investigation of the relationships between basin morphology tectonic uplift and denudation from the study of an active fold belt in the siwalik hills central nepal the present study investigates correlations between an extensive range of geomorphic properties that can be estimated from a digital elevation model and the uplift rate on geological timescales the analysis focuses on an area in the siwalik hills central nepal where lithology and climate can be considered as uniform this area undergoes rapid tectonic uplift at rates of up to 15 mm yr 1 which are derived from the geometric pattern of a fault bend model of fold growth the selected geomorphic properties can be divided in two categories depending on whether or not the vertical dimension is taken into account none of the planar properties are significantly correlated to uplift rate unlike those that include the vertical dimension such as the mean elevation of basins hypsometric curve and hypsometric integral and relief defined by the amplitude factor of length scaling analysis correlation between relief and uplift rate is observed for all length scales of topography shorter than 600 m which suggests that all orders of the streams are able to adjust to the tectonic signal simple mass balance considerations imply that the average elevation is only 10 of surface uplift suggesting that a dynamic equilibrium has been reached quite rapidly using a simple two process model for erosion we find that fairly high diffusion coefficients order of 10 m 2 yr 1 and efficient transport of the material by rivers are required this unusually high value for mass diffusivity at small length scales may be obtained by either a very efficient linear diffusion or by landsliding actually both processes may be active which appears likely given the nature of the unconsolidated substratum and the favorable climatic conditions local relief in the study area may therefore be used to predict either uplift or denudation but the prediction is calibrated only for that specific climatic and lithologic conditions and cannot be systematically applied to other contexts

landsliding and the evolution of normal fault bounded mountains much of the tectonic and climatic history in high relief regions such as the mountains of the western u s basin and range province is contained in the morphology of hillslopes drainage networks and other landforms that range in scale from 10 1 to 101km to understand how these landforms evolve we have developed a numerical landscape evolution model that combines a detailed tectonic displacement field with a set of physically based geomorphic rules bedrock landsliding long recognized as a significant geomorphic process in mountainous topography is for the first time explicitly included in the rule set in a series of numerical experiments we generate synthetic landscapes that closely resemble mountainous topography observed in the basin and range the production of realistic landscapes depends critically on the presence of bedrock landslides and landsliding yields rates of long term erosion that are comparable in magnitude to those of fluvial erosion the erosive efficiency of bedrock landsliding implies that hillslopes may respond very quickly to changes in local base level and that fluvial erosion is the rate limiting process in steady state experimental landscapes our experiments generate power law distributions of landslide sizes somewhat similar to both field and laboratory observations thus even a simple model of bedrock landsliding is capable of quantitatively reproducing mountainous topography and landslide distributions and represents a significant step forward in our understanding of the evolution of normal fault bounded ranges

modes of operation of thermo mechanically coupled ice sheets by richard c a hindmarsh geoffrey s boulton grant institute of geology university of edinburgh west mains road edinburgh eh9 3jw scotland u k and kolumban hutter lnstitut fur mechanik technische hochschule oarmstadt hochschulstrasse i 0 6100 oarmstadt federal republic of germany dimensionless lapse rate a dimensionless model of thermo mechanically coupled ice sheets is used to analyse the operation of the system three thermal processes are identified i dissipation having a maximum time scale of thousands of years ii advection having a time scale of tens of thousands of years and iii conduction having a time scale of loo 000 years kinematical processes occur on two time scales i a marginal advective time scale of thousands of years and ii a diffusive time scale of tens of thousands of years dominant in the divide area

a numerical study of plane ice sheet flow l ecoulement bidimensionnel stationnaire d une nappe de glace reposant sur le sol est analyse numeriquement au moyen du modele simplifie de morland et hutter la glace se comporte comme un fluide visqueux non lineaire dependant de la temperature la nappe de glace est supposee longue et mince

styles of subglacial glaciotectonic deformation within the context of the anglian ice sheet a substantial proportion of the glacial sediments of east anglia england are chaotic and highly deformed this has been attributed to a direct glaciotectonic deformation or b slumping of glaciomarine sediments we show that interpretation a leads to a classification of subglacial glaciotectonic deposits into sites of constructional deformation where little or no erosion of the preglacial deposits has occurred and styles of deformation are preserved moving up in the sequence and excavational deformation where substantial erosion of preglacial sediments has occurred and deformational styles are superimposed on each other by using glaciological and geotechnical theory we show that the expression of these deformational styles in east anglia is a natural response to the stress fields imposed by the ice sheet and that in consequence their spatial relationships are explicable the depth of deformation is related to the geotechnical properties of till and substrate the stress and effective stress fields and found to be consistent with observations

supply and removal of sediment in a landslide dominated mountain belt central range taiwan abstract a strong coupling between hillslope and valley systems is often inferred for mountain landscapes dominated by bedrock landsliding we reveal the nature of this link using data sets on landsliding and sediment transport from two montane catchments draining the eastern central range of taiwan here the magnitude frequency distribution of landslides can be modeled by a robust power law but this scale invariance is not mirrored in the sediment discharge at the mountain front instead downstream sediment loads reflect a complex response to both sediment supply and ambient hydraulic conditions the rivers do not transport significant amounts of sediment unless it is provided by hillslope mass wasting in the catchment removal of landslide debris is a function of the transport capacity of the stream at the site of entry thus there is a dual supply and transport control on sediment loads in bedrock floored streams over a monitoring period of 25 yr the bulk of the sediment leaving the mountain bel

middle late miocene 10 ma formation of the main boundary thrust in the western himalaya three independent data sets from northwestern india and pakistan suggest initial displacement along 1000 km of the main boundary thrust prior to 10 ma at least 5 m y earlier than previously reported regionally extensive changes in the depositional characteristics and rates of the foreland basinfill between 11 and 9 5 ma are interpreted to reflect new hinterland loading due to the formation of the main boundary thrust sediment accumulation rates sandstone siltstone ratios and thickness and amalgamation of individual sandstone bodies all substantially increase after 11 ma in well dated stratigraphic sections from pakistan to nepal across the indo gangetic foreland basin in the himachal pradesh reentrant of northwestern india a newly discovered 8 7 ma conglomerate derived from the hanging wall of the main boundary thrust indicates that source area uplift and denudation must have occurred prior to 9 ma and probably prior to 10 ma assuming a gravelprogradationrateof3cm yr threeapatitefission trackages fromstructuresattheleadingedgeofthemainboundarythrustin the kohat region of northwest pakistan indicate that rapid cooling below 105 c1 2 m y earlier these data indicate that the main boundary thrust in the western himalaya formed synchronously along strike in the middle late miocene has a displacement rate of 10 mm yr and has a displacement history that is coeval with late displacement on the main central thrust

fabric analysis using the intercept method abstract the intercept method is a convenient yet powerful method for the numerical analysis of fabrics for geological applications it can be used when a set of objects e g pores grains or aggregates of a specific mineral lineaments etc with unique characteristics can be identified in an image the intercept method analyses boundaries of objects as a population of lines how intercepts are definined is therefore important in order to establish the significance of the intercept counts when the method is applied to digital images the subdivision of the information into square pixels must also be considered we apply a linear filter which minimizes the effects of grid anisotropy on the counting of intercepts analysis of the resulting rose of intercept counts by a fourier series considerably facilitates the interpretation of the data a rose of directions can readily be derived for the phase boundaries if a population of objects can be assumed to have started with an originally isotropic orientation and to have been deformed passively the fourier components of the rose of intercepts also give the sectional strain the fourier components also permit an objective test of the validity of describing a fabric as resulting from a homogeneous strain on an initially isotropic section two computer programs for the counting and analysis of intercepts on digital images are presented and applied in four examples which illustrate the usefulness of the intercept method for magmatic fabric measurement strain analysis and lineament analysis we also illustrate the importance of fourier series decomposition of rose of intercept counts for fabric interpretations intercept results are also shown to be compatible with those derived by other methods eigenvector surfor inertia tensor autocorrelation methods

neogene foreland basin deposits erosional unroofing and the kinematic history of the himalayan fold thrust belt western nepal sedimentological and provenance data from the lower miocene pliocene dumri formation and siwalik group in western nepal provide new information about the timing of thrust faulting and the links between erosional unroofing of the himalaya and the cenozoic 87 sr 86 sr record of the ocean in western nepal the dumri formation is an 750 1300 m thick fluvial sandstone and overbank mudstone unit the siwalik group is 4200 m thick and consists of a lower member 850 m of 2 12 m thick fluvial channel sandstones and oxidized calcareous paleosols a middle member 2400 m of very thick 20 m channel sandstones and mainly organic rich histosols and an upper member 1000 m composed of gravelly braided river deposits paleocurrent data indicate that middle miocene pliocene rivers in western nepal flowed southward transverse to the thrust belt throughout deposition of the siwalik group no evidence was found for an axial fluvial trunk system i e the paleo ganges river in siwalik group sandstones a major increase in fluvial channel size is recorded by the transition from lower to middle siwalik members at 10 8 ma probably in response to an increase in seasonal discharge modal petrographic data from sandstones in the dumri formation and the siwalik group manifest an upsection enrichment in potassium feldspar carbonate lithic fragments and high grade metamorphic minerals modal petrographic analyses of modern river sands provide some control on potential source terranes for the miocene pliocene sandstones the dumri formation was most likely derived from erosion of sedimentary and low grade metasedimentary rocks in the tibetan tethyan himalayan zone during early miocene emplacement of the main central thrust the presence in dumri sandstones of plagioclase grains suggests exposure of crystalline rocks of the greater himalayan zone perhaps in response to tectonic unroofing by extensional detachment faults of the south tibetan detachment system during deposition of the lower siwalik group 15 11 ma emplacement of the dadeldhura thrust sheet one of the synformal crystalline thrust sheets of the southern himalaya on top of the dumri formation supplied abundant metasedimentary lithic fragments to the foreland basin a steady supply of plagioclase grains and high grade minerals was maintained by deeper erosion into the main central thrust sheet from 11 ma to the present k feldspar sand increased steadily suggesting that granitic source rocks became widely exposed during deposition of the upper part of the lower siwalik group this provenance change was caused by erosion of passively uplifted granites and granitic orthogneisses in the dadeldhura thrust sheet above a large duplex in the lesser himalayan rocks since the onset of deposition of the conglomeratic upper siwalik group 4 5 ma fault slip in this duplex has been fed updip and southward into the main boundary and main frontal thrust systems we obtained 113 u pb ages on detrital zircons from modern rivers and siwalik group sandstones that cluster at 460 530 ma 850 1200 ma 1 8 2 0 ga and 2 5 ga an abundance of cambrian ordovician grains in the siwalik group suggests sources of siwalik detritus in the granites of the dadeldhura thrust sheet and possibly the greater himalayan orthogneisses the older ages are consistent with sources in the greater and lesser himalayan zones an overall upsection increase in zircons older than 1 7 ga suggests increasing aerial exposure of lesser himalayan rocks none of the detrital zircons even in the modern river samples yielded a cenozoic age that might suggest derivation from the cenozoic greater himalayan leucogranites but this may be attributable to the inheritance problems that characterize the u pb geochronology of the leucogranites when compared with recent studies of the 87 sr 86 sr composition of paleosol carbonate nodules and detrital carbonate in paleosols from the siwalik group the provenance data suggest that erosion and weathering of metamorphosed carbonate rocks in the lesser himalayan zone and cambrian ordovician granitic rocks of the crystalline thrust sheets in central and eastern nepal may have played a significant role in elevating the 87 sr 86 sr ratio of middle miocene synorogenic sediments in the indo gangetic foreland basin and the bengal fan as well as global seawater

the structural evolution of active fault and fold systems in central otago new zealand evidence revealed by drainage patterns abstract central otago in new zealand is an area of active continental shortening in which a peneplain surface cut into schist has been deformed by folds which are developed above buried reverse faults we use the drainage patterns in this region to demonstrate various processes in fold and fault growth and interaction that would be difficult to identify by other means in particular we show 1 how simple asymmetric folds can develop into box folds 2 how apparently continuous ridges were formed by the coalescing of quite separate propagating fold and fault segments 3 evidence for the relative ages or relative uplift rates of adjacent structures and 4 evidence for the propagation direction of folds or faults as they grow the few quantitative estimates we obtain for fault propagation rates suggest an increase in length of 10 50 m per earthquake on faults about 20 km long these estimates are very uncertain but are similar in magnitude to an estimate made in nevada for a normal fault of similar size and are also similar to predicted estimates from theoretical growth models they raise the question of whether fault growth earthquake recurrence rates and climate change can interact to produce semi regular discrete features in an active landscape

geometrical modeling of fault related folds a pseudo three dimensional approach abstract area balanced cross sections of ramp anticlines and tip line folds in thin skinned fold thrust belts can be quantitatively constructed using plane strain fault bend and fault propagation fold models cross sections alone however are somewhat inadequate for interpreting along strike changes in thrust related fold geometry complex interplay between along strike changes in fault slip and fault shape determines a range of geometric configurations which are difficult to visualize without the aid of quantitative three dimensional models we extend conventional two dimensional folding theories into the third dimension by allowing for continuous variations in fault slip and fault shape along strike geometric equations relating map view angles between fold axial surface traces are derived for a case assuming a uniform fault geometry and an along strike displacement gradient the resulting folds possess no unique fold axis but rather are characterized by multiple fold hingelines each corresponding to bends in the fault for typical displacement gradients observed in the field analysis of the models suggests that discernible differences exist between map patterns of fault bend folds and fault propagation folds whereas little difference exists between map patterns of similar and parallel fault bend folds map view angles between axial surface traces for parallel fault bend folds and fault propagation folds are a function of both ramp cutoff angle and fault slip whereas these same angles for similar fault bend folds are solely a manifestation of fault slip examples assuming uniform shortening and continuously varying fault geometry along strike are also presented fault geometry is varied along strike to simulate oblique and lateral ramps depending upon model type and fault shape parameter modified resulting folds exhibit either cylindrical or non cylindrical fold geometries closure is generated by a decrease in fold amplitude where there is a decrease in ramp cutoff angle along strike or where a fault laterally cuts up section in either instance closure develops without varying shortening along strike model map patterns resemble actual thrust related fold geometries and can be compared to maps of real structures to help predict subsurface geology

control of detachment dip on drainage development in regions of active fault propagation folding we have studied drainage development on active folds by using a numerical model that couples the kinematics of fault propagation folding to a physical description of surface processes modeling results suggest that the dip of the underlying detachment exerts a major control on drainage development for a nonzero detachment dip the lateral displacement gradient sets up an axial slope behind the growing fold that diverts drainage so that the fold is shielded from rivers with large incision capacity in this case the characteristic fault segment length rather than the relative rates of fold uplift and incision may control the spacing of transverse streams we apply our model to the foothills of the nepal himalayas and suggest that striking differences in drainage patterns between central and western nepal are controlled to a first order by variations in the dip of the underthrusting indian plate

introduction to special section on tectonics and topography this special section on tectonics and topography developed from an american geophysical union chapman conference convened in the late summer of 1992 in snowbird utah the intent of the gathering was to assemble a diverse group of earth scientists working on ultimately the same problem the interaction between crustal and surficial processes or euphemistically tectonics and topography through numerous and enthusiastic conversations it became clear to us prior to the chapman conference that many people were working on the interaction between tectonics and topography but about half of these were either unaware of the others or were aghast at the way in which the others simplified their subject and vice versa for the other half to our delight the conference brought forth a wonderful array of scientists from virtually all subdisciplines of geology and the desire for mutual help and a sympathetic ear was palpable

seismotectonics of the himalayan collision zone geometry of the underthrusting indian plate beneath the himalaya fault plane solutions and well determined focal depths of medium sized earthquakes topography and landsat imagery in conjunction with seismicity maps cross sections and available geological information are used to investigate the present tectonics of the himalayan continental collision zone most of the accurately located epicenters of events along the himalayan arc 78 e 95 e that occurred between 1961 and 1981 are concentrated in a narrow zone about 50 km wide lying between the northerly dipping main boundary mbt and main central mct thrusts most of these events are located just south of the mct though the epicenters of the events are in general well located their depths as determined by teleseismic travel time data are very unreliable events with accurately determined depths obtained from identification of surface reflected phases define a simple planar zone from about 10 km and 20 km depth with an apparent dip of about 15 this result is all the more remarkable considering that the events used were located along about an 1800 km length of the himalyan arc except for one all available focal mechanisms of events within this zone indicate shallow 30 north dipping thrusts this shallow north dipping zone apparently defines a part of the detachment that separates the underthrusting indian plate from the lesser himalayan crustal block the spatial extent and the geometry of this interplate thrust zone strongly indicate that the mbt and nearby subsidiary surface and blind thrusts rather than the mct are currently the most active structures of the himalayan arc we suggest that the great himalayan earthquakes m 8 occur along the same detachment surface as defined by the thrust type medium sized events events located to the south of the mbt and beneath the ganges foredeep show normal faulting with t axes perpendicular to the himalayan trend the above results suggest that the indian continental plate is underthrusting the himalayan crustal blocks in a relatively coherent and simple geometry and that this geometry is not much different from that observed along oceanic subduction zones the november 19 1980 earthquake that occurred near the mct near 88 5 e shows a predominantly strike slip focal mechanism one of the nodal planes of this mechanism is transverse to the himalayan structural grain and moreover this plane has a trend similar to that of the recently mapped yadong gulu rift in the tethyan himalaya and in southern tibet just northeast of the earthquake we interpret this predominantly left lateral strike slip mechanism to indicate a possible genetic relationship between transverse structural features in the underthrusting indian plate the kishangang basement fault and the upper himalayan blocks and tibet

geomorphic criteria to determine direction of lateral propagation of reverse faulting and folding fault related folds develop above active faults and as these faults propagate laterally so do the folds they produce geomorphic criteria useful in evaluating rates and direction of lateral propagation of active folds in the direction of propagation are 1 decrease in drainage density and degree of dissection 2 decrease in elevation of wind gaps 3 decrease in relief of the topographic profile along the crest 4 development of characteristic drainage patterns 5 deformation of progressively younger deposits or landforms and 6 decrease in rotation and inclination of forelimb all these criteria are consistent with lateral propagation but do not prove it the presence of more than one wind or water gap formed by the same stream however is strong evidence of lateral propagation rates of lateral propagation of folding may be several times the rate of uplift and fault slip lateral propagation of anticlinal folds allows for a new explanation of how drainage may develop across active fold belts development of drainage across an active fold belt is probably a function of relatively long structurally controlled drainage diversion parallel to fold axes and development of relatively short antecedent stream reaches around the nose plunge panel of a fold water and or wind gaps form as uplift drainage diversion and stream capture associated with fold growth continue

coupled thermomechanical response of an axisymmetric cold ice sheet this study proposes a detailed model for ice sheets having a radially symmetric geometry governing relations are derived from first principles of physics and the interactive effects of mechanical and thermal energy are taken into account the resulting model is a nonlinear partial differential equation relating the temperature velocity and stress fields boundary conditions of accumulation rate geothermal flux sliding laws and surface temperatures having been specified and the model determines a steady state response in terms of ice sheet profile and internal temperatures velocities and stresses we describe the computational methodology for numerically determining solutions from which one can assess the credibility of the model itself and to the extent that it is credible examine effects such as perturbing of external temperatures accumulation rates and so forth piloting examples illustrate this

the reconstruction of former ice sheets and their mass balance characteristics using a non linearly viscous flow model on a utilise un modele d ecoulement visqueux non lineaire pour etudier l influence des caracteristiques du bilan de masse des conditions aux limites sur le fond rocheux et de la topographie sous glaciaire sur l etendue et la forme des calottes glaciaires le but est de rendre possible l utilisation des traces geologiques de l extension glaciaire passee pour reconstituer les paleoclimats

structural evolution of the irish variscides the effects of the variscan orogeny on the upper palaeozoic succession are recognizable throughout central and southern ireland the deformation can be zoned using structural styles and trends which illustrate a northward decrease in deformation intensity coupled with increasing control by pre variscan structures in the basement the most intense deformation occurs to the south of the dingle dungarvan line where shortening approaches 50 as a result of layer parallel shortening followed by buckling and finally thrusting the deformation is considered to have developed above a shallow dipping sole thrust which has been located offshore by the swat seismic reflection profiles cross faults which are common throughout the orogen resulted from n s shear couples this is consistent with their origin as accommodation structures due to differential shortening along strike the original shape of the munster basin has exerted a control on the distribution and overall facing of the major structures the sole thrust extends to the north of the dingle dungarvan line and the structural zonation of the orogen is due to variations in the stratigraphic level of this thrust the leinster massif is considered to be essentially autochthonous slip on the sole thrust decays towards the massif which acted as a rigid passive block and was partially responsible for the arcuate trend of the orogen

hillslope development in areas of active tectonics tectonic and geomorphic displacements of the earth s surface control topographic profile development therefore their analysis should be combined in the model presented here transient finite difference solutions to the continuity equation for material transport determine geomorphic displacements the material transport rate is a function of distance from the divide to the power m local slope to the power n and a rate constant values of m and n may be adjusted to simulate processes varying from rainsplash and soil creep i e diffusive m 0 n 1 to slope wash and river flow m 0 n 0 the actual geomorphic displacements may be transport or weathering limited depending on soil profile development superimposed edge dislocations in an elastic half plane are used to model tectonic displacements slip along a normal or reverse fault of any dip depth and down dip length may be incremental earthquake or continuous aseismic creep considering climate and material properties constant the ratio of the transport capacity rate constant to the fault slip rate roughly determines form this model extends existing morphologic diffusion erosion analyses to include other geomorphic conditions and processes transport or weathering limited conditions material flux boundary conditions and the development of gullies and knickpoints and more heterogeneous spatial and temporal distributions of tectonic displacement such as those due to slip along buried thrust faults we advocate calibration of these parameters and processes to provide a quantitative approach to modeling landform development determining deformation rates and inferring earthquake hazards

seismotectonics of the nepal himalaya from a local seismic network the national seismological network of nepal consists of 17 short period seismic stations operated since 1994 it provides an exceptional view of the microseismic activity over nearly one third of the himalayan arc including the only segment between longitudes 78 e and 85 e that has not produced any m 8 earthquakes over the last century it shows a belt of seismicity that follows approximately the front of the higher himalaya with most of the seismic moment being released at depths between 10 and 20 km this belt of seismicity is interpreted to reflect interseismic stress accumulation in the upper crust associated with creep in the lower crust beneath the higher himalaya the seismic activity is more intense around 82 e in far western nepal and around 87 e in eastern nepal western nepal between 82 5 and 85 e is characterized by a particularly low level of seismic activity we propose that these lateral variations are related to segmentation of the main himalayan thrust fault the major junctions between the different segments would thus lie at about 87 e and 82 e with possibly an intermediate one at about 85 e these junctions seem to coincide with some of the active normal faults in southern tibet lateral variation of seismic activity is also found to correlate with lateral variations of geological structures suggesting that segmentation is a long lived feature we infer four 250 400 km long segments that could produce earthquakes comparable to the m 8 4 bihar nepal earthquake that struck eastern nepal in 1934 assuming the model of the characteristic earthquake the recurrence interval between two such earthquakes on a given segment is between 130 and 260 years

geomorphic evidence for tear faults accommodating lateral propagation of an active fault bend fold wheeler ridge california abstract wheeler ridge is the topographic expression of an actively growing fault bend fold developed in the hanging walls of a system of mostly north vergent blind thrusts the eastern tip of the fold system has migrated laterally by approximately 3 km during the last 120 ka perpendicular to the regional shortening direction comparison of surface topography and structural cross sections indicates that eastern wheeler ridge comprises three fold segments whose topographic expression steps southward towards the eastern end of the structure lateral propagation of the fold is associated with numerous tear faults expressed as fault scarps that face towards the eastern end of the fold we suggest that these tear faults accommodate earthquake related processes that collectively build plunge of the fold and lateral propagation on blind thrusts we envisage that the fold will grow primarily by thrust faulting events with similar displacement s along strike that are terminated abruptly at tear faults build displacement over multiple earthquake cycles and then step eastward to form a new tear fault formation of especially large tear faults is inferred to be in response to an increase in rock strength governed by lithology the high fault displacement vs length d l for wheeler ridge may be related to high strain rates across a restraining bend in the nearby san andreas strike slip fault system

a theoretical treatment of the sliding of glaciers in the absence of cavitation a model is proposed for the description of glacier sliding which includes the nonlinearity of the flow law for ice the model describes coupled flow problems in the basal ice and a thin water film together with a temperature problem in the underlying bedrock to determine the sliding law relating basal velocity to basal stress the sliding theory should be formulated as a boundary layer to the larger scale bulk ice flow dimensional analysis indicates that the regelative component of ice velocity may be neglected provided roughness is absent at the smallest wavelengths and then the ice flow effectively uncouples from the other problems in this case with the crucial but unrealistic assumption that the flow law for temperate ice is independent of the moisture content there exist complementary variational principles that describe the functional form of the sliding law and give bounds on the magnitude of the 9roughness9 coefficient these principles are valid for nonlinear stress strain rate relations and for non vanishing bedrock corrugation and indicate how the basal velocity is determined by two parameters that together describe the degree of roughness of the bed specific estimates are then given finally the main weakness in the model as a predictor of quantitatively accurate results is pointed out that is that the variation of moisture within the basal layer and the resultant effect on the flow law are neglected a valid description of this phenomenon does not yet appear to be available

bedrock incision rock uplift and threshold hillslopes in the northwestern himalayas the topography of tectonically active mountain ranges reflects a poorly understood competition between bedrock uplift and erosion dating of abandoned river cut surfaces in the northwestern himalayas reveals that the indus river incises through the bedrock at extremely high rates 2 12 mm yr 1 in the surrounding mountains the average angles of hillslopes are steep and essentially independent of erosion rate suggesting control by a common threshold process in this rapidly deforming region an equilibrium is maintained between bedrock uplift and river incision with landsliding allowing hillslopes to adjust efficiently to rapid river down cutting

the siwaliks of western nepal i geometry and kinematics abstract the siwalik group which forms the southern zone of the himalayan orogen constitutes the deformed part of the neogene foreland basin situated above the downflexed indian lithosphere it forms the outer part of the thin skinned thrust belt of the himalaya a belt where the faults branch off a major decollement md that is the external part of the basal detachment of himalayan thrust belt this decollement is located beneath 13 ma sediments in far western nepal and beneath 14 6 ma sediments in mid western nepal i e above the base of the siwalik group unconformities have been observed in the upper siwalik member of western nepal both on satellite images and in the field and suggest that tectonics has affected the frontal part of the outer belt since more than 1 8 ma several north dipping thrusts delineate tectonic boundaries in the siwalik group of western nepal the main dun thrust mdt is formed by a succession of 4 laterally relayed thrusts and the main frontal thrust mft is formed by three segments that die out laterally in propagating folds or branch and relay faults along lateral transfer zones one of the major transfer zones is the west dang transfer zone wdtz which has a north northeast strike and is formed by strike slip faults sigmoid folds and sigmoid reverse faults the width of the outer belt of the himalaya varies from 25 km west of the wdtz to 40 km east of the wdtz the wdtz is probably related to an underlying fault that induces a a change of the stratigraphic thickness of the siwalik members involved in the thin skinned thrust belt and particularly of the middle siwalik member b an increase from west to east of the depth of the decollement level and c a lateral ramp that transfers displacement from one thrust to another large wedge top basins duns of western nepal have developed east of the wdtz the superposition of two decollement levels in the lower siwalik member is clear in a large portion of the siwalik group of western nepal where it induces duplexes development the duplexes are formed either by far travelled horses that crop out at the hangingwall of the internal decollement thrust id to the south of the main boundary thrust or by horses that remain hidden below the middle siwaliks or lesser himalayan rocks most of the thrusts sheets of the outer belt of western nepal have moved toward the s sw and balanced cross sections show at least 40 km shortening through the outer belt this value probably under estimates the shortening because erosion has removed the hangingwall cut off of the siwalik series the mean shortening rate has been 17 mm yr in the outer belt for the last 2 3 ma

evolution of the santa cruz mountains california through tectonic growth and geomorphic decay i describe a strategy for modeling a specific large scale topographic feature that recognizes the discrepancy between tectonic and geomorphic scales and allows for important feedbacks between these processes in the geomorphic model the largest channel in each 4 4 km cell is explicitly treated the channel incision being driven by a stream power rule hillslopes internal to each cell respond rapidly to incision of the local channel prompting their treatment in the model as steady state forms i present analytic expressions for the local relief which is controlled by the local channel incision rate and rate constants associated with the dominant hillslope process either diffusive or landsliding initial conditions include any preexisting topography and a lightly etched a channel network the model is illustrated using the santa cruz mountains california i show that an earlier hypothesis that the santa cruz mountains are due to advection of topography past an uplift source related to a restraining bend in the san andreas fault remains viable when more specific geomorphic processes are considered the local tectonics are represented by crustal thickening necessitated by conservation of mass in the bend right lateral translation of crust with respect to the bend and flexural response to the distributed load the model produces reasonable geographic and statistical distributions of topography using rates of tectonic and geomorphic processes that are within the range of those measured locally this argues that this restraining bend is a long lived feature of the san andreas fault the slightly higher crest and shorter length of the southern santa cruz mountains may potentially be explained by slightly faster slip of the crust west of the fault with respect to the bend than of that to the east deflection is minimal beneath these relatively small mountains disallowing the interesting feedback of erosionally driven uplift the predicted pattern of exhumation could be used to guide the search for fission track sites that could aid in the testing of the model

basal sliding relations deduced from ice sheet data l analyse de l etat d equilibre isotherme d une masse glaciaire proposee par morland et johnson 1980 1982 est appliquee a des profils de surface connus au groenland et a la calotte glaciaire de l ile devon avec les distributions correspondantes des bilans de masse pour determiner la force tractrice tangentielle b la pression au fond b et la vitesse tangentielle b dans chaque cas on utilise en retour ces estimations sur ce fond pour construire par correlation aux moindres carres des representations polynomiales pour une fonction de la profondeur b dans la forme adoptee pour la loi de glissement b b b 1 m avec m 1

contemporary tectonics of the himalayan frontal fault system folds blind thrusts and the 1905 kangra earthquake the sub himalayan fold thrust belt consists of deformed late cenozoic and older deposits south of the main boundary thrust mbt in pakistan east of the indus river the sub himalaya comprises the potwar plateau and the salt range which is thrust southward over the jhelum river floodplain along the salt range thrust although an estimated 9 14 mm a 1 shortening has been taken up on the salt range thrust during the last 2 ma the range front scarp does not show signs of recent faulting shortening may be shifting southward to the lilla overpressured anticline which rises from the jhelum floodplain as a fault propagation fold farther east shortening is partitioned among several anticlines underlain by foreland and hinterland dipping blind thrusts southeast of the main deformation zone the pabbi hills overpressured anticline is best explained as a fault propagation fold throughout the potwar plateau and salt range thrusts and folds rise from a basal decollement horizon in eocambrian evaporites the pakistani part of the decollement horizon could generate large earthquakes only if these evaporites die out northward at seismogenic depths in india and nepal the sub himalaya is narrower reflecting the absence of evaporites and a steeper slope of the basement towards the hinterland the southern boundary of the sub himalaya is the himalayan front fault discontinuous because part of the shortening is expressed at the surface by folding broad alluvial synclinal valleys dun valleys are bounded on the south by rising barrier anticlines of siwalik molasse the 1905 kangra earthquake m8 produced uplift on the mohand anticline and the dehra dun valley suggesting that this earthquake occurred on a decollement horizon above basement downdip from the fold if so the kangra event is the largest known earthquake on a blind thrust expressed at the surface as a fold

displacement gradients on fault surfaces abstract the maximum displacements and the dimensions of single faults are systematically related and the variation in displacement from a maximum at the centre of a fault to zero at an elliptical tip line loop is described by a simple theoretically derived expression these relationships are used to examine the theoretical ranges and distributions of rates of change of displacement on fault surfaces the dimensions and maximum displacement of a fault can be estimated from a limited number of displacement gradient measurements estimates are further constrained by a knowledge of the effective shear modulus of the rocks containing a fault an understanding of displacement gradients can be applied to problems commonly encountered in mining operations and in the interpretation and use of seismic reflection data in addition to field problems a comparison of theoretical displacement gradients with measured displacement gradients from british coalfield faults shows that i fault surface ellipticities are between 1 5 and 2 5 and ii gross displacement gradients expressed as the ratio of maximum displacement long axis radius of the fault ellipse range from 0 02 to 0 002 the measured displacement gradients indicate that the shear moduli of the faulted rocks range from 3 to 11 gpa which is within the known range for coal measure rocks displacement gradients along an array of fault segments are similar to those of single faults for analysis of displacement gradients it is necessary to sum the discontinuous and continuous components of displacement in extreme cases all the displacement may be accommodated by continuous deformation

propagation of the thrust system and erosion in the lesser himalaya geochemical and sedimentological evidence centre de recherches pe trographiques et ge ochimiques bp 20 54501 vandoeuvre les nancy franceabstractsedimentological and nd isotope data of two sections of the sub himalaya of westernnepal are used as new constraints for understanding the erosion history of the himalaya throughout the deposition of the middle and upper members of the siwalik group thelesser himalaya contribution to the total detrital input progressively increased from lessthan 20 to 40 the increasing proportion of lesser himalaya sediments started at ca 10 8 ma and is associated with a coarsening of the maximum grain size at both micro scopic and macroscopic scales thin skinned tectonics of the lesser himalaya thrust sys tem would have controlled the exhumation of the lesser himalaya rocks and would havebegun at 12 10 ma taking into account the delay for denudation together with otherstudies these data restrict the onset of movement on the lesser himalaya thrust systemto less than 3 m y along more than 1750 km this short time frame implies a ratio oflateral propagation rate to shortening rate far too high for the propagation of a singlecrustal thrust thus we suggest instead the simultaneous initiation of several thrusts aheadof the main central thrust at ca 12 ma we suggest that a rapid rise of the tibetanplateau at this time has transformed the himalaya to an overcritical thrust wedge thathas propagated forward to return to a stable state this regional rising could be the primecause of the increase of sediment in ux at ca 11 ma around the himalaya keywords siwalik formations neodymium grain size erosion thrust sheets himalaya

growth and lateral propagation of fault related folds in the siwaliks of western nepal rates mechanisms and geomorphic signature 1 we study the controls on drainage development in tectonically active regions using a numerical tectonic surface processes model combining tectonic uplift caused by fault related folding with erosion by fluvial incision hillslope diffusion and landsliding our model shows the fundamental control exerted by the dip of the detachment underlying the folds on drainage evolution when the detachment is horizontal the relative rates of tectonic uplift and fluvial incision control the evolution for a nonzero dip in contrast the lateral displacement gradient associated with fold propagation sets up a lateral slope behind the active structure which deflects the stream network we also demonstrate the importance of landsliding for the attainment of realistic and steady state topography in contrast discontinuous tectonic movements do not appear to influence the system we apply our model to the dundwa fault related fold ridge siwalik nepal we estimate a remarkably low mean propagation rate for different segments of the structure this finding together with the structure and morphology of the ridge leads us to propose that the ridge developed by linkage of several component segments the drainage evolution predicted when modeling this scenario compares favorably with field observations our models provide insights into the dynamics of fault related fold propagation in particular as a result of fault segment linkage estimates of propagation rate of these structures may be strongly scale dependent and the observed morphology and drainage patterns on fault related fold ridges may be controlled by fault geometry rather than by the relative rates of tectonics and surface processes

gps measurements of present day convergence across the nepal himalaya the high elevations of the himalaya and tibet result from the continuing collision between india and asia which started more than 60 million years ago1 4 from geological and seismic studies of the slip rate of faults in asia5 it is believed that approximately one third of the present day convergence rate between india and asia 58 4mmyr 1 is responsible for the shortening uplift and moderate seismicity of the himalaya great earthquakes also occur infrequently in this region releasing in minutes the elastic strain accumulated near the boundary zone over several centuries and accounting for most of the advance of the himalaya over the plains of india the recurrence time for these great earthquakes is determined by the rate of slip of india beneath tibet which has hitherto been estimated indirectly from global plate motions6 from the slip rates of faults in asia7 8 from seismic productivity9 and from the advance of sediments on the northern ganges plain10 here we report geodetic measurements using the global positioning system gps of the rate of contraction across the himalaya which we find to be 17 52 2 mm yr 1 from the form of the deformation field we estimate the rate of slip of india beneath tibet to be 20 5 2 mmyr 1 strain sufficient to drive one or more great himalayan earthquakes with slip similar to that accompanying the magnitude 8 1 bihar nepal 1934 earthquake may currently be available in western nepal

a healing reloading feedback control on the growth rate of seismogenic faults spatial and temporal variations in the growth rates of faults are explained in terms of a stress feedback mechanism operating in the seismogenic upper crust it is based on the idea that seismic rupture of a fault perturbs the surrounding stress field advancing the occurrence of future earthquakes on some faults that are optimally oriented while relaxing stress levels on others if post slip healing is geologically rapid then the earthquakes that are thus induced will contribute to reloading along the earlier rupture zone because of the symmetry of the optimal geometry a positive feedback is set up so that even in areas that are undergoing uniform tectonic straining some faults develop higher displacement rates and grow more rapidly while others experience reduced rates or become inactive using a thin plate elastic model for lithospheric scale faulting it is shown that this healing reloading feedback mechanism drives rapid localisation and the formation of major through going faults moving at plate boundary velocities enhanced displacement rates compared to an isolated fault develop shortly after the onset of deformation along those faults which are optimally positioned in the overall fault population thus the formation of a new plate boundary fault zone is predetermined and is a consequence of rather than the precursor of preferentially high displacement rates also fault segments located at points of rupture symmetry e g the central portion of a fault zone are reloaded more frequently and develop higher displacement rates and consequently have longer segment lengths and or larger displacement to length ratios episodic fault movement through time is a general feature of the model these predictions are consistent with available field observations over a wide range of scales thus elastic brittle failure and healing appear to be important rheological components of the lithosphere on long time scales 104 106 y as well as on the time scale of earthquake recurrence

morphotectonic evidence from lateral propagation of an active frontal fold pakuashan anticline foothills of taiwan abstract the pakuashan anticline is uniquely suited for study of the forward and lateral growth of fault related folds the pakuashan ridge development arises from the late quaternary uplift of the most external thrust zone of the western foothills of taiwan from kaoshiung to taichung recent and active westward thrusting occurs at the front of the foothills the pakuashan anticline trending n 150 e in the northern part to n 000 in the southern part has been active throughout the quaternary period this activity is marked by geological structures tectonic geomorphology and seismicity a multisource and multiscale approach to study of the continental collision setting has been undertaken to combine tectonics sedimentology and geomorphology studies of fracture patterns allow identification of two main features of stress orientations a wnw ese compression direction and e w and n s extension directions quantitative geomorphic parameters have been used to define the morphotectonic evolution and to infer tectonic style along the mountain front geomorphic evidence provides significant information on the processes that govern lateral propagation of an active anticline quaternary terraces are uplifted tilted and folded over the pakuashan ridge drainage systems in areas of active compression give information on the thrust zone structures and their development steep drainage and high local relief indicate that the pakuashan anticline forms a well defined zone of high uplift especially in the southern part the two main controls on drainage in that area are rock strength in the hanging wall and propagation of the deformation towards the south

growth of faults by accumulation of seismic slip the accommodation of large strains in the upper crust is largely achieved by the accumulation of displacement on faults observation shows that as a fault accumulates displacement it grows in size i e its surface area and its length increase here we address the question for an increase in the amount of displacement on a fault by how much would the length of the fault change it is argued by cowie and scholz 1992a that the displacement on a fault is linearly related to the length of the fault this simple result is expanded upon in this paper by assuming that a fault accumulates displacement by repeated earthquakes two different approaches are presented the first approach considers the balance that must be achieved between the energy available for deformation and the work involved in creating new fault surface area as the fault grows the available energy is provided by changes in strain energy when the fault slips the second approach is to construct a geometrical model for fault growth using the scaling relationship between the slip during a single earthquake and the length of the rupture the total displacement on a fault is the sum of the slips contributed by many earthquakes the usefulness of these two approaches is that the growth of a fault over geologic time can be described by parameters that can be obtained from earthquake and fault data the models presented here predict that 1 the maximum amount a fault can grow in a single earthquake that ruptures the entire fault is of the order of 1 of its previous length and 2 the work of faulting may account for 10 of the total energy available during an earthquake the energy lost from the system is accounted for by work done against friction and seismic radiation consequences of fault growth for the segmentation and thus seismogenic potential of a fault over geologic time are discussed using predictions of the theory

antarctic ice streams a review an ice stream is a part of an inland ice sheet that flows rapidly through the surrounding ice the distinction between an ice sheet and an outlet glacier is clear in principle but muddy in practice many antarctic glaciers are intermediate in character the ross ice streams which flow through the west antarctic inland ice into the ross ice shelf are distinct in character differing even from other ice streams in the marine ice sheet of west antarctica their surface elevation profiles are low their bed slopes are low and smooth and their driving stresses diminish monotonically downglacier in transverse profile they are broader in relation to ice thickness and exhibit shallower subglacial troughs than other ice streams many models for the fast sliding of glaciers have been applied to the ross ice streams most have included in some form a reduction in basal drag resulting from a lesser effective than glaciostatic pressure at the bed most models are only semi physical at best so need to be tuned by fitting to assumed parameters at some place or places in the real ice sheet the recent discovery of a very small effective pressure beneath one ice stream consequently has led to some gross errors in the velocities predicted by the models the difficulty may be resolved if it is true as recent experiments suggest that ice stream b and by extrapolation other ross ice streams as well slide on a deforming bed that absorbs most or all of the differential motion between the ice and the bedrock