the karoo igneous province a problem area for inferring tectonic setting from basalt geochemistry tholeiitic basalts and associated intrusives are the major component of the karoo igneous province they are of mesozoic age and constitute one of the world s classic continental flood basalt cfb provinces it has been argued that most karoo basalts have not undergone significant contamination with continental crust and that their lithospheric mantle source areas were enriched in incompatible minor and trace elements during the proterozoic the only exceptions to this are late stage morb like dolerites near the present day continental margins which are considered to be of asthenospheric origin when data for the southern karoo basalts are plotted on many of the geochemical discriminant diagrams which have been used to infer tectonic setting essentially all of them would be classified as calc alkali basalts cab s or low k tholeiites virtually none of them plot in the compositional fields designated as characteristic of within plate basalts there is little likelihood that the compositions of the karoo basalts can be controlled by active subduction at the time of their eruption and no convincing evidence that a subduction component has been added to the subcontinental lithospheric mantle under the entire area in which the basalts crop out it must be concluded that the mantle source areas for cab s and the southern karoo basalts have marked similarities in contrast the data for northern karoo basalts largely plot in the within plate field on geochemical discriminant diagrams available data suggest that the source composition and or the restite mineralogy and degree of partial melting are different for southern and northern karoo basalts there is no evidence for any difference in tectonic setting between the southern and northern karoo basalts at the time they were erupted this appears to be clear evidence that specific mantle source characteristics and or magmatic processes can vary within a single cfb province to an extent that renders at least some geochemical discriminant diagrams most unreliable for classifying tectonic environment with respect to continental volcanic rocks

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

c o sr isotopic stratigraphy of cap carbonates overlying marinoan age glacial diamictites in the paraguay belt brazil abstract the neoproterozoic paraguay belt located along the southeastern border of the amazon craton and eastern border of the rio apa block comprises a thick passive margin succession of glaciomarine turbidites carbonates and siliciclastic sedimentary rocks that were deformed during the brasiliano pan african orogeny the carbonate sequence comprises a 1300 m thick succession of platformal carbonate rocks araras formation which directly overlie marinoan glacial diamictites puga formation the late neoproterozoic age of this carbonate sequence is indicated by the presence of cloudina lucianoi and corumbela werneri found in the upper part of the corumba group southern paraguay belt carbon and sr isotope data were obtained on marine carbonate samples across three different sections of the araras formation 13 c pdb values of the carbonate from the border of the basin range between 10 5 and 2 7 including a short stratigraphic interval of cap dolomite 22 m directly overlying diamictites the isotopic profile across the basal 200 m of deep shelf laminated microcrystalline limestone and clay limestone cap carbonate that overlie glacial diamictites also show negative 13 c pdb values 5 3 and 0 6 in contrast with the lower portion of the carbonate sequence a 800 m thick unit of shallow water dolostones forming the upper part of the carbonate sequence present uniform positive 13 c pdb values ranging from 1 9 to 2 4 dolostone and arenaceous dolostone of the upper parts of the araras formation have high 13 c pdb values 4 1 to 9 6 which are followed by an abrupt decrease in 13 c values down to 1 0 the carbon isotope profile of the paraguay belt is quite different from the bambui group an other important neoproterozoic sequences in central brazil which also overlie glacial sediments negative 13 c pdb values are found only within the lowermost meters of these sequence and are followed by an important and extensive positive carbon isotope excursion values ranging from 6 9 to 16 0 on the other hand the 13 c pdb profile observed across the araras formation exhibits an approximately 200 m thick section of carbonates with low 13 c pdb values carbonates of the araras formation have 87 sr 86 sr ratios ranging from 0 70753 to 0 70803 comparable to cap carbonates that overly marinoan glacial deposits from other neoproterozoic sequences

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

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

closure of the paleotethys in the external hellenides constraints from u pb ages of magmatic and detrital zircons crete abstract paleotethys was a highly mobile oceanic realm pinching into the supercontinent pangea between gondwana and eurasia in the late paleozoic early mesozoic published paleotethyan reconstructions reveal that the time of paleotethys closure and the position of its suture are highly debated we present new magmatic and detrital zircon ages separated from pre alpine basement and permian to triassic cover rocks exposed in the external hellenides of crete these age data reveal variscan and cimmerian docking of microplates along the southern margin of laurasia and help to constrain the time of paleotethys closure 46 of detrital zircons from quartzite in the variscan basement are pennsylvanian with concordant ages at 321 2 ma 310 3 ma and 300 3 ma the basement is unconformably overlain by arc related volcanics of the tyros unit magmatic zircons of which yielded a concordant u pb zircon age at 285 2 ma thus the metasediments of the basement interpreted as former trench sediments were deposited metamorphosed and exhumed in latest carboniferous to early permian times 302 283 ma magmatic activity during this late variscan phase is also indicated by igneous boulders within olenekian meta conglomerates of the tyros unit which yielded concordant u pb zircon ages at 291 2 and 310 2 ma the late variscan orogenic phase is attributed to the collision of the gondwana derived southern minoan terrane smt with laurasia subsequent to northward subduction of paleotethys lithosphere and visean collision of the northern minoan terrane nmt magmatic activity ceased during the late permian but revived in the lower triassic as is indicated by felsic volcanics 249 2 ma concordant u pb zircon and by detrital zircons 242 3 240 5 ma 237 3 ma concordant u pb zircon of the tyros unit at the same time the variscan chain was exhumed and removed as is shown by the detritus in the lower to middle triassic tyros sediments which includes high grade metamorphic rocks and detrital zircons with u pb ages ranging from 280 to 335 ma a significant change in the detrital components occurred in the ladinian when the variscan basement with its permo triassic cover was thrust on top of clastic sediments today represented by the phyllite quartzite unit s str the phyllite quartzite unit s str shows cadomian and older but no variscan detritus because of its position along the northern margin of the cimmerian ribbon continent thus in the eastern mediterranean paleotethys was closed during the ladinian and the related suture in the external hellenides is situated between the variscan basement active margin in the north and the phyllite quartzite unit s str passive margin in the south carnian crustal extension led to subsidence of the variscan cimmerian chain most parts of which merged below sea level this is the reason why 90 of the detritus of the carnian tyros beds are not related to the variscan but to the cadomian and grenvillian basement of the e gondwana derived cimmerian ribbon continent

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

the variscan orogeny in chios greece carboniferous accretion along a palaeotethyan active margin new stratigraphic and structural data on the turbiditic succession of chios volissos turbidites suggest that this clastic wedge formed during the early carboniferous these turbidites fed long distance by erosion of the variscan orogen were most likely deposited in a palaeotethyan remnant ocean basin they were severely deformed and structurally thickened at anchi metamorphic conditions before the close of the palaeozoic both contractional and layer parallel extensional structures high strain rates and broken in matrix fabrics along thick shear zones may suggest deformation at the toe of an accretionary wedge stratigraphic petrographic and structural data indicate that chios represents the westernmost part of palaeotethys which escaped the carboniferous collision between gondwana and laurasia but received great volumes of terrigenous sediments from the variscan belt favouring the growth of a large accretionary prism

the east variscan shear zone geochronological constraints from the capo ferro area ne sardinia italy abstract the exhumation of the variscan high grade metamorphic complex of sardinia was driven by large scale asymmetric shear zones which reached middle crustal levels at 325 1 3 u pb monazite ma exhumation was accompanied by heating and partial melting the migmatization process was followed by the emplacement of syntectonic granites dated with u pb on zircon at 318 3 ma and 317 2 ma which produced contact metamorphism at 315 1 3 ma u pb monazite in mylonitic silurian orthogneisses the nw se dextral shear zone of the capo ferro area ne sardinia is affected by partial melting by granite emplacement and shows remarkable petrotectonic analogies to and can thus be related to the east variscan shear zone a major intra continental shear zone which was active in the southern european variscides from 325 ma to 315 ma and that has been precisely dated with the u pb monazite system the emplacement of migmatitic gneisses along large shear zones within this time interval is not restricted to sardinia but has also been observed in southern corsica in the french massif central in the western alps and in central spain this pattern of shear zones is related to a complex geodynamic scenario linked to the oblique convergence of the gondwana and laurussia plates during the middle upper carboniferous

the nature of triassic extension related magmatism in greece evidence from nd and pb isotope geochemistry the widespread triassic volcanic rocks of greece dismembered during the hellenide orogeny are used to interpret the nature of triassic rifting four assemblages of volcanic rocks are distinguished on geochemical criteria 1 a predominant subalkaline basalt andesite dacite series with a high proportion of pyroclastic rocks 2 minor shoshonites 3 alkali basalt and 4 morb the stratigraphic and palaeogeographic distribution of these rock types is synthesized new pb and nd isotopic data are used to discriminate between hypotheses suggesting that either subduction or extension was responsible for the triassic volcanism in the subalkaline basalt assemblage e nd is negative with depleted mantle model ages 1 5 ga pb isotopic compositions are mostly close to the very distinctive compositional field of cenozoic extensional rocks of the aegean area with very high 207 pb 204 pb for relatively low 206 pb 204 pb ratios these isotopic data confirm interpretations based on trace elements that subalkaline basalts were predominantly derived from melt depleted peridotite in the sub continental lithospheric mantle as a result of extension small areas of enriched hydrous mantle partially melted to yield shoshonitic magmas nd and pb isotopic compositions of the alkali basalts are quite different from those in other rock types and suggest a himu mantle source component derived from a small plume which also influenced morb compositions distribution of these various rock types is used to constrain palaeogeographic reconstruction of triassic micro continental blocks

a new seismic tomography of aigion area gulf of corinth greece from the 1991 data set summary an ew three dimensional delay traveltime tomography is performed to image the intermediate structure of the western gulf of corinth a large data set collected in 1991 during a two month passive tomographic experiment has been reanalysed for the reconstruction of detailed vp and vs images an improved tomography method based on an accurate traveltime computation is applied to invert simultaneously delayed p and s first arrival traveltimes for both velocity and hypocentre parameters we perform different synthetic tests to analyse the sensitivity of tomography results to the model parametrization and to the starting 1 d model selection the analysis of the retrieved vp and vs models as well as deduced vp vs and vp vs images allows us to interpret and delineate the distribution of lithological variation porosity crack content and fluid saturation in the upper 9 11 km of the crust beneath the gulf the tomographic models image a rather complex crustal structure which is characterized by a vertical change in both velocity features and seismicity distribution we identify a shallower zone of the crust 0 5 km depth in which velocity distributions seem to be controlled by the still active n s extensional regime and a deeper zone 7 11 km depth which matches the seismogenic zone the correlation between this latter and a specific unit of the hellenic mountain structure the pyllite quartzite series allows us to suggest a possible explanation for seismicity concentration in a narrow band at 7 9 km depth finally the occurrence of clusters showing low angle normal fault mechanisms in areas characterized by high vp vs values indicates a possible role of fluids in triggering brittle creep along the identified low angle normal faults

a new passive tomography of the aigion area gulf of corinth greece from the 2002 data set we present the results of a tomographic study performed in the framework of the 3f corinth project the aim of this work is to better understand the rifting process by imaging the crustal structure of the western gulf of corinth forty nine stations were deployed for a period of six months allowing us to monitor the microseismicity delayed p and s first arrival times have been simultaneously inverted for both hypocenter locations and 3 d velocity distributions we use an improved linearized tomography method based on an accurate finite difference travel time computation to invert the data set the obtained vp and vs models confirm the presence of a two layer vertical structure characterized by a sharp velocity gradient lying at 5 7 km depth which may be interpreted as a lithological contrast the shallower part of the crust down to 5 km depth is controlled by the n s extension and lacks seismicity the deeper part 7 13 km depth matches the seismogenic zone and is characterized by faster and more heterogeneous anomalies in this zone the background seismicity reveals a low angle active surface dipping about 20 toward the north and striking wnw ese the position of this active structure is consistent with both high vp vs and low vp vs anomalies identified at 8 12 km depth and suggesting a highly fracturated and fluid saturated zone both the geometry of the active structure beneath the gulf and the presence of fluids at 8 12 km depth are in accordance with a low angle detachment model for the western part of the gulf of corinth

permian circumpacific deep water faunas from the western tethys sicily italy new evidences for the position of the permian tethys abstract circumpacific deep water faunas characterized mainly by radiolarians paleopsychrospheric ostracods and conodonts were found in the early middle and late permian deposits pertaining to the sicanian paleogeographic domain of western sicily near the western end of the eurasiatic tethys similar permian deep water faunas are known from the phyllite unit of crete and from oman in the latter area partly above oceanic crust all these occurrences indicate the presence of a late paleozoic tethys ocean immediately north of gondwana

permian triassic palaeogeography of the external hellenides abstract in the external hellenides two crustal types are developed 1 a variscan basement overlain by the tyros unit and 2 a presumably panafrican basement overlain by the phyllite quartzite group late carboniferous early triassic and the plattenkalk group permian oligocene if we accept this assumption the plattenkalk and the phyllite quartzite groups fit well into the permian triassic palaeogeography of the northeastern margin of afro arabia during the permian triassic there is evidence for a siliciclastic fluvio marine system in the western sinai and nile area whereas the early triassic shelf of eastern sinai and israel is dominated by a carbonate platform the western facies association of the plattenkalk and the phyllite quartzite groups represents the northward extension of a fluvio marine system the siliciclastics of the plattenkalk group formed part of a marine delta those of the phyllite quartzite group indicate a turbidite fan system the units of middle and eastern crete represent the seaward continuation of the carbonate platform plattenkalk group and a deep sea realm free of turbidite influx phyllite quartzite group during the permian triassic the different crustal segments were brought together due to dextral strike slip movements of gondwana against the southern margin of the european plate as indicated by a uniform shallow marine facies of the carnian norian

finite frequency tomography in a crustal environment application to the western part of the gulf of corinth in this paper we investigate finite frequency effects in crustal tomography we developed an inversion procedure based on an exact numerical computation of the sensitivity kernels in this approach we compute the 3d travel time sensitivity kernels by using 1 graph theory and an additional bending to estimate accurately both rays and travel times between source receiver and diffraction points and 2 paraxial ray theory to estimate the amplitude along theses rays we invert both the velocity and the hypocentre parameters using these so called banana doughnut kernels and the lsqr iterative solver we compare the ray theoretical and the finite frequency tomography to image the intermediate structures beneath the gulf of corinth greece which has long been recognized as the most active continental rifting zone in the mediterranean region our dataset consists of 451 local events with 9233 p first arrival times recorded in the western part of the gulf aigion area in the framework of the 3f corinth european project previous tomographic images showed a complex velocity crustal model and a low dip surface that may accommodate the deformation accurate velocity models will help to better constrain the rifting process which is still a subject of debate the main results of this study show that finite frequency tomography improves crustal tomographic images by providing better resolved images of the 3d complicated velocity structure because the kernels spread the information over a volume finite frequency tomography results in a sharpening of layer boundaries as we observed for the shallower part of the crust down to 5 km depth beneath the gulf of corinth

the atlas east variscan elbe shear system and its role in the formation of the pull apart late palaeozoic basins the variscan orogeny occurred as a result of the late devonian to late carboniferous collision and accretion of gondwana derived microcontinents and continental masses with those of laurussia the irregular boundaries of the colliding continents caused isochronous transpressional and transtensional tectonics accompanied by a complex pattern of intracontinental shear zones at the scale of the southern european variscides these shear zones and their configuration controlled the subsequent evolution of permian to middle triassic paleogeography the geographic distribution from morocco to the eastern alps of the late carboniferous permian up to triassic basins most of which are considered as pull apart basins was related with the development of the late palaeozoic intracontinental shear network our analysis of the stratigraphic tectonic and volcanic features of the late carboniferous permian continental basins across the laurussia gondwana boundary reveals the role of the east variscan shear zone during this time as a precursory lineament for the development of the permian to triassic rifting of pangaea and the following opening of oceanic basins e g the neothetyan ocean

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

the inception of the maliac ocean regional geological constraints on the western circum rhodope belt northern greece abstract considerable debate surrounds the early alpine paleogeographic significance of the circum rhodope belt crb greece in particular whether it was a rifted section of the southern euroasian foreland or of the late mesozoic back arc system of the vardar ocean or of both in the attempt to clarify the complex nature of the crb nine new lithostratigraphic formations with its members in consideration of the spatiotemporal and regional geological context are constrained by tracing the changes in regional plate interactions and linked magmato sedimentary record the analysis of the stratigraphic data in combination with the highly complex yet poorly constrained regional geology is employed to reconstruct the late permian triassic and latest jurassic developments of this part of the southern european foreland a continuum between the rifting and triassic opening of the maliac ocean the carnian eo cimmerian docking is supported by the consolidated new data embedded into a collection of the nine distinct lithostratigraphic formations the terminal vardar plate approach during the latest jurassic earliest cretaceous and the convergence with the investigated portion positioned to the south of the balkan hellenic foreland thrust belt configured this regional neotethyan flysch trough whereas the paleocene eocene east west shortening and nappe stacking ejected the incoherent early alpine system referred to as the crb the ejected permo triassic including the early jurassic ophiolite bearing formations of the crb are matched to be in line with those of the regional southeast european importance in fact the results place the western crb or eastern maliac ocean to lie at the distal southern margin promontory of the eurasian craton aligned with the early and late paleozoic crystalline terranes paleozoic to lower triassic variscan basement segment of the pelagonian unit veles series and lower complex of the serbo macedonian unit vertiskos unit

correlation between the anisotropy of the magnetic susceptibility strain and x ray texture goniometry in phyllites from crete greece abstract a comparison of petrofabric and strain in phyllitic rocks from crete greece has been undertaken using anisotropy of magnetic susceptibility ams at room and low temperature the projected dimension strain method pds and x ray texture goniometry xtg of phyllosilicates the magnetic at room and low temperature and strain fabrics show similar orientations with the minimum axes of the ams ellipsoid coincident with the flattening direction of a pronounced solution mass transfer deformation smt the kmax axes of the ams and the maximum stretching axis of the deformation overlap in the nne ssw direction parallel to the axes of macroscopic folds the quantitative relationship between the absolute values of strain and ams is unclear nevertheless a degree of correlation is found between the woodcock diagram values of ams and strain sx values discriminate better the clustered samples of kmax whereas the girdled samples of kmax are better differentiated with the sz values the stacking of the octahedral layers in micas correlates with sx in the clustered kmax axes fabrics the lack of a clear quantitative relationship is probably related to the generation of new phyllosilicates during folding and smt deformation

braided rivers perspectives and problems progress towards a fuller understanding of the dynamics and deposits of braided rivers demands an interdisciplinary approach to a host of unresolved problems although many advances have been made within recent years in interpreting the mechanics of flow transport of sediment and sedimentary architecture of braided rivers many key issues remain to be addressed in particular several areas demand attention the mechanisms of braid bar initiation confluence diffluence dynamics the nature of sedimentary facies over a range of grain sizes and the influence of flow stage and aggradational regime upon the depositional architecture over a range of channel scales this paper focuses upon these issues and highlights several areas of fruitful future interdisciplinary collaboration 1993 the geological society

effect of bed morphology on flow mixing length at river confluences mixing processes at river confluences have an important bearing on problems such as pollutant dispersal and the management of river systems but remain poorly understood previous studies1 11 have indicated that flow mixing downstream of a confluence is a slow process typically completed at distances greater than 100 times the channel width bed morphology is now emerging as a critical factor flume experiments have shown that the width to depth ratios12 and height discordance13 14 between the confluent channels should influence mixing rates12 14 but the magnitude of these effects in real rivers is not known here we report measure ments from three river confluences of moderate size which show that bed discordance can markedly increase mixing rates for the rivers studied mixing is complete at distances five to ten times shorter than those reported previously

fluvial herring bone cross stratification in a modern tributary mouth bar coonamble new south wales australia herring bone cross stratification occurs in tributary mouth bar sediments less than 150 yr old in warrena creek near its confluence with the castlereagh river some 2000 river kilometres from the sea in northern new south wales these streams have low gradients with straight to anastomosing channels which become sinuous and distributive downstream channel beds are sand but banks are almost exclusively mud which is burrowed and extensively penetrated by roots herring bone cross stratification results from flow reversals in warrena creek during flood events flow direction depends upon discharge and stage in the creek relative to that in the adjacent river the lithofacies resemble inter tidal deposits and could easily be misidentified on the basis of herring bone cross stratification in an ancient sedimentary sequence herringbone cross stratification should be regarded as diagnostic of depositional environments in which current directions are principally determined by reversals of water surface gradient rather than by regional slope flow reversal phenomena may be a characteristic of very low gradient fluvial systems

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

fault deformation mechanisms and fault rocks in micritic limestones examples from corinth rift normal faults abstract a multidisciplinary study investigates the influence of different parameters on fault rock architecture development along normal faults affecting non porous carbonates of the corinth rift southern margin here some fault systems cut the same carbonate unit pindus and the gradual and fast uplift since the initiation of the rift led to the exhumation of deep parts of the older faults this exceptional context allows superficial active fault zones and old exhumed fault zones to be compared our approach includes field studies micro structural optical microscope and cathodoluminescence geochemical analyses 13 c 18 o trace elements and fluid inclusions microthermometry of calcite sin kinematic cements our main results in a depth window ranging from 0 m to about 2500 m are i all cements precipitated from meteoric fluids in a close or open circulation system depending on depth ii depth in terms of p t condition determines the development of some structures and their sealing iii lithology marly levels influences the type of structures and its cohesive non cohesive nature iv early distributed rather than final total displacement along the main fault plane is the responsible for the fault zone architecture v petrophysical properties of each fault zone depend on the variable combination of these factors

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

measurements in a braided river chute and lobe 1 flow pattern sediment transport and channel change this paper and its companion ashworth et al this issue discuss measurements of channel change and associated flow and sediment transport processes in a representative chute and bar reach within a proglacial gravel bed river the sunwapta river in the canadian rockies during a week in which water discharge through the reach increased then decreased a sediment wave passed into and partly along the reach at first the chute aggraded then sediment was eroded from the chute and deposited in a prograding lobe to one side of the original bar head between two distributaries measurements of velocity shear stress and gravel transport rate revealed day to day changes in the divergent pattern of flow and sediment transport the off center location of the lobe reflected an initial asymmetry in the pattern of flow and sediment transport but as the lobe grew the flow and transport gradually switched away to the other distributary width averaged bed load transport rates do not agree well with average rates inferred from bar head deposition or from volumes and spatial patterns of scour and fill the indirect estimates are considered more reliable than those based on direct sampling for necessarily brief durations by the end of the study the new lobe had almost accreted onto the original bar head supporting the idea that most braid bars are of compound origin temporal and spatial patterns in grain size distributions of the bed bed load and deposited sediment are discussed in the companion paper

sedimentary evidence from the south mediterranean region sicily crete peloponnese evia used to test alternative models for the regional tectonic setting of tethys during late palaeozoic early mesozoic time