unconformity of red sandstones in north vietnam field evidence for indosinian orogeny in northern indochina we re examine several unconformity sites in northwestern vietnam where the indosinian orogenesis has been defined field observations in the black river song da region near son la demonstrate that red sandtones and conglomerates reportedly of cretaceous age occur unconformably above schistosed epimetamorphic middle triassic sediments a mesozoic unconformity thus really exists in this region though the age of deformation is poorly contrained upper triassic to upper cretaceous in most places however the unconformity is strongly deformed folded triassic limestones are often thrusted onto the cretaceous rocks this implies that tertiary deformations probably due to the india asia collision have been strong in the northern part of the indochina block

linking morphodynamic response with sediment mass balance on the colorado river in marble canyon issues of scale geomorphic setting and sampling design 1 measurements of morphologic change are often used to infer sediment mass balance such measurements may however result in gross errors when morphologic changes over short reaches are extrapolated to predict changes in sediment mass balance for long river segments this issue is investigated by examination of morphologic change and sediment influx and efflux for a 100 km segment of the colorado river in grand canyon arizona for each of four monitoring intervals within a 7 year study period the direction of sand storage response within short morphologic monitoring reaches was consistent with the flux based sand mass balance both budgeting methods indicate that sand storage was stable or increased during the 7 year period extrapolation of the morphologic measurements outside the monitoring reaches does not however provide a reasonable estimate of the magnitude of sand storage change for the 100 km study area extrapolation results in large errors because there is large local variation in site behavior driven by interactions between the flow and local bed topography during the same flow regime and reach average sediment supply some locations accumulate sand while others evacuate sand the interaction of local hydraulics with local channel geometry exerts more control on local morphodynamic response than sand supply over an encompassing river segment changes in the upstream supply of sand modify bed responses but typically do not completely offset the effect of local hydraulics thus accurate sediment budgets for long river segments inferred from reach scale morphologic measurements must incorporate the effect of local hydraulics in a sampling design or avoid extrapolation altogether

structural and geochronological constraints on the tectonic evolution of the dulong song chay tectonic dome in yunnan province sw china abstract the dulong song chay tectonic dome lies on the border of china se yunnan province and northern vietnam and consists of two tectonic and lithologic units a core complex and a cover sequence separated by an extensional detachment fault these two units are overlain unconformably by late triassic strata the core complex is composed of gneiss schist and amphibolite shrimp zircon u pb dating results for the orthogneiss yield an age of 799 10 ma which is considered to be the crystallization age of its igneous protolith formed in an arc related environment a granitic intrusion within the core complex occurred with an age of 436 402 ma which probably formed during partial closure of paleotethys within the core complex metamorphic grades change sharply from upper greenschist low amphibolite facies in the core to low greenschist facies in the cover sequence there are two arrays of foliation within the core complex detachment fault and the cover sequence s 1 and s 2 the pervasive s 1 is the axial plane of intrafolial s 0 folds d 1 deformation related to this foliation is characterized by extensional structures the strata were structurally thinned or selectively removed along the detachment faults indicating exhumation of the dulong song chay tectonic dome the major extension occurred at 237 ma determined by shrimp zircon u pb and 39 ar 40 ar isotopic dating techniques regionally simultaneous tectonic extension was associated with pre indosinian collision between the south china and indochina blocks the s 2 foliation appears as the axial plane of nw striking s 1 buckling folds formed during a compressional regime of d 2 d 2 is associated with collision between the south china and indochina blocks along the jinshajiang ailao shan suture zone and represents the indosinian deformation the dulong granites intruded the dulong song chay dome at 144 2 140 2 and 116 10 ma based on 39 ar 40 ar measurement on muscovite and biotite the dome was later overprinted by a conjugate strike slip fault and related thrust fault which formed a vortex structure contemporaneously with late cenozoic sinistral movement on the ailao shan red river fault

gully annealing by aeolian sediment field and remote sensing investigation of aeolian hillslope fluvial interactions colorado river corridor arizona usa abstract processes contributing to development of ephemeral gully channels are of great importance to landscapes worldwide and particularly in dryland regions where soil loss and land degradation from gully erosion pose long term land management problems whereas gully formation has been relatively well studied much less is known of the processes that anneal gullies and impede their growth this study of gully annealing by aeolian sediment spanning 95 km along the colorado river corridor in glen marble and grand canyon arizona usa employed field and remote sensing observations including digital topographic modelling results indicate that aeolian sediment activity can be locally effective at counteracting gully erosion gullies are less prevalent in areas where surficial sediment undergoes active aeolian transport and have a greater tendency to terminate in active aeolian sand although not common examples exist in the record of historical imagery of gullies that underwent infilling by aeolian sediment in past decades and evidently were effectively annealed we thus provide new evidence for a potentially important interaction of aeolian hillslope fluvial processes which could affect dryland regions substantially in ways not widely recognized moreover because the biologic soil crust plays an important role in determining aeolian sand activity and so in turn the extent of gully development this study highlights a critical role of geomorphic ecologic interactions in determining arid landscape evolution

a new model for the indochina and south china collision during the late permian to the middle triassic article i nfo a comparative analysis of indochina and south china during the early paleozoic indicates the former may be extended to include north vietnam part of the qinzhou tectonic zone and southern hainan island these three regions were traditionally regarded as parts of south china separated from indochina by the song ma suture in central north vietnam a new suture called the dian qiong suture is proposed here approximately along the southern margin of the present nanpanjiang basin this suture is linked to its eastern counterpart in hainan island through a nne trending dextral transform fault zone along the eastern margin of the nanpanjiang basin the conventional song ma suture originally constitutes its western extension and was translated to its present location by sinistral displacement along the red river fault zone during the tertiary upper paleozoic deep water turbidites and associated mid oceanic ridge basalts along the suture in the nanpanjiang basin illustrate a coeval ocean between indochina and south china the ocean was consumed by south directed subduction beneath indochina during the late permian to the middle triassic this suture zone was finally re shaped by the indentation between the active margin of indochina and the irregular passive continental margin of south china during the late triassic as well as displacement along the red river fault zone during the cenozoic

relations between rainfall runoff induced erosion and aeolian deposition at archaeological sites in a semi arid dam controlled river corridor process dynamics in fluvial based dryland environments are highly complex with fluvial aeolian and alluvial processes all contributing to landscape change when anthropogenic activities such as dam building affect fluvial processes the complexity in local response can be further increased by flood and sediment limiting flows understanding these complexities is key to predicting landscape behavior in drylands and has important scientific and management implications including for studies related to paleoclimatology landscape ecology evolution and archaeological site context and preservation here we use multi temporal lidar surveys local weather data and geomorphological observations to identify trends in site change throughout the 446 km long semi arid colorado river corridor in grand canyon arizona usa where archaeological site degradation related to the effects of upstream dam operation is a concern using several site case studies we show the range of landscape responses that might be expected from concomitant occurrence of dam controlled fluvial sand bar deposition aeolian sand transport and rainfall induced erosion empirical rainfall erosion threshold analyses coupled with a numerical rainfall runoff soil erosion model indicate that infiltration excess overland flow and gullying govern large scale centimeter to decimeter scale landscape changes but that aeolian deposition can in some cases mitigate gully erosion whereas threshold analyses identify the normalized rainfall intensity defined as the ratio of rainfall intensity to hydraulic conductivity as the primary factor governing hydrologic driven erosion assessment of false positives and false negatives in the dataset highlight topographic slope as the next most important parameter governing site response analysis of 4 years of high resolution four minute weather data and 75 years of low resolution daily climate records indicates that dryland erosion is dependent on short term storm driven rainfall intensity rather than cumulative rainfall and that erosion can occur outside of wet seasons and even wet years these results can apply to other similar semi arid landscapes where process complexity may not be fully understood copyright 2015 john wiley sons ltd

the devonian vertebrates placodermi sarcopterygii from central vietnam and their bearing on the devonian palaeogeography of southeast asia abstract new vertebrate remains are described from the terrigenous facies of the givetian dong tho formation in quang binh province central vietnam they are referred to three placoderm groups the antiarchi arthrodira and acanthothoraci and to a sarcopterygian the antiarchs are represented by a new form vukhuclepis lyhoaensis n g n sp which is regarded here as closely related to the yunnanolepididae an endemic group from the lower devonian of the south china block additional material of the phlyctaeniid arthrodire lyhoalepis duckhoai tong dzuy thanh janvier and doan nh t truong is described the sarcopterygian is regarded here as a dipnomorph and may be closely related to the lower middle devonian youngolepididae of the south china block this material confirms the close ties between the indochina and south china blocks as early as the middle devonian

north directed triassic nappes in northeastern vietnam east bac bo abstract a careful re examination of the previous tectonic descriptions and the acquisition of new structural and kinematic data have been undertaken in northeastern vietnam the structure of the area consists of a system of slightly metamorphosed but ductilely deformed nappes including recumbent folds formed during the triassic prior to the unconformable deposition of the upper triassic terrigenous sediments these results confirm the previous interpretation of preyunnanaises nappes represented by middle upper paleozoic foliated limestone resting through a flat mylonitic contact over an intermediate song mien unit deprat 1915 the latter made essentially of deformed lower triassic sedimentary and volcanic formations which previously filled the song hien rift bourret 1922a bourret 1922b in the external part of the belt the triassic strata conformably overlie slightly deformed and unmetamorphosed upper paleozoic rocks forming an autochthonous domain alternatively the same triassic succession can be seen resting tectonically through decollement zones directly over the middle paleozoic marbles to form a distinct but subsidiary allochthonous unit our data show that the song chay orthogneiss and its lower paleozoic sedimentary country rocks belong to the main nappe they also demonstrate that this nappe was transported to the n ne a newly discovered ophiolitic melange of supposed oceanic nature and triassic age lying along the song chay fault north of the nui con voi could hypothetically represent the rooted zone of the nappe a tertiary overprinting event in particular accommodated by strike slip movements likely accounts for the present orocline of ne vietnam

understanding mesozoic accretion in southeast asia significance of triassic thermotectonism indosinian orogeny in vietnam results from a zircon u pb study of the metamorphic basement of vietnam reveal that a large part of the continental crust was affected by a short lived episode of ductile deformation and high temperature metamorphism between 258 6 ma and 243 5 ma although coincident with final stages of north south china collision qinling orogenesis the thermotectonism in vietnam was caused by accretion of sibumasu to indochina south china this accretion event indosinian orogeny has regional significance because it contributed to the final stages of north south china collision an aspect not explicitly recognized in qinling orogenic models

the structural setting of the song ma region vietnam and the indochina south china plate boundary problem abstract the song ma song da region of northern vietnam contains the song ma anticlinorium a polydeformed polymetamorphosed early palaeozoic island arc forearc terrane accreted to the south china plate in siluro devonian times the song ma anticlinorium is not an indosinian subduction zone and nor is the post triassic post cretaceous song ma fault the song ma fault is one of many northwest trending post cretaceous high angle reverse oblique slip faults and thrusts responsible for shortening and strike slip transposition of northern vietnam folds penecontemporaneous with this faulting include the song ma anticlinorium which is formed probably of thrust ramp folds the faulting and penecontemporaneous folding developed during oligocene sinistral strike slip on the song hong fault and was reactivated during pliocene quaternary times we conclude that the present plate boundary is a broad deformation zone between da nang and the song hong and that parts of the south china plate extend well to the south of the song hong fault

effects of river regulation on aeolian landscapes colorado river southwestern usa 1 connectivity between fluvial and aeolian sedimentary systems plays an important role in the physical and biological environment of dryland regions this study examines the coupling between fluvial sand deposits and aeolian dune fields in bedrock canyons of the arid to semiarid colorado river corridor southwestern usa by quantifying significant differences between aeolian landscapes with and without modern fluvial sediment sources this work demonstrates for the first time that the flow and sediment limiting effects of dam operations affect sedimentary processes and ecosystems in aeolian landscapes above the fluvial high water line dune fields decoupled from fluvial sand supply have more ground cover biologic crust and vegetation and less aeolian sand transport than do dune fields that remain coupled to modern fluvial sand supply the proportion of active aeolian sand area also is substantially lower in a heavily regulated river reach marble grand canyon arizona than in a much less regulated reach with otherwise similar environmental conditions cataract canyon utah the interconnections shown here among river flow and sediment aeolian sand transport and biologic communities in aeolian dunes demonstrate a newly recognized means by which anthropogenic influence alters dryland environments because fluvial aeolian coupling is common globally it is likely that similar sediment transport connectivity and interaction with upland ecosystems are important in other dryland regions to a greater degree than has been recognized previously

structural evolution of the day nui con voi metamorphic complex implications on the development of the red river shear zone northern vietnam the day nui con voi dncv metamorphic complex in north vietnam is the southernmost high grade metamorphic zone along the nw se trending red river shear zone rrsz in indochina the rrsz was considered as a classical large scale continental strike slip fault that had played a significant role in the continental extrusion of southeast asia since the collision of india and eurasia earlier ideas determined the rrsz as a steep shear zone that penetrated the entire lithosphere both metamorphism and structures within rocks along the dncv metamorphic complex have been previously thought to be formed syn tectonically by left lateral shearing of the rrsz during the oligocene miocene continental escape tectonics however our meso and microstructural re examination of this region shows that these metamorphic rocks were formed during earlier tectonic episodes unrelated to strike slip shearing high angle to near orthogonal overprinting fabrics indicated that this region recorded three episodes of ductile deformation followed by brittle faulting events with different intensity spanning from the triassic to the tertiary d1 is preserved as nw se striking upright folds under garnet grade regional metamorphism during the triassic indosinian orogeny as south china block amalgamated with the indochina block the large scale horizontal d2 folds with a dominant top to n nw bottom to s sw sense of shear and sub horizontal fold axial planes suggest that the dncv metamorphic complex remained at midcrustal depths since the indosinian orogeny the youngest ductile deformation event d3 refolded d2 recumbent folds into a dome and uplifted the dncv as lower temperature fabrics s3 indicated steep mylonite zones with left lateral kinematic indicators and brittle faulting were developed on both limbs of the dome along the steep song hong and song chay faults during left lateral movement of the rrsz our new spatial temporal and kinematic correlations of metamorphic fabrics within the dncv metamorphic complex support the suggestion that the rrsz developed after regional metamorphism and remained purely a crustal fault the complicated deformation history recorded within the dncv metamorphic complex provides an alternative interpretation and suggests that crustal scale strike slip faults such as the rrsz need not root from the mantle

direct dating of left lateral deformation along the red river shear zone china and vietnam 1 exposures of high grade midcrustal rocks within the red river shear zone rrsz which separates the indochina and south china blocks exhibit clear evidence of left lateral ductile deformation assuming that the south china sea represents a pull apart basin formed at the southeastern termination of the rrsz it has been argued that seafloor magnetic anomalies constrain the timing of sinistral slip accommodated by the rrsz between 32 and 17 ma at a rate of 4 cm yr while 40ar 39ar thermochronometry indicates that left lateral slip occurred along the rrsz between 25 and 17 ma the timing of earlier high temperature deformation has not been directly constrained in situ th pb ion microprobe dating of monazite inclusions in garnets allows direct assessment of the timing of amphibolite grade metamorphism and synchronous left lateral shearing results from northern segments of the rrsz in yunnan china indicate that synkinematic garnet growth occurred between 34 and 21 ma and are the first to document late oligocene metamorphism and left lateral shearing data from the southern rrsz within vietnam are complicated by tertiary overprinting of rocks that experienced amphibolite facies metamorphism during the indosinian orogeny 220 ma the period during which sinistral deformation is now constrained to have occurred along the rrsz i e 34 17 ma is essentially coincident with spreading of the south china seafloor 32 17 ma this temporal and kinematic link between left lateral shearing along the rrsz and opening of the south china sea supports the view that indochina was extruded from asia as a block along lithospheric scale strike slip faults

ar ar and fission track ages in the song chay massif early triassic and cenozoic tectonics in northern vietnam the song chay massif is the northeasternmost metamorphic complex in vietnam to the east of the red river shear zone it shows a large antiformal structure involving orthogneisses and migmatites overlain on its northern flank by muscovite bearing marbles an e w striking fault bounds the dome to the south kinematic indicators along a s n section reveal top to the n shear sense along the interface between the orthogneissic core and the overlying metasediments radiometric ages were obtained by the 40ar 39ar method using purified mica separates across the dome ages range from 236 ma at the southern edge to 160 ma in the core attesting to a strong imprint in the early triassic time a clear difference is seen between these mesozoic ages and the eocene to miocene ages from 40 to 24 ma that obtained in the nearby red river shear zone using the same method these data show that the song chay massif was already high in the crust when the high temperature deformation of the red river shear zone took place the final exhumation of the song chay orthogneiss constrained by fission track analysis on samples along the same transect occurred during the early miocene and could be interpreted as the consequence of a first normal sense of motion along the fault which bounds the massif to the south timing is similar to that of exhumation in the red river shear zone

impact of reach geometry on stream channel sensitivity to extreme floods predicting spatial and temporal variations in bank erosion due to extreme floods presents a long standing challenge in geomorphology we develop two methodologies for rapid regional scale assessments of stream reaches susceptible to channel widening the first proposes that channel widening occurs when unit stream power exceeds a critical threshold 300 w m2 the second is motivated by the observation that widening often occurs at channel bends we introduce a new metric the bend stress parameter which is proportional to the centripetal force exerted on a concave bank we propose that high centripetal forces generate locally high bank shear forces and enhance channel bank erosion we test both metrics using the geomorphic signature of tropical storm irene 2011 on the white and the saxtons rivers vermont specifically we test if reaches where significant channel widening occurred during irene required one or both metrics to exceed threshold values we observe two distinct styles of channel widening where unit stream power and bend stress parameter are high widening is usually due to bank retreat elsewhere widening is usually due to the stripping of the upstream end of mid channel islands excluding widening associated with the stripping of the heads of mid channel islands almost all the widening 98 occurred along reaches identified as susceptible to widening the combined metrics identify up to one quarter of the reaches lacking susceptibility to channel widening copyright 2014 john wiley sons ltd

the south china block indochina collision where when and how this study uses new field observations and existing studies to shed new light on the origin and significance of two nw se striking orogens in nw and ne vietnam we conclude that the architecture of each belt is a stack of ne directed nappes formed either under deep ductile synmetamorphic conditions or under shallow depth in the sw and ne parts respectively the song ma zone and song chay ophiolitic melange represent two ophiolitic sutures however the late permian song da and babu mafic rocks are not ophiolites but intraplate basalts related to the emeishan plume a late triassic unconformity the 225 205 ma postorogenic plutonism and the 250 230 ma syntectonic metamorphism support an early to middle triassic age for these tectonic events both nw and ne belts are due to sw directed subduction with arc magmatism ocean closure and continental collision though two contemporary s dipping subduction zones might explain the structural evolution of the two belts a single convergent system offset by the tertiary red river fault is preferred as this s directed subduction better accounts for the late permian intraplate magmatism this scenario is discussed in the general geodynamic framework of se asia

sedimentary evidence for a rapid kilometer scale crustal doming prior to the eruption of the emeishan flood basalts biostratigraphic and sedimentologic investigations in 67 sections have been carried out for the middle permian maokou formation that immediately underlies the emeishan flood basalts in southwest china the results suggest a domal crustal thinning before the emplacement of the emeishan large igneous province variably thinned carbonates in the maokou formation are capped by a subaerial unconformity which is generally manifested by karst paleotopography paleoweathering zone or locally by relict gravels and basal conglomerates provenance analysis indicates that these gravels and conglomerates were mainly derived from the uppermost maokou formation therefore the stratigraphic thinning likely resulted from differential erosion due to regional uplift iso thickness contours of the maokou formation delineate a subcircular uplifted area in accordance with the crustal doming caused by a starting mantle plume as predicted by experimental and numerical modeling the duration of this uplift is estimated to be less than 3 myr and the magnitude of uplift is greater than 1000 m the sedimentary records therefore provide independent supporting evidence for the mantle plume initiation model for the generation of the emeishan flood basalts

structural development of the lower paleozoic belt of south china genesis of an intracontinental orogen abstract an extensive structural study of wuyishan and surrounding areas south china brings data on the structures formed prior to the devonian unconformity building the lower paleozoic belt an ordovician tectonic event was responsible for i south directed structures in the wuyishan proper and its southern border related to both thin and thick skinned tectonics accompanied by metamorphism and crustal thickening ii north directed structures to the west of ganjiang fault and north of jiangshan shaoxing fault where only thin skinned tectonics is visible the southward deformation accommodating the shortening includes emplacement of thrust sheets involving deep crustal material and some mantle peridotite and likely repetition of continental material both responsible for crustal thickening the orogeny was due to the underthrusting of the southern part of the south china block beneath the northern part of this block closing the pre existing nanhua rift created at around 850 800 ma and involving the rift sedimentary fill in the southward thrusting the syn metamorphic tectonic piling was followed by anatexis and granite emplacement at around 440 390 ma our new u th pb epma monazite dating gives ages of 433 9 ma and 437 5 ma early silurian for the main anatectic event and a younger age of 412 5 ma late silurian early devonian for a late re heating process the lower paleozoic belt is an intracontinental orogen without oceanic suture record it shows some similarities with the cenozoic european pyrenees chain with a strong asymmetry

constraining assembly time of some blocks on eastern margin of pangea using permo triassic non marine tetrapod records abstract the supercontinent pangea was mainly formed during the permian but when it reached maximum land is unsure because the configuration of the east asian blocks during the permo triassic is still highly debated fossil tetrapods provide one of the best calibrations to the time of connection between continents but the data of permo triassic tetrapods have rarely been applied in the previous reconstructions here we review the oldest records of non marine tetrapods on the east asia blocks and use them to discuss the timing of connection between continents during the permian and triassic the distribution of seymouriamorpha shows the kazakhstanian was connected to the baltica by land from the cisuralian the diverse dashankou fauna indicates that the alxa block amalgamated to pangea at least in the guadalupian older than 266 ma the questionable footprints and late permian dicynodont assemblage from the north china show the north china craton may have been connected to the main part of pangea from the guadalupian and became part of pangea at least by 256 ma the late permian laotian tetrapods support the hypothesis that the south china block already collided with the north china and the indochina blocks by the end of the permian and there was a land route for the migration of non marine tetrapods

geomorphic process from topographic form automating the interpretation of repeat survey data in river valleys the ability to quantify the processes driving geomorphic change in river valley margins is vital to geomorphologists seeking to understand the relative role of transport mechanisms e g fluvial aeolian and hillslope processes in landscape dynamics high resolution repeat topographic data are becoming readily available to geomorphologists by contrasting digital elevation models derived from repeat surveys the transport processes driving topographic changes can be inferred a method termed mechanistic segregation unfortunately mechanistic segregation largely relies on subjective and time consuming manual classification which has implications both for its reproducibility and the practical scale of its application here we present a novel computational workflow for the mechanistic segregation of geomorphic transport processes in geospatial datasets we apply the workflow to seven sites along the colorado river in the grand canyon where geomorphic transport is driven by a diverse suite of mechanisms the workflow performs well when compared to field observations with an overall predictive accuracy of 84 across 113 validation points the approach most accurately predicts changes due to fluvial processes 100 accuracy and aeolian processes 96 with reduced accuracy in predictions of alluvial and colluvial processes 64 and 73 respectively our workflow is designed to be applicable to a diversity of river systems and will likely provide a rapid and objective understanding of the processes driving geomorphic change at the reach and network scales we anticipate that such an understanding will allow insight into the response of geomorphic transport processes to external forcings such as shifts in climate land use or river regulation with implications for process based river management and restoration copyright 2017 john wiley sons ltd

marine derived 87sr 86sr in coal a new key to geochronology and palaeoenvironment elucidation of the india eurasia and china indochina collisions in yunnan china abstract coal has formed in terrestrial and coastal marine environments from sub polar to equatorial regions since the devonian it contains detailed long term records of contemporaneous environment climate and subsequent modifications however in general direct chronological information in coal has been sparse the coal investigated in the present study is from the mile intermontane basin yunnan province china north of an arm of the mesozoic tethys ocean the coal contains marine geochemical signatures and syngenetic gypsum common in coastal marine sediments the gypsum contains marine derived sr and hence has geochronological potential the 87sr 86sr record 0 708350 0 708591 in the mile coal agrees with time calibrated 87sr 86sr records of marine planktonic foraminifera obtained from core dsdp 588c 22 25 18 27 ma early miocene the peat of the mile coal was deposited over 4 6 ma which possibly is the longest deposition of a coal bed in the world to have been found today although this duration should include the period of non peat deposition or erosion if present during the time of the 4 6 ma during this period the regional geological structures were determined by the india eurasia collision which resulted in transform faults with extensive rift structures including the mile rhomb shaped graben this structural setting enabled the flow of seawater from the south china sea to reach inland graben structures including that of the mile basin where peat was deposited subsequent deformation caused by the south china indochina collision changed the regional structural and geographical hydrological patterns this affected the hydrology of the mile basin and resulted in its uplift to its present day elevation of 1350 m this study is to our knowledge the first to use the marine derived 87sr 86sr indicator and chronometer in coal at present marine influenced peats generated in coastal salt marshes extend from the arctic ocean alaska and siberia in the north to patagonia and new zealand in the south while mangrove forests abound in equatorial and low latitude coastal areas the 87sr 86sr record of marine influenced coal in this study area provides a key for the determination of age duration rate of geological processes in the inland basin associated with the closure of the tethys ocean results indicate that this method has potential for providing a temporal framework for geological events and processes in other areas these may be found near marine shore lines across the globe dating back to the devonian moreover the 87sr 86sr signal in marine influenced coal can be used for correlation with the well established 87sr 86sr chronology of marine planktonic foraminifera in turn the 87sr 86sr signal provides a basis for correlating terrestrial records of climate and environment contained in this type of coal and associated sediments with those of marine sediments such as those based on 18o in planktonic foraminifera

morphodynamic signatures of braiding mechanisms as expressed through change in sediment storage in a gravel bed river previous flume based research on braided channels has revealed four classic mechanisms that produce braiding central bar development chute cutoff lobe dissection and transverse bar conversion the importance of these braiding mechanisms relative to other morphodynamic mechanisms in shaping braided rivers has not yet been investigated in the field here we exploit repeat topographic surveys of the braided river feshie uk to explore the morphodynamic signatures of different mechanisms of change in sediment storage our results indicate that when combined the four classic braiding mechanisms do indeed account for the majority of volumetric change in storage in the study reach 61 total chute cutoff traditionally thought of as an erosional braiding mechanism appears to be the most common braiding mechanism in the study river but was more the result of deposition during the construction of diagonal bars than it was the erosion of the chute three of the four classic mechanisms appeared to be largely net aggradational in nature whereas secondary mechanisms including bank erosion channel incision and bar sculpting were primarily net erosional although the role of readily erodible banks in facilitating braiding is often conceptualized we show that bank erosion is as or more important a mechanism in changes in sediment storage than most of the braiding mechanisms and is the most important secondary mechanism 17 of total change the results of this study provide one of the first field tests of the relative importance of braiding mechanisms observed in flume settings

how do gravel bed rivers braid sedimentary processes and bed forms leading to the onset of braiding were observed in small scale hydraulic models of gravel bed streams the laboratory streams had a variety of combinations of constant discharge and slope but identical bed material particle size distributions from initially straight channels braiding occurred by four different processes deposition and accumulation of a central bar chute cutoff of point bars conversion of single transverse unit bars to mid channel braid bars and dissection of multiple bars in these experiments the chute cutoff mechanism was the most common but the predominant braiding mechanism depends upon sediment mobility excess bed shear stress and the bed form regime at very low excess bed shear stress the central bar process dominates but at higher excess bed shear stress slip face unit bars are more common bed scour at confluences is more pronounced and propogation of alternate convergence scour and divergence deposition is more likely thus chu

the effects of bioturbation on soil processes and sediment transport abstract plants and animals exploit the soil for food and shelter and in the process affect it in many different ways for example uprooted trees may break up bedrock transport soil downslope increase the heterogeneity of soil respiration rates and inhibit soil horizonation in this contribution we review previously published papers that provide insights into the process of bioturbation we focus particularly on studies that allow us to place bioturbation within a quantitative framework that links the form of hillslopes with the processes of sediment transport and soil production using geometrical relationships and data from others work we derive simple sediment flux equations for tree throw and root growth and decay

first evidence for archean continental crust in northern vietnam and its implications for crustal and tectonic evolution in southeast asia southeast asia or in a more strict sense the indochinese continent was previously considered to be composed entirely of proterozoic to phanerozoic rocks and reliable evidence for archean crust was lacking gneisses from the cavinh complex south of the red river shear zone northern vietnam however show archean nd model ages of 3 4 3 1 ga zircon separates from the rocks yielded u pb dates of 2 8 2 5 ga the first convincing evidence for the presence of a late archean complex in southeast asia given that the red river shear zone was propagating in the south china block with a left lateral offset of 600 km the cavinh complex can be correlated with the late archean kangding complex in the western margin of the yangtze craton southwestern china the cavinh complex therefore represents one of the oldest crustal nuclei of the south china block

gradients in stream power influence lateral and downstream sediment flux in floods locations of landslides bank failures and floodplain deposition during recent intense flooding in vermont and colorado usa were spatially nonuniform indicating that some reaches are more prone to these types of geologic hazards these three key flood effects signal redistribution of sediment across landscapes reflecting hillslope channel coupling and the sources and or sinks of material we show that spatial gradients in total stream power provide critical additional information beyond at a point magnitudes for predicting which reaches are likely to be susceptible to these hazards during floods field tests in four rivers watershed areas 0 8 180 km 2 indicate that downstream increases in coincide with erosion and mass wasting into channels and downstream decreases in are associated with floodplain deposition our analytical approach supported by field evidence predicts geologic hazards and more broadly sources and sinks of material along rivers these are critical concerns from a practical and theoretical standpoint

intracontinental subduction a possible mechanism for the early palaeozoic orogen of se china the early palaeozoic orogen of se china consists of three lithotectonic elements from top to bottom a sedimentary upper unit a metamorphic lower unit and a gneissic basement the boundaries between these units are flat lying south directed ductile decollements the lower one is coeval with an amphibolite facies metamorphism m1 the belt is reworked by migmatite granite domes high temperature metamorphism m2 and granitic plutons related to post orogenic crustal melting we date here the syn m1 ductile shearing at 453 7 ma by u th pb method on monazite previous ages and our new 40 ar 39 ar ages of biotites and muscovites show that the metamorphic rocks experienced syn m2 exhumation from 440 to 400 ma the early palaeozoic orogen of se china is an intracontinental belt in which decollements accommodated the north directed subduction of the cathaysian continent this orogen is an example of intracontinental subduction that was not preceded by oceanic subduction

permo triassic granitoids in the northern part of the truong son belt nw vietnam geochronology geochemistry and tectonic implications abstract the northern segment of the truong son belt of northwestern vietnam is composed of three granitoid complexes chieng khuong dien bien and song ma respectively our new la icp ms u pb dating results of magmatic zircons from five granitoid samples in the dien bien and the chieng khuong complexes reveal the existence of three age populations of magma emplacement and crystallization from 276 to 202 ma the new data show that the quartz diorite of the chieng khuong complex is dated to be 271 ma however the previously named dien bien complex exposed at different localities has various ages from 281 to 202 ma of magmatic crystallization the chieng khuong quartz diorite exhibits high mg value 60 7 and enriched sr nd isotopic features i sr 0 713822 end t 11 34 with subduction related geochemical features e g depletion in nb ta and ti and enrichment in rb and la the dien bien gabbroic diorite at muong lay is characterized by relatively depleted isotopic data i sr 0 705050 end t 0 09 and extensive depletion in nb and ta by contrast the dien bien granodiorite and quartz monzonite at dien bien are high k calc alkaline to calc alkaline with high mg values 45 5 to 54 4 and they display enriched sr nd isotopic data i sr 0 714912 to 0 716430 end t 9 58 to 8 85 and post collisional geochemical characteristics e g enrichment in total alkali lile and lree and depletion in eu sr p ti and nb under the framework of regional tectonic evolution the permo triassic magmatism along the northern truong son belt is thus grouped into an early stage from ca 280 to 250 ma and a late stage from 229 to 202 ma the early permian early triassic arc magmatism 280 270 ma and 250 245 ma which is widely distributed in the truong son belt provides reliable information on the continuous subduction of the song ma oceanic plate beneath the indochina block furthermore the magmatism of the dien bien complex at dien bien was formed during post collisional extension at ca 229 to 202 ma diachronous suturing of the eastern tethyan evolution is characterized by the differences in the ages of subduction post collisional evolution along the ailaoshan and the song ma belts

emeishan large igneous province sw china in recent years there have been major advances in our understanding of the emeishan large igneous province lip of sw china following publication of a number of lip focused investigations of the terrain and associated rocks this paper reviews the current state of knowledge the volcanic and upper intrusive portion of the province is relatively small 0 3 106 km3 even when offset fragments eroded sections and buried portions are included in the volume calculation the most reliable radiometric age dates zircon u pb shrimp from an associated layered intrusive body several kilometers in area indicate generation at 259 ma consistent with the end guadalupian end middle permian stratigraphic age in addition several ar ar dating studies have been carried out mainly on the volcanic rocks with a number of reported dates 253 251 ma late permian but a consensus is emerging that these ages are problematic because they are in conflict with the stratigraphic data possibly due to a monitor standard miscalibration the ar ar investigations have also yielded a large number of secondary ages which are clustered at 175 142 98 and 42 ma and these are inferred to record sub regional tectonic events that affected the western yangtze block as east asia was assembled and later deformed by india s collision indentation into asia magnetostratigraphic data and field observations suggest that the bulk of the volcanic sequence formed within 1 2 my the geochemistry of the volcanic rocks and bio lithostratigraphic studies of the underlying maokou formation suggests a mantle plume generated the province the basalts can be classified into low and high ti groups with different parental magmas the low ti basalts are confined to the western part of the province and are overlain by the high ti basalts the low ti magmas formed at shallow mantle depths in the spinel garnet field transition zone 60 80 km whereas the high ti magmas formed by low degrees of partial melting within the deeper garnet stability field this observation suggests a deepening of the melting column as the lithosphere thickened due to under plating and a transition from peak basalt generation to the waning stage outstanding issues which might focus future studies are also outlined

fish suggests continental connections between the indochina and south china blocks in middle devonian time a yunnanolepiform antiarch placoderm fish is recorded from the givetian dong tho formation of quang binh province central vietnam this and other fishes from the same locality occur in marginal marine detrital facies with plant remains and lingulid brachiopods that indicate a nearshore to deltaic environment yunnanolepiform antiarchs were hitherto known exclusively from the lower devonian of the south china block the new occurrence of this group well south of the song ma suture suggests close links between the indochina and south china blocks in middle devonian time the massive sandstone exposures of the dong tho formation may be a southern extension of the do son sandstone formation of the hai phong area which is located on the south china block

indosinian nw trending shear zones within the truong son belt vietnam 40ar39ar triassic ages and cretaceous to cenozoic overprints abstract several nw trending ductile and partly mylonitic shear zones cross the truong son belt of central vietnam along the song ma and song ca valleys and north of the kontum block from da nang to aluoi and khe sanh high grade metamorphic rocks of amphibolite facies showing a retrograde evolution and consisting of ortho and paragneisses metavolcanics amphibolites marbles and quartzites are exposed along these structures they display a homogeneous deformation pattern characterized by a generally steeply dipping foliation and a near horizontal to gently plunging mineral stretching lineation indicating a strike slip tectonic regime of deformation along the southernmost fault zone at least various and consistent kinematic indicators including s c structures asymmetric tails of porphyroclasts prove that the strain and metamorphism have been generated by a phase of dominantly non coaxial deformation with a dextral sense of shear 40ar 39ar dating applied on the high grade metamorphic rocks minerals occurring along these zones provide plateau cooling ages closely around 245 ma establishing that this event took place in the lowermost triassic as an early phase of the indosinian orogeny the existence of indosinian movements in vietnam as they have been defined by previous authors during the early century is now accurately confirmed and this is the first insight in the occurrence of ductile strike slip tectonics of indosinian age along nw se fault zones well expressed in central vietnam is a cretaceous thermal and deformational overprint marked by epimetamorphism which took place between 90 and 120 ma as attested by low temperature degassing ages this cretaceous event is not found further north in the song ma zone where younger ages appear as a result of the influence of cenozoic shear movements along the red river fault which displays 20 30 ma ages on the northern flank of song ca in the bu khang phu hoat core complex comparable oligocene lower miocene ages 20 35 ma yielded by biotite and phlogopite reflect a strong overprinting and attest to a rapid uplift of the basement

genetic relationship between the emeishan plume and the bauxite deposits in western guangxi china constraints from u pb and lu hf isotopes of the detrital zircons in bauxite ores abstract the formation processes and alumina sources of the large and super large bauxite deposits that have developed in west guangxi in the yangtze block have been debated for a long time shrimp zircon u pb dates combined with in situ hf isotopic data of the detrital zircons from permian bauxite ores in the western guangxi province china provide new constraints on the genesis of the bauxites in west guangxi u pb dating ages of the detrital zircons cluster around 256 ma and 261 ma this is consistent with the emplacement age of the emeishan plume thus it has been determined that the detrital zircons come from magmatic rocks related to the emeishan plume and that the emeishan plume has significant control on the formation of bauxites in addition this work presents new evidence for the evolution of the emeishan plume most of the e hf t values of the zircons around 256 ma and 261 ma are negative varying widely from 1 3 to 16 1 this data indicates the melting of yangtze block basement rocks and the mixing of magmas generated both from the mantle and from the crust occurred at the periphery of the plume in the main phase of emeishan lip magmatism this study verified that the plume induced uplift exposed carbonates as well as the mafic and felsic rocks generated by the emeishan plume to long periods of intense weathering under humid tropical conditions the weathering remnants of the high alumina magmatic rocks and carbonates were deposited on or transported to the paleokarst surface and converted to bauxites

detrital zircon u pb ages and hf isotopes of lower middle devonian to middle jurassic sandstones in the qinfang basin southern south china block constraints on provenance and tectonic setting abstract the affinity and tectonic setting of the qinfang basin have long been an enigma here we conduct detrital zircon u pb geochronology and hf isotope analysis of lower middle devonian to middle jurassic samples from the basin along with published age data and paleontology from the basin and its east and west sides this study constrains the sediment provenance and tectonic setting of the basin sandstones of devonian jurassic exhibit an age distribution with major age groups at 2700 2300 ma absence for jurassic 1200 800 ma 600 400 ma and 334 246 ma for jurassic alone zircons peak 2500 ma were sourced from the yangtze block while 1200 800 ma grains may originate from the yunkai massif and jiangnan orogen 600 400 ma zircons came from yunkai massif and or cambrian to silurian sediments in the basin carboniferous early triassic 334 246 ma zircons were probably supplied by an older sequence in the great youjiang basin and the darongshan shiwandashan igneous rocks the qinfang basin and its two sides are considered to share a common east gondwana source in silurian the similarity in age spectra between silurian devonian samples in the basin and ailaoshan belt suggests that the basin is a remnant of the paleo tethyan ailaoshan ocean however the distinct age distribution between the basin and indochina block shows that the basin is not a part of the indochina block by middle jurassic following the final closure of the ailaoshan ocean in the late triassic the adjacent western area may be still tectonically active thereby feeding the basin with detritus of early permian

recirculating flow and sedimentation in the colorado river in grand canyon arizona debris fans debouching into the bottom of grand canyon create rapids and flow separation in the colorado river the patterns of flow and the behavior of recirculation zones formed by flow separation are consistent throughout the canyon s length zones of recirculating flow occur along the margin of channel expansions recirculation zones are comprised of one primary eddy secondary eddies and areas of unorganized low velocity may exist upstream from the primary eddy the longest recirculation zones are formed by channel constrictions of low width to depth ratio recirculation zones increase in length with increasing discharge sand bars form beneath recirculation zones especially near separation and reattachment points reattachment bars project upstream from the reattachment point and underlie primary eddies separation bars mantle the downstream parts of the debris fans and form beneath secondary eddies and low velocity areas sediment that forms reattachment bars is dominated by sizes characteristic o

the jinshajiang ailaoshan suture zone china tectonostratigraphy age and evolution abstract the jinshajiang suture zone is important for enhancing our understanding of the evolution of the paleo tethys and its age tectonic setting and relationship to the ailaoshan suture zone have long been controversial based on integrated tectonic biostratigraphic chemostratigraphic and isotope geochronological studies four tectono stratigraphic units can be recognized in the jinshajiang suture zone the eaqing complex the jinshajiang ophiolitic melange the gajinxueshan group and the zhongxinrong group isotope geochronology indicates that the redefined eaqing complex composed of high grade metamorphic rocks might represent the metamorphic basement of the jinshajiang area or a remnant micro continental fragment eaqing complex protolith rocks are pre devonian and probably of early middle proterozoic age and are correlated with those of the ailaoshan complex two zircon u pb ages of 340 3 and 294 3 ma separately dated from the shusong and xuitui plagiogranites within the ophiolitic assemblage indicate that the jinshajiang oceanic lithosphere formed in latest devonian to earliest carboniferous times the oceanic lithosphere was formed in association with the opening and spreading of the jinshajiang oceanic basin and was contiguous and equivalent to the ailaoshan oceanic lithosphere preserved in the shuanggou ophiolitic melange in the ailaoshan suture zone the latter yielded a u pb age of 362 41 ma from plagiogranite the re defined gajinxueshan and zhongxinrong groups are dated as carboniferous to permian and latest permian to middle triassic respectively on the basis of fossils and u pb dating of basic volcanic interbeds the gajinxueshan group formed in bathyal slope to neritic shelf environments and the zhongxinrong group as bathyal to abyssal turbidites in the jinshajiang ailaoshan back arc basin latest permian earliest middle triassic synorogenic granitoids with ages of 238 18 and 227 5 255 8 ma respectively and an upper triassic overlap molasse sequence indicate a middle triassic age for the jinshajiang ailaoshan suture formed by collision of the changdu simao block with south china

the song ma anticlinorium northern vietnam the structure of an allochthonous terrane containing an early palaeozoic island arc sequence abstract the song ma anticlinorium displays evidence for four deformations the first two accompanied by high grade to greenschist facies metamorphism and the third by production of syn low grade metamorphic sheath folds the fourth occurred during post cretaceous folding and thrusting which produced a thrust fold complex across northern vietnam whilst the song ma anticlinorium may contain a metamorphosed island arc volcano sedimentary sequence this palaeozoic sequence cannot represent the active plate boundary in triassic cenozoic or recent times the song ma anticlinorium is not an indosinian subduction zone but is an allochthonous terrane which accreted to the south china plate in siluro devonian times

changhsingian late permian conodonts from son la northwest vietnam and their stratigraphic and tectonic implications abstract late permian conodonts are for the first time reported from vietnam pa sa sb sc and m elements of the changhsingian conodont species hindeodus julfensis sweet are reported from a 40 cm thick limestone in the middle part of the yenduyet formation near son la nw vietnam the occurrence of h julfensis indicates a changhsingian age that is consistent with an interpreted early changhsingian age for a brachiopod fauna slightly higher in the sampled section the son la section is located in the song da rift zone and overlies basaltic volcanics considered equivalent to the emeishan large igneous province basalts that are plume related the permian triassic boundary in vietnam is yet to be precisely located biostratigraphically but proxy chemostratigraphic data indicate its likely position in sections at nhi tao and lung cam n vietnam and correlation with the global stratotype section and point at meishan south china the recovered conodonts have a conodont colour alteration index of 5 and have been heated to c 600 c but they do not show any evidence of textural alteration due to regional metamorphism such as micro folding or stretching that would indicate any direct effects of the compressional indosinian orogeny

u pb zircon geochronology and geochemistry from ne vietnam a tectonically disputed territory between the indochina and south china blocks volcanoplutonic complexes in ne vietnam have recently been interpreted as intraplate products of the emeishan plume alternatively mafic ultramafic rocks have been considered as dismembered palaeotethyan ophiolites juxtaposed along a tectonic melange zone new u pb zircon geochronological and geochemical datasets presented here suggest a complex geological history that records collision between the indochina south china blocks mafic ultramafic rocks exposed within a tectonic melange song hien tectonic zone include sub alkaline pillow basalts that define two geochemically distinct ophiolitic suites sh 1 n morb like sh 2 transitional e morb like both suites have geochemical signatures suggestive of crustal contamination compatible with a volcanic passive margin rift setting we suggest that sh 1 basalts may correlate with the devonian carboniferous jinshajiang ailaoshan song ma branch of the palaeotethys and form part of the associated dian qiong belt whereas sh 2 basalts are co magmatic with middle late permian mafic ultramafic intrusive rocks dolerites gabbros peridotites that developed in a rift basin most likely on the margin of the down going south china plate during west vergent subduction beneath indochina during continental orogenesis and thrust stacking these ophiolitic rocks were juxtaposed with other lithotectonic blocks within the song hien tectonic zone post collisional relaxation led to the development of a rift basin song hien rift comprising late permian triassic volcano sedimentary strata including 270 265 ma terrigenous sandstones 252 ma mudstones and c 254 248 ma felsic effusives granites and granodiorites were emplaced across ne vietnam between c 252 and 245 ma in a syn to post collisional setting the late permian early triassic felsic magmatic rocks best correlate with coeval rocks in sw guangxi and the central and western ailaoshan fold belts china and the truong son fold belt vietnam together they signal the final to post collisional stages of indochina south china collision we demonstrate that the analysed magmatic rocks in the lo gam song hien domains of ne vietnam are not genetically linked to the emeishan large igneous province in the yangtze block of south china as has been previously widely proposed

tectonic deformation of the southeastern tip of the indochina peninsula during its southward displacement in the cenozoic time abstract the terminal collision of india with asia brought about significant tectonic deformation into the asian continent in order to depict these deformational features in the southeastern margin of the indochina peninsula late cretaceous red beds of the dak rium formation were sampled from da lat area 11 7 n 108 3 e the kontum massif vietnam thermal demagnetization procedure revealed the presence of characteristic remanent magnetization with unblocking temperatures around 680 c in most samples primary nature of this magnetization is ascertained through positive fold test a tilt corrected mean direction calculated for the dak rium formation is dec inc 11 4 35 4 ks 331 7 95 1 7 n 21 with a corresponding paleopole at 76 5 n 161 2 e a95 1 7 compared with the european apwp a negative inclination flattening 15 2 4 9 is observed indicating southward displacement of the kontum massif by 9 2 4 9 since late cretaceous since comparable amount of southward displacement has been reported from 16 localities of the indochina and shan thai blocks it seems a reasonable assumption to predict that the indochina peninsula underwent a southward displacement of 5 1 2 4 570 km 270 km as a composite unit including the shan thai block indochina block and kontum massif on the contrary 27 0 9 8 counter clockwise rotation of the kontum massif with respect to the indochina block indicate a significant amount of internal deformation along the tip of south moving indochina peninsula this rotation of the kontum massif is attributed to 32 17 ma left lateral motion along the east vietnam boundary fault which makes up the 1000 km long seaward continuation of the ailao shan red river shear zone tectonic deformation of the indochina peninsula was thus partly accommodated by local block rotation e g counterclockwise rotation of the kontum massif and to some extent by internal tectonic deformation of the shan thai block

the geological evolution of the tibetan plateau the geological evolution of the tibetan plateau is best viewed in a context broader than the india eurasia collision zone after collision about 50 million years ago crust was shortened in western and central tibet while large fragments of lithosphere moved from the collision zone toward areas of trench rollback in the western pacific and indonesia cessation of rapid pacific trench migration 15 to 20 million years ago coincided with a slowing of fragment extrusion beyond the plateau and probably contributed to the onset of rapid surface uplift and crustal thickening in eastern tibet the latter appear to result from rapid eastward flow of the deep crust probably within crustal channels imaged seismically beneath eastern tibet these events mark a transition to the modern structural system that currently accommodates deformation within tibet

colorado river sediment transport 1 natural sediment supply limitation and the influence of glen canyon dam analyses of flow sediment transport bed topographic and sedimentologic data suggest that before the closure of glen canyon dam in 1963 the colorado river in marble and grand canyons was annually supply limited with respect to fine sediment i e sand and finer material furthermore these analyses suggest that the predam river in glen canyon was not supply limited to the same degree and that the degree of annual supply limitation increased near the head of marble canyon the predam colorado river in grand canyon displays evidence of four effects of supply limitation 1 seasonal hysteresis in sediment concentration 2 seasonal hysteresis in sediment grain size coupled to the seasonal hysteresis in sediment concentration 3 production of inversely graded flood deposits and 4 development or modification of a lag between the time of a flood peak and the time of either maximum or minimum depending on reach geometry bed elevation analyses of sediment budgets provide additional support for the interpretation that the predam river was annually supply limited with respect to fine sediment but it was not supply limited with respect to fine sediment during all seasons in the average predam year sand would accumulate and be stored in marble canyon and upper grand canyon for 9 months of the year from july through march when flows were dominantly below 200 300 m3 s this stored sand was then eroded during april through june when flows were typically higher after closure of glen canyon dam because of the large magnitudes of the uncertainties in the sediment budget no season of substantial sand accumulation is evident because most flows in the postdam river exceed 200 300 m3 s substantial sand accumulation in the postdam river is unlikely

la icpms zircons u pb dating of permo triassic and cretaceous magmatisms in northern vietnam geodynamical implications abstract in northeastern vietnam the major tectonic episode responsible for nappes emplacement is triassic these allochtonous structures are intruded by granitic melts two post tectonic massifs showing no sign of deformation have been dated by the la icpms zircon u pb techniques dating reveals a multiphase history with zircon cores showing evidence of proterozoic magmatism the emplacement of the phia bioc granite intrusive in allochtonous units is 248 245 ma an age which assesses a younger limit for the major nappes tectonic this tectonic could be synchronous of the tectonometamorphic strike slip faulting events 250 245 ma defined in the truong son belt as the indosinian orogen the phia bioc intrusion is probably linked with the intra plate magmatism of the emeishan large igneous province or with magmatism associated with the paleotethys closure the age of the phia oac granite intrusion in displaced units is much younger at 87 3 1 2 ma this granite is probably linked to the magmatic activity produced during the paleo pacific plate subduction under the se asia continental plate during the mesozoic although the cenozoic red river fault system is close to these two plutons this last thermotectonic episode has not been strong enough to disturb the u pb system zircons rims do not show any tertiary magmatic or metamorphic overprint

the permo triassic paleotethys in mainland southeast asia and adjacent parts of china aspects of the geodynamic evolution of mainland southeast asia and china are discussed in consideration of the permo triassic paleotethys problem it is shown that all sutures in thailand vietnam and yunnan already closed during the paleozoic era and that the entire region must be regarded as part of the northern continents the permo triassic paleotethys must be expected south of tibet and in burma

chime dating of monazite xenotime zircon and polycrase protocol pitfalls and chemical criterion of possibly discordant age data abstract this paper outlines the chime chemical th u total pb isochron method dating method which is based on precise electron microprobe analyses of th u and pb in th and u bearing accessory minerals such as monazite xenotime zircon and polycrase the age mapping technique that is applicable to young monazite and zircon is also described chime dating consists of analyzing multiple spots within homogeneous age domains that show sufficient compositional variation and then these data are used to construct a pseudo isochron from which an age can be obtained via regression this method when coupled with discrimination of possibly concordant age data by chemical criteria such as the ca si th u pb s ratio for monazite and ca and s contents for zircon has the potential advantage of significant precision and the ability to work with minerals that have a significant initial common pb component this technique can identify two or more homogeneous domains that are separated by age gaps smaller than the error on individual spot age analysis many features that are insignificant in major element analysis can have major impact in the acquisition of trace element data critical factors include the roles of collimator slit detector gas background estimation accelerating voltage probe current x ray interferences and count rate in affecting the accuracy and a way to apply the th and u interference correction without pure th and u oxides or synthesized pure thsio4 the age mapping procedure for young monazite and zircon includes acquiring pbm or pbm intensity of individual pixels with multiple spectrometers correcting background with background maps computed from a measured background intensity by the intensity relationships determined in advance of the measurement calibrating of intensity with standards and calculating of ages from the th u and pb concentrations this technique provides age maps that show differences in age domains on the order of 20 ma with in monazite as young as 100 ma the effect of sample damage by irradiation of intense and prolonged probe measurement is also described

geochronology and isotope analysis of the late paleozoic to mesozoic granitoids from northeastern vietnam and implications for the evolution of the south china block abstract in northeastern vietnam late paleozoic and permo triassic granitic plutons are widespread but their tectonic significance is controversial in order to understand the regional magmatism and crustal evolution processes of the south china block scb this study reports integrated in situ u pb hf o and sr nd isotope analyses of granitic rocks from five plutons in northeastern vietnam zircon sims u pb ages of six granitic samples cluster around in two groups 255 228 ma and 90 ma bulk rock end t ranges from 11 to 9 7 suggesting that continental crust materials were involved in their granitic genesis in situ zircon hf o isotopic measurements for the granitic samples yield a mixing trend between the mantle and supracrustal derived melts it is suggested that the granitic rocks were formed by re melting of the continental crust these new data are compared with the paleozoic and mesozoic granitic rocks of south china we argue that northeastern vietnam belongs to the south china block though still speculated an ophiolitic suture between ne vietnam and south china so called babu ophiolite appears unlikely the late paleozoic to mesozoic magmatism in the research area provides new insights for the magmatic evolution of the south china block

discussion on the role of the red river shear zone yunnan and vietnam in the continental extrusion of se asia journal vol 163 2006 1025 1036 p h leloup p tapponnier r lacassin write in his recent paper searle 2006 acknowledges that the 1000 km long ailao shan red river shear zone is a large miocene left lateral shear zone but speculates that left lateral slip started after 21 ma and claims that the total finite offset remains unknown from this he concludes that continental extrusion was only a relatively minor tectonic factor during the india asia collision as long argued by other workers e g england houseman 1986 cobbold davy 1988 dewey et al 1989 houseman england 1993 we summarize below the field and geochronological evidence that makes us maintain a viewpoint in better accordance with facts timing of left lateral shear along the ailao shan red river shear zone the ailao shan red river shear zone is composed mostly of high grade metamorphic rocks and deformed granitoids with ubiquitous evidence for left lateral shear parallel to the belt e g tapponnier et al 1986 1990 leloup et al 1993 1995 2001 the crystallization of the granitoids has been dated between 22 and 35 ma fig 1d f for example by scharer et al 1990 1994 and zhang scharer 1999 leading those workers to propose that left lateral shear started at least c 35 ma ago in contrast searle 2006 claims that all the deformed granitoids found within the shear zone predate left lateral shear and that their crystallization ages should thus be interpreted to provide an upper limit for the onset of deformation rather than a lower limit thus suggesting a maximum age of 21 ma for this deformation a clear understanding of the p t history and in situ deformation history of the intrusions as well as of their relationships with surrounding paragneisses is fundamental for interpreting correctly the geochronological

exhumation history of the red river shear zone in northern vietnam new insights from zircon and apatite fission track analysis abstract a new set of zircon and apatite fission track ages from the ailao shan and day nui con voi dncv metamorphic massifs of the red river shear zone rrsz and neighboring rocks in northern vietnam is presented a complex along strike diachronous denudation history is revealed the southern sector of the dncv cooled to about 100 c by the late oligocene whereas its central compartment was affected by the later thermotectonic evolution of the song chay dome to the e of the rrsz whose final exhumation occurred during the early miocene the northern sector of the rrsz is characterized by the 35 ma phan si pang pre to synkinematic intrusion fission track ages from a vertical section within the phan si pang granite indicate rapid exhumation and cooling the paleozoic tectonic block to the west of the rrsz fission track ages between 40 and 30 ma was exhumed and cooled earlier than the fault mylonite belt fission track ages of 30 ma and younger and also than the eastern block its structural level is consistent with field observations that suggest the rrsz in northern vietnam to be a transtensional system with a regional ne sw oriented extension component

restoration of cenozoic deformation in asia and the size of greater india a long standing problem in the geological evolution of the india asia collision zone is how and where convergence between india and asia was accommodated since collision proposed collision ages vary from 65 to 35 ma although most data sets are consistent with collision being underway by 50 ma plate reconstructions show that since 50 ma 2400 3200 km west to east of india asia convergence occurred much more than the 450 900 km of documented himalayan shortening current models therefore suggest that most post 50 ma convergence was accommodated north of the indus yarlung suture zone we review kinematic data and construct an updated restoration of cenozoic asian deformation to test this assumption we show that geologic studies have documented 600 750 km of n s cenozoic shortening across and north of the tibetan plateau the pamir hindu kush region accommodated 1050 km of n s convergence geological evidence from tibet is inconsistent with models that propose 750 1250 km of eastward extrusion of indochina approximately 250 km of indochinese extrusion from 30 to 20 ma of indochina suggested by se asia reconstructions can be reconciled by dextral transpression in eastern tibet we use our reconstruction to calculate the required size of greater india as a function of collision age even with a 35 ma collision age the size of greater india is 2 3 times larger than himalayan shortening for a 50 ma collision the size of greater india from west to east is 1350 2600 km consistent with robust paleomagnetic data from upper cretaceous paleocene tethyan himalayan strata these estimates for the size of greater india far exceed documented shortening in the himalaya we conclude that most of greater india was consumed by subduction or underthrusting without leaving a geological record that has been recognized at the surface

detrital zircon ages and heavy mineral composition along the gulf of tonkin implication for sand provenance in the yinggehai song hong and qiongdongnan basins abstract neogene reservoir sand in the yinggehai song hong ygh shb and qiongdongnan basins qdnb was derived from the red river hainan island and the vietnamese truong son belt we investigate the sand dispersal pathways through comparisons of detrital zircon geochronology and heavy mineral analyses of modern vietnamese river sand holocene to recent near coastal sand from hainan island and vietnam and offshore neogene sand and we evaluate the methodologies eleven distinct zircon age clusters characterize the gulf of tonkin provenances red river sand contains the most complex age pattern with 35ma 88ma 155ma 250ma 440ma peaks and broader pre cambrian clusters the 88ma peak uniquely occurs in red river sand northern truong son belt sand contains 35ma 250ma 440ma peaks and pre cambrian clusters comparable to the red river the lack of cretaceous zircons and a high proportion of 250ma relative to 440ma zircons distinguishes the northern truong son belt from the red river the southern truong son belt is characterized by low proportions of 250ma relative to 440ma zircons and the absence of cretaceous and cenozoic zircons hainan island sand is identified by the presence of 100 ma zircons and the absence of cenozoic and 88 ma zircons the zircon age patterns suggest simple although dynamic sand dispersal pathways with mixed sand sources over much of the basins likely resulting from sediment reworking and down stream river merging induced by neogene sea level fluctuations sand in the western ygh shb was mainly sourced from the truong son belt the red river sourced the northern and central ygh shb and qdnb and hainan sand was primarily deposited along the island margin heavy mineral compositions in the gulf of tonkin primarily reflect sediment compositional maturity epidote amphibole pyroxene and garnet indicate immaturity whereas a high zircon content tends to indicate high maturity the use of heavy minerals as provenance indicator should proceed with the utmost caution

differentiating the relative importance of land cover change and geomorphic processes on fine sediment sequestration in a logged watershed abstract timber harvest often results in accelerated soil erosion and subsequent elevated fine

geochronology of polygenetic monazites constrained by in situ electron microprobe th u total lead determination implications for lead behaviour in monazite abstract monazite grains are generally concordant and with an electron microprobe give th u total pb isochron ages in agreement with conventional u pb data monazites from four samples of post archean migmatite from the ivory coast record two main ages an archean age at 2 80 ga partly overprinted by a late migmatization event at 2 03 ga the in situ electron microprobe determination has also shown a third event that systematically appears at 2 72 ga for all the monazites from two of the four studied rocks the three events at 2 80 2 72 and 2 03 ga recorded by the th u total pb system in monazite were each obtained on independent homogeneous parts of a single grain with no sign of pb diffusion thus the oldest monazite remained closed during a high grade resetting migmatization at a temperature estimated at around 700 c and also remained closed during the growth of secondary monazite conventional u pb data isotope dilution on monazite from the same rocks did not allow precise age determinations because the representative points were highly up to 83 discordant and did not fit well along a chord in the concordia diagram the u pb isotope dilution method gave a lower intercept age of 2029 25 ma in agreement with the electron microprobe age although with an unusually high uncertainty due to a poor fit of the data the pb evaporation method gave an old archean age at 2830 7 ma and proterozoic ages ranging from 2417 10 to 2074 7 ma thus although the oldest archean age obtained with the pb evaporation and electron microprobe methods is similar within the analytical error the proterozoic ages obtained by the pb evaporation method are significantly older than the th u total pb proterozoic age obtained with the electron microprobe this discrepancy is presumed to be due to the influence in the pb evaporation method of radiogenic lead contained within archean zones of the monazite grains in other words it means that migmatization took place later than 2074 7 ma and probably around 2030 ma the second archean event at 2 72 ga revealed by high spatial resolution of the electron microprobe could not be determined by other methods the th u total pb method appears to be an inexpensive alternative method for dating simple monazites with a precision close to 20 ma even if the u pb isotopic dilution technique remains the reference method moreover owing to the very high spatial resolution 1 m of the electron microprobe it appears to have no competitor for dating complex polygenetic monazites especially when more than two events are recorded homogeneous zones of different ages coexist in a single grain with no major indication of lead diffusion a procedure for age and error calculations is defined

structure from motion photogrammetry a low cost effective tool for geoscience applications abstract high resolution topographic surveying is traditionally associated with high capital and logistical costs so that data acquisition is often passed on to specialist third party organisations the high costs of data collection are for many applications in the earth sciences exacerbated by the remoteness and inaccessibility of many field sites rendering cheaper more portable surveying platforms i e terrestrial laser scanning or gps impractical this paper outlines a revolutionary low cost user friendly photogrammetric technique for obtaining high resolution datasets at a range of scales termed structure from motion sfm traditional softcopy photogrammetric methods require the 3 d location and pose of the camera s or the 3 d location of ground control points to be known to facilitate scene triangulation and reconstruction in contrast the sfm method solves the camera pose and scene geometry simultaneously and automatically using a highly redundant bundle adjustment based on matching features in multiple overlapping offset images a comprehensive introduction to the technique is presented followed by an outline of the methods used to create high resolution digital elevation models dems from extensive photosets obtained using a consumer grade digital camera as an initial appraisal of the technique an sfm derived dem is compared directly with a similar model obtained using terrestrial laser scanning this intercomparison reveals that decimetre scale vertical accuracy can be achieved using sfm even for sites with complex topography and a range of land covers example applications of sfm are presented for three contrasting landforms across a range of scales including an exposed rocky coastal cliff a breached moraine dam complex and a glacially sculpted bedrock ridge the sfm technique represents a major advancement in the field of photogrammetry for geoscience applications our results and experiences indicate sfm is an inexpensive effective and flexible approach to capturing complex topography

strong inheritance of texture between perovskite and post perovskite in the d layer flow in the deep mantle is thought to create textures in the high pressure mineral post perovskite laboratory simulations of the transformation between lower pressure perovskite and post perovskite show that post perovskite can also inherit textures from the perovskite phase and vice versa

evolution of early triassic outer platform paleoenvironments in the nanpanjiang basin south china and their significance for the biotic recovery detailed microfacies and paleoenvironmental analyses were conducted through the early triassic interval of the outer platform ammonoid rich series of the nanpanjiang basin luolou formation guangxi province south china extensive investigations on outcrops and on thin sections reveal that the widely reported well diversified latest permian fauna dominated by abundant corals calcareous algae calcareous sponges crinoids brachiopods gastropods ostracods and foraminifera is abruptly replaced by decimated early triassic communities essentially composed of low diversity bivalves microgastropods ostracods and rare foraminifera along with this drastic change in faunal assemblages the recolonization of devastated habitats by calcimicrobial frameworks is observed in the aftermath of the end permian mass extinction calcimicrobialites of griesbachian age are followed by mixed ammonoid and conodont rich carbonate siliciclastic series consisting of dark suboxic laminated mudstones intercalated with organic rich shales of dienerian to late smithian age the carbonate siliciclastic series are interrupted by a brief carbonate episode during the early middle smithian massive carbonate production resumes only from the early spathian onward and persists throughout the spathian the initial deposition of both carbonate episodes is coeval with i re establishment of well oxygenated conditions ii a marked early smithian to extreme early spathian ammonoid conodont diversification iii a significant increase in the skeletal grain diversity and abundance and iv peak carbon isotope values a short interval of black organic rich shales precedes the onset of these two distinct carbonate events which are both marked by a positive carbonate carbon isotope excursion known from other tethyan marine localities the comparison between the early triassic stratigraphic evolution of tethyan outer platform paleoenvironments and platform basin margin settings of the nanpanjiang basin suggests a causal connection between large fluctuations of the global carbon cycle climate shifts sea level changes oxygen availability in the oceans and the biological rediversification in the wake of the end permian biotic crisis

combining geomorphometry feature extraction techniques and earth surface processes research the way forward abstract in recent years the wealth of technological development revolutionised our ability to collect data in geosciences due to the unprecedented level of detail of these datasets geomorphologists are facing new challenges giving more in depth answers to a broad er range of fundamental questions across the full spectrum of the earth s and planetary processes this contribution builds on the existing literature of geomorphometry the science of quantitative land surface analysis and feature extraction translate land surface parameters into extents of geomorphological elements it provides evidence of critical themes as well as emerging fields of future research in the digital realm supporting the likely effectiveness of geomorphometry and feature extractions as they are advancing the theoretical empirical and applied dimension of geomorphology the review further discusses the role of geomorphometric legacies and scientific reproducibility and how they can be implemented in the hope that this will facilitate action towards improving the transparency and efficiency of scientific research and accelerate discoveries in geomorphology in the current landscape substantial changes in landforms ecosystems land use hydrological routing and direct anthropogenic modifications impact systems across the full spectrum of geomorphological processes although uncertainties in the precise nature and likelihood of changes exist geomorphometry and feature extraction can aid exploring process regimes and landscape responses taken together they can revolutionise geomorphology by opening the doors to improved investigations crossing space and time scales blurring the boundaries between traditional approaches and computer modelling and facilitating cross disciplinary research ultimately the exploitation of the available wealth of digital information can help to translate our understanding of geomorphic processes which is often based on observations of past or current conditions into the rapidly changing future

role of the red river shear zone yunnan and vietnam in the continental extrusion of se asia the 1000 km long nw se striking left lateral ailao shan red river shear zone runs from the southeastern corner of tibet to the gulf of tonkin and the south china sea it has been used as the prime example of a lithospheric scale strike slip fault that has accommodated between 500 and 1000 km of southeastwards extrusion of indo china away from the indian plate indentor central to the model of continental extrusion is that such faults cut through the entire lithosphere that shear heating resulted in high grade metamorphism and local anatexis and that the ages of sheared granites along the fault also date the timing of strike slip shearing however structural data from the red river shear zone clearly show that vertical strike slip faulting post dated metamorphism and granite emplacement most granites along the shear zone are mantle related granodiorites or within plate alkali granites formed prior to shearing along the red river shear zone left lateral kinematic indicators are ubiquitous within the red river mylonites but they are always lower temperature fabrics formed after peak sillimanite metamorphism and after granite crystallization it is suggested that left lateral strike slip shearing along the red river shear zone started after 21 ma not at 35 ma as previously thought and the fault was purely a crustal structure none of the geological features used to propose the 500 1000 km offsets are robust and the total finite offset remains unknown

electron microprobe monazite geochronology of magmatic events examples from variscan migmatites and granitoids massif central france u th pb dating of monazite with the electron probe microanalyser epma is increasingly documented as a reliable geochronological method offering high spatial resolution this method has been applied on monazite from the cevennes migmatites and granitoids from the southeast of the french massif central measurements were performed on separated grains after systematic back scattered electron bse imaging monazites from migmatites record two main ages i a protolith age of about 550 543 ma obtained on inherited cores and ii a migmatization event between 329 5 and 323 3 ma recorded by monazite rims and all other monogenetic grains monazite from the peraluminous rocles pluton yields a 318 3 ma age finally three granite dykes are dated at 333 6 318 5 and 311 5 ma the older dyke is the most deformed of them and is interpreted as linked to the migmatization event the two other dykes are geochronologically petrologically and structurally coeval with the rocles pluton the data constrain the timing of crustal melting following variscan thickening in the northern cevennes area migmatization of ordovician protoliths took place at 329 323 ma and was shortly followed by intrusion of leucogranite at 318 311 ma the study shows that epma dating of monazite can be successfully used to resolve a close succession of regional melting events

permian tectonic framework and palaeogeography of se asia abstract the principal continental lithospheric terranes now comprising se asia and that existed in the permian are the south china indochina super terrane and the sibumasu simao west burma and sw borneo semitau terranes the simao terrane is here regarded a separate terrane derived from south china by back arc spreading in the lower carboniferous rather than as an extension of indochina the main palaeo tethys ocean in thailand is identified as being represented by the chiang mai and sra kaeo sutures and the eastern boundary of the sibumasu terrane is thus interpreted to lie farther west in thailand than previously inferred the nan uttaradit suture is interpreted as representing a marginal back arc basin and contiguous with the ailaoshan and jinshajiang sutures in sw china the south china indochina super terrane and the simao terrane were located within the palaeo tethys in equatorial latitudes during the permian where they together with north china exhibit cathaysian floras and faunas the sibumasu terrane was located on the nw australian margin of gondwana until the sakmarian major shifts in faunal affinities from gondwana province faunas to an independent sibumasu province and then to assimilation into the cathaysian province records the separation and rapid northward drift of sibumasu after the late sakmarian the principal permian palaeogeographic models for the region are assessed for internal consistencies and conflicts

long and complex thermal history of the song chay metamorphic dome northern vietnam by multi system geochronology multi system geochronology was used to unravel the long and complex thermal history of the song chay range northern vietnam a high grade granitic and metamorphic dome in the vicinity of the cenozoic ailao shan red river fault zone it was considered to be proterozoic south china basement but its geological history was basically unknown scattered field observations suggest three episodes of high temperature deformation firstly at the time of granite emplacement secondly a decollement with top to the north shear and thirdly anticlinal doming of the foliations formed during the two first stages p t estimates suggest that metamorphism coeval with the second deformation phase culminated at 580 ca nd 4 5 kbar 16 km depth multi system geochronology is applied to a two micas granite sample slightly deformed within the decollement u pb dating of zircon yields an age of 428 5m a 2s interpreted as the time of granite crystallization within the south china caledonian belt rb sr on white micas and biotite yields ages of 206 10 ma and 176 5 3 ma respectively 2s whereas 39ar 40 ar ages of the same minerals are 210 9a nd 190 8m a 2s these ages suggest an upper triassic episode of rapid cooling interpreted as due to doming a few million years after the end of movement on the decollement the k feldspar irregular 39ar 40ar age spectrum can to the first order be explained by a cooling history with two episodes of rapid cooling one at 140 ma and a second around 41 to 25 ma apatite fission tracks central age 33 6 3 6 ma 1s confirms a tertiary rapid cooling event interpreted as the final exhumation of the song chay dome 2000 elsevier science b v all rights reserved

displacement along the red river fault constrained by extension estimates and plate reconstructions significant e w extension and or compression must have been generated by displacements along the red river fault rrf since its curvature does not match a small circle centered at the euler pole for the indochina south china plate pair the amount of extension perpendicular to the rrf offshore vietnam depends on the magnitude of left lateral displacement along the rrf in general the larger the left lateral displacement along the fault the smaller the amount of e w extension all purely strike slip models of the opening of the south china sea that assume large displacements 250 km along the rrf encounter major problems because they imply little extension or even considerable shortening offshore east vietnam this is inconsistent with the presence of large elongated basins offshore vietnam using a plate tectonic model we compare continental extension values implied by different magnitudes of displacement along the rrf with crustal stretching estimates derived from 2 d profiles modeled from gravity data we utilize 2 d gravity forward models to restore the extended continental margin crust to its original position prior to extension we find that substantial amounts of extension for offshore vietnam can only be modeled assuming moderate displacements along the rrf compatible with the presence of a southward subducting proto south china sea the total amount of ene wsw extension offshore northern vietnam constrained by our 2 d gravity profiles and gravity inversion increases southward from 36 to 89 km along the yinggehai basin these values of ene wsw extension are consistent with 250 km of left lateral displacement along the rrf

the effects of topographic surveying technique and data resolution on the detection and interpretation of geomorphic change abstract change detection of high resolution topographic data is commonly used in river valleys to quantify reach and site scale sediment budgets by estimating the erosion deposition volume and to interpret the geomorphic processes driving erosion and deposition field survey data are typically collected as point clouds that are often converted to gridded raster datasets and the ultimate choice of grid resolution is left to the user this choice may have important implications for both the quantification and interpretation of geomorphic change here we used concurrent topographic data collected by terrestrial laser scanning tls and structure from motion sfm photogrammetry to quantify the influence of grid resolution and sampling technique on a the sediment budget and b the presence and role of geomorphic processes i e alluvial colluvial aeolian and fluvial transport driving topographic change at four sites along the colorado river in grand canyon arizona usa we found that while both techniques produced similar estimates for site scale sediment budgets the magnitude of detected topographic change was dampened at coarser pixel resolutions an overall decrease in the areal extent of erosion and deposition were observed respectively when coarsening pixel size from 5 cm to 1 m among all sites coarser resolution data tended to affect interpretation of landscape change along the margins of river valleys for example when changing from 5 cm to 1 m pixel resolution the inferred contribution of aeolian changes to total site scale geomorphic change increased in area by 7 9 whereas the inferred contribution of alluvial and colluvial processes decreased in area by 97 9 and 88 2 respectively more generally we found that coarsening pixel sizes disproportionately attributed geomorphic change to one or more of the most common processes operating at a site we also found that coarsening pixel resolution amplified the net sediment imbalance at the site scale driving the imbalance at erosional sites further into erosion and vice versa for depositional sites our results have implications both for point cloud data collection and for raster dataset processing we argue that selecting the finest obtainable resolution is not always warranted to accurately quantify and interpret geomorphic change because remote sensing technique topographic data resolution and analysis procedure can be optimized to capture the spatial scale of those processes driving landscape change however in landscapes at or near sediment equilibrium i e equal amounts of erosion and deposition the finest obtainable topographic data resolution is warranted to avoid amplifying sediment imbalance and erroneously inferring that sites are trending toward erosion or deposition

modeling the topography of shallow braided rivers using structure from motion photogrammetry abstract recent advances in computer vision and image analysis have led to the development of a novel fully automated photogrammetric method to generate dense 3d point cloud data this approach termed structure from motion or sfm requires only limited ground control and is ideally suited to imagery obtained from low cost non metric cameras acquired either at close range or using aerial platforms terrain models generated using sfm have begun to emerge recently and with a growing spectrum of software now available there is an urgent need to provide a robust quality assessment of the data products generated using standard field and computational workflows to address this demand we present a detailed error analysis of sub meter resolution terrain models of two contiguous reaches 1 6 and 1 7 km long of the braided ahuriri river new zealand generated using sfm a six stage methodology is described involving i hand held image acquisition from an aerial platform ii 3d point cloud extraction modeling using agisoft photoscan iii georeferencing on a redundant network of gps surveyed ground control points iv point cloud filtering to reduce computational demand as well as reduce vegetation noise v optical bathymetric modeling of inundated areas and vi data fusion and surface modeling to generate sub meter raster terrain models bootstrapped geo registration as well as extensive distributed gps and sonar based bathymetric check data were used to quantify the quality of the models generated after each processing step the results obtained provide the first quantified analysis of sfm applied to model the complex terrain of a braided river results indicate that geo registration errors of 0 04 m planar and 0 10 m elevation and vertical surface errors of 0 10 m in non vegetation areas can be achieved from a dataset of photographs taken at 600 m and 800 m above the ground level these encouraging results suggest that this low cost logistically simple method can deliver high quality terrain datasets competitive with those obtained with significantly more expensive laser scanning and suitable for geomorphic change detection and hydrodynamic modeling