paleomagnetism of the sierra de las animas complex southern uruguay its implications in the assembly of western gondwana global paleogeographies in the neoproterozoic are poorly known in particular the assembly of gondwana at the end of the proterozoic is controversial in order to constrain the kinematic evolution of the rio de la plata craton a paleomagnetic study was carried out in the vendian to cambrian sierra de las animas magmatic complex exposed in southeastern uruguay 34 7 s 55 3 w one hundred and forty seven samples from 23 sites were studied these included two sites on the vendian glaciogenic sediments of the playa hermosa formation after stepwise af and thermal cleaning a characteristic remanence was isolated at several sites according to the available isotopic data for the complex and the correlation between remanence directions and lithologic type and age two paleomagnetic poles were computed for the sierra de las animas complex sa1 338 1 e 5 9 n dp 19 6 dm 26 7 n 7 sites mean declination 44 5 mean inclination 58 0 95 18 1 with a likely age around 520 ma shows a distribution of vgps that suggests a remanence acquisition that spanned a long time during a period of fast apparent polar wander for gondwana sa2 250 9 e 16 9 s dp 15 9 dm 21 5 n 6 sites mean declination 95 3 mean inclination 58 5 95 14 5 corresponds to a likely age around 550 ma and falls close to a pole of similar age from the congo craton this suggests that the rio de la plata craton was at that time part of gondwana and confirms previous postulates that a single apwp can be defined for the supercontinent since 550 ma a preliminary mean geomagnetic pole obtained from the playa hermosa formation ph 198 4 e 43 0 s dp 8 6 dm 16 0 n 6 samples mean declination 226 0 mean inclination 24 2 95 15 0 is consistent with a previous pole of 595 ma from the campo alegre ca lavas and may suggest if confirmed that the glaciogenic deposits were produced at low to intermediate latitudes despite high uncertainties a new speculative model on the process of assembly of gondwana is proposed

new method for estimation of discharge a new technique for drawing isovel patterns in an open or closed channel is presented it is assumed that the velocity at each arbitrary point in the conduit is affected by the hydraulic characteristics of the boundary while any velocity profile can be applied to the model a power law formula is used here in addition to the isovels patterns the energy and momentum correction factors and the ratio of mean to maximum velocity v umax and the position of the maximum velocity are calculated to examine the results obtained the model was applied to a pipe with a circular cross section a comparison between the profiles of the proposed model and the available power law profile indicated that the two profiles were coincident with each other over the majority of the cross section furthermore the predicted isovels were compared with velocity measurements in the main flow direction obtained along the centerline and lateral direction of a rectangular flume the estimated discharge based on meas

estimation of flow direction in meandering compound channels abstract the flow in the main channel of a meandering compound channel does not occur in the ridge direction because of the effect of the upstream floodplain flows this study proposes a model for estimating the flow direction in the depth averaged two dimensional domain depth averaged flow angles between the entrance and the apex sections detailed velocity measurements were performed in the region between the meander entrance section and apex section in a large scale meandering compound channel the vertical size of the secondary current cell is highly related to the depth averaged flow angle thus the means of the local flow angles above the secondary current cell and within the cell are separately discussed the experimental measurements indicate that the mean local flow angle above the cell is equal to the section angle whereas the mean local flow angle within the cell is equal to zero the proposed model is validated using published data from five sources good agreement is obtained between the predictions and measurements indicating that the proposed model can accurately estimate the depth averaged flow direction in the meandering compound channels finally the limitations and application ranges of the model are discussed

flow resistance and momentum flux in compound open channels new formulations are presented for flow resistance and momentum flux in compound open channels as implemented in the st venant equations these formulations facilitate a physically enhanced approach to evaluating conveyance roughness stage discharge relationship and unsteady flood routing in compound open channels an analysis using steady flow data from the well controlled experiments at the large scale flood channel facility hr wallingford demonstrates the ability of the present approach to properly resolve the discontinuity of overall roughness across the main channel bankfull level also the proposed formulations are shown to be conducive to obviating the long standing computational difficulty in unsteady flood routing due to small flow depths over flat and wide floodplains the present work should find general applications in one dimensional computation of river flows

estimates of flow resistance and eddy viscosity coefficients for 2d modelling on vegetated floodplains the problem of quantifying the effects of flexible plants on flow resistance and eddy viscosity by vegetated floodplains is first addressed with a one dimensional 1d approximation based upon the so called lateral distribution method the estimates so obtained are then tested with two dimensional 2d numerical simulations based on the full shallow water equations through the use of the computational code telemac 2d data obtained on a physical model of the bes os river spain whose floodplains were covered with plastic ornamental plants to mimic the effect of flexible vegetation is used for the validation of the numerical results additionally the values of flow resistance estimated numerically with the 1d and 2d simulations are compared with values obtained in a rectangular flume under flow conditions slope water depth and artificial lining similar to those used on the reduced model it is then established that as more physical mechanisms are included in the mathematical model used to study the problem the ratio between the floodplain and the main channel flow resistance coefficient increases the approach demonstrates that whenever enough flow data is available the lateral distribution method delivers values of flow resistance and eddy viscosity which are highly consistent with 2d numerical modelling this finding could mean considerable savings in the burdensome task of specifying flow resistance and turbulence dissipation values for 2d modelling of large compound channel systems copyright 2004 john wiley sons ltd

paleomagnetism of the callander complex and the cambrian apparent polar wander path for north america the 7 km2 circular callander alkaline complex was emplaced into anorthositic and granitic gneisses of the grenville province in the canadian shield about 575 5 ma ago at the start of the cambrian the complex has not been subsequently metamorphosed or tilted detailed alternating field and thermal step demagnetization of 252 specimens from 29 sites led to the identification of a characteristic a magnetization component with a direction of d 82 2 i 82 7 95 3 1 k 83 n 26 sites in 5 sites of mesocratic to leucocratic syenite from the core of the complex in 5 sites of fenitized host rock from its aureole and in 16 sites of lamprophyre from radiating dikes isothermal remanent magnetization tests show that the a component is retained by both magnetite and hematite in a complete spectrum of domain sizes a reversals test suggests and a contact test shows the a component to be primary its pole position at 46 3 s 121 4 e dp 5 9 dm 6 1 does not fall on published but poorly defined

turbulent flow across a natural compound channel 1 the measurements and primary analysis of turbulent flow across a compound channel river severn england are presented the velocity was measured using a three dimensional acoustic doppler velocimeter in combination with a directional current meter the statistical flow structure is examined against existing analytical formulations derived for single channel flows based on laboratory studies the existence of a vertically two layer structure around the interface between the main channel and the floodplain is demonstrated indicating 1 a vertical shear dominated flow zone near the bed and 2 away from the bed a transverse shear dominated flow zone with enhanced turbulent mixing the temporal spectra clearly reveal the occurrence of anisotropic turbulence both in the main channel and over the floodplain the present findings necessitate the resolution of both transverse and vertical structures for advanced modeling of compound channel flows the measured data can be used to assess the performance of mathematical river models

a paleomagnetic study of the layered mafic intrusion at sept iles quebec a paleomagnetic study of gabbroic and anorthositic members of the sept iles layered mafic intrusion has isolated a remanence with direction d 333 i 29 remanence a and a corresponding pole of 141 e 20 n the rocks are cut by numerous diabase dykes of unknown age the remanence carried by the dykes has direction d 188 i 85 remanence b with a corresponding pole of 116 e 59 s another remanence statistically identical to b d 186 i 85 is found at dyke contacts and in the local host rock its associated pole 115 e 61 s is identical to the one derived from the dykes this overprint is significantly different from remanence a and is obviously related to dyke emplacement the location of the pole derived from remanence a is in excellent agreement with many other cambrian poles from a variety of locations throughout cratonic north america the rocks at sept iles have been previously dated radiometrically at 540 ma thus all evidence indicates that remanence a is an original

phanerozoic hot spot traces and paleogeographic reconstructions of the siberian continent based on interaction with the african large low shear velocity province abstract we review intraplate magmatism in siberia and its folded surroundings from 480 ma to the present we describe several large igneous provinces lips and the intervals in which they were continuously formed within the limits of the siberian continent the altay sayan early paleozoic magmatic area 598 446 ma the altay sayan lip 408 393 ma the viluy lip 380 350 ma the barguzin vitim lip 310 275 ma the late paleozoic rift system of central asia 318 242 ma the siberian traps and west siberian rift system 250 249 ma the east mongolian and west trans baikalian lip 228 195 ma and a number of various aged late mesozoic and cenozoic rift zones and magmatic areas from 160 ma to the present day following lawver and muller 1994 kharin 2000 lawver et al 2002 and chernysheva et al 2005 we accept the position of the icelandic hot spot under the siberian trap area at the permo triassic boundary that enables us to estimate the geographic coordinates of the siberian trap location at 250 ma they are the same as iceland today presently the icelandic hot spot is situated above the african large low shear velocity province llsvp that indicates a hot mantle plume we suggest a set of paleogeographic reconstructions of the siberian continent for which we evaluate the paleolatitude based on the paleomagnetic data and estimate the paleolongitude position by placing siberia above the african llsvp furthermore we estimate the geographic coordinates for other ancient hot spots in the framework of the african llsvp that we consider to be responsible for the intraplate magmatism during different time periods in the phanerozoic eon available rare element and isotopic characteristics of the intraplate magmatic rocks of siberia enable us to determine three primary sources moderately depleted mantle prema enriched mantle em i and em ii of the mantle origin magma we propose that the model explains the interaction of the hot mantle plume including hot spots plume tails with the siberian intraplate magmatism areas throughout the phanerozoic eon

comment on new palaeomagnetic result from vendian red sediments in cisbaikalia and the problem of the relationship of siberia and laurentia in the vendian by s a pisarevsky r a komissarova and a n khramov pisarevsky et al 2000 document a poorly dated pole from vendian age sedimentary rocks in siberia and argue for a lowlatitude position of both laurentia and siberia during the interval from 650 to 550 ma their argument is based at least in part on a reinterpretation of palaeomagnetic poles from laurentia because previous studies e g symons chiasson 1991 meert et al 1994 torsvik et al 1996 argued for a high latitude position for laurentia beginning at 580 ma perhaps earlier the new data provided by pisarevsky et al 2000 are a welcome addition to the siberian palaeomagnetic database however given the broad age range of their result we feel that the authors over interpret the extant data in an effort to rescue a controversial positioning of siberia against the northern margin of laurentia sears price 2000 pelechaty 1996 hoffman 1991 dalziel 1997 condie rosen 1994 frost et al 1998 therefore we wish to address the following points in the manuscript that we feel are contentious

paleomagnetism of the catoctin volcanic province a new vendian cambrian apparent polar wander path for north america the vendian cambrian segment of the lauretian apparent polar wander path apwp has been poorly constrained and the subject of some controversy the catoctin volcanic province in central virginia is well dated at 570 35 ma rb sr and 597 18 ma u pb and therefore presented an excellent paleomagnetic target for resolving the laurentian vendian cambrian apwp a total of 206 samples from 32 sites were collected from the catoctin basalts feeder dikes and sills the study revealed three ancient directions of magnetization the youngest c component fails the fold test and yielded a characteristic in situ direction of d 147 i 44 k 21 95 9 the corresponding paleopole falls along the middle ordovician segment of the laurentian apwp and we consider this component to be the result of a taconic remagnetization the second component the b component is carried by hematite exhibits dual polarities and passes a fold test the tilt corrected b component characteristic direction is d 92 i 17 k 16 95 13 the corresponding paleopole at 4 s 193 e falls near a well established late cambrian 505 ma pole for laurentia and we consider this component to be a remagnetization during a late cambrian tectonic event in the central appalachians the third component isolated in the catoctin basalts the a component yields a tilt corrected mean of d 68 i 84 k 59 95 9 this component passes a fold and reversal test a suite of samples was collected from two catoctin feeder dikes and surrounding country rocks that yield a positive baked contact test the a pole at 43 s 128 e falls significantly away from previously proposed vendian poles for laurentia a reevaluation of previous paleomagnetic studies from coeval rock units reveals similarly steep directions and leads us to propose a new apwp this new apw track indicates that laurentia was located near the pole during the interval 615 580 ma and drifted rapidly 16 cm yr 1 toward its late cambrian equatorial position

the formation and fate of internal waves in the south china sea internal oceanic waves are subsurface gravity waves that can be enormous and travel thousands of kilometres before breaking but they are difficult to study here observations of such waves in the south china sea reveal their formation mechanism extreme turbulence relationship to the kuroshio current and energy budget internal waves are the underwater version of more familiar surface waves they can be enormous and travel thousands of kilometres before breaking the south china sea is known to be home to the largest internal waves in the world s oceans but their size generation mechanisms and role in the regional energy budget are unknown matthew alford and colleagues now present the results from the iwise observational campaign and reveal that internal waves more than 200 metres high break in the south china sea and create turbulence that is orders of magnitude larger than in the open ocean and that wave formation is influenced by the kuroshio current these results now allow for a complete energy budget of the south china sea and for a more accurate incorporation of internal waves into climate models internal gravity waves the subsurface analogue of the familiar surface gravity waves that break on beaches are ubiquitous in the ocean because of their strong vertical and horizontal currents and the turbulent mixing caused by their breaking they affect a panoply of ocean processes such as the supply of nutrients for photosynthesis1 sediment and pollutant transport2 and acoustic transmission3 they also pose hazards for man made structures in the ocean4 generated primarily by the wind and the tides internal waves can travel thousands of kilometres from their sources before breaking5 making it challenging to observe them and to include them in numerical climate models which are sensitive to their effects6 7 for over a decade studies8 9 10 11 have targeted the south china sea where the oceans most powerful known internal waves are generated in the luzon strait and steepen dramatically as they propagate west confusion has persisted regarding their mechanism of generation variability and energy budget however owing to the lack of in situ data from the luzon strait where extreme flow conditions make measurements difficult here we use new observations and numerical models to 1 show that the waves begin as sinusoidal disturbances rather than arising from sharp hydraulic phenomena 2 reveal the existence of 200 metre high breaking internal waves in the region of generation that give rise to turbulence levels 10 000 times that in the open ocean 3 determine that the kuroshio western boundary current noticeably refracts the internal wave field emanating from the luzon strait and 4 demonstrate a factor of two agreement between modelled and observed energy fluxes which allows us to produce an observationally supported energy budget of the region together these findings give a cradle to grave picture of internal waves on a basin scale which will support further improvements of their representation in numerical climate predictions

two dimensional solution for straight and meandering overbank flows this paper presents a practical method to predict depth averaged velocity and shear stress for straight and meandering overbank flows an analytical solution to the depth integrated turbulent form of the navier stokes equation is presented that includes lateral shear and secondary flows in addition to bed friction the novelty of the present approach is not only its inclusion of the secondary flows in the formulation but also its applicability to straight and meandering channels the analytical solution is applied to a number of channels at model and field scales and is also compared with other available methods such as that of shiono and knight and the lateral distribution method the present formulation gives much better predictions of velocity and shear stress particularly in those cases where the secondary flows are dominant

velocity and turbulence measurements for two overbank flow events in river severn field measurements of velocity and turbulence have been carried out in a study reach of the river severn at lower farm near shrewsbury during overbank flow acoustic doppler velocity meters have been used for the field measurements of velocity and turbulence particularly in the interface region between river channel and floodplain the values of local shear velocity and roughness length for the reach under study were calculated using measured velocity data the distributions of turbulent intensities and the reynolds stresses are also presented the variation of horizontal shear stress in the vertical direction deviates from linear for the main channel floodplain interaction region due to the existence of a lateral shear and momentum transfer from the floodplain towards the main channel comparisons are made between the field data and previous experimental data from the flood channel facility

petrogenesis of the 723 ma coronation sills amundsen basin arctic canada implications for the break up of rodinia the coronation mafic sills and associated natkusiak flood basalts in arctic canada are part of the 723 ma franklin igneous events a flood basalt and dyke swarm province that extends across the southern arctic of north america and greenland two representative sills 70 and 25 m thick respectively outcrop near coronation gulf in nunavut they are composed of clinopyroxene plagioclase olivine and fe ti oxides and have tholeiitic composition similar to the natkusiak basalts their end values range between 4 2 and 5 9 and have la yb n ratios of 1 5 2 these rocks are mantle derived and were affected by minor crustal contamination compositional variations indicate that the coronation sills have experienced extensive fractionation including flow differentiation during emplacement and show increased differentiation from west to east across the province a feature similar to the mackenzie dyke swarm however the mackenzie dyke swarm and the franklin igneous events have distinct chemical composition the franklin rocks are related to a mantle plume located near victoria island at the center of the dyke swarm and could represent an offshoot from the superplume related to the break up of rodinia the absence of such dykes and sills in siberia argues against locating siberia against northern laurentia in a rodinia reconstruction 2003 elsevier b v all rights reserved

the age of the sept iles layered mafic intrusion canada implications for the late neoproterozoic cambrian history of southeastern canada the large sept iles layered mafic intrusion of eastern canada has been precisely dated by u pb zircon methods at 565 4 ma consideration of other magmatic events at that time in southeastern canada reveals three different phases of magmatism the first phase was composed of two tholeiitic dike swarms at 615 ma and 590 ma the second including the sept iles intrusion comprised a widespread series of mostly alkaline intrusions emplaced around 575 565 ma along the rift faults of the st lawrence graben the last phase was composed predominantly of alkali and transitional basalts now exposed in the appalachians erupted during the latest neoproterozoic and early cambrian the second and third phases are best accounted for by the arrival of a major mantle plume near the town of sept iles 565 million years ago the widespread but volumetrically minor alkali magmatism that peaked 10 m y earlier is typical of other plumes elsewhere the plume model is reinforced by sedimentological evidence for a crusta

lateral depth averaged velocity distributions and bed shear in rectangular compound channels this paper presents a method to predict depth averaged velocity and bed shear stress for overbank flows in straight rectangular two stage channels an analytical solution to the depth integrated navier stokes equation is presented that includes the effects of bed friction lateral turbulence and secondary flows the shiono and knight method accounts for bed shear lateral shear and secondary flow effects via three coefficients f and respectively a novel boundary condition at the internal wall between the main channel and the adjoined floodplain is proposed and discussed along with other conventional boundary conditions the analytical solution using the novel boundary condition gives good prediction of both lateral velocity distribution and bed shear stress when compared with experimental data for different aspect ratios

exchange processes induced by large horizontal coherent structures in floodplain vegetated channels mangrove forests along the mekong delta estuaries are usually observed to degrade together with the increasing extension of fish farms in this limited width condition the formation of coherent structures their interactions with mangroves the role of the width of the mangrove forest and their effects on the exchange processes between the open channel and the adjacent floodplain are still not well examined in order to obtain more insight a unique laboratory experiment of a vegetated compound channel mimicking estuarine mangroves has been conducted the results show that in a compound vegetated channel with a very gentle transverse slope the vegetated shear layer dynamics resembled that associated with vegetation rather than that associated with a depth differential furthermore the flow field under the effect of large horizontal coherent structures lhcss shows a spatially and temporally cycloid motion with associated flow events namely sweep ejection stagnant and reverse flows it is also suggested that the coherent structures can have an influence on a broader area than the vegetation area into which the eddy structures can penetrate in terms of the exchange processes the momentum transfer and the intensity of transverse fluctuations induced by the lhcss can be related to this phenomenon consequently decreasing the mangrove width can significantly affect the pattern of the lhcss disturb the transverse exchange processes induced by these structures and thereby changing the shear layer creating unfavorable conditions for sedimentation inside forests and for river bank stability

turbulent open channel flows with variable depth across the channel the flow of water in straight open channels with prismatic complex cross sections is considered lateral distributions of depth mean velocity and boundary shear stress are derived theoretically for channels of any shape provided that the boundary geometry can be discretized into linear elements the analytical model includes the effects of bed generated turbulence lateral shear turbulence and secondary flows experimental data from the science and engineering research council serc flood channel facility are used to illustrate the relative importance of these three effects on internal shear stresses new experimental evidence concerning the spatial distribution of reynolds stresses yx and zx is presented for the particular case of compound or two stage channels in such channels the vertical distributions of zx are shown to be highly nonlinear in the regions of strongest lateral shear and the depth averaged values of yx are shown to be significantly different from the depth mean apparent shear stresses the importance of secondary flows in the lateral shear layer region is therefore established the influence of both reynolds stresses and secondary flows on eddy viscosity values is quantified a numerical study is undertaken of the lateral distributions of local friction factor and dimensionless eddy viscosity the results of this study are then used in the analytical model to reproduce lateral distributions of depth mean velocity and boundary shear stress in a two stage channel the work will be of interest to engineers engaged in flood channel hydraulics and overbank flow in particular

secondary current effects on cohesive river bank erosion 1 the emphasis of this research was placed on the development of a methodology that allows 1 an adequate representation of the distribution of the near bank shear stress s when secondary currents are present and 2 estimation of the critical erosional strength cr and other sediment erodibility parameters for fluvial erosion these two factors combine to form the overarching objective of this investigation which was carried out in union flat creek a stream with pronounced cross sectional irregularities located within the loess region of the palouse basin of washington state united states the study has shown that the presence of the secondary currents increases the magnitude of the depth averaged sidewall shear stress at least by a factor of 2 0 it was also found that the ratio of the maximum to the depth averaged sidewall shear stress is greater than 5 these findings suggest therefore that use of the may be a good approximation of the fluid shear stress magnitude for simple channel geometries but not for natural channels characterized by complex geometries the measured critical erosional strength cr 4 16 pa was comparable with the findings of other laboratory and field studies that also focused on palouse sediments

paleomagnetic directions in late precambrian glaciomarine sediments of the mirbat sandstone formation oman abstract a paleomagnetic study has been carried out in the late precambrian glaciomarine mirbat sandstone formation in the sultanate of oman limited availability and geographical distribution of outcrops restricted the sampling to six sites of these three were in laminated fine grained clay to siltstones and three in laminated carbonates one of these sites was rejected the remaining five sites represented by a total of 21 samples displayed a similar magnetic component during progressive thermal demagnetization between 0 and 500 c the site mean pole calculated from this component is at 86 5 n 328 1 e p 11 2 m 19 7 this low temperature component is interpreted as the direction of the average recent geomagnetic field in oman two sites 10 samples revealed a consistent magnetic component above 500 c the sample mean declination of 68 4 and inclination of 18 4 k 46 4 95 7 2 are interpreted as the characteristic remanent magnetization the late precambrian arabian pole position computed from the samples at these two sites lies at 23 3 n 141 8 e p 3 9 m 7 5 rotated into west african coordinates the resulting pole position is at 31 6 n 153 2 e in good agreement with published poles from gondwana of late precambrian age 550 ma the resulting paleolatitude is about 9 4 indicating a low latitude of deposition for the glaciomarine deposits of the mirbat sandstone formation