kinematic model for the main central thrust in nepal we present a kinematic model for the himalayan thrust belt that satisfies structural and metamorphic data and explains recently reported late miocene pliocene geochronologic and thermochronologic ages from rocks in the main central thrust zone in central nepal at its current exposure level the main central thrust juxtaposes a hanging wall flat in greater himalayan rocks with a footwall flat in lesser himalayan rocks of the ramgarh thrust sheet which is the roof thrust of a large lesser himalayan duplex sequential emplacement of the main central early miocene and ramgarh middle miocene thrust sheets was followed by insertion of thrust sheets within the lesser himalayan duplex and folding of the main central and ramgarh thrusts during late miocene pliocene time thorium lead th pb ages of monazite inclusions in garnets from central nepal record the timing of coeval progressive metamorphism of lesser himalayan rocks in the footwall of the main central thrust although this model does not rule out minor late stage reactivation of the main central thrust major late miocene reactivation is not required

strain softening along the mct zone from the sikkim himalaya relative roles of quartz and micas abstract in the darjeeling sikkim himalaya two distinct faults form the main central thrust mct the structurally higher mct1 and the lower mct2 each has accommodated translation greater than 100 km the lower mct2 places greater himalayan amphibolite grade paro lingtse gneiss over lesser himalayan greenschist grade daling metapelites the mct2 is folded by the underlying lesser himalayan duplex and is exposed at different structural positions of the fold at pelling the mct2 zone is exposed as a 373 m thick nw dipping fault zone that exposes 19 m of hanging wall mylonitized lingtse gneiss the lingtse protolith shows evidence of amphibolite grade plastic deformation features in quartz and feldspar within the hanging wall mylonite zone hwmz quartz and feldspar have undergone grain size reduction by different deformation mechanisms and feldspars are sericitized suggesting the presence of fluids during deformation we estimate a temperature of 300 c within the fault zone during fluid assisted retrogression and deformation reaction softening of feldspars produced a large proportion of intrinsically weak matrix this in combination with development of a strong foliation defined by parallel mica grains resulted in strain softening along the mct2 zone and concentrated the deformation along a thin zone or zones

isotopic constraints on the structural relationships between the lesser himalayan series and the high himalayan crystalline series garhwal himalaya nd and sr isotope systematics may provide important constraints on the location of major thrust systems that separate lithologically similar sedimentary sequences the potential of the technique is illustrated by this isotopic study of the main central thrust system of the himalaya nd isotope data from the garhwal himalaya indicate that metasedimentary rocks from the vaikrita group nd 14 to 19 correlate closely with those from the high himalayan crystalline series which constitutes the hanging wall lithologies of the main central thrust in contrast metasedimentary rocks from the munsiari group nd 23 to 28 show marked similarities to the lesser himalayan series in the footwall of the main central thrust sr isotopes support the correlations in that the vaikrita group shows partial reequilibration at 500 ma whereas the munsiari group has not undergone sr isotope homogenization since 1800 ma thus the vaikrita thrust that juxtaposes these two formations is recognized as the main central thrust in garhwal himalaya the thrust coincides approximately with the location of the kyanite isograd confirming that inverted metamorphism is characteristic of both hanging wall and footwall of the main central thrust along the tons thrust known locally as the srinagar thrust 50 km south of the main central thrust low grade quartzarenites with nd sr isotope and trace element characteristics typical of lesser himalayan formations have been emplaced on phyllites and siltstones with geochemical characteristics of the high himalayan crystalline series the field relationships most probably result from out of sequence thrusting in which lesser himalayan series rocks to the north were emplaced over low grade equivalents of the high himalayan crystalline series preserved in the external part of the orogen this study establishes the value of isotope data for lithostratigraphic correlations within orogenic belts

a penultimate glacial monsoon record from hulu cave and two phase glacial terminations oxygen isotope records of three stalagmites from hulu cave china extend the previous high resolution absolute dated hulu asian monsoon record from the last to the penultimate glacial and deglacial periods the penultimate glacial monsoon broadly follows orbitally induced insolation variations and is punctuated by at least 16 millennial scale events we confirm a weak monsoon interval between 135 5 1 0 and 129 0 1 0 ka prior to the abrupt increase in monsoon intensity at asian monsoon termination ii based on correlations with both marine ice rafted debris and atmospheric ch 4 records we demonstrate that most of marine termination ii the full rise in antarctic temperature and atmospheric co 2 and much of the rise in ch 4 occurred within the weak monsoon interval when the high northern latitudes were probably cold from these relationships and similar relationships observed for termination i we identify a two phase glacial termination process that was probably driven by orbital forcing in both hemispheres affecting the atmospheric hydrological cycle and combined with ice sheet dynamics

dynamic recrystallization mechanisms and their transition in the daling thrust dt zone darjeeling sikkim himalaya abstract the daling thrust dt delineates a zone of intense shear localization in the lesser himalayan sequence lhs of the darjeeling sikkim himalaya from microstructural studies of deformed quartzite samples we show a transition in the dynamic recrystallization mechanism with increasing distance from the dt dominated by grain boundary bulging blg recrystallization closest to the dt and progressively replaced by sub grain rotation sgr recrystallization away from the thrust the transition is marked by a characteristic variation in the fractal dimension d of grain boundaries estimated from the area perimeter method for the blg regime d 1 046 which decreases significantly to a value as low as 1 025 for the sgr regime using the available thermal data for blg and sgr recrystallization we infer increasing deformation temperatures away from the dt in the hanging wall based on the quartz piezometer our estimates reveal strong variations in the flow stress 59 00 mpa to 16 00 mpa over a distance of 1 2 km from the dt deformation mechanism maps constructed for different temperatures indicate that the strain rates 10 12 s 1 to 10 14 s 1 comply with the geologically possible range finally we present a mechanical model to provide a possible explanation for the cause of stress intensification along the dt

miocene dextral shearing between himalaya and tibet the main central thrust zone is the major structural feature of the himalayas it was active during miocene time and accommodated at least 100 km and possibly as much as 300 km of convergence between india and tibet we present evidence that late in the tectonic history another large shear zone located north of the high himalayas the north himalayan shear zone underwent a phase of dextral strike slip motion the most plausible explanation for this phase of motion is that it reflects the onset of extension in tibet it is usually thought that extension began about 2 5 ma the phase of dextral shear we report occurred between 25 and 15 ma if our explanation for this movement is correct it places the onset of extension at least 12 m y earlier than previously thought

quantification of holocene asian monsoon rainfall from spatially separated cave records abstract a reconstruction of holocene rainfall is presented for southwest china an area prone to drought and flooding due to variability in the east asian monsoon the reconstruction is derived by comparing a new high resolution stalagmite 18 o record with an existing record from the same moisture transport pathway the new record is from heshang cave 30 27 n 110 25 e 294 m and shows no sign of kinetic or evaporative effects so can be reliably interpreted as a record of local rainfall composition and temperature heshang lies 600 km downwind from dongge cave which has a published high resolution 18 o record wang y j cheng h edwards r l he y q kong x g an z s wu j y kelly m j dykoski c a li x d 2005 the holocene asian monsoon links to solar changes and north atlantic climate science 308 854 857 by differencing co eval 18 o values for the two caves secondary controls on 18 o e g moisture source moisture transport non local rainfall temperature are circumvented and the resulting 18 o signal is controlled directly by the amount of rain falling between the two sites this is confirmed by comparison with rainfall data from the instrumental record which also allows a calibration of the 18 o proxy the calibrated 18 o record provides a quantitative history of rainfall in southwest china which demonstrates that rainfall was 8 higher than today during the holocene climatic optimum 6 ka but only 3 higher during the early holocene significant multi centennial variability also occurred with notable dry periods at 8 2 ka 4 8 4 1 ka 3 7 3 1 ka 1 4 1 0 ka and during the little ice age this holocene rainfall record provides a good target with which to test climate models the approach used here of combining stalagmite records from more than one location will also allow quantification of rainfall patterns for past times in other regions

temperature and strain gradients through lesser himalayan rocks and across the main central thrust south central bhutan implications for transport parallel stretching and inverted metamorphism in order to understand mass and heat transfer processes that operated during himalayan orogenesis we collected temperature finite and incremental strain and kinematic vorticity data through a 5 km thickness of lesser and greater himalayan rocks in southern bhutan this transect crosses two major shear zones the main central thrust mct and shumar thrust st raman spectroscopy on carbonaceous material and garnet biotite thermometry are integrated with deformation temperatures from quartz petrofabrics these data define inverted field gradients that correspond in structural position with the mct and st which are separated by sections in which temperatures remain essentially constant temperatures increase from 400 500 c to 700 750 c between 675 m below and 200 m above the mct this defines a 269 44 c km inverted gradient interpreted to have formed via high magnitude 100 250 km shearing on a discrete mct zone delineated by the limits of inverted metamorphism temperatures increase from 300 400 c to 400 530 c across the st which is attributed to differences in maximum burial depth of hanging wall and footwall rocks strain and vorticity data indicate that lesser and greater himalayan rocks were deformed by layer normal flattening transport parallel lengthening and foliation normal shortening increase from 38 71 and 36 49 respectively between 2 3 1 0 km below the mct the mct acted as a stretching fault with translation on the order of 100 s of km accompanied by transport parallel stretching of footwall and hanging wall rocks on the order of 10 s of km this demonstrates that stretching accommodated between major shear zones can make a significant contribution to cumulative mass transfer

orogen parallel deformation of the himalayan midcrust insights from structural and magnetic fabric analyses of the greater himalayan sequence annapurna dhaulagiri himalaya central nepal the metamorphic core of the himalaya greater himalayan sequence ghs in the annapurna dhaulagiri region central nepal recorded orogen parallel stretching during mid crustal evolution anisotropy of magnetic susceptibility ams and field based structural analyses suggest that mid crustal deformation of the amphibolite facies core of the ghs occurred under an oblate sub oblate strain regime with associated formation of low angle northward dipping foliation magnetic and mineral stretching lineations lying within this foliation from the top of the ghs record right lateral orogen parallel stretching we propose that oblate strain within a mid crustal flow accommodated oblique convergence between india and the arcuate orogenic front without need for strain partitioning in the upper crust oblate flattening may have also promoted orogen parallel melt migration and development of melt depleted regions between km3 scale leucogranite culminations at 50 100 km intervals along orogen strike following cessation of flow continued oblique convergence led to upper crustal strain partitioning between orogen perpendicular convergence on thrust faults and orogen parallel extension on normal and strike slip faults in the annapurna dhaulagiri himalaya orogen parallel stretching lineations are interpreted as a record of transition from mid crustal orogen perpendicular extrusion to upper crustal orogen parallel stretching our findings suggest that mid crustal flow and upper crustal extension could not be maintained simultaneously and support other studies from across the himalaya which propose an orogen wide transition from mid crustal orogen perpendicular extrusion to upper crustal orogen parallel extension during the mid miocene the 3d nature of oblate strain and orogen parallel stretching cannot be replicated by 2d numerical simulations of the himalayan orogen

the east asian monsoon during mis 2 expressed in a speleothem δ18o record from jintanwan cave hunan china abstract stalagmite j1 from jintanwan cave hunan china provides a precisely dated decadally resolved 18o proxy record of paleoclimatic changes associated with the east asian monsoon from 29 5 to 14 7 ka and from 12 9 to 11 0 ka at the time of the last glacial maximum lgm the east asian summer monsoon weakened and then strengthened in response to changes in northern hemisphere insolation as the ice sheets retreated the east asian summer monsoon weakened especially during heinrich event h1 when atmospheric and oceanic teleconnections transferred the climatic changes around the north atlantic to the monsoonal regions of eastern asia a depositional hiatus between 14 7 and 12 9 ka leaves the deglacial record incomplete but an abrupt shift in 18o values at 11 5 ka marks the end of the younger dryas and the transition into the holocene comparisons of the j1 record to other chinese speleothem records indicate synchronous climatic changes throughout monsoonal china further comparisons to a speleothem record from western asia socotra island and to greenland ice cores support hemispherical scale paleoclimatic change spectral and wavelet analyses reveal centennial and decadal scale periodicities that correspond to solar frequencies and to oscillations in atmospheric and oceanic circulation

development of crystallographic preferred orientation and microstructure during plastic deformation of natural coarse grained quartz veins 1 the microstructure and crystallographic preferred orientation cpo of quartz were quantified in 17 samples of natural monomineralic tabular veins the veins opened and were deformed up to shear strain 15 in a small temperature window about 25 c above 500 c as established by ti in quartz thermometry the veins filled a set of fractures within the adamello tonalite southern alps italy and localized homogeneous simple shear during postmagmatic cooling the local square millimeter scale and bulk square centimeter cpo were investigated by computer integrated polarization microscopy cip and x ray texture goniometry weakly deformed veins wdv 1 consist of millimeter to centimeter sized crystals with a strong cpo showing a c axis girdle slightly inclined mostly with the shear sense to the foliation xy plane and a strong maximum close to the lineation x moderately deformed veins mdv 2 3 consist of elongated nonrecrystallized ribbon grains and most have a cpo showing a strong y maximum of c axes some with weak extension into a yz girdle strongly deformed veins sdv 4 to 15 are pervasively to completely recrystallized to fine 34 40 m grain size aggregates with a strong cpo similar to that of mdv the slip systems during plastic deformation were dominantly prism a with subordinate rhomb and basal a slip recrystallization occurred rather abruptly for 3 4 in contrast to dislocation creep experiments in quartz and other minerals a steady state recrystallized fabric is achieved at early stages of deformation 4 as there is no evidence with increasing strain of strengthening of the cpo of rotation of the fabric skeleton or of change in grain size wdv represent weakly deformed relicts of veins with an initial cpo believed to have developed during crystal growth but unsuitably oriented for prism a slip during subsequent shear mdv and sdv appear to derive from veins different from wdv where the vein crystals grew with orientation favorable for prism a slip the relationship between the initial growth cpo and the kinematic framework suggests that veins opened at a temperature close to that at which there is a switch between the activity of prism c and prism a slip with the temperature of growth causing growth of crystals well oriented for slip the initial cpo of veins from which quartz mylonites are commonly derived plays a critical role in the fabric evolution the strong growth and strain induced cpos of these sheared veins inhibited significant reworking during lower temperature stages of pluton cooling when basal a slip would have been dominant

tectonometamorphic evolution of the himalayan metamorphic core between the annapurna and dhaulagiri central nepal the metamorphic core of the himalaya in the kali gandaki valley of central nepal corresponds to a 5 km thick sequence of upper amphibolite facies metasedimentary rocks this greater himalayan sequence ghs thrusts over the greenschist to lower amphibolite facies lesser himalayan sequence lhs along the lower miocene main central thrust mct and it is separated from the overlying low grade tethyan zone tz by the annapurna detachment structural petrographic geothermobaro metric and thermochronological data demonstrate that two major tectonometamorphic events charac terize the evolution of the ghs the first eohimalayan episode included prograde kyanite grade metamorphism during which the ghs was buried at depths greater than c 35 km a nappe structure in the lowermost tz suggests that the eohimalayan phase was associated with underthrusting of the ghs below the tz a c 37 ma 40ar 39ar hornblende date indicates a late eocene age for this phase the second neohimalayan event corresponded to a retrograde phase of kyanite grade recrystallization related to thrust emplacement of the ghs on the lhs prograde mineral assemblages in the mct zone equilibrated at average t 880 k 610 c and p 940 mpa 35 km probably close to peak of meta morphic conditions slightly higher in the ghs final equilibration of retrograde assemblages occurred at average t 810 k 540 c and p 650 mpa 24 km indicating re equilibration during exhumation controlled by thrusting along the mct and extension along the annapurna detachment these results suggest an earlier equilibration in the mct zone compared with higher levels as a consequence of a higher cooling rate in the basal part of the ghs during its thrusting on the colder lhs the annapurna detachment is considered to be a neohimalayan synmetamorphic structure representing extensional reactivation of the eohimalayan thrust along which the ghs initially underthrust the tz within the upper ghs a metamorphic discontinuity across a mylonitic shear zone testifies to significant late to post metamorphic out of sequence thrusting the entire ghs cooled homogeneously below 600 700 k 330 430 c between 15 and 13 ma middle miocene suggesting a rapid tectonic exhumation by movement on late extensional structures at higher structural levels

crustal architecture of the himalayan metamorphic front in eastern nepal abstract the himalayan metamorphic front consists of two basinal sequences deposited on the indian passive margin the mesoproterozoic lesser himalayan sequence and the neoproterozoic cambrian greater himalayan sequence the current paradigm is that the unconformity between these two basinal sequences coincides with a crustal scale thrust that has been called the main central thrust and that this acted as the fundamental structure that controlled the architecture of the himalayan metamorphic front geological mapping of eastern nepal and eight detailed stratigraphic kinematic strain and metamorphic profiles through the himalayan metamorphic front define the crustal architecture in eastern nepal the unconformity does not coincide with a discrete structural or metamorphic discontinuity and is not a discrete high strain zone in recognition of this we introduce the term himalayan unconformity to distinguish it from high strain zones in the himalayan metamorphic front the fundamental structure that controls orogen architecture in eastern nepal occurs at higher structural levels within the greater himalayan sequence and we suggest the name high himal thrust this 100 400 m thick mylonite zone marks a sharp deformation discontinuity associated with a steep metamorphic transition and separates the upper plate from the lower plate in the himalayan metamorphic front the high t moderate p metamorphism at 20 24 ma in the upper plate reflects extrusion of material between the high himal thrust and the south tibet detachment system at the top of the section the lower plate is a broad schistose zone of inverted diachronous moderate t high p metamorphic rocks formed between 18 and 6 ma the high himal thrust is laterally continuous into sikkim and bhutan where it also occurs at higher structural levels than the himalayan unconformity and main central thrust as originally defined to the west in central nepal the upper plate lower plate boundary has been placed at lower structural levels coinciding with the himalayan unconformity and has been named the main central thrust above the originally defined main central thrust or ramgarh thrust

the geometry and kinematics of the main boundary thrust and related neotectonics in the darjiling himalayan fold and thrust belt west bengal india abstract the main boundary thrust mbt is well exposed in a section of the tista river valley in the darjiling himalayan fold and thrust belt the fault trace trends e w across the n s flowing tista and thrusts gondwana permian age sandstones over lower siwalik miocene age sandstones the mbt has been folded into a fault propagation synform antiform pair by tectonic activity along a younger south kalijhora thrust skt in its footwall that transports lower siwalik rocks over middle siwalik sandstones the skt cuts through the fault propagation structure preserving the mbt fault propagation fold pair in its hanging wall the motion along the skt rotated the bedding and the limbs of the folded mbt in its hanging wall footwall imbrication in front of the skt further rotated the folded mbt footwall imbrication continued to about 30 km south of the study area in the middle siwalik and younger section the region has been subsequently reactivated as evident from uplifted gravel beds and strath terraces along the folded trace of the mbt fault zone and also in the footwall of the skt the reactivation resulted in the formation of connective splays off the existing faults probably forming horses of a connecting splay duplex in the sub surface within the siwalik section the motion along these splays passively folded overlying beds and formed uplifted gravel beds and strath terraces along the local drainage the rotations resulting from footwall imbrication and reactivated splays probably caused the southern limb of the fault propagation synform to become overturned the duplexing also raised the river bed and arrested the flow of the river tista causing the river to pond and deposit its load resulting in the formation of a bar upstream of the mbt trace the tectonic reactivation of the area points to taper building activities in a subcritical wedge probably in response to monsoon induced excessive erosion in the middle of the darjiling sikkim tibet himalayan thrust wedge

geochronologic and thermobarometric constraints on the evolution of the main central thrust central nepal himalaya the main central thrust mct juxtaposes the high grade greater himalayan crystallines over the lower grade lesser himalaya formation an apparent inverted metamorphic sequence characterizes the shear zone that underlies the thrust garnet bearing assemblages sampled along the marysandi river and darondi khola in the annapurna region of central nepal show striking differences in garnet zoning of mn ca mg and fe above and below the mct thermobarometry of mct footwall rocks yields apparent inverted temperature and pressure gradients of 18 c km 1 and 0 06 km mpa 1 respectively pressure temperature p t paths calculated for upper lesser himalaya samples that preserve prograde compositions show evidence of decompression during heating whereas garnets from the structurally lower sequences grew during an increase in both pressure and temperature in situ i e analyzed in thin section ion microprobe ages of monazites from rocks immediately beneath the greater himalayan crystallines yield ages from 18 to 22 ma whereas late miocene and pliocene monazite ages characterize rocks within the apparent inverted metamorphic sequence a lesser himalayan sample collected near the garnet isograd along the marysandi river transect contains 3 3 0 1 ma monazite ages p 0 72 gpa t 535 c this remarkably young age suggests that this portion of the mct shear zone accommodated a minimum of 30 km of slip over the last 3 ma i e a slip rate of 10 mm yr 1 and thus could account for nearly half of the convergence across the himalaya in this period the distribution of ages and p t histories reported here are consistent with a thermokinematic model in which the inverted metamorphic sequences underlying the mct formed by the transposition of right way up metamorphic sequences during late miocene pliocene shearing

orogen parallel ductile extension and extrusion of the greater himalaya in the late oligocene and miocene 1 predominant stretching structures in the greater himalayan crystalline complex ghc trend perpendicular to the belt and are linked to the southward exhumation or emplacement of the ghc between the south tibet detachment std and the main central thrust however our field investigations in southern tibet reveal the widespread presence of gently dipping shear zones with a penetrative orogen parallel stretching lineation which separates the tethyan himalayan sequence and the underlying ghc the shear zones are well preserved in the upper part of the ghc south to and structurally lower than the std field criteria microstructures and quartz fabrics indicate top to the east shearing in the yadong shear zone eastern ghc coexistence of top to the east and top to the west shearing in the nyalam shear zone central ghc but top to the west shearing in the pulan shear zone western ghc characteristic microstructures and slip systems of quartz in the high grade ghc rocks resulted from the lateral flow under upper amphibolite up to 650 700 c to greenschist facies conditions u pb ages of metamorphic zircon rims by sensitive high resolution ion microprobe shrimp and laser ablation multi collector inductively coupled plasma mass spectrometry la mc icp ms analyses yield 28 26 ma for the initiation of the yadong and nyalam shear zones and 22 15 ma for the activation of the pulan shear zone in addition 40ar 39ar cooling ages of biotite and muscovite suggest cessation of ductile sharing at 13 11 ma on the yadong shear zone which is coeval with the activation of the std combined with previous studies we propose that initiation of orogen parallel extension marks the transition from burial crustal thickening to exhumation of the ghc due to lateral crustal thickness gradients in a thickened crust orogen parallel gravitational collapse occurred within the convergent himalayan orogen in the late oligocene miocene this tectonic denudation triggered and enhanced partial melting and ductile extrusion of the ghc in the miocene

deformation temperatures and flow vorticities near the base of the greater himalayan series sutlej valley and shimla klippe nw india abstract we report new deformation temperature and flow vorticity data from the base of the greater himalayan series ghs exposed in the sutlej valley and shimla klippe of nw india we focus on three groups of transects across the hanging wall of the main central thrust mct in order of relative foreland hinterland positions they are the shimla klippe western and eastern sutlej transects deformation temperatures indicated by quartz c axis fabric opening angles increase both from foreland to hinterland at a given structural distance above the mct and up structural section from the mct within individual transects deformation temperatures in the immediate hanging wall to the mct are estimated at 510 535 535 550 and 610 c on the shimla western sutlej and eastern sutlej transects respectively the steepest inferred field gradients in deformation temperatures are recorded adjacent to the mct and progressively decrease up structural section following a power law relationship comparison with temperature estimates based on multi mineral phase equilibria data suggests that penetrative shearing occurred at close to peak metamorphic conditions vorticity analyses indicate that shearing along the base of the ghs occurred under sub simple shear conditions wm values of 0 9 1 0 with a minor component of pure shear

records of the evolution of the himalayan orogen from in situ th pb ion microprobe dating of monazite eastern nepal and western garhwal abstract in situ th pb monazite ages from rocks collected along two transects the dudh kosi everest eastern nepal and the bhagirathi river garhwal himalaya india perpendicular to the main central thrust mct suggest a striking continuity of tectonic events across the himalaya the youngest age reported in this study 5 9 0 2 ma mswd 0 4 from matrix monazite grains collected beneath the mct in the garhwal region is consistent with several age data from rocks at similar structural levels in central nepal providing support for widespread late miocene mct activity the lateral parallelism of orogenic events is further manifested by the 20 7 0 1 ma age of a high himalayan leucogranite from an injection complex that outcrops along the dudh kosi everest transect resembling ages of these bodies reported elsewhere the youngest monazite grain analyzed along the dudh kosi everest transect is 10 3 0 8 ma the absence of 7 3 ma monazite ages in eastern nepal may reflect a different nappe structure which obscures the reactivated ramp equivalent exposed in the garhwal and central nepal garnets from the mct hanging wall greater himalayan crystallines and footwall lesser himalaya display different major element zoning and the patterns are useful for constraining the location of the thrust system that separates the two lithologies pressure temperature paths for two upper lesser himalayan garnets that contain monazite inclusions indicate the utility of an in situ methodology to constrain the metamorphic evolution of the shear zone along the dudh kosi everest transect upper lesser himalayan monazite grains from three rocks record a clear signal at 14 5 0 1 ma mswd 8 4 and the 23 ma age that characterizes the hanging wall is notably absent monazites collected within a large scale greater himalayan crystallines fold yield the 14 ma age consistent with the structure forming due to mct related compression paleo mesoproterozoic 1407 35 ma matrix monazite grains are found within an augen gneiss unit located beneath the mct whereas cambro ordovician 436 8 548 17 ma inclusions are preserved within garnets of the greater himalayan crystallines the presence of 45 2 2 1 ma grains from lower structural levels of the greater himalayan crystallines indicates the unit realized conditions conducive for monazite growth during the eocene

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

millennial scale variability in the asian monsoon evidence from oxygen isotope records from stalagmites in southeastern china abstract two stalagmites from xiangshui x3 and yaoba don yb1 caves in southeastern china provide high resolution 18o time series that exhibit prominent millennial scale fluctuations in the intensity and character of the asian monsoon for the period from 20 000 to 50 000 yr b p timing of these fluctuations established by u series disequilibrium 230th 234u correlates with dansgaard oeschger events 2 13 and heinrich events h2 h5 recorded in the gisp2 ice core indicating a climatic link between asian monsoon circulation and air temperatures over the north atlantic for much of the last glaciation although the exact mechanisms linking climatic fluctuations in the north atlantic to those in eastern china have yet to be identified climatic signals associated with changes in global ice volume and air temperatures over greenland may be transferred to asia by atmospheric mechanisms that affect the strength of the siberian high pressure cell and the amount of snow cover on the tibetan plateau which force the intensity of the asian monsoon another mechanism linking climate of the north atlantic to the asian monsoon relates to variation in oceanic circulation millennial scale fluctuations in thermohaline circulation in the north atlantic may have affected ocean currents in the tropical western pacific ocean which is the moisture source for the east asian monsoon despite the similarity of these paleoclimatic records and the implication of global teleconnections the magnitude and timing of millennial scale events at different locations in china reveal regional variations in climatic conditions comparisons of the 18o curves from xiangshui and yaoba don cave stalagmites with those from qixin cave and hulu cave show general concordance between millennial scale events albeit with some notable differences among all the records the well studied hulu cave records show 18o values that are lower than those of xiangshui and yaoba don caves reflecting geographical differences hulu cave is located near the eastern coast at a relatively low elevation dominated by the east asian monsoon by comparison xiangshui cave and yaoba don cave are further inland on the eastern slope of the yunnan guizhou plateau which receives precipitation from both the east asian monsoon and quasi stationary frontal systems rainfall contributed by the east asian summer monsoon is relatively diminished in this region by these geographic and atmospheric circulation conditions resulting in higher 18o values

tracking basement cross strike discontinuities in the indian crust beneath the himalayan orogen using gravity data relationship to upper crustal faults the himalaya is the result of the on going convergence and collision of india and asia two plates with complex and protracted geological evolutions the internal configuration and processes that govern the rise of the himalaya and tibetan plateau are crucial to understand continental collision zones however knowledge of the prior configuration of the colliding plates is equally important since inherited preorogenic basement structures can undeniably influence the development of the orogenic architecture throughout the orogen s cycle of collision and eventual collapse three northeast trending palaeotopographic ridges of faulted precambrian indian basement underlie the ganga basin south of the himalaya oil and natural gas commission 1968 valdiya 1976 gahalaut kundu 2012 our study godin harris 2014 illustrates a crustal scale fault origin for these ridges and succeeds in determining how far north beneath the himalayan system they extend and how they ultimately govern the location of upper crustal faults in southern tibet spectrally filtered egm2008 bouguer gravity data and edges in its horizontal gradient at different source depths gravity worms over northern peninsular india the himalaya and southern tibet reveal several continuous himalayan cross strike discontinuities interpreted to represent crustal faults fig 1 gravity lineaments in peninsular india coincide with edges of the precambrian basement ridges and megakinks up to 100 km wide develop in foreland cover sequences between the interpreted basement faults the interpreted basement faults project northward beneath the himalayan system and southern tibet our results suggest that several active himalayan cross strike faults such as the ones related to many graben in southern tibet are rooted in the underplated indian lower crust or step en echelon along interpreted basement faults our interpretation thus suggests that south tibet graben are spatially related to deepseated crustal scale faults rooted in the underplated indian crust these major discontinuities partition the himalayan range into distinct zones and could ultimately contribute to lateral variability in tectonic evolution along the orogen s strike

holocene stalagmite δ18o records in the east asian monsoon region and their correlation with those in the indian monsoon region we compare well dated holocene stalagmite delta o 18 records from the east asian summer monsoon easm region and from the indian summer monsoon ism region we found that the pattern of latitudinal change of speleothem delta o 18 values from the indian monsoon region to the east asian monsoon region is almost identical to that of modern precipitation this suggests that the modern moisture transport route from the indian ocean to the east asian monsoon region has existed since at least the early holocene the delta o 18 records from both regions exhibit a remarkably similar trend of variation in that the values are more negative during the early holocene rapidly become heavy from the mid holocene and are heaviest during the late holocene the stalagmite delta o 18 changes in the east asian monsoon region are statistically well correlated with those in the indian monsoon region both over the entire holocene and in detail over the last 2000 years however in contrast to the obvious consistency of the delta o 18 values in all of the speleothem records both instrumental and historical climate records indicate significant spatial variations in rainfall over eastern china the early holocene strong easm pattern referred from speleothem delta o 18 in the east asian monsoon region is quite different to that of other paleoclimatic records such as holocene paleosol development in the chinese loess plateau eolian activity in the northern chinese sandlands and lake sediments in easm dominated region in china in which the strongest easm was documented during the mid holocene these findings suggest that the speleothem delta o 18 record from the east asian monsoon region may not record easm variability but rather that it is controlled by variations in the isotopic composition of precipitation which is determined mainly by rainfall variability in the ism region

late miocene movement within the himalayan main central thrust shear zone sikkim north east india in the sikkim region of north east india the main central thrust mct juxtaposes high grade gneisses of the greater himalayan crystallines over lower grade slates phyllites and schists of the lesser himalaya formation inverted metamorphism characterizes rocks that immediately underlie the thrust and the large scale south tibet detachment system stds bounds the northern side of the greater himalayan crystallines in situ th pb monazite ages indicate that the mct shear zone in the sikkim region was active at c 22 14 15 and 12 10 ma whereas zircon and monazite ages from a slightly deformed horizon of a high himalayan leucogranite within the stds suggest normal slip activity at c 17 and 14 15 ma although average monazite ages decrease towards structurally lower levels of the mct shear zone individual results do not follow a progressive younging pattern lesser himalaya sample kbp1062a records monazite crystallization from 11 5 0 2 to 12 2 0 1 ma and peak conditions of 610 25 c and 7 5 0 5 kbar whereas in the mct shear zone rock chg14103 monazite crystallized from 13 8 0 5 to 11 9 0 3 ma at lower grade conditions of 525 25 c and 6 1 kbar the p t t results indicate that the shear zone experienced a complicated slip history and have implications for the understanding of mid crustal extrusion and the role of out of sequence thrusts in convergent plate tectonic settings

the age and rate of displacement along the main central thrust in the western bhutan himalaya abstract in western bhutan the main central thrust mct is broadly folded creating multiple exposures of the fault surface over a 70 km across strike distance this unusual map pattern presents a unique opportunity to map the mct and document both the magnitude and age of displacement in situ th pb sims and la icp ms geochronology of metamorphic monazite from the immediate hanging wall of the mct indicates that prograde monazite growth in greater himalayan gh rocks continued until 20 8 1 1 ma whereas crystallization of in situ melts characterized by high y monazite overgrowths occurred during cooling from ca 15 10 ma prograde monazite growth at 15 ma in lesser himalaya lh rocks in the immediate footwall requires that lh footwall strata began to be buried at this time and the mct had reached its southernmost exposed extent by combining prograde monazite ages in the immediate hanging wall and footwall the duration of mct displacement is bracketed between 20 8 1 1 and 15 0 2 4 ma immediately north of our study area a published estimate of shearing along the outer south tibetan detachment std argues for displacement between 20 and 15 ma coeval with the age range for mct displacement that we document in this study however retrograde monazite grains as young as 10 ma suggest that gh rocks were cooling until 10 ma 5 myr later than motion on the outer std immediately to the north this cooling was either the result of continued displacement on the mct or growth of a duplex that passively folded the mct using a sequential reconstruction we estimate a total displacement of 230 km which is the sum of displacements on the mct and the structurally lower paro thrust over a duration of 5 8 2 6 myr this indicates a horizontal shortening rate of 4 0 3 2 1 3 cm yr which exceeds present rates estimated from geodetic measurements across the himalaya and mct displacement rates c 2 cm yr inferred from petrologic and thermal models in central nepal but is indistinguishable from plate convergence rates calculated for eastern bhutan between 23 and 20 ma 3 3 0 7 cm yr our study highlights that displacement on the mct alone achieved plate velocity rates in western bhutan and that the age and rate of mct displacement varied significantly across the himalaya

seasonal variability of oxygen and hydrogen stable isotopes in precipitation and cave drip water at guilin southwest china the interpretation of climatic information from stalagmites has traditionally been a complex research problem with oxygen isotopes playing a particularly important role in global climate change studies this study investigates the relationship between oxygen isotope composition of the atmospheric in precipitation and cave drip water at panlong cave in southwest china on seasonal timescales of variability time series seasonal variability was derived from panlong cave in guilin by collecting daily precipitation samples for stable isotope analysis during 2012 results indicate that 18o of precipitation contains a clear seasonal variation whereby higher values are mainly distributed during winter and lower values during summer seasonal variations in water sources affect the precipitation 18o values drip water 18o also displayed a seasonal cycle which is attenuated relative to 18o of precipitation drip water time series display seasonal cycle ranges from 1 5 to 3 5 relative to vienna standard mean ocean water which mainly follow the precipitation 18o seasonal cycle seasonal variation in drip water 18o supports interpretations of the stalagmite 18o record as a paleoclimate proxy sensitive to the local environment this monitoring experiment revealed that drip water must be transported through the epikarst in approximately 1 5 months during cold periods and 0 5 months during warm periods different residence time percolation is mainly affected by the atmospheric precipitation amount depending on whether soil moisture reaches saturation

the transfer of seasonal isotopic variability between precipitation and drip water at eight caves in the monsoon regions of china abstract this study presents new stable isotope data for precipitation 18op and drip water 18od from eight cave sites in the monsoon regions of china mrc with monthly to bi monthly sampling intervals from may 2011 to april 2014 to investigate the regional scale climate forcing on 18op and how the isotopic signals are transmitted to various drip sites the monthly 18op values show negative correlation with surface air temperature at all the cave sites except shihua cave which is opposite to that expected from the temperature effect in addition although the monthly 18op values are negatively correlated with precipitation at all the cave sites only three sites are significant at the 95 level these indicate that due to the various vapor sources a large portion of variability in 18op in the mrc cannot be explained simply by either temperature or precipitation alone all the thirty four drip sites are classified into three types based on the 18od variability about 82 of them are static drips with little discernable variation in 18od through the whole study period but the drip rates of these drips are not necessary constant their discharge modes are site specific and the oxygen isotopic composition of the stalagmites growing from them may record the average of multi year climatic signals which are modulated by the seasonality of recharge and potential effects of evaporation and in some cases infiltration from large rainfall events about 12 of the thirty four drip sites are seasonal drips although the amplitude of 18od is narrower than that of 18op the monthly response of 18od to coeval precipitation is not completely damped and some of them follow the seasonal trend of 18op very well these drips may be mainly recharged by present day precipitation mixing with some stored water thus the stalagmites growing under them may record portions of the seasonal climatic signals embedded in 18op about 6 of the thirty four drip sites are medium variability drips with constant and relatively low 18od values in the wet season but with variable and relatively high 18od values in the dry season reflecting flow switching in the karst or evaporation inside the cave

argon behaviour in an inverted barrovian sequence sikkim himalaya the consequences of temperature and timescale on 40ar 39ar mica geochronology abstract 40 ar 39 ar dating of metamorphic rocks sometimes yields complicated datasets which are difficult to interpret in terms of timescales of the metamorphic cycle single grain fusion and step heating data were obtained for rocks sampled through a major thrust sense shear zone the main central thrust and the associated inverted metamorphic zone in the sikkim region of the eastern himalaya this transect provides a natural laboratory to explore factors influencing apparent 40 ar 39 ar ages in similar lithologies at a variety of metamorphic pressure and temperature p t conditions the 40 ar 39 ar dataset records progressively younger apparent age populations and a decrease in within sample dispersion with increasing temperature through the sequence the white mica populations span 2 9 ma within each sample in the structurally lower levels garnet grade but only 0 3 ma at structurally higher levels kyanite sillimanite grade mean white mica single grain fusion population ages vary from 16 2 3 9 ma 2 to 13 2 1 3 ma 2 from lowest to highest levels white mica step heating data from the same samples yields plateau ages from 14 27 0 13 ma to 12 96 0 05 ma biotite yield older apparent age populations with mean single grain fusion dates varying from 74 7 11 8 ma 2 at the lowest structural levels to 18 6 4 7 ma 2 at the highest structural levels the step heating plateaux are commonly disturbed temperatures 600 c at pressures of 0 4 0 8 gpa sustained over 5 ma appear to be required for white mica and biotite ages to be consistent with diffusive open system cooling at lower temperatures and or over shorter metamorphic timescales more 40 ar is retained than results from simple diffusion models suggest diffusion modelling of ar in white mica from the highest structural levels suggests that the high temperature rocks cooled at a rate of 50 80 c ma 1 consistent with rapid thrusting extrusion and exhumation along the main central thrust during the mid miocene

the δ18o and δ13c records in an aragonite stalagmite from furong cave chongqing china a 2000 year record of monsoonal climate abstract a 7 cm long aragonite stalagmite fr0510 1 from furong cave chongqing was dated by 210pb and 230th methods revealing a 2000 year record of climate history under the influence of the east asian monsoon the fr0510 1 record resembles dongge cave da record on 10 100 year scales but quite different from the wanxiang cave wx42b record indicating that while stalagmite 18o record represents local regional moisture change spatial variability of the monsoonal rainfall over eastern china must take into account during the past 2000 years climate in chongqing was relatively wet in the intervals of 50 bc ad 250 ad 1150 1450 and ad 1600 1950 and relatively dry during the periods of ad 250 1150 and ad 1450 1600 dry conditions were prevailing over the medieval warm period whereas wet climates were dominant during the most time of the little ice age in chongqing area

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

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

lessons learned from oxygen isotopes in modern precipitation applied to interpretation of speleothem records of paleoclimate from eastern asia abstract variability in oxygen isotope ratios collected from speleothems in chinese caves is often interpreted as a proxy for variability of precipitation summer precipitation seasonality of precipitation and or the proportion of 18o to 16o of annual total rainfall that is related to a strengthening or weakening of the east asian monsoon and in some cases to the indian monsoon we use modern reanalysis and station data to test whether precipitation and temperature variability over china can be related to changes in climate in these distant locales we find that annual and rainy season precipitation totals in each of central china south china and east india have correlation length scales of 500 km shorter than the distance between many speleothem records that share similar long term time variations in 18o values thus the short distances of correlation do not support though by themselves cannot refute the idea that apparently synchronous variations in 18o values at widely spaced 500 km caves in china are due to variations in annual precipitation amounts we also evaluate connections between climate variables and 18o values using available instrumental measurements of 18o values in precipitation these data from stations in the global network of isotopes in precipitation gnip show that monthly 18o values generally do not correlate well with either local precipitation amount or local temperature and the degree to which monthly 18o values do correlate with them varies from station to station for the few locations that do show significant correlations between 18o values and precipitation amount we estimate the differences in precipitation amount that would be required to account for peak to peak differences in 18o values in the speleothems from hulu and dongge caves assuming that 18o scales with the monthly amount of precipitation or with seasonal differences in precipitation insofar as the present day relationship between 18o values and monthly precipitation amounts can be applied to past conditions differences of at least 50 in mean annual precipitation would be required to explain the 18o variations on orbital time scales which are implausibly large and inconsistent with published gcm results similarly plausible amplitudes of seasonal cycles in amounts or in seasonal variations in 18o values can account for less than half of the 4 5 difference between glacial and interglacial 18o values from speleothems in china if seasonal cycles in precipitation account for the amplitudes of 18o values on paleoclimate timescales they might do so by extending or contracting the durations of seasons a frequency modulation of the annual cycle but not by simply varying the amplitudes of the monthly rainfall amounts or monthly average 18o values amplitude modulation allowing that several processes can affect seasonal variability in isotopic content we explore the possibility that one or more of the following processes contribute to variations in 18o values in chinese cave speleothems different source regions of the precipitation which bring different values of 18o in vapor different pathways between the moisture source and the paleorecord site along which exchange of 18o between vapor surface water and condensate might differ a different mix of processes involving condensation and evaporation within the atmosphere or different types of precipitation each may account for part of the range of 18o values revealed by speleothems and each might contribute to seasonal differences between past and present that do not scale with monthly or even seasonal precipitation amounts

how does the mid crust accommodate deformation in large hot collisional orogens a review of recent research in the himalayan orogen abstract the presence of hot weak crust is a central component of recent hypotheses that seek to explain the evolution of continent continent collisions and in particular may play an important role in accommodating the 3000 km of convergence within the himalaya tibetan collision over the last 55 myr models that implicate flow of semi viscous midcrustal rocks south toward the front of the himalayan orogen channel flow are able to account for many geologic observations in the himalaya while alternative models of collision particularly thrust wedge taper demonstrate that much of the observed geology could have formed in the absence of a low viscosity mid crustal layer several recent studies synthesized here have prompted a shift from initial assumptions that channel flow and thrust wedge taper processes are by definition mutually exclusive these new studies reveal the presence of several tectonometamorphic discontinuities in the midcrust that appear to reflect a continuum of deformation in which both channel and wedge type processes operate in spatially and temporally distinct domains within the orogen and further that the system may migrate back and forth between these types of behavior this continuum of deformation styles within the collisional system is of crucial importance for explaining the evolution of the himalayan orogen and hence for understanding the evolution of earth s many continent continent collision zones

structure and strain variation at mid crustal levels in a transpressional orogen a review of kaoko belt structure and the character of west gondwana amalgamation and dispersal abstract the neoproterozoic cambrian kaoko belt is an orogen scale 800 180 km transpressional system important in the amalgamation of west gondwana mid crustal transpression at amphibolite to granulite facies conditions is dominated by two major 400 km exposed strike slip shear zones bounding a 20 40 km wide high grade orogen core to the east a deeply buried nappe dominated escape zone has inverted metamorphic sequence and verges outwards onto a platformal foreland to the west an arc like neoproterozoic coastal terrane was amalgamated and variably reworked during transpression the major purros and three palms mylonite zones have calculated shear displacements on the order of 120 180 km these shear zones are moderately to steeply dipping mylonite zones of 1 5 km width are arcuate and curvilinear in map view and show along strike variation in slip kinematics also highly curved in vertical section the shear zones define a flower to half flower geometry for the orogen core an oblique network of mylonitic shear zones akin to riedel shears links the major shear zones and defines regional scale shear lozenges internally deformed by tight upright folding and shear fabrics these shear zones create domains in the orogen core with varying dominance of pure shear in shear lozenges and simple shear in shear zones however absence of dip slip domains and the smoothly continuous traces of sub horizontal to shallow and acute oblique stretching lineations across all parts of the belt preclude marked kinematic partitioning and the internal part of the belt resembles large scale triclinic shear clast aspect ratios boudin train extension sheath fold aspect geometry degree of rotation of planes producing flanking folds composite s c foliations pressure fringes on pyrite and garnet porphyroclasts provide a semi quantitative measure of strain intensity average strain ratios are x z 40 1 for the major shear zones x z 12 1 for the orogen core x z 8 1 for the escape zone and x z 3 1 for the coastal terrane a more continuous pattern of strain intensity across the whole belt is mapped using a qualitative foliation intensity index foliation traces have a sigmoidal pattern in the orogen core swinging from sub parallel to the boundary shear zones to higher acute angles in the internal parts deformation character also varies from upright open folding in amphibolite facies domains in the north upright tight chevron folding in a low grade central domain to a high grade domain of tight to isoclinal inter folded basement and cover with inclination decreasing towards the south the kaoko belt is a well exposed sector of an extensive 3000 km long broad 400 km arcuate orogenic system adamastor orogen that amalgamated west gondwana bringing the south american sao francisco and rio de la plata cratons and african kalahari and congo cratons components together though a complex system most sectors involved oblique collision and accretion of magmatic arcs of 660 610 ma age followed by peak metamorphism and main phase transpressional orogenesis between 585 and 560 ma with shear zones remaining active until 530 ma this e w amalgamation immediately pre dates the final n s amalgamation of gondwana along the kuunga orogen between 535 and 510 ma the large scale adamastor orogen consisting of kaoko dom feliciano ribeira aracuai and west congo mobile belts also shows broadly similar and symmetric structural architecture throughout the high grade thermally softened core partitioned intense wrench dominated strains and networks of transcurrent shear zones that dip inwards with listric form either side of the internal zone containing amalgamated arcs and high grade core are nappe fold and thrust belts that rework attenuated passive margin basement and adamastor ocean sediments and structures verge outward at moderate to high angles onto both foreland margins the kaoko belt well illustrates the highly partitioned nature of transpressional systems in general and patterns in common throughout the greater adamastor orogen such as metamorphic zonation and heterogeneous distribution of deformation style flow regime and highly variable degrees of reworking strain and recrystallization this highly partitioned and steep structural grain localized lithospheric extension and rifting 415 ma later during breakup and dispersal of gondwana

seasonal variations of deuterium and oxygen 18 isotopes and their response to moisture source for precipitation events in the subtropical monsoon region deuterium and oxygen 18 are common environmental tracers in water used to investigate hydrological processes such as evaporation and groundwater recharge and to trace moisture source in this study we collected event precipitation from 01 january 2010 to 28 february 2011 at a site in changsha yangtze river basin to estimate the influence of moisture source and atmospheric conditions on stable isotope compositions the local meteoric water line established as d 8 45 0 13 18o 17 7 0 9 r2 0 97 n 189 had a higher slope and intercept than global meteoric water line temperature 18o exhibited complex correlations with positive correlations during nov apr superior to during jun sep which was attributed to distinctive moisture sources but vague the overall period amount effect examined throughout the year linear regressions between 18o and d value in different precipitation event size classes revealed progressively decreasing slope and intercept values with decreasing precipitation amount and increasing vapour pressure deficit indicating that small rainfall events 0 5 mm were subject to secondary evaporation effects during rainwater descent in contrast snowfall and heavy precipitation events exhibited high slope and intercepts for the regression equation between 18o and d high concentrations of heavy isotopes were associated with precipitation events sourced from remote westerly air masses degenerated tropical marine air masses from the bay of bengal bob and inland moisture in the pre monsoon period as determined from backward trajectories assessed in the hysplit model meanwhile low concentrations of heavy isotopes were found to correspond with remote maritime moisture from bob the south china sea and the west pacific at three different air pressures in summer monsoon and post monsoon using hysplit and records of typhoon paths these findings suggest that stable isotope compositions in precipitation events are closely associated with the meteorological conditions and respond sensitively to moisture source in subtropical monsoon climates copyright 2013 john wiley sons ltd

δ18o characteristics of meteoric precipitation and its water vapor sources in the guilin area of china the stable isotopic composition of meteoric precipitation is an important component of global and regional water cycle research stable isotope data can help reconstruct paleoclimate related to ice cores lake sediments and stalagmites we investigated the daily variation in isotopic composition of meteoric precipitation from 2008 to 2012 in the guilin region of china 18o of meteoric precipitation ranged from 14 21 to 2 38 with an average value of 5 78 vienna standard mean ocean water vsmow meteoric precipitation in the summer half year may through october increased with a relatively low 18o value of 8 04 vsmow average of 261 groups and 56 03 vsmow of average d accounting for 67 6 of the total annual meteoric precipitation meteoric precipitation in the winter half year november through april decreased with a relatively high 18o value of 2 89 average of 210 groups and 9 23 of average d accounting for 32 4 of total meteoric precipitation the local meteoric water line lmwl equation in guilin area has local climate characteristics by combining environmental isotope data of precipitation using a backwards trajectory water vapor sources of the meteoric precipitation in guilin area are inferred and traced our results show that the isotopic composition of meteoric water vapor sources in guilin area are related to monsoon type source of precipitation cloud masses and precipitation properties the isotopic composition of meteoric precipitation in the summer half year may through october was mainly affected by the summer monsoon or summer typhoons namely controlled by water vapor source from the bay of bengal and the south china sea and the second the west pacific and the 18o value of meteoric precipitation was strongly negative there was a significant negative correlation between the 18o values of meteoric precipitation and the amount of precipitation and temperature in the summer half year the amount effect of the meteoric precipitation often concealed the temperature effect the isotopic composition of meteoric precipitation in the winter season or winter half year was affected by the water vapor source of the warm moist air masses from the west pacific and continental cold air masses from the siberia mongolia winter monsoon or local evaporation vapor circulation and the 18o value of meteoric precipitation was relatively positive and indicated that the water vapor of meteoric precipitation along the water vapor trajectory was affected by the evaporation as well as the local water vapor evaporation the research results have showed that different sources of water vapor has a significant influence on the 18o variation of meteoric precipitation therefore analysis of 18o in the meteoric precipitation especially its seasonal variation characteristics of analysis can conversely reveal the water vapor sources of local meteoric precipitation

a new kinematic evolutionary model for the growth of a duplex an example from the rangit duplex sikkim himalaya india abstract the lesser himalayan duplex lhd is a prominent structure through much of the lesser himalayan fold thrust belt in the darjeeling sikkim himalaya a component of the lhd is exposed in the rangit window as the rangit duplex rd the rd consists of ten horses of the upper lesser himalayan sequence gondwana buxa upper daling the duplex varies from hinterland dipping in the north through an antiformal stack in the middle to foreland dipping in the south the ramgarh thrust rt is the roof thrust and based on a balanced cross section the main himalayan sole thrust is the floor thrust at a depth of 10 km and with a dip of 3 5 n retrodeformation suggests that the rd initiated as a foreland dipping duplex with the early ramgarh thrust as the roof thrust and the rt as the floor thrust the rt became the roof thrust during continued duplexing by a combination of footwall imbrication and concurrent rt reactivation this kinematic history best explains the large translation of the overlying mct sheets the restoration suggests that rd shortening is 125 km and the original gondwana basin extended 142 km northward of its present northernmost exposures within the window

detection of ground motions using high rate gps time series monitoring surface deformation in real time help at planning and protecting infrastructures and populations manage sensitive production i e seveso type and mitigate long term consequences of modifications implemented we present rt shake an algorithm developed to detect ground motions associated with landslides sub surface collapses subsidences earthquakes or rock falls rt shake detects first transient changes in individual gps time series before investigating for spatial correlation s of observations made at neighbouring gps sites and eventually issue a motion warning in order to assess our algorithm on fast seconds to minute large from 1cm to meters and spatially consistent surface motions we use the 1hz geonet gnss network data of the tohoku oki mw9 0 2011 as a test scenario we show the delay of detection of seismic wave arrival by gps records is of 10 seconds with respect to an identical analysis based on strong motion data and this time delay depends on the level of the time variable noise nevertheless based on the analysis of the gps network noise level and ground motion stochastic model we show that rt shake can narrow the range of earthquake magnitude by setting a lower threshold of detected earthquakes to mw6 5 7 if associated with a real time automatic earthquake location system

the asian monsoon over the past 640 000 years and ice age terminations oxygen isotope records from chinese caves characterize changes in both the asian monsoon and global climate here using our new speleothem data we extend the chinese record to cover the full uranium thorium dating range that is the past 640 000 years the record s length and temporal precision allow us to test the idea that insolation changes caused by the earth s precession drove the terminations of each of the last seven ice ages as well as the millennia long intervals of reduced monsoon rainfall associated with each of the terminations on the basis of our record s timing the terminations are separated by four or five precession cycles supporting the idea that the 100 000 year ice age cycle is an average of discrete numbers of precession cycles furthermore the suborbital component of monsoon rainfall variability exhibits power in both the precession and obliquity bands and is nearly in anti phase with summer boreal insolation these observations indicate that insolation in part sets the pace of the occurrence of millennial scale events including those associated with terminations and unfinished terminations records of the asian monsoon have been extended to 640 000 years ago and confirm both that the 100 000 year ice age cycle results from integral numbers of precessional cycles and that insolation influences the pacing of major millennial scale climate events prior records of the asian monsoon have revealed cyclic variations over hundreds of thousands of years probably driven by variations in insolation caused by the precession of earth s orbit hai cheng and colleagues now provide a speleothem record from chinese cave samples that extends earlier records to 640 000 years ago close to the maximum age possible with uranium thorium dating this spectacular record confirms that the characteristic 100 000 year ice age cycle corresponds to an integral number four or five of precession cycles and that insolation influences millennial scale variations in monsoon strength

spatial variations in deformation mechanisms along the main central thrust zone implications for the evolution of the mct in the darjeeling sikkim himalaya abstract in the darjeeling sikkim himalaya we recognize two distinct mct sheets the structurally higher mct1 and the lower mct2 microstructural studies from three different segments along the transport direction of the mct2 fault zone suggest that the fault has undergone strain softening by different mechanisms the geometry of the tapered crystalline orogenic wedge resulted in variation of overburden along the mct2 strain softening by different deformation mechanisms accommodated translation of 100 km along a thin mct2 fault zone as the mylonitic trailing part of the mct2 in pelling had the greatest overburden deformation took place by dislocation creep in quartz and by microfracturing in feldspar reaction softening of feldspar produced an intrinsically weak matrix that primarily controlled the deformation resulting in a strain softening fault zone at soreng mct2 zone under intermediate crustal conditions finer grained recrystallized quartz and micaceous matrix deformed by grain size sensitive diffusion creep mechanisms resulting in strain softening the fault rocks at sivitar had the least overburden and record a prominent mineralogical change from the protolith strain softening occurred by pressure solution slip possibly by a combination of grain size reduction by cataclasis and an increase in fluid activity

shear zone evolution and timing of deformation in the neoproterozoic transpressional dom feliciano belt uruguay abstract new structural microstructural and geochronological u pb la icp ms ar ar k ar rb sr data were obtained for the dom feliciano belt in uruguay the main phase of crustal shortening metamorphism and associated exhumation is recorded between 630 and 600 ma this stage is related to the collision of the rio de la plata and congo cratons at ca 630 ma which also involved crustal reworking of minor crustal blocks such as the nico perez terrane and voluminous post collisional magmatism subsequent orogen parallel sinistral shearing gave rise to further deformation up to ca 584 ma and resulted from the onset of the convergence of the kalahari craton and the rio de la plata congo cratons sinistral shear zones underwent progressive strain localization and retrograde conditions of deformation during crustal exhumation dextral ene striking shear zones were subsequently active at ca 550 ma coeval with further sinistral shearing along n to nne striking shear zones the tectonothermal evolution of the dom feliciano belt thus recorded the collision of the rio de la plata and congo cratons which comprised one of the first amalgamated nuclei of gondwana and the subsequent incorporation of the kalahari craton into western gondwana

east asian monsoon variability since the mid holocene recorded in a high resolution absolute dated aragonite speleothem from eastern china abstract paleoclimatic conditions associated with the east asian monsoon are reconstructed for the mid to late holocene using 18o values 819 measurements from a precisely dated 10 230th dates aragonite stalagmite from lianhua cave hunan province china 18o values 1 6 to 7 0 are interpreted to reflect the precipitation amount effect related to the strength of summer monsoonal circulation throughout this period the intensity of east asian summer monsoonal circulation declined in response to changes in insolation and a southward shift in the intertropical convergence zone itcz punctuating this first order trend are abrupt millennial to decadal scale fluctuations the most prominent of which occurred around 3337 5 yr bp and records a dramatic weakening in summer monsoonal circulation that coincides with the beginning of neoglacial conditions comparisons between the 18o values of stalagmite a1 and previously published data from stalagmite hs 4 from heshang cave and stalagmites d4 and da from dongge cave china demonstrate a similar regional response to changes in east asian monsoonal circulation several spectral analysis techniques and wavelet analysis were applied to these stalagmite records to evaluate the nature and character of cyclicity in the east asian monsoon during the holocene multi taper method mtm lomb scargle and single spectrum analysis ssa results reveal common periodicities that are unlikely to be artifacts of any single technique the periodicities expressed in the stalagmite 18o records occur at both solar and non solar frequencies and support previous interpretations that variability in monsoonal circulation responds to both solar forcing and internal climatic mechanisms wavelet analysis demonstrates that some of these frequencies are intermittent and recur at semi regular intervals which may indicate long term cyclicity that is unconstrained by the chronology of the record

water fluxed melting of the continental crust a review abstract water fluxed melting also known as fluid or water present melting is a fundamental process in the differentiation of continents but its importance has been underestimated in the past 20 years during which research efforts focused mostly on dehydration melting reactions involving hydrate phases in the absence of a separate aqueous phase the presence of a free aqueous phase in anatectic terranes influences all major physical and chemical aspects of the melting process from melt volumes viscosity and ability to segregate from rock pores to melt chemical and isotopic composition a review of the literature shows that melting due to the fluxing of aqueous fluids is a widespread process that can take place in diverse tectonic environments active tectono magmatic processes create conditions for the release of aqueous fluids and deformation driven transient high permeability channels capable of fluxing high temperature regions of the crust where they trigger voluminous melting water fluxed melting can be either congruent in regions at the water saturated solidus or incongruent at suprasolidus p t conditions incongruent melting reactions can give rise to peritectic hornblende or to nominally anhydrous minerals such as garnet sillimanite or orthopyroxene in this case the presence of an aqueous phase is indicated by a mismatch between the large melt fraction generated and the much smaller fractions predicted in its absence the relatively small volumes of aqueous fluids compared to that of rocks imply that melting reactions are generally rock buffered fluids tend to move upwards and down temperature however there are cases in which pressure gradients drive fluids up temperature potentially fluxing suprasolidus terranes crustal regions at conditions equivalent to the water saturated solidus represent a natural impediment to the up temperature migration of aqueous fluids because they are consumed in melting reactions in this case continued migration into supra solidus terranes take place through the migration of water rich melts thus melts become the transport agent of water into supra solidus terranes and responsible for water fluxed melting other processes such as the relatively rapid fluid migration through fractures also allow regional aqueous fluids to by pass the water saturated solidus fluid trap and trigger melting above solidus conditions when aqueous fluids or hydrous melts flux rocks at supra solidus conditions they equilibrate with the surroundings through further melting decreasing water activity and giving rise to undersaturated melts it is in these conditions that hornblende or anhydrous peritectic phases are stabilized unlike dehydration melting the melt fraction generated in this case is not limited by the water contained in hydrous minerals but by the volume of water added to the system unlike melting at the water saturated solidus these melts are capable of rising without freezing and do give rise to upper crustal granitic bodies

exhumation of the main central thrust from lower crustal depths eastern bhutan himalaya geothermometry and mineral assemblages show an increase of temperature structurally upwards across the main central thrust mct however peak metamorphic pressures are similar across the boundary and correspond to depths of 35 45 km garnet bearing samples from the uppermost lesser himalayan sequence lhs yield metamorphic conditions of 650 675 c and 9 13 kbar staurolite kyanite schists about 30 m above the mct yield p t conditions near 650 c 8 10 kbar kyanite bearing migmatites from the greater himalayan sequence ghs yield pressures of 10 14 kbar at 750 800 c top to the south shearing is synchronous with and postdates peak metamorphic mineral growth metamorphic monazite from a deformed and metamorphosed proterozoic gneiss within the upper lhs yield u pb ages of 20 18 ma staurolite kyanite schists within the ghs a few metres above the mct yield monazite ages of c 22 1 ma we interpret these ages to reflect that prograde meta morphism and deformation within the main central thrust zone mctz was underway by c 23 ma u pb crystallization ages of monazite and xenotime in a deformed kyanite bearing leucogranite and kyanite garnet migmatites about 2 km above the mct suggest crystallization of partial melts at 18 16 ma higher in the hanging wall south verging shear bands filled with leucogranite and pegmatite yield u pb crystallization ages for monazite and xenotime of 14 15 ma and a 1 2 km thick leuco granite sill is 13 4 0 2 ma thus metamorphism plutonism and deformation within the ghs con tinued until at least 13 ma p t conditions at this time are estimated to be 500 600 c and near 5 kbar from these data we infer that the exhumation of the mct zone from 35 to 45 km to around 18 km occurred from 18 to 16 to c 13 ma yielding an average exhumation rate of 3 9 mm year 1 this process of exhumation may reflect the ductile extrusion by channel flow of the mctz from between the overlying tibetan plateau and the underthrusting indian plate coupled with rapid erosion

pacific east asian teleconnection how does enso affect east asian climate observational evidence is presented to show a teleconnection between the central pacific and east asia during the extreme phases of enso cycles this pacific east asian teleconnection is confined to the lower troposphere the key system that bridges the warm cold events in the eastern pacific and the weak strong east asian winter monsoons is an anomalous lower tropospheric anticyclone cyclone located in the western north pacific the western north pacific wind anomalies develop rapidly in late fall of the year when a strong warm or cold event matures the anomalies persist until the following spring or early summer causing anomalously wet dry conditions along the east asian polar front stretching from southern china northeastward to the east of japan kuroshio extension using atmospheric general circulation and intermediate models the authors show that the anomalous philippine sea anticyclone results from a rossby wave response to suppressed convective heating which is induced by both the in situ ocean surface cooling and the subsidence forced remotely by the central pacific warming the development of the anticyclone is nearly concurrent with the enhancement of the local sea surface cooling both the anticyclone and the cooling region propagate slowly eastward the development and persistence of the teleconnection is primarily attributed to a positive thermodynamic feedback between the anticyclone and the sea surface cooling in the presence of mean northeasterly trades the rapid establishment of the philippine sea wind and sst anomalies implies the occurrence of extratropical tropical interactions through cold surge induced exchanges of surface buoyancy flux the central pacific warming plays an essential role in the development of the western pacific cooling and the wind anomalies by setting up a favorable environment for the anticyclone sst interaction and midlatitude tropical interaction in the western north pacific

timing duration and inversion of prograde barrovian metamorphism constrained by high resolution lu hf garnet dating a case study from the sikkim himalaya ne india abstract we investigated lu hf isotopic systematics in garnets from gradually higher grade metamorphic rocks from the first appearance of garnet at c 500 c to biotite dehydration melting at c 800 c in the sikkim himalaya india exceptionally precise lu hf ages obtained for the barrovian metasedimentary sequence in the lesser himalaya lh correspond to the time of early garnet formation on a prograde path and show remarkable correlation with increasing metamorphic grade and decreasing structural depth the youngest age is reported for the garnet zone 10 6 0 2 ma and then the ages become progressively older in the staurolite 12 8 0 3 ma kyanite 13 7 0 2 ma and sillimanite 14 6 0 1 ma zones the oldest age of 16 8 0 1 ma was recorded at the top of the sequence in the zone marking the onset of muscovite dehydration melting directly below the main central thrust mct these ages provide a tight constraint on the timing and duration of the barrovian sequence formation which lasted about 6 ma the age pattern is clearly inverted with respect to structural depth but shows normal correlation with the metamorphic grade i e earlier garnet growth in higher grade rocks the timing of the peak metamorphic conditions estimated for the garnet and kyanite zones suggests that thermal climax occurred in all zones within a relatively short time span about 10 13 ma ago the metamorphic inversion in the sikkim himalaya must therefore have occurred after the metamorphic peak was attained in all grades lu hf garnet ages in the overthrust higher himalaya hh continue to be older but do not show a clear progression there is about 6 ma jump to 22 6 0 1 ma within the mct zone and even older ages of 26 28 ma were obtained for migmatites from the lower part of the hh the lu hf system in garnets from the hh records the time of melting at or near the thermal peak rather than dates initial garnet growth as in the lh the age pattern obtained in this study provides precise time constraints on the continental collision models which must take into account early and prolonged anatexis in the hh progressive delay in initial garnet growth in different barrow zones in the lh nearly contemporaneous metamorphic peak in all grades and post peak inversion of the coherent lh block during exhumation

pressure temperature time path discontinuity in the main central thrust zone central nepal metapelites collected in central nepal reveal a discontinuity in metamorphic pressuretemperature time p t t paths near the base of the main central thrust zone despite an absence of obvious structural breaks garnets in the structurally lowest rocks grew with increasing t and p loading whereas garnets 1 3 km upsection grew with increasing t but decreasing p exhumation monazite grains in structurally lower rocks yield ionmicroprobe th pb ages of 8 9 ma structurally higher monazite grains range from 10 to 22 ma the p t t paths confirm previous interpretations that footwall metamorphism in part resulted from thrust reactivation ca 8 ma but also reflect thermal relaxation following older 20 ma or older thrust movement the main central thrust zone formed during pulses of movement that resulted in progressive transfer of material from the lower to upper plate

a detailed comparison of asian monsoon intensity and greenland temperature during the allerod and younger dryas events abstract an annual layer counted and 230th dated stalagmite oxygen isotope record from qingtian cave in hubei province central china provides an asian monsoon am history across the allerod to younger dryas yd transition with an average 2 5 year resolution seasonal 18o profiles indicate that the calcite 18o is a sensitive proxy for am changes and the close similarity between the qingtian and other cave records from eastern china suggests a large scale regional coherence of monsoonal precipitation 18o variations associated with the temperature changes in high northern latitudes the annually resolved chronology with a u th age uncertainty of less than 100 yr defines the timing duration and transition of the early allerod the intra allerod cold period iacp the late allerod and the start of the yd the 160 yr long iacp with two brief reversals is clearly shown in both 18o and lamina thickness records the early and late allerod separated by the iacp are characterized by several decadal to centennial cycles of 18o variations each punctuated by sub cycles these decadal to centennial monsoon variations correlate with the greenland temperature changes supporting a model simulation that the decadal north atlantic oscillation nao and north pacific oscillation npo are coupled via atmospheric circulation under glacial boundary conditions perhaps affecting tropical subtropical monsoon changes however the monsoon transition between the late allerod and yd lasted 380 yr longer than the analogous greenland temperature shift by at least 130 yr stuiver m grootes p m gisp2 oxygen isotope ratios quat res 53 2000 277 284 this implicates other links besides the direct link between greenland and the am which is now well documented one possibility is the influence by southern hemisphere climate via cross equatorial air flow an z s the history and variability of the east asian paleomonsoon climate quat sci rev 19 2000 171 187

geometry and kinematics of the darjeeling sikkim himalaya india implications for the evolution of the himalayan fold thrust belt abstract the darjeeling sikkim himalaya lies in a zone where the convergence vector between the indian and eurasian plates is almost perpendicular to the himalayan deformation front current outcrop pattern preserves high grade hanging wall rocks of the main central thrust mct and the pelling thrust pt within 5 km of the mountain front u pb shrimp analysis on zircon grains from mct sheet record two 206pb 238u age groups of ca 2575 and ca 943 ma and from the pt 1835 13 ma the lesser himalayan duplex in the footwall of the mct is composed of two duplex systems a structurally higher hinterland dipping duplex and a lower duplex system that varies from hinterland dipping in the north to an antiformal stack in the middle and foreland dipping duplex in the south the foreland dipping horses and reactivation of the roof thrust resulted in large translation of the overlying mct and pelling thrust sheets the duplex also resulted in a plunge culmination forming a broad n s trending anticline a transport parallel restorable balanced cross section reveals a minimum shortening of 450 km 81 south of the south tibetan detachment system and an average long term shortening rate 20 mm yr although the study area lies in a zone of high convergence angle with the deformation front the surface structures do not record the greatest shortening of the orogen the shillong plateau and a tranverse zone lying southeast of the study area have accommodated a part of the convergence related shortening thereby under representing the shortening in the sikkim himalayan fold thrust belt lateral variations in the original width of the lesser himalayan basin may have played a role in partitioning the shortening in the fold thrust belt

isotopic and structural constraints on the location of the main central thrust in the annapurna range central nepal himalaya five isotope enhanced geologic transects in the southern annapurna range of central nepal elucidate structural geometries near the main central thrust whole rock nd isotopes and u pb ages of detrital zircons unambiguously distinguish greater himalayan hanging wall and lesser himalayan footwall metasedimentary rocks nd 0 values for lower lesser himalayan rocks typically range from 20 to 26 whereas greater himalayan rocks usually have nd 0 values of 19 to 12 lower lesser himalayan rocks yield detrital zircons with an age peak at ca 1880 ma and no detrital zircons younger than ca 1550 ma in contrast greater himalayan rocks yield detrital zircons with a prominent broad peak of ages at ca 1050 ma and no detrital zircons younger than ca 600 ma the protolith boundary between greater and lesser himalayan rocks is up to 1 km farther south than usually mapped on the basis of lithology field and microstructural observations indicate the presence of a top to the south ductile shear zone superimposed on this boundary confirming this shear zone as the main central thrust no evidence exists for large scale structural mixing of greater and lesser himalayan rocks along the main central thrust in the annapurna range

defining the himalayan main central thrust in nepal an inverted metamorphic field gradient associated with a crustal scale south vergent thrust fault the main central thrust has been recognized along the himalaya for over 100 years a major problem in himalayan structural geology is that recent workers have mapped the main central thrust within the greater himalayan sequence high grade metamorphic sequence along several different structural levels some workers map the main central thrust as coinciding with a lithological contact others as coincident with the kyanite isograd up to 1 3 km structurally up section into the tertiary metamorphic sequence without supporting structural data some workers recognize a main central thrust zone of high ductile strain up to 2 3 km thick bounded by an upper thrust mct 2 vaikrita thrust and a lower thrust mct 1 munsiari thrust some workers define an upper lesser himalaya thrust sheet that shows similar p t conditions to the greater himalayan sequence others define the main central thrust either on isotopic nd sr differences differences in detrital zircon ages or as being coincident with a zone of young

enso variability reflected in precipitation oxygen isotopes across the asian summer monsoon region abstract oxygen isotope signals 18 o from paleo archives are important proxies for past asian summer monsoon asm climate reconstruction however causes of interannual variation in the 18 o values of modern precipitation across the asm region remain in argument we report interannual 18 o variation in southern tibetan plateau precipitation based on long term observations at lhasa these data together with precipitation 18 o records from five global network of isotopes in precipitation gnip stations and two ice core 18 o records were used to define a regional metric of asm precipitation 18 o asmoi back trajectory analyses for rainy season precipitation events indicate that moisture sources vary little between years with relatively high and low 18 o values a result that is consistent for the south lhasa southeast bangkok and east asm regions hong kong in contrast 18 o values at these three locations are significantly correlated with convection in the estimated source regions and along transport paths these results suggest that upstream convection rather than moisture source change causes interannual variation in asm precipitation 18 o values contrasting values of the asmoi in el nino and la nina years reveal a positive isotope el nino southern oscillation enso response e g high values corresponding to warm phases which we interpret as a response to changes in regional convection we show that the isotope enso response is amplified at high elevation sites and during la nina years these findings should improve interpretations of paleo 18 o data as a proxy for past asm variation and provide new opportunities to use data from this region to study paleo enso activity

quantitative kinematic flow analysis from the main central thrust zone nw himalaya india implications for a decelerating strain path and the extrusion of orogenic wedges quantitative kinematic indicators from the main central thrust zone mctz in the nw himalaya have been used to characterize the type of flow during deformation different generations of tension gashes have been rotated by variable angles with respect to the mylonitic foliation forming associated fringe folds these record the late stage brittle ductile flow and reveal that a strong pure shear component of deformation occurred throughout the mctz to characterize earlier deformation increments fabrics from highly deformed quartz ribbons were analyzed well developed shape and lattice preferred orientation patterns show a systematic change of the glide systems and suggest inverted palaeotemperatures within the mctz investigations of the c axes patterns reveal a strong asymmetry at the top of the mctz whereas the samples from the base of the mctz show almost perfectly symmetrical type i crossed girdles deformation within the mctz probably started close to simple shear flow at higher temperatures which progressively became a more general shear during cooling and ended in a pure shear dominated flow during the final stages of brittle ductile deformation i e a decelerating strain path using the higher himalaya crystalline as an example a kinematic model for the extrusion of crustal wedges above major thrust zones is suggested

contrasting the indian and east asian monsoons implications on geologic timescales the surface winds over the arabian sea and south china sea are meaningful indicators for the strength of the indian monsoon and east asian monsoon respectively paleo monsoon variability has been studied through analysis of sediment records from these two monsoon regions to facilitate interpretation of these records we focus on the impacts of internal and external forcing of the monsoon system by contrasting the annual cycle and interannual variability of two subsystems the monsoon over the indian sector 40 105 e and over the east asian sector 105 160 e differences in the annual cycle within these subsystems arise primarily from the different land ocean configurations that determines atmospheric response to the solar forcing thus factors that drive intensities of the monsoonal annual cycle share common features with the external geographic and orbital forcing that controls paleomonsoon variability we show that the differences in interannual variations between the two monsoon subsystems are primarily due to internal factors of the coupled atmosphere ocean land system such as remote impacts of el nin o la nin a and local monsoon ocean interactions the mechanisms that operate on interannual to interdecadal timescales may differ fundamentally from that on geologic orbital timescales the low level flows over the east asia and australia are essentially established by geographic forcing the amplification of the australia summer monsoon during increased solar precession is likely caused by an enhanced east asian winter monsoon rather than following an enhanced indian summer monsoon as on the interannual timescale it is also found that el nin o influences the low level flow moderately over the arabian sea but to a greater extent over the south china sea as such large changes in the pacific thermal conditions may significantly alter the intensity of the east asian monsoon but not the indian monsoon

quantifying internal strain and deformation temperature in the eastern himalaya bhutan implications for the evolution of strain in thrust sheets abstract quartz microstructural analysis together with quantification of 3 d crystal plastic strain of quartz clasts normalized fry and rf methods constrain deformation temperature ranges and internal strain magnitude orientation within himalayan thrust sheets in bhutan lesser himalayan lh thrust sheets display an inverted deformation temperature gradient from 250 to 310 c at the main boundary thrust to 500 670 c at the main central thrust mct attributed primarily to stacking of lh thrust sheets deformed at progressively higher temperatures toward the hinterland the hot iron effect of the mct hanging wall only affects the upper few hundred meters of hinterland lh thrust sheets frontal thrust sheets exhibit 7 layer parallel shortening lps strain all other thrust sheets exhibit layer normal flattening lnf strain with 1 9 1 8 1 0 lh rocks and 2 1 1 8 1 0 greater and tethyan himalayan rocks median ellipsoids x parallel to lineation we propose that lps strain developed foreland ward of the thrust deformation front and that lnf strain resulted from later tectonic loading the lps to lnf transition occurs at minimum temperatures for quartz plasticity ca 250 270 c lnf ellipsoids are folded along with strata within thrust sheets indicating that internal strain preceded thrust imbrication and translation at the scale of 10 s of meters structural distance strain magnitude of hanging wall rocks does not increase near thrusts which supports the existence of discrete faults with large translations as generally depicted in balanced cross sections strain magnitudes in low grade greater and tethyan himalayan rocks in central bhutan indicate that the top to the north component of channel flow extrusion is most likely less than 19 km

holocene east asian summer monsoon records in northern china and their inconsistency with chinese stalagmite δ18o records abstract monsoon precipitation over china exhibits large spatial differences it has been found that a significantly enhanced east asian summer monsoon easm is characterized by increased rainfall in northern china and by reduced rainfall in southern china and this relationship occurs on different time scales during the holocene this study presents results from a diverse range of proxy paleoclimatic records from northern china where precipitation variability is traditionally considered as an easm proxy our aim is to evaluate the evolution of the easm during the holocene and to compare it with all of the published stalagmite 18 o records from the asian monsoon region in order to explore the potential mechanism s controlling the chinese stalagmite 18 o we found that the intensity of the easm during the holocene recorded by the traditional easm proxy of moisture or precipitation records from northern china is significantly different from the chinese stalagmite 18 o records the easm maximum occurred during the mid holocene challenging the prevailing view of an early holocene easm maximum mainly inferred from stalagmite 18 o records in eastern china in addition all of the well dated holocene stalagmite 18 o records covering a broad geographical region exhibit a remarkably similar trend of variation and are statistically well correlated on different time scales thus indicating a common signal however in contrast with the clear consistency in the 18 o values in all of the cave records both instrumental and paleoclimatic records exhibit significant spatial variations in rainfall on decadal to centennial time scales over eastern china in addition both paleoclimatic records and modeling results suggest that holocene east asian summer monsoon precipitation reached a maximum at different periods in different regions of china thus the stalagmite 18 o records from the easm region should not be regarded as a reliable indicator of the strength of the east asian summer monsoon furthermore modern observations indicate that the moisture for precipitation in the east asian monsoon region is mainly derived from the indian ocean the moisture transport route from the indian ocean to the east asian monsoon region during the holocene is almost identical to that of modern precipitation therefore the strong correlation of 18 o records in the easm and indian summer monsoon ism regions and the similarity of the pattern of latitudinal changes in 18 o values in stalagmites and in modern meteoric precipitation along the water vapor transport route further suggests that the stalagmite 18 o records from the easm region are essentially a signal of the isotopic composition of precipitation which is determined mainly by changes in the 18 o of atmospheric vapor in the upstream source region over the indian ocean and indian monsoon region via the upstream depletion mechanism finally the main trends of the stalagmite 18 o records are strongly correlated with the known evolution of the ism and therefore these records reflect the history of the ism rather than that of the easm our findings support the conclusion that easm variability is mainly controlled by northern hemisphere summer insolation and was strongly modulated by ice volume during the last deglaciation and early holocene which delayed the response of the easm maximum to peak insolation forcing

diachronous deformation along the base of the himalayan metamorphic core west central nepal geologic mapping combined with microstructural and geochronologic analyses from the lower himalayan metamorphic core in west central nepal record along strike similarity in flow style despite variability in the timing of metamorphism and deformation the main central thrust zone at the base of the himalayan metamorphic core varies in thickness tectonostratigraphy and metamorphic gradient along the 250 km of strike length studied in situ u th pb geochronology of monazite sampled from an along strike transect at the top of the high strain zone records eocene oligocene prograde metamorphism followed by miocene retrograde metamorphism the timing of prograde and retrograde metamorphism and the muscovite 40 ar 39 ar dates gradually decrease along strike from northwest to southeast this age trend is punctuated by an abrupt 3 8 m y decrease in the age of prograde and retrograde metamorphism and muscovite 40 ar 39 ar dates near the marsyandi river in central nepal quartz crystallographic preferred orientation fabrics from a parallel transect along the base of the high strain zone document similar flow style at 440 c in central nepal muscovite 40 ar 39 ar ages interpreted to approximate the age of deformation at this structural level decrease from ca 7 to 4 ma along strike from northwest to southeast diachronous deformation and metamorphism along strike in west central nepal demonstrate the necessity of incorporating more than a single transect into tectonic models along strike tectonometamorphic variability in west central nepal spatially corresponds to faults in the indian basement bounding the subsurface faizabad ridge highlighting the possible influence of inherited basement faults on the geometry of the basal himalayan detachment the main himalayan thrust as well as the tectonic evolution of the structurally overlying himalayan metamorphic core this study highlights the potential influence of inherited structures on the overlying orogenic wedge and the probability of along strike diachroneity of deformation in the himalaya

dynamic recrystallization of feldspar a mechanism for ductile shear zone formation feldspar aggregates experimentally deformed in the dislocation creep regime undergo dynamic recrystallization because recovery is difficult due to the limited climb of dislocations recrystallizing aggregates have a lower strength because of the cyclic production of small strain free grains and they develop a strong preferred orientation consistent with that observed in mylonites thus recrystallization accommodated dislocation creep may be responsible for the grain size reduction and strain softening that lead to the formation of many mylonites and ductile shear zones

tectonic evolution of the central annapurna range nepalese himalayas the metamorphic core of the himalayan orogen or greater himalayan sequence is a northward tapering prism bound at the bottom by a n dipping family of thrust faults the main central thrust system and at the top by a n dipping family of normal faults the south tibetan detachment system research in the central annapurna range of nepal demonstrates a close temporal and spatial association between contractional and extensional deformation on these bounding fault systems and within the metamorphic core throughout much of the early miocene the main central thrust system is represented here by a 2 to 3 km thick zone of high strain that developed during two or more episodes of movement most of its displacement was concentrated along the chomrong thrust a sharp late metamorphic discontinuity that places middle amphibolite facies rocks of the greater himalayan sequence on top of lower amphibolite facies rocks of the lesser himalayan sequence the earliest demonstrable movement on this thrust system occurred 22 5 ma the most recent movement may be as young as pliocene the oldest element of the south tibetan detachment system in this area is the deorali detachment which appears to have been active at the same time as the earliest shortening structures of the main central thrust system fabrics related to the deorali detachment are disrupted by a previously unrecognized sw vergent thrust structure the modi khola shear zone the effect of this structure which is constrained to be between 22 5 and 18 5 ma was to shorten rock packages that had been extended previously during movement on the deorali detachment transition back to a local extensional regime after 18 5 ma was marked by development of the machhupuchhare detachment and related splays geologic evidence for rapid two way transitions between contraction and extension in the annapurna range indicates that extensional deformation in convergent settings does not only represent gravitational collapse at the end of an orogenic cycle it also appears to be an important factor in mountain range development

prism and basal plane parallel subgrain boundaries in quartz a microstructural geothermobarometer empirical data on quartz subgrain patterns from various metamorphic rocks show that at least up to 10 kbar in the stability field of low quartz prismatic subgrain boundaries are dominant whereas basal subgrain boundaries are not developed in the high quartz stability field both prismatic and basal subgrain boundaries occur and form typical rectangular chessboard patterns the likely reason behind the different occurrence of these subgrain patterns is that in high quartz prismatic glide becomes as easy as or probably even easier than basal glide the two types of subgrain patterns can be clearly distinguished by optical microscopy consequently the occurrence of chessboard subgrain patterns in quartz represents a practicable geothermobarometer the possibilities of its application are far reaching and include the specification of deformation conditions at high grade metamorphism the recognition of syntectonic intrusions and the distinction between pluton emplacement at lower and at higher crustal levels

a high resolution absolute dated late pleistocene monsoon record from hulu cave china oxygen isotope records of five stalagmites from hulu cave near nanjing bear a remarkable resemblance to oxygen isotope records from greenland ice cores suggesting that east asian monsoon intensity changed in concert with greenland temperature between 11 000 and 75 000 years before the present yr b p between 11 000 and 30 000 yr b p the timing of changes in the monsoon as established with 230th dates generally agrees with the timing of temperature changes from the greenland ice sheet project two gisp2 core which supports gisp2 s chronology in this interval our record links north atlantic climate with the meridional transport of heat and moisture from the warmest part of the ocean where the summer east asian monsoon originates

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

holocene variability of the east asian summer monsoon from chinese cave records a re assessment oxygen isotope records from stalagmites in caves in southern china interpreted as proxy rainfall records reflecting the intensity of the east asian summer monsoon indicate gradual monsoon weakening for the last 9000 years as also documented for the indian monsoon coupled with high precision dating the speleothem proxy records have been used to test monsoon links with orbital forcing solar changes iceberg discharges in the north atlantic ocean currents and atmospheric methane however these benchmark cave records do not match other published dated e asian proxy rainfall record specifically here independently calibrated rainfall records from loess palaeosol magnetic properties and cave oxygen isotope intercomparisons which show variable e asian monsoon intensity through the entire holocene the strong correlation of the cave records with the extraregional indian monsoon record yet their mismatch with these other dated chinese rainfall records might be reconciled if the speleothem isotope variations reflect not changes in holocene rainfall amount but in rainfall source declining holocene influence of isotopically lighter indian monsoon sourced moisture over china would have resulted in increasing proportions of isotopically heavier rainfall sourced from the more oceanic e asian monsoon individual speleothems may thus regionally record holocene changes in indian monsoon intensity and isotopic influence conversely the other chinese proxy records described here reflect changes in rainfall amount and thus in e asian summer monsoon intensity for the holocene the e asian and the indian monsoon responses to orbital forcing are likely to have differed specifically due to e asian internal feedbacks and the seasonal contrasts between the two monsoon systems

how to measure the strength of the east asian summer monsoon defining the intensity of the east asian summer monsoon easm has been extremely controversial this paper elaborates on the meanings of 25 existing easm indices in terms of two observed major modes of interannual variation in the precipitation and circulation anomalies for the 1979 2006 period the existing indices can be classified into five categories the east west thermal contrast north south thermal contrast shear vorticity of zonal winds southwesterly monsoon and south china sea monsoon the last four types of indices reflect various aspects of the leading mode of interannual variability of the easm rainfall and circulations which correspond to the decaying el nino while the first category reflects the second mode that corresponds to the developing el nino the authors recommend that the easm strength can be represented by the principal component of the leading mode of the interannual variability which provides a unified index for the majority of the existing indices this new index is extremely robust captures a large portion 50 of the total variance of the precipitation and three dimensional circulation and has unique advantages over all the existing indices the authors also recommend a simple index the reversed wang and fan index which is nearly identical to the leading principal component of the easm and greatly facilitates real time monitoring the proposed index highlights the significance of the mei yu baiu changma rainfall in gauging the strength of the easm the mei yu which is produced in the primary rain bearing system the east asian ea subtropical front better represents the variability of the easm circulation system this new index reverses the traditional chinese meaning of a strong easm which corresponds to a deficient mei yu that is associated with an abnormal northward extension of southerly over northern china the new definition is consistent with the meaning used in other monsoon regions worldwide where abundant rainfall within the major local rain bearing monsoon system is considered to be a strong monsoon

age of initiation of collision between india and asia a review of stratigraphic data abstract the collision of india with asia is perhaps the most profound tectonic event to have occurred in past 100 ma it is responsible for the uplift of the himalayas and tibetan plateau and has been argued to have been responsible for geological geochemical and climatological consequences of global extent yet the age of initiation of this collision remains poorly constrained the literature is replete with estimates that range from the late cretaceous 65 ma to latest eocene

a high resolved record of the asian summer monsoon from dongge cave china for the past 1200 years abstract two annually laminated and 230th dated stalagmite oxygen isotope 18o records from dongge cave china provided a high resolution asian summer monsoon asm history for the past 1200 years a close similarity between annual band thickness and stable isotope analyses 13c and 18o suggests the calcite 18o is most likely a proxy associated with asm precipitation the two duplicated stalagmite 18o records show that the asm varies at a periodicity of 220 years concordant with a dominant cycle of solar activity a period of strong asm activity occurred during the sporer minimum 1450 1550 a d followed by a striking drop circa 1580 a d potentially consistent with the social unrest in the final decades of china s ming dynasty 1368 1644 a d centennial scale changes in asm precipitation over the last millennium match well with changes in tropical atlantic sea surface temperatures ssts and south american summer monsoon precipitation our findings suggest that variations in low latitude monsoon precipitation are probably driven by shifts in the mean position of the intertropical convergence zone itcz which is further mediated by solar activity and tropical ssts

fabric controls on strain accommodation in naturally deformed mylonites the influence of interconnected micaceous layers abstract we present microstructural analyses demonstrating how the geometrical distribution and interconnectivity of mica influences quartz crystallographic preferred orientation cpo development in naturally deformed rocks we use a polymineralic qtz pl kfs bt ms grt tur mylonite from the zanskar shear zone a section of the south tibetan detachment nw himalaya to demonstrate how quartz cpo intensity decreases from quartz dominated domains to micaceous domains independently of whether or not quartz grains are pinned by mica grains we then use a bimineralic qtz ms mylonite from the main central thrust nw himalaya to show how increasing mica grain connectivity is concomitant with a systematic weakening of quartz cpo our results draw distinctions between cpo weakening due to i second phase drag leading to ineffective recovery in quartz and ii increased transmission and localisation of strain between interconnected mica grains in the latter case well connected micaceous layers take up most of the strain weakening the rock and preventing straining of the stronger quartz matrix our findings suggest that rock weakening in quartz rich crustal rocks is influenced not only by the presence of mica rich layers but also the degree of mica grain connectivity which allows for more effective strain localization through the entire rock mass

india asia collision and the cenozoic slowdown of the indian plate implications for the forces driving plate motions the plate motion of india changed dramatically between 50 and 35 ma with the rate of convergence between india and asia dropping from 15 to 4 cm yr this change is coincident with the onset of the india asia collision and with a rearrangement of plate boundaries in the indian ocean on the basis of a simple model for the forces exerted upon the edges of the plate and the tractions on the base of the plate we perform force balance calculations for the precollision and postcollision configurations we show that the observed euler poles for the indian plate are well explained in terms of their locations and magnitudes if 1 the resistive force induced by mountain building in the himalaya tibet area is 5 6 10 12 n m 2 the net force exerted upon the indian plate by subduction zones is similar in magnitude to the ridge push force 2 5 10 12 n m and 3 basal tractions exert a resisting force that is linearly proportional to the plate velocity in the hot spot reference frame the third point implies an asthenospheric viscosity of 2 5 10 19 pa s assuming a thickness of 100 150 km synthetic euler poles show that crustal thickening in the tibetan plateau was the dominant cause of the cenozoic slowdown of the indian plate

high p t polymetamorphism dehydration melting and generation of migmatites and granites in the higher himalayan crystalline complex sikkim india the higher himalayan crystalline complex hhc in sikkim india consists of pelitic migmatites interlayered with calc silicate rocks and minor metabasites microstructural relationships between the mineral phases and deformational fabric elements and zoning characteristics of garnet indicate a prolonged and complex polymetamorphic history for the hhc the pelitic rocks in the upper part of the hhc contain the assemblage plagioclase quartz garnet k feldspar biotite sillimanite and are devoid of muscovite most of the mineral phases grew syn to post tectonically mineral growth coincided with development of a pervasive fabric s 2 during prograde metamorphism m 2 in the early stages of the collisional event some garnet grains display texturally distinct cores and rims which are separated by calcic plagioclase this texture suggests an earlier metamorphic episode m l m may represent pre himalayan metamorphism and decompression of the hhc later the collisional event led to renewed burial of the hhc and m 2 reactions m 2 is reflected by dehydration melting of muscovite and biotite to form granitic melts which either crystallized in situ to form leucosomes or migrated from their source regions to form larger granitic bodies geothermobarometric estimates for peak m 2 conditions indicate p 10 12 kbar t 800 850 c a subsequent metamorphic event m 3 occurred because of 5 kbar of decompression m 3 is recorded by the breakdown of porphyroblastic garnet in all hhc lithologies higher temperature and pressure estimates come from progressively higher structural levels of the complex the thermal gradient of 5 5 c km is anomalous and may be a consequence of thermal buffering during melting however the pressure gradient of 0 25 kbar km resembles a normal lithostatic gradient which suggests that the hhc in sikkim represents an inverted barrovian sequence this inverted zonation of the hhc is probably the result of large scale structural inversion and or tectonic juxtaposition because of ductile shearing

structure of the main central thrust zone and extrusion of the high himalayan deep crustal wedge kishtwar zanskar himalaya the main central thrust is a crustal scale ductile shear zone between 1 5 and 3 km wide which places the oligocene miocene metamorphic rocks of the high himalayan zone south or sw over the unmetamorphosed or weakly metamorphosed rocks of the lesser himalaya the high strain zone of the main central thrust is coincident with an inverted metamorphic field gradient from biotite to kyanite grade over a structural thickness of 1500 m kyanite grade rocks metamorphosed at 9 5 10 kbar were exhumed from depths of 33 37 km along the main central thrust hanging wall and emplaced over lesser himalayan rocks never buried deeper than 10 12 km exhumation of the deepest buried kyanite grade rocks occurred along the zone above the main central thrust zone approximately 45 km width 28 km structural thickness of the high himalaya exposes sillimanite grade gneisses migmatites and 19 5 21 5 ma old leucogranites formed at pressures between 4 5 and 7 kbar and depths of 16 25 km a ne dipping normal fault ductile shear zone at the top of the slab zanskar shear zone shows condensed but right way up isograds from sillimanite to chlorite grade over 2 400 metres structural thickness 40 ar 39 ar geochronology shows that most of the high himalayan slab had cooled below 350 c by 16 ma supporting models linking the two bounding faults of the high himalaya both kinematically and temporally there is no evidence of melting along the main central thrust zone and the himalayan leucogranites were generated 10 30 km structurally above the main central thrust frictional heating along the main central thrust could not therefore have played any role in generating the leucogranites thus far there is little evidence for late miocene reactivation along the main central thrust as seen elsewhere along the main central thrust in nepal

orbital scale timing and mechanisms driving late pleistocene indo asian summer monsoons reinterpreting cave speleothem δ18o 1 southeast china cave 18o often interpreted as a pure east asian summer monsoon proxy lags maximum northern hemisphere summer insolation by 2 9 0 3 kyrs at the precession cycle the arabian sea summer monsoon stack lags by 8 1 kyr consistent with 13 other indian and east asian summer monsoon proxies from marine lake and terrestrial archives this 5 kyr phase difference cannot be attributed to age control inadequacies in the marine chronology it requires reconciliation in the context of proxy interpretation both of these lags are incompatible with a direct response to northern hemisphere summer insolation implicating additional forcing mechanisms analysis of heterodynes in the cave 18o spectrum demonstrates that variance contained in the arabian sea summer monsoon proxies also resides in the cave 18o record this variance is subtracted from the cave 18o record yielding a residual that is highly coherent and in phase with precession minima reflecting the impact of winter temperature change on cave 18o meteorological precipitation under cold conditions thus we argue that the timing of light cave 18o peaks cannot be interpreted as reflecting the timing of strong summer monsoons alone the 2 9 kyr precession band phase lag of cave 18o reflects the combined influence of summer monsoon forcing with a phase lag of 8 kyrs relative to precession minima and winter temperature forcing that is in phase with precession minima this interpretation is consistent with modern seasonality in the amount and isotopic composition of rainfall in southeast china

climatic and environmental controls on speleothem oxygen isotope values variations in speleothem oxygen isotope values d 18 o result from a complicated interplay of environmental controls and processes in the ocean atmosphere soil zone epikarst and cave system as such the controls on speleothem d 18 o values are extremely complex an understanding of the processes that control equilibrium and kinetic fractionation of oxygen isotopes in water and carbonate species is essential for the proper interpretation of speleothem d 18 o as paleoclimate and paleoenvironmental proxies and is best complemented by study of site specific cave processes such as infiltration flow routing drip seasonality and saturation state and cave microclimate among others this review is a process based summary of the multiple controls on d 18 o in the atmosphere soil epikarst and speleothem calcite illustrated with case studies primary controls of d 18 o in the atmosphere include temperature and relative humidity through their role in the multiple isotope effects variability and modifications of water d 18 o values in the soil and epikarst zones are dominated by evaporation mixing and infiltration of source waters the isotopically effective recharge into a cave system consists of those waters that participate in precipitation of caco3 resulting in calcite deposition rates which may be biased to time periods with optimal dripwater saturation state recent modeling experimental and observational data yield insight into the significance of kinetic fractionation between dissolved carbonate phases and solid caco3 and have implications for the hendy test to assist interpretation of speleothem d 18 o time series quantitative and semi quantitative d 18 o climate calibrations are discussed with an emphasis on some of the difficulties inherent in using modern spatial and temporal isotope gradients to interpret speleothems as paleoclimate proxy records finally several case studies of globally significant speleothem paleoclimate records are discussed that show the utility of d 18 o to reconstruct past climate

dynamic recrystallization of quartz correlation between natural and experimental conditions quartz veins in the eastern tonale mylonite zone italian alps were deformed in strike slip shear due to the synkinematic emplacement of the adamello pluton a temperature gradient between 280 c and 700 c was effected across this fault zone the resulting dynamic recrystallization microstructures are characteristic of bulging recrystallization subgrain rotation recrystallization and grain boundary migration recrystallization the transitions in recrystallization mechanisms are marked by discrete changes of grain size dependence on temperature differential stresses are calculated from the recrystallized grain size data using paleopiezometric relationships deformation temperatures are obtained from metamorphic reactions in the deformed host rock flow stresses and deformation temperatures are used to determine the strain rate of the tonale mylonites through integration with several published flow laws yielding an average rate of approximately 10 14s 1 to 10 12s 1 the deformation conditions of the natural fault rocks are compared and correlated with three experimental dislocation creep regimes of quartz of hirth tullis linking the microstructures of the naturally and experimentally deformed quartz rocks a recrystallization mechanism map is presented this map permits the derivation of temperature and strain rate for mylonitic fault rocks once the recrystallization mechanism is known

timing duration and transitions of the last interglacial asian monsoon thorium 230 ages and oxygen isotope ratios of stalagmites from dongge cave china characterize the asian monsoon and low latitude precipitation over the past 160 000 years numerous abrupt changes in 18o 16o values result from changes in tropical and subtropical precipitation driven by insolation and millennial scale circulation shifts the last interglacial monsoon lasted 9 7 1 1 thousand years beginning with an abrupt less than 200 years drop in 18o 16o values 129 3 0 9 thousand years ago and ending with an abrupt less than 300 years rise in 18o 16o values 119 6 0 6 thousand years ago the start coincides with insolation rise and measures of full interglacial conditions indicating that insolation triggered the final rise to full interglacial conditions

spatial and temporal variability in the stable isotope systematics of modern precipitation in china implications for paleoclimate reconstructions the stable isotopic composition of materials such as glacial ice tree rings lake sediments and speleothems from low to mid latitudes contains information about past changes in temperature t and precipitation amount p however the transfer functions which link 18op to changes in t or p d 18op dt and d 18op dp can exhibit significant temporal and spatial variability in these regions in areas affected by the southeast asian monsoon past variations in 18o and d of precipitation have been attributed to variations in monsoon intensity storm tracks and or variations in temperature proper interpretation of past 18op variations here requires an understanding of these complicated stable isotope systematics since temperature and precipitation are positively correlated in china and have opposite effects on 18op it is necessary to determine which of these effects is dominant for a specific region in order to perform even qualitative paleoclimate reconstructions here we evaluate the value of the transfer functions in modern precipitation to more accurately interpret the paleorecord the strength of these transfer functions in china is investigated using multiple regression analysis of data from 10 sites within the global network for isotopes in precipitation gnip 18op is modeled as a function of both temperature and precipitation the magnitude and signs of the transfer functions at any given site are closely related to the degree of summer monsoon influence 18op values at sites with intense summer monsoon precipitation are more dependent on the amount of precipitation than on temperature and therefore exhibit more negative values in the summer in contrast 18op values at sites that are unaffected by summer monsoon precipitation exhibit strong relationships between 18op and temperature the sites that are near the northern limit of the summer monsoon exhibit dependence on both temperature and amount of precipitation comparison with simple linear models 18op as a function of t or p and a geographic model 18op as a function of latitude and altitude shows that the multiple regression model is more successful at reproducing 18op values at sites that are strongly influenced by the summer monsoon the fact that the transfer function values are highly spatially variable and closely related to the degree of summer monsoon influence suggests that these values may also vary temporally since the southeast asian monsoon intensity is known to exhibit large variations on a number of timescales annual to glacial interglacial and the magnitude and sign of the transfer functions is related to monsoon intensity we suggest that as monsoon intensity changes the magnitude and possibly even the sign of the transfer functions may vary therefore quantitative paleoclimate reconstructions based on 18op variations may not be valid

chinese stalagmite δ 18 o controlled by changes in the indian monsoon during a simulated heinrich event carbonate cave deposits in india and china are assumed to record the intensity of monsoon precipitation because the 18 o of the carbonate tracks the isotopic signature of precipitation these records show spatially coherent variability throughout the last ice age and suggest that monsoon strength was altered during the millennial scale climate variations known as dansgaard oeschger events and during the heinrich cooling events here we use a numerical climate model with an embedded oxygen isotope model to assess what caused the shifts in the oxygen isotope signature of precipitation during a climate perturbation designed to mimic a heinrich event our simulations show that a sudden increase in north atlantic sea ice extent during the last glacial period leads to cooling in the northern hemisphere reduced precipitation over the indian basin and weakening of the indian monsoon the precipitation is isotopically heavier over india and the water vapour exported to china is isotopically enriched our model broadly reproduces the enrichment of 18 o over northern india and east asia evident in speleothem records during heinrich events we therefore conclude that changes in the 18 o of cave carbonates associated with heinrich events reflect changes in the intensity of indian rather than east asian monsoon precipitation