structure and flow induced variability of the subtidal salinity field in northern san francisco bay abstract the structure of the salinity field in northern san francisco bay and how it is affected by freshwater flow are discussed two datasets are examined the first is 23 years of daily salinity data taken by the u s bureau of reclamation along the axis of northern san francisco bay the second is a set of salinity transects taken by the u s geological survey between 1988 and 1993 central to this paper is a measure of salinity intrusion x2 the distance from the golden gate bridge to where the bottom salinity is 2 psu using x2 to scale distance the authors find that for most flow conditions the mean salinity distribution of the estuary is nearly self similar with a salinity gradient in the center 70 of the region between the golden gate and x2 that is proportional to x 12 analysis of covariability of q and x2 showed a characteristic timescale of adjustment of the salinity field of approximately 2 weeks the steady state response deduced from the x2 time series implies that x2 is proportional

upwelling asthenosphere beneath western arabia and its regional implications two distinct phases of continental magmatism are evident in western arabia the first from about 30 to 20 ma produced tholeiitic to transitional lavas emplaced along nw trends the second from about 12 ma to recent produced transitional to strongly alkalic lavas emplaced along n s trends the older phase is attributed to passive mantle upwelling during extension of the red sea basin whereas the younger phase is attributed to active mantle upwelling but was facilitated by minor continental extension perpendicular to plate collision the younger magmatic phase is largely contemporaneous with a major period of crustal uplift to produce the west arabian swell after about 14 ma a variety of evidence suggests that the west arabian swell is thermally supported by hot upwelling asthenosphere in contrast to the distinct asymmetry of uplift and magmatism on opposing sides of the red sea basin these processes were symmetric across a n s line marking the central axis of the west arabian swell this axis coincides with two fundamental features the ha il rutbah arch in the north and the makkah madinah nafud mmn volcanic line in the south the symmetry of magmatism is demonstrated by petrochemical evidence that the mmn harrats were derived by greater degrees of partial melting at shallower depths than those harrats lying to the west and east of the mmn line the potential temperature of the asthenosphere is estimated to be about 1436 c beneath the mmn line and about 1354 c beneath the flanking more undersaturated harrats the source of upwelling is either a mantle plume centrally located beneath the west arabian swell or an elongated and extended lobe of hot asthenosphere emanating from the ethiopian mantle plume convective flow may have been channelled along a preexisting regional flexure in the continental lithosphere which concentrated hot asthenosphere beneath the central axis of the afroarabian dome this crest of mantle upwelling underlies the mmn line in the north and the danakil depression and ethiopian rift system in the south it also passes through the red sea basin at the midpoint of an unusual doubly propagating rift system where axial seafloor spreading began 4 5 m y ago the nw trend of the red sea basin was well established by crustal attenuation during the older magmatic phase the pliocene invasion of this basin by a n s zone of mantle upwelling has resulted in seafloor spreading parallel to the preexisting structure along a rift system that has continued to propagate away from its eccentric mantle source in two opposing directions

continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

exchange between long island sound and adjacent waters 1 we exploit archived salinity data averaged by month for the years 1995 2001 to evaluate exchange of salt between long island sound and shelf waters and treat long island sound as a control volume with eastern and western boundaries at the race and the east river respectively the use of inverse methods to solve a system of equations expressing conservation of salt and water provides estimates of model parameters together with uncertainty estimates and measures of model performance we explore the use of a single exchange coefficient acting on an axial salinity gradient to represent exchange at the race and find that it is necessary to allow the coefficient to depend weakly on variability of freshwater discharge r into the sound i e 0 2 1 600 200 m2 s using established estimates of the mean circulation at the western boundary of the sound we determine a dispersion coefficient in the upper east river of 300 200 m2 s explicitly modeling the advective exchange flow gravitational circulation at the race using a coefficient that varies as the one third power of the normalized discharge indicates that gravitational circulation accounts for less than 10 of the up estuary salt flux in eastern long island sound

crustal constraints on the uplift mechanism of the western ghats of india summary over 800 receiver functions rfs from three broadband and five short period stations deployed in the northern segment of the western ghats of india were analysed to examine the crustal constraints on the rift flank uplift mechanism of the ghats estimates of moho depths mean shear velocities and poisson s ratios were determined and used as constraints to model the rfs the study reveals that the crust beneath the ghats varies in thickness between 31 and 39 km and is neither uniformly thin nor thick as required by existing hypotheses for uplift mechanism the poisson s ratio is 0 26 and the mean shear velocity is 3 65 km s 1 the uncertainties in the estimates of the moho depths and poisson s ratios are 2k m and 0 01 respectively the crust beneath the ghats is 3 4 km thinner than that beneath the adjoining low lying coastal plains suggesting block tectonics modelling the rfs using crustal constraints reveals sub moho low velocity zones lvzs with velocity reductions of 0 5 0 65 km s 1 which are confined to shallow depths of about 50 km beneath the ghats the persistent relief of the ghats may be attributed to the buoyancy forces of the possibly rift related shallow upper mantle lvzs beneath the ghats

the effects of channels and shoals on exchange between the chesapeake bay and the adjacent ocean the role of bathymetric changes in determining the transport of water and salt in the lower chesapeake bay lcb was investigated using high resolution acoustic doppler current profiler adcp and conductivity temperature depth profiles a cross channel transect was repeated eight times during neap tides on october 6 7 1993 which illustrated the lateral structure of the longitudinal and transverse flow fields and the intratidal variations in the flow structure across the lcb amplitude and phase of the m2 tidal component as well as the mean flow velocity were calculated using least squares fitting at every point of a uniform grid obtained from the adcp data the results differ from the classical two layer pattern of estuarine circulation modified by coriolis effects but are consistent with recent hydrographic observations semidiurnal flow was highest over the navigational channels and lateral gradients were strongest in regions of sharp bathymetric changes the phase lag of the semidiurnal flow also showed lateral and vertical gradients that represented advances at the bottom with respect to the surface and over the shoals in relation to the channels the section of the water column measured indicated a mean outflow of 0 7 104 m3 s and a mean inflow of 1 3 104 m3 s the apparent gain of water by the estuary during the period of observation can be explained by meteorologically forced net barotropic inflow the depth averaged mean longitudinal flow consisted of inflow in the navigational channels and outflow over the shoals the mean transverse flow showed near surface convergence over the channels we propose a possible explanation for the observed flow and density structure as follows the barotropic and baroclinic forcing interact with the bathymetry to extend the inflow from the bottom to the surface thereby inducing a transverse circulation that yields near surface convergence over the channels

gravitational circulation and its response to the variation in river discharge in the seto inland sea japan 1 in this study gravitational circulation and its response to the variation in river discharge have been investigated in the seto inland sea japan monthly hydrographic surveys have been conducted from 2002 to 2006 in hiuchi nada a basin located between two straits horizontal salinity gradients along the channel varied as the 9 10 power of the river discharge into one end of the basin significant volume fluxes which were proportional to the horizontal salinity gradients were found the relationship between river discharge r m3 s 1 and the volume flux q m3 s 1 was represented as q 13 9r0 88 when river discharge increased vertical density gradients were intensified and then along channel horizontal density gradients were gradually enhanced due to vertical mixing at the western strait these gradients drove gravitational circulations along the channel the gravitational circulations were not uniform across the channel and accompanied by horizontal circulations in the northern part of the basin low salinity water at the eastern end of the basin was directed toward the west and high salinity water was directed toward the east the eastward flows arising from the intrusion of relatively heavy water at the western strait extended from the bottom to the surface in the southern part low salinity water was directed toward the west the westward flows arising from the intrusion of relatively light water at the eastern strait concentrated in the middle layer and formed a plume like structure accompanied by the eastward flow under the plume these situations were responsible for the development of the bulk exchange flux

effects of bathymetry friction and rotation on estuary ocean exchange an analytical model that includes pressure gradient friction and the earth s rotation in both components of the flow is used to study the transverse structure of estuarine exchange flows and the nature of transverse circulation in estuaries of arbitrary bathymetry analytical results are obtained for generic bathymetry and also over real depth distributions and are compared with observations this study extends previous efforts on the topic of transverse structure of density induced exchange flows in three main aspects 1 the analytical model explores any arbitrary bathymetry 2 the results reflect transverse asymmetries relative to a midchannel centerline associated with the effects of the earth s rotation and 3 the transverse circulation produced by the analytical model is examined in detail analytical results over generic bathymetry show in addition to the already reported dependence of exchange flow structure on the ekman number two new features first the transverse structure of along estuary flows shows the earth s rotation effects even in relatively narrow systems thus producing transverse asymmetries in these flows the asymmetries disappear under strongly frictional high ekman number conditions thus illustrating the previously documented pattern of inflow in channels and outflows over shoals for typical estuaries second transverse flows resemble a sideways gravitational circulation when frictional effects are apparent ekman number greater than 0 1 responding to a transverse balance between pressure gradient and friction these transverse flows reverse direction under very weak friction and reflect coriolis deflection of along estuary flows that is geostrophic dynamics all examples of observed flows are satisfactorily explained by the dynamics included in the analytical model

residual currents in a semi enclosed bay of the seto inland sea japan 1 on the basis of the princeton ocean model a robust diagnostic model is developed to calculate the residual currents in the hiuchi nada a semi enclosed bay of the seto inland sea the input water temperature and salinity data are from six hydrographic surveys in 2002 and the model results in the summer show a clockwise eddy in the western part of the bay and an anticlockwise eddy in the eastern part the magnitude of two eddies is 10 cm s the modeled flow pattern is consistent with early observations derived from moored arrays the two eddies are persistent throughout the summer because winds are usually weak and cannot influence their existence the tide induced residual currents are appreciable only in the close vicinity of the kurushima strait and bisan strait the two straits connecting the bay to the other bays and have little affect on the two eddies that are inside the bay thus the two density driven eddies in the hiuchi nada are the basic pattern of circulation in the summer using the calculated residual currents we examined the intrusion route of waters from the kurushima strait and bisan strait with passive tracer experiments the results show that the water in the kurushima strait intrudes into the bay mainly through the middle and bottom layers while the water in the bisan strait intrudes mainly through the surface layer furthermore the tracer cannot reach the center of the anticlockwise eddy in the eastern part of the bay this indicates that the eddy prevents water exchange between its center and outer edges and provides favorable conditions for the formation of oxygen deficient waters in the central part of the eddy

relationship among o mg isotopes and the petrography of two spinel bearing compound chondrules the petrological properties and o and al mg isotopic compositions of two spinel bearing chondrules from the allende cv chondrite were investigated using scanning electron microscopy and secondary ion mass spectrometry a coarse spinel grain in a barred olivine bo chondrule is less enriched in 16o 17o 5 17o 17o 0 52 18o whereas smaller spinel grains in a plagioclase rich chondrule member of a compound chondrule are extremely 16o rich 17o 17 and the spinels have a strongly serrated character the petrological features and 16o enrichments of the spinels in the plagioclase rich chondrule indicate that spinels originating in coarse grained ca al rich inclusions cais were incorporated into chondrule precursors and survived the chondrule forming event the degree of 16o excesses among minerals within each chondrule is correlated to the crystallization sequences this evidence suggests that the o isotopic variation among minerals may have resulted from incomplete exchange of o isotopes between 16o rich chondrule melt and 16o poor nebular gas aqueous alteration also has changed the o isotope compositions in the mesostasis the feldspathic mesostasis in the bo chondrule shows a disturbed mg al isochron indicating that the bo chondrule experienced secondary alteration while plagioclase in the plagioclase rich chondrule member of the compound chondrule shows slight 26mg excesses corresponding to 26al 27al 0 4 6 4 0 2 10 6 nepheline formed by secondary alteration shows no detectable excess the al mg isotopic system of these chondrules was disturbed by aqueous alteration and thermal metamorphism on the allende parent body

joint inversion for three dimensional s velocity mantle structure along the tethyan margin abstract for purposes of studying the lateral heterogeneity as well as for ultimately predicting seismograms for this region we construct a new 3 d s velocity model by jointly inverting a variety of different seismic data we jointly invert regional waveforms surface wave group velocity measurements teleseismic s arrival times and crustal thickness estimates from receiver functions refraction lines and gravity surveys these data types have complementary resolving power for crust and mantle structures vertical and lateral variations shallow and deep mantle features local and global structure therefore a joint inversion of these data sets might help unravel the complexity of this tectonically diverse area these measurements are made from a combination of mantle investigation of the deep suture between europe and africa midsea program for array seismic studies of the continental lithosphere passcal geoscope geofon global seismographic network gsn international deployment of accelerometeres ida mednet national networks and local deployments throughout the study region which extends from the western mediterranean region to the hindu kush and encompasses northeastern africa the arabian peninsula the middle east and part of the atlantic ocean for reference we have fitted the waveforms of regional s and rayleigh waves from over 3800 seismograms using partitioned waveform inversion we include over 3000 crustal thickness estimates from receiver functions gravity measurements and refraction profiles we include rayleigh wave group velocities for hundred thousands of paths transecting the region we have over 3000 teleseismic s arrival times measured through cross correlation and over 170000 from picks originally reported to the international seismological centre isc

crustal structure of the arabian plate new constraints from the analysis of teleseismic receiver functions an edited version of this paper was published by elsevier science copyright 2005 elsevier science see also http dx doi org 10 1016 j epsl 2004 12 020 http atlas geo cornell edu saudiarabia publications al damegh 202005 htm

a study of mg and k isotopes in allende cais implications to the time scale for the multiple heating processes the measurements of magnesium and potassium isotopic compositions of refractory minerals in allende calcium aluminum rich inclusions cais 7r 19 1 hn3 1 and egg3 were taken by secondary ion mass spectrometry sims the 7r 19 1 contains 16 o rich and 16 o poor melilite grains and define a single isochron corresponding to an initial 26 al 27 al ratio of 6 6 1 3 10 5 the al mg isochron o isotope measurements and petrography of melilite in 7r 19 1 indicate that 16 o poor melilite crystallized within 0 4 myr after crystallization of 16 o rich melilite suggesting that oxygen isotopic composition of the cai forming region changed from 16 o rich to 16 o poor within this time interval the 16 o poor melilite is highly depleted in k compared to the adjacent 16 o rich melilite indicating evaporation during remelting of 7r 19 1 we determined the isochron for 41 ca 41 k isotopic systematics in egg3 pyroxene with 4 1 2 0 10 9 2 as an initial ratio of 41 ca 40 ca which is at least two times smaller than the previous result sahijipal et al 2000 the ratio of 41 ca 40 ca in the egg3 pyroxene grain agrees within error with the value obtained by hutcheon et al 1984 no evidence for the presence of 41 k excess decay product of a short lived radionuclide 41 ca was found in 7r 19 1 and hn3 1 we infer that the cai had at least an order of magnitude lower than canonical 41 ca 40 ca ratio at the time of the cai formation

seismic structure and composition of the crust beneath the southern scandes norway abstract new results on p and s wave seismic velocity structure in southern norway indicate that the crust has an average poisson s ratio of 0 25 is predominantly of felsic intermediate composition and lacks a significant mafic lower crust a crustal scale refraction seismic study magnus rex mantle investigations of norwegian uplift structure refraction experiment acquired data along three 300 to 400 km long active source seismic profiles across the southwest scandinavian domain in southern norway the youngest section of the fennoscandian shield moho depths in the domain are 36 40 km thinning towards the continental shelf and oslo graben the high vp lower crust beneath the southwest scandinavian domain vp 7 km s is around 4 km thick crustal structure in the adjacent svecofennian domain differs significantly moho depths reach 50 km and an up to 24 km thick high vp lower crust is present strong p and s wave arrivals are recorded from magnus rex from which poisson s ratio for the crust is calculated the unusually strong s wave arrivals allow rare insight into crustal poisson s ratio structure that is not commonly available from active source data the thinness of the mafic lower crust in southern norway is in significant contrast to other parts of the fennoscandian shield and delamination or convective removal of the lower crust is proposed tectonic events that may have triggered detachment of the lower crust include extensional collapse after the caledonian orogeny and permian extension

upper mantle seismic anisotropy and lithospheric decoupling the lithosphere underlying stable continental shield regions is thought to extend to a depth of 200 km compressional wave velocity models derived from a recent approximately ns refraction profile in northern australia include a pronounced high velocity zone at a depth of around 200 250 km conventional petrological interpretations in terms of mineralogical or compositional changes cannot readily explain this feature calculated velocities for the garnet pyrolite model using a derived geotherm fit the velocities above and below this high velocity zone but cannot account for the high velocity 8 6 km s 1 zone itself this feature is explained in terms of velocity anisotropy which might reflect mechanical decoupling of the continental lithosphere from the underlying mantle

investigating the p wave velocity structure beneath harrat lunayyir northwestern saudi arabia using double difference tomography and earthquakes from the 2009 seismic swarm 1 in 2009 a swarm of more than 30 000 earthquakes occurred beneath the harrat lunayyir lava field in northwest saudi arabia this event was just one of several seismic swarms to occur in this region over the past decade surface deformation associated with the seismicity modeled in previous studies using interferometric synthetic aperture radar insar data is best attributed to the intrusion of a 10 km long dyke however little is known about the velocity structure beneath harrat lunayyir making assessment of future seismic and volcanic hazards difficult in this study we use local double difference tomography to generate a p wave velocity model beneath harrat lunayyir and to more precisely locate earthquakes from the 2009 seismic swarm a pronounced fast velocity anomaly centered at 15 km depth with a shallower extension to the n nw is interpreted as an area of repeated magmatic intrusion the crust surrounding the fast intrusion is slower than that suggested by broader scale models for the arabian shield the largest magnitude events occurred early in the swarm concentrated at shallow depths 2 8 km beneath northern harrat lunayyir and these events are associated with the dyke intrusion later deep earthquakes 15 km beneath the southern end of the study region as well as a group of intermediate depth events connecting the shallow and deep regions of seismicity occurred these later events likely represent responses to the local stress conditions following the intrusion our results are unique since harrat magma systems are rarely imaged and our observations coupled with the seismic history in this region suggest that future volcanic intrusions beneath harrat lunayyir are likely

crustal imprints of precambrian orogenesis in western laurentia crustal low velocity zones lvzs have been reported in active orogens such as the himalayas and the andes but rarely in stable cratonic regions in this study we provide compelling evidence for a significant midcrustal lvz beneath eastern central alberta an integral part of the precambrian canadian shield covered by thick phanerozoic sedimentary deposits this 200 km wide over 10 km thick midcrustal lvz is well resolved by shear velocity inversions using p to s receiver functions from more than 4600 earthquakes it is generally overlain by a high velocity upper crust in the depth range of 8 15 km especially in western central alberta which coincides with the previously documented winagami reflection sequence we interpret the lvz to be of granitic composition potentially in connection with the crystallization of partially molten crust during the paleoproterozoic eon in addition to the precambrian tectonic history of western laurentia which featured plate convergence conducive to crustal melting our crustal model is further supported by 1 a moderate spatial correlation between the lvz and heat flow and 2 shear velocities consistent with that of granite the well preserved winagami reflection sequence and the lvz are potential evidence of distinct episodes of magmatism and crust modification in the precambrian basement of the western canada sedimentary basin the existence of a broad crustal lvz suggests extensive subduction orogenesis and crustal melting during the precambrian assembly of the north american craton

moho depth and crustal composition in southern africa abstract we present new results on structure thickness and composition of the crust in southern africa based on 6300 seismic receiver functions at 85 stations application of hk stacking to the entire sase dataset and use of multi frequency bands improve resolution substantially we observe a highly heterogeneous crustal structure with short wavelength variations in thickness h vp vs ratio composition and moho sharpness which defines 20 blocks that do not everywhere coincide with surface tectonic features in the zimbabwe craton the tokwe block has h 35 38 km and vp vs 1 74 1 79 whereas the thicker crust in the tati block h 47 51 km may be related to deformation of the archean crust along the cratonic margin two distinct crustal blocks with similar crustal thickness 42 46 km but significantly different vp vs ratios are recognized in the limpopo belt extreme values of 1 90 1 94 at the dyke swarms in eastern limpopo and 1 84 at the olifants river dyke swarm and easternmost bushveld intrusion complex bic indicate voluminous magmatic intrusions in the whole crust we find no evidence for magmatic intrusions in the central inferred part of bic where the crust is thick 45 50 km and vp vs is low 1 68 1 70 this thick crustal root may have deflected rising magmas to form the two bic lobes most of central kaapvaal has thin 35 40 km crust and vp vs 1 74 these characteristics are similar to the tokwe block in zimbabwe craton and may indicate delamination of pre existing lower crust which is further supported by a very sharp moho transition the exposed cross section in the vredefort impact crater is non representative of cratonic crust due to shallow moho 34 km and high vp vs 1 80 attributed to shock metamorphism high vp vs 1 76 is typical of the witwatersrand basin and anomalously low vp vs 1 66 1 67 marks the kaapvaal kheis namaqua transition highly heterogeneous crust both in thickness and vp vs ratio is typical of the namaqua natal and cape fold belts

seismic imaging of mantle transition zone discontinuities beneath the northern red sea and adjacent areas summary the dramatic asymmetry in terms of surface elevation cenozoic volcanisms and earthquake activity across the red sea is an enigmatic issue in global tectonics partially due to the unavailability of broad band seismic data on the african plate adjacent to the red sea here we report the first comprehensive image of the mantle transition zone mtz discontinuities using data from the egyptian national seismic network and compare the resulting depths of the 410 and 660 km discontinuities with those observed on the arabian side our results show that when a standard earth model is used for time to depth conversion the resulting depth of thediscontinuities increases systematicallytowards theaxisoftheafro arabiandome aad from both the west and east relative to the westernmost area the maximum depression of the 410 km discontinuity is about 30km and that of the 660 km discontinuity is about 45km the observed systematic variations can best be explained by a model involving a hydrated mtz and an upper mantle low velocity zone beneath the aad models invoking one or more mantle plumes originated from the mtz or the lower mantle beneath the study area are not consistent with the observations

archean and proterozoic crustal evolution evidence from crustal seismology seismic velocity models for archean and proterozoic provinces throughout the world are analyzed the thickness of the crust in archean provinces is generally found to be about 35 km except at collisional boundaries whereas proterozoic crust has a significantly greater thickness of about 45 km and has a substantially thicker high velocity 7 0 km s layer at the base we consider two models that may explain these differences the first model attributes the difference to a change in the composition of the upper mantle the higher temperatures in the archean mantle led to the eruption of komatiitic lavas resulting in an ultradepleted lithosphere unable to produce significant volumes of basaltic melt proterozoic crust developed above fertile mantle and subsequent partial melting resulted in basaltic underplating and crustal inflation in the second model convection in the hot archean mantle is considered to have been too turbulent to sustain stable long lived subduction zones by the proterozoic the mantle had cooled sufficiently for substantial island and continental arcs to be constructed and the high velocity basal layer was formed by basaltic underplating

lithospheric and upper mantle structure of the indian shield from teleseismic receiver functions receiver functions derived from teleseismic p waveforms of 297 earthquakes recorded at broadband station hyderabad hyb are analysed to investigate the lithospheric and upper mantle structure in the vicinity of the station located in the northeastern part of the archaean dharwar craton the internal structure of the 33 2 km thick crust is simple it can be approximated by a single layer with s velocity linearly increasing with depth from 3 4 to 4 0 km s the average being 3 7 0 2 km s the crustal poisson s ratio is 0 25 0 01 a seismic discontinuity at 90 km depth is inferred from a signal particularly prominent on the sh component and possibly indicative of a layer of depth localized anisotropy in the lithospheric mantle the two principal mantle discontinuities are at depths of 406 and 659 km which are close to the global averages

crustal structure of the indian shield new constraints from teleseismic receiver functions teleseismic earthquake waveform data from 10 broadband stations spread over the indian shield and in operation since 1997 were analyzed to infer the crustal structure using the receiver function technique the south indian shield is characterized by a 33 39 km thick and remarkably simple crust with an average poisson s ratio close to 0 25 the archaean crust is devoid of any prominent intra crustal discontinuities the velocity contrast at the well developed moho is large resulting in very clear p to s conversions as well as first order multiples in contrast the predominantly proterozoic crust in the northern part of the shield exhibits a complex character due to the presence of additional seismic discontinuities moho conversions which are considerably weaker compared to the archaean terrains indicate crustal thicknesses of more than 40 km

global scale analysis of the mantle pds phases a global scale analysis of travel times of the mantle pds phases converted from p to s from the discontinuity at a depth d beneath the receiver is presented most of the data are related to p 410 s and p 660 s recorded by the broadband stations of geoscope incorporated research institutions for seismology iris and other networks in order to provide a good global scale coverage published measurements at 37 stations are complemented by our measurements at 45 stations lateral resolution of our data is many times higher than that of ss and its precursors previously used for similar purposes but we have fewer data in the oceans lateral variations of travel times of the major pds phases are strongly correlated with the lateral p and s velocity variations in the uppermost mantle as revealed by tomographic studies the scaling coefficient between the teleseismic s and p residuals as found from the variations of travel times of the major pds phases is around 3 7 for north america and around 3 3 for the rest of the world almost all the differential travel times t p 660 s t p 410 s are in the range of 1 s relative to the average value of 24 0 s if the lateral variations of the velocities within the mantle transition zone mtz can be neglected the corresponding variations of thickness of the mtz are in the range of 10 km relative to the standard depth of 250 km and do not show significant correlation with either velocities in the upper mantle above the mtz or with the variations of thickness of the mtz as found in some recent studies of precursors to ss there is a weak but marginally significant positive correlation between the variations of thickness of the mtz inferred from our data and the s velocity variations in the mtz as given by su et al 1994 this correlation suggests that both variables contain temperature dependent components lateral variability of the mtz is also manifested by extreme weakness of p 410 s at several locations this may indicate either anomalous topography on 410 km discontinuity or strong broadening of the otherwise sharp discontinuity at some stations there are arrivals that can be interpreted as p 520 s this interpretation implies that the strengh and or sharpness of 520 km discontinuity as well as its depth are laterally variable in a broad range

upper mantle structure beneath the arabian peninsula and northern red sea from teleseismic body wave tomography implications for the origin of cenozoic uplift and volcanism in the arabian shield upper mantle structure between 150 and 400 km depth is imaged beneath the arabian shield and northern red sea by modeling p and s traveltime residuals from teleseismic events recorded on the saudi arabia national digital seismic network the 1995 1997 saudi arabian passcal experiment and three permanent stations rayn eil and mrni relative traveltime residuals were obtained using a multichannel cross correlation method and inverted for upper mantle structure using vandecar s inversion method the resulting images reveal a low velocity region 1 5 for the p model and 3 for the s model trending nw se along the western side of the arabian shield and broadening to the northeast beneath the makkah madinah nafud volcanic line we attribute the low velocities to a mantle thermal anomaly that could be as large as 330 k and that is associated with the cenozoic uplift of and volcanic centers on the shield our tomographic images are not consistent with models invoking separate mantle upwellings beneath the northern and southern regions of the shield and instead favor single plume or superplume models we also find little evidence for low velocities beneath the northern red sea suggesting that there might not be a geodynamic link between rifting in the red sea and plateau uplift and volcanism in the shield

buoyancy driven current during cooling periods in ise bay japan 1 the cooling effect on buoyancy driven current is studied in the coastal embayment of ise bay japan by applying knowledge derived from studies of convection in a rotating fluid this convection process is highly sensitive to the presence of inhomogeneities in the preexisting stratification and to differential buoyancy loss on freshwater input the bay water is divided into two regions that differ in their response to the cooling from the surface one region represents a river plume which in effect shields the deeper regions from the cooler surface in the other region convective mixing is active although the mixed water forms and becomes extensive in this convection region the mixed water hardly develops in the river plume region the subsequent differential deepening of the mixed water causes gravitational circulation and yields a mixed water lens which has a reversed dome shape and rotates anticyclonically in a thermal wind balance in the mixed water lends nonvanishing stratification remains because of the rotational effect the location of the mixed water lens and associated anticyclonic circulation determines the basin wide circulation during cooling periods

mantle discontinuities beneath the deccan volcanic province abstract mantle discontinuities beneath the deccan volcanic province dvp are imaged using about 900 seismograms from a network of six broadband stations deployed in the dvp along a 350 km long north south profile paralleling the west coast of india teleseismic receiver function analysis of the data enabled distinct identification of the moho 410 and 660 km discontinuities presence of sub moho low velocity zones with velocity reductions in the range of 0 1 0 4 km s have been brought out through modeling of receiver function stacks at individual stations the arrival times of p410s and p660s are normal as predicted by the iasp91 model and the mantle transition zone thickness is 252 km which is close to the global average this study reveals that the upper mantle beneath dvp south of the narmada rift does not exhibit any anomalous signatures associated with a mantle plume a shield like lithosphere coupled with the presence of an upper mantle low velocity zone confined to shallow depths beneath dvp imposes constraints on the mechanisms feasible for deccan volcanism the present study does not favour the plume impact plume incubation models instead necessitates searching for clues offshore west coast of india and exploring alternate mechanisms

crustal structure and upper mantle stratigraphy of the arabian shield 1 analysis of receiver functions from an eight station network on the saudi arabian shield indicates that the moho depths vary between 35 38 km the crust appears to be rather felsic in nature as evidenced by poisson s ratios of 0 25 0 27 a prominent negative phase following the moho conversion is suggestive of a zone of velocity inversion indication of mantle stratification is observed beneath the coastal region of arabia by way of an additional upper mantle discontinuity at about 240 km the conversions from the 410 and 660 km discontinuities are significantly delayed suggesting a slow and hot upper mantle

micro distribution of oxygen isotopes in a refractory inclusion from the allende meteorite abstract isotope ratios of oxygen have been measured on a fassaite crystal and the surrounding area of a coarse grained cai from the allende meteorite using secondary ion mass spectrometry values of 18 o smow for fassaite spinel anorthite grains and alteration products are 27 5 38 2 5 6 and 4 respectively which are consistent with the results obtained on mineral separates the o isotopic distributions are homogeneous either within individual single crystals of fassaite and anorthite or among the spinel grains within the analytical error no gradual variation in o isotopic composition was detected even at the boundary between different phases or at cracks inside a phase with a spatial resolution of 10 m these observations strongly suggest that the partial exchange of o isotopes in the minerals in the cai with the surrounding gas through diffusion is an unlikely origin for the observed inter mineral heterogeneity of o isotopes in the allende cais

evidence for crustal low shear wave speed in western saudi arabia from multi scale fundamental mode rayleigh wave group velocity tomography abstract we investigate the crustal and upper mantle shear velocity structure of saudi arabia by fundamental mode rayleigh wave group velocity tomography and shear wave velocity inversion the seismic dataset is compiled using 140 stations of the saudi national seismic network snsn operated by the saudi geological survey sgs we measure rayleigh wave group velocities at periods of 8 40 s from regional earthquakes after obtaining 1 d shear wave velocity models by inverting group velocities at each grid node we construct a 3 d shear velocity model for saudi arabia and adjacent regions by interpolating the 1 d models our 3 d model indicates significant lateral variations in crustal and lithospheric thickness as well as in the shear wave velocity over the study region in particular we identify zones of reduced shear wave speed at crustal levels beneath the cenozoic volcanic fields in the arabian shield the inferred reductions of 2 5 in shear wave speed may be interpreted as possibly indicating the presence of partial melts however their precise origin we can only speculate about our study also reveals an upper mantle low velocity zone lvz below the arabian shield supporting the model of lateral mantle flow from the afar plume further geophysical experiments are needed to confirm or refute the hypothesis that partial melts may exist below the cenozoic volcanism in western saudi arabia and to build a comprehensive geodynamic geological model for the evolution and present state of the lithosphere of the arabian plate and the red sea

ray path interpretation of the crustal structure beneath saudi arabia abstract interpretation of a long range seismic refraction line in saudi arabia has shown that beneath the arabian shield velocity generally increases with depth from about 6 km s 1 at the surface to about 7 km s 1 at the top of the crust mantle transition zone the base of this transition zone moho occurs at 37 44 km in depth intracrustal discontinuities can also be recognized the most important being in the 10 20 km depth range and separating the upper from the lower crust laterally the variations in the intracrustal discontinuities and the total crustal thickness can be correlated with previously defined tectonic regions beneath the red sea shelf and coastal plain the crust including 4 km of sediments is only 15 17 5 km thick with the aid of both seismic and gravity data an abrupt steeply dipping transition from the crust of the red sea shelf and coastal plain to that of the arabian shield has been derived with a jump of more than 20 km in moho depth this appears to be the major discontinuity between the red sea depression and the arabian continental shield

seismic velocity and q structure of the middle eastern crust and upper mantle from surface wave dispersion and attenuation observed velocities and attenuation of fundamental mode rayleigh waves in the period range 7 82 sec were inverted for shear wave velocity and shear wave q structure in the middle east using a two station method additional information on q structure variation within each region was obtained by studying amplitude spectra of fundamental mode and higher mode rayleigh waves we obtained models for the turkish and iranian plateaus region 1 areas surrounding and including the black and caspian seas region 2 and the arabian peninsula region 3 the effect of continent ocean boundaries and mixed paths in region 2 may lead to unrealistic features in the models obtained there at lower crustal and upper mantle depths shear velocities are similar in all three regions shear velocities vary significantly in the uppermost 10 km of the crust being 3 21 2 85 and 3 39 km s for regions 1 2 and 3 respectively q models obtained from an inversion of interstation attenuation data show that crustal shear wave q is highest in region 3 and lowest in region 1 q s for the upper 10 km of the crust are 63 71 and 201 for regions 1 2 and 3 respectively crustal q s at 30 km depth for the three regions are about 51 71 and 134 the lower crustal q values contrast sharply with results from stable continental regions where shear wave q may reach one thousand or more these low values may indicate that fluids reside in faults cracks and permeable rock at lower crustal as well as upper crustal depths due to convergence and intense deformation at all depths in the middle eastern crust

saudi arabian seismic refraction profile a traveltime interpretation of crustal and upper mantle structure abstract the crustal and upper mantle compressional wave velocity structure across the southwestern arabian shield has been investigated by a 1000 km long seismic refraction profile the profile begins in mesozoic cover rocks near riyadh on the arabian platform trends southwesterly across three major precambrian tectonic provinces traverses cenozoic rocks of the coastal plain near jizan and terminates at the outer edge of the farasan bank in the southern red sea more than 500 surveyed recording sites were occupied and six shot points were used including one in the red sea two dimensional ray tracing techniques used to analyze amplitude normalized record sections indicate that the arabian shield is composed to first order of two layers each about 20 km thick with average velocities of about 6 3 km s and 7 0 km s respectively west of the shield red sea margin the crust thins to a total thickness of less than 20 km beyond which the red sea shelf and coastal plain are interpreted to be underlain by oceanic crust a major crustal inhomogeneity at the northeast end of the profile probably represents the suture zone between two crustal blocks of different composition elsewhere along the profile several high velocity anomalies in the upper crust correlate with mapped gneiss domes the most prominent of which is the khamis mushayt gneiss based on their velocities these domes may constitute areas where lower crustal rocks have been raised some 20 km two intracrustal reflectors in the center of the shield at 13 km depth probably represent the tops of mafic intrusives the mohorovicic discontinuity beneath the shield varies from a depth of 43 km and mantle velocity of 8 2 km s in the northeast to a depth of 38 km and mantle velocity of 8 0 km s depth in the southwest near the shield red sea transition two velocity discontinuities occur in the upper mantle at 59 and 70 km depth the crustal and upper mantle velocity structure of the arabian shield is interpreted as revealing a complex crust derived from the suturing of island arcs in the precarnbrian the shield is currently flanked by the active spreading boundary in the red sea

super deep low velocity layer beneath the arabian plate 1 s and p receiver functions reveal indications of a low s velocity layer at 350 410 km depths beneath the arabian plate a similar layer was previously found beneath the kaapvaal craton in southern africa and tunguska basin of the siberian platform we hypothesize that the boundary at 350 km depth may separate dry mantle root of the arabian plate from the underlying wet mantle layer this boundary is not found beneath the gulf of aden where the root is destroyed by sea floor spreading

precambrian crustal structure in africa and arabia evidence lacking for secular variation we review the thickness and shear wave velocity structure of precambrian crust in africa and arabia where over 90 of the landmass is comprised of archean and proterozoic terranes and examine the data for evidence of secular variation the data come from many published 1d shear wave velocity profiles obtained by jointly inverting receiver functions and surface wave dispersion measurements 35 new 1d shear wave velocity profiles for locations in eastern africa and a new map of crustal thickness for africa and arabia derived from modeling satellite gravity data we find for both archean and proterozoic terranes a similar range of crustal thicknesses 33 45 km similar mean crustal shear wave velocities 3 6 3 7 km s and similar amounts of heterogeneity in lower crustal structure as reflected in the thickness of lowermost crust with shear wave velocities 4 0 km s there is little evidence for secular variation in crustal structure indicating that there may have been few changes over much of earth s history in the processes that form the continental crust post formation tectonic events also may have modified many of the terranes to such an extent that secular trends arising from crustal genesis may be difficult to recognize

exchange fluxes between the ria de vigo and the shelf a bidirectional flow forced by remote wind 1 in the ria de vigo western coast of the iberian peninsula the seasonal pattern associated with shelf winds at these latitudes produces upwelling downwelling episodes that affect the residual circulation the aim of this paper is to go deeply into the interaction between remote winds and the dynamics of the ria aided by an inverse model in the galician r ias the box models inverse models have been developed from a two dimensional 2 d point of view since the classical baroclinic residual circulation in positive estuaries has a vertical distribution freshwater outflow through the surface layer and inflow through the bottom layer however recent work in the ria de vigo has shown the existence of a 3 d residual circulation in the seaward region of the ria during upwelling downwelling conditions a coastal jet enters leaves the ria through the northern mouth and leaves enters through the southern mouth the inverse model developed in this paper is a 3 d kinematic box model based on tracer and volume balances with the constraint of nonnegative vertical exchange coefficients the fluxes obtained confirm during some periods the 3 d residual circulation overlying the classical 2 d scheme it is shown how the 3 d pattern affects the inner ria causing transversal differences in the flow field in addition the model results have been helpful in the discussion of a possible barotropic bidirectional flow associated with remote winds the results have set the basis for the existence of a bidirectional flow originated by a barotropic interaction with the shelf upwelling

surface wave waveform anomalies at the saudi seismic network this paper describes surface wave waveforms observed at the saudi arabian seismic network we identify records corresponding to events occuring in the kuril islands region as anomalous and discuss possible causes for the waveform distortions most likely the waveform anomalies are caused by constructive and destructive interference of laterally refracted wave packets some waveform anomalies are probably of instrumental origin and we suspect a calibration error for the vertical component at station riyd for station afif we find a significant clockwise rotation of the horizontal components with respect to geographic north by approximately 4 degrees

the upper mantle discontinuities correlated or anticorrelated the discontinuities in the earth s upper mantle at 410km and 660km are frequently considered as phase changes which are exothermic and endothermic respectively from this follows that at laterally changing temperatures both discontinuities move in opposing vertical directions that means they are negatively correlated we attempt to test this hypothesis using travel times of seismic waves converted at both discontinuities underneath seismic stations the broadband stations used are located in areas of high e g western us and low heat flow e g central asia sclater et al 1980 we observe a clear positive correlation of the conversion times of both discontinuities which can however be explained with known lateral velocity changes above 410km from the scatter of our data we conclude that the limit of a possibly existing and negatively correlated topography of the two discontinuities is about 28km this is in contrast to recently claimed observations of about 35 km anticorrelated topography in the western pacific revenaugh jordan 1991 stammler kind 1992

the distribution of moho depths beneath the arabian plate and margins abstract in this study three new maps of moho depths beneath the arabian plate and margins are presented the first map is based on the combined gravity model eigen 06c which includes data from satellite missions and ground based studies and thus covers the whole region between 31 e and 60 e and between 12 n and 36 n the second map is based on seismological and ground based gravity data while the third map is based only on seismological data both these maps show gaps due to lack of data coverage especially in the interior of the arabian plate beneath the interior of the arabian plate the moho lies between 32 and 45 km depth below sea level there is a tendency for higher pn and sn velocities beneath the northeastern parts of the plate interior with respect to the southwestern parts of the plate interior across the northern destructive margin with the eurasian plate the moho depths increase to over 50 km beneath the zagros mountains across the conservative western margin the dead sea transform dst moho depths decrease from almost 40 km beneath the highlands east of the dst to about 21 23 km under the southeastern mediterranean sea this decrease seems to be modulated by a slight depression in the moho beneath the southern dst the constructive southwestern and southeastern margins of the arabian plate also show the moho shallowing from the plate interior towards the plate boundaries a comparison of the abruptness of the moho shallowing between the margins of the arabian plate the conjugate african margin at 26 n and several atlantic margins shows a complex picture and suggests that the abruptness of the moho shallowing may reflect fundamental differences in the original structure of the margins

the lithospheric shear wave velocity structure of saudi arabia young volcanism in an old shield abstract we investigate the lithospheric shear wave velocity structure of saudi arabia by conducting h stacking analysis and jointly inverting teleseismic p receiver functions and fundamental mode rayleigh wave group velocities at 56 broadband stations deployed by the saudi geological survey sgs the study region the arabian plate is traditionally divided into the western arabian shield and the eastern arabian platform the arabian shield itself is a complicated melange of crustal material composed of several proterozoic terrains separated by ophiolite bearing suture zones and dotted by outcropping cenozoic volcanic rocks locally known as harrats the arabian platform is primarily covered by 8 to 10 km of paleozoic mesozoic and cenozoic sedimentary rocks our results reveal high vp vs ratios in the region of harrat lunayyir which are interpreted as solidified magma intrusions from old magmatic episodes in the shield our results also indicate slow velocities and large upper mantle lid temperatures below the southern and northern tips of the arabian shield when compared with the values obtained for the central shield we argue that our inferred patterns of lid velocity and temperature are due to heating by thermal conduction from the afar plume and possibly the jordan plume and that volcanism in western arabia may result from small scale adiabatic ascent of magma diapirs

effect of the earth s rotation on the circulation in regions of freshwater influence recent surveys in various regions of freshwater influence have shown considerably different flow patterns in each region an analytical model including viscous effects and the earth s rotation is proposed to examine the along channel flow pattern and to explain the differences model results show that the flow pattern is strongly dependent on the ekman number e with a large ekman number e 1 the system is governed by gravitational circulation and thus horizontal density gradients in along channel direction are important the whole water column is in the ekman layer and consequent jet inflow penetrates the surface over a deep depression if the bottom topography varies in a cross channel direction with an intermediate ekman number e 0 1 this jet inflow concentrates in the lower layer in which the viscosity still plays an important role the flow in the upper layer is on the other hand determined by the geostrophic balance the contribution by the geostrophic flow becomes larger so that the cross channel density gradients are important when die ekman number is small e 0 01 since the ekman layer is clung to the bottom the jet inflow exists only in the thin bottom layer the hydrographic and acoustic doppler current profiler surveys were conducted in ise bay japan both the density structure and the flow pattern were different from those observed in many drowned river valleys a strong jet inflow existed in the lower layer over the depression while the flow in the upper layer suggested anticyclonic circulation the estimated ekman number is 0 07 in the bay and thus the observed pattern is consistent with the model result when e 0 1

texture of mantle lithosphere along the dead sea rift recently imposed or inherited seismic anisotropy a property linked to the texture of the mantle rock should be distributed with depth along the trace of the dead sea rift dsr owing to a combination of present day and ancient tectonics using data from four permanent and one temporary seismic observatories we evaluate birefringence splitting of 91 teleseismic core refracted shear waves primarily sks phases we find significant levels of birefringence in the bulk of observed phases we also find that birefringence parameters fast directions and delays vary as a function of source receiver geometry notably the pattern of this directional variation in birefringence is quite similar at all sites we have examined we interpret observed birefringence in sks phases in terms of one and two layer models single layer models for all stations exhibit a fast polarization oriented 12 19 east of north with anisotropy sufficient to generate 1 3 s time delay we find strong evidence for at least two distinct anisotropic layers for the two layer models the upper layers resemble the single layer models showing near north fast polarizations and delays on the order of 1 s three out of four sites show fast polarizations in the lower layer that strike 50 80 cw from north with time delays 0 3 0 6 s one site at the northern end of the dsr displays a higher degree of anisotropy in the lower layer and a more northerly fast polarization overall the lower layers at all sites appear to be consistent with the deformation caused by plate motion relative to the asthenosphere the fabric in the upper layer is sub parallel to the present day transcurrent motion on the dsr but also matches the typical orientation of lithospheric seismic anisotropy in the arabian shield our overall conclusion is that the impact of the dsr on the rock fabric of the mantle lithosphere is probably quite weak

a multistep approach for joint modeling of surface wave dispersion and teleseismic receiver functions implications for lithospheric structure of the arabian peninsula 1 we present a multiple step procedure for joint modeling of surface wave group velocity dispersion curves and teleseismic receiver functions for lithospheric velocity structure the method relies on an initial grid search for a simple crustal structure followed by a formal iterative inversion an additional grid search for shear wave velocity in the mantle and finally forward modeling of transverse isotropy to resolve love rayleigh surface wave dispersion discrepancy it considers longer period surface wave group velocity swgv dispersion allowing for the resolution of deeper structure compared to previous joint inversions the grid search for simple crustal structure is facilitated using a library of precomputed receiver functions and swgv dispersion curves the iterative inversion improves fit to the data by increasing the number of layers in the crust when necessary in order to fit the swgv for periods greater than about 50 s we perform a grid search over mantle velocities including the mantle lid and low velocity zone keeping the crustal structure fixed to the values from the previous step in some cases a clear love rayleigh discrepancy prevents a simultaneous fit of the group velocities with an isotropic model the love rayleigh discrepancy can be resolved by allowing shear wave transverse isotropy with a vertical symmetry axis vsh vsv differences in the uppermost mantle the method is applied to 10 stations in the arabian peninsula sampling various tectonic environments including active continental rifting and stable regions the resulting shear velocity models confirm rapid crustal thinning of the arabian shield toward the red sea however we do not find strong evidence for crustal thickening toward the arabian platform our results suggest that the mantle lithosphere thickness varies regionally but that the mantle shear velocities beneath the arabian shield and red sea coast are generally anomalously low furthermore our results indicate the presence of strong polarization anisotropy up to about 10 in the lithospheric upper mantle in the vicinity of as well as farther away from the red sea our modeling yields vsv vsh in the southwestern part of the arabian peninsula consistent with vertical flow and vsh vsv in the northwestern part of the arabian peninsula and the continental interior consistent with horizontal flow indicating that the mantle flow pattern is not uniform along the axis of the red sea