inherited smnd isotope components preserved in monazite inclusions within garnets in leucogneiss from east antarctica and implications for closure temperature studies abstract application of the sm nd dating technique using garnet can be hampered by the presence of minor microscopic to submicroscopic ree rich inclusions in particular monazite and to a lesser extent zircon an acid leaching method using a mixture of hno 3 hcl has been used to tackle the monazite contamination problem with this leaching technique we demonstrate that inherited monazite inclusions have been preserved in garnet formed under high grade metamorphic conditions even very low levels of monazite contamination 0 001 wt have a profound effect on the sm nd system in the garnet analysis of a garnet from a leucogneiss dyke in the northern prince charles mountains eastern antarctica reveals that unleached garnet concentrates and the monazite inclusions as represented by the leachates define an apparent isochron of 1150 30 ma mswd 1 88 which represents a mixing age of inclusions preserving their inherited sm nd isotopic systematics in contrast analyses of the leached garnet concentrates yield much younger ages of 630 to 555 ma model calculations show that the presence of as little as 0 01 wt of monazite inclusions would swamp the sm nd isotope signature of the garnet concentrates this very low level of contamination is beyond the reach of physical purification processes and without leaching contamination is inevitable in inclusion bearing garnets such contamination can lead to older apparent ages than those of garnet formation and thus has profound implications for sm nd as well as u pb garnet chronometry

isotopic constraints on the thermal history of the wind river range wyoming implications for archean metamorphism precise u pb monazite and 40ar 39ar hornblende ages have been obtained from three locations in the high grade archean core of the wind river range wyoming monazites from metapelites in the paradise basin medina mountain and crescent lake have u pb ages of 2718 1 2633 5 and 2657 2 ma respectively hornblendes from amphibolites and granulites from the same locations yield plateau 40ar 39ar isotope ages of 2652 11 2572 9 and 2527 8 ma respectively and are interpreted as cooling ages from the last thermal event the three localities experienced similar peak pressure temperature conditions the timing of high grade metamorphism in the paradise basin is older than the emplacement of large subjacent batholiths at 2 63 2 67 ga calculated cooling rates based on monazite hornblende pairs of 3 4 1 0 c ma for paradise basin 3 8 1 2 c ma for medina mountain and 1 7 0 4 c ma for crescent lake cannot be used to rule out reheating during subsequent pluton emplacement the markedly slowe

on the dense bubble clouds and near bottom turbulence in the surf zone 1 measurements of high frequency 100 khz acoustical backscatter in the surf zone have identified the presence of a wall formed by dense assemblages of bubbles generated by breaking gravity waves and persisting for periods up to 30 min these bubbles are advected by alongshore and cross shore rip currents and are controlled by the local wave field turbulence dissolution and buoyancy during the summer of 2000 the location of the bubble wall south of scripps pier california was tracked for a 6 week period distinct categories of bubble wall defined by the backscatter intensity data are observed distinguished by the presence or absence of rip current activity the results suggest that presence or absence of long period waves 10 20 s may play a role in controlling the bubble wall type during periods without rip currents the location of the bubble wall is controlled by the incoming wave field simultaneous turbulence dissipation measurements 0 4 m above the seafloor show clear dependence on the local water depth beneath incoming waves the acoustical backscatter data are interpreted using in situ bubble and turbulence measurements in conjunction with a simple model based on bubble sorting by buoyancy dissolution and turbulence

lu hf geochronology of mississippian high pressure metamorphism in the acatlan complex southern mexico abstract since the first discovery of eclogite in the acatlan complex in southern mexico the age of the high pressure metamorphism has been a matter of debate several attempts to date high pressure metamorphic rocks of the acatlan complex have been made using the u pb and 40 ar 39 ar methods the resulting dates however do not correspond unambiguously to the time of eclogite facies metamorphism in this study the age of high pressure metamorphism in the acatlan complex has been determined by lu hf garnet whole rock geochronology this paper presents four high precision 4 to 7 point garnet whole rock isochrons of amphibolitized eclogite from the piaxtla suite and the asis lithodeme in the acatlan complex the four dates agree within uncertainties yielding a weighted mean of 352 5 1 6 ma which we interpret to be the age of eclogite facies metamorphism in the piaxtla suite and the asis lithodeme marking active subduction in the acatlan complex in the carboniferous mississippian high pressure metamorphism in turn might be related to the closure of the rheic ocean and the assembly of pangea

numerical modelling of hydrodynamic and morphodynamic response of a meso tidal estuary inlet to the impacts of global climate variabilities abstract tidal inlets a common feature along coastlines globally can be significantly affected by the impacts of global climate variabilities computational models provide the best opportunity to assess future changes to the dynamics of inlet systems in this paper the morphodynamic response of a gravel dominated meso tidal estuary inlet to sea level rise slr is discussed based on three future slr scenarios it uses a process based computational coastal area model the study s test site is the meso tidal deben estuary inlet in the uk it is very morphodynamically active and has a unique sediment environment is used as the test site of this study the modelling results reveal that the morphological response of deben inlet is sensitive to the slr scenario rising sea levels give rise to increased hydrodynamic and morphodynamic activities at and around the inlet the ebb delta which is a prominent morphodynamic feature of this inlet shows greater instability as a result of increased sea levels it is possible that the inlet may deviate significantly from its current morphodynamic regime in the future as a result of the changes imposed by higher sea levels

drifter measurements in a laboratory rip current 1 this paper investigates circulation in a laboratory rip current system using large numbers of lagrangian drifters with supplementary current meter and water level measurements these represent the most dense measurements to date of laboratory rip circulation over a large spatial area overall circulation is found to be very unsteady with strong changes in circulation apparent at many length and time scales drifter and current meter measurements are compared in the rip channel and stokes drift is found to be a significant component of drifter velocities in this region estimated generation of circulation is compared with computed bottom friction dissipation with good agreement the mean cellular circulation about a dividing streamline is found to be surprisingly small when compared with computed rates of generation of circulation finally the divergence of volume flux computed using drifters is found to be significant with depth varying wave induced mass transport depth varying eulerian velocities and velocity drifter concentration effects the likely sources of error

the role of inclusions in u pb and sm nd garnet geochronology stepwise dissolution experiments and trace uranium mapping by fission track analysis the u pb and sm nd dating of garnet are important tools for understanding rates of tectonometamorphic processes and have been widely applied in studies of metamorphic terranes however the budgets of uranium lead samarium and neodymium in garnet separates from metamorphic rocks may be dominated by contributions from inclusions of monazite or zircon a combined fission track and stepwise dissolution technique is proposed for evaluating the role of inclusions of monazite and zircon in the budgets of uranium lead samarium and neodymium in garnets used for u pb and sm nd chronology variations of th u u nd and sm nd ratios between successive dissolution steps reveal the contributions of monazite and zircon inclusions the stepwise dissolution procedure does not induce any apparent artifacts on 207pb 206pb and sm nd ages the technique has been applied to garnets from three metamorphic terranes almandine garnet samples from the high grade pikwitonei granulite domain manitoba canada and the wind river range wyoming usa have the majority of their uranium samarium neodymium and radiogenic lead pb hosted by micrometer scale inclusions of monazite fission track densities reveal that uranium is 108 times more abundant in inclusions than in garnets stepwise dissolution of the samples shows that neodymium and uranium are associated with the dissolution of monazite in contrast grossularandradite garnets from cascade slide adirondack highlands ny usa show little variation in fission track density and similar isotopic ratios between dissolution steps indicating that the budgets of uranium samarium neodymium and pb are not significantly influenced by inclusions the demonstrated success of these techniques leads us to recommend similar procedures as a routine matter in u pb and sm nd garnet geochronology of metasedimentary rocks

observations of nearshore circulation rip currents a sector scanning doppler sonar was deployed at the end of the scripps institution of oceanography pier aimed in toward the surf zone estimates of acoustic backscatter intensity and radial velocity were obtained seaward of the break point over a wedge with an arc of about 45 and a radius of 200 to 400 m the acoustic intensity is proportional to some combination of the bubble density and the suspended sediment in the water well defined jets of water extending seaward from the surf zone or rip currents were observed these rip currents occurred episodically recurred aperiodically one to four times per hour or so and lasted several minutes each the water transported offshore within the rips was rich in suspended sediment and bubbles producing distinctive bright patterns that can be easily recognized in successive images of backscatter intensity from the sonar fluid velocities were estimated from the doppler shift of the sonar signal approaching 0 7 m s in some of the events simultaneously a transect of temperature density and turbidity within the rip current was obtained with conductivity temperature depth ctd and optical backscatter strength obs sensors these data describe the ejection of warm turbid water from the surf zone and provide a comparison between the acoustic intensity and the optical turbidity of the fluid the horizontal extent of the sonar measurements and the vertical resolution of the ctd obs measurements complement each other in the study of the dynamics of the nearshore environment exchange rates are not fully resolved here since only one seaward jet is contained within the field of view however these observations suggest that rip currents could cause significant exchange between the nearshore and offshore waters

strontium isotope zoning in garnet implications for metamorphic matrix equilibration geochronology and phase equilibrium modelling in principle garnet growth rates may be calculated from 87 rb 86 sr and 87 sr 86 sr measurements in garnet subsamples and the surrounding rock matrix because of low rb sr garnet should passively record the matrix decay of 87 rb to 87 sr as a progressive increase in 87 sr 86 sr from core to rim this concept was tested by collecting rb sr data for five garnet grains from four major orogenic belts eastern vermont c 380 ma western new hampshire c 320 ma southern chile c 75 ma and northwestern italy c 35 ma both normal sr isotope zoning increasing 87 sr 86 sr from core to rim and inverse sr zoning decreasing 87 sr 86 sr from core to rim were observed garnet and matrix iso tope data commonly yielded grossly inaccurate model ages incomplete rb and sr equilibration among matrix minerals is invoked to explain the deviations between theoretical v measured zoning patterns and the age disparities initially the reactive matrix is dominated by rapidly equilibrating rb rich mica which imparts high 87 sr 86 sr values in garnet cores progressive participation of slower equilibrating sr rich plagioclase buffers or even reduces 87 sr 86 sr possibly leading to flat or decreas ing 87 sr 86 sr from garnet cores to rims unusually high 87 sr 86 sr in garnet in combination with bulk matrix compositions causes erroneously young apparent ages so metamorphic ages growth rates and associated heating and loading rates are likely suspect although rb sr may be the most suscep tible because of the profound disparities between mica and feldspar zircon reactivity might influence the lu hf system by up to a few per cent the sm nd system seems generally immune to these effects pseudosection analysis and conventional garnet geochronology which presume complete matrix equilibration during metamorphism may require modification to account for differences between whole rock v reactive matrix compositions

open ocean surface wave measurement using doppler sonar in october november 1983 doppler sonars mounted on the research platform flip were used to scatter 75 khz sound from the underside of the sea surface at low angle as well as from the interior of the mixed layer surface gravity waves were seen in velocity estimates from the surface scattering sonar even though the wave conditions were unusually calm valid measurements of sea surface motion were obtained over the range interval 600 to 1400 m from flip a second sonar which transmitted horizontally and scattered from the interior of the mixed layer also sensed surface waves although with amplitude reduced in proportion to the decay of the motions with depth wave number frequency spectra of the observed motions are consistent with linear theory the existence emergence of this technology enables the synthesis of kilometer long surface wave arrays in the open sea without the cost and logistical support usually associated with large aperture arrays in addition the doppler acoustic approach can provide information on lower frequency surface currents and on the spatial variations in these low frequency currents such as langmuir cells which might affect wave propagation

acoustic attenuation in very shallow water due to the presence of bubbles in rip currents an experiment was performed just off the research pier at the scripps institute of oceanography to determine the acoustic effects of small bubbles in very shallow water 6 m depth the distance offshore was 300 m the propagation lengths were 2 10 m and the frequency range was from 39 to 244 khz during the experiment rip currents passed through the field of measurement instruments these rip currents were laden with bubbles created in the surf between the instruments and the shore the effects of these rip currents on the spatial distributions of the resulting acoustic attenuation are discussed from the attenuation data the bubble distributions are calculated using a new iterative approach caruthers et al in press j acoust soc am that is based on the well known resonant bubble approximation calculated bubble distributions varied from an essentially uniform lack of bubbles during quiescent periods to highly inhomogeneous and dense bubbly regions within rip events such observed distributions were consistent with measurements made by other investigators during the experiment

bubble transport in rip currents a number of remote and in situ acoustical sensors were mounted in the surf zone off scripps pier la jolla california during march 1997 and used to characterize rip currents and their ability to transport bubbles offshore from the surf zone the bubble size distributions and air fractions at the injection point were measured using an acoustical resonator and a conductivity sensor right in the surf zone the offshore moving bubble plume was tracked with a multibeam 100 khz doppler sidescan sonar giving the spatial extent and offshore variability in the rip events as well as information about the surface wave field furthermore three acoustical resonators were mounted at 2 5 m depth 55 60 m farther offshore to probe the bubble field 300 400 s after injection these results are interpreted using models that include bubble buoyancy and dissolution rip current advection and the turbulent boundary layer which is represented by the christoffersen and jonsson 1985 formulation

isotopic constraints on the evolution of metamorphic conditions in the jijal patan complex and the kamila belt of the kohistan arc pakistan himalaya

a model for the horizontal directionality of breaking wave noise in the surf zone a model is presented for the horizontal directionality of the noise generated by individual breaking waves in the surf zone the model is based on the interaction between sound radiated by ringing bubbles created in a breaking wave crest and the population of acoustically quiescent bubbles left on the seaward side of the wave in the shallow water waveguide the effect of the quiescent bubbles is to absorb sound from all but the very ends of the breaking crest resulting in the formation of acoustic hot spots the model calculations are in good agreement with observations of the horizontal directionality of noise from individual breaking waves in the surf zone

isotopic chronometry of zoned garnets growth kinetics and metamorphic histories basic information on the chronological and pressure temperature evolution of regional metamorphic terrains may in principle be derived from metamorphic garnets because of the similarly low diffusivities of sm nd and major cations in this mineral we report here smnd and rbsr isotopic and major element data on prograde garnets from regionally metamorphosed pelites from newfoundland the garnets preserve a prograde major element zonation as well as a sympathetic variation in sm nd ratio smnd data for separated portions of the garnet from core to rim provide both upper limits on the time for garnet growth and demonstrate synchronous growth of different garnet grains on a hand specimen scale the rbsr data on the same garnet fractions are in general agreement with these results but in some cases cannot be interpreted in terms of growth a minimum heating rate of 3k ma 1 is derived by combining the estimates for garnet growth time with the apparent temperature interval over which the garnet grew deduced from the major element zonation this value is similar to the minimum suggested by theoretical models for the thermal evolution of thickened continental crust the growth rate is within the range of 1 3 19 mm ma 1 set respectively by the isotopic data and the likely upper limit for heating rate during regional metamorphism these growth rates appear too slow to be controlled by surface reaction and suggest that other factors such as transport may be rate limiting in this case the limits set on the effective diffusion coefficient for material transport to the growth site 0 4 6 1 10 17 m2 s 1 suggest that grain boundary diffusion is probably the transport mechanism for supply of material to the growing garnet

the nature and evolution of the main central thrust structural and geochronological constraints from the sikkim himalaya ne india the main central thrust mct is a prominent continental scale fault within the himalaya its definition has been the topic of some debate in the literature after a brief consideration of the state of discussion to clarify the definition we use in this work we report features from the field to the microstructural scale of a particularly well exposed section in sikkim ne india the nature of the protoliths as well as the overlying and underlying rocks is characterized in terms of e nd the dates of motion on the fault are constrained using u pb geochronology of zircon and monazite from pegmatitic dikes that cross cut the deformation fabric it is found that the mechanism of deformation recorded in the fault zone rocks is different compared to that found in the overlying greater himalayan gh or the underlying lesser himalayan lh rocks the gh and lh have different protolith characteristics as well combined with existing data on p t history dates of metamorphism and cooling and exhumation rates of the gh and the lh our measurements show that major motion on this fault occurred before 20 ma at 450 700 c but after peak metamorphism of rocks 750 800 c in this zone isolated events occurred in this zone as late as 11 ma possibly in the brittle domain this underscores the pulsed nature of movement over an extended period on such major faults and the related difficulties in dating fault movement determination of the rates of movement and designating a fault plane as in or out of sequence within a propagating deformation front

infragravity rip current pulsations 1 the origins of rip current pulsations within the infragravity frequency band 0 004 0 04 hz are determined from measurements made with arrays of colocated pressure and velocity sensors deployed on a beach with persistent rip current channels the observations indicate significant energy in cross shore infragravity velocities that varies spatially in the alongshore due to the presence of rip channels infragravity velocities are smaller within the rip channel than on the shore connected shoal owing to differences in water depth rip current pulsations at infragravity frequencies are linked to standing infragravity motions but not to the ponding and subsequent release of water by wave group pumping

analysis of surf zone turbulence measurements of turbulent kinetic energy k under surf zone waves are analyzed to show how k varies over depth between breaker point and shoreline and how k depends on the beach slope it is found that the variation of k over depth is remarkably weak large values being measured a few percent of the depth above the bottom a simple model for the dissipation mechanism makes it possible to derive an empirical formula for the time averaged k that accurately describes all the data considered reliable

single mineral pbpb dating a new stepwise leaching procedure applied to ca bearing silicates enables the selective recovery of radiogenic and common pb from a mineral and thus makes single phase pbpb dating possible this permits independent dating of rock forming minerals within a paragenetic sequence and the direct combination of age data with petrological results furthermore the stepwise leaching technique has the advantage over multi phase pbpb dating in that it does not require initial pb isotope equilibrium among the phases but may be used to test it the presence of microscopic solid inclusions which complicate dating of single phases if they are not in equilibrium with the host can be traced via the behaviour of the 208pb 204pb ratio through the various leaching steps this constitutes a major advantage over conventional bulk phase pbpb dating in addition the precision of an isochron age can be improved relative to that of the bulk mineral approach since the 207pb 204pb and 206pb 204pb data spread is significantly enhanced therefore the technique may both be used to date younger phases and or minerals with less favourable u pb ratios and to extend the applicability of self consistent p t t modelling

comparison of laser ablation icp ms and isotope dilution ree analyses implications for sm nd garnet geochronology abstract high precision garnet chronometry using the sm nd u pb rb sr or lu hf isotope systems is gaining widespread application for constraining the prograde and high temperature history of metamorphic belts however due to the low concentrations of the chronometrically important elements garnet analyses are often susceptible to incorporation of small quantities of accessory phases such as zircon monazite and allanite here we assess the extent to which such inclusions affect garnet analyses by comparing in situ analyses of sm and nd concentrations in the crystal lattice obtained by laser ablation inductively coupled mass spectrometry la icp ms with those obtained by isotope dilution id on separates of the same garnets the results show that the garnets studied here are largely free from inclusions in general however the low concentrations of sm and nd in the garnet lattice mean that the incorporation of even very small quantities of accessory phases will result in a significant change in the measured garnet sm and nd concentration simple modelling of the effect of such inclusions on age information shows that garnet inclusion disequilibrium and the extent to which inclusions decrease the apparent parent daughter ratio are critical such modelling demonstrates that the incorporation of old lree enriched minerals such as monazite will yield an apparently lower age than the true garnet age however this study indicates that erroneous age information can be identified if duplicate garnet analyses are performed in addition la icp ms is shown to be a useful technique in verifying the purity of samples for isotope analysis

air entrainment processes and bubble size distributions in the surf zone abstract a new optical instrument was deployed in the surf zone in a trial experiment to measure bubble size distributions and visualize air entrainment and bubble formation mechanisms within breaking surf images of bubbles and the evolving air water mixture inside and beneath breaking wave crests are presented the images resolve features of the air water mixture to length scales of hundreds of microns across a 3 7 cm field of view two qualitatively different large scale air entrainment processes are observed first intrusions of air and water thought to be created by jets penetrating the water s surface fragment into plumes of bubbles second an air cavity trapped by the overturning wave crest is observed to disintegrate into bubbles the timescale for the evolution from a compacted air water mass to individual bubbles was on the order of 90 ms or less for both of these processes in addition small scale air filaments hundreds of microns wide and millimeters long have been discovered beneath wave

new geochronological constraints on the thermal and exhumation history of the lesser and higher himalayan crystalline units in the kullu kinnaur area of himachal pradesh india new geochronological petrological and structural data from the beas sutlej area of himachal pradesh india are used to reconstruct the tectonothermal and exhumation history of this part of the himalayan orogen sm nd garnet ages at 40 5 1 3 ma obtained on a pegmatoid from the inverse metamorphic high himalayan crystalline hhc in the malana parbati area probably mark local melting during initial decompression ongoing exhumation in ductilely deformed leuco gneiss is constrained by sm nd garnet ages at 29 1 ma and white mica rb sr ages around 24 20 ma while bt rb sr ages indicate a drop of regional metamorphic temperatures below 300 c between 15 and 12 ma the deep sutlej gorge exposes medium grade paragneisses and proterozoic orthogneisses of the lesser himalayan crystalline lhc overthrust by the hhc along the main central thrust mct mica cooling ages in the hhc are in the range of 14 11 ma above the extruded wedge of the hhc the leo pargil leucogranite and associated dykes intrude the haimanta unit hu below the weakly metamorphic palaeo mesozoic sediments of the tethyan himalayas th the leo pargil leucogranite yielded a mean sm nd garnet age of 19 1 ma and rb sr muscovite and biotite cooling ages between 16 4 and 11 6 ma marked young extrusion of lhc units resulted in differentiated exhumation cooling of more frontal parts of the orogen very young ductile deformation of lhc gneisses near wangtu is constrained by late kinematic pegmatite intrusions crosscutting the main mylonitic foliation sm nd garnet and rb sr muscovite ages of these pegmatites range between 7 9 0 9 and 5 5 0 1 ma published apatite ft ages down to 0 6 ma also document accelerated diachronous sub recent exhumation of different parts of the orogen together with geochronological data from the literature the new results demonstrate that the hhc and the hu were deformed by shortening and crustal thickening during the eohimalayan phase late eocene oligocene followed by a strong thermal overprint and intrusions of granitoids during the neohimalayan phase early to middle miocene the lhc experienced amphibolite facies metamorphic conditions in the late miocene prior to extrusion between the hhc and the very low grade lesser himalayan sediments in conjunction with climate changes young tectonic activity in this central part of the himalayan orogen may have strongly influenced fluvial incision and erosion and therefore contributed to the accelerated uplift as indicated by extensive accumulation of late miocene to early pleistocene fluviatile lacustrine sediments in the zanda basin the transhimalayan headwaters of the sutlej in western tibet

longshore current and wave height modulation at tidal frequency inside the surf zone data were acquired continuously during the 19 day delilah nearshore experiment with a specific objective of examining variability of the longshore current at tidal frequencies it is hypothesized that breaking wave heights inside the surf zone are strong functions of the depth which are modulated by the tidal variations and since radiation stress is a function of the wave height longshore currents are forced at the tidal frequency inside the surf zone the measured longshore current variations at tidal frequency are the same order of magnitude as the mean longshore current variations for moderate wave height conditions indicating that the tide is a dominant mechanism associated with longshore current variability simulations of the magnitude and phase of the longshore current variability with tide elevation using the model by thornton and guza 1986 are used to explain observations the measured tidal elevation and longshore current are in phase in the inner surf zone and out of phase in the outer surf zone as predicted by the model verifying the hypothesis

nearshore circulation over transverse bar and rip morphology with oblique wave forcing there is a paucity of field data to describe the transition in nearshore circulation between alongshore meandering and rip current systems a combination of in situ current meters and surf zone drifters are used to characterize the nearshore circulation over a transverse bar and rip morphology at pensacola beach florida in the presence of relatively low energy oblique waves current speeds vary in response to the relative wave height ratio hs h which defines the degree and extent of breaking over the shoal in the absence of wave breaking the nearshore circulation was dominated by an alongshore current driven by the oblique waves as waves begin to break across the shoal 0 2 0 5 the meandering current is replaced by an unsteady rip circulation that moves offshore between the shoals before turning alongshore in the direction of wave advance outside the surf zone the increase in wave dissipation is associated with an increase in very low frequency vlf variations in the current speed across the shoal and in the rip channel that caused the circulation to oscillate between an offshore and an alongshore flow the unsteady nature of the nearshore circulation is responsible for 55 of all surf zone exits under these more dissipative conditions in contrast only 29 of the drifters released from the shoal exited the surf zone and bypassed the adjacent shoal with the alongshore meandering current while the currents had a low velocity maximum of 0 4 m s 1 and would not pose a significant hazard to the average swimmer the results of this study suggest that the transverse bar and rip morphology is sufficient to create an alongshore variation in wave dissipation that forces alongshore meandering and low energy rip circulation systems under oblique wave forcing copyright 2013 john wiley sons ltd

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

discussion on the role of the red river shear zone yunnan and vietnam in the continental extrusion of se asia journal vol 163 2006 1025 1036 p h leloup p tapponnier r lacassin write in his recent paper searle 2006 acknowledges that the 1000 km long ailao shan red river shear zone is a large miocene left lateral shear zone but speculates that left lateral slip started after 21 ma and claims that the total finite offset remains unknown from this he concludes that continental extrusion was only a relatively minor tectonic factor during the india asia collision as long argued by other workers e g england houseman 1986 cobbold davy 1988 dewey et al 1989 houseman england 1993 we summarize below the field and geochronological evidence that makes us maintain a viewpoint in better accordance with facts timing of left lateral shear along the ailao shan red river shear zone the ailao shan red river shear zone is composed mostly of high grade metamorphic rocks and deformed granitoids with ubiquitous evidence for left lateral shear parallel to the belt e g tapponnier et al 1986 1990 leloup et al 1993 1995 2001 the crystallization of the granitoids has been dated between 22 and 35 ma fig 1d f for example by scharer et al 1990 1994 and zhang scharer 1999 leading those workers to propose that left lateral shear started at least c 35 ma ago in contrast searle 2006 claims that all the deformed granitoids found within the shear zone predate left lateral shear and that their crystallization ages should thus be interpreted to provide an upper limit for the onset of deformation rather than a lower limit thus suggesting a maximum age of 21 ma for this deformation a clear understanding of the p t history and in situ deformation history of the intrusions as well as of their relationships with surrounding paragneisses is fundamental for interpreting correctly the geochronological

prograde temperature time evolution in the barrovian type locality constrained by sm nd garnet ages from glen clova scotland abstract the timing of garnet growth during metamorphism associated with the grampian orogeny in the sillimanite zone of the barrovian type locality in glen clova scotland was determined by sm nd geochronology two high precision garnet whole rock ages were achieved by employing hf partial dissolution of garnet separates to optimize purity multiple garnet growth generations were identified on the basis of the geochronology and detailed textural and chemical data an early stage at 472 9 2 9 ma during d 2 deformation under garnet zone conditions c 500 550 c and a later stage at 464 8 2 7 ma during or slightly after d 3 deformation mostly under sillimanite zone conditions peak temperature of c 660 c but possibly including some growth during kyanite zone conditions when combined with recently published garnet ages from the kyanite and garnet zones the data suggest that peak metamorphic temperatures in at least these three of barrow9s zones were achieved roughly contemporaneously the difference between garnet zone and sillimanite zone peak temperature attainment is 2 8 3 7 ma the near contemporaneity of peak temperature attainment in different metamorphic zones requires an additional source of heat beyond thermal relaxation of a variably over thickened crust we suggest that local igneous intrusions with synmetamorphic ages provided that additional heat

the influence of high u th inclusions on the u th pb systematics of almandine pyrope garnet results of a combined bulk dissolution stepwise leaching and sem study abstract the isotope systematics of garnet are increasingly being used to define prograde thermal histories in metamorphic belts this approach to geochronology has the distinct advantage that time information is directly related to the pressure temperature deformation history recorded by the same mineral one potential problem with the technique however is that the inventories of radiogenic isotopes apparently measured in garnet separates may be located in inclusions rather than the garnet lattice itself inclusions of old refractory minerals such as zircon may bias the ages to the extent that they are rendered meaningless in terms of dating garnet growth establishing pt histories and constraining tectonic mechanisms in this contribution we present u th pb data obtained on bulk dissolutions of garnet from the himalaya as well as the results of a detailed sem and u th pb stepwise leaching study in order to further clarify these issues our experiment has the advantage that the age difference between the garnets and potential inclusions is very large so that the influence of the latter is highlighted the u th pb data from the bulk dissolutions are highly complex and yield apparent ages ranging from 1000 ma to 30 ma the stepwise leaching study demonstrates that this complexity results from small inclusions of allanite and zircon detected with the sem the bulk of the radiogenic 206pb in these samples was released only by treatment at high pressure in a bomb in addition the 208pb 206pb ratio of the radiogenic pb released in earlier leach steps is inconsistent with the available constraints on the th u ratio of garnet lattices as measured by sims in this case the u pb ages of these garnets is dependent purely on the age of the included high u th phases and the degree to which they have isotopically re equilibrated with the whole rock during the subsequent metamorphism that grew garnet simple mass balance arguments show that in this particular instance very small amounts of these inclusions are required amounts that escape detection during conventional sample preparation these data also have implications for current thinking on the closure temperature for the sm nd and u pb systems in garnet

surf zone eddies coupled with rip current morphology 1 energetic very low frequency vlf frequencies 0 004 hz surf zone eddies szes were observed on a beach composed of shore connected shoals with quasi periodic 125 m incised rip channels at sand city monterey bay california incident waves consisted of predominantly shore normal narrow banded swell waves szes were located outside the gravity region in alongshore wave number ky spaced within the vlf band and did not appear to exist in higher frequency bands the szes were significant urms vlf 0 25 m s and constant in intensity within the surf zone shore connected shoals and rip channels and rapidly decreased offshore the alongshore and cross shore sze velocity variances were similar in magnitude vlf sze velocities were not forced by vlf surface elevations and were not well correlated with rip current flows r2 0 18 there is an indication that the szes were related to wave forcing with the szes statistically correlated with incoming sea swell wave height r2 0 49 f ky spectral estimates illustrate a strong relationship between rip channel spacing and sze cross shore velocities ky 0 008 m 1 and minimal sze alongshore velocity variation ky 0 m 1 data analysis suggests that the szes are not simply instabilities of an unstable rip current jet a simple conceptual model suggests that sze f ky spectra can be explained by the entire rip current circulation cells oscillating predominantly in the cross shore and slightly in the alongshore

mean lagrangian flow behavior on an open coast rip channeled beach a new perspective article i nfo the accepted view of rip currents is that they are an efficient mechanism for transporting material out of the surf zone previous rip current campaigns on natural beaches have focused on eulerian measurements with sparse in situ pressure and current meter arrays here for the first time spatially synoptic estimates of rip current flow patterns vorticity and lagrangian transport behavior are measured in the field using a fleet of 30 position tracking surfzone drifters during multiple rip current occurrences on an open coast beach in monterey ca contrary to the classic view shepard et al 1941 the rip current flow field consisted of semi enclosed large scale vortices that retained the drifters and resulted in a high number of lagrangian observations that are temporally and spatially repeated approximately 19 of the drifters deployed in the rip currents exited the surf zone per hour on average during the experiments the observed surf zone retention of drifters is consistent with measurements from different open coast beach rip current systems 14 at meso macrotidal truc vert france and 16 at macrotidal perranporth united kingdom the three hour average cross shore rip current velocity at monterey was 30 cm s with peak time averaged velocities of 40 60 cm s depending on wave and tidal conditions drifters that episodically exited the surf zone were transported approximately 2 surf zone widths offshore at 20 cm s published by elsevier b v

models of garnet differential geochronology abstract rayleigh distillation models are developed to describe theoretical growth zoning of lu hf rb sr sm and nd in typical garnet crystals from metapelites and metabasites effects of diffusion limited transport within the matrix and intracrystalline diffusion are also considered qualitatively theoretical zoning profiles show strong depletions of lu in garnet rims compared to cores but virtually invariant hf rb sr sm and nd profiles generally consistent with natural profiles for lu and hf and previously published models theoretical isochron diagrams for lu hf exhibit distinctive arcuate distributions and high mswds consistent both with himalayan data and with expectations that garnet growth durations exceed chronologic resolution by as much as an order of magnitude predicted isochron diagrams for sm nd and rb sr exhibit vertical arrays for garnet and high mswds that are generally lower than for lu hf in metapelites inherent chronologic resolution for bulk separates is best for lu hf in metapelites and rb sr but analytical considerations favor rb sr or sm nd for chronologic zoning studies diffusion limited transport in the rock matrix strongly influences zoning profiles but does not change the main trends on isochron diagrams intracrystalline diffusion will initially rotate lu hf isochrons to steeper slopes giving older apparent ages the natural himalayan data indicate growth of garnet in one rock from the greater himalayan sequence at 34 ma consistent with previously measured monazite ages from the same rock data from another himalayan rock suggest polymetamorphism that includes a paleozoic component

mean currents and sediment transport in a rip channel measurements of hydrodynamics and suspended sediment transport were carried out along a cross shore transect in a rip channel a tidally induced threshold for rip flow initiation was found this threshold depended upon the degree of wave energy dissipation and could be identified through a critical value of the ratio of significant wave height to water depth s in the rip neck with normally incident waves rip current velocities were well correlated with velocities predicted from a simple model involving the onshore mass transport in asymmetric breaking waves crossing the bar hydrodynamics and sediment transport were tidally modulated with strong offshore directed rip currents and associated offshore sediment transport due to the mean flow at low tide at high tide when the rip was inactive the tendency was for weak onshore directed mean flows and or oscillatory incident waves transporting smaller amounts of sediment landward the net result was a gradual lowering of the topographic relief along the rip channel with net deposition seaward of the rip outside the surf zone transport was mainly landward due to oscillatory incident waves