zircon u pb ages and hf isotope compositions of migmatite from the north dabie terrane in china constraints on partial melting migmatite gneisses are widespread in the dabie orogen but their formation ages are poorly constrained eight samples of migmatite including leucosome melanosome and banded gneiss were selected for u pb dating and hf isotope analysis most metamorphic zircon occurs as overgrowths around inherited igneous cores or as newly grown grains morphological and internal structure features suggest that their growth is associated with partial melting according to the hf isotope ratio relationships between metamorphic zircon and inherited cores three formation mechanisms for metamorphic zircon can be determined which are dissolution reprecipitation of pre existing zircon breakdown of zr bearing phase other than zircon in a closed system and crystallization from externally derived zr bearing melt four samples contain magmatic zircon cores yielding upper intercept u pb ages of 807 35 768 12 ma suggesting that the protoliths of the migmatites are neoproterozoic in age the migmatite zircon yields weighted mean two stage hf model ages of 2513 97 894 54 ma indicating reworking of both juvenile and ancient crustal materials at the time of their protolith formation the metamorphic zircons give u pb ages of 145 2 120 2 ma the oldest age indicates that partial melting commenced prior to 145 ma which also constrains the onset of extensional tectonism in this region to pre 145 ma the youngest age of 120 ma was obtained from an undeformed granitic vein indicating that deformation in this area was complete at this time two major episodes of partial melting were dated at 139 1 and 123 1ma the first episode of partial melting is obviously older than the timing of post collision magmatism corresponding to regional extension the second episode of partial melting is coeval with the widespread post collision magmatism indicating the gravitational collapse and delamination of the orogenic lithospheric keel of the dabie orogen which were possibly triggered by the uprising of the cretaceous mid pacific superplume

zircon hf isotopic constraints on the sources of the indus molasse ladakh himalaya india 1 india asia collision has resulted in the largest orogenic event on earth since at least 500 ma and this has had a profound influence on the drainage patterns of asia on the asian monsoon climate on global oceanography as well as on faunal extinctions despite this importance understanding of the paleogene evolution of this system is limited u pb and lu hf isotopic analyses of detrital zircons from the clastic sedimentary rocks of the indus molasse located within the indus suture of the ladakh himalaya india show that most have mesozoic cenozoic ages and juvenile hf isotopic compositions this suggests that the main source of these sediments is the transhimalayan batholith with minor flux from the lhasa karakoram terrane and indian plate the lower indus molasse chogdo formation is inferred to have been deposited at 60 49 ma whereas the upper choksti formation was deposited later than 45 41 ma and contains a greater contribution from the lhasa terrane the transhimalayan batholith was eroded throughout indus molasse sedimentation but increasing exhumation of lhasa karakoram terrane indicates its progressive uplift after initial collision and requires formation of apaleo indus river shortly after collision

interactions of thiols with sedimentary particulate phase studies of 3 mercaptopropionate in salt marsh sediments from shelter island new york abstract 3 mercaptopropionate 3 mpa is a major hydrophilic thiol in salt marsh sediments from shelter island new york it is associated mainly with the sedimentary particulate phase although the mechanism of its binding has not been clear in this study we show that humic substances coated on clay minerals are involved in binding 3 mpa through di and poly sulfide linkages with increasing depth the abundance of 3 mpa in the fulvic acid fraction decreases with a concomitant increase in the humic acid fraction suggesting that with depth fulvic bound 3 mpa is converted into humic bound 3 mpa we modeled the depth variation with a multi g model which suggests that there are two types of reactive components in both fulvic acid bound and humic acid bound 3 mpa one changing about 10 times more rapidly than the other these two 3 mpa pools probably reflect different types of sulfur bridging with the polysulfide bridged pool changing more rapidly than the disulfide bridged pool as free thiol 3 mpa is attacked relatively rapidly by anaerobic bacteria its degradation following firstorder kinetics however in the sediment core we studied humic bound 3 mpa was present throughout the sediment column down to 22 cm corresponding to an age of about 90 years these results suggest that thiols can escape bacterial degradation in sediments by binding to humic substances the sedimentary distribution of 3 mpa is an important illustration of the interplay of biological and chemical processes in the fate of thiols in anoxic marine sediments

protracted fluid induced melting during barrovian metamorphism in the central alps the timing and dynamics of fluid induced melting in the typical barrovian sequence of the central alps has been investigated using zircon chronology and trace element composition multiple zircon domains in leucosomes and country rocks yield u pb ages spanning from 32 to 22 ma the zircon formed during alpine melting can be distinguished from the inherited and detrital cores on the basis of their age th u 0 1 and trace element composition ti in zircon thermometry indicates crystallization temperatures around 620 700 c their composition allows discriminating between 1 zircon formation in the presence of early garnet 2 zircon in equilibrium with abundant l mree rich accessory phases allanite titanite and apatite typical of metatonalites and 3 zircon formed during melting of metasediments in feldspar dominated assemblages the distribution of zircon overgrowths and ages indicate that repeated melting events occurred within a single barrovian metamorphic cycle at roughly constant temperature that in the country rocks zircon formation was limited to the initial stages of melting whereas further melting concentrated in the segregated leucosomes that melting occurred at different times in samples a few meters apart because of the local rock composition and localized influx of the fluids and that leucosomes were repeatedly melted when fluids became available the geochronological data force a revision of the temperature time path of the migmatite belt in the central alps protracted melting over 10 my followed the fast exhumation of alpine eclogites contained within the same region and preceded fast cooling in the order of 100 c ma to upper crustal levels

models for recession flows in the upper blue nile river stream flow recessions are commonly characterized by the exponential equation or in the alternative power form equation of a single linear reservoir the most common measure of recession is the recession constant k which relates to the power function form of the recession equation for a linear reservoir however in reality it can be seen that the groundwater dynamics of even the simplest of aquifers may behave in a non linear fashion in this study three different storage outflow algorithms single linear non linear and multiple linear reservoir were considered to model the stream flow recession of the upper blue nile the recession parameters for the linear and non linear models were derived by the use of least squares regression procedures whereas for the multiple linear reservoir model a second order autoregressive ar 2 model was applied first in order to determine the parameters by the least squares method the modelling of the upper blue nile recession flow performed shortly after the wet season when interflow and bank storage may be contributing considerably to the river flow showed that the non linear reservoir model simulates well with the observed counterparts the variation related to preceding flow on a recession parameter of the non linear reservoir remains significant which was obtained by stratification of the recession curves although a similar stratification did not show any systematic variation on the recession parameters for the linear and multiple linear reservoir models copyright 2004 john wiley sons ltd

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

the integration of thermal infrared imaging discharge measurements and numerical simulation to quantify the relative contributions of freshwater inflows to small estuaries in atlantic canada nutrient fluxes from developed catchments are often a significant factor in the declining water quality and ecological functioning in estuaries determining the relative contributions of surface water and groundwater discharge to nutrient sensitive estuaries is required because these two pathways may be characterized by different nutrient concentrations and temporal variability and may thus require different remedial actions quantifying the volumetric discharge of groundwater which may occur via diffuse seepage or springs remains a significant challenge in this contribution the total discharge of freshwater including groundwater to two small nutrient sensitive estuaries in prince edward island canada is assessed using a unique combination of airborne thermal infrared imaging direct discharge measurements in streams and shoreline springs and numerical simulation of groundwater flow the results of the thermal infrared surveys indicate that groundwater discharge occurs at discrete locations springs along the shoreline of both estuaries which can be attributed to the fractured sandstone bedrock aquifer the discharge measured at a sub set of the springs correlates well with the area of the thermal signal attributed to each discharge location and this information was used to determine the total spring discharge to each estuary stream discharge is shown to be the largest volumetric contribution of freshwater to both estuaries 83 for trout river estuary and 78 for mcintyre creek estuary however groundwater discharge is significant at between 13 and 18 of the total discharge comparison of the results from catchment scale groundwater flow models and the analysis of spring discharge suggest that diffuse seepage to both estuaries comprises only about 25 of the total groundwater discharge the methods employed in this research provide a useful framework for determining the relative volumetric contributions of surface water and groundwater to small estuaries and the findings are expected to be relevant to other fractured sandstone coastal catchments in atlantic canada copyright 2009 john wiley sons ltd

evaluation of aquifer thickness by analysing recession hydrographs application to the oman ophiolite hard rock aquifer for more than a century hydrologists and hydrogeologists have been investigating the processes of stream and spring baseflow recession for obtaining data on aquifer characteristics the maillet formula librairie sci a hermann paris 1905 218 an exponential equation widely used for recession curve analysis is an approximate analytical solution for the diffusion equation in porous media whereas the equation proposed by boussinesq c r acad sci 137 1903 5 j math pure appl 10 1904 5 that depicts baseflow recession as a quadratic form is an exact analytical solution other formulas currently used involve mathematical functions with no basis on groundwater theory only the exact analytical solutions can provide quantitative data on aquifer characteristics the efficiency of the two methods was compared on the basis of recession curves obtained with a 2d cross sectional finite differences model that simulates natural aquifers simulations of shallow aquifers with an impermeable floor at the level of the outlet show that their recession curves have a quadratic form thus the approximate maillet solution largely overestimates the duration of the influenced stage and underestimates the dynamic volume of the aquifer moreover only the boussinesq equations enable correct estimates of the aquifer parameters numerical simulations of more realistic aquifers with an impermeable floor much deeper than the outlet proves the robustness of the boussinesq formula even under conditions far from the simplifying assumptions that were used to integrate the diffusion equation the quadratic form of recession is valid regardless of the thickness of the aquifer under the outlet and provides good estimates of the aquifer s hydrodynamic parameters nevertheless the same numerical simulations show that aquifers with a very deep floor provide an exponential recession thus in that configuration the maillet formula also provides a good fit of recession curves even if parameter estimation remains poor in fact the recession curve appears to be closer to exponential when flow has a very important vertical component and closer to quadratic when horizontal flow is dominant as a consequence aquifer permeability anisotropy also changes the recession form the combined use of the two fitting methods allows one to quantify the thickness of the aquifer under the outlet

eocene break off of the neo tethyan slab as inferred from intraplate type mafic dykes in the gaoligong orogenic belt eastern tibet article i nfo eocene 40 42 ma basaltic dykes in the gaoligong tengliang belt eastern tibet are characterized by high na2 o 2 4 in contrast with the widespread post collisional potassic and ultrapotassic rocks in the tibetan plateau despite the ubiquitous negative nb anomalies these dykes have relatively high nb and zr contents making them distinct significantly from the gangdese arc magmas all these together with the positive nb anomaly in some samples indicate an intraplate affinity for the gaoligong tengliang dykes specifically the gaoligong dykes represent the asthenosphere derived melts which has been contaminated to various degrees by the lithosphere mantle derived melts whereas the tengliang samples were directly derived from an enriched lithosphere mantle the thin lithosphere b80 km inferred from basalt geochemistry is unusual in tibet thus demanding a peculiar mechanism to thin lithosphere a slab break off model is preferred given the geochemical contrast between pre 40 ma and post 40 ma magmas in tibet and the temporal correlation among this intraplate magmatism the termination of gangdese arc magmatism and regional thermally driven metamorphism therefore the occurrence of intraplate type magmas in the gaoligong orogenic belt likely represents magmatic expression of the detachment of subducting neo tethyan slab from the indian continental plate during the eocene in the light of the slab break off concept and thermo mechanical modeling the eocene slab break off furthermore suggests the onset of the india asia collision between 52 and 57 ma

tibetan tectonic evolution inferred from spatial and temporal variations in post collisional magmatism cenozoic magmatism on the tibetan plateau shows systematic variations in space and time that must be considered in models concerning tibetan tectonic evolution after the india asia collision which started in the early tertiary and terminated the gangdese arc magmatism in the lhasa terrane made of the southern tibetan plateau widespread potassium rich lavas and subordinate sodium rich basalts were generated from f50 to 30 ma in the qiangtang terrane of northern tibet subsequent post collisional magmatism migrated southwards producing ultrapotassic and adakitic lavas coevally between f26 and 10 ma in the lhasa terrane then potassic volcanism was renewed to the north and has become extensive and semicontinuous since f13 ma in the western qiangtang and songpan ganze terranes such spatial temporal variations enable us to elaborate a geodynamic evolution model which depicts when and how the indian continental lithospheric mantle started thrusting under asia by involving rollback and breakoff of the subducted neo tethyan slab followed by removal of the thickened lhasa lithospheric root we propose that only after the lithospheric removal which occurred at f26 ma could the indian mantle lithosphere have commenced its northward underthrusting and henceforth served as a pivotal control to the himalayan tibetan orogenesis consequently the tibetan plateau is suggested to have risen diachronously from south to north whereas the southern part of the plateau may have been created and maintained since the late oligocene the northern plateau would have not attained its present day elevation and size until the mid miocene when the lower part of the western qiangtang and songpan ganze lithospheres began to founder owing to the push of the underthrust indian mantle lithosphere d 2004 elsevier b v all rights reserved

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

confined unconfined flow in a horizontal confined aquifer mixed confined and unconfined groundwater flow occurs in a bounded initially dry aquifer when the hydraulic head at the side boundary suddenly rises above the elevation of the aquifer s top boundary the flow problem as modelled by the boussinesq equation is non trivial because of the involvement of two moving boundaries the transformed equation based on a similarity transformation can however be dealt with more easily here we present an approximate analytical solution for this flow problem the approximate solution is compared with an exact numerical solution and found to be a very accurate description for describing the mixed confined and unconfined flow in the confined aquifer c 2002 elsevier science b v all rights reserved

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

cenozoic volcanism and tectonic evolution of the tibetan plateau abstract cenozoic volcanism on the tibetan plateau which shows systematic variations in space and time is the volcanic response to the india asia continental collision the volcanism gradually changed from na rich k rich to potassic ultrapotassic adakitic compositions along with the india asia collision shifting from contact collision i e soft collision or syn collision to all sided collision i e hard collision the sodium rich and potasium rich lavas with ages of 65 40 ma distribute mainly in the lhasa terrane of southern tibet and subordinately in the qiangtang terrane of central tibet the widespread potassic ultrapotassic lavas and subordinate adakites were generated from 45 to 26 ma in the qiangtang terrane of central tibet subsequent post collisional volcanism migrated southwards producing ultrapotassic and adakitic lavas coevally between 26 and 8 ma in the lhasa terrane then potassic and minor adakitic volcanism was renewed to the north and has become extensive and semicontinuous since 20 ma in the western qiangtang and songpan ganze terranes such spatial temporal variations provide important constraints on the geodynamic processes that evolved at depth to form the tibetan plateau these processes involve roll back and break off of the subducted neo tethyan slab followed by removal of the thickened lhasa lithospheric root and consequently northward underthrusting of the indian lithosphere the tibetan plateau is suggested to have risen diachronously from south to north whereas the southern part of the plateau may have been created and maintained since the late oligocene the northern plateau would have not attained its present day elevation and size until the mid miocene when the lower part of the western qiangtang and songpan ganze lithospheres began to founder and detach owing to the persistently northward push of the underthrust indian lithosphere

zircon u pb chronology of the nyingtri group southern lhasa terrane tibetan plateau implications for grenvillian and pan african provenance and mesozoic cenozoic metamorphism abstract the nyingtri group of the lhasa terrane in southern tibet consists dominantly of metasedimentary rocks and orthogneiss these rocks have a similar mineral paragenesis of plagioclase k feldspar biotite quartz sillimanite garnet staurolite muscovite amphibole indicating amphibolite facies metamorphic conditions inherited detrital zircons from the metasedimentary rocks show magmatic features and yield widely variable 206pb 238u ages ranging from 3300 to 50 ma the data define two prominent age populations 1200 1000 and 600 500 ma indicating that the source of the nyingtri group preserves the records of both grenville and pan african magmatic thermal events inherited magmatic zircon cores from the orthogneiss yield a crystallization age of 496 ma limiting the depositional age of the metasedimentary sequence to cambrian or older overgrowth rims on the detrital zircons from one metasedimentary rock yield a metamorphic age of 32 ma on the basis of these results together with the

exhumation during crustal folding in the namche barwa syntaxis geological observation in the eastern end of the himalaya shows that the asia india suture is folded metamorphic rocks derived from india occur structurally below the suture in the core of a regional antiform isotopic and fission track dating establish cooling exhumation of rocks from c 30 km depth within the last 4 myr we argue that exhumation is caused by 10 mm yr 1 erosion coeval with crustal scale folding

paleogene crustal anatexis and metamorphism in lhasa terrane eastern himalayan syntaxis evidence from u pb zircon ages and hf isotopic compositions of the nyingchi complex in the eastern himalayan syntaxis the southern lhasa terrane is dominated by middle to high grade metamorphic rocks nyingchi complex which are intruded by felsic melts u pb zircon dating and zircon hf isotopic composition of these metamorphic and magmatic rocks provide important constraints on the tectono thermal evolution of the lhasa terrane during convergent process between indian and asian continents u pb zircon data for an orthogneiss intruding the nyingchi complex yield a protolith magma crystallization age of 83 4 1 2 ma with metamorphic ages of 65 46 ma this orthogneiss is characterized by positive ehf t values of 8 3 and young hf model ages of 0 6 ga indicating a derivation primarily from a depleted mantle or juvenile crustal source zircons from a quartz diorite yield a magma crystallization age of 63 1 0 6 ma with ehf t values of 8 2 to 2 7 suggesting that this magma was sourced from partial melting of older crustal materials zircon cores from a foliated biotite granite show ages ranging from 347 to 2690 ma with age peaks at 347 403 ma 461 648 ma and 1013 1183 ma their zircon ehf t values range from 30 6 to 6 9 both the u pb ages and hf isotopic composition of the zircon cores are similar to those of detrital zircons from the nyingchi complex paragneiss implying that the granite was derived from anatexis of the nyingchi complex metasediments the zircon rims from the granite indicate crustal anatexis at 64 4 0 7 ma and subsequent metamorphism at 55 1 1 3 and 41 4 2 3 ma our results suggest that the late cretaceous magmatism in the southern lhasa terrane resulted from neo tethys oceanic slab subduction and we infer that paleocene crustal anatexis and metamorphism were related to the thermal perturbation caused by rollback of the northward subducted neo tethyan oceanic slab

tectono metamorphic evolution of the karakorum metamorphic complex dassu askole area ne pakistan exhumation of mid crustal ht mp gneisses in a convergent context the south karakorum margin east of the himalayan syntaxis consist of an e w elongated zone of young 10 3 ma high grade metamorphic rocks m2 and related migmatitic domes this late tectono metamorphic event post dates the palaeogene 55 37 ma phase of thickening of the belt featured by nw se structures and associated m1 amphibolite facies metamorphism 0 7 gpa 700 c this m2 metamorphism is characterised by low pressure high temperature conditions coeval with migmatite formation in response to a thermal increase of c 150 c compared to m1 culminating at a temperature of c 770 c and a pressure of 0 5 0 6 gpa rapid exhumation of migmatitic domes at a rate of 5 mm yr 1 was accommodated by vertical extrusion in the core of e w crustal scale folds these crustal scale folds formed in response to n s syn collisional shortening and were enhanced by thermal weakening of the migmatised continental crust m2 metamorphism is spatially and temporarily associated with granitoids showing a mantle affinity firmly suggesting that this could be the advective heat source for the granite and syenite generation and the subsequent migmatisation of the mid crustal level such relationships between a mantle related magmatism and a high temperature metamorphism in a convergent shortening context are suggestive of the breakoff of the subducted indian slab since 20 ma

mesozoic and cenozoic volcanic rocks from central and southern tibet 39ar 40ar dating petrological characteristics and geodynamical significance field relations radiometric ages and mineralogical and chemical characteristics of the orogenic volcanic rocks from the lhasa block tibet are studied volcanic activity started before the deposition of the albian aptian sedimentary units takena formation and xigaze group volcanism is volumetrically limited in the northern part of the lhasa block basalts andesites and dacites are present while ignimbrites are scarce radiometric ages of these rocks range between 110 and 80 ma conversely in the southern part of the lhasa block volcanism is largely developed gangdese belt there paroxysmal activity occurred later than in the north between 60 and 50 ma lingzizong formation this volcanism is mainly represented by andesites and ignimbrites a younger episode at about 15 10 ma took place in this southern region the mineralogical and geochemical characteristics indicate that as a whole the volcanic rocks from the lhasa block belong to a calc alkaline continental margin series enriched in k20 and other incompatible elements subordinate potassic alkaline lavas are present in the northern margin of the gangdese belt some basalts exhibit a tholeiitic affinity daquin area southern region the geodynamic significance of these cenozoic volcanic rocks lies in the relative motion of india and the lhasa block after accretion of the latter to the northern qantang block a geodynamic model based on changes of the type of subduction of the indian oceanic crust beneath the lhasa block for the period 120 40 ma is proposed to account for the abundance age and magmatic affinity of the volcanic products

the onset of india asia continental collision early steep subduction required by the timing of uhp metamorphism in the western himalaya ultrahigh pressure uhp rocks in the nw himalaya are some of the youngest on earth and allow testing of critical questions of uhp formation and exhumation and the timing of the india asia collision initial collision of india with asia is widely cited as being at 55f1 ma based on a paleomagnetically determined slowdown of india s plate velocity and as being at ca 51 ma based on the termination of marine carbonate deposition even relatively small changes in this collision age force large changes in tectonic reconstructions because of the rapid india asia convergence rate of 134 mm a at the time of collision new u pb shrimp dating of zircon shows that indian rocks of the tso morari complex reached uhp depths at 53 3f0 7 ma given the high rate of indian subduction this dating implies that indian continental crust arrived at the asian trench no later than 57f1 ma providing a metamorphic age for comparison with previous paleomagnetic and stratigraphic estimates india s collision with asia may be compared to modern processes in the timor region in which initiation of collision precedes both the slowing of the convergence rate and the termination of marine carbonate deposition the indian uhp rocks must have traveled rapidly along a short hence steep path into the mantle early continental subduction was at a steep angle probably vertical comparable to modern continental subduction in the hindu kush despite evidence for modern day low angle subduction of india beneath tibet oceanic slab break off likely coincided with exhumation of uhp terranes in the western himalaya and led to the initiation of low angle subduction and leucogranite generation d 2005 elsevier b v all rights reserved

geomorphic signatures on brutsaert base flow recession analysis this paper addresses the signatures of catchment geomorphology on base ow recessioncurves its relevance relates to the implied predictability of base ow features which arecentral to catchment scale transport processes and to ecohydrological function movingfrom the classical recession curve analysis method originally applied in the finger lakesregion of new york a large set of recession curves has been analyzed from swissstream ow data for these catchments digital elevation models have been preciselyanalyzed and a method aimed at the geomorphic origins of recession curves has beenapplied to the swiss data set the method links river network morphology epitomized bytime varying distribution of contributing channel sites with the classic parameterization ofrecession events this is done by assimilating two scaling exponents and b

shallowing and steepening subduction zones continental lithospheric loss magmatism and crustal flow under the central andean altiplano puna plateau

the detrital record of orogenesis a review of approaches and techniques used in the himalayan sedimentary basins the sediment archive of material eroded from an active tectonic region and stored in adjacent basins can provide a valuable record of hinterland tectonism especially when information in the source region itself is obscured by later metamorphism or removed by tectonism or erosion using the sediment record to document tectonism is a well established approach but more recently there has been a burgeoning of the number of isotopic techniques which can be applied to detrital material in particular single grain analyses thus the scope for application of detrital studies to a number of different tectonic problems has widened considerably in this review the example of sediments eroded from the himalayan orogen and preserved in the suture zone basin foreland basin remnant ocean basins and deep sea fans is used to illustrate the approach techniques as diverse as petrography heavy mineral xrf and sr nd studies single grain dating by ar ar u pb and fission track methodologies and single grain sm nd and pb isotopic analyses are described the paper documents how the sediment record can be used to determine the thermal and tectonic evolution of the orogen constrain mechanisms of continental deformation exhumation rates and palaeodrainage

u pb zircon constraints on the tectonic evolution of southeastern tibet namche barwa area the eastern syntaxis of the himalayas is expressed in the crust as a pronounced southward bend in the orogen the change in strike of geologic features coincides with the high topography of the namche barwa region the exposure of granulite grade metamorphic rocks and a 180 degree bend in the yalu tsangpo we have conducted a geochronologic and geochemical investigation of several suites of granitoids collected from the namche barwa massif and subjacent terranes of southeastern tibet ranging from cm scale dikes and sills to larger outcrop scale intrusions u pb shrimp rg zircon ages establish at least five magmatic episodes 400 to 500 ma 120 ma 40 to 70 ma 18 to 25 ma and 3 to 10 ma these episodes broadly correlate to spatial patterns in sample localities as follows 400 to 500 ma ages occur in zircon cores collected from within the massif proper 120 ma granites related to early gangdese arc plutonism are primarily located northeast of namche barwa later 40 70 ma gangdese activity is expressed in granites west of namche barwa 18 to 25 ma granites occur both along the suture zone west of gyala peri and directly north of namche barwa along the area of the jiali fault zone and are attributed both to shearing within the jiali fault zone and to an early miocene gangdese thrust event exceptionally young 10 ma zircon ages are clustered near the core of the massif along the yalu tsangpo gorge trace element geochemical data indicates the presence of both fluid present and fluid absent melts with a fluid absent decompression melting regime dominating near the core of namche barwa

analytical approximation for the recession of a sloping aquifer an approximation is obtained for the recession of a sloping aquifer the analytical approximation can provide a useful tool to analyze data and obtain physical properties of the aquifer in contrast to the case of a horizontal aquifer when plotting the time derivative of the flux versus the flux on a log scale the result shows that the flux derivative reaches a minimum value and that the curve can have a slope of unity as often observed illustration of the application of the analytical results to the mahantango creek data is also discussed

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

a slab breakoff model for the neogene thermal evolution of south karakorum and south tibet on the south karakorum margin neogene high temperature medium pressure ht mp gneisses define an east west trending thermal anomaly these rocks have been heated from 600 to 750 c during a slight pressure drop from 0 7 to 0 5 gpa their retrogressive path cross cuts the relaxed geotherm of tectonically thickened crust such a p t evolution occurs only if an advective source of heat is involved involvement of an advective heat source is also implied by the occurrence of neogene granitoids and lamprophyres within the ht mp gneiss area these rocks are strongly enriched in large ion lithophile elements relative to primitive mantle and show negative high field strength element anomalies we interpret these geochemical characteristics to be the result of melting of metasomatized asian lithospheric mantle the nd and sr isotopic compositions of the south karakorum neogene magmatic rocks nd 12 to 7 and 87sr 86sr 0 705 0 725 further suggest they could have originated from mixing between asian variously metasomatized mantle and precambrian crust by contrast the origin of the youngest magmatic rocks 10 myr here exemplified by the hemasil syenite and associated lamprophyres requires involvement of a depleted mantle the combined hf nd signature of these rocks hf 10 4 11 5 and nd 3 4 4 3 suggests that the source of the hemasil syenite could have been depleted mantle contaminated by oceanic sediments likely during the earlier subduction of the tethyan ocean neogene magmatic rocks with the same geochemical characteristics and evolution as those of south karakorum have previously been described in south tibet based on their location and the geochemical evolution of their source region we here propose that the neogene magmatic and metamorphic evolution of the south asian margin was controlled by slab breakoff of the subducting indian continental margin starting at about 25 ma this model is supported by available geophysical data from south karakorum and south tibet

late cretaceous gangdese intrusions of adakitic geochemical characteristics se tibet petrogenesis and tectonic implications article i nfo the gangdese batholith emplaced from the cretaceous to eocene in southern tibet has been widely regarded as the major constituent of an andean type convergent margin resulting from northward subduction of the neo tethyan oceanic lithosphere under asia while the gangdese batholith consists predominantly of calc alkaline rocks we identify from the eastern part of the batholith a suite of epidote bearing granodiorites that shows adakitic geochemical characteristics marked with apparently higher la yb and sr y and lower y and hree than other gangdese rocks and common arc magmas shrimp zircon u pb analyses of two of the samples yielded 206 pb 238 u dates of 80 4 1 1 and 82 7 1 6 2 ma which constrain the emplacement ages of the adakitic rocks trace element modeling suggests that these rocks originated from partial melting of a garnet amphibolite source that on the basis of the sr and nd isotope data isr 0 7044 0 7048 end t 3 2 to 0 9 we interpret to be a newly underplated mafic lower crust rather than the subducted neo tethyan oceanic crust this juvenile crust was produced by cretaceous basaltic underplating above the mantle wedge and then thickened by the tectonic contraction owing to flattening of the neo tethyan subduction a process that also led to the adakitic magmatism our interpretation involving a late cretaceous flat slab subduction and related orogenesis in southern tibet is consistent with petrographic data such as the occurrence of magmatic epidote and muscovite rimmed with resorption texture in the granodiorites which indicate deep seated emplacement followed by rapid tectonic exhumation

causes and consequences of protracted melting of the mid crust exposed in the north himalayan antiform gneiss domes exposed by the north himalayan antiform in southern tibet provide evidence for protracted melting of the mid crust during convergent tectonics anatexis was initiated during underthrusting and recurred during the southwards extrusion of a low viscosity crustal sheet sr nd isotope systematics establish that the cores of the domes dominated by cambrian gneiss form a window of the high himalayan crystalline series hhcs indian plate combined trace element and isotopic analyses from 4 two mica granites that intrude both core and the metasedimentary mantle of the domes indicate an origin from melting of the high himalayan crystalline series by the incongruent breakdown of muscovite and to a lesser extent biotite at temperatures from 680 to 760 c u pb dating of accessory phases extends the known period of granite magmatism from the late oligocene to mid miocene a ca rich garnet bearing leucogranite of oligocene age 27 5 ma provides a rare insight into the pre miocene history of the underthrust indian plate reflecting prograde melting at greater depths during a period of crustal thickening the origin of the miocene granites 15 10 ma can be modeled from the exhumation of an extruding channel at a rate of ca 1 mm year constrained by ar isotope data melting over a protracted time scale along the leading edge of the indian plate less than 50 km from the suture accelerated horizontal shortening within the collision zone and provided a positive feedback mechanism between magmagenesis and deformation

on the use of the boussinesq equation for interpreting recession hydrographs from sloping aquifers 1 the method of recession analysis proposed by brutsaert and nieber 1977 remains one of the few analytical tools for estimating aquifer hydraulic parameters at the field scale and beyond in the method the recession hydrograph is examined as dq dt f q where q is aquifer discharge and f is an arbitrary function the observed function f is parameterized through analytical solutions to the one dimensional boussinesq equation for unconfined flow in a homogeneous and horizontal aquifer while attractive in its simplicity as originally presented it is not applicable to settings where slope is an important driver of flow or where hydraulic parameters vary greatly with depth we compare analytical solutions to the linearized one dimensional boussinesq equation for a sloping aquifer to numerical solutions of the full nonlinear equation the behavior of the nonlinear boussinesq equation is also assessed when the aquifer is heterogeneous wherein the lateral saturated hydraulic conductivity k varies as a power law with height z above the impermeable layer k zn n constant 0 all of the analytical solutions differ in key aspects from the nonlinear solution when plotted as dq dt f q and thus are inappropriate for a brutsaert and nieber type analysis however new analytical solutions for a sloping aquifer are derived empirically from the numerical simulations that are applicable during the late period of recession when the recession curve converges to dq dt aqb where b 2n 1 n 1 and a is a function of the dimensions and hydraulic properties of the aquifer

early eocene warming events and the timing of terrestrial faunal exchange between india and asia the timing of initiation of continent continent collision between asia and india is controversial but this major tectonic event is generally thought to have occurred in the early eocene ca 50 ma new and independent data from strontium isotopes stable carbon isotopes microfossil biostratigraphy and mammal fossils from an early eocene marginal marine sequence cambay shale at the vastan lignite mine of western india indicate that terrestrial faunal exchanges and therefore continental collision between asia and the indian subcontinent took place before 53 7 ma this age coincides with the second eocene thermal maximum etm2 a short lived warming episode that followed the paleocene eocene thermal maximum petm ca 55 5 ma our data also document for the first time a clear record of the etm2 in terrestrial organic material from a low latitude site which is represented by a 3 4 carbon isotope excursion cie in lignite and dispersed organic carbon 13c values the magnitude of the cie at this location closely matches that observed in marine cores from the arctic ocean which supports an interpretation that this hyperthermal event though of lower magnitude was similar in character to that of the petm being a global phenomenon that affected both terrestrial and marine ecosystems

zircon u pb and hf isotope constraints on the mesozoic tectonics and crustal evolution of southern tibet the first in situ hf and u pb isotope analyses of zircon separates from mesozoic granites in southern tibet identify a significant previously unknown stage of magmatism igneous zircons n 34 from a granite within the gangdese batholith show a weighted mean 206 pb 238 u age of 188 1 1 4 ma and e hf t the parts in 10 4 deviation of initial hf isotope ratios between the zircon sample and the chondritic reservoir values between 10 4 and 16 8 suggesting predominantly early jurassic intrusive activity with a juvenile mantle contribution of 40 inherited zircons from two cretaceous s type granites in the northern magmatic belt 23 delineate a slightly older 206 pb 238 u age cluster between 188 and 210 ma these zircons have e hf t values from 3 9 to 13 7 yielding crustal hf model ages from ca 1 4 to 2 1 ga suggesting a major episode of crustal growth in proterozoic time and remelting of this crust in early jurassic time combining these with literature data we interpret the jurassic gangdese magmatism as an early product of the neo tethyan subduction that played a long lasting role in the tectonic evolution of southern tibet prior to the india asia collision

the eastern himalayan syntaxis major tectonic domains ophiolitic melanges and geologic evolution abstract geologic mapping in the eastern himalayan syntaxis confirmed the three regional tectonic elements outlined by previous geologic workers our studies however show that the indus yarlung tsangpo suture iys is a continuous melange zone that forms an inverted u in map view around the namche barwa antiform the namche barwa and nyainqentanglha crystalline complexes lie below and above the iys suture respectively and both were parts of the northern indian plate basement rocks with petrologically and geochronologically correlative protoliths both units were deformed metamorphosed and intruded at the end of the proterozoic the zhibai formation the lower part of the namche barwa group extends along the northwest slope of the himalaya and is mainly composed of highly deformed aluminous felsic gneiss containing sporadic boudins of high pressure granulite in the namche barwa antiform the upper part of the namche barwa group includes a calcareous rock assemblage characteristized by marble and diopside bearing calcsilicate rocks interlayed with felsic gneiss petrochemical studies show that the iys contains lenses of oceanic crustal rocks originated from fore arc trench island arc and back arc basin environments which implies they were derived from a ssz type supra subduction zone ophiolite our field mapping identifies the jiali parlung tsangpo remnant suture jps that lies north of the namche barwa antiform as a possible branch of the neo tethyan oceanic realm subduction of the mesozoic neo tethyan oceanic plate resulted in both mesozoic and cenozoic granite intrusions in the northwest trending gangdise magmatic belt along the southern edge of asia uplift and exhumation have been the most recent dominant tectonic processes in the late cenozoic for the high himalayan crystalline rocks namche barwa group in the core of the namche barwa antiform

cretaceous tertiary carbonate platform evolution and the age of the india asia collision along the ladakh himalaya northwest india abstract the india asia collision resulted in the formation and uplift of the himalaya and the enhanced uplift of the tibetan plateau the transition from marine to continental facies within the indus yarlung tsangpo suture zone and along the northern margin of the indian plate provides the most accurate method of dating the closure of the tethys ocean separating the indian and asian plates other indirect methods of dating the collision such as paleomagnetism dating the uhp metamorphism along the north margin of india dating the youngest subduction related granites along the southern margin of asia and dating the postorogenic indus molasse group deposits within the suture zone cannot provide such a precise or reliable age of collision ophiolite obduction onto the indian passive margin occurred during the latest cretaceous and predated initial collision of the two continental plates unconformities occur beneath the late maastrichtian marpo formation and beneath the danian stumpata formation on the

power law catchment scale recessions arising from heterogeneous linear small scale dynamics 1 power law recession behavior has typically been attributed to the hydraulics of subsurface flows in the landscape e g the boussinesq equation many catchments exhibit enormous heterogeneity in their characteristics such as the permeability of soils hillslope length and gradient and soil mantle thickness in the paper we demonstrate using a simple linear flow model that the between hillslope heterogeneity alone can give rise to observed apparently nonlinear recession slope curves the degree of nonlinearity represented by the exponent fitted to the recession slope curve is shown to increase with the degree of heterogeneity of catchment hydraulic properties in sufficiently heterogeneous catchments the recession is also sensitive to the history of recharge events tending to increase when the past events have built up a storage of water in the slow parts of the catchment the models based on the catchment heterogeneity were fitted to data from the panola mountain research watershed yielding a good fit and allowing us to infer how the heterogeneity in catchment properties varies with scale the range of exponents explained by this theory overlaps with the range explained by the hydraulics of subsurface flow which raises doubts about our ability to extract catchment hydraulic parameters on the basis of recession curves so long as the heterogeneity is not accounted for in general the results presented here have enabled us to show how the heterogeneity of landscape characteristics is upscaled and manifested in the apparent nonlinear relationship at the catchment scale

timing of granulite facies metamorphism in the eastern himalayan syntaxis and its tectonic implications abstract we present geochronological evidence in the eastern himalayan syntaxis namche barwa for high pressure hp granulite facies metamorphism and explain its importance for understanding both the deep continental subduction of the indian plate beneath asia and its subsequent exhumation the timing of peak and retrograde metamorphism in part constrains these processes but is debated we present zircon u pb and trace element data on granulite facies rocks zircon cores and rims from a weakly retrograded mafic granulite p 14 18 kbar t 800 c yield u pb ages of 24 0 0 3 ma and 18 8 0 3 ma respectively zircon cores and rims from an orthogneiss the host of the mafic granulite yield u pb ages of 490 3 ma and 24 2 0 4 ma respectively an amphibolitized mafic granulite gives a u pb zircon age of 17 0 0 4 ma combined with petrography zircon cl images th u ratios and ree patterns we suggest that the peak metamorphic age for the hp granulite is at 24 ma and subsequent moderate and low pressure retrograde metamorphism occurred at 19 17 ma indicating reasonably rapid exhumation for the hp granulite the ages of detrital zircons from a metasedimentary rock another host rock of the mafic granulite range from 0 6 to 2 0 ga with peak at 0 8 1 2 ga the protolith depositional ages for the metasedimentary rock are constrained to be between 490 and 600 ma our data suggest that the granulite terrane in the eastern himalayan syntaxis has an affinity with the greater himalayan series the hp granulite facies metamorphic events in the eastern himalayan syntaxis are distinct from ultrahigh pressure uhp metamorphic events in the western himalayan syntaxis in both age and depth of burial however the metamorphic history of the eastern and western himalayan syntaxes becomes similar after 24 ma the namche barwa granulites appear to result from the underthrusting of the indian plate lithologies beneath the lhasa block during progressive collisional processes followed by extrusion and or exhumation that result from a slab breakoff of the indian plate during the miocene

the timing of partial melting barrovian metamorphism and granite intrusion in the naxos metamorphic core complex cyclades aegean sea greece abstract the relationship between partial melting and granite intrusion in a classic barrovian metamorphic terrane has been assessed thirteen samples were dated by shrimp u pb zircon geochronology from the island of naxos greece one of the aegean metamorphic core complexes the effect of partial melting during peak barrovian metamorphism on naxos is recorded by fine

cenozoic behind arc volcanism in the bolivian andes south america implications for mantle melt generation and lithospheric structure we develop a model of arc and behind arc volcanism as well as constraining the evolution of the lithospheric mantle structure beneath the bolivian andes during the last 25 ma using new geochronological and geochemical data on behind arc mafic volcanic rocks together with a regional isotopic study of geothermal helium emissions and a comparison with seismic tomographic images of the same region helium isotopes measured in natural gas emissions in geothermal and mineral water springs suggest recent mantle melting in a wide zone extending at c 20 s from a few tens of kilometres west of the volcanic arc to over 300 km behind the arc where the subducted slab is at depths increasing from c 100 to c 250 km during the neogene there has been behind arc mafic felsic magmatism right across the high andes between 17 and 22 s in the altiplano and western margin of the eastern cordillera modelling of plio pleistocene basalts and basaltic andesites c 15 depleted mid ocean ridge basalt morb source at depths between c 75 km and c 110 km for mafic volcanic rocks that crop out c 65 km and c 90 km for those that crop out 100 km behind the arc ree inversions for oligo miocene shoshonites and basalts c 25 21 ma suggest that they originated at significantly shallower depths c 45 km to c 100 km from a slightly enriched morb source eruption of shoshonitic lavas also occurred in the altiplano between 13 and 11 ma and widespread behind arc intermediate felsic volcanism has been more or less continuous since the early miocene all this suggests that since c 25 ma there has been a thin c 25 ma by the opening up of a mantle wedge and inflow of hot asthenosphere as a consequence of the steepening of the subducted slab and detachment of hydrated and weakened mantle lithosphere and possibly mafic lower crust

variation of baseflows in the headstreams of the tarim river basin during 1960 2007 summary the tarim river is the largest inland river of china and a major water resource to the vast arid region in northwest china for its four major headstreams which contribute 98 of the tarim river s streamflow we investigated the dynamics of baseflow in the past 50 years the digital filtering method was used to separate baseflow from surface flow after which the baseflow index bfi was calculated and analyzed the major findings of this study include 1 baseflows of the four headstreams have increased considerably over the past 50 years the large baseflow index bfi usually occurred in the wet years but the change rate was irregular because of the increasing recharge from snow and glacial meltwater 2 the annual baseflow variation of the four headstreams appeared to have cycles of 3 5 years 10 14 years and 25 years 3 the baseflow and bfi showed obvious seasonal variation the lowest baseflow and bfi typically occurred in december and january and both increased gradually until the maximums reached in august or july 4 the responses of runoff and baseflow to climatic factors were different precipitation possessed a great impact on runoff whereas temperature possessed a great impact on baseflow baseflow is an important source to the tarim river and is affected by the increasing snow and glaciers melt as a result of temperature rise for the four headstreams we identified temperature thresholds in the time series of bfi and calculated regression relationships between temperature and bfi in the past 50 years the thresholds and the regression relationships would help to assess and predict the impact of climate change on headwater inflow to the tarim river

evolutionary model of the himalaya tibet system geopoem based on new modelling geological and geophysical data a two dimensional thermo mechanical laboratory modelling of continental subduction was performed the subducting continental lithosphere includes a strong brittle upper crust a weak ductile lower crust and a strong upper mantle the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is denser than the asthenosphere the lithospheric layers are composed of material whose strength is sensitive to and inversely proportional to temperature throughout the experiment the model surface was maintained under relatively low temperature and the model base at higher temperature the subduction rate satisfied the peclet criterion modelling confirms that the continental crust can be deeply subducted and shows that slab break off delamination and tectonic underplating are fundamental events with drastic consequences on the subsequent evolution of the convergent system combining these results with previous purely mechanical modelling we elaborate a new evolutionary model for the himalaya tibet convergent system the principal successive stages are 1 subduction of the indian continental lithosphere to 200 250 km depth following subduction of the tethys oceanic lithosphere 2 failure and rapid buoyancy driven uplift of the subducted continental crust from ca 100 km depth to some depth that varies along the mountain belt 20 30 km on average 3 break off of the indian subducted lithospheric mantle with the attached oceanic lithosphere 4 subduction underplating of the indian lithosphere under asia over a few to several hundred kilometers 5 delamination roll back and break off of the indian lithospheric mantle 6 failure of the indian crust in front of the mountain belt formation of the main central thrust and underthrusting of the next portion of indian lithosphere beneath tibet for a few hundred kilometers at the beginning of stage 6 the crustal slice corresponding to the crystalline himalayas undergoes erosion activated uplift and exhumation fl 2000 elsevier science b v all rights reserved

eocene neotethyan slab breakoff in southern tibet inferred from the linzizong volcanic record abstract formation of the linzizong volcanic successions in the lhasa terrane southern tibet has long been related to northward subduction of the neotethyan oceanic slab under asia here we report new 40ar 39ar age results for the volcanic rocks recovered from a large area 29 n to 32 n and 85 e to 93 e that together with literature data delineate two discrete stages of volcanism these are a widespread cretaceous stage and an intense but spatially confined paleogene stage the latter occurring only in the southern part of the lhasa terrane resulted in the linzizong volcanic successions our data furthermore suggest southward migration and intensification of the volcanism in the lhasa terrane with magmatic flare ups at ca 50 ma while the volcanic successions consist dominantly of calc alkaline rocks typical of arc lava geochemistry those formed during the flare up period show significant compositional variations from low k tholeiitic through calc alkaline to shoshonitic magma suites these observations enable us to interpret the volcanic southward migration and following flare ups as the consequences of rollback and breakoff of the subducted neotethyan slab that occurred ahead and in the early stage respectively of the india asia collision our interpretation that involves a major eocene tectonomagmatic activity and concomitant topographic uplift in southern tibet is consistent with himalayan metamorphic constraints regional sedimentary records and seismic tomography

drying front in a sloping aquifer nonlinear effects 1 the profiles for the water table height h x t in a shallow sloping aquifer are reexamined with a solution of the nonlinear boussinesq equation we demonstrate that the previous anomaly first reported by brutsaert 1994 that the point at which the water table h first becomes zero at x l at time t t c remains fixed at this point for all times t t c is actually a result of the linearization of the boussinesq equation and not as previously suggested brutsaert 1994 verhoest and troch 2000 a result of the dupuit assumption rather by examination of the nonlinear boussinesq equation the drying front i e the point x f at which h is zero for times t greater than or equal to t c actually recedes downslope as physically expected this points out that the linear boussinesq equation should be used carefully when a zero depth is obtained as the concept of an average depth loses meaning at that time

analytical methods for estimating saturated hydraulic conductivity in a tile drained field determination of field scale hydraulic properties is required for many hydrologic predictions four analytical methods for determining the saturated hydraulic conductivity k on the field scale for a field drained by a network of parallel drains are considered these methods all derived from the boussinesq equation relate 1 time rate of change of discharge to discharge 2 relative discharge to time 3 time rate of change of water table height to height and 4 relative water table height to time though all four methods eliminate the need to know precisely the beginning time of the recession curve which is in practice ambiguous methods 2 and 4 do not require taking time derivates of observations which introduces noise the mean field scale k based on several recession events on two 1 ha fields using all methods was 4 m d this is near the median k of 5 m d calculated from 40 soil cores taken at various depths within the same fields but five times less than the mean k of the soil cores 20 m d q 2003 elsevier b v all rights reserved

reworked precambrian metamorphic basement of the lhasa terrane southern tibet zircon titanite u pb geochronology hf isotope and geochemistry abstract due to the paucity of exposure the formation and evolution of the precambrian basement of the lhasa terrane remain poorly known here we report zircon and titanite in situ u pb ages bulk rock geochemical and zircon hf isotopic data on the orthogneisses from the dongjiu area of the southern lhasa subterrane slt southern tibet geochemical data suggest that the protoliths of the biotite amphibole gneiss and biotite gneiss are granodiorite and granite respectively inherited magmatic zircon cores from these orthogneisses give protolith crystalline ages of 1520 1506 ma whereas the overgrown zircon rims give metamorphic ages of 605 590 ma the mesoproterozoic granitic rocks have bulk rock end t values of 3 6 to 0 1 and zircon core ehf t values of 4 5 to 2 6 which give similar tdm2 ages of 2 35 2 05 ga and 2 54 2 10 ga respectively suggesting their derivation from partial melting of paleoproterozoic crustal material the granitic rocks are also local provenance for the mesoproterozoic detrital zircons in the paleozoic strata in the lhasa terrane titanite in situ u pb ages further indicate that the dongjiu orthogneiss experienced more recent metamorphism at 26 ma the mineral assemblage and thermobarometry calculations indicate that the oligocene metamorphism occurred under medium pressure mp amphibolite facies conditions 5 4 7 2 kbar 691 765 c we propose that the dongjiu gneisses represent the precambrian metamorphic basement of the lhasa terrane but have been intensively reworked by metamorphism in the slt in response to the continued india asia convergence since the collision

zircon u pb geochronology and hf isotopic constraints on petrogenesis of the gangdese batholith southern tibet abstract during the mesozoic cenozoic northward neotethyan subduction and subsequent india asia collision gave rise to the extensive transhimalayan magmatism that stretches from burma and western yunnan through southern tibet to the ladakh and kohistan complexes to understand the age distribution and petrogenesis of the gangdese batholith the largest intrusive exposure along the transhimalayan magmatic belt fifty granitic samples were selected for in situ zircon u pb and hf isotopic analyses the u pb data suggest four discrete stages of magmatic activity i e 205 152 109 80 65 41 and 33 13 ma respectively with the 65 41 ma stage being the most prominent the hf isotopic data indicate that the gangdese batholith is overwhelmed by positive e hf t values which are comparable to those of the kohistan ladakh batholiths in the west but differ markedly from those of the chayu burma batholiths in the east most of the gangdese granites show similar and young hf model ages 1000 300 ma indicating their derivation from juvenile crust however those formed in the 65 41 ma stage exhibit more heterogeneous hf isotopic ratios with negative e hf t values being observed in some granites younger than 50 ma suggesting the involvement of old indian continental crust in their petrogenesis this age may thus mark the onset of the india asia collision the newly established zircon u pb age and hf isotope database of the gangdese batholith can be used as a powerful tracer or fingerprint when studying the source to sink relation of the sediments eroded from the southern tibetan plateau

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

some analytical solutions of the linearized boussinesq equation with recharge for a sloping aquifer subsurface flow from a hillslope can be described by the hydraulic groundwater theory as formulated by the boussinesq equation several attempts have been made to solve this partial differential equation and exact solutions have been found for specific situations in the case of a sloping aquifer brutsaert 1994 suggested linearizing the equation to calculate the unit response of the hillslope in this paper we first apply the work of brutsaert by assuming a constant recharge to the groundwater table the solution describes the groundwater table levels and the outflow in function of time then an analytical expression is derived for the steady state solution by allowing time to approach infinity this steady state water table is used as an initial condition to derive another analytical solution of the boussinesq equation this can then be used in a quasi steady state approach to compute outflow under changing recharge conditions subsurface flow from a hillslope can be described by the hydraulic groundwater theory as formulated by the boussinesq equation several attempts have been made to solve this partial differential equation and exact solutions have been found for specific situations in the case of a sloping aquifer brutsaert 1994 suggested linearizing the equation to calculate the unit response of the hillslope in this paper we first apply the work of brutsaert by assuming a constant recharge to the groundwater table the solution describes the groundwater table levels and the outflow in function of time then an analytical expression is derived for the steady state solution by allowing time to approach infinity this steady state water table is used as an initial condition to derive another analytical solution of the boussinesq equation this can then be used in a quasi steady state approach to compute outflow under changing recharge conditions

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

the significance of cenozoic magmatism from the western margin of the eastern syntaxis southeast tibet the major and trace element geochemistry sr nd bulk rock isotopes u pb zircon chronology and lu hf isotopic compositions are described for three granitic bodies which intrude the nyingchi gneisses lhasa terrane along the western margin of the eastern himalayan syntaxis the bayi two mica granite and lunan granite granodiorite were intruded at 22 1 and 25 4 0 3 ma respectively whereas the confluence biotite granite was emplaced at 49 1 0 4 ma all share strong depletions in y and hree requiring a garnet bearing source both during and following the eocene collision of the indian plate with the lhasa terrane the isotope geochemistry of these intrusives e nd t 3 to 5 87sr 86sr t 0 706 0 707 indicates a crustal source within the lhasa terrane sr nd systematics of the garnet bearing nyingchi gneisses together with the u pb and lu hf isotopic ratios of detrital zircons recovered from this unit identifies it as a potential melt source the combined element and isotope geochemistry of the plutons indicate a mixed source the gneisses provide the older component whereas the gangdese batholith provides a younger siliceous component the involvement of garnet bearing crustal material in melt sources from the cretaceous 80 ma to the miocene 20 ma is consistent with the presence of a thicker continental crust in the eastern lhasa terrane as is the presence of magmatic epidote in several plutons which indicates a regional deepening level of exposure eastwards post collision crustal melting is synchronous with proposed slab break off during the early miocene suggesting advective heating by rising asthenospheric melts

zircon u pb and hf isotopic constraints from eastern transhimalayan batholiths on the precollisional magmatic and tectonic evolution in southern tibet abstract before the indian collision with asia northward subduction of the neo tethyan oceanic lithosphere produced an andean type convergent margin in south asia characterized by arc magmatism starting from the early jurassic and lasting until the eocene the magmatic arc is now represented by widespread intrusive bodies or the so called transhimalayan batholiths in the lhasa terrane of southern tibet that have been divided into two main magmatic suites i e the northern plutonic belt and the southern gangdese batholith their temporal distribution concerning how exactly the magmatic suites correlate eastwards and then southeastwards around the eastern himalayan syntaxis however remains poorly constrained here we report the first combined zircon u pb and hf isotopic study of the transhimalayan batholiths from the eastern part of the lhasa terrane 95 97 5 e and 28 5 30 n se tibet zircon u pb dating results of 24 granitoids indicate that the rocks were emplaced principally in the early cretaceous 133 110 ma and subordinately in the paleocene 66 57 ma long after an older granite intrusion in the earliest jurassic 198 ma the zircon ehf t values range from 5 to 20 yielding hf crustal model ages tdmc between 0 8 and 2 4 ga that peak at 1 7 ga suggesting a major episode of crustal growth in the proterozoic and a predominantly or even exclusively crustal source for the granitoid petrogenesis these zircon u pb and hf isotopic constraints together with whole rock geochemical characteristics allow us to correlate them to s type granitoids in the northern plutonic belt rather than to the i type gangdese batholith from the central part of the southern lhasa terrane in contrast to the gangdese magmatism that formed with a significant juvenile mantle input related to the neo tethyan subduction these s type granitoids show geochemical similarities to the north american cordilleran interior batholiths that did not form directly from a subduction setting thus we attribute the petrogenesis of the widespread early cretaceous granitoids in the northern belt and eastern himalayan batholiths to a postcollisional regime due to the late jurassic early cretaceous continental collision between the lhasa and qiangtang terranes under this framework the s type magmatism was generated in response to collision induced crustal thickening which may have capabilities of causing crustal anatexis by itself in the region however continued interplay with the neo tethyan subduction zone processes such as back arc extension could also have played a crucial and long lasting role in the magma generation

early cretaceous subduction related adakite like rocks of the gangdese belt southern tibet products of slab melting and subsequent melt peridotite interaction abstract the limited geochronology and geochemistry data available for the early cretaceous igneous rocks of the southern gangdese belt southern tibet has resulted in the proposal of conflicting geodynamic models for the generation of the widespread cretaceous igneous rocks in the middle and northern parts of the belt to explore this issue we present shrimp u pb zircon data and geochemical and sr nd pb hf isotopic data for the mamen andesites from the southern margin of the gangdese belt the mamen andesites emplaced at 136 5 ma are sodic na2o k2o 1 2 2 3 and have geochemical characteristics typical of adakites i e high al2o3 high la yb ratios and sr contents low y and hree contents and positive eu anomalies except for high cr ni and mgo contents the andesites have initial 87sr 86sr t ratios of 0 70413 0 70513 positive end t values of 3 7 5 8 and 206pb 204pb t ratios of 18 37 18 51 207pb 204pb t ratios of 15 59 15 65 and 208pb 204pb t ratios of 38 43 38 72 in situ hf isotopic analyses of zircons that had previously been dated by shrimp yielded positive initial ehf t values ranging from 11 0 to 15 5 a model calculation using trace element and sr nd pb isotopic data indicates that several percent of subducted sediment is required to generate the mamen andesites which were derived via the partial melting of subducted neo tethyan slab morb sediment fluid and subsequently hybridized by peridotite in the mantle wedge our data indicate that the neo tethyan oceanic crust was subducted northward beneath the gangdese belt during the early cretaceous at a high angle our results are inconsistent with a tectonic model that advocates the low angle or flat slab subduction of neo tethyan oceanic crust in generating the widespread cretaceous magmatism recorded in the gangdese belt

petrologic case for eocene slab breakoff during the indo asian collision the greater himalayan sequence is the metamorphic core of the himalaya and has been a focus of considerable study yet its petrologic evolution remains controversial pre oligocene metamorphism was nearly obliterated by miocene metamorphism and melting which many workers ascribe to shear heating unusually high concentrations of radioactive elements accompanying burial and or several kilometers of miocene exhumation due to extensional faulting sparse oligocene to eocene ages are often assigned to a tectonically unspecified event an alternative slab breakoff model subduction of greater himalayan rocks to 100 km depth in the eocene followed by buoyant extrusion due to decoupling of the oceanic lithosphere has also been proposed based on rare eocene eclogites but without explanation of miocene melting or metamorphic petrogenesis we argue that slab breakoff readily explains the eocene eclogites miocene partial melts and late eocene k rich magmas in southeastern tibet and that metamorphic and plutonic ages help define the timing and rates of breakoff and extrusion this model implies that 1 much of the greater himalayan sequence was subducted to depths greater than commonly considered 2 fluid absent decompression melting at 30 35 km depth was the consequence of as much as 100 km of extrusion rather than radioactive heating or a smaller crustal level extensional or erosional event and 3 eruption of eocene k rich tibetan plateau lavas has no implication for topography of the tibetan plateau

the source of granitic gneisses and migmatites in the antarctic peninsula a combined u pb shrimp and laser ablation hf isotope study of complex zircons zircons gneisses and migmatites collected from the antarctic peninsula have different core rim hafnium isotope ratio relationships depending on whether evidence for zircon dissolution is present or absent two samples contain inherited zircon that is partially dissolved in these samples the 176hf 177hf rations of the inherited zircon and new magmatic zircon rims are on average indistinguishable and consistent with in situ melting in such cases the hafnium isotopic composition of the melt was probably strongly influenced by the dissolved zircon component at the source variation in 176hf 177hf within the magmatic zircon rims from grain to grain suggests that hf isotopes were only partially homogenized during melt migration alternatively zircon growth may have taken place within small volumes of partial melt other samples do not preserve textural evidence for zircon dissolution during melt generation in these samples the 176hf 177hf values of the inherited zircon and new magmatic zircon rims are different the zircon rims apparently suggest a source of less evolved hafnium than that contained within the inherited zircon whether this relates to a separate juvenile source or alternatively is derived from minerals other than zircon at the source cannot be resolved inherited zircon irrespective of age has been strongly influenced by the reworking of a juvenile late mesoproterozoic source suggesting that such crust underlies the antarctic peninsula our results therefore suggest that hf isotope analyses provide great potential for future studies investigating the source and processes involved in the generation of crustal melts

anatomy and global context of the andes main geologic features and the andean orogenic cycle the andes make up the largest orogenic system developed by subduction of oceanic crust along a continental margin subduction began soon after the breakup of rodinia in late proterozoic times and since that time it has been intermittently active up to the present the evolution of the pacifi c margin of south america during the paleozoic occurred in the following stages 1 initial proterozoic rifting followed by subduction and fi nal re amalgamation of the margin in early cambrian times as depicted by the puncoviscana and tucavaca basins and related granitoids in southern bolivia and northern argentina 2 a later phase of rifting in the middle cambrian and subsequent collisions in middle ordovician times of parautochthonous terranes derived from gondwana such as paracas arequipa and antofalla and exotic terranes originating in laurentia such as cuyania chilenia and chibcha 3 fi nal permian collision between south america and north america to form pangea during the alleghanides orogeny leaving behind rifted pieces of laurentia as the tahami and tahuin terranes in the northern andes and other poorly known orthogneisses in the cordillera real of ecuador in the late permian early triassic and 4 amalgamation of the mejillonia and patagonia terranes in early permian times representing the last convergence episodes recorded in the margin during the gondwanides orogeny these rifting episodes and subsequent collisions along the continental margin were the result of changes of the absolute motion of gondwana related to global plate reorganizations during proterozoic to paleozoic times generalized rifting during pangea breakup in the triassic concentrated extension in the hanging wall of the sutures that amalgamated the paleozoic terranes the opening of the indian ocean in early jurassic times was associated with a new phase of subduction along the continental margin the northeastward absolute motion of western gondwana produced a negative trench roll back velocity that controlled subduction under an extensional regime until late early cretaceous times the northern andes of venezuela colombia and ecuador record a series of collisions of island arcs and oceanic plateaus from the early cretaceous to the middle miocene as a result of interaction with the caribbean plate the remaining central and southern andes record periods of orogenesis and mountain building alternating with periods of quiescence and absence of deformation as recorded in parts of the oligocene based on the generalized occurrence of fl at slab subduction episodes through time as recorded in most ramos v a 2009 anatomy and global context of the andes main geologic features and the andean orogenic cycle in kay s m ramos v a and dickinson w r eds backbone of the americas shallow subduction plateau uplift and ridge and terrane collision geological society of america memoir 204 p 31 65 doi 10 1130 2009 1204 02 for permission to copy contact editing geosociety org 2009 the geological society of america all rights reserved

cenozoic volcanism in tibet evidence for a transition from oceanic to continental subduction geochronological k ar or ar ar major and trace element sr nd pb isotopic and mineral chemical data are presented for newly discovered cenozoic volcanic rocks in the western qiangtang and central lhasa terranes of tibet alkali basalts of 65 45ma occur in the western qiangtang terrane and represent primitive mantle melts as indicated by high mgnumbers 100 mg mg fe 54 65 cr 204 839 ppm and ni 94 218 ppm contents and relatively low ratios of sr sr 0 7046 0 7061 pb pb 18 21 18 89 pb pb 15 49 15 61 and pb pb 38 42 38 89 and high ratios of nd nd 0 5124 0 5127 in contrast younger volcanic rocks in the western qiangtang terrane 30ma and the central lhasa terrane 23 13 and 8ma are potassic to ultrapotassic and interpreted to have been derived from an enriched mantle source they are characterized by very high contents of incompatible trace elements negative ta nb and ti anomalies and radiogenic pb isotopic compositions pb pb 18 43 19 10 pb pb 15 64 15 83 pb pb 39 14 39 67 sr sr 0 7088 0 7092 and nd nd 0 5122 ratios of the western qiangtang terrane potassic lavas are similar to those of 45 29ma potassic volcanic rocks in the north central qiangtang terrane whereas sr sr 0 7167 0 7243 and nd nd 0 5119 ratios of central lhasa terrane lavas are similar to those of 25 16ma ultrapotassic volcanic rocks in the western lhasa terrane the 65 45ma alkali basalts in the western qiangtang terrane along with widespread calc alkaline volcanic rocks of this age in the lhasa terrane may be related to roll back of a previously shallow north dipping slab of tethyan oceanic lithosphere beneath tibet subduction as opposed to convective thinning of continental lithosphere is favored to explain potassic volcanism in tibet because of its occurrence in distinct east west trending belts 45 29ma in the qiangtang terrane 25 17ma in the northern lhasa terrane 16 8ma in the southern lhasa terrane and temporal and spatial relationships with major thrust systems

channel flow and the himalayan tibetan orogen a critical review the movement of a low viscosity crustal layer in response to topographic loading provides a potential mechanism for 1 eastward flow of the asian lower crust causing the peripheral growth of the tibetan plateau and 2 southward flow of the indian middle crust to be extruded along the himalayan topographic front thermomechanical models for channel flow link such extrusion to focused orographic precipitation at the surface isotopic constraints on the timing of fault movement anatexis and thermobarometric evolution of the exhumed garnet to sillimanite grade metasedimentary rocks support mid crustal channel flow during the early to mid miocene exhumed metamorphic assemblages suggest that the dominant mechanism of the viscosity reduction that is a requirement for channel flow was melt weakening along the upper surface defined by the south tibetan detachment system and strain softening along the base bounded by the main central thrust neotectonic extrusion bounded by brittle quaternary faults south of the main central thrust is positively correlated with the spatial distribution of precipitation across a north south transect suggesting climate tectonic linkage over a million year time scale a proposed orogen wide eastward increase in extrusion rate over 20 ma reflects current precipitation patterns but climate tectonic linkage over this time scale remains equivocal

mantle contributions to crustal thickening during continental collision evidence from cenozoic igneous rocks in southern tibet the tibetan crust is twice as thick as average continental crust crustal compression and shortening as a result of indian asian collision isoftenconsideredtobetheprimarycauseforthecrustalthickening inthispaper weshowthatmagmaticcontributionisalsoimportant we come to this conclusion by documenting the paleogene linzizong volcanic succession lvs its coeval granitoid batholiths and the miocene adakitic rocks along the gangdese magmatic belt in southern tibet it has been widely accepted that the indian asian collision proceeded from a soft phase at 65 70 ma to a hard phase at 45 40 ma followed by continued post collisional convergence to the present in response to the collision and post collision convergence are a series of tectono magmatic events recorded in the gangdese magmaticbelt theseinclude 1 thesyn collisionallvsvolcanism 65 40ma andtheemplacementofsoutherngangdesebatholiths a peakageof 50ma 2 aperiod 40mato25ma thatismagmaticallyquiescent yettectonicallydominatedbyactivecompressionand crustal shortening and 3 the emplacements of post collisional adakitic rocks 25 12 ma potassic ultrapotassic volcanics 25 10 ma and peraluminous muscovite bearing granites between 24 and 18 ma these three major events contribute in different ways to the crustal thickness phase i formation of the lower juvenile crust from 65 ma to 50 ma with crustal thickening largely concentrated at 50 40maviainputofmantle derivedmagmas phaseii crustalthickeningbytectonicshorteningat 40 25ma andphaseiii retaining crustal thickness but thinning of the lithospheric mantle since 25 ma in response to crustal extension and upwelling and lateral flow of asthenosphericmantle weemphasizethatcollision inducedcrustalthickeningtookplacemainlyintheperiodof 50 40maand 25ma i e the period between the late stage of the lvs volcanism and the beginning of the adakitic rock emplacement most of the lvs rocks and the collision related granitoids in southern gangdese have end 0 attesting to the significance of mantle input most likely through remelting of mantle derived basaltic rocks including the subducted neo tethyan ocean crust the petrologic and geochemical characteristics of the miocene potassic adakitic rocks support the idea that the lower portion of the thickened tibetan crust is mafic and is genetically associated with the earlier lvs magmatism we estimate that the mantle material input contributed about 30 of the total thickness of the present day tibetan crust by assuming a pre collision crustal thickness of 35 km then the tectonic contribution would be about 20 km

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

cenozoic structural and metamorphic evolution of the eastern himalayan syntaxis namche barwa combined geological and geochronological investigations of the eastern himalayan syntaxis in the namche barwa region of tibet reveal the first order elements of its cenozoic tectonic evolution the syntaxis is characterized by a northeast plunging antiform and is bounded by two northeast striking strike slip shear zones a left slip shear zone on the western side and a right slip shear zone on the eastern side these strike slip shear zones are linked by east westtrending thrusts and served either as 1 a roof thrust to a large duplex system or 2 transfer faults to a south directed thrust system that accommodated northward indentation of a folded indian plate an east west trending pop up structure in the core of the antiform juxtaposes a granulite bearing complex over sillimanite bearing gneisses of gangdese affinity to the north and of indian affinity to the south previous studies suggest that mafic granulites in the complex record at least two episodes of metamorphism at v800 c the first at high pressures 14 15 kbar followed by a second event at 8 10 kbar zircons from mafic granulites yield four populations of concordant u pb ion microprobe ages two groups are at v65 ma and v160 ma and likely crystallized during andean type gangdese magmatism prior to the indo asian collision a third cluster at v40 ma exhibits very low th u ratios and is interpreted to have crystallized in the presence of fluids associated with a high pressure granulite facies metamorphic event during the early stages of the indo asian collision subsequent to high pressure metamorphism in the western himalaya syntaxis between v50 and 43 ma a fourth cluster of zircons yields ages between 11 and 25 ma and th u ratios that decrease systematically with decreasing age we interpret the youngest zircon age v11 ma to represent the timing of moderatepressure high grade metamorphism with the older ages and higher th u ratios being a result of mixing with a restitic igneous component this interpretation coupled with a v 8m a 40 ar 39 ar age on hornblende from a metadiorite within the core of the antiform suggests that the namche barwa syntaxis has been characterized by rapid cooling and exhumation since at least late miocene time despite its contrasting structural setting the miocene and younger metamorphism and cooling history in the namche barwa syntaxis are strikingly similar to those of the nanga parbat syntaxis of the western himalaya fl 2001 elsevier science b v all rights reserved

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

volcanic rocks of the 1985 tibet geotraverse lhasa to golmud volcanic rocks encountered during the tibet geotraverse have been studied in the field in thin section and by major and trace element geochemistry in order to determine their most probable original eruptive environment rocks from a total of eleven distinct volcanic provinces were studied in this way they provide evidence for an active continental margin or post collision province of probable devonian early carboniferous age in the northern kunlun mountains an active continental margin of late carboniferous age in the southern lhasa terrane permian continental rifts in the central qiangtang and central kunlun terranes triassic volcanic arcs in the southern lhasa and northern qiangtang terranes a triassic active continental margin dyke swarm in the northern kunlun mountains a jurassic post collision or back arc rifting province in the southern qiangtang terrane a jurassic island arc in the northern lhasa terrane a cretaceous post collision province in the northern lhasa terrane possibly extending into the southern qiangtang terrane and a palaeogene active continental margin in the southern lhasa terrane an oligocene trachyte plug in the northern qiangtang terrane was the only evidence encountered during the geotraverse of volcanism post dating the palaeogene india eurasia collision however the composition of this plug coupled with new and published analyses from miocene volcanics in the southern lhasa terrane and from the pliocene recent volcanic province of northwest tibet places important constraints on models for post collision underplating of tibet by continental lithosphere any underplating is likely to have been a much later than the start of collision b directed beneath tibet from the north as well as the south and c limited in extent

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

when and where did india and asia collide timing of the collision between india and asia is the key boundary condition in all models for the evolution of the himalaya tibetan orogenic system thus it profoundly affects the interpretation of the rates of a multitude of associated geological processes ranging from tibetan plateau uplift through continental extrusion across eastern asia as well as our understanding of global climate change during the cenozoic although an abrupt slowdown in the rate of convergence between india and asia around 55 ma is widely regarded as indicating the beginning of the collision most of the effects attributed to this major tectonic episode do not occur until more than 20 ma later refined estimates of the relative positions of india and asia indicate that they were not close enough to one another to have collided at 55 ma on the basis of new field evidence from tibet and a reassessment of published data we suggest that continent continent collision began around the eocene oligocene boundary 34 ma and propose an alternative explanation for events at 55 ma

the role of collisional tectonics in the metallogeny of the central andean tin belt abstract the inner arc of the central andes broadly corresponding to the eastern cordillera is the location of a rich tertiary and triassic sn w ag base metal metallogenic province commonly referred to as the bolivian tin belt we propose that the tertiary metallogeny which generated most of the tin ores was a direct consequence of discrete collisions between the south american plate and the nazca slab and sub slab mantle during the ongoing andean orogeny evidence supporting this proposal include 1 the coincidence of the tin province and the inner arc in a marked hump in the andean orogen which may represent tectonic indentation 2 the symmetry of the tin province with respect to the bolivian orocline the axis of which corresponds to the direction of highest compression 3 the relative symmetry of the magmatism and tin mineralization with respect to this axis 4 the concurrent timing of mineralization and compressional pulses 5 the similar host rock geochemistry and ore lead isotope data testifying to a common crustal reservoir and 6 the striking similarity of the igneous suites associated with the ore deposits to those from typical collisional orogens a number of studies have called upon a persistent tin anomaly to explain the metallogeny of the region we propose instead that the latter is better explained by periodic compressional interaction between the farallon nazca oceanic plate and the south american continent this led to the generation of peraluminous magmas which during fractional crystallization exsolved the fluids responsible for the voluminous sn w mineralization

a note on groundwater flow along a hillslope 1 particular cases of groundwater flow along a hillslope are studied using the groundwater hydraulic theory e g dupuit approximation described by the boussinesq equation analytical solutions are found as simple transformations of known similarity solutions in particular the expressions for phreatic surface level and volume flux and the corresponding loop rating curve for the flow of a groundwater mound as well as the travelling wave solutions of the boussinesq equation are described in detail

dating the indian continental subduction and collisional thickening in the northwest himalaya multichronology of the tso morari eclogites multichronometric studies of the low temperature eclogitic tso morari unit ladakh india place timing constraints on the early evolution of the northwest himalayan belt several isotopic systems have been used to date the eclogitization and the exhumation of the tso morari unit lu hf sm nd rb sr and ar ar a ca 55 ma age for the eclogitization has been obtained by lu hf on garnet omphacite and whole rock from mafic eclogite and by sm nd on garnet glaucophane and whole rock from high pressure metapelites these results agree with a previously reported u pb age on allanite and together these ages constrain the subduction of the indian continental margin at the paleocene eocene boundary during exhumation the tso morari rocks underwent thermal relaxation at about 9 3 kbar characterized by partial recrystallization under amphibolite facies conditions ca 47 ma as dated by sm nd on garnet calcic amphibole and whole rock from metabasalt rb sr on phengite apatite and whole rock and ar ar on medium si phengite from metapelites ar ar analyses of biotite and low si muscovite from metapelites which recrystallized at 5 kbar toward the end of the exhumation show that the tso morari unit was at upper crustal levels ca 30 ma these results indicate variable exhumation rates for the tso morari unit beginning with rapid exhumation while the indian margin subduction was still active and later proceeding at a slower pace during the crustal thickening associated with the himalayan collision

middle cretaceous back arc formation and arc evolution along the asian margin the shyok suture zone in northern ladakh nw himalaya abstract the shyok suture zone of the ladakh palaeovolcanic arc is made of cretaceous volcanosedimentary formations intruded by granodioritic plutons field observations show a tectonic stacking of litho units from different volcanic arc and back arc environments in the western part skardu area the shyok suture zone can be subdivided into two groups 1 the northern group which consists of olistolith basaltic blocks and tuffs the basalts are lree depleted with a lile enrichment and a slight nb depletion suggesting a back arc basin origin 2 the southern group which consists predominantly of andesites that show lree enrichment a flat hree pattern strong nb ta depletion and lile enrichment the volcanic rocks of the southern group appear to have island arc tholeiite iat to calc alkaline affinities in the eastern part of the suture zone nubra shyok area albian to cenomanian age silicoclastic sediments predominate these sediments correspond to a large detrital platform built on the northern part of the ladakh arc at the top these sediments interlayer with basaltic to andesitic lavas these lavas appear to be more differentiated and calc alkaline in nature than the skardu southern group lavas but show similar volcanic arc affinities no evidence of a back arc basin was found in this area our data from these two areas shows a northwest southeast evolution from back arc to arc formations in northern ladakh opening of this back arc basin occurred on the northwestern side of the ladakh arc this back arc was progressively closing eastward the arc itself was more mature in the east resembling the southern tibetan continental arc and was characterised by more continental sedimentation subsequent himalayan tectonometamorphic evolution closure of the back arc basin and deformation along the shyok suture reflects an early asymmetrical geometry along the asian margin and kohistan ladakh arc

sudden drawdown and drainage of a horizontal aquifer drainage of a saturated horizontal aquifer following a sudden drawdown is reanalyzed using the boussinesq equation the effect of the finite length of the aquifer is considered in detail an analytical approximation based on a superposition principle yields a very good estimate of the outflow when compared to accurate numerical solutions an illustration of the new analytical approach to analyze basin scale field data is used to demonstrate possible field applications of the new solution

cretaceous tertiary shortening basin development and volcanism in central tibet the geologic map pattern of the qiangtang terrane in central tibet defines a 600 km long and up to 270 km wide east plunging structural culmination it is characterized by early mesozoic blueschist bearing melange and upper paleozoic strata in the core and mainly triassic jurassic strata along the limbs in the western qiangtang terrane 84 e the culmination is unconformably overlain by weakly deformed mid cretaceous volcanic flows and tuffs along the bangong suture to the south 32 n 84 e mid cretaceous nonmarine red beds and volcanic rocks lie unconformably on jurassic suture zone rocks these relationships demonstrate that west central tibet was above sea level during the mid cretaceous and experienced significant denudation prior to mid cretaceous time growth of the qiangtang culmination is inferred to have initiated during southward emplacement of a thrust sheet of early mesozoic melange and upper paleozoic strata during the early cretaceous lhasa qiangtang collision the north south width of the inferred thrust sheet provides a minimum slip estimate of 150 km at 84 e decreasing eastward to 70 km at 87 e paleogene deformation in the qiangtang terrane is characterized by widely distributed mainly north dipping thrust faults that cut eocene oligocene red beds and volcanic rocks in their footwall along the bangong suture the north dipping shiquanhe gaize amdo thrust system cuts 64 and 43 m y old volcanic tuffs in its footwall and accommodated 40 km of post mid cretaceous shortening the tertiary south dipping gaize siling co backthrust bounds the southern margin of the bangong suture and marks the northernmost limit of mid cretaceous marine strata in central tibet cretaceous deformation and denudation in central tibet is attributed to northward underthrusting of the lhasa terrane beneath the qiangtang terrane along the bangong suture this model implies that 1 cretaceous strata along the bangong suture and in the northern lhasa terrane were deposited in a flexural foreland basin system and derived at least in part from the qiangtang terrane and 2 the central tibetan crust was thickened substantially prior to the indo asian collision although its magnitude is poorly known tertiary shortening in the qiangtang terrane is more prevalent than in the lhasa terrane this difference may be attributed to the presence of underthrust melange in the deeper central tibetan crust which would have made it weaker than the lhasa terrane during the indo asian collision

baseflow separation based on analytical solutions of the boussinesq equation a technique for baseflow separation is presented based on similarity solutions of the boussinesq equation the method makes use of the simplifying assumptions that a horizontal impermeable layer underlies a dupuit aquifer which is drained by a fully penetrating stream the value of the baseflow maximum as well as the baseflow recession hydrograph can be estimated by fitting the analytical solutions of the boussinesq equation to the observed discharge values for individual flood events for the rising limb of the baseflow hydrograph a linear function is assumed for simplicity the method is first demonstrated on three watersheds for a total of five flood events and extensively applied over a three year period for the mahantango creek pennsylvania watershed the proposed method reduces some of the subjective aspects long associated with baseflow separation techniques

a base flow separation algorithm based on the linearized boussinesq equation for complex hillslopes 1 the separation of base flow from discharge records is an important issue in hydrology the objective of this paper is to develop a base flow separation algorithm based on groundwater theory in which the shape of the aquifer is taken into account and for which the parameters can be estimated from river discharge measurements for this purpose an analytical solution to the linearized boussinesq equation for realistic aquifer shapes and temporally variable recharge rates is derived this solution is combined with a mass balance at the land surface in order to construct the base flow separation algorithm the parameters of the base flow filter are calculated on the basis of the same analytical solution comparison of the results of the new algorithm to existing base flow separation methods shows the need for a physical basis for base flow filters

timing of himalayan ultrahigh pressure metamorphism sinking rate and subduction angle of the indian continental crust beneath asia coesite relics were discovered as inclusions in clinopyroxene in eclogite and as inclusions in zircon in felsic and pelitic gneisses from higher himalayan crystalline rocks in the upper kaghan valley north west himalaya the metamorphic peak conditions of the coesite bearing eclogites are estimated to be 27 32 kbar and 700 770 c using garnet pyroxene phengite geobarometry and garnet pyroxene geothermometry respectively cathodoluminescence cl and backscattered electron bse imaging distinguished three different domains in zircon inner detrital core widely spaced euhedral oscillatory zones and thin broadly zoned outermost rims each zircon domain contains a characteristic suite of micrometre sized mineral inclusions which were identified by in situ laser raman microspectroscopy core and mantle domains contain quartz apatite plagioclase muscovite and rutile in contrast the rim domains contain coesite and minor muscovite quartz inclusions were identified in all coesite bearing zircon grains but not coexisting with coesite in the same growth domain rim domain 206 pb 238 u zircon ages reveal that the quartz bearing mantle domains and the coesite bearing rim were formed at c 50 ma and 46 2 0 7 ma respectively these facts demonstrate that the continental materials were buried to 100 km within 7 9 myr after initiation of the india asia collision palaeomagnetic data from the indian oceanic floor supports an initial india asia contact at 55 53 ma combination of the sinking rate of 1 1 1 4 cm year 1 with indian plate velocity of 4 5 cm year 1 suggests that the indian continent subducted to about 100 km depth at an average subduction angle of 14 19

position of the lhasa terrane prior to india asia collision derived from palaeomagnetic inclinations of 53 ma old dykes of the linzhou basin constraints on the age of collision and post collisional shortening within the tibetan plateau the lhasa terrane comprises the southern eurasian margin since at least mid cretaceous therefore paleomagnetic data from the lhasa terrane are crucial to reconstruct the pre collisional southern margin of eurasia previous paleomagnetic data from the cretaceous takena formation provide a rough constraint on the pre collisional paleolatitudinal position of the southern eurasian margin however paleogene paleomagnetic data from the lhasa terrane which can provide more direct constraints are very limited in addition reliability of the existing data is ambiguous because of a possible later overprint in our new study 17 sites were drilled from mafic dykes intruded in the t1 unit of the linzizong formation of the linzhou basin near lhasa for paleomagnetic studies figure 1 the dykes were ar 40 ar 39 dated to c 53 ma by yue and ding 2006 from ten sites a higher coercivity component demagnetized between 20 and 100 mt could be isolated detailed rockmagnetic analyses reveal ti rich titanomagnetite as the remanence carrier which suggests that the rock is not much altered and the remanent magnetization is likely of primary origin this is supported by a positive fold test the occurrence of normal and reverse polarity directions within single sites the subvolcanic character of the dykes and the within site scatter as well as the scatter between site mean directions indicate that paleosecular variation is recorded within sites tilt correction was performed using the bedding of overlying fluvial lacustrine sediments and tilt angles of ignimbrite columns postdating the dyke emplacement bedding corrected directions give an overall mean direction of d i 12 3 27 2 95 10 6 k 21 7 n 10 corresponding to a paleolatitude of 13 2 5 8 n at a reference location on the suture zone 20 n 90 e comparison with previous cretaceous data mainly from the takena formation yields a stable position of the lhasa terrane during cretaceous and early eocene this indicates that no large north south crustal shortening occurred along the southern eurasian margin between late cretaceous and early eocene the difference between expected paleolatitudes determined from the apwp of eurasia and observed paleolatitudes reveals relative northward movement of the lhasa block of ca 1847 763 km since early eocene this is attributed to indentation of india into asia and implies a considerable amount of north south crustal shortening two models of greater india were used to determine the extended northern margin of india ali and aitchison 2005 proposed a greater india based on bathymetric features in the eastern indian ocean and patzelt and others 1996 determined the paleolatitude of northern india using paleomagnetic investigations on paleocene sediments of the zongpu formation at gamba and duela intersection of the pre collisional southern eurasian margin and the northern margin of india yields a collision age of the indian and eurasian continents of c 5349 ma an initial collision within this time range is in accordance with previous studies based on paleomagnetic or geologic data e g patriat and achache 1984 klootwijk and others 1992 guillot and others 2003 a significantly later collision age at the eocene oligocene boundary as proposed by aitchison and others 2007 is not in agreement with our new and previous paleomagnetic data from the lhasa block