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

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

40ar 39ar multigrain dating of detrital white mica of sandstones of the surma group in the sylhet trough bengal basin bangladesh abstract the sylhet trough a subbasin of the bengal basin in northeastern bangladesh is bordered by two orogens the himalaya to the north and the indo burman ranges to the east detrital white micas from six sandstone samples of the neogene subsurface surma group were analysed by the 40ar 39ar method the ages of the individual steps in the ar ar release diagrams range from 25 up to 92 ma these ages represent cooling ages rather than formation ages because they originate from large mica flakes the low temperature step ar ar ages of detrital white mica with plateau type character are diagnostic for the youngest cooling events and cluster between 25 and 36 ma except one age of 51 ma the medium and high temperature steps of the ar release diagrams are characterized by staircase type pattern these various age discordances are interpreted as a mixture of micas with different ages the 40ar 39ar results point to heterogeneous ages of the source region which seems to be typical of the early stages of a rapidly uplifting collision orogen the narrow age range of the low temperature steps can be correlated with early cooling ages known from the high himalayan crystalline basement the ar ar data indicate that the major sediment input in the sylhet trough during sedimentation of the surma group has been derived from the rising himalaya orogen

climatic control on rapid exhumation along the southern himalayan front abstract along the southern himalayan front shf areas with concentrated precipitation coincide with rapid exhumation as indicated by young mineral cooling ages twenty new young 80 of the annual precipitation aft cooling ages reveal a coincidence between rapid erosion and exhumation that is focused in a 50 70 km wide sector of the himalaya rather than encompassing the entire orogen assuming simplified constant exhumation rates the rocks of two age vs elevation transects were exhumed at 1 4 0 2 and 1 1 0 4 mm a with an average cooling rate of 40 50 c ma during pliocene quarternary time following other recently published hypotheses regarding the relation between tectonics and climate in the himalaya we suggest that this concentrated loss of material was accommodated by motion along a back stepping thrust to the south and a normal fault zone to the north as part of an extruding wedge climatically controlled erosional processes focus on this wedge and suggest that climatically controlled surface processes determine tectonic deformation in the internal part of the himalaya

an early pliocene thermal disturbance of the main central thrust central nepal implications for himalayan tectonics since the beginning of the collision between india and asia at about 50 ma the convergence in the himalaya has largely been taken up along major thrust zones in this study samples of the lesser himalaya formations up to 10 km below the main central thrust mct and the greater himalaya sequence up to 12 km above the mct have been analyzed by the 40ar 39ar and u pb methods to investigate the thermal history of the mct the ages can be summarized as follows 1 the youngest ages from muscovites 3 1 ma biotites 3 4 ma and hornblendes 4 1 ma all come from within 1 km of the mct 2 there is a marked asymmetry of ages between the footwall and the hanging wall of the mct the maximum mica age in the hanging wall greater himalaya sequence is 13 ma whereas a muscovite 5 km below the mct in the footwall has an age spectrum with a gradient from 400 to 1400 ma 3 five k feldspars from 6 2 to 11 9 km above the mct all give minimum ages in the range 3 0 6 4 ma and 4 a 206pb 238u age on a brannerite from the greater himalayan sequence is 4 8 ma structural and petrologic observations preclude the possibility that this age distance distribution reflects faulting within the greater himalaya sequence and production of the necessary thermal energy by shear heating requires unrealistically high shear stresses infiltration of hot fluids through the mct zone appears to be the best hypothesis to explain these data simple numerical simulations which account for heat transfer by advection within the fluid flow zone and by conduction outside it indicate that the observed age distribution could have been produced by infiltration of hot fluids through the mct zone at circa 5 to 4 ma within the following range of conditions the fluids heated rocks to temperatures in the range 470 to 510 c for less than 1 million years in a region narrower than the entire mct zone the temperature of the thermal disturbance inferred from the 40ar 39ar data is consistent with petrologic data from albitic alteration assemblages in the greater himalaya sequence and the u pb age of the brannerite is similar to the youngest 40ar 39ar hornblende age this scenario hydrothermal heating culminating at about 4 ma is similar to the model of le fort 1981 for the generation of the high himalayan leucogranites in the late oligocene early miocene by dehydration of the footwall rocks of an active thrust in the younger instance we interpret that fluids derived from the footwall rocks of the main boundary thrust migrated upward through the lesser himalaya formations and were subsequently channeled along the mct producing the thermal disturbance

the tectonostratigraphy and structure of the eastern nepal himalaya six years of geological research in eastern nepal has resulted in a new geological map of the eastern nepal himalaya which includes the region stretching from the sikkhim border in the east to the kathmandu valley in the west and from the summits of the higher himalaya in the north to the ganges plain in the south this research has permitted the determination of the tectonostratigraphy and structure of one section of the central himalayan arc south of the tibetan plateau the eastern nepal himalaya can be divided into three distinct thrust bound tectonic packages 1 the higher himalayan thrust sheet composed of the higher himalayan crystallines 2 the lesser himalayan thrust sheet composed of the lesser himalayan series and 3 the sub himalayan imbricate zone composed of sedimentary rocks belonging to the siwalik group the higher himalayan thrust sheet of eastern nepal has been thrust over the lesser himalayan metasediments a minimum of 140 km and possibly as much as 175 210 km along the main central thrust mct the lesser himalayan thrust sheet is overlain by the mct and is underlain by the main detachment fault mdf and the main boundary thrust mbt out of sequence thrust faults in the hanging wall of the mbt have breached and offset the presently inactive mct the sub himalayan imbricate zone is an emergent imbricate fan bounded by the mbt to the north and the main frontal thrust mft to the south and is underlain by the mdf which lies at a depth of between 5 km and 7 km a balanced cross section constructed across the higher lesser and sub himalaya of eastern nepal shows that the eastern nepal himalayan orogenic wedge has undergone a minimum of between 210 and 280 km of horizontal north south tectonic shortening since the initiation of the mct the lesser and sub himalaya have absorbed 70 km of north south shortening by thrusting along the basal mdf of which the sub himalayan imbricate zone has accommodated 25 km the sun kosi thrust has accommodated about 10 km and the mbt has accommodated the remaining 35 km of shortening since the initiation of the mct between 15 ma and 25 ma shortening across the eastern nepal himalaya has occurred at an average rate of 8 4 18 6 mm per year the structural geometry of the eastern nepal himalaya suggests an overall piggyback sequence of thrusting with motion transferred from the mct to the underlying mdf and its emergent splay thrust the mbt and with the mbt rotated to its present steep orientation by imbricate thrusting within the sub himalaya

metamorphic response to collision in the central himalayan orogen abstract closure of the neotethys ocean and high angle continent continent collision between india and asia after about 55 ma resulted in low angle subduction of the indian plate below the tibetan plateau and by 30 ma established an arcuate 2300 km long shallow north dipping metamorphic fold thrust belt in the foreland this himalayan metamorphic front quickly established an upper plate lower plate paired metamorphic architecture centred on the median high himal thrust that largely controlled subsequent evolution the upper plate is a thick slab of high t moderate p high grade migmatized metamorphic rocks whereas the lower plate is an inverted series of moderate t high p schists in two crustal wedges the broad main central thrust zone and at lowest structural levels the footwall below the basal main central thrust spatial and temporal patterns of metamorphic response in the evolving himalayan metamorphic front has been characterized in a large scale integrated structural metamorphic study based on 8 profiles across eastern nepal metamorphic response at all structural levels is established using a large dataset n 160 of internally consistent quantitative pt determinations petrology of metapelite samples semi quantitative p t paths metamorphic mapping and metamorphic field gradients these results are integrated with previously published metamorphic studies structural profiles and metamorphic chronology from these datasets the architecture and evolution of the himalayan metamorphic front is constrained by rock kinematics metamorphic field gradients showing discontinuities and diachronous metamorphism with contrasting p t evolutions at different structural levels each of the three panels constituting the himalayan metamorphic front upper plate main central thrust zone and footwall experienced distinctly different tectono metamorphic histories crustal processes operating during metamorphism and exhumation differ between the upper and lower plates the upper plate experienced long lived metamorphism starting from at least 28 38 ma and tracking low p t clockwise p t paths that culminated at 19 27 ma in peak high grade condition with 27 31 c km thermal regimes protracted high grade conditions produced significant partial melt which facilitated gravity driven southward extrusion involving internal ductile flow processes southward extrusion of the upper plate was accommodated by coeval reverse movement on the high himal thrust and top down to the north normal reactivation of the south tibet detachment system between 22 10 ma transport of this thick slab to the south resulted in further prograde burial of the lower plate below culminating in peak metamorphism at the highest pressures attained in the lower plate rocks the lower plate consists of at least two deeply buried 7 5 9 4 kb crustal wedges both of which experienced steep p t hairpin clockwise p t paths with isothermal decompression the main central thrust zone experienced 20 26 c km metamorphism at 12 22 ma and was exhumed along the basal main central thrust after 11 ma whereas loading by the main central thrust zone gave rise to further prograde burial of the footwall which experienced 16 21 c km metamorphism at 4 10 ma and was exhumed along the main boundary thrust at 3 9 ma in contrast to the upper plate both crustal wedges were exhumed by a process of out wedging which involved upthrusting along basal and internal thrusts with coeval extensional slip in the hanging walls this three stage foreland propagating lateral exhumation history resulted in telescoping of the himalayan metamorphic front in concert with peak metamorphic events at different structural levels during main phase orogenesis and is not the result of superimposed retrograde reactivation of the belt

testing for the presence of a terrane boundary within neoproterozoic dalradian to cambrian siliceous turbidites at callander perthshire scotland the southern highland group dalradian and keltie water grit formation which includes the lower cambrian leny limestone form an inverted 1 4 km thick largely arenaceous sequence at callander the grits have the same detrital mineralogy throughout mainly quartz plagioclase an 1 3 muscovite and biotite chlorite formed from detrital biotite during low grade regional metamorphism t 40 ar 39 ar laser fusion dating of detrital muscovites yields an age spectrum with a peak at 1600 1800 ma in the dalradian rocks similar old ages occur in the keltie water grits but are diluted by ages of 507 886 ma we interpret these new data as showing that the rocks were most likely deposited as a single sequence possibly with a disconformity in neoproterozoic to early cambrian times before the onset of grampian orogenesis in the early palaeozoic no major structural or stratigraphical breaks have been identified and there is no direct evidence for the presence of two separate terranes

garnet provenance studies juxtaposition of laurentian marginal terranes and timing of the grampian orogeny in scotland a study of the composition and zoning patterns of detrital garnets from ordovician greywackes from the northern belt of the southern uplands terrane of scotland reveals characteristics of the metamorphic sources very similar to the dalradian supergroup of the scottish grampian terrane the radiometric cooling and uplift ages of the dalradian metamorphic zones and the depositional ages of the southern uplands greywackes support the hypothesis for local provenance detrital metamorphic garnet identical to dalradian garnet has also been identified in the upper ordovican sandstones of the highland border complex these observations do not support proposals that the grampian midland valley and southern uplands terranes were exotic to each other in the late ordovician time these new results together with a review of published age dates clarify the late ordovician palaeogeography for this part of the laurentian margin the distances between the grampian midland valley and southern uplands terranes may have been similar to the present day it is concluded that large rivers flowed out of the uplifting mountainous grampian terrane and across the midland valley into a southern uplands trench during the late ordovician time the main orogeny i e mountain building in the grampian terrane was therefore post cambrian producing the first high mountains and resultant flysch in the caradoc

spatiotemporal variation in exhumation of the crystallines in the nw himalaya india constraints from fission track dating analysis abstract during himalayan orogeny coeval thrusting along the main central munsiari thrust mct mt and extension along the south tibetan detachment system stds are widely responsible for rapid exhumation of the higher himalayan crystalline hhc zone apatite and zircon fission track data along the kaliganga and darma valleys in the kumaon himalaya serve to document the shallow bedrock exhumation history of the hhc taking into account sample location within the hhc with respect to the mct mt the apatite fission track aft data sets along the darma 1 0 0 1 to 2 8 0 3 ma and kaliganga 1 4 0 2 to 2 4 0 3 ma which are sharing same structural setting and rock types and being separated by 40 km show very similar patterns of exhumation histories since plio quaternary in the kumaon himalaya data sets along darma and kaliganga are very similar to data set of adjacent traverse 50 km away along the goriganga valley studied by patel and carter 2009 whole data sets within the hhc in kumaon himalaya provide clear evidence for plio quaternary tectonic activity along the vaikrita thrust vt precipitation in this region exerts a strong influence on erosional surface processes fluvial erosional unloading along the himalaya is focused on the high mountainous region of the hhc where the orographic barrier forces out the maximum percentage of annual rainfall ft cooling ages reveal coincidence between rapid erosion and exhumation that is focused in a 25 30 km wide sector of the hhc rather than covering the entire orogen similarity of aft age pattern and exhumation rates along all three major traverses goriganga darma and kaliganga indicates that the region has been experiencing constant rate of crustal uplift and erosion since long time comparison of fission track ages from the kumaon himalaya with other segments of the nw himalaya shows spatiotemporal variation in exhumation it is described due to the development of local structures such as dome window in jammu and kashmir and himachal himalayas and quaternary active thrusting along the vt and mct mt in the garhwal kumaon himalaya

post collisional rapid exhumation and erosion during continental sedimentation the example of the late variscan salvan dorenaz basin western alps the salvan dorenaz intramontane basin formed between ca 308 293 ma as an asymmetric graben along crustal scale transtensional fracture zones within the aiguilles rouges crystalline massif western alps and represents a feature of the post collisional evolution of the variscan orogens it contains 1 5 1 7 km of continental clastic deposits which were eroded from granitic volcanic and metamorphic rocks textural and compositional immaturity of the sandstones and the numerous lithic fragments with low chemical and physical stability suggest only short range transport 40ar 39ar analyses of detrital muscovite are interpreted to represent cooling of the crystalline basement below the respective closure temperatures ages from detrital muscovite range between ca 280 330 ma 40ar 39ar white mica plateau ages from granitic boulders range between 301 312 ma and suggest rapid cooling the very short time interval recorded between the 40ar 39ar cooling ages and the stratigraphic age of the host sediment suggests that considerable portions of the upper crust were removed prior to the formation of the basin late variscan granitic boulders document surface exposure and erosion of late carboniferous granites during early stages of the infilling of the basin therefore unroofing of basement units magmatic activity and formation of the fault bounded salvan dorenaz basin were acting concomitantly and are highly suggestive of extensional tectonics when compared with other orogens this situation seems specific to the variscan especially the exclusively young ages of detrital material however modern analogous may exist

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

a method for evaluating basement exhumation histories from closure age distributions of detrital minerals we have developed a two dimensional thermokinetic model that predicts the closure age distributions of detrital minerals from pervasively intruded and differentially exhumed basement using this model we outline a method to determine the denudation history of orogenic regions on the basis of closure age distributions in synorogenic to postorogenic forearc strata at relatively high mean denudation rates of 0 5 km m y 1 sustained over millions of years magmatic heating events have minimal influence upon the age distributions of detrital minerals such as k feldspar that are moderately retentive of radiogenic ar at lower rates however the effects of batholith emplacement may be substantial we have applied the approach to detrital k feldspars from forearc strata derived from the deeply denuded peninsular ranges batholith prb agreement of the denudation history deduced from the detrital k feldspar data with thermochronologic constraints from exposed prb basement lead us to conclude that exhumation histories of magmatic arcs should be decipherable solely from closure age distributions of detrital minerals whose depositional age is known c 1999 american geophysical union

detrital muscovite 40ar 39ar ages from carboniferous sandstones of the british isles provenance and implications for the uplift history of orogenic belts major progradations of clastic sediments are recorded in the sedimentary record of the famennian and the visean namurian of the united kingdom and surrounding waters we have determined 40ar 39ar ages of 162 detrital muscovites from 11 coarse sandstones which were deposited between 370 and 465 ma spanning both progradations detrital mica ages are dominated by a peak at 415 ma with minor peaks at 440 ma and 390 ma the 415 ma muscovites are derived from the unroofing of the scandian nappes during the compressional phase of the caledonian orogeny in scandinavia the 440 ma muscovites record pre scandian orogenic activity which is rarely preserved in the orogenic record thermochronological evidence suggests that episodic postorogenic uplift and exhumation events kept the scandian orogen a major topographic feature and likely a sediment source for over 100 million years after nappe emplacement and implicates tectonic rather than climatic control on the clastic sediment progradations the near total absence of detrital muscovites with ages 415 ma suggests that the scandian nappes had not been entirely eroded despite repeated uplift during the postorogenic extension the river s which supplied the sediments probably ran parallel to the strike of the major scandian thrusts along the length of the caledonian orogen in a manner analogous to the major river systems of contemporary orogenic highlands

p wave velocity tomography from local earthquakes in western mexico in western mexico the subduction of the rivera and cocos plates beneath the north america plate has deformed and fragmented the overriding plate forming several structural rifts and crustal blocks to obtain a reliable subsurface image of the continental crust and uppermost mantle in this complex area we used p wave arrivals of local earthquakes along with the fast marching method tomography technique we followed an inversion scheme consisting of 1 the use of a high quality earthquake catalog and corrected phase picks 2 the selection of earthquakes using a maximum location error threshold 3 the estimation of an improved 1 d reference velocity model and 4 the use of checkerboard testing to determine the optimum configuration of the velocity nodes and inversion parameters surprisingly the tomography results show a very simple vp distribution that can be described as being controlled by geologic structures formed during two stages of the separation of the rivera and cocos plates the earlier period represents the initial stages of the separation of the rivera and cocos plates beneath western mexico the later period represents the more advanced stage of rifting where the rivera and cocos plates had separated sufficiently to allow melt to accumulate below the colima volcanic complex during the earlier period 14 or 10 1 6 ma ne sw oriented structures lineaments such as the southern colima rift were formed as the two plates separated during the second period 1 6 ma to the present the deformation is attributed to magma generated within and above the tear zone between the rivera and cocos plates rising beneath the region of the colima volcanic complex the rising magma fractured the overlying crust forming a classic triple rift junction geometry this triple rift system is confined to the mid to lower crust perhaps indicating that this rifting process is still in an early stage this fracturing along with fluid circulation and associated heat advection within the fractures can easily explain the observed distribution of vp as well as many of the results of previous seismological studies also surprisingly we find no evidence at deep crustal depths to support either a trenchward migration of the volcanic arc or toroidal asthenospheric flow through the slab tears bounding the jalisco block to the nw and se

links between orogenic wedge deformation and erosional exhumation evidence from illite age analysis of fault rock and detrital thermochronology of syn tectonic conglomerates in the spanish pyrenees abstract we present new geochronologic data illuminating the tectonic and erosional history of the orogenic wedge exposed in the south central pyrenees spain we interpret illite age analyses from four fault gouges that record thrust belt development and document the importance of out of sequence thrusting fault activity occurs in pulses with slip occurring contemporaneously on multiple faults throughout the wedge new apatite fission track data from syn orogenic sediments of the sis conglomerate body reveal ages of 48 to 42 ma with no consistent variation upsection in strata deposited from 41 to 30 ma we interpret these data as well as thermal modeling of track length distributions to imply rapid cooling in the interior of the pyrenean wedge during the middle eocene the record of fault activity and erosion suggests that orogenic wedges may not evolve in a steady fashion but generally exhibit significant changes in rates of deformation and exhumation the observed correlation in the timing of tectonism and erosional exhumation provides evidence for links between tectonic and surface processes

uniform permian 40ar 39ar detrital mica ages in the eastern qaidam basin nw china where is the source new single grain 40ar 39ar detrital white mica ages from the lulehe section at the eastern qaidam basin yield uniform permian ages between 250 3 and 279 3 ma throughout the whole cenozoic sequence this is inconsistent with the present hinterland which is composed of early palaeozoic metamorphic units with subordinate early palaeozoic and few permian granites the new data indicate that permian tectonic units are likely more widespread at the north eastern margin of the tibetan plateau as known at present particularly within the qilian mountains the preferred explanation is that the qaidam block represents a rigid indenter which indented during late tertiary times into early palaeozoic orogenic units this is consistent with recent findings of a nw trending sinistral permian ductile shear zone and a dextral nw trending tertiary fault system close to the north eastern margin of the qaidam basin

sr isotope evolution of seawater the role of tectonics we use a high resolution seawater sr isotopic evolution curve for the last 100 m y in conjunction with modern riverine sr flux measurements and also geologic tectonic and geochronological data to make the case for a close relationship between seawater sr isotopic composition and the india asia continental collision using a simple seawater sr budget model we begin by showing that the sr flux associated with alteration of seafloor basalts is too small and does not have the right time evolution to account for much of the seawater sr isotopic curve of the last 100 m y the flux of dissolved sr carried by rivers originating in the himalaya tibet region on the other hand is presently a significant fraction of the global sr budget we calculate how this riverine flux would have had to change with time in order to match the observed seawater sr isotopic curve and find that the riverine flux remains relatively constant prior to the collision of india with asia but then increases very significantly after collision we note that the period of most rapid change in seawater sr isotopic ratio from 20 ma to 15 ma is also a period of exceptionally high erosion in parts of the himalayas and the tibetan plateau as further evidence that sr derived from the collision of india with asia plays a major role in the sr isotopic evolution of seawater we show that the total amount erosion of the himalaya tibetan plateau since collision which we calculate separately represents a total amount of sr that is very nearly the same as the cumulative amount required by the sr isotopic change of seawater since collision the relationship between erosion and riverine sr flux allows us to use the sr isotopic evolution of seawater to reconstruct a history of erosion since collision and we find that the erosion rate accelerates with time since collision with the present having the largest rate when we apply the sr budget model to the entire phanerozoic using a new compilation of deformed continental area versus time we find that we can account for the large scale structure of the seawater sr isotopic curve but fail to reproduce several local maxima and minima especially in the period 100 300 ma the present high s7sr s6sr of seawater and similar highs in the devonian and cambrian do correlate with extensive deformation on the continents

variations in the strontium isotopic ratio of seawater during the miocene stratigraphic and geochemical implications a composite strontium isotopic seawater curve was constructed for the miocene between 24 and 6 ma by combining 87sr 86sr measurements of planktonic foraminifera from deep sea drilling project sites 289 and 588 site 289 with its virtually continuous sedimentary record and high sedimentation rates 26 m m y was used for constructing the oligocene to mid miocene part of the record which included the calibration of 63 biostratigraphic datums to the sr seawater curve using the timescale of cande and kent 1992 across the oligocene miocene boundary a brief plateau occurred in the sr seawater curve 87sr 86sr values averaged 0 70824 which is coincident with a carbon isotopic maximum cm o m from 24 3 to 22 6 ma during the early miocene the strontium isotopic curve was marked by a steep rise in 87sr 86sr that included a break in slope near 19 ma the rate of growth was about 60 ppm m y between 22 5 and 19 0 ma and increased to over 80 ppm m y between 19 0 and 16 ma beginning at 16 ma between carbon isotopic maxima cm3 and cm4 of woodruff and savin 1991 the rate of 87sr 86sr growth slowed and 87sr 86sr values were near constant from 15 to 13 ma after 13 ma growth in 87sr 86sr resumed and continued until 9 ma when the rate of 87sr 86sr growth decreased to zero once again the entire miocene seawater curve can be described by a high order function and the first derivative d87sr 86sr dt of this function reveals two periods of increased slope the greatest rate of 87sr 86sr change occurred during the early miocene between 20 and 16 ma and a smaller but distinct period of increased slope also occurred during the late miocene between 12 and 9 ma these periods of steepened slope coincide with major phases of uplift and denudation of the himalayan tibetan plateau region supporting previous interpretations that the primary control on seawater 87sr 86sr during the miocene was related to the collision of india and asia the rapid increase in 87sr 86sr values during the early miocene from 20 to 16 ma imply high rates of chemical weathering and dissolved riverine fluxes to the oceans in the absence of another source of co2 these high rates of chemical weathering should have quickly resulted in a drawdown of atmospheric co2 and climatic cooling through a reversed greenhouse effect the paleoclimatic record however indicates a warming trend during the early miocene culminating in a climatic optimum between 17 and 14 5 ma we suggest that the high rates of chemical erosion and warm temperatures during the climatic optimum were caused by an increase in the contribution of volcanic co2 from the eruption of the columbia river flood basalts crfb between 17 and 15 ma the decrease in the rate of crfb eruptions at 15 ma and the removal of atmospheric carbon dioxide by increased organic carbon burial in monterey deposits eventually led to cooling and increased glaciation between 14 5 and 13 ma the crfb hypothesis helps to explain the significant time lag between the onset of increased rates of organic carbon burial in the monterey at 17 5 ma as marked by increased 13c values and the climatic cooling and glaciation during the middle miocene as marked by the increase in 18o values which did not begin until 14 5 ma

provenance of oligocene synorogenic sediments of the ligurian alps nw italy inferences on belt age and cooling history mineral chemistry 40ar 39ar geochronology on white micas and apatite fission track thermochronology aftt are applied here to study the provenance of the synorogenic molare formation lowermost unit of the tertiary piedmont basin clastic sequence the molare formation was deposited during transgression onto the ligurian alps nappe stack in the early oligocene depositional facies show that clastic distribution remained transversal with local sources located just landward from the coastline phengite mineral chemistry together with 40ar 39ar data clearly shows two distinctive source areas each one mirroring the composition of the basement directly beneath the clastic sequence amphibole mineral chemistry allows second order provenance distinctions within each sector reflecting heterogeneous metamorphic evolution of the bedrock complexes integrated 40ar 39ar dating and aftt suggest that following a fast cooling exhumation episode of the ligurian alps during the oligocene very little net uplift has since occurred this is due to a period of general subsidence from the oligocene late miocene followed by comparable uplift from late miocene pliocene to the present in general our data provide an image of the ligurian alps during the oligocene which is very similar to the present day one

the thakkhola mustang graben in nepal and the late cenozoic extension in the higher himalayas abstract the thakkhola mustang graben is located at the northern side of the dhaulagiri and annapurna ranges in north central nepal the structural pattern is mainly characterised by the n020 040 thakkhola fault system responsible for the development of the half graben a detailed study of the substrate and the sedimentary fill in several outcrops indicates polyphased faulting pre sedimentation faulting miocene with a mainly nnw sse to n s compressional stress expressed in the substratum by n020 040 and n180 n010 sinistral and n130 140 dextral conjugate strike slip faults syn sedimentation faulting pliocene pleistocene characterised by a w e to wnw ese extensional stress and tectonic subsidence of the half graben during the tetang period pliocene probably followed by a doming of the tetang deposits and a short period of erosion cf pliocene planation surface and unconformity between the tetang and thakkhola formations the thakkhola period pleistocene is characterized by a w e to wnw ese extensional stress and a major subsidence of the half graben post sedimentation recurrent extensional faulting and n s and ne sw normal faults in the late quaternary terrace formations geodynamic interpretation of the faulting is discussed in relation to the following 1 the geographic situation of the thakkhola mustang half graben in the southern part of tibet and its setting in the tethyan series above the south tibetan detachment system stds 2 the geodynamic conditions of the convergence between india and eurasia and the dextral east west shearing between the high himalayas and south tibet 3 the possible relations between the sinistral thakkhola and the dextral karakorum strike slip faults in a n s compressional stress regime during the miocene

the crustal assembly of southern mongolia new structural lithological and geochronological data from the nemegt and altan ranges abstract the gobi altai region is an ideal setting for studying processes of continental growth and subsequent intracontinental and intraplate deformation including terrane accretion and dispersal ophiolite obduction crustal reactivation and intraplate mountain building to assess the diverse tectonic evolutionary models of the gobi altai and the wider region more field data and geochronological data are required to constrain the tectonic evolution of individual terranes and the relationship of adjacent crustal domains to each other throughout time in this paper we present new lithological structural and 40ar 39ar age data which constrain the crustal evolution across a previously unreported late paleozoic terrane boundary in the gobi altai nemegt and altan nuruu are topographically linked mountain ranges that were formed by miocene recent uplift at a right stepping restraining bend along the left lateral gobi tien shan fault system in southern mongolia ordovician carboniferous arc rocks and an ophiolite are exposed in the mountain ranges and form a small part of the east west arcuate trans altai zone field observations of rock types and structures combined with petrographic data are used to distinguish metamorphosed volcano sedimentary arc rocks in altan nuruu and western nemegt nuruu from arc rocks in central and eastern nemegt nuruu these distinct sequences are correlated with the dzolen and edrengin terranes in the trans altai zone along strike to the west integration of field data 40ar 39ar age data and published studies are used to describe a polyphase deformation history that includes late carboniferous ophiolite obduction mid permian to late triassic shortening and lateral terrane redistribution cretaceous rifting and late cenozoic intraplate mountain building

rapid exhumation since at least 13 ma in the himalaya recorded by detrital apatite fission track dating of bengal fan iodp expedition 354 and modern himalayan river sediments abstract apatite fission track analysis of middle bengal fan sediments iodp expedition 354 and modern himalayan river sediments shows that most of the detrital apatites are very young compared to their depositional ages independent of their uranium content bengal fan apatites display an average central age lag time as short as 2 26 1 6 myr since at least 13 ma such lag times reflect a mean exhumation rate on the order of at least 1 3 km myr the occurrence of detrital apatites with relatively short aft lag times since at least 13 ma indicates that there have always been areas of rapid erosional exhumation supplying detrital apatites to the fluvial system and delivering them to the paleo ganges and or brahmaputra plains and finally to the bengal fan it also supports that temporary storage of detrital apatites in the floodplains or delta has always been negligible since at least 13 ma comparison of the aft data of the bengal fan with those of the central and eastern proximal neogene himalayan foreland basin shows that both paleo ganga and brahmaputra catchments provided apatites with similar short lag time to the distal bengal fan basin in the modern drainage system of the bengal fan the apatites with young fission track cooling ages are principally derived from areas where the topography has a sharp relief controlled by threshold hillslope processes and stream power resulting in landslide erosion as a coupled response to tectonic and fluvial forcing by analogy with the modern erosion processes in the himalayan range we suggest that over the past 13 ma apatites were mainly derived from areas of sharp relief where river stream power was high and hill slopes close to the threshold angle as the exhumation signal is rather consistent since the late miocene the detrital apatite fission track data are either not sensitive enough to detect rapid climatically controlled changes in exhumation rates or overall long term erosion rates on the orogen scale are not strongly affected by climatic variations such as the variability of the indian summer monsoon given the already rapid exhumation rates controlled by tectonics the impact of climate variability on surface erosion rates cannot be detected with our data especially in the case of erosion processes dominated by threshold hillslope model

the tectonic evolution of western central iran seen through detrital white mica abstract a first order survey of 40ar 39ar dating of detrital white mica from jurassic to pliocene sandstones has been carried out in order to reveal the tectonic evolution of blocks in central iran the central iran block was believed to represent a stable precambrian block our results indicate that 1 only a very small proportion of precambrian but abundant paleozoic and mesozoic detrital white mica indicate the phanerozoic mostly mesozoic age of metamorphic crust exposed in central iran the oldest but scarce detrital white mica grains have ages ranging from 524 to 826 ma heralding a late precambrian and cambrian crystalline basement or cannibalism from older clastic successions 2 jurassic and cretaceous sandstones from the west and east of the chapedony fault yield different age spectra with a dominance of variscan ages ca 308 385 ma in the biabanak unit west of the chapedony fault compared to coeval sandstones from the block east of the chapedony fault where variscan ages are subordinate and cimmerian ages predominate the micas from the biabanak unit are most likely derived from the variscan accretionary complex exposed in the anarak jandaq areas further northwest this result underlines the importance of a major block boundary identified as the chapedony fault which is in extension of a fault previously proposed 3 two stages of cimmerian events are visible in our data set from cretaceous and paleogene sandstones a cluster around 170 ma and at ca 205 ma these clusters suggest a two stage cimmerian evolution of the largely amphibolite grade metamorphic posht e badam and boneh shurow complexes 4 the youngest micas in paleogene conglomerates have an age of ca 100 ma and are most likely derived from the base of the posht e badam complex no record of the uplifted eocene chapedony metamorphic core complex has been found in eocene and pliocene clastic rocks

valley incision and the uplift of mountain peaks simple end member models describe the effect of isostatically compensated valley incision on the elevation of mountain peaks these models allow calculation of the maximum portion of mountain peak elevations attributable to the development of observed relief and predict general downstream patterns of ridgetop uplift in response to erosion by fluvial processes analyses of topographic profiles across major drainages of the central sierra nevada and the tibetan plateau indicate that isostatically compensated valley incision could account for at most 5 10 of the present elevation of mountain peaks in contrast analyses of topographic profiles across the himalaya show that as much as 20 30 of the present elevation of himalayan peaks could be explained by isostatically compensated valley incision the degree to which valley incision is compensated locally however depends on the flexural rigidity of the lithosphere the size of the range and its tectonic boundary conditions these results show that while a tectonic control is required for late cenozoic surface uplift of the sierra nevada a significant portion of the elevation of himalayan peaks may reflect late cenozoic valley incision on the margin of the tibetan plateau

neogene syn tectonic sedimentation in the eastern margin of arakan bengal basins and its implications on for the indian asian collision in western myanmar abstract the arakan basin is one of the major sedimentary basins formed in the frontal part of the himalayan orogenic belt since the late cenozoic defining one of the four major sedimentary basins of myanmar it is geomorphologically and tectonically differentiated from the others the study area along the westernmost edge of myanmar is separated from the arakan yoma indo burman ranges by a narrow coastal strip and is bordered by the bay of bengal to the west regional stratigraphic correlation and the geological age of the siliciclastic sequences were established based on planktonic foraminiferal zonation deep marine slope and shelf environments during early to middle miocene ca 21 5 11 ma and a southward prograded shelf delta environment during late miocene to pliocene time were determined the early miocene underthrusting along the himalayan front is well documented by the forced regressive sedimentation patterns in the slope and shelf systems sediments of which derived from the paleo ganges brahmaputra river systems in the bengal arakan basins sequential evolution of the miocene successions manifests in forced regressive wedged systems tracts these evolved through slope by passing and slumping and following deep marine channel in filling began to accumulate an increasing sediment load due to the rapid fall of sea level by the uplift in the hinterlands during the early to early middle miocene the formation of a shelf delta systems marks a dramatic shift in the evolution of the southward prograding delta systems following a eustatic sea level low in the foreland areas erosional off loading with foreland uplifting caused a wide active fluvial systems and formed transverse rivers distally in the late middle to late miocene

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

evidence for the multiphase nature of the india asia collision from the yarlung tsangpo suture zone tibet recent investigations in southern tibet enable the testing and refinement of existing models for india asia collision presently available data indicate that marine deposition continued in the southern central portion of tibet until at least the end of the eocene subduction related magmatism continued until the mid oligocene after which rapid uplift of the plateau was initiated mass wasting of sediments into molasse basins did not commence until the latest oligocene the implications are that existing models based on less precise age constraints invoking india asia collision at 55 ma are either flawed or collision began at a different time recent work has produced sufficient data to allow the recognition of two different collisional events along the suture between india and asia features related to each event require separate interpretation and no collisional continuum should be assumed in southern tibet a collision between the northern margin of india and a south facing intra oceanic island arc occurred at around 55 ma whereas continent continent collision between india and asia did not occur until at least 20 million years later

detrital zircon fission track ages from the lower cenozoic sediments nw himalayan foreland basin clues for exhumation and denudation of the himalaya during the india asia collision detrital zircon fission track ft ages from the lower cenozoic sub himalayan foreland basin reflect the progressive effects of crustal thickening and exhumation on the himalayan source rocks as a consequence of the india asia collision the oldest stratum the transgressive marine paleocene eocene subathu formation 57 41 5 ma contains ca 50 ma detrital zircon p1 peak which was derived from the indus tsangpo suture zone and the ladakh batholith of the asian plate a dominant 302 4 21 9 ma peak with a few 520 ma grains in this formation has been derived by erosion of the zircon partial annealing zone zpaz of 240 180 c as the first imprint of the collision this zone affected the himalayan proterozoic basement and its tethyan sedimentary cover since the detritus in the subathu has been derived both from the indian and asian plates the possible suturing of these plates took place during the subathu sedimentation a sudden change in the provenance is recorded in the detrital zircon ft cooling ages in the oligo miocene dagshai and kasauli formations which have dominant 30 and 25 ma p1 peaks respectively we interpret a distinct unconformity spanning 10 m y between the subathu and dagshai formations since ca 30 ma molassic sedimentation coincides with shifting of the source rocks to the himalayan metamorphic belt this belt has sequentially undergone three distinct cooling and exhumation pulses after the ultrahigh pressure high pressure uhp hp metamorphism 53 50 ma in the extreme north and two widespread m1 and m2 metamorphisms 40 30 and 25 15 ma in the middle parts these events appear to be largely responsible for the deposition of the ca 30 ma zircon himalayan peak and ca 25 and 15 ma young himalayan peaks respectively the latter appears within the lower siwalik subgroup 13 11 ma during the lower siwalik deposition pre himalayan peaks gradually decrease with the intensification of the himalayan events in source rocks in spite of uninterrupted fluvial sedimentation in the dagshai kasauli lower siwalik sequences since 30 ma breaks of 5 7 m y in the zircon ft ages reveal pulsative cooling and exhumation in the well identified source areas although cooling and exhumation of the himalayan source rocks remained almost uniform during the eocene source heterogeneity is reflected in fluvial sedimentation since 37 ma from pakistan to nepal in response to the india asia collision

tracing patterns of erosion and drainage in the paleogene himalaya through ion probe pb isotope analysis of detrital k feldspars in the indus molasse india abstract the indus molasse is a pre and syn tectonic sedimentary sequence situated in the indus suture zone of the western himalaya spanning in time the collision of india and asia this deposit is well placed to record the evolving uplift and erosion history of the early himalayan orogen nd isotope analyses from clay extracted from shales interbedded within the dominantly alluvial sequence indicate a low negative nd 1 64 to 0 72 in the basal paleocene chogdo formation slightly more negative than measured values from the transhimalaya and kohistan dras arc up section nd becomes more negative as low as 10 05 indicating influence of a different more enriched source ion microprobe pb isotopic analyses of single k feldspars help constrain this source as being either the lhasa or karakoram blocks with westward paleo current flow favoring the former 207pb 204pb ratios are too low to be consistent with known indian plate sources a conclusion supported by the lack of muscovite or garnet that would be indicative of a high himalayan contribution given the known age of rapid cooling of the high himalaya at 20 ma and the lack of exposure of suitable lithologies prior to that time an age of sedimentation prior to 20 ma is inferred the post collisional change in paleo flow and provenance is suggested to reflect the initiation of the indus river during the early eocene this study demonstrates the power of combined bulk sediment and single grain analyses in resolving provenance in tectonically complex settings

evolution of miocene fluvial environments eastern potwar plateau northern pakistan the miocene pliocene siwalik group records changing fluvial environments in the himalayan foreland basin the nagri and dhok pathan formations of this group in the eastern potwar plateau northern pakistan comprise relatively thick tens of metres sandstone bodies and mudstones that contain thinner sandstone bodies metres thick and palaeosols thick sandstone bodies extend for kilometres normal to palaeoflow and are composed of large scale stratasets storeys stacked laterally and vertically adjacent to each other sandstone bodies represent single or superimposed braided channel belts and large scale stratasets represent channel bars and fills channel belts had widths of km bankfull discharges on the order of 103 cumecs and braiding parameter up to about 3 individual channel segments had bankfull widths maximum depths and slopes on the order of 102 m 101 m and 10 4 respectively and sinuosities around 1 1 these rivers are comparable to many of those flowing over the megafans of the modern indo gangetic basin and a similar depositional setting is likely thin sandstone bodies within mudstone sequences extend laterally for on the order of 102 m and have lobe wedge sheet and channel form geometries they represent crevasse splays levees and floodplain channels mudstones are relatively bioturbated disrupted and represent mainly floodbasin and lacustrine deposition mudstones and sandstones are extremely disrupted in places showing evidence of prolonged pedogenesis these mature palaeosols are m thick and extend laterally for km lateral and vertical variations in the nature of their horizons apparently depend mainly on deposition rate the 500 m thick nagri formation has a greater proportion and thicker sandstone bodies than the overlying 700 m thick dhok pathan formation the thick sandstone bodies and their large scale stratasets thicken and coarsen through the nagri formation then thin and fine at the base of the dhok pathan formation compacted deposition rates increase with sandstone proportion 0 53 mm year for nagri 0 24 mm year for dhok pathan and palaeosols are not as well developed where deposition rates are high within both formations there are 100 m scale variations representing on the order of 105 years in the proportion and thickness of thick sandstone bodies and tens of m scale alternations of thick sandstone bodies and mudstone sandstone strata that represent on the order of 104 years formation scale stratal variations extend across the potwar plateau for at least 100 km although they may be diachronous however 100 m and smaller scale variations can only be traced laterally for up to tens of km alluvial architecture models indicate that increases in the proportion and thickness of thick sandstone bodies can be explained by increasing channel belt sizes mainly average deposition rate and avulsion frequency on a megafan comparable in size to modern examples 100 m scale variations in thick sandstone body proportion and thickness could result from regional shifts in the position of major channels possibly associated with fan lobes on a single megafan or with separate megafans however such variations could also be related to local changes in subsidence rate or changes in sediment supply to the megafan system formation scale and 100 m scale stratal variations are probably associated with interelated changes in tectonic uplift sediment supply and basin subsidence increased rates of hinterland uplift sediment supply and basin subsidence recorded by the nagri formation may have resulted in diversion of a relatively large river to the area alternatively changing river sizes and sediment supply rates may be related to climate changes affecting the hinterland possibly linked to tectonic uplift climate during deposition of the siwalik group was monsoonal although the deposits contain no direct evidence for climate change independent evidence indicates global cooling throughout the miocene and the possibility of glacial periods e g around 10 8 ma corresponding to base of nagri formation if the higher himalayas were periodically glaciated a mechanism would exist for varying sediment supply to megafans on time scales of 104 105 years although eustatic sea level changes are related to global climatic change they are not directly related to siwalik stratigraphic changes because the shoreline was many 100 km away during the miocene

the early stages of the alpine collision an image derived from the upper eocene lower oligocene record in the alps apennines junction area the upper eocene lower oligocene sediments deposited in the eastern part of the tertiary piedmont basin in northern italy provide a complete record of the unroofing of the alpine orogenic prism during the early stages of exhumation in the ligurian sector from late priabonian till late rupelian time the sediments in the study area were derived from two different sources one characterised by white micas with sib6 5 pfu and permian 40 ar 39 ar ages 270 ma and the other characterised by white micas with sin7 pfu and eocene oligocene 40 ar 39 ar ages 32 50 ma the first source is considered to be indicative of low pressure metamorphic rocks that covered the hp rocks of the ligurian alps and were completely eroded by chattian time from this time on the study area started to record the first input from western alpine sources characterised by a larger span of ages with a more frequent eoalpine signal thus sediments deposited in the eastern part of the tertiary piedmont basin contain the only available evidence of rocks belonging to high crustal levels in the alpine orogenic prism that were not affected by the alpine overprint these data also provide time constraints to the poorly dated first conglomerates deposited in this area 40 ar 39 ar geochronology reveals a minimum age of 33f1 4 ma for the pianfolco conglomerates in the type locality and of 31 4f3 5 ma for the borbera conglomerates d 2004 elsevier b v all rights reserved

unroofing of the himalayas a view from apatite fission track analysis of bengal fan sediments apatite fission track analysis of early miocene to quaternary distal bengal fan sediments from odp leg 116 provides a unique perspective on denudation processes associated with the tertiary himalayan orogeny these sediments yield pooled apatite fission track ages that are 0 to 10 m y older than depositional ages mean track lengths range from 13 2 to 14 8 mu m these young ages and long track lengths indicate that rocks in the upper crust of bengal fan source areas cooled rapidly ge 15 degrees c m y implying denudation rates of several hundreds of meters per million years these observations support the hypothesis that nearly continuous unroofing of a large source area similar to the dimensions of the present himalayan and southern tibetan plateau deformation front and an efficient transport system similar to the present himalaya ganges brahmaputra system supplied sediment to the bengal fan since the early miocene

volume balance of erosional loss and sediment deposition related to himalayan uplifts the volume of sediment in the foreland basins deltas and fans which are a consequence of the collision of india and asia appears to require the removal of a vertical section c 35km high if the sediments were entirely derived from the himalaya it is argued that this is implausible and that it is likely that the himalayan hinterland e g karakoram and tibet was an important source of sediment prior to c 20 ma when significant himalayan uplift commenced

distorted drainage basins as markers of crustal strain east of the himalaya extensive lithospheric deformation on the eastern margin of the india eurasia collision has brought three major rivers into exceptionally close proximity for over 300 km the salween mekong and yangtze rivers are only tens of kilometers apart 10 times closer than rivers of comparable length elsewhere in the world we view deeply incised river valleys as displacement markers that move with the underlying crust over time scales of at least tens of millions of years accordingly the close spacing of these major rivers and their nearly parallel trends reflect the large crustal strains near the eastern himalayan syntaxis these river characteristics are consistent with two primary phases of deformation in this tectonic setting ne sw shortening of 0 5 oriented radially from the eastern syntaxis followed by dextral shear strain of 6 1 along a north trending zone pervasive shearing of this magnitude throughout the 150 km wide region of anomalous drainage would account for 1000 km of northward motion of india relative to southern china directly east of this region another 1000 km of northward motion is likely to have been accommodated by pervasive shearing through a zone hundreds of kilometers wide finally we draw attention to the remarkable accord between the arcuate paths of each of the three great rivers in the broader southeastern tibet region and small circles that curve around the eastern himalayan syntaxis

two stage erosion and deposition in a continental margin setting an 40ar 39ar laserprobe study of offshore detrital white micas in the norwegian sea detrital white micas from a restricted region of the norwegian sea have been analysed by the 40ar 39ar laserprobe examining the ages of individual white mica grains from offshore triassic jurassic and cretaceous sandstones reveals a narrow range of detrital white mica ages 369 424 ma moreover there is an inverse relationship between sediment age and detrital white mica age with the triassic sands containing a larger proportion of younger white micas and the cretaceous sands containing the oldest white micas in addition residence times of the detritus which is the age of the sediment subtracted from the age of the detrital white mica range from c 150 to c 300 ma for the triassic and cretaceous sands respectively the inverse age relationships combined with the large residence times for these stratigraphic levels can best be explained by an erosion deposition model involving at least two stages taking into account previous provenance studies in the lofoten region of the norwegian sea these new data suggest that detritus shed initially from the exhuming caledonian nappes of nw norway in late silurian early devonian times was deposited in intramontane basins that were themselves subsequently unroofed and re eroded from permo triassic times this caused the recycling of the caledonian detritus from the devono carboniferous basins and led to the inverted detrital age distribution ovserved in the triassic cretaceous sediments examined in this study while sedimentary recycling is not a surprising feature for mesozoic basins in the north and norwegian seas this study documents a previously untapped method 40ar 39ar laserprobe provenance studies by which to identify its effect and extent

petrogenesis of the eocene and mio pliocene alkaline basaltic magmatism in meseta chile chico southern patagonia chile evidence for the participation of two slab windows the meseta chile chico mcc 46 48s is the westernmost exposure of eocene lower basaltic sequence lbs 55 40 ma k ar ages and mio pliocene upper basaltic sequence ubs 16 4 ma k ar ages flood basalt volcanism in patagonia the mcc is located south of the lago general carrera buenos aires lgcba southeast from the present day chile triple junction ctj east of the actual volcanic gap between southern south volcanic zone and austral volcanic zone ssvz and avz respectively and just above the inferred location of the south chile ridge segment subducted at 6 ma scr 1 erupted products consist of mainly ne normative olivine basalt with minor hy normative tholeiites basalt trachybasalt and basanite mcc lavas are alkaline 42 7 53 1 wt sio2 3 8 wt na2o k2o and relatively primitive ni 133 360 ppm cr 161 193 ppm co 35 72 ppm 4 16 5 mgo wt they have a marked oib like signature as shown by their isotopic compositions 87 sr 86 sro 0 70311 0 70414 and qnd 4 7 5 1 and their incompatible trace elements ratios ba la 10 20 la nb 0 46 1 09 ce pb 15 52 27 5 sr lab25 reflecting deep mantle origin ubs primitive lavas have characteristics similar to those of the eocene lbs basalts while ubs intermediate lavas show geochemical imprints la nbn1 sr lan25 low ce pb nb u compatible with contamination by arc slab derived and or crustal components we propose that the genesis and extrusion of

precise tracing of exhumation and provenance using 40ar 39ar geochronology of detrital white mica the example of the central alps

tibet uplift and erosion abstract the 5 km high tibetan plateau is an outstanding topographic feature on the earth today its horziontal extent elevation and location cause significant effects on modern atmospheric circulation and climate so the history of uplift of the surface of tibet is linked to cenozoic climate changes at local regional and perhaps global scales geological and geophysical studies of the plateau are contributing data on the present and past deformation of the tibetan lithosphere that has formed the plateau primarily during the cenozoic the principal of isostasy then can be used to estimate the elevation history of the surface for a given deformation history different parts of tibet probably had different uplift histories but presently available data are not sufficient for distinguishing many contrasts in one scenario cretaceous and early cenozoic north south distributed shortening of tibetan crust and mantle lithosphere probably caused significant uplift of the surface relative to sea level to perhaps half of the present elevation by the early miocene thinning of the high density mantle portion of the lithosphere during the miocene may then have allowed the thick tibetan crust to rise close to its present elevation perhaps higher before 8 ma since then slow east west extension of tibet probably reduced the crustal thickness slightly and may have caused the elevation of the plateau to decrease during the late cenozoic erosion of tibet unlike narrow mountain belts has been unable to match the uplift of a broad plateau orographic precipitation and efficient river networks concentrate erosion on the edges while the interior is protected from significant erosion despite its lofty elevation the southern edge of the plateau the himalaya has suffered a minimum of 25 km of denudation since the miocene while central tibet shows little or no sign of major erosion since that time the gangdese arc in southern tibet was rapidly eroded during the mid miocene when 4 km of rock apparently were removed from the surface as shown by mineral cooling ages this pulse of erosion was probably caused by a combination of local thrust system movement and changes in base level and precipitation due to relative elevation changes between the gangdese and the himalaya to the south the modern long wavelength flatness of tibet is unlikely to have been caused by erosion and indicates viscous flow at some level of the lithosphere has been acting to level the plateau surface

crustal channel flows 2 numerical models with implications for metamorphism in the himalayan tibetan orogen 1 results from a thermal mechanical model ht1 that includes midcrustal channel flow are compatible with many features of the himalayan tibetan system radioactive self heating and rheological weakening of thickened model orogenic crust lead to the formation of a hot low viscosity midcrustal channel and a broad plateau channel material corresponding to the greater himalayan sequence ghs flows outward from beneath the plateau in response to topographically induced differential pressure at the plateau flank it is exhumed by focused surface denudation and juxtaposed with cooler newly accreted material corresponding to the lesser himalayan sequence lhs the model channel is bounded by coeval thrust and normal sense ductile shear zones interpreted to represent the main central thrust mct zone and south tibetan detachment system respectively inverted metamorphism associated with the model mct zone results from distributed ductile shear along the mct and extrusion of the hot channel a variety of model p t t path styles resembling those observed in the ghs and lhs are produced for points traveling through contrasting tectonic regimes that coexist in different parts of the model predicted times of peak metamorphism cooling and erosion of metamorphic facies are generally compatible with observations although model ghs cooling ages are too young the times of m1 and m2 metamorphic events observed in the ghs correspond to model times of maximum burial and maximum heating respectively the results highlight the need to integrate tectonics and metamorphism in continental collision models and demonstrate the importance of lateral transport of both heat and material in large hot orogens

mineralogy and geochemistry of surface sediments from the bengal fan indian ocean abstract the mineralogy and composition of 75 sediment samples from the bengal fan andaman sea and the eastern flanks of the upper nicobar fan have been determined sediments from the upper and middle bengal fan are mainly terrigenous and derived from the ganges brahmaputra river system whereas sediments from the andaman sea and the eastern flank of the upper nicobar fan vary from terrigenous to largely biogenic for the terrigenous sediments the continental influence is illustrated by the distribution of the major elements excluding ca which decrease away from the source the ti al and fe al ratios of the sediments are higher in samples from the cast coast of india than from the fans indicating the influence of material from the deccan basalts as well as from mafic rocks of the indian peninsula in the clay mineral fraction of upper bengal fan sediments illite and chlorite are the dominant constituents with minor amounts of smectite and traces of kaolinite illite is mainly a weathering product of lithic material from the himalaya and is transported by the ganges brahmaputra river system smectite is the dominant constituent of the sediments adjacent to the indian peninsula

evidence for long term diffuse deformation of the lithosphere of the equatorial indian ocean the presence of large earthquakes east west striking folds and thrust faults in sediments and east west striking undulations of wavelength 200 km in topography and gravity shows that the equatorial indian ocean is the locus of unusual deformation1 2 3 4 5 6 7 8 this deformation has been interpreted as a diffuse boundary between two tectonic plates9 10 11 12 13 seismic stratigraphy and deep sea drilling at two locations in the bengal fan indicate that the deformation began 7 5 8 0 myr ago3 14 15 here however we show using plate reconructions that motion across this diffuse oceanic plate boundary began more than 10 myr earlier than previously inferred and that the amount of north south convergence across the boundary through the central indian basin has been significantly greater than the convergence estimated from seismic profiles the relative plate velocity accommodated across the central indian basin has varied with time and has been as fast as 6 mm yr 1 about half the separation rate of earth s slowest spreading mid ocean ridge the earliest interval of measurable motion which began more than 18 myr ago may coincide with rapid denudation of the tibetan plateau from 21 myr to 15 17 myr ref 16 the present motion across the central indian basin began no earlier than 11 myr following an earlier interval of slower motion from 18 to 11 myr and may have begun at 8 myr when the tibetan plateau is thought to have attained its maximum elevation16 17

erosion as a driving mechanism of intracontinental mountain growth in nature mountains can grow and remain as localized tectonic features over long periods of time 10 m y by contrast according to current knowledge of lithospheric rheology and neglecting surface processes any intracontinental range with a width that exceeds that which can be supported by the strength of the lithosphere should collapse within a few tens of millions of years for example assuming a quartz dominated crustal rheology the relief of a range initially 3 km high and 300 400 km wide is reduced by half in about 15 m y as a result of lateral spreading of its crustal root we suggest that surface processes might actually prevent such a subsurface collapse removal of material from topographic heights and deposition in the foreland oppose spreading of the crustal root and could eventually drive a net influx of material toward the orogeny we performed a set of numerical experiments in order to validate this hypothesis a section of a lithosphere with a brittle elasto ductile rheology initially loaded by a mountain range is submitted to horizontal shortening and to surface processes if erosion is intense material is removed more rapidly than it can be supplied by crustal thickening below the range and the topography is rapidly smoothed for example a feature 3 km high and 300 400 km wide is halved in height in about 15 m y for an erosion coefficient k 10 3 m 2 yr the erosion rate is of the order of a few 0 1 mm yr this regime might be called erosional collapse if erosion is not active enough the crustal root spreads out laterally and subsurface collapse occurs in the third intermediate regime removal of the material by erosion is dynamically compensated by isostatic rebound and inward flow in the lower crust so that the range can grow in this mountain growth regime the range evolves toward a characteristic graded shape that primarily depends on the erosion law the erosion rate may be high e g 0 5 0 9 mm yr close to the rate of tectonic uplift e g 0 7 1 1 mm yr and few times higher than the rate of topographic uplift 0 15 0 2 mm yr these experiments show that surface processes can favor localized crustal shortening and participate in the development of an intracontinental mountain surface processes must therefore be taken into account in the interpretation and modeling of long term deformation of continental lithosphere conversely the mechanical response of the lithosphere must be accounted for when large scale topographic features are interpreted and modeled in terms of geomorphologic processes

comparison of late pleistocene and modern glacier extents in central nepal based on digital elevation data and satellite imagery abstract late pleistocene and modern ice extents in central nepal are compared to estimate equilibrium line altitude ela depressions new techniques are used for determining the former extent of glaciers based on quantitative objective geomorphic analyses of a 90 m resolution digital elevation model dem for every link of the drainage network valley form is classified as glacial or fluvial based on cross valley shape and slope statistics down valley transitions from glacial to fluvial form indicate the former limits of glaciation in each valley landsat multispectral scanner imagery for the same region is used to map current glacier extents for both full glacial and modern cases elas are computed from the glacier limits using the dem and a toe to headwall altitude ratio of 0 5 computed ela depressions range from 100 900 m with a modal value of 650 m and a mean of 500 m values consistent with previously published estimates for the central himalaya but markedly smaller than estimates for many other regions we suggest that this reflects reduced precipitation rather than a small temperature depression consistent with other evidence for a weaker monsoon under full glacial conditions

episodic rapid uplift in the himalaya revealed by 40ar 39ar analysis of detrital k feldspar and muscovite bengal fan detrital k feldspar and muscovite samples from ocean drilling program leg 116 cores have been dated by the sup 40 ar sup 39 ar technique and have depositional ages from 0 to 18 ma from 4 to 13 individual k feldspars and 1 to 12 individual muscovites have been dated from 7 stratigraphic levels in every level at least one k feldspar and one muscovite yielded a minimum age identical within uncertainty to the age of deposition these results indicate that a significant portion of the material in the bengal fan is first cycle detritus derived from the himalaya therefore the substantial amount of sediment deposited in the distal fan in early to middle miocene time can be ascribed to a significant pulse of uplift and erosion in the collision zone at this time moreover these data indicate that throughout the neogene some part of the himalayan orogen was undergoing rapid erosion 1 to 10 mm yr this erosion must have been less than or equal to uplift relative to sea level the lack of granulite facies rocks in the eastern himalaya and tibetan plateau suggests to us that very rapid uplift must have been distributed in brief pulses over different parts of the mountain belt these datamore are incompatible with tectonic models in which the himalaya and tibetan plateau are uplifted either uniformly over the past 40 m y or mostly within the past 2 to 5 m y less

thrust tectonics crustal shortening and the structure of the far eastern nepal himalaya balanced and restored structural sections across the far eastern nepal himalaya have been constructed in order to determine the structure and evolution of the himalayan orogenic wedge and the amount of tectonic shortening the region has undergone since the initiation of thrusting along the main central thrust mct the far eastern nepal himalaya is comprised of three distinct thrust bound tectonic packages the higher himalayan crystalline thrust sheet the lesser himalayan metasediment thrust package and the sub himalayan imbricate fan the higher himalayan crystallines consisting of kyanite and sillimanite bearing gneisses intruded by the miocene jannu leucogranites have been thrust over the lesser himalayan metasediments along the mct for a distance of 140 km to 175 km the lesser himalayan metasediments are a 12 km thick unit consisting primarily of phyllites metaquartzites and mylonitic augen gneisses in which garnet biotite and chlorite metamorphic zones are exposed in progressively deeper structural levels the lesser himalayan metasediment thrust package is underlain by a decollement the main detachment fault mdf which lies at a calculated depth of between 6 and 10 km underneath the mahabharat lekh and at a calculated depth of 20 to 25 km north of the tamar khola dome the tamar khola dome overlies a footwall ramp along the mdf where the mdf cuts upsection through the lesser himalayan metasediments the lesser himalayan thrust package probably has an internal structure aproximating a hinterland dipping duplex with the mct and the mdf corresponding to the roof and floor thrusts respectively both the tamar khola thrust an out of sequence breach thrust and the main boundary thrust mbt are splay thrusts off of the mdf the sub himalaya consisting of nonmetamorphosed sedimentary rocks displays an emergent imbricate fan geometry and is underlain by the southern continuation of the mdf which lies at a depth of 5 5 km to 6 km beneath the siwalik hills folding and thrusting within the lesser himalayan thrust package and the sub himalayan imbricate fan have accomodated 45 to 70 km of tectonic shortening total north south shortening across the higher lesser and sub himalaya of far eastern nepal south of the tibetan plateau has been of the order of 185 km to 245 km and has occurred at an average rate of 7 4 mm to 15 3 mm per year since the initiation of the mct between 16 and 25 ma

flexural and stratigraphic development of the west indian continental margin subsidence curves for 27 wells from the western continental margin of india show a characteristic late oligocene to early miocene 24 5 ma rapid increase in subsidence rate superposed on the long lived slow subsidence typical of the thermal subsidence phase of passive continental margins by subtracting a best fit negative exponential subsidence from the observed subsidence curves we obtain an estimate of the distribution and magnitude of the excess subsidence affecting the neogene development of the west indian margin the magnitude of this excess subsidence increases seaward from the coast ranging from a few meters to 2000 m near the shelf edge we examine the following hypotheses to explain the distribution and timing of this excess subsidence 1 modification of basin stratigraphy due to the effects of lithospheric in plane compression 2 creation of accommodation space on the margin by flexural effects associated with indus fan loading and 3 rapid growth of the continental margin and associated flexural effects of the three hypotheses tested the least important mechanism to account for the observed excess subsidence is that of variations of lithospheric in plane force principally because maximum in plane compression within the indo australian plate was only achieved in the late miocene because indus fan sediment deposition began in the late oligocene to early miocene we investigated three dimensional flexural effects associated with fan loading as a cause of the excess subsidence beginning at 24 ma the distribution and magnitude of modeled flexural deflection however are not consistent with the observed excess subsidence interpretation of seismic reflection data indicates that the margin has aggraded and prograded by 100 km basin ward since the oligocene therefore we evaluate the flexural effects of this margin growth by estimating the amount of space infilled by margin progradation and aggradation since 24 ma and computing the resulting deflection this deflection matches the distribution and magnitude of observed excess subsidence along the margin in addition the distribution of the flexural bulge predicted from the combined deflections due to indus fan and margin loading is spatially coincident with the distribution of exposed marine terraces and drainage divides in the saurastra peninsula and the regions surrounding the gulfs of cambay and kutch respectively available gravity seismic reflection refraction and well data are consistent with our prediction of a 4000 to 5000 m thick sediment load developed during the neogene along the outer margin we propose that flexural deformation due to sedimentary loading provides a potential tectonic feedback mechanism that affects coastal and fluvial depositional processes as regions in close proximity to the load are depressed regions farther from the load experience uplift i e the peripheral bulge which is sufficient to cause subaerial exposure of large portions of the shelf and to modify existing drainage networks this feedback represents a mechanism for inducing relative sea level changes without invoking glacial eustasy

stratigraphic evidence for the uplift of the tibetan plateau between 1 1 and 0 9 myr ago uplift of the tibetan plateau is manifest not only in widespread denudation but also by an increased deposition rate of sediment near or far from the exhumed regions our results indicate that the mass accumulation rate mar of eolian dust increased between 1 1 and 0 9 myr ago we associate this increase in mar and median grain size with uplift of the tibetan plateau and its adjacent regions during this period this middle pleistocene uplift can also be evidenced by the age of volcanism in the marginal region the existence of thick conglomerate deposits surrounding the uplifted plateau and the increased sedimentation rate of lacustrine deposits in the qaidam basin northeastern tibetan plateau between 1 1 and 0 9 myr ago the correlation between the loess and marine records indicates that after 0 9 myr ago these two records correlate well this good correlation probably suggest that the middle pleistocene upheaval event not only brought the plateau into the cryosphere but also enhanced the coupling of regional scale chinese loess transportation and deposition to the global ice volume variations through its effects on glacial grinding rock denudation and east asian monsoonal circulation

reconstructing early himalayan tectonic evolution and paleogeography from tertiary foreland basin sedimentary rocks northern india the latest paleocene middle eocene subathu formation and the oligocene miocene dagshai and kasauli formations of the indian foreland basin record the early evolution of the himalayan fold thrust belt sandstone petrography of the subathu formation shows a predominantly recycled sedimentary source with a distinct ophiolitic and volcanic influence that was drastically reduced by the time of deposition of the dagshai formation sandstones in the dagshai and kasauli formations consist predominantly of metapelitic detritus the metamorphic grade of metapelitic lithic grains increases with time from dominantly very low grade at the base of the dagshai formation to dominantly low grade in the kasauli sandstones mudstone geochemistry documents the presence of a mafic ultramafic source during the time of deposition of the subathu formation that becomes significantly less important by the time of deposition of the dagshai formation compositions of subathu formation detrital spinels show they were either derived from both mid ocean ridge basalt type and arc type ophiolites or from an ophiolite of composite origin and that southerly derivation from the deccan trap continental flood basalts is unlikely kasauli formation garnet compositions suggest derivation from medium grade metamorphic rocks to amphibolite facies subathu formation composition reflects provenance influence from the indus suture zone during latest paleocene middle eocene time indicating initiation of continent continent collision and development of the foreland basin by this time suture zone influence was drastically reduced by the time of deposition of the dagshai formation when the embryonic thrust belt provided a barrier sufficient to partially separate the suture zone from the himalayan foreland basin the first appearance of himalayan metamorphic detritus occurs in the dagshai formation at the close of oligocene time whereas the kasauli formation records erosion to deeper metamorphic levels during earliest miocene time the occurrence of garnet and higher grade metamorphic lithic grains during early neogene time is coincident with the timing of displacement along the main central thrust and south tibetan detachment zone composition of early foreland basin sediments from pakistan balakot murree and kamlial formations to nepal bhainskati and dumri formations and bangladesh kopili barail formations surma group indicates diachronous arrival of ophiolitic to low grade metamorphic detritus derived respectively from the indus suture and early himalayan thrust sheets in the north this is consistent with progressively later closure of neotethys along the suture from latest paleocene time in the west to eocene time or even later in the east

pedostratigraphy and paleomagnetism of a 7 0 ma eolian loess red clay sequence at lingtai loess plateau north central china and the implications for paleomonsoon evolution abstract a 305 m thick loess red clay sequence was discovered recently at lingtai which is located in the middle part of the chinese loess plateau it consists of a complete pleistocene loess soil sequence with a thickness of about 175 m and 130 m of tertiary red clay deposits the red clay sequence at lingtai is the thickest one presently known in the loess plateau field observations show that the lingtai red clay contains over 110 couplets of pedogenic b horizons and horizontal carbonate nodule horizons and that it can be regarded as an extremely thick soil complex paleomagnetic studies of 680 samples suggest that the basal age of the lingtai loess red clay sequence is about 7 05 ma grain size analysis of samples taken at 3 3 cm intervals indicates that the red clay has the same sedimentological characteristics as those observed in the loess thus suggesting a wind blown origin of the tertiary red clay and continuous atmospheric dust deposition in the loess plateau during the last 7 05 ma the pedogenic characteristics of the paleosols within the pleistocene loess and the b horizons in the red clay suggest that the east asia summer monsoon in the latest miocene may have already been slightly stronger than that during the holocene this implies that at about 7 5 ma bp the tibetan plateau could have been uplifted to a critical height in maintaining the east asia summer monsoon system observations of the loess red clay sequence also suggest that the long term changes in the east asia summer monsoon strength have been nonlinear since the latest miocene

triassic sedimentary filling and closure of the eastern paleo tethys ocean new insights from detrital zircon geochronology of songpan ganzi yidun and west qinling flysch in eastern tibet

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

exhumation and tilting of the western metamorphic belt of the coast orogen in southern southeastern alaska the uplift history of the western metamorphic belt in southern southeastern alaska can be traced using emplacement depths and cooling ages of mid cretaceous plutons which intruded the metamorphic belt during the period 101 ma to 89 ma during the period 101 ma to 94 ma the rocks of the metamorphic belt now at the surface were buried to a depth of approximately 30 km pressure data indicate that by 89 ma the western portion of the metamorphic belt was at a shallower crustal level 23 km within the study area while the eastern side remained at a depth of approximately 30 km cooling curves constructed from minerals in mid cretaceous plutonic rocks indicate that the western side of the metamorphic belt cooled earlier than the eastern side an observation consistent with the inferred uplift history southwestward transport of metamorphic belt rocks within the hanging walls of thrust and high angle faults may have provided one mechanism for uplift during mid to late cretaceous time the eastern portion of the western metamorphic belt cooled rapidly at 55 ma probably in response to a period of rapid uplift the coast shear zone an early tertiary high angle fault with west side up displacement bounds the east side of the metamorphic belt we suggest tertiary uplift occurred by tilting of the metamorphic belt due to movement along this fault

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

laser ablation rb sr dating by online chemical separation of rb and sr in an oxygen filled reaction cell abstract the rb sr beta decay dating system is one of the most attractive tools in geochronology as rb is sufficiently abundant in common k bearing minerals like biotite muscovite and k feldspar this allows dating of a wide variety of rocks e g volcanic magmatic metamorphic sedimentary and hydrothermal environments without the need of preconcentration as is often required for u pb dating of zircon however this advantage was to date negatively counteracted by the lack of a suitable in situ technique as beta decay systems by nature have isobaric interferences of the daughter isotope by their respective parent isotope a reaction cell sandwiched between two quadrupoles within an inductively coupled plasma mass spectrometer icp ms allows exactly this the online chemical separation of two different elements coupled to a laser ablation la system in situ rb sr dating is therefore possible if a suitable reaction gas within the reaction cell can be found that separates sr from rb we present here a simple procedure in which rb sr ages can be obtained from a suite of individual phases in regular thin sections using the most established reaction gas oxygen o 2 it is possible to react part of the ablated sr to sro while no rbo is formed measurements of feldspars and micas with a spot size of 80 m were calibrated against glass standards nist srm 610 bcr 2g and biotite from la posta california 91 6 ma results are presented for a variety of magmatic rocks with well established thermal records a sample each from the klokken syenodiorite greenland 1161 ma the ulvo alkaligabbro sweden 1256 ma and a pegmatite from the bohus granite sweden 920 ma obtained in situ rb sr isochron ages are accurate 87 sr 86 sr ratios are accurate the methodology outlined in this study has significant implications for rb sr studies due to the high spatial resolution 1 quality of measurements can be better controlled by avoiding alterations and inclusions 2 large sample numbers can be investigated simply by using regular thin sections 3 several mineral generations can be targeted that would not be distinguishable in mineral separates and 4 isotope zonation within single crystals can be revealed this will open the rb sr dating system to new fields of study for example thermochronologic studies can use 87 sr 86 sr and 87 rb 86 sr zonation within crystals to better work out cooling and or reheating paths further applications are in the field of provenance studies and temporal evolution of shear zones

late cenozoic environmental changes and uplift on the northern side of the central himalaya a reappraisal from field data abstract the extreme view that uplift of the tibetan plateau and the adjacent himalayan range occurred mainly during the quaternary cannot be further sustained as discussed in the light of available field data palaeobiogeography sedimentary environments and tectonic evidence comparison with other independent sources such as the loess records of central china and data on the post orogenic sedimentation and geochronology of the himalayan and south asian oceanic fans shows that the tibetan plateau certainly reached an elevation close to its present one before the climatic changes of the quaternary as such this elevated himalaya tibet ensemble was probably one of the major causes of the late tertiary general cooling of the northern hemisphere as suggested by global modelling of climate palaeoenvironmental changes observed during the quaternary were certainly governed by orbital parameters yet they were also regionally influenced by both the uplift of the greater himalaya with respect to tibet and by the disruption of the northern himalaya and south tibet into horsts and grabens uncertainty still surrounds the periods during which critical elevations were achieved to induce both the monsoon circulation and e w extension

weathering flux and co2 consumption determined from palaeosol sequences across the eocene oligocene transition abstract rates of base cation depletion are determined for late eocene and early oligocene alluvial palaeosols pacific northwest of north america from pedogenic mass balance geochemistry and time estimates of palaeosol sequences pedogenic strain volume change and mass transfer of base cations silica and other pertinent elements are calculated from the geochemical differences between palaeosol horizons and parent material parent material compositions are determined by identification of the least altered alluvial strata in the sequence time spans of sequences of palaeosols are estimated from 40ar 39ar age dates of interbedded tuff beds and from time of formation estimates of palaeosols based on their degree of development palaeosol sequences from the mid latitude setting of central oregon document a significant decrease in the degree of weathering of individual palaeosols across the eocene oligocene transition as the climate became cooler and drier this decrease in the degree of weathering is accompanied by an increase in the accumulation rate of alluvial soils as indicated by sedimentation rates of stratigraphic packages according to these calculations and estimates the weathering flux of base cations in the john day basin measured in grams per 1000 years per square centimetre increased across the eocene oligocene transition these results when attributed to climate change indicate that weathering flux at this local increased as climate changed from warm humid late eocene conditions to cooler drier oligocene conditions furthermore the extrapolation of these weathering flux results to other mid latitude settings with similar climate change history may help to explain the severe climate change of the eocene oligocene boundary it is proposed that in some mid latitude settings greater quantities of less weathered silicate material were exposed during the early oligocene than during the late eocene thus causing an increase in flux of weathered product increased weathering flux during this time in certain regions of the mid latitudes may have increased both co2 consumption and bicarbonate transport to the ocean and contributed to atmospheric pco2 drawdown and global cooling

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

crustal volume balance during the india eurasia collision and altitude of the tibetan plateau a working hypothesis a working hypothesis is developed for the two stages of evolution of the tibetan plateau during the continental collision between india and eurasia the first phase was dominated by crustal shortening and thickening in what is now tibet while indochina was expelled southeastward away from eurasia at the end of this phase at 17 ma the estimated tibetan crustal thickness was 51 km the second phase has been dominated by thickening of tibet by the influx of crustal material from india with no associated crustal shortening the crustal thickening that is inferred to have occurred by this process is only feasible because leakage of crust from beneath the plateau is restricted by neighboring archean cratons that block lower crustal flow this leakage to the east and south from tibet to beneath yunnan is estimated as 20 30 of the influx at present but was much smaller at the start of this second phase of deformation it is suggested that at 11 ma the force exerted against the tibetan upper crust by this flowing material first exceeded the threshold that this brittle layer could resist and tibet began to extend eastward any satisfactory working hypothesis for the tibetan plateau must explain its dimensions its 17 ma ending of shortening and 11 ma start of extension and volcanism the eastward slip sense on its active normal and strike slip faults and the ratios of their displacements to present day slip rates which imply that their slip rates have gradually increased this working hypothesis accounts for these features and predicts a smooth and continuing increase in altitude