temporal spatial distribution and tectonic implications of the batholiths in the gaoligong tengliang yingjiang area western yunnan constraints from zircon u pb ages and hf isotopes abstract considerable progress has recently been made regarding temporal and spatial distribution of magmatism in the lhasa terrane however the eastward and southeastward correlation of these tibetan magmatic suites in western yunnan and burma remains poorly constrained this paper reports zircon u pb dating and hf isotopic compositions of granites in the gaoligong tengliang yingjiang area west yunnan it reveals three episodes of plutonism and more importantly a southwestward magmatic migration the gaoligong batholiths in the northeast were mainly emplaced during early cretaceous 126 121 ma and comprised predominantly s type granites with negative zircon ehf values ehf 2 12 the tengliang granites situated southwest of the gaoligong belt were emplaced in late cretaceous 68 76 ma and also displayed a strong peraluminous affinity and negative ehf 5 14 indicating a provenance from a proterozoic sedimentary source with little mantle contribution the youngest phase of magmatism 52 66 ma occurred in yingjiang southwestmost of the study area it is composed of s type granites ehf 2 12 in east yingjiang and i type granites ehf 4 6 in west yingjiang near the china burma border the late cretaceous early cenozoic plutons in the tengliang and yingjiang area are thus considered as the northern continuation of the late cretaceous magmatic arc west which comprises i type granites and andesitic rocks and of the belt of predominant s type granites east in burma thailand and malaysia such a chemical polarity of the dual i type and s type granites is strongly reminiscent of the northern american cordillera indicating a cordilleran style continental margin during the late cretaceous early cenozoic while the magmatic arc was related to eastward subduction of the neo tethys beneath the asian continent the s type granites represented the melting products of thickened crust in the hinterland in response to subduction induced decrease in lithospheric strength and compressive plate convergence forces and to a less degree to the collision between burma and sundaland blocks the gaoligong early cretaceous granites which bear strong similarities in lithology geochemistry and emplacement age to those in the northern magmatic belt in the lhasa terrane are also the magmatic expression of crustal thickening this crustal thickening may have stemmed from the collision between the lhasa block and the qiangtang block in late jurassic and early cretaceous the magmatism in western yunnan thus recorded a long term subduction of the neo tethyan plate enhanced by continental collisions at different time

lateral extrusion of lower crust from under high topography in the isostatic limit where there is isostasy the rocks of the lower continental crust are subject to an effective lateral pressure gradient equal to the gradient of topographic load whether compensation is in crustal roots or in the mantle the result is a poiseuille flow planar channel flow in the weak lower crust which removes crust from under mountains and smooths and levels the topography assuming cubic power law creep the flux of crust is proportional to the third power of the topographic gradient to the usual arrhenius term to the tenth power of the absolute temperature of the lower crust and to the negative fifth power of the geothermal gradient the result is that any initial condition with an isolated high tends eventually toward a state where the topography is pancake shaped with a flat central plateau and steeper flanks spreading outward when adjacent pancakes merge most of the relief is eliminated flow may either roughen or smooth the moho depending on whether there are lateral density contrasts in the mantle lithosphere or not although it is difficult to find analytic solutions for this evolution it is easily simulated numerically using various published flow laws for plausible lower crustal rocks lateral extrusion is shown to be insignificant under the oceans marginally significant under shields and platforms important under elevated plains and dominant beneath high plateaus and or hot delaminated regions in particular the basin and range province of the western united states cannot maintain short wavelength 100 km moho relief of more than 1 km for times greater than 10 20 m y at most if the tibetan plateau of china has been delaminated as recently suggested then it may flatten even faster reducing 100 km wavelength topographic features to 200 m relief in no more than 0 03 0 13 m y and leaving only features with wavelengths over 400 km at the present therefore the lower crust of tibet may resemble a hydraulic reservoir as previously suggested because the flatness of the moho is self maintained in these regions information on past tectonics is continually lost and the present moho shape cannot be used to balance cross sections for times in the past the flatness of the moho today is not a constraint on the mechanisms of extension or shortening in these regions nor is it evidence for underplating by intrusions it also follows that very intense and inhomogeneous dilational strains may have occurred locally as in the metamorphic core complexes of the basin and range province

paleomagnetic versus gps determined tectonic rotation around eastern himalayan syntaxis in east asia abstract northward indentation of the indian plate has brought about significant tectonic deformation into east asia a record of long term tectonic deformation in this area for the past 50 m yr particularly the vertical axis rotation is available through paleomagnetic data in order to depict rotational deformation in this area with respect to eurasia we compiled reliable paleomagnetic data sets from 79 localities distributed around eastern himalayan syntaxis in east asia this record delineates that a zone affected by clockwise rotational deformation extends from the southern tip of the chuan dian fragment to as far as the northwestern part of the indochina peninsula a limited zone that experienced a significant amount of clockwise rotation after an initial india asia collision is now located at 23 5 n 101 e far away from an area 27 5 n 95 5 e where an intense rotational motion has been viewed by a snapshot of gps measurements this discrepancy in clockwise rotated positions is attributed to southeastward extrusion of the tectonic blocks within east asia as a result of ongoing indentation of the indian plate a quantitative comparison between the gps and paleomagnetically determined clockwise rotation further suggests that following an initial india asia collision the crust at 30 n 94 e paleoposition was subjected to southeastward displacement together with clockwise rotation which eventually reached to present day position of 23 5 n 101 e implying a crustal displacement of about 1000 km during the past 50 m yr

surface deformation and lower crustal flow in eastern tibet field observations and satellite geodesy indicate that little crustal shortening has occurred along the central to southern margin of the eastern tibetan plateau since about 4 million years ago instead central eastern tibet has been nearly stationary relative to southeastern china southeastern tibet has rotated clockwise without major crustal shortening and the crust along portions of the eastern plateau margin has been extended modeling suggests that these phenomena are the result of continental convergence where the lower crust is so weak that upper crustal deformation is decoupled from the motion of the underlying mantle this model also predicts east west extension on the high plateau without convective removal of tibetan lithosphere and without eastward movement of the crust east of the plateau

spreading structure of the troodos ophiolite cyprus some paleomagnetic constraints it has been suggested that grabens on the northern flank of the troodos ophiolite formed by listric faulting in an oceanic environment and represent axial valleys fossilized by ridge jumping this paleomagnetic survey of the solea graben was carried out to constrain the structural interpretation an analysis was devised which restores both the dikes sampled to vertical and the primary remanent magnetizations to a previously derived structurally corrected remanent magnetization direction typically two solutions for the original dike orientation and the pole and angle of net rotation are obtained for each dike sampled these solutions can be separated if a locally consistent original dike strike is assumed reasons are discussed for the selection of the solution that gives an original northwesterly dike strike results are consistent with a graben rotations are up to 78 at the sites studied rotation vectors are subhorizontal and subparallel to the original dike strike suggestive of an axial process for the development of the structure however on the basis of field observations an alternative mode of origin is proposed invoking ephemeral extreme upper crustal extension by listric faulting on the west flank of an intermediate to fast spreading axis the grabens being formed by antithetic faulting

eocene break off of the neo tethyan slab as inferred from intraplate type mafic dykes in the gaoligong orogenic belt eastern tibet article i nfo eocene 40 42 ma basaltic dykes in the gaoligong tengliang belt eastern tibet are characterized by high na2 o 2 4 in contrast with the widespread post collisional potassic and ultrapotassic rocks in the tibetan plateau despite the ubiquitous negative nb anomalies these dykes have relatively high nb and zr contents making them distinct significantly from the gangdese arc magmas all these together with the positive nb anomaly in some samples indicate an intraplate affinity for the gaoligong tengliang dykes specifically the gaoligong dykes represent the asthenosphere derived melts which has been contaminated to various degrees by the lithosphere mantle derived melts whereas the tengliang samples were directly derived from an enriched lithosphere mantle the thin lithosphere b80 km inferred from basalt geochemistry is unusual in tibet thus demanding a peculiar mechanism to thin lithosphere a slab break off model is preferred given the geochemical contrast between pre 40 ma and post 40 ma magmas in tibet and the temporal correlation among this intraplate magmatism the termination of gangdese arc magmatism and regional thermally driven metamorphism therefore the occurrence of intraplate type magmas in the gaoligong orogenic belt likely represents magmatic expression of the detachment of subducting neo tethyan slab from the indian continental plate during the eocene in the light of the slab break off concept and thermo mechanical modeling the eocene slab break off furthermore suggests the onset of the india asia collision between 52 and 57 ma

genesis of high potassium calc alkaline peraluminous i type granite new insights from the gaoligong belt granites in southeastern tibet plateau abstract silicic magmas primarily felsic i and s type granite sensu lato play a key role in understanding the compositional evolution and differentiation of the continental crust however the genetic models of felsic i type granites remain controversial herein we investigate the gaoligong granite sensu lato belt in southern tibet and compare its features with an extensive dataset that we compiled on high potassium calc alkaline peraluminous i type granite from typical granitic belts around the word with a view to gain insights on the genesis of felsic i type granites the early cretaceous granites from gaoligong belt mostly belong to high k calc alkaline peraluminous i type series zircon lu hf ehf t 16 3 0 9 and whole rock sr nd isotopic data end t 14 6 3 0 on these rocks show highly variable and evolved nature suggesting that the granites were generated by partial melting of lower crustal materials with possible involvement of upper crustal materials followed by mixing leading to marked variation in source compositions compilation of the dataset and comparison with felsic i and s type granites show the following salient features 1 possibly distinct trends of al2o3 p2o5 k2o rb sr pb th u and ga vs mgo feot between the felsic i type granites and those s types and 2 there are notably higher and variable zircon hf ehf t 10e units and whole rock sr nd end t 5e units isotopic compositions of i type granites than those of the s types comparison with experimental liquids and other typical cases around the world indicate that the felsic i type granites from gaoligong are similar to those secondary i type granites where mixing processes and extensive partial melting of the heterogeneous lower crust by a mantle like magma control the variable components the range of compositional diversity is significantly higher for the studied felsic i type granites than those typical s type granites from north queensland our study provides further insights into the application of geochemical and isotopic proxies to differentiate felsic i and s type granites

quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation abstract a new method of analysing isothermal remanent magnetisation irm acquisition curves based on cumulative log gaussian analysis robertson and france phys earth planet inter 82 1994 223 234 is proposed it is based on the curve fitting of the irm acquisition curve versus the logarithm of the applied field with i the acquisition curve on a linear scale ii the acquisition curve expressed as a gradient and iii the acquisition curve on a probability scale even when a sample is not saturated its magnetic properties can be defined although with less certainty the number of magnetic components required for an optimal fit to a measured irm acquisition curve is evaluated statistically the method discriminates on the basis of different mineral coercivity therefore additional rock magnetic tests are still required to separate minerals with similar coercivities

post 4 ma initiation of normal faulting in southern tibet constraints from the kung co half graben the timing of e w extension of the tibetan plateau provides a test of mechanical models of the geodynamic evolution of the india asia convergence zone in this work we focus on the kung co half graben southern tibet china bounded by an active n s normal fault with a minimum vertical offset of 1600 m to estimate the onset of normal faulting we combined high and medium temperature u pb ar ar and low temperature u th he thermochronometry of the kung co pluton a two mica granite of the northern himalayan granitic belt that outcrop in the footwall of the fault biotite and muscovite ar ar ages are close from each other 16 ma 0 2 ms and 15 0 4 ma bt which is typical of fast cooling the zircon and apatite u th he ages range from 11 3 to 9 6 ma and 9 9 to 3 7 ma respectively these he ages are indicative of 1 fast initial cooling from 11 3 to 9 ma gradually decreasing with time and 2 a high geothermal gradient 400 c km close to the surface at 10 ma the kung co pluton was emplaced at about 22 ma u pb on zircon at less than 10 km depth and 520 545 c subsequent to its shallow emplacement the pluton underwent fast thermal re equilibration ending around 7 5 ma followed by a period of slow cooling caused either by the end of the thermal re equilibration or by very slow exhumation 0 02 0 03 mm yr from 7 5 ma to at least 4 ma in either case the data suggest that the exhumation rate increased after 4 ma we infer this increase to be related to the initiation of the kung co normal fault a critical examination of previously published data show that most n s tibetan normal faults may have formed less than 5 ma ago rather than in the miocene as assumed by several authors such a young age implies that e w extension is not related to the neogene south tibetan magmatism 25 to 8 ma consequently models relating e w extension to magmatism such as convective removal of the lower lithosphere may be inappropriate we rather think that this extension is related with local accommodation of boundary forces and displacements 2007 elsevier b v all rights reserved

two phase growth of high topography in eastern tibet during the cenozoic high topography in eastern tibet is thought to have formed in response to weak lower crust flowing towards the plateau margin thermochronologic analyses of rocks exposed at the eastern plateau margin record periods of mountain growth early in the indo asian collision implying that crustal flow alone could not have created the high topography

paleomagnetic evidence for clockwise rotation of the simao region since the cretaceous a consequence of india asia collision samples were collected for paleomagnetic analysis from 36 sites from six formations west of the red river fault in southwestern yunnan province china on the simao block 23 4 n 100 8 e in an attempt to examine the effects of the india asia collision on the asian continent paleomagnetic results from rocks ranging from cretaceous to miocene in age indicate a large clockwise rotation of these sites with respect to stable eurasia since the late cretaceous the tectonic corrected data from cretaceous samples mean direction of two formations are d 112 6 i 34 7 95 7 5 k 9 1 n 45 for eocene samples the data are d 84 7 i 38 9 95 7 6 k 12 0 n 32 and for miocene samples they are d 21 1 i 35 5 95 7 1 k 15 3 n 29 the samples from the paleocene and pliocene are either remagnetized or too weak to provide stable directions the cretaceous sites were sampled from folded beds and these sites pass the fold test the amount of rotation seen in this study r 86 1 9 9 as a mean value for the cretaceous r 76 8 11 for the eocene and r 14 7 10 0 for the miocene is significantly larger than that documented in previous studies from the lower cretaceous of northwestern and central yunnan province and may indicate that differential rotation has taken place in the region this rotation can be explained by the northward motion and collision of india with asia in the paleocene which caused continuous deformation of southwestern china in the form of strike slip faulting and clockwise rotation it is concluded that the rotation and deformation resulting from this collision probably continued from the paleocene until at least the miocene

first global positioning system results in northern myanmar constant and localized slip rate along the sagaing fault we present the first recorded global positioning system gps data from myanmar measured at the northern tip of the sagaing fault this area is in a very complex geodynamic context where rigid and semirigid plates interact the 12 gps sites measured in 2005 and 2008 in northern myanmar show that the slip rate is 18 mm yr and is localized along a single active narrow fault trace 20 km wide the same rate was previously demonstrated and remeasured 500 km southward in central myanmar despite the geodynamic regional complexity induced by interaction between the sunda and india plates the burma microplate and the highly deformable eastern himalayan syntaxis the slip rate remains surprisingly constant along this fault however the modeled locking depth varies from 20 km in central myanmar to 5 km in the north

intersections of remanence small circles new tools to improve data processing and interpretation in palaeomagnetism summary intersections of remanence small circles are increasingly used in palaeomagnetism to determine the palaeofield direction from synfolding remanences the approach small circle intersection sci method presupposes that remanence small circles from sites tilted to different directions intersect in a common point which is assumed to represent the palaeofield direction the sci method is not restricted to synfolding remanences it can be applied also to derive the palaeofield direction from pre folding remanences in addition to tilt correction and as a cross check for fold tests however the sci method has specific requirements and does not work in all cases the suitability of data sets and reliability of palaeofield estimates have to be assessed carefully and always against the background of local geology this paper examines the occurrence and distribution of sci as a tool of judgement first a data set is most suitable and a palaeofield estimate by sci most reliable when most of the theoretically possible intersections are realized and in addition clearly grouping in a narrow region second the small circles that greatly determine the result of sci are identified by the highest number of intersections they have with the others these criteria allow a systematic investigation of sci estimates upon stability and dependencies moreover scis can be an indicator of irregular unexpected tectonic settings when the intersections disperse strongly although significant bedding strike differences are given relative rotations or other unexpected displacements are likely to have occurred if so the sci estimate will be invalid and probably biased however also fisher mean calculation and most fold tests will be affected in this case the scis can help to separate the data into subsets valid for fisher statistics and sci methods the other possibility is to use tentative reconstructions to infer the true sequence of tilts and rotations and correct the data adequately

new insights into the cenozoic lateral extrusion of crustal blocks on the southeastern edge of tibetan plateau evidence from paleomagnetic results from paleogene sedimentary strata of the baoshan terrane in order to characterize the lateral extrusion of crustal material during the cenozoic we conducted a paleomagnetic study of the eocene oligocene zhushan formation and paleocene muguahe formation in the central part of the baoshan terrane bst on the southeastern edge of tibetan plateau a primary magnetic component and a remagentized component with the miocene acquisition age were isolated from the muguahe formation and zhushan formation respectively these data indicate that the bst did not commence clockwise rotation in the period of the paleocene and the oligocene and that the bst experienced 80 clockwise rotation relative to east asia since the miocene combining with the documented history of crustal boundary strike slip faults on the southeastern edge of the tibetan plateau shows that the bst and tengchong terrane tct firstly underwent latitudinal crustal shortening in the oligocene and the eastward extension was the main type of motion of the crustal material during this period which induced the initial strike slip movement of their boundary faults since the miocene the main form of crustal motion of bst tct and simao terrane was gradually transformed into clockwise rotation which possibly indicates that the fold and thrusting fault systems induced crustal shortening and thickening in the southeastern edge of the tibetan plateau have reached extremity and the southeastern part of the tibetan plateau have already been uplifted to the similar elevation with today s elevation in the miocene

strain and rotation rate from gps in tibet anatolia and the altiplano 1 deformation measured by regional gps networks in continental plateaus reflects the geologic and tectonic variability of the plateaus for two collisional plateaus tibet and anatolia and one noncollisional the altiplano we analyze the regional strain and rotation rate by inverting gps velocities to calculate the full two dimensional velocity gradient tensor to test the method we use gridded velocities determined from an elastic block model for the eastern mediterranean middle east region and show that to a first order the deformation calculated directly from the gps vectors provides an accurate description of regional deformation patterns principal shortening and extension rate axes vertical axis rotation and two dimensional 2 d volume strain dilatation are very consistent with long term geological features over large areas indicating that the gps velocity fields reflect processes responsible for the recent geologic evolution of the plateaus differences between geological and gps descriptions of deformation can be attributed either to gps networks that are too sparse to capture local interseismic deformation or to permanent deformation that accrues during strong earthquakes the altiplano has higher internal shortening magnitudes than the other two plateaus and negative 2 d dilatation everywhere vertical axis rotation changes sign across the topographic symmetry axis and is due to distributed deformation throughout the plateau in contrast the collisional plateaus have large regions of quasi rigid body rotation bounded by strike slip faults with the opposite rotation sense from the rotating blocks tibet and anatolia are the mirror images of each other both have regions of positive dilatation on the outboard sides of the rotating blocks positive dilatation in the aegean correlates with a region of crustal thinning whereas that in eastern tibet and yunnan province in china is associated with an area of vertical uplift rollback of the hellenic trench clearly facilitates the rotation of anatolia rollback of the sumatra burma trench probably also enables rotation about the eastern syntaxis of tibet

late cenozoic to recent transtensional deformation across the southern part of the gaoligong shear zone between the indian plate and se margin of the tibetan plateau and its tectonic origin abstract within the southeastern part of the tibetan plateau the gaoligong shan shear zone forms the boundary between the tengchong and baoshan blocks and is one of the main cenozoic strike slip fault systems of the eastern himalayan syntaxis the mountain ranges rise to a maximum elevation of 3623 m with a width of 20 km and a length of 400 km the core of the shear zone consists of a narrow belt of mylonitic rocks with right lateral strike slip fabrics formed during the northward movement of india relative to eurasia in oligocene and miocene time to the south the mountains decreases in elevation and end against the wanding fault the latter is one of a series of ne sw striking active left lateral strike slip faults within the southeastern part of the tibetan plateau the cause and timing for the change in elevation of the gaoligong shan is unknown but based on our recent structural and fission track geochronological work the decrease in elevation is the result of east west extension across the southern part of the gaoligong shear zone inferred from the late cenozoic and active normal faults along its eastern and western boundaries southward the eastern boundary fault system merges with a series of splays of the wanding strike slip fault while the western boundary fault system is bent into parallelism with the wanding fault this geometric relationship suggests that the normal faulting along the eastern and western boundary of the gaoligong shear zone was caused by transfer of the ne sw left lateral movement along the wanding fault into e w extension on faults to the northeast the resultant normal faulting caused the lowering of the topography in the southern part of the shear zone fission track data from 11 samples from the mylonitic rocks in the footwall of the ruili oblique slip fault left lateral thrust slip yielded ages between 8 4 and 0 9 ma coeval with volcanism within the jietou and mangbang basins the left lateral strike slip movement along the wanding and ruili faults is interpreted to have accommodated differential clockwise rotation of the gaoligong shear zone and its adjacent tectonic units during late cenozoic to recent time during the northward movement of the indian plate with respect to the southeastern margin of the tibetan plateau along the right lateral sagaing fault

contemporary crustal deformation around the southeast borderland of the tibetan plateau 1 we derive a detailed horizontal velocity field for the southeast borderland of the tibetan plateau using gps data collected from the crustal motion observation network of china between 1998 and 2004 our results reveal a complex deformation field that indicates that the crust is fragmented into tectonic blocks of various sizes separated by strike slip and transtensional faults most notably the regional deformation includes 10 11 mm yr left slip across the xianshuihe fault 7 mm yr left slip across the anninghe zemuhe xiaojiang fault zone 2 mm yr right slip across a shear zone trending northwest near the southern segment of the lancang river fault and 3 mm yr left slip across the lijiang fault deformation along the southern segment of the red river fault appears not significant at present time the region south and west of the xianshuihexiaojiang fault system whose eastward motion is resisted by the stable south china block to the east turns from eastward to southward motion with respect to south china resulting in clockwise rotation of its internal subblocks active deformation is detected across two previously unknown deformation zones one is located 150 km northwest of and in parallel with the longmenshan fault with 4 6 mm yr right slip and another is continued south southwestward from the xiaojiang fault abutting the red river fault with 7 mm yr left slip while both of these zones are seismically active the exact locations of faults responsible for such deformation are yet to be mapped by field geology comparing our gps results with predictions of various models proposed for tibetan plateau deformation we find that the relatively small sizes of the inferred microblocks and their rotation pattern lend support to a model with a mechanically weak lower crust experiencing distributed deformation underlying a stronger highly fragmented upper crust

jurassic to miocene magmatism and metamorphism in the mogok metamorphic belt and the india eurasia collision in myanmar situated south of the eastern himalayan syntaxis at the western margin of the shan thai terrane the highgrade mogok metamorphic belt mmb in myanmar occupies a key position in the tectonic evolution of southeast asia the first sensitive high resolution ion microprobe u pb in zircon geochronology for the mmb shows that strongly deformed granitic orthogneisses near mandalay contain jurassic 170 ma zircons that have partly recrystallized during 43 ma high grade metamorphism a hornblende syenite from mandalay hill also contains jurassic zircons with evidence of eocene metamorphic recrystallization rimmed by thin zones of 30 9 plus or minus 0 7 ma magmatic zircon the relative abundance of jurassic zircons in these rocks is consistent with suggestions that southern eurasia had an andean type margin at that time mid cretaceous to earliest eocene 120 to 50 ma i type granitoids in the mmb myeik archipelago and western myanmar confirm that prior to the collision of india an up to 200 km wide magmatic belt extended along the eurasian margin from pakistan to sumatra metamorphic overgrowths to zircons in the orthogneiss near mandalay date a period of eocene 43 ma high grade metamorphism possibly during crustal thickening related to the initial collision between india and eurasia at 65 to 55 ma this was followed by emplacement of syntectonic hornblende syenites and leucogranites between 35 and 23 ma similar syntectonic syenites and leucogranites intruded the ailao shan red river shear belt in southern china and vietnam during the eocene oligocene to miocene and the wang chao and three pagodas faults in northern thailand that most likely link with the mmb were also active at this time the complex history of eocene to early miocene metamorphism deformation and magmatism in the mmb provides evidence that it may have played a key role in the network of deformation zones that accommodated strain during the northwards movement of india and resulting extrusion or rotation of indochina

velocity field in asia inferred from quaternary fault slip rates and global positioning system observations we perform a joint inversion of quaternary strain rates and 238 global positioning system gps velocities in asia for a self consistent velocity field the reference frames for all geodetic velocity observations are determined in our inversion procedure india in moves relative to eurasia eu about a pole of rotation at 29 78 n 7 51 e 0 353 myr 1 which yields a velocity along the himalaya within india that is 73 76 of the magnitude of the in eu nuvel 1a velocity and a vector azimuth that is 8 10 clockwise of nuvel 1a in eu vector azimuth relative to eurasia south china moves at 9 11 mm yr in the direction 110 120 with a pole position 64 84 n 156 74 e 0 12 myr 1 amurian block motion has a pole position in a similar location but at a slower rate 64 61 n 158 23 e 0 077 myr 1 and most of the amurian eurasia motion is accommodated by extension across lake baikal tarim basin moves relative to eurasia about a pole of rotation at 39 24 n 98 2 e 0 539 myr 1 and 16 18 mm yr of shortening is accommodated across the west central tien shan there is distributed e w extension throughout both southern and north central tibet within southern tibet between the longitudes of 77 e to 92 e the deformation field accommodates 16 19 mm yr of e w extension we compare predicted seismic moment rates with those observed in this century in asia total observed seismic moment rates within the entire area of central and east asia 2 2 107 km2 in this century are 2 26 0 7 1020 n m yr 1 as compared with a predicted total rate of 2 03 0 066 1020 n m yr 1 comparisons between observed and predicted moment rates within 42 subregions reflect the generally unstable process of inferring long term seismic moment rates from a catalog of limited duration 94 years an observation period of 10 000 years would be required to reduce uncertainties in observed seismic moment rate to the same size as the uncertainties in model tectonic moment rates inferred from the joint inversion of gps and quaternary rates of strain we show that in general a better correlation with model tectonic moment rate is inferred from the seismicity catalog by considering the numbers of earthquakes above a cutoff magnitude mb 5 0 for the period january 1 1965 to january 1 1999

geochemical and geochronological characteristics of late cretaceous to early paleocene granitoids in the tengchong block southwestern china implications for crustal anatexis and thickness variations along the eastern neo tethys subduction zone abstract the tengchong block of southwestern china is key to tracing the eastward subduction of neo tethys and collision between indian and asian continents the block contains a magmatic belt that represents the southeastward continuation of the gangdese belt produced by the eastward subduction of eastern neo tethyan oceanic lithosphere in this paper we present geochemical and geochronological data of late cretaceous to early paleocene 64 ma granitic rocks of the guyong and husa batholiths in the tengchong block these can be subdivided into high silica peraluminous granites and low silica metaluminous granodiorites and all belong to the high k calc alkaline series are enriched in lile and depleted in hfse the guyong granitoids have high initial sr ratios of 0 706511 0 711753 negative e nd t values of 9 2 to 11 6 two stage model ages of 1 39 1 55 ga and pb isotopic compositions that indicate a crustal affinity the husa granodiorites also have high initial sr ratios of 0 716496 negative e nd t value of 16 5 two stage model age of 1 89 ga variable e hf t values of 3 4 to 18 1 and pb isotopic compositions similar to lower crustal values these geochemical and isotopic data indicate that the guyong granitoids were likely derived from partial melting of ancient crustal metapelite or mixed pelite greywacke sources while the husa granodiorites were derived from the partial melting of lower crustal mixed sources involving metasedimentary and metaigneous rocks to understand the thermal state and architecture of the late cretaceous to early paleocene magmatic arc crust the crust derived intermediate to acidic igneous rocks of the southern central lhasa and tengchong blocks and eastern himalayan syntaxis are compared we infer that partial melting of crust occurred at great depth in the southern lhasa block intermediate depths in the eastern himalayan syntaxis and shallow depths in the central lhasa and tengchong block sr y ratios indicate that the tengchong block was characterized by thin crust while the eastern himalayan syntaxis and northernmost part of southern lhasa block contained crust of normal thickness and the southernmost part of the southern lhasa block contained thickened crust

paleomagnetic study of cretaceous rocks from pu er western yunnan china evidence of internal deformation of the indochina block abstract cretaceous red sandstones and siltstones were collected at 31 sties for paleomagnetic investigation from the jingxing and nanxin formation around pu er 23 0 n 101 0 e western part of yunnan province china in order to examine quantitative amount of tectonic deformation of the simao terrane due to collision of india with asia after stepwise thermal demagnetization the high temperature component magnetization with unblocking temperature of around 680 c is isolated positive field tests show that the high temperature component magnetization from 25 sites is primary for the cretaceous formations a tilt corrected mean direction is d 59 9 i 45 2 with 95 5 1 corresponding to a paleopole at 35 8 n 173 1 e with a 95 5 6 comparison with cretaceous paleomagnetic pole of the whole china block indicates that amount of clockwise rotation in the pu er area is evaluated to be 46 9 with respect to the whole china block which is consistent with large amount of rotation in the mengla and jinggu areas 47 and 66 in the central part of the simao terrane clockwise rotation of the central part is larger by more than 20 compared to the yunlong and yongping areas of northern part of the simao terrane suggesting that the simao terrane experienced internal deformation the southward displacement of the pu er area expected from inclination data is 7 5 6 6 with respect to the chuan dian fragment which is a similar estimate to other areas in the simao terrane and the indochina block while the simao terrane displaced southward by 800 km together with clockwise rotation of 30 as a whole further clockwise rotation occurred in the central part

direct dating of left lateral deformation along the red river shear zone china and vietnam 1 exposures of high grade midcrustal rocks within the red river shear zone rrsz which separates the indochina and south china blocks exhibit clear evidence of left lateral ductile deformation assuming that the south china sea represents a pull apart basin formed at the southeastern termination of the rrsz it has been argued that seafloor magnetic anomalies constrain the timing of sinistral slip accommodated by the rrsz between 32 and 17 ma at a rate of 4 cm yr while 40ar 39ar thermochronometry indicates that left lateral slip occurred along the rrsz between 25 and 17 ma the timing of earlier high temperature deformation has not been directly constrained in situ th pb ion microprobe dating of monazite inclusions in garnets allows direct assessment of the timing of amphibolite grade metamorphism and synchronous left lateral shearing results from northern segments of the rrsz in yunnan china indicate that synkinematic garnet growth occurred between 34 and 21 ma and are the first to document late oligocene metamorphism and left lateral shearing data from the southern rrsz within vietnam are complicated by tertiary overprinting of rocks that experienced amphibolite facies metamorphism during the indosinian orogeny 220 ma the period during which sinistral deformation is now constrained to have occurred along the rrsz i e 34 17 ma is essentially coincident with spreading of the south china seafloor 32 17 ma this temporal and kinematic link between left lateral shearing along the rrsz and opening of the south china sea supports the view that indochina was extruded from asia as a block along lithospheric scale strike slip faults

new constraints on the structure thermochronology and timing of the ailao shan red river shear zone se asia new structural petrographic and 40ar 39ar data constrain the kinematics of the asrr ailao shan red river shear zone in the xuelong shan xls geochronological data reveal triassic early tertiary and oligo miocene thermal events the latter event 33 26 ma corresponds to cooling during left lateral shear in the fansipan fsp range thrusting of the sapa nappe linked to left lateral deformation and cooling of the fsp granite occurred at 35 ma rapid cooling resumed at 25 29 ma as a result of uplift within the transtensive asrr in the daynuiconvoi dncv foliation trends nw se but is deflected near large scale shear planes stretching lineation is nearly horizontal on steep foliations shear criteria indicate left lateral shear sense zones with flatter foliations show compatible shear senses petrographic data indicate decompression from 6 5 kbar during left lateral shear temperatures 700 c 40ar 39ar data imply rapid cooling from above 350 c to below 150 c between 25 and 22 ma without diachronism along strike along the whole asrr cooling histories show two main episodes 1 rapid cooling from peak metamorphism during left lateral shear 2 rapid cooling from greenschist conditions during right lateral reactivation of the asrr in the nw part of the asrr xls diancang shan we link rapid cooling 1 to local denudations in a transpressive environment in the sw part ailao shan and dncv cooling 1 resulted from regional denudation by zipper like tectonics in a transtensive regime the induced cooling diachronism observed in the ailao shan suggests left lateral rates of 4 to 5 cm yr from 27 ma until 17 ma dncv rocks always stayed in a transtensive regime and do not show cooling diachronism the similarities of deformation kinematics along the asrr and in the south china sea confirms the causal link between continental strike slip faulting and marginal basin opening

40ar 39ar dating of the jiali and gaoligong shear zones implications for crustal deformation around the eastern himalayan syntaxis abstract we conducted a comprehensive 40ar 39ar geochronological study of the jiali and gaoligong shear zones to obtain a better understanding of crustal deformation and tectonic evolution around the eastern himalayan syntaxis ehs the new age data reveal that the main phase of deformation in the jiali and gaoligong shear zones occurred from 22 to 11 ma and from 18 to 13 ma respectively structural data collected during this study indicate that the jiali shear zone underwent a change in shear sense from sinistral to dextral during its movement history based on a comparison with the deformation histories of other major shear zones in the region we argue that the initial sinistral motion recorded by the jiali shear zone was coincident with that of the ailao shan red river shear zone which marked the northern boundary of the southeastward extrusion of the indochina block during the early miocene from the middle miocene 18 ma the jiali shear zone changed to dextral displacement becoming linked with the dextral gaoligong shear zone that developed as a consequence of continued northward indentation of the indian continent into asia since this time the jiali and gaoligong shear zones have been united defining the southwestern boundary of the ehs during clockwise rotation of the eastward extruding tibetan block as revealed by recent gps data the temporal change in regional deformation pattern from southeastward block extrusion to clockwise rotation of crustal fragments may have played an important role in the development of the eastern himalayan drainage system around the ehs

present day crustal motion within the tibetan plateau inferred from gps measurements 1 using the measurements of 726 gps stations around the tibetan plateau we determine the rigid rotation of the entire plateau in a eurasia fixed reference frame which can be best described by an euler vector of 24 38 0 42 n 102 37 0 42 e 0 7096 0 0206 ma the rigid rotational component accommodates at least 50 of the northeastward thrust from india and dominates the eastward extrusion of the northern plateau after removing the rigid rotation to highlight the interior deformation within the plateau we find that the most remarkable interior deformation of the plateau is a glacier like flow zone which starts at somewhere between the middle and western plateau goes clockwise around the eastern himalayan syntaxis ehs and ends at the southeast corner of the plateau with a fan like front the deformation feature of the southern plateau especially the emergence of the flow zone could be attributed to an eastward escape of highly plastic upper crustal material driven by a lower crust viscous channel flow generated by lateral compression and gravitational buoyancy at the later developmental stage of the plateau the first order feature of crustal deformation of the northeastern plateau can be well explained by a three dimensional elastic half space dislocation model with rates of dislocation segments comparable to the ones from geological observations in the eastern plateau although gps data show no significant convergence between the eastern margin of the plateau and the sichuan basin a small but significant compressional strain rate component of 10 5 2 8 nstrain yr exists in a relatively narrow region around the eastern margin in addition a large part of the eastern plateau northeast of the ehs is not undergoing shortening along the northeastward convergence direction of the ehs but is stretching

the onset of india asia continental collision early steep subduction required by the timing of uhp metamorphism in the western himalaya ultrahigh pressure uhp rocks in the nw himalaya are some of the youngest on earth and allow testing of critical questions of uhp formation and exhumation and the timing of the india asia collision initial collision of india with asia is widely cited as being at 55f1 ma based on a paleomagnetically determined slowdown of india s plate velocity and as being at ca 51 ma based on the termination of marine carbonate deposition even relatively small changes in this collision age force large changes in tectonic reconstructions because of the rapid india asia convergence rate of 134 mm a at the time of collision new u pb shrimp dating of zircon shows that indian rocks of the tso morari complex reached uhp depths at 53 3f0 7 ma given the high rate of indian subduction this dating implies that indian continental crust arrived at the asian trench no later than 57f1 ma providing a metamorphic age for comparison with previous paleomagnetic and stratigraphic estimates india s collision with asia may be compared to modern processes in the timor region in which initiation of collision precedes both the slowing of the convergence rate and the termination of marine carbonate deposition the indian uhp rocks must have traveled rapidly along a short hence steep path into the mantle early continental subduction was at a steep angle probably vertical comparable to modern continental subduction in the hindu kush despite evidence for modern day low angle subduction of india beneath tibet oceanic slab break off likely coincided with exhumation of uhp terranes in the western himalaya and led to the initiation of low angle subduction and leucogranite generation d 2005 elsevier b v all rights reserved

kinematics and ar 40 ar 39 geochronology of the gaoligong and chongshan shear systems western yunnan china implications for early oligocene tectonic extrusion of se asia the gaoligong and chongshan shear systems glss and csss in western yunnan china have similar tectonic significance to the ailaoshan red river shear system asrrss during the cenozoic tectonic development of the southeastern tibetan syntaxis to better understand their kinematics and the cenozoic tectonic evolution of se asia this paper presents new kinematic and 40 ar 39 ar geochronological data for these shear systems all the structural and microstructural evidence indicate that the glss is a dextral strike slip shear system while the csss is a sinistral strike slip shear system and both were developed under amphibolite to greenschist grade conditions the 40 ar 39 ar dating of synkinematic minerals revealed that the strike slip shearing on the glss and csss at least began at 32ma possibly coeval with the onset of other major shear systems in se asia the latestage shearing on the glss and csss is dated at 27 29ma by the biotite 40 ar 39 ar ages consistent with that of the wang chao shear zone wcsz but 10ma earlier than that of the asrrss the dextral gaoligong shear zone within the glss may have separated the india plate from the indochina block during early oligocene combined with other data in western yunnan we propose that the baoshan southern indochina block escaped faster southeastward along the csss to the east and the glss to the west than the northern indochina block along the asrrss accompanying with the obliquely northward motion of the india plate during early oligocene 28 36ma during 28 17ma the northern indochina block was rotationally extruded along the asrrss relative to the south china block as a result of continuously impinging of the india plate

evidence for mechanical coupling and strong indian lower crust beneath southern tibet how surface deformation within mountain ranges relates to tectonic processes at depth is not well understood the upper crust of the tibetan plateau is generally thought to be poorly coupled to the underthrusting indian crust because of an intervening low viscosity channel here however we show that the contrast in tectonic regime between primarily strike slip faulting in northern tibet and dominantly normal faulting in southern tibet requires mechanical coupling between the upper crust of southern tibet and the underthrusting indian crust such coupling is inconsistent with the presence of active channel flow beneath southern tibet and suggests that the indian crust retains its strength as it underthrusts the plateau these results shed new light on the debates regarding the mechanical properties of the continental lithosphere and the deformation of tibet

reconstruction of the deformed collision zone between india and asia by backward motion of lithospheric blocks 1 on the basis of a synthesis of tectonic data available on the india asia collision we present a first attempt to reconstruct the evolution of the collision zone assuming that the deformation of the lithosphere is localized along narrow shear zones and that the interiors of mantle blocks in between remain relatively undeformed we define block contours from the fault pattern and move back the blocks along their boundary faults along convergent or extensional boundaries the crust is assumed to shorten or stretch coherently step by step we go backward in time to finally reach the collision onset for each time step we find a solution compatible with the data set available and the position of the adjacent blocks for each block the search for compatibility at the scale of the entire collision zone allows for solving the kinematics of regions with fewer data and suggests plausible scenarios for regions where data is lacking for each step we calculate large scale displacement maps and determine euler poles for each block for the most recent time step the map proposed is compared to gps motions the deformation budget implies that extrusion absorbed 30 of the convergence between india and siberia during the entire collision span but varied with time accounting for as little as 3 or as much as 60 of this convergence at different epochs

detrital zircon provenance comparison between the paleocene eocene nangqian xialaxiu and gongjue basins new insights for cenozoic paleogeographic evolution of the eastern tibetan plateau abstract paleogeographic reconstructions of terranes can greatly benefit from the provenance analysis of sediments a series of cenozoic basins provide key sedimentary archives for investigating the growth of the tibetan plateau yet the provenance of the sediments in these basins has never been constrained robustly here we report sedimentary petrological and detrital zircon geochronological data from the paleocene eocene nangqian xialaxiu and gongjue basins sandstone detrital modes and zircon morphology suggest that the samples collected in these two basins were sourced from recycled orogen detrital zircon geochronology indicates that sediments in the nangqian xialaxiu basin are characterized by two distinct age populations at 220 280 ma and 405 445 ma in contrast three predominant age populations of 207 256 ma 423 445 ma and 1851 1868 ma and two subordinate age populations of 50 ma and 2500 ma are recognized in the gongjue basin comparison with detrital zircon ages from the surrounding terranes suggests that sediments in the nangqian xialaxiu basin come from the neighboring thrust belts whereas sediments from the gongjue basin are predominantly derived from the distant songpan ganzi terrane with minor contribution from the surrounding areas a three stage cenozoic evolution of the eastern tibetan plateau is proposed during the paleocene the nangqian xialaxiu basin appeared as a set of small intermontane sub basins and received plentiful sediments from the neighboring mountain belts during the eocene the gongjue basin kept a relatively low altitude and was a depression at the edge of a proto plateau since the oligocene the tibetan plateau further uplifted and the marginal gongjue basin was involved in the tibetan interior orogeny indicating the eastward propagation of the tibetan plateau

clockwise rotation of the red river fault inferred from paleomagnetic study of cretaceous rocks in the shan thai malay block of western yunnan china more than 150 samples were collected at 23 sites from the lower cretaceous jingxing formation near the city of yongping 25 5 n 99 5 e which is located on the west side of the red river fault sixteen sites have characteristic directions with a high temperature component above 500 c the high temperature component of magnetization from twelve sites is of pre fold origin and reveals clockwise deflection in declination d 42 0 i 51 1 and 95 15 7 the easterly declination of more than 40 at yongping is consistent with the cretaceous paleomagnetic direction d 45 of chuxiong 25 n 101 5 e which is located on the east side of the red river fault both areas were subjected to about 25 clockwise rotation with respect to the eastern part of the yangtze block since cretaceous time we conclude that the red river fault was rotated through25 16 with respect to the eastern part of the yangtze block associated with rotation of the yongping and chuxiong areas removal of the rotation from the red river fault indicates that the latter and the jinsha suture formed a straight line along n55 w s55 e within the asian continent from vietnam to the tibetan plateau in the present coordinate system during the cretaceous the linear feature of the red river fault and jinsha suture was probably deformed to the present curved shape as a result of collision of the indian continent with the asian continent

early mississippian evaporites of coastal tropical wetlands extensive evaporites in lower mississippian successions from palaeoequatorial regions are commonly used as evidence for an arid to semi arid palaeoclimate however in this study detailed studies of evaporites and their context refute this interpretation detailed sedimentological and petrographical analysis of the lower mississippian of northern britain is combined with archived log data from more than 40 boreholes across southern scotland northern england and northern ireland and published literature from canada two key cores from the tweed basin and the northern margin of the northumberland solway basin contain 178 evaporite intervals and reveal twelve distinct forms of gypsum and anhydrite across seven facies that are associated with planar laminated siltstone and intercalated thin beds of ferroan dolostone nodular gypsum and anhydrite typically in intervals less than 1 to 2 m thick are integral components of the succession nodular evaporite occurs within about 1 m of a palaeosurface but most evaporite deposits represent ephemeral brine pans to semi permanent hypersaline lakes or salinas on a floodplain that was subjected periodically to storm surges introducing marine waters formation of evaporites under a strongly seasonal climate in a coastal wetland is supported by palaeosol types and geochemical proxies and from palaeobotanical evidence published previously although 65 of modern equatorial areas experience a strongly seasonal climatic regime salinas and sabkhas are a minor component today in comparison with the evidence from these lower mississippian successions this implies that the earliest terrestrial environments were complex and dynamic providing a diverse range of habitats in which the early tetrapods became terrestrialized and represent a setting that is rarely preserved in the geological record

paleomagnetic study of cretaceous rocks from the yangtze block central yunnan china implications for the india asia collision abstract more than 200 samples for this study were collected at 30 sites from the cretaceous matoushan and puchanghe formations around the city of chuxiong 25 n 101 5 e twenty four sites have characteristic directions with a high temperature component above 350 c paleomagnetic directions from 21 sites of the cretaceous formations reveal clockwise deflection in declination d 44 6 i 41 3 and 95 10 7 the reliability of the direction is ascertained through a positive fold test and the presence of normal and reversed polarities comparison with paleomagnetic data from other areas indicates that the chuxiong area was rotated clockwise through26 17 with respect to the eastern part of the yangtze block whereas it was subjected to more than 44 of clockwise rotation with respect to the neighboring blocks of lhasa 56 21 and western sichuan 44 16 since cretaceous time the clockwise rotation of chuxiong is a reflection of rotational motion of the large region including the southeastern qiangtang terrane and the jinsha suture red river fault paleomagnetic data and tectonic features suggest that the rotation occurred prior to the folding in the central yunnan province probably during the paleogene this large rotation is a spectacular example of the deformation in the southeastern part of the asian continent due to the collision of india

polystage deformation of the gaoligong metamorphic zone structures 40ar 39ar mica ages and tectonic implications abstract the gaoligong metamorphic zone is located southeast of the eastern himalayan syntaxis in western yunnan china the zone is characterized by four stages of deformation d1 d4 d1 structures record early compressive deformation during the indosinian orogeny which formed tight to isoclinal f1 folds of bedding with a penetrative s1 foliation developed parallel to fold axial planes mid crustal horizontal shearing during d2 resulted in overprinting of d1 structures d1 and d2 structures are associated with granulite facies metamorphism d3 doming resulted in late crustal thickening and the development of a regional nw se trending f3 antiform synchronous with or slightly subsequent to d3 deformation the zone experienced d4 ductile strike slip shearing resulting in its exhumation to shallow crustal levels and retrograde metamorphism granitic d4 mylonites predominantly yield 40 ar 39 ar mica ages of 15 16 ma indicating that d4 dextral strike slip shearing occurred in the miocene weakly deformed leucogranite and protomylonite yield 40 ar 39 ar ages of 10 11 ma suggesting that ductile strike slip shearing continued to the late miocene the new 40 ar 39 ar data indicate that escape related deformation along the gaoligong strike slip shear zone occurred in the miocene in association with recent geophysical studies and on the basis of the structural crystal preferred orientation cpo and geochronological data presented in this paper we suggest that the gaoligong metamorphic zone formed in response to intracontinental transpression in the southeast of tibet characterized as intense deformation and metamorphism at middle upper crustal levels

dynamics of the tengchong volcanic region in the southeastern tibetan plateau a numerical study abstract the constraints of surface heat flow and lithospheric structure were used to investigate the dynamics of the tengchong volcanic region based on thermal mechanical modeling three different boundary conditions representing three types of tectonic movements were modeled to discuss the thermal state and lithospheric deformation the compressive model shows that the subduction of the indian plate causes the upwelling of asthenosphere beneath the tengchong block in this process upwelling of the asthenosphere causes thinning of the lithosphere beneath the tengchong block however the surface heat flow of this model is not consistent with the observations the regional extension model shows that under the extension the asthenosphere upwells along the preexisting weak suture zone in the southeastern tibetan plateau the thickness of lithosphere is the thinnest along the suture zone and the surface heat flow is the highest however the extension time was found to generally need 10 15 myr to conform to the surface heat flow however this is not consistent with accepted geochronological data when a boundary condition from compression to extension is imposed both the surface heat flow and the lithospheric structure fit well with the observations the results show that the subduction of the indian plate leads to the upwelling of the asthenosphere which causes a thinned lithosphere beneath the tengchong block since the late miocene 8 5 ma the extension or extrusion has been predominant the thermal erosion of the lithosphere caused by upwelling of the asthenosphere renders the lithosphere even thinner along the bangong nujiang suture zone the upwelling of the asthenosphere provides an upward driving force and high temperature environment for the tengchong volcanic region these findings suggest that the origin of the tengchong volcanic region is caused by both the subduction of the indian plate and regional extension across the bangong nujiang suture zone

west directed thrusting south of the eastern himalayan syntaxis indicates clockwise crustal flow at the indenter corner during the india asia collision abstract whether continental deformation is accommodated by microplate motion or continuum flow is a central issue regarding the nature of cenozoic deformation surrounding the eastern himalayan syntaxis the microplate model predicts southeastward extrusion of rigid blocks along widely spaced strike slip faults whereas the crustal flow model requires clockwise crustal rotation along closely spaced semi circular right slip faults around the eastern himalayan syntaxis although global positioning system gps data support the crustal flow model the surface velocity field provides no information on the evolution of the india asia orogenic system at million year scales in this work we present the results of systematic geologic mapping across the northernmost segment of the indo burma ranges located directly southeast of the eastern himalayan syntaxis early research inferred the area to have experienced either right slip faulting accommodating northward indentation of india or thrusting due to the eastward continuation of the himalayan orogen in the cenozoic our mapping supports the presence of dip slip thrust faults rather than strike slip faults specifically the northern indo burma ranges exposes south to west directed ductile thrust shear zones in the hinterland and brittle fault zones in the foreland the trends of ductile stretching lineations within thrust shear zones and thrust sheets rotate clockwise from the northeast direction in the northern part of the study area to the east direction in the southern part of the study area this clockwise deflection pattern of lineations around the eastern himalayan syntaxis mirrors the clockwise crustal rotation pattern as suggested by the crustal flow model and contemporary gps velocity field however our finding is inconsistent with discrete strike slip deformation in the area and the microplate model

correction to conjugate strike slip faulting along the bangong nujiang suture zone accommodates coeval east west extension and north south shortening in the interior of the tibetan plateau correction

kinematics and dynamics of the southeastern margin of the tibetan plateau summary on the southeastern margin of the tibetan plateau lies a large region which seismicity and gps data show to be actively deforming this paper describes the active faulting in the region and how it relates to the velocity field observed with gps in places the velocity field is accommodated by rotations about vertical axes and most or all of the strain at the surface in the region appears to be released seismically gps velocities are then compared to velocities calculated using a model for deformation driven by gravitational driving forces using rheologies estimated from experimentally derived mineral flow laws the model provides velocities that are in good agreement with observed gps velocities it is not possible to uniquely determine the rheology or flow velocity at depth and there are two forms of model solution which match the observed horizontal surface velocities in one of these vertical planes deform by pure shear and in the other vertical gradients of horizontal velocity are present within the crust two distinct regions of normal faulting earthquakes are present in the region and have mechanisms which are most easily explained by gravity driven deformation

distributed deformation in southern and western tibet during and after the india asia collision field and radiometric data are used to describe and date strain and stress states in southern longitude 88 o to 91oe latitude 28 o to 30on and western tibet longitude 79 o to 82oe latitude 30 o to 34on we factorize deformation into syncollisional and postcollisional and we present stretching lineation and displacement orientation maps two sections across the indian shelf sequence and stress orientations calculated from mesoscale fault slip data in southern tibet syncollisional stretching and displacement directions trend 90 46 o and displacement is top to south synkinematic low grade metamorphism is dated at 50 ma at one locality in the indian shelf sequence underlying the main mantle thrust of the indus yarlung suture this implies paleocene onset of continental collision for the investigated section postcollisional structures comprise a backthrust group which includes foreland and hinterland directed thrusts reverse and strike slip faults and folds it dominates postcollisional deformation is concentrated along the indus yarlung suture and portrays n s compression otrend of 8o 17 o o2 of 97o 17o a strike slip group consists of conjugate strike slip faults is concentrated in east trending narrow highly deformed zones and indicates that n s compression is locally compensated by e w extension oof 15o 29 o o3 of 103o 30o synkinematic muscovite dates postcollisional deformation as late early miocene 17 5 ma at one locality at the suture strike slip and oblique normal 03 of 600 23 o oof 144o 21 o and normal o3 114o 16 o faulting dated between late miocene and recent and including active deformation represents dominant e w and minor n s extension due to e w stretching of southern tibet and oroclinal bending along the himalayan arc restoring syncollisional and postcollisional deformation yields a minimum of 67 258 km shortening across the indian shelf sequence incorporating recently published contraction estimates across the eastern himalaya yields minimum shortening between undeformed india and the indus yarlung suture of 66 536 km the himalaya tibet orogenic system south of the indus yarlung suture had an initial width of 811 km in the southern tibetan section in western tibet imbrication of an ophiolite sequence of the bangong nujiang suture is top to south stretching lineation trend of 15o 18o and o3 of active deformation trends ese faulting along the shiquanhe fault zone which transfers displacement from the northern part of the karakorum fault to a system of rifts in western central tibet indicates dextral strike slip alternating with sinistral oblique normal faulting and block rotations around vertical axes during a prolonged shearing history the indian shelf sequence south of mount kailas shows top to south imbrication stretching lineation trend of 52o 60o both indian shelf rocks and oligocene miocene kailas conglo merates record backthrusting and backfolding oof 33 o and recent e w extension 03 of 85o 28o

late cenozoic evolution of the eastern margin of the tibetan plateau inferences from 40ar 39ar and u th he thermochronology high topography in central asia is perhaps the most fundamental expression of the cenozoic indo asian collision yet an understanding of the timing and rates of development of the tibetan plateau remains elusive here we investigate the cenozoic thermal histories of rocks along the eastern margin of the plateau adjacent to the sichuan basin in an effort to determine when the steep topographic escarpment that characterizes this margin developed temperature time paths inferred from 40 ar 39 ar thermochronology of biotite multiple diffusion domain modeling of alkali feldspar 40 ar release spectra and u th he thermochronology of zircon and apatite imply that rocks at the present day topographic front of the plateau underwent slow cooling 30 50 c m y coincident with exhumation from inferred depths of 8 10 km at denudation rates of 1 2 mm yr samples from the interior of the plateau continued to cool relatively slowly during the same time period 3 c m y suggesting limited exhumation 1 2 km however these samples record a slight increase in cooling rate from 1 to 3 c m y at some time during the middle tertiary the tectonic significance of this change remains uncertain regardless late cenozoic denudation in this region appears to have been markedly heterogeneous with the highest rates of exhumation focused at the topographic front of the plateau margin we infer that the onset of rapid cooling at the plateau margin reflects the erosional response to the development of regionally significant topographic gradients between the plateau and the stable sichuan basin and thus marks the onset of deformation related to the development of the tibetan plateau in this region the present margin of the plateau adjacent to and north of the sichuan basin is apparently no older than the late miocene or early pliocene 5 12 ma

paleomagnetic analysis of eastern tibet implications for the collisional and amalgamation history of the three rivers region sw china abstract a comparative analysis of paleolatitudes and latitudinal displacements was undertaken for the south china simao baoshan shan thai indochina qiangtang lhasa and himalayan blocks by using available paleomagnetic data to determine the collisional and amalgamation history of the main blocks in the three rivers region western yunnan the results show that 1 the simao block is a south china derived block 2 the baoshan and shan thai blocks were subjected to rapid northward drift during the period from late carboniferous to late permian 3 the baoshan block collided with the simao block in the late permian and continued to drift rapidly northwards together with the south china and shan thai blocks until the late triassic 4 the paleo tethys separating the baoshan and simao blocks was possibly opened in the early silurian 5 the meso tethys ranged in age from early permian time to early cretaceous time and reached its greatest width of about 42 latitude in the late triassic

tectonic deformation around the eastern himalayan syntaxis constraints from the cretaceous palaeomagnetic data of the shan thai block summary lower to middle cretaceous red sandstones were sampled at four localities in the lanpin simao fold belt of the shan thai block to describe its regional deformational features most of the samples revealed a characteristic remanent magnetization with unblocking temperatures around 680 c primary natures of magnetization are ascertained through positive fold test a tilt corrected formation mean direction for the jingdong 24 5 n 100 8 e locality which is located at a distance of 25 km from the ailaoshan red river fault revealed northerly declination with steep inclination dec inc 8 3 48 8 95 7 7 n 13 however mean directions obtained from the zhengyuan 24 0 n 101 1 e west zhengyuan 24 0 n 101 1 e and south mengla 21 4 n 101 6 e localities indicate an easterly deflection in declination such as dec inc 61 8 46 1 95 8 1 n 7 dec inc 324 2 49 4 95 6 4 n 4 and dec inc 51 2 46 4 95 5 6 n 13 respectively the palaeomagnetic directions obtained from these four localities are incorporated into a palaeomagnetic database for the shan thai block when combined with geological geochronological and gps data the processes of deformation in the shan thai block is described as follows subsequent to its rigid block clockwise rotation of about 20 in the early stage of india asia collision the shan thai block experienced a coherent but southward displacement along the red river fault prior to 32 ma this block was then subjected to a north south compressive stresses during the 32 27 ma period which played a key role in shaping the structure of chongshan lancang chiang mai belt following this some local clockwise rotational motion has occurred during the pliocene quaternary time in central part of the shan thai block as a result of internal block movements along the reactivated network of faults

an early india asia contact paleomagnetic constraints from ninetyeast ridge odp leg 121 new paleomagnetic results from sedimentary rock and basement of the ninetyeast ridge ocean drilling program leg 121 sites 756 758 detail the northward movement of the indian plate for the past 80 m y analysis of the combined paleolatitude age profile indicates a distinct reduction in india9s northward movement rate at 55 ma interpreted as completion of suturing of greater india and asia india9s northward motion slowed from 18 19 5 cm yr to 4 5 cm yr for the location of site 758 comparison of this profile with paleomagnetic data from the wider himalayan region indicates that initial contact between northwestern greater india and southern asia was already established by cretaceous tertiary time this supports a possible causal link between the india asia convergence and the deccan traps extrusion

cenozoic clockwise rotation of the tengchong block southeastern tibetan plateau a paleomagnetic and geochronologic study abstract paleomagnetic data from 50 35 ma likely 40 ma mafic dykes in yunnan southeastern tibetan plateau cutting 115 ma granitoids record the rotation of the tengchong lhasa block around the east himalayan syntaxis ehs ti rich titanomagnetite and magnetite carry a primary magnetic component group1 a magnetic overprint resides in magnetite group2 likely induced by low grade metamorphism between 30 and 10 ma the tilt corrected overall mean directions are d i 89 8 35 1 for group1 and d i 33 3 41 4 for group2 these data imply a clockwise rotation of 87 87 3 12 5 of the tengchong block since remanence acquisition at 40 ma with respect to stable eurasia the average rotation rate of 2 18 0 31 myr is at the upper limit of the present day rotation rates around the ehs obtained from gps velocities and quaternary strain rates the remagnetized group2 indicates a poorly defined rotation of 31 31 2 32 9 our key results the detection of 87 clockwise rotation with high rotation rates following the india asia collision and a decrease in rotation rates during the miocene suggest that first the tengchong block rotated rapidly around the ehs synchronous with eastward tectonic escape of lithospheric blocks the later slower clockwise rotation similar to those recorded geodetically may be related to viscous flow of tibetan crust

continuous deformation of the tibetan plateau from global positioning system data global positioning system velocities from 553 control points within the tibetan plateau and on its margins show that the present day tectonics in the plateau is best described as deformation of a continuous medium at least when averaged over distances of 100 km deformation occurs throughout the plateau interior by ese wnw extension and slightly slower nne ssw shortening relative to eurasia material within the plateau interior moves roughly eastward with speeds that increase toward the east and then flows southward around the eastern end of the himalaya crustal thickening on the northeast ern and eastern margins of the plateau occurs over a zone 400 km wide and cannot be the result of elastic strain on a single major thrust fault shortening there accommodates much of india s penetration into eurasia a description in terms of movements of rigid blocks with elastic strain associated with slip on faults between them cannot match the velocity field

low temperature magnetic behavior of titanomagnetites titanium substitution has a significant effect on the temperature dependence 5 300 k of low temperature saturation remanence and of initial susceptibility in synthetic and natural titanomagnetite fe3 xtixo4 0 x 0 4 and electronic and lattice relaxation phenomena over the entire titanomagnetite compositional range these features produce diagnostic magnetic behavior useful for titanomagnetite identification in natural samples moreover the apparent similarities between the thermal decay of low temperature remanence of titanomagnetite with pyrrhotite pure magnetite and superparamagnetic phases can lead to complications in interpreting low temperature remanence data when titanomagnetite is present

the impact of scotian shelf water cross over on the plankton dynamics on georges bank a 3 d experiment for the 1999 spring bloom the 1999 march seawifs images detected an intense phytoplankton bloom on the southern flank of georges bank gb the bloom covered a large portion of the southern flank between the 60 and 200 m isobaths and later extended to and connected with an even larger patch near the northeast peak nep and browns bank a three dimensional 3 d model experiment was conducted to examine the cause of the bloom and the impact of scotian shelf water on the spring phytoplankton bloom dynamics the finite volume coastal ocean model fvcom provided the hydrodynamic fields for lagrangian particle trajectory tracer and biological model experiments process oriented modeling experiments showed that the formation and maintenance of the phytoplankton bloom on the southeastern flank of gb is related to the weak stratification caused by the transport of the colder but fresher scotian shelf water across the northeast channel nec with sufficient nutrients from the slope the bloom could result from in situ growth of phytoplankton near the slope where the stabilizing salinity front is located the model results suggest that the timing and location of the phytoplankton bloom on the southern flank of gb is sensitive to the spatial distribution of temperature and salinity on the bank the flow fields across the nec and the location of the salinity front near the shelf break r 2006 elsevier ltd all rights reserved

dual polarity directions in basaltic andesitic dykes reversal record or self reversed magnetization summary rock magnetic analyses were performed on basaltic andesitic dyke samples from the lhasa block which were previously used for palaeomagnetic investigations of the india asia collision the aim of the study is to prove whether antipodal directions separated in these samples represent different polarities of the earth s magnetic field or a self reversal magnetization reflected light microscopy domain pattern observations and energy dispersive x ray analyses revealed two generations of titanomagnetite tm grains one consisting of largesized 20 m to more than 100 m and inhomogeneous ti poor tm and another with small smaller than 10 m and rather homogeneous ti rich tm grains partial thermoremant magnetization experiments and reflected light microscopy show that magnetostatic or superexchange interaction between these two phases is unlikely temperature dependence of saturation magnetization and low temperature curves of isothermal remanent magnetization disprove a possible n type behaviour of the samples a self reversal due to ionic reordering can be ruled out as it requires a high degree of oxidation of the ti rich tms which was not observed in summary a self reversal magnetization is very unlikely in the studied dyke samples and therefore the observed antipodal directions most probably represent a record of different polarity epochs of the earth s magnetic field this conclusion implies that the time of remanence acquisition in the studied dykes was sufficiently long to average out paleosecular variation supporting the significance and reliability of the palaeomagnetic results

timing of india asia collision geological biostratigraphic and palaeomagnetic constraints 1 a range of ages have been proposed for the timing of india asia collision the range to some extent reflects different definitions of collision and methods used to date it in this paper we discuss three approaches that have been used to constrain the time of collision the time of cessation of marine facies the time of the first arrival of asian detritus on the indian plate and the determination of the relative positions of india and asia through time in the qumiba sedimentary section located south of the yarlung tsangpo suture in tibet a previous work has dated marine facies at middle to late eocene by far the youngest marine sediments recorded in the region by contrast our biostratigraphic data indicate the youngest marine facies preserved at this locality are 50 6 52 8 ma in broad agreement with the timing of cessation of marine facies elsewhere throughout the region double dating of detrital zircons from this formation by u pb and fission track methods indicates an asian contribution to the rocks thus documenting the time of arrival of asian material onto the indian plate at this time and hence constraining the time of india asia collision our reconstruction of the positions of india and asia by using a compilation of published palaeomagnetic data indicates initial contact between the continents in the early eocene we conclude the paper with a discussion on the viability of a recent assertion that collision between india and asia could not have occurred prior to 35 ma

multi system geochronological and isotopic constraints on age and evolution of the gaoligongshan metamorphic belt and shear zone system in western yunnan china abstract the gaoligongshan metamorphic belt located east of the eastern himalayan syntaxis ehs in the yunnan province southwestern china is a compelling region in which to determine temporal constrains on crustal dynamic processes related to the himalayan orogeny we applied multi system geo and thermochronology 900 c to zircon u pb data reveal three magmatic events at 486 ma 136 ma and 76 ma similar ages are found in orthoderivative rocks of the neighboring tengchong and baoshan blocks and the gangdese batholith suggesting that the southern gaoligongshan is composed of an assemblage of lhasa and qiangtang terrane derived rocks muscovite rb sr ages of 35 21 ma are coeval with the onset of lateral crustal displacement along major shear zones in eastern tibet and indochina and with the post collision volcanic activity in western yunnan biotite rb sr and mica 40 ar 39 ar ages provide evidence that mylonitization along the gaoligongshan shear zone and crustal rotation of the tengchong and baoshan blocks proceeded during the miocene between 19 and 12 ma when the rocks rapidly cooled through the 350 280 c temperature range almost identical 40 ar 39 ar ages reported for the karakorum jiali parlung fault system in western tibet suggest that the gaoligongshan shear zone is the southeastern continuation of this fault recording the eastward extrusion of tibet and crustal movement around the ehs the final stage of exhumation of the gaoligongshan occurred between 8 and 5 ma at an average exhumation rate of 3 km ma as documented by apatite fission track and apatite u th sm he data this rapid exhumation was triggered by crustal root delamination and opening of the andaman sea our results clearly show that the complex tectonothermal evolution of the gaoligongshan was influenced by tibetan extrusion and escape tectonics associated with lower crustal flow around the ehs and the southeastward movement of indochina and back arc extension in response to andaman seafloor spreading

tectonic evolution of the mogok metamorphic belt burma myanmar constrained by u th pb dating of metamorphic and magmatic rocks 1 the mogok metamorphic belt mmb extends for over 1500 km along the western margin of the shan thai block from the andaman sea north to the eastern himalayan syntaxis previous geochronology has suggested that a long lasting jurassic early cretaceous subduction related event resulted in emplacement of granodiorites and orthogneisses 171 120 ma and a poorly constrained tertiary metamorphic event on the basis of new u pb isotope dilution thermal ionization mass spectrometry and u th pb laser ablation multicollector inductively coupled plasma mass spectrometer geochronology presented here we propose two tertiary metamorphic events affected the mmb in burma the first was a paleocene event that ended with intrusion of crosscutting postkinematic biotite granite dikes at 59 ma a second metamorphic event spanned late eocene to oligocene at least from 37 possibly 47 to 29 ma this resulted in the growth of metamorphic monazite at sillimanite grade growth of zircon rims at 47 43 ma sillimanite muscovite replacing older andalusite and synmetamorphic melting producing garnet and tourmaline bearing leucogranites at 45 5 0 6 ma and 24 5 0 7 ma these data imply high temperature sillimanite muscovite metamorphism peaking at 680 c and 4 9 kbar between 45 and 33 ma to around 606 656 c and 4 4 4 8 kbar at 29 3 0 5 ma the later metamorphic event is older than 24 5 0 3 ma the age of leucogranites that crosscut all earlier fabrics our structural and geochronological data suggest that the mmb links north to the unexposed middle or lower crust rocks of the lhasa terrane south tibet and east to high grade metamorphic core complexes in northwest thailand

internal crustal deformation in the northern part of shan thai block new evidence from paleomagnetic results of cretaceous and paleogene redbeds in order to describe the internal deformation of the northeast part of the shan thai block resulting from the penetration of the indian plate into asia a paleomagnetic study was carried out on the cretaceous and paleogene strata in the southern part of the simao terrane high temperature magnetic components with unblocking temperatures around 680 degrees c were isolated and positive fold test shows its primary nature of magnetization the tilt corrected site mean directions of the lower cretaceous and upper cretaceous from the menglun area are d s 46 2 degrees i s 45 9 degrees k 11 0 alpha 95 10 6 degrees and d s 33 2 degrees i s 30 9 degrees k 68 4 alpha 95 8 2 degrees respectively the site mean directions of the upper cretaceous and lower paleogene for the mengban area are d s 50 5 degrees i s 31 0 degrees k 207 6 alpha 95 6 4 degrees and d s 43 5 degrees i s 23 0 degrees k 26 1 alpha 95 13 4 degrees respectively the results of the lower cretaceous and upper paleogene from the mengla area are d s 46 9 degrees i s 42 2 degrees k 27 3 alpha 95 7 7 degrees and d s 41 8 degrees i s 23 8 degrees k 38 9 degrees alpha 95 5 8 degrees respectively in comparison with previous paleomagnetic data of the simao terrane the rotational deformation can be divided into four stages the cretaceous and paleocene sediments of the simao terrane experienced intensive folding before the early oligocene during the early oligocene to early miocene the shan thai and indochina blocks experienced southeastward extrusion with similar to 20 degrees clockwise rotation relative to the south china block during the middle to late miocene the shan thai blocks experienced more 15 degrees clockwise rotation since the late miocene the activity of strike slip faults surrounding the simao terrane enhanced the internal differential rotations of the terrane these deformation processes finally resulted in a complex internal deformation of the simao terrane c 2013 elsevier b v all rights reserved

miocene initiation and acceleration of extension in the south lunggar rift western tibet evolution of an active detachment system from structural mapping and u th he thermochronology 1 ongoing extension in tibet may have begun in the middle to late miocene but there are few robust estimates of the rates timing or magnitude of neogene deformation within the tibetan plateau we present a comprehensive study of the seismically active south lunggar rift in southwestern tibet incorporating mapping u pb geochronology and zircon u th he thermochronology the south lunggar rift is the southern continuation of the north lunggar rift and comprises a 50 km n s central horst bound by two major normal faults the west dipping south lunggar detachment and the east dipping palung co fault the sld dips at the rangefront 20 w and exhumes a well developed mylonite zone in its footwall displaying fabrics indicative of normal sense shear the range is composed of felsic orthogneiss mafic amphibolite and leucogranite intrusions dated at 16 and 63 ma zircon u th he cooling ages are oligocene through late pliocene with the youngest ages observed in the footwall of the sld we tested 25 000 unique thermokinematic forward models in pecube against the structural and u th he data to fully bracket the allowable ranges in fault initiations accelerations and slip rates we find that normal faulting in the slr began in the middle miocene with horizontal extension rates of 1 mm a 1 and in the north accelerated at 8 ma to 2 5 3 0 mm a 1 as faulting commenced on the sld cumulative horizontal extension across the slr ranges from 10 km in the south to 19 21 km in the north

a new late cretaceous to present apwp for asia and its implications for paleomagnetic shallow inclinations in central asia and cenozoic eurasian plate deformation based on a compilation of 533 cretaceous to present day palaeomagnetic poles obtained from both sedimentary and igneous rocks we present a new analysis of the so called asian inclination anomaly and demonstrated the anomaly to be twofold a 2nd order anomaly characterized by high palaeolatitudes in indochina and low palaeolatitudes over tibet and central asia is superimposed on a 1st order anomaly characterized by cenozoic low palaeolatitudes found all over northeastern asian stable blocks the analysis herein convincingly shows that the europe apparent polar wandering path apwp can no longer be used to interpret palaeomagnetic data east of the urals including interpretation of asian tertiary deformation related to the india asia collision we thus construct a new apwp for east asia based on palaeopoles from blocks assumed to be stable this new apwp is consistent with and reinforces previous analyses of asian tectonics such as the age 55 ma and locus 5 10 n of the indo asian collision the lateral extrusion of se asian continental blocks and the intracontinental shortening in central asia possible origins of the 1st order palaeolatitude anomaly are 1 a geomagnetic origin due to long lasting non dipolar contribution to the magnetic field and 2 a tectonic hypothesis in which a newly defined east asia plate was located 10 farther south than expected from the current europe apwp based on a set of six new reconstructions from 90 ma to present we show that our tectonic model reconciles geophysical geological and tectonic observations throughout eurasia from siberia to europe including kinematics in the arctic ocean up to northwestern arctic alaska beyond possible occurrences of non dipolar field contribution and or local inclination flattening in the sedimentary data our model leads us to conclude that cenozoic tectonics is the dominant contributor to the observed 1st order 10 low palaeolatitude anomaly over asia during the tertiary

the formation of mountain range curvature by gravitational spreading this paper presents a mechanism by which mountain ranges can form curved range fronts gravitational spreading of mountain ranges that have been thrust onto rigid lowlands will result in the formation of curvature provided that enough gravity driven flow occurs to dominate the shape of the topography whether this mechanism can operate during the lifetime of a given mountain range depends upon the viscosity of the range the square of the along strike length of the range and the cube of the elevation of the range the curvature of the southern edge of the tibetan plateau is consistent with formation by gravitational spreading provided that the viscosity is similar to that previously estimated using other independent methods the low elevation and young age of the zagros mountains mean that large scale curvature has not had time to develop the short along strike extent and possibly low viscosity of the sulaiman ranges in pakistan may have allowed the ranges to form their distinctive arcuate shape the formation of range front curvature plays an important role in controlling the tectonic evolution of the interiors of the ranges with arc parallel extension becoming progressively more important as range front curvature develops

geodynamics of the southeastern tibetan plateau from seismic anisotropy and geodesy ongoing plate convergence between india and eurasia provides a natural laboratory for studying the dynamics of continental collision a fi rst order process in the evolution of continents regional climate and natural hazards in southeastern tibet the fast directions of seismic anisotropy determined using shear wave splitting analysis correlate with the surfi cial geology including major sutures and shear zones and with the surface strain derived from the global positioning system velocity fi eld these observations are consistent with a clockwise rotation of material around the eastern himalayan syntaxis and suggest coherent distributed lithospheric deformation beneath much of southeastern tibet at the southeastern edge of the tibetan plateau we observe a sharp transition in mantle anisotropy with a change in fast directions to a consistent e w direction and a clockwise rotation of the surface velocity surface strain fi eld and fault network toward burma around the eastern himalayan syntaxis the coincidence between structural crustal features surface strain and mantle anisotropy suggests that the deformation in the lithosphere is mechanically coupled across the crust mantle interface and that the lower crust is suffi ciently strong to transmit stress at the southeastern margin of the plateau in yunnan province a change in orientation between mantle anisotropy and surface strain suggests a change in the relationship between crustal and mantle deformation lateral variations in boundary conditions and rheological properties of the lithosphere play an important role in the geodynamic evolution of the himalayan orogen and tibetan plateau and require the development of three dimensional models that incorporate lateral heterogeneity

paleomagnetic results from cretaceous and jurassic rocks of south and southwest yunnan evidence for large clockwise rotations in the indochina and shan thai malay terranes abstract cretaceous and jurassic redbeds have been sampled from south and southwest yunnan china for paleomagnetic study the paleomagnetic directions resolved from the cretaceous rocks indicate that the xiaguan area which is located on the red river fault has not been rotated with respect to stable eurasia since at least late cretaceous times and that the jinggu mengla area 200 400 km farther south has been rotated by 46 65 clockwise the jurassic results reveal the same rotation pattern at face value the differences between the observed cretaceous paleolatitudes and those predicted from the eurasian apparent polar wander path apwp indicate a southward extrusion of the region but the amount of extrusion is not large enough to be resolved by paleomagnetic methods comparison with mesozoic paleomagnetic data from central yunnan northern thailand and northwestern borneo suggests that the red river fault is not a demarcation between unrotated and significantly rotated regions and that sundaland did not respond to the india asia collision as a single coherent unit the existing paleomagnetic data from east asia appear to indicate that the crust in central tibet and tien shan north of tarim directly in front of the indian indentor has been substantially shortened and thickened whereas the crust in the syntaxial areas has probably been largely squeezed out to further evaluate the relative importance of crustal thickening and extrusion in the tectonic evolution of the tibetan plateau paleomagnetic investigation should be intensified in the bounding zones to the indian indentor and be expanded into the cenozoic sedimentary rocks in tibet and surrounding regions

cretaceous tertiary geology of the gangdese arc in the linzhou area southern tibet abstract knowledge of the cretaceous tertiary history of upper crustal shortening and magmatism in tibet is fundamental to placing constraints on when and how the tibetan plateau formed in the lhasa terrane of southern tibet the widely exposed angular unconformity beneath uppermost cretaceous lower tertiary volcanic bearing strata of the linzizong formation provides an excellent geologic and time marker to distinguish between deformation that occurred before vs during the indo asian collision in the linzhou area located 30 km north of the city of lhasa a 3 km thick section of the linzizong formation lies unconformably on cretaceous and older rocks that were shortened by both northward and southward verging structures during the late cretaceous the linzizong formation dips northward in the footwall of a north dipping thrust system that involves triassic jurassic strata and a granite intrusion in the hanging wall u pb zircon geochronologic studies show that the linzizong formation ranges in age from 69 ma to at least 47 ma and that the hanging wall granite intrusion crystallized at 52 ma coeval with dike emplacement into footwall cretaceous strata 40ar 39ar thermochronologic studies suggest slow cooling of the granite between 49 and 42 ma followed by an episode of accelerated cooling to upper crustal levels beginning at 42 ma the onset of rapid cooling was coeval with the cessation of voluminous arc magmatism in southern tibet and is interpreted be a consequence of either 1 tertiary thrusting in this region or 2 regional rock uplift and erosion following removal of overthickened gangdese arc lower crust and upper mantle or break off of the neo tethyan oceanic slab

cenozoic deformation in western yunnan china myanmar border in the present paper we describe ductile and brittle deformation styles in western yunnan and ne myanmar using field data and landsat 7 imagery we show that this complex area located at the northern termination of sunda plate three rivers area was wedged during the tertiary between the left lateral ailao shan chong shan metamorphic belts to the east and the right lateral shan scarp gaoligong metamorphic belt in west this triangular region therefore underwent the effects of these continental size ductile strike slip faults separating major blocks with a dominant ew to ene compression since the late miocene date of the reversal of motion along the rrf the incipient eastward motion of the sunda block and the persisting right lateral motion along its western boundary sagaing fault created n s compression and e w to wnw extension underlined by left lateral transtension along the wanding nanting fault zones at the same time the diangcan shan situated along strike the ailao shan metamorphic belt was slightly impinged by the blocks extruded from the syntaxis and exhumed again from the early pliocene in accordance with this late and still active state of stress q 2004 elsevier ltd all rights reserved

conjugate strike slip faulting along the bangong nujiang suture zone accommodates coeval east west extension and north south shortening in the interior of the tibetan plateau this is the published version copyright 2002 american geophysical union all rights reserved

paleomagnetism of late jurassic to early cretaceous red beds from the cardamom mountains southwestern cambodia tectonic deformation of the indochina peninsula abstract late jurassic to early cretaceous red beds of the phuquoc formation were sampled at 33 sites from the sihanoukville and koah kong areas of the phuquoc kampot som basin southwestern cambodia two high temperature remanent components with unblocking temperature ranging 650 670 c and 670 690 c were identified the magnetization direction for the former component d 5 2 i 18 5 with 95 3 1 in situ reveals a negative fold test that indicates a post folding secondary nature however the latter component carried by specular hematite is recognized as a primary remanent magnetization a tilt corrected mean direction of d 43 4 i 31 9 95 3 6 was calculated for the primary component at 11 sites corresponding to a paleopole of 47 7 n 178 9 e a 95 3 6 when compared with the 130 ma east asian pole a southward displacement of 6 0 3 5 and a clockwise rotation of 33 1 4 0 of the phuquoc kampot som basin as a part of the indochina block with respect to east asia were estimated this estimate of the clockwise rotation is 15 larger than that of the khorat basin which we attribute to dextral motion along the wang chao fault since the mid oligocene the comparison of the herein estimated clockwise rotation with the counter clockwise rotation reported from the da lat area in vietnam suggests the occurrence of a differential tectonic rotation in the southern tip of the indochina block during the southward displacement of the indochina block the non rigid lithosphere under its southern tip moved heterogeneously while the rigid lithosphere under the khorat basin moved homogeneously

permian tectonic framework and palaeogeography of se asia abstract the principal continental lithospheric terranes now comprising se asia and that existed in the permian are the south china indochina super terrane and the sibumasu simao west burma and sw borneo semitau terranes the simao terrane is here regarded a separate terrane derived from south china by back arc spreading in the lower carboniferous rather than as an extension of indochina the main palaeo tethys ocean in thailand is identified as being represented by the chiang mai and sra kaeo sutures and the eastern boundary of the sibumasu terrane is thus interpreted to lie farther west in thailand than previously inferred the nan uttaradit suture is interpreted as representing a marginal back arc basin and contiguous with the ailaoshan and jinshajiang sutures in sw china the south china indochina super terrane and the simao terrane were located within the palaeo tethys in equatorial latitudes during the permian where they together with north china exhibit cathaysian floras and faunas the sibumasu terrane was located on the nw australian margin of gondwana until the sakmarian major shifts in faunal affinities from gondwana province faunas to an independent sibumasu province and then to assimilation into the cathaysian province records the separation and rapid northward drift of sibumasu after the late sakmarian the principal permian palaeogeographic models for the region are assessed for internal consistencies and conflicts

paleomagnetic results from the upper carboniferous of the shan thai malay block of western yunnan china the upper carboniferous basaltic woniusi formation was sampled at 21 sites from two limbs of a syncline near baoshan city 25 2 n 99 3 e and from a monocline near yongde county 23 9 n 99 2 e for paleomagnetic study the study area is generally thought to be part of the shan thai malay microplate thermal demagnetization reveals a single polarity characteristic remanent magnetization chrm which is carried by both magnetite and hematite with the former being the dominant mineral in basalt and the latter being the principal carrier in associated tuffaceous basalt and sandstone the tilt corrected formation mean from baoshan d 209 2 i 61 0 95 6 7 confirms the results reported by previous workers and a positive fold test constrains the chrm acquisition to be prefolding the paleomagnetic inclinations determined from this study are similar to that derived from the devonian rocks from the same area by other workers indicating a paleolatitude of about 42 for the baoshan area during the devono carboniferous time the paleomagnetic declinations however present a large difference between the two sampling localities for the upper carboniferous as well as between the sampling sites for the devonian and for the upper carboniferous the paleo declination difference can be partly explained by the apparent polar wander of gondwana if the shan thai malay microplate was rigidly attached to gondwanaland during this time interval the large part of it has to be ascribed to tectonic rotations between the sampling sites this fact suggests that extreme caution should be exerted when trying to establish an apparent polar wander path for and to reconstruct the paleo geographic position of the shan thai malay microplate

intraplate deformation of the indian subcontinent 1 while deformation at the earth s surface primarily occurs along tectonic plate boundaries major earthquakes have shaken regions deep within continental interiors three of the largest m 7 5 historic intraplate earthquakes occurred within the indian subcontinent suggesting the possibility of significant intraplate deformation we consider surface velocities determined from new gps data collected at 29 continuous gps stations and 41 survey mode gps stations in india between 1995 and 2007 to find a north south shortening rate of 0 3 0 05 nanostrain yr 1 which may be accommodated by 2 1 mm yr of more localized convergence across central india southward motions at 4 7 mm yr of sites on the shillong plateau in northeast india reflect rapid shortening and high earthquake hazard associated with active thrust faults bounding the plateau the width and magnitude of the elastic strain accumulation field across the himalaya varies little from 76 90 longitude but the strain is more broadly distributed and convergence rates are higher along the eastern 200 km of the range

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

testing the influence of high voltage mineral liberation on grain size shape and yield and on fission track and 40ar 39ar dating mineral liberation and separation are fundamental for modern thermochronology the fragmentation of rocks by high voltage electrical pulses called electrical fragmentation is a new technology only available since the development of a lab sized machine a few years ago the proposed advantages e g disintegration along grain boundaries compete with suspected negative effects of the locally occurring high temperatures up to 104 k on the physical properties of the liberated minerals comparison of electrical fragmentation with conventional mechanical fragmentation jaw crusher revealed similar amounts of liberated apatite and zircon from the same quantity of granite but 45 production of fines 80 m for mechanical fragmentation versus 5 for electrical fragmentation electrical fragmentation yielded larger sized apatite grains 180 250 m compared to 80 125 m for mechanical liberation but no differences in shape factors elongation roundness compactness the liberation of idiomorphic crystals from coarse grained rocks is ameliorated by the new method as shown by the liberation of mm sized idiomorphic biotite crystals from granite degassing curves of 40ar from electrically and mechanically liberated biotites from the same sample are nearly identical maximum difference six percentage points 40ar 39ar ages of biotites from three samples two mechanically one electrically fragmented from the same intrusion are identical within 2 error length measurements of apatite fission tracks gave identical results within 2 error regardless of fragmentation method used in summary the positive aspects of electrical fragmentation have been confirmed in this study while effects negative for thermochronology are not observed

new constraints to the onset of the india asia collision paleomagnetic reconnaissance on the linzizong group in the lhasa block china abstract to better understand the onset of the india asia collision a paleomagnetic study has been conducted on paleogene volcanic rocks and sediments of the linzizong group 64 44 ma from the linzhou and namling basins of the lhasa block at the southern margin of the asian continent following detailed rock magnetic investigations and progressive thermal or hybrid thermal and alternating field demagnetization stable characteristic remanent magnetizations chrms were successfully defined from the dianzhong 64 60 ma nianbo and pana 60 44 ma formations these chrms are of dual polarity and pass a fold test with some of them in volcanic rocks carried by oxidation induced hematite we thus interpret them as primary remanences paleomagnetic results from the bulk of the collection imply that the southern leading edge of asia i e the lhasa block remained essentially stationary in low latitudes at 10 n without paleomagnetically detectable tectonic rotation and latitudinal variation during the formation of the linzizong group consequently a large scale post collisional northward convergence in the tibetan plateau and central asia is very likely to have occurred after formation of the linzizong group and the definitive paleoposition of the lhasa block during the period embracing the interval between 64 and 44 ma provides further constraints on the latitudinal position of the initial contact between india and asia this evidence rules out the model for a younger 35 ma india asia collision and implies that the initial india asia collision occurred between 65 and 50 ma given that greater india comprised the present indian continent together with a 950 km extension beyond its northern margin prior to the rifting of gondwana in the middle jurassic we conclude that the initial india asia collision most probably occurred between 55 and 60 ma

tertiary paleomagnetic data from northwestern yunnan china further evidence for large clockwise rotation of the indochina block and its tectonic implications abstract eocene red sandstones were collected at 15 sites for paleomagnetic study from two areas lanping and yunlong of the western part of yunnan province china in the northern part of indochina the high temperature magnetization component with an unblocking temperature of around 680 c is isolated after stepwise thermal demagnetization characteristic directions from nine accepted sites reveal a positive fold test at the 99 confidence limit indicating that the high temperature magnetization component is possibly of eocene age a tilt corrected mean direction from nine accepted sites is d 266 1 i 39 8 with 95 11 2 corresponding to a paleopole at 14 5 n 169 7 e with a95 10 9 a comparison with previously reported tertiary paleomagnetic data indicates that the lanping basin of the northern part of the indochina block rotated clockwise through 91 15 with respect to the yangtze craton this large magnitude clockwise rotation is probably caused by the superimposition of regional and local rotations our previous middle cretaceous paleomagnetic results from the yunlong area indicate that about 40 of clockwise rotation out of 90 is explained by tectonic rotation due to a relatively rigid body rotation of the simao terrane during indentation of india the remaining component of rotation is probably ascribed to further eocene post eocene rotational motion of the tectonically independent small blocks within the lanping basin associated with fault movement

late cenozoic evolution of the central longmen shan eastern tibet insight from u th he thermochronometry 1 this article presents u th he thermochronological data from the longmen shan belt eastern tibet located between the songpan garze terrane and the yangtze craton this mountain range is one of the steepest margins of the tibetan plateau and an important area for the comprehension of the mechanisms that control the dynamics of such plateau borders in terms of spatial distribution of deformation or timing of topographic building we describe several age elevation transects and perform forward modeling of our data to derive quantitative information on the exhumation of the range a major phase of exhumation started at 8 11 ma with an average rate of 0 65 mm a a1 comparison of zircon and apatite ages indicates that the eastern part of the range may have experienced a significant decrease in exhumation since 2 3 ma we use the distribution of finite exhumation across the major faults of the area to quantify their dip slip throw rate over the last 10 ma the beichuan fault which was activated during the 2008 sichuan earthquake is the major active structure of the longmen shan since the late miocene with an average thrusting slip rate between 0 4 and 1 mm a a1 conversely over the same time period only minor dip slip activity occurred on the wenchuan fault zone this distribution in space and time of exhumation and deformation is discussed and compared to the different proposed models for the geodynamical evolution of the eastern tibetan margin it also provides an important long term perspective to put in context the destructive 2008 sichuan earthquake that struck the central longmen shan

tectonics and paleomagnetic rotation pattern of yunnan 24 n 25 n china gaoligong fault shear versus megablock drift

formation of the maturin foreland basin eastern venezuela thrust sheet loading or subduction dynamic topography 1 the maturin basin in eastern venezuela is considered a good example of a peripheral foreland basin earthquake and tomographic data indicate that eastern venezuela is affected by the oblique subduction of the south american plate underneath the caribbean plate new forward flexural isostatic modeling of eastern venezuela has been carried out in order to determine whether the maturin basin was generated purely by thrust sheet loading from the serrania and monagas foreland thrust belts a sequence of forward models from middle miocene to present was generated for 3 profiles across the serrania del interior thrust belt the monagas foreland thrust belt and the maturin foreland basin the predictions of these models are constrained using seismic reflection and well data the flexural isostatic modeling shows that thrust sheet loading associated with the serrania del interior and monagas foreland thrust belts is insufficient to generate the observed subsidence within the maturin basin dynamic fluid flow modeling of subduction related dynamic topography of eastern venezuela has been used to investigate the influence of south american plate subduction on the generation of the accommodation space observed in the maturin basin fluid flow modeling of subduction related dynamic topography suggests that the subduction of the south american lithospheric mantle caused downward deflection of the south american crust affecting the maturin basin and the serrania thrust belt this modeling suggests that the maturin basin subsidence has two components 55 related to thrust sheet loading and 45 driven by continental subduction

relative motion across the eastern tibetan plateau contributions from faulting internal strain and rotation rates abstract a kinematic model comprising 14 rotating elastic plastic blocks is used to represent the modern deformation of eastern tibet and neighboring regions block rotations fault slip rates and permanent strain rates within the blocks are constrained by inverting gps velocities slip vector azimuths derived from earthquakes and fault slip rates derived from geology the calculated internal strain rates of blocks in eastern tibet amounts to 10 to 30 10 9 yr in contrast to relatively low rates

geomagnetic secular variation and the statistics of palaeomagnetic directions in this study we examine the role of palaeosecular variation psv in the use of statistics for palaeomagnetic studies and we provide new reliability criteria for palaeomagnetic poles or directions we first conclude that fisher statistics should not be applied to average palaeomagnetic directions but to virtual geomagnetic pole vgp distributions instead secondly we strongly advocate that typical properties of geomagnetic field behaviour are taken into account in the assessment of palaeomagnetic data sets the latitude dependent properties e s k provide useful guidelines for the reliability of a palaeomagnetic data set a reliable assessment of these properties depends on the sufficient number of palaeomagnetic samples being taken therefore as an additional instrument of assessing data sets we provide a n dependent a95 envelope bounded by an upper limit a95max and a lower limit a95min that helps to ascertain whether or not a distribution has sufficiently well sampled psv and therefore geomagnetic field behaviour applying these criteria is indispensable for studies of geomagnetic behaviour or for studies aiming at using tk03 gad for inclination error correction through the elongation inclination e i method for palaeomagnetic studies aimed at geological reconstructions they form helpful guidelines and increase the confidence in the rocks having faithfully recorded the field an analysis of published eastern mediterranean data shows that the vast majority of studies do not conform to the van der voo criteria in particular with respect to n and a95 we have provided criteria that are on the one hand more lenient lower n may still provide relevant information and on the other hand more strict for high n the criterion of a95 16 should be adapted to a requirement of lower a95 e g a95 80

assembly of the pamirs age and origin of magmatic belts from the southern tien shan to the southern pamirs and their relation to tibet 1 magmatic rocks and depositional setting of associated volcaniclastic strata along a north south traverse spanning the southern tien shan and eastern pamirs of kyrgyzstan and tajikistan constrain the tectonics of the pamirs and tibet the northern pamirs and northwestern tibet contain the north facing kunlun suture the south facing jinsha suture and the intervening carboniferous to triassic karakul mazar subduction accretion system the latter is correlated with the songpan garze hoh xi system of tibet the kunlun arc is a composite early paleozoic to late paleozoic triassic arc arc formation in the pamirs is characterized by 370 320 ma volcanism that probably continued until the triassic the cryptic tanymas suture of the southern northern pamirs is part of the jinsha suture a massive 227 ma batholith stitches the karakul mazar complex in the pamirs there are striking similarities between the qiangtang block in the pamirs and tibet like tibet the regional structure of the pamirs is an anticlinorium that includes the muskol and sares domes like tibet the metamorphic rocks in these domes are equivalents to the karakul mazar songpan garze system granitoids intruding the qiangtang block yield 200 230 ma ages in the pamirs and in central tibet the stratigraphy of the eastern pshart area in the pamirs is similar to the bangong nujiang suture zone in the amdo region of eastern central tibet but a triassic ocean basin sequence is preserved in the pamirs arc type granitoids that intruded into the eastern pshart oceanic basin arc sequence 190 160 ma and granitoids that cut the southern qiangtang block 170 160 ma constitute the rushan pshart arc cretaceous plutons that intruded the central and southern pamirs record a long lasting magmatic history their zircons and those from late miocene xenoliths show that the most distinct magmatic events were cambro ordovician 410 575 ma triassic 210 250 ma likely due to subduction along the jinsha suture middle jurassic 147 195 ma subduction along rushan pshart suture and mainly cretaceous middle and late cretaceous magmatism may reflect arc activity in asia prior to the accretion of the karakoram block and flat slab subduction along the shyok suture north of the kohistan ladakh arc respectively before india and asia collided the pamir region from the indus yarlung to the jinsha suture was an andean style plate margin our analysis suggests a relatively simple crustal structure for the pamirs and tibet from the kunlun arc in the north to the southern qiangtang block in the south the pamirs and tibet likely have a dominantly sedimentary crust characterized by karakul mazar songpan garze accretionary wedge rocks the crust south of the southern qiangtang block is likely of granodioritic composition reflecting long lived subduction arc formation and cretaceous cenozoic underthrusting

zircon u pb ages in myanmar magmatic metamorphic events and the closure of a neo tethys ocean abstract previous isotopic age studies have concluded that the main metamorphism in the mogok metamorphic belt mmb in myanmar was tertiary and caused by collision with or underthrusting by india we present here results of zircon u pb age determinations on 18 samples from myanmar largely on intrusive rocks from the north trending 550 km long shan scarps segment of the sickle shaped mmb the oldest determination is 491 ma on orthogneiss north of mandalay interpreted as a protolith age and similar to the stratigraphic age of the bawdwin volcanics augen gneiss at kyaukse has a 114 ma protolith age our zircon u pb ages on undeformed intrusive rocks in the mmb range from cretaceous to miocene the oldest 128 ma on a diorite near yebokson implies an earliest cretaceous minimum age for regional metamorphism here younger ages on undeformed intrusions include 91 ma on diorite at mokpalin 72 ma on the weakly foliated garnet bearing nattaung granite 44 and 48 ma respectively on the mec and sedo granites and 17 20 ma on granite dykes for the 128 ma diorite sr and nd isotopes indicative of a continental crust component and mapped continuity of metamorphic host rocks suggest correlation with mid early cretaceous plutons in the eastern transhimalayas zircon u pb ages for the mec sedo and other granites within the mmb and slate belt help to define a late cretaceous to early eocene age range for mostly ilmenite series peraluminous granites within the western granite belt and more restricted western tin belt of myanmar and southern thailand in the wuntho popa arc west of the dextral sagaing fault our new u pb zircon age of 105 ma on diorite at salingyi supports previous k ar ages 106 93 ma for pre tertiary intrusions in the cretaceous this arc may have occupied the andaman sea and continued northwards through the mokpalin diorites and west of the mmb to the 103 80 ma i type plutons in the gandise batholith of southern tibet we interpret the mmb and slate belt as part of a western myanmar block separated in the jurassic from the shan plateau to the east by the southwestward continuation of the bangong nujiang luxi neo tethys i ocean we relate most intrusive and metamorphic events in and near the mmb to westward subduction of this ocean and end jurassic collision of the plateau with the overriding western myanmar slate belt block to orogenic polarity reversal and early cretaceous westward translation of the plateau sequence over the suture zone and to eastward subduction of the ancestral indian ocean or neo tethys ii beneath myanmar with generation of the wuntho popa arc beginning before the late cretaceous the mmb includes meta plateau rocks near mandalay but meta slate belt rocks to the north and south within the mmb previous monazite xenotime thorite ages of eocene oligocene and oligocene early miocene cooling ages on micas with no deformation in eocene granites suggest a high temperature thermal event related to intrusion of these and other granites during a previously proposed regional extension our data imply that the main fabric forming metamorphic event in the mmb pre dated the india asia collision

position of the lhasa terrane prior to india asia collision derived from palaeomagnetic inclinations of 53 ma old dykes of the linzhou basin constraints on the age of collision and post collisional shortening within the tibetan plateau the lhasa terrane comprises the southern eurasian margin since at least mid cretaceous therefore paleomagnetic data from the lhasa terrane are crucial to reconstruct the pre collisional southern margin of eurasia previous paleomagnetic data from the cretaceous takena formation provide a rough constraint on the pre collisional paleolatitudinal position of the southern eurasian margin however paleogene paleomagnetic data from the lhasa terrane which can provide more direct constraints are very limited in addition reliability of the existing data is ambiguous because of a possible later overprint in our new study 17 sites were drilled from mafic dykes intruded in the t1 unit of the linzizong formation of the linzhou basin near lhasa for paleomagnetic studies figure 1 the dykes were ar 40 ar 39 dated to c 53 ma by yue and ding 2006 from ten sites a higher coercivity component demagnetized between 20 and 100 mt could be isolated detailed rockmagnetic analyses reveal ti rich titanomagnetite as the remanence carrier which suggests that the rock is not much altered and the remanent magnetization is likely of primary origin this is supported by a positive fold test the occurrence of normal and reverse polarity directions within single sites the subvolcanic character of the dykes and the within site scatter as well as the scatter between site mean directions indicate that paleosecular variation is recorded within sites tilt correction was performed using the bedding of overlying fluvial lacustrine sediments and tilt angles of ignimbrite columns postdating the dyke emplacement bedding corrected directions give an overall mean direction of d i 12 3 27 2 95 10 6 k 21 7 n 10 corresponding to a paleolatitude of 13 2 5 8 n at a reference location on the suture zone 20 n 90 e comparison with previous cretaceous data mainly from the takena formation yields a stable position of the lhasa terrane during cretaceous and early eocene this indicates that no large north south crustal shortening occurred along the southern eurasian margin between late cretaceous and early eocene the difference between expected paleolatitudes determined from the apwp of eurasia and observed paleolatitudes reveals relative northward movement of the lhasa block of ca 1847 763 km since early eocene this is attributed to indentation of india into asia and implies a considerable amount of north south crustal shortening two models of greater india were used to determine the extended northern margin of india ali and aitchison 2005 proposed a greater india based on bathymetric features in the eastern indian ocean and patzelt and others 1996 determined the paleolatitude of northern india using paleomagnetic investigations on paleocene sediments of the zongpu formation at gamba and duela intersection of the pre collisional southern eurasian margin and the northern margin of india yields a collision age of the indian and eurasian continents of c 5349 ma an initial collision within this time range is in accordance with previous studies based on paleomagnetic or geologic data e g patriat and achache 1984 klootwijk and others 1992 guillot and others 2003 a significantly later collision age at the eocene oligocene boundary as proposed by aitchison and others 2007 is not in agreement with our new and previous paleomagnetic data from the lhasa block