s wave residuals from earthquakes in the tibetan region and lateral variations in the upper mantle abstract mean s wave residuals from 46 earthquakes within and on the margins of the tibetan plateau exhibit systematic lateral variations that do not correlate well with elevation or with simple aspects of the geologic history the earliest s waves come from earthquakes in western tibet the karakorum and the western himalaya and the latest come from earthquakes in north central tibet although s waves from earthquakes in the himalaya tend to be early the east west variation in residuals across tibet is at least as large as the north south difference between the himalaya and northern tibet if the variations in residuals are a reflection of temperature variations in the upper mantle associated with convection then upwelling beneath north central tibet seems to be flanked by downwelling in western eastern and probably southern tibet this convective flow might reflect the detachment and removal of thickened mantle lithosphere beneath tibet

miocene high sr y magmatism south tibet product of partial melting of subducted indian continental crust and its tectonic implication abstract post collisional 26 10 ma high sr y and hree depleted intrusive rocks occur in an e w trending belt paralleling the yarlung zangbo suture in south tibet their petrogenesis has been a subject of debate here we present chronological geochemical and sr nd hf isotopic data for two newly discovered large volume high sr y granitoids the pagu granodiorite and the nanmuqie granite and three high sr y porphyries intruding the granitoids in the lhasa terrane la icp ms zircon u pb dating for the pagu granodiorite and nanmuqie granite yielded identical magma crystallization ages of 14 0 14 4 ma which is indistinguishable from their associated high sr y porphyries 14 2 14 6 ma the granitoid was intruded at middle crust depth whereas the porphyry was intruded at upper crust depth indicating that the lhasa terrane has experienced a rapid crustal uplift during the high sr y magma emplacement zircon hf isotopic and whole rock sr nd isotopic compositions for these granitoids and porphyries suggest that their magmas were dominantly derived from partial melting of crustal materials the granitoids and porphyries have ehf t values overlapping with the mafic granulites in the himalayan terrane indian plate their sr nd isotopic compositions show two endmember mixing between the himalayan mafic lower crust and the ultrapotassic lavas the lhasa lower crust we suggest that the high sr y magmas in the lhasa terrane could be derived from partial melting of subducted indian mafic lower crust with incorporation of the ultrapotassic lava and or the lhasa lower crust components our study suggests a new model for the high sr y magma generation in the lhasa terrane and provides a line of geochemical evidence that the indian continental crust was subducted beneath the southern lhasa terrane in the early middle miocene

numerical model predictions of autogenic fluvial terraces and comparison to climate change expectations terraces eroded into sediment alluvial and bedrock strath preserve an important history of river activity river terraces are thought to form when a river switches from a period of slow vertical incision and valley widening to fast vertical incision and terrace abandonment consequently terraces are often interpreted to reflect changing external drivers including tectonics sea level and climate in contrast the intrinsic unsteadiness of lateral migration in rivers may generate terraces even under constant rates of vertical incision without external forcing to explore this mechanism we simulate landscape evolution by a vertically incising meandering river and isolate the age and geometry of autogenic river terraces modeled autogenic terraces form for a wide range of lateral and vertical incision rates and are often paired and longitudinally extensive for intermediate ratios of vertical to lateral erosion rate autogenic terraces have a characteristic reoccurrence time that scales with the time for relief generation there is a preservation bias against older terraces due to reworking of previously visited parts of the valley evolving spatial differences in bank strength between bedrock and sediment reduce terrace formation frequency and length favor pairing and can explain sublinear terrace margins at valley boundaries age differences and geometries for modeled autogenic terraces are consistent in cases with natural terraces and overlap with metrics commonly attributed to terrace formation due to climate change we suggest a new phase space of terrace properties that may allow differentiation of autogenic terraces from terraces formed by external drivers

origin of paired high pressure ultrahigh temperature orogens a ridge subduction and slab window model introductionmetamorphism at elevated pressure temperature conditions and distinct uid regimes generates ultrahigh pres sure uhp high pressure hp andultrahigh temperature uht rocks harley 2004 brown 2006 2007 2009 kelsey 2008 santosh andomori 2008a b santos et al 2009 santosh et al 2009a zhang et al 2009 although previous studies havecorrelated these di erent orogens todistinct tectonic settings recent inves tigations record high pressure anduht conditions from the same tec tonic environment within the sameorogenic belt and in some cases evenwithin the same samples mostoftheseexamples come from major collisionalzones along which continental blocksweresuturedatdi erenttimesinearthhistory including from the neoarch ean hp uht rocks of the limpopobeltinsouthafrica tsunogaeandvanreenen 2006 the palaeoproterozoichp guo et al 2004 obrien et al 2005 and uht santosh et al 2007a b 2008 2009b belts in thenorth china craton and the hp uht belts in the neoproterozoiceastern ghats belt as well as thecambrian gondwana suture in south ern india shimpo et al 2006 tsuno gaeet al 2008a santoshet al 2009c notably the thermal regimes for thesepaired hp and uht rocks are similar 1000 c and they also share acommon uid history characterizedby co

miocene volcanism in the lhasa block tibet spatial trends and geodynamic implications miocene ultrapotassic up shoshonitic p and calc alkaline ca volcanism is recognized widely over the tibetan plateau this volcanism occurred after the end of india s oceanic lithosphere subduction beneath the plateau ambiguities about the distribution and significance of this volcanism arise in part from a paucity of geochronologic and geochemical data we have sampled two volcanic fields in the lhasa block 1 the eastern edge of the zabuye salt lake central part of the block 2 the southwestern edge of the yangbajin graben major and trace elements display high k2o 6 06 6 54 over a wide range of sio2 75 03 55 30 na2o 1 32 3 30 and lree hree enrichment with a significant rare earth element fractionation 266 la yb n 6 42 as well as negative nb ta and ti anomalies these features indicate that our samples are chemically identical to the post collision miocene up to p volcanic rocks already identified all over the tibetan plateau and the southern plateau specifically but distinct from the ca rocks found in the lhasa block the miocene volcanics found in the lhasa block present greater chemical variation than the rest of the tibetan plateau 40 ar 39 ar ages on both sanidine and biotite gave ages ranging from 16 16 g 0 12 ma to 16 01 g 0 12 ma and 10 60 g 0 14 ma to 10 88 g 0 17 ma 2 c excluding systematic errors relative to fish canyon sanidine at 28 02 ma for zabuye salt lake and yangying geothermal field volcanics respectively these results are consistent with the range of previously published ages from this part of the tibetan plateau the presence of n s trending dikes and the location of the volcanic fields suggest that local e w extension in the lhasa block was coeval we propose that the synchronous up p and ca volcanism in the lhasa block between 23 and 8 ma is the result of progressively deepening northward subduction of indian continental lithosphere producing asthenospheric upwelling convective thinning delamination of lithospheric mantle fueled by the hot asthenosphere induced partial melting of an enriched sub continental mantle lithosphere as well as eclogitic lower crust this mechanism could also explain the local e w extension observed in the same period 18 10 ma we also propose that lateral migration of the deepening indian continental lithosphere mantle slab could explain the observed younger volcanism in the eastern part of the lhasa terrane

lithologic and tectonic controls on bedrock channel form at the northwest himalayan front 1 recognition that channel form reflects a river s ability to erode rock and transport material has spawned stream power models that estimate incision patterns by approximating energy dissipation within a channel these models frequently assume that channel width scales as a power law with drainage area partly because drainage area is easily extracted from digital elevation models dems however this assumption is often confounded by local variations in rock strength and rock uplift rate that can cause channel constriction downstream here we investigate the morphological response to spatial changes in rock strength and rock uplift rate of 10 bedrock channels traversing the mohand range along the northwest himalayan front we present a new method to continuously measure and compare channel width slope and other hydraulic parameters that integrate satellite imagery and dem analysis our method corrects for an 13 overestimation of average channel gradient from a 90 m resolution dem that arises from short circuits of fine scale meanders we find that channels 1 narrow 1 km upstream from knickpoints formed by an increase in rock strength 2 adjust laterally more than vertically in response to downstream decreases rock erodibility and uplift rate and 3 meander where shear stresses are high and channel widths are low we attribute these results to a high ratio of sediment supply to transport capacity which enhances lateral erosion relative to vertical incision our results suggest that substrate strength and sediment supply substantially influence channel form and that channel width should be explicitly measured when interpreting tectonic signals from bedrock channel morphology

timing of tibetan uplift constrained by analysis of volcanic rocks the tibetan plateau has had a central role in the development of recent models for the mechanics of mountain belts1 2 and cenozoic global climate change3 the present elevation and extensional deformation of the plateau probably result from uplift owing to convective thinning of the underlying lithospheric mantle1 2 an age for the uplift would provide a valuable constraint on these models but because recently proposed indicators of uplift are all climate dependent they are equivocal possibly reflecting global cooling rather than regional uplift4 here we present new geochemical data on post mid miocene lavas from the plateau which show that the lavas were derived from the lithospheric mantle simple thermal arguments indicate that the generation of such magmas also necessitates thinning of the lithospheric mantle thus volcanism is coincident with uplift providing a climate independent means of dating the onset of uplift dating by the laser 40ar 39ar technique places the beginning of this volcanism and therefore the time of uplift of the tibetan plateau at 13 myr ago

adakites from continental collision zones melting of thickened lower crust beneath southern tibet adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones here we report the first example of such magmas from southern tibet in an active continental collision environment the tibetan adakites were emplaced from ca 26 to 10 ma and their overall geochemical characteristics suggest an origin by melting of eclogites and or garnet amphibolites in the lower part 50 km of thickened tibetan crust this lower crustal melting required a significantly elevated geotherm which we attribute to removal of the tectonically thickened lithospheric mantle in late oligocene time the identification of collision type adakites from southern tibet lends new constraints to not only the himalayan tibetan orogenesis how and when the indian lithosphere started underthrusting asia can be depicted but also the growth of the early continental crust on earth that consists dominantly of the tonalite trondhjemite granodiorite suites marked by adakitic geochemical affinities

tectonic implications of felsic tuffs within the lower miocene gangrinboche conglomerates southern tibet felsic tuffs are interbedded with the gangrinboche conglomerates adjacent to the yarlung tsangpo suture zone in southern tibet laser ar ar dating of mineral separates indicates they are of early miocene age such tuffs are most likely an eruptive manifestation of geochemically indistinguishable coeval felsic adakitic intrusions that are widely reported across southern tibet the considerable lateral e w extent of the gangrinboche conglomerates and their depositional setting indicates sediment accumulation in an overall n s compressional regime and thereby places important constraints on the tectonic setting in which magmatism initiated the conglomerates were folded and truncated during back thrusting associated with development of the north directed great counter thrust as n s trending rifts associated with e w extension of the tibetan plateau cut both the conglomerates and this thrust system it can be inferred that post collisional volcanism is unlikely to have been genetically linked to later e w extension early miocene slab break off beneath tibet provides a model that appears to be consistent with petrogenesis of the associated magmatic suite which requires a lowermost crust or lithospheric mantle generation molasse accumulation and uplift and emplacement of north himalayan gneiss domes

numerical simulations of bedrock valley evolution by meandering rivers with variable bank material bedrock river valleys are fundamental components of many landscapes and their morphologies from slot canyons with incised meanders to wide valleys with strath terraces may record environmental history several formation mechanisms for particular valley types have been proposed that involve changes in climatic and tectonic forcing but the uniqueness of valley evolution pathways and the long term stability of valley morphology under constant forcing are unknown and are not predicted in existing numerical models for vertically incising rivers because rivers often migrate more rapidly through alluvium than through bedrock we explore the hypothesis that the distribution of bank materials strongly influences river meandering kinematics and can explain the diversity of bedrock river valley morphology simulations using a numerical model of river meandering with vector based bank material tracking indicate that channel lateral erosion rate in sediment and bedrock vertical erosion rate and initial alluvial belt width explain first order differences in bedrock valley type that bedrock bound channels can evolve under steady forcing from alluvial states and that weak bedrock and low vertical incision rates favor wide shallow valleys while resistant bedrock and high vertical incision rates favor narrow deep valleys during vertical incision sustained planation of the valley floor is favored when bedrock boundaries restrict channel migration to a zone of thin sediment fill the inherent unsteadiness of river meandering in space and time is enhanced by evolving spatial contrasts in bank strength between sediment and bedrock and can account for several valley features including strath terraces and underfit valleys commonly ascribed to external drivers

controlling factors in the distribution and development of incised meanders in the central colorado plateau a study of the distribution and geometry of incised meanders in 64 reaches encompassed approximately 600 km of the green colorado and san juan rivers in the central colorado plateau the sinuosity average planform size and average cross sectional symmetry of each reach were determined by map measurements and by spectral analysis of the curvature series for each reach as determined from interpolations of the digitized traces of the channels possible controlling variables examined including average channel gradient drainage area average bedrock erodibility and bedrock structure were compiled for each reach using available maps gradients in the studied reaches are significantly correlated with bedrock type sinuous incised channels are generally found in low gradient reaches in the san juan river the channel is sinuous where it flows against the bedrock dip and generally straight in reaches where flow is downdip this correlation is weak in the green and colorado rivers the average meander size of the sinuous reaches as described by the median curvature value for each reach is generally less in steeper reaches than in low gradient reaches although the relation of bend size to controlling variables is much less clear than for sinuosity or cross section shape most meander cross sections in the area are relatively symmetrical but highly ingrown forms are also present in general symmetric bends are associated with resistant bedrock units whereas ingrown forms develop in massive sandstone and in highly erodible bedrock gradient significantly influences the distribution of ingrown bends with asymmetric meanders concentrated in reaches of low average gradient this correlation is stronger than that between cross section shape and lithology itself incised meanders of the central colorado plateau are probably at least partly inherited from ancestral streams of unknown age that flowed across the area before the present canyons were cut correlation between meander distribution and regional structures suggests that the general location of low gradient sinuous reaches has probably not changed during the incision of the present canyons incised meanders however are clearly able to modify their geometry in response to changes in bedrock resistance as indicated by the strong correlation between bedrock type and cross section symmetry one mechanism of modification is abandonment of bends which is documented by 18 cutoff meanders in the study area

the climatic signature of incised river meanders climate controls landscape evolution but quantitative signatures of climatic drivers have yet to be found in topography on a broad scale here we describe how a topographic signature of typhoon rainfall is recorded in the meandering of incising mountain rivers in the western north pacific spatially averaged river sinuosity generated from digital elevation data peaks in the typhoon dominated subtropics where extreme rainfall and flood events are common and decreases toward the equatorial tropics and mid latitudes where such extremes are rare once climatic trends are removed the primary control on sinuosity is rock weakness our results indicate that the weakness of bedrock channel walls and their weakening by heavy rainfall together modulate rates of meander propagation and sinuosity development in incising rivers

high sr y and la yb ratios the meaning of the adakitic signature abstract the name of adakite is used to describe a far too large group of rocks whose sole common feature is high sr y and la yb ratios however such a signature can be achieved via different processes melting of a high sr y and la yb source deep melting with abundant residual garnet fractional crystallization or afc or interactions of felsic melts with the mantle causing selective enrichment in lree and sr over hree a database of the compositions of adakitic rocks including high silica and low silica adakites martin h smithies r h rapp r p moyen j f champion d c 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 2 1 24 continental adakites and archaean adakites was assembled geochemical modeling of the potential processes is used to interpret it and reveals that 1 the classical model of slab melting provides the best explanation for the genesis of high silica adakites 2 low silica adakites are explained by garnet present melting of an adakite metasomatized mantle i e at depths greater than 2 5 gpa 3 continental adakites is a term encompassing a huge range of rocks with a corresponding diversity of petrogenetic processes and most of them are different from both low and high silica adakites 4 archaean adakites show a bimodal composition range with some very high sr y examples similar to part of the ttg suite reflecting deep melting 2 0 gpa of a basaltic source with a relatively high sr y while lower sr y rocks formed by shallower 1 0 gpa melting of similar sources comparison with the archaean ttg suite highlights the heterogeneity of the ttgs whose composition spreads the whole combined range of hsa and archaean adakites pointing to a diversity of sources and processes contributing to the ttg suite

adakitic rocks from slab melt modified mantle sources in the continental collision zone of southern tibet abstract major trace element and sr nd pb isotopic data are presented for newly discovered adakitic rocks in the western gangdese belt southern tibet the miocene 26 10 ma adakitic rocks from the southern tibetan continental collision zones exhibit distinct differentiation trends typical of arc magmas these rocks display geochemical affinities similar to those of cretaceous 136 80 ma adakitic rocks derived from the partial melting of subducted neotethyan slab in southern tibet the whole rock geochemical and isotope characteristics of the post collision adakitic rocks reveal that their magmas likely originated from an upper mantle region previously metasomatized by slab melts during the cretaceous subduction event an interesting observation is that the e w trending belt of adakitic rocks along the yarlung tsangpo suture zone occupies a fore arc position reflecting a geotectonic setting compatible with the genesis of adakitic magmas the widespread occurrences of cretaceous adakitic rocks in this region are interpreted to testify to the former location of the adakite metasomatized mantle our favoured interpretation is that the spatial isotopic variation in the post collision adakitic rocks is mostly linked to a westward increase in sediment input in the tibetan mantle region under the framework of our paleo subduction model a slab break off event that initiated at around 25 ma would have allowed an asthenospheric upwelling beneath southern tibet which was instrumental in generating the post collision adakitic magmatism in southern tibet

mesozoic and cenozoic volcanic rocks from central and southern tibet 39ar 40ar dating petrological characteristics and geodynamical significance field relations radiometric ages and mineralogical and chemical characteristics of the orogenic volcanic rocks from the lhasa block tibet are studied volcanic activity started before the deposition of the albian aptian sedimentary units takena formation and xigaze group volcanism is volumetrically limited in the northern part of the lhasa block basalts andesites and dacites are present while ignimbrites are scarce radiometric ages of these rocks range between 110 and 80 ma conversely in the southern part of the lhasa block volcanism is largely developed gangdese belt there paroxysmal activity occurred later than in the north between 60 and 50 ma lingzizong formation this volcanism is mainly represented by andesites and ignimbrites a younger episode at about 15 10 ma took place in this southern region the mineralogical and geochemical characteristics indicate that as a whole the volcanic rocks from the lhasa block belong to a calc alkaline continental margin series enriched in k20 and other incompatible elements subordinate potassic alkaline lavas are present in the northern margin of the gangdese belt some basalts exhibit a tholeiitic affinity daquin area southern region the geodynamic significance of these cenozoic volcanic rocks lies in the relative motion of india and the lhasa block after accretion of the latter to the northern qantang block a geodynamic model based on changes of the type of subduction of the indian oceanic crust beneath the lhasa block for the period 120 40 ma is proposed to account for the abundance age and magmatic affinity of the volcanic products

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

the roles of flood magnitude and duration in controlling channel width and complexity on the green river in canyonlands utah usa abstract predictions of river channel adjustment to changes in streamflow regime based on relations between mean channel characteristics and mean flood magnitude can be useful to evaluate average channel response however because these relations assume equilibrium sediment transport their applicability to cases where streamflow and sediment transport are decoupled may be limited these general relations also lack the specificity that is required to connect specific characteristics of the streamflow and sediment regime with the dynamics of channel morphological change that create channel complexity which is often of ecological interest we integrate historical records of channel change observations of scour and fill during a snowmelt flood measurements of sediment transport and predictions from a two dimensional streamflow model to describe how annual peak flow magnitude and peak flow duration interact with the upstream sediment supply to control channel form for a 15 km study reach on the regulated green river in canyonlands national park utah two major decadal scale episodes of channel narrowing have occurred within the study area for each of these episodes the reduction in average channel width was consistent with the change predicted by hydraulic geometry relations as a function of average flood magnitude however channel narrowing occurred during periods of exceptionally low annual floods the most recent episode of channel narrowing occurred between 1988 and 2009 during low flow cycles when the 5 yr mean peak flow was less than 60 of the long term 1959 2016 mean peak flow these findings together with findings from previous studies demonstrate that decreases in peak flow magnitude caused by streamflow regulation climate change or a combination of those factors have driven episodes of channel narrowing on the green river observations of streamflow sediment transport and morphologic change coupled with predictions from a two dimensional streamflow model indicate that peak flow magnitudes of at least 75 of the long term mean peak flow are required to transport bed material sand in suspension in all regions of the multi thread channel and that the 2 month duration of the snowmelt flood played an important role in creating conditions necessary to maintain channel conveyance these results indicate that detailed characterizations of channel response such as these are needed to predict how river channels will respond to changes in streamflow regime that affect annual peak flow magnitude and duration

streamflow regulation and multi level flood plain formation channel narrowing on the aggrading green river in the eastern uinta mountains colorado and utah abstract the style and degree of channel narrowing in aggrading reaches downstream from large dams is dependent upon the dominant geomorphic processes of the affected river the magnitude of streamflow regulation and the post dam sediment transport regime we measured different magnitudes of channel adjustment on the green river downstream from flaming gorge dam ut usa that are related to these three factors bankfull channel width decreased by an average of about 20 in the study area in reaches with abundant debris fans and eddy deposited sand bars the amount of channel narrowing was proportional to the decrease in specific stream power the fan eddy dominated reach with the greatest decrease in stream power narrowed by 22 while the reach with the least decrease in stream power narrowed by 11 in reaches with the same magnitude of peak flow reduction meandering reaches narrowed by 15 to 22 and fan eddy dominated reaches narrowed by 11 to 12 specific stream power was not significantly affected by flow regulation in the meandering reaches in the diverse array of reach characteristics and deposit types found in the study area all pre and post dam deposits are part of a suite of topographic surfaces that includes a terrace that was inundated by rare pre dam floods an intermediate bench that was inundated by rare post dam floods and a post dam floodplain that was inundated by the post dam mean annual flood analysis of historical photographs and tree ring dating of tamarix sp shows that the intermediate bench and post dam floodplain are post dam landforms in each reach type although these two surfaces occur at different levels they are forming simultaneously during flows of different magnitude and while the relative elevation and sedimentologic characteristics of the deposits differ between meandering reaches and reaches with abundant debris fans and eddies both reach types contain deposits at all of these topographic levels the process of channel narrowing varied between fan eddy dominated and meandering reaches in the meandering reaches where stream power has not changed narrowing was accomplished by essentially the same depositional processes that operated prior to regulation in fan eddy dominated reaches where significant reductions in stream power have occurred channel narrowing has been accompanied by a change in dominant depositional processes mid channel sand deposits are aggrading on deposits that in the pre dam era were active gravel bars these deposits are creating new islands and decreasing the presence of open framework gravel bars in eddies bare sand bars are replaced with vegetated bars that have a simpler topography than the pre dam deposits

late cretaceous gangdese intrusions of adakitic geochemical characteristics se tibet petrogenesis and tectonic implications article i nfo the gangdese batholith emplaced from the cretaceous to eocene in southern tibet has been widely regarded as the major constituent of an andean type convergent margin resulting from northward subduction of the neo tethyan oceanic lithosphere under asia while the gangdese batholith consists predominantly of calc alkaline rocks we identify from the eastern part of the batholith a suite of epidote bearing granodiorites that shows adakitic geochemical characteristics marked with apparently higher la yb and sr y and lower y and hree than other gangdese rocks and common arc magmas shrimp zircon u pb analyses of two of the samples yielded 206 pb 238 u dates of 80 4 1 1 and 82 7 1 6 2 ma which constrain the emplacement ages of the adakitic rocks trace element modeling suggests that these rocks originated from partial melting of a garnet amphibolite source that on the basis of the sr and nd isotope data isr 0 7044 0 7048 end t 3 2 to 0 9 we interpret to be a newly underplated mafic lower crust rather than the subducted neo tethyan oceanic crust this juvenile crust was produced by cretaceous basaltic underplating above the mantle wedge and then thickened by the tectonic contraction owing to flattening of the neo tethyan subduction a process that also led to the adakitic magmatism our interpretation involving a late cretaceous flat slab subduction and related orogenesis in southern tibet is consistent with petrographic data such as the occurrence of magmatic epidote and muscovite rimmed with resorption texture in the granodiorites which indicate deep seated emplacement followed by rapid tectonic exhumation

temporal geochemical trends in kerguelen archipelago basalts evidence for decreasing magma supply from the kerguelen plume abstract the kerguelen archipelago a 39 ma to recent volcanic plutonic complex is interpreted to be a manifestation of the kerguelen plume most 85 of the surface area is covered by flood basalts ranging in age from 29 to 25 ma the youngest 25 ma studied flood basalts are in the southeast se province of the archipelago a composite 460 m section of this southeast flood basalt dominantly consists of evolved 3 to 6 mgo alkalic basalt and trachybasalt with a few interbedded highly evolved lavas trachyandesites a 40 70 m conglomerate which contains lignite beds and a trachytic breccia tuff unit all of the lavas in this composite section have sr and nd isotopic ratios that are typical of the kerguelen plume e g 80 of the 115 analyzed archipelago lavas with 2 3 mgo have 87 sr 86 sr i 0 70515 12 and 143 nd 144 nd i 0 51259 5 these ranges include the southeast flood basalts pb isotopes however are more variable these 25 ma lavas have high 206 pb 204 pb at 18 4 to 18 6 relative to other archipelago lavas the temporal trend of the archipelago flood basalt from older 29 ma transitional basalts to younger 25 ma alkalic basalt with an increasing proportion of highly evolved lavas and intra bedded sediments in the relatively young southeast section indicates a a temporal decrease in extent of melting and b a decreasing supply of magma from the plume to the crust these temporal trends are attributed to increasing lithosphere thickness as the plume evolved from a spreading ridge centered plume at 43 ma to its intraplate setting supporting evidence for this interpretation is a the absence of a morb geochemical signature in these 25 ma lavas and b the relatively low abundances of heavy rare earth elements in these southeast lavas which reflect partial melting within the garnet stability field

the rate and pattern of bed incision and bank adjustment on the colorado river in glen canyon downstream from glen canyon dam 1956 2000 closure of glen canyon dam in 1963 transformed the colorado river by reducing the magnitude and duration of spring floods increasing the magnitude of base flows and trapping fine sediment delivered from the upper watershed these changes caused the channel downstream in glen canyon to incise armor and narrow this study synthesizes over 45 yr of channel change measurements and demonstrates that the rate and style of channel adjustment are directly related to both natural processes associated with sediment deficit and human decisions about dam operations although bed lowering in lower glen canyon began when the first cofferdam was installed in 1959 most incision occurred in 1965 in conjunction with 14 pulsed high flows that scoured an average of 2 6 m of sediment from the center of the channel the average grain size of bed material has increased from 0 25 mm in 1956 to over 20 mm in 1999 the magnitude of incision at riffles decreases with distance downstream from the dam while the magnitude of sediment evacuation from pools is spatially variable and extends farther downstream analysis of bed material mobility indicates that the increase in bed material grain size and reduction in reach average gradient are consistent with the transformation of an adjustable bed alluvial river to a channel with a stable bed that is rarely mobilized decreased magnitude of peak discharges in the post dam regime coupled with channel incision and the associated downward shifts of stage discharge relations have caused sandbar and terrace erosion and the transformation of previously active sand bars and gravel bars to abandoned deposits that are no longer inundated erosion has been concentrated in a few pre dam terraces that eroded rapidly for brief periods and have since stabilized the abundance of abandoned deposits decreases downstream in conjunction with decreasing magnitude of shift in the stage discharge relations in the downstream part of the study area where riffles controlling channel elevation have not incised channel narrowing has resulted from decreased magnitude of peak discharges and minor post dam deposition these physical changes to the aquatic and riparian systems have supported the establishment and success of an artifact ecosystem dominated by non native species models for the channel response downstream from large dams typically consider factors such as the degree of sediment deficit the pre dam surface and subsurface grain size and the magnitude of post dam average flows these results suggest that it is also necessary to consider 1 the possibility of variable responses among different channel elements and 2 the potential importance of exceptional flows resulting from management decisions

multi channel patterns of bedrock rivers an example from the central narmada basin india abstract an anomalous multiple channel pattern in bedrock is observed on a predominantly downcutting reach of the narmada river the multi channel reach 800 2750 m in width and 8500 m in length is bounded by major faults and is underlain by granite and gneiss bedrock geomorphological investigations reveal differences among the upstream middle and downstream sub reaches of the multi channel study area whereas the upstream sub reach is dominated by deep flows and fine sediments the lower sub reach is characterized by a steep gradient and rapids the middle sub reach is the widest and is marked by thickly forested islands and boulder berms the characteristics of the three different sub reaches suggest control by the interactions of lithology flood processes and tectonics estimations of hack s 1973 stream gradient index values indicate considerable variations for the sl values along the length of narmada river the highest value of gradient index sl 797 is associated with the multi channel reach implying lithologic or tectonic control given the dimensions of the reach and its channels it appears that the present hydrological regime is inadequate to produce the feature we hypothesize that the multi channel pattern development in bedrock was initiated by block or domal uplift enhanced gradients and extreme floods permitted the system to exploit linear weaknesses in the bedrock leading to the development of anabranches and establishment of multiple channels in bedrock abrupt changes in the channel planform and morphology at the study site indicate that the river is adjusting its channel geometry width depth gradient and plan configuration to a new equilibrium channel morphology through the action of the extreme floods characteristic of this fluvial environment

a vector based method for bank material tracking in coupled models of meandering and landscape evolution sinuous channels commonly migrate laterally and interact with banks of different strengths an interplay that links geomorphology and life and shapes diverse landscapes from the seafloor to planetary surfaces to investigate feedbacks between meandering rivers and landscapes over geomorphic timescales numerical models typically represent bank properties using grids however this approach produces results inherently dependent on grid resolution herein we assess existing techniques for tracking landscape and bank strength evolution in numerical models of meandering channels and show that grid based models implicitly include unintended thresholds for bank migration that can control simulated landscape evolution building on stratigraphic modeling techniques we develop a vector based method for land surface and subsurface material tracking that overcomes the resolution dependence inherent in grid based techniques by allowing high fidelity representation of bank material properties for curvilinear banks and low channel lateral migration rates we illustrate four specific applications of the new technique 1 the effect of resistant mud rich deposits in abandoned meander cutoff loops on meander belt evolution 2 the stratigraphic architecture of aggrading alluvial meandering channels that interact with cohesive bank and floodplain material 3 the evolution of an incising meandering river with mixed bedrock and alluvial banks within a confined bedrock valley and 4 the effect of a bank height dependent lateral erosion rate for a meandering river in an aggrading floodplain in all cases the vector based approach overcomes numerical artifacts with the grid based model because of its geometric flexibility the vector based material tracking approach provides new opportunities for exploring the coevolution of meandering rivers and surrounding landscapes over geologic timescales

epeirogenic controls on canadian river incision and landscape evolution great plains of northeastern new mexico abstract the canadian river has carved a deep bedrock canyon into the great plains of northeastern new mexico in response to a complex interaction of post laramide epeirogenic rock uplift processes along the jemez lineament and downstream baselevel fall caused by late tertiary and quaternary evaporite dissolution broad slow bulging along the jemez lineament a lithospheric scale structure with prolific late cenozoic volcanism along its trend helps drive incision in the canyon canadian river canyon fluvial terraces mapped and correlated according to soils elevation and rock type criteria reveal longitudinal profiles that appear broadly warped across the jemez lineament and converge to the modern channel profile both upstream and downstream of the canyon similarly the modern channel long profile is distinctly and broadly convex through its great plains reach a unique feature among major rivers draining the eastern flank of the southern rocky mountain front radiocarbon dated charcoal from terrac

geomorphic and sedimentary responses of the bull creek valley southern high plains usa to pleistocene and holocene environmental change abstract fluvial geomorphology and stratigraphy often reflect past environmental and climate conditions this study examines the response of bull creek a small ephemeral creek in the oklahoma panhandle to environmental conditions through the late pleistocene and holocene fluvial terraces were mapped and their stratigraphy and sedimentology documented throughout the course of the main valley based on their elevations terraces were broadly grouped into a late pleistocene fill terrace t3 and two holocene fill cut terrace sets t2 and t1 terrace systems are marked by similar stratigraphies recording the general environmental conditions of the time sedimentary sequences preserved in terrace fills record the transition from a perennial fluvial system during the late glacial period and the younger dryas to a semiarid environment dominated by loess accumulation and punctuated by flood events during the middle to late holocene the highest rates of aeolian accumulation within the valley occurred during the early to middle holocene our data provide significant new information regarding the late pleistocene and holocene environmental history for this region located between the well studied southern and central high plains of north america

landscape evolution valley excavation and terrace development following abrupt postglacial base level fall many high latitude fluvial systems are adjusting to base level changes since the last glaciation channels that experienced base level fall may still be incising often through glacial diamictons tills these tills can be quite competent behaving more like weak bedrock than unconsolidated sediment and erode at a fast pace thus providing a unique opportunity to test models of channel incision and knickpoint migration in transient systems here we integrate light detection and ranging lidar topography strath terrace chronology and numerical modeling to determine knickpoint migration and incision history of the le sueur river in central minnesota usa results indicate that the le sueur river is best modeled as a detachment limited channel with downstream coarsening related to lag clasts from tills playing a critical factor in longitudinal profile development the le sueur river meanders as it incises so we coupled the best fit incision model to a meander model to determine valley excavation history the excavation history was used to determine a natural background erosion rate prior to land use changes associated with settlement and agricultural expansion in the mid 1800s we compared background fine sediment silt and clay erosion rates with historic decadal average annual suspended loads results show that modern fine sediment contributions from sources associated with valley excavation are three times higher than modeled presettlement loads recent changes in hydrology associated with land use and climate change have increased flows in rivers leading to higher sediment loads not just from field erosion but from increased bank and bluff erosion in the deeply incised valleys

a lithologic control on active meandering in bedrock channels topographic evidence requires that some rivers actively meander in bedrock yet the way in which rivers can erode laterally and meander within bedrock banks is not well understood lateral channel migration and especially lateral channel migration via active bedrock meandering is commonly responsible for the preservation of unpaired strath terraces a process level understanding of lateral channel migration and active meandering in bedrock rivers is key to interpreting the climatic and tectonic significance of unpaired strath terraces and the planform shape of bedrock rivers in this study we compare erosional processes in two adjacent bedrock channels in the santa cruz mountains california the main differences between these channels are that pescadero creek actively meanders within mudstone while butano creek is straight and incises sandstone laboratory rock strength and slake durability tests show that while the two lithologies have similar tensile strengths before drying the meander supporting mudstone loses strength dramatically when dried and rewet slakes while the sandstone does not the slaked mudstone bank rock was easily detached without the need for bed load tools during in situ erosion tests while mudstone that had not dried and sandstone were not detachable the depth of bank rock detached solely from rewetting of previously dried mudstone ranges between 1 and 8 mm which is well in excess of annual background erosion in the santa cruz mountains in addition hand samples of the mudstone rapidly disintegrated upon wetting and drying in the laboratory whereas sandstone hand samples remained intact in the meandering stream there is a consistent pattern of scoured bedrock exposed to drying and slaking along the outside cutbank of meander bends and forced bars that grade into soil and vegetation which protect the bedrock from slaking along the inside of bends additionally in the meandering stream subaerially exposed mudstone clasts are often found to be disintegrating on the surface of bars taken together these observations suggest that slaking allows for bedrock meandering in two fundamental ways first by rapidly disintegrating coarse hillslope derived sediment that is deposited in the channel slaking suppresses the negative feedback on lateral channel migration that would otherwise result from the buildup of talus along a retreating bedrock valley wall on the outside of a meander bend second at cutbanks where scour exposes bare bedrock to drying slaking weakens a layer of bank rock to the point where it can be eroded by clear water flows in these ways slaking enables erosion into bedrock banks in response to curvature driven fluid shear stress perturbations as in alluvial rivers