hydrographic structure of the convection regime in the gulf of lions winter 1987 abstract deep winter convection in the northwestern mediterranean gulf of lions and the subsequent formation of mediterranean deep water were observed using advanced oceanographic instrumentation during a six week long experiment in 1987 the severe 1987 european winter forced an intense outbreak of the mistral a cold dry wind blowing down the rhone valley in mid january surface cooling and evaporation were of sufficient intensity to cause an initial episode of deep convection shortly before the experiment described here began however several more mistral events took place during the experiment during several cruises into the area ctd and absolute horizontal velocity profiles were measured in the mixed area over the rhone fan as well as across the southern front of this region in addition continuous records of shipboard meteorological and oceanographic parameters air temperature surface salinity etc were made an early february mistral apparently did not produce enough surface cooling to

interpentadal variability of temperature and salinity at intermediate depths of the north atlantic ocean 1970 1974 versus 1955 1959 we have composited historical hydrographic observations for the world ocean for two pentads 1955 1959 and 1970 1974 and have objectively analyzed these data for the purpose of studying the temporal variability of temperature and salinity in the north atlantic ocean the subtropical gyre 500 1300 m of the north atlantic was colder and fresher during 1970 1974 compared to 1955 1959 the relatively large changes in temperature and salinity on constant depth surfaces in the subtropical gyre were due to upward displacements of isopycnal surfaces at intermediate depths the eastern portion of the subarctic gyre was also colder and fresher during the 1970 1974 pentad in contrast the western portion of the subarctic gyre exhibited higher temperatures and salinities during 1970 1974 compared to 1955 1959 characteristic differences in temperature and salinity for both gyres were 0 5 c and 0 025 respectively analysis of changes in potential vorticity indicates that in the subtropical gyre potential vorticity increased in all density layers above the 26 5 surface suggesting that convection in this region may have been shallower in 1970 1974 compared to 1955 1959 below the 26 5 surface potential vorticity decreased through the 27 3 surface indicating that deeper than normal convection may have formed the water masses in these layers at 250 m depth relatively large changes in temperature and salinity occurred along the line of maximum climatological mixed layer depth that extends northeast to southwest across the north atlantic ocean

isotopic constraints on the structural relationships between the lesser himalayan series and the high himalayan crystalline series garhwal himalaya nd and sr isotope systematics may provide important constraints on the location of major thrust systems that separate lithologically similar sedimentary sequences the potential of the technique is illustrated by this isotopic study of the main central thrust system of the himalaya nd isotope data from the garhwal himalaya indicate that metasedimentary rocks from the vaikrita group nd 14 to 19 correlate closely with those from the high himalayan crystalline series which constitutes the hanging wall lithologies of the main central thrust in contrast metasedimentary rocks from the munsiari group nd 23 to 28 show marked similarities to the lesser himalayan series in the footwall of the main central thrust sr isotopes support the correlations in that the vaikrita group shows partial reequilibration at 500 ma whereas the munsiari group has not undergone sr isotope homogenization since 1800 ma thus the vaikrita thrust that juxtaposes these two formations is recognized as the main central thrust in garhwal himalaya the thrust coincides approximately with the location of the kyanite isograd confirming that inverted metamorphism is characteristic of both hanging wall and footwall of the main central thrust along the tons thrust known locally as the srinagar thrust 50 km south of the main central thrust low grade quartzarenites with nd sr isotope and trace element characteristics typical of lesser himalayan formations have been emplaced on phyllites and siltstones with geochemical characteristics of the high himalayan crystalline series the field relationships most probably result from out of sequence thrusting in which lesser himalayan series rocks to the north were emplaced over low grade equivalents of the high himalayan crystalline series preserved in the external part of the orogen this study establishes the value of isotope data for lithostratigraphic correlations within orogenic belts

pressure temperature evolution in the kathmandu and gosainkund regions central nepal abstract the kathmandu and gosainkund regions in central nepal are occupied by a metamorphic sequence of upper amphibolite to granulite facies metasedimentary rocks on the basis of structure and lithology three units are distinguished the gosainkund crystalline nappe gcn corresponding to the southward continuation of the higher himalayan crystallines hhc of langtang and the kathmandu crystalline nappe kcn representing an out of sequence thrust sheet in the lesser himalaya the boundary between the two nappes corresponds to the main central thrust mct the gcn and the kcn are thrust over the lesser himalaya along the mct and the mahabharat thrust mt respectively structural and petrographical data demonstrate that the metamorphic assemblage in the three units is related to movements on the mct and mt with a well preserved inverted thermal gradient in the mct zone both conventional and thermocalc thermobarometric methods are used to calculate pressure and temperature the upper lesser himalayan sample records syn mct metamorphic conditions of 750 150 mpa and 566 136 c the kcn samples record p t conditions from 900 140 to 720 420 mpa and 700 60 to 484 194 c while the gcn samples equilibrated at upper amphibolite to granulite facies conditions from 890 260 to 583 198 mpa and 754 40 to 588 100 c error estimation quoted at 2 the final equilibration of the upper section of the gcn occurred with a pressure and temperature drop of 170 mpa and 60 c respectively associated with nnw normal ductile shearing the good preservation of the metamorphic conditions in the different units with no field and petrographic evidence of a metamorphic discontinuity across the mct and mt zones suggests that the final exhumation of the nappe piles occurred along the mbt

warming trends in the continental shelf of malaga bay alboran sea 1 temperature time series from 1992 to 2001 ecomalaga project show an intense warming trend in deep layers 200m depth of malaga bay 0 02 c yr data from medatlas show that this trend is 0 005 c yr throughout the whole century another striking change in malaga bay is the disappearance of the cold western intermediate water wiw after 1992 this could be the responsible of temperature changes in the malaga continental shelf

metamorphic response to collision in the central himalayan orogen abstract closure of the neotethys ocean and high angle continent continent collision between india and asia after about 55 ma resulted in low angle subduction of the indian plate below the tibetan plateau and by 30 ma established an arcuate 2300 km long shallow north dipping metamorphic fold thrust belt in the foreland this himalayan metamorphic front quickly established an upper plate lower plate paired metamorphic architecture centred on the median high himal thrust that largely controlled subsequent evolution the upper plate is a thick slab of high t moderate p high grade migmatized metamorphic rocks whereas the lower plate is an inverted series of moderate t high p schists in two crustal wedges the broad main central thrust zone and at lowest structural levels the footwall below the basal main central thrust spatial and temporal patterns of metamorphic response in the evolving himalayan metamorphic front has been characterized in a large scale integrated structural metamorphic study based on 8 profiles across eastern nepal metamorphic response at all structural levels is established using a large dataset n 160 of internally consistent quantitative pt determinations petrology of metapelite samples semi quantitative p t paths metamorphic mapping and metamorphic field gradients these results are integrated with previously published metamorphic studies structural profiles and metamorphic chronology from these datasets the architecture and evolution of the himalayan metamorphic front is constrained by rock kinematics metamorphic field gradients showing discontinuities and diachronous metamorphism with contrasting p t evolutions at different structural levels each of the three panels constituting the himalayan metamorphic front upper plate main central thrust zone and footwall experienced distinctly different tectono metamorphic histories crustal processes operating during metamorphism and exhumation differ between the upper and lower plates the upper plate experienced long lived metamorphism starting from at least 28 38 ma and tracking low p t clockwise p t paths that culminated at 19 27 ma in peak high grade condition with 27 31 c km thermal regimes protracted high grade conditions produced significant partial melt which facilitated gravity driven southward extrusion involving internal ductile flow processes southward extrusion of the upper plate was accommodated by coeval reverse movement on the high himal thrust and top down to the north normal reactivation of the south tibet detachment system between 22 10 ma transport of this thick slab to the south resulted in further prograde burial of the lower plate below culminating in peak metamorphism at the highest pressures attained in the lower plate rocks the lower plate consists of at least two deeply buried 7 5 9 4 kb crustal wedges both of which experienced steep p t hairpin clockwise p t paths with isothermal decompression the main central thrust zone experienced 20 26 c km metamorphism at 12 22 ma and was exhumed along the basal main central thrust after 11 ma whereas loading by the main central thrust zone gave rise to further prograde burial of the footwall which experienced 16 21 c km metamorphism at 4 10 ma and was exhumed along the main boundary thrust at 3 9 ma in contrast to the upper plate both crustal wedges were exhumed by a process of out wedging which involved upthrusting along basal and internal thrusts with coeval extensional slip in the hanging walls this three stage foreland propagating lateral exhumation history resulted in telescoping of the himalayan metamorphic front in concert with peak metamorphic events at different structural levels during main phase orogenesis and is not the result of superimposed retrograde reactivation of the belt

warming salting and origin of the tyrrhenian deep water 1 data collected from 1996 to 2001 down to 3 500 m in the tyrrhenian sub basin with ship handled and moored instruments show 5 year t and s trends 0 016 c yr 0 008 yr that are the largest ever evidenced in mediterranean deep waters this is not consistent with the usual hypothesis that tyrrhenian deep water tdw is a mixture of eastern water flowing from the sicily channel and western water flowing from the sardinia channel partly since both are reported to encounter lower trends we argue that tdw might result from a dense water formation process occurring within the tyrrhenian itself in a region never reported up to now east of the bonifacio strait whatever the validity of our hypothesis climatic changes are occurring in the whole sea and are efficiently specified with long time series

numerical simulation of the interannual variability of the mediterranean sea upper ocean circulation numerical simulations reveal that variations in wind stress and heat fluxes can induce significant interannual fluctuations in the circulation of the upper layers of the mediterranean from january 1980 to november 1988 the atmosphere shows changes in the structure and magnitude of the surface winds and in the air temperatures which induce modifications in the upper ocean structure and currents the model prediction of the interannual fluctuations of the sicily strait baroclinic westward volume transport is in agreement with observations and the variability is explained as a function of the wind curl forcing in the region the current anomalies persist for many months after a winter atmospheric anomalous disturbance has occurred over the basin the eastern mediterranean basin is the area where the interannual ocean response is most pronounced

upper crustal shortening and forward modeling of the himalayan thrust belt along the budhi gandaki river central nepal a balanced cross section along the budhi gandaki river in central nepal between the main central thrust including displacement on that fault and the main frontal thrust reveals a minimum total shortening of 400 km minimum displacement on major orogen scale structures include 116 km on the main central thrust 110 km on the ramgarh thrust 95 km on the trishuli thrust and 56 km in the lesser himalayan duplex the balanced cross section was also incrementally forward modeled assuming a generally forward breaking sequence of thrusting where early faults and hanging wall structures are passively carried from the hinterland toward the foreland the approximate correspondence of the forward modeled result to observe present day geometries suggest that the section interpretation is viable and admissible in the balanced cross section the trishuli thrust is the roof thrust for the lesser himalayan duplex the forward model and reconstruction emphasize that the lesser himalayan duplex grew by incorporating rock from the footwall and transferring it to the hanging wall along the main himalayan thrust as the duplex developed the lesser himalayan ramp migrated southward the movement of lesser himalayan thrust sheets over the ramp pushed the lesser himalayan rock and the overburdens of the greater and tibetan himalayan rock toward the erosional surface this vertical structural movement caused by footwall collapse and duplexing in combination with erosion exhumed the lesser himalaya

thermal and kinematic modeling of bedrock and detrital cooling ages in the central himalaya 1 we introduce a new method that convolves detrital mineral cooling ages with digital elevation models to test numerical models of erosion in collisional orogens along a trans himalayan transect in central nepal we develop a kinematic and thermal model to predict variations in bedrock cooling ages in modern himalayan topography the model assumes a thermal steady state and utilizes a simple ramp and flat style decollement representing the main himalayan thrust the model also assumes a topographic steady state such that overthrusting is balanced by erosion to maintain a constant topographic profile erosion rates display strong spatial variations as a function of the angle between the slope of the topographic surface and the trajectories of rock particles approaching the surface to predict the detrital cooling age signal we combine the distribution of bedrock cooling ages within a catchment with the rate of erosion and distribution of muscovite predicted cooling age distributions are compared with detrital 40ar 39ar muscovite data to assess varying tectonic and erosion scenarios such cooling age distributions are very sensitive to how much of the total plate convergence is expressed as erosion of the overthrusting plate the best fit model assigns 4 6 km myr 1 of overthrusting equivalent to as much as 1 5 2 km myr 1 of vertical erosion to the asian plate although a trade off exists between ramp geometry along the decollement and the best fit rate only a narrow range of ramp dips decollement depths and erosionally compensated overthrusting rates are compatible with the observed detrital ages

recent changes in deep water formation and spreading in the eastern mediterranean sea a review abstract observations of the last decade testify that the characteristics of the deep thermohaline circulation in the eastern mediterranean sea have changed thoroughly the source of the most dense waters of the basin has moved from the adriatic to the aegean sea this new source has proved to be very efficient since the estimated formation rate for the period 1989 95 was more than 1 sv about three times more than the dense water formation rate of the adriatic sea these new waters with hydrological characteristics that are warmer and more saline have replaced almost 20 of the older deep waters of the basin and have uplifted the deep isopycnals by about 500 m this major event can be attributed to important meteorological anomalies in the eastern mediterranean and to changes of circulation patterns the extended dry period of 1988 93 and the exceptionally cold winters of 1987 and 1992 93 created favorable conditions for increased formation of dense water in the aegean furthermore changes in the circulation patterns in the intermediate water range levantine intermediate water liw and cretan intermediate water themselves possibly linked to meteorological anomalies appear to have played an important role in the redistribution of salt as a result of an interruption to exchanges between the ionian and levantine basin the salinities in the latter started to rise high salinity waters were diverted into the aegean malanotte rizzoli p manca b b ribera dacala m theocharis a 1998 the eastern mediterranean in the 80s and in the 90s the big transition emerged from the poem bc observational evidence rapport du commission international de la mer medittanee 35 174 175 and the westward transport of liw was reduced an additional effect of the deep water discharge from the aegean and the resulting uplifting of mid depth waters was to lower salinities in the liw layer this effect is most strongly felt in the ionian sea a 3 d primitive equation numerical model for the eastern mediterranean with a 20 km grid size is used to simulate the observed changes and understand the basic mechanisms which caused them under appropriate atmospheric forcing the model successfully reproduces the main characteristics of the transient these results indicate that the observed changes can be at least partially explained as a response of the eastern mediterranean and more specifically of the aegean to atmospheric forcing variability

the western mediterranean deep water a proxy for climate change reconstructions of mediterranean ocean temperature fields back to 1950 show a proxy relationship between heat content changes in the north atlantic and the western mediterranean deep water wmdw formed in the gulf of lions in winter because of consistent air sea heat fluxes over these areas strongly correlated to the north atlantic oscillation nao

turbulent mixing in the eddy transport of western mediterranean intermediate water to the alboran sea western mediterranean intermediate water wiw is formed in winter in the north western mediterranean wiw identifiable as a distinct temperature minimum layer between atlantic mediterranean interface waters and the denser levantine intermediate water is carried down the east coast of spain in anticyclonic mode water eddies or weddies eventually reaching the alboran sea a previous detailed analysis of a weddy in the vicinity of the almeria oran front indicated that it could have accounted for 10 of a winter s production of wiw but this analysis was unable to consider turbulent dissipation in this study we present microstructure measurements across a similar observation of wiw in the vicinity of the almeria oran front and show that this figure could be conservative by 15 50 due to the turbulent dissipation associated with a weddy

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

water masses and circulation in the western alboran sea and in the straits of gibraltar abstract some new elements of alboran sea circulation and water masses in close connection with flows through the straits of gibraltar are presented a general description is given of water masses found in 1981 during mediprod iv cruise from the gulf of cadiz to the gulf of lions followed by a brief review of the conditions found in the straits of gibraltar based on 1981 and 1961 observations the hydrological structure vertical and horizontal and surface and bottom currents typical of the western alboran sea are analysed we present some detailed observations about vertical currents in the deep waters near the straits and finally we analyse the characteristics of the mediterranean overflow through the straits our conclusions in part confirm those of bryden and stommel 1982 and in part shed light on the general pattern of the mediterranean currents entering the alboran sea from the east however considering the suggestion of stommel bryden and mangelsdorf 1973 that the deep currents in the interior of the western mediterranean basin are basically controlled by a bernoulli flow induced by the intense currents in the straits and the weak vertical stratification in the mediterranean we suggest that an interaction of a large scale circulation pattern and the topography of the alboran basin which is much shallower than the western mediterranean basin may play a major role in controlling hydraulically the deep mediterranean waters overflow a detailed water mass analysis of 1981 data and comparison with previous data revealed a time and space variability in the mediterranean waters the scales of this variability are unknown but the s variations make it difficult to make assumptions as to the depth of origin of the deep waters found in the alboran sea which flow into the atlantic ocean over the gibraltar sill

southward extrusion of tibetan crust and its effect on himalayan tectonics the tibetan plateau is a storehouse of excess gravitational potential energy accumulated through crustal thickening during india asia collision and the contrast in potential energy between the plateau and its surroundings strongly influences the modern tectonics of south asia the distribution of potential energy anomalies across the region derived from geopotential models indicates that the himalayan front is the optimal location for focused dissipation of excess energy stored in the plateau the modern pattern of deformation and erosion in the himalaya provides an efficient mechanism for such dissipation and a review of the neogene geological evolution of southern tibet and the himalaya shows that this mechanism has been operational for at least the past 20 million years this persistence of deformational and erosional style suggests to us that orogens like other complex systems can evolve toward steady state configurations maintained by the continuous flow of energy the capacity of erogenic systems to self organize into temporally persistent structural and erosional patterns suggests that the tectonic history of a mountain range may depend on local energetics as much as it does on far field plate interactions

the metamorphic signature of contemporaneous extension and shortening in the central himalayan orogen data from the nyalam transect southern tibet abstract geological relationships and geochronological data suggest that in miocene time the metamorphic core of the central himalayan orogen was a wedge shaped body bounded below by the n dipping main central thrust system and above the n dipping south tibetan detachment system we infer that synchronous movement on these fault systems expelled the metamorphic core southward toward the indian foreland thereby moderating the extreme topographic gradient at the southern margin of the tibetan plateau reaction textures thermobarometric data and thermodynamic modelling of pelitic schists and gneisses from the nyalam transect in southern tibet 28 n 86 e imply that gravitational collapse of the orogen produced a complex thermal structure in the metamorphic core amphibolite facies metamorphism and anatexis at temperatures of 950 k and depths of at least 30 km accompanied the early stages of displacement on the main central thrust system our findings suggest that the late metamorphic history of these rocks was characterized by high t decompression associated with roughly 15 km of unroofing by movement on the south tibetan detachment system in the middle of the metamorphic core roughly 7 8 km below the basal detachment of the south tibetan system the decompression was essentially isothermal near the base of the metamorphic core roughly 4 6 km above the main central thrust the decompression was accompanied by about 150 k of cooling we attribute the disparity between the p t paths of these two structural levels to cooling of the lower part of the metamorphic core as a consequence of continued and probably accelerated underthrusting of cooler rocks in the footwall of the main central thrust at the same time as movement on the south tibetan detachment system

effective radium concentration across the main central thrust in the nepal himalayas effective radium concentration ec ra of 622 rock samples from 6 different sites in the nepal himalayas was measured in the laboratory using radon accumulation experiments these sites located from lower dolpo in western nepal to eastern nepal are divided into 9 transects which cut across the main central thrust zone mct zone separating low grade meta morphic lesser himalayan sequence lhs units to the south and higher grade metamorphic greater himalayan sequence ghs units to the north this boundary remains difficult to define and is the subject of numerous debates ec ra values range from 0 03 0 03 to 251 6 4 0 bq kg a1 and appear to be representative of the formation and clearly related to the local lithology for example for the upper trisuli and langtang valleys site in central nepal the most studied place with 350 available ec ra values lhs rocks are characterized by a mean value of 5 3 1 3 bq kg a1 while ghs rocks of formations i and ii show significantly lower values with a mean value of 0 69 0 11 bq kg a1 thus leading to a lhs ghs ec ra ratio of 7 8 2 2 this behavior was systematically confirmed by other transects ratio of 7 9 2 2 in all other sites with a threshold ec ra value separating lhs from ghs of 0 8 bq kg a1 thus bringing forward a novel method to characterize within the mct shear zone which rocks belong to the ghs and lhs units in addition ulleri augen gneiss belonging to lhs rocks occurred in several transects and were characterized by high ec ra values 17 9 4 3 bq kg a1 easy to distinguish from the ghs gneisses characterized by low ec ra values at the bottom of the ghs thus providing a further argument to locate the mct the measurement of ec ra data thus provides a cost effective method which can be compared with neodymium isotopic anomalies or estimates of the peak metamorphic temperature this study therefore shows that the measurements of ec ra provides additional information to discriminate different geological formations and can be particularly useful in areas where geology mapping is not straightforward or still remains controversial

trends and time variability in the northern continental shelf of the western mediterranean 1 different works appearing during the last decade show that the western mediterranean has suffered a temperature and salinity increase during the 20th century most of these works analyze long term trends in levantine intermediate water liw and western mediterranean deep water wmdw those dealing with changes in shallow and coastal waters are very scarce it is still an open question whether these changes are due to meteorological factors occurring in the western mediterranean or if they are caused by the salinity increase of the water masses contributing to deep water formation in this work we analyze the data obtained in the last decade of the 20th century and longer time series in some cases within the frame of six projects aimed at the systematic collection of hydrographic data at fixed stations in the northern sector of the western mediterranean mainly in coastal areas we detect strong warming trends in those stations located in the continental shelf and probably with no influence of the liw this result could indicate that changes observed in the wmdw cannot be explained only on the basis of changes imported from the eastern basin another striking result is that these trends are an order of magnitude higher than those reported for the rest of the century indicating that the 1990s have been an exceptionally warm decade on the other hand time series affected by the liw show a salinity increase and in some cases this is not accompanied by the corresponding temperature increment indicating that the liw salinity increase could also be a factor to be taken into account

recent advances in observing the physical oceanography of the western mediterranean sea the mediterranean sea has been investigated intensively since the early nineties using modern techniques and collaborative approaches this overview summarizes some of the resulting advances that were made concerning the physical oceanography of the western mediterranean the water mass formation processes are now much better understood and have been quantified to a large extent the boundary conditions of the system in terms of surface fluxes and strait transports can be determined with improved accuracy thus enabling future investigation of interannual variability the dynamics of the surface and intermediate layers have revealed a variety of eddy and mesoscale processes that are important for the circulation and spreading of water masses the deep circulation is being investigated with lagrangian techniques tracers and floats first results show a large component of the deep water originating from the tyrrhenian sea and intense cyclonic and anticyclonic eddy flows

crustal structure of the himalayan orogen at 90 east longitude from project indepth deep reflection profiles international deep profiling of tibet and the himalaya indepth deep seismic reflection profiles show that the indian lithosphere is underthrusting the central himalaya along a gently north dipping decollement that is traceable northward beneath the tethyan belt to 28 6 n and to a depth of about 45 km the decollement carries in its hanging wall a near crustal thickness slice of internally deformed indian continental basement and cover represented in outcrop by the greater himalayan belt and structurally overlying tethyan belt geometric relationships suggest that the decollement probably ramps downward beneath the northern tethyan belt to near the base of the crust and that the hanging wall crustal slice was detached from indian mantle lithosphere that presently underlies southern tibet to the north the north himalayan anticlinorium kangmar dome appears to be a large duplex ramp anticline in the hanging wall of the decollement the south tibetan detachment appears to have been imaged in two areas in the wagye la area it appears to follow the tethyan belt greater himalayan belt contact northward in the subsurface and to have been folded over the kangmar dome in the zherger la area it appears to have been active relatively recently as a 30 north dipping normal fault that cuts deeply into the greater himalayan belt allochthon palinspastic reconstruction indicates a minimum of about 326 km of shortening of the indian basement across the himalaya since the initiation of the main central thrust hence indian continental lithosphere largely stripped of its overlying crust can extend northward beneath the tibetan plateau to at least 32 n where earthquake seismological observations show that the properties of the upper mantle change markedly e g mcnamara et al 1995 taken together the seismic reflection and earthquake seismological observations lend support for the view that the indian mantle lid mechanically underplates roughly the southern half of the tibetan plateau the indepth reflection data do not show that indian crust attached to this lithosphere extends north of southernmost tibet kangmar dome nor do they negate this possibility similarly the reflection data do not yield direct evidence for or against lower crustal subduction beneath the himalaya

isotopic and structural constraints on the location of the main central thrust in the annapurna range central nepal himalaya five isotope enhanced geologic transects in the southern annapurna range of central nepal elucidate structural geometries near the main central thrust whole rock nd isotopes and u pb ages of detrital zircons unambiguously distinguish greater himalayan hanging wall and lesser himalayan footwall metasedimentary rocks nd 0 values for lower lesser himalayan rocks typically range from 20 to 26 whereas greater himalayan rocks usually have nd 0 values of 19 to 12 lower lesser himalayan rocks yield detrital zircons with an age peak at ca 1880 ma and no detrital zircons younger than ca 1550 ma in contrast greater himalayan rocks yield detrital zircons with a prominent broad peak of ages at ca 1050 ma and no detrital zircons younger than ca 600 ma the protolith boundary between greater and lesser himalayan rocks is up to 1 km farther south than usually mapped on the basis of lithology field and microstructural observations indicate the presence of a top to the south ductile shear zone superimposed on this boundary confirming this shear zone as the main central thrust no evidence exists for large scale structural mixing of greater and lesser himalayan rocks along the main central thrust in the annapurna range

kinematic history of the main central thrust zone in the langtang area nepal abstract this paper describes for the first time evidence for a northeastward brittle extensional movement of the main central thrust mct zone overprinting the southward ductile thrust movement the mct zone is the major tectonic boundary between the lesser himalayan metasedimentary sequence lhs and higher himalayan crystalline sequence hhs and contributes to the kinematic and tectonic evidence of continent continent collision in the neogene the langtang area 50 km north of kathmandu is a matter of interest in considering the movement of the kathmandu nappe because the general trend of foliation of the hhs and the underlying mct zone is turned from wnw ese to n s or nne ssw the major tectono lithostratigraphic units of the mct zone in the study area are divided into three the lower and middle units included in the lhs and the upper unit even included in the hhs characteristic mesoscopic quartz lenses are developed in the schists of the lower unit forming asymmetric boudins this asymmetry indicates an extensional top to the ne shear sense however a thrust sense of top to the sw is also preserved in microscopic ductile shear bands and mica fish the extensional shear movement in the lower unit took place after the major thrust movement in the mct zone as a negative inversion the middle unit contains ulleri type augen gneiss syabru bensi augen gneiss which is widely distributed in the mct zone in the other parts of the nepalese himalaya asymmetric clast tails or clast pressure shadows and asymmetric boudins observed in the syabru bensi augen gneiss give a dextral thrust oblique sense of shear this dextral component is regarded as movement of a n s trending lateral ramp during the overthrusting of the kathmandu nappe the upper unit above the mct is commonly made up of kyanite bearing medium pressure type amphibolite facies pelitic gneiss asymmetric deformational features such as mica fish and shear bands in the pelitic gneisses commonly show a thrust top to the w sense of movement after the emplacement of the kathmandu nappe 9 6 ma brittle extensional shear occurred only in the crystalline schists of the lower unit presumably associated with layer parallel slip this extensional movement in the lower unit is probably caused by the change in strike of foliations e g from a general trend of e w in the other areas to n s as seen in the study area this is because n s contraction during continued continent continent collision produced e w extension normal to the contraction axis n s thus the relative east directed movement on normal faults took place only in the n s trending domain of the mct zone another possibility for the extensional movement is squeezing of the upper part of the lhs between the mct above and an out of sequence thrust or the main detachment fault below

eddy transport of western mediterranean intermediate water to the alboran sea during the second cruise of the eu funded omega project the towed undulating vehicle seasoar was deployed to survey the upper 350 m of the water column in the eastern alboran sea and extreme western algerian basin with an effective along track resolution of 4 km the data sets enabled a detailed description of the different upper ocean water types and the fronts that separate them the almeria oran front forms at the eastern boundary of the alboran sea gyre system in the upper 150 200 m of the water column and separates waters of predominantly atlantic origin from those formed in the western mediterranean sea below these surface waters but above the levantine intermediate water western mediterranean intermediate waters believed to be formed to the north of the balearic sea are normally observed in this region however to our knowledge this is the first time a discrete eddy of western mediterranean intermediate water a weddy has been described in the extreme western algerian basin repeated surveys of the region allowed us to observe the evolution of the eddy over a period of 40 d a climatological analysis of historical data in the medar medatlas database provides evidence for the repeatability of this observation and the significance of the estimated transport

petrological constraints on the tectonic setting of the kathmandu nappe in the langtang gosainkund helambu regions central nepal himalaya the gosainkund helambu region in central nepal occupies a key area for the development of himalayan kinematic models connecting the well investigated langtang area to the north with the kathmandu nappe kn whose interpretation is still debated to the south in order to understand the structural and metamorphic architecture of the greater himalayan sequence ghs in this region a detailed petrological study was performed focusing on selected metapelite samples from both the gosainkund helambu and langtang transects the structurally lowest sample investigated belongs to the lesser himalayan sequence its metamorphic evolution is characterized by a narrow hairpin p t path with peak p t conditions of 595 25 c 7 5 1 kbar all of the other samples here investigated belong to the ghs along the langtang section two tectono metamorphic units have been distinguished within the ghs the lower greater himalayan sequence l ghs characterized by peak p t conditions at 728 11 c 10 0 5 kbar corresponding to a t depth ratio of 22 1 c km 1 and the structurally higher upper greater himalayan sequence with peak metamorphic conditions at 780 20 c 7 8 0 8 kbar corresponding to a t depth ratio of 31 4 c km 1 this confirms the existence of a main tectono metamorphic discontinuity within the ghs as previously suggested by other authors the results of petrological modelling of the metapelites from the gosainkund helambu section show that this region is entirely comprised within a sub horizontal and thin l ghs unit the estimated peak metamorphic conditions of 734 19 c 10 0 8 kbar correspond to a uniform t depth ratio of 23 3 c km 1 the metamorphic discontinuity identified along the langtang transect and dividing the ghs in two tectono metamorphic units is located at a structural level too high to be intersected along the gosainkund helambu section our results have significant implications for the interpretation of the kn and provide a contribution to the more general discussion of the himalayan kinematic models we demonstrate that the structurally lower unit of the kn known as sheopuri gneiss can be correlated with the l ghs unit this result strongly supports those models that correlate the kn to the tethyan sedimentary sequence and that suggest the merging of the south tibetan detachment system and the main central thrust on the northern side of the kn moreover we speculate that in this sector of the himalayan chain the most appropriate kinematic model able to explain the observed tectono metamorphic architecture of the ghs is the duplexing model or hybrid models which combine the duplexing model with another end member model

rising temperature and salinity fields to the north of the almeria oran front during the past decade 1 time series of temperature and salinity profiles in the south western mediterranean sea are analyzed from june 1996 to november 2006 previous studies have addressed the detection of changes on isobaric levels in this paper we decompose thermohaline changes into those caused by vertical displacements of the isopycnals more likely linked to changes in the dynamics of the region and dominant winds and those changes on isopycnal surfaces the latter give a more accurate idea about changes in water mass properties due to ocean atmosphere interaction our analysis reveals that there has been a warming and salting on isopycnal surfaces the warming during this 10 year period is quite intense around 0 032 c yr 1 and the salinity increase is around 0 009 yr 1 the depth of isopycnals shows an upward trend rising 45 m over the study period compensating for the warming on isobaric surfaces

the origin of helium in deep sedimentary aquifers and the problem of dating very old groundwaters noble gases inert elements having isotopes produced by the decay of long half life radionuclides offer a powerful approach for tracing fluid circulation and dating groundwaters the u th 4he water ages calculated from the accumulation rate in water of radiogenic 4he produced by decay of u and th contained in the aquifer rocks is frequently higher than the hydrological ages this discrepancy is generally interpreted by two contrasting models i heterogeneities of the aquifers which allow water stagnation and accumulation of large amounts of radiogenic 4he or ii addition of 4he produced in deeper regions of the continental crust in this contribution we propose that the apparent contrast between u th 4he ages and hydrological ages in the paris basin reflects the mixing of different types of water having different residence times we show using the helium isotopic signatures of waters that this mixing occurs between three aquifers the middle jurassic the triassic and the palaeozoic basement which have contrasting helium contents and heterogeneous chemical compositions and permeabilities the difference of radiogenic 4he 4 ratios between the aquifers of triassic and middle jurassic strongly suggests that a significant fraction of helium is produced in the aquifer rocks this implies residence times for groundwaters circulating in the middle jurassic carbonate aquifer much longer than those obtained from hydrological studies independent fluid age estimates based on the ground palaeotemperatures recorded in the same groundwaters by the atmospherederived noble gases seem to confirm the presence of very old groundwaters in the paris basin the estimation of the residence time of ancient groundwaters is the most challenging problem in basin hydrology e g chapman 1987 typical water residence times of 104 108 years outreach the range of half lives of most of the geochronometers available in hydrology e g 3h 14c and prolonged water rock interactions alter the chemical and isotopic signature of water preventing the identification of the fluid sources on the other hand hydrological modelling faces serious problems of estimating space and time variations of critical parameters such as the permeability due to the petro physical heterogeneities of the aquifers and fluid velocity estimates can be valid only on a local scale e g alexander et al 1987 because noble gases are inert and have isotopes produced by natural decay of long halflife radionuclides these elements have received increasing attention as tracers of groundwater movements in deep aquifers e g marine 1979 andrews et al 1982 1985 bottomley et at 1984 1990 torgersen clarke 1985 zaikowski et al 1987 ballentine et al 1991 stute et al 1992 marty et al 1993 o nions ballentine 1993 groundwaters contain noble gases derived from the atmosphere the crust and the mantle fig 1 studies on water dating are focused mainly on the crustal derived radiogenic noble gas component for obvious reasons although the mantle derived component has the capacity of tracing fluid sources oxburgh et al 1986 and the atmosphere derived component can provide independent estimates on the water residence times pinti et al 1997 in continental environments mantle derived noble gases are generally observed in regions of active volcanism or extensional tectonics ballentine et al 1991 the source is probably sub continental magma intrusion and underplating with successive episodic and tectonically controlled advective transport in the crust torgersen 1993 mantle derived helium can be easily identified from its isotopic signature indeed the mantle is enriched in primordial 3he as indicated by 3he 4he ratios greater than 10 5 whereas the atmospheric ratio is l a x 10 6 and crustalderived radiogenic helium has 3he 4he ratios in the range of 10 8 consequently the geographical distribution of the 3he 4he ratios in sedimentary fluids can be used to identify the fluids pinti d l marty b 1998 the origin of helium in deep sedimentary aquifers and the problem of dating very old groundwaters in parnell j ed 1998 dating and duration of fluid flow and fluid rock interaction geological society london special publications 144 53 68 by guest on january 5 2014 http sp lyellcollection org downloaded from

chronology of tectonic events in the crystalline core of the himalaya langtang national park central nepal the main central thrust mct is an important intracontinental subduction zone that accommodated a significant proportion of shortening between india and asia during tertiary himalayan orogenesis argon 40 argon 39 geochronology indicates at least two distinct periods of thrust movement on the mct in the langtang national park region of central nepal ductile deformation and associated mylonitization characterizing the earlier event are constrained by 40ar 39ar dating of muscovites to have occurred sometime before 5 8 ma a later period of brittle deformation resulting in the juxtaposition of rocks of different lithology within the mct zone occurred at approximately 2 3 ma based on 40ar 39ar dating of neoblastic muscovites from the brittle fault zones the hanging wall of the mct contains amphibolite to upper amphibolite grade gneisses and small leucogranite bodies assigned to the greater himalayan sequence argon 40 argon 39 cooling ages of muscovite and biotite from the gneisses range from 4 6 to 9 7 ma these dates contrast with previously obtained 16 21 ma u pb monazite and zircon ages for metasedimentary rocks from the same structural levels parrish et al 1992 indicating relatively slow cooling over the early to middle miocene interval for much of the mct hanging wall however one 19 3 ma biotite 40ar 39ar cooling age for a sample from the uppermost portion of the hanging wall is only slightly younger than u pb monazite ages for nearby anatexites possibly suggesting rapid cooling of the uppermost greater himalayan sequence by tectonic denudation associated with the structurally higher south tibetan detachment system the general consistency of 40ar 39ar ages throughout the 11 km thick greater himalayan sequence suggests rapid cooling in late miocene time probably due to an increase in erosion rate related to ramping on the structurally lower main boundary thrust an alternative possibility would be massive hydrothermal resetting in late miocene time of the entire sequence

a structural analysis of the main central thrust zone langtang national park central nepal himalaya the main central thrust mct is one of the most tectonically significant structures in the himalayan orogen detailed geologic mapping and structural analysis of the mct in the langtang national park region of central nepal reveals that this segment of the fault zone experienced multiple episodes of south directed movement under both brittle and ductile conditions during the tertiary period early mid miocene movement resulted in the development of mylonitic fabrics synchronous with amphibolite facies metamorphism the mean orientation of the dominant mylonitic foliation is n28 w 38 ne an associated mineral stretching lineation plunges 40 to n40 e kinematic indicators suggest hanging wall movement to the southwest relative to the footwall along the north dipping fault it is not possible to constrain the magnitude of high temperature displacement on the mct at the longitude of langtang late stage structures in the mct zone at langtang include a series of imbricate brittle thrust faults that separate different lithostratigraphic units and correspond to metamorphic discontinuities we interpret this fault system as a duplex structure muscovite 40 ar 39 ar cooling ages from the mct zone range from 8 9 6 9 ma because the nominal closure temperature of ar diffusion in muscovite approximately 625 k is higher than the apparent temperature conditions under which late brittle deformation occurred we suggest that brittle deformation was a latest miocene pliocene phenomenon another major himalayan fault the main boundary thrust mbt was developing to the south of langtang at approximately the same time we speculate that brittle faulting within the mct zone may have initiated as the mct zone was transported over a ramp in the mbt

a study of an intense density front in the eastern alboran sea the almeria oran front abstract studies of satellite imagery and space shuttle photographs of the western mediterranean have indicated that the main path of inflowing atlantic water is around two large anticyclonic gyres in the alboran sea and along the algerian coast these studies have also shown that a strong ocean front is present between almeria spain and oran algeria which is part of the easternmost segment of the eastern alboran gyre based on these satellite studies the first in situ investigation of the front called here the almeria oran front was conducted in march 1986 as part of the winter campaign of the western mediterranean circulation experiment wmce analyses of the resulting data show that the almeria oran front is a large scale density front formed by the convergence of two distinct water masses and controlled by the geographic position and strength of the eastern alboran gyre physical and biochemical data indicate that the front is limited to the upper 300 m with a strong southward baroclinic jet

yadong cross structure and south tibetan detachment in the east central himalaya 89 90 e the yadong cross structure ycs occurring at 89 30 east longitude in the himalaya is the largest across strike discontinuity in the geologic structure and topography of the high himalaya between the himalayan syntaxes it is manifest by a plan view left offset of the topographic crest of the range and a coincident apparent left strike offset of the south tibetan detachment system stds of about 70 km new field mapping indicates that the stds intersects the west side of the ycs at zherger la and further suggests that the ycs is the surface expression of a large north northeast striking west facing lateral ramp in the himalayan thrust system the greater himalayan allochthon is apparently draped across this lateral ramp resulting in a north northeast striking monoclinal flexure of the allochthon manifest in the chomolhari range superimposed steep west northwest directed normal shear appears to have obliterated earlier northerly directed stds shear fabric along along the ycs segment of the tethyan belt greater himalayan belt contact recent seismicity suggests that the greater himalayan allochthon is still moving southward along the lateral ramp u pb monazite dates on leucogranites in the footwall of the stds on opposite sides of the ycs are suggestive of a south to north decrease in the crystallization age of these granites the age pattern is consistent with southward extrusion of the greater himalayan allochthon and suggests the possibility of determining an average slip rate for the stds by dating granites in the immediate footwall of the stds that are substantially separated in north south section an average slip rate of about 7 mm yr between 23 and 12 ma is suggested by the data presented here

climatic changes in the aegean sea influence the eastern mediterranean thermohaline circulation 1986 1997 recent changes of water mass characteristics in the south aegean sea have considerably influenced the eastern mediterranean thermohaline circulation a combination of salinity increase 1987 92 and temperature drop 1992 94 caused massive dense water formation and strong outflow towards the deep and bottom parts of the eastern mediterranean this climatic shift the most important since the existence of observations in the basin is a combined effect of extreme meteorological events superimposed on large scale trends it has initiated a series of modifications in the hydrology and dynamics of the entire eastern mediterranean with possible influence on the mediterranean outflow in the north atlantic ocean