methanotrophic microbial communities associated with bubble plumes above gas seeps in the black sea bubbles evolving from active gas seeps can be traced by hydroacoustic imaging up to 1000 m high in the black sea water column although methane concentrations are not distinguishable between the water column above the deep seep and reference sites atmospheric noble gas measurements clearly show the constant input of gases mainly methane via seepage into the black sea archaea anme 1 anme 2 and methanotrophic bacteria detected with specific 16s rrna targeted oligonucleotide probes are related to active gas seeps in the oxic and anoxic water column it is suggested that methane seeps have a much greater influence on the black sea methane budget than previously acknowledged and that anme 1 and anme 2 are injected via gas bubbles from the sediment into the anoxic water column mediating methane oxidation our results show further that only minor amounts of methane evolving from black sea gas seeps reach the atmosphere due to the very effective microbial barrier hence only major thermodynamically and or tectonically triggered gas hydrate dissociation has the potential to induce rapid climate changes as suggested by the clathrate gun hypothesis

atmospheric methane flux from bubbling seeps spatially extrapolated quantification from a black sea shelf area bubble transport of methane from shallow seep sites in the black sea west of the crimea peninsula between 70 and 112 m water depth has been studied by extrapolation of results gained through different hydroacoustic methods and direct sampling ship based hydroacoustic echo sounders can locate bubble releasing seep sites very precisely and facilitate their correlation with geological or other features at the seafloor here the backscatter strength of a multibeam system was integrated with single beam data to estimate the amount of seeps m2 for different backscatter intensities resulting in 2709 vents in total direct flux measurements by submersible revealed methane fluxes from individual vents of 0 32 0 85 l min or 14 5 37 8 mmol min at ambient pressure and temperature conditions a conservative estimate of 30 mmol min per site was used to estimate the flux into the water to be 1219 1355 mmol s the flux to the atmosphere was calculated by applying a bubble dissolution model taking release depth temperature gas composition and bubble size spectra into account the flux into the atmosphere 3930 4533 mol d or into the mixed layer 6186 6899 mol d from the 21 8 km2 large study area is three times higher than independently measured fluxes of dissolved methane for the same area using geochemical methods 1030 2495 mol d the amount of methane dissolving in the mixed layer is 2256 2366 mol d this close match shows that the hydroacoustic approach for extrapolating the number of seeps m2 and the applied bubble dissolution model are suitable to extrapolate methane fluxes over larger areas

interaction between a bubble plume and the near field in a stratified lake a steady state bubble plume model is evaluated using full scale temperature salinity and dissolved oxygen data collected in a swiss lake the data revealed a plume generated near field environment that differed significantly from the ambient far field water column properties a near field torus of reduced stratification developed around the plume the extent of which is on the same lateral scale as the horizontal dislocations generated by persistent first mode seiching the plume fallback water was found to penetrate much deeper than expected thereby maintaining reduced vertical gradients in the near field torus the plume entrains a portion of the fallback water leading to short circuiting which generates a complex plume lake interaction and reduces far field downwelling relative to the upward plume flow as the integral plume model incorporates the entrainment hypothesis it is highly sensitive to the near field environmental conditions after identifying appropriate near field boundary conditions the plume model predictions agree well with the field observations

intermediate and deep currents of the black sea obtained from autonomous profiling floats abstract float trajectories observed at three depths 200 750 and 1550 m in the black sea during a 1 5 yr period offer new insights on the circulation below the permanent pycnocline the float observations for the first time provided direct quantitative evidence for strong currents and a well organized flow structure at intermediate and deep layers in contrast to prior ideas of a rather sluggish deep circulation of the black sea the magnitudes of intermediate and deep currents are typically about 5 10 of the surface currents reaching as much as 5 cm s 1 at 1550 m adjacent to the steep topographic slope on the periphery the combination of float and sea level height altimeter data suggests that a well defined cyclonic circulation extends from the surface to the bottom without reversal in its direction deep currents are steered by the steep topographic slope and well correlated with surface currents at seasonal and longer time scales

tectono metamorphic evolution of syros and sifnos islands cyclades greece abstract miocene exhumation of metamorphic rocks in the aegean sea is partly a consequence of post orogenic extension if the post orogenic mechanism of exhumation is rather well understood the earlier syn orogenic eocene exhumation is still largely enigmatic previous authors have argued in terms of extension or compression new structural and petrological data on sifnos and syros islands show that exhumation of high pressure low temperature hp lt rocks involves crustal scale extensional ductile shear zones during the eocene we observe a continuum of top to the ne and e ductile shear from the eocene in the blueschist facies to the miocene in the greenschist facies this deformation is distributed in the eclogites and blueschists whereas it is rather localised along ductile shear zones in the greenschists eclogites which are preserved only at the top of the structural pile are exhumed with a cold retrograde p t path in the lower part of the structural pile we observe a progressive retrogression of eclogites in blueschist then greenschist facies this lower part of pile is subsequently exhumed with p t paths showing a nearly isothermal decompression before cooling p t t deformation data suggest that the cycladic blueschists are progressively exhumed by a continuum of accretion at the base of the orogenic wedge and by a partly non coaxial extensional deformation above distributed during the syn orogenic stage then localised during the post orogenic stage we then compare the mechanism of syn orogenic exhumation of crete and the cyclades and we discuss a simple geodynamic scenario for the aegean domain and the external hellenides which accounts for 1 the southward migration of the hellenic trench and arc during the cenozoic 2 the p t t deformation data for the cycladic blueschists and the phyllite quartzite nappe and 3 the transition from syn orogenic to post orogenic in the cyclades

vorticity analysis in calcite tectonites an example from the attico cycladic massif attica greece abstract although calcite tectonites are widespread in nature their use to quantify flow vorticity is limited we use new micro structural petrofabric and vorticity data to analyse the kinematics of flow in outcrop scale calcite mylonite zones these zones are genetically related to a crustal scale ne directed ductile thrust basal thrust that emplaced the blueschist over the basal unit during the exhumation of the attico cycladic massif calcite microstructures reveal that the last stage of deformation occurred at temperatures 200 300 c achieved by mild heating which is possibly related with the reburial of the basal thrust s footwall vorticity analyses were based on the degree of asymmetry of calcite c axis fabrics as well as on the assumption that the orientation of the long axes of calcite neoblasts within an oblique foliation delineates the direction of instantaneous stretching axis both methodological approaches provide consistent estimates with a simple shear component between 55 and 82 w n 0 76 0 96 the use of the stress axis 1 orientation recorded by twin c axis pairs to quantify vorticity generally gives significantly lower simple shear component comparison of our vorticity estimates with previous estimates inferred from quartz fabrics and rigid porphyroclasts reveals that exhumation related deformation in the nappe pile was steady state

strain path partitioning within thrust sheets microstructural and petrofabric evidence from the moine thrust zone at loch eriboll northwest scotland abstract quartz c axis fabrics and microstructures have been investigated within a suite of quartzites collected from the loch eriboll area of the moine thrust zone and are used to interpret the detailed processes involved in fabric evolution the intensity of quartz c axis fabrics is directly proportional to the calculated strain magnitude a correlation is also established between the pattern of c axis fabrics and the calculated strain symmetry two kinematic domains are recognized within one of the studied thrust sheets which outcrops immediately beneath the moine thrust within the upper and central levels of the thrust sheet coaxial deformation is indicated by conjugate mutually interfering shear bands globular low strain detrital quartz grains whose c axes are aligned sub parallel to the principal finite shortening direction z and quartz c axis fabrics which are symmetric both in terms of skeletal outline and intensity distribution with respect to mylonitic foliation and lineation non coaxial deformation is indicated within the more intensely deformed and recrystallized quartzites located near the base of the thrust sheet by single sets of shear bands and c axis fabrics which are asymmetric with respect to foliation and lineation tectonic models offering possible explanations for the presence of kinematic strain path domains within thrust sheets are considered

using helium and other noble gases in ocean sediments to characterize active methane seepage off the coast of new zealand abstract recently developed analytical techniques to determine the abundances of noble gases in sediment pore water allow noble gas concentrations and isotope ratios to be measured easily and routinely in lacustrine sediments we applied these techniques for the first time to ocean sediments to investigate an active cold methane seepage system located in the south pacific off the coast of the north island of new zealand using 3he 4he ratios determined in the sediment pore water the results show that more 3he rich fluids are released in the vicinity of the pacific australian subduction zone than at the forearc stations located closer to the new zealand coast however the he isotope signature in the sediment column indicates that only a minor part of the he emanating from deeper strata originates from a depleted mantle source hence most he in the pore water is produced locally by the radioactive decay of u and th in the sediment minerals or in the underlying crustal rocks such an occurrence of isotopically heavy crustal he also suggests that the source of the largest fraction of methane is a near surface geochemical reservoir this finding is in line with a previous 13c study in the water column which concluded that the emanating methane is most likely of biological origin and is formed in the upper few meters of the sediment column moreover the prevalence of isotopically heavy he agrees well with the outcome of other previous studies on island arc systems which indicate that the forearc regions are characterized by crustal he emission whereas the volcanic arc region is characterized by the presence of mantle he associated with rising magma

geological and morphological setting of 2778 methane seeps in the dnepr paleo delta northwestern black sea abstract the dnepr paleo delta area in the nw black sea is characterized by an abundant presence of methane seeps during the expeditions of may june 2003 and 2004 within the eu funded crimea project detailed multibeam seismic and hydro acoustic water column investigations were carried out to study the relation between the spatial distribution of methane seeps sea floor morphology and sub surface structures 2778 new methane seeps were detected on echosounding records in an area of 1540 km 2 all seeps are located in the transition zone between the continental shelf and slope in water depths of 66 to 825 m the integration of the different geophysical datasets clearly indicates that methane seeps are not randomly distributed in this area but are concentrated in specific locations the depth limit for the majority of the detected seeps is 725 m water depth which corresponds more or less with the stability limit for pure methane hydrate at the ambient bottom temperature 8 9 c in this part of the black sea this suggests that where gas hydrates are stable they play the role of buffer for the upward migration of methane gas and thus prevent seepage of methane bubbles into the water column higher up on the margin gas seeps are widespread but accurate mapping illustrates that seeps occur preferentially in association with particular morphological and sub surface features on the shelf the highest concentration of seeps is found in elongated depressions pockmarks above the margins of filled channels on the continental slope where no pockmarks have been observed seepage occurs along crests of sedimentary ridges there seepage is focussed by a parallel stratified sediment cover that thins out towards the ridge crests on the slope seepage also appears in the vicinity of canyons bottom flanks and margins or near the scarps of submarine landslides where mass wasting breaches the fine grained sediment cover that acts as a stratigraphic seal the seismic data show the presence of a distinct gas front which has been used to map the depth of the free gas within the sea floor sediments the depth of this gas front is variable and locally domes up to the sea floor where the gas front approaches the seafloor gas bubbles were detected in the water column a regional map of the sub surface depth of the gas front emphasises this gas front versus seep relationship the integration of all data sets indicates that the spatial distribution of methane seeps in the dnepr paleo delta is mainly controlled by the gas hydrate stability zone as well as by stratigraphic and sedimentary factors

the geological significance of 40ar 39ar and rb sr white mica ages from syros and sifnos greece a record of continuous re crystallization during exhumation the attic cycladic crystalline belt in the central aegean region records a complex structural and metamorphic evolution that documents cenozoic subduction zone processes and exhumation a prerequisite to develop an improved tectono metamorphic understanding of this area is dating of distinct p t d stages to evaluate the geological significance of phengite ages of variably overprinted rocks 40ar 39ar and rb sr analyses were undertaken on transitional blueschist greenschist and greenschist facies samples from the islands of syros and sifnos white mica geochronology indicates a large age variability 40ar 39ar 41 27 ma rb sr 34 20 ma petrologically similar samples have either experienced greenschist facies overprinting at different times or variations in ages record variable degrees of greenschist facies retrogression and incomplete resetting of isotopic systematics the 40ar 39ar and rb sr data for metamorphic rocks from both islands record only minor localized evidence for miocene ages c 21 ma that are well documented elsewhere in the cyclades and interpreted to result from retrogression of high pressure mineral assemblages during lower pressure metamorphism field and textural evidence suggests that heterogeneous overprinting may be due to a lack of permeability and or limited availability of fluids in some bulk compositions and that retrogression was more or less parallel to lithological layering and or foliation as a result of possibly deformation enhanced channelized fluid ingress published and new 40ar 39ar and rb sr data for both islands indicate apparent age variations that can be broadly linked to mineral assemblages documenting transitional blueschist to greenschist and or greenschist facies metamorphism the data do not record the timing of peak hp metamorphism but may accurately record continuous partial resetting of isotopic systematics and or re crystallization of white mica during exhumation and greenschist facies retrogression the form of 40ar 39ar phengite age spectra are complex with the lowest temperature steps yielding middle to late miocene ages the youngest rb sr ages suggest maximum ages of 20 6 0 8 ma syros and 22 5 0 6 ma sifnos for the timing of greenschist facies overprinting the results of this study further accentuate the challenges of interpreting isotopic data for white mica from polymetamorphic terranes particularly when mixing of populations and or incomplete resetting of isotopic systematics occurs during exhumation these data capture the full range of isotopic age variations in retrogressed hp rocks documented in previous isotopic studies and can be interpreted in terms of the geodynamic evolution of the aegean

tectonics of the syros blueschists cyclades greece from subduction to aegean extension 1 on syros high pressure metamorphism affects a lithological pile that is composed of from base to top 1 the komito vari granitic basement 2 a margin sedimentary sequence that is predominantly made of marbles and schists the pyrgos and kastri units and 3 the kambos metaophiolitic melange the tectonic history occurred in three main stages during the first stage in the mid eocene the kambos oceanic unit was thrust southward on top of the sedimentary pile top to the south southwest ductile senses of shear are synchronous with prograde high pressure metamorphism and associated with this thrusting event the second stage corresponds to a top to the northeast ductile shear that affects the whole metamorphic pile and is synchronous with the metamorphic retrogression from eclogite to greenschist facies however the kambos oceanic unit remained partly undeformed as shown by significant volumes containing undeformed lawsonite pseudomorphs no major extensional detachment related to this exhumation event outcrops on the island the localized semibrittle to brittle deformation of the third stage is associated with the postmetamorphic development of 1 a ramp flat extensional system at the island scale whose southward minimum displacement is estimated at approximately 7 km and 2 two sets of steeply dipping strike slip faults with a normal component trending either east west or around north south indicating that the mean stretching and shortening directions are trending nne ssw and ese wnw respectively this sequence of major tectonic events and their relationship to metamorphism are interpreted within the framework of the subduction of the pindos ocean and then of the adria continental passive margin

1300 m high rising bubbles from mud volcanoes at 2080m in the black sea hydroacoustic characteristics and temporal variability a mud volcano area in the deep waters 2000 m of the black sea was studied by hydroacoustic measurements during several cruises between january 2002 and june 2004 gas bubbles in the water column give strong backscatter signals and thus can be detected even in great water depths by echosounders as the 38 khz ek500 scientific split beam system that was used during the surveys because of their shape in echograms and to differentiate against geochemical plumes and real upwelling bubble water plumes we call these hydroacoustic manifestations of bubbles in the water column flares digital recording and processing of the data allows a 3d visualization and data comparison over the entire observation period without artefacts caused by changing system settings during our surveys we discovered bubble release from three separate mud volcanoes dvurechenskiy dmv vodianitskiy vmv and the nameless seep site nss in about 2080 m water depth simultaneously bubble release was observed between 9 june 2003 and 5 june 2004 the most frequently surveyed dmv was found to be inactive during very intensive studies in january 2002 the first activity was observed on 27 june 2002 which finally ceased between 5 and 15 june 2004 after a period of continuously decreasing activity this observed 2 yr bubble release period at a mud volcano may give an indication for the duration of active periods the absence of short term variations within days or hours may indicate that the bubble release from the observed mud volcanoes does not undergo rapid changes the recorded echograms show that bubbles rise about 1300 m high through the water column to a final water depth of about 770 m which is 75 m below the phase boundary of pure methane hydrate in the black sea with a release depth from 2068 m and a detected rise height of 1300 m the flare at vmv is among the deepest and highest reported so far and gives evidence of highly extended bubble life times up to 108 min in deep marine environments to better understand how a methane bubble gas analyses of the pore water and gas hydrate gave 99 4 methane can rise so high without dissolving we applied a recently developed bubble dissolution model that takes into account a decreased mass transfer due to an immediately formed gas hydrate rim using the hydroacoustically determined bubble rising speeds 19 22 cm s at the bottom 12 14 cm s at the flare top and the relation between the rising speed of dirty gas hydrate rimmed bubbles and the bubble size we could validate that a gas hydrate rimmed bubble with a diameter of 9 mm could survive the 1300 m rise through the water column before it is finally dissolved a diameter of about 9 mm is reasonable for bubbles released at seep sites and the coincidence between the observed bubble rising speed and the model approach of a 9 mm bubble supports the assumption of gas hydrate rimmed bubbles

pressure temperature evolution of blueschist facies rocks from sifnos greece and implications for the exhumation of high pressure rocks in the central aegean the blueschist and greenschist units on the island of sifnos cyclades were affected by eocene high pressure hp metamorphism using conventional geothermobarometry the hp peak metamorphic stage was determined at 550 600 c and 20 kbar close to the blueschist and the eclogite facies transition the retrograde p t paths are inferred with phase diagrams pseudosections based on a quantitative petrogenetic grid in the model system na2o cao feo mgo al2o3 sio2 h2o reveal coeval decompression and cooling for both the blueschist and the greenschist unit the conditions of the metamorphic peak and those of the retrograde stages conform to a similar metamorphic gradient of 10 12 c km 1 for both units the retrograde overprint can be assigned to low pressure blueschist to hp greenschist facies conditions this result cannot be reconciled with the prograde barrovian type event which affected parts of the cyclades during the oligocene to miocene instead the retrograde overprint is interpreted in terms of exhumation directly after the hp stage without a separate metamorphic event constraints on the exhumation mechanism are given by decompression cooling paths which can be explained by exhumation in a fore arc setting during on going subduction and associated crustal shortening back arc extension is only responsible for the final stage of exhumation of the hp units

natural marine seepage blowout contribution to atmospheric methane the release of methane sequestered within deep sea methane hydrates is postulated as a mechanism for abrupt climate change however whether emitted seabed methane reaches the atmosphere is debatable we observed methane emissions for a blowout from a shallow 22 m hydrocarbon seep the emission from the blowout was determined from atmospheric plume measurements simulations suggest a 1 1 gas loss to dissolution compared to 10 loss for a typical low flux bubble plume transfer to the atmosphere primarily was enhanced by the rapid upwelling flows induced by the massive discharge this mechanism could allow methane suddenly released from deeper 250 m waters to contribute significantly to atmospheric methane budgets copyright 2006 by the american geophysical union

noble gas anomalies related to high intensity methane gas seeps in the black sea dissolved noble gases and tritium were analyzed at a series of high intensity methane gas seeps in the black sea to study the transport and gas exchange induced by bubble streams in the water column these processes affect marine methane emissions to the atmosphere and are therefore relevant to climate warming the seep areas investigated are located in the dnepr paleo delta west of crimea and in the sorokin trough mud volcano area south east of crimea noble gas concentration profiles at active seep sites revealed prominent anomalies compared to reference profiles that are unaffected by outgassing supersaturations of the light noble gases helium and neon observed relatively close to the sea floor are interpreted as effects of gas exchange between the water and the rising bubbles depletions of the heavy noble gases argon krypton and xenon that were detected above an active bubble releasing mud volcano appear to be related to the injection of fluids depleted in noble gases that undergo vertical transport in the water column due to small density differences in both cases the noble gas anomalies clearly document seep specific processes which are difficult to detect by other methods helium is generally enriched in the deep water of the black sea due to terrigenic input although exceptionally high helium concentrations observed in one seep area indicate a locally elevated helium flux most of the seeps studied seem to be negligible sources of terrigenic helium noble gas analyses of sediment pore waters from the vicinity of a mud volcano showed large vertical gradients in helium concentrations the helium isotope signature of the pore waters points to a crustal origin for helium whereas the deep water of the black sea also contains a small mantle type component

oceanic methane layers the hydrocarbon seep bubble deposition hypothesis bubble plumes from hydrocarbon seeps drive upwelling flows in the water column that can disappear if the bubbles dissolve this may lead to formation of a layer enriched in gases and substances transported by the bubbles a process we term bubble deposition a review of observed dissolved methane layers in the north sea showed their existence in an area of active seeping pockmarks at a height of 20 30 m above the sea bed well below the thermocline to test the bubble deposition hypothesis rising seep bubbles were simulated numerically the model predicted a dissolution depth consistent with the observed methane layer for 2700 m radius bubbles the model also predicted that bubbles smaller than 3400 m dissolved subsurface decreasing to 2000 m for a 10 cm s 1 upwelling flow we speculate that this layer may be attractive to marine organisms although north sea seeps are not oily this mechanism also applies to oily bubbles from hydrocarbon seeps or a leaking undersea gas oil pipeline thus bubble deposition can create a subsurface oil layer which rises far slower than either the bubble stream or droplets entrained in the stream

interfering folds in constrictional deformation abstract experiments on buckle folding indicate that in pure constriction 2 3 2 3 there is an association of domes and basins with nonplane noncylindrical folds the areas of domes and basins are reduced with increasing deformation there is a dominant fold trend perpendicular to 3 with a less dominant trend perpendicular to 3 folds at an angle to these two sets may also occur the divergent folds link up in smooth arcs or join up at a dome or a basin over a relatively large domain the interference pattern produced by general constriction can be recognized by the association of domes and basins with nonplane noncylindrical folds by occurrence of hair pin bends of hinge lines of open folds by occurrence of amoeboid outcrop patterns and by absence of a consistent overprinting relation among different sets of folds layers subparallel to the 3 axis of general constriction may give rise to two or more sets of coaxial cylindrical folds with orthogonal axial surfaces

ductile extrusion of the higher himalayan crystalline in bhutan evidence from quartz microfabrics quartz textures measured from deformed quartz tectonites within the lesser himalaya and higher himalaya crystalline of bhutan show similar patterns orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense the asymmetry of c axis and a axis patterns consistently indicates top to the south shearing the obliquity of the texture and the inferred finite strain plane strain to moderately constrictional suggest the strain regime had a combination of rotational and irrotational strain path in most of the samples from the bhutan himalaya the inferred deformation mechanisms suggest moderate to high temperature conditions of deformation that produced the observed crystallographic preferred orientation much higher temperature of deformation is indicated in the quartz veins from a leucogranite the observed ductile deformation is pervasively developed in the rocks throughout the investigated area the intensity of deformation increases only slightly in the vicinity of the main central thrust simultaneous southward shearing within a large part of the higher himalaya crystalline near and above the main central thrust and normal faulting across the south tibetan detachment is explained by the tectonically induced extrusion of a ductily deforming wedge the process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes channel flow accounts for some observed phenomena in the himalayan orogen such as inverted metamorphic sequences near the main central thrust not related to an inversion of isotherms and the syntectonic emplacement of leucogranites into the extruding wedge locally leading to an inversion of isotherms due to heat advection

attenuation of diffusive noble gas transport in laminated sediments of the stockholm archipelago a sediment core has been sampled at a spatial resolution of about two centimeters for noble gas analysis of the sediment pore water this high resolution sampling allowed an excess of dissolved atmospheric gases to be identified in the pore water at a sediment depth of 40 cm sedimentological evidence indicates that the observed gas excess is related to a past mass movement that trapped and conserved the gas surplus for more than a century the measured noble gas concentrations support the hypothesis that depending on the sediment texture or particular structures in the sediment the transport of noble gases in the pore space can be significantly attenuated compared to molecular diffusion in bulk water the atmospheric noble gas abundance in the pore water allows the deposition of sediments originally located near the air water interface to be identified although the nature of the physical processes that limit the fluid transport in the pore space could not be fully assessed our observations indicate that noble gas concentration profiles can be archived for unexpectedly long times in specific sedimentary sequences this finding represents a significant step forward in the interpretation of noble gas concentrations in sediment pore waters in terms of past environmental conditions

arcuate lineation trends in a deep level ductile thrust belt syros greece abstract the metamorphic rocks of blueschist facies on the aegean island of syros show structures which indicate deformation in a ductile thrust zone at 40 50 km depth the associated dominant linear fabric shows systematic variations in orientation across the island tracing out a series of arcuate patterns on various scales differing microstructural development along these lineation arcs suggests that they are the result of localized and variable superimposed wrench shears in the overall thrust regime the angular relations of these microstructures show a close correspondence with those predicted theoretically from the superimposition of thrust and wrench shears and a possible irrotational component

constraints on fluid origins and migration velocities along the marmara main fault sea of marmara turkey using helium isotopes abstract fluids venting from the submarine portion of the marmara main fault part of the north anatolian fault system turkey were sampled in ti bottles deployed by submersible the fluids consist of mixtures of fault derived gases fault related cold seep fluids and ambient seawater these components can readily be distinguished using the isotopes of he and the he ne ratios 3he 4he ratios range between 0 03 0 1 and 4 9 0 4 ra indicating that both crustal and mantle derived sources of helium are sampled by the fault the dominant gas in all the samples analyzed is methane with the abundance of co2 below detection 2 in the mantle rich high 3he 4he fluids this is in contrast to nearly all mantle derived fluids where the c species are dominated by co2 while high ch4 co2 ratios may reflect organic or inorganic reactions within the crust which reduce mantle derived co2 to methane this is not a priori necessary we show that simple dilution of mantle fluids with methane produced within local sediments could result in the high 3he 4he methane rich gases currently emanating from the fault this observation is supported by an anticorrelation between 3he 4he and c 3he which is consistent with addition of c and 4he simultaneously to the fluids the highest 3he 4he ratios were found in the tekirdag basin at the foot of the escarpment bordering the western sea of marmara where seismic data are consistent with the presence of a fault network at depth which could provide conduits permitting deep seated fluids to rise to the surface the lack of recent volcanism or any evidence of underlying magmatism in the area along with low temperature fluids strongly suggests that the 3he rich helium in these fluids was derived from the mantle itself with the marmara main fault providing a high permeability conduit from the mantle to the surface assuming that the mantle source to the fluids originally had a 3he 4he ratio of 6 ra the minimum fluid velocities considering only vertical transport and no mixing with parentless 4he implied by the high 3he 4he ratios are of the order of 1 100 mm yr 1

dissolved noble gases in the porewater of lacustrine sediments as palaeolimnological proxies this paper presents results from the numerical modelling of the transport of atmospheric noble gases he ne ar kr xe tritiated water and 3 he produced by radioactive decay of 3 h in unconsolidated lacustrine sediment two case studies are discussed 1 the evolution of 3 h and 3 he concentrations in the sediment porewater of lake zug switzerland from 1953 up to the present and 2 the response of dissolved atmospheric noble gas concentrations in the sediment porewater of a subtropical lake to an abrupt climatic change that occurred some 10 kyr before the present 1 modelled 3 h and 3 he porewater concentrations are compared with recent data from lake zug an estimate of the effective diffusion coefficients in the sediment porewater is derived using an original approach which is also applicable also to lakes for which the historical 3 h and 3 he concentrations in the water column are unknown 2 the air water partitioning of atmospheric noble gases is sensitive to water temperature and salinity and thus provides a mechanism by which these environmental variables are recorded in the concentrations of atmospheric noble gases in lakes we investigate the feasibility of using noble gas concentrations in the porewater of lacustrine sediments as a proxy for palaeoenvironmental conditions in lakes numerical modelling shows that heavy noble gases in sediment porewater because of their comparatively small diffusion coefficients and the strong temperature sensitivity of their equilibrium concentrations can preserve concentrations corresponding to past lake temperatures over times on the order of 10 kyr noble gas analysis of sediment porewaters therefore promises to yield valuable quantitative information on the past environmental states of lakes copyright 2005 elsevier ltd

subduction and exhumation related structures in the cycladic blueschists insights from south evia island aegean region greece 1 detailed geological mapping structural investigation and amphibole chemistry analyses in southern evia aegean sea greece allow us to place new constraints on the internal structural architecture and tectonic evolution of the cycladic blueschists we show that the early deformation history was related to ese directed thrusting resulting in the stacking of the styra and ochi nappes which constitute the cycladic blueschist unit in evia these early thrust movements initiated just before and proceeded at peak conditions of the eocene high pressure metamorphism subsequent constrictional deformation gave rise to e w trending upright folding accomplished at the early exhumation stage the main ductile stage exhumation occurred during a single deformation phase associated with the decompression of blueschist rocks from the stability field of crossite to that of actinolite this phase was characterized by localization of ductile deformation into a series of major tens of meters thick ene directed shear zones which cut up section in their transport direction and restack the early thrust and fold sequence locally bringing the structurally lower styra nappe over the higher ochi nappe it is suggested that these zones operated as thrusts rather than normal sense shear zones as has been previously argued and were possibly formed during the oligocene ene ward extrusion of the blueschists brittle ductile ne dipping normal faulting of post early miocene age was probably responsible for the final exhumation of rocks

heterogeneous constrictional deformation in a ductile shear zone resulting from the transposition of a lineation parallel fold abstract we use new micro structural petrofabric strain and vorticity data to analyze the deformation path in a mesoscopic quartz mylonite zone the mylonite zone resulted from the complete transposition of a stretching lineation parallel isoclinal fold symmetric cleft girdle quartz c axis fabrics were recorded in the middle domain which occupies the inner limbs of the precursor isoclinal fold while asymmetric cleft and crossed girdle fabrics were observed in the upper and lower domains that represent the outer limbs constrictional strain with increasing k values towards the middle domain is inferred from petrofabric and 3d strain data oblique grain shape fabrics yield vorticity estimates of 0 72 0 90 in the zone however in the middle domain pure shear dominated deformation is suggested by orthorhombic crystallographic fabrics strain rate is constant throughout the zone a strain decrease towards the zone center implies that deformation ceased earlier in the middle domain the data indicates that fold transposition and subsequent mylonitization started as pure shear dominated constrictional deformation and progressively changed to simple shear dominated plane strain during this flow path the asymmetric quartz c axis fabrics likely developed by depopulation of cleft girdle maxima rather than from the synthetic rotation of fabric maxima itself

eohimalayan fold and thrust belt implications for the geodynamic evolution of the nw himalaya india 1 in the himalayas the eocene deformation phase is called the eohimalayan event by means of a forward and backward balanced cross section along a classical traverse through the tethyan himalaya we constrain eohimalayan sw vergent folding and sw directed thrusting in a belt above a basal thrust at 10 km depth this thrust is located at the structural position of the southern tibetan detachment system with a ne directed normal sense displacement suggesting miocene reactivation of a major eocene contractional fault stepwise palinspastic restoration demonstrates that this eocene contractional fault has been subsequently openly folded due to southwestward propagation of the himalayan thrust system

helium systematics of cold seep fluids at monterey bay california usa temporal variations and mantle contributions 1 we report helium isotope ratios 3he 4he as well as helium and neon abundance results for submarine cold seep fluids from extrovert cliff in monterey bay california samples were collected in copper tubing attached to submarine flux meters operating in continuous pumping mode following instrumentation recovery the tubing was sectioned to produce for the first time a high resolution time series of dissolved he and ne variations over a time span of several days noble gas concentrations are variable and appear affected by interaction with a hydrocarbon phase within the aquifer however it is still possible to resolve the he signal into components associated with air equilibration excess air entrainment and terrigenic fluxes both crustal and mantle derived the mantle he contribution reaches 25 30 in some samples up to 2 3 ra where ra air 3he 4he our quasi continuous he ne record shows remarkable fluctuations over time scales of only a few hours and reflects the combined effects of gas stripping by hydrocarbons and an episodic input of mantle derived fluids

himalayan hinterland verging superstructure folds related to foreland directed infrastructure ductile flow insights from centrifuge analogue modelling abstract the orogenic superstructure ss and infrastructure is constitute two levels of a mountain belt with contrasting structural styles in the nepal himalaya n verging back folds which oppose the orogenic vergence dominate the ss competing explanations for these folds are tested using centrifuge analogue models modelling suggests that ss folding occurs during bulk shortening accompanied by is thickening before is flow focused erosion then instigates is lateral flow and stretching decoupling of the ss and transposition of the lower ss into a detachment zone decoupling at the is ss interface separates an ss dominated by older folds and an is characterised by younger horizontal transposition and stretching of early folds extrusive ductile flow of the is locally modifies fold vergence in the ss the fold asymmetry is thus controlled by the efficiency of coupling between is and ss a low viscosity at the is ss interface favours complete decoupling and hinders modification of fold vergence whereas a higher viscosity is ss interface favours fold vergence modification modelling supports a tectonic scenario in which himalayan hinterland verging folds are the product of early shortening of the ss followed by local modification of fold geometry when the is subsequently stretches and flows during focused erosion and melt enhanced is weakening

structural and thermochronometric evidence for multi stage exhumation of southern syros cycladic islands greece abstract the attic cycladic metamorphic core complex province was formed during miocene to recent regional back arc extension in response to slab rollback and gravitational collapse the development of several metamorphic core complexes along extensional low angle detachment faults resulted in the exhumation of lower plate rocks and the tectonic denudation of the upper plate that is only sparsely exposed throughout the aegean sea one of these areas is in southern syros where the tectonic configuration includes three tectonometamorphic units separated by two detachment faults the lower plate cycladic blueschist unit underwent late cretaceous eocene hp lt metamorphism and subsequent greenschist facies overprint during oligo miocene times the tectonically overlying greenschist facies upper unit is a tectonic sliver between the cycladic blueschist unit and the structurally highest unit the vari unit which is dominated by quartzofeldspathic gneisses that are affected by four distinct deformation events d1 d4 the late cretaceous upper greenschist facies d1 episode is related to late cretaceous eocene subduction while d2 d4 fabrics were formed in response to late cenozoic extension and exhumation field relations and structural data suggest that the late miocene brittle vari detachment juxtaposes both vari unit and upper unit against the cycladic blueschist unit mylonitic rocks observed at the base of vari unit along the contact with upper unit and zircon and apatite u th he thermochronometry corroborate this scenario with lower plate cycladic blueschist unit rocks recording rapid cooling at 8 10 ma in contrast cooling ages from the vari unit cluster at 13 15 ma indicating rapid exhumation coeval with activity along the livadi detachment on the neighboring island of tinos these data document new evidence that vari unit was affected by a major middle miocene phase of exhumation and an older semi brittle episode both unrelated to late miocene detachment faulting revealing that crustal extension occurred in multiple stages and in a protracted episodic fashion similar to other parts of the cyclades

fate of rising methane bubbles in stratified waters how much methane reaches the atmosphere there is growing concern about the transfer of methane originating from water bodies to the atmosphere methane from sediments can reach the atmosphere directly via bubbles or indirectly via vertical turbulent transport this work quantifies methane gas bubble dissolution using a combination of bubble modeling and acoustic observations of rising bubbles to determine what fraction of the methane transported by bubbles will reach the atmosphere the bubble model predicts the evolving bubble size gas composition and rise distance and is suitable for almost all aquatic environments the model was validated using methane and argon bubble dissolution measurements obtained from the literature for deep oxic saline water with excellent results methane bubbles from within the hydrate stability zone typically below 500 m water depth in the ocean are believed to form an outer hydrate rim to explain the subsequent slow dissolution a model calibration was performed using bubble dissolution data from the literature measured within the hydrate stability zone the calibrated model explains the impressively tall flares 1300 m observed in the hydrate stability zone of the black sea this study suggests that only a small amount of methane reaches the surface at active seep sites in the black sea and this only from very shallow water areas 100 m clearly the black sea and the ocean are rather effective barriers against the transfer of bubble methane to the atmosphere although substantial amounts of methane may reach the surface in shallow lakes and reservoirs

evolution of a passive crustal scale detachment syros aegean region insights from structural and petrofabric analyses in the hanging wall abstract new detailed micro structural investigations quartz petrofabric analyses and geological structural mapping in southeast syros cycladic massif aegean region allow us to place new constraints on the tectonic evolution of the uppermost unit which occupies the immediate hanging wall of the crustal scale vari detachment we show that the eocene oligocene deformation history in the hanging wall of this detachment is associated with sw directed ductile shearing this history includes an early distributed constrictional deformation expressed by transport parallel upright folds l tectonites and cleft girdles quartz c axis fabrics that were formed at temperatures 500 c ductile deformation progressively localized at the bottom of the uppermost unit leading to the formation of a greenschist facies mylonitic zone under plane strain conditions inferred from type i cross girdles quartz c axis fabrics the ongoing mylonitization was also associated with temporally increasing pure shear component of deformation coupled with cooling from 500 c to 400 c we suggest that the vari detachment represents a passive normal sense roof fault resulted from the ne directed ductile extrusion of the blueschist unit footwall at middle eocene oligocene times in the middle miocene the activation of the brittle ssw directed late vari detachment enhanced the brittle exhumation of both the blueschist unit and the vari detachment

noble gas signals in corals predict submarine volcanic eruptions abstract deep water corals growing in close proximity to the 2011 submarine eruption at tagoro el hierro island canary archipielago spain have revealed their ability to record the magmatic helium 3 he signal during a period of months prior to the eruption with magmatic 3 he 4 he ratios of 3 6 5 0 r a this is similar to the range of he isotope values 3 10 ra obtained from olivine samples from basinites erupted during the 2011 tagoro eruption whip like black corals anthozoa antipatharia antipathidae stichopathes growing on basaltic rocks trapped 3 he within their skeletons we used a theoretical growth rate to estimate the time of trapping of 3 he between release from the magmatic source at depth to trapping by the aquatic organisms our findings suggest that magmatic 3 he emission occurred a few months before the beginning of the seismic activity in the region the latter occurring four months in advance of the beginning of the eruption this discovery implies that corals living in submarine volcanic areas may act as archives of noble gases released before the beginning of an eruption and that the continuous monitoring of 3 he may help to constrain the arrival time of a subaqueous eruption with potential applications in volcano eruption forecasting

correction to noble gases and radiocarbon in natural gas hydrates by gisela winckler werner aeschbach hertig johannes holocher rolf kipfer ingeborg levin christian poss gregor rehder erwin suess and peter schlosser

rheological weakening of olivine orthopyroxene aggregates due to phase mixing part 2 microstructural development to understand the processes involved in phase mixing during deformation and the resulting changes in rheological behavior we conducted torsion experiments on samples of iron rich olivine plus orthopyroxene the experiments were conducted at a temperature t of 1200 c and a confining pressure p of 300 mpa using a gas medium deformation apparatus samples composed of olivine plus 26 orthopyroxene were deformed to outer radius shear strains up to 26 in samples deformed to lower strains of 4 elongated olivine and pyroxene grains form a compositional layering already by this strain mixtures of small equant grains of olivine and pyroxene begin to develop and continue to evolve with increasing strain the ratios of olivine to pyroxene grain size in deformed samples follow the zener relationship indicating that pyroxene grains effectively pin the grain boundaries of olivine and inhibit grain growth due to the reduction in grain size the dominant deformation mechanism changes as a function of strain the microstructural development forming more thoroughly mixed fine grained olivine pyroxene aggregates can be explained by the difference in diffusivity among me fe or mg o and si with transport of meo significantly faster than that of sio2 these mechanical and associated microstructural properties provide important constraints for understanding rheological weakening and strain localization in upper mantle rocks

devonian orogen parallel opposed extension in the central norwegian caledonides late orogenic early to middle devonian extension in the scandinavian caledonides was unidirectional in western norway new data from two detachment zones hoybakken and kollstraumen north of the more trondelag fault complex show that bidirectional opposed orogen parallel extension dominated in this region at this time the fault complex acted as a transfer zone for the hoybakken detachment extension and uplift in central norway triggered significant magmatic activity in contrast to the lack of granite intrusions during exhumation of western norway

high precision lu hf geochronology of eocene eclogite facies rocks from syros cyclades greece abstract garnet bearing high pressure low temperature hp lt metamorphic rocks of the cycladic blueschist unit have been investigated by lu hf geochronology eclogites from syros island yield precise ages of 52 2 0 3 ma 51 4 0 4 ma and 50 2 ma preserved major and trace element growth zoning in garnet suggests that the closure temperature of the lu hf system in garnet was higher than the estimated peak metamorphic temperature of 500 c hence lu hf ages most likely reflect garnet growth in the studied samples our new lu hf grt ages are in excellent agreement with previously published u pb shrimp ages of metamorphic zircon and the maximum reported white mica rb sr and ar ar ages from syros garnet growth in all samples apparently took place over a narrow time interval because the lu hf ages cluster tightly even though the distribution of lu between cores and rims varies among samples we find no evidence for garnet generations that are significantly older or younger than the given isochron ages this in conjunction with the agreement among the ages of metamorphic zircon the maximum white mica ages and the lu hf ages strongly suggests that the 52 ma age dates a single hp event that affected these rocks

carbon release from submarine seeps at the costa rica fore arc implications for the volatile cycle at the central america convergent margin 1 we report total dissolved inorganic carbon dic abundances and isotope ratios as well as helium isotope ratios 3he 4he of cold seep fluids sampled at the costa rica fore arc in order to evaluate the extent of carbon loss from the submarine segment of the central america convergent margin seep fluids were collected over a 12 month period at mound 11 mound 12 and jaco scar using copper tubing attached to submarine flux meters operating in continuous pumping mode the fluids show minimum 3he 4he ratios of 1 3 ra where ra is air 3he 4he consistent with a small but discernable contribution of mantle derived helium at mound 11 13c co2 values between 23 9 and 11 6 indicate that dic is predominantly derived from deep methanogenesis and is carried to the surface by fluids derived from sediments of the subducting slab in contrast at mound 12 most of the ascending dissolved methane is oxidized due to lower flow rates giving extremely low 13c co2 values ranging from 68 2 to 60 3 we estimate that the carbon flux co2 plus methane through submarine fluid venting at the outer fore arc is 8 0 105 g c km 1 yr 1 which is virtually negligible compared to the total sedimentary carbon input to the margin and the output at the volcanic front unless there is a significant but hitherto unidentified carbon flux at the inner fore arc the implication is that most of the carbon being subducted in costa rica must be transferred to the deeper mantle i e beyond the depth of arc magma generation

thrust or detachment exhumation processes in the aegean insight from a field study on ios cyclades greece 1 whether back arc extension in the aegean is symmetric or asymmetric at the regional scale is a key question to the understanding of the dynamics of back arc extension new field observations structural analysis of finite and instantaneous strain and their relationship with metamorphism on ios island cyclades lead us to investigate the significance of the south cycladic shear zone scsz two successive tectonometamorphic events have been documented in the hanging wall and the footwall of the scsz first penetrative top to the south shearing is observed in the upper cycladic blueschists and the lower cycladic basement close to their contact it is expressed in the hanging wall by shear bands and pressure shadows associated to blueschist facies minerals contemporaneous with overthrusting of the cycladic blueschists over the basement unit second large extensional top to the north shear zones lately crosscut the whole stack these constraints allow us to reinterpret the kinematics of the aegean domain during synorogenic and postorogenic periods when replaced in the reconstructed geometry of the hellenic eocene orogenic wedge the observed top to the south thrusting is coeval with extensional shear zones observed in syros that could represent the roof of the cycladic blueschists an extrusion structure is then proposed for the exhumation of the cycladic blueschists in the subduction zone top to the north asymmetric extension together with the andros tinos mykonos and paros naxos detachments can be correlated to oligo miocene extension in the back arc domain

coaxial flattening at deep levels of orogenic belts evidence from blueschists and eclogites on syros and sifnos cyclades greece this work presents new structural data from a high pressure low temperature hp lt metamorphic terrane exposed on the islands of syros and sifnos cyclades greece the structure and the metamorphism of a relatively coherent hp lt rock section were studied in order to elucidate how strain was accommodated at deep crustal levels during the formation and exhumation of hp lt rocks at least three deformation phases associated with eclogite and blueschist facies conditions p 8 15 kbar t 400 550 degreesc were recognised the earliest deformation fabric s1 preserved as inclusion trails within garnet porphyroblasts is aligned to define a sub vertical schistosity at present orientation which is frequently orthogonal to the flat matrix schistosity s2 and may indicate that deep crustal thickening involved upright folding the currently dominant fabric in the hp rock section s2 is usually moderately dipping and locally contains nw trending glaucophane lineations symmetric pressure shadows and eclogitic boudins the symmetric structures associated with this fabric seem to indicate coaxial vertical thinning although the existence of non coaxial structures out of the study area cannot be excluded glaucophane bearing shear bands s3 with top to nw sense of shearing locally crosscut the earlier structures the latest recognised fabric d4 is scarce and often absent within the hp rocks it is associated with top to ne kinematic criteria that formed at greenschist facies conditions p 4 7 kbar t 400 450 degreesc based on these observations it is suggested that partitioning of strain occurred at different crustal levels and at different times deep crustal deformation was governed by thickening via upright folding followed by coaxial vertical thinning whereas non coaxial shearing occurred when the rocks were already exhumed to relatively shallow crustal levels the earliest fabrics d1 to d3 pertain to alpine orogenesis and possibly to syn orogenic extension whereas the latest correspond to whole crust back are extension c 2002 elsevier science ltd all rights reserved

structural development of high pressure metamorphic rocks on syros island cyclades greece abstract new data obtained during re mapping of the northern part of syros cyclades greece allow the refinement of existing concepts about the structural evolution of this island a metabasite belt and its metasedimentary envelope near kampos and san michali were mapped at a 1 5000 scale with special emphasis on the structural inventory the hp lt rocks p 1 5 gpa t 500 c at 50 ma exposed are intensely deformed by at least two isoclinal folding events relative age relations between deformation and peak metamorphism indicate that isoclinal folding took place before or during peak metamorphism later deformation stages include miocene non penetrative upright kink folding and crenulation transpressional strike slip faulting and open upright cylindrical folding followed by the development of steep normal faults taking into account recent zircon u pb geochronological constraints lithostratigraphic observations and published data on the neogene structural evolution of the aegean we present a history for the rocks of syros beginning with the formation of the oceanic crust represented by the metabasites to the present

prism and basal plane parallel subgrain boundaries in quartz a microstructural geothermobarometer empirical data on quartz subgrain patterns from various metamorphic rocks show that at least up to 10 kbar in the stability field of low quartz prismatic subgrain boundaries are dominant whereas basal subgrain boundaries are not developed in the high quartz stability field both prismatic and basal subgrain boundaries occur and form typical rectangular chessboard patterns the likely reason behind the different occurrence of these subgrain patterns is that in high quartz prismatic glide becomes as easy as or probably even easier than basal glide the two types of subgrain patterns can be clearly distinguished by optical microscopy consequently the occurrence of chessboard subgrain patterns in quartz represents a practicable geothermobarometer the possibilities of its application are far reaching and include the specification of deformation conditions at high grade metamorphism the recognition of syntectonic intrusions and the distinction between pluton emplacement at lower and at higher crustal levels

pycnocline and deep mixing in the black sea stable isotope and transient tracer measurements abstract the peculiar mixing processes at the pycnocline and deep regions of the black sea are reviewed in addition to the wind stirring and convective mixing active in the upper pycnocline the other important mechanism that results in limited ventilation of the anoxic waters of the black sea is the mediterranean dense water inflow from the bosphorus modified by the entrainment of surface and intermediate waters introduced into the interior through double diffusive intrusions this inflow aided by the surface ekman flux divergence boundary processes and internal wave breaking is the main mechanism for the mixing and renewal of the sub pycnocline waters in the black sea interior a review of these mixing processes is complemented by results from isotope measurements with improved accuracy and reduced noise compared to earlier experiments measurements of the stable isotopes oxygen 18 18 o and deuterium 2 h confirm the origin of the water masses in the black sea and in the turkish straits including the sea of marmara to be a continuous mixture with variable fractions of salty mediterranean waters with inflowing fresh waters tritium 3 h measurements confirm very little penetration of the transient signal to the sub pycnocline and deep waters of the black sea in comparison to the better ventilated mediterranean waters filling the lower layer of the marmara sea the comparison of stable and transient tracer isotopes shows the effects of fresh waters originating from the north western shelf and the difference between the renewal mechanisms of the marmara and black seas as well as those between the upper and lower pycnocline of the black sea

the partitioning of deformation in flowing rock masses abstract most geological structures owe their development to heterogeneous and or non steady flow flow partitioning refers to the division of the instantaneous velocity field into components related to translation strain and rotation flow partitioning during the development of a geological structure usually changes with time and different histories of flow are involved at different points in the developing structure heterogeneity leads to the development of flow domains with different characteristics notably the degree of non coaxiality of deformation and different partitionings of the rotational component of flow into shear induced vorticity and spin during non steady flow there is often continual repartitioning of vorticity between the spin and shear induced components spin is an important aspect of flow leading to the development of structure this paper discusses the partitioning of deformation in a flowing rock mass coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries non coaxial flow is favoured by suitably oriented material anisotropy and or the presence of well defined boundary constraints involving parallel relative displacements a developing anisotropy e g a crystallographic fabric in a shear zone can induce a change from coaxial to non coaxial flow other effects are related to the scale of approximation for example bulk shear induced vorticity can be locally converted to spin flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response the factors that affect flow partitioning e g spin versus non coaxial deformation are therefore of interest

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred