crustal structure and earthquake focal depths beneath northeastern india and southern tibet summary we use broad band teleseismic data recorded at eight sites along a north south profile from karimganj 24 84 n 92 34 e south of the eastern shillong plateau to bomdilla 27 27 n 92 41 e in the eastern lesser himalaya to determine the seismic characteristics of the crust in northeastern india we also analyse data from the chinese digital seismic network station at lhasa and indepthii stations located on the southern tibetan plateau north of our profile to extend the seismic images of the crust further northwards although the northeastern indian stations and the tibetan stations do not lie along a linear profile across the himalaya the wellrecognized uniformity of the himalaya along strike make this comparison of the two profiles meaningful receiver functions calculated from these data show that the crust is thinnest 35 38 km beneath the shillong plateau receiver functions at cherrapunji on the southern edge of the shillong plateau have a strong azimuthal dependence those from northern backazimuth events show that the moho beneath the southernmost shillong plateau is at a depth of 38 km while receiver functions from southern backazimuth events indicate that the moho beneath the northernmost bengal basin is at a depth of 44 km receiver functions from sites on the brahmaputra valley demonstrate that the moho is deeper by 5 7 km than below the shillong plateau a result which agrees with the hypothesis that the shillong plateau is supported by shearing stress on two steep faults that cut through the crust further north of the eastern himalayan foredeep the moho dips gently northwards reaching a depth of 48 km beneath bomdilla in the lesser himalaya and 88 km below lhasa in tibet using the crustal velocity models obtained from receiver function inversions we redetermined focal depths of wellrecorded earthquakes across this part of the indo tibetan collision zone and find all of these to occur within the crust hence we find no evidence for bimodal depth distribution of earthquakes beneath this region of northeastern india

evolving strain partitioning in the eastern himalaya the growth of the shillong plateau the shillong plateau is the only raised topography up to 2000 m elevation in the himalayan foreland it is proposed to have had a major influence on strain partitioning and thus tectonics in the eastern himalaya additionally its position on the trajectory of the summer monsoon means it has influenced the regional climate with reduced erosion rates proposed over geological timescales in its lee the timing of surface uplift of the plateau has been difficult to determine exhumation rates have been calculated over geological timescales but these seem at variance with estimates based upon extrapolating the present day velocity field measured with gps and it has thus been suggested that exhumation and surface uplift are decoupled we determine the timing of surface uplift using the sedimentary record in the adjacent surma basin to the south which records the transition from a passive margin with southward thickening sedimentary packages to a flexural basin with north thickening strata due to loading by the uplifting plateau our method involves using all available 2d seismic data for the basin coupled to well tie information to produce isochore maps and construct a simple model of the subsidence of the surma basin in order to assess the timing and magnitude of flexural loading by the shillong plateau we conclude that the major period of flexural loading occurred from the deposition of the tipam formation 3 5 2 ma onwards which is likely to represent the timing of significant topographic growth of the shillong plateau our isochore maps and seismic sections also allow us to constrain the timing of thinning over the north south trending anticlines of the adjacent basin bounding indo burman ranges as occurring over this same time interval the combined effect of the uplift of the shillong plateau and the westward encroachment of the indo burman ranges to this region served to sever the palaeo brahmaputra drainage connection between himalayan source and surma basin sink at the end of tipam formation times 2 ma

late miocene pleistocene evolution of india eurasia convergence partitioning between the bhutan himalaya and the shillong plateau new evidences from foreland basin deposits along the dungsam chu section eastern bhutan the shillong plateau is a unique basement cored uplift in the foreland of the eastern himalaya that accommodates part of the india eurasia convergence since the late miocene it was uplifted in the late pliocene to 1600 m potentially inducing regional climatic perturbations by orographically condensing part of the indian summer monsoon ism precipitations along its southern flank as such the eastern himalaya shillong plateau ism is suited to investigate effects of tectonics climate and erosion in a mountain range broken foreland system this study focuses on a 2200 m thick sedimentary section of the siwalik group strategically located in the lee of the shillong plateau along the dungsam chu at the front of the eastern bhutan himalaya we have performed magnetostratigraphy constrained by vitrinite reflectance and detrital apatite fission track dating combined with sedimentological and palynological analyses we show that 1 the section was deposited between 7 and 1 ma in a marginal marine deltaic transitioning into continental environment after 5 ma 2 depositional environments and paleoclimate were humid with no major change during the depositional period indicating that the orographic effect of the shillong plateau had an unexpected limited impact on the paleoclimate of the bhutanese foothills and 3 the diminution of the flexural subsidence in the basin and or of the detrital input from the range is attributable to a slowdown of the displacement rates along the main boundary thrust in eastern bhutan during the latest miocene pleistocene in response to increasing partitioning of the india eurasia convergence into the active faults bounding the shillong plateau

structure of the riddleville basin from cocorp seismic data and implications for reactivation tectonics seismic reflection data collected during cocorp s traverse of the southern appalachians reveal that the triassic riddleville basin is a half graben defined by strong north dipping basal reflections truncated on the north by a major south dipping normal fault lithologies observed in similar basins suggest that these high amplitude continuous reflections may be from lacustrine sediments or basaltic layers the high angle border fault soles into the augusta fault a major south dipping low angle paleozoic thrust geometric relations suggest that formation of the riddleville basin reactivated this older appalachian fault

the brahmaputra tale of tectonics and erosion early miocene river capture in the eastern himalaya the himalayan orogen provides a type example on which a number of models of the causes and consequences of crustal deformation are based and it has been suggested that it is the site of a variety of feedbacks between tectonics and erosion within the broader orogen fluvial drainages partly reflect surface uplift different climatic zones and a response to crustal deformation in the eastern himalaya the unusual drainage configuration of the yarlung tsangpo brahmaputra river has been interpreted either as antecedent drainage distorted by the india asia collision and as such applied as a passive strain marker of lateral extrusion latest neogene tectonically induced river capture or glacial damming induced river diversion events here we apply a multi technique approach to the neogene paleo brahmaputra deposits of the surma basin bengal basin bangladesh to test the long debated occurrence and timing of river capture of the yarlung tsangpo by the brahmaputra river we provide u pb detrital zircon and rutile isotopic sr nd and hf and petrographic evidence consistent with river capture of the yarlung tsangpo by the brahmaputra river in the early miocene we document influx of cretaceous paleogene zircons in early miocene sediments of the paleo brahmaputra river that we interpret as first influx of material from the asian plate transhimalayan arc indicative of yarlung tsangpo contribution prior to capture the predominantly precambrian paleozoic zircons indicate that only the indian plate was drained contemporaneous with transhimalayan influx reflecting the river capture we record arrival of detrital material affected by cenozoic metamorphism as indicated by rutiles and zircons with cenozoic u pb ages and an increase in metamorphic grade of detritus as recorded by petrography we interpret this as due to a progressively increasing contribution from the erosion of the metamorphosed core of the orogen whole rock sr nd isotopic data from the same samples provide further support to this interpretation river capture may have been caused by a change in relative base level due to uplift of the tibetan plateau assuming such river capture occurred via the siang river in the early miocene we refute the tectonic aneurysm model of tectonic erosion coupling between river capture and rapid exhumation of the eastern syntaxis since a time interval of at least 10 ma between these two events is now demonstrated this work is also the first to highlight u pb dating on detrital rutile as a powerful approach in provenance studies in the himalaya in combination with zircon u pb chronology

post magmatic cooling of the elberton granite bearing on the late paleozoic tectonothermal history of the georgia inner piedmont the elberton granite is an undeformed pluton which intrudes polydeformed gneisses along the eastern margin of the georgia inner piedmont magmatic biotite and muscovite display internally concordant 40 ar 39 ar release spectra which record plateau dates of 235 241 m y biotite from adjacent gneisses and from recrystallized xenoliths yield similar plateau ages hornblende from country rock amphibolite and from recrystallized xenoliths record well defined 40 ar 39 ar plateau ages of 267 276 m y the 40 ar 39 ar ages are markedly younger than the 320 20 m y crystallization age of the pluton u pb zircon ross and bickford 1980 and are interpreted to date times of post magmatic cooling through temperatures required for argon retention in the various mineral species examined because similar ages are recorded by equivalent minerals from the country rock terrane post magmatic cooling must have closely followed post metamorphic cooling of the inner piedmont 40 ar 39 ar date

multiscale analysis of geomorphological and geological features in high resolution digital elevation models using the wavelet transform article i nfo at the end of the 1990s the emergence of high resolution 1 m digital elevation models dems settled the context of high precision geomorphological analysis these new elevation models permitted to reveal struc tures that remained heretofore undetectable earth scientists henceforth benefit from a source of data with a textural detail that was never attained before despite its richness this information must be treated efficient ly to extract features helping geomorphologists to analyze the processes occurring at the surface such pro cesses are complex localized and naturally multiscale recently space frequential descriptors as wavelets have been proposed successfully for the analysis of dems the wavelet transform is widely used in image pro cessing since it allows us to decompose a signal into a weighted sum of atoms with joint space frequency lo calization such a decomposition facilitates a coherent navigation from scale to scale but also permits to detect heretofore undiscerned phenomena at different scales this is appealing in geomorphology where structural components related to a given phenomenon are well determined in these sub spaces specifi ct o the scale continuum in this paper we propose a filtering procedure of the wavelet decomposition as an ap proach for the analysis of geomorphological multiscale structures this filtering procedure enhances the high pass information contained at each scale the proposed bottom up approach is applied here to a case study to detect geomorphological structural elements in a valley of the swiss jura it demonstrates that the proposed filtering procedure is an efficient tool for geomorphological multiscale generalization

thermal and tectonic consequences of india underthrusting tibet the tibetan plateau is the largest orogenic system on earth and has been influential in our understanding of how the continental lithosphere deforms beneath the plateau are some of the deepest 100 km earthquakes observed within the continental lithosphere which have been pivotal in ongoing debates about the rheology and behaviour of the continents we present new observations of earthquake depths from the region and use thermal models to suggest that all of them occur in material at temperatures of t 600 1c thermal modelling combined with experimentally derived flow laws suggests that if the indian lower crust is anhydrous it will remain strong beneath the entire southern half of the tibetan plateau as is also suggested by dynamic models in northwest tibet the strong underthrust indian lower crust abuts the rigid tarim basin and may be responsible for both the clockwise rotation of tarim relative to stable eurasia and the gradient of shortening along the tien shan

miocene rise of the shillong plateau and the beginning of the end for the eastern himalaya abstract a common feature of convergent plate boundaries is the self organization of strain exhumation and topography along discrete arcuate boundaries deviations from this geometry can represent first order changes in stress applied at a plate boundary that must affect how strain is partitioned within the interior of an orogen the simplicity of the himalayan fold and thrust belt seen along its central portion breaks down along the eastern extremity of the arc where the 400 km long shillong plateau has developed this change in strain partitioning affects nearly 25 of the arc and has not previously been considered to be important to the orogen s development new low temperature thermochronometry data suggest this structure initiated in mid to late miocene time significantly earlier than was previously estimated from the sedimentary record alone development of the shillong plateau may be linked to a number of kinematic changes within the himalayan and burman collision zones that occur at the same time these events include the onset of e w extension in central tibet eastward expansion of high topography of the tibetan plateau onset of rotation of crustal fragments in southeastern tibet and re establishment of eastward subduction beneath the indo burman ranges we suggest that the coincidence of these tectonic events is related to the dismemberment of the eastern himalayan arc signifying a change in regional stress applied along the india eurasia burma plate boundaries discrepancies between vertical long term faulting rates and geodetically derived far field convergence rates suggest that the collisional boundary in the eastern himalayan system may be poorly coupled due to introduction of oceanic and transitional crust into the eastern plate boundary the introduction of dense material into the plate boundary late in the orogen s history may explain regional changes in the strain field that affect not only the himalaya but also the deformation field more than 1000 km into the tibetan plateau

shallow marine to fluvial transition in the siwalik succession of the kameng river section arunachal himalaya and its implication for foreland basin evolution abstract an understanding of the depositional environment and paleogeography of the siwalik foreland basin are crucial in interpreting the basin configuration sediment transport pathways and its evolutionary history this study examines the sedimentology of the siwalik succession of the kameng river valley arunachal himalaya northeastern india the facies characteristics of the fine grained well sorted sediments of the dafla formation and its complex polymodal paleocurrent pattern in this section reveals deposition in a variety of open marine to deltaic environment the overlying subansiri formation characterized by coarse grained thick multistoried sandstone and showing more consistent sw ward paleocurrent indicate deposition from a large axial braided river system the proposed redefinition of the boundary between the lower siwalik dafla and the middle siwalik subansiri formations implies their transition at around 7 5 ma instead of 10 5 ma suggested earlier the revised age of the transition is consistent with the age of arrival of the transhimalayan sediments at 7 ma and also denotes the time of marine to fluvial transition in this area presence of marine sediments in the kameng section with similar records further west indicates the existence of an extensive seaway in the eastern himalaya during the lower siwalik time the extant paleodrainage reconstructions have been recast on the basis of new data on the sedimentology and paleocurrent from this section it is inferred that the changing sea level uplifting shillong plateau and drainage evolution in the eastern himalayan foreland during the middle miocene time controlled the marine to fluvial transition in the basin

the shillong plateau and the great 1897 assam earthquake previous analysis of triangulation data of the survey of india concluded that the great 1897 assam earthquake occurred on a south dipping fault near the northern edge of the shillong plateau which was named the oldham fault this attribution has been questioned on geological and geodetic grounds we refine the triangulation data adding recently discovered observations and demonstrate that they require average slip of 25 5 m on a fault that dips south at 40 beneath the plateau the best fitting solution to the geodetic observations gives a rupture length of 79 km however the chedrang fault immediately to the west of the oldham fault appears to have slipped as a subvertical tear fault during or shortly after the 1897 earthquake with over 10 m of down to the west normal sense slip this observation suggests that the western end of the main rupture approached within a few kilometers of the chedrang fault giving a length of 95 km for the rupture this range of parameters gives a magnitude 8 15 mw 8 35 for the earthquake the triangulation data cannot be satisfied by slip on the north dipping faults that border the southern edge of the shillong plateau nor by slip on a south dipping fault that has been postulated in the brahmaputra valley gps velocities show that up to 5 mm yr of shortening is taken up across the plateau and its borders this suggests via moment frequency relations that the interval between great earthquakes in the region is several thousand years but that earthquakes of magnitude 7 or greater should occur roughly once per century

40ar 39ar thermochronology and alleghanian development of the southernmost appalachian piedmont alabama and southwest georgia 40ar 39ar age spectra of hornblende muscovite and microcline and total fusion ages of biotite from metamorphic rocks of the inner piedmont pine mountain and uchee belts are reported mineral cooling ages from the eastern part of the inner piedmont are as follows hornblende 320 ma muscovite 296 ma biotite 293 ma and microcline diffusional release patterns tmax 267 ma tmin 234 ma a 347 ma hornblende spectrum from the highest inner piedmont structural level sampled is the oldest date determined and implies earlier passage of this level through the 500 c isotherm most release spectra from pine mountain belt units are discordant with little or no apparent geologic meaning modified saddle shaped release patterns for hornblende indicate extraneous argon with a maximum age of 358 ma muscovite from the pine mountain belt cover sequence is 286 ma plateau age and one from the underlying grenville basement is 277 ma correlation age indicating cooling below the 350 c isotherm plateau ages on uchee belt rocks are as follows hornblende from 297 to 288 ma muscovite 285 ma biotite 276 ma and microcline tmax 261 ma tmin 230 ma muscovite fish from a bartletts ferry fault zone phyllonite have a plateau age of 283 ma the 40ar 39ar results combined with other geologic data indicate that 1 a large part of the southern and inner piedmonts of alabama and southwest georgia experienced a late paleozoic amphibolite facies thermal and deformational event contemporaneous with the alleghanian orogeny observed in the foreland 2 the tectonic development of this event characterized by initial crustal thickening followed by right slip and normal slip movements is grossly similar to that described for the amphibolite facies alleghanian belt in the eastern piedmont of south carolina and georgia and 3 extensional movements along the flanks of the pine mountain window occurred between ca 277 ma and the late triassic early jurassic and thus may reflect latest alleghanian extensional collapse or mesozoic rifting

clockwise rotation of the brahmaputra valley relative to india tectonic convergence in the eastern himalaya naga hills and shillong plateau gps data reveal that the brahmaputra valley has broken from the indian plate and rotates clockwise relative to india about a point a few hundred kilometers west of the shillong plateau the gps velocity vectors define two distinct blocks separated by the kopili fault upon which 2 3 mm yr of dextral slip is observed the shillong block between longitudes 89 and 93 e rotating clockwise at 1 15 myr and the assam block from 93 5 e to 97 e rotating at 1 13 myr these two blocks are more than 120 km wide in a north south sense but they extend locally a similar distance beneath the himalaya and tibet a result of these rotations is that convergence across the himalaya east of sikkim decreases in velocity eastward from 18 to 12 mm yr and convergence between the shillong plateau and bangladesh across the dauki fault increases from 3 mm yr in the west to 8 mm yr in the east this fast convergence rate is inconsistent with inferred geological uplift rates on the plateau if a 45 n dip is assumed for the dauki fault unless clockwise rotation of the shillong block has increased substantially in the past 4 8 myr such acceleration is consistent with the reported recent slowing in the convergence rate across the bhutan himalaya the current slip potential near bhutan based on present day convergence rates and assuming no great earthquake since 1713 a d is now 5 4 m similar to the slip reported from alluvial terraces that offsets across the main himalayan thrust and sufficient to sustain a mw 8 0 earthquake in this area

transient basin as indicator of tectonic expressions in bedrock landscape approach based on matlab geomorphic tool transient profiler abstract the tectonic and erosional processes sculpture the earth s surface producing a landscape that preserves the evolutionary history of rock uplift and incision the bedrock channels act as an efficient gradation agent where the river profile adjusts to changing tectonic climatic or topological e g river capture boundary conditions and develops a transient basin a suite of slope break knickpoints at comparable elevations in the channel profiles of the transient basin act as important markers we developed a set of comprehensive and interactive matlab tools called a transient profiler to extract the geomorphic indices from the digital elevation model for a basin characterization and landscape evolution period the paleo channel reconstruction from the relict reach in the three studied transient drainage basins palar umiam and dibang suggest a varying surface uplift from 514 1246 and 1981 m with a respective relief gain of 158 230 and 95 respectively we found that the horizontal knickpoint retreat rate is faster in the himalayan thrust system as compared to the shillong pop up system whereas the vertical knickpoint retreat rate is faster in the latter

a meta analysis of soil erosion rates across the world abstract over the last century extraordinary efforts have been devoted to determining soil erosion rates in units of mass per area and time under a large range of climatic conditions and land uses and involving various measurement methods we undertook a meta analysis of published data from more than 4000 sites worldwide the results show that there is extraordinarily high variability in erosion rates with almost any rate apparently possible irrespective of slope climate scale land use land cover and other environmental characteristics however detailed analysis revealed a number of general features including positive relationships of erosion rate with slope and annual precipitation and a significant effect of land use with agricultural lands yielding the highest erosion rates and forest and shrublands yielding the lowest despite these general trends there is much variability that is not explained by this combination of factors but is related at least partially to the experimental conditions our analysis revealed a negative relationship between erosion rate and the size of the study area involved significant differences associated with differing measurement methods with direct sediment measurement yielding the lowest erosion rates and bathymetric radioisotope and modeling methods yielding the highest rates and a very important effect of the duration of the experiment our results highlight that when interpreting erosion rates the experimental conditions involved must be taken into account even so the data suggest that only order of magnitude approximations of erosion rates are possible and these retain a very large degree of uncertainty consequently for practical purposes such as calculation of global sediment budgets empirical equations are not a substitute for direct measurements our results also show that a large proportion of the experiments have been short term less than 3 years which reduces dramatically the reliability of the estimated erosion rates given the highly non normal behavior of soil erosion time dependency despite the efforts already made more long term measurement experiments need to be performed especially in regions of the world that are under represented in global datasets in addition protocols need to be established for standardizing the measurement methods and reporting the results to enable data to be compared among diverse sites

sedimentation and tectonics of the sylhet trough bangladesh the sylhet trough a sub basin of the bengal basin in northeastern bangladesh contains a thick fill 12 to 16 km of late mesozoic and cenozoic strata that record its tectonic evolution stratigraphic sedimentologic and petrographic data collected from outcrops cores well logs and seismic lines are here used to reconstruct the history of this trough the sylhet trough occupied a slope basinal setting on a passive continental margin from late mesozoic through eocene time subsidence may have increased slightly in oligocene time when the trough was located in the distal part of a foreland basin paired to the indo burman ranges oligocene fluvial deltaic strata barail formation were derived from incipient uplifts in the eastern himalayas subsidence increased markedly in the miocene epoch in response to western encroachment of the indo burman ranges miocene to earliest pliocene sediments of the surma group were deposited in a large mud rich delta system that may have drained a significant proportion of the eastern himalayas subsidence rates in the sylhet trough increased dramatically 3 8 times from miocene to pliocene pleistocene time when the fluvial tipam sandstone and dupi tila formation were deposited this dramatic subsidence change is attributed to south directed overthrusting of the shillong plateau on the dauki fault for the following reasons 1 pliocene and pleistocene strata thin markedly away from the shillong plateau consistent with a crustal load emplaced on the northern basin margin 2 the shillong plateau is draped by mesozoic to miocene rocks but pliocene and younger strata are not represented suggesting that the massif was an uplifted block at this time 3 south directed overthrusting of the shillong plateau is consistent with gravity data and with recent seismotectonic observations sandstone in the tioam has a marked increase in sedimentary lithic fragments compared to older rocks reflecting uplift and erosion of the sedimentary cover of the shillong plateau if the dauki fault has a dip similar to that of other himalayan overthrusts then a few tens of kilometers of horizontal tectonic transport would be required to carry the shillong plateau to its present elevation uplift of the shillong plateau probably generated a major 300 km westward shift in the course of the brahmaputra river

plio pleistocene exhumation of the eastern himalayan syntaxis and its domal pop up the eastern termination of the himalayan orogen forms a structural syntaxis that is characterised by young from 10 to 7 km in elevation in the core of the syntaxis very high erosion rates documented in sediment downstream of the gorge at the foot of the himalaya contribute 50 of total detritus to the sediment load of the brahmaputra the initiation of very high rates of exhumation has been attributed either to the extreme erosive power of a river flowing across a deforming indentor corner and the associated positive feedback or to the geometry of the indian plate indentor with the corner being thrust beneath the asian plate resulting in buckling which accommodates shortening both processes may be important the northern third of the syntaxis corresponds to a steep domal pop up structure bounded by the india asia suture on three sides and a thrust zone to the south within the dome greater himalaya rocks equilibrated at 800 c and 25 30 km depth during the miocene with these conditions potentially persisting into the latest miocene and possibly the pliocene with modest decompression prior to 4 ma this domal pop up corresponds to the area of youngest bedrock ages on a wide variety of thermochronometers and geochronometers in this paper we review the extensive scientific literature that has focused on the eastern syntaxis and provide new chronological data on its bedrock and erosion products to constrain the age of inception of the very rapid uplift and erosion we then discuss its cause with the ultimate aim to reconstruct the exhumation history of the syntaxis and discuss the tectonic context for its genesis we use zircon and rutile u pb white mica ar ar and zircon fission track dating methods to extract age data from bedrock brahmaputra modern sediments including an extensive compilation of modern detrital chronometry from the eastern himalaya and neogene palaeo brahmaputra deposits of the surma basin bangladesh numerical modelling of heat flow and erosion is also used to model the path of rocks from peak metamorphic conditions of 800 c to 250 c our new data include u pb bedrock rutile ages as young as 1 4 ma from the namche barwa massif and 0 4 ma from the river downstream of the syntaxis combined with existing data our new data and heat flow modelling show that i the detrital age signature of the modern syntaxis is unique within the eastern himalayan region ii the rocks within the domal pop up were 575 75 c only 1 2 myr ago iii the neogene surma basin does not record evidence of the rise and erosion of the domal pop up until latest pliocene pleistocene time iv pleistocene exhumation of the north easternmost part of the syntaxis took place at rates of at least 4 km myr with bedrock erosion of 12 21 km during the last 3 ma v the inception of rapid syntaxial exhumation may have started as early as 7 ma or as late as 3 ma and vi the yarlung tsangpo is antecedent and subsequently distorted by the developing antiform together our data and modelling demonstrate that the domal pop up with its exceptional erosion and topographic relief is likely a pleistocene feature that overprinted earlier structural and metamorphic events typical of himalayan evolution

evolutionary multi scale analysis of river bank line retreat using continuous wavelet transforms jamuna river bangladesh in this study continuous wavelet transforms are used to explore spatio temporal patterns of multi scale bank line retreat along a 204 km reach of the jamuna river bangladesh a sequence of eight bank line retreat series derived from remotely sensed imagery for the period 1987 1999 is transformed using the morlet mother wavelet bank erosion is shown to operate at several characteristic spatial and temporal scales local erosion and bank line retreat are shown to occur in short well defined reaches characterised by temporal persistence at the same location and separated by relatively stable reaches in contrast evidence of downstream propagation of bank line retreat patterns is evident at larger spatial scales the intensity of localised bank line retreat i e at scales of 0 20 km is strongly related to the magnitude of monsoonal peak discharge but this relationship weakens as the spatial scale of erosion increases the potential of continuous wavelet analysis to enhancing our understanding of morphological evolution in complex fluvial systems with multi channel planforms is discussed

generation segregation ascent and emplacement of alleghanian plutons in the southern appalachians the carboniferous permian alleghanian orogeny in the southern appalachians produced a distinctive 45 m y long intrusive event that formed a ne trending band of dispersed composite plutons covering 10 500 km 2 of these plutons 75 are metaluminous biotite or amphibole biotite granitoids with lower crustal sources and 20 are peraluminous biotite muscovite garnet or cordierite biotite granitoids with more evolved crustal sources the remaining 5 are gabbroids intermediate rocks and possible alkaline granitoids the granitoid to gabbroid intermediate rock area ratio is 180 1 the exposed intrusive rocks were produced at a relatively slow rate of 2 km 3 linear km m y along the orogen indicating an inability of the source to produce much melt inefficient melting mechanisms or difficulty of magma segregation and ascent the absence of spatial trends in rock composition and age of emplacement the small amount but relative abundance of the granitoids and coincidence with con

timing and mechanism of the rise of the shillong plateau in the himalayan foreland island arcs provide insights into the origin and recycling of continental crust but questions remain concerning source metasomatism the depth of differentiation the potential role of amphibole fractionation and the time scales involved basaltic andesites on batan island philippines contain 150 ma peridotite fragments encased in hornblendite and gabbroic rinds produced via melt rock reaction the peridotites have some of the lightest 7li and 57fe values yet measured in mantle rocks these values are too fractionated to have been created solely by equilibrium partial melting and thus require a combination of melt depletion and slab fluid addition and may be derived from diffusion modified melt channel wall rocks stable isotope signals are easily modified by diffusive equilibration between peridotite and host magma so the preservation of light 7li and 57fe here suggests magma ascent rates of 10 km yr 1 we show that melt wall rock reactions at 25 30 km depth led to the crystallization of amphibole plagioclase followed by gabbroic fractionation at 7 km depth the former provides a location and mechanism for the cryptic amphibole fractionation observed in these and perhaps many other arc lavas and may obviate the requirement for delamination of cumulates

climatic forcing of erosion landscape and tectonics in the bhutan himalayas a fundamental objective in studies of climate erosion tectonics coupling is to document convincing correlation between observable indicators of these processes on the scale of a mountain range the eastern himalayas are a unique range to quantify the contribution of tectonics and climate to long term erosion rates because uniform and steady tectonics have persisted for several million years while monsoonal precipitation patterns have varied in space and time specifically the rise of the shillong plateau the only orographic barrier in the himalayan foreland has reduced the mean annual precipitation downwind in the eastern bhutan himalaya at the miocene pliocene transition apatite fission track aft analyses of 45 bedrock samples from an e w transect along bhutan indicate faster long term erosion rates outside of the rain shadow in the west 1 0 1 8 mm yr than inside of it in the east 0 55 0 85 mm yr furthermore an aft vertical profile in the latter segment reveals a deceleration in erosion rates sometime after 5 9 ma in this drier segment of bhutan there are remnants of a relict landscape formed under a wetter climate that has not yet equilibrated to the present climatic conditions uplift and preservation of the paleolandscape are a result of a climate induced decrease in erosion rates rather than of an increase in rock uplift rate this study documents not only a compelling spatial correlation between long term erosion and precipitation rates but also a climatically driven erosion rate change on the scale of the eastern himalayas a change that in turn likely influences that region s recent tectonic evolution

tectonics exhumation and drainage evolution of the eastern himalaya since 13 ma from detrital geochemistry and thermochronology kameng river section arunachal pradesh the exhumation history of the central himalaya is well documented but lateral variations in exhumation remain poorly constrained in this study we identify sediment source areas and examine the late neogene exhumation history of the eastern himalaya from the synorogenic sedimentary record of its foreland basin we present nd and hf isotopic data as well as apatite and zircon fission track analyses from the miocene pliocene siwalik group along the recently dated kameng river section in arunachal pradesh northeastern india our isotopic data show that siwalik group sediments deposited between 13 7 and 2 6 ma in arunachal pradesh were mainly derived from higher himalayan source rocks in contrast sediments deposited between ca 7 and 3 ma have far less negative end and ehf values that require involvement of the gangdese batholith and yarlung suture zone source areas via the brahmaputra river system consequently these sediments should also record incision of the namche barwa massif by this river source area exhumation rates of himalayan derived sediments determined from detrital zircon fission track data were on the order of 1 8 km m y in the fastest exhuming areas these rates are very similar to those calculated for the central himalaya and have been relatively constant since ca 13 ma our results do not support the hypothesis of a major change in exhumation rate linked to either local or regional climate change or to shillong plateau uplift during the miocene as reported elsewhere the zircon fission track data further suggest that exhumation of the namche barwa massif between 7 and 3 ma was much slower than the 10 km m y rate recorded in the recent past detrital apatite fission track data indicate deformation of the siwaliks due to forward propagation of the frontal thrust since around 1 ma

drainage evolution and exhumation history of the eastern himalaya insights from the nicobar fan northeastern indian ocean abstract the eastern himalayan syntaxis where the yarlung tsangpo sharply bends is one of the areas experiencing most rapid exhumation on earth the rapid exhumation is often regarded as the result of capture of the yarlung tsangpo by the brahmaputra river however both the timing of integration of the yarlung tsangpo brahmaputra river and initiation of the rapid syntaxial exhumation are debated as the ultimate sedimentary trap of the yarlung tsangpo brahmaputra river the nicobar fan is a window to look into the drainage evolution and exhumation history of the eastern himalaya international ocean discovery program expedition 362 drilled the nicobar fan for the first time recovering fan sediments dating back to the early miocene 19 ma we apply trace elements and sr nd isotopes to investigate the provenance of the sediments in the nicobar fan with the aim of constraining the timing of integration of the yarlung tsangpo brahmaputra river and initiation of the rapid syntaxial exhumation the geochemical and sr nd isotope compositions indicate an eastern himalayan source dominated by the greater himalaya with significant gangdese arc contribution and primarily carried by the brahmaputra river flux of gangdese arc material appears to have been continuous from the base of the nicobar fan suggesting that the yarlung tsangpo brahmaputra river has been established at least since 19 ma synchronously with the sharp rise in sedimentation rate the abrupt change of geochemical and isotope compositions at 9 2 ma indicates an increase in erosion of the greater himalaya as the result of initiation of rapid exhumation in the broad syntaxial region the proportion of greater himalayan material increased again at 3 5 1 7 ma consistent with a younger pulse of rapid exhumation focused in the core of the syntaxis since 3 5 ma our results show that initiation of the rapid syntaxial exhumation postdated integration of the yarlung tsangpo brahmaputra river by at least 10 m y therefore tectonic uplift rather than river capture could be responsible for the initiation of the rapid syntaxial exhumation

steady erosion rates in the himalayas through late cenozoic climatic changes sediment accumulation rates and thermal trackers suggest a substantial and global increase in erosion rates over the past few million years that increase is commonly associated with the impact of the northern hemisphere glaciation but methodological biases have led researchers to debate this hypothesis here we test whether himalayan erosion rates increased by measuring beryllium 10 10be in the sediment of the bengal bay seabed sediment originated from rocks that produced 10be under the impact of cosmic rays during erosion near surface thus the 10be concentrations indicate erosion rates the 10be concentration of the bengal bay sediment depends on the contributions of the ganga and brahmaputra rivers their sediments have distinct 10be concentrations because of distinct elevations and erosion in their drainage basins variable contributions could thus complicate erosion rate calculation we traced these contributions by a provenance study using the strontium sr and neodymium nd isotopic sediment compositions within uncertainties of 30 our reconstructed past erosion rates show no long term increase for the past six million years this stability suggests that climatic changes during the late cenozoic have an undetectable impact on the erosion patterns in the himalayas at least on the ten thousand to million year timescales accounted for by our dataset long term himalayan erosion rates remained stable through the global climatic changes of the past six million years according to the cosmogenic nuclide composition of terrestrial sediments recovered from the bay of bengal

crustal structure and tectonics of bangladesh new constraints from inversion of receiver functions abstract an understanding of the sedimentary and crustal structure of the bengal basin and of the tectonics deforming it remains elusive due to lack of seismic data from bangladesh taking advantage of recently available data from 11 seismic stations deployed over bangladesh we determine the crustal structure beneath each station using 2768 high quality receiver functions rfs inversion of the rfs reveals a highly variable thickness of the overlying sediments beneath the bengal basin the thickness of the sediments increases dramatically across the hinge zone of the early cretaceous passive margin from 3 to 17 km the thick sediments partly represent progradation of the continental margin due to the influx of clastic sediments from the himalayas the moho shallows across the region this reflects thinning of the crystalline crust from 38 km in the indian craton to 34 km at the hinge zone to 4 0 km s at lower crustal levels supports an influence of the kerguelen plume igneous activity during rifting we invert data for a station near the dauki fault which marks the southern boundary of the uplifted shillong plateau for dip and anisotropic effects our results show the dauki as a north dipping thrust fault at jaflong with a dip angle of 32 and strike 110 close to its surficial expression a strong anisotropy 15 and the sense of shear plunge 24 trend 79 compliment the dipping geometry and deformation seems to be related to the initiation of the dauki fault

temporal variations in runoff and sediment yield associated with soil surface roughness under different rainfall patterns abstract soil surface roughness ssr is one of the topographic factors affecting soil erosion currently temporal variation characteristics of runoff and sediment yield of ssr is still unknown in this study ridge tillage rt and linear slope ck were used to simulate different ssr artificial rainfall experiments including increased and decreased rainfall series were conducted in two runoff plots on a slope gradient of 15 the rainfall intensities were 1 0 1 5 and 2 0 mm min 1 and the rainfall duration were 60 40 and 30 min respectively a method combining of the pin meter and photography was used to measure micro topographic changes before and after each run the results showed that the ssr values of rt ranged from 175 94 mm to 181 50 mm and ssr values of ck ranged from 75 66 mm to 79 80 mm the temporal variations of runoff and sediment yield associated with ssr had positive long range correlation and varied periodically every 9 16 min under different rainfall patterns there was a critical value on the spatial heterogeneity of ssr that affecting soil erosion ssr reduced the frequency of periodic oscillations of runoff and sediment yield and then reduced the turbulence of runoff and sediment yield during the rainfall which had good effect of soil and water conservation when ssr exceeded the certain critical condition ssr served as the source of erosion by increasing the periodic oscillation energy increased the turbulence of runoff and sediment yield which led to a large amount of soil and water loss the findings will help elucidate soil erosion mechanisms in sloping farmland and provide a new insight for understanding of temporal variations in runoff and sediment yield

controls on the geometry and evolution of thin skinned fold thrust belts and applications to the makran accretionary prism and indo burman ranges the formation of fold thrust belts at convergent margins is a dynamic process accretion of weak sediments to the front of the overriding plate results in crustal thickening and continued flexural subsidence of the underthrusting plate fold thrust belts are often treated as a coulomb wedge having self similar geometries with a critical taper and either a rigid or isostatically compensated base in this paper we build upon this work by developing a new dynamic model to investigate both the role of the thickness and material properties of the incoming sediment and the flexure in the underthrusting plate in controlling the behaviour and evolution of fold thrust belts our analysis shows that the evolution of fold thrust belts can be dominated by either gravitational spreading or vertical thickening depending on the relative importance of sediment flux material properties and flexure we apply our model to the makran accretionary prism and the indo burman ranges and show that for the makran flexure must be considered in order to explain the dip of the sediment basement interface from seismic reflection profiles in the indo burman ranges we show that incoming sediment thickness has a first order control on the variations in the characteristics of the topography from north to south of the shillong plateau

subduction initiation and the rise of the shillong plateau abstract discrepancies between geodetically and geologically estimated thrust fault slip rates are generally viewed as a methodological problem even when slip rate is steady over geological time a discrepancy may exist because each method is sensitive to different deformation processes however this offers a tool to estimate the partitioning of convergence between footwall and hanging wall deformation and therefore a way to discriminate among orogenic styles here we investigate one such discrepancy for the shillong plateau a basement cored contractional orogen within the himalayan foreland basin using a regional block model to explain the modern geodetic velocity field and explicit uncertainty analysis of the geologic rates we show that this discrepancy cannot be reconciled simply by invoking uncertainties in individual methods our results indicate that the shillong plateau is not an ongoing forward break of the bhutan himalayas as was believed until recently instead the observed inter plate convergence and plateau uplift in this region may be driven primarily by an attempt of the negatively buoyant indian passive margin lithosphere the surma basin south of the plateau to initiate subduction as a result the uplift history of the plateau which constrains the geologic rate is significantly lower than expected given the geodetic convergence rate we propose that this convergence is largely accommodated by the transport of the footwall into the mantle this geodynamic scenario has important regional seismotectonic implications 1 the cold and brittle sinking passive margin may have enabled the deep extent 30 km and therefore large magnitude of the mw 8 shillong earthquake of 1897 2 the collapse of the indian lithosphere into the mantle may have created the anomalously deep 20 km surma basin and 3 this subsidence may also drive accelerated post miocene westward propagation of the indo burman wedge we propose that the shillong plateau is the only modern example of passive margin collapse and can serve as a natural laboratory to study the earliest phase of subduction

inferences on late stage evolution of the russell lake allochthon and the soapstone ridge complex in georgia southern appalachians based on chlorite geothermometry

exhumation of high pressure pelitic schist lake murray spillway south carolina evidence for crustal extension during alleghanian strike slip faulting

reconstructing the end of the appalachian orogeny in contrast to crustal deformation observed in the actively forming himalayas where shallowly dipping crustal detachments extend over hundreds of kilometers prior work on the paleozoic southern appalachian orogeny inferred that the final continental collision occurred on a steeply dipping crustal suture permitting collision models that are dominated by strike slip motion here we use scattered seismic phases to instead reveal the appalachian alleghanian crustal suture as a low angle 15 southward dipping interface that soles into a flat lying mid crustal detachment the observed suture geometry implies more than 300 km of head on shortening across a plate boundary structure similar to the himalayan mid crustal detachment indicating that this mode of deformation has been fundamental in continental collisions over hundreds of millions of years

lithological control on the geomorphic evolution of the shillong plateau in northeast india abstract the shillong plateau in northeast india is a block of raised topography in the himalayan foreland which consists of crystalline basement rocks partially covered by a cretaceous to miocene sedimentary succession it is dominated by a mature low relief landscape surrounded by high relief fluvially dissected margins particularly along its southern flank which is bounded by the dauki thrust fault we use river profiles and geological relationships to show that the low relief plateau is a topographic expression of a re exposed basement palaeosurface following the stripping of sedimentary cover by scarp retreat we show that initiation of the wave of incision does not require surface rupture on the dauki fault or an increase in fault slip rate at the end of the miocene as suggested by previous studies we propose that incision has been spatially controlled by the slope of the basement palaeosurface likely moderated by an incision threshold river profiles in the shillong plateau cannot be interpreted as simple records of surface uplift the observed heterogeneous spatial pattern of steepness is a function of a dynamic landscape response to the erosion of layered lithology with contrasting erodibility such dynamics have implications for fluvial geomorphology highlighting that near horizontal lithological contacts can strongly influence river profiles and topography even when no longer physically preserved the topography of the northern shillong plateau is controlled by the structure of basement rocks and is reminiscent of stable cratonic interior landscapes consistent with its surface exposure during late cretaceous times

effect of microrelief on water erosion and their changes during rainfall soil surface roughness contains two elementary forms depressions and mounds which affect water flow on the surface differently while depressions serve as temporary water storage mounds divert water away from their local summits although roughness impacts on runoff and sediment production have been studied almost no studies have been designed explicitly to quantify the evolution of depressions and mounds and how this impacts runoff generation and sediment delivery the objectives of this study were to analyze how different surface forms affect runoff and sediment delivery and to measure the changes in surface depressions and mounds during rainfall events a smooth surface was used as the control both mounds and depressions delayed the runoff initiating time but to differing degrees and slightly reduced surface runoff when compared to the runoff process from the smooth surface surface mounds significantly increased sediment delivery whilst depressions provided surface storage and hence reduced sediment delivery however as rainfall continued and rainfall intensity increased the depression effect on runoff and erosion gradually decreased and produced even higher sediment delivery than the smooth surface depressions and mounds also impacted the particle size distribution of the discharged sediments many more sand sized particles were transported from the surface with mounds than with depressions the morphology of mounds and depressions changed significantly due to rainfall but to different extents the difference in change had a spatial scale effect i e erosion from each mound contributed to its own morphological change while sediments deposited in a depression came from a runoff contributing area above the depression hence a much greater source area than a single mound the results provide a mechanistic understanding of how soil roughness affects runoff and sediment production copyright 2015 john wiley sons ltd

40ar 39ar constraints on chronology of augusta fault zone movement and late alleghanian extension southern appalachian piedmont south carolina and georgia

pollen and spores date origin of rift basins from texas to nova scotia as early late triassic palynological studies of the nonmarine newark supergroup of eastern north america and of rift basins in the northern gulf of mexico facilitate correlation with well dated marine sections of europe new information emphasizes the chronological link between the newark basins and a gulf of mexico basin and their common history in the rifting of north america from pangea shales from the subsurface south georgia basin are shown to be of late karnian age early late triassic the known time of earliest sedimentation in the culpeper basin is extended from norian late late triassic to mid karnian and the date of earliest sedimentation in the richmond and deep river basins is moved to at least earliest karnian perhaps ladinian the subsurface eagle mills formation in texas and arkansas has been dated palynologically as mid to late karnian the oldest parts of the newark supergroup and the eagle mills formation mostly began deposition in precursor rift basins that formed in ladinian to early karnian time in the southern newark basins sedimentation apparently ceased in late karnian but continued in the northern basins well into the jurassic until genesis of the atlantic ended basin sedimentation

fault gouge dating in the southern appalachians usa illite age analysis iaa the method of comparing radiometric ages of successive size fractions with varying percentages of detrital illite has been successfully applied to several rock types including fault gouge shales and argillaceous limestones iaa results are presented for fi ve fault rocks including four clay gouges and one cataclasite from the exhumed southern appalachian foreland fold thrust belt eastern united states determining detrital versus authigenic illite is now an established procedure utilizing x ray analysis to quantify illite polytypes less established however is how to interpret 40 ar 39 ar ages as a function of diagenetic grade both a total gas age incorporating the recoiled argon fraction after irradiation and a retention age omitting the recoiled fraction are obtained for a sample we relate their respective use to diagenetic grade and specifi cally the crystallite thickness of illite when measured crystallite sizes are equal to or smaller than that calculated from ar recoil we use total gas ages here 5 nm while for crystallites greater than 10 nm we use retention ages the four clay gouge ages are all the same within error and much younger than that of the cataclasite we conclude that a major period of frontal appalachian fault activity occurred during the early permian cisuralian 276 280 ma and that these foreland faults were active simultaneously as part of an internally deforming regional thrust wedge

variable exhumation rates and variable displacement rates documenting recent slowing of himalayan shortening in western bhutan abstract we link exhumational variability in space and time to the evolving geometry of the himalayan fold thrust belt in western bhutan by combining new and published geochronologic and thermochronologic data we document the burial age peak temperatures and complete cooling history from 20 ma to the present over an across strike distance of 125 km these integrated cooling curves highlight windows of fast exhumation that vary spatially and temporally we propose that pulses of fast exhumation are a result of structures that facilitate the vertical motion of material illustrated in sequentially restored cross sections due to a range of permissible geometries at depth we explore and evaluate the impact of geometry on kinematics and rates of deformation the linked cooling history and cross sections provide estimates of both magnitude and timing of thrust sheet displacement and highlight temporal variability in potential shortening rates structural and chronologic data illustrate a general north to south progression of himalayan deformation with emplacement of the main central thrust mct paro thrust and shumar thrust by 12 to no later than 9 ma two different geometries and kinematic scenarios for the lesser himalayan duplex are proposed a north to south propagating duplex system requires that the southern portion of that system south of the mct deformed and cooled by 9 ma leaving only the southernmost thrust sheets including the main boundary and main frontal thrusts to deform between 9 and 0 ma this limited post 9 ma shortening would necessitate a marked slowdown in convergence accommodated on the main himalayan thrust a two tiered duplex system which allows for the paro window duplex and the southern baxa duplex to form simultaneously permits duplex formation and accompanying exhumation until 6 ma limited cooling from 200 c to the surface post 6 ma suggests either a decrease in shortening rates from 6 to 0 ma or that duplex formation and exhumation are temporally decoupled our combined cooling curves highlight that the youngest cooling ages may not mark the fastest thrusting rates or the window of fastest exhumation instead temporal variations in exhumation are best viewed through identifying transients in exhumation rate we suggest that the strongest control on exhumation magnitude and variability is fold thrust belt geometry particularly the locations and magnitudes of footwall ramps which can change over 10 s of km distance balanced cross sections predict the location and magnitude of these ramps and how they vary in space and time providing an untapped potential for testing permissible cross section geometries and kinematics against measured cooling histories

the tectonics and paleo drainage of the easternmost himalaya arunachal pradesh india recorded in the siwalik rocks of the foreland basin the siwalik sedimentary rocks of the himalayan foreland basin preserve a record of himalayan orogenesis paleo drainage evolution and erosion this study focuses on the still poorly studied easternmost himalaya siwalik record located directly downstream of the namche barwa syntaxis we use luminescence palaeomagnetism magnetostratigraphy and apatite fission track dating to constrain the depositional ages of three siwalik sequences the sibo outcrop upper siwalik sediments at ca 200 800 ka the remi section middle and upper siwalik rocks at 0 8 8 8 2 4 ma and the siang section middle siwalik rocks at 9 3 1 5 to 13 5 1 5 ma cretaceous paleogene detrital zircon and apatite u pb ages characteristic of the transhimalayan gangdese batholiths that crop out northwest of the syntaxis are present throughout the sibo remi and siang successions confirming the existence of a yarlung brahmaputra connection since at least the late miocene a ca 500 ma zircon population increases up section most strikingly sometime between 3 6 to 6 6 ma at the expense of transhimalayan grains we consider the ca 500 ma population to be derived from the tethyan or greater himalaya and we interpret the up section increase to reflect progressive exhumation of the namche barwa syntaxis early cretaceous zircon and apatite u pb ages are rare in the sibo remi and siang successions but abundant in modern siang river sediments zircons of this age range are characteristic of the transhimalayan bomi chayu batholiths which crop out east of the syntaxis and are eroded by the parlung river a modern tributary of the siang river we interpret the difference in relative abundance of early cretaceous zircons between the modern and ancient sediments to reflect capture of the parlung by the siang after 800 ka

downstream evolution of the thermochronologic age signal in the brahmaputra catchment eastern himalaya implications for the detrital record of erosion the namche barwa massif in the eastern himalayan syntaxis is characterized by very rapid exhumation and provides a significant proportion of the sediment flux carried by the brahmaputra river we present new detrital zircon fission track zft and muscovite 40ar 39ar mar data from modern sediments of rivers draining the eastern himalaya the cooling age populations for both thermochronometers contain a characteristic 2 ma signature related to the rapid exhumation of namche barwa which can be traced hundreds of kilometers downstream from their source into the brahmaputra foreland despite dilution from downstream tributary catchments to estimate present day erosion in the catchments we apply a mixing model based on linear inversion of the binned age distributions the inversion predicts relative erosion rates in the syntaxial region that are an order of magnitude higher than those in upstream catchments and about twice as high as those in the southern himalayan catchments consistent to first order with previous estimates of erosion rate patterns in the eastern himalaya a comparison of the observed downstream evolution of age distributions with a sediment flux model suggests that the zft signal can be explained by dilution from himalayan tributaries only whereas the mar signal is also affected by selective sequestering possibly through winnowing of micas as they are transported downstream nevertheless thermochronological ages 2 ma provide a diagnostic signal of syntaxial exhumation in the sedimentary record of the eastern himalaya this study suggests the most robust signal to be recorded in the most proximal deposits with respect to the syntaxis

a paleo brahmaputra subsurface lithofacies analysis of miocene deltaic sediments in the himalayan bengal system bangladesh abstract the bengal foreland basin contains a succession of up to 16 km of dominantly deltaic deposits eroded from the eastern himalayas and the indo burman ranges and carried by major river systems similar to the present day ganges and brahmaputra analysis of electric logs and core descriptions acquired during oil and gas exploration in bangladesh allows construction of lithofacies maps which constrain depositional patterns of miocene strata compilations of sand thickness and sand shale ratio of the miocene surma group show that lower to middle miocene strata of the bhuban formation accumulated in a large elongate trough sand thickness and percentage both decrease markedly away from this depocenter which describes a large scale bend running initially westward from rashidpur northeast bengal basin and curving southward toward the bengal fan middle to upper miocene strata of the boka bil formation show a similar geographic trend in deposition of coarsest and thickest sediment but the major depocenter had shifted northward relative to that of the bhuban formation by some 30 km passing near beani bazar these trends suggest that deltaic deposits of the surma group filled the sylhet trough of the northeast bengal basin from the east published seismic data from western bangladesh show that additional large channels also contributed materials to the bengal basin from the northwest during the late miocene but these channels resulted in very little accumulation in the northwestern part of the basin probably due to restricted subsidence of underlying continental crust this study suggests that there was a major drainage system similar to the modern brahmaputra river during miocene time which carried orogenic sediments eroded from the uplifted terranes of the eastern himalayas and indo burman ranges to the eastern bengal delta

reprocessing and enhanced interpretation of the initial cocorp southern appalachians traverse abstract reprocessing of the 1978 1980 cocorp southern appalachian seismic reflection data has produced improved images of structures related to the emplacement of the blue ridge inner piedmont allochthon the results enhance and extend the interpretation presented previously that the blue ridge and inner piedmont are allochthonous above a shallow and shallow dipping detachment that can be followed from outcrop at the blue ridge valley and ridge transition to at least beneath the carolina terrane the continuity of reflections in the new images supports the interpretation that the southern appalachian detachment is not rooted on the east side of the inner piedmont but rather projects as a low angle detachment or zone of decoupling to beneath the coastal plain an implication of this geometry is that terranes such as the carolina terrane between autochthonous north america and the alleghanian suture beneath the coastal plain are detached thin flakes

alleghanian development of the goat rock fault zone southernmost appalachians temporal compatibility with the master decollement the goat rock and associated bartletts ferry fault zones which mark the eastern margin of the pine mountain grenville basement massif are controversial due to the suggestion that they are rare exposed segments of the late paleozoic southern appalachian master decollement the controversy in part stems from reported middle paleozoic acadian radiometric dates postulated as the time of movement along these fault zones ultramylonite samples from the type area at goat rock dam yield a 287 plus minus 15 ma rb sr isochron interpreted as the time of sr isotopic rehomgenization during mylonitization this date is corroborated by late pennsylvanian early permian sup 40 ar sup 39 ar mineral ages on hornblende 297 288 ma and muscovite 285 278 ma from neomineralized and dynamically recrystallized rocks within and straddling the fault zone these late pennsylvanian early permian dates indicate the time of right slip movement alleghenian along the goat rock fault zone which is compatible with the timing suggested by cocorp for thrusting along the southern appalachian master decollement

coupling erosion and topographic development in the rainiest place on earth reconstructing the shillong plateau uplift history with in situ cosmogenic 10 be abstract the uplift of the shillong plateau in northeast india between the bengal floodplain and the himalaya mountains has had a significant impact on regional precipitation patterns strain partitioning and the path of the brahmaputra river today the plateau receives the highest measured yearly rainfall in the world and is tectonically active having hosted one of the strongest intra plate earthquakes ever recorded despite the unique tectonic and climatic setting of this prominent landscape feature its exhumation and surface uplift history are poorly constrained we collected 14 detrital river sand and 3 bedrock samples from the southern margin of the shillong plateau to measure erosion rates using the terrestrial cosmogenic nuclide 10be the calculated bedrock erosion rates range from 2 0 to 5 6 m my 1 whereas catchment average erosion rates from detrital river sands range from 48 to 214 m my 1 these rates are surprisingly low in the context of steep tectonically active slopes and extreme rainfall moreover the highest among these rates which occur on the low relief plateau surface appear to have been affected by anthropogenic land use change to determine the onset of surface uplift we coupled the catchment averaged erosion rates with topographic analyses of the plateau s southern margin we interpolated an inclined pre incision surface from minimally eroded remnants along the valley interfluves and calculated the eroded volume of the valleys carved beneath the surface the missing volume was then divided by the volume flux derived from the erosion rates to obtain the onset of uplift the results of this calculation ranging from 3 0 to 5 0 ma for individual valleys are in agreement with several lines of stratigraphic evidence from the brahmaputra and bengal basin that constrain the onset of topographic uplift specifically the onset of flexural loading and the transgression from deltaic to marine deposition ultimately our data corroborate the hypothesis that surface uplift was decoupled from the onset of rapid exhumation which occurred several millions of years earlier

insights into the mode of the south georgia rift extension in eastern georgia usa abstract the south georgia rift sgr lies oblique to the east coast margin of north america and across the alleghenian suture between laurentia and africa in southern georgia regionally the sgr can be divided into a southwest compartment and a northeast compartment across the jacksonville structure that is located in the vicinity of that suture analytical and numerical models are used to characterize the mode of rifting in the northeast compartment borehole cocorp seismic and regional geophysical information from the compartment that were used previously to infer the geometry of the basin are reassessed with the use of those models to analyze the lithospheric conditions influencing triassic extension this approach led to the interpretation of core complex mode extension and to the proposal of a model of progressive rifting the model shows how the riddleville and main sgr basins are associated and how changes in structural style of those two basins resulted from changing lithospheric conditions during extension the core complex model also indicated that extension was influenced by distributed deformation of a younger warmer and less stable lithosphere adjacent to the permian suture whereas extension in other east coast rifts that lie subparallel to structural fabric was probably localized by preexisting zones of weakness

speleogenesis of the world s longest cave in hybrid arenites krem puri meghalaya india abstract meghalaya nw india is one of the regions in the world with highest recorded rainfalls because of these peculiar climatic conditions and intense solutional weathering karstic caves are widely reported in the numerous limestone areas of this part of india likewise the extremely high rainfall and the tropical monsoon climate have fostered the formation of several caves in silicate rich lithologies the most important of these caves is krem puri over 25 5 km long characterized by a fracture controlled labyrinth forming a bi dimensional maze with mostly phreatic morphologies and some recent still active vadose entrenchments a detailed petrographic study of the host rocks has shown this cave to be formed along a set of quartzarenite hybrid arenite and calcarenite layers in a mostly siliciclastic sedimentary succession xrf elemental analysis of the different rock strata guiding the speleogenesis confirms that the early stage of cave development is confined mainly in the calcium carbonate rich layers geochemical analyses carried out on water flowing in the cave drips streams have shown that carbonate dissolution is the dominant chemical weathering process contributing to krem puri water solutes whereas the dissolution of quartz feldspars and clays play only a secondary role in rock weathering the morphology of the cave points to a lithology guided dissolution of the carbonate rich layers and piping of the residual quartz sand in phreatic conditions when the surface river network was not yet evolved and the water table was at the level of or higher than the cave this phreatic speleogenetic phase predates the start of river network entrenchment that started not earlier 5 ma ago and probably accelerated around 3 5 ma which deactivated the karst maze with a low denudation rate of 0 15 mm yr this active phreatic cave level which currently lies beneath a more or less 120 m overburden was originally covered by a sedimentary sequence at least 1 2 km thick after its decoupling from the underground drainage system denudation slowly brought the surface close to the maze system gradually introducing infiltration of vadose waters into the conduits this caused the formation of the vadose entrenchments probably

formation of a rain shadow o and h stable isotope records in authigenic clays from the siwalik group in eastern bhutan we measure the oxygen and hydrogen stable isotope composition of authigenic clays from himalayan foreland sediments siwalik group and from present day small stream waters in eastern bhutan to explore the impact of uplift of the shillong plateau on rain shadow formation over the himalayan foothills stable isotope data from authigenic clay minerals 2 m suggest the presence of three paleoclimatic periods during deposition of the siwalik group between 7 and 1 ma the mean 18o value in paleometeoric waters which were in equilibrium with clay minerals is 2 5 lower than in modern meteoric and stream waters at the elevation of the foreland basin we discuss the factors that could have changed the isotopic composition of water over time and we conclude that a the most likely and significant cause for the increase in meteoric water 18o values over time is the amount effect specifically a decrease in mean annual precipitation b the change in mean annual precipitation over the foreland basin and foothills of the himalaya is the result of orographic effect caused by the shillong plateau s uplift the critical elevation of the shillong plateau required to induce significant orographic precipitation was attained after 1 2 ma c by applying scale analysis we estimate that the mean annual precipitation over the foreland basin of the eastern bhutan himalayas has decreased by a factor of 1 7 2 5 over the last 1 3 million years 2018 american geophysical union all rights reserved