concurrent evolution of 3he 4he ratio in the earth s mantle reservoirs for the first 2 ga the differences in 3he 4he ratios between mid oceanic ridge basalts morbs and ocean island basalts oibs require the existence of at least two distinct mantle reservoirs that have been preserved over long periods of the earth s history however it is unclear how the 3he 4he ratios have evolved in the mantle over time and when these reservoirs acquired their present day signatures here we report the results of our model calculation for helium evolution in the mantle and show that from the time of the earth s formation to the late archean 2 ga the 3he 4he ratios in both the morb and oib mantle reservoirs evolved in a rather homogeneous fashion in contrast to previous models our model suggests that the distinction in 3he 4he ratios between the morb type source and the plume type source is a relatively recent characteristic of the earth s mantle

the primordial noble gases in the earth a key constraint on earth evolution models abstract we made an extensive compilation of noble gas data obtained from morb oceanic island basalts oib and diamonds on the basis of this data base we discuss the primordial noble gases in the earth and its implications on mantle evolution we conclude that in the contrary to a common assumption the primordial ne ar kr and xe in the earth are distinctly different from the solar noble gases the primordial noble gases were derived from the solar noble gas through rayleigh distillation like mass dependent fractionation the primordial ar kr xe are identical with the atmospheric components but ne observed in the mantle derived materials differs significantly from the atmospheric ne we suggest that ne was further fractionated in the atmosphere while the abundances of ar kr and xe in the mantle derived materials are well correlated to each other he and ne in morb is totally uncorrelated with the heavier noble gases this indicates that he and ne in morb is decoupled from the heavier noble gases in the mantle source he ne ar systematics indicate that he was open to the morb and oib mantle sources and ne in the morb mantle source while he is excess in the morb mantle source it is deficient in the oib mantle source relative to the closed system values however it is not known whether this apparent complementary characteristic corresponds to some fundamental mantle dynamics or accidental coincidence

late pliocene pleistocene changes in mass accumulation rates of eolian deposits on the central chinese loess plateau 1 late pliocene pleistocene changes in the mass accumulation rates mars of eolian deposits on the central chinese loess plateau clp are reconstructed from measured bulk sediment densities combined with sedimentation rates calculated for two loess red clay sequences the reconstructed mars demonstrate that over the past 3600 kyr distinct aridity humidity fluctuations occurred over glacial interglacial timescales and these were superimposed on a gradual long term drying trend comparisons of the clp mar record with the mar and 18o records for marine sediments suggest that the development of northern hemisphere ice sheets since the late pliocene has played a part in increasing the aridity of central asia in addition the phased uplift of the himalaya tibetan plateau evidently had a profound influence on the aridity of central asia this was mediated by variations in the east asian monsoonal circulation correlations among dust records from ice cores marine sediments and terrestrial loess over the last several glacial cycles provide compelling evidence that glacial periods are characterized by higher and more variable dust fluxes than interglacial intervals synchronous changes in the mar grain size and 18o records reveal dynamic linkages between asian interior aridity paleowind intensity and ice volume after the onset of major northern hemisphere glaciations

geophysical insights into the transition fault debate propagating strike slip in response to stalling yakutat block subduction in the gulf of alaska on the basis of faulting mapped on seismic reflection and bathymetric data seismicity current plate motions and evidence that the yakutat block may be anomalously thick we propose a tectonic model for yakutat pacific interactions including the often debated transition fault to the east deformation associated with the queen charlotte fairweather fault system is extending offshore facilitating westward propagation of strike slip motion along the eastern segment of the transition fault to the west the oblique slip pamplona zone and transition faults merge at an embayment in the continental margin where a north south dextral strike slip fault within the pacific plate illuminated by the 1987 1992 earthquake swarm intersects the pacific yakutat tectonic boundary these fault patterns are consistent with modern plate motions and reflect a plate boundary reorganization that may be caused by resistance to subduction by the yakutat block a possible moderate sized oceanic plateau

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

kurosegawa terrane as a transform fault zone in southwest japan abstract the kurosegawa terrane intervening in the jurassic early cretaceous accretionary complexes along the pacific side of the sw japanese islands is a serpentinite melange zone it contains various kinds of exotic rocks for example granitoids metamorphic rocks siluro devonian deposits and is intimately associated with cretaceous forearc basin deposits the terrane is regarded as a key to clarify the mesozoic geotectonic history of the western circum pacific orogenic belts the current model in which the formation of the kurosegawa terrane is attributed to nappe movement or sinistral strike slip faulting can explain neither the mode of occurrence of the kurosegawa terrane we observed in eastern kii peninsula nor the array of evidence obtained from the ryoke terrane southward to the shimanto terrane we suggest a new hypothesis in which the kurosegawa terrane was a transform fault zone that originated because of oceanic ridge subduction along the southern margin of the coeval accretionary prism butsuzo t l in the late early cretaceous our model is mainly based on new geological evidence from the kurosegawa terrane in eastern kii peninsula where the deepest erosion level is exposed due to neotectonic uplift

travel time of accreted igneous assemblages in western pacific orogenic belts and their associated sedimentary rocks abstract accreted igneous assemblages in orogenic belts maybe divided into three types depending on whether they derive from seamounts ocean ridges or subduction related ophiolites seamount type basalts are associated with shallow water sediments mostly reefoidal limestones ocean ridge type basalts are generally overlain by pelagic cherts subduction related ophiolitic eruptives often underlain by gabbroic and ultramafic rocks are associated with hemipelagic mudstones the age of such diverse eruptive lithologic assemblages reflects the time taken for them to have traveled from their locus of generation to their place of accretion at a continental margin this relationship has been established for each type of accretionary complex examples being taken mostly from japan and the western pacific rim in order to represent evolutionary processes at a typical active plate margin in general the seamount types are older ridge types are of intermediate age and the ophiolitic types are by far the youngest usually close to zero age seamount type basalts are accreted by shallower scraping of the seamount s sediment apron together with fragments of seamount basalt ridge type by peeling due to permeability contrast and the ophiolitic types by deeper scraping as a consequence of an inflected temperature gradient accordingly it is concluded that the ophiolitic rocks are generated close to the trench and may be accreted as a result of ridge subduction

imbricate architecture of the upper paleozoic to jurassic oceanic cache creek terrane central british columbia upper paleozoic to lower jurassic oceanic rocks of the cache creek terrane near fort st james in central british columbia form a stack of thrust sheets cut by steeply dipping strike slip faults paleontologically dated upper paleozoic strata include bioclastic shallow water limestone and ribbon chert isotopically dated permian rocks consist of tonalite sills and stocks and rhyolite flows intercalated with basalt flows paleontologically dated lower mesozoic rocks include greywacke sandstone siltstone argillite ribbon chert conglomerate limestone and basalt tuff trembleur ultramafite unit of the cache creek complex in places part of an ophiolite suite forms thrust sheets and klippen that overlie lower mesozoic sedimentary rocks sedimentological lithochemical paleontological petrological and textural comparisons with other areas and established models demonstrate that cache creek terrane is an accretionary complex a structurally stacked assemblage of rocks that originated in diverse and

late cenozoic uplift in southern asia and the american west rationale for general circulation modeling experiments geologic evidence suggests significant increases in uplift rates and absolute elevation in several areas during the late cenozoic especially in the last 10 m y regions of major uplift include the tibetan plateau and himalayan mountains of southern asia and the sierra nevada colorado plateau basin and range rocky mountains and high plains of the american west uplift occurred at a scale large enough to affect regional and global climate in a number of ways several of which we evaluate by general circulation model experiments in accompanying papers

modulation of orbitally forced enso variation by tibetan plateau topography abstract the modulation of the orbitally induced variation in the el nino southern oscillation enso by tibetan plateau tp topography was investigated with four fully coupled ocean atmosphere model experiments under the modern boundary conditions the modeled orbitally induced enso change was strongly affected by the presence of the tp specifically in the experiments with orbital forcing at 6 ka in comparison to the counterpart experiments with modern orbital forcing it was shown that the orbitally forced enso variability was slightly weakened by 3 in the presence of the tp but increased significantly by 13 in the simulation of the absence of the tp this difference was primarily attributed to the anomalous zonal winds over the tropical central to eastern pacific in response to the increased summertime incoming insolation which was intrinsically related to the different changes in the asian monsoon and tropical pacific walker circulation under the absence of modulation by the tp the anomalous westerly winds dominated the equatorial pacific and favored the development of a stronger el nino event and in turn the further intensification of la nina due to the stronger heat discharge during the recession of the stronger el nino in contrast the simulation with the presence of the tp showed the enhanced easterly wind anomalies in the boreal summer over the tropical pacific which further reduced increased the magnitude of the el nino la nina event in the subsequent winter these results suggest that the tp is a potentially crucial boundary condition for evaluating the changes in the enso dynamics on orbital scales during the cenozoic

the tibetan plateau as amplifier of orbital scale variability of the east asian monsoon 1 asian monsoon climate variability at geological time scales is modulated by both the earth s orbital changes and tectonic uplift of the tibetan plateau tp here using bandpass filtered versions of previously published high resolution geological records from chinese loess we show that the orbital scale variability of the east asian summer monsoon easm has increased dramatically since the late pliocene climate model simulations indicate that the increase in the variability of the northern easm at both precession and tilt periods may be at least partially attributed to the uplift of the tp

ophiolite accretion in the lachlan orogen southeastern australia slivers of dismembered cambrian ophiolites are preserved in major fault zones of the turbidite dominated western and central lachlan orogen of southeastern australia geometrical chronological and metamorphic constraints indicate that these slivers did not undergo classic tethyan style obduction but were incorporated into evolving turbidite wedges as offscraped slices imbricated fault slivers or duplexes and as blocks in melange the processes involved include decoupling of the oceanic crustal stratigraphy predominantly above the moho formation and metamorphism of serpentinite talc matrix melange up to blueschist conditions and accretion via underplating or offscraping during underthrusting these processes have produced major large scale fault zones within the chevron folded turbidites that show marked along strike variation in structural complexity that relate to magnitude of displacement the fault zones may have formed preferentially where topographic highs occurred in the oceanic crust promoting fold and fault nucleation within irregularities in the layering these fault zones have predominantly brittle character due to their formation under a low geothermal gradient duplex like structures in the fault zones and the inferred development through buckling of then faulting of the oceanic lithosphere have analogues in modern oceanic environments e g indian ocean and young subduction systems e g sulu sea southeast asia they are interpreted to have formed due to convergence in a non collisional intra oceanic setting during closure of a cambrian backarc basin

transient simulation of the tibetan plateau modulated distinct orbital scale precipitation variation in east and south asia abstract the insolation change caused by the earth s orbital forcing is known to play an important role in regulating variations of the asian monsoon at various time scales geological records and numerical simulation revealed that the climate change in the asian monsoon region was in phase or out of phase with the boreal summer insolation at the orbital scale using the community climate system model version 3 and the orbital acceleration technique we conducted two long term transient simulations with and without the tibetan plateau tp orography tp1 and tp0 experiments the main objective of this study was to examine the influence of the tp uplift on the orbital scale precipitation in east and south asia and the anti phase relationship between southern and northern east asia sea and nea during the past 150 kyr the results further confirm that the uplift of the tp significantly affects the asian atmospheric circulation and precipitation and is an amplifier of the asian summer monsoon at the precessional scale in general the presence of the tp causes the rain belt and atmospheric circulation front to shift northward in particular we found that the anti phase relationship of precipitation between nea and sea at the precessional band is also controlled by the uplift of the tp simulations in tp1 and tp0 experiments show that the inverse variations in atmospheric circulation including the 850 hpa wind field and vertical pressure velocity caused by the uplift of the tp can directly lead to the out of phase jja precipitation at the precessional scale between nea and sea future research to collect various climatic proxies with high spatial and temporal resolutions at least since the cenozoic is required to support the simulation results of this study

hydrogen isotope ratios of recent lacustrine sedimentary n alkanes record modern climate variability abstract hydrogen isotope ratios were measured on n alkanes n c12 to n c31 extracted from recent lake surface sediments along a n s european transect to test if modern climate variability is recorded in these biomarkers d values of the n alkanes are compared to d values of meteoric water from the iaea gnip database spanning a range from 119 in northern sweden to 41 in southern italy to lake water d values and to mean annual temperatures varying between 2 0 c in the north and 13 7 c in the south d values of the short chained n alkanes n c12 to n c20 excluding algal derived n c17 and n c19 are higher in the north and lower in the south the isotopic fractionation e for hydrogen between meteoric water and the short chained n alkanes is increasing from n to s by more than 100 and is significantly correlated to mean annual temperature for n c16 and n c18 this suggests that these n alkanes may originate from a different source in the northern lakes possibly due to petroleum contamination or are synthesized using a different biochemical pathway the n c17 and n c19 alkanes of algal origin the n c21 and n c23 alkanes originating from water plants and the long chain n alkanes n c25 n c27 n c29 and n c31 of terrestrial origin clearly correlate with d values of meteoric water lake water and mean annual temperature indicating that they excellently record the d value of meteoric water the mean hydrogen isotope fractionation ec17 w of 157 sd 13 between n c17 and meteoric water is fairly constant over the wide range of different climates and lake environments suggesting only minor influence of environmental factors on this biochemical fractionation this suggests that d values of n c17 are suitable to reconstruct the isotopic composition of source water the mean fractionation between the long chain n alkanes and water is 128 sd 12 the mean difference of 31 between both e values is likely due to evaporative enrichment of deuterium in the leaf water if this is the only influence on the enrichment the difference between the d values of terrestrial and aquatic compounds might be suitable to reconstruct terrestrial evapotranspiration of the lake environment

tectonic melange as fault rock of subduction plate boundary abstract an assemblage of quantitative data sets is examined to evaluate tectonic melange as a plate boundary fault rocks in subduction zone the research object is the latest cretaceous mugi melange in the shimanto belt southwest japan systematic age younging from pelagic to terrigenous through hemipelagic sediments is well documented even though original stratigraphy is disrupted systematic shear fabric consistent with ancient plate convergence is reconstructed the melange was formed at temperatures of 130 200 c by cataclastic comminution of sandstone layers accompanied by tensile cracking and plastic deformation and the dehydration of clayey shale matrix with subsequent peeling off and underplating of the oceanic basement the temperature setting for the mugi melange indicates around the up dip limit of the seismogenic zone therefore includes various fault rocks suggestive of earthquake fault pseudotachylyte fluidized ultracataclasite with heating evidence amorphous silica and so on these suggest that fluid induced lubrication was dominated localized cataclastic shear which is a candidate of small earthquake or very low frequency earthquake is also recognized especially in sandstone blocks dominated portion in melange these observations are consistent with the melange being a fault rock along the plate boundary that records various types of earthquakes in a subduction zone the quantitative examination of the mugi melange suggests several criteria to define the tectonic melange of the plate boundary fault in subduction zone from other melanges in orogenic belt

impacts of grain size sorting and chemical weathering on the geochemistry of jingyuan loess in the northwestern chinese loess plateau abstract major and trace elemental compositions of loess samples collected from the jingyuan section in the northwestern chinese loess plateau clp were analyzed to investigate the potential impacts of grain size sorting and chemical weathering on the loess geochemistry and to extract appropriate geochemical indices for better evaluating the east asian monsoon variability based on variations of major and trace elements in different grain size fractions seventeen elements were classified into three types 1 si and na display higher contents with the increased particle sizes 2 al fe mg k mn zn rb cr v are mainly enriched in fine size fractions 3 ti ba zr p ca and sr show irregular variations among different size fractions comparison of al normalized elemental ratios with zr rb and rb sr ratios two commonly employed indicators for grain size sorting and pedogenic weathering indicates that si al zr al ti al variations match well with zr rb and grain size results whilst ca al sr al p al ratios display similar variability as that of rb sr ratio comparison of loess based proxies e g elemental ratios magnetic susceptibility grain size of jingyuan section with speleothem and ice core records confirms that elemental ratios of high resolution loess sequences developed in the northwestern clp can be employed to address fluctuations of the winter monsoon induced grain size sorting and summer monsoon related weathering and pedogenesis at glacial interglacial and millennial timescales

the subduction plate interface rock record and mechanical coupling from long to short timescales abstract short and long term processes at or close to the subduction plate interface e g mineral transformations fluid release seismicity and more generally deformation might be more closely related than previously thought increasing evidence from the fossil rock record suggests that some episodes of their long geological evolution match or are close to timescales of the seismic cycle this contribution uses rocks recovered episodically from subduction zones together with insights from thermomechanical modelling to provide a new dynamic vision of the nature structure and properties of the plate interface and to bridge the gap between the mechanical behavior of active subduction zones e g coupling inferred from geophysical monitoring and fossil ones e g coupling required to detach and recover subducted slab fragments based on critical observations and an exhaustive compilation of worldwide subducted oceanic units for which the presence near the plate interface rock types pressure temperature t p gradients thickness and timing of detachment can be assessed the present study demonstrates how long term mechanical coupling exerts a key control on detachment from the slab and potential rock recovery critical assessment of rock t p characteristics indicates that these fragments can indeed be used as natural probes and provide reliable information on subduction interface dynamics down to 2 8 gpa rock clusters are identified at depths of 30 55 60 and 80 km with some differences between rock types data also reveal a first order evolution with subduction cooling in the first 5 myr which is interpreted as reflecting a systematic trend from strong to weak mechanical coupling after which subduction is lubricated and mostly inhibits rock recovery this contribution places bounds on the plate interface constitution regular thickness

southern hemisphere forcing of pliocene δ18o and the evolution of indo asian monsoons monsoon phase relationships consistent with direct orbital forcing across the entire indo asian region including marine and terrestrial proxies from the chinese loess plateau the south china sea and the arabian sea strong pliocene summer and winter monsoons were in phase with one another strengthened at obliquity minima and precession minima the summer monsoon was also strengthened at precession maxima yielding a semiprecession spectral signal strong pliocene monsoons at orbital extremes indicate a direct response to fast physics processes including sensible heating and cooling of the asian landmass and for the summer monsoon the export of latent heat from the southern indian ocean as northern hemisphere ice volume grew into the pleistocene the timing of strong winter and summer monsoons drifted apart becoming influenced by the combined effects of fast physics and slow physics ice volume variables the phase of strong winter monsoons shifted toward ice maxima and the phase of strong summer monsoons shifted toward ice minima

tectonic slicing of subducting oceanic crust along plate interfaces numerical modeling multikilometer sized slivers of high pressure low temperature metamorphic oceanic crust and mantle are observed in many mountain belts these blueschist and eclogite units were detached from the descending plate during subduction large scale thermo mechanical numerical models based on finite difference marker in cell staggered grid technique are implemented to investigate slicing processes that lead to the detachment of oceanic slivers and their exhumation before the onset of the continental collision phase in particular we investigate the role of the serpentinized subcrustal slab mantle in the mechanisms of shallow and deep crustal slicing results show that spatially homogeneous serpentinization of the sub moho slab mantle leads to complete accretion of oceanic crust within the accretionary wedge spatially discontinuous serpentinization of the slab mantle in form of unconnected patches can lead to shallow slicing of the oceanic crust below the accretionary wedge and to its deep slicing at mantle depths depending on the patch length slab angle convergence velocity and continental geothermal gradient p t paths obtained in this study are compared to natural examples of shallow slicing of the crescent terrane below vancouver island and deeply sliced crust of the lago superiore and saas zermatt units in the western alps

orbital forcing of arctic climate mechanisms of climate response and implications for continental glaciation progress in understanding how terrestrial ice volume is linked to earths orbital configuration has been impeded by the cost of simulating climate system processes relevant to glaciation over orbital time scales 10 3 10 5 years a compromise is usually made to rep resent the climate system by models that are averaged over one or more spatial dimensions or by three dimensional models that are limited to simulating par ticular snapshots in time we take advantage of the short equilibration time 310 years of a climate model consisting of a three dimensional atmosphere coupled to a simple slab ocean to derive the equilibrium climate response to accelerated variations in earths orbital configuration over the past 165 000 years prominent decreases in ice melt and increases in snowfall are sim ulated during three time intervals near 26 73 and 117 thousand years ago ka when aphelion was in late spring and obliquity was low there were also significant decreases in ice melt and increases in snowfall near 97 and 142 ka when eccentricity was relatively large aph elion was in late spring and obliquity was high or near its long term mean these glaciation friendly time intervals correspond to prominent and secondary phases of terrestrial ice growth seen within the marine d 18 o record both dynamical and thermal effects contribute to the increases in snowfall during these periods through increases in storm activity and the fraction of precipi tation falling as snow the majority of the mid to high latitude response to orbital forcing is organized by the properties of sea ice through its influence on radiative feedbacks that nearly double the size of the orbital forcing as well as its influence on the seasonal evolution of the latitudinal temperature gradient

lateral growth of the continental crust through deep level subduction accretion a re evaluation of central greek rhodope abstract the rhodope massif of northern greece and southern bulgaria has been variably interpreted as a fragment of pre alpine possibly hercynian or precambrian age continental crust that was actively involved in european margin convergent tectonics during the mesozoic and tertiary alternatively we propose that for central greek rhodope the lithological associations and tectonothermal history are consistent with progressive south directed mesozoic crustal growth through deep level subduction accretion a process which involved the near continuous transferral of material of oceanic and continental margin affinity from footwall to hanging wall within a north directed palaeotethyan subduction system central greek rhodope comprises a sequence of schists gneisses and migmatites of wide compositional variation that have undergone intense tectonic intermixing extensive occurrences of marble are associated with amphibolites variably retrogressed eclogites meta gabbros and ultramafic rocks quartzo feldspathic gneisses are abundant throughout the area and are associated with psammitic meta greywacke sequences pelites and quartzites the presence of intercalated marble slivers and thin manganiferous and iron rich siliceous bands is significant the various components exposed in the subduction accretion complex were first underthrust northwards to deep crustal levels subjected to high pressure eclogite facies metamorphism and underplated to the base of the overriding eurasian plate this latter accretion event was characterised by intense southwest directed ductile thrusting and pervasive amphibolite facies metamorphic overprinting it was accompanied by migmatisation and intracrustal melt generation and followed by subduction related magmatism deep level subduction accretion leads to southward growth of the continental margin which in turn leads to subduction retreat allowing the encroachment of the magmatic arc from the north subsequent extensional tectonic exhumation brought the complex up to its present position within the southern margin of eurasia this new interpretation may have implications for other high grade terrains within convergent and collisional systems that have been assumed to represent ancient or pre existing continental crust if so then the importance of deep level subduction accretion as a mechanism of continental crustal growth has hitherto been underestimated

sensitivity of eurasian climate to surface uplift of the tibetan plateau experiments with climate models are used to illustrate how earth s climate could be expected to change if surface uplift occurred the experiments contrast two climate simulations one with all continents at sea level and one with present day mountains and plateaus the simulations in the eurasia sector show that surface uplift increased elevation produces marked increases in the intensity of summer and winter monsoons of asia with development of wetter climates to the south and east of the tibetan plateau and drier climates to the north and west of the plateau there is a general cooling in northern polar latitudes comparison of simulations from two different models shows that the magnitude of the simulated changes depends upon how the models calculate the land surface hydrology and the ocean temperature the simulated changes produced by surface uplift agree in a general way with many geologic indicators of late cenozoic climate change the results suggest that surface uplift of mountains and platea

enso modulation by mountain uplift how climate changes will modify the behavior of el nino southern oscillation enso is one of the important questions in future climate projections an investigation under different climate forcing gives us a good insight on the mechanism of enso variability and its changes in this paper sensitivity on enso by progressive mountain uplift is investigated with an atmosphere ocean coupled general circulation model we used eight different mountain heights 0 no mountain 20 40 60 80 100 control run 120 and 140 land sea distribution is the same for all experiments and all mountains in the world are uniformly varied systematic changes in precipitation and circulation fields as well as sst are obtained with progressive mountain uplift in the summertime the precipitation area moved inland of the asian continent with mountain uplift while the pacific subtropical anticyclone and associated trade winds became stronger the western pacific warm pool and enso also systematically changed when the mountain height is low a warm pool is located over the central pacific due to weak trade winds in the pacific the model enso is strongest its frequency longest and is most periodic in the no mountain run the model enso becomes weaker shorter and less periodic when the mountain height increases strengthening the mean state trade winds and narrowing meridional extent of equatorial wind and ocean response by mountain uplift would be responsible for enso modulation

depth of oceanic crust underplating in a subduction zone inferences from fluid inclusion analyses of crack seal veins fluid inclusions in crack seal veins are analyzed in accreted melange now on land the vein i inclusions are observed in the necked parts of sandstone blocks in melange and the vein ii inclusions developed in shale below the thrust fault which cuts the melanges the pressure and temperature estimations from the inclusions within vein i show that the cracks were sealed at 125 195 8c under 92 144 mpa of fluid pressure vein ii formed in cracks that might have opened in a damage zone caused by the thrust fault as it broke through oceanic crust and into the melange these veins contain fluid inclusions trapped at 135 245 8c under 107 149 mpa of fluid pressure the depth 4 6 km below the seafloor and temperature estimates are consistent with the conditions where an aseismic decollement within sedimentary rocks steps down into the oceanic basement so that a thin section of oceanic crust underplates in the hanging wall such a step down site is the updip limit of the seismogenic zone in the modern nankai trough

lip reading recognizing oceanic plateaux in the geological record basaltic oceanic plateaux are important features in the geological record not only do they record ancient mantle plume activity but they also are believed to be important building blocks in the formation of the continental crust in this paper we review the salient features of two cretaceous oceanic plateaux the ontong java and the caribbean colombian thick sequences of predominantly homogeneous basalt the occurrence of high mgo basalt including komatiites and an apparent absence of sheeted dyke complexes in addition pyroclastic deposits may be scarce we then explore ways of distinguishing plateaux from basaltic sequences erupted in different tectonomagmatic settings continental flood basalt provinces island arcs back arc basins ocean islands and mid ocean ridges using these criteria potential archaean and proterozoic oceanic plateaux are reviewed and identified finally we explore how these remnant oceanic plateaux became incorporated into the continents by reviewing the proposed accretion mechanisms for the cretaceous caribbean colombian oceanic plateau on the basis of evidence from south america and the tonalites of the southern caribbean island of aruba

reconstruction of ocean plate stratigraphy in the gwna group nw wales implications for the subduction accretion process of a latest proterozoic trench forearc abstract the gwna group in anglesey island and lleyn peninsula wales consists of a latest proterozoic volcano sedimentary trench melange which has a complicated accretionary structure and is poorly constrained by isotopic ages the melange contains oceanic trench rocks including pillow basalts cherts mudstones and sandstones which have not previously been interpreted as ocean plate stratigraphy ops we reconstructed imbricated ops at 5 localities in the coastal lleyn peninsula in order to constrain the depositional u pb age of the upper clastic sediments detrital zircons separated from 9 clastic sediments were analyzed with a nu attom single collector inductively coupled plasma mass spectrometer the ages indicate that there are two gwna groups maximum depositional ages of 1 at 608 601 ma and 2 at 564 539 ma that were deposited between the late neoproterozoic and the middle cambrian contemporaneously with dated calc alkaline arc magmatism and regional metamorphism in the anglesey lleyn complex the age spectra of the detrital zircons show a prominent peak at ca 650 600 ma and several proterozoic and archean ages to account for the older ages we integrated our new isotopic data with published radiometric and fossil ages and conclude that the clastic sediments at the top of the ops were deposited in a trench on the western active margin of avalonia when it was close to the amazonian craton and that the gwna group ops began to be incorporated into an accretionary wedge in an active subduction zone in the latest proterozoic

the asian monsoon over the past 640 000 years and ice age terminations oxygen isotope records from chinese caves characterize changes in both the asian monsoon and global climate here using our new speleothem data we extend the chinese record to cover the full uranium thorium dating range that is the past 640 000 years the record s length and temporal precision allow us to test the idea that insolation changes caused by the earth s precession drove the terminations of each of the last seven ice ages as well as the millennia long intervals of reduced monsoon rainfall associated with each of the terminations on the basis of our record s timing the terminations are separated by four or five precession cycles supporting the idea that the 100 000 year ice age cycle is an average of discrete numbers of precession cycles furthermore the suborbital component of monsoon rainfall variability exhibits power in both the precession and obliquity bands and is nearly in anti phase with summer boreal insolation these observations indicate that insolation in part sets the pace of the occurrence of millennial scale events including those associated with terminations and unfinished terminations records of the asian monsoon have been extended to 640 000 years ago and confirm both that the 100 000 year ice age cycle results from integral numbers of precessional cycles and that insolation influences the pacing of major millennial scale climate events prior records of the asian monsoon have revealed cyclic variations over hundreds of thousands of years probably driven by variations in insolation caused by the precession of earth s orbit hai cheng and colleagues now provide a speleothem record from chinese cave samples that extends earlier records to 640 000 years ago close to the maximum age possible with uranium thorium dating this spectacular record confirms that the characteristic 100 000 year ice age cycle corresponds to an integral number four or five of precession cycles and that insolation influences millennial scale variations in monsoon strength

seismic evidence for active underplating below the megathrust earthquake zone in japan determining the structure of subduction zones is important for understanding mechanisms for the generation of interplate phenomena such as megathrust earthquakes the peeling off of the uppermost part of a subducting slab and accretion to the bottom of an overlying plate underplating at deep regions has been inferred from exhumed metamorphic rocks and deep seismic imaging but direct seismic evidence of this process is lacking by comparing seismic reflection profiles with microearthquake distributions in central japan we show that repeating microearthquakes occur along the bottom interface of the layer peeling off from the subducting philippine sea plate this region coincides with the location of slow slip events that may serve as signals for monitoring active underplating

plumes and hotspots as sources of greenstone belts abstract greenstone belts that are older than 0 18 ga formed under circumstances that generate a buoyant garnet poor root because they do not generate enough depletion in the lithospheric mantle juvenile arcs involving average temperature early precambrian mantle were not a significant source of early precambrian greenstone belts instead early precambrian greenstone belts originated from mantle plumes and hotspots we focus on the origin of highly deformed morb like greenstone belts that are underlain by tonalitic to trondhjemitic plutons and surrounded by metamorphosed accretionary wedge like sediments these greenstone belts could form as the consequence of subducting a ridge segment within 500 km of a hotspot located beneath the continent as has happened on the taitao peninsula in chile the tonalitic to trondhjemitic plutons result from partial melting of oceanic crust thickened by extra melt derived from the hotspot the morb like rocks result from decompressive partial melting within the slab window produced by the subducting ridge the stratigraphy and tectonic setting of the recently emplaced taitao ophiolite are a possible phanerozoic analog of some morb like archean and early proterozoic greenstone belts

interpreting vegetation change in tropical arid ecosystems from sediment molecular fossils and their stable isotope compositions a baseline study from the pilbara region of northwest australia detection of source diagnostic molecular fossils biomarkers within sediments can provide valuable insights into the vegetation and climates of past environments however hot and arid regions offer particularly challenging interpretive frameworks for reconstructions because baseline data are scarce organic matter is generally very low and in the inland tropics in particular sediments are also often subject to flooding and drought here we investigated whether biomarkers and compound specific 13c values could be extracted from a late holocene sediment record from the fortescue marsh pilbara northwest australia to allow interpretation of past catchment vegetation and hydroclimate the low total carbon tc content 1 4 was a major challenge for the molecular analyses over the 2000 years old sequence nevertheless they revealed that the dominant hydrocarbon features e g long chain n alkanes indicative of terrestrial plants e g c4 grasses riparian and other c3 plants encompassed the last 1300 yrs and that low abundance of products from aquatic sources e g n c17 were detected in the uppermost sediments only when permanently inundated conditions prevailed recent decades similarly the lower 13c values i e a difference of 2 3 of long chain n alkanes in upper sediments reflected a vegetation response to the emergence of wetter conditions through the late holocene in the region based on the diverging dominant source contributions obtained from the molecular distributions and arid based bayesian mixing model 13c of n c27 33 alkanes results less arid conditions may have favoured the input of 13c depleted n alkanes from the eucalyptus c3 dominant riparian vegetation the deepest sediments 700 ce however had a tc content of 0 4 and no organic compounds were detected consistent with local and regional records of hyperarid conditions these results demonstrate that n alkanes can provide a molecular and stable isotopic fingerprint of important and perhaps underappreciated ecological processes in modern tropical arid environments for future paleoclimate investigations

noble gas constraints on the evolution of the atmosphere mantle system abstract we present a model on the evolution of atmosphere mantle system assuming a significant degassing from the less depleted mantle based on the recent results of the he ar systematics proposed by matsuda and marty 1995 the degassing fluxes of noble gases are represented by the concentrations in the mass flow which follows the model of porcelli and wasserburg 1995a porcelli and wasserburg 1995b however we have not assumed the steady state and have assumed the mass transfer as an exponential function of time the degassed amount from the less depleted mantle is related to the mixed amount of mass between the less depleted mantle and the depleted mantle the amount of mixing is constrained from the k abundances in each mantle reservoir consequently the degassed amount from the less depleted mantle is also constrained the following conclusions are obtained from this model the initial elemental ratio 3 he 36 ar is likely to be in cosmic abundances and the initial concentration of 36 ar in the earth is constrained to be 1 10 12 atoms g the present fraction of 36 ar in the depleted mantle reservoir is calculated to be at most 2 4 10 3 of the initial inventory the 40 ar 36 ar ratio in this reservoir is estimated to be less than 1 3 1 4 10 5 the less depleted mantle reservoir also degassed and the present amount of 36 ar is calculated to be at most 1 3 10 1 of the initial inventory the 40 ar 36 ar ratio in the less depleted mantle reservoir is estimated to be less than 3 7 10 4 to explain the difference between the neon isotopic ratios in the atmosphere and those observed in the mantle rocks isotopic fractionation during the escape from the atmosphere and or significant contribution of late accreted materials possessing planetary ne is necessary in our model ne degassing from both mantle reservoirs would have provided at least 50 90 times the amount of 22 ne in the present atmosphere therefore the fraction supplied by late accreted material would be negligible which is contrary to the model proposed by porcelli and wasserburg 1995b we conclude that isotopic fractionation model during escape from the atmosphere provides the best fit to the observations

accretion subduction erosion and tectonic extrusion during late paleozoic to mesozoic orogenesis in ne china abstract the high pressure heilongjiang complex records a long and complicated evolution from the late paleozoic to mesozoic in ne china which is still subject to controversy in this paper we present a new detailed study including structural geochemical and geochronological analyses ar ar dating on metamorphic minerals u pb sims dating on zircons two main periods of oceanward accretionary growth are identified between 305 ma and 195 ma and around 112 101 ma these periods of accretionary growth were associated with the emplacement of voluminous magmatism slab roll back with continental back arc extension and growth of a forearc accretionary prism between these periods of accretionary growth a major phase of regional subduction erosion took place between 195 ma and 142 ma at the eastern edge of the songliao block the main effects of this subduction erosion phase were that regional jurassic early cretaceous extrusion overlapped with mesozoic accretion and erosion in ne china which disturbed the initial orogenic architecture with a review of relevant zircon ages of this accretionary orogen we unravel the complicated geodynamic history of accretion subduction erosion and tectonic extrusion that shaped ne china from the late paleozoic to mesozoic

dominant control of the south asian monsoon by orographic insulation versus plateau heating heat emitted from the tibetan plateau as dry heat and water vapour has long been assumed to be the main driver of the south asian summer monsoon but new work suggests that in fact it is the neighbouring mountains that are the major influence william boos and zhiming kuang use an atmospheric model to show that flattening the tibetan plateau has little effect on the monsoon so long as the himalayas and surrounding mountain ranges remain the plateau does boost rainfall locally along its southern edge but it is the build up of hot moist air over india insulated from colder drier air by the himalayas that drives large scale monsoon circulation the elevation of the tibetan plateau is thought to cause its surface to serve as a heat source that drives the south asian summer monsoon potentially coupling uplift of the plateau to climate changes on geologic timescales here however an atmospheric model is used to show that flattening of the tibetan plateau has little effect on the monsoon provided that the narrow orography of the himalayas and adjacent mountain ranges is preserved the tibetan plateau like any landmass emits energy into the atmosphere in the form of dry heat and water vapour but its mean surface elevation is more than 5 km above sea level this elevation is widely held to cause the plateau to serve as a heat source that drives the south asian summer monsoon potentially coupling uplift of the plateau to climate changes on geologic timescales1 2 3 4 5 observations of the present climate however do not clearly establish the tibetan plateau as the dominant thermal forcing in the region peak upper tropospheric temperatures during boreal summer are located over continental india south of the plateau here we show that although tibetan plateau heating locally enhances rainfall along its southern edge in an atmospheric model the large scale south asian summer monsoon circulation is otherwise unaffected by removal of the plateau provided that the narrow orography of the himalayas and adjacent mountain ranges is preserved additional observational and model results suggest that these mountains produce a strong monsoon by insulating warm moist air over continental india from the cold and dry extratropics these results call for both a reinterpretation of how south asian climate may have responded to orographic uplift and a re evaluation of how this climate may respond to modified land surface and radiative forcings in coming decades

assembling north china craton within the columbia supercontinent the role of double sided subduction abstract an evaluation of recent s wave receiver functions s wave velocities and two versions of p wave tomographic images along various transects in the north china craton provides some clues on the subduction collision history of the different crustal blocks and their final amalgamation within the paleoproterozoic columbia supercontinent interpretation of a n s seismic section of the craton suggests thick slab debris sinking to various depths in the mantle the w e seismic corridors show the preservation of a thick 200 km lithospheric root tectosphere beneath the ordos block and its variable and extensive erosion towards the yanliao block eastern block this zone is characterized by layers with marked velocity contrast and suggests repeated stacking of the remnants of underplated and accreted paleoproterozoic oceanic lithosphere the present day lithosphere asthenosphere boundary beneath this region probably marks the erosional plane along which decratonization occurred through subduction erosion from the east and thermal and material erosion by upwelling asthenosphere from below resulting in the partial destruction of the tectosphere and its thinning towards the east within the asthenosphere below the yanliao block younger and thinner slabs predominate in the absence of any prominent thick high velocity layers these younger slabs define a westward polarity and constitute a mega scale duplex formed by underplating through phanerozoic subduction process particularly the pacific plate subduction from the east the lithologic associations within the inner mongolia suture zone dividing the yinshan block to the north and ordos block to the south correspond to an accreted ocean plate stratigraphic sequence with the tonalite trondhjemite granodiorite ttg gneisses charnockites and calc alkaline granites representing a continental arc built up through subduction from the north the seismic transects bring out a contrasting polarity in the subduction regime with an oblique east to southward subduction of the yinshan block and a westward subduction of the yanliao block here i propose a double sided subduction history for the ncc similar to the ongoing subduction process in the western pacific such double sided subduction is considered to promote rapid amalgamation of continental fragments within supercontinents and the subduction polarities and mantle dynamics of ncc are therefore considered to be critical in evaluating the final assembly of the paleoproterozoic supercontinent columbia

structural anatomy of the early paleozoic laohushan ophiolite and subduction complex implications for accretionary tectonics of the proto tethyan north qilian orogenic belt northeastern tibet

diapirism and structural thickening in an early palaeozoic subduction complex southeastern new south wales australia abstract a major problem in the study of subduction complexes has been to distinguish between processes of mud diapirism deformation of unlithified sediments frontal accretion and underplating in the batemans bay area cambro ordovician rocks of the south coast of new south wales eastern australia contain common mud rich melanges that occur amongst an imbricated succession of early to middle ordovician quartz turbidites late ordovician black mudstone and chert cambrian limestone and mafic volcanics an early set of fluidal obliquely trending folds are restricted to the late ordovician black mudstone and chert and probably formed by gravitational down slope movement on a lower trench slope features indicative of deformation of unlithified sediments are still preserved and include mud injections delicate small scale faulting and the absence of grain scale deformation tight to isoclinal north south folds with axial planar west dipping cleavage overprint all units and formed during the younger main phase deformation that post dated early deformation in the prism the melange has been strongly overprinted by this main phase deformation early melange formation is attributed to mud diapirism involving intrusion along early accretionary faults bedding planes and in pipe like features resulting in chaotic outcrop and map patterns

deep plate serpentinization triggers skinning of subducting slabs most of the present day ocean floor is continuously being consumed in subduction zones but large fragments of oceanic crust ophiolites have also been recognized on land the process by which oceanic crust is separated from the subducting slab remains enigmatic and several competing hypothesis have been proposed in the past based on numerical experiments we suggest that serpentinized mantle formed in the outer rise regions of subduction zones may provide a mechanically weak horizon within which basal detachment of the oceanic crust is feasible deformation of this serpentinized layer may lead to decoupling and separation of oceanic crust from the downgoing slab fragments of the former oceanic crust can underplate the accretionary wedge or be exposed on continental crust whereas the skinned lithospheric part of the slab subducts into the mantle

bainang terrane yarlung tsangpo suture southern tibet xizang china a record of intra neotethyan subduction accretion processes preserved on the roof of the world the bainang terrane an intra oceanic island arc subduction complex into which tethyan oceanic rocks were accreted during the cretaceous is preserved within the yarlung tsangpo suture zone of tibet the lithostratigraphic succession established from field mapping records a long history of sedimentation in different portions of the central tethyan domain from late triassic to mid cretaceous time these rocks are preserved within a south verging imbricate thrust stack of thin 1 km thick northward younging tectonic slices five lithotectonic units were mapped in the terrane and these units are assigned to two distinct tracts the northern tract which accumulated on the north side of neotethys was probably separated from its southern counterpart by a mid ocean ridge detailed radiolarian biostratigraphy is used to constrain the timing of depositional events within each tract oceanic plate stratigraphy of the northern tract records its northward travel and mid cretaceous late aptian approach towards a south facing intra oceanic subduction zone rocks in the southern tract developed closer to the indian subcontinent and experienced thermotectonic subsidence and mid jurassic basic alkaline intraplate magmatism they were probably accreted late in the cretaceous variations in structural style across the terrane indicate deformation at different depths and vertical growth of the wedge rather than lateral accretion the overall tectonostratigraphy of the terrane reflects its development in a remote intra oceanic setting

modeling the influence of a reduced equator to pole sea surface temperature gradient on the distribution of water isotopes in the early middle eocene proxy based climate reconstructions suggest the existence of a strongly reduced equator to pole temperature gradient during the azolla interval in the early middle eocene compared to modern changes in the hydrological cycle as a consequence of a reduced temperature gradient are expected to be reflected in the isotopic composition of precipitation d 18o the interpretation of water isotopic records to quantitatively reconstruct past precipitation patterns is however hampered by a lack of detailed information on changes in their spatial and temporal distribution using the isotope enabled version of the national center for atmospheric research ncar atmospheric general circulation model community atmosphere model v 3 isocam3 relationships between water isotopes and past climates can be simulated here we examine the influence of an imposed reduced meridional sea surface temperature gradient on the spatial distribution of precipitation and its isotopic composition in an early middle eocene setting as a result of the applied forcings the eocene simulation predicts the occurrence of less depleted high latitude precipitation with d values ranging only between 0 and 140 compared to present day 0 to 300 comparison with early middle eocene age isotopic proxy data shows that the simulation accurately captures the main features of the spatial distribution of the isotopic composition of early middle eocene precipitation over land in conjunction with the aspects of the modeled early middle eocene climate hence the included stable isotope module quantitatively supports the existence of a reduced meridional temperature gradient during this interval

the structure and kinematics of an imbricate stack of oceanic rocks in the jurassic accretionary complex of central japan an oblique subduction model the jurassic accretionary complex of the mino belt in the takayama area central japan includes a major unit characterized by abundant bedded chert and basalt slabs this unit has an imbricate structure represented by the repetition of stratigraphic layers composed of sandstone black mudstone felsic tuff bedded chert siliceous claystone limestone and basalts shear zones subparallel to the trend of the slabs occur in the siliceous claystones and basalts deformation structures in the shear zones suggest that phyllosilicate rich layers within the accretionary complex were preferentially weakened due to grain size reduction by cataclasis and pressure solution accommodated deformation restored shear directions in the siliceous claystones and basalts indicate dextral kinematics subparallel to the trench axis in contrast mesoscopic asymmetric folding of the bedded chert indicated from the restored shear was oblique or normal to the trench axis to interpret the two distinct shear directions a model is proposed in which the constituent rocks of this unit were obliquely subducted dextrally along the east asian continental margin due to the oblique subduction slip vectors were apparently partitioned into trench orthogonal components in the shallower part of the subduction zone and trench parallel components in the deeper part

dynamic role of tectonic melange during interseismic process of plate boundary mega earthquakes abstract tectonic melange has a key role in subduction zones because its thick pile forms the plate boundary itself therefore the plate boundary process is nearly identical to the melange forming process we examined three tectonic melanges in the shimanto belt in southwest japan to decipher their progressive deformation process with subduction especially within the seismogenic zone here we report detailed observation of sandstone blocks and the strain history of the shale matrix of a sediment dominated tectonic melange the necessity of tectonic melange accompanying fossil seismogenic decollement is unveiled several deformation processes continue by turn until the depth of the down dip limit of the seismogenic zone is reached the results support the space time partition of deformation in terms of seismic behavior and suggest a possible candidate for a geological consequence of recently observed slow earthquakes

paleozoic accretionary orogenesis in the eastern beishan orogen constraints from zircon u pb and 40ar 39ar geochronology abstract the continental growth mechanism of the altaids in central asia is still in controversy between models of continuous subduction accretion versus punctuated accretion by closure of multiple oceanic basins the beishan orogenic belt located in the southern altaids is a natural laboratory to address this controversy key questions that are heavily debated are the closure time and subduction polarity of former oceans the emplacement time of ophiolites and the styles of accretion and collision this paper reports new structural data u pb and ar ar ages from the eastern beishan orogen that provide information on the accretion process and tectonic affiliation of various terranes our geochronological and structural results show that the younging direction of accretion was northwards and the subduction zone dipped southwards under the northern margin of the shuangyingshan micro continent this long lived and continuous accretion process formed the hanshan accretionary prism our field investigations show that the emplacement of the xiaohuangshan ophiolite was controlled by oceanic crust subduction beneath the forearc accretionary prism of the shuangyingshan mazongshan composite arc to the south moreover we address the age and terrane affiliation of lithologies in the eastern beishan orogen through detrital zircon geochronology of meta sedimentary rocks we provide new information on the ages subduction polarities and affiliation of constituent structural units as well as a new model of tectonic evolution of the eastern beishan orogen the accretionary processes and crustal growth of central asia were the result of multiple sequences of accretion and collision of manifold terranes

melange and its seismogenic roof decollement a plate boundary fault rock in the subduction zone an example from the shimanto belt japan 1 the mugi melange located in western shikoku of the shimanto belt shows systematic y p deformation fabrics formed by microshear and pressure solution that penetrate throughout the melange pile magnetic susceptibility ellipsoids obtained from the anisotropy of magnetic susceptibility ams are highly oblate maximum and minimum axes of the ellipsoids are consistent with the shear orientation of the melange and the mean pole of p surfaces respectively this agreement suggests that the mugi melange was formed as a result of underthrusting of trench filling sediment vitrinite reflectance ranges from 2 52 to 3 08 which corresponds to a maximum paleotemperature of 180 200 c pseudotachylyte evidence of a seismogenic slip was found in the upper boundary roof fault of the mugi melange however there is not a thermal gap between the melange and the overlying coherent piles and the temperature from vitrinite reflectance gradually rises downward from the coherent piles to the melange beyond the boundary fault which suggests that paleoisotherms parallel the boundary fault orientation the isotherms in the seismogenic zone are estimated as subparallel to the plate boundary decollement therefore the setting of the cataclastic boundary fault which includes pseudotachylyte appears to be a major plate boundary thrust or a subhorizontal splay fault a probable geologic setting that accounts for the mugi melange and the seismogenic roof fault is partitioning of the slip along the plate boundary fault in space and time interseismic slip in the melange and seismic slip along the roof fault

a new source of water in seismogenic subduction zones 1 seismogenic plate boundary faults at accretionary margins e g the nankai margin southwest japan may occur where the uppermost part of subducting oceanic crust composed of basaltic rocks is in contact with the overriding plate of a lithified accretionary prism the plate boundary faults in ancient accretionary complexes typically record high velocity slip under fluid rich conditions although previous studies have emphasized the mechanical significance of fluids in terms of dynamic slip weakening the source of fluid in seismogenic subduction zones remains poorly constrained in this work we focus on the hydrous smectite in the uppermost oceanic crust an alteration product of intact basalt before arrival at the trench axis a comparison between 1 new mineralogical data on basalt drillcore recovered by integrated ocean drilling program iodp expedition 322 at site c0012 a reference site for subduction input to the nankai trough and 2 mineralogical data on basalt within ancient oceanic crust embedded in a fossil accretionary complex of the shimanto belt southwest japan suggests that progressive smectite chlorite conversion would liberate bound fluids at a rate of 0 34 to 0 65 10 14 s 1 along the plate interface this rate of fluid production appears to be more than an order of magnitude greater than that from other possible sources including from overlying sediments via smectite illite conversion and the expulsion of pore fluids and may facilitate seismic slip along plate boundary faults

sedimentary evidence of intense hurricane strikes from new jersey nine vibracores from the backbarrier marsh at whale beach new jersey reveal three large scale overwash deposits associated with historic and prehistoric storms the uppermost and smallest overwash fan was deposited in the ash wednesday northeaster of march 5 8 1962 a second more substantial overwash fan between 100 and 35 cm depth dates to the late eighteenth or early nineteenth century this fan was most likely deposited during the 1821 hurricane the only intense hurricane to make landfall in new jersey in the past 350 yr a third larger overwash fan between 200 and 300 cm depth was deposited between a d 1278 and 1438 and is likely the result of a prehistoric intense hurricane strike the combination of historical and stratigraphic evidence indicates that two intense hurricanes winds 50 ms 21 have likely made landfall on the southern new jersey coast in the past 700 yr resulting in an annual landfall probability of 0 3

the formation and fate of large oceanic igneous provinces abstract large igneous provinces are conspicuous features of late phanerozoic geology and include continental flood basal rifted continental margin volcanic sequences and oceanic plateaus the latter are formed in an environment which typically recycles back into the mantle on a time scale of 200 m y but because comparisons have recently been made between oceanic plateaus and precambrian greenstone belt sequences new questions arise about their formation their fate and their preservation here we review some critical aspects of three oceanic plateaus ontong java kerguelen and the caribbean colombian abducted plateau and comment on their make up and the factors governing their preservation with particular relevance to ancient terranes many large igneous provinces can be linked to mantle plumes where plumes ascend beneath spreading ridges their energy is transformed into a large melt volume producing over thickened plateau crust where the spreading rate is low in relation to magma supply the plateau may become subaerial e g iceland but with fast spreading the plateau remains submarine thicker lithosphere may result in plume incubation before magma extrusion and there are many intermediate situations where plumes could readily break through thin lithosphere oceanic or continental because magma supply exceeds extension rate plateaus may be characterised by thick sequences of flows and sills rather than the sheeted dykes typical of phanerozoic ophiolites precambrian greenstones could represent imbricated oceanic plateaus or plumes penetrating thin continental lithosphere the initial high temperature and the buoyant nature of the depleted refractory keel of plateaus contributes to their preservation relative to normal oceanic crust when they collide with active margins they choke the subduction zone causing subduction flip or backstep and the development of extensive talc alkaline arc volcanism on top of the plateau sequences however after x 100 m y they are potentially negatively buoyant so if fluids become available to promote tmnsfotmation of the deeper zones to eclogite they may be able to spontaneously subduct

an important source of4he and3he in diamonds a large data base has recently accumulated on the concentrations of helium isotopes in diamonds mined from various regions it was noted earlier ozima et al 1985 1 lal et al 1989 2 that the frequency distribution of the4he concentrations is a fairly narrow one whereas that of3he concentrations is a broad one with no pronounced peaks the ratios 3he 4he on the other hand show a broad maximum around 2 ra ra equals atmospheric 3he 4he ratio 1 40 10 6 with a slow decrease over two orders of magnitude on either side does this imply that the diamonds sample a wide variety of helium reservoirs having a range of 3he 4he ratios but somehow attain similar4he concentrations we propose that in a majority of the diamonds studied 4he is primarily due to implantation of radiogenic alpha particles from the host material after emplacement in the crust usually kimberlite and that the concentrations of4he in diamonds often get appreciably altered by this process thus the4he trapped in the diamond at the time of its crystallization is usually overwhelmed by the implanted helium and the measured 3he 4he ratios do not generally correspond to any sources in the mantle however the implanted4he resides in the outer 16 m of the diamond and the intrinsic4he and 3he 4he ratios in the diamond can be studied if its outer layers are removed the wider implications of diamond being the target material for nuclear reaction products from the host material are discussed radiogenic3he produced in the host material is also implanted in the diamond but this contribution is small on a gross basis however since the depth of implantation of3he is greater than that of4he some of the very high 3he 4he ratios observed in diamonds could be due to the implantation of radiogenic3he the radiogenic reactions in the host material can also contribute to appreciable21ne in diamonds

modeling the time dependent response of the asian summer monsoon to obliquity forcing in a coupled gcm a phasemap sensitivity experiment to study the time dependent response of the asian summer monsoon to obliquity forcing we analyze a 284 000 year long transient simulation produced by a fully coupled global climate model gcm using a new phase mapping phasemap approach here we focus on understanding the phase response of monsoonal circulation to insolation forcing at the earth orbital obliquity band 41 kyr our results show that the east asian summer monsoon easm can be divided into two geographic regions the north east asian summer monsoon neasm and the south east asian summer monsoon seasm the indian summer monsoon ism and the seasm are in phase at the obliquity band strengthened with an increase in obliquity from obliquity minima omin to obliquity maxima omax the neasm is out of phase with the ism and seasm weakened with an increase in obliquity from omin to omax we hypothesize that the inverse phase between the neasm and the ism at the obliquity band results from an ism neasm teleconnection linked to the formation mechanism of the bonin high

last glacial maximum climate over china from pmip simulations article i nfo using the results of 25 climate models under the framework of the paleoclimate modelling intercomparison project pmip and available proxydata thisstudyexaminesthe regionalclimate ofchina during the lastglacial maximum lgm 21 000 years ago compared to the baseline climate results show that annual surface temperature decreasedby2 00 7 00 c inchina during thatperiod withanaverage of 4 46 c forthe ensemble mean of all models annual precipitation and evaporation during the lgm were 5 40 less than the baseline values with an average reduction of 20 0 60 mm day and a reduction of 21 0 41 mm day at the national scale based on results from 15 out of the 25 models these models were selected for their ability to simulate the modern precipitation climatology and for the availability of suitable evaporation data both the geographical distribution and magnitude of changes in surface temperature precipitation and evaporation during the lgm varied with the seasons and with the models particularly at the sub regional scale model data comparisons revealed that the 25 models successfully reproduced the surface cooling trend during the lgm but they failed to reproduce its magnitude in all four regions of comparison particularly in the hexi corridor and in north and northeastchina thesimulationswithcomputedseasurface temperatures ssts wereinbetteragreementwith proxy estimates of surface temperature than those with prescribed ssts on the other hand large scale lgm minus baseline anomalies in annual precipitation minus evaporation agreed well in a qualitative manner with lake status based reconstructions of changes in annual water budgets in east china and the region of 35 42 n 74 97 e on the eastern qinghai tibetan plateau drier climates from the 15 models agreed with pollen based reconstructions for most parts of west china excluding the qinghai tibetan plateau the simulations with computed prescribed ssts are consistent inconsistent with reconstructed moister conditions

peeling oceanic crust in subduction zones thin slabs of oceanic pillow basalt formed at ridge axes with associated pelagic sediments are common features in ancient accretionary complexes estimates of the average thicknesses of scraped off basaltic sections of oceanic crust in accretionary prisms vary from a few tens of metres to 300 m with a mean thickness of 80 100 m in the complexes studied the permeability contrast within the uppermost part of the oceanic crust is an important factor governing the thickness of delaminated oceanic crust this mechanism is an effective means of separating oceanic crust altered at low temperatures from the remainder of the downgoing slab and thus controlling geochemical budgets in subduction zones

reconstruction of prehistoric landfall frequencies of catastrophic hurricanes in northwestern florida from lake sediment records abstract sediment cores from western lake provide a 7000 yr record of coastal environmental changes and catastrophic hurricane landfalls along the gulf coast of the florida panhandle using hurricane opal as a modern analog we infer that overwash sand layers occurring near the center of the lake were caused by catastrophic hurricanes of category 4 or 5 intensity few catastrophic hurricanes struck the western lake area during two quiescent periods 3400 5000 and 0 1000 14 c yr b p the landfall probabilities increased dramatically to ca 0 5 per yr during an hyperactive period from 1000 3400 14 c yr b p especially in the first millennium a d the millennial scale variability in catastrophic hurricane landfalls along the gulf coast is probably controlled by shifts in the position of the jet stream and the bermuda high

plateaux ancient and modern geochemical and sedimentological perspectives on archaean oceanic magmatism unequivocal examples of oceanic plateaux older than about 2 1 ga have yet to be identified from the geological record the most likely explanation for this is the partial dismemberment of ancient plateaux at convergent plate margins and their subsequent juxtaposition in accretionary complexes with arc volcanic sedimentary and plutonic rocks a review of cretaceous plateaux in the eastern indian ocean suggests that certain geochemical isotopic criteria e g positive nb anomalies relative to primitive mantle employed by petrologists to identify modern oceanic plateau lavas are of little practical use in distinguishing ancient plateau lavas from arc and volcanic rifted margin basalts the extreme chemical and isotopic heterogeneity of indian ocean plateaux together with large variations in area and volume suggest that these edifices represent one extreme of a spectrum of oceanic plateau compositions the other extreme represented by iceland and ontong java is characterised by relatively homogeneous melt compositions reflecting steady state melting conditions and the absence of a continental lithosphere contaminant to plateau magmas crustal thickness and compositional estimates for archaean oceanic plateaux using the mckenzie bickle method give a mean thickness of 43 km and mean mgo content of 19 wt archaean plateaux were thus compositionally distinct from surrounding ocean floor basaltic komatiite and notably more magnesian than plateaux such as iceland mean for tertiary basalts 6 3 wt mgo and ontong java mean for core samples 6 9 wt mgo by analogy with cretaceous tertiary ultramafic complexes in western colombia the deeper portions of these plateaux may have consisted of noritic rocks underlain by iherzolite pyroxenite gabbronorite and dunite the general absence of such rocks in archaean terrains suggests that these portions of plateaux are only rarely preserved the search for remnants of ancient oceanic plateaux has identified examples of archaean submarine lavas in the canadian shield 2 7 ga southern abitibi belt superior province and kaapvaal craton of southern africa 3 5 3 3 ga barberton and pietersberg belts chemical and isotopic data suggest that komatiite tholeiite units from these areas were derived by partial melting of a plume or asthenospheric mantle source but do not distinguish adequately the tectonic environment rifted continental margin ocean floor oceanic plateau in which these rocks were erupted however the uplift histories of these putative plateau fragments as recorded in sediments bounding and intercalated with the komatiite basalt sequences appear to be broadly comparable to those of the best documented phanerozoic oceanic plateaux although further stratigraphical investigations of presumed plateaux ancient and modern are required to test this hypothesis we believe that the sedimentary record offers the best evidence yet for the formation of oceanic plateaux as far back as the mid archaean 3 5 ga

a transient simulation of precession scale spring dust activity over northern china and its relation to mid latitude atmospheric circulation abstract dust aerosol plays an important role in energy and water cycles of the earth system across multiple spatial and temporal scales and it is also a key factor in determining air quality in arid and semi arid areas of asia spring is the season of peak dust activity i e emission transport and sedimentation in asia however on the earth s orbit scale spring dust activity in northern china nc and its relationship with atmospheric circulation have seldom been investigated we present the results of an analysis of the evolution characteristic on the astronomical timescale and dynamic mechanism of spring dust activity in nc based on a transient numerical simulation over the past 150 kyr spanning multiple precessional cycles we found that within the precessional band spring dust transport is in phase with insolation the upper westerly jet on the north nwj of the tibetan plateau tp surface wind and heating of the tp whereas it is out of phase with siberian high sh atmospheric precipitation and soil water during the high insolation stage the nwj is strengthened by the thermal forcing of the tp which causes a northward shift of the location of the westerly jet so that dust activity is enhanced by the propagation of upper level waves meanwhile during the high insolation stage the instability of the lower atmosphere is increased due to the strengthening of the northerly wind associated with the weakening of the sh and surface warming together with their higher interaction with the nwj giving rise to dust mobilization in the asian arid and semi arid source areas and vice versa rainfall and soil moisture have little influence on the dust cycle on the orbital scale although modern observations show they have an important inhibitory effect on dust emission

mobility and diagenesis of pb and 210pb in peat peatlands long have been considered to preserve the record of atmospheric deposition of anthropogenic contaminants such as pb in the past two decades sup 210 pb has been widely used to data recent strata of peat and to calculate accumulation rates the assumption that pb and sup 210 pb are immobile and not subject to diagenesis in peat has been questioned but not rigorously tested the authors attempted to determine if pb is mobile in peatlands and if pb profiles are altered by diagenic processes by constructing a mass balance for pb about a small peatland by comparing inventories concentrations and accumulation rates of pb and sup 210 pb in peatlands across northeastern north america and by examining the relationship between concentrations of pb in bog waters and peat in numerous sites the results clearly demonstrate that pb and sup 210 pb are mobilized by the organic rich waters of peatlands profiles of pb and sup 210 pb at depths below the water table do not preserve the record of atmospheric deposition and inventories of pb and sup 210 pb are depleted in peatland hollows concentrations of pb in bog waters are regulated by the concentration of pb in the peat and the concentration of dissolved organicmore carbon the mass balance for one bog indicated that over the specific three year period of study more than 30 of inputs of pb were not retained within the peat as a result of this mobility dates based on sup 210 pb can be biased and inaccurate by as much as 30 years dates based on sup 210 pb should be verified by other techniques especially when the inventory of sup 210 pb is less than that expected from local rates of deposition less

greater frequency variability of landfalling tropical cyclones at centennial compared to seasonal and decadal scales assessing risk from tropical cyclones and predicting the impact of this hazard under a human altered climate is based exclusively upon the behaviour of these events over the past 50 100 yr and often less critical to these determinations is an understanding of the full extent of the natural variability of this hazard the coarse resolution of millennial scale sedimentary records brevity of the instrumental register imprecision of longer historical accounts and lack of any long term high resolution records has led to the assumption that the total variability of tropical cyclone behaviour is encompassed within the seasonal to multi decadal oscillations observed to date we present a near 800 yr long annual resolution isotope record of tropical cyclones in northeast australia which displays marked centennial scale regimes our record demonstrates that the frequency variability of intense landfalling cyclones is greatest at centennial scale compared to seasonal and decadal oscillations switching between centennial scale regimes in this record occurred rapidly 10 20 yr highlighting the importance of accounting for this phenomenon in coastal planning and risk assessment our study highlights the importance of obtaining high resolution multi century records of tropical cyclone activity in order to more accurately assess future impacts of this hazard to human society

deformation and hydrofracture in a subduction thrust at seismogenic depths the rodeo cove thrust zone marin headlands california we have investigated the fabric and the deformational processes of an exhumed subduction zone thrust active at seismogenic depths the rodeo cove thrust zone which outcrops north of the golden gate bridge of san francisco imbricates two basalt chertsandstone sequences belonging to the marin headlands terrane franciscan complex the thrust outcrop is a 200 m thick complex zone that displays a range of stratal disruption from incipient deformation to a broken formation in the central part of the outcrop dominated by basaltic lithologies where zones of concentration of deformation have been mapped disruption is made by variably dense discrete fault systems synthetic to the main thrust r and p fractures these faults are marked by cataclasites with a shaly matrix that shows a scaly foliation defi ned by chlorite and pumpellyite which also constrain the depth of faulting 8 10 km t 200 250 c within the seismogenic zone the central part of the fault also features the densest system of carbonate fi lled veins veins occur in the broken formation matrix and fragments in both cases parallel to the foliation the veins are either folded truncated or pressure solved along the cleavage cementation and hardening of shear surfaces of the fault core may have caused the distribution as opposed to localization of subsequent slip events the fault core may have developed in basaltic rocks because of their inherently high permeability and propensity to transmit overpressure from deeper levels of the subduction zone our analysis has shown that accretionary deformation is strongly controlled by injection of overpressured fl uids occurring through systems of multiple dilatant fractures grossly parallel to the decollement zone the crosscutting relationships between veining and foliation suggest that fl uid injection is cyclic and consequently that large transient variations in permeability and cohesion may occur the repeated injection of veins parallel to the fault zone may be explained by cyclic changes of the stress or by difference in tensional strength parallel to and perpendicular to the foliation both of which would require extremely high fl uid pressure we interpret the features of the rodeo cove thrust zone as evidence of the seismic cycle and hypothesize a compressional stress fi eld in the interseismic phase and an extensional stress fi eld in the immediately postseismic phase

the internal structure of oceanic plateaus inferences from obducted cretaceous terranes in western colombia and the caribbean although the structure of mantle plume derived oceanic plateaus has recently been assessed using remote geophysical techniques combined with petrological modelling it is nevertheless desirable to test whether these theoretical rock types dunites gabbros and basalts actually exist and to establish their geochemical nature oceanic plateaus may have initially formed above or near sea level during a short vigorous pulse and thereafter commonly subside to abyssal depths as the lithosphere cools thus making sampling of their deeper levels extremely difficult however the cretaceous age colombian caribbean oceanic plateau was partially accreted against the south american continent so making the imbricated segments available for study during the process of plateau accretion and imbrication it is predominantly the basaltic layers which are obducted but parts of the sequence down to layered and banded gabbros with associated pyroxenites and dunites sometimes foliated can be exposed where the imbricate thrusting brings up deeper levels most of the upper crustal sequence in western colombia is composed of basaltic pillowed and massive flows and sills that are chemically uniform and undepleted relative to normal mid ocean ridge basalts komatiites and more abundant picrites are found at intervals and appear to occur near the base of the sequence in these zones both depleted and moderately enriched basalt and komatiite compositions occur and may result from dynamic partial melting and mixing processes associated with the high temperature part of the plume it is possible to integrate these compositional characteristics into a general model for oceanic plateau structure where the rate of magma supply is in excess of that which can be accommodated by normal spreading processes thus leading to extrusion of flows and the emplacement of sills and high level magma chambers but relatively few dykes the dense and chemically heterogeneous ultramafic magmas intrude the base of the pile and undergo fractionation to form ultramafic cumulates whereas the well mixed basaltic magmas are erupted to form the homogeneous plateau basalts based on these observations a possible structure of the caribbean colombian plateau is proposed which is compatible with geophysical models for other less well exposed oceanic plateaus

anti phased response of northern and southern east asian summer precipitation to enso modulation of orbital forcing abstract the timing of orbital scale asian monsoon changes as a direct response of northern summer insolation or remarkably lagged by southern insolation is still unclear in particular various monsoon records obtained in the east asian monsoon region show distinct phase relationships indicating additional forcing feedback mechanisms here monsoon proxies covering the past several precession cycles either from cave stalagmites or from land ocean deposits are first reviewed to present the nearly inverse precipitation responses to the precession forcing between southern and northern east asia modern meteorological observations show that different modes of tropical pacific sea surface temperature sst could lead to an out of phase interannual change in the east asian summer precipitation this enso influence is also found in the precession scale monsoon variability from the long term transient modeling which can explain the phase differences among monsoon proxies at precession maxima the east asian summer monsoon strengthens causing more precipitation in the north and less precipitation in the south the sst precipitation teleconnection is closely associated with a high pressure anomaly due to surface cooling over northwestern pacific therefore the timing of asian paleo monsoon might be significantly influenced by the internal ocean feedbacks and one can not expect all the monsoon proxies are consistently responded to the external insolation forcing

a 350 000 year summer monsoon multi proxy stack from the owen ridge northern arabian sea five summer monsoon proxies from the northern arabian sea are combined using stacking and principal components analysis pca to create two very similar multi proxy records of summer monsoon variability the five individual proxies all respond to monsoon variability but are largely independent in terms of the processes that complicate their interpretation as summer monsoon indicators e g preservation dissolution diagenesis sediment reworking as such stacking and pca average out non monsoon variance yielding a more pure monsoon signal these stacked and pca records hereafter summer monsoon stack and summer monsoon factor allow evaluation of relative monsoon strength through time as well as the relative concentration of variance within orbital bands these two parameters are less reliable when estimated from individual proxy records in fact the summer monsoon factor smf accounts for only 33 of the total variance in the five records suggesting that relative amplitude variations in each individual proxy time series are influenced by non monsoon processes the summer monsoon stack sms and smf are spectrally very similar dominated by variance in the 41 k y obliquity and 23 k y precession bands there is very little variance at the 100 k y eccentricity band associated with large scale changes in global ice volume indeed equally strong monsoons occur in both glacial and interglacial intervals within the 23 k y precession cycle monsoon maxima fall at 121 relative to precession minima june 21 perihelion maximum northern hemisphere nh summer insolation this phase falls midway between 18o minima 78 and december 21 perihelion 180 indicating that two mechanisms exert equal influence in determining the timing of strong summer monsoons within the precession band 1 sensible heating of the asian plateau which is maximized at times of ice volume minima 78 and 2 latent heat export from the southern subtropical indian ocean which is maximized at times of december 21 perihelion 180 the seasonal cycle at december 21 perihelion is characterized by warm southern hemisphere sh summers followed by cold sh winters a combination that preconditions the ocean to export latent heat during the boreal summer monsoon season summer monsoon winds transport this latent heat into asia where it is released during precipitation enhancing the asian monsoon low within the 41 k y obliquity cycle monsoon maxima are in phase with obliquity maxima this indicates that two mechanisms quite similar to those in the precession band influence the timing of strong summer monsoons in the obliquity band 1 sensible heating of the asian plateau but with no ice volume delay and 2 latent heat export from the southern subtropical indian ocean which is maximized at times of obliquity maxima again the seasonal cycle at obliquity maximum is characterized by warm sh summers followed by cold sh winters ideal for maximizing latent heat export during the boreal summer monsoon

underplating process from melange formation to duplexing example from the cretaceous shimanto belt kii peninsula southwest japan accretionary complexes are considered to form through off scraping and underplating which results in the lateral and vertical growth of an accretionary prism accretion of the upper part of the oceanic crust and the formation of melange are important components of the accretionary process although the origin of melange is considered controversial in order to better understand the origin of melange and its role in accretion in particular during underplating we have studied the relationship between the formation of melange and duplexing in the miyama assemblage here the melange is composed dominantly of shale basalt chert and sandstone this study stresses the following features 1 the structure of the miyama assemblage appears to be a composite suite of landward and seaward dipping duplexes 2 the melange fabric displays a systematic asymmetry which may have resulted through shearing along the decollement and 3 facing of horses within duplexes changes from north to south coinciding with a change in the sense of shear inferred from the melange fabric which suggests that it formed prior to duplexing

early cretaceous wedge extrusion in the indo burma range accretionary complex implications for the mesozoic subduction of neotethys in se asia the indo burma range ibr of myanmar the eastern extension of the yarlung tsangpo neotethyan belt of tibet in china contains melanges with serpentinite greenschist facies basalt chert sericite schist silty slate and unmetamorphosed triassic sandstone mudstone and siltstone interbedded with chert in the east and farther north high pressure blueschist and eclogite blocks in the naga hills melange our detailed mapping of the mindat and magwe sections in the middle ibr revealed a major 18 km antiformal isocline in a melange in which greenschist facies rocks in the core decrease in grade eastwards and westwards symmetrically outwards to lower grade sericite schist and silty slate and at the margins to unmetamorphosed sediments and these metamorphic rocks are structurally repeated in small scale imbricated thrust stacks in the mindat section the lower western boundary of the isoclinal melange is a thrust on which the metamorphic rocks have been transported over unmetamorphosed sediments of the triassic pane chaung group and the upper eastern boundary is a normal fault these relations demonstrate that the ibr metamorphic rocks were exhumed by wedge extrusion in a subduction generated accretionary complex along strike to the north in the naga hills is a comparable isoclinal melange in which central eclogite lenses are succeeded outwards by layers of glaucophane schist and glaucophanite and to lower grade greenschist facies sericite schist and slate towards the margins in the natchaung area from west to east unmetamorphosed triassic sediments overlie quartzites sericite schists actinolite schists and meta volcanic amphibolites derived from morb type basalt which are in fault contact with peridotite olivine in the peridotite has undulatory extinction suggesting deformation at 600 700 c similar to the peak temperature of the amphibolite these relations suggest generation in a metamorphic sole the amphibolites have u pb zircon ages of 119 3 ma and 115 ma which are close to the zircon ages of nearby calc alkaline granite and diorite which belong to an active continental margin arc that extends along the western side of the shan thai block the ibr accretionary complex and the active continental margin arc were generated during early cretaceous 115 128 ma subduction of the neotethys ocean

imbricated ocean plate stratigraphy and u pb zircon ages from tuff beds in cherts in the ballantrae complex sw scotland ocean plate stratigraphy basalt chert clastics of the balcreuchan group in the ballantrae complex is repeated by layer parallel thrusts to form duplex structures south of bennane head 5 km north of ballantrae five tuff beds all in chert have similar u pb zircon ages of ca 470 10 ma the geometrical polarity of the duplexes and the zircon ages provide new constraints on the tectonic evolution of the accretionary wedge of the ballantrae complex northwestward thrusting of the duplexes suggests that subduction was from the northwest to the southeast and this polarity at the leading edge of the main ophiolite body is consistent with the northward younging of the volcanic arc rocks of the ballantrae complex

using argon as a temporal tracer of large scale geologic processes abstract argon ar has proven to be one of the most useful elements in understanding the timing and duration of geological processes because of its chemical and physical properties ar is a particularly sensitive indicator of temperature variations associated with large scale geological events ar mobility in minerals is commonly described using solid state diffusion theory volume diffusion which has been used extensively to model diffusion profiles and closure temperatures in minerals because 40 ar is the radiogenic daughter product of radioactive 40 k and therefore occurs in many common rock forming minerals ar has been used effectively as a temporal tracer to understand a wide variety of geological phenomena ar isotopes have been applied with great success in three broad fields of earth science the origin and evolution of the earth s atmosphere the tectonothermal history of geologic terranes and the timing and duration of large scale tectonic events from ar isotopic data the earth s atmosphere is now known to be mainly secondary in nature although evolutionary models have undergone a recent paradigm shift due to geophysical evidence moving away from the earlier notion of a chemically layered upper and lower mantle to a whole earth mantle structure that may be chemically inhomogeneous although recent models are growing in sophistication by integrating additional information and measurements reflecting the behaviour of other noble gases through time the temporal evolution of the earth s atmosphere still remains ambiguous because measured compositions of ancient atmosphere are sparse and remain difficult to obtain 40 ar 39 ar thermochronology is one of the most commonly used tools in tectonothermal investigations because of the large range of ar closure temperatures recorded by various common k bearing minerals although more recent thermochronological models such as multidiffusion domain mdd modelling remain controversial the combination of 40 ar 39 ar thermochronology with both higher and lower temperature thermochronological methods utilizing other isotopic systems makes this an extremely versatile method of understanding the temperature time evolution of geologic regions recent developments involving the numerical modelling of ar diffusion coupled with the geodynamic modelling of orogens have led to new ways of estimating the timing and duration of a complete tectonic cycle of subduction and exhumation by applying these new methods to rocks from the bergen arcs in norway studies have shown that this cycle can be so rapid that the subducted and exhumed crust can remain relatively cool never achieving thermal equilibrium with its ambient environment throughout the entire process in summary the versatility of the 40 ar 39 ar dating method combined with the effective application of various techniques e g laser spot dating and step heating and a detailed understanding of the diffusional behaviour of ar has proven to be a powerful research tool in elucidating the timing and duration of many large scale earth system processes

reconstructing tropical cyclone frequency using hydrogen isotope ratios of sedimentary n alkanes in northern queensland australia a peat record from quincan crater queensland australia spanning the past 200 years was used to test if hydrogen isotope ratios of leaf wax long chain n alkanes derived of higher plants can be used to reconstruct past tropical cyclone activity queensland is frequently impacted by tropical cyclones with on average 1 2 hits per year the most abundant n alkanes in the peat are c 29 and c 31 possible sources for long chain n alkanes in the peat core are ferns and grasses which grow directly on the peat layer and the tropical forest growing on the crater rim hydrogen isotope ratios of c 27 c 29 and c 31 n alkanes vary between 155 and 185 parts per thousand vsmow with the largest variability in the upper 30 cm of the record for the period 1950 2000 ad the variability in delta d of c 29 alkanes resembles a smoothed record of historical tropical cyclone frequency occurring within a 500 km radius from the site this suggests that the high number of tropical cyclones occurring in this period strongly impacted the delta d signal and on average resulted in more depleted values of precipitation in the period before 1900 ad the variability in the hydrogen isotope record is relatively small compared to the period 1950 2000 ad this might be the result of lower variability of tropical cyclones during this time period more likely however is that it results from the increasing age span per sampled interval resulting in a lower temporal resolution average delta d values between 1900 and 2000 ad are around 167 parts per thousand which is similar to average values found for the period between 1800 and 1900 ad this suggests that on average tropical cyclone frequency did not change during the past 200 years this study demonstrates the potential of stable hydrogen isotope ratios of long chain n alkanes for the reconstruction of past tropical cyclone frequency

δd values of individual n alkanes from terrestrial plants along a climatic gradient implications for the sedimentary biomarker record abstract the compound specific hydrogen isotope composition d of sedimentary n alkanes is increasingly used as a palaeoclimate proxy to explore the origin of the hydrogen isotope signal of n alkanes from higher terrestrial plant biomass in sedimentary organic matter of small lakes we have analysed the n alkane composition and d values of 31 plant biomass samples birch betula beech fagus oak quercus alder alnus hornbeam carpinus and myrtle myrtus as well as non stomata containing sphagnum cladonia and moss from 14 sites along a climatic gradient from northern finland to southern italy eleven of the sites were previously investigated for the n alkane d values in adjacent lake sediments and are compared to the leaf wax n alkane d values the distribution of terrestrial n alkanes in lake sediments was not found to be characteristic of the dominant vegetation around the lake evidently n alkane distributions within the same species at different sites were found to be variable these findings complicate the use of certain n alkane distributions in sediments as a palaeoenvironmental indicator d values of plant biomass n alkanes show a significant positive correlation with the precipitation d values biomass n alkane d values are in general less negative than the associated sedimentary n alkanes of terrestrial origin possibly due to the stronger evapotranspiration during the summer when sampling took place this suggests that n alkanes in leaves show seasonal variations and the sedimentary organic matter receives the lighter isotope signal of the autumn when most leaves are incorporated into the sediment the variability of the d values from non stomata containing plants is much higher than for broadleaf trees since stomata can regulate water loss through evapotranspiration the average chain length acl of n alkanes from deciduous tree leaves increases from north to south with increasing acl the hydrogen isotopic fractionation e between source water and n alkanes also increases our results prove that terrestrial n alkane d values record the d value of precipitation modified by the amount of isotope enrichment in the leaf water due to plant anatomy stomata vs no stomata and site specific meteorological conditions such as evapotranspiration relative humidity and soil moisture therefore terrestrial n alkanes in lake sediments can deliver information on changes in these parameters over geological timescales in lake sediments where aquatic n alkanes such as n c 17 record the lake water d value the isotopic difference between terrestrial and aquatic n alkanes could therefore be used to reconstruct changes in evapotranspiration

compound specific d h ratios and molecular distributions of higher plant leaf waxes as novel paleoenvironmental indicators in the chinese loess plateau abstract we measured d h ratios of individual long chain leaf wax n alkanes isolated from a loess profile spanning the last 130 ka from xifeng china the d values show large variation 140 to 200 for the past 130 ka and are highly correlated with changes in magnetic susceptibility ms an accepted proxy for past monsoon circulation in the chinese loess plateau peaks in high d values 140 to 160 coincide with low ms values for loess sections formed under relatively dry conditions marine isotope stages miss 2 and 4 whereas peaks in low d values 190 to 200 coincide with high ms values for paleosol sections formed under relatively wet conditions miss 1 3 and 5 we attribute the isotopic variations to the combined effects of varying aridity which controls the soil and leaf water evaporation and temperature and monsoon intensity which alter the d h ratios of precipitation we also discovered a strong negative correlation between the molecular distributions ratios of c31 c29 and c31 c27 n alkanes and the d values of n alkanes based on studies of modern plants and the xifeng loess sequence the complex relationship between ms and monsoon circulation has been difficult to understand mechanistically however the climatic controls on the d h ratios of individual long chain n alkanes and c31 c29 n alkane ratios can be readily demonstrated and calibrated from modern plants and surface soils therefore these new molecular and isotopic proxies have potential as quantitative proxies for paleoenvironmental reconstruction in the chinese loess plateau

quartz vein formation by local dehydration embrittlement along the deep tremorgenic subduction thrust interface hydrothermal quartz veins are ubiquitous in exhumed accretionary complexes including the namibian damara belt here subduction related deformation occurred at temperatures 550 c and vein geometry is consistent with plate interface shear low effective normal stresses and mixed mode deformation quartz vein 18o values relative to standard mean ocean water smow range from 9 4 to 17 9 n 30 consistent with precipitation from metamorphic fluids a dominant subset of quartz veins away from long lived high strain zones and basaltic slivers have 18o values in a smaller range of 14 9 1 requiring precipitation from a fluid with 18o of 12 1 at 470 550 c this uniform fluid isotope value is consistent with progressive local breakdown of chlorite allowing extensive hydrofracture at temperatures typical of the plastic regime in active subduction zones brittle deformation within the plastic regime is inferred from observations of tectonic tremor a noise like seismic signal including overlapping low and very low frequency earthquakes which occurs below the seismogenic zone both tremor and hydrothermal veins correlate with zones of inferred high fluid pressure could represent a mixture of shear and dilatant failure and may therefore be controlled by episodic hydrofracturing within a dominantly plastic and aseismic regime

argon solubility in sio2 melt under high pressures a new experimental result using laser heated diamond anvil cell abstract we have measured the solubility of argon in sio2 melt under high pressure by melting a sio2 crystal at the argon pressure as high as 19 gpa using laser heated diamond anvil cell lhdac sio2 was successfully melted at the pressure of about 12 gpa and recovered with a completely glassy phase above 15 gpa the sample contained a large amount of crystalline sio2 although it was completely melted and spherical due to the surface tension this may arise from the change in the coordination number of si in sio2 melt at around 12 15 gpa which is close to the triple point of coesite stishovite and liquid sio2 argon content in the recovered sio2 was measured by energy dispersive spectroscopy and mass spectrometry it increased at pressures up to approximately 5 7 gpa and exhibited a constant value up to at least 12 gpa this implies that at these conditions the noble gas behaves as an incompatible element the result is completely different from those of previous lhdac studies which claimed that the increased solubility then dramatically drops above the pressure of about 5 gpa here we report the new details of experimentally determined pressure dependence of noble gas solubility by lhdac and discuss the experimental difficulties and the structure change of silicate melt under high pressures

the impact of regional uplift of the tibetan plateau on the asian monsoon climate abstract the climatic effect of tibetan plateau tp uplift is a hot topic in paleoclimate research in this paper the results of a set of numerical simulations are reported to analyze the impact of regional tp uplift on the asian monsoon climate the focus is on the impact of the regional uplift in the west and east parts on south asian summer monsoon as well as the impact of regional uplift on east asian winter monsoon the results show that the uplifts of the himalaya and the central southern tp have little effect on summer precipitation increasing over south asia whereas the further uplifts of the tp in the west and east parts including the uplifts of both the eastern and western tp have important effects on the strengthening of the south asian summer monsoon the uplift of the eastern tp leads to low level cyclonic circulation anomalies appearing in the bay of bengal accompanied by anomalous upward movement and in turn precipitation markedly increases in the bay of bengal whereas the uplift of the western tp produces low level cyclonic circulation anomalies around the uplifted region and intensifies the moisture transport from the tropical ocean and thus precipitation further increases in south asia especially in the northern arabian sea the strengthened summer monsoon in south asia is coupled with the intensification in diabatic heating and moisture convergence for the east asian winter monsoon the intensification of winter monsoon winds is related more to the regional uplifts than the uplift of the whole tp the uplifts of the central southern tp the northern tp and other northern topography have greater effects on the strengthening of the winter monsoon winds

inverse phase oscillations between the east asian and indian ocean summer monsoons during the last 12 000 years and paleo el nino abstract the inverse phase variations between the east asian and indian ocean summer monsoons on the interannual timescale result from the el nino southern oscillation activity in the tropical pacific which potentially provides a new way for studying paleo enso here we reconstruct a 12 000 yr proxy record for the east asian summer monsoon from 13 c time series of the plant cellulose of the hani peat bog in the northeastern china the comparison of it with the peat proxy record of the indian ocean summer monsoon suggests that the el nino conditions are coincident with strong east asian summer monsoons weak indian summer monsoons and drift ice events in the north atlantic at both orbital and millennial time scales the orbital scale inverse phase relationships between the monsoons indicate the occurrence of a long term enso like pattern confirming the sensitivity of the monsoon systems associated with enso to insolation forcing the nine inverse phase variations on the millennial time scales may suggest the nine el nino like patterns superimposing the long term enso like pattern the inverse phase variations between the monsoons also show close correspondence to the drift ice events at high northern latitudes in every case when the abrupt ice rafted debris events occurred in the north atlantic the inverse phase relationship established and the el nino like pattern occurred in the tropical pacific correspondingly the discussions on the influence of ocean thermohaline circulation on these global linkages have been made

implications of basement rock alteration in the nankai trough japan for subduction megathrust slip behavior abstract the nankai trough is an exceptionally well studied convergent margin known to host damaging megathrust earthquakes as well as various forms of slow fault slip outcrop studies of exhumed analogues for the modern subduction thrust as well as seismic reflection images of the active subduction zone suggest that the basaltic basement participates in plate boundary shearing at depths of a few km and greater we obtained altered basaltic upper basement from the modern nankai trough seaward of the trench recovered by drilling on iodp expedition 333 we performed laboratory friction experiments on bare surfaces and gouge powders of this material in addition to and in combination with other materials for comparison the altered basalt exhibits predominantly velocity strengthening frictional behavior indicating a tendency for stable slip for bare surface experiments few instances of velocity weakening friction occur and are restricted to slip rates of

onset of asian desertification by 22 myr ago inferred from loess deposits in china the initial desertification in the asian interior is thought to be one of the most prominent climate changes in the northern hemisphere during the cenozoic era1 2 3 4 but the dating of this transition is uncertain partly because desert sediments are usually scattered discontinuous and difficult to date here we report nearly continuous aeolian deposits covering the interval from 22 to 6 2 million years ago on the basis of palaeomagnetic measurements and fossil evidence a total of 231 visually definable aeolian layers occur as brownish loesses interbedded with reddish soils this new evidence indicates that large source areas of aeolian dust and energetic winter monsoon winds to transport the material must have existed in the interior of asia by the early miocene epoch at least 14 million years earlier than previously thought3 5 regional tectonic changes and ongoing global cooling are probable causes of these changes in aridity and circulation in asia

evolution and variability of the asian monsoon system state of the art and outstanding issues the asian monsoon is comprised of the indian and east asian subsystems these two components are linked to one another in varying degrees by regions of strong sensible heating indo asian landmass and strong latent heat export the western pacific warm pool and the southern subtropical indian ocean variability within the indian and east asian subsystems interactions among them and the extent to which they interact with other climate phenomena e g enso are current topics of modern and paleoclimate research this work provides an overview of past and current paleomonsoon research on tectonic to interannual time scales with a primary focus on marine sediment records while past and current work has contributed greatly to our understanding of paleomonsoon variability at all time scales additional efforts are required to make further progress in two critical areas 1 additional efforts are needed in terms of proxy development and evaluation requiring concerted efforts at long term sediment trap deployments in key monsoon influenced regions as well as development of adequate and widely available core top databases these are necessary to assess the impact of modern oceanographic and seafloor processes on potential monsoon proxies 2 additional effort is also needed in acquiring a sufficient geographic distribution of downcore records to assess linkages among the subsystems and their role in the context of extratropical climate change these records should have high sedimentation rates including varved sections this requires substantial survey support to identify the most appropriate coring and drilling targets c 2004 elsevier ltd all rights reserved

noble gas constraints on the layered structure of the mantle several chemical models of mantle structure have been proposed1 6 based on isotopic data such as sr nd hf and pb in volcanic rocks from both oceanic and continental areas the most popular of these are two chemical models of the vertically layered mantle 1 that it is a depleted mantle from which mid oceanic ridge basalt morb is derived and 2 that it is a fertile mantle from which a mantle plume may arise1 5 however these models are almost always based on solid elements such as sr nd hf and pb isotopes and do not fully reflect the noble gas data because noble gases are liable to be lost from the solid earth when a mantle differentiation occurs on a global scale in the upper mantle the data on noble gas isotopes could give constraints on the structure of the mantle that differ from those by solid element isotopes based on a consideration 3he 4he and 40ar 36ar ratios of recent samples i argue here that the observed data suggest that mixing occurs between the source materials of a mantle plume and of morb the data seem to favour the model which assumes that the source of a typical mantle plume is deeper than that of morb

glacial interglacial indian summer monsoon dynamics the modern indian summer monsoon ism is characterized by exceptionally strong interhemispheric transport indicating the importance of both northern and southern hemisphere processes driving monsoon variability here we present a high resolution continental record from southwestern china that demonstrates the importance of interhemispheric forcing in driving ism variability at the glacial interglacial time scale as well interglacial ism maxima are dominated by an enhanced indian low associated with global ice volume minima in contrast the glacial ism reaches a minimum and actually begins to increase before global ice volume reaches a maximum we attribute this early strengthening to an increased cross equatorial pressure gradient derived from southern hemisphere high latitude cooling this mechanism explains much of the nonorbital scale variance in the pleistocene ism record