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

quaternary channel focused rapid incision in the phung chu arun river in central himalaya implications for a quaternary capture event abstract the phung chu arun river one of the few major trans himalaya rivers consists of a large drainage area captured in the northern slope of the himalaya and provides a key to understand tectonic surface processes and their interplay in topographic development of the himalaya in this study we carried out thermochronologic and topographic analysis along the phung chu arun river in central himalaya to reveal its incision development apatites from a downstream section along its major tributary yield very young fission track and u th he ages of 0 8 1 5 ma these cooling ages combined with previously reported data for higher elevations reveal an abrupt change of the slope in the age elevation plot suggesting an accelerated channel incision initiated at between 1 5 and 2 5 ma the age elevation relationship yields a high apparent exhumation rate aer of 2 6 3 8 mm a during quaternary which is spatially focused along the mainstream of the kharta chentang reaches and the associated nearby tributaries topographic analysis suggests that this channel focused incision acceleration was probably facilitated by a quaternary capture event in upper drainage of the palaeo arun river this capture event greatly enhanced the effective discharge draining southward causing an accelerated incision wave along the mainstream and tributaries of the palaeo arun river furthermore thrusting at physiographic transition zone is also required to sustain high channel gradient and the long term rapid incision channel morphology presenting common knick points in major tributaries suggests that the drainage basin in ama drime range area likely remain in pre steady state adjustment of this capture event

shisha pangma leucogranite south tibetan himalaya field relations geochemistry age origin and emplacement the shisha pangma pluton forming most of the xixabangma 8027 m massif in south tibet is one of the 20 larger leucogranite intrusives that mark the highest structural levels of the himalayan metamorphic core the pluton occurs immediately below the shisha pangma detachment a strand of the south tibetan detachment std system a low angle 30 north dipping normal fault placing paleozoic black slates atop sillimanite grade pelites and calc silicate rocks k feldspar augen gneisses containing fibrolite and sillimanite paragneisses along the footwall show strong internal s c fabrics indicative of down to the north extension the shisha pangma leucogranite is a heterogeneous polyphase intrusion with an earlier foliated biotite rich phase and a later tourmaline muscovite rich phase typically containing the assemblage kfs pl qtz ms tur gt bt sil ap the highly peraluminous granites have high 87sr 86sr ratios 0 738 0 750 typical of pelite derived anatectites nd depleted mantle model

channel profiles around himalayan river anticlines constraints on their formation from digital elevation model analysis 1 we present a comparison between measured and numerically modeled channel profiles of rivers in two important drainage basins of central nepal the kali gandaki and the arun drainage basins modeled channel profiles are based on a simple stream power approach using best fit exponents defining the nonlinearities in the relative contributions of local channel gradient and water flux to erosion rate our analysis of the stream power in the whole river network confirms the work of other authors that a 50 to 80 km wide zone roughly corresponding to the high himalayan topography is subjected to rapid rock uplift we suggest a model where the uplift of this zone is driven by erosion and isostatic response so that centers of maximum uplift are located within the main channels of the north south draining rivers we also suggest that the rate of uplift slows down with increasing distance to the main channels such a spatial distribution of the uplift leads ultimately to the formation of river anticlines as observed along all major himalayan rivers we propose that the formation of river anticlines along south draining himalayan rivers was accelerated by a sudden increase of the drainage area and discharge when the rivers captured orogen parallel drainages on the north side of the range this may follow successive headward cutting into the tibetan plateau the model is confirmed by differences between main channels and east west running tributaries time dependent numerical models predict that capture events cause strongly elevated erosion rates in the main channel

structure and geochronology of the southern xainza dinggye rift and its relationship to the south tibetan detachment system abstract the xainza dinggye rift is one of several north south trending rifts in central and southern tibet created by cenozoic east west extension during indo asian convergence the southern part of the rift cuts through the tethyan and high himalayas in the tethyan himalaya this rift consists of an early domal structure and a late normal fault developed during the progressive deformation the dome is cored by leucogranitic plutons that intruded during extension muscovite 40ar 39ar ages of the mylonitic leucogranite indicate that extension in the tethyan himalaya began at 8 ma or before in the high himalaya the rift is controlled by a normal fault dipping to the southeast this fault has a structural constitution similar to a detachment fault its lower block is made up of mylonitic high himalayan gneiss intruded by early mylonitic leucogranite sills and late less deformed biotite bearing leucogranite dikes mica 40ar 39ar ages of these leucogranites and the retrograded metamorphosed gneiss of the lower block range from 13 to 10 ma in the study area the south tibetan detachment system stds is a ductile shear zone composed of mylonitic leucogranite that is intruded by less deformed leucogranite and overlain by low grade metamorphic rocks mica 40ar 39ar ages of leucogranites in the shear zone and schist from the detachment hanging wall indicate a protracted deformation history of the stds from 19 to 13 ma the xainza dinggye rift is younger than the stds because it offsets the stds this north south trending rift belongs to a different tectonic system from the east west striking stds and may be caused by geological process related to india asia convergence this temporal and spatial relationship of the stds to the rift may indicate an important change in tectonic regime at 13 ma in the building of the plateau

calcimetry as a useful tool for a better knowledge of flow pathways in the soultz sous forets enhanced geothermal system the soultz sous forets granite located in the rhine graben france has been chosen for the european enhanced geothermal system egs three wells have been drilled to a minimum depth of 5000 m in order to reach a temperature of 200 c at soultz the main fracture network in the basement is orientated approximately n s with moderate to steep dip its geometrical relationship with the in situ stress field suits fluid circulation hence the egs project however fractures are firmly sealed due to natural fluids fossil and or possibly present the main sealing minerals as identified during previous studies are clay minerals calcite and quartz however some fractured zones remain permeable due to the high porosity developed in the wall rocks through hydrothermal alteration a circulation test was carried out from july to december 2005 to test the performance of the soultz geothermal reservoir this test showed a significant difference in the production rate between the two outflow wells gpk2 and gpk4 in order to improve the fluid circulation within the underground heat exchanger chemical stimulations have been scheduled in this framework quantifying the calcite content of the granite provides a basis for identifying the calcite rich zones as well as choosing the most suitable chemical stimulation measurement of calcite ponderal concentration was carried out in the cutting samples of the 3 deep wells gpk2 gpk3 gpk4 between 4000 and 5000 m depth using manocalcimetry based on detailed measurements in the 3 wells this study shows that calcite is not only a very common and ubiquitous hydrothermal mineral but that its content seems to be also spatially correlated with most of the fracture zones bearing natural flow the conductivity of these natural flow pathways seems anti correlated with the calcite content calcimetry has proved to be an easy efficient and low cost method for a better knowledge of hydrothermal sealing of the soultz reservoir this method can be applied with great benefit to all types of basement reservoirs oil gas water heat overlain by sedimentary rocks for a better understanding of the fracture network permeability

basin formation in the high himalaya by arc parallel extension and tectonic damming zhada basin southwestern tibet 1 the late miocene pleistocene zhada basin in southwestern tibet provides a record of subsidence and basin formation within an active collisional thrust belt the 800 m thick basin fill is undeformed and was deposited along an angular unconformity on top of tethyan strata that were previously shortened in the himalayan fold thrust belt modal sandstone petrographic data conglomerate clast count data and detrital zircon u pb age spectra indicate a transition from detritus dominated by a distal northern source to a local southern source this transition was accompanied by a change in paleocurrent directions from uniformly northwestward to basin centric at the same time the depositional environment in the zhada basin changed from a large braided river to a closed basin lake sedimentation in the zhada basin was synchronous with displacement on the qusum and gurla mandhata detachment faults which root beneath the basin and exhume midcrustal rocks along the northwestern and southeastern flanks of the basin respectively these observations indicate that accommodation for zhada basin fill was produced by a combination of tectonic subsidence and damming as midcrustal rocks were evacuated from beneath the zhada basin in response to arc parallel slip on crustal scale detachment faults

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

alteration of nuclear glass in contact with iron and claystone at 90 c under anoxic conditions characterization of the alteration products after two years of interaction abstract the present study investigates the alteration of a fractured glass block in contact with iron and callovo oxfordian claystone at 90 c under anoxic and water saturated conditions the alteration rates and the nature of glass alteration products at the different compact interfaces glass clay glass iron and in cracks were assessed by solution chemistry and microscopic scale techniques scanning electron microscopy coupled with energy dispersive x ray microscopy microraman spectroscopy and x ray absorption fine structure spectroscopy a significant but modest two fold increase in glass alteration in contact with steel was observed leading to an average alteration rate over the experiment of about 0 007 0 014 g m2 d this rate is significantly lower than forward rate r0 in clay equilibrated groundwater 1 7 g m2 d indicating that a decrease of the alteration rate was not hindered by the steel presence the corrosion alteration interface was made up of successive layers of corrosion products in contact with iron a layer of fe silicates and an altered glass layer enriched in fe characterization of the glass block in direct contact with claystone revealed that the thickness of altered glass was much more important than at the glass iron interface the altered glass layer in contact with clay was slightly enriched in fe and mg and depleted in alkali cations altered glass layers in cracks were usually limited to fringes thinner than 2 m with a thickness decreasing from the crack mouth indicating that alteration is controlled by transport in the cracks the fractures were partially filled with calcite and lanthanide hydroxocarbonate precipitates these results contribute to the understanding of nuclear vitrified waste iron corrosion products interactions in a deep geological repository

mineralogy and composition of matrix and chondrule rims in carbonaceous chondrites abstract the degree of compositional variation of fine grained minerals displayed by the members within any carbonaceous chondrite group i e ci cm cv cr is a direct reflection of the range of aqueous alteration assemblages present matrix and fine grained chondrule rims within any particular carbonaceous chondrite are mineralogically nearly identical to one another but not necessarily similar in bulk elemental composition even though they have subsequently experienced post accretional secondary processing aqueous alteration under identical conditions this may indicate preferential alteration of one component brecciation exchange of components between rims and matrix and or accretion of matrix and rims from mineralogically or compositionally distinct materials we propose that co chondrites experienced parent body conditions of low f o2 low water rock ratios and temperatures below 50 c cr chondrites experienced higher water rock ratios potentially higher temperatures 150 c and a wide range of f o2 the alteration mineralogy of cv chondrites indicates water rock ratios at the high end at least of the range for cr chondrites essebi and mac 87300 cm chondrites experienced temperatures below 50 c low f o2 and low water rock ratios except eet 83334 which probably experienced relatively higher f o2 and b 7904 and y 86720 which experienced postalteration temperatures in the range 500 700 c most ci chondrites experienced temperatures between 50 and 150 c relatively high water rock ratios and variable f o2 with a late stage increase in the latter due possibly to brecciation and consequent release of methane y 82162 witnessed post alteration heating possibly as high as 400 c

fine grained rim mineralogy of the cold bokkeveld cm chondrite abstract a chrysotile like phase cronstedtite polygonal serpentine pentlandite and finely intergrown tochilinite comprise the fine grained rim fgr mineralogy of the cold bokkeveld cm chondrite transmission electron microscope images combined with compositional data indicate reaction among cronstedtite the chrysotile like phase and polygonal serpentine the mg mg fe ratios of the cronstedtite are higher than those reported for the less altered murchison cm chondrite cronstedtite grains exhibit layer separations particularly at their boundaries the fgrs surround different chondrule types but have similar bulk compositions and mineralogy ca is depleted in the fgrs relative to the bulk cm chondrite the fgrs display non uniform thicknesses especially where they coat embayed chondrule areas and they exhibit grain size coarsening outward from the chondrules they enclose fgr formation in cold bokkeveld is most plausibly explained by multiple accretionary episodes during which progressively coarser dust was deposited onto chondrules presumably in the solar nebula the compositional and mineralogic data are consistent with aqueous alteration on the parent body

tectonic evolution of the qinghai tibet plateau abstract the qinghai tibet plateau composed of several continental slivers within the eastern tethyan domain is one of the pivotal sites to examine to better understand the theory of plate tectonics and the orogenic evolution on earth this plateau is generally inferred to be a collage of several continental blocks that rifted from gondwanaland and subsequently accreted to the asian continent however recent recognition of over twenty ophiolite melange zones and their associated island arcs indicates that the traditional model of tectonic evolution requires revision based on 177 recently finished 1 250 000 scale geological maps and related studies we summarize the main tectonic context of the qinghai tibet plateau and propose a new integrated model to account for the new findings the complex orogen of the immense qinghai tibet plateau consisting of multiple island arc basin systems that developed at different stages while surrounded by the north china yangtze tarim and indian plates is emphasized the entire orogen surrounded by suture zones that mark the locations of oceanic closure is investigated by examining i the first order tectonic units and ophiolitic melanges including arc arc continent collision zones and ii their internally enclosed blocks as the second order tectonic units therefore the qinghai tibet plateau is divided into three major orogenic systems namely from northeast to southwest the early paleozoic qinling qilianshan kunlunshan qin qi kun the late paleozoic triassic qiangtang sanjiang and the late paleozoic to cenozoic gangdese himalaya orogenic systems which are separated by the kangxiwa muzitagh maqin mianxian and the bangong shuanghu changning menglian sutures respectively we propose that the formation and evolution of the qinghai tibet plateau to have been intrinsically related to those of the eastern tethys recorded by the longmu co shuanghu ophiolite melange zone the southern qiangtang paleozoic accretionary arc basin system the bangong nujiang suture zone and their associated composite island arc basin systems the present day bangong shuanghu changning menglian suture system marks the final closure of the tethyan ocean the yarlung zangbo ocean opened as a back arc basin in response to the southward subduction of the tethyan ocean lithosphere in the middle triassic and closed as a result of the india asia collision at the end of cretaceous followed by the northward indention of the indian plate that resulted in significant intra continental deformation and plateau uplift in the cenozoic

a late miocene pliocene origin for the central himalayan inverted metamorphism abstract perhaps the best known occurrence of an inverted metamorphic sequence is that found immediately beneath the himalayan main central thrust mct generally thought to have been active during the early miocene however in situ 208 pb 232 th dating of monazite inclusions in garnet indicates that peak metamorphic recrystallization of the mct footwall occurred in this portion of the central himalaya at only ca 6 ma the apparent inverted metamorphism appears to have resulted from activation of a broad shear zone beneath the mct which tectonically telescoped the young metamorphic sequence this explanation may resolve some outstanding problems in himalayan tectonics such why the mct and not the more recently initiated thrusts marks the break in slope of the present day mountain range it also renders unnecessary the need for exceptional physical conditions e g high shear stress to explain the generation of the himalayan leucogranites

mineralogy of fine grained rims in the alh 81002 cm chondrite abstract the mineralogy of fine grained rims in alh 81002 a relatively primitive cm carbonaceous chondrite has been determined by using transmission electron microscopy the most abundant phase is mg rich serpentine which occurs in small 20 nm crystals with cylindrical or fibrous morphologies cronstedtite an fe rich phyllosilicate occurs as relatively large 100 to 2500 nm platy crystals some cronstedtite is coherently intergrown with tochilinite in many cases cronstedtite has been partially altered to serpentine the compositions and textures of these two phases provide a mineralogical and morphologic alteration sequence that parallels the known compositional trend for cm chondrite matrix accessory minerals embedded within the phyllosilicates include chlorite pentlandite gypsum olivine kamacite taenite and chromite regions containing only anhydrous minerals also occur the hydrated and anhydrous regions are in direct contact with each other suggesting that the rims accreted material from multiple reservoirs

correlated alteration effects in cm carbonaceous chondrites three parameters are proposed to determine the relative extent of alteration in cm chondrites the mineralogic alteration index monitors the relative progress of coupled substitutions in the progressive alteration of cronstedtite to mg serpentine and increases with increasing alteration to calculate values of this index an algorithm has been developed to estimate the average matrix phyllosilicate composition in individual cm chondrites the second parameter is the volume percent of isolated matrix silicates which decreases with progressive alteration due to mineral hydration finally the volume percent of chondrule alteration monitors the extent of chondrule phyllosilicate production and increases as alteration proceeds these parameters define the first cm alteration scale that relies on multiple indicators of progressive alteration the following relative order of increasing alteration is established by this model murchison bells pollen murray mighei nogoya cold bokkeveld the relative degree of aqueous processing cochabamba and boriskino experienced is less precisely constrained although both fall near the middle of this sequence a comparison between the mineralogic alteration index and literature values for the whole rock chemistry of cm chondrites reveals several correlations a positive nearly linear correlation between bulk h content and progressive cm alteration suggests an approximately constant production rate of new phyllosilicates relative to the mineralogical transition from cronstedtite to mg serpentine the abundance of trapped planetary 36ar decreases systematically in progressively altered cm chondrites suggesting the wholesale destruction of primary noble gas carrier phase s by aqueous reactions because low temperature fluid rock reactions are generally associated with large isotopic mass fractionation factors we also compared our model predictions with 18o values for bulk cm samples although some of these data are poorly resolved the order of increasing 18o values approximates the order of increasing alteration predicted by our model parameters multiple correlations between diverse alteration parameters strongly suggest that a different cm chondrites experienced similar kinds of processes and conditions and b cm materials experienced in situ alteration on the cm parent body or bodies

cm chondrites exhibit the complete petrologic range from type 2 to 1 we have characterized the most phyllosilicate rich members of the cm chondrite group based upon petrographic and compositional factors we conclude that these particular meteorites have experienced pervasive aqueous alteration far beyond that witnessed by typical cms the lack of anhydrous silicates cai and except as relicts chondrules merits the classification of type cmi for three meteorites eet 83334 alh 88045 and one kaidun lithology still other cms notably alh 83100 eet 90047 and yamato 82042 are clearly intermediate between types 1 and 2 brecciated cm chondrites like cold bokkeveld contain all of these varied lithologies the cm chondrites thus exhibit the complete petrologic range from 2 through 1 our results show that progressive aqueous alteration on the parent cm asteroid s was locally accompanied by significant increases in temperature to a peak of 450 c for the kaidun lithology fo2 and locally degree of chemical leaching all well beyond the conditions recorded by typical cm2s the most altered cms also are commonly deformed displaying a distinct lineation probably due to static rather than dynamic forces

iron clay interactions detailed study of the mineralogical transformation of claystone with emphasis on the formation of iron rich t o phyllosilicates in a step by step cooling experiment from 90 c to 40 c abstract iron minerals water interactions are of primary importance in the contexts of underground structure engineering e g reactive barriers or deep geological storage and for the understanding of secondary alteration processes in primitive meteorites to improve our understanding of these systems we determine the mineralogical transformations induced by the association of iron and silicates during a cooling through an experimental simulation of iron clay interactions with a step by step procedure in the range of 90 c to 40 c the run products and solutions are well characterised by means of different techniques x ray diffraction scanning and transmission electron microscopy manocalcimetry inductively coupled plasma optical emission spectrometry and ion chromatography and the thermodynamic data concerning fe bearing phyllosilicates are well tested comparing the modelling and experimental results therefore the main mineralogical modifications observed include the remarkable formation of cronstedtite and greenalite as well as the formation of magnetite at all temperatures along with a significant dissolution of quartz mixed layer illite smectite clays illite affecting more than 70 of each mineralogical phase and a partial alteration of chlorite kaolinite and dolomite the experimental results confirm the reaction path predicted by thermodynamic modelling i e the formation of iron rich t o phyllosilicates cronstedtite and greenalite and magnetite at the expense of metal iron and silicates both the experimental and thermodynamic results presented in this study provide important constraints to well predict the impact of nuclear waste canister corrosion in a claystone media and to better understand secondary alteration processes which could also affect the mineralogical and chemical composition of primitive meteorites

multiple precursors of secondary mineralogical assemblages in cm chondrites we report a petrographic and mineralogical survey of tochilinite cronstedtite intergrowths tcis in paris a new cm chondrite considered to be the least altered cm identified to date our results indicate that type i tcis consist of compact tochilinite cronstedtite rims surrounding fe ni metal beads thus confirming kamacite as the precursor of type i tcis in contrast type ii tcis are characterized by complex compositional zoning composed of three different fe bearing secondary minerals from the outside inwards tochilinite cronstedtite and amakinite type ii tcis present well developed faces that allow a detailed morphological analysis to be performed in order to identify the precursors the results demonstrate that type ii tcis formed by pseudomorphism of the anhydrous silicates olivine and pyroxene hence there is no apparent genetic relationship between type i and type ii tcis in addition the complex chemical zoning observed within type ii tcis suggests that the alteration conditions evolved dramatically over time at least three stages of alteration can be proposed characterized by alteration fluids with varying compositions 1 fe and s rich fluids 2 s poor and fe and si rich fluids and 3 s and si poor fe rich fluids the presence of unaltered silicates in close association with euhedral type ii tcis suggests the existence of microenvironments during the first alteration stages of cm chondrites in addition the absence of mg bearing secondary minerals in paris tcis suggests that the mg content increases during the course of alteration

geomorphology how tibet might keep its edge the stability of the margins of the himalayan tibetan mountain belt constitutes a puzzle repeated damming of major tibetan rivers by glaciers so controlling river erosion is a possible explanation much attention has focused on how and when the tibetan plateau formed but far less research has been done on the controls on river incision into the plateau itself oliver korup and david montgomery address this issue which also relates to the question of why the tibetan plateau is still there at all despite hosting one of the deepest gorges on earth the plateau edge is extremely well defined and according to widely accepted stream power theory should be heavily dissected by one of asia s most powerful rivers the yarlung tsangp or yarlung zangbo korup and montgomery propose that the southeastern edge of the plateau was preserved in part by the presence of glaciers that were large enough during the holocene to stall aggressive river downcutting along the plateau margin

comment on hydrothermal preparation of analogous matrix minerals of cm carbonaceous chondrites from metal alloy particles by y peng and y jing earth planet sci lett 408 2014 252 262

tibetan plateau river incision inhibited by glacial stabilization of the tsangpo gorge much attention has focused on how and when the tibetan plateau formed but far less research has been done on the controls on river incision into the plateau itself oliver korup and david montgomery address this issue which also relates to the question of why the tibetan plateau is still there at all despite hosting one of the deepest gorges on earth the plateau edge is extremely well defined and according to widely accepted stream power theory should be heavily dissected by one of asia s most powerful rivers the yarlung tsangp or yarlung zangbo korup and montgomery propose that the southeastern edge of the plateau was preserved in part by the presence of glaciers that were large enough during the holocene to stall aggressive river downcutting along the plateau margin a considerable amount of research has focused on how and when the tibetan plateau formed in the wake of tectonic convergence between india and asia1 although far less enquiry has addressed the controls on river incision into the plateau itself2 widely accepted theory3 predicts that steep fluvial knick points river reaches with very steep gradients in the eastern himalayan syntaxis at the southeastern plateau margin should erode rapidly4 driving a wave of incision back into the plateau preservation of the plateau edge thus presents something of a conundrum that may be resolved by invoking either differential rock uplift matching erosional decay5 6 7 or other mechanisms for retarding bedrock river incision8 9 in this region where high stream power excludes the potential for aridity as a simple limit to dissection of the plateau10 here we report morphologic evidence showing that quaternary depression of the regional equilibrium line altitude where long term glacier mass gain equals mass loss was sufficient to repeatedly form moraine dams on major rivers such damming substantially impeded river incision into the southeastern edge of the tibetan plateau through the coupled effects of upstream impoundment and interglacial aggradation such glacial stabilization of the resulting highly focused river incision centred on the tsangpo gorge could further contribute to initiating and accentuating a locus of rapid exhumation known as tectonic anaeurysm6

some analytical methods for converting thermochronometric age to erosion rate 1 an analytical method is presented for converting thermochronometric ages to surface erosion or equivalently exhumation rate the method incorporates the two most important thermal processes during cooling by erosion the dependence of closure temperature on cooling rate and the advection of heat by rock motion toward the earth s surface two thermal models are considered 1 a steady state model valid for low erosion rates and 2 an eroding half space model which has no steady state but captures the transient increase of geothermal gradient with erosion in each case it is assumed that data consist of one or more thermochronometric ages present day surface geothermal gradient and topographic information including the elevation at which the age was obtained analytical solutions are provided to derive the erosion rate from these data either as an explicit expression for the steady case or as a root finding problem for the transient case a graphical method for plotting age against erosion rate and geothermal gradient is presented as a method for solving the root finding problem and for tracking analytical errors in observations of age and surface geothermal gradient the graphical method is also appropriate for comparing data from different elevations or from different thermochronometric systems examples are provided using synthetic data or published data from the literature

development of active low angle normal fault systems during orogenic collapse insight from tibet active north trending rifts in tibet vary significantly in their character as a function of extension magnitude most rifts are characterized by internally drained basins bounded by high angle normal faults with paleogene or older rocks in the footwall however the central part of the yadong gulu rift near lhasa and the newly documented lunggar rift in west central tibet are bounded by low angle normal faults detachments with mylonitic rocks and miocene granites in the footwall and exhibit active basin incision and intrabasin topographic highs in areas of inferred maximum extension we suggest that tibetan rifts initiate as high angle normal fault and half graben or graben basin systems and evolve in response to increasing extension and footwall isostatic rebound into detachment systems that are active at uppermost crustal levels and above which rift basin fill is being uplifted and eroded