lithospheric uplift in eastern gondwana evidence for a long lived mantle plume system recent work has demonstrated that modern drainage systems on rifted continental margins may be the product of ancient lithospheric doming in areas of flood basalt magmatism this uplift is thought to reflect the existence of widespread mafic underplating and excess melt generation at rift zones here i show that uplift due to the initiation of a thermal anomaly plume head in the asthenosphere may be evidenced by the formation of discrete rift structures and persistent paleodrainage sediment dispersal patterns for up to 150 m y prior to the commencement of igneous activity and final continental breakup volcanism thus occurs as a comparatively late stage geochemical response to the long term residence of hot plume material beneath continental mantle lithosphere

influence of man made cuts on the stability of pyroclastic covers campania southern italy a numerical modelling approach intense rainfall in may 1998 and december 1999 caused disastrous landslides in the sarno quindici and cervinara areas campania southern italy the landslides began with slips of the local pyroclastic covers mantling the carbonate relief and then evolved into debris flows avalanches the study discussed in this paper used a numerical modelling approach to assess the influence of man made cuts on the stability conditions of pyroclastic covers the model that was developed took into account initial failure conditions in order to better simulate the impact of man made cuts along the slopes with or without water seepage into the permeable pumiceous layers of the pyroclastic multilayer numerical analysis of stress strain field clearly showed that tracks or geomorphological discontinuities had a negative impact on the multilayer stability conditions consequently preservation of this vulnerable environment requires correct forest management practices

a study of critical state on an unsaturated silty soil critical state models for unsaturated soils have been proposed in recent years however the proposed models have been based on limited experimental data compacted specimens have generally been used for research and the complications of soil fabric resulting from the compaction procedures have brought difficulties into the interpretation of fundamental soil behavior there is a need to undertake further laboratory research studies on unsaturated soils particularly to obtain a fundamental understanding of the shear strength and critical state of unsaturated soils by testing soil specimens with simple soil structures suction controlled triaxial drained shear tests on an unsaturated silt were carried out as part of this research program specimens were prepared by gradually consolidating the initially slurried soil the resulting specimens had a relatively simple soil fabric and stress history the results showed that applying suction to an initially saturated specimen has a similar influence on the stre

stability characteristics of deep water replacement in the strait of georgia abstract it has been suggested that low frequency current fluctuations in the southern strait of georgia are the result of baroclinic instability however data extracted from cyclesonde and fixed current meter moorings suggest that the conditions for baroclinic instability are highly variable in space and time it has been recently discovered that there are summertime bottom intensified gravity currents with fortnightly and monthly periods associated with the introduction of salty waters from the juan de fuca strait during periods of neap tides these currents are the dominant mechanism for deep water renewal in the strait of georgia it is argued that these currents are baroclinically unstable and that the stability characteristics are reasonably consistent with the observed structure of the low frequency current fluctuations the episodic nature of these unstable bottom flows may help to explain the spatial and temporal variability of the low frequency current fluctuations observed in the strait of geo

the meridional flow of source driven abyssal currents in a stratified basin with topography part i model development and dynamical properties the equatorward flow of source driven grounded deep western boundary currents within a stratified basin with variable topography is examined the model is the two layer quasigeostrophic qg equations describing the overlying ocean coupled to the finite amplitude planetary geostrophic pg equations describing the abyssal layer on a midlatitude plane the model retains subapproximations such as classical stommel arons theory the nof abyssal dynamical balance the so called planetary shock wave balance describing the finite amplitude induced westward propagation of abyssal anomalies and baroclinic instability the abyssal height field can possess groundings in the reduced gravity limit a new nonlinear steady state balance is identified that connects source driven equatorward abyssal flow as predicted by stommel arons theory and the inertial topographically steered deep flow described by nof dynamics this model is solved explicitly and the meridional structure of the predicted grounded abyssal flow is described in the fully baroclinic limit a variational principle is established and is exploited to obtain general stability conditions for meridional abyssal flow over variable topography on a plane the baroclinic coupling of the pg abyssal layer with the qg overlying ocean eliminates the ultraviolet catastrophe known to occur in inviscid pg reduced gravity models the baroclinic instability problem for a constantvelocity meridional abyssal current flowing over sloping topography with present is solved and the stability characteristics are described

descent and modification of the overflow plume in the denmark strait bulk properties of the denmark strait overflow dso plume observed in velocity and hydrography surveys undertaken in 1997 and 1998 are described despite the presence of considerable short term variability it is found that the pathway and evolution of the plume density anomaly are remarkably steady bottom stress measurements show that the pathway of the plume core matches well with a rate of descent controlled by friction the estimated entrainment rate diagnosed from the rate of plume dilution with distance shows a marked increase in entrainment at approximately 125 km from the sill leading to a net dilution consistent with previous reports of a doubling of overflow transport measured by current meter arrays the entrainment rate increase is likely related to the increased topographic slopes in the region compounded by a decrease in interface stratification as the plume is diluted and enters a denser background

origin of the rajmahal traps and the 85 e ridge preliminary reconstructions of the trace of the crozet hotspot the 85 e ridge is a buried aseismic ridge approximately parallel to and west of the ninetyeast ridge in the northeastern indian ocean it was previously shown to be of probable volcanic origin emplaced on very young oceanic crust but no satisfactory model of emplacement of the rocks was offered we propose a model of origin of the rajmahal traps of northeastern india the 85 e ridge and afanasy nikitin seamount as the trace of the hotspot that now lies beneath the crozet islands in the southern indian ocean this reconstruction places the kerguelen hotspot which formed the ninetyeast ridge at the triple junction between greater india australia and antarctica before the breakup of eastern gondwana

three dimensional stability evaluation of a preexisting landslide with multiple sliding directions by the strength reduction technique landslide 1 is located on the right bank of the downstream side of the dam for hongjiadu hydropower station in guizhou province china in view of its close proximity to important workings the stability of the slide is of great concern ground investigation showed that the slide is potentially unstable and has two sliding directions reinforcement mainly by piles was proposed as the stabilization measure but two key questions were raised during the design of the piles 1 how could the stability of the slope be evaluated as there are two major sliding directions 2 how could the shear strength parameters of the slip band be determined as it contains a significant proportion of coarse particles in this paper we demonstrate how these issues can be addressed by the strength reduction technique first the strength parameters of the slip band are back analyzed by assuming a factor of safety of unity second the major sliding directions are determined by carrying out finite difference analyses third

on the baroclinic instability of cold core coupled density fronts on a sloping continental shelf a theory is presented to describe the linear baroclinic instability of coupled density fronts on a sloping continental shelf the new baroclinic model equations used to study the instability process correspond to an intermediate lengthscale dynamical balance specifically the frontal dynamics while geostrophic is not quasigeostrophic because frontal height deflections are not small in comparison with the frontal scale height the evolution of the frontal height is strongly coupled to the geostrophic pressure in the surrounding slope water through the hydrostatic balance which expresses the continuity of the dynamic pressures across the frontal interface the deeper surrounding slope water evolves quasi geostrophically and is coupled to the front by baroclinic vortex tube stretching compression associated with the perturbed density front allowing the release of mean frontal potential energy and the topographic vorticity gradient associated with the sloping bottom it is shown that the baroclinic stability characteristics are principally determined by a so called non dimensional interaction parameter denoted p which physically measures the ratio of the destabilizing baroclinic vortex tube stretching compression to the stabilizing topographic vorticity gradient for a given along front mode wavenumber it is shown that a minimum p is required for instability several other general stability results are presented necessary conditions for instability growth rate and phase speed bounds the existence of a high wavenumber cutoff and a semicircle theorem for the unstable modes the linear stability equations are solved exactly for a parabolic coupled density front and a detailed description of the spatial and temporal characteristics of the instabilities is given for physically realistic parameter values the instabilities are manifested as amplifying topographic rossby waves in the slope water and on the density front the unstable perturbations take the form of amplifying anticyclones which have maximum amplitude on the offshore side

antarctic bottom water from the adelie and george v land coast east antarctica 140 149 e 1 we report on observations of dense shelf water overflows and antarctic bottom water aabw formation along the continental margin of the adelie and george v land coast between 140 e and 149 e vertical sections and bottom layer water mass properties sampled during two rvib nathaniel b palmer hydrographic surveys nbp00 08 december 2000 january 2001 and nbp04 08 october 2004 describe the spreading of cold dense shelf water on the continental slope and rise from two independent source regions the primary source region is the adelie depression exporting high salinity dense shelf water through the adelie sill at 143 e an additional eastern source region of lower salinity dense shelf water from the mertz depression is identified for the first time from bottom layer properties northwest of the mertz sill and mertz bank 146 e 148 e that extend as far as the buffon channel 144 75 e in summer regional analysis of satellite derived ice production estimates over the entire region from 1992 to 2005 suggests that up to 40 of the total ice production for the region occurs over the mertz depression and therefore this area is likely to make a significant contribution to the total dense shelf water export concurrent time series from bottom mounted microcats and adcp instruments from the mertz polynya experiment april 1998 to may 1999 near the adelie sill and on the upper continental slope 1150 m and lower continental rise 3250 m to the north describe the seasonal variability in downslope events and their interaction with the ambient water masses the critical density for shelf water to produce aabw is examined and found to be 27 85 kg m 3 from the adelie depression and as low as 27 80 kg m 3 from the mertz depression this study suggests previous dense shelf water export estimates based on the flow through the adelie sill alone are conservative and that other regions around east antarctica with similar ice production to the mertz depression could be contributing to the total aabw in the australian antarctic basin

eddy formation in the denmark strait overflow on the basis of observations along the continental slope of east greenland a primitive equation open boundary model is used to study the formation of eddies in the denmark strait overflow the eddies are produced by vortex tube stretching initiated by cross slope components of the velocity field a sequence of cyclones and anticyclones is formed the cyclones are located along the deeper part of the slope and the anticyclones are along the shallower slope the eddies are almost barotropic with horizontal and vertical velocities of 30 cm s 1 and 0 1 cm s 1 respectively the cyclones migrate along the isobaths with a slight downhill component and intensify on their path until they become influenced by the flat deep basin their translation speed is typically 20 30 cm s 1 and the local change in relative vorticity is nearly entirely determined by vortex tube stretching and vorticity advection the overflow water flows mainly between the cyclonic and anticyclonic eddy chains the overflow is strongly accelerated by the eddies the existence of anticyclonic eddies along the shallower part of the east greenland continental slope predicted by the numerical model has been confirmed by observations

failure mechanisms in loess and the effects of moisture content changes on remoulded strength abstract investigations into the failure mechanisms of loess slopes in the loess region of north central china have shown that a majority of mass movements are associated with shallow slab slides predominantly moving under tension the effects of soil moisture content are of paramount importance when dry vertical slopes in loess may exceed 15 m in height but under conditions close to saturation the structural strength of loess rapidly decreases causing failure of the slopes mass failure of loess occurs as collapse due to hydroconsolidation and disintegration of the loess fabric as a result of liquefaction or fluidization and can be associated with progressive failure processes local destruction of the openwork fabric of the loess can lead to small plastic movements within the loess deposits this is as close as loess can come to creep

boomerang seamount the active expression of the amsterdam st paul hotspot southeast indian ridge during a survey of the axis of the southeast indian ridge seir we discovered a 1100 m tall volcanically active submarine volcano boomerang seamount near the spreading center on the amsterdam st paul asp plateau the summit of the volcano is 650 m below sea level and has a 200 m deep 2 km wide circular caldera samples of very fresh volcanic glass dated by the 210po 210pb technique at 5 months old were collected from the floor of the caldera in march 1996 the volcano is 18 km northeast of amsterdam island near the intersection of a long spreading segment and the boomerang transform fault it is built on the stationary antarctic plate where widely scattered volcanic activity thickens the crust continuing to build the plateau water column profiles reveal a 1 7 c temperature anomaly and a 0 3 v stepped nephelometer water column turbidity anomaly within the caldera nearly an order of magnitude larger than other hydrothermal plume anomalies we measured these anomalies suggest hydrothermal activity within the caldera volcanic glass compositions at two sample sites on the volcano summit are similar to each other and to amsterdam and st paul island basalts but have some important differences as well k2o tio2 ratios in boomerang seamount glasses are similar to st paul island samples but differ significantly from amsterdam island samples rare earth element patterns in lavas from boomerang amsterdam and st paul are similar sr nd and pb isotope ratios in samples from the boomerang caldera floor are similar to samples from amsterdam island however another sample from boomerang seamount deviates from a seir st paul amsterdam mixing trend and shows evidence of mixing with a kerguelen like source component the geochemical complexity of these three closely spaced volcanic edifices on the asp plateau suggests that the boomerang seamount source is heterogeneous on a very small scale

stability assessment of jinlong village landslide sichuan the jinlong village landslide is located in zitong county sichuan and approximately 150 km east of wenchuan the landslide has exhibited creep deformation for nearly 30 years field investigations indicated that the slope deformation was caused by the combined effects of unfavorable topographic and geological conditions and earthquakes the sliding surface is along a contact between silty clay and mudstone the wenchuan earthquake accelerated the creep causing bulging of the ground surface shear test results of the silty clay near the sliding surface indicated that the residual strength is consistent with the stability state of the landslide the factors of safety were calculated using the limit equilibrium method lem and the method of fast lagrangian analysis of continua in two dimensions the results using the lem indicate that the stability is clearly affected by seismic shaking and when the peak ground acceleration is 0 08g the slope reaches the limit state the shear failure surface given by numerical simulation develops on the contact between the clay and silty mudstone which confirmed the assumed sliding surfaces location using the lem

flood basalt from mont tourmente in the central kerguelen archipelago the change from transitional to alkalic basalt at 25 ma the surface of the cenozoic kerguelen archipelago constructed on the kerguelen plateau in the southern indian ocean is dominantly flood basalt with the objective of understanding the cenozoic history of the kerguelen mantle plume the age and geochemical characteristics of this flood basalt province are being determined by studying stratigraphic sections of basalt flows at several locations sections from the nw north central east and se parts of the archipelago have been studied here we report results for a 597 m succession of lavas from mont tourmente from the plateau central a region of the archipelago that has not been studied in detail mont tourmente lavas range from 26 ma dominantly transitional basalts in the lower 80 of the section to 25 3 ma dominantly alkalic basalts in the upper part of the section the timing of this change from transitional to alkalic volcanism within the mont tourmente section is consistent with that defined by the older 28 29 ma transitional basalts in the north and the 25 ma alkalic lavas erupted in the east this change in basalt composition may be related to migration of the archipelago away from the plume or to increasing lithosphere thickness over the 5 myr of flood basalt volcanism the alkalic and transitional tourmente lavas are nearly homogeneous in isotopic ratios of sr nd and pb they have lower 47sr 46sr and higher 112nd 111nd compared with lavas from the eastern sections which have been proposed to be characteristic of the kerguelen plume consequently the mont tourmente isotopic data may reflect heterogeneity within the plume or a constant proportion of a depleted component mixed with the high 87sr 86sr and low 112nd 111nd plume in contrast to many of the cretaceous kerguelen plateau lavas there is no evidence in trace element abundances or sr and nd isotopic ratios that the cenozoic kerguelen archipelago lavas were influenced by continental lithosphere

analysis of satellite gravity and bathymetry data over ninety east ridge variation in the compensation mechanism and implication for emplacement process 1 we investigate the mode of compensation emplacement history and deep density structure of the ninety east ridge indian ocean using spectral analyses and forward modeling of satellite gravity and bathymetry data we find that the northern 0 10 n and the southern 20 30 s parts of the ridge are flexurally compensated with an effective elastic thickness 15 km whereas the central part 0 20 s is locally compensated furthermore we find that for a part of central block 10 20 s over osborn knoll the compensation depth is unreasonably very high 30 40 km therefore we favor a model with subsurface loading and interpret this to be due to underplating of mafic material at the base of the crust a hypothesis that is supported by seismic results and direct modeling of gravity data along some profiles these results suggest that the northern and southern parts of ninety east ridge were emplaced off to a ridge axis compared to the central one which might have been emplaced on or near a spreading center locally compensated large topography thick underplated crust in the central part near osborn knoll might result from an interaction of a hot spot with the extinct wharton spreading ridge

mixed bottom friction kelvin helmholtz destabilization of source driven abyssal overflows in the ocean source driven ocean currents that flow over topographic sills are important initiation sites for the abyssal component of the thermohaline circulation these overflows exhibit vigorous space and time variability over many scales as they progress from a predominately gravity driven downslope flow to a geostrophic along slope current observations show that in the immediate vicinity of a sill grounded abyssal ocean overflows can possess current speeds greater than the local long internal gravity wave speed with bottom friction and downslope gravitational acceleration dominating the flow evolution it is shown that these dynamics lead to the mixed frictionally induced and kelvin helmholtz instability of grounded abyssal overflows within the overflow the linearized instabilities correspond to bottom intensified baroclinic roll waves and in the overlying water column amplifying internal gravity waves are generated the stability characteristics are described as functions of the bottom drag coefficient and slope froude bulk richardson and reynolds numbers associated with the overflow and the fractional thickness of the abyssal current compared to the mean depth of the overlying water column the marginal stability boundary and the boundary separating the parameter regimes in which the most unstable mode has a finite or infinite wavenumber are determined when it exists the high wavenumber cutoff is obtained conditions for the possible development of an ultraviolet catastrophe are determined in the infinite reynolds number limit an exact solution is obtained which fully includes the effects of mean depth variations in the overlying water column associated with a sloping bottom for parameter values characteristic of the denmark strait overflow the most unstable mode has a wavelength of about 19 km a geostationary period of about 14 hours an e folding amplification time of about 2 hours and a downslope phase speed of about 74 cm s 1

discussion of temporal evolution of the kerguelen plume geochemical evidence from 38 to 82 ma lavas forming the ninetyeast ridge by f a frey and d weis

two stage eastward diachronous model of india eurasia collision constraints from the intraplate tectonic records in northeast indian ocean abstract the onset of timing and the model of the india eurasia collision are strongly debated based on continental works without systematic marine tectonic constraints two aseismic ridges the laccadives maldives chagos ridge lmcr formed after the deccan volcanism in western india and the ninetyeast ridge ner formed after the emplacement of the rajmahal traps in eastern india are prominent features in the northeast indian ocean the onset of a major and steady increase in magma production rates along the aseismic ridge associated with the significant slowdown of the northward moving indian plate is a likely indicator of the onset of the india eurasia collision using gravity derived crustal thickness we calculated the magma production rates along the lmcr and the ner respectively the steady increasing magma production rate along the lmcr and a southwestward jump of the central indian ridge constrained the soft india island arc and hard india eurasia collisions between western india and eurasia to 50 52 ma and 41 ma respectively the steady increasing magma production rate along the ner and the formation of the mammerickx microplate constrained the soft collision between eastern india and eurasia to 47 49 ma and the extinction of the wharton ridge constrained the hard collision between eastern india and eurasia to 38 ma both the soft and hard collisions between western india and eurasia are 3 myr earlier than the soft and hard collisions between eastern india and eurasia respectively with systematic marine tectonic constraints a two stage eastward diachronous model of the india eurasia collision was proposed

topographic controls on evolution of shallow landslides in pastoral wairarapa new zealand 1979 2003 abstract this study aims to identify topographic influence on shallow landslides and their evolution during 1979 2003 in the pastoral lower north island new zealand landslide affected sites were mapped from aerial photographs and an aster satellite image the two landslide maps were overlaid in a geographic information system arcgis to detect the changes that have taken place in landslide affected areas the change map and the landslide distribution maps were overlaid with three topographic layers of slope gradient slope position and slope aspect that had been derived from a digital elevation model in arcgis the overlay analysis revealed that landslides affected area increased from 23 75 ha in 1979 to 110 97 ha in 2003 the distribution of landslide affected sites is inversely correlated with slope gradient but positively with slope position northern aspects are the most vulnerable to landsliding containing about 78 of the total affected sites moderate slope gradients suffered the most severe landsliding in the lower to middle slopes the overall magnitude and frequency of landsliding is also related to northern and eastern octants on the moderate slope gradients in the low to moderate slope positions although the same topographic setting is conducive to landsliding in both 1979 and 2003 the rate of landsliding varies with topographic setting hence topography is critical not only to landsliding but also to the evolution of sites already affected by landslides such an understanding over large spatial scales is possible only with the use of multitemporal remote sensing data and gis that are able to speed up data collection and analysis

seismic evidence of continental margin influence on the ninetyeast ridge in the bay of bengal the ninetyeast ridge ner one of the most enigmatic features in the indian ocean is covered by thick bengal fan sediment north of 9 n we present seismic reflection data on the eastern flank of the ner at 10 n that show the presence of 4 5 km thick sediments beneath the bengal fan sediments these sediments can be imaged up to 60 km beneath the andaman fore arc accretionary wedge suggesting that the decollement lies above these sediments the presence of thick sediments above the northernmost segment of the ner suggests that this segment was close to a continental margin during its emplacement we propose that these sediments were deposited soon after the breakup of india and antarctica between 130 and 100 ma and might act as source rocks for oil and gas generation beneath recent bengal fan sediments furthermore subducting thick sediments can significantly change the seismogenic behavior of the andaman subduction zone

temporal geochemical trends in kerguelen archipelago basalts evidence for decreasing magma supply from the kerguelen plume abstract the kerguelen archipelago a 39 ma to recent volcanic plutonic complex is interpreted to be a manifestation of the kerguelen plume most 85 of the surface area is covered by flood basalts ranging in age from 29 to 25 ma the youngest 25 ma studied flood basalts are in the southeast se province of the archipelago a composite 460 m section of this southeast flood basalt dominantly consists of evolved 3 to 6 mgo alkalic basalt and trachybasalt with a few interbedded highly evolved lavas trachyandesites a 40 70 m conglomerate which contains lignite beds and a trachytic breccia tuff unit all of the lavas in this composite section have sr and nd isotopic ratios that are typical of the kerguelen plume e g 80 of the 115 analyzed archipelago lavas with 2 3 mgo have 87 sr 86 sr i 0 70515 12 and 143 nd 144 nd i 0 51259 5 these ranges include the southeast flood basalts pb isotopes however are more variable these 25 ma lavas have high 206 pb 204 pb at 18 4 to 18 6 relative to other archipelago lavas the temporal trend of the archipelago flood basalt from older 29 ma transitional basalts to younger 25 ma alkalic basalt with an increasing proportion of highly evolved lavas and intra bedded sediments in the relatively young southeast section indicates a a temporal decrease in extent of melting and b a decreasing supply of magma from the plume to the crust these temporal trends are attributed to increasing lithosphere thickness as the plume evolved from a spreading ridge centered plume at 43 ma to its intraplate setting supporting evidence for this interpretation is a the absence of a morb geochemical signature in these 25 ma lavas and b the relatively low abundances of heavy rare earth elements in these southeast lavas which reflect partial melting within the garnet stability field

crustal architecture and isostatic compensation of the comorin ridge central indian ocean implications for the breakup of east gondwana abstract the comorin ridge located off the southwest margin of sri lanka is an important geological feature with significant relevance to understand the breakup of eastern gondwanaland earlier geophysical studies over this ridge provided several contrasting views due to which its origin and nature of emplacement could not be established very well in this study an integrated interpretation of bathymetry gravity geoid and ship borne magnetic data is carried out to bring out a well constrained evolution model for the ridge results of 3 d gravity inversion of crustal bouguer anomalies and joint gravity magnetic modeling indicate that the southern part of the ridge is underlain by 7 12 km thick crust intruded with volcanic rocks and the presence of 13 18 km of anomalously thicker crust below the northern part of the ridge further crustal models spatial variations in geoid topography ratio gtr and the effective elastic thickness te value obtained in this study suggest a two stage emplacement model for the evolution of the ridge the observed lower te values 7 8 km in the southern part 0 5 n 3 5 n and its proximity to the c34ny spreading anomaly suggest its emplacement on the oceanic lithosphere due to marion plume activity along a fracture zone during 88 84 ma on the other hand relatively higher te values 12 5 15 km in the northern part 3 5 n 6 5 n with absence seafloor spreading magnetic anomalies suggests that it could be a marginal basement high formed at the continent ocean boundary cob during the second phase of rifting between india and sri lanka 94 ma

formation of hp lt rocks and their tectonic implications in the western tianshan orogen nw china geochemical and age constraints the western tianshan high pressure low temperature metamorphic belt in northwestern china formed within a paleozoic accretionary wedge on the south side of the yili central tianshan plate it comprises various lithologies including mafic metavolcanic rocks metavolcaniclastic rocks metagreywacke marble and serpentinite the metavolcanics comprise eclogite omphacitite epidosite and blueschist the trace and rare earth element characteristics of these rocks indicate morb to oib affinities suggesting a seamount like setting for the protoliths of these rocks the end value of the eclogite 8 9 indicates that the protolith was derived from a long term depleted mantle i e an n morb source supporting the trace and rare earth element characteristics of these samples sm nd isochron dating of 343 44 ma omp gln grt wr and an age of 346 3 ma grt gln were determined using an eclogite boudin within blueschist a well defined 40ar 39ar plateau age of 344 1 ma was obtained for crossite from an omphacite phengite bearing blueschist and is concordant with the sm nd isochron age for the eclogite suggesting that neither argon loss nor excess argon affected the crossite a well defined 40ar 39ar plateau age of 331 2 ma was determined for phengite from the same sample the high pressure metamorphic rocks were formed by the b type subduction of a paleozoic south tianshan ocean about 344 ma ago they were however exhumed to higher crustal levels during the collision of the tarim and yili central tianshan plates about 331 ma ago similar to an alpine type tectonic regime

a view on some hydrological triggering systems in landslides abstract in this paper different types of hydrological triggering systems for debris flows shallow and deeper landslides are described the generation of surface run off and high peak discharges in first order alpine catchments is an important triggering mechanism for debris flows failure conditions in shallow landslides can occur when at a critical depth which is determined by the cohesion of the soil and the slope angle the moisture content in the soil becomes close to saturation resulting in a considerable reduction of soil strength deeper landslides 5 20 m depth are in most cases triggered by positive pore pressures on the slip plane induced by a rising ground water level the assessment of meteorological threshold conditions for shallow landslides 1 2 m needs more detailed meteorological information than for deeper landslides in the analyses of the hydrological triggering systems of deeper landslides the presence of a permeable top layer and fissures has to be taken into consideration

structural and engineering geology of the east gate landslide purcell mountains british columbia canada the east gate landslide is a prehistoric landslide that was reactivated in january 1997 the slope failure took place in the lower greenschist metasedimentary units of the precambrian horsethief creek group the grizzly creek thrust is a regional overturned fault that coincides with the location of the headscarp of the east gate landslide four discontinuity sets were recognised from detailed engineering geological mapping of the headscarp and surrounding area the main scarp of the section reactivated in 1997 was sub divided into three structural domains based on its position within the landslide lithology and orientation of the discontinuity sets limit equilibrium techniques finite difference flac and distinct element udec codes were used to investigate the failure mechanism of the 1997 event the results of the field observations and numerical models suggest that the 1997 failure involved a complex mechanism incorporating components of rock slumping bi planar and pseudo circular failure that was controlled by both the orientation of the discontinuity sets and reduced rock mass quality due to tectonic deformation

temporal evolution of the kerguelen plume geochemical evidence from 38 to 82 ma lavas forming the ninetyeast ridge abstract basaltic basement has been recovered by deep sea drilling at seven sites on the linear ninetyeast ridge in the eastern indian ocean studies of the recovered lavas show that this ridge formed from 82 to 38 ma as a series of subaerial volcanoes that were created by the northward migration of the indian plate over a fixed magma source in the mantle the sr nd and pb isotopic ratios of lavas from the ninetyeast ridge range widely but they largely overlap with those of lavas from the kerguelen archipelago thereby confirming previous inferences that the kerguelen plume was an important magma source for the ninetyeast ridge particularly important are the 81 ma ninetyeast ridge lavas from dsdp site 216 which has an anomalous subsidence history coffin 1992 these lavas are feti rich tholeiitic basalts with isotopic ratios that overlap with those of highly alkalic upper miocene lavas in the kerguelen archipelago the isotopic characteristics of the latter which erupted in an intraplate setting have been proposed to be the purest expression of the kerguelen plume weis et al 1993a b despite the overlap in isotopic ratios there are important compositional differences between lavas erupted on the ninetyeast ridge and in the kerguelen archipelago the ninetyeast ridge lavas are dominantly tholeiitic basalts with incompatible element abundance ratios such as la yb and zr nb which are intermediate between those of indian ocean morb mid ocean ridge basalt and the transitional to alkalic basalts erupted in the kerguelen archipelago these compositional differences reflect a much larger extent of melting for the ninetyeast ridge lavas and the proximity of the plume to a spreading ridge axis this tectonic setting contrasts with that of the recent alkalic lavas in the kerguelen archipelago which formed beneath the thick lithosphere of the kerguelen plateau from 82 to 38 ma there was no simple systematic temporal variation of sr nd and pb isotopic ratios in ninetyeast ridge lavas therefore all of the isotopic variability cannot be explained by aging of a compositionally uniform plume although class et al 1993 propose that some of the isotopic variations reflect such aging we infer that most of the isotopic heterogeneity in lavas from the ninetyeast ridge and kerguelen archipelago can be explained by mixing of the kerguelen plume with a depleted morb like mantle component however with this interpretation some of the youngest 42 44 ma lavas from the southern ninetyeast ridge which have206pb 204pb ratios exceeding those in indian ocean morb and kerguelen archipelago lavas require a component with higher206pb 204pb such as that expressed in lavas from st paul island

collapsibility and deformation characteristics of deep seated loess in china abstract lin z g and wang s j 1988 collapsibility and deformation characteristics of deep seated loess in china eng geol 25 271 282 as part of a comprehensive research program laboratory tests were conducted on samples of deep seated loess from different locations of the loessial regions of northern china including shaanxi gansu shanxi and honan provinces the information thus acquired on some aspects of the engineering properties of the samples such as the unit collapse pressure relationships the initial collapse pressure versus the preconsolidation pressure and deformation pressure characteristics is interesting and may prove useful to geotechnical engineers who work on projects on water poor plateaux or high river terraces in loessial regions throughout the world

numerical simulations of the baroclinic dynamics of density driven coupled fronts and eddies on a sloping bottom numerical simulations of the baroclinic dynamics of density driven coupled fronts and eddies are described the simulations are based on a two layer intermediate length scale model which filters out barotropic instability and focuses on the subinertial baroclinic evolution of density driven flows within the context of allowing finite amplitude height variations in the lower layer the baroclinic destabilization of a bottom trapped coupled front on a sloping bottom is described in the overlying fluid the instability takes the form of amplifying topographic rossby waves in the coupled front the perturbations to the downslope incropping are preferentially amplified compared to those on the upslope incropping the perturbations to the downslope incropping develop into downslope propagating plumes which eventually evolve into relatively coherent along slope propagating domes we discuss the propagation characteristics of these domes we also simulate the evolution of density driven eddies or domes the first eddy simulation we describe is for an initial eddy configuration which satisfies a zero topographic rossby wave condition in the upper layer we show that these traveling solutions remain remarkably coherent over a period of about 40 eddy circulation times or about 250 days for typical continental slope values for a sufficiently large initial eddy height upper layer fluid parcels can be transported in the along slope direction by the baroclinic eddy we also simulate the evolution of an initial eddy configuration which does not satisfy a zero topographic rossby wave condition in the upper layer a relatively intense cyclonic circulation develops in the overlying fluid over the traveling dome as does a topographic rossby wave tail however even these solutions remain surprisingly coherent over many eddy circulation times

erosion rates in the southern oregon coast range evidence for an equilibrium between hillslope erosion and sediment yield the relationship of hillslope erosion rates and sediment yield is often poorly defined because of short periods of measurement and inherent spatial and temporal variability in erosion processes in landscapes containing hillslopes crenulated by alternating topographic noses and hollows estimates of local hillslope erosion rates averaged over long time periods can be obtained by analysing colluvial deposits in the hollows hollows act as local traps for a portion of the colluvium transported down hillslopes and erosion rates can be calculated using the age and size of the deposits and the size of the contributing source area analysis of colluvial deposits in nine oregon coast range hollows has yielded average colluvial transport rates into the hollows of about 35 cm3cm 1yr 1 and average bedrock lowering rates of about 007 mm yr for the last 4000 to 15000 yr these rates are consistent with maximum bedrock exfoliation rates of about 0 09 mm yr calculated from six of the hollows supporting the interpretation that exfoliation rates limit erosion rates on these slopes sediment yield measurements from nine coast range streams provide similar basin wide denudation rates of between 0 05 and 0 08 mm yr 1 suggesting an approximate steady state between sediment production on hillslopes and sediment yield in addition modern sediment yields are similar in basins varying in size from 1 to 1500 km2 suggesting that erosion rates are spatially uniform and providing additional evidence for an approximate equilibrium in the landscape

cyclogenesis in the denmark strait overflow plume a densely spaced hydrographic survey of the northern irminger basin together with satellite tracked near surface drifters confirm the intense mesoscale variability within and above the denmark strait overflow in particular the drifters show distinct cyclonic vortices over the downslope edge of the outflow plume growing perturbations such as these can be attributed to the baroclinic instability of a density current a primitive equation model with periodic boundaries is used to simulate the destabilization of an idealized dense filament on a continental slope that resembles the northeastern irminger basin unstable waves evolve rapidly if the initial temperature profile is perturbed with a sinusoidal anomaly that exceeds a certain cutoff wavelength as the waves grow to large amplitudes isolated eddies of both signs develop anticyclones form initially within the dense filament and are rich in overflow water in contrast cyclones form initially with their center in the ambient water but wrap outflow water around their center thus containing a mixture of both water types the nonlinear advection of waters that were originally located within the front between both water masses contributes most significantly to the stronger intensification of the cyclones in comparison with anticyclones the frontal waters carry positive relative vorticity into the center of the cyclone the process bears therefore some resemblance to atmospheric frontal cyclogenesis after saturation there is a bottom jet of overflow water that is confined by counterrotating eddies anticyclones upslope and cyclones downslope of the overflow core the parameter dependence of the maximum growth rate is studied and the implications of eddy induced mixing for the water mass modification is discussed

a framework for unsaturated soil behaviour a framework is put forward which explains the shear behaviour of an unsaturated soil in terms of the total stresses and suctions acting within the soil it is based on the critical state model for

magma production rate along the ninetyeast ridge and its relationship to indian plate motion and kerguelen hot spot activity the ninetyeast ridge a linear trace of the kerguelen hot spot in the indian ocean was emplaced on a rapidly drifting indian plate magma production rates along the ridge track are computed using gravity derived excess crustal thickness data the production rates change between 2 and 15 m3 s over timescales of 3 16 myr major variations in magma production rates are primarily associated with significant changes in the indian plate velocity with low production phases linked to high plate velocity periods the lowest magma production rate 2 m3 s at 62 ma is associated with the rapid northward drift of indian plate under the influence of the reunion mantle plume the contemporaneous slowing of the african plate coincides with increase in magma production rate along the walvis ridge in the atlantic ocean the present study suggests that variations in the indian plate motion and frequent ridge jumps have a major role in controlling the magma production particularly on long period cycles 16 myr short period variations 5 myr in magma productions may be associated with intrinsic changes in the plume possibly due to the presence of solitary waves in the plume conduit

the mechanical behaviour of an unsaturated compacted silty clay a comprehensive experimental study has been carried out to investigate the volumetric and shear strength behaviour of a compacted silty clay which exhibited collapse on wetting behaviour the experiments consisted mainly of suction monitored triaxial tests carried out in a new apparatus the test results are interpreted on the basis of an elasto plastic framework using the conventional stress variable approach net stress and suction as well as an approach that takes into account the degree of saturation within two modified stress variables the evidence for critical states was examined based on the stress dilatancy relationships it was found that a critical state was observed experimentally in cases where the samples contracted over much of their stress paths but was not truly established in other cases relationships between the shear strength and specific volume at the ultimate state were established for samples over a wide range of degrees of saturation from fully saturated to the air dried state

isotopic constraints on the origin of hawaiian lavas from the maui volcanic complex hawaii strontium neodymium and lead isotopic data for samples from five volcanoes of the maui volcanic complex hawaii span nearly the total compositional range previously recognized for the hawaiian islands these data reveal a systematic compositional continuum for hawaiian tholeiites which is interpreted to result from partial melting of a variably mixed two component mantle plume source in contrast post shield building alkalic cap lavas define distinct compositional trends which appear to result from time and volume dependent binary mixing between hawaiian plume melts and a depleted mantle component

three dimensional simulations of overflows on continental slopes abstract three dimensional features and instabilities of dense overflows from marginal seas onto continental slopes are investigated using a three dimensional primitive equation numerical ocean model the numerical simulations reveal important instability and three dimensional features of the overflow plumes that have not been included in previous simulations with a one dimensional streamtube model and a two dimensional plume model it is shown that the large primary plume breaks into a number of smaller subplumes on the offshore side of the plume due to instabilities manifested as growing topographic rossby waves over the slope the observed high temporal and spatial variabilities in the denmark strait overflow could be caused by the inherent dynamic instabilities as revealed by the numerical simulations the simulations indicate that the initial overflow velocity and width the properties of the source water the planetary rotation and the slope steepness play major roles in determining the scales of

synoptic sections of the denmark strait overflow we report on a rapid high resolution survey of the denmark strait overflow dso as it crosses the sill the first such program to incorporate full water column velocity profiles in addition to conventional hydrographic measurements seven transects with expendable profilers over the course of one week are used to estimate volume transport as a function of density our observations reveal the presence of a strongly barotropic flow associated with the nearly vertical front dividing the arctic and atlantic waters the seven section mean transport of water denser than 27 8 is 2 7 0 6sv while the mean transport of water colder than 2 0 c is 3 8 0 8 sv although this is larger than the 2 9 sv of 2 c water measured by a 1973 current meter array we find that a sampling of our sections equivalent to the extent of that array also measures 2 9sv of cold water both the structure and magnitude of the measured flow are reproduced well by a high resolution numerical model of buoyancy driven exchange with realistic topography

plateaux ancient and modern geochemical and sedimentological perspectives on archaean oceanic magmatism unequivocal examples of oceanic plateaux older than about 2 1 ga have yet to be identified from the geological record the most likely explanation for this is the partial dismemberment of ancient plateaux at convergent plate margins and their subsequent juxtaposition in accretionary complexes with arc volcanic sedimentary and plutonic rocks a review of cretaceous plateaux in the eastern indian ocean suggests that certain geochemical isotopic criteria e g positive nb anomalies relative to primitive mantle employed by petrologists to identify modern oceanic plateau lavas are of little practical use in distinguishing ancient plateau lavas from arc and volcanic rifted margin basalts the extreme chemical and isotopic heterogeneity of indian ocean plateaux together with large variations in area and volume suggest that these edifices represent one extreme of a spectrum of oceanic plateau compositions the other extreme represented by iceland and ontong java is characterised by relatively homogeneous melt compositions reflecting steady state melting conditions and the absence of a continental lithosphere contaminant to plateau magmas crustal thickness and compositional estimates for archaean oceanic plateaux using the mckenzie bickle method give a mean thickness of 43 km and mean mgo content of 19 wt archaean plateaux were thus compositionally distinct from surrounding ocean floor basaltic komatiite and notably more magnesian than plateaux such as iceland mean for tertiary basalts 6 3 wt mgo and ontong java mean for core samples 6 9 wt mgo by analogy with cretaceous tertiary ultramafic complexes in western colombia the deeper portions of these plateaux may have consisted of noritic rocks underlain by iherzolite pyroxenite gabbronorite and dunite the general absence of such rocks in archaean terrains suggests that these portions of plateaux are only rarely preserved the search for remnants of ancient oceanic plateaux has identified examples of archaean submarine lavas in the canadian shield 2 7 ga southern abitibi belt superior province and kaapvaal craton of southern africa 3 5 3 3 ga barberton and pietersberg belts chemical and isotopic data suggest that komatiite tholeiite units from these areas were derived by partial melting of a plume or asthenospheric mantle source but do not distinguish adequately the tectonic environment rifted continental margin ocean floor oceanic plateau in which these rocks were erupted however the uplift histories of these putative plateau fragments as recorded in sediments bounding and intercalated with the komatiite basalt sequences appear to be broadly comparable to those of the best documented phanerozoic oceanic plateaux although further stratigraphical investigations of presumed plateaux ancient and modern are required to test this hypothesis we believe that the sedimentary record offers the best evidence yet for the formation of oceanic plateaux as far back as the mid archaean 3 5 ga

isotopic evidence for the origin of the manihiki and ontong java oceanic plateaus abstract pb nd and sr isotopic results for lavas of the cretaceous ontong java and manihiki oceanic plateaus fall well within the modern day oceanic island or hotspot field the data provide no evidence of old continental basements but indicate a major involvement of kerguelen type or em i like mantle in the sources of both plateaus which appear to have probably been formed at least in part by hotspots however the presently active hotspots that pacific plate reconstructions suggest might have been possible plateau sources lack kerguelen type isotopic compositions either these hotspots did not participate in the formation of the two plateaus or if they did kerguelen type material must have been volumetrically much more important early in their existence two hypotheses for the origins of these plateaus which involve hotspot sources are consistent with the sparse available geochemical geochronological and geophysical data the first holds that the plateaus formed cataclysmically in association with surfacing plume heads the second posits a relatively steady but robust hotspot at or near a ridge crest and requires a much longer period of formation a near ridge origin appears to be indicated by evidence that most of the pacific plateaus were built largely on relatively young ocean crust however we suggest that a near ridge origin is also compatible with the plume head concept in that plume heads appear very likely to become associated with spreading axes through their influence on rift propagation which should be substantially greater than for ordinary hotspots in either case the lack of hotspot tracks seamount chains attached to the two plateaus would be a consequence of ridge migration or rift propagation in a near ridge setting

intershield geochemical differences among hawaiian volcanoes implications for source compositions melting process and magma ascent paths as hawaiian volcanoes develop their lavas systematically change in composition and isotopic ratios of sr nd and pb these trends provide important constraints for understanding plume related volcanism as a volcano migrates away from the hotspot there are also geochemical differences between hawaiian shields in particular lavas from adjacent shields such as kilauea and mauna loa on hawaii and koolau and waianae on oahu have significant differences in abundances of some major and incompatible elements and isotopic ratios of sr nd and pb some incompatible element abundance ratios such as zr nb and sr nb are correlated with intershield differences in sr and nd isotope ratios but these isotopic ratios are not correlated with intershield differences in major element composition or even parent daughter abundance ratios such as rb sr and sm nd moreover at kilauea and mauna loa the intershield differences have apparently persisted for a relatively long time perhaps 100 ka these intershield geochemical differences provide important constraints on plume volcanism specifically i each volcano must have distinct magma ascent paths from the region of melt segregation ii the 25 50 km distance between adjacent but geochemically distinct shields requires that the sources vary on a similar scale and that the melt production region is similarly restricted the absence of correlations between lava compositions and radiogenic isotope ratios provides evidence for significant differences in melting process such as each shield forming by a different mean extent of melting with melt segregation at different mean pressures two types of models are consistent with the intershield geochemical differences i a relatively large radius ca 40 km plume conduit with a systematic spatial distribution of geochemical heterogeneities or ii a small radius less than 20 km plume conduit composed of geochemically distinct diapirs because relatively small radius diapirs of limited vertical extent are too small to create the large hawaiian shields a possible alternative is a continuous conduit containing solitary waves which transport geochemically distinct packets of material

petrogenesis of the flood basalts forming the northern kerguelen archipelago implications for the kerguelen plume the thick 20 km crust of the kerguelen archipelago formed as the tectonic setting of the kerguelen plume changed from an oceanic ridge centered location at 43 ma to its present location beneath the antarctic plate the uppermost crust is dominantly flood basalt with a thickness of up to 10 km inverse isochron 40ar 39ar ages for upper and lower lavas in a 630 m section of basalt flows from mont bureau are 30 4 and 29 0 ma re os isotopic systematics are consistent with this age most of the lavas in two stratigraphic sections mont bureau and mont rabouillere from the northern part of the archipelago have sr nd and pb isotopic characteristics similar to the youngest upper miocene to pleistocene lavas erupted in the southeast part of the archipelago i e initial 87sr 86sr 0 7050 143nd 144nd 10 mgo alkalic lavas in the southeast province of the archipelago in these northern sections it is confined to transitional lavas with

baroclinic characteristics of frictionally destabilized abyssal overflows received 11 october 2002 and in revised form 4 april 2003 observations show that the near sill dynamics of dense abyssal overflows is variable and is governed to a significant extent by a balance between rotation bottom friction and downslope acceleration due to gravity numerical simulations indicate that the near sill downslope velocities are comparable to the phase group velocities of long internal gravity waves this suggests the possibility that overflows can become supercritical and destabilized by bottom friction a theory is presented fo rt he frictional destabilization of rotating abyssal overflows and the accompanying baroclinic coupling with the overlying ocean this mode of transition allows for the formation of downslope and alongslope propagating periodic bores or pulses in the overflow and the generation of amplifying long internal gravity waves in the overlying ocean and may help to explain aspects of the observed variability which seem unrelated to purely inertial baroclinic instability

structure and variability of the denmark strait overflow model and observations we report on a combined modeling and observational effort to understand the denmark strait overflow dso four cruises over the course of 3 years mapped hydrographic properties and velocity fields with high spatial resolution the observations reveal the mean path of the dense water as well as the presence of strong barotropic flows energetic variability and strong bottom friction and entrainment a regional sigma coordinate numerical model of interbasin exchange using realistic bottom topography and an overflow forced only by an upstream reservoir of dense fluid is compared with the observations and used to further investigate these processes the model successfully reproduces the volume transport of dense water at the sill as well as the 1000 m descent of the dense water in the first 200 km from the sill and the intense eddies generated at 1 3 day intervals hydraulic control of the mean flow is indicated by a region supercritical to long gravity waves in the dense layer located approximately 100 km downstream of the sill in both model and observations in addition despite the differences in surface forcing both model and observations exhibit similar transitions from mostly barotropic flow at the sill to a bottom trapped baroclinic flow downstream indicating the dominant role of the overflow in determining the full water column dynamics

a stochastic hydro geomorphological model for shallow landsliding due to rainstorm abstract a prediction model of shallow landsliding is proposed it considers not only the deterministic aspects containing slope stability saturated throughflow and a soil regolith depth development but also the stochastic aspects of intensity and duration of rainfall it turns out that the probability of saturated throughflow which is the direct trigger mechanism to shallow landsliding can be expressed by a log normal distribution the short term probability of landsliding is defined as the excess probability that the depth of saturated throughflow surpasses the critical value the average recurrence interval t av of landsliding can be calculated as the expected value this model was applied to a test field where a lot of shallow landsliding occurred at a heavy rainstorm in 1988 then a dem of 5 m grid interval was utilized to calculate t av at every grid point consequently it was found that the percentage of the landslide grid number to the total grid number for every t av rank increases when t av decreases therefore it is confirmed that t av is an index of the susceptibility to shallow landsliding and the distribution map of t av can be regarded as a kind of hazard map the spatial distribution of t av reveals its significant dependence on the topography

ridge jumps and propagations in the south atlantic ocean the normal pattern of mid ocean ridge divergent plate boundaries comprises straight spreading segments with narrow zones of crustal accretion bounded by transform faults where these segments are offset normally the configuration of spreading segments and transform faults is stable for long periods but in places on fast and medium rate spreading ridges the transform offsets have propagated along the ridge axis as some spreading segments have grown at the expense of adjacent ones1 7 although such behaviour has hitherto not been well documented on slow spreading ridges we have mapped multiple ridge jumps and propagating rifts in two areas of the slow spreading 18 mm yr 1 half rate south atlantic ridge crest we attribute this anomalous behaviour in the south atlantic to the presence of hotspots near our study areas causing elevated asthenospheric mantle temperatures and lateral mantle flow with consequent excess volcanism and disruption of the normally well focused melt delivery processes at the ridge crest

geochemical characteristics of the youngest volcano mount ross in the kerguelen archipelago inferences for magma flux lithosphere assimilation and composition of the kerguelen plume mount ross is the youngest 1 0 1 ma volcanic edifice recognized in the kerguelen archipelago which is located on the northern part of the submarine kerguelen plateau lava types range from basaltic trachyandesite to trachyte and extensive glaciation has exposed an intrusive core ranging from gabbro to syenite mount ross rocks are not as silica undersaturated as the basanite to phonolite upper miocene lavas erupted in the southeast province of the archipelago however both lava suites are characterized by relatively high 87sr 86sr 0 7051 0 7054 and 0 7054 0 7058 respectively and low 206pb 204pb 18 02 18 14 and 18 06 18 27 respectively the abundant trachytes and phonolites in the age range from lower miocene to pleistocene on the kerguelen archipelago indicate that the supply of basaltic magma has been low for the last 20 m y despite the prolonged time period required for extensive mineral fractionation there is no evidence for assimilation of geochemically distinctive lithosphere i e most of the trachytes and syenites have initial isotopic ratios within the narrow range of the more mafic rocks volcanic and plutonic rocks of diverse composition and age 0 1 30 ma in the kerguelen archipelago have similar initial isotopic ratios consequently we infer that the kerguelen plume is characterized by 87sr 86sr 0 7051 0 7058 143nd 144nd 0 51263 0 51249 and 206pb 204pb 18 02 18 27

formation and development of the structure of collapsing loess in china abstract gao g 1988 formation and development of the structure of collapsing loess in china eng geol 25 235 245 it is well known that in china the regional variations in microstructure of loess are in phase with the overall tendency for collapsibility gradually decreasing from the northwest to the southeast these differences are consistent with variations in climatic conditions this indicates that the structure of collapsing loess is closely related to the climatic condition and geological environment based on the microstructure analyses it can be assumed that the formation and development of the structure of collapsing loess in china has undergone five stages the loessization stage the formation stage the development stage the degeneration stage and the clayization stage the five stages of loess regularly exist in different regions of the loess plateau this also shows that under the modern climatic condition the collapsing loess in the northwest of the loess plateau in china is still developing however the feature of collapsing loess is gradually disappearing in the southeast of the loess plateau

occurrence of landsliding on slopes where flowsliding had previously occurred an investigation in a loess platform north west china abstract loess slides in south jingyang platform shaanxi province usually occur on slopes where loess flowslides had previously occurred numerical modeling shows that flowslide occurrence causes conditions that increase shear stress and water content in the subsequent slopes both of these factors move the soil stress states towards its theoretical failure line under these conditions only small increases in water content can cause soil collapse this leads to the backscarps of former flowslides being much more prone to landsliding wetting test results show that the loess can collapse suddenly as the water content gradually increases therefore the wetting process induced by continued irrigation may result in the subsequent slides during constant water content tests of the loess material soil shear strength increases with the development of specimen deformation therefore slide mobilization which may be a relatively slow process is dependent on whether the wetting conditions exist finally a slope evolution model for loess platforms is proposed in this study

contrasting geophysical and geochemical signatures of a volcano at the axis of the wharton fossil ridge n e indian ocean the styx volcano is a prominent seamount 20 30 km diameter 2500 m high located at the axis of the wharton fossil spreading center n e indian ocean where seafloor spreading stopped 40 ma ago gravity modeling shows that it was emplaced on a weak lithosphere in agreement with an on axis origin when seafloor spreading was active the rocks dredged at the summit of the volcano are extreme typical alkaline basalts indicating a deep enriched mantle source we show here that most of this seamount was emplaced during the final stage of the seafloor spreading in the wharton basin we then discuss how alkaline magmatism can be emplaced at the summit of the styx

spatial variations in isostatic compensation mechanisms of the ninetyeast ridge and their tectonic significance the ninetyeast ridge ner one of the longest linear volcanic features on the earth offers an excellent opportunity of understanding the isostatic response to the interactions of mantle plume with the migrating mid ocean ridge bathymetry geoid and gravity ship borne and satellite data along 72 closely spaced transects and 17 overlapping grids on the ner are analyzed and modeled to determine the effective elastic thickness te beneath the entire ridge the results of 2 d and 3 d flexural modeling of the ner show large spatial variations in te values ranging from 4 to 35 km suggesting that the ridge was compensated along its length by different isostatic mechanisms the southern south of 22 s latitude and northern north of 2 n latitude parts of the ner have te values of 10 and 23 km respectively revealing that the southern part was emplaced on a lithosphere of intermediate strength possibly on flank of the indian plate whereas the northern part was emplaced in an intraplate setting in contrast in the central part of the ner between latitudes 22 s and 2 n highly variable te values 4 22 km are estimated the scattered te values in the central ner suggest that this part may have evolved due to the occurrence of more frequent ridge jumps caused by the interaction of kerguelen hot spot with rapid northward migration of the wharton spreading ridge residual mantle bouguer anomaly rmba map of the ner and adjacent basins reveals that the entire length of the ner is associated with a significant negative anomaly up to 200 mgal indicating the presence of thickened crust or less dense mantle beneath the ridge 3 d crustal thickness map of the ner generated by inversion of the rmba data shows a thick crust ranging from 15 to 19 km the present study clearly shows that ner possesses a highly segmented isostatic pattern with the occurrence of sub crustal underplating or sub surface loading

lithosphere structure and upper mantle characteristics below the bay of bengal the oceanic lithosphere in the bay of bengal bob formed 80 120 ma following the breakup of eastern gondwanaland since its formation it has been affected by the emplacement of two long n s trending linear aseismic ridges 85oe and ninetyeast and by the loading of ca 20 km of sediments of the bengal fan here we present the results of a combined spatial and spectral domain analysis of residual geoid bathymetry and gravity data constrained by seismic reflection and refraction data self consistent geoid and gravity modeling defined by temperature dependent mantle densities along a n s transect in the bob region revealed that the depth to the lithosphere asthenosphere boundary lab deepens steeply from 77 km in the south to 127 km in north with the greater thickness being anomalously thick compared to the lithosphere of similar age beneath the pacific ocean the geoid topography ratio gtr analysis of the 85 e and ninetyeast ridges indicate that they are compensated at shallow depths effective elastic thickness te estimates obtained through admittance coherence analysis as well as the flexural modeling along these ridges led to the conclusions i 85 e ridge was emplaced in off ridge environment te 10 15 km ii the higher te values of 25 km over the afanasy nikitin seamount ans reflect the secondary emplacement of the seamount peaks in off ridge environment iii that the emplacement of the ninetyeast ridge north of 2 n occurred in an off ridge environment as indicated by higher te values 25 30 km furthermore the admittance analysis of geoid and bathymetry revealed that the admittance signatures at wavelengths 800 km are compensated by processes related to upper mantle convection

the meridional flow of source driven abyssal currents in a stratified basin with topography part ii numerical simulation abstract a numerical simulation is described for source driven abyssal currents in a 3660 km 3660 km stratified northern hemisphere basin with zonally varying topography the model is the two layer quasigeostrophic equations describing the overlying ocean coupled to the finite amplitude planetary geostrophic equations describing the abyssal layer on a midlatitude plane the source region is a fixed 75 km 150 km area located in the northwestern sector of the basin with a steady downward volume transport of about 5 6 sv sv 106 m3 s 1 corresponding to an average downwelling velocity of about 0 05 cm s 1 the other parameter values are characteristic of the north atlantic ocean it takes about 3 2 yr for the abyssal water mass to reach the southern boundary and about 25 yr for a statistical state to develop time averaged and instantaneous fields at a late time are described the time averaged fields show an equatorward flowing abyssal current with distinct up and downslope groundings with decr

mesoscale variability in denmark strait the pv outflow hypothesis abstract the outflow through denmark strait shows remarkable mesoscale variability characterized by the continuous formation of intense mesoscale cyclones just south of the sill these cyclones have a diameter of about 30 km and clear signatures at the sea surface and in currents measured near the bottom they have a remnant of arctic intermediate water aiw in their core the authors hypothesis is that these cyclones are formed by stretching of the high potential vorticity pv water column that outflows through denmark strait the light upper layer of the outflow the east greenland current remains on the surface in the irminger sea while the dense overflow water descends the east greenland continental slope the midlevel waters mostly aiw could thus be stretched by more than 100 which would induce very strong cyclonic relative vorticity the main test of this new hypothesis is by way of numerical experiments carried out with an isopycnal coordinate ocean model configured to have a marginal sea

irrigation induced loess flow failure in heifangtai platform north west china because the construction of dams and hydro electric power stations of liujiaxia and yanguoxia across the yellow river in west gansu province north west china inhabitants in the reservoir areas were resettled in heifangtai loess platform irrigation thus started in 1968 perched water table was significantly raised and resulted in many loess flow failures in the past 30 years this type of landslide was often featured by long runout distance high speed and recurrence laboratory tests along icu and csd stress paths were performed it is shown that the loess has a strong strain softening behavior and instability can occur in drained condition before undrained liquefaction the failure mechanism of loess flow can be interpreted as the following process with increasing perched water table collapse in loess was first triggered in drained condition and led to undrained loading to adjacent soil in the lower saturated part of the loess platform under gravity loading applied by upper dry loess undrained flow failure mobilized eventually

stability investigation and stabilization of a heavily fractured and loosened rock slope during construction of a strategic hydropower station in china abstract this paper presents a case history of the 205 m high intake slope at the huangjinping hjp hydropower station on the dadu river deformation and cracks developed on the cut slope during excavation and the deformation extended onto the natural slope above the cut giving rise to serious safety concerns research was conducted to study the engineering geological conditions and the deformation characteristics of the slope during excavation in situ monitoring data and possible causes of cracking were analyzed the results show that the zone of loosened rock mass extended much deeper into the rock mass than it did at other sites in the region because of the tectonic history of the site and cutting the empirically designed cable anchors with lengths of 40 m or 45 m were insufficient to stabilize the slope while the remedial cable anchors as long as 100 m successfully stabilized the slope the analytic method and procedure used in this paper could be useful to similar projects

instability of abyssal currents in a continuously stratified ocean with bottom topography a theory is developed for the baroclinic destabilization of density driven abyssal flows over topography in a rotating environment the dominant instability mechanism being studied is the release of available potential energy caused by gradual downhill slumping of the abyssal current the present model assumes a two layer configuration and allows for intersections of the interface with the bottom i e true fronts as well as continuous stratification in the ambient fluid the linear instability problem in a channel for a current with parabolic cross section is solved and the perturbation growth rate and most unstable wavenumber are both shown to increase with current thickness a similar trend is evident as the stratification number is increased or the current width is decreased the instability manifests itself in the overlying ocean as an amplifying topographic rossby wave alternating positive negative pressure anomalies in the upper layer are accompanied by a wavelike deformation of the abyssal current that is most pronounced on the downslope side upper layer vortical features have a distinct vertically tapered shape and are to be interpreted as bottom intensified eddies long term evolution of the flow is elucidated in a series of simulations employing the fully nonlinear governing equations it is found that even though the linear instability calculation relates to a periodic current the instability characteristics are still valid to a good approximation for the case of a source flow the abyssal current breaks up into a series of plumes that penetrate downslope into the deeper ocean producing strong current fluctuations not unlike those observed in denmark strait overflow water furthermore introduction of more realistic topography into the numerical simulation leads to the development of coherent baroclinic vortex pairs whose upper layer component is strongly cyclonic

the production of north atlantic deep water sources rates and pathways updating an earlier account by dickson et al 1990 this paper reviews the initial development phase of north atlantic deep water nadw production from the points where the dense inflows from nordic seas cross the greenland scotland ridge to the point off south greenland where the buildup of new production appears almost complete in particular three long term current meter arrays totaling 91 instruments and set at 160 km intervals south from the denmark strait sill are used to validate earlier short term arrays by others and in combination with these earlier arrays to describe the downstream evolution of mean speed depth and entrainment the variability of the overflow current in space and time and the likely contribution of the other three main constituents of nadw production at densities greater than 27 8 from the points of overflow 5 6 sv the transport within this range increases by entrainment and confluence with other contributory streams to around 13 3 sv at cape farewell while recirculating elements prevent us from determining the net southgoing transport a nadw transport of this order appears consistent with recent estimates of net abyssal flow passing south through the north and south atlantic