the madden julian oscillation barotropic dynamics and north pacific tropical cyclone formation part i observations low level barotropic dynamics may help to explain the modulation of eastern and western north pacific tropical cyclones by the madden julian oscillation mjo during northern hemisphere summer the mjo is characterized by alternating periods of westerly and easterly 850 mb zonal wind anomalies across the tropical pacific ocean when mjo 850 mb wind anomalies are westerly small scale slow moving eddies grow through barotropic eddy kinetic energy eke conversion from the mean flow these growing eddies together with strong surface convergence 850 mb cyclonic shear and high mean sea surface temperatures create a favorable environment for tropical cyclone formation periods of strong mjo easterlies over the pacific are characterized by lesser eke and negligible eddy growth by barotropic conversion the term 2 x is a leading contributor to low level barotropic eke conversion during mjo westerly 2 u9 u periods across the pacific indicating the importance of zonal variations in the westerly jet for producing concentrations of eddy energy this mechanism can be described as wave accumulation associated with variations of the low level zonal flow the conversion term 2 y contributes a smaller portion of the total conversion u9y9 u over the eastern pacific but is of comparable importance to 2 x during westerly mjo events in the 2 u9 u western pacific

decadal changes in tropical cyclone activity over the western north pacific in the late 1990s a pronounced decadal change in tropical cyclone tc activity over the western north pacific wnp in the late 1990s was identified based on a comparison of the two epochs that occurred before and after the late 1990s the tc genesis number exhibited an evident decrease over the southern wnp s wnp 5 20 n 105 170 e and an increase over the northern wnp n wnp 20 25 n 115 155 e which partly corresponded to a significant northward migration in the seasonal mean latitudinal location of tc genesis i e from 17 2 n to 18 7 n after the late 1990s the northwestward moving track became the most dominant track mode accompanied by the weakening of both the westward moving track and the northeastward recurving track meanwhile the tc occurrence frequency tcf experienced evident increases over southeastern china and the okinawa islands while prominent decreases occurred over the south china sea the philippine sea japan and east of japan changes in the tcf were determined by tc genesis changes tc track shifts and variations in regional tc durations which were all ascribed to the decadal change in tropical indo pacific sea surface temperature the full picture on the decadal changes in the wnp tc activity revealed in this study may provide useful guidance for regional tc seasonal forecasts and future projections

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

tropical indian ocean influence on northwest pacific tropical cyclones in summer following strong el nino abstract in the summer following a strong el nino tropical cyclone tc number decreases over the northwest nw pacific despite little change in local sea surface temperature the authors analysis suggests el nino induced tropical indian ocean tio warming as the cause the tio warming forces a warm tropospheric kelvin wave that propagates into the western pacific inducing surface divergence off the equator the tropospheric kelvin wave suppresses convection and induces an anomalous anticyclone over the nw pacific both anomalies unfavorable for tcs the westerly vertical shear associated with the warm kelvin wave reduces the magnitude of vertical shear in the south china sea and strengthens it in the nw pacific an east west variation that causes tc activity to increase and decrease in respective regions these results help improve seasonal tc prediction

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

indian ocean capacitor effect on indo western pacific climate during the summer following el nino significant climate anomalies persist through the summer june august after el nino dissipates in spring over the equatorial pacific they include the tropical indian ocean tio sea surface temperature sst warming increased tropical tropospheric temperature an anomalous anticyclone over the subtropical northwest pacific and increased mei yu baiu rainfall over east asia the cause of these lingering el nino effects during summer is investigated using observations and an atmospheric general circulation model gcm the results herein indicate that the tio warming acts like a capacitor anchoring atmospheric anomalies over the indo western pacific oceans it causes tropospheric temperature to increase by a moist adiabatic adjustment in deep convection emanating a baroclinic kelvin wave into the pacific in the northwest pacific this equatorial kelvin wave induces northeasterly surface wind anomalies and the resultant divergence in the subtropics triggers suppressed convection and the anomalous anticyclone the gcm results support this kelvin wave induced ekman divergence mechanism in response to a prescribed sst increase over the tio the model simulates the kelvin wave with low pressure on the equator as well as suppressed convection and the anomalous anticyclone over the subtropical northwest pacific an additional experiment further indicates that the north indian ocean warming is most important for the kelvin wave and northwest pacific anticyclone a result corroborated by observations these results have important implications for the predictability of indo western pacific summer climate the spatial distribution and magnitude of the tio warming rather than simply whether there is an el nino in the preceding winter affect summer climate anomalies over the indo western pacific and east asia

constraining the age of liuqu conglomerate southern tibet implications for evolution of the india asia collision zone abstract controversy over the depositional age and provenance of the liuqu conglomerate along the major structural indus yarlung suture zone in south tibet clouds our understanding of the process of the india asia collision here we report low temperature thermochronometric data apatite fission track apatite and zircon u th he for the liuqu conglomerate in the xigaze area our new data constrain its depositional age to latest oligocene early miocene time indicating that rather than having formed immediately following paleogene india asia collision or collision between india and an intra oceanic arc as previously proposed the conglomerate was probably deposited in an intermontane basin at a slightly later time than the gangrinboche group to the north the liuqu conglomerate should therefore not be used as a key horizon in models constraining the early stages of india asia collision our data together with previous studies suggest that the liuqu conglomerate was sourced from the xigaze forearc basin indus yarlung suture zone as well as the tethyan himalaya furthermore our data indicate that exhumation of the liuqu conglomerate commenced at

palynological stratigraphy of the late cretaceous and cenozoic collision related conglomerates at qiabulin xigaze xizang tibet and its bearing on palaeoenvironmental development abstract the stratigraphy age depositional environment and correlation of the collision related variegated sandstone and conglomerate beds exposed along the southern side of the gangdise mountains to the north of the yarlung zangbo suture in the xigaze region xizang tibet have been much debated generally referred to as the dagzhuka formation these beds of great thickness have until recently not been reported to contain any convincing fossils two distinctly different palynological assemblages indicating late cretaceous and oligocene early miocene ages respectively have now been recovered from lithologically distinguishable units of the sedimentary succession at qiabulin to the west of the town of xigaze this means that instead of one stratigraphic unit as has been generally accepted hitherto at least two are represented as a result the dagzhuga formation is considered here to refer only to the cenozoic deposits and the qiabulin formation previously regarded as a synonym is resurrected to accommodate the cretaceous beds these two formations were juxtaposed by tectonic movements our finding indicates that the stratigraphic successions on the southern laurasian continental margin of xizang reflect changes from a deep forearc basin through shallow littoral environments to a mountainous landscape during the late cretaceous and early cenozoic this interpretation compares well with those documented from the gamba and tingri districts to the south of the yarlung zangbo suture

atlantic seasonal hurricane frequency part i el nino and 30 mb quasi biennial oscillation influences abstract this is the first of two papers on atlantic seasonal hurricane frequency in this paper seasonal hurricane frequency as related to e1 nino events during 1900 82 and to the equatorial quasi biennial oscillation qbo of stratospheric zonal wind from 1950 to 1982 is discussed it is shown that a substantial negative correlation is typically present between the seasonal number of hurricanes hurricane days and tropical storms and moderate or strong 15 cases el nin off the south american west coast a similar negative anomaly in hurricane activity occurs when equatorial winds at 30 mb are from an easterly direction and or are becoming more easterly with time during the hurricane season this association of atlantic hurricane activity with el nino can also be made with the southern oscillation index by contrast seasonal hurricane frequency is slightly above normal in non el nino years and substantially above normal when equatorial stratospheric winds blow from a westerly direction and or are bec

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

interannual variation in the tropical cyclone formation over the western north pacific abstract the interannual variation in tropical cyclone genesis frequency over the western north pacific was examined for the active tropical cyclone including summer and fall during 1979 94 an emphasis was put on the possible effect of the interannual variation of atmospheric circulation and monsoon trough on tropical cyclone occurrence the major findings of this study are the following a distinct increase decrease of tropical cyclone genesis frequency occurs north of the climatological location of the monsoon trough in the philippine sea during summers june august with anomalous cold warm sea surface temperature sst over the nino3 region the interannual variation of tropical cyclone genesis in this region results from the appearance of an anomalous cyclonic anticyclonic cell situated in a summer teleconnection wave train emanating from the western tropical pacific and progressing along the rim of the north pacific in addition to the north south interannual variation there is also a long

on the relationship between the qbo and tropical deep convection abstract the height and amount of tropical deep convection are examined for a correlation with the stratospheric quasi biennial oscillation qbo a new 23 yr record of outgoing longwave radiation olr and a corrected 17 yr record of the highly reflective cloud hrc index are used as measures of convection when binned by phase of the qbo zonal means and maps of olr and hrc carry a qbo signal the spatial patterns of the maps highlight the qbo signal of olr and hrc in typically convective regions spectral analysis of zonal mean olr and hrc near the equator reveals significant peaks at qbo frequencies rotated empirical orthogonal function reof analysis is used to determine if enso variations of convection are aliased into the observed qbo signals some analyses are repeated using the olr record after enso reof modes have been removed yielding very similar results compared to the original analyses it appears that the qbo signal is distinct from the enso signal although the relative brevity of the

paleomagnetic constraints on the paleolatitude of the lhasa block during the early cretaceous implications for the onset of india asia collision and latitudinal shortening estimates across tibet and stable asia abstract interbedded volcano sedimentary sequences are well exposed in the northern part of the lhasa block in southern tibet zircon u pb dating results from two samples indicate that the emplacement age of the duoni formation volcanic flows is 120 2 0 5 ma paleomagnetic results from 235 progressively demagnetized volcanic rock samples 25 sites and 41 sandstone samples 5 sites indicate that the dominant remanence carriers are ti poor titanomagnetite and ti poor titanohematite in the volcanic samples and ti rich titanomagnetite in the sandstone samples rock magnetic investigations systematic demagnetization behavior positive fold test results and direct petrographic identifications all indicate that the paleodirections recorded by the chemically stable magnetic particles are primary thermal remanent magnetization in the volcanic flows and primary detrital remanent magnetization in the sandstones the tilt corrected chrm mean direction is d i 356 4 16 4 with 95 6 3 n 19 corresponding to a paleopole position of p 66 9 n p 281 2 e with a95 6 1 combined with previously published results the geochronological dating and paleomagnetic analysis indicate a paleolatitude of 13 1 2 7 n for the southern margin of the lhasa block during the early cretaceous therefore the southernmost margin of the eurasian continent likely remained at the low middle paleolatitude of 13 1 2 7 n between the early cretaceous and the paleocene based on comparisons to results from the tethyan himalayan block and the reference poles from stable india and eurasia the low middle paleoposition of the lhasa block during the early cretaceous through early paleocene suggests that the initial contact between india and asia occurred at ca 59 3 ma under the assumption of a rigid eurasian plate this timing implies that a total collision related latitudinal convergence of 1450 400 km 13 1 3 7 n has been accommodated by folding thrust faulting normal faulting crustal thickening intracontinental subduction in tibet and central asia and southeastward continental extrusion of the indo china block from the eastern syntaxis between the lhasa block and stable asia

paleocene eocene foreland basin evolution in the himalaya of southern tibet and nepal implications for the age of initial india asia collision siliciclastic sedimentary rocks derived from the southern lhasa terrane sitting depositionally upon rocks of the northern indian passive continental margin provide an estimate of the age of initial contact between the continental parts of the indian and asian plates we report sedimentological sedimentary petrological and geochronological data from upper cretaceous paleocene strata in the sangdanlin section located along the southern flank of the indus yarlung suture zone in southern tibet this is probably the most proximal and therefore the oldest record of the india asia collision these strata were deposited by high density turbidity currents or concentrated density flows and suspension settling of pelagic biogenic debris in a deep marine setting an abrupt change from quartz arenitic to feldspatholithic sandstone compositions marks the transition from indian to asian sediment provenance the abrupt compositional change is accompanied by changes in u pb ages of detrital zircons diagnostic of a sediment provenance reversal from indian to asian sources the timing of the transition is bracketed between 60 ma and 58 5 0 6 ma by detrital zircon u pb ages and zircon u pb ages from a tuffaceous bed in the upper part of the section in the context of a palinspastically restored regional paleogeographic framework data from the sangdanlin section combined with previously published data from the northern tethyan himalaya and the frontal nepalese lesser himalaya and subhimalaya suggest that a flexural wave migrated 1300 km southward across what is now the himalayan thrust belt from paleocene time to the present

tectonic setting origin and obduction history of the spontang ophiolite ladakh himalaya nw india abstract the spontang ophiolite forms the highest tectonic thrust slice above the mesozoic early tertiary continental margin of the north indian plate in the ladakh zanskar himalaya detailed field mapping combined with geochemical analysis has defined two major units a full ophiolite sequence spontang ophiolite overlain by an upper unit consisting of 500 m thick basalt andesite volcanic and volcano sedimentary rocks of island arc affinity spong arc the spontang ophiolite comprises a harzburgitic mantle sequence gabbroic and ultramafic cumulates isotropic gabbros and highly tectonized sheeted dikes feeding pillow lavas with a few uncommon highly fractionated plagiogranites a separate lherzolitic peridotite unit affected by possible transform related shearing was thrust over the harzburgites in the west possibly during the early stages of subduction initiation beneath the ophiolite whole rock geochemistry and u pb geochronology show that the ophiolite formed at a normal mid ocean ridge sp

paleogene carbonate microfacies and sandstone provenance gamba area south tibet stratigraphic response to initial india asia continental collision abstract the paleogene marine strata in the gamba area of south tibet comprise carbonates of the zongpu formation and siliciclastic rocks of the enba and zhaguo formations documenting the final stages of marine deposition in the tethyan himalaya the 350 m thick zongpu formation was dated as late danian to ypresian based on larger benthic foraminifers thirteen distinct microfacies identify three sedimentary environments mudstone wackestone with udoteacean algae bioclastic peloidal packstone packstone with rotaliids and green algae floatstone with alveolina and orbitolites were deposited in restricted lagoonal environments bioclastic packstone and grainstone with rotaliids were deposited in high energy shoal environments floatstones with nummulitids or alveolinids were deposited in shallow open marine environments the zongpu formation was accumulated on a carbonate ramp it documents two deepening upward sequences separated by an unconformity corresponding to the palaeocene eocene boundary and marked by a conglomerate with limestone clasts the overlying enba formation comprises greenish grey calcareous shales intercalated with litho quartzose sandstones in the upper part and capped by subaerial litho quartzose red beds of the zhaguo formation petrographic analysis detrital zircon geochronology and hf isotopic data indicate that detritus in the enba and zhaguo formations deposited on the indian passive margin was derived from the asian active margin in the north these clastic units were thus deposited after the onset of the india asia continental collision in the early himalayan foreland basin major lithological and paleoenvironmental changes occurred at three stratigraphic levels the jidula zongpu boundary 62 ma the paleocene eocene boundary 56 ma and the zongpu enba boundary 51 ma our provenance study confirms that the india asia collision was already under way during the deposition of the enba member i e by 51 ma and along with facies analysis and general palaeogeographic considerations indicates that neo tethys was still wide open during the early middle paleocene it is thus argued consistently with previous studies that the paleocene eocene disconformity documented in the gamba area as in the northwestern tethyan himalaya is likely to record flexural uplift consequent to initial underthrusting of the indian continental margin beneath asia at or just a little earlier than 56 ma

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

two stages of granulite facies metamorphism in the eastern himalayan syntaxis south tibet petrology zircon geochronology and implications for the subduction of neo tethys and the indian continent beneath asia the eastern himalayan syntaxis in southeastern tibet consists of the lhasa terrane high himalayan rocks and indus tsangpo suture zone the lhasa terrane constitutes the hangingwall of a subduction zone whereas the high himalayan rocks represent the subducted indian continent our petrological and geochronological data reveal that the lhasa terrane has undergone two stages of medium p metamorphism an early granulite facies event at c 90 ma and a late amphibolite facies event at 36 33 ma however the high himalayan rocks experienced only a single high p granulite facies metamorphic event at 37 32 ma it is inferred that the late cretaceous c 90 ma medium p metamorphism of the southern lhasa terrane resulted from a northward subduction of the neo tethyan ocean and that the oligocene 37 32 ma high p 1 8 1 4 gpa rocks of the high himalayan and coeval medium p 0 8 1 1 gpa rocks of the lhasa terrane represent paired metamorphic belts that resulted from the northward subduction of the indian continent beneath asia our results provide robust constraints on the mesozoic and cenozoic tectonic evolution of south tibet

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

reconciling himalayan midcrustal discontinuities the main central thrust system the occurrence of thrust sense tectonometamorphic discontinuities within the exhumed himalayan metamorphic core can be explained as part of the main central thrust system this imbricate thrust structure which significantly thickened the orogenic midcrustal core comprises a series of thrust sense faults that all merge into a single detachment the existence of these various structures and their potential for complex overprinting along the main detachment may help explain the contention surrounding the definition mapping and interpretation of the main central thrust the unique evolution of specific segments of the main central thrust system along the orogen is interpreted to be a reflection of the inherent basement structure and ramp position and structural level of exposure of the mid crust this helps explain the variation in the timing and structural position of tectonometamorphic discontinuities along the length of the mountain belt

general circulation model sensitivity experiments with pole centered supercontinents although high co2 levels appear to be required to explain most times of ice free climates in earth history previous calculations with a two dimensional energy balance model ebm have suggested that the low thermal inertia associated with pole centered supercontinents might allow seasonally snow free conditions to develop in the absence of high co2 this conclusion has been challenged by other ebm calculations with lower geographic resolution but more explicit model physics to address this problem more directly we have undertaken a series of general circulation model gcm simulations with a new model genesis developed at the national center for atmospheric research for idealized pole centered supercontinents we obtain mid summer south polar temperatures of 22 c for the present orbital configuration and 15 c with a modified orbital configuration favorable to cool summers these results support the two dimensional ebm calculations however their application to real world situations of the paleozoic is hampered by the fact that due to extremely high sea levels and decreased solar luminosity the best time periods for applying this concept 420 360 ma have supercontinent areas too small to yield an ice free climate for all orbital configurations snow free conditions with summer temperatures at the pole of 25 c are simulated for a hot orbital configuration for one realistic geography configuration carboniferous 300 ma these results indicate that while the basic idea of snow free pole centered supercontinents is still theoretically possible the real world application to specific past time periods requires some modifications our reframed hypothesis states that for realistic geography and high co2 levels pole edged supercontinents could have permanent snow cover but pole centered supercontinents would be snow free in summer

constraints on the collision and the pre collision tectonic configuration between india and asia from detrital geochronology thermochronology and geochemistry studies in the lower indus basin pakistan abstract knowledge of the timing of india asia collision is a fundamental prerequisite for understanding the evolution of the himalayan tibetan orogen and its role in global climate oceanic chemistry and ecological evolution despite much active research the basic pre collision tectonic configuration and the timing of terminal india asia suturing remain debated for example debates regarding when and how the intervening kohistan ladakh arc was sutured with india and asia still remain elusive some models propose the arc collided with asia at about 100 ma with india asia collision at ca 55 ma whilst a newer model proposed the arc s collision with india at 50 ma and subsequently with asia at 40 ma another example is the recent proposition that an oceanic greater india basin separated the tethyan himalaya microcontinent from the remaining indian plate until 20 25 ma with the consumption of this oceanic basin marking the final collision at this time these controversies relate to whether the commonly documented 50 ma contact represents the terminal india asia suturing or the amalgamation between various arcs or microcontinents with india or asia here we present an integrated provenance study of geochronology thermochronology and geochemistry on the late cretaceous pleistocene sediments from the lower indus basin on the indian plate the detrital zircon u pb and fission track data show a reversal in sediment source from a pure indian signature to increasing inputs from the suture zone and the asian plate between the middle paleocene and early oligocene the nd and sr isotopes narrow down this change to 50 ma by revealing input of asian detritus and the establishment of a nd sr isotopic pattern similar to the present day indus fan by 50 ma with no significant variations up section contrary to what might be expected if later major collisions had occurred our isotopic data indicate that greater india was occupied by a fluvial deltaic system analogous to the present day indus and named as the paleo indus which has been transporting asian detritus southward across the suture zone and kohistan ladakh arc since 50 ma suggesting no other ocean basins intervened between india and asia after this time in this region our data require that in the west the india asia collision were accomplished by 50 ma

influence of stratospheric quasi biennial oscillation on tropical cyclone tracks in the western north pacific 1 the possible influence of the stratospheric quasibiennial oscillation qbo on tropical cyclone tc passages in the western north pacific wnp is examined using tc data recorded by the tokyo typhoon center and the qbo index derived from reanalysis data the influence is observed to be significant the number of tcs approaching the east china sea is large during the westerly phase of the qbo however during the easterly phase the number of tcs approaching the eastern offshore of japan is large this difference in the tc tracks is found to be related to the background flow change associated with the qbo however the total number of tc events over the wnp and the sum of the number of tcs approaching the aforementioned two regions appear to be unrelated to the qbo phases citation ho c h h s kim j h jeong and s w son 2009 influence of stratospheric quasi biennial oscillation on tropical cyclone tracks in the western north pacific geophys res lett 36 l06702 doi 10 1029 2009gl037163

geology of the cenozoic indus basin sedimentary rocks paleoenvironmental interpretation of sedimentation from the western himalaya during the early phases of india eurasia collision this study reassesses the stratigraphy sedimentology and provenance of the indus basin sedimentary rocks deposited within the indus tsangpo suture zone itsz during the early phases of india eurasia collision using field observations biostratigraphy and petrographic and isotopic analyses we create a paleodepositional reconstruction within the paleotectonic setting of the early phases of india eurasia collision we then re examine existing constraints to the timing of india eurasia collision previously interpreted from the earliest occurrence of mixed indian and eurasian derived detritus in the succession from mid cretaceous to early paleocene times the jurutze and sumda formations were deposited within an arc bounded marine basin between the dras and kohistan ladakh island arcs the 51 ma aged deltaic chogdo formation then filled the basin until deposition of the 50 8 49 4 ma aged nummulitic limestone during a marine incursion before continental facies developed in an evolving intermountain basin with the deposition of the paleogene indus group within these systems sediment was sourced from the eurasian margin to the north and was transported southward into the suture zone in this section we see no unequivocal evidence of indian plate input to the sedimentary succession and thus no evidence of mixed indian eurasian derived detritus indicative of india asia collision until the upper stratigraphic horizons of the indus group when facies are representative of an axial northwesterly flowing river system we suggest that the paleo indus river was initiated within the itsz during late oligocene early miocene times sedimentation of the indus group continued until the late miocene

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

paleocene eocene record of ophiolite obduction and initial india asia collision south central tibet 1 uppermost cretaceous to eocene marine sedimentary sequences occur both to the south and north of the yarlung zangbo suture in south central tibet they consist of indian margin strata of the northern tethyan himalaya and asian margin strata of the gangdese forearc both assemblages are characterized by major changes in depositional environment and sedimentary provenance at 65 ma and an appearance of detrital chromium rich spinel of ophiolite affinity tio2 generally 0 1 wt during the paleocene ophiolitic melange exposed along the suture could have provided a source for detrital spinel the melange occurs in the hanging wall of a north dipping south directed mylonitic shear zone which includes a tectonic sliver of mafic schist amphibole from the schist yields 40ar 39ar ages of 63 ma which we attribute to cooling during slip along the shear zone and southward obduction of the melange melange obduction was coeval with the development of an angular unconformity within the gangdese forearc basin to the north between late maastrichtian time and 62 ma upper paleocene to middle eocene sandstones in the northern tethyan himalaya yield 200 120 ma u pb detrital zircon ages and 190 170 ma 40ar 39ar detrital mica ages these detrital grains were most likely sourced from regions north of the yarlung zangbo suture suggesting that onset of india asia collision in south central tibet is middle eocene or older in age collectively our results support previous suggestions that oceanic rocks were obducted onto the northern margin of india during latest cretaceous earliest tertiary time coeval changes in gangdese forearc sedimentation raise the possibility that this obduction event marks onset of tectonic interaction between india and asia at 65 ma alternatively in concert with the conventional view of eocene collision initiation the obducted oceanic rocks may be of intraoceanic origin while coeval changes in gangdese forearc sedimentation may be a consequence of an increase in the rate of ocean continent convergence following the demise of the intraoceanic subduction zone

impact of the cross tropopause wind shear on tropical cyclone genesis over the western north pacific in may the role of the 50 to 200 hpa zonal wind shear in modulating tropical cyclone tc genesis over the western north pacific wnp in may is investigated in this study concurrent with the strong cross tropopause shear over the key region 0 5 n 160 180 e suppressed convection was observed over the tropical wnp especially over the south china sea and the philippines the monsoon trough mt was confined westward however enhanced convection occurred in the weak shear years and the mt extended eastward this cross tropopause wind shear is negatively correlated with tc genesis in may with a decreased increased number of tcs corresponding to strong weak cross tropopause wind shear this cross tropopause wind shear can be treated as the combined impacts of the el nino southern oscillation enso events and the stratospheric quasi biennial oscillation qbo when decaying el nino events coupled with the easterly phase of the qbo were noted the cross tropopause wind shear was stronger with weakened convection and an enhanced western pacific subtropical high was observed tcs are rarely generated during these years in contrast the modulation of the qbo westerly phase on decaying la nina events is limited affected by the qbo westerly phase tc genesis in the may following la nina events is only slightly enhanced the energy analysis indicates that the combined impacts of the decaying el nino events and the qbo easterly phase might suppress the barotropic eddy kinetic energy conversion in may whereas the decaying la nina events and the qbo west phase act in an opposite manner

extrusion vs duplexing models of himalayan mountain building 1 discovery of the pabbar thrust confirms duplex dominated growth of the northwestern indian himalaya since mid miocene ongoing himalayan growth is generally thought to be dominated by duplexing and or extrusion processes these models may be tested by reconstructing himalayan fold thrust belt growth since the middle miocene however our knowledge of basic structural geometry remains too fragmentary to resolve the issue even in areas with rich stratigraphic diversity such as the northwestern indian himalaya in this region a primary outstanding question involves the uncertain relationship of the berinag thrust and the tons thrust structures with displacements of 80 km and 40 km respectively the uncertain geometry and kinematics allow for the complete range of duplexing or extrusion processes for the integrated kinematic history since the middle miocene to address this issue field mapping and kinematic analysis were performed to reconstruct the deformation of the lesser himalayan sequence in the northwest indian himalaya our results reveal a new discovery a 450 m thick top to southwest shear zone termed the pabbar thrust the pabbar thrust placed the outer lesser himalayan sequence the tons thrust hanging wall directly on the berinag group the berinag thrust hanging wall this discovery requires that the berinag thrust and tons thrust are in fact the same structure and discrete duplexing processes dominated growth of the northwest indian himalaya for the past 10 15 million years along strike extension of these kinematics and corresponding geometries is consistent with the observed orogenic framework and resolves a stratigraphic continuity problem across the india west nepal border where prior work suggests that structures are continuous but stratigraphy does not match

influence of the monsoon trough on westward propagating tropical waves over the western north pacific part ii energetics and numerical experiments abstractpart i of this study examined the modulation of the monsoon trough mt on tropical depression td type mixed rossby gravity mrg and equatorial rossby er waves over the western north pacific based on observations this part investigates the interaction of these waves with the mt through a diagnostics of energy conversion that separates the effect of the mt on td mrg and er waves it is found that the barotropic conversion associated with the mt is the most important mechanism for the growth of eddy energy in both td mrg and er waves the large rotational flows help to maintain the rapid growth and tilted horizontal structure of the lower tropospheric waves through a positive feedback between the wave growth and horizontal structure the baroclinic conversion process associated with the mt contributes a smaller part for td mrg waves but is of importance comparable to barotropic conversion for er waves as it can produce the tilted vertical structure the growth rates of the waves are much larg

reconstructing the greater indian margin a balanced cross section in central nepal focusing on the lesser himalayan duplex across much of the himalaya rocks in the lesser himalayan duplex are covered by roof thrusts of other paleoproterozoic lesser himalayan rocks or greater himalayan rocks however in central nepal between the main central thrust and the main boundary thrust lesser himalayan rocks are exposed in structurally complex relationships within the lesser himalayan duplex we present two balanced cross sections with different stratigraphy involved in the duplex one with the basal kuncha formation and one without this unit to test stratigraphic assumptions both cross sections have roof thrust sheets consisting of the main central thrust ramgarh munsiari thrust and trishuli thrust folded over the hinterland dipping lesser himalayan duplex dissected by late faults cross section 1 has a shortening estimate from the main central thrust to the main boundary thrust including motion on the main central thrust of 359 km or 78 cross section 2 has a shortening estimate of 349 km or 76 over the same region because the amount and percentage of shortening are not significantly different between the two cross sections the different stratigraphic assumptions did not change the shortening results this similarity suggests that many of the choices made when constructing a cross section may be less important than researchers previously thought

tectonostratigraphic subdivisions of the himalaya a view from the west 1 indian plate rocks in the central himalaya have traditionally been divided into orogen parallel fault bound tectonostratigraphic zones a straightforward westward extrapolation of these zones has proved problematic in part because of a lack of consensus on the existence or significance of major faults within the metamorphic zone of the indian plate in pakistan where more than 10 locations for the main central thrust mct have been proposed we address this ambiguity by systematically tracing established central himalayan tectonostratigraphy around the western himalayan syntaxis and across pakistan this exercise reveals the following stratigraphic and structural relationships 1 there is a westward decrease in neogene shortening across the himalayan fold and thrust belt such that there is no age equivalent thrust in pakistan with displacement and metamorphic juxtaposition equivalent to the central himalayan mct 2 shortening across the fold and thrust belt in western pakistan is concentrated in the unmetamorphosed foreland as opposed to the metamorphic zone in the central himalaya 3 lesser himalayan higher himalayan and tethyan rocks are in stratigraphic order within the metamorphic zone of pakistan which appears to be the metamorphic equivalent of kashmir tethyan stratigraphy 4 the combination of early paleozoic and late paleozoic tectonism in pakistan has locally eliminated upper proterozoic higher himalayan rock and lower to middle paleozoic tethyan rock from the metamorphic zone of pakistan 5 late cretaceous and or early paleocene proto himalayan deformation in the pakistan foreland telescoped and eroded stratigraphy prior to the main phase of himalayan orogeny 6 tectonostratigraphic zones are offset in eastern pakistan by the transverse jhelum balakot fault 7 there is no evidence within the indian plate of pakistan for a large scale normal fault system comparable to the south tibetan detachment system 8 stratigraphy as well as the age and tectonic setting of deformation and metamorphism must be taken into account when drawing tectonostratigraphic zones

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

recent investigations of the 0 5 ma geomagnetic field recorded by lava flows we present a synthesis of 0 5 ma paleomagnetic directional data collected from 17 different locations under the collaborative time averaged geomagnetic field initiative tafi when combined with regional compilations from the northwest united states the southwest united states japan new zealand hawaii mexico south pacific and the indian ocean a data set of over 2000 sites with high quality stable polarity and declination and inclination measurements is obtained this is a more than sevenfold increase over similar quality data in the existing paleosecular variation of recent lavas psvrl data set and has greatly improved spatial sampling the new data set spans 78 s to 53 n and has sufficient temporal and spatial sampling to allow characterization of latitudinal variations in the time averaged field taf and paleosecular variation psv for the brunhes and matuyama chrons and for the 0 5 ma interval combined the brunhes and matuyama chrons exhibit different taf geometries notably smaller departures from a geocentric axial dipole field during the brunhes consistent with higher dipole strength observed from paleointensity data geographical variations in psv are also different for the brunhes and matuyama given the high quality of our data set polarity asymmetries in psv and the taf cannot be attributed to viscous overprints but suggest different underlying field behavior perhaps related to the influence of long lived core mantle boundary conditions on core flow psv as measured by dispersion of virtual geomagnetic poles shows less latitudinal variation than predicted by current statistical psv models or by previous data sets in particular the brunhes data reported here are compatible with a wide range of models from those that predict constant dispersion as a function of latitude to those that predict an increase in dispersion with latitude discriminating among such models could be helped by increased numbers of low latitude data and new high northern latitude sites tests with other data sets and with simulations indicate that some of the latitudinal signature previously observed in vgp dispersion can be attributed to the inclusion of low quality insufficiently cleaned data with too few samples per site our matuyama data show a stronger dependence of dispersion on latitude than the brunhes data the taf is examined using the variation of inclination anomaly with latitude best fit two parameter models have axial quadrupole contributions of 2 4 of the axial dipole term and axial octupole contributions of 1 5 approximately 2 of the octupole signature is likely the result of bias incurred by averaging unit vectors