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

controls on the strength of coupling among climate erosion and deformation in two sided frictional orogenic wedges at steady state 1 many important insights regarding the coupling among climate erosion and tectonics have come from numerical simulations using coupled tectonic and surface process models however analyses to date have left the strength of the coupling between climate and tectonics uncertain and many questions unanswered we present an approximate analytical solution for two sided orogenic wedges obeying a frictional rheology and in a condition of flux steady state that makes explicit the nature and sensitivity of the coupling between climate and deformation we make the simplifying assumption that the wedge grows in a self similar fashion consistent with airy isostasy such that topographic taper is invariant with orogen width tectonic influx rate and climate we illustrate first how and why the form of the erosion rule matters to orogen evolution and then derive a physically based orogen scale erosion rule we show that steady state orogen width crest elevation and crustal thickness are controlled by the ratio of accretionary flux to erosional efficiency to a power dictated by the erosion process remarkably we show that for most combinations of parameters in the erosion law rock uplift rate is more strongly controlled by erosional efficiency than it is by the accretionary flux further assuming frontal accretion with no underplating the spatial distribution of erosional efficiency dictates the relative rock uplift rates on the pro wedge and retro wedge and the time averaged trajectories of rocks through the orogen the restriction to invariant frictional properties is conservative in these respects systems subject to positive feedback between erosion and rheology will exhibit even stronger coupling among climate erosion and deformation than shown here

mechanics of fold and thrust belts and accretionary wedges cohesive coulomb theory a critically tapered fold and thrust belt or submarine accretionary wedge is one that is on the verge of coulomb failure everywhere including its base where frictional sliding along a decollement is assumed to be occurring cohesion within a wedge can add significantly to the overall strength near the toe the effect of this is to decrease the near toe taper leading to a critical topographic profile that is concave upward if the decollement is planar we obtain an approximate self consistent solution for the state of stress within a thin skinned cohesive critical coulomb wedge and determine the relationship between the wedge taper and its strength and basal friction the theory is then applied to the presently deforming fold and thrust belt of western taiwan fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step up angles of thrust faults from the basal decollement are used to constrain the taiwan wedge strength parameters an attractive assertion fully consistent with all the observations is that the mechanics of fold and thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory in particular if byerlee s law b 0 85 is adopted as the coefficient of sliding friction on the base we find a coefficient of internal friction 0 9 1 0 in the wedge and a wedge cohesion so 5 20 mpa other solutions having strengths and ambient stresses up to 4 times lower than this can also however satisfy the data

the dynamical structure of squall line type thunderstorms abstract the structure of squall line type thunderstorms is investigated with special emphasis on the upshear sloping updraft which is often observed in these storms to aid in the investigation a new version of the convective richardson number is formed which adds the negative buoyancy due to liquid water loading to the thermal buoyancy of the lifted parcel in the calculation of the buoyant energy this modified convective richardson number is easily calculated using a pre storm sounding and has predictive value for thunderstorm type while it is widely accepted that the updraft s upshear slope is caused by the ascending parcels partially conserving their horizontal momentum toward the rear of the storm this paper presents a new theory for the production of this slope in this theory the vorticity production due to the liquid water distribution leads to an upshear slope of the updraft downdraft interface a stable slope is reached when the loading mechanism is balanced by the forces exerted by the env

growth and erosion of fold and thrust belts with an application to the aconcagua fold and thrust belt argentina 1 the development of topography within and erosional removal of material from an orogen exerts a primary control on its structure we develop a model that describes the temporal development of a frontally accreting critically growing coulomb wedge whose topography is largely limited by bedrock fluvial incision we present general results for arbitrary initial critical wedge geometries and investigate the temporal development of a critical wedge with no initial topography increasing rock erodibility and or precipitation decreasing mass flux accreting to the wedge front increasing wedge sole out depth decreasing wedge and basal decollement overpressure and increasing basal decollement friction lead to narrow wedges large power law exponent values cause the wedge geometry to quickly reach a condition in which all material accreted to the front of the wedge is removed by erosion we apply our model to the aconcagua fold and thrust belt in the central andes of argentina where wedge development over time is well constrained we solve for the erosional coefficient k that is required to recreate the field constrained wedge growth history and these values are within the range of independently determined values in analogous rock types using qualitative observations of rock erodibilities within the wedge we speculate that power law exponents of 1 3 m 0 4 and 2 3 n 1 characterize the erosional growth of the aconcagua fold and thrust belt this general model may be used to understand the development of mountain belts where orogenic wedges grow as they deform at their coulomb failure limit

orogeny and orography the effects of erosion on the structure of mountain belts a numerical model of the coupled processes of tectonic deformation and surface erosion in convergent orogens is developed to investigate the nature of the interaction between these processes crustal deformation is calculated by a two dimensional finite element model of deformation in response to subduction and accretion of continental crust erosion operates on the uplifted surface of this model through fluvial incision which is taken to be proportional to stream power the relative importance of the tectonic and erosion processes is given by a dimensionless erosion number relating convergence velocity rock erodibility and precipitation rate this number determines the time required for a system to reach steady state and the final topographic shape and size of a mountain belt fundamental characteristics of the model orogens include asymmetric topography with shallower slopes facing the subducting plate and an asymmetric pattern of exhumation with the deepest levels of exhumation opposite to subduction these characteristics are modified when the regional climate exhibits a dominant wind direction and orographically enhanced precipitation on one side of the mountain belt the two possible cases are dominant wind in the direction of motion of the subducting plate and dominant wind direction in the opposite direction of the subducting plate velocity models of the former case predict a broad zone of exhumation with maximum exhumation in the orogen interior models of the latter case predict a focused zone of exhumation at the margin of the orogen and at high erosion number a reversal in the topographic asymmetry natural examples of these two cases are presented the southern alps of new zealand exhibits the climate and exhumation asymmetry characteristic of wind in the direction opposite to motion of the subducting plate the asymmetry of topography suggests that erosion is not efficient enough to have reversed the topographic asymmetry the contrasting example of dominant wind in the direction of subduction motion is provided by the olympic mountains of washington state in this case exhumation of deep levels of the cascadia accretionary wedge shows a broad domal pattern consistent with the observed orographic precipitation

critical taper model of fold and thrust belts and accretionary wedges the fold and thrust belts and submarine accretionary wedges that lie along compressive plate boundaries are one of the best understood deformational features of the earth s upper crust although there is considerable natural variation among the many fold and thrust belts and accretionary wedges that have been recognized and explored several features appear to be universal in cross section fold and thrust belts and accretionary wedges occupy a wedge shaped deformed region overlying a basal detachment or decollement fault the rocks or sediments beneath this fault show very little deformation the decollement fault characteristically dips toward the interior of the mountain belt or in the case of a submarine wedge toward the island arc the topography in contrast slopes toward the toe or deformation front of the wedge deformation within the wedge is generally dominated by imbricate thrust faults verging toward the toe and related fault bend folding two north american fold and thrust belts that exhibit these features are shown in figure 1 neither of these two examples is tectonically active today the southern canadian fold and thrust belt was active during the late jurassic and cretaceous 150 100 ma whereas the southern appa lachians were deformed during the late carboniferous to permian alle ghenian orogeny 300 250 ma figure 2 shows two examples that are currently active the taiwan fold and thrust belt produced by the sub duction of the eurasian plate beneath the philippine sea plate suppe 1981 1987 and the barbados accretionary wedge produced by the sub

orographic precipitation and the relief of mountain ranges 1 the influence of climate on the relief of mountain ranges has long been supposed but has received little quantitative investigation the simplified system of a longitudinal river profile in a bedrock channel coupled to a physically based model of orographic precipitation is used to explore how orographically influenced patterns of precipitation affect the relief of fluvially sculpted mountain ranges it is shown that the impact on the orography depends on the nature of the precipitation regime significant changes in relief also occur for imposed temperature changes of realistic magnitude although the ability of the relief to respond to any given change depends sensitively on its response time and the timescale of the forcing

on steady states in mountain belts the dynamic system of tectonics and erosion contains important feedback mechanisms such that orogenic systems tend toward a steady state this concept is often invoked but the nature of the steady state is commonly not specified we identify four types of steady state that characterize the orogenic system and illustrate these cases by using numerical model results and natural examples these types are 1 flux steady state 2 topographic steady state 3 thermal steady state and 4 exhumational steady state they refer to the erosional flux the topography the subsurface temperature field and the spatial pattern of cooling ages respectively models suggest that the topography will reach a steady mean form at the scale of an orogenic belt but perfect topographic steady state is unlikely to be achieved at shorter length scales thermal steady state is a precondition for exhumational steady state and in the case of temperature dependent deformation topographic steady state exhumational steady state is characterized by reset age zones spatially nested according to closure temperature as illustrated in natural systems from new zealand the cascadia accretionary margin and taiwan

a fluid model for the shape of accretionary wedges in this paper the accretionary wedges landward of most oceanic trenches are modelled assuming a newtonian fluid rheology it is shown that lubrication theory is applicable and that the addition of material to the wedges can be neglected in determining the shape of the wedges the predicted shape of the wedges is in good agreement with bathymetric profiles across the kurile ryukyu and aleutian wedges from the shape of the above wedges it is deduced that accretionary sediments have viscosities between 1018 and 1019 poise

channel initiation and the problem of landscape scale since the 1940s it has been proposed that landscape dissection into distinct valleys is limited by a threshold of channelization that sets a finite scale to the landscape this threshold is equal to the hillslope length that is just shorter than that necessary to support a channel head a field study supports this hypothesis by showing that an empirically defined topographic threshold associated with channel head locations also defines the border between essentially smooth undissected hillslopes and the valley bottoms to which they drain this finding contradicts assertions that landscapes are scale independent and suggests that landscape response to changes in climate or land use depends on the corresponding changes in the threshold of channelization

feedbacks among climate erosion and tectonics in a critical wedge orogen the interactions among climate erosion and tectonics have long been of interest to geologists but have not yet been united into a single theoretical framework in this study representations of orographic precipitation and fluvial erosion are combined with the concept of a critical wedge orogen the idealized framework captures the basic system dynamics it also allows for a formal analysis of the precipitation and tectonic interactions in terms of feedback factors and gains and so permits a quantitative comparison of their relative strengths the constraint of self similar growth in a critical wedge orogen acts as a tectonic governor whereby changes in orogen size are strongly damped we determine that this negative tectonic feedback is stronger than the precipitation feedback which may be negative or positive depending on whether precipitation increases or decreases with orogen size for an extreme positive feedback unconstrained runaway growth is possible ultimately leading to plateau formation when orographic precipitation leads to a significant rain shadow there is a strong partitioning of rock uplift rates that favors the wet windward flank of the orogen this flank also dominates the response of the whole orogen to changes in climate or tectonic forcing except in the case where a large fraction of the eroded material is recycled into the wedge finally it is demonstrated that the response time of the orogen depends on the feedbacks and is proportional to the gain of the system

climate and tectonic controls on glaciated critical taper orogens abstract in this work we present the results of a new analytical model that examines the coupling between glacial erosion and orogen development surface processes are assumed to be glacially dominated and tectonic activity is controlled by critical wedge mechanics in these circumstances we find that orogen width is strongly dependent on both the rate of accretion and on the rate of precipitation the orogen size is linked to tectonic and climate changes via proportionality constants the orogen width scales with the rate of accretion to between the 2 3rd and 2nd powers and with the rate of precipitation to the 1 3rd and 5 4th powers the value of the proportionality constants varies with the taper angle and the relative rates of ice deformation and sliding in all cases the sensitivities are higher than those calculated for fluvially eroding critical wedge orogens analytical solutions are supported by the results of a numerical flow line model the flow line model further predicts that uplift rates will be highest at and just below the equilibrium line altitude if glacial ablation is largely a calving process rock uplift in the wedge will be strongly focused towards the toe the predicted response time of glaciated orogens to changes in climate and tectonic forcing is dependent upon the constants of erosion the rheology and the rate of precipitation these analyses predict that actual orogens have variable e folding response times for example approximately 1 5 myr for the southern alps of new zealand and the olympic mountains of washington state and approximately 5 5 myr for the european alps

rheological dependence of extension in wedge models of convergent orogens abstract although several mechanical models of extension in contractional orogens have been proposed in many cases these models have been based on viscous constitutive laws lack of more complicated rheological models has primarily been due to the technical difficulty of solving for deformation with complex rheological behavior and boundary conditions a finite element model is presented that allows for solution of the wedge mechanical problem with viscous non linear viscous and coulomb plastic rheologies this model is used to investigate extension in a doubly vergent wedge model in which deformation is primarily contractional driven by a reversal of basal traction proposed models showing extension in the crust at shallow crustal levels in spite of overall contraction and contraction at depth are confirmed but only for high argand number low crustal viscosity at low argand number little or no extension is observed non linear stress dependence power law flow also suppresses extension but not entirely some extension is observed at high effective argand number however with a coulomb plastic rheology no extension is observed for a wide range of parameter values no extension is observed for models with viscous deformation at depth and plastic deformation near the surface this suggests that the extension is a function of the rheologic model not the effective strength of the crust coeval extension and contraction in convergent orogens with steady external forcing therefore requires viscous deformation to very shallow depths

oblique convergence of the crust driven by basal forcing implications for length scales of deformation and strain partitioning in orogens summary we investigate the effects of oblique convergence on crustal thickening by modelling the crust as a thin viscous sheet crustal deformation is driven by basal velocity boundary conditions representing the kinematics of the underlying convergent mantle lithosphere which is assumed to detach and subduct the crustal sheet is coupled to the basal velocity field by a weak simple shear layer which exerts a traction on the base of the sheet the use of basal velocity boundary conditions with a thin sheet model is shown to be valid provided coupling with the base is weak and deformation extends over a horizontally wide zone in comparison to the thickness of the sheet the effect of the mantle lithosphere on crustal deformation can be shown to depend on the non dimensional ampferer number am which represents the relative strengths of the crust and the weak basal shear zone analytical and numerical model results for small amounts of normalized convergence show that the ratio of horizontal length scales for normal and transcurrent crustal deformation is 2 for both linear and non linear viscous rheologies the length scales depend on the relative strength of the crust and the basal shear zone convolved with a forcing length scale from the boundary conditions for obliquely incident velocities the length scale ratio is between 2 and 1 the exact value depending on the power law exponent of the rheology and the relative magnitude of the normal and transverse velocity components driving deformation the model results for the ratio of normal to transcurrent deformation length scales are different from those found by using a side driven viscous thin sheet model england houseman sonder 1985 sonder england houseman 1986 we propose that this difference may be used as a test to distinguish the primary driving mechanism for large scale orogenic deformation if the deformation mechanism corresponds to pure shear of the whole lithosphere without significant basal tractions we expect the deformation length scale to be determined by the scale of the applied boundary conditions and buoyancy forces in the crust the normal transcurrent length scale ratio for this case in early stages of deformation will be approximately 4 alternatively if the results presented here are more appropriate then we expect the deformation length scale to be determined by the intrinsic relative strengths of the crust and underlying basal shear zone the corresponding normal transcurrent length scale ratio for this case in early stages of deformation will be between 2 and 1 on the basis of a comparison of the predicted length scales of deformation with the length scale over which the velocity boundary conditions are applied we suggest that long narrow linear orogens such as the andes are controlled by basal boundary conditions whereas the himalayan tibetan orogen has dimensions consistent with either basally or side driven deformation

a thin viscous sheet model for continental deformation summary for the purposes of describing its large scale and long term deformation the continental lithosphere is regarded as a continuum obeying a newtonian or a power law rheology the flow of a thin sheet of power law material overlying an inviscid substrate is studied under the assumption that vertical gradients of the horizontal velocity are negligible a numerical model is used to investigate the deformation of such a sheet under conditions approximating those of continent continent collision the material flows in response to forces applied to its boundaries for example the indenting of one continent by another and to forces in its interior arising from gradients in crustal thickness the horizontal divergence of the flow produces changes in the crustal thickness and hence a time dependent form to the flow itself for a given set of boundary conditions the flow depends on the stress exponent in the power law rheology n and on the argand number which is a measure of the ratio between the stress arising from crustal thickness contrasts and the stress required to deform the material at the ambient strain rates when the effective viscosity of the medium is very high ar o crustal thickness variations do not influence the flow if the material is newtonian n l the deformation associated with an influx of material approximating an indenter is of much greater lateral dimension than the width of the indenter whereas when material has a power law rheology n 3 5 are used the deformation is confined to a region of lateral extent comparable to that of the indenter as ar increases the forces arising from crustal thickness contrasts exert more influence on the flow and the maximum crustal thickness that can be sustained by a given influx of material is related by a simple expression to the effective viscosity of the medium at the ambient strain rates in the limit of a very weak medium ar 10 the lithosphere is unable to sustain appreciable crustal elevation contrasts the results of these numerical experiments show that systems in which the effective viscosities are such that the maximum deviatoric stresses are between 1 kbar

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

response time of glaciers as a function of size and mass balance 1 theory a simple interpretation of the traditional definitions of glacier and ice sheet response time e g thickness divided by mass balance rate h b suggests that larger glaciers respond more slowly than small glaciers to a perturbation in climate however with reasonable choices for mass balance behavior a scaling analysis shows that the response time of valley glaciers decreases as a function of increasing size when other variables are held constant in essence this is because larger valley glaciers push further into the ablation zone and ablation increases more rapidly than the thickness so that h b gets smaller ice sheets have different mass balance regimes than valley glaciers and as they grow larger the ablation does not increase faster than the thickness therefore as ice sheets grow in surface area the response time increases for both ice sheets and valley glaciers the response time also depends on a mass balance index which is defined as the slope of the balance curve as a function of horizontal distance along the glacier surface rather than elevation the response time decreases as the balance index increases so for valley glaciers an increase in the balance index and an increase in glacier size have opposite effects on the response time the balance index is typically larger for smaller valley glaciers therefore a small glacier will typically respond faster than a large glacier but this quicker response is because of mass balance considerations and not because of the dynamic characteristics of the glacier arising from its small size as often assumed

erosion and exhumation in accretionary orogens experimental and geological approaches 1 this study examines the role of erosion on exhumation and deformation history of accretionary orogens through experimental and geological methods fault propagation kinematics and exhumation rates in an eroded thrust wedge are analyzed through scaled sandbox simulations by testing different parameters friction rheologic layering and sediment input output ratio a model coulomb wedge is submitted to erosion under flux steady state conditions that is the volume of eroded material remains equal to the volume of newly accreted material maintaining a constant surface slope during shortening results show that the way of exhumation depends on the internal dynamics of thrust wedges and conversely on how this dynamics is modified by erosion in the eroded thrust wedges the diversity of exhumation patterns is controlled by the mode of fault propagation depending on the basal friction high or low the vertical component of exhumation is generally higher for the wedges with high basal friction than for low friction wedges the uplift of material occurs along a cluster of subvertical thrusts in the middle part of the eroded thrust wedge with low basal friction the material is exhumed along a series of inclined 20 50 thrusts in the rear of the high friction wedge the zone of maximum exhumation is generally localized in the middle part of the thrust wedge and migrates toward the backstop with continued shortening the vertical exhumation rate increases with time and the material accreted later is rapidly transferred to the main exhumation zone compared to the material accreted during the early stages the exhumation occurs at rates 2 times faster in the wedges with half the thickness than in full thickness wedges the total quantity of eroded material at the end of convergence constitutes 36 50 of initial model area for the thrust wedges with low basal friction and 20 40 for the high basal friction wedges the extent of basal underplating increases with total shortening the area of basal underplated material constitutes up to 30 and 40 of the eroded thrust wedge area for the models with low and high basal friction respectively presence of decollements in the accreted series allows underplating of thrust units developing an anticlinal stack whose growth and location is favored by erosion our results are compared to different present day active and ancient convergent orogens himalaya taiwan southern alps of new zealand and cascadia orogen allowing the different specific exhumation processes to be characterized

large scale geomorphology classical concepts reconciled and integrated with contemporary ideas via a surface processes model linear systems analysis is used to investigate the response of a surface processes model spm to tectonic forcing the spm calculates subcontinental scale denudational landscape evolution on geological timescales 1 to hundreds of million years as the result of simultaneous hillslope transport modeled by diffusion and fluvial transport modeled by advection and reaction the tectonically forced spm accommodates the large scale behavior envisaged in classical and contemporary conceptual geomorphic models and provides a framework for their integration and unification the following three model scales are considered micro meso and macroscale the concepts of dynamic equilibrium and grade are quantified at the microscale for segments of uniform gradient subject to tectonic uplift at the larger meso and macroscales which represent individual interfluves and landscapes including a number of drainage basins respectively the system response to tectonic forcing is linear for uplift geometries that are symmetric with respect to baselevel and which impose a fully integrated drainage to baselevel for these linear models the response time and the transfer function as a function of scale characterize the model behavior numerical experiments show that the styles of landscape evolution depend critically on the timescales of the tectonic processes in relation to the response time of the landscape when tectonic timescales are much longer than the landscape response time the resulting dynamic equilibrium landscapes correspond to those envisaged by hack 1960 when tectonic timescales are of the same order as the landscape response time and when tectonic variations take the form of pulses much shorter than the response time evolving landscapes conform to the penck type 1972 and to the davis 1889 1899 and king 1953 1962 type frameworks respectively the behavior of the spm highlights the importance of phase shifts or delays of the landform response and sediment yield in relation to the tectonic forcing finally nonlinear behavior resulting from more general uplift geometries is discussed a number of model experiments illustrate the importance of fundamental form which is an expression of the conformity of antecedent topography with the current tectonic regime lack of conformity leads to models that exhibit internal thresholds and a complex response

controls on the patterns of topography and erosion rate in a critical orogen 1 we solve for the steady state patterns of erosion rate and topography in a critical wedge to understand the mutual adjustment of tectonics erosion and topography in the absence and presence of spatial variations in precipitation rate we consider steady state systems in which tectonics favors a critical topographic form assumed to be the mean elevation across the range and in which surface erosion by rivers and hillslopes operates on a two dimensional landscape we find that 1 a nonuniform critical topographic form implies a nonuniform pattern of ridge valley relief and hence a nonuniform pattern of erosion rate and 2 when the system is forced by local variations in precipitation rate maintenance of the critical topographic form requires a local response of rock uplift that greatly dampens changes in topography we apply these concepts to the western side of the olympic mountains of washington state where mean elevation ridge valley relief and precipitation rates increase from the coast to the topographic crest of the range we find that the main control on the erosion rate pattern is the pattern in mean elevation and the amount of precipitation in contrast the pattern of precipitation is only a minor control as a whole our work demonstrates an approach for developing the theoretical context that is necessary for interpreting spatial associations between patterns in topography precipitation and erosion in natural orogens

critical form and feedbacks in mountain belt dynamics role of rheology as a tectonic governor 1 the rheology that governs deformation within a convergent orogen also controls its topographic form or in other words the covaration of its height or thickness and its width under conditions pertaining to small orogens and for rheologies ranging from linear viscous to coulomb plastic we show that this topographic form is insensitive to the distributions of fluxes into and out of the wedge and is thus a fundamental property of the system it can therefore be thought of as a critical topographic form directly analogous to the well studied case of a coulomb plastic rheology which predicts a constant critical taper angle independent of its size the tendency of the system to evolve toward a critical topographic form can be regarded as a tectonic governor that strongly damps the response of the orogen to changes in climate or tectonic forcing scaling relationships can be derived for the variation of orogen width height and exhumation rate as a function of accretionary flux and precipitation rate this study explores how sensitive these scaling relationships are to the assumed rheology it is found that the scaling relationships vary by less than a factor of 2 across the range of typical geological rheologies from coulomb friction to power law viscous to linear viscous these scaling relationships provide a first order representation of the behavior of convergent wedges and are relatively insensitive to the underlying assumptions

current strain regime in the western alps from continuous global positioning system measurements 1996 2001 four to six years of continuous measurements at 10 permanent global positioning system sites in the western alps show horizontal residual velocities of 2 mm yr with respect to stable europe uncertainties range from 0 3 to 1 4 mm yr these velocities and the associated strain rate field indicate that the central part of the range is currently dominated by east west extension whereas the southern part shows north south to northwest southeast compression the geodetic and seismotectonic data are consistent with a model where strain is essentially controlled by the counterclockwise rotation of the adriatic microplate with respect to eurasia this rotation together with the arcuate shape of the contact between the adriatic microplate and the alps induces dextral shear kinematic boundary conditions across the western alps with an additional divergence component in their central part and in switzerland and a convergence component in their southern part

constraints on extension of continental lithosphere the extension of the continental lithosphere is considered in terms of the deformation of a thin viscous sheet with a temperature dependent rheology which is governed primarily by the strength of the upper mantle the lithosphere is subjected either to a constant strain rate or to a strain rate decreasing with time corresponding to the stretching of a region of fixed volume with a constant velocity boundary condition the progress of the extension is influenced by the decrease in strength of the lithosphere owing to its attenuation and by an increase in strength resulting from cooling the relative importance of these two effects depends on two parameters a dimensionless strain rate and the total strain for a given strain rate the extending lithosphere goes through two phases as the strain increases in the first the effect of thinning the lithosphere is predominant and the average strength of the lithosphere remains close to and perhaps slightly below its original strength in the second the effect of cooling of the lithosphere leads to very rapid increase in strength as a function of strain resulting presumably in an effective limit on the degree of stretching that the lithosphere experiences for the case of strain rate decreasing with time when the initial strain rates are of order 10 15s 1 this maximum extension is between 100 and 150 for a strain rate of 10 l4 s 1 the maximum lies around 400 to 500 at higher strain rates than this very large extension may occur before cooling significantly affects the strength of the lithosphere this model accords well with the constraints available from the continental margins of the north atlantic

analogue model of gravitational collapse and surface extension during continental convergence most mountain belts occur where continents collide so it is not surprising that the dominant form of surface deformation is shortening but there is also abundant geological evidence for extension in the central parts of many mountain belts at the same time as shortening occurs elsewhere previous models for extension require temporal changes in the thermal structure of the lithosphere1 2 the rate of convergence2 3 the strength of the crust3 or the geometry of accretion4 here we present a simple model in which no such changes are required for surface extension during the convergent thickening of a viscous crusta layer convergence is driven by the motion of two plates at the base of the layer in the area where one plate subducts under the other the surface begins to extend soon after the start of convergence eventually stretching by more than 60 per cent extension occurs because gravity drives horizontal flow faster at the free surface than in the centre of a viscous layer that is fixed at its base in real mountain belts mid crustal weaknesses may allow the depth dependent motion required for surface extension

length scales for continental deformation in convergent divergent and strike slip environments analytical and approximate solutions for a thin viscous sheet model the deformation of a thin viscous layer that has a moving boundary is investigated for comparison with zones of deformation in the continental lithosphere exact analytical solutions for the case of a newtonian fluid and approximate solutions for the case of fluid with power law rheology show that when the imposed velocity vector is normal to the boundary congressional or extensional regime the deformation field decays away from the boundary with a characteristic length scale 1 3 to 1 10 the wavelength of the imposed boundary velocity distribution for n between 1 and 10 where n is the stress strain exponent in the rheology in contrast when the imposed velocity vector is parallel to the boundary transcurrent regime the length scale of the deformation field is approximately 4 times smaller in each case these length scales decrease approximately as n the difference in length scales arises even in the absence of any buoyancy forces acting on thickened or thinned crust such forces would modify the ratio of length scales but not sufficiently to affect this result

sediment and rock strength controls on river incision into bedrock recent theoretical investigations suggest that the rate of river incision into bedrock depends nonlinearly on sediment supply challenging the common assumption that incision rate is simply proportional to stream power our measurements from laboratory abrasion mills support the hypothesis that sediment promotes erosion at low supply rates by providing tools for abrasion but inhibits erosion at high supply rates by burying underlying bedrock beneath transient deposits maximum erosion rates occur at a critical level of coarse grained sediment supply where the bedrock is only partially exposed fine grained sediments provide poor abrasive tools for lowering bedrock river beds because they tend to travel in suspension experiments also reveal that rock resistance to fluvial erosion scales with the square of rock tensile strength our results suggest that spatial and temporal variations in the extent of bedrock exposure provide incising rivers with a previously unrecognized degree of freedom in adjusting to changes in rock uplift rate and climate furthermore we conclude that the grain size distribution of sediment supplied by hillslopes to the channel network is a fundamental control on bedrock channel gradients and topographic relief

messinian climate change and erosional destruction of the central european alps at the end of the miocene the european alps ceased outward expansion and tectonic uplift and exhumation shifted into the orogen interior this shift is consistent with a change from orogenic construction to orogenic destruction reflecting an increase in the ratio of erosional flux to accretionary flux the coincidence of this change with an increase in sediment yield from the alps suggests a climate driven increase in erosional flux the timing of deformation and sediment release from the southern alps indicates that the tectonic change occurred synchronous with the last phase of the messinian salinity crisis we attribute the increase in erosional flux to a climatic shift to wetter conditions throughout europe likely augmented by the base level fall that occurred during the mediterranean dessication this climate change is represented in the stratigraphic record by the lago mare deposits of the mediterranean salinity crisis

asymmetric growth of the pyrenees revealed through measurement and modeling of orogenic fluxes orogenic asymmetry is ubiquitous in terms of topography structure and metamorphism but the relative influences of climate and tectonics in determining such asymmetry remain contentious this study demonstrates that for the case of the european pyrenees orogenic asymmetry is not defined by inherited structural asymmetry nor by orographic asymmetry of precipitation but is the simple consequence of the inherent tectonic growth of a doubly vergent orogen the pyrenees record approximately 60 myr of upper crustal accretion erosion and sedimentation that developed from late cretaceous to early miocene times approximately 165 km of plate convergence was accommodated by four phases of orogenic growth that can be summarized in terms of their generic orogenic development as follows 1 early inversion of passive margin extensional faults initiating the pro wedge 2 growth by a combination of frontal accretion and underplating to the pro wedge symmetrical and steady record of erosion and foreland basin sedimentation 3 cessation of pro wedge frontal accretion with propagation of the retro wedge deformation front 4 accelerated growth of pro wedge antiformal stack induced by underplating accelerated erosion over antiformal stack and increase in sediment discharge to pro foreland basin stabilization of outer pro wedge by sediment blanketing discrete element model experiments are used to explore the natural tendency of the system towards asymmetry model output demonstrates that during active frontal accretion to the pro wedge of an orogen the maxima in erosional denudation is offset towards the pro wedge relative to the maximum depths of exhumation this asymmetry is fundamentally driven by the progressive reduction in advection velocities of material within the orogen from the pro wedge into the retro wedge hence for an efficient erosional regime a dynamic coupling between erosion and structural uplift of the pro wedge will reduce activity of the retro wedge and hence drive asymmetry in terms of low temperature thermochronometry erosion and sedimentation the models also demonstrate the tendency for pro wedge growth through frontal accretion to slow however the cessation of frontal accretion as recorded by the south pyrenean thrust belt phases 3 and 4 requires additional forcing it is proposed that the thick wedge top sedimentation recorded at this time increased taper angles and hence stabilized the wedge accretionary influx into the orogen at this time was accomplished through underplating these processes governing asymmetry do not require a spatially variable climate or a laterally heterogeneous underthrust plate

steady state erosion of critical coulomb wedges with applications to taiwan and the himalaya 1 orogenic structure appears to be partially controlled by the addition to and removal of material from the mountain belt by tectonic accretion and geomorphic erosion respectively we developed a coupled erosion deformation model for orogenic wedges that are in erosional steady state and deform at their coulomb failure limit erosional steady state is reached when all material introduced into the wedge is removed by erosion that is limited by the rate at which rivers erode through bedrock we found that the ultimate form of a wedge is controlled by the wedge mechanical properties sole out depth of the basal decollement erosional exponents basin geometry and the ratio of the added material flux to the erosional constant as this latter ratio is increased wedge width and surface slopes increase we applied these models to the taiwan and himalayan orogenic wedges and found that despite a higher flux of material entering the former the inferred ratio was larger for the latter calculated values for the erodibility of each wedge showed at least an order of magnitude lower value for the himalaya relative to taiwan these values are consistent with the lower precipitation regime in the himalaya relative to taiwan and the exposure of crystalline rocks within the himalayan orogenic wedge independently determined rock erodibility estimates are consistent with the accretionary wedge sediments and metasediments and the crystalline and high grade metamorphic rocks exposed within taiwan and the himalaya respectively therefore differences in rock type and climate apparently lead to key differences in the erosion and hence orogenic structure of these two mountain belts

modeling precipitation over ice sheets an assessment using greenland the interaction between ice sheets and the rest of the climate system at long time scales is not well understood and studies of the ice ages typically employ simplified parameterizations of the climate forcing on an ice sheet it is important therefore to understand how an ice sheet responds to climate forcing and whether the reduced approaches used in modeling studies are capable of providing robust and realistic answers this work focuses on the accumulation distribution and in particular considers what features of the accumulation pattern are necessary to model the steady state response of an ice sheet we examine the response of a model of the greenland ice sheet to a variety of accumulation distributions both observational datasets and simplified parameterizations the predicted shape of the ice sheet is found to be quite insensitive to changes in the accumulation the model only differs significantly from the observed ice sheet for a spatially uniform accumulation rate and the most important factor for the successful simulation of the ice sheet s shape is that the accumulation decreases with height according to the ability of the atmosphere to hold moisture however the internal ice dynamics strongly reflects the influence of the atmospheric circulation on the accumulation distribution

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