palaeomagnetic results from palaeocene basalts from mongolia reveal no inclination shallowing at 60 ma in central asia summary we present the results of a palaeomagnetic study of 277 cores drilled at 35 sites in 32 basaltic flows from three early palaeocene volcanic regions in the gobi desert mongolia at sumber uul 62 2 ma 42 6 n 104 0 e tulga 62 0 ma 43 2 n 104 1 e and khuts uul 57 1 ma 43 2 n 104 6 e localities samples from sumber uul 62 2 0 9 ma and khuts uul 57 1 0 8 ma localities were dated using the k ar cassignol gillot technique stepwise thermal and alternating field demagnetizations isolated a stable a component of magnetization carried by single domain sd to nearly sd magnetite we interpret this a component to be the primary magnetization of these basaltic lava flows the sumber uul and tulga data were combined and recomputed at the sumber uul locality because of their similar ages the palaeopoles computed from the a components lie at 85 2 n 92 5 e dp dm 3 9 4 9 n 14 flows for sumber uul tulga average age 62 1 5 9 ma and 69 6 n 148 0 e dp dm 6 3 7 3 n 14 flows for khuts uul average age 57 1 0 8 ma the palaeomagnetic inclinations are steeper than expected at the sites and consequently our palaeopoles occupy a near sided position with respect to the 60 ma reference apparent polar wander path apwp pole for europe besse courtillot 2002 however they appear to fully conform to the new high resolution apwp poles for the 65 42 ma period of moreau et al 2007 following these authors we interpret this anomalous near sided position of our poles as arising from a rapid true polar wander tpw event in the palaeocene highlighted by a cusp at anomalies 26 25 61 56 ma inexistent in the european apwp of besse courtillot 2002 we conclude that our new data do not reveal any anomalous shallow inclinations in the central asia palaeocene effusive rocks which is consistent with the late jurassic early cretaceous age of mongol okhotsk ocean closure and amalgamation of amuria and siberia forming a rigid entity since then

magnetic remanence and fabric properties of laboratory deposited hematite bearing red sandstone redeposition experiments of disintegrated hematite bearing sandstone have confirmed a tangent relationship between inclinations of the drm and ambient magnetic field pronounced variations of drm intensities with field inclination are attributed to processes acting during the depositional formation of laminated sediments coinciding azimuthal distributions of remanence and maximum susceptibility directions reflect intrinsic properties of flaky hematite grains in which the magnetic moment is parallel to the direction of maximum susceptibility this parallelism probably depends on magnetic domain configurations

paleomagnetism of early paleogene marine sediments in southern tibet china implications to onset of the india asia collision and size of greater india article i nfo we report a paleomagnetic study of paleocene marine sediments in the gamba area of the tethyan himalayan terrane southern tibet which aims to accurately locate the position of the northern margin of greater india and further constrain timing of initial contact between india and asia following detailed rock magnetic and paleomagnetic experiments on a total of 675 drill core samples collected from the zongpu and upper jidula formations characteristic remanent magnetizations chrms were successfully isolated from most samples following alternating field af or integrated thermal and af demagnetization the chrms are of dual polarity and pass fold and reversal tests indicating a pre folding origin together with detailed biostratigraphic investigation of the sampling section a magnetic polarity sequence is constructed from data at 167 sampled horizons and satisfactorily correlates with polarity chrons c24r to c27r dating the zongpu formation between 56 2 and 61 8 ma on the geomagnetic polarity time scale the chrm directions from the zongpu formation are grouped stratigraphically into 33 sites and yield two paleopoles of 71 6 n 277 8 e a95 2 5 and 67 3 n 266 3 e a95 3 5 for the time intervals comprising 56 to 59 ma and 59 to 62 ma respectively these new paleopoles imply that the tethyan himalayan terrane was sited at low latitudes in the northern hemisphere during the interval 62 56 ma suggestingthatinitial contact between thetethyanhimalayaand lhasa terranes was established before60 5 1 5 ma and verylikelyoccurred nearthecretaceous tertiary boundary at least in the central partof the suturezone the results alsoindicate that atleast 1500 480 kmof post collisionalcrustal shortening occurred within the himalayas from the analysis of available paleomagnetic data obtained on both sides of the suture zone we propose a conceptional collision model for the india asia collision

new paleomagnetic results of paleocene volcanic rocks from the lhasa block tectonic implications for the collision of india and asia abstract a combined geochronological and paleomagnetic investigation has been performed on paleocene volcanic sequences in the lhasa block near the localities of mendui 30 1 n 90 9 e a total of 15 sites have been sampled from rhyolitic tuffs stepwise thermal demagnetizations successfully isolated high unblocking temperature characteristic directions the tilt corrected mean direction is d i 359 0 26 1 with 95 9 2 and n 14 sites corresponding to a paleopole at 73 6 n 274 3 e with a 95 7 3 positive fold tests suggest a primary origin for the characteristic remanence in order to provide a more accurate pole we propose to combine site mean directions from this study and achache et al s 1984 the combined average palaeomagnetic direction from early paleocene volcanic rocks is d 356 6 i 25 9 21 7 95 6 8 after tilt correction n 22 sites corresponding to a pole at 73 2 n 282 4 e with a 95 5 4 the paleomagnetic results yield a paleolatitude of 13 6 5 4 n for the southern margin of eurasia at 55 ma compared with expected paleomagnetic directions from the stable india and eurasia blocks significant crustal shortening of 1400 600 km and 2000 550 km respectively may have occurred between the southern margin of eurasia and the stable india and within eurasia since the collision of india and eurasia

middle late miocene 10 ma formation of the main boundary thrust in the western himalaya three independent data sets from northwestern india and pakistan suggest initial displacement along 1000 km of the main boundary thrust prior to 10 ma at least 5 m y earlier than previously reported regionally extensive changes in the depositional characteristics and rates of the foreland basinfill between 11 and 9 5 ma are interpreted to reflect new hinterland loading due to the formation of the main boundary thrust sediment accumulation rates sandstone siltstone ratios and thickness and amalgamation of individual sandstone bodies all substantially increase after 11 ma in well dated stratigraphic sections from pakistan to nepal across the indo gangetic foreland basin in the himachal pradesh reentrant of northwestern india a newly discovered 8 7 ma conglomerate derived from the hanging wall of the main boundary thrust indicates that source area uplift and denudation must have occurred prior to 9 ma and probably prior to 10 ma assuming a gravelprogradationrateof3cm yr threeapatitefission trackages fromstructuresattheleadingedgeofthemainboundarythrustin the kohat region of northwest pakistan indicate that rapid cooling below 105 c1 2 m y earlier these data indicate that the main boundary thrust in the western himalaya formed synchronously along strike in the middle late miocene has a displacement rate of 10 mm yr and has a displacement history that is coeval with late displacement on the main central thrust

palaeomagnetism and 40ar 39ar geochronology of upper palaeogene volcanic rocks from central tibet implications for the central asia inclination anomaly the palaeolatitude of tibet and post 50 ma shortening within asia summary palaeomagnetic results from upper palaeogene volcanics from three localities in the qiangtang terrane of central tibet record a maximum likelihood palaeolatitude of 28 7 3 7 n these are the first volcanic based palaeomagnetic results from the plateau interior that appear to average secular variation well enough to yield a reliable palaeolatitude estimate this palaeolatitude estimate is significantly lower than the 36 3 n predicted for central tibet by late palaeogene eurasia reference poles consistent with the so called central asia inclination anomaly but it is consistent with values predicted from upper palaeogene reference poles for mongolia the mean inclination of coeval sedimentary rocks from central tibet is 15 5 shallower than our volcanic results similar to the magnitude of sedimentary inclination shallowing estimated from elongation inclination analysis of remanent directions in clastic rocks from the same region we conclude that most of the inclination anomaly observed in volcanic rocks and that which remains after removal of sedimentary flattening in clastic rocks probably is due to the combined effect of reference poles that are not representative of stable asia and by cenozoic intracontinental shortening within central asia on the order of several hundred not more than 1000 kilometers our compilation of palaeomagnetic data from the qiangtang terrane further suggest that the southern margin of the lhasa terrane the southern margin of pre india asia was located as far south as 20 n throughout eocene time these palaeolatitudes in conjunction with the upper crustal shortening record for asia indicate that only approximately one third of the post 50 ma india asia convergence is partitioned in asian lithosphere low latitudes also imply that much of the proto tibetan plateau occupied southerly tropical to subtropical latitudes which has important implications for the weathering and erosion of the tibetan plateau palaeoaltimetry proxies and early cenozoic climate dynamics

tibetan tectonic evolution inferred from spatial and temporal variations in post collisional magmatism cenozoic magmatism on the tibetan plateau shows systematic variations in space and time that must be considered in models concerning tibetan tectonic evolution after the india asia collision which started in the early tertiary and terminated the gangdese arc magmatism in the lhasa terrane made of the southern tibetan plateau widespread potassium rich lavas and subordinate sodium rich basalts were generated from f50 to 30 ma in the qiangtang terrane of northern tibet subsequent post collisional magmatism migrated southwards producing ultrapotassic and adakitic lavas coevally between f26 and 10 ma in the lhasa terrane then potassic volcanism was renewed to the north and has become extensive and semicontinuous since f13 ma in the western qiangtang and songpan ganze terranes such spatial temporal variations enable us to elaborate a geodynamic evolution model which depicts when and how the indian continental lithospheric mantle started thrusting under asia by involving rollback and breakoff of the subducted neo tethyan slab followed by removal of the thickened lhasa lithospheric root we propose that only after the lithospheric removal which occurred at f26 ma could the indian mantle lithosphere have commenced its northward underthrusting and henceforth served as a pivotal control to the himalayan tibetan orogenesis consequently the tibetan plateau is suggested to have risen diachronously from south to north whereas the southern part of the plateau may have been created and maintained since the late oligocene the northern plateau would have not attained its present day elevation and size until the mid miocene when the lower part of the western qiangtang and songpan ganze lithospheres began to founder owing to the push of the underthrust indian mantle lithosphere d 2004 elsevier b v all rights reserved

new insights into the india asia collision process from cretaceous paleomagnetic and geochronologic results in the lhasa terrane abstract to further constrain the india asia collisional process a combined paleomagnetic and geochronologic study has been carried out on the upper cretaceous jingzhushan formation redbeds and the lower cretaceous dianzhong formation volcanic rocks dated at 121 117 ma from the cuoqin area in the central lhasa terrane stepwise thermal demagnetization successfully isolated reliable characteristic remanent magnetization chrm directions that include dual polarity and pass positive fold tests at 95 and 99 confidence levels indicating prefolding primary magnetizations the tilt corrected site mean direction for 33 redbed sites is d 316 8 i 30 2 with 95 5 4 corresponding to a paleopole at 49 0 n 344 3 e with a 95 5 3 and the other for 12 volcanic sites is d 350 5 i 25 5 with 95 7 7 corresponding to a paleopole at 70 5 n 292 9 e with a 95 7 4 our new paleomagnetic results together with reliable cretaceous paleomagnetic data obtained from the lhasa terrane demonstrate that the southern margin of asia was located at 15 1 n during the cretaceous comparison with the apparent polar wander paths apwp of india and the cretaceous paleocene paleopoles of the himalayan terrane suggests that the india asia collision was likely a complex process which consists of the collision of the lhasa and himalayan terranes at 54 0 2 1 ma the longtime subduction of an intra oceanic basin between the himalayan terrane and the indian craton from 54 0 to 40 4 ma and the collision of the himalayan terrane and the indian craton at 40 4 4 1 ma comparing with the late cretaceous average pole of the east asia apwp reveals that a latitudinal convergence of 780 240 km has taken place between the lhasa terrane and east asia the hexi corridor since the india asia collision the amount of latitudinal shortening deduced from paleomagnetic data is very consistent with the 600 1000 km accommodated by the cenozoic fold and thrust belts between the lhasa terrane and hexi corridor

palaeomagnetism of cretaceous to tertiary sediments from southern tibet evidence for the extent of the northern margin of india prior to the collision with eurasia abstract palaeomagnetic studies have been carried out on mid cretaceous to latest palaeocene sediments from the tethyan himalaya on the northern margin of the indian plate near the localities of gamba 28 3 n 88 5 e and duela 28 0 n 89 2 e in southern tibet a total of 61 sites more than 700 cores have been sampled mainly from limestones zongshan formation jidula formation member ii zongpu formation sandstones jidula formation member iii and graywackes gamba group member ii with ages well defined from detailed lithological and biostratigraphic analyses carriers of characteristic remanences are magnetite in limestones and graywackes and hematite in sandstones reliable palaeodirections were obtained from 35 sites 356 specimens positive fold tests antiparallel normal and reverse polarities a northward trend of the palaeolatitudes with decreasing age and absence of any metamorphic overprint indicate a primary origin for the characteristic remanences the results have been compared with expected palaeodirections according to available data for the indian subcontinent based on its present day geometry the late maastrichtian sites fourteen sites 156 specimens and the middle late palaeocene sites fourteen sites 113 specimens yield palaeolatitudes of 5 7 s and 4 0 n for the sampling localities in southern tibet respectively these palaeolatitudes are much further north than expected and imply that crustal shortening between the tethyan himalaya and the indian shield since the time of india eurasia collision has occurred on the order of 1500 km at the longitude of gamba duela comparison of our observed palaeolatitudes with palaeolatitudes for the lhasa block during cretaceous and palaeocene times indicate that initial contact of the northern indian margin with the lhasa block was already established at 65 60 ma with suturing completed at about 55 50 ma

reconstructing greater india paleogeographic kinematic and geodynamic perspectives abstract key in understanding the geodynamics governing subduction and orogeny is reconstructing the paleogeography of greater india the indian plate lithosphere that subducted since tethyan himalayan continental collision with asia here we discuss this reconstruction from paleogeographic kinematic and geodynamic perspectives and isolate the evolution scenario that is consistent with all three we follow recent constraints advocating a 58 ma initial collision and update a previous kinematic restoration of intra asian shortening with a recently proposed model that reconciles long debated large and small estimates of indochina extrusion our new reconstruction is tested against paleomagnetic data and against seismic tomographic constraints on paleo subduction zone locations the resulting restoration shows 1000 1200 km of post collisional intra asian shortening leaving a 2600 3400 km wide greater india from a paleogeographic sediment provenance perspective eocene sediments in the lesser himalaya and on undeformed india may be derived from tibet suggesting that all greater indian lithosphere was continental but may alternatively be sourced from the contemporaneous western indian orogen unrelated to india asia collision a quantitative kinematic paleomagnetic perspective prefers major cretaceous extension and a greater india basin opening within greater india but data uncertainty may speculatively allow for minimal extension finally from a geodynamic perspective assuming a fully continental greater india would require that subduction rates close to 20 cm yr was driven by a down going lithosphere crust assemblage more buoyant than the mantle which seems physically improbable we conclude that the greater india basin scenario is the only sustainable one from all three perspectives we infer that old pre collisional lithosphere rapidly entered the lower mantle sustaining high subduction rates whilst post collisional continental and young greater india basin lithosphere did not inciting the rapid india asia convergence deceleration 8 myr after collision subsequent absolute northward slab migration and overturning caused flat slab subduction tibetan shortening arc migration and arc volume decrease

persistently low asian paleolatitudes implications for the india asia collision history 1 paleomagnetism provides independent paleolatitude constraints on the india asia convergence however implied cenozoic latitudinal convergence within asia thousands of km largely exceeds geologic estimates of tectonic shortening hundreds of km this discrepancy may result from a notoriously low bias in paleomagnetically determined cenozoic paleolatitudes in asia we provide here new paleomagnetic data from cenozoic mongolian volcanic rocks and from chinese paleogene sediments corrected from the depositional bias of inclination shallowing these results combined with similar asian data sets confirm that paleolatitudes are still 5 10 lower than predicted by the paleomagnetic apparent polar wander path apwp for asia between 50 and 20 ma inclination shallowing being excluded from the selected data sets we investigate the likeliness of other proposed mechanisms for this discrepancy 1 more southerly positions of asia than expected by the apwp due to apwp inaccuracies rather than eurasian non rigidity or 2 non dipolar geomagnetic field contributions fully explaining this discrepancy by only one of these mechanisms would imply either unrealistically large 10 apwp inaccuracies or unrealistically large octupolar field contributions up to 16 a combination of these mechanisms is found more likely to have produced the observed latitudinal discrepancy but their respective contributions cannot be quantified given the still relatively low amount and poor quality of cenozoic paleomagnetic data from stable cratons of asia india and europe by allowing for reasonable time dependant non dipolar contributions and a slight 5 apwp bias the latitudinal discrepancy can be resolved and the excessive amounts of intra asian shortening decrease to values in line with tectonic shortening from structural studies

the timing of india asia collision onset facts theories controversies abstract the timing of initial collision between india and asia has remained controversial for half a century this paper attempts to review this crucial and hotly debated argument describing first the different methods used to constrain the age of collision and discussing next the rationale results inferences and problems associated with each we conclude that stratigraphy represents the best direct way to unravel collision chronology other methods focusing on the magmatic metamorphic or paleomagnetic record provide additional fundamental constraints but cannot provide a robust direct estimate of collision onset initial collision in the central eastern himalaya is dated directly at the middle paleocene 59 1 ma by the abrupt change in sediment provenance recorded in trench settings the quasi synchronous unconformities documented along both tethyan passive margin of india and active margin of asia from tibet to zanskar ladakh confirm that orogeny was underway at the close of the paleocene 56 ma well before the disappearance of marine seaways in the himalaya during the early middle eocene 50 45 ma sedimentary evolution and provenance changes in marine to fluvio deltaic successions are recorded synchronously within error from the western to the central eastern himalaya failing to provide conclusive evidence for diachronous collision these coherent observations are hard to reconcile with three widely cited hypotheses invoking either paleogene arc continent collision or late cretaceous ophiolite obduction or the protracted existence of a greater india basin which are all not favored after discussing the geological evidence critically point by point a scenario no more complex than the one involving solely the passive continental margin of india and the active continental margin of asia is needed to explain the geological evolution of the nascent himalaya the collision between the tethys himalaya and the transhimalayan arc trench system does represent the collision between india and asia because the yarlung zangbo ophiolite is the forearc basement of the asian active margin its obduction onto india could not have preceded the initial closure of neo tethys ophiolite obduction began when collision began in the middle paleocene

precollisional latitude of the northern tethyan himalaya from the paleocene redbeds and its implication for greater india and the india asia collision

new paleomagnetic results from the lhasa block revised estimation of latitudinal shortening across tibet and implications for dating the india asia collision knowing how and when the tibetan plateau formed is of global concern as high topography modifies climate circulation patterns and alters species migration paths previous paleomagnetic work suggested that intracontinental shortening from deformation related to the india asia collision played a major role in building the tibetan plateau however these studies were derived from red bed facies which are often afflicted by inclination shallowing leading to overestimates of continental shortening we report new paleomagnetic data from the lhasa block the southern continental margin of asia before the india asia collision a high temperature characteristic remanent magnetization chrm was isolated from 43 sites of late cretaceous red beds 32 sites of late cretaceous lava flows and nine sites of eocene tuffs all the chrm directions are significantly different from the present day geomagnetic field and the mean directions of red beds lava flows and tuffs pass the fold test the distribution patterns of chrm directions derived from volcanic rocks are consistent with a secular variation model suggesting that secular variation was likely averaged out in contrast the red bed chrm directions exhibit an east west elongated distribution pattern indicative of inclination flattening the mean chrm inclination of the red beds 23 5 degrees 2 5 degrees is consistent with previous results but significantly shallower than that of 41 9 degrees 4 4 degrees carried by the coeval lava flows an elongation inclination analysis of the red bed chrm directions yielded an unflattened mean inclination of 39 7 degrees with 95 confidence limits between 37 7 degrees and 41 9 degrees the paleolatitudes of the late cretaceous volcanic rocks and inclination corrected sediments suggest about 10 degrees less shortening occurred between lhasa and siberia than previous estimates with total shortening lying between 3 3 degrees 2 6 degrees and 7 3 degrees 5 4 degrees since the late cretaceous the mean chrm inclination derived from the eocene tuffs 51 8 degrees 5 2 degrees is indistinguishable from the expected reference direction our new results imply that india asia convergence was accommodated mainly through consumption of the india plate and yield a much younger age ca 43 ma for the initial india asia collision than previously estimated by paleomagnetic data c 2010 elsevier b v all rights reserved

stratigraphy structure and tectonic evolution of the himalayan fold thrust belt in western nepal regional mapping stratigraphic study and 40ar 39ar geochronology provide the basis for an incremental restoration of the himalayan fold thrust belt in western nepal tectonostratigraphic zonation developed in other regions of the himalaya is applicable with minor modifications in western nepal from south to north the major structural features are 1 the main frontal thrust system comprising the main frontal thrust and two to three thrust sheets of neogene foreland basin deposits 2 the main boundary thrust sheet which consists of proterozoic to early miocene lesser himalayan metasedimentary rocks 3 the ramgarh thrust sheet composed of paleoproterozoic low grade metasedimentary rocks 4 the dadeldhura thrust sheet which consists of medium grade metamorphic rocks cambrian ordovician granite and granitic mylonite and early paleozoic tethyan rocks 5 the lesser himalayan duplex which is a large composite antiformal stack and hinterland dipping duplex and 6 the main central thrust zone a broad ductile shear zone the major structures formed in a general southward progression beginning with the main central thrust in late early miocene time eocene oligocene thrusting in the tibetan himalaya north of the study area is inferred from the detrital unroofing record on the basis of 40ar 39ar cooling ages and provenance data from synorogenic sediments emplacement of the dadeldhura thrust sheet took place in early miocene time the ramgarh thrust sheet was emplaced between 15 and 10 ma the lesser himalayan duplex began to grow by 10 ma simultaneously folding the north limb of the dadeldhura synform the main boundary thrust became active in latest miocene pliocene time transport of its hanging wall rocks over an 8 km high footwall ramp folded the south limb of the dadeldhura synform thrusts in the subhimalayan zone became active in pliocene time the minimum total shortening in this portion of the himalayan fold thrust belt since early miocene time excluding the tibetan zone is 418 493 km the variation depending on the actual amounts of shortening accommodated by the main central and dadeldhura thrusts the rate of shortening ranges between 19 and 22 mm yr for this period of time when previous estimates of shortening in the tibetan himalaya are included the minimum total amount of shortening in the foldthrust belt amounts to 628 667 km this estimate neglects shortening accommodated by small scale structures and internal strain and is therefore likely to fall significantly below the actual amount of total shortening

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

paleomagnetic constraints on the geodynamic history of the major blocks of china from the permian to the present paleomagnetic study of china and its environs has been the center of a major international effort for the last 10 years in this paper we critically review all available paleomagnetic poles of upper permian to tertiary age from the main blocks of china with the goal of placing constraints on models of the formation and the subsequent deformation of the region after selecting reliable poles by applying objective criteria we divide our analysis into first order motions of blocks and second order deformation within blocks for first order analysis apparent polar wander paths are constructed for the major blocks we discuss the compatibilities and contradictions between the geological and paleomagnetic records a sequence of paleogeographic configurations taking into account geological constraints but remaining within paleomagnetic uncertainties is presented in general the major blocks were probably in contact throughout the permian and triassic but the jurassic was the key age during which most of the movement toward china s present configuration took place our reconstructions include certain details which are suggested by the paleomagnetic record but whose geological signatures seem to have been hidden by subsequent events during the cretaceous chinese poles agree with poles from other continents transferred onto eurasia at the second order we observe that for almost each period with sufficient data the paleomagnetic poles are streaked along a small circle centered on the sampling region indicating that much of china has been affected by small 20 differential rotations this we interpret as deformation caused in part by the extrusion of the chinese blocks away from the indian collision the complete annotated list of poles is given as an appendix

magnetostratigraphy of tertiary sediments from the hoh xil basin implications for the cenozoic tectonic history of the tibetan plateau summary we conducted an integrated palaeomagnetic and stratigraphic study on a 5452 8 m thick sedimentary sequence of the hoh xil basin in northern qinghai tibet plateau to obtain a chronostratigraphic framework for these sediments a total of 966 individual oriented palaeomagnetic samples spaced at stratigraphic intervals were collected from six measured sections in the hoh xil basin magnetic directions in these samples were obtained by progressive thermal mainly and alternating field demagnetization experiments most samples exhibit two components of magnetization the lower unblocking temperature component is an overprint resembling the present day geocentric axial dipole field direction at the sampling locality the most stable characteristic remanence chrm appears to be an early chemical remanent magnetization residing mainly in haematite the positive results of fold and reversal tests indicate that the chrm is a record of the palaeomagnetic field close to the time of formation of these sediments further evidence for the magnetization of these sediments acquired close to their time of deposition is the fact that patterns of magnetic reversals can be matched with the established polarity timescale on the basis of the distinct interval of magnetic reversal zones and biostratigraphic datums 13 magnetozones can be recognized at the hoh xil basin that range from chrons c11 to c23 30 1 51 0 ma the age of the fenghuoshan group is palaeomagnetically dated as 51 31 ma early eocene middle early oligocene and the age of the yaxicuo group is between 31 and 30 ma middle early oligocene late early oligocene the new palaeomagnetic data from the fenghuoshan group suggest that it has undergone no significant rotation since the oligocene in contrast declination data from the yaxicuo group in wudaoliang area imply a vertical axis clockwise rotation 29 1 8 5 since the late oligocene the tertiary palaeomagnetic pole position of the hoh xil basin implies a significant northward convergence of the hoh xil basin 1600 km with respect to eurasia siberia since early eocene late oligocene time our results are consistent with the pattern of disturbingly low palaeolatitudes derived from a large number of high quality palaeomagnetic studies of tertiary rocks from sites that reach all the way from eastern china to kyrgyzstan future work is needed to separate the influences of sedimentary inclination shallowing and tectonic shortening

shallow bias in neogene palaeomagnetic directions from the guide basin ne tibet caused by inclination error summary too shallow inclinations have frequently been observed in cenozoic sedimentary strata in central asia and new palaeomagnetic results obtained by us from the guide basin in ne tibet are no exception we use a statistical analysis technique developed by tauxe and kent tk03 gad which is based on a geomagnetic field model that predicts distributions of palaeomagnetic directions and show that the too shallow neogene mean inclination 44 from 627 sites can be corrected to a value of 58 which closely matches the inclination predicted for the area we conclude that syn to post depositional flattening is the most likely cause for the widely observed inclination bias in central asia

the apparent polar wander path for the north china block since the jurassic summary a palaeomagnetic study was carried out on sedimentary rocks from datong shanxi province and on volcanic rocks from chifeng inner mongolia and zhangjiakou hebei province china all of these sites belong to the stable part of the north china block ncb after detailed demagnetization experiments reliable palaeomagnetic directions were obtained for middle jurassic late cretaceous and late tertiary these data define the apparent polar wander path for the ncb since the jurassic free from the possible effect of the movement along tanlu fault since the mesozoic comparisons with reliable palaeomagnetic poles obtained in siberia and south china indicate that the middle jurassic poles for north and south china are almost the same while the siberian pole is displaced from them to the west by about 18 13 on the other hand the cretaceous poles for the three areas are not significantly different from each other consequently it is suggested that the amalgamation of north china and south china was almost complete in the middle jurassic and that their accretion to siberia continued until the late cretaceous

paleomagnetic results from the early cretaceous zenong group volcanic rocks cuoqin tibet and their paleogeographic implications article i nfo article history a paleomagnetic study has been carried out on the early cretaceous zenong group volcanic rocks ca 110 130 ma near cuoqin town in the central lhasa terrane stepwise thermal demagnetization up to 680 c isolated a stable high temperature component htc including antipodal dual polarities the htc directions passed both a fold test at 99 confidence and a reversal test at 95 confidence suggesting a primary origin the tilt corrected average direction for 18 sites is d 327 0 i 35 7 59 3 95 4 5 which corresponds to a paleopole at 58 2 n 341 9 e a95 4 6 yielding a paleolatitude of 19 8 4 6 n for the study area our results combined with previous paleomagnetic data suggest that southern tibet was at a mean paleolatitude of 20 n during the cretaceous and paleogene when compared with the apparent polar wander paths of asia the difference suggests that only ca 870 km north south crustal shortening has accumulated between the lhasa terrane and stable asia since ca 110 ma when comparing the paleolatitudes of the lhasa terrane and india the results indicate that the width of the neotethys was 6720 690 km during the early cretaceous

eocene early miocene foreland basin development and the history of himalayan thrusting western and central nepal sedimentologic petrographic and u pb detrital zircon ages from middle eocene through early miocene sedimentary rocks in the lesser himalayan zone of western and central nepal indicate that a peripheral foreland basin system had developed in the eastern himalayan collision zone by middle eocene time the shallow marine eocene bhainskati formation accumulated in a back bulge depozone between a southward migrating forebulge and the indian craton migration of the forebulge through this region during eocene oligocene time produced a regional unconformity that spans 15 20 myr by early miocene time the forebulge unconformity was onlapped by the distal fringes of the southward migrating foredeep depozone represented by fluvial deposits of the dumri formation continued southward migration of the foredeep during the neogene accommodated the fluvial siwalik group light mineral provenance data and u pb detrital zircon ages suggest that the bhainskati was derived partly from tethyan sedimentary rocks of the tibetan himalayan zone during initial growth of the himalayan fold thrust belt the dumri was derived from metasedimentary and crystalline rocks of the greater himalayan zone during emplacement of the main central thrust and contemporaneous tectonic unroofing by normal faulting along the south tibetan detachment system the lesser himalayan crystalline thrust sheets were emplaced soon after deposition of the dumri formation 15 10 ma paleocurrent and lithofacies data from the dumri formation indicate deposition by west southwestward flowing rivers that drained into the indus portion of the himalayan foreland basin system during the early miocene thick channel sandstones in the lower dumri may represent the early miocene counterpart of the modern ganges river eastward diversion of the ganges drainage system to near its present location had occurred by 15 ma as the high standing aravalli range on the northern indian shield approached the front of the fold thrust belt assuming reasonable values for the flexural rigidity of indian lithosphere the time span of the forebulge unconformity yields a velocity of 14 33 mm yr for the southward migration of the fold thrust belt relative to india this range of values is consistent with neogene and present day estimates and suggests that only one third to one half of india eurasia convergence has been accommodated by shortening in the himalayan fold thrust belt since the onset of collision

isotopic constraints on the age and provenance of the lesser and greater himalayan sequences nepalese himalaya the main central thrust of the central himalaya juxtaposes the lesser himalayan sequence footwall with the greater himalayan sequence hanging wall both of which are regarded as components of the northern passive margin of india considerable uncertainty surrounds their respective ages and sedimentary derivation u pb and sm nd isotopic studies of samples from the langtang area central nepal demonstrate important distinctions between the two rock packages the greater himalayan sequence had a sedimentary provenance that included a major source of 0 8 1 0 ga zircons implying a late proterozoic age the source of the precambrian part of the lesser himalayan sequence contained 1 87 2 60 ga zircons and its depositional age is probably middle proterozoic nd isotopic characteristics of the two sequences are different at 21 ma the greater himalayan rocks had end values between 14 6 and 18 5 while their lesser himalayan counterparts had values between 21 4 and 25 9 giving rise to major differences in their model ages the lesser and greater himalayan sequences at langtang are interpreted as correlative to the lower and upper parts of the vindhyan supergroup currently exposed south of the gangetic plain in northern india on the basis of available age data the exposed hanging wall of the main central thrust in central and eastern nepal contains no archean early proterozoic basement if the fault does root into the basement at depth it must do so farther north than recently published cross sections indicate this implies a total displacement on the fault that is substantially greater than the 140 210 km required by tectonic overlap in the eastern himalayas

late cretaceous to early paleogene paleomagnetic results from sikhote alin far eastern russia implications for deformation of east asia abstract welded tuffs in the bogopol and sijanov groups were sampled at 27 sites from 12 caldera formations in the sikhote alin mountain range around kavalerovo 44 3 n 135 0 e for chronological and paleomagnetic studies k ar age dates show that the welded tuffs erupted between 66 ma and 46 ma all sites yield reliable paleomagnetic directions with unblocking temperatures higher than 560 c the high temperature component at 12 sites and the medium temperature component at 3 sites in the bogopol group show reversed polarity d 193 7 i 57 6 95 8 1 the high temperature component at 11 sites in the sijanov group showed both reversed and normal polarities and its mean direction reveals no detectable deflection from north d 2 9 i 59 6 95 11 2 the combined paleomagnetic direction of the two groups yields a paleomagnetic pole of 250 5 e 84 1 n a 95 8 8 which falls near cretaceous paleomagnetic poles from outer mongolia inner mongolia the north china block and the south china block the sikhote alin area appears not to have been subjected to detectable motion with respect to east asia since about 50 ma this implies that the sikhote alin area behaved as an integral part of east asia during the opening of the japan sea at about 15 ma however significant separation between the paleomagnetic poles of east asia and europe during the jurassic paleogene implies a major relative movement between these two blocks since the paleogene

tertiary deformation history of southeastern and southwestern tibet during the indo asian collision geologic mapping and geochronological analysis in southwest kailas area and southeast zedong area tibet reveal two major episodes of tertiary crustal shortening along the classic indus tsangpo suture in the yalu river valley the older event occurred between ca 30 and 24 ma during movement along the north dipping gangdese thrust the development of this thrust caused extensive denudation of the gangdese batholith in its hanging wall and underthrusting of the xigaze forearc strata in its footwall examination of timing of major tectonic events in central asia suggests that the initiation of the gangdese thrust was approximately coeval with the late oligocene initiation and development of north south shortening in the eastern kunlun shan of northern tibet the nan shan at the northeastern end of the altyn tagh fault the western kunlun shan at the southwestern end of the altyn tagh fault and finally the tian shan north of the tarim basin such regionally synchronous initiation of crustal shortening in and around the plateau may have been related to changes in convergence rate and direction between the eurasian plate and the indian and pacific plates the younger thrusting event along the yalu river valley occurred between 19 and 10 ma along the south dipping great counter thrust system equivalent to the locally named renbu zedong thrust in southeastern tibet the backthrust system in south central tibet and the south kailas thrust in southwest tibet the coeval development of the great counter thrust and the north himalayan granite gneiss dome belt is consistent with their development being related to thermal weakening of the north himalayan and south tibetan crust due perhaps to thermal relaxation of an already thickened crust created by the early phase of collision between india and asia or frictional heating along major thrusts such as the main central thrust beneath the himalaya

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

palaeomagnetic estimates of crustal shortening in the himalayan thrusts and zangbo suture this article reports new palaeomagnetic results from precisely dated late palaeocene limestones from the former northern margin of the indian subcontinent now in tibet between the major himalayan thrusts and the india tibet suture zone convergence in the suture and intracontinental thrusting can be estimated separately and are respectively found to be of the order of 550 km 650 km and 400 km 400 km comparison with kinematic models derived from magnetic anomalies constrains the age of the collision at about 50 myr

statistical significance of the fold test in palaeomagnetism summary the significance of the change in direction dispersion on applying the fold test in palaeomagnetism is used as the criterion for deciding whether or not the fold test has statistical significance a set of tables is compiled which can be used for quick application of the test

new cretaceous and early tertiary paleomagnetic results from xining lanzhou basin kunlun and qiangtang blocks china implications on the geodynamic evolution of asia we present the results of a paleomagnetic study of 360 cores drilled at three different areas from the tibetan plateau 13 sites from lower cretaceous red beds in the xining lanzhou basin around xining and lanzhou cities southwest of the qilian mountains 36 2 n 103 5 e 13 sites from cretaceous red beds in the kunlun block near maqin 34 5 n 100 1 and 9 sites from paleocene lower eocene red beds in the qiangtang block near fenghuoshan 34 5 n 92 8 e thermal demagnetization of the samples allowed us to isolate a high temperature component which passes both positive reversal and fold tests for all formations the corresponding paleopoles lie at 50 3 n 195 5 e a95 4 6 for xining lanzhou 80 1 n 281 2 e dp 7 8 dm 12 7 for maqin and 62 6 n 210 5 e dp 3 9 dm 6 8 for fenghuoshan we discuss these in the frame of a new paleogeographic reconstruction of the cretaceous paleoposition of the blocks forming the asian mosaic we conclude that the xining lanzhou area could not be part of the north china block but rather was associated with the tarim qaidam assemblage paleomagnetic data argue in favor of a qaidam kunlun tarim junggar assemblage in the cretaceous significantly to the south of its current position with respect to the asian continent siberia mongolia and north china the large n s convergence 800 500 km implied since the cretaceous appears to be far larger than could be absorbed in the altay range to the north and qilian shan to the east of the order of 300 km part of this motion could have occurred along a large left lateral strike slip fault system which may connect with the mongol okhotsk suture to the northeast verifying this hypothesis will require new geologic and paleomagnetic data from these remote regions

new paleomagnetic results from northern china collision and suturing with siberia and kazakhstan abstract we report new results from our paleomagnetic studies of the inner mongolia autonomous region in northeastern china and the xinjiang autonomous region in northwestern china primary magnetizations are isolated in tertiary cretaceous late jurassic and late permian rocks and secondary components of magnetization of late jurassic and recent ages are seen in strata of inner mongolia western junggar basin and part of the tarim block suffered widespread complete tectono thermal remagnetization at about 280 ma permian the paleomagnetic evidence suggests that the sampled areas in inner mongolia have been part of the north china block at least since late permian time and the junggar block collided with the tarim block by late permian time our own paleomagnetic results together with available data from other parts of eastern eurasia suggest a tectonic model in which kazakhstan europe and siberia collided with each other and formed the europe siberia kazakhstan plate eur sib kaz in late permian time to the east mongolia outer mongolia inner mongolia and the north china block were also wedged together forming the mongolia north china plate mon nc by this time these two composite continental plates were not a stable unit in the late permian the geometrical relationship of the paleomagnetic poles for these two plates suggests that collision initially occurred in the late permian near the western end 50 n 75 e of the southern siberian fold belt and progressed eastward as the mon nc plate rotated and clockwise about 117 relative to the eur sib kaz plate during the early mesozoic suturing and rotation were probably complete by the late jurassic significant relative motion between the tarim and siberia blocks continued from early triassic to late jurassic as tarim moved northeastward 1400 km relative to siberia

simplification of the anisotropy based inclination correction technique for magnetite and haematite bearing rocks a case study for the carboniferous glenshaw and mauch chunk formations north america summary an anisotropy based inclination correction was applied to the carboniferous glenshaw formation from southwestern pennsylvania a combination of low temperature thermal demagnetization followed by alternating field demagnetization isolated a palaeomagnetic remanence direction similar to that previously reported for these rocks the inclination correction was conducted by fitting the samples anisotropy of anhysteretic remanence aar to the theoretical correction curves for a magnetite remanence that yielded a remanence individual magnetic particle anisotropy the a factor equal to 1 86 direct measurement of individual particle anisotropy in magnetic grain extracts yielded an a value of 2 in good agreement with the curve fitting technique the inclination corrected formation mean directions for these two a factors were statistically indistinguishable this result shows that curve fitting is an easier but accurate method of applying an anisotropy based inclination correction than direct measurement of the individual grain anisotropy in magnetic grain extracts the corrected glendale formation magnetite based palaeopole is very similar to haematite based palaeopoles from the carboniferous canadian maritimes a new technique is described that extracts the detrital haematite grains from red sedimentary rocks the measured haematite a factors from the carboniferous mauch chunk and cretaceous kapusaliang formations yield values consistent with susceptibility individual particle anisotropies a determined previously by curve fitting techniques extending the use of curve fitting for individual particle anisotropy determination to haematite bearing rocks however the corrected mauch chunk formation palaeopole is significantly different from the corrected glendale formation palaeopole calling the accuracy of the mauch chunk palaeopole into question tectonic strain during folding may have added to the compaction strain in the rocks leading to an overcorrection of the inclination a new inclination corrected palaeopole for the north american carboniferous is reported for the glenshaw formation at 28 6un 119 9ue

late miocene topographic inversion in southwest tibet based on integrated paleoelevation reconstructions and structural history article investigations of the deformation history of the himalayan orogen support interpretations of rapid and striking changes in the landscape of the tibetan himalaya and high himalaya we examine this issue by integrating oxygen isotope based paleoelevation reconstructions of the zada basin in southwestern tibet with information on the structural evolution of the high and tibetan himalaya between 79 e and 84 30 e 18 opsw values were calculated from 18 occ values from pristine fluvial miocene gastropod shells analyses comparing the most negative 18 osw and reconstructed 18 opsw values to 18 osw versus elevation relationships based on both thermodynamic models and an empirical data set suggest a decrease in the mean watershed elevation of 1 to 1 5 km since the late miocene geologic mapping and structural data from crustal scale fault systems in the zada region and regions to its east indicate a phase of arc normal shortening and vertical thickening since the middle miocene followed by ongoing arc parallel extension and vertical thinning these results suggest that regions in this part of the orogen transitioned from undergoing arc normal shortening to arc parallel extension in the late miocene and that arc parallel extensional structures root deeply within the himalayan thrust wedge when combined with data on the distribution age and provenance of sedimentary basins our geologic mapping structural data and paleoelevation results suggest that this transition from shortening to extension was accompanied by a topographic inversion from mountains to basins in b4 m y these observations can be explained by a foreland propagating fault system that accommodates outward radial expansion of the himalayan orogen

new early cretaceous paleomagnetic data from volcanic and red beds of the eastern qaidam block and its implications for tectonics of central asia abstract in order to better understand anomalously low paleomagnetic inclinations in the cretaceous rocks of central asia a combined geochronologic and paleomagnetic investigation has been performed on cretaceous basalt sequences intercalated with red beds from zeku area in the eastern qaidam block northeastern tibetan plateau potassium argon k ar dating indicates that the basalt sequences are of early cretaceous age 89 9 2 8 to 112 1 2 4 ma isothermal remanent magnetization irm thermal demagnetization of a three component irm and curie point experiment suggest that magnetite and hematite dominate in red beds magnetite in basalt flows stepwise thermal demagnetization successfully isolated stable characteristic remanent magnetization chrm from 7 lava flows and 12 red beds sites the chrms of red bed and basalt sites pass fold test and likely represent primary remanent magnetization the tilt corrected mean direction from red bed sites basalts plus baked red bed site is d s 18 4 i s 55 3 s 69 1 95 5 5 n 11 sites d s 16 1 i s 52 0 s 42 6 95 8 6 n 8 corresponding to a pole at 75 2 n 182 3 e with a 95 7 07 76 9 n 194 9 e with a 95 10 0 respectively the consistent inclination recorded in red beds and basalts implies no significant inclination shallowing caused by deposition compaction in red beds from the studied area compared with the early cretaceous poles from the north china block ncb or eurasia insignificant post early cretaceous northward motion may have occurred between the qaidam block and the ncb on the basis of the existing regional paleomagnetic data the qaidam block has not experienced wholescale vertical axis rotation since the early cretaceous with respect to ncb

structure paleogeographic inheritance and deformation history of the southern atlas foreland fold and thrust belt of tunisia 1 structural analysis of the southern tunisian atlas was carried out using field observation seismic interpretation and cross section balancing it shows a mix of thick skinned and thin skinned tectonics with lateral variations in regional structural geometry and amounts of shortening controlled by nw se oblique ramps and tear faults it confirms the role of the late triassic early jurassic rifting inheritance in the structuring of the active foreland fold and thrust belt of the southern tunisian atlas in particular in the development of nw se oblique structures such as the gafsa fault the late triassic early jurassic structural pattern is characterized by a family of first order nw se trending normal faults dipping to the east and by second order e w trending normal faults limiting a complex system of grabens and horsts these faults have been inverted during two contractional tectonic events the first event occurred between the middle turonian and the late maastrichtian and can be correlated with the onset of the convergence between africa and eurasia the second event corresponding to the principal shortening tectonic event in the southern atlas started in the serravalian tortonian and is still active during the neogene the southern atlas foreland fold and thrust belt propagated on the evaporitic decollement level infilling the late triassic early jurassic rift the major eocene atlas event described in hinterland domains and in eastern tunisia did not deform significantly the southern tunisian atlas which corresponded in this period to a backbulge broad depozone

paleomagnetism of cretaceous to lower tertiary rocks from southwestern sichuan a revisit abstract a re study has been carried out on the cretaceous to lower tertiary rocks from southwestern sichuan province on the south china block the characteristics remanent magnetization chrm resolved from the paleocene eocene leidashu formation is of dual polarity and very shallow inclination the anomalouslylow paleolatitude 6 9 6 3 n indicated by the inclination is not supported by geological evidence from this region and is probably due to inclination error related to depositional and or post depositional processes a positive fold test as well as the prevalence of normal polarity suggests that the chrm of the mid upper cretaceous xiaoba formation might be acquired during the cretaceous long normal superchron clns the tilt corrected mean direction of the xiaoba formation d i 8 1 38 8 95 6 6 n 18 sites is not significantly different from a preliminary result d i 357 6 31 6 95 8 6 n 12 samples we reported earlier for the collective xiaoba and leidashu formations the resultant paleopole 81 5 n 220 9 e a95 7 1 is coincident with other cretaceous poles for the chinese blocks comparison with the eurasian apparent polar wander path apwp indicates neither significant rotation nor latitudinal translation of the study area relative to eurasia the newly obtained mean direction for the xiaoba formation is believed to be more reliable than our earlier result since it is based on a larger sample size and is apparently free from effects of the shallow inclination observed in the leidashu formation and should supercede our previous preliminary result the paleopole derived from sevenentes of the lower cretaceous feitianshan formation 69 0 n 204 6 e a95 4 3 deviates from the track from the late jurassic to late cretaceous poles the deviation might be reduced by further sampling

paleogeographic and tectonic evolution of southern tibet since middle cretaceous time new paleomagnetic data and synthesis during the second french chinese paleomagnetic campaign in tibet in the summer of 1981 we sampled the middle cretaceous red beds of the takena formation and the cenozoic volcanics of the lingzizong formation a detailed analysis of about 400 specimens of red beds reveals a stable component of normal magnetization n in the north of the lhasa block north of the nyainqentanglha range and two stable components in the south lt ht where the red beds are more strongly folded and are unconformably overlain by the lingzizong volcanics all three components are carried by hematite blocking temperatures range from 400 to 600 c for lt and up to the neel temperature of hematite for both n and ht lt is shown to postdate part of the folding of the takena whereas n and ht both indicate a positive fold test at the 99 probability level in addition a small less than 10 of the natural remanent magnetization antiparallel component is superimposed in most samples and may be consistent with a model of long term acquisition of a chemical remanent magnetization for the red beds analysis of the volcanics reveals a single stable component with mixed polarities carried either by hematite or magnetite the paleomagnetic directions indicate that 1 the lhasa block stood at 12 5 3 n by upper cretaceous and at 13 5 6 5 n in the paleocene eocene thus confirming the limited displacement of the margin of eurasia between albian aptian deposition of the red beds and eocene onset of the india asia collision these values imply crustal shortening by as much as 1900 850 km north of the block since the upper cretaceous and by 1400 1200 km south of the block since the onset of the collision 2 southern tibet formed a single tectonic unit by middle cretaceous time and 3 differential rotations are observed within the block and are related to the collision these rotations are discussed in the frame of a recent propagating extrusion tectonics model this study confirms the close paleogeographic relationship between indochina khorat plateau and southern tibet prior to the india asia collision

tertiary paleomagnetism of north and south china and a reappraisal of late mesozoic paleomagnetic data from eurasia implications for the cenozoic tectonic history of asia abstract to contribute to the understanding of the cenozoic tectonic history of china and eastern eurasia in general we collected paleomagnetic samples from tertiary rocks in both north and south china our miocene paleomagnetic data for northern china are from four widely separated basalt sections in the inner mongolia autonomous region and shandong province both normal and reversed polarities of the characteristic magnetization are present in the stratigraphie sequence and the mean normal and reversed directions are closely antiparallel to each other the fact that the same characteristic direction was carried by both magnetite and hematite suggests that the hematite was probably formed by auto oxidation processes during primary cooling of the basalts the paleomagnetic data from these four areas also pass a regional generalized fold test the corresponding mean miocene paleomagnetic pole for northern china is longitude 261 6 e latitude 88 3 n a 95 6 4 our early tertiary paleomagnetic results for south china are from two sandstone sections around the city of nanning 22 7 n 108 3 e in guangxi zhuang autonomous region high temperature characteristic directions isolated in 31 samples from five sites pass both tilt and reversal tests the paleomagnetic pole position determined from the five sites lies at longitude 228 0 e latitude 83 6 n a 95 8 5 the tertiary paleomagnetic poles for north and south china are statistically indistinguishable from each other and thus confirm that north and south china were fully assembled at least by the tertiary our new tertiary paleomagnetic data are consistent with the hypothesis that the major continental blocks of north and south china were in the same position as today during the tertiary a critical evaluation and selection of paleomagnetic results from the late jurassic onward for siberia europe and north and south china further corroborate this hypothesis most previously existing jurassic and cretaceous paleomagnetic poles from europe siberia are of uncertain reliability and should not be used for tectonic reconstruction using poles derived from transferring reliable data from other continents we show that the previous paleomagnetic inferences of large eastward displacements between these chinese blocks during the cenozoic are no longer warranted the available paleomagnetic data that meet our acceptance criteria are also of interest concerning the boundary conditions of the propagating extrusion model for asian tectonics additional mesozoic and cenozoic paleomagnetic data from siberia and north and south china are still needed to resolve the smaller second order displacements that appear to characterize the cenozoic tectonic history of this part of asia

palaeolatitude and age of the indo asia collision palaeomagnetic constraints summary ongoing controversies on the timing and kinematics of the indo asia collision can be solved by palaeomagnetically determined palaeolatitudes of terranes bounding the indo asia suture zone we show here based on new palaeomagnetic data from the linzizong volcanic rocks 54 47 ma near the city of lhasa that the latitude of the southern margin of asia was 22 8 4 2 n when these rocks were deposited this result combined with revised palaeomagnetic results from the northernmost sedimentary units of greater india and with apparent polar wander paths of india and eurasia palaeomagnetically constrain the collision to have occurred at 46 8 ma 95 per cent confidence interval these palaeomagnetic results are consistent with tomographic anomalies at 15 25 n that are interpreted to locate the tethyan oceanic slab that detached following collision and with independent 56 46 ma collision age estimates inferred from the timing of slowing down of india high pressure metamorphism the end of marine sedimentation and the first occurrence of suture zone and arc detritus on the greater indian margin when compared with apparent polar wander paths of india and eurasia the 46 ma onset of collision at 22 8 4 2 n implies 2900 600 km subsequent latitudinal convergence between india and asia divided into 1100 500 km within asia and 1800 700 km within india

cretaceous and tertiary paleomagnetic results from southeast china and their tectonic implications abstract we report cretaceous and tertiary paleomagnetic data from fujian guangxi and guangdong provinces in south china in guangxi province we sampled early cretaceous redbeds of the xinlong formation at eight localities separated by up to 100 km the high temperature component obtained by thermal demagnetization is of dual polarity and passes the fold test and implies 18 8 counterclockwise rotation and insignificant poleward displacement with respect to cratonic localities in sichuan province a tertiary basin lying in a southeast trending fault zone is also rotated counterclockwise by 40 8 both results can be explained very well by a simple block rotation model for the cenozoic tectonic evolution of western guangxi province that is driven by left lateral shear on southeast trending faults the timing and sense of shear are the same as for india asia collision models involving the extrusion of indochina along the red river fault zone which lies several hundred kilometers to the southwest but the magnitude of fault displacement in guangxi is much less in the coastal provinces of fujian and guangdong we find late cretaceous directions that are rotated clockwise with respect to sichuan province combined with similar clockwise rotations found by two other studies of coeval rocks from the same region the mean rotation for the coastal provinces is 12 8 this could indicate a coherent block rotation of the region driven by extrusion tectonics however the limited number of sampling localities also allows the possibility that the directions reflect local rotations due to dextral shearing driven by circum pacific tectonics

paleomagnetism of the oligocene kangtuo formation red beds central tibet inclination shallowing and tectonic implications abstract a paleomagnetic study on the red beds of the oligocene kangtuo formation fm was carried out in the gerze basin of the lhasa terrane a total of 700 samples were collected from 37 sites stepwise thermal demagnetization revealed that the main magnetic carrier is hematite the natural remnant magnetization nrm consists of two components a low temperature component ltc is identified below 300 c and is interpreted to be a recent viscous overprint whereas a high temperature component htc unblocks at 665 690 c and is interpreted to be the primary magnetization the htc distributions show a clear east west elongated distribution which is considered as reflecting inclination flattening of deposited magnetic remanence carriers after inclination calibration using the e i method the htc could pass both a reversal test and a fold test at 95 confidence level showing the mean direction at ds 340 3 is 44 2 with k 63 0 and 95 3 1 corresponding to a paleopole at 71 7 n 339 3 e a95 3 1 and the paleolatitude of the sampling site at 25 9 3 1 n the paleolatitude is consistent with that expected from the coeval pole of the qiangtang terrane obtained from volcanic rocks suggesting that there has been no paleomagnetically discernable latitudinal motion between the qiantang and lhasa terranes since 30 ma comparing our new data with the apparent polar wander paths apwps of east asian blocks cogne et al 2013 europe and india besse and courtillot 2002 we have reached the following conclusions 1 there is no significant paleolatitudinal difference observed between the lhasa terrane and other central and northern asian terranes at 30 ma 2 the observed paleolatitude of the kangtuo fm is 8 0 4 9 lower than the expected paleolatitude deduced from the data of stable europe highlighting the asian inclination anomaly phenomenon but is 4 6 5 1 higher than that deduced from the data of the india plate likely presenting a 506 561 km shortening between the india and lhasa terrane since the oligocene it is inferred that this amount of shortening has been absorbed mainly by the himalaya orogenic belt

inclination shallowing in eocene linzizong sedimentary rocks from southern tibet correction possible causes and implications for reconstructing the india asia collision a systematic bias towards low palaeomagnetic inclination recorded in clastic sediments that is inclination shallowing has been recognized and studied for decades identification understanding and correction of this inclination shallowing are critical for palaeogeographic reconstructions particularly those used in climatemodels and to date collisional events in convergent orogenic systems such as those surrounding the neotethys here we report palaeomagnetic inclinations from the sedimentary eocene upper linzizong group of southern tibet that are 20 lower than conformable underlying volcanic units at face value the palaeomagnetic results from these sedimentary rocks suggest the southern margin of asia was located 10 n which is inconsistent with recent reviews of the palaeolatitude of southern tibet we apply two different correction methods to estimate the magnitude of inclination shallowing independently from the volcanics the mean inclination is corrected from 20 5 to 40 0 within 95 per cent confidence limits between 33 1 and 49 5 by the elongation inclination e i correction method an anisotropy based inclination correction method steepens the mean inclination to 41 3 3 3 after a curve fitting determined particle anisotropy of 1 39 is applied these corrected inclinations are statistically indistinguishable from the well determined 40 3 4 5o mean inclination of the underlying volcanic rocks that provides an independent check on the validity of these correction methods our results show that inclination shallowing in sedimentary rocks can be corrected careful inspection of stratigraphic variations of rock magnetic properties and remanence anisotropy suggests shallowing was caused mainly by a combination of syn and post depositional processes such as particle imbrication and sedimentary compaction that vary in importance throughout the section palaeolatitudes calculated from palaeomagnetic directions from eocene sedimentary rocks of the upper linzizong group that have corrected for inclination shallowing are consistent with palaeolatitude history of the lhasa terrane and suggest that the india asia collision began at 20 n by 45 55 ma

structural evolution and sequence of thrusting in the tethyan fold thrust belt and indus yalu suture zone southwest tibet regional mapping of a north south traverse from the india nepal china border junction to mount kailas in southwest tibet combined with previously published geochronologic and stratigraphic data is the basis for an incremental restoration of the tethyan fold thrust belt and deformation along the indus yalu suture zone from north to south the major structural features are 1 the indus yalu suture zone composed of five south dipping thrust faults involving rocks interpreted to represent parts of the former indian passive margin and asian active margin 2 the tethyan fold thrust belt composed of a dominantly north dipping system of imbricate thrust faults involving precambrian through upper cretaceous strata and 3 the kiogar jungbwa thrust sheet a linelength cross section reconstruction indicates a minimum of 176 km of north south horizontal shortening partitioned by the tethyan fold thrust belt 112 km and indusyalu suture zone 64 km sequential restoration of the cross section shows that the locus of shortening prior to the late oligocene occurred significantly south possibly 60 km of the indus yalu suture zone within the tethyan fold thrust belt and that a significant amount of unsubducted oceanic lithosphere was present south of the suture in southwest tibet until that time an implication of this result is that postcollision oligocene miocene high k calcalkaline magmatism may be explained by melting due to active subduction of oceanic crust beneath the kailas magmatic complex until the late oligocene a regional profile across the tibetan himalayan orogen from

detrital remanence inclination errors and anhysteretic remanence anisotropy quantitative model and experimental results summary we suggest that inclination errors in detrital remanent magnetization drm may be recognized and corrected by measurement of the anisotropy of anhysteretic remanent magnetization arm arm anisotropy reflects directional variations in the remanence capacity of relatively fine grained magnetic particles in rocks or sediments generally the same particles that carry the stable component of drm relative vertical and horizontal drm magnitudes are controlled by this directional remanence capacity as well as by the alignment efficiency of the particle magnetic moments which in turn is governed by the relative intensities of the vertical and horizontal components of the ambient magnetic field thus the respective vertical and horizontal components of palaeointensity and therefore palaeofield inclination may be obtained by normalizing the measured drm components by parallel arm intensities synthetic sediments containing silica kaolin and sized magnetite powders have previously been found to acquire experimental drm with an inclination error that is a direct function of kaolin concentration further experiments have now shown that arm anisotropy in the synthetic sediments is also a direct function of kaolin content and thus a correlation exists between drm inclination errors and arm anisotropy multiplying the measured drm vector by the inverse of the arm anisotropy matrix yields an improved estimate of the actual laboratory depositional field in all cases for samples with acicular 0 5 m magnetite this procedure reduces a mean inclination error of over 5 to less than 0 5 samples with subequant magnetite of slightly larger grain size 0 75 m exhibit similar arm anisotropy 10 per cent but somewhat larger inclination errors 10 we interpret this in terms of stronger horizontal alignment but weaker particle anisotropies for the larger more equant magnetites

shortening budgets and the role of continental subduction during the india asia collision abstract the india asia collision provides a remarkable example of diffused deformation because the zone of compressive strain extends far into the hinterland of the himalaya molnar and tapponnier s science 189 1975 419 concept of india as a rigid indenter responsible for the diffused deformation has been widely accepted perhaps uncritically this review gives a reappraisal of landmark papers by dewey et al philos trans r soc london a 327 1988 379 eclogae geol helv 82 1989 717 which discussed the shortening budget in se asia and the evidence for the amount and the rate of the convergence between india and asia taking place after the initial collision of these continents at roughly 55 50 ma evidence for the total convergence comes from magnetic anomaly palaeomagnetic and volumetric balancing studies the results are consistent in showing about 1800 2100 km convergence between india and asia in the western sector 2475 km in the central and 2750 2800 km in the eastern sector the rate of convergence between india and asia over the period since initial collision is roughly 5 cm year however the possible long term rate of convergence within the himalaya is considerably less i e 1 5 2 0 cm year a fraction of the total convergence this shortfall can be made up by adding to the rate for the himalaya alone the convergence rates in other parts of the diffused zone of compressive strain i e tibet tien shan altai this works well for the post oligocene but not for the earlier post collisional history when the areal extent of thrust fold tectonics was much more limited possibly the apparent shortening deficit before the miocene was made up by strike slip faulting but with the notable exception of the red river fault evidence for widespread pre miocene strike slip faults is not convincing summation of the amount of shortening in thrust fold regimes in the himalaya and its hinterland falls short of that required by the total convergence figures quoted above the deficit arises firstly because of the likelihood that pre collisional crustal shortening of uncertain areal extent in tibet reduces tibet s contribution 40 according to dewey et al philos trans r soc london a 327 1988 379 eclogae geol helv 82 1989 717 to the shortening budget for the cenezoic revised shortening budgets are presented on the assumption that most of the lhasa block southern tibet was already thickened prior to collision this thickening is difficult to quantify secondly balanced cross sections from various parts of the himalaya give 60 70 shortening that is only about 500 km roughly half of the shortening proposed by dewey et al the conclusion of le pichon et al tectonics 11 1992 1085 that only indian mid upper crust is involved in the himalayan thrust sheets is supported but it needs to be reassessed in the light of the recognition of eocene oligocene thickening in the himalaya this early crustal thickening antedates the sequence of thrusts which started with the main central thrust and therefore the shortening estimates based on balanced cross sections probably underestimate the total shortening in the himalaya the prospect of substantial underthrusting of india beneath tibet minimizes the amount of horizontal shortening of tibetan lithosphere during the cenozoic the conclusion is that either the total convergence deduced from geophysical and volumetric work is too large or the deficit must be made up by shortening mechanisms other than thrust fold regimes i e strike slip faulting which serves to move rocks out of the way of india

paleomagnetic evidence for post cretaceous internal deformation of the chuan dian fragment in the yangtze block a consequence of indentation of india into asia abstract paleomagnetic samples of paleocene eocene red sandstones were collected at 36 sites from the jiangdihe 4 and zhaojiadian formations around the yongren 26 1 n 101 7 e and dayao areas 25 7 n 101 3 e these areas are located in the chuxiong basin of the chuan dian fragment southwestern part of the yangtze block after stepwise thermal demagnetization a high temperature component with unblocking temperature of about 680 c is isolated from 26 sites the primary nature of this magnetization is ascertained through positive fold and reversal tests at 95 confidence level the tilt corrected mean paleomagnetic directions for the yongren and dayao areas are d 17 2 i 26 6 with 95 5 8 and d 16 5 i 31 1 with 95 4 8 respectively easterly deflected declinations from this study are consistent with those reported from other areas of the chuxiong basin indicating its wide presence in the cretaceous eocene formations of the said basin comparison with declination values expected from the cretaceous eocene apwp of eurasia indicates that the magnitude of clockwise rotation systematically increases toward the southeast within the chuxiong basin as well as in the chuan dian fragment this trend of the differential tectonic rotation in the chuan dian fragment is consistent with curvature of the xianshuihe xiojiang fault system deformation of the chuxiong basin can fairly be associated with the formation of eastward bulge in the southern part of the chuan dian fragment during southward displacement the chuan dian fragment was probably subjected to tectonic stresses as a result interaction with the yangtze and indochina blocks which resulted into east west extension and north south shortening

oligo miocene magnetostratigraphy and rock magnetism of the xishuigou section subei gansu province western china and implications for shallow inclinations in central asia magnetostratigraphy of 222 remanent directions together with late oligocene to early miocene mammal and charophyte paleontology suggest that 2179 m of the xishuigou section subei gansu province china were deposited from 26 to 19 ma stratigraphic patterns of bulk susceptibility anisotropy of magnetic susceptibility parameters and natural and anhysteretic remanent magnetization intensities demonstrate that 1 faulting does not significantly affect the record 2 sediment deposition was relatively continuous 3 sediment source changed around 23 ma and 4 rapid uplift near subei occurred at 21 ma subei rotated 27 5 counterclockwise with respect to the 20 ma pole from the eurasian synthetic apparent polar wander path folding and rotation of the section took place after 19 ma the paleolatitude of subei is 14 less than at present and 19 3 less than predicted from the reference pole both rock magnetic and paleomagnetic data sets suggest that the unusually low paleolatitude is the result of synsedimentary inclination shallowing a phenomenon which has likely affected other paleomagnetic data from central asia

53 43 ma deformation of eastern tibet revealed by three stages of tectonic rotation in the gongjue basin the gongjue basin from the eastern qiangtang terrane is located in the transition region where the regional structural lineation curves from east west oriented in tibet to north south oriented in yunnan in this study we sampled the red beds in the basin from the lower gongjue to upper ranmugou formations for the first time covering the entire stratigraphic profile the stratigraphic ages are bracketed within 53 43 ma by new detrital zircon u pb ages constraining the maximum deposition age to 52 5 1 5 ma rock magnetic and petrographic studies indicate that detrital magnetite and hematite are the magnetic carriers positive reversals and fold tests demonstrate that the characteristic remanent magnetization has a primary origin the gongjue and ranmugou formations yield mean characteristic remanent magnetization directions of d s i s 31 0 21 3 and d s i s 15 9 22 0 respectively the magnetic inclination of these characteristic remanent magnetizations is significantly shallowed compared to the expected inclination for the locality however the elongation inclination correction method does not provide a meaningful correction likely because of syn depositional rotation rotations relative to the eurasian apparent polar wander path occurred in three stages stage i 33 3 3 4 clockwise rotation during the deposition of the gongjue and lower ranmugou formations stage ii 26 9 3 7 counterclockwise rotation during deposition of the lower and middle ranmugou formation and stage iii 17 7 3 3 clockwise rotation after 43 ma the complex rotation history recorded in the basin is possibly linked to sinistral shear along the qiangtang block during india indentation into asia and the early stage of the extrusion of the northwestern indochina blocks away from eastern tibet

geometry and crustal shortening of the himalayan fold thrust belt eastern and central bhutan we present a new geologic map of eastern and central bhutan and four balanced cross sections through the himalayan fold thrust belt major structural features from south to north include 1 a single thrust sheet of subhimalayan rocks above the main frontal thrust 2 the upper lesser himalayan duplex system which repeats horses of the neoproterozoic cambrian baxa group below a roof thrust shumar thrust carrying the paleoproterozoic daling shumar group 3 the lower lesser himalayan duplex system which repeats horses of the daling shumar group and neoproterozoic ordovician jaishidanda formation with the main central thrust mct acting as the roof thrust 4 the structurally lower greater himalayan section above the mct with overlying tethyan himalayan rock in stratigraphic contact in central bhutan and structural contact above the south tibetan detachment in eastern bhutan and 5 the structurally higher greater himalayan section above the kakhtang thrust cross sections show 164 267 km shortening in subhimalayan and lesser himalayan rocks 97 156 km structural overlap across the mct and 31 53 km structural overlap across the kakhtang thrust indicating a total of 344 405 km of minimum crustal shortening 70 75 our data show an eastward continuation of lesser himalayan duplexing identified in northwest india nepal and sikkim which passively folded the overlying greater himalayan and tethyan himalayan sections shortening and percent shortening estimates across the orogen although minima do not show an overall eastward increase which may suggest that shortening variations are controlled more by the original width and geometry of the margin than by external parameters such as erosion and convergence rates

paleomagnetic results from the early cretaceous lakang formation lavas constraints on the paleolatitude of the tethyan himalaya and the india asia collision abstract to better constrain the early cretaceous paleogeographic position of the tethyan himalaya and the india asia collision process a paleomagnetic study was performed on the lakang formation lava flows in the cuona area in the southeastern tethyan himalaya stepwise thermal and alternating field demagnetizations successfully isolated reliable characteristic remanent magnetization chrm directions that include antipodal dual polarities and pass positive fold tests at the 99 confidence level and reversal tests at 95 confidence level indicating prefolding primary magnetizations the distribution patterns of chrm directions from the lakang formation lava flows are consistent with young lava flows at similar latitudes suggesting that secular variation has likely been averaged out the tilt corrected site mean direction for 31 sites is d 261 6 i 68 5 with 95 3 6 which provides a paleopole at 26 8 s 315 2 e a 95 5 7 corresponding to a paleolatitude of 52 2 5 7 s for the study area comparison of the paleolatitude observed from the lakang formation lava flows with that expected from the apparent polar wander paths of india at 130 ma show a paleolatitude difference of 2 1 230 km indicating that neither a great north south continental crustal shortening occurred between the indian craton and the tethyan himalaya after 130 ma nor that a wide ocean separated them at that time comparison with reliable cretaceous paleocene paleomagnetic results observed from the tethyan himalaya and the lhasa terrane indicates that the latitudinal width of the neo tethyan ocean could have been up to 7000 km at 134 130 ma and an extension should have existed between the indian craton and the tethyan himalaya during the late cretaceous and paleocene furthermore reliable paleomagnetic results suggest that the india asia collision was a dual collision process consisting of a first collision of the tethyan himalaya with the lhasa terrane asia at 54 9 2 3 ma and a final continent continent collision of the indian craton with the tethyan himalaya at 40 0 3 3 ma

propagation of the thrust system and erosion in the lesser himalaya geochemical and sedimentological evidence centre de recherches pe trographiques et ge ochimiques bp 20 54501 vandoeuvre les nancy franceabstractsedimentological and nd isotope data of two sections of the sub himalaya of westernnepal are used as new constraints for understanding the erosion history of the himalaya throughout the deposition of the middle and upper members of the siwalik group thelesser himalaya contribution to the total detrital input progressively increased from lessthan 20 to 40 the increasing proportion of lesser himalaya sediments started at ca 10 8 ma and is associated with a coarsening of the maximum grain size at both micro scopic and macroscopic scales thin skinned tectonics of the lesser himalaya thrust sys tem would have controlled the exhumation of the lesser himalaya rocks and would havebegun at 12 10 ma taking into account the delay for denudation together with otherstudies these data restrict the onset of movement on the lesser himalaya thrust systemto less than 3 m y along more than 1750 km this short time frame implies a ratio oflateral propagation rate to shortening rate far too high for the propagation of a singlecrustal thrust thus we suggest instead the simultaneous initiation of several thrusts aheadof the main central thrust at ca 12 ma we suggest that a rapid rise of the tibetanplateau at this time has transformed the himalaya to an overcritical thrust wedge thathas propagated forward to return to a stable state this regional rising could be the primecause of the increase of sediment in ux at ca 11 ma around the himalaya keywords siwalik formations neodymium grain size erosion thrust sheets himalaya

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

thrust tectonics crustal shortening and the structure of the far eastern nepal himalaya balanced and restored structural sections across the far eastern nepal himalaya have been constructed in order to determine the structure and evolution of the himalayan orogenic wedge and the amount of tectonic shortening the region has undergone since the initiation of thrusting along the main central thrust mct the far eastern nepal himalaya is comprised of three distinct thrust bound tectonic packages the higher himalayan crystalline thrust sheet the lesser himalayan metasediment thrust package and the sub himalayan imbricate fan the higher himalayan crystallines consisting of kyanite and sillimanite bearing gneisses intruded by the miocene jannu leucogranites have been thrust over the lesser himalayan metasediments along the mct for a distance of 140 km to 175 km the lesser himalayan metasediments are a 12 km thick unit consisting primarily of phyllites metaquartzites and mylonitic augen gneisses in which garnet biotite and chlorite metamorphic zones are exposed in progressively deeper structural levels the lesser himalayan metasediment thrust package is underlain by a decollement the main detachment fault mdf which lies at a calculated depth of between 6 and 10 km underneath the mahabharat lekh and at a calculated depth of 20 to 25 km north of the tamar khola dome the tamar khola dome overlies a footwall ramp along the mdf where the mdf cuts upsection through the lesser himalayan metasediments the lesser himalayan thrust package probably has an internal structure aproximating a hinterland dipping duplex with the mct and the mdf corresponding to the roof and floor thrusts respectively both the tamar khola thrust an out of sequence breach thrust and the main boundary thrust mbt are splay thrusts off of the mdf the sub himalaya consisting of nonmetamorphosed sedimentary rocks displays an emergent imbricate fan geometry and is underlain by the southern continuation of the mdf which lies at a depth of 5 5 km to 6 km beneath the siwalik hills folding and thrusting within the lesser himalayan thrust package and the sub himalayan imbricate fan have accomodated 45 to 70 km of tectonic shortening total north south shortening across the higher lesser and sub himalaya of far eastern nepal south of the tibetan plateau has been of the order of 185 km to 245 km and has occurred at an average rate of 7 4 mm to 15 3 mm per year since the initiation of the mct between 16 and 25 ma

implications of shortening in the himalayan fold thrust belt for uplift of the tibetan plateau 1 recent research in the himalayan fold thrust belt provides two new sets of observations that are crucial to understanding the evolution of the himalayan tibetan orogenic system first u pb zircon ages and sm nd isotopic studies demonstrate that 1 greater himalayan medium to high grade metasedimentary rocks are much younger than true indian cratonic basement and 2 these rocks were tectonically mobilized and consolidated with the northern margin of gondwana during early paleozoic orogenic activity these observations require that greater himalayan rocks be treated as supracrustal material in restorations of the himalayan fold thrust belt rather than as indian cratonic basement in turn this implies the existence of greater himalayan lower crust that is not exposed anywhere in the fold thrust belt second a regional compilation of shortening estimates along the himalayan arc from pakistan to sikkim reveals that 1 total minimum shortening in the fold thrust belt is up to 670 km 2 total shortening is greatest in western nepal and northern india near the apex of the himalayan salient and 3 the amount of himalayan shortening is equal to the present width of the tibetan plateau measured in an arc normal direction north of the indus yalu suture zone this information suggests that a slab of greater indian lower crust composed of both indian cratonic lower crust and greater himalayan lower crust with a north south length of 700 km may have been inserted beneath the tibetan crust during the cenozoic orogeny we present a modified version of the crustal underthrusting model for himalayan tibetan orogenesis that integrates surface geological data recent results of mantle tomographic studies and broadband seismic studies of the crust and upper mantle beneath the tibetan plateau previous studies have shown that incremental mesozoic and early cenozoic shortening had probably thickened tibetan crust to 45 55 km before the onset of the main cenozoic orogenic event thus the insertion of a slab of greater indian lower crust with maximum thickness of 20 km tapering northward could explain the cenozoic uplift of the tibetan plateau the need for tibetan crust to stretch laterally as the greater indian lower crust was inserted may explain the widespread east west extension in the southern half of the plateau our reconstruction of the himalayan fold thrust belt suggests that indian cratonic lower crust of presumed mafic composition and high strength should extend approximately halfway across the tibetan plateau to the banggong suture from there northward we predict that the tibetan plateau is underlain by more felsic and therefore weaker lower crust of greater himalayan affinity two slab break off events are predicted by the model the first involved neotethyan oceanic lithosphere that foundered 45 35 ma and the second consisted of greater indian lithosphere most likely composed of greater himalayan material that delaminated and foundered 20 10 ma asthenospheric upwelling associated with the break off events may explain patterns of cenozoic volcanism on the tibetan plateau although the model predicts a northward migrating topographic front due solely to insertion of greater indian lower crust the actual uplift history of the plateau was complicated by early middle tertiary shortening of tibetan crust

paleomagnetism of upper cretaceous rocks from south china paleomagnetic study of upper cretaceous redbeds from two areas in south china was undertaken in an effort to constrain better the history of tectonic movements in southeastern asia from the nanjing area 32 n l19 e high temperature characteristic directions which pass a fold test were obtained from ten sites 43 samples in the upper cretaceous puko and yanzijing formations the paleomagnetic pole position 76 3 n 172 6 e a95 10 3 agrees well with a late cretaceous reference pole for eurasia and thus confirms that this part of south china was fully sutured to eurasia by the end of the cretaceous with no paleomagnetically resolvable subsequent relative rotation in western sichuan 26 5 n 102 3 e high temperature characteristic directions of normal and reverse polarity were obtained from twelve samples 48 specimens in the upper cretaceous to lower paleocene xiaoba and leidashu formations a provisional paleomagnetic pole position 80 9 n 296 8 e a95 7 7 differs significantly from a late cretaceous eurasia reference pole in terms of apparent tectonic movement the difference in paleopole positions would indicate a 13 10 3 northward translation and 22 11 2 counterclockwise rotation of western sichuan relative to eurasia confirmation of this reconnaisance result which would suggest that western sichuan was pushed ahead of an impinging india is needed

paleogene high elevations in the qiangtang terrane central tibetan plateau abstract the timing of uplift of the tibetan plateau remains controversial with estimates varying from eocene to more recent than pliocene in particular the paleoaltimetry of the qiangtang terrane on the central tibetan plateau is completely unknown here we present new stable isotope results of fluvial lacustrine carbonate cement pedogenic carbonate and marl from the kangtuo and suonahu formations deposited between 51 and 28 ma in the heihuling bamaoqiongzong area of the northern qiangtang terrane the lithofacies associations indicate that the kangtuo formation was deposited in alluvial fan and fluvial floodplain environments and the suonahu formation was deposited in near shore lacustrine playa lake and channelized fluvial environments carbon and oxygen isotope values coupled with the sedimentary facies interpretations point to evaporation and low respiration rates in the eocene oligocene paleosols suggesting an arid climate in the high qiangtang area at the time the 18opsw values of paleo surface water reconstructed from the lowest i e the least evaporated 18oc pdb values of the unaltered authigenic carbonates are used to make the minimum estimates of the average paleoelevation of the drainage basin the paleoelevation of the northern qiangtang terrane is reconstructed as above 5000 m by at least the middle oligocene 28 ma similar to the present elevation in this area the aridity and the positive shift in oxygen isotope values of surface waters in our study area may suggest that the high lhasa terrane established by the middle oligocene blocked the northward transport of tropical moisture maintenance of high elevation 5000 m from at least the oligocene to the present suggests that the qiangtang crust was already thickened by that time and that underthrusting of india beneath asia since then has continued to provide additional material while at the same time driving asian lower crust eastward by crustal flow

restoration of cenozoic deformation in asia and the size of greater india a long standing problem in the geological evolution of the india asia collision zone is how and where convergence between india and asia was accommodated since collision proposed collision ages vary from 65 to 35 ma although most data sets are consistent with collision being underway by 50 ma plate reconstructions show that since 50 ma 2400 3200 km west to east of india asia convergence occurred much more than the 450 900 km of documented himalayan shortening current models therefore suggest that most post 50 ma convergence was accommodated north of the indus yarlung suture zone we review kinematic data and construct an updated restoration of cenozoic asian deformation to test this assumption we show that geologic studies have documented 600 750 km of n s cenozoic shortening across and north of the tibetan plateau the pamir hindu kush region accommodated 1050 km of n s convergence geological evidence from tibet is inconsistent with models that propose 750 1250 km of eastward extrusion of indochina approximately 250 km of indochinese extrusion from 30 to 20 ma of indochina suggested by se asia reconstructions can be reconciled by dextral transpression in eastern tibet we use our reconstruction to calculate the required size of greater india as a function of collision age even with a 35 ma collision age the size of greater india is 2 3 times larger than himalayan shortening for a 50 ma collision the size of greater india from west to east is 1350 2600 km consistent with robust paleomagnetic data from upper cretaceous paleocene tethyan himalayan strata these estimates for the size of greater india far exceed documented shortening in the himalaya we conclude that most of greater india was consumed by subduction or underthrusting without leaving a geological record that has been recognized at the surface

the origin of himalayan anatexis and inverted metamorphism models and constraints abstract the key to comprehending the tectonic evolution of the himalaya is to understand the relationships between large scale faulting anatexis and inverted metamorphism the great number and variety of mechanisms that have been proposed to explain some or all of these features reflects the fact that fundamental constraints on such models have been slow in coming recent developments most notably in geophysical imaging and geochronology have been key to coalescing the results of varied himalayan investigations into constraints with which to test proposed evolutionary models these models fall into four general types 1 the inverted metamorphic sequences within the footwall of the himalayan thrust and adjacent hanging wall anatexis are spatially and temporally related by thrusting 2 thrusting results from anatexis 3 anatexis results from normal faulting and 4 apparent inverted metamorphism in the footwall of the himalayan thrust is produced by underplating of right way up metamorphic sequences we review a number of models and find that many are inconsistent with available constraints most notably the recognition that the exposed crustal melts and inverted metamorphic sequences not temporally related the generalization that appears to best explain the observed distribution of crustal melts and inverted metamorphic sequences is that due to specific petrological and tectonic controls episodic magmatism and out of sequence thrusting developed during continuous convergence juxtaposing allochthonous igneous and metamorphic rocks this coincidental juxtaposition has proven to be something of a red herring unduly influencing attention toward finding a causal relationship between anatexis and inverted metamorphism

a stable southern margin of asia during the cretaceous paleomagnetic constraints on the lhasa qiangtang collision and the maximum width of the neo tethys

new late cretaceous paleomagnetic data from volcanic rocks and red beds from the lhasa terrane and its implications for the paleolatitude of the southern margin of asia prior to the collision with india abstract in order to improve estimation of the paleolatitude of the southern margin of asia during the late cretaceous as well as the constraints on the intracontinental convergence within asia due to the india asia collision we performed a combined geochronological and paleomagnetic investigation of late cretaceous red bed sequences and intercalated volcanic rocks from the linzhou basin in the lhasa terrane 29 9 n 91 1 e the results of potassium argon k ar dating indicated that the basalt sequences are of late cretaceous age 68 75 ma stepwise thermal demagnetization successfully isolated high unblocking temperature characteristic directions from 10 red bed sites and 11 lava flow sites positive fold and reversal tests indicate a primary origin for the characteristic remanence of the red beds and basalt sites the consistent inclination recorded in the red beds and basalt lava flows indicates that no significant compaction related inclination shallowing has occurred in the red beds from the studied section the tilt corrected mean direction is d i 0 5 20 2 with 95 6 4 and n 21 sites corresponding to a paleopole at 70 5 n 269 6 e with a 95 4 9 the paleomagnetic results yield a paleolatitude of 10 4 4 9 n for the southern margin of asia in the late cretaceous compared with expected paleomagnetic directions from the tethyan himalaya and asia significant crustal shortening of 17 7 5 5 1960 610 km and 9 4 4 7 1040 520 km respectively may have occurred between the southern margin of asia and the tethyan himalaya and within asia since the late cretaceous based on previous cretaceous and paleocene data from the lhasa terrane the latitude of the southern margin of asia was located at a stable paleolatitudinal position 10 15 n during the cretaceous and paleocene

paleomagnetic tests of tectonic reconstructions of the india asia collision zone several solutions have been proposed to explain the long standing kinematic observation that postcollisional upper crustal shortening within the himalaya and asia is much less than the magnitude of india asia convergence here we implement these hypotheses in global plate reconstructions and test paleolatitudes predicted by the global apparent polar wander path against independent and the most robust paleomagnetic data our tests demonstrate that 1 reconstructed 600 750 km postcollisional intra asian shortening is a minimum value 2 a 52 ma collision age is only consistent with paleomagnetic data if intra asian shortening was 900 km a 56 58 ma collision age requires greater intra asian shortening 3 collision ages of 34 or 65 ma incorrectly predict late cretaceous and paleogene paleolatitudes of the tibetan himalaya th and 4 cretaceous counterclockwise rotation of india cannot explain the paleolatitudinal divergence between the th and india all hypotheses regardless of collision age require major cretaceous extension within greater india

palaeomagnetic results from the tibetan plateau palaeomagnetic measurements were carried out on 1325 oriented samples collected from 246 sites on a traverse of the tibetan plateau from lhasa to golmud in 1985 crossing the lhasa terrane qiangtang terrane and kunlun terrane high blocking temperature high coercivity statistically grouped magnetizations were isolated from the following units lhasa terrane cretaceous takena formation mid cretaceous nagqu volcanics mid cretaceous qelico volcanics qiangtang terrane norian batang group volcanics kimmeridgian yanshiping group paleocene to eocene fenghuoshan group kunlun terrane visean to namurian dagangou formation dykes of the triassic igneous province the triassic data from the kunlun terrane triassic and lower tertiary data from the qiangtang terrane and the cretaceous data from the lhasa terrane indicate palaeolatitudes ca 20 s of their present position within the eurasian frame of reference a possible interpretation is that the terranes successively accreted to eurasia and remained in the southern position until the convergence of india drove them northward via a process of tectonic shortening and or displacement of continental crust the carboniferous data from the kunlun terrane are consistent with moderate southern hemisphere latitude well separated from eurasia which was in the northern hemisphere at this time implying the existence of ocean crust between these blocks during the carboniferous

paleomagnetism indicates no neogene vertical axis rotations of the northeastern tibetan plateau 1 paleomagnetic data were obtained from 108 paleomagnetic sites collected in cretaceous to tertiary red beds from seven localities distributed in three general regions adjacent to the altyn tagh fault at the northern edge of the tibetan plateau in the hexi corridor 12 sites in oligocene strata at yaoquanzi 39 97 n 97 68 e yield a mean paleomagnetic direction i 33 0 d 8 6 95 6 0 with concordant declination and 36 sites in early cretaceous mudstones of the longshou shan 39 09 n 100 50 e provide a concordant paleomagnetic direction i 53 4 d 6 8 95 3 8 across the nan shan fold thrust belt miocene paleomagnetic directions from 39 sites distributed among four localities have a concordant mean declination i 40 6 d 7 2 95 5 8 in the altyn tagh range 21 sites in oligocene strata at xorkoli 38 93 n 91 43 e yield a paleomagnetic direction with concordant declination i 49 7 d 5 0 95 5 4 these results combined with existing regional paleomagnetic data indicate that 1 the hexi corridor along with the north china block has not undergone tectonic vertical axis rotation since at least early cretaceous time and is separated from the xining lanzhou basin and eastern tibet by an important post early cretaceous tectonic boundary 2 the nan shan fold thrust belt and the qaidam basin have not experienced wholesale vertical axis rotation during neogene time and 3 absence of vertical axis rotations in areas adjacent to the altyn tagh fault indicates that sinistral shear strain between the tarim basin and the northern tibetan plateau is concentrated on the fault these results are consistent with asian tectonic models that combine distributed lithospheric deformation and thickening with narrow and weak shear zones

palaeomagnetism and k ar ages of neogene rocks of northern taiwan tectonics of the arc junction of ryukyu and luzon arcs summary more than 100 block samples were collected for palaeomagnetic and geochronological studies from 14 sites in miocene and plio pleistocene volcanic rocks in the western foothills province of northern taiwan in an attempt to understand the tectonic evolution of the junction of the ryukyu and the luzon arcs potassiumargon dating yields ages of 7 9 f 0 5 ma and 10 1 f 0 6 ma for two samples from the miocene chiopanshan volcanic series and 1 2 f 0 1 ma for a sample from the plio pleistocene chilung volcanic group the palaeomagnetic direction from seven sites of the miocene volcanics shows no deflection of declination value d 5 y i 43 8 cxqs 14 0 the direction from the volcanic rock of 1 2 ma is d 2 5 and i 53 3 the reliability of the direction is ascertained through thermal demagnetization positive fold test and the presence of normal and reversed polarities comparison of neogene palaeopoles between northern taiwan and china indicates that little relative tectonic movement has occurred in northern taiwan since 20ma these results establish that the arcuate shape of geological belts in northern taiwan is not due to tectonic bending but to an inherited geometry reconstruction of the 10ma palaeoposition of taiwan and the south ryukyu arc implies that the cusp shape at the arc junction has already existed prior to 10 ma

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

paleomagnetism and u pb zircon geochronology of lower cretaceous lava flows from the western lhasa terrane new constraints on the india asia collision process and intracontinental deformation within asia to better understand the india asia collisional process and intracontinental deformation within asia a paleomagnetic study has been conducted on the qushenla formation lava flows dated at 132 120 ma from the yanhu area in the western lhasa terrane stepwise thermal demagnetization isolates stable characteristic remanent magnetizations which include dual polarity and pass fold tests at a 99 confidence level indicating primary magnetizations the tilt corrected site mean direction for 51 sites is d 28 2 i 34 5 and k 74 3 with 95 2 3 corresponding to a paleopole at 61 4 n 192 9 e a95 2 1 our new paleomagnetic data combined with previous cretaceous volcanic paleomagnetic data from the lhasa terrane show that the precollisional southern margin of asia was at 16 8 n with a relatively e w alignment and that the lhasa terrane did not experience significantly discrepant north south movement although local vertical axis rotations did take place after the cretaceous comparison with the apparent polar wander paths apwps of india and the cretaceous paleocene paleopoles of the tethyan himalaya shows that the india asia collision was at 54 3 ma and the greater india during the paleocene had a large northern extent of 2000 km 18 1 beyond its present northern margin comparison with the cretaceous eurasian apwp indicates that a latitudinal convergence of 1000 km has taken place between the lhasa terrane and eurasia since the india asia collision the amount of north south shortening deduced from cretaceous paleomagnetic data is consistent with that accommodated by the cenozoic fold and thrust belts between the lhasa terrane and the hexi corridor

paleomagnetic and paleointensity study of an oligocene miocene lava sequence from the hannuoba basalts in northern china abstract a combined geochronologic paleomagnetic and paleointensity investigation has been performed on a lava sequence from the hannuoba basalts in northern china potassium argon k ar dating indicates that the lava sequence is of late oligocene to middle miocene age 24 15 ma rock magnetic measurements of thermomagnetic curves hysteresis properties saturation isothermal remanent magnetization sirm and back field demagnetization on representative samples from each lava flow in conjunction with reflected light microscopy observations show the primary carriers of remanence to be fine grained pseudo single domain titanomagnetite with various titanium contents stepwise thermal and alternating field demagnetization successfully isolated stable characteristic remanent magnetization from 40 out of the 43 lava flows which record 24 paleomagnetically independent directions data excluding directions regarded as abnormal transitional yield a mean direction of d i 6 8 56 3 with 95 5 2 and 51 6 corresponding to a paleomagnetic pole at 232 3 e 83 7 n with dp 5 4 and dm 7 5 consistent with previously published paleomagnetic poles for the north china block the modified thellier method with systematic partial thermoremanent magnetization ptrm checks and ptrm tail checks was conducted on 72 selected samples reliable paleointensity estimates from four lava flows yield a mean virtual dipole moment of 5 6 1 6 1022 a m2 suggesting moderately low intensity of the geomagnetic field this is in good accord with reliable paleointensity datasets for the late oligocene to middle miocene 30 15 ma but lower than the average for middle miocene to pliocene age 15 2 ma it thus appears that at some point in the mid miocene there was a transition where the field intensity changes between these two states although many more data are needed to confirm this

new tertiary paleomagnetic poles from mongolia and siberia at 40 30 20 and 13 ma clues on the inclination shallowing problem in central asia we report results of a paleomagnetic study of 490 cores from 59 sites corresponding to 52 distinct basaltic flows from mongolia and siberia khaton sudal 39 4 ma 44 5 n 101 4 e taatsyn gol 1 31 5 ma 45 4 n 101 3 e 2 28 0 ma 45 5 n 101 1 e ust bokson 19 9 ma 52 1 n 100 3 e and taatsyn gol 3 12 7 ma 45 5 n 101 0 e stepwise thermal and alternating field demagnetizations isolated a stable high temperature component htc of magnetization in most specimens which we interpret as the primary magnetization of these basaltic lava flows the four corresponding paleopoles appear consistent with coeval paleopoles from other asian effusive formations however except for the 12 7 ma paleopole the paleopoles are systematically far sided from the european apparent polar wander path apwp with respect to site locations corresponding to anomalously shallow inclinations in tertiary asian effusive formations in the hypothesis of a dipolar magnetic field in the tertiary this indicates a 1000 1500 km position of the siberia craton and amuria block farther south than expected at 40 and 30 ma tectonically this interpretation implies decoupling and relative rotations between the western and eastern parts of eurasia between the cretaceous and present we show that if siberia were located more to the south the 15 20 paleolatitude anomaly generally observed in sedimentary formations from central asia reduces to a more reasonable average value of 7 which could result from the superimposition of shallowing mechanisms due to sedimentary processes and northward motion of asian blocks under the effect of ongoing penetration of india into eurasia in the tertiary

cretaceous tertiary shortening basin development and volcanism in central tibet the geologic map pattern of the qiangtang terrane in central tibet defines a 600 km long and up to 270 km wide east plunging structural culmination it is characterized by early mesozoic blueschist bearing melange and upper paleozoic strata in the core and mainly triassic jurassic strata along the limbs in the western qiangtang terrane 84 e the culmination is unconformably overlain by weakly deformed mid cretaceous volcanic flows and tuffs along the bangong suture to the south 32 n 84 e mid cretaceous nonmarine red beds and volcanic rocks lie unconformably on jurassic suture zone rocks these relationships demonstrate that west central tibet was above sea level during the mid cretaceous and experienced significant denudation prior to mid cretaceous time growth of the qiangtang culmination is inferred to have initiated during southward emplacement of a thrust sheet of early mesozoic melange and upper paleozoic strata during the early cretaceous lhasa qiangtang collision the north south width of the inferred thrust sheet provides a minimum slip estimate of 150 km at 84 e decreasing eastward to 70 km at 87 e paleogene deformation in the qiangtang terrane is characterized by widely distributed mainly north dipping thrust faults that cut eocene oligocene red beds and volcanic rocks in their footwall along the bangong suture the north dipping shiquanhe gaize amdo thrust system cuts 64 and 43 m y old volcanic tuffs in its footwall and accommodated 40 km of post mid cretaceous shortening the tertiary south dipping gaize siling co backthrust bounds the southern margin of the bangong suture and marks the northernmost limit of mid cretaceous marine strata in central tibet cretaceous deformation and denudation in central tibet is attributed to northward underthrusting of the lhasa terrane beneath the qiangtang terrane along the bangong suture this model implies that 1 cretaceous strata along the bangong suture and in the northern lhasa terrane were deposited in a flexural foreland basin system and derived at least in part from the qiangtang terrane and 2 the central tibetan crust was thickened substantially prior to the indo asian collision although its magnitude is poorly known tertiary shortening in the qiangtang terrane is more prevalent than in the lhasa terrane this difference may be attributed to the presence of underthrust melange in the deeper central tibetan crust which would have made it weaker than the lhasa terrane during the indo asian collision

combined paleomagnetic and geochronological study on cretaceous strata of the qiangtang terrane central tibet abstract a combined paleomagnetic and geochronological investigation has been performed on cretaceous rocks in southern qiangtang terrane 32 5 n 84 3 e near gerze central tibetan plateau a total of 14 sites of volcanic rocks and 22 sites of red beds have been sampled our new u pb geochronologic study of zircons dates the volcanic rocks at 103 8 0 46 ma early cretaceous while the red beds belong to the late cretaceous rock magnetic experiments suggest that magnetite and hematite are the main magnetic carriers after removing a low temperature component of viscous magnetic remanence stable characteristic remanent magnetization chrm was isolated successfully from all the sites by stepwise thermal demagnetization the tilt corrected mean direction from the 14 lava sites is d 348 0 i 47 3 k 51 0 95 5 6 corresponding to a paleopole at 79 3 n 339 8 e a 95 5 7 and yielding a paleolatitude of 29 3 5 7 n for the study area the chrm directions isolated from the volcanic rocks pass a fold test at 95 confidence suggesting a primary origin the volcanic data appear to have effectively averaged out secular variation as indicated by both geological evidence and results from analyzing the virtual geomagnetic pole vgp scatter the mean inclination from the late cretaceous red beds however is 13 1 shallower than that of the 100 ma volcanic rocks after performing an elongation inclination analysis on 174 samples of the red beds a mean inclination of 47 9 with 95 confidence limits between 41 9 and 54 3 is obtained which is consistent with the mean inclination of the volcanic rocks the site mean direction of the late cretaceous red beds after tilt correction and inclination shallowing correction is d 312 6 i 47 7 k 109 7 95 3 0 n 22 sites corresponding to a paleopole at 49 2 n 1 9 e a 95 3 2 yielding a paleolatitude of 28 7 3 2 n for the study area the chrm of the red beds also passes a fold test at 99 confidence indicating a primary origin comparing the paleolatitude of the qiangtang terrane with the stable asia there is no significant difference between our sampling location in the southern qiangtang terrane and the stable asia during 100 ma and late cretaceous our results together with the high quality data previously published suggest that an 550 km n s convergence between the qiangtang and lhasa terranes happened after 100 ma comparison of the mean directions with expected directions from the stable asia indicates that the gerze area had experienced a significant counterclockwise rotation after 100 ma which is most likely caused by 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