topography relief and trmm derived rainfall variations along the himalaya 1 along the southern himalayan topographic front the indian summer monsoon modulates erosive processes and rates to investigate the influence of topography and relief on rainfall generation and resultant erosion we processed satellite rainfall amounts for the last 8 years 1998 2005 from the tropical rainfall measurement mission trmm based upon a spatial resolution of 5 5 km for the himalaya we identify 1 the spatial distribution of rainfall and 2 the large scale relationships between topography relief and rainfall locations our results show two distinct rainfall maxima along strike in the himalaya the first outer rainfall peak occurs along the southern margin of the lesser himalaya within a narrow band of mean elevation 0 9 0 4 km and mean relief 1 2 0 2 km the second discontinuous inner band typically occurs along the southern flank of the greater himalaya elevation and relief both 2 1 0 3 km

evolving strain partitioning in the eastern himalaya the growth of the shillong plateau the shillong plateau is the only raised topography up to 2000 m elevation in the himalayan foreland it is proposed to have had a major influence on strain partitioning and thus tectonics in the eastern himalaya additionally its position on the trajectory of the summer monsoon means it has influenced the regional climate with reduced erosion rates proposed over geological timescales in its lee the timing of surface uplift of the plateau has been difficult to determine exhumation rates have been calculated over geological timescales but these seem at variance with estimates based upon extrapolating the present day velocity field measured with gps and it has thus been suggested that exhumation and surface uplift are decoupled we determine the timing of surface uplift using the sedimentary record in the adjacent surma basin to the south which records the transition from a passive margin with southward thickening sedimentary packages to a flexural basin with north thickening strata due to loading by the uplifting plateau our method involves using all available 2d seismic data for the basin coupled to well tie information to produce isochore maps and construct a simple model of the subsidence of the surma basin in order to assess the timing and magnitude of flexural loading by the shillong plateau we conclude that the major period of flexural loading occurred from the deposition of the tipam formation 3 5 2 ma onwards which is likely to represent the timing of significant topographic growth of the shillong plateau our isochore maps and seismic sections also allow us to constrain the timing of thinning over the north south trending anticlines of the adjacent basin bounding indo burman ranges as occurring over this same time interval the combined effect of the uplift of the shillong plateau and the westward encroachment of the indo burman ranges to this region served to sever the palaeo brahmaputra drainage connection between himalayan source and surma basin sink at the end of tipam formation times 2 ma

cosmic ray labeling of erosion surfaces in situ nuclide production rates and erosion models a number of in situ cosmogenic radionuclides and stable nuclides have been measured in natural exposed rock surfaces with a view to study their in situ production and rock erosion rates 1 the in situ radionuclides can be used for a high resolution tomography of the erosional history of an exposed surface two stable nuclides 3he 21ne and five radionuclides 10be 26al 36cl 14c 39ar having half lives in the range of 300 1 5 106 yr half life are measurable in many rock types a prerequisite for the application of the in situ nuclides for the study of erosional histories of surfaces is a knowledge of their production rates under different irradiation conditions altitude latitude irradiation geometry and shielding relative nuclide production rates can be determined fairly accurately using the extensive available data on cosmic ray neutrons 2 absolute nuclide production rates cannot generally be predicted with any accuracy because of lack of data on excitation functions of nuclides unless some normalization is possible as was done in the case of several cosmic ray produced isotopes in the atmosphere 3 based on a recent natural calibration experiment in which erosion free surfaces exposed to cosmic radiation for 11 000 yrs were sampled the absolute production rates of 10be and 26al in quartz have been accurately estimated for mountain altitudes in sierra nevada 4 the absolute production rates of 10be and 26al in quartz can therefore be estimated fairly accurately for any given latitude and altitude some measurements of 14c in rocks of low erosion rate 5 similarly allow an estimate of its production rate attempts made to measure the in situ production rates of 3he in rocks have not yet led to a convergent production rate in view of the importance of knowing the production rates of isotopes of he ne and ar i present here theoretical estimates of their production rates based on available cross section data i discuss the information that can be extracted from the study of the in situ nuclides in rocks useful parameters characterizing the exposure history of a rock surface are 1 the effective surface exposure age and 2 the time averaged erosion rate the implications of these parameters for single and multiple nuclide studies are discussed in terms of the erosion models considered

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

stable isotope based paleoaltimetry abstractthe quantitative estimation of paleoaltitude has become an increasing focus of earth scientists because surface elevation provides constraints on the geodynamic mechanisms operating in mountain belts as well as the influence of mountain belt growth on regional and global climate the general observation of decreasing 18o and 2h values in rainfall as elevation increases has been used in both empirical and theoretical approaches to estimate paleoelevation these studies rely on the preservation of ancient surface water compositions in authigenic minerals to reconstruct the elevation at the time the minerals were forming in this review we provide a theory behind the application of stable isotope based approaches to paleoaltimetry we apply this theory to test cases using modern precipitation and surface water isotopic compositions to demonstrate that it generally accords well with observations examples of the application of paleoaltimetry techniques to himalaya tibet and the andes are discussed

miocene rise of the shillong plateau and the beginning of the end for the eastern himalaya abstract a common feature of convergent plate boundaries is the self organization of strain exhumation and topography along discrete arcuate boundaries deviations from this geometry can represent first order changes in stress applied at a plate boundary that must affect how strain is partitioned within the interior of an orogen the simplicity of the himalayan fold and thrust belt seen along its central portion breaks down along the eastern extremity of the arc where the 400 km long shillong plateau has developed this change in strain partitioning affects nearly 25 of the arc and has not previously been considered to be important to the orogen s development new low temperature thermochronometry data suggest this structure initiated in mid to late miocene time significantly earlier than was previously estimated from the sedimentary record alone development of the shillong plateau may be linked to a number of kinematic changes within the himalayan and burman collision zones that occur at the same time these events include the onset of e w extension in central tibet eastward expansion of high topography of the tibetan plateau onset of rotation of crustal fragments in southeastern tibet and re establishment of eastward subduction beneath the indo burman ranges we suggest that the coincidence of these tectonic events is related to the dismemberment of the eastern himalayan arc signifying a change in regional stress applied along the india eurasia burma plate boundaries discrepancies between vertical long term faulting rates and geodetically derived far field convergence rates suggest that the collisional boundary in the eastern himalayan system may be poorly coupled due to introduction of oceanic and transitional crust into the eastern plate boundary the introduction of dense material into the plate boundary late in the orogen s history may explain regional changes in the strain field that affect not only the himalaya but also the deformation field more than 1000 km into the tibetan plateau

shallow marine to fluvial transition in the siwalik succession of the kameng river section arunachal himalaya and its implication for foreland basin evolution abstract an understanding of the depositional environment and paleogeography of the siwalik foreland basin are crucial in interpreting the basin configuration sediment transport pathways and its evolutionary history this study examines the sedimentology of the siwalik succession of the kameng river valley arunachal himalaya northeastern india the facies characteristics of the fine grained well sorted sediments of the dafla formation and its complex polymodal paleocurrent pattern in this section reveals deposition in a variety of open marine to deltaic environment the overlying subansiri formation characterized by coarse grained thick multistoried sandstone and showing more consistent sw ward paleocurrent indicate deposition from a large axial braided river system the proposed redefinition of the boundary between the lower siwalik dafla and the middle siwalik subansiri formations implies their transition at around 7 5 ma instead of 10 5 ma suggested earlier the revised age of the transition is consistent with the age of arrival of the transhimalayan sediments at 7 ma and also denotes the time of marine to fluvial transition in this area presence of marine sediments in the kameng section with similar records further west indicates the existence of an extensive seaway in the eastern himalaya during the lower siwalik time the extant paleodrainage reconstructions have been recast on the basis of new data on the sedimentology and paleocurrent from this section it is inferred that the changing sea level uplifting shillong plateau and drainage evolution in the eastern himalayan foreland during the middle miocene time controlled the marine to fluvial transition in the basin

extreme rainfalls in eastern himalaya and southern slope of meghalaya plateau and their geomorphologic impacts this paper presents the detailed rainfall characteristics of 3 key areas located in the eastern monsoon india the margin of darjeeling himalaya the margin of bhutanese himalaya and the cherrapunji region at the southern slope of meghalaya upland all these areas are sensitive to changes but differ in annual rainfall totals 2000 4000 mm 4000 6000 m and 6000 23 000 mm respectively and in the frequency of extreme rainfalls therefore the response of geomorphic processes is different also due to various human impact in the darjeeling himalaya the thresholds may be passed 2 3 times in one century and the system may return to the former equilibrium at the margin of western bhutanese himalaya in 1990s the clustering of three events caused an acceleration in the transformation and formation of a new trend of evolution especially in the piedmont zone in the cherrapunji of meghalaya region in the natural conditions the effects of dozens of extreme rainfalls every year were checked by the dense vegetation cover after deforestation and extensive land use the fertile soil was removed and either the exposed bedrock or armoured debris top layer protect the surface against degradation and facilitate only rapid overland flow a new sterile system has been formed

the control mechanisms of erosion and weathering at basin scale from cosmogenic nuclides in river sediment the study of a sample of river sediment enables the determination of spatially averaged denudation rates that provide an exceptional perspective on erosion and weathering processes that have taken place within a landscape these measurements are done with in situ produced cosmogenic nuclides e g 10be 26al mostly in quartz from alluvial sediment cosmogenic nuclides are produced when secondary cosmic rays interact with the very uppermost layer of the earth s surface they are produced within a characteristic depth scale of about 1 m which means that the measured concentrations record an integrated denudation history while material passed through this depth interval depending on the denudation rate the resulting integration time scales are 103 to 105 years and one obtains a robust long term estimate of natural denudation that is relatively insensitive to short term changes the last 10 years have seen significant research activity using these methods and an array of fascinating tectono geomorphologic and geochemical insights are emerging amongst these is the ability to identify the physical and chemical processes with which a landscape responds to tectonic activity or climate change a compilation of world wide denudation rates in non glaciated areas that however does not yet include some of the world s most active mountain belts has resulted in the following findings some of which have been unexpected 1 no obvious relationship between precipitation or mean annual temperature and total denudation is apparent 2 topographic relief alone does not result in high rates of denudation 3 denudation rates are high in areas of landscape rejuvenation that is triggered and controlled by tectonic activity faulting escarpment formation and retreat rifting surface uplift 4 rates of weathering using a combination of cosmogenic nuclides and zirconium normalised cation loss balances co vary primarily with physical erosion rates and much less with temperature or precipitation 5 in some areas of high land use short term rates from river load gauging exceed those from cosmogenic nuclides by several orders of magnitude which serves to highlight the severity of geomorphic change caused by human action in the future the control mechanisms over denudation will be determined on all spatial scales ranging from the single soil section to entire river basins the same analysis can be done back through time on well dated terraces lake records and marine sediment cores which is possible with 10be for the past 1 2 my the rates obtained will be used to develop a quantitative understanding of tectonic geomorphologic and geochemical landscape processes which in turn is a prerequisite to design and calibrate models of the response of landscapes to tectonic climate and anthropogenic forcing

the shillong plateau and the great 1897 assam earthquake previous analysis of triangulation data of the survey of india concluded that the great 1897 assam earthquake occurred on a south dipping fault near the northern edge of the shillong plateau which was named the oldham fault this attribution has been questioned on geological and geodetic grounds we refine the triangulation data adding recently discovered observations and demonstrate that they require average slip of 25 5 m on a fault that dips south at 40 beneath the plateau the best fitting solution to the geodetic observations gives a rupture length of 79 km however the chedrang fault immediately to the west of the oldham fault appears to have slipped as a subvertical tear fault during or shortly after the 1897 earthquake with over 10 m of down to the west normal sense slip this observation suggests that the western end of the main rupture approached within a few kilometers of the chedrang fault giving a length of 95 km for the rupture this range of parameters gives a magnitude 8 15 mw 8 35 for the earthquake the triangulation data cannot be satisfied by slip on the north dipping faults that border the southern edge of the shillong plateau nor by slip on a south dipping fault that has been postulated in the brahmaputra valley gps velocities show that up to 5 mm yr of shortening is taken up across the plateau and its borders this suggests via moment frequency relations that the interval between great earthquakes in the region is several thousand years but that earthquakes of magnitude 7 or greater should occur roughly once per century

influence of thrust belt geometry and shortening rate on thermochronometer cooling ages insights from thermokinematic and erosion modeling of the bhutan himalaya advancements in thermochronology and numerical modeling offer the potential to associate the age of thermochronometric samples to both exhumational and deformational processes however understanding how these components are related in compressional systems requires linking the geometry and magnitude of fault slip to the distribution and amount of erosion to address this we apply a 2 d thermokinematic model to a forward modeled balanced cross section to quantify the cooling history in fold thrust belt settings the restored cross section provides a kinematic path of rocks and structures necessary to reproduce the surface geology by assigning ages to displacement amounts we produced a range of potential velocity vectors used to calculate heat transport erosion and rock cooling we test the predicted ages against a suite of previously published thermochronometric data from the bhutan himalaya to explore the utility of the data to constrain the timing rate and geometry of fault motion as well as variations in the exhumation rate we evaluate the cooling history associated with a constant rate of shortening of 18 mm yr rates that are 2 0 1 5 0 75 and 0 5 times the constant rate and rates that vary with time to determine which kinematic history best matches the measured cooling ages the combination of relatively old apatite fission track and zircon u th he measured ages and younger 15 9 ma 40ar 39ar ages from white mica is best matched with faster rates relative to constant rates between 11 5 and 8 ma and slower than constant rates from 17 to 11 5 ma and 8 ma to present

erosion of the rwenzori mountains east african rift from in situ produced cosmogenic10be 1 high relief and steep topography are thought to result in high erosion rates in the rwenzori mountains of the albert rift east africa where more than 3 km of relief have formed during uplift of the rwenzori fault block overall low denudation rates prevail we measured in situ derived cosmogenic denudation rates of 28 2 to 131 mm kyr in mountainous catchments and rates of 7 8 to 17 7 mm kyr on the adjacent low relief east african plateau these rates are roughly an order of magnitude lower than in other settings of similar relief we present an extensive geomorphological analysis and find that denudation rates are positively correlated with relief hillslope gradient and channel steepness indicating that river incision controls erosional processes in most upper headwater reaches above quaternary ela levels 4500 m a s l glacial imprinting inherited from several older and recent minor glaciation stages prevails in regions below 4500 m a s l however mild climatic conditions impede frost shattering favor dense vegetation and minimize bare rock areas and associated mass wasting we conclude that erosion of the rwenzori mountains is significantly slower than corresponding rates in other mountains of high relief due to a combination of factors extremely dense mountain cloud forest vegetation high rock strength of gneiss and amphibolite lithologies and low internal fracturing due to the extensional tectonic setting this specific combination unique to this extensional tropical setting leads to unexpected low erosion rates that cannot outpace post pliocene ongoing rock uplift of the rwenzori fault block

rainfall variability in the himalayan orogen and its relevance to erosion processes rainfall is an important driver of erosion processes the mean rainfall rate is often used to account for the erosive impact of a particular climate however for some erosion processes erosion rate is a nonlinear function of rainfall e g due to a threshold for erosion when this is the case it is important to take into account the full distribution of rainfall instead of just the mean in light of this we have characterized the variability of daily rainfall over the himalayan orogen using high spatial and temporal resolution rainfall data sets we find significant variations in rainfall variability over the himalayan orogen with increasing rainfall variability to the west and north of the orogen by taking into account variability of rainfall in addition to mean rainfall rate we find a pattern of rainfall that from a geomorphological perspective is significantly different from mean rainfall rate alone using these findings we argue that short term rainfall variability may help explain observed short and long term erosion rates in the himalayan orogen

rapid long term erosion in the rain shadow of the shillong plateau eastern himalaya abstract geodynamic models of collisional orogens suggest that precipitation gradients profoundly influence spatial patterns of exhumation and deformation in active collisional mountain ranges a basic tenet of this hypothesis is that in unglaciated areas spatial patterns of long term precipitation erosion and exhumation should be correlated a correlation of this type has been observed in the eastern himalaya where uplift of the shillong plateau by pliocene time drastically reduced monsoonal rainfall in the himalayan range downwind existing apatite fission track data suggest that the resulting precipitation gradient caused a twofold gradient in long term erosion rates across an area with similar geology suggesting a strong influence of climate on the region s geomorphic and tectonic evolution we extend this dataset by presenting 53 new bedrock apatite and zircon fission track ages from deeper within the rain shadow we expected latest miocene to pliocene apatite ages similar to previously published ages from neighboring areas in the rain shadow instead apatites as young as 1 3 0 2 ma and zircons as young as 4 5 1 0 ma 2 demonstrate that spatial gradients in precipitation do not correlate with variations in long term erosion and crustal strain as predicted by geodynamic models thermal kinematic modeling of these data suggests that local exhumation patterns reflect gradients in rock uplift dictated by fault kinematics in this rapidly deforming area despite a dramatic precipitation gradient these findings both highlight the need to better understand how erosive processes scale with precipitation amount and intensity in such settings and suggest a disconnect between the predictions of orogen scale geodynamic models and the relationship between erosion and tectonics at the regional scale

clockwise rotation of the brahmaputra valley relative to india tectonic convergence in the eastern himalaya naga hills and shillong plateau gps data reveal that the brahmaputra valley has broken from the indian plate and rotates clockwise relative to india about a point a few hundred kilometers west of the shillong plateau the gps velocity vectors define two distinct blocks separated by the kopili fault upon which 2 3 mm yr of dextral slip is observed the shillong block between longitudes 89 and 93 e rotating clockwise at 1 15 myr and the assam block from 93 5 e to 97 e rotating at 1 13 myr these two blocks are more than 120 km wide in a north south sense but they extend locally a similar distance beneath the himalaya and tibet a result of these rotations is that convergence across the himalaya east of sikkim decreases in velocity eastward from 18 to 12 mm yr and convergence between the shillong plateau and bangladesh across the dauki fault increases from 3 mm yr in the west to 8 mm yr in the east this fast convergence rate is inconsistent with inferred geological uplift rates on the plateau if a 45 n dip is assumed for the dauki fault unless clockwise rotation of the shillong block has increased substantially in the past 4 8 myr such acceleration is consistent with the reported recent slowing in the convergence rate across the bhutan himalaya the current slip potential near bhutan based on present day convergence rates and assuming no great earthquake since 1713 a d is now 5 4 m similar to the slip reported from alluvial terraces that offsets across the main himalayan thrust and sufficient to sustain a mw 8 0 earthquake in this area

effect of melt squeezing rate on shear stress along a simulated fault in gabbro during frictional melting 1 high velocity friction experiments were performed on gabbro at an initial constant slip rate of 1 32 m s and normal stress of 1 2 mpa under these conditions the sliding surfaces melted due to frictional heating once steady state conditions shear strength and shortening rate of the sample in the presence of melt were achieved normal stress was increased gradually up to 4 5 mpa to simulate enhanced squeezing of melt from the slipping zone at low normal stress shear stress increased only slightly with increasing normal stress when normal stress increased above 2 5 mpa however shear stress showed a linear increase with increasing normal stress we propose that the deformation mechanism changes from melt separated viscous shear to a mixed lubrication solid solid contact frictional melting due to a reduction in the thickness of the melt layer associated with an increase in the melt squeezing rate at high normal stresses

in situ development of high elevation low relief landscapes via duplex deformation in the eastern himalayan hinterland bhutan prior studies have proposed tectonic and climatic mechanisms to explain surface uplift throughout the bhutan himalaya while the resulting enigmatic low relief landscapes elevated above deeply incised canyons are a popular setting to test ideas of interacting tectonic and climatic forces when and why these landscapes formed is still debated we test the idea that these landscapes were created by a spatially variable and recent increase in rock uplift rate associated with the formation of structural duplexes at depth we utilize a new suite of erosion rates derived from detrital cosmogenic nuclide techniques geomorphic observations and a landscape evolution model to demonstrate the viability of this hypothesis low relief landscapes in bhutan are eroding at a rate of 70 m ma while basins from surrounding steep landscapes yield erosion rates of 950 m ma demonstrating that this portion of the range is in a transient period of increasing relief applying insights from our erosion rates we explore the influence of an active duplex on overlying topography using a landscape evolution model by imposing a high rock uplift rate in the middle of a mountain range our simulations show that low relief landscapes with thick alluvial fills form upstream of convex knickpoints as rivers adjust to higher uplift rates downstream a pattern consistent with geologic geomorphic and thermochronometric data from bhutan with our new erosion rates reconstructed paleo river profiles and landscape evolution simulations we show that the low relief landscapes were formed in situ as they were uplifted 800 m in the past 0 8 1 ma

transient basin as indicator of tectonic expressions in bedrock landscape approach based on matlab geomorphic tool transient profiler abstract the tectonic and erosional processes sculpture the earth s surface producing a landscape that preserves the evolutionary history of rock uplift and incision the bedrock channels act as an efficient gradation agent where the river profile adjusts to changing tectonic climatic or topological e g river capture boundary conditions and develops a transient basin a suite of slope break knickpoints at comparable elevations in the channel profiles of the transient basin act as important markers we developed a set of comprehensive and interactive matlab tools called a transient profiler to extract the geomorphic indices from the digital elevation model for a basin characterization and landscape evolution period the paleo channel reconstruction from the relict reach in the three studied transient drainage basins palar umiam and dibang suggest a varying surface uplift from 514 1246 and 1981 m with a respective relief gain of 158 230 and 95 respectively we found that the horizontal knickpoint retreat rate is faster in the himalayan thrust system as compared to the shillong pop up system whereas the vertical knickpoint retreat rate is faster in the latter

sedimentation and tectonics of the sylhet trough bangladesh the sylhet trough a sub basin of the bengal basin in northeastern bangladesh contains a thick fill 12 to 16 km of late mesozoic and cenozoic strata that record its tectonic evolution stratigraphic sedimentologic and petrographic data collected from outcrops cores well logs and seismic lines are here used to reconstruct the history of this trough the sylhet trough occupied a slope basinal setting on a passive continental margin from late mesozoic through eocene time subsidence may have increased slightly in oligocene time when the trough was located in the distal part of a foreland basin paired to the indo burman ranges oligocene fluvial deltaic strata barail formation were derived from incipient uplifts in the eastern himalayas subsidence increased markedly in the miocene epoch in response to western encroachment of the indo burman ranges miocene to earliest pliocene sediments of the surma group were deposited in a large mud rich delta system that may have drained a significant proportion of the eastern himalayas subsidence rates in the sylhet trough increased dramatically 3 8 times from miocene to pliocene pleistocene time when the fluvial tipam sandstone and dupi tila formation were deposited this dramatic subsidence change is attributed to south directed overthrusting of the shillong plateau on the dauki fault for the following reasons 1 pliocene and pleistocene strata thin markedly away from the shillong plateau consistent with a crustal load emplaced on the northern basin margin 2 the shillong plateau is draped by mesozoic to miocene rocks but pliocene and younger strata are not represented suggesting that the massif was an uplifted block at this time 3 south directed overthrusting of the shillong plateau is consistent with gravity data and with recent seismotectonic observations sandstone in the tioam has a marked increase in sedimentary lithic fragments compared to older rocks reflecting uplift and erosion of the sedimentary cover of the shillong plateau if the dauki fault has a dip similar to that of other himalayan overthrusts then a few tens of kilometers of horizontal tectonic transport would be required to carry the shillong plateau to its present elevation uplift of the shillong plateau probably generated a major 300 km westward shift in the course of the brahmaputra river

formation of a rain shadow o and h stable isotope records in authigenic clays from the siwalik group in eastern bhutan we measure the oxygen and hydrogen stable isotope composition of authigenic clays from himalayan foreland sediments siwalik group and from present day small stream waters in eastern bhutan to explore the impact of uplift of the shillong plateau on rain shadow formation over the himalayan foothills stable isotope data from authigenic clay minerals 2 m suggest the presence of three paleoclimatic periods during deposition of the siwalik group between 7 and 1 ma the mean 18o value in paleometeoric waters which were in equilibrium with clay minerals is 2 5 lower than in modern meteoric and stream waters at the elevation of the foreland basin we discuss the factors that could have changed the isotopic composition of water over time and we conclude that a the most likely and significant cause for the increase in meteoric water 18o values over time is the amount effect specifically a decrease in mean annual precipitation b the change in mean annual precipitation over the foreland basin and foothills of the himalaya is the result of orographic effect caused by the shillong plateau s uplift the critical elevation of the shillong plateau required to induce significant orographic precipitation was attained after 1 2 ma c by applying scale analysis we estimate that the mean annual precipitation over the foreland basin of the eastern bhutan himalayas has decreased by a factor of 1 7 2 5 over the last 1 3 million years 2018 american geophysical union all rights reserved

timing and mechanism of the rise of the shillong plateau in the himalayan foreland island arcs provide insights into the origin and recycling of continental crust but questions remain concerning source metasomatism the depth of differentiation the potential role of amphibole fractionation and the time scales involved basaltic andesites on batan island philippines contain 150 ma peridotite fragments encased in hornblendite and gabbroic rinds produced via melt rock reaction the peridotites have some of the lightest 7li and 57fe values yet measured in mantle rocks these values are too fractionated to have been created solely by equilibrium partial melting and thus require a combination of melt depletion and slab fluid addition and may be derived from diffusion modified melt channel wall rocks stable isotope signals are easily modified by diffusive equilibration between peridotite and host magma so the preservation of light 7li and 57fe here suggests magma ascent rates of 10 km yr 1 we show that melt wall rock reactions at 25 30 km depth led to the crystallization of amphibole plagioclase followed by gabbroic fractionation at 7 km depth the former provides a location and mechanism for the cryptic amphibole fractionation observed in these and perhaps many other arc lavas and may obviate the requirement for delamination of cumulates

plateau pop up in the great 1897 assam earthquake the great assam earthquake of 12 june 1897 reduced to rubble all masonry buildings within a region of northeastern india roughly the size of england and was felt over an area exceeding that of the great 1755 lisbon earthquake1 hitherto it was believed that rupture occurred on a north dipping himalayan thrust fault propagating south of bhutan2 3 4 5 but here we show that the northern edge of the shillong plateau rose violently by at least 11 m during the assam earthquake and that this was due to the rupture of a buried reverse fault approximately 110 km in length and dipping steeply away from the himalaya the stress drop implied by the rupture geometry and the prodigious fault slip of 18 7 m explains epicentral accelerations observed to exceed 1g vertically and surface velocities exceeding 3 m s 1 ref 1 this quantitative observation of active deformation of a pop up structure confirms that faults bounding such structures can penetrate the whole crust plateau uplift in the past 2 5 million years has caused the indian plate to contract locally by 4 2 mm yr 1 reducing seismic risk in bhutan but increasing the risk in northern bangladesh

constraints on the tectonic and landscape evolution of the bhutan himalaya from thermochronometry the observed geomorphology and calculated thermal histories of the bhutan himalaya provide an excellent platform to test ideas regarding the influence of tectonics and climate on the evolution of a convergentmountain range however little consensus has been reached regarding the late cenozoic history of the bhutan himalaya some researchers have argued that observed geologic relationships show slowing deformation rates such that the range is decaying from a geomorphic perspective while others see the range as growing and steepening we suggest that a better understanding is possible through the integrated interpretation of geomorphic and thermochronometric data from the comparison of predictions from models of landscape evolution and thermal kinematic models of orogenic systems new thermochronometric data throughout bhutan aremost consistent with a significant decrease in erosion rates from2 to 3 km ma down to 0 1 0 3 km ma around 6 4ma we interpret this pattern as a decrease in rock uplift rates due to the activation of contractional structures of the shillong plateau an uplifted region approximately 100 km south of bhutan however low relief fluvial landscapes throughout the bhutanese hinterland record a late pulse of surface uplift likely due to a recent increase in rock uplift rates constraints from our youngest thermochronometers suggest that this increase in rock uplift and surface uplift occurred within the last 1 75ma these results imply that the dynamics of the bhutan himalaya and shillong plateau have been linked during the late cenozoic with structural elements of both regions active in variable ways and times over that interval

climatic forcing of erosion landscape and tectonics in the bhutan himalayas a fundamental objective in studies of climate erosion tectonics coupling is to document convincing correlation between observable indicators of these processes on the scale of a mountain range the eastern himalayas are a unique range to quantify the contribution of tectonics and climate to long term erosion rates because uniform and steady tectonics have persisted for several million years while monsoonal precipitation patterns have varied in space and time specifically the rise of the shillong plateau the only orographic barrier in the himalayan foreland has reduced the mean annual precipitation downwind in the eastern bhutan himalaya at the miocene pliocene transition apatite fission track aft analyses of 45 bedrock samples from an e w transect along bhutan indicate faster long term erosion rates outside of the rain shadow in the west 1 0 1 8 mm yr than inside of it in the east 0 55 0 85 mm yr furthermore an aft vertical profile in the latter segment reveals a deceleration in erosion rates sometime after 5 9 ma in this drier segment of bhutan there are remnants of a relict landscape formed under a wetter climate that has not yet equilibrated to the present climatic conditions uplift and preservation of the paleolandscape are a result of a climate induced decrease in erosion rates rather than of an increase in rock uplift rate this study documents not only a compelling spatial correlation between long term erosion and precipitation rates but also a climatically driven erosion rate change on the scale of the eastern himalayas a change that in turn likely influences that region s recent tectonic evolution

tidal fluvial interaction in the guadalquivir river estuary spatial and frequency dependent response of currents and water levels tidal fluvial interaction in the gre

steady erosion rates in the himalayas through late cenozoic climatic changes sediment accumulation rates and thermal trackers suggest a substantial and global increase in erosion rates over the past few million years that increase is commonly associated with the impact of the northern hemisphere glaciation but methodological biases have led researchers to debate this hypothesis here we test whether himalayan erosion rates increased by measuring beryllium 10 10be in the sediment of the bengal bay seabed sediment originated from rocks that produced 10be under the impact of cosmic rays during erosion near surface thus the 10be concentrations indicate erosion rates the 10be concentration of the bengal bay sediment depends on the contributions of the ganga and brahmaputra rivers their sediments have distinct 10be concentrations because of distinct elevations and erosion in their drainage basins variable contributions could thus complicate erosion rate calculation we traced these contributions by a provenance study using the strontium sr and neodymium nd isotopic sediment compositions within uncertainties of 30 our reconstructed past erosion rates show no long term increase for the past six million years this stability suggests that climatic changes during the late cenozoic have an undetectable impact on the erosion patterns in the himalayas at least on the ten thousand to million year timescales accounted for by our dataset long term himalayan erosion rates remained stable through the global climatic changes of the past six million years according to the cosmogenic nuclide composition of terrestrial sediments recovered from the bay of bengal

spatial variability of 10be derived erosion rates across the southern peninsular indian escarpment a key to landscape evolution across passive margins abstract the persistence of significant topography in ancient tectonically inactive orogenic belts remains one of the outstanding questions in geomorphology in southern peninsular india the impressive topographic relief of the western ghat mountains in tectonic quiescence since at least ca 65 ma has raised important questions concerning the long term mechanism of topographic evolution quantifying the distribution of erosion in space and time is critical to understanding landscape evolution although the long term erosion rates are reasonably well known the short term erosion rates and the relative importance of factors controlling erosion in southern peninsular india are less well constrained we present a new suite of catchment averaged and local erosion rates using in situ produced 10be concentrations in river sediments and exposed bedrock samples in southern peninsular india catchment averaged erosion rates vary from 9 6 0 8 m ma 1 in the highlands to 114 3 13 8 m ma 1 on the escarpment side bedrock erosion rates range from 2 4 0 2 m ma 1 in the ridge top to 143 4 25 4 m ma 1 in active channel beds of the highlands catchment averaged erosion rates derived from the across escarpment westward draining catchments are significantly higher than those derived from the eastward draining over highland catchments the difference indicates that long term down wearing of the highland proceeds at lower rates than in the escarpment zones catchment averaged erosion rates are moderately correlated with mean hillslope angles and local relief whereas they are strongly correlated with catchment averaged channel steepness index this suggests that topographic steepness is the major control on the spatial variability of erosion while strong rainfall gradient is of minor importance in this area 10be derived average erosion rates in highlands are consistent with previous long term erosion rate estimated from thermochronometry these results collectively point to large scale steady state topography only decaying slowly with time steady state likely reflects the balance between erosion and isostatically driven uplift of the southern peninsular india

correlation of himalayan exhumation rates and asian monsoon intensity although most data suggest that the india eurasia continental collision began 45 55 myr ago the architecture of the himalayan tibetan orogen is dominated by deformational structures developed in the neogene period 23 myr ago the stratigraphic record and thermochronometric data indicate that erosion of the himalaya intensified as this constructional phase began and reached a peak around 15 myr ago it remained high until 10 5 myr ago and subsequently slowed gradually to 3 5 myr ago but then began to increase once again in the late pliocene and pleistocene epochs here we present weathering records from the south china sea bay of bengal and arabian sea that permit asian monsoon climate to be reconstructed back to the earliest neogene these indicate a correlation between the rate of himalayan exhumation as inferred from published thermochronometric data and monsoon intensity over the past 23 myr we interpret this correlation as indicating dynamic coupling between neogene climate and both erosion and deformation in the himalaya although the india eurasia collision initiated 50 myr ago major deformation and exhumation of the himalaya did not begin until the early neogene 23 myr ago this coincides with the increased intensity of the asian monsoons as indicated by weathering records from the south china sea bay of bengal and arabian sea and hints at a dynamic coupling between climate and both erosion and deformation in the himalaya

expression of active tectonics in erosional landscapes understanding the manner and degree to which topography in active mountain ranges reflects deformation of the earth s surface remains a first order goal of tectonic geomorphology a substantial body of research in the past decade demonstrates that incising channel systems play a central role in setting relationships among topographic relief differential rock uplift rate and climatically modulated erosional efficiency this review provides an introduction to the analysis and interpretation of channel profiles in erosional mountain ranges we show that existing data support theoretical expectations of positive monotonic relationships between channel steepness index a measure of channel gradient normalized for downstream increases in drainage area and erosion rate at equilibrium and that the transient response to perturbations away from equilibrium engenders specific spatial patterns in channel profiles that can be used to infer aspects of the forcing these aspects of channel behavior lay the foundation for a series of case studies that we use to illustrate how focused quantitative analysis of channel morphology can provide insight into the spatial and temporal dynamics of active deformation although the complexities of river response to climate lithology and uplift patterns mean that multiple interpretations of topographic data alone will always possible we show that application of stream profile analysis can be a powerful reconnaissance tool with which to interrogate the rates and patterns of deformation in active mountain belts

erosion rates of the bhutanese himalaya determined using in situ produced 10be abstract western bhutan provides an ideal setting to understand the interplay between uplift erosion and fluvial sediment transport in an active mountain environment using in situ produced 10 be 49 samples and 26 al 5 samples in fluvial sediment from nested catchments throughout the puna tsang chhu drainage basin we examine erosion rates in different geomorphic environments including two high relief regions a glacierized zone in the north and a high rainfall zone in the south as well as remnants of an uplifted lower relief paleosurface between them the erosion rates roughly mirror this north south zonation lower rates avg 388 32 m my 1 n 16 prevail in the low relief zone roughly coinciding with lower relief terrain where mean annual precipitation is 1500 mm yr 1 the highest rates avg 956 160 m my 1 n 13 are in the south 27 10 27 35 n where rainfall is 4000 mm yr 1 high rates avg 700 62 m my 1 n 15 also occur in the northern glacierized region 27 70 28 10 n all 49 purified mineral separates used in this study contain measurable amounts of native 9 be up to 900 g violating the assumption of negligible 9 be that is commonly made in the isotope dilution method used to quantify 10 be to correct for this native 9 be we use high precision replicate measurements of 9 be in each sample to calculate 10 be concentrations from measured isotopic ratios neglecting native 9 be would have led to erosion rate overestimates from 400 the pervasive nature of 9 be in these samples underscores the importance of quantifying the native 9 be concentration in mineral separates used for cosmogenic 10 be analysis

fluvial dissection isostatic uplift and geomorphological evolution of volcanic islands gran canaria canary islands spain abstract digital analysis of torrential gullies barrancos deeply incised into the volcanic island of gran canaria canary islands allows us to extract the longitudinal profiles and pre incision surfaces for individual basins from which morphometric parameters length elevation area slope have been calculated other derived parameters such as ridgeline profiles maximum incision values volume removed by fluvial erosion geophysical relief and isostatic uplift have also been computed based on k ar ages for the island well constrained incision uplift rates have been calculated by means of the combination of different methodological approaches commonly used in orogens and large mountain ranges the geomorphological and morphometric analyses reveal that the island is clearly divided into four environmental quadrants determined by the combination of a couple of key factors the age of the volcanic surfaces and the climatic conditions these factors determine a young sector covered with plio quaternary platform forming lavas finished at 1 9 1 5 ma evolving under contrasting wet ne to dry se climates and an older sector conserving the residual surfaces of the miocene shield building 14 5 8 7 ma at the ridgelines also subjected to wet nw and dry sw climates incision is related to the age zonation of the island maximum incisions

subduction initiation and the rise of the shillong plateau abstract discrepancies between geodetically and geologically estimated thrust fault slip rates are generally viewed as a methodological problem even when slip rate is steady over geological time a discrepancy may exist because each method is sensitive to different deformation processes however this offers a tool to estimate the partitioning of convergence between footwall and hanging wall deformation and therefore a way to discriminate among orogenic styles here we investigate one such discrepancy for the shillong plateau a basement cored contractional orogen within the himalayan foreland basin using a regional block model to explain the modern geodetic velocity field and explicit uncertainty analysis of the geologic rates we show that this discrepancy cannot be reconciled simply by invoking uncertainties in individual methods our results indicate that the shillong plateau is not an ongoing forward break of the bhutan himalayas as was believed until recently instead the observed inter plate convergence and plateau uplift in this region may be driven primarily by an attempt of the negatively buoyant indian passive margin lithosphere the surma basin south of the plateau to initiate subduction as a result the uplift history of the plateau which constrains the geologic rate is significantly lower than expected given the geodetic convergence rate we propose that this convergence is largely accommodated by the transport of the footwall into the mantle this geodynamic scenario has important regional seismotectonic implications 1 the cold and brittle sinking passive margin may have enabled the deep extent 30 km and therefore large magnitude of the mw 8 shillong earthquake of 1897 2 the collapse of the indian lithosphere into the mantle may have created the anomalously deep 20 km surma basin and 3 this subsidence may also drive accelerated post miocene westward propagation of the indo burman wedge we propose that the shillong plateau is the only modern example of passive margin collapse and can serve as a natural laboratory to study the earliest phase of subduction

a signature of transience in bedrock river incision rates over timescales of 10 4 10 7 years measured rates of river incision into bedrock are commonly interpreted as proxies for rates of rock uplift see refs 1 and 2 for example and indices of the strength of climatic forcing of erosion over time see refs 3 and 4 for example this approach implicitly assumes that river incision rates are in equilibrium with external forcings over a wide range of timescales here we directly test this assumption by examining the temporal scaling of bedrock river incision from 155 independent measurements of river incision compiled from 14 sites of these sites 11 exhibit a negative power law dependence of bedrock river incision rate on measurement interval a relationship that is apparent over timescales of 10 4 10 7 years and is independent of tectonic and geomorphic setting thus like rates of sediment accumulation rates of river incision into bedrock exhibit non steady state behaviour even over very long measurement intervals non steady state behaviour can be explained by episodic hiatuses in river incision triggered by alluvial deposition if such hiatuses have a heavy tailed length distribution regardless of its cause the dependence of incision rate on measurement interval complicates efforts to infer tectonic or climatic forcing from changes in rates of river incision over time or from comparison of rates computed over different timescales

rapid erosion of the southern african plateau as it climbs over a mantle superswell we present new sedimentary flux data confirming that a large pulse of erosion affected the southern african plateau in the late cretaceous and is likely to be related to a major uplift episode of the plateau this short phase of erosion i e less than 30 myr in duration has commonly been difficult to reconcile with a mantle origin for the plateau anomalous uplift given its size the rise of the african superplume is likely to have lasted much longer here we demonstrate by using a simple model for fluvial erosion that tilting of the continent as it rides over a wide dynamic topography high cannot only cause rapid uplift of the plateau but also trigger continent wide drainage reorganization leading to substantial denudation in a relatively short amount of time the amplitude and short duration of the sedimentary pulse are best reproduced by assuming a strong erodibility contrast between the karoo sedimentary and volcanic rocks and the underlying basement we also present a new compilation of paleoclimate indicators that shows a transition from arid to very humid conditions approximately at the onset of the documented erosional pulse suggesting that climate may have also played a role in triggering the denudation the diachronism of the sedimentary flux between the eastern and western margins of the plateau and the temporal and geographic coincidence between the uplift and kimberlite eruptions are however better explained by our tilt hypothesis driven by the migration of the continent over a fixed source of mantle upwelling

fluvial bevelling of topography controlled by lateral channel mobility and uplift rate rivers crossing zones of active uplift can bevel broad alluvial platforms experiments suggest that competition between lateral channel mobility and uplift rate controls the ability of a river to flatten the landscape

multi phased uplift of the southern margin of the central anatolian plateau turkey a record of tectonic and upper mantle processes abstract uplifted neogene marine sediments and quaternary fluvial terraces in the mut basin southern turkey reveal a detailed history of surface uplift along the southern margin of the central anatolian plateau from the late miocene to the present new surface exposure ages 10be 26al and 21ne of gravels capping fluvial strath terraces located between 28 and 135 m above the goksu river in the mut basin yield ages ranging from ca 25 to 130 ka corresponding to an average incision rate of 0 52 to 0 67 mm yr published biostratigraphic data combined with new interpretations of the fossil assemblages from uplifted marine sediments reveal average uplift rates of 0 25 to 0 37 mm yr since late miocene time starting between 8 and 5 45 ma and 0 72 to 0 74 mm yr after 1 66 to 1 62 ma together with the terrace abandonment ages the data imply 0 6 to 0 7 mm yr uplift rates from 1 6 ma to the present the different post late miocene and post 1 6 ma uplift rates can imply increasing uplift rates through time or multi phased uplift with slow uplift or subsidence in between longitudinal profiles of rivers in the upper catchment of the mut and ermenek basins show no apparent lithologic or fault control on some knickpoints that occur at 1 2 to 1 5 km elevation implying a transient response to a change in uplift rates projections of graded upper relict channel segments to the modern outlet together with constraints from uplifted marine sediments show that a slower incision uplift rate of 0 1 to 0 2 mm yr preceded the 0 7 mm yr uplift rate the river morphology and profile projections therefore reflect multi phased uplift of the plateau margin rather than steadily increasing uplift rates multi phased uplift can be explained by lithospheric slab break off and possibly also the arrival of the eratosthenes seamount at the collision zone south of cyprus

lithological control on the geomorphic evolution of the shillong plateau in northeast india abstract the shillong plateau in northeast india is a block of raised topography in the himalayan foreland which consists of crystalline basement rocks partially covered by a cretaceous to miocene sedimentary succession it is dominated by a mature low relief landscape surrounded by high relief fluvially dissected margins particularly along its southern flank which is bounded by the dauki thrust fault we use river profiles and geological relationships to show that the low relief plateau is a topographic expression of a re exposed basement palaeosurface following the stripping of sedimentary cover by scarp retreat we show that initiation of the wave of incision does not require surface rupture on the dauki fault or an increase in fault slip rate at the end of the miocene as suggested by previous studies we propose that incision has been spatially controlled by the slope of the basement palaeosurface likely moderated by an incision threshold river profiles in the shillong plateau cannot be interpreted as simple records of surface uplift the observed heterogeneous spatial pattern of steepness is a function of a dynamic landscape response to the erosion of layered lithology with contrasting erodibility such dynamics have implications for fluvial geomorphology highlighting that near horizontal lithological contacts can strongly influence river profiles and topography even when no longer physically preserved the topography of the northern shillong plateau is controlled by the structure of basement rocks and is reminiscent of stable cratonic interior landscapes consistent with its surface exposure during late cretaceous times

discharge discharge variability and the bedrock channel profile long term bedrock incision is driven by daily discharge events of variable magnitude and frequency with ineffective events below an incision threshold we explore theoretically how this short term stochastic behavior controls long term steady state incision rates and bedrock channel profiles combining a realistic frequency magnitude distribution of discharge with a deterministic detachment limited incision model in which incision rate is a power function of basal shear stress above a critical shear stress our model predicts a power law relationship between steady state slope and drainage area consistent with observations the exponent of this power law is independent of discharge mean and variability while the amplitude factor which controls mountain belt relief is a power law function of mean runoff with an exponent of 0 5 and a complex function of runoff variability in accordance with evidence that incision occurs between 6 and 20 of time in rapidly incising rivers 1 mm yr our model predicts that channel steepness is virtually insensitive to runoff variability runoff variability can only decrease channel steepness for very slow incision rates and or weak lithologies the relationship between channel steepness and incision rate is always a power law whose exponent depends on the channel cross sectional geometry and runoff variability this contradicts models neglecting discharge stochasticity in which the steepness incision scaling is set by the incision law exponent our results suggest that changes in climate variability cannot explain an increase in bedrock incision rates during the late cenozoic within the context of a detachment limited model

stress transfer and connectivity between the bhutan himalaya and the shillong plateau abstract within the northern indian plate the shillong plateau is a peculiar geodynamic terrane hosting significant seismic activity outboard the himalayan belt this activity is often used as an argument to explain apparent reduced seismicity in the bhutan himalayas although current geophysical and geodetic data indicate that the bhutan himalayas accommodate more deformation than the shillong plateau we aim to quantify the extent to which the two geodynamic regimes are connected and potentially interact through stress transfers we compiled a map of major faults and earthquakes in the two regions and computed co seismic stress transfer amplitudes our results indicate that the bhutan himalayas and the shillong plateau are less connected than previously suggested major earthquakes in either of the two regions mainly affect transverse faults connecting them causing up to 40 bar coulomb stress change however this effect is clearly less on thrust faults of the either region up to 1 bar only the mw 8 25 1897 assam earthquake that affected the shillong plateau did not cause a stress shadow on the main himalayan thrust in bhutan as previously suggested similarly the mw 8 0 5 1714 bhutan earthquake had negligible impact on stress accumulation on thrust faults bounding the shillong plateau furthermore the main process shaping the regional stress patterns continues to be interseismic loading with complex boundary conditions in a diffuse deformation field involving the bengal basin and indo burman ranges while both the bhutan himalayas and the shillong plateau exhibit a compressional regime their stress evolutions are more weakly connected than hypothesized although our modelling suggests lateral increase in stress interactions from west less to east more in the bhutan himalayas a clearer picture will only emerge with better constrained fault geometries slip rates crustal structure and seismicity catalogues in the entire region of distributed deformation

spatially averaged long term erosion rates measured from in situ produced cosmogenic nuclides in alluvial sediment spatially averaged erosion rates of small catchments can be accurately inferred from the concentrations of cosmogenic nuclides in stream sediment here we test this technique at two catchments by comparing erosion rates inferred from cosmogenic nuclides with rates of alluvial fan deposition over the past 16 000 years these two independent estimates agree within one standard error demonstrating that cosmogenic nuclide signatures of stream sediment can be used to measure spatially averaged long term erosion rates using this technique we show that long term erosion rates are an exponential function of average hillslope gradient at these sites

collision of the ganges brahmaputra delta with the burma arc implications for earthquake hazard abstract we take a fresh look at the topography structure and seismicity of the ganges brahmaputra delta gbd burma arc collision zone in order to reevaluate the nature of the accretionary prism and its seismic potential the gbd the world s largest delta has been built from sediments eroded from the himalayan collision these sediments prograded the continental margin of the indian subcontinent by 400 km forming a huge sediment pile that is now entering the burma arc subduction zone subduction of oceanic lithosphere with 20 km sediment thickness is fueling the growth of an active accretionary prism exposed on land the prism starts at an apex south of the gbd shelf edge at 18 n and widens northwards to form a broad triangle that may be up to 300 km wide at its northern limit the front of the prism is blind buried by the gbd sediments thus the deformation front extends 100 km west of the surface fold belt beneath the comilla tract which is uplifted by 3 4 m relative to the delta this accretionary prism has the lowest surface slope of any active subduction zone the gradient of the prism is only 0 1 rising to 0 5 in the forearc region to the east this low slope is consistent with the high level of overpressure found in the subsurface and indicates a very weak detachment since its onset the collision of the gbd and burma arc has expanded westward at 2 cm yr and propagated southwards at 5 cm yr seismic hazard in the gbd is largely unknown intermediate size earthquakes are associated with surface ruptures and fold growth in the external part of the prism however the possibility of large subduction ruptures has not been accounted for and may be higher than generally believed although sediment clogged systems are thought to not be able to sustain the stresses and strain weakening behavior required for great earthquakes some of the largest known earthquakes have occurred in heavily sedimented subduction zones a large earthquake in 1762 ruptured 250 km of the southern part of the gbd suggesting large earthquakes are possible there a large but poorly documented earthquake in 1548 damaged population centers at the northern and southern ends of the onshore prism and is the only known candidate for a rupture of the plate boundary along the subaerial part of the gbd burma arc collision zone

cosmogenic nuclide evidence for low weathering and denudation in the wet tropical highlands of sri lanka 1 some of the lowest weathering and erosion rates in any mountain range in the world have been measured using cosmogenic nuclides in the steep humid tropical highlands of sri lanka the total preanthropogenic denudation rates were measured in creek sediments and soil samples from unperturbed rain forest sites bedrock from mountain crests and bedrock from inselbergs denudation rates are in the range of 5 30 t km 2 yr 1 2 11 mm ky 1 these rates average denudation over the last 50 250 ky weathering exports in rivers draining the mountainous central highlands show that silicate weathering rates are also low varying from 5 to 20 t km 2 yr 1 today 2 7 mm ky 1 but they represent a significant fraction of the total denudation all these observations run contrary to the conventional geomorphologic and geochemical wisdom that would predict rapid erosion for highlands of high relief temperatures and precipitation we speculate that the high relief in sri lanka represents the remnant of a geomorphic block that was uplifted during rifting at 130 ma or even earlier and that was reduced to the interior of the island by rapid receding of escarpments after continental breakup it is possible that throughout this history hillslopes where not exposing bare bedrock were protected by thick weathered profiles such clay rich layers would inhibit silicate weathering by shielding bedrock from weathering agents in the absence of landscape rejuvenation physical erosion rates are low and fresh mineral surfaces are not being supplied the observation that wet steep tropical highlands can have low rates of rock weathering and erosion has some potentially profound implications for the long term controls of atmospheric co2 budgets high temperature and precipitation which are much invoked though controversial agents for silicate dissolution and co2 drawdown become ineffective in promoting weathering in areas that are not tectonically active

estimating rates of denudation using cosmogenic isotope abundances in sediment we propose as a testable hypothesis a basin scale approach for interpreting the abundance of in situ produced cosmogenic isotopes an approach which considers explicitly both the isotope and sediment flux through a drainage basin unlike most existing models which are appropriate for evaluating in situ produced cosmogenic isotope abundance at discrete points on earth s surface our model is designed for interpreting isotope abundance in sediment because sediment is a mixture of materials in favourable cases derived from throughout a drainage basin we suggest that measured isotope abundances may reflect spatially averaged rates of erosion we investigate the assumptions and behaviour of our model and conclude that it could provide geomorphologists with a relatively simple means by which to constrain the rate of landscape evolution if a basin is in isotopic steady state and if sampled sediments are well mixed

large sedimentation rate in the bengal delta magnetostratigraphic dating of cenozoic sediments from northeastern bangladesh a 4 3 km thick section of clastic sediments of the surma basin northeastern bangladesh that is exposed along the shari river was sampled for a magnetostratigraphic study miocene through pleistocene ages have been discussed previously however there has been no firm dating through biostratigraphy or radiometric or other methods primary paleomagnetic remanent magnetizations of normal or reversed polarity were determined for most of the 300 collected samples on the basis of assumptions of 1 constant sedimentation rates on average throughout the deposition of the sedimentary sequence 2 no major gaps in sedimentation and 3 a fairly young 2 ma age for the topmost dupi tila deposits we find good agreement between the measured profile and the geomagnetic polarity time scale if the entire sedimentary sequence is between 4 9 and 1 4 ma in age the upper marine shales an important seismic marker horizon are dated as 3 5 ma and are therefore 1 5 m y younger than previously assigned by biostratigraphic results the upper marine shales presumably correspond to the last pre pleistocene sea level highstand the dating of rock units below the upper marine shales is tentative because of a large gap in exposure the inferred sedimentation rate and thus the rate of delta subsidence of 1 2 m k y is much larger than previously assumed it appears to be one of the highest sedimentation rates in earth history that was sustained for millions of years

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

coupling erosion and topographic development in the rainiest place on earth reconstructing the shillong plateau uplift history with in situ cosmogenic 10 be abstract the uplift of the shillong plateau in northeast india between the bengal floodplain and the himalaya mountains has had a significant impact on regional precipitation patterns strain partitioning and the path of the brahmaputra river today the plateau receives the highest measured yearly rainfall in the world and is tectonically active having hosted one of the strongest intra plate earthquakes ever recorded despite the unique tectonic and climatic setting of this prominent landscape feature its exhumation and surface uplift history are poorly constrained we collected 14 detrital river sand and 3 bedrock samples from the southern margin of the shillong plateau to measure erosion rates using the terrestrial cosmogenic nuclide 10be the calculated bedrock erosion rates range from 2 0 to 5 6 m my 1 whereas catchment average erosion rates from detrital river sands range from 48 to 214 m my 1 these rates are surprisingly low in the context of steep tectonically active slopes and extreme rainfall moreover the highest among these rates which occur on the low relief plateau surface appear to have been affected by anthropogenic land use change to determine the onset of surface uplift we coupled the catchment averaged erosion rates with topographic analyses of the plateau s southern margin we interpolated an inclined pre incision surface from minimally eroded remnants along the valley interfluves and calculated the eroded volume of the valleys carved beneath the surface the missing volume was then divided by the volume flux derived from the erosion rates to obtain the onset of uplift the results of this calculation ranging from 3 0 to 5 0 ma for individual valleys are in agreement with several lines of stratigraphic evidence from the brahmaputra and bengal basin that constrain the onset of topographic uplift specifically the onset of flexural loading and the transgression from deltaic to marine deposition ultimately our data corroborate the hypothesis that surface uplift was decoupled from the onset of rapid exhumation which occurred several millions of years earlier

speleogenesis of the world s longest cave in hybrid arenites krem puri meghalaya india abstract meghalaya nw india is one of the regions in the world with highest recorded rainfalls because of these peculiar climatic conditions and intense solutional weathering karstic caves are widely reported in the numerous limestone areas of this part of india likewise the extremely high rainfall and the tropical monsoon climate have fostered the formation of several caves in silicate rich lithologies the most important of these caves is krem puri over 25 5 km long characterized by a fracture controlled labyrinth forming a bi dimensional maze with mostly phreatic morphologies and some recent still active vadose entrenchments a detailed petrographic study of the host rocks has shown this cave to be formed along a set of quartzarenite hybrid arenite and calcarenite layers in a mostly siliciclastic sedimentary succession xrf elemental analysis of the different rock strata guiding the speleogenesis confirms that the early stage of cave development is confined mainly in the calcium carbonate rich layers geochemical analyses carried out on water flowing in the cave drips streams have shown that carbonate dissolution is the dominant chemical weathering process contributing to krem puri water solutes whereas the dissolution of quartz feldspars and clays play only a secondary role in rock weathering the morphology of the cave points to a lithology guided dissolution of the carbonate rich layers and piping of the residual quartz sand in phreatic conditions when the surface river network was not yet evolved and the water table was at the level of or higher than the cave this phreatic speleogenetic phase predates the start of river network entrenchment that started not earlier 5 ma ago and probably accelerated around 3 5 ma which deactivated the karst maze with a low denudation rate of 0 15 mm yr this active phreatic cave level which currently lies beneath a more or less 120 m overburden was originally covered by a sedimentary sequence at least 1 2 km thick after its decoupling from the underground drainage system denudation slowly brought the surface close to the maze system gradually introducing infiltration of vadose waters into the conduits this caused the formation of the vadose entrenchments probably

discrimination of fluvial eolian and neotectonic features in a low hilly landscape a dem based morphotectonic analysis in the central pannonian basin hungary abstract the godoll hills a low relief terrain within the central pannonian basin in hungary is characterised by moderate tectonic deformation rates although typical tectonic landforms are not clearly recognisable in the study area this paper succeeded in discriminating between tectonically controlled landforms and features shaped by fluvial erosion or deflation with no tectonic control dem based morphometric parameters including elevation slope and surface roughness enabled the delineation of two nw se trending spearhead shaped ridges separated by a wide rectilinear valley of the same strike although directional statistics suggested possible tectonic control of nw se striking landforms precise morphometry completed with an analysis of subsurface structures rejected their tectonic preformation deflation plays a significant role in shaping the area and the presence of two large scale yardangs separated by a wind channel is proposed in temperate continental areas of europe no deflational landforms of such scale have been described so far suggesting that pleistocene wind power in periglacial areas was more significant than it was previously thought characteristic drainage patterns and longitudinal valley profiles enabled the recognition of areas probably affected by neotectonic deformation a good agreement was observed between locations of quaternary warping predicted by the morphometric study and subsurface structures revealed by the tectonic analysis zones of surface uplift and subsidence corresponded to anticlinal and synclinal hinges of fault related folds in low relief and slowly deforming areas where exogenous forces may override tectonic deformation only the integrated application of morphometric and subsurface structural indications could assure correct interpretation of the origin of various landforms while a morphometric study alone could have led to misinterpretation of some morphometric indices apparently suggesting tectonic preformation on the other hand the described morphological expression of subsurface structures could verify quaternary age of the deformation