precessional forcing of lacustrine sedimentation in the late cenozoic chemeron basin central kenya rift and calibration of the gauss matuyama boundary the fluviolacustrine sedimentary sequence of the chemeron formation exposed in the barsemoi river drainage tugen hills kenya contains a package of five successive diatomite fluvial cycles that record the periodic development of freshwater lakes within the axial portion of the central kenya rift the overwhelming abundance in the diatomite of planktonic species of the genera aulacoseira and stephanodiscus and the virtual absence of benthic littoral diatoms and detrital material indicate areally extensive deep lake systems a paleomagnetic reversal stratigraphy has been determined and chronostratigraphic tie points established by 40 ar 39 ar dating of intercalated tuffs the sequence spans the interval 3 1 2 35 ma and bears a detailed record of the gauss matuyama paleomagnetic transition the 40 ar 39 ar age for this boundary of 2 589 0 003 ma can be adjusted to concordance with the astronomical polarity time scale apts on the basis of an independent calibration to 2 610 ma 29 kyr older than the previous apts age the diatomites recur at an orbital precessional interval of 23 kyr and are centered on a 400 kyr eccentricity maximum it is concluded that these diatomite fluvial cycles reflect a narrow interval of orbitally forced wet dry climatic conditions that may be expressed regionally across east africa the timing of the lacustrine pulses relative to predicted insolation models favors origination of moisture from the northern africa monsoon rather than local circulation driven by direct equatorial insolation this moisture event at 2 7 2 55 ma and later east african episodes at 1 9 1 7 and 1 1 0 9 ma are approximately coincident with major global climatic and oceanographic events

vertical averages of rheology of the continental lithosphere relation to thin sheet parameters abstract we calculate vertically averaged rheologies for continental lithosphere subject to biaxial states of stress corresponding to extensional compressional and strike slip regimes we assume that brittle deformation occurs by byerlee s law and that ductile deformation occurs by steady state creep mechanisms under these conditions the vertically averaged rheology may be approximated by a single power law over the range of strain rates from 10 13 to 10 17 s 1 this is true not only when either brittle or ductile behavior dominates the strength of the lithospere but also when the contributions of brittle and ductile behavior are comparable the value of the power law exponent n and the strength of the lithosphere depend on the depth and stress difference at the brittle ductile transition the temperature at the moho and the geothermal gradient in the range of vertically integrated stress between 1012 and 1013 n m 1n reaches a minimum of between 5 and 10 under other circumstancesn increases rapidly as brittle behavior dominates over ductile behavior the strength of the lithosphere depends most strongly on the thermal state of the upper mantle but also is influenced by the stress supported in brittle portions of the lithosphere

east african lake evidence for pliocene millennial scale climate variability late cenozoic climate history in africa was punctuated by episodes of variability characterized by the appearance and disappearance of large freshwater lakes within the east african rift valley in the baringo bogoria basin a well dated sequence of diatomites and fluviolacustrine sediments documents the precessionally forced cycling of an extensive lake system between 2 70 ma and 2 55 ma one diatomite unit was studied using the oxygen isotope composition of diatom silica combined with x ray fluorescence spectrometry and taxonomic assemblage changes to explore the nature of climate variability during this interval data reveal a rapid onset and gradual decline of deepwater lake conditions which exhibit millennial scale cyclicity of 1400 1700 yr similar to late quaternary dansgaard oeschger events these cycles are thought to reflect enhanced precipitation coincident with increased monsoonal strength suggesting the existence of a teleconnection between the high latitudes and east africa during this period such climatic variability could have affected faunal and floral evolution at the time

contemporary crustal deformation in east asia constrained by global positioning system measurements global positioning system gps measurements collected since the early 90 s allow us to derive geodetic velocities at 16 permanent stations in east asia and 68 campaign mode sites in north china the resulting velocity field shows the following 1 contrary to the early inferences that the shanxi rift has accommodated significant right slip motion our results suggest that the rift system at least in its northern part in north china is under ese wnw extension at a rate of 4 2 mm yr the velocity field also suggests that an ese wnw trending left lateral shear zone deforming at a rate of 2 1 mm yr may exist along the north rim of north china at the latitude of 40 n separating actively extending north china in the south from relatively stable mongolia in the north 2 central and east china move at a rate of 8 11 mm yr east southeast with respect to siberia implying that the overall east southeastward motion is the dominant mode of deformation in east china 3 the india plate moves at a rate of 6 1 mm yr slower than the nuvel 1a model prediction relative to the eurasia plate 4 significant eastward motion 20 2 mm yr with respect to siberia occurs in southeastern tibet about half of this eastward motion 11 mm yr is absorbed by structures along the eastern boundary of the tibetan plateau

dynamics of the india eurasia collision zone we present simple new dynamic calculations of a vertically averaged deviatoric stress field over a depth average of 100 km for asia from geodetic geologic topographic and seismic data a first estimate of the minimum absolute magnitudes and directions of vertically averaged deviatoric stress is obtained by solving force balance equations for deviatoric stresses associated with gravitational potential energy differences within the lithosphere plus a first order contribution of deviatoric stresses associated with stress boundary conditions this initial estimate of the vertically averaged deviatoric stress field is obtained independent of assumptions about the rheology of the lithosphere absolute magnitudes of vertically averaged deviatoric stresses vary between 5 and 40 mpa assuming bulk viscous behavior for the lithosphere the magnitudes of deviatoric stresses together with the magnitudes of strain rates inferred from quaternary fault slip rate and gps data yield vertically averaged effective viscosities for tibet of 0 5 5 1022 pa s compared with 1 2 5 1023 pa s in more rigid areas elsewhere in the region a forward modeling method that solves force balance equations using velocity boundary conditions allows us to refine our estimates of the vertically averaged effective viscosity distribution and deviatoric stress field the total vertically averaged deviatoric stress and effective viscosity field are consistent with a weak lower crust in tibet they are consistent with some eastward motion of tibet and south china lithosphere relative to eurasia and they confirm that gravitational potential energy differences have a profound effect on the spatially varying style and magnitude of strain rate around the tibetan plateau our results for the vertically averaged deviatoric stress argue for a large portion of the strength of the lithosphere to reside within the seismogenic upper crust to get deviatoric stress magnitudes there to be as high as 100 300 mpa in accord with laboratory and theoretical friction experiments indicating that stress drops in earthquakes are small fractions of the total deviatoric stress

zeolitic diagenesis of late quaternary fluviolacustrine sediments and associated calcrete formation in the lake bogoria basin kenya rift valley late quaternary fluviolacustrine siltstones mudstones and claystones loboi silts on the northern margins of the saline alkaline lake bogoria in the kenya rift valley contain up to c 40 authigenic analcime and minor natrolite the zeolitic sediments are reddish brown and up to 1 m thick the amount of analcime increases upward in the profile but decreases with distance from the lake the altered sediments show many pedogenic features including zeolitic root mats rootmarks concretions and carbonate rhizoliths residual patches of calcrete locally cap the zeolitic rocks the profile is interpreted as an exhumed palaeosol and land surface on the former margins of the lake the analcime occurs as submicroscopic 0 5 2 5 m subhedral and euhedral crystals which have an average si a 1 ratio of 2 33 as determined by x ray microanalysis or 2 18 d value of 639 analcime peak the analcime formed in lake marginal sediments soils by reaction of silicate detritus with na2co3 rich pore waters concentrated close to the land surface by evaporative pumping and evapotranspiration poorly ordered clay minerals were probably the main reactants authigenic illite may have been a by product of the reactions chemical analyses suggest that pore waters supplied some of na and possibly k and sio2 the associated calcrete and rhizoliths were formed during or shortly after the main period of zeolitic alteration the ca2 may have originated from infiltrating dilute runoff and groundwater authigenic smectite was precipitated in open porosity following analcime formation the zeolitic alteration at lake bogoria provides a relatively recent analogue for lake marginal zeolites found in many ancient saline alkaline lake sediments

chronostratigraphy of the baringo tugen barsemoi hspdp btb13 1a core 40ar 39ar dating magnetostratigraphy tephrostratigraphy sequence stratigraphy and bayesian age modeling the baringo tugen barsemoi 2013 drillcore btb13 acquired as part of the hominin sites and paleolakes drilling project recovered 228 m of fluviolacustrine sedimentary rocks and tuffs spanning a 3 29 2 56 ma interval of the highly fossiliferous and hominin bearing chemeron formation tugen hills kenya here we present a bayesian stratigraphic age model for the core employing chronostratigraphic control points derived from 40ar 39ar dating of tuffs from core and outcrop 40ar 39ar age calibration of related outcrop diatomaceous units and core magnetostratigraphy the age model reveals three main intervals with distinct sediment accumulation rates an early rapid phase from 3 2 to 2 9 ma a relatively slow phase from 2 9 to 2 7 ma and the highest rate of accumulation from 2 7 to 2 6 ma the intervals of rapid accumulation correspond to periods of high earth orbital eccentricity whereas the slow accumulation interval corresponds to low eccentricity at 2 9 2 7 ma suggesting that astronomically mediated climate processes may be responsible for the observed changes in sediment accumulation rate lacustrine transgression regression events as delineated using sequence stratigraphy dominantly operate on precession scale particularly within the high eccentricity periods a set of erosively based fluvial conglomerates correspond to the 2 9 2 7 ma interval which could be related to either the depositional response to low eccentricity or to the development of unconformities due to local tectonic activity age calibration of core magnetic susceptibility and gamma density logs indicates a close temporal correspondence between a shift from high to low frequency signal variability at 3 ma approximately coincident the end of the mid piacenzian warm period and the beginning of the cooling of world climate leading to the initiation of northern hemispheric glaciation c 2 7 ma btb13 and the baringo basin records may thus provide evidence of a connection between high latitude glaciation and equatorial terrestrial climate toward the end of the pliocene

biogenic silica deposition in lake malawi east africa over the past 150 000years abstract we report a 150 000 year record of the abundance of biogenic silica bsi in drill cores from the northern and central basins of lake malawi the periods of highest diatom burial at both sites occurred around 65 69 and 51 60 thousand years ago ka after the termination of prolonged intense drought in the region these peaks are believed to reflect elevated delivery of dissolved silica to the lake due to acceleration of chemical weathering in the drainage basin the droughts that preceded these bsi depositional episodes occurred at precessional frequency corresponding to times of minimum austral spring or summer insolation in the region prior to 60 ka subsequent arid spells have not been as severe due to the reduced amplitude of precessional forcing as a result of the superimposed effect of eccentricity however biogenic silica records do not show significant sensitivity to precessional forcing the bsi burial flux in the north basin displays strong millennial scale variability since 50 ka with peak values occurring during cold times in the northern hemisphere as reflected in the greenland ice core records we observe no relationship between bsi and deposition of volcanic tephras in this system the central basin core also displays millennial scale variability in bsi abundance during the past 50 ky of comparable magnitude to that in the north basin but without systematic correlation to the north basin bsi or greenland ice core records in general we conclude that bsi profiles from the north and probably south basins are more readily interpreted in terms of regional climate dynamics than are bsi profiles from more central locations in long narrow tropical rift lakes diatom burial rates are suspected to be too influenced by silica limitations imposed by diatom production and burial in the upwelling systems at either end of these low latitude lacustrine systems

δ13c records from fish fossils as paleo indicators of ecosystem responses to lake levels in the plio pleistocene lakes of tugen hills kenya abstract the carbon isotopic ratios of organic matter in fish fossils from diatomites and other lake beds in the hspdp drill core from tugen hills kenya 2 56 3 29 ma reflect trophic resource uses and can indicate the dietary habitats of fish in the paleolake this information offers insight into how fish communities responded to lake level fluctuations during the plio pleistocene in the east african rift valley we have compared this record with fish fossil isotopes from both a previously published study of a lake malawi drill core 139 ka present and core top modern ca 1978 samples collected at the water sediment boundary from lake turkana kenya of known environmental provenance both the lake malawi drill core fossils 7 2 to 27 5 vpdb and modern lake turkana samples 16 to 24 6 vpdb have 13c values indicating a mix of near shore and deep water pelagic species in contrast the 13c values for the tugen hills core fossils vary only between 20 and 27 vpdb the absence of 13c values greater than 19 suggests none of these fossils are derived from near shore benthic habitats the lack of shallow water benthic lacustrine fish fossils through the tugen hills lake cycles may indicate that the rate of change from low lake stands to deeper lake phases was very rapid and shallow water communities were not established for long enough to leave a fish fossil record at the core site these results strongly suggest that lake level responses to climate variability in the baringo basin of the east african rift were very abrupt during the plio pleistocene transition

velocity field in asia inferred from quaternary fault slip rates and global positioning system observations we perform a joint inversion of quaternary strain rates and 238 global positioning system gps velocities in asia for a self consistent velocity field the reference frames for all geodetic velocity observations are determined in our inversion procedure india in moves relative to eurasia eu about a pole of rotation at 29 78 n 7 51 e 0 353 myr 1 which yields a velocity along the himalaya within india that is 73 76 of the magnitude of the in eu nuvel 1a velocity and a vector azimuth that is 8 10 clockwise of nuvel 1a in eu vector azimuth relative to eurasia south china moves at 9 11 mm yr in the direction 110 120 with a pole position 64 84 n 156 74 e 0 12 myr 1 amurian block motion has a pole position in a similar location but at a slower rate 64 61 n 158 23 e 0 077 myr 1 and most of the amurian eurasia motion is accommodated by extension across lake baikal tarim basin moves relative to eurasia about a pole of rotation at 39 24 n 98 2 e 0 539 myr 1 and 16 18 mm yr of shortening is accommodated across the west central tien shan there is distributed e w extension throughout both southern and north central tibet within southern tibet between the longitudes of 77 e to 92 e the deformation field accommodates 16 19 mm yr of e w extension we compare predicted seismic moment rates with those observed in this century in asia total observed seismic moment rates within the entire area of central and east asia 2 2 107 km2 in this century are 2 26 0 7 1020 n m yr 1 as compared with a predicted total rate of 2 03 0 066 1020 n m yr 1 comparisons between observed and predicted moment rates within 42 subregions reflect the generally unstable process of inferring long term seismic moment rates from a catalog of limited duration 94 years an observation period of 10 000 years would be required to reduce uncertainties in observed seismic moment rate to the same size as the uncertainties in model tectonic moment rates inferred from the joint inversion of gps and quaternary rates of strain we show that in general a better correlation with model tectonic moment rate is inferred from the seismicity catalog by considering the numbers of earthquakes above a cutoff magnitude mb 5 0 for the period january 1 1965 to january 1 1999

phytoliths pollen and microcharcoal from the baringo basin kenya reveal savanna dynamics during the plio pleistocene transition abstract as part of the hominin sites and paleolakes drilling project hspdp phytoliths pollen and microcharcoal were examined from the 228 m 3 29 to 2 56 ma baringo tugen hills barsemoi drill core btb13 a total of 652 samples were collected at 10 to 32 cm intervals corresponding to sub millennial to millennial scale temporal resolution microcharcoal was well preserved throughout the core and often peaked in abundance 5 kyr before and after insolation peaks phytolith preservation varied between excellent to total dissolution in alternating intervals throughout the core pollen was rarely preserved these combined datasets indicate that prior to 3 1 ma woody cover fluctuated between open savanna 80 cover at typically precessional 19 23 kyr periodicities during the mid piacenzian warm period mpwp 3 26 3 01 ma intervals with exceptionally high microcharcoal abundance suggest that regional turnover from wooded to open habitats was driven in part by fire after 3 1 ma low elevation woody cover likely never exceeded 40 with oscillations between mesic tall grass vs xeric short grass savanna at precessional periodicities mesic c4 tall grass panicoideae peaked in abundance during insolation maxima whereas xeric c4 short grass chloridoideae peaked during insolation minima the onset of northern hemisphere glaciation nhg at 2 75 ma coincided with the appearance of deep lake phases and increases in grass density and fire frequency spectral analysis and intervals with well preserved phytoliths indicate that precession and interhemispheric insolation gradients influenced vegetation via their effects on equatorial precipitation and fire this study fills a crucial gap in pliocene vegetation reconstructions from the east african rift valley and its associated hominin localities it also provides orbitally resolved regional vegetation data useful in paleodata model comparisons for the onset of the mpwp which is often used as an analog for future warming and nhg

multi basin depositional framework for moisture balance reconstruction during the last 1300 years at lake bogoria central kenya rift valley multi proxy analysis of sediment cores from five key locations in hypersaline alkaline lake bogoria central kenya rift valley has allowed reconstruction of its history of depositional and hydrological change during the past 1300 years analyses including organic matter and carbonate content granulometry mineralogical composition charcoal counting and high resolution scanning of magnetic susceptibility and elemental geochemistry resulted in a detailed sedimentological and compositional characterization of lacustrine deposits in the three lake basins and on the two sills separating them these palaeolimnological data were supplemented with information on present day sedimentation conditions based on seasonal sampling of settling particles and on measurement of physicochemical profiles through the water column a new age model based on 210pb 137cs and 14c dating captures the sediment chronology of this hydrochemically complex and geothermally fed lake an extensive set of chronological tie points between the equivalent high resolution proxy time series of the five sediment sequences allowed transfer of radiometric dates between the basins enabling inter basin comparison of sedimentation dynamics through time the resulting reconstruction demonstrates considerable moisture balance variability through time reflecting regional hydroclimate dynamics over the past 1300 years between ca 690 and 950 ad the central and southern basins of lake bogoria were reduced to shallow and separated brine pools in the former occasional near complete desiccation triggered massive trona precipitation between ca 950 and 1100 ad slightly higher water levels allowed the build up of high pco2 leading to precipitation of nahcolite under still strongly evaporative conditions lake bogoria experienced a pronounced highstand between ca 1100 and 1350 ad only to recede again afterwards for a substantial part of the time between ca 1350 and 1800 ad the northern basin was probably disconnected from the united central and southern basins throughout the last two centuries lake level has been relatively high compared to the rest of the past millennium evidence for increased terrestrial sediment supply in recent decades due to anthropogenic soil erosion in the wider bogoria catchment is a reason for concern about possible adverse impacts on the unique ecosystem of lake bogoria this article is protected by copyright all rights reserved

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

three dimensional modeling of lacustrine diatom habitat areas improving paleolimnological interpretation of planktic benthic ratios three dimensional modeling of lake morphometry enables the calculation of lake volume planar surface area and basin surface area which are critical components of a conceptual model of how planktic and benthic habitat areas change with changing lake level we have applied three dimensional modeling to foy lake montana and compared model results of changing ratios of planktic and benthic habitat areas at all possible lake levels to sediment cores recovered from the lake the model allows us to produce semiquantitative depth reconstructions and greatly improves lake level reconstruction of this morphometrically complex basin the conceptual model also was modified to examine the influence of resuspension versus gravity redistribution of diatomaceous material on the sedimentary record comparison of the model results with both long and short sedimentary records from foy lake montana suggests that planktic benthic ratios may be a better predictor of mean lake depth than maximum lake depth the model developed here is simple enough to be easily implemented has few requirements can be adapted to suit any lake basin and can be augmented to examine a variety of different lake processes that affect habitat availability

geodetic evidence for a low slip rate in the altyn tagh fault system the collision between india and asia has been simulated with a variety of computational models that describe or predict the motions of the main faults of east asia geological slip rate estimates of 20 30 mm yr 1 suggest that the largest of these faults the 2 000 km long altyn tagh fault system on the northern edge of the tibetan plateau absorbs as much of the indo asian convergence signal as do the himalayas partly by oblique slip and partly by contraction and mountain growth however the predictions of dynamic models for asian deformation and the lower bounds of some geological slip rates estimates 3 9 mm yr 1 refs 7 8 suggest that the altyn tagh system is less active here we report geodetic data from 89 91 degrees e that indicate left lateral shear of 9 5 mm yr 1 and contraction of 3 1 mm yr 1 across the altyn tagh system this result combined with our finding that at 90 degrees e tibet contracts north south at 9 1 mm yr 1 supports the predictions of dynamic models of asian deformation

seasonal signals in lake malawi sediments the sedimentary record of lake malawi one of the large rift valley lakes of east africa contains thick sequences of biologically undisturbed finely laminated muds and silts laminated sequences were recovered in cores from the extensive diatom ooze and silty diatom clay facies of the lake basin that is permanently anoxic below 250 m the sequences consist of light dark laminae couplets with average thickness of 1 mm the light laminae are completely dominated by the diatom genus melosira with a relatively minor contribution from the genus stephanodiscus the dark laminae consist of 50 diatoms equal proportions of melosira and stephanodiscus few nitzschia and occasional littoral diatom genera and 50 terrestrial plant debris mineral grains and fine clay and organic material based on a 210pb and 14c calculated sedimentation rate of 1 mm yr 1 for lake malawi basin sediments and the well defined seasonal variations in fluvial input and diatom productivity levels our data suggest that the lamination couplets are annual varves representing biannual sedimentation sequences the light laminae result from diatom blooms occurring during the dry windy season april october when the upper 250 m of the lake is well mixed localized upwelling occurs and the epilimnion becomes enriched with nutrients the dark laminae represent sedimentation in the rainy low wind velocity season november march when runoff and allochthonous terrestrial input are at a maximum and primary productivity is at a minimum

magnetic fingerprint of southern portuguese speleothems and implications for paleomagnetism and environmental magnetism environmental magnetism of speleothems is still in its early stage of development here we report on our investigation of the environmental and paleomagnetic information that has been recorded in speleothems and what are the factors that control its preservation and reliability to address these issues we used a multidisciplinary approach including rock magnetism petrography scanning electron microscopy stable carbon and oxygen isotope compositions and major and trace element concentrations we applied this to a set of samples from different stages of speleothem evolution present day dripwater glass plates a weathered stalactite a fresh stalagmite cave sediments and terra rossa soils these samples come from the penico and excentricas caves located in two distinct aquifers of the algarve region south portugal our results show that the main magnetic carriers of the speleothems under study are primary detrital and consist of maghemite and magnetite similarities in coercivity and temperature dependence of the studied set of samples suggest that iron oxides are inherited from the terra rossa soils that cap the cave and were transported to the speleothems by dripwater hence they represent a regional environmental signature interestingly a stable and probably detrital remanent magnetization could be isolated in the fresh stalagmite whereas the weathered stalactite yielded chaotic magnetic directions and very low remanent intensities we propose that these low intensities can be the result from i different remanence acquisition mechanisms between stalagmite and stalactite and or ii iron dissolution by fungal activity we also suggest that magnetic properties and color and the content in detrital elements in the fresh speleothem inform about environmental processes acting on the interface of rock soil atmosphere while oxygen isotope composition and alkaline earth element concentrations inform about calcite water interaction processes these results provide a better understanding of how environmental information is recorded in speleothems and what the factors are that control the reliability of the paleomagnetic and paleo environmental signal

human evolution in a variable environment the amplifier lakes of eastern africa abstract the development of the cenozoic east african rift system ears profoundly re shaped the landscape and significantly increased the amplitude of short term environmental response to climate variation in particular the development of amplifier lakes in rift basins after three million years ago significantly contributed to the exceptional sensitivity of east africa to climate change compared to elsewhere on the african continent amplifier lakes are characterized by tectonically formed graben morphologies in combination with an extreme contrast between high precipitation in the elevated parts of the catchment and high evaporation in the lake area such amplifier lakes respond rapidly to moderate precessional forced climate shifts and as they do so apply dramatic environmental pressure to the biosphere rift basins when either extremely dry or lake filled form important barriers for migration mixing and competition of different populations of animals and hominins amplifier lakes link long term high amplitude tectonic processes and short term environmental fluctuations east africa may have become the place where early humans evolved as a consequence of this strong link between different time scales

a 10 000 year high resolution diatom record from pilkington bay lake victoria east africa a new diatom record from lake victoria s pilkington bay subsampled at 21 to 25 year intervals and supported by 20 ams dates reveals a 10 000 calendar year environmental history that is supported by published diatom and pollen data from two nearby sites with their chronologies adjusted here to account for newly documented ancient carbon effects in the lake these three records provide a coherent finely resolved reconstruction of holocene climate change in equatorial east africa after an insolation induced rainfall maximum ca 8800 8300 cal yr b p precipitation became more seasonal and decreased abruptly ca 8200 and 5700 yr b p in apparent association with northern deglaciation events century scale rainfall increases occurred ca 8500 7000 5800 and 4000 yr b p conditions after 2700 yr b p were generally similar to those of today but major droughts occurred ca 1200 600 yr b p during europe s medieval warm period

diatom paleolimnology of late pliocene baringo basin kenya paleolakes abstract kenya s baringo tugen hills barsemoi drill site is one of six localities across kenya and ethiopia from which the hominin sites and paleolakes drilling project has obtained sediment cores in an effort to investigate the role of environmental forcing in shaping human evolution the baringo basin site features extensive exposures of the chemeron formation which contains 100 fossil vertebrate localities including five hominin sites the 228 m drill core dating from 3 29 to 2 56 ma is characterized by fluvio lacustrine sediments including multiple diatomites with evidence of variable degrees of later pedogenic modification in the lower part of the core 3 29 3 04 ma diatoms were preserved only in very low abundance consistent with predominantly fluvial or lake marginal environments in contrast five diatomites and two additional diatom rich intervals were deposited after 3 04 ma reflecting a major shift in the basin hydrology planktonic freshwater species dominated these diatom rich intervals whereas periphytic taxa were present in proportions less than 2 suggesting that these intervals represent open water deposition during lake highstands littoral or saline assemblages are largely absent throughout the core instead we observed a pattern of increasing diatom frustule dissolution at the tops and bottoms of diatomite units indicating increased alkalinity during the transgressive regressive phases a na bearing zeolite analcime indicative of saline waters precipitated in clastic dominated intervals between diatomites suggesting extreme environmental variability between lake highstands and lowstands diatom assemblages were consistently dominated by a few species belonging to the genera aulacoseira and stephanodiscus which were at times co dominant we infer that assemblages dominated by aulacoseira represent a well mixed lake with abundant supply of silica when stephanodiscus was dominant which occurred more frequently in the later freshwater phases we infer incomplete mixing and reduced silica flux to the epilimnion upper water layer

a study of azimuthal p residuals and shear wave splitting across the kunlun range northern tibetan plateau abstract the kunlun fault system is an east west transcurrent fault bordering the tibetan plateau that has experienced left lateral motion since 3 ma a linear array of one component and three component seismological stations was deployed across the kunlun range for 4 months p travel time residuals define three basic units along the profile the north kunlun unit the south kunlun unit and the bayan har unit p residuals are a function of azimuth p residuals are fast in the southern stations for southern azimuths and in the northern stations for northern azimuths measurements of shear wave splitting of the s scs and sks phases have been performed on deep events delay times and fast polarization directions have been computed by finding the maximum of the cross correlation between the two horizontal components rotated at different angles delay times from 0 40 s to 2 52 s remain in the range of those found by mcnamara et al 1994 j geophys res 99 13655 13665 at two sites along the same profile p residual travel times and the splitting of s waves are discussed in relation to models of the geological evolution of this region the northern tibetan plateau appears as a juxtaposition of different terranes the azimuthal p residual pattern implies that p wave anisotropy is about 3 in the lithosphere on both sides of the kunlun fault and that fast velocities are oriented outwards from the kunlun fault plane s wave splitting is observed far from the fault and implies that the lithosphere beneath northern tibet has undergone an important deformation

paleolimnological records of climatic change in north america lacustrine fossil records provide long time series of data on limnological and climatic conditions these data are useful for establishing natural patterns of climate variability and for generating testable hypotheses about atmospheric circulation and climate ecosystem linkages shoreline features can indicate past lake level fluctuations that may reflect changes in moisture balance but often these records are discontinuous and are evidence of only extreme conditions the organisms geochemistry and sedimentology of lake sediments may provide a more continuous sequence of direct and indirect lake climate interactions in the past the most clearly interpretable paleolirnnological records of climatic change are those that use several lines of evidence to corroborate a climatic hypothesis and are from sites near an ecotone or in regions of extreme climate in all cases hydrologic setting mediates a lake s response to climate and must be considered in interpreting sedimentary sequences discussion of the impact of recent and future climatic changes necessitates a baseline against which the magnitude of observed or predicted changes can be measured commonly we look to recent history as a reference point for comparison however the landscape of recent decades or in some cases even centuries has been altered greatly by human activities thus to determine whether recent or future climatic change is unique or unusual we need long natural preanthropogenic time series against which observed or predicted climatic states and variability can be compared paleolimnological archives are a tool for looking at environmental patterns prior to anthropogenic impact and show a range of climatic states far greater than those documented by written records former strand lines of pluvial lakes bonneville and lahontan in the western u s for example testify to moisture extremes unsurpassed in recent centuries or even millennia in addition one can us the paleolimnological record as an independent climatic proxy against which patterns and rates of terrestrial response to climatic change can be compared here i present a broad overview of paleoclimatic reconstruction in north america from lacustrine records with a focus on the late quaternary the review is limited to climatic inference from paleolirnnological records a term i use in a restrictive sense to refer to records of past lake states physical biological and geochemical as opposed to records of past stream hydrology e g ely et al 1993 knox 1993 or terrestrial conditions i summarize paleolimnological tools available for reconstruction of past climate the capabilities and limitations of these tools

observing plate motions in s e asia geodetic results of the geodyssea project this paper presents the final geodetic results of the geodyssea project the gps data from a 42 station network observed during two field campaigns 1994 1996 were analyzed by four groups using different software packages and analysis strategies the precision of both campaign coordinate solutions was found to be 4 7 mm for the horizontal and 1 cm for the vertical component the campaign solutions were merged into one unique solution which was accurately mapped into the itrf 96 reference frame the global accuracy of this solution with respect to itrf 96 is 1 cm while the resolution of the relative horizontal velocities is estimated to be at the level of 2 3 mm yr this solution was used as the basis for all scientific interpretations which are published in separate papers the velocity estimates of a part of the network provided the first direct measurement of a relative motion of the sundaland block with respect to eurasian plate

on the relation between seismic anisotropy and finite strain because seismic anisotropy in the upper mantle is due primarily to lattice preferred orientation lpo of olivine crystals induced by deformation observations of anisotropy can provide constraints on mantle deformation and flow in this paper we use a previously published theory for deformation induced lpo ribe and yu 1991 to quantify the relation between seismic anisotropy and deformation for two model compositions a pure olivine dunite aggregate and a harzburgite comprising 70 olivine and 30 enstatite analytical and numerical solutions of the governing equations show that the predicted lpo and hence also the seismic anisotropy depends only on the finite strain and not on the deformation path by which that strain is produced the magnitude of the anisotropy depends only on the ratios c1 c2 and c2 c3 where c1 c2 c3 are the lengths of the principle axes of the finite strain ellipsoid and the directions of maximum and minimum compressional wave velocity vp coincide with the axes c1 and c3 respectively we calculate the percent anisotropies for compresional and shear waves propagating along the three principle axes of the finite strain ellipsoid as functions of the axial ratios c1 c2 and c2 c3 for our two model compositions however inferences of the magnitude of finite strain obtained using these results are likely to be minimum estimates because our theory neglects the effects of dynamic recrystallization finally we present as a byproduct of this study a new direct method for calculating the finite strain produced by an arbitiary deformation history

comparison of the hydrological and hydrochemical evolution of lake naivasha kenya during three highstands between 175 and 60 kyr bp abstract three diatomite beds exposed in the ol njorowa gorge south of lake naivasha central kenya rift document three major lake level highstands between 175 and 60 kyr bp diatom transfer function estimates of hydrological and hydrochemical parameters suggest that a deep and large freshwater lake existed during the highstands at 135 and 80 kyr bp in contrast a shallower but more expanded freshwater lake existed at 110 kyr bp the best analog for the most extreme highstand at 135 kyr bp is the highstand during the early holocene humid period from 10 to 6 kyr bp the environmental conditions as reconstructed from diatom assemblages suggest long lasting episodes of increased humidity during the high lake periods this contrasts to the modern situation with a relatively shallow lake naivasha characterized by rapid water level fluctuations within a few decades the most likely cause for the variable hydrological conditions since 175 kyr bp is orbitally driven insolation changes on the equator and increased lateral moisture transport from the ocean

tropical east african climate change and its relation to global climate a record from lake tanganyika tropical east africa over the past 90 kyr abstract forcing mechanisms of tropical climate in continental areas remain poorly understood due in large part to a lack of continuous long term high fidelity records sediment core t97 52v from lake tanganyika provides new insight into the timing and mechanisms behind east african climate change over the past 90 kyr this record is particularly important because other than a recently recovered scientific drill core from lake malawi there are no other continuous well dated records from east africa prior to 60 ka the high resolution age model presented here provides a large degree of age certainty for the past 45 kyr and our suite of proxies allows a thorough examination of lake tanganyika s dynamics from core stratigraphy and chemical analyses we present evidence of a lake level drop greater than 400 m sometime prior to 90 ka much greater than that inferred for the lgm suggesting a period of intense aridity sometime around 100 ka additionally core t97 52 v preserves evidence of worm burrows detected by x radiographic imagery as indicated by burrow shaped deposits of iron oxide indicating a shallow lake at the time of deposition of that material intermittently high lake levels between 78 ka and 72 ka developed at the same time as a weakened asian monsoon and a pluvial phase in northeast brazil suggesting a global reorganization of climate possibly forced by a reduction in orbital eccentricity over the past 60 ka this core preserves the same events recorded in a core collected 100 km away in the southern basin of lake tanganyika including an unexplained increase in biogenic silica at 37 ka suggesting that this vast lake is responding coherently across both major bathymetric basins to regional and global climate forcing

the late pleistocene and holocene stratigraphy and paleolimnology of lakes kivu and tanganyika abstract evidence from diatoms chemistry and sedimentology shows that lakes kivu and tanganyika experienced late pleistocene low stands 86 to 300 m for kivu and 250 to 350 m for tanganyika both lakes had higher 2 10 fold sedimentation rates prior to 11 500 yr b p about 9900 yr b p upwelling became important in southern lake tanganyika which had been deepening for at least several millenia by 9400 yr b p kivu had become deep and dilute with overflow via the ruzizi river to lake tanganyika southern lake tanganyika became more stably stratified about 5200 yr b p volcanism and hydrothermal activity in kivu began about 5000 yr b p and the salinity reached 3 ppt and the lake became strongly stratified the ruzizi river outflow was interrupted from 3500 yr b p until recently the sedimentary record implies that sudden hydrothermal activity and sublacustrine volcanism in kivu has probably been the cause of its impoverished fauna these results show that high lake levels in the eastern rift of africa from 10 000 to 7000 yr b p were paralleled in the western rift given the great volumes of these lakes climate has not caused great variations in salinity the diatom floras however have been remarkably sensitive to changes in stratification and nutrient supply volcanism has been a major force in the limnological and biological evolution of lake kivu while both kivu and tanganyika have responded to changes in precipitation and wind stress

crustal structure of the himalayan orogen at 90 east longitude from project indepth deep reflection profiles international deep profiling of tibet and the himalaya indepth deep seismic reflection profiles show that the indian lithosphere is underthrusting the central himalaya along a gently north dipping decollement that is traceable northward beneath the tethyan belt to 28 6 n and to a depth of about 45 km the decollement carries in its hanging wall a near crustal thickness slice of internally deformed indian continental basement and cover represented in outcrop by the greater himalayan belt and structurally overlying tethyan belt geometric relationships suggest that the decollement probably ramps downward beneath the northern tethyan belt to near the base of the crust and that the hanging wall crustal slice was detached from indian mantle lithosphere that presently underlies southern tibet to the north the north himalayan anticlinorium kangmar dome appears to be a large duplex ramp anticline in the hanging wall of the decollement the south tibetan detachment appears to have been imaged in two areas in the wagye la area it appears to follow the tethyan belt greater himalayan belt contact northward in the subsurface and to have been folded over the kangmar dome in the zherger la area it appears to have been active relatively recently as a 30 north dipping normal fault that cuts deeply into the greater himalayan belt allochthon palinspastic reconstruction indicates a minimum of about 326 km of shortening of the indian basement across the himalaya since the initiation of the main central thrust hence indian continental lithosphere largely stripped of its overlying crust can extend northward beneath the tibetan plateau to at least 32 n where earthquake seismological observations show that the properties of the upper mantle change markedly e g mcnamara et al 1995 taken together the seismic reflection and earthquake seismological observations lend support for the view that the indian mantle lid mechanically underplates roughly the southern half of the tibetan plateau the indepth reflection data do not show that indian crust attached to this lithosphere extends north of southernmost tibet kangmar dome nor do they negate this possibility similarly the reflection data do not yield direct evidence for or against lower crustal subduction beneath the himalaya

occurrence of zeolites in sedimentary rocks an overview zeolites have been known since the mid 1750s but prior to the early 1950s most reported occurrences of zeolites were in fracture fillings and amygdules in igneous rocks particularly basaltic lava flows indeed most of the large attractive zeolite specimens in museum collections were obtained from lavas in recent years zeolites have been recognized as important rock forming constituents in low grade metamorphic rocks and in a variety of sedimentary rocks most zeolites in sedimentary rocks are finely crystalline that is they occur as microscopic or submicroscopic crystals and they are therefore of little appeal to mineral collectors however deposits of this type are voluminous and have great geologic significance and economic potential zeolites are among the most common authigenic silicate minerals that occur in sedimentary rocks and they form in sedimentary rocks of diverse lithology age and depositional environment about twenty species of zeolites have been reported from sedimentary rocks but only eight zeolites commonly make up the major part of zeolitic rocks these are analcime chabazite clinoptilolite erionite heulandite laumontite mordenite and phillipsite this chapter will consider chiefly the zeolites in sedimentary rocks with emphasis on volcaniclastic deposits which contain the largest concentrations of zeolites the occurrence of zeolites in lava flows is mentioned only briefly journal articles on the occurrence and origin of natural zeolites have multiplied at a rapid rate since the mineralogy and geology of natural zeolites was first published in 1977 as volume 4 of the mineralogical society of america s reviews in mineralogy the present review will highlight areas of more recent research on the occurrence and origin of zeolites and some of the coexisting minerals a wide variety of zeolites has been identified in sedimentary deposits with the most common being clinoptilolite analcime heulandite laumontite and phillipsite less abundant zeolites include chabazite erionite mordenite natrolite

the active tectonics of the eastern himalayan syntaxis and surrounding regions source parameters of 53 moderate sized earthquakes obtained from the joint inversion of regional and teleseismic distance long period body waves provide the data set for an analysis of the style of deformation and kinematics in the region of the eastern himalayan syntaxis focal mechanisms of eastern himalayan events show oblique thrust consistent with the n ne directed movement of the indian plate as it underthrusts a boundary that strikes at an oblique angle to the direction of convergence earthquakes near the sagaing fault show strike slip mechanisms with right lateral slip earthquakes on its northern splays however indicate predominant thrusting evidence that the dextral motion on the sagaing fault which accommodates a portion of the lateral motion between india and southeast asia terminates in a zone of thrust faulting at the eastern himalayan syntaxis remaining motion between india and southeast asia is accommodated in a zone of distributed shear in east burma and yunnan manifested by strike slip and oblique normal faulting east west extension crustal thinning and clockwise rotation of crustal blocks we determined strain rates throughout the region with a moment tensor summation using 25 years modern and 85 years modern and historic of earthquake data we matched the observed strains with a fifth order polynomial function and from this we determined both the velocity field and rotations with respect to a specified region velocities calculated relative to south china stationary show that the entire area extending from 20 n 36 n within deforming asia yunnan western sichuan and east tibet constitutes a distributed dextral shear zone with clockwise rotations up to 1 7 m y maximum in the region of the eastern syntaxis proper integrated strains across this zone relative to south china stationary show 38 mm yr 12mm yr of north directed motion at the himalaya remaining plate motion relative to south china fixed must be taken up by the underthrusting of india beneath the lesser himalaya strike slip motion on the sagaing fault and intraplate ne directed shortening within ne india as well as ne directed shortening within the eastern syntaxis proper 10 mm yr 2 mm yr of relative right lateral motion between india and southeast asia is absorbed in the region between the sagaing and red river faults 94 e 100 e it is the clockwise vorticity relative to south china associated with the deformation in yunnan east tibet and western sichuan that provides the relative north directed motion of 38 12 mm yr at the himalaya not all of the deformation is accommodated in right lateral shear between india and south china and between east tibet and south china velocity gradients exist that are parallel to the trend of the shear zone relative to a point within western sichuan 32 n 100 e the velocity field shows that the yunnan crust is moving s se at rates of 8 10 mm yr relative to south china there is no eastward expulsion of crustal material beyond the eastern margin of the tibetan plateau

mapping the descent of indian and eurasian plates beneath the tibetan plateau from gravity anomalies the collision of india with asia has produced a complicated continental continental plate boundary involving folding and faulting of variable trends and styles within and along the margins of the tibetan plateau numerous lines of evidence including the development of two scales of folding in tibet suggest that the lowermost crust is behaving in a ductile fashion this weak lower crust might then decouple the fundamental plate tectonic motions in the uppermost mantle from the complex pattern of surface faulting in this study we use bouguer gravity anomalies to map out the geometry of indian and eurasian plate interactions in the mantle beneath the plateau based on both the inferred geometry of the moho and lateral variations in lithospheric strength determined from mechanical modeling in our preferred model the lithosphere beneath tibet consists of two distinct units 1 the underthrust to the north indian plate which sutures with the eurasian plate in the upper mantle below the yarlung zangpo suture or the gangdese igneous belt 200 400 km north of the main boundary thrust and 2 the underthrust to the south eurasian plate a subducting slab of indian upper mantle extends about 200 km into the asthenosphere north from the mantle suture and exerts a bending moment of about 3 5 1017 n on the indian plate thus the mantle lithosphere appears to be behaving in the simple fashion of converging oceanic plates while the more buoyant continental crust deforms under high gravitational potential in a complex pattern controlled by its lateral and vertical strength heterogeneity

large scale crustal deformation of the tibetan plateau the topography velocity and strain fields calculated from a three dimensional newtonian viscous model for large scale crustal deformation are generally in good agreement with results from geological geodetic and earthquake studies in and around the tibetan plateau provided that the model rheology incorporates a weak zone within the deep crust beneath the plateau equivalent to a viscosity of 1012 pa s within a 250 m thick channel or 1018 pa s within a 15 km thick channel model studies and observations show a plateau at 5 km elevation with steep topographic gradients across the southern and northern plateau margins and more gentle gradients across the southeastern and northeastern margins rapid shortening strain is concentrated along the lower portions of the northern and southern plateau margins at rates 20 mm yr model results show north south shortening 10 mm yr in reasonable agreement with gps data 5 8 mm yr of north south shortening across the northern two thirds of the plateau and east west stretching 10 15 mm yr across the eastern half of the high plateau in reasonable agreement with seismic geologic and gps data upper crustal material moves eastward from the plateau proper into a lobe of elevated topography that extends to the south and east clockwise rotation of material around the east himalayan syntaxis at rates up to 10 mm yr occurs partly as a result of dextral shear between indian and asian mantle at depth and partly as a result of gravitational spreading from the high plateau to the south and east there is little difference in model surface deformation for assumptions of moderately weak or extremely weak lower crust except in southern and northern tibet where margin perpendicular extension occurs only for the case of an extremely weak lower crust our results suggest that the tibetan plateau is likely to have gone through a two stage development the first stage produced a long narrow high orogen whose height may have been comparable to the modern plateau the second stage produced a plateau that grew progressively to the north and east east west stretching eastward plateau growth and dextral rotation around the east himalayan syntaxis probably did not begin until well into the second stage of plateau growth perhaps becoming significant after 20 m y of convergence

active faulting and tectonics of burma and surrounding regions this study of the active tectonics of burma and surrounding regions is based mostly on an interpretation of landsat imagery and on fault plane solutions for shallow and intermediate earthquakes fold axes in the indoburman ranges and an east dipping inclined zone of intermediate depth earthquakes suggest that a slab of oceanic lithosphere was recently subducted to the east under the indoburman ranges fault plane solutions of shallow earthquakes however do not show underthrusting at the present time instead p axes are oriented roughly north south parallel not perpendicular to the fold axes in the ranges p axes for fault plane solutions of intermediate shocks also trend north south parallel to the strike of the subducted slab and may indicate that the hanging slab is being dragged north by india through the surrounding asthenosphere the sagaing fault probably accommodates most of the right lateral slip of india past indochina but india s northward movement also causes internal deformation within burma thailand and yunnan differences between the tectonic fabric seen on the landsat images and the tectonic regime compatible with the fault plane solutions throughout the region imply major changes in the directions of relative motion in the orientations of strain and in the style of deformation in the late cenozoic presumably these changes reflect the transition from subduction to collision and strike slip motion as india penetrated into eurasia past burma large parts of burma and thailand may have rotated several tens of degrees clockwise in this process

crustal deformation in the baikal rift from gps measurements three years and four campaigns of global positioning system gps measurements 1994 1997 in the baikal rift zone largest active continental rift system in eurasia show crustal extension at a rate of 4 5 1 2 mm yr in a wnw ese direction a comparison with moment release of large historical earthquakes suggests that elastic strain is currently accumulating in the baikal rift zone along active faults that currently have the potential for a m 7 5 earthquake the gps derived extension rate in the baikal rift zone is at least two times greater than the prediction of most deformation models of asia this result could reflect the dynamic contribution of the pacific eurasia subduction to intracontinental deformation in asia in addition to the effect of the india eurasia collision

abrupt change in tropical african climate linked to the bipolar seesaw over the past 55 000 years 1 the tropics play a major role in global climate dynamics and are vulnerable to future climate change we present a record of east african climate since 55 ka preserved in lake malawi sediments that indicates rapid shifts between discrete climate modes related to abrupt warming d o events observed in greenland although the timing of the malawi events cannot be determined exactly our age model implies that they occur prior to their greenland counterparts consistent with southward excursions of the intertropical convergence zone during greenland stadials the magnitude of each of the events recorded in malawi sediments corresponds to the scale of the subsequent greenland warming this suggests that a tropical component of climate sets a template for abrupt high northern latitude climate fluctuations associated with the bipolar seesaw

zeolites in closed hydrologic systems zeolites were first described from vugs and fissures in basaltic flows in the mid eighteenth century cronstedt 1756 and they are now known to be widespread in a variety of geological environments more zeolite species occur in vesicles and fractures of basaltic rocks than in any other geologic setting however zeolites from sedimentary rocks in particular represent the most important occurrences both in terms of aerial extent of deposits and in terms of the abundance of certain zeolite species the introduction of new analytical techniques in the latter part of the 20th century provided a significant stimulus to zeolite research these techniques allowed identification of large quantities of zeolites in widespread deposits of sedimentary rocks and in particular in sediments from saline alkaline lakes of the western united states sheppard and gude 1968 1969a 1973 based on geological and hydrologic environments zeolite occurrences in sedimentary rocks can be classified in the following framework a saline alkaline lakes b alkaline soils and land surfaces c deep sea sediments d open hydrologic systems e as products of hydrothermal alteration and f burial diagenetic or low metamorphic environments from a broader perspective zeolite deposits can be classified into two main groups namely closed hydrologic systems and open hydrologic systems the work of sheppard and gude 1968 1969a 1973 and later surdam 1977 discussing not only the mineralogy of the deposits but also the geology hydrology and chemistry of the depositional basins represent milestones in the study of closed hydrologic system formation of zeolites surdam 1977 considered closed hydrologic basins in two different tectonic settings a block faulted regions in arid and semiarid regions and b trough valleys associated with rifting examples of the first setting are the closed lakes of the basin and range province of the western united states whereas the

the mantle flow field beneath western north america although motions at the surface of tectonic plates are well determined the accompanying horizontal mantle flow is not we have combined observations of surface deformation and upper mantle seismic anisotropy to estimate this flow field for western north america we find that the mantle velocity is 5 5 1 5 centimeters per year due east in a hot spot reference frame nearly opposite to the direction of north american plate motion west southwest the flow is only weakly coupled to the motion of the surface plate producing a small drag force this flow field is probably due to heterogeneity in mantle density associated with the former farallon oceanic plate beneath north america

constraining the extent of crust mantle coupling in central asia using gps geologic and shear wave splitting data abstract we have obtained constraints on mechanical crust mantle coupling for tibet and yunnan indo china by comparing the observed surface deformation field inferred from gps and quaternary fault slip rate data with the mantle deformation field inferred from several sks shear wave splitting data sets we first determined whether the anisotropy is dominantly asthenospheric or lithospheric by testing simple models of both types against the observed values of the fast polarization direction which is assumed to be parallel to the horizontal projection of the direction of maximum shear for asthenospheric flow we solved for a best fitting uniform sub asthenospheric velocity model for eurasia the fit however was not satisfactory rms misfit 20 solving for separate uniform flow fields in each region improves the fit 15 for tibet 11 for yunnan although the resulting flow fields are inconsistent with several geophysical and geological constraints and thus considered unlikely we then considered lithospheric models for tibet vertically coherent deformation i e maximum shear direction from surface deformation is parallel to yields an improved match 11 for left lateral shear both the goodness of fit and the dominance of left lateral surface faulting in tibet argue for a lithospheric source of anisotropy the misfit for yunnan is large for either right 53 or left lateral 49 shear which argues for complete crust mantle decoupling in yunnan we show that the fast polarization directions throughout both the tibet and yunnan region can be fit by a single lithospheric dynamic model in which there is strong coupling between crust and mantle beneath tibet but a complete decoupling between crust and mantle beneath yunnan crust this dynamic model predicts left lateral maximum shear directions within the mantle that align with fast polarization directions in both regions 9 these maximum shear directions within the mantle align with the left lateral maximum shear directions in the crustal deformation field in tibet but are not in yunnan our results have the following implications first the coherence between crust and mantle deformation in tibet implies strong crust mantle mechanical coupling since this is the only way that crustal buoyancy forces required to account for the surface deformation field can be transmitted into the mantle this behavior is consistent with a uniform strength lithosphere or strong crust but not with a substantially weaker crust for example it is inconsistent with the popular jelly sandwich rheology and thus precludes behavior such as large scale lower crustal flow in tibet the crust mantle coherence is also incompatible with mantle delamination second crust mantle decoupling within the yunnan lithosphere argues that mantle deformation there is controlled only by boundary conditions crustal buoyancy forces are not transmitted into the mantle beneath yunnan moreover the dynamic model for the mantle shows that the yunnan crust is moving south with respect to the mantle at rates as high as 30 mm yr third there is a fundamental rheological lithospheric transition between tibet and yunnan that may provide a key to understanding this significant orogen

subduction of asian lithospheric mantle beneath tibet inferred from models of continental collision the relative northward motion of the indian subcontinent that followed the onset of continental collision with asia has produced extensive deformation of the earth s crust giving rise to the world s highest mountains in the himalayan chain and the world s largest high elevation region the tibetan plateau the formation of the broad mountain belt implies that contrary to the original tenets of plate tectonics the lithospheric plates have experienced widespread deformation far from the plate boundary1 several models have been proposed2 6 to explain the manner in which this post collisional deformation is distributed within the continental lithosphere of the indian and asian plates here we propose an alternative model in which subduction of the asian lithospheric mantle develops following the collision of india our model is supported by numerical calculations of crustal deformation and thickening and is consistent with available geological and geophysical data7 9 this picture suggests that lithospheric mantle is not deformed along with the crust and would imply that continental collision zones are more analogous to oceanic subduction zones than was previously believed

late pleistocene paleohydrography and diatom paleoecology of the central basin of lake malawi africa analysis of sedimentary diatom assemblages 10 to 144 ka form the basis for a detailed reconstruction of the paleohydrography and diatom paleoecology of lake malawi lake level fluctuations on the order of hundreds of meters were in ferred from dramatic changes in the fossil and sedimentary archives many of the fossil diatom assemblages we observed have no analog in modern lake malawi cyclotelloid diatom species are a major component of fossil assemblages prior to 35 ka but are not found in significant abundances in the modern diatom communities in lake malawi salinity and alkalin ity tolerant plankton has not been reported in the modern lake system but frequently dominant fossil diatom assemblages prior to 85 ka large stephanodiscoid species that often dominate the plankton today are rarely present in the fossil record prior to 31 ka similarly prior to 31 ka common central basin aulacoseiroid species are replaced by species found in the shallow well mixed southern basin surprisingly tychoplankton and periphyton were not common throughout prolonged lowstands but tended to increase in relative abundance during periods of inferred deeper lake environments a high resolution lake level reconstruction was generated by a principle component analysis of fossil diatom and wetsieved fossil and mineralogical residue records prior to 70 ka fossil assemblages suggest that the central basin was periodically a much shallower more saline and or alkaline well mixed environment the most significant reconstructed lowstands are 600 m below the modern lake level and span thousands of years these conditions contrast starkly with the deep dilute dysaerobic environments of the modern central basin after 70 ka our reconstruction indicates sustained deeper water environments were common marked by a few brief but significant lowstands high amplitude lake level fluctuations appear related to changes in insolation seismic reflection data and additional sediment cores recovered from the northern basin of lake malawi provide evidence that supports our reconstruction