thermal structure and seismicity of subducting lithosphere it has recently been suggested that the mechanical behaviour of the oceanic and continental uppermost mantle depends on temperature alone and that there is no strong evidence for compositional effects it seems in the majority of cases that if uppermost mantle temperatures exceed 600 c any deformation that occurs does so in the absence of significant seismicity this observation apparently applies irrespective of whether that mantle material underlies an ocean or a continent this paper investigates whether the same behaviour exists for earthquakes occurring in descending lithospheric slabs namely whether there is some limiting temperature above which deformation occurs without significant seismicity and whether that limiting temperature is consistent with the behaviour seen in the uppermost mantle beneath oceans and continents by re examining the thermal structure in subducting slabs accounting for the temperature dependence of the relevant physical parameters teleseismically recorded seismicity is shown to be limited to material having potential temperatures less than not vert similar570 600 c or homologous temperatures less than not vert similar0 42 the exceptions to this pattern occur in the subhorizontally subducting regions of the nazca plate in which temperatures are probably affected by the relatively cold overriding plate

coevolution of bed surface patchiness and channel morphology 2 numerical experiments in gravel bed rivers bed topography and the bed surface grain size distribution evolve simultaneously but it is not clear how feedbacks between topography and grain sorting affect channel morphology in this the second of a pair of papers examining interactions between bed topography and bed surface sorting in gravel bed rivers we use a two dimensional morphodynamic model to perform numerical experiments designed to explore the coevolution of both free and forced bars and bed surface patches model runs were carried out on a computational grid simulating a 200 m long 2 75 m wide straight rectangular channel with an initially flat bed at a slope of 0 0137 over five numerical experiments we varied a whether an obstruction was present b whether the sediment was a gravel mixture or a single size and c whether the bed surface grain size feeds back on the hydraulic roughness field experiments with channel obstructions developed a train of alternate bars that became stationary and were connected to the obstruction freely migrating alternate bars formed in the experiments without channel obstructions simulations incorporating roughness feedbacks between the bed surface and flow field produced flatter broader and longer bars than simulations using constant roughness or uniform sediment our findings suggest that patches are not simply a by product of bed topography but they interact with the evolving bed and influence morphologic evolution

lithospheric structure across oblique continental collision in new zealand from wide angle p wave modeling 1 oblique convergence between the australian and pacific plates in south island new zealand has resulted in crustal thickening and distributed deformation within both plates we measure this thickening and image the deformation with seismic wide angle data along a 600 km long transect that spans the plate boundary p wave arrival times from 34 000 rays are used to construct a two dimensional model for seismic velocity with depth crustal velocities average 6 1 and 6 2 0 23 km s for the australian and pacific sides of the boundary respectively upper mantle velocities average 8 1 0 25 km s distributed deformation is indicated by the following observations a reduction in upper crustal and midcrustal velocities by 4 west of the plate boundary ascribed to flexural bending stresses a velocity reduction of 8 immediately east of the alpine fault linked to high pore fluid pressures 17 km of crustal thickening to form a 44 1 4 km deep crustal root that is offset 10 20 km se of the highest mountains a velocity reversal below the midcrust in the eastern part of the crustal root caused either by heating or an artifact from seismic anisotropy strong thickening of a 7 0 0 4 km s fast lower crust within the crustal root and a low velocity zone in the australian upper mantle due to sampling the slow orientation of upper mantle anisotropy

direct evidence of active deformation in the eastern indian oceanic plate conventional plate tectonics theory postulates that plates only deform on their boundaries to the contrary there is ample evidence of intraplate deformation in the equatorial indian ocean west of the ninetyeast aseismic ridge prior to this study no direct evidence of deforma tion east of the ninetyeast ridge was available we present the results of a multipurpose geo physical cruise showing that intraplate deformation also occurs in this area long at least 1000 km left lateral north south strike slip faults are active and reactivate fossil fracture zones this style of deformation is strikingly different from the east west folds and reverse faults that affect the region west of the ninetyeast ridge contrasting processes of convergence at the northern plate boundaries can account for the two styles of deformation west of the ninetyeast ridge there is a continent continent collision and east of the ridge oceanic lithosphere subducts along the sumatra trench the ninetyeast aseismic ridge therefore appears to be a mechanical border separating two distinct deformed areas

experimental and numerical evidence for intrinsic nonmigrating bars in alluvial channels alternate bars in straight alluvial channels are migrating or nonmigrating the currently accepted view is that they are nonmigrating if the width to depth ratio is at the value of resonance or if the bars are forced by a persistent local perturbation we carried out 2 d numerical computations and a long duration mobile bed flume experiment to investigate this view we find that nonmigrating bars can also occur in straight channels without resonant width to depth ratio or steady local perturbation they appear to be an intrinsic response of the alluvial river bed this finding bears on explanations for meandering of alluvial rivers for which nonmigrating bars are seen as a prerequisite we find however that the intrinsic tendency of a straight channel to form meanders usually has a different origin the identified intrinsic nonmigrating bars can only become the dominant mechanism for incipient meandering if the erodibility of the banks is very low

ridge torques and continental collision in the indian australian plate the excess gravitational potential energy of a collisional orogen that may be supported by focusing distributed ridge torques in a plate depends on the ratio l of the effective lengths of ridge systems and collisional fronts in the indian australian plate the net torque due to the asymmetric distribution of mid ocean ridges and young oceanic lithosphere along the southern boundary is 8 5 10 25 n m and is capable of supporting an excess potential energy of the himalayan collision zone of up to 7 5 10 12 j m 2 the concept that collision in the indian australian plate may be sustained by ridge torques suggests that collisional driving stresses may be sensitive to l in the indian australian plate l changed significantly during progressive collision from 55 to 45 ma during amalgamation of the indian and australian plates at 45 ma and during the development of the new guinea orogen through the late oligiocene and miocene such changes in l may help explain the termination of spreading between india and australia following the onset of the himalayan collision and the early miocene normal faulting in the high himalaya

present day stress orientations adjacent to active strike slip faults california and sumatra present day stress directions interpreted from well bore breakouts adjacent to two crastal scale active strike slip faults the san andreas fault in california and the great sumatran fault in sumatra indicate that the maximum horizontal stress direction sh is oriented at a high angle 70 90 to both faults the regionally defined stress fields spanning these faults show that adjacent young or actively growing folds have formed orthogonal to sh and are therefore in the thrust fault orientation these observations indicate a decoupling of the strike slip and compressional components of the deformation within these broadly transpressive zones borehole breakouts in 118 wells in western california indicate a regionally consistent stress pattern with sh generally oriented ne sw nearly perpendicular 80 90 to the strike of the san andreas fault the orientation of sh nearly perpendicular to the san andreas fault implies low shear stress on the fault and is consistent with geological interpretations of the coast and transverse ranges indicating active compressional deformation fault plane solutions for recent dip slip style earthquakes principal stress directions determined from inversion of earthquake focal mechanisms and induced hydraulic fracture orientations a stress trajectory map for western california is computed using an iterative statistical algorithm in which observed directional data such as breakout directions are used to obtain a model regional stress field analysis of well bore breakouts in 25 wells within the central and southern oil districts of sumatra indicates that the regionally defined sh adjacent to the active great sumatran strike slip fault is oriented at a high angle 70 80 to the fault this orientation of sh is consistent regionally with geologic stress indicators and focal mechanisms of dip slip earthquakes preliminary analysis of the stress field in the vicinity of the philippine and alpine faults suggests sh is also oriented at a high angle to these active strike slip faults similarly the minimum horizontal stress sh is oriented at a high angle to the the kane and dead sea transforms the observation of sh and sh in the vicinity of active strike slip faults being oriented nearly perpendicular and parallel to the faults suggests that large crustal scale strike slip faults may in general be inherently weak surfaces

shear stresses on megathrusts implications for mountain building behind subduction zones 1 shear stresses on a subduction megathrust play an important role in determining the forces available for mountain building adjacent to a subduction zone in this study the temperatures and shear stresses on megathrusts in 11 subduction zones around the pacific rim hikurangi tonga izu ogasawara western nankai northeastern japan aleutians western alaska cascadia northern chile southern chile and se asia northern sumatra have been determined the main constraint is that vertical normal stresses beneath the highlands behind the subduction zone are nearly equal to horizontal normal stresses in the plane of a trench or arc normal section for a typical brittle and ductile megathrust rheology frictional shear stress gz for depth z and ductile shear stress a exp b rt at temperature t where a b are rheological parameters treated as constants rheological constants common to all the megathrusts crust mantle b are determined by simultaneously solving for the force balance in the overlying wedge and megathrust thermal structure using a simplex minimization algorithm taking account of the induced mantle corner flow at depth 65 15 km 2 and constant radiogenic heating 0 65 0 3 w m 3 2 throughout the crust the a constants are solved individually for each subduction zone assuming that the maximum depth of interplate slip earthquakes marks the brittle ductile transition the best fit solution shows two groupings of megathrusts with most subduction zones having a low mean shear stress in the range 7 15 mpa crust 0 032 0 006 mantle 0 019 0 004 and unable to support elevations 2 5 km for a typical frictional sliding coefficient 0 5 the low effective coefficients of friction suggest high pore fluid pressures at 95 lithostatic pressure tonga and northern chile require higher shear stresses with crust 0 095 0 024 mantle 0 026 0 007 suggesting slightly lower pore fluid pressures at 81 lithostatic ductile shear in the crust is poorly resolved but in the mantle appears to show a strong power law dependency with b 36 18 kj mol 1 amantle values are sensitive to the precise value of b but are in the range 1 20 kpa the power law exponent n for mantle flow is poorly constrained but is likely to be large n 4 the brittle ductile transition in the crust occurs at temperatures in the range 370 c 512 c usually close to the base of the crust and in the mantle at much lower temperatures 180 c 300 c possibly reflecting a marked change in pore fluid pressure or quasi ductile and subfrictional properties in subduction zones where the subducted slab is older than 50 ma a significant proportion of the integrated shear force on the megathrust is taken up where it cuts the mantle and temperatures are 300 c in much younger subduction zones the stress transmission is confined mainly to the crust the shear stresses particularly in the crust may be kept low by some sort of lubricant such as abundant water rich trench fill which lowers the frictional sliding coefficient or effective viscosity and or raises pore fluid pressure the unusual high stress subduction zone in northern chile lacks significant trench fill and may be poorly lubricated with a mean shear stress 37 mpa required to support elevations 4 km in the high andes however where the crust is thin in sediment starved and poorly lubricated subduction zones such as tonga the mean shear stress will still be low sediment may lubricate megathrusts accommodating underthrusting of continental crust such as in the himalayas or eastern central andes which have a low mean shear stress 15 mpa

long term sea level fluctuations driven by ocean basin dynamics earth9s long term sea level history is characterized by widespread continental flooding in the cretaceous period 145 to 65 million years ago followed by gradual regression of inland seas however published estimates of the late cretaceous sea level high differ by half an order of magnitude from 40 to 250 meters above the present level the low estimate is based on the stratigraphy of the new jersey margin by assimilating marine geophysical data into reconstructions of ancient ocean basins we model a late cretaceous sea level that is 170 85 to 270 meters higher than it is today we use a mantle convection model to suggest that new jersey subsided by 105 to 180 meters in the past 70 million years because of north america9s westward passage over the subducted farallon plate this mechanism reconciles new jersey margin based sea level estimates with ocean basin reconstructions

mantle dynamics uplift of the tibetan plateau and the indian monsoon convective removal of lower lithosphere beneath the tibetan plateau can account for a rapid increase in the mean elevation of the tibetan plateau of 1000 m or more in a few million years such uplift seems to be required by abrupt tectonic and environmental changes in asia and the indian ocean in late cenozoic time the composition of basaltic volcanism in northern tibet which apparently began at about 13 ma implies melting of lithosphere not asthenosphere the most plausible mechanism for rapid heat transfer to the midlithosphere is by convective removal of deeper lithosphere and its replacement by hotter asthenosphere the initiation of normal faulting in tibet at about 8 3 ma suggests that the plateau underwent an appreciable increase in elevation at that time an increase due solely to the isostatic response to crustal thickening caused by india s penetration into eurasia should have been slow and could not have triggered normal faulting another process such as removal of relatively cold dense lower lithosphere must have caused a supplemental uplift of the surface folding and faulting of the indo australian plate south of india the most prominent oceanic intraplate deformation on earth began between about 7 5 and 8 ma and indicates an increased north south compressional stress within the indo australian plate a tibetan uplift of only 1000 m if the result of removal of lower lithosphere should have increased the compressional stress that the plateau applies to india and that resists india s northward movement from an amount too small to fold oceanic lithosphere to one sufficient to do so the climate of the equatorial indian ocean and southern asia changed at about 6 9 ma monsoonal winds apparently strengthened northern pakistan became more arid but weathering of rock in the eastern himalaya apparently increased because of its high altitude and lateral extent the tibetan plateau provides a heat source at midlatitudes that should oppose classical symmetric hadley circulation between the equator and temperate latitudes and that should help to drive an essentially opposite circulation characteristic of summer monsoons for the simple case of axisymmetric heating no dependence on longitude of an atmosphere without dissipation theoretical analyses by hou lindzen and plumb show that an axisymmetric heat source displaced from the equator can drive a much stronger meridianal monsoonlike circulation than such a source centered on the equator but only if heating exceeds a threshold whose level increases with the latitude of the heat source because heating of the atmosphere over tibet should increase monotonically with elevation of the plateau a modest uplift 1000 2500 m of tibet already of substantial extent and height might have been sufficient to exceed a threshold necessary for a strong monsoon the virtual simultaneity of these phenomena suggests that uplift was rapid approximately 1000 m to 2500 m in a few million years moreover nearly simultaneously with the late miocene strengthening of the monsoon the calcite compensation depth in the oceans dropped plants using the relatively efficient c4 pathway for photosynthesis evolved rapidly and atmospheric co2 seems to have decreased suggesting causal relationships and positive feedbacks among these phenomena both a supplemental uplift of the himalaya the southern edge of tibet and a strengthened monsoon may have accelerated erosion and weathering of silicate rock in the himalaya that in turn enhanced extraction of co2 from the atmosphere thus these correlations offer some support for links between plateau uplift a downdrawing of co2 from the atmosphere and global climate change as proposed by raymo ruddiman and froehlich mantle dynamics beneath mountain belts not only may profoundly affect tectonic processes near and far from the belts but might also play an important role in altering regional and global climates

numerical study of alternate bars in alluvial channels with nonuniform sediment rivers typically present heterogeneous bed material but the effects of sediment nonuniformity on river bar characteristics are still unclear this work investigates the impact of sediment size heterogeneity on alternate bars with a morphodynamic numerical model the model is first used to reproduce a laboratory experiment showing alternate bar formation with nonuniform bed material subsequently the influence of sediment size heterogeneity on alternate bars is investigated distinguishing hybrid from free bars definition based on the presence absence of morphodynamic forcing considering the results of nine scenarios in four of them a transverse obstacle is used to generate forcing the computations are carried out with the telemac mascaret system solving the two dimensional shallow water equations with a finite element approach accounting for horizontal and vertical sediment sorting processes the results show that sediment heterogeneity affects free migrating and hybrid bars in a different way the difference lies in the presence absence of a migration front so that distinct relations between bed topography bed shear stress and sediment sorting are obtained sediment sorting and associated planform redistribution of bed roughness only slightly modify free migrating bar morphodynamics whereas hybrid bars are greatly impacted with decreased amplitude and increased wavelength increased sediment size heterogeneity increases the degree of sediment sorting while the sorting pattern remains the same for both free and hybrid bars moreover it produces averagely higher longer and faster free bars while in the case of hybrid bars their wavelength is increased but no general trend can be determined for their amplitude

early pre 8 ma fault activity and temporal strain accumulation in the central indian ocean the diffuse deformation zone in the central indian ocean is the classical example of distributed deformation of the oceanic lithosphere where shortening between the indian and capricorn plates is manifest as reverse faulting 5 10 km spaced faults and long wavelength 100 300 km folding the onset of this deformation is commonly regarded as a key far field indicator for the start of major uplift of the himalayas and tibet some 4000 km further to the north due to increased deviatoric stresses within the wider india asia area there has been disagreement concerning the likely timing for the onset of deformation between plate motion inversions and seismic reflection based studies in the present study fault displacement data from seismic reflection profiles within the central indian ocean demonstrate that compressional activity started much earlier than previously thought at around 15 4 13 9 ma from reconstructions of fault activity histories we show that 12 of the total reverse fault population had been activated and 14 of the total strain accumulated prior to a sharp increase in the deformation rate at 8 0 7 5 ma there is no evidence for any regional unconformity before 8 0 7 5 ma early shortening was accommodated by activity on single isolated fault blocks total strain estimates derived are more variable and complex than those predicted from plate inversion and they do not show simple west to east increase

the existence of a thin low viscosity layer beneath the lithosphere the horizontal temperature gradient at the base of the lithosphere at an oceanic fracture zone where plate of different ages is juxtaposed is expected to drive a local circulation the characteristics of which can be constrained by the amplitude wavelength and age dependence of the geoid two dimensional numerical models of convection in a fluid layer overlain by a solid conducting lid have been used to generate theoretical geoid profiles at right angles to the fracture zone only a thin low viscosity layer provides a reasonable fit to the data the best model so far obtained has a fluid layer 150 km thick with viscosity 1 5 1019 pa s under a 75 km lid such a layer which is incapable of transmitting strong horizontal shear stresses could provide the decoupling mechanism between plate and deep mantle flow required to balance the forces on the plates

development of sediment slug upstream from the czorsztyn reservoir southern poland and its interaction with river morphology abstract the effects of base level rising upstream of dam reservoirs on in channel sedimentation and interaction of the stored sediments with the gravel bed channel morphology have received little attention so far previous studies however suggested that the feedback mechanism between in channel sedimentation and bank erosion may affect channel morphology here the pattern of the bar area bank erosion and morphology of the gravel bed dunajec river upstream from the czorsztyn reservoir cr constructed in 1997 in southern poland were analyzed from aerial images 1982 2012 and lidar data 2013 in the part of the post dam period with a large flood the average bar area increased significantly in the backwater section and at some distance upstream and then extended in the upstream direction at an average rate of above 40 m y reaching 2 2 km upstream from the cr in 2012 the bar area variation was 40 and 77 explained by local bank erosion in the periods of large and low to moderate floods respectively the sum of bank erosion from the post dam period explained 80 of the variation in the present width depth ratio significantly increased in the backwater section the results showed that the large floods in 1997 in conjunction with backwater inundation initiated intensive bank erosion and bar growth subsequently in the period with low and moderate floods the localized bar bank interaction connected with the flow divergence around the deposited bar led to localized bank erosion and additional bar growth promoting bend development these processes propagating in the upstream direction were facilitated by the existence of a large amount of easily remobilized sediment stored in the floodplain connected with the sedimentation zone from the end of the nineteenth century bend development was controlled by valley confinement causing downstream bend translation in the narrower valley confined section and its extension in the wider unconfined section of past sediment storage the results obtained from this site specific location imply that the occurrence of similar past sediment storage zones of easily remobilized coarse sediments may favor intensive bank erosion and bar growth during large floods and in the case of low transport competence backwater zones of gravel bed rivers where the remobilized sediments cause localized bar growth it may promote morphologically effective bar bank interaction in the period with low and moderate floods

hydraulic validation of two dimensional simulations of braided river flow with spatially continuous adcp data 1 gravel bed braided rivers are characterized by shallow branching flow across low relief complex and mobile bed topography these conditions present a major challenge for the application of higher dimensional hydraulic models the predictions of which are nevertheless vital to inform flood risk and ecosystem management this paper demonstrates how high resolution topographic survey and hydraulic monitoring at a density commensurate with model discretization can be used to advance hydrodynamic simulations in braided rivers specifically we detail applications of the shallow water model delft3d to the rees river new zealand at two nested scales a 300 m braid bar unit and a 2 5 km reach in each case terrestrial laser scanning was used to parameterize the topographic boundary condition at hitherto unprecedented resolution and accuracy dense observations of depth and velocity acquired from a mobile acoustic doppler current profiler adcp along with low altitude aerial photography were then used to create a data rich framework for model calibration and testing at a range of discharges calibration focused on the estimation of spatially uniform roughness and horizontal eddy viscosity h through comparison of predictions with distributed hydraulic data results revealed strong sensitivity to h which influenced cross channel velocity and localization of high shear zones the high resolution bed topography partially accounts for form resistance and the recovered roughness was found to scale by 1 2 1 4 d84 grain diameter model performance was good for a range of flows with minimal bias and tight error distributions suggesting that acceptable predictions can be achieved with spatially uniform roughness and h

field experiment on alternate bar development in a straight sand bed stream alternate bars in rivers and streams develop as a result of differences in length scales involved in the adjustment of flow and sediment transport to irregularities of the bed the amount of field evidence supporting theoretical insights is highly limited here we present results from a large scale field experiment in a 600 m long straight reach over a period of almost 3 years the channel was allowed to evolve autogenously from initially flat bed conditions subject to discharge variation alternate bars developed within 8 months from the start of the experiment the initial stages of bar development included bar growth both in wavelength and amplitude and bar migration the latter was too limited to classify the bars as being migrating bars therefore we classify the bars as nonmigrating bars toward the end of the experiment the regular alternate bar pattern evolved into an irregular pattern and bar amplitude started to decrease from the start of the experiment we observed a declining channel slope from 1 8 m km 1 initially to 0 9 m km 1 halfway the experiment after which it stabilized we applied two bar theories to establish their predictive capacity both bar theories predicted the development of alternate bars under the constructed channel conditions in response to the declining channel slope both theories predicted a decreasing likelihood for the development of alternate bars our study shows that under field conditions the applied bar theories may predict the initial stages of bed development

cenozoic evolution of neotethys and implications for the causes of plate motions africa north america eurasia plate circuit rotations combined with red sea rotations and new estimates of crustal shortening in iran define the cenozoic history of the neotethyan ocean between arabia and eurasia the new constraints indicate that arabia eurasia convergence has been fairly constant at 2 to 3 cm yr since 56 ma with slowing of africa eurasia motion to 1 cm yr near 25 ma coeval with the opening of the red sea ocean closure occurred no later than 10 ma and could have occurred prior to this time only if a large amount of continental lithosphere was subducted suggesting that slowing of africa significantly predated the arabia eurasia collision these kinematics imply that africa s disconnection with the negative buoyancy of the downgoing slab of lithosphere beneath southern eurasia slowed its motion the slow steady rate of northward subduction since 56 ma contrasts with strongly variable rates of magma production in the urumieh dokhtar arc implying magma production rate in continental arcs is not linked to subduction rate

effects of plate bending and fault strength at subduction zones on plate dynamics for subduction to occur plates must bend and slide past overriding plates along fault zones because the lithosphere is strong significant energy is required for this deformation to occur energy that could otherwise be spent deforming the mantle we have developed a finite element representation of a subduction zone in which we parameterize the bending plate and the fault zone using a viscous rheology by increasing the effective viscosity of either the plate or the fault zone we can increase the rates of energy dissipation within these regions and thus decrease the velocity of a plate driven by a given slab buoyancy we have developed a simple physical theory that predicts this slowing by estimating a convecting cell s total energy balance while taking into account the energy required by inelastic deformation of the bending slab and shearing of the fault zone the energy required to bend the slab is proportional to the slab s viscosity and to the cube of the ratio of its thickness to its radius of curvature the distribution of dissipation among the mantle lithosphere and fault zone causes the speed of a plate to depend on its horizontal length scale using the observation that earth s plate velocities are not correlated to plate size we can constrain the lithosphere viscosity to be between 50 and 200 times the mantle viscosity with higher values required if the fault zone can support shear tractions 50 mpa over 300 km these subduction zone strengths imply that the mantle fault zone and lithosphere dissipate about 30 10 and 60 of a descending slab s potential energy release if the slab is 100 km thick the lithospheric component is highly dependent on slab thickness it is smaller for thin plates but may be large enough to prevent bending in slabs that can grow thicker than 100 km ubduction zone strength should be more stable than mantle viscosity to changes in mantle temperature so the controlling influence of subduction zones could serve to stabilize plate velocities over time as the earth cools because the details of convergent plate boundaries are so important to the dynamics of plate motion numerical models of mantle flow should treat subduction zones in a realistic way

location of the nubia somalia boundary along the southwest indian ridge the northern boundary between the nubian west african and somalian east african plates is marked by the existence of the east african rift which is the locus of roughly east west stretching due to the separation of these plates south of 20 s any expression of deformation or seismicity due to the relative motion of these two presumably distinct plates vanishes or is extremely subtle although the boundary must continue until it intersects another plate boundary until recently there were no observational limits on the location of that intersection and the recent limits were very broad being thousands of kilometers wide here from an analysis of the locations of the old edge of magnetic anomaly 5 which is over 11 ma seafloor we show that the main locus of deformation between nubia and somalia near the southwest indian ridge over the past 11 m y can be no wider than a few hundred kilometers from 100 km west of the du toit fracture zone to 50 km east of the andrew bain fracture zone complex deformation is most likely concentrated along a closely spaced set of several fracture zones known collectively as the andrew bain fracture zone complex which is only 100 km wide the displacement along the nubia somalia boundary over the past 11 m y is 23 km 6 km 95 confidence limits indicating a displacement rate of 2 mm yr much slower than typical rates of seafloor spreading and subduction but comparable to that along the owen fracture zone and dalrymple trough which separate the arabian plate from the indian plate and along the azores gibraltar line which separates the eurasian plate from the nubian plate

a late cenozoic regional mesofracture system in southern england and northern france the upper cretaceous and palaeogene rocks of s england and n france are cut by a nw trending system of fractures characterized by swarms of vertical extension joints accompanied by conjugate sets of hybrid and shear joints and normal mesofaults many major photolineaments exhibit the same nw trend as the fractures and are particularly conspicuous in n france where they are coincident with rectilinear drainage channels suggesting a relationship between fracture orientation and physiography in s england this relationship is absent possibly because of the influence of additional fracture systems related to major flexures trending obliquely to the nw se direction these major e w trending flexures and associated fracture systems were formed during the oligocene to early miocene 9helvetic9 phase of deformation and are cross cut obliquely by fractures in the nw trending system the nw trending fractures and lineaments are parallel to neotectonic normal faults in the lower rhine embayment and become less well developed to the west with increasing distance from these major structures the lower rhine faults were reactivated and propagated into the quaternary cover as a consequence of ne sw regional tension generated during the late neogene to recent 9jura9 phase of nw se alpine convergence the nw trending extensional structures in s england and n france are considered to be of about the same age and related to the same stress regime as the neotectonic normal faults of the lower rhine embayment however because fractures of the nw system cutting palaeogene sediments in se england are truncated by the erosion surface beneath the plio pleistocene red crag deposits it is possible that in england fracture initiation did not continue into quaternary times

new constraints on the tectonic evolution of the eastern indian ocean abstract from marine magnetic anomaly studies a fossil spreading ridge is identified beneath the nicobar fan in the northwestern wharton basin several north south trending transform faults offset this ridge left laterally east of the 86 e transform fault our findings show that this ridge which was part of the plate boundary between the indian and australian plates ceased its spreading shortly after formation of magnetic anomaly 20 45 6m y b p since the breakup of australia and antarctica probably occurred sometime between 110 and 90 m y b p we suggest that the indian australian and antarctic plates were moving relative to one another from about 90 to 45 m y b p a triple junction would have existed in the southeastern indian ocean during that period of time at anomaly 19 time 45m y b p the junction became inactive and australia and india became a single plate the northwest southeast trending southeast indian ridge was formed by connecting the india antarctica spreading center with the australia antarctica spreading center its activity has continued to the present time

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

detailed simulation of morphodynamics 3 ripples and dunes we present a 3 d physics based high resolution modeling approach to the dynamics of underwater ripples and dunes the flow is modeled by large eddy simulation on a cartesian grid with local refinements the sediment transport is modeled by computing pickup transport over the bed transport in the water column and deposition of rigid spherical particles in a lagrangian framework the morphological development of the bed is modeled by a sediment balance equation in which the pickup and deposition from the sediment motion submodels appear as source and sink terms the model realistically replicated the formation and migration of dunes model results showed a good agreement with data from five flume experiments we subsequently applied the model to investigate the effect of sediment grain size on ripples finer sediments were found to yield more superimposed ripples than coarser sediments moreover under the same hydrodynamic conditions the finer sediments yielded two dimensional bed forms whereas for coarser sediment irregularities increased we extended the tests to pronounced 3 d morphologies by simulating the development of local scour at a pier the results agreed well with experimental data the model contributes to unraveling the complex problem of small scale morphodynamics and may be used in a wide range of applications for instance to develop more reliable parameterizations of small scale processes for application in large scale morphodynamic models

a tectono geomorphological perspective of micro earthquake swarm activity prone areas around bhavnagar and jamnagar in saurashtra western india abstract areas around bhavnagar and jamnagar in eastern and northern parts of saurashtra respectively are identified previously as prone to recurrent earthquake swarm activity sparse seismotectonic studies so far have not revealed any linkage with known faults in bhavnagar area while no structures are known to occur in the area around jamnagar we carried out detailed tectono geomorphic studies with contrasting geomorphic settings with a view to provide an insight into the swarm activity from a geomorphological perspective the study is based on extensive field studies and quantitative geomorphic analysis of landscape and drainage the area around bhavnagar is divisible into two tectono geomorphic domains the e w trending sihor fault zone located to the west of bhavnagar domain i and the terrain to its south which shows undulating landscape controlled by n s trending structural trends domain ii the quadrant in the sw between these two domains consists of southward titled trappean hills which form source area for drainages flowing northward through sihor fault zone and those flowing eastward through domain ii late quaternary possibly holocene tectonic activity along the sihor fault is indicated by the northward tilt of alluvial plain surface knickpoint in rivers and small scale minor faults along the fault zone prominent decrease in depth of fluvial incision away from the fault zone and youthful landscape indicated by drainage basin analysis domain ii comprises the largely erosional pediplain developed over trappean and tertiary rocks to the south of bhavnagar the drainages show an early stage of landscape development due to ongoing tectonic activity the area around jamnagar is dominantly a peneplain formed over subhorizontal trappean lava flows the peneplain is a manifestation of the prolonged period of non erosion and deposition during the cenozoic the rivers show youthful landscape in the central part of the peneplain which includes large knickpoints rapids anomalous river bends steepening of channel gradient and large potholes this ene wsw trending zone of anomalous fluvial geomorphic anomalies points to the existence of a fault or fold in the subsurface the structure appears to be active as indicated by the denser cluster of earthquake swarm along this zone we infer that the generation of earthquake swarm activity comprising low to moderate magnitude shocks is caused by the sheared rocks along the sihor fault near bhavnagar and the ene wsw trending subsurface structure near lalpur which provide avenues for downward percolation of rainwater to deeper levels causing loss the frictional resistance along the weak planes

alternating bar instabilities in unsteady channel flows over erodible beds channel flows over erodible beds are susceptible to instabilities of the coupled fluid and sediment flow equations the most dangerous mode usually takes the form of a migrating alternating bar instability propagating in the flow direction previous theories have assumed that the underlying flow is steady and here the theory is extended to the unsteady situation stability characteristics are calculated for large amplitude oscillations superimposed on a mean flow in addition it is found that the basic instability is convective and we address the receptivity problem for bars induced by flow oscillations interacting with spatial variations associated with seepage channel width variations etc the effect of unsteadiness in the weakly nonlinear situation is also discussed a mechanism which allows flow oscillations to interact with migrating bars to produce a sinusoidal structure fixed in space which might be relevant to meander formation is discussed

modelling free forming meander evolution in a laboratory channel using three dimensional computational fluid dynamics abstract the present study investigated the use of computational fluid dynamics cfd in predicting the formation development and migration of free forming meander bends the three dimensional cfd model computed water flow and sediment transport in alluvial channels and predicted vertical and horizontal bed changes different algorithms and parameters were tested to provide an insight into the application range of cfd when modelling free forming meander formation the computational domain was discretized by an unstructured grid a control volume method was used for the discretization of the navier stokes equations for the flow calculation and of the convection diffusion equation for the sediment transport calculation turbulence was modelled by the k e turbulence model the simulation was started from an initially straight grid with neither sediment feed nor any perturbation at the inflow boundary the model computed the river bed evolution over a real time period of 3 d results were compared with laboratory experiments and showed that the cfd model can predict many of the characteristics of the alluvial meander formation and migration however some limitations and uncertainties exist that have to be clarified in future investigations

formation of stationary alternate bars in a steep channel with mixed size sediment a flume experiment alternate bars were formed by sediment transport in a flume with froude modelled flow and relative roughness characteristic of gravel boulder channels with steep slopes the flume 0 3 m wide 7 5 m long was filled with a sand gravel mixture which was also fed into the top of the flume at a constant rate under constant discharge channel slope was set at 0 03 initially coarse particles accumulated on incipient bar heads near one side of the flume and diverted flow and bedload transport across the flume toward a pool scoured against the opposite flume wall downstream sorting in the pool directed coarse particles onto the next bar head downstream alternate sequences of pools and coarse bar heads were thereby linked down the entire flume by interactions of sediment sorting flow and channel morphology during episodes of bar construction unsorted bedload invaded interior bar surfaces and was deposited persistent deposition of coarse particles on bar heads prevented downstream migration of bars by inhibiting bar head erosion and bedload transport over bars likely factors leading to bar head stabilization in modelled gravel bed channels are coarse mixed size sediment steep channel gradients and shallow depths

is there a genetic relationship between selected regional joints and contemporary stress within the lithosphere of north america on the appalachian plateau of new york ene striking joints parallel the mid continent contemporary stress field and do not correlate with other structures joints within ohio indiana michigan and west virginia also parallel the mid continent maximum horizontal compression as indicated by hydraulic fracture measurements in northern indiana where the contemporary stress field deviates significantly from the orientation of the mid continent stress the attitude of the joints deviates accordingly in addition to joints structures on several scales from subcontinent size fracture systems to microfabrics in shales apparently parallel the mid continent stress field from this i conclude that these joints are mode i cracks whose orientation corresponds to the contemporary tectonic stress field in itself the formation of these structures constitutes a tectonic event that has often been overlooked in favor of tectonic events that produce the more spectacular but less pervasive faults and folds

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

viscosity and thickness of the asthenosphere detected from the fennoscandian uplift the earth s response to the deglaciation in fennoscandia is modelled using a layered viscous model with an elastic lithosphere the modelled tilting of palaeoshorelines at particular locations peripheral to the former ice load and the pattern of the present rate of uplift consistent with the observations strongly suggest a low viscosity asthenosphere earth models without a low viscosity asthenosphere give significant mismatch to the present rate of uplift and are unable to mimic the observed tilting of palaeoshorelines models with a viscosity of the lower mantle of 1 0 2 0 1021 pa s and a viscosity of the upper mantle of 0 7 1 0 1021 pa s overlain by low viscosity asthenosphere with a thickness ranging from 25 to 100 km fit equally well with the observations it is suggested strongly that the asthenosphere has a thickness of less than 150 km and a viscosity of less than 7 0 1019 pa s

experimental study of the response of a gravel streambed to increased sediment supply if increased sediment supply to a river channel exceeds its transport capacity deposition necessarily occurs as the bed adjusts to accommodate the increased supply both the mean and spatial patterns in bed elevation and grain size may change and an ability to understand their relative importance is needed to predict bed response we report on an experiment in a field scale flume in which sediment supply is increased to a gravel bed with alternate bars sediment was recirculated in the experiments but augmented in two steps after which the bed was allowed to reach a new steady state the transport rate at the end of the experiment was three times larger than at the start high resolution sediment flux and topographic measurements grain size derived from photographs and hydrodynamic modeling allow us to document the topographic and textural response of the bed to increased sediment supply the spatial patterns of bed topography and texture were forced by the flume setup and the initial and final steady states included long stationary alternate bars with associated grain size sorting the transient bed contained several scales of shorter wavelength migrating bedforms superimposed on and temporarily replacing the stationary alternate bars bed topography and textural patterns adjusted to increased sediment supply over different timescales bed slope and mean stress increased directly with sediment supply rate to produce a new transport steady state in a time about 2 5 times the minimum needed to deposit the required sediment wedge indicating a trap efficiency of about 40 for the aggrading wedge adjustments in local topography and sorting primarily in the form of smaller migrating bars continued for a period approximately equal to that required to initially reach transport steady state copyright 2013 john wiley sons ltd

changes in stress pattern around epicentral region of bhuj earthquake of 26 january 2001 1 the 26 january 2001 bhuj earthquake mw 7 6 was one of the deadly indian earthquakes that took about 20 000 lives the region was seismically quite in the last 50 years in the present paper we have made efforts to understand the general nature of stress field and also spatial and temporal stress field around bhuj prior and after the earthquake by analyzing lineaments extracted from remote sensing data our results demonstrate n to ne orientation of stress showing a high level of correlation between the shmax deduced from the lineament and earthquake focal mechanism the present results show the use of lineaments derived from remote sensing data in the evaluation of seismic hazards of any region and will be useful in updating world stress map

geophysics of the pitman fracture zone and pacific antarctic plate motions during the cenozoic multibeam bathymetry and magnetometer data from the pitman fracture zone fz permit construction of a plate motion history for the south pacific over the past 65 million years reconstructions show that motion between the antarctic and bellingshausen plates was smaller than previously hypothesized and ended earlier at chron c27 61 million years ago the fixed hot spot hypothesis and published paleomagnetic data require additional motion elsewhere during the early tertiary either between east antarctica and west antarctica or between the north and south pacific a plate reorganization at chron c27 initiated the pitman fz and may have been responsible for the other right stepping fracture zones along the ridge an abrupt 8 clockwise rotation in the abyssal hill fabric along the pitman flowline near the young end of chron c3a 5 9 million years ago dates the major change in pacific antarctic relative motion in the late neogene

the use of a morphological acceleration factor in the simulation of large scale fluvial morphodynamics abstract numerical simulation of fluvial morphodynamic processes can identify important dynamics at time and space scales difficult to observe in the field however simulations involving large spatial scales and or the long timescales characteristic of morphodynamic processes are often untenable because of long computation times the morphological acceleration factor morfac applies a scalar multiplier to the sediment continuity equation and is often applied in morphodynamic simulations to reduce computational time although the use of morfac in coastal simulations is relatively common its applicability in field scale fluvial models is generally confined to steady flow simulations over reach scale spatial domains here we explore the viability of using morfac to simulate large scale long term morphodynamics in a gravel bed river we use delft3d to simulate a 60 d period with a significant discharge event in the nooksack river washington we systematically adjust morfac values ranging from 5 to 50 to compare with a baseline condition of no acceleration model results suggest that morfac based modification of the inflow hydrograph time series reduces peak flow magnitudes and flood wave celerities higher morfac values result in greater flood wave attenuation and lower celerity reducing the simulated morphological impact at locations farther downstream in general relative error compared to the model baseline increases farther downstream because of the altered flood wave propagation however error analysis also shows a positive relationship with channel slope and a negative relationship with channel width hence errors increase in the steeper confined reaches of the upper part of the domain even for the lowest morfac values absolute cumulative volume change errors are on the order of 10 indicating that the use of morfac in fluvial simulations is best restricted to short term and or smaller scale modeling efforts wider channel sections with lower gradients are shown to produce lower morfac related errors therefore morphological acceleration as examined here is potentially more appropriate for limited reaches rather than large scale domains

influences of holocene sea level regional tectonics and fluvial gravity and slope currents induced sedimentation on the regional geomorphology of the continental slope off northwestern india abstract regional geomorphology derived from echosounding and bathymetric data lines of a total of 12 000 km depth to basement regional deep seated tectonic features surficial sediment distribution and holocene sedimentation rates of the continental margin off northwestern india are presented results indicate that the shelf edge over most of the region shelf break between 80 and 154 m water depth is covered with relict carbonate facies and partially buried bioherms of holocene age particularly off bombay and saurashtra regions significant changes in the depth and orientation of the shelf edge adjacent to saurashtra and the bombay high areas in relict carbonate facies are recorded at locations shelf edge depth is much more up to 150 m than the early holocene eustatic sea level still stand in this region about 100 m the upper boundary of the slope also varies considerably in depth about 1450 m in the vicinity of the indus fan having high sedimentation and 2900 m further southwards the gradient of the slope off the kachchh is steepest and some regional notches are observed at the upper slope the gentler slope off saurashtra and the bombay high area has large benches at depths of 180 230 m width 2 10 km and 650 780 m well below the holocene sea level the bombay high area has slope breaks between 400 and 600 m whereas off saurashtra steep breaks in the slope occur between 560 and 960 m depth further southwards at the foot of the slope elevations and depressions are present variations in the geomorphology of the slope off the northwestern continental margin of india are attributed to 1 late pleistocene holocene sea levels 2 regional tectonics 3 fluvial sedimentation and 4 gravity and slope currents induced sediment dynamics the locations under influences of slumping turbidity and tectonic movements with altered in situ sequence of sedimentations are delineated

mechanism for joint saturation in mechanically layered rocks an example from southern israel abstract over 700 joint spacings were measured along a 190 m scanline within a single limestone bed of the turonian gerofit formation southern israel by investigating joints belonging to a single systematic set confined to a bed of uniform thickness one can factor out the major influences of bed thickness lithology and mechanical properties on joint set development joint spacing distributions and fracture spacing ratios fsr vary dramatically along the length of the scanline and correlate directly with throughgoing fracture zones belonging to the same tectonic stress regime regions cross cut by fracture zones display narrower median joint spacings fsr of 1 3 and unimodal joint spacing distributions where fracture zones are absent joint spacings have wider medians fsr of 0 8 and multimodal distributions because fracture zones most likely represent areas of high strain and laboratory experiments demonstrate a systematic decrease in fracture spacing with increasing strain regions of the bed cross cut by fracture zones are likely at a more advanced stage of joint set development and hence closer to saturation than other regions joint saturation in the gerofit formation may be achieved by a shift in scale from single layer to multi layer jointing as the development of throughgoing joints prevents the formation of additional bedding confined joints

alternate bar response to sediment supply termination 1 sediment supply is widely held to be one of the primary controls on bar topography in alluvial channels yet quantitative linkages between sediment supply and bar topography are not well developed we explore the conditions under which alternate bars form and how they respond to the elimination of sediment supply in two linked laboratory experiments the first set of experiments was conducted in a 28 m long 0 86 m wide flume channel using a unimodal sand gravel mix the second set of experiments was conducted at field scale in a 55 m long 2 74 m wide channel using a unimodal gravel mixture in both experiments alternate bars and patchy surface grain size distributions developed under steady flow and sediment supply conditions the cessation of the sediment supply induced a reduction in the surface grain size heterogeneity and the bars were eliminated in both flumes mean boundary shear stress had declined but were capable of moving sediments after the bars disappeared albeit at relatively small rates compared to when the bars were present in the smaller flume the previously stationary bars migrated out of the flume and were not replaced with new bars a nearly featureless bed formed with limited surface grain size heterogeneity a slightly coarsened surface and a slightly reduced slope in the larger flume the formation of alternate bars was induced by an imposed upstream flow constriction and as such the bars did not migrate termination of sediment supply led to progressive erosion of bed topography and loss of the bars coarsening of the bed surface loss of bed texture patchiness and significant slope reduction the original alternate bar topography redeveloped when the sediment supply was restored once sufficient deposition had occurred to reconstruct the original channel slope this shows that the bar loss was reversible by establishing the previous conditions and highlights the importance of sediment supply for bar formation the role of sediment supply in bar formation and stability is not often recognized in stream restoration our results suggest that the loss of sediment supply can significantly affect alternate bar topography and that considerable volumes of sediment may be needed restore channel bars

ridge forces absolute plate motions and the intraplate stress field torque poles are calculated for a variety of possible forces acting on the plates including ridge push slab pull and collisional resistance these torque poles are then compared to the directions of absolute plate motions there is a strong correlation between ridge torque poles and the azimuth of absolute plate motions for the north american south american pacific cocos and eurasian plates simple slab pull torques correlate well with absolute motion azimuths for the pacific nazca and cocos plates and moderately well with the absolute motion azimuth of the indoaustralian plate collisional resistance torque poles correlate with the absolute motion azimuth of the eurasian plate only the correlations are presented as further evidence that the absolute reference frame for plate motion is determined by the surface plates themselves torque poles for various forces are also compared with several long wavelength features of the global intraplate stress field that also tend to be aligned with absolute motion directions in general ridge torque directions agree well with the orientations of maximum horizontal stresses for stable north america western europe and south america and provide an alternative explanation for the alignment in terms of ridge push forces rather than basal drag collisional resistance forces can also explain the alignment of stresses in western europe for the indoaustralian plate the torque pole for collisional resistance forces is consistent with the general pattern of stresses in at least the western half of the plate but is not a good predictor of the entire data set for the plate other processes in addition to collisional resistance must be important for the indo australian plate ridge push forces may account for a significant portion of the long wavelength features of the intraplate stress field especially away from continental collisions such a conclusion is consistent with negative buoyancy of the slab being an important component of the driving mechanism as previously suggested slab forces may be largely balanced within the subducted slab itself and thus have limited effect on deformation of the surface plates

numerical simulation of large scale bed load particle tracer advection dispersion in rivers with free bars asymptotic characteristics of the transport of bed load tracer particles in rivers have been described by advection dispersion equations here we perform numerical simulations designed to study the role of free bars and more specifically single row alternate bars on streamwise tracer particle dispersion in treating the conservation of tracer particle mass we use two alternative formulations for the exner equation of sediment mass conservation the flux based formulation in which bed elevation varies with the divergence of the bed load transport rate and the entrainment based formulation in which bed elevation changes with the net deposition rate under the condition of no net bed aggradation degradation a 1 d flux based deterministic model that does not describe free bars yields no streamwise dispersion the entrainment based 1 d formulation on the other hand models stochasticity via the probability density function pdf of particle step length and as a result does show tracer dispersion when the formulation is generalized to 2 d to include free alternate bars however both models yield almost identical asymptotic advection dispersion characteristics in which streamwise dispersion is dominated by randomness inherent in free bar morphodynamics this randomness can result in a heavy tailed pdf of waiting time in addition migrating bars may constrain the travel distance through temporary burial causing a thin tailed pdf of travel distance the superdiffusive character of streamwise particle dispersion predicted by the model is attributable to the interaction of these two effects

experimental and numerical findings on the long term evolution of migrating alternate bars in alluvial channels migrating alternate bars form in alluvial channels as a result of morphodynamic instability extensive literature can be found on their origin and short term development but their long term evolution has been poorly studied so far in particular it is not clear whether migrating bars eventually reach a dynamic equilibrium as in previous studies bars were observed to elongate with time we studied the long term evolution of alternate bars by performing two independent long duration laboratory experiments and some numerical tests with a physics based depth averaged model in a straight flume with constant water flow and sediment recirculation migrating bars followed a cyclic variation they became gradually longer and higher for a while then quickly much shorter and lower in one case all migrating bars simultaneously vanished almost completely only to reform soon after at the same time steady bars two to three times as long progressively developed from upstream gradually suppressing the migrating bars we also observed simultaneous vanishing of migrating bars in an annular flume experiment this time at intervals of 6 8 d numerical simulations of long alluvial channels with constant flow rate and fixed banks show periodic vanishing of a few migrating bars at a time occurring at regular spacing under constant flow rates migrating bars appear as a transition phenomenon of alluvial channels having a cyclic character these observations however might hold only for certain morphodynamics conditions which should be further investigated

late cretaceous cenozoic deformation of northeast asia abstract the plate tectonic paradigm implies rigid plates and narrow plate boundaries in contrast diffuse plate boundaries are common both in the oceans and continents r g gordon annu rev earth planet sci 26 1998 615 642 and their history is difficult to constrain especially in remote tectonically complex areas such as northeast asia m e chapman s c solomon j geophys res 81 1976 921 930 here we show how extensive north atlantic marine magnetic r macnab et al eos 76 1995 449 458 and gravity data d t sandwell w h f smith j geophys res 102 1997 10039 10054 can be used to unravel with tight confidence limits successive periods of deformation over 80 million years along the diffuse continental eurasian north american plate boundary a period of compression in the late cretaceous 14 mm yr in the laptev sea to 20 mm yr in kamchatka led to thrusting in the verkhoyansk mountains and was followed by extension from 68 to 40 ma when 400 km of extension was accommodated by the formation of a series of grabens including the moma rift system since 40 ma time varying compression and transpression along the moma rift system created strike slip faults thrusts and folds at rates up to 6 3 mm yr in the laptev sea region 600 km of extension from latest late cretaceous to present created the laptev sea and lena rift systems the deformation predicted by our model fits most geological features formed in the laptev sea and central northeast asia during late cretaceous cenozoic times the most recent deformation late miocene pliocene is not very well constrained since our model lacks data younger than 11 ma the deformation that occurred in kamchatka reflects a complex tectonic setting and our model s predictions are only tentative

regional and local stress field orientation inferred from quantitative analyses of extension joints case study from southern italy 1 an intense tectonic activity in eastern sicily and southern calabria italy is well documented by the differential uplift of late quaternary coastlines and by the record of the strong historical earthquakes the extensional belt that crosses this area is dominated by a well established wnw ese oriented stretching direction however this area is largely lacking of any structural analysis for defining the tectonics at a more local scale the analysis of systematic extension joint sets affecting pleistocene deposits presented in this paper allows to infer the causative tectonic stress tensor by means of a quantitative inversion technique local perturbations of the first order regional stress field are consequently recognized such perturbations are interpreted as due to interferences between large active faults and their particular geometrical setting these results contribute to the understanding of the quaternary tectonic evolution and the present day stress regime

experiments on bar formation in a straight flume 2 graded sediment laboratory experiments have been carried out in a large laboratory flume using a strongly bimodal sediment mixture in a range of flow conditions such that the initially flat bed of the flume became unstable enhancing the development of alternate bars in order to elucidate the effect of grain sorting the present experiments are compared with a previous set of experiments performed in the same flume under similar hydraulic conditions but using a nearly uniform sand with a mean geometric grain diameter equal to that characterizing present experiments the comparison suggests that sediment heterogeneity may appreciably affect bed form characteristics the development of smallscale ripples and mesoscale dunes sediment waves tended to be inhibited thus allowing a decrease in flow resistance owing to the bimodal character of the adopted mixture at low values of the bed shear stress a condition of partial transport was attained for which only the finer grained portion of the mixture was observed to move while the coarsergrained fractions remained essentially immobile throughout the experimental run however complete mobilization of all size fractions was observed to occur in runs carried out at higher slopes i e at higher bed shear stress in order to generate an alternate bar pattern selective transport of individual grain size fractions coupled with the characteristic bar topography pattern led to an intense longitudinal sorting which accreted the coarser particles on bar crests furthermore bar migration caused through scour and fill a significant vertical sorting as a consequence of these sorting processes and in accordance with previous experimental observations bar height turns out to be invariably damped with respect to uniform sediment experiments the trend exhibited by the wavelengths is less clear and suggests that in the present experiments sorting effects do not enhance the clear shortening of bar wavelengths typically observed in other series of flume experiments carried out with weakly bimodal mixtures

constraints on indian plate motion since 20 ma from dense russian magnetic data implications for indian plate dynamics we use more than 230 000 km of russian marine magnetic and bathymetric data from the carlsberg and northern central indian ridges comprising one of the most geographically extensive dense shipboard surveys anywhere in the ocean basins to describe in detail seafloor spreading since 20 ma along the trailing edge of the indian plate india somalia plate rotations for 1 myr intervals over the past 20 myr are derived from inversions of more than 6600 crossings of 20 magnetic reversals and 1400 crossings of fracture zones that offset these two ridges statistical analysis of the numerous data indicates that outward displacement of reversal boundaries due to finite seafloor emplacement widths and correlated noise for anomaly crossings from individual spreading segments constitute two distinct sources of systematic bias in the locations of magnetic anomaly crossings contrary to the often made assumption that random gaussian distributed noise dominates the error budget seafloor spreading rates slowed gradually by 30 from 20 ma to 10 1 ma about a relatively stationary pole of rotation from 11 ma to 9 ma the rotation axis migrated several angular degrees toward the plate boundary modestly increasing the spreading gradient along the plate boundary india somalia kinematic data for times since 9 ma are consistent with remarkably steady motion with no evidence for a change in either the rotation pole or rate of angular opening within the few percent precision of our data the timing and nature of changes in india somalia motion since 20 ma closely resemble those for the capricorn somalia plate pair indicating that india and capricorn plate motions are strongly coupled we speculate that the slowdown in seafloor spreading at the trailing edges of the indo capricorn composite plate from 20 ma to 10 1 ma resulted from the increasing amount of work that was needed to build topography in the himalayan collisional zone the transition to stable india somalia and capricorn somalia seafloor spreading at 10 9 ma corresponds well with the onset at 8 ma of folding and faulting across an equatorial plate boundary separating the indian and capricorn plates suggesting that the latter may have played a fundamental role in restoring equilibrium between the torques that were driving and resisting the northward motions of the indian and capricorn plates

tectonic stress field in the indian subcontinent a map of maximum horizontal compressive stress orientation in the indian subcontinent has been prepared using orientations derived from three different stress indicators borehole elongation breakouts in situ hydraulic fracturing measurements and earthquake focal mechanisms most part of the subcontinent appears to be characterized by a compressional stress regime thrust and strikeslip faulting imposed by plate boundary forces although shmax orientations do not in general show clear correlation with the direction of motion of the indian plate four provinces are recognized on the basis of regionally consistent orientations these are the midcontinent stress province the southern shield the bengal basin and the assam wedge their boundaries have been determined taking into consideration regional tectonics and seismicity central and northern india including the shillong plateau stretching up to the great himalaya pakistan and nepal are included in a broad midcontinent stress province characterized by nne ene oriented shmax the mean orientation of shmax in this province is n23 e subparallel to the direction of compression expected to arise from the net resistive forces at the himalayan collision zone suggesting that it is largely determined by the tectonic collision processes much of southern india mysore plateau and the high grade metamorphic terrain south of the plateau appears to be part of a second stress province characterized by nw oriented shmax these appear close to those of the intraplate stress field prevailing in the central indian ocean a third stress province was recognized in the bengal basin including parts of west bengal tripura manipur and mizoram in northeastern india and most of bangladesh this province extends eastward from the marginal fault in the western margin of the bengal basin to the indo burma subduction zone and is bounded on the north by the e w striking dauki fault shmax within the sedimentary pile of the bengal basin is oriented in e w direction while p axes of earthquakes within the basement and the crust beneath the basin and within the subducted slab of the indian plate beneath the indo burman ranges generally trend north n30 e shmax orientations within the sedimentary pile of the basin are parallel to the local approximately e w direction of the convergence of indian and burmese plates suggesting a casual relation to the resistive forces at the subduction zone in the indo burma region interestingly the stress field in the basement and the crust beneath the bengal basin and in the subducted slab is similar to the one prevailing in the midcontinent stress province assam wedge stress region occupies the northeastern corner of the indian plate including upper assam arunachal pradesh and much of nagaland this region subducts beneath a sharply bent continental collision boundary consisting of the northeastern limb of the himalayan and northern limb of the indo burman fold belts as a result the stress field in this province is depth differentiated and most likely responsible for the absence of consistent shmax directions

global azimuthal seismic anisotropy and the unique plate motion deformation of australia differences in the thickness of the high velocity lid underlying continents as imaged by seismic tomography have fuelled a long debate on the origin of the roots of continents1 2 3 4 5 some of these differences may be reconciled by observations of radial anisotropy between 250 and 300 km depth with horizontally polarized shear waves travelling faster than vertically polarized ones2 this azimuthally averaged anisotropy could arise from present day deformation at the base of the plate as has been found for shallower depths beneath ocean basins6 such deformation would also produce significant azimuthal variation owing to the preferred alignment of highly anisotropic minerals7 here we report global observations of surface wave azimuthal anisotropy which indicate that only the continental portion of the australian plate displays significant azimuthal anisotropy and strong correlation with present day plate motion in the depth range 175 300 km beneath other continents azimuthal anisotropy is only weakly correlated with plate motion and its depth location is similar to that found beneath oceans we infer that the fast moving australian plate contains the only continental region with a sufficiently large deformation at its base to be transformed into azimuthal anisotropy simple shear leading to anisotropy with a plunging axis of symmetry may explain the smaller azimuthal anisotropy beneath other continents

evidence for long term diffuse deformation of the lithosphere of the equatorial indian ocean the presence of large earthquakes east west striking folds and thrust faults in sediments and east west striking undulations of wavelength 200 km in topography and gravity shows that the equatorial indian ocean is the locus of unusual deformation1 2 3 4 5 6 7 8 this deformation has been interpreted as a diffuse boundary between two tectonic plates9 10 11 12 13 seismic stratigraphy and deep sea drilling at two locations in the bengal fan indicate that the deformation began 7 5 8 0 myr ago3 14 15 here however we show using plate reconructions that motion across this diffuse oceanic plate boundary began more than 10 myr earlier than previously inferred and that the amount of north south convergence across the boundary through the central indian basin has been significantly greater than the convergence estimated from seismic profiles the relative plate velocity accommodated across the central indian basin has varied with time and has been as fast as 6 mm yr 1 about half the separation rate of earth s slowest spreading mid ocean ridge the earliest interval of measurable motion which began more than 18 myr ago may coincide with rapid denudation of the tibetan plateau from 21 myr to 15 17 myr ref 16 the present motion across the central indian basin began no earlier than 11 myr following an earlier interval of slower motion from 18 to 11 myr and may have begun at 8 myr when the tibetan plateau is thought to have attained its maximum elevation16 17

evidence for early 44 ma himalayan crustal thickening tethyan himalaya southeastern tibet abstract comprehensive understanding of himalayan orogenesis is limited in part by a poor knowledge of the eohimalayan episode a phase of tectonic activity predating the better understood neohimalayan crustal thickening associated with coaxial deformation along the main central thrust the cambo ordovician to tertiary metasedimentary sequences outcropping within the tethyan himalaya are the structurally highest units of the himalayan fold and thrust belt they and are inferred to have been the first to have accreted to asia a compilation of data from transects along the length of the main himalayan arc shows that the tethyan sequences have experienced at least five deformation events although the timing of these episodes is poorly constrained emplacement of a series of undeformed granitoid bodies following the second of these events which accounts for the majority of crustal thickening in the tethyan himalayan units is constrained by u pb zircon dating to be older than 44 1 1 2 ma thus significant crustal thickening had occurred along the length of the proto himalayan arc by the mid eocene or within 10 to 20 myr of the initiation of himalayan orogenesis

topography boundary forces and the indo australian intraplate stress field the relative contribution of topographic e g ridge push continental margins and elevated continental crust and plate boundary e g subduction and collisional forces to the intraplate stress field in the indo australian plate iap is evaluated through a finite element analysis two important aspects of the iap intraplate stress field are highlighted in the present study 1 if substantial focusing of the ridge push torque occurs along the collisional boundaries i e himalaya new guinea and new zealand many of the first order features of the observed stress field can be explained without appealing to either subduction or basal drag forces and 2 it is possible to fit the observed shmax maximum horizontal stress orientation and stress regime information with a set of boundary conditions that results in low tectonic stress magnitudes e g tens of megapascals averaged over the thickness of the lithosphere throughout the plate this study therefore presents a plausible alternative to previous studies of the iap intraplate stress field which predicted very large tectonic stress magnitudes hundreds of megapascals in some parts of the plate in addition topographic forces due to continental margins and elevated continental material were found to play an important role in the predicted stress fields of continental india and australia and the inclusion of these forces in the modeling produced a significant improvement in the fit of the predicted intraplate stresses to the available observed stress information in these continental regions a central focus of this study is the relative importance of the boundary conditions used to represent forces acting along the northern plate margin we note that a wide range of boundary conditions can be configured to match the large portion of the observed intraplate stress field and this nonuniqueness continues to make modeling the iap stress field problematic while our study is an important step forward in understanding the sources of the iap intraplate stress field a more complete understanding awaits a better understanding of the relative magnitude of the boundary forces acting along the northern plate margin

a revised identification of the oldest sea floor spreading anomalies between australia and antarctica we propose that magnetic anomalies south of australia and along the conjugate margin of antarctica that were originally identified as anomalies 19 to 22 may be anomalies 20 to 34 the original anomaly identification has two troublesome aspects 1 it does not account for an extra anomaly between anomalies 20 and 21 and 2 it provides no explanation for the rough topography comprising the diamantina zone with our revised identification there is no extra anomaly and the diamantina zone is attributed to a period of very slow spreading 4 5mm yr half rate between 90 and 43 m y the ages bounding the interval of slow spreading 90 and 43 m y correspond to times of global plate reorganizations our revised identification opens up the possibility that part of the magnetic quiet zone south of australia formed during the cretaceous long normal polarity interval breakup of australia and antarctica probably occurred sometime between 110 and 90 m y b p the breakup unconformity identified by falvey in the otway basin may correspond to a eustastic sea level change

fluvial islands first stage of development from nonmigrating forced bars and woody vegetation interactions abstract fluvial islands can develop from the channel bed by interactions between pioneer trees and bars although vegetation recruitment and survival is possible on all bar types it is easier for trees to survive on nonmigrating bars developed from a change in channel geometry or to the presence of a steady perturbation this field study details the first stages of development of a vegetated mid channel nonmigrating or forced bar and its evolution toward an island form over six years analysis of bed topographical changes vegetation density and roughness scour and fill depths sediment grain size and architecture and excess bed shear stress highlighted a specific signature of trees on topography and grain size segregation two depositional processes combining the formation of obstacle marks and upstream shifting deposition of sediments led to the vertical accretion of the vegetated bar during the first stage of the bar accretion bedload sediment supply coming from surrounding channels during floods was identified as a key process modulated by the presence of woody vegetation and a deflection effect induced by the preexisting topography grain size segregation between vegetated and bare areas was also highlighted and interpreted as an important process affecting the development of surrounding channels and the degree of disconnection and hence the speed of development of a growing island the heterogeneity of bedload supply can explain why sediment deposition and density of trees are not strictly related a general conceptual model detailing the first stages of evolution from a bar to an established island is proposed for relatively large lowland rivers

effect of a shallow low viscosity zone on small scale instabilities under the cooling oceanic plates we have studied the role of a low viscosity zone in the uppermost mantle on convective instabilities in the cooling oceanic plates the upper mantle is represented by three layers in a finite element model a conducting lid overlies a low viscosity zone which in turn overlies a layer of constant viscosity extending to the base of the upper mantle at 675 km in depth the viscosity contrast the rayleigh number the relative fluid layer thicknesses and the conducting lid thickness are varied if the rayleigh number of the low viscosity zone exceeds a critical value then convection will be confined to the low viscosity zone for a period which depends on the viscosity contrast and the rayleigh number however the scale of the convection will eventually grow and extend throughout the upper mantle we compare our model to the small scale geoid and topography anomalies observed in the central pacific which are 50 80 cm in geoid and 250 m in topography 10 40 m y after plate formation and whose wavelength 150 500 km increases with age the magnitude early onset time and lifetime of the anomalies suggest a viscosity contrast of greater than 2 orders of magnitude the increase in wavelength with time also suggests a high rayleigh number 106 and the initial 150 250 km wavelength indicates a low viscosity zone thickness of 75 125 km small scale convection does not alter the slope of the depth age curve but does elevate the depth age curve by 250 m it is not until the onset of long wavelength convection that the depth age curve departs from age predicted by a cooling half space these results agree with the depth age behavior observed in the pacific where the depth age curve is elevated in the region of small scale anomalies and departs from age at 70 m y the models also predict heat flow values on old ocean floor which are higher than the plate model 50 55 mw m2 but which are in agreement with heat flow measurements in the atlantic and pacific

numerical modelling of alternate bar formation development and sediment sorting in straight channels a two dimensional shallow water hydro sediment morphodynamic model is applied to investigate alternate bar formation development and sediment sorting in straight channels the model is coupled explicitly incorporating the flow sediment bed interactions by using the full mass and momentum conservation equations which are numerically solved by a well balanced version of the finite volume slope limiter centred slic scheme the model is first tested against a flume experiment on alternate bars formed over a uniform sediment bed which clearly exhibits processes of bar formation migrating and finally approaching an equilibrium state then it is applied to another flume experiment on alternate bars due to non uniform sediment transport the computational results are evaluated with a focus on the longitudinal and vertical sediment sorting it is argued for the first time that the inconsistent sediment sorting patterns observed in previous studies are determined by different sediment transport conditions i e full versus partial transport when a condition of full transport is achieved under which all size fractions are fully mobilized and transported the longitudinal surface sediment shows a sorting pattern of coarse on head and fine in pool and the vertical substrate sediment exhibits an immobile fine coarse structure upwards in contrast for a partial transport condition under which only finer fraction participates in the transport process an opposite longitudinal pattern i e fine on head and coarse in pool and a different vertical structure i e immobile coarse fine are observed concurrently numerical experiments with specified conditions show that the critical aspect ratio for the formation of migrating alternate bars is approximately equal to 12 with the increase of the aspect ratio the bar length grows gradually while the bar height increases rapidly for moderate values of the aspect ratio and then keeps nearly stable the bar celerity however is weakly sensitive to the variation of this ratio copyright 2016 john wiley sons ltd

palaeo altimetry of the late eocene to miocene lunpola basin central tibet the elevation history of the tibetan plateau provides direct insight into the tectonic processes associated with continent continent collisions here we present oxygen isotope based estimates of the palaeo altimetry of late eocene and younger deposits of the lunpola basin in the centre of the plateau which indicate that the surface of tibet has been at an elevation of more than 4 kilometres for at least the past 35 million years we conclude that crustal but not mantle thickening models combined with plate kinematic solutions of india asia convergence are compatible with palaeo elevation estimates across the tibetan plateau

cenozoic plate driving forces past studies of plate driving forces have concluded that the forces due to subducted slabs in the upper mantle and those due to the thickening of the oceanic lithosphere are the principal driving forces we reexamine the balance of driving forces for the present day and extend our analysis through the cenozoic using an analytical torque balance method which accounts for interactions between plates via viscous coupling to the induced mantle flow we use an evolving mantle density heterogeneity field based on the last 200 myr of subduction to drive plate motions an approach which has proven successful in predicting the present day mantle heterogeneity field we find that for plausible upper mantle viscosities the forces due to subducted slabs in the cenozoic and mesozoic account for in excess of 90 of plate driving forces and those due to lithospheric thickening for less than 10

age of initiation of collision between india and asia a review of stratigraphic data abstract the collision of india with asia is perhaps the most profound tectonic event to have occurred in past 100 ma it is responsible for the uplift of the himalayas and tibetan plateau and has been argued to have been responsible for geological geochemical and climatological consequences of global extent yet the age of initiation of this collision remains poorly constrained the literature is replete with estimates that range from the late cretaceous 65 ma to latest eocene

populus nigra l establishment and fluvial landform construction biogeomorphic dynamics within a channelized river populations of the riparian pioneer species populus nigra l which establish on alluvial bars within river channels modulate sediment dynamics and fluvial landforms dense cohorts of p nigra have colonized gravel point bars along the channelized river garonne france during the last 20 years and have enhanced the vertical lateral and longitudinal development of the bars for this period the geomorphic characteristics of two wooded point bars on this laterally stable river are closely linked to the spatial distribution and intensity of establishment and resistance of different cohorts of p nigra furthermore p nigra colonization dynamics were controlled by engineer effects of this same species this relationship is illustrated by a significant correlation between key geomorphic and biological variables measured in situ and characterized with a set of four aerial photographs taken between 2000 and 2010 the development of wooded point bars which are discrete biogeomorphic units over the studied period appear to result from a specific biogeomorphic positive feedback of matter aggregation and vegetation establishment related to sediment trapping and stabilization by pioneer engineer plants we propose a conceptual model of biogeomorphic unit construction for channelized lateral stable rivers we consider the resultant biogeomorphic units as functional from an ecological point of view because p nigra enhances at the cohort scale i its own inherent capacity to resist hydrogeomorphic disturbances and ii its resilience capacity as a result of successful colonization especially downstream of mature poplar stands

india asia collision and the cenozoic slowdown of the indian plate implications for the forces driving plate motions the plate motion of india changed dramatically between 50 and 35 ma with the rate of convergence between india and asia dropping from 15 to 4 cm yr this change is coincident with the onset of the india asia collision and with a rearrangement of plate boundaries in the indian ocean on the basis of a simple model for the forces exerted upon the edges of the plate and the tractions on the base of the plate we perform force balance calculations for the precollision and postcollision configurations we show that the observed euler poles for the indian plate are well explained in terms of their locations and magnitudes if 1 the resistive force induced by mountain building in the himalaya tibet area is 5 6 10 12 n m 2 the net force exerted upon the indian plate by subduction zones is similar in magnitude to the ridge push force 2 5 10 12 n m and 3 basal tractions exert a resisting force that is linearly proportional to the plate velocity in the hot spot reference frame the third point implies an asthenospheric viscosity of 2 5 10 19 pa s assuming a thickness of 100 150 km synthetic euler poles show that crustal thickening in the tibetan plateau was the dominant cause of the cenozoic slowdown of the indian plate

toward a self consistent high resolution absolute plate motion model for the pacific the hot spot hypothesis postulates that linear volcanic trails form as lithospheric plates move relative to stationary or slowly moving plumes given geometry and ages from several trails one can reconstruct absolute plate motions apm that provide valuable information about past and present tectonism paleogeography and volcanism most apm models have been designed by fitting small circles to coeval volcanic chain segments and determining stage rotation poles opening angles and time intervals unlike relative plate motion rpm models such apm models suffer from oversimplicity self inconsistencies inadequate fits to data and lack of rigorous uncertainty estimates in addition they work only for fixed hot spots newer methods are now available that overcome many of these limitations we present a technique that provides high resolution apm models derived from stationary or moving hot spots given prescribed paths the simplest model assumes stationary hot spots and an example of such a model is presented observations of geometry and chronology on the pacific plate appear well explained by this type of model because it is a one plate model it does not discriminate between hot spot drift or true polar wander as explanations for inferred paleolatitudes from the emperor chain whether there was significant relative motion within the hot spots under the pacific plate during the last 70 m y is difficult to quantify given the paucity and geological uncertainty of age determinations evidence in support of plume drift appears limited to the period before the 47 ma hawaii emperor bend and apart from the direct paleolatitude determinations may have been somewhat exaggerated

dating the indian continental subduction and collisional thickening in the northwest himalaya multichronology of the tso morari eclogites multichronometric studies of the low temperature eclogitic tso morari unit ladakh india place timing constraints on the early evolution of the northwest himalayan belt several isotopic systems have been used to date the eclogitization and the exhumation of the tso morari unit lu hf sm nd rb sr and ar ar a ca 55 ma age for the eclogitization has been obtained by lu hf on garnet omphacite and whole rock from mafic eclogite and by sm nd on garnet glaucophane and whole rock from high pressure metapelites these results agree with a previously reported u pb age on allanite and together these ages constrain the subduction of the indian continental margin at the paleocene eocene boundary during exhumation the tso morari rocks underwent thermal relaxation at about 9 3 kbar characterized by partial recrystallization under amphibolite facies conditions ca 47 ma as dated by sm nd on garnet calcic amphibole and whole rock from metabasalt rb sr on phengite apatite and whole rock and ar ar on medium si phengite from metapelites ar ar analyses of biotite and low si muscovite from metapelites which recrystallized at 5 kbar toward the end of the exhumation show that the tso morari unit was at upper crustal levels ca 30 ma these results indicate variable exhumation rates for the tso morari unit beginning with rapid exhumation while the indian margin subduction was still active and later proceeding at a slower pace during the crustal thickening associated with the himalayan collision

experiments on bar formation in a straight flume 1 uniform sediment laboratory experiments have been carried out in a large laboratory flume using a nearly uniform sand and under controlled steady flow conditions such as to ensure the development of alternate bars on a ripple and or dune covered bed the interaction between small scale and large scale bed forms is found to enhance a modulation in time and in space of the bar pattern moreover the flow depth decrease experienced by the flow field when approaching bar fronts leading to a progressive reduction of the dimensions of small scale bed forms may influence appreciably the overall flow resistance experimental values of bar wavelength and of bar celerity are compared with the theoretical estimates obtained from a model developed within the classical framework of linear stability analysis in particular the model accounts for the local variability of friction coefficient and water level for the secondary helical flow effects and for the influence of longitudinal slope on sediment transport quantitatively satisfactory predictions of bar wavelength appear to be possible in spite of the fact that in various runs a correct estimation of flow resistance and flow discharge appears to be relatively difficult

alternate bars in a sandy gravel bed river generation migration and interactions with superimposed dunes a field study was carried out to investigate the development of alternate bars in a secondary channel of the loire river france as a function of discharge variations we combined frequent bathymetric surveys scour chains and stratigraphical analysis of deposits with measurements and modelling of flow dynamics the channel exhibited migrating bars non migrating bars and superimposed dunes possible mechanisms of bar initiation were found to be chutes associated with changes of bank direction and instability resulting from interactions between existing bars during the fall in water level after floods we propose that the reworking of bar sediments during low flows high width to depth ratio reinforced by high values of the shields mobility parameter can explain the formation or re generation of new alternate migrating bars during a subsequent flood the migration pattern of the bars was found to be cyclic and to depend mainly on i channel layout and ii the dynamics of superimposed dunes with heights and lengths depending on location and discharge value for instance the hysteresis affecting the steepness of dunes influences the flow resistance of the dunes as well as the celerity of migrating bars during flood events we compare the findings from the field with results from theoretical studies on alternate bars this gives insight in the phenomena occurring in the complex setting of real rivers but it also sheds light on the extent to which bar theories based on idealized cases can predict those phenomena copyright 2014 john wiley sons ltd

effect of orogeny plate motion and land sea distribution on eurasian climate change over the past 30 million years the eurasian climates of today 10 million and 3o million years ago are simulated using an atmospheric general circulation model that incorporates realistic continental geography and epicontinental sea distributions the resulting climates compare well with various palaeoclimate records the retreat of the paratethys an epicontinental sea shifts the central asian climate from temperate to continental conditions and plays as important a role as uplift of the himalayan tibetan plateau in driving the asian monsoon changes

late quaternary neotectonic joints confirmation of the connection between jointing and contemporary tectonic stress 1 identification of neotectonic joints has been on the basis of correlating the orientations of vertical joints to the horizontal maximum compressive stress direction as determined from geophysical measurements to date no study has explicitly linked neotectonic joint azimuths in late quaternary sediments to maximum horizontal stress trajectories as determined geophysically late quaternary coastal aeolianites from southeastern coastal saurashtra western india host neotectonic joints the morphology of the neotectonic joints matches earlier contentions regarding their vertical orientations the joints are planar irregular show step overs between interacting joints and refraction while traversing across palaeosols aeolianite boundaries comparisons with geophysically determined maximum horizontal stress directions show that these neotectonic joints propagated in to the same direction as the sh of the contemporary tectonic stress field confirming the utility of neotectonic joints as inexpensive tools for measuring maximum horizontal stress however multiple joint sets suggest that even the neotectonic stress field can be complex