response of the strickland and fly river confluence to postglacial sea level rise 1 the sand bed portion of the fly river system in papua new guinea was influenced significantly by postglacial sea level rise but this response differed along the three main low gradient branches of the system the main stem lower fly river and its two tributary reaches the middle fly river and the strickland river a numerical model for valley development over 1000 to 10 000 year timescales is presented that can account for the coupled morphodynamic evolution of the main stem and its two tributaries it is shown that the underlying theory includes both advective and diffusive terms for morphodynamic evolution in contrast to other purely diffusive models for valley development the advective terms arise due to the inclusion of backwater in the theory the model is applied to the fly river system over the 20 000 years since glacial low stand of sea level results imply that the relatively low sediment supply to the middle fly river compared with that to the strickland probably prevented the middle fly from keeping pace with aggradation along the downstream lower fly river the middle fly may still be responding to this forcing it appears unlikely that the middle fly river was incised more than 10 m prior to the onset of base level rise this implies that the low stand lower fly river passed through a zone characterized by channel slopes much steeper than the present slope somewhere between the junction with the strickland river and the low stand delta

gravitational instabilities triggered by fluid overpressure and downslope incision insights from analytical and analogue modelling fluid overpressure at the base of low permeability strata reduces effective stress allowing for gravitational sliding of the overlying cover the force driving sliding is the slope parallel component of the weight of the cover whereas the resisting forces are the friction at the base of the cover and the buttressing resistance to shortening which can be critically reduced by incision at the base of the slope we developed an analytical model and undertook a series of analogue experiments to better understand the evolution of a sedimentary cover sliding above a low permeability layer subjected to fluid overpressure where a downslope buttress was present the sliding sheet length decreased with increasing pore fluid pressure in the absence of such buttress the slide s length increased exponentially with increasing pore fluid pressure another important difference dealt with geometry and kinematics buttressed slides consisted of one large slope parallel mass rigidly translated and bounded by downslope thrusts and upslope normal faults with increasing pore fluid pressure the contractional structures propagated upslope by contrast non buttressed slides showed intense strain deformation started with normal faults forming near the incision then propagating upslope throughout the slide s evolution

effects of lateral friction on the structural evolution of fold and thrust belts insights from sandbox experiments with implications for the origin of landward vergent thrust wedges in cascadia various factors are inferred to influence the structural evolution of fold and thrust belts or thrust wedges but the effects of lateral friction such as that exerted by the opposite block of a strike slip fault in nature or by sidewalls in analogue models have rarely been given sufficient attention and hence are still far from being fully understood up to now seaward vergent and dually vergent thrust wedges have been satisfactorily explained by the classic coulomb wedge theory and can be well reproduced in analogue and numerical models in contrast landward vergent thrust wedges which are uncommon in nature e g those occurring in the cascadia subduction zone have not yet been adequately explained by the classic coulomb wedge theory and can only be reproduced in a few analogue models under particular conditions in order to look closer at the effects of lateral friction on the structural evolution of fold and thrust belts and to explore the origin of landward vergent thrust wedges we conducted a series of sandbox models under different conditions of lateral friction and various ratios of initial average lateral frictional stress to basal shear stress s d results of our models revealed that in the presence of a weak or ductile basal decollement the direction of lateral friction can dictate not only the deformation propagation direction and the surface slope direction but also the structural vergence of fold and thrust belts fold and thrust belts normally show a structural vergence in the same direction as the direction of lateral friction but a deformation propagation direction and surface slope direction opposite to the direction of lateral friction according to the results of models and the relationships between structural vergence and lateral friction direction occurring both in cascadia and the analogue models we propose that an extremely high s d ratio due to an extremely weak basal decollement and landward directed lateral friction exerted by the opposite blocks of basement involved strike slip faults are probably the two key factors responsible for the origin of landward vergent thrust wedges in cascadia this work illustrates that effects of lateral friction should be properly taken into account both in the kinematic analysis and designs of models to simulate fold and thrust belts in the presence of weak or ductile basal decollements

double evaporitic decollements influence of pinch out overlapping in experimental thrust wedges abstract this article focuses on how deformation and displacements are transferred between two decollements located at different stratigraphic levels by means of analogue modeling using brittle viscous sand silicone systems we present results from ten analogue models in which we varied key parameters such as the amount of horizontal offset or overlap between the pinch outs of the upper and lower decollements the total amount of shortening and the planform geometry of the upper decollement results indicate that i structures root onto the basal and upper decollement defining an inner and an outer domain and ii the offset overlap of the decollements controls the geometry of the transition zone located between the two decollements the propagation of deformation into the foreland both in space and time and the deformation style and kinematics in the different domains of the model when the pinch out of the upper decollement is at an angle with the backstop oblique structures form and the geometry and propagation mode of the structures change progressively along strike we compare our experimental results with other silicone sand analogue models and with the natural examples of the southern pyrenees where upper triassic and eocene oligocene syn tectonic evaporites acted as basal and upper decollements respectively

effects of bed discordance on flow dynamics at open channel confluences although a discordance in bed elevation between mainstream and tributary channels is a common feature of most river confluences this height differential is largely ignored in many models of junction flow dynamics here we present new laboratory flume data that demonstrate the significant influence of bed discordance on flow at an acute angled confluence a difference in bed height between mainstream and tributary channels is shown to obliterate flow deflection at the bed and create a distortion of the mixing layer between the flows resulting in fluid upwelling at the downstream junction corner this upwelling is responsible for the absence of a flow separation zone near the bed at the downstream junction corner and the lack of a zone of marked flow acceleration in the postconfluence channel these experiments reveal complex flow fields generated by bed discordance and suggest the need for major revisions to current models of river channel confluence flow dynamics and their application to sediment routing at these sites

bed morphology and sedimentology at the confluence of unequal depth channels abstract spatial and temporal variations in the bed geometry and bed material size of a sand bed river confluence with unequal depth channels were monitored during a sequence of floods additionally two other sand bed confluences were surveyed to test the replicability of these observations the confluences studied here have only one avalanche face which corresponds to the front of a tributary mouth bar in the shallower channel and do not exhibit a marked scour zone three distinct morpho sedimentological zones are present 1 an area around the upstream corner of the junction where the sediments are generally finer than the mean 2 a maximum depth zone with coarser than average particles and 3 a bar at the downstream junction corner where grain size is finer than the mean and decreases slightly downstream changes in relative discharge between the two channels had little effect on the grain size of the downstream junction corner bar but exerted a strong influence on the position of the maximum depth zone and the front of the tributary mouth bar the downstream junction corner bar showed little evidence of the separation zone which is commonly observed at the confluences of laboratory channels the contrasting depths of the approach channels at the sites studied here may be partly responsible for the absence of the separation zone

abrupt holocene climate transitions in the northern north atlantic region recorded by synchronized lacustrine records in iceland abstract two high sediment accumulation rate icelandic lakes the glacial lake hvitarvatn and the non glacial lake haukadalsvatn contain numerous tephra layers of known age which together with high resolution paleomagnetic secular variations allow synchronization with a well dated marine core from the shelf north of iceland a composite standardized climate record from the two lakes provides a single time series that efficiently integrates multi proxy data that reflect the evolution of summer temperatures through the holocene the first order trends in biogenic silica bsi 13c and c n rise relatively abruptly following deglaciation reaching maximum values shortly after 8 ka following a complex minimum between 8 7 and 8 0 ka the holocene thermal maximum htm in the lakes is marked by all proxies with a sharp transition out of the 8 ka cold event into peak summer warmth by 7 9 ka and continuing warm with some fluctuations until 5 5 ka decreasing summer insolation after the htm is reflected by incremental cooling initially 5 5 ka with subsequent cold perturbations recorded by all proxies 4 3 to 4 0 ka and 3 1 to 2 8 ka the strongest disturbance occurred after 2 ka with initial summer cooling occurring between 1 4 and 1 0 ka followed by a more severe drop in summer temperatures after 0 7 ka culminating between 0 5 and 0 2 ka following each late holocene cold departure bsi re equilibrated at a lower value independent of the sediment accumulation rate some of the abrupt shifts may be related to icelandic volcanism influencing catchment stability but the lack of a full recovery to pre existing values after the perturbation suggests increased periglacial activity decreased vegetation cover and glacier growth in the highlands of iceland the similarity in timing direction and magnitude of our multi proxy records from glacial and non glacial lakes and from the adjacent marine shelf suggests that our composite record reflects large scale shifts in ocean atmosphere circulation throughout the northern north atlantic

numerical simulation of three dimensional time averaged flow structure at river channel confluences current confluence research emphasizes three broad controls on flow structure generation 1 planform curvature 2 topographic steering and 3 anisotropic turbulence associated with flow separation and shear layer dynamics the relative importance of these processes in explaining observed flow structures is controversial a situation that may be related to the fact that different investigators have examined different confluence configurations this paper uses a three dimensional numerical model with a fully elliptic solution a free surface treatment and a turbulence model based on a renormalized group rng to help to provide a physically based explanation of the controls upon flow structure generation for both a laboratory rectangular and a field confluence the confluence of the kaskaskia river and copper slough and to identify the particular conditions under which particular flow structures are observed results suggest that an analogy with back to back meanders is possible for symmetrical configurations but that there will be progressive divergence from this state as confluence asymmetry increases in asymmetric situations a dual cell structure may be limited to the immediate vicinity of the junction because of the effects of streamline curvature and topographic steering these differences can be explained by consideration of the dynamic pressure field which may be specific to each confluence configuration as such this study partially reconciles differing views over what controls time averaged flow structures in river channel confluences although further research into the interaction of these processes with instantaneous velocity fluctuations is required

on playfair s law of accordant junctions geomorphologists since playfair have apparently accepted that fluvial confluences will be morphologically accordant in distinction to the hanging junctions of glaciated troughs a view reinforced by gilbert s law of declivities the issue has been largely ignored despite field instances of discordant fluvial junctions and widespread engineering practice which favours discordant confluences in non erodible materials six categories of fluvial junctions are defined accordant hanging discordant barred impeded disrupted and disturbed the study focuses principally on the first two classes and defines discordance purely morphologically in terms of relative depth of incision of the confluent thalwegs likely controls of accordance discordance are identified as flow magnitudes synchroneity of flow regimes channel boundary materials and debris calibre field investigations in malawi france west malaysia and new zealand in small 100 km2 basins on a variety of rock types are described barred junctions are relatively rare but complex in character a high proportion of confluences are distinctly discordant and there is a general continuum from accordant cases tectonic control is improbable and the principal cause identified is the degree of difference in channel forming discharges estimated by stream magnitude and channel area some evidence for lithologic control is also apparent it appears that many confluences behave as if the tributary junction is non erodible and not in the fashion predicted by the law of declivities it is argued that the observed relation is a necessary result of the general down stream changes in hydraulic geometry limited comparison with glacial trough confluences reveals a difference in degree but not in nature although the relationship between discordance and channel size is affected by cross section geometry a 10 fold difference in the cross sectional areas of channels meeting at a junction produces on average a 2 4 fold difference in mean channel depths for fluvial cases and a 4 fold difference for glacial troughs the role of varying flow regimes seems to require attention nevertheless it is maintained that playfair s law is a misleading description of the morphology of fluvial junctions which behaves fundamentally in the same fashion as glacial troughs

use of phosphorus mapping in assessing coastal activity zones of an icelandic multi period site of vatnsfjorður this paper presents the results of phosphorus mapping conducted on a number of coastal activity zones on the multi period archaeological farm site of vatnsfjor ur northwest iceland the aim of the study was to detect the exact levels and extents of shorelines contemporary with the archaeological site s activities and to use sea level change to establish a relative chronology of coastal activity zones absolute dating of the coastal zones and sea level changes was achieved by integrating an existing sea level curve with a novel tephrochronology based curve created for the purpose of this research results were projected onto a detailed digital terrain model of the area in order to reconstruct the extent of the coastline contemporary with human activity in the respective zones a significant component of the research was an attempt to develop the existing approach to phosphorus mapping results interpretation this has resulted in an improved methodology that can be applied to the dynamic and challenging environments of coastal sites worldwide

causes of rapid mixing at a junction of two large rivers rio parana and rio paraguay argentina 1 airborne and satellite observations show that when large rivers join they can take hundreds of kilometers to mix completely but on occasion may mix very rapidly application of established semitheoretical analyses shows that long mixing lengths should be expected the first measurements of mixing processes at a large river junction rio parana and rio paraguay argentina combined width 2 8 km are presented at two occasions first when they mix in 400 km and second when mixing is complete in only 8 km downstream of the junction for the case of slower mixing at a point surveys showed that mixing driven by turbulent shear associated with a near vertical shear layer was restricted to close to the junction to 0 272 multiples of the postconfluence width downstream transect surveys showed penetration of more turbid water from the rio paraguay underneath the rio parana but this was insufficient to promote more rapid mixing there was no clear channel scale circulation present and slow mixing was compounded by reverse topographic forcing on the mainstream rio parana side of the river this kept more turbid water on the rio paraguay side of the river close to the bed in the case of rapid mixing we found clear channel scale circulation the momentum ratio between the combining flows reinforced the effects of the discordance in bed height between the tributaries at the confluence and allowed penetration of more turbid rio paraguay water further across the channel width deeper within the flow the importance of the interaction between momentum ratio and bed morphology at channel junctions makes mixing rates at the confluence dependent upon basin scale hydrological response which is more likely to differ between large confluent rivers than small rivers as a result of the different climatic topographic zones that they may capture

stratification of parent material in european volcanic and related soils studied by laser diffraction grain sizing and chemical analysis abstract sedimentary stratification identified by field descriptions of 12 soils derived wholly or partly from volcanic materials was compared with results obtained by chemical analyses contents and ratios of lithogenic elements contents of heavy metals and detailed grain size data after removal of organic matter and amorphous fractions the finest sedimentary stratification was indicated by cluster analysis of ratios of lithogenic elements and by the grain size analyses sedimentary stratification in the field is difficult to determine or describe when 1 the deposits are strongly weathered 2 soil horizon boundaries coincide with stratification boundaries or 3 stratification is very fine nevertheless field descriptions are a reasonably good way of differentiating parent materials though they often underestimate the number of sedimentary layers

prediction of thrusting sequences in accretionary wedges 1 the main objective is to determine the three stages of the life of a thrust in an accretionary wedge which are the onset of thrusting along its ramp the development with the construction of the relief and the arrest because of the onset of another thrusting event a simple kinematics is proposed for the geometry of the developing thrust fold based on rigid regions separated by velocity discontinuities along which work is dissipated according to the coulomb criterion the evolution of the thrust fold satisfies mechanical equilibrium and is optimized at every time of the three stages to provide the least upper bound in tectonic force according to the maximum strength theorem the development of the thrust or its arrest because of the initiation of another thrust is decided by selecting the event which leads to the least upper bound in tectonic force the approach is first validated by proving that the critical slope angle c for the classical triangular wedge is properly captured it is shown that a perturbation in the form of an extra relief in this perfectly triangular geometry leads to the onset of thrusting with the ramp or the back thrust outcropping either at the back or to the front of the perturbation respectively for a range of slope dip close to the critical angle c the study of normal thrust sequences from the rear to the front in the wedge toe reveals that weakening of the ramp accounted for by changing its friction angle from an initial to a smaller final value is necessary for each thrust to have a finite life span this life span is longer with a larger relief buildup for more pronounced weakening decreasing the decollement friction angle results in an increase in the number of thrusts in the sequence each thrust creating milder relief the normal sequence is ended with the first out of sequence thrust which occurs earlier for smaller weakening over the ramp the proposed methodology is partly used to construct an inverse method proposed to assess the likeliness for the transfer of activity from the active to the incipient thrust in a section of nankai s accretionary wedge the inverse method provides the initial friction angle over the incipient ramp and the final friction angle over the fully active ramp from the geometry of the corresponding thrusts and the topography it is shown that the friction angle over the incipient ramp is most likely to be larger then the one over the active ramp justifying a key hypothesis needed to predict discrete sequences of thrusting

investigation of controls on secondary circulation in a simple confluence geometry using a three dimensional numerical model recent research into river channel confluences has identified confluence geometry and particularly bed discordance as a control on confluence flow structures and mixing processes and this has been illustrated using both field measurements in natural confluences and laboratory measurements of simplified confluences generalization of the results obtained from these experiments is limited by the number of confluence geometries that can be examined in a reasonable amount of time this limitation may be overcome by numerical models in which confluence geometry is more readily varied and data acquired more rapidly this paper aims to i validate the application of a three dimensional numerical model to a simple confluence geometry ii simulate the effects of different boundary condition values upon flow structures and iii interpret the implications of these simulations for river channel confluence dynamics the model used in this research solves the three dimensional form of the navier stokes equations and is used to simulate the flow in a parallel confluence of unequal depth channels and to investigate the effect of different combinations of velocity and depth ratio between the two tributaries the results generally agree with empirical evidence that secondary circulation is generated in the absence of streamline curvature but only for specific combinations of depth and velocity ratio this research shows how understanding of the interaction of these controls is enhanced if pressure gradients are considered the velocity ratio is the prime determinant of the cross stream pressure gradient that initiates cross stream velocities however for significant secondary circulation to form cross stream velocities must lead to significant transfer of fluid in the cross stream direction this depends on the vertical extent of the cross stream pressure gradient which is controlled by the depth ratio in this study strong secondary circulation occurred for a depth differential of 25 or more as long as the velocity in the shallower tributary was at least as great as that in the deeper channel this provides an important context for interpretation of previous work and for the design of new experiments in both the field and the laboratory 1998 john wiley sons ltd

the occurrence of obtuse junction angles and changes in channel width below tributaries along the mekong river south east asia classic descriptions of drainage patterns suggest that confluence angle is determined by the shape of the drainage basin unless constraining factors such as the geological structure affect stream flow downstream changes in channel width below tributary junctions have long been associated with tributary inputs of flow and sediment analysis of tributary junction geometry and channel width changes in large rivers and over large reaches is sparse the lower mekong basin exhibits a generally dendritic drainage network despite flowing through a diverse array of geological settings publicly available spot 5 imagery from google earth was used to identify and catalogue junction geometries and downstream changes in channel width below tributary junctions along a 2200 km reach of the mekong river of the 284 junctions identified the majority 66 2 were acute however 12 4 8 were found to be normal 90 and 75 30 were found to be obtuse this latter number is in contrast to previous studies over similar spatial scales which found little evidence of obtuse junctions meander extension of the incoming tributary and deflection of the tributary across bedrock shoulders were found to be the dominant geomorphological causes of obtuse tributary junctions the relationship between the width of the tributary channels and the width of the mainstem upstream and downstream of the confluences was analysed it was observed that over the whole reach a slight narrowing occurred immediately below tributary junctions although the changes themselves were small the slight net narrowing is shown to be statistically significant the observed relationship is shown to vary considerably with geology the geological control suggests that complex factors play important roles in determining changes to channel width across large systems and that simple cause effect relationships do not hold in such complicated geological settings

erosional effects on terrestrial resources over the last millennium in reykjanes southwest iceland abstract the study presents the effect of soil erosion on vegetation soil accumulation sa sa rate sar soil quality soil mass and the soil organic carbon soc pool in brown andosols and histosols in a 24 km 2 area in southwest iceland undisturbed prehistoric soils were distinguished from disturbed historic soils using tephrochronology soil erosion has been severe during historic time last 1135 yr resulting in the increase of the soil mass deposited in soils covered by vegetation by a factor of 7 3 9 2 and net loss of soil in unvegetated areas the sar correlated positively with soc sequestration soc is easily transported and given the extensive accumulation of soil the net effect of burial and subsequent reduction in decomposition is to increase soc storage nevertheless the increased accumulation and soil depletion has decreased soil quality including the soc and reduced soil resistance to erosion with the depleted soc contributing to enrichment of atmospheric co 2 the initial terrestrial disturbance was triggered by anthropogenic land use during the medieval warm period followed by volcanic activity approximately three centuries later the combination of harsh climate during the little ice age and drastic anthropogenic perturbations has led to land degradation at a catastrophic scale

flow and sediment dynamics in channel confluences confluences with relatively low discharge and momentum flux ratios where a small steep tributary with a high supply of poorly sorted sediment joins a large low gradient main channel commonly occur in nature but they have not yet been investigated measurements of the three dimensional velocity field turbulence sediment transport bed material grain size and morphology are reported in a laboratory setting that is representative of confluences on the upper rhone river switzerland the difference between the low flow depth in the steep tributary and the higher flow depth in the main channel creates a marked bed discordance in the tributary zone due to this bed discordance the tributary flow penetrates into the main channel mainly in the upper part of the water column whereas the main channel flow is hardly hindered by the tributary in the lower part of the water column giving rise to a two layer flow structure in the confluence zone in confluences with high supply of coarse sediment from the tributary the development of a deposition bar downstream from the confluence reduces the flow area and causes flow acceleration that contributes to an increase in sediment transport capacity the sediment supplied by the tributary is mainly sorted and transported on the face of the bar by the near bed flow originating from the main channel the sediment transport capacity is further increased by the three dimensionality of the flow which is characterized by maximum velocities occurring near the bed and by a considerable increase in turbulent kinetic energy generated in the shear layer at the interface of the flows originating from the main channel and the tributary a conceptual model is proposed for the hydro morpho sedimentary processes and compared to existing conceptual models for confluences with different characteristics

mixing layer distortion at the confluence of channels of different depth channel confluences have received considerable recent attention in the fields of geomorphology1 3 sedimentology4 9 and hydraulic engineering10 13 where channels join rapid changes in fluid velocity turbulence intensity channel hydraulic geometry and bed geometry may occur14 17 previous models of junction dynamics have tended to assume that the confluent channels are of equal depth10 18 a condition that may be found only rarely in natural junctions more often the depths of confluent channels are different19 this discordance is commonly the result of the forma tion at the mouth of each confluent channel of bars that possess steep avalanche faces which dip into a central junction scour1 2 5 20 here we suggest that when channels of different depth join the mixing layer that forms at their confluence is distorted by interacting with a separation zone which forms in the lee of the mouth of the shallower channel this mixing layer distortion imparts a complex three dimensional flow to the junction promoting vertical fluid upwelling and significantly enhancing rapid mixing within the flow of the shallow tributary while retarding the rate of mixing across the flow of the deeper channel suspended sediments or contaminants within the deeper channel may be transferred rapidly across the flow of the shallower tributary upwelling downstream of the junction

geochemistry of historical age silicic tephras in iceland the major element chemistry of nine silicic tephras of historical age from iceland is assessed as a key step in the development of the recent tephrochronology of the north atlantic region the tephras include the largest such layers produced by each of the five central volcanoes hekla o raefajokull eyjafjallajokull torfajokull and askja since the ninth century ad h 1104 o1362 e 1821 landnam tephra c 870 a 1875 and four other tephras h 1158 h 1510 h 1947 o1727 the determination of grain discrete major element chemistry of the glass fraction is a fundamental stage in the identification and correlation of tephra and allows links to be made between icelandic source areas with precise dating evidence and distal deposits elsewhere in the north atlantic region although major element data can be used to discriminate between tephra layers produced by the different central volcanoes on its own it cannot be used to identify all the holocene layers produc

experimental modelling of orogenic wedges a review abstract experimental modelling applied to the study of orogenic wedge dynamics has been a subject of fruitful research for more than 30 years although the technique dates back as far as the early xix th century on one hand several first order parameters controlling the structural evolution of mountain belts have been intensively investigated using the classic tectonic sandbox models the main parameters are the properties of the basal decollement the deforming material the backstop and fluxes kinematics and surface processes on the other hand the morphological evolution of a mountain relief subjected to changing tectonic or climatic forcing has been addressed using another kind of approach called geomorphic models nowadays the literature is extremely rich particularly for the sandbox technique so that it becomes difficult to have an exhaustive view of the effects of the above parameters on mountain evolution in this article we propose a detailed review of the main results obtained using both tectonic and geomorphic approaches our goal is to provide an almost complete state of the art in the experimental study of relief dynamics to guide present and future researchers in their understanding of mountain belt evolution

experimental insights on the geometry and kinematics of fold and thrust belts above weak viscous evaporitic decollement abstract this paper discusses the differences in mechanical properties and kinematics between fold and thrust belts detaching on evaporitic decollements and those detaching on stronger detachments physical experiments are described that model shortening of a thick brittle cover overlying a weak viscous decollement to gain a better understanding of these differences we tested the influence of 1 the decollement layer thickness and 2 the presence of a deformable backstop on the hinterland side and of a decollement pinch out on the foreland side because of the very low shear strength of the viscous decollement folds and thrusts did not propagate according to a piggy back sequence but by centripetal back and forth propagation additional shortening was accommodated by growth of all existing structures fold symmetry and thrust vergence varied between experiments models confined between two rigid vertical endwalls always deformed by symmetric folding and thrusting in models having a deformable backstop and a foreland decollement pinch out forethrusting initially dominated folds were asymmetric and fault blocks rotated in models having a thick decollement layer folds kept growing asymmetrically diapirism also depended on the initial decollement geometry diapirs formed only in models having a deformable backstop and were restricted to the hanging wall of a fault related fold finally the implications of such kinematics on the thermal history are discussed as well as the likelihood of hydrocarbon maturation and preservation

reconstructing late holocene environmental change in iceland using high resolution tephrochronology high resolution records of sediment accumulation are necessary to evaluate subtle temporal and spatial variations in sediment flux especially in the context of decadal scale human environment inte

hydrodynamics of mountain river confluences and its relationship to sediment transport mountain confluences are characterized by narrow and steep tributaries that provide important sediment load to the confluence whereas themain channel supplies the dominant flow discharge this results in a marked bed discordance between the tributary and main channel which induces key differences in the hydromorphodynamics of the confluence when compared to concordant bed and some common types of discordant bed confluences the processes of initiation and maintenance of the morphology of confluences are still unknown and research linking morphodynamics and hydrodynamics of river confluences is required to understand these phenomena in this paper eddy resolving simulations based on laboratory experiments made in a live bed model of a mountain river confluence are used to provide a detailed description of flow hydrodynamics and implications for morphodynamics the test case study corresponds to a confluence with an angle of 70 numerical simulations are performed for two extreme bathymetric conditions those at the start of the experiment and when equilibriummorphological conditions are reached results of the simulations and experimental observations are used tomake inferences on erosion mechanisms during the initial and final states of the erosion deposition process the relationship between the coherent structures present in the near bed region in the instantaneous and mean flow fields and sediment entrainment transport is described the present paper demonstrates the critical role played by large scale turbulence generated in the shear layers forming on the side of the tributary flow as it enters the main channel by the main vortex forming over the discordant bed region surrounding the downstream end of the tributary as well as by several near bed vortices induced by the deflection of the tributary flow by the incoming flow in the main channel the predicted patterns of bed shear stress are linked to pathways of sediment movement

imaging skeletal remains with ground penetrating radar comparative results over two graves from viking age and medieval churchyards on the stora seyla farm northern iceland detailed ground penetrating radar surveys were conducted at separate viking age and medieval churchyards on the stora seyla farm in skagafjor ur northern iceland surveying over a previously unknown site ca ad 1000 that is located just a few meters above the skagafjor ur valley bottom delineated the remnants of a buried circular turf wall that encloses a church structure and several graves the radar profiles over the graves contain strong hyperbolic reflections that emanated from the skeletal remains over one of the graves an air filled void within the chest cavity had been detected as noted by reflections with normal polarity which indicated a boundary towards increasing microwave velocity during excavation the soil surrounding an intact rib cage collapsed thus confirming the presence of the void in general the skeletal remains were very well preserved and yielded strong reflections which permitted the orientation of the body to be determined conversely the radar profiles over a grave from a more recent churchyard ca ad 1200 show ground disturbance but lack hyperbolic reflections upon excavation only teeth were recovered the poor preservation of the skeletal remains is attributed to increase contact with infiltrating groundwater from an overlying gravel layer interpretations were aided by time slice overlay imaging forward modeling and analysis of the reflection coefficient

impact of an interbedded viscous decollement on the structural and kinematic coupling in fold and thrust belts insights from analogue modeling abstract fold and thrust belts ftbs can be segmented both across and along strike because of various factors including tectonic and stratigraphic inheritance in this study we investigated along across strike structural interactions in a ftb propagating toward a foreland which displays contrasted lithological sequences a set of analogue models was performed in a compressional box where a single viscous level of varying width was interbedded within a frictional series the tectonic interaction between the viscous and the frictional provinces was tested both along and across strike results indicate that a frictional province influences the along strike tectonic evolution of an adjacent viscous province this influence decreases when the width of the viscous province increases the frictional provinces control the taper structural style obliquity of the structures trend and kinematics of the shallow deformation front of the viscous province results evidence how far a frictional province can impact the deformation of an adjacent viscous province for frictional viscous wedges it appears that the critical taper theory which is generally applied in 2 d should be likely considered in terms of 3 d moreover the kinematics of the deep deformation front shows mutual influences between the adjacent viscous and frictional provinces experimental results are compared to natural examples in the kuqa basin southern tian shan china and the salt range pakistan and give an insight to a better understanding of the dynamics of fold and thrust belts bearing a viscous decollement such as salt

experimental insights on the geometry and kinematics of fold and thrust belts above weak viscous evaporitic decollement reply to comments by hemin koyi and james cotton

clay minerals of four soils formed in eolian and tephra materials in iceland abstract clay minerals in a vitricryand and three haplocryands derived from eolian and tephra materials in different parts of iceland and in glacial till samples representative of underlying strata were studied by a combination of methods the soils are primarily used as rangelands and are highly susceptible to wind erosion allophanes imogolite and poorly crystalline ferrihydrite are abundant 700 g kg of the clay throughout the soils including a horizons this clay mineralogy would give rise to stable low density aggregates that influence the susceptibility of the soils to wind erosion it also affects chemical properties such as high phosphate retention of the soils even of the cryands with relatively low clay contents 160 275 g kg imogolite was found in some but not all horizons of all soils no particular relationship of imogolite to measured si al molar ratios of the oxalate oxalic acid soluble fractions of the clays 0 51 to 1 13 was evident layer silicate minerals and laminar opaline silicas are either not present or occur in small quantities plagioclase augite and or their weathering residues were found in the clay and silt separates dark colored glass is predominant in the fine sand throughout the soils and the tills one till sample had a clay mineral composition similar to the eolian andic soils while the other contained smectite and possibly a spherical halloysite smectite intermediate but not allophane and imogolite as major weathering products the clay mineral composition of these soils and tills was compared with those derived from basic tephras in other regions

iron and silicon isotope behaviour accompanying weathering in icelandic soils and the implications for iron export from peatlands abstract incipient warming of peatlands at high latitudes is expected to modify soil drainage and hence the redox conditions which has implications for fe export from soils this study uses fe isotopes to assess the processes controlling fe export in a range of icelandic soils including peat soils derived from the same parent basalt where fe isotope variations principally reflect differences in weathering and drainage in poorly weathered well drained soils non peat soils the limited fe isotope fractionation in soil solutions relative to the bulk soil 57fesolution soil 0 11 0 12 is attributed to proton promoted mineral dissolution in the more weathered poorly drained soils peat soils the soil solutions are usually lighter than the bulk soil 57fesolution soil 0 41 0 32 which indicates that fe has been mobilised by reductive mineral dissolution and or ligand controlled dissolution the results highlight the presence of fe organic complexes in solution in anoxic conditions an additional constraint on soil weathering is provided by si isotopes the si isotope composition of the soil solutions relative to the soil 30sisolution soil 0 92 0 26 generally reflects the incorporation of light si isotopes in secondary aluminosilicates under anoxic conditions in peat soils the largest si isotope fractionation in soil solutions relative to the bulk soil is observed 30sisolution soil 1 63 0 40 and attributed to the cumulative contribution of secondary clay minerals and amorphous silica precipitation si supersaturation in solution with respect to amorphous silica is reached upon freezing when al availability to form aluminosilicates is limited by the affinity of al for metal organic complexes therefore the precipitation of amorphous silica in peat soils indirectly supports the formation of metal organic complexes in poorly drained soils these observations highlight that in a scenario of decreasing soil drainage with warming high latitude peatlands fe export from soils as fe organic complexes will increase which in turn has implications for fe transport in rivers and ultimately the delivery of fe to the oceans

volcanic soils of iceland abstract the parent materials of all icelandic soils are volcanic in origin and most of the soils are classified as andosols iceland has the largest area of andosols in europe and may have 5 of all andosols in the world a new classification scheme based on the world reference base world reference base for soil resources world soil resources reports 84 fao rome is used to provide an overview of icelandic volcanic soils the data were acquired as part of a recently initiated effort to build a database for icelandic soils but are also partly based on data from the eu cost 622 action a literature review and unpublished icelandic research the main soil types in iceland are histosols histic andosols gleyic andosols brown andosols vitrisols and leptosols in the icelandic language a single term is used to refer to each soil type this system reflects the dominant influence of eolian deposition and drainage conditions on the formation of histosols and andosols the parent materials of icelandic soils are mixtures of tephra layers and eolian sediments consisting mostly of volcanic glass the steady flux of eolian sediments from unstable sandy deserts constantly recharges the surface with basaltic tephra materials the histosols 20 c have appreciable amounts of andic materials because of the eolian deposition the andosols histic gleyic and brown andosols show characteristic properties of soils dominated by andic materials with allophane contents of up to 30 ferrihydrite commonly accounts for 5 15 of the soil the contents of both allophane and ferrihydrite generally decrease with increasing organic c content of soil horizons as organic matter inhibits allophane formation and iron tends to become reduced and lost in poorly drained locations the vitrisols are soils of desert surfaces with low organic content but they usually meet criteria for andic materials with 30 glass and al 1 2fe o 0 4 cryoturbation is intense in iceland resulting in a range of geomorphic surface features the cryoturbation is related to both climatic factors and the andic soil properties including high water retention and rapid hydraulic conductivity

three dimensional structure of flow at a confluence of river channels with discordant beds this paper presents three dimensional data of the mean and turbulent structure of flow collected at a natural confluence of rivers with discordant beds to 1 describe the three dimensional flow field of a natural junction of channels 2 assess the role of changes in bed morphology occurring during transport effective events on the structure of flow at a confluence and 3 examine how the three dimensional structure of flow varies with changes in the ratio of momentum flux between the two confluent streams three dimensional measurements of velocity were reconstructed from the measurements obtained with an array of four two component electromagnetic current meters six detailed velocity profiles were taken at five cross sections in a wide range of flow conditions the mean field of flow is characterised by 1 the acceleration of flow in the downstream portion of the post confluence channel but by lower velocities upstream in the mixing layer area 2 a stagnation zone at the apex of the junction 3 a zone of flow deviation and strong fluid upwelling close to the avalanche face and at the margin of the tributary mouth bar and 4 reduced velocities over the depositional bar at the downstream junction corner the position and extent of these zones vary with changes in the ratio of momentum flux very high intensity of turbulence peaks up to 50 and turbulent kinetic energy were observed in the mixing layer region distortion of the mixing layer characteristic of flow where bed discordance is present between the two tributary channels was evident from mean and turbulent flow data this field study suggests that the effects of bed discordance on flow sediment transport and the resultant bed morphology must be incorporated into conceptual and numeric models of these sites of complex flow

ferrihydrite and allophane in four andepts from hawaii and implications for their classification abstract the mineralogy and composition of the waimea kukaiau akaka and hilo soils on the island of hawaii were studied using methods which included moessbauer spectroscopy and chemical dissolution the hilo soil which experiences 3000 mm year rainfall and the akaka soil 3800 mm year rainfall both contain large amounts about 25 of iron oxides of short range order together with other iron oxides gibbsite and allophane the iron oxides of short range order have characteristics similar to those of the reactive mineral ferrihydrite which is known to occur in a wide range of soils these characteristics however in two respects x ray diffraction and acid oxalate solubility do not rule out the presence or admixture of the much less known feroxyhite another closely related iron oxide of short range order most samples from these two soils dry irreversibly to give hard aggregates a property probably related to the presence of the iron oxides of shortrange order both the hilo and the akaka soils are classified as hydrandepts in the current usda soil taxonomy in the new proposed order of andisols these two soils would become hydrudands it is desirable to recognise the presence of short range order iron oxides in these soils the kukaiau soil currently classified as a dystrandept which occurs in a lower rainfall zone 1500 mm year contains about 25 allophane and lesser amounts of ferrihydrite other iron oxides and gibbsite the waimea soil occurs in the lowest rainfall zone 1000 mm year and the dominant mineral is halloysite but some allophane ferrihydrite and hematite are also present this soil is currently classified as a eutrandept under the andisol proposal the kukaiau and waimea soils would be a hapludand and a haplustand respectively quartz mica and other layer silicates were present in all four of the soils suggesting that there have been additions of eolian material

modeling of thrust fronts above ductile and frictional detachments application to structures in the salt range and potwar plateau pakistan series of scaled sandbox models are used to simulate the development of thin skinned simultaneous shortening above adjacent ductile and frictional substrates these models simulate the evolution of the potwar plateau and salt range in pakistan where pale

hydro morphodynamic evolution in a 90 movable bed discordant confluence with low discharge ratio confluences with low discharge and momentum ratios where narrow steep tributaries with high sediment load join a wide low gradient main channel that provides the main discharge are often observed in high mountain regions such as in the upper rhone river catchment in switzerland few existing studies have examined the hydro morphodynamics of this type of river confluence while considering sediment discharge in both confluent channels this paper presents the evolution of the bed morphology and hydrodynamics as observed in an experimental facility with a movable bed for that purpose one experiment was carried out in a laboratory confluence with low discharge and momentum ratios where constant sediment rates were supplied to both flumes during the experiment bed topography and water surface elevations were systematically recorded when the bed topography reached a steady state so called equilibrium and the outgoing sediment rate approximated the incoming rate flow velocity was measured at 12 different points distributed throughout the confluence and the grain size distribution of the bed surface was analyzed typical morphodynamic features of discordant confluences such as a bank attached bar and a flow deflection zone are identified in this study nevertheless the presence of a marked scour hole in the discordant confluence and distinct flow regimes for the tributary and main channel differ from results obtained in previous studies strong acceleration of the flow along the outer bank of the main channel is responsible for the scour hole this erosion is facilitated by the sediment discharge into the confluence from the main channel which inhibits bed armoring in this region the supercritical flow regime observed in the tributary is the hydrodynamic response to the imposed sediment rate in the tributary copyright 2015 john wiley sons ltd

characterization of a weathering sequence of soils derived from volcanic ash in cameroon taxonomic mineralogical and agronomic implications the characteristics of eight pedons developed in quaternary basaltic pyroclasts in western cameroon under humid tropical conditions are reported the soil weathering sequence consists of an andept tropept udalf udult association andic properties physico chemical characteristics and weathering stage clearly distinguish the taxonomic units the total reserve in bases trb and the clay content measured after adequate h2o dispersion with na resins both appear to be suitable weathering indices clay mineralogy of non andic soils shows direct relationships between interstratified halloysite smectite content cec of the clay fraction and weathering stage the weathering stage of the investigated soils is reflected not only in their components and classification but also in their nutrient status and banana crop performance

turbulent flow structure at a discordant river confluence asymmetric jet dynamics with implications for channel morphology only a handful of field studies have examined turbulent flow structure at discordant confluences the dynamics of flow at such confluences have mainly been examined in the laboratory this paper reports results of a field based investigation of turbulent flow structure at a discordant river confluence these results support the hypothesis that flow at a discordant alluvial confluence with a velocity ratio greater than two exhibits jet like characteristics scaling analysis shows that the dynamics of the jet core are quite similar to those of free jets but that the complex structure of flow at the confluence imposes strong effects that can locally suppress or enhance the spreading rate of the jet this jet like behaviour of the flow has important implications for morphodynamic processes at these types of confluences the highly energetic core of the jet at this discordant confluence is displaced away from the riverbed thereby inhibiting scour however helical motion develops adjacent to the jet particularly at high flows which may promote scour numerical experiments demonstrate that that the presence or absence of a depositional wedge at the mouth of tributary can strongly influence detachment of the jet from the bed and the angle of the jet within the confluence

changes in channel size at river confluences with coarse bed material a field survey of thirty stream junctions from a small watershed together with data collected by miller 1958 allowed us to investigate morphometric adjustments occurring at confluences the model proposed by roy and woldenberg 1986 was slightly modified and used as a tool for morphometric analysis two parameters are necessary in order to evaluate the rate of change in channel size at a confluence the area ratio channel capacity above the confluence channel capacity below the confluence and the discharge ratio discharge of the minor tributary discharge of the major tributary our data show that total channel capacity tends to decline below most confluences a reduction in cross sectional area implies an increase in average flow velocity this interpretation is consistent with lyell s observations and with results from recent flume experiments best and reid 1984

squeeze box in a centrifuge abstract a squeeze box which operates in a centrifuge is presented in the experimental set up the centrifugal body force mimics the role of gravity in nature and at the same time drives a hydraulic press which simulates lateral forces the rate of lateral shortening is controlled by varying the loads placed on the hydraulic press utilization of a squeeze box in a centrifuge enhances the range of scaling and enables the use of coherent model materials without violating similitude requirements in the traditional squeeze box which operates under normal gravity similitude requirements severely limit the choice of model materials as a test of the apparatus simple experiments were carried out comparing models shortened in a centrifuge with similar models shortened under normal gravity the realistic form of decollement structures e g folds faults and diapirs shortened in a centrifuge in these tests as well as in other tests which will be reported later show the potential of this new technique for rigorous modelling of geodynamic systems which evolve under the influence of both gravity and a push from the rear the set up can also be used as a simple shear box in a centrifuge after a slight adjustment

hydraulic geometry and changes in flow velocity at a river confluence with coarse bed material a survey of flows was conducted at a river confluence with coarse bed material bridges were installed on both tributaries at the confluence and farther downstream on the receiving stream at these stations flow velocities were measured over a dense grid for seven conditions ranging from very low flows to the bankfull stage hydraulic geometry relationships established at all four stations revealed that flow is accelerated through the confluence as stage rises at bankfull discharge average velocity is 1 6 times higher at the confluence than on either tributary flow acceleration occurs at and above intermediate flow stages and is concentrated at the centre of a linear pool located at the confluence the development of a zone of high shear stress is also associated with the cell of high flow velocity flow acceleration is dissipated at the exit of the pool where water surges over boulder ribs the acceleration is not related to the development of flow separation zones as observed by best and reid 1984 for wide junction angles nor is it explained by the reduction of the friction exerted by the banks acceleration is associated with the plan geometry of the confluence with the lateral slopes which permit water to converge and with a reduction in grain roughness at the confluence owing to the curvature of the tributary and to the acute angle of entry relative power losses through the confluence decrease with increasing stages

analogue modeling of large transport thrust faults in evaporites floored basins example of the chazuta thrust in the huallaga basin peru abstract the huallaga basin is a deformed foreland basin located in north peru the basin comprises several syntectonic depocenters the most significant is the biabo syncline located at the back of the chazuta thrust a long flat floored thrust detaching on an evaporitic decollement which has accommodated more than 40 km of horizontal displacement the hangingwall of the chazuta thrust has remained remarkably intact with little or no internal deformation and has incorporated a large volume of evaporites at its base in order to unravel the formation and evolution of this thrust we conducted a series of physical experiments that tested the role of various parameters the goal is to investigate a system in which most of the deformation is accommodated in the frontal part of the chain chazuta thrust whereas deformation of the thrust sheet itself remains minor results from our experimental investigations suggest that the three key parameters that have allowed for such a long lived large slip frontal thrust to operate are 1 the wedge shaped syn kinematic sedimentation 2 the presence of the biabo syncline which acted as a bulldozer pushing the evaporites forward forcing their distal inflation and 3 the erosion at the front that favored farther advance of the frontal thrust dragging passively large volumes of evaporites along with it

development and application of high resolution petrography on resin impregnated holocene peat columns to detect and analyse tephras cryptotephras and other materials we describe the potential for high resolution detection observation and chemical analysis of tephras and cryptotephras in freeze dried and resin impregnated peat sections special attention is drawn to tephra grain alteration products sensitive to standard lab treatment but preserved using an impregnation technique developed here resulting blocks and thin sections provide a continuous archive of tephras that can be investigated through a wide range of non destructive techniques encompassing polarising and fluorescence microscopy scanning electron microscopy with without elemental analysis and xrf based core scanning

statistical analysis of an experimental compressional sand wedge the quasi static deformation of dry sand is widely used as an analogue to the brittle deformation of the upper crust the quantitative comparison of analogue to natural tectonics or to mechanical predictions requires identifying sources of biases and estimating the intrinsic variability of the experimental results we develop experimental and statistical methods that fulfill these requirements we consider an initially perfect wedge resting on a flat layer made of a uniform dry sand in a rectangular glass box the box is shortened lengthwise by translating one of its end walls towards the other the lateral walls can remain fixed or be translated with the moving end wall upon shortening the wedge is thrusted above the flat layer forming classical fore and backthrusts as essentially plane strain structures lifetimes locations and dips of all thrusts constitute seven quantifiable output parameters called observables in addition to the shortening forces monitored at both end walls during shortening up to seventy measurements of each observable were performed in seven final state cross sections of ten experiments a three step statistical analysis allows us to prove that first the observables vary independently justifying their modeling with independent distributions second the ergodic hypothesis holds meaning that along strike variations can be used to infer the intrinsic experimental variability measurements can thus be repeated on successive cross sections in each experiment third our data set is free from bias due to friction on the lateral walls or due to the finite length of the box we then construct statistical models of each observable using either gauss or laplace distributions for example forethrusts dip at 38 3 2 and backthrusts at 41 3 3 we finally show how to apply these statistical models to experiments using a different initial geometry the statistical methods presented here are applicable to experiments with different setups materials and observables although the ergodic hypothesis is relevant only to plane strain experiments

hydrodynamic characteristics and channel morphodynamics at a large asymmetrical confluence with a high sediment load main channel abstract previous studies of confluences have concentrated mainly on confluences of small streams confluences with simple geometric configurations and confluences of curving rivers however the confluences of natural rivers especially large confluences of high sediment load rivers generally have complex geometric confluent boundaries and various incoming flow and sedimentation conditions in this study several field measurements of the three dimensional velocity components water surface elevation bed topography and suspended sediment particle sizes reveal the complex hydrodynamic characteristics and bed morphology of the asymmetrical confluence of the high sediment load yellow river and its clear tributary the fen river the results show that the water surface morphology of this confluence is elevated in the stagnation zone mixing interface and the opposite side of the tributary whereas the converse is true at the downstream junction corner the geomorphologic features of the bed show a small scour hole and a long narrow scour belt under conventional hydrological conditions or a larger scour hole under hydraulic conditions when the value of the momentum flux ratio is much greater than one in contrast to the confluences of small streams and curving rivers this confluence does not exhibit continuous flow separation or helical motion but has pronounced regions such as a stagnation zone flow acceleration and deflection areas a shear layer mixing interface and a flow recovery zone the position and size of these regions vary with the momentum flux ratio the particle size of suspended sediment at the confluence is normally distributed and the changing sedimentation process corresponds to the scour and siltation state of the bed in general the particle size of the suspended sediment is small in the stagnation zone and the downstream channel near the tributary but large in the acceleration zone and shear layer these results are helpful to better understand the hydrodynamic and morphodynamics characteristics of confluences with complex geometric configurations and junction conditions

geochemistry dispersal volumes and chronology of holocene silicic tephra layers from the katla volcanic system iceland at least 12 silicic tephra layers silk tephras erupted between ca 6600 and ca 1675 yr bp from the katla volcanic system have been identified in southern iceland in addition to providing significant new knowledge on the holocene volcanism of the katla system which typically produces basaltic tephra the silk tephras form distinct and precise isochronous marker horizons in a climatically sensitive location close to both the atmospheric and marine polar fronts with one exception the silk tephras have a narrow compositional range with sio2 between 63 and 67 geochemically they are indistinguishable from ocean transported pumice found on beaches in the north atlantic region although they differ significantly from the silicic component of the north atlantic ash zone one naazo volumes of airborne silk tephra range from 0 05 to 0 3 km3 we present new isopach maps of the six largest layers and demonstrate that they originate within the katla caldera the apparently stable magma system conditions that produced the silk tephras may have been established as a consequence of the eruption of the silicic component of naazo ca 10 3 ka and disrupted by another large scale event the tenth century ad eldgja eruption ca 1 ka despite the current long repose silicic activity of this type may occur again in the future presenting hitherto unknown hazards copyright 2001 john wiley sons ltd

holocene volcanic activity at grimsvotn bardarbunga and kverkfjoll subglacial centres beneath vatnajokull iceland assessment of potential future eruptive behaviour of volcanoes relies strongly on detailed knowledge of their activity in the past such as eruption frequency magnitude and repose time the eruption history of three partly subglacial volcanic systems grimsvotn bardarbunga and kverkfjoll was studied by analysing tephra from soil profiles around the vatnajokull ice cap which extend back to 7 6 ka well known regional holocene marker tephra e g h3 h4 h5 were utilized to correlate profiles stratigraphic positions and geochemical compositions were used for fine scale correlation of basaltic tephra around vatnajokull ice cap 345 tephra layers were identified of which 70 originated from grimsvotn bardarbunga or kverkfjoll the eruption frequency of each volcanic system was estimated grimsvotn has been the most active with an average of 7 eruptions 100 years range 4 14 during prehistoric time before 870 ad bardarbunga has been the second most active with 5 eruptions 100 years range 1 8 and kverkfjoll has remained essentially calm with 0 3 eruptions 100 years but showing periodic activity with repose times of 1000 years all three volcanic systems experienced lulls in activity from 5 ka to 2 ka referred to as the mid holocene low this reduced eruption frequency appears to have resulted from a decrease in magma generation and delivery from the mantle plume rather than from changes in ice load glacier thickness in prehistoric time there was a time lag of 1000 3000 years between a peak of activity at volcanoes directly above the mantle plume versus at volcanoes located in the non rifting part of the eastern volcanic zone closer to the periphery of the island this time space relationship suggests that a significant future increase in volcanism can be expected there following increased levels of volcanism above the plume

sediment transport and bed morphology at river channel confluences river channel confluences form important morphological elements of every river system being points at which rapid changes in flow sediment discharge and hydraulic geometry must be accommodated this article presents results of a quantitative investigation of sediment transport at channel confluences accomplished through both scaled laboratory flume simulation and complementary monitoring of a natural channel confluence bed morphology at channel confluences is characterized by three distinct elements avalanche faces at the mouth of each confluent channel a deep central scour and a bar within the separation zone formed at the downstream junction corner these elements are controlled predominantly by the confluence angle and the ratio of discharges between the tributary and mainstream channels as confluence angle and discharge ratio increase the sediment contributions from the confluent channels are progressively segregated in their paths through the junction with sediment being transported around rather than through the centre of the confluence this segregation of sediment loads is accompanied by the retreat of the main channel avalanche face from the confluence an increase in the scour depth a change in the orientation of the scour and an increase in the size of the separation zone bar field measurements closely replicate the flume simulation a model of sediment transport and bed morphology links these features to the fluid dynamics of these sites an understanding of confluence dynamics is important not only in considerations of channel morphology and design criteria but must form the basis for the interpretation of confluence sediments in the ancient record

the icelandic rofabard soil erosion features soil erosion and desertification are severe problems in iceland erosion processes are numerous and more than one can occur at each site resulting in many erosional forms erosion forms and an erosion severity scale are the basis for a recent national survey of erosion in iceland one of the most distinctive erosion forms in iceland is an erosion escarpment termed rofabard in icelandic rofabards are formed in thick but non cohesive andosols that overlie more cohesive materials such as glacial till or lava the relatively loose andosols beneath the root mat are undermined creating escarpments or rofabards the rofabards retreat as a unit with a fully vegetated and rich ecosystem on top but leaving barren desert in their place rofabards are common within a 20 000 km2 area the agricultural research institute and soil conservation service erosion database suggests that erosion associated with rofabards has denuded 15 000 30 000 km2 of land that was previously fully vegetated and had fertile andosols but is now mostly desert erosion rates associated with rofabards are reported as the loss of vegetated land with andosol mantle measured as hectares per square kilometre per year this measure of erosion has more meaning for icelandic landscapes than the traditional tonnes per hectare per year estimated losses of andosol cover in rofabard areas for the whole country are currently about 230 ha a 1 this rate is about 10 times lower than the rate needed to cause estimated losses of andosol mantle in rofabard areas since settlement 1125 years ago during peak years of soil erosion losses were probably several thousand hectares per year but the erosion rates slowed down as extensive andosol areas have become barren deserts copyright 2000 john wiley sons ltd

magnesium retention on the soil exchange complex controlling mg isotope variations in soils soil solutions and vegetation in volcanic soils iceland understanding the biogeochemical cycle of magnesium mg is not only crucial for terrestrial ecology as this element is a key nutrient for plants but also for quantifying chemical weathering fluxes of mg and associated atmospheric co2 consumption requiring distinction of biotic from abiotic contributions to mg fluxes exported to the hydrosphere here mg isotope compositions are reported for parent basalt bulk soils clay fractions exchangeable mg seasonal soil solutions and vegetation for five types of volcanic soils in iceland in order to improve the understanding of sources and processes controlling mg supply to vegetation and export to the hydrosphere bulk soils 26mg 0 40 0 11 are isotopically similar to the parent basalt 26mg 0 31 whereas clay fractions 26mg 0 62 0 12 exchangeable mg 26mg 0 75 0 14 and soil solutions 26mg 0 89 0 16 are all isotopically lighter than the basalt these compositions can be explained by a combination of mixing and isotope fractionation processes on the soil exchange complex successive adsorption desorption of heavy mg isotopes leads to the preferential loss of heavy mg from the soil profile leaving soils with light mg isotope compositions relative to the parent basalt additionally external contributions from sea spray and organic matter decomposition result in a mixture of mg sources on the soil exchange complex vegetation preferentially takes up heavy mg from the soil exchange complex 26mgplant exch 0 50 0 09 and changes in 26mg in vegetation reflect changes in bioavailable mg sources in soils this study highlights the major role of mg retention on the soil exchange complex amongst the factors controlling mg isotope variations in soils and soil solutions and demonstrates that mg isotopes provide a valuable tool for monitoring biotic and abiotic contributions of mg that is bioavailable for plants and is exported to the hydrosphere

long term storage and age biased export of fluvial organic carbon field evidence from west iceland terrestrial organic carbon oc plays an important role in the carbon cycle but questions remain regarding the controls and timescale s over which atmospheric co remains sequestered as particulate oc poc motivated by observations that terrestrial poc is physically stored within soils and other shallow sedimentary deposits we examined the role that sediment storage plays in the terrestrial oc cycle specifically we tested the hypothesis that sediment storage impacts the age of terrestrial poc we focused on the efri haukadalsa river catchment in iceland as it lacks ancient sedimentary bedrock that would otherwise bias radiocarbon based determinations of poc storage duration by supplying pre aged petrogenic poc our radiocarbon measurements of riverine suspended sediments and deposits implicated millennial scale storage times comparison between the sample types suspended and deposits suggested an age offset between transported suspended sediments and stored deposits poc at the time of sampling which is predicted by theory for the sediment age distribution in floodplains we also observed that poc in suspended sediments is younger than the predicted mean storage duration generated from independent geomorphological data which suggested an additional role for oc cycling consistent with this we observed interparticle heterogeneity in the composition of poc by imaging our samples at the microscale using x ray absorption spectroscopy specifically we found that particles within individual samples differed in their sulfur oxidation state which is indicative of multiple origins and or diagenetic histories altogether our results support recent coupled sediment storage and oc cycling models and indicate that the physical drivers of sediment storage are important factors controlling the cadence of carbon cycling

flow and particle paths at a natural river confluence with coarse bed material abstract an investigation of flow and particle trajectories was conducted at a gravel bed confluence where the junction angle is gradually reduced from 60 to 20 as the tributary joins the main channel a shallow and elongated scour zone is present at the mouth of the tributary three flow stages ranging from 1 7 to 1 2 bankfull were recorded flow velocity vectors were measured from five bridges installed across the channels sediment transport paths were determined by following the progression of seeded marked particles through the confluence from may to october 1986 the results show that flow structure is responsive to stages at lower stages less than 1 3 bankfull flow velocity vectors are controlled by local bed topography and specifically by the steeper slopes around the scour zone at higher stages velocity vectors become aligned with the plan geometry of the confluence for all stages surface flow velocities are higher in the scour zone bed shear stress computed from the law of the wall for the deepest flow is higher at the bottom of the scour zone compared to its sides head and exit the trajectories of particles seeded on the tributary pass through the scour zone particles coming from the main channel displayed a dual pattern first moving laterally towards the scour zone early in the season then following paths that are consistent with the plan geometry of the confluence this shift in pattern reflects an increase in the peak discharges during the survey period these results highlight the role of stage in controlling the flow dynamics and particle paths at the confluence

magnetic and geochemical characterization of andosols developed on basalts in the massif central france identification of andosols is primarily based upon the content of their colloidal constituents clay and metal humus complexes and on the determining of andic properties this needs time and cost consuming geochemical analyses our primary aim of this study is to describe the magnetic and geochemical properties of soils rich in iron oxides derived from strongly magnetic volcanic basement in this case andosols secondary aim is to explore links between magnetic and chemical parameters of andic soils with respect to genesis factors parent material age precipitation and thickness of the soil profile six pedons of andic properties developed on basaltic lavas were analyzed down to parent rock by a set of magnetic and geochemical methods magnetic data of soil and rock samples reflect the type concentration and particle size distribution of ferrimagnetic minerals geochemical data include soil reaction ph in h2o cation exchange capacity organic carbon and different forms of extractable iron and aluminum content our results suggest the following 1 magnetic measurements of low field mass specific magnetic susceptibility can be a reliable indicator for estimating andic properties and in combination with thermomagnetic curves may be suitable for discriminating between alu andic and sil andic subtypes 2 in the studied andosols strong relationships were found between a magnetic grain size parameters precipitation and exchangeable bases b concentration of ferrimagnetic particles and degree of crystallization of free iron and c parameters reflecting changes in magneto mineralogy and soil genesis parent material age soil depth

bias due to side wall friction in sand box experiments abstract the principal concern of this paper is with the accuracy and validity of experimental sand box models of geological structures and of thrust faults in particular as influenced by side wall friction side wall friction can produce undesirable artifacts in particular in experiments aimed at reproducing plane deformations the importance of such artifacts is revealed in the present study by the results of experiments that are affected in opposite ways by side wall friction two distinct experimental configurations have been chosen for this purpose for a set up that comprises a body of sand placed between vertical side walls on a basal plate against a back wall and a front wall all made of glass in configuration m the bottom plate and attached front wall move relative to the side walls towards a fixed back wall while in configuration f the back wall moves relative to the side walls and attached bottom plate towards a fixed front wall fixed fixed relative to the side walls in both configurations thrust traces on the top surface of the sand become curved near the side walls because of wall friction the outcome of altogether 31 experiments in the two configurations shows that the magnitude of the experimental bias depends on the ratio s l s b of the area of contact of the sand body with the side walls s l to its area of contact with the basal surface s b the bias is the difference in thrust locations in the two configurations it is measured on photographs taken through the side walls during the experiments away from the side walls the bias is estimated by the thrust curvatures observed in top views for s l s b 0 1 i e very little contact with side walls there is no identifiable bias furthermore a 2d theoretical solution reproduces well the results for s l s b increasing to 0 25 curvatures increase yielding rather different cross sections at the side walls but still similar cross sections in the central part of the sand body for s l s b 0 3 thrust planes occupy different regions of the box for each configuration although their curvature is decreasing at s l s b 0 9 curvatures have almost disappeared but the location of the thrusts and the length of the decollement depend strongly on the experimental configuration departures from the 2d theoretical solution are clearly stronger with configuration m than f another important conclusion is that the absence of curvature of the thrust traces is not a proof of the 2d character of the experiment mechanical equilibrium of the total forces recorded at both front and back walls during shortening shows that the thrust locations are controlled by the difference between basal and side wall shear forces

effect of the sediment discharge on the equilibrium bed morphology of movable bed open channel confluences abstract the objective of this study was to characterize the influence of sediment discharge on the morphology of movable bed asymmetric open channel confluences for this purpose fifteen experiments were performed on rough turbulent flows through an experimental confluence by running a single combination of confluent discharges and using a single bed material with three different sediment loads in the main channel combined with five different sediment loads in the tributary channel the experiments were run until equilibrium was reached physical interpretations of the influence of the sediment amount fed into the system on the most important variables and parameters of the confluent flows and confluence morphology were provided morphological variables considered included the height of the tributary mouth bar the depth of the scour hole located in the vicinity of the tributary mouth the deviation angle of the tributary flow and the dimensions of the bank attached bar the hydrodynamic parameters of the confluent flows studied included the unit momentum flux the unit stream power or the ratio of confluent unit momentum fluxes the crucial importance of the sediment load entering the system was highlighted since it was documented that i both channels upstream of the confluence adapted their respective bed slope flow depth and average flow velocity to respond to the independent sand amounts entering the system ii the characteristic dimensions and shape of the most important confluence features changed consistently with the imposed changes on the sediment discharge ratio and concentration iii for a given water discharge ratio the unit momentum flux ratio depended on the sediment amounts flowing through the confluence instead of being an independent parameter