reply to the comment by zhang et al on first finding of a type and adakitic magmatism association in songpan garze fold belt eastern tibetan plateau implication for lithospheric delamination we greatly thank zhang et al 2008 this issue for their comments on our recent paper zhang et al 2007 regarding the tectonic setting of the indosinian granitoids in the songpan garze fold belt as it gives us an opportunity to further clarify our viewpoints the discrepancy between zhang et al 2008 this issue and zhang et al 2007 h f zhang r parrish l zhang w c xu h l yuan s gao and q g crowley a type granite and adakitic magmatism association in songpan garze fold belt eastern tibetan plateau implication for lithospheric delamination lithos 97 2007 pp 323 335 article pdf 1219 k view record in scopus cited by in scopus 7 zhang et al 2007 is focused on the interpretation of the tectonic setting of the indosinian a type granite 211 1 ma and adakitic magmatism 216 221 ma in the songpan garze fold belt we emphasize that these magmatic rocks formed in post collisional tectonic setting while zhang et al 2008 this issue suggest that they were related to the northward subduction of the jinsajian tethys

geochronological and geochemical constraints on mesozoic suturing in east central tibet 1 this paper reports isotopic major and minor element geochemistry of igneous and metamorphic rocks from the kokoxili and yushu regions of central and eastern tibet the first region lies along the kunlun suture which separates the bayan har songpan ganze songpan terrane from the tarim and qaidam blocks two kokoxili granitoids yield u pb zircon dates of 217 10 and 207 3 ma late triassic which represent the time of emplacement and rb sr isochron dates of 195 3 and 190 3 ma early jurassic which are interpreted as cooling ages the geochemical signatures of these granitoids suggest that they are related to subduction continuing into the late triassic in the yushu area three samples help constrain the age of the jinsha suture which separates the songpan terranes from the qiangtang blocks a leucocratic granite and an orthogneiss in the suture zone yield u pb zircon dates of 206 7 and 204 1 ma respectively and a paragneiss south of it a u pb monazite date of 244 4 ma the existence of coeval magmatism in both the jinsha and kunlun sutures suggests that the two subduction zones were simultaneously active combining isotopic dating with structural evidence on subduction polarity and paleomagnetic reconstructions we propose that the kunlun and qinling block boundaries which were distinct in the permian subsequently formed a continuous late triassic northward subducting plate margin our data suggest that the jinsha suture correlates with the benzilan and nan uttaradit sutures which together belong to a major late triassic subduction zone

indosinian deformation of the songpan garze fold belt northeast tibetan plateau abstract deformation of the songpan garze fold belt southwest china occurred progressively during the indosinian orogeny ca 230 200 ma two phases of south directed deformation are identified d 1 associated with pervasive thickening of the songpan garzi sedimentary pile and detachment of that portion of the fold belt onlapping the yangtze passive margin and d 2 a localized phase of deformation associated with transpressive shortening of the fold belt s southeast margin an arcuate form surface in the southeast fold belt reflects the distribution of d 1 and d 2 structural phases and the diachronous migration of these phases into the fold belt this migration accommodated lateral growth and cumulative sinistral transpression within the fold belt s eastern margin an enigmatic metamorphic complex the so called danba antiform is constrained to have formed during the early stages of the d 2 deformation this was associated with diapiric upwelling of buoyant migmatised basement and domal distortion of the overlying metasedimentary pile

sources and petrogenesis of late triassic zhiduo volcanics in the northeast tibet implications for tectonic evolution of the western jinsha paleo tethys ocean abstract the western jinsha suture representing a branch of the paleo tethys makes the junction between the songpan ganzi complex and the north qiangtang block nqb however the triassic tectonic evolution of the western jinsha paleo tethys ocean is poorly constrained this paper shows new la icp ms zircon u pb geochronology major and trace elements and sr nd hf isotopic compositions of the zhiduo dacitic volcanic rocks in the northern margin of the nqb la icp ms zircon u pb dating displays that they erupted between 227 2 and 228 1 ma these rocks exhibit high silica medium k calc alkalic signature and low mgo tio2 p2o5 cr and ni compositions they are enriched in light rare earth elements and large ion lithophile elements with moderate to strong negative eu anomalies eu eu 0 24 0 56 and depleted in heavy rare earth elements and high field strength elements they possess relatively low initial 87sr 86sr ratios 0 7055 0 7084 positive whole rock end t 0 2 to 1 4 and magmatic zircon ehf t values focus on 9 2 2 7 with young nd and hf model ages t2dm nd 885 983 ma t2dm hf 467 1035 ma these features are comparable to those of typical arc volcanic rocks and indicate that they were probably derived from partial melting of a juvenile crust source the eruption of these volcanic rocks could be attributed to southward subduction of the western jinsha paleo tethys ocean in combination with previous studies we propose that the subduction of the western jinsha paleo tethys ocean initiated at the beginning of the late triassic ca 228 ma and closed at the end of the late triassic ca 208 ma in addition there is a universal northeastward younging trend of arc magmas in the northern margin of the nqb which may indicate a model involving the initial shallow subduction and subsequent slab roll back

delamination and detachment of a lithospheric root abstract orogens can be formed by shortening of the lithosphere on time scales much shorter than that of thermal diffusion for time scales of a few ma the major part of the uplift of an orogen which may occur during the transition from crustal shortening to extension may be due to delamination of the mantle lithosphere rather than to convective thinning the negative buoyancy forces of a lithospheric root may cause lateral shortening while the lithospheric root delaminates peels off from the crust detaches and is replaced by hot upwelling asthenosphere this provides a large input of heat into the crust geological observations show that the late orogenic phase may be characterized by volcanic activity high temperature low pressure metamorphism extension high heat flow and a rapid within a few ma increase of the topography we present dynamically and thermally consistent numerical models of the evolution of an orogenic root mantle rheology is based on laboratory data of olivine while models with the rheology of olivine do not allow for delamination due to its very high viscosity the incorporation of a quasi brittle rheology is necessary for modelling delamination of the mantle lithosphere successful delamination models are always associated with a rapid change in topography we examined the parameter range of our models and found different regimes of behaviour of the orogenic root of the mantle lithosphere no delamination delamination delamination and full detachment for a fixed viscosity of the upper crust of 1023 pa s delamination occurs for a range of viscosities of 1023 pa s to 1020 pa s for the lower crust depending on the thickness of the lithospheric root but full detachment of the delaminated lithospheric slab only occurs if the viscosity of the lower crust is greater than approximately 1021 pa s hot asthenosphere is welling up into the gap opening during delamination further implications of our models are lithospheric roots or unsupported slabs of at least 100 170 km depth extent are needed to provide sufficient negative buoyancy to allow delamination and detachment in all delamination models significant amounts of lower crustal material are subducted into the mantle in applying our models to selected orogens we argue that mantle delamination may have played an important role in the variscan and the himalayan orogenies

tectonometamorphic evolution of the eastern tibet plateau evidence from the central songpan garze orogenic belt western china the songpan garze orogenic belt northeastern tibet plateau introduction experienced polyphase deformation and metamorphism that is best the songpan garze orogenic belt sgob occupies developed in the danba domal metamorphic terrane ddmt the northeastern portion of the tibet plateau fig 1a in which three tectonometamorphic events can be identified the first which comprises the tectonically distinct tethyan event d1 m1 is characterized by prograde sericiteto kyanite grade himalayan domain between india and eurasia and is barrovian metamorphism during late indosinian 205 190 ma characterized by a polyphase continent continent colcrustal thickening and shortening a subsequent early yanshanian lisional history dewey et al 1988 the tibet plateau 165 ma sillimaniteto migmatite grade event m2 developed has been the subject of a number of studies which have during predominantly east west compression d2 a final greenfocused on the himalayas themselves and on the major schist facies event m3 is best developed in shear zones of probable strike slip faults within or fault zones along the margin himalayan age p t conditions during m1 varied from 3 5 of the tibet plateau e g dirks et al 1994 roger et al kbar and 410 530 c biotite zone to 5 3 8 kbar and 1995 arne et al 1997 meng zhang 1999 in 570 600 c staurolite and kyanite zones and during m2 from comparison little work has focused on the interior of the 4 8 6 3 kbar and 640 680 c sillimanite zone to 5 8 6 2 sgob as the sgob resulted from the closure of the kbar and 660 725 c migmatite zone clockwise p t t segpalaeo tethys and subsequent continental collision and ments were inferred for the staurolite kyanite and sillimanite zones convergence associated with interaction between the timuscovite dehydration melting during m2 was largely responsible bet indochina south china and north china blocks for the generation of migmatites and locally voluminous pegmatites xu et al 1992 the multiphase deformational and metathe polyphase tectonometamorphic evolution of the eastern tibet morphic evolution of the sgob is critical for underplateau as documented in the danba area resulted from interactions standing the mesozoic tertiary tectonometamorphic between the indian tibet and the south and north china blocks evolution of the tibet plateau the eastern tibet plateau experienced limited uplift during the metamorphic studies of the sgob are hampered by its mesozoic followed by large scale uplift and rapid cooling during generally low grade lower greenschist facies however the tertiary himalayan orogeny poorly documented medium pressure barrovian type metamorphic complexes as high as upper amphibolite grade occur locally and are associated with basementcored structural domes mattauer et al 1992 the best examples are the danba domal metamorphic terrane

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

convective instability of thickening mantle lithosphere mantle lithosphere being colder and therefore denser than the underlying mantle is prone to convective instability that can be induced by horizontal shortening numerical experiments on a cold layer with imposed horizontal shortening are carried out to examine the relative importance of mechanical thickening thermal diffusion and gravitational instability in deforming the layer this analysis is then used to develop a method for determining which of these styles dominates for a layer thickening at a given rate if viscosity is non newtonian the imposition of shortening decreases the lithospheric strength which causes perturbations to the lithosphere s temperature structure to grow exponentially with time once these perturbations become sufficiently large they then grow super exponentially with time eventually removing the lithospheric base because lithospheric viscosity is highly temperature dependent at most only the lower 30 per cent of the lithosphere participates in the downwelling associated with this initial super exponential growth event after this event however a downwelling develops that removes material advected into the region of downwelling by horizontal shortening the magnitude of this persistent downwelling depends on the rate and duration of shortening if the total amount of shortening does not exceed 50 per cent doubling of crustal thickness then this downwelling extends to a depth three to four times the thickness of undeformed lithosphere and forms a sheet significantly thinner than the width of the region undergoing shortening once shortening stops this downwelling is no longer replenished by the shortening process and should then detach due to its inherent gravitational instability the hottest 60 per cent of the mantle lithosphere could be removed in such an event which would be followed by an influx of hot buoyant asthenosphere that causes rapid surface uplift because more cold material is removed after the cessation of shortening than by the initial gravitational instability the former has a potentially greater influence on surface uplift the tibetan interior is thought to have been shortened by about 50 per cent in 30myr and afterwards at 8ma experienced a period of rapid uplift that may have resulted from the removal of a large downwelling finger of cold lithosphere generated by shortening

lithospheric scale folding numerical modelling and application to the himalayan syntaxes we describe the eastern and western himalayan syntaxes which are large scale antiforms situated at geodynamically similar locations and the metamorphic evolution of which is coeval in the india asia collisional history to understand the mechanical plausibility of the structural interpretation we present two dimensional finite element modelling of lithospheric folding the models reveal the coeval development of adjacent synformal basins analogous to the peshawar and kashmir basins on both sides of the western syntaxis similarities between geological data and calculated models indicate that lithospheric buckling is a basic response to large scale continental shortening and an efficient mountain building process

detrital zircon provenance of the late triassic songpan ganzi complex sedimentary record of collision of the north and south china blocks the majority of the songpan ganzi triassic flysch sequence is believed to have derived from denudation of the dabie and sulu ultrahigh pressure uhp metamorphic belt in eastern china e g nie et al 1994 1 zhou and graham 1996 2 however intense debate still exists on the sources of

u pb zircon dating geochemical and sr nd hf isotopic compositions of motuo quartz monzonite implication for the genesis and diversity of the high ba sr granitoids in orogenic belt abstract early paleogene granitoids in southern lhasa subterrane have been widely investigated and many petrogenesis and geodynamic models have been proposed in the past few years however contemporaneous granitoids in the motuo tectono magmatic belt southeast lhasa terrane are still limitedly studied here we present the petrology zircon u pb geochronology whole rock geochemistry and sr nd hf isotope data of the damu and 52 k quartz monzonite in the motuo area la icp ms u pb zircon dating shows that they have magma crystallization ages of 49 and 69 ma respectively the damu quartz monzonite sio2 63 76 68 33 wt is high k calc alkaline k2o 2 54 4 02 wt with k2o na2o 0 59 1 09 and metaluminous to weakly peraluminous a cnk 0 99 1 07 the 52 k quartz monzonite sio2 61 12 66 12 wt shows slightly higher k2o contents 3 80 5 28 wt with k2o na2o 1 03 1 45 and metaluminous series a cnk 0 96 1 00 the analyzed samples are characterized by high ba 850 2573 ppm sr 534 986 ppm contents and fractionated ree patterns la yb n 22 72 and sm yb n 4 55 8 24 these geochemical features are comparable with those of high ba sr granite they display weakly evolved sr nd hf compositions whole rock 87sr 86sr 0 0 7068 to 0 7086 end t 4 20 to 3 41 and zircon ehf t 5 2 to 0 9 geochemical and sr nd hf isotopic data reflect that the damu and 52 k quartz monzonite represent residual magma from afc processes of lithospheric mantle derived mafic melts the over thickened lower crust in the eastern lhasa terrane had been delaminated during ca 83 70 ma which led to the replacement of ancient lithospheric mantle by the juvenile lithospheric mantle the juvenile mantle wedge in the study area was suspected to be metasomatized by melts that were derived from the foundering arc root rather than the subducted sediments thus the early paleogene high ba sr magmas from the se lhasa terrane may provide evidence for recycling of continental lithosphere by density destruction

structural evidence for the permo triassic tectonic evolution of the yidun arc eastern tibetan plateau abstract deformation across the yidun arc occurred as a two phase process i early triassic deformation of paleozoic sequences resulting from closure of the jinsha jiang suture and ii late triassic deformation of middle to late triassic rocks as a result of final basin closure deformation in the western yidun arc is dominantly west vergent a north south striking west directed thrust the jinsha thrust forms the boundary between the western yidun arc and the eastern qiangtang block the jinsha thrust incorporates within it ophiolitic material of the jinsha jiang suture and the structural geometry across the jinsha thrust suggests that early triassic subduction was east dipping along the jinsha jiang suture deformation within middle to late triassic sequences of the eastern yidun arc eastern qiangtang block and western songpan garze fold belt is thin skinned being restricted largely to a single generation of upright folds and associated thrusts late triassic westwards subduction underneath the combined western yidun arc qiangtang block resulted in arc volcanism in the eastern yidun arc coeval arc volcanics in the northern margin of the qiangtang block suggest that the suture along the northern margin of the qiangtang block may be correlated with the garze litang suture rather than the jinsha jiang suture

high sr y and la yb ratios the meaning of the adakitic signature abstract the name of adakite is used to describe a far too large group of rocks whose sole common feature is high sr y and la yb ratios however such a signature can be achieved via different processes melting of a high sr y and la yb source deep melting with abundant residual garnet fractional crystallization or afc or interactions of felsic melts with the mantle causing selective enrichment in lree and sr over hree a database of the compositions of adakitic rocks including high silica and low silica adakites martin h smithies r h rapp r p moyen j f champion d c 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 2 1 24 continental adakites and archaean adakites was assembled geochemical modeling of the potential processes is used to interpret it and reveals that 1 the classical model of slab melting provides the best explanation for the genesis of high silica adakites 2 low silica adakites are explained by garnet present melting of an adakite metasomatized mantle i e at depths greater than 2 5 gpa 3 continental adakites is a term encompassing a huge range of rocks with a corresponding diversity of petrogenetic processes and most of them are different from both low and high silica adakites 4 archaean adakites show a bimodal composition range with some very high sr y examples similar to part of the ttg suite reflecting deep melting 2 0 gpa of a basaltic source with a relatively high sr y while lower sr y rocks formed by shallower 1 0 gpa melting of similar sources comparison with the archaean ttg suite highlights the heterogeneity of the ttgs whose composition spreads the whole combined range of hsa and archaean adakites pointing to a diversity of sources and processes contributing to the ttg suite

exhumation of the dabie shan ultra high pressure rocks and accumulation of the songpan ganzi flysch sequence central china the presence of coesite and diamond bearing ultra high pressure uhp metamorphic rocks in the dabie and sulu regions central china suggests that a 100 km thick crustal section 4 x 106 km6 in volume has been denuded this volume is comparable to that represented by the 10 15 km thick middle to upper triassic flysch rocks in the songpan ganzi region regional geology and radiometric dates are compatible with an interpretation that the majority of these sedimentary rocks were derived from denudation of the orogenic belt between north and south china blocks following their latest paleozoic to triassic collision this correlation implies a high denudation rate of 4 mm yr that may have been in part promoted by tropical precipitation and extreme topographic relief for as long as 25 m y

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

provenance of the triassic songpan ganzi flysch west china abstract the songpan ganzi basin of western china is a triangular structure on the northeast corner of the qinghai tibet plateau filled with a great thickness of triassic flysch surrounded to the north by the eastern kunlun orogenic belt that extends farther eastwards as the qinling dabie belt to the southwest by the ganzi litang ophiolite and yidun volcanic arc and to the east by the longmenshan thrust belt we have carried out the most extensive petrographic and geochemical study so far undertaken on typical turbidites from the songpan ganzi basin including major element trace element rb sr and sm nd isotopic analyses in the baryan har region on the northern side of the basin petrographic study identifies the sandstones as typical graywackes whose matrix is fine grained detrital material recrystallized by greenschist facies metamorphism chemical weathering indices confirm that the fine grained turbidites were little altered during transport and deposition so compositions accurately represent their source areas major trace and isotope analyses displayed on various discriminant diagrams indicate that the flysch from the baryan har region is derived from the eastern kunlun belt to the north a conclusion supported by published paleocurrent directions comparison of the baryan har turbidites with published results from western sichuan on the eastern side of the songpan ganzi basin suggests that the latter is derived from a source area of varied sedimentary rocks and a few igneous rocks the most likely source is south qinling to the northeast presently available data do not support derivation of material in the songpan ganzi basin from the yidun volcanic arc or the dabie ultra high pressure metamorphic belt the present provenance studies on the triassic songpan ganzi flysch support a westward opening model for the paleotethyan ocean and reveal different evolutional histories for the qilian qinling dabie and eastern kunlun orogens

u pb dating on single detrital zircon grains from the triassic songpan ganze flysch central china provenance and tectonic correlations the songpan ganze flysch belt central china covers a huge triangular area of more than 200 000 km and is bounded by the continental blocks of south china north china and the tibetan plateau detrital zircons extracted from three flysch samples collected in the central part of the belt were analyzed grain by grain using the u pb method two samples of middle triassic sandstones collected at different locations in the belt provide identical results which suggests similar source regions the detrital zircons yield a wide range of ages and indicate their principal derivation from mid proterozoic 1 8 2 0 ga source rocks with minor contribution from late archean ca 2 5 2 6 ga material the discordance and pb loss patterns from low u zircons indicate disturbances during a subsequent event which may be of caledonian age as suggested by concordant zircon grains at ca 420 and 450 ma one sample collected within the palang shan pass zone provides concordant zircon grains at around 230 ma 231 1 ma and 233 1 ma these triassic ages are synchronous to flysch deposition and suggest intense geological activity calc alkaline volcanism at that time in the area close to the basin the data support an origin of the clastic material mainly from a northeastern landmass corresponding to the southern margin of the sino korean craton to a lesser degree inputs from the yangtze craton and possibly from the northern margin of the basin kunlun arc are also detected the age spectrum from the upper triassic sandstone is significantly different and shows predominance of sinian ca 760 ma grains probably derived from the yangtze craton this change in the source region is interpreted as reflecting the tectonic evolution of this area and in particular as being linked to the late triassic collision between south china and north china in the middle triassic while subduction of the songpan sea northward beneath the north china plate was still taking place continental subduction of south china in the dabie region was responsible for uplift of the overriding plate i e the sino korean craton which supplied large volumes of sediments during the late triassic clockwise rotation of the south china block uplifted the indo sinian part of the qinling belt and closed the basin as the accretionary wedge was thickening along the southern margin of north china detritus derived from this source region were unable to reach the flysch basin the age spectrum presented by detrital zircons indicates

a type granite and adakitic magmatism association in songpan garze fold belt eastern tibetan plateau implication for lithospheric delamination abstract the songpan garze fold belt covers a huge triangular area 200 000 km2 confined by the south china yangtze north china and tibetan plateau continental blocks in the songpan garze fold belt triassic adakitic granitoids have been identified however whether there are triassic a type granites is unclear here we report our first finding of an a type granite nianbaoyeche which occurs in the central part of the songpan garze fold belt the nianbaoyeche granite 820 km2 is characterized by arfvedsonite in its mineral assemblage using both la icpms and tims u pb zircon dating methods we obtain a magma crystallization age of 211 1 ma which is slightly younger than triassic adakitic granitoids 216 221 ma in the songpan garze fold belt the nianbaoyeche granite is enriched in si k na rb ree hfse nb ta zr hf with elevated feotot feotot mgo and ga al ratios but is depleted in al mg ca ba and sr the ree compositions show moderately fractionated patterns with la yb n 2 67 7 54 and eu eu 0 09 0 34 these geochemical characteristics indicate that the nianbaoyeche granite has an a type affinity geochemical data and u pb zircon age combined with regional studies show that the nianbaoyeche granite formed in a post collisional tectonic setting sr nd isotopic data for the granite exhibit isr 0 7090 0 7123 and end t 2 72 to 4 26 with tdm 1 15 1 51 ga suggesting that the magma has a dominantly crustal source though a minor contribution from the mantle cannot be ruled out melting to produce an a type granite may have resulted from triassic lithospheric delamination after triassic crustal thickening of the songpan garze fold belt due to convergence between the yangtze north china and north tibet continental blocks the lithospheric delamination model also helps to explain the triassic adakitic magma generation in the songpan garze belt we conclude that association of a type granite and adakitic granitoids in post collisional environment could be a useful indicator of lithospheric delamination

a temporal link between the emeishan large igneous province sw china and the end guadalupian mass extinction abstract previous studies have suggested that there were two mass extinction events in the late permian one that occurred at the permo triassic p t boundary 251 ma and a second smaller mass extinction that occurred 5 8 myr earlier at the end of the guadalupian many workers have argued that there is a causal relationship between large scale volcanic activity and mass extinctions the major mass extinction event at the p t boundary coincides with the outpouring of huge quantities of lava that formed the siberian flood basalt province in russia courtillot et al earth planet sci lett 166 1999 177 195 and wignall earth sci rev 53 2001 1 33 suggested that the earlier late permian mass extinction coincided with the eruption of the lavas that formed the emeishan flood basalt efb province in sw china however the age of eruption of the efb lavas is poorly constrained using the sensitive high resolution ion microprobe to analyze zircons we have established the age of the xinjie intrusion believed to be a feeder to the main phase of efb volcanism to be 259 3 ma hence the formation of the efb is coincident with a proposed extinction event at 256 259 ma this result supports a temporal link between the emeishan large igneous province and the end guadalupian mass extinction

tectonic evolution of the early mesozoic blueschist bearing qiangtang metamorphic belt central tibet this is the published version copyright 2003 american geophysical union all rights reserved

petrogenesis of cretaceous adakitic and shoshonitic igneous rocks in the luzong area anhui province eastern china implications for geodynamics and cu au mineralization both adakitic and shoshonitic igneous rocks in the luzong area anhui province eastern china are associated with cretaceous cu au mineralization the shaxi quartz diorite porphyrites exhibit adakite like geochemical features such as light rare earth element lree enrichment heavy ree hree depletion high al2o3 mgo sr sr y and la yb values and low y and yb contents they have low end t values 3 46 to 6 28 and high 87sr 86sr i ratios 0 7051 0 7057 sensitive high resolution ion microprobe shrimp zircon analyses indicate a crystallization age of 136 3 ma for the adakitic rocks most volcanic rocks and the majority of monzonites and syenites in the luzong area are k rich or shoshonitic and were also produced during the cretaceous 140 125 ma they are enriched in lree and large ion lithophile elements and depleted in ti and nb and ba and exhibit relatively lower end t values ranging from 4 65 to 7 03 and relatively higher 87sr 86sr i ratios varying between 0 7057 and 0 7062 the shoshonitic and adakitic rocks in the luzong area have similar pb isotopic compositions 206pb 204pb 17 90 18 83 207pb 204pb 15 45 15 62 and 208pb 204pb 38 07 38 80 geological data from the luzong area suggest that the cretaceous igneous rocks are distributed along ne fault zones e g tanlu and yangtze river fault zones in eastern china and were likely formed in an extensional setting within the yangtze block the shaxi adakitic rocks were probably derived by the partial melting of delaminated lower crust at pressures equivalent to crustal thickness of 50 km i e 1 5 gpa possibly leaving rutile bearing eclogitic residue the shoshonitic magmas in contrast originated mainly from an enriched mantle metasomatized by subducted oceanic sediments they underwent early high pressure 1 5 gpa fractional crystallization at the boundary between thickened 50 km lower crust and lithospheric mantle and late low pressure 1 5 gpa fractional crystallization in the shallow 50 km crust the adakitic and shoshonitic rocks appear to be linked to an intra continental extensional setting where partial melting of enriched mantle and delaminated lower crust was probably controlled by lithospheric thinning and upwelling of hot asthenosphere along ne fault zones e g tanlu and yangtze river fault zones in eastern china both the shoshonitic and adakitic magmas were fertile with respect to cu au mineralization

triassic continental subduction in central tibet and mediterranean style closure of the paleo tethys ocean the qiangtang metamorphic belt qmb in central tibet is one of the largest and most recently documented high pressure hp to near ultrahigh pressure near uhp belts on earth lu hf ages of eclogite and blueschist facies rocks within the qmb are 244 223 ma indistinguishable from the age of uhp metamorphism in the qinling dabie orogen results of a u pb detrital zircon study suggest that protoliths of the qmb include upper paleozoic qiangtang continental margin strata and sandstones that were derived from a paleozoic arc terrane that developed within the paleo tethys ocean to the north we attribute qmb hp metamorphism to continental collision between the qiangtang terrane and a paleo tethys arc terrane this collision and the coeval south china north china collision may have slowed convergence between laurasia and gondwana derived terranes and initiated mediterranean style rollback and backarc basin development within much of the remnant paleo tethys ocean realm

lithospheric weakening during retroforeland basin formation tectonic evolution of the central south alpine foredeep beginning with the late cretaceous convergence between europe and adria caused the southward subduction of the european plate beneath adria contractional movements at high crustal levels were partly accommodated by southward thrusting southern alps causing flexure of the loaded adriatic plate and the formation of the late cretaceous to late miocene south alpine foredeep with respect to the alpine subduction system the south alpine foredeep is a retroforeland basin in the late paleogene dip values of the base foredeep varied between 3 and 5 the shallower dips were found along profiles crossing domains which had undergone extension during previous late triassic to middle jurassic rifting with ongoing convergence the base foredeep along the entire basin steepened and dip values in the tortonian are of about 7 8 the increase in dip goes hand in hand with an increase in the curvature of the loaded plate this suggests a progressive weakening of the flexed plate modeled effective elastic thickness te values derived from the analysis of plate curvatures decrease from 15 20 km in the late paleogene to 5 km in the tortonian preconvergence values obtained from modeling studies were even higher in the 24 27 km range a practically complete decoupling of the upper middle crust from its mantle substratum enabled the increase in curvature of the hinge zone the progressive weakening of the south alpine lithosphere around the bulge zone is correlated with the its upper plate position with respect to the alpine subduction system

crustal structure beneath the eastern margin of the tibetan plateau and its tectonic implications 1 two crustal cross sections through the eastern margin of the tibetan plateau are jointly determined from deep seismic sounding the e w trending line aa passes through the western sichuan plateau including the songpan garze terrane and the longmenshan fault belt and ends in the sichuan basin a part of the yangtze craton line bb has a trend of nne and crosses the songpan garze terrane two dimensional crustal structures along the profiles were jointly determined by the additional use of existing deep seismic sounding data our seismic velocity models indicate that the western sichuan plateau and the sichuan basin have crustal thicknesses of 62 and 43 km average crustal p wave velocities of 6 27 and 6 45 km s and lower crustal vp 6 5 km s thicknesses of 27 and 15 km respectively density models constructed from the seismic velocity models are consistent with observed bouguer gravity anomalies we infer that collision between the tibetan plateau and the yangtze craton has caused thickening of the lower crust and uplift of the western sichuan plateau we detect a low velocity layer in the upper crust of the western sichuan plateau but observe no equivalence in the sichuan basin west dipping thrusts may detach into this low velocity layer the seismic phase pmp in the western sichuan plateau has low amplitude suggesting high attenuation in the lower crust qp of 100 300 we suggest that the high attenuation is a consequence of lower crustal flow caused by the large lower crustal thickness beneath the western sichuan plateau

the wulonggou metaluminous a2 type granites in the eastern kunlun orogenic belt nw china rejuvenation of subduction related felsic crust and implications for post collision extension abstract the wulonggou pluton is located in wulonggou area eastern segment of the eastern kunlun orogenic belt nw china and consists of mainly alkali feldspar granites covering an area of about 150 km2 petrogenesis of these granitoids has been investigated through an integrated study of petrography zircon u pb ages whole rock geochemistry and hf nd isotopic compositions u pb dating of magmatic zircons indicated these granites crystallized during 426 424 ma in the middle silurian the granites display high sio2 75 26 77 55 wt k2o na2o 7 98 9 03 wt extremely low mgo 0 04 0 19 wt cao 0 28 0 61 wt and tio2 0 05 0 09 wt contents showing metaluminous calcic alkali and ferroan features enrichment in rb and some hfses zr u nb ta and y depletion in sr ba p and ti mostly right inclined ree curve flat hree patterns high 10 000 ga al and intensively negative eu anomalies exhibiting an a2 type granite affinity with y nb 1 2 mostly the primitive magma of these large quantities of granites was generated under a high temperature low pressure reduced and anhydrous environment indicating intense upwelling of asthenosphere combining with the positive uniform zircon ehf t values of 0 2 to 3 8 and decoupled end t values of 4 9 to 2 1 at t 424 ma it can be concluded that subduction related juvenile materials probably calc alkaline granitoids are the source of these a type granites geochemical studies of wulonggou granites spatial and temporal distributions of regional magmatism metamorphism and sedimentary records throughout the eastern kunlun orogen belt jointly indicate that the whole orogenic belt was in a typical post collision extension setting and experienced an isostatic uplift during the middle silurian triggered by delamination after the convergence of the northeastern margin of gondwana

spheroidal structures in arfvedsonite lujavrite ilimaussaq alkaline complex south greenland an example of macro scale liquid immiscibility abstract lujavrites are meso to melanocratic agpaitic nepheline syenites and are the most highly evolved rocks of the ilimaussaq complex south greenland spheroidal bodies measuring up to 20 cm in diameter occur locally in arfvedsonite lujavrite they consist of a core rich in analcime and arfvedsonite and a rim rich in analcime aegirine and a low temperature potassium feldspar the host lujavrite is dominated by albite and arfvedsonite the igneous lamination of the host continues undisturbed through rim and core of spheroids and the arfvedsonite crystals in host rim and core have identical chemical compositions there are sharp contacts between host and rim and rim and core host rim and core contain the same accessory minerals and have almost identical chemical compositions the main differences are high h 2 o and na 2 o but low sio 2 in the core and high fe 2 o 3 in the rim the spheroids are proposed to have been formed by separation of immiscible interstitial h 2 o rich globules of magma from the host lujavrite magma at a late stage in the crystallization of the lujavritic melt

preliminary assessment of sand blows in the southern mississippi embayment trenching of sand blows in two liquefaction fields on late to mid holocene alluvium within the southern mississippi embayment was conducted to determine the origin of the sand blows and their possible relationship to new madrid seismic zone events trench stratigraphy and crosscutting relationships show evidence of multiple events of seismically induced sand venting these two fields of moderate to intense liquefaction are 175 km and 240 km southwest of the southern limit of similarly intense sand blows within the new madrid seismic zone nmsz liquefaction field the southernmost field ashley county arkansas is 25 km in diameter and sand blows are densely spaced sand blows are more sparsely spaced in the larger 30 km by 45 km northern field desha county arkansas various correlations of liquefaction events between these two fields and between each of these fields and the nmsz can be accommodated by our radiocarbon and infrared stimulated luminescence irsl ages of sediments predating and postdating vented sand deposits in our trenches these ages show a major liquefaction event at both the ashley and desha county fields between 4600 and 5500 years ago and a major event at the ashley county field approximately 700 years ago the early venting episode is older than the documented nmsz chronology and so a nmsz seismic source cannot be assessed the age of the later venting episode falls between two postulated nmsz events so it may reflect seismicity from a different source zone our trench logs show at least three other moderate to minor sand venting episodes at the ashley county field and two at the desha county field and one may be the same event at both fields about 2200 years ago if these sand blows formed during large nmsz events 175 km northeast the area of strong ground motion for this seismic source has been underestimated alternatively if these liquefaction fields formed due to local seismicity then additional significant seismic source zones are present in the mississippi embayment outside the nmsz each of the two fields sits astride seismicity alignments associated with the margins of a previously identified active fault block between the arkansas river fault zone that projects through the desha county field and the saline river fault zone that projects through the ashley county field

conflicting tectonics contraction and extension at middle and upper crustal levels along the cordilleran late jurassic arc southeastern california effects of mid mesozoic contraction followed closely in time by extension are present in mid to upper crustal plutonic rocks in the chuckwalla mountains of the eastern transverse ranges california late jurassic movement along a steeply dipping right lateral mylonitic shear zone is bracketed between 159 and 147 ma via u pb dated plutons depth of emplacement vs time data based on hornblende geobarometry and u pb geochronology of mesozoic plutons indicate that a period of dramatic uplift affected the chuckwalla mountains during the late jurassic contrasting sharply with data from the then nearby san gabriel mountains subsequent latest jurassic extensional tectonics is suggested by alkalic plutonism nearly synchronous intrusion of the late jurassic independence dike swarm and possibly by deposition of the mccoy mountains formation we conclude that both contraction and extension were significant at upper and mid crustal depths in the chuckwalla mountains region during the late jurassic and speculate that the combined influence of oblique convergence and the opening of the gulf of mexico may have caused the contrasting and nearly contemporaneous tectonic modes

partial detachment of a lithospheric root under the southeast carpathians toward a better definition of the detachment concept the southeast corner of the carpathian mountains romania provides a unique opportunity to study the very final and shortlived stage of plate convergence when a piece of subducted lithosphere finally detaches from the overriding plate and begins to sink into the mantle research efforts commonly focus on trying to image the cold lithospheric slab tomographic techniques and on trying to understand the tearing process from the earthquakes it generates here this issue is tackled from a different perspective emphasizing that the southeast carpathians are extremely low considering crustal thickness the crust there is almost as thick as the crust of the alps 50 55 km but the elevation is less than onehalf of the elevation of the alps it is interesting that the apennines italy are just the opposite they are rebounding after slab breakoff to reach elevations higher than expected from their crustal thickness 30 35 km like the crust of most passive margin coasts it is thus proposed that although seismologic studies indicate that the cold mantle body under the southeast carpathians is already partly detached from the overriding lithosphere it is still viscously coupled to the lithosphere and pulls it downward by utilizing residual topography analysis observed topography minus the contribution of the crust to it the subcrustal load is quantified and interpreted in terms of mantle lithosphere thickness extending to a depth of 300 km a particularly interesting finding is that the seismically active region is shifted 50 km east of the place of maximum pull down indicating that lithospheric tearing is now occurring at one side of the detaching root and not above it this interpretation is consistent with the idea of lateral delamination of the lower lithosphere

triassic granitoids in the eastern songpan ganzi fold belt sw china magmatic response to geodynamics of the deep lithosphere the songpan ganzi fold belt sgfb sw china was developed from a passive continental margin into an orogenic belt with the consumption of the paleo tethys during the evolution of the sgfb numerous late triassic granitic plutons formed and exhibited a progressive development from adakite i type granite high ba sr granite a type granite and monzonite representative late triassic plutons were studied to unravel the bewildering evolution of the eastern sgfb the menggu pluton 224 3 ma consists of granites with high alkali k2o na2o 7 85 10 4 wt and adakitic characteristics sr y 19 38 the endt values 2 77 to 5 03 initial 87sr 86sr ratios 0 7050 0 7063 and low nb ta ratios 8 10 are indicative of an origin by partial melting of amphibolitic lower crust rocks from the niuxingou pluton 215 3 ma are richer in k than na k2o na2o 1 1 1 5 and contain high sr 1006 1662 ppm and ba 1277 2009 ppm typical of shoshonite and high ba sr granite they have less enriched endt values 0 08 to 2 04 and less radiogenic 87sr 86sri ratios 0 7047 0 7048 and formed from a mixed melt derived from upwelling asthenosphere and the overlying metasomatised lithospheric mantle the taiyanghe pluton 205 3 ma consists of monzonites with high al2o3 20 wt but low mgo 0 94 1 39 wt the rocks are richer in na than k k2o na2o 0 7 contain high large ion lithophile element lile 681 834 ppm sr and 2142 9453 ppm ba and display strongly fractionated ree patterns la yb n 35 63 these features together with their enriched nd sr isotopic compositions endt 4 78 to 6 50 87sr 86sri 0 7074 0 7090 suggest that the monzonite probably formed from low degrees of partial melting of metasomatised lithospheric mantle although a continuous compressional regime during the mid and late triassic has been invoked for the sgfb the generation of crustally derived adakitic and shoshonitic plutons reflect thickening in response to an arc continental collision accompanied by fracturing of the lithosphere and an extensional regime in the deep lithosphere in the late triassic the 205 ma taiyanghe pluton was emplaced simultaneously with a rapid uplift of the lithosphere when surface deposits changed from deep water turbidite to tidal flat sediments it was therefore generated during decompression probably related to the rapid removal of the overthickened lithospheric mantle the triassic magmatism in the eastern sgfb is therefore important for probing geodynamic processes in the deep lithosphere

a new approach to geographic partitioning of probabilistic seismic hazard using seismic source distance with earthquake extreme and perceptibility statistics an application to the southern balkan region

high mg adakites from kadavu island group fiji southwest pacific evidence for the mantle origin of adakite parental melts we report here the compositions of primitive high mg adakite lavas from the kadavu island group fiji two distinctive high mg adakite compositions are present on kadavu and both are strongly enriched in sr and la with high sr y 120 and la yb 30 values the east kadavu high mg adakite is relatively enriched in k2o and depleted in tio2 and nb our results from kadavu suggest that there is an adakite magma series that is the result of complex open system magmatic processes including magma mixing between relatively low sio2 mantle derived melts of high mg adakite composition and high sio2 low mg adakite magma the results from kadavu island suggest that adakite magma suites worldwide are likely to include primitive high mg compositions despite the lack of fi eld evidence in many cases

timing of granite emplacement and cooling in the songpan garze fold belt eastern tibetan plateau with tectonic implications abstract new u pb and rb sr geochronology on syn and post orogenic granites provide constraints on the timing of major tectonic events in the songpan garze fold belt west sichuan china the ma nai granite was probably syn kinematic with the main deformation and yields an age of 197 6 ma that is interpreted as an upper age limit of the indosinian event zircons and apatites from the post kinematic rilonguan granite also yield jurassic ages 195 6 and 181 4 ma the post orogenic markam massif gives two ages of 188 1 and 153 3 ma both granites are undeformed and cut structures in the triassic sedimentary rocks these results demonstrate that the major deformation and decollement tectonics in the songpan garze fold belt occurred prior to the early jurassic the wide range of ages obtained for post kinematic granites from early jurassic to late jurassic suggests that locally magmatic activity persisted for a long time at least 50 ma after the indosinian compressional tectonism no tertiary ages have been obtained suggesting that these granites were not affected strongly by the india asia collision

modes of raising northeastern tibet probed by explosion seismology abstract new wide angle reflection and refraction seismic data provide constraints on the structure of the upper lithosphere and test models of its evolution to raise the northeastern part of the tibetan plateau amplitudes observed for reflections from the crust mantle boundary are sufficiently large to suggest that there is no significant partial melt in the deep crust the data show an increase of the crustal thickness between terranes from north of the kun lun fault into the qang tang of central tibet and a contrast among their intracrustal images and compositions in the north p and s velocities are consistent with a dominantly felsic composition and show that only the upper crust thickened south of the kun lun fault a thicker crust made of two layers could result from the superposition of the originally thin crust of the bayan har terrane on the lower part of the crust of the domain to the north which upper crust it shoved and thickened different modes of crustal thickening either by thickening of individual layers or superpositions and imbrication among them appear to work jointly to raise the topography

adakites without slab melting high pressure differentiation of island arc magma mindanao the philippines abstract new geochemical data for pleistocene magmatic rocks from the surigao peninsula eastern mindanao the philippines demonstrate typical adakitic traits including elevation of sr y and depletion of the heavy rare earth elements 87sr 86sr and 143nd 144nd ratios of the adakites do not support melting of the subducted philippine sea plate but resemble pliocene arc lavas generated in the same subduction zone excepting the heavy rare earth elements the adakites and arc lavas also possess similar ratios of incompatible elements suggesting that the adakites were ultimately derived from melting of the mantle wedge the wide range of sio2 in the adakites and its strong correlation with trace element concentrations and ratios indicate two possible mechanisms for generating the adakitic signature 1 adakitic melt was produced from basaltic arc magma by fractional crystallisation of a garnet bearing assemblage 2 solidified basaltic rock containing garnet melted to yield adakitic magma in either case the basaltic precursor was generated from fluid modified mantle then differentiated within the garnet stability field in surigao this requires differentiation within mantle the surigao example suggests that any subduction zone has the potential to produce adakitic magma if basalt crystallises at sufficient depth this has important implications for the geodynamics of modern and ancient subduction zones that have generated similar rocks

late triassic granitoids of the eastern margin of the tibetan plateau geochronology petrogenesis and implications for tectonic evolution abstract late triassic granitoids in the songpan garze fold belt sgfb on the eastern margin of the tibetan plateau formed at 230 to 220 ma and can be divided into two groups group 1 are high k calc alkaline rocks with adakitic affinities k adakites with sr 400 ppm y n 32 105 and high k 2 o na 2 o 1 group 2 are ordinary high k calc alkaline i types with lower sr 18 ppm and weakly fractionated ree patterns la yb n 0 50 to 0 94 and initial 87 sr 86 sr 0 70528 to 0 71086 but group 1 rocks have higher e nd t 1 01 to 4 84 than group 2 3 11 to 6 71 calculated initial pb isotope ratios for both groups are 206 pb 204 pb 18 343 to 18 627 207 pb 204 pb 15 610 to 15 705 and 208 pb 204 pb 38 269 to 3759 group 1 magmas were derived through partial melting of thickened and then delaminated ttg type eclogitic lower crust with some contribution from juvenile enriched mantle melts group 2 magmas were generated by partial melting of shallower lower crustal rocks the inferred magma sources of both groups suggest that the basement of the sgfb was similar to the exposed kangding complex and that the sgfb was formed in a similar manner to the south china basement here passive margin crust was greatly thickened and then delaminated all within a very short time interval 20 myr such post collisional crustal thickening could be the tectonic setting for the generation of many adakitic magmas especially where there is no spatial and temporal association with subduction

numerical simulations of the mantle lithosphere delamination 1 sudden uplift extension and increased igneous activity are often explained by rapid mechanical thinning of the lithospheric mantle two main thinning mechanisms have been proposed convective removal of a thickened lithospheric root and delamination of the mantle lithosphere along the moho in the latter case the whole mantle lithosphere peels away from the crust by the propagation of a localized shear zone and sinks into the mantle to study this mechanism we perform two dimensional 2 d numerical simulations of convection using a viscoplastic rheology with an effective viscosity depending strongly on temperature depth composition crust mantle and stress the simulations develop in four steps 1 we first obtain classical sublithospheric convection for a long time period 300 myr yielding a slightly heterogeneous lithospheric temperature structure 2 at some time in some simulations a strong thinning of the mantle occurs progressively in a small area 100 km wide this process puts the asthenosphere in direct contact with the lower crust 3 large pieces of mantle lithosphere then quickly sink into the mantle by the horizontal propagation of a detachment level away from the asthenospheric conduit or by progressive erosion on the flanks of the delaminated area 4 delamination pauses or stops when the lithospheric mantle part detaches or when small scale convection on the flanks of the delaminated area is counterbalanced by heat diffusion we determine the parameters crustal thicknesses activation energies and friction coefficients leading to delamination initiation step 2 we find that delamination initiates where the moho temperature is the highest as soon as the crust and mantle viscosities are sufficiently low delamination should occur on earth when the moho temperature exceeds 800 c this condition can be reached by thermal relaxation in a thickened crust in orogenic setting or by corner flow lithospheric erosion in the overriding lithosphere of subduction zones