seismic anisotropy of the northeastern tibetan plateau from shear wave splitting analysis abstract we present shear wave splitting results obtained from the analysis of teleseismic sks skks and pks phases recorded by 70 permanent seismographic stations located in the northeastern tibetan plateau we identify a contrast in the splitting pattern complexity beneath different parts of ne tibet in the western and northern part anisotropy observations are well explained by a single layer of anisotropy with a fast anisotropic direction trending nww see or nw se in xining and its adjacent area the anisotropy shows strong azimuthal dependence of splitting parameters that can be modeled by two anisotropic layers the fast direction for the upper layer lies in the n75 95 e range which is consistent with the surface movement direction determined from gps and could be associated with middle to lower crustal flow the fast direction in the lower layer is in the n105 125 e range and similar to the direction observed in the western and northern part where only a single layer is required these nww see or nw se fast feature could be related to the current orogenesis induced from the india eurasia collision or flow in the asthenosphere related to the absolute motion of eurasia comparison between the anisotropy patterns expected from proposed models with our shear wave splitting observation suggests that no unique geodynamic model can reconcile all splitting measurements for such a complex region

crustal deformation of the eastern tibetan plateau revealed by magnetotelluric imaging the ongoing collision of the indian and asian continents has created the himalaya and tibetan plateau through a range of deformation processes these include crustal thickening detachment of the lower lithosphere from the plate delamination and flow in a weakened lower crust 1 6 debate continues as to which of these processes are most significant 7 in eastern tibet large scale motion of the surface occurs but the nature of deformation at depth remains unresolved a large scale crustal flow channel has been proposed as an explanation for regional uplift in eastern tibet 6 8 9 but existing geophysical data 10 11 do not constrain the pattern of flow magnetotellurics uses naturally occurring electromagnetic waves to image the earth s subsurface here we present magnetotelluric data that image two major zones or channels of high electrical conductivity at a depth of 20 40 km the channels extend horizontally more than 800 km from the tibetan plateau into southwest china the electrical properties of the channels imply an elevated fluid content consistent with a weak crust 12 13 that permits flow on a geological timescale these findings support the hypothesis that crustal flow can occur in orogenic belts and contribute to uplift of plateaux our results reveal the previously unknown complexities of these patterns of crustal flow many previous studies of the indiaasia

seismic anisotropy beneath the lau back arc basin inferred from sscs scs splitting data we determine upper mantle anisotropy just beneath active basins by comparing scs and sscs waveform splitting we have measured sscs scs shear wave splitting with data from the spanet and gsn seismic networks the observed sscs scs shear wave splitting suggests that there is a large lateral variation of anisotropy beneath the lau back arc basin scs and sscs waves which sample the southern part of the lau basin show small splitting in contrast the northern part of the lau basin which is characterized to be tectonically active is observed to have large seismic anisotropy with a wnw ese direction the cause of anisotropy may be related to the mantle dynamics of the active basin

brazilian and african passive margins of the central segment of the south atlantic ocean kinematic constraints the thinning of passive continental margins is usually explained by models using pure stretching or simple shear these models imply hypothetical extensional structures and large horizontal movements between the two conjugate margins more than 250 km for the brazilian and angolan margins refraction reflection data together with the most recent and tightest pre opening fit using continental and oceanic geological and geophysical constraints show that the substratum of the sag basin is divided into an autochthonous part upper continental crust and an allochthonous part exhumed material the thinning process which evolves in an elevated position of the system until at least the break up seems to be depth dependent and to mainly concern the lower middle crust which we postulate is exhumed this exhumation does not explain the entire thinning of the system horizontal motions cannot alone explain the formation of the huge angolan brazilian basin similar observations are made on the whole central segment of the south atlantic ocean part of the lower continental crust is still missing and it seems most improbable that the continental crust maintains its integrity throughout the thinning process

kinematic model of active deformation in central asia the velocity field of present day deformation in central asia is modelled using a set of four rotating blocks siberia tarim tibet india on a spherical earth a best fit is inverted on the basis of estimated shortening rates across the main thrust zones himalayas tien shan and measured slip rates along the principal strike slip faults altyn tagh and karakorum separating those blocks the fit to the data implies that nearly all the present convergence between india and asia can be accounted for by slip partitioning on these four zones with as much as 50 absorbed by northeastwards extrusion of tibet this suggests that localised deformation governs the present mechanical behaviour of the central asian lithosphere

surface deformation and lower crustal flow in eastern tibet field observations and satellite geodesy indicate that little crustal shortening has occurred along the central to southern margin of the eastern tibetan plateau since about 4 million years ago instead central eastern tibet has been nearly stationary relative to southeastern china southeastern tibet has rotated clockwise without major crustal shortening and the crust along portions of the eastern plateau margin has been extended modeling suggests that these phenomena are the result of continental convergence where the lower crust is so weak that upper crustal deformation is decoupled from the motion of the underlying mantle this model also predicts east west extension on the high plateau without convective removal of tibetan lithosphere and without eastward movement of the crust east of the plateau

isostasy of aseismic tectonic units in the south atlantic ocean and geodynamic implications abstract mechanical isostasy of the lithosphere in the south atlantic ocean was studied using information on gravity anomalies and bathymetry with additional constraints imposed by the altimetric geoid the isostatic responses admittances over the walvis ridge rio grande rise and trindade seamount chain were computed using a three dimensional algorithm the eastern walvis ridge and the rio grande rise have the same response which is well explained by an airy model of isostasy the other features are regionally supported a variation in the thickness of the elastic plate was found along the western walvis ridge a high value of the elastic plate thickness 20 km was found under the trindade chain geodynamic implications are discussed in the light of these results

a recipe for microcontinent formation accreted slivers of continental margins are common in the geologic record but the processes that lead to their formation are poorly understood we observe an association of plume related microcontinent isolation and subsequent long term asymmetries in oceanic crustal accretion based on four recent examples the seychelles in the indian ocean jan mayen in the norwegian greenland sea and the east tasman plateau and the gilbert seamount complex in the tasman sea these microcontinents formed by rerifting of a young continental margin 25 m y old in the vicinity of a mantle plume stem followed by asymmetric seafloor spreading two dimensional numerical stochastic basin modeling suggests that a yield strength minimum along the landward edge of a rifted margin thermally enhanced by heating from a mantle plume may cause a spreading ridge to jump onto this zone of weakness this action isolates a passive margin segment the association of large igneous provinces and microcontinents should be useful for identifying similar events in the geological record

anisotropy and crustal thickness of northern tibet new constraints for tectonic modelling in 1993 a seismological array was installed on a profile along the golmud lhasa highway from the qaidam basin to central tibet a travel time analysis for local and regional earthquakes was performed in order to better constrain the crustal structure obtained by teleseismic tomography more information about the crustal thickness and anisotropy comes from the analysis of moho converted p to sv waves the two methods supplement each other and confirm the existence of crust thinner than expected accross the northern edge of the tibetan plateau and qaidam p to sv splitting analysis yields a striking similarity between the fast polarization direction and the velocity field of present day deformation as inferred from kinematic modelling the two observations the thinning of the crust and the clockwise rotation of the fast polarization direction emphasize the leading role of eastward extrusion for the absorption of the large amount of crustal shortening occuring in central asia

mantle deformation and tectonics constraints from seismic anisotropy in the western united states abstract we have examined shear wave splitting in teleseismic shear waves sks skks s from 15 stations in the western united states based on analysis of 123 records from 67 events the varied past and present tectonic styles in this region subduction transform faulting extension and stable domains are expected to cause a wide variety of anisotropic behavior and therefore make it an excellent natural laboratory fast polarization azimuths vary from e w to nearly n s and time delays t range from being barely detectable less than 0 6 s to 1 6 s most stations yielded consistent measurements independent of station event geometry the exceptions were stations situated very close to the san andreas fault which yielded well constrained but inconsistent splitting parameters these have been successfully modeled by two anisotropic layers with different horizontal symmetry axes the upper layer has a fast direction parallel to the fault and t of about 1 s the lower layer with t from 0 6 to 0 9 s is oriented e w near the san francisco bay area and ne sw in the mojave desert other measurements of e w fast are observed as far east as western nevada with large delay times of 1 3 1 5 s but disappear to the north stations in the northwestern basin and range have values of oriented at about 70 with delay times ranging from 0 7 to 1 2 s varies in other regions of the basin and range from 70 in eastern nevada to 20 in the transition zone between the colorado plateau and the basin and range with t of 1 0 s measurements are geographically coherent and in many cases changes correspond to geologic or tectonic boundaries the e w fast feature extends across several tectonic provinces but disappears near the southern boundary of the gorda plate and may therefore be related to the dynamics of plate interactions regional variations in the fast directions of anisotropy within the basin and range suggest that the present extension has not been strong enough to align the fast directions over a great depth extent fast polarization directions within the transition zone between the basin and range and colorado plateau are nearly parallel to the geologic and geophysical boundaries between the two regions and to other geophysical boundaries the size of the delay times suggests that in many of the areas studied either the mantle is almost twice as anisotropic as we estimate or the anisotropy and therefore mantle deformation extends into the sublithospheric mantle we used the two layer method to examine the effect of a postulated lower layer with fast direction parallel to the absolute motion of the north american plate and found the best estimate of contribution from a uniform layer is 0 1 s of t although a contribution of up to 0 7 s cannot be ruled out a value as low as 0 1 s suggests either that olivine is not strongly aligned in a horizontal direction in this layer or that the layer thickness is of the order of 10 km

evidence for mechanically coupled lithosphere in central asia and resulting implications the recent dramatic increase in seismic anisotropy and surface global positioning system gps data for central asia permits a comprehensive examination of the mantle s role in mountain building a joint analysis of 178 shear wave splitting and 2000 gps observations using a new technique reveals that the crust and lithospheric mantle deform coherently arguing for crust mantle mechanical coupling during deformation the observed spatial variations in anisotropy refl ect the large scale pattern of lithospheric deformation as well as a change in deformational style from simple shear on the tibetan plateau transitioning to pure shear in surrounding regions

application of a seasonal climate model to cenozoic glaciation mathematical modelling of climate has matured as a discipline to the point that it can be useful in palaeoclimatology as an example a new two dimensional energy balance model is described and applied to several problems of current interest the model includes the seasonal cycle and the detailed land sea geographical distribution by examining the changes in the seasonal cycle when external perturbations are forced upon the climate system it is possible to construct hypotheses about the origin of mid latitude ice sheets and polar ice caps in particular the model predicts a rather sudden potential for glaciation over large areas when the earth s orbital elements are only slightly altered similarly the drift of continents or the change of atmospheric carbon dioxide over geological time can induce radical changes in continental ice cover with the advance of computer technology and our improved understanding of the individual components of the climate system it should be possible to test these ideas in far more realistic models in the near future

crustal thickness mapping of the central south atlantic and the geodynamic development of the rio grande rise and walvis ridge abstract the origin of the rio grande rise and walvis ridge within the central south atlantic and its implications for the separation of south america and africa during the cretaceous are controversial the recent report of the discovery from submersible sampling of continental material of proterozoic age on the rio grande rise suggests that the existing explanation involving ocean ridge mantle plume interaction or simply excess on ridge magmatism for the formation of the rio grande rise and walvis ridge needs to be re examined we use gravity anomaly inversion to map crustal thickness for the central south atlantic area encompassing the rio grande rise walvis ridge and adjacent south american and african rifted continental margins we show that the rio grande rise consists of three distinct bodies of anomalously thick crust western central and eastern separated by normal thickness oceanic crust the central rio grande rise forms a large elliptical body with maximum crustal thickness of 25 km the walvis ridge also has a maximum crustal thickness of 25 km but has a narrower and more linear geometry we use plate reconstructions to restore maps of crustal thickness and magnetic anomaly to early cretaceous times to examine the development of the rio grande rise and walvis ridge these restorations together with ages of magmatic addition suggest that the central rio grande rise and walvis ridge formed a single body between 90 and 80 ma located on the ocean ridge plate boundary similar to iceland today on the basis of crustal thickness mapping the plate restorations and the magmatic ages we propose that the rio grande rise was fragmented into its 3 parts and separated from walvis ridge by at least 4 ocean ridge jumps during the opening of the south atlantic ocean between approximately 90 and 50 ma plate reconstructions of crustal thickness showing rotated structural lineaments imply that the separation of eastern rio grande rise and walvis ridge was highly complex involving simultaneous crustal extension and magmatic addition we propose that the continental material reported on the rio grande rise if not drop stones was isolated from the main continental land mass and transported into the ocean by these ridge jumps during the cretaceous formation of the south atlantic

shear wave splitting in eastern and central china implications for upper mantle deformation beneath continental margin eastern china located at the east margin of the eurasia plate is an ideal place to study the deformation pattern of upper mantle beneath continental margin we investigated the upper mantle anisotropy beneath eastern china by applying teleseismic shear wave splitting measurements we examined shear wave splitting measurements at 33 permanent stations from cdsn and gsn iris networks in eastern and central china as well as splitting parameters from a previous regional study the splitting observations are characterized by apparent diversity of anisotropy pattern in adjacent tectonic domains including the southeast china orogenic belt and the yangtze craton and the eastern and central north china craton based upon the straightforward relationships between seismic anisotropy and the development of lattice preferred orientation of mineral in upper mantle rocks we interpret the splitting results in terms of tectonic fabric within the upper mantle our interpretation implies that the upper mantle anisotropy beneath eastern china remained the effects of tectonic rejuvenation occurred during the late mesozoic to cenozoic we proposed that there are possibly two kinds of mechanisms involved in the upper mantle deformation beneath eastern china depending on the roles of thick root of cratonic lithosphere one mechanism is the deflecting the asthenospheric flow by the thick root of craton and the other one is the extensional asthenospheric flow causing the craton lost part of its thick root 2007 elsevier b v all rights reserved

the rio grande fracture zone in the western south atlantic and its tectonic implications abstract a broad zone of linear mappable basement structures is observed north and northeast of the rio grande rise in the south atlantic ocean these structures lie along the same flow line as the sa o paulo ridge the florianopolis high and onshore lineaments suggesting that they all comprise the same tectonic trend the rio grande fracture zone the morphology developed along this fracture zone during the early opening of the south atlantic ocean formed a barrier to open ocean circulation during the aptian and allowed the formation of extensive evaporite deposits to the north of it

a positive climatic feedback mechanism for himalayan glaciation abstract glaciation at elevated sites in low latitudes has the potential to play an important role in the global radiation budget because the magnitudes of incoming and reflected shortwave radiation are so large a climatic mechanism is presented which provides a positive feedback for glaciation in the himalayan mountains a higher bare surface albedo in the himalayas induces an atmospheric circulation response that consists of cooling and descent over the himalayas and warming and a contrasting air mass uplift to the west over the persian gulf and the arabian peninsula although annual mean rainfall over the subcontinent south of the himalayas decreases the summer southwesterly monsoon winds are stronger over the mountains in northwestern india and carry more moisture to higher elevations where it is precipitated as snow conversely in wintertime the northeasterly monsoon winds are weakened over the himalayas and surface wind convergence and snowfall are increased there increased snowfall in the mountains expands the surface area of the high albedo region in both seasons leading to a positive feedback the peak amplitude of this feedback is ultimately regulated by springtime melting rates

post younger dryas climate interval linked to circumpolar vortex variability isotopic evidence from fayetteville green lake new york the late glacial holocene transition in the north atlantic european sectors has long been known to be a period of rapid climate change there is however a continued need for acquiring and developing paleoclimate archives spanning this interval from continental settings here we report on a lacustrine fayetteville green lake isotope record sampled at a 10 year resolution from the ne usa over the late glacial holocene interval 14 600 8000 cal year bp based on prior isotopic and hydrologic research from green lake the 18o calcite values predominantly reflect winter moisture source and thus winter atmospheric patterns furthermore we use historic ad 1948 1980 winter circulation data and 18o calcite values from varved sediments to examine the relationship between the circumpolar vortex latitude and isotopes which results in a strong r 0 79 r2 0 63 negative relationship using the linear regression from the isotope vortex relationship we model the winter vortex latitude for the late glacial holocene transition over the ne usa in addition we identify an interval from 11 600 to 10 300 cal year bp the post younger dryas climate interval wherein the mean winter vortex over the ne usa was expanded by 6 latitude 36 1 n i e 630 km from its mean historic position between ad 1948 1998 41 8 n renewal of more vigorous thermohaline circulation following the younger dryas cold event may have forced the post younger dryas climate interval increased poleward heat transport due to an active oceanic conveyor would have strengthened the thermal contrast between the ne usa and the north atlantic thereby enhancing atmospheric pressure gradients and firmly establishing the semi permanent winter trough over the ne usa consequently storms tracked more frequently up the east coast of the united states from the gulf of mexico and atlantic regions delivering precipitation with relatively high 18o values to the ne usa alternatively the relative dominance of seasonal precipitation may have changed resulting in less total winter precipitation low 18o causing lake waters to become relatively enriched in high 18o summer precipitation both hypotheses are good candidates for testing by global circulation modeling gcm the termination of the post yd interval at 10 300 cal year bp and the beginning of the holocene was likely forced by a gradual decrease in thermohaline circulation and poleward heat transport as freshwater input from melting of high latitude ice sheets increased

the thermal regime of south african continental margins abstract we present thermal data from 100 oil exploration boreholes on the continental margins of south africa near surface temperature profiles are constrained with temperatures acquired during reservoir tests corrected bottom hole temperatures and seafloor temperatures the thermal conductivity profiles are estimated from geophysical well logs using a neural network method available data allow us to determine 41 new heat flow values computed from depth ranges greater than 1000 m located on the western and southern continental margins of south africa we believe that the variations in surface heat flow are not controlled by the contribution of the sediments or the basement but rather by the mantle heat flow an upper bound of 30 mw m 2 for the mantle heat flow on the interior of the continent can be inferred from the lowest available estimates located near the coastline on the southern continental margin on both continental margins heat flow values reach a maximum of about 60 mw m 2 100 km away from the shore and consistently decrease further away towards values typically encountered in old oceanic domains 50 mw m 2 this indicates that 1 the mantle heat flow increases over a short distance at the edge of the continent which is consistent with studies on the effect of continents on mantle convection and 2 it may increase to a value greater than heat flow on the adjacent ocean as a possible consequence of the effect of the transition between thin oceanic lithosphere and thick and cold continental lithosphere on mantle convection

evolution of the walvis ridge rio grande rise hot spot system implications for african and south american plate motions over plumes crystallization ages of volcanic rocks dredged or drilled from the walvis ridge ten sites and the rio grande rise one site have been determined by the 40ar 39ar incremental heating technique the fundamentally age progressive distribution of these basement ages suggests a common hot spot source for volcanism on the island of tristan da cunha along the walvis ridge and rio grande rise and for the formation of the continental flood basalts located in namibia africa and brazil south america the walvis ridge rio grande rise volcanic system evolved along a section of the south atlantic spreading axis as the african and south american plates migrated apart astride or in close proximity to an upwelling plume reconstructions of the spatial relationship between the spreading axis the tristan hot spot and the evolving walvis ridge rio grande rise volcanic feature show that at about 70 ma the spreading axis began to migrate westward away from the hot spot the resulting transition to intraplate hot spot volcanism along the walvis ridge and associated termination of rio grande rise formation also involved a northward migration of previously formed african seafloor over the hot spot rotation parameters for african motion over fixed hot spots i e absolute motion have been recalculated such that the predicted trail of the tristan hot spot agrees with the distribution of radiometric and fossil basement ages along the walvis ridge african absolute motion has been extended to the south and north american plates by the addition of relative motion reconstruction poles

tomographic inversion of pn travel times in china 1 pn travel times are useful for studying crustal and uppermost mantle structure and regional tectonics because they are affected by crustal velocity and thickness as well as uppermost mantle velocity and anisotropy we obtained 57 740 pn travel time picks from 5433 earthquakes and 307 stations from chinese national and provincial earthquake bulletins and the international seismological center bulletins to invert for pn velocity variation and anisotropy and station delays in china our inversion reveals significant features that correlate with surface geology the main results are as follows 1 the pn velocities show a mosaic of very fast and very slow anomalies mirroring the heterogeneous geology of china at the surface the pn velocities are high beneath the major basins in the west sichuan qaidam west tarim tulufan and junggar and low in areas of active volcanoes myanmar and western yunnan and quaternary volcanism in northern tibet in seismically active areas in north china and tien shan and in the southern part of south china the hainan plume 2 the pn anisotropy beneath the major basins in the west is generally weak strong anisotropy is found beneath high deformation regions the tibetan plateau southeastern margin of the tibetan plateau western tien shan and part of north china suggesting the anisotropy is likely related to recent large scale tectonic activity 3 a large area of north china shows prominent low pn velocity beneath archean basement with thin crust our observations are consistent with rifting lithospheric thinning and mantle upwelling in the region the pn anisotropy is consistent with a dextral simple shear in the nne direction in the lithosphere mantle during the last and ongoing major deformation period 4 the pn velocity in northern tibet is generally lower than that in the south southern tibet has significant e w structure low velocity anomalies can be traced from northern tibet across southwestern tibet and south central tibet to near the india plate anisotropy is absent beneath much of the himalaya block but consistent anisotropy with e w fast direction is present beneath the lhasa block and large anisotropy up to 4 is observed in low velocity regions of the northern and western tibet complex station delays in the eastern margin of the plateau suggest that the whole crust may be highly deformed the anisotropy pattern in the southeastern margin of the tibetan plateau suggests a mantle lithospheric deformation similar to the clockwise rotation of material observed at the surface 5 crustal thicknesses inferred from our station delays are consistent with previous models which correlate well with surface topography

basalt magmatism along the passive continental margin of se brazil campos basin the se brazil passive continental margin is characterized by tholeiitic magmatism that is particularly widespread in the marginal campos basin facing the inland flood basalts of the adjacent parana basin campos magmatism is represented by early cretaceous ec 134 122 ma flood basalts and minor upper cretaceous early tertiary basalt flows and intrusives which were emplaced in a basin with attenuated crustal thickness 20 km petrography mineral chemistry wholerock geochemistry and sr nd isotope composition emphasize that the ec campos basalts have suffered extensive seawater interaction which caused enrichment in mgo feo total k2o rb and ba and depletion in sio2 and cao while zr nb y and ree remained virtually constant in samples with loss on ignition values less than 4 wt in general campos basalts have bulkrock chemistry similar to those of the inland parana tholeiites 140 130 ma with relatively low concentrations of incompatible elements and tio2 16 morb mid ocean ridge basalt mantle in simple binary mixing models on the whole the early cretaceous campos basalts appear as an easterly younger extension of the northern parana volcanism and probably erupted during early stages of the major riftingprocesses which caused continental thinning it is notable that in the campos marginal basin both the basalt magmatism contemporaneous with the continental break up as well as that which occurred after the s america africa separation appears substantially related to subcontinental lithosphere and a dupal like oib ocean island basalt e g tristan da cunha source components

seismic polarization anisotropy beneath the central tibetan plateau sks and skks shear waves recorded on the indepth iii seismic array deployed in central tibet during 1998 1999 have been analyzed for the direction and extent of seismic polarization anisotropy the 400 km long nnw trending array extended south to north from the central lhasa terrane across the karakoram jiali fault system and banggong nujiang suture to the central qiangtang terrane substantial splitting with delay times from 1 to 2 s and fast directions varying from e w to ne sw was observed for stations in the qiangtang terrane and northernmost lhasa terrane no detectable splitting was observed for stations located farther south in the central lhasa terrane the change in shear wave splitting characteristics occurs at 32 n approximately coincident with the transcurrent karakoram jiali fault system but 40 km south of the surface trace of the banggong nujiang suture this location is also near the southernmost edge of a region of high sn attenuation and low upper mantle velocities found in previous studies the transition between no measured splitting and strong anisotropy 2 2 s delay time is exceptionally sharp 15 km suggesting a large crustal contribution to the measured splitting the e w to ne sw fast directions are broadly similar to the fast directions observed farther east along the yadong golmud highway suggesting that no large scale change in anisotropic properties occurs in the east west direction however in detail fast directions and delay times vary over lateral distances of 100 km in both the n s and e w direction by as much as 40 and 0 5 1 s respectively the onset of measurable splitting at 32 n most likely marks the northern limit of the underthrusting indian lithosphere which is characterized by negligible polarization anisotropy taken in conjunction with decades of geophysical and geological observations in tibet the new anisotropy measurements are consistent with a model where hot and weak upper mantle beneath northern tibet is being squeezed and sheared between the advancing indian lithosphere to the south and the tsaidam and tarim lithospheres to the north and west resulting in eastward flow and possibly thickening and subsequent detachment due to gravitational instability in northern tibet crustal deformation clearly follows this large scale deformation pattern

on the reconstruction of pleistocene ice sheets a review abstract pleistocene ice sheets can be reconstructed through three separate approaches 1 evidence based on glacial geological studies such as erratic trains till composition crossing striations and exposures of multiple tills nonglacial sediments 2 reconstructions based on glaciological theory and observations these can be either two or three dimensional models they can be constrained by known ice margins at specific times or they can be open ended with the history of growth and retreat controlled by parameters resting entirely within the model 3 glacial isostatic rebound after deglaciation provides a measure of the distribution of mass ice across a region a best fit ice sheet model can be developed that closely approximates a series of relative sea level curves within an area of a former ice sheet in addition the model should also provide a reasonable sea level fit to relative sea level curves at sites well removed from glaciation this paper reviews some of the results of a variety of ice sheet reconstructions and concentrates on the various attempts to reconstruct the ice sheets of the last wisconsin weischelian wurm devensian glaciation evidence from glacial geology suggests flow patterns at variance with simple single domed ice sheets over north america and europe in addition reconstruction of ice sheets from glacial isostatic sea level data suggests that the ice sheets were significantly thinner than estimates based on 18 ka equilibrium ice sheets cf denton and hughes 1981 the review indicates it is important to differentiate between ice divides which control the directions of glacial flow and areas of maximum ice thickness which control the glacial isostatic rebound of the crust upon deglaciation recent studies from the laurentide ice sheet region indicate that the center of mass was not over hudson bay that a major ice divide lay east of hudson bay so that flow across the hudson bay and james bay lowlands was from the northeast that hudson bay was probably open to marine invasions two or three times during the wisconsin glaciation and that the laurentide ice sheet was thinner than an equilibrium reconstruction would suggest

a new greenland deep ice core the polar ice sheets are rich sources of information on past atmospheric conditions including paleoclimates a new deep ice core has been drilled in south greenland comparison of the oxygen isotopic profile with that from camp century and with a deep sea foraminifera record indicates that the new core reaches back to about 90 000 years before present in a continuous sequence the details in the wisconsin part of the ice core records seem to be climatically significant and the general trends reveal all of the relevant emiliani stages recorded in deep sea cores the redated camp century record suggests a dramatic termination of the eem sangamon interglacial

lithospheric structure of tibet and western north america mechanisms of uplift and a comparative study we seek to determine the mantle lid thickness of the lithosphere and to constrain the mechanisms that are responsible for the cenozoic uplift of the tibetan plateau located behind the himalaya collision zone and the western united states located behind the san andreas transform convergent plate boundary selected first p arrivals of international seismological centre isc data at distances less than 22 are analyzed by the two station method to constrain the upper mantle structure of the basin and range br colorado plateau cp great plains gp and tibetan plateau tp the data are presented as plots of apparent velocity versus distance to the second station we show that these observations of apparent velocities versus distance as well as those that concern the attenuation of sn provide estimates of the mantle lid thicknesses we obtain the following mantle lid thicknesses br 20 40 km cp 35 50 km gp 150 195 km and tp 135 180 km sn is not observed for paths beneath the br and cp while sn efficiently propagates beneath the gp sn is also observed beneath most of tibet except in the north central part of the tp a possible interpretation of our observations is that indian continental lithosphere underthrusts only about the southern 2 3 of tibet

atmospheric circulation patterns during glacial inception a possible candidate abstract models of atmospheric circulation in the north atlantic sector during glacial inception can be expanded to a hemispheric scale with the aid of diagnostic studies of the present climate the present greenland above ga atmospheric circulation type may be a candidate for the atmospheric circulation type required during glacial inception the pattern is an amplification with only minor phase shifts of the present average winter circulation pattern in the extratropical northern hemisphere southerly flow in the northwest atlantic is associated with warm ocean temperatures low sea ice in the davis strait and increased precipitation over northeast canada evidence from modeling of the present climate indicates that the ga pattern could be maintained by increased snow cover over eastern north america enhanced snow cover due to decreased northern hemisphere summer insolation could cause a similar response on an ice age time scale

greenland ice core signal characteristics an expanded view of climate change the last millenium of earth history is of particular interest because it documents the environmental complexities of both natural variability and anthropogenic activity we have analyzed the major ions contained in the greenland ice sheet project 2 gisp 2 ice core from the present to 674 a d to yield an environmental reconstruction for this period that includes a description of nitrogen and sulfur cycling volcanic emissions sea salt and terrestrial influences we have adapted and extended mathematical procedures for extracting sporadic e g volcanic events secular trends and periodicities found in the data sets finally by not assuming that periodic components signals were stationary and by utilizing evolutionary spectral analysis we were able to reveal periodic processes in the climate system which change in frequency turn on and turn off with other climate transitions such as that between the little ice age and the medieval warm period

teleconnections in the geopotential height field during the northern hemisphere winter abstract contemporaneous correlations between geopotential heights on a given pressure surface at widely separated points on earth referred to as teleconnections in this paper are studied in an attempt to identify and document recurrent spatial patterns which might be indicative of standing oscillations in the planetary waves during the northern hemisphere winter with time scales on the order of a month or longer a review of existing literature on the subject reveals the existence of at least four such patterns the north atlantic and north pacific oscillations identified by walker and bliss 1932 a zonally symmetric seesaw between sea level pressures in polar and temperature latitudes first noted by lorenz 1951 and what we will refer to as the pacific north american pattern which has been known to operational long range forecasters in this country since the 1950 s a data set consisting of nmc monthly mean sea level pressure and 500 mb height analyses for a 15 year period is used as a basis fo

biostratigraphy and late cenozoic paleoceanography of the arctic ocean foraminiferal lithostratigraphic and isotopic evidence detailed studies of benthonic foraminifera stable isotopes and lithofacies in cores from the southeastern alpha ridge central arctic ocean reveal some new aspects of arctic ocean paleoceanography high ratios of benthonic to planktonic foraminifera are found in most of the quaternary sediment units and ratios of 1 1 appear to characterize the arctic deep water sediments benthonic foraminifera in the carbonate mud unit m show a succession of calcareous species reflecting increased influx of norwegian sea bottom water to the arctic ocean during the past 0 4 m y foraminiferal and lithological data indicate less uniform sedimentation during a warmer interval from 0 4 to 0 6 ma when most of the silty lutite unit l was deposited at the cesar site lower pleistocene units j to i contain less limestone and more dolomite and they contain a uniform faunal assemblage with low numbers of calcareous foraminifera upper pliocene units h to ab contain rare limestone and relatively large amounts of do lomite and quartz sand middle to upper pliocene units ab to a3 are marked by abundant sand sized ferromanganese coated particles which in many cases have a silt nucleus hence much of the coarse sand in these units does not indicate increased ice rafting the pliocene sediments mostly contain a low diversity assemblage of agglutinated foraminifera but a mixed calcareous arenaceous fauna occurs in a short interval above the matuyama gauss boundary 2 4 ma stable isotopic curves occur within sequences which broadly correspond to stages 1 9 of the global record below stage 9 the record is discontinuous strong vertical mixing apparently prevailed during most of the pliocene and early pleistocene then decreased during the past 0 4 m y owing to damping by a perennial ice cover isotopic and foraminiferal data however suggest that an interval of perennial sea ice also occurred during the late pliocene at the time of the earliest glacial event recorded in the north atlantic

a quantitative evaluation of the contribution of crustal rocks to the shear wave splitting of teleseismic sks waves abstract the seismic elastic properties of the crust have been modelled using hypothetical polycrystals with typical crustal compositions and commonly measured petrofabrics this modelling allows us to better evaluate and quantify the crustal contribution to splitting of teleseismic sks waves mafic rocks such as anorthosite or pyroxene bearing gabbros have complex s waves properties i e with small shear wave anisotropies

evidence for mechanical coupling and strong indian lower crust beneath southern tibet how surface deformation within mountain ranges relates to tectonic processes at depth is not well understood the upper crust of the tibetan plateau is generally thought to be poorly coupled to the underthrusting indian crust because of an intervening low viscosity channel here however we show that the contrast in tectonic regime between primarily strike slip faulting in northern tibet and dominantly normal faulting in southern tibet requires mechanical coupling between the upper crust of southern tibet and the underthrusting indian crust such coupling is inconsistent with the presence of active channel flow beneath southern tibet and suggests that the indian crust retains its strength as it underthrusts the plateau these results shed new light on the debates regarding the mechanical properties of the continental lithosphere and the deformation of tibet

multiple deglaciations of the hudson bay lowlands canada since deposition of the missinaibi last integlacial formation abstract the stratigraphic record in the james and hudson bay lowlands indicates that the sequence of glacial events at the geographical center of the 12 6 10 6 km 2 laurentide ice sheet may have been more complex than hitherto imagined isoleucine epimerization ratios of in situ and transported shells recovered from till and associated marine and fluvial sediments cluster into at least 4 discrete groups two alternative explanations of the data are offered of which we strongly favor the first hypothesis 1 setting the age of the last interglacial marine incursion the bell sea at 130 000 yr b p results in a long term average diagenetic temperature for the lowlands of 0 6 c using this temperature enables us to predict the age of shells intermediate in age between the last interglaciation and the incursion of the tyrrell sea 8000 yr ago between these two interglacial marine inundations hudson bay is predicted to have been free of ice along its southern shore about 35 000 75 000 and 105 000 yr ago based on amino acid ratios from shells occurring as erratics in several superimposed tills and fluvial sediments these results suggest 1 that traditional concepts of ice sheet build up and decay must be reexamined 2 that high sea levels may have occurred during the wisconsin glaciation and 3 that a critical reappraisal is required of the open ocean 18 o record as a simple indicator of global ice volume an alternative hypothesis 2 is also examined it is based on the assumption that the 35 000 yr old deposits calculated on the basis of hypothesis 1 date from the last interglaciation this in effect indicates that the missinaibi formation commonly accepted as sediments of the last interglaciation are about 500 000 yr old and that the effective diagenetic temperature in the lowlands during approximately the last 130 000 yr has been close to 6 c we argue for rejection of this alternative hypothesis

seismic anisotropy and implications for mantle deformation beneath the ne margin of the tibet plateau and ordos plateau the northeastern tibet and the ordos plateau connects east china and west china which are dominated by horizontal extension and compression respectively knowledge of seismic anisotropy beneath this transitional region can provide important constraints on deformation pattern of the crust and lithosphere mantle during an orogeny process we measured sks wave splitting parameters from the new installed regional networks and obtained a detailed anisotropy map for 119 broadband stations beneath the ne margin of tibet the observed wnw fast polarization direction is parallel to the surface geological features the coherence between the observed geodetic motion of the crust and fast axis direction suggests that the vertical coherent deformation of the lithosphere is the dominate source for the observed seismic anisotropy further east to the qinling orogenic belt the fast axis direction tracks change in strike of the faults with the fast direction aligning almost ew we suggest that both the lithosphere mantle and east extrusion of mantle material contribute to the observed anisotropy small value of delay time is observed interior the thick rooted ordos plateau suggesting a rigid and stable lithosphere with little deformation the deviation of fast polarization direction beneath the adjacent rifting regions might be caused by the edge flow induced by sharp changes in the thickness of lithosphere our results propose that the upper crust and mantle lithosphere beneath the northeastern tibet are at least partly coupled or subjected to the same boundary conditions which is inconsistent with the channelized lower crustal flow model

how are vertical shear wave splitting measurements affected by variations in the orientation of azimuthal anisotropy with depth splitting measurements of teleseismic shear waves such as sks have been used to estimate the amount and direction of upper mantle anisotropy worldwide these measurements are usually made by approximating the anisotropic regions as a single homogeneous layer and searching for an apparent fast direction and an apparent splitting time t by minimizing the energy on the transverse component of the back projected seismogram in this paper we examine the validity of this assumption in particular we use synthetic seismograms to explore how a vertically varying anisotropic medium affects shear wave splitting measurements we find that weak heterogeneity causes observable effects such as frequency dependence of the apparent splitting parameters these variations can be used in principle to map out the vertical variations in anisotropy with depth through the use of frechet kernels which we derive using perturbation theory in addition we find that measurements made in typical frequency bands produce an apparent orientation direction that is consistently different from the average of the medium and weighted towards the orientation of the anisotropy in the upper portions of the model this tendency of the measurements to mimic the anisotropy at the top part of the medium may explain why shear wave splitting measurements tend to be correlated with surface geology when the heterogeneity becomes stronger multiple scattering reduces the amplitude of the tangential component seismogram and the associated splitting time so that a null result may be obtained despite the fact that the waves have travelled through a strongly anisotropic medium regardless of the amount of vertical heterogeneity we find that there is very little dependence on backazimuth for the measured fast axis direction or splitting time if the top and bottom halves of the medium average to similar fast axis directions if however the average fast axis direction in the top half of the model differs from that in the bottom half splitting time measurements will show a significant dependence on backazimuth but fast axis direction measurements will remain relatively constant

comparison of the rift and post rift architecture of conjugated salt and salt free basins offshore brazil and angola namibia south atlantic abstract this study presents a regional comparison between selected 2d seismic transects from large conjugated salt and salt free basins offshore southern brazil campos basin santos basin pelotas basin and southwest africa kwanza basin northern and southern namibe basin walvis basin tectonic stratigraphic interpretation of the main rift and post rift units free air gravity data and flexural isostatic backstripping were used for a comprehensive basin to basin documentation of key mechanisms controlling the present day differences in conjugated and neighbouring south atlantic basins a significant variation in the tectonic sedimentary architecture along strike at each margin and between the conjugated basins across the south atlantic reflects major differences in 1 the structural configuration of each margin segment at transitional phase between rifting and breakup as emphasized in the highly asymmetric settings of the large santos salt basin and the conjugated salt free southern namibe basin 2 the post breakup subsidence and uplift history of the respective margin segment which caused major differences for example between the campos and espirito santo basins and the conjugated northern namibe and kwanza basins 3 variations in the quantity and distribution of post breakup margin sediments which led to major differences in the subsidence history and the related present day basin architecture for example in the initially rather symmetric siliciclastic pelotas and walvis basins and 4 the deposition of aptian evaporites in the large rift and sag basin provinces north of the rio grande rise and walvis ridge highly contrasting the siliciclastic basins along the margin segments south of the ridges the resulting present day architecture of the basins can be generally classified as i moderately symmetric salt free and magma rich in the northern part of the southern segment i highly asymmetric salt bearing and magma poor vs salt free and magma rich in the southern part of the central segment and iii moderately symmetric to asymmetric salt rich and magma poor in the central part of the central segment of the south atlantic

the kaapvaal craton seismic anisotropy petrophysical analyses of upper mantle kimberlite nodules a dense network of seismic stations has been deployed on the kaapvaal craton south africa to investigate the upper mantle seismic structures in order to bring independent petrophysical constraints we analyze a direct sampling of the cratonic upper mantle and determine the seismic properties of 48 mantle nodules brought up to the earth s surface by kimberlite eruptions seismic properties of these nodules are calculated from the olivine and pyroxene crystal preferred orientations and the single crystal elastic constants despite variations in the nodules compositions microstructures and crystallographic preferred orientations seismic anisotropy is rather homogeneous throughout the craton mean s wave anisotropy is weak 2 64 which is compatible with the small measured sks wave splitting mean delay time of 0 62 s

lithospheric thickness of the chinese continent abstract we invert for the upper mantle temperatures of the chinese continent in the depth range of 70 240 km from a recent s velocity model the depth where temperatures intersect a mantle adiabat with a potential temperature of 1300 c is in close correspondence with the top of the seismic low velocity zone for most regions this correspondence implies that seismic lithosphere estimated from short time scale seismic information may be equivalent to the long time scale geodynamical lithosphere defining the 1300 c adiabat as coinciding with the lithospheric base we estimate the seismic thermal lithosphere thickness the estimated thickness shows obvious dependence on the tectonic settings beneath eastern china which mainly belongs to the circum pacific tectonic domain it has a thickness of 100 km and beneath the qinghai tibet plateau and south to the tarim craton which belong to the tethyan tectonic domain it has a thickness of 160 220 km the lithospheric thicknesses of the three large para platforms cratons range from 170 km for the western yangtze 140 km for tarim and 100 km for sino korean the three cratons may have been reshaped by phanerozoic tectonic activities and are thinner than most cratons in other continents

evidence of explosive seafloor volcanic activity from the walvis ridge south atlantic ocean 1 hydrophones moored in the north atlantic ocean recorded a sequence of explosive volcano acoustic signals originated at the walvis ridge in the south atlantic ocean 365 explosive signals were detected from the walvis ridge beginning 24 november 2001 continuing through march 2002 the largest swarm began on 19 december at 2329 gmt and lasted 1 25 hrs producing 32 locatable events swarm locations are centered on the northern flank of an unnamed seamount 32 96 s 5 22 w northwest of wust seamount these signals are interpreted as volcanogenic explosions due to similarities with acoustic signals recorded from a confirmed submarine eruption in the caribbean in 2001 kick em jenny volcano the observations presented suggest recent magmatic activity along the walvis ridge may be unrelated to the tristan da cunha mantle plume furthermore these events lend support for an extensional fracture zone model resulting in the recurrence of volcanic activity along older segments of large scale sea floor lineaments

pleistocene sea levels from raised coral reefs of haiti the elevations and ages of a sequence of three uplifted pleistocene coral reefs on the northwest peninsula of haiti have been determined with the assignment of a sea level of 6 meters relative to the present day at 130 000 years before present and constant uplift of the reefs the data indicate that sea level stood 10 and 13 meters at 108 000 and 81 000 years before present respectively these results are in substantial agreement with those reported for barbados and new guinea and support the hypothesis of constant uplift for each area sea level data from raised reefs indicate that the interglacial marine oxygen isotope oscillations during oxygen isotope stage 5 are a result of 30 percent ice volume effects and 70 percent temperature effects

anisotropy beneath california shear wave splitting measurements using a dense broadband array we have determined the shear wave splitting parameters for a dense network of broad band stations in the western united states using high quality sks and skks waveforms with particularly high spatial resolution in the southern california region covered by the trinet seismic network the alignment of most fast polarization directions can be explained by plate tectonic extensional and compressional events we find that the overall pattern of fast directions agrees well with the pn anisotropy model by hearn that images the uppermost mantle furthermore the measured fast directions are generally orthogonal to the maximum horizontal compressive stress directions as determined from shallow crustal stress indicators world stress map this suggests that the pattern of anisotropy in the western us is predominantly uniform throughout the crust and upper mantle and that a 100 150 km thick layer as estimated from the sks delay time assuming 4 per cent anisotropy of anisotropic material has experienced coherent strain conditions and has undergone a similar deformation history a more detailed investigation reveals small scale lateral variations in anisotropy that are manifested by minor differences in splitting parameters between closely located stations as well as between sks and skks for the same station event pairs we also identify a contrast in splitting parameters between central the greater bay area and southern california in central california our measurements show evidence for variation of splitting parameters with backazimuth while in southern california the pattern of measurements can be fit adequately with a single layer anisotropy model this contrast dominates any consistent effect of the san andreas fault saf we can fit the variation of splitting parameters as a function of polarization azimuth for some stations in the vicinity of the saf better with a two layer anisotropy model than a single layer model with one thin layer having a fast direction parallel to the saf however many alternative models which could incorporate dipping axes of anisotropy lateral variation of anisotropy or a more continuous variation of fast direction with depth would be able to produce a similar fit

extension in the graben systems around the ordos china and its contribution to the extrusion tectonics of south china with respect to gobi mongolia the graben basins around the ordos block are major tectonic features of late cenozoic extension in north china this paper gives a synthetic view of the basic characteristics of the graben basins and of the kinematics of the graben bounding faults this view is based on the available geological data spot imagery analysis field observations and focal mechanisms of earthquakes from this region these graben basins have been grouped into two major systems the yinchuan hetao graben system along the northern and northwestern margins of the ordos and the weihe shanxi graben system along the southern and eastern margins of the ordos crustal deformation along these graben systems involves normal slip on ne sw striking graben bounding faults normal right lateral and normal left lateral slip on nne ssw and wnw ese striking faults respectively this fault pattern is consistent with nw se trending extension the amount and rate of deformation resulting from this extension have been estimated from geological data using a cross sectional area balance and the extensional factor method deduced from the tilt of faulted blocks the results of these estimates show that the uppermost pliocene quaternary extension rate in a nw se direction is in the order of 1 6 mm yr 1 across the weihe graben 0 5 mm yr 1 across the central shanxi graben on the southeastern margin of the ordos block and 3 1 mm yr 1 across the linhe graben on the northwestern margin of the ordos block the extension across these graben systems is accompanied with counterclockwise rotation of continental blocks in north china ordos taihang shan north china plain a kinematic model is proposed in which the late cenozoic extension and block rotation in north china are interpreted as the effect of the eastward termination of the left lateral slip haiyuan fault and kunlun taibai fault system the ordos block rotates counterclockwise as a result of the push of tibet through the left lateral strike slip haiyuan fault and the liupan shan thrust and fold belt the extension along the weihe shanxi graben system may represent the effect of the left lateral slip on the kunlun taibai qinling fault system the rough estimates of the extensional deformation around the ordos block indicate that this contributes to about 5 2 mm yr 1 of left lateral movement in a n125 e direction between south china and gobi mongolia this motion adds to the left lateral slip on the qinling fault zone about 7 2 mm yr 1 in a n90 e direction between the qinling shan and the hua shan north china thus the eastward motion of south china with respect to gobi mongolia is partitioned between normal faulting around the ordos and strike slip faulting in the qinling shan it is estimated to be in the order of 12 4 mm yr 1 in a n105 e direction during the quaternary

high resolution mantle tomography of china and surrounding regions 1 a high resolution p wave tomographic model of the crust and mantle down to 1100 km depth under china and surrounding regions is determined by using about one million arrival times of p pp pp and pcp waves from 19 361 earthquakes recorded by 1012 seismic stations the subducting pacific slab is imaged clearly as a high velocity zone from the oceanic trenches down to about 600 km depth and intermediate depth and deep earthquakes are located within the slab the pacific slab becomes stagnant in the mantle transition zone under east china the western edge of the stagnant slab is roughly coincident with a surface topographic boundary in east china the active changbai and wudalianchi intraplate volcanoes in northeast china are underlain by significant slow anomalies in the upper mantle above the stagnant pacific slab these results suggest that the active intraplate volcanoes in ne china are not hot spots but a kind of back arc volcano associated with the deep subduction of the pacific slab and its stagnancy in the transition zone under the mariana arc however the pacific slab penetrates directly down to the lower mantle the active tengchong volcano in southwest china is related to the eastward subduction of the burma microplate the subducting indian and philippine sea plates are also imaged clearly the indian plate has subducted down to 200 300 km depth under the tibetan plateau with a horizontal moving distance of about 500 km high velocity anomalies are revealed in the upper mantle under the tarim basin ordos and sichuan basin which are three stable blocks in china

late quaternary stratigraphy and paleoecology of northwest labrador sea abstract sedimentological study of 20 piston cores from the northwestern labrador sea shows that thin bedded distal turbidites are important constituents of the quaternary strata on the continental slope and rise sedimentation on the outer shelf ca 500 900 m is dominated by ice rafting and hemipelagic deposition detailed study of two outer shelf cores reveals a stratigraphic record extending to oxygen isotopic stage 9 studies of foraminifera and dinoflagellates show that subarctic surface waters penetrated into the northern labrador sea during the early middle glacial stages and pollen data indicate that northward advection of warm air masses occurred at the same time microfossil data indicate that late glacial stages were characterized by year around sea ice cover and arctic surface water and by the dominance of cold dry arctic air masses transitions from glacial to interglacial stages are marked by the presence of low salinity melt water these data support models which postulate that open water in the labrador sea played an important role in supplying moisture to laurentide and greenland ice sheets

colorado plateau magmatism and uplift by warming of heterogeneous lithosphere the forces that drove rock uplift of the low relief high elevation tectonically stable colorado plateau are the subject of long standing debate while the adjacent basin and range province and rio grande rift province underwent cenozoic shortening followed by extension the plateau experienced approximately 2 km of rock uplift without significant internal deformation here we propose that warming of the thicker more iron depleted colorado plateau lithosphere over 35 40 myr following mid cenozoic removal of the farallon plate from beneath north america is the primary mechanism driving rock uplift in our model conductive re equilibration not only explains the rock uplift of the plateau but also provides a robust geodynamic interpretation of observed contrasts between the colorado plateau margins and the plateau interior in particular the model matches the encroachment of cenozoic magmatism from the margins towards the plateau interior at rates of 3 6 km myr 1 and is consistent with lower seismic velocities and more negative bouguer gravity at the margins than in the plateau interior we suggest that warming of heterogeneous lithosphere is a powerful mechanism for driving epeirogenic rock uplift of the colorado plateau and may be of general importance in plate interior settings

conjugate divergent margins an introduction the main objective of this book is to provide a global overview of divergent margins based on geological and geophysical interpretation of sedimentary basins along the south central and north atlantic conjugate margins from plate tectonics and crustal scales to a more detailed description of stratigraphical and structural elements that are responsible for petroleum plays these themes are complemented by geodynamic concepts based on physical and numerical models and by comparisons with present day embryonic margins which are succinctly discussed in some papers

gravity modeling of the ocean continent transition along the south atlantic margins 1 gravity magnetic and seismic data have been used to examine changes in crustal structure of conjugate portions of the south atlantic volcanic margins south of the walvis ridge rio grande rise hot spot tracks we have constructed 18 seismically constrained crustal scale gravity models of the ocean continent transition 12 across the african margin and 6 across the south american margin we attribute changes in character of the gravity anomalies to variations in crustal thickness sedimentation and magmatic underplating to investigate variations in the lower crust and upper mantle we have removed the relatively well constrained contributions to the gravity field from shallow sources the resulting residual anomalies delineate distinct crustal domains associated with crustal thinning and regions of underplating that correspond well with those independently identified from limited seismic refraction data near the hot spot tracks we find underplated crust to be symmetrically distributed with a horizontal extent of 240 km for each margin further south however the underplated region is asymmetric with significantly more beneath the african than the corresponding south american margin residual gravity anomalies indicate that the width of underplated crust decreases from 240 km to 150 km at 33 s on the african side and to 100 km at 38 s on the south american side the wider underplated region on the african side suggests an asymmetric rifting mechanism in which greater melts accumulate beneath an upper plate margin our results are consistent with recently proposed asymmetric rifting models that involve strain softening of a coupled cold frictional upper lithosphere at low rifting velocities such models also predict greater symmetry for areas with more heat such as those associated with the hot spot

shear wave splitting in the southern margin of the ordos block north china 1 the southern margin of the ordos block in north china marks the transition from a stable cratonic block to active transextensional deformation in the weihe graben driven by the northeastward expansion of the tibet plateau we deployed 29 portable digital stations across this boundary and studied the upper mantle anisotropy using sks splitting the anisotropy is weak t 0 5 s at stations within the ordos block and in the north weihe graben but strong t 1 23 0 39 s in the south weihe graben and qinling orogen further south the directions of the fast s wave are 98 7 subparallel to the orientations of surface structures and the gps site velocities relative to stable asian continent we suggest that this e w oriented anisotropy is the result of both the collision between the north and south china blocks in the mesozoic and the eastward upper mantle flow driven by the indo asian collision

continuous deformation of the tibetan plateau from global positioning system data global positioning system velocities from 553 control points within the tibetan plateau and on its margins show that the present day tectonics in the plateau is best described as deformation of a continuous medium at least when averaged over distances of 100 km deformation occurs throughout the plateau interior by ese wnw extension and slightly slower nne ssw shortening relative to eurasia material within the plateau interior moves roughly eastward with speeds that increase toward the east and then flows southward around the eastern end of the himalaya crustal thickening on the northeast ern and eastern margins of the plateau occurs over a zone 400 km wide and cannot be the result of elastic strain on a single major thrust fault shortening there accommodates much of india s penetration into eurasia a description in terms of movements of rigid blocks with elastic strain associated with slip on faults between them cannot match the velocity field

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

upper mantle flow beneath and around the hangay dome central mongolia mongolia represents the northernmost area affected by the india asia collision and it is actively deformed along transpressive belts closely associated with large scale strike slip faults the active and past mantle flow beneath this region is however poorly known in order to investigate deep mantle deformation beneath central mongolia and its relation with the surrounding major structures such as the siberian craton the gobi altay belt and the baikal rift a ns trending profile of broadband seismic stations has been deployed in the summer 2003 from the southern siberian craton to the gobi altay range crossing the entire hangay dome mantle flow is deduced from the splitting of teleseismic shear waves such as sks phases in eastern mongolia the permanent station uln in ulaanbaatar reveals the presence of two anisotropic layers the upper one being oriented ne sw close to the trend of lithospheric structures and the lower one nw se close to the trend of eurasia absolute plate motion along the ns profile in central mongolia seismic anisotropy deduced from sks splitting reveals a homogeneous nw se trending structure fully consistent with the observations made in the altay sayan in western mongolia the observed delay times of 1 5 to more than 2 0 s favor consistent mantle flow over large mantle thicknesses since the lithosphere is less than 100 km thick beneath central mongolia and since the observed fast directions are parallel to the trend of the lithospheric structures but also close to the trend of the absolute plate motion we propose that both the lithosphere and the asthenosphere may join their anisotropic effects beneath central mongolia to explain the large delay times although gps vectors represent the instantaneous displacement of the earth s surface and sks splitting the time and vertical integration of finite strain at depth we use the opportunity of the dense geodetic measurements available in this region to discuss the anisotropy pattern in term of present day deformation in the eurasia fixed reference frame gps and sks both depict a similar trend beneath central mongolia suggesting a lithospheric block escaping toward the east that could orient olivine a axes in the upper mantle within a transpressive tectonic regime a different behaviour is observed in western mongolia the gps vectors trend ns close to the regional compression direction whereas the fast sks directions trend ew suggesting a tectonic regime close to a mode of axial shortening generating the development of an ew trending foliation at depth we therefore propose that mongolia is a place where active and frozen lithospheric deformation may add their effects together with the sublithospheric flow in the three sources of anisotropy inferred a fundamental role is played by the siberian craton that acted as an undeformable core of the continent through time the frozen paleozoic and mesozoic structures wrap around the craton building up the fast anisotropic direction pattern the present day sublithospheric flow induced by the plate motion is likely deflecting around its deep roots finally the present day tectonic regime appears to be controlled by the presence of the craton to the north

seismic anisotropy beneath stable continental interiors a robust knowledge of seismic anisotropy beneath the continents is essential to our understanding of plate tectonic theory as anisotropy provides a unique constraint on the character of past and present deformation in the lithosphere and sublithospheric mantle this review paper summarizes the range of techniques currently available to image seismic anisotropy with passive source seismic data and addresses current issues surrounding the observation and interpretation of continental seismic anisotropy to this end we present case studies for four regions where seismic anisotropy has been extensively investigated in recent years eastern north america the canadian shield australia and southern africa based on this full suite of results we infer that stable continental regions generally contain seismic anisotropy that is clearly located within both the lithosphere and the sublithospheric mantle usually to depths of at least 200 km and perhaps more an implication of these results is that tectonic plates are at most only partially coupled to the underlying mantle the results from these case studies also demonstrate that while remarkable progress in seismic anisotropy imaging has been achieved in recent years it is clear that much more work will be required to adequately understand the origin of continental seismic anisotropy we suggest that a more robust characterization of anisotropic parameters can only be achieved by integrating complementary seismic datasets and by incorporating constraints from key related datasets from mineral physics magnetotellurics gravity and geodesy

active thrusting and folding in the qilian shan and decoupling between upper crust and mantle in northeastern tibet fieldwork south of the city of gaotai gansu province china shows that active shortening of surface sediments in the foothills of the yumu shan a large fore mountain of the qilian shan at the northeastern edge of tibet involves both overthrusting and flexural slip folding north of this mountain we found and mapped a prominent north facing thrust scarp that offsets a holocene fan sloping gently 3 4 to the north part of this scarp appears to be related to the m 7 5 180 a d earthquake that may have led to the demise of the han dynasty city of luo tuo chen in the hexi corridor a set of 10 100 150 m long profiles measured across this scarp 3 2 m high on the average can be made to fit the diffusion degraded morphology of a surface break related to the 180 a d event using a value of about 3 3 m 2 10 3 yr for the mass diffusivity of fanglomerates in this part of gansu province smaller mountain facing scarps on a terrace capped foothill result from bedding slip concurrent with active folding of underlying steeply northdipping plioquaternary sandstone and conglomerate beds holocene uplift rates along the yumu shan which is only one of the qilian shan ranges are estimated to be between 0 4 and 1 9 mm yr which implies that much of the mountain formed in the quaternary the periclinal structure of the plioquaternary envelope under which the paleozoic core of the yumu shan plunges towards the west suggests that the whole 3200 m high mountain is a basement ramp anticline mountains striking parallel to the yumu shan with similar structure and comparable or greater sizes north and south of the hexi corridor probably also correspond to recent crustal ramp anticlines this implies that the wide mountainous upper crustal wedge making the northeastern edge of the tibet qinghai plateau is detached from the underlying lower crust and upper mantle

neglecting ice atmosphere interactions underestimates ice sheet melt in millennial scale deglaciation simulations dynamic and thermodynamic interactions between the atmosphere and underlying ice sheets are generally not represented in the traditional one way boundary condition forcing used to drive ice sheet models this shortcoming is investigated through a series of idealized millennial scale deglaciation simulations designed to isolate the mechanisms regulating the deglaciation timescale of the laurentide ice sheet sensitivity experiments indicate that the conventional use of one way non interactive atmospheric forcing fields leads to an unrealistically insensitive melt response in the ice sheet model even when atmospheric carbon dioxide is set to modern preindustrial levels and earth s angle of obliquity is set to its early holocene value a more realistic deglaciation timescale is obtained only through the application of a new two way interactive asynchronous ice atmosphere coupling scheme and a seasonal ice albedo parameterization that accounts for the observed darkening of ice in the moist summertime ablation zone

cenozoic rifting and volcanism in eastern china a mantle dynamic link to the indo asian collision abstract the indo asian continental collision is known to have had a great impact on crustal deformation in south central asia but its effects on the sublithospheric mantle remain uncertain studies of seismic anisotropy and volcanism have suggested that the collision may have driven significant lateral mantle flow under the asian continent similar to the observed lateral extrusion of asian crustal blocks here we present supporting evidence from p wave travel time seismic tomography and numerical modeling the tomography shows continuous low velocity asthenospheric mantle structures extending from the tibetan plateau to eastern china consistent with the notion of a collision driven lateral mantle extrusion numerical simulations suggest that at the presence of a low viscosity asthenosphere continued mass injection under the indo asian collision zone over the past 50 my could have driven significant lateral extrusion of the asthenospheric mantle leading to diffuse asthenospheric upwelling rifting and widespread cenozoic volcanism in eastern china

seismic evidence for significant lateral variations in lithospheric thickness beneath the central and western north china craton abstract we have migrated teleseismic s receiver functions to construct detailed lithospheric structure images that cover the three constituent parts of the north china craton ncc our images show that in contrast to the eastern ncc where significantly thinned lithosphere 60 100 km is widespread the central and western ncc are characterized by the coexistence of both preserved thick and dramatically thinned lithosphere the thick lithosphere 200 km is present beneath the stable ordos plateau and the thinned lithosphere up to 80 km is found in the late cenozoic yinchuan hetao and shaanxi shanxi rift areas with sharp changes occurring over a lateral distance of

a diagnostic study of recurrent meteorological anomalies appearing in a 15 year simulation with a gfdl general circulation model abstract the spatial structure and temporal characteristics of prominent anomalies occurring in a 15 year simulation with a gfdl spectral general circulation model are examined using empirical orthogonal functions teleconnection patterns composite charts lagged correlation functions and frequency spectra despite the absence of any nonseasonal perturbation in the prescribed forcing such as sea surface temperature insolation and cloud cover the simulated circulation exhibits an appreciable degree of temporal variability on monthly time scales the standing oscillation in the northern hemisphere winter which accounts for the largest fraction of this variance has a coherent three dimensional structure in the middle and upper troposphere this preferred mode of oscillation is characterized by a wavelike pattern with multiple centers of action the corresponding anomaly pattern at the sea level is dominated by north south pressure seesaws over the north atlantic and north pacific the flow patterns assoc

velocity depth ambiguity and the seismic structure of large igneous provinces a case study from the ontong java plateau summary the seismic structure of large igneous provinces provides unique constraints on the nature of their parental mantle allowing us to investigate past mantle dynamics from present crustal structure to exploit this crust mantle connection however it is prerequisite to quantify the uncertainty of a crustal velocity model as it could suffer from considerable velocity depth ambiguity in this contribution a practical strategy is suggested to estimate the model uncertainty by explicitly exploring the degree of velocity depth ambiguity in the model space in addition wide angle seismic data collected over the ontong java plateau are revisited to provide a worked example of the new approach my analysis indicates that the crustal structure of this gigantic plateau is difficult to reconcile with the melting of a pyrolitic mantle pointing to the possibility of large scale compositional heterogeneity in the convecting mantle

upper mantle deformation beneath the north american pacific plate boundary in california from sks splitting in order to constrain the vertical and lateral extent of deformation and the interactions between lithosphere and asthenosphere in a context of a transpressional plate boundary we performed teleseismic shear wave splitting measurements for 65 permanent and temporary broadband stations in central california we present evidence for the presence of two anisotropic domains 1 one with clear e w trending fast directions and delay times in the range 1 5 to 2 0 s and 2 the other closely associated with the san andreas fault system with large azimuthal variations of the splitting parameters that can be modeled by two anisotropic layers the upper of the two layers provides fast directions close to the strike of the main californian faults and averaged delay times of 0 7 s the lower layers show e w directions and delay times in the range 1 5 to 2 5 s and thus can be compared to what is observed in stations that require a single layer we propose the e w trending anisotropic layer to be a 150 to 200 km thick asthenospheric layer explained by the shearing associated with the absolute plate motion of the north american lithosphere the shallower anisotropic layer ought to be related to the dynamics of the san andreas fault system and thus characterized by a vertical foliation with lineation parallel to the strike of the faults localized in the lithosphere we also propose that the anisotropic layer associated with each fault of the san andreas fault system is about 40 km wide at the base of the lithosphere

two anisotropic layers in central orogenic belt of north china craton abstract we present evidence of two layers of anisotropy in earth s upper mantle beneath the central orogenic belt of the north china craton from teleseismic shear wave splitting analysis a one layer model is in disagreement with the well constrained azimuth dependent splitting parameters for stations in the central orogenic belt however two layers of anisotropy improve the fit to the data at the bottom layer the fast polarization directions are subparallel to the absolute plate motion indicating the presence of the current strain within the subcratonic asthenosphere related to the interactions between the surrounding plates in contrast at the top layer the fast polarization directions are consistent with the strikes of the local surface faults confirming a combined contribution of the past orogenies in the lithosphere

mantle plumes continental magmatism and asymmetry in the south atlantic melt production rates are estimated along the ocean tracks rio grande rise walvis ridge and the onshore parana etendeka flood basalt province for the mantle plume currently associated with tristan da cunha in the south atlantic assuming that the volcanic edifice is locally compensated with a thickened crust the maximum oceanic rate 0 3 km3 a 1 is similar to the value for the continental flood basalt calculated using the surface volume of lava and a duration of 5 m y however this latter estimate is conservative and the appropriate rate is probably at least a factor of 5 higher the maximum mantle potential temperature for the ocean plume tracks is estimated to have been 1450 1540 c assuming a dry peridotite source using a similar model temperatures in excess of 1600 c are required to explain the observed magma volumes onshore this is because there is no significant extension in the region where the oldest lavas occur it is argued that the observed volumes of melt can be generated with small amounts of extension and with plume temperatures of 1500 c if melting in the lithosphere is strongly influenced by the relative enrichment of volatiles moreover the model predicts the inferred change to an asthenosphere dominated source when significant extension occurs recent geochonological data have shown that the continental volcanics are 138 ma in the northwest of the parana province and become progressively younger to the southeast oceanic melt production rates decrease by a factor of 3 5 towards the mid ocean ridge tristan da cunha intriguingly melt production rates on the rio grande rise vary to more than twice that of the walvis ridge for oceanic crust of similar age this asymmetry is also seen in the continental regions most obviously in the preserved volume of lava in the parana and etendeka but also in the topography of the underplated continental margin and the orientation of dykes and oblique extension structures these observations reflect the combined roles of the plume temperature lithospheric thickness crustal tectonics and magma source region composition during the evolution of the south atlantic over the last 150 ma

geodynamics of the shanxi rift system china the pliocene quaternary shanxi rift system of northern china transsects the domal axis of the shanxi highlands and is defined by an s shaped string of asymmetric basins extending from huailai yanqing in the north via taiyuan to yuncheng in the south this rift system has a length of about 1200 km and a width of 20 80 km it is characterized by strong earthquakes its central nne trending transtensional segment forms the main element of the shanxi rift system basin subsidence commenced during the early pliocene along nne oriented master faults characterized by large strike slip rates 5 68 7 mm yr crustal extension amounts to some 1 4 km basins are separated by push up blocks destructive earthquakes m 7 8 are unevenly distributed the ene trending northern and southern branches of the shanxi rift system are characterized by orthogonal crustal extension along ene striking normal faults amounting to about 8 km these areas are characterized by a basin and range type structural style and weak seismic activity the shanxi rift system developed on a some 40 km thick thermally cool precambrian crustal segment that was overprinted during mesozoic diastrophic events there is only minor rift related volcanic activity the moho is pulled up by a few kilometers only under the axial rift zone sub moho compressional velocities are in the 8 0 8 1 km s range the shanxi rift system evolved in response to the build up of regional stress fields related to the collision of india and eurasia its localization involved reactivation of pre existing fracture systems

the evolution of the rio grande rise in the southwest atlantic ocean abstract the rio grande rise a major aseismic rise in the western south atlantic ocean is composed of two portions with distinct morphologies and geological histories the western portion of the rise wrgr is a large elliptical bulge with its crest at a mean depth of about 2000 m numerous guyots protrude from its main platform the crust underlying this portion of the rise is older than 80 m y b p and was formed at a spreading center probably situated close to the sea level a widespread volcanic event affected this region during the eocene and generated several volcanic islands located at the center of the present wrgr this eocene volcanism is responsible for the unusual high elevation of this region and for the generation of the observed guyots much less is known about the eastern portion of the rio grande rise ergr it has a north south trend parallel to the present south atlantic spreading center is bounded by fracture zones is in a conjugate position to part of the walvis ridge in the eastern south atlantic and may represent an abandoned spreading center

geodynamics of the southeastern tibetan plateau from seismic anisotropy and geodesy ongoing plate convergence between india and eurasia provides a natural laboratory for studying the dynamics of continental collision a fi rst order process in the evolution of continents regional climate and natural hazards in southeastern tibet the fast directions of seismic anisotropy determined using shear wave splitting analysis correlate with the surfi cial geology including major sutures and shear zones and with the surface strain derived from the global positioning system velocity fi eld these observations are consistent with a clockwise rotation of material around the eastern himalayan syntaxis and suggest coherent distributed lithospheric deformation beneath much of southeastern tibet at the southeastern edge of the tibetan plateau we observe a sharp transition in mantle anisotropy with a change in fast directions to a consistent e w direction and a clockwise rotation of the surface velocity surface strain fi eld and fault network toward burma around the eastern himalayan syntaxis the coincidence between structural crustal features surface strain and mantle anisotropy suggests that the deformation in the lithosphere is mechanically coupled across the crust mantle interface and that the lower crust is suffi ciently strong to transmit stress at the southeastern margin of the plateau in yunnan province a change in orientation between mantle anisotropy and surface strain suggests a change in the relationship between crustal and mantle deformation lateral variations in boundary conditions and rheological properties of the lithosphere play an important role in the geodynamic evolution of the himalayan orogen and tibetan plateau and require the development of three dimensional models that incorporate lateral heterogeneity

new insights of the tectonic and sedimentary evolution of the rio grande rise south atlantic ocean abstract oceanic plateaus such as the rio grande rise rgr comprise crucial pieces of information about the opening of the south atlantic ocean and the evolution of associated lithospheric plate margins a detailed seismostratigraphic assessment of the volcano sedimentary package of the rgr was conducted in its western portion based on correlations between seismic data and dsdp site 516f the approximately 330 km long seismic transect is characterized by a complex array of normal faults related to extensional tectonics from late cretaceous to the miocene and probably into the present day along the cruzeiro do sul rift the bathymetric and seismic data reveal a the presence of deep faults that rise from the basement to the sea floor b the occurrence of several seamounts with tops at a water depth below 1000 m and c prominent graben structures that represent unidentified volcanic intrusions indicative of the reactivation of lower miocene faults by younger extensional events based on integration of older data with the results presented here we propose a five stage tectono sedimentary model for the western portion of the rgr 1 the first basaltic flow began to form the rio grande rise in the coniacian santonian 2 extensional movements caused rifting during the uplift of the large volcanic structures 3 volcanic islands emerged above sea level increasing the deposition of volcanic breccia and ash layers 4 after the volcanism ceased thermal subsidence took place over the entire rise with intense erosion and sedimentation and 5 the uppermost sedimentary layers were deposited in pelagic conditions and offset by sub vertical normal faults

shear wave splitting and mantle flow associated with the deflected pacific slab beneath northeast asia 1 a total of 361 sks and five local s wave splitting measurements obtained at global and regional seismic network stations in ne china and mongolia are used to infer the characteristics of mantle fabrics beneath northeast asia fast polarization directions at most of the stations in the western part of the study area are found to be consistent with the strike of local geological features the dominant fast directions at the eastern part beneath which seismic tomography and receiver function studies revealed a deflected slab in the mantle transition zone mtz are about 100 from north which are almost exactly the same as the motion direction of the eurasian plate relative to the pacific plate and are independent of the direction of local geological features the splitting times at those stations are about 1 s which correspond to a layer of about 150 km thickness with a 3 anisotropy the shear wave splitting observations complemented by the well established observation that most of the eastern part of the study area is underlain by a lithosphere thinned by delamination in the paleozoic era can be best explained by the preferred alignment of metastable olivine associated with the subduction of the deflected pacific slab in the mtz or by back arc asthenospheric flow in the mantle wedge above the slab

land ocean correlations during the last interglacial glacial transition baffin bay northwestern north atlantic a review abstract evidence from terrestrial sections ice cores and marine cores are reviewed and used to develop a scenario for environmental change in the area of the extreme northwest north atlantic during marine isotope stages 5 and 4 the critical physical link between the landbased glacial chronology and marine events in baffin bay is the presence of carbonate rich drift along the baffin bay coast of bylot island and a detrital carbonate facies facies b in baffin bay sediments cores from baffin bay labrador sea can be dated by means of oxygen isotope variations and by peaks in the abundance of volcanic glass shards one occurrence of facies b is dated between late stage 5 and stage 4 and we correlate this event with the eclipse glaciation of bylot island and the ayr lake stade of the foxe glaciation of baffin island kogalu aminozone in contrast on west greenland amino acid racemization evidence suggests that the greenland ice sheet developed throughout stage 4 and reached a maximum in stage 3 svartenhuk advance 40 ka the oxygen isotope record in the devon island ice cap northwest baffin bay indicates that baffin bay was largely open during marine isotope stage 5 analyses of shallow water molluscan and foraminiferal assemblages deep water foraminifera pollen from iand sections and deep sea cores and dinoflagellates from marine cores indicate that interglacial conditions prevailed during much of the stage 5 4 glaciation

seismic anisotropy beneath tibet evidence for eastward extrusion of the tibetan lithosphere strong seismic anisotropy beneath tibet has recently been reported from the study of sks shear wave splitting the fast split waves are generally polarized in an easterly direction close to the present day direction of motion of the tibetan crust relative to stable eurasia as deduced from holocene slip rates on the major active faults in and around tibet this correlation may be taken to suggest that the whole tibetan lithosphere is being extruded in front of indenting india and that the anisotropic layer is the deforming asthenosphere that accommodates the motion of the tibetan lithosphere relative to the fixed mantle at depth uncertainties about this motion are at present too large to bring unambiguous support to that view assuming that this view is correct however a simple forward model is used to compute theoretical delay times as a function of the thickness of the anisotropic layer the observed delay times would require a 50 100 km thick anisotropic layer beneath south central tibet and an over 200 km thick layer beneath north central tibet where particularly hot asthenosphere has been inferred this study suggests that the asthenospheric anisotropy due to present absolute block motion might be dominant under actively deforming continents

the southern oscillation part i global associations with pressure and temperature in northern winter abstract we describe the global correlations between a measure of the southern oscillation and sea level pressure and surface air temperature in the northern winter the stability of these correlations were tested on the northern hemisphere for an 80 year period and it turned out that most stable correlation coefficients were found over india the north pacific ocean the rocky mountains and the central and western north atlantic ocean on the southern hemisphere most records are too short for a similar test but the following may tentatively be said about the southern oscillation in middle and high southern latitudes when pressure is low in lower latitudes over the south pacific ocean it tends to be high at higher latitudes of that ocean high over east antarctica and low in the belt of westerlies in the indian and south atlantic oceans in the zonal average on both hemispheres the pressure gradients in this extreme of the oscillation tend to be steeper at lower latitudes and flatter at higher latitud

linear and nonlinear aspects of snow albedo feedbacks in atmospheric models namias hypothesis that anomalous snowcover on the eastern side of the north american continent can generate an anomalous east coast low pressure system and an anomalous inland high pressure system is consistent with the time averaged anomalous response from a nonlinear primitive equation channel model with an idealized flat land sea arrangement an attempt to understand and describe this anomalous response in the nonlinear model as a linear response to anomalous diabatic heating was largely unsuccessful primarily because the anomalous eddy fluxes were also important this unsuccessful attempt to describe the nonlinear model s time averages by linear theory then motivated several comparisons between linear and nonlinear severely truncated quasi geostrophic models it was also found in these models that the eddy fluxes were extremely important for forcing or dissipating the stationary eddies

structure of the upper mantle and transition zone beneath southeast asia from traveltime tomography 1 tomographic images of the mantle beneath east asia were obtained from the inversion of traveltime data from global and regional seismograph networks and from temporary arrays on and around the tibetan plateau our results are consistent with previous studies but the unprecedented resolution of mantle heterogeneity provides new insight into the large scale tectonic framework of the continental india asia collision in the western part of the study region and subduction of the oceanic lithosphere in the east in the realm of continental collision west of 100 e a relatively slow p wave speed characterizes the upper mantle beneath much of the tibetan plateau but the wave speed is high beneath cratonic india the southern and western part of the tibetan plateau hindu kush and the tian shan in the subduction realm east of 110 e the main structures are i pronounced low wave speed anomalies at a depth of between 100 and 400 km beneath asia s southeastern seaboard and the back arc regions of ongoing subduction ii narrow fast anomalies in the upper mantle beneath major subduction zones and iii widespread fast anomalies at a depth of 500 700 km beneath the sea of japan the northern part of the philippine sea plate and southeastern china if the latter anomalies represent stagnant slabs their fragmented nature and large lateral extent suggest that they are produced by different episodes of subduction beneath western pacific island arcs along the old se margin of asia or during the mesozoic collision of cratonic units in southeast asia attribution to ancient subduction systems implies that slab fragments can reside in the transition zone for at least several tens of millions of years shallow slow anomalies beneath the red river fault region connect to deep anomalies beneath the south china fold belt and south china sea suggesting a causal relationship between the evolution of the continental lithosphere of sw china and deeper mantle processes between the collision and the subduction realms tomography reveals high wave speed continental roots beneath the western part of the north china craton ordos block and the south china or yangtze craton sichuan basin to a depth of 300 km

regional variations within the parana flood basalts southern brazil evidence for subcontinental mantle heterogeneity and crustal contamination abstract continental flood volcanism of the parana basin lower cretaceous is represented by two pyroxene tholeiitic basalts 90 vol the northern parana province npp is dominated by basalts high in tio2 and incompatible elements htib while the southern parana province spp is dominated by basalts low in tio2 and incompatible elements ltib npp basalts show relatively small variations of initial 120 ma 87 sr 86 sr r0 and 143 nd 144 nd nd ratios r0 0 7051 0 7062 and nd 0 5124 0 5125 respectively relative to those occurring in spp r0 0 7046 0 7120 and nd 0 5122 0 5128 respectively the latter basalts show significant positive correlations between r0 vs sio2 k2o rb and ba and negative correlations between r0 vs cr ni and mg value believed to be due to crustal granitic contamination in general the effects of contamination are pronounced in spp and tend to vanish towards npp 18o values range from 6 5 to 10 0 in the basalts from npp and spp and essentially reflect water magma interactions uncontaminated ltib and htib basalts from npp are isotopically distinct from the uncontaminated analogues from spp ltib r0 0 7055 vs 0 7046 nd 0 5124 vs 0 5128 respectively all chemical data consistently indicate distinct sources and a large scale mantle heterogeneity the npp mantle source is expected to have relatively high content of enriched components possibly related to small volume melts and metasomatic fluids tentatively the last stabilization age of the parana mantle heterogeneity is 0 5 1 0 ga old the existence of high and low tio2 basalt suites in both parana and karoo provinces in brazil and southern africa respectively indicates a large scale heterogeneity in the subcontinental mantle and suggests that basalt generation occurred in the lithospheric mantle

continental material in the shallow oceanic mantle how does it get there unusual compositions of some oceanic basalts have been attributed to their sources containing continental lithosphere detached during the breakup of gondwana however the processes of how such continental lithospheric material is detached and transported into the ocean basin have not been constrained here we identify walvis ridge where it has been argued that deep sea drilling project dsdp site 525a contains continental material as a unique location to constrain these processes absolute plate motion relative to the tristan mantle plume and relative plate motion between africa and south america of the african plate are oblique to one another such that tectonic detachment versus hotspot related thermal erosion should sample spatially separated continental units of different age we present isotopic compositions of xenoliths representing the neo proterozoic lithosphere at the inferred site for tectonic detachment during continental breakup and show that this process does not explain the walvis ridge dsdp site 525a mantle source rather thermal erosion of ancient cratonic mantle by the tristan mantle plume is indicated a convective return flow is required to transport the eroded subcontinental lithospheric mantle to the site of plume activity some 50 m y later and provides constraints on the direction and velocity of mantle flow in the upper mantle