crustal reworking at nanga parbat pakistan metamorphic consequences of thermal mechanical coupling facilitated by erosion within the syntaxial bends of the india asia collision the himalaya terminate abruptly in a pair of metamorphic massifs nanga parbat in the west and namche barwa in the east are actively deforming antiformal domes which expose quaternary metamorphic rocks and granites the massifs are transected by major himalayan rivers indus and tsangpo and are loci of deep and rapid exhumation on the basis of velocity and attenuation tomography and microseismic magnetotelluric geochronological petrological structural and geomorphic data we have collected at nanga parbat we propose a model in which this intense metamorphic and structural reworking of crustal lithosphere is a consequence of strain focusing caused by significant erosion within deep gorges cut by the indus and tsangpo as these rivers turn sharply toward the foreland and exit their host syntaxes the localization of this phenomenon at the terminations of the himalayan arc owes its origin to both regional and local feedbacks between erosion and tectonics

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

40ar 39ar ages in mantle xenolith phlogopites determining the ages of multiple lithospheric mantle events and diatreme ascent rates in southern africa and malaita solomon islands kimberlites are extraordinary natural phenomena ascending through the earth s lithosphere entraining xenoliths to erupt at the surface within hours to days of their inception deep within the lithospheric mantle with the realization that some ar ar phlogopite grain core ages may be indicative of geological events we have undertaken high spatial resolution ar ar dating of phlogopites in xenoliths and megacrysts from kimberley monastery and letseng in southern africa and malaita in the solomon islands to est whether other mantle phlogopite cores may yield meaningful ages modelling of ar diffusive loss profiles from phlogopite grain boundaries to cores provides information on both the eruption age and the duration of outgassing within the kimberlite magma and hence yields estimates on diatreme ascent rates the ascent durations are very similar for all of the southern african pipes studied yielding durations of 0 9 6 9 days assuming an average kimberlite magma temperature of 1000 c these can be compared to estimates from phlogopite xenoliths from siberian diamond bearing kimberlites yielding ascent durations of 2 15 hours assuming the same magma temperature

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

river profiles along the himalayan arc as indicators of active tectonics abstract longitudinal profiles along sixteen major transverse himalayan rivers add important constraints to models of active continental subduction and its evolution these profiles are characterized by a zone of relatively high gradient that cannot be associated with differential resistence to erosion in all cases the base of the zone of increased gradients correlates with 1 the topographic front between the lesser and high himalayas 2 the narrow belt of intermediate magnitude thrust earthquakes 3 the main central thrust zone mct these features define a small circle in the central portion of the himalayan arc these correlations suggest that the discontinuity in the river profiles and the other features are controlled by a major tectonic boundary between the rising high himalayas and the lesser himalayas no sharp increases in gradient are observed near the main boundary thrust mbt except on a few rivers such as the jhelum or kundar where the mbt lies close to both the mct and the seismic belt thus it is unlikely that the mbt is a major tectonic boundary the diversion of river courses along the mbt and around anticlines in the sub himalayas has probably been caused by aggradation near the rosion deposition boundary upstream of uplifts in the mahabharat range and sub himalayas a parallel is drawn between the himalayas and new guinea based on the hypothesis that continent arc collision of the type occurring in northern australia preceded continent continent collision in the himalayas the present sedimentary tectonic phase in new guinea resembles the subathu paleocene eocene phase in the himalayas incipient counterparts of the major himalayan structures including the mct and the mbt are recognized in new guinea the drainage patterns in the himalayas and in new guinea bear a similar relation to major structures this suggests that 1 the tectonic evolution of the himalayas has been rather uniform since early stages of collision and 2 the himalayan drainage was also formed at these early stages and is therefore antecedent to the rise of the high himalayas

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

87sr 86sr in kimberlitic carbonates by ion microprobe hydrothermal alteration crustal contamination and relation to carbonatite carbonates in a 30 cm wide zoned kimberlite dyke from the de beers mine kimberley s africa were studied by cathodoluminescence and electron microprobe techniques and their 87sr 86sr ratios were measured using an aei im20 ion microprobe primary carbonates including calcite dendrites rhombohedral calcites in segregation vesicles and mosaic dolomite have high sr 0 69 1 35 wt sro and ba 0 24 0 44 bao and 87sr 86sr ratios in the range 0 7046 to 0 7056 secondary sparry calcite in amygdales and veins is characterised by low ba 0 05 bao and 87sr 86sr near 0 72 rhombohedral calcite 0 5 cm from a contact with 2 900 my old biotite gneiss has minor element chemistry like that of primary carbonate but an elevated 87sr 86sr ratio of 0 7103 possibly indicating crustal contamination in a boundary layer of the kimberlite magma amygdale like segregations of carbonate and or serpentine originated as gas cavities and were not formed by liquid immiscibility they are now filled either by secondary calcite or by minerals precipitated from residual kimberlite liquid however dendritic calcite and primary dolomite and calcite with high sr ba and low 87sr 86sr demonstrate shared chemical characteristics between these carbonates and carbonatite the primary kimberlite magma had initial 87sr 86sr close to 0 7046

quantitative testing of bedrock incision models for the clearwater river nw washington state 1 the incision of rivers in bedrock is thought to be an important factor that influences the evolution of relief in tectonically active orogens at present there are at least six competing models for incision of bedrock rivers but these models have received little quantitative testing we statistically evaluate these models using observations from the clearwater river in northwestern washington state which crosses the actively rising forearc high of the cascadia margin a previous study has used fluvial terraces along the clearwater to estimate bedrock incision rates over the last 150 kyr they show that incision rates have been steady over the long term 50 kyr consistent with other evidence based on isotopic cooling ages for steady long term 1 myr erosion rates the steady state character of the river allows us to use the relatively simple time invariant solutions for the various incision models and also to estimate long term sediment discharge along the river which is a critical variable for some incision models an interesting feature of the clearwater river is that it has a downstream decrease in the rate of incision from 0 9 mm yr in the headwater to 0 1 mm yr at the coast none of the incision models including the shear stress model successfully accounts for this relationship this result may be due to the simple way in which these models are used commonly without consideration for the distribution of discharge with time and the variable capacity of the river channel to contain peak flows along its course we suggest some general improvements for the incision models and also guidelines for selecting those rivers that will allow good discrimination between competing models

u pb ages sr nd isotope geochemistry and petrogenesis of kimberlites kamafugites and phlogopite picrites of the alto paranaiba igneous province brazil abstract the kimberlites kamafugites and phlogopite picrites of alto paranaiba igneous province apip southern brazil span a range between 91 and 78 ma with new in situ more tightly constrained u pb ages the kimberlites show porphyritic texture with olivine xeno and phenocrysts plus phlogopite fe ti cr oxides and perovskite microcrysts set in a carbonate rich matrix the kamafugites are feldspar free rocks represented by ugandites and mafurites ugandites and mafurites are porphyritic and contain olivine and clinopyroxene phenocrysts in a fine grained groundmass composed by clinopyroxene perovskite apatite magnetite phlogopite leucite and or analcime in ugandites and by olivine clinopyroxene amphibole phlogopite perovskite magnetite kalsilite and ba zeolites in mafurites phlogopite picrites have a pseudo fluidal porphyritic texture with olivine phenocrysts and abundant phlogopite microcrysts in a groundmass composed by olivine spinel apatite perovskite calcite and rare garnet new in situ sr and nd isotopic data on perovskites 87 sr 86 sr i 0 70467 0 70565 and 143 nd 144 nd i 0 51222 0 51233 fall within the known ranges of apip rocks 87 sr 86 sr i 0 70431 0 70686 143 nd 144 nd i 0 51205 0 51280 the apip magmas derived from a source assemblage made up of an old metasomatized mica carbonate garnet lherzolite that did not suffer interaction with convective mantle nor with any hypothetical melts derived from a trindade mantle plume geochemical modelling show that low degree melting f 0 5 2 of such a source can produce compositions resembling the apip rocks the geochemical and isotopic composition of the magmas the calculated degrees of partial melting the composition of the calculated source and the absence of a hot spot track from goias to alto paranaiba igneous provinces can be explained with the presence of chemically and mineralogically heterogeneous mantle sources that melted at different pressures

the petrogenesis of group 2 ultrapotassic kimberlites from finsch mine south africa abstract major trace element and radiogenic isotope results are presented for a suite of hypabyssal kimberlites from a single pipe at the finsch mine south africa these are group 2 kimberlites characterised by abundant phlogopite serpentine diopside they are ultrabasic sio2 6 9 igneous rocks they exhibit a wide range in major element chemistry with sio2 27 6 41 9 wt and mgo 10 4 33 4 wt 87 sr 86 sr i 0 7089 to 0 7106 nd is 6 2 to 9 7 and they have unradiogenic 207pb 204pb i contents which ensure that they plot below the pb ore growth curve they have high incompatible and compatible element contents a striking positive array between y and nb which indicates that garnet was not involved in the within suite differentiation processes and a negative trend between k nb and nb contents which suggests that phlogopite was involved in addition some elements exhibit an unexpected order of relative incompatibility for different trace elements which suggests that the intra kimberlite variations are not primarily due to variations in the degree of partial melting the effects of fractional crystallization are difficult to establish because for the most part they have been masked by the entrainment of 50 60 mantle peridotite thus the finsch kimberlites are interpreted as mixtures of a melt component and entrained garnet peridotite with no evidence for significant contamination with crustal material the melt component was characterised by high incompatible element contents which require both very small degrees of partial melting and source regions with higher incompatible element contents than depleted or primitive mantle since the melt component was the principal source of incompatible elements in the kimberlite magma the enriched nd sr and pb isotope ratios of the kimberlite are characteristic of the melt source region the melt fractions were therefore derived from ancient trace elements enriched portions of the upper mantle most probably situated within the sub continental mantle lithosphere and different from the low 87sr 86sr garnet peridotite xenoliths found at finsch within the sub continental mantle lithosphere old incompatible element enriched source regions for the kimberlite melt fraction are inferred to have been overlain by depleted mantle material which became entrained in the kimberlite magma

zircon ion microprobe studies bearing on the age and evolution of the witwatersrand triad abstract the ventersdorp and witwatersrand supergroups and the dominion group collectively referred to as the witwatersrand triad represent some of the oldest and economically most important volcano sedimentary basins within the kaapvaal craton of southern africa precise age constraints which are vital to models aimed at understanding the formation mineralisation and evolution of these basins have been hampered by analytical and geological problems these problems have been overcome by this ion microprobe study on zircons from volcanic rocks within the three basins felsic and mafic volcanics from the ventersdorp supergroup yield ages of 2709 4 ma and 2714 8 ma respectively constraining the timing of the closure of the witwatersrand basin a maximum age for the witwatersrand basin is indicated by the age of 3074 6 ma for volcanics from the underlying dominion group thus a period of c 350 ma was available for the accumulation of the full witwatersrand basin succession unfortunately dating within the witwatersrand basin itself is not complete and future work could show that extended time breaks may exist within the basin these new data show that previous age determinations on the ventersdorp witwatersrand and dominion basins were inaccurate and that the economically important witwatersrand basin must now be recognised as archaean

late triassic sinistral shear in the east gobi fault zone mongolia abstract tectonic studies of the east gobi fault zone in southeastern mongolia reveal multiple distinct intracontinental deformation events postdating late paleozoic arc accretion and continental amalgamation metamorphic tectonites of the tsagan subarga and tavan har blocks previously mapped as precambrian basement comprise a sinistral shear zone dominated by steeply dipping northeast striking foliations field observations and petrographic analyses indicate that the protoliths of the metamorphic tectonites are paleozoic arc volcanic and sedimentary sequences 40ar 39ar step heating analyses of minerals from pre syn and late to post kinematic lithologies bracket the timing of ductile sinistral shear as late triassic the main phase of distributed deformation associated with cooling through amphibolite upper greenschist facies conditions occurred ca 225 ma and shear zone activity waned ca 210 ma cooling rates inferred from the 40ar 39ar data are on the order of 40 20 c myr 1 apparent differences for the two basement blocks may reflect subsequent differential uplift during late jurassic cretaceous rifting relatively rapid late triassic cooling suggests a transtensional component to the deformation and is coincident with core complex formation in northern china late triassic intraplate deformation in southeastern mongolia is likely the result of far field forces associated with collision between mongolian arcs and the siberian craton i e closure of the mongol okhotsk ocean and or collisions associated with closure of the paleotethys the ductile shear zone has been documented over 250 km along strike and has been modified by subsequent brittle deformation events in the mesozoic and cenozoic

miocene to holocene exhumation of metamorphic crustal wedges in the nw himalaya evidence for tectonic extrusion coupled to fluvial erosion the himalayan crystalline core zone exposed along the sutlej valley india is composed of two high grade metamorphic gneiss sheets that were successively underthrusted and tectonically extruded as a consequence of the foreland directed propagation of crustal deformation in the indian plate margin the high himalayan crystalline sequence hhcs is composed of amphibolite facies to migmatitic paragneisses metamorphosed at temperatures up to 750 c at 30 km depth between eocene and early miocene during early miocene combined thrusting along the main central thrust mct and extension along the sangla detachment induced the rapid exhumation and cooling of the hhcs whereas exhumation was mainly controlled by erosion since middle miocene the lesser himalayan crystalline sequence lhcs is composed of amphibolite facies para and orthogneisses metamorphosed at temperatures up to 700 c during underthrusting down to 30 km depth beneath the mct the lhcs cooled very rapidly since late miocene as a consequence of exhumation controlled by thrusting along the munsiari thrust and extension in the mct hanging wall this renewed phase of tectonic extrusion at the himalayan front is still active as indicated by the present day regional seismicity and by hydrothermal circulation linked to elevated near surface geothermal gradients in the lhcs as recently evidenced in the himalayan syntaxes active exhumation of deep crustal rocks along the sutlej valley is spatially correlated with the high erosional potential of this major trans himalayan river this correlation supports the emerging view of a positive feedback during continental collision between crustal scale tectono thermal reworking and efficient erosion along major river systems

coupling of rock uplift and river incision in the namche barwa gyala peri massif tibet geodynamic modeling demonstrates the strong potential for erosion to influence the pattern and style of deformation in active mountain belts but field studies yield conflicting views on the importance of erosion in influencing orogenesis here we compare patterns in river power inferred excess fluvial transport capacity topographic relief precipitation and mineral cooling ages to assess the coupling between surface erosion and rock uplift within the vicinity of the namche barwa gyala peri massif an active antiformal structure within the eastern himalayan syntaxis our rich and dense data set reveals a tight spatial correspondence of fluvial incision potential high relief and young cooling ages the spatial coincidence is most easily explained by a sustained balance between rock uplift and denudation driven by river incision over at least the last 1 m y the yarlung tsangpo brahmaputra river is the largest and most powerful river in the himalaya and two lines of evidence point to its active role in the dynamic interaction of local erosion rock uplift thermal weakening of the lithosphere and deformation 1 whereas along the rest of the himalayan front high relief and high rock uplift rates are essentially continuous the high relief and rapid exhumation in the syntaxis is restricted to a bull s eye pattern exactly where the largest river in the himalaya the yarlung tsangpo brahmaputra has the most energy per unit area available to erode its channel and transport sediment 2 the location of rapid incision on the yarlung tsangpo brahmaputra has been pinned for at least 1 m y and without compensatory uplift of the namche barwa gyala peri massif during this time the river would have eroded headward rapidly incising deeply into tibet

fluvial incision and tectonic uplift across the himalayas of central nepal the pattern of fluvial incision across the himalayas of central nepal is estimated from the distribution of holocene and pleistocene terraces and from the geometry of modern channels along major rivers draining across the range the terraces provide good constraints on incision rates across the himalayan frontal folds sub himalaya or siwaliks hills where rivers are forced to cut down into rising anticlines and have abandoned numerous strath terraces farther north and upstream in the lesser himalaya prominent fill terraces were deposited probably during the late pleistocene and were subsequently incised the amount of bedrock incision beneath the fill deposits is generally small suggesting a slow rate of fluvial incision in the lesser himalaya the terrace record is lost in the high range where the rivers are cutting steep gorges to complement the terrace study fluvial incision was also estimated from the modern channel geometries using an estimate of the shear stress exerted by the flowing water at the bottom of the channel as a proxy for river incision rate this approach allows quantification of the effect of variations in channel slope width and discharge on the incision rate of a river the determination of incision rates requires an additional lithological calibration the two approaches are shown to yield consistent results when applied to the same reach or if incision profiles along nearby parallel reaches are compared in the sub himalaya river incision is rapid with values up to 10 15 mm yr it does not exceed a few millimeters per year in the lesser himalaya and rises abruptly at the front of the high range to reach values of 4 8 mm yr within a 50 km wide zone that coincides with the position of the highest himalayan peaks sediment yield derived from the measurement of suspended load in himalayan rivers suggests that fluvial incision drives hillslope denudation of the landscape at the scale of the whole range the observed pattern of erosion is found to closely mimic uplift as predicted by a mechanical model taking into account erosion and slip along the flat ramp flat geometry of the main himalayan thrust fault the morphology of the range reflects a dynamic equilibrium between present day tectonics and surface processes the sharp relief together with the high uplift rates in the higher himalaya reflects thrusting over the midcrustal ramp rather than the isostatic response to reincision of the tibetan plateau driven by late cenozoic climate change or late miocene reactivation of the main central thrust

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

can kimberlites be generated from an ordinary mantle kimberlites are characterized by a high concentration of incompatible elements including light rare earths however nd isotopic evidence indicates that their source regions do not have a long history of enrichment if kimberlites can be generated from non enriched mantle sources by simple partial melting processes this implies that crystal liquid partition coefficients for some trace elements are lower for kimberlitic liquids than for basalts or andesites from a comparison of clinopyroxene megacrysts regarded as equilibrated with kimberlite at depth with existing kimberlite data we argue here that low crystal liquid partition coefficients for the rare earths are plausible

petrology of the hypabyssal kimberlite of the kroonstad group ii kimberlite orangeite cluster south africa evolution of the magma within the cluster abstract we report here a detailed petrographic study accompanied by the first whole rock major and trace element geochemical analyses as well as the first sr and nd isotope data for the kroonstad group ii kimberlite cluster hypabyssal kimberlite is described from the lace voorspoed and besterskraal north kimberlite pipes the lace kimberlite is a monticellite phlogopite kimberlite with mineralogy and geochemistry more similar to typical group i kimberlites the voorspoed kimberlite is an aegirine and sanidine bearing kimberlite and along with the geochemistry shows a more evolved composition relative to the lace kimberlite the besterskraal north kimberlite is a highly evolved type group ii kimberlite containing abundant k richterite sanidine aegirine and leucite the magma evolutionary trend is linked to a variation in olivine macrocryst abundance the lace unevolved magma contains abundant olivine macrocrysts and has not been affected by significant fractionation whereas the voorspoed and besterskraal north kimberlite contain far fewer olivine macrocrysts which indicates that significant fractionation has affected these magmas en route to the surface we evaluate the potential for fractionation to result in differentiation of a common parent magma or magma generated from a common source en route to the surface this process is complicated by the fact that olivine macrocrysts are essentially xenocrysts within the magma and the loss of these xenocrysts will not affect the primary magma composition furthermore we present two alternate hypotheses for the evolution of the magmas a contamination en route to the surface and b varying degrees of partial melting at the source the evolutionary trend is also linked with diamond characteristics in particular diamond parcels at the voorspoed pipe between 1906 and 1911 were deficient in large stones which indicate that fractionation has likely also removed the large stones from the voorspoed magma along with the olivine macrocrysts

distribution of active rock uplift along the eastern margin of the tibetan plateau inferences from bedrock channel longitudinal profiles 1 current models of long term river incision into bedrock suggest that the local rate of differential rock uplift should exert a primary control on the gradient of channel longitudinal profiles however discrimination of this effect from the influence of variations in substrate erodibility sediment flux precipitation and transient changes in profile shape has proved difficult in practice here we investigate the controls on the spatial distribution of bedrock channel gradients adjacent to the sichuan basin in an effort to assess the degree and nature of active deformation along this margin of the tibetan plateau analysis of river longitudinal profiles utilizing a channel steepness index a measure of profile gradient normalized for drainage area reveals a zone of anomalously steep channels adjacent to the topographic front of the plateau margin channel profiles are systematically less steep in their headwater reaches on the plateau and in their lower reaches east of the plateau margin comparison of steepness indices to mapped lithologic variations reveals that lithology has only a limited influence on channel gradient in this field area we observe no systematic relationship between steepness indices and upstream drainage area channels of all size are steeper near the plateau margin we argue that these systematic changes are not readily explained as a consequence of increased sediment flux or of orographic precipitation we are led to conclude that steep channel profiles along the topographic front of the plateau reflect active differential rock uplift between this region and the foreland