meso tethyan oceanic sutures and their deformation abstract on the basis of comparative stratigraphie and paleontologic analysis supported by some key paleomagnetic data and interpretations it is shown that during the mesozoic and the cenozoic the eurasian continent grew by accretion of microcontinents these microcontinents separated basins with oceanic crust from the main ocean during the late cretaceous and the early cenozoic the collision of the microcontinents with eurasia resulted in the closure of the basins and meso tethyan oceanic sutures originated in the region under consideration from the carpathians to tibet there are two main meso tethyan sutures the carpathian lesser caucasus and the afghan tibet suture the above mentioned main structures also had branches which remained as sutures of small basins the kamennopotock interpontide nain baft basins and others in the west carpathians the carpathian lesser caucasus suture is overlain by a widespread gemeric tatric allochton from the west carpathians the suture passes through the pannonian basin into the vardar ophiolite zone and farther to the ophiolites of the izmir ankara zone being displaced along the north anatolian right lateral strike slip fault the main suture passes from the eastern pontides into the lesser caucasus where it is marked by ophiolites of the amasia area the shirak bozum and zangezur ridges sunsequently it can be observed through the iranian qara dagh mountains to lake urumiyeh and the north anatolian strike slip fault being again shifted along the strike slip fault the carpathian lesser caucasus meso tethyan suture ends in the western zagros near the cenozoic neo tethyan suture the afghan tibet meso tethyan suture is situated in the pamirs in the rushan pshart zone the east prolongation of the suture has been displaced along the pamir karakorum right lateral strike slip fault in tibet west of the pamirs this suture is also displaced along strike slip faults and continues in the farahrud zone in afghanistan and than passes through the zabol baluch and daz murian ophiolite zones and approaches the neo tethyan suture the paleomagnetic data allow us to reconstruct the location of those sutures in the late cretaceous early cenozoic if one compares the present position of the carpathian lesser caucasus and afghan tibet meso tethyan sutures with the late cretaceous early cenozoic reconstructions of these sutures it is possible to trace the inner deformation of the alpine belt according to these data the carpathian lesser caucasus suture moved northwards over 1200 km ahead of the arabian turkish syntaxis with the displacement direction across the fold belt at the border of the dinarides and hellenides the displacement decreases to 400 500 km and its direction is along the fold belt the amplitude of the suture displacement increases in the pannonian region and again decreases towards the eastern alps the displacement of the afghan tibet suture was 2000 km during the alpine deformation a great ensemble of cenozoic nappes folds and structural arcs of the pamir punjab and arabian turkish syntaxes and carpathian loop is the result of lateral shortening of the alpine belt through the collision

inclination variation in the late jurassic to eocene red beds from southeast asia lithological to locality scale approach summary shallowpalaeomagneticinclinationshavebeenfrequentlyreportedfromtheredbedsofcentral and southeast asia to trace the origin of this phenomenon inclination variation in southeast asia is examined on lithological to locality scale lithological aspect of this variation has been studied using the early cretaceous bailong and cangxi formations of the bazhong area 32 1 n 106 7 e the northern sichuan basin samples from 36 sites consisting 18 pairs of successive sandstones and mudstones layers were collected for this purpose from a synclinal structure stepwise thermal demagnetization of most samples revealed the presence of stable characteristic remanent magnetization which is generally unblocked by 680 c positive fold and reversal tests suggest a primary origin for this component yielding the early cretaceous palaeomagnetic direction of declination inclination 20 9 26 5 ks 37 2 95 4 4 n 30 10 pairs of sandstone and mudstone layers show almost identical inclinations isandstones 23 3 3 7 and imudstones 24 7 2 4 but 27 shallower than that expected from the eurasian apparent polar wander paths apwps indicating that no lithological variation in inclination has occurred location wise variation in inclination shallowing is examined through palaeomagnetic data from late jurassic to eocene red beds distributed around southeast asia based on these investigations no inclination shallowing is observed in the eastern part of the south china block scb whereas large degree of shallowing is observed in the sichuan and xining ninhe basins variation in inclination shallowing from one sedimentary basin to another could probably be caused by changes in the depositional environment the eastern part of the scb where no inclination shallowing have been observed is characterized by basin and range type tectono geological setting in contrast the foreland basins are bounded to the north by east west striking high altitude orogenic belt swift developmentofthesebasinsasaresultofflexuralsubsidenceisthemostlikelyreasonforinclination shallowing

change from late tertiary compression to quaternary extension in southern tibet during the india asia collision field studies in southern tibet show that subsequent to folding resulting from the india asia collision 50 40 ma reverse faults and backthrusts formed as a result of nne ssw and nnw sse trending compressions they affect conglomerates of oligocene miocene age and are probably contemporaneous with shearing on the himalayan main central thrust and folding of paleogene basins along the northern border of southern tibet 25 16 ma rift systems postdate compressional structures they were active during the pleistocene and probably during the pliocene 5 2 ma change from a compressional to an extensional regime in tibet probably occurred during the late miocene 10 5 ma this change is tentatively correlated with uplift inferred by chinese workers from paleogene to present day variations of the tibetan vegetation recent and active faulting in the high plateau results from a wnw ese striking extension this is in agreement with tibetan focal mechanisms the himalayan lowlands are subjected to a nne ssw trending compression as demonstrated by superficial deformations and seismicity therefore the horizontal maximum compressive stress trajectory is roughly parallel to india asia convergence hmax being 1 in the lowlands and 2 in the high plateau this change results from the increase of the vertical principal stress value due to the weight of the elevated mass zz is 3 in the lowlands and 1 in the high plateau thus the third principal stress axis is 2 in the lowlands and 3 in the high plateau allowing extension to occur in the wnw ese direction extensional tectonics are significantly different in southern and northern tibet southern tibet being constrained between the himalayan syntaxes the e w extensional principal deviatoric stress value 3 is of rather small magnitude and the n s trending 2 value is extensional as demonstrated by analysis of recent faults normal components prevail along all the azimuths in the field this favors formation of rift systems of large axial extension on the contrary north of the himalayan syntaxes northern tibet flows easily eastward the e w extensional principal deviatoric stress value 3 is of higher magnitude and the n s stress 2 is compressional as shown by inversion of focal mechanism data this favors strike slip components on faults which deviate from the n s direction as expressed in the field by shorter n s trending graben linked with normal strike slip faults these faults permit some n s shortening in northern tibet without thickening in response to india asia collision in high plateaus which are subjected to lateral compression such as the andean altiplano and tibet the extensional direction is basically controlled by the convergence direction being roughly orthogonal to the latter

seismic and aseismic weakening effects in transtension field and microstructural observations on the mechanics and architecture of a large fault zone in se tibet

magmatism and metamorphism in the lhasa block tibet a geochronological study u pb systematics in small fractions of zircon monazite and sphene 5 to 20 grains each have been analyzed in order to decipher the age and origin of magmatism in the lhasa block and to date the tectono metamorphic events additionally we analyzed a few minerals for rb sr the results show that the granites and an ignimbrite were formed essentially through anatexis of continental crust the magma source regions of which consisted of palaeozoic to precambrian materials up to 2 b y in age the following episodes can be distinguished 1 an early tertiary plutonism volcanism in the southern lhasa block yangbajing region strongly related to intracontinental tectono metamorphism 2 a late jurassic to early cretaceous plutonism in the central and northern lhasa block and 3 a middle jurassic low grade metamorphism affecting the basement at the northern margin 171 6 m y ago the early tertiary magmatism and metamorphism in the south are probably a direct result of tethys subduction indian plate

the xigaze ophiolite tibet a peculiar oceanic lithosphere the xigaze ophiolite which outcrops along the yarlung zangbo river southern tibet locally displays a complete ophiolitic sequence from marine sedimentary cover over basaltic volcanics to the north to fresh cr diopside rich harzburgites to the south in contrast with other ophiolites the mafic part of the sequence is particularly thin it is almost devoid of cumulate gabbros consisting of a diabase sill complex covered by lava flows or pillow lavas the ultramafic unit is poor in residual harzburgites and dunites and consists dominantly of fresh cr diopside rich harzburgites the origin of this peculiar ophiolite of oceanic crust is discussed

what was the paleogene latitude of the lhasa terrane a reassessment of the geochronology and paleomagnetism of linzizong volcanic rocks linzhou basin tibet the paleogene latitude of the lhasa terrane southern tibet can constrain the age of the onset of the india asia collision estimates for this latitude however vary from 5 n to 30 n and thus here we reassess the geochronology and paleomagnetism of paleogene volcanic rocks from the linzizong group in the linzhou basin the lower and upper parts of the section previously yielded particularly conflicting ages and paleolatitudes we report consistent 40ar 39ar and u pb zircon dates of 52 ma for the upper linzizong and 40ar 39ar dates 51 ma from the lower linzizong are significantly younger than u pb zircon dates 64 63 ma suggesting that the lower linzizong was thermally and or chemically reset paleomagnetic results from 24 sites in lower linzizong confirm a low apparent paleolatitude of 5 n compared to the upper part 20 n and to underlying cretaceous strata 20 n detailed rock magnetic analyses end member modeling of magnetic components and petrography from the lower and upper linzizong indicate widespread secondary hematite in the lower linzizong whereas hematite is rare in upper linzizong volcanic rocks of the lower linzizong have been hydrothermally chemically remagnetized whereas the upper linzizong retains a primary remanence we suggest that remagnetization was induced by acquisition of chemical and thermoviscous remanent magnetizations such that the shallow inclinations are an artifact of a tilt correction applied to a secondary remanence in lower linzizong we estimate that the paleogene latitude of lhasa terrane was 20 4 n consistent with previous results suggesting that india asia collision likely took place by 52 ma at 20 n

an early pliocene thermal disturbance of the main central thrust central nepal implications for himalayan tectonics since the beginning of the collision between india and asia at about 50 ma the convergence in the himalaya has largely been taken up along major thrust zones in this study samples of the lesser himalaya formations up to 10 km below the main central thrust mct and the greater himalaya sequence up to 12 km above the mct have been analyzed by the 40ar 39ar and u pb methods to investigate the thermal history of the mct the ages can be summarized as follows 1 the youngest ages from muscovites 3 1 ma biotites 3 4 ma and hornblendes 4 1 ma all come from within 1 km of the mct 2 there is a marked asymmetry of ages between the footwall and the hanging wall of the mct the maximum mica age in the hanging wall greater himalaya sequence is 13 ma whereas a muscovite 5 km below the mct in the footwall has an age spectrum with a gradient from 400 to 1400 ma 3 five k feldspars from 6 2 to 11 9 km above the mct all give minimum ages in the range 3 0 6 4 ma and 4 a 206pb 238u age on a brannerite from the greater himalayan sequence is 4 8 ma structural and petrologic observations preclude the possibility that this age distance distribution reflects faulting within the greater himalaya sequence and production of the necessary thermal energy by shear heating requires unrealistically high shear stresses infiltration of hot fluids through the mct zone appears to be the best hypothesis to explain these data simple numerical simulations which account for heat transfer by advection within the fluid flow zone and by conduction outside it indicate that the observed age distribution could have been produced by infiltration of hot fluids through the mct zone at circa 5 to 4 ma within the following range of conditions the fluids heated rocks to temperatures in the range 470 to 510 c for less than 1 million years in a region narrower than the entire mct zone the temperature of the thermal disturbance inferred from the 40ar 39ar data is consistent with petrologic data from albitic alteration assemblages in the greater himalaya sequence and the u pb age of the brannerite is similar to the youngest 40ar 39ar hornblende age this scenario hydrothermal heating culminating at about 4 ma is similar to the model of le fort 1981 for the generation of the high himalayan leucogranites in the late oligocene early miocene by dehydration of the footwall rocks of an active thrust in the younger instance we interpret that fluids derived from the footwall rocks of the main boundary thrust migrated upward through the lesser himalaya formations and were subsequently channeled along the mct producing the thermal disturbance

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

unusual resinite rich coals found in northeastern greenland and along the norwegian coast petrographic and geochemical composition abstract middle jurassic coals unusually rich in resinite liptinite are known in northeastern greenland from a few outcrops and from several erratic pieces in addition since the mid 1800 s a number of loose pieces of resinite rich coal the so called kvaefjord coal have been found along the norwegian coast in or close to the shore zone in places where it would have been impossible for the coal to originate from passing ships this study presents a comprehensive organic petrographic and geochemical investigation of two new pieces of resinite rich coal found on store koldewey in northeastern greenland a resinite rich core sample from the norwegian island andoya and two pieces of the norwegian kvaefjord coal the coals are thermally immature and contain an extraordinary high content of liptinite and in particular resinite 34 2 93 6 vol the two coal pieces from store koldewey and the core sample from andoya are compositionally comparable to the resinite liptinite rich middle jurassic coals that crop out on hochstetter forland and kuhn o in northeastern greenland the two loose pieces of kvaefjord coal are exceptionally rich in resinite and appear to be slightly different from the other coals albeit they are compositionally related the documentation of in situ resinite liptinite rich coals on andoya and the finding of loose pieces along the atlantic coast of norway indicate that formation of these unusual coals occurred regionally in the pre rift north atlantic region probably in peat forming mires with a yet unknown strongly resin producing vegetation established at the margins of a gradually northward prograding embayment formed during middle jurassic rifting between greenland and norway the unique maceral composition makes the coals excellent potential petroleum source rocks with hi values 400 mg hc g toc but the sketchy knowledge of the regional distribution of the coals combined with the probably restricted areal extent of the coal deposits limits their significance as an additional source rock in the north atlantic area

widespread late mesozoic to recent remagnetization of paleozoic and lower triassic sedimentary rocks from south china kent d v zeng x zhang w y and opdyke n d 1987 widespread late mesozoic to recent remagnetization of paleozoic and lower triassic sedimentary rocks from south china in d v kent and m krs editors laurasian paleomagnetism and tectonics tectonophysics 139 133 143 paleomagnetic results are described from eighteen formations sampled in the nanjing jiangsu province and yichang hubei province areas of south china the marine platform carbonates and elastics range in age from early cambrian to early triassic progressive alternating field and or thermal demagnetization analyses reveal predominantly single component magnetizations sometimes of both normal and reverse polarity these magnetizations isolated in 457 of the 549 samples studied typically have directions in geographic coordinates coincident with late mesozoic to recent fields a secondary origin for the magnetizations is supported by negative fold tests in several units a lower carboniferous and possibly a lower triassic limestone may have escaped the pervasive remagnetization if the remanences isolated are indeed prefolding a large counterclockwise tectonic rotation of the nanjing area is implied by comparison with available carboniferous and triassic paleopoles reported from elsewhere in south china this rotation may have been associated with mesozoic left lateral motion on the tancheng lujiang fault zone

palaeomagnetic data about southern tibet xizang i the cretaceous formations of the lhasa block summary a palaeomagnetic study of middle to late cretaceous redbeds from linzhou basin lhasa block north of the yarlung zangbo suture zone gives a stable palaeomagnetic direction of magnetization with a positive fold test six sites 57 samples d 333 i 38 k 78 95 8 pole 64 n 348 e we discuss the problem of a possible remagnetization but consider that this direction of magnetization gives a good approximation for the palaeolatitude of the lhasa block during middle to late cretaceous time results from more recent tibetan formations are also presented late cretaceous to palaeocene sediments and volcanics give a lower palaeolatitude of 10 n and but more recent andesites have emplaced about 30 n close to the present day latitude an interpretation is proposed whereby the lhasa block which was a part of asia in the early cretaceous has undergone first a southward motion accompanied by an anticlockwise rotation and then after the palaeocene a northward motion under the constraint of the colliding india

paleomagnetic constraints on the late cretaceous and cenozoic tectonics of southeastern asia abstract many features of the cenozoic tectonic history of central and southeastern asia can be understood as direct consequences of the thrust and penetration of india into asia recent indentation experiments with plasticine tapponnier et al 7 have extended this idea and have led to the prediction of a pattern of large rotations and displacements of continental blocks that can be tested by paleomagnetism the available cretaceous and cenozoic paleomagnetic data from this part of the world have been reviewed and a new apwp for eurasia has been constructed for reference the negligible rotation of south china and large clockwise rotation of indochina are consistent with the model i e with an history of large scale left lateral strike slip motion along the altyn tagh and red river faults data from malaya and borneo can be reconciled with the model although in a less straightforward fashion the large counter clockwise rotation of south tibet implies that it rotated in sympathy with india during the collision and suggests that future indentation experiments should include this feature finally a middle cretaceous reconstruction of the south margin of asia is proposed one interesting result is the restored continuity of geological features in tibet and indochina with active subduction of oceanic indian plate crust taking place to the south at subtropical latitudes

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

palaeomagnetism and k ar dating of pleistocene volcanic rocks along the altyn tagh fault northern border of tibet summary two basaltic andesite lava flows were sampled in the puru area 36 15 n 81 30 e north of the altyn tagh fault the sampling sites were distributed between 13 km and 18km from the fault the two lavas are dated by k ar methods to be 1 05 f 0 11 and 1 44 f 0 06 ma their palaeomagnetic directions show reversed polarity of the matuyama chron declination values for nine sampling sites on the upper lava flow more than 14 km from the fault are concordant with little variance d 166 3 i 50 8 2 2 but declination at a site nearer the fault 13 km from it d 173 7 displays a counter clockwise deflection of 7 4 f 5 2 from that of the other sites this palaeomagnetic rotation is ascribed to tectonic rotation due to left lateral slip on the altyn tagh fault during the last 1 0myr the area beyond 13 km from the fault however does not provide any evidence for the large rotation 60 expected from the known strike slip movement of 30 mm yr this implies that little deformation penetrates deep into the tarim craton from the fault the lack of deformation of features along the altyn tagh fault is explained by the non rigid behaviour of the narrow zone of the continental lithosphere near the fault and or by low stress acting on the fault

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

position of the lhasa block south tibet during the late cretaceous the latitude of the lhasa block xizang south tibet during the late cretaceous early tertiary is of prime importance for understanding the collision of india and eurasia and the mechanism of the formation of the qinghai xizang plateau during the first field expedition of the french chinese collaboration several geological formations have been sampled in southern tibet between lhasa 29 40 n 91 09 e and dingri 28 35 n 86 38 e we present here the first results obtained for middle cretaceous to palaeocene red beds from the lhasa block north of the yarlung zang bo suture zone recognized as the main suture between the indian and the eurasian plates they indicate a palaeolatitude of the lhasa block of 20 n during the late cretaceous while india was situated 30 s

formation and evolution of strike slip faults rifts and basins during the india asia collision an experimental approach the processes which have governed the formation and evolution of large tertiary strike slip faults during the penetration of india into eastern asia are investigated by means of plane strain indentation experiments on layered plasticine models the steady state deformation of plasticine obeys a power creep flow law e c t n the stress exponent n is between 6 and 9 at 25 c uniaxial plane strain tests on cubic specimens show that the growth of faults in layered plasticine results from strain softening a process observed for strain rates ranging from 3 5 10 5 to 3 6 10 3 s 1 fault or shear zones form after only 7 10 bulk strain subsequent deformation is controlled by the geometry of the fault pattern rather than the physical properties of the plasticine a series of nine plane strain indentation experiments shows the influence of boundary conditions as well as that of the internal structure of the plasticine model on the faulting sequence the ubiquity of strain softening in experimental deformation of a variety of rocks as well as the widespread occurrence of shear zones in nature suggest that long term deformation of the continental lithosphere may also be primarily influenced by the geometry of large faults which rapidly develop with increasing strain the deformation and faulting sequence observed in the plasticine indentation experiments may thus be compared to collision induced strikeslip faulting in asia particularly to total offsets and rates of movements on the faults the experiments simulate the evolution of the western ends of the strike slip faults which have probably been analogous to trench fault fault triple junctions the experiments also illustrate mechanisms for the formation of extensional basins such as the south china sea north china basin and andaman sea near active continental margins the basins which appear to absorb terminal offsets along major strike slip faults near such margins may result from mismatch between the sharply angular shape of the deformed continental edge and the more regularly curved trench along which the smoothly flexed oceanic lithosphere subducts the existence of distinct phases of strike slip extrusion corroborates the idea that the discontinuities in time which typify intracontinental tectonics and orogenic cycles may often result from strain localization and the ensuing discontinuous non steady state deformation of the continental lithosphere

tectonic evolution of the high himalaya in upper lahul nw himalaya india the upper lahul region in the nw himalaya is located in the transition zone between the high himalayan crystalline hhc to the sw and the tethyan zone sedimentary series to the ne the tectonic evolution of these domains during the himalayan orogeny is the consequence of a succession of five deformation events an early dl phase corresponds to synmetamorphic ne verging folding this deformation created the tandi syncline which consists of permian to jurassic tethyan metasediments cropping out in the core of a large scale synformal fold within the hhc paragneiss this tectonic event is interpreted as related to a ne directed nappe stacking shikar beh nappe probably during the late eocene to the early oligocene a subsequent d2a phase caused sw verging folding in the hhc this deformation is interpreted as contemporaneous with late oligocene to early miocene sw directed thrusting along the main central thrust in the tethyan zone a d2b phase is marked by a decollement thrust a system of reverse faults and gentle folds associated with sw directed tectonic movements this deformation is related to an imbricate structure characteristic of a shallow structural level and developed in the frontal part of a nappe affecting the tethyan zone units of se zanskar nyimaling tsarap nappe a later d3 phase generated the chandra dextral shear zone cdsz a large scale ductile dextral strike slip shear zone located in the transition zone between the hhc and the tethyan himalaya the cdsz most likely represents a part of a system of early miocene extensional and or dextral strike slip shear zones observed at the hhc tethyan zone contact along the entire himalaya a final d4 phase induced large scale doming and ne verging back folding

paleomagnetic and plate tectonic constraints on the movement of tibet abstract the paleomagnetic results from tibet north of the yarlung zang bo suture zone show that tibet was at about 15 20 n in middle cretaceous time it then moved south down to 7 10 n in the late cretaceous paleogene the oceanic crust of the xigaze ophiolites was magnetized at 13 n but thereafter migrated further south this movement is compared with the relative movement of india and asia as deduced from magnetic anomalies and paleomagnetism experimental models on deformation help us to explain how tibet moved during the late cretaceous under the constraint of the africa arabia indenter and during the upper tertiary under the constraint of the indian indenter

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

structure and evolution of the himalaya tibet orogenic belt the 1981 french chinese expedition to tibet focused on the lhasa block extending earlier coverage 400 km north of the tsangpo suture the lhasa block stood between 10 and 15 n latitude over most of the upper cretaceous and eocene and if gondwanian in origin had detached from gondwana by early permian seismic profiles reveal a complex moho topography resulting both from multiple continental thrusting and large scale strike slip faulting subduction related granitoids representing mixtures of mantle and crustal components and anatectic granitoids have been analysed and dated this study emphasizes the role of smaller blocks in the accretion of the continental mosaic

late cretaceous to middle tertiary basin evolution in the central tibetan plateau changing environments in response to tectonic partitioning aridification and regional elevation gain located in the bangong suture zone between the lhasa and qiangtang terranes of central tibet the nima basin records cretaceous late miocene sediment accumulation during a period of drastically changing paleogeography and paleoelevation the bangong suture zone originally formed during late jurassic early cretaceous time as the lhasa terrane collided with the qiang tang terrane during early to middle cretaceous time the region lay at northern near equatorial paleolatitudes near sea level by aptian time the nima basin was above sea level and was strongly influenced by nearby volcanic activity and crustal shortening in the reactivated bangong suture zone in the southern nima basin an 50 m y late cretaceous eocene depositional hiatus correlates with major crustal shortening and ensuing voluminous ignimbrite eruptions in the lhasa terrane in the northern nima basin deposition continued during latest cretaceous time recording arid paleoclimate in evaporitic lacustrine and eolian dunefield deposits by oligocene time the nima basin comprised two independent depocenters that accumulated coarse grained alluvial fluvial lacustrine evaporitic and fan delta deposits in close association with reactivated thrusts in the bangong suture zone carbon and oxygen isotope data from oligocene paleosol carbonate reported elsewhere indicate that regional paleoelevation during the late oligocene was 4 6 km as it is today overall the depositional record of the nima basin combined with ongoing structural and geochronologic studies demonstrates that the bangong suture zone was reactivated during middle cretaceous and middle tertiary time that the intervening 50 m y interval was a time of regional upper crustal shortening in the lhasa terrane followed by regional ignimbrite eruptions and that arid paleoclimate and high paleoelevation were established by the late cretaceous and late oligocene respectively within the context of other data sets from the lhasa terrane the record of deposition in the nima basin is consistent with low angle subduction of neotethyan oceanic lithosphere and reactivation of the bangong suture zone during the early cretaceous followed by shortening within the gangdese retroarc and northern lhasa terrane thrust belts during the late cretaceous lithospheric delamination dripping and regional magmatic flare up during latest cretaceous through early tertiary time and underthrusting of indian lower crust and lithosphere as far north as the bangong suture zone during late oligocene time

paleomagnetism of the moenkopi formation colorado plateau basis for long term model of acquisition of chemical remanent magnetism in red beds paleomagnetic studies of moenkopi samples from several sites on the colorado plateau including a complete 427 m section at sinbad valley western colorado reveal the following 1 the presence of both normal and reversed triassic components in many of the same samples at many localities 2 the close correlation of the magnetic polarity and intensity of magnetic remanence with the lithologic characteristics in the sinbad valley section 3 a streaked pattern of the remanence directions in any one member in the sinbad valley section similar to that expected from polar wander 4 a lack of progressive change from the base upward and 5 the common intermediate remanence directions of weak intensity suggesting the presence of two or more nearly balanced anti parallel components throughout the formation the results of our investigation combined with previously reported petrographic and paleomagnetic studies of the moenkopi formation have led us to conclude that the principal remanence carried by the formation is chemical remanent magnetism crm acquired diagenetically over a geologically long time interval from the available data we have formulated the following model to explain how the crm was acquired through natural processes acting intermittently at geologically reasonable rates when initially deposited the moenkopi sediments consisted primarily of first cycle detritus containing a few percent of iron bearing accessory minerals such as biotite hornblende magnetite and ilmenite at some time after deposition under favorable subsurface conditions intrastratal alteration began involving breakdown of iron bearing silicate and oxide minerals partial replacement of some iron rich silicate grains by hematite martitization of magnetite growth of hematite pods and blebs in ilmenite and biotite and precipitation of crystalline hematite in both primary and secondary voids the authigenesis spanned at least two polarity events and probably continued over several tens of millions of years stratigraphic units with grossly similar lithologies underwent similar diagenetic alterations at rates and to extents that were different from other stratigraphic units having different lithologies because of variations in the rate and duration of alteration and variations in the length of normal versus reversed polarity events any given lithologic unit tended to be dominated by one polarity the model given above seems to be capable of explaining not only those magnetic and petrographic features found in the moenkopi formation but also those of most other red beds

field evidence for active normal faulting in tibet crustal thickening does not now occur in southern tibet field observations made during the chinese french expedition of 1980 support an earlier hypothesis based on a combined analysis of satellite images and fault plane solutions of earthquakes immediately north of the highest peaks of the himalayas the tectonic regime is dominated by east west extension we have mapped a large number of north south normal faults which sharply cut the glacial and post glacial morphology and followed several recent earthquake breaks along faults detected on satellite and aerial photographs microtectonic measurements in the quaternary suggest that the maximum principal stress is vertical

geological interpretation of dekorp 2 s a deep seismic reflection profile across the saxothuringian and possible implications for the late variscan structural evolution of central europe abstract a 250 km long deep seismic section the first of its kind in west germany s dekorp project is interpreted in terms of late variscan collisional tectonics the structural model used is based on the intracontinental thrust tectonics concept of modern collision orogens the crustal profile across the saxothuringian zone seems to indicate north facing ramps antiformal stacks and culminations within the crystalline basement of the internal mid european variscides that have been only partially overprinted by permo mesozoic crustal stretching

age constraints on the india asia collision derived from secondary remanences of tethyan himalayan sediments from the tingri area abstract we report paleomagnetic results from the zhepure shan formation and the youxia formation of the tingri section 28 42 n 86 43 e and the longjiang section 28 27 n 86 40 e in southern tibet a lower coercive component demagnetized below 30 mt and a higher coercive component demagnetized between 30 mt and 100 mt were separated rockmagnetic analyses reveal magnetite as the remanence carrier the differences in coercivity are probably due to a wider grain size spectrum and therefore different domain states remanence directions of the low coercive component are highly scattered and were not considered for further analysis directions of the high coercive component of members a c of the zhepure shan fm are fairly well grouped within sites k 8 for most sites whereas those of member d and the overlying youxia fm are highly dispersed applied fold tests indicate a postfolding remanence for members a c the latest possible time for remanence acquisition of members a c is constrained to a mean of 48 ma by comparing the determined paleolatitudes to the curve of expected paleolatitudes versus age calculated from the apwp of india deformation of the rocks is likely related to india asia collision and therefore the remanence age yields a constraint on the latest possible age of the india asia collision in consideration of an extended pre collisional northern indian margin greater india the determined values of 48 ma mean and 37 ma youngest possible age within 95 confidence limits underestimate the age of the continental collision

preliminary conclusions of the royal society and academia sinica 1985 geotraverse of tibet the 1985 chinese british expedition to the tibetan plateau attempted to solve the question of the origin of the very thick crustal rocks in this region continuing northwards movement of the indian plate over the past 38 myr has given rise to severe folding and thrust faulting causing crustal thickening by internal deformation previous collisions of microplate terranes derived from gondwanaland occurred during mesozoic times but the kun lun terrane of northern tibet was already part of laurasia by the carboniferous

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

a preliminary upper triassic paleomagnetic pole for the khorat plateau thailand consequences for the accretion of indochina against eurasia we report new paleomagnetic results from the upper triassic norian huai hin lat formation at the base of the khorat group in central thailand five sites of conglomerate sandstone and limestone reveal a stable component of magnetization d 43 i 43 95 7 5 we argue that this component is of primary origin on the basis of a positive fold test observation of both normal and reversed directions and a reported magnetostratigraphy from middle triassic to cretaceous the new triassic pole yields together with published younger poles a mesozoic apparent polar wander path for indochina this unit is found to have remained at tropical northern latitudes since the upper triassic comparison with eurasian poles reveals 1650 850 km of sw ne convergence and 15 10 of counterclockwise rotation between 205 and 160 ma two mechanisms are discussed to account for this motion the first one implies the closure of an oceanic basin to the north of the south china block the second one involves post welding intracontinental deformation

tectonic environment and geodynamic significance of the neo cimmerian donqiao ophiolite bangong nujiang suture zone tibet the bangong nujiang suture zone is underlain by a discontinuous and roughly ew ophiolite belt around donqiao gyanco ophiolitic and metasedimentary palaeozoic series form nappes thrusted over jurassic flysch series all these series have undergone a late symmetrical folding accompanied by generally northward reverse faulting although strongly dismembered a complete ophiolitic sequence comprising harzburgites with large podiform type chromite deposits wehrlites cumulate gabbros sheeted dolerite dykes foliated garnet bearing amphibolites and pillow lavas has been found

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

neotethys and the india asia collision insights from a palaeomagnetic study of the dazhuqu ophiolite southern tibet since 1999 independently derived geophysical and geological models have been published arguing for an intra oceanic subduction system along essentially the entire width of the india eurasia collision belt this idea conflicts with earlier proposals where in the eastern part of the convergence zone neotethyan mid ocean ridge generated lithosphere directly north of the indian craton was consumed beneath eurasia in tibet in an andean type plate configuration new palaeomagnetic data are reported from three barremian aptian 120 ma sequences of chert siliceous mudstones and volcaniclastic rocks these rocks form the uppermost part of the dazhuqu supra subduction zone ophiolite terrane which crop out along substantial portions of the india eurasia yarlung tsangpo suture zone in southern tibet the declination data provide little regionally useful tectonic information they are dominated by local rotations presumably related to the dazhuqu terrane s initial obduction onto the india plate in the palaeocene and subsequent movement s as india later collided and indented into eurasia the inclination data are however useful because they yield consistent sub equatorial formation sites 2 7 s 3 0 7 9 n 2 7 1 4 n 5 7 which correspond with the location of the neotethyan intra oceanic subduction system inferred from the seismic tomographic data at the time these dazhuqu terrane rocks formed the intra tethyan subduction zone would have been about 2500 km south of eurasia

geomagnetic reversal frequency since the late cretaceous models of geomagnetic reversals as a stochastic or gamma renewal process have generally been tested for the heirtzler et al 1 magnetic polarity time scale which has subsequently been superseded examination of newer time scales shows that the mean reversal frequency is dominated in the cenozoic and late cretaceous by a linearly increasing trend on which a rhythmic fluctuation is superposed subdivision into two periods of stationary behavior is no longer warranted the distribution of polarity intervals is visibly not poissonian but lacks short intervals the labrecque et al 2 polarity time scale shows the positions of 57 small wavelength marine magnetic anomalies which may represent short polarity chrons after adding these short events the distribution of all polarity intervals in the age range 0 40 myr is stationary and does not differ significantly from a poisson distribution a strong asymmetry develops in which normal polarity chrons are poisson distributed but reversed polarity chrons are gamma distributed with indexk 2 this asymmetry is of opposite sense to previous suggestions and results from the unequal distribution of the short polarity chrons which are predominantly of positive polarity and concentrated in the late cenozoic if short wavelength anomalies arise from polarity chrons the geomagnetic field may be more stable in one polarity than the other alternative explanations of the origin of short wavelength marine magnetic anomalies cast doubt on the inclusion of them as polarity chrons however the observed behavior of reversal frequency suggests that core processes governing geomagnetic reversals possess a long term memory

a quasi linear structure of the southern margin of eurasia prior to the india asia collision first paleomagnetic constraints from upper cretaceous volcanic rocks near the western syntaxis of tibet we report the first combined geochronologic and paleomagnetic study of volcanic rocks from the shiquanhe and yare basins at the westernmost lhasa terrane which aims to provide an accurate constraint on the shape and paleoposition of the southern margin of asia prior to the india asia collision three new 40ar 39ar ages of 92 5 2 9 ma 92 4 0 9 ma and 79 6 0 7 ma determined by fresh matrix or feldspar from lava flows suggest a late cretaceous age for the investigated units characteristic remanent magnetizations have been successfully isolated from 38 sites which pass positive fold and or reversal conglomerate tests and are hence interpreted as primary in origin the two paleopoles obtained from yare and shiquanhe yield consistent paleolatitudes of 13 6 n 9 6 n and 14 2 n 2 7 n respectively for a reference site of 31 5 n 80 e indicating that the southern margin of asia near the western syntaxis was located far south during the late cretaceous time a reconstruction of the lhasa terrane in the frame of eurasia with paleomagnetic data obtained from its western and eastern parts indicates that the southern margin of eurasia probably had a quasi linear orientation prior to the collision formerly trending approximately 315 e this is compatible with the shape of the neo tethys slab observed from seismic tomographic studies our findings provide a solid basis for evaluating cenozoic crustal shortening in the asian interior and the size of greater india near the western syntaxis

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

paleomagnetism of eocene red beds in the eastern part of the qiangtang terrane and its implications for uplift and southward crustal extrusion in the southeastern edge of the tibetan plateau abstract a primary magnetic component with the acquisition time of 56 0 43 2 ma was isolated between 580 c and 685 c from the eocene gonjo and ranmugou formations in the eastern part of the qiangtang terrane tibetan plateau the tilt corrected site mean direction is d s 35 5 i s 29 3 k 45 9 95 3 2 the site mean inclination increased from 29 3 to 41 6 after multiple inclination shallowing corrections giving a paleopole of 57 9 n 192 1 e a 95 2 9 comparison of the paleogene paleomagnetic data for the qiangtang terrane and lhasa terrane reveals that both terranes experienced latitudinal crustal shortening before 54 43 ma indicating the uplift of southern and central tibet in the early eocene subsequently since 35 4 2 4 ma the north of the qiangtang terrane experienced 1300 410 km of crustal shortening indicating the uplift of the northern tibetan plateau the lhasa terrane and qiangtang terrane have not experienced further crustal shortening since the late eocene and the southeastern part of tibet cannot have provided abundant crustal material to accommodate the significant crustal southward extrusion in the southeastern edge of the tibetan plateau the crust of the tengchong terrane and shan thai block did not experience significant southward extrusion since the late oligocene early miocene the indochina block was situated in the north of the qiangtang terrane before the oligocene and since the early oligocene the indochina block began to experience southward extrusion from the north of the qiangtang terrane which absorbed part of the crustal shortening in the north of the qiangtang terrane

crustal recycling of metamorphic basement late palaeozoic granitoids of northern chile 22 s implications for the composition of the andean crust upper palaeozoic silicic magmatism is widespread in the central introduction andes but its origin is poorly constrained we investigated wholethe magmatic arc in the puna altiplano plateau of the rock chemical and isotopic composition of upper palaeozoic grancentral andes rests on a continental crust of a maximum itoids and their early palaeozoic high grade country rocks in thickness of 70 km crustal thickening is mainly atthe chilean coastal cordillera and precordillera at 22 s in tributed to shortening of the crust and not to juvenile comparison with an upper cretaceous granitoid the age of the magmatic additions from the subduction zone see review late palaeozoic granitoids from a rb sr isochron of 300 ma by allmendinger et al 1997 this implies a major is consistent with k ar cooling ages of hornblende and biotite contribution of pre mesozoic material to the present similar major and trace element patterns as well as nd and pb thick crust because mesozoic juvenile additions to the isotopic composition of upper palaeozoic granitoids and gneisses crust are focused in the mesozoic magmatic arcs mainly point to a source of the granitoids that is similar to the gneisses at in the chilean coastal cordillera fig 1 e g scheuber outcrop sr isotope ratios of the upper palaeozoic granitoids are et al 1994 lucassen et al 1996a which is not involved less radiogenic than those in many of the gneisses we propose a in the crustal thickening the last major reorganization of stratification of the early palaeozoic crust with a rb deficient the crust before the cenozoic was in the early palaeozoic granulitic mid lower crust resulting in less radiogenic sr compared orogeny between 500 ma the age of peak metawith the upper crust based on the interpretation of the p t t morphism and 400 ma the age of the final exhumation history and isotopic composition of the lower palaeozoic meta damm et al 1990 1994 lucassen et al 1996b unmorphic basement and of the isotopic composition of the late published data 1998 becchio et al 1997 the widepalaeozoic granitoids and younger magmatic rocks nd isotopic spread upper palaeozoic granitoid magmatism starting composition is identical in lower and upper crust and in crustal at 300 ma was assigned to arc magmatism e g brown melts from the late palaeozoic to recent the cretaceous granitoid 1991 bahlburg herve 1997 or to melting of the evolved from partial melts of a mantle derived source with concrust at a passive margin breitkreuz zeil 1984 and siderable contamination by the old crustal component the crust references therein the metamorphic history of the prethat formed in the early palaeozoic is the major source of material mesozoic crust was studied in detail before lucassen et for the cenozoic tectonic thickening of the andean crust al 1999a unpublished data 1998 summary of previous

tibetan tectonics from 40ar 39ar analysis of a single k feldspar sample ar 39ar data on an alkali feldspar sample from the quxu pluton gangdese batholith southern tibet allow a detailed assessment of unroofing and uplift history between 35 and 18 ma the 39ar arrhenius plot for this sample shows departures from a linear relationship between the effective diffusion parameter log d r2 and reciprocal temperature which we interpret to be the result of a distribution of distinct diffusion domain sizes we use an alternative way of plotting the arrhenius data that exhibits domain size versus cumulative 39ar released during step heating the 4 ar 39ar age spectrum of the sample has features such as local age plateaus that are most easily explained in terms of the distinctive closure age of particular domains the fact that the same distribution of diffusion domain sizes explains both the arrhenius data and the age spectrum is an indication that the diffusion properties operating in the laboratory are those of the sample while it was in its natural environment modelling of the age spectrum with a distribution of domain sizes results in the recovery of a continuous cooling history segment rather than a single time temperature datum we demonstrate the robustness of the cooling curve determination by showing the large misfits to the age spectrum that arise from relatively small changes in the cooling history the best fit cooling curve for the quxu sample shows a decreasing rate of cooling in the time interval 35 18 ma followed by a very rapid cooling beginning at about 18 ma we have used a thermal model for the conductive cooling of an unroofing pluton to estimate the rate of unroofing required to explain the quxu cooling curve and find that in the 35 20 ma time interval the primary control of the thermal evolution is the conductive loss of magmatic heat with little or no unroofing unroofing rates of approximately 0 05 mm yr followed by a brief period 5 ma of very rapid unroofing with rates of order 2 mm yr

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

tectonic and sedimentary basin evolution of the eastern bangong nujiang zone tibet a reading cycle the ophiolite bearing bangong nujiang zone bnz traversing central tibet from east to west separates the qiangtang block in the north from the lhasa block in the south their stratigraphic development indicates that both blocks once formed a continuous continental platform until the late triassic following late paleozoic triassic rifting ocean crust formed between both blocks during the late triassic creating the dongqiao naqu basin dnb among other basins yu et al 1991 the analysis of the rift flank sequences reveals that rifting was dominated by transtension the basin was shortened by post mid cretaceous transpression thus the overall basin evolution represents a reading cycle despite some active margin processes which gave this cycle a special imprint major basin parts were preserved despite transpressional shortening suggesting that the eastern bnz represents a remnant basin our understanding of the dnb solves the prior problem of viewing the bnz as a mid late jurassic collisional suture although typical collision related deformation thickening mountain building as well as related molasse formation are lacking our model also explains the scattered linear ophiolite distribution by local transpression of remnant oceanic basin floor without having to consider problematic long range ophiolite thrusting

the himalayan arc large scale continental subduction oroclinal bending and back arc spreading abstract palaeomagnetic results from the himalayan arc and southern tibet compared with simulated apparent polar wander paths for the indian plate show a consistent pattern of rotations of the himalayan arc relative to the indian shield varying gradually from 45 clockwise in the northwestern himalaya to slightly counterclockwise in the lhasa region this pattern is consistent with continental underthrusting of greater india beneath the tibetan plateau since the early miocene over at least 650 km at the longitude of western nepal and oroclinal bending since the latest miocene available palaeomagnetic observations support the steady state model for the formation of the himalayan arc with refinements as follows 1 collision between greater india s northern boundary and southern asia occurred at equatorial latitudes with progressive suturing from palaeocene in the northwestern himalaya until early eocene in the eastern himalaya 2 continuing convergence and indentation of greater india into southern asia over about 2000 km up to the early miocene resulted in southeastward extrusion of indochina and 3 neogene counterclockwise rotational underthrusting of greater india along the main central thrust with pliocene quaternary oroclinal bending of the himalayan arc

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

role of lithospheric strength heterogeneities in the tectonics of tibet and neighbouring regions unlike rigid oceanic plates continents show diffuse deformation for example the tibetan plateau that has accommodated much of the convergence of india with asia we interpret the tertiary deformation of asia as the three dimensional strain of a non linear viscous continental lithosphere lateral strength contrasts modify this deformation stress concentration around the relatively strong tarim basin gives rise to local crustal thickening in the tien shan to the north and to strike slip in the altyn tagh along its southern margin

distorted drainage basins as markers of crustal strain east of the himalaya extensive lithospheric deformation on the eastern margin of the india eurasia collision has brought three major rivers into exceptionally close proximity for over 300 km the salween mekong and yangtze rivers are only tens of kilometers apart 10 times closer than rivers of comparable length elsewhere in the world we view deeply incised river valleys as displacement markers that move with the underlying crust over time scales of at least tens of millions of years accordingly the close spacing of these major rivers and their nearly parallel trends reflect the large crustal strains near the eastern himalayan syntaxis these river characteristics are consistent with two primary phases of deformation in this tectonic setting ne sw shortening of 0 5 oriented radially from the eastern syntaxis followed by dextral shear strain of 6 1 along a north trending zone pervasive shearing of this magnitude throughout the 150 km wide region of anomalous drainage would account for 1000 km of northward motion of india relative to southern china directly east of this region another 1000 km of northward motion is likely to have been accommodated by pervasive shearing through a zone hundreds of kilometers wide finally we draw attention to the remarkable accord between the arcuate paths of each of the three great rivers in the broader southeastern tibet region and small circles that curve around the eastern himalayan syntaxis

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

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

can a primary remanence be retrieved from partially remagnetized eocence volcanic rocks in the nanmulin basin southern tibet to date the india asia collision paleomagnetic dating of the india asia collision hinges on determining the paleogene latitude of the lhasa terrane southern tibet reported latitudes range from 5 n to 30 n however leading to contrasting paleogeographic interpretations here we report new data from the eocene linzizong volcanic rocks in the nanmulin basin which previously yielded data suggesting a low paleolatitude 10 n new zircon u pb dates indicate an age of 52 ma negative fold tests however demonstrate that the isolated characteristic remanent magnetizations with notably varying inclinations are not primary rock magnetic analyses end member modeling of isothermal remanent magnetization acquisition curves and petrographic observations are consistent with variable degrees of posttilting remagnetization due to low temperature alteration of primary magmatic titanomagnetite and the formation of secondary pigmentary hematite that unblock simultaneously previously reported paleomagnetic data from the nanmulin basin implying low paleolatitude should thus not be used to estimate the time and latitude of the india asia collision we show that the paleomagnetic inclinations vary linearly with the contribution of secondary hematite to saturation isothermal remanent magnetization we tentatively propose a new method to recover a primary remanence with inclination of 38 1 35 7 40 5 95 significance and a secondary remanence with inclination of 42 9 41 5 44 4 95 significance the paleolatitude defined by the modeled primary remanence 21 n 19 8 n 23 1 n is consistent with the regional compilation of published results from pristine volcanic rocks and sedimentary rocks of the upper linzizong group corrected for inclination shallowing the start of the tibetan himalaya asia collision was situated at 20 n and took place by 50 ma

a paleomagnetic study of mesozoic sediments from the junggar and turfan basins northwestern china mesozoic sedimentary rocks have been collected from the karamay area northwestern junggar block and in the turfan basin northeastern tarim block for a paleomagnetic study with the aim of testing the rigid nature of these blocks and to better constrain their paleogeographic relationships within central asia prior to the collision with india the results from the karamay area are disappointing lower cretaceous grey sandstones having apparently been recently remagnetized this remagnetization is carried by magnetite and yields highly clustered normal polarity directions both a medium mtc and a high htc unblocking temperature component are isolated from flat lying or weakly tilted jurassic red sandstones the mtc could be carried by maghemite and the htc is carried by hematite the two components are statistically identical and seem to correspond to an ancient magnetization however there is no fold test and comparison with previous data from junggar and the reference apwp of siberia does not allow us to decide between a middle jurassic primary magnetization and an upper jurassic or cretaceous remagnetization stronger constraints are obtained from upper jurassic to lower eocene fine grained red sandstones from anticlines that have been thrust southwards in the turfan area of south tien shan after the elimination of a recent overprint that is probably related to weathering a high coercivity high unblocking temperature component carried essentially by hematite is isolated all directions from the jurassic to the lower eocene which yield positive fold and reversal tests are statistically indistinguishable this is compatible with a primary origin for these magnetizations similar to what has previously been found in junggar tarim and western tibet the corresponding turfan pole is not statistically distinct from that of tarim for the same period in accordance with the notion of a large quasi rigid tarim block remaining attached to eurasia in a rather constant position and at a constant latitude throughout the second half of the mesozoic this new datum can therefore be included to derive a new mean pole for the tarim that is valid for the period from the middle jurassic to the end of the cretaceous 68 4 n 224 5 e a95 3 1

paleomagnetism in the tajikistan continental shortening of european margin in the pamirs during indian eurasian collision abstract a stable and well defined magnetization withd 350 i 55 5 and 95 4 carried by hematite is isolated in the red beds of the tajik basin the fold test is positive 99 probability and this magnetization predates the late eocene early oligocene folding phase in the tajik basin the coordinates of the vgp are 82 n 323 e dp 4 dm 6 with a paleolatitude of dushanbe 38 38n 68 51e of35 6 5 a high blocking temperature component with weak intensity and reverse polarity has been demonstrated in the site of ragoon but it has not been isolated this component predates the long cretaceous normal period and is compatible with the older stratigraphic age attributed to the formation valanginian these arguments indicate that this component is likely of primary origin the other component of higher intensity and clearly isolated in all the sites is a pre folding remagnetization the process of remagnetization is attributed to the tectonic events such as fluids migration and slight heating during a few million years which took place just before the first late eocene early oligocene phase of folding an anticlockwise rotation of 26 encountered by the region of dushanbe is in good agreement with the indentation model of collision leading to anticlockwise rotation west of the indian wedge and clockwise rotation to the east a comparison of the paleolatitude of dushanbe computed from the direction of magnetization and the paleolatitude expected from the apparent polar wander path for eurasia gives an estimate of the post collision continental shortening north of the tajik basin this shortening did not exceed a few hundred kilometers and was probably between 100 and 300 km an estimate of the roles of the different classical mechanisms of crustal shortening is discussed and a paleogeographic reconstruction giving the location of the southern margin of eurasia at 45 ma is proposed the paleolatitude deduced from the prefolding remagnetization of the red beds of the tajik basin is then compared with the results obtained by soviet workers in tertiary sediments of the same area revealing strong contradictions in interpretation this contradiction emphasizes the problem of the rigidity of european plate

tibet uplift and erosion abstract the 5 km high tibetan plateau is an outstanding topographic feature on the earth today its horziontal extent elevation and location cause significant effects on modern atmospheric circulation and climate so the history of uplift of the surface of tibet is linked to cenozoic climate changes at local regional and perhaps global scales geological and geophysical studies of the plateau are contributing data on the present and past deformation of the tibetan lithosphere that has formed the plateau primarily during the cenozoic the principal of isostasy then can be used to estimate the elevation history of the surface for a given deformation history different parts of tibet probably had different uplift histories but presently available data are not sufficient for distinguishing many contrasts in one scenario cretaceous and early cenozoic north south distributed shortening of tibetan crust and mantle lithosphere probably caused significant uplift of the surface relative to sea level to perhaps half of the present elevation by the early miocene thinning of the high density mantle portion of the lithosphere during the miocene may then have allowed the thick tibetan crust to rise close to its present elevation perhaps higher before 8 ma since then slow east west extension of tibet probably reduced the crustal thickness slightly and may have caused the elevation of the plateau to decrease during the late cenozoic erosion of tibet unlike narrow mountain belts has been unable to match the uplift of a broad plateau orographic precipitation and efficient river networks concentrate erosion on the edges while the interior is protected from significant erosion despite its lofty elevation the southern edge of the plateau the himalaya has suffered a minimum of 25 km of denudation since the miocene while central tibet shows little or no sign of major erosion since that time the gangdese arc in southern tibet was rapidly eroded during the mid miocene when 4 km of rock apparently were removed from the surface as shown by mineral cooling ages this pulse of erosion was probably caused by a combination of local thrust system movement and changes in base level and precipitation due to relative elevation changes between the gangdese and the himalaya to the south the modern long wavelength flatness of tibet is unlikely to have been caused by erosion and indicates viscous flow at some level of the lithosphere has been acting to level the plateau surface

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

the tibetan side of the india eurasia collision results are reported from the 1980 joint french chinese field expedition in tibet the area covered was from the high himalaya in the south to the region of nagqu 250 km north of yangbajain ophiolites in the zangbo valley represent remnants of the crust of an ocean basin which lay adjacent to the gangdise granodiorite belt in the late mesozoic the ophiolites were thrust to the south onto the cretaceous melange and triassic flysch but the age of this event is unclear in the tertiary sediments of the indian margin and the xigaze basin were folded with steeply dipping cleavage which indicates some north south shortening of the crust however the tertiary volcanic sequence on the lhasa block and further north shows comparatively little folding and thrusting in the quaternary deformation of tibet was characterized by east west extension

further palaeomagnetic data from chitral eastern hindukush evidence for an early india asia contact the eastern hindukush forms part of an elongate belt central domain collage of cimmerian microcontinents that encircles the northern part of the indian subcontinent a gondwanan origin is commonly assumed for this belt but a laurasian origin for the chitral region has been argued on palaeontological talent and mawson 1979 and palaeomagnetic klootwijk and conaghan 1979 grounds the laurasian view was based on a pilot study we undertook of upper devonian pisolitic ironstones from a thrust sheet at kur gh spur in chitral preliminary results showed a characteristic magnetization component d 318 i 6 5 n 7 block samples k 14 95 16 5 indicating an equatorial palaeoposition this component was thought to be of primary origin and was interpreted in terms of a late devonian laurasian affinity of the kur gh spur rocks this controversial conclusion has been tested in the present more comprehensive study of the thrust pile of sedimentary rocks in the resh n kur gh b ni region of chitral and the primary origin of the characteristic magnetization component refuted thermal demagnetization of 333 block samples from middle to upper devonian variegated sediments permian quartz flysch permo triassic carbonates and mid cretaceous redbeds showed two interpretable components a softer component of recent origin a and a harder characteristic component b of both normal and reverse polarity whose mean direction d 314 1 i 6 0 n 4 thrust sheets k 198 2 95 6 5 is comparable to the characteristic component observed in our preliminary study however the universal presence of this component throughout the thrust pile proves its overprint origin which we attribute to initial india asia contact palaeomagnetic information pertinent to the controversy of a laurasian versus a gondwanan origin of the chitral region has not been obtained in this further study because primary magnetizations could not be identified beyond doubt hence we retract herewith our original conclusion of a late devonian laurasian affinity of the chitral region on the basis of the palaeomagnetic evidence the secondary component b comprises a suite of secondary magnetizations acquired at equatorial to low northern palaeolatitudes and is attributed to initial contact between greater india and southern asia component b has been observed previously in the himalayan tibetan region both north and south of the indus tsangpo suture zone identification is herein extended to the hindukush region north of the northern kohistan or shyok suture zone which is a western continuation of the indus tsangpo suture comparison of this suite of collision at tributed equatorial palaeolatitude data from the india asia convergence zone with new palaeolatitude constraints from the ninetyeast ridge on the northward movement of the indian plate constrained additionally by a recent minimal estimate of the palaeogeographic northern extent of greater india indicates that initial contact between northwestern greater india and southern asia was established at or before the cretaceous tertiary boundary the overprint origin of component b at about this time is further supported by observations by zeitler 1985 on rocks from the sampled area in chitral of partially reset zircon fission track ages around 68 55 ma the nw se declination axis of component b indicates a 60 70 counterclockwise rotation of the sampled thrust pile with respect to eurasia and a counterclockwise rotation between 10 and 30 with respect to india some of the recent field components a show a comparable rotation and indicate that the tectonic activity that led to the formation of the hindukush pamir karakorum syntaxial zone has continued into recent times

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

the cretaceous tertiary deformation of the lhasa block and its implications for crustal thickening in tibet the strong tertiary deformation of the northern margin of the indian plate and of the tsangpo suture zone is not matched by comparable deformation within the lhasa block yet the southern margin of the block was the site of large scale intrusive activity the gangdese or transhimalayan batholith throughout the late cretaceous and was therefore presumably weaker than its surroundings this poses a problem for any approach that interprets the deformation of tibet in terms of diffuse crustal thickening however there is evidence that the lhasa block was in elevation as well as in magma type an andean margin by the end of the cretaceous and we suggest that the buoyancy force associated with the elevated crust was sufficient to inhibit compressional deformation within the southern lhasa block this suggestion is tested quantitatively by treating the continental lithosphere as a thin viscous sheet containing an isostatically compensated elevation contrast when subjected to boundary conditions representing the collision between india and asia the sheet accommodates convergence by diffuse deformation over a region comparable in size with the tibetan plateau in the absence of an elevation contrast this deformation consists of approximately 100 thickening strain over the region a precollision elevation contrast of 3000 m in the lhasa block would have resulted in its experiencing roughly 40 rather than 100 postcollisional thickening smaller elevation contrasts would have resulted in greater thickening and vice versa these calculations have implications for the mode of crustal thickening in northern tibet as the available evidence suggests that the topographic expression of the earlier mesozoic tectonic activity in southern asia was erased before the india asia collision we expect that there has been widespread and relatively homogeneous crustal shortening migrating northward from the lhasa block during the tertiary

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

the contact between the higher himalaya crystallines and the tibetan sedimentary series miocene large scale dextral shearing space and time evolution of the synmetamorphic structures across the metamorphic pile have been studied in several areas along the himalayan belt east and central nepal garhwal zanskar from one area to the other the evolution is very similar 1 at the base of the pile in the main central thrust mct shear zone the stretching lineation penetrative and regularly oriented n0 e to n30 e indicates the mct transport direction very constant all along the belt from the eohimalayan main metamorphic development up to the late metamorphic movements 2 at the top of the pile at the contact between the crystalline unit and its sedimentary cover gravity driven structures are confirmed north vergent folds ductile normal faulting however there are numerous local indications of late miocene syn to late emplacement of the leucogranitic plutons dextral shearing 3 in between across the medium part of the pile the stretching lineation shows a conspicuous progressive regional clockwise rotation clearly indicated by the strain trajectories mapped in nepal and garhwal points 1 and 2 show that the crystalline sedimentary boundary despite its apparent structural and metamorphic continuity is not only a normal fault but also an important dextral shear zone which has acted since the upper miocene as the main southern limit of the eastward extruding tibetan block

large scale crustal deformation of the tibetan plateau the topography velocity and strain fields calculated from a three dimensional newtonian viscous model for large scale crustal deformation are generally in good agreement with results from geological geodetic and earthquake studies in and around the tibetan plateau provided that the model rheology incorporates a weak zone within the deep crust beneath the plateau equivalent to a viscosity of 1012 pa s within a 250 m thick channel or 1018 pa s within a 15 km thick channel model studies and observations show a plateau at 5 km elevation with steep topographic gradients across the southern and northern plateau margins and more gentle gradients across the southeastern and northeastern margins rapid shortening strain is concentrated along the lower portions of the northern and southern plateau margins at rates 20 mm yr model results show north south shortening 10 mm yr in reasonable agreement with gps data 5 8 mm yr of north south shortening across the northern two thirds of the plateau and east west stretching 10 15 mm yr across the eastern half of the high plateau in reasonable agreement with seismic geologic and gps data upper crustal material moves eastward from the plateau proper into a lobe of elevated topography that extends to the south and east clockwise rotation of material around the east himalayan syntaxis at rates up to 10 mm yr occurs partly as a result of dextral shear between indian and asian mantle at depth and partly as a result of gravitational spreading from the high plateau to the south and east there is little difference in model surface deformation for assumptions of moderately weak or extremely weak lower crust except in southern and northern tibet where margin perpendicular extension occurs only for the case of an extremely weak lower crust our results suggest that the tibetan plateau is likely to have gone through a two stage development the first stage produced a long narrow high orogen whose height may have been comparable to the modern plateau the second stage produced a plateau that grew progressively to the north and east east west stretching eastward plateau growth and dextral rotation around the east himalayan syntaxis probably did not begin until well into the second stage of plateau growth perhaps becoming significant after 20 m y of convergence

paleomagnetism of cretaceous rocks from eastern qiangtang terrane of tibet thermal demagnetization has successfully isolated high unblocking temperature characteristic directions from paleomagnetic samples of lower and middle cretaceous redbed formations from eastern qiangtang terrane tibet the tilt corrected mean direction for the lower cretaceous is d i 57 7 50 2 with 95 10 9 and n 12 sites and that for the middle cretaceous is d i 39 7 51 6 with 95 8 5 and n 11 sites both formations pass the fold test and the latter possesses two polarities the magnetization predates the eocene and was probably acquired during cretaceous time comparison of the mean directions with expected directions from the stable interior of eurasia indicates significant clockwise rotation but little paleolatitudinal change for the sampling area deformation and rotation of these rocks are presumably related to the collision of india with asia and its subsequent northward indentation into asia existing paleomagnetic data from tibet and its environs seems to be compatible with a crustal shortening thickening model dewey et al 1988 in that the continental crust directly north of the indian indenter was substantially shortened north south whereas in the vicinity of the western and eastern syntaxes latitudinal displacement was minimal but tectonic rotation was very important more data are needed to further differentiate between this model and the lateral eastward crustal extrusion model molnar and tapponier 1975

paleomagnetic study on the triassic rocks from the lhasa terrane tibet and its paleogeographic implications abstract we present paleomagnetic results from the newly discovered early middle and late triassic marine sediments of the lhasa terrane orientated samples were collected from 32 sites 330 samples on the north side of the dibu co lake 84 7 e 30 9 n coqen county in the western region of the lhasa terrane rock magnetic data revealed that most of the samples were dominated by magnetite and or pyrrhotite the stepwise demagnetization curves illustrated three components a low temperature component component a near the present day field pdf a secondary remanent magnetization component b that may be from the cretaceous period and a high temperature component component c the component c were isolated from the early middle triassic rocks in 8 sites 47 specimens and from the late triassic rocks in 6 sites 37 specimens the component c of the early middle triassic rocks passed a reversal test b class 95 confidence level and a fold test 99 confidence level that of the late triassic rocks passed a fold test 95 confidence level the corresponding paleopoles for the early middle and late triassic periods of the lhasa terrane were at 18 9 n 208 4 e with a 95 3 9 and 19 6 n 211 8 e with a 95 10 7 respectively we suggest that the lhasa terrane maintained a relative stable latitude 16 5 3 9 s and 18 4 10 7 s in the southern hemisphere during the triassic period before moving northwards and amalgamating with the main body of eurasia the qiangtang and lhasa terranes which were located at the mid low latitudes of the southern hemisphere might have been isolated between eurasia and gondwanaland since the early triassic period the meso tethys potentially represented by the bangong nujiang suture zone bns between the lhasa and qiangtang terranes opened up in the early middle triassic period and expanded during the entire course of the triassic period

upb geochronology of gangdese transhimalaya plutonism in the lhasa xigaze region tibet abstract a series of different plutons from the lhasa xigaze segment of the gangdese transhimalaya belt has been studied by high resolution u pb analyses of zircon using zircon fractions of 5 100 grains each selected upon specific grain characteristics for two diorites located east of xigaze dazhuka the zircons yield concordant ages of 93 4 1 0 and 94 2 1 0 m y respectively also concordant ages of 41 1 0 4 and 41 7 0 4 m y have been obtained for two granodiorites collected southwest of lhasa qushui the precision on the ages of two granites from the xigaze and lhasa area is limited by two factors the presence of inherited radiogenic lead and the occurrence of subsequent lead loss however some concordant zircons detected in both granites define approximate ages of about 67 and 53 m y respectively the inherited lead components show that melting of precambrian material was involved in magma genesis the u pb ages substantiate a magmatic activity lasting from mid cretceous cenomanian to eocene lutetian time such a period of plutonism at the southern margin of eurasia as well as the occurrence of magma generation from continental crust suggest that the gangdese range results from the subduction of tethys oceanic lithosphere indian plate underneath eurasia eurasian plate if this model is true the collision of india with eurasia along the lhasa xigaze sector postdates the emplacement of the 41 m y old gangdese granodiorites i e the collision occurred after lutetian time

greater india basin hypothesis and a two stage cenozoic collision between india and asia cenozoic convergence between the indian and asian plates pro duced the archetypical continental collision zone comprising the himalaya mountain belt and the tibetan plateau how and where india asia convergence was accommodated after collision at or be fore 52 ma remains a long standing controversy since 52 ma the two plates have converged up to 3 600 35 km yet the upper crustal shortening documented from the geological record of asia and the himalaya is up to approximately 2 350 km less here we show that the discrepancy between the convergence and the shortening can be explained by subduction of highly extended continental and oceanic indian lithosphere within the himalaya be tween approximately 50 and 25 ma paleomagnetic data show that this extended continental and oceanic greater india promontory resulted from 2 675 700 km of north south extension between 120 and 70 ma accommodated between the tibetan himalaya and cratonic india we suggest that the approximately 50 ma india asia collision was a collision of a tibetan himalayan microconti nent with asia followed by subduction of the largely oceanic greater india basin along a subduction zone at the location of the greater himalaya the hard india asia collision with thicker and contiguous indian continental lithosphere occurred around 25 20 ma this hard collision is coincident with far field deforma tion in central asia and rapid exhumation of greater himalaya crys talline rocks and may be linked to intensification of the asian monsoon system this two stage collision between india and asia is also reflected in the deep mantle remnants of subduction imaged with seismic tomography

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

impacts of palaeogeography and sea level changes on mid cretaceous climate climate is often depicted as globally warm during the middle part of the cretaceous however major tectonic events such as the break up of gondwana or sea level changes inducing the flooding of vast continental areas occurred from 120 to 95 ma resulting in drastic climatic re organisations in order to investigate the impact of plate tectonics and sea level change on climate we performed climatic simulations during the cenomanian 95 ma and aptian 120 ma using an atmospheric general circulation model problems with models lie in our knowledge of a few key factors for examples the middle cretaceous sea surface temperature gradient from equator to pole remains an open question and it is not clear yet how flat it was two sensitivity experiments for each period prescribing a flatter sst gradient based on oxygen isotopic data have been done in order to test its climatic consequences we discuss separately the effect of palaeogeography and meridional sst gradient impacts in the northern hemisphere the flooding of large continental areas due to sea level rise leads to a more equable climate in the mid latitudes however the impact of epicontinental seas varies seasonally from site to site due to local feedbacks in the southern hemisphere the breakup of gondwana deeply affects the climate of this region moderating the seasonal thermal variations in addition we test the impact of a flatter sst gradient on climate and we show that simulations with flat sst gradients compare well with field data we analyse the remaining model data discrepancies in terms of missing feedbacks co2 vegetation but also of new measurements that enable to infer a better meridional variation of sea surface temperature

outward growth of the tibetan plateau during the cenozoic a review abstract the surface uplift history of the tibetan plateau tp offers a key testing ground for evaluating models of collisional tectonics and holds important implications for processes ranging from global cooling to the onset of the asian monsoon various models have been proposed to reveal the surface uplift history of the tp but controversies remain we evaluate these models using data from sedimentology and stratigraphy structural geology magmatism exhumation and paleoaltimetry studies structural analyses indicate that thrust belts which spread from the central tp outward toward its surrounding margins accommodated most of the india asia convergence and facilitated crustal shortening and thickening in the central tp eocene adakitic rocks located in the qiangtang and the lhasa blocks likely were generated by partial melting of an eclogitic source paleogene 50 30 ma potassic rocks only occur in the qiangtang block whereas late oligocene late miocene 26 8 ma potassic rocks occur both in the qiangtang and lhasa blocks low temperature thermochronologic ages in the central tp are older than 40 35 ma whereas those in the margins are younger than 20 ma mostly late miocene and pliocene pleistocene in age independent paleoaltimetry estimates suggest that the lhasa and qiangtang terranes attained their current elevations during the eocene most likely due to the initial collision between india and lhasa whereas the hoh xil basin area to the north and himalayas to the south were still low even below sea level in the latter case we argue for an inside out growth pattern for the tibetan plateau the tp grew southward and northward from a nucleus of high topography and is likely to continue expanding along the mazar tagh fault to the northwest the kuantai shan hei shan longshou shan to the northeast the longquan shan to the east and the shillong plateau to the south if the northward force of india would not diminished

39 ar 40 ar dating of the trans himalayan calc alkaline magmatism of southern tibet the age of the trans himalayan calk alkaline magmatism has not been clearly established on the basis of geological control it is possible to infer a pre late cretaceous age for the plutonites at the southern edge of the batholith belt and a late cretaceous or younger age for the lava series on the top and or to the north of the plutons k ar radiometric data previously obtained by chinese groups agree with these geological ages the 39ar 40ar method has been used to analyse more accurately some of the rocks which have been sampled during the joint french chinese field work in southern tibet in the summer of 1980 the incremental degassing of argon gives generally more information about the thermal history of the sample using this method we now show that the earlier basic plutons of the southern edge of the trans himalayan belt have ages of 90 110 myr albian to cenomanian whereas the lavas of the upper unconformable series are clearly younger 60 myr palaeocene the ladakh and kohistan magmatic belts further west are probable equivalents of the trans himalaya north of the indus suture their precise ages are also poorly known orbitolinas of middle cretaceous aptian age have been found in the drosh yasin series which lie unconformably on dioritic plutons in the northern edge of the belt the few radiometric data that are available give ages in fairly good agreement with those we have obtained in the trans himalaya so we conclude that a subduction zone has been active during the cretaceous between india and asia the only outstanding question is whether this active margin was at that time close to india or to asia or somewhere in between

the tectonic history of kohistan and its implications for himalayan structure the tectonic history of kohistan northern pakistan involves two collisional events cleavage and folding developed at 90 100 ma along the northern suture between the kohistan island arc and the asian plate at the same time there was major folding and shearing of the lower part of the kohistan arc approximately 100 km south of the suture this deformation was followed by ocean subduction south of the kohistan arc generating the kohistan calc alkaline batholith with subsequent ocean closure during the eocene and obduction of the kohistan arc together with the adjacent part of the asian plate over the indian continental crust the construction of balanced cross sections through the imbricated upper part of the indian continental crust in the footwall to this southern suture indicates a minimum displacement of 470 km requiring the western himalayan hinterland to be underlain by a large wedge of indian middle to lower crust there is some shortening of the overriding kohistan and asian plates by thrusts and shear zones but it is insufficient to satisfy the palaeomagnetic data there must be major crustal shortening involving thrusts in the hindu kush and pamirs north of kohistan the post eocene thrust direction which for most of pakistan is towards 160 is almost perpendicular to that immediately to the east in the himalayan belt generating complex refolded thrust patterns in the hazara syntaxis and large scale folding and rapid uplift with associated brittle faulting and seismic activity adjacent to the nanga parbat syntaxis these different thrust trends indicate that major thrust movement as well as the folds and deformation fabrics cannot always be related to plate movement vectors but are modified by or develop from complex rotations during place collision or from the gravitational spreading of a thickened crust a regional approach is required to recognize and correctly attribute the various components in thrust displacements

acceleration and deceleration of india asia convergence since the cretaceous roles of mantle plumes and continental collision 1 a strong 50 35 ma decrease in india asia convergence is generally ascribed to continent continent collision however a convergence rate increase of similar magnitude occurred between 65 50 ma an earlier increase occurred at 90 ma both episodes of accelerated convergence followed upon arrival of a mantle plume below and emplacement of a large igneous province lip on the indian plate we here first confirm these convergence rate trends reassessing the indo atlantic plate circuits then using two different numerical models we assess whether plume head arrival and its lateral asthenospheric flow may explain the plate velocity increases and whether decreased plume flux and increasing continent plume distance may explain deceleration even without continental collision the results show that plume head arrival can indeed lead to absolute indian plate motion accelerations on the order of several cm yr followed by decelerations on timescales similar to the reconstructed fluctuations the 90 ma increase could potentially be explained as response to the morondova mantle plume alone the 65 50 ma convergence rate increase however is larger than can be explained by plume head spreading alone we concur with previous hypotheses that plume induced weakening of the indian continental lithosphere asthenosphere coupling and an increased slab pull and ridge push efficiency are the most likely explanations for the large convergence rate increase the post 50 ma decrease is best explained by orogeny related increased trench resistivity decreased slab pull due to continental subduction and possibly restrengthening of lithosphere asthenosphere coupling upon plume demise

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

did a large departure from the geocentric axial dipole hypothesis occur during the eocene evidence from the magnetic polar wander path of eurasia abstract low paleolatitudes are very frequent in eocene to oligocene formations particularly in the tethyan mobile belt these anomalies and their possible sources are discussed they are neither due to unreliable results nor to inclination errors in sedimentary rocks neither can large tectonic movements explain these discrepancies the most probable causes having been rejected therefore the best explanation consists in an alternative magnetic pole position for eurasia located at 69 n 215 e it is proposed that the mean magnetic field of the earth was dipolar but that its axis has swung from one stable position to another during the middle tertiary and was therefore sometimes different from the rotation axis

resolving the problem of shallow magnetizations of tertiary age in asia insights from paleomagnetic data from the qiangtang kunlun and qaidam blocks tibet china and a new hypothesis we present new paleomagnetic results obtained at 39 sampling sites from five sections of tertiary red bed formations two eocene formations from the qiangtang block of tibet xialaxiu locality 32 8 n 96 6 e and the xining basin of qaidam xining locality 36 5 n 102 0 e and three neogene formations from the xining basin jungong locality 34 7 n 100 7 e and the kunlun block tuoluo lake and west yushu localities 35 3 n 98 6 e and 33 2 n 96 7 e respectively thermal demagnetization of the rocks isolated a high temperature component that we interpret as the primary magnetization in four localities the paleopoles lie at 52 6 n 352 e dp dm 6 0 10 7 for xialaxiu 61 6 n 211 3 e dp dm 9 7 16 1 for xining 66 0 n 228 6 e dp dm 3 6 6 9 for jungong and 53 9 n 205 4 e dp dm 5 6 10 0 for west yushu as in previous studies of tertiary formations from asia the inclinations we obtained are shallower by 18 to 26 than the magnetic field computed from the eurasian apparent polar wander path apwp at 10 and 20 ma for neogene rocks and at 40 and 60 ma for eocene rocks on the basis of a compilation of eocene data from the south china block tibet central asia and kyrgyzstan we conclude that this inclination anomaly reflects erroneous predictions of positions of the siberian craton when based on the apwp of eurasia the main reason for this discrepancy might be nonrigid behavior of the eurasian plate in the tertiary combination of this with intracontinental shortening of asia under the penetration of india provides a full explanation for the anomaly verification of this new interpretation of the inclination anomaly will require new geologic and paleomagnetic data from the northern parts of these remote regions in mongolia and siberia

mineral chemistry and k ar ages of plutons across the karakoram fault in the shyok nubra confluence of northern ladakh himalaya india abstract petrographic analyses of plutons across the karakoram fault in the shyok nubra confluence of northern ladakh area revealed that the rocks consist of diorite granodiorite granite and leucogranite pyroxene and biotite chemistries by epma indicate that most of the diorite granodiorite and granite are calc alkaline and formed in an arc related tectonic setting these calc alkaline plutons yield k ar biotite ages from 49 3 1 1 to 51 2 1 1 for ladakh batholith in khardung la and from 60 8 1 3 to 65 8 1 4 ma for granites and diorites in hundar area the k ar ages of the hundar igneous complex are within the age range of 46 70 ma of the ladakh batholith near leh town and the neighbor sites indicating the hundar igneous complex as a part of the ladakh arc their ages are also coincident with the khardung volcanics 67 4 and 60 5 ma the calc alkaline granites from panamik of karakoram block yield k ar biotite ages of 95 7 2 1 and 96 7 2 1 ma suggesting that the granites are member of the plutons 90 120 ma occurring in northern pakistan tangtse in ladakh and western tibet that emplaced during subduction of the neo tethyan oceanic plate under the karakoram block the leucogranite and granodiorite in the panamik region yield k ar biotite and muscovite ages ranging from 9 18 0 21 to 9 45 0 21 ma the leucogranites are the s type related to post collision tectonic setting and are considered to have emplaced in relating to activation of the karakoram fault

paleomagnetism of the upper cretaceous red beds from the eastern edge of the lhasa terrane new constraints on the onset of the india eurasia collision and latitudinal crustal shortening in southern eurasia abstract the late cretaceous location of the lhasa terrane is important for constraining the onset of india eurasia collision however the late cretaceous paleolatitude of the lhasa terrane is controversial a primary magnetic component was isolated between 580 c and 695 c from upper cretaceous jingzhushan formation red beds in the dingqing area in the northeastern edge of the lhasa terrane tibetan plateau the tilt corrected site mean direction is d s i s 0 9 24 3 k 46 8 95 5 6 corresponding to a pole of plat plon 71 4 273 1 with a 95 5 2 the anisotropy based inclination shallowing test of hodych and buchan 1994 demonstrates that inclination bias is not present in the jingzhushan formation the cretaceous and paleogene poles of the lhasa terrane were filtered strictly based on the inclination shallowing test of red beds and potential remagnetization of volcanic rocks the summarized poles show that the lhasa terrane was situated at a paleolatitude of 13 2 8 6 n in the early cretaceous 10 8 6 7 n in the late cretaceous and 15 2 5 0 n in the paleogene reference point 29 0 n 87 5 e the late cretaceous paleolatitude of the lhasa terrane 10 8 6 7 n represented the southern margin of eurasia prior to the collision of india eurasia comparisons with the late cretaceous to paleogene poles of the tethyan himalaya and the 60 ma reference pole of east asia indicate that the initial collision of india eurasia occurred at the paleolatitude of 10 8 6 7 n since 60 5 1 5 ma reference point 29 0 n 87 5 e and subsequently 1300 910 km post collision latitudinal crustal convergence occurred across the tibet the vast majority of post collision crustal convergence was accommodated by the cenozoic folding and thrust faulting across south eurasia

lhasa block and bordering sutures a continuation of a 500 km moho traverse through tibet a north south wide angle fan profile through the yarlung zangbo suture across the lhasa block and through the bangong nujiang suture shows several significant and sharp changes in crustal thickness both sutures appear as vertical 20 km steps in the moho and may have been the loci for eastward strike slip motion of the tibetan lithosphere