controls on turbulent mixing in a strongly stratified and sheared tidal river plume abstractconsiderable effort has been made to parameterize turbulent kinetic energy tke dissipation rate e and mixing in buoyant plumes and stratified shear flows here a parameterization based on kunze et al is examined which estimates e as the amount of energy contained in an unstable shear layer ri ric that must be dissipated to increase the richardson number ri n2 s2 to a critical value ric within a turbulent decay time scale observations from the tidal columbia river plume are used to quantitatively assess the relevant parameters controlling e over a range of tidal and river discharge forcings observed e is found to be characterized by kunze et al s form within a factor of 2 while exhibiting slightly decreased skill near ri ric observed dissipation rates are compared to estimates from a constant interfacial drag formulation that neglects the direct effects of stratification this is found to be appropriate in energetic regimes when the bulk averaged richardson number rib is less than

sedimentation from buoyant muddy plumes in the presence of interface mixing an experimental study a series of flume experiments were conducted to study the sediment removal rates characterized by an effective settling velocity from buoyant muddy plumes overriding clear saltwater at steady state conditions under different richardson numbers and initial suspended sediment concentrations the experiments were all carried out in the subcritical regime leading to cusps style instabilities at the interface of the two fluids flocculation in the experiments was not suppressed yet flocs remained small while in the plume layer data from the experiments allowed for calculation of the individual floc settling velocity ws f and the overall effective settling velocity ws eff i e that velocity needed to predict the downward flux with cws eff results showed that ws eff was greater than the floc settling velocity in all runs ws eff was found to increase with plume velocity and interface mixing but not with plume concentration the difference between the effective and floc settling velocities was therefore attributed to turbulent diffusion brought on by mixing at the interface and not to convective sedimentation or leaking the turbulence enhanced settling velocity ws t was found to be a strong function of the richardson number and conceptual and empirical equations are presented to predict ws t as a function of the plume surface velocity ups and richardson number ri this analysis showed that ws t upsri 2 in the runs with low richardson number ws t was approximately equal to the floc settling velocity leading to an overall doubling of the effective settling velocity relative to that of the individual particles this article is protected by copyright all rights reserved

shear instability in a highly stratified estuary abstract shear instability is found to be the principal mechanism of vertical exchange within the pycnocline of a salt wedge estuary a field program involving high resolution velocity and density measurements as well as high frequency acoustic imagery allowed direct comparison of instantaneous richardson number distributions to the occurrence of shear instability the theoretical stability threshold of 0 25 is consistent with the measurements based on estimates of gradients that contain the mean as well as fluctuations due to internal waves an effective stability threshold based on mean gradients is found to be approximately one third reflecting a significant contribution of internal wave shear the integral effect of the mixing process is to homogenize the gradients of velocity and density producing linear profiles of these quantities across the pycnocline a turbulent prandtl number of unity is suggested by the vertical distributions of velocity and density during periods of active vertical mixin

turbulence and entrainment in a buoyant surface plume in winter tidal outflow from the leschenault estuary forms a buoyant sheared stratified plume in koombana bay on each outgoing tide the turbulence in the plume and the local entrainment into the plume are examined based on measurements collected in an intensive field program coherency measurements between horizontally separated sensors are used to examine the anisotropy of the turbulence the effects of viscosity and buoyancy on the turbulence are studied using an activity diagram which is also an indicator of the mixing efficiency and total mixing the osborne cox model is tested and found to be valid in the plume

radial spreading of buoyant surface plumes in coastal waters shallow buoyant surface plumes in coastal waters sometimes develop by nearly radial spreading from a continuous source such as at a river mouth such plumes often display a ring structure where buoyant water is concentrated between fronts at both the leading and trailing edges of the expanding ring these rings often occur in multiple concentric forms a model of the radial time dependent spreading of a shallow buoyant layer over a motionless deep ambient layer is developed it includes fronts as discontinuities and treats the remainder of the plume flow as inviscid gravitational spreading controlled by nonlinear internal waves calculations by the method of characteristics are presented that show the evolution of a clear ring structure the combination of rapid spreading in the radial geometry together with reflection of internal waves off the frontal boundary at the plume leading edge is shown to be necessary for ring formation a mechanism is suggested for the formation of multiple rings through the shear flow instability inherent in radial spreading

tectonics of western tibet between the tarim and the indus a tarim to indus traverse provides insight into the tectonics of western tibet the kunlun was the site of a mid paleozoic collision at least three phases of post paleozoic accretion have thickened the blanket of sediments that covers western tibet sizeable parts of western qiangtang have remained stable however since the mid mesozoic since the neogene deformation and volcanism have been localized near the edges of the plateau strike slip motion along the karakorum and altyn tagh faults has been coeval with overthrusting in the himalayas and kunlun such slip partitioning and the volcanism appear to result simply from northward subduction of india and southward subduction of the tarim as tibet is extruded eastwards by india s penetration into asia

zircon fission track ages from the gasherbrum diorite karakoram range northern pakistan the gasherbrum peaks in the himalaya of pakistan reach elevations of 8000 m the relief between the peaks and the adjacent valley baltoro glacier is in excess of 3000 m eight samples of the early cretaceous gasherbrum diorite at elevations between 4880 and 7165 m on gasherbrum iv were collected for fission track dating zircon fission track ages from the gasherbrum diorite vary from early cretaceous to middle tertiary in age there is no consistent pattern between age and elevation the cretaceous ages indicate that these rocks were never deeply buried i e heated to temperatures in excess of 175 c to reset the zircons during cenozoic time these results also indicate that the uplift of this part of the himalaya has been either very rapid and recent or very slow since early cretaceous time this latter possibility is not consistent with the high relief at gasherbrum and what is known about regional tectonics gasherbrum iv zircons currently at approximately 4880 m have never been at depths greater than 6 km and less than 3 km of material has been removed from the top of the range by erosion since the early cretaceous rapid uplift has occurred very recently and erosion rates have notmore been able to keep pace with this uplift less

ebb tide dynamics and spreading of a large river plume abstract momentum balances in the near field region of a large tidally pulsed river plume are examined the authors concentrate on a single ebb tide of the columbia river plume using the regional ocean modeling system roms configured to hindcast flow conditions on the washington and oregon shelves and in the columbia river estuary during ebb plume interior streamwise balances are largely between advection pressure gradient and frictional forces stream normal balances in this region reduce to centrifugal cross stream pressure gradient and coriolis terms i e the gradient wind balance commonly assumed in river plume bulge investigations temporal derivatives are most important at the plume front and as the ebb progresses winds were light and contributed little to the force balance midebb stress and vertical salt flux were largest at a midplume depth where stratification and vertical shear were also high consistent with shear induced mixing internal stress slows the spreading plume consi

analyses of internal solitary waves generated at the columbia river plume front using sar imagery 1 internal solitons generated at the columbia river plume front during ebb tide are displayed on a synthetic aperture radar sar image taken on 26 july 2004 at 14 24 22 utc scale analyses suggest that these internal solitons belong to the finite depth category a theoretical model relating the radar backscatter cross section to dynamic parameters of internal solitons in the continuous stratification is developed using the model and background stratification data collected by the river influences on shelf ecosystem rise project in july 2004 we extract the soliton dynamic parameters of the half width amplitude phase speed and average energy flux per crest length along a typical section across the internal solitons the observed stratification is simulated by using a theoretical model parameterized by pycnocline depth thickness and maximum buoyancy frequency the half width amplitude phase speed and energy flux of the leading soliton at the section are 119 3 m 9 3 m 0 69 m s 1 and 417 4 w m 1 with the uncertainty ranges of 14 7 m 2 6 m 0 10 m s 1 and 281 6 w m 1 respectively sensitivity analyses reveal that the extraction of the soliton half width from the sar image is not affected by the stratification the amplitude is less influenced by the pycnocline thickness than the depth and is independent of the maximum buoyancy frequency

initial expansion of the columbia river tidal plume theory and remote sensing observations 1 analysis of the columbia river tidal plume using lagrangian frontal equations provides a concise description of the evolution of frontal depth h velocity u reduced gravity g and frontal internal froude number fr because the estuary mouth is narrow the initial radial plume motion is supercritical fr 1 for up to 12 hours understanding this supercritical phase is vital because plume properties change rapidly with strong ecosystem impacts to analyze this expansion analytical and numerical models the latter with three mixing formulations were tested model results are compared to synthetic aperture radar images to verify that the predicted frontal properties are realistic lagrangian theory provides especially simple constraints independent of the mixing model on spatial variations in fr and ug h for parameters representative of the columbia river plume the plume spreads 10 35 km and thins to 25 60 of its initial depth before becoming subcritical after liftoff fr increases as the front accelerates and thins it then decreases to unity g decreases but more slowly than h and u h controlled by a balance between spreading and mixing first decreases then increases the strength of vertical mixing and the mixing efficiency ef ratio of buoyancy flux to dissipation both play a significant role in determining plume properties and it is important to include both the horizontal gradient in g and surface slope component of the pressure gradient a model with an ellison turner type entrainment scheme predicts the plume trajectory better than one that assumes a constant interfacial gradient richardson number

a traverse through the western kunlun xinjiang china tentative geodynamic implications for the paleozoic and mesozoic the northern part of the western kunlun southern margin of the tarim basin represents a sinian rifted margin to the south of this margin the sinian to paleozoic proto tethys ocean formed south directed subduction of this ocean beneath the continental southern kunlun block during the paleozoic resulted in the collision between the northern and southern kunlun blocks during the devonian the northern part of the paleo tethys ocean located to the south of the southern kunlun was subducted to the north beneath the southern kunlun during the late paleozoic to early mesozoic this caused the formation of a subduction accretion complex including a sizeable accretionary wedge to the south of the southern kunlun a microcontinent or oceanic plateau which we refer to as uygur terrane collided with the subduction complex during the late triassic both elements together represent the kara kunlun final closure of the paleo tethys ocean took place during the early jurassic when the next southerly located continental block collided with the kara kunlun area from at least the late paleozoic to the early jurassic the tarim basin must be considered a back arc region the kengxiwar lineament which connects the karakorum fault in the west and the ruogiang xingxingxia altyn tagh fault zone in the east shows signs of a polyphase strike slip fault along which dextral and sinistral shearing occurred

late pleistocene stratigraphy of iodp site u1396 and compiled chronology offshore of south and south west montserrat lesser antilles marine sediments around volcanic islands contain an archive of volcaniclastic deposits which can be used to reconstruct the volcanic history of an area such records hold many advantages over often incomplete terrestrial data sets this includes the potential for precise and continuous dating of intervening sediment packages which allow a correlatable and temporally constrained stratigraphic framework to be constructed across multiple marine sediment cores here we discuss a marine record of eruptive and mass wasting events spanning 250 ka offshore of montserrat using new data from iodp expedition 340 as well as previously collected cores by using a combination of high resolution oxygen isotope stratigraphy ams radiocarbon dating biostratigraphy of foraminifera and calcareous nannofossils and clast componentry we identify five major events at soufriere hills volcano since 250 ka lateral correlations of these events across sediment cores collected offshore of the south and south west of montserrat have improved our understanding of the timing extent and associations between events in this area correlations reveal that powerful and potentially erosive density currents traveled at least 33 km offshore and demonstrate that marine deposits produced by eruption fed and mass wasting events on volcanic islands are heterogeneous in their spatial distribution thus multiple drilling coring sites are needed to reconstruct the full chronostratigraphy of volcanic islands this multidisciplinary study will be vital to interpreting the chaotic records of submarine landslides at other sites drilled during expedition 340 and provides a framework that can be applied to the stratigraphic analysis of sediments surrounding other volcanic islands

the karakorum block in central asia from ordovician to cretaceous abstract the karakorum block records a predominantly marine ordovician to cretaceous sedimentary history six major sedimentary cycles are recognised the oldest sediments early ordovician transgress over a crystalline basement the 1 km thick ordovician silurian mostly shaly succession contains rare carbonate intercalations in the early devonian a wide peritidal platform spread over the craton sedimentation rates were low 10 20 m ma and upper devonian probable lower carboniferous carbonates and clastics rest unconformably on the early devonian carbonates no evidence has been found for either late carboniferous or glaciogenic deposits sedimentation rates increased during the permian up to 50 m ma with thicknesses between 1 and 2 5 km three steps are identified 1 a huge alluvial to marginal marine terrigenous prism aggraded during the asselian sakmarian 2 the artinskian murgabian is characterised by local emergence and erosion linked to extension with block rotations 3 greater differentiation occurred from late permian to middle triassic when a peritidal carbonate flat developed in the southwest facing a deeper basin to the northeast carbonate sediments prevailed throughout the carnian norian the permo triassic evolution is interpreted as the passive margin stage of the karakorum block which previously belonged to the perigondwanian fringe when like mega lhasa it drifted northward on the tethyan transit plate mega lhasa is considered as a collage of blocks possibly separated by thinned crust or short lived seas with ocean crust quartzo lithic sandstones with grains of mafic volcanics and serpentinite overlain with gentle unconformity by pliensbachian toarcian red sandstones with sedimentary and metasedimentary clasts record the eo cimmerian deformation in the karakorum this orogenic episode was over by the aalenian when a shallow water carbonate ramp aggraded onto the previously emergent area the pre barremian upper jurassic and lower cretaceous are poorly documented in post barremian times the sedimentary succession of n karakorum was severely deformed in huge thrust sheets including slabs of crystalline basement a coarse conglomerate of mid cretaceous age sealed the thrust sheet edifice finally isolated campanian pelagic mudstones are recorded anticlockwise rotation of mega lhasa followed the late triassic docking of the iranian spur using the latter as pivot the rotation was completed during the middle jurassic when se pamir shaksgam and karakorum joined the qiangtang more closely assembling mega lhasa as it approached the asian margin

distribution and characteristics of mixed transitional mid permian late artinskian ufimian marine faunas in asia and their palaeogeographical implications abstract the permian marine biogeography of the asian region is characterised by the development of three broad realms boreal tethyan and gondwanan in the early early permian asselian to early artinskian there seems to have existed sharp biogeographical boundaries with no transitional zones between the realms presumably due to the effect of gondwanan glaciation and sharp climatic zonation the mid permian late artinskian to ufimian of the asian region is distinguished by the rise of two broad transitional zones in which mixed marine faunas flourished the northern transitional zone north and northeast china sikhote alin central and northeast japan is characterised by an admixture of both warm water cathaysian and cold temperate boreal genera the southern transitional zone arabian peninsula iran afghanistan the karakorum southeast and central pamir salt range central and northwest tibet shan thai terrane timor and western irian jaya on the other hand is distinguished by incorporating faunal elements of both gondwanan and cathaysian origin the mixed faunas of both transitional zones share several antitropically distributed genera e g monodiexodina lytvolasma and spiriferella and are succeeded by palaeotropical tethyan faunas of late permian age it has been postulated that the mixed mid permian fauna of the northern transitional zone was probably formed in an epicontinental sea which was connected to both the asian tethys and the arctic which facilitated the migration of cathaysian and boreal forms into the epicontinental sea and subsequent faunal mixing the formation of the mixed faunas of the southern transitional zone on the other hand is probably related to the breaking off of the cimmerian blocks from northern gondwana and subsequent drifting into a lower latitudinal zone with geographical proximity to both gondwana and cathaysia during the mid permian

reply to the comments on middle cretaceous back arc formation and arc evolution along the asian margin the shyok suture zone on northern ladakh nw himalaya by weinberg and dunlap

microstructure observations of turbulent mixing in a partially mixed estuary part ii salt flux and stress abstract turbulent mixing in the water column was observed with a microstructure profiler in the hudson river estuary during two cruises in summer and fall 1995 the focus is on the estimation of turbulent salt flux and turbulent stress from measured viscous dissipation rates e and on the tidal and fortnightly variability of these fluxes in estimating eddy viscosity km and eddy diffusivity k the authors follow measurement modeling techniques of busch and osborn while prescribing a variable flux richardson number rf dependent upon the gradient richardson number ri it is argued that a steady state production dissipation balance holds in the turbulent kinetic energy budget all turbulence characteristics varied strongly over semidiurnal tidal cycles and over the fortnightly cycle subject to complications arising from nontidal flows the strongest mixing occurred during flood on neap tides and during ebb on spring tides in the lower part of the water column during floods and spring ebb km

reconstruction of the altyn tagh fault based on u pb geochronology role of back thrusts mantle sutures and heterogeneous crustal strength in forming the tibetan plateau 1 reconstructing deformation along the northwestern margin of the tibetan plateau is critical for evaluating the relative importance of microplate versus continuum models of the indo asian collision questions regarding this margin s evolution are as follows 1 what is the total offset along the sinistral altyn tagh strike slip system 2 how has that offset been absorbed in the western kunlun shan 3 why does the n s width of the plateau vary along strike ion microprobe u pb zircon apparent ages of 17 plutons from nw tibet together with regional geologic observations define a discrete e w trending boundary between two tectonic belts that has been offset along the altyn tagh system by 475 70 km kinematic arguments indicate that this offset cannot be the result of north directed thrusting in the western kunlun shan therefore we propose that south directed faulting in the tianshuihai thrust belt both offset the tectonic boundary and produced the asymmetry in the plateau shortening appears to have been absorbed in the upper crust by thin skinned thrusting in the middle lower crust by east directed ductile flow and or subduction and in the mantle by north dipping subduction factors controlling the formation of the south directed thrust system appear to be the contrast between the rigid tarim and the weaker songpan ganzi flysch belt and a mantle suture inherited from late paleozoic subduction the evolution of western tibet leads to a view of continental deformation that integrates elements of the microplate model e g plate like mantle and crust mantle decoupling with aspects of the continuum model weak crustal flow beneath the plateau

distal ash hurricane pyroclastic density current deposits from a ca 2000 yr b p plinian style eruption of mount pelee martinique distribution grain size characteristics and implications for future hazard plinian style activity has not occurred on any volcano in the lesser antilles since european settlement however pumice rich deposits from this eruptive style are found throughout the lesser antilles and such eruptions were witnessed by pre columbian native american populations one of these eruptive events from mount pelee on martinique occurring between 1800 and 2200 yr b p produced pumice and ash flow deposits that followed the main drainage channels on the volcano ash hurricane deposits that both mantle the volcano s flanks slopes up to 25 and extended at least 20 km from the crater we examined the latter in the context of modern developments in understanding transport and depositional mechanics of pyroclastic density currents based on grain size analysis they are subdivided into proximal and distal deposits the distal deposits become progressively more depleted in fines and enriched in crystals with distance from the crater we conclude these are deposits of dilute or inertia dominated pyroclastic overcurrents which decoupled from their underflows as they surmounted the topographic barrier formed by the older pitons du carbet volcanic center a combination of topography and ingestion of air and water vapor from tropical vegetation perhaps caused the considerable expansion and liftoff and resulting extreme crystal enrichment their areal distribution suggests much of the northern half of martinique must have been severely affected and the zone of devastation extended at least to the northern outskirts of the capital fort de france although their final runout distance may have extended significantly further south major coignimbrite plumes dispersing ash and aerosols over thousands of kilometers would today pose hazards to aviation

syn kinematic emplacement of the pangong metamorphic and magmatic complex along the karakorum fault n ladakh this paper investigates the age p t conditions and kinematics of karakorum fault kf zone rocks in the nw part of the himalaya karakorum belt granulite to greenschist facies assemblages were developed within the kf zone during strike slip shearing the granulites were formed at high temperature 800 c 5 5 kbar were subsequently retromorphosed into the amphibolite facies 700 750 c 4 5 kbar and the greenschist facies 350 400 c 3 4 kbar the tangtse granite emplaced syn kinematically at the contact between a lt and the ht granulite facies intrusion occurred during the juxtaposition of the two units under amphibolite conditions microstructures observed within the tangtse granite exhibit a syn magmatic dextral s c fabric compiled u pb and ar ar data show that in the central kf segment granulite facies metamorphism occurred at a minimum age of 32 ma subsequent amphibolite facies metamorphism at 20 18 ma further shearing under amphibolite facies 650 500 c was recorded at 13 6 0 9 ma and greenschist facies mica growth at 11 ma these data give further constrains to the age of initiation and depth of the karakorum fault the granulite facies conditions suggest that the kf accommodating the lateral extrusion of tibet could be at least a crustal or even a lithosphere scale shear zone comparable to other peri himalayan faults

tectonic evolution of the qiangtang block northern tibet during the late cisuralian late early permian evidence from fusuline fossil records abstract the tectonic evolution of the qiangtang block in the qinghai tibetan plateau has been a controversial subject for a long time in this paper the discovery of new stratigraphic and fusuline fossil evidence from the permian sequences qudi and lugu formations of the qiangtang block is reported and the palaeogeographical position and tectonic history of this block during the late cisuralian late early permian are discussed the qudi formation is typified by thick turbidite deposits and contains artinskian fusulines such as pseudofusulina and chalaroschwagerina the fusulines were deposited as grains involved in debris flow deposits suggesting a synchronicity with the depositional time of the turbidites the subsequent lugu formation is dominated by seamount type carbonates with an irregular basalt base fusulines cancellina pseudodoliolina and parafusulina in the base of the carbonates confirm its age as middle kungurian the transition from the turbidite qudi formation to the seamount lugu formation is here interpreted to be a continuous depositional process recording the qiangtang block s separation from the indian plate this separation signaled the opening of the neotethys ocean between the qiangtang block and the indian plate palaeogeographically the qiangtang block s separation is comparable with the baoshan block s separation in the east and central pamir s separation in the west by contrast the ultimate opening of the neotethys ocean by the separation of india pakistan and northern oman is apparently much later than this event recorded in the qiangtang block consequently it is interpreted that the opening of the neotethys ocean in the whole northern gondwanan margin is a diachronous series of events

geologic offsets across the northern karakorum fault implications for its role and terrane correlations in the western himalayan tibetan orogen abstract while the 1000 km long karakorum right slip fault is one of the most prominent structures in the himalayan tibetan orogen considerable disagreement exists as to the total magnitude of offset across the fault and the role it has played in accommodating convergence between india and asia using satellite images i have correlated a well defined carbonate unit the late triassic early jurassic aghil formation across the northern portion of the karakorum fault from the southwest pamir to the tianshuihai terrane of western tibet the northern and southern exposure limits of the aghil formation have 149 167 km of separation which i interpret to represent the magnitude of displacement on the northern karakorum fault these values overlap with estimates correlating the bangong nujiang and shyok sutures across the central karakorum fault and maximum offsets of the miocene baltoro granite and rule out correlations of the bangong nujiang suture and the rushan pshart suture the displacement yields a geologic slip rate of 10 8 1 3 mm yr using an initiation age of 14 7 1 ma or 6 89 0 8 mm yr with an initiation age of 23 1 ma this result supports previous work showing limited offset across the karakorum fault and indicates that the fault that has not accommodated either large magnitudes i e 100 s of km of eastward lateral extrusion of tibet or hundreds of kilometers of offset between terranes of the western and central portions of the orogenic belt

timing origin and emplacement dynamics of mass flows offshore of se montserrat in the last 110 ka implications for landslide and tsunami hazards eruption history and volcanic island evolution 1 mass flows on volcanic islands generated by volcanic lava dome collapse and by larger volume flank collapse can be highly dangerous locally and may generate tsunamis that threaten a wider area it is therefore important to understand their frequency emplacement dynamics and relationship to volcanic eruption cycles the best record of mass flow on volcanic islands may be found offshore where most material is deposited and where intervening hemipelagic sediment aids dating here we analyze what is arguably the most comprehensive sediment core data set collected offshore from a volcanic island the cores are located southeast of montserrat on which the soufriere hills volcano has been erupting since 1995 the cores provide a record of mass flow events during the last 110 thousand years older mass flow deposits differ significantly from those generated by the repeated lava dome collapses observed since 1995 the oldest mass flow deposit originated through collapse of the basaltic south soufriere hills at 103 110 ka some 20 30 ka after eruptions formed this volcanic center a 1 8 km3 blocky debris avalanche deposit that extends from a chute in the island shelf records a particularly deep seated failure it likely formed from a collapse of almost equal amounts of volcanic edifice and coeval carbonate shelf emplacing a mixed bioclastic andesitic turbidite in a complex series of stages this study illustrates how volcanic island growth and collapse involved extensive large volume submarine mass flows with highly variable composition runout turbidites indicate that mass flows are emplaced either in multiple stages or as single events

bottom stress estimates from vertical dissipation rate profiles on the continental shelf abstract measurements of the near bottom distribution of the turbulent dissipation rate on the continental shelf west of vancouver island are used to calculate bottom stress a free failing vertical profiler with microstructure shear probes was used to measure the dissipation rate from near the surface to within 0 15 m of the bottom the shear probes measure velocity gradients at scales within the viscous subrange of the turbulence and therefore directly measure the rate at which kinetic energy is dissipated by viscosity friction velocities are computed from the formula uast z where the dissipation rate is measured in the constant stress layer the technique is more reliable than estimates of the dissipation rate obtained by fitting spectral slopes to velocity spectra at scales in the inertial subrange near bottom current measurements indicate that the bottom stress values obtained from the turbulent measurements are well correlated with the current magnitude an estimate of the drag coeff

buoyant gravity currents released from tropical instability waves abstracttwo extremely sharp fronts with changes in sea surface temperature 0 4 c over lateral distances of 1 m were observed in the equatorial pacific at 0 140 w and at 0 75 n 110 w in both c

shoshonitic and ultrapotassic post collisional dykes from northern karakorum sinkiang china abstract high k calc alkaline shoshonitic and ultrapotassic post collisional dykes of neogene age have been found in the remote and little known region of northern karakorum located around the shaksgam valley north of the k2 gasherbrum range china the dykes derive from more or less comparable basic magma s and display rather unusual petrographic and geochemical characters with respect to the other k rich rocks the geochemical data are consistent with derivation of the basic magma s from small degrees of partial melting of garnet lherzolites previously enriched in incompatible elements of crustal origin possibly during the subduction of the indian plate beneath karakorum the spectrum of compositions reflects fractional crystallization governed by an early removal of clinopyroxene phlogopite plagioclase garnet amphibole followed by the precipitation of abundant alkali feldspar amphibole apatite quartz additionally assimilation of crustal rocks during magma ascent contributed to the unusual compositional characteristics and is suggested by the abundance of corroded quartz plagioclase xenocrysts and by the occasional presence of granitic xenoliths an apparent connection exists between magmatism and tectonism in the complex karakorum fault zone kfz it is suggested that during the neogene the strike slip kfz and some adjacent post metamorphic faults transiently behaved as extensional fault systems down to deep levels triggering ascent and emplacement of the k rich magma the subsequent re activation of a compressive and transcurrent regime determined the rapid and recent uplift of the more primitive lamprophyres occurring in the plutonites and metamorphites of the upper baltoro glacier and k2 gasherbrum range relative to the more fractionated and contaminated lamprophyres injecting the shallower rocks of the shaksgam valley area

relating river plume structure to vertical mixing abstract the structure of a river plume is related to the vertical mixing using an isohaline based coordinate system salinity coordinates offer the advantage of translating with the plume as it moves or expanding as the plume grows this coordinate system is used to compare the relative importance of different dynamical processes acting within the plume and to describe the effect each process has on the structure of the plume vertical mixing due to inertial shear in the outflow of a narrow estuary and wind mixing are examined using a numerical model of a wind forced river plume vertical mixing and the corresponding entrainment of background waters is greatest near the estuary mouth where inertial shear mixing is large this region is defined as the near field with the more saline far field plume beyond wind mixing increases the mixing throughout the plume but has the greatest effect on plume structure at salinity ranges just beyond the near field wind mixing is weaker at high salinity classes tha

a geological transect from kun lun to karakorum sinkiang china the western termination of the tibetan plateau preliminary note a geological geophysical expedition ev k2 cnr 1988 visited the area from west kun lun to karakorum k2 gasherbrum seven tectonic units including sedimentary magmatic and metamorphic rocks were distinguished in this area the northernmost are suggested to belong to the kun lun and qiangtang microplates the sedimentary sequence of shaksgam is proved to extend from the permian to the jurassic with carboniferous and cretaceous ages more doubtful this sequence shows intermediate affinities between the karakorum and the qiangtang the two southernmost units belong to the karakorum microplate the karakorum fault zone comprises a complex pattern of faults and thrusts with brittle deformation and uplifting of granitoid bodies

new record of bathonian callovian calcareous nannofossils in the eastern karakoram block a possible clue to understanding the dextral offset along the karakoram fault we report the first record of bathonian callovian calcareous nannofossils from a marine sedimentary sequence of the eastern karakoram block in northern india the calcareous mudstones and packstones occasionally bearing red chert nodules yielded calcareous nanofossils and middle jurassic choffatia furcula ammonoids middle to upper jurassic nannofossil assemblage is dominated by representatives of the genus watznaueria the occurrence of ansulasphaera helvetica whose range is upper bathonian upper callovian indicates a correlation with nannofossil zones nj12 13 the occurrence of cyclagelosphaera wiedmannii further infer an upper bathonian callovian age these specimens show affinities with those found in a similar sedimentary formation exposed in north karakoram this suggests the existence of a narrow and elongated sedimentary basin oriented in a nw se direction at a latitude of c 25 30 n at that time the karakoram block was situated near the already welded qiangtang block of asia the northern and eastern karakoram blocks were connected during middle jurassic the activity and dextral offset of the karakoram fault separated the jurassic sedimentary formations of the northern and eastern karakoram blocks by c 150 km

evidence against quaternary slip on the northern karakorum fault suggests kinematic reorganization at the western end of the himalayan tibetan orogen abstract while the right slip karakorum fault is one of the most prominent structures in the himalayan tibetan orogen its evolution and role in accommodating deformation related to the collision between india and asia is controversial different models interpret the karakorum fault as either 1 a long lived stable feature analogous to a plate boundary structure or 2 an evolving fault system with limited displacement 150 ka overly the trace of the two strands of the karakorum fault and show no evidence of tectonic disturbance 2 older late pliocene to early quaternary loess deposits which overlie the trace of the fault are either undisturbed or are cut by small normal fault scarps with no evidence of strike slip displacement 3 east west trending bedrock incised stream valleys up to 500 m deep cross both strands of the fault with no sign of lateral deflection i suggest that termination of slip along the northern segment of the karakorum fault occurred in the pliocene due to southward propagation of the altyn tagh fault system along the longmu co gozha co fault and impingement of the karakorum fault this study suggests that large intracontinental strike slip faults embedded within orogenic plateaus are transient features which should be expected to have complicated kinematic histories and short life spans relative to the life of an orogenic belt

observations of the frontal region of a buoyant river plume using an autonomous underwater vehicle to characterize the transitional region from the near field to far field of a river plume entering coastal waters we conducted four surveys using an autonomous underwater vehicle auv to target the outflow of the new river inlet north carolina during maximum ebb tide the utilization of a mobile sensor to synoptically observe current velocity data in tandem with natural river plume tracers e g colored dissolved organic matter salinity was essential in understanding the mechanisms driving the observed circulation and mixing patterns within these waters we find that this region is regularly impacted by two primary processes 1 the interaction of an old dredged channel plume with the main discharge and 2 the recirculation of the discharge plume by an eddy that persistently forms between the old channel and main discharge location wind driven processes in the nearshore can enhance the interaction of these two plumes resulting in unstable regions where mixing of the merged plume with the receiving waters is accelerated we also conduct comparisons between auv velocity observations from two surveys and their corresponding velocity outputs from a parallelized quasi 3 d model we conclude that the ability to observe the estuarine outflow transitional region at near synoptic temporal scales and resolutions discussed in this paper is key in providing the mechanisms driving local circulation which is essential for proper parameterization of high resolution numerical coastal models

long term changes in explosive and effusive behaviour at andesitic arc volcanoes chronostratigraphy of the centre hills volcano montserrat volcanism on montserrat lesser antilles arc has migrated southwards since the formation of the silver hills 2 5 ma and has formed three successively active volcanic centres the centre hills volcano was the focus of volcanism from 1 0 4 ma before activity commenced at the currently active soufriere hills volcano the history of activity at these two volcanoes provides an opportunity to investigate the pattern of volcano behaviour on an andesitic arc island over the lifetime of individual volcanoes here we describe the pyroclastic stratigraphy of subaerial exposures around central montserrat identifying 11 thick 1 m pumiceous units derived from sustained explosive eruptions of centre hills from 0 8 0 4 ma over 10 other less well exposed pumiceous units have also been identified the pumice rich units are interbedded with andesite lava breccias derived from effusive dome forming eruptions of centre hills the stratigraphy indicates that large up to magnitude 5 explosive eruptions occurred throughout the history of centre hills alongside effusive activity this behaviour at centre hills contrasts with soufriere hills where deposits from sustained explosive eruptions are much less common and restricted to early stages of activity at the volcano from 175 130 ka subsequent eruptions at soufriere hills have been dominated by andesitic effusive eruptions the bulk composition petrography and mineral chemistry of volcanic rocks from centre hills and soufriere hills are similar throughout the history of both volcanoes except for occasional transient departures to different magma compositions which mark shifts in vent location or dominant eruption style for example the final recorded eruption of centre hills before the initiation of activity at soufriere hills was more silicic than any other identified eruption on montserrat and the basaltic south soufriere hills episode marked the transition to the current stage of predominantly effusive soufriere hills activity the compositional stability observed throughout the history of centre hills and soufriere hills suggests that a predominance towards effusive or explosive eruption styles is not driven by major compositional shifts of magma but may reflect local changes in long term magma storage conditions that characterise individual episodes on 105 year timescales of volcanism on montserrat

quantification of the intrusive magma fluxes during magma chamber growth at soufriere hills volcano montserrat lesser antilles magma fluxes in the crust control the thermal viability and mechanical stability of magma chambers we estimated the magma fluxes required to generate the negative seismic velocity anomaly observed below soufriere hills volcano montserrat growth of a magma body by accretion of andesitic sills was simulated numerically and the resulting temperatures and melt fractions were used to calculate a synthetic anomaly of seismic wave velocity which was filtered to be comparable with the velocity anomaly obtained from a tomographic experiment petrology indicates that before it was reheated remobilized and erupted the temperature of the magma residing in the chamber was about 850 c we ran simulations where convection is assumed to be low and heat transfer is mostly by conduction and simulations where convection is assumed to be vigorous enough to rapidly cool the magma chamber to 850 c in both cases magma chamber growth over the last 350 years results in tomography anomalies that are too strong unless the magma was emplaced at an unlikely low melt fraction 0 5 good fits between the modelled and the observed velocity anomaly were obtained with sills 2 5 km in radius emplaced over 6000 150 000 years depending on the temperature and melt fraction of the emplaced magma because of a trade off between intrusion dimensions and emplacement durations the volumetric magma fluxes are restricted to 7 10 4 and 5 10 3 km3 a 1 the velocity anomaly can be reproduced with a chamber containing high melt fraction magma or with a mush of crystals and melt the range of magma ages in the modelled magma chamber is much wider than the crystal residence time of the erupted andesite this suggests that the eruption taps small pockets of recently assembled magma and that the velocity anomaly is mostly due to a non eruptible mush

the role of turbulence stress divergence in decelerating a river plume 1 turbulence controls the composition of river plumes through mixing and alters the plume s trajectory by diffusing its momentum while believed to play a crucial role in decelerating river source waters the turbulence stress in a near field river plume has not previously been observationally quantified in this study finely resolved density velocity and turbulence observations are combined with a control volume technique to describe the momentum balance in the columbia river s near field plume during 10 tidal cycles that encompass both large and small river flow turbulence stress varies by 2 3 orders of magnitude both within a given ebb and between ebbs with different tidal or river forcing its magnitude scales with the strength of the instantaneous ebb outflow i e high stresses occur during peak flow of strong ebbs during these periods the momentum equation is represented by a balance between stress divergence and plume deceleration as the flow relaxes the stress divergence weakens and other terms pressure gradient and coriolis may become appreciable and influence plume deceleration while the momentum balance could not be closed during these weaker flow periods during strong tidal pulses the time scale for decay based on observed stress is significantly less than a tidal half period indicating that stress divergence plays a fundamental role in the initial deceleration of the plume

observations of shear and vertical stability from a neutrally buoyant float measurements of 1 to 5 m shear and strain from a neutrally buoyant float are used to examine the statistics and causes of shear instability in a 9 day record unstable conditions defined as vz n 2 where vz vz are due to higher than average shear rather than lower than average buoyancy frequency and there are no instances of unstable buoyancy frequency or overturns on the 0 5 m spacing of the thermistors shear is dominated by upward propagating near inertial motions unstable events were more frequent when a near inertial wave packet occupied the water parcel tracked by the float groups of unstable events occur roughly every 5 7 hours at scales 2 5 m and much less often at larger scales events typically last 10 min one buoyancy period or longer these time scales appear to be controlled by vertical advection of 1 m fine structure past the float sensors so do not represent the intrinsic time scales of instability turbulent and mixing quantities are estimated from these fine scale measurements giving a dissipation rate of e 2 3 16 10 10 w kg and an eddy diffusivity of k 3 10 6 m2 s in agreement with direct microstructure estimates therefore the float adequately resolves those quantities needed to estimate turbulent dissipation and could be used to monitor turbulent mixing in the ocean

observations of turbulence in a partially stratified estuary the authors present a field study of estuarine turbulence in which profiles of reynolds stresses were directly measured using an adcp throughout a 25 h tidal day the dataset that is discussed quantifies turbulent mixing for a water column in northern san francisco bay that experiences a sequence of states that includes a weak ebb and flood that are stratified followed by a strong and eventually unstratified ebb and flood these measurements show that energetic turbulence is confined to a bottom mixed layer by the overlying stratification examination of individual reynolds stress profiles along with profiles of richardson number and turbulent froude number shows that the water column can be divided into regions based on the relative importance of buoyancy effects using the measured turbulence production rate p the dissipation rate e is estimated the observed turbulence had values of e nn 2 20 all of the time and e nn 2 200 most of the time suggesting that the observed motions were buoyancy affected turbulence rather than internal waves however at times turbulent froude numbers in much of the upper water column were less than one indicating important stratification effects taken as a whole the data show that stratification affects the turbulent velocity variance q2 most severely that is observed reductions in u9w9 are largely associated with small values of q2 rather than with a dramatic reduction in the efficiency with which turbulent motions produce momentum fluxes finally the dataset is compared to predictions made using the popular mellor yamada level 2 5 closure these comparisons show that the model tends to underestimate the turbulent kinetic energy in regions of strong stratification where the turbulence is strongly inhomogeneous and to overestimate the turbulent kinetic energy in weakly stratified regions the length scale does not appear to compensate for these errors and as a result similar errors are seen in the eddy viscosity predictions it is hypothesized that the underestimation of q2 is due to an inaccurate parameterization of turbulence self transport from the near bed region to the overlying stratification

carboniferous triassic subduction and accretion in the western kunlun china implications for the collisional and accretionary tectonics of the northern tibetan plateau a newly defined 250 km by 500 km carboniferous triassic subduction accretion complex the mazar accretionary prism in the western kunlun comprises two subduction complexes and a forearc basin succession 1 the bazar dara subduction complex contains imbricated blocks of sandstone arenite limestone and metavolcanic rocks in a matrix of weakly metamorphosed triassic deep sea turbidites the metavolcanic rocks include basalt diabase spilite and andesitic porphyry trace element geochemistry shows that pillow and amygdaloidal basalts are oceanic island tholeiites this zone has ordovician to permian fossils and is situated on the older more highly deformed and metamorphosed side of the prism adjacent to the sailiyak magmatic arc 2 the heweitan subduction complex is composed of blocks of limestone turbidite and radiolarite in a slate phyllite matrix intercalated with calc alkalic volcanic rocks this complex has permian to triassic fossils and is situated on the younger less deformed and metamorphosed side of the prism adjacent to the suture zone 3 the qitai forearc basins are infilled with turbidites late triassic intercalated with carbonates these rocks overlie the accretionary prism the accretion related structure is dominated by large scale northeast dipping thrusts and subvertical cleavage stitched by 215 190 ma granites there is an overall decrease in metamorphic grade and deformation intensity from the arc to the suture zone across the bazar dara and heweitan subduction complexes the mazar accretionary prism formed by subduction accretion processes during closure of the paleotethyan ocean and the final docking of the gondwanan karakoram qiangtang block to the cathaysian eurasian kunlun block

particle resuspension in the columbia river plume near field 1 measurements of suspended sediment concentration velocity salinity and turbulent microscale shear in the near field region of the columbia river plume are used to investigate the mechanisms of sediment resuspension and entrainment into the plume an east west transect was occupied during spring and neap tide periods in august 2005 and may 2006 corresponding to low and high river discharge conditions respectively during the high discharge period the plume is decoupled from the bottom and fine sediment resuspended from the bottom does not leave the benthic boundary layer the primary modes of sediment transport associated with the plume are advection of sediment from the estuary and removal of sediment from the plume by gravitational settling and turbulent mixing in contrast the plume is much less stratified during low discharge conditions and large resuspension events are observed that entrained sediment through the water column and into the plume our measurements indicate that two factors control the magnitude and timing of sediment resuspension and entrainment the supply of fine sediment on the seabed and the relative influence of tidal turbulence compared with buoyancy input from the river the latter is quantified in terms of the estuary richardson number rie the magnitude of vertical turbulent sediment flux is correlated with rie during the low flow period when there is a sufficient supply of bottom sediment in the near field region such sediment resuspension may be an important mechanism for the delivery of bioavailable micronutrients to the plume during the summer

observations at the tidal plume front of a high volume river outflow 1 we present shipboard observations of very strong convergence vertical velocities and mixing and near bed impacts associated with the leading edge front of the tidally pulsed columbia river plume with upwelling favorable winds and riverflow of 4900 m3s 1 the plume propagates as a buoyant gravity current with a rotary bore like vertical frontal circulation and downwelling as strong as 0 35 m s 1 in waters as deep as 65 m near bed currents intensify to as much as 1 0 m s 1 after frontal passage and are often associated with elevated acoustic backscatter mixing is locally strong with an eddy diffusivity of o 0 2 m2s 1 50 m behind the front and t s diagrams imply plume mixing with 10 m deep ocean water these observations indicate that the leading edge front of a surface advected plume can cause exchanges of a nutrients between cold subsurface shelf waters and the river plume and b nutrients and sediments across the sediment water interface

granites of western karakorum and northern kohistan pakistan a composite mid cretaceous to upper cenozoic magmatism abstract the petrography the chemical mineralogical typology and the ages of six plutonic units four from the karakorum axial batholith darkot pass ghamu bar batura and hunza units and two of northern kohistan gindai and nomal plutons are defined and compared based on field data and analyses of up to 78 samples major elements ree rb s sbnd sr and k ar isotopes the karakorum axial batholith is a composite body three major intrusive stages occurred 1 around mid cretaceous times ca 110 95 ma with the emplacement of subalkaline i e monzonitic darkot pass and calc alkaline hunza ghamu bar units 2 during palaeogene maybe up to 43 ma batura subalkaline unit a strong tectonommetamorphic event recorded in the gneissification of the cretaceous intrusives occurred between these two stages it may be of palaeocene age p t estimates of the highest metamorphic grade rocks of the hunza unit have yielded values of 580 640 c and 5 0 5 kbar and 3 during upper miocene ca 9 ma baltoro subalkaline unit debon et al 1986c in addition a conspicuous network of aplo pegmatitic dykes emplaced into the hunza area possibly from the eocene up to the upper cenozoic with a maximum during the middle miocene ca 15 ma most of these major magmatic stages are met again among the acidic intrusives and dykes of northern kohistan the first one as blastomylonitic tholeiitic plutons nomal ca 102 ma petterson and windley 1985 the second one as subalkaline plutons gindai ca 59 ma and a third one as leucocratic dykes of oligocene age ca 30 ma petterson and windley 1985 these data may be related to the geodynamic evolution of the nw part of the india eurasia suture zone thus allowing better constraints on the major steps of this evolution the partly synchronous closures by n dipping subduction of the two tethys branches which are assumed to have encircled the kohistan arc in upper mesozoic times may have generated both the karakorum and the kohistan intrusives of cretaceous and palaeogene ages the northern branch very likely closed before the southern one at the time of the second intrusive stage palaeogene a kohistan and karakorum had already collided were welded and had suffered the same major tectonometamorphic event b subduction of the southern tethys floor beneath the welded kohistan karakorum was still active c however collision between india and kohistan karakorum may have already begun particularly at the level of the nanga parbat promontory finally it is emphasized that the intrusive processes continued in the karakorum long after the collision e g baltoro granite

a new scenario for the last magmatic eruption of la soufriere of guadeloupe lesser antilles in 1530 a d evidence from stratigraphy radiocarbon dating and magmatic evolution of erupted products la soufriere of guadeloupe is a dangerous volcano characterized over the last decade by moderate seismic and fumarolic unrest in the last 15 000 years it has experienced phreatic and magmatic eruptions and unusually numerous flank collapse events sometimes associated with a magmatic eruption we propose a new age of 1530 a d and a new eruptive scenario for the last magmatic eruption on the basis of a novel statistical analysis of radiocarbon age dates and new field and geochemical data this eruption is the only magmatic eruption likely to have occurred in guadeloupe during the last 1400 years the eruption mainly involved an andesitic magma which in the first phase of the eruption partially mixed with a slightly more differentiated magma stored in a small and shallow magma chamber ascent of magma to the surface generated a partial collapse of the hydrothermally altered edifice that increased the magma discharge and led to a sub plinian phase with scoria fallout and column collapse pyroclastic flows followed by near vent pyroclastic scoria fountains the eruption ended with growth of a lava dome our revised interpretation of the last magmatic eruption of la soufriere constitutes the most likely key to a future magmatic eruption scenario for this volcano which displays strong evidence of unrest since 1992

prehistoric stratigraphy of the soufriere hills south soufriere hills volcanic complex montserrat west indies abstract the soufriere hills and south soufriere hills volcanoes are morphologically distinct but petrologically similar the history of the complex is subdivided into seven episodes the soufriere hills subunit i 175 ka has deposits of pelean style with ignimbrites of plinian style abundant near its top this is followed by deposits from the south soufriere hills 130 ka that represent open crater st vincent style activity soufriere hills subunits ii v 112 000 400 bp return to more characteristic pelean activity at 22 000 bp block and ash flow deposits containing abundant clasts of vesicular andesite dominate age dating shows that renewed activity began just before european settlement in 1632 ce this activity including the ongoing eruption is typical of the dominant eruptive style of this volcano for the last 175 ka

construction of volcanic records from marine sediment cores a review and case study montserrat west indies detailed knowledge of the past history of an active volcano is crucial for the prediction of the timing frequency and style of future eruptions and for the identification of potentially at risk areas subaerial volcanic stratigraphies are often incomplete due to a lack of exposure or burial and erosion from subsequent eruptions however many volcanic eruptions produce widely dispersed explosive products that are frequently deposited as tephra layers in the sea cores of marine sediment therefore have the potential to provide more complete volcanic stratigraphies at least for explosive eruptions nevertheless problems such as bioturbation and dispersal by currents affect the preservation and subsequent detection of marine tephra deposits consequently cryptotephras in which tephra grains are not sufficiently concentrated to form layers that are visible to the naked eye may be the only record of many explosive eruptions additionally thin reworked deposits of volcanic clasts transported by floods and landslides or during pyroclastic density currents may be incorrectly interpreted as tephra fallout layers leading to the construction of inaccurate records of volcanism this work uses samples from the volcanic island of montserrat as a case study to test different techniques for generating volcanic eruption records from marine sediment cores with a particular relevance to cores sampled in relatively proximal settings i e tens of kilometres from the volcanic source where volcaniclastic material may form a pervasive component of the sedimentary sequence visible volcaniclastic deposits identified by sedimentological logging were used to test the effectiveness of potential alternative volcaniclastic deposit detection techniques including point counting of grain types component analysis glass or mineral chemistry colour spectrophotometry grain size measurements xrf core scanning magnetic susceptibility and x radiography this study demonstrates that a set of time efficient non destructive and high spatial resolution analyses e g xrf core scanning and magnetic susceptibility can be used effectively to detect potential cryptotephra horizons in marine sediment cores once these horizons have been sampled microscope image analysis of volcaniclastic grains can be used successfully to discriminate between tephra fallout deposits and other volcaniclastic deposits by using specific criteria related to clast morphology and sorting standard practice should be employed when analysing marine sediment cores to accurately identify both visible tephra and cryptotephra deposits and to distinguish fallout deposits from other volcaniclastic deposits

late pleistocene tephrochronology of marine sediments adjacent to montserrat lesser antilles volcanic arc the recent history of the soufriere hills volcano montserrat lesser antilles volcanic arc is deduced using data obtained from a submarine core collected in 2002 the core contains concentrations of ash and several tephra layers which are identified by the abundance of glass shards dense and poorly vesiculated particles and scoria the tephra layers have been dated using micropalaeontology and stable isotope stratigraphy tephra layers in a marine sediment core off the coast of montserrat record the volcanic history of south soufriere hills soufriere hills volcano back to 250 ka eight layers are composed of dense juvenile ash related to dome eruptions five of which can be directly correlated to dated domes or related pyroclastic flow sequences on land six layers are composed of pumiceous glassy ash and relate to significant explosive eruptions a marker sequence of basalt tephra layers is dated at 124 147 ka and is correlated with construction of the south soufriere hills basaltic stratocone pelagic sediments between the main tephra layers have low abundances of volcanogenic components c 10 4 years of dormancy or low activity

evidence for carbonate platform failure during rapid sea level rise ca 14 000 year old bioclastic flow deposits in the lesser antilles bioclastic flow deposits offshore from the soufriere hills volcano on montserrat in the lesser antilles were deposited by the largest volume sediment flows near this active volcano in the last 26 kyr the volume of these deposits exceeds that of the largest historic volcanic dome collapse in the world which occurred on montserrat in 2003 these flows were most probably generated by a large submarine slope failure of the carbonate shelf comprising the south west flank of antigua or the east flank of redonda adjacent islands that are not volcanically active the bioclastic flow deposits are relatively coarse grained and either ungraded or poorly graded and were deposited by non cohesive debris flow and high density turbidity currents the bioclastic deposit often comprises multiple sub units that cannot be correlated between core sites some located just 2 km apart multiple sub units in the bioclastic deposit result from either flow reflection stacking of multiple debris flow lobes and or multi stage collapse of the initial landslide this study provides unusually precise constraints on the age of this mass flow event that occurred at ca 14 ka few large submarine landslides have been well dated but the slope failures that have been dated are commonly associated with periods of rapid sea level change

sedimentology palaeoecology and palaeoenvironmental evolution of the 1985 lhasa to golmud geotraverse vertical and horizontal measurements of almost 30 km of sections were made along the geotraverse route at 113 localities ranging in age from ordovician to tertiary over 280 palaeocurrent measurements were taken and 200 thin sections were studied ordovician strata occur only in the kunlun terrane w here thick metamorphosed sequences of elastics and carbonates occur these are tentatively interpreted as platform margin and slope deposits during carboniferous times in the kunlun terrane transgressive late dinantian marine limestones with tropical to subtropical eurasian reefoidal faunas overlie fluviatile redbeds derived from an unroofed orogenic belt the lhasa terrane contains shelf basin elastics with low diversity faunas succeeded by thick late carboniferous early permian glacio marine mixtites deposited by iceberg meltout permian carbonate ramp and shelf facies with reefoidal developments occur over both the lhasa and qiangtang terranes with coal bearing elastics and fluviatile redbeds also occurring in the latter

timing and emplacement dynamics of newly recognised mass flow deposits at 8 12ka offshore soufriere hills volcano montserrat how submarine stratigraphy can complement subaerial eruption histories this contribution describes two mass movement deposits total volume 0 5 km3 identified in seven marine cores located 8 to 15 km offshore southern montserrat west indies the deposits were emplaced in the last 35 ka and have not previously been recognised in either the subaerial or distal submarine records age constraints provided by radiocarbon dating show that an explosive volcanic eruption occurred at ca 8 12 ka emplacing a primary eruption related deposit that overlies a large 0 3 km3 reworked bioclastic and volcaniclastic flow deposit formed from a shelf collapse between 8 and 35 ka the origin of these deposits has been deduced through the correlation of marine sediment cores component analysis and geochemical analysis the 8 12 ka primary volcanic deposit was likely derived from a highly erosive pyroclastic flow from the soufriere hills volcano that entered the ocean and mixed with the water column forming a water supported density current previous investigations of the eruption record suggested that there was a hiatus in activity at the soufriere hills volcano between 16 and 6 ka the ca 8 12 ka eruptive episode identified here shows that this hiatus was shorter than previously hypothesised and thus highlights the importance of obtaining an accurate and complete marine record of events offshore from volcanic islands and incorporating such data into eruption history reconstructions comparisons with the submarine deposit characteristics of the 2003 dome collapse also suggests that the 8 12 ka eruptive episode was more explosive than eruptions from the current eruptive episode

structure and dynamics of the columbia river tidal plume front 1 time dependent buoyant plumes form at the outflow of tidally dominated estuaries when estuary discharge velocity exceeds plume internal wave speed c a sharp front forms at the plume s leading edge that expands from the time dependent source using observations of the columbia river tidal plume from multiple tidal cycles we characterize time evolving plume structure and quantify front speed uf plume internal wave speed c front curvature and ultimate extent we identify three distinct stages of propagation 1 initially the plume is strongly influenced by shallow bathymetry near the river mouth 2 as the front advances offshore the plume detaches from the bottom and expands as a freely propagating gravity current with relatively constant uf c and frontal froude number f uf c ambient currents explain intracycle variability in uf and winds alter front shape variability in ambient stratification associated with previous cycles plume remnants leads to complex fronts and internal waves 3 finally the plume decelerates adjusts toward geostrophy and may radiate additional internal waves using a simple kinematic model we suggest that constant frontal propagation speed uf 0 9 0 1 m s during stage 2 is primarily controlled by linearly increasing volume flux from the columbia river mouth as this discharge rate subsides the plume expands as a fixed volume with decreasing front speed stage 3 the plume s final extent is controlled by the rossby radius which scales with a length based on the total volume discharged this provides an integral description of plume front evolution based on the time dependent estuary discharge

new insights into landslide processes around volcanic islands from remotely operated vehicle rov observations offshore montserrat submarine landslide deposits have been mapped around many volcanic islands but interpretations of their structure composition and emplacement are hindered by the challenges of investigating deposits directly here we report on detailed observations of four landslide deposits around montserrat collected by remotely operated vehicles integrating direct imagery and sampling with sediment core and geophysical data these complementary approaches enable a more comprehensive view of large scale mass wasting processes around island arc volcanoes than has been achievable previously the most recent landslide occurred at 11 5 14 ka deposit 1 1 7 km3 and formed a radially spreading hummocky deposit that is morphologically similar to many subaerial debris avalanche deposits hummocks comprise angular lava and hydrothermally altered fragments implying a deep seated central subaerial collapse inferred to have removed a major proportion of lavas from an eruptive period that now has little representation in the subaerial volcanic record a larger landslide deposit 2 10 km3 occurred at 130 ka and transported intact fragments of the volcanic edifice up to 900 m across and over 100 m high these fragments were rafted within the landslide and are best exposed near the margins of the deposit the largest block preserves a primary stratigraphy of subaerial volcanic breccias of which the lower parts are encased in hemipelagic mud eroded from the seafloor landslide deposits south of montserrat deposits 3 and 5 indicate the wide variety of debris avalanche source lithologies around volcanic islands deposit 5 originated on the shallow submerged shelf rather than the terrestrial volcanic edifice and is dominated by carbonate debris