volcanism in the afar rift sustained by decompression melting with minimal plume influence continental breakup is caused by some combination of heating and stretching1 2 the afar rift system in africa is an example of active continental rifting where a mantle plume probably weakened the lithosphere through thermal erosion and magma infiltration however the location and degree of plume influence today are debated2 3 here we use seismic s to p receiver functions to image the mantle structure beneath afar we identify the transition between the lithosphere and underlying asthenosphere at about 75 km depth beneath the flanks of the continental rift however this boundary is absent beneath the rift itself and we instead observe a strong increase in seismic velocities with depth at about 75 km we use geodynamic modelling to show that the velocity increase at this depth is best explained by decompression melting of the mantle in the absence of a strong thermal plume so although the absence of mantle lithosphere beneath the rift implies a plume may have once been active we conclude that the influence of a thermal plume directly beneath afar today is minimal

heat flow in indian gondwana basins and heat production of their basement rocks abstract temperatures have been measured in eight boreholes ranging from 260 to 800 m in depth in five gondwana basins of the damodar and son valleys with the aid of about 250 thermal conductivity determinations on core samples from these holes heat flow has been evaluated measurements of radioactive heat generation have been made on samples of precambrian gneisses constituting the basement for the sonhat son valley and chintalapudi godavari valley basins heat flow values from all of the damodar valley basins are within the narrow range of 69 79 mw m 2 the value from the sonhat basin 107 mw m 2 is significantly higher the generally high heat flows observed in gondwana basins of india cannot be attributed to the known tectonism or igneous activity associated with these basins the plots of heat flow vs heat generation for three gondwana basins jharia sonhat and chintalapudi are on the same line as those of three regions in the exposed precambrian crystalline terrains in the northern part of the indian shield this indicates that the crust under exposed regions of the precambrian crystalline rocks as well as the gondwana basins form an integral unit as far as the present day geothermal character is concerned

bar dynamics in a sandy braided river insights from sediment numerical simulations abstract bar dynamics in sandy braided rivers are keys to better understand the morphological processes of braided rivers and sediment distributions limited by insufficient quantitative information and satellite images that only show the larger scale morphology and do not reveal the sand bars in the deeper channels most previous studies have qualitatively assessed bar dynamics but the ways in which the hydraulic parameters of sandy braided rivers control bar dynamics remains poorly understood here we present sediment numerical simulations on sandy braided rivers to investigate the quantitative controls of river parameters on bar dynamics three bar dynamic forms downstream migration lateral variations and bar tail elongation are observed unit bars are prone to downstream migration following formation and the migration rate shows a deceasing trend during braided river evolution this gradually decreasing migration rate is proven to be strongly connected to the decreasing flow velocity of braided channels that confine bar growth the lateral variation in bar dynamics exhibits narrowing migration and widening of sectional morphologies we suggest that braided channel curvature dominantly controls lateral sediment transportation around the bar and thus results in several lateral variation forms of bars the bar tail elongation difference between the two sides of the bar is associated with bar asymmetry strong bar asymmetry can make apparent differences in bar tail elongation

thin lithosphere asthenosphere boundary beneath eastern indian craton the lithosphere asthenosphere boundary lab separates the hard and rigid outer layer of the earth lithosphere and the weaker hotter and deeper part of the upper mantle asthenosphere and plays a pivotal role in plate tectonics however its definitive detection especially beneath the cratons is proving elusive one of the geophysical tools used to map the lab beneath the cratons is through magnetotelluric mt observations the resistivity at boundary falls in the range of 5 25 m and can be explained by the presence of a small amount of water in the asthenosphere possibly inducing partial melt here we report thickness of the lab in one of the oldest dated ancient cratons of india eastern indian craton eic of 3 3 gyr from mt studies the two prominent phase sensitive strike directions one each for crust and mantle and the presence of resistive continental lower crust act as a window to mantle in resolving deeper electrical conductivity structures beneath eic our results show that the lab beneath the eic is at 95 km the region is interesting as the electrical properties of the crust and mantle and the moho depth are similar to those of the slave craton canada 4 0 gyr but the depth of the lab beneath the eic is half that of the slave craton as cratonic signatures depicted by ultrapotassic rocks from gondwana coal fields close to eic are preserved at least till early cretaceous 117 ma it is likely that himalayan orogeny could have played a major role in delamination of the lithospheric roots of the eic in addition to attendant seismicity

geomorphic effects of wood quantity and characteristics in three italian gravel bed rivers abstract in channel wood is a fundamental component of the riverine system its nature abundance and distribution as well as the role of wood in trapping sediment have been reported by many authors however a lack of knowledge still exists on how the geomorphic effects quantity and characteristics of in channel wood may be altered by different human pressures for this reason in channel wood was surveyed in the brenta piave and tagliamento gravel bed rivers northeastern italy which are altered by different degrees of human pressures both single pieces of wood 0 1 m diameter and or 1 m long and accumulations of large wood were measured on cross sectional transects within the active channels overall 3430 8 4 13 9 and 10 7 elements ha in the brenta piave and tagliamento rivers respectively of isolated pieces and 591 9 8 15 0 and 11 0 wood accumulations ha in the brenta piave and tagliamento rivers respectively accumulations were surveyed in the study sites in the brenta and piave rivers which feature the greater human pressures logs appear in a worse state of conservation in the less disturbed tagliamento river the logs appear to be smaller and in a better state of conservation with higher capacity for resprouting in addition higher geomorphic interactions were found between wood and sediments in the tagliamento river because of its ability to create geomorphic effects in channel wood represents an important source of complexity that can increase habitat diversity in river systems a better knowledge of the role of human disturbances on the characteristics and abundance of large wood in river systems could help in developing better river management and the practical application of river ecology

complex variations in sediment transport at three large river bifurcations during discharge waves in the river rhine river bifurcations strongly control the distribution of water and sediment over a river system a good understanding of this distribution process is crucial for river management in this paper an extensive data set from three large bifurcations in the dutch rhine is presented containing data on bed load transport suspended bed sediment transport dune development and hydrodynamics the data show complex variations in sediment transport during discharge waves the objective of this paper is to examine and explain these measured variations in sediment transport it is found that bend sorting upstream of the bifurcations leads to supply limitation particularly in the downstream branch that originates in the outer bend of the main channel tidal water level variations lead to cyclical variations in the sediment distribution over the downstream branches lags in dune development cause complex hysteresis patterns in flow parameters and sediment transport all bifurcations show evidence of sediment waves which probably are intrinsic bifurcation phenomena the complex transport processes at the three bifurcations cause distinct discontinuities in the downstream fining trend of the river differences among the studied river bifurcations are mainly due to differences in sediment mobility shields value because the variations in sediment transport are complex and poorly correlated with the flow discharge prediction of the sediment distribution with existing relationships for one dimensional models is problematic

morphological perspective on the sedimentary characteristics of a coarse braided reach tagliamento river ne italy abstract in order to understand heterogeneity distribution of hydraulic properties many studies have proposed models of coarse braided river deposits however they often focus either on the surface near surface or on ancient deposit analysis furthermore the link between morphological surface and sedimentological outcrops information has not been fully explored yet this publication aims to characterize the morphodynamics of a braided gravel bed reach of the tagliamento river ne italy and to assess its relationship with the subsurface heterogeneity the morphological analysis is based on a lidar derived dem aerial and satellite photographs from 1998 to 2011 as well as field observations a water stage time series allows the morphological changes to be related to the discharge dynamics the sedimentological knowledge on coarse braided deposits was mainly gained from the observation of analog pleistocene coarse deposits in switzerland the main geomorphological elements are identified in terms of their topographic signature and genesis setting apart the trichotomy water vegetation bar the braidplain is characterized by higher lying zones with dentritic gully drainage networks and active zones where most of the morphology reworking occurs furthermore two morphologies mark the active zones a low discharge morphology low discharge incisions and channels slip face lobes etc superimposed on a high discharge morphology gravel sheets scours etc each morphological element is related to a depositional i e sedimentological element whose preservation potential in the subsurface is assessed as a function of the river bed aggradation dynamic the settings that impact the subsurface flow in terms of fast pathway and connectivity are addressed

sm nd isotopic and geochemical study of the archean tonalite amphibolite association from the eastern indian craton we report the results of a sm nd isotopic major element and rare earth element ree study of the older metamorphic group omg tonalite amphibolite association of the eastern indian craton the older metamorphic tonalite gneisses omtg have been previously dated to be 3 8 ga using sm nd isotope systematies and 3 2 3 4 ga by rb sr and pb pb dating the results of this study indicate that the protoliths of the omg amphibolites are 3 3 ga isochron age 3 30 0 06 ga nd 0 9 0 7 and therefore the omtg which intrude into the associated amphibolites cannot be any older than 3 3 ga the amphibolites display light ree enrichment ce yb n 2 2 6 7 la 30 100 x chondrite and nearly flat heavy ree patterns tb lu n 1 2 1 9 the basaltic parents of the amphibolites were probably generated by the partial melting of a spinel lherzolite mantle strong linear relationships between the amphibolites and tonalites in 147sm 144nd 143nd 144nd space isochron age 3 29 0 04 ga nd 0 8 0 8 imply that they are genetically related the tonalites display fractionated ree patterns la 100 300 x chondrite with moderate heavy ree depletions tb lu n 1 9 3 4 the isotopic major element and ree data are consistent with the derivation of the omtg from partial melting of omg amphibolites or equivalent rocks at amphibolegarnet stabilization depths an initial nd t value of 0 9 0 7 for the amphibolites indicates the presence of a slightly depleted mantle source at 3 3 ga with 147sm 144nd between 0 20 and 0 22 it is suggested that the growth of continental crust in the eastern indian craton occurred in response to magmatic underplating in a plume setting

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

effect of sampling time in the laboratory investigation of braided rivers we focus on the measurement of the bed elevation of braided networks in flume experiments in particular the effect of the survey frequency on the measurement accuracy is studied to this aim an innovative measurement system is adopted it consists of a laser ultrasonic sensor and can survey the bed elevation under owing water this measurement system was used to profile a flume transect with a frequency of 4 minutes without stopping the water discharge by this technique the topography of a single transect was continuously acquired during the evolution of a braided river model twelve braided rivers generated with different experimental conditions were studied the main results are i there exists a threshold survey frequency 4 8 minutes in our analysis which guarantees that the morphological evolution of the braiding channels is fully measured ii if this threshold frequency of survey is exceeded significant errors occur in the balance of the eroded deposited sediments and in the evaluation of the bed elevation dynamics and iii these errors depend on the river stream power

group velocity dispersion characteristics and one dimensional regional shear velocity structure of the eastern indian craton abstract in the past three years a semi permanent network of fifteen 3 component broadband seismographs has become operational in the eastern indian shield region occupying the archean 2 5 3 6 ga singhbhum odisha craton soc and the proterozoic 1 0 2 5 ga chotanagpur granitic gneissic terrane cggt the reliable and accurate broadband data for the recent 2015 nepal earthquake sequence from 10 broadband stations of this network enabled us to estimate the group velocity dispersion characteristics and one dimensional regional shear velocity structure of the region first we measure fundamental mode rayleigh and love wave group velocity dispersion curves in the period range of 7 70 s and then invert these curves to estimate the crustal and upper mantle structure below the eastern indian craton eic we observe that group velocities of rayleigh and love waves in soc are relatively high in comparison to those of cggt this could be attributed to a relatively mafic rich crust mantle structure in soc resulting from two episodes of magmatism associated with the 1 6 ga dalma and 117 ma rajmahal volcanisms the best model for the eic from the present study is found to be a two layered crust with a 14 km thick upper crust uc of average shear velocity vs of 3 0 km s and a 26 km thick lower crust lc of average vs of 3 6 km s the present study detects a sharp drop in vs 2 to 3 at 120 260 km depths underlying the eic representing the probable seismic lithosphere asthenosphere boundary lab at 120 km depth such sharp fall in vs below the lab indicates a partially molten layer further a geothermal gradient extrapolated from the surface heat flow shows that such a gradient would intercept the wet basalt solidus at 88 103 km depths suggesting a 88 103 km thick thermal lithosphere below the eic this could also signal the presence of small amounts of partial melts thus this 2 3 drop in vs could be attributed to the presence of partial melts in the upper mantle related to the earlier volcanic episodes viz back arc volcanism associated with the archean proterozoic subduction 1 6 ga dalma volcanism and 117 ma rajmahal volcanism the main result of our modeling provides evidences for the absence of keel or thick lithosphere below the eic

the genesis of archaean chromitites from the nuasahi and sukinda massifs in the singhbhum craton india abstract the chromitite deposits of the nuasahi and sukinda massifs are part of layered ultramafic bodies which occur within archaean low grade metamorphic rocks of the iron ore group iog in the singhbhum craton of the indian shield the chromitite seams are interlayered with dunite and associated with orthopyroxenite detailed electron microprobe study reveals very high mg numbers 0 66 0 82 and cr numbers 0 75 0 87 for chromite in massive chromitite from the seams high fo content fo 92 95 for olivine in dunite and high en content en 89 94 for orthopyroxene in orthopyroxenite our study suggests that the original igneous compositions for these minerals are preserved in the respective monomineralic rocks and can be used to evaluate the primary magmatic petrogenesis of the rocks the primitive compositions of the minerals in monomineralic rocks suggest high degree of partial melting of the source mantle during the mesoarchaean the parental magma from which the massive chromitites crystallized was of siliceous high mg basaltic or boninitic compositions similar to the compositions of spatially associated chromite bearing siliceous high mg basalts of the iron ore group the parental magma may have generated due to the interaction of a depleted mantle with a fluid enriched melt possibly derived in response to the dehydration of a subducting slab monomineralic chromitite layers formed either in response to mixing of magmas or due to the suppression of silicate mineral crystallization in response to elevated h 2 o concentrations in the parent magmas the parental magmas that produced chromitites may have intruded into the volcano sedimentary greenstone belts of a supra subduction zone setting

morphological effects of different channel forming discharges in a gravel bed river the study analyses the morphological response of a gravel bed river to discharges of different magnitude from moderate events that occur several times a year to a 12 year flood and so defines the range of formative discharges for single morphological units channels bars islands and a range of magnitude of morphological activity from the threshold discharges for gravel transport and minor bar modification up to flows causing major morphological changes the study was conducted on the tagliamento river a large gravel bed river in north eastern italy using two different methods analysis of aerial photographs and field observation of painted gravel particles the available photographs five flights from august 1997 to november 2002 and the two commissioned flights june 2006 and april 2007 do not define periods with a single flood event but the intervals are short enough 11 to 22 months to have a limited number of flood events in each case the fieldwork which involved cross section survey grain size analysis and observation of painted sediments complemented the aerial surveys by allowing analysis of channel response to single flood events substantial morphological changes e g bank erosion of several tens of metres up to more than 100 m associated with flood events with a recurrence interval between 1 1 year and 12 years have been documented multiple forming discharges were defined based on the activity of different morphological units discharges equal to 20 50 of the bankfull discharge are formative for the channels whereas the bankfull discharge 1 1 year flood in this case of the tagliamento river is formative for low bars larger floods but still relatively frequent with a recurrence interval less than five years are required for full gravel transport on high bars and significant morphological changes of islands copyright 2009 john wiley sons ltd

stability morphology and surface grain size patterns of channel bifurcation in gravel cobble bedded anabranching rivers this study presents the first detailed field based analysis of the morphology of bifurcations within anabranching cobble gravel rivers bifurcations divide the flow of water and sediment into downstream anabranches thereby influencing the characteristics of the anabranches and the longevity of river islands the history morphology bed grain size and flow vectors at five bifurcations on the renous river new brunswick canada were studied in detail the angles of bifurcations within five anabranching rivers in the miramichi basin were investigated the average bifurcation angle was 47 within the range of values cited for braided river bifurcations bifurcation angle decreased when anabranches were of similar length shields stresses in channels upstream of bifurcations were lower than reported values for braided rivers stable bifurcations displayed lower shields stresses than unstable bifurcations contrary to experimental results from braided river bifurcations bifurcations in anabranching rivers are stabilized by vegetation that slows channel migration and helps to maintain a uniform upstream flow field the morphology of stable bifurcations enhances their stability a large bar shaped like a shallow ramp that increases in elevation to floodplain level forms at stable bifurcations floodplains at stable bifurcations accrete upstream at rates between 0 9 and 2 5 m a 1 bars may also form within the entrance of an anabranch downstream of the bifurcation node these bars are associated with bifurcation instability forming after a period of stability or an avulsion channel abandonment occurs when a bar completely blocks the entrance to one anabranch the stability of channels upstream of bifurcations and the location of bars at bifurcations influence bifurcation stability and the maintenance of river anabranching in the long term copyright 2006 john wiley sons ltd

bifurcation dynamics and avulsion duration in meandering rivers by one dimensional and three dimensional models at river bifurcations water and sediment are divided over two branches the dynamics of the bifurcation determine the long term evolution centuries of the downstream branches potentially leading to avulsion but the dynamics are poorly understood the long term evolution can only be studied by one dimensional models because of computational costs for such models a relation describing the sediment division is necessary but only few relations are available and these remain poorly tested so far we study the division of sediment and the morphodynamics on a timescale of decades to centuries by idealized three dimensional modeling of bifurcations with upstream meanders and dominantly bed load transport an upstream meander favors one bifurcate with more sediment and the other with more water leading to destabilization the bifurcations commonly attain a highly asymmetrical division of discharge and sediment after a few decades to a few centuries depending on combinations of the relevant parameters although past work on avulsions focused on slope advantage we found that bifurcations can be quasibalanced by opposing factors such as a bifurcate connected to the inner bend with a downstream slope advantage nearly balanced bifurcations develop much slower than unbalanced bifurcations which explains the observed variation in avulsion duration in natural systems which branch becomes dominant and the timescale to attain model equilibrium are determined by the length of the downstream bifurcates the radius of the upstream bend a possible gradient advantage for one bifurcate and notably the width depth ratio the latter determines the character of the bars which may result in overdeepening and unstable bars the distance between the beginning of the upstream bend and the bifurcation determines the location of such bars and pools which may switch the dominant bifurcate in fact when the bifurcation is quasibalanced by opposing factors any minor disturbance or a different choice of roughness or sediment transport predictor may switch the dominant bifurcate the division of sediment is nearly the same as the division of flow discharge in most runs until the discharge division becomes very asymmetrical so that a bifurcate does not close off entirely this partly explains the sustained existence of residual channels and existence of anastomosing rivers and the potential for reoccupation of old channel courses we develop a new relation for sediment division at bifurcations in one dimensional models incorporating the effect of meandering the flow and sediment divisions predicted by two existing relations and the new relation for one dimensional models are in qualitative agreement with the three dimensional model these one dimensional relations are however of limited value for wider rivers because they lack the highly three dimensional bar dynamics that may switch the direction of bifurcation evolution the potential effects of bed sediment sorting bank erosion and levee formation on bifurcation stability and avulsion duration are discussed

the rapid drift of the indian tectonic plate the breakup of the supercontinent gondwanaland into africa antarctica australia and india about 140 million years ago and consequently the opening of the indian ocean is thought to have been caused by heating of the lithosphere from below by a large plume whose relicts are now the marion kerguelen and reunion plumes plate reconstructions based on palaeomagnetic data suggest that the indian plate attained a very high speed 18 20 cm yr 1 during the late cretaceous period subsequent to its breakup from gondwanaland and then slowed to approximately 5 cm yr 1 after the continental collision with asia approximately 50 myr ago the australian and african plates moved comparatively less distance and at much lower speeds of 2 4 cm yr 1 refs 3 5 antarctica remained almost stationary this mobility makes india unique among the fragments of gondwanaland here we propose that when the fragments of gondwanaland were separated by the plume the penetration of their lithospheric roots into the asthenosphere were important in determining their speed we estimated the thickness of the lithospheric plates of the different fragments of gondwanaland around the indian ocean by using the shear wave receiver function technique we found that the fragment of gondwanaland with clearly the thinnest lithosphere is india the lithospheric roots in south africa australia and antarctica are between 180 and 300 km deep whereas the indian lithosphere extends only about 100 km deep we infer that the plume that partitioned gondwanaland may have also melted the lower half of the indian lithosphere thus permitting faster motion due to ridge push or slab pull

shear wave velocity structure in the northern basin and range province from the combined analysis of receiver functions and surface waves a new method based on the joint inversion of receiver functions and surface wave phase velocities results in well determined shear velocity structures that are consistent with the compressional wave structure gravity heat flow and elevation data in the northern basin and range this new inversion method takes advantage of average velocity information present in the surface wave method and differential velocity information contained in the receiver function method thus min imizing the nonuniqueness problem that results from the velocity depth trade off an unusually thick 38 kin and relatively faster crust and upper mantle are found in central and eastern nevada compared to the thin 28 to 34 km and slower crust and upper mantle of the western basin and range we interpret the regions of thicker and faster crust and upper mantle as zones that have undergone less cenozoic ex tension relative to the surrounding regions to the west and north the thick crust and consequently greater depth to the dense mantle material is consistent with the gravity low pattern in central and eastern nevada simple gravity modeling shows both local and regional isostatic compensation occur within 40 km of the surface indicating a near classical airy type of compensation in the province we analyze in detail the shear wave s wave velocity model derived from the receiver functions at station bmn and compressional wave p wave velocity models derived from the 1986 passcal experiment in northwestern nevada the most interesting feature of these models is the presence of negative velocity gradients in the s wave model with no corresponding velocity decrease in the p wave models between depths of 10 and 24 km this combined velocity model may be explained by high pore fluid pressures at these depths this model favors a layered fluid porosity model proposed in the literature to explain extensive middle to lower crust conti nental seismic reflections and high electrical conductivity an upper mantle grada tional low velocity zone is present between 32 and 38 km in the s wave model this upper mantle shear wave low velocity zone is consistent with partial melt which may be the source material for magmatic underplating in this region

alternative model of thrust fault propagation a widely accepted explanation for the geometry of thrust faults is that initial failures occur on deeply buried planes of weak rock and that thrust faults propagate toward the surface along a staircase trajectory we propose an alternative model that applies gretener s beam failure mechanism to a multilayered sequence invoking compatibility conditions which demand that a thrust propagate both upsection and downsection we suggest that ramps form first at shallow levels and are subsequently connected by flat faults this hypothesis also explains the formation of many minor structures associated with thrusts such as backthrusts wedge structures pop ups and duplexes and provides a unified conceptual framework in which to evaluate field observations

how do gravel bed rivers braid sedimentary processes and bed forms leading to the onset of braiding were observed in small scale hydraulic models of gravel bed streams the laboratory streams had a variety of combinations of constant discharge and slope but identical bed material particle size distributions from initially straight channels braiding occurred by four different processes deposition and accumulation of a central bar chute cutoff of point bars conversion of single transverse unit bars to mid channel braid bars and dissection of multiple bars in these experiments the chute cutoff mechanism was the most common but the predominant braiding mechanism depends upon sediment mobility excess bed shear stress and the bed form regime at very low excess bed shear stress the central bar process dominates but at higher excess bed shear stress slip face unit bars are more common bed scour at confluences is more pronounced and propogation of alternate convergence scour and divergence deposition is more likely thus chu

modelling bedload yield in braided gravel bed rivers abstract this paper outlines an approach for estimating the annual bedload yield of a braided channel this procedure is based on the extension of theory of flow and sediment transport in braided rivers recently presented by paola 1996 the revised approach accounts explicitly for the relationship between increasing discharge and varying channel hydraulics and is suitable for use in obtaining bedload transport rate estimates over a range of discharges integration of such estimates using flow duration data allows annual bedload yield to be determined model parameterisation is achieved using topographic survey data for the waimakariri river new zealand comparison of modelled bedload yield with values estimated from repeated topographic surveys indicates that the model is able to accurately predict both the medium term c 30 years mean annual bedload yield of the waimakariri at crossbank the section 17 8 km upstream of the river mouth and also short term fluctuations in bedload yield associated with varying annual flow statistics streamwise patterns of volumetric erosion and deposition determined for a 45 km length of the waimakariri using the model are also in broad agreement with trends identified in topographic survey data for the period 1961 1997 however significant deviations between modelled and surveyed volumes of cut and fill are evident at some locations comparison of model performance with conventional applications of bedload transport equations which tend to underestimate transport rates for braided channels suggests that the approach presented here may represent a significant improvement this is the case because the model quantifies the relationship between braid intensity and spatial variability in flow hydraulics at a cross section output from the model suggests that braided rivers may transport a significant proportion of their annual bedload at lower discharges than those indicated by earlier theoretical approaches results also provide quantitative support for the argument that increased intensity of braiding may promote higher rates of bedload transport in gravel bed rivers

the response of braided planform configuration to flow variations bed reworking and vegetation the case of the tagliamento river italy morphological features of braided rivers bars channels and pools experience major changes in area shape and spatial distribution as a response to i the pulsation of discharge during a flood and ii the bed evolution induced by floods in this work at a station relationships between water level and planform configuration were investigated on the tagliamento river a large gravel bed braided river in northeast italy over a 2 year study period comprising three bankfull events and several small to medium floods the analysis was performed on two 1 km long reaches characterized by different riparian vegetation cover ground based images with an hourly temporal resolution were acquired using software controlled digital cameras bars channels pools and vegetated patches were manually digitized on more than 100 rectified images sequences of constant level images spanning the study period were used to quantify the impact of floods on the stability of at a station relationships and on the turnover rate of water bodies the analysis shows that wetted area increased almost linearly with water level in both reaches the average number of branches per cross section peaked at intermediate flow levels increasing from 2 at low flow up to 6 7 the number of branches displayed the largest fluctuations over time with significant changes produced also by moderate floods turnover rates were high in both reaches with more than 30 of wetted areas at low flow converting into bare gravel in less than 2 months vegetation colonization was found to limit the mobility of the low flow channels over time by concentrating the flow in fewer deeper anabranches the number of channels per cross section was 30 40 less in the vegetated reach and the proportion of low flow water bodies in the same position after 12 months increased from 3 to 14 copyright 2012 john wiley sons ltd

active width of gravel bed braided rivers previous analyses have identified the active width of braided rivers the bed area over which bed load flux and short term morphological change occurs as an important element of braiding dynamics and predictions of bed load flux here we compare theoretical predictions of active width in gravel bed braided rivers with observations from sunwapta river and from a generic physical model of gravel braided rivers to provide general observations of the variation in active width and to develop an understanding of the causes of variation bed topography was surveyed daily along a 150 m reach of the pro glacial sunwapta river for a total of four weeks during summer when flow was above threshold for morphological activity in the laboratory detailed digital elevation models dems were derived from photogrammetric survey at regular intervals during a constant discharge run from the field and flume observations there is considerable local and circumstantial variation in active width but also a general trend in average active width with increasing discharge there is also a clear relationship of active width with active braiding index number of active branches in the braided channel network and with dimensionless stream power which appears to be consistent across the range of data from field and physical models thus there is a link between active width and the river morphology and dynamics and the possibility of a general relationship for estimating active width from channel pattern properties or reach scale stream power values from which approximate bedload flux calculations may be made the analysis also raises questions about differences between hydraulically based numerical model computations of instantaneous active width and observation of time integrated morphological active width understanding these differences can give insight into the nature of bedload transport in braided rivers and the relationship to morphological processes of braiding copyright 2011 john wiley sons ltd

changes in braided river morphology resulting from aggradation and degradation forty three experiments were completed in a large flume 1 8 x 18 3 m to examine the effects of sediment load sediment size and channel gradient on braided river morphology the laboratory results were supported by field research and analogs from the published literature subaerially exposed braid bars were formed by the dissection and further accretion of stalled linguoid dunes which are actively migrating bedforms bar forming processes and channel dynamics were the same in both sand and gravel bed channels however linguoid dunes stalled and were transformed into braid bars more frequently in gravel bed channels and therefore gravel bed channels were more intensely braided aggradation resulted in the development of more bars rather than in an increase in the size of preexisting braid bars the number of actively migrating linguoid dunes also increased with sediment load in the highly mobile sand bed channels conversely in degrading channels the number of braid bars decreased and average bar

precambrian tectonic evolution of eastern india a model of converging microplates the precambrian formations of the singhbhum and chotanagpur region of the indian peninsular shield are tectonically classified and their implications in the context of plate tectonics are reviewed on the basis of the stratigraphic structural petrologic geochemical geophysical and geochronologic data that have accumulated through extensive research in the region in recent years it is shown that the essential elements in tectonic settings geological facies and structural and metamorphic characters of the singhbhum orogenic belt and the reactivated chotanagpur plateau are elegantly interpretable in terms of interaction of two converging microplates named here as the singhbhum and chotanagpur plates a detailed correlation of the tectonic evolution with the different stages of a proposed model of plate motions is attempted in the paper the study reveals three cycles of plate motions with intervening periods of quiescence during the first cycle 2000 1600 ma the singhbhum plate moved northward and collided with the chotanagpur plate this led to the tectonic emplacement of the dalma ophiolite belt and development of the f1 folds and thrusts and m1 metamorphism during the second cycle 1550 1170 ma a clockwise rotation of the singhbhum plate towards the ne generated the f2 folds and a transcurrent sinistral shear zone obduction of the continental lithosphere of this plate occurred during the third cycle 1000 850 ma as a result of its continued impingement on the chotanagpur plate in the nnw direction this is documented by the evolution of the f3 folds m3 metamorphism and the singhbhum thrust zone the quiescence periods allowed time for isostatic readjustments viz uplifts intrusions of basic dyke swarms erosion and paralic sedimentation

lithospheric mantle deformation beneath the indian cratons abstract the nature of deformation of the deep continental roots beneath the archean early proterozoic terrains opens the question whether these ancient terrains have had stable roots since the precambrian or whether recent plate motions have deformed them in view of this we make an attempt to study the thermal structure beneath the cratonic regions of the indian shield which vary in lithospheric thickness from 65 km in the singhbhum craton to 148 km in the archean dharwars the average depth of 104 km to the top of the underlying asthenosphere is consistent with other termination methods and is in fact less than half the 200 400 km depth found in other stable areas of the earth similarly the average reduced heat flow of about 35 mw m2 and moho temperature of about 550 c range 400 730 c for the indian cratons are also much higher than their counterparts elsewhere our study indicates a large scale deformation of the cratonic mantle lithosphere beneath the indian shield since the mesoproterozoic c

life of a bifurcation in a gravel bed braided river channel bifurcation is a key element in braided rivers determining the water and sediment distribution and hence controlling the morphological evolution recent theoretical and experimental findings as well as field observations showed that bifurcations in gravel bed braided rivers are often asymmetrical and highly unstable in this paper field data are presented on a bifurcation in the tagliamento river northeast italy the planform configuration of the bifurcation and its temporal evolution was monitored by an automatic digital camera during a series of seven floods with different magnitudes this remote sensing technique allowed a high temporal resolution pictures were acquired every hour that was proved to be essential in a highly dynamic system as the one considered here digitized maps of the channels provided information on the location of the bifurcation the width of the anabranches the angle between them along with the occurrence and migration of sediment bars data were acquired at two different water levels giving the possibility to compare low and high flow conditions the monitored bifurcation is largely unstable and shows sudden changes in the water distribution mainly driven by the bar migrating in the upstream channel and entering the distributaries a relationship between width asymmetry and flood magnitude was observed confirming previous analyses moreover recent theoretical findings were applied in order to test the possibility to estimate general trends in bifurcation evolution the analysis pointed out the relevance of a correct assessment of the characteristic temporal scales as the bifurcation evolves on a timescale similar to that of bar migration and flood duration understanding the interactions between these processes is therefore crucial in order to increase the ability to model and predict the morphological evolution of a braided network copyright 2012 john wiley sons ltd

shear wave splitting in eastern indian shield detection of a pan african suture separating archean and meso proterozoic terrains abstract based on fast axes orientation and delay time t measurements at fifteen broadband stations the archean eastern indian craton eic is classified into three main tectonic zones viz singhbhum odisha craton soc 82 8 and t 1 5 0 2 s chotanagpur granitic gneissic terrain cggt 63 9 and t 1 4 0 3 s and eastern ghats mobile belt egmb 70 10 and t 1 3 0 2 s the results in the archean province of eic depict a moderate to large delay times 1 0 1 9 s with azimuths that reveals an upper mantle anisotropy with a mean orientation of 74 5 5 6 sub parallel to the e w trending structural grain of the archean craton evidencing frozen lithospheric anisotropy the direction of anisotropy n39 e in northern meso proterozoic cggt is found to be consistent with the direction of current absolute plate motion of indian plate regional variation of observed orientation of fast axes near the flanking paleo and late archean orogenic belts is attributed to the tectonic control rather than current plate motion nevertheless estimated large mean delay times 1 41 0 27 s do also suggest a contribution from the anisotropy associated with asthenospheric mantle flow most interestingly our study detects a transition zone coinciding with the location of damodar graben separating south late archean and north meso proterozoic cggt across which the fast axes orientation changes from e w 71 to ne 39 which could be evidencing the signatures of the eastward extension of pan african 550 500 ma suture as also observed in antarctica

the missing dimension effects of lateral variation on 1 d calculations of fluvial bedload transport most calculations of bedload transport in rivers including those in numerical models of aggradation and degradation are 1 d all hydraulic and transport rate calculations are averaged over the channel width because bedload transport laws are nonlinear width averaged calculations will underestimate the true bedload flux if there is any local spatial variation in either the bed or the flow this paper analyses the effects on bedload transport capacity of spatial variation in applied and critical c shear stress separately and in combination a simple but versatile statistical model is used to represent variability in with allowance for differences between sand and gravel bed rivers and for below bankfull flow bedload flux is shown to increase greatly with the variance of especially in gravel bed rivers variability in c through bed patchiness may increase reduce or make little difference to bedload flux depending on the correlation between and c simple width averaging leads to severe underestimation of bedload transport in most conditions some alternatives are considered the findings have implications for sediment routing models srms but further research is needed to explore the issue fully

assessment of morphological changes induced by flow and flood pulses in a gravel bed braided river the tagliamento river italy large gravel bed braided rivers show a highly dynamic evolution where even moderate flow pulses can produce significant morphological adjustment an estimation of network variations is needed by river authorities as well as by ecologists for better managing river ecosystems however few field investigations have been performed to measure the effect of unsteady discharges in braided rivers in this paper we report on the morphological changes induced by six different floods in a 1 km long reach of the tagliamento river italy topographic surveys were performed after each significant flood and an automatic digital camera continuously monitored the planimetric configuration repeated measurements of cross section profiles allowed the quantification of the transported sediments volume and in particular provided a basis to estimate the percentage of the floodplain subject to morphological evolution data analysis showed a good relationship between peak flow level active width and mean bed level changes similarly lateral channel shifting is likely to increase for larger floods monitoring of a range of discharges up to bankfull conditions revealed a fundamental difference between flow pulses events on the average occurring one or more times per year and flood pulses with a return period larger than two years the morphological evolution in the former case is mainly limited to a few active branches and is locally driven by bank erosion at the bend apex on the other hand larger floods induce a complete reworking of the network configuration where bifurcations and confluences play a crucial role single morphological units like branches nodes bars are no longer recognizable after this type of event braided rivers are therefore characterized by the occurrence of two different spatial and temporal scales that force the ecosystem dynamics

crustal structure of the western bengal basin from joint analysis of teleseismic receiver functions and rayleigh wave dispersion teleseismic data recorded at the broadband seismological observatory of the indian institute of technology iit kharagpur are analyzed to determine the seismic characteristics of the crust beneath the western bengal basin receiver functions calculated from the teleseismic p waveform for a range of back azimuths show little variation in the moho ps arrival time indicating a nearly one dimensional crustal structure beneath kharagpur transforming the time domain receiver functions into the h v p v s domain for a stack of all receiver functions reveals a 38 2 km thick crust with a v p v s of 1 73 0 01 receiver functions stacked in narrow bins of back azimuth and great circle arc distance were inverted jointly with group velocity dispersion data for periods from 15 to 45 sec joint inversion results reveal a near surface sedimentary layer of 1 5 km thickness a 13 km thick upper crust and a 23 km thick lower crust with the moho at a depth of 37 5 km the average crustal s wave velocity is 3 7 km sec the top 0 5 km of the sedimentary layer has very low s wave velocity and a large impedance contrast with the underlying layer this represents the thin layer of unconsolidated sediments and weathered laterites near the surface as seen from deep seismic sounding dss studies across the region the s wave velocity structure of the crust beneath kharagpur is the first such result for the western bengal basin and is broadly similar to that of the indian shield crust a comparison of the crustal thickness and lower crustal velocities between kharagpur and agartala seperated by the eocene hinge zone shows remarkable differences crustal models beneath agartala reveal 36 km thick crust with the presence of a high velocity lower crust 6 km thick possibly oceanic in nature which is absent beneath kharagpur the hinge zone therefore separates the continental crust of the western bengal basin from the eastern deep basinal oceanic crust both being overlain by sediments derived from the himalayas

mechanics of river mouth bar formation implications for the morphodynamics of delta distributary networks 1 in this paper we use observational data and numerical modeling to present a new explanation for the formation of river dominated delta networks observational data from deltas throughout the world show that distributary channel widths depths and lengths decrease nonlinearly with successive bifurcations trends in width and depth are an outcome of hydraulic geometry scaling the trend in channel length is a consequence of delta growth analyses of serial maps show that the positions of bifurcations are the fossilized locations of river mouth bars also called middle ground and distributary mouth bar in front of old delta channel mouths therefore the trend in channel length can be explained through the mechanics of river mouth bar formation and evolution we use delft3d a coupled hydrodynamic and morphodynamic model to simulate the process of river mouth bar formation within an expanding turbulent jet in front of distributary channel mouths our results describe in detail the formation and evolution of a river mouth bar system and demonstrate that the distance to the river mouth bar is proportional to jet momentum flux and inversely proportional to grain size therefore channel length decreases down delta because with each successive bifurcation the jet momentum flux decreases these results can be used to predict the future evolution of river dominated deltas and can be used to aid in hydrocarbon exploration of these depositional environments

a method for estimating the mean bed load flux in braided rivers prediction of bed load flux remains a significant problem in understanding braided river morphodynamics for geomorphic and engineering applications two sets of data from laboratory experiments on braided networks performed at the university of trento italy and at the university of alberta canada provide the basis for development of a dimensionless bed load function and for testing a simple predictive model measured total sediment transport rates time averaged at equilibrium channel configuration collapse to a single dimensionless relationship based on dimensionless stream power bed load fluxes predicted by the parker and bagnold functions and cross section average hydraulic parameters under predict the bed load flux particularly at low shear stress this is consistent with previous observation and theory demonstrating the significant influence of transverse variability of the hydraulic parameters in controlling and predicting bed load flux a simple method for adjusting for this lateral variation is proposed for computing the sediment transport rate using topographic cross sections of braided rivers results show good agreement with the measured values suggesting that simple assumptions combined with the mean morphology of the channel may be sufficient to estimate mean bed load flux model predictions also highlight the significance of active bed width rather than bed shear stress as a fundamental ingredient in the prediction of the bed load rate and therefore as an important morphological property to be modelled the model also predicts similar behaviour in the at a station variability of bed load and in the importance of variation of active width relative to bed shear stress in the transport process

measurements in a braided river chute and lobe 1 flow pattern sediment transport and channel change this paper and its companion ashworth et al this issue discuss measurements of channel change and associated flow and sediment transport processes in a representative chute and bar reach within a proglacial gravel bed river the sunwapta river in the canadian rockies during a week in which water discharge through the reach increased then decreased a sediment wave passed into and partly along the reach at first the chute aggraded then sediment was eroded from the chute and deposited in a prograding lobe to one side of the original bar head between two distributaries measurements of velocity shear stress and gravel transport rate revealed day to day changes in the divergent pattern of flow and sediment transport the off center location of the lobe reflected an initial asymmetry in the pattern of flow and sediment transport but as the lobe grew the flow and transport gradually switched away to the other distributary width averaged bed load transport rates do not agree well with average rates inferred from bar head deposition or from volumes and spatial patterns of scour and fill the indirect estimates are considered more reliable than those based on direct sampling for necessarily brief durations by the end of the study the new lobe had almost accreted onto the original bar head supporting the idea that most braid bars are of compound origin temporal and spatial patterns in grain size distributions of the bed bed load and deposited sediment are discussed in the companion paper

passive source seismic imaging of the crust and upper mantle underlying the archean singhbhum craton eastern india abstract an improved image of crust mantle structure has been obtained through the joint inversion of p to s receiver functions and fundamental mode group velocity data 7 70 s of rayleigh and love waves at fifteen broadband stations in the eastern indian craton our modelling reveals a marked crustal thinning of 5 10 km below the singhbhum odisha craton soc while the modelled moho depths show a flat crust with a thickness of 42 km below the chotanagpur granitic gneissic terrain cggt the modelled lithosphere is thin 90 km beneath the soc and deepens on both north and south sides to 100 km with a sharp drop of 4 4 in shear velocity across the lithosphere asthenosphere boundary the amount of melt required in the asthenosphere to account for this drop in shear velocity is 1 the isotopic constituents of the dalma volcanic rocks contains asthenospheric component we suggest that the modeled crustal and lithospheric thinning might have been resulted from the earlier magmatism or plume episodes that led to lithospheric delamination below the soc our modeling also detects a common elongated feature extending between latitude 21 23 n and longitude 85 86 4 e which is characterized by a marked crustal 5 10 km as well as lithospheric 10 15 km thinning and a significant drop in vs 2 1 to 4 4 in the upper mantle below the soc this mapped area coincides with the horse shoe shaped broad singhbhum synclinorium known as iron ore group iog basin that hosts huge iron ore and to some extent manganese deposits of precambrian age this lithospheric feature is inferred to have played a key role in forming the passage for melts to carry various iron rich minerals and metals to the uppermost oceanic crust during the archean plume associated with magmatism episode

group velocity tomography of the indo eurasian collision zone we present results of a rayleigh and love wave group velocity dispersion study of the indo eurasian collision zone group velocity dispersion curves are measured and combined to produce dispersion maps for 10 70 s period rayleigh waves from 4054 paths and for 15 70 s love waves from 1946 paths group velocity maps benefit from the inclusion of data recorded at a large number of stations within india an advantage over previous global studies this has the largest impact at short periods as a result of the improved path length distribution synthetic tests are used to estimate resolution which ranges from 3 to 5 on the continents for rayleigh wave maps and from 5 to 7 5 for love wave maps group velocities correspond well with known geological and tectonic features and show good correlation with sediment thickness at short periods the cratons of the indian shield can be distinguished in the short period and midperiod group velocities group velocities are slow across tibet until 70 s whereas the cratonic cores of the indian shield appear as a high velocity anomaly at 70 s dispersion curves extracted from the rayleigh wave group velocity maps are inverted for shear wave velocity as a function of depth for profiles across india and tibet the relationship between shear velocity contours and the moho indicated by receiver function studies has been used to obtain a first order estimate of crustal thickness across the collision zone results suggest a slow tibetan midcrust and low sub moho velocities beneath the central and northeastern tibetan plateau

interaction between migrating bars and bifurcations in gravel bed rivers 1 in the present work we investigate the interaction between migrating alternate bars and the dynamics of river bifurcations laboratory experiments are carried out to study a y shaped bifurcation with fixed banks and erodible bed composed of well sorted sand the problem is also analyzed by developing a theoretical one dimensional model results show the occurrence of regular fluctuations in the discharge distribution at the bifurcation node which are strictly related to bar migration the effectiveness of bars in conditioning the bifurcation behavior increases with bar amplitude and decreases with bar migration speed four qualitatively different behaviors of the system are observed as the controlling parameters of the flow are varied within a range significant for gravel bed rivers the theoretical predictions are in good qualitative agreement with the experimental observations

the role of vegetation and large wood on the topographic characteristics of braided river systems riparian vegetation and large wood play a crucial role in shaping rivers on the one hand vegetation tends to increase bank stability reduce channel width and reduce the braiding index in gravel bed rivers conversely large wood tends to increase channel dynamics and promote avulsions the effects of vegetation and large wood have been rarely simulated together in flume experiments in this paper we present a series of experimental runs conducted in a large flume facility using cylindrical wooden dowels and alfalfa seedlings to represent logs and vegetation respectively in order to investigate their role on determining the topographical nature of braided river systems a terrestrial laser scanner was used to measure in detail the topography of the channels and bars and distribution of the elevations and second order structure functions were used to explore the topography that large wood and vegetation imposed on the braided pattern in the flumes results show that vegetation reduced the braiding index and produced wider and deeper channels the standard deviation of the bed elevations was higher than in flumes without vegetation large wood alone was not effective in changing the braiding index or the standard deviation of the elevations when vegetation and large wood acted together in the flume the braiding index reached a minimum value and the skewness of the distribution of elevation increased revealing shallower pools if compared with runs conducted with only vegetation

dating the oldest greenstone in india a 3 51 ga precise u pb shrimp zircon age for dacitic lava of the southern iron ore group singhbhum craton abstract this article reports a precise 3506 8 2 3 ma u pb shrimp zircon age for dacitic lava in a well preserved low grade metamorphic and low strained greenstone belt succession of the southern iron ore group singhbhum craton india this age makes the succession the oldest known greenstone belt succession in india and one of the oldest low strain greenstone successions in the world after the 3 51 ga coonterunah group of the pilbara craton western australia and the moderately deformed 3 54 ga theespruit formation of the barberton greenstone belt kaapvaal craton south africa the geochemical composition of the dacitic lava and related volcanic rocks suggests that they formed in a volcanic arc setting the succession also contains a major 120 m thick oxide facies banded iron formation that distinguishes it from the slightly older successions of the pilbara and kaapvaal cratons this banded iron formation may well be one of the oldest and most well preserved and together with associated volcanics

evidence of underplating from seismic and gravity studies in the mahanadi delta of eastern india and its tectonic significance 1 we have imaged the crustal configuration of the mahanadi delta of eastern india by modeling both wide angle seismic and bouguer gravity data this delta has undergone several stages of rifting subsidence sedimentation and uplift it is one of the important gondwana sedimentary basins of india where widespread volcanic activity occurred during the early cretaceous along the rift zones this volcanic activity corresponds to the breakup of greater india from east gondwana e g present antarctica and australia we have derived a five layer crustal model with velocities of 6 0 6 5 6 0 7 0 and 7 5 km s having estimated densities of 2 7 2 8 2 65 2 9 and 3 05 g cm3 respectively the velocity density relation along with heat flux and other geological geochronological information indicates typical rift related evolution of the delta with a midcrustal low velocity 6 0 km s and low density 2 65 g cm3 zone and a 10 km thick high velocity 7 5 km s and high density 3 05 g cm3 layer at the base of the crust presumably due to underplating the moho upwarping or crustal thinning in the rift zone and emplacement of thick high velocity material at the base of the crust strongly suggests basaltic underplating probably due to the kerguelen hot spot activity these activities are synchronous with 117 ma rajmahal volcanism in northeast india and the lambert graben in east antarctica and are closely associated with the gondwana breakup in the india antarctica sector the rifting stages are closely correlated with the sedimentation phases of the lower and upper gondwana deposits in the mahanadi delta which evolved parallel to the lambert graben in east antarctica

lithostructural and chronological constraints for tectonic restoration of proterozoic accretion in the eastern indian precambrian shield abstract the central indian tectonic zone citz is a 700 km long orogenic belt in north central india that demarcates the accretion zone between the northern and southern indian crustal domains however the timing of accretion remains speculative the chottanagpur gneiss complex cgc the north singhbhum mobile belt nsmb and the singhbhum craton sc are three crustal units located at the eastern extremity of the citz the cgc dominated by blastoporphyritic granitoids intrusive into amphibolite to granulite facies gneisses is juxtaposed against the greenschist facies northern metaflysch belt in the nsmb n nsmb north of the dalma ophiolite belt dob along the northern shear zone nsz both south cgc and n nsmb exhibit correlated increase in strain and prograde metamorphism towards the nsz laser ablation inductively coupled plasma mass spectrometry u pb zircon and u th pb monazite chemical ages from the southern parts of the cgc lie between 1 1 and 0 9 ga monazite from n nsmb are of similar age in contrast zircon from central parts of the cgc away from the nsz preserves older concordant ages up to 2 6 ga and the 1 0 0 90 ga age population is scarcely represented south of the dob the 1 0 ga ages are absent in s nsmb instead monazites from all samples in s nsmb record a tightly constrained 1 59 1 56 ga high p metamorphic event arguably a result of northward subduction of the metaflysch belt these two terranes were in turn accreted to the cgc at 1 0 0 90 ga this accretion at the leading edge of the amalgamated sc s nsmb block led to the emplacement of expansive granitoids in the cgc apparently the polychronous citz orogen marks the gradual closure of a major ocean basin separating the crustal domains of the northern and southern indian blocks during the meso and the early neoproterozoic era