gravity anomalies and flexure of the lithosphere at ascension island summary ascension island in the northern south atlantic forms the summit of a volcanic edifice 60 km in diameter which places a substantial load on the underlying young oceanic lithosphere an analysis of a combined data set of recent and historical surface gravity and bathymetry measurements on and around the island suggests that the lithosphere responds to this load by flexure equivalent to that of an elastic plate only z3 km thick and that the mean density of the volcanic edifice is 2500 kg m 3 a steep gravity gradient across the island cannot be explained by a simple flexural model and must be attributed to lateral density variations within the volcano itself the effective elastic thickness is considerably less than the expected z 12 km mechanical thickness of the 6 ma lithosphere loaded by the volcano and less even than zero age elastic thicknesses commonly observed at slow spreading ridges with axial rift valleys the unusually small elastic thickness may be attributed to the combined effects of the high curvature beneath the island which produces bending stresses that are limited by the yield stress envelope localized heating of the lithosphere during emplacement of the island and crustal thickening when these factors are taken into account the observed flexure is consistent with rheological models based on experimental rock mechanics

seismic structure of the mid atlantic ridge 8 9 s 1 the mid atlantic ridge at 8 9 s is characterized by a transition from axial valley to axial high and recent episodes of ridge jumping and ridge propagation we present constraints on the structure of 0 4 ma crust in this region on the basis of the analysis of wide angle seismic data from a grid of profiles across and parallel to the current and abandoned spreading centers a 350 800 m thick oceanic layer 2a interpreted as high porosity extrusive basalts is underlain by a 2 0 2 5 km layer 2b with velocities which increase with age and decrease in the vicinity of the pseudofaults layer 3 velocities are uniform across the area except for a possible localized anomaly at the ridge axis the crustal thickness varies from 6 7 km near the pseudofaults formed by ridge propagation to 9 10 km at the segment center of the recently 0 3 ma abandoned spreading center seismically determined crustal thickness and density variations and age related lithospheric cooling can plausibly account for all observed variations in gravity across the area and there is no requirement for the thicker crust at the segment center to be underlain by hot mantle the transition from axial valley to axial high occurs at a crustal thickness of 8 km

a multichannel seismic study of lithospheric flexure across the hawaiian emperor seamount chain existing models assume that the thickened crust beneath seamounts is the result of a surface volcanic load flexing an elastic plate new results suggest that flexed oceanic crust beneath the hawaiian emperor seamount chain is underlain by a 4 km thick deep crustal body we intepret the body as a deep crustal sill complex associated with the tholeiitic stage of volcano building along the chain

seasonal water use patterns of juniperus ashei on the edwards plateau texas based on stable isotopes in water summary naturally occurring stable isotopes of oxygen and hydrogen were used to determine the seasonal water sources of juniperus ashei and its relationship with soil water content on the edwards plateau of central texas honey creek state natural area the patchy nature of juniper stands rooting in the thin soils over karstified carbonate bedrock imparts large spatial heterogeneity in soil water content and isotopic composition largely by differential shading but also by canopy interception there is a seasonal decrease in the oxygen isotope composition of juniper xylem water by 2 during the transition from hot dry summer conditions to the cool moist winter prior to this shift the xylem water isotope composition is similar to local springs presumed to represent local groundwater after this shift the xylem water composition is similar to soil water below 10 cm these results suggest a change in juniper water use from a dominantly deep water source during the summer to a dominantly soil source during the winter over the study period but the timing of this transition will likely vary from year to year

ocean island densities and models of lithospheric flexure estimates of the effective elastic thickness te of the oceanic lithosphere based on gravity and bathymetric data from island loads are commonly significantly lower than those based on the wavelength of plate bending at subduction zones the anomalously low values for ocean islands have been attributed to the finite yield strength of the lithosphere to erosion of the mechanical boundary layer by mantle plumes to pre existing thermal stresses and to overprinting of old volcanic loads by younger ones a fifth possible contribution to the discrepancy is an incorrect assumption about the density of volcanic loads we suggest that load densities have been systematically overestimated in studies of lithospheric flexure potentially resulting in systematic underestimation of effective elastic thicknesses and overestimation of the effects of hotspot volcanism we illustrate the effect of underestimating load density with synthetic examples and an example from the marquesas islands this effect combined with the other effects listed above in many cases may obviate the need to invoke hotspot reheating to explain low apparent elastic thicknesses

finite difference calculation of traveltimes in three dimensions the traveltimes of first arriving seismic rays through most velocity structures can be computed rapidly on a three dimensional numerical grid by finite difference extrapolation head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 11 percent are found between the results of this technique and ray tracing for a complex but smooth model this scheme has proven useful for earthquake location and shows promise as an inexpensive well behaved substitute for ray tracing in forward modeling and kirchhoff inversion applications

a seismic study of lithospheric flexure in the vicinity of tenerife canary islands seismic data have been used to determine the crustal and upper mantle structure of tenerife canary islands a volcanic island of tertiary age located on 140 ma oceanic crust reflection data show that oceanic basement dips gently towards the island forming a flexural moat which is infilled by 2 3 km of well stratified material the moat is characterised by a major angular unconformity which we attribute to volcanic loading of pre existing oceanic crust and overlying sediments and the subsequent infilling of the flexure by material that was derived at least in part from the islands refraction data show that the flexed oceanic crust has a mean thickness of 6 41 0 42 km and upper and lower crustal velocities of 4 8 5 4 km s 1 and 6 7 7 3 km s 1 respectively the flexure which has been verified by gravity modelling can be explained by a model in which tenerife and adjacent islands have loaded a lithosphere with a long term 106 yr elastic thickness of approximately 20 km seismic and gravity data suggest that up to 1 5 105 km3 of magmatic material has been added to the surface of the flexed oceanic crust which assuming an age of 6 16 ma for the shield building stage on tenerife implies a magma generation rate of about 0 006 to 0 02 km3 a 1 this rate is similar to estimates from other african oceanic islands e g reunion and cape verdes but is significantly less than that which has been calculated at hawaii there is no evidence in either the seismic or gravity data that any significant amount of magmatic material has underplated the flexed oceanic crust the crustal and upper mantle structure at tenerife therefore differs from other oceanic islands such as hawaii and marquesas where 4 km of underplated material have been reported

focusing of eruptions by fracture wall erosion lithospheric flexural stresses beneath volcanic loads are horizontally strongly compressive towards the top of the lithosphere thus while magma transport through the brittle lithosphere occurs via fractures the fracture paths under the volcanic center are predicted by stress trajectories to be horizontal and thus unable to supply melt to the volcanic edifice where eruptions are observed moreover the magnitude of the compressive stresses under large loads would close down any vertical magma paths both problems may be resolved by additional stresses due to melting or thermomechanical erosion of fracture walls developing over the life span of the volcano fractures form and close frequently in the seismogenic zone of the lithosphere with each fracture eroding away a small amount of material the total amount of material removed makes the stress field more tensile thereby facilitating the long lived and vertically oriented magma pathways necessary to build discrete volcanic structures

structure and evolution of the hengill grensdalur volcanic complex iceland geology geophysics and seismic tomography recent geological and geophysical research indicates the presence of three volcanic systems in the hengill grensdalur area and progressive westerly migration of the accretionary plate boundary the hengill system comprises the active hengill central volcano and a nne trending fissure swarm that is the present locus of crustal accretion in the area extinct volcanic systems are identified with the mountain hromundartindur and the grensdalur area and associated nne trending zones the grensdalur system was formerly the locus of accretion in the area a widespread geothermal area encompasses the whole area and is fueled by at least three distinct heat sources associated with the three volcanic systems a tomographic study of the upper 5 km of crust using local earthquakes imaged three bodies with velocities up to 15 higher than the average background velocities and volumes of several tens of cubic kilometers one of these underlies the grensdalur volcano and a second underlies the olkelduhals area within the hromundartindur system these are interpreted as intrusions that are the solidified magma reservoirs of their respective volcanic systems and the heat sources of those parts of the geothermal area above them the third high velocity body underlies the extinct basalt shield husmuli which is not associated with geothermal resources that body is interpreted as a cold intrusion that is the frozen magma conduit that fed the surface eruptive site a low velocity body with a volume of a few cubic kilometers was imaged in the depth range 2 4 km beneath the northern part of the presently active hengill central volcano this volume may contain partial melt and represent the heat source or part of the heat source fueling the hengill field heat balance calculations show that subsurface magmatism continued in the extinct grensdalur volcano long after volcanic activity and crustal accretion migrated from it to the hengill system

pn velocities beneath continental collision zones the turkish iranian plateau summary tomographic imaging of the uppermost mantle velocity in the middle east shows normal pn velocities 8 0 8 2 kms beneath the black sea and the southern caspian sea in contrast low pn velocities 7 9 km s i are found beneath the aegean sea and much of the turkish iranian plateau a region of exceptionally low pn velocities c7 6 km s beneath the lesser caucasus mountains and along the turkish iranian border coincides with a region of high sn attenuation and extensive neogene volcanism all these features suggest that near solidus conditions exist within the uppermost mantle beneath the turkish iranian plateau such conditions may result either from the decompression melting associated with upwelling convection cells or by the infiltration of water released from subducted lithosphere into the mantle above the presence of a partially melted uppermost mantle weakens the lithosphere beneath the turkish iranian plateau thus allowing it to become the locus of deformation in the arabian eurasian collision zone

seasonal water use strategy of cyclobalanopsis glauca in a karst area of southern china subtropical evergreen and deciduous broadleaved mixed forests occur on shallow soils or even barren areas with carbonate bedrock in karst areas of southern china the nature of seasonal water sources is poorly understood in the complex environment of these deeply rooted forests these forests grow well despite frequent seasonal droughts and limited water availability the authors hypothesized that trees here mainly adsorb water from deep root systems that also recharge the surface soil at least during the dry season to test this hypothesis seasonal isotopic hydrogen and oxygen compositions 18o and d of surface different subsurface well and xylem water available to cyclobalanopsis glauca were compared in a subtropical mixed forest near guilin guangxi china levels of 18o and d in soil water decreased with depth in the 0 30 cm soil horizon and increased below 30 cm depth in march july and october however in january the soil water 18o and d values decreased generally with soil depth deep root systems of trees were suspected to redistribute groundwater to shallow surface soils providing the main stable water source for adult c glauca during the growing season march october however isosource outputs indicated that c glauca extracts water mainly from shallow soil throughout the sampling year in the cold and dry season january winter rains recharging soil water may provide most of the water available for transpiration the two tier water structure in this karst environment connected by deep roots appears to help form a closed water circulation of groundwater soil plant atmosphere

three dimensional tomographic inversion of combined reflection and refraction seismic traveltime data summary a tomographic inversion method is presented for the determination of 3 d velocity and interface structure from a wide range of body wave seismic traveltime data types it is applicable to refraction wide angle reflection normal incidence and multichannel seismic data and is best suited to a combination of these that provides good independent constraints on seismic velocities and interface depths the inversion process seeks a layer interface minimum structure model that is able to explain the given data satisfactorily by inverting to minimize data misfit and model roughness norms simultaneously this regularized inversion and the use of smooth functions to describe velocities and depths allows the highly non linear tomographic problem to be approximated as a series of linear steps the inversion process begins by optimizing the fit to the data of a highly smoothed initial model in each subsequent step structure is allowed to develop in the model with successively greater detail evolving until a satisfactory fit to the data is obtained parameter uncertainties for the final model are then estimated using an a posteriori covariance matrix analysis smooth layer interface models are parametrized using regular grids of velocity and depth nodes from which spline interpolated interface surfaces and velocity fields are defined forward modelling is achieved using ray perturbation theory and a two point ray tracing method that is optimized for a large number of closely spaced shot or receiver points the method may be used to generate 1 and 2 d models from for example vertical seismic profile data or 2 d surveys in which the 3 d geometry of a survey is correctly accounted for the ability of the method to resolve typical target structures is tested in a synthetic salt dome inversion from a set of noisy traveltime data the model converges quickly to a well resolved final model from different starting models the application of this method to real data is demonstrated with a combined 3 d inversion of refraction and reflection data which provide p wave velocity constraints on the methane hydrate stability zone in the cascadia margin offshore vancouver island

crustal evolution over the last 2 m y at the mid atlantic ridge oh 1 segment 35 n we present the crustal and mantle velocity structure along the strike of the eastern rift mountains at 35 n on the mid atlantic ridge these results were obtained by an inversion of 1800 pg pn and 450 pmp travel times and by gravity modeling as commonly observed at slow spreading mid ocean ridges thicker crust 9 km occurs at the segment midpoint while thinner crust 7 km is found toward the segment ends this along strike variation occurs primarily in the lower crust which is 7 km thick at the segment center and 4 6 km thick at the segment ends in contrast the thickness of the upper crust is relatively constant along strike at the segment ends relatively low velocities extend for 10 15 km along strike and from the seafloor to 4 km depth these low velocities may indicate an attenuated melt supply and or fracturing and alteration within the shallow to mid crust directly beneath a cluster of three seamounts at the segment center is a region of relatively high velocity 0 5 km s in the mid crust this feature may correspond to a frozen magma chamber that fed the overlying volcanoes a synthesis of these results with those from two companion experiments along the rift valley and the conjugate flank provide a detailed record of crustal accretion and evolution at this segment specifically the crustal velocity structures of each flank are nearly identical and they exhibit a thinner and 16 faster upper crust than is observed on axis the lower crust is remarkably similar in all three settings except for a low velocity body on axis which is interpreted as a partially molten zone the maximum crustal thickness is also similar in all three profiles but north of the segment center zero age crust is nearly 4 km thinner than beneath the eastern flank and 2 km thinner than beneath the western flank these differences may indicate that segment centered mantle upwelling varies on a timescale of 2 m y

field hydrological monitoring of a sloping shallow pyroclastic deposit many mountainous areas in campania southern italy are characterized by steep slopes covered by unsaturated volcanic deposits shallow landslides are frequently triggered by intense and persistent

field validation of isotopic procedures for determining sources of water used by plants in a semi arid environment abstract methods for using stable isotopes of water 2 h 18 o for determining the sources of water transpired by plants in a semi arid field situation are validated a comparison of the isotopic composition of the soil water in zones of high soil water potential and hence high plant water availability with that in plants shows that the overall approach is subject to a randomly distributed error of 5 in 2 h isotopic composition with no indication of any bias this includes errors involved in the extraction of water from plants and soil in spatial heterogeneity in soil profiles in heterogeneity within plant canopies in the time of travel of water through the plant and in assumptions about fractionation of isotopes during water uptake by plants these errors are generally smaller than natural variations of isotope thus allowing the isotopic method to be used similarly an estimate for the error in 18 o is is 1 a compartmental model was used to quantify errors associated with an inversion model using isotopic data of water coming from two soil layers the error in the estimation of the fraction of water taken from each compartment was 20 for many hydrological purposes this error is acceptable this is the first study estimating the errors associated with the method

hydraulic properties of karst fractures filled with soils and regolith materials implication for their ecohydrological functions abstract in regions with shallow soils underlain by fractured bedrock hydrological processes of substrate are no longer only limited to soil layers however ecohydrological functions of bedrock fractures especially those filled with different soil and regolith materials have not been fully understood in this study we aimed to evaluate water transport and supplying capacities of the filled fractures through investigating hydraulic properties and explore their dominant influencing factors three typical fractures a vertical one filled with fine textured soil vsf a vertical one filled with soil and regolith materials vsrf and a non vertical one filled with coarse textured soil nsf were selected from a large newly excavated trench on a karst hillslope of southwest china stratified samples of fracture fillings were collected to measure saturated hydraulic conductivity k s water retention curves and basic physicochemical properties additionally twenty soil profiles in different topographic locations in the same study area were also analyzed in order to support the results derived from the filled fractures all fractures exhibited extremely high k s 87 149 mm h 1 in surface soil 0 10 cm which allowed rapid infiltration of rainwater into subsurface subsurface water transport was smooth when underlying fracture fillings were loamy with relatively high k s about 10 mm h 1 like in nsf but was blocked when they were clayey with low k s 1 like in vsf and vsrf all fracture fillings had abilities to hold water and available water was the most in nsf which provided an extra water source for plant growth in shallow soil regions the ecohydrological functions of the filled fractures often depend on the properties and layering of fracture fillings which was supported by those obtained from the twenty soil profiles these results are helpful for better understanding of the ecohydrological processes and functions of fractures filled with different materials in karst regions

structure and water storage capacity of a small karst aquifer based on stream discharge in southwest china summary karst spring stream discharge reflects the global configuration of the aquifer however quantitative description of the aquifer structure such as effective porosity neff and water storage capacity by the discharge analysis is difficult because of the complex conduit fracture system this study attempted to quantify the characteristics of karst aquifer based on discharge recession and time series analysis methods three recession models including modified maillet mangin and boussinesq models were evaluated to choose the most suitable one for analyzing the aquifer structure and auto correlation and cross correlation functions were applied to study the aquifer response in both year and rainfall event time scales the results showed that the modified maillet model was more suitable in the study catchment with mangin model overestimating and boussinesq model underestimating the discharge the neff was 3 73 for the total aquifer and it was 0 07 0 33 and 3 33 for the conduit fracture and matrix respectively based on a case study of a rainfall event with precipitation of 68 mm the water volumes drained by the three media were 25 43 33 40 and 41 17 respectively this indicates that although conduit network is not very developed with lower neff it is still an important water transmissive element draining more than a quarter of water after the rainfall event the memory time of the aquifer was 4 days for the year scale and 8 h for the rainfall event 68 mm scale this demonstrates that the aquifer has a well developed drainage system with a quick response to the rainfall the above results provide further insights for hydrological processes modeling and water resources management for the small catchment in karst regions

seismic structure of the crust and upper mantle in the peace river arch region canada four intersecting 300 km long reversed refraction lines within northwestern alberta and northeastern british columbia have been interpreted for crustal and upper mantle seismic velocity structure in the peace river arch pra region of the western canada sedimentary basin the data have been modeled using a two dimensional ray trace forward modeling approach based on asymptotic ray theory to match travel times and amplitudes of first and coherent later arrivals in addition an inversion of first arrival travel times along a fan shot profile has been performed to constrain crustal thickness northwest of the arch in a region not sampled by the reversed profiles the major features of the structural models are 1 weak to moderate lateral variations in crustal structure with no evidence of significant layering or thick low velocity zones within the crust 2 an average sub basement rms crustal velocity of 6 6 km s average upper mantle velocity of 8 25 km s and average crustal thickness of 40 km 3 regional variations in structure which appear related to the dominant n s trending cratonic structure including crustal thickness upper crustal and upper mantle velocities and pmp character and 4 subtle variations in structure that may be associated with the e w trending devonian axis of the pra including a shallowing of high lower crustal velocities thickening of the crust and an anisotropic pmp character beneath the arch and a weak trend in the dip of intracrustal reflectors away from the center of the pra region evidence from the refraction and other geophysical data for the presence of a local crustal expression of the pra is weak but suggests a thermal as opposed to flexural origin for its anomalous vertical movements

seismic imaging of hotspot related crustal underplating beneath the marquesas islands volcanism in active continental rift zones1 2or on rifted continental margins3is frequently associated with unusually high lower crustal seismic velocities indicating the presence of large igneous intrusions at the base of the crust the only comprehensive investigation of crustal underplating at an oceanic hotspot beneath hawaii4 6 has yielded controversial results7 here we report the results of seismic refraction experiments across the marquesas islands hotspot trace which show that the island chain is underlain by crust 15 17 km thick and that a large lower crustal region 275 km wide and 2 8 km thick has seismic velocities 7 3 7 75 km s 1 indicative of crustal underplating wide angle reflections occur near the base of the normal lower crustal velocities and at the base of the underplating complex we interpret these reflectors as the relict pre hotspot moho and the current post hotspot moho respectively the high velocity material may be purely intrusive or may consist of a mixture of intrusive and preexisting rocks the volume of the high velocity body is impressive amounting to nearly twice the volume of volcanics erupted at the surface

isostatic compensation of tectonic features of the mid atlantic ridge 25 27 30 s we investigate the isostatic compensation of tectonic features along the mid atlantic ridge 25 27 30 s through a detailed examination of the relationship between bathymetry and gravity anomalies the study area includes three ridge segments spreading at an average full rate of 35 mm yr their flanks out to about 6 m y old crust and the intervening rio grande and moore fracture zones in a three dimensional analysis of gravity and sea beam bathymetry data we focus on crustal thickness variations and mantle density anomalies by removing from the observed fields the predicted contribution of simple crustal and mantle models positive residual gravity anomalies over the northern wall of the rio grande fracture zone indicate that in spots the crust is 2 3 km thinner than average there in contrast to the moore fracture zone where little thinning is observed the greater than average depths of the fracture zones as a whole are not locally compensated by thin crust but the deepest basins within the active transform parts of the fracture zones are partially compensated by thin crust or cooler mantle a narrow linear ridge that crosses one of the inactive branches of the moore fracture zone had been suggested to be the product of an episode of excess volcanism but is found to be underlain by thinner not thicker crust there is no indication of thicker crust beneath topographic highs at the inside corners of ridge transform intersections presumably these highs are dynamically maintained as must be the median valleys characteristic of most the length of the ridge segments judging from the absence of axial residual gravity anomalies there is a residual gravity low associated with an unusually shallow 15 to 20 km section of the ridge segment between the rio grande and moore offsets where the median valley virtually disappears our analysis suggests that while part of the anomalous elevation of this section of the ridge axis is attributable to excess volcanism and a thicker crust much of the elevation contrast is simply caused by the diminution of the dynamic mechanism responsible for median valley formation two dimensional fourier transforms of the gravity and bathymetry fields show that seafloor topography is strongly aligned parallel to either ridges or transforms while density anomalies are more randomly oriented this implies that the tectonic processes that control the gross ridge transform topography are not the dominant control on the magmatic processes that determine the upper mantle and crustal density structure statistical analysis of the coherence between the gravity and bathymetry fields indicates that the average effective elastic plate thickness in the study area is about 6 km

temporal and spatial variability of seafloor spreading processes in the northern south atlantic the paper reports the results of an aeromagnetic study of the mid atlantic ridge in the northern south atlantic the study consists of 38 000 km of low altitude magnetics tracks centered on the ridge covering nearly 700 000 km2 south of ascension island most tracks extend out to crust of anomaly 5b age large areal coverage coupled with high resolution allows comparison of crustal generation processes over space along ridge strike and time across strike temporal heterogeneities revealed in the data include a pattern of change in spreading rate and direction of plate motion the total spreading rate increased sharply at 10 ma and then decreased between 6 ma and the present the time variation of spreading rate is similar to that seen in other areas such as the north atlantic and the southern and eastern pacific the plate boundary has evolved over time in response to the shift in plate motion direction through fragmentation and rotation of ridge segments between fracture zones in several cases the location of the spreading axis has shifted through jumps or propagation across fracture zones the periods of plate boundary modification correlate with the increase and decrease in spreading rate spatial heterogeneities are caused by the influence of the ascension hotspot on the mid atlantic ridge in the northern portion of the study area and include two apparent seamount chains formed by the hotspot ascension island and several of the seamounts appear to have been produced some distance away from the hotspot location presently about 225 km southeast of ascension island by channeled subaxial magma flow beneath the spreading center the seamount chains therefore do not mark hotspot tracks large local variations in spreading rate and degree of asymmetry along strike are also found

crustal structure gravity anomalies and flexure of the lithosphere in the vicinity of the canary islands summary simple elastic plate models have been used to calculate the flexure of the lithosphere caused by volcanic loading at the canary islands and sediment loading at the moroccan continental margin by comparing the calculated flexure to observations based on seismic refraction and free air gravity anomaly data constraints have been placed on the long term 106 years elastic thickness of the lithosphere te the best fit between the calculated and observed flexure in the vicinity of the canary islands is for te 20 km this value of te also explains seismic reflection data in regions that flank the island provided that the lithosphere underlying the moroccan margin is sufficiently weak te 5 km for sediment loading to contribute little to the island flexure a backstripping study in which gravity data are used to constrain the value of te supports the suggestion that the lithosphere underlying the moroccan margin like its conjugate at the baltimore canyon trough is weak although sediment loading at the moroccan margin appears therefore to have exerted little influence on the structure of the canary islands there is evidence that island flexure may have influenced the stratigraphic development of the moroccan margin especially in the region of the slope anticline a te of 20 km is about 15 km lower than would be expected on the basis of the thermal age of the oceanic lithosphere that underlies the canary islands similar low values have been reported from oceanic islands in the pacific superswell region of french polynesia where they have been attributed to re heating of the lithosphere by one or more hotspots a general e w age progression of the volcanic rocks suggests that the canary islands were also generated by a hotspot they lack however the topographic swell and gravity geoid high which usually accompanies these features one possibility is that the low te values are the result of a pre existing weakness in the oceanic crust a te of 20 km is large enough however for a significant part of the mantle to still be involved in the support of the island loads a more likely explanation is thermal weakening by a hotspot which has been localized enough to reduce te but not to produce a swell or gravity geoid high irrespective of its cause the low te suggests that oceanic lithosphere does not necessarily progressively increase its strength with age so that even 140 ma old lithosphere is vulnerable and can in some regions be significantly weakened by later thermal and mechanical processes

simultaneous velocity and interface tomography of normal incidence and wide aperture seismic traveltime data summary a tomographic inversion technique that inverts traveltimes to obtain a model of the subsurface in terms of velocities and interfaces is presented it uses a combination of refraction wide angle reflection and normal incidence data it simultaneously inverts for velocities and interface depths and it is able to quantify the errors and trade offs in the final model the technique uses an iterative linearized approach to the non linear traveltime inversion problem the subsurface is represented as a set of layers separated by interfaces across which the velocity may be discontinuous within each layer the velocity varies in two dimensions and has a continuous first derivative rays are traced in this medium using a technique based on ray perturbation theory and two point ray tracing is avoided by interpolating the traveltimes to the receivers from a roughly equidistant fan of rays the calculated traveltimes are inverted by simultaneously minimizing the misfit between the data and calculated traveltimes and the roughness of the model this smoothing regularization stabilizes the solution of the inverse problem in practice the first iterations are performed with a high level of smoothing as the inversion proceeds the level of smoothing is gradually reduced until the traveltime residual is at the estimated level of noise in the data at this point a minimum feature solution is obtained which should contain only those features discernible over the noise the technique is tested on a synthetic data set demonstrating its accuracy and stability and also illustrating the desirability of including a large number of different ray types in an inversion

susceptibility of mid ocean ridge volcanic islands and seamounts to large scale landsliding 1 with a view to assessing the incidence of large scale landsliding a morphologic database was created for volcanic islands and seamounts on young oceanic lithosphere the database included 44 mid ocean ridge seamounts jasper seamount and the islands of ascension bouvet guadalupe and several of the galapagos and azores islands supplemented with published reports from a further five volcanic edifices the data reveal that major landslides are common on edifices taller than 2500 m but are rare in shorter edifices implying a threshold of instability at around 2500 m a number of causes of this threshold are discussed for example many structures taller than 2500 m are or were originally volcanic islands and therefore their flanks probably include extensive weak hyaloclastite built up from lava sea interactions around coasts compaction of hyaloclastites in larger edifices lead to regions of low permeability which may help to explain the more deeply seated slope failures it is intriguing that the threshold also coincides with the edifice height at which volcanic ridges become observable a stage at which dike intrusion is probably common because slope oversteepening or excess pore pressure associated with dike intrusions have been proposed elsewhere as landslide triggers the current indications of landsliding reveal no observable relation to rainfall as landslides occur equally in wet and dry climates and no relation to tectonic setting as there are relatively few major landslides in the seismically active azores group

crustal structure flexure and subsidence history of the hawaiian islands seismic reflection profile data have been used to estimate the long term 106 years mechanical properties of the oceanic lithosphere underlying the hawaiian islands the data show prominent reflectors associated with the top of the oceanic crust reflector 2 the m discontinuity and the base of the crust reflector 3 the two way travel time to the reflectors has been depth converted using velocity depth functions derived from seismic refraction expanding spread profile data and compared to calculated depths based on simple three dimensional elastic plate models a good fit to the reflector depths was obtained for a model with an effective elastic thickness of the lithosphere te of 40 km the main discrepancies with the refraction data occur beneath oahu and molokai where the predicted depth of the base of the flexed crust is too shallow and beneath the southeast flank of hawaii where it is too deep we attribute the thickened crust beneath oahu to ponding of magma at the base of the crust following the last phases of tholeiitic shield building and the thinned crust beneath the flanks of hawaii to crustal melting beneath the youngest part of the chain the effective te of 40 km is higher than expected for 80 m y oceanic lithosphere possibly due to the effects of subsurface loads that act at or near the base of the flexed crust according to free air gravity anomaly and bathymetry data te does not change significantly along the island chain retaining a similar value for hawaii as it does for oahu we have used this result to estimate the pattern of subsidence and uplift that would be expected along the hawaiian islands because of flexural loading of hawaii the observed depth of the haleakala wave cut terrace northeast of maui can be generally explained by the flexure model the main discrepancies between observed and the predicted depths occur northwest of hawaii where they suggest a somewhat higher value of te possibly due to incomplete adjustment of the lithosphere we suggest that the addition of loads to the end of a volcanic chain generated as a plate moves over a mantle hotspot is accommodated by progressive flexing downstream causing subsidence of preexisting volcanos that lie in the flexural moat and uplift of islands on the flanking bulge the progressive flexure model predicts a characteristic pattern of offlap and onlap in the regions flanking the hawaiian islands that is generally similar to stratigraphic relationships observed along previously published seismic profiles of the moat a progressive flexure model may also apply to the case of a continental plate moving over a mantle hotspot and could explain the observations of 1 a clinoform pattern of stratigraphy in some flood basalt provinces and 2 a seaward dipping reflector sequence at some passive continental margins

crustal and upper mantle seismic structure and lithospheric flexure along the society island hotspot chain summary four wide angle and refraction lines have been shot along the society island hotspot track a 280 km long profile covers oceanic crust to the east of the islands the active hotspot region and the apron of tahiti the other profiles cover the region between moorea and huahine and the western archipelago of huahine raiatea bora bora and adjacent oceanic crust traveltime modelling provided lateral variations in crustal structure along the hotspot chain removal of the contribution of the uppermost low velocity layer which represents the volcanic edifices and debris suggests that the islands and atolls sit on normal oceanic crust and are surrounded by a large volcaniclastic apron seismic profiles running between raiatea and huahine and between raiatea and moorea cover a region where extreme down bending due to volcanic loading is expected a nearly flat crust mantle boundary indicates regional isostasy and clearly supports a stiff plate seismic structural models have been compared with calculated boundaries based on simple 3 d elastic plate models a good fit to the seismic crust mantle boundary was obtained for models with an effective elastic thickness of the lithosphere of 20 25 km these values are in accordance with a model in which the elastic thickness depends on the thermal age of the lithosphere and therefore contradict models that suggest that the region of french polynesia has been thermally rejuvenated amplitude modelling of crustal and upper mantle arrivals suggests that crust on the hotspot trail has been modified with respect to the pre hotspot crust to explain the observations we suggest that during the ongoing late stage volcanic activity the lower crust has been intruded by sills and some melts are trapped in the crust mantle boundary region if late stage volcanism and plutonism continue prolonged crustal growth may form a large underplate or subcrustal plutonic core complex along the hotspot chain

the pliocene seamount series of la palma canary islands a pliocene submarine series of alkali basaltic pillow lavas hyaloclastites and breccias a a sheeted dike swarm b and a basal suite of gabbro and ultramafic rocks c from la palma canary islands is interpreted as a cross section through an uplifted seamount this series has been tilted to its present orientation of 50 230 plunge and azimuth probably by upwarping due to intrusions in the central portion of the island the basal plutonic complex c also includes intrusives coeval with up to 2000 m of younger subaerial alkali basaltic lavas unconformably overlying the submarine series the plutonic suite c is overlain abruptly by more than 1800 m of sills b 0 4 1 m thick on average with minor screens of lavas and breccias extrusives a form a 1750 m thick sequence of pillow lavas breccias and hyaloclastites the clastic rocks increase in abundance upward and are of four main types 1 breccias consisting of partly broken pillows formed nearly in situ 2 heterolithologic pillow fragment breccias 3 hyaloclastites composed dominantly of highly vesicular lapilli and ash sized shards the latter thought to have formed by near surface explosive eruptions and been subsequently transported downslope by mass flows 2 and 3 being interpreted to have been resedimented and 4 pillow scoria breccias from the upper 700 m of the extrusive section consisting of amoeboidal highly vesicular pillows and lava stringers and local bombs probably formed by cracking and bleeding of gas rich expanding pillow lava and some shallow submarine subaerial lava fountaining the extrusive series is chemically and mineralogically crudely zoned with the most differentiated rocks metatrachytes and mugearites at the base and most picritic lavas occurring near the top of the series subsequent to emplacement the entire extrusive and intrusive series has been hydrothermally altered the lower part to greenschist and the upper part to smectite zeolite facies mineral assemblages the la palma succession combined with evidence from surface studies of seamounts suggests that seamounts are formed by intrusive and extrusive processes in approximately equal portions the nature of eruptive clastic and depositional mechanisms changes drastically during growth of a seamount if the critical depth for major magmatic degassing is surpassed and especially if magmatic explosive processes can occur at very shallow water depth the critical depth depending on magma and thus volatile composition changes in slopes of a seamount influence depositional processes based on these factors at least three major depositional sites develop as a seamount grows summit flank and apron facies nonexplosive extrusive processes prevail in the deep water stage dominantly producing pillow lavas 75 these consist of individual pillow volcanoes up to 200 m high with large pillows near the base and decreasing pillow size toward the top of a volcano pillow breccias and pillow fragment breccias comprise approximately 20 of this facies the deep water flank and apron facies are characterized by debris flow deposits with possibjy rather dense matrix material the shallow water shoaling stage is reached when the seamount top reaches the critical depth for drastic increase in exsolution of magmatic volatiles about 800 m for the alkali basaltic seamount of la palma resulting in formation of mainly clastic rocks 70 in situ pillow rind breccias and scoriaceous amoeboidal breccias and pillows are formed in the summit regions of the seamount by repeated expansion and leaking of frothy pillow lava possibly by lava fountaining resedimented heterolithologic pillow fragment breccias lapilli breccias and hyaloclastites are deposited on the flanks and aprons of the seamount pillow lavas comprise 30 of these deposits

crustal structure of northern gran canaria canary islands deduced from active seismic tomography seismic p wave travel times collected during meteor cruise m24 are inverted to derive a three dimensional model of the p wave velocity structure of the northern part of gran canaria canary islands the data consist of 6689 p wave travel times from 1487 offshore air gun shots which were recorded by both land based seismometers and ocean bottom hydrophones the crustal structure is well imaged by the data set as demonstrated by analysis of the resolution and tests with synthetic data the volcanic island is characterized by generally high p wave velocities 5 5 km s and a heterogeneous structure with large lateral velocity variations high p wave velocities are found around the centers of the miocene shield volcanoes in the vicinity of aguimes san nicolas and agaete as well as the center of the pliocene roque nublo volcano the velocity structure suggests a high percentage of dense intrusive rocks some of the intrusive rocks were emplaced during the eruption of 1000 km3 of miocene felsic magmas following the basaltic shield phase the velocity structure beneath la isleta peninsula and its submarine continuation is interpreted as a volcanic rift zone with abundant dikes the velocities decrease to 5 km s north of the coastline a high velocity zone thinning away from the central edifice is interpreted as the massive island flank extending up to 50 km off the coast which is underlain by prevolcanic neogene jurassic sediments the igneous part of the oceanic crust exhibits an anomalous structure with a relatively small thickness 3 km layer 3 and a 2 4 km thick layer 2 probably reflecting a modification of the crust due to long lasting magmatic intrusive activity during the evolution of the canary islands the moho north of gran canaria is found at a depth of 15 km the structure of gran canaria and the adjacent ocean basin is thought to be the result of a diffuse mantle upwelling under a slowly moving plate

densities and porosities in the oceanic crust and their variations with depth and age we have used drilling and downhole logging results from deep sea drilling project holes 395a and 418a measured properties of core samples from the seafloor and from ophiolites and seismic velocity structure models of the oceanic crust to constrain empirical statistical relationships between in situ densities porosities and seismic velocities and to estimate densities and porosities in the oceanic crust with the following principal results our review of drilling results and studies of ophiolites suggests that the velocity structure of the oceanic crust is more closely related to changes in porosity and alteration or metamorphic grade than to its igneous structure the transition from layer 2a 3 7 0 4 km s 1 to 2b 5 2 0 4 km s 1 occurs within the extrusive pile the top of layer 2c 6 2 0 2 km s 1 is associated with the top of the sheeted dike complex and with the first appearance of greenschist facies alteration products ophiolite studies have shown that the top of layer 3 occurs within the sheeted dikes where the metamorphic grade of alteration products increases from greenschist to amphibolite facies a comparison of reprocessed and corrected downhole logs from holes 395a and 418a indicates that in situ densities and fractional porosities differ by 0 2 mg m 3 and 0 07 respectively these differences are consistent with prevailing views of the progressive alteration of the upper oceanic crust statistical relationships between in situ density porosity and slowness are highly linear and indicate that the density slowness relation estimated from laboratory samples pb 3 81 0 02 6 0 0 1 s applies to in situ properties as well we obtained a similar empirical relationship for porosities from the hole 418a log data f 0 35 0 03 2 37 0 15 s r2 0 84 s e 0 015 applying these empirical models to seismic velocity structure models yields the following crustal densities and porosities layer 2 2 62 2 69 mg m 3 0 10 0 12 and layer 3 2 92 2 97 mg m 3 the estimated average density of the oceanic crust is 2 86 0 03 mg m 3 densities and porosities estimated from a seismic crustal structure model for old atlantic crust agree very well with the logging data from hole 418a suggesting that in situ densities and porosities can be estimated from seismic velocities high density and porosity gradients are characteristic of the upper oceanic crust increasing densities reflect both a progressive decrease of porosity and increasing grain density with depth based on the seismic properties of layer 2a we estimate that the layer thins at an approximate rate 20 to 45 m m y 1 while the velocity increases by 0 040 km s 1 m y 1 the corresponding rates of change of density and porosity are 0 016 0 009 mg m 3 m y 1 and 0 005 0 003 m y 1 respectively

three dimensional seismic refraction tomography a comparison of two methods applied to data from the faeroe basin this paper presents a comparison of two tomographic methods for determining three dimensional 3 d velocity structure from first arrival travel time data the first method is backprojection in which travel time residuals are distributed along their ray paths independently of all other rays the second method is regularized inversion in which a combination of data misfit and model roughness is minimized to provide the smoothest model appropriate for the data errors both methods are nonlinear in that a starting model is required and new ray paths are calculated at each iteration travel times are calculated using an efficient implementation of an existing method for solving the eikonal equation by finite differencing both inverse methods are applied to 3 d ocean bottom seismometer obs data collected in 1993 over the faeroe basin consisting of 53 479 travel times recorded at 29 obss this is one of the most densely spaced large scale 3 d seismic refraction experiments to date different starting models and values for the free parameters of each tomographic method are tested a new form of backprojection that converges more rapidly than similar methods compares favorably with regularized inversion but the latter method provides a simpler model for little additional computational expense when applied to the faeroe basin data bounds on two model features are assessed using regularized inversion with combined smoothness and flatness constraints an inversion of synthetic data corresponding to 100 data recovery from the real experiment shows a marked improvement in lateral resolution at deeper depths and demonstrates the potential of currently feasible 3 d refraction experiments to provide well resolved long wavelength velocity models the similarity of the final models derived from the two tomographic methods suggests that the results from the new form of backprojection can be relied on when limited computational resources rule out regularized inversion

imaging the crustal magma sources beneath mauna loa and kilauea volcanoes hawaii three dimensional seismic p wave traveltime tomography is used to image the magma sources beneath mauna loa and kilauea volcanoes hawaii high velocity bodies 6 4 km s in the upper 9 km of the crust beneath the summits and rift zones of the volcanoes correlate with zones of high magnetic intensities and are interpreted as solidified gabbro ultramafic cumulates from which the surface volcanism is derived the proximity of these high velocity features to the rift zones is consistent with a ridge spreading model of the volcanic flank southeast of the hilina fault zone along the south flank of kilauea low velocity material

jasper seamount structure seafloor seismic refraction tomography the velocity structure of jasper seamount was modeled using one and three dimensional inversions of p wave travel times the results represent the first detailed seismic images of a submerged intraplate volcano two seismic refraction experiments were completed on jasper seamount incorporating ocean bottom seismometers and navigated seafloor shots the p wave travel times were first used to compute a one dimensional velocity profile which served as a starting model for a three dimensional tomographic inversion the seamount p velocities are significantly slower than those observed in typical oceanic crust at equivalent subbasement depths this suggests that jasper seamount is constructed predominantly of extrusive lavas with high average porosity the velocity models confirm morphological predictions jasper seamount is a shield volcano with rift zone development high seismic velocities were detected beneath the large radial ridges while low velocities characterize the shallow summit and flanks comparisons between p velocity models of jasper seamount and the island of hawaii reveal that these two shield volcanoes are not structurally proportional jasper seamount is far smaller than hawaii yet both volcanoes exhibit an outer extrusive layer of similar thickness this suggests that seamount size influences the intrusive extrusive proportions density equilibrium between melt and country rock may explain this behavior

soil structure and nutrient contents in underground fissures in a rock mantled slope in the karst rocky desertification area near surface karst fissures are filled with soils these soils are often derived from the erosion and transport of surface soil but have different properties than the surface soil in karst rocky desertification regions this study was undertaken to determine the soil structure and nutrient contents in underground fissures with different grades of rocky desertification no potential mild moderate and severe based on field surveys near surface karst fissures were selected in a typical karst plateau area soil particle compositions particularly nutrient characteristics were determined through field sampling and laboratory experiments the results indicated that the higher the rock desertification grade was the higher the soil sand 0 02 2 mm content and the lower the clay 0 002 mm content the soil in both limestone and dolomite fissures was basically neutral to weakly alkaline the surface soil in dolomite fissures presented a higher soil organic matter content than that in limestone fissures but the opposite was true for the subsurface soil from the surface to the lower layer the total nitrogen content generally decreased and the total phosphorus content generally decreased in dolomite fissures but exhibited a more complex pattern in limestone fissures the total phosphorus content of the surface soil was higher than that of the subsurface soil but the total potassium content was lower there was no significant difference among different soil layers near surface fissures are a good place to promote vegetation restoration projects because they provide good soil structure and nutritional conditions for plant growth

water source utilization by woody plants growing on dolomite outcrops and nearby soils during dry seasons in karst region of southwest china summary in the seasonally dry karst region of southwest china woodland vegetation is frequently associated with rocky outcrops we used stable isotope techniques to determine the water sources of two woody plant species the semideciduous tree rademachera sinica and the deciduous shrub alchornea trewioides across three surface types continuous and isolated dolomite outcrops as well as adjacent surfaces with soils main water sources for r sinica growing on the continuous outcrops shifted from deep water sources e g water in the saturated zone on march 23 2009 the late dry season of 2008 2009 to rainwater stored in rock fissures of the unsaturated zone on november 8 2009 the early dry season of 2009 2010 r sinica growing on nearby thin soils exhibited a similar shift of main water sources from deep water sources in the late dry season to shallow soil water 0 30 cm in the early dry season however they extracted deep water sources from the bottom of nearby outcrops rather than from deeper layers below the soil surface main water sources for r sinica growing on the isolated outcrops shifted from previous one to two months ago rainwater in the late dry season to the mixture of recent within the last one month and previous rainwater in the early dry season while using little or no deep water sources in contrast a trewioides growing on these two kinds of outcrops always relied on recent rainwater while those growing on soils always relied on shallow soil water the shift of water sources for tree species especially those growing on continuous rock outcrops and nearby soils may allow them to maintain normal transpiration throughout the year which increases the amount of evapotranspiration in the watershed and further enhances the water storage capacity of the study area

spatial distribution of hotspot material added to the lithosphere under la reunion from wide angle seismic data wide angle seismic lines recorded by ocean bottom and land seismometers provide a pseudo three dimensional investigation of the crust and upper mantle structure around the volcanically active hotspot island of la reunion the submarine part of the edifice has fairly low seismic velocities without evidence for intrusives an upper unit with a velocity depth gradient is interpreted as made of material erupted subaerially then transported and compacted downslope between this unit and the top of the oceanic plate imaged by normal incidence seismic reflection a more homogeneous unit indicated by shadow zones on several wide angle sections may correspond to lavas of a different nature extruded underwater in the earlier phase of volcanism coincident wide angle and normal incidence reflections document that the oceanic plate is not generally downwarping toward the island but doming instead toward its southeastern part with limited evidence for some intracrustal intrusion deeper in the lithosphere the presence of a layer of intermediate velocity between the crust and mantle is firmly established it is interpreted as resulting from the advection of hotspot magmatic products possibly partially molten and of a composition for which the crust is a density barrier the extensive wide angle coverage constrains the extent of this body it does not show the elongated shape expected from plate drift above a steady hotspot supply alternative propositions can hence be considered for example that la reunion is caused by a solitary wave of hotspot material or by a young hotspot the size of the underplate 140 km wide and up to 3 km thick corresponds to less than half the volume of the edifice on top of the plate

ridge plume interactions or mantle heterogeneity near ascension island between the ascension and bode verde fracture zones an 200 km length of the axis of the mid atlantic ridge has a large positive residual depth anomaly and an axial high morphology that suggest a plume influence new wide angle seismic and gravity data from this area show that 1 the crustal thickness increases from 6 km at the propagating rift marking the northern limit of the anomalous zone to 10 km at the center of the zone and 2 the residual depth anomalies may be largely explained by airy isostatic support for these crustal thickness variations these observations combined with the lack of long wavelength gravity and residual depth anomalies associated with the proposed ascension plume suggest either the presence of a weak and intermittent plume or melting of a series of small mantle heterogeneities

three dimensional crustal structure of ascension island from active source seismic tomography we present a crustal velocity and moho depth model of the ascension volcanic edifice using one of the most densely sampled 3 d wide angle data sets for volcanic islands currently available we invert traveltimes of first and second arrivals by seeking a layer interface minimum structure model and then test the resulting velocity model by gravity modelling and a 3 d checkerboard resolution test within the shallow extrusive part of the crust two main high velocity regions coincide with the highest topography on land and the gravity maximum off the west coast of the island respectively these features are connected with a high velocity intrusive core that is created either within or on the top of oceanic layer 3 and is interpreted as a possible relic magma chamber the thickness of the surface low velocity region is similar to that observed at hawaii jasper seamount and great meteor seamount suggesting a similar process of edifice construction the mean density of the volcanic edifice is significantly less than that of normal oceanic crust and than load densities typically assumed in studies of flexure as a result of seamount loading there is no simple flexural model that explains the shape of the moho beneath the island perhaps because of the long lived volcanism and the proximity of the island to the mid atlantic ridge mar and the ascension fracture zone there is no evidence for magmatic underplating beneath ascension island

crustal structure of ascension island from wide angle seismic data implications for the formation of near ridge volcanic islands abstract the study of the internal structure of volcanic islands is important for understanding how such islands form and how the lithosphere deforms beneath them studies to date have focused on very large volcanic edifices e g hawaiian islands marquesas but less attention has been paid to smaller islands which are more common ascension island a 4 km high volcanic edifice with a basal diameter of 60 km is located in the equatorial atlantic 8 s 90 km west of the mid atlantic ridge on 7 ma oceanic lithosphere we present results of a wide angle seismic profile crossing the island revealing a crustal thickness of 12 13 km an overthickened layer 3 7 km thick and little evidence of lithospheric flexure together these results suggest ascension island may be older than previously assumed and may have begun forming at an on axis position around 6 7 ma this hypothesis is further supported by the presence of a young 1 4 km high edifice directly adjacent to the mid atlantic ridge with a volume about 1 7 that of ascension island possibly representing the earliest stages of seamount formation excess magmatism appears to be related to the tectonic setting at the ridge fracture zone intersection

spatial distribution of rock fragments on steep hillslopes in karst region of northwest guangxi china abstract rock fragments 5 mm in diameter at the soil surface and within the topsoil have a large effect on the intensity of various hydrologic and geomorphic processes however little information is available on the spatial distribution of rock fragments in subtropical regions the objective of this paper was to investigate the relationship between the spatial distribution of rock fragments and landforms on two different steep karst hillslopes in northwest guangxi southwest china on the first hillslope a disintegrated landslide failure with the presence of several large rock outcrops 2 m in height the spatial distribution of rock fragment cover had no obvious relationship with topographic position except that the mean cover percentage of small rock fragments 5 20 mm decreased from bottom to top on the second hillslope an avalanche slope without the presence of large rock outcrops the mean total rock fragment cover 5 600 mm increased from bottom 5 to top 21 with decreasing variability and rock fragments with various sizes 5 20 20 75 and 75 250 mm showed a similar increasing trend the mean total rock fragment cover increased linearly with slope gradient on the second hillslope and tended to increase and then decrease with gradient but their relationship was not obvious on the first hillslope this indicated that the spatial distribution of surface rock fragment cover had a close relationship with the presence of large rock outcrops and slope gradient however the median diameter d50 of the surface rock fragments had an increase decrease trend with slope gradient but there was no obvious relationship on both hillslopes with low overland flow therefore the dominant factor for the spatial distribution of rock fragment cover and size at the soil surface might not be soil erosion by water but slope gradient vegetation and geomorphologic condition of the slope the mean total volumetric rock fragment content 5 250 mm within the topsoil 10 20 cm thick increased linearly from bottom 16 to top 39 with slope gradient on the first hillslope and had a logarithmic increase from bottom 10 to top 27 with gradient on the second hillslope this suggested that rock fragment content within the topsoil was mostly controlled by slope gradient and topographic positions and had not a close relationship with the presence of large rock outcrops

a theoretical model of hotspot volcanism control on volcanic spacing and patterns via magma dynamics and lithospheric stresses many linear island chains are thought to be the result of the steady motion of the lithospheric plates over stationary hotspots the occurrence of discrete nearly regularly spaced volcanoes rather than continuous ridges are assumed to be caused by the interaction of flexural stresses due to the volcanic loads with magma percolation a parameter study is performed on a simple model that incorporates these effects in addition to dike wall erosion it is demonstrated that the strength of the erosional feedback determines whether the model generates discrete volcanoes or a continuous ridge the intervolcanic spacing depends not only on the elastic thickness of the lithosphere but also on the magma pressure at the base of the lithosphere the size of the eruptive region of the individual volcanoes is controlled by the elastic response of the lithosphere to magma overpressurization if an initial off axis edifice is introduced the model is able to preserve this asymmetry and produce an alternating series of volcanoes a small initial perturbation grows over time resulting in double lines or wider patterns depending on the width of the magma source region single lines of volcanoes therefore indicate very narrow magma source regions

use of tree rings as indicator for groundwater level drawdown caused by tunnel excavation in zhongliang mountains chongqing southwest china tunnel excavation causes geological hydrological environmental and social changes the effects of groundwater level changes caused by tunnelling on tree growth have been poorly understood dendrochronology was used to evaluate the impact of groundwater level drawdown on tree growth tree cores of masson pines were collected from the areas affected by tunnel construction in the zhongliang mountains of chongqing in south western china to study the effects of tunnel excavation on the growth rate of trees by comparing with tree cores collected from unaffected areas excavation and early operation of the first tunnel in the zhongliang mountains from 1968 to 1984 caused a groundwater table drawdown in both karst aquifer and non karst aquifer the lowered groundwater table significantly reduced the growth rate of pine trees and the low growth rate remained for 15 years the effect was experienced up to at least 1 km from the tunnel axis the decline in tree growth was higher in karst than in non karst areas though the effects on the trees of the karst areas were lagging the high precipitation in 1998 contributed to groundwater recovery after which the tree growth recovered moderately but not to the original level groundwater leakage of the recently excavated tunnels did not affect pine trees heavily probably because the pines had adapted to the new hydrogeological conditions and new strategies in tunnel inflow management were adopted in the recently excavated tunnels the use of tree growth rate as an indicator of groundwater table change in tunnelling areas offers a new option to study environmental impacts and the extent of tunnelling effects

the effect of vegetation on infiltration in shallow soils underlain by fissured bedrock abstract mean annual infiltration above the high level waste repository proposed to be sited at yucca mountain nevada has a large impact on assessments of repository performance ongoing investigations of infiltration processes have identified the relatively horizontal caprock environment above portions of the repository as a potentially large source of infiltrating waters due to shallow permeable soils above a moderately welded tuff with large soil filled fissures the combination of shallow soils and fissured bedrock allows rapid penetration of wetting pulses to below the rooting zone plant uptake can strongly reduce net infiltration in arid environments with high water storage capacity and despite the low water storage capacity there is a relatively high vegetation density in this environment the apparent discrepancy between high vegetation density and low water storage motivates the study of plant hydrologic interactions in this semiarid environment field observations were coupled with plant and landscape scale models to provide insight into plant hydrologic interactions several lines of evidence including i linear plant growth features observed on aerial photographs ii comparisons of plant cover within the fissured environment and comparable environments lacking fissures and iii direct excavations all suggest that the widely spaced soil filled fissures are conducive to plant growth even when fissures are buried at soil depths exceeding 30 cm results from a mechanistic simulation model for root growth into fissures suggest that the additional sheltered plant available soil water within fissures provides a competitive advantage for plant establishment therefore plants that germinate above a fissure are more likely to survive in turn developing linear features above fissures having established that plants preferentially root within soil filled fissures in the caprock environment a set of simulations were performed to examine the hydrologic consequence of plant roots within fissures at the landscape scale the response to three rainfall amounts was simulated for the largest storm fluxes at the fissure bottom peaked at 1 4 weeks after the storm when plant uptake was not active but were eliminated when fissures had active vegetation when plants were active within a fissure uptake eliminated net infiltration in the fissure regardless of the size of the storm two plant related mechanisms reduced total flux through the plant filled fissures i transpiration during fissure flow and ii wetting pulse retardation due to drier fissures prior to rain the first mechanism appears to be dominant in these simulations results suggest that transpiration may strongly limit net infiltration i e total deep percolation flux escaping the plant root zone significant infiltration can occur however when plants are dormant so that most infiltration would be expected to occur during winter

spatial and temporal variability in the stable isotope systematics of modern precipitation in china implications for paleoclimate reconstructions the stable isotopic composition of materials such as glacial ice tree rings lake sediments and speleothems from low to mid latitudes contains information about past changes in temperature t and precipitation amount p however the transfer functions which link 18op to changes in t or p d 18op dt and d 18op dp can exhibit significant temporal and spatial variability in these regions in areas affected by the southeast asian monsoon past variations in 18o and d of precipitation have been attributed to variations in monsoon intensity storm tracks and or variations in temperature proper interpretation of past 18op variations here requires an understanding of these complicated stable isotope systematics since temperature and precipitation are positively correlated in china and have opposite effects on 18op it is necessary to determine which of these effects is dominant for a specific region in order to perform even qualitative paleoclimate reconstructions here we evaluate the value of the transfer functions in modern precipitation to more accurately interpret the paleorecord the strength of these transfer functions in china is investigated using multiple regression analysis of data from 10 sites within the global network for isotopes in precipitation gnip 18op is modeled as a function of both temperature and precipitation the magnitude and signs of the transfer functions at any given site are closely related to the degree of summer monsoon influence 18op values at sites with intense summer monsoon precipitation are more dependent on the amount of precipitation than on temperature and therefore exhibit more negative values in the summer in contrast 18op values at sites that are unaffected by summer monsoon precipitation exhibit strong relationships between 18op and temperature the sites that are near the northern limit of the summer monsoon exhibit dependence on both temperature and amount of precipitation comparison with simple linear models 18op as a function of t or p and a geographic model 18op as a function of latitude and altitude shows that the multiple regression model is more successful at reproducing 18op values at sites that are strongly influenced by the summer monsoon the fact that the transfer function values are highly spatially variable and closely related to the degree of summer monsoon influence suggests that these values may also vary temporally since the southeast asian monsoon intensity is known to exhibit large variations on a number of timescales annual to glacial interglacial and the magnitude and sign of the transfer functions is related to monsoon intensity we suggest that as monsoon intensity changes the magnitude and possibly even the sign of the transfer functions may vary therefore quantitative paleoclimate reconstructions based on 18op variations may not be valid

characterization of recharge processes in shallow and deeper aquifers using isotopic signatures and geochemical behavior of groundwater in an arsenic enriched part of the ganga plain abstract arsenic concentrations in groundwater extracted from shallow aquifers in some areas of the ganga plain in the states of bihar and uttar pradesh exceed 50 g l 1 and locally reach levels in the 400 g l 1 range the study covered 535 km2 of active flood plain of the river ganga in bihar where a two tier aquifer system has been delineated in a multi cyclic sequence of quaternary sand clay sandy clay and silty clay all 250 m below ground surface the research used isotopic signatures 18o 2 3h 14c and major chemical constituents hco 3 so 4 2 no 3 cl ca 2 mg 2 na k as total of groundwater to understand the recharge processes and groundwater circulation in the aquifers values of 18o and 2 combined with 3h data indicate that the recharge to the as enriched top 40 m of the deposits is modern