tectonic control on 10 be derived erosion rates in the garhwal himalaya india erosion in the himalaya is responsible for one of the greatest mass redistributions on earth and has fueled models of feedback loops between climate and tectonics although the general trends of erosion across the himalaya are reasonably well known the relative importance of factors controlling erosion is less well constrained here we present 25 10 be derived catchment averaged erosion rates from the yamuna catchment in the garhwal himalaya northern india tributary erosion rates range between 0 1 and 0 5 mm yr 1 in the lesser himalaya and 1 and 2 mm yr 1 in the high himalaya despite uniform hillslope angles the erosion rate data correlate with catchment averaged values of 5 km radius relief channel steepness indices and specific stream power but to varying degrees of nonlinearity similar nonlinear relationships and coefficients of determination suggest that topographic steepness is the major control on the spatial variability of erosion and that twofold to threefold differences in annual runoff are of minor importance in this area instead the spatial distribution of erosion in the study area is consistent with a tectonic model in which the rock uplift pattern is largely controlled by the shortening rate and the geometry of the main himalayan thrust fault mht our data support a shallow dip of the mht underneath the lesser himalaya followed by a midcrustal ramp underneath the high himalaya as indicated by geophysical data finally analysis of sample results from larger main stem rivers indicates significant variability of 10 be derived erosion rates possibly related to nonproportional sediment supply from different tributaries and incomplete mixing in main stem channels

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

orogen response to changes in climatic and tectonic forcing despite much progress many questions remain regarding the potential dynamic coupling between atmospheric and lithospheric processes in the long term evolution of mountain belts as a complement to recent efforts to discover the interrelationships among climate topography erosion and rock deformation under conditions of mass flux steady state we explore orogen response to changes in climate and tectonic influx we derive an analytical model that predicts a powerful climatic control on orogen evolution and distinct potentially diagnostic responses to climatic and tectonic perturbations due to isostatic compensation the near surface rock uplift rate during transients is tightly coupled to climate modulated erosional efficiency system response is approximately exponential with a characteristic response timescale that is inversely proportional to the climate and lithology modulated erosional efficiency and is largely insensitive to initial conditions tectonic influx and both the sign and magnitude of perturbations

temporally and spatially uniform rates of erosion in the southern appalachian great smoky mountains we measured 1 0 be in fluvial sediment samples n 27 from eight great smoky mountain drainages 1 330 km 2 results suggest spatially homogeneous sediment generation on the 10 4 10 5 yr time scale and 100 km 2 spatial scale at 73 11 t km 2 yr 1 equivalent to 27 4 m m y of bedrock erosion this rate is consistent with rates derived from fission track long term sediment budget and sediment yield data all of which indicate that the great smoky mountains and the southern appalachians eroded during the mesozoic and cenozoic at 30 m m y in contrast unroofing rates during the paleozoic orogenic events that formed the appalachian mountains were higher 10 2 m m y erosion rates decreased after termination of tectonically driven uplift enabling the survival of this ancient mountain belt with its deep crustal root as an isostatically maintained feature in the contemporary landscape

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

geomorphic constraints on surface uplift exhumation and plateau growth in the red river region yunnan province china field observations digital elevation model dem data and longitudinal profi le analysis reveal a perched low relief upland landscape in the red river region yunnan province china which correlates to an uplifted regional low relief landscape preserved over the eastern margin of the tibetan plateau as with other major rivers of the plateau margin the red river has deeply incised the lowrelief upland landscape which we interpret to be the remnants of a pre uplift or relict landscape we examine longitudinal river profi les for 97 tributaries of the red river most profi les consist of three segments separated by sharp knickpoints an upper lowgradient channel segment a steeper middle channel segment and a very steep lower channel segment upper channel segments correspond to the relict landscape and have not yet experienced river incision steeper middle and lower segments indicate onset of rapid two phase river incision on the basis of which changes in external forcings such as climate or uplift can be inferred in terms of two end member scenarios two phase incision could be the result of pulsed plateau growth in which relatively slow uplift during the fi rst phase is followed by rapid uplift during the second phase or it could refl ect adjustments of the main channel to changing climate conditions against the backdrop of steady plateau growth reconstruction of the paleo red river indicates 1400 m river incision 1400 1500 m surface uplift and a maximum of 750 m vertical displacement across the northern red river fault elevating the northern ailao shan range above the surrounding relict landscape on the basis of stratigraphic constraints incision along the red river likely began in pliocene time

effects of bedrock landslides on cosmogenically determined erosion rates abstract the successful quantification of long term erosion rates underpins our understanding of landscape formation the topographic evolution of mountain ranges and the mass balance within active orogens the measurement of in situ produced cosmogenic radionuclides crns in fluvial and alluvial sediments is perhaps the method with the greatest ability to provide such long term erosion rates although deep seated bedrock landsliding is an important erosional process in active orogens its effect on crn derived erosion rates is largely unquantified we present a numerical simulation of cosmogenic nuclide production and distribution in landslide dominated catchments to address the effect of bedrock landsliding on erosion rates results of the simulation indicate that the temporal stability of erosion rates determined from crn concentrations in sediment decreases with increased ratios of landsliding to bedrock weathering rates within a given catchment area and that as the frequency of landsliding increases larger catchment areas must be sampled in order to accurately evaluate long term erosion rates in addition results of this simulation suggest that sediment sampling for crns is the appropriate method for determining long term erosion rates in regions dominated by mass wasting processes whereas bedrock surface sampling for crns may generally underestimate long term erosion rates response times of crn concentrations to changes in erosion rate indicate that climatically driven cycles of erosion may be detected but that complete equilibration of crn concentrations to new erosional conditions may take tens of thousands of years comparison of simulated crn derived erosion rates with a new data set of such rates from the nepalese himalaya underscore the conclusions drawn from the model results

control of detachment geometry on lateral variations in exhumation rates in the himalaya insights from low temperature thermochronology and numerical modeling 1 the himalayan range is commonly presented as largely laterally uniform from west to east however geological structures topography precipitation rate convergence rates and low temperature thermochronological ages all vary significantly along strike here we focus on the interpretation of thermochronological data sets in terms of along strike variations in geometry and kinematics of the main crustal detachment underlying the himalaya the main himalayan thrust mht we report new apatite fission track aft ages collected along north south transects in western and eastern central nepal at the latitudes of the annapurna and langtang massifs respectively aft ages are consistently young 3 ma along both n s transects in the high relief zone of the higher himalaya and increase 4 to 6 ma toward the south in the lesser himalaya we compare our new data to published low temperature thermochronological data sets for nepal and the bhutan himalaya we use the full data set to perform both forward and inverse thermal kinematic modeling with a modified version of the pecube code in order to constrain potential along strike variations in the kinematics of the himalayan range our results show that lateral variations in the geometry of the mht in particular the presence or absence of a major crustal scale ramp strongly control the kinematics and exhumation history of the orogen

neotectonics of the central nepalese himalaya constraints from geomorphology detrital 40ar 39ar thermochronology and thermal modeling 1 the southern flanks of the central nepalese himalaya correspond to a sharp transition in landscape morphology and bedrock mineral cooling ages that suggests a change in rock uplift rate this transition can be explained by either 1 accretion of footwall material to the hanging wall across a ramp in the decollement separating india from eurasia thereby enhancing rock uplift rates above the zone of accretion or 2 out of sequence surface thrust faulting at the physiographic transition here we use geomorphic data 649 new detrital 40ar 39ar cooling ages and a simple thermokinematic model to evaluate which of these tectonic configurations is most appropriate for the central nepalese himalaya we first define and delineate the physiographic transition in central nepal using maps of knickpoints river steepness indices local relief and the distribution of thick alluvial fill deposits we then report new detrital 40ar 39ar data from two trans himalayan transects each of which suggests a rapid northward increase in the total amount of exhumation across the physiographic transition thermokinematic modeling suggests that either of the two developmental scenarios for the transition is plausible but that an accretion model is viable only under an extremely narrow range of conditions we contend that the physiographic and thermochronologic data in our study area are most simply explained by recent out of sequence surface thrusting within the lesser himalayan metasedimentary sequence approximately 15 30 km south of the mapped surface trace of the main central thrust system an important finding of this work is that there are substantial along strike variations in physiography and thermal history that reflect along strike changes in the degree and location of out of sequence surface thrusting

landscape form and millennial erosion rates in the san gabriel mountains ca abstract it has been long hypothesized that topography as well as climate and rock strength exert first order controls on erosion rates here we use detrital cosmogenic 10 be from 50 basins ranging in size from 1 to 150 km 2 to measure millennial erosion rates across the san gabriel mountains in southern california where a strong e w gradient in relief compared to weak variation in precipitation and lithology allow us to isolate the relationship between topographic form and erosion rate our erosion rates range from 35 to 1100 m ma and generally agree with both decadal sediment fluxes and long term exhumation rates inferred from low temperature thermochronometry catchment mean hillslope angle increases with erosion rate until 300 m ma at which point slopes become invariant with erosion rate although this sort of relation has been offered as support for non linear models of soil transport we use 1 d analytical hillslope profiles derived from existing soil transport laws to show that a model with soil flux linear in slope but including a slope stability threshold is indistinguishable from a non linear law within the scatter of our data catchment mean normalized channel steepness index increases monotonically though non linearly with erosion rate throughout the san gabriel mountains even where catchment mean hillslope angles have reached a threshold this non linearity can be mostly accounted for by a stochastic threshold incision model though additional factors likely contribute to the observed relationship between channel steepness and erosion rate these findings substantiate the claim that the normalized channel steepness index is an important topographic metric in active ranges

tectonic control of yarlung tsangpo gorge revealed by a buried canyon in southern tibet the himalayan mountains are dissected by some of the deepest and most impressive gorges on earth constraining the interplay between river incision and rock uplift is important for understanding tectonic deformation in this region we report here the discovery of a deeply incised canyon of the yarlung tsangpo river at the eastern end of the himalaya which is now buried under more than 500 meters of sediments by reconstructing the former valley bottom and dating sediments at the base of the valley fill we show that steepening of the tsangpo gorge started at about 2 million to 2 5 million years ago as a consequence of an increase in rock uplift rates the high erosion rates within the gorge are therefore a direct consequence of rapid rock uplift

beyond threshold hillslopes channel adjustment to base level fall in tectonically active mountain ranges numerous empirical and model based studies argue that in general hillslopes and river channels increase their gradients to accommodate high rates of base level fall to date however few data sets show the dynamic range of both these relationships needed to test theoretical models of hillslope evolution and river incision here we utilize concentrations of 10 be in quartz extracted from river sand on the eastern margin of the tibetan plateau to explore relationships among short term 10 2 10 5 a erosion rate hillslope gradient and channel steepness our data illustrate nonlinear behavior and a threshold in the relationship between erosion rate and mean hillslope gradient confi rming the generalization that hillslopes around the world are limited by slope stability and cease to provide a metric for erosion at high rates 0 2 mm a the relationship between channel steepness index and erosion rate is also nonlinear but channels continue to steepen beyond the point where threshold hillslopes emerge up to at least 0 6 mm a demonstrating that channel steepness is a more reliable topographic metric than mean hillslope gradient for erosion rate and that channels ultimately drive landscape adjustment to increasing rates of base level fall in tectonically active settings

cover deformation above a blind duplex an example from west virginia u s a abstract pretectonic cover above blind thrust systems responds to underlying thrusting by combinations of forethrusting backthrusting and coupling the role of these responses was examined in the hanging rock cacapon mountain anticlinorium of the central appalachians this cover fold consists of middle ordovician and younger sedimentary rocks above a blind horse of cambro ordovician carbonates the cover has abundant folding and subsidiary faulting cover cleavage in carbonate and fine grained clastic rocks formed before regional folding in response to layer parallel shortening lps this 15 lps also produced grain suturing and fluid inclusion trails in quartzarenites the lps formed during cover forethrusting accommodating older blind horses in the hinterland with the formation of the underlying horse cover coupling occurred with bending folding additional secondary buckling and cleavage rotation by flexural flow with perhaps minor flattening this cover coupling was insufficient to accommodate horse formation no evidence exists for backthrusting but forelandwards abundant subsurface evidence exists for cover forethrusting with imbricate thrusting this thrusting accommodates horse formation and differs in style from the lps of the older forethrusting event so for this blind system the cover responded with a combination of coupling and forethrusting

formation of internally drained contractional basins by aridity limited bedrock incision 1 low internal relief aridity and internal drainage characterize the puna altiplano plateau and the tibetan plateau and the tarim basin structurally these areas are reverse fault bounded terrains with intervening wedge top basins that store thick accumulations of sediment for millions of years orographic barriers along the margins of these basins are oriented normal to moisture bearing winds resulting in regional aridity the combination of this aridity rapid shortening and uplift rates and resistant exposed lithologies coincides with the location of these internally drained basins we hypothesize that these large internally drained areas persist indefinitely when uplift overwhelms the fluvial systems and defeats the channel network to test this hypothesis we developed models of channel defeat to examine the threshold conditions required to fragment the channel network the results of these models suggest that low erodibility rocks moderate to high uplift rates and low precipitation favor defeat of orogen traversing channels most coupled tectonic landscape development models prevent the development of internal drainage and so may overestimate the voracity of erosion in regimes that favor internal drainage profoundly influencing predicted orogenic growth patterns

fluvial landscape response time how plausible is steady state denudation whether or not steady state topography and denudation are probable states depends on the timescale of system response to tectonic and climatic perturba tions relative to the frequency of those perturbations this paper presents analytical derivations of algebraic relations for the response time of detachment limited fluvial bedrock channel systems both to tectonic and climatic perturbations detachment limited fluvial erosion is described by the stream power incision model and the derivations are limited to the applicability of that model all factors likely to influence system response time that are not adequately captured by the stream power incision model will tend to increase the response time the calculations presented thus provide minimum estimates of landscape response time and therefore over predict the prob ability of attaining and sustaining steady state topography and denudation the central range of taiwan is used as a case study to estimate response times in a landscape often argued to be in steady state model parameters are fit to modern stream profiles by assuming that the topography represents a quasi steady state form estimated response times generally range from 0 25 to 2 5 ma depending on the non linearity of the incision rule and the magnitude and type of perturbation thus it may be reasonably argued that steady state topography and denudation are likely to prevail during periods of climatic stability response time is sufficiently short compared with plate tectonic timescales however rapid climatic fluctuation in the quaternary appears to preclude the attainment of steady state conditions in modern orogens motivation

scaling and elevation in river networks the analysis of large river networks obtained from digital elevation models has given insight into the variation of channel slope with scale investigators have recently suggested that channel slopes are self similar with magnitude or area as a scaling parameter our data indicates otherwise in particular the variance of channel slope is larger than that predicted by simple self similarity this suggests multiscaling the scaling exponent for the standard deviation is approximately half the corresponding exponent in the relationship of the slope mean to magnitude or area a model for channel slopes based on a point process of elevation drops along the channel is presented the model reproduces observed multiscaling properties when the density of elevation increments is related to area or magnitude as a

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

the age and rate of displacement along the main central thrust in the western bhutan himalaya abstract in western bhutan the main central thrust mct is broadly folded creating multiple exposures of the fault surface over a 70 km across strike distance this unusual map pattern presents a unique opportunity to map the mct and document both the magnitude and age of displacement in situ th pb sims and la icp ms geochronology of metamorphic monazite from the immediate hanging wall of the mct indicates that prograde monazite growth in greater himalayan gh rocks continued until 20 8 1 1 ma whereas crystallization of in situ melts characterized by high y monazite overgrowths occurred during cooling from ca 15 10 ma prograde monazite growth at 15 ma in lesser himalaya lh rocks in the immediate footwall requires that lh footwall strata began to be buried at this time and the mct had reached its southernmost exposed extent by combining prograde monazite ages in the immediate hanging wall and footwall the duration of mct displacement is bracketed between 20 8 1 1 and 15 0 2 4 ma immediately north of our study area a published estimate of shearing along the outer south tibetan detachment std argues for displacement between 20 and 15 ma coeval with the age range for mct displacement that we document in this study however retrograde monazite grains as young as 10 ma suggest that gh rocks were cooling until 10 ma 5 myr later than motion on the outer std immediately to the north this cooling was either the result of continued displacement on the mct or growth of a duplex that passively folded the mct using a sequential reconstruction we estimate a total displacement of 230 km which is the sum of displacements on the mct and the structurally lower paro thrust over a duration of 5 8 2 6 myr this indicates a horizontal shortening rate of 4 0 3 2 1 3 cm yr which exceeds present rates estimated from geodetic measurements across the himalaya and mct displacement rates c 2 cm yr inferred from petrologic and thermal models in central nepal but is indistinguishable from plate convergence rates calculated for eastern bhutan between 23 and 20 ma 3 3 0 7 cm yr our study highlights that displacement on the mct alone achieved plate velocity rates in western bhutan and that the age and rate of mct displacement varied significantly across the himalaya

structural evolution of connecting splay duplexes and their implications for critical taper an example based on geometry and kinematics of the canyon range culmination sevier belt central utah abstract critical taper in an evolving orogenic wedge can be maintained by structural thickening in the back of the wedge this is often achieved by folding and uplift of pre existing large internal thrust sheets by the continued growth and or reactivation of duplexes in the hinterland the growth of a blind imbricate complex off a lower thrust provides another mechanism for folding a higher thrust individual splays in a blind imbricate complex may have fault propagation folds and or fault bend folds associated with them if the splays reach the overlying thrust they serve as connecting splays between the lower thrust floor and the upper thrust roof thereby forming a connecting splay duplex csd this type of duplex geometry requires periodic reactivation of the pre existing roof thrust the canyon range thrust in central utah is an internal thrust that is folded into an overturned antiform synform pair and overrides its own synorogenic conglomerates the same conglomerates are infolded into the core of the synform and preserve critical timing information regarding the history of folding and thrust reactivation the folding is caused by the development of a footwall antiformal stack in the core of the canyon range antiformal culmination the geometry and timing relationships suggest that the structure grew as a csd regional timing relationships suggest that the growth of the csd and reactivation of the thrust took place during the later part of the evolution of the orogenic wedge and may have helped the wedge maintain critical taper during thrusting in the external fold and thrust belt

debris flow dependent variation of cosmogenically derived catchment wide denudation rates catchment wide denudation rates cwdrs obtained from cosmogenic nuclides are an efficient way to determine geomorphic processes quantitatively in alpine mountain ranges over holocene time scales these rate estimations assume steady geomorphic processes here we use a time series 3 yr in the aare catchment central swiss alps to test the impact of spatially heterogeneous stochastic sediment supply on cwdrs our results show that low frequency high magnitude debris flow events significantly perturb cosmogenic nuclide 10 be 14 c concentrations and thus cwdrs the 10 be concentrations decrease by a factor of two following debris flow events resulting in a doubling of inferred cwdrs the variability indicates a clear time and source dependency on sediment supply with restricted area weighted mixing of sediment accordingly in transient environments it is critical to have an understanding of the history of geomorphic processes to derive meaningful cwdrs we hypothesize that the size of debris flows their connectivity with the trunk stream and the ability of the system to sufficiently mix sediment from low and high order catchments control the magnitude of cwdr perturbations we also determined in situ 14 c in a few samples in conjunction with 10 be these data suggest partial storage for colluvium of a few thousand years within the catchment prior to debris flow initiation

channel changes in badlands stripping of the vegetation and soil from a 13 hectare site in virginia underlain by coastal plain sediments created a rapidly evolving badland topography two types of channels developed 1 sand bed alluvial channels were graded to transport the bed material load supplied from slope erosion with available runoff but they also generally eroded their beds slowly and 2 steeper bedrock floored channels incised rapidly in bedrock channels the erosion rate was proportional to the 4 9ths power of drainage area and the 2 3rds power of gradient these exponents are consistent with a model in which the erosion rate is proportional to the bed shear during high flows due to rapid mass wasting and reduced runoff the alluvial channels became as much as 50 steeper during the winter than the summer with an attendant yearly cycle of winter aggradation and summer entrenchment the gradients their seasonal variability and their downstream hydraulic geometry were consistent with the predictions of total load transport formulas for sand beds and high loads the hydraulic geometry of alluvial channels in the virginia badlands were similar to that on the morrison formation in the western united states

10be derived himalayan denudation rates and sediment budgets in the ganga basin abstract the himalayas represent the archetype of mountain building due to active continental collision and are considered in many studies as the locus of intense interactions between climate denudation and tectonics estimates of modern denudation rates across the entire range remain however relatively sparse in this study in situ produced cosmogenic 10 be concentrations were measured in detritic quartz in order to determine basin scale denudation rates for the central part of the himalayan range river sand was sampled over several years in the main trans himalayan rivers from the himalayan front to the ganga outlet in bangladesh the calculated 10 be denudation rates of the trans himalayan river basins range from 0 5 to 2 4 mm yr 1 average 1 3 mm yr 1 and vary by up to a factor of 3 between sampling years these denudation rates strongly contrast with the 0 007 mm yr 1 denudation rate of southern tributary basins draining the indian craton this work also shows that in the ganga basin no systematic evolution of average 10 be concentrations is observed during floodplain transfer implying that distal samples can be used to estimate the integrated denudation rate of the whole central himalayan range samples from the ganga in bangladesh display remarkably low variability in 10 be concentration implying an average himalayan denudation rate of 1 0 1 1 mm yr 1 however within the floodplain several samples suggest a recent perturbation of sediment transport dynamics with a recent increase in the relative sediment contribution from southern tributaries the himalayan sediment flux deduced from the 10 be denudation rate of the range is 610 230 mt yr 1 this flux is consistent within uncertainty with sediment fluxes derived from sediment gauging the similarity of the two flux estimates suggests that himalayan erosion fluxes have remained stable over the last centuries even if the large uncertainties associated with each method hamper more precise assessments

how steep are the himalaya characteristics and implications of along strike topographic variations differences in the topography of central nepal and bhutan are characterized by map and profile patterns of elevation slope relief and stream gradients nepal exhibits a narrow hinterland zone of extreme relief steep slopes and channels and deep fluvial dissection and a wide foreland zone of lower relief active shortening on the main frontal thrust occurs far outboard of the steep topography in contrast bhutan has two high relief zones separated by a narrow low relief step in the topographic and river profiles the southern zone has the higher relief rising abruptly from the main frontal thrust to a series of 4000 m peaks whereas the northern zone consists of low relief high elevation plateau like terrain cut by widely spaced steep walled valleys spatial relationships between these geomorphic features and geologic contrasts are used to examine scenarios of tectonic and surface process interactions the observations favor a model of along strike segmentation in which timing differences in the varying balance of uplift and erosion produce spatial differences of topographic and geologic patterns although other interpretations are possible

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

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

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

climate and other factors in the development of river and interfluve profiles in bhutan eastern himalayas abstract the longitudinal profiles of the main n s aligned rivers and the crests of the interfluve mountain ranges of bhutan have been plotted against latitude the river profiles are highly variable even between branches of the same system the main rivers in eastern bhutan are antecedent and rise in tibet they have irregular concave bed profiles in deep steeply sided valleys the rivers further west rise on the southern slopes of the high himalaya they have stepped profiles with steep concave sections in gorges through the southern mountains and one or more concave sections upstream separated by knickpoints all of the n s interfluve ranges rise steeply from the piedmont some then dip to major passes before again rising irregularly northwards to the high himalaya whilst others continue to climb northwards as irregular monoclines the combination of various types of river and interfluve profiles creates a range of valley forms the heterogeneity means that it is not possible to generalise about a typical bhutanese river interfluve or valley relief profile there is no indication that the rivers of bhutan have more knickpoints than those of the central and western himalayas rainfall and runoff data soils and natural vegetation have been examined for indications of significantly drier conditions in eastern bhutan the rainfall data show an eastwards decrease in the southern foothills probably due to the rainshadow cast by the meghalaya plateau to the south but mean annual totals are about or above three metres throughout and the whole of this zone has a wet climate there is no marked e w climatic trend in the drier interior of bhutan we attribute the general topographic structure of bhutan and the variability of river and interfluve profiles and valley forms more to tectonic factors than to climatic variation

numerical and analytical models of cosmogenic radionuclide dynamics in landslide dominated drainage basins 1 in many tectonically active regions on earth landslides dominate sediment delivery to channels while cosmogenic radionuclides crn represent a valuable tool to document basin averaged erosion rates a better understanding of crn dynamics in these systems is required before erosion rates can be reliably inferred from crn concentrations the stochastic nature of landslides results in variability of crn concentrations in both alluvial and hillslope samples that is both difficult to document and poorly understood to guide sampling strategies we developed a 2 d plan view model to simulate the influence of landslides on crn dynamics we employ a sediment mixing model in which concentrations of crns from material eroded by both landslides and steady processes are combined to evaluate how best to infer erosion rates from crn concentrations this model is further used to test a new analytical solution for the distribution of crn concentrations on hillslopes in a landslide prone system results suggest that for landscapes dominated by deep 5 m landslides only large drainage basins 100 km2 provide consistently reliable estimates of erosion rates from crn concentrations for landscapes dominated by shallow landslides 0 7 2 m smaller basins can produce reliable estimates results further suggest that the distribution of concentrations from hillslope samples could be used to deduce the relative contribution of material derived from landslide and steady processes these findings underscore the need to consider carefully the sources of natural variability in sampling and analyzing crns if properly done the variability can reveal useful information about geomorphic processes in landslide prone areas

ductile extrusion of the higher himalayan crystalline in bhutan evidence from quartz microfabrics quartz textures measured from deformed quartz tectonites within the lesser himalaya and higher himalaya crystalline of bhutan show similar patterns orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense the asymmetry of c axis and a axis patterns consistently indicates top to the south shearing the obliquity of the texture and the inferred finite strain plane strain to moderately constrictional suggest the strain regime had a combination of rotational and irrotational strain path in most of the samples from the bhutan himalaya the inferred deformation mechanisms suggest moderate to high temperature conditions of deformation that produced the observed crystallographic preferred orientation much higher temperature of deformation is indicated in the quartz veins from a leucogranite the observed ductile deformation is pervasively developed in the rocks throughout the investigated area the intensity of deformation increases only slightly in the vicinity of the main central thrust simultaneous southward shearing within a large part of the higher himalaya crystalline near and above the main central thrust and normal faulting across the south tibetan detachment is explained by the tectonically induced extrusion of a ductily deforming wedge the process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes channel flow accounts for some observed phenomena in the himalayan orogen such as inverted metamorphic sequences near the main central thrust not related to an inversion of isotherms and the syntectonic emplacement of leucogranites into the extruding wedge locally leading to an inversion of isotherms due to heat advection

has focused denudation sustained active thrusting at the himalayan topographic front the geomorphic character of major river drainages in the himalayan foothills of central nepal suggests the existence of a discrete west northwest trending break in rock uplift rates that does not correspond to previously mapped faults the 40 ar 39 ar thermochronologic data from detrital muscovites with provenance from both sides of the discontinuity indicate that this geomorphic break also corresponds to a major discontinuity in cooling ages samples to the south are proterozoic to paleozoic whereas those to the north are miocene and younger combined these observations virtually require recent pliocene holocene motion on a thrust sense shear zone in the central nepal himalaya 20 30 km south of the main central thrust field observations are consistent with motion on a broad shear zone subparallel to the fabric of the lesser himalayan lithotectonic sequence the results suggest that motion on thrusts in the toe of the himalayan wedge may be synchronous with deeper exhumation on more hinterland structures in central nepal we speculate that this continued exhumation in the hinterland may be related to intense sustained erosion driven by focused orographic precipitation at the foot of the high himalaya

southward extrusion of tibetan crust and its effect on himalayan tectonics the tibetan plateau is a storehouse of excess gravitational potential energy accumulated through crustal thickening during india asia collision and the contrast in potential energy between the plateau and its surroundings strongly influences the modern tectonics of south asia the distribution of potential energy anomalies across the region derived from geopotential models indicates that the himalayan front is the optimal location for focused dissipation of excess energy stored in the plateau the modern pattern of deformation and erosion in the himalaya provides an efficient mechanism for such dissipation and a review of the neogene geological evolution of southern tibet and the himalaya shows that this mechanism has been operational for at least the past 20 million years this persistence of deformational and erosional style suggests to us that orogens like other complex systems can evolve toward steady state configurations maintained by the continuous flow of energy the capacity of erogenic systems to self organize into temporally persistent structural and erosional patterns suggests that the tectonic history of a mountain range may depend on local energetics as much as it does on far field plate interactions

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

geomorphic limits to climate induced increases in topographic relief recognition of the potential for strong dynamic coupling between atmospheric and tectonic processes has sparked intense cross disciplinary investigation and debate on the question of whether tectonics have driven long term climate change or vice versa it has been proposed that climate change might have driven the uplift of mountain summits through an isostatic response to valley incision because isostasy acts to compensate mean elevations the debate hinges on the question of whether climate change can significantly increase topographic relief or more precisely increase the volume of missing mass between summits and ridges here we show that in tectonically active mountain ranges geomorphic constraints allow only a relatively small increase in topographic relief in response to climate change thus although climate change may cause significant increases in denudation rates potentially establishing an important feedback between surficial and crustal processes neither fluvial nor glacial erosion is likely to induce significant isostatic peak uplift

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

exhumation of the main central thrust from lower crustal depths eastern bhutan himalaya geothermometry and mineral assemblages show an increase of temperature structurally upwards across the main central thrust mct however peak metamorphic pressures are similar across the boundary and correspond to depths of 35 45 km garnet bearing samples from the uppermost lesser himalayan sequence lhs yield metamorphic conditions of 650 675 c and 9 13 kbar staurolite kyanite schists about 30 m above the mct yield p t conditions near 650 c 8 10 kbar kyanite bearing migmatites from the greater himalayan sequence ghs yield pressures of 10 14 kbar at 750 800 c top to the south shearing is synchronous with and postdates peak metamorphic mineral growth metamorphic monazite from a deformed and metamorphosed proterozoic gneiss within the upper lhs yield u pb ages of 20 18 ma staurolite kyanite schists within the ghs a few metres above the mct yield monazite ages of c 22 1 ma we interpret these ages to reflect that prograde meta morphism and deformation within the main central thrust zone mctz was underway by c 23 ma u pb crystallization ages of monazite and xenotime in a deformed kyanite bearing leucogranite and kyanite garnet migmatites about 2 km above the mct suggest crystallization of partial melts at 18 16 ma higher in the hanging wall south verging shear bands filled with leucogranite and pegmatite yield u pb crystallization ages for monazite and xenotime of 14 15 ma and a 1 2 km thick leuco granite sill is 13 4 0 2 ma thus metamorphism plutonism and deformation within the ghs con tinued until at least 13 ma p t conditions at this time are estimated to be 500 600 c and near 5 kbar from these data we infer that the exhumation of the mct zone from 35 to 45 km to around 18 km occurred from 18 to 16 to c 13 ma yielding an average exhumation rate of 3 9 mm year 1 this process of exhumation may reflect the ductile extrusion by channel flow of the mctz from between the overlying tibetan plateau and the underthrusting indian plate coupled with rapid erosion

a short duration pulse of ductile normal shear on the outer south tibetan detachment in bhutan alternating channel flow and critical taper mechanics of the eastern himalaya in easternmost bhutan the south tibetan detachment std is a ductile shear zone that juxtaposes the radi or sakteng klippe of the tethyan sedimentary series from underlying high grade greater himalayan rocks in situ la icpms u th pb analysis of metamorphic monazite from the immediate footwall and hanging wall of the std within the shear zone at the base of the klippe constrains north vergent normal shear to between 25 and 20 ma coeval thrusting on the main central thrust during this time supports a phase of channel flow viscous wedge model activity lasting only ca 3 ma geochronologic data from the eastern himalaya indicate alternating mechanisms for extrusion of the metamorphic core of the orogen from the late oligocene through to the late miocene switching from channel flow viscous wedge behavior to critical taper frictional wedge behavior each phase lasting approximately only 2 to 5 ma the tectonic evolution of the eastern himalaya is comparable to central and western himalayan tectonics during the early miocene but during the middle miocene metamorphism and magmatism in the eastern himalaya migrated towards the orogenic hinterland a process not widely documented elsewhere in the himalaya thus highlighting the need for an orogenic model in three spatial dimensions

preliminary stratigraphic and structural architecture of bhutan implications for the along strike architecture of the himalayan system article i nfo preliminary mapping and stratigraphic correlation of lesser himalayan rock in eastern bhutan using field characteristics u pb detrital zircon dating and nd geochemistry define the first order stratigraphic architecture of the indian passive margin sequence in the eastern himalaya we use this new image of the lateral and vertical relationships of the original stratigraphy to determine the structural framework of the eastern himalayan fold thrust belt in bhutan we propose that lesser himalayan rock in bhutan can be divided into lower lesser himalayan rocks with a paleoproterozoic detrital zircon signal and upper lesser himalayan rocks with detrital zircon signals of 1000 500 ma the 500 ma detrital grains are from rocks in the frontal portions of the fold thrust belt north of the main boundary thrust as well as directly in the footwall of the main central thrust in the hinterland our preliminary stratigraphic study coupled with mapping allows us to construct a composite balanced cross section through the kuru chu valley in eastern bhutan which provides the first image of the geometry and amount of shortening through bhutan the main frontal thrust tilts a 6 km of thick section of neogene foreland basin deposits these deposits are separated from folded and faulted upper lesser himalayan rocks by the main boundary thrust north of the main boundary thrust we propose two duplex systems the southernmost duplex contains 9 repeated sections of upper lesser himalayan units including the permian gondwana sequence and the cambrian or younger baxa group the northernmost duplex is located in the footwall of the main central thrust and is comprised of two repeated sections of the proterozoic shumar and daling formations the southern boundary of this northern duplex is the shumar thrust which acts as a roof thrust for the southern duplex system and may correlate to the ramgarh thrust in nepal and india we propose that the development of the duplex systems passively folded overlying tethyan and greater himalayan rocks minimum shortening for this part of the himalayan fold thrust belt is 359 km all of which has occurred from 22 ma to present defining a long term shortening rate of 16 mm yr

river incision or diversion in response to bedrock uplift introductionmany researchers have examined the long term behavior of alluvialrivers i e schumm et al 1987 snow and slingerland 1990 paola et al 1992 on long time scales alluvial rivers act as sediment transport systemsthat broadly deposit sediment over low areas in their basins i e tucker andslingerland 1996 the behavior of bedrock rivers is less well understood although most workers agree that bedrock erosion is some function of streampower howard and kerby 1983 seidl and dietrich 1992 howard et al 1994 accordingly researchers addressing a river s response to bedrock up lift equate incision with relatively high stream power and diversion with rela tively low stream power e g burbank et al 1996 however these studiesassume that the erosion rate of the bedrock channel across the mountainrange controls the eventual outcome they have not examined the behavior ofthe alluvial river upstream and downstream of the uplift we propose to lookat the entire erosional and depositional river system and examine the long term response of a coupled alluvial bedrock river system to tectonic uplift our motivation is general and aims at a greater understanding offluvial and tectonic interactions but our model is necessarily both specificand simplified the tectonic uplift of a bedrock massif into the path of a later ally unconstrained alluvial river such a scenario has two possible out comes either the river maintains its original course and incises through theuplift or the river diverts and forms a new course around the uplift a sig nificant complication in the analysis of this scenario is that the evolvingriver system is partial alluvial and partial bedrock channeled because thebehaviors of these two river types are different modeling requires couplingdiverse systems we construct an analytical model to perform a first orderanalysis of this coupled system our objective is to identify the specific fac tors that determine the final outcome of river diversion or incision our model focuses on long time scales 10

dynamic reorganization of river basins river networks evolve as migrating drainage divides reshape river basins and change network topology by capture of river channels we demonstrate that a characteristic metric of river network geometry gauges the horizontal motion of drainage divides assessing this metric throughout a landscape maps the dynamic states of entire river networks revealing diverse conditions drainage divides in the loess plateau of china appear stationary the young topography of taiwan has migrating divides driving adjustment of major basins and rivers draining the ancient landscape of the southeastern united states are reorganizing in response to escarpment retreat and coastal advance the ability to measure the dynamic reorganization of river basins presents opportunities to examine landscape scale interactions among tectonics erosion and ecology

relation between rock uplift and denudation from cosmogenic nuclides in river sediment in the central alps of switzerland 1 a north south traverse through the swiss central alps reveals that denudation rates correlate with recent rock uplift rates in both magnitude and spatial distribution this result emerges from a study of in situ produced cosmogenic 10be in riverborne quartz in central alpine catchments as a prerequisite we took care to investigate the potential influence of shielding from cosmic rays due to snow glaciers and topographic obstructions to calculate a possible memory from last glacial maximum lgm glaciation and to identify a watershed size that is appropriate for systematic sampling mean denudation rates are 0 27 0 14 mm a for the alpine foreland and 0 9 0 3 mm a for the crystalline central alps the measured cosmogenic nuclide derived denudation rates are in good agreement with post lgm lake infill rates and are about twice as high as denudation rates from apatite fission track ages that record denudation from 9 to 5 ma in general denudation rates are high in areas of high topography and high crustal thickness the similarity in the spatial distribution and magnitude of denudation rates and those of rock uplift rates can be interpreted in several ways 1 postglacial rebound or climate change has introduced a transient change in which both uplift and denudation follow each other with a short lag time 2 the amplitude of glacial to interglacial changes in both is small and is contained in the scatter of the data 3 both are driven by ongoing convergence where their similarity might hint at some form of long term quasi steady state or 4 enhanced continuous quaternary erosion and isostatic compensation of the mass removed accounts for the distribution of present day rock uplift

infilling of the younger kathmandu banepa intermontane lake basin during the late quaternary lesser himalaya nepal a sedimentological study the kathmandu and banepa basins central nepal are located in a large syncline of the lesser himalayas the older kathmandu lake evolved during the pliocene and early pleistocene the younger kathmandu lake which is the focus of this study is infilled with late quaternary sediments three formations arranged in stratigraphical order the kalimati gokarna and thoka formations formed during the infilling stage of this lacustrine basin structural and textural sedimentological analyses a chemical survey across the basin and mineralogical investigations of fine grained sediments form the basis of this palaeogeographical study the basin under investigation was covered by a perennial freshwater lake before 30 000 yr bp the lake was infilled with alluvial and fluvial sediments delivered mainly from the mountains north of the basin a fairly low gradient was favourable for the formation of diatomaceous earths carbonaceous mudstones and siltstones which were laid down in the centre of the lake and in small ponds towards the basin edge lacustrine sediments gave way to deltaic deposits spread across the delta plain crevasse splays and anastomosing rivers mainly delivered suspended load for the widespread siltstones and mudstones the proximal parts of the alluvial fluvial sedimentary wedge contain debris flows that interfinger with fine grained floodplain deposits three highstands of the water level 30 000 yr bp 28 000 19 000 yr bp 11 000 4000 yr bp have been recognised in the sedimentary record of the younger kathmandu lake in the late quaternary second order water level fluctuations are assumed to be triggered by local processes damming by tectonically induced landslides first order water level fluctuations are the result of climatic changes copyright 2003 john wiley sons ltd

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

mechanics of low relief detachment folding in the bajiaochang field sichuan basin china using three dimensional 3 d seismic reflection data we apply new methods of two dimensional 2 d and 3 d structural restoration based on mechanical constraints to gain insights into the development of the bajiaochang anticline sichuan basin china this structure forms the trap for the bajiaochang field which is reported to contain substantial gas reserves within thick upper triassic delta front and lakeshore facies siliciclastics the basal detachment lies in a middle triassic evaporite unit above which layers are folded and faulted to various intensities based on their mechanical strengths the new restoration methods enable us to test a variety of fault displacement directions and to resolve that the bajiaochang structure grew primarily by dip slip and contractional folding moreover we quantify the styles and amounts of shortening within various tectonic levels of the structure large amounts of shortening and ductile thickening at the basal detachment level are consistent with general detachment fold models however we demonstrate that slip on the basal detachment is much larger than would be predicted by these models implying that substantial slip on the basal detachment extends beyond the structure into the foreland moreover structural thickening in the core of the fold is localized by a series of small ramps in the basal detachment that produce structural imbrication this implies that the bajiaochang detachment fold grew by a mechanism akin to duplexes which has important implications for the manner in which shortening and strain are partitioned within different mechanical layers of the fold

predictions of steady state and transient landscape morphology using sediment flux dependent river incision models 1 recent experimental and theoretical studies support the notion that bed load in mountain rivers can both enhance incision rates through wear and inhibit incision rates by covering the bed these effects may play an important role in landscape evolution and in particular the response of river channels to tectonic or climatic perturbation we use the channel hillslope integrated landscape development child numerical model with two different bedrock incision models that include the dual role of the sediment flux to explore the transient behavior of fluvial landscapes both models predict that steady state channel slopes increase in landscapes with higher rock uplift rates however the incision models predict different transient responses to an increase in uplift rate and the behavior of each incision model depends on both the magnitude of change in uplift rate and the local drainage area in some cases the transient channel behavior is indistinguishable from that predicted for transport limited alluvial rivers in other cases knickpoints form in some or all of the drainage network as predicted by the detachment limited stream power model in all cases the response in the lower parts of the network is highly dependent on the response in the upper parts of the network as well as the hillslopes as the upper parts of the network send more sediment downstream channel incision rates may rise or fall and slopes in the lower parts of the channel may in fact decrease at times during the transient adjustment to an increase in rock uplift rate in some cases channel incision in the upper parts of the network ceases during the transient while the hillslopes adjust to the new uplift rate drainage density may also change as a function of uplift rate our results suggest that if the sediment flux strongly controls bedrock incision rates then 1 the transient fluvial response will take longer than predicted by the detachment limited stream power model 2 changes in channel slope may be much more complex than predicted by the detachment limited stream power model and 3 changes in the fluvial system will be closely tied to sediment delivery from the hillslopes importantly our results outline quantitative differences in system behavior produced by competing models and provide a framework for identifying locations in natural systems where differences in channel morphology can be used to discern between competing fluvial erosion models

middle late quaternary paleoclimate of northern margins of the saharan arabian desert reconstruction from speleothems of negev desert israel abstract speleothems in arid and hyper arid areas of negev desert israel are used in paleoclimate reconstruction of northern margins of saharan arabian desert focused on the following objectives 1 precise u th dating of the timing of speleothem growth as an indicator of periods of humid climate i e positive effective precipitation 2 the origin of rainfall using the speleothem 18 o and changes in spatial pattern of speleothem deposition and speleothem thickness along a north south transect 3 changes of vegetation cover based on speleothem 13 c variations during the last 350 ka major humid periods referred to herein as negev humid periods nhp occurred in the central and southern negev desert at 350 310 ka nhp 4 310 290 ka nhp 3 220 190 ka nhp 2 and 142 109 ka nhp 1 nhp 4 nhp 2 and nhp 1 are interglacial events whereas nhp 3 is associated with a glacial period during nhp 1 2 and 3 the thickness and volume of the speleothems decrease from the north to the south and in the most southern part of the region only a very thin flowstone layer formed during nhp 1 with no speleothem deposition occurring during nhp 2 and 3 these data imply that the eastern mediterranean sea was the major source of the rainfall in northern and central negev more negative speleothem 18 o values relative to central parts of israel soreq cave are attributed to rayleigh distillation because of the increasing distance from the mediterranean sea speleothem deposition during the nhp 4 in the southern negev was more intensive than in most of the central negev suggesting the prominence of the tropical rain source decrease in speleothem 13 c during nhp events indicates growth of the vegetation cover nevertheless the ranges of 13 c values show that the vegetation remained semi desert c4 type throughout the nhps with an additional significant carbon fraction coming from the host rock and the atmosphere these observations together with small thickness of the speleothem layers favor that nhp events consisted of clusters of very short humid episodes interspersed with long droughts nhp events were contemporaneous with climate periods with monsoon index of 51 cal cm 2 day and with the formation of sapropel layers in the mediterranean sea such simultaneous intensification of the monsoon and atlantic mediterranean cyclones is probably related to the weakening of the high pressure cell above sub tropical atlantic ocean which enabled more rainfall to penetrate into the saharan arabian desert from the north and south the contemporaneous occurrence of the nhp events and the increased monsoon rainfall could have opened migration corridors creating climatic windows of opportunity for dispersals of hominids and animals out of the african continent

dynamic equilibrium among erosion river incision and coastal uplift in the northern and central apennines italy erosion river incision and uplift rates in the northern and central apennines italy since 0 9 ma are determined from new cosmogenic nuclide data beryllium 10 concentrations in modern and middle pleistocene sediments indicate erosion rates from 0 20 to 0 58 mm yr these rates are similar to estimates of sediment yield 0 12 0 44 mm yr river incision 0 35 mm yr and uplift 0 01 1 0 mm yr rates inferred from other methods that integrate landscape process rates since the early pleistocene these rates of landscape change are significantly lower than long term exhumation rates of 1 2 mm yr since ca 4 5 ma inferred from thermochronometry collectively these data suggest that hillslope erosion and river incision rates in the northern and central apennines have balanced local uplift rates for 1 my but that exhumation rates have slowed significantly since emergence of the mountain chain in the pliocene this condition of dynamic equilibrium was potentially achieved within ca 3 ma similar to some model predictions of hillslope and fluvial system adjustment

evidence for a wide and gently dipping main himalayan thrust in western bhutan the main himalayan thrust mht is the source of great earthquakes that have been documented along the range its geometry is a key parameter that influences accommodation of tectonic loading and earthquake magnitudes along the himalayan arc although seismic images are available for both the western and the central part of the range this geometry remains poorly constrained for the bhutanese himalayas here we address this issue using a 10be cosmogenic nuclides denudation transect across western bhutan we observe a wide low denudation rate domain between 50 km and 110 km from the front followed by a strong northward increase using a joint inversion of denudation rates gps data and holocene uplift rates we interpret this pattern as a consequence of a flat ramp transition along the mht compared to central nepal and sikkim this location of the ramp suggests a wider decollement with implications for greater seismogenic potential of the mht in western bhutan

40ar 39ar pressure temperature and fission track constraints on the age and nature of metamorphism around the main central thrust in the eastern bhutan himalaya ar 39 ar data from a profile across the main central thrust in the eastern bhutan himalaya indicate muscovite cooling ages of 14 1 0 2 ma from a sample in the immediate hanging wall of the thrust and 11 2 ma from about 400 m structurally higher in the hanging wall these two ages are repeated by two samples from 2 1 and 4 7 km vertical distance from the thrust within the hanging wall respectively a single apatite fission track age from the immediate hanging wall of the thrust gives an age of 3 1 0 6 ma pressure temperature estimates give temperatures around 6508c and 6 5 kbar for the highest sample collected samples closer to the main central thrust give also temperatures between 600 and 6508c at the same pressure indicating possibly a slight temperature decrease with proximity to the thrust however uncertainties are large and the parageneses are thermodynamically too highly variant to place much significance on their interpretation the 40 ar 39 ar cooling age data are consistent with a repetition of the sequence in the hanging wall of the thrust they confirm the data of davidson et al 1997 metamorphic reactions related to decompression and synkinematic intrusion of leucogranite high himalayan crystallines bhutan journal of metamorphic geology 15 593 612 and are consistent with a more rapid exhumation of deeper levels towards the centre of the high himalayan crystalline complex despite the large uncertainties the pt data shown here are also consistent with this interpretation the apatite fission track results reveal low temperature cooling and final exhumation of the main central thrust at the same time as in nepal q 2001 elsevier science ltd all rights reserved

in situ low relief landscape formation as a result of river network disruption the relict landscapes of southeast tibet are being formed in situ as a result of river drainage reorganization that renders rivers unable to balance tectonic uplift so these landscapes may not provide an unaltered record of past geomorphic conditions high elevation low relief surfaces or relict landscapes are generally thought to be uplifted old landscapes that preserve past tectonic and environmental conditions rong yang et al have tested current models of landscape evolution using digital topographic analysis of so called relict landscapes in southeast tibet and yunnan province china dramatic scenery boasting up to 3 km of relief encompassing the gorges of the yangtze mekong and salween rivers they find that the region may not provide an unaltered record of past geomorphic conditions rather these landscapes form in situ as a result of river drainage reorganization that renders rivers unable to balance tectonic uplift landscapes on earth retain a record of the tectonic environmental and climatic history under which they formed landscapes tend towards an equilibrium in which rivers attain a stable grade that balances the tectonic production of elevation and with hillslopes that attain a gradient steep enough to transport material to river channels equilibrium low relief surfaces are typically found at low elevations graded to sea level however there are many examples of high elevation low relief surfaces often referred to as relict landscapes1 2 or as elevated peneplains3 these do not grade to sea level and are typically interpreted as uplifted old landscapes preserving former more moderate tectonic conditions4 here we test this model of landscape evolution through digital topographic analysis of a set of purportedly relict landscapes on the southeastern margin of the tibetan plateau one of the most geographically complex climatically varied and biologically diverse regions of the world we find that in contrast to theory the purported surfaces are not consistent with progressive establishment of a new steeper river grade and therefore they cannot necessarily be interpreted as a remnant of an old low relief surface we propose an alternative model supported by numerical experiments in which tectonic deformation has disrupted the regional river network leaving remnants of it isolated and starved of drainage area and thus unable to balance tectonic uplift the implication is that the state of low relief with low erosion rate is developing in situ rather than preserving past erosional conditions

formation of fluvial hanging valleys theory and simulation 1 although only recently recognized hanging tributary valleys in unglaciated tectonically active landscapes are surprisingly common stream power based river incision models do not provide a viable mechanism for the formation of fluvial hanging valleys thus these disequilibrium landforms present an opportunity to advance our understanding of river incision processes in this work we demonstrate that thresholds apparent in sediment flux dependent bedrock incision rules provide mechanisms for the formation of hanging valleys in response to transient pulses of river incision we simplify recently published river incision models in order to derive analytical solutions for the conditions required for hanging valley formation and use these results to guide numerical landscape evolution simulations analytical and numerical results demonstrate that during the response to base level fall sediment flux dependent incision rules may create either temporary or permanent hanging valleys these hanging valleys form as a consequence of 1 rapid main stem incision oversteepening tributary junctions beyond some threshold slope or 2 low tributary sediment flux response during the pulse of main stem incision thus limiting the tributary s capacity to keep pace with main stem incision the distribution of permanent and temporary hanging valleys results from four competing factors the magnitude of base level fall the upstream attenuation of the incision signal the lag time of the sediment flux response and the nonsystematic variation in tributary drainage areas within the stream network the development of hanging valleys in landscapes governed by sediment flux dependent incision rules limits the transmission of base level fall signals through the channel network ultimately increasing basin response time

geometry and crustal shortening of the himalayan fold thrust belt eastern and central bhutan we present a new geologic map of eastern and central bhutan and four balanced cross sections through the himalayan fold thrust belt major structural features from south to north include 1 a single thrust sheet of subhimalayan rocks above the main frontal thrust 2 the upper lesser himalayan duplex system which repeats horses of the neoproterozoic cambrian baxa group below a roof thrust shumar thrust carrying the paleoproterozoic daling shumar group 3 the lower lesser himalayan duplex system which repeats horses of the daling shumar group and neoproterozoic ordovician jaishidanda formation with the main central thrust mct acting as the roof thrust 4 the structurally lower greater himalayan section above the mct with overlying tethyan himalayan rock in stratigraphic contact in central bhutan and structural contact above the south tibetan detachment in eastern bhutan and 5 the structurally higher greater himalayan section above the kakhtang thrust cross sections show 164 267 km shortening in subhimalayan and lesser himalayan rocks 97 156 km structural overlap across the mct and 31 53 km structural overlap across the kakhtang thrust indicating a total of 344 405 km of minimum crustal shortening 70 75 our data show an eastward continuation of lesser himalayan duplexing identified in northwest india nepal and sikkim which passively folded the overlying greater himalayan and tethyan himalayan sections shortening and percent shortening estimates across the orogen although minima do not show an overall eastward increase which may suggest that shortening variations are controlled more by the original width and geometry of the margin than by external parameters such as erosion and convergence rates

himalayan metamorphic sequence as an orogenic channel insight from bhutan the bhutan himalayas differ from the rest of the himalayas in two major ways i low grade sedimentary rocks lie above the greater himalayan sequence as klippen i e erosional remains of the south tibetan detachment and ii an out of sequence thrust the kakhtang thrust lies structurally above the klippen and it doubles the exposed thickness of the greater himalayan sequence our field observations and geochronological data constrain the main kinematic events in the bhutan himalayas crystallisation ages of leucogranite dykes deformed by the main central thrust and the south tibetan detachment indicate that these two structures operated together between 16 and 22 ma the out of sequence kakhtang thrust was active at 10 14 ma and was concurrent with reactivation of the south tibetan detachment restoration of the bhutan himalayas prior to the out of sequence thrusting shows that the greater himalayan sequence was the core of a long low viscosity crustal channel extending under the tibetan plateau we propose that the gravity driven southward extrusion of the channel material from underneath the tibetan plateau caused the inverted metamorphic sequence in the lesser himalayan sequence and in the greater himalayan sequence this process also led to occurrences of present day surface rocks that were derived from variable distances down dip but from similar crustal depths such an exhumation pattern can explain the similar peak pressures for the greater himalayan sequence along the length of the himalayas

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

variable shortening rates in the eastern himalayan thrust belt bhutan insights from multiple thermochronologic and geochronologic data sets tied to kinematic reconstructions 1 we present data on the burial displacement and exhumation history of the himalayan fold thrust belt in eastern bhutan these data document the magnitude and timing of displacement of large discrete structures and highlight temporal variability in shortening rates eight new40ar 39ar ages from white mica 32 new zircon u th he ages 7 new apatite fission track ages and 1 new u pb zircon la mc icp ms metamorphic rim growth age are combined with published cooling ages and deformation temperatures and incremental shortening magnitudes from restorations of two published balanced cross sections to illustrate the kinematic and temporal development of the bhutan thrust belt integrating these data from 23 ma to the present illustrates rapid horizontal shortening rates 28 35 mm yr between 23 20 ma and 15 10 ma separated by more moderate rates 10 23 mm yr shortening rates decrease significantly to 7 10 mm yr and possibly as low as 3 4 mm yr from 10 to 0 ma this decrease is interpreted to represent the onset of strain partitioning in the eastern part of the himalayan tibetan orogenic system between shortening in the bhutan thrust belt uplift of the shillong plateau and deformation and outward growth of the northern and eastern tibetan plateau within estimated error horizontal shortening rates during emplacement of the main central thrust sheet and during construction of the upper lesser himalayan duplex approached india asia tectonic velocities thus for periods of time between 23 20 ma and 15 10 ma the bhutan thrust belt may have absorbed nearly all india asian convergence at this longitude

holocene glacier fluctuations and migration of neolithic yak pastoralists into the high valleys of northwest bhutan abstract here we present geomorphologic palaeoenvironmental and archaeo botanical data which elucidate the late pleistocene and holocene glacial history of the high mountain locked himalayan valleys in northwest bhutan and provide one of the earliest proofs of human activity yet known for the high himalaya range in this area difficult to access close linkage between climatic change glacier fluctuations and human migration patterns has been discovered glacier systems in the studied area are characterized by avalanching and debris mantled glacier snouts with the significant local influence of the indian summer monsoon causing decoupling of glacier responses from temperature changes but supporting the idea of monsoonal forcing geomorphologic mapping together with optically stimulated luminescence osl and radiocarbon dating of ice proximal sediments has been used to construct a local glacial chronology local ice stream networks developed during the early holocene ca 10 000 9000 a ago and during the early part of the mid holocene 6710 90 4680 155 cal a bp at which times there were ice advances of about 5 km from the modern glacier termini at such times the intensity of pro and periglacial processes would have intensified and ice dammed lakes were probably common as well rendering human colonization of the high valleys in northwest bhutan impossible an abrupt shift to dry climatic conditions on the tibetan plateau between 5000 and 4500 a bp coincided with glacial decay and the onset of morphodynamically stable conditions on the broad valley floors of the high valleys in this part of the himalaya palynological data suggest that the sudden disappearance of juniper and rhododendron pollen the immediate onset of pollen input from cereals confirmed by detailed sem analysis and a clear pattern of over grazing trampling and peat deterioration can be linked to human arrival in the valleys at ca 4280 130 cal a bp extensive charcoal horizons dating to 4745 250 and 4680 155 cal a bp are interpreted as evidence for human use of fire and forest clearances and agree spatially and temporally with the pollen based picture charcoal occurrences as old as 6710 90 cal a bp might be linked to yet earlier exploration of these himalayan valleys during phases of low glacial activity we provide an account of the colonization of these high valleys in response to glacial and monsoonal change and argue that the most likely founder societies come from the tibetan plateau where yak and barley based pastoralism and neolithic settlements are known to have existed since the mid holocene

variability in the vertical hyporheic water exchange affected by hydraulic conductivity and river morphology at a natural confluent meander bend river confluences and their associated tributaries are key morphodynamic nodes that play important roles in controlling hydraulic geometry and hyporheic water exchange in fluvial networks however the existing knowledge regarding hyporheic water exchange associated with river confluence morphology is relatively scarce on the 14th and 15th of january 2016 the general hydraulic and morphological characteristics of the confluent meander bend cmb between the juehe river and the haohe river in the southern region of xi an city shaanxi province china were investigated the patterns and magnitudes of vertical hyporheic water exchange vhwe were estimated based on a one dimension heat steady state model while the sediment vertical hydraulic conductivity kv was calculated via in situ permeameter tests the results demonstrated that six hydrodynamic zones and their extensions were observed at the cmb during the test period these zones were likely controlled by the obtuse junction angle and low momentum flux ratio influencing the sediment grain size distribution of the cmb the vhwe patterns at the test site during the test period mostly showed upwelling flow dominated by regional groundwater discharging into the river the occurrence of longitudinal downwelling and upwelling patterns along the meander bend at the cmb was likely subjected to the comprehensive influences of the local sinuosity of the meander bend and regional groundwater discharge and finally formed regional and local flow paths additionally in dominated upwelling areas the change in vhwe magnitudes was nearly consistent with that in kv values and higher values of both variables generally occurred in erosional zones near the thalweg paths of the cmb which were mostly made up of sand and gravel this was potentially caused by the erosional and depositional processes subjected to confluence morphology furthermore lower kv values observed in downwelling areas at the cmb were attributed to sediment clogging caused by local downwelling flow the confluence morphology and sediment kv are thus likely the driving factors which cause local variations in the vhwe of fluvial systems

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

geologic constraints on bedrock river incision using the stream power law denudation rate in unextended terranes is limited by the rate of bedrock channel incision often modeled as work rate on the channel bed by water and sediment or stream power the latter can be generalized as ka m s n where k represents the channel bed s resistance to lowenng whose variation with lithology is unknown a is drainage area a surrogate for discharge s is local slope and m and n are exponents whose values are debated we address these uncertainties by simulating the lowering of ancient river profiles using the finite difference method we vary m n and k to match the evolved profile as closely as possible to the corresponding modern river profile over a time period constrained by the age of the mapped paleoprofiles we find at least two end member incision laws ka 0 3 0 5 s for australian rivers with stable base levels and k f a 0 1 0 2 s n for rivers in kauai subject to abrupt base level change the long term lowering rate on the latter expression is a function of the frequency and magnitude of knickpoint erosion characterized by k f incision patterns from japan and california could follow either expression if they follow the first expression with m 0 4 k varies from 10 7 10 6 m 0 2 yr for granite and metamorphic rocks to 10 5 10 4 m 0 2 yr for volcaniclastic rocks and 10 4 10 2 m 0 2 yr for mudstones this potentially large variation in k with lithology could drive strong variability in the rate of long term landscape change including denudation rate and sediment yield

intraplate deformation of the indian subcontinent 1 while deformation at the earth s surface primarily occurs along tectonic plate boundaries major earthquakes have shaken regions deep within continental interiors three of the largest m 7 5 historic intraplate earthquakes occurred within the indian subcontinent suggesting the possibility of significant intraplate deformation we consider surface velocities determined from new gps data collected at 29 continuous gps stations and 41 survey mode gps stations in india between 1995 and 2007 to find a north south shortening rate of 0 3 0 05 nanostrain yr 1 which may be accommodated by 2 1 mm yr of more localized convergence across central india southward motions at 4 7 mm yr of sites on the shillong plateau in northeast india reflect rapid shortening and high earthquake hazard associated with active thrust faults bounding the plateau the width and magnitude of the elastic strain accumulation field across the himalaya varies little from 76 90 longitude but the strain is more broadly distributed and convergence rates are higher along the eastern 200 km of the range

variable exhumation rates and variable displacement rates documenting recent slowing of himalayan shortening in western bhutan abstract we link exhumational variability in space and time to the evolving geometry of the himalayan fold thrust belt in western bhutan by combining new and published geochronologic and thermochronologic data we document the burial age peak temperatures and complete cooling history from 20 ma to the present over an across strike distance of 125 km these integrated cooling curves highlight windows of fast exhumation that vary spatially and temporally we propose that pulses of fast exhumation are a result of structures that facilitate the vertical motion of material illustrated in sequentially restored cross sections due to a range of permissible geometries at depth we explore and evaluate the impact of geometry on kinematics and rates of deformation the linked cooling history and cross sections provide estimates of both magnitude and timing of thrust sheet displacement and highlight temporal variability in potential shortening rates structural and chronologic data illustrate a general north to south progression of himalayan deformation with emplacement of the main central thrust mct paro thrust and shumar thrust by 12 to no later than 9 ma two different geometries and kinematic scenarios for the lesser himalayan duplex are proposed a north to south propagating duplex system requires that the southern portion of that system south of the mct deformed and cooled by 9 ma leaving only the southernmost thrust sheets including the main boundary and main frontal thrusts to deform between 9 and 0 ma this limited post 9 ma shortening would necessitate a marked slowdown in convergence accommodated on the main himalayan thrust a two tiered duplex system which allows for the paro window duplex and the southern baxa duplex to form simultaneously permits duplex formation and accompanying exhumation until 6 ma limited cooling from 200 c to the surface post 6 ma suggests either a decrease in shortening rates from 6 to 0 ma or that duplex formation and exhumation are temporally decoupled our combined cooling curves highlight that the youngest cooling ages may not mark the fastest thrusting rates or the window of fastest exhumation instead temporal variations in exhumation are best viewed through identifying transients in exhumation rate we suggest that the strongest control on exhumation magnitude and variability is fold thrust belt geometry particularly the locations and magnitudes of footwall ramps which can change over 10 s of km distance balanced cross sections predict the location and magnitude of these ramps and how they vary in space and time providing an untapped potential for testing permissible cross section geometries and kinematics against measured cooling histories

climatic limits on landscape development in the northwestern himalaya the interaction between tectonism and erosion produces rugged landscapes in actively deforming regions in the northwestern himalaya the form of the landscape was found to be largely independent of exhumation rates but regional trends in mean and modal elevations hypsometry frequency distribution of altitude and slope distributions were correlated with the extent of glaciation these observations imply that in mountain belts that intersect the snowline glacial and periglacial processes place an upper limit on altitude relief and the development of topography irrespective of the rate of tectonic processes operating

initiation and reactivation of faults during movement over a thrust fault ramp numerical mechanical models abstract with numerical continuum models we investigate the effects of fault geometry fault friction material properties and anisotropy on the initiation and reactivation of faults in the hanging wall of a thrust fault ramp the models use an elastic plastic frictional dilatant cohesion softening material in which deformation may localize as shear bands with a sharp lower fault bend backthrust shear bands propagate up from the concentration of differential stress at the fault bend with a rounded lower fault bend bedding parallel shear bands develop above the fault bend in the center of the layer where differential stress is highest friction on the ramp enhances the development of backthrusts and causes them to initiate at shallower dips increased overburden decreases the amount of localization within shear bands and the spacing between them shear bands that develop at the lower fault bend are weaker than the surrounding material and are thus potential sites of reactivation during subsequent deformation because of this potential reactivation the style of deformation at the upper fault bend depends on the deformation that accumulates at the lower fault bend at the upper fault bend backthrusts are reactivated as extension faults and are crosscut by more steeply dipping extension faults bedding parallel shear bands are reactivated at the upper fault bend with top to the hinterland sense of shear low angle extension faults are listric into the reactivated bedding parallel shear bands producing hinterland verging extensional duplexes above the upper flat

three holocene and late pleistocene glacial stages inferred from moraines in the lingshi and thanza village areas bhutan abstract moraines formed by valley glaciers in lingshi area northwestern bhutan constitute three distinct stages with contrasts in the spatial distribution and size of moraines and surface features such as clasts soils and vegetation moraines in the thanza village area northern bhutan show three stages with similar characteristics to those in the lingshi area using morphostratigraphic criteria these three stages correlate with some glacial stages in the khumbu himal eastern nepal dates from moraines in the khumbu himal provide a tentative chronological sequence of these three stages valley glaciers in bhutan expanded during the following periods 1 the takaphu and raphsthreng stages are the little ice age and late holocene glacial advances 2 the jykhuje and tenchey stages represent the late glacial and or early holocene glacial advances and 3 the lingshi dzong and lunana stages are coincident with the last glacial period

implications of sediment flux dependent river incision models for landscape evolution 1 developing a quantitative understanding of the factors that control the rate of river incision into bedrock is critical to studies of landscape evolution and the linkages between climate erosion and tectonics current models of long term river network incision differ significantly in their treatment of the role of sediment flux we analyze the implications of various sediment flux dependent incision models for large scale topography in an attempt 1 to identify quantifiable and diagnostic differences between models that could be detected from topographic data or from the transient responses of perturbed systems and 2 to explain the apparent ubiquity of mixed bedrock alluvial channels in active orogens although certain forms of the various models can be discarded as inconsistent with morphological data we find that the relative intrinsic concavity indices of detachment and transport limited systems defined herein largely dictate whether the various models can be tied to distinctive steady state morphologies preliminary data suggest that no such diagnostic differences may exist and other methods must be developed to test models accordingly we develop and explore differences in the scaling behavior of topographic relief and the extent of detachment versus transport limited channels as a function of rock uplift rate that may allow discrimination among various models further we explore potentially diagnostic differences in the rates and patterns of transient channel response to changes in rock uplift rate in addition to general differences between detachment and transport limited systems our analysis identifies an interesting hysteresis in landscape evolution hybrid channels at the threshold between detachment and transport limited conditions are expected to act as detachment limited systems in response to an increase in rock uplift rate or base level fall and as transport limited systems in response to a decrease in rock uplift rate especially during postorogenic topographic decline the analyses presented set the stage for field studies designed to test quantitatively the various river incision models that have been proposed

thrust timing growth of structural culminations and synorogenic sedimentation in the type sevier orogenic belt western united states most of the regional shortening in the type area of the sevier orogenic belt in central utah was accommodated by displacement on the canyon range neocomian aptian pavant aptian albian paxton cenomanian campanian and gunnison late campanian paleocene thrust systems inception of each thrust system generated synorogenic sediment associated with frontal thrust tip anticlines or triangle zones and older thrust sheets that were elevated above major ramps farther toward the hinterland the sevier culmination a large antiformal duplex cored by crystalline basement rocks developed during paxton and gunnison thrusting west of and structurally beneath the canyon range and pavant thrusts growth of the sevier culmination was coeval with reactivation of the canyon range and pavant thrust systems and produced a second culmination in proterozoic lower cambrian rocks in the canyon range these structural highs provided much of the sediment to the adjacent foreland basin from late cenomanian to late paleocene time and may have helped to maintain critical taper in the thrust belt