teleseismic earthquake swarms at ultraslow spreading ridges indicator for dyke intrusions earthquakes at mid ocean ridges reflect the active magmatic and tectonic processes that form new oceanic crust studies of large earthquakes observed on land and smaller earthquakes observed locally or regionally by ocean bottom seismometers or autonomous underwater hydrophones have greatly contributed to our understanding of the structure and active spreading processes at the mid ocean ridges of the atlantic and pacific ocean opening with velocities in excess of 25 mm yr 1 at spreading rates below 20 mm yr 1 full rate the appearance and the accretion processes of mid ocean ridges undergo fundamental changes as the melt supply is drastically reduced the active spreading processes at these so called ultraslow spreading ridges are still poorly known as the main representatives the arctic ridge system and the southwest indian ridge are poorly accessible and neither autonomous underwater hydrophone nor ocean bottom seismometer records of local seismicity are available in an attempt to compare on a large scale the accretion style of ultraslow spreading ridge sections i analyse the teleseismically recorded seismicity in 11 sections of the arctic ridge system and the southwest indian ridge spanning altogether 7200 km epicentres located within 30 35 km of the rift axis were extracted from the catalogue of the international seismological centre for a time period of 35 yr on the basis of a single link cluster analysis i identified 27 swarms with eight or more events these swarms occur almost exclusively at centres of focussed magmatism suggesting that the swarms are probably initiated by magmatism normal faults along several tens of kilometres surrounding the volcanic centres react in large earthquakes m 5 to dyke emplacement the routine generation of large earthquakes in the cold brittle lithosphere of ultraslow spreading ridges makes the teleseismic record a valuable means to study ultraslow accretion processes and to provide a global framework for the interpretation of the limited local and regional seismicity studies

how many vent fields new estimates of vent field populations on ocean ridges from precise mapping of hydrothermal discharge locations abstract decades of exploration for venting sites along spreading ridge crests have produced global datasets that yield estimated mean site spacings of 12 220 km this conclusion demands that sites where hydrothermal fluid leaks from the seafloor are improbably rare along the 66 000 km global ridge system despite the high bulk permeability of ridge crest axes however to date exploration methods have neither reliably detected plumes from isolated low temperature particle poor diffuse sources nor differentiated individual closely spaced clustered within a few kilometers sites of any kind here we describe a much lower mean discharge spacing of 3 20 km revealed by towing real time oxidation reduction potential and optical sensors continuously along four fast and intermediate rate 55 mm yr spreading ridge sections totaling 1470 km length this closer spacing reflects both discovery of isolated sites discharging particle poor plumes 25 of all sites and improved discrimination at a spatial resolution of 1 km among clustered discrete and diffuse sources consequently the number of active vent sites on fast and intermediate rate spreading ridges may be at least a factor of 3 6 higher than now presumed this increase provides new quantitative constraints for models of seafloor processes such as dispersal of fauna among seafloor and crustal chemosynthetic habitats biogeochemical impacts of diffuse venting and spatial patterns of hydrothermal discharge

apparent polar wander paths of the major chinese blocks since the late paleozoic toward restoring the amalgamation history of east eurasia abstract high quality paleomagnetic poles a k a paleopoles are essential for quantitative lithospheric plate reconstruction however the current paleomagnetic database of the major chinese blocks including the north china block ncb the south china block scb and the tarim block tb since the late paleozoic era contain outdated and low quality data here we update the database by adding recently published high quality paleopoles and rejecting low quality outdated ones the database includes 288 paleopoles published 1980 2014 90 of them published after 2000 following the van der voo selection criteria 75 paleopoles each with a quality factor q smaller than 4 were rejected in the first round of selection we then removed another 59 paleopoles that have been locally rotated or shallowed since the acquisition of stable remanence eventually 154 paleopoles were selected and adopted to calculate the new apparent polar wander paths apwps for the major chinese blocks we found comparable clastic and igneous paleopoles at intervals when a quantitative comparison was available as such comparison was not possible during most periods epochs our conclusions reflect an unclear extent of inclination errors in chinese clastic paleopoles new models of the chinese apwps with and without inclination error corrections were computed from carefully selected paleopoles version 1 running mean v1rm paths were calculated at the period epoch level version 2 running mean v2rm paths were computed in a set sliding time window of 20 or 30 myr spline paths were calculated with the same time windows along with a smoothing parameter of 50 using a recent global reconstruction model and up to date geological observations new models of the chinese apwps allowed us to re evaluate the coalescence history of east eurasia since the late paleozoic era four major tectonic events were confirmed 1 the tb accreted with the kazakhstan orocline during amalgamation of the west altaids during the middle late permian period ca 265 250 ma 2 the suturing of the ncb and the scb likely occurred in a scissor like pattern and had been accomplished no later than the middle jurassic period ca 180 160 ma 3 the amalgamation between the ncb and the tb along with the microblocks between the two might have been achieved during the late jurassic early cretaceous periods ca 160 140 ma 4 the mongol okhotsk ocean should have been closed no later than the early cretaceous period ca 140 120 ma

multiple zircon growth during fast exhumation of diamondiferous deeply subducted continental crust kokchetav massif kazakhstan diamondiferous rocks from the kokchetav massif kazakhstan represent deeply subducted continental crust in order to constrain the age of ultra high pressure uhp metamorphism and subsequent retrogression during exhumation zircons from diamondiferous gneisses and metacarbonates have been investigated by a combined petrological and isotopic study four different zircon domains were distinguished on the basis of transmitted light microscopy cathodoluminescence trace element contents and mineral inclusions mineral inclusions and trace element characteristics of the zircon domains permit us to relate zircon growth to metamorphic conditions domain 1 consists of rounded cores and lacks evidence of uhp metamorphism domain 2 contains diamond coesite omphacite and titanian phengite inclusions providing evidence that it formed at uhp metamorphic conditions p 43 kbar t 950 c domain 3 is characterised by low pressure mineral inclusions such as garnet biotite and plagioclase which are common minerals in the granulite facies overprint of the gneisses p 10 kbar t 800 c this multi stage zircon growth during cooling and exhumation of the diamondiferous rocks can be best explained by zircon growth from zr saturated partial melts present in the gneisses domain 4 forms idiomorphic overgrowths and the rare earth element pattern indicates that it formed without coexisting garnet most probably at amphibolite facies conditions p 5 kbar t 600 c the metamorphic zircon domains were dated by shrimp ion microprobe and yielded ages of 527 5 528 8 and 526 5 ma for domains 2 3 and 4 respectively these indistinguishable ages provide evidence for a fast exhumation beyond the resolution of shrimp dating the mean age of all zircons formed between uhp metamorphic conditions and granulite facies metamorphism is 528 3 ma indicating that decompression took place in less than 6 ma hence the deeply subducted continental crust was exhumed from mantle depth to the base of the crust at rates higher than 1 8 cm year we propose a two stage exhumation model to explain the obtained p t t path fast exhumation on top of the subducted slab from depth 140 to 35 km was driven by buoyancy and facilitated by the presence of partial melts a period of near isobaric cooling was followed by a second decompression event probably related to extension in a late stage of continental collision

seismic character of volcanic activity at the ultraslow spreading gakkel ridge never before has a volcanic eruption on a slow or ultraslow spreading mid ocean ridge been both observed seismically and confirmed on the seafloor during the first half of 1999 a long lived volcanic spreading event occurred on the ultraslow spreading gakkel ridge in the arctic ocean the seismicity associated with this event was unprecedented in duration and magnitude for a seafloor eruption sonar images from the u s s hawkbill which passed over the area within four months of the start of activity are consistent with the presence of a large recently erupted flow and a volcanic peak directly in the area of seismic activity seismic activity began in mid january and continued vigorously for three months a reduced rate of activity persisted for an additional four months or more in total 252 events were large enough to be recorded on global seismic networks although a limited number of volcanic spreading events have been observed globally the duration and magnitude of the gakkel ridge swarm when compared with volcanic seismicity at ridges spreading at intermediate and fast spreading rates suggest that a negative power law relationship may exist between these parameters and spreading rate fault activation in response to magmatic emplacement appears to have occurred over a broad region suggesting that magma may have been tapped from mantle depths the slow migration of the largest magnitude events along the axis of the rift valley suggests multiple magmatic pulses at depth in combination with bathymetric setting and sidescan sonar confirmation the seismic data for this event have provided a unique look at the scale and character of eruption processes at ultraslow spreading rates

deep subduction and rapid exhumation role of crustal strength and strain weakening in continental subduction and ultrahigh pressure rock exhumation 1 the exhumation of crustal ultra high pressure uhp material depends on temporal and spatial variations in its detachment within the subduction channel this dependence is investigated using numerical models with variable initial crustal strengths representing a range of initial crustal compositions and parameterized strain weakening representing a range of processes that reduce effective crustal viscosity during deformation competition between down channel shear traction favoring subduction and up channel buoyancy favoring exhumation is expressed as the exhumation number e which can vary with time and position along the channel exhumed lower strength crust which resists subduction owing to weak down channel traction records peak conditions 38 kbar given sufficient strain weakening exhumation proceeds at 60 km ma 1 indicating that buoyancy e 1 drives exhumation in these models in all models exhuming uhp material forms a deforming ductile plume with a range of possible structural relationships predicted between exhumed uhp and hp materials

the remarkable longevity of submarine plumes implications for the hydrothermal input of iron to the deep ocean the longevity of submarine plumes generated at sea floor hydrothermal systems constrains the hydrothermal input of chemical species into the deep ocean decades of observations of episodic event plumes suggest that a key process governing the dynamics of an hydrothermal cloud spreading out laterally from a buoyant rising plume is the production of internal layering here we use analog experiments on turbulent hot particle laden plumes and clouds to show that this layering occurs where particle diffusive convection driven by the differential diffusion of heat and small mineral precipitates gives rise to a large scale double diffusive instability where hydrothermal clouds are enriched in fine minerals this particle diffusive convection can extend the longevity of an event plume to 2 yr after its emplacement the very long residence time imposed by diffusive convective effects enables complete dissolution of fine sulfide and sulfate minerals we develop a new theoretical model that includes both sedimentation and dissolution processes to quantify the potential amount of iron produced by the dissolution of iron sulfide minerals settling through the cloud by diffusive convection a key prediction is that the concentration of dissolved iron in hydrothermal clouds can reach up to 19 3 nm19 3 nm which represents about 5 of the global hydrothermal discharge if these results are representative of all hydrothermal vent fields hydrothermal systems could provide 75 of the global budget of dissolved iron in the deep ocean regionally this flux is expected to scale in magnitude with mid ocean ridge heat flow consistent with observations and global ocean models

on the rise of turbulent plumes quantitative effects of variable entrainment for submarine hydrothermal vents terrestrial and extra terrestrial explosive volcanism 1 the maximum height reached by a turbulent plume rising in a stratified environment is a key tool to estimate the flux released at its source particularly for large scale flows because flux can often be very hard to measure directly this height is known to be mainly controlled by the stratification of the ambient fluid source buoyancy flux and the efficiency of turbulent mixing between the plume and the external fluid the latter effect has been only superficially explored in spite of its fundamental control on the dynamics here we show that commonly used one dimensional models incorporating a constant entrainment coefficient do not provide satisfying predictions we propose a new model allowing for variable entrainment which gives excellent predictions of maximum heights reached by laboratory plumes in stratified environments we then apply our formalism to natural plumes produced by explosive volcanic eruptions under terrestrial paleo martian and venusian conditions and by submarine hydrothermal activity at mid ocean ridges source mass discharge rates deduced from maximum volcanic column heights for terrestrial eruptions are found to be greater than those estimated with the commonly used constant entrainment parameterization by a factor of 2 in the paleo martian atmosphere existing models overestimate plume heights by 14 27 in the current atmosphere of venus the maximum height reached by a volcanic plume is also found to be smaller than previously estimated for large eruption rates the source heat flux released by the tag field atlantic ocean deduced from several submarine hydrothermal plumes is found greater by a factor 3 with our model

a comparison of the evolution of diamondiferous quartz rich rocks from the saxonian erzgebirge and the kokchetav massif are so called diamondiferous gneisses magmatic rocks abstract the petrography and chemical composition of minerals of quartz rich diamondiferous rocks from the kokchetav massif especially the zonation of garnet were studied and compared with diamondiferous quartzofeldspathic rocks from the saxonian erzgebirge many compositional and textural features were found to be similar for instance microdiamonds are enclosed systematically in a specific intermediate growth zone of garnet in these rocks on the basis of experimental data a magmatic scenario was constructed to check if the quartz rich diamondiferous rocks are of magmatic origin by this the p t paths derived here for the kokchetav rocks and the textural observations it is concluded that the minerals of the diamondiferous rocks have crystallized from silicate melts these melts originated by anatexis of deeply submerged metasediments erzgebirge at t as high as 1200 c kokchetav massif at 50 100 c lower t and ascended from at least 200 km depth relics of the pre anatectic evolution are still present for instance as garnet cores after ascent and emplacement of the magma in deep portions of thickened continental crust kokchetav massif 45 50 km close to 800 c erzgebirge 55 60 km at 30 50 c lower t considerable quantities of white and or dark micas formed by peritectic reactions from melt for instance garnets could be resorbed at this stage and biotite grew instead after the magmatic stage retrogression took place much stronger in the kokchetav massif this was accompanied by deformation transforming broadly the magmatic texture of quartz rich diamondiferous rocks from the kokchetav massif to a gneissic texture

recycling of crustal materials through study of ultrahigh pressure minerals in collisional orogens ophiolites and mantle xenoliths a review abstract newly recognized occurrences of ultrahigh pressure uh p minerals including diamonds in ultrahigh temperature uh t felsic granulites of orogenic belts in chromitites associated with ophiolitic complexes and in mantle xenoliths suggest the recycling of crustal materials through deep subduction mantle upwelling and return to the earth s surface this circulation process is supported by crust derived mineral inclusions in deep seated zircons chromites and diamonds from collision type orogens from eclogitic xenoliths in kimberlites and from chromitities of several alpine himalayan and polar ural ophiolites some of these minerals contain low atomic number elements typified by crustal isotopic signatures ophiolite type diamonds in placer deposits and as inclusions in chromitites together with numerous highly reduced minerals and alloys appear to have formed near the mantle transition zone in addition to ringwoodite and inferred stishovite a number of nanometric minerals have been identified as inclusions employing state of the art analytical tools reconstitution of now exsolved precursor uh p phases and recognition of subtle decompression microstructures produced during exhumation reflect earlier uh p conditions for example tibetan chromites containing exsolution lamellae of coesite diopside suggest that the original chromitites formed at p 9 10 gpa at depths of 250 300 km the precursor phase most likely had a ca ferrite or a ca titanite structure both are polymorphs of chromite and at 2000 c would have formed at minimum pressures of p 12 5 or 20 gpa respectively some podiform chromitites and host peridotites contain rare minerals of undoubted crustal origin including zircon feldspars garnet kyanite andalusite quartz and rutile the zircons possess much older u pb ages than the time of ophiolite formation these uh p mineral bearing chromitite hosts evidently had a deep seated evolution prior to extensional mantle upwelling and partial melting at shallow depths to form the overlying ophiolite complexes these new findings together with stable isotopic and inclusion characteristics of diamonds provide compelling evidence for profound underflow of both oceanic and continental lithosphere recycling of surface organic carbon into the lower mantle and ascent to the earth s surface through mantle upwelling intensified study of uh p granulite facies lower crustal basement and ophiolitic chromitites should allow a better understanding of the geodynamics of subduction and crustal cycling

dating the indian continental subduction and collisional thickening in the northwest himalaya multichronology of the tso morari eclogites multichronometric studies of the low temperature eclogitic tso morari unit ladakh india place timing constraints on the early evolution of the northwest himalayan belt several isotopic systems have been used to date the eclogitization and the exhumation of the tso morari unit lu hf sm nd rb sr and ar ar a ca 55 ma age for the eclogitization has been obtained by lu hf on garnet omphacite and whole rock from mafic eclogite and by sm nd on garnet glaucophane and whole rock from high pressure metapelites these results agree with a previously reported u pb age on allanite and together these ages constrain the subduction of the indian continental margin at the paleocene eocene boundary during exhumation the tso morari rocks underwent thermal relaxation at about 9 3 kbar characterized by partial recrystallization under amphibolite facies conditions ca 47 ma as dated by sm nd on garnet calcic amphibole and whole rock from metabasalt rb sr on phengite apatite and whole rock and ar ar on medium si phengite from metapelites ar ar analyses of biotite and low si muscovite from metapelites which recrystallized at 5 kbar toward the end of the exhumation show that the tso morari unit was at upper crustal levels ca 30 ma these results indicate variable exhumation rates for the tso morari unit beginning with rapid exhumation while the indian margin subduction was still active and later proceeding at a slower pace during the crustal thickening associated with the himalayan collision

a crustal model of the ultrahigh pressure dabie shan orogenic belt china derived from deep seismic refraction profiling we present a new crustal cross section through the east west trending ultrahigh pressure uhp dabie shan orogenic belt east central china based on a 400 km long seismic refraction profile data from our profile reveal that the cratonal blocks north and south of the orogen are composed of 35 km thick crust consisting of three layers upper middle and lower crust with average seismic velocities of 6 0 6 0 2 km s 6 5 6 0 1 km s and 6 8 6 0 1 km s the crust reaches a maximum thickness of 41 5 km beneath the northern margin of the orogen and thus the present day root beneath the orogen is only 6 5 km thick the upper mantle velocity is 8 0 6 0 1 km s modeling of shear wave data indicate that poisson s ratio increases from 0 24 6 0 02 in the upper crust to 0 27 6 0 03 in the lower crust this result is consistent with a dominantly felsic upper crustal composition and a mafic lower crustal composition within the amphibolite or granulite metamorphic facies our seismic model indicates that eclogite which is abundant in surface exposures within the orogen is not a volumetrically significant component in the middle or lower crust much of the triassic structure associated with the formation of the uhp rocks of the dabie shan has been obscured by post triassic igneous activity extension and large offset strike slip faulting nevertheless we can identify a high velocity 6 3 km s zone in the upper 5 km depth crustal core of the orogen which we interpret as a zone of ultrahigh pressure rocks a north dipping suture and an apparent moho offset that marks a likely active strike slip fault

theory applied to the mid atlantic ridge hydrothermal plumes the finite difference approach a system of differential equations describing the entrainment of ambient seawater during the rise of a buoyant hydrothermal plume has been solved by an explicit finite difference method chosen for its ease of application an equation for tracer concentration has been added so that concentrations in the neutrally buoyant plume can be calculated for those elements whose background profiles vary over the depth range of the buoyant plume the height of plume rise can be related directly to the thermal and hydrothermal vent flux for the tag vent field 26 n mar five plume layers have been identified the calculated heat flux is 500 940 mw and the fluid flux is 0 210 0 392 m3 s 1 the fluid flux to the neutrally buoyant plume is 1460 2740 m3 s 1 calculated plume concentrations of mn compared with measurements made at tag in 1988 indicate that there is either 50 removal of manganese in the buoyant plume or that low temperature vent fluids contribute about 50 of the total thermal output of the tag vent field

timing of himalayan ultrahigh pressure metamorphism sinking rate and subduction angle of the indian continental crust beneath asia coesite relics were discovered as inclusions in clinopyroxene in eclogite and as inclusions in zircon in felsic and pelitic gneisses from higher himalayan crystalline rocks in the upper kaghan valley north west himalaya the metamorphic peak conditions of the coesite bearing eclogites are estimated to be 27 32 kbar and 700 770 c using garnet pyroxene phengite geobarometry and garnet pyroxene geothermometry respectively cathodoluminescence cl and backscattered electron bse imaging distinguished three different domains in zircon inner detrital core widely spaced euhedral oscillatory zones and thin broadly zoned outermost rims each zircon domain contains a characteristic suite of micrometre sized mineral inclusions which were identified by in situ laser raman microspectroscopy core and mantle domains contain quartz apatite plagioclase muscovite and rutile in contrast the rim domains contain coesite and minor muscovite quartz inclusions were identified in all coesite bearing zircon grains but not coexisting with coesite in the same growth domain rim domain 206 pb 238 u zircon ages reveal that the quartz bearing mantle domains and the coesite bearing rim were formed at c 50 ma and 46 2 0 7 ma respectively these facts demonstrate that the continental materials were buried to 100 km within 7 9 myr after initiation of the india asia collision palaeomagnetic data from the indian oceanic floor supports an initial india asia contact at 55 53 ma combination of the sinking rate of 1 1 1 4 cm year 1 with indian plate velocity of 4 5 cm year 1 suggests that the indian continent subducted to about 100 km depth at an average subduction angle of 14 19

arctic ocean mn stratigraphy genesis synthesis and inter basin correlation abstract across the arctic ocean late quaternary deep marine sediments are characterized by the occurrence of brownish layers intercalated with yellowish to olive gray sediments these layers show enhanced levels of bioturbation peaks in mn content and typically contain elevated abundances of planktonic and benthic micro and nannofossils it was early surmised that these layers were deposited under interglacial conditions and that their cyclical downcore occurrence could be correlated to the global benthic oxygen isotope curve however the synchronicity of mn layers with interglacial conditions and the underlying mechanisms responsible for their formation remain controversial here we compile and synthesize findings of the last decades with several recent studies that shed light on issues such as the sources of mn to the arctic ocean the processes and pathways for mn to the deep sea the chemical processes active in the sediment and the spatial and temporal distribution of mn rich layers in arctic deep marine sediments budget calculations show that about 90 of mn input to the arctic ocean originates from arctic rivers or coastal erosion two sources effectively shut down during mid to late quaternary glacial intervals by continental ice sheets blocking or re directing the rivers and vast subaerial exposure of the shelf areas thus the strong late quaternary interglacial glacial cyclicity in mn content is clearly an input related signal and only secondarily influenced by chemical processes in the water column and in the sediment on the shelves the mn undergoes repeated geochemical recycling caused by the high organic carbon content in the sediments before it is ultimately exported to the deep basins where scavenging processes in the water column effectively bring the mn to the sea floor in the form of mn oxyhydr oxides the close synchronicity with enhanced bioturbation and elevated micro and nannofossil abundances shows that the mn peaks are preserved at a stratigraphic level closely corresponding to the interglacial intervals however under certain biogeochemical conditions mn oxyhydr oxides may diagenetically become both dissolved and re precipitated deep in the sediments as shown by pore water analyses and x ray radiograph studies dissolution is particularly conspicuous in late quaternary sediments from the lomonosov ridge where in rapidly deposited coarse grained intervals diamictons with elevated total organic carbon toc contents mn appears almost completely removed from within the glacial sediments and also the surrounding interglacial sediments correspondingly bundles of closely spaced mm thick mn rich horizontal bands are observed in sediment otherwise devoid of indicators for interglacial conditions suggesting that these bands were purely formed by diagenetic processes redistributing the mn from deeper sediment layers this type of diagenetic mn redistribution within the sediment can be recognized in xrf core scanner data combined with sedimentological information from x ray radiographs while pore water data are highly promising if clear diagenetic features in the sediment are missing with this increasing ability to recognize intervals where a diagenetic overprint exists in the mn record the recently improved understanding of the mn cycle in the arctic ocean provides a conceptual paleoenvironmental framework in which carefully applied mn stratigraphy can provide a powerful correlation tool when combined with other paleoceanographic proxies and sedimentological data

the oldest uhp eclogites of the world age of uhp metamorphism nature of protoliths and tectonic implications coesite bearing eclogitic rocks from the pan african belt in northern mali were dated by multichronological methods rb sr sm nd and ar ar rb sr and sm nd isotope analyses on whole rock and mineral separates of an omphacite kyanite micaschist and a mafic eclogite yielded concordant ages of about 620 ma this is interpreted as the time of the eclogitisation which represents the oldest ultrahigh pressure uhp metamorphic event so far recorded in the continental crust the nearly identical rb sr and sm nd ages suggest very rapid exhumation as observed in many phanerozoic uhp rocks 40ar 39ar dating of phengites from two uhp micaschists gave much older ages of 1045 and 760 ma phengite from a quartzite collected from the same site yielded a plateau date of 623 3 ma which is in agreement with the rb sr and sm nd dates the two 40ar 39ar ages older than 625 ma testify once more the recurrent problem of excess ar in ultrahigh pressure phengites the occurrence of the mali uhp eclogites indicates that by the late precambrian the earth has cooled enough to sustain the formation and preservation of deeply subducted uhp metamorphic rocks the new ages of ca 620 ma also constrain the amalgamation in nw gondwana to have occurred in the latest proterozoic but not in the middle cambrian this is supported by several recent chronological data for the southern segment of the major suture zone in the dahomeyide orogenic belt

hydrothermal venting in magma deserts the ultraslow spreading gakkel and southwest indian ridges 1 detailed hydrothermal surveys over ridges with spreading rates of 50 150 mm yr have found a linear relation between spreading rate and the spatial frequency of hydrothermal venting but the validity of this relation at slow and ultraslow ridges is unproved here we compare hydrothermal plume surveys along three sections of the gakkel ridge arctic ocean and the southwest indian ridge swir to determine if hydrothermal activity is similarly distributed among these ultraslow ridge sections and if these distributions follow the hypothesized linear trend derived from surveys along fast ridges along the gakkel ridge most apparent vent sites occur on volcanic highs and the extraordinarily weak vertical density gradient of the deep arctic permits plumes to rise above the axial bathymetry individual plumes can thus be extensively dispersed along axis to distances 200 km and 75 of the total axial length surveyed is overlain by plumes detailed mapping of these plumes points to only 9 10 active sites in 850 km however yielding a site frequency fs sites 100 km of ridge length of 1 1 1 2 plumes detected along the swir are considerably less extensive for two reasons an apparent paucity of active vent fields on volcanic highs and a normal deep ocean density gradient that prevents extended plume rise along a western swir section 10 23 e we identify 3 8 sites so fs 0 3 0 8 along a previously surveyed 440 km section of the eastern swir 58 66 e 6 sites yield fs 1 3 plotting spreading rate us versus fs the ultraslow ridges and eight other ridge sections spanning the global range of spreading rate establish a robust linear trend fs 0 98 0 015us implying that the long term heat supply is the first order control on the global distribution of hydrothermal activity normalizing fs to the delivery rate of basaltic magma suggests that ultraslow ridges are several times more efficient than faster spreading ridges in supporting active vent fields this increased efficiency could derive from some combination of three dimensional magma focusing at volcanic centers deep mining of heat from gabbroic intrusions and direct cooling of the upper mantle and nonmagmatic heat supplied by exothermic serpentinization

40ar 39ar constraints on the tectonic history and architecture of the ultrahigh pressure sulu orogen new 40ar 39ar ages are presented from the giant sulu ultrahigh pressure uhp terrane and surrounding areas combined with u pb ages sm nd ages rb sr ages inclusion relationships and geological relationships they help define the orogenic events before during and after the triassic collision between the sino korean and yangtze cratons in the qinling microcontinent tectonism occurred between 2 0 and 1 4 ga the uhp metamorphism occurred in the yangtze craton between 240 and 222 ma its thermal effect on the qinling microcontinent was limited to partial resetting of k feldspar 40ar 39ar ages subsequent unroofing at rates of 5 25 km myr 1 brought the uhp terrane to crustal levels where it underwent a relatively short amphibolite facies metamorphism the end of that metamorphism is marked by 40ar 39ar ages in the 219 210 ma range implying cooling at crustal depths at rates of 50 200 c myr 1 ages in the 210 170 ma range may reflect protracted cooling or partial resetting by jurassic or cretaceous magmatism jurassic 166 149 ma plutonism was followed by cooling at rates of c 15 c myr 1 suggesting relatively deep crustal conditions whereas cretaceous 129 118 ma plutonism was succeeded by cooling at rates of c 50 c myr 1 suggesting relatively shallow crustal depths

factors in the preservation of coesite the importance of fluid infiltration the survival of coesite in ultrahigh pressure uhp rocks is most commonly attributed to rapid exhumation continuous cooling during uplift and inclusion in strong phases that can sustain a high internal over pressure during decompression exceptions to all of these criteria exist perhaps less attention has been paid to the role of fluid infiltration in the preservation of coesite we used infrared spectroscopy to measure water contents of coesite and coesite pseudomorphs in a variety of uhp rocks in all cases oh concentrations in coesite are below the detection limit of 100 ppm h2o the silica phases surrounding coesite however show varying amounts of h2o this is most spectacularly observed in pyrope quartzites from the dora maira massif that contain at least three phases of silica replacing coesite also distinguished by varying color of cathodoluminescence cl palisade textured quartz 100 ppm h2o red violet cl mosaic quartz which is actually chalcedony up to 0 4 wt h2o yellow brown cl and a rare highly hydrated silica phase interpreted to be opal 7 wt h2o dark blue cl very similar signatures are observed in a grospydite xenolith from the roberts victor kimberlite the quartz replacing coesite in other uhp samples studied contains on the order of 500 ppm h2o or less and most measurements are under the detection limit of our technique we infer that palisade quartz forms under dry or nearly dry conditions and at high temperatures during dilation of the host phase the formation of hydrous silica phases such as chalcedony and opal however must take place at much lower temperatures after cracking of the host phase which allows external fluids to infiltrate delay of fluid infiltration to low temperatures where kinetics are slow even in the presence of water is the most critical factor in the preservation of coesite

prograde high to ultrahigh pressure metamorphism and exhumation of oceanic sediments at lago di cignana zermatt saas zone western alps pelagic metasediments and morb type metabasalts of the former tethyan oceanic crust at cignana valtournanche italy experienced uhp metamorphism and subsequent exhumation during the early to late tertiary maximum pt conditions attained during uhp metamorphism were 600 630 c 2 7 2 9 gpa which resulted in the formation of coesite glaucophane eclogites in the basaltic layer and of garnet dolomite aragonite lawsonite coesite phengite bearing calc schists and garnet phengite coesite schists with variable amounts of epidote talc dolomite na pyroxene and na amphibole in the overlying metasediments during subduction the rocks followed a prograde hp uhp path which in correspondance with the jurassic age of the tethyan crust reflects the thermal influence of relatively old and cold lithosphere and of low to moderate shear heating inflections on the prograde metamorphic path may correspond to thermal effects that arise from a decrease in shear heating due to brittle plastic transition in the quartz aragonite dominated rocks induced convection in the asthenospheric mantle wedge and or heat consumption by endothermic reactions over a restricted pt segment during subduction after detachment from the downgoing slab some 50 70 ma before present the cignana crustal slice was first exhumed to ca 60 km and concomitantly cooled to ca 550 c tracing back the uhp hp prograde path displaced by 50 80 c to higher temperatures exhumation at this stage is likely to have occurred in the benioff zone while the subduction of cool lithosphere was going on subsequently the rocks were near isothermally exhumed to ca 30 km followed by concomitant decompression and cooling to surface conditions at 500 c 1 gpa during this last stage the uhpm slice arrived at its present tectonic position with respect to the overlying greenschist facies combin zone in contrast to the well preserved hp uhpm record of the coesite glaucophane eclogites the hp uhp assemblages of the metasediments have been largely obliterated during exhumation relics from which the metamorphic evolution of the rocks during prograde hp metamorphism and the uhp stage can be retrieved are restricted to rigid low diffusion minerals like garnet dolomite tourmaline and apatite

white mica 40ar 39ar ages of erzgebirge metamorphic rocks simulating the chronological results by a model of variscan crustal imbrication

the relationship between near axis hydrothermal cooling and the spreading rate of mid ocean ridges recent seismic observations and new crustal thermal models suggest that near axis 1 km from the spreading axis hydrothermal cooling increases linearly with increasing spreading rate we present here the first corroborating evidence from near axis hydrothermal plume observations comprehensive surveys of plume distributions have now been conducted on multi segment portions of ridge crest with full spreading rates from 20 to 150 km myr these surveys find thatph the fraction of ridge crest length overlain by hydrothermal plumes and full spreading rate us are related byph us where 0 004 myr km we recast this large scale spatial variability into long term temporal variability by postulating that any vent field site is hydrothermally active for an us fraction of geologic time we then use this relationship and the mean of eight studies that have determined instantaneous heat flux ha at vent field scales 75 45 mw km to estimate the time averaged heat flux ha asha us applying these scaled values ofha to the distribution of mid ocean ridge spreading rates yields a global near axis heat flux ha of 8 8 1011 w this value is 10 of the total oceanic hydrothermal heat flux and agrees with a crustal cooling model derived value of ha of 9 1011 w our estimate of ha implies a mean global3he heat ratio of 2 4 8 3 10 13 cm3 j stp and high temperature hydrothermal fluid flow of 1 8 1013 kg yr

enstatite sapphirine crack related assemblages in ultrahigh pressure pyrope megablasts dora maira massif western alps throughout the ultrahigh pressure uhp metamorphic unit of the dora maira massif western alps pyrope megablasts contain the typical assemblage clinochlore kyanite talc rutile phlogopite ellenbergerite as prograde inclusions in the upper part of the uhp unit in val gilba some megablasts xmg 0 89 0 98 contain in addition polymineralic inclusions consisting of various combinations of enstatite gedrite sapphirine clinochlore talc magnesiostaurolite and rare corundum or spinel we present evidence that these assemblages developed from cracks running across the megablasts and are therefore of late origin post dating the highest pressure stage enstatite xmg 0 94 0 99 contains 0 7 to typically 3 wt al2o3 but up to 8 4 wt in the presence of sapphirine sapphirine xmg 0 96 0 998 be free shows the largest compositional variations with si contents ranging from 1 7 to at least 2 1 atoms pfu thereby clearly exceeding the 2 2 1 stoichiometry the late stage talc contains up to 4 wt al2o3 0 35 wt na2o and 0 6 wt f gedrite 1 1 2 9 wt na2o and up to 0 36 wt f the successive development within pyrope of alternative hydrous assemblages involving first enstatite plus an al rich phase kyanite sapphirine magnesiostaurolite clinochlore then a gedrite compositionally close to pyrope then talc plus an al rich phase sapphirine corundum is a clear record of decompression however the temperature conditions implied under the assumption of high h2o activity are 100 to 150 c higher than and so inconsistent with existing constraints on the decompression path these constraints are in particular the stability of talc phengite in the matrix assemblage during decompression and the absence of regional evidence for a granulite facies event this inconsistency can only be alleviated if h2o activity inside the garnet megablast was or became considerably reduced with respect to that in the matrix fluid influx into an opening fracture in garnet sealing of the fracture by breakdown products of pyrope and continued evolution under closed system conditions may have led to increasing solute concentration and such low h2o activity within the garnet megablast driving the microsystem toward fluid absent conditions micrometre size inclusions of ca sulfate and crandallite type compounds in minerals of these reactive areas may be evidence for such residual brines and suggest that these were phosphate and sulfate rather than halide dominated this finding is additional evidence for the very local control that fluid composition and h2o activity may have on the occurrence of granulite facies assemblages regardless of temperature it highlights the role of deformation here fracturing in triggering reactions in otherwise unreactive systems it also shows how carefully inclusion to host relationships have to be considered post growth reaction within the host being more common than hitherto reported

time of formation and peak of variscan hp ht metamorphism of quartz feldspar rocks in the central erzgebirge saxony germany the variscan erzgebirge represents an antiform with a core of gneisses and mica schists surrounded by a phyllitic mantle the gneiss eclogite unit geu in the central part is a composite tectonometamorphic assemblage characterized by a hp ht imprint and comprises migmatitic para and orthogneisses ht mylonites hp granulites eclogites and garnet peridotites it is tectonically sandwiched between two major units with distinctly lower pt histories the geu experienced a characteristic kinked retrograde pt path after hp ht equilibration with 1 strong near isothermal decompression at high temperatures 2 extensive re equilibration at medium pressures followed 3 by rapid cooling during continued uplift we dated zircons pb pb evaporation from granitoid orthogneisses and metapelites of the geu the orthogneisses contain euhedral long prismatic zircons of igneous origin that provided protolith ages between 470 and 524 ma metapelites retain well preserved granulite facies mineral assemblages and contain spherical multifaceted metamorphic zircons that grew near the peak of hp ht metamorphism inclusions of prograde hp phengite 15 kbar and rutile are included in one such zircon metamorphic zircons of three samples from different localities yielded identical 207pb 206pb ages of 340 5 0 7 ma 341 2 0 5 ma and 341 6 0 5 ma respectively consideration of these zircon ages with published 39ar 40ar white mica ages suggests fast cooling and uplift rates in excess of 50 c ma and 4 km ma this is typical for large scale extensional tectonic unroofing of the ultra deep part of a fossil thickened variscan continental crust 60 km during continuing continental collision and orogenic collapse

bulk properties of hot smoker plumes the hot buoyant fluid from a submarine vent rises as a plume entraining fluid from the stratified ocean until a height is reached where the driving buoyant force vanishes and shortly thereafter the diluted plume fluid intrudes into the ocean interior at its equilibrium depth while a large positive heat flux issues from the vent the potential temperature anomaly of the horizontally spreading fluid at the equilibrium height depends on the oceanic stratification of both salinity and potential temperature and on the salinity of the source fluid at the vent for example in the atlantic ocean the oceanic stratification causes the spreading plume fluid to be cooler and fresher than the surrounding seawater of the same density while in the pacific the spreading fluid is relatively warm here the well known solution for a turbulent plume rising in a linearly stratified environment is used to obtain simple formulae for 1 the potential temperature anomaly of the horizontally spreading fluid 2 the heat flux associated with this injection of diluted vent water at this density level and 3 the nondimensional solution for the evolution of tracer properties e g salinity and potential temperature in the rising plume

metamorphic evolution of diamond bearing and associated rocks from the kokchetav massif northern kazakhstan representative diamond bearing gneisses and dolomitic marble eclogite and ti clinohumite bearing garnet peridotite from unit i at kumdy kol and whiteschist from unit ii at kulet eastern kokchetav massif northern kazakhstan were studied diamond bearing gneisses contain variable assemblages including grt bt qtz pl kfs zo chl tur cal and minor ap rt and zrn abundant inclusions of diamond graphite chlorite or calcite phengite clinopyroxene k feldspar biotite rutile titanite calcite and zircon occur in garnet diamond bearing dolomitic marbles consist of dol di grt phl inclusions of diamond dolomite graphite biotite and clinopyroxene were identified in garnet whiteschists carry the assemblage ky tlc grt rt garnet shows compositional zoning and contains abundant inclusions of talc kyanite and rutile with minor phlogopite chlorite margarite and zoisite inclusions and zoning patterns of garnet delineate the prograde p t path inclusions of quartz pseudomorphs after coesite were identified in garnet from both eclogite and gneiss other ultrahigh pressure uhp indicators include na bearing garnet up to 0 14 wt na2o with omphacitic cpx in eclogite occurrence of high k diopside up to 1 56 wt k2o and phlogopite in diamond bearing dolomitic marble and cr bearing kyanite in whiteschist these uhp rocks exhibit at least three stages of metamorphic recrystallization the fe mg partitioning between clinopyroxene and garnet yields a peak temperature of 800 1000 c at p 40 kbar for diamond bearing rocks and about 740 780 c at 28 35 kbar for eclogite whiteschist and ti bearing garnet peridotite the formation of symplectitic plagioclase amphibole after clinopyroxene and replacement of garnet by biotite amphibole or plagioclase mark retrograde amphibolite facies recrystallization at 650 680 c and pressure less than about 10 kbar the exsolution of calcite from dolomite and development of matrix chlorite and actinolite imply an even lower grade greenschist facies overprint at c 420 c and 2 3 kbar a clockwise p t path suggests that supracrustal sediments together with basaltic and ultramafic lenses apparently were subjected to uhp subduction zone metamorphism within the diamond stability field tectonic mixing may have occurred prior to uhp metamorphism at mantle depths during subsequent exhumation and juxtaposition of many other tectonic units intense deformation chaotically mixed and mylonitized these lithotectonic assemblages

discovery of abundant hydrothermal venting on the ultraslow spreading gakkel ridge in the arctic ocean submarine hydrothermal venting along mid ocean ridges is an important contributor to ridge thermal structure and the global distribution of such vents has implications for heat and mass fluxes from the earth s crust and mantle and for the biogeography of vent endemic organisms previous studies have predicted that the incidence of hydrothermal venting would be extremely low on ultraslow spreading ridges ridges with full spreading rates 2 cm yr 1 which make up 25 per cent of the global ridge length and that such vent systems would be hosted in ultramafic in addition to volcanic rocks here we present evidence for active hydrothermal venting on the gakkel ridge which is the slowest spreading 0 6 1 3 cm yr 1 and least explored mid ocean ridge on the basis of water column profiles of light scattering temperature and manganese concentration along 1 100 km of the rift valley we identify hydrothermal plumes dispersing from at least nine to twelve discrete vent sites our discovery of such abundant venting and its apparent localization near volcanic centres requires a reassessment of the geologic conditions that control hydrothermal circulation on ultraslow spreading ridges

microseismicity of the ultraslow spreading gakkel ridge arctic ocean a pilot study summary the active mid ocean ridge of the arctic ocean named gakkel ridge is the slowest spreading ridge of the global system of mid oceanic ridges with full spreading rates declining from about 12 5 to 6 mm yr 1 from west to east geological models of seafloor spreading predict a decreasing intensity of magmatic processes with decreasing spreading rate in summer 2001 the multidisciplinary arctic mid ocean ridge expedition amore2001 discovered robust magmatism at western gakkel ridge an amagmatic section further east and pronounced volcanic centres at eastern gakkel ridge during amore2001 an attempt was made at recording the microearthquake activity of the ridge which allows important insights into the character and dynamics of active crustal accretion at the ridge axis due to the permanent ice cover of the arctic ocean the use of ocean bottom seismometers bears the risk of instrument and data loss in this pilot study we used for the first time drifting ice floes as platforms for small seismological arrays the arrays consisted of four three component seismometers equipped with gps devices and arranged as a triangle with a central seismometer and a side length of about 1 km three such arrays were deployed in different rift segments and recorded the seismic activity continuously for 5 11 days at a sampling rate of 100 hz the array technique allowed to distinguish clearly between icequakes and earthquakes and to localize the earthquake source to within few kilometres or less depending on epicentral distance we intensively discuss the detection capabilities and the location accuracy of this single array on a drifting ice floe earthquake magnitudes could not be calculated in our pilot study but are estimated to be significantly smaller than magnitude 2 by comparison with a regional earthquake of known magnitude furthermore we analyse the characteristics of the recorded seismic events ranging from long waveforms of regional events to short local events with reverberations in the water column all of the arrays recorded numerous microearthquakes in the central rift valley and on its flanks which we interpret as tectonic earthquakes a swarm of microearthquakes was localized with high accuracy underneath the crest of a volcanic ridge in the rift valley and is proposed to have magmatic origin the pilot study was thus successful in detecting localizing and interpreting microearthquakes below magnitude 2 at gakkel ridge however we suggest improvements of the method for a comprehensive microearthquake survey of gakkel ridge which should aim at an understanding of active magmatism and faulting at ultraslow spreading ridges

slow exhumation of uhp terranes titanite and rutile ages of the western gneiss region norway abstract u pb ages of titanite and rutile were obtained from the central western gneiss region norway to assess the style and timing of exhumation and cooling of the western gneiss uhp terrane approximately half of the titanite ages are concordant the majority of which yield a limited age range from 393 to 390 ma the other titanite data are discordant and define discordia arrays with upper intercept ages of either 938 ma or 1 6 ga and a lower intercept of 389 ma concordant rutile analyses range from 385 to 392 ma both titanite and rutile ages young wnw toward the core of the orogen and are 4 ma older than 40 ar 39 ar muscovite ages corresponding to a cooling rate of 90 c ma a well defined boundary between concordant and discordant titanite ages in combination with the wnw increasing p t gradient and the similarity between muscovite cooling ages in the east and eclogite ages in the west suggests that the wgr remained coherent throughout its exhumation history and was progressively unroofed from east to west a 390 2 0 8 ma titanite in the soroyane uhp domain indicates that exhumation occurred at a vertical rate of 7 mm yr for 12 ma these rates are slower than estimates from smaller uhp terranes but similar to other large uhp terranes suggesting that there may be fundamental differences in the mechanisms controlling the evolution of large uhp terranes that undergo protracted subduction and exhumation and smaller uhp terranes that undergo rapid subduction and exhumation

crustal structure and continental dynamics of central china a receiver function study and implications for ultrahigh pressure metamorphism abstract the qinling tongbai hong an dabie sulu orogenic belt records the tectonic history of paleozoic convergence between the south china and north china blocks in this study the distribution of crustal thickness and p and s wave velocity ratio vp vs is obtained by using the h k stacking technique from the dabie sulu belt in central china our results show marked differences in the crustal structure between the dabie and sulu segments of the ultrahigh pressure uhp orogen the lower crust in the dabie orogenic belt is dominantly of felsic intermediate composition whereas the crust beneath the sulu segment is largely intermediate mafic the crust of the dabie orogenic belt is thicker by ca 3 5 km as compared to that of the surrounding region with the presence of an orogenic root the crustal thickness is nearly uniform in the dabie orogenic belt with a generally smooth crust mantle boundary a symmetrically thickened crust in the absence of any deep structural features similar to that of the yangtze block suggests no supportive evidence for the proposed northward subduction of the yangtze continental block beneath the north china block we propose that the collision between the yangtze and north china blocks and extrusion caused crustal shortening and thickening as well as delamination of the lower crust resulting in asthenospheric upwelling and lower crustal uhp metamorphism along the dabie orogen our results also reveal the presence of a se to nw dipping moho in the north china block beneath the tran north china orogen and eastern block suggesting the fossil architecture of the northwestward subduction of the kula plate

the fate of subducted continental margins two stage exhumation of the high pressure to ultrahigh pressure western gneiss region norway thermobarometry suggests that ultrahigh pressure uhp to high pressure hp rocks across the western gneiss region ponded at the moho following as much as 100 km of exhumation through the mantle and before exhumation to the upper crust eclogite across the c 22 000 km 2 study area records minimum pressures of c 8 18 kbar and temperatures of c 650 780 c one orthopyroxene eclogite yields an uhp of c 28 5 kbar and evidence of former coesite has been found c 50 km farther east than previously known despite this widespread evidence of uhp to hp thermobarometry of metapelite and garnet amphibolite samples reveals a surprisingly uniform supra barrovian amphibolite facies overprint at c 11 kbar and c 650 750 c across the entire area chemical zoning analysis suggests that garnet in these samples grew during heating and decompression presumably during the amphibolite facies event these data indicate that the norwegian uhp hp province was exhumed from mantle depths of c 150 km to lower crustal depths where it stalled and underwent a profound high temperature overprint the ubiquity of late stage supra barrovian metamorphic overprints suggests that large scale collisional uhp terranes routinely stall at the continental moho where diminishing body forces are exceeded by boundary forces significant portions of the middle or lower crust worldwide may be formed from uhp terranes that were arrested at the moho and never underwent their final stage of exhumation

exploring the ocean for hydrothermal venting new techniques new discoveries new insights abstract enumerating active hydrothermal fields on the seafloor has been a challenge since their discovery almost 40 years ago high temperature hydrothermal fields are readily discoverable primarily by detecting mineral laden plumes but low temperature particle poor vent fields resist discovery decades of exploration for vent fields have covered though often cursorily about one third of the global lengths of both oceanic spreading ridges osrs and volcanic arcs identifying some 630 active vent fields about 80 of these fields are on osrs and the spatial frequency of those fields is currently estimated as 0 5 5 100 km generally increasing with spreading rate over the last decade however a few detailed surveys have added sensors capable of detecting ephemeral chemical tracers oxidation reduction potential in addition to standard sensors that detect quasi conservative optical tracers such as light backscattering this approach has revealed a new view of the distribution of venting fields along fast spreading 55 mm yr osrs studies of four such ridge sections totaling 1470 km length suggest that the present inventory of vent fields may underestimate the true global population of vent fields on fast spreading osrs by a factor of 3 6 this increase implies that ridge axes are unexpectedly leaky reservoirs from which hydrothermal fluids escape at far more sites than presently assumed that the supply of dissolved hydrothermal iron which may be fertilizing the primary production of the southern ocean is higher than now calculated and that present estimates of recoverable sulfide tonnage from ridge axes may be too low along slow spreading ridges which account for 60 of the global osr length and 86 of known sulfide tonnage expansive axial valleys present special exploration challenges that will not be easily overcome

partial transformation of gabbro to coesite bearing eclogite from yangkou the sulu terrane eastern china an ultra high pressure uhp metamorphic slab at yangkou beach near qingdao in the sulu region of china consists of blocks of eclogite facies metagabbro metagranitoid ultramafic rock and mylonitic orthogneisses enclosed in granitic gneiss a gradational sequence from incipiently metamorphosed gabbro to completely recrystallized coesite eclogite formed at ultra high pressures was identified in a single 30 m block metagabbro is preserved in the core whereas coesite eclogite occurs along the block margins the metagabbro contains an igneous assemblage of pl aug opx qtz bt ilm ti mag it shows relict magmatic textures and reaction coronas fine grained garnet developed along boundaries between plagioclase and other phases primary plagioclase broke down to ab ky ms zo grt amp augite is rimmed by sodic augite or omphacite whereas orthopyroxene is rimmed by a corona of cum act and omp qtz layers or only omp qtz in transitional rocks augite and orthopyroxene are totally replaced by omphacite and the lower pressure assemblage ab ky phn zo grt coexists with domains of omp jd70 73 ky phn in pseudomorphs after plagioclase both massive and weakly deformed coesite bearing eclogites contain omp ky grt phn coe qtz rt and preserve a faint gabbroic texture coesite inclusions in garnet and omphacite exhibit limited conversion to palisade quartz some intergranular coesite and quartz pseudomorphs after coesite also occur assemblages of the coronal stage transitional and uhp peak occurred at about 540 50 c at c 13 kbar 600 800 c at 15 25 kbar and 800 850 c at 30 kbar respectively garnet from the coronal through the transitional to the eclogite stage rocks show a decrease in almandine and an increase in grossular pyrope components garnet in low grade rocks contains higher mno and lower pyrope components the growth textures of garnet within pseudomorphs after plagioclase or along grain boundaries between plagioclase and other phases are complex the application of garnet zoning to estimate p t should be carried out with caution some garnet enclosing quartz aggregates as inclusions shows radial growth boundaries these quartz aggregates as well as other primary and low p phases persisted metastably at uhp conditions due to sluggish reactions resulting from the lack of fluid during prograde and retrograde p t evolution

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

geochemical observations within the water column at the co2 rich hydrothermal systems hatoma knoll and yonaguni knoll iv in the southern okinawa trough the okinawa trough is one of three known hydrothermal sites worldwide where liquid carbon dioxide is emitted from the seafloor into the water column in march 2008 investigations were performed at two active areas yonaguni knoll iv and hatoma knoll in order to identify impacts of hydrothermal venting on the water column chemistry vertical profiles of ph and redox potential eh were recorded and discrete water samples were taken for the analysis of total carbon dioxide ct and helium 3he 4he anomalies with respect to reference stations ct ph and 3he with respect to saturation with the atmosphere 3heexcess were used to characterize the impact of hydrothermal vents these data indicate that the flux of co2 into the water column is dominated by hot hydrothermal co2 rich vents located in close proximity to the liquid co2 emission sites bubbles and droplets sampled at the cold gas outlets at hatoma knoll differed considerably from the water column regarding co2 3he ratios and thus provide additional evidence that cold liquid phase co2 is of minor importance for the total co2 flux at both hydrothermal systems although hydrothermal vents at back arc basins are known to emit large amounts of acids other than co2 the correlation between ph and ct at both research areas clearly suggests that the observed ph reduction is mainly caused by the addition of co2 deviating ct 3he and ct ph ratios and the prevailing water currents indicate a yet undiscovered vent site at the flank of a seamount in the northeast

thermo mechanical modeling of subduction of continental lithosphere abstract we consider two dimensional thermo chemical mantle convection models to investigate the deformation of the continental lithosphere that follows the oceanic lithosphere into the subduction zone the models account for the compositional buoyancy forces by considering lithospheric plates with distinct crustal layers continental convergence results in crust mantle detachment in the subducting plate and crustal thickening in both subducting and overriding plates the depth of detachment ranges from 90 to 200 km depending on the strength of the lithosphere and the density of the continental crust as the crust thickens the convergence velocity in the collision zone decreases and the locus of subduction gradually shifts toward the interior of the subducting plate in models with greater viscosity the subducting mantle lithosphere maintains its integrity and does not break up in models with weaker rheology the positive buoyancy of the thickened crust can overcome the strength of the subducting lithosphere and causes the oceanic slab to break off and sink into the mantle the breakoff occurs over a time interval of 10 20 million years which is roughly the time needed for the crust to reach its maximum thickness crustal detachment takes place over a wide range of lithospheric strength suggesting that crustal buoyancy has an important role in the dynamics of continental subduction

argon oxygen and boron isotopic evidence documenting 40 ar e accumulation in phengite during water rich high pressure subduction metasomatism of continental crust abstract the luliang shan area of the north qaidam high pressure hp to ultrahigh pressure uhp metamorphic terrane in northwestern china features thick garnet and phengite rich metasomatic selvages that formed around gneiss hosted mafic eclogite blocks during hp conditions here we present new 40 ar 39 ar 18 o and 11 b results from a previously studied 30 m 18 sample traverse that extends from the host gneiss into a representative eclogite block previous thermobarometry and new mica quartz oxygen isotope thermometry from the traverse reveal that the phengite rich selvage formed at temperatures similar to those recorded by the eclogites at peak pressure quartz and white mica 18 o data from the selvage cannot be explained by simple mixing of gneiss and eclogite and indicate a fluid rock ratio 1 during regional scale infiltration of high 18 o ca 14 fluids heavy 18 o overgrowths of metamorphic zircon over lighter 18 o detrital grains indicate that the gneiss was similarly affected starkly contrasting boron content and 11 b compositions for the host gneiss and the selvage also cannot be explained by local scale devolatilization of the gneiss to form the selvage instead the boron systematics are best attributed to two distinct phases of fluid infiltration 1 low boron selvage phengite with 11 b from 10 to 30 grew under hp conditions and 2 tourmaline and boron rich muscovite with generally positive 11 b crystallized in the host gneiss under subsequent lower pressure epidote amphibolite facies conditions as the luliang shan gneiss terrane was exhumed past shallower portions of the subduction channel consistent with observations made worldwide we were able to identify uptake of excess argon 40 ar e in phengite as a high pressure phenomenon phengite 40 ar 39 ar ages from massive eclogite exceed the ca 490 ma zircon u pb age of eclogite metamorphism by a factor of 1 5 however phengite ages from the more permeable schistose selvage were even older exceeding the time of eclogite formation by a factor of 1 7 in contrast lower pressure retrograde muscovite present within the host gneiss and in discrete shear zones cutting the selvage yield 40 ar 39 ar ages that were younger than the time of hp metamorphism and consistent with regional cooling age patterns our observation of high 40 ar e concentrations in phengite from schistose rocks infiltrated by regionally extensive fluids at hp conditions runs contrary to widely held expectations conventional wisdom dictates that low phengite fluid partition coefficients for argon d phg fluid ar 10 3 to 10 5 coupled with the dry closed systems conditions that are widely reported to characterize hp metamorphism of continental crust explains why high concentrations of 40 ar e partitions are able to accumulate within phengite we alternatively propose that phengite fluid partition coefficients for argon increase linearly with pressure to values as high as 10 2 to allow phengites to accumulate large amounts of 40 ar e from aqueous fluids under hp to uhp conditions

exhumation of ultrahigh pressure metamorphic oceanic crust from lago di cignana piemontese zone western alps the structural record in metabasites the metamorphic and deformational history of coesite bearing ultrahigh pressure metamorphic oceanic crust from lago di cignana valtoumanche western alps as recorded along the decompression and cooling path has been resolved to derive constraints on the physical state of the crust during exhumation the following history is proposed eclogites derived from pillow basalts were deformed by dislocation creep of pyroxene at a depth of more than 80 km in the stability field of coesite the deformation was progressively localised into shear bands over the first 40 km of subsequent exhumation the rocks remained essentially unmodified any deformation must have been localised beyond the limits of the present outcrop retrogression first in a closed system started without concomitant deformation at a depth of about 40 km and temperatures of 550 to 500 c subsequently the system size increased fluid infiltration focussed along tensile fractures is documented by the formation of veins vein shapes and orientations indicate high pore fluid pressures and low differential stress after cooling to about 350 450 c at depths of 12 to 18 km localised ductile deformation transformed the retrogressed eclogites into greenschists for that stage the microstructure of the quartz veins indicates deformation by dislocation creep at falling temperatures and increasing differential stress final cooling to below ca 300 c took place at depths of 7 to 12 km as indicated by the density of fluid inclusions later deformation in the brittle field cannot be correlated with the p t path

high temperature geochronology constraints on the tectonic history and architecture of the ultrahigh pressure dabie sulu orogen 1 new u pb zircon and th pb monazite ages are presented from the giant sulu ultrahigh pressure uhp terrane combined with sm nd ages rb sr ages inclusion relationships and geologic relationships they help define the timing of peak recrystallization the timing of subsequent amphibolite facies metamorphism and the architecture of the dabie sulu suture zone between the collided sino korean and yangtze cratons the data indicate a 15 myr record of uhp recrystallization the first clearly documented for a giant uhp terrane this requires that continental subduction in the dabie sulu orogen involved multiple uhp tectonic or recrystallization events a 244 236 ma precursor uhp event seen only in the dabie shan was followed by a second 230 220 ma main uhp event which was itself terminated by a 220 205 ma amphibolite facies overprint older eclogite facies events seen in the qinling segment of this orogenic belt raise the possibility that these rocks have undergone u hp metamorphism three or four times but at present there is no geochronological evidence in the dabie sulu area to support this the subduction of the lower yangtze plate did not proceed in a simple fashion the ages of inherited zircon cores demonstrate that a ribbon continent of yangtze affinity escaped subduction and became wedged against the sino korean plate hanging wall

analysis and modeling of hydrothermal plume data acquired from the 85 e segment of the gakkel ridge 1 we use data from a ctd plume mapping campaign conducted during the arctic gakkel vents agave expedition in 2007 to constrain the nature of hydrothermal processes on the gakkel ridge at 85 e thermal and redox potential eh anomalies were detected in two discrete depth intervals 2400 2800 m interval 1 and 3000 3800 m interval 2 the spatial and temporal patterns of the signals indicate that the interval 1 anomalies were most likely generated by a single large high temperature t 100 c vent field located on the fault terraces that form the ne axial valley wall in contrast the interval 2 anomalies appear to have been generated by up to 7 spatially distinct vent fields associated with constructional volcanic features on the floor of the axial valley many of which may be sites of diffuse low temperature t 10 c discharge numerical simulations of turbulent plumes rising in a weakly stratified arctic ocean water column indicate that the high temperature field on the axial valley wall has a thermal power of 1 8 gw similar to the trans atlantic geotraverse and rainbow fields in the atlantic ocean whereas the sites on the axial valley floor have values ranging from 5 to 110 mw

a model of an atlantic and pacific hydrothermal plume a turbulent entrainment model of a hydrothermal plume was modified to include the effects of vertical structure in the background temperature and salinity fields owing to the stably stratified salinity field in the pacific relatively warm salty water spreads at an equilibrium level where the plume density equals the ambient density in contrast the unstable salinity profile in the atlantic gives the plume added buoyancy and it continues to rise to an equilibrium level where it is relatively cool and fresh these attributes result from the mixing between the plume and the surrounding seawater at each level below the spreading layer the equilibrium temperature anomaly is not directly proportional to the source strength but depends on the mixing and background salinity gradient the net entrainment of surrounding seawater is greater in the atlantic plume model than in the pacific plume model for the same buoyancy flux recent data from the tag hydrothermal field at 26 n on the mid atlantic ridge and data from a vent field on the endeavor segment of the juan de fuca ridge are compared to model results the model predicts the penetration height and s characteristics of the atlantic plume and it explains some of the differences between the observed atlantic and pacific hydrothermal plumes

coesite in clinopyroxene in the caledonides and its implications for geodynamics coesite is reported for the first time from the caledonide orogen it occurs as inclusions in clinopyroxene in the dolomite eclogite at grytting norway and provides valuable new evidence to support the hypothesis of extremely high pressure 30 kbar in certain norwegian eclogites this finding has implications for the geodynamics and kinetics of the concepts of microscale intracrystalline tectonic overpressure and of macroscale local or regional tectonic overpressure and for the hypotheses of subduction and obduction involving a more than 100km vertical component of transport

seismicity and structure of the 85 e volcanic complex at the ultraslow spreading gakkel ridge from local earthquake tomography accretion mechanisms at ultraslow spreading ridges are still only poorly understood due to difficult survey conditions for seismic experiments at these ridges melts gets focused in distinct magmatic centres with thin crust in between at the 85 e 85 n volcanic complex at gakkel ridge arctic ocean where a recent spreading episode has been observed perennial sea ice cover challenges traditional investigations of seismic structure and microseismicity we used an unusual survey set up to gather seismological data during 16 days in july 2007 using seismometer arrays mounted on ice floes despite only 12 stations the drift of the ice floes over the survey area resulted in a multitude of crossing rays in the rift valley the data included 303 microearthquakes of which 248 events could be confidently located we compiled a 1d velocity model by localizing a subset of these earthquakes with a suite of randomly created velocity models in this model the moho is placed at 7 km depth below seafloor inferring a thick basaltic crust using 124 events which were recorded by at least two arrays we inverted for p wave velocity structure in a local earthquake tomography resolution tests indicate reliable results in the central rift valley illuminating the thermal structure underneath the asgard volcanic chain in the aftermath of its most recent spreading episode our results show microearthquake activity down to 16 km beneath seafloor inferring a cold lithosphere most hypocentres cluster at the centre of the rift valley at the site the asgard volcanic chain this may mean that existing thermal models for this class of ridges have to be refined an area of decreased seismic velocities crosses the rift valley at this location and microearthquake activity is located at its eastern fringe where the velocity gradient is highest we therefore speculate that the reduced velocities may be caused by warm intruded material and that the observed seismicity predominatly reflects the relaxation of thermal stresses following recent intrusion

preservation exhumation of ultrahigh pressure subduction complexes abstract ultrahigh pressure uhp metamorphic terranes reflect subduction of continental crust to depths of 90 140 km in phanerozoic contractional orogens rocks are intensely overprinted by lower pressure mineral assemblages traces of relict uhp phases are preserved only under kinetically inhibiting circumstances most uhp complexes present in the upper crust are thin imbricate sheets consisting chiefly of felsic units serpentinites dense mafic and peridotitic rocks make up less than 10 of each exhumed subduction complex roundtrip prograde retrograde p t paths are completed in 10 20 myr and rates of ascent to mid crustal levels approximate descent velocities late stage domical uplifts typify many uhp complexes sialic crust may be deeply subducted reflecting profound underflow of an oceanic plate prior to collisional suturing exhumation involves decompression through the p t stability fields of lower pressure metamorphic facies scattered uhp relics are retained in strong refractory watertight host minerals e g zircon pyroxene garnet typified by low rates of intracrystalline diffusion isolation of such inclusions from the recrystallizing rock matrix impedes back reaction thin aspect ratio ductile deformed nappes are formed in the subduction zone heat is conducted away from uhp complexes as they rise along the subduction channel the low aggregate density of continental crust is much less than that of the mantle it displaces during underflow its rapid ascent to mid crustal levels is driven by buoyancy return to shallow levels does not require removal of the overlying mantle wedge late stage underplating structural contraction tectonic aneurysms and or plate shallowing convey mid crustal uhp decollements surfaceward in domical uplifts where they are exposed by erosion unless these situations are mutually satisfied uhp complexes are completely transformed to low pressure assemblages obliterating all evidence of profound subduction

geochemistry and age of ultrahigh pressure metamorphic rocks from the kokchetav massif northern kazakhstan isotopic and geochemical data of the zerenda series metamorphic rocks from the kokchetav massif are reported some of these rocks contain microdiamond inclusions in garnets and other indicators of ultrahigh pressure metamorphism p 40 kbar t 900 1000 c the diamond bearing rocks exhibit distinctive geochemical characteristics compared to typical crustal rocks the ree patterns range from lree depleted to slightly lree enriched chondrite normalized la yb n 0 1 5 4 with a negative eu anomaly they are depleted in incompatible elements e g sr ba u th with respect to the upper crust in contrast non diamondiferous rocks of the zerenda series exhibit normal crustal geochemistry all rocks of the zerenda series have very radiogenic lead isotopes the measured values 238u 204pb compared with those calculated for the interval between crust formation and ultrahigh pressure uhp metamorphism suggest a decrease by factors of up to 200 during the uhp metamorphism the sm nd mineral isochrons from the diamond bearing rocks and other rock types of the zerenda series give a middle cambrian 524 535 ma age of metamorphism the nd model ages show that crust formation occurred about 2 3 ga ago significant fractionation of sm and nd and loss of incompatible elements may be due to partial melting of the protoliths the ar ar age determinations of secondary biotite and muscovite from the diamond bearing rocks yield an age of 517 5 ma this cooling age requires a short time interval between uhp metamorphism and uplift to a crustal level ultrahigh pressure metamorphism might be a significant source of pb for the mantle we propose that the radiogenic pb of the oceanic array is the contamination traces of numerous uhp events beside the geological aspect we demonstrate a method of dating a high grade metamorphic terrain using nd isotopes we compare whole rock isochrons and mineral isochrons and in this way get some insight into the behaviour of the sm nd system during very high grade metamorphic events

coesite inclusions and prograde compositional zonation of garnet in whiteschist of the hp uhpm kokchetav massif kazakhstan a record of progressive uhp metamorphism coarse grained whiteschist containing the assemblage garnet kyanite phengite talc quartz coesite is an abundant constituent of the ultrahigh pressure metamorphic uhpm belt in the kulet region of the kokchetav massif of kazakhstan garnet displays prograde compositional zonation with decreasing spessartine and increasing pyrope components from core to rim cores were recrystallized at t 380 c inner to 580 c outer at p 10 kbar garnet ilmenite geothermometry margarite quartz stability and mantles at t 720 760 c and ph20 34 36 kbar coesite graphite stability phengite geobarometer kfmash system reaction equilibria textural evidence indicates that rims grew during decompression and cooling within the qtz stability field silica inclusions quartz and or coesite of various textural types within garnets display a systematic zonal distribution cores contain abundant inclusions of euhedral quartz type 1 inclusions inner mantle regions contain inclusions of polycrystalline quartz pseudomorphs after coesite type 2 with minute dusty micro inclusions of chlorite and more rarely talc and kyanite in their cores intense radial and concentric fractures are well developed in the garnet intermediate mantle regions contain bimineralic inclusions with coesite cores and palisade quartz rims type 3 which are also surrounded by radial fractures subhedral inclusions of pure coesite without quartz overgrowths or radial fractures type 4 occur in the outer part of the mantle garnet rims are silica inclusion free type 1 inclusions in garnet cores represent the low p low t precursor stage to uhpm recrystallization and attest to the persistence of low p assemblages in the coesite stability field coesites in inclusion types 2 3 and 4 are interpreted to have sequentially crystallized by net transfer reaction kyanite talc garnet coesite h2o and were sequestered within the garnet with progressively decreasing amounts of intragranular aqueous fluid during the retrograde evolution of the rock all three inclusion types diverged from the host garnet p t path at the coesite quartz equilibrium and followed a trajectory parallel to the equilibrium boundary resulting in inclusion overpressure coesite in type 2 inclusions suffered rapid intragranular h2o catalysed transformation to quartz and ruptured the host garnet at about 600 c when inclusion p 27 kbar garnet host p 9 kbar instantaneous decompression to the host garnet p t path passed through the kyanite talc chlorite quartz reaction equilibrium resulting in the dusty micro assemblage in inclusion cores type 3 inclusions suffered a lower volumetric proportion transformation to quartz at the coesite quartz equilibrium and finally underwent rupture and decompression when t 400 c facilitating coesite preservation type 4 coesite inclusions are interpreted to have suffered minimal transformation to quartz and proceeded to surface temperature conditions along or near the coesite quartz equilibrium boundary

kinematics and geometry of active detachment faulting beneath the trans atlantic geotraverse tag hydrothermal field on the mid atlantic ridge newly acquired seismic refraction and microearthquake data from the trans atlantic geotraverse tag segment of the mid atlantic ridge at 26 n reveal for the first time the geometry and seismic character of an active oceanic detachment fault hypocenters from 19 232 microearthquakes observed during an eight month ocean bottom seismometer deployment form an 15 km long dome shaped fault surface that penetrates to depths 7 km below the seafloor on a steeply dipping 70 interface a tomographic model of compressional wave velocities demonstrates that lower crustal rocks are being exhumed in the detachment footwall which appears to roll over to a shallow dip of 20 5 and become aseismic at a depth of 3 km outboard of the detachment the exhumed lithosphere is deformed by ridge parallel antithetical normal faulting our results suggest that hydrothermal fluids at the tag field exploit the detachment fault to extract heat from a region near the crust mantle interface over long periods of time

exhumation of the ultrahigh pressure tso morari unit in eastern ladakh nw himalaya a case study exhumation processes of the ultra high pressure uhp tso morari dome nw himalaya are investigated using structural petrological and geochronological data the uhp tso morari unit is bounded by the low grade metamorphic indus suture zone to the ne and mata unit to the sw three deformation phases d1 d2 and d3 are observed only d3 is common to the uhp unit and the surrounding units in the uhp unit the first deformation phase d1 produced upright folds under eclogitic conditions 20 kbar 580 60 c d1 is overprinted by d2 structures related to a nw se trending open anticline this phase is characterized by blueschist mineral associations and corresponds to the quasi isothermal decompression from a depth of 90 km eclogitic conditions up to 30 40 km the final exhumation phase of the tso morari unit is dominated by tectonic denudation and erosion d3 associated with a slight temperature increase radiochronological analyses indicate that the uhp exhumation process began during the eocene exhumation was fast during d1 d2 and slowed down through d3 in oligocene time the change in the deformation style from d1 d2 to d3 in the tso morari unit coincides with changes in the exhumation rates and in the metamorphic conditions these changes may reflect the transition from an exhumation along the subduction plane in a serpentinized wedge to the vertical uplift of the tso morari unit across the upper crust

effusive and explosive volcanism on the ultraslow spreading gakkel ridge 85 e 1 we use high definition seafloor digital imagery and multibeam bathymetric data acquired during the 2007 arctic gakkel vents expedition agave to evaluate the volcanic characteristics of the 85 e segment of the ultraslow spreading gakkel ridge 9 mm yr 1full rate our seafloor imagery reveals that the axial valley is covered by numerous small volume order 1000 m3 lava flows displaying a range of ages and morphologies as well as unconsolidated volcaniclastic deposits with thicknesses up to 10 cm the valley floor contains two prominent volcanic lineaments made up of axis parallel ridges and small cratered volcanic cones the lava flows appear to have erupted from a number of distinct source vents within the 12 15 km wide axial valley only a few of these flows are fresh enough to have potentially erupted during the 1999 seismic swarm at this site and these are associated with the oden and loke volcanic cones we model the widespread volcaniclastic deposits we observed on the seafloor as having been generated by the explosive discharge of co2 that accumulated in possibly deep crustal melt reservoirs the energy released during explosive discharge combined with the buoyant rise of hot fluid lofted fragmented clasts of rapidly cooling magma into the water column and they subsequently settled onto the seafloor as fall deposits surrounding the source vent

burial rates during prograde metamorphism of an ultra high pressure terrane an example from lago di cignana western alps italy abstract estimation of burial rates and duration of prograde metamorphism of ultra high pressure uhp rocks t 590 630 c p 2 7 2 9 gpa of the zermatt saas ophiolite from lago di cignana italy may be made through combined lu hf and sm nd garnet geochronology in conjunction with petrologic estimates of the prograde p t path we report a lu hf garnet omphacite whole rock isochron age of 48 8 2 1 ma from the uhp locality at lago di cignana which stands in contrast to the sm nd age of 40 6 2 6 ma amato et al earth planet sci lett 171 1999 425 438 obtained from the same sample and mineral material the sm nd and lu hf ages as well as other ages determined on metamorphic garnet zircon and white mica amato et al earth planet sci lett 171 1999 425 438 mayer et al eur union geosci 10 1999 abstr 809 rubatto et al contrib mineral petrol 132 1998 269 287 dal piaz et al int j earth sci 90 2001 668 684 from lago di cignana and elsewhere in the zermatt saas ophiolite lie within a range of 50 38 ma which we interpret to encompass the duration of prograde metamorphism and possibly the duration of garnet growth the difference in measured sm nd and lu hf ages from cignana can be accounted for by expected core to rim variations in lu hf sm and nd the measured yttrium content in garnet which may be a proxy for lu is highest in garnet core and lowest in the mineral rim generally following a profile that is predicted by rayleigh fractionation preferential enrichment of lu in the core produces a lu hf age that is weighted toward the older garnet core sm nd ages as predicted by rayleigh fractionation of sm and nd during garnet growth however reflect later grown garnet as compared to lu hf ages the difference in sm nd and lu hf ages from a single sample should therefore be a minimum estimate for the duration of garnet growth and prograde metamorphism so long as sm nd and lu hf blocking temperatures were not exceeded for a long period of time based on the 12 myr duration of prograde garnet growth estimated in this study burial rates for rocks at lago di cignana were on the order of 0 23 0 47 cm yr these values correlate with continuous shortening rates of 0 4 1 4 cm yr between the european plate and the african adriatic promontory between 50 and 38 ma which is on the order of that calculated for plate velocities from plate reconstructions suggesting that the zermatt saas ophiolite may have remained well coupled to the down going slab to uhp conditions

rapid exhumation of the zermatt saas ophiolite deduced from high precision smnd and rbsr geochronology sm nd isotope data from garnets in ultrahigh pressure coesite bearing eclogite from the western alps were obtained to determine the age of peak metamorphism and exhumation rates of deeply buried oceanic crust in the zermatt saas ophiolite complex we report an exceptionally well constrained sm nd isochron age of 40 62 6 ma from the lago di cignana eclogite difficulties in dating alpine eclogites using this method were overcome by using an hf acid leaching procedure on garnet to remove mineral inclusions the zermatt saas rocks reached greenschist facies conditions around 38 2m a on the basis of rb sr whole rock phengite isochrons these data combined with existing estimates of the pressure and temperature conditions of the eclogite give an estimate for the initial exhumation rate of 10 to 26 km m y depending on the inferred pressure for the recrystallization of phengite followed by slow exhumation of 0 3 km m y from 34 to 14 ma the initial exhumation rates at lago di cignana are among the highest yet determined for high pressure terranes and are well constrained because the age has been determined directly on the high pressure assemblage the rapid exhumation rates presented here place critical constraints on collisional tectonic models 1999 elsevier science b v all rights reserved

pre collisional high pressure metamorphism and nappe tectonics at active continental margins a numerical simulation the evolution of a subduction channel and orogenic wedge is simulated in 2d for an active continental margin with p t paths being displayed for selected markers in our simulation subduction erosion affects the active margin and a structural pattern develops within a few tens of millions of years with four zones from the trench into the forearc i an accretionary complex of low grade metamorphic sedimentary material ii a wedge of nappes with alternating upper and lower crustal provenance and minor interleaving of oceanic or hydrated mantle material iii a megascale melange composed of high pressure hp and ultra high pressure uhp metamorphic rocks extruded from the subduction channel and iv the upward tilted frontal part of the remaining lid the p t paths and time scales correspond to those typically recorded in orogenic belts the simulation shows that hp uhp metamorphism of continental crust does not necessarily indicate collision but that the material can be derived from the active margin by subduction erosion and extruded from the subduction channel beneath the forearc during ongoing subduction

35 ma old ultrahigh pressure metamorphism and evidence for very rapid exhumation in the dora maira massif western alps abstract ion microprobe shrimp data on zircons from various rock types of an ultrahigh pressure uhp metamorphic whiteschist type pyrope quartzite lens of the dora maira massif dmm consistently show domains giving a late eocene age of 35 4 1 0 ma which is taken as the age of uhp metamorphism these domains partially replace the older oscillatory zoning pattern of the zircons formed during primary magmatic crystallization at about 275 ma zircons of a metagranitic country rock next to the uhp metamorphic lens have these primary features best preserved all zircons measured also yield intermediate ages between 275 and 35 ma with a statistical concentration between 260 and 210 ma thus the uniformity of the initial zircon population both in the lens and the country rock evidences a common protolith that is a granite intruded during the late herynian while the intermediate ages are at least partly due to incomplete resetting of the zircons during uhp metamorphism those in the 260 210 ma range may be related to rifting episodes in the permotriassic the mg rich chemistry of the whiteschist lenses is due to local metasomatic alterations of the granite perhaps by fluids derived from evaporitic sediments dating as early as the permotriassic as well the more pervasive resetting of zircon ages during uhp metamorphism in the pyrope quartzite lenses is explained by the ubiquity of fluids and or melts produced during subduction by a series of dehydration reactions that occurred only in the more hydrous mg rich protoliths and not in their drier granitic neighborhood fission track ages determined partly on the same zircon samples yielding 29 9 1 4 ma mark the time when the uhpm unit had reached about 290 c at a shallow location within the crust thus exhumation over a vertical distance of about 120 km must have occurred within about 5 6 ma indicating an average uplift rate of about 20 24 km ma and an average cooling rate of about 85 100 c ma the radiometric data obtained do not lend any support to an eo alpine cretaceous subduction event so that deep subduction and immediately following exhumation all took place during early to mid tertiary time this scenario seems to apply to large portions of the western and central alps as well calling for drastic geodynamic reinterpretations of these parts of the alps

revel a model for recent plate velocities from space geodesy 1 we present a new global model for recent plate velocities revel describing the relative velocities of 19 plates and continental blocks the model is derived from publicly available space geodetic primarily gps data for the period 1993 2000 we include an independent and rigorous estimate for gps velocity uncertainties to assess plate rigidity and propagate these uncertainties to the velocity estimates the velocity fields for north america eurasia and antarctica clearly show the effects of glacial isostatic adjustment and australia appears to depart from rigid plate behavior in a manner consistent with the mapped intraplate stress field two thirds of tested plate pairs agree with the nuvel 1a geologic 3 myr average velocities within uncertainties three plate pairs caribbean north america caribbean south america and north america pacific exhibit significant differences between the geodetic and geologic model that may reflect systematic errors in nuvel 1a due to the use of seafloor magnetic rate data that do not reflect the full plate rate because of tectonic complexities most other differences probably reflect real velocity changes over the last few million years several plate pairs arabia eurasia arabia nubia eurasia india move more slowly than the 3 myr nuvel 1a average perhaps reflecting long term deceleration associated with continental collision several other plate pairs including nazca pacific nazca south america and nubia south america are experiencing slowing that began 25 ma the beginning of the current phase of andean crustal shortening

performance of localization algorithms for teleseismic mid ocean ridge earthquakes the 1999 gakkel ridge earthquake swarm and its geological interpretation in 1999 an unusual earthquake swarm at the 85 e 85 n volcanic centre on the ultraslow spreading gakkel ridge arctic ocean was detected teleseismically the swarm lasted over 9 months and counted 252 events with mb 3 1 it represents the strongest and largest ever recorded mid ocean ridge earthquake swarm and it occurred at a site where spreading rates are only about 10 mm yr 1 we relocated the earthquake swarm comparing the performance of three different localization algorithms 1 the absolute least squares routine hyposat 2 the absolute probabilistic routine nonlinloc and 3 the relative least squares routine mlocate the epicentres as calculated by each algorithm mostly did not agree within their error ellipses thus the choice of location algorithm proved more critical than for example the choice of a local velocity model we compiled a set of well localized events which closely agree in at least two routines mostly mlocate and nonlinloc we conclude that the earthquake swarm of 1999 was related to a spreading episode and shows a complex interplay of tectonic and magmatic events our geological interpretation revealed three phases in swarm activity in the first phase from january 17 up to february 1 fracturing of the crust took place either as a result of or enabling magmatic intrusion seismicity in the second phase from february 2 to april 6 expanded along and across axis it showed signs of magmatic interaction but a clear dyke migration pattern is absent at the beginning of the third phase a distinct break in the event rate suggested a change in the physical process either an adjustment of the stress field to the new regime or a transition to an effusive stage

the rainbow vent fluids 36 14 n mar the influence of ultramafic rocks and phase separation on trace metal content in mid atlantic ridge hydrothermal fluids fluids were collected from the rainbow vent field 36 14 n on the mid atlantic ridge mar during the 1997 diving flores cruise this vent field in ultramafic rocks at a depth of 2300 m is composed of 10 black smokers emitting acidic ph 2 8 fluids at 365 c the low ph of the hot temperature rainbow fluids likely results from seawater ultramafic rock interaction that releases h ions into reducing hydrothermal fluids fluid chemistry is strongly influenced by phase separation generating cl rich brines clem 750 mm strongly enriched with mn fe co ni cu zn ag cd cs pb y and rare earth elements ree ree and transition metal abundance particularly fe and mn in the rainbow fluids is dramatically higher than in other mar fluids the abundance of trace element and ree enrichment is due to the greater solubility of these elements that is strongly favored by cl complexation at low ph and high temperature conditions chondrite normalized ree patterns show strong lree enrichment with evidence of the typical eu anomaly this ree partitioning suggests that either 1 ultramafic rocks represent only a part of rocks leached during hydrothermal alteration and or 2 that the unique rainbow fluid temperature ph and redox state issued from the ultramafic character of leached substratum can produce unique ree partitioning

modeling the behavior of the continental mantle lithosphere during plate convergence in studying orogenic processes the mechanism of viscous rayleigh taylor type removal of gravitationally unstable lithosphere is often invoked to explain the behavior of the mantle lithosphere using numerical models we consider this mechanism and explore alternate styles of deep lithospheric deformation during tectonic convergence the numerical experiments incorporate a mix of viscous and plastic rheologies to model the mechanical evolution of the lithosphere asthenosphere system our results suggest that there are a number of deformational modes of the model mantle lithosphere 1 a dripping or rayleigh taylor type instability 2 an asymmetric underthrusting or subduction 3 symmetric ablative plate consumption 4 slab breakoff the failure and detachment of the strong lithosphere and 5 mixed modes with combinations of these processes the development of the modes is controlled by the rate of convergence associated with the background tectonic regime the density field and the rheology of the mantle lithosphere it is important to determine whether these modes occur in the earth beneath collisional orogens

petrology of ultrahigh pressure rocks from the southern su lu region eastern china abstract in the su lu ultrahigh p terrane eastern china many coesite bearing eclogite pods and layers within biotite gneiss occur together with interlayered metasediments now represented by garnet quartz jadeite rock and kyanite quartzite in addition to garnet omphacite rutile coesite other peak stage minerals in some eclogites include kyanite phengite epidote zoisite talc nyboite and high al titanite the garnet quartz jadeite rock and kyanite quartzite contain jadeite quartz garnet rutile zoisite apatite and quartz kyanite garnet epidote phengite rutile omphacite assemblages respectively coesite and quartz pseudomorphs after coesite occur as inclusions in garnet omphacite jadeite kyanite and epidote from both eclogites and metasediments study of major elements indicates that the protolith of the garnet quartz jadeite rock and the kyanite quartzite was supracrustal sediments most eclogites have basaltic composition some have experienced variable crustal contamination or metasomatism and others may have had a basaltic tuff or pyroclastic rock protolith the su lu ultrahigh p rocks have been subjected to multi stage recrystallization and exhibit a clockwise p t path inclusion assemblages within garnet record a pre eclogite epidote amphibolite facies metamorphic event ultrahigh p peak metamorphism took place at 700 890 c and p 28 kbar at c 210 230 ma the symplectitic assemblage plagioclase hornblende epidote biotite titanite implies amphibolite facies retrogressive metamorphism during exhumation at c 180 200 ma metasedimentary and metamafic lithologies have similar p t paths several lines of evidence indicate that the supracrustal rocks were subducted to mantle depths and experienced in situ ultrahigh p metamorphism during the triassic collision between the sino korean and yangtze cratons

size and exhumation rate of ultrahigh pressure terranes linked to orogenic stage abstract a growing set of data indicates a stark contrast between the evolution of two types of ultrahigh pressure uhp terranes large terranes that evolved slowly over 10 30 myr and small terranes that formed and were exhumed on timescales of

a barotropic inverse tidal model for the arctic ocean 1 we describe high resolution 5 km grid linear dynamics and inverse models of arctic ocean barotropic tides obtained with the osu tidal inversion software otis package the 8 constituent dynamics based model uses the latest ibcao bathymetry and open boundary forcing from the recent tpxo 6 2 global barotropic tidal solution this model performs significantly better than the present benchmark arctic tidal model 14 km grid by z kowalik and a proshutinsky as judged by comparisons with 300 coastal tide gauges the greatest improvements are found in the canadian arctic archipelago nares strait and the baffin bay and labrador sea and can be explained by the higher resolution of the new model in these topographically complex regions the new arctic inverse model assimilates coastal and benthic tide gauges and topex poseidon and ers altimetry for further improvements of the 4 dominant constituents m2 s2 k1 and o1

early to mid silurian extrusion wedge tectonics in the central scandinavian caledonides we present evidence for an extrusion wedge in the scandian fold thrust belt of the central scandinavian caledonides seve nappe complex rb sr multimineral geochronology in synkinematic assemblages indicates simultaneous movements at the normal sense roof shear zone and at the reverse sense floor shear zone between 434 ma and 429 ma a sm nd age of 462 ma from a mylonitic garnet mica schist documents prograde garnet growth and possible incipient subduction pressure temperature pseudosection calculations provide evidence for eclogite facies metamorphic conditions and nearly isothermal decompression at 670 50 c from 17 5 to 14 5 kbar in garnet kyanite mica schists during reverse sense shearing and from 15 to 11 kbar in garnet mica schists during normal sense shearing these data and the presence of decompression related pegmatites dated at 434 ma and 429 ma indicate that the seve nappes form a large scale extrusion wedge this wedge extends along strike for at least 150 km and marks an early stage of ultrahigh pressure metamorphism exhumation and orogenic wedge formation in this part of the scandinavian caledonides predating the major post 415 ma ultrahigh pressure exhumation processes in southwestern norway

seismic imaging of the downwelling indian lithosphere beneath central tibet a tomographic image of the upper mantle beneath central tibet from indepth data has revealed a subvertical high velocity zone from 100 to 400 kilometers depth located approximately south of the bangong nujiang suture we interpret this zone to be downwelling indian mantle lithosphere this additional lithosphere would account for the total amount of shortening in the himalayas and tibet a consequence of this downwelling would be a deficit of asthenosphere which should be balanced by an upwelling counterflow and thus could explain the presence of warm mantle beneath north central tibet

multiple ash layers in late quaternary sediments from the central indian basin abstract we have investigated three sediment cores collected from water depths 5000 m along the transect 76 30 e in close proximity to a fracture zone in the central indian basin cib the cores yielded five volcanic horizons of which four have visual and dispersed shards rhyolitic glass shards of bubble wall platy angular and blocky types were retrieved from various stratigraphic horizons in the cores the abundance of glass shards composition of bulk sediments and 230thexcess ages of the host sediments were used to distinguish the volcanic horizons of the four volcanic horizons three are now newly reported and correspond to ages of 85 107 109 and 142 146 ka while the fourth horizon is of 70 75 ka by using trace element ratios and cr and nb based normative calculations cryptotephra has been identified for the first time from the cib sediment the cryptotephra forms the fifth ash horizon and is of 34 ka a comparison with the published data on volcanic tephra in and around the indian ocean indicate the shard rich horizon srh of 70 75 ka to resemble the younger toba tuffs ytt while the other volcanic horizons that were deposited during different time periods do not correlate with any known marine or terrestrial records these tephra layers have produced a tephrostratigraphic framework across the tectonically and volcanically complex regions of the cib due to the lack of terrestrial equivalents of these tephra it is hypothesized that the newly found volcanic horizons may have been derived from submarine volcanic eruptions multiple layers of submarine volcaniclastic deposits found at water depths as great as 5300 m reaffirm the growing belief that submarine phreatomagmatic eruptions are much more common in the intraplate region of the indian ocean than previously reported

from magmatic generation to uhp metamorphic overprint and subsequent exhumation a rapid cycle of plate movement recorded by the supra subduction zone ophiolite from the north qaidam orogen abstract the rapid subduction and exhumation of high pressure hp and ultrahigh pressure uhp rocks exposed in collisional belts play a critical role in understanding the evolution of geodynamics and plate tectonics in subduction zone environments herein ophiolitic fragments eclogites and amphibolites from a subduction channel exposed in the dulan terrane north qaidam orogen were investigated garnet compositional zoning from the studied eclogites records only a single cycle of subduction and exhumation new uranium lead u pb zircon ages of the current study combined with previous rutile u pb ages from eclogites indicate a rapid cycle of events starting with the generation of the magmatic protolith at ca 446 ma its subsequent subduction reaching a depth of ca 100 110 km at ca 435 ma and its exhumation into shallow portions of the crust which occurred at ca 416 ma thus a rapid 30 million years myr protolith generation subduction exhumation cycle could be recorded according to an ophiolitic rock association with island arc basalt iab like and mid ocean ridge basalt morb to ocean island basalt oib like trace element compositions the slab derived components determined by sr nd hf isotopes and the short time span of only about 11 myr between photolith generation and uhp metamorphism in eclogites the protolith of eclogite is best explained as a supra subduction zone ophiolite this finding likely records the closure of the south qilian ocean and the initiation of a continent arc collision at ca 446 ma furthermore starting with subduction erosion during continent arc collision in the early palaeozoic ophiolitic and arc related materials become subducted in the north qaidam it was inferred that such a rapid succession of events from protolith formation to uhp overprint and final exhumation could be explained by movements of rock material in a subduction channel thus this study provides a new view in understanding the mechanics of different episodes of subduction systems in north qaidam uhp metamorphic belt

metamorphic rocks seek meaningful cooling rate interpreting 40ar 39ar ages in an exhumed ultra high pressure terrane 40ar 39ar dating of metamorphic white mica is commonly used to elucidate the timing of cooling and or exhumation of metamorphic terranes in theory so long as the dated mineral originally crystallised above its closure temperature the yielded 40ar 39ar ages should not vary significantly within or between samples and should vary only due to grain size 3 ma variation depending on the estimated cooling rate infra red laser single grain fusion and ultra violet laser spot muscovite and or phengite data from mafic eclogite bodies and their host gneisses in the nordfjord region of the western gneiss complex wgc norway show that the ages yielded both within and between white micas from the same mafic eclogite sample vary by up to 284 ma ages yielded within the same pelitic gneiss sample vary by up to 28 ma overall the 40ar 39ar phengite dates from mafic eclogites range from 750 to 440 ma and are significantly older than the previously reported 405 400 ma u pb zircon age constraining peak metamorphism in the nordfjord wgc the felsic host gneiss samples which recrystalised during a post peak exhumation related amphibolite facies overprint yield single grain fusion and laser spot 40ar 39ar white mica dates spanning 420 to 385 ma encompassing previous estimates of the timing of the entire metamorphic cycle these data suggest that ar is heterogeneously distributed both within grains and between grains on the cm to sub mm scale in all samples and imply restricted ar transport and inefficient removal during the metamorphic cycle open system zero grain boundary ar concentration behaviour is an inherent assumption in the commonly applied closure temperature formulation but unless this can be demonstrated linking 40ar 39ar ages to temperature is no longer trivial the high spatial and chronological precision afforded by uv laser ablation mean that intra grain ar concentration distributions may be used to assess open vs closed system behaviour the data presented here suggest that open system behaviour cannot be assumed in metamorphic systems even those in which the pressure temperature and time conditions in theory allow for efficient diffusion during the metamorphic cycle high spatial resolution 40ar 39ar data and robust comparison with numerical models of open system behaviour provide the greatest future potential for unravelling the complexities of metamorphic exhumation timescales

observations and modeling of a hydrothermal plume in yellowstone lake acoustic doppler current profiler and conductivity temperature depth data acquired in yellowstone lake reveal the presence of a buoyant plume above the deep hole hydrothermal system located southe

evidence of recent volcanic activity on the ultraslow spreading gakkel ridge seafloor spreading is accommodated by volcanic and tectonic processes along the global mid ocean ridge system as spreading rate decreases the influence of volcanism also decreases and it is unknown whether significant volcanism occurs at all at ultraslow spreading rates 1 5 cm yr 1 here we present three dimensional sonar maps of the gakkel ridge earth s slowest spreading mid ocean ridge located in the arctic basin under the arctic ocean ice canopy we acquired this data using hull mounted sonars attached to a nuclear powered submarine the uss hawkbill sidescan data for the ultraslow spreading approximately 1 0 cm yr 1 eastern gakkel ridge depict two young volcanoes covering approximately 720 km2 of an otherwise heavily sedimented axial valley the western volcano coincides with the average location of epicentres for more than 250 teleseismic events detected in 1999 suggesting that an axial eruption was imaged shortly after its occurrence these findings demonstrate that eruptions along the ultraslow spreading gakkel ridge are focused at discrete locations and appear to be more voluminous and occur more frequently than was previously thought

petrology and geochronology of eclogites from the western segment of the altyn tagh northwestern china abstract the eclogites from the western segment of the altyn tagh occur as lens or boundins within quartzofeldspathic gneisses or pelitic gneisses characterized by amphibolite facies parageneses the petrographic features and reaction textures testify to three main metamorphic stages 1 the peak eclogite facies stage p 15 kbar t 665 880 c 2 the granulitic facies stage p 11 14 kbar t 670 800 c and 3 the amphibolite facies stage p 6 3 9 5 kbar t 619 738 c during decompression the trace element rare earth element and sm nd isotopic data suggest that most of the eclogites have protolith features resembling transitional type t type mid ocean ridge basalt morb the well preserved eclogite was selected for sm nd and u pb isotopic dating the sm nd isotopic data yield a whole rock garnet omphacite isochron of 500 10 ma age the u pb isotopic measurement of zircons from the same eclogite shows that the four grain populations are near concordant and are well plotted on concordia giving a weighted mean age of 503 9 5 3 ma the two different geochronological methods yield a similar age which is interpreted as the peak metamorphic age of eclogites and reflects the existence of a mountain root related to continental subduction during early paleozoic moreover based on the comparison with similar rocks in the northern margin of the qaidam basin a displacement of 350 400 km for the altyn tagh fault have been estimated

geometry and structural evolution of ultra high pressure and high pressure rocks from the dora maira massif western alps italy abstract the crystalline nappes of the dora maira massif western alps essentially made of continental material from the upper crust show petrological relics of an ultra high pressure uhp to high pressure hp cold eclogite eoalpine metamorphism they also display relics of uhp hp structures preserved in boudins and or within large uhp porphyroclasts in a retrograde greenschist facies regional deformation fabric the greenschist facies overprint has the character of a shallow dipping mylonitic foliation s m bearing a penetrative stretching lineation l m which roughly parallels the axes of coeval isoclinal folds shear sense markers indicate a w verging overthrust mechanism the uhp and hp relic structures arc of variable nature the coexistence of equant and inequant either symmetric or asymmetric fabrics indicates that the deformation at uhp hp conditions was strongly heterogeneous and partitioned this is also supported by the local preservation of hercynian magmatic fabrics the uhp and hp deformation involved at least locally rotational components although less intensive than during the latter retrograde stage the regional structural evolution is envisaged as follows i the eoalpine subducted crust was subdivided into lenticular bodies surrounded by uhp hp shear zones the main part of the exhumation processes remains unconstrained due to the sparseness and late rotation of the uhp hp structural relics conflicting models are possible depending on the interpretation of the early sense of movement normal vs reverse along the faults that limit the lens shaped units and ii the late heterogeneous regional greenschist deformation can be attributed to the eocene collapse of the alpine orogenic wedge

rheological and geodynamic controls on the mechanisms of subduction and hp uhp exhumation of crustal rocks during continental collision insights from numerical models while subduction of crustal rocks is increasingly accepted as a common scenario inherent to convergent processes involving continental plates and micro continents its occurrence in each particular context as well as its specific mechanisms and conditions is still debated the presence of ultra high pressure uhp terranes is often interpreted as a strong evidence for continental subduction subduction of continental crust since the latter is seen as the most viable mechanism of their burial to uhp depths yet if one admits nearly lithostatic pressure conditions in the subduction interface or channel the presumed links of continental subduction to exhumation of high and ultra high pressure hp uhp units also remain a subject of controversy despite the fact that recent physically consistent thermo mechanical numerical models of convergent processes suggest that subduction can create specific mechanisms for uhp exhumation we hence review and explore possible scenarios of subduction of continental crust and their relation to exhumation of hp and uhp rocks as inferred from last generation of thermo mechanical numerical models accounting for thermo rheological complexity and structural diversity of the continental lithosphere the inferences from these models are matched with the petrology data in particular with p t t paths allowing for better understanding of subtleties of both subduction and burial exhumation mechanisms numerical models suggest that exhumation and continental subduction are widespread but usually transient processes that last for less than 5 10 myr while long lasting 10 15 myr subduction can take place only in rare cases of fast convergence of cold strong lithospheres e g india the models also show that tectonic heritage can play a special role in subduction exhumation processes in particular when thicker continental terrains are embedded in subducting oceanic plate exhumation of uhp terranes results in the formation of versatile metamorphic belts and domes and in series of slab roll back and exhumation events with remarkably different p t t records

ultrarapid exhumation of ultrahigh pressure diamond bearing metasedimentary rocks of the kokchetav massif kazakhstan the diamond bearing ultrahigh pressure kokchetav massif recrystallized at eclogite facies conditions deep in the mantle at 180 km depth at 535f3 ma and yet new 40 ar 39 ar ages suggest that it may have been exhumed to crustal depths as indicated by closure of mica to ar loss by f529 ma these data indicate a possible exhumation rate of tens of kilometers per million years i e a rate that is comparable to rates of horizontal plate motion and subduction d 2003 elsevier b v all rights reserved

the late jurassic tithonian a greenhouse phase in the middle jurassic early cretaceous cool mode evidence from the cyclic adriatic platform croatia well exposed mesozoic sections of the bahama like adriatic platform along the dalmatian coast southern croatia reveal the detailed stacking patterns of cyclic facies within the rapidly subsiding late jurassic tithonian shallow platform interior over 750 m thick ca 5 6 myr duration facies within parasequences include dasyclad oncoid mudstone wackestone floatstone and skeletal peloid wackestone packstone shallow lagoon intraclast peloid packstone and grainstone shoal radial ooid grainstone hypersaline shallow subtidal intertidal shoals and ponds lime mudstone restricted lagoon fenestral carbonates and microbial laminites tidal flat parasequences in the overall transgressive lower tithonian sections are 1 4ae5 m thick and dominated by subtidal facies some of which are capped by very shallow water grainstone packstone or restricted lime mudstone laminated tidal caps become common only towards the interior of the platform parasequences in the regressive upper tithonian are dominated by peritidal facies with distinctive basal oolite units and well developed laminate caps maximum water depths of facies within parasequences estimated from stratigraphic distance of the facies to the base of the tidal flat units capping parasequences were generally 4 m and facies show strongly overlapping depth ranges suggesting facies mosaics parasequences were formed by precessional 20 kyr orbital forcing and form parasequence sets of 100 and 400 kyr eccentricity bundles parasequences are arranged in third order sequences that lack significant bounding disconformities and are evident on accommodation fischer plots of cumulative departure from average cycle thickness plotted against cycle number or stratigraphic position modelling suggests that precessional sea level changes were small several metres as were eccentricity sea level changes or precessional sea level changes modulated by eccentricity supporting a global hot greenhouse climate for the late jurassic tithonian within the overall cool mode of the middle jurassic to early cretaceous

rheology of the plate interface dissolution precipitation creep in high pressure metamorphic rocks abstract subduction zone models invoke deformation to be concentrated along the plate interface in a region of particularly low temperature geophysical observations do not provide constraints on temperature stress and deformation patterns with desired resolution in contrast the record of high pressure metamorphic rocks exhumed from subduction zones provides details on p t history deformation mechanisms and stress state albeit not readily correlated with the former dynamic situation on larger scale here we review available information on dissolution precipitation creep dpc in high pressure metamorphic rocks which if representative for subduction zones in general can pose constraints on conditions rheology and flow patterns along the plate interface the key observations and conclusions are that 1 deformation is typically highly inhomogeneous and localized into shear zones 2 stresses are generally too low to drive crystal plastic deformation 3 microfabrics suggest dissolution precipitation creep to be the predominant deformation mechanism 4 an aqueous fluid at quasi lithostatic pressure is available throughout allowing for tensile fracturing and crack healing or sealing 5 low stress combined with high strain rates required for localized deformation at typical subduction rates implies low viscosity and 6 contribution of shear heating to the thermal budget of subduction zones should be moderate the dominant deformation mechanism dpc is reviewed in some detail including experimental and theoretical approaches various examples of dpc in high pressure metamorphic rocks are illustrated emphasizing the role of interphase boundaries as sites of dissolution rheology governed by dpc is proposed to control interplate coupling and development of a subduction channel with return flow being a likely candidate for rapid exhumation of high pressure metamorphic rocks

dispersal of volcaniclasts during deep sea eruptions settling velocities and entrainment in buoyant seawater plumes abstract we use tank experiments to measure settling rates of deep sea volcaniclastic material recovered from the arctic 85 e gakkel ridge and pacific juan de fuca ridge loihi seamount oceans we find that clast size and shape exert a strong influence on settling velocity with velocities of 30 cm s for large 8 mm blocky clasts compared to velocities of 2 5 cm s for small

diapiric exhumation of earth s youngest uhp eclogites in the gneiss domes of the d entrecasteaux islands papua new guinea abstract the woodlark rift in papua new guinea hosts the world s youngest 2 8 ma eclogite facies rocks and extensional deformation has played a key role in exhuming these u hp rocks at rates of 20 mm yr during the eocene papuan arc continent collision australian plate derived continental rocks were subducted to u hp depths there they remained for up to 30 m y until the pliocene when asthenospheric circulation ahead of the west propagating woodlark spreading ridge introduced heat and fluids this caused rocks to break away from the paleosubduction channel recrystallize in the eclogite facies and rise as rayleigh taylor instabilities the diapirs ascended adiabatically undergoing partial melting which lowered their viscosity and increased buoyancy u hp crust ponded near the moho at 2 4 ma thickening the crust to 40 km 11 kb domal uplifts emerged above sea level and these are still underlain by an unusually thick crust 26 km for a rift that has stretched by factor of 3 since 6 ma after ponding they acquired a flat lying foliation during amphibolite facies retrogression vertical shortening accompanied the gravitationally driven outflow of ponded lower crust the weak material was extended parallel to the rift margin thinning ductilely by w k 0 2 and was mechanically decoupled from and orthogonal to plate motion in the rift top e shear fabrics suggest that this flow was westward perhaps driven by isostatic stresses towards a strongly thinned rift corridor ahead of the woodlark spreading ridge at 100 c per m y and were mechanically incorporated into the rift s upper crust final exposure was by normal faulting and minor erosion such exhumation may also apply to other u hp terranes where less evidence for moho ponding is preserved

plate velocity exhumation of ultrahigh pressure eclogites in the pakistan himalaya u pb ages of zircon and allanite from the coesite bearing ultrahigh pressure uhp units in the kaghan valley northern pakistan demonstrate that peak uhp metamorphism along the northern margin of the indian plate occurred at 46 4 0 1 ma at peak pressure temperature conditions of 27 5 kbar 100 km and 720 770 c much lower pressure retrogressive growth of titanite took place between 46 4 and ca 44 ma indicating that the eclogites were exhumed to 35 km depth at or before 44 ma implying very rapid exhumation rates within the mantle of 30 80 mm yr or more comparable to rapid plate velocities once entrained in the base of the crust the eclogites followed a slower cooling history from 45 ma similar to the amphibolite facies gneisses of the pakistan himalaya

rheologic controls on slab dynamics several models have been proposed to relate slab geometry to parameters such as plate velocity or plate age however studies on the observed relationships between slab geometry and a wide range of subduction parameters show that there is not a simple global relationship between slab geometry and any one of these other subduction parameters for all subduction zones numerical and laboratory models of subduction provide a method to explore the relative importance of different physical processes in determining subduction dynamics employing 2 d numerical models with a viscosity structure constrained by laboratory experiments for the deformation of olivine we show that the observed range in slab dip and the observed trends between slab dip and convergence velocity subducting plate age and subduction duration can be reproduced without trench motion i e slab roll back for locations away from slab edges successful models include a stiff slab that is 100 1000 times more viscous than previous estimates from models of plate bending the geoid and global plate motions we find that slab dip in the upper mantle depends primarily on slab strength and plate boundary coupling with a small dependence on subducting plate age once the slab sinks into the lower mantle the primary processes controlling slab evolution are 1 the ability of the stiff slab to transmit stresses up dip 2 resistance to slab descent into the higher viscosity lower mantle and 3 subduction induced flow in the mantle wedge corner

extension of thickened and hot lithospheres inferences from laboratory modeling 1 the extension of a previously thickened lithosphere is studied through a series of analogue experiments the models deformed in free and boundary controlled gravity spreading conditions that simulate the development of wide rift type and core complex type structures in models the development of structures mainly depends on boundary velocity and therefore on bulk strain rate wide rifts are of tilted block type at high strain rate and of horst and graben type at low strain rate the development of metamorphic core complex type structures is enhanced by low strain rates and by the presence of weak heterogeneities within the ductile crust core complexes result from a necking instability of the upper crust creating a graben which further widens allowing the rise and exhumation of a ductile layer dome an upward convex detachment flat on top of the dome and steeper on dome limb appears not to be the primary cause of the core complex development but its consequence

slow subduction of a thick ultrahigh pressure terrane 1 high precision lu hf and sm nd ages reported here cover a large portion of the western gneiss region wgr ultrahigh pressure uhp terrane norway and range from 413 9 3 7 to 397 1 4 8 ma collectively new and existing geochronologic data for eclogites demonstrate that eclogite facies metamorphism occurred over a large area 60 000 km2 for an unexpectedly long period of 20 ma a thermal model using slow subduction 2 4 mm a 1 of a relatively warm and thick slab can reproduce the observed p t t history of the wgr uhp terrane indicating that large uhp terranes with protracted peak or near peak histories probably reflect slow subduction of relatively thick crustal sections