paleoproterozoic tectonic transition from collision to extension in the eastern cathaysia block south china evidence from geochemistry zircon u pb geochronology and nd hf isotopes of a granite charnockite suite in southwestern zhejiang abstract the badu complex and associated paleoproterozoic granitoids are among the oldest known rocks in the cathaysia block in south china the paleoproterozoic units of the badu complex are dominantly composed of metapelitic rocks and meta greywackes here we report la ss icp ms laser ablation split stream inductively coupled plasma mass spectrometry zircon u pb data from a newly discovered garnet bearing granite which show an emplacement age of 1929 15 ma and metamorphism at 1872 34 ma we also report u pb ages of 1886 16 ma 1858 7 ma 1848 11 ma from a gneissic granodiorite and two charnockites respectively the garnet bearing granite is peraluminous with a cnk range from 1 1 to 1 3 the rock shows relatively high sio2 k2o and rb contents and low total ree sr cao and ferromagnesian components typical of leucogranites the whole rock nd two stage model age tdm2 nd of this rock is ca 2 7 ga zircon hf crustal model ages tdmc hf peak at about 2 7 ga and abundant inherited zircons occur with u pb ages in the range of 2044 to 2803 ma evidences from zircon u pb age and hf isotope compositions whole rock nd isotopes and whole rock major and trace elements suggest a metasedimentary protolith and that the garnet bearing granite leucogranite was derived by partial melting in a thickened crust at about 1 93 ga the gneissic granodiorite and charnockites show indistinguishable major and trace element features as well as zircon hf and whole rock nd isotope compositions indicating that they were generated from the same source rocks the gneissic granodiorite and charnockites are ferroan calc alkalic and metaluminous with a cnk range from 0 80 to 0 98 they display relatively low sio2 contents and ga al ratios suggesting their a type affinity their zircon tdmc hf age peak is 2 9 ga and whole rock tdm2 nd ages range from 2 8 to 2 9 ga these high temperature rocks were generated possibly through the partial melting of ancient amphibolites 2 9 ga with heat input from asthenosphere upwelling and magmatic underplating in an extensional setting at about 1 85 1 88 ga combined with data from previous studies we suggest three episodes of major crustal growth in this region at 1 8 1 9 ga 2 7 ga and 2 9 ga during the 1 8 1 9 ga event extensive reworking also took place from the 1 93 ga syn collision leucogranite garnet bearing granite the 1 88 1 85 ga post collision a type granites and charnockites the granite charnockite suite and the1 85 1 77 ga intraplate basaltic rocks li 1997 and xiang et al 2008 in the region a paleoproterozoic orogenic cycle can be deciphered in the cathaysia block the rapid transition of tectonic regime from collision to post collisional extension can be correlated to the assembly of the supercontinent columbia

tectonothermal evolution of the mayuan assemblage in the cathaysia block implications for neoproterozoic collision related assembly of the south china craton the mayuan assemblage in the cathaysia block southeast china consists of felsic paragneiss pelitic schist greenschist amphibolite marble calcsilicate and quartzite that underwent three episodes of deformation d1 d3 and four episodes of metamorphism m1 m4 in the early neoproterozoic the m1 assemblage consists of mineral inclusions defining an early foliation s1 within porphyroblasts represented by chlorite muscovite biotite plagioclase quartz in pelitic schist and actinolite chlorite epidote albite quartz enclosed in amphibolite m2 coincides with the development of the regional schistosity s2 and represents the formation of the porphyroblasts and growth of matrix minerals resulting in development of prograde metamorphic zones chlorite biotite garnet staurolite and kyanite zones m3 is simultaneous with the third phase of deformation d3 and produced sillimanite bearing mineral assemblages in pelitic schist and hornblende bearing assemblages in amphibo lite the last metamorphic episode m4 gave rise to the retrogressive assemblage chlorite muscovite in pelitic rock and actinolite chlorite epidote in amphibolite the sequence of mineral assemblages and history of metamorphic reactions built from the petrogenetic grid of pelites suggest a near isothermal decompression clockwise p t path for the mayuan pelitic schists using the tweequ software program the garnet biotite thermometer and garnet muscovite biotite plagioclase barometer yield p t conditions for m1 of 5 5 to 6 0 kb and 450 to 500 c and conditions for the garnet staurolite and kyanite zones of m2 of 6 0 to 7 0 kb and 550 to 600 c 6 0 to 7 5 kb and 600 c and 11 0 to 11 5 kb and 600 c the p t conditions of m3 were estimated at 570 to 625 c and 4 0 to 4 5 kb using the muscovite biotite thermometer and hornblende plagioclase geothermo barometer the garnet chlorite thermometer yields temperatures of 300 to 400 c for m4 but the pressures of m4 cannot be quantitatively estimated because of the lack of a suitable geobarometer these p t estimates also define an near isothermal decompression clockwise p t path which is involved in initial crustal thickening followed by rapid exhumation and final cooling and is related to amalgamation of the cathaysia and yangtze blocks to form the south china craton

four climate regimes on a land planet with wet surface effects of obliquity change and implications for ancient mars abstract series of numerical experiments are performed using a general circulation model to gain insights on the hydrologic cycle on ancient mars since the state of the ancient mars atmosphere is not well constrained we did not try to simulate an ancient mars climate under warm and wet condition in stead we used an idealized model and tried to extract general features of the hydrologic cycle by modeling an ideal land planet that has no ocean on its surface four different climate regimes warm upright warm oblique frozen upright and frozen oblique regimes are recognized depending on the inclination of the spin axis obliquity and average surface temperature the period of active hydrologic cycle suggested from the geomorphology on mars seems to be consistent with that at the warm oblique regime which appears at warm above freezing environment with high obliquity higher than about 30 condition

evolution of terrestrial proto co2 atmosphere coupled with thermal history of the earth abstract the effect of volatile exchange between surface reservoirs and the mantle on the evolution of proto co2 atmosphere on the earth is investigated using a global carbon cycle model coupled with thermal evolution of the mantle carbon is assumed to circulate among five reservoirs atmosphere ocean continents seafloor and mantle and the carbon flux of each reservoir is calculated under varying conditions such as an increase in solar luminosity continental growth and a decrease in tectonic activity with time we consider processes such as continental weathering carbonate precipitation in the ocean carbonate accretion to the continents metamorphism of carbonates followed by co2 degassing through arc volcanism carbon regassing into the mantle and co2 degassing from the mantle the degassing rate of volatiles from the mantle is assumed to be proportional to the volatile concentration in the mantle multiplied by the mantle degassing volume the mantle degassing volume is determined by the seafloor spreading rate and the melt generation depth in the mantle we use a parameterized convection model to calculate the thermal evolution of the mantle from which we estimate the seafloor spreading rate and the melt generation depth in the mantle numerical simulations suggest that the amount of surface carbon at the present time would be in a steady state this is because the response time of the carbon cycle system against the perturbation for surface carbon is short being estimated at about 900 million years under the present conditions thus the present amount of surface carbon would not be affected by the initial amount of surface carbon the seafloor spreading rate would have been almost constant throughout the earth s history to explain apparent constancy of the carbon isotope ratio in the mantle after about 3 5 billion years ago the surface carbon might probably have been circulated between surface reservoirs and the mantle once or twice after the archean period in any event co2 in the proto atmosphere on the earth is suggested to have decreased with the growth of continents resulting in stabilization of the terrestrial environment against the increase in solar luminosity

shisha pangma leucogranite south tibetan himalaya field relations geochemistry age origin and emplacement the shisha pangma pluton forming most of the xixabangma 8027 m massif in south tibet is one of the 20 larger leucogranite intrusives that mark the highest structural levels of the himalayan metamorphic core the pluton occurs immediately below the shisha pangma detachment a strand of the south tibetan detachment std system a low angle 30 north dipping normal fault placing paleozoic black slates atop sillimanite grade pelites and calc silicate rocks k feldspar augen gneisses containing fibrolite and sillimanite paragneisses along the footwall show strong internal s c fabrics indicative of down to the north extension the shisha pangma leucogranite is a heterogeneous polyphase intrusion with an earlier foliated biotite rich phase and a later tourmaline muscovite rich phase typically containing the assemblage kfs pl qtz ms tur gt bt sil ap the highly peraluminous granites have high 87sr 86sr ratios 0 738 0 750 typical of pelite derived anatectites nd depleted mantle model

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

petrogenesis of the early paleozoic strongly peraluminous granites in the western south china block and its tectonic implications abstract strongly peraluminous sp granites have a cnk molecular al 2 o 3 cao na 2 o k 2 o ratios 1 1 indicating a predominant origin from the partial melting of metasedimentary rocks however an increasing number of studies have documented that mantle derived magmas can also be involved in the petrogenesis of some sp granites this is the case in the dulong batholith in the southeastern yunnan province southwestern south china block which is typically composed of sp granitic rocks with high a cnk values 1 1 zircon u pb dating of four samples from this batholith yielded consistent crystallization ages of ca 430 ma synchronous with the widespread late orogenic magmatism including s and i type granites and mafic igneous rocks in the wuyi yunkai orogen south china block scb all the granites have fractionated ree patterns la yb n 1 28 12 3 and conspicuous negative eu anomalies eu eu 0 07 0 43 with a similar depletion in hfse nb zr hf p ba and sr suggesting that these granitic magmas had a dominantly crustal source likely the neoproterozoic metasedimentary rocks i e the danzhou or banxi groups that are dominated by pelitic rocks with minor interlayered siltstones in the western yangtze block however their zircon hf isotopic results also reveal an important input of the mantle derived melts into their parental magma taken together their geochemical and isotopic compositions reflect a derivation by magma mixing between the crust and mantle derived 10 in volume magmas their chemical variations resulted from the fractionation process during the emplacement from the magma chamber after magmatic mixing in conjunction with regional data within the scb it is most likely that the magmatism younger than 435 ma was generated in a post collisional extensional regime related to the partial delamination of an overthickened lithospheric mantle root without lower crust

a hybrid origin for lachlan s type granites the murrumbidgee batholith example the murrumbidgee batholith is typical of other composite s type granite batholiths in the lachlan fold belt lfb of southeastern australia consisting of discrete peraluminous cogenetic granite suites however it contains a unique suite murrumbucka at its southern extremity that has chemical and petrological features transitional with metaluminous i type granites previous detailed structural and metamorphic studies have shown that the batholith is tilted northward exposing subvolcanic plutons in the north and root zones in the south originally located at depths of not vert similar10 km a transition from mafic foliated sheet like granites in the south to felsic generally less foliated homogeneous granites in the north is consistent with magma ascent via subvertical structurally controlled sheets to emplacement in an overlying magma chamber in the inferred root zones the murrumbucka suite hosts migmatitic metasedimentary and gabbroic rocks both of which have transitional contacts and show evidence for mafic felsic interaction with the murrumbucka suite the migmatites extend southward to become part of the high t low p cooma metamorphic complex which contains a core of heterogeneous remobilised diatexitic granite cooma suite lenses of which also occur throughout the southern deeper parts of the murrumbidgee batholith the composition of the most mafic rocks from the batholith lies on a chemical tie line between cooma suite granites and the gabbros for almost all elements this chemical coincidence is interpreted to reflect derivation of parental murrumbidgee s type granite magmas by bulk mixing between a felsic crustal and mafic mantle derived component consistent with sr and nd isotopic results and field observations based on the tie lines and simple numerical modelling the murrumbucka suite is estimated to be a 50 50 mix of mantle and crustal derived magmas whereas the more peraluminous and widespread clear range suite which is very typical of lachlan s type granites is modelled as a 40 60 mix chemical variation trends diverge from the mixing lines and suggest that much of the fractionation process operated after mixing mixing occurred within sheets during the ascent of the magmas whereas fractionation was the dominant process generating chemical diversity during emplacement in the magma chamber generation of the s type granites probably occurred in an extensional tectonic environment

structural development of the lower paleozoic belt of south china genesis of an intracontinental orogen abstract an extensive structural study of wuyishan and surrounding areas south china brings data on the structures formed prior to the devonian unconformity building the lower paleozoic belt an ordovician tectonic event was responsible for i south directed structures in the wuyishan proper and its southern border related to both thin and thick skinned tectonics accompanied by metamorphism and crustal thickening ii north directed structures to the west of ganjiang fault and north of jiangshan shaoxing fault where only thin skinned tectonics is visible the southward deformation accommodating the shortening includes emplacement of thrust sheets involving deep crustal material and some mantle peridotite and likely repetition of continental material both responsible for crustal thickening the orogeny was due to the underthrusting of the southern part of the south china block beneath the northern part of this block closing the pre existing nanhua rift created at around 850 800 ma and involving the rift sedimentary fill in the southward thrusting the syn metamorphic tectonic piling was followed by anatexis and granite emplacement at around 440 390 ma our new u th pb epma monazite dating gives ages of 433 9 ma and 437 5 ma early silurian for the main anatectic event and a younger age of 412 5 ma late silurian early devonian for a late re heating process the lower paleozoic belt is an intracontinental orogen without oceanic suture record it shows some similarities with the cenozoic european pyrenees chain with a strong asymmetry

petrogenetic interpretation of granitoid rock series using multicationic parameters abstract granitoid rock compositions from a range of tectonic environments are plotted on a multicationic diagram devised by de la roche and his coworkers this shows that there is a systematic change through an orogenic cycle which leads progressively to the ultimate development of alkaline magmas possible source materials and mechanisms of magma generation are considered from analysis of mineral compositional vectors these suggest that most granitoid series result from a two stage process first fractional crystallisation of clinopyroxene olivine and calcic plagioclase from a basic source with tholeiitic affinities produces a magma of intermediate composition this magma then undergoes periodic mixing with a felsic magma followed by in situ fractionation to generate individual intrusions within granitoid series

formation of the jurassic south china large granitic province insights from the genesis of the jiufeng pluton abstract the magma source and thermal regime for the formation of large granitic provinces lgps have been the subjects of much debate as mineralogical geochemical and isotopic variations of plutonic rocks preserve records of crustal thermal evolution the jiufeng pluton was comprehensively studied to gain insights into material and heat sources and tectonic process in the formation of the south china lgp sims zircon u pb analyses of a biotite granite 158 9 1 3 ma three two mica granites stage i 159 7 1 2 ma stage ii 158 6 1 2 ma and 156 8 1 2 ma and a muscovite granodiorite 159 0 4 4 ma of the jiufeng pluton suggest that they are coeval and were emplaced in a short period of no more than 5 3 m y limited variations in whole rock nd isotope end t 12 6 to 9 5 and zircon hf o isotopes ehf t 12 0 to 6 3 18 o 6 8 9 4 suggest that the jiufeng pluton was mainly derived from melting of a common metasedimentary source possibly with a minor basaltic contribution we consider the geochemical variations of the jiufeng pluton are primarily a result of incremental assembly of magma batches produced from rapid step like transition from fluid saturated to fluid absent melting of the source the muscovite granodiorite with high na 2 o 3 80 wt k 2 o na 2 o 1 is interpreted to have been produced by fluid saturated melting at low temperature 650 c high p 2 o 5 0 09 0 17 wt zircon saturation temperature t zr 769 816 c and la yb ratios 8 4 55 8 of the stage ii two mica granite support its formation from high temperature 800 c biotite dehydration melting whereas lower p 2 o 5 zr 685 742 c and la yb

age provenance and tectonic setting of neoproterozoic to early paleozoic sequences in southeastern south china block constraints on its linkage to western australia east antarctica abstract zircon u pb hf isotopes and whole rock geochemical data are applied to investigate the age provenance and tectonic setting of the late neoproterozoic and overlying sequences from the cathaysia block of south china in order to understand the tectonothermal evolution of this block and its linkage to supercontinents new zircon u pb ages in combination with previous fossil data indicate the lower and upper louziba groups were deposited between 655 and 635 ma cryogenian and 635 and 542 ma ediacaran respectively low cia 65 57 and high icv 1 64 1 05 values for the louziba sandstones are suggestive of immature sediments that experienced mild to weak chemical weathering in their source areas in contrast early paleozoic sandstones have higher cia 80 69 and lower icv 0 95 0 84 values indicative of mature sediments with intensive chemical weathering despite the different degrees of chemical weathering all sedimentary samples show positive correlation between tio2 and al2o3 and negative correlation in plot of la yb n vs zr but lack any correlation between la yb n and p2o5 these data demonstrate sorting of resistant minerals such as rutile monazite and zircon did not affect the chemical signatures of sampled detritus consequently geochemical features in combination with detrital age spectra indicate that the louziba group and overlying sequences record sedimentation in collisional and passive settings with the detritus being derived dominantly from intermediate to felsic igneous rocks late mesoproterozoic ca 1300 1000 ma detritus in the lower louziba group were probably derived primarily from the windmill islands bunger hills 1300 1050 ma and albany fraser belt musgrave 1300 1050 ma regions in east antarctica western australia increasing input of early neoproterozoic 990 950 ma detritus in the uppermost louziba group indicates a greater contribution from the eastern ghats ca 990 950 ma in eastern india and north prince charles mountains ca 990 950 ma in east antarctica this source change was synchronous with the formation of the kuunga 560 530 ma and pinjarra 560 520 ma orogens during assembly of gondwana the sediments of the huanglian formation of the uppermost louziba group likely recorded this event overlying paleozoic sequences then were deposited on passive margin with detritus being derived dominantly from an early neoproterozoic source e g eastern india and east antarctica

geochronology and geochemistry of igneous rocks from the kongling terrane implications for mesoarchean to paleoproterozoic crustal evolution of the yangtze block abstract the kongling terrane is a high grade metamorphic complex exposed in the northern part of the yangtze block it mainly consists of ttg gneisses amphibolites migmatites and metasedimentary rocks the amphibolites occur as enclaves or layers in the ttg gneisses and both the amphibolites and the ttg gneisses are intruded by granitic and diabasic dykes u pb ages of zircons from the amphibolite migmatite granite and diabase are determined by la icp ms igneous zircons from an amphibolite have an upper intercept age of 2945 13 ma mswd 1 2 while metamorphic zircons yield an upper intercept age of 2870 13 ma mswd 0 9 igneous zircons from a migmatite and a granite have weighted average 207pb 206pb age and upper intercept age of 1987 5 ma mswd 1 7 and 1985 6 ma mswd 1 3 respectively igneous zircons from a diabase dyke yield an upper intercept age of 1783 23 ma mswd 0 86 the protoliths of the amphibolites are tholeiitic basalts and exhibit slightly depleted to variably enriched lree patterns with moderate enrichment of rb and ba and depletion in nb and zr similar to oceanic island arc volcanics they have high cr co ni mgo and feot content with negative nd t values indicating that the tholeiitic basalts resulted from high degree batch melting of enriched mantle wedge the granites are strongly peraluminous with a cnk 1 1 and they have relatively low hree and y and negative nd t values detailed elemental and isotopic data suggest that they formed by partial melting of recycled products of the ttg gneisses amphibolites and older crustal material in a continental collisional setting the diabases are characterized by relatively high al2o3 feot and tio2 contents and low zr ba ratios their high initial 87sr 86sr ratios and negative nd t values indicate that they were derived from enriched lithospheric mantle in extensional setting therefore the kongling terrane occurred as an oceanic island arc at ca 2 95 ga and experienced earliest metamorphism at ca 2 87 ga which probably contributed to the formation of the high grade metamorphic terrane this terrane experienced a strong collisional event at 2 0 ga which may have led to cratonization over all of the yangtze block and also experienced lithospheric extension during 1 85 1 78 ga the global 2 0 ga paleoproterozoic collisional events were related to the assembly of supercontinent columbia

the relationship between calc alkaline volcanism and within plate continental rift volcanism evidence from scottish palaeozoic lavas lower carboniferous lavas from the midland valley and adjacent regions of scotland are mildly alkaline and intraplate in nature the sequence is dominated by basalt and hawaiite although mugearite benmoreite trachyte and rhyolite are also present basic volcanic rocks display the lil element and lree enrichment typical of intraplate alkali basalt terrains low initial87sr 86sr 0 7029 0 7046 high end 0 4 to 5 6 and moderately radiogenic206pb 204pb 17 77 18 89 ratios are also comparable with alkali basalts from other continental rifts and oceanic islands when the carboniferous lavas are compared with subduction related lavas of old red sandstone age erupted in and around the midland valley ca 50 ma earlier at 410 ma remarkable similarities are apparent significant overlap occurs in nd and pb isotopic compositions sr isotopic compositions are however more radiogenic in the older subduction related lavas this combined with high k and rb concentrations in ors lavas may be explained by the incorporation of a sediment component derived from the subducted slab which by lower carboniferous times had been lost from the mantle source region by convection a pronounced negative nb anomaly in the ors subduction related lavas may be explained by the retention of a nb bearing phase in the mantle during hydrous melting of the mantle wedge above the subduction zone allowing for the effects of the added component from the subducted slab there appears to be no necessity to invoke separate mantle source regions for the two suites of lavas both may have been derived from chemically similar portions of mantle if volcanic arc lavas are derived from the mantle wedge the implication is that such a source lies at relatively shallow depth within the upper mantle the same may therefore apply to the carboniferous continental rift basalts this evidence combined with the fact that there is no evident hot spot trail across the midland valley despite a long period of within plate volcanism and rapid plate movements during the carboniferous suggests that the alkali basalt magmatism is not the product of a deep seated mantle plume rather the volcanism appears to owe more to passive rifting and to diapiric upwelling from a source region within the uppermost mantle

elemental and sr nd isotope geochemistry of microgranular enclaves from peralkaline a type granitic plutons of the emeishan large igneous province sw china abstract microgranular enclaves are common within intermediate to felsic granitic rocks that have i and s type affinity however they are rare within alkaline anorogenic granitoids of a type affinity the permian 260 ma emeishan large igneous province elip of southwest china contains two peralkaline silica saturated a type granitic plutons that host microgranular enclaves the enclaves from the baima pluton are intermediate in composition and have lower sio 2 and higher tio 2 cao and mg sio 2 57 2 to 63 0 wt tio 2 0 8 to 1 8 wt cao 1 7 to 3 3 wt mg 28 to 44 than their host sio 2 62 6 to 67 8 wt tio 2 0 5 to 1 4 wt cao 0 4 to 1 8 wt mg 15 to 31 the enclaves from the taihe pluton are more felsic sio 2 63 8 to 71 3 wt tio 2 0 3 to 0 6 wt cao 0 6 to 2 3 wt mg 8 to 22 but are still less evolved than their host sio 2 69 8 to 75 1 wt tio 2 0 2 to 0 6 wt cao 0 4 to 0 8 wt mg 3 to 12 in both cases the enclaves have very similar end t values baima end t 2 8 to 3 2 taihe end t 1 0 to 2 0 as their hosts baima end t 3 0 to 3 2 taihe end t 1 5 to 1 9 the major and trace element trends of the enclave host pairs suggest that fractional crystallization occurred and that element diffusion was likely minimal the enclaves are interpreted as entrained accumulations of early formed crystals of a silicic magma which was originally produced by fractional crystallization of a mafic magma

the oldest known rocks in south western china shrimp u pb magmatic crystallisation age and detrital provenance analysis of the paleoproterozoic dahongshan group abstract the paleoproterozoic dahongshan group outcrops along the present day southwest margin of the yangtze craton it is the oldest rock found in the region and forms the basement for the late mesoproterozoic sedimentary successions the dahongshan group is a sequence of volcanic and sedimentary rocks which have undergone structural deformation and metamorphism to upper greenschist lower amphibolite facies some of the original sedimentary features such as unimodal cross bedding in quartzite and rounded conglomerate clasts are still preserved suggesting deposition in a fluvial environment abundant carbonate rocks also found within the sedimentary sequence indicate periods of shallow marine or lagoonal conditions a weighted mean shrimp 206pb 207pb age of 1675 8 ma for a tuffaceous schist unit confirms the paleoproterozoic formation age of the group shrimp and lam icp ms u pb analysis of detrital zircon grains from metasedimentary units indicate a late archean to paleoproterozoic ca 2780 1860 ma source region the age spectrum of the detrital zircons is similar to that of the late archean and early paleoproterozoic basement rocks previously identified in the yangtze craton but includes a population of paleoproterozoic grains ca 2400 2100 ma which have no known source region in the exposed yangtze craton

a possible buried paleoproterozoic collisional orogen beneath central south china evidence from seismic reflection profiling abstract the south china craton consisting of the yangtze and cathaysia blocks has figured importantly in the neoproterozoic supercontinent of rodinia however its lack of grenville age structures and metamorphism and the presence of paleoproterozoic high grade metamorphism and s type magmatism raise questions about the timing and nature of the yangtze cathaysia collision here we present results of a 400 km long high resolution seismic reflection profile across the purported suture between the yangtze and cathaysia blocks the seismic profile reveals folded and thrust imbricated seismic reflectors which we interpret to represent the relics of the yangtze cathaysia collisional orogen the inferred orogen was extended in the neoproterozoic resulting in its burial by neoproterozoic flysch strata geochronological data suggest that this buried orogen was formed in the paleoproterozoic 2 0 1 9 ga likely associated with the assembling of the columbia supercontinent these results call for major revision of the models for the formation of the south china craton and its role in the assembling of the rodinia and the columbia supercontinents

detecting low frequency earthquakes within non volcanic tremor in southern taiwan triggered by the 2005 mw8 6 nias earthquake 1 we use a matched filter technique to detect 41 low frequency earthquakes lfes within 700 s of triggered tremor signals in the southern central range in taiwan during the surface waves of the 2005 mw8 6 nias earthquake off the coast of northern sumatra the depth distributions of lfes after double difference relocations concentrate at the depth range of 12 38 km below the background seismicity and above the moho depth inferred from receiver function studies the locations of lfes are close to the downward extension of the steep dipping chaochou lishan fault with only modestly high vp vs ratios 1 75 1 85 our observation indicates that at least portions of triggered tremor consists of many lfes similar to ambient tremor observed at other major plate boundary faults

geochronological and geochemical study of palaeoproterozoic gneissic granites and clinopyroxenite xenoliths from nw fujian se china implications for the crustal evolution of the cathaysia block abstract u pb zircon dating of gneissic granite samples in northwestern fujian province southeast china gave neoarchean to palaeoproterozoic ages 2 6 1 9 ga for the rounded zircon cores and palaeoproterozoic upper intercept ages 1851 21 ma 1857 29 ma respectively for zoned rims that have oscillatory growth zoning and yield e hf t values between 8 8 and 3 7 and t dm model ages between 2 1 and 2 6 ga these gneissic granites are peraluminous a cnk 0 94 1 28 characterized by high sio 2 68 72 wt al 2 o 3 14 15 wt and low cao mgo feo t tio 2 and p 2 o 5 contents they also possess relatively low ree contents and moderate lree hree fractionation and display pronounced negative anomalies in nb ta sr p and ti all these features indicate that the precursor magmas were likely formed by partial melting of sedimentary rocks clinopyroxenite xenoliths with gneissosity structures are found in these gneissic granites and they have high mgo ni and cr but low contents of tio 2 and are characterized by high cao al 2 o 3 4 1 5 3 low al 2 o 3 tio 2 ratios 8 7 9 5 and hree depletion these features are similar to the typical al depleted type komatiites their geochemical characteristics such as high gd yb n and low hree y zr and hf suggest partial melting of the upper mantle source with residual garnet the negative nb ta and ti anomalies indicate a strong arc related affinity our results and recently published data for granitic and mafic basement rocks in this region reveal that 1 85 ga was an important period for the evolution of the cathaysia block possibly related to the columbia supercontinent assembly large amount of granitic magma was emplaced probably as a result of partial melting of old neoarchean to palaeoproterozoic materials

zircon u th pb hf isotopes of the basement rocks in northeastern cathaysia block south china implications for phanerozoic multiple metamorphic reworking of a paleoproterozoic terrane abstract the cathaysia block records a complex tectonic history the understanding of which is central to the debate on the evolution of the south china block and its position in various supercontinent assemblies here we investigate two key formations from this block the mayuan group in the northern fujian province northeastern cathaysia block and its equivalent the badu group in southwestern zhejiang province previous studies traced the paleoproterozoic records from the badu group whereas a neoproterozoic age was proposed for the mayuan group the rocks sampled in this study from both groups show similar mineral assemblages of garnet sillimanite biotite plagioclase quartz k feldspar muscovite graphite as well as high contents of sio 2 and al 2 o 3 typical of amphibolite to granulite facies metapelitic rocks zircon u pb data yield two discordia intercept ages of 1990 ma and 2450 ma from one sample and discordia intercept ages of 3 5 ga 2 5 ga 1 86 ga and 233 ma from another in the badu group zircons in two samples from the mayuan group yield intercept ages of 1859 ma and 249 ma in one sample and 2 6 ga 1 87 ga 257 ma and a weighted mean 206 pb 238 u age of 248 ma in the other the ca 1 86 1 87 ga and 230 250 ma ages are interpreted to represent the time of metamorphic reworking because zircon grains of these ages tend to have low th u ratios flat hree patterns and unzoned internal texture as revealed by cathodoluminescene cl images these results confirm that the badu group is a paleoproterozoic lithostratigraphic unit and also suggest that at least part if not all of the mayuan group is paleoproterozoic evidence for paleozoic metamorphic reworking that is considered to have affected the whole of cathaysia block is not revealed in this study in contrast our data clearly show obvious mesozoic metamorphic reworking at ca 230 250 ma zircon hf t values range from 19 to 11 with a peak at 5 5 and show t dm c hf ranging from 1 9 to 4 1 ga with a peak at ca 2 7 3 0 ga suggesting that a major crustal growth took place during this time this interpretation is consistent with the previously suggested crustal growth peaks of 2 7 ga and 2 9ga a synthesis of the reliable geochronological data gathered so far on phanerozoic metamorphic reworking of the northeastern cathaysia block reveals that the imprints of these tectonothermal events is differently distributed in the different zones rocks metamorphosed during the paleozoic tectonothermal event dominantly occur in the western zone whereas those reworked by high grade metamorphism during the mesozoic tectonothermal event mainly outcrop in the eastern zone our study alerts the previous notion of a uniform distribution of the reworked rocks by high grade metamorphism all across the northeastern cathaysia block and provides new insights on the evolution of the south china block

crustal generation of the himalayan leucogranites abstract detailed studies of the himalayan two mica leucogranites such as the manaslu pluton indicate that they have very uniform mineralogical petrological and structural characteristics one can relate their occurrence to the thickest zones of the underlying tibetan slab in these zones migmatization attains its greatest development and vertical extension the granite is emplaced at first along the main disharmonic plane above the main central thrust mct at the top of the tibetan slab infrastructure ductile deformation of the granite is variable the granite being syn to late kinematic with regard to the functioning of the mct major elements are very homogeneous except for na and k implying that p t conditions of melting were relatively uniform the melted material was of a similar composition over a vast volume and the percentage of melting was small 10 15 trace elements are highly variable some are characteristic for very evolved material ta rb cs u or show the link with the tibetan slab ba sr whilst others are problematic th ree ree and th abundances being much less in the granite than in the tibetan slab imply that they have been extracted during one of the main stages of formation possibly by monazite radiogenic pb sr nd and stable o isotopes are consistent with the origin of the granite from the tibetan slab however the heterogeneous sr isotopic ratios make age dating difficult and imply poor mixing or little fluid interaction during its evolution h isotope data indicate that magmatic compositions of the main body of the manaslu granite have been preserved late or post magmatic alterations are extremely local in the main pluton to the north another belt of two mica granites occurs whose characteristics are very similar to the high himalaya belt they were probably generated in a similar way during this recent intracontinental evolution

zircon u pb ages and hf isotope compositions of the chencai migmatite central zhejiang province south china constraints on the early palaeozoic orogeny u pb ages and hf isotope compositions of zircons from the chencai complex in zhejiang province have been determined to provide constraints on mechanisms of migmatization and tectonic evolution related to the early palaeozoic orogeny in the cathaysia block south china zircons from leucosome samples of migmatites are characterized by nebulous overgrowths enclosing inherited cores or occur as newly formed grains with weak zoning five samples gave weighted mean ages ranging from 438 3 ma to 432 4 ma which are interpreted as recording the time of anatexis of a regional tectono thermal event their e hf t values range from 21 4 to 4 8 with peak at 11 with corresponding t dm2 ages of 1 73 2 77 ga with peak at c 1 9 2 3 ga suggesting that the protoliths formed by reworking of ancient crust evolved from late palaeoproterozoic early archaean crust mantle differentiation the migmatization was spatially and temporally associated with reported 460 435 ma metamorphism with a clockwise pressure temperature p t path and was most likely controlled by crustal thickening driven by the early palaeozoic orogenesis the t dm2 ages of the chencai complex are consistent with those of the wuyi yunkai structural belt in the cathaysia block but distinct from those with peak at 2 7 3 0 ga of the badu complex which lacks early palaeozoic tectono thermal records the data support the suggestion that a postulated geological entity instead of the east domain the badu complex being its main part of the cathaysia block was probably involved in the early palaeozoic orogeny

constraining timing and tectonic implications of neoproterozoic metamorphic event in the cathaysia block south china abstract the cathaysia block of the south china craton includes a proterozoic basement that experienced a prolonged precambrian crustal evolution but to date lacks evidence of proterozoic metamorphic ages at lichuan and jianning in the wuyi domain of the eastern cathaysia block proterozoic rock units include migmatized paragneiss of the wanyuan group and minor amphibolite of the tianjingping formation which are enveloped by schist of mayuan group and all are intruded by paleozoic and mesozoic igneous rocks detrital zircon grains from the wanyuan paragneiss display metamorphic rims that yield concordant weighted average 206pb 238u ages of 860 6 ma and 435 5 ma along with variably disconcordant ages with lower intercept ages of 442 41 ma the zircon core ages range from 3015 ma to 851 ma with three major age populations at 930 865 ma 1850 1200 ma and 2650 2400 ma detrital zircon grains from mayuan schist samples at jianning generally lack core rim structures and yield three main age populations at 860 736 ma 1835 1775 ma and 2720 2500 ma metamorphic ages of ca 860 ma and ca 435 ma for the wanyuan paragneiss along with the youngest detrital zircon constrain the depositional age of the protolith to ca 865 860 ma whereas the mayuan group is younger and probably deposited after ca 736 ma characteristics of detrital zircon age populations along with regional geological data suggest accumulation of the wanyuan group in a convergent and or collisional setting metamorphism and a possible subduction collision process within the cathaysia block at around 860 ma suggest it was not a unified block in early neoproterozoic the growth of ca 440 ma metamorphic rims is likely related to granitic magmatism such as that exposed in the lichuan region the sparse evidence for early neoproterozoic metamorphism likely reflects widespread overprinting by the paleozoic tectonothermal event at around 440 ma

a synthesis of the geologic evolution of taiwan abstract the island arc of taiwan is composed of cenozoic geosynclinal sediments more than 10 000 m thick lying on a pre tertiary metamorphic basement pleistocene to miocene andesitic islands surround the main island and are related mostly to arc magmatism the penghu island group in the taiwan strait is covered with pleistocene flood basalt neogene shallow marine clastic sediments are exposed mainly in the western foothills with pleistocene andesitic extrusives at the northern tip and the northeastern offshore islands a thick sequence of paleogene to miocene argillitic to slaty metaclastic rocks underlies the western central range and forms the immediate sedimentary cover on the pre tertiary metamorphic complex to the east which represents an older mesozoic arc trench system the coastal range in eastern taiwan is a neogene andesitic magmatic arc including also a large variety of volcaniclastic and turbiditic sediments cenozoic taiwan is the site of arc continent collision where the luzon arc on the philippine sea plate overrides the chinese continental margin on the eurasian plate east and northeast of taiwan the polarity of subduction changes whereby the oceanic philippine sea plate is subducting beneath the ryukyu arc system on the eurasian plate continent arc collision in taiwan island is anomalous and may occur in a broad belt of deformation rather than along a well defined plate boundary or subduction zone

u pb and hf isotopic study of zircons from migmatised amphibolites in the cathaysia block implications for the early paleozoic peak tectonothermal event in southeastern china abstract amphibolites occur in a number of localities in the cathaysia block some of them have been migmatised and their protoliths represent basaltic magmas erupted in various tectonic settings four migmatised amphibolites were collected from jiangxi and fujian provinces cathodo luminescence images of zircons extracted from the representative amphibolites show unzoned or sector zoned structure la icp ms analysis indicates that most zircons have high th u ratios and yield u pb zircon ages of 446 5 435 2 434 4 and 423 2 ma respectively lu hf isotopic analysis on these zircons gives hf model ages ranging from 900 to 1200 ma based on lithological observations and previously published geochronological data we interpret that these u pb ages record an important tectonothermal event that led to the migmatization this early paleozoic caledonian tectonothermal event in the southeastern china has a great tectonic implication for the evolutionary history of the cathaysia block

detrital zircon u pb hf isotopes and whole rock geochemistry of neoproterozoic cambrian successions in the cathaysia block of south china implications on paleogeographic reconstruction in supercontinent abstract paleogeographic reconstruction of the cathaysia block of south china and its link with orogeny associated with rodinia and gondwana remain enigmatic leading to controversial models proposed for supercontinental configuration this study presents detailed provenance analysis of late neoproterozoic to early paleozoic sedimentary successions from the cathaysia block to trace the sedimentary provenance that was essentially linked to previously adjacent continents blocks zircon u pb dating combined with published paleontological evidence suggests an ediacaran depositional age range 633 542 ma for the lechangxia group while deposition of the bacun group most likely commenced during early cambrian the older strata were sourced mainly from recycling of quartz rich sedimentary rocks while the younger one received sediments dominantly from felsic igneous rocks high sio2 contents 54 6 79 3 wt and k2o na2o ratios 0 69 35 7 detrital zircon age distribution and abundant monocrystalline quartz indicate a passive setting for ediacaran and early cambrian successions in great contrast the middle cambrian strata was likely deposited in collisional setting considering relatively lower sio2 contents 67 9 74 5 wt and k2o na2o ratios 0 97 1 63 plus absence of old zircons this tectonic transition is consistent with the change from quartz arenite to arkose that could be related to a disconformity between early and middle cambrian strata dominant late mesoproterozoic detritus 1250 1050 ma in the lechangxia group was possibly sourced from older grenville orogens 1300 1050 ma in western australia and east antarctica because rocks of these ages are rare in the cathaysia block similarly abundant early neoproterozoic clastic 1000 950 ma in the late ediacaran and early cambrian successions imply an increasing input from younger grenville orogens 990 950 ma in east antarctica and eastern india thus the cathaysia block was a part of the east gondwana assembly and was linked to eastern india east antarctica and western australia during ediacaran and cambrian the source change was coeval with the pinjarra orogeny 550 520 ma and the early stage of the kuunga orogeny 570 550 ma in middle cambrian strata the middle neoproterozoic sediments 850 700 ma were mainly derived from intrinsic sources likely to be the wuyishan terrane this provenance variation can be linked to the middle cambrian uplift of the cathaysia block in synchronous with the later phase of the kuunga orogeny 530 480 ma

spatial heterogeneity of tectonic stress and friction in the crust new evidence from earthquake focal mechanisms in taiwan we performed inversions of earthquake focal mechanisms in central taiwan to investigate the heterogeneity of the stress field and fault strength and temporal variations of stress parameters friction and pore pressure associated with the 1999 chi chi earthquake we divided the focal mechanism data into two groups before and after the chi chi earthquake and analysed them separately with the assumption of a uniform stress field the friction coefficient is mostly within a range of 0 2 0 4 in central taiwan which is lower than the commonly quoted laboratory result 0 6 0 85 the low friction coefficient is also inferred by the rotation of principal stress axes after the chi chi earthquake by contrast if we assume that the friction is constant and failures occur on optimally oriented planes we find that the resulting stress orientations must be spatially variable however a large dispersion of stress orientations is not seen in borehole breakouts and fault slip data implying a constant friction model might be ruled out our analysis suggests that either the distribution of the coefficient of friction or pore pressure changed during the 1999 chi chi earthquake we infer that the pore pressure probably rose in the chi chi rupture area and northern longitudinal valley and dropped in the areas south of the coseismic rupture area after the main shock

tectonics and surface effects of the supercontinent columbia abstract assembly of the supercontinent columbia at about 1 85 1 90 ga coincided with several events that affected the entire earth the oldest worldwide network of orogenic belts formed at the same time although some granite granodiorite gg suites had formed earlier the gg suites became common in the 1 8 1 9 ga orogenic belts these suites succeeded the older tonalite trondhjemite granodiorite ttg suites which were not produced after 1 8 ga changes on the earth s surface at 1 8 1 9 ga include rapid increase in the concentration of oxygen in the atmosphere and oceans and probably the evolution of eukaryotes all of these surface changes occurred as columbia accreted and the assembly of columbia may have contributed to the drastic changes in the earth s surface environment as well as to the evolution of primitive life forms

remote triggering of non volcanic tremor around taiwan summary we perform a systematic survey of triggered deep non volcanic tremor beneath the central range cr in taiwan for 45 teleseismic earthquakes from 1998 to 2009 with mw 7 5 and epicentral distance 1000 km to the broad band station tpub triggered tremors are visually identified as bursts of high frequency 2 8 hz non impulsive and long duration seismic energy that are coherent among many seismic stations and modulated by the teleseismic surface waves out of the 45 earthquakes we identified nine teleseismic events associated with nine tremor sources in the southern and five in the northern cr most of the tremor sources are located within the depth range of 15 25 km in the lower crust above the moho we find that the amplitudes of the surface waves play an important role in determining the triggering potential and the apparent triggering threshold is 0 1 cm s 1 or 7 8 kpa however such threshold is partially controlled by the background noise level which could prevent weaker tremor triggered by surface waves with smaller amplitudes from being identified the amplitudes of the triggered tremor show a positive correlation with the amplitudes of the triggering surface waves consistent with the predictions by the clock advance model in addition to amplitudes other factors such as frequency contents and incidence angles also affect the triggering potential we find that intermediate period 30 10 s surface waves could trigger modulate tremors suggesting that long period 30 s surface waves are not always required in long range triggering tremors appear to be triggered by both love and rayleigh waves when the incidence angles are parallel to the strike of the cr all six events triggered tremor primarily during the rayleigh waves for strike normal incidence only the 2001 mw7 8 kunlun earthquake showed predominant love wave triggering this observation can be qualitatively explained by a simple coulomb failure for a left lateral shear on the low angle detachment fault beneath the southern cr

geochronology and petrogenesis of the early paleozoic i type granite in the taishan area south china middle lower crustal melting during orogenic collapse abstract the early paleozoic wuyi yunkai orogeny represents the first extensive tectonothermal event in south china since the neoproterozoic break up of the rodinia supercontinent the early paleozoic igneous rocks in the south china block scb are important for understanding the processes responsible for the orogeny the taishan batholith in the inner cathaysia block consists dominantly of granodiorite and hornblende bearing granite zircon u pb dating of four samples gives a consistent crystallization age of ca 436 ma synchronous with widespread late orogenic s type granitic intrusions in the wuyi yunkai orogen the taishan granitoid rocks are metaluminous or peraluminous a cnk 0 94 1 15 and have variable sio 2 61 5 74 9 wt mgo 0 4 2 4 wt fe 2 o 3 1 8 7 4 wt and cao 1 1 5 5 wt with gradually decreasing p 2 o 5 from mafic to felsic components thus the taishan batholith displays the petrological and geochemical characteristics of i type granites additionally all samples have strongly negative e nd t values 11 to 9 suggesting an overall ancient source the early paleozoic i type granitoid rocks in the taishan area were derived from partial melting of ancient middle to lower crust at a high thermal gradient 35 c km and underwent extensive fractional crystallization of hornblende plagioclase and biotite the wuyi yunkai orogenic collapse may have been responsible for middle to lower crustal melting of the cathaysia block the early paleozoic i and s type granitoid magmatism lack significant input of a juvenile component indicating that the processes accompanying orogenic collapse such as asthenosphere upwelling and basaltic underplating contributed heat to trigger widespread early paleozoic felsic magmatism in the scb an intraplate orogenic event instead of continental or arc collision may be the preferred model for the wuyi yunkai orogeny

ambient tremors in a collisional orogenic belt deep seated tectonic tremors have been regarded as an observation tied to interconnected fluids at depth which have been well documented in worldwide subduction zones and transform faults but not in a collisional mountain belt in this study we explore the general features of collisional tremors in taiwan and discuss the possible generation mechanism in the 4 year data we find 231 ambient tremor episodes with durations ranging from 5 to 30 min in addition to a coseismic slip induced stress change from nearby major earthquake increased tremor rate is also highly correlated with the active normal faulting earthquake swarms at the shallower depth both the tremor and earthquake swarm activities are confined in a small area where the high attenuation high thermal anomaly the boundary between high and low resistivity and localized veins on the surfaces distributed suggesting the involvement of fluids from metamorphic dehydration within the orogen

the xiong er volcanic belt at the southern margin of the north china craton petrographic and geochemical evidence for its outboard position in the paleo mesoproterozoic columbia supercontinent abstract the xiong er volcanic belt covering an area of more than 60 000 km 2 along the southern margin of the north china craton has long been considered an intra continental rift zone and recently interpreted as part of a large igneous province formed by a mantle plume that led to the breakup of the paleo mesoproterozoic supercontinent columbia however such interpretations cannot be accommodated by lithology mineralogy geochemistry and geochronology of the volcanic rocks in the belt lithologically the xiong er volcanic belt is dominated by basaltic andesite and andesite with minor dacite and rhyolite different from rock associations related to continental rifts or mantle plumes which are generally bimodal and dominated by mafic components however they are remarkably similar to those rock associations in modern continental margin arcs in some of the basaltic andesites and andesites amphibole is a common phenocryst phase suggesting the involvement of h 2 o rich fluids in the petrogenesis of the xiong er volcanic rocks geochemically the xiong er volcanic rocks fall in the calc alkaline series and in most tectono magmatic discrimination diagrams the majority of the xiong er volcanic rocks show affinities to magmatic arcs in the primitive mantle normalized trace element diagrams the xiong er volcanic rocks show enrichments in the lile and lree and negative nb ta ti anomalies similar to arc related volcanic rocks produced by the hydrous melting of metasomatized mantle wedge nd isotope compositions of the xiong er volcanic rocks suggest that 5 15 older crust has been transferred into the upper lithospheric mantle by subduction related recycling during archean to paleoproterozoic time available shrimp and la icp ms u pb zircon age data indicate that the xiong er volcanic rocks erupted intermittently over a protracted interval from 1 78 ga through 1 76 1 75 ga and 1 65 ga to 1 45 ga though the major phase of the volcanism occurred at 1 78 1 75 ga such multiple and intermittent volcanism is inconsistent with a mantle plume driven rifting event but is not uncommon in ancient and existing continental margin arcs taken together the xiong er volcanic belt was most likely a paleo mesoproterozoic continental magmatic arc that formed at the southern margin of the north china craton similar paleo mesoproterozoic continental magmatic arcs were also present at the southern and southeastern margins of laurentia the southern margin of baltica the northwestern margin of amonzonia and the southern and eastern margins of the north australia craton which are considered to represent subduction related episodic outbuilding on the continental margins of the paleo mesoproterozoic supercontinent columbia therefore in any configuration of the supercontinent columbia the southern margin of the north china craton could not have been connected to any other continental block as proposed in a recent configuration but must have faced an open ocean whose lithosphere was subducted beneath the southern margin of the north china craton

insolation driven 100 000 year glacial cycles and hysteresis of ice sheet volume comprehensive climate and ice sheet models show that insolation and internal feedbacks between the climate the ice sheets and the lithosphere asthenosphere system explain the 100 000 year period on which the northern hemisphere ice sheets grow and shrink for the past million years earth s climate has been dominated by a 100 000 year cycle of glaciation involving the advance and retreat of the northern hemisphere ice sheets variations in insolation the solar radiation reaching earth s surface caused by constant changes in earth s orbit and orientation to the sun are closely associated with glaciations but the physical mechanisms driving the cycle have remained unclear now ayako abe ouchi and colleagues demonstrate that insolation variations interact with the size of the ice sheets and the solid earth to control the 100 000 year cycles their model shows that internal feedbacks between insolation climate and the elevation of the land beneath the ice sheet allows ice sheets to grow gradually until a maximum size is reached at that point the lowered ice sheet elevation caused by the delayed rebound of the solid earth means that a small increase in insolation can destroy the ice sheet in just a few thousand years the growth and reduction of northern hemisphere ice sheets over the past million years is dominated by an approximately 100 000 year periodicity and a sawtooth pattern1 2 gradual growth and fast termination milankovitch theory proposes that summer insolation at high northern latitudes drives the glacial cycles3 and statistical tests have demonstrated that the glacial cycles are indeed linked to eccentricity obliquity and precession cycles4 5 yet insolation alone cannot explain the strong 100 000 year cycle suggesting that internal climatic feedbacks may also be at work4 5 6 7 earlier conceptual models for example showed that glacial terminations are associated with the build up of northern hemisphere excess ice 5 8 9 10 but the physical mechanisms underpinning the 100 000 year cycle remain unclear here we show using comprehensive climate and ice sheet models that insolation and internal feedbacks between the climate the ice sheets and the lithosphere asthenosphere system explain the 100 000 year periodicity the responses of equilibrium states of ice sheets to summer insolation show hysteresis11 12 13 with the shape and position of the hysteresis loop playing a key part in determining the periodicities of glacial cycles the hysteresis loop of the north american ice sheet is such that after inception of the ice sheet its mass balance remains mostly positive through several precession cycles whose amplitudes decrease towards an eccentricity minimum the larger the ice sheet grows and extends towards lower latitudes the smaller is the insolation required to make the mass balance negative therefore once a large ice sheet is established a moderate increase in insolation is sufficient to trigger a negative mass balance leading to an almost complete retreat of the ice sheet within several thousand years this fast retreat is governed mainly by rapid ablation due to the lowered surface elevation resulting from delayed isostatic rebound14 15 16 which is the lithosphere asthenosphere response carbon dioxide is involved but is not determinative in the evolution of the 100 000 year glacial cycles

topographic isostatic evaluation of new generation goce gravity field models 1 we use gravity implied by the earth s rock equivalent topography ret and modeled isostatic compensation masses to evaluate the new global gravity field models ggms from european space agency esa s gravity field and steady state ocean circulation explorer goce satellite gravimetry mission the topography is now reasonably well known over most of the earth s landmasses and also where conventional ggm evaluation is prohibitive due to the lack or unavailability of ground truth gravity data we construct a spherical harmonic representation of earth s ret to derive band limited topography implied gravity and test the somewhat simplistic airy heiskanen and pratt hayford hypotheses of isostatic compensation but which did not improve the agreement between gravity from the uncompensated ret and goce the third generation goce ggms based on 12 months of space gravimetry resolve the earth s gravity field effectively up to spherical harmonic degree 200 220 90 100 km resolution such scales could not be resolved from satellites before goce from the three different goce processing philosophies currently in use by esa the time wise and direct approaches exhibit the highest sensitivity to short scale gravity recovery being better than the space wise approach our topography implied gravity comparisons bring evidence of improvements from goce to gravity field knowledge over the himalayas africa the andes papua new guinea and antarctic regions in attenuated form goce captures topography implied gravity signals up to degree 250 80 km resolution suggesting that other signals originating e g from the crust mantle boundary and buried loads are captured as well which might now improve our knowledge on the earth s lithosphere structure at previously unresolved spatial scales

the crust of cathaysia age assembly and reworking of two terranes abstract u pb dating and hf isotope data from detrital zircons have been used to analyse the crustal evolution of the eastern and western parts of the cathaysia block in se china zircons from the oujiang river in eastern cathaysia indicate that the basement is dominantly paleoproterozoic 1850 1870 ma 2100 2400 ma in age with minor archean components it was extensively reworked in jurassic cretaceous time 100 155 ma to produce the widespread yanshanian magmatic suite both the 1850 1870 ma event and the yanshanian magmatism show wide ranges in hf isotope composition consistent with mixing between crustal and juvenile magmas marked downstream changes in the relative proportions of zircon age populations emphasize the care required in using detrital zircon data to estimate continental growth rates zircons from the north river indicate that the crust of western cathaysia was generated mainly during neoproterozoic time although it contains some archean 2500 3500 ma to mesoproterozoic components this crust was strongly reworked during caledonian ca 450 ma indosinian ca 240 ma and early yanshanian ca 160 ma thermal events there is little evidence for juvenile crustal growth in any of these events although the yanshanian granitoids of the eastern and western parts of the cathaysia give similar mean hf model ages consistent with published nd model ages this is a coincidence they have different crustal sources the distinct patterns of crustal evolution suggest that eastern and western cathaysia may represent separate microcontinents accreted to the older yangtze craton and transposed by extensive strike slip faults

multi stage crust mantle interaction in se china temporal thermal and compositional constraints from the mesozoic felsic volcanic rocks in eastern guangdong fujian provinces abstract the mesozoic geology of southeastern se china was characterized by emplacement of voluminous felsic magmas we present zircon u pb hf isotopic compositions and whole rock major and trace elemental compositions of a series of felsic volcanic rocks from the eastern guangdong and fujian provinces the volcanic rocks have been divided into three formations the douling nanyuan and shimaoshan fms zircon u pb dating defines three eruption cycles 168 145 ma for the douling fm 143 130 ma for the nanyuan fm and 104 95 ma for the shimaoshan fm in situ hf isotope analyses on dated zircon yield an ehf t range of 13 8 to 6 9 and a tdm hf range from 1400 to 486 ma with a peak at ca 930 ma the majority of the mesozoic felsic lavas have more radiogenic hf than that of metamorphic basement rocks and related igneous rocks requiring the involvement of a juvenile component in their origin the mesozoic felsic magmas are predominantly dacites and rhyolites with only minor basalt and andesite and show relatively homogeneous hf isotopic compositions of zircon in single sample combined these data indicated that neither differentiation of a mantle derived magma nor mixing of magmas from different sources was a dominant petrogenetic process both the ehf t of zircon and the magma temperature increase from the douling and nanyuan fms to the shimaoshan fm all these facts imply an increasing contribution of contemporaneous underplated hot and dry mantle derived magmas as both a heat source inducing crustal melting and as a source of material melt that variably mixed with the local crustal melts the felsic crustal melt component was generated at amphibolite facies conditions with a residual assemblage of plagioclase hornblende garnet zircon our results suggest a progressive role of crust mantle interaction in generating the episodic felsic volcanic eruption during the mesozoic

inner edge of habitable zones for earth sized planets with various surface water distributions when planets receive insolation above a certain critical value called the runaway threshold liquid surface water vaporizes completely which forms the inner edge of the habitable zone because land planets can emit a large amount of radiation from the dry tropics they have a higher runaway threshold than aqua planets do here we systematically investigated the runaway threshold for various surface water distributions using a three dimensional dynamic atmosphere model the runaway threshold for the meridionally uniform surface water distribution increases from the typical value for the aqua planet regime 130 s0 to one for the land planet regime 155 s0 as the dry surface area increases where s0 is the present earth s insolation although this result is similar to the previous work considering zonally uniform surface water distributions the runaway threshold for the land planet regime is quite low compared to that of the previous work this is because a part of the tropical atmosphere is always wet for the meridionally uniform case we also considered the surface water distributions determined by the earth s mars and venus topographies we found that their runaway thresholds are close to that for the meridionally uniform cases and the amount of water at the boundary between an aqua and land planet regime is around 10 of the earth s ocean this clearly shows that the runaway threshold is not determined uniquely by the luminosity of the central star but it has a wide range caused by the surface water distribution of the terrestrial water planet itself

major lithologies of the high grade zhoutan terrane within the cathaysia block and their tectonic implications for the neoproterozoic paleozoic south china abstract pre devonian crystalline basement of the cathaysia block was united through the wuyi yunkai orogeny the uplifted orogenic core is dominantly composed of paleozoic high grade terranes occurring in a northeast southwest trending belt the zhoutan terrane in the middle part of this belt is one of the key terranes to reveal the tectonic architecture of the south china block however how to correlate it with other high grade terranes what kind of rock records are preserved in this terrane and their tectonic implications for the wuyi yunkai orogen are still unresolved this paper carries out an integrated study on the petrology zircon u pb geochronology geochemistry and zircon lu hf isotopic compositions of the main rock types in the zhoutan terrane with a purpose of better understanding the wuyi yunkai orogeny mafic granulite noritic of high t and low p conditions occurs as lenses within the zhoutan formation the zircon u pb geochronological data give magmatic and metamorphic ages of 460 450 ma and 430 ma respectively neoproterozoic and paleoproterozoic 1 89 ga xenocrysts also occur meta pelites of the zhoutan formation experienced high t p amphibolite facies metamorphism and ages of their detrital zircons are dominantly neoproterozoic 810 820 ma with some subordinate paleo to mesoproterozoic and archean signatures amphibole gneiss layers of this formation give a paleozoic metamorphic age of 450 ma consistent with the high p amphibolite facies metamorphism of the neighboring longyou terrane fine grained garnet mica schist of the wanyuan formation consists of greenschist to amphibolite facies mineral assemblage and detrital zircons of these meta sediments display both neoproterozoic and early precambrian ages at 770 ma 820 ma 850 ma 1 84 ga and 2 48 ga metamorphosed calc silicates of the wanyuan formation show similar detrital zircon age peaks of 624 ma 780 790 ma 810 ma 840 850 ma 1 8 1 98 ga and 2 48 ga and a paleozoic metamorphic age of 447 ma the cizhu pluton is a composite granitoid body intruding the zhoutan and wanyuan formations with emplacement ages ranging at least from 485 ma to 446 ma ehf t values of zircons from the mafic granulite range from 10 9 to 8 5 indicating its protolith derived from enriched mantle sources with or without crustal contaminations the cizhu magmatic event is a crustal reworking event as indicated by the coherently negative zircon ehf t values and ancient hf crustal model ages of 1530 1830 ma zircon ehf t values of the metamorphosed calc silicates show large variations ranging from 21 2 to 11 consistent with its complex provenances the deposition of the sedimentary rocks of the zhoutan and wanyuan formations is consistent with the development of the neoproterozoic paleozoic nanhua rift the development of continental margin sequences like the zhoutan and wanyuan formations the evolution of the paleozoic metamorphism from early higher p t metamorphic stage to later lower p t stage as well as the generation of the paleozoic mafic ultramafic rocks and the intermediate cizhu pluton are best explained by collision to post collision settings we therefore conclude that the zhuotan terrane as well as other high grade terranes witnessed a whole geological cycle from early rifting to the closure of a rifted basin nanhua basin and the final lithospheric delamination following the rejoining of continental blocks flanking the rift

automated detection and location of cascadia tremor 1 a new autonomous seismic location and detection methodology enables real time opportunities for high resolution spatio temporal monitoring of non volcanic tremor combining a unique cross correlation technique with epicenter clustering analysis in northern cascadia automatically yields thousands of tremor epicenters from the may 2008 episodic tremor and slip ets event while routinely detecting and locating inconspicuous inter ets tremor bursts although ets events in this area produce about two weeks of continuous tremor we find a nearly equal amount of tremor during the last 15 month inter ets period the resulting ets and inter ets epicenters occur in the slow slip region where the plate interface is 30 45 km deep and have a sharp well resolved updip boundary about 75 km east of the downdip edge of the seismogenic megathrust zone this ability to track tremor with high spatio temporal resolution facilitates automatic tremor monitoring and the mapping of the transition zone and regions of locked zone stress accumulation

texture and geochemistry of pyrite from the gaoche gold deposit in south china implications of ore forming processes abstract the gaoche lode type gold deposit is hosted in the neoproterozoic metasedimentary rocks at the northeastern cathaysia block in south china the deposit comprises three ore bodies that are controlled by the ne trending faults four generations of pyrite and associated paragenesis minerals have been identified the py0 formed in the early sericite quartz alteration period unrelated to the gold mineralization contains abundant silicate inclusions and has low au 0 0 03 ppm ag 0 79 6 4 ppm and as 0 93 402 ppm the py1 shows oscillatory zonings and has high invisible au 0 01 1 8 ppm ag 0 01 106 ppm and as 0 46 1163 ppm representing the first stage mineralization the py2 also has high invisible au 0 08 8 9 ppm ag 0 06 37 ppm and as 5 5 10275 ppm reflecting pseudomorphic dissolution of the py1 and reprecipitation at the second stage mineralization the py3 lacks mineral inclusions but is chemically similar to the py2 available 34s values for the mineralization related pyrites range from 11 4 to 7 2 and their variation suggests the fluids were originally released from magmas followed by fluid rock interaction under redox states at shallow depth in addition the py1 and py2 show high lead isotope compositions which lie above the orogen curve in the pb pb isotope plots indicating that the ore forming materials were probably sourced from the metamorphic basement rocks in combination with the regional geology and mineralization the gaoche gold deposit is suggested to have been genetically associated with regional metamorphism and deformation in relation to the western subduction of the paleo pacific oceanic slab during the mesozoic

zircon u pb and hf isotopic study of mesozoic felsic rocks from eastern zhejiang south china geochemical contrast between the yangtze and cathaysia blocks abstract the jiangshan shaoxing fault zone jsfz in zhejiang province has been proposed to represent a suture between the yangtze and cathaysia blocks in south china in this study in situ zircon u pb and hf isotopic analysis and whole rock major and trace element measurement of early to middle cretaceous felsic rocks across the fault zone were conducted to constrain the nature of the fault zone twelve cretaceous granitoid bodies were sampled from the nw and se sides of the fault zone respectively with composition ranging from diorite to granite sio2 56 2 76 6 wt these granitoids yielded u pb ages ranging from 135 100 ma with a systematic variation in zircon hf isotopic compositions ehf t 6 9 to 7 0 in the nw side vs 1 9 to 12 9 in the se side the tdm2 values for the granitoids from the nw side are 0 34 to 1 33 ga with two peaks at ca 876 and 1170 ma respectively whereas those from the se side are 0 70 to 1 62 ga with a single peak at ca 1126 ma the hf isotopic disparity for the two sides may indicate a fundamental difference in the lower crustal compositions of the yangtze and cathaysia blocks supporting that the jsfz is possibly a suture zone between the two blocks our results together with the available geological data suggest that the mesoproterozoic materials are important for both the yangtze and cathaysia basement and the neoproterozoic magmatic activities were important in the yangtze block possibly related to the break up of the rodinia supercontinent but less significant in the cathaysia block this may imply that the two blocks have not completely juxtaposed in the neoproterozoic

ca 830 ma back arc type volcanic rocks in the eastern part of the jiangnan orogen implications for the neoproterozoic tectonic evolution of south china block abstract the volcanic rocks in the eastern part of jiangnan orogen provide an important suite to investigate magma petrogenesis and tectonic evolution of south china block we carried out a combined study of geochronology and geochemistry for the bimodal association of basalts and rhyolitic tuffs from the shuangqiaoshan group in south anhui province zircons from a basalt and two rhyolitic tuff samples yield crystallization ages of 829 12 ma 832 9 ma and 832 10 ma respectively the basalts are characterized by left sloping ree distribution patterns enriched large iron lithosphere elements liles e g rb ba k and depleted high field strength elements hfses e g nb ta relative to those in n morb resembling those of modern central lau back arc basin basalts babb in addition they have positive nd t values 4 6 to 5 7 with initial 87sr 86sr ratios in the range of 0 70706 0 70719 these geochemical and isotopic signatures indicate that the basalts were derived from an n morb like source modified by slab derived fluids the rhyolitic tuffs have high sio2 72 9 78 3 wt contents and high 10 000 ga al ratios mostly 2 6 taking into account their aluminous nature a cnk 1 1 they are affinitive to aluminous a type granites they invariably show a relative enrichment in light ree and strong negative eu anomalies eu eu 0 16 0 31 and negative anomalies in hfse sr and p compared with the neighboring elements in their trace element spidergrams their nd t values range from 2 1 to 0 1 detailed elemental and isotopic data suggest that the rhyolitic tuffs formed by partial melting of mafic lower crust triggered by asthenosphere upwelling the occurrence of bimodal volcanic rocks with the basalts showing geochemical affinity to intra oceanic babb appears to support the development of a neoproterozoic back arc basin in the eastern part of jiangnan orogen at ca 830 ma combined with the published data from central and western part of jiangnan orogen it is inferred that the back arc basin developed in the entire jiangnan orogen during 850 810 ma considering the age of the strata overlying the folded basement sequence we suggest that the final collision of the yangtze and cathaysia blocks happened during 820 810 ma along the jiangnan orogen

shrimp u pb zircon geochronology and geochemistry of metavolcanic and metasedimentary rocks in northwestern fujian cathaysia block china tectonic implications and the need to redefine lithostratigraphic units abstract northwestern fujian province is one of the most important pre palaeozoic areas in the cathaysia block of south china metavolcano sedimentary and metasedimentary rocks of different types ages and metamorphic grades granulite to upper greenschist facies are present and previously were divided into several formations and groups tectonic contacts occur between some units whereas deformed unconformities have been reported between others new shrimp u pb zircon ages presented here indicate that the original lithostratigraphy and the old group and formation terminology should be abandoned thus the tianjingping formation was not formed in the archaean or palaeoproterozoic as previously considered but must be younger than its youngest detrital zircons 1790 ma but older than regional metamorphism 460 ma besides magmatic zircon ages of 807 ma obtained from metavolcano sedimentary rocks of the nanshan formation and 751 728 ma for the mamianshan group many inherited and detrital zircons with ages ranging from 1 0 to 0 8 ga were also found in them these ages indicate that the geological evolution of the study area may be related to the assembly and subsequent break up of the rodinia supercontinent the new zircon results poorly constrain the age of the mayuan group as neoproterozoic to early palaeozoic 728 458 ma and not palaeoproterozoic as previously thought many older inherited and detrital zircons with ages of 3 6 2 8 2 7 2 6 2 5 2 0 1 8 and 1 6 ga were found in this study a 3 6 ga detrital grain is the oldest one so far identified in northwestern fujian province as well as throughout the cathaysia block nd isotope tdm values of eight volcano sedimentary and clastic sedimentary rock samples centre on 2 73 1 68 ga being much older than the formation ages of their protoliths and thus showing that the recycling of older crust played an important role in their formation these rocks underwent high grade metamorphism in the early palaeozoic 458 425 ma during an important tectono thermal event in the cathaysia block