio per diagrams have been used i to determine whether members of a rock suite are co genetic ii to identify the minerals involved in differentiation processes and iii to evaluate the extent to which those mineral are involved the axis coefficients of each diagram are chosen such that sorting of minerals or combinations of minerals will generate unique and predictable trends unfortunately selection of the optimal combination of axis coefficients is a difficult task especially if the system being investigated has a large number of phases or complicated solid solution minerals our work has established a formal set of rules and matrix operations which facilitate the determination of per diagram axes coefficients this methodology can be used to determine the unit molar vector displacement caused by the addition or subtraction of a specific mineral given a set of axis coefficients it can also be used to create per diagrams on which minerals have predetermined vector displacements by designating all vector displacements to be parallel axis coefficients for assemblage test diagrams can be determined to test the following hypothesis the observed chemical variation is due to the addition or removal of a specific set of minerals alternatively by designating all vector displacements to be mutually perpendicular phase discrimination diagrams can be created which test whether the observed chemical variations require a specific phase to be involved in differentiation phase discrimination diagrams also provide a means to estimate the extent of that involvement this methodology facilitates construction of powerful yet simple per diagrams which provide an effective means of testing alternative differentiation hypotheses

the geology of the darien panama and the late miocene pliocene collision of the panama arc with northwestern south america the geology of the darien province of eastern panama is presented through a new geologic map and detailed biostratigraphic and paleobathymetric analysis of its upper cretaceous to upper miocene sediments the sequence of events inferred from the stratigraphic record includes the collision of the panama arc the southwestern margin of the caribbean plate and south american continent three tectonostratigraphic units underlie the darien region 1 precollisional upper cretaceous eocene crystalline basement rocks of the san blas complex form a series of structurally complex topographic massifs along the northeastern and southwestern margins of the darien province these rocks formed part of a 20 m y submarine volcanic arc developed in a pacific setting distant from the continental margin of northwestern south america the northerly basement rocks are quartz diorites granodiorites and basaltic andesites through dacites to rhyolites indicating the presence of a magmatic arc the southerly basement rocks are an accreted suite of diabase pillow basalt and radiolarian chert deposited at abyssal depths precollisional arc related rocks of eocene to lower miocene age consist of 4000 m of pillow basalts and volcaniclastics and biogenic calcareous and siliceous deep water sediments they consist of the eocene oligocene darien formation the oligocene porcona formation and the lower middle miocene clarita formation postcollisional deposits are mostly coarse to fine grained siliciclastic sedimentary rocks and turbiditic sandstone of upper middle to latest miocene age this 3000 m thick sedimentary sequence is deformed as part of a complexly folded and faulted synclinorium that forms the central chucunaque tuira basin of the darien the sedimentary package reveals general shallowing of the basin from bathyal to inner neritic depths during the 12 8 7 1 ma collision of the panama arc with south america the sediments are divided into the upper middle miocene tapaliza formation the lower upper miocene tuira and membrillo formations the middle upper miocene yaviza formation and the middle to upper miocene chucunaque formation the precollisional open marine units of late cretaceous middle miocene age are separated from the overlying postcollisional sequence of middle to late miocene age by a regional unconformity at 14 8 12 8 ma this unconformity marks the disappearance of radiolarians the changeover of predominantly silica deposition from the atlantic to the pacific the initiation of the uplift of the isthmus of panama and the onset of shallowing upward coarser clastic deposition this pattern is also recorded from the southern limon basin of caribbean costa rica to the atrato basin of northwestern colombia by the middle late miocene neritic depths were widespread throughout the darien region and a regional unconformity suggests completion of the central american arc collision with south america by 7 1 ma no pliocene deposits are recorded from either the darien or the panama canal basin and no sediments younger than 4 8 ma have been identified in the atrato basin of colombia suggesting rapid uplift and extensive emergence of the central american isthmus in the latest miocene northward movement of the eastern segment of the panama arc along a now quiescent panama canal zone fault during eocene oligocene time may have dislocated the pre collision arc since collision the portion west of this fault chorotega block has remained stable without rotation to the east in the darien region compression has been accommodated through formation of a panama microplate with convergent boundaries to the north north panama deformed belt and south south panama deformed belt and suturing with south america along the atrato valley deformation within the microplate has been accommodated in the darien province by several major left lateral strike slip faults that were active until the early pliocene since when the plate has behaved rigidly

spatial and temporal patterns of exhumation across the venezuelan andes implications for cenozoic caribbean geodynamics 1 the venezuelan andes formed by complex geodynamic interaction between the caribbean plate the panama arc the south american plate and the continental maracaibo block we study the spatial and temporal patterns of exhumation across the venezuelan andes using 47 new apatite fission track aft ages as well as topographic analyses this approach permits the identification of at least seven tectonic blocks escalante cerro azul trujillo caparo sierra nevada sierra la culata and el carmen blocks with contrasting exhumation and cooling histories the sierra nevada sierra la culata and el carmen blocks located in the central part of the venezuelan andes and separated by the bocono fault system cooled rapidly but diachronously during the late miocene pliocene major surface uplift and exhumation occurred in the sierra nevada block since before 8 ma a second phase of uplift and exhumation affected the el carmen and sierra la culata blocks to the north of the bocono fault during the late miocene pliocene the highest topography and steepest relief of the belt coincides with these blocks the caparo and trujillo blocks located at the northeastern and southwestern ends of the orogen cooled more slowly from the oligocene to the late miocene these blocks are characterized by significantly lower mean elevations and slightly lower mean slopes than the central blocks unraveling the cooling history of the individual blocks is important to better understand the control of preexisting faults and regional caribbean geodynamics on the evolution of the venezuelan andes our data indicate a strong control of major preexisting fault zones on exhumation patterns and temporal correlation between phases of rapid exhumation in different blocks with major tectonic events e g collision of the panama arc rotation of the maracaibo block

variability over time in the sources of south portuguese zone turbidites evidence of denudation of different crustal blocks during the assembly of pangaea this study combines geochemical and geochronological data in order to decipher the provenance of carboniferous turbidites from the south portuguese zone sw iberia major and trace elements of 25 samples of graywackes and mudstones from the mertola visean mira serpukhovian and brejeira moscovian formations were analyzed and 363 u pb ages were obtained on detrital zircons from five samples of graywackes from the mira and brejeira formations using la icpms the results indicate that turbiditic sedimentation during the carboniferous was marked by variability in the sources involving the denudation of different crustal blocks and a break in synorogenic volcanism the visean is characterized by the accumulation of immature turbidites mertola formation and the base of the mira formation inherited from a terrane with intermediate to mafic source rocks these source rocks were probably formed in relation to devonian magmatic arcs poorly influenced by sedimentary recycling as indicated by the almost total absence of pre devonian zircons typical of the gondwana and or laurussia basements the presence of carboniferous grains in visean turbidites indicates that volcanism was active at this time later serpukhovian to moscovian turbiditic sedimentation mira and brejeira formations included sedimentary detritus derived from felsic mature source rocks situated far from active magmatism the abundance of precambrian and paleozoic zircons reveals strong recycling of the gondwana and or laurussia basements a peri gondwanan provenance is indicated by zircon populations with neoproterozoic cadomian avalonian and pan african zircon forming events paleoproterozoic and archean ages the presence of late ordovician and silurian detrital zircons in brejeira turbidites which have no correspondence in the gondwana basement of sw iberia indicates laurussia as their most probable source

an autochthonous avalonian basement source for the latest ordovician brenton pluton in the meguma terrane of nova scotia u pb hf isotopic constraints and paleogeographic implications the ediacaran ordovician meguma supergroup was thrust over avalonia basement prior to the intrusion of post acadian ca 370 ma s type granitic batholiths this has led to two main hypotheses regarding the original location of the meguma terrane a continental rise prism bordering either nw africa or avalonia on the other hand the pre acadian ca 440 ma brenton pluton has yielded the following u pb la icp ms zircon data 1 448 3 ma population peak inferred to be the intrusive age and 2 ca 550 and 700 ma inherited ages common to both avalonia and nw africa in contrast hf isotopic analyses of zircon yielded model ages ranging from 814 to 1127 ma with most between 940 and 1040 ma such ages are typical of avalonia and not nw africa the ages of the inherited zircons found within the brenton pluton suggest that it was probably derived by partial melting of sub meguma mid crustal avalonian rocks upon which the meguma supergroup was deposited although avalonia is commonly included in the peri gondwanan terranes off nw africa or amazonia paleomagnetic data faunal provinciality and hf data suggest that during the ediacaran early cambrian it was an island chain lying near the tropics ca 20 30 s and was possibly a continuation of the bolshezemel volcanic arc accreted to northern baltica during the ediacaran timanide orogenesis this is consistent with the similar derital zircon population in the ediacaran cambrian meguma supergroup and the dividal group in northeastern baltica

mechanical model for the tectonics of doubly vergent compressional orogens a mechanical model of crustal shortening and deformation driven by the relative convergence of rigid underlying mantle plates explains many features of convergent orogens results based on numerical models and supported by sandbox models show that a coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth 1 block uplift bounded by step up shear zones 2 development of a low taper wedge over the underthrusting mantle plate and 3 development of a low taperwedge overlying the overthrusting mantle plate and verging in the opposite direction when modified by isostasy basal viscous flow surface erosion and denudation and sedimentation the resultant model orogens exhibit a variety of styles with characteristics in common with small rapidly denuded orogens large orogens with plateaus and extensional characteristics and active subduction margins with doubly vergent accretionary wedges and deformed fore arc basins

late palaeozoic formation and development of the st marys basin mainland nova scotia canada a prolonged record of intracontinental strike slip deformation during the assembly of pangaea

u pb zircon ages and provenance of the white rock formation of the rockville notch group meguma terrane nova scotia canada evidence for the sardian gap and west african origin the white rock formation is the lowermost formation of the rockville notch group an assemblage of silurian devonian rocks preserved in five areas along the northwestern margin of the meguma terrane of nova scotia the formation consists mainly of mafic and felsic metavolcanic rocks interlayered with and overlain by marine metasedimentary rocks felsic metatuff has now been dated from four locations near both the bottom and top of the volcanic pile and yielded a narrow age range with errors of about 446 434 ma these dates confirm a 30 ma hiatus after deposition of the early ordovician hellgate formation in the underlying halifax group this hiatus is coeval with the sardian gap in the lower palaeozoic of peri gondwanan europe the metavolcanic metasedimentary assemblage is overlain by mainly metasiltstone with abundant quartzite and metaconglomerate lenses some of the latter were previously interpreted to be ordovician tillite an interpretation no longer viable new detrital zircon data from metas

areal distribution and age of low k high alumina olivine tholeiite magmatism in the northwestern great basin field petrographic chemical and geochronologic information has led to the identification and characterization of a widespread low k high alumina olivine tholeiite haot magma type in the northwestern great basin this basalt covers at least 22 000 km 2 and is estimated to represent a total volume of at least 650 km 3 the time period over which haot lavas were erupted extends from late miocene to holocene 10 5 0 m y b p this interval overlaps with the timing of snake river cascade and northwestern basin and range volcanism but distinguishes haot from the main pulse of columbia river volcanism 15 m y b p furthermore three major pulses of haot magmatism are suggested from the geochronology of this study 0 to 2 5 m y b p 3 5 to 6 m y b p and 7 to 10 m y b p the distinctive holocrystalline nonporphyritic and diktytaxitic texture the low incompatible element concentrations and the high mgo feo of haot serve to distinguish this basalt from other basalts of the northwestern united states the low incompatible element signature accentuates the similarities between haot mid ocean ridge basalts and back arc basin basalts these similarities combined with the haot chronology support the idea that the processes giving rise to extensional tectonism and haot magmatism in the northwestern great basin are similar to those acting in active back arc spreading regions

current states of stress in the northern andes as indicated by focal mechanisms of earthquakes abstract systematic inversion of double couple focal mechanisms of shallow earthquakes in the northern andes reveals relatively homogeneous patterns of crustal stress in three main regions the first region presently under the influence of the caribbean plate includes the northern segment of the eastern cordillera of colombia and the western flank of the central cordillera north of 4 n it is characterized by wnw ese compression of dominantly reverse type that deflects to nw se in the merida andes of venezuela where it becomes mainly strike slip in type a major bend of the eastern thrust front of the eastern cordillera near its junction with the merida andes coincides with a local deflection of the stress regime sw ne compression suggesting local accommodation of the thrust belt to a rigid indenter in this area the second region includes the sw pacific coast of colombia and ecuador currently under the influence of the nazca plate in this area approximately e w compression is mainly reverse in type it deflects to wsw ene in the northern andes south of 4 n where it is accommodated by right lateral displacement of the romeral fault complex and the eastern front of the northern andes the third and most complex region is the area of the triple junction between the south american nazca and caribbean plates it reveals two major stress regimes both mainly strike slip in type the first regime involves sw ne compression related to the interaction between the nazca and caribbean plates and the panama micro plate typically accommodated in an e w left lateral shear zone the second regime involves nw se compression mainly related to the interaction between the caribbean plate and the north andes block which induces left lateral displacement on the uramita and romeral faults north of 4 n deep seismicity about 150 170 km concentrates in the bucaramanga nest and cauca valley areas the inversion reveals a rather homogeneous attitude of the minimum stress axis which dips towards the e this extension is consistent with the present plunge of the nazca and caribbean slabs suggesting that a broken slab may be torn under gravitational stresses in the bucaramanga nest this model is compatible with current blocking of the subduction in the western northern andes inhibiting the eastward displacement of slabs which are forced to break and sink in to the asthenosphere under their own weight

identifying minerals controlling the chemical evolution of igneous rocks beyond pearce element ratio diagrams abstract a new method for identifying mineral controls on the chemical evolution of magmas has statistical power and interpretative reliability exceeding those of the conserved denominator method conserved denominator or pearce element ratio analysis compares two chemical variables that have each been divided by the same third chemical variable the common divisor introduces induced correlations that tend to reduce statistical power the method tends to assert the presence of mineral controls where none exist no available method can remove or even quantify the effects of these induced correlations by recasting chemical data into a new form standard regression analysis serves to evaluate the same geochemical hypotheses evaluated by common denominator analysis this new approach devolves from simple geometric relationships among compositional variables is consistent with a linear mixing paradigm and does not introduce induced correlations furthermore tests for co control by minerals with common cations e g olivine and orthopyroxene are easily formulated with this new method case studies involving olivine and olivine plagioclase control as well as synthetic data studies illustrate and verify this new method

the acadian fold belt in the meguma terrane nova scotia cross sections fold mechanisms and tectonic implications 1 structural data and cross sections from the eastern part of the devonian carboniferous fold belt of the meguma terrane give insight into its structural development and crustal tectonics the sandy goldenville formation the active competent member during buckling formed a multilayer several kilometers thick that lay beneath the denser softer muds and silts of the halifax formation cross sections reveal 2 orders of folds 11 18 km wavelength folds are interpreted as resulting from buckling under the influence of gravity 4 6 km wavelength folds reflect the thickness of the multilayer with buckle shortening in the range 32 44 depth to detachment derived from the sections 11 13 5 km below present erosion surface is such that given the depth of the detachment below the paleosurface during the main phase pre devonian granite of folding it would likely have been a relatively soft and thick shear zone that would have influenced profile geometry of the main phase folds at this stage of fold belt development granite production that may have been triggered by mantle delamination during continuing convergence accompanying terminal pangean assembly and ocean basin closure was followed by rapid exhumation later localized deformation late devonian carboniferous in the fold belt took place as the meguma terrane was displaced along the terrane bounding cobequid chedabucto fault system the characteristic asymmetric cross sections of structures of this phase are attributed to cooling and strengthening of the detachment that was a consequence of the postgranite uplift and exhumation

episodic orogenic front migration in the northern andes constraints from low temperature thermochronology in the eastern cordillera colombia 1 new thermochronometric data from the eastern cordillera of the colombian andes reveal diachronous exhumation associated with cenozoic contractional deformation in this sector of the northern andes we present a comprehensive account of exhumation patterns along a 150 km long across strike transect between 4 and 6 n by integrating 29 new apatite fission track aft ages and 17 new zircon fission track zft ages with sparse published thermochronological data from this area our data reveal episodic eastward migration of the orogenic front at an average rate of 2 5 2 7 mm a during the late cretaceous cenozoic we identify three major stages of orogen propagation 1 slow propagation 0 5 3 1 mm a until early eocene 2 rapid orogenic advance 4 0 18 0 mm a during middle late eocene which accounts for 86 of the orogen s present width and 3 slow orogen propagation 1 2 2 1 mm a from oligocene to holocene times our data demonstrate that in the course of changes in plate kinematics the presence of inherited crustal anisotropies such as the former rift bounding faults of the eastern cordillera favor a nonsystematic progression of foreland basin deformation through time by preferentially concentrating accommodation of slip and thrust loading along these zones of weakness

morphology and dynamics of inflated subaqueous basaltic lava flows during eruptions onto low slopes basaltic pahoehoe lava can form thin lobes that progressively coalesce and inflate to many times their original thickness due to a steady injection of magma beneath brittle and viscoelastic layers of cooled lava that develop sufficient strength to retain the flow inflated lava flows forming tumuli and pressure ridges have been reported in different kinds of environments such as at contemporary subaerial hawaiian type volcanoes in hawaii la reunion and iceland in continental environments states of oregon idaho washington and in the deep sea at juan de fuca ridge the galapagos spreading center and at the east pacific rise this study these lava have all undergone inflation processes yet they display highly contrasting morphologies that correlate with their depositional environment the most striking difference being the presence of water lava that have inflated in subaerial environments display inflation structures with morphologies that significantly differ from subaqueous lava emplaced in the deep sea lakes and rivers their height is 2 3 times smaller and their length being 10 15 times shorter based on heat diffusion equation we demonstrate that more efficient cooling of a lava flow in water leads to the rapid development of thicker by 25 cooled layer at the flow surface which has greater yield strength to counteract its internal hydrostatic pressure than in subaerial environments thus limiting lava breakouts to form new lobes hence promoting inflation buoyancy also increases the ability of a lava to inflate by 60 together these differences can account for the observed variations in the thickness and extent of subaerial and subaqueous inflated lava flows

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

contiguous rather than discrete paleozoic histories for the avalon and meguma terranes based on detrital zircon data upper ordovician lower devonian strata of the meguma terrane in the canadian appalachians contain zircon populations including an important mesoproterozoic zircon population 1 0 1 4 ga similar to those in coeval strata of avalonia and strongly suggest contiguous rather than discrete histories for these terranes throughout the paleozoic that these terranes were juxtaposed throughout the early paleozoic is indicated by the absence of a cambrian ordovician accretionary event the lack of intervening suture zone ophio litic units and the similarity of avalonian and meguma basement nd isotope signatures in early paleozoic igneous suites as avalonia had accreted to laurentia baltica by the early silurian these data suggest that the meguma terrane like avalonia resided along the same northern margin of the rheic ocean at that time these conclusions have implications for reconstructions of the northern gondwanan margin in the early paleozoic and imply that the silurian devonian acadian orogeny in maritime canada occurred in an andean type setting and was not related to collision of the meguma terrane with the laurentian margin

spatial and temporal relationships between compression strike slip and extension in the central venezuelan andes clues for plio quaternary tectonic escape abstract the geometry of tectonic structures attributed to the neogene quaternary time interval is described in the active setting of the venezuelan andes our methodology is based on the analysis of radar satellite and digital elevation model imagery complemented by structural fieldwork and the compilation of seismotectonic data to make a structural analysis on a regional scale radar images provide first class data for morphostructural analysis in areas of dense vegetation and frequent cloud covering like the venezuelan andes we focused our analysis in the burbusay rio momboy and bocono faults corner located in the central part of the belt we have described three stages of deformation during the neogene quaternary the first one mio pliocene in age is a nw se compression responsible for the uplift of the venezuelan andes the second tectonic stage corresponds to a strike slip regime of deformation marked by shearing along the bocono burbusay and valera faults which separates two triangular wedges in the larger trujillo block this strike slip faulting dominated compressional extensional tectonic regime allowed the trujillo crustal block to move towards the ne wrenching has therefore started at some point between the pliocene and the quaternary these two tectonic events are consistent with ongoing strain partitioning in the venezuelan andes the third stage corresponds to extensional deformation limited to the trujillo block and is still active today extension is associated with the motion of crustal blocks moving relative to each other probably above the upper lower crust boundary such extensional deformation can be understood considering that the crust extends and stretches at the same time as it moves towards the ne the combination of both horizontal lateral motion and extension is characteristic of a tectonic escape process the northeastward escape of the trujillo block which belongs to the larger north andes block occurs as a result of the combination of the nw se intracontinental convergence between the south american plate and the maracaibo block and the presence to the north of the caribbean oceanic plate considered as a free boundary we have showed that the kinematics of the caribbean plate offers not only a favorable environment but may also be considered as the driving force of the tectonic escape of the north andes block

40ar 39ar ages for detrital white mica in meguma terrane nova scotia canada implications for provenance of the goldenville and halifax groups single grain 40 ar 39 ar ages are reported for detrital white mica along with conventional step heating data for whole rocks from low grade metasandstone samples from the goldenville halifax and rockville notch groups in the meguma terrane of southern nova scotia the majority 166 of single grains from 11 samples yielded ages between ca 615 and 350 ma and the remaining 12 grains yielded ages between ca 1900 and 870 ma the late neoproterozoic paleozoic age distributions are consistent with derivation of sediments from the rapidly uplifted flanks of an active rift where partial to complete resetting of white mica ages occurred at ca 520 500 ma preceding sediment deposition the ca 615 ma ages may be relics of the original detrital white mica that existed in the source rocks prior to the rifting event ages from the upper silurian white rock formation appear to reflect this same ca 520 500 ma event suggesting that sediments in the white rock formation were recycled from the goldenville and halifax groups the older precambrian ages are inferred to represent white mica in the source region likely amazonia the whole rock age spectra are discordant with pronounced age gradients and no well defined age plateaus initial gas released from five of the samples at low laboratory extraction temperatures ca 450 500 c yielded ages of ca 260 300 ma not seen in the single grain data whereas gas released at the highest extraction temperatures yielded ages in the range ca 510 530 ma possibly reflecting the principal result obtained from the single grain data