mineral parageneses in metamorphic rocks exposed along tailuko gorge central mountain range taiwan field petrographic and microprobe investigations of metaclastic rocks calcareous schists marbles chloritic calcareous meta volcanic units and schists paragneisses which crop out along the eastern portion of the central east west cross island highway in taiwan demonstrate that metamorphic intensity gradually increases eastward the lower greenschist facies slate formation on the w contains completely recrystallized pure albitic plagioclase but at least some of the white micas chlorites probably represent relict detrital flakes neo blastic biotite and epidote occur sporadically in the pihou formation and increase dramatically eastward concomitantly the abundance of carbonaceous matter decreases to zero in the eastern tailuko zone and the amount of chlorite white mica diminishes somewhat epidote becomes more aluminous at higher metamorphic grade eastward phengites change progressively to more muscovitic compositions as the proportion of biotite increases a close approach to chemical equilibrium for the pre cenozoic complexly deformed metamorphic basement assemblages is suggested by regular systematic major and minor element partitioning between analysed coexisting phases fractionation is less pronounced on the e reflecting higher temperatures estimated physical conditions of recrystallization with h2o and co2 moderate are t 325 75 c p 3 kbar w t 425 75 c p 4kbar e the gradual eastward increase in metamorphic intensity from the slate formation through the pihou formation and the three tailuko zones as well as the relict precursor textures in the pre cenozoic layered basement rocks indicate that the observed paragenetic sequence could represent a synchronous neogene recrystallization event probably accompanying the plio pleistocene collision of the asiatic continental margin and the luzon coastal range andesitic arc

a quaternary solution model for white micas based on natural coexisting phengite paragonite pairs a thermodynamic model for the quaternary white mica solid solution with end members muscovite mg celadonite paragonite feceladonite ms mgcel pg fecel is presented the interaction energies for the mgcel pg join the fecel pg join and the ternary interactions were obtained from natural coexisting phengite paragonite pairs phengite paragonite pairs were selected based on the criteria that their chemical compositions may be represented as a linear combination of the model end member compositions and that the respective formation conditions 350 650 c 4 21kbar are accurately known previously published excess free energy expressions were used for the ms pg ms mgcel and ms fecel binaries the suggested mixing model was tested by calculating multicomponent equilibrium phase diagrams this proved to be particularly well suited to reproduce compositional variations of white micas from amphibolite facies metapelites

triassic paragonite and garnet bearing epidote amphibolite from the hida mountains japan paragonite and garnet bearing high grade epidote amphibolite pgea in the ise area of the hida mountains japan is characterized by the high pressure hp epidote amphibolite facies parageneses m1 garnet hornblende clinozoisite paragonite quartz rutile paragonite and garnet of the peak m1 stage are locally replaced by retrograde albite oligoclase and chlorite m2 respectively phase equilibria constrain peak metamorphic conditions of p 1 1 1 4 gpa and t 530 570 c and a decompressional p t path for this rock mineral parageneses of prograde epidote amphibolite facies are comparable to some hp rocks from the hongan region of western dabie but differ from other hp mafic schists with cooling ages of c 330 ma in the hida mountains new paragonite k ar dating for the pgeayields a triassic cooling event at 210 ma that is coeval with regional cooling and exhumation of the sulu dabie qinling sdq belt both petrological and geochronological data of the triassic hp epidote amphibolite in hida mountains support our earlier hypothesis that the sdq belt extends across the korean peninsula to sw japan

high pressure metamorphism in the chinshuichi area yuli belt eastern taiwan abstract tectonic blocks and slabs of mafic ultramafic rocks are distributed discontinuously in the yuli metamorphic belt of taiwan the blocks include rare omphacite metagabbros and garnet epidote blueschists in the wanjung and juisui tamayen areas respectively such high pressure hp mineral assemblages have been attributed to a mid miocene subduction event however the surrounding psammitic pelitic and chloritic schists are the dominant greenschist facies lithologies of the yuli belt in the chinshuichi area tectonic blocks are enclosed in garnet bearing metapelites suggesting elevated pressures in this area we recently discovered meta plagiogranite containing the assemblage glaucophane omphacite xjd up to 0 39 rutile quartz indicating p t conditions near 13 kbar 550 c new equilibrium phase modeling of a garnet paragonite mica schist and compositional isopleths for peak assemblage minerals garnet and phengite si 3 33 3 37 pfu indicate metamorphic conditions of 15 5 17 kbar 530 550 c these p t estimates are higher than previously reported in the yuli belt and suggest that both tectonic blocks and host metapelites underwent hp metamorphism the juxtaposition of tectonic blocks and metapelites apparently occurred during the formation of a subduction accretionary complex followed by exhumation facilitated by a collisional event these new findings imply that hp metamorphism was not limited to tectonic blocks and instead played a significant role attending orogenesis in eastern taiwan

cooling and uplift patterns in the lepontine alps south central switzerland and an age of vertical movement on the insubric fault line 96 new fission track ft apatite and zircon k ar and rb sr biotite and muscovite ages are presented for 19 samples mainly acid gneisses from a 40 km traverse through the lepontine alps in the maggia valley south central switzerland plotting measured mineral ages against assumed system closure temperatures yields cooling rates for each sample the entire profile shows a fairly uniform late neogene recent mean uplift rate of 0 5 mm a confirmed by a gradient of ft apatite age with elevation cooling from higher temperatures occurred earlier in the south where uplift rates of 2 2 mm a in the steep belt root zone indicate 9 km early miocene uplift of the northern pennine block this uplift started before 23 ma and is interpreted as resulting from a major phase of backthrusting along the insubric line and as dating the formation of the mylonite belt estimated cooling rates constrain the timing of lepontine mid tertiary metamorphism 3 schematic models are proposed which also consider published rb sr k ar mica and hornblende and u pb monazite ages slow cooling differential initial heating and subsequent cooling of different parts of the central alps and post 38 ma cooling with syntectonic metamorphism at 27 ma are postulated as alternative interpretations of isotopic data and geologic evidence from extrapolation between k ar and rb sr mica ages and apatite ft ages 240 50 c is proposed as the closure temperature for the retention of fission tracks in zircon

prograde and retrograde metamorphism of hematitebearing basic schists in the sanbagawa belt in central shikoku the mineral assemblages of hematite bearing basic schists in intermediate high pressure metamorphism are temperature dependent for assemblages with excess hematite albite muscovite and quartz the paragenetic relations can be dealt with in terms of a four component system without omitting or grouping major components in the sanbagawa belt in central shikoku the dominant amphibole in the hematite bearing basic schists changes from winchite via crossite and barroisite to hornblende the stability of amphibole is described chemographically within a pseudoternary system with another excess phase epidote many amphiboles are chemically heterogeneous owing to retrograde reactions which produced low t p amphibole around the prograde amphibole the examination of amphibole zoning makes it possible to draw a retrograde p t trajectory which passes on the lower pressure side of the prograde one

zircon radiation damage ages abstract the suggestion that the accumulation of radiation damage in zircons could be used as a dating method was first proposed in the 1950s in the original technique radiation damage was determined by x ray diffraction however this is not suitable for the small sample analysis needed in modern geochronology it is now possible to measure radiation damage and u and th contents on micron sized areas on the polished surface of zircon grains using raman spectroscopy and sims mass spectrometry this opens the way for a reassessment of the potential of radiation damage ages and the purpose of this contribution is to re examine the technique through its application to zircons from two granitoids from the archean yilgarn craton and ancient detrital zircons from the jack hills in western australia the three examples show internally consistent radiation damage ages that are also in accord with independent geochronological evidence the 420 110 ma radiation damage age for the darling range granite is coincident with 500 400 ma biotite rb sr ages in the same region of the yilgarn craton likewise a tonalite from the youanmi terrane in the craton with a 1650 150 ma zircon radiation damage age lies within the domain of a 1600 ma event recorded by biotite rb sr ages the jack hills zircons have a 1120 130 ma radiation damage age that is explained by a complex damage accumulation and annealing history culminating in a mild heating event indicated by biotite ar ar ages of about 1140 ma the positive results for the three case histories suggest that radiation damage ages could play a useful role in dating low temperature thermal events

mesozoic and cenozoic tectonics of the northern edge of the tibetan plateau fission track constraints abstract fission track analysis on zircons and apatites yields new information about the timing of deformation of the northern tibetan plateau ages on zircons ranging from 221 22 to 96 4 ma are indicative of a general late triassic early jurassic cooling probably driven by the collision between the qiantang and kunlun blocks mid jurassic slow cooling is recorded also in the apatites in regions not affected by later cenozoic deformation this jurassic denudation was followed by a period of sedimentation during the cretaceous except along the altyn tagh fault atf zone and in some restricted areas of the western and eastern qilian shan this long and relatively quiet period ended at about 40 10 ma along the major altyn tagh and kunlun strike slip fault zones which were activated by the india asia collision this first movement along lithospheric faults resulted in the eastward extrusion of the tibet plateau which was followed in late oligocene miocene times by a major compression event initiating the formation of the high relief of north tibet a final compressional event took place at 9 5 ma and is well correlated with high sedimentation rates in the basins of this region this compression induced continental subduction in the kunlun ranges the altun shan belt and possibly the qilian shan belt

stratigraphic architecture magnetostratigraphy and incised valley systems of the pliocene pleistocene collisional marine foreland basin of taiwan lithofacies analysis magnetostratigraphy and seismic profiles of pliocene pleistocene foreland basin deposits of taiwan provide a framework to evaluate the stratigraphic development of a collisional marine foreland basin we have recognized several scales of stratigraphic packages and unconformities in deposits of the taiwan foreland basin small scale 20 to 150 m thick stratigraphic sequences contain upward shallowing marine lithofacies successions that are bracketed by thin coquina sandstones we interpret the small scale stratigraphic packages as parasequences in the traditional sequence stratigraphy model the thin coquina sandstones representing marine flooding intervals the average duration of individual small scale packages was in the range of 37 5 k y on the basis of our magnetostratigraphy these sequences are interpreted as the product of eustatic sea level change possibly related to the orbital time series of obliquity intermediate scale stratigraphic sequences are 150 to 1000 m thick and are bounded by unconformities that are well exposed in outcrop and can be clearly identified in seismic sections the unconformity surfaces have several hundred meters of relief and represent periods of major fluvial valley incision in the foreland basin one of the unconformities is locally an angular one that we interpret as representing a growth structure that formed during structural uplift of the proximal margin of the foreland basin at ca 1 25 ma across this angular unconformity there were marked increases in rates of sediment accumulation and tectonic subsidence in the foreland basin other major unconformities that bound intermediate scale stratigraphic sequences are high relief disconformities these unconformities may be the product of eustatic changes because there has been little change in rates of sediment accumulation and tectonic subsidence across these unconformities the duration of individual intermediate scale packages ranges from 100 000 to 700 000 yr on the basis of magnetostratigraphy and biostratigraphy we interpret the intermediate scale sequences as sequences in the traditional sequence stratigraphy model our analysis of the pliocene pleistocene deposits of the taiwan foreland basin has several implications for understanding the stratigraphic evolution of this collisional marine foreland basin 1 deposition in the taiwan foreland basin appears to have been punctuated by at least five episodes of erosion and major fluvial valley incision large volumes of sediment were eroded from the proximal margin of the foreland basin and transported to more distal parts of the foreland basin or to depocenters outside the foreland basin system during all stages of basin development 2 the presence of high relief unconformities and growth structures in the pliocene pleistocene foreland basin deposits suggests a well developed wedge top depozone in the foreland basin system 3 the pliocene pleistocene strata of the foreland basin of taiwan record 2 3 m y of deposition on the basis of our magnetostratigraphy sediment accumulation rate was on the order of 950 m m y during the earlier stages of basin development during the later stages of basin development sediment accumulation rate increased to 1900 m m y sediment accumulation rates in the collisional marine foreland basin of taiwan are much higher than previously published rates from more extensively studied retroarc foreland basins and collisional nonmarine foreland basins

pressure temperature and time indicators in mafic schist their application to reconstructing the polymetamorphic history of vermont mineral chemistry and overgrowth relationships in mafic schist from vermont are shown to be sensitive indicators of pressure temperature and relative time and to be good chronicles of the paleozoic history of this polymetamorphic terrane within the common assemblage amphibole chlorite epidote plagioclase quartz ti phase carbonate k mica fe 3 oxide electron microprobe analyses show that increasing metamorphic grade as defined by intercalated pelitic schist is recorded by an increase in the edenite glaucophane and tschermakite contents of amphibole in the anorthite content of plagioclase and in the substitution of al vi f 2 ti al iv for fe 2 mg mn si in biotite chlorite and muscovite with increasing pressure the glaucophane component of amphibole increases for medium pressure metamorphism the albite oligoclase gap is in the garnet zone where amphibole has between 1 2 and 1 8 formula proportion al iv and al vi fe 3 ti cr this gap is below the garnet isograd in low pressure mafic schist where al iv and al vi fe 3 ti cr in the amphibole are both less than 0 6 mineral growth periods observed are characterized by metamorphic grade and facies series and are assigned to two ordovician taconic and two devonian acadian events silurian devonian mafic schist in northeastern vermont records two periods of low pressure devonian metamorphism these events are expressed by medium pressure and low pressure metamorphism in the cambrian and ordovician rocks to the west and south in the pre silurian rocks high pressure and medium pressure metamorphism is assigned to the ordovician the observed high pressure metamorphism is confined to a 110 by 40 km area along the green mountain anticlinorium axis in north central vermont

the effects of el nino and la nina on snow and avalanche patterns in british columbia canada and central chile el nino and la nina affect global climate and atmospheric circulation to determine winter temperature and precipitation patterns both winter temperatures and the associated precipitation patterns have effects on mountain snow deposition and snow avalanche occurrences approximately 25 000 slab avalanches from 30 winters were analyzed in relation to snowfall patterns contrasted for el nino and la nina winters for two avalanche areas with different snow climates in british columbia bc canada la nina winters were shown to produce more snow more avalanches and a higher percentage of dry avalanches than wet avalanches the data and analysis show that the avalanche patterns depend on the altitude and snow climate analysis of snowfall and accident data from the andes of chile suggests behavior opposite to bc el nino winters in central chile produced the most snow and by inference the most avalanches this paper is the first to show the links of el nino and la nina to snow avalanche activity

natural long term annealing of the zircon fission track system in vienna basin deep borehole samples constraints upon the partial annealing zone and closure temperature abstract fission track ft analysis of zircon from cretaceous and tertiary sandstone samples in vienna basin boreholes has provided constraints for the thermal stability of the zircon ft system over a geological timescale confined track lengths and ages were measured for samples from depths of 1 5 7 5 km the deepest with a present environmental temperature of 200 c mean track lengths range from 10 3 to 10 8 m indistinguishable within error from the unannealed reference length of 10 7 0 1 m 1 standard error thus showing no sign of systematic downhole reduction central zircon ages are consistently older than stratigraphic ages of parent rocks with no single grain ages significantly younger than their respective stratigraphic ages such ft age and length evidence strongly suggests that the tracks have not been significantly annealed since sediment deposition because the present geothermal regime as well as sample burial depths have been near constant for at least the past 5 m y the lower temperature limit of the zircon ft partial annealing zone is 200 c for a heating duration of the order of 5 10 m y such evidence from long term natural annealing is compatible with a zircon ft partial annealing zone cf 200 350 c derived by the extrapolation of laboratory annealing results based on onfined track length measurements but cannot alone discriminate between different annealing models for the zircon ft closure temperature a perhaps over simplified concept these results are consistent with the oft quoted temperature of 250 c for cooling rates of 10 100 c m y

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

the succession of small shelly fossils especially conoidal microfossils from english precambrian cambrian boundary beds small shelly fossils especially conoidal microfossils have potential for biostratigraphic correlation of precambrian cambrian boundary rocks where evolutionary successions of taxa can be recognized a succession of hyoliths brachiopods and species of sunnaginia torellella eccentrotheca and rhombocorniculum can be recognized in the home farm member of nuneaton and or the comley limestone of shropshire england revised ranges of taxa are given and sunnaginia neoimbricata n sp s parva n sp s angulata n sp eccentrotheca grandis n sp and several new records are described from nuneaton and comley the similarity in the sequence of small shelly fossils in england and siberia indicates that a correlation can be achieved between the two areas it is suggested that the lower tommotian to lower atdabanian correlates with the hartshill formation of nuneaton of which the condensed home farm member spans the mid tommotian to lowest atdabanian while the upper atdabanian to lower botomian correlates with the condensed comley limestone of shropshire

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

chronology of early cambrian biomineralization data on the first appearances of major animal groups with mineralized skeletons on the siberian platform and worldwide are revised and summarized herein with references to an improved carbon isotope stratigraphy and radiometric dating in order to reconstruct the cambrian radiation popularly known as the cambrian explosion with a higher precision and provide a basis for the definition of cambrian stages 2 to 4 the lophotrochozoa and probably chaetognatha were first among protostomians to achieve biomineralization during the terreneuvian epoch mainly the fortunian age fast evolutionary radiation within the lophotrochozoa was followed by radiation of the sclerotized and biomineralized ecdysozoa during stage 3 the first mineralized skeletons of the deuterostomia represented by echinoderms appeared in the middle of cambrian stage 3 the fossil record of sponges and cnidarians suggests that they acquired biomineralized skeletons in the late neoproterozoic but diversification of both definite sponges and cnidarians was in parallel to that of bilaterians the distribution of calcium carbonate skeletal mineralogies from the upper ediacaran to lower cambrian reflects fluctuations in the global ocean chemistry and shows that the cambrian radiation occurred mainly during a time of aragonite and high magnesium calcite seas

confined fission track length measurement of zircon assessment of factors affecting the paleotemperature estimate abstract we carried out a series of laboratory annealing experiments in order to understand the factors affecting observed confined track lengths in thermally annealed zircon the variation of confined fission track lengths according to both experimental and analytical conditions was determined on zircons from nisatai dacite it was found that etching temperature track etching criteria measurement criteria of horizontal confined tracks hct s and the orientation of hct s significantly control the observed track lengths if samples are highly annealed i e 600 c for 1 hr annealing the first three factors may be related to the presence of gap zones in an annealed track whilst the fourth is caused by anisotropic annealing and etching characteristics for reliable thermal history reconstruction therefore it should be the minimum requirement to document explicitly the experimental and analytical conditions of these factors in zircon fission track analysis

lower cambrian halkieriids from oxfordshire uk lower cambrian sediments from the withycombe farm borehole oxfordshire yield isolated halkieriid sclerites all bear prominent ridges and most appear to be either cultrates or palmates although distinction between the two in the available material is generally difficult two other sclerites are more elongate one may represent a spinose sclerite while the other is possibly derived from the siculate zone of the original scleritome adjacent horizons of the withycombe formation yield disc like fossils superficially similar to hyolith opercula or monoplacophorans these are tentatively identified as deriving from the same halkieriid scleritome two main types are identified and these may correspond to the anterior and posterior shells of halkieria evangelista from the lower cambrian sirius passet fauna of north greenland the age of the withycombe formation within the cambrian is not entirely resolved but faunal evidence suggests an equivalence to the cuslett formation of southeast newfoundland which corresponds to either the upper tommotian or lower atdabanian of the standard siberian section

on steady states in mountain belts the dynamic system of tectonics and erosion contains important feedback mechanisms such that orogenic systems tend toward a steady state this concept is often invoked but the nature of the steady state is commonly not specified we identify four types of steady state that characterize the orogenic system and illustrate these cases by using numerical model results and natural examples these types are 1 flux steady state 2 topographic steady state 3 thermal steady state and 4 exhumational steady state they refer to the erosional flux the topography the subsurface temperature field and the spatial pattern of cooling ages respectively models suggest that the topography will reach a steady mean form at the scale of an orogenic belt but perfect topographic steady state is unlikely to be achieved at shorter length scales thermal steady state is a precondition for exhumational steady state and in the case of temperature dependent deformation topographic steady state exhumational steady state is characterized by reset age zones spatially nested according to closure temperature as illustrated in natural systems from new zealand the cascadia accretionary margin and taiwan

joint vp and vs tomography of taiwan implications for subduction collision orogeny abstract the taiwan orogen sitting at the pre rifted eurasian margin and bracketed by two sub orthogonal subduction systems is subject to complicated geotectonics motivating numerous seismic tomographic studies in local and regional scale most of them obtained p wave velocities vp or with vp vs ratio jointly but few were for s wave velocities vs with unprecedented amount of s wave data in this study a new set of well tuned vp vs and vp vs models were determined by an elaborate joint inversion scheme integrating the data of p and s wave travel times s p times and the borehole logging data for near surface correction into one system this allowed us to revisit the subduction collision process in detail by comparing vp vs and vp vs ratio simultaneously with enhanced slab imaging of philippine sea plate psp under northeast taiwan we are able to discriminate the plausible missing luzon forearc basement along the plate boundary and propose a skateboarding edge model in which the westernmost psp rides on the east subducted forearc basement and subducts northward underneath the eurasian lithosphere the underlain forearc basement along with the psp subduction then results in the anomalous shallow double seismic zone observed only near taiwan island meanwhile at around 121 e the north subducting psp seems dipping west to collide with the deep eurasian lithosphere shaping an irregular corner wedge structure in addition we evaluate our model with previous ones by waveform modeling approach and show the current performance of travel time tomography in taiwan region

provenance evolution during arc continent collision sedimentary petrography of miocene to pleistocene sediments in the western foreland basin of taiwan taiwan is an example of an orogen involving a doubly vergent critical wedge critical wedge theory predicts asymmetrical tectonic development of the pro and retro sides of an orogen whereas the consequences for exhumation peak metamorphism and erosion have been extensively investigated the implications for sandstone petrography preserved on either side of an orogen have not been adequately documented plio pleistocene sandstones from the western foothills of taiwan were deposited in a rapidly subsiding foreland basin recording the collision between a volcanic arc and the asian passive margin new data on the evolution of sedimentary petrography and clay minerals preserved in miocene to pleistocene sandstone of the western pro foreland basin record the history of taiwan s orogenesis sandstone petrography of the eastern basin on the retro side of taiwan s orogenic wedge had been studied previously comparison of the provenance record from the western and eastern basins illustrates the opposing signature of unroofing and recycling held to be characteristic of an asymmetric orogenic wedge sandstone petrography can be used in ancient sedimentary basins to complement other indications of the polarity of subduction

survival of graphitized petrogenic organic carbon through multiple erosional cycles abstract graphite forms the endpoint for organic carbon metamorphism it is extremely resilient to physical biological and chemical degradation carbonaceous materials cm contained within sediments collected across taiwan and from the gaoping submarine canyon were analyzed using raman spectroscopy to determine the crystallinity this allowed the erosional and orogenic movements of petrogenic organic carbon ocpetro during the taiwanese orogeny to be deduced after automatically fitting and classifying spectra the distribution of four groups of cm within the sediments provides evidence that many forms of ocpetro have survived at least one previous cycle of erosion transport and burial before forming rocks in the western foothills of the island there is extensive detrital graphite present in rocks that have not experienced high grade metamorphism and graphite flakes are also found in recently deposited marine sediments off taiwan the tectonic and geological history of the island shows that these graphite flakes must have survived at least three episodes of recycling therefore transformation to graphite during burial and orogeny is a mechanism for stabilizing organic carbon over geological time removing biospheric carbon from the active carbon cycle and protecting it from oxidation during future erosion events

a ternary solid solution model for omphacite and its application to geothermobarometry of eclogites from the middle adula nappe central alps switzerland a ternary solid solution model for omphacite with the end members jadeite naalsi2o6 diopside camgsi2o6 and hedenbergite cafesi2o6 was derived from experimental data from the literature the subregular solution model fitted by linear programming is best suited to omphacites with very little aegirine component in common eclogites applying this solution model to the calculation of equilibrium phase diagrams of eclogites from the adula nappe central alps switzerland results in large stability fields for common eclogite assemblages garnet omphacite quartz h2o kyanite within this field the compositions of garnet and omphacite show very little variation a precise determination of the peak pressure and temperature is not possible the occurrence of amphibole overgrowing the peak pressure assemblage in fresh eclogite suggests retrograde re equilibration still under eclogite facies conditions the computation of isopleths for garnet and pyroxene end members allows the estimation of the pressure and temperature conditions of this re equilibration event 19 21 kbar c 700 c

an inverse method of modeling thermal histories from apatite fission track data abstract apatite fission track ft ages and track length distributions are important sources of information about the thermal histories of rocks recent advances in the understanding of track annealing in apatite provide a solution to the forward problem of predicting ft ages and track length distributions that result from a given thermal history in this paper a method is presented to address the inverse problem estimation of the thermal history from ft data the inverse method uses an iterative approach the downhill simplex to systematically modify a starting thermal history to achieve satisfactory statistical agreement between the predicted and observed ft age and track length distribution however because of analytical uncertainties a unique thermal history does not exist monte carlo simulations are used to take into account the uncertainties in the data and yield a spectrum of possible thermal histories the results are based on the isothermal annealing model of laslett et al 1 chem geol isot geosci sect vol 65 but because the method uses forward predictions and not an analytical formula specific to a single annealing model alternative annealing models could be used within the same framework this method is an improvement in interpreting fission track data because the range of thermal histories permitted by the data can be evaluated to demonstrate the method it is applied to several sets of test data created by forward modeling of idealized thermal histories the results show that the age at which the sample cooled through its closure temperature and the most general features of the thermal history are revealed however data from partially annealed samples have wide spectra that reflect large uncertainties in the form of the thermal history for such samples ft data alone are not likely to provide strong tests of geologic hypotheses conversely geologic information may play a crucial role in constraining the thermal history spectrum in application to uplift related cooling this inverse method can provide much more realistic assessments of the cooling rate and its uncertainty than are otherwise possible

statistical modelling of annealing kinetics of fission tracks in zircon reassessment of laboratory experiments abstract we remodel the annealing kinetics of fission tracks fts in zircon to fit data from 43 laboratory experiments in three earlier publications using spontaneous ft lengths in zircon from the nisatai dacite heating temperature and time are in the ranges of 350 912 c and 10 3 104 h respectively the data sets illustrate a transition in annealing process at a certain temperature zone in order to express the annealing behavior involving the possible transitional process with improved goodness of fit two types of models are constructed the hybrid linear model that consists of the fanning linear model at high temperature and the parallel linear model at low temperature connected with a transitional temperature zone and the parallel curvilinear model that approximates smoothly the transition in the annealing process the validity of the extrapolation of the model to geological time scale is confirmed by comparison with some deep borehole core data the length estimates by the hybrid linear model agree well with the borehole data whilst the estimates by the parallel curvilinear model show a significant disagreement with highly annealed borehole samples although this model provides good agreement with the experimental data for the hybrid linear model estimates of the partial annealing zone corresponding to isothermal heating for 106 and 107 years are 281 352 c and 262 330 c where the top and bottom of the partial annealing zone are defined by the mean length reduction ratio of 0 4 and 0 8 respectively

assessment of overetching factor for confined fission track length measurement in zircon abstract the effect of prolonged etching provides a fundamental bias in observed confined track lengths in minerals particularly those having highly anisotropic etching characteristics such as zircon in this paper the anisotropic prolonged etching factor around the ends of horizontal confined tracks hct s which are practically used for track length analysis was assessed quantitatively by step etch of spontaneous tracks in zircon the observed etching characteristics are two fold 1 the track etch rate for the direction perpendicular to the crystallographic c axis is significantly smaller than that parallel to it and 2 for a particular crystallographic direction the growth rate of track length is consistently greater than the bulk etch rate measured as the growth rate of track width for the direction perpendicular to that of interest to further constrain the etching characteristics of hct s the shape of their ends were compared and contrasted with that of track etch pits on a prismatic surface in contrast to the diamond shape of surface tracks the ends of hct s are generally convex outward with the core of etched track longest regardless of track orientation this suggests that at the ends of hct s the core of track has a growth rate greater than the surrounding bulk region probably with decreasing rate outward hence the traditional model of track etching dividing a latent track into the etchable and unetchable ranges should be refined etch rate of a latent track decreases rather continuously toward both ends this model is concordant with the ionization pattern in a mineral detector produced by nuclear fission and explains reasonably the experimental results because the correction of such a complicated overetching factor for an individual sample is extremely laborious for routine analysis it is recommended that hct s nearly perpendicular to the crystallographic c axis e g 70 should be used for track length measurement nevertheless the reduction in number of tracks to be measured may be rather small since hct s with smaller angles to the c axis are less detectable due to the etching anisotropy

towards zircon fission track thermochronology reference framework for confined track length measurements abstract horizontal confined track hct lengths and angles to the crystallographic c axis were measured for both unannealed spontaneous and induced fission tracks ft in zircons to establish a reference framework of hct length measurement induced track lengths were analysed on eleven zircon separates from various rock types including four age standards and spontaneous lengths on three zircon standards were analysed in general the observed hct lengths in each sample are slightly anisotropic with shorter tracks having greater angles to the c axis in addition all samples show large anisotropy in the number of observed hct s particularly for induced tracks as an observed mean hct length is controlled by the combined influence of these two anisotropies selective measurement of tracks that have specified azimuth angles to the c axis e g 60 90 is recommended in annealing studies or thermochronological applications mean spontaneous and induced hct lengths of 60 90 are approximately concordant for individual age standards although slightly controlled by the difference in overetching effects due to the different etching criteria adopted this indicates that natural track annealing at an ambient surface temperature at geological timescales is negligible in zircon in contrast to the case in apatite mean lengths of induced hct s do not differ significantly between the zircons analysed therefore the overall mean and standard deviation values of hct lengths for 60 90 azimuth angles i e 10 50 0 04 1 standard error m and 0 76 0 03 m for spontaneous tracks and 10 73 0 04 and 0 63 0 03 m for induced tracks respectively offer reliable references for hct length measurement in zircon ft thermochronology

the north atlantic oscillation and snow avalanching in iceland 1 the number of snow avalanche cycles occurring in iceland is correlated to a major source of variability in climate the north atlantic oscillation a new index for this oscillation is produced that reflects the cumulative effect of snow build up in a starting zone an analysis of the time series for this index shows that recent avalanche tragedies in iceland are linked to a rise in this index consequently estimations of avalanche return periods and thus avalanche hazard zones may need re evaluation because the oscillation has an impact upon climate throughout the northern hemisphere this may be necessary in a number of other countries

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

pleistocene onset of rapid punctuated exhumation in the eastern central range of the taiwan orogenic belt the taiwan orogenic belt is often treated as a steady southward propagating orogenic system with an essentially constant erosion rate of 4 6 mm yr over the past 5 m y we present 4 new age elevation transects from the central range based on 19 new and 86 previously published fission track and u th he dates of completely reset detrital zircon and apatite grains the age elevation curves and thermal models imply slow cooling prior to ca 2 1 5 ma at exhumation rates of 0 1 mm yr an increase in exhumation rates from ca 2 1 5 ma to ca 0 5 ma 2 4 mm yr and possibly a further acceleration in exhumation from ca 0 5 ma to present 4 8 mm yr three transects from three different latitudes in the eastern central range yield similar results each showing punctuated exhumation with progressively faster rates

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

precambrian cambrian boundary global stratotype ratified and a new perspective of cambrian time the precambrian cambrian boundary global stratotype is the base of the phycodes pedum zone at fortune head southeastern newfoundland the initial two stage radiation of the phanerozoic fauna the appearance of phanerozoic aspect trace producers and subsequent origin of diverse skeletalized metazoans was earliest cambrian fossil assemblages with deep complex burrows or originally mineralized skeletal fossils with exception of cloudina are early cambrian or younger correlation into avalon indicates that strata once referred to the proterozoic e g manykaian or nemakit daldynian and rovno stages are lowest cambrian and that the siberian tommotian stage is middle lower cambrian the thick sequence and complex epeirogenic history of the placentian series as compared with the trilobite bearing branchian series suggest that most early cambrian time is represented by the subtrilobitic lower cambrian

provenance evolution and unroofing history of a modern arc continent collision evidence from petrography of plio pleistocene sandstones eastern taiwan abstract plio pleistocene sandstones from the coastal range of eastern taiwan provide a record of shifting source areas and unroofing events in adjacent arc and accretionary wedge terranes during the early stages of are continent collision in latest miocene and early pliocene time arc derived sediments were deposited in a precollisional forearc basin between the luzon volcanic arc and the manila trench sandstones of this age are rich in zoned plagioclase volcanic lithic fragments and pelagic foraminifera and contain only minor quartz and sedimentary metasedimentary lithic fragments in contrast lower pliocene to lower pleistocene sandstones are poor in feldspar and volcanic fragments and contain abundant sedimentary and metasedimentary lithic fragments this reflects a provenance shift f om the volcanic arc to the uplifted meta sedimentary accretionary wedge of proto taiwan which resulted from the early pliocene onset of arc continent collision a refined classification of sedimentary and metasedimentary lithic fragments is introduced here and used to interpret the early evolution of the collisional orogen lithic fragments are rich in pelitic minerals and are divided into 1 sedimentary ls mudstone shale siltstone chert and argillite 2 low grade metamorphic lm1 slate slatey siltstone and quartzite and 3 medium grade metamorphic lm2 phyllite schist phyllitic quartzite and quartz mica albite aggregate a good correlation is observed between these lithic fragment types and bedrock lithologies exposed in the present day central range of taiwan based on this correlation the change in relative abundance of sedimentary and metasedimentary lithic fragments through time has been used to interpret an unroofing sequence for the collisional fold and thrust belt lower pliocene taiwan derived sandstones are rich in sedimentary lithic fragments ls which were shed from the accretionary wedge during the earliest stages of arc continent collision from early pliocene to early pleistocene time sandstones became progressively depleted in ls lithic fragments and enriched first in lm1 and later in lm2 fragments as deeper levels in the metamorphic complex were uplifted and exposed to erosion regional metamorphism of biotite grade rocks lm2 occurred at about 4 5 ma at depths of 12 15 km ernst 1983 and were uplifted to the surface by about 1 4 ma this study these ages give an uplift rate of 4 to 5 km m y which is in good agreement with independent estimates based on fission track studies raised coral reefs quaternary elevation changes and present day denudation rates thus it appears that sandstone detrital modes of clastic orogenic sequences can be used to interpret specific aspects of unroofing and tectonic evolution in nearby active mountain belts

natural long term annealing of the zircon fission track system around a granitic pluton we measured 24 ages and 14 length distributions of fission tracks in zircons separated from basement sandstones around a granitic pluton to investigate natural long term annealing behaviors of the zircon fission track zft system samples were collected from three cretaceous coherent units of the shimanto accretionary complex shikoku japan along an eastward traverse at distances of between 1 6 and 10 6 km from the intrusion contact of 15 ma takatsukiyama granite exposing about 5 km in diameter sampling was carried out around the expected position of the transition zone of track retention i e the fossil partial annealing zone fpaz based on previous regional zft age distributions approaching the contact original zft ages of 100 ma show a continuous reduction to 15 ma at 3 km distance their single grain age distributions accordingly change from widespread multimodal patterns reflecting provenance ages to unimodal ones characterizing the secondary heating by the granite intrusion mean and distributions of zft lengths also show notable variations within the fpaz as a result of mixing both preexistent tracks that were shortened by the miocene heating and unannealed ones having original long lengths that have been formed and accumulated after the cooling of the pluton these facts along with observed track segmentation and annealing anisotropy suggest that the shortening patterns of tracks in nature a heating duration of 105 106 years estimated from thermal modeling are indistinguishable from those previously demonstrated at laboratory timescales this basically validates the conventional approach of extrapolating laboratory based kinetic parameters to geologic timescales the observed fpaz and thermal modeling results in conjunction with laboratory heating data previously reported and recent constraints from an ultradeep borehole favor a relatively narrow temperature range for the partial annealing zone i e 230 to 330 c for a heating duration of 106 years

late cenozoic exhumation and erosion of the taiwan orogenic belt new insights from petrographic analysis of foreland basin sediments and thermochronological dating on the metamorphic orogenic wedge abstract late cenozoic orogen derived sediments recorded in the pro and retro foreland basins on both sides of the taiwan orogenic wedge provide important constraints on sediment provenance evolution and further outline the exhumation and erosion histories of the orogenic belt we present 172 sandstone petrographic data from seven representative regions on the pro and retro foreland basins which we coupled with previous thermochronological data from the pro wedge of the hsuehshan range and the retro wedge of the backbone range to elucidate the source sink relationships between a source orogenic wedge and its associated foreland basin our data analysis and estimation provide evidence that the initial depositional age of orogen derived sediments in pro and retro foreland basins is consistent with the exhumation and erosion histories of the adjacent orogen indicating that the backbone and hsuehshan ranges started to build up at 12 and 5 6 ma respectively the diachronous onset of orogenic sediment deposition and orogenic uplift also suggest a tendency for progressively southward and westward migration of the foreland basin and asymmetric exhumation and erosion of the orogenic wedge over time due to oblique collision the timing of the onset of the taiwan orogeny formed the retro wedge of the backbone range at 12 ma which is much older than the previously determined onset age of taiwan mountain building of 6 ma we divide the taiwan orogenic evolution into two stages the first stage comprises the extrusion and exhumation of the subduction wedge formed as the yuli belt of the backbone range prior to the collision during the middle miocene the second stage comprises the arc continent collision producing doubly vergent orogenic wedge extrusion leading to a diachronous uplift history of the hsuehshan and backbone ranges since late miocene

thermochronology of the torlesse accretionary complex wellington region new zealand the torlesse complex comprises several mesozoic accretionary prism complexes together forming continental basement over large parts of new zealand this study focuses on the thermal history of relatively low grade graywacke rocks exposed in a transect in southern north island that crosses the structural grain of the torlesse complex including its older and younger parts zircon fission track ft ages for the late triassic rakaia terrane which is the most inboard of the accretionary complexes are partially annealed some possibly reset and may indicate early cretaceous 134 10 ma cooling from maximum temperatures tmax probably related to imbrication of younger complexes of the pahau terrane numerical modeling of the zircon ft ages and published 40ara 39ar muscovite and biotite ages for the rakaia terrane suggest tmax values of 265 310 c and exhumation from depths of 10 12 km the rocks underlying the aorangi range and involving the youngest accretionary complex have experienced much lower tmax values of 210 and 110 c bracketed by reset apatite ft ages and detrital zircon ft ages the occurrence of a circa 100 ma component of zircon ft ages in both the weakly and highly indurated rocks beneath the aorangi range as well as in remnants of an overlying albian accretionary slope basin whatarangi formation imply multistorey accretion and incorporation of sediment into the youngest prism this circa 100 ma zircon ft age component also places a maximum age on the termination of mesozoic subduction beneath the new zealand region the occurrence of reset apatite ft ages across the whole of the wellington transect indicates that at least 4 km of exhumation occurred during the late miocene

a thermomechanical wedge model of taiwan constrained by fission track thermochronometry abstract we present 31 new apatite fission track aft ages for the island of taiwan that when combined with existing aft and zircon fission track zft data provide regional spatial coverage of the island with respect to low temperature thermochronometry the overall pattern of zft and aft ages in taiwan exhibits unreset ages in the southern and western portions of the island and reset ages predominantly in the central range and eastern taiwan this pattern supports interpretations of the orogen kinematics as reflecting a crustal scale wedge with a southward propagating collision zone in this model new material is accreted to the wedge from the west and is transferred to the east with the greatest exhumation occurring along the eastern margin as recorded in the reset ages in the east and unreset ages in the west the southward propagating collision is consistent with reset ages in the north where erosional exhumation has been ongoing for longer and unreset ages in the south where the younger collision implies less time for erosional exhumation despite the variation in the age of the collision along the strike of the island the widths of the aft and zft reset age zones remain nearly constant between 23 00 n to 24 00 n and 23 20 n to 24 00 n respectively suggesting that the orogen is in an exhumational steady state over these regions with respect to the aft and zft thermochronometers we use the fission track data in conjunction with observations of crustal structure crustal fabric and heat flow measurements to constrain a time dependent two dimensional thermomechanical model of orogen evolution by accounting for the heat transfer tectonic and erosion processes needed to predict aft and zft ages we are able to investigate the relationship between the measured ages and the tectonic characteristics of the orogen with our model we conclude that 1 roughly half of the material accretion in taiwan occurs through underplating over an approximately 40 km wide region 2 current average erosion rates are 3 3 mm yr in the eastern central range and 2 3 mm yr over the whole island 3 the collision has been propagating southward at a rate between 20 and 51 km ma over the past 2 3 ma and 4 central taiwan is in a topographic thermal and exhumational steady state

glacier area changes in the central chilean and argentinean andes 1955 2013 14 to improve our knowledge of glacier area changes in the central chilean and argentinean andes 32 9 s 33 4 s two new glacier inventories from 1989 to 2013 14 are compared with a reinterpreted inventory from 1955 comparisons show glacier area retreat of 30 3 since 1955 decreasing from 134 to 94 km2 in 2013 14 whilst the annual rate of area loss showed a small increase insignificant between the periods of 1955 1989 and 1989 2013 14 separate analysis of the 1989 and 2013 14 inventories including a larger sample revealed a higher rate of glacier change compared with the smaller samples of these inventories additionally an analysis at 5 year intervals for six major glaciers 1955 2013 indicates large variability in response times and area loss magnitudes glacier olivares alfa for example lost 63 of its ice area while the juncal norte glacier lost only 10 1955 2013 the findings from this study improve our current knowledge base concerning widespread glacier decline in the southern andes and furthers monitoring efforts in this poorly described region of the world a region containing vital water resources for populated areas in south america

a reinterpretation of the metamorphic yuli belt evidence for a middle late miocene accretionary prism in eastern taiwan the yuli metamorphic belt has been the topic of petrological and geochronological studies for over 40 years and has been interpreted as a cretaceous melange our study utilizes zircon u pb dating of schist and exotic blueschist blocks in the yuli belt these new ages indicate that these metamorphic rocks are actually middle miocene in age and may represent the deeper structural levels of an accretionary prism several distinctive detrital zircon u pb age populations are recognized from 14 siliceous schists of melange hosted rocks that are similar in age population to the cretaceous eocene oligocene and miocene strata of taiwan the wide range of ages is interpreted as a product mixing of various sedimentary strata prior to metamorphism three blueschists of a volcanic arc protolith enclosed within the host rocks yield crystallization ages of 15 4 0 4 15 5 0 3 and 16 0 0 2 ma based on zircon u pb dating in consideration of the new data regarding the cretaceous miocene host rocks and the middle miocene exotic blueschist blocks it strongly suggests that the yuli belt formed at deeper levels of an accretionary wedge during subduction of south china sea oceanic crust at the middle late miocene subsequently the rapid uplift of the metamorphic belt was probably related to doubly vergent wedge extrusion due to the pliocene arc continent collision

organic geochemistry of vienna basin model for hydrocarbon generation in overthrust belts the vienna basin is a tertiary pull apart basin in the alpine carpathian overthrust belt thickness of the sedimentary sequence including neogene basin fill mesozoic overthrust complex and mesozoic autochthonous strata can exceed 10 000 m 33 000 ft source rocks for oil and gas are in the autochthonous jurassic sequence beneath the overthrust we can assume that much of the source potential of these rocks was preserved during thrusting and most of the hydrocarbon generation took place after thrusting during neogene burial the vertical stacking of hydrocarbon pools indicates predominantly vertical migration through major faults results of geochemical studies have influenced exploration strategy in the vienna basin as well as in the northern alps and the foreland basin the vienna basin could thus be considered a model for hydrocarbon generation in other overthrust belts

from submarine continental accretion to arc continent orogenic evolution the thermal record in southern taiwan constraining the timing of initial collision is critical for understanding how long term plate convergence is accommodated at collisional plate boundaries reevaluation of the age of collision from orogenic thermal evolution requires distinguishing the onset of submarine continental accretion from earlier oceanic subduction and subsequent subaerial orogenic evolution we present new thermochronological constraints from the first age elevation relationship transect in taiwan and zircon and apatite fission track ages from sediments in the western foreland our data reveal the onset of cooling from at least 7 1 1 3 ma at a minimum rate of 21 c m y in the submarine sedimentary wedge followed by a marked acceleration of subaerial exhumation after ca 3 2 0 6 ma at an average rate of 1 7 km m y our data reflect the effect of margin architecture as taiwan evolved from submarine accretion of the distal extremely thinned continental margin to crustal thickening of the proximal margin and orogenic development

late cenozoic metamorphism and mountain building in taiwan a review abstract the orogenic wedge of taiwan results from the late miocene collision of the chinese passive margin and the luzon volcanic arc because of the southward propagation of the collision over time and because of its extreme rates of deformation and erosion it has appeared as an ideal place to investigate mountain building processes here we review existing metamorphic and thermochronological data on late cenozoic burial and exhumation most of the island has been extensively investigated in particular in terms of zircon or apatite fission track and u th he thermochronology peak metamorphic temperatures have also been quantified from the raman spectroscopy of carbonaceous material along three transects in central taiwan the different datasets appear consistent and indicate that cooling rates and exhumation are more intense within the hsueshan range and within the tananao complex lateral variations from south to north are also observed and they can be interpreted in terms of different stages of exhumation and mountain building existing models provide a framework to analyze the data and discuss the limits of some previous interpretations in particular in terms of exhumational steady state or of recent acceleration of uplift and exhumation these models are also discussed in light of available data it appears that all of them fit thermochronological data within the tananao complex whatever the kinematics the depth of the wedge or the mechanism responsible for exhumation and that exhumation of the hsueshan range has been ignored in most models future models need therefore to consider the taiwan wedge as a whole additional constraints on peak metamorphic conditions on kinematics of shortening and exhumation or on the geometry of the wedge at depth need to be gathered to further unravel mountain building processes at the scale of the whole orogen

annealing kinetics of fission tracks in zircon an experimental study a series of laboratory annealing experiments on zircon fission tracks has been carried out under heating conditions of 350 750 c for 10 1 103 hr i e 4 5 min 40 days variation in the confined lengths of spontaneous fission tracks was determined using zircon grains from nisatai dacite the fading contours of normalized mean track length r on the arrhenius diagram showed as sets of straight lines we performed a series of model fittings called the parallel and fanning models in order to describe the decrease in r with increasing temperature or heating time the lowest temperature limit of the zircon partial annealing zone zpaz was defined as r 0 95 and the highest as r 0 4 which approximately corresponded to the total fading of surface tracks extrapolation of the results of the laboratory experiments to the geological timescale gives for a heating duration of 106 yr estimated values of the zpaz of 210 320 c 60 c 2 with the parallel model 190 350 c 50 c 2 with the fanning model critical temperature t0 and 170 390 c 50 c 2 with the fanning model t0 the temperatures of the zpaz decrease by 20 c for an annealing duration that is an order of magnitude longer because the estimated closure temperature of zircon fission track analysis approximately corresponds to the middle of the zpaz these results support the previously estimated closure temperature of 240 c by varying the etching time it was revealed that significant removal of radiation damage occurs at r 0 93

early post rift sequence stratigraphy of a mid tertiary rift basin in taiwan insights into a siliciclastic fill up wedge abstract in order to scrutinize the development process and control of an early post rift siliciclastic fill up wedge sequence stratigraphy was applied to the oligo miocene coastal shelf strata of a mid tertiary rift basin in taiwan four sequences at the million year 3rd order scale constrained by microfossil biohorizons and radioactive dates were identified and correlated regionally the lower sequence belongs to the fault bounded oligocene syn rift wedge the middle two comprise the lower miocene early post rift wedge onlapping onto the mesozoic basement highs the upper sequence is the lower miocene late post rift drape on the continental margin the middle two sequences record a process of active topographic transformation through successive coastal shelf progradations at first the remnant rift topographic low received drastic progradations from both the inner and outer highs during the later progradations principally from the inner high the topographic low was filled up for the successive drape this wedge formation was characteristically short lived and high in sedimentation rate when accommodation space and sediment supply both increased the initial accommodation space of the remnant rift deep shelf was augmented by rapid thermal subsidence and long term eustatic sea level rise the sediment supply in spite of a decrease in provenance exposure due to onlapping was promoted by periodic exposures due to 3rd order eustatic sea level falls and additionally by a warm and humid climate these falls are age concordant with the sequence boundaries and thus evidently important for the sequence boundary formations and sediment dispersal

thermal modeling of continental subduction and exhumation constrained by heat flow and seismicity in taiwan abstract the thermal evolution of crustal exhumation subsequent to the subduction of the continental margin is modeled through numerical solutions to the two dimensional heat conduction equation the boundary conditions used in the modeling are basically constrained by available geophysical and geological observations in the taiwan area temperature distributions are calculated at one million year intervals over a 5 ma duration although this study presents a tentative estimation based on some reasonable assumptions the preliminary results not only explain the sharp change in seismicity within the crust but also agree with the general heat flow pattern on the surface of taiwan more specifically both the high heat flow and the aseismic belt beneath the eastern central range which are probably attributed to higher temperatures within the crust are modeled by crustal exhumation after the subduction of the continental margin however the occurrence of deeper earthquakes east of the central range is a result of the lower thermal gradient which is due to the subduction of the cold continental crust into the uppermost mantle in summary preliminary thermal modeling supports the possibility of a tectonic model of active continental subduction and crustal exhumation for the taiwan orogen

the thermal evolution of corsica as recorded by zircon fission tracks abstract new zircon fission track zft ages from corsica record multiple thermal events that can be tied to the structural evolution of the western mediterranean region the corsican zircons have a wide scatter of zft grain ages 243 14 ma which together define several age domains western corsica consists largely of stable hercynian basement characterized by zft ages in the range 161 114 ma we interpret these ages late jurassic early cretaceous as the product of a long lived tethyan thermal event related to continental rifting and subsequent drifting during the separation of the european and african plates and the formation of the liguro piemontese ocean basin in contrast to hercynian corsica alpine corsica northeast corsica experienced widespread deformation and metamorphism in late cretaceous tertiary time dated samples from alpine corsica range in age from 112 to 19 ma and all are reset or partially reset by one or more alpine thermal events the youngest zft grain ages are from the northernmost alpine corsica and define an age population at 24 ma that indicates cooling after tertiary thermal events associated with the alpine metamorphism and the opening of the liguro provencal basin a less well defined zft age population at 72 ma is present in both alpine corsica and hercynian basement rocks the thermal history of these rocks is not clear one interpretation is that the zft population at 72 ma reflects resetting during a late cretaceous event broadly synchronous with the early alpine metamorphism another interpretation is that this peak is related to variable fission track annealing and partial resetting during the tertiary alpine metamorphic event across central to north eastern corsica this partial age resetting supports the presence of a fossil zft partial annealing zone and limits the peak temperature in this area below 300 c for both the affected pre alpine and alpine units

submicron polyphase inclusions in garnet from the tananao metamorphic complex taiwan a key to unravelling otherwise unrecognized metamorphic events kyanite and staurolite occur in the tananao metamorphic complex as submicron inclusions in almandine rich garnet from a metamorphosed palaeosol weathering horizon near hoping eastern taiwan quartz rutile brookite and zircon are also found as associated submicron inclusions in garnet employing the reaction ilmenite kyanite quartz almandine rutile and the breakdown of staurolite and quartz as thermobarometers these submicron scale minerals formed at 8 3 8 8 kbar and 660 690 c this p t estimate is different from that i e 5 7 kbar and 530 550 c derived from matrix minerals which include almandine rich garnet muscovite chlorite chloritoid plagioclase quartz and ilmenite these results suggest that submicron inclusions in garnet like materials may record portions of the otherwise undocumented prograde path or provide information about previous metamorphic events and thus yield new insights into orogenic belts

inversion of apatite fission track data for thermal history information apatite fission track length distributions and ages provide an integrated record of the low temperature 20 150 c thermal history of the host rock over geologic time scales the information content of these data is quantified by applying a stochastic optimization technique simulated annealing to determine the range of thermal histories compatible with synthetic apatite fission track data given a mathematical description of fission track annealing the goodness of fit between the observed and predicted data parameters and a slowly decreasing control parameter are treated as analogous to the free energy and temperature respectively of a thermodynamic system on this basis a boltzmann type probability distribution is defined and used to accept or reject random thermal history perturbations a priori information used to constrain the model space may include the temperature range in which the solutions are sought present day temperature the complexity of the thermal history and the maximum duration of the data sensitive thermal history this approach allows determination of the range of thermal histories compatible with an observed confined track length distribution and associated fission track age and it provides an estimate of the probability distribution of temperatures at a specific time results for synthetic data sets for diverse model thermal histories suggest that the resolution of thermal history is generally poor parts of the thermal history to which the data are most sensitive can be resolved to no better than about 10 c for ideal data bounds on acceptable time temperature space are sensitive to the assumed complexity of the thermal history but are relatively insensitive to typical uncertainties associated with the data application of this approach to real data is fundamentally limited by the uncertainty associated with extrapolating annealing models to geologic conditions

rejuvenation of kar systems for minerals in the taiwan mountain belt the late mesozoic amphibolites and granitic rocks in the basement complex of the taiwan mountain belt are overprinted by late tertiary greenschist facies metamorphism the kar isotope systems of late mesozoic horn blende muscovite biotite and microcline have been reset by this metamorphic event as reflected in three systematic variations 1 a regional decrease in40ar 39ar dates with increasing metamorphic overprinting 2 a unique sequence of decreasing dates among minerals i e hornblende muscovite biotite microcline in the same sample or within closely spaced samples and 3 a positive correlation between the grain size fractions and their40ar 39ar dates e g smaller grain size fractions of a given mineral yield younger dates in the lower greenschist facies area hornblendes and coarse grained muscovites still retain most of their radiogenic argon and display fairly flat40ar 39ar age spectra with plateau dates of 82 95 ma which record a rapid cooling during the nanao orogeny a late mesozoic tectonothermal event in contrast all microclines are completely reset and yield young plateau dates of 1 6 and 1 7 ma reflecting the rapid uplift associated with the penglai orogeny most of the partially reset minerals e g muscovites from the upper greenschist facies area and biotites from the lower greenschist facies area yield geologically meaningless40ar 39ar integrated dates that fall between the ages of the two tectonothermal events and exhibit disturbed age spectra which are caused mainly by mixing of argon released from two generations of phases excess argon and partial argon loss after isolating possible recrystallization and mixing effects the regional variation in the40ar 39ar dates for the earlier generation phases could be simulated by a volume diffusion model coupled with a thermal model for the taiwan mountain belt

new zircon fission track analysis constraints on tectonic mineralization epochs in the ganzi litang gold belt eastern qinghai tibet plateau abstract the ganzi litang gold belt as a part of the sanjiang tethys orogen is located between the yidun block and songpan ganzi block terrane in the eastern qinghai tibetan plateau and experienced oceanic slab subduction continent arc collision and intracontinental convergence since the indosinian orogeny a series of gold ore deposits have been found within the region this paper describes the tectonic evolution and episodes of gold mineralization within this belt using zircon fission track ft thermochronology a total of 15 zircon ft analysis results were obtained that can be divided into four age groups 217 213 ma 190 160 ma 155 93 ma and 72 67 ma representing four distinct episodes of gold mineralization of which the middle two episodes predominate eruption of the auriferous volcanic rocks that comprise the belt started at 217 213 ma when the ganzi litang meso tethys ocean began to subduct the mineralization episode at 190 160 ma corresponds to the formation of auriferous volcanic rocks during intracontinental collisional convergence of the ganzi litang belt finally the episodes at 155 93 ma and 72 67 ma are respectively related to the closing times of the bangong nujiang and yarlung zangbu oceans corresponding to tectonic quiet episodes

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

simultaneous mountain building in the taiwan orogenic belt the obliquity of the arc continent collision in taiwan is thought to have produced a southward propagating orogenic system starting as early as 8 ma we combine 62 new zircon fission track ages with timing of rapid subsidence in the foreland basin to identify the timing of the onset of exhumation and orogenesis the oldest completely reset zircon fission track ages and the onset of rapid tectonic subsidence in both the northern and southern parts of the foreland basin are ca 5 6 ma we propose that north south rifting in the south china sea created a north trending continental margin before the collision rather than the commonly assumed northeast trending margin consequently both this north trending continental margin and the luzon arc of the philippine sea plate were subparallel resulting in an initial collision that was simultaneous along strike

biostratigraphy of the vendian cambrian beds and the lower cambrian boundary in siberia general agreement exists that the position of the base of the cambrian precambrian cambrian boundary should coincide with a biostratigraphic marker that can be widely traced and is linked to the evolution of the small shelly fauna on the siberian platform the presence of distinct facies during the vendian cambrian interval has made biostratigraphic correlation based on palaeontological data highly contentious a solution to this problem appears to exist with the recognition of an almost uninterrupted profile in the aldan uchur watershed where key sections expose the eastern and transitional facial region on this basis we present a general biostratigraphic scheme for the vendian cambrian across the southern siberian platform that connects separate facies further correlation with key sections in northern siberia leads to the recognition of three biostratigraphic zones for the vendian cambrian interval these are anabarites trisulcatus zone purella antiqua zone aldanocyathus sunnaginicus zone the abundance of chinese type small shelly fossils in the region of the eastern facies suggest correlation of the a sunnaginicus zone with the siphogonuchites paragloborilus zone of china and the p antiqua zone with the circotheca anabarites protohertzina zone in places at the base of the circotheca anabarites protohertzina zone in china analogies to the a trisulcatus zone can be identified it is proposed that in siberia the base of the a sunnaginicus zone base of tommotian stage be taken as the base of the cambrian this would correspond with marker b of the meishucunian in china