regional retrograde metamorphism of a high grade terrain the willyama complex broken hill australia abstract retrograde metamorphism has been a major influence in the development of the presently observed lithologies of the willyama complex broken hill two broad types of retrogression are distinguished pseudomorphous and kinematic retrogression the former type of retrogression involves replacement of prograde phases without complete loss of the high grade fabric hence the prograde assemblage can frequently be inferred kinematic retrogression involves the development of a new schistose fabric and like pseudomorphous retrogression is commonly related to f3 deformation retrogression was initiated during the waning stage of prograde metamorphism and was accompanied by an influx of aqueous fluid at similar pressure but lower temperature conditions than prograde metamorphism the source of the water is believed to be the crystallization of cooling partial melts the regional nature of this willyama style of retrogression is attributed to the metasediment rich nature of the sequence the high geothermal gradient and the considerable amount of in situ partial melting

the modern carbonate ramp slope of central west florida abstract a holistic investigation of the modern carbonate ramp slope of central west florida has been completed via analysis of approximately 1 500 km of air gun seismic reflection profiles tied to two drillcores and bottom samples recovered at 92 stations our primary goal has been to characterize modern depositional processes and products to compare and contrast results with modern rimmed platform slopes and to develop a model for application to the rock record the modern depositional environment of the west florida ramp slope was initiated 12 15 ma when depositional style changed from seaward prograding clinoforms to a pelagic slope front fill system bound along the shelf margin by contour parallel flow of the loop current a seaward thickening wedge 0 250 m of late miocene to recent pelagic carbonate ooze has been deposited across antecedent mid miocene topography in general the sea floor dips 1 2 seaward from water depths of about 200 2 000 m and slopes are relatively smooth however contemporary submarine geomorphology is controlled in part by earlier slope failures the primary control of modern sedimentation along the west florida slope is loop current flow which acts as a dynamic oceanographic boundary between outer shelf and upper slope winnows bottom sediments catalyzes early submarine cementation and stimulates pelagic production of calcium carbonate the axis of loop current circulation has also migrated laterally in response to climatically controlled sea level fluctuations which has resulted in a relatively wide 25 km band of winnowed sands and hardgrounds along the shelf margin climatic oscillations have also resulted in cyclic deposition of pelagically derived aragonite pteropods on the ramp slope that is out of phase with bank derived aragonite cycles of rimmed platform slope there are three major facies belts that parallel modern isobaths 1 a hardground facies of intraclastic grainstones and rhodolith rudstones as well as algal ridges along the outer shelf at water depths of approximately 200 400 m 2 a winnowed sand facies of planktonic forams intraclasts glauconite and reworked phosphorite grains as well as deep water coral mounds along the shelf margin at depths of approximately 400 600 m and 3 a bioturbated pelagic ooze facies along the ramp slope at depths of approximately 600 2 000 m these facies patterns geometries and grain types along with a general paucity of sediment gravity flow deposits readily distinguish this carbonate ramp slope from better known rimmed platform slopes of the bahamas

the p t deformation path for a mid proterozoic low pressure terrane the reynolds range central australia the proterozoic low pressure high temperature lpht terrane of the reynolds range occurs in a 130 km long nw trending belt in the central part of the arunta block central australia the reynolds range has been affected by two mid proterozoic tectonic cycles di and dii associated with two metamorphic events mi and mii di mi effects are restricted to the older of two sedimentary successions the lander rock beds which are separated from the younger reynolds range group by an angular unconformity the dominant structural metamorphic features formed during dii mii affected both sedimentary successions and the various granites that intruded them and reworked most di mi effects the dii deformation history can be subdivided into one prograde two peak and one retrograde stage average p t calculations in the southeastern half of the range indicate a peak metamorphic pressure of 4 1 0 3 kbar because the calculated values are derived from the same stratigraphic level corresponding to the base of the reynolds range group which is exposed throughout the area it is likely that pressures were similar in the entire range in fact however the peak metamorphic temperature shows a dramatic increase from greenschist facies c 400 c in the northwest to granulite facies 740 60 c in the southeast indicating that mii was associated with anomalously high heat flows the p t path is anticlockwise with isobaric cooling from the metamorphic peak indicated by corona textures however the evidence of a prograde increase in pressure is indirect and based on the compressional nature of the structures peak metamorphic mineral assemblages and retrograde mineral assemblages in amphibolite facies shear zones show the same metamorphic zonation suggesting they formed in response to the same thermal event if this is true the implication is that a thermal perturbation external to the crust was maintained for a considerable period of time 110 ma based on zircon dating as it is not clear whether proterozoic asthenosphere active thermal perturbations operated for this long the alternative interpretation must be considered namely that the peak metamorphic events are separate from the shear zone event associated with reheating of the area

thermal stresses and the bimodal distribution of elastic thickness estimates of the oceanic lithosphere observations of plate flexure beneath seamounts and at deep sea trenches suggest that the mechanical strength of the oceanic lithosphere increases with age the mechanical thickness h seems to follow the depth to a particular isotherm in the cooling plate model a detailed review of available estimates of elastic thickness te shows that the strength deduced from studies at seamounts is significantly different from the strength inferred at trenches and fracture zones whereas te beneath seamounts approximately follows the 400 c isotherm h at trenches follows the depth to the 700 800 c isotherms the cause of this bimodal distribution is not fully understood thermal bending stresses due to cooling of the lithosphere have been suggested to play an important role in the flexural deformation at transform faults and fracture zones and their distribution with depth is consistent with intraplate seismicity this paper evaluates the effects of thermal stresses on deformation beneath seamounts and at trenches and how their interaction with flexural stresses may modify plate strength the results indicate that thermal bending stresses will bias the moment curvature relationship such that a plate of a certain strength will appear relatively weaker beneath a seamount positive curvature but relatively stronger at a trench negative curvature this systematic bias appears to be in qualitative agreement with published te estimates for old lithosphere the theory predicts a diminishing amount of bias the general lack of sufficient te determinations from old ocean basins and the uncertainties associated with modeling of trenches prohibit a detailed analysis of this part of the te versus age relationship i conclude that the scatter in the elastic thickness estimates appears to be caused by a combination of lithospheric reheating during seamount emplacement bias introduced by thermal stresses and variable amount of yielding of the plate beneath seamounts in proportion to seamount size and lithospheric age at the time of loading

seamount loading and stress in the ocean lithosphere 2 viscoelastic and elastic viscoelastic models analytical solutions are presented for the deformation and stress state of a horizontally stratified earth subject to normal loads of size and wavelength that are characteristic of seamounts the models investigated include layered elastic plates homogeneous viscoelastic plates and elastic over viscoelastic models in all cases the composite models overlie an inviscid half space the principal advantage of the laminated elastic models over homogeneous elastic plates is that they can result in a substantial reduction in the maximum stress differences in those parts of the plate least capable of supporting large stress differences authors

early holocene sea level record from submerged fossil reefs on the southeast florida margin massive fossil outlier reefs are preserved seaward of the modern shelf and reef tract along the southeast florida margin thermal ionization mass spectrometric tims u th dating of 16 pristine acropora palmata and head corals cored from two transects document early holocene reef growth from 8 9 to 5 0 ka from approximately 13 5 to 7 m msl mean sea level these samples fill a gap in the florida keys sea level database and clarify the timing of a significant decrease in the rate of sea level rise a portion of this interval represented by a gap in the caribbean record of a palmata reefs has been interpreted as reef drowning during an inferred catastrophic sea level rise event of 45 mm yr or a 6 5 m rise between 7 6 and 7 2 ka attributed to west antarctic ice sheet instability and changes in marine ice extent between 8 and 7 ka continuous in situ shallow water reef growth in florida during this interval precludes the occurrence of exceedingly rapid rates of sea level rise and is consistent with the north atlantic record of deglaciation from 9 to 7 ka gaps in the early holocene sea level records for florida and the caribbean are thus more likely to be artifacts of limited sampling and or core coverage and not necessarily a result of drowning

uplift by thermal expansion of the lithosphere summary green s function solutions are derived for the surface heat flow and the uplift caused by conductive heating when axially symmetric variations in heat flow occur at the base of the lithosphere the solutions determined for different space time dependence of the heat flux into the lithosphere illustrate that 1 when the lateral variations in heat flux occur over short distances in comparison to the lithospheric thickness the amplitude of the uplift and of the anomalous surface heat flow are attenuated in comparison to the changes that would follow uniform variations this effect is slightly more important for the surface heat flow which spreads out over larger distances 2 there is a time lag between the uplift which starts instantly and the change in the surface heat flow it is suggested that the model could be applied to the study of plateaux uplift of moderate amplitude such as the black hills or the adirondacks and possibly to the heating event and uplift that may have preceded the subsidence of intracontinental basins

west florida shelf response to upwelling favorable wind forcing kinematics the barotropic responses of the west florida continental shelf to idealized upwelling favorable alongshore and offshore wind stresses are studied using the three dimensional time dependent primitive equation princeton ocean model pom when forced with uniform winds the shelf circulation evolves quickly to a quasi steady state a southeastward alongshore wind lowers sea level along the coast and drives a southeastward coastal jet with a relatively weak northwestward return flow farther offshore a southwestward offshore wind lowers sea level along the west florida coast and raises sea level along the panhandle coast two independent circulation gyres are set up in association with these regionally different sea level distributions an anticyclonic gyre off the west florida coast consisting of a southeastward coastal jet and a broad return flow over the middle shelf and a cyclonic gyre off the panhandle coast consisting of a strong northwestward coastal jet and an adjacent narrow southeastward undercurrent these gyres are separated by the big bend region in both cases alongshore and offshore wind forcing the circulations are fully three dimensional with opposing surface and bottom boundary layer flows accounting for the across shelf transports the shapes of the coastline and the isobaths are important determinants of the shelf wide responses several locally maximum upwelling regions are identified for geometrical reasons these include the panhandle coast south of apalachicola bay the west florida coast south of tampa bay along the florida keys and at the shelf break where topographic rossby waves are evident

rate and mechanism in prograde metamorphism for a given rate of heat input into a prograde metamorphic sequence the extent of overstep of reaction temperature disequilibrium depends on the slowest of three sequential steps a surface detachment of reactant minerals b transport of material to the site of mineral growth and c nucleation and growth of the product mineral we have developed analytical expressions which enable determination of the rates of mineral dissolution and growth and of advective and diffusive mass transport during metamorphism the dissolution and growth steps are linear functions of the driving force g of the overall reaction while diffusion may take place either through a grain boundary fluid film or through the disorganized grain boundary itself

deglacial sea level record from tahiti corals and the timing of global meltwater discharge the timing of the last deglaciation is important to our understanding of the dynamics of large ice sheets1 and their effects on the earth s surface2 3 moreover the disappearance of the glacial ice sheets was responsible for dramatic increases in freshwater fluxes to the oceans which probably disturbed the ocean s thermohaline circulation and hence global climate4 7 sea level increases bear witness to the melting of continental ice sheets but only two such records from barbados8 9 and new guinea10 11 corals have been accurately dated but these corals overlie active subduction zones where tectonic movements are large and often discontinuous especially in new guinea so the apparent sea level records may be contaminated by a complex tectonic component here we date fossil corals from tahiti which is far from plate boundaries and thus is likely to be tectonically relatively stable and remote from the locations of large former ice sheets the resulting record indicates a large sea level jump shortly before 13 800 calendar years bp which corresponds to meltwater pulse 1a in the barbados coral records8 9 the timing of this event is more accurately constrained in the tahiti record revealing that the meltwater pulse coincides with a short and intense climate cooling event12 15 that followed the initiation of the bolling allerod warm period12 16 but preceded the younger dryas cold event by about 1 000 years

fringing reef growth on a terrigenous mud foundation fantome island central great barrier reef australia the leeward fringing reef at fantome island central great barrier reef province is a carbonate body which has developed under the influence of terrigenous sedimentation the reef flat is up to 1000 m wide and is surfaced by mobile sand and gravel with almost all live corals restricted to the seaward rim the reef slope has coral columns and heads on the upper part but below 5 m water depth it is a muddy substrate with scattered mounds of branching corals three high recovery cores show the reef is up to 10 m thick and developed over a gently sloping terrace of weathered pleistocene alluvium three post glacial stratigraphic units are recognised i carbonate reef top unit of coral rudstone and framestone including sinuluriu spiculite 2 lower slope unit of coral floatstone in a terrigenous muddy matrix and 3 transgressive basal unit of skeletal arkosic sand the acid insoluble content of matrix and of individual corals increases downwards coral growth rates decrease downwards reflecting slower growth in muddier environments radiocarbon dating shows that the reef prograded seaward at almost stable sea level an average vertical accumulation rate of 6 7 mm yr is indicated two age reversals are interpreted as material transported by storms or by erosion in response to a late holocene sea level fall the carbonate reef top unit has developed adjacent to and is environmentally compatible with a muddy terrigenous lower slope unit terrigenous influx has not changed during the holocene and terrigenous content of sediments is controlled by deposition on the reef slope of fine sediment winnowed from the reef flat and concentration of coarse sediment in the transgressive basal sheet

rock buffered fluid rock interaction in deformed quartz rich turbidite sequences eastern australia quartz vein 18o 16o ratios across a 500 km transect through the lachlan fold belt of southeastern australia are remarkably uniform 1 1 5 permil at both local centimeter to meter and regional over 104 km2 scales they define isotopic zones that correlate with the tectonic divisions of the cambrian through devonian dominantly ordovician quartz rich turbidites determined by regional mapping from west to east these divisions are 1 stawell 18o 14 7 1 1 47 samples 2 bendigo ballarat 18o 17 5 1 3 204 samples 3 melbourne 18o 19 0 1 6 80 samples 4 tabberabbera 18o 16 3 1 9 12 samples and 5 omeo 18o 14 4 1 0 26 samples isotope profiles across zone boundaries and intrazone fault zones particularly within the bendigo ballarat zone show steps indicating abrupt changes across faults with little or no evidence of fluid mixing within the fault zones 18o values of veins are insensitive to relative age type of vein and immediate host rock lithologies the 18o values of coexisting vein and host rocks show nonequilibrium relationships which can be explained in terms of rock buffering under conditions of low fluid rock ratios water rock 1 limited d h determinations on fluid inclusions fall mainly in the range 70 and 100 with one value as low as 140 these low deuterium values when considered in the context of paleolatitude may have been inherited from deutrium depleted detrital minerals and do not necessarily require the direct penetration of meteoric fluids to midcrustal depths the work suggests that vein formation is possible in regions where the integrated water rock w r ratio is very low w r 1 as long as a pervasive fluid phase is present this fluid appears to achieve a steady state isotopic composition on scales of hundreds of meters and once a quasi steady state has been reached the isotopic compositions of the resultant quartz veins are rather insensitive to the diachronous and nonisothermal conditions under which vein growth occurs advective cycling of the fluid on this scale is by episodic dilatancy pumping in fracture networks associated with localized faulting

fluid evolution and transport during metamorphism evidence from the llano uplift texas a field petrologic and stable isotopic investigation of the marbles and calc silicates of the 1 15 b y valley spring gneiss documents the dilution of internally evolved co2 rich fluids by externally derived aqueous fluids introduced along channelways reaction textures within calcsilicates record the evolution through time of initially co2 rich fluids toward increasingly more aqueous compositions assemblage zonations within calc silicates require equilibration within local gradients of the mole fraction of co2 in the fluid and suggest that the infiltration of aqueous fluids was largely channelized along more permeable lithologies localized depletions in 13c and 18o corroborate petrologic evidence for channelized infiltration isotopic compositions reflect both devolatilization and the introduction of low 18o fluids estimated minimum oxygen equivalent fluid to rock ratios are near unity both mineralogical and stable isotopic systematics document the essential role of infiltration in driving decarbonation reactions during calc silicate formation the calc silicate assemblages which equilibrated with fluids of the lowest mole fraction of co2 record isotopic exchange equilibrium with fluids of 18o typical of those derived from normal granites as do the granitic aplites and pegmatites which transect most calcsilicate occurrences thus the infiltrating fluids are believed to be genetically linked to the intrusion of a suite of granitic plutons emplaced after the peak of regional metamorphism

thermoelastic bending of the lithosphere implications for basin subsidence summary thermoelastic stresses are capable of producing significant lithospheric deflection a systematic approach to modelling this effect is presented in which the lithosphere is presumed to behave as a thin elastic or viscoelastic plate on a fluid substrate and lateral variations in basal heat flow induce both vertical buoyant loads and thermoelastic bending moments the amount of uplift or subsidence produced by a given heat source depends on a number of factors including the strength duration and lateral extent of the thermal anomaly and the thickness density rigidity and viscosity of the plate the mechanical response of the plate is characterized by two distinct length scales one for shearing the other for bending and the deformation produced depends critically on the scaled width of the heat source the plate acts as a low pass spatial filter in response to thermal loads but has a narrow band pass filter response to applied moments the temporal response to an abrupt change in basal temperature or heat flow also depends on the ratio of the thermal diffusion time for the plate versus the viscoelastic relaxation time of the material a rapid localized increase in basal heat flow will initially produce a central depression and a peripheral uplift thus while it is often assumed that the main depositional phase in platform basins is associated with the cooling and contraction of the lithosphere following an earlier thermal doming event it is shown that if conditions are right a basin may also form in direct response to the initial heating event either way subsequent thermal equilibration and viscous stress relaxation will tend to modify the initial deflection profile

submerged reef terraces in the maldivian archipelago indian ocean abstract sea level changes have shaped the world s carbonate platform margins and continental shelves leaving typical geomorphic imprints such as drowned reef terraces in this paper we present the results of 112 scuba diving transects across seven different maldivian atolls and one multibeam survey around male island the capital of maldives we report on the occurrence of drowned reef terraces down to 120 m depth in total we identified six levels of submerged terraces that we consider as indicative of periods of time with stable or slowly rising sea level that can be attributed either to deceleration of the last deglacial sea level rise or to late quaternary sea level highstands we compare our dataset to the depth of reef terraces reported globally and we discuss the reasons why common global submerged terrace levels are difficult to identify in the field record

reef drowning during the last deglaciation evidence for catastrophic sea level rise and ice sheet collapse elevations and ages of drowned acropora palmata reefs from the caribbean atlantic region document three catastrophic metre scale sea level rise events during the last deglaciation these catastrophic rises were synchronous with 1 collapse of the laurentide and antarctic ice sheets 2 dramatic reorganization of ocean atmosphere circulation and 3 releases of huge volumes of subglacial and proglacial meltwater this correlation suggests that release of stored meltwater periodically destabilized ice sheets causing them to collapse and send huge fleets of icebergs into the atlantic massive inputs of ice not only produced catastrophic sea level rise drowning reefs and destabilizing other ice sheets but also rapidly reduced the elevation of the laurentide ice sheet flipping atmospheric circulation patterns and forcing warm equatorial waters into the frigid north atlantic such dramatic evidence of catastrophic climate and sea level change during deglaciation has potentially disastrous implications for the future especially as the stability of remaining ice sheets such as in west antarctica is in question

global ocean response to orbital forcing in the holocene 1 global upper ocean evolution in the holocene is studied in two coupled ocean atmosphere models under orbital forcing conditions at 3 6 8 and 11 ka the annual mean sea surface temperature sst changes in the early to mid holocene are found to be forced mainly by the annual mean insolation forcing with an overall symmetric response of colder equator 1 c in the arctic this sst change is consistent with a synthesis of mid holocene paleo sst records in contrast the temperature response in the thermocline is dominated by an antisymmetric pattern with a cooling warming in the northern southern hemisphere midlatitudes the thermocline response is determined predominantly by surface water subduction and ultimately the insolation forcing in local late winter

crystallization of melts pegmatite intrusion and the inverian retrogression of the scourian complex north west scotland partial melting of tonalitic gneisses in the 2 7 ga badcallian granulite facies metamorphic episode in the scourian complex of north west scotland produced a suite of granitic to trondhjemitic liquids on cooling and excavation of the complex these melts underwent fractional crystallization and the residual liquids eventually became water saturated comparison with experimental data suggests that water saturation would have occurred in these melts at around 620 700 c from the retrograde p t time path followed by the complex it is estimated that h2o dominated fluids were exsolved from these melts at c 2 5 ga it is proposed that these fluids were the cause of the 2 5 ga inverian retrogression of the scourian complex and that water saturated melts formed during the crystallization of the leucogneisses were intruded as a suite of pegmatites the timing of pegmatite intrusion is consistent with this proposition as are the temperature estimates timing distribution and nature of the inverian phase of metamorphism it is likely that the crystallization of melts is an important process in bringing about hydrous retrogressive metamorphic episodes in a number of other basement terrains such as west greenland and australia

comments on on the vulnerability of lithospheric plates to mid plate volcanism reply to comments by p r vogt by h n pollack i g gass r s thorpe and d s chapman

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

the retrograde p t t path for low pressure granulites from the reynolds range central australia petrological constraints and implications for low p high t metamorphism several aspects of the petrogenesis of low pressure granulite facies rocks from the reynolds range central australia are contentious including a the shape of the retrograde p t time path and whether it is an artefact of repeated thermal events at different p t conditions b the type of regional metamorphism and c the causes of metamorphism granulite facies rocks from the reynolds range group experienced three major periods of mineralogical equilibration metapelitic rocks underwent dehydration melting reactions to form migmatites under peak m2 p t conditions of c 5 0 5 3 kbar and c 750 800 c metapsammitic rocks that did not melt during m2 show spectacular garnet orthopyroxene intergrowths that developed at c 3 5 3 7 kbar and c 700 750 c after penetrative regional deformation but prior to amphibolite facies rehydration in discrete strike parallel zones rehydration occurred within the sillimanite stability field at p t conditions close to the granite solidus c 3 2 3 4 kbar and 650 700 c subsequently the terrane cooled into the andalusite stability field geochronological constraints suggest that a peak m2 conditions were reached at c 1594 ma b the garnet orthopyroxene intergrowths in unmelted metapsammites probably developed between c 1594 ma and c 1586 ma and c upper amphibolite facies rehydration occurred between c 1586 ma and 1568 ma the lack of petrological evidence for multiple dehydration and rehydration of the rocks suggests that the three episodes of mineralogical recrystallization can be linked to yield a single continuous retrograde p t t path of minor initial decompression c 1 5 kbar from the m2 peak followed by cooling c 100 c to the granite solidus over a period of c 26 ma late kyanite bearing shear zones that dissect the terrane are unrelated to this event and formed during the c 300 400 ma alice springs orogeny the shape of the p t t path and the duration of m2 metamorphism suggests that advective heating was not the major cause of high grade metamorphism and that some other longer lived heat source such as the burial of anomalously radiogenic pre tectonic granites is required

petrology of upper amphibolite facies marbles from the east humboldt range nevada usa evidence for high temperature retrograde hydrous volatile fluxes at mid crustal levels

development of small carbonate banks on the south florida platform margin response to sea level and climate change abstract geophysical and coring data from the dry tortugas tortugas bank and riley s hump on the southwest florida margin reveal the stratigraphic framework and growth history of these carbonate banks the holocene reefs of the dry tortugas and tortugas bank are approximately 14 and 10 m thick respectively and are situated upon pleistocene reefal edifices tortugas bank consists of the oldest holocene corals in the florida keys with earliest coral recruitment occurring at 9 6 cal ka growth curves for the tortugas bank reveal slow growth

the influence of deformation on the formation of axial planar leucosomes and the segregation of small melt bodies within the migmatitic napperby gneiss central australia in the napperby gneiss central australia a distinctive group of elongate leucosomes occurs along the axial planes of mesoscopic crenulations that have harmonic two dimensional kink band like geometries the leucosomes truncate a fabric defined by biotite quartz k feldspar plagioclase sillimanite and are chiefly composed of k feldspar and quartz with or without plagioclase they are characterized by accessory garnet or aggregates of intergrown ilmenite and magnetite produced during the melting reactions bi plag q sill g ksp melt and bi plag q ru ilm mt ksp melt the absence of garnet and intergrown ilmenitemagnetite aggregates in the gneiss surrounding the crenulation zones indicates that these melting reactions preferentially occurred in the deformation zones in spite of a homogeneous distribution of reactant phases in the gneiss this means that strain partitioning and partial melting are coupled processes in which the garnet and ilmenite magnetite producing melt reactions initially occurred in high strain zones where the melting reaction activation energy was first overstepped this was a consequence of an additional strain induced matrix free energy component on top of the temperature effect once reaction product nucleation had occurred the deformation zones became the sites for continued melting which was energetically favoured over nucleating garnet and ilmenite magnetite elsewhere the melt reactions producing garnet and ilmenite magnetite resulted in the formation of grain supported leucosomes with a higher melt component than the surrounding gneiss and the leucosomes were therefore relatively incompetent in a differential stress field diffusional processes will cause elongation of the leucosome in a plane normal to 1 parallel to the axial planes of folds if sufficient melting has occurred an interconnected leucosome network will result creating a potentially important buoyancy driven melt drainage system in the napperby gneiss interconnected leucosome bearing crenulation zones are mostly vertical which is an ideal orientation for buoyancy driven diffusive melt migration however melt segregations remained small due to a melt impermeable lithological boundary overlying the gneiss

christmas island lagoonal lakes models for the deposition of carbonate evaporite organic laminated sediments the atoll of christmas island now known as kiritimati in the kiribati republic central pacific lies at about 28n in the intertropical convergence zone much of the surface area of the atoll ca 360 km 2 is occupied by numerous lakes in which carbonate evaporite calcium sulfate halite and organic layers are deposited observations suggest that deposition of these different laminae is controlled by climatic and biologic factors it is thought that periodic climatic variations such as el nino southern oscillations enso events which bring heavy rainfall to the atoll result in the succession of the precipitation of carbonate minerals during periods after dilution of hypersaline waters by heavy rains followed by evaporitic minerals carbonate calcium sulfate halite when salinity increases through evaporation thick up to 5 cm microbial essentially cyanobacterial mats develop continuously on the lake bottom surfaces providing the sediment with an important total organic carbon 2 5 organic contribution in the form of an internal geometrically structured network in which the authigenic minerals precipitate the high bioproductivity of these microbial populations is reflected in low d 13 c values of sedimentary organic carbon 214 to 217 interpreted as being the result of high atmospheric co2 demand geochim cosmochim acta 56 1992 335 the well laminated organic layers present in the sediment profile result from the death and burial of microbial populations at the time of severe climatic events storms heavy rainfall these lagoonal lakes provide a model for the deposition of carbonate and organic matter in an evaporitic environment the high ratio of deposited carbonate vs sulfate 1 chloride when compared to low ratio in evaporitic salinas results from both a lack of limitation of calcium magnesium and carbonate ions in a carbonate reef environment and active processes of high mg calcite precipitation organomineralization q 2001 elsevier science b v all rights reserved

steep oxygen isotope gradients at marble metagranite contacts in the northwest adirondack mountains new york usa products of fluid hosted diffusion steep gradients in oxygen isotope ratios are recorded from two contacts between grenville marbles calcite 18o 21 24 and metamorphosed granitic intrusives whole rock 18o 11 12 in the northwest adirondacks oxygen and carbon isotope fractionations between coexisting minerals in the marbles and the metagranites are consistent with isotopic equilibration at peak metamorphic temperatures the 18o profiles are sigmoidal with inflection points at the contacts and are developed over 5 6 m quantitative fluid infiltration diffusion modeling suggests that the isotopic gradients are best explained as the result of fluid hosted diffusion for porosities of 10 3 to 10 6 the profiles could have formed in 103 107 years via an interconnected grain boundary fluid the granites were probably emplaced at shallow crustal levels prior to the regional grenville metamorphism and isotopic exchange is most likely to have occurred at that time contact metamorphism at these localities involved devolatilisation of the marbles and the time estimates may reflect the periods over which fluid producing reactions took place and an enhanced porosity was maintained in the marbles there is a progressive increase in the percentage of diopside and k feldspar at the expense of phlogopite and tremolite towards the metagranites these mineralogical changes probably occurred due to the temperature increase associated with granite intrusion and diffusion of h2o from the granite the two localities lie on either side of the regional metamorphic amphibolite to granulite facies transition the preservation of isotopic profiles formed during contact metamorphism suggests that there was little pervasive fluid flow at this crustal level during the regional metamorphism or at any time thereafter steep isotopic gradients are present at orthogneiss marble contacts elsewhere in the adirondacks suggesting that this conclusion may be generally applicable to this terrain

fluid migration and vein formation during deformation and greenschist facies metamorphism at ormiston gorge central australia during the alice springs orogeny deformation at ormiston gorge central australia occurred under lower to middle greenschist facies conditions dolomites of the bitter springs formation and quartzites metagreywackes and metapelites of the heavitree quartzite contain abundant early syn and post tectonic veins however though vein densities locally approach 15 the distribution of veins and the oxygen isotope geochemistry of wallrocks and veins suggest that fluid movement was on a local scale the heavitree quartzite contains quartz veins that even along the main thrust plane have similar 18o values 13 5 16 9 o to those of their wallrocks 13 6 16 9 o with 18o vein wallrock values of 0 6 to 0 4 o in contrast the bitter springs formation contains predominantly dolomite veins that have 18o values of 23 4 to 27 7 o these differences are observed even at the boundary between the heavitree and bitter springs rocks implying that significant fluid exchange between these rocks has not occurred or that fluid flow was channelled through areas outside those sampled for this study by contrast with the heavitree quartzite 18o values of wallrocks in individual samples of the bitter springs formation are significantly higher 23 3 29 1 o than those of the veins with 18o vein wallrock values up to 4 o average of 2 1 o these systematic differences in 18o values most likely result from oxygen isotope fractionation caused by fluid immiscibility or disequilibrium dissolution smaller differences in 13c values between some dolomite veins and wallrocks 13c vein wallrock up to 1 9 o average of 0 5 o are also explained by these processes this study indicates that large volumes of veins may be produced by repeated fracturing and fluid migration within particular rock units without involving large volumes of externally derived fluids

shrimp ionprobe dating of short lived proterozoic tectonic cycles in the northern arunta inlier central australia structural and metamorphic evidence from low p high t metamorphic terranes within the northern arunta inlier central australia indicates that two discrete tectonic cycles affected the region each cycle was associated with compressive deformation coincident with peak granulite facies metamorphism and granite intrusion shrimp u th pb analysis of zircons from early syn and late tectonic granitoids allows the age of the two tectonic cycles to be directly measured the age of the first cycle d1 is determined by emplacement of the mount stafford granite dated at or before 1818 15 ma after the first two foliations of the cycle had formed but before the axial plane cleavage associated with later upright folding the regional extent of this tectono magmatic event is confirmed by an emplacement age of 1818 8 ma measured on the large discordant harverson granite 40 km away the second cycle d2 consisted of four distinct deformation stages d2a 2d the ages of which are constrained by 1785 1775 ma old early syn and late tectonic granitoids the warimbi granite emplaced at or before 1785 22 ma and the granitic protolith to the napperby gneiss inferred to be 1780 10 ma pre date or formed during the earliest foliation s2a zircon considered to have grown at peak granulite facies metamorphic conditions during generation of the dominant s2b foliation gives an age of 1774 6 ma in the possum creek charnockite leucogranite in the napperby gneiss which post dates s2b and large scale upright nw se trending f2d folds yields an age of 1775 12 ma and confirms that the entire tectonic cycle d2a 2d formed in a rapid 10 ma period younger zircons which formed at 1659 6 ma in the napperby gneiss cannot be related to any local structural event but are similar in age to a major magmatic metamorphic episode in the southern arunta black and shaw 1992 150 km away they may be the peripheral manifestation of that event further zircon growth at 1600 ma in the napperby gneiss and possum creek charnockite was caused by hydrothermal fluid fluxing associated with post tectonic pegmatite emplacement these dykes were deformed by amphibolite facies retrograde shear zones ion probe dating of zircon stratigraphy from the granitoids has confirmed the discrete successive tectonic cycles identified by earlier structural and metamorphic studies for the northern arunta inlier at least those cycles or orogenies are associated with short lived 10 ma thermal pulses related to granite magmatism not prolonged thermal perturbations spanning 100 ma or more cf dirks and wilson 1990 dirks and hand 1991 such short lived pulses are intimately associated with granite intrusion and may be a general feature of low p high t metamorphism

petrogenesis of cordierite orthoamphibole assemblages from the springton region south australia textural evidence for the partial breakdown of staurolite biotite and andalusite biotite assemblages to cordierite orthoamphibole implies high temperature metasomatic depletion of k2o in semi pelitic rocks from springton south australia the origin of the reaction textures is discussed with reference to k2o t diagrams derived from the topologically equivalent k2o h2o diagram showing both discontinuous and fe mg continuous reactions the involvement of fluids in the metasomatic process is implied by the scale of k2o removal and suggests that the outcrop pattern of cordierite gedrite rocks reflects at least in part a heterogeneous distribution of advecting fluids in the metamorphic pile at high temperatures

polyphase metamorphic fluid flow in the lower calcsilicate unit reynolds range central australia abstract lower calcsilicate unit metasediments and napperby gneiss metagranites at conical hill in the reynolds range central australia underwent at least two periods of fluid flow the ingress of surface derived fluids 18 o 2 h 18 o values in both the lower calcsilicate unit and the napperby gneiss fluid flow during cooling from the peak of 1600 ma regional metamorphism formed quartz garnet veins and caused epidotisation of the lower calcsilicate unit the fluids in this later event were water rich x co 2

on the causes of 18 o depletion and 18 o 16 o homogenization during regional metamorphism the east humboldt range core complex nevada previous stable isotope studies at lizzies basin revealed that metasedimentary rocks are 18o depleted relative to protolith values particularly in the lower parts of the section lower zone where the rocks are also isotopically homogeneous on a scale of hundreds of meters quartz 18o 9 0 to 9 6 per mil in contrast metasedimentary rocks at higher levels at lizzies basin upper zone are less 18o depleted and more heterogeneous in 18o in order to understand more fully the isotopic evolution of this terrane a series of detailed meter scale traverses across various metamorphic and igneous lithologies were completed at lizzies basin and at the structurally higher angel lake locality traverses in the lizzies basin lower zone and in the lower parts of angel lake angel lake lower sequence across various silicate lithologies including abundant granitoids reveal similar degrees of homogeneity although the average 18o values are higher at angel lake in contrast traverses which include substantial thicknesses of marble and calc silicate gneiss and very little granitoid have more heterogeneous quartz 18o values 11 9 to 13 4 per mil and also have a higher average 18o 12 9 per mil than observed elsewhere the scale of 18o 16o homogeneity in quartz observed at lizzies basin and angel lake meters to hundreds of meters requires fluid mediated isotope exchange which accompanied tertiary metamorphism there is a correlation between the degree of 18o depletion in metasedimentary rocks 18o 16o homogenization between lithologies and the proportion of granitoids leucogranites in particular within any part of the section and a corresponding anticor relation with the proportion of marble this points to a causal relationship whereby the leucogranites as well as the tertiary hornblende diorite and biotite monzogranite acted as both a relatively low 18o reservoir and a source of fluids to enhance exchange while the marbles hindered isotope deplction and homogenization by acting as relatively high 18o reservoirs and impermeable layers material balance calculations help delineate the plausible mechanisms of exchange between granitoids and metasediments single pass infiltration of magmatic fluids from the granitoids is not capable of reproducing all of the observations fluidmediated exchange by convective recirculation of magmatic fluids on a scale of meters is the mechanism which explains all of the observations the generalized model for the isotopic evolution of the east humboldt range core complex provides and excellent opportunity to establish the main causes and controlling factors of 18o depletion and 18o 16o homogenization during regional metamorphism

vermetid reefs and their use as palaeobathymetric markers new insights from the late miocene of the mediterranean southern italy crete abstract vermetids are a poorly known family of reef building gastropods characterized by an extremely irregular growth of the shell vermetid reefs are known from the badenian middle miocene to present day and their main importance is related to their use as sea level indicators strictly associated to the intertidal or immediate subtidal zone during their evolution two different vermetid genera have been recognized as reef builders petaloconchus was the major component of vermetid reefs until the holocene when for still unknown reasons it was almost completely replaced by dendropoma in the mediterranean basin petaloconchus reefs have been frequently found in close association to late miocene coral bioconstructions new finding of petaloconchus reefs within the slope facies of the salento messinian reef complex southern italy together with new data from late tortonian early messinian reefs of crete greece provide further information on the palaeoecology of these peculiar bioconstructions stratigraphical and sedimentological contexts of salento and crete vermetid reefs have been described together with an account of their size shape and inner structure biotic vermetids serpulids bryozoans and coralline algae and abiotic components early marine cements and intra reef sediment have been identified by microfacies analysis and their percentage abundance has been reported these data allow the description of vermetid reefs from two different depositional settings along the shelf edge and for the first time within the coral reef slope shelf edge and slope petaloconchus reefs show the same inner structure and composition but they differ by their shape by the density of the framework and by the relative proportion of the associated reef building organisms serpulids bryozoans and coralline algae additional information on their palaeobathymetric setting have been acquired from the presence of other reef building biota porites corals and halimeda green algae which occur in close proximity to slope vermetid reefs together with observations on some anatomical feature of petaloconchus shells results clearly indicate for petaloconchus reefs a palaeobathymetric range spanning from the upper subtidal zone down to 30 50 m of depth the present study confirms vermetid reefs as important tools for the reconstructions of ancient sea level but great attention has to be paid to the identification of the main reef building vermetid actually while a great number of present day dendropoma reefs can be associated to the mean sea level late miocene vermetid reefs from salento and crete indicate for petaloconchus bioconstructions a wider palaeobathymetric range

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

the role of nutrient availability in bioerosion consequences to carbonate buildups abstract zooxanthellate organisms which are among the major carbonate producers on coral reefs are highly adapted to nutrient deficient conditions and tend to be outcompeted by filamentous or fleshy algae if nutrients are abundant reef dwelling bioeroding organisms on the other hand seem to increase in abundance with increasing availability of nutrient and food resources maximum rates of calcium carbonate production in a reef system are comparable in magnitude to maximum rates of bioerosion the dynamic interplay between accretion destruction of coral reefs is therefore likely to be strongly influenced by nutrient availability nutrient availability may also influence development of carbonate facies in reef systems that develop in oligotrophic waters skeletal sands and framework should dominate carbonate buildups in systems that develop in mesotrophic waters enhanced bioerosion should reduce much of the skeletal material to carbonate muds increasing the prevalence of mud rich facies eutrophic conditions should result in bioeroded hardgrounds and carbonate depositional hiatuses when carbonate accumulation rates fail to equal or exceed rates of local relative sea level rise reefs drown bored and lithified pavements with crusts of ferromanganese oxides phosphates or glauconite often separate shallow reef from overlying deeper water deposits nondeposition bioerosion and evidence for low redox potentials at the sediment water interface all implicate excess nutrient availability as a factor in reef drowning some carbonate depositional hiatuses and bioerosional surfaces are local phenomena evidence of responses to local increases in nutrient availability resulting from submergence of soils runoff or topographic upwelling others appear to reflect regional or worldwide events because many carbonate producing organisms have life history strategies specialized to nutrient deficient conditions they are unable to compete when nutrients become plentiful events that triggered ocean turnover or sharp increases in the rate of deep ocean circulation would have increased sea surface nutrient availability worldwide such eutrophication could have caused widespread extinctions in reef communities because eutrophication promotes bioerosion details of the fossil records of those events are probably lost in bioerosional hiatuses

relations among net transfer reaction progress 18 o 13 c depletion and fluid infiltration in a clinohumite bearing marble a siliceous dolomitic marble xenolith within a mid crustal jurassic diorite exhibits mineralogical and stable isotopic evidence for infiltration of water rich fluid adjacent to endoskarn which bounds the xenolith forsterite has been replaced by clinohumite as a result of a devolatilization reaction driven by addition of aqueous fluid and extraction of heat isotopic compositions of calcite also record infiltration of aqueous fluid concentrated near contacts with endoskarn marble calcite 18osmow values range from 19 5 at the center of the xenolith to 12 6 adjacent to endoskarn calcite 13cpdb decreases sympathetically from 3 0 to 1 4 the calculated equilibrium composition of coexisting c o h f fluid and a quantitative characterization of the whole rock reaction which produced clinohumite and consumed graphite are used to place inequality contraints on the composition of the infiltrated fluid continnum mechanical transport models based on the resulting fluid compositions suggest that a small time integrated darey flux of 44 cm3 cm2 coupled with diffusive transport in the fluid was sufficient to produce both the isotopic shifts and the net transfer reactions evidenced in the xenolith the calculations demonstrate the importance of graphite as an indicator of time integrated flux the maximum possible ch4 content of the infiltrated fluid is sufficiently high to impart a 2 0 uncertainty in the 13c of the fluid the isotopic composition of the fluid is consistent with a magmatic origin when this uncertainty is taken into account

carbonate platform facies models various types of carbonate platforms are characterized by distinctive profiles facies and evolutionary sequences ramps may be homoclinal or distally steepened and may have fringing or barrier shoal water complexes of ooid pellet sands or skeletal banks homoclinal ramps pass seaward into deeper water without major break in slope and lack deep water breccias distally steepened ramps may be low energy and characterized by widespread shallow subwave base mud blankets or high energy with coastal beach dune complexes and widespread skeletal sand blankets slope facies may contain abundant breccias of slope derived clasts rimmed shelves have relatively flat tops and marked break in slopes at the high energy shallow shelf edge where they pass into deep water such shelves may be aggraded with peritidal facies extending over much of the shelf or incipiently drowned depending on magnitude of sea level fluctuations they may be accretionary or bypass types that include gullied slope escarpment and high relief erosional forms intrashelf basins occur on some shelves controlling distribution of reservoir and source beds isolated platforms are surrounded by deeper water and may be located on rifted continental margins or on submarine volcanoes most have high relief rimmed margins platforms that have been subjected to rapid sea level rise may be incipiently drowned and characterized by raised rims elevated patch or pinnacle reefs and widespread subwave base carbonate or fine clastic blankets completely drowned shelves develop where the shelf is submerged to subphotic depths terminating shallow water deposition and commonly resulting in blanketing of the shelf by deeper water facies some margins show extensive down to basin faulting that is contemporaneous with carbonate deposition or associated with thick prograding clastic sequences the various types of platforms change in response to variations in sedimentation subsidence or sea level rise and may form distinctive evolutionary sequences the relatively few models presented appear to accommodate most geological examples some of which contain major reservoir facies

migration of reaction and isotopic fronts in infiltration zones assessments of fluid flux in metamorphic terrains abstract criteria are presented for recognising fluid infiltration driven reactions in metamorphic rocks and quantifying the associated fractionations of oxygen and carbon isotopic compositions in medium or high grade metamorphic rocks with local equilibrium between fluid and solid phases reactions or isotopic changes driven by infiltrating fluid are expected to migrate as relatively sharp fronts with limited diffusive broadening fluid leaving such reaction fronts is compositionally buffered by the reacting phases in suitably reactive rocks a series of reaction fronts will develop each driven by infiltration of fluid buffered by the upstream reaction knowledge of the spatial arrangement of such fronts allows calculation of the time integrated fluid fluxes total fluid volume per unit area distinctive stable isotope fractionations are predicted adjacent to infiltration driven reactions with the distribution and magnitude of the fractionation effects dependent upon the relative velocities of the reaction and isotopic fronts the various possibilities are illustrated by reference to a hypothetical infiltration driven decarbonation reaction the calculated isotopic fractionations do not in general resemble those predicted by the open or closed system water rock ratio calculations

sapphirine kornerupine rocks from the reynolds range central australia constraints on the uplift history of a proterozoic low pressure terrain sapphirine kornerupine bearing rocks from the reynolds range northern territory australia preserve spectacular metamorphic reaction textures that provide valuable insights into the regional metamorphic uplift history the rocks occur in pods that are several meters in diameter within high temperature low pressure 750 to 800 c and 4 to 5 kbar granulite facies exposures of the early proterozoic lander rock beds a laterally extensive sequence of folded pelitic and quartzose metasediments the pods are not associated with large volumes of partial melts and are likely to have formed by metasomatism near the peak of m2 metamorphism the rocks in the pods consist of high temperature mg and al rich minerals such as boron free korneurpine and are coarse grained 0 5 to 15 cm non foliated and locally nearly monomineralic the growth of the coarse minerals in the pods largely post dated the high grade regional metamorphic d2 fabric and completely reconstructed the precursor rocks the retrograde metamorphic reaction textures show that the early retrogression from the m2 granulite facies conditions was characterized not by isobaric cooling as previously proposed but by nearly isthermal decompression these data imply that the reynolds range did not follow a simple anticlockwise p t t path because rocks such as these preserve information from a only restricted portion of the metamorphic history and can preserve evidence of decompression reactions more clearly than many more ordinary lithologies they can be especially important for discerning metamorphic p t t paths

fluid rock history of granulite facies humite marbles from ambasamudram southern india an extensive humite bearing marble horizon within a supracrustal sequence at ambasamudram southern india was studied using petrological and stable isotopic techniques to define its metamorphic history and fluid characteristics at peak metamorphic temperatures of 775 73 c based on calcite graphite carbon isotope thermometry the mineral assemblages suggest layer by layer control of fluid compositions clinohumite calcite bearing assemblages suggest xco2 0 9 at 790 c and local wollastonite scapolite grossular bearing zones formed at xco2 of c 0 3 retrograde reaction textures such as scapolite quartz symplectites after feldspar and calcite and replacement of dolomite diopside or tremolite dolomite after calcite forsterite or calcite clinohumite are indicative of retrogression under high xco2 conditions calcite preserves late proterozoic carbon and oxygen isotopic signatures and the marble lacks evidence for extensive retrograde fluid infiltration while during prograde metamorphism the possible infiltration of aqueous fluids did not produce significant isotopic resetting isotopic zonation of calcite and graphite grains was likely produced by localized co2 fluid infiltration during retrogression contrary to the widespread occurrence of humite marbles related to retrograde aqueous fluid infiltration the ambasamudram humite marbles record a prograde to peak metamorphic humite formation and retrogression under conditions of low xh2o

phasing of deglacial warming and laurentide ice sheet meltwater in the gulf of mexico evidence is emerging that the tropical climate system played a major role in global climate change during the last deglaciation however existing studies show that deglacial warming was asyn chronous across the tropical band complicating the identification of causal mechanisms the orca basin in the northern gulf of mexico is ideally located to record subtropical atlantic sea surface temperature sst warming in relation to meltwater input from the laurentide ice sheet paired d 18 o and mg ca data on the planktonic foraminifer globigerinoides ruber from core en32 pc6 are used to separate deglacial changes in sst and d 18 o of sea water sst as calculated from mg ca data increased by 3 8c from ca 17 2 to 15 5 ka in association with heinrich event 1 and was not in phase with greenland air temperature subtracting tem perature effects from d 18 o values in g ruber reveals two excur sions representing laurentide meltwater input to the gulf of mex ico one of 1 5 from ca 16 1 to 15 6 ka and a second major spike of 2 5 from ca 15 2 to 13 0 ka that encompassed melt water pulse 1a and peaked ca 13 8 ka during the bolling allerod conversion to salinity through the use of a laurentide meltwater end member of 225 indicates that near surface salinity de creased by 2 4 during these spikes these results suggest that gulf of mexico sst warming preceded peak laurentide ice sheet decay and the bolling allerod interval by 2 k y and that heat was retained in the subtropical atlantic during heinrich event 1 consistent with modulation of deglacial climate by thermohaline circulation

two dimensional patterns of metamorphic fluid flow and isotopic resetting in layered and fractured rocks pa one dimensional advection dispersion models predict that characteristic 18o vs distance and 18o vs 13c profiles should be produced during isothermal metamorphic fluid flow under equilibrium conditions however the patterns of isotopic resetting in rocks that have experienced fluid flow are often different from the predictions two dimensional advection dispersion simulations in systems with simple geometries suggest that such differences may be as a result of fluid channelling and need not indicate disequilibrium high dispersivities or polythermal flow the patterns of isotopic resetting are a function of 1 the permeability contrast between more permeable layers channels and less permeable layers matrix 2 the width and spacing of the channels 3 the width and spacing of discrete fractures and 4 the orientation of the pressure gradient with respect to layering in fractured systems the efficiency of isotopic transport depends on the fracture aperture and the permeability of the surrounding rock resetting initially occurs along and immediately adjacent to the fractures but with time isotopic resetting because of flow through the rock as a whole increases in importance application of the one dimensional advection dispersion equations to metamorphic fluid flow systems may yield incorrect estimates of fluid fluxes intrinsic permeabilities dispersivities and permeability contrasts unless fluid flow occurred through zones of high permeability that were separated by relatively impermeable layers

crustal structure in the elko carlin region nevada during eocene gold mineralization ruby east humboldt metamorphic core complex as a guide to the deep crust the deep crustal rocks exposed in the ruby east humboldt metamorphic core complex northeastern nevada provide a guide for reconstructing eocene crustal structure 50 km to the west near the carlin trend of gold deposits the deep crustal rocks in the footwall of a west dipping normal sense shear system may have underlain the pinon and adobe ranges about 50 km to the west before tertiary extension close to or under part of the carlin trend eocene lakes formed on the hanging wall of the fault system during an early phase of extension and may have been linked to a fluid reservoir for hydrothermal circulation the magnitude and timing of paleogene extension remain indistinct but dikes and tilt axes in the upper crust indicate that spreading was east west to northwest southeast perpendicular to a paleozoic and mesozoic orogen that the spreading overprinted high geothermal gradients associated with eocene or older crustal thinning may have contributed to hydrothermal circulation in the upper crust late eocene eruptions upper crustal dike intrusion and gold mineralization approximately coincided temporally with deep intrusion of eocene sills of granite and quartz diorite and shallower intrusion of the harrison pass pluton into the core complex rocks stacked mesozoic nappes of metamorphosed paleozoic and precambrian rocks in the core complex lay at least 13 to 20 km deep in eocene time on the basis of geobarometry studies in the northern part of the complex the presently exposed rocks had been even deeper in the late mesozoic to 30 km depths before losing part of their cover by eocene time nappes in the core plunge northward beneath the originally thicker mesozoic tectonic cover in the north part of the core complex mesozoic nappes and tectonic wedging likely occupied the thickened midlevel crustal section between the deep crustal core complex intrusions and nappes and the overlying upper crust these structures as well as the subsequent large displacement cenozoic extensional faulting and flow in the deep crust would be expected to blur the expression of any regional structural roots that could correlate with mineral belts structural mismatch of the mineralized upper crust and the tectonically complex middle crust suggests that the carlin trend relates not to subjacent deeply penetrating rooted structures but to favorable upper crustal host rocks aligned within a relatively coherent regional block of upper crust

fluid rock interaction and thermal evolution during thrusting of an alpine metamorphic complex tinos island greece this study examines the fluid rock interaction and thermal evolution along a thrust that juxtaposes calcite rich marbles of high p t metamorphic unit of the attic cycladic massif greece on top of a lower grade dolomite marble unit the tertiary thrust represents a major phase of tectonic movement related to the decompression of the alpine orogen in the hellenides the stable isotope signatures of the thrust plane and adjacent sections of the footwall and hanging wall rocks are characterized by significant carbon and oxygen isotope depletions the depletion is most pronounced in calcite but is almost entirely missing in coexisting dolomite the isotopic patterns in the thrust zone can be explained by the infiltration of an externally derived water rich h2o co2 ch4 fluid x c x co 2 x ch 4 0 05 at water rock ratios on the order of 0 1 to 0 5 by weight the fluid induced calcite recrystallization is viewed as an important rheological control during thrusting the temperature evolution of the footwall hanging wall and mylonitic tectonic contact was determined by calcite dolomite solvus thermometry histograms of calcite dolomite temperatures are interpreted as indicating a heating of the footwall dolomite marble during the thrusting of the hotter upper plate conversely the hanging wall marble unit was cooled during the thrusting the calcite dolomite thermometry of the thrust plane gives temperatures intermediate between the initial temperatures of the lower and upper marble units and this leads to the conclusion that conductive heat transfer rather than fluid infiltration controlled the thermal evolution during thrusting

fluid absent melting and the roles of fluids in the lithosphere a slanted summary abstract the presence or absence nature and compositions of lithospheric fluids are crucial factors in the evolution of igneous and metamorphic rocks despite this confusion still surrounds perceptions of the degrees of fluid involvement in many geological settings however experimental and field investigations constrain these roles and we are able to make some valid generalizations studies of prograde metamorphism have highlighted the spatial heterogeneity of fluid concentrations in many crustal environments this commonly indicates internal lithological control of volatile activities under fluid present conditions crustal fluids do not appear to mix readily fluid flow is generally structurally or lithologically controlled the transition from amphibolite to granulite facies is characterized by a decrease in ah2o as well as a temperature increase in many terranes these phenomena correlate with the appearance of migmatites some granulites are not migmatitic but have chemical and mineralogical characteristics of melt depleted restites this indicates efficient extraction of melts incongruent melting reactions involving the breakdown of crystalline hydrates in crustal rocks produce granitoid melts plus denser restites that are nearly or actually anhydrous experimental and theoretical treatments of fluid absent melting show that micas and amphiboles decompose at 700 to 900 c for mid to lower crustal pressures the melts resemble the compositions of s and i type granitoids pervasive aqueous fluids could not generally have been present during this melting since exposed granulite terranes are not characterized by high melt proportions and the magmas themselves are water undersaturated likewise the general presence of very h2o poor pervasive fluids is unlikely since this would allow only subsolidus dehydration at normally attainable crustal temperatures in this case the amphibolite granulite transition is possible at relatively low temperatures and the resultant rocks would not contain evidence of in situ melting the granulite granite connexion and effective crustal differentiation seem viable only during high temperature fluid absent metamorphism data on continental geotherms show that the lower crust is not entirely granulitic granulites are produced in regions of anomalously high heat flow the mantle connexion thus inferences on the nature of the lower crust should not be entirely based on studies of exposed granulite terranes mineral assemblages in eclogites also indicate widespread internal buffering of volatile activities and probable fluid absent conditions mantle derived basaltic magmas too are volatile bearing and it is plausible that volatile fixing phases also occur in the mantle volatiles may largely be introduced into the mantle through subduction processes and only released during the melting attending the breakdown of hydrous minerals as in the crust

continental collision tectonics evidence from the himalaya and the neighbouring regions abstract the evolution of the himalaya and the tectonics of neighbouring regions are generally believed to be due to continent to continent collision of the eurasian and the indian plates during the last decade many new data have been obtained and examined and a suite of models has been developed to explain the evolution of the himalaya and the neighbouring regions these models could broadly be classified into three categories the subdivision of the himalaya into five main structural units is now well accepted a tectogenetic diagram of the himalaya is briefly discussed ophiolites are reported to occur all along the indus tsangpo suture zone results of geochemical analysis of an ophiolite sequence from dras in kashmir himalaya are presented surface wave dispersion and attenuation investigations have revealed the crustal and upper mantle structure below the himalaya and tibet plateau the finding of low velocity layers is consistent with the concept that horizontal compression has been responsible for creating these weak zones and the inferred elevated temperatures in the vicinity of the moho discontinuity below tibet earthquake focal mechanism solutions and interpretation of landsat imagery indicate that convergence of the indian plate with eurasia continues and is accommodated partly by crustal shortening and partly by lateral mass movement at the northwest and the northeast margins of the indian plate palaeomagnetic data from the himalaya belt and central asia can be interpreted in support of molnar and tapponnier s model of continental collision

thermal isostasy response of a moving lithosphere to a distributed heat source spreading ridges and hot spot swells are identified by their high surface heat flow shallow seafloor and high geopotential to understand these and other thermotectonic features the oceanic lithosphere is modeled as a thermomechanical boundary layer moving through a three dimensional time independent heat source the heat source mimics the heat advection associated with a spreading ridge or hot spot without introducing the nonlinearities of these flow processes the fourier transforms of three green s functions response functions which relate the three observable fields to their common heat source are determined analytically each of these reponse functions is highly anisotropic because the lithosphere is moving with respect to the source however the ratio of the gravity response function to the topography response function i e gravity topography transfer function is nearly isotropic and has a maximum lying between the flexural wavelength and 2 times the thickness of the thermal boundary layer the response functions are most useful for determining the surface heat flow seafloor topography and geopotential for complex lithospheric thermal structures in practice these three observables are calculated by multiplying the fourier transform of the heat source by the appropriate response function and inverse transforming the products almost any time independent thermotectonic feature can be modeled using this technique included in this report are examples of spreading ridges and thermal swells although more complex geometries such as ridges offset by transform faults and rrr type triple junctions can also be modeled because forward modeling is both linear and computationally simple the inverse of this technique could be used to infer some basic characteristics of the heat source directly from the observed fields

an extended episode of early mesoproterozoic metamorphic fluid flow in the reynolds range central australia the products of metamorphic fluid flow are preserved in zones within the marbles and metamorphosed semipelites of the upper calcsilicate unit in the granulite portion of the late palaeoproterozoic reynolds range group northern arunta block central australia the zones of retrogression characterized by minerals such as wollastonite grossular and clinohumite local resetting of oxygen isotopic compositions and local major element metasomatism were channelways for water rich fluids derived from granulite facies metapelites u th pb isotopic ages measured by the shrimp ion microprobe on zircon and monazite from a granulite facies semipelite an early semiconcordant aluminous quartz rich fluid flow segregation and a late discordant quartz rich segregation record some of the extended thermal history of the area zircon cores from the semipelite show its likely protolith to be an igneous rock 1812 11 ma old itself derived from a source containing zircon as old as 2 2 ga low th u overgrowths on the zircon grew during granulite facies metamorphism at 1594 6 ma monazite cooled to its blocking temperature at 1576 8 ma zircon cores from the semiconcordant segregation are dominantly 2 3 ga old indicating that the source of the fluids was not the particular metamorphosed semipelite studied two generations of low th u overgrowths on the zircon give indistinguishable ages for the older and younger of 1589 8 and 1582 8 ma respectively the monazite age is the same 1576 12 ma zircon from the late discordant segregation gave 1568 4 ma fluid flow occurred for at least 18 3 ma and ended 26 3 ma after the peak of metamorphism suggesting a very slow cooling rate of 3 c ma 1 the last regional high grade metamorphism in the reynolds range occurred at 1 6 ga not 1 78 ga as previously thought the high grade event at 1 78 ga is a separate event that affected only the basement to the reynolds range group

deglacial meltwater pulse 1b and younger dryas sea levels revisited with boreholes at tahiti reconstructing sea level changes during the last deglaciation provides a way of understanding the ice dynamics that can perturb large continental ice sheets the resolution of the few sea level records covering the critical time interval between 14 000 and 9 000 calendar years before the present is still insufficient to draw conclusions about sea level changes associated with the younger dryas cold event and the meltwater pulse 1b mwp 1b we used the uranium thorium method to date shallow living corals from three new cores drilled onshore in the tahiti barrier reef no significant discontinuity can be detected in the sea level rise during the mwp 1b period the new tahiti sea level record shows that the sea level rise slowed down during the younger dryas before accelerating again during the holocene

axisymmetric finite element analysis of an elastic lithosphere subjected to thermal loading summary an axisymmetric finite element was used to model the lithosphere as an infinite 40 km thick circular elastic plate on an elastic foundation asthenosphere subjected to concentrated thermal loading thermal loading is produced by a point heat source of 4 184 lo8 watts located 60 km beneath the plate within the asthenosphere increased nodal temperatures within the lithospheric finite element mesh were obtained from a solution to the steady state heat conduction equation for a point source in a semi infinite half space vertical as well as radial thermal expansion coefficients of 1 33 x lo k were utilized in the solutions thermoelastic loads as well as isostatic uplift forces are applied as nodal loads in the finite element lithosphere model thermoelastic loads are produced from uneven heating of the lithosphere and differential expansion isostatic uplift forces result from radial expansion and the diminished density of heated lithosphere columns uplift of the lithosphere due to the point heat source and vertical expansion is reduced because of radial expansion and flexure this occurs because material at the base of the lithosphere is closer to the point heat source and experiences greater radial expansion for sediment infdl thermoelastic loading reduces the surface uplift produced by isostatic uplift forces from 3 11 to 192m thermoelastic lithosphere models yield significantly higher radial stress profiles than density models without thermoelastic stresses the neutral plane for radial stress is displaced towards the surface in the thermoelastic models due to greater radial expansion of basal lithosphere material which is closer to the point heat source the finite element model has several advantages over the classical thin plate approach stresses and strains can be evaluated at any point within the lithosphere this capability was used in this study to determine vertical lithosphere expansion thickening non linear variation of stress with depth and the stress neutral plane position

calculation of oxygen isotope fractionation in hydroxyl bearing silicates abstract the modified increment method has been applied to the calculation of oxygen isotope fractionation factors for hydroxyl bearing silicate minerals the order of 18o enrichment obtained in common rock forming minerals is pyrophyllite kaolinite tourmaline talc prehnite topaz illite phengite lepidolite muscovite staurolite epidote glaucophane serpentine chlorite tremolite hornblende phlogopite biotite humite norbergite ilvaite hydroxyl bearing silicates are enriched in 18o relative to hydroxyl groups but depleted in 18o relative to anhydrous counterparts three sets of self consistent fractionation factors between quartz and the hydroxyl bearing silicate minerals between calcite and the silicate minerals and between the silicate minerals and water have been calculated for a temperature range of 0 1200 c the fractionation factors calculated for mineral pairs are applicable to isotopic geothermometry in igneous metamorphic and sedimentary petrology they can be used as a test of isotopic equilibrium or disequilibrium in natural mineral assemblages over all temperature ranges of geological interest the difference in oxygen isotope composition between the hydroxyl bearing mineral and the oh group is quantitatively demonstrated to be temperature dependent and therefore can be used as a single mineral geothermometer

three dimensional sedimentary framework of the carbonate ramp slope of central west florida a sequential seismic stratigraphic perspective the sequential post early cretaceous stratigraphic evolution of the carbonate ramp slope of central west florida has been determined via the integration of 1 500 km of seismic reflection profiles with drillcores piston cores and rock dredge hauls our goal has been to gain insight into and increase our knowledge and understanding of carbonate ramps in general as well as to develop a model for potential application to the rock record our results which focus on the neogene indicate that the three dimensional sedimentary framework of the west florida ramp slope has developed in response to the interplay of a number of processes including 1 regional tectonic subsidence 2 long term sea level change 3 short term sea level oscillations 4 large scale gravity collapse 5 a major paleoceanographic event 6 fluctuating intensity of oceanic circulation 7 lateral migration of ocean currents and 8 open ocean upwellang and pelagic carbonate production during the early cretaceous seismic sequence vi the continental margin of west florida was a large shallow water carbonate evaporate platform that had persisted since late stages of continental rifting in the jurassic during the mid cretaceous seismic sequence v terrigenous sediment nutrient influx terminated platform growth and deposited a layer of neritic marl for the next 60 m y during a long term eustatic highstand in the late cretaceous paleogene seismic sequence iv the present west florida slope was an open ocean deep water marginal plateau that accumulated pelagic carbonates following a eustatic lowstand in the late oligocene 30 m y ago the carbonate ramp slope of central west florida began to take shape as a low relief ramp margin built seaward into the early and middle miocene seismic sequences iii and ii in the latest early miocene large scale gravity collapse generated a slope basin that was rapidly filled by prograding clinoforms in the middle miocene 12 15 m y ago intensification of oceanic circulation truncated antecedent clinoforms blocked off shelf sediment transport and stimulated pelagic carbonate production this oceanographic event resulted in the succession of seaward prograding clinoforms by a slope front fill pelagic depositional system that has survived numerous sea level oscillations into the present seismic sequence i

role of membrane stresses in the support of planetary topography the principal purpose of this paper is to examine whether membrane stresses can support topographic loads on planetary elastic lithospheres it is found that the ability of a spherical shell to support loads through membrane stresses is determined by the nondimensional parameter ed gr2 where d is the thickness of the elastic lithosphere is the density difference between the mantle and crust and r is the radius of the planetary body when this parameter is large membrane stresses can fully support topographic loads without flexure and when it is small the influence of membrane stresses can be neglected solutions of the equation governing the behavior of a spherical shell are obtained for a topographic load expressed in terms of spherical harmonics spherical harmonic expansions of the measured gravity and topography for mars and the moon are compared with the theory it is found that for mars the support of topography is primarily due to membrane stresses for n 8 and for the moon for n 17 for mars the data for 4 n 7 give 0 5 for the moon the data have considerable scatter that is attributed to the mascons but generally correlate with 0 5 if bending stresses are neglected the governing equation for the deflection of the spherical shell is legendre s equation a general solution is obtained for an axisymmetric load this solution is applied to the tharsis region on mars the 60 65 compensation of this region requires that 0 6 the well defined fracture pattern surrounding the tharsis region is attributed to tensional membrane stresses

high temperature retrogression of granulite facies marbles form the reynolds range group central australia phase equilibria isotopic resetting and fluid fluxes reynolds range group rocks underwent granulite facies metaintroduction morphism m2 at 1 6 ga 5 kbar 750 800 c and were the uplift history of high grade metamorphic belts comsubsequently retrogressed in narrow strike parallel zones at 1 59 monly involves partial re equilibration of mineral as1 57 ga within these zones metacarbonates that initially equisemblages to lower temperature and pressure conditions librated at x 2 0 8 during m2 were mineralogically reset this re equilibration is facilitated by fluid rock interby the infiltration of water rich fluids x 2 0 02 0 3 at action and new mineral assemblages are often best 650 700 c and 3 4 kbar do carb values of the retrogressed developed in discrete zones of extensive retrogression metacarbonates were variably reset during fluid infiltration with the e g van reenen 1986 cartwright 1988 peters lowest values 10 13 suggesting that the fluids that caused wickham 1994 cartwright buick 1995 petrological retrogression were exsolved from segregated partial melts themselves studies of retrograde mineral assemblages may provide derived from the underlying granulite facies metapelites minimportant information about the tectonics and hydroeralogical and isotopic resetting were locally accompanied by silica geology of the lower and middle crust metasomatism the mineralogically reset marbles record time inhowever although there have been many recent quantegrated fluid fluxes of typically 10 10 m m and metasomatic titative studies of metamorphic fluid rock interaction in rocks record time integrated fluid fluxes as high as 10 10 m contact aureoles cartwright valley 1991 ferry m for upward flow of high temperature fluids through the marbles dipple 1991 young morrison 1992 jamveit et al over a distance of 200 m in 18 ma the observed mineralogical 1992 davis ferry 1994 and during prograde regional and isotopic resetting and metasomatism require intrinsic permetamorphism bickle baker 1990a 1990b ferry meabilities between 10 and 10 m that vary across strike 1992 stern et al 1992 leger ferry 1993 cartwright on a centimetre to metre scale indicating that fluid flow was strongly et al 1995 buick cartwright 1996 there have been channelled relatively few studies of high temperature retrogression vennemann stuart smith 1992 peters wickham 1994 cartwright buick 1995 therefore the origins of the retrograde fluids and the absolute timing of

crustal structure of the ruby mountains and southern carlin trend region nevada from magnetotelluric data abstract we have collected about 150 magnetotelluric mt soundings in northeastern nevada in the region of the ruby mountains metamorphic core complex uplift and southern carlin mineral trend in an effort to illuminate controls on core complex evolution and deposition of world class gold deposits the region has experienced a broad range of tectonic events including several periods of compressional and extensional deformation which have contributed to the total expression of electrical resistivity most of the soundings reside in three east west profiles across increasing degrees of core uplift to the north bald mountain harrison pass and secret pass latitudes one short cross line was also taken to assess an east west structure to the north of the northern profile model resistivity cross sections were derived from the mt data using a 2 d inversion algorithm which damps departures of model parameters from an a priori structure geological interpretation of the resistivity combines previous seismic potential field and isotope models structural and petrological models for regional compression and extension and detailed structural stratigraphic interpretations incorporating drilling for petroleum and mineral exploration to first order the resistivity structure is one of a moderately conductive phanerozoic sedimentary section fundamentally disrupted by intrusion and uplift of resistive crystalline rocks late devonian and early mississippian shales of the pilot and chainman formations together form an important conductive marker sequence in the stratigraphy and show pronounced increases in conductance conductivity thickness product from east to west these increases are attributed to graphitization caused by elko sevier era compressional shear deformation and possibly by intrusive heating the resistive crystalline central massifs adjoin the host stratigraphy across crustal scale steeply dipping fault zones the zones provide pathways to the lower crust for heterogeneous upper crustal induced electric current flow resistive core complex crust appears steeply bounded under the middle of the neighboring grabens and not to deepen at a shallow angle to arbitrary distances to the west the numerous crustal breaks imaged with mt may contribute to the low effective elastic thickness t e estimated regionally for the great basin and exemplify the mid crustal steeply dipping slip zones in which major earthquakes nucleate an east west oriented conductor in the crystalline upper crust spans the east humboldt range and northern ruby mountains the conductor may be related to nearby graphitic metasediments with possible alteration by middle tertiary magmatism lower crustal resistivity everywhere under the profiles is low and appears quasi one dimensional it is consistent with a low rock porosity

melt production during granulite facies anatexis experimental data from primitive metasedimentary protoliths we conducted fluid absent partial melting experiments at 0 5 and 1 0 gpa in the temperature range 750 to 1000 c to investigate the influence of bulk rock mg 100mg mg fe and the effects of additional tio2 on the granulite grade anatectic evolution of relatively magnesian metapelites and metagreywackes in these experiments melting began between 780 and 830 c by the incongruent breakdown of biotite to produce quartz saturated granulite facies residual mineral assemblages in equilibrium with h2o undersaturated granitic melt the glass quenched melt compositions produced in this study vary little generally the glasses have compositions similar to those of many natural strongly peraluminous leucogranites the solidus temperatures in both rock types increase with increasing mg but are unaffected by the presence or absence of a tio2 component at 0 5 gpa the metapelites melted at temperatures up to 50 c lower than the equivalent metagreywackes but at 1 gpa there was no discernible difference this study suggests that the fluid absent solidus has a steep positive dp dt slope in metapelites and steep negative dp dt slope in metagreywackes the pattern of melt production with increasing temperature is strongly controlled by the upper limit of biotite stability in tio2 free compositions this was found to increase by 15 to 20 c in the metapelites and by 30 to 40 c in the metagreywackes as a function of increasing mg from 49 to 81 the presence of a tio2 component increases the upper limit of biotite stability by 50 c in the metapelites and by 80 c in the metagreywackes over that observed in the equivalent tio2 free compositions in consequence in the tio2 free samples large pulses of melt up to 35 wt are produced over narrow temperature ranges as little as 15 c in these experiments between 830 and 875 c in the tio2 bearing samples the major pulse of melt production occurs more gradually between 830 and 900 c

free air gravity over the hoggar massif northwest africa evidence for alteration of the lithosphere abstract analysis of the free air gravity field over the broadly elevated hoggar region in northwest africa suggests that the uplift is similar in origin to the midplate hotspot swells observed on the seafloor the hoggar massif is a dome of precambrian basement approximately 1 km high 1000 km wide and capped by late tertiary alkali basalt volcanos gravity measurements reveal a broad free air high with maximum amplitude of approximately 40 mgal which is coincident with the areal extent of the hoggar using the reasonable assumption that the hoggar is in local isostatic balance calculations indicate that the amplitude of the gravity anomaly is best explained if the isostatic root of the elevated area is 60 km below ground surface this root depth is similar to that observed for both the hawaiian swell in the pacific and the bermuda rise in the atlantic and is shallower than the expected base of the lithosphere the gravity data suggest that the lithosphere beneath the hoggar has been reduced in density perhaps by reheating as inferred for oceanic swells

the geological evolution of the reynolds range central australia evidence for three distinct structural metamorphic cycles the reynolds range is affected by three distinct structural metamorphic cycles di mi dii mii anddiii miii which represent the local expressions of a series of orogenic events that affected large sections of the arunta block central australia the tectonic cycles recognized in the reynolds range correlate extremely well with similar events in the nearby anmatjira range di mi took place around 1820 ma and only affected the nw reynolds range dii mii occurred between 1760 and 1650 ma and involved ne sw shortening with identical timing relationships between the deformations and metamorphism along the entire reynolds range dii commenced with the formation of a fabric sii1 preserved as inclusion trails in porphyroblasts sii1 formed early during a progressive event dii1 2 which involved the formation and tightening of upright nw se trending isoclinal folds a penetrative subvertical nw se trending fabric sii2 developed axial planar to these folds the general symmetry of dii2 microstructures suggests that fii2 folds were formed during progressive coaxial deformation under plane strain conditions dii3 structures are associated with conjugate sets of crenulation bands within the macroscopic crenulation bands large sections of earlier folds and fabrics are rotated to a near recumbent position the dominant set of crenulation bands is upright and trends ene wsw while the conjugate to this set trends nw se no penetrative fabric formed during dii3 dii4 is associated with a system of amphibolite grade ne dipping thrusts interconnected by numerous smaller shear zones the sense of movement is ne over sw during dii essentially penetrative coaxial deformation dii2 evolved into essentially localized non coaxial deformation dii3 dii4 dii deformation occurred concomitant with a low pressure granulite facies metamorphic event mii 1730 ma peak metamorphic sillimanite cordierite orthopyroxene ilmenite spinel garnet k feldspar quartz assemblages define sii2 oriented sillimanite biotite assemblages that formed early during dii3 and unoriented andalusite chlorite muscovite assemblages that formed late during dii3 indicate that cooling of mii was initially isobaric during dii4 thrusting staurolite garnet biotite quartz and biotite kyanite quartz assemblages defining the mylonitic fabric overprint the late dii3 andalusite assemblages dii4 shear movement is therefore accompanied by a slight rise in pressure dii4 shear zones gradually increase in grade eastward along the reynolds range this trend coincides with the variation in peak metamorphic grade attained during mii indicating that the rocks had not yet fully cooled when dii4 thrusting commenced diii miii is late devonian to carboniferous in age and caused minor differential movement 100 m and retrogression along dii4 shear zones

magnitude and timing of episodic sea level rise during the last deglaciation a succession of elevated ridge deposits on the south florida margin was mapped using high resolution seismic and side scan sonar imaging in water depths ranging from 50 to 124 m the ridges are interpreted to be subtidal shoal complexes and paleoshorelines eolian dune or beach formed during the last sea level transgression oolitic and skeletal grainstones and mixed skeletal peloidal ooid packstones were recovered using a research submersible all of the grains are of shallow water or intertidal origin and both marine and nonmarine cements were identified formation and preservation of these features are attributed to episodic and rapid changes in the rate of the deglacial sea level rise at the onset of the termination 1a 18o excursion this high resolution record of sea level change appears to be related to deglacial processes operating on submillennial time scales and supports increasing evidence of rapid episodic fluctuations in ice volume climate and ocean circulation patterns during glacial interglacial transitions

stable isotope evidence for regional scale fluid migration in a barrovian metamorphic terrane vermont usa we measured the c and o isotopic composition of carbonate minerals in the waits river formation eastern vermont to determine the extent of fluid infiltration during regional metamorphism over an approx 2000 km2 area in a deep seated 25 km barrovian terrane from a petrologic study of this terrane ferry proposed the existence of a large regional metamorphic hydrothermal system with two first order features 1 on the scale of the entire terrane fluid flow was focused into the axes of two major antiforms of regional extent 2 on a smaller scale about 100 km2 flow was further focused around synmetamorphic granitic plutons that intruded along the axes of the antiforms we find isotopic evidence for both the regional hydrothermal activity along the antiforms and the more intense fluid flow around synmetamorphic plutons the evidence for hydrothermal activity around the plutons is large heavy isotope depletions up to 6 9 in 18ocarb and 13ccarb in diopside zone rocks adjacent to the plutons these isotopic shifts are greater than can be explained solely by prograde metamorphic reactions we find two lines of evidence for the more diffuse regional flow that was focused into axes of the antiforms first 18ocarb and 13ccarb within individual outcrops become increasingly homogeneous with increasing grade towards antiform axes indicating that the rocks equilibrated with a permeating fluid second there are depletions in 18o near the margins of the waits river formation which can be interpreted as a dispersed advective infiltration front displaced toward the antiform axes these fronts were modelled using eq 13 of bickle and baker and imply time integrated fluid of 105 106 cm3 cm2 which are consistent with values derived by ferry from measured progress of prograde devolatilization reactions

dating the detachment fault system of the ruby mountains nevada significance for the kinematics of low angle normal faults 1 the mechanics of low angle normal faulting and metamorphic core complexes continue to be a subject of debate we investigate the conditions timing and kinematics of slip in the late upper crustal stages of core complex evolution of the ruby mountains detachment fault at the well exposed secret pass locality with an x ray diffraction xrd and ar ar study of clay gouge samples from three separate faults two from the low angle detachment system and one from a high angle normal fault that soles into the main detachment fault system xrd analysis and modeling of xrd analysis show that authigenic illite rich illite smectite i s in gouge at secret pass is distinguishable from clay phases in hanging wall rocks because the i s in the gouges contains only one water layer as opposed to the more common two water i s phases found in both the hanging wall and footwall ar ar ages for the monomineralic one water i s found in the hanging wall high angle fault the main detachment and a low angle normal fault structurally above the main detachment are 11 6 0 1 ma 12 3 0 1 ma and 13 8 0 2 ma respectively the not quite flat ar ar spectra indicate the gouge illites grew over some interval of time and not in discrete events the nearly overlapping ages indicate that gouge formation and thus the last major period of activity on the detachment were at 11 13 ma and were active coevally as part of a kinematically linked fault system with the main detachment active at dips 45 and possibly as low as 22

rapid sea level rise and ice sheet response to 8 200 year climate event 1 the largest abrupt climatic reversal of the holocene interglacial the cooling event 8 6 8 2 thousand years ago ka was probably caused by catastrophic release of glacial lake agassiz ojibway which slowed atlantic meridional overturning circulation amoc and cooled global climate geophysical surveys and sediment cores from chesapeake bay reveal the pattern of sea level rise during this event sea level rose 14 m between 9 5 to 7 5 ka a pattern consistent with coral records and the ice 5g glacio isostatic adjustment model there were two distinct periods at 8 9 8 8 and 8 2 7 6 ka when chesapeake marshes were drown as sea level rose rapidly at least 12 mm yr 1 the latter event occurred after the 8 6 8 2 ka cooling event coincided with extreme warming and vigorous amoc centered on 7 9 ka and may have been due to antarctic ice sheet decay

partial melting and the formation of granulite facies assemblages in namaqualand south africa abstract dehydration melting reactions in which water from a hydrous phase enters the melt leaving an anhydrous solid assemblage are the dominant mechanism of partial melting of high grade rocks in the absence of externally derived vapour equilibria involving melt and solid phases are effective buffers of ah2 o the element partitioning observed in natural rocks suggests that dehydration melting occurs over a temperature interval during which for most cases ah2o is driven to lower values the mass balance of dehydration melting in typical biotite gneiss and metapelite shows that the proportion of melt in the product assemblage at t 850 c is relatively small 10 20 and probably insufficient to mobilize a partially melted rock body granulite facies metapelite biotite gneiss and metabasic gneiss in namaqualand contain coarse grained discordant unfoliated anhydrous segregations surrounded by a finer grained foliated matrix that commonly includes hydrous minerals the segregations have modes consistent with the hypothesis that they are the solid and liquid products of the dehydration melting reactions bt sil qtz pi grt crd kfs l metapelite bt qtz pl opx kfs l biotite gneiss and hbl qtz opx cpx pl l metabasic gneiss the size shape distribution and modes of segregations suggest only limited migration and extraction of melt growth of anhydrous poikiloblasts in matrix regions development of anhydrous haloes around segregations and formation of dehydrated margins on metabasic layers enclosed in migmatitic metapelites all imply local gradients in water activity also they suggest that individual segregations and bodies of partially melted rock acted as sinks for soluble volatiles the preservation of anhydrous assemblages and the restricted distribution of late hydrous minerals suggest that retrograde reaction between hydrous melt and solids did not occur and that h2o in the melt was released as vapour on crystallization this model combined with the natural observations suggests that it is possible to form granulite facies assemblages without participation of external fluid and without major extraction of silicate melt

discovery of a submerged relic reef and shoreline off grand cayman further support for an early holocene jump in sea level abstract drilling close to the base of a submerged sea cliff on the terraced eastern shelf of grand cayman has revealed a relic acropora palmata reef at a depth of 21 m below msl ten cores from its crest are principally composed of cobble sized clasts of a palmata set in a matrix of cemented skeletal grainstone the clasts have a distinctive succession of encrusters that indicate rapid burial a photophilic association of crustose coralline algae foraminifera and vermetid gastropods superimposed by a cryptic association of sclerosponges foraminifera and serpulids in addition to rapid burial u th thermal ionization mass spectrometer tims dating of coral clasts within 1 m of the relic reef surface indicates minor temporal mixing with ages between 8 9 and 8 1 ka such mixing and rapid burial is consistent with a hurricane deposit and is identical to deposits found on the crests of modern reefs in relation to its age the preservation of a 18 5 m intertidal notch cut into the submerged sea cliff on the western shelf of grand cayman implies that the crest of the relic reef has been lowered 1 5 2 m by marine abrasion bioerosion at a rate of 0 25 mm yr 1 reconstructing this eroded section using average holocene accretion rates indicates that the reef likely ceased accreting at 7 6 ka at a depth of 19 m comparing these data with other relic reefs in the caribbean indicates that the relic reef on grand cayman died within 160 years of relic reefs on barbados st croix st thomas and north florida this narrow interval of reef demise also coincides with the time when modern reefs were establishing themselves some 4 9 m higher upslope a fact that can only be resolved by invoking a rapid 6 m jump in sea level 7 5 ka ago such a jump would also account for two unexplained events around this time the restricted interval of global delta initiation and the catastrophic flooding of the glacially lowered black sea

global glacial isostasy and the surface of the ice age earth the ice 5g vm2 model and grace abstract the 100 kyr quasiperiodic variation of continental ice cover which has been a persistent feature of climate system evolution throughout the most recent 900 kyr of earth history has occurred as a consequence of changes in the seasonal insolation regime forced by the influence of gravitational n body effects in the solar system on the geometry of earth s orbit around the sun the impacts of the changing surface ice load upon both earth s shape and gravitational field as well as upon sea level history have come to be measurable using a variety of geological and geophysical techniques these observations are invertible to obtain useful information on both the internal viscoelastic structure of the solid earth and on the detailed spatiotemporal characteristics of glaciation history this review focuses upon the most recent advances that have been achieved in each of these areas advances that have proven to be central to the construction of the refined model of the global process of glacial isos

strange bedfellows a deep water hermatypic coral reef superimposed on a drowned barrier island southern pulley ridge sw florida platform margin abstract the southeastern component of a subtle ridge feature extending over 200 km along the western ramped margin of the south florida platform known as pulley ridge is composed largely of a non reefal coastal marine deposit modern biostromal reef growth caps southern pulley ridge spr making it the deepest hermatypic reef known in american waters subsurface ridge strata are layered lithified and display a barrier island geomorphology the deep water reef community is dominated by platy scleractinian corals leafy green algae and coralline algae up to 60 live coral cover is observed in 60 75 m of water although only 1 2 of surface light is available to the reef community vertical reef accumulation is thin and did not accompany initial ridge submergence during the most recent sea level rise the delayed onset of reef growth likely resulted from several factors influencing gulf waters during early stages of the last deglaciation 14 kyr b p including cold low salinity waters derived from discrete meltwater pulses high frequency sea level fluctuations and the absence of modern oceanic circulation patterns currently reef growth is supported by the loop current the prevailing western boundary current that impinges upon the southwest florida platform providing warm clear low nutrient waters to spr the rare discovery of a preserved non reefal lowstand shoreline capped by rich hermatypic deep reef growth on a tectonically stable continental shelf is significant for both accurate identification of late quaternary sea level position and in better constraining controls on the depth limits of hermatypic reefs and their capacity for adaptation to extremely low light levels

fluid rock interaction during low pressure polymetamorphism of the reynolds range group central australia marbles and metapelites from the reynolds range group central australia were regionally metamorphosed at low pressure during m at 6 go m ranged in grade from gnenschist to granulite fades along the length of the reynolds range and overprinted l 78 ga granites and their contact aureoles in the reynolds range group metasediments at all m 2 grades the marbles and metapelites have highly variable oxygen isotope ratios marbles 5i8o carb 14 20 o metapelites b180 6 14 o similarly l 78 ga granites have highly variable oxygen isotope ratios 8i8o 5 13 o with the lowest values occurring at the granite margins in all rock types the lowest oxygen isotope values are consistent with the infiltration of channelled magmatic and or meteoric fluids the variable lowering of oxygen isotope values resulted from pre mi2 contact metamorphism and fluid rock interaction around the l 78 ga granites in contrast mineral assemblages in the marbles define a trend of increasing xco with increasing grade from 0 05 gnenschist fades to 0 71 0 granulite fades this together with the lack of regionally systematic resetting of oxygen isotope ratios implies that there was little fluid rock interaction during prograde regional metamorphism key word law pressure polymetamorphism fluids stable isotopes petrology

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

new perspectives on the geology and origin of the florida middle ground carbonate banks west florida shelf usa abstract bathymetric and seismic surveys of the florida middle ground area on the west florida shelf aid in identifying the structure and origin of these unusual banks recent cores recovered from the area by other workers provide a useful context for interpreting geophysical data data indicate that high relief banks may be largely biohermal whereas low relief banks are likely composed of clastic sediments with a vermetid gastropod carbonate caprock the most seaward ridges reveal subtle internal aggradational stratal geometries indicating a possible origin as a subtidal shoal or paleoshoreline deposited during or immediately following the younger dryas bank locations appear to be related to the antecedent geology specifically the occurrence of the highly weathered miocene surface high resolution multibeam bathymetry and backscatter data reveal varied morphologies that suggest the influence of karst processes linear trends suggestive of wave reworked intertidal hardbottom poorly lithified and benthic fauna mantled beach ridges or sand waves or perhaps surface water flow patterns during lowstand exposure circular depressions on low banks are of the same scale and morphology as modern shallow karst features in the everglades an episodically active sand wave field occurs in the northern study area and provides sediment to the system scoured depressions surrounding carbonate banks also indicate intense episodic current activity originating from shelf and wave currents a partial modern analog for some of the morphology of this system may be the ten thousand islands and portions of the florida everglades data indicate that the banks represent a brief period of reef growth possibly mixed coral and vermetid constrained by two rapid sea level rise events and impacted by changing water quality and trophic conditions