nucleation process with dilatant hardening on a fluid infiltrated strike slip fault model using a rate and state dependent friction law 1 dilatancy and fluid movements in a fault zone significantly affect the mode of the nucleation process and are one of the key factors for understanding the physical mechanisms of some short term precursory phenomena we investigated a model of the earthquake nucleation processes on a vertical fluid infiltrated strike slip fault in a three dimensional elastic half space using a dieterich ruina rate and state dependent friction law and an evolution equation for plastic porosity because of dilatancy porosity is thought to increase with slip velocity we assume that plastic porosity depends logarithmically on the state variable in the friction law the results of numerical simulations show that porosity in the fault zone increases with increasing slip velocity in the nucleation zone then pore fluid pressure in the fault zone drops significantly resulting in dilatant hardening with increasing dilatancy coefficient or with decreasing pore compressibility larger pore fluid pressure drop is observed during the nucleation process which in turn increases the size of the nucleation zone we observe that when a dilatancy coefficient is large a quasi static nucleation zone extends widely with a low stress drop finally slip velocity is accelerated in a localized area in the quasi static nucleation zone which is a place where a dynamic rupture originates the size of the accelerated slip zone is significantly smaller than that of the quasi static nucleation zone numerical results using a larger dilatancy coefficient can explain the extension of foreshock activity which is occasionally observed in large earthquakes

a criteria based methodology for determining the mechanism of transverse drainage development with application to the southwestern united states the study of how rivers cross obstructing mountains once popular in the early twenti eth century has seen a dramatic resurgence in the last decade since hutton s scholarly introduction to a possible cause for trans verse drainage however no single study has compiled all of the various criteria that can be used to discriminate among the four pos sible mechanisms of antecedence superimpo sition overfl ow and piracy this paper pres ents the fi rst such compilation and related methodology to apply these criteria both in tabular and graphical formats as well as an online interactive tool in the data repository combining nominal and ordinal data sources this methodology generates an objective reproducible assessment for the mechanism most likely to have established the trans verse drainage at fi ordinal levels of con fi dence when applied to southwestern u s sites randomly selected through an objec tive spatial procedure four general observa tions emerged on the relationship between the development of transverse drainage and landscape evolution 1 streams persisting through lengthy periods of extension develop antecedent canyons 2 in order to reestab lish through fl owing channels streams over fl ow closed basins as active extension wanes 3 following a drop in base level related to the newly developed trunk channels streams tributary to the trunk channels incise into basin fi ll deposits sometimes leading to the development of superimposed drainages 4 tributaries eroding headward in response to the integration of two or more closed basins can capture and redirect drainage this permits transverse drainage through both piracy and superimposition upstream of the capture event because extant criteria use nominal and ordinal data almost entirely considerable potential exists to refi ne this approach through future strategies that incorporate interval data future use of a criteria based method has the potential to inform on prior geomorphic studies by pro viding a new perspective with which to study how basins evolve in active tectonic regions the analysis of related basin sedimentation the hydrological and biological aspects of drainage evolution and transverse drainage found in martian crater fi elds

stream profile adjustment to crustal warping nonlinear results from a simple model three numerical experiments have been performed modeling response of a greatly simplified stream system to initiation of crustal tilting and domal uplift model results show signs of nonlinear behavior in varying degrees these signs include river profile adjustments that have forms quite different from the geometries of crustal movements cases of channel erosion following deposition at the same location and spatial transition points between opposing adjustments i e erosion and deposition that fail to coincide with boundaries or axes of crustal movement particular nonlinear effects wax or wane as the system adjusts to a new dynamic equilibrium with the continuing crustal movement the modeled features of stream adjustment are not in so extreme a class as the commonly discussed complex response of fluvial systems but they do serve to suggest some caution in studies that employ stream profile information as an indicator of crustal movement

distinguishing allogenic from autogenic causes of bed elevation change in late quaternary alluvial stratigraphic records abstract allogenic and autogenic mechanisms both cause changes in the bed elevation of rivers and thereby influence the characteristics of alluvial stratigraphic records asrs allogenic forcing mechanisms can be grouped into five categories whose relative influence varies with timescale climate tectonism base level land use land cover and direct human modification of channels late quaternary asrs are influenced by the greatest range of allogenic forcing variables with climate among the most important autogenic mechanisms of bed elevation change are ubiquitous throughout fluvial systems and are always time transgressive autogenic bed elevation change propagates through drainage networks at predictable rates and results in a time space envelope within which its effects are capable of operating asrs that can be correlated over geographical areas large enough and time intervals small enough to exist outside this envelope most likely result from allogenic forcing this formulation represents a quantitative and geographic set of threshold criteria for distinguishing between autogenic and allogenic mechanisms over late quaternary timescales 10 2 to 10 5 years in tectonically stable regions climate change is the dominant allogenic mechanism and therefore the first order control on the morphology sedimentology pedologic characteristics and chronology of alluvial stratigraphic records that meet or exceed the threshold criteria for demonstrating allogenic causality

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

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

the occurrence of obtuse junction angles and changes in channel width below tributaries along the mekong river south east asia classic descriptions of drainage patterns suggest that confluence angle is determined by the shape of the drainage basin unless constraining factors such as the geological structure affect stream flow downstream changes in channel width below tributary junctions have long been associated with tributary inputs of flow and sediment analysis of tributary junction geometry and channel width changes in large rivers and over large reaches is sparse the lower mekong basin exhibits a generally dendritic drainage network despite flowing through a diverse array of geological settings publicly available spot 5 imagery from google earth was used to identify and catalogue junction geometries and downstream changes in channel width below tributary junctions along a 2200 km reach of the mekong river of the 284 junctions identified the majority 66 2 were acute however 12 4 8 were found to be normal 90 and 75 30 were found to be obtuse this latter number is in contrast to previous studies over similar spatial scales which found little evidence of obtuse junctions meander extension of the incoming tributary and deflection of the tributary across bedrock shoulders were found to be the dominant geomorphological causes of obtuse tributary junctions the relationship between the width of the tributary channels and the width of the mainstem upstream and downstream of the confluences was analysed it was observed that over the whole reach a slight narrowing occurred immediately below tributary junctions although the changes themselves were small the slight net narrowing is shown to be statistically significant the observed relationship is shown to vary considerably with geology the geological control suggests that complex factors play important roles in determining changes to channel width across large systems and that simple cause effect relationships do not hold in such complicated geological settings

geomorphic evidence for plio quaternary compression in the andean foothills of the southern neuquen basin argentina the southern central andes foothills are made of imbricate thick and thin skinned fold and thrust belts they were formed during cretaceous and miocene compressive periods by successive eastward propagation episodes of the orogenic front we analyze remnant terraces and alluvial deposits of the neuqun river and its tributaries to study the plio quaternary tectonic regime in the southern neuqun basin topographic profiles of remnant terraces show a crustal scale tens of kilometers anomaly above the chihuido south anticline this anomaly is accompanied by sedimentary aggradations downstream and upstream of the anticline another anomaly in the terrace profiles correlated with a knickpoint in the current neuqun river is observed farther east near aelo we interpret these anomalies to be the consequence of a plio quaternary uplift of the chihuidos and aelo anticlines folding and surface faulting of plio quaternary alluvial deposits show that this uplift is concomitant with the reactivation of the miocene orogenic front and of the predominantly thin skinned agrio fold and thrust belt from these data we infer that the southern neuqun basin is still shortening under a compressive stress regime and that the andean orogenic front is migrating farther to the east extensional features in the study area correspond mainly to thin skinned gravitational gliding resulting from crustal uplift thus we argue that the tectonic compressive regime is preserved in the southern neuqun basin throughout the plio quaternary though the slab may have steepened during this period

onset and growth of tributary dammed lakes 1 we propose and test a theory describing the onset and growth of tributary dammed lakes using a diffusion equation constrained by backwater effects we show how tributary sediment influx can cause either a cuspate river aggradation or the formation of a lake upstream of the tributary junction this lake expands up valley and drowns previous deposits to leave a downstream facing lake bed of inclination milder than the angle of stability we obtain exact similarity solutions for such river and lake bed profiles and validate them quantitatively using laboratory experiments the resulting profiles are in qualitative accord with those of natural tributary dammed rivers

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

natural oscillations in coupled geomorphic systems an alternative origin for cyclic sedimentation internally oscillating cycles of cutting and filling occur naturally in many coupled geomorphic systems as a demonstration we have modeled stream entrenchment and backfilling where alluvial systems couple to uplifting mountain blocks this simple model tracksbedrockstreamerosionandtransportofalluviumtobaselevel internaloscillations arise from coupling of a kinematic wave model for mountain erosion to a diffusion model for the alluvial basin decaying periodic cutting andfilling occur on time scales of 10 5 yr the largest magnitudes are found at the coupling point model results suggest that cyclic sedimentation may result from the tintinnabulation of single perturbations applied to natural systems and need not record cyclic changes in climate tectonics or base level

rate and processes of river network rearrangement during incipient faulting the case of the cahabon river guatemala deeply incised river networks are generally regarded as robust features that are not easily modified by erosion or tectonics although the reorganization of deeply incised drainage systems has been documented the corresponding importance with regard to the overall landscape evolution of mountain ranges and the factors that permit such reorganizations are poorly understood to address this problem we have explored the rapid drainage reorganization that affected the cahabon river in guatemala during the quaternary sediment provenance analysis field mapping and electrical resistivity tomography ert imaging are used to reconstruct the geometry of the valley before the river was captured dating of the abandoned valley sediments by the be 10 al 26 burial method and geomagnetic polarity analysis allow us to determine the age of the capture events and then to quantify several processes such as the rate of tectonic deformation of the paleovalley the rate of propagation of post capture drainage reversal and the rate at which canyons that formed at the capture sites have propagated along the paleovalley transtensional faulting started 1 to 3 million years ago produced ground tilting and ground faulting along the cahabon river and thus generated differential uplift rate of 0 3 0 1 up to 0 7 0 4 mm y 1 along the river s course the river responded to faulting by incising the areas of relative uplift and depositing a few tens of meters of sediment above the areas of relative subsidence then the river experienced two captures and one avulsion between 700 ky and 100 ky the captures breached high standing ridges that separate the cahabon river from its captors captures occurred at specific points where ridges are made permeable by fault damage zones and or soluble rocks groundwater flow from the cahabon river down to its captors likely increased the erosive power of the captors thus promoting focused erosion of the ridges valley fill formation and capture occurred in close temporal succession suggesting a genetic link between the two we suggest that the aquifers accumulated within the valley fills increased the head along the subterraneous system connecting the cahabon river to its captors and promoted their development upon capture the breached valley experienced widespread drainage reversal toward the capture sites we attribute the generalized reversal to combined effects of groundwater sapping in the valley fill axial drainage obstruction by lateral fans and tectonic tilting drainage reversal increased the size of the captured areas by a factor of 4 to 6 at the capture sites 500 m deep canyons have been incised into the bedrock and are propagating upstream at a rate of 3 to 11 mm y 1 deepening at a rate of 0 7 to 1 5 mm y 1 at this rate 1 to 2 million years will be necessary for headward erosion to completely erase the topographic expression of the paleovalley it is concluded that the rapid reorganization of this drainage system was made possible by the way the river adjusted to the new tectonic strain field which involved transient sedimentation along the river s course if the river had escaped its early reorganization and had been given the time necessary to reach a new dynamic equilibrium then the transient conditions that promoted capture would have vanished and its vulnerability to capture would have been strongly reduced

geomorphic response to an active transpressive regime a case study along the chaman strike slip fault western pakistan the chaman left lateral strike slip fault bounds the rigid indian plate boundary at the western end of the himalayan tibetan orogen and is marked by contrasting topographic relief deformed landforms along the fault provide an excellent record for understanding this actively evolving intra continental strike slip fault the geomorphic response of an active transpessional stretch of the chaman fault was studied using digital elevation model dem data integrated with advanced spaceborne thermal emission and reflection radiometer aster visible and near infrared short wave infrared vnir swir and images from geoeye 1 geologic and geomorphic mapping helped in reconstructing the late quaternary landscape history of this transpessional strand of the chaman strike slip fault and the associated spinatizha thrust fault in western pakistan topographic analysis of a part of the transpression the thrust bounded roghani ridge revealed northward growth of the spinatizha fault with the presence of three water gaps and two corresponding wind gaps geomorphic indices including stream length gradient index mountain front sinuosity valley floor width to valley height ratios and entrenchment of recent alluvial fan deposits were used to define the lateral growth and direction of propagation of the spinatizha fault left lateral displacement along chaman fault and uplift along the spinatizha fault was defined using topographic analysis of the roghani ridge and geomorphic mapping of an impressive alluvial fan the bostankaul fan the landforms and structures record slip partitioning along the indian plate boundary and account for the convergence resulting from the difference in the chaman fault azimuth and orientation of the velocity vector of the indian plate copyright 2012 john wiley sons ltd

drainage reorganization during mountain building in the river system of the eastern cordillera of the colombian andes the eastern cordillera of colombia is a thick skinned thrust fold belt that is characterized by two topographic domains 1 the axial zone a high altitude plateau the sabana de bogota 2500 masl with low local relief and dominated by longitudinal rivers and 2 the cordillera flanks where local relief exceeds 1000 m and transverse rivers dominate on the basis of an analysis of digital topography and river parameters combined with a review of paleodrainage data we show that the accumulation of shortening and crustal thickening during the andean orogeny triggered a process of fluvial reorganization in the cordillera owing to a progressive increase of the regional slope the drainage network evolves from longitudinal to transverse dominated a process that is still active at present this study provides the idea of progressive divide migration toward the inner part of the mountain belt by which the area of the sabana de bogota plateau is decreasing the flanks increase in area and ultimately transverse rivers will probably dominate the drainage of the cordillera

differentiating pleistocene tectonically driven and climate related fluvial incision the sanggan river datong basin north china the sanggan river is an alluvial river flowing through a graben basin system of the northern shanxi rift zone north china during pleistocene times the river reach in the datong basin was affected successively by various external variables such as invasion by basaltic flow along valley faulting and climatic change therefore it provides excellent constraints for differentiating tectonically driven and climate related fluvial incision in the context of tectonic subsidence based on equilibrium profile analysis k ar dating of basalts 0 74 0 41 ma studies of the river terrace and of stream action history we present a conceptual model for differentiating fault driven and climate related fluvial incision by the river the results show that fluvial incision induced by tectonic lowering of the base level due to along valley movement on the sanggan river fault is equal to fault displacement the amount of post basalt fluvial incision of the reach upstream from the lava dam is 23 to 25 m of which the fault driven and climate related incisions are 15 m and 8 to 10 m respectively the former predominating over the latter the total amount of incision in the lava dam reach is 40 to 47 m of which the fault driven and climate related incisions are 10 m and 30 37 m respectively here the latter is predominant over the former since 0 41 0 10 ma the rate of fluvial incision of the lava dam reach of the river has reached 98 115 m ma which is 1 5 2 times as great as those of the reaches upstream and downstream from the lava dam the higher rate of fluvial incision can be attributed to high water levels supplied by the onset and maintenance of backwater conditions in the reach upstream from the lava dam due to the long period of warm and humid climate in this region plucking abrasion and knickpoint migration appear to be the primary erosional processes in the lava dam reach

estimating uplift rate histories from river profiles using african examples 1 we describe and apply a method for estimating uplift rate histories from longitudinal river profiles our strategy is divided into three parts first we develop a forward model which calculates river profiles from uplift rate histories height variation along a river profile is controlled by uplift rate and moderated by the erosional process we assume that the erosional process can be represented by a combination of advection and diffusion which are parameterized using four erosional constants second we have posed and solved the geologically more interesting inverse problem which uplift rate history minimizes the misfit between calculated and observed river profiles the inverse algorithm has been tested on synthetic river profiles which demonstrates that uplift rate histories can be reliably retrieved our tests show that the erosional process is dominated by advection i e knickpoint retreat and that changes in lithology and discharge play a secondary role in determining the transient form of a river profile finally we have inverted river profiles from a series of african topographic swells namely the bie south african namibian hoggar and tibesti domes fits between calculated and observed river profiles are excellent calculated uplift rate histories suggest that these domes grew rapidly during the last 30 40 million years uplift rate histories vary significantly from dome to dome but cumulative uplift histories agree closely with independent geologic estimates

shortening structural relief and drainage evolution in inverted rifts insights from the atlas mountains the eastern cordillera of colombia and the pyrenees the atlas eastern cordillera and pyrenees are thick skinned thrust fold belts formed by tectonic inversion of rift basins in continental settings a comparison of shortening between them shows a gradation from 20 25 in the central high atlas to 25 30 in the eastern cordillera and c 40 in the pyrenees accordingly there is a structural variation from interior zones with low structural relief and isolated basement massifs in the first two cases to an axial culmination of stacked basement thrust sheets in the pyrenees this results in marked topographic and drainage variation the high atlas and eastern cordillera contain axial plateaus dominated by structure controlled longitudinal rivers and orogen flanks with slope controlled transverse rivers whereas the pyrenees show a two sided wedge profile dominated by transverse rivers in spite of singularities exhibited by each orogen we propose that this spatial variation can be understood as reflecting different degrees of evolution in mountain building rapidly incising transverse rivers are capturing earlier longitudinal streams of the atlas and eastern cordillera thus reducing their axial plateaux which will eventually disappear into a transverse dominated drainage this pattern of landscape evolution may be characteristic of inversion orogens as they develop from initial stages of inversion to full accretion

can erosion drive tectonics can climate modulated erosion affect the location rate and style of tectonic deformation in earth s crust and thus seismic hazards particularly in the himalaya there are clear correlations between rugged topography and high rates of rainfall erosion and rock uplift 1 2 but a vigorous cause and effect debate continues with new papers presenting evidence for 1 4 and against 5 7 erosional control of tectonics see the figure although much field data are consistent with predicted associations among topography erosion climate and deformation unambiguous evidence has proven elusive 8 by focusing on a simple diagnostic test wang et al show on page 978 1 of this issue 9 that the zone of rugged topography and extreme erosion rates where the tsangpo brahmaputra river cuts through the namche barwa massif see the figure often argued to reflect a dynamic tectonic response to river incision 2 records instead a passive landscape response to tectonics over the past 2 to 2 5 million years the data threaten to undermine one of the most dramatic examples of a proposed tectonic response to erosion 1 lookup volpage 346 978 iss 6212

fluvial archives a valuable record of vertical crustal deformation abstract the study of drainage network response to uplift is important not only for understanding river system dynamics and associated channel properties and fluvial landforms but also for identifying the nature of crustal deformation and its history in recent decades geomorphic analysis of rivers has proved powerful in elucidating the tectonic evolution of actively uplifting and eroding orogens here we review the main recent developments that have improved and expanded qualitative and quantitative information about vertical tectonic motions the effects of horizontal deformation are not addressed channel long profiles have received considerable attention in the literature and we briefly introduce basic aspects of the behaviour of bedrock rivers from field and numerical modelling perspectives before describing the various metrics that have been proposed to identify the information on crustal deformation contained within their steady state characteristics then we review the literature dealing with the transient response of rivers to tectonic perturbation through the production of knickpoints propagating through the drainage network inverse modelling of river profiles for uplift in time and space is also shown to be very effective in reconstructing regional tectonic histories finally we present a synthetic morphometric approach for deducing the tectonic record of fluvial landscapes as well as the erosional imprint of tectonic forcing sedimentary deposits such as fluvial terrace staircases are also considered as a classical component of tectonic geomorphology we show that these studies have recently benefited from rapid advances in dating techniques allowing more reliable reconstruction of incision histories and estimation of incision rates the combination of progress in the understanding of transient river profiles and larger more rigorous data sets of terrace ages has led to improved understanding of river erosion and the implications for terrace profile correlation i e extrapolation of local data to entire profiles finally planform changes in fluvial systems are considered at the channel scale in alluvial rivers and regional level in terms of drainage reorganisation examples are given of how numerical modelling can efficiently combine with topographic data to shed new light on the dis equilibrium state of drainage systems across regional drainage divides

glacier and landslide feedbacks to topographic relief in the himalayan syntaxes despite longstanding research on the age and formation of the tibetan plateau the controls on the erosional decay of its margins remain controversial pronounced aridity and highly localized rock uplift have traditionally been viewed as limits to the dissection of the plateau by bedrock rivers recently however glacier dynamics and landsliding have been argued to retard headward fluvial erosion into the plateau interior by forming dams and protective alluvial fill here we report a conspicuous clustering of hundreds of natural dams along the indus and the tsangpo rivers where these cross the himalayan syntaxes the indus is riddled by hundreds of dams composed of debris from catastrophic rock avalanches forming the largest concentration of giant landslide dams known worldwide whereas the tsangpo seems devoid of comparable landslide dams in contrast glacial dams such as river blocking moraines in the headwaters of both rivers are limited to where isolated mountain ranges intersect the regional snowline we find that to first order high local topographic relief along both rivers corresponds to conspicuously different knickzones and differences in the type and potential longevity of these dams in both syntaxes glacier and landslide dams act as a negative feedback in response to fluvial dissection of the plateau margins natural damming protects bedrock from river incision and delays headward knickpoint migration thereby helping stabilize the southwestern and southeastern margins of the tibetan plateau in concert with the effects of upstream aridity and localized rock uplift

modelling centennial sediment waves in an eroding landscape catchment complexity sediment flux dynamics in fluvial systems have often been related to changes in external drivers of topography climate or land cover it is well known that these dynamics are non linear recently model simulations of fluvial activity and landscape evolution have suggested that self organization in landscapes can also cause internal complexity in the sedimentary record in this contribution one particular case of self organization is explored in the sabinal field study area spain where several dynamic zones of sedimentation and incision are observed along the current river bed whether these zones can be caused by internal complexity was tested with landscape evolution model lem lapsus landscape process modelling at multi dimensions and scales during various 500 year simulations zones of sedimentation appear to move upstream and downstream in eroding river channels waves these waves are visualized and characterized for a range of model settings under constant external forcing and the self organizing process behind their occurrence is analysed results indicate that this process is not necessarily related to simplifications in the model and is more generic than the process of bed armouring that has recently been recognized as a cause for complexity in lem simulations we conclude that autogenic sediment waves are the result of the spatial propagation in time of feedbacks in local transport limited deposition and detachment limited erosion conditions

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

analogue modeling of transverse drainage mechanisms fifteen physical experiments on a stream table replicated four general mechanisms proposed to explain transverse drainage development namely antecedence superimposition overflow and piracy the experimental design encompassed multiple strategies to effectively replicate the different mechanisms these experiments roughly emulate natural fluvial processes including aggradation lateral erosion degradation knickpoint retreat ponding and overflow the experiments address landscape morphology and depositional field criteria associated with antecedence overflow and piracy antecedence experiments showed that defeat of streams by a tectonically growing structure could result from a combination of upstream ponding and diversion into a trunk drainage insufficiently fast retreating knickpoints and downstream deposition of eroded sediments piracy can result from stream aggradation until it overtops a low divide by headward erosion of a stream through a divide and by sapping processes for piracy driven by headward erosion asymmetrically sloped ridges allow the drainage divide across the ridge to retreat from headward erosion but a symmetric divide hinders the ability of a stream to retreat across the ridge apex aggradationally driven piracy is potentially hindered because the slope across the divide needs to be sufficiently steep compared to the uncaptured stream otherwise deposition occurs across the divide and a levee is formed which prevents capture for transverse drainages produced by overflow knickpoints retreat across the overflow outlet producing a high probability of downstream flooding a positive feedback develops where added discharge to the drainage from lake water increases erosion and knickpoint retreat further eroding and undermining the basin outlet and releasing even more lake water the result is rapid incision and dramatic flooding downstream of the bedrock high during lake outlet erosion two superimposition experiments were unsuccessful most of the field criteria for antecedence overflow and piracy were either directly replicated or implied based on observations during each experimental run

transient landscape evolution of basement cored uplifts example of the kyrgyz range tian shan 1 the kyrgyz range located on the northern margin of the western tian shan illustrates long term 106 107 year transient landscape evolution in response to an active basement cored rock uplift eastward propagation of range uplift has progressively exposed resistant bedrock capped by a tilted formerly planar pre cenozoic unconformity we develop an approximate stream power based analytic model of transient river profile incision into progressively exposed and resistant bedrock to explore the patterns of channel development into the unconformity surface this analysis shows that the unconformity can be preserved as a geomorphic marker defined by upland headwaters and interfluvial areas though channels are not at equilibrium with rock uplift prominent knickpoints are not predicted to develop on main stem channels however knickpoints are predicted to develop on tributaries upstream of their junctions with the trunk stream because of differential erosion rates initial channel slope and transient channel form are both sensitive to the n value of the stream power model and could prove useful for calibration of n from field data accumulation of catchment area a proxy for discharge into larger catchments develops a positive feedback where larger drainages with higher stream power at a given slope undermine and capture adjacent drainage area a simple model of competitive fluvial incision illustrates the role of tributary junction position in maximizing stream power expended upon the trunk stream examples from the kyrgyz range illustrate the effects of tributary junction position on fluvial relief and we propose that adjustments to the tributary network through stream capture are ongoing within this landscape even after several kilometers of exhumation

paleocurrent and fabric analyses of the imbricated fluvial gravel deposits in huangshui valley the northeastern tibetan plateau china abstract gravel deposits on fluvial terraces contain a wealth of information about the paleofluvial system in this study flow direction and provenance were determined by systematic counts of more than 2000 clasts of imbricated gravel deposits in the xining region northeastern tibetan plateau china these gravel deposits range in age from the modern huangshui riverbed to miocene aged deposits overlain by eolian sediments our major objectives were not only to collect first hand field data on the fluvial gravel sediments of the xining region but also to the reconstruct the evolution of the fluvial system these data may offer valuable information about uplift of the northeastern tibetan plateau during the late cenozoic era reconstructed flow directions of the higher and lower gravel deposits imply that the river underwent a flow reversal of approximately 130 180 in addition the lithological compositions in the higher gravel deposits differ significantly from the lower terraces suggesting that the source areas changed at the same time eolian stratigraphy overlying the gravel deposits and paleomagnetic age determination indicate that this change occurred sometime between 1 55 ma and 1 2 ma we suggest that tectonic activity could explain the dramatic changes in flow direction and lithological composition during this time period therefore this study provides a new scenario of fluvial response to tectonic uplift a reversal of flow direction in addition field observation and statistical analyses reveal a strong relationship between rock type size and roundness of clasts

geomorphic impacts of a 100 year flood kiwitea stream manawatu catchment new zealand abstract the geomorphic impacts of a 100 year flood are assessed in the kiwitea stream 254 km 2 a tributary within the manawatu river catchment new zealand using sequential aerial photographs and reach based morphological sediment budgeting channel expansion and avulsion eroded in excess of one million cubic metres of sediment over 1 km 2 of floodplain along a 30 km long reach of kiwitea stream channel transformation was spatially discontinuous and predominantly associated with large scale bank erosion in response to a flood over 5 times bigger than the mean annual flood annual recurrence interval ari 100 years total energy expenditure of this flood in the kiwitea was 14 900 10 3 j the spatial discontinuity of channel transformation relates to valley floor and channel configurations high stream powers generated in confined channels at bends produced catastrophic channel transformation where flood flows dissipated overbank stream powers and the extent of channel transformation were reduced hydrologic hydraulic and geomorphic variables can be invoked to thus explain the variability of geomorphic impacts encountered during this event

evidence for post early eocene tectonic activity in southeastern ireland the role played by cenozoic deformation in denudation and landscape development in ireland has historically been difficult to assess because of the lack of widespread pre glacial cenozoic deposits onshore here we combine analysis of apatite fission track data and geomorphic observations to place constraints on the timing kinematics and magnitude of onshore deformation in southeastern ireland relationships between apatite fission track central age and elevation for samples from the wicklow and blackstairs mountains and tullow lowland suggest that these rocks record an exhumed apatite partial annealing zone which after cooling was dismembered by differential vertical displacements of up to several hundred metres we use inverted models of sample thermal history to show that samples across the region experienced very similar thermal histories up to and including a cooling event in late paleocene or early eocene time this effectively rules out strongly spatially heterogeneous denudation and implies that differential rock uplift occurred in post early eocene time the central age elevation relationships define at least three spatial domains with internally consistent apatite fission track data separated by known faults or topographic escarpments geomorphic analysis of these structures shows that patterns of catchment incision and sinuosity as well as the presence of antecedent drainage are best explained by differential vertical displacements at or near the domain boundaries the kinematics and magnitudes of these displacements are consistent with those implied by the apatite fission track results and are compatible with other examples of known cenozoic deformation from ireland and the adjacent continental margin

response of bedrock channel width to tectonic forcing insights from a numerical model theoretical considerations and comparison with field data the morphology of bedrock river channels is controlled by climatic and tectonic conditions and substrate properties knowledge of tectonic controls remains scarce this is partly due to slow tectonic rates and long response times of natural channels and partly due to the difficulty in isolating and constraining tectonic forcing conditions in the field to study the effect of tectonic forcing on channel geometry we have developed a numerical model of the cross sectional evolution of a detachment limited channel its predictions are matched by an analytical model based on the assumption of the minimization of potential energy expenditure using these models we illustrate how local tectonics can alter the observed width discharge scaling and discuss published field data in light of our findings except for one case the models fail to correctly describe field observations of well constrained cases this implies that the shear stress stream power family of models is too simple to describe the behavior of natural channels additional complexities such as sediment effects and discharge variability exert a strong control on channel morphology and need to be taken into account in the modeling of channel dynamics and steady state

high altitude plio quaternary fluvial deposits and their implication on the tilt of a horst western anatolia turkey abstract this study investigates the origin and regional tectonic implications of high altitude plio quaternary fluvial deposits developed over the bozdag horst which is an important structural element within the horst graben system of western anatolia turkey a total of 23 deposits occur near the modern drainage divide comprising fluvial to occasionally lacustrine deposits the deposits are all elongated in n s direction with a width length ratio of 1 10 the largest of them is of 13 km in length with a maximum observable thickness of about 100 110 m morphological lithological deformational characteristics of these deposits and the drainage system of the area all suggest that the deposits were formed due to uplift and southward tilting of the bozdag horst this tilting which is estimated as 1 2 to 2 2 caused accumulation of the stream load along channels flowing from south to north all the deposits were later dissected by the same streams with the exception of one deposit which still preserves its original lake form these deposits are of quaternary age which corresponds to the latest n s directed extensional tectonic phase in the region