oligocene miocene burial and exhumation of the southernmost gangdese mountains from sedimentary and thermochronological evidence abstract the kailas conglomerates crop out ubiquitously along the southernmost boundary of the gangdese batholith they unconformably overlie the gangdese batholith and are displaced by the great counter thrust gct fault forming a fault contact with the xigaze forearc basin the associated subduction complex and the tethyan himalayan sequence these strata furnish a record of uplift and paleoenvironmental change in the indus yarlung suture zone during the oligocene miocene our new and previously published low temperature thermochronometric data from the gangdese batholith and the kailas conglomerates indicate a period of rapid exhumation beginning approximately 17 15 ma centered on the southern margin of the gangdese batholith whereas regional uplift commenced significantly earlier during the deposition of the kailas conglomerates based on the presence of an abrupt facies transition from deep water lacustrine deposits to red alluvial fan or fluvial deposits the period of rapid exhumation probably lagged behind the initiation of faster uplift while the related changes in the depositional environment were most likely recorded immediately in the basin stratigraphy subsequently the kailas conglomerates were buried in association with the development of the north directed great counter thrust while rapid exhumation was facilitated by efficient incision by the paleo yarlung river at approximately 17 15 ma

constant elevation of southern tibet over the past 15 million years the uplift of the tibetan plateau an area that is 2 000 km wide to an altitude of about 5 000 m has been shown to modify global climate and to influence monsoon intensity mechanical and thermal models for homogeneous thickening of the lithosphere make specific predictions about uplift rates of the tibetan plateau but the precise history of the uplift of the plateau has yet to be confirmed by observations here we present well preserved fossil leaf assemblages from the namling basin southern tibet dated to approximately 15 myr ago which allow us to reconstruct the temperatures within the basin at that time using a numerical general circulation model to estimate moist static energy at the location of the fossil leaves we reconstruct the elevation of the namling basin 15 myr ago to be 4 689 895 m or 4 638 847 m depending on the reference data used this is comparable to the present day altitude of 4 600 m we conclude that the elevation of the southern tibetan plateau probably has remained unchanged for the past 15 myr

separate episodes of eclogite and blueschist facies metamorphism in the aegean metamorphic core complex of ios cyclades greece

thermochronology of the south cyclades shear zone ios greece effects of ductile shear in the argon partial retention zone micas and potassium feldspars from the south cyclades shear zone ios cyclades greece yield varied and complex 40ar 39ar apparent age spectra a correlation exists between 40ar 39ar apparent ages and the relative timing of different episodes of recrystallization and grain growth as indicated by fabric and microstructural analysis the 40ar 39ar apparent age spectra record the effects of variation in the degree of recrystallization and grain growth and partial to complete resetting of argon systematics in potassium bearing minerals during hercynian m0 alpine m1 and late oligocene early miocene m2 metamorphism deformation was strongly partitioned within the shear zone and this led to localized recrystallization and heterogeneous resetting of argon systematics within preexisting minerals modeling suggests the oligo miocene thermal events were of insufficient magnitude and or duration to completely reset the isotopic systematics in these samples our data lead to the concept of the argon partial retention and resetting zone prz defined as that portion of the crust where temperatures are insufficient to completely reset argon systematics within preexisting potassium bearing minerals within the prz some radiogenic argon in preexisting potassium bearing minerals will be outgassed and only partially retained tectonic exhumation of the prz involves movement on crustal scale ductile shear zones accompanied by strongly partitioned deformation and localized recrystallization recrystallization leads to resetting of argon systematics and thus will result in heterogeneous 40ar 39ar age distributions within these ductile shear zones e g in the south cyclades shear zone

palaeo moisture evolution in monsoonal central asia during the last 50 000 years abstract the late quaternary climate history of monsoonal central asia was inferred from 75 palaeoclimatic records which provide information about moisture conditions in the last 50 ka or part of this period wet conditions occurred during middle and late marine isotope stage 3 while the last glacial maximum lgm was characterized by dry climate conditions in the region a stepwise climate amelioration is suggested by the climate records following the lgm several climate signals of this period which were reported from high latitude ice core records are preserved in archives from monsoonal central asia as well during the early holocene high effective moisture was inferred from most records from the area dominated by the indian monsoon e g the tibetan plateau suggesting that holocene optimal climate conditions occurred there during this period in contrast areas which are dominated by the south east asian monsoon se monsoon and the westerlies in north western and north central china mongolia do not uniformly show an early holocene climate optimum for this area optimal conditions prevailed during the mid holocene these apparent contradictions can possibly be explained by the regional uplift and descent of air masses in the holocene during the early holocene strengthened insolation possibly caused an enhanced low level convergence over the tibetan plateau which led to the intensification of the summer monsoon the strong air uplift caused intensified precipitation and air divergence in the upper troposphere over the tibetan plateau the areas adjacent to the north therefore experienced an intensified descent of air masses and consequently increased aridity the majority of the palaeoclimatic records suggest reduced effective moisture since the late holocene in the region

miocene burial and exhumation of the india asia collision zone in southern tibet response to slab dynamics and erosion the india asia collision zone in southern tibet preserves a record of geodynamic and erosional processes following intercontinental collision apatite fission track and zircon and apatite u th he data from the oligocene miocene kailas formation within the india asia collision zone show a synchronous cooling signal at 17 1 ma which is younger than the ca 26 21 ma depositional age of the kailas formation constrained by u pb and 40 ar 39 ar geochronology and requires heating burial after ca 21 ma and subsequent rapid exhumation data from the gangdese batholith underlying the kailas formation also indicate miocene exhumation the thermal history of the kailas formation is consistent with rapid subsidence during a short lived phase of early miocene extension followed by uplift and exhumation driven by rollback and northward underthrusting of the indian plate respectively significant removal of material from the india asia collision zone was likely facilitated by efficient incision of the paleo indus river and paleo yarlung river in response to drainage reorganization and or intensification of the asian monsoon

synchronous fluvial response to surface uplift in the eastern tibetan plateau implications for crustal dynamics synchronous response of river erosion to surface uplift is often assumed by studies using thermochronometer constrained erosion histories to estimate changes in landscape and tectonics we have tested this assumption by studying river incision histories at two sites heishui and lianghekou located in the westernmost inland part of the deeply incised eastern tibetan plateau apatite fission track age elevation plots for the two sites show clear late miocene inflection points marking the time when apparent erosion rates increased fourfold to eightfold to 300 800 m myr this timing is further refined as 10 12 ma by thermal history modeling and overlaps within analytical uncertainties with that recorded from the eastern tibetan plateau margin 150 200 km to the east demonstrating that river incision propagated rapidly from the margin to the interior in less than 2 ma such synchronous regional river incision is consistent with regional surface uplift generated by crustal extrusion along a detachment at depth

landscape and drainage evolution during the cenozoic in the salinas grandes basin andean plateau of nw argentina abstract endorheic basins are distinctive features of the modern andean plateau these basins record thick neogene to modern evaporite successions that are considered as the onset of internal drainage and aridity some isolated older paleogene evaporites and intermontane depocenters have been also described across the argentine puna suggesting that some degree of hydrological isolation would have pre dated the main miocene pliocene plateau uplift evaporites beds however are not conclusive evidence for endorheism given that their formation depends not only on the tectonically driven paleo drainage but also on other sensitive variables as climate and salinity of the sources here we discuss the paleo drainage evolution across the eastern puna plateau at 23 24 s combining landscape numerical modeling with seismic stratigraphy seismic observations suggest an intermontane configuration as early as the late paleogene and relief formation to the east and west basin margins using different combinations of precipitation rates our landscape models allowed us to reproduce the modern elevations observed in the study region as well as published thermochronological data our results suggest that endorheism would have started in the paleogene and would have dominated most 70 of the cenozoic basin history models show the recurrence of endorheism interrupted by intermittent phases of drainage connectivity exorheism to the east foreland in the upper half eocene and the early miocene according to these results the formation of closed basins in the puna region would have pre dated the main miocene pliocene uplift stage of the andean plateau

the genesis of dedolomites a discusion based on reactive transport modeling abstract dedolomites are rocks formed by the replacement of dolomite with calcite which is driven by the infiltration of ca rich water this process has been described in the literature as either increasing preserving or decreasing porosity of the initial dolostone according to textures observed in prades ne spain dedolomitization took place due to two simultaneous processes non pseudomorphic replacement of dolomite with calcite at the wall rock of fractures and pseudomorphic replacement in the bulk rock a multicomponent reactive transport model is used to analyze the two dedolomitization processes observed diffusion of ions from the fractures was the driving force for non pseudomorphic replacement in the wall rock the dissolution of dolomite and precipitation of calcite took place in accord with a 1 2 stoichiometry as a consequence of the mineral molar volume the porosity was sealed and replacement limited to a rim on both sides of the fractures according to calculations the development of a rim took less than 500 years advection of ions from the top of the dolomite bed was the driving force for pseudomorphic replacement in the bulk rock due to the absence of empty volume for expansion replacement was governed by the condition of equal volumetric rate for dolomite dissolution and calcite precipitation according to calculations pseudomorphic replacement of 1 m of dolostone was completed after about 10 5 years for reasonable values of flow parameters as the infiltrating water was also undersaturated with respect to calcite one stage of calcite dissolution postdated dedolomitization this process was responsible for the widening of open fractures where the dissolution front followed the dedolomitization front into the dolomite matrix intensive calcite dissolution was also developed following the advective flux from the top of the bed downwards drusy calcite is observed to precipitate in the pore spaces which consist of intergrains fractures and dissolution cavities this process is not predicted in detail by reactive transport calculations although it may be due to calcite supersaturation of the solution resulting from pseudomorphic dedolomitization

the brahmaputra tale of tectonics and erosion early miocene river capture in the eastern himalaya the himalayan orogen provides a type example on which a number of models of the causes and consequences of crustal deformation are based and it has been suggested that it is the site of a variety of feedbacks between tectonics and erosion within the broader orogen fluvial drainages partly reflect surface uplift different climatic zones and a response to crustal deformation in the eastern himalaya the unusual drainage configuration of the yarlung tsangpo brahmaputra river has been interpreted either as antecedent drainage distorted by the india asia collision and as such applied as a passive strain marker of lateral extrusion latest neogene tectonically induced river capture or glacial damming induced river diversion events here we apply a multi technique approach to the neogene paleo brahmaputra deposits of the surma basin bengal basin bangladesh to test the long debated occurrence and timing of river capture of the yarlung tsangpo by the brahmaputra river we provide u pb detrital zircon and rutile isotopic sr nd and hf and petrographic evidence consistent with river capture of the yarlung tsangpo by the brahmaputra river in the early miocene we document influx of cretaceous paleogene zircons in early miocene sediments of the paleo brahmaputra river that we interpret as first influx of material from the asian plate transhimalayan arc indicative of yarlung tsangpo contribution prior to capture the predominantly precambrian paleozoic zircons indicate that only the indian plate was drained contemporaneous with transhimalayan influx reflecting the river capture we record arrival of detrital material affected by cenozoic metamorphism as indicated by rutiles and zircons with cenozoic u pb ages and an increase in metamorphic grade of detritus as recorded by petrography we interpret this as due to a progressively increasing contribution from the erosion of the metamorphosed core of the orogen whole rock sr nd isotopic data from the same samples provide further support to this interpretation river capture may have been caused by a change in relative base level due to uplift of the tibetan plateau assuming such river capture occurred via the siang river in the early miocene we refute the tectonic aneurysm model of tectonic erosion coupling between river capture and rapid exhumation of the eastern syntaxis since a time interval of at least 10 ma between these two events is now demonstrated this work is also the first to highlight u pb dating on detrital rutile as a powerful approach in provenance studies in the himalaya in combination with zircon u pb chronology

the tectonic significance of triassic dolomite and cargneule in the gran paradiso region western alps the interrelationships of dolomite anhydrite gypsum and cargneule a yellow brown dedolomitised breccia are discussed with specific reference to the gran paradiso region western alps the dolomite is a highly deformed rock which occurs along the soles of both early and late alpine thrusts evidence suggests that cargneule formed late in the geological history of the region it is suggested that dolomite anhydrite water acted as an important decollement horizon recent exposure to the surface hydration of anhydrite to gypsum and dedolomitisation resulted in cargneule formation

insights into the dynamics of an orogenic wedge from lubrication theory implications for the himalayan tectonics abstract using lubrication theory we develop a mechanical model to evaluate the dynamic relation between an orogenic wedge and the overriding plate the model suggests that the subducting plate motion produces a dynamic pressure in the wedge which supports the gravity load of the overriding plate lying above it stable condition a drop in the dynamic pressure results in a rotational collapse of the overriding plate unstable condition we present an analytical solution to provide an estimate of the dynamic pressures in orogenic wedges the lubrication model is also implemented in computational fluid dynamics cfd simulations to perform real scale numerical experiments considering mainly three variables 1 plate convergence angle 2 subduction rate us and 3 viscosity ratio r between the overriding plate and wedge the overriding plates attain mechanical stability when us exceeds a threshold value us otherwise they become unstable as us

deformation history of a subducted continental crust gran paradiso western alps continuing crustal shortening during exhumation eclogite facies continental basement in the western alps outcrops as tectonic windows below the oceanic units in the gran paradiso massif eclogite facies assemblages in mafic rocks display a n s stretching lineation da1 the main phase foliation formed under epidote amphibolite facies conditions da2 and defines a regional dome structure for the whole gran paradiso massif structural data including new detailed mapping microstructural analyses and metamorphic studies in the northern part of the gran paradiso cogne valley reveal the occurrence of major thrusts in this continental basement associated with the da2 deformation e w trending stretching lineations and fold axes are associated with this second and main deformation nappe stacking of the different units was achieved by top to the west shearing during exhumation of the eclogite facies basement the antiformal doming of the gran paradiso basement is thus produced by the nappe stacking during exhumation under epidote amphibolite facies

miocene subsidence and surface uplift of southernmost tibet induced by indian subduction dynamics the indus yarlung suture of southernmost tibet marks the initial collisional zone the ongoing india asia collision and yet more than 30 million years after the onset of collision a thick detrital sedimentary unit was deposited just north of the suture the kailas formation the mechanism permitting subsidence of the deep intracontinental kailas basin in a compressional tectonic regime remains uncertain we present new apatite 16 11 ma and zircon 24 19 ma fission track aft and zft ages from the gangdese batholith just north of the kailas basin zft analysis of modern river sand from the northern gangdese magmatic arc indicates an exhumation at 27 3 1 3 ma thermal modeling indicates that the batholith experienced reheating between 28 and 20 ma coeval with deposition in the kailas basin between 26 and 21 ma followed by overall rapid cooling between 20 and 17 ma we interpret this thermal history as a phase of regional oligocene miocene sedimentary burial followed by exhumation by modeling mantle dynamics in the geodynamic framework of the india asia collision we show that transient dynamic topography over the relative southward folding of the indian slab is consistent with burial and exhumation of the gangdese magmatic arc during oligocene miocene time the northward migration of the indian continent relative to its own stati onary slab created a wave of dynamic topography that caused subsidence in the overriding plate north of the himalaya followed by a phase of surface uplift since 27 ma of the northern gangdese magmatic arc during latest oligocene early miocene time the dynamic deflection center was in the kailas area and it progressively relocated southward to its present position at the ganges basin

tectonic control of yarlung tsangpo gorge revealed by a buried canyon in southern tibet the himalayan mountains are dissected by some of the deepest and most impressive gorges on earth constraining the interplay between river incision and rock uplift is important for understanding tectonic deformation in this region we report here the discovery of a deeply incised canyon of the yarlung tsangpo river at the eastern end of the himalaya which is now buried under more than 500 meters of sediments by reconstructing the former valley bottom and dating sediments at the base of the valley fill we show that steepening of the tsangpo gorge started at about 2 million to 2 5 million years ago as a consequence of an increase in rock uplift rates the high erosion rates within the gorge are therefore a direct consequence of rapid rock uplift

thermal evolution of the gangdese batholith southern tibet a history of episodic unroofing the gangdese batholith southern tibet was part of an andean type arc at the southern margin of asia prior to the collision of india and asia at approximately 50 to 40 ma fission track and 40ar 39ar analyses of 28 rocks from 10 gangdese granitoid plutons along an 250 km length of the batholith in the lhasa region provide a detailed understanding of the age and the postcrystallization erosional and tectonic history of these rocks these data suggest a range of ages for these plutons of 94 to 42 ma with the majority being of tertiary age the postcrystallization cooling histories of all of these plutons are characterized by marked discontinuities we conclude that most of these discontinuities and all of them after 40 ma reflect tectonic changes that produced brief pulses of rapid erosion which were distributed in both space and time in addition to the initial cooling of hot magma against cold country rock all of the rocks we studied showed evidence for at least one subsequent episode of rapid cooling dropping many tens of degrees in a few million years conversely these plutons all experienced intervals during which they cooled very slowly or not at all these slow cooling intervals lasted from 5 to 50 million years our data indicate that since the collision between india and asia began response to continued convergence has been quite variable in even this relatively small area the data reported here are consistent with a recently proposed model of oligo miocene crustal shortening along the gangdese thrust system in this area

cenozoic low temperature cooling history of the northern tethyan himalaya in zedang se tibet and its implications abstract major structures and climatic change during the cenozoic especially miocene interacted to shape the giant tibetan plateau whose formation was triggered by convergence of indian and asian plates in this work we report a low temperature thermochronology data set from the northern tethyan himalaya and indus yarlung suture zone in the zedang area where structures are defined by two parallel thrusts the zhongba gyantse thrust to the south and great counter thrust to the north eighteen samples from the region constrain their low temperature cooling and exhumation histories and further elucidate the evolution of the southern tibetan plateau thermal history modelling reveals eocene and miocene cooling episodes based on the temporal and spatial patterns eocene cooling is probably linked to crustal thickening resulting in the eocene anatexis in the northern tethyan himalaya the rapid pronounced cooling episode between ca 20 ma and 15 ma in the luobusha area reflects the activity of the great counter thrust gct the ca 14 10 ma cooling event of the yardoi dome can probably be linked to rapid erosional denudation during that period and pronounced cooling commencing in the middle miocene was probably caused by incision of branches of the yarlung zangbo river a further cooling episode commencing in late miocene time ca 5 ma in luobusha area and yardoi dome may reflect initial activity of the cona graben cutting through the region

exhumation of high pressure rocks driven by slab rollback rocks metamorphosed under high pressure hp and ultra high pressure uhp conditions in subduction zones come back to the surface relatively soon after their burial and at rates comparable to plate boundary velocities in the mediterranean realm their occurrence in several belts related to a single subduction event shows that the burial exhumation cycle is a recurrent transient process using the calabria apennine and aegean belts as examples we show that the exhumation of hp rocks is associated in time and space with the subduction of small continental lithosphere blocks that triggers slab rollback creating the necessary space for the exhumation of the buoyant continental crust that was deeply buried just before the buoyancy force of the subducted crust increases until this crust detaches from the downgoing slab it then exhumes at a rate that depends directly on the velocity of trench retreat to become part of the overriding plate heated from below by the asthenosphere that flows into the opening mantle wedge the exhumed crust weakens and undergoes core complex type extension responsible for a second stage of exhumation at a lower rate the full sequence of events that characterizes this model crust mantle delamination slab rollback and trench retreat hp rock exhumation asthenosphere heating and core complex formation arises entirely from the initial condition imposed by the subduction of a small continental block no specific condition is required regarding the rheology and erosion rate of hp rocks the burial exhumation cycle is transient and can recur every time a small continental block is subducted

paleocene to pliocene low latitude high elevation basins of southern tibet implications for tectonic models of india asia collision cenozoic climate and geochemical weathering the elevation history of the tibetan plateau promises insight into the mechanisms and dynamics that develop and sustain high topography over tens of millions of years we present the first nearly continuous cenozoic elevation history from two sedimentary basins on the southern tibetan plateau within the latest cretaceous to eocene gangdese arc oxygen isotope and 47 clumped isotope compositions of nonmarine carbonates allow us to constrain carbonate formation temperature and reconstruct the paleoprecipitation record of the eocene to pliocene oiyug basin and paleocene to eocene penbo basin we exploit the systematic decrease of surface temperature and meteoric water 18o values with elevation to derive paleoelevation estimates for these basins minimally altered and unaltered pedogenic and lacustrine carbonates from the oiyug basin yield 47 cdes relative to the carbon dioxide equilibrium scale cdes values of 0 625 to 0 755 which correspond to temperatures of 1 30 c using a 47 thermometer for low temperature carbonates similarly the penbo basin yielded 47 cdes values of 0 701 to 0 726 corresponding to temperatures of 6 12 c the apparent evidence for survival of primary clumped isotope values in the face of substantial burial and heating is an important result for the field of carbonate clumped isotope thermometry our paleoelevation estimates for the eocene to pliocene oiyug basin 6 5 4 1 km support previous evidence that high elevations were attained in southern tibet by at least ca 30 ma stable isotope results allow for the possibility of significant topographic subsidence during the miocene as a result of regional extension in the penbo basin our paleoelevation estimates for the paleocene to eocene nianbo formation 4 4 1 3 1 7 km and eocene pana formation 4 1 1 2 1 6 km extend the altitude record of the southern tibetan plateau to pre india asia collision these results support the lhasaplano model of an andean type continental margin tectonic system the rise of the himalayas and tibet is often invoked to understand isotopic proxies for global chemical weathering in the cenozoic and has constrained the debate on the nature of co2 climate weathering feedbacks the nature of the tibetan paleoelevations from pre to postcollision as presented here indicates that high relief at low latitude prevailed on the asian margin much earlier than previously thought thus high topography alone at low latitude is not sufficient to account for the cenozoic weathering proxy record

improved spatial resolution of elemental maps through inversion of la icp ms data laser ablation inductively coupled plasma mass spectrometry la icp ms provides the spatial distribution of elements within crystals and therefore can constrain the rates of geological processes spatial resolution of la icp ms is limited by the requirement to ablate sufficient material to surpass the detection limit of the instrument too little material and the concentration cannot be measured too much material from the same spatial location and the possibility of depth dependent variations in concentration increases because of this requirement and typical analytical setup this commonly places a lower bound on the diameter of an ablation spot size of approximately 20 m for elements with ppm concentration here we present a means to achieve sub spot size resolution using inverse methods we discretize the space sampled in an analysis into pixels and note that the average concentration of the pixels sampled by a spot equals the measured concentration as multiple overlapping spots sample some of the same pixels we can combine discrete expressions for each spot as a system of linear equations through linear inversion with smoothness constraints we can solve for unknown pixel concentrations we highlight this approach with two natural examples in which diffusive processes are important magmatic ascent speeds and u th he noble gas thermochronometry in these examples accurate results require that the true concentration gradients can be recovered from la icp ms data we show that the ability to infer rapid rates of magma ascent is improved from months to weeks and that we are able to interpret previously un interpretable thermochronometric data

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

the andean type gangdese mountains paleoelevation record from the paleocene eocene linzhou basin abstract paleoelevation reconstruction using oxygen isotopes is making a significant contribution to understanding the cenozoic uplift of the himalayas and the tibetan plateau this paper presents new oxygen and carbon isotopic compositions from well dated tertiary paleosols lacustrine calcareous carbonates and marls from the nianbo 60 54 ma and upper pana formations 51 48 ma of the linzizong group in the linzhou penbo basin the sediments of the nianbo formation which are 180 m thick were deposited in alluvial fans braided rivers fan deltas and on nearshore to offshore lacustrine settings whereas those of the upper pana formation are 100 m thick and are comprised predominantly of proximal alluvial fan and braided river deposits correlations between the lithofacies and stable isotopic compositions suggest that the basin was mainly a hydrologically open environment it is confirmed that the 18 o c and 13 c c values from nianbo and pana formations have not yet been reset by late stage diagenesis based on petrographic examination oxygen isotope of the fossil ostracodes and tectonic deformation of strata the paleoelevations are reconstructed using the corrected most negative paleosurface water 18 o psw values these imply that the linzhou area had attained an elevation of 4500 400 m during the period of the indo asian collision i e achieved a near present elevation and may form an andean type mountain range stretching the gangdese arc before collision the gangdese mountains probably maintained high elevations since at least the paleocene and could play a crucial role in the climate change in the interior of the tibetan plateau during the early cenozoic the paleogeomorphic scenario of the eocene tibet is proposed to exist at two high mountains in excess of 4500 m that sandwiched a low elevation basin

post glacial entrenchment and knickpoint migration of the yarlung tsangpo gorge southeastern tibetan plateau abstract the formation of the yarlung tsangpo river system is commonly thought to be controlled by tectonic processes initiated several million years ago however climate change may also play an important role in guiding drainage evolution here we reconstruct the drainage history of the yarlung tsangpo gorge using field observations age constraints from optically stimulated luminescence and radiocarbon dating paleosalinity reconstruction and grain size analysis of lacustrine deposits and river valley characteristics field observations clay boron derived paleosalinity reconstruction and grain size analysis of lacustrine deposits demonstrate that a closed inland saline lake existed upstream of the yarlung tsangpo gorge implying that cutting of the gorge postdated this paleolake optically stimulated luminescence and radiocarbon dating suggest that the paleolake formed at least 51 ka and terminated after 13 ka with a shift in global climate following the last glacial maximum the paleolake drained by river capture and the gorge entrenchment commenced at the village of jiala northwest of the namche barwa peak approximately 13 10 ka ago the evolution of the yarlung tsangpo occurred on a remarkably short timescale of just tens of thousands of years in response to drainage system reorganization the yarlung tsangpo incised at an average rate of at least 26 mm yr the rapid fluvial incision initiated at jiala 13 ka ago propagated rapidly upstream at 2 12 m yr the yarlung tsangpo knickpoint has not stabilized but will likely continue to migrate upstream

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

exhumation history of the gangdese batholith southern tibetan plateau evidence from apatite and zircon u th he thermochronology abstractto test previously suggested exhumation histories of the gangdese batholith in the central part of the transhimalayan plutonic belt we conducted paired apatite and zircon u th he thermochronological investigations of the yarlung zangbo gorge in the central part of the batholith age elevation relationships and multisystem thermochronometers showed three periods of accelerated exhumation 46 48 22 18 and 11 8 ma combining these data with previously published thermochronological ages and synthesizing these ages with regional geological events provides an entire exhumation history the cretaceous early paleogene exhumation of the gangdese batholith was probably caused by both the northward subduction of the neo tethys and the collision between the lhasa and qiangtang blocks the early miocene rapid exhumation might be a response to shortening caused by the gangdese thrust or erosion driven by dynamic uplift following lithospheric delamination in contrast the late miocene exhumation is coi

river network evolution as a major control for orogenic exhumation case study from the western tibetan plateau the westernmost tibetan plateau despite being internally drained has a high topographic relief here using apatite u th sm he and 4he 3he thermochronometry we reconstruct the exhumation history of the rutog batholith during the neogene thermal modeling in 1d using the qtqt program indicates that relatively slow cooling occurred from 30 ma to 19 ma which we interpret as an exhumation rate of 10 m ma this was followed by two pulses of moderate cooling from 19 to 17 ma and 11 to 9 ma that correspond to a total exhumation of about 1500 m cooling since 9 ma has been negligible this differs from exhumation patterns in central tibet but reveals timing similarities with externally drained portions of southern tibet we interpret our cooling constraints as recording two different transitions in western tibet from an externally to an internally drained system since the oligocene external drainage allowed this part of the tibetan plateau unlike internally drained portions of central tibet to record regional scale processes the first cooling event at about 20 ma was likely related to a major geodynamic event such as slab breakoff that induced contemporaneous potassic and ultrapotassic magmatism the second rapid cooling pulse from 11 ma to 9 ma and subsequent negligible cooling was most likely controlled by a local factor such as indus and shyok river network reorganization caused by dextral motion of the karakorum fault we discuss these interpretations and their limitations in this contribution

peneplain formation in southern tibet predates the india asia collision and plateau uplift the uplift history of tibet is crucial for understanding the geodynamic and paleoclimatologic evolution of asia however it remains controversial whether tibet attained its high elevation before or after india collided with asia 50 m y ago here we use thermochronologic and cosmogenic nuclide data from a large bedrock peneplain in southern tibet to shed light on the timing of the uplift the studied peneplain which was carved into cretaceous granitoids and jurassic metasediments is located in the northern lhasa block at an altitude of 5300 m thermal modeling based on u th he ages of apatite and zircon and apatite fission track data indicate cooling and exhumation of the granitoids between ca 70 and ca 55 ma followed by a rapid decline in exhumation rate from 300 m m y to 10 m m y between ca 55 and ca 48 ma since then the peneplain has been a rather stable geomorphic feature as indicated by low local and catchment wide erosion rates of 6 11 m m y and 11 16 m m y respectively which were derived from cosmogenic 10 be concentrations in bedrock grus and stream sediment the prolonged phase of erosion and planation that ended ca 50 ma removed 3 6 km of rock from the peneplain region likely accomplished by laterally migrating rivers the lack of equivalent sediments in the northern lhasa block and the presence of a regional unconformity in the southern lhasa block indicate that the rivers delivered this material to the ocean this implies that erosion and peneplanation proceeded at low elevation until india9s collision with asia induced crustal thickening surface uplift and long term preservation of the peneplain

post collisional exhumation of the indus yarlung suture zone and northern tethyan himalaya saga sw tibet abstract the cretaceous eocene sedimentary sequences of the sangdanlin section at saga sw tibet within the india asia collisional zone have been studied in detail for constraining the timing of initial collision however the post depositional history has not been investigated here we report new low temperature thermochronology data along the classic sangdanlin section to reveal the cooling exhumation history of the indus yarlung suture zone iysz and northern tethyan himalaya samples from the iysz all yield latest oligocene miocene ages with most zircon u th he ages ranging between 18 and 27 ma and apatite fission track aft ages of 12 9 ma thermal history modelling indicates a consistent middle late miocene cooling episode related to post great counter thrust gct incision of the yarlung river to the south samples from the northern tethyan himalaya mesozoic sequences yield older zhe ages of 32 36 ma signifying activity of the tethyan himalayan fold thrust zone while two granites linked to formation of the north himalayan gneiss domes yield late miocene aft ages the regional low temperature thermochronology data reveal different cooling patterns for tributaries on the northern and southern sides of the yarlung river this points to the strong influence of early miocene tectonism along the gct as a major controlling factor on river drainage reorganization at this time

pressure temperature evolution of blueschist facies rocks from sifnos greece and implications for the exhumation of high pressure rocks in the central aegean the blueschist and greenschist units on the island of sifnos cyclades were affected by eocene high pressure hp metamorphism using conventional geothermobarometry the hp peak metamorphic stage was determined at 550 600 c and 20 kbar close to the blueschist and the eclogite facies transition the retrograde p t paths are inferred with phase diagrams pseudosections based on a quantitative petrogenetic grid in the model system na2o cao feo mgo al2o3 sio2 h2o reveal coeval decompression and cooling for both the blueschist and the greenschist unit the conditions of the metamorphic peak and those of the retrograde stages conform to a similar metamorphic gradient of 10 12 c km 1 for both units the retrograde overprint can be assigned to low pressure blueschist to hp greenschist facies conditions this result cannot be reconciled with the prograde barrovian type event which affected parts of the cyclades during the oligocene to miocene instead the retrograde overprint is interpreted in terms of exhumation directly after the hp stage without a separate metamorphic event constraints on the exhumation mechanism are given by decompression cooling paths which can be explained by exhumation in a fore arc setting during on going subduction and associated crustal shortening back arc extension is only responsible for the final stage of exhumation of the hp units

erosion history of the tibetan plateau since the last interglacial constraints from the first studies of cosmogenic 10be from tibetan bedrock abstract the cosmogenic 10be exposure histories of in situ bedrock surfaces from the tibetan plateau indicate low erosion rates of

phase relations of epidote blueschists phase diagrams have been computed for minerals in the system ncmash using the optimised thermodynamic data base of berman 1988 augmented by data on glaucophane and pumpellyite to illustrate the relationships between rocks containing epidote and sodic amphibole and those containing assemblages characteristic of neighbouring metamorphic facies the stoichiometries of stable bounding equilibria yield a simple list of mineral parageneses that serves to define an epidote blueschist facies and surrounding metamorphic facies the calculations suggest that when iron free the pairs zoisite glaucophane and clinozoisite glaucophane do not possess a stability field the natural fe bearing paragenesis of epidote sodic amphibole occupies a closed field in the pt diagram whose size increases dramatically as the ratio fe3 al in the minerals is increased where garnet is involved in bounding reactions such as with respect to eclogite and garnet bearing albite epidote amphibolite the epidote blueschist field decreases in size as the ratio fe2 mg is increased the classic blueschist to greenschist transition reaction is best developed in rocks with large fe3 and fe2 contents and is possibly metastable in mgal rich rocks in the latter a wedge of albite epidote actinolite barroisite amphibolite separates the epidote blueschist from the greenschits paragenesis field a first occurrence of garnet divides the epidote blueschist field into a lower temperature garnet free zone and a higher temperature garnet bearing zone although the temperatures calculated for this garnet isograd seem high the locations of all the facies boundaries are influenced by h2o activity however even minor amounts of co2 in the fluid phase x co2 0 005 render the epidote sodic amphibole paragenesis metastable so that facies boundaries cannot be shifted by varying the co2 content of the fluid phase below 600 c no mutual boundaries exist between the amphibolite facies and the epidote blueschist and eclogite facies respectively the calculated low pressure limit for the eclogite paragenesis pyralspite omphacite jd 50 quartz typical metabasite compositions is 12 to 14 kbars with h2o activity 0 9 or 1 5 to 2 kbar lower for jd 30

westernmost grand canyon incision testing thermochronometric resolution the timing of carving of grand canyon has been debated for over 100 years with competing endmember hypotheses advocating for either a 70 ma old or 6 ma young grand canyon several geological constraints appear to support a young canyon model but thermochronometric measures of cooling history and corresponding estimates of landscape evolution have been in debate in particular 4he 3he thermochronometric data record the distribution of radiogenic 4he from the 238u 235u and 232th decay series within an individual apatite crystal and thus are highly sensitive to the thermal history corresponding to landscape evolution however there are several complicating factors that make interpreting such data challenging in geologic scenarios involving reheating here we analyze new data that provide measures of the cooling of basement rocks at the base of westernmost grand canyon and use these data as a testbed for exploring the resolving power and limitations of 4he 3he data in general we explore a range of thermal histories and find that these data are most consistent with a young grand canyon a problem with the recovered thermal history however is that burial temperatures are under predicted based on sedimentological evidence a solution to this problem is to increase the resistance of alpha recoil damage to annealing thus modifying he diffusion kinetics allowing for higher temperatures throughout the thermal history this limitation in quantifying radiation damage and hence crystal retentivity introduces non uniqueness to interpreting time temperature paths in rocks that resided in the apatite helium partial retention zone for long durations another source of non uniqueness is due to unknown u and th distributions within crystals we show that for highly zoned with a decrease in effective u of 20 ppm over the outer 80 of the radius of the crystal the 4he 3he data could be consistent with an old canyon model to reduce this non uniqueness we obtain u and th zonation information for separate crystals from the same rock sample through la icp ms analysis the observed u and th distributions are relatively uniform and not strongly zoned thus supporting a young canyon model interpretation of the 4he 3he data furthermore we show that for the mapped zonation the difference between predicted 4he 3he data for a uniform crystal and a 3d model of the crystal are minimal highlighting that zonation is unlikely to lead us to falsely infer an old grand canyon

coupled numerical analytical approach to landscape evolution modeling the earth s topography is shaped by surface processes that operate on various scales in particular river processes control landscape dynamics over large length scales whereas hillslope processes control the dynamics over smaller length scales this scale separation challenges numerical treatments of landscape evolution that use space discretization large grid spacing cannot account for the dynamics of water divides that control drainage area competition and erosion rate and slope distribution small grid spacing that properly accounts for divide dynamics is computationally inefficient when studying large domains here we propose a new approach for landscape evolution modeling that couples irregular grid based numerical solutions for the large scale fluvial dynamics and continuum based analytical solutions for the small scale fluvial and hillslope dynamics the new approach is implemented in the landscape evolution model dac divide and capture the geometrical and topological characteristics of dac s landscapes show compatibility with those of natural landscapes a comparative study shows that even with large grid spacing dac predictions fit well an analytical solution for divide migration in the presence of horizontal advection of topography in addition dac is used to study some outstanding problems in landscape evolution i the time to steady state is investigated and simulations show that steady state requires much more time to achieve than predicted by fixed area calculations due to divides migration and persistent reorganization of low order streams ii large scale stream captures in a strike slip environment are studied and show a distinct pattern of erosion rates that can be used to identify recent capture events iii three tectono climatic mechanisms that can lead to asymmetric mountains are studied each of the mechanisms produces a distinct morphology and erosion rate distribution application to the southern alps of new zealand suggests that tectonic advection precipitation gradients and non uniform tectonic uplift act together to shape the first order topography of this mountain range copyright 2014 john wiley sons ltd

rb sr isotopic and compositional retentivity of muscovite during deformation abstract permian metapegmatite muscovite from the upper austroalpine matsch unit in southern tyrol italy was investigated regarding its rb sr and compositional retentivity during cretaceous upper greenschist facies deformation the data imply that microstructurally relic permian magmatic muscovite largely maintained its major and trace element compositions during deformation whereas the rb sr geochronometer is strongly affected by a net loss of sr lower sr concentrations of muscovite correlate with higher 87rb 86sr and 87sr 86sr ratios in most samples the muscovite grain size and magnetic fractions with the lowest 87rb 86sr and 87sr 86sr ratios preserve a permo triassic muscovite whole rock rb sr apparent age interpreted as to reflect formation during or cooling after pegmatite emplacement contrastingly muscovite fractions with higher 87rb 86sr and 87sr 86sr ratios are arranged along a roughly linear array with a positive correlation of the 87rb 86sr and 87sr 86sr ratios in the 87rb 86sr vs 87sr 86sr space they yield successively lower muscovite whole rock rb sr apparent ages we explain the variations in the rb sr isotopic character of microstructurally relic muscovite by a presumably deformation related loss of sr during the cretaceous event contemporaneously only very limited amounts of isotopically different sr from the matrix reservoir might possibly have entered the muscovite consequently the rb sr of the relic muscovite is affected by a net loss of sr the results imply that at temperatures of

differential exhumation histories between qulong and xiongcun porphyry copper deposits in the gangdese copper metallogenic belt insights from low temperature thermochronology abstract the qulong and xiongcun porphyry deposits in the gangdese copper molybdenum metallogenic belt gcmb are both giant porphyry deposits they were located in the eastern and the central gcmb about 400 km apart and were generated in the miocene and the jurassic respectively in order to reveal why the two asynchronous deposits could occur in the same metallogenic belt the in situ zircon u pb zircon and apatite fission track zft and aft and apatite u th he ahe dating were conducted their associated exhumation amounts and histories were compared and finally the prospecting significance was evaluated ahe ages of the qulong deposit indicate it underwent an amount of at least 1 3 2 8 km exhumation since 13 8 13 1 ma however aft data and modeling results reveal that the xiongcun deposit experienced two stages of rapid cooling at 20 13 ma and 13 8 ma respectively the temperature decreases from 160 c to 40 c after 20 ma and the corresponding exhumation amount is about 4 6 km integrated the regional thermochronological data exhumation amounts of 4 6 km and

evolution of north himalayan gneiss domes structural and metamorphic studies in mabja dome southern tibet field structural and metamorphic petrology investigations of mabja gneiss dome southern tibet suggest that contractional extensional and diapiric processes contributed to the structural evolution and formation of the domal geometry the dome is cored by migmatites overlain by sillimanite zone metasedimentary rocks and orthogneiss metamorphic grade diminishes upsection and is defined by a series of concentric isograds evidence for three major deformational events two older penetrative contractional and extensional events and a younger doming event is preserved metamorphism migmitization and emplacement of a leucocratic dike swarm were syntectonic with the extensional event at mid crustal levels metamorphic temperatures and pressures range from 500 8c and 150 450 mpa in chloritoid zone rocks to 705 65 8c and 820 100 mpa in sillimanite zone rocks we suggest that adiabatic decompression during extensional collapse contributed to development of migmatites diapiric rise of low density migmatites was the driving force at least in part for the development of the domal geometry the structural and metamorphic histories documented in mabja dome are similar to kangmar dome suggesting widespread occurrence of these events throughout southern tibet q 2004 elsevier ltd all rights reserved

subduction and exhumation related structures in the cycladic blueschists insights from south evia island aegean region greece 1 detailed geological mapping structural investigation and amphibole chemistry analyses in southern evia aegean sea greece allow us to place new constraints on the internal structural architecture and tectonic evolution of the cycladic blueschists we show that the early deformation history was related to ese directed thrusting resulting in the stacking of the styra and ochi nappes which constitute the cycladic blueschist unit in evia these early thrust movements initiated just before and proceeded at peak conditions of the eocene high pressure metamorphism subsequent constrictional deformation gave rise to e w trending upright folding accomplished at the early exhumation stage the main ductile stage exhumation occurred during a single deformation phase associated with the decompression of blueschist rocks from the stability field of crossite to that of actinolite this phase was characterized by localization of ductile deformation into a series of major tens of meters thick ene directed shear zones which cut up section in their transport direction and restack the early thrust and fold sequence locally bringing the structurally lower styra nappe over the higher ochi nappe it is suggested that these zones operated as thrusts rather than normal sense shear zones as has been previously argued and were possibly formed during the oligocene ene ward extrusion of the blueschists brittle ductile ne dipping normal faulting of post early miocene age was probably responsible for the final exhumation of rocks

4he 3he thermochronometry using classical diffusion theory we present a mathematical technique for the determination of 4he concentration profiles in minerals this approach should prove useful for constraining the low temperature cooling histories of individual samples and for correcting u th he ages for partial diffusive loss the calculation assumes that the mineral of interest contains an artificially produced and uniform distribution of 3he obtained by proton irradiation shuster et al earth planet sci lett 217 2004 19 32 in minerals devoid of natural helium this isotope allows measurement of he diffusion coefficients in minerals with measurable radiogenic he it permits determination of 4he profiles arising during ingrowth and diffusion in nature the 4he profile can be extracted from stepwise degassing experiments in which the 4he 3he ratio is measured the evolution of the 4he 3he ratio as a function of cumulative 3he released can be compared with forward models to constrain the shape of the profile alternatively we present a linear inversion that can be used to directly solve for the unknown 4he distribution the inversion incorporates a standard regularization technique to filter the influence of random measurement errors on the solution using either approach we show that stepwise degassing data can yield robust and high resolution information on the 4he profile profiles of radiogenic he are a sensitive function of the time temperate t t path that a cooling sample experienced thus by step heating a proton irradiated sample it is possible to restrict the sample s acceptable t t paths the sensitivity of this approach was explored by forward modeling 4he profiles resulting from a range of realistic t t paths using apatite as an example results indicate that 4he profiles provide rich information on t t paths especially when the profiles are coupled with u th he cooling ages on the same sample samples that experienced only moderate diffusive loss have 4he concentration profiles that are rounded at the edge but uniform in the core of the diffusion domain such profiles can be identified by nearly invariant 4he 3he ratios after the first few to few tens of percent of 3he have been extracted by step heating we show how such data can be used to correct u th he ages for partial diffusive loss

differential late cenozoic uplift across the dongjiu milin fault zone in the eastern himalayan syntaxis revealed by low temperature thermochronology abstract the eastern himalayan syntaxis ehs as one of the most tectonically active regions on the earth is one of the best places to explore the relationship between tectonics and surface exhumation process the uplift and exhumation history are often by one of two leading models a stationary or localized tectonic aneurysm or a northeasterly migrating crustal antiform previous researchers have placed great emphasis on the core zone of namche barwa antiform however the dongjiu milin fault zone dmfz which forms the northwestern boundary of the antiform and plays an important role for deformation process in the ehs is still poorly understood in this study we focus on differential late cenozoic uplift across the dmfz we collected 10 bedrock samples in two vertical transects one on the northwest side of the dmfz on the lhasa block and one on the southeast side on the namche barwa antiform and conducted apatite fission track and zircon fission track analyses ten samples collected from the two vertical transects yield aft ages between 3 2 million years ago ma and 10 3 ma and zft ages between 9 9 ma and 12 9 ma a finite element analysis reveals that the two sides of the dmfz have undergone quite different uplift histories and processes in the late cenozoic the lhasa block is uniformly uplifted while the namche barwa antiform is nonuniformly uplifted similar to a buckle fold the phenomenon revealed in this study is best explained by a mechanism whereby the ehs is characterized by the northeasterly migration of an antiform

tectonic juxtaposition of blueschists and greenschists in sifnos island aegean sea implications for the structure of the cycladic blueschist belt abstract in the cyclades kilometre thick high pressure rock sequences displaying blueschist and eclogite facies mineral assemblages overlie a rock sequence which was thoroughly overprinted in the greenschist and amphibolite facies during its exhumation both rock sequences subducted and metamorphosed at high pressures in eocene times have been considered by previous workers to have been exhumed as a coherent rock unit in contrast it is suggested here that the preserved high pressure rock sequences were exhumed more rapidly and prior to the underlying greenschists significant metamorphic structural and geochronological discontinuities exist across the blueschist greenschist contact and field evidence suggests that the high pressure metamorphic rocks in sifnos are tectonically juxtaposed above the greenschists these two rock sequences were juxtaposed by a low angle fault subsequent to the oligocene miocene greenschist facies overprint published geochronological data and petrological criteria are used to show that the high pressure sequence cooled below 350 c when the rocks now immediately underlying it suffered a greenschist facies overprint at temperatures of ca 450 c the section inferred to absorb this temperature difference is now missing and it is suggested that it has been cut out by the low angle fault

geochemistry and isotopic study of southern bay of bengal sediments implications for provenance and paleoenvironment during the middle miocene abstract the bay of bengal bob the largest bay in the world is known to receive large amounts of fresh water and suspended particulate matter from different sources the sediments deposited in the bay serve as a repository for paleoenviromental reconstruction studies the miocene epoch is an important interval in geologic time as it is marked by major climatic events that controlled ocean productivity redox conditions and provenance of sediments however limited work has been carried out to establish paleoenviromental changes and provenance of sediments during the miocene here we present detailed geochemical and radiogenic sr nd isotopic results from predominantly middle miocene sediments of ocean drilling program odp site 758a located in the southern bob lat 5 23 n long 90 21 e water depth 2925 m to understand paleoredox conditions paleoproductivity and provenance the results suggest that these sediments are strongly linked to climatically modulated surface processes that affected the region the distribution of redox sensitive element proxies u v cr at interval 17 8 ma points to an oxic depositional environment the major trace and rare earth elements distribution supports prevalence of the indian summer monsoon ism at 12 10 ma productivity proxies babio porg show a decreasing trend during colder events east antarctic ice sheet expansion eais and advent of late miocene cooling lmc events the trace and rare earth element concentrations are generally low except at intervals where glass shards were found the 87sr 86sr and end 0 values indicate that the sediments belong to the high himalayan sedimentary series hhss throughout the middle miocene however prior to initiation of the ism these sediments underwent meteoric alteration in the lowlands before deposition in the bob our records suggest that sediments at odp site 758a may have been introduced into the bob through the irrawaddy yarlung tsangpo drainage system during the entire period of this study the irrawaddy yarlung tsangpo system drained the indus tsangpo suture zone southern tibet the eastern himalayas and the arakan coast before debouching in the bob

middle miocene paleoaltimetry of southern tibet implications for the role of mantle thickening and delamination in the himalayan orogen the stable isotope composition of pedogenic and early diagenetic carbonates from the oiyug basin of southern tibet allows model estimates of the paleoaltimetry of the tibetan plateau for the middle miocene pedogenic calcium carbonate nodules have average d 18 occ values of 219 6 whereas nodular lacustrine dolomites range in composition from 27 6 to 25 5 the most negative of the carbonate isotope values can be used to constrain the oxygen isotope composition of paleoprecipitation from which model estimates of paleoaltimetry can be made model results indicate that the southern tibetan plateau achieved elevations of 5200 11370 2605 m by at least 15 ma our results are identical within uncertainty to previous workers paleoelevation estimates based on oiyug basin fossil floral physiognomy this is the first time that two paleoaltimeters have been directly compared and are in accord collectively these data strongly support tectonic models in which thickening of mantle lithosphere beneath the domain of crustal thickening and subsequent detachment of the mantle lithosphere plays an indiscernible role in the elevation history of this part of the himalaya tibet orogenic system

tectonic mode switches and the nature of orogenesis abstract the birth and death of many mountain belts occurs in lithosphere that over rides major subduction zones here the tectonic mode shortening versus extension can abruptly switch even during continuous and otherwise smooth convergence if the hinge line of the foundering slab rapidly retreats i e rolls back the foundering slab creates a gravitational potential well into which the orogen collapses this motion coupled with stress guides can pull the orogen apart a slowing of roll back or of hinge retreat means that the subduction flexure may subsequently begin to be pushed back or be pushed over by the advancing orogen the consequence of such changes in relative motion is that orogenic belts are affected by abrupt tectonic mode switches the change from push to pull leads to a sudden change from horizontal extension to horizontal shortening potentially throughout the entire mass of the orogenic lithosphere that over rides the subducting slab the sequencing of these tectonic mode switches affects the thermal evolution of the orogen and thus fundamentally determines the nature of orogenesis this insight led to us to our quite different views as to how orogens work it is evident that orogens affected by abrupt push pull mode switches are characterized by high pressure metamorphism whereas orogens affected by abrupt pull push mode switches are characterized by high temperature metamorphism magmatism and anatexis

crustal tilting and differential exhumation of gangdese batholith in southern tibet revealed by bedrock pressures abstract the exhumation history of the gangdese batholith southern tibet bears on how magmatism and tectonism interact with surface processes in a long lived magmatic orogen in this study we applied al in hornblende barometry across the eastern gangdese batholith to obtain pluton emplacement pressures our results together with existing bedrock pressure data reveal the regional paleo depth pattern across the gangdese batholith the western part of the batholith near lhasa exposes plutons emplaced at 1 2 kbar whereas the eastern part near nyingchi exposes crust recording pressures typically of 6 12 kbar we coupled pressure data with new and published u pb zircon ages to constrain the exhumation history of the gangdese batholith the results show that since 100 ma the upper crust experienced limited exhumation except a pronounced oligocene miocene pulse in contrast the middle lower crust experienced a complex exhumation and burial history reflecting major tectonic events including the development of continental arc and continent continent collision since ca 10 ma the eastern nyingchi sector experienced fast exhumation total exhumation 40 km which was likely related to the exhumation of the eastern himalayan syntaxis the lhasa sector experienced comparatively limited exhumation total exhumation

d h of late miocene meteoric waters in western australia paleoenvironmental conditions inferred from the δd of u th he dated cid goethite abstract nineteen u th he ages were determined for eight samples from a core drilled in an ore grade channel iron deposit cid of the robe pisolite robe formation of mesa j in western australia with one exception uncorrected ages of the analyzed aliquots range from 6 7 0 4 ma to 30 2 3 1 ma while molar ratios of th u range from 0 42 to 5 06 the exception is an aliquot with an apparent age of 2 7 ma and th u of 5 70 a three component mixing model involving one generation of goethite and two generations of hematite suggests that the age of crystallization of the oolitic goethites is 7 1 ma if so the goethites have effectively been closed systems for 7 million years and should preserve a stable hydrogen isotope record of late miocene rainfall in the vicinity of mesa j cenozoic movement of the australian continent had placed mesa j and environs in the subtropics at a paleolatitude of about 29 s during the late miocene al adjusted d values of oolitic goethite in the eight cid samples range from 153 to 146 and imply that the d of the late miocene meteoric waters ranged from 61 to 53 with an average of 56 these relatively negative d values might indicate that near coastal late miocene rain was derived primarily from summer season tropical cyclones with storm tracks that extended into the subtropics of western australia the postulated late miocene tropical cyclones would have occurred more often and or exhibited greater intensity at a paleolatitude of 29 s than is the case for modern sites at approximately 30 s on the west coast of australia e g perth higher fluxes of meteoric water in the miocene summers would have facilitated dissolution and removal of bif sourced silica with concomitant enrichment in oxidized fe moreover wetter late miocene summers could have promoted multiple cycles of microbially mediated dissolution and recrystallization of fe iii oxides in the aerobic systems the oolitic textures may be indicative of such recycling however the oolitic goethites of mesa j were closed systems after 7 ma therefore the climate in the vicinity of mesa j seems to have changed in the late miocene to conditions that did not favor widespread recycling of fe iii oxides perhaps changing from seasonally wet to the modern dry climate

deformation induced resetting of rb sr and 40ar 39ar mineral systems in a low grade polymetamorphic terrane eastern alps austria a paradox in dating metamorphic events in low grade polymetamorphic terranes is exemplified by the eastern lower austroalpine nappes of the eastern alps here the last metamorphic event best recorded in post variscan cover rocks is dated as late cretaceous in age c 80 85 ma by white mica rb sr and 40 ar 39 ar systems within the underlying polymetamorphic basement 40 ar 39 ar and rb sr ages of phengitic white mica record only early and or late variscan ages 375 270 ma indicating that the alpine greenschist facies metamorphic overprint virtually caused no rejuvenation of variscan mineral ages based on these results the timing of a penetrative ductile top to wnw simple shear deformation recorded within both basement and cover rocks was contradictory deformation within the post variscan cover rocks had to be alpine in age whereas phengite 40 ar 39 ar ages from basement mylonites yield variscan ages to date this deformation directly we isolated different mineral size fractions 63 30 and 30 10 m from highly strained shearbands within the wechsel basement nappe a resulting rb sr errorchron pointed to an upper cretaceous age c 85 ma for the deformation consistent with the timing of alpine metamorphism in the cover rocks coarser grained white mica 150 300 im from similar basement mylonites do not reflect any alpine overprint of either k ar and or rb sr systems it follows that dynamic re and or neocrystallization induced by ductile deformation within the shearbands was the dominant process by which the rb sr system locally virtually re equilibrated this is valid even for overprinting metamorphic conditions below the temperatures required for ar diffusional loss in phengitic white mica c 40 ar 39 ar should be considered with caution

cretaceous to cenozoic evolution of the northern lhasa terrane and the early paleogene development of peneplains at nam co tibetan plateau abstract highly elevated and well preserved peneplains are characteristic geomorphic features of the tibetan plateau in the northern lhasa terrane north northwest of nam co the peneplains were carved in granitoids and in their metasedimentary host formations we use multi method geochronology zircon u pb and u th he dating and apatite fission track and u th he dating to constrain the post emplacement thermal history of the granitoids and the timing and rate of final exhumation of the peneplain areas la icp ms u pb geochronology of zircons yields two narrow age groups for the intrusions at around 118 ma and 85 ma and a third group records paleocene volcanic activity 63 58 ma in the nam co area the low temperature thermochronometers indicate common age groups for the entire nam co area zircon u th he ages cluster around 75 ma apatite fission track ages around 60 ma and apatite u th he ages around 50 ma modelling of the thermochronological data indicates that exhumation of the basement blocks took place in latest cretaceous to earliest paleogene time by middle eocene time the relief was already flat documented by a thin alluvial sediment sequence covering a part of the planated area the present day horst and graben structure of the peneplains is a late cenozoic feature triggered by e w extension of the tibetan plateau the new thermochronological data precisely bracket the age of the planation to early eocene i e between ca 55 and 45 ma the erosional base level can be deduced from the presence of early cretaceous zircon grains in eocene strata of bengal basin the sediment generated during exhumation of the nam co area was transported by an early cenozoic river system into the ocean suggesting that planation occurred at low elevation

the zedong window a record of superposed tertiary convergence in southeastern tibet determining the timing magnitude and location of deformation due to the indo asian collision is widely acknowledged as an important step in understanding how the lithosphere responds during continental collision a puzzling result of geological investigations of the lhasa block over the past 2 decades has been the apparent lack of significant tertiary deformation there perhaps the most important structural feature of the lhasa block is the south directed gangdese thrust system which developed along its southern edge the thrust system which separates the andean type batholith of southern asia from rocks of indian affinity is obscured at most locations across southeastern tibet by back thrusts of the younger north directed renbu zedong thrust system the best documented site where both thrusts are exposed occurs near zedong zedong window systematic geochronologic analyses were conducted in this area u pb zircon dating of three samples of a synkinematically to postkinematically deformed hanging wall granitoid the yaja granodiorite cut by the gangdese thrust indicates a crystallization age of 30 4 0 4 ma 2 thus placing an upper bound on the initiation of the thrust u pb zircon dating of granitoid samples structurally higher in the gangdese hanging wall yields emplacement ages of 42 5 1 0 ma and 48 9 0 8 ma 2 similar to other magmatic complexes within the gangdese arc geochemical results are consistent with these plutons forming in the same environment as precollisional intrusions within the gangdese batholith suggesting a significant post collisional input of juvenile heat the 40ar 39ar thermochronology of samples from a vertical section through the hanging wall within the yaja granodiorite coupled with results of a numerical thermal model indicate an average slip rate along the gangdese thrust of 7 mm yr between 30 and 23 ma and a minimum displacement of 50 km farther east in the zedong window thermal effects produced by later north directed thrust sheets of the renbu zedong system appear to have obscured thermal history signatures in the gangdese hanging wall related to earlier south directed thrusting thermochronological results from this region indicate that thermal overprinting related to the north directed thrusting occurred between 25 and 10 ma consistent with previous estimates

crustal scale geometry and kinematics of late orogenic extension in the central aegean cyclades and ewia island abstract the aegean continental domain is known to be the site of widespread back arc extension since at least the middle miocene overprinting structures related to the mesozoic cenozoic hellenic orogeny features attributed to early thrusting include the overall ductile deformation within two broad belts that have suffered hp lt metamorphism across the aegean this study summarizes the results of a structural study in the central aegean area cyclades and evvia island examining in particular the relationship between ductile and brittle deformation on a regional scale extension appears to be responsible for most of the ductile deformation within hp rock units that have experienced penetrative greenschist facies and higher grade metamorphic overprinting on each island studied progressive extensional deformation has occurred through the development of a major normal sense detachment zone down to depths of about 18 25 km the geometry of the extensional system on the scale of the central aegean is described taking into account the characteristic features of the regional scale ductile strain field at least two probably three subparallel major north dipping detachment zones are identified trending nw se in the northwestern cyclades and e w in the southeastern cyclades both with initial dips estimated at around 30 45 the present geometry of the extensional system implies that these detachment zones and related metamorphic core complexes interfere with each other two main evolutionary models are considered which can account for such a kinematic link between parallel detachment zones structural data favour the sequential development of synthetic major detachments zones in a direction opposite to the sense of dip of the detachments early oligocene lower miocene detachment type extension in the area studied is related to a late orogenic post thickening back arc setting

india asia convergence insights from burial and exhumation of the xigaze fore arc basin south tibet the composite fore arc syncollisional xigaze basin in south tibet preserves a key record of india asia collision new apatite fission track and zircon u th he data from an n s transect across the preserved fore arc basin sequence near xigaze show a consistent northward late cretaceous to middle miocene younging trend while coexisting apatite u th sm he ages are all miocene corresponding detrital zircon u pb data are also reported for constraining the cretaceous depositional ages of the xigaze basin sequence in the region thermal history modeling indicates that the basin experienced northward propagating episodic exhumation along with a northward migration of the depocenter and a pre existing cenozoic syncollisional basin sequence which had been removed in the southern part fore arc exhumation commenced in the late cretaceous 89 2 ma following transition to a syncollisional basin in the paleocene sedimentation in the central and northern xigaze basin continued until the latest eocene 34 4 ma ongoing folding and thrusting e g great counter thrusts caused by progressive plate convergence during late oligocene early miocene time resulted in regional uplift and considerable basin denudation which fed two fluvial basins along its northern and southern flanks and exposed the basement ophiolite subsequent incision of the yarlung river resulted in miocene cooling in the region different episodes in the exhumation history of the xigaze basin caused by thrusting of an accretionary wedge and ophiolitic basement can be linked to changes in india asia convergence rates and the changing subduction pattern of the indian and neo tethyan slabs

late cenozoic uplift of southeastern tibet the age of surface uplift in southeastern tibet is currently unknown but the initiation of major river incision can be used as a proxy for the timing of initial uplift the topographically high eastern plateau and gently dipping southeastern plateau margin are mantled by an elevated low relief relict landscape that formed at a time of slow erosion at low elevation and low tectonic uplift rates prior to uplift of the eastern tibetan plateau thermochronology from deep river gorges that are cut into the relict landscape shows slow cooling between ca 100 and ca 10 20 ma and a change to rapid cooling after ca 13 ma with initiation of rapid river incision at 0 25 0 5 mm yr between 9 and 13 ma a rapid increase in mean elevation of eastern tibet beginning at this time supports tectonic climate models that correlate the lateral eastern expansion of high topography in tibet with the late miocene intensification of the indian and east asian monsoons

low temperature thermochronology of the indus basin in central ladakh northwest india implications of miocene pliocene cooling in the india asia collision zone the india asia collision zone in ladakh northwest india records a sequence of tectono thermal events in the interior of the himalayan orogen following the intercontinental collision between india and asia in early cenozoic time we present zircon fission track and zircon and apatite u th he thermochronometric data from the indus basin sedimentary rocks that are exposed along the strike of the collision zone in central ladakh these data reveal a postdepositional miocene pliocene 22 4 ma cooling signal along the india asia collision zone in northwest india our zircon fission track cooling ages indicate that maximum basin temperatures exceeded 200 c but stayed below 280 300 c in the stratigraphically deeper marine and continental strata thermal modeling of zircon and apatite u th he cooling ages suggests postdepositional basin cooling initiated in early miocene time by 22 20 ma occurred throughout the basin across zircon u th he partial retention temperatures from 20 10 ma and continued in the pliocene time until at least 4 ma we attribute the burial of the indus basin to sedimentation and movement along the regional great counter thrust the ensuing miocene pliocene cooling resulted from erosion by the indus river that transects the basin an approximately coeval cooling signal is well documented east of the study area along the collision zone in south tibet our new data provide a regional framework upon which future studies can explore the possible interrelationships between tectonic geodynamic and geomorphologic factors contributing to miocene pliocene cooling along the india asia collision zone from nw india to south tibet

eocene tectonometamorphism on serifos western cyclades deduced from zircon depth profiling geochronology and mica thermochronology abstract phengite bearing metasedimentary sequences preserving relict glaucophane from serifos western aegean are indicative of high pressure conditions during cycladic blueschist metamorphism depth profiling and conventional sims u pb geochronology and o isotope analysis on zircon from highly strained orthogneisses at the base of the sequence reveal xenocrystic zircon with varying degrees of recrystallization one end member population has needle like or prismatic morphologies is internally oscillatory zoned with little to no rim material created during recrystallization these zircons yield triassic to carboniferous ages elevated th u and 18 o mostly ranging between 4 and 11 in marked contrast a separate zircon population has a spongy structure created by complete recrystallization of the pre existing crystal these zircons possess low th u and flat hree patterns and yield eocene ages with 18 o 7 chondrite normalized ree profiles are extremely depleted within this spectrum other zircons show variable states of recrystallization and mixed ages regression of u pb data from three mylonitic orthogneiss samples that exhibit near complete recrystallization and similar age depth profiles yields c 40 ma ages these new data are the first geochronological constraints that successfully demonstrate an eocene metamorphic event in the western cyclades and new 40 ar 39 ar phengitic mica ages of 38 32 ma presented here suggest the metamorphic wedge was exhumed into the shallow crust shortly thereafter

rapid incision of the mekong river in the middle miocene linked to monsoonal precipitation the uplift of orogenic plateaus has been assumed to be coincident with the fluvial incision of the gorges that commonly cut plateau margins the mekong river which drains the eastern qiangtang terrane and southeastern tibetan plateau is one of the ten largest rivers in the world by water and sediment discharge when the mekong river was established remains highly debated with estimates that range from more than 55 to less than 5 million years ago despite being a key constraint on the elevation history of the tibetan plateau here we report low temperature thermochronology data from river bedrock samples that reveal a phase of rapid downward incision 700 m of the mekong river during the middle miocene about 17 million years ago long after the uplift of the central and southeastern tibetan plateau however this coincides with a period of enhanced east asian summer monsoon precipitation over the region compared with the early miocene using stream profile modelling we demonstrate that such an increase in precipitation could have produced the observed incision in the mekong river in the absence of an obvious tectonic contribution we suggest that the rapid incision of the tibetan plateau and the establishment of the mekong river in the middle miocene may be attributed to increased erosion during a period of high monsoon precipitation incision of the mekong river that occurred after the uplift of the tibetan plateau may have been driven by a period of high monsoon precipitation as suggested by age data from river bedrock samples and stream profile modelling

thrust or detachment exhumation processes in the aegean insight from a field study on ios cyclades greece 1 whether back arc extension in the aegean is symmetric or asymmetric at the regional scale is a key question to the understanding of the dynamics of back arc extension new field observations structural analysis of finite and instantaneous strain and their relationship with metamorphism on ios island cyclades lead us to investigate the significance of the south cycladic shear zone scsz two successive tectonometamorphic events have been documented in the hanging wall and the footwall of the scsz first penetrative top to the south shearing is observed in the upper cycladic blueschists and the lower cycladic basement close to their contact it is expressed in the hanging wall by shear bands and pressure shadows associated to blueschist facies minerals contemporaneous with overthrusting of the cycladic blueschists over the basement unit second large extensional top to the north shear zones lately crosscut the whole stack these constraints allow us to reinterpret the kinematics of the aegean domain during synorogenic and postorogenic periods when replaced in the reconstructed geometry of the hellenic eocene orogenic wedge the observed top to the south thrusting is coeval with extensional shear zones observed in syros that could represent the roof of the cycladic blueschists an extrusion structure is then proposed for the exhumation of the cycladic blueschists in the subduction zone top to the north asymmetric extension together with the andros tinos mykonos and paros naxos detachments can be correlated to oligo miocene extension in the back arc domain

age and thermal history of eo and neohimalayan granitoids eastern himalaya abstract four distinct granitoid suites outcrop across a n s transect in the eastern himalaya between the main central thrust mct and the indus tsangpo suture the arunachal and tsona leucogranites outcrop respectively in the mct hanging wall and adjacent the south tibetan detachment both crystallized at about 19 ma and appear to be lateral equivalents of the high himalayan leucogranites in the central tethyan himalaya the dala granitoids form a suite of undeformed plutons emplaced into deformed tethyan himalayan sequence metasediments in the northern tethyan himalaya the yala xiangbo granitoids are part of an igneous complex in the core of the yala xiangbo dome both the dala and yala xiangbo granitoids have eocene magmatic ages 44 1 1 2 ma and 42 5 ma respectively that are notably older than the vast majority of post collisional granitoids along the main himalayan arc pelitic units from the core of the yala xiangbo dome record two episodes of amphibolite grade metamorphism and monazite growth coinciding with well documented episodes of eohimalayan and neohimalayan metamorphism the thermochronological histories of the dala and yala xiangbo units suggest that the eastern tethyan himalaya was tectonically quiescent from ca 40 ma until ca 15 ma miocene exhumation of the dala and yala xiangbo units and all north himalayan domes studied to date appears best explained by a structural control such as concurrent north directed motion on the great counter thrust we hypothesize that an extension of the ninety east ridge may have been responsible for eocene high heat flow beneath the eastern himalaya and explain the localization of magmatism there

comparisons of the kinematics and deep structures of the zagros and himalaya and of the iranian and tibetan plateaus and geodynamic implications 1 we compare the geologic histories the deep structures and the present day kinematics of deformation of the himalaya and the adjacent tibetan plateau with those of the zagros and iranian plateau to test geodynamic processes of continental collision shortly after india and arabia collided with eurasia horizontal shortening manifested itself by folding and thrust faulting of sedimentary rock detached from india s and arabia s underlying crystalline basement subsequently slip on thrust faults stacked slices of india s basement to build the himalaya on india s northern margin such faulting has not yet developed in the zagros where collision is more recent and arabia penetrates into eurasia more slowly than india does so that postcollision convergence with eurasia is less the greater elevation thicker crust and more marked heterogeneity of the upper mantle beneath the tibetan than beneath the iranian plateau also reflect a more advanced stage of development moreover while thrust or reverse faulting and crustal shortening continue on the margins of both plateaus normal faulting suggesting horizontal extension and crustal thinning occurs within tibet but not in iran hence the balance of forces that built the high tibetan plateau must have changed apparently some time since 15 ma removal of tibetan mantle lithosphere could have altered that balance if mantle lithosphere beneath the iranian plateau has been removed however the change in force balance has been too small to initiate normal faulting low seismic wave speeds in the uppermost mantle just beneath the moho of both plateaus suggest to us that lithosphere beneath both is thin consistent with late cenozoic removal of it but alternative explanations might account for these low speeds despite its apparently thin and hence presumably weak mantle lithosphere much of central iran undergoes little deformation it illustrates how a crustal block can behave rigidly not necessarily because it is strong but because deviatoric stresses can be small whereas differences between the two regions clearly depend on the amount that arabia and india have penetrated into eurasia which scales with both the dates of collision and rates of convergence we see no differences in the operative processes that depend on the present day rates of convergence

erosion in southern tibet shut down at 10 ma due to enhanced rock uplift within the himalaya exhumation of the southern tibetan plateau margin reflects interplay between surface and lithospheric dynamics within the himalaya tibet orogen we report thermochronometric data from a 1 2 km elevation transect within granitoids of the eastern lhasa terrane southern tibet which indicate rapid exhumation exceeding 1 km ma from 17 16 to 12 11 ma followed by very slow exhumation to the present we hypothesize that these changes in exhumation occurred in response to changes in the loci and rate of rock uplift and the resulting southward shift of the main topographic and drainage divides from within the lhasa terrane to their current positions within the himalaya at 17 ma steep erosive drainage networks would have flowed across the himalaya and greater amounts of moisture would have advected into the lhasa terrane to drive large scale erosional exhumation as convergence thickened and widened the himalaya the orographic barrier to precipitation in southern tibet terrane would have strengthened previously documented midcrustal duplexing around 10 ma generated a zone of high rock uplift within the himalaya we use numerical simulations as a conceptual tool to highlight how a zone of high rock uplift could have defeated transverse drainage networks resulting in substantial drainage reorganization when combined with a strengthening orographic barrier to precipitation this drainage reorganization would have driven the sharp reduction in exhumation rate we observe in southern tibet

thermochronologic evidence for plateau formation in central tibet by 45 ma the timing of tibetan plateau development remains elusive despite its importance for evaluating models of continental lithosphere deformation and associated changes in surface elevation and climate we present new thermochronologic data biotite and k feldspar 40 ar 39 ar apatite fi ssion track and apatite u th he from the central tibetan plateau lhasa and qiangtang terranes the data indicate that over large regions rocks underwent rapid to moderate cooling and exhumation during cretaceous to eocene time this was coeval with 50 upper crustal shortening suggesting substantial crustal thickening and surface elevation gain thermal modeling of combined thermochronometers requires exhumation of most samples to depths of 3 km between 85 and 45 ma followed by a decrease in erosional exhumation rate to low values of 0 05 mm yr the thermochronological results when interpreted in the context of the deformation and paleoaltimetric history are best explained by a scenario of plateau growth that began locally in central tibet during the late cretaceous and expanded to encompass most of central tibet by 45 ma

thermal structure and exhumation history of the lesser himalaya in central nepal the lesser himalaya lh consists of metasedimentary rocks that have been scrapped off from the underthrusting indian crust and accreted to the mountain range over the last 20 myr it now forms a significant fraction of the himalayan collisional orogen we document the kinematics and thermal metamorphism associated with the deformation and exhumation of the lh combining thermometric and thermochronological methods with structural geology peak metamorphic temperatures estimated from raman spectroscopy of carbonaceous material decrease gradually from 520 550 c below the main central thrust zone down to less than 330 c these temperatures describe structurally a 20 50 c km inverted apparent gradient the ar muscovite ages from lh samples and from the overlying crystalline thrust sheets all indicate the same regular trend i e an increase from about 3 4 ma near the front of the high range to about 20 ma near the leading edge of the thrust sheets about 80 km to the south this suggests that the lh has been exhumed jointly with the overlying nappes as a result of overthrusting by about 5 mm yr for a convergence rate of about 20 mm yr this implies underthrusting of the indian basement below the himalaya by about 15 mm yr the structure metamorphic grade and exhumation history of the lh supports the view that since the mid miocene the himalayan orogen has essentially grown by underplating rather than by frontal accretion this process has resulted from duplexing at a depth close to the brittle ductile transition zone by southward migration of a midcrustal ramp along the main himalayan thrust fault and is estimated to have resulted in a net flux of up to 150 m 2 yr of lh rocks into the himalayan orogenic wedge the steep inverse thermal gradient across the lh is interpreted to have resulted from a combination of underplating and post metamorphic shearing of the underplated units

miocene initiation and acceleration of extension in the south lunggar rift western tibet evolution of an active detachment system from structural mapping and u th he thermochronology 1 ongoing extension in tibet may have begun in the middle to late miocene but there are few robust estimates of the rates timing or magnitude of neogene deformation within the tibetan plateau we present a comprehensive study of the seismically active south lunggar rift in southwestern tibet incorporating mapping u pb geochronology and zircon u th he thermochronology the south lunggar rift is the southern continuation of the north lunggar rift and comprises a 50 km n s central horst bound by two major normal faults the west dipping south lunggar detachment and the east dipping palung co fault the sld dips at the rangefront 20 w and exhumes a well developed mylonite zone in its footwall displaying fabrics indicative of normal sense shear the range is composed of felsic orthogneiss mafic amphibolite and leucogranite intrusions dated at 16 and 63 ma zircon u th he cooling ages are oligocene through late pliocene with the youngest ages observed in the footwall of the sld we tested 25 000 unique thermokinematic forward models in pecube against the structural and u th he data to fully bracket the allowable ranges in fault initiations accelerations and slip rates we find that normal faulting in the slr began in the middle miocene with horizontal extension rates of 1 mm a 1 and in the north accelerated at 8 ma to 2 5 3 0 mm a 1 as faulting commenced on the sld cumulative horizontal extension across the slr ranges from 10 km in the south to 19 21 km in the north

activation of the nyainqentanghla shear zone implications for uplift of the southern tibetan plateau neogene extension of the tibetan plateau is manifested as a series of north south trending graben the most prominent of which is the yadong gulu rift the nyainqen tanghla shan a ne sw trending mountain range 100 km nw of lhasa bounds the western margin of the yangbajian graben the central segment of the yadong gulu rift the eastern edge of the nyainqentanghla massif is marked by a low angle 25 o detachment fault shear zone of amphibolite grade mylonites the noar 3 ar thermal history results from samples collected along two deeply incised valleys within the massif reveal that a rapid cooling event propagated from 8 ma in the core of the range to 4 ma within the high strain zone at the eastern boundary assuming that faulting initiated at high angle 60o thermal histories were fit to a numerical simulation of slip on a normal fault to yield estimates of both the age of fault initiation and the slip rate history the form of the isotopically derived thermal histories are similar to general form predicted by the thermal model and suggests that significant movement began at 8 1 ma in the southern valley goring la and proceeded at an average slip rate of 3 mm yr between 8 and 3 ma a more complex history is required to fit the data from the northern valley balum chun but the timing of initiation and average slip rate are similar to the goring la result numerical simulations in which the fault angle is varied indicate that the isotopically derived temperature histories are inconsistent with slip occurring at low angle 40o because the extension direction of the yangbajian graben is representative of most rifts on the southern tibetan plateau our data suggest that crustal thickness and elevation reached close to their present values by 8 1 ma a carbon isotopic shift in pedogenic carbonates from the siwalik formation at about 7 5 ma appears to reflect intensification of the asian monsoon and by inference that the plateau had attained an important threshold elevation by that time formation of a diffuse plate boundary in the indian oceanic lithosphere beginning at 7 5 8 0 ma is also consistent with this history we suggest that the plateau had attained a threshold area and elevation by 8 1 ma sufficient to trigger these three independent manifestations

spatial coincidence of rapid inferred erosion with young metamorphic massifs in the himalayas a spatially distributed rate of erosion index ei based on models of bedrock river incision documents a strong spatial correspondence between areas of high erosion potential and young metamorphic massifs as well as structural highs throughout the himalayas the ei is derived from slopes and drainage areas calculated from a hydrologically cor rected digital elevation model gtopo30 combined with precipitation data iiasa to generate synthetic annual stream discharges these variables drive three generalized pro cess models to produce ei maps that while differing in detail provide an internally con sistent spatially continuous index of large scale erosion rates the large spatial variation in potential erosion rates in the himalayas suggested by the ei patterns contrasts with the uniform convergence of the indian subcontinent if these ei gradients persist through time they support the emerging view of a positive feedback between localized rapid ero sion and upward advection of lower crust

tertiary structural evolution of the gangdese thrust system southeastern tibet structural and thermochronological investigations of southern tibet xizang suggest that intracontinental thrusting has been the dominant cause for formation of thickened crust in the southernmost tibetan plateau since late oligocene two thrust systems are documented in this study the north dipping gangdese system gts and the younger south dipping renbu zedong system rzt west of lhasa the gangdese thrust juxtaposes the late cretaceous forearc basin deposits of the lhasa block the xigaze group over the tethyan sedimentary rocks of the indian plate whereas east of lhasa the fault juxtaposes the late cretaceous eocene andean type arc the gangdese batholith over tethyan sedimentary rocks near zedong 150 km southeast of lhasa the gangdese thrust is marked by a 200 m thick mylonitic shear zone that consists of deformed granite and metasedimentary rocks a major south dipping backthrust in the hanging wall of the gangdese thrust puts the xigaze group over tertiary conglomerates and the gangdese plutonics north of xigaze and west of lhasa a lower age bound for the gangdese thrust of 18 3 0 5 ma is given by crosscutting relationships the timing of slip on the gangdese thrust is estimated to be 27 23 ma from 40 ar 39 ar thermochronology and a displacement of at least 46 9 km is indicated near zedong the age of the gangdese thrust gt is consistent with an upper age limit of 24 ma for the initiation of movement on the main central thrust in places the younger renbu zedong fault is thrust over the trace of the gt obscuring its exposure the rzt appears to have been active at circa 18 ma but had ceased movement by 8 1 ma the suture between india and asia has been complexly modified by development of the gts rzt and locally strike slip and normal fault systems

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

late pliocene onset of the cona rift eastern himalaya confirms eastward propagation of extension in himalayan tibetan orogen abstract several competing hypotheses have been proposed to explain east west extension observed across the himalayan orogen based primarily on observations from the western and central himalaya they make predictions for the temporal and spatial patterns of deformation in the eastern himalaya these tectonic models include radial spreading or oroclinal bending of the himalayan arc oblique convergence of the indian continent tearing or lateral detachment of the indian slab and eastward flow of lithosphere here for the first time we constrain the activity history of the cona rift the only north trending rift currently recognized in the eastern himalaya based on biotite and k feldspar 40ar 39ar and zircon and apatite u th he thermochronology to evaluate these proposed rifting mechanisms low temperature thermochronological results suggest that the cona rift is the youngest rift system in the himalayas after a slow phase of exhumation since 14 ma 0 2 0 13 mm yr normal faulting initiated here at 3 0 2 3 ma with a fault slip rate of 3 8 1 6 mm yr and a horizontal extension magnitude of 2 5 km analysis of rifting across the himalayas shows that rift initiation ages young eastward which is matched by eastward decreasing rift extension magnitudes monotonically eastward younging rift development is consistent with the tectonic model involving eastward lithospheric flow driven by indian slab dynamics and coupled asynchronous gravitational potential energy gradients

miocene faulting at plate tectonic velocity in the himalaya of central nepal abstract combination of geochemical zoning in metamorphic garnet and monazite plus in situ th pb isotopic dating of monazite yields p t conditions ages and convergence rates for the main central thrust mct and affiliated faults in central nepal inferred rates were 1 5 0 9 cm yr langtang thrust 19 ma 2 2 0 7 cm yr main central thrust 15 ma and 7 3 cm yr ramgarh thrust 10 ma the lower values are similar to modern convergence rates across the himalaya but the ramgarh thrust may have briefly absorbed all indo asian convergence at 10 ma when foreland and marine sedimentation rates markedly increased and at least one major strike slip fault in tibet experienced a hiatus in movement variable rates of convergence across the himalaya on myr timescales imply myr variations in strain rates throughout all components of the indo asian orogen

oligocene miocene bu khang extensional gneiss dome in vietnam geodynamic implications a large oligocene miocene extensional gneiss dome in vietnam has implications for the kinematics of the ailao shan red river shear zone and rifting of the south china sea a major northeast dipping extensional shear zone separates the bu khang dome from less metamorphosed units above shearing deformation occurred during the exhumation of high pressure rocks 40 ar 39 ar ages on micas reveal oligocene miocene ages from 36 to 21 ma after a high pressure episode of probable oligocene age the area was subjected to northeast southwest extension during the opening of the south china sea a kinematic model involving left lateral shear and block rotation within a north south dextral shear zone integrates our observations in the bu khang dome with south china sea rifting

palaeo altimetry of the late eocene to miocene lunpola basin central tibet the elevation history of the tibetan plateau provides direct insight into the tectonic processes associated with continent continent collisions here we present oxygen isotope based estimates of the palaeo altimetry of late eocene and younger deposits of the lunpola basin in the centre of the plateau which indicate that the surface of tibet has been at an elevation of more than 4 kilometres for at least the past 35 million years we conclude that crustal but not mantle thickening models combined with plate kinematic solutions of india asia convergence are compatible with palaeo elevation estimates across the tibetan plateau

miocene development of alpine glacial relief in the patagonian andes as revealed by low temperature thermochronometry abstract apatite thermochronometry and synthetic maps of ages and rates for thermochronometric data are used to estimate the timing of incision of valley relief in the andes central patagonia offers a unique location to study the feedbacks between long term climate topography and erosion due to the high relief and well resolved mid latitude glacial history new apatite u th he ages from two vertical transects and two 4he 3he release spectra in the fjord network around 47 s reveal fast cooling 15 30 c ma from 10 to 5 ma samples currently at the surface cooled below 35 c by 5 ma indicating slow cooling and little erosion in those regions since 5 ma we show that these very low temperature thermochronometric data are useful indicators of changes in topography and insensitive to deep thermal processes such as migration of the chile triple junction map based predictions of the thermochronometric signatures of disparate topographic scenarios show the distribution of sample data necessary to resolve the timing of relief change comparisons to predicted cooling ages and rates indicate that our new apatite he data are most consistent with a pulse of early glacial incision with much of the observed valley relief in patagonia carved between 10 and 5 ma early onset of glaciation in patagonia is supported by glacial till with bracketing ages of 7 4 and 5 ma we therefore conclude that the observed thermochronometric signal of fast cooling from 10 to 5 ma is likely due to an increase in valley relief coinciding with these early glaciations in the andes in other glaciated areas at lower latitudes studies have found a dramatic increase in valley relief at 1 ma this timing has generated the idea that incision of glacial valleys may be related to the mid pleistocene transition when the global glacial cycle changed from 40 to 100 ka periods our results from a higher latitude indicate an alternative that glacial valleys incised rapidly after the onset of alpine glaciation

very slow erosion rates and landscape preservation across the southwestern slope of the ladakh range india erosion rates are key to quantifying the timescales over which different topographic and geomorphic domains develop in mountain landscapes geomorphic and terrestrial cosmogenic nuclide tcn methods were used to determine erosion rates of the arid tectonically quiescent ladakh range northern india five different geomorphic domains are identified and erosion rates are determined for three of the domains using tcn 10be concentrations along the range divide between 5600 and 5700 m above sea level asl bedrock tors in the periglacial domain are eroding at 5 0 0 5 to 13 1 1 2 meters per million years m m y principally by frost shattering at lower elevation in the unglaciated domain erosion rates for tributary catchments vary between 0 8 0 1 and 2 0 0 3 m m y bedrock along interfluvial ridge crests between 3900 and 5100 m asl that separate these tributary catchments yield erosion rates 850 ky respectively these results highlight the persistence of very slow erosion in different geomorphic domains across the southwestern slope of the ladakh range which on the scale of the orogen records spatial changes in the locus of deformation and the development of an orogenic rain shadow north of the greater himalaya copyright 2014 john wiley sons ltd

controls on cenozoic exhumation of the tethyan himalaya from fission track thermochronology and detrital zircon u pb geochronology in the gyirong basin area southern tibet the gyirong basin southern tibet contains the record of miocene pliocene exhumation drainage development and sedimentation along the northern flank of the himalaya the tectonic controls on basin formation and their potential link to the south tibetan detachment system stds are not well understood we use detrital zircon zft and apatite aft fission track analysis together with detrital zircon u pb dating to decipher the provenance of gyirong basin sediments and the exhumation history of the source areas results are presented for nine detrital samples of gyirong basin sediments aft zft and u pb two modern river sediment samples zft and aft and six bedrock samples zft from transect across the gyirong fault bounding the basin to the east the combination of detrital zircon u pb and fission track data demonstrates that the gyirong basin sediments were sourced locally from the tethyan sedimentary sequence this provenance pattern indicates that deposition was controlled by the gyirong fault active since 10 ma whose vertical throw was probably 5000 m rather than being controlled by normal faults associated with the stds the detrital thermochronology data contain two prominent age groups at 37 41 and 15 18 ma suggesting rapid exhumation at these times a 15 18 ma phase of rapid exhumation has been recorded widely in both southern tibet and the himalaya a possible interpretation for such a major regional exhumation event might be detachment of the subducting indian plate slab during the middle miocene inducing dynamic uplift of the indian plate overriding its own slab

spatial and temporal trends in exhumation of the eastern himalaya and syntaxis as determined from a multitechnique detrital thermochronological study of the bengal fan the bengal fan provides a neogene record of eastern and central himalaya exhumation we provide the first detrital thermochronological study apatite and rutile u pb mica ar ar zircon fission track of sediment samples collected during international ocean discovery program iodp expedition 354 to the mid bengal fan our data from rutile and zircon fission track thermochronometry show a shift in lag times over the interval 5 59 3 47 ma the oldest sample with a lag time of 6 m y has a depositional age of 5 59 4 50 ma and the zircon and rutile populations then show a static peak until 12 ma this interval from 5 59 4 50 ma to 12 ma is most easily interpreted as recording passive erosion of the greater himalaya however single grains with lag times of 4 m y but with high analytical uncertainty are recorded over this interval for sediments older than 10 ma these grains were derived from the greater himalaya which was exhuming rapidly until ca 14 ma in sediments younger than 10 ma these grains could represent slower yet still rapid exhumation of the syntaxial antiform to the south of the massif lag times 1 m y are again recorded from 14 5 ma to the base of the studied section at 17 ma reflecting a period of greater himalayan rapid exhumation mica 40ar 39ar and apatite u pb data are not sensitive to syntaxial exhumation we ascribe this to the paucity of white mica in syntaxial lithologies and to high levels of common pb resulting in u pb ages associated with unacceptably high uncertainties respectively

metamorphic core complexes of cordilleran type in the cyclades aegean sea greece the islands of ios and naxos in the cycladic archipelago greece contain elongate domes over which a pervasive north to north northeast stretching lineation is warped these islands may be metamorphic core complexes of cordilleran type involving middle crustal rocks drawn upward and outward from underneath the sedimentary basins now 100 km to the south from ca 25 ma onward during uplift the crystalline rocks now exposed on the islands were deformed noncoaxially in the deeper levels of a major shallow dipping ductile shear zone this shear zone may have connected with low angle normal faults in its upper levels because it is probable that its later history involved the metamorphic complexes being dragged out from under a stretching and fracturing upper plate composed mainly of unmetamorphosed sedimentary and ophiolite suite rocks this model can be extended to explain the exhumation of the high pressure low temperature metamorphic rocks on crete which with the cyclades forms a distended paired metamorphic belt separated by an active sedimentary basin

high and dry in central tibet during the late oligocene the time at which the tibetan plateau rose to its present high elevation remains controversial with estimates ranging from 40 ma to more recent than 7 ma new stable isotope analyses of modern and accurately dated ancient paleosol carbonate in the nima basin of central tibet point to an arid climate and high paleoelevation 4 5 5 km comparable to today s setting by 26 ma oxygen isotope values of ancient 26 ma soil carbonate are both very negative and indistinguishable after modest corrections for changes in global climate from the lowest least evaporated oxygen isotope values of modern soil carbonates in the area substantial enrichments in oxygen 18 in paleolacustrine carbonates as well as high carbon isotope values from paleosol carbonates indicate considerable lake evaporation and low soil respiration rates respectively and both are consistent with the present arid climate of the nima area blockage of tropical moisture by the himalaya and perhaps the gangdese shan probably has contributed strongly to the aridity and very negative oxygen isotope values of soil carbonate and surface water in the nima area since at least the late oligocene the maintenance of high elevation since at least 26 ma suggests that any flow of lower crust from beneath central tibet must have been balanced by coeval northward insertion of indian crust beneath the plateau