late miocene coesite eclogite exhumed in the woodlark rift late miocene pliocene eclogites were exhumed in the woodlark rift of eastern papua new guinea an actively extending region west of the woodlark basin seafl oor spreading center we report the discovery of coesite in late miocene eclogite from the lower plate of one of the d entrecasteaux islands metamorphic core complexes within the woodlark rift zircon crys tallization temperatures 650 675 c and 238 u 206 pb age ca 8 ma and rutile thermometry 695 743 c combined with garnet pyroxene thermometry 600 760 c and garnet pyroxene phengite barometry 18 27 kbar indicate that the coesite eclogite was exhumed from mantle depths 90 km to the earth s surface at plate tectonic rates cm yr 1 this late miocene coesite eclogite is the youngest exhumed ultrahigh pressure uhp rock on earth and its preservation ahead of the westward propagating seafl oor spreading center forces reevaluation of models for uhp exhumation as well as the geologic and tectonic evolution of the woodlark rift

intraseasonal variability over tropical africa during northern summer abstract the intraseasonal variability over africa during northern summer was analyzed using 25 years of ncep ncar reanalysis and satellite data the dominant pattern of variability was one of enhanced deep convection over the whole african monsoon region it appeared to arise at least partly as a remote response to the intraseasonal madden julian oscillation over the warm pool sector twenty days prior to the maximum in convection over africa there was no signal over africa but convection was reduced over the equatorial warm pool an equatorial kelvin wave response to this change in warm pool convection propagated eastward and an equatorial rossby wave response propagated westward and between them they completed a circuit of the equator and met up 20 days later over africa where the negative midtropospheric temperature anomalies in the kelvin and rossby waves favored deep convection over west africa the kelvin wave component contained lower tropospheric westerly anomalies that acted to increase t

the role of the indian monsoon onset in the west african monsoon onset observations and agcm nudged simulations in spring the inland penetration of the west african monsoon wam is weak and the associated rainband is located over the guinean coast then within a few days deep convection weakens considerably and the rainband reappears about 20 days after over the sahel where it remains until late september signalling the summer rainy season over the period 1989 2008 a teleconnection induced by the indian monsoon onset is shown to have a significant impact on the wam onset by performing composite analyses on both observational data sets and atmospheric general circulation model simulations ensembles where the model is nudged to observations over the indian monsoon sector the initiation of convective activity over the indian subcontinent north of 15 n at the time of the indian monsoon onset results in a westward propagating rossby wave establishing over north africa 7 15 days after a back trajectory analysis shows that during this period dry air originating from the westerly subtropical jet entrance is driven to subside and move southward over west africa inhibiting convection there at the same time the low level pressure field over west africa reinforces the moisture transport inland after the passage of the wave the dry air intrusions weaken drastically hence 20 days after the indian monsoon onset convection is released over the sahel where thermodynamic conditions are more favourable this scenario is very similar in the observations and in the nudged simulations meaning that the indian monsoon onset is instrumental in the wam onset and its predictability at intraseasonal scale

strain histories from the eastern central range of taiwan a record of advection through a collisional orogen abstract in the eastern central range of taiwan there is a regional variation in the orientation of maximum finite stretch across the slate belt with down dip maximum stretch found in the western central range and along strike maximum stretch in the eastern central range incremental strain histories from syntectonic fibers in pyrite pressure shadows indicate a progressive change in extension direction from down dip to along strike during deformation there is a corresponding temporal variation in stretching direction shown in samples from the eastern edge of the central range a pattern that mimics the regional west to east spatial variation these observed temporal and spatial strain distributions are used to evaluate the kinematics associated with slaty cleavage development during advection through the taiwan orogenic system the subduction zone beneath the island of taiwan is influenced by two types of obliquity that have the potential to generate the observed along strike stretching first the plate motion vector of the philippine sea plate relative to the eurasian plate is slightly oblique to the regional strike of the mountain range which could result in partitioning of strike slip shearing into the interior of the collision second the north south luzon volcanic arc on the philippine sea plate is obliquely oriented relative to the northeast southwest edge of the eurasian continental margin which could result in lateral extrusion of the ductile core of the range incremental strain histories in cleavage parallel samples represent a time for space equivalence where the stretching direction is fixed relative to the position within the mountain belt architecture e g the topographic divide and temporal variations in the eastern central range reflect lateral advection through the strain field in response to accretionary and erosional fluxes incremental strain histories in cleavage perpendicular samples show both clockwise and counter clockwise rotation of strain indicating the possibility of bulk coaxial strain and lateral extrusion

modulation of daily rainfall in southern vietnam by the madden julian oscillation and convectively coupled equatorial waves abstractrainfall extremes have a large socioeconomic relevance for southern vietnam more than 30 million people live in this low lying flood prone region in southeast asia in this study the influence of the madden julian oscillation mjo and convectively coupled equatorial waves on the modulation of daily rainfall during the rainy season may october is evaluated and quantified using an extensive station database and the gridded asian precipitation highly resolved observational data integration toward evaluation of water resources aphrodite product for different phases of the equatorial waves the mjo kelvin and equatorial rossby er waves significantly modulate daily rainfall in vietnam south of 16 n the mjo shows the most coherent signals across the region followed by er waves whose influence is strongest in central vietnam kelvin waves only affect the southern parts of vietnam for all waves the frequency of occurrence of intense daily rainfall larger than 25 mm is significantly enhanced

convection activity over the guinean coast and central africa during northern spring from synoptic to intra seasonal timescales this study proposes an overview of the main synoptic medium range and intraseasonal modes of convection and precipitation in northern spring march june 1979 2010 over west and central africa and to understand their atmospheric dynamics it is based on daily national oceanic and atmospheric administration outgoing longwave radiation and cloud archive user service tb convection data daily trmm and global precipitation climatology project rainfall products and daily era interim reanalysis atmospheric fields it is first shown that mesoscale convective systems can be modulated in terms of occurrences number and intensity at such time scales based on empirical orthogonal function analyses on the 2 90 day filtered data it is shown that the main mode of convective and rainfall variability is located along the guinean coast with a moderate to weak extension over central africa corresponding regressed deseasonalised atmospheric fields highlight an eastward propagation of patterns consistent with convectively coupled equatorial kelvin wave dynamics then a singular spectrum analysis combined with a hierarchical ascendant classification enable to define objectively the main spectral bands of variability within the 2 90 day band and highlight three main bands 2 8 8 22 and 20 90 day within these three bands space time spectral decomposition is used to identify the relative impacts of convectively coupled equatorial kelvin rossby and inertia gravity waves as well as madden julian oscillation mjo signal it confirms that eastward propagating signals convectively coupled equatorial kelvin wave and mjo are highly dominant in these convection and precipitation variability modes over the guinean coast during northern spring so while rain producing individual systems are moving westward their activity are highly modulated by sub regional and regional scales envelops moving to the east this is a burning issue for operational forecasting centers to be able to monitor and predict such eastward propagating envelops of convective activity

the dynamics of the west african monsoon part v the detection and role of the dominant modes of convectively coupled equatorial rossby waves abstract this study is the last in a series of papers addressing the dynamics of the west african summer monsoon at intraseasonal time scales between 10 and 90 days the signals of convectively coupled equatorial rossby er waves within the summer african monsoon have been investigated after filtering noaa outgoing longwave radiation olr data within a box delineated by the dispersion curves of the theoretical er waves two families of waves have been detected in the 10 100 day periodicity band by performing a singular spectrum analysis on a regional index of er filtered olr for each family the first eof mode has been retained to focus on the main convective variability signal within the periodicity band of 30 100 days an er wave pattern with an approximate wavelength of 13 500 km has been depicted this er wave links the mjo mode in the indian monsoon sector with the main mode of convective variability over west and central africa this confirms the investigations carried out in previous studies wi

intraseasonal convective perturbations related to the seasonal march of the indo pacific monsoons abstract the seasonal evolution of the 20 90 day intraseasonal variability isv of the convection in the indo pacific region presents intriguing features related to monsoon dynamics i a sharp isv maximum in may for the southern bay of bengal and in june for the eastern arabian sea ii a maximum isv over the west pacific in july september when the isv over the northern indian ocean is weaker iii a persistent isv north of australia from december to march and iv a weak isv over continental regions the source of these behaviors is investigated from time series of satellite observations meteorological reanalysis and from an ocean mixed layer depth mld climatology for the northern indian ocean sharp isv maxima in may and june when the mld is still small 20 30 m are related to an abrupt surface cooling associated with the setting of the monsoon low level jet the isv of the convection is smaller over these regions during july september when the mld is deepened 60 70 m by the monsoon lo

the isotopic composition of yb and the precise and accurate determination of lu concentrations and lu hf ratios by isotope dilution using mc icpms 1 we report yb isotopic data for the purpose of accurately correcting measurements of lu and lu hf ratios during multicollector inductively coupled plasma mass spectrometer mc icpms analysis the yb isotopic ratios agree with recent results of chu et al 2002 and amelin and davis 2004 when corrected for mass fractionation to the same reference values protocols for lu measurement involving simultaneous subtraction of yb interference and application of a fractionation correction based on yb are described it is shown that lu concentrations and lu hf ratios determined by mc icpms using this methodology are more precise and potentially more accurate than results based on analysis of lu by thermal ionization mass spectrometry tims a number of mc icpms lu hf isotopic analyses of standard rocks and other geological samples previously analyzed by tims are presented these document reproducibility of lu hf within 0 2 where most discrepancies can be assigned to the unknown lu mass fractionation during tims analysis

two stage exhumation of ultrahigh pressure metamorphic rocks from the western dabie orogen central china abstract zircon trace element u pb age and lu hf isotope composition were determined for two eclogite samples from the xinxian ultrahigh pressure uhp unit in the western dabie orogen to constrain its exhumation processes cathodoluminescence imaging reveals that most zircons are metamorphic while some have inherited magmatic cores in one sample the magmatic cores have high th u and lu hf ratios high trace element contents and a clear negative eu anomaly consistent with their igneous genesis they yield an upper intercept age of ma and a positive ehf t value of 8 0 arguing for reworking of early mesoproterozoic crust during the middle neoproterozoic in the western dabie orogen the metamorphic zircons contain mineral inclusions of garnet omphacite rutile and quartz they are characterized by low nb ta y and heavy rare earth elements hree contents a nearly flat hree pattern and an insignificant negative eu anomaly these indicate that the metamorphic zircons formed under high pressure

first record and timing of uhp metamorphism from zircon in the xitieshan terrane implications for the evolution of the entire north qaidam metamorphic belt the xitieshan terrane is one of four metamorphic terranes in the north qaidam metamorphic belt which is an early paleozoic high pressure to ultrahigh pressure hp uhp metamorphic belt in nw china however conclusive evidence and precise timing of uhp metamorphism in the xitieshan terrane have not been well documented in this study we report an integrated study of zircon grains from an amphibolite in the xitieshan terrane in terms of mineral inclusions trace elements and u pb age systematics the first record of coesite is reported as an inclusion in a metamorphic zircon which provides unambiguous evidence for the uhp metamorphism of the xitieshan terrane the metamorphic zircon domains have weak or no zoning very low th u ratios insignificant eu anomalies and flat hree patterns zircon grains contain inclusions of garnet omphacite and rutile in addition to the coesite inclusion these inclusions indicate that the metamorphic zircon grains formed under uhp metamorphic conditions the metamorphic zircon grains were dated by the sims and la icpms methods and yield weighted mean ages of 432 14 and 441 9 ma respectively combined with previous results the hp uhp metamorphism of the xitieshan terrane may have lasted 460 440 ma with the peak uhp metamorphism at 441 9 ma a compilation of the reported geochronological data reveals that all four terranes of the north qaidam metamorphic belt might have experienced coeval uhp metamorphism during the early paleozoic 420 450 ma and thus may have suffered a coherent subduction uhp metamorphism and exhumation cycle

exhumation of high pressure rocks driven by slab rollback rocks metamorphosed under high pressure hp and ultra high pressure uhp conditions in subduction zones come back to the surface relatively soon after their burial and at rates comparable to plate boundary velocities in the mediterranean realm their occurrence in several belts related to a single subduction event shows that the burial exhumation cycle is a recurrent transient process using the calabria apennine and aegean belts as examples we show that the exhumation of hp rocks is associated in time and space with the subduction of small continental lithosphere blocks that triggers slab rollback creating the necessary space for the exhumation of the buoyant continental crust that was deeply buried just before the buoyancy force of the subducted crust increases until this crust detaches from the downgoing slab it then exhumes at a rate that depends directly on the velocity of trench retreat to become part of the overriding plate heated from below by the asthenosphere that flows into the opening mantle wedge the exhumed crust weakens and undergoes core complex type extension responsible for a second stage of exhumation at a lower rate the full sequence of events that characterizes this model crust mantle delamination slab rollback and trench retreat hp rock exhumation asthenosphere heating and core complex formation arises entirely from the initial condition imposed by the subduction of a small continental block no specific condition is required regarding the rheology and erosion rate of hp rocks the burial exhumation cycle is transient and can recur every time a small continental block is subducted

impact of the madden julian oscillation on the summer west african monsoon in amip simulations at intraseasonal timescales convection over west africa is modulated by the madden julian oscillation mjo in this work we investigate the simulation of such relationship by 11 state of the art atmospheric general circulation models runs with prescribed observed sea surface temperatures in general the atmospheric model intercomparison project simulations show good skill in capturing the main characteristics of the summer mjo as well as its influence on convection and rainfall over west africa most models simulate an eastward spatiotemporal propagation of enhanced and suppressed convection similar to the observed mjo although their signal over west africa is weaker in some models in addition the ensemble average of models composites gives a better performance in reproducing the main features and timing of the mjo and its impact over west africa the influence on rainfall is well captured in both sahel and guinea regions thereby adequately producing the transition between positive and negative rainfall anomalies through the different phases as in the observations furthermore the results show that a strong active convection phase is clearly associated with a stronger african easterly jet aej but the weak convective phase is associated with a much weaker aej our analysis of the equatorial waves suggests that the main impact over west africa is established by the propagation of low frequency waves within the mjo and rossby spectral peaks results from the simulations confirm that it may be possible to predict anomalous convection over west africa with a time lead of 15 20 day

propagation mechanisms for the madden julian oscillation the madden julian oscillation mjo is examined using 20 years of outgoing long wave radiation and national centers for environmental prediction national center for atmospheric research re analysis data two mechanisms for the eastward propagation and regeneration of the convective anomalies are suggested the first is a local mechanism operating over the warm pool region at the phase of the mjo with a dipole structure to the convection anomalies there is enhanced tropical convection over the eastern indian ocean and reduced convection over the western pacific over the equatorial western indian ocean the equatorial rossby wave response to the west of the enhanced convection includes a region of anomalous surface divergence associated with the anomalous surface westerlies and pressure ridge this lends to suppress ascent in the boundary layer and shuts off the deep convection eventually leading to a convective anomaly of the opposite sign over the indonesian sector the equatorial kelvin wave response to the east of the enhanced convection includes a region of anomalous surface convergence into the anomalous equatorial surface easterlies and pressure trough which will tend to favour convection in this region the indonesian sector is also influenced by an equatorial rossby wave response of opposite sign to the west of die reduced convection over the western pacific which also has a region of anomalous surface convergence associated with its anomalous equatorial surface easterlies and pressure trough hence convective anomalies of either sign tend to erode themselves from the west and initiate a convective anomaly of opposite sign via their equatorial rossby wave response and expand to the east via their equatorial kelvin wave response the second mechanism is global involving an anomaly completing a circuit of the equator enhanced convection over the tropical western pacific excites a negative mean sea level pressure m s l p anomaly which radiates rapidly eastward as a dry equatorial kelvin wave at approximately 35 m s 1 over the eastern pacific it is blocked by the orographic barrier of the andes and central america for several days before propagating through the gap at panama after rapidly propagating as a dry equatorial kelvin wave over the atlantic the m s l p anomaly is delayed further by the east african highlands before it reaches the indian ocean and coincides with the development of enhanced convection at the start of the next mjo cycle

a systematic comparison of tropical waves over northern africa part i influence on rainfall abstractlow latitude rainfall variability on the daily to intraseasonal time scale is often related to tropical waves including convectively coupled equatorial waves the madden julian oscillation

impact of the indian part of the summer mjo on west africa using nudged climate simulations observational evidence suggests a link between the summer madden julian oscillation mjo and anomalous convection over west africa this link is further studied with the help of the lmdz atmospheric general circulation model the approach is based on nudging the model towards the reanalysis in the asian monsoon region the simulation successfully captures the convection associated with the summer mjo in the nudging region outside this region the model is free to evolve over west africa it simulates convection anomalies that are similar in magnitude structure and timing to the observed ones in accordance with the observations the simulation shows that 15 20 days after the maximum increase decrease of convection in the indian ocean there is a significant reduction increase in west african convection the simulation strongly suggests that in addition to the eastward moving mjo signal the westward propagation of a convectively coupled equatorial rossby wave is needed to explain the overall impact of the mjo on convection over west africa these results highlight the use of mjo events to potentially predict regional scale anomalous convection and rainfall spells over west africa with a time lag of approximately 15 20 days

structural geology of a classic thrust belt earthquake the 1999 chi chi earthquake taiwan mw 7 6 abstract we document the structural context of the 1999 chi chi earthquake m w 7 6 in western taiwan which is one of the best instrumented thrust belt earthquakes the main surface break and large slip 3 10 m is on two segments of the shallow otherwise aseismic bedding parallel chelungpu sanyi thrust system which shows nearly classic ramp flat geometry with shallow detachments 1 6 km in the pliocene chinshui shale and mio pliocene kueichulin tungkeng formations however rupture is complex involving at least six faults including a previously unknown deeper thrust 8 10 km on which the rupture began we compare the coseismic displacements with a new 3d map of the chelungpu sanyi system the displacements are spatially and temporally heterogeneous and well correlated with discrete geometric segments of the 3d shape of the fault system geodetic displacement vectors are statistically parallel to the nearest adjacent fault segment and are parallel to large scale oblique fault corrugations the displacement magnitudes are heterogeneous at several scales which requires in the long term other non chi chi events or significant aseismic deformation the chelungpu thrust has a total displacement of 14 km but the area of largest chi chi slip 10 m is on a newly propagated north chelungpu chinshui detachment 0 3 km total slip which shows abnormally smooth rupture dynamics

late miocene to early pliocene blueschist from taiwan and its exhumation via forearc extraction blueschist facies rocks in the yuli belt of taiwan s central range record ongoing subduction of the eurasian plate we present a prograde lu hf garnet whole rock age of 5 1 1 7 ma from a retrogressed blueschist in the yuli belt this age is considerably younger than the previously assumed age of 14 8 ma for high pressure metamorphism in the yuli belt and represents the youngest lu hf garnet age ever recorded for blueschist facies metamorphism the age sheds new light on the palaeogeographic origin and exhumation scenario of the yuli belt we propose that the yuli belt originated from the ocean continent boundary of the chinese passive margin it was subducted eastward during collision with the luzon island arc and rapidly exhumed when the forearc lithosphere was removed from above the continental slab by discrete subduction extraction this process reduces the pressure above the continental slab and may prompt the ascent of subducted crust into the opening gap thus it can control the exhumation of high pressure rocks

diffusion limited ree uptake by eclogite garnets and its consequences for lu hf and sm nd geochronology garnets from the zermatt saas fee eclogites contain narrow central peaks for lu yb tm er and at least one additional small peak towards the rim the ree sm eu gd tb dy are depleted in the cores but show one prominent peak close to the rim these patterns cannot be modeled using rayleigh fractionation accompanied by mineral breakdown reactions instead the patterns are well explained using a transient matrix diffusion model where ree uptake is limited by diffusion in the matrix surrounding the porphyroblast observed profiles are well matched if a roughly linear radius growth rate is used the secondary peaks in the garnet profiles are interpreted to reflect thermally activated diffusion due to temperature increase during prograde metamorphism the model predicts anomalously low 176lu 177hf and 147sm 144nd ratios in garnets where growth rates are fast compared to diffusion of the ree and these results have important implications for lu hf and sm nd geochronology using garnet

early history of earth s crust mantle system inferred from hafnium isotopes in chondrites the 176lu to 176hf decay series has been widely used to understand the nature of earth s early crust mantle system1 2 3 4 5 6 the interpretation however of lu hf isotope data requires accurate knowledge of the radioactive decay constant of 176lu as well as bulk earth reference parameters a recent calibration of the value calls for the presence of highly unradiogenic hafnium in terrestrial zircons with ages greater than 3 9 gyr implying widespread continental crust extraction from an isotopically enriched mantle source more than 4 3 gyr ago7 but does not provide evidence for a complementary depleted mantle reservoir here we report lu hf isotope measurements of different solar system objects including chondrites and basaltic eucrites the chondrites define a lu hf isochron with an initial 176hf 177hf ratio of 0 279628 0 000047 corresponding to 1 983 0 033 10 11 yr 1 using an age of 4 56 gyr for the chondrite forming event this value indicates that earth s oldest minerals were derived from melts of a mantle source with a time integrated history of depletion rather than enrichment7 the depletion event must have occurred no later than 320 myr after planetary accretion consistent with timing inferred from extinct radionuclides8 9 10

crust mantle boundaries in the taiwan luzon arc continent collision system determined from local earthquake tomography and 1d models implications for the mode of subduction polarity reversal abstract in order to better understand the mode of subduction polarity reversal in the taiwan luzon arc continent collision zone we mapped its crust mantle boundaries using local earthquake tomography by contouring surfaces of constant vp 7 5 km s 1 we identified three moho discontinuities and the plate interface that juxtaposes eurasian lower crust against mantle lithosphere of the philippine sea plate the plate interface dips to the east under southeastern taiwan and steepens progressively towards north until it becomes vertical at 23 7 n from there it continues northward in a vertical orientation until the limit of the tomographic model inhibited further mapping for the moho additional depth constraints were derived from 1d models using p wave arrivals of local earthquakes the mohos of the eurasian and philippine sea plates are disconnected across the plate interface beneath southern taiwan the eurasian moho dips to the east at 50 60 following the orientation of the plate boundary and continuous with the benioff zone towards north the eurasian moho steepens into subvertical again together with the plate boundary the philippine sea plate moho exhibits a synform like crustal root interpreted as the base of the magmatic luzon arc towards the north this root deepens from 30 to 70 km underneath the ryukyu trench in northernmost taiwan the hinge of the vertically subducting eurasian slab steps westward out of the thrust belt leaving the deformation front to the east inactive and giving way to the north dipping philippine sea plate a subhorizontal moho at 30 35 km depth overlies the north dipping philippine sea slab and is interpreted as a newly formed moho established after westward rollback and delamination of the subducting eurasian slab in combination these data support a model of progressive subduction polarity reversal in which a tear within the eurasian lithosphere propagated southwestward deactivating the deformation front

seasonality and meridional propagation of the mjo abstract the madden julian oscillation mjo is known to have a substantial impact on the variability of the asian australian summer monsoons an important but not well understood aspect of the mjo monsoon connection is the meridional propagation of bands of enhanced or reduced precipitation that are especially pronounced during the northern summer in this study the nature of the seasonality of the mjo is examined with a focus on the meridional propagation using both observations and simulations with an atmospheric general circulation model agcm a key result is that the agcm when forced with idealized eastward propagating equatorial dipole heating anomalies reproduces the salient features of the observed seasonality in the precipitation and wind fields associated with the mjo including meridional propagation into the indian and australian summer monsoon regions an analysis of the simulations and observations shows that the off equatorial precipitation anomalies are initiated by surface fricti

the influence of the mjo on upstream precursors to african easterly waves abstractthe madden julian oscillation mjo produces alternating periods of increased and reduced precipitation and african easterly wave aew activity in west africa this study documents the influence of the mjo on the west african monsoon system during boreal summer using reanalysis and brightness temperature fields mjo related west african convective anomalies are likely induced by equatorial kelvin and rossby waves generated in the indian ocean and west pacific by the mjo which is consistent with previous studies the initial modulation of tropical african convection occurs upstream of west africa near the entrance of the african easterly jet aej previous studies have hypothesized that an area to the east of lake chad is an initiation region for aews called the trigger region in this study this area exhibits significant intraseasonal convection and wave activity anomalies prior to the wet and dry mjo phases in the west african monsoon region in the trigger region cold tropospheric tempera

tectonic erosion and the removal of forearc lithosphere during arc continent collision evidence from recent earthquake sequences and tomography results in eastern taiwan a very common but important observation of collided and accreted volcanic arcs is that the forearc regions of these arcs are largely or even entirely missing the processes and mechanisms responsible for the removal and transport of the forearc materials from the collisional belts are thus important issues in understanding tectonics and crustal growth the young and ongoing collision between the luzon volcanic arc and the eurasian continental margin that forms the island of taiwan provides a rare opportunity to examine these processes and mechanisms as they occur from observations of a new detailed 3 d tomography combined with relocated hypocenters of two earthquake sequences occurred in 2003 and 2006 we found that the luzon forearc lithosphere initially underthrusts westward after the collision began as the collision proceeds the forearc basement then subducts eastward beneath the colliding and accreting luzon arc along a major fault system in eastern taiwan thus the luzon forearc lithosphere appears to be removed by tectonic erosion and is being transported eastward into the mantle our results from the active taiwan orogen will provide important insights for interpreting rock records from many old arc continent collisional belts in the world

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

northwestward propagation of the intraseasonal oscillation in the western north pacific during the boreal summer structure and mechanism abstract the convection of the intraseasonal timescale in the western north pacific during the boreal summer tends to propagate northwestward in the philippine sea to near 20 n and then continues propagating westward the formation of enhanced convection in the western north pacific is a result of the merging of a convective system moving eastward along the equator and a westward propagating low level convergence anomaly which is located to the east of a vortex in the subtropics a positive feedback between the anomalous circulation and convection leads to a rapid enhancement of the system the strengthened southwesterly associated with the vortex enhances evaporation over the oceans e g the eastern indian ocean the bay of bengal and the south china sea and transports moisture northeastward the moisture converges at the northwestern corner of the convection and results in a potentially unstable atmosphere the result is the northwestward propagation of the coupled circulation convection system in

the west african monsoon dynamics part i documentation of intraseasonal variability intraseasonal variability in the west african monsoon is documented by using daily gridded datasets of rainfall and convection and reanalyzed atmospheric fields over the period 1968 90 rainfall and convection over west africa are significantly modulated at two intraseasonal timescales 10 25 and 25 60 day leading to variations of more than 30 of the seasonal signal a composite analysis based on the dates of the maximum minimum of a regional rainfall index in wet dry sequences shows that these sequences last on average 9 days and belong to a main quasiperiodic signal of about 15 days a secondary periodicity of 38 days is present but leads to a weaker modulation during a wet dry sequence convection in the itcz is enhanced weakened and its northern boundary moves to the north south while the speed of the african easterly jet decreases increases the speed of the tropical easterly jet increases decreases and the monsoon flow becomes stronger weaker all these features being similar to the ones associated with interannual variability characterizing wet and dry years this modulation of convection at intraseasonal timescales is not limited to west africa but corresponds to a westward propagating signal from eastern africa to the western tropical atlantic an enhanced monsoon phase is associated with stronger cyclonic activity in the low levels over the sahel associated with stronger moisture advection over west africa five days before the full development of the wet phase a stronger cyclonic circulation at 20 e induces enhanced southerly winds along 25 e where convection enhances while another westwardpropagating cyclonic circulation is located at 0 this atmospheric pattern is linked to the enhancement of the subsiding branch of the northern hadley cell at 35 n northerly advection of drier air over west africa and to increased dry convection in the heat low at 20 n it propagates westward leading to a zonally extended area of enhanced monsoon winds over west africa consistent with the occurrence of the wet phase

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

implication of the madden julian oscillation in the 40 day variability of the west african monsoon abstract madden julian oscillations mjos are extracted over the indo pacific basin using a local mode analysis the convective perturbations are then projected over a larger domain to evaluate their remote consequences over the west african monsoon wam intraseasonal variability rather weak 4 6 w m 2 convective fluctuations occurring in phase with those over the southern indian basin are found over africa confirming the results of matthews in reverse 40 day fluctuations in the wam similarly detected and projected over a widened area demonstrate that a large majority of these events are embedded in the larger scale patterns of the mjo the regional amplitude of intraseasonal perturbations of the west african convection is not statistically associated with the amplitude of the mjo over the indian basin but is instead closely related to background vertical velocity anomalies over africa possibly embedded in changes in the regional walker type circulation subsiding motion over africa is recorded

mjo related sst variations over the tropical eastern pacific during northern hemisphere summer abstract the madden julian oscillation mjo significantly modulates eastern tropical pacific sea surface temperatures ssts during northern hemisphere summer typical sst variations during an mjo life cycle are 0 4 0 5 c considerably higher variations occur with the strongest events 1 2 c the magnitudes of these variations are comparable to those associated with west pacific mjo variability during nh winter sst anomalies to the north of 4 n where the strongest modulation of east pacific convection by the mjo occurs are 180 out of phase with equatorial anomalies these sst variations are responsible for variations in surface saturation equivalent potential temperatures of greater than 3 c the highest ssts lead maximum convection by about 10 days and ssts begin to cool with the onset of enhanced convection off equatorial sst anomalies are most likely due to variations in surface latent heat flux although shortwave flux anomalies are important in the heart of the mjo convective region decrea

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

geochemistry of high grade metamorphic rocks of the north qaidam mountains and their geological significance the high grade metamorphic basement in the north qaidam mountains nqd is composed mainly of jinning age metasedimentary rocks and crustally derived granites possibly accompanied by minor paleoproterozoic and archaean material based on their geochemistry the metasedimentary rocks are divided into two groups type i which makes up the bulk of the high grade metamorphic basement in the nqd is characterized by high k2o low na2o relatively high total rare earth elements treez114 357 ppm and weak fractionation of light lree and heavy hree rare earth elements la yb nz8 27 these rocks also have strong negative eu anomalies eu eu z0 3 0 6 type ii rocks crop out mainly in the wulan area and are characterized by relatively high mgo 2 2 4 1 wt lower total ree and strong fractionation of lree and hree la yb nz29 48 both types of metasedimentary rocks have similar nd isotope compositions with tdmz1 9 2 2 ga and 3ndzk4 1 to k8 5 the granites are compositionally similar to the type i metasedimentary rocks the high grade metamorphic basement of the nqd is similar to that in the qilian orogenic belt and the yangtze platform the central qilian nqd terrane was separated from the dunhuang alxa terrane before the jinning period and the two terranes belong to the south china block and north china craton respectively q 2006 elsevier ltd all rights reserved

shrimp u pb dating trace elements and the lu hf isotope system of coesite bearing zircon from amphibolite in the sw sulu uhp terrane eastern china abstract in this study we link mineral inclusion data trace element analyses u pb age and hf isotope composition obtained from distinct zircon domains of complex zircon to unravel the origin and multi stage metamorphic evolution of amphibolites from the sulu ultrahigh pressure uhp terrane eastern china zircon grains separated from amphibolites from the ccsd mh drill hole g12 and niushan outcrop g13 were subdivided into two main types based on cathodoluminescence cl and laser raman spectroscopy big dusty zircons with inherited cores and uhp metamorphic rims and small clear zircons weakly zoned grey white luminescent inherited cores preserve mineral inclusions of cpx pl ap qtz indicative of a mafic igneous protolith dark grey luminescent overgrowth rims contain the coesite eclogite facies mineral inclusion assemblage coe grt omp phe ap and formed at t 732 839 c and p 3 0 4 0 gpa in contrast white luminescent small clear zircons preserve mineral inclusions formed during retrograde hp quartz eclogite to lp amphibolite facies metamorphism t 612 698 c and p 0 70 1 05 gpa inherited zircons from both samples yield shrimp 206 pb 238 u ages of 695 520 ma with an upper intercept age of 800 31 ma the uhp rims yield consistent triassic ages around 236 225 and 239 225 ma for g12 and g13 with weighted means of 229 3 and 231 3 ma respectively small clear zircons from both samples give 206 pb 238 u ages around 219 210 ma with a weighted mean of 214 3 ma interpreted as the age of retrograde quartz eclogite facies metamorphism matrix amphibole from both samples indicate ar ar ages of 209 0 7 and 207 0 7 ma respectively probably dating late amphibolite facies retrogression the data suggest subduction of neoproterozoic mafic igneous rocks to uhp conditions in middle triassic 230 ma times and subsequent exhumation to an early hp 214 ma and a late lp stage 208 ma over a period of 16 and 6 myr respectively thus early exhumation from a mantle depth of 120 100 km to about 60 km occurred at an average rate of 0 3 cm y while subsequent exhumation to a middle crustal level took place at approximately 0 54 cm y these exhumation rates are considerably slower than those obtained for uhp rocks in the dora maira and kokchetav massifs 2 3 cm y based on similar p t estimates and trace element and hf isotope compositions sulu amphibolites can be identified as retrograde uhp eclogites the ehf 800 of 8 implies a significant input from the depleted mantle to the sulu dabie terrane during the middle neoproterozoic overgrown rims are characterized by a distinct trace element composition with low lu hf and th u and significantly higher 176 hf 177 hf ratios than inherited cores consistent with formation during after garnet re crystallization and fractionation of the lu hf system during uhp metamorphism the combined dataset suggests homogenization of the 176 hf 177 hf ratio within the metamorphic mineral assemblage and during protolith formation observed variations are explained by mixing of material from both domains during laser ablation e g due to partial recrystallization of inherited cores

blueschists eclogites and subduction zone tectonics insights from a review of late miocene blueschists and eclogites and related young high pressure metamorphic rocks abstract in this paper late miocene blueschist and eclogite belts including the world s youngest blueschist belt in timor tanimbar island chain eastern indonesia and the world s youngest coesite bearing eclogite papua new guinea together with selected cenozoic high pressure and ultrahigh pressure metamorphic rocks are reviewed from a synthesis of the geology metamorphism and chronology of these rocks the formation and exhumation process are evaluated and the significance on tectonics at convergent plate boundaries is discussed

paradigms new and old for ultrahigh pressure tectonism regional ultrahigh pressure uhp metamorphic terranes exhibit a spectrum of lithological structural and petrological characteristics that result from the geodynamic processes that formed and exhumed them at least six geodynamic processes can be envisioned to have carried continental rocks to mantle depths i continental margin subduction ii microcontinent subduction iii sediment subduction iv intracontinental subduction v subduction erosion and vi foundering of a crustal root most of these processes have been investigated through numerical or analog models and most have been invoked for one or more specific occurrences of uhp rocks at least six geodynamic processes can be envisioned to have exhumed uhp continental rocks i eduction ii microplate rotation iii crustal stacking iv slab rollback v channel flow and vi trans mantle diapirs most of these processes have also been investigated through numerical or analog models and all have been invoked to explain the exhumation of at least one uhp terrane more detailed and systematic field investigations are warranted to assess the predictions of numerical models and more sophisticated and realistic numerical models are required to replicate and explain the petrological structural and chronological data obtained from uhp terranes

the south atlantic convergence zone intensity form persistence and relationships with intraseasonal to interannual activity and extreme rainfall abstract the characteristics of intensity geographical location and persistence of the south atlantic convergence zone sacz during the austral summer are investigated intensity and spatial features of the sacz are identified by performing a factor analysis of structural properties of outgoing longwave radiation olr data the first two leading factors explain 65 of the total variance of structural properties and characterize the sacz according to intensity and location oceanic versus continental an index is constructed based on the magnitude of the factor scores to identify intense weak and oceanic continental sacz the intense sacz category is associated with negative olr anomalies over a large area of tropical south america extending from the western amazon to the atlantic ocean the weak sacz category is observed with positive olr anomalies over tropical south america and negative olr anomalies over southeastern south america oceanic and continental aspects of the sacz are related to a

subduction collisional processes between the eurasian and philippine sea plates constraints from thermal age paths of the taiwan orogen abstract records of the thermal age path of orogenic rocks are crucial in understanding subduction and collisional processes between a continent and an arc tectonic events and thermochronological data of the taiwan orogen have been extensively investigated we use the zircon u pb youngest ages as the initial subduction age the raman spectroscopy of carbonaceous material 40ar 39ar age of mica porphyroblasts whole rock lu hf isochron age and thermal results as prograde peak metamorphic condition during subduction and basal accretion and the apatite fission track data for cooling age of the taiwan orogen to illuminate the subduction and collisional processes between the eurasian plate and the luzon arc the hsuehshan range nappe in the west of the taiwan orogen whose subduction was initiated at 12 ma reached peak metamorphism with a temperature of 250 c to 490 c at a depth of 8 km during the interval 6 2 5 ma mainly by basal superimposed accretion the isothermal dwelling during peak metamorphism condition of the hsuehshan range suggests that tectonic underplating through basal superimposed accretion contribute significantly to the formation of the taiwan orogen and led to a sudden increase of subduction and decollement depths and slow downward thickening of the orogenic wedge and mountain root growth located in the east of the taiwan orogen the yuli beltsubduction was initiated at 15 4 16 ma and attained peak metamorphism at a depth of 40 km with a temperature of 550 560 c and a pressure of 10 5 13 kbar during the interval 12 2 4 4 ma due to basal superimposed accretion the exhumation of the hsuehshan range commenced at 2 5 ma which is later than that of the yuli belt at 3 4 ma such distinct and different thermal age paths of the hsuehshan range and the tananao complex indicate that different tectonic wedges in the taiwan orogen are formed through different subduction and accretional processes

the 30 70 day oscillations in the tropical atlantic 1 evidence is presented for intraseasonal oscillations in the trade winds of the northern and southern tropical atlantic the pattern of intraseasonal oscillations in surface pressure and wind speed in the northern hemisphere resembles that of the north atlantic oscillation nao like the nao these oscillations have a strong seasonal cycle with a maximum amplitude in boreal winter spring winds in the southern tropical atlantic are related to the southern hemisphere equivalent of the nao exhibiting a weaker seasonality with peak amplitudes in austral winter the madden julian oscillation mjo which originates over the indian ocean also exerts an influence on tropical atlantic surface winds particularly between about 10 n and 10 s where intraseasonal wind speed oscillations in the atlantic are strong they force changes in sea surface temperature sst through latent heat loss from the ocean potentially affecting lower frequency climate variations

active break cycles diagnosis of the intraseasonal variability of the asian summer monsoon in this study various diagnostics have been applied to daily observed outgoing longwave radiation olr and ecmwf reanalysis era products to provide a comprehensive description of the active break cycles associated with the asian summer monsoon and to address the differing behaviour of the two dominant time scales of intraseasonal variability 10 20 days and 30 60 days composite analysis of olr based on filtered daily all india rainfall air for the 40 day 30 60 days intraseasonal mode indicates that during active phases convection is significantly enhanced over the indian continent extending over the bay of bengal maritime continent and equatorial west pacific while convection is suppressed over the equatorial indian ocean and northwest tropical pacific resulting in a quadrapole structure over the asian monsoon domain in response to this heating pattern the large scale hadley lateral and the two east west transverse tropical circulations are enhanced there is also a significant impact on the extra tropical circulation through excitation and propagation of rossby waves in contrast the 15 day mode is more regional to the monsoon domain and has a prominent east west orientation in convection only the local hadley circulation over the monsoon region is modulated by this mode the evolution of these two modes as revealed by pop principal oscillation pattern analysis shows that the 40 day mode originates over the equatorial indian ocean once formed it has poleward propagation on either side of the equator and eastward propagation into the equatorial west pacific from the equatorial west pacific northward propagation over the west pacific and westward propagation into the indian longitudes are prominent the propagative features are complex and interactive and are responsible for the quadrapole structure in convection seen from the composites the interannual variability assessed from the pop coefficient time series indicates that the 40 day mode is strong during the onset phase of the monsoon in all the years but systematic propagation over the entire season depends crucially on the activity of the oceanic tcz tropical convergence zone the pop analysis of the 15 day mode indicates that this event originates over the equatorial west pacific associated with westward propagating rossby waves amplifies over the northwest tropical pacific and modulates both the continental and oceanic tczs over indian longitudes simultaneously this mode is pronounced during the established phase of the monsoon due to the complexity in the propagational features of both the intraseasonal modes it is concluded that understanding the subseasonal variability of one regional component of the asian summer monsoon asm requires understanding the entire asm system

evolutionary model for the taiwan collision based on physical modelling abstract 2 d and 3 d physical modelling of lithospheric convergence in the luzon taiwan ryukyu region is performed with properly scaled laboratory models the lithospheric model consists of two parts continental the asian plate ap and oceanic the philippine sea plate psp the oceanic lithosphere has one layer while the continental lithosphere includes both mantle and crustal layers the continental margin is covered by sediments a low viscosity asthenosphere underlies the lithosphere the opposing luzon and ryukyu subduction zones are initiated by inclined cuts made within the psp the subduction collision is driven by a piston pre collisional intraoceanic subduction along the luzon and ryukyu boundaries results in the formation of a transform zone between them with two tear faults at the ends the psp undergoes strong compression along this zone subduction of the chinese margin under the luzon boundary further increases the compression compressive stresses reach the yield limit of the psp in the arc area which is a weak zone in the experiments the plate fails at the western side of the arc along an eastward dipping fault the longitudinal valley fault underthrusting of the frontal wedge of the psp along this fault results in the closure of the fore arc basin and is then blocked the psp fails at the opposite side of the luzon arc along the westward dipping fault the failure releases lithospheric compression in this region and results in the initiation of southward propagating subduction of the psp under northeastern taiwan the incipient subduction zone becomes part of the southeastward retreating ryukyu subduction zone which allows the okinawa back arc rift to propagate into taiwan the taiwan collision thus includes the following succession of major processes over time or from south to north 1 an e w shortening of the psp in the luzon arc 2 a failure of this plate at the western side of the arc and the formation of the eastward dipping longitudinal valley fault the transient plate boundary 3 a closure of the fore arc basin and a rapid uplift of the orogen 4 a failure of the psp at the eastern side of the luzon arc partly overthrusting the orogen and the initiation of westward wn ward subduction of the psp 5 and finally back arc rifting in the rear of this incipient subduction zone i e in northern taiwan all these processes commence with some delay with respect to the preceding ones and propagate southwards

influence of the madden julian oscillation on east african rainfall i intraseasonal variability and regional dependency the influence of the madden julian oscillation mjo on rainfall amounts over equatorial east africa kenya and northern tanzania is analysed for the period 1979 95 at the intraseasonal pentad time scale the two rainy seasons march to may and october to december are considered intraseasonal wet events in east africa are embedded in large scale zonal circulation anomaly patterns along the equator showing distinct eastward propagation it is further found that these wet events display a clear phasing with respect to the mjo cycle this phasing is expressed as out of phase variations between the highland and the coastal areas such a pattern is suggested to reflect different rain causing mechanisms mjo phases leading to wet spells in the western highland region are those associated with the development of large scale convection in the africa indian ocean region these events are unambiguously related to deep convection fuelled by low level westerly moisture advection mjo phases leading to wet spells in the eastern coastal region are often those associated with overall suppressed deep convection in the africa indian ocean region however these phases induce moisture advection from indian ocean the possible role of stratiform rainfall or relatively shallow convection in the coastal wet spells observed in this phase is discussed the contrasting rainfall conditions found in the two regions for the two opposite mjo phases are strongly correlated with the pressure gradient between the indian and atlantic oceans copyright 2006 royal meteorological society

a systematic comparison of tropical waves over northern africa part ii dynamics and thermodynamics abstractthis study presents the first systematic comparison of the dynamics and thermodynamics associated with all major tropical wave types causing rainfall modulation over northern tropical afric

erosion rates and orogenic wedge kinematics in taiwan inferred from fission track thermochronometry new apatite and zircon fission track ages and previously published thermochronometric data are used to evaluate erosion rates and particle paths within the active taiwan arc continent collision we present 20 new apatite fission track ages and 6 new zircon fission track ages apatite and zircon ages are all reset in the northern and eastern parts of taiwan although the region of reset apatite ages is larger we interpret this pattern as resulting from crustal accretion at the western margin of the orogenic wedge combined with southward propagation of the collision zone a onedimensional thermal model including erosion provides prediction of the fission track ages the distribution of reset ages is best explained with an erosion rate of 4 6 mm yr given a propagation velocity of 60 mm yr this erosion rate implies that nearly 25 km of material has been eroded from northern taiwan the lack of reset 40 ar 39 ar ages from muscovite and biotite suggests that rockparticle paths have a large horizontal component a result consistent with an eroding orogenic wedge model

the madden julian oscillation s influence on african easterly waves and downstream tropical cyclogenesis abstractthe influence of the madden julian oscillation mjo over tropical africa and atlantic is explored during the northern hemisphere summer months the mjo is assessed by using real time multivariate mjo rmm indices these indices divide the active convective signal of the mjo into 8 phases convection associated with the mjo is enhanced over tropical africa during rmm phases 8 1 and 2 convection becomes suppressed over tropical africa during the subsequent rmm phases phases 3 7 african convective signals are associated with westward propagating equatorial rossby waves the mjo modulates african easterly wave aew activity aew activity is locally enhanced during rmm phases 1 3 and suppressed during rmm phases 6 8 enhanced aew activity occurs during periods of enhanced convection over tropical africa consistent with stronger or more frequent triggering of aews as well as more growth associated with latent heat release enhanced aew activity occurs during the low level westerly wind phase of

temporal variations of the tropical 40 50 day oscillation abstract in recent years there has been a great deal of interest in a quasi periodic tropical oscillation of zonal winds which was first reported by madden and julian an attempt to determine the temporal variation of the oscillation parameters is presented here using a 4 year duration global time series and a 25 year station time series we find that although the nonseasonal variations are large any seasonal cycle in the oscillation amplitude and frequency must be very small the small seasonal signal in the oscillation frequency seems to argue against explanations for the time scale based on doppler shifted traveling waves

using raman spectroscopy of carbonaceous materials to track exhumation of an active orogenic belt an example from eastern taiwan abstract the burial and exhumation history of an orogen can provide insights into the thermal history of rocks in the mountain belt asynchronous collision will exhume materials to the surface at different times and expose different metamorphic grades along the orogenic axis by tracking the thermal signals of eroded materials it is possible to compare spatial patterns of exhumation along the mountain belt previous studies such as bulk mineralogy of sandstones crystallinity of clay minerals and fission track thermochronology all suggest that the pliocene to pleistocene meta sedimentary deposits within the preserved basins of the coastal range reflect the unroofing history of the central range in eastern taiwan in this study modern sediments from six drainages of the east central range liwu mugua laku laku luye jinlun and hsuhai rivers and three basin sections of meta sedimentary deposits from the coastal range shuilien chimei and madagida sections were sampled to reconstruct the thermal signals of each drainage along the modern orogenic axis and reconstruct temporal variations of thermal signals in preserved plio pleistocene sedimentary rocks the temperature index t i of modern sediments determined by raman spectroscopy of carbonaceous materials are medium to high 0 4 0 8 in conjunction with bio and magneto stratigraphy our data suggest that the northern shuilien area has been receiving detritus of medium metamorphic grade from the exhumed orogeny in the north from 3 4 ma and higher grade since 2 ma meanwhile the southern madagida section has been mostly taking in juvenile detritus from the south part of the orogen until 1 ma the central chimei section has started collecting mature detritus since 1 24 ma the data suggest that the plio pleistocene basins contain a good record of orogen evolution

exhumation of high and ultrahigh pressure metamorphic rocks by slab extraction exhumation of high and ultrahigh pressure metamorphic rocks in collisional orogens may be explained by upward extrusion of these rocks erosion of their overburden or extensional thinning of the overburden some high pressure terranes such as the adula nappe in the central alps fit none of these scenarios we propose an additional way in which part of the overburden may be removed it may sink off into the deeper mantle slab extraction structural and metamorphic relationships in and around the adula nappe indicate that the emplacement of this alpine high to ultrahigh pressure nappe to 3 2 gpa in a pile of lower pressure nappes resulted from the interaction of two subduction zones that accommodated the closure of two ocean basins ultimately leading to the extraction of the intervening slab in terms of mechanics the cause of the exhumation is in this case not the buoyancy of the high pressure rocks but the negative buoyancy of the extracted slab

evolution of early crust in chondritic or non chondritic earth inferred from u pb and lu hf data for chemically abraded zircon from the itsaq gneiss complex west greenland zircon grains in rocks collected from the itsaq gneiss complex southwest greenland were analyzed for u pb and lu hf in the same grain using isotope dilution thermal ionization mass spectrometry tims and multicollector inductively coupled plasma mass spectrometry mc icp ms grains were pretreated using chemical abrasion or air abrasion to assure that only zircon material unaffected by the migration of parent and daughter elements was analyzed the data are consistent with derivation of all studied rocks from a single enriched mantle source or mafic crustal protolith with 176lu 177hf of 0 022 0 003 that was repeatedly melted and produced tonalitic magmas the assessment of the primary mantle source from which this mafic protolith was derived at or before 3 85 ga greatly depends on the assumed composition of the bulk silicate earth using the currently accepted lu hf bulk earth parameters based on the analysis of chondrites yields ehf t of 0 to 1 for the 3 80 3 86 ga rocks suggesting that

high precision lu hf geochronology of eocene eclogite facies rocks from syros cyclades greece abstract garnet bearing high pressure low temperature hp lt metamorphic rocks of the cycladic blueschist unit have been investigated by lu hf geochronology eclogites from syros island yield precise ages of 52 2 0 3 ma 51 4 0 4 ma and 50 2 ma preserved major and trace element growth zoning in garnet suggests that the closure temperature of the lu hf system in garnet was higher than the estimated peak metamorphic temperature of 500 c hence lu hf ages most likely reflect garnet growth in the studied samples our new lu hf grt ages are in excellent agreement with previously published u pb shrimp ages of metamorphic zircon and the maximum reported white mica rb sr and ar ar ages from syros garnet growth in all samples apparently took place over a narrow time interval because the lu hf ages cluster tightly even though the distribution of lu between cores and rims varies among samples we find no evidence for garnet generations that are significantly older or younger than the given isochron ages this in conjunction with the agreement among the ages of metamorphic zircon the maximum white mica ages and the lu hf ages strongly suggests that the 52 ma age dates a single hp event that affected these rocks

dynamics of the west african monsoon part iv analysis of 25 90 day variability of convection and the role of the indian monsoon this paper is part of a series of studies addressing the dynamics of the west african summer monsoon at intraseasonal time scales between 10 and 90 days the dominant mode of 25 90 day convective variability in the african monsoon was investigated starting from previous results involving the excitation of dry equatorial kelvin and rossby waves by a negative diabatic heat source located over the warm pool this evolution is consistent with a significant contribution by a convectively coupled equatorial rossby wave and the mjo on the other hand convectively coupled kelvin waves as well as the dry kelvin wave signal have a very weak impact however there is more to the global control of the african summer monsoon than convectively coupled wave dynamics the active break cycle of the indian monsoon controlled by a northward moving dipole of diabatic heating in the indian sector can also influence the african monsoon through atmospheric teleconnections simulations performed with a dry primitive equation model show that this influence may be transferred through the northern indian heat source which excites a rossby cyclonic circulation propagating westward over north africa that is cut off by the northward arrival of the equatorial indian heat source and the associated intrusion of an anticyclonic ridge low level westerly winds and moisture advection within the itcz consequently increase over africa the mean time lag between an active phase over india and over africa is about 15 20 days

influence of the madden julian oscillation on southern african summer rainfall composite maps of outgoing longwave radiation olr anomalies over the madden julian oscillation mjo cycle show marked intraseasonal fluctuations over southern africa south of 15 s large scale convective clusters are seen to propagate eastward and then northward over the continent mainly between 10 and 20 s the corresponding response of the rainfall field presents the alternation over the cycle of dry and humid phases which are both significant moisture flux anomalies indicate an intraseasonal modulation of the midtropospheric easterly flow over the congo basin at 700 hpa these fluctuations are coupled to meridional flux anomalies that extend from the tropical to the subtropical austral latitudes and favor occurrences of wet or dry conditions over the domain though statistically significant the influence of the mjo on southern africa is however not homogeneous spatially and only the tropical areas exhibit sharp periodicities in the 30 60 day period range the olr dipole observed in previous studies at the interannual and synoptic time scales between the hinterland parts of southern africa and the southwestern indian ocean in the north of madagascar is investigated next as it also shows strong fluctuations at the intraseasonal time scale the study points out that the dipole is partly influenced by the mjo though the strongest periodicities are found for slightly longer periods 35 80 days than those typically associated with the oscillation the forcing of the mjo on the olr dipole though significant remains thus partial

the boreal summer intraseasonal oscillation relationship between northward and eastward movement of convection the summertime intraseasonal oscillation iso is an important component of the south asian monsoon lagged regressions of intraseasonally filtered 25 80 days outgoing longwave radiation olr reveal that centers of convection move both northward and eastward from the central equatorial indian ocean subsequent to the initiation of an iso eastward movement of convection is also seen at indian subcontinent latitudes 10 8 208n based on the regression results the summertime iso convection signal appears as a band tilting northwestward with latitude and stretching from the equator to about 208n viewed along any meridian convection appears to propagate northward while equatorial convection propagates to the east to examine the robustness of the connection between eastward and northward movement individual isos are categorized and composited relative to the strength of the large scale eastward component of convection in the central equatorial indian ocean it is found that the majority of isos that exhibit northward movement onto the indian subcontinent 42 out of 54 isos or 78 also exhibit eastward movement into the western pacific ocean it is also found that when convection in the central indian ocean is not followed within 10 20 days by convection in the western pacific ocean 12 out of 54 isos or 22 the independent northward movement of convection in the indian ocean region is somewhat stunted the link between the eastward and northward movement of convection is consistent with an interpretation of the summertime iso in terms of propagating equatorial modes the northward moving portion of convection is forced by surface frictional convergence into the low pressure center of the rossby cell that is excited by equatorial iso convection a similar convergence pattern is seen for the northern winter iso but it does not generate poleward movement due to relatively cool ssts underlying the surface convergence

trace element signature and u pb geochronology of eclogite facies zircon bergen arcs caledonides of w norway secondary ion mass spectrometry sims u pb and trace element data are reported for zircon to address the controversial geochronology of eclogite facies metamorphism in the lindas nappe bergen arcs caledonides of w norway caledonian eclogite facies overprint in the nappe was controlled by fracturing and introduction of fluid in the proterozoic sveconorwegian granulite facies meta anorthosite norite protolith zircon grains in one massive eclogite display a core rim structure sveconorwegian cores have trace element signatures identical with those of zircon in the granulite protolith i e 0 31 th u 0 89 heavy rare earth element hree enrichment and negative eu anomaly weakly zoned to euhedral oscillatory zoned caledonian rims are characterized by th u 0 13 low lree content minimum normalized abundance for pr or nd variable enrichment in hree and no eu anomaly a decrease of ree towards the outermost rim especially hree is documented this signature reflects co precipitation of zircon with garnet and clinozoisite in a feldspar absent assemblage and consequently links zircon to the eclogite facies overprint the rims provide a mean 206pb 238u crystallization age of 423 4 ma this age reflects eclogite forming reactions and fluid rock interaction this age indicates that eclogite facies overprint in the lindas nappe took place at the onset of the scandian silurian collision between laurentia and baltica

basal accretion a major mechanism for mountain building in taiwan revealed in rock thermal history abstract deep tectonic processes are key integral components in the evolution of mountain belts while observations of their temporal development are generally obscured by thermal resetting retrograde alteration and structural overprinting here we recorded an integrated rock time temperature history for the first time in the pro wedge part of the active taiwan arc continent collision starting from sedimentation through cleavage forming state to its final exhumation the integrated thermal and age results from the raman spectroscopy of carbonaceous material rscm method zircon u pb laser ablation dating and in situ 40ar 39ar laser microprobe dating suggest that the basal accretion process was crucial to the development of the taiwanese orogenic wedge the basal accretion process commenced early in the mountain building history 6 ma and gradually migrated to greater depths as constrained by persistent plate convergence and cleavage formation under nearly isothermal state at similar depths until 2 5 ma recorded in the early accreted units such development essentially contributed to mountain root growth by the increased depth of the wedge detachment and the downward wedge thickening during the incipient to full collision stages in the taiwan mountain belt

intraseasonal behavior of clouds temperature and motion in the tropics abstract the spectral character of tropical convection is investigated in an 11 yr record of outgoing longwave radiation from the advanced very high resolution radiometer to identify interaction with the tropical circulation along the equator in the eastern hemisphere the space time spectrum of convection possesses a broad peak at wave numbers 1 3 and eastward periods of 35 95 days significantly broader than the dynamical signal of the madden julian oscillation mjo this quasi discrete convective signal is associated with a large scale anomaly that propagates across and modulates time mean or climatological convection over the equatorial indian ocean and western pacific outside that region the convective signal is small even though under amplified conditions coherence can be found east of the date line and in the subtropics having a zonal scale of approximately wavenumber 2 anomalous convection propagates eastward at some 5 m s 1 and suppresses as well as reinforces climatological convection

microdiamond discovered in the seve nappe scandinavian caledonides and its exhumation by the vacuum cleaner mechanism when a continent collides with an island arc or other continent continental crust of the subducted continent may be buried to depths exceeding 100 km and exposed to pressures that can cause formation of coesite and diamond this process leads to substantial density increase in sio2 rich rocks and in turn to a reduction of the buoyancy of the subducted material which should inhibit exhumation nevertheless coesite and diamond bearing continental crustal rocks are known from several occurrences worldwide we report on the discovery of microdiamond in kyanite garnet gneiss from allochthonous metasediments of the seve nappe complex in the scandinavian caledonides our discovery calls for general reconsideration of existing exhumation models of deeply subducted continental crust we propose that the diamond bearing rocks were subducted in an arc continent collision setting and their exhumation was facilitated by local pressure reduction resulting from extraction of the forearc lithospheric block

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

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c

a model for the boreal summer intraseasonal oscillation abstract the tropical intraseasonal oscillation iso exhibits pronounced seasonality the boreal summer iso is more complex than its winter counterpart due to the coexistence of equatorial eastward off equatorial westward and northward propagating low frequency modes and their interactions based on observational evidence and results obtained from numerical experiments a mechanism is proposed for the boreal summer iso in which the northern hemisphere summer monsoon nhsm circulation and moist static energy distribution play essential roles with a climatological july mean basic state the life cycle of model low frequency waves consists of four processes an equatorial eastward propagation of a coupled kelvin rossby wave packet an emanation of moist rossby waves in the western pacific a westward propagation and amplification of the rossby waves in south asian monsoon regions and a reinitiation of the equatorial disturbances over the central indian ocean the life cycle spans about one month and p

the west african monsoon onset in 2006 sensitivity to surface albedo orography sst and synoptic scale dry air intrusions using wrf in order to test the sensitivity of the transitional phase of the 2006 west african monsoon wam onset to different mechanisms weather research and forecasting wrf model simulations have been carried out addressing the role of the saharan heat low shl and its sensitivity to the albedo field and to the northern africa orography and the role of the sea surface temperature sst in the eastern tropical atlantic and mediterranean lowering albedo over the desert region induces a northward location of the inter tropical convergence zone itcz while removing mountains in north africa reduces rainfall over west africa shifting sst forward by 15 days leads to a northward location of the itcz before the wam onset however none of these factors modifies the timing of the wam onset in 2006 the transitional phase of the 2006 wam onset has been examined in more detail the enhancement of shl intensity combined with the development of the oceanic cold tongue in the guinea gulf leads to low level moisture flux divergence in the itcz reducing rainfall and increasing low level humidity over the sahel however weakening of convection can be clearly attributed to dry air intrusions in mid levels originating from the subtropical westerly jet and associated with rossby wave pattern over north africa sensitivity tests on the synoptic scale forcing outside of the wrf model domain confirm the dominating role of large scale dynamics to control the transitional phase of the wam onset and its timing however it is shown that the regional factors can modulate this larger scale forcing

observed changes in the lifetime and amplitude of the madden julian oscillation associated with interannual enso sea surface temperature anomalies the madden julian oscillation mjo is analyzed using the reanalysis zonal wind and satellite outgoing longwave radiation based indices of wheeler and hendon for the 1974 2005 period the average lifetime of the mjo events varies with season 36 days for events whose central date occurs in december and 48 days for events in september the lifetime of the mjo in the equinoctial seasons march may and october december is also dependent on the state of el nino southern oscillation enso during october december it is only 32 days under el nino conditions increasing to 48 days under la nina conditions with similar values in northern spring this difference is due to faster eastward propagation of the mjo convective anomalies through the maritime continent and western pacific during el nino consistent with theoretical arguments concerning equatorial wave speeds the analysis is extended back to 1950 by using an alternative definition of the mjo based on just the zonal wind component of the wheeler and hendon indices a rupture in the amplitude of the mjo is found in 1975 which is at the same time as the well known rupture in the enso time series that has been associated with the pacific decadal oscillation the mean amplitude of the mjo is 16 larger in the postrupture 1976 2005 compared to the prerupture 1950 75 period before the 1975 rupture the amplitude of the mjo is maximum minimum under el nino la nina conditions during northern winter and minimum maximum under el nino la nina conditions during northern summer after the rupture this relationship disappears when the mjo enso relationship is analyzed using all year round data or a shorter dataset as in some previous studies no relationship is found

u pb zircon geochronology of coesite bearing eclogites from the southern dulan area of the north qaidam uhp terrane northwestern china spatially and temporally extensive uhp metamorphism during continental subduction coesite bearing eclogites from 100 km 2 in the southern dulan area north qaidam mountains nqm of western china contain zircon that records protolith crystallization and ultra high pressure uhp metamorphism sensitive high resolution ion microprobe mass spectrometer and laser ablation inductively coupled plasma mass spectrometry u pb analyses from cathodoluminescence cl dark zircon cores in a coesite bearing eclogite yield an upper intercept age of 838 50 ma and oscillatory zoned cores in a kyanite bearing eclogite gave a weighted mean 206 pb 238 u age of 832 20 ma these zircon cores yield steep heavy rare earth element hree slopes and negative eu anomalies that suggest a magmatic origin thus c 835 ma is interpreted as the eclogite protolith age unzoned cl grey or bright zircon and zircon rims from four samples yield weighted mean ages of 430 4 438 2 446 10 and 446 3 ma flat hree patterns without eu anomalies and contain inclusions of garnet omphacite rutile phengite and rare coesite these ages are interpreted to record 16 5 myr of uhp metamorphism these new uhp ages overlap the age range of both eclogite and paragneiss from the northern dulan area suggesting that all uhp rock types in the dulan area belong to the same tectonic unit our results are consistent with slow continental subduction but do not match oceanic subduction and diapiric exhumation uhp model predictions these new data suggest that similar to eclogites in other hp uhp units of the nqm and south altyn tagh protoliths of the eclogites in the dulan area formed in a continental setting during the neoproterozoic and then subducted to mantle depth together with continental materials during the early palaeozoic