glider observations of enhanced deep water upwelling at a shelf break canyon a mechanism for cross slope carbon and nutrient exchange using underwater gliders we have identified canyon driven upwelling across the celtic sea shelf break in the vicinity of whittard canyon the presence of this upwelling appears to be tied to the direction and strength of the local slope current which is in itself highly variable during typical summer time equatorward flow an unbalanced pressure gradient force and the resulting disruption of geostrophic flow can lead to upwelling along the main axis of two small shelf break canyons as the slope current reverts to poleward flow the upwelling stops and the remnants of the upwelled features are mixed into the local shelf water or advected away from the region the upwelled features are identified by the presence of sub pycnocline high salinity water on the shelf and are upwelled from a depth of 300 m on the slope thus providing a mechanism for the transport of nutrients across the shelf break onto the shelf

internal tide coherence and decay over a wide shelf sea 1 a quasi synoptic hydrography and velocity section is used to determine the structure and the decay rate of the internal tide it across the broad continental shelf of the celtic sea in these observations the it is coherent over more than 170 km about five wavelengths with an estimated shoreward energy decay scale of 42 km the inferred it wavelength averaged dissipation rate near the shelf edge is estimated as 2 08 10 7 wkg 1 in close agreement with tidally and vertically averaged measurements from the region these results provide the firstin situ evidence of it coherence over many wavelengths in a shelf sea

internal tides in a dendritic submarine canyon abstract submarine canyons are a common geomorphological feature along continental slopes worldwide and often found to be hotspots of internal tide activity however the majority of well studied submarine canyons are simple linear incisions or have meandering morphology internal tide energetics in branching dendritic canyons has not previously been investigated here we present a high resolution 500 m numerical modelling study of the internal tide within whittard canyon a large dendritic submarine canyon system that incises the celtic sea continental slope a modified version of the princeton ocean model is used to simulate the m 2 semidiurnal internal tide in the whittard canyon region verified against a hydrographic dataset collected by an autonomous ocean glider much of the internal tide energy entering whittard canyon originates to the southeast along the celtic sea shelf break internal tide generation also occurs within the canyon itself but is in part compensated by areas of negative energy conversion depth integrated internal tide energy fluxes exceed 8 kw m 1 in the eastern limb of the canyon the internal tide is topographically steered through the major limbs and along canyon energy flux is bottom intensified suggesting topographic focusing the down canyon extent of bottom intensification closely corresponds to the point that along canyon slope becomes near critical to the semidiurnal internal tide energetically the multiple limbs of whittard canyon behave differently some are net sources of internal tide energy whilst others are net sinks internal tide energy dissipation also varies between the canyon limbs bulk dissipation rates are 2 1 7 7 10 8 w kg 1 in addition the effect of bathymetric resolution on internal tide generation and propagation is investigated by progressively smoothing the model domain decreasing the bathymetric resolution reduces internal tide generation and energy dissipation in both whittard canyon and the model domain as a whole however internal tide energy flux into the canyon is not consistently changed at least 1 5 km resolution bathymetry is required to adequately resolve the semidiurnal internal tide field in this region of complex topography

indian punch rifts tibet the strikes of late cenozoic normal faults in tibet fan systematically from a northwesterly to northeasterly direction from west to east they exhibit an axis of symmetry that bisects the himalayan arc and trends subparallel to india asia motion the two dimensional stress field inferred from the spatial pattern of normal fault orientations can be explained solely by collisional stresses localized along a southern part of the himalayan arc continued insertion of indian crust into tibet can explain the persistence of extension and may be increasing plateau crustal thickness and elevation coeval with rifting

indepth iii seismic data from surface observations to deep crustal processes in tibet 1 during the summer of 1998 active source seismic data were collected along a transect running 400 km nnw sse across the central tibetan plateau as the third phase of project indepth international deep profiling of tibet and the himalaya the transect extends northward from the central lhasa block across the jurassic bangong nujiang suture bns at 89 5 e to the central qiangtang block a seismic velocity model for the transect to 25 km depth produced by inversion of p wave first arrivals on 3000 traces shows 1 a 50 km wide region of low velocity at least 5 less than surrounding velocities extending to the base of the model at the bns 2 sedimentary cover for the southern qiangtang block that is 3 5 km thick 3 a distinct interface between sedimentary cover and qiangtang basement or underplated jurassic melange in the central qiangtang block and 4 evidence that the bangoin granite extends to a depth of at least 15 km the bns has little geophysical signature and appears unrelated to the 5 km northward shallowing of the moho which is associated with the bns in central tibet geophysical data along the main indepth iii transect show little evidence for widespread crustal fluids in contrast to the seismic bright spots found in southern tibet and to magnetotelluric evidence of fluid accumulations in eastern tibet a comparison between the global average and tibetan velocity depth functions offers constraints for models of plateau uplift and crustal thickening taken together with the weak geophysical signature of the bns these velocity depth functions suggest that convergence has been accommodated largely through pure shear thickening accompanied by removal of lower crustal material by lateral escape likely via ductile flow although we cannot resolve the details we believe lateral lower crustal flow has overprinted or destroyed evidence in the deep crust for the earlier assembly of tibet as a series of accreted terranes

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

dynamics of advection driven upwelling over a shelf break submarine canyon 1 the response over a submarine canyon to a several day upwelling event can be separated into three phases an initial transient response a later much longer steady advection driven response and a final relaxation phase for the advection driven phase over realistically steep deep and narrow canyons with near uniform flow and stratification at rim depth we have derived scale estimates for four key quantities observations from 5 real world canyon studies and 3 laboratory studies are used to validate the scaling and estimate the scalar constant for each scale based on 4 geometric parameters of the canyon the background stratification the coriolis parameter and the incoming current we can estimate 1 the depth of upwelling in the canyon to within 15 m 2 the deep vorticity to within 15 and 3 the presence absence of a rim depth eddy can be determined based on laboratory data 4 the total upwelling flux can also be estimated the scaling analysis shows the importance of a rossby number based on the radius of curvature of isobaths at the upstream mouth of the canyon this rossby number determines the ability of the flow to cross the canyon isobaths and generate the pressure gradient that drives upwelling in the canyon other important scales are a rossby number based on the length of the canyon which measures the ability of the flow to lift isopycnals and a burger number based on the width of the canyon that determines the likelihood of an eddy at rim depth generally long canyons with sharply turning upstream isobaths strong incoming flow small coriolis parameter and weak stratification have the strongest upwelling response

deep seismic reflection evidence for continental underthrusting beneath southern tibet the himalaya and adjacent tibetan plateau constituting earth s largest region of elevated topography and anomalously thick crust formed as a consequence of cenozoic collision between india and asia itself considered the archetypal continent continent collision1 3 here we report the first results from an attempt to image the structure of the crust beneath this region using deep seismic reflection profiling our 100 km long profile acquired in the tethyan himalaya shows a mid crustal reflection that prob ably marks the active thrust fault along which the indian plate is underthrusting southern tibet upper crustal reflections with geo metries suggestive of large scale structural imbrication of the upper crust and moho reflections from the base of the double normal thickness crust underlying the region these results lend substantial support to the view that crustal thickening beneath southernmost tibet was accomplished by wholesale underthrusting of indian continental crust beneath the structurally imbricated upper crust comprising the tethyan himalaya

detection of widespread fluids in the tibetan crust by magnetotelluric studies magnetotelluric exploration has shown that the middle and lower crust is anomalously conductive across most of the north to south width of the tibetan plateau the integrated conductivity conductance of the tibetan crust ranges from 3000 to greater than 20 000 siemens in contrast stable continental regions typically exhibit conductances from 20 to 1000 siemens averaging 100 siemens such pervasively high conductance suggests that partial melt and or aqueous fluids are widespread within the tibetan crust in southern tibet the high conductivity layer is at a depth of 15 to 20 kilometers and is probably due to partial melt and aqueous fluids in the crust in northern tibet the conductive layer is at 30 to 40 kilometers and is due to partial melting zones of fluid may represent weaker areas that could accommodate deformation and lower crustal flow

north south extension within the convergent himalayan region recent work by burg et al 1984 indicates the presence of east west striking gently north dipping normal faults in the higher himalayas and southern tibet that formed during postcollisional convergence of india and tibet these faults extend for at least 800 km along strike we interpret these normal faults as probable late miocene extensional features with perhaps several tens of kilometres of down to the north displacement a simple elastic model suggests that these normal faults may have formed during gravitational collapse of the miocene topographic front between india and tibet in this interpretation gravitational collapse occurred by southward motion relative to india and tibet of a wedge of crustal rocks bounded above by gently north dipping normal faults and below by thrust faults that probably dip north north south extension produced in this way is probably confined to upper crustal levels only and does not reflect regional extension of the entire lithosphere

intense variable mixing near the head of monterey submarine canyon abstract a microstructure survey near the head of monterey submarine canyon the first in a canyon confirmed earlier inferences that coastal submarine canyons are sites of intense mixing the data collected during two weeks in august 1997 showed turbulent kinetic energy dissipation and diapycnal diffusivity up to 103 times higher than in the open ocean dissipation and diapycnal diffusivity within 10 km of the canyon head were among the highest observed anywhere e 1 1 10 6 w kg 1 k 1 0 10 2 m2 s 1 mixing occurred mainly in an on axis stratified turbulent layer with thickness and intensity increasing from neap to spring tide strain spectra showed a gentler than k 1z rolloff suggesting that critical reflection and scattering may push energy into high wavenumbers dissipation dependence on shear appears to be much weaker in the canyon than in the open ocean with indications that the dependence maybe as low as e s coastal canyons may account for a small but significant fraction of the int

causes and consequences of protracted melting of the mid crust exposed in the north himalayan antiform gneiss domes exposed by the north himalayan antiform in southern tibet provide evidence for protracted melting of the mid crust during convergent tectonics anatexis was initiated during underthrusting and recurred during the southwards extrusion of a low viscosity crustal sheet sr nd isotope systematics establish that the cores of the domes dominated by cambrian gneiss form a window of the high himalayan crystalline series hhcs indian plate combined trace element and isotopic analyses from 4 two mica granites that intrude both core and the metasedimentary mantle of the domes indicate an origin from melting of the high himalayan crystalline series by the incongruent breakdown of muscovite and to a lesser extent biotite at temperatures from 680 to 760 c u pb dating of accessory phases extends the known period of granite magmatism from the late oligocene to mid miocene a ca rich garnet bearing leucogranite of oligocene age 27 5 ma provides a rare insight into the pre miocene history of the underthrust indian plate reflecting prograde melting at greater depths during a period of crustal thickening the origin of the miocene granites 15 10 ma can be modeled from the exhumation of an extruding channel at a rate of ca 1 mm year constrained by ar isotope data melting over a protracted time scale along the leading edge of the indian plate less than 50 km from the suture accelerated horizontal shortening within the collision zone and provided a positive feedback mechanism between magmagenesis and deformation

erosion and tectonics at the margins of continental plateaus the topography across the eastern margin of the central andean plateau north of 18 s beni region bears a strong resemblance to the topography of the southern margin of the tibetan plateau nepal himalaya with both regions featuring a steep frontal slope and high peaks at the plateau edge in contrast the topography of the eastern margin of the central andean plateau south of 18 s pilcomayo region tapers toward the foreland more gently and has no line of high peaks at the margin both the himalayan and the beni regions have been the sites for large amounts of denudation and in both regions geologic evidence suggests that erosion has been sufficiently vigorous for the physiographic plateau margin to have retreated toward the plateau interior during the neogene we hypothesize that the steep frontal slope and high peaks of the beni region and himalayan front largely reflect the high orographic precipitation and high erosion rates occurring in these regions and that the more gentle topography of the semiarid pilcomayo region reflects a tectonic landform only slightly modified by erosion we propose that orographic precipitation impinging on a plateau margin will generally tend to drop moisture low on the slope eroding back the plateau while enhancing or maintaining the steep long wavelength slope a numerical model coupling orographic precipitation erosion and tectonic uplift demonstrates the plausibility of this hypothesis the erosional efflux in both the beni and nepal himalaya have been considerable and simple mass balance calculations for the himalaya suggest that during the neogene the erosional mass efflux has generally outpaced the tectonic mass influx this contrasts with the apparent prior domination of tectonic influx and may reflect a decrease in the rate of tectonic addition during the same period and or increased late cenozoic erosion rates

tectonic history of the altyn tagh fault system in northern tibet inferred from cenozoic sedimentation the active left slip altyn tagh fault defines the northern edge of the tibetan plateau to determine its deformation history we conducted integrated research on cenozoic stratigraphic sections in the southern part of the tarim basin fission track ages of detrital apatites existing biostratigraphic data and magnetostratigraphic analysis were used to establish chronostratigraphy whereas composition of sandstone and coarse clastic sedimentary rocks was used to determine the unroofing history of the source region much of the detrital grains in our measured sections can be correlated with uplifted sides of major thrusts or transpressional faults implying a temporal link between sedimentation and deformation the results of our studies together with existing stratigraphic data from the qaidam basin and the hexi corridor suggest that crustal thickening in northern tibet began prior to 46 ma for the western kunlun shan thrust belt at ca 49 ma for the qimen tagh and north qaidam thrust systems bounding the north and south margins of the qaidam basin and prior to ca 33 ma for the nan shan thrust belt these ages suggest that deformation front reached northern tibet only 10 5 m y after the initial collision of india with asia at 65 55 ma because the aforementioned thrust systems are either termination structures or branching faults of the altyn tagh left slip system the altyn tagh fault must have been active since ca 49 ma the altyn tagh range between the tarim basin and the altyn tagh fault has been a long lived topographic high since at least the early oligocene or possibly late eocene this range has shed sediments into both the tarim and qaidam basins while being offset by the altyn tagh fault its continuous motion has made the range act as a sliding door which eventually closed the outlets of westward flowing drainages in the qaidam basin this process has caused large amounts of oligocene miocene sediments to be trapped in the qaidam basin the estimated total slip of 470 70 km and the initiation age of 49 ma yield an average slip rate along the altyn tagh fault of 9 2 mm yr remarkably similar to the rates determined by gps global positioning system surveys this result implies that geologic deformation rates are steady state over millions of years during continental collision

the zedong window a record of superposed tertiary convergence in southeastern tibet determining the timing magnitude and location of deformation due to the indo asian collision is widely acknowledged as an important step in understanding how the lithosphere responds during continental collision a puzzling result of geological investigations of the lhasa block over the past 2 decades has been the apparent lack of significant tertiary deformation there perhaps the most important structural feature of the lhasa block is the south directed gangdese thrust system which developed along its southern edge the thrust system which separates the andean type batholith of southern asia from rocks of indian affinity is obscured at most locations across southeastern tibet by back thrusts of the younger north directed renbu zedong thrust system the best documented site where both thrusts are exposed occurs near zedong zedong window systematic geochronologic analyses were conducted in this area u pb zircon dating of three samples of a synkinematically to postkinematically deformed hanging wall granitoid the yaja granodiorite cut by the gangdese thrust indicates a crystallization age of 30 4 0 4 ma 2 thus placing an upper bound on the initiation of the thrust u pb zircon dating of granitoid samples structurally higher in the gangdese hanging wall yields emplacement ages of 42 5 1 0 ma and 48 9 0 8 ma 2 similar to other magmatic complexes within the gangdese arc geochemical results are consistent with these plutons forming in the same environment as precollisional intrusions within the gangdese batholith suggesting a significant post collisional input of juvenile heat the 40ar 39ar thermochronology of samples from a vertical section through the hanging wall within the yaja granodiorite coupled with results of a numerical thermal model indicate an average slip rate along the gangdese thrust of 7 mm yr between 30 and 23 ma and a minimum displacement of 50 km farther east in the zedong window thermal effects produced by later north directed thrust sheets of the renbu zedong system appear to have obscured thermal history signatures in the gangdese hanging wall related to earlier south directed thrusting thermochronological results from this region indicate that thermal overprinting related to the north directed thrusting occurred between 25 and 10 ma consistent with previous estimates

along isopycnal variability of spice in the north pacific two hydrographic surveys in the gulf of alaska and the north pacific subtropical gyre are presented both surveys are roughly perpendicular to lateral temperature gradients and were collected in the summer when there was a shallow mixed layer and a seasonal thermocline isopycnal displacements and horizontal velocities are dominated by internal waves spice anomalies along isopycnals are examined to diagnose lateral stirring mechanisms the spectra of spice anomaly gradients along near surface isopycnals roughly follow power laws of kx0 6 0 2 variance spectra power laws of kx 1 4 0 2 and in most cases the spectra become redder at depth the near surface spectra are possibly consistent with the predictions of quasi geostrophic turbulence theory when surface buoyancy effects are accounted for but the spectra at depth are inconsistent with any quasi geostrophic theory probability distributions of spice gradients exhibit a large peak at low gradients and long tails for large gradients symptomatic of fronts vertical coherence of the spice signal falls off with a decorrelation depth scale that has a maximum of about 80 m at 100 km wavelengths and depends on horizontal wavelength with a power law of approximately kx 1 2 lateral decorrelation length scales are 20 40 km close to the baroclinic rossby radius lateral stirring occurs over large scales with average lateral displacements of about 200 km in the upper 75 m decreasing to 100 km at greater depths the depth variation of the statistics indicates that time history of tracer stirring on each isopycnal is important or that there are unconsidered depth dependent stirring mechanisms

miocene to holocene exhumation of metamorphic crustal wedges in the nw himalaya evidence for tectonic extrusion coupled to fluvial erosion the himalayan crystalline core zone exposed along the sutlej valley india is composed of two high grade metamorphic gneiss sheets that were successively underthrusted and tectonically extruded as a consequence of the foreland directed propagation of crustal deformation in the indian plate margin the high himalayan crystalline sequence hhcs is composed of amphibolite facies to migmatitic paragneisses metamorphosed at temperatures up to 750 c at 30 km depth between eocene and early miocene during early miocene combined thrusting along the main central thrust mct and extension along the sangla detachment induced the rapid exhumation and cooling of the hhcs whereas exhumation was mainly controlled by erosion since middle miocene the lesser himalayan crystalline sequence lhcs is composed of amphibolite facies para and orthogneisses metamorphosed at temperatures up to 700 c during underthrusting down to 30 km depth beneath the mct the lhcs cooled very rapidly since late miocene as a consequence of exhumation controlled by thrusting along the munsiari thrust and extension in the mct hanging wall this renewed phase of tectonic extrusion at the himalayan front is still active as indicated by the present day regional seismicity and by hydrothermal circulation linked to elevated near surface geothermal gradients in the lhcs as recently evidenced in the himalayan syntaxes active exhumation of deep crustal rocks along the sutlej valley is spatially correlated with the high erosional potential of this major trans himalayan river this correlation supports the emerging view of a positive feedback during continental collision between crustal scale tectono thermal reworking and efficient erosion along major river systems

vivaldi 1991 a study of the formation circulation and ventilation of eastern north atlantic central water abstract a synoptic hydrographic data set comprising 32 full depth ctd casts and 2500 ctd seasoar profiles to 500 m is used to describe the s properties and circulation of central water east of the mid atlantic ridge and between 39 n and 54 n eastward transport of 20 10 6 m 3 s 1 in the north atlantic current turns entirely northwards to the west of 54 n 20 w this transport consists in the upper layers of western north atlantic water freshened at temperatures below 10 c by mixing with subarctic intermediate water northern and southern branches of the north atlantic current are well defined and both turn northwards west of 20 w a further 10 10 6 m 3 s 1 of eastern north atlantic water forms and recirculates anticyclonically to the west of spain south of the north atlantic current and north of 40 n eastern north atlantic water is most weakly stratified east of 20 w and there is clear correlation between weakly stratified pycnostads and positive salinity anomalies relative to western north atlantic water thus eastern north atlantic water is a winter mode water in which strong winter cooling has increased the density and hence also the salinity anomaly at a given temperature near the southern entrance to the rockall trough there is evidence that salinities are also increased by mediterranean water influence circulation south of the north atlantic current is complex there is no evidence for direct ventilation southwards across 40 n where water properties s potential vorticity and cfc 113 and historical data all indicate westward ventilation east of 24 w with weak southward ventilation occurring further west in the vicinity of the azores the circulation pattern suggested is remarkably similar to that proposed by helland hansen and nansen in 1926 the eastern north atlantic geophysiske publicajoner 4 1 76 with anticyclonic circulation of colder eastern north atlantic water north of 40 n meeting warmer water from south of 40 n circulating cyclonically north of the azores current the distribution of pycnostads and s properties between 20 w and 35 w north of the azores indicates alternate bands of western and eastern north atlantic water moving eastward and westward respectively including evidence for westward motion immediately south of the southern branch of the north atlantic current possibly by westward propagation of anticyclonic eddies containing deep pycnostads

the spatial distribution and annual cycle of upper ocean thermohaline structure 1 observations of the spatial distribution and persistence of thermohaline structure are presented and show how advection and diffusion affect a passive tracer more than two years of underwater glider observations in the central subtropical north pacific showed thermohaline variability over horizontal scales from 5 to 1300 km thermohaline fluctuations along isopycnals spice fluctuations were elevated in layers throughout the water column with the largest fluctuations near the surface and subtropical frontal regions fluctuations were uncorrelated between the layers but stirred by the same velocity field spice variance had local extrema in the vertical because of differences in source water properties and the influence of neighboring water masses spice variance spanned about three orders of magnitude along deeper isopycnals with larger variance where different water masses met and where velocity and vorticity variance were elevated horizontal wave number spectra of spice had slopes of 2 everywhere in the upper 1000 m submesoscale spice fluctuations had slopes in physical space near the ratio of the coriolis parameter to the buoyancy frequency f n consistent with predictions of quasi geostrophic theory in the mixed layer thermohaline structure had a significant annual cycle with smaller interannual differences thermohaline fluctuations left behind during restratification and isolated from the mixed layer decayed with time because of diffusion along isopycnals horizontal diffusivity estimates in the remnant mixed layer were 0 4 m2 s 1 at 15 28 km wavelengths and 0 9 m2 s 1 at 35 45 km wavelengths

upb geochronology of gangdese transhimalaya plutonism in the lhasa xigaze region tibet abstract a series of different plutons from the lhasa xigaze segment of the gangdese transhimalaya belt has been studied by high resolution u pb analyses of zircon using zircon fractions of 5 100 grains each selected upon specific grain characteristics for two diorites located east of xigaze dazhuka the zircons yield concordant ages of 93 4 1 0 and 94 2 1 0 m y respectively also concordant ages of 41 1 0 4 and 41 7 0 4 m y have been obtained for two granodiorites collected southwest of lhasa qushui the precision on the ages of two granites from the xigaze and lhasa area is limited by two factors the presence of inherited radiogenic lead and the occurrence of subsequent lead loss however some concordant zircons detected in both granites define approximate ages of about 67 and 53 m y respectively the inherited lead components show that melting of precambrian material was involved in magma genesis the u pb ages substantiate a magmatic activity lasting from mid cretceous cenomanian to eocene lutetian time such a period of plutonism at the southern margin of eurasia as well as the occurrence of magma generation from continental crust suggest that the gangdese range results from the subduction of tethys oceanic lithosphere indian plate underneath eurasia eurasian plate if this model is true the collision of india with eurasia along the lhasa xigaze sector postdates the emplacement of the 41 m y old gangdese granodiorites i e the collision occurred after lutetian time

on subinertial flow in submarine canyons effect of geometry shelf break canyons on the west coast of canada and the united states have been observed to be regions of enhanced upwelling during southward currents compared to the surrounding shelf break most shelf break canyons from oregon north cross only part of the continental shelf cutting from the shelf break toward the coast but end on the continental shelf well below the mixed layer juan de fuca canyon on the other hand cuts the continental shelf from the slope to and actually continues into the strait of juan de fuca this difference in geometry has a very strong effect on the subinertial flow around the canyon model canyon shapes which include convergent bathymetric contours are constructed and motivated for juan de fuca canyon and a typical shelf break canyon geostrophic analytic solutions show that the in canyon flow in juan de fuca canyon is generated by first order geostrophic dynamics whereas in the majority of canyons of which astoria is an example in canyon flow is generated by higher order effects this difference is postulated to lead to the observed very deep upwelling over juan de fuca canyon compared to more moderate episodic upwelling over astoria canyon

internal tides in monterey submarine canyon abstract the m2 internal tide in monterey submarine canyon is simulated using a modified version of the princeton ocean model most of the internal tide energy entering the canyon is generated to the south on sur slope and at the head of carmel canyon the internal tide is topographically steered around the large canyon meanders depth integrated baroclinic energy fluxes are up canyon and largest near the canyon axis up to 1 5 kw m 1 at the mouth of the upper canyon and increasing to over 4 kw m 1 around monterey and san gregorio meanders the up canyon energy flux is bottom intensified suggesting that topographic focusing occurs net along canyon energy flux decreases almost monotonically from 9 mw at the canyon mouth to 1 mw at gooseneck meander implying that high levels of internal tide dissipation occur the depth integrated energy flux across the 200 m isobath is order 10 w m 1 along the majority of the canyon rim but increases by over an order of magnitude near the canyon head where internal ti

flow and mixing in juan de fuca canyon washington we report breaking internal lee waves strong mixing and hydraulic control associated with wind driven up canyon flow in juan de fuca canyon washington unlike the flow above the canyon rim which shows a tidal modulation typical on continental shelves the flow within the canyon is persistently up canyon during our observations with isopycnals tilted consistent with a geostrophic cross canyon momentum balance as the flow encounters a sill near the canyon entrance at the shelf break it accelerates significantly and undergoes elevated mixing on the upstream and downstream sides of the sill on the downstream side a strong lee wave response is seen with displacements of o 100 m and overturns tens of meters high the resulting diffusivity is o 10 2 m2 s 1 sufficient to substantially modify coastal water masses as they transit the canyon and enter the salish sea estuarine system

distribution of chalcophile and platinum group elements among pyrrhotite pentlandite chalcopyrite and cubanite from the noril sk talnakh ores implications for the formation of platinum group minerals in most magmatic sulfide deposits platinum group elements pge are found both within the structure of the base metal sulfides bms pyrrhotite po pentlandite pn chalcopyrite ccp and cubanite cbn and as platinum group minerals pgm tellurium as bi sb and sn tabs are essential elements in many of these pgm the potential role of tabs in collecting pge and thus forming a pge deposit has not been closely investigated we have determined the concentrations of a full suite of chalcophile elements in po pn ccp and cbn using laser ablation inductively coupled plasma mass spectrometry on samples from the noril sk talnakh ni deposits in these deposits the po rich ore is thought to represent monosulfide solid solution mss cumulate of the initial sulfide liquid and the ccp rich ore a mixture of the fractionated sulfide liquid and intermediate solid solution iss the bms from the po rich ore contain lower concentrations of tabs pd pt and au and higher concentrations of mo ru rh re os and ir than bms from the ccp rich ores this observation is consistent with experimental results which show that tabs pd pt and au are incompatible with mss whereas the other elements are compatible in mss counter intuitively in the po rich ore the bulk of the pd and tabs is hosted by bms this is because during crystallization although only a small amount of the incompatible elements partitioned into the bms the fractionated liquid has migrated away thus the po rich ores represent mss adcumulates therefore as the po rich ores contain very little trapped liquid fraction the bms host the bulk of pd and tabs in contrast in the ccp rich ore the bulk of au pd pt and tabs is present as pgm or electrum grains this is because more trapped liquid is present and as tabs au pd and pt are not compatible with iss they concentrated into the very last sulfide liquid and crystallized as intergrowths of pd pt tabs pgm the tabs then do not appear to collect pd pt and au but rather all elements are concentrated in the most fractionated sulfide liquid by crystal fractionation

tectonics and structural zonation of southern tibet china the yarlung zangbo suture zone is composed of several thrust sheets with distinct stratigraphies states of deformation and metamorphic grades successive deformations have been produced by persistent north south shortening since obduction of the ophiolites by late cretaceous palaeocene times a possible decoupling of the crust may be associated with the intracontinental thrust system which has formed in the subducted indian plate this article emphasizes the role of shear deformations in the building of a collision belt

the fleuve manche the submarine sedimentary features from the outer shelf to the deep sea fans bathymetric seismic and coring surveys have been carried out since the beginning of the 1990s on the outer shelf the slope and the continental rise of the western approaches margin most of the sedimentary features studied belong to a sedimentary system built up since the oligocene and fed by a palaeoriver the fleuve manche that flowed in the english channel during eustatic lowstands a geomorphological map details the spatial distributions of the palaeovalleys on the outer shelf the drainage basin like organised canyons on the slope and the channel levee and lobe complexes on the rise the obvious channel canyon connections and the assessed palaeovalley canyon links demonstrate that the fleuve manche was the single source for terrigeneous fluxes of the armorican deep sea fan when it was one of two sources of the celtic deep sea fan amongst parameters controlling the sedimentation two factors play a major role i subsidence controlled the possibly messinian riverine incision the turbidite activities during lowstands as well as marine infilling of the seawards part of the fleuve manche ii quaternary sedimentary supplies were influenced by the melting ice sheets and the connections with the onshore catchment basins copyright 2003 john wiley sons ltd

normal faulting in central tibet since at least 13 5 myr ago tectonic models for the evolution of the tibetan plateau interpret observed east west thinning of the upper crust to be the result of either increased potential energy of elevated crust1 or geodynamic processes that may be unrelated to plateau formation2 3 4 5 6 a key piece of information needed to evaluate these models is the timing of deformation within the plateau the onset of normal faulting has been estimated to have commenced in southern tibet between about 14 myr ago7 and about 8 myr ago8 and in central tibet about 4 myr ago9 here however we report a minimum age of approximately 13 5 myr for the onset of graben formation in central tibet based on mineralization ages determined with rb sr and 40ar 39ar data that post date a major graben bounding normal fault these data along with evidence for prolonged activity of normal faulting in this and other tibetan grabens support models that relate normal faulting to processes occurring beneath the plateau thinning of the upper crust is most plausibly the result of potential energy increases resulting from spatially and temporally heterogeneous changes in thermal structure and density distribution within the crust and upper mantle beneath tibet this is supported by recent geophysical and geological data10 11 12 13 14 15 16 17 which indicate that spatial heterogeneity exists in both the tibetan crust and lithospheric mantle

nyainqentanglha shear zone a late miocene extensional detachment in the southern tibetan plateau a major low angle ductile shear zone containing s c mylonites involves metamorphosed granitic rocks at the southeastern edge of the nyainqentanglha mountain range southern tibet prominent triangular facet geomorphology is developed by valley erosion of the detachment surface defined by the top of the southeast dipping mylonitec shear zone kinematic criteria consistently indicate a top to southeast sense of shear the ductile shearing deformation is inferred from isotopic cooling ages to have occurred during the interval 11 5 ma late miocene we interpret the shear zone as a regional extensional detachment its development indicates when the extensional tectonics in this area started which in turn may mark the time when the maximum sustainable surface elevation and perhaps crustal thickness was reached in southern tibet

uplift of tibetan plateau a history of the elevation and crustal thickness of the tibetan plateau since the continental collision at 45 ma has been computed using a kinematic model based on plate tectonic reconstructions and conservation of crustal volume in our reconstructions the indochina block was attached to the southern margin of eurasia before being extruded to the southeast as evidenced by the opening of the south china sea this opening affected the mass input into tibet so that tibet did not uplift to an elevation higher than 1 km until early miocene when spreading of the south china sea ceased the model then gives a steady rise of tibet from middle miocene to its present elevation

tibetan satellite magnetic low evidence for widespread melt in the tibetan crust the tibetan plateau is associated with a pronounced satellite magnetic low forward modeling of this anomaly demonstrates that the crust underlying the tibetan plateau is weakly magnetic compared to the crust on either side and that the boundaries of the weakly magnetic region coincide closely with the topographic edges of the plateau himalaya kun lun because there are no obvious changes in the bulk magnetization of the crust associated with the principal terrane boundaries within the plateau banggong and yarlung zangbo sutures the low likely manifests hot crust beneath tibet forward modeling of the magnetic anomaly with simple assumptions suggests that the curie isotherm is likely to reside in the upper crust across the tibetan plateau 15 km depth implying that granite minimum melt temperature 600 650 c is also achieved in the upper crust across the plateau 16 18 km depth this inference is consistent with the shallow depth extent of earthquakes in tibet and with the recent suggestion that melt may be areally widespread within the tibetan crust

flow modification and shelf slope exchange induced by a submarine canyon off the northeast spanish coast we present results from a study of the modifications induced by the short timescale response of a shelf break baroclinic jet to the presence of a downstream canyon the field site was the canyon of palamos located in the northeastern edge of spain to determine the short time scale variability of the area three intensive surveys were carried out at approximately 10 day intervals data indicate that the jet s location usually over the slope is subjected to significant onshore offshore displacements analysis of the data has shown the importance of a small scale temporal variability induced by river runoff and the climatology of the area this variability generates on off shore displacements of the jet which interacts with the head mouth of the canyon the different interactions with the canyon induced by these displacements produce different regimes of shelf slope exchange in the canyon domain that strongly modify the biomass patterns in the area