new empirical polyaxial criterion for rock strength abstract a new criterion is presented for prediction of intact rock and rock mass failure under polyaxial state of stresses a comprehensive database of the results of uniaxial triaxial and polyaxial tests on intact rock was utilized for evaluation of the new criterion and comparison of its accuracy with the most accurate and frequently used criteria analysis of 195 individual data groups of brittle failure in the form of 3 1 for twelve different rock types showed that the proposed criterion lead to determination coefficients higher than 0 99 in most cases it also gave the lower values of root mean squared errors relative to mohr coulomb and hoek brown criteria in fitting the normalized strength data for each rock type the criteria were used to fit a typical series of triaxial strength data including brittle and ductile behavior it showed that the new criterion can maintain its accuracy over a wider range of stresses in the absence of rock mass strength data applicability of the new criterion for rock mass was verified by fitting it to typical hoek brown failure envelopes regression analysis of the polyaxial strength data in the form of 3 2 1 for six rock types showed that the new criterion predict the strength more accurately than the modified wiebols cook and you criteria in all cases

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

provenance of lower cretaceous wolong volcaniclastics in the tibetan tethyan himalaya implications for the final breakup of eastern gondwana abstract lower cretaceous volcanic lithic arenites widely distributed in the tethyan himalaya provide insights into the continental breakup of eastern gondwana in southern tibet they are represented by the wolong volcaniclastics the volcanic rocks that contributed clastic material to the lower parts of this unit were predominantly alkali basalts whereas rhyolitic dacitic volcanism becomes the predominant source of the upper strata geochemical analyses of basaltic grains and of detrital cr spinels from the wolong volcaniclastics demonstrate the alkaline character of the volcanism and suggest within plate tectonic setting for the volcanism zircon u pb ages confirm that this volcanism continued from 140 ma to 119 ma hf isotope data on these early cretaceous zircons indicate that their parental magmas were mantle derived but in the later stage of magmatic activity mantle derived magmas were mixed with partial melts derived from the continental crust the lower cretaceous volcaniclastics occur along a broad belt paralleling the northern margin of greater india the onset of volcaniclastic deposition in the himalayas appears to become progressively younger toward the west but it ended synchronously during the late albian 102 ma the low volume of volcanic rocks and their intra plate tectonic setting suggest that they are the result of decompressional melting along extensional deep seated fractures cross cutting the continental crust and reflect changes in regional intra plate tectonic stresses when greater india began to separate from the australia antarctica supercontinent

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

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

age of initiation of the india asia collision in the east central himalaya abstract we document the stratigraphy and provenance of the lower tertiary terrigenous sections in the zhepure shan region of the tethyan himalaya southern tibet using petrographic and geochemical whole rock and single grain techniques the cretaceous early tertiary shelf deposits of shallow marine carbonates and siliciclastics of the former indian passive margin near the western end of the zhepure shan are conformably overlain by lower tertiary clastic rocks sandstones in the jidula formation paleocene mostly contain monocrystalline quartz grains of cratonic origin in contrast significant amounts of immature framework grains with a distinct ophiolitic and volcanic arc influence are present in the youxia early eocene and shenkeza post early eocene formations major trace and rare earth element concentrations in both sandstones and shales complement the petrographic data and indicate that the source of the jidula formation consisted primarily of quartzose basement rocks probably of indian con

first report of lower permian basalts in south tibet tholeiitic magmatism during break up and incipient opening of neotethys abstract the neo tethys ocean began to form at early permian times when continental flood basalts were emplaced in various areas of the newly formed indian passive margin exposed today in the so called tibetan sedimentary zone of the himalaya lower permian mafic volcanic rocks which have long been known from various himalayan localities from kashmir to arunachal pradesh are here for the first time reported to occur also in south tibet bhote kosi basalts of the gyirong county the basalts unconformably overlie lowermost permian diamictites with locally intervening black shales and debris flow deposits and are followed in turn by chert bearing quartzarenites and silty to phosphatic marls yielding brachiopods of roadian wordian age the age of the lavas can thus be bracketed as late early permian post sakmarian and pre roadian the geochemistry of these subalkalic tholeiites akin to morbs testifies to their similarity not only with the adjacent nar tsum spilites of central nepal but also with the panjal traps and abor volcanics of the western and eastern himalayas respectively the geochemical signature of lower permian volcanic rocks is in fact uniform all along the himalayan range and markedly different from that of basaltic rhyolitic alkalic products sporadically emplaced during the previous rifting stage rift volcanism in the tethys himalaya began in the early carboniferous and came to an end in sakmarian times in the early permian initial submergence of the rift shoulders and sediment starvation were followed by tholeiitic magmatism which is therefore interpreted as following break up and incipient sea floor spreading in the neotethys ocean roughly contemporaneous emplacement of continental flood basalts of similar geochemical signature along a 2000 km long rift axis would in fact suggest extensive mantle melting at the transition from continental rifting to break up and opening of the neotethys between northern gondwana and the peri gondwanian blocks

the tethyan himalaya palaeogeographical and tectonic constraints from ordovician palaeomagnetic data abstract to test whether the tethyan himalaya were part of the northern margin of india in the early palaeozoic we have produced the first primary palaeomagnetic data bedding corrected declination 267 5 inclination 63 0 95 10 pole latitude 20 2 n longitude 28 6 e from low metamorphic grade ordovician red beds in the tethyan himalaya shian formation the palaeomagnetic data are of excellent quality and a statistically positive fold test combined with a comparison with late cambrian ordovician gondwana poles suggests a primary hematite bearing magnetization acquired between 470 and 500 ma this is in excellent agreement with stratigraphic faunal and provenance age estimates and the palaeomagnetic data demonstrate that the tethyan himalaya must have been located in proximity to the indian craton during early ordovician times and are therefore consistent with a continuous margin at that time the shian formation pole overlaps with 470 500 ma gondwana poles but an even better fit can be obtained by invoking a post ordovician clockwise rotation of 13 4 such a rotation is similar in both sense and magnitude to clockwise rotations recorded in primary triassic sequences as well as palaeogene palaeomagnetic overprint data from the tethyan himalaya rotations of the tethyan himalaya compared with cratonic india are thus probably all of post late eocene age triassic and early ordovician data do not imply any crustal shortening between tethyan himalaya and cratonic india however in the early ordovician india was rotated 90 compared with its present orientation and any enlargement of india would not be detected by palaeomagnetic data

latest marine horizon north of qomolangma mt everest implications for closure of tethys seaway and collision tectonics the newly discovered marine horizon at zhepure syncline north of qomolangma revises timing of the closure of the eastern neo tethys seaway in the central himalayas the marine pengqu formation conformably overlies a lutetian shallow water carbonate platform and comprises shales interbedded with sandstones deposited in a neritic shelf environment the strata are dated by nannofossils and foraminifera as late early lutetian to late priabonian age np15 np20 deposited 47 34 ma indicating that the final closure of the tethys seaway in this region occurred at 34 ma the newly discovered strata provide evidence about emplacement of a major thrust sheet south of southern tibet prior to 37 ma which affected the regional climate

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

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

ionospheric convection response to slow strong variations in a northward interplanetary magnetic field a case study for january 14 1988 we analyze ionospheric convection patterns over the polar regions during the passage of an interplanetary magnetic cloud on january 14 1988 when the interplanetary magnetic field imf rotated slowly in direction and had a large amplitude using the assimilative mapping of ionospheric electrodynamics amie procedure we combine simultaneous observations of ionospheric drifts and magnetic perturbations from many different instruments into consistent patterns of high latitude electrodynamics focusing on the period of northward imf by combining satellite data with ground based observations we have generated one of the most comprehensive data sets yet assembled and used it to produce convection maps for both hemispheres we present evidence that a lobe convection cell was embedded within normal merging convection during a period when the imf by and bz components were large and positive as the imf became predominantly northward a strong reversed convection pattern afternoon to morning potential drop of around 100 kv appeared in the southern summer polar cap while convection in the northern winter hemisphere became weak and disordered with a dawn to dusk potential drop of the order of 30 kv these patterns persisted for about 3 hours until the imf rotated significantly toward the west we interpret this behavior in terms of a recently proposed merging model for northward imf under solstice conditions for which lobe field lines from the hemisphere tilted toward the sun summer hemisphere drape over the dayside magnetosphere producing reverse convection in the summer hemisphere and impeding direct contact between the solar wind and field lines connected to the winter polar cap the positive imf bx component present at this time could have contributed to the observed hemispheric asymmetry reverse convection in the summer hemisphere broke down rapidly after the ratio by bz exceeded unity while convection in the winter hemisphere strengthened a dominant dawn to dusk potential drop was established in both hemispheres when the magnitude of by exceeded that of bz with potential drops of the order of 100 kv even while bz remained northward the later transition to southward bz produced a gradual intensification of the convection but a greater qualitative change occurred at the transition through by bz 1 than at the transition through bz 0 the various convection patterns we derive under northward imf conditions illustrate all possibilities previously discussed in the literature nearly single cell and multicell distorted and symmetric ordered and unordered and sunward and antisunward

stratigraphy of the upper cretaceous and lower tertiary strata in the tethyan himalayas of tibet tingri area china the 1500 m thick marine strata of the tethys himalaya of the zhepure mountain tingri tibet comprise the upper albian to eocene and represent the sedimentary development of the passive northern continental margin of the indian plate investigations of foraminifera have led to a detailed biozonation which is compared with the west tethyan record five stratigraphic units can be distinguished the gamba group upper albian lower santonian represents the development from a basin and slope to an outer shelf environment in the following zhepure shanbei formation lower santonian middle maastrichtian outer shelf deposits continue pebbles in the top layers point to beginning redeposition on a continental slope intensified redeposition continues within the zhepure shanpo formation middle maastrichtian lower paleocene the series is capped by sandstones of the jidula formation danian deposited from a seaward prograding delta plain the overall succession of these units represents a sea level high at the cenomanian turonian boundary followed from the turonian to danian by an overall shallowing upward megasequence this is followed by a final transgression regression cycle during the paleocene and eocene documented in the zhepure shan formation upper danian lutetian and by upper eocene continental deposits the section represents the narrowing and closure of the tethys as a result of the convergence between northward drifting india and eurasia the plate collision started in the lower maastrichtian and caused rapid changes in sedimentation patterns affected by tectonic subsidence and uplift stronger subsidence and deposition took place from the middle maastrichtian to the lower paleocene the final closure of remnant tethys in the tingri area took place in the lutetian

palaeomagnetic results from the tibetan sedimentary series of the manang area north central nepal summary the remanent magnetization of palaeozoic and mesozoic carbonates from the tibetan sedimentary series tss of the manang area north of the annapurna range north central nepal has been investigated the tss forms part of the tectonic unit between the indian eurasian plate boundary in the north and the main central thrust mct in the south apart from a present field direction three remanence components have been separated a normal polarity post folding remanence negative fold test is carried by pyrrhotite the core corrected mean direction for five sites with 47 specimens from silurian devonian rocks is decl 312 9 incl 58 3 95 17 6 k 19 8 the origin of this component is not understood a more precisely determined reversed polarity post folding remanence 99 per cent negative fold test is also carried by pyrrhotite the core corrected mean direction for 12 sites with 229 specimens from carboniferous triassic rocks is decl 196 4 incl 65 9 95 3 2 k 183 one site of lower carinan age shows in addition to the pyrrhotite component a second stable remanence direction carried by magnetite the fold test is 99 per cent positive and there is a high probability that the remanence is of detrital origin the bedding corrected direction for one site with 40 specimens is decl 334 3 incl 54 1 95 4 2 k 30 2 palaeolatitude 34 6 s the direction of the magnetite component coincides with other triassic palaeodirection data from the literature and the inclination suggests a palaeoposition of the tss at the northern margin of india at this time the northern extent of india in the triassic cannot be reconstructed from the data because of the strong anticlockwise rotation 48 of stable india since this time the inclination of the reverse polarity pyrrhotite component is 19 steeper than expected from the maximum northern position of the area this suggests a regional northward tilt along the mct which is supported by steep inclinations found in other areas declination data for the reverse polarity pyrrhotite component and for the magnetite component deviate clockwise from expected values for the area by 23 34 calculated from the indian apwp utilizing klootwijk conaghan powell s 1985 rotational underthrusting model the minimum magnitude of continental underthrusting between the tss and stable india at the longitude of central nepal 84 e has been calculated from the declination of our reverse polarity pyrrhotite component a possible range of 520 1100 km results from consideration of different tectonic models

ostracod recovery after permian triassic boundary mass extinction the south tibet record lower to middle triassic ostracods from the tulong section south tibet are described here for the first time samples from the first two stages of the early triassic griesbachian and dienerian are barren of ostracods the following stage smithian revealed low diversity ostracod faunas a substantial diversification in taxa began at the base of the fourth stage spathian and developed into the first stage of the middle triassic anisian furthermore exploration of additional feeding modes is developed in the spathian and anisian with notable occurrence of filter feeding taxa high abundance of filter feeding ostracods in spathian and lower anisian units indicates that benthic habitats became oligotrophic these habitats typically harbored ostracod faunas of mesozoic affinities suggesting that the evolutionary turnover of ostracods was fostered by the declining input of nutrients from the spathian on marked faunal similarities with other tethyan areas mainly the northern part of tethys are observed during spathian and anisian times

dimension of greater india in the early mesozoic paleomagnetic constraints from triassic sediments in the tethyan himalaya abstract the northern dimension of greater india defined as the continental part of india from which the himalayan fold thrust belt was derived is difficult to determine due to major tectonic deformations of the himalaya during the tertiary caused by the india eurasian collision paleomagnetic data from the tethyan himalaya can potentially help determine this dimension however the tectonic deformations also led to widespread remagnetization of rocks there and thus previously published paleomagnetic poles from the lower mesozoic himalayan rocks may not accurately reflect their original paleomagnetic directions to better understand the triassic history of india in a gondwana configuration we report new paleomagnetic data after tilt correction d 345 5 i 43 0 95 12 0 k 17 3 n 10 sites and the corresponding paleopole at 34 5 n 282 2 e with a95 11 7 of triassic limestone and sandstone from the tulong group in the tethyan himalaya the primary features of the magnetization are supported by the positive fold test our new triassic paleomagnetic pole overlaps with 230 250 ma poles from india suggesting the close affinity between the tethyan himalaya and cratonic india since the triassic assuming that the tethyan himalaya was a part of the great india since triassic the differences between the tethyan himalaya paleolatitude obtained by this study and those previously reported for the cratonic india imply 1300 km dimension of the tethyan himalaya has been absorbed after the eurasia india collision

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

stratigraphy and sedimentary history of the nepal tethys himalaya passive margin abstract the sedimentary history of the nepal tethys himalaya began with deposition of thick carbonates in the cambro ordovician followed by a mixed siliciclastic carbonate epicontinental succession recording two major deepening events in the early silurian and late devonian fossiliferous carbonate ramp deposits in the tournaisian were disconformably followed by white quartzose sandstones and black mudrocks with locally intercalated diamictites derived from sedimentary rocks and deposited in asymmetric tectonic basins rift stage break up in the mid early permian locally associated with effusion of tholeiitic lava flows was followed by a transgressive sandy to shaly locally coal bearing or bioclastic unit capped by condensed pelagic carbonates in the middle to late permian juvenile ocean stage subsidence of the cooling stretched crust led close to bathyal water depths in the olenekian but then slowed down in the middle triassic to increase again sharply in the late triassic owing to renewed extensional tectonic activity and sediment loading during up and out building of the indian continental terrace deposition of tropical platform carbonates finally became widespread in the middle liassic mature passive margin stage the initial fragmentation of gondwana in the middle jurassic led to rejuvenation of the indian craton and deposition of quartzo feldspathic hybrid arenites capped by condensed oolitic ironstones deposited at warm subtropical latitudes in the late bathonian middle callovian next a discontinuous pelagic grey marly limestone unit was followed by the ammonoid rich offshore spiti shale in the late jurassic the final disintegration of gondwana began in the berriasian when quartzose siliciclastics derived again from the rejuvenated indian craton and partly from recycling of older clastic successions were followed by thick deltaic to shelf volcaniclastics documenting eruption of alkali basalts in the valanginian followed in the hauterivian to albian by more felsic differentiates such as the trachyandesites exposed in the lesser himalaya 120 km to the south a widespread drowning episode fostered by waning volcaniclastic supply during a global eustatic rise is documented by a major glauconitic horizon deposited at middle southern latitudes in the late albian overlain by scaglia like pelagic limestones in the latest albian the final part of sedimentary history during the rapid northward flight of india and its collision with eurasia is not documented anywhere in nepal due to later erosion of upper cretaceous to lower tertiary strata

provenance of the upper cretaceous eocene deep water sandstones in sangdanlin southern tibet constraints on the timing of initial india asia collision abstractthe first arrival of sedimentary material from asia onto the indian continental margin provides a minimum constraint on the timing of initial india asia collision a combination of petrology detrital cr spinel geochemistry and zircon u pb dating of the upper cretaceous to eocene deep water succession at the sangdanlin section southern tibet provides evidence for major provenance change within the strata the upper cretaceous paleocene denggang formation quartzarenites contain zircons with dominant proterozoic ordovician u pb ages with an additional age peak of early cretaceous which we interpret to be derived from the northern indian margin by contrast the lithic sandstones of the early to middle eocene sangdanlin and zheya formations are dominated by zircons younger than 200 ma showing one major peak at 80 125 ma and two subdominant peaks at 54 70 and 180 196 ma comparable to those from the gangdese magmatic arc cr spinels in the sangdanlin and zheya formations are abundant and char

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

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

tectonic evolution of the himalayan thrust belt in western nepal implications for channel flow models we present a new geologic map of western nepal and three balanced regional cross sections in the himalayan thrust belt the minimum shortening between the south tibetan detachment and the main frontal thrust is 485 743 km and suggests that total himalayan shortening may exceed 900 km all rocks involved in the thrust belt are of upper crustal affinity implying that a comparable length of indian lower crust and mantle lithosphere was subducted beneath tibet major structural features are the subhimalayan thrust system lesser himalayan imbricate zone dadeldhura thrust sheet lesser himalayan duplex ramgarh thrust sheet main central thrust sheet and a north dipping normal sense shear zone possibly related to the south tibetan detachment these structures are continuous along the entire nepalese portion of the himalayan thrust belt new 40 ar 39 ar ages from the ramgarh thrust zone greater himalayan rocks and the lower part of the tethyan sequence support a kinematic model in which major thrust systems in nepal propagated southward from early miocene time onward the geometry and kinematic history of the thrust belt in western nepal are not compatible with recent models for southward ductile extrusion of greater himalayan rocks in a mid crustal channel instead the thrust belt in western nepal behaved like a typical forward propagating thrust system involving unmetamorphosed brittlely deformed rocks in its frontal part and ductilely deformed higher grade metamorphic rocks in its hinterland region although our results do not support published versions of the channel flow model they provide additional geological and geo chronological data that will assist future attempts to develop geodynamic models for the himalayan tibetan orogenic system

where did rotational shortening occur in the himalayas inferences from palaeomagnetic remagnetisations in metacarbonates of the lesser lh and tethyan th himalayas of kumaon garhwal n india characteristic remanent magnetisations carried by pyrrhotite unblocking temperatures 250 330 c and magnetite demagnetising spectra 15 50 mt have been identified negative fold tests indicate remanence acquisition after the main folding phase which is of short wavelength character and occurs during the early orogenese of the himalayas a thermal or thermochemical origin of magnetisation is likely and the age of remanence acquisition is indicated to be about 40 ma by 40 k 39 ar cooling and 40 ar 39 ar crystallisation ages in the kumaon lh a long wavelength tilting is indicated by a distribution of the remanence directions along a small circle in n s direction steepening of the remanence directions in the th related to ramping on the main central thrust mct was not observed in contrast to other related studies in the alaknanda valley of lh a 38 8 ma age of remanence acquisition is supported by comparison of observed inclinations to the apparent polar wander path of india clockwise rotation of 20 3 11 7 lh alaknanda valley and 11 3 8 5 th with respect to the indian plate is observed indicating that there is no significant evidence for rotational shortening along the mct since about 40 ma our results suggest that most of rotational underthrusting and oroclinal bending has not been accommodated by the mct but by the main thrusts south of it the latest miocene pliocene age of the main boundary thrust indicates that oroclinal bending is a late orogenic process