crustal scale mass transfer geotherm and topography at convergent plate boundaries end member cases for the fate of the crust at convergent lithospheric plate boundaries accommodated by subduction are considered combining i plate kinematics slab retreat vs slab advance and ii mechanical coupling decoupling within the subducting slab slab advance is accommodated by thickening of the overriding lithospheric mantle whereas slab retreat is associated with thinning crust mantle mechanical coupling results in continental subduction whereas decoupling leads to continental accretion the thermal and topographic evolutions of the associated orogenic belts are functions of the crust mantle thickness ratio resulting from the combined effects of crust mantle coupling decoupling and slab advance retreat crustal accretion associated with slab advance results in crustal thickening and contributes to an increase in radioactive heat production and buoyancy which is impeded by concomitant thickening of the conductive and dense lithospheric mantle in contrast crustal accretion combined with slab retreat results in thickening simultaneous with thinning of the lithospheric mantle this peculiar situation is the most favourable for generating a high temperature and buoyant orogenic belt

surface wave tomography of the western united states from ambient seismic noise rayleigh and love wave phase velocity maps summary we present the results of rayleigh wave and love wave phase velocity tomography in the western united states using ambient seismic noise observed at over 250 broad band stations from the earthscope usarray transportable array and regional networks all available threecomponent time series for the 12 month span between 2005 november 1 and 2006 october 31 have been cross correlated to yield estimated empirical rayleigh and love wave green s functions the love wave signals were observed with higher average signal to noise ratio snr than rayleigh wave signals and hence cannot be fully explained by the scattering of rayleigh waves phase velocity dispersion curves for both rayleigh and love waves between 5 and 40 speriod were measured for each interstation path by applying frequency time analysis the average uncertainty and systematic bias of the measurements are estimated using a method based on analysing thousands of nearly linearly aligned station triplets we find that empirical green s functions can be estimated accurately from the negative time derivative of the symmetric component ambient noise cross correlation without explicit knowledge of the source distribution the average traveltime uncertainty is less than 1 s at periods shorter than 24 s we present rayleigh and love wave phase speed maps at periods of 8 12 16 and 20 s the maps show clear correlations with major geological structures and qualitative agreement with previous results based on rayleigh wave group speeds

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

tutorial on seismic interferometry part 1 basic principles and applications seismic interferometry involves the crosscorrelation of responses at different receivers to obtain the green s function between these receivers for the simple situation of an impulsive plane wave propagating along the x axis the crosscorrelation of the responses at two receivers along the x axis gives the green s function of the direct wave between these receivers when the source function of the plane wave is a transientas in exploration seismology or a noise signalas in passive seismology then the crosscorrelation gives the green s function convolved with the autocorrelation of the source function direct wave interferometry also holds for 2d and 3d situations assuming the receivers are surrounded by a uniform distribution of sources in this case the main contributions to the retrieved direct wave between the receivers come from sources in fresnel zones around stationary points the main application of direct wave interferometry is the retrieval of seismic surface wave responses from ambient noise and the subsequent tomographic determination of the surfacewave velocity distribution of the subsurface seismic interferometry is not restricted to retrieving direct waves between receivers in a classic paper claerbout shows that the autocorrelation of the transmission response of a layered medium gives the plane wave reflection response of that medium this is essentially 1d reflected wave interferometry similarly the crosscorrelation of the transmission responses observed at two receivers of an arbitrary inhomogeneous medium gives the 3d reflection response of that medium one of the main applications of reflected wave interferometry is retrieving the seismic reflection response from ambient noise and imaging of the reflectors in the subsurface a common aspect of direct and reflected wave interferometry is that virtual sources are created at positions where there are only receivers without requiring knowledge of the subsurface medium parameters or of the positions of the actual sources

crustal structure and seismic anisotropy near the san andreas fault at parkfield california summary receiver functions rfs from station pkd located 3 km sw of the san andreas fault saf samples the salinian terrane near parkfield crustal multiples indicate a 26 km thick crust with avp vs of 1 88 which is slightly lower 1 83 for the upper and middle crust in the west for the mid crust arrivals are observed at times corresponding to recently imaged seismic reflectors and may correspond to a layer of metasedimentary rocks below the base of the granitic batholith exposed at the surface for the lower crust rfs display strong polarity reversals with backazimuth and a change in moho amplitude that require strong seismic anisotropy 15 per cent in a low velocity high vp vs possibly serpentinite or fluid filled schist layer that has a ene dipping 35 rock fabric similar patterns of amplitude variations and polarity reversal observed in rfs for some southern california stations located west of the saf support the hypothesis that the cause of these data characteristics is a regionally prevalent lower crustal anisotropy the orientation of this anisotropic fabric is inconsistent with the recent san andreas sense of shear and is most likely a fossilized fabric of past eastward directed farallon plate subduction

metamorphic rocks in central tibet lateral variations and implications for crustal structure insights about lateral variations in the age composition and structure of the central tibetan crust are provided by geologic investigations of metamorphic rocks in the qiangtang terrane previous studies have shown that a tectonic melange of triassic age with blueschist and eclogite bearing blocks within a greenschist facies matrix is exposed over an e w distance of 600 km in the central qiangtang terrane new mapping shows that the melange extends over a n s distance of 150 km nearly to the trace of the early mesozoic jinsha suture in the north the melange exposed structurally beneath upper paleozoic to mesozoic strata in the footwalls of early mesozoic normal faults is composed mostly of paleozoic metasedimentary and crystalline rocks these findings support the hypothesis that a large part of the central and northern qiangtang terrane crust is composed of supracrustal rocks the duguer range

origin of high potassium talc alkaline i type granitoids many i type granitoid magmas are generated through partial melting of older metaigneous rocks and the compositions of such melts are broadly calc alkaline and metaluminous these melts are granitic to tonalitic and result from thermal extremes in their lower crustal source regions data on the experimental partial melting of common crustal rocks suggest that high k i type granitoid magmas can be derived only from the partial melting of hydrous calc alkaline to high k calc alkaline mafic to intermediate metamorphic rocks in the crust because of their low k2o contents metabasaltic rocks of all kinds are unsuitable as sources and models that propose mixing of mantle derived basaltic magmas and crustal melts are also inadequate there is no requirement that i type calc alkaline magmatism be related in any way to subduction processes

upper mantle p wave tomography across the longmenshan fault belt from passive source seismic observations along aba longquanshan profile abstract a passive seismic experiment across the longmenshan lms fault belt had been conducted between august 2006 and july 2007 for the understanding of geodynamic process between the eastern tibet and sichuan basin we herein collected 3677 first p arrival times with high precision from seismograms of 288 teleseismic events so as to reconstruct the upper mantle velocity structure our results show that the depth of the lithosphere asthenosphere boundary lab changes from 70 km beneath eastern tibet to about 110 km beneath longquanshan sichuan basin which is consistent with the receiver function imaging results the very thin mantle part of the lithosphere beneath eastern tibet may suggest the lithosphere delamination due to strong interaction between the tibetan eastward escaping flow and the rigid resisting sichuan basin which can be further supported by the existences of two high velocity anomalies beneath lab in our imaging result we also find there are two related low velocity anomalies beneath the lms fault belt which may indicate magmatic upwelling from lithosphere delamination and account for the origin of tremendous energy needed by the devastating wenchuan earthquake

crustal response to continental collisions between the tibet indian south china and north china blocks geochronological constraints from the songpan garze orogenic belt western china we report shrimp u th pb monazite conventional u pb titanite sm nd garnet and rb sr muscovite and biotite ages for metamorphic rocks from the danba domal metamorphic terrane in the eastern songpan garze orogenic belt eastern tibet plateau these ages are used to determine the timing of polyphase metamorphic events and the subsequent cooling history the oldest u th pb monazite and sm nd garnet ages constrain an early barrovian metamorphism m1 in the interval c 204 190 ma coincident with extensive indosinian granitic magmatism throughout the songpan garze orogenic belt a second higher grade sillimanite grade metamorphic event m2 recorded only in the northern part of the danba terrane was dated at c 168 158 ma by a combination of u th pb monazite and titanite and sm nd garnet ages it is suggested that m1 was a thermal event that affected the entire orogenic belt while m2 may represent a local thermal perturbation rb sr muscovite ages range from c 138 100 ma whereas rb sr biotite ages cluster at c 34 24 ma these ages document regional cooling at rates of c 2 3 c myr 1 following the m1 peak for most of the terrane however those parts of the terrane affected by the higher temperature m2 event e g the migmatite zone experienced initially more rapid c 8 c myr 1 cooling after peak m2 before joining the regional slow cooling path defined by the rest of the terrane at c 138 ma regional slow cooling between c 138 and c 30 ma is thought to be the result of post tectonic isostatic uplift after extensive crustal thickening caused by collision of the south and north china blocks the clustering of biotite rb sr ages marks the onset of rapid uplift across the entire terrane commencing at c 30 20 ma this cooling history is shared with many other regions of the tibet plateau suggesting that uplift of the tibet plateau including the songpan garze orogenic belt occurred predominantly in the last c 30 myr as a response to the continuing northwards collision of india with eurasia

generation and emplacement of triassic granitoids within the songpan ganze accretionary orogenic wedge in a context of slab retreat accommodated by tear faulting eastern tibetan plateau china the eastern songpan ganze accretionary orogenic wedge northern tibetan plateau is characterized by extensive magmatism coeval with middle to upper triassic closure of the paleotethys ocean along a double subduction system traced by the kunlun anyemaqen suture to the north and the jinsha litang suture to the south new field petrographic and geochemical data on mesozoic plutons intrusive into triassic metasediments of the eastern songpan ganze wedge are presented and integrated in a review of available data for the region three types of granitoids are distinguished i high k calc alkaline granite to granodiorite yanggon maoergai markam sheng meng xue sheng taiyanghe menggu manai dusong xian tagong and jiulong plutons ii high k alkaline granite nyanbaoyeche and rilong plutons and the niuxingou shoshonitic syenite and iii peraluminous s type granite markam and manai leucogranites elemental and isotopic signatures of these granitoids are consistent with the implication of both crustal yangtze craton and songpan ganze metasediments and mantle asthenospheric mantle and metasomatised lithospheric mantle sources based on i the ages of the plutons that are concomitant with the end of the northern and southern subductions of the paleo tethys ii the position of the plutons intrusive in the crustal tectonically accreted units decoupled from the downgoing plate of this double subduction system iii the diversity of their petrological and geochemical signatures we propose that magma emplaced in the songpan ganze accretionary orogenic wedge were generated at the end of the paleotethys closure in a context of slab retreat accommodated by a tear fault along the passive margin of the south china block indeed we argue that this context is the most favourable to trigger synchronous partial melting of i the suprasubduction enriched mantle wedge owing to lowering of the mantle solidus by metasomatism ii the upwelled undepleted asthenospheric mantle into the tear fault owing to decompression iii the metasomatized lithospheric continental mantle of the yangtze craton owing to the temperature increase associated to the asthenospheric upwelling and iv the rocks of the songpan ganze accretionary wedge composed of metasediments and yangtze continental margin basement owing to the combined effects of increased radioactive heat production and mantle heat flux we propose that the concentration of plutons along the eastern margin of the songpan ganze results from focussed migration and emplacement of the granitic magmas guided by the development of a tear fault along the former passive eastern margin of the yangtze craton as a consequence of the paleotethys slab retreat

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

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

enriched archean lithospheric mantle beneath western churchill province tapped during paleoproterozoic orogenesis ultrapotassic rocks of the christopher island formation baker lake basin were emplaced across an enormous area 240 000 km 2 minimum of the western churchill province ca 1 83 ga these rocks extend across the snowbird zone a geophysical feature postulated by others to represent a paleoproterozoic suture that welded the rae and hearne domains minette dikes and flows of the rae and hearne domains display identical nd 1830 values and incompatible element patterns and thus appear to have originated from a common lithospheric mantle source christopher island nd model ages cluster at 2 8 ga and nd data from one archean lamprophyre and three 2 45 to ca 2 2 ga mafic suites suggest that enriched lithospheric mantle sources beneath both the rae and hearne domains existed well before ca 1 83 ga inconsistent with paleoproterozoic suturing along the snowbird zone in contrast to commonly invoked models that envisage melting of local enriched domains christopher island ultrapotassic rocks appear to have originated from an extensive reservoir we suggest that such a reservoir was created during an archean metasomatic event perhaps owing to flat subduction and that it remained in nearly complete isolation until tapped during paleoproterozoic extension related to squeezing of western churchill crust between flanking wopmay and trans hudson orogens

cenozoic and mesozoic metamorphism in the longmenshan orogen implications for geodynamic models of eastern tibet new zircon u pb and mica 40ar 39ar dating combined with structural studies in the longmenshan orogen confirm that most of the upper crustal deformation in the eastern margin of tibet is mesozoic however at lower structural levels apatite u pb and monazite electron microprobe dating reveals a previously unknown domain of cenozoic ca 65 ma barrovian type metamorphism and deformation this discovery shows that the crust in the eastern margin of tibet was already a substantial thickness around the time of the india asia collision associated deformation has a n s oriented stretching lineation implying that deformation was not driven by topographic gradients in the tibetan plateau the observed moderate amounts of distributed postmetamorphic e w shortening can probably explain the present thickness of the continental crust in the area these results do not support models of crustal thickening caused by solid state lateral flow of midcrustal metamorphic rocks

implications of crustal property variations for models of tibetan plateau evolution shear coupled teleseismic p waves sampling the interior of the tibetan plateau provide evidence of systematic variations in crustal structure the crust thins by up to 20 km from south to north with a concomitant increase in poisson s ratio from normal values in the south to unusually high values in the north this suggests that the crust of the northern plateau is partially melted due to high temperatures these changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained

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

layered crustal anisotropy around the san andreas fault near parkfield california the rheology of the earth s crust controls the long term and short term strength and stability of plate boundary faults and depends on the architecture and physical properties of crustal materials in this paper we examine the seismic structure and anisotropy of the crust around the san andreas fault saf near parkfield california using teleseismic receiver functions these data indicate that the crust is characterized by spatially variable and strongly anisotropic upper and middle crustal layers with a moho at 35 km depth the upper layer is 5 10 km thick and is characterized by strong 30 anisotropy with a slow axis of hexagonal symmetry where the plane of fast velocity has a strike parallel to that of the saf and a dip of 40 we interpret this layer as pervasive fluid filled microcracks within the brittle deformation regime the 10 15 km thick midcrustal layer is also characterized by a weak axis of hexagonal symmetry with 20 anisotropy but the dip direction of the plane of fast velocity is reversed the midcrustal anisotropic layer is more prominent to the northeast of the san andreas fault we interpret the mid crustal anisotropic layer as fossilized fabric within fluid rich foliated mica schists when combined with various other geophysical observations our results suggest that fault creep behavior around parkfield is favored by intrinsically weak and overpressured crustal fabric

crustal mass budget and recycling during the india asia collision abstract the long lasting collision between india and asia has resulted in crustal and lithospheric deformation of both continents providing unequalled opportunities to evaluate their long term mechanisms of deformation we have quantified a crustal mass budget for the collision by comparing the present day and initial crustal volumes of the indian and asian continents involved in the collision initial crustal thickness was estimated from the non deformed parts of the continents whereas their initial extent was mapped from their contours at the onset of collision using seismic tomography we quantify the portion of the initial crustal thickness of both continents stored within the currently thickened crust or redistributed due to extrusion for the indian continent this portion amounts to about 19 km far lower than the mean observed present day crustal thickness of the indian craton of about 38 km we conclude that between 40 and 50 of indian crust has been recycled into the mantle by continental subduction corresponding to a decoupling level at about 15 19 km depth for the asian continent the estimated crustal thickness stored during collision is about 33 km close to the initial asian crustal thickness we estimate that only 3 of the asian crust was recycled into the mantle this corresponds to one episode of continental subduction occurring most probably soon after the initiation of collision along the bangong suture we identify the related slab using seismic tomography the strong contrast between india and asia implies an age dependent capacity of the continental crust to be recycled into the mantle this result has to be taken into account for further analysis of global crustal recycling during earth s history

passive seismic imaging with directive ambient noise application to surface waves and the san andreas fault in parkfield ca summary this study deals with surface waves extracted from microseismic noise in the 0 1 0 2 hz frequency band with passive seismic correlation techniques for directive noise we explore the concept of passive seismic noise tomography performed on three component sensors from a dense seismic network from the nine component correlation tensor a rotation algorithm is introduced that forces each station pair to re align in the noise direction a necessary condition to extract unbiased traveltime from passive seismic processing after rotation is performed the new correlation tensor exhibits a surface wave tensor from which rayleigh and love waves can be separately extracted for tomography inversion methodological aspects are presented and illustrated with group speed maps for rayleigh and love waves and ellipticity measurements made on the san andreas fault in the parkfield area california usa

blueschist bearing metamorphic core complexes in the qiangtang block reveal deep crustal structure of northern tibet a 500 km long belt of metamorphic exposures in the qiangtang block provides an opportunity to study the internal structure of northern tibetan crust metamorphic rocks exposed at two widely separated areas along this belt consist of blueschist bearing melange and are bounded by late triassic early jurassic domal low angle normal faults we propose that this melange was underplated to the qiangtang block and was subsequently exhumed by detachment faulting both the underplating and the exhumation occurred during early mesozoic southward subduction of oceanic lithosphere along the jinsha suture this model predicts that the deeper crust of much of northern tibet consists of accretionary melange in contrast to the continental crystalline crust of southern tibet and may account for north south variations of cenozoic tectonism in tibet

origin of potassic c type adakite magmas experimental and field constraints the postcollisional mesozoic c type or high k adakitic intrusions k adakites of china lack either temporal or spatial association with subduction and have k2o na2o around unity otherwise their geochemistry is very similar to ordinary adakites new experimental data geological observations and theoretical considerations lead us to suggest that these k adakites were high t 1050 and probably 1075 c rather hydrous magmas 6 wt h2o produced by fluid absent partial melting of metatonalites meta andesites or possibly potassic metabasalts at pressures exceeding 2 gpa their peculiar chemistry is a consequence of both the elevated k2o na2o in the protolith and the very high pressure of partial melting the most likely tectonic setting is one of extreme crustal thickening followed by delamination of the eclogitic keel and partial melting of this continental crustal material at mantle depths with high mantle heat flow during orogenic collapse

collision leading to multiple stage large scale extrusion in the qinling orogen insights from the mianlue suture abstract the geologic framework of the phanerozoic qinling dabie orogen was built up through two major suturing events of three blocks from north to south these include the north china craton including the north qinling block the qinling dabie microblock and the south china craton including the bikou block separated by the shangdan and mianlue sutures the mianlue suture zone contains evidence for mesozoic extrusion tectonics in the form of major strike slip border faults surrounding basement blocks a late paleozoic ophiolite and a ca 240 200 ma thrust belt that reformed by 200 150 ma thrusts during a type intracontinental subduction the regional map pattern shows that the blocks are surrounded by complexly deformed devonian to early triassic metasandstones and metapelites forming a regional scale block in matrix melange fabric five distinct tectonic units have been recognized in the belt 1 basement blocks including two types of precambrian basement crystalline and transitional 2 continental margin slices including early paleozoic strata and late paleozoic fluviodeltaic sedimentary rocks proximal and distal fan clastics reflecting the development of a north facing rift margin on the edge of the south china plate 3 out of sequence oceanic crustal slices including strongly deformed postrift deep water sedimentary rocks sheeted dikes basalts and mafic ultramafic cumulates of a late paleozoic ophiolite suite developing independent of the rift margin in a separate basin 4 out of sequence island arc slices 5 accretionary wedge slices all the tectonic units were deformed during three geometrically distinct deformation episodes d 1 d 2 and d 3 during 240 200 ma units 2 4 involved southward thrusting and vertical then southward extrusion of about 20 km of horizontal displacement above the autochthonous basement during the d 1 episode thrust slices 20 km south of the mianlue suture are related to this vertical extrusion due to the same rock assemblages ages and kinematics the d 2 and d 3 episodes folded all the units in a thick skinned style about east west d 2 and west northwest d 3 axes in the mianlue suture zone an early foreland propagating sequence of accretion of late paleozoic rocks deposited above the yangtze craton is not involved in d 1 deformation but is temporally equivalent to the d 2 and d 3 deformation in the mianlue suture two stages of strike slip faulting mainly occurred at the end of d 2 and d 3 respectively during d 2 deformation the bikou block was obliquely indented to the ese into the mianlue suture rather than being thrust over the mianlue suture from the north as a part of the qinling dabie microblock during d 3 deformation however the bikou block was bounded by the south boundary fault of the mianlue suture and the yangpingguan fault on the south these faults are coeval strike slip faults but of opposite senses and accommodated minor southwestward extrusion of the bikou block into songpan ganze orogen the other basement blocks north of the mianlue suture were extruded eastward by about 20 km of lateral displacement based on the offset of the wudang dome during the d 3 episode due to the northeastward indentation of the hannan complex of the south china craton post d 3 emplacement of granite cutting across the strike slip faults such as the mianlue suture provides a minimum age of 200 ma for d 3 deformation therefore based on insights from the evolution of the mianlue suture the d 2 and d 3 episodes in the mianlue suture and its neighbors are not responsible for and associated with the two stage extrusion of the dabie uhp hp terranes from the foping dome to the present erosional surface more than 350 km

nonvolcanic tremors deep beneath the san andreas fault we have discovered nonvolcanic tremor activity i e long duration seismic signals with no clear p or s waves within a transform plate boundary zone along the san andreas fault near cholame california the inferred epicentral region of the 1857 fort tejon earthquake moment magnitude 7 8 the tremors occur between 20 to 40 kilometers depth below the seismogenic zone the upper 15 kilometers of earth s crust where earthquakes occur and their activity rates may correlate with variations in local earthquake activity

body wave imaging of earth s mantle discontinuities from ambient seismic noise ambient seismic noise correlations are widely used for high resolution surface wave imaging of earth s lithosphere similar observations of the seismic body waves that propagate through the interior of earth would provide a window into the deep earth we report the observation of the mantle transition zone through noise correlations of p waves as they are reflected by the discontinuities associated with the top 410 kliometers km and the bottom 660 km of this zone our data demonstrate that high resolution mapping of the mantle transition zone is possible without using earthquake sources

the jinshajiang ailaoshan suture zone china tectonostratigraphy age and evolution abstract the jinshajiang suture zone is important for enhancing our understanding of the evolution of the paleo tethys and its age tectonic setting and relationship to the ailaoshan suture zone have long been controversial based on integrated tectonic biostratigraphic chemostratigraphic and isotope geochronological studies four tectono stratigraphic units can be recognized in the jinshajiang suture zone the eaqing complex the jinshajiang ophiolitic melange the gajinxueshan group and the zhongxinrong group isotope geochronology indicates that the redefined eaqing complex composed of high grade metamorphic rocks might represent the metamorphic basement of the jinshajiang area or a remnant micro continental fragment eaqing complex protolith rocks are pre devonian and probably of early middle proterozoic age and are correlated with those of the ailaoshan complex two zircon u pb ages of 340 3 and 294 3 ma separately dated from the shusong and xuitui plagiogranites within the ophiolitic assemblage indicate that the jinshajiang oceanic lithosphere formed in latest devonian to earliest carboniferous times the oceanic lithosphere was formed in association with the opening and spreading of the jinshajiang oceanic basin and was contiguous and equivalent to the ailaoshan oceanic lithosphere preserved in the shuanggou ophiolitic melange in the ailaoshan suture zone the latter yielded a u pb age of 362 41 ma from plagiogranite the re defined gajinxueshan and zhongxinrong groups are dated as carboniferous to permian and latest permian to middle triassic respectively on the basis of fossils and u pb dating of basic volcanic interbeds the gajinxueshan group formed in bathyal slope to neritic shelf environments and the zhongxinrong group as bathyal to abyssal turbidites in the jinshajiang ailaoshan back arc basin latest permian earliest middle triassic synorogenic granitoids with ages of 238 18 and 227 5 255 8 ma respectively and an upper triassic overlap molasse sequence indicate a middle triassic age for the jinshajiang ailaoshan suture formed by collision of the changdu simao block with south china

u pb zircon ages constrain the architecture of the ultrahigh pressure qinling dabie orogen china new shrimp and tims zircon ages 40ar 39ar ages and eclogite locations contribute significantly to our understanding of the ultrahigh pressure dabie shan 1 the geographic extent of the yangtze craton that was subducted to ultrahigh pressure extends to the northern edge of the dabie shan 2 the northern half of the dabie shan is a magmatic complex intruded over a 10 myr interval between 137 and 126 ma that accommodated 100 n s stretching of the pre existing collisional architecture 3 granitic orthogneisses and enclosing ultrahigh pressure paragneisses have indistinguishable zircon populations the population of triassic zircon ages ranges from 219 to 245 ma leading us to question the prevailing assumption that 219 ma zircons formed at ultrahigh pressure and to propose instead that they reflect late retrogression at crustal pressures following the bulk of exhumation

nonvolcanic tremor evolution and the san simeon and parkfield california earthquakes nonvolcanic tremors occur adjacent to locked faults and may be closely related to the generation of earthquakes monitoring of the san andreas fault in the parkfield california region revealed that after two strong earthquakes tremor activity increased in a nearly dormant tremor zone increased and became periodic in a previously active zone and has remained elevated and periodic for over 4 years static shear and coulomb stress increases of 6 to 14 kilopascals from these two earthquakes are coincident with sudden increases in tremor rates the persistent changes in tremor suggest that stress is now accumulating more rapidly beneath this part of the san andreas fault which ruptured in the moment magnitude 7 8 ft tejon earthquake of 1857

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

constraining p wave velocity variations in the upper mantle beneath southeast asia we have produced a p wave model of the upper mantle beneath southeast se asia from reprocessed short period international seismological centre isc p and pp data short period p data of the annual bulletin of chinese earthquakes abce and long period pp p data we used 3d sensitivity kernels to combine the datasets and mantle structure was parameterized with an irregular grid in the best sampled region our data resolve structure on scale lengths less than 150 km the smearing of crustal anomalies to larger depths is reduced by a crustal correction using an a priori 3d model our tomographic inversions reveal high velocity roots beneath the archean ordos plateau the sichuan basin and other continental blocks in se asia beneath the himalayan block we detect high seismic velocities which we associate with subduction of indian lithospheric mantle this structure is visible above the 410 km discontinuity and may not connect to the remnant of the neo tethys oceanic slab in the lower mantle our images suggest that only the southwestern part of the tibetan plateau is underlain by indian lithosphere and thus that the upper mantle beneath northeastern tibet is primarily of asian origin our imaging also reveals a large scale high velocity structure in the transition zone beneath the yangtze craton which could have been produced in multiple subduction episodes the low p wave velocities beneath the hainan island are most prominent in the upper mantle and transition zone they may represent counter flow from the surrounding subduction zones and may not be unrelated to processes beneath eastern tibet 2005 elsevier b v all rights reserved

three dimensional surface displacement of the 2008 may 12 sichuan earthquake china derived from synthetic aperture radar evidence for rupture on a blind thrust summary the sichuan earthquake mw 7 9 struck the longmen shan lms range front china on 2008 may 12 affecting an area of moderate historical seismicity where little active shortening has been previously reported recent studies based on space geodesy have succeeded in retrieving the far field surface displacements caused by the earthquake but the near field 25 km from the faults coseismic surface displacement is still poorly constrained thus shallow fault geometry and shallow coseismic slip are still poorly resolved here for the first time for this earthquake we combine c and l band synthetic aperture radar offsets data from ascending and descending tracks to invert for the 3 d surface displacement in the near coseismic field of the sichuan earthquake our data coupled with a simple elastic dislocation model provide new results strongly suggesting the presence of a blind thrust striking along the range front and being active at depth during the earthquake the presence of a rupture on a blind thrust brings new evidence for an out of sequence thrusting event and new elements for interpreting the tectonic strain partitioning in the lms which has important implications both for seismic hazard assessment and long term evolution of the mountain belt

miocene emplacement and deformation of the konga shan granite xianshui he fault zone west sichuan china geodynamic implications the presently active sinistral xianshui he strike slip fault xsh is a lithospheric scale strike slip fault in the eastern himalaya in the study area this fault affects the eastern edge of the konga shan granitic massif where it has caused both brittle and ductile deformation a rbsr isochron and nd and pb isotope study of three samples and a upb zircon study of a single sample were completed on the granite upb data indicate a granite emplacement age of 12 8 1 4 ma the rbsr isochrons show that the granite emplacement and the deformational event were synchronous at around 12 10 ma minimum age for deformation the nd and pb isotope compositions of whole rocks and k feldspars indicate the involvement of proterozoic continental crust which is confirmed by upb systematics indicating inherited zircons sinistral faulting along the xsh began at the latest at 12 ma and marks the extrusion toward the east of the west sichuan and south china blocks following their extrusion along the red river fault zone between 50 and 21 ma

three dimensional velocity structure seismicity and fault structure in the parkfield region central california this study examines the three dimensional velocity structure in a 60 by 80 km region containing the parkfield segment of the san andreas fault we use local earthquake and shot p arrival times in an iterative simultaneous inversion for velocity and hypocentral parameters using the three dimensional model we relocated 5251 events that occurred from 1969 to 1991 as well as the 1966 aftershocks and computed 664 fault plane solutions the san andreas fault saf characterized by a sharp across fault velocity gradient is the primary feature in the velocity solution there is a 5 20 lateral change in velocity over a 4 km width the contrast being sharper where there is better resolution the model also shows significant variations in the velocity and in the complexity of the velocity patterns along the saf the largest across fault velocity difference is below middle mountain where a large volume of low velocity material impinges on the saf from the northeast this material is inferred to be overpressured and may be key to understanding the unusual behavior in the parkfield preparation zone a 20 km long high velocity slice is imaged northeast of the saf near gold hill its along fault length corresponds to the length of the maximum slip in 1966 the relocated seismicity shows that the san andreas fault is a planar vertical fault zone at seismogenic depths ninety percent of the fault plane solutions that are on or near the saf were right lateral strike slip on subvertical fault planes that parallel the saf thus the surface fault complexities do not appear to extend to depth and therefore do not explain the rupture character at parkfield at parkfield variations in material properties play a key role in fault segmentation and deformation style our observations suggest that there may be a general relation between increasing velocity and increasing ability of the rocks to store strain energy and release it as brittle failure

three dimensional compressional wavespeed model earthquake relocations and focal mechanisms for the parkfield california region we present a new three dimensional 3d compressional wavespeed v p model for the parkfield region taking advantage of the recent seismicity associated with the 2003 san simeon and 2004 parkfield earthquake sequences to provide increased model resolution compared to the work of eberhart phillips and michael 1993 epm93 taking the epm93 3d model as our starting model we invert the arrival time data from about 2100 earthquakes and 250 shots recorded on both permanent network and temporary stations in a region 130 km northeast southwest by 120 km northwest southeast we include catalog picks and cross correlation and catalog differential times in the inversion using the double difference tomography method of zhang and thurber 2003 the principal v p features reported by epm93 and michelini and mcevilly 1991 are recovered but with locally improved resolution along the san andreas fault saf and near the active source profiles we image the previously identified strong wavespeed contrast faster on the southwest side across most of the length of the saf and we also improve the image of a high v p body on the northeast side of the fault reported by epm93 this narrow body is at about 5 to 12 km depth and extends approximately from the locked section of the saf to the town of parkfield the footwall of the thrust fault responsible for the 1983 coalinga earthquake is imaged as a northeast dipping high wavespeed body in between relatively low wavespeeds 5 km sec extend to as much as 10 km depth we use this model to derive absolute locations for about 16 000 earthquakes from 1966 to 2005 and high precision double difference locations for 9 000 earthquakes from 1984 to 2005 and also to determine focal mechanisms for 446 earthquakes these earthquake locations and mechanisms show that the seismogenic fault is a simple planar structure the aftershock sequence of the 2004 mainshock concentrates into the same structures defined by the pre 2004 seismicity confirming earlier observations waldhauser et al 2004 that the seismicity pattern at parkfield is long lived and persists through multiple cycles of mainshocks online material 3d v p model and earthquake relocations

thermal history vs fabric anisotropy in granitoids emplaced at different crustal levels an example from calabria southern italy the sila and serre granitoids of calabria were emplaced in the late carboniferous at depths ranging from 6 to 23 km in a postcollisional extensional regime their fabric which developed during and after final crystallization up to the solid state strongly increases in intensity with emplacement depth this relationship is attributed to the thermal history of the calabrian basement cooling histories of granitoids constrained by geological data and rb sr cooling ages on micas demonstrate that residence times of rocks at temperatures greater than those of the brittle ductile transition vary greatly as a function of initial emplacement depth this explains why shallow level granitoids that remained for about 10 myr at temperatures above those of the brittle domain retain their original magmatic fabric by contrast the strong fabrics of the deep seated granitoids are explained by solid state strain overprint that lasted more than 100 myr at temperatures above those of the brittle domain

contrasting rift and subduction related plagiogranites in the jinshajiang ophiolitic melange southwest china and implications for the paleo tethys 1 the jinshajiang ophiolitic melange zone in southwest china represents a remnant of the eastern paleo tethys ocean field geochronological and geochemical studies have identified two distinct suites of plagiogranites within the melange the dongzhulin trondhjemite and jiyidu tonalite which represent rift and subduction settings respectively related to opening and closing of the ocean shrimp u pb analysis on zircons extracted from the dongzhulin trondhjemite yields a mean 206pb 238u age of 347 7 ma ree and isotopic characteristics suggest an origin from low pressure partial melting of an amphibolitic protolith highly variable hf isotopic compositions for zircons from this body may indicate a heterogenous source involving both depleted mantle and enriched continental components this together with geologic relations suggests formation near an embryonic spreading center in a continent ocean transition setting the jiyidu tonalite has a u pb zircon age of 283 3 ma and geochemical data indicates high sr y la yb n nb ta and low y and marked heavy ree depletion these signatures suggest derivation from low degree partial melting of subducted slab at pressure high enough to stabilize garnet and rutile a slab melt origin is also supported by in situ hf and o data for zircon that show isotopic compositions comparable with typical altered oceanic crust thus the crystallization age of the jiyidu high sr y tonalite provides a constraint for the subduction of the jinshajiang ocean floor the rift related dongzhulin trondhjemite and subduction related jiyidu high sr y tonalite constrain the timing and setting of opening and closing of this segment of the paleo tethys ocean

ages of ultrahigh pressure metamorphism and protolith orthogneisses from the eastern dabie shan u pb zircon geochronology the dabie shan contains two rare geologic features one the occurrence of ultra high pressure metamorphic rocks peak p 2 7 gpa uhp in the east central part of this belt and two some of these uhp rocks are characterized by anomalously 18o depleted oxygen isotopic compositions geochronological data are presented that constrain the age of the uhp metamorphism the ages of protoliths and potentially the age of hydrothermal alteration believed to be responsible for the anomalous oxygen isotopic compositions the age of uhp metamorphism in the eastern dabie shan is dated at 218 5 1 7 ma using u pb in zircon separated from host gneisses this age is identical within uncertainty with a less precise age using u pb in zircon separated from uhp eclogite 225 5 3 6 ma and with previously published ages from uhp eclogites of the dabie shan map relationships strongly support an in situ as opposed to exotic orgin of the uhp eclogites in this region the identical ages of uhp metamorphism in the gneisses and eclogites provides additional support for this interpretation the age of the protolith gneisses is dated as 772 5 9 5 ma based on the upper intercept of the zircons which accords well with other dates of granitic gneisses in the qinling belt the association of mafic and felsic magmatism is interpreted to reflect rifting along the northern margin of the yangtze block at this time these upper intercept ages may date the hydrothermal circulation of 18o depleted meteoric water which gave rise to the anomalously 18o depleted uhp protoliths of the dabie shan and sulu region the coincidence of these protolith ages with the early sinian glaciation of south china is consonant with the suggestion that the highly anomalous oxygen isotopic compositions in this region date from this time the upper intercept age of the eclogite is imprecisely determined at 447 82 79 ma but is younger than the host gneisses and is not characterized by anomalous oxygen isotopic compositions

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

earthquake locations and three dimensional fault zone structure along the creeping section of the san andreas fault near parkfield ca preparing for safod 1 arrival time data from 453 local earthquakes and 6 explosions at parkfield ca are inverted for earthquake locations and three dimensional vp and vp vs structure the structure is dominated by the velocity contrast across the saf with the southwest side about 20 25 faster consistent with previous studies nearly all the earthquakes occur almost directly beneath the fault trace we find high vp vs anomalies that correlate with low resistivity features in a magnetotelluric model that are interpreted to represent fluids we locate a magnitude 2 earthquake that is a potential target event for the final stage of safod drilling yielding a depth of 3 1 km below surface and an epicenter 100 m southwest of the fault trace nonlinear analyses indicate 95 confidence relative and absolute uncertainties on the order of 500 700 m vertically and 200 300 m horizontally for this target earthquake

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

subduction of asian lithospheric mantle beneath tibet inferred from models of continental collision the relative northward motion of the indian subcontinent that followed the onset of continental collision with asia has produced extensive deformation of the earth s crust giving rise to the world s highest mountains in the himalayan chain and the world s largest high elevation region the tibetan plateau the formation of the broad mountain belt implies that contrary to the original tenets of plate tectonics the lithospheric plates have experienced widespread deformation far from the plate boundary1 several models have been proposed2 6 to explain the manner in which this post collisional deformation is distributed within the continental lithosphere of the indian and asian plates here we propose an alternative model in which subduction of the asian lithospheric mantle develops following the collision of india our model is supported by numerical calculations of crustal deformation and thickening and is consistent with available geological and geophysical data7 9 this picture suggests that lithospheric mantle is not deformed along with the crust and would imply that continental collision zones are more analogous to oceanic subduction zones than was previously believed

an integrated perspective of the continuum between earthquakes and slow slip phenomena slow slip a mechanism by which faults can relieve stress was thought to be distinct from earthquakes however a global review of slow slip phenomena suggests that instead there is a continuum between the two types of event

mesozoic plutons of the yidun arc sw china u pb geochronology and hf isotopic signature abstract the yidun arc is a triassic volcanic arc located between the songpan garze fold belt and the qiangtang block southwest china to constrain the age of a number of the major granitic plutons from the yidun arc laser ablation icp ms u pb analysis of zircon was conducted hafnium isotope data was also acquired through laser ablation multicollector icpms analysis of zircon with the aim of gaining insight into the age and nature of the source region of the plutons three age groups have been identified from seven granite samples early middle triassic 245 to 229 ma late triassic 219 to 216 ma and cretaceous 105 to 95 ma hafnium analysis shows the triassic granites to have negative and variable e hf values and mesoproterozoic 1 6 ga depleted mantle model ages which is interpreted to reflect derivation from an isotopically heterogeneous largely crustal source the cretaceous granite shows higher and less variable e hf values and slightly younger model ages 1 3 ga and is interpreted to be derived from melting of a more homogeneous crustal source a depleted mantle model age of 1 5 ga is calculated from the pooled triassic and cretaceous samples the source region for these magmas may be tentatively correlated with mesoproterozoic material of the yangtze craton which has been suggested to underlie the yidun arc however further work is necessary to demonstrate this suggestion