a carbon isotope reference curve for ca 1700 1575 ma mcarthur and mount isa basins northern australia abstract shallow marine paleo and mesoproterozoic sedimentary successions are widely distributed in several major basins across northern australia the successions are only gently deformed and their stratigraphy is relatively continuous thus offering an ideal opportunity to document secular variations in carbon isotopes marine carbonate intervals from two of these major basins the mcarthur and mount isa basins have been sampled to document secular variation in 13ccarb from approximately 1700 to 1575 ma in all cases the samples have been tied to a well established sequence stratigraphy which along with u pb shrimp zircon dates provides a time resolution of the order of 1 m y the data presented here thus provide the most comprehensive and best dated 13ccarb stratigraphy yet obtained from such ancient rocks diagenesis occurred early in the carbonate rocks from both basins with the result that fluid movements were restricted and primary carbon isotopic signatures were retained the 13ccarb values from both basins vary within a very narrow range around a mean of 0 6 with extreme values seldom lying further than 1 from the mean that is the curves are essentially flat the results of this study combined with earlier studies on younger rocks imply that the global ocean reached a state of equilibrium in the mid paleoproterozoic and remained stable for much of the following billion years current models of the ocean suggest that to maintain the carbon mass balance relatively low levels of tectonic activity would be required which in turn suggests that the availability of nutrients such as phosphorus was stable and low prolonged nutrient stability may therefore have exerted a major influence upon the evolution of the biosphere over this time interval

first evidence of high pressure metamorphism in the cover series of the southern menderes massif tectonic and metamorphic implications for the evolution of sw turkey abstract the menderes massif occupies a large part of western turkey it is tectonically overlain by nappes of the izmir ankara suture zone including the bornova flysch zone on its northern flank the afyon zone on its eastern border and the lycian nappes on its southern flank the discovery of magnesiocarpholite kyanite assemblages in the mesozoic cover sequence of the southern menderes massif leads to the very first consideration that a major high pressure hp metamorphic event affected the massif during its alpine evolution estimations of minimum pressure temperature conditions about 10 12 kbar and 440 c suggest a minimum burial of 30 km for the metasedimentary rocks of the menderes massif similar to that for the eocene dilek blueschists which belong to the cycladic complex this finding of high pressure rocks within the menderes cover series as well as the recent description of low grade high pressure parageneses in the basal thrust sheets of the overlying lycian nappe complex has strong implications for the tectono metamorphic evolution of these units the characteristics of this high pressure metamorphism and the deformation features observed in the southern menderes massif allow consideration of a fast burial during alpine subduction and nappe stacking three high pressure units were piled up during the eocene collision of the sakarya continent and the tauride menderes block the lowermost unit consists of the imbricated core and cover of the menderes massif the intermediate unit is the cycladic blueschist complex dilek and samos regions and the uppermost unit is made of the lycian nappes the retrogression of high pressure parageneses from the menderes massif follows an isothermal decompression path and the associated deformation is characterized by a severe stretching oriented n s to ne sw during the high pressure event and its exhumation we propose a tectono metamorphic model in three main stages 1 the southward translation of the high pressure lycian nappes and the burial of the menderes and cycladic metasediments to a depth of at least 30 km to allow their eocene high pressure metamorphism 2 a northward continuum of deformation responsible for top to the north fabrics associated with the main menderes metamorphism peak paleocene eocene and its progressive cooling late eocene north verging folding thrusting imbrication of core and cover series and n s to ne sw intense stretching associated with the menderes high pressure metamorphism and its exhumation these top to the north to top to the northeast movements are probably related to the northward backthrusting of the lycian nappes reactivating the contact between the menderes massif and the lycian nappes as a top to the ne shear zone that allowed exhumation of the lycian high pressure rocks 3 oligo miocene n s extension during which the final exhumation of the metamorphic rocks along kilometre scale low angle detachments occurred

weakening associated with the diaspore corundum dehydration reaction in metabauxites an example from naxos greece abstract metabauxite lenses embedded in marble on naxos consist of diasporites below the 420 c isograd and dehydrate into corundum rich rocks with increasing grades of metamorphism while the diasporites are essentially undeformed the corundum rich rocks are strongly deformed even though both diasporites and corundum rich rocks are much stronger than the surrounding intensely deformed marbles the observed structures can be explained as an effect of high fluid pressures during the prograde diaspore corundum dehydration reaction which causes dramatic temporary weakening of the metabauxites to a strength comparable to that of the surrounding deforming marbles deformation of the metabauxite is thus largely restricted to the time span the phase transformation occurred allowing the dehydrating bauxite mass to deform together with the surrounding marbles

from transtension to transpression along the northern caribbean plate boundary off cuba implications for the recent motion of the caribbean plate abstract marine geophysical surveys using seabeam single channel seismic reflection gravimetric and magnetic measurements have been conducted along a segment of the northern caribbean transcurrent plate boundary seacarib ii cruise the data allow a better definition of the geometry and the tectonic regime of this major strike slip area they support the following results 1 1 along the southern cuban margin the oriente fault displays a discontinuous trace mainly composed of dextral offset en echelon segments some pull apart basins are located between fault segments cabo cruz basin chivirico and baitiquiri basins in the windward passage area the plate boundary enters into the tortue channel and is not connected with the subduction front off northern hispaniola 2 2 the eastern part of the oriente deep and the santiago promontory are characterised by active compressional tectonics they form the santiago deformed belt described here for the first time this deformed belt can be divided longitudinally into three main segments each one characterised by a particular tectonic style its development is related to a transpressional mechanism along the left lateral oriente strike slip fault our observations suggest that a tectonic and kinematic reorganisation occurred recently in this area probably in the late pliocene which may be compared with the recent geological events recorded on land in the northern caribbean domain the precise knowledge of both geometry and structures along the oriente strike slip fault south of cuba provides new constraints for the recent kinematic evolution along the northern caribbean transcurrent plate boundary it leads us to infer the existence of a convergence component associated with the slip component along the oriente transform fault

tectono metamorphic evolution of the siliciclastic units in the middle tuscan range inner northern apennines mg carpholite bearing quartz veins related to syn metamorphic syn orogenic foliation abstract this paper accounts for a new finding of high pressure low temperature hp lt metamorphism documented in continental derived rocks exposed in the central part of the middle tuscan range representing the easternmost location in the tyrrhenian region where blueschist facies metamorphism has been documented it describes the development mechanism of mg carpholite bearing quartz veins in the framework of the deformational events described for the inner northern apennines the hp lt peak metamorphism can be estimated in p 1 1 gpa and t 370 420 c as supported by the mineralogical assemblage consisting of quartz muscovite chloritoid pyrophylite hematite chlorite paragonite ti oxides documented in the aluminous metasediments pelites and quartzose sandstones of the triassic verrucano succession this paragenesis defines the s 1 schistosity an axial planar tectonic foliation associated to map and mesoscale isoclinals folds f 1 s 1 schistosity is a composite foliation resulted from a non coaxial progressive deformation associated to the de 1 collisional event late oligocene early miocene mg carpholite bearing quartz veins developed as tension gashes in en echelon arrays within brittle ductile shear zones the development of quartz veins was guaranteed by the high volumes of si rich fluids produced by the dynamic recrystallization process on the siliciclastic rocks

evolution of synmetamorphic veins and their wallrocks through a western alps transect no evidence for large scale fluid flow stable isotope major and trace element systematics quartz rich synfolial veins and wallrocks from different areas of a western alps cross section are examined in an attempt to constrain the scale and mechanisms of fluid flow through metamorphic terrains different portions of this cross section underwent distinct p t evolutions as reflected by the mineralogy of veins a in the greenschist external dauphinois domain veins are characterized by quartz and calcite together with an aluminosilicate pyrophyllite na k bearing phyllosilicates and a femg bearing silicate chlorite b in narrow domains of the brianconnais and piemontais zones with a non eclogitic hp lt evolution followed by rapid cooling veins are characterized by quartz calcite ca and fe mg bearing silicates lawsonite and fe mg carpholite respectively and c in the major part of the brianconnais andpiemontais zones where low grade blueschistfacies metamorphism is followed by a greenschist facies evolution veins are characterized by the replacement of earlier lawsonite and carpholite by na and k bearing micas in order to constrain the provenance of fluids involved in vein formation we determined stable isotope c 0 h and major and trace element compositions in minerals and whole rocks for a large set of veins and wallrocks these rocks are compared with some non metamorphic shales from the northern paris basin considered as protoliths of the alpine schists stable isotope compositions of calcites are regionally distinct 6r80 28 0 smow in the northern paris basin 26 0 in the brianconnais zone 22 0 in the piemontais zone and 18 0 in the dauphinois zone within each studied region calcite g o values are quite homogeneous regardless of rock type vein or wallrock this feature is explained as an isotopic homogenization with the most abundant lithology in each zone rock buffered system where the overall ratio of calcite of marine origin to detrital silicate minerals determines the final isotopic composition of the whole rock w r after equilibration during alpine metamorphism g o gd values of analysed hydrated minerals and whole rocks si80 15 to t 25 0 and 6d 60 0 smow lie within the field of alpine cover rocks and of fluids in equilibrium with these rocks interaction with an external o or d depleted fluid such as hydrothermal waters meteoric water or basement fluid can be ruled out major and trace element systematics indicate an enrichment in si ca p sr y ta and locally eu in the veins with respect to the wallrock related to massive crystallization of quartz and carbonates together with phosphate and oxide potential fluid sources chemical composition of the fluid phase and the mechanisms of fluid flow during vein formation are discussed fluid flow takes place within tectonic units at both hectometric and decimetric scale fluid may also have migrated along and across the major tectonic contacts but this process is poorly documented

the triple junction of the north america cocos and caribbean plates seismicity and tectonics the triple junction of the north america cocos and caribbean plates is ambiguously defined mainly because the north america caribbean plate boundary does not clearly continue beyond its known surface trace the motagua fault zone in western guatemala to intersect the middle america trench well located regional shallow earthquakes h 70 km show that there is no intermediate or large magnitude seismic activity associated with a presumed extension of the north america caribbean plate boundary to the west beyond its well defined surface trace there is however a clear zone of shallow seismic activity from the western section of the fault system through southern mexico fault plane solutions for these events indicate a left lateral strike slip displacement which is in good agreement with surface faulting we suggest that these strike slip faults together with the salina cruz fault in the isthmus of tehuantepec mark the boundaries of a broad zone of deformation in southern mexico and northern central america which takes up the interactions of the three plates in this sense no single point constitutes the triple junction the geologic record suggests that the motagua fault zone developed because the westernmost portion of the caribbean plate became locked against north america

geologic evaluation of plate kinematic models for the north american caribbean plate boundary zone heubeck ch and mann p 1991 geologic evaluation of plate kinematic models for the north american caribbean plate boundary zone tectonophysics 191 l 26 four published plate kinematic models for the present day north american caribbean plate boundary predict significantly different fault behavior along this dominantly strike slip plate boundary zone pbz using a computer graphics system we compare the deformational pattern predicted by the four kinematic models with geologic and seismic observations along the entire length of the pbz from central america to the northern lesser antilles our results indicate that none of the kinematic models predict all the geologic and seismic observations across the entire pbz however all four models predict mapped fault character and observed earthquake focal mechanisms along certain segments of the pbz we conclude that the present day deformation within the north american caribbean plate boundary zone cannot be described by rotation about a single pole instead the caribbean plate appears to be segmented into at least three major sub plates or blocks each of which requires an individual pole of rotation to describe its motion relative to the north american plate sub plate boundaries correspond to two poorly studied zones of diffuse deformation within the caribbean plate honduras depression of central america and beata ridge of the caribbean sea

formation of quartz veins by local dissolution and transport of silica a simple model is proposed for the often thick quartz veins observed in the modum complex in southern norway the formation of these veins cannot easily be explained by silica imported by hot ascending fluids the proposed model has dissolution in the host rock adjacent to the veins as the source for silica the suggested process for vein formation is represented by a reaction diffusion equation and the process is studied in terms of a damkohler number estimates for the growth rate of quartz cement are derived the estimates for the growth rate can be used to constrain poorly known parameters of the vein formation process like for instance the degree of supersaturation in the host rock

local origin of high pressure vein material in eclogite facies rocks of the zermatt saas zone switzerland prograde dehydration reactions within metabasalts at the blueschist eclogite facies transition produce kyanite veins usually with quartz sometimes with omphacite or chloritoid of up to several meters length and 20 cm width bulk rock analyses show clear depletion haloes for al2o3 and sio2 near the veins mass balance calculations support the suggestion that the vein material must have been derived from the immediate surroundings these veins are interpreted as local segregation phenom ena and not as fluid dehydration channels of the eo alpine subduction zone calcula tions have been performed for a stagnant vein formation model that considers the rate of the dehydration reactions k responsible for providing the h2o for al and si transport and the rate of diffusion d of these from wall rock to growing vein the model is satisfied for quite a wide range of values for k d and upper limits on the duration of diffusional vein formation of about 1 ma are indicated

synmetamorphic veining origin of andalusite bearing veins in the vedrette di ries contact aureole eastern alps italy discordant andalusite biotite quartz bearing veins occur in the contact aureole of the vedrette di ries pluton italian eastern alps never outside the area of contact metamorphic andalusite development andalusite veins are found only within andalusite bearing hornfelses and vein biotite occurs wherever host rock garnet is partially replaced by biotite veins formed during contact metamorphism synchronously with the crystallization of andalusite and biotite within host rocks their pegmatitic structure and their orientation suggest that vein parageneses crystallized within fluid filled cavities that opened by hydraulic fracturing a mechanism of synmetamorphic veining is proposed to explain rock failure and subsequent mineral deposition within veins during hydrofracturing induced by dehydration reactions in response to heating in the aureole fissures were immediately filled with locally derived fluids the lack of large scale flux together with high fluid pressures required by hydrofracturing suggest fluid in the cavities was a virtually stagnant passive medium and that mass transport toward fractures was driven by intergranular diffusion because temperature and pf values within veins are similar to those in the host rock vein assemblages are interpreted as the stable high t side of reactions taking place within pelitic schists at the time when fractures opened once nucleation of product phases occurred chemical components released by dissolution of reactant minerals were driven to precipitation sites by chemical potential gradients since nucleation was favoured at the strained grains of vein walls andalusite and biotite simultaneously grew in vein and host rock the proposed genetic model contrasts with generally adopted metasomatic mechanisms for the genesis of al2sio5 bearing veins in not requiring large fluid rock ratios or a highly aggressive fluid composition the mechanism of synmetamorphic veining may be particularly useful in the interpretation of vein occurrences in medium and deep crustal rocks which have undergone extensive devolatilization

glacial facies associations in a neoproterozoic back arc setting zavkhan basin western mongolia diamictites many of glacial origin are globally distributed in the neoproterozoic recently two relatively thin diamictites in the maikhan uul member at the base of the neoproterozoic tsagaan oloom formation from the zavkhan basin of western mongolia have been identified as being of glacial origin the mongolian diamictites form a series of backstepping units within the transgressive systems tract of two major depositional sequences associated with sea level changes in each case the diamictites of the transgressive systems tract are abruptly overlain by deeper water upward shoaling highstand systems tracts consisting of thinly bedded sandstones and shales in sequence 1 and thinly bedded dark carbonates in sequence 3 the fact that the sequences conform closely to depositional models established at other localities suggests that all are related to major ice ages and that the depositional sequences they have generated provide a valuable tool for global correlation in this part of the stratigraphic column available stratigraphic and isotope geochemical information presented by brasier et al 1996 this issue suggests that both diamictites are likely to be of sturtian age a riftogenic setting and sturtian age for the diamictites provide a link with eastern australia and western america it is possible therefore that these diamictites formed during the breakup of a supercontinental assembly including siberia australia and laurentia c 750 725 ma bp

transformation kinetics of polycrystalline aragonite to calcite new experimental data modelling and implications experimental data are used to model the transformation rate of polycrystalline aragonite grain diameter 80 m to calcite optimized values for nucleation and growth rates were obtained by numerically fitting the overall transformation rates from 280 to 380 c and 0 10 mpa to an expression for a grain boundary nucleated and interface controlled transformation the nucleation rate is 4 5 orders of magnitude faster than for calcite nucleated within aragonite grains and the growing in rate is slower below 300 c than for calcite growing in aragonite single crystals the activation enthalpy for growth in polycrystalline aggregate is 247kj mol compared to 163 kj mol for growth in single crystals permanent deformation of the phases limits the elastic strain energy due to the 7 volume change and reduces the coherency of the calcite aragonite interace theoretical expressions are used to extrapolate the data for nucleation and growth to other temperatures and data from 0 10 to 400 mpa are used to evaluate the effect of pressure on the grain boundary nucleation rate because of permanent deformation of the phases the effective strain energy for nucleation is 0 55 kj mol which is less than a quarter of the value for purely elastic deformation these data are used to predict the percent transformation for various p t t paths without heating during uplift partial preservation of aragonite in dry blueschist facies rocks can occur if the calcite stability field is entered at 235 c and the kinetic data are also consistent with published p t t paths which include heating during uplift the predicted percent transformation is relatively insensitive to variations in the initial grain size of the aragonite but strongly dependent on the effective strain energy

sequence stratigraphy and depositional controls in late proterozoic early cambrian sediments of amadeus basin central australia the amadeus basin is an isolated intracratonic basin at the center of the australian continent which because of its location and geometry provides an ideal opportunity to investigate depositional controls to this end more than 6 000 km of seismic data in conjunction with a field and well log program have been used in a study of the late proterozoic early cambrian arumbera sandstone the arumbera sandstone was deposited as two major depositional sequences in a shallow marine and deltaic or coastal plain setting similar to that of the devonian catskill delta of north america sediment was supplied from the southwest by heavily laden braided streams during the deposition of the first sequence when sea level was at its maximum sedimentation appears to have been largely aggradational as sediment supply balanced or exceeded basin subsidence the sediments were probably deposited in a coastal or delta plain setting where small scale deltas prograded across the carbonate platform sediments of the julie formation during the accumulation of the second sequence sea level fell rapidly exposing large areas of sediment deposited during the first cycle and restricting edimentation to two steep sided rapidly subsiding subbasins deposition was largely progradational as major deltaic complexes developed on the southern and southwestern margins of the subbasins between the major delta complexes a depositional environment similar to that of the first sequence predominated in the deep subbasins the arumbera sandstone forms a continuous depositional unit across the proterozoic cambrian boundary the unconformable nature of that boundary over much of the earth possibly could be the result of a major eustatic sea level drop the final geometry of the arumbera sandstone and the nature of its sediments were determined largely by local subsidence rates subsidence briefly outstripped sediment supply during a stage of crustal extension as the basin evolved during deposition of sequence 1 depositional space and facies distribution were however fine tuned by salt tectonics and major sea level cycles as at passive margins sequence boundaries appear to be controlled by sea level cycles other variables such as climate the lack of vascular plants and the uplift of the source area were also factors in controlling deposition

semiquantitative modeling of strain and kinematics along the caribbean north america strike slip plate boundary zone recent structural and geophysical studies conducted along the northern caribbean plate boundary at sea and on land have led to a precise description of the geometry and the tectonic regimes along this major transcurrent zone which separates the caribbean and north american plates in order to interpret its tectonic features in terms of plate motion we use a simple numerical model of strike slip faulting to test previously proposed kinematic models and to compute new motion parameters we show that none of the previously proposed models correctly accounts for the observed deformation along the whole plate boundary on the basis of the deformation pattern obtained from geological data we compute a motion parameters set that integrate rigid plate rotation and a plate boundary zone deformation component our results show that the deformation along the northern caribbean plate boundary zone is controled by regional kinematics i e the caribbean north america relative motion rather than by local effects e g small block rotation intraplate deformation

syntectonic fluid flow along thrust faults example of the south pyrenean fold and thrust belt abstract during compressive events deformation in sedimentary basins is mainly accommodated by thrust faulting and related fold growth thrust faults are generally rooted in the basement and may act as conduits or barriers for crustal fluid flow most of recent studies suggest that fluid flow through such discontinuities is not apparent and depends on the structural levels of the thrust within the fold and thrust belt in order to constrain the paleofluid flow through the jaca thrust sheet top basin paleogene southwest pyrenean fold and thrust belt this study compares on different thrust faults located at different structural levels the microstructures in the different fault zones studied are similar and consist of pervasive cleavage calcite shear veins sv1 extension veins ev1 and late dilatation veins ev3 in order to constrain the nature and the source of fluids involved in fluid rock interactions within fault zones a geochemical approach based on oxygen and carbon stable isotope and trace element compositions of calcite from different vein generations and host rocks was adopted the results suggest a high complexity in the paleohydrological behaviors of thrust faults providing evidence for a fluid flow compartmentalization within the basin previous studies in the southern part of the axial zone north of the jaca basin indicates a circulation of deep metamorphic water probably derived from the paleozoic basement along fault zones related to the major basement gavarnie thrust in contrast in northern part of the jaca basin the monte perdido thrust fault is affected by a closed hydrological fluid system involving formation water during its activity the jaca and cotiella thrust faults in turn both located more to the south in the basin are characterized by a composite fluid flow system indeed stable isotope and trace element compositions of the first generations of calcite veins suggest a relatively closed paleohydrological system whereas the late calcite vein generations which are probably associated with the late tectonic activity of the basin support a contribution of both meteoric and marine waters based on these results a schematic fluid flow model is presented this model allows visualization of three main fluid flow compartments along a n s transect

on the arrangement of solid inclusions in fibrous veins and the role of the crack seal mechanism the arrangement of solid inclusions in veins was examined in three different rock types and tectonic settings common to all samples is a new type of sinusoidal inclusions associated with inclusion bands oriented parallel to the vein wall and inclusion trails at high angle to the vein wall crosscutting relationships of inclusion bands and the sinusoidal inclusions imply that inclusion bands do not reflect individual crack seal increments in our veins inclusion bands are not a sufficient criterion to infer the crack seal mechanism further evidence for solid inclusions not formed by the classic crack seal mechanism is given by inclusion bands in gypsum crystals grown in soft clay during incorporation solid inclusions can become folded or boudinaged depending on their orientation with respect to the opening direction the complex displacement field within the vein is recorded by curved crystal fibres which track the opening direction based on the arrangement of solid inclusions within the vein we suggest that our veins grew continuously during syntectonic vein formation solid inclusions formed by a fluctuation of adhesion at the vein wall interface rather than by crack sealing and vein growth was at least partly driven by the force of crystallisation q 2005 elsevier ltd all rights reserved

raman spectra of carbonaceous material in metasediments a new geothermometer metasedimentary rocks generally contain carbonaceous material cm deriving from the evolution of organic matter originally present in the host sedimentary rock during metamorphic processes this organic matter is progressively transformed into graphite s s and the degree of organisation of cm is known as a reliable indicator of metamorphic grade in this study the degree of organisation of cm was systematically characterised by raman microspectroscopy across several mesozoic and cenozoic reference metamorphic belts this degree of organisation including within sample heterogeneity was quantified by the relative area of the defect band r2 ratio the results from the schistes lustres western alps and sanbagawa japan cross sections show that 1 even through simple visual inspection changes in the cm raman spectrum appear sensitive to variations of metamorphic grade 2 there is an excellent agreement between the r2 values calculated for the two sections when considering samples with an equivalent metamorphic grade and 3 the evolution of the r2 ratio with metamorphic grade is controlled by temperature t along the tinos cross section greece which is characterised by a strong gradient of greenschist facies overprint on eclogite facies rocks the r2 ratio is nearly constant consequently the degree of organisation of cm is not affected by the retrogression and records peak metamorphic conditions more generally analysis of 54 samples representative of high temperature low pressure to high pressure low temperature metamorphic gradients shows that there is a linear correlation between the r2 ratio and the peak temperature t c 445 r2 641 whatever the metamorphic gradient and probably the organic precursor the raman spectrum of cm can therefore be used as a geothermometer of the maximum temperature conditions reached during regional metamorphism temperature can be estimated to 50 c in the range 330 650 c a few technical indications are given for optimal application

contrasted metamorphic evolutions in thrusted cover units of the brianconnais zone french alps a model for the conservation of hp lt metamorphic mineral assemblages the evolution of organic matter silicate and fluid phases in cover units of the three structural zones of the vanoise area allows one to distinguish differentp t cooling paths these paths depend on whether the cover has been transported western and northern vanoise or not southern vanoise all units first underwent a common high pressure low temperature hp lt metamorphic stage 300 c 6 kbar of blueschist type fe mg carpholite chloritoid facies the cover units transported on the extermal colder zone coal measure series metamorphosed in albite chlorite facies preserved their hp mineralogy fe mg carpholite lawsonite and organic matter content oils wet gases and kerogen while the unit which remained in contact with its more thermally conductive basement polymetamorphic now shows extensive greenschist facies overprinting breakdown of fe mg carpholite and lawsonite appearance of chlorite pyrophyllite chloritoid and clinozoisite absence of oils and wet gases these two tectonic evolutions might serve as a more general model for the preservation of hp lt mineral assemblages

neoproterozoic stratigraphy of the centralian superbasin australia abstract the basement of central australia includes the arunta block and the 1200 1100 ma musgrave block succeeded by 1075 1000 ma bimodal volcanic rifts and dolerite dykes a regional hiatus at 1000 800 ma interpreted as reflecting a central post volcanic underplated upland was followed at 800 ma by a second swarm of dykes in the musgrave block gawler craton and stuart shelf associated with the crustal sagging that initiated the centralian superbasin the centralian superbasin is defined here to encompass the neoproterozoic fill of the amadeus georgina ngalia officer and savory basins the intra cratonic superbasin was disrupted internally 540 600 ma ago petermann ranges orogeny by a central uplift and associated thrusts and by mid carboniferous 320 ma tectonism alice springs orogeny to form the structural basins listed above the stratigraphy and sedimentology of the relatively well known areas can be used as a basis for predicting those of the poorly known regions new results from acritarch biostratigraphy and isotope chemostratigraphy in conjunction with conventional lithostratigraphy and sequence analysis allow better stratigraphic resolution than previously possible field observations in the recently discovered savory basin in western australia provide a basis for correlations with the adjacent basins

fluid flow mineral reactions and metasomatism a general model that relates fluid flow along a temperature gradient to chemical reaction in rocks can be used to quantitatively interpret petrologic and geochemical data on metasomatism from ancient flow systems in terms of flow direction and time integrated fluid flux the model is applied to regional metamorphism quartz veins and a metasomatized ductile fault zone

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

trace element rich brines in eclogitic veins implications for fluid composition and transport during subduction primary and pseudosecondary fluid inclusions occur in oscillatory and sector zoned omphacite in eclogitic veins from the monviso ophiolitic complex in the western alps the inclusions contain aqueous brines and daughter crystals of halite sylvite calcite dolomite albite anhydrite and or gypsum barite baddeleyite rutile sphene fe oxides pyrite and monazite this daughter mineral suite indicates high solubilites of na k ca mg fe zr ti p ba ce la th and s species and provides direct evidence for transport of high fieldstrenght large ion lithophile and light rare earth elements as dissolved species during subduction fluid inclusion heterogeneities preserved within and between adjacent grains in the veins however suggest that the scale of fluid equilibration was small a crack seal geometry in some of the veins implies that fluid release in pulses rather than steady flow controlled mineral deposition and growth in the veins from these observations we develop a model of fluid release and entrapment in which pulses of fluid are associated in time with increments of shear and tensile failure the rate of fluid release and the reduction in porosity both depend on the rate of plastic flow vein fluids may initially be derived from decreptitation of early fluid inclusions in the host eclogites small scale fluid heterogeneities implied by the fluid inclusions in the veins are best interpreted in terms of limited fluid flow and hence limited metasomatism we conclude that element recycling into the mantle wedge during subduction will depend at least as strongly on fluid transport mechanisms as on element solubilities in the fluid phase at monviso despite evidence for high trace element solubilities in saline brines the elements were not removed from the downgoing slab prior to teaching depths of 40 km

neogene north american caribbean plate boundary across northern central america offset along the polochic fault abstract the polochic fault was a segment of the north american caribbean plate boundary across central america in the neogene its 130 km of left slip was previously determined by matching structures and stratigraphie outcrop patterns of northwest and central guatemala across the fault additional support for the model and the youthfulness of the recorded offset comes from an essentially perfect match of major geomorphic features across the fault a reconstruction process which eliminates 123 km of left slip brings together rivers and drainage divides that existed before the polochic became active with the reconstruction carried across the isthmus on an east west fault the regional structural geology assumes the coherent pattern of a continuous orogenic belt whose geometry is compatible with the model of collisional tectonics centered on the motagua suture zone confined within this belt narrowed to some 60 km by the reconstruction lie the major laramide thrusts folds and tectonically emplaced serpentinites of guatemala crystalline rocks of guatemala re join the chiapas massif and paleozoic sedimentary rocks exposed in the core of an almost continuous anticlinorium extend from southern chiapas to lake izabal the polochic does not bend in eastern guatemala but continues eastward to the motagua fault where it dies westward drift of the northern block resulted in rifting which extended from eastern guatemala into the caribbean along the cayman trough the honduras depression may represent an element of a triple junction along with the polochic and izabal cayman rift the polochic continues westward into the pacific ocean and offsets the middle america trench the polochic has offset the miocene volcanic belt of northern central america confirming the previous estimate of a neogene time of movement about 300 km of relative east west neogene displacement has been recorded on the mid cayman rise only 130 km of which can be accounted for across the polochic it is suggested that cumulative extension on north south faults south of the motagua fault zone between the trench and the honduras depression might make up that difference

neotectonics of hispaniola plate motion sedimentation and seismicity at a restraining bend abstract the pattern of neogene faulting volcanism and sedimentation in hispaniola helps to resolve a problem that arises in attempting to determine the direction of caribbean plate motion from earthquakes alone namely how well do earthquake mechanisms define plate motion hispaniola occupies a compressional north step or restraining bend in the generally east west trending motagua cayman trough puerto rico trench fault system which marks an active plate boundary zone pbz of left lateral strike slip motion between the north america and caribbean plates four areas are distinguishable in hispaniola from field mapping and from the interpretation of satellite imagery and conventional aerial photographs two of these areas consist of active left lateral strike slip fault systems that roughly define the northern and southern coasts of the island the structure of both fault systems suggest a roughly east west trending direction of relative plate motion consistent with previous findings the intervening area consists of en echelon mountain ranges thrust up at the restraining bend from the early miocene a pleistocene volcanic province within this area is interpreted as defining a diffuse extensional fault termination of the southern strike slip fault zone the fourth area in eastern hispaniola is interpreted as being underthrust by discontinuous fragments of lithosphere in response to convergence at the restraining bend it appears possible that thinner young island arc lithosphere may be more prone to subduction than to escape or flaking processes that are more important in older more buoyant lithosphere

on the significance of aragonite occurrences in the western alps aragonite occurrences from two areas of the western alps are described it is shown that aragonite has been formed under blueschist metamorphic conditions in the western vanoise while it has been precipitated under sub surface conditions in the queyras region an uplift p t path of the aragonite bearing rocks of the western vanoise is constructed using two independent methods 1 temperature estimates using the sr content of aragonite in the successive veins and 2 the kinetics of the aragonitecalcite solid state transition the uplift p t path has an unusual shape with an important temperature decrease 100 to 150 c for a pressure decrease of 0 2 gpa following blueschists metamorphism p 0 7 gpa t 300 c thermal models show that this unusual p t path of the western vanoise can be explained if one maintains a low temperature between 125 and 175 c at the base of the tectonic unit containing the aragonite bearing rocks during part of its burial history followed by the whole of its exhumation a tectonic scenario is proposed to account for the observed and modelled p t path

reply to comment on the 1946 hispaniola earthquake and the tectonics of the north america caribbean plate boundary zone northeastern hispaniola by r m russo and a villasenor

from transcurrent faulting to frontal subduction a seismotectonic study of the northern caribbean plate boundary from cuba to puerto rico recent marine and onland geophysical and geological investigations along the northern caribbean plate boundary call for a review of its tectonic interpretation in the light of a new compilation of the seismological data avalaible from cuba to puerto rico we show that the shallow seismicity in the northeastern caribbean is concentrated along an east west trending lineament corresponding to the trace of the major strike slip fault system the most intense seismicity is located around restraining bends such as southern cuba and northern hispaniola the stress and strain distribution deduced from focal mechanisms and microtectonic analysis lead us to infer a small n s convergence component associated with the major eastward strike slip motion of the caribbean plate versus north america earthquake distribution and focal mechanisms suggest the existence of a lithospheric slab inherited from the frontal subduction under the lesser antilles diping down under puerto rico and eastern hispaniola we propose a model in which this slab is disconnected from the atlantic oceanic lithosphere by transcurrent faulting along the plate boundary

silica mobility and fluid movement during metamorphism of the connemara schists ireland quartz veins are developed in a wide range of metasediment types in the upper amphibolite facies rocks of connemara and attest to considerable migration of silica contrary to common assumptions there is clear evidence that these veins do not primarily result from movement of fluid to regions of lower p t down the regional geothermal gradient under amphibolite facies conditions a dilute chloride fluid moving down temperature has the potential to alter 60g of plagioclase to muscovite for each gram of vein quartz precipitated while cooling over the temperature interval from 650 to 500 c the absence of significant metasomatic effects in the vein walls effectively precludes a simple origin from such through flowing externally derived fluids the oxygen isotopic composition of matrix quartz shows considerable differences between different rock types quartzite pelite and marble with a range of 18osmow from c 11 5 quartzite to 18 5 marble in each rock type vein quartz compositions closely match those of the matrix quartz these results demonstrate the importance of local segregation processes in the formation of veins and suggest that fluid convection cells were not developed during metamorphism on a scale larger than the individual sedimentary formations if at all both oxygen isotope data and the absence of metasomatism indicate that veins form primarily by segregation of quartz from the host lithologies with only a relatively minor component of through flow of externally derived fluid veins are clearly not the major pathways of metamorphic dewatering it is proposed that abundant veins in the predominantly pelitic ballynakill formation formed during peak metamorphic d3 folding because the formation was embrittled by high fluid pressures but was capped by impermeable marble hence the pelitic formation fractured repeatedly and the pore fluid drained through the fractures to form veins while irreversible loss through the rest of the succession was a much less important process in the central mountains of connemara rather pure unreactive quartzites are cut by widely spaced laterally extensive quartz veins that are axial planar to d3 folds these veins may mark pathways whereby metamorphic fluid made its way through the massive impermeable quartzite from lower parts of the nappe pile but here too oxygen isotope data indicate considerable segregation of locally derived quartz reflecting the importance of pumping of fluid between wail rocks and fractures relative to the component of through flow

seamarc ii mapping of transform faults in the cayman trough caribbean sea seamarc ii maps of the southern wall of the cayman trough between honduras and jamaica show zones of continuous well defined fault lineaments adjacent and parallel to the wall both to the east and west of the cayman spreading axis these lineaments mark the present active traces of transform faults which intersect the southern end of the spreading axis at a triple junction the swan islands transform fault to the west is dominated by two major lineaments that overlap with right stepping sense across a large push up ridge beneath the swan islands the fault zone to the east of the axis named the walton fault is more complex containing multiple fault strands and a large pull apart structure the walton fault links the spreading axis to jamaican and hispaniolan strike slip faults and it defines the southern boundary of a microplate composed of the eastern cayman trough and western hispaniola the presence of this microplate raises questions about the veracity of caribbean plate velocities based primarily on cayman trough opening rates

cyclic fluid flow through a regionally extensive fracture network within the kodiak accretionary prism two types of periodic textures observed in veins from the kodiak accretionary prism attest to cyclic fluid flow through a regionally extensive fracture network buried at 8 12 km depth 1 crack seal microstructures with bands of mica inclusions and 2 collapse microstructures with jagged bands of residue embedded within euhedral crystals of quartz the difference in texture reflects the closure of cracks crack seal microstructures record the complete chemical sealing of the crack after each fracture event whereas the collapse features record longer fluid filled periods followed by more rapid draining of fractures collapse features consist of pressure solution selvages trapped within veins and in the wall rock adjacent to euhedral growth terminations the high concentrations of immobile elements in these selvages indicate that these fractures closed by collapse and penetration of quartz crystals into wall rock analysis of chemical composition on either side of four large euhedral growth veins and whole rock analysis of slates across the kodiak formation reveal local depletion of silica adjacent to veins but no evidence for long distance silica transport within the system both crack seal and collapse textures are observed in a regionally extensive vein system that displays a regular geometry with thin closely spaced 0 5 3 cm near vertical crack seal veins that connect vertically and laterally with thicker euhedral growth veins arranged in widely spaced 500 mm southeast dipping en echelon sets the mesoscopic distribution and textural variability of the vein network suggests that the development of the vein system involved early nucleation and growth of vertical hydrofractures as the fracture density increased arrays of fractures locally provided zones of weakness and southeast dipping brittle ductile shear zones nucleated these en echelon cracks remained open and provided small reservoirs of fluid textures show that en echelon fractures remain open but periodically grow by upward and downward propagation crack tips are then sealed with locally derived silica and fluid drains back into en echelon fracture arrays this local fluid movement is punctuated by less frequent events where the system links up over a greater distance fractured reservoirs become interconnected and the fluid within reservoirs is drained upward or laterally periodic inflation and deflation of en echelon arrays may reflect periodic slip on crosscutting faults and rupture of the seals that separate reservoirs

empirical calibration of the oxygen isotope fractionation between quartz and fe mg chlorite abstract an empirical calibration has been made of the oxygen isotope fractionation between chlorite and quartz using samples from localities in metamorphic terrains where the formation temperature has been constrained by independent methods e g fluid inclusion homogenization temperatures mineral assemblages and petrology of the host rocks to the veins thermodynamic modeling and raman spectrometry textural equilibrium of chlorite and quartz was a pre requisite and the chemical composition of chlorite was determined the chlorite analyzed had a range in fe mg varying between 0 35 and 0 7 except for pure mg chlorite fe mg substitution within the range measured does not change the oxygen isotope fractionation between quartz and chlorite measurably the results support a linear correlation for chlorite quartz fractionation over the temperature range given by this study 240 550 c 1000 ln 0 01323 0 002 t c 10 97 0 92 the presently established geothermometer can be applied to quartz chlorite fractionations in low grade metamorphic rocks and may be extrapolated to diagenetic systems

deformation mass transfer and mineral reactions in an eclogite facies shear zone in a polymetamorphic metapelite monte rosa nappe western alps this study analyses the mineralogical and chemical transformations associated with an alpine shear zone in polymetamorphic metapelites from the monte rosa nappe in the upper val loranco n italy in the shear zone the pre alpine assemblage plagioclase biotite kyanite is replaced by the assemblage garnet phengite paragonite at eclogite facies conditions of about 650 c at 12 5 kbar outside the shear zone only minute progress of the same metamorphic reaction was attained during the alpine metamorphic overprint and the pre alpine mineral assemblage is largely preserved textures of incomplete reaction such as garnet rims at former grain contacts between pre existing plagioclase and biotite are preserved in the country rocks of the shear zone reaction textures and phase relations indicate that the alpine metamorphic overprint occurred under largely anhydrous conditions in low strain domains in contrast the mineralogical changes and phase equilibrium diagrams indicate water saturation within the alpine shear zones shear zone formation occurred at approximately constant volume but was associated with substantial gains in silica and losses in aluminium and potassium changes in mineral modes associated with chemical alteration and progressive deformation indicate that plagioclase biotite and kyanite were not only consumed in the course of the garnet and phengite producing reactions but were also dissolved congruently during shear zone formation a large fraction of the silica liberated by plagioclase biotite and kyanite dissolution was immediately re precipitated to form quartz but the dissolved aluminium and potassium bearing species appear to have been stable in solution and were removed via the pore fluid the reaction causes the localization of deformation by producing fine grained white mica which forms a mechanically weak aggregate

the chixoy polochic fault and its associated fractures in western guatemala in western guatemala post cretaceous volcanic and sedimentary rocks locally cover the trace of the chixoy polochic fault no single throughgoing fault cuts these units however a complicated series of en echelon lineations identified from aerial photographs and field mapping represent the accommodations of displacements along the underlying fault comparison of fracture trends in the chixoy polochic fault zone with models from experimental studies and field examples of strike slip faults suggest that left lateral displacement along the chixoy polochic fault in this area is probably no more than a few kilometres since the accumulation of the overlying beds of probable post mid tertiary age earlier lateral displacements of large magnitude are not precluded although none have been documented by field study

tectonic and metamorphic evolution of the temsamane units external rif northern morocco implications for the evolution of the rif and the betic rif arc located at an intermediate position in the external rif nappe pile the temsamane units northern morocco are characterized by an abnormally intense metamorphism and a penetrative ductile deformation we present new metamorphic data showing that in spite of their external position in the rif part of the temsamane units underwent medium pressure low temperature mp lt metamorphism at c 7 9 kbar and 330 430 c possibly during the oligocene structural data show that the exhumation of these units during middle to late miocene times was characterized by an intense approximately east west stretching and by top to the west shear senses we tentatively propose two possible origins for the mp lt temsamane units 1 an internal origin related to the subduction and the hp lt event recorded in the internal rif alboran domain or 2 an external origin implying a second subduction system within the external rif parallel to and almost contemporaneous with that of the alboran domain this tectonometamorphic evolution of the temsamane units is set within the context of the external rif evolution at a larger scale we show that the exhumation history of the temsamane units which strongly resembles that documented in the core of the internal betics is compatible with the westward slab retreat occurring during the middle to late miocene in the betic rif region

age and spreading history of the cayman trough as determined from depth heat flow and magnetic anomalies the opening of the cayman trough records aspects of relative motion between the north american and caribbean plates essential to accurate reconstruction of caribbean evolution assessment of opening rates based upon basement subsidence analysis as well as magnetic anomaly interpretation shows that the trough may have opened at one third to one half previously proposed rates depths to basement show that the crust has subsided away from the present axis in two stages a younger stage characterized by substantial and asymmetric greater to the east subsidence preceded by an older stage of slight increases of depth with distance depth to age relations calculated from subsidence curves which accommodate lateral as well as vertical cooling of the accreting slab indicate that the trough has opened at an overall rate of less than 15 mm yr since 25 30 ma and at 20 30 mm yr prior to 30 ma marine magnetic anomalies show lineations perpendicular to the trough including a distinct central anomaly but the pattern is neither well defined nor symmetrical about the present axis however partial anomaly sequences can be identified on both sides of the axis and the distribution of these sequences indicates that spreading may have involved frequent short ridge jumps mostly eastward the overall rate of opening as indicated by this interpretation of the magnetics is about 15 and 30 mm yr since and prior to 26 ma both subsidence and magnetics indicate that the trough opened by at least 45 50 ma

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

development of the late proterozoic to mid paleozoic intracratonic amadeus basin in central australia a key to understanding tectonic forces in plate interiors the intracratonic amadeus basin in central australia is a complex composite basin covering 17 000 km with at least nine distinct episodes of evolution between 900 and 300 ma these have been identified in neighboring central australian basins and are characterized by intervals of renewed subsidence followed by intervals of erosion and changes in basin shape several unconformities separating the tectonostratigraphic sequences represent periods of mild regional uplifts and correlate with major episodes of compressive tectonism at the evolving margin of the early australian plate in the absence of direct geological evidence for thermal events in the amadeus basin after the initial period of subsidence ended possibly at about 800 ma the correlations found between events in the amadeus basin those in the neighboring interconnected basins and tectonic events at the continental or plate margins emphasize that regional horizontal stress fields contribute significantly to primary basin forming mechanisms this suggests that stresses generated at continental or plate margins may propagate to the plate interior with the lithosphere acting as a stress guide throughout the history of the amadeus basin both periods when compressional stresses were dominant and when extensional tectonics controlled subsidence can be identified there were also transitional periods when neither compression nor extension dominated so that other processes masked the effects of weak horizontal stresses the most recent compressional thrust belt concentrated at the northern margin between 300 and 400 ma and has partly obscured the record of earlier basin shape and size

ediacaran fossil preservation taphonomy and diagenesis of a discoid biota from the amadeus basin central australia abstract aspects of the preservation of the enigmatic soft bodied ediacara biota in relatively coarse grained siliciclastic sediments remain unresolved the taphonomic model proposed by gehling in 1999 is here validated through examination of previously unpublished discoid fossils from the shallow marine deltaic arumbera sandstone in the amadeus basin within the flinders style preservation the model predicts stalked frondose organisms attached by a discoid holdfast to a microbial mat encrusted sea bed with sedimentation as periodic storm event beds during a terminal neoproterozoic taphonomic window a pyrite sole veneer provided critical fossil rigidity during burial the sequence of sedimentation events presented here advances a slightly modified gehling model no direct evidence of pyrite is detected instead a thin haematite clay coating covers fossil hyporelief surfaces nevertheless an original pyrite sole veneer is indirectly validated by tracing its diagenetic history through to today s haematite detailed petrography demonstrates reducing conditions during eodiagenesis that remained anoxic mildly alkaline through mesodiagenesis thus the pyrite sole veneer survived deepest burial meteoric water penetrated the deep regolith during prolonged telodiagenesis creating oxic mildly acidic pore water so pyrite oxidised and hydrolised to haematite the applicability of gehling s model is higher where marine shelf sediments subsided continuously in shallower or eroded areas that experienced intermittent meteoric eodiagenesis diagenetic history was different and ediacaran fossil preservation unlikely diagenetic history therefore has crucial implications for ediacaran fossil preservation

fracturing and fluid flow in a sub decollement sandstone or a leak in the basement crack seal texture within fracture cements in the triassic el alamar formation ne mexico shows that the fractures opened during precipitation of quartz cements later overlapping calcite cements further occluded pore space previous workers defined four systematic fracture sets a oldest to d youngest with relative timing constrained by crosscutting relationships quartz fluid inclusion homogenization temperatures are higher within set b 148 20 c than in set c 105 12 c these data and previous burial history modelling are consistent with set c forming during exhumation fluid inclusions in set c quartz have higher salinity than those in set b 22 9 v 14 2 wt nacl equivalent respectively and set c quartz cement is more enriched in 18 o 20 2 v 18 7 vsmow under most assumptions about the true temperature during fracture opening the burial duration the amount of cement precipitated and fluid flow patterns it appears that the fracture fluid became depleted in 18 o and enriched in 13 c this isotopic evolution combined with increasing salinity suggests that throughout fracture opening there was a gravity driven influx of fluid from upsection jurassic evaporites which form a regional decollement fracture opening amid downward fluid motion suggests that fracturing was driven by external stresses such as tectonic stretching or unloading rather than increases in fluid pressure

metamorphic veining and mass transfer in a chemically closed system a case study in alpine metabauxites western vanoise in western vanoise french alps karstic pockets of triassic to jurassic metabauxites embedded in carbonate rocks and containing several generations of metamorphic veins were studied during blueschist facies metamorphism a cumulative amount of 13 vol of water is inferred to have been produced locally by successive dehydration reactions and part of this fluid remained in the bauxitic lenses during most of the metamorphic cycle field and geochemical evidence show that these rocks have been isolated from large scale fluid flow closed system behaviour it is proposed that the internally derived fluid has promoted the opening of fluid filled open spaces as attested by the euhedral habits of vein minerals and served as medium for mass transfer from rock to vein indeed the vein infill is obviously the result of chemical interactions at the millimetre to centimetre scale between the rock minerals and the locally produced aqueous fluid two vein types can be distinguished based on mineralogical and textural features i some veins are filled with newly formed products of either prograde chloritoid or retrograde chlorite metamorphic reactions in this case fluid filled open spaces seem to offer energetically favourable nucleation growth sites ii the second vein type is infilled with cookeite or pyrophyllite that were present in the host rock prior to the vein formation in this closed chemical system the components for the vein infill minerals have been transferred from rock to vein through the fluid in a dissolution transport precipitation process possibly stress assisted these different vein generations all contain al rich mineral infills suggesting that al was a mobile element cm scale during metamorphism in these hp rocks fluid flow may have been restricted and if so mass transfer occurred by diffusion in an almost stagnant fluid metamorphic veins can be seen as witnesses of fluid and mass redistribution that partly accommodate the rock deformation lateral segregation

tectonic interpretation of the morphology of honduras the morphotectonics of honduras have been studied using existing geologic and topographic maps and imaging done from satellites and the following interpretations have been made during the late oligocene to early miocene a wide strike slip boundary existed in western honduras and eastern guatemala and produced east west trending basins containing the coarse clastic rocks of the subinal formation marine geophysical studies of the continental shelf of honduras show that during the rest of the miocene and the early pliocene the boundary was inactive but motion resumed in the late pliocene to pleistocene and produced horsts on the north coast of honduras in northwestern honduras the east northeast trending alignment of the jocotan copan and charnelecon river valleys may result from crustal flexuring rather than from faulting as previously proposed because the postulated fault the jocotan chamelecon fault cannot be reconciled with available geologic maps the northwest and northeast flowing rivers of central and eastern honduras define domains of strike slip faulting that have probably rotated 30 from an original set of conjugate fractures widespread deposition of conglomerates suggests that strike slip movements began in the mid cretaceous and reoccurred at intervals culminating in the development of pull apart basins in western honduras during the late pliocene at the present time no further strike slip movement is possible as the faults defining each domain lie parallel and perpendicular to the middle america trench new conjugate fractures however cut honduras along north south and east northeast trends and have also separated into domains in the domain of north south fractures easterly directed ductile extension of the lithosphere beneath western honduras and guatemala is opening up rifts but the domain of east northeast fractures is characterized by strike slip motion that has produced left lateral pull aparts in central honduras early workers identified no rift valley extending from the gulf of fonseca to the caribbean and there is no fresh evidence upon which to base the existence of one today the notion of the honduras depression which has persisted over the last half century stems from misinterpretation of the maps and writings of karl sapper

interaction between sea level changes and depositional tectonics implications for hydrocarbon prospectivity in the western coastal swamp depobelt niger delta basin nigeria this paper presents the potential depositional framework of sandstones and unconsolidated sand reservoirs seals and traps in the western part of the coastal swamp depobelt of the niger delta basin nigeria through integration of sequence stratigraphy and seismic imaging the integration of well logs and biostratigraphic foraminifera and palynological data with three dimensional seismic volume established the sequence stratigraphic framework which has improved our understanding of the impact of sea level changes in the distribution of sand facies four third order seismic depositional sequences sequences 1 4 of serravallian deposits were identified based on the vail stratigraphic concept depositional processes structures thickness and seismic signatures vary in each sequence sequence 1 dominated by mud and very fine grained sand facies represents the outer shelf shelf margin setting and has experienced shale tectonics ductile deformation sequences 2 and 3 consist of alternating sand shale facies and have experienced gravitational tectonics although sequence 2 represents middle shelf inner shelf setting sequence 3 represents inner shelf transitional setting sequence 4 consists of thick fluvial coastal plain sand facies and represents deltaic plain or continental sediments these deposits reflect postgravitational tectonics within the third order sequences are fourth order sequences that recorded the interaction between small scale transgressive regressive cycles and depositional tectonics the potential reservoirs within these sequences are stacked channel sandstones shoreface sandstones and heteroliths the juxtaposition of the potential reservoirs against transgressive shales provides potential seals and traps structural interpretation shows synthetic and antithetic faults in addition event maps at various ages document structural closures that indicate the existence of possible hydrocarbon leads and prospects

origin of salinity in produced waters from the palm valley gas field northern territory australia abstract the chemical composition and evolution of produced waters associated with gas production in the palm valley gas field northern territory has important implications for issues such as gas reserve calculations reservoir management and saline water disposal the occurrence of saline formation water in the palm valley field has been the subject of considerable debate there were no occurrences of mobile water early in the development of the field and only after gas production had reduced the reservoir pressure was saline formation water produced initially this was in small quantities but has increased dramatically with time particularly after the initiation of compression in november 1996 the produced waters range from highly saline up to 300 000 mg l tds with unusual enrichments in ca ba and sr to low salinity fluids that may represent condensate waters the sr isotopic compositions of the waters 87sr 86sr 0 7041 0 7172 are also variable but do not correlate closely with major and trace element abundances although the extreme salinity suggests possible involvement of evaporite deposits lower in the stratigraphic sequence the sr isotopic composition of the high salinity waters suggests a more complex evolutionary history the formation waters are chemically and isotopically heterogeneous and are not well mixed the high salinity brines have sr isotopic compositions and other geochemical characteristics more consistent with long term residence within the reservoir rocks than with present day derivation from a more distal pool of brines associated with evaporites if the high salinity brines entered the reservoir during the devonian uplift and were displaced by the reservoir gas into a stagnant pool which has remained near the reservoir for the last 300 400 ma then the size of the brine pool is limited at a minimum it might be equivalent to the volume displaced by the reservoired gas

mineral growth in rocks kinetic rheological models of replacement vein formation and syntectonic crystallization abstract we model three cases of coupling between mineral growth kinetics and mechanical response of the rock i dispersed spherical crystals growing by replacement in a hydrostatically stressed elastic rock ii growth of veins or vein networks accommodated by viscous relaxation of surrounding rock and iii syntectonic crystallization in a rock undergoing bulk pure shear such models for the microscopic environment of mineral growth together with additional assumptions or knowledge about rheological behavior and aggregate geometry provide refined estimates of the behavior of a macroscopic volume element which could be combined with geochemical reaction transport models crucial in the models are the various consequences pressure solution creep fracturing of the local stress that is necessarily generated by mineral growth in rocks other than in pores in the first model the dispersed spherical crystals of mineral a are assumed to grow within a spherical volume of rock consisting of mineral b the mineralized zone mz itself embedded in elastic rock the macroscopic stress in the mz and the far field stress in the surrounding rock are uniform and hydrostatic mineral growth of the a crystals is driven by supersaturation with respect to mineral a is accommodated by replacement of b grains and leads to an expansion of the mz described by an infinitesimal strain the radial growth rate of a spherical crystal of mineral a with replacement of mineral b is da dt k a r tln a k b v 0 b k b v 0 b k a v 0 a where ka and kb are kinetic constants r is the gas constant t the temperature in kelvin and v0a and v0b are specific volumes reference saturation states of mineral a a 1 and host mineral b b 1 are specified at the far field hydrostatic stress 0 the microscopic environment of each crystal of a is modeled by a representative volume element rve consisting of a sphere of mineral a embedded in a spherical shell of mineral b in each rve stress is neither uniform nor hydrostatic the model links the local microscopic stress with the macroscopic stress in the mz and surrounding rock the second model refers to veins that make room for themselves by growing not to veins that form by cementation of previously opened or opening cracks if the growth of mineral a as propagating veins or as an interconnected vein network takes place by deformation and expansion of the mz with the deformation itself driven by the stress arising from the supersaturation driven growth the surrounding rock undergoes deformation and is treated as viscous the rate of growth in vein width w is dw dt 2 k a rtln a sv 0 a 1 m where m 16 kav0ac2 3b3 is the medium viscosity c is the vein radius and each vein is treated as centered in a spherical rve of radius b s is an effective tensile stress required for vein propagation for a vein network in which veins surround equant polyhedra of rock of radius b we set c b and drop the term in s for simplicity veins may also widen by replacement the ratio of vein widening by expansion to that by replacement is dw dt expansion dw dt replacement 3 b 16 3 b 16 k a v 0 a k b v 0 b k b v 0 b k a v 0 a both mechanisms of accommodation contribute equally when 3b 16kbv0b at higher viscosity replacement is dominant the incipient growth of a cylindrical crystal with circular cross section in a medium deforming in pure shear simulates syntectonic crystallization both dissolution and deformation of the host accommodate growth in the model the crystal tends to grow faster in the direction of maximum rate of extension in this direction the host mineral may either dissolve to accommodate growth or precipitate to form a pressure shadow accommodation by host dissolution is greatest in the direction of maximum rate of shortening

fluid generation and evolution during exhumation of deeply subducted uhp continental crust petrogenesis of composite granite quartz veins in the sulu belt china

diachronous uplift and recycling of sedimentary basins during cenozoic tectonic transpression northeastern caribbean plate margin abstract four marine sedimentary sequences of late cretaceous to pleistocene age crop out in the 320 km long 1 to 30 km wide peralta rio ocoa belt of hispaniola haiti and dominican republic the four sedimentary sequences are dominated by marine turbiditic rocks ranging up to 11 5 km in apparent thickness and exhibit mainly northwest to southeast belt parallel paleocurrents the three younger sequences are generally in fault or unconformable contact with the underlying sequence from northwest to southeast the four belts become progressively wider younger less metamorphosed less folded and faulted and lower in topographic elevation we interpret the three younger sequences as a syn tectonic stratigraphic record of diachronous northwest to southeast transpressional closure of a coniacian danian back arc basin represented by the oldest exposed sedimentary sequence at the northwest end of the belt and inferred at depth beneath the three younger sequences using the stratigraphic record we infer the following three stages in the closure of the back arc basin and the overlying younger basins stage one latest cretaceous to early late eocene closure a minimum of 11 km of paleocene turbidites and limestone in the padre las casas area and a minimum of 11 5 km of early eocene to early late eocene pelagic limestone mudstone sandstone and siltstone in the peralta and sierra el numero areas was deposited in an elongate basin derived from the first stage of latest cretaceous early late eocene closure and erosion of the coniacian danian back arc basin to the northwest paleocene eocene turbidites in both areas contain large amounts of reworked campanian paleocene microfauna and exhibit northwest to southeast belt parallel paleocurrents syn deformational features in eocene sedimentary rocks in the peralta and sierra el numero area indicate that convergent deformation accompanied sedimentation witschard and dolan 1990 stage two middle eocene to early miocene closure up to 8 6 km of middle eocene to early miocene turbidites debris flows and olistostromes of the rio ocoa group of the dominican republic were deposited in an elongate basin derived from middle eocene to early miocene closure uplift and erosion of the peralta group to the west and northwest turbidites contain reworked eocene faunas and lithologies similar to those in the underlying peralta group and exhibit northwest to southeast belt parallel paleocurrents erosion of late cretaceous island arc and overlying paleogene carbonate rocks of the cordillera central provided additional belt perpendicular sources of coarser grained conglomerate and olistostromes following deposition of the rio ocoa group both the peralta and the rio ocoa group were deformed in a southwest verging fold and thrust belt during the early miocene stage three middle miocene to recent closure up to 1 5 km of middle miocene to pleistocene sandstone conglomerate and reefal limestone of the ingenio caei group of the dominican republic were deposited in an elongate basin above a pronounced angular unconformity developed on the older deformed rocks of the rio ocoa group sediments of the ingenio caei group were derived from middle miocene to recent closure uplift and erosion of the rio ocoa and peralta groups to the northwest onshore exposures of the ingenio caei group can be correlated with submerged strata seen on seismic reflection profiles across the offshore san pedro basin recent submarine clastic sedimentation in the san pedro basin is dominated by northwest to southeast influx of clastic turbidites derived from active erosion of onshore exposures of the rio ocoa and ingenio caei groups middle miocene to recent sediments of the san pedro basin are being actively shortened by northward to northeastward directed underthrusting along the muertos trench underthrusting at the muertos trench is suggested to represent ongoing closure of the back arc basin thought to be deeply buried beneath the san pedro basin tectonic transpression along the closing back arc basin marks one edge of an actively deforming microplate within the north america caribbean strike slip plate boundary zone

intraplate deformation in central australia the link between subsidence and fault reactivation abstract central australia has experienced two intraplate orogenic events involving significant north south shortening the late neoproterozoic to early cambrian petermann orogeny and the devonian to carboniferous alice springs orogeny in each event pre existing structures inherited from mesoproterozoic terrain amalgamation were reactivated and basement rocks exhumed from beneath thick sedimentary successions accumulated in the centralian superbasin the pattern of fault reactivation during these events shows a striking similarity to the pattern of subsidence in the overlying basin immediately prior to the petermann orogeny the centralian superbasin was thickest in the vicinity of the musgrave block the region in which deformation was subsequently localised at the same time crustal scale faults elsewhere in central australia that were covered by a relatively thin sheet of sediment remained inactive despite being favourably oriented to accommodate the north south shortening between the petermann and alice springs orogenies subsidence patterns shifted such that fault systems in the arunta block and also those in the southern musgrave block were buried by significant thicknesses of sediment whereas the major structures that were exhumed during the petermann orogeny were not significantly buried during the alice springs orogeny reactivation once again occurred along the most deeply buried faults even in the instances where those faults had remained inactive during the earlier petermann orogeny importantly the major petermann aged structures that were not buried during renewed subsidence remained inactive during the alice springs orogeny the record of reactivation implies that the presence of pre existing crustal scale faults alone was insufficient to localise deformation rather fault reactivation appears to have required a priming process that modulated the strength of the lithosphere on a regional scale the correspondence between the distribution of basement fault reactivation and subsidence patterns during both the petermann and alice springs orogenies implies a link between relatively thick sedimentation and long term lithospheric weakening we show that this link is compatible with the thermal effects of a thick sedimentary blanket in the context of central australia the mechanical impact of basin formation is likely to be enhanced by the presence of regionally elevated heat production in the proterozoic basement

p t deformation fe3 fe2 mapping at the thin section scale and comparison with xanes mapping application to a garnet bearing metapelite from the sambagawa metamorphic belt japan quantitative x ray maps of composition from a chlorite k white mica albite quartz and garnet bearing thin section from a sambagawa blueschist facies metapelite were combined with a multiequilibrium calculation method to calculate a p t fe3 fe2 deformation map at the millimetre scale the studied sample was chosen because elongated chlorite crystallization tails pressure shadows rimmed by phengite are present which is an appropriate assemblage for the quantification of the p t evolution chlorite temperature and fe3 content maps were calculated by successive iterations for each pixel analysis of fe3 until convergence of the four chlorite quartz h2o equilibria that can be written using the fe and mg amesite clinchlore daphnite and sudoite chlorite end members the calculated map of fe2 fe3 in chlorite is in good qualitative agreement with the in situ mapping of this ratio using xanes x ray absorption near edge structure techniques the temperature map indicates that high temperature chlorite zones with low fe3 contents alternate with lower temperature zones and higher fe3 contents in the crystallization tail late fractures perpendicular to the elongation axis of the tail are filled by very low temperature chlorite 250 c showing fe3 fetotal up to 0 4 groups of chlorite and mica pixels were then identified based on compositional and structural criteria and a p tdeformation map was calculated using representative analyses of these groups the calculated p tdeformation map suggests that in contrast to chlorite the composition of most mica did not change significantly during exhumation mica reequilibrated in late ew shear bands only ew shearing was already active at 0 1 gpa 500 c which corresponds to the peak temperature and probably pressure conditions at reduced redox conditions the intensity of deformation probably decreased with decrease in temperature to 350 400 c at this temperature a second main deformation event corresponding to a further ew stretching occurred and was still active below 250 c and more oxidizing conditions these results indicate that the scale at which p t data can be obtained is now close to the scale of observation of structural geologists a close link between deformation and mineral reaction is therefore possible at the microscopic scale which provides information about the relationship between deformation and mineral reactivity the modalities of deformation with time and the p t conditions at which it occurred

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

subduction of aseismic ridges beneath the caribbean plate implications for the tectonics and seismic potential of the northeastern caribbean normal seafloor entering the puerto rico and northern lesser antillean trenches in the northeastern caribbean is interrupted by a series of aseismic ridges on the north and south american plates these topographic features lie close to the expected trend of fracture zones created about 80 110 m y ago when this seafloor was formed at the mid atlantic ridge the northernmost of the ridges that interact with the lesser antillean subduction zone the barracuda ridge intersects the arc in a region of high seismic activity some of this seismicity including a large shock in 1974 occurs within the overthrust plate and may be related to the deformation of the caribbean plate as it overrides the ridge a large bathymetric high the main ridge is oriented obliquely to the puerto rico trench and intersects the subduction zone north of the virgin islands in another cluster of seismic activity along the inner wall of the trench data from a seismic network in the northeastern caribbean indicate that this intersection is also characterized by both interpolate and intraplate seismic activity magnetic anomalies bathymetric trends and the pattern of deformed sediments on the inner wall of the trench strongly suggest that the main and barracuda ridges are parts of a formerly continuous aseismic ridge a segment of which has recently been overridden by the caribbean plate reconstruction of mid miocene to recent plate motions also suggest that at least two aseismic ridges and possibly fragments of the bahama platform have interacted with the subduction zone in the northeastern caribbean the introduction of these narrow segments of anomalous seafloor into the subduction zone has segmented the arc into elements about 200 km long these ridges may act as tectonic barriers or asperities during the rupture processes involved in large earthquakes they also leave a geologic imprint on segments of the arc with which they have interacted a 50 km landward jump of the locus of island arc volcanism occurred in late miocene time along the northern half of the lesser antilles we postulate that the subduction of a segment of seafloor of anomolously thick crust being more buoyant than adjacent seafloor resulted in a marked shoaling in the dip of the descending slab and therefore a shift of the locus of volcanism in the region near western puerto rico and eastern hispanolia plio pleistocene interaction with a similar feature in this case a part of the bahama platform about 3 4 m y ago led to a jump in the locus of subduction as evidenced by a gap in the downgoing seismic zone that segment of the bahama platform interferred with the subduction process and was subsequently sutured onto the caribbean plate when the boundary jumped about 60 km to the northeast the maximum size of historic shallow earthquakes along the lesser antillean arc varies from about 7 0 7 5 in the center of the arc where the dip of the shallow part of the plate boundary is steep to 8 0 8 5 along the northern part of the arc where the dip is shallow the interaction of anomalous seafloor as along the northern lesser antilles can lead to the development of a wider than normal zone of interplate contact and hence to earthquakes that are larger than those associated with more typical seafloor entering subduction zones major seismic gaps and regions of high seismic potential currently exist along the northern lesser antilles and to the north of puerto rico both gaps are bounded by anomalous features on the downgoing plate the intersection of these features with the plate boundary created large asperities that may be good places to search for precursors to future large earthquakes a great shock in 1787 may have ruptured an existing seismic gap north of puerto rico between 65 and 67 w thus that gap can be expected to eventually rupture again in a great shock and not to accommodate plate motion by totally aseismic processes

primary diagenetic copper carbonate at the malbunka copper deposit amadeus basin northern territory australia abstract the malbunka copper deposit located about 220 km west of alice springs in the northern territory of australia may be a rare example of primary formation of copper carbonate mineralization this deposit consists of unusual azurite disks up to 25 cm diameter and lesser amounts of secondary azurite crystals and malachite carbon isotope values of the copper carbonate minerals are consistent with formation from groundwater dissolved inorganic carbon oxygen isotope thermometry formation temperature estimates are 5 16 c above ambient temperatures suggesting the copper carbonates formed at a depth between 0 3 and 1 6 km in the amadeus basin azurite fluid inclusion waters are rich in boron chlorine and other elements suggestive of dilute oil basin formation fluids in addition presence of euhedral tourmaline with strong chemical zonation suggest that this was a low temperature diagenetic setting the strong correlation of structures associated with hydraulic fracturing and rich copper carbonate mineralization suggest a strongly compartmentalized overpressure environment it is proposed that copper carbonates of the malbunka deposit formed when deep copper rich formation fluids were released upward by overpressure induced failure of basin sediments permitting mixing with carbonate rich fluids above this work bears directly upon exploration for a new type of primary copper deposit through understanding of the conditions of genesis

indian derived sediments deposited in australia during gondwana assembly abstract sedimentary exchanges across continents during the collisional assembly and lifespan of supercontinents provide a powerful way of testing the assembling process and configuration of supercontinents the ord basin in north western australia contains mid cambrian shallow marine and deltaic sedimentary successions that post date the ca 650 520 ma paterson petermann orogeny ppo but are synchronous with the ca 530 470 ma kurgiakh orogeny in north india we report new u pb ages and hf isotopes of detrital zircons from the mid cambrian ord basin that suggest indian derived sedimentary input to australian cambrian basins compilations of published ediacaran ordovician samples from the centralian superbasin indicate that during the ca 650 520 ma ppo the 1200 1100 ma musgrave province in central australia was exhumed and fed sediments to the syn ppo centralian superbasin soon after the ppo termination the musgrave province likely remained a topographic high and continued to supply detritus to the amadeus and officer basins that were fragmented from the centralian superbasin supplemented by the 630 ma granites of the paterson orogen conversely we conclude that the mid cambrian ord basin did not record similar sedimentary provenance from the musgrave province rather indian derived detritus travelled along the northeastern gondwanan continental margin via longshore currents and in turn incorporated paterson granitic detritus during the assembly of gondwana including the kurgiakh orogeny in north india a broad provenance comparison between potential eastern gondwanan source regions and relevant asian neighbours suggests that south china and south qiangtang are affiliated with northern india but lhasa appears to have had a mixed affiliation between india and australia

le chaˆtelet gold bearing arsenopyrite deposit massif central france mineralogy and geochemistry applied to prospecting abstract the abundant data available on the geochemistry and mineralogy of the herynian gold deposit of le cha telet massif central france formed the basis for an analysis of the complex signature of a gold deposit and its hydrothermal alteration haloes this study describes the deposit and its environment at various scales the sulfide paragenesis of the le cha telet deposit resulted from several stages one of which was an event responsible for the deposition of au bearing arsenopyrite quartz veins the au is contained within the arsenopyrite lattice hydrothermal activity associated with these veins produced two superimposed alteration haloes an outer halo of kaolinite tosudite siderite alteration that developed at the expense of the plagioclase minerals and an inner halo of illite siderite alteration that affected all the minerals the hydrothermal alteration was followed by silicification of the wall rocks and then deposition of the au bearing arsenopyrite the hydrothermal alteration related to the event also generated geochemical signatures such as li b lree as sb au and w anomalies mineralogical studies and geochemical analyses of the hydrothermal facies show that 1 the anomalous li is trapped mainly in the tosudite lattice and to a lesser degree in the illite lattice and 2 the b is probably located in tosudite and illite lattices arsenic sb and au are contained in the arsenopyrite lattice during weathering the lithochemical signatures determined by the analytical techniques used in this study lose some of their specific characteristics depending on the original concentrations at deposit scale 0 25 1 0 km 2 b and li in the soill profiles indicate zones of hydrothermal alteration whereas as and au indicate zones of mineralization at a regional scale the association of au li and b in stream sediments is characteristics of this type of mineralization therefore in the hydrothermal setting described at le cha telet au as li and b appear at all scales to be dependable pathfinders for au bearing hydrothermal systems of possible economic interest

fluid evolution in fracturing black shales appalachian basin opening mode veins in cores drilled from the mudrocks overlying and underlying the major silurian salt decollement in the appalachian plateau tioga and lawrence counties pennsylvania have mineralogic and isotopic compositions generally matching those of their host mudrocks suggesting opening and filling amid little cross stratal fluid motion calcite and most trace minerals probably entered the veins via dissolution reprecipitation from nearby host rock consistent with this interpretation are the observations that 1 trace minerals within the veins including quartz pyrite and dolomite are invariably also present within the layers hosting the veins with vein cement minerals generally reflecting the abundance and solubility of minerals in the host rock and 2 carbon and oxygen isotopic compositions of vein filling calcite are similar to those of calcite within the host rock with vein filling 18o slightly depleted and 13c slightly enriched modeling the fluid isotopic evolution assuming vein opening and filling amid immobile connate formation water accounts for these minor but systematic differences which are attributable to increasing temperature and hydrocarbon maturation an exception to the above trend is barite which despite its low solubility is systematically enriched in veins with respect to the host rock it is unclear whether barite precipitation resulted from the influx of external fluids perhaps deriving from silurian salt or from barium mobilized at depth from local clays or organic material

dehydration of dioctahedral aluminous phyllosilicates thermodynamic modelling and implications for thermobarometric estimates we propose a solid solution model for dioctahedral aluminous phyllosilicates accounting for the main compositional variations including hydration observed in natural smectites interlayered illite smectite illites and phengites from diagenetic to high grade metamorphic conditions the suggested formalism involves dehydrated micas and hydrated pyrophyllite like thermodynamic end members with these end members the equilibrium conditions of quartz water k bearing mica like phyllosilicates of fixed 2 1 composition are represented by a line in p t space along which the interlayer water content varies the relevant thermodynamic properties required for the calculation of equilibrium conditions were derived using a set of 250 natural data of known maximal temperature and pressure conditions which covers a range between 25 c and few mpa to 800 c and 5 gpa the temperatures calculated at fixed pressure with our model are in fair agreement with those reported in the literature for the 250 natural data at low temperature and pressure the amount of interlayer water in k deficient phengite and illite is predicted to reach 100 of the apparent vacancies which is consistent with previous values reported in the literature although the amount of interlayer water is predicted to decrease with pressure and temperature it is calculated to be significant in k deficient phengite from lt hp pelites metamorphosed at about 350 c 10 kbar the presence of molecular water in the interlayer site of such phengites has been confirmed by ftir mapping its implications for p t estimates are discussed

kinematic thermal and petrological model of the central alps lepontine metamorphism in the upper crust and eclogitisation of the lower crust abstract seismic and seismological studies as well as gravimetric models indicate that a slab of european lithospheric mantle and lower crust is currently underthrust below the apulian crust we assume a simple kinematic model in which the lower and upper subducted european crusts are decoupled along a decollement the lower crust goes into subduction without deformation the upper crust deforms by pure shear with a horizontal compressional axis the total erosional flux is adjusted to balance upper crust input so that the belt keeps the same geometry different distributions of erosion being used the computed temperature field is steady state if the kinematic model applies during a minimum time of 40 myr for a convergence rate of 8 mm yr equilibrium mineral assemblages and densities are determined from the computed p t conditions for a granodioritic chemical composition of the upper crust and an andesitic composition of the lower crust assuming local isostasy the density model fits the average topographic profile across the central alps the p t t paths obtained for the part of the upper crust initially at depths 10 to 16 km are compatible with the medium pressure oligocene metamorphism in the lepontine dome the peak calculated temperature for the deepest non subducted crustal rocks is 600 c for a pressure of 0 8 gpa near the lower limit of high pressure amphibolites we thus propose that the lepontine metamorphism corresponds to the steady state thermal regime however either faster erosion rates in the internal part of the belt or tectonic denudation are required for exhumation of the deeper portion of the belt the computed temperature field implies eclogitisation of the lower crust at a depth of 55 to 60 km we conclude that the moho limiting the deepest part of the root may correspond to the eclogitisation phase change lower crust eclogites have a density comparable to or higher than that of the mantle depending on their chemical composition 3 37 for andesitic eclogites 3 56 for gabbroic eclogites thus andesitic eclogites may stay in gravitational equilibrium in the mantle below the root whereas gabbroic eclogites are gravitationally unstable and should sink

sequence stratigraphy and the interpretation of neoproterozoic earth history abstract the application of sequence stratigraphy to neoproterozoic successions is important for improving the resolution of time correlation within individual sedimentary basins and potentially at a global scale the methodology is illustrated in this paper by reference to two contrasting examples from the flinders ranges adelaide geosyncline of south australia where the younger part of the neoproterozoic to earliest cambrian succession 770 ma to 540 ma has been divided into thirteen unconformity bounded depositional sequences one of the most prominent sequence boundaries at or near the base of the wonoka formation is characterized by a series of buried canyons as much as 1 km deep high resolution sequence stratigraphic studies at umberatana syncline continue to support the view that the canyons were cut subaerially and filled by fluvial and shallow water sediments in contrast to the wonoka canyons sequence boundaries interpreted at the base of the nuccaleena formation seacliff sandstone and near the top of the abc range quartzite are relatively subtle with only limited evidence for erosion and valley incision four sequence boundaries at the level of the sturtian and marinoan varanger glacial deposits and in the vicinity of the precambrian cambrian boundary are thought to correlate with surfaces in the amadeus basin of central australia other prominent sequence boundaries including the wonoka canyons and surfaces within the upper part of the wonoka formation and at the base of the ediacara member of the rawnsley quartzite correspond with a relatively condensed section in the amadeus basin and their lateral persistence beyond the adelaide geosyncline is therefore difficult to evaluate given the lack of precision in biostratigraphy and isotope geochemistry in neoproterozoic rocks and in a marked departure from phanerozoic practice we recommend placement of a terminal proterozoic gssp at a sequence boundary a prime candidate in australia is the sequence boundary at the base of the nuccaleena formation seacliff sandstone immediately above the marinoan glacial rocks in the adelaide geosyncline and its likely correlative at or near the base of the gaylad sandstone in the amadeus basin

fluid systems above basement shear zones during inversion of pre orogenic sedimentary basins external crystalline massifs western alps abstract in the inner part of the external alps inherited liassic basins were buried and inverted during the oligo miocene collisional phase of the alpine orogeny in northern oisans during crustal shortening the basement was locally sheared while the cover was disharmonically folded above the main basement shear zones that did not propagate into the cover in this contribution we analyze the witnesses of paleo fluid circulations associated with these crustal deformations focusing particularly on bourg d oisans and mizoen basins external western alps on the basis of structural and microstructural observations coupled to geochemical analyses cathodoluminescence o and c stable isotopes trace elements of vein versus host rock minerals we show that in the cover fluids mainly circulated over short distances closed system however trace element data also show that percolation of small amounts of basement derived fluids occurred over several tens of meters in cover rocks right above basement shear zones indeed the three successive vein sets recognized in the field display enrichments in basement derived ni co and cr which indicate that fluid transfer from the basement was efficient since the beginning of basin inversion therefore confirming the synchronous deformation of cover and basement fluid temperatures and pressures are estimated microthermometry coupled to 18o of vein minerals to about 250 400 c and 2 5 kbar for veins that most likely formed at or close to metamorphic peak conditions these results coupled to literature data are finally integrated into a model of fluid circulation evolution through progressive deformation of the whole external western alps

isotope chemostratigraphy of marbles in northeastern mozambique apparent depositional ages and tectonostratigraphic implications marbles are minor but characteristic components of metasedimentary units within nappes in the pan african mozambique belt in ne mozambique metasedimentary units remain largely undated and carbon and strontium isotope stratigraphy of marbles has been used for indirect dating of the depositional history in this part of the mozambique belt sixty nine samples from nine occurrences of dolomite calcite and magnesite marbles in the montepuez xixano lalamo ocua and nampula metamorphic complexes were analysed for major and trace elements and a subset of 39 samples for c o and sr isotopes the least altered 13c values range from 3 5 to 7 1 v pdb and 87sr 86sr ratios from 0 70504 to 0 70671 these values are considered as the best proxy to seawater composition at the time of deposition the apparent deposition ages derived from available seawater evolution curves range from c 1250 to c 660 ma an age of 1250 910 ma is obtained from a tripartite marble unit in the montepuez complex which is exposed in the montepuez quarries five other age groups are represented by marble units with apparent depositional ages of 800 750 ma xixano north 800 660 ma montepuez west c 750 ma nampula c 740 ma xixano south and lalamo and 740 670 ma montepuez east the data suggest that i pan african nappes in ne mozambique include neoproterozoic and probable mesoproterozoic sediments ii neoproterozoic rocks of the xixano and nampula complexes might have different ancestry and were tectonically juxtaposed during the neoproterozoic pan african orogeny

calibration and testing of an empirical chloritoid chlorite mg fe exchange thermometer and thermodynamic data for daphnite a compilation of literature data on the fe mg composition of coexisting chlorite and chloritoid from metapelites metamorphosed at various p t conditions shows that the logarithm of the fe mg partitioning lnkd varies linearly with the inverse of temperature from about 2 4 at 300 c to about 1 3 at 600 c in contrast no trend was observed with pressure and the molar volumes of mg and fe chlorite end members suggest that the pressure dependence of lnkd is not significant therefore the chloritoid chlorite mg fe exchange reaction is a potential thermometer and has been empirically calibrated using the analyses of 112 chloritoid chlorite pairs from 28 different localities temperatures estimated using the chl cld thermometer were checked against independent estimates for 20 samples not involved in the calibration beni mzala window morocco and the results are in fair agreement with independent temperature estimates however the analytical uncertainties and errors are too large to obtain reliable temperature estimates for extremely mg rich or fe rich compositions the chl cld thermometer is unreliable at xmg cld 0 8 at 700 c and xmg cld 0 9 at 300 c using the results of the empirical calibration we calculated new thermodynamic data for daphnite implementing these data it becomes possible to estimate t and p conditions of metamorphism for the invariant chlorite chloritoid quartz aluminosilicate assemblage that is widespread in low grade metapelites these estimates appear to be relevant only in the stability field of kyanite whereas the uncertainties on the calculated pressure conditions are very large in the stability field of kaolinite and pyrophyllite

li rich zincostaurolite and its decompression related breakdown products in a diaspore bearing metabauxite from east samos greece an emp and sims study li h rich zincostaurolite locally formed at a metabauxite marble contact along the eastern coast of samos during early alpine high p low t metamorphism the staurolite which probably grew from gahnite cookeite kaolinite pyrophyllite diaspore and minor fe oxide occurs as prismatic crystals up to several mm long in a calcite matrix it has an unusual composition with 7 5 12 9 wt zno 1 0 4 9 wt feo 0 66 0 82 wt li 2 o up to 2 6 wt nio and 0 59 wt coo and 0 32 wt mgo calculated structural formulae point to high h 2 o contents 4 14 h atoms per 48 oxygen atoms during late alpine uplift of the samos rocks the staurolite was at greenschist facies conditions variably replaced by bluish cobaltoan gahnite x zn 0 82 white na ca li mica ni rich chlorite x ni 0 42 0 59 zincohogbomite x zn 0 68 0 80 diaspore and fe hydr oxide detailed emp work and determination of li in staurolite and mica by sims indicates systematic zn fe mg ni co li partitioning between staurolite and its decomposition products implying local scale chemical equilibrium balanced reaction equations based on mineral chemical data indicate that the breakdown of staurolite occurred largely isochemically with only introduction of water na and ca into the reacting system the breakdown of high p staurolite during uplift and decompression is thought to be related to the sensitivity of its complex crystal chemistry to changes in physico chemical conditions

exceptional preservation of fossil plant spores in high pressure metamorphic rocks it is commonly believed that high grade metamorphism erases all traces of fossils and more generally of biogenicity in rocks to address this issue we report in this study x ray spectromicroscopy raman microspectroscopy and electron microscopy observations at the nm scale of fossil lycophytes megaspores found in triassic metasedimentary rocks from the french alps these rocks have undergone high pressure metamorphic conditions not vert similar 14 kbar not vert similar 360 c during the alpine orogeny corresponding to a burial of not vert similar 35 km during subduction despite such metamorphism the fossil megaspores are remarkably well preserved texturally an unusual mineralogical zonation consisting of magnetite ankerite and calcite is superposed on the structures of the spore walls in parallel chemical heterogeneities of the organic matter om composing the spore walls were detected we discuss different interpretations for our observations in particular a scenario in which these features are remnants of the original structural heterogeneities in the spores and provide insights on the fossilization processes under high grade metamorphic conditions whatever the processes leading to such heterogeneities our observations oppose the usual assumption that high grade metamorphism erases all indications of biogenicity in rocks and open new avenues for investigation at the nm scale of the evolution of organic matter during diagenesis and metamorphism

nd isotopic and geochemical constraints on provenance of sedimentary rocks in the eastern officer basin australia implications for the duration of the intracratonic petermann orogeny sm nd isotopic and geochemical data from neoproterozoic to cambrian sedimentary rocks in the intracratonic eastern officer basin in central australia highlight the evolving provenance roles of the basement complexes that underlie and bound the basin initial e nd values of around 12 for the basal units indicate that both were largely derived from the late archaean to mesoproterozoic gawler craton which bounds the basin to the south at c 720 ma an influx of juvenile glacially derived sediment indicates partial uplift of the mesoproterozoic musgrave block along the basin9s northern margin in a regime interpreted to be broadly extensional at around 600 ma synchronous with the development of a foreland architecture there was a large influx of musgrave block derived sediments this is interpreted to mark the onset of the intracratonic petermann orogeny which was a long lived event or series of events spanning more than 70 ma subsequent to c 600 ma the nd isotopic composition of sequences within the officer basin indicates an increasing contribution from the gawler craton despite up to 45 km of denudation of part of the musgrave block this suggests that the majority of sediment derived from the petermann orogen bypassed the eastern officer basin for much of the history of the petermann orogeny

the neotectonic setting of puerto rico the island of puerto rico in the northeast caribbean lies within a broad deformation zone between the caribbean and north american plates the simplest model for the tectonic setting of puerto rico has major strike slip movement on nearly east west lines in the vicinity of the puerto rico trench coupled to a small counterclockwise rotation of a puerto rico block within the broader plate boundary zone this simple model is attractive because it predicts the tectonic regime south of puerto rico and provides an explanation for a possible component of extension across the puerto rico trench west of 65 5 w gloria long range sidescan sonar data and seismic reflection profiles have been used to test this model by mapping the major tectonic features across the plate boundary north and south of puerto rico to the north the new data help to resolve between conflicting models of underthrusting or strike slip motion at the puerto rico trench no direct evidence of compression is seen although evidence for normal and strike slip movement is abundant this combined with regional considerations leads us to conclude that the main east west trending part of the puerto rico trench between 65 5 w and 68 w lies within a strike slip regime although oblique convergence occurs both to the east and west where the plate boundary trends east southeast to the south of puerto rico underthrusting of the caribbean plate beneath the island decreases from west to east and it is ultimately replaced by extension in the virgin islands basin east of 65 w

crystal growth during a single stage opening event and its implications for syntectonic veins abstract the crystallization of a fluid filled vein or cavity and interpretation of the microstructure in a deformed rock is an important problem for geologists dynamic crystallization experiments using ice and an aqueous solution have been used as an analogue system for single stage opening events and have shown that the presence or absence of a deviatoric stress is an important factor in the development of texture as a result of crystallization from a fluid the textures can vary from 1 coarse crystal aggregates involving static growth where there is no significant macroscopic deviatoric stress at the time of crystallization the free face growth model to 2 polycrystalline aggregates involving static growth which develop from a mixture of skeletal crystals and fluid in a stressed environment in the latter situation the crystals and fluid coexist until the macroscopic stress can be transmitted between impinging crystals and at this stage fast dynamic recrystallization and grain boundary migration occurs this recrystallization has been termed the contact growth model and the resulting metamorphic texture is compared with examples in natural quartz veins during a single stage vein opening uncomplicated by intragranular deformation either free face and or contact growth textures can develop from the fluid filled void depending upon the kinematic environment