forearc strike slip displacement as an alternative to subduction erosion with examples from mexico and california sinistral nacimiento fault slip on the nacimiento fault of the central coast ranges of california has been variably interpreted as dextral sinistral or reverse the currently prevailing interpretation is that the nacimient

geochronology stratigraphy and geochemistry of cindery tuff in pliocene hominid bearing sediments of the middle awash ethiopia cindery tuff is a subalkaline rhyolitic air fall deposit that was probably produced by a mixed magma eruption it is a distinctive datable regional isochronous marker bed within the pliocene sediments of the middle awash district and is stratigraphically situated between two new fossil hominid discoveries based on 40ar 39ar analyses of plagioclase rhyolitic glass and basaltic glass as well as fission track analyses of zircons we estimate its age to be 3 8 4 0 myr this implies that associated hominid skull fragments are at least 3 9 myr old

geochronology and distribution of silicic volcanic rocks of plio pleistocene age from the central sector of the main ethiopian rift abstract plio pleistocene silicic volcanoes calderas and their eruptive products are common throughout the main ethiopian rift mer a compllation of k ar and 40 ar 39 ar dating results for the central sector of the mer demonstrate that the most voluminous and widespread ignimbrites of this region are pliocene in age between 4 2 and 3 5 ma these units occur in and around 1 the awasa and wagebeta calderas 2 the guraghe and munesa escarpments where a major buried caldera is believed to occur in the adjacent rift floor 3 the addis ababa nazret area and 4 the foothills and shoulders of the northern sector of the main ethiopian rift the older units are overlain by less volumious and in most cases rift bound late pliocene and pleistocene pyroclastic rocks the pliocene ignimbrites in this area are likely to be the near source equivalents of widespread a ash fall units associated with fossil hominid deposits of the turkana and afar sedimentary basins

earth science lower mantle may be rich in silica there have been conflicting claims about the composition of earth s lower mantle the latest brillouin spectroscopy data suggest that this section of the planet s interior may contain more silica than the upper mantle see letter p 90

application and limitation of fission track geochronology to quarternary tephras abstract zircon and glass are the principal materials used for fission track dating of quaternary tephras this is because zircon is a common accessory mineral in rhyolitic tephras and it usually contains sufficient uranium to produce detectable quantities of spontaneous fission tracks over the time span of the quaternary volcanic glass though uranium poor is typically the dominant component of rhyolitic tephras comprising up to 99 of some distal tephra fall deposits low uranium contents however result in low spontaneous track densities that make the age measurement difficult and laborious a condition exacerbated by very young glasses in addition fission tracks in glass shards are unstable and can partially anneal at ambient temperatures therefore unless the glass age is corrected by plateau or isothermal heating experiments partial annealing causes a reduction in the measured age compared to the true age of the sample fission track dating has a large subjective component that can contribute systematic errors to the analysis microlite inclusions and some structural defects resemble fission tracks and if misidentified and included in the count of spontaneous tracks the calculated age would be too old the most prevalent source of systematic error is contamination in the natural environment for many tephras in the stratigraphic record are reworked tuffaceous sediments these deposits may contain detrital components that could mask the primary age of the tephra however by using the external detector fission track method it is possible to calculate ages of single mineral grains and to distinguish contaminants from primary phases on the basis of their age distribution this grain discrete capability is the major advantage of the fission track method over bulk geochronological systems

sound velocity of mgsio3 post perovskite phase a constraint on the d discontinuity abstract the discovery of a post perovskite phase transition in mgsio 3 has significant implications for seismological observations in the d region at the bottom of earth s mantle the d discontinuity which is manifested as a sharp positive seismic wave velocity jump 200 300 km above the core mantle boundary at pressure of 119 125 gpa is one of the most enigmatic seismic features in this region whether this velocity increase may be due to the formation of a post perovskite phase at the d discontinuity has not however been directly addressed by experiments here we present the results of aggregate sound velocity measurements of the mgsio 3 post perovskite phase by brillouin spectroscopy in the diamond anvil cell dac up to a pressure of 172 gpa in combination with infrared laser annealing of the sample based on these results and our recent high pressure velocity measurements on perovskite the aggregate shear wave velocity contrast across the perovskite to post perovskite phase transition is at most 0 5 this contrast is much smaller than typically observed across the d discontinuity indicating that the formation of an isotropic aggregate of the post perovskite phase provides an insufficient velocity increase to explain the d discontinuity lattice preferred orientation lpo of post perovskite is likely to be crucial for explaining the d discontinuity

single crystal elasticity of mg0 9fe0 1 o to 81 gpa ferropericlase is the second most abundant mineral in earth s lower mantle and knowledge of its elastic properties at relevant conditions is needed to adequately interpret seismic observations in terms of the mineralogy and thermal state of the deep earth here we report the complete elastic tensor of mg sub 0 9 fe sub 0 1 o ferropericlase to 81 2 gpa at room temperature measured by brillouin spectroscopy and x ray diffraction in the diamond anvil cell our data indicate that the spin transition of iron between 45 and 63 gpa dramatically affects the longitudinal and off diagonal elements of the elastic stiffness tensor c sub 11 and c sub 12 whereas it leaves the shear constant c sub 44 almost unaffected based on our results the spin transition markedly changes the pressure and temperature dependence of the compressional and bulk sound velocities and must be taken into account in any attempt to match average radial seismic velocity profiles with mineral physics observations the different pressure dependence of compressional and bulk and shear sound velocities across the high spin to low spin hs ls transition implies that the spin transition might be best observed in the v sub p v sub s ratio and its pressure and temperature derivative we also point out the possibility that the spin transitionmore leads to an anti correlated temperature dependence of shear and compressional wave speeds in certain pressure temperature regimes in earth s lower mantle less

mantle tomography and its relation to temperature and composition we propose a new method to constrain lateral variations of temperature and composition in the lower mantle from global tomographic models of shear and compressional wave speed we assume that the mantle consists of a mixture of perovskite and magnesio wustite in a first stage we directly invert vp and vs anomalies for variations of temperature and composition using the appropriate partial derivatives or sensitivities of velocities to temperature and composition however uncertainties in the tomographic models and in the sensitivities are such that variations in composition are completely unconstrained inferring deterministic distributions of temperature and composition being currently not possible we turn to a statistical approach which allows to infer several robust features comparison between synthetic and predicted ratios of the relative shear to compressional velocity anomalies indicates that the origin of seismic anomalies cannot be purely thermal but do not constrain the amplitude of the variations of temperature and composition we show that we can estimate these variations using histograms of the relative vp and vs anomalies at a given depth we computed histograms for a large variety of cases and found that at the bottom of the mantle variations in the volumic fraction of perovskite from 14 to 10 are essential to explain seismic tomography in the mid mantle anomalies of perovskite are not required but moderate variations up to 6 can explain the observed distributions equally well these trade offs between anomalies of temperature and composition cannot be resolved by relative velocity anomalies alone an accurate determination of temperature and composition requires the knowledge of density variations as well we show that anomalies of iron can then also be resolved 2003 elsevier b v all rights reserved

petrogenesis of a basalt rhyolite tephra from the west central afar ethiopia the cindery tuff is an unusual tephra fall deposit that contains evidence for the mixing of basaltic and rhyolitic liquids prior to eruption it contains clear rhyolitic glass shards together with brown basaltic glass spheres and a broadly bimodal phenocryst assemblage brown glasses are ferrobasaltic in composition and are similar to the voluminous pliocene tholeiites of the surrounding west central afar volcanic field both are enriched in the light rare earth and incompatible elements and possess higher 87sr 86sr and lower 143nd 144nd than morb rhyolitic glasses are subalkaline and compared to the basaltic glasses are strongly depleted in the compatible elements and enriched in the incompatible elements both glass types have similar incompatible element and isotopic ratios and with the rhyolite glass showing a 2 fold parallel enrichment in rare earth element abundances over the basaltic glass these observations suggest that the two glasses are genetically related

plio pleistocene volcanism and sedimentary facies changes at gadeb prehistoric site ethiopia volcanic ashes resulting from plinian type eruptions are regularly interbedded with fluvio lacustrine successions of the plio pleistocene upper webi shebele basin in the southeastern uplands of ethiopia pumiceous and vitric ashes of rhyolitic composition dominate interbedded with the lacustrine succession are also scoriaceous ashes of alkali olivine basalt composition and minor amounts of mixed scoria the majority of ashes interbedded with the lake sequence is primary air fall material that underwent only minor degrees of reworking within the overlying fluviatile sequence primary ashes decrease in abundance and crystalrich volcanoclastic deposits crystal tuffs take their place

ages of tuff beds at east african early hominid sites and sediments in the gulf of aden the early hominids of east africa were dated by determining the ages of tuff beds at the sites despite much research using palaeomagnetic and k ar dating techniques some of those ages are still controversial1 2 to obtain independent age estimates for these tephra layers we have examined cores from dsdp sites 231 and 232 in the gulf of aden fig 1a which consist mainly of calcareous nannofossil ooze but also contain rare tephra horizons3 dated by interpolation from the established nannofossil stratigraphy fig 1b chemical analysis confirms that the identity and sequence of these horizons is the same as that at the east african sites we conclude that the age of the tulu bor tuff is 3 4 myr and hence that the hadar hominid specimens are also 3 4 myr old

is low spin fe2 present in earth s mantle abstract recently x ray spectroscopy indicated that the low spin ls electronic configuration of fe 2 and fe 3 ions substituting in silicate perovskite of various compositions and also in magnesiowustite is stable at room temperature above pressures of 49 to 120 gpa j badro g fiquet f guyot j p rueff v v struzhkin g vanko g monaco iron partitioning in earth s mantle toward a deep lower mantle discontinuity science 300 2003 789 791 j badro j p rueff g vanko g monaco g figuet f guyot electronic transition in perovskite possible non convecting layers in the lower mantle science 305 2004 383 386 j li v v struzhkin h k mao j shu r j hemley y fei b mysen p dera v prakapenka g shen electronic spin state of iron in lower mantle perovskite proc natl acad sci 101 2004 14027 14030 j m jackson w sturhahn g shen j zhao m y hu d errandonea j d bass y fei a synchrotron mossbauer study of mg fe sio 3 perovskite up to 120 gpa am mineral 90 2005 199 205 j f lin v v struzhkin s d jacobsen m y hu p chow j kung h liu h k mao r j hemley spin transition of iron in magnesiowustite in the earth s lower mantle nature 436 2005 377 380 simple thermodynamic relationships combined with crystal field theory provide a minimum clausius clapeyron slope of 0 23 to 0 4 gpa k for the high spin hs to low spin transition of fe ions in perovskite consistent with lower mantle temperatures stabilizing the disordered hs state this computation of p t utilizes experimentally determined parameters only p and t at the transition and is supported by microstructural analysis of the change in volume and entropy considerations similarly p t min 0 18 to 0 31 gpa k for magnesiowustite high spin fe 2 should be stable in silicates throughout earth s mantle for compositions of fe mg fe 0 1 consistent with disorder prevailing at high temperature although partial conversion of octahedral fe 3 to ls may occur similarly partial conversion of octahedral fe 2 in magnesiowustite to the ls is possible for fe rich compositions which transform at relatively low pressures

sound velocity of mgsio3 perovskite to mbar pressure abstract brillouin scattering measurements of the aggregate shear wave velocities in mgsio 3 perovskite were conducted at high pressure conditions relevant to the earth s lowermost mantle approaching 1 mbar infrared laser annealing of samples in a diamond anvil cell allowed us to obtain high quality brillouin spectra and to drastically extend the upper limit of pressure for brillouin measurements we found that the pressure derivative of the shear modulus d g d p g of mgsio 3 perovskite is 1 56 0 04 which is distinctly lower than that of previous lower pressure experiments below 9 gpa extrapolation of the high pressure shear velocities of perovskite to ambient pressure are in excellent agreement with earlier room pressure brillouin measurements the shear modulus shear velocity and longitudinal velocity at ambient pressure were determined to be g 0 172 9 15 gpa v s 6 49 3 and v p 10 85 3 km sec the mineralogical model that provides a best fit to global seismological 1 d velocity profiles is one that contains 85 90 vol perovskite in the lower mantle the results of this study are the first to demonstrate that the elastic wave velocities for a near adiabatic lower mantle with a bulk composition dominated by magnesium silicate perovskite are consistent with the average lower mantle seismic velocity structure the large pressure range over which acoustic measurements of mgsio 3 perovskite performed in this study has thus allowed us to put tighter constraints on compositional models of the earth s lower mantle

revisions of k ar ages for the hadar hominid site ethiopia conventional k ar age measurements are reported for two volcanic units from the hominid bearing hadar formation these ages represent revision of previously published ages for the bkt 2 tephra and the kadada moumou basalt bkt 2 is revised from 2 65 to 2 9 myr bp and the kadada moumou basalt is revised from 3 0 to 3 6 myr bp

pressure and temperature dependence of elastic wave velocity of mgsio3 perovskite and the composition of the lower mantle abstract simultaneous measurements of compressional p and shear s wave velocities and unit cell volume density have been conducted on mgsio 3 perovskite to 9 2 gpa and 873 k using ultrasonic interferometry in conjunction with x ray diffraction finite strain analysis of current data allowed us to determine the elastic moduli and their pressure derivatives independent of the pressure measurement the results are k s 253 2 gpa g 173 1 gpa k s p 4 4 1 and g p 2 0 1 the temperature derivatives of the bulk and shear moduli were tightly constrained from acoustic velocity measurements yielding k s t 0 021 2 gpa k and g t 0 028 2 gpa k p and s wave velocities and density profiles for a pyrolite compositional model consisting of magnesium silicate perovskite and magnesiowustite are calculated at lower mantle depths along a 1600 k adiabatic geotherm the results agree with those of seismic model prem within 0 5 0 5 and 0 3 respectively from 800 to 2600 km the effect of fe and mg partitioning between magnesium silicate perovskite and magnesiowustite on the calculated velocities and density is negligibly small compared to the effects of composition and temperature simultaneous constraints from p and s wave velocities and density support for a homogeneous lower mantle with pyrolitic composition

orientation of logs in the taupo ignimbrite as an indicator of flow direction and vent position the taupo ignimbrite is an unwelded rhyolitic ignimbrite that originated about 1 820 yr ago from an eruptive vent within lake taupo in the taupo volcanic zone new zealand this ignimbrite is rich in carbonized plant matter including many logs the logs lie subhorizontally throughout all levels of the ignimbrite but are especially common in the lower half regionally the logs show a strong radial orientation indicating flow directions and position of the source vent log orientation is thus a reliable indicator of vent position

palynology stratigraphy and palaeoenvironment of a pliocene hominid site 2 9 3 3 m y at hadar ethiopia abstract the pliocene hadar formation 2 9 3 3 m y is well known for its abundant fossil fauna and hominid remains fossil pollen data obtained from 27 distinct stratigraphic layers are presented together with tectonic and sedimentary data comparison with modern pollen data from the awash basin 67 surface and fluviatile samples indicates that montane evergreen bushland presently known from above 1500 m occupied the hadar basin 500 m and was associated with a humid montane forest a downfaulting of about 1000 m presumably happened after 2 9 m y whereas a shift toward more arid grasslands occurred in the middle of the hadar formation

transrotation induced by crustal blocks moving through restraining double bends with southern california examples abstractwithin the san andreas system of southern california vertical axis clockwise rotations have been attributed to 1 capture of the monterey microplate during which oceanic lithosphere beneath north america was transferred to the pacific plate imparting basal traction and inducing northwestward motion of overlying north american crust and 2 torsional force couples imposed at the ends of rotating panels in contrast some crustal blocks that are smaller and or weaker than crust on opposite sides of faults have experienced clockwise transrotation as they have moved through restraining double bends 1 rapid 40 clockwise rotation of the eastern transverse ranges 6 0 ma starting with the southernmost parts and progressing northward 2 rapid 23 clockwise rotation of the morales formation in the cuyama valley 3 0 ma and 3 continuing clockwise rotation of the westernmost transverse ranges the magnitude of rotation matches the angle between the strikes of fault segments on either side of

elasticity of single crystal periclase at high pressure and temperature the effect of iron on the elasticity and seismic parameters of ferropericlase in the lower mantle

palaeoenvironmental and palaeobiogeographical significance of the fossil rodent assemblages of laetoli pliocene tanzania abstract the study of the laetoli rodent assemblages and comparison with other extant and fossil communities of africa by different statistical methods provide new palaeoenvironmental and palaeobiogeographical results firstly it is demonstrated that at 3 7 myr the laetoli assemblage belonged to a southern savanna domain and that its modern equivalent is the kalahari rodent fauna namibia south west africa secondly these analyses show that the rodent faunas from laetoli hadar omo member b and f ethiopia and olduvai bed i tanzania all of which are situated in the rift valley are clearly distinct assemblages each fauna is characterized by a strong endemism and seems to have evolved independently a tentative explanation of this observation is proposed by the examination of the structure of the rift valley and its evolution during plio pleistocene times moreover the different phases of speciation and migration of the rodent faunas can also be correlated with the tectonic events related to the formation of the rift valley

subduction related shoshonitic and ultrapotassic magmatism a study of siluro ordovician syenites from the scottish caledonides syenites are important or predominant components of several plutonic complexes emplaced between 456 and 415 ma along the nw margin of the caledonian orogenic belt adjacent to the lewisian foreland in w and nw scotland although there are in detail chemical differences between the syenites from each centre they form a well defined compositional group overall ratios amongst their trace elements especially very high values of la nb are quite different from those trachytes and syenites formed by fractional crystallisation of ocean island basalts and their continental equivalents emplaced in regions of anorogenic crustal tension instead the scottish caledonian syenites closely resemble chemically the fractional crystallisation residua of potassic subduction related magmas such as the shoshonitic series a comendite minor intrusion from a swarm associated with the loch borralan and loch ailsh syenitic complexes is remarkably similar in composition to recent obsidian from the shoshonitic volcano of lipari in the aeolian arc published sr and pb isotopic ratios preclude a significant component of either upper proterozoic moine schists or lower crust granulite facies archaean lewisian or proterozoic grenvillian gneisses in all these syenites except in local syenitic facies of the glenelg ratagain complex fractional crystallisation appears to be the mechanism by which the liquids which formed these syenites evolved from basic parental magmas the phases involved in this process may have included plagioclase alkali feldspar pyroxene amphibole biotite garnet fe ti oxide sphene allanite apatite zircon and zirconolite and therefore all the ratios amongst even the so called incompatible elements may have changed during the evolution of the leucocratic magmas nevertheless a detailed study of the glen dessarry complex shows that the changes are insufficient to disguise the geochemical nature of the parental magmas these appear to be picritic shoshonite mgo 15 ni 400ppm la yb 20 la nb 5 for the glen dessarry loch borralan and loch ailsh syenites and picritic ultrapotassic magmas mgo 15 ni 400 ppm la yb 60 la nb 7 such as would crystallise to minettes for the loch loyal and glenelg ratagain syenites mafic shoshonites were erupted amongst the 410 ma lome lavas of this region and also occur as widespread approximately contemporaneous volcanic feeder plugs minettes of similar age are also common as dykes in nw scotland numerous large tonalite granite complexes with minor diorites and gabbros were emplaced in w and nw scotland between 435 and 400 ma these are generally acknowledged to be mixtures of magmas from crustal and mantle sources their more mafic members show compositional features such as very high ba and sr which group them with the rare syenites and the lorne lavas of the same region into a distinctive geochemical province within which shoshonitic and allied magmas were the mantle derived component two models are presented to account for the generation of strongly potassic subduction related magmas several hundred km behind a nw directed subduction zone or during the period immediately following continental collision

volcanic ash beds recorders of upper cenozoic silicic pyroclastic volcanism in the western united states stratigraphic relationships and tephrochronologic information for volcanic ash beds 4 0 0 1 m y of the western interior allow the recognition of at least 68 different ash falls the average reoccurrence rate based on the succession of the isotopically dated ash beds is near one eruption every 57 000 years the ash beds were grouped into two chemical types rhyolitic and dacitic the main property that distinguishes dacitic from rhyolitic ashes is the larger ca and mg and smaller k content in dacitic ashes dacitic volcanic ashes were defined to have more than 0 55 wt ca rhyolitic ashes were subdivided further into two types w and g type based on the presence of biotite in the w type and its absence in g type w type rhyolitic ashes are chalky white have less than 0 55 wt fe have colorless glass shards of pumiceous habit g type rhyolitic ashes are light to medium gray have from 0 55 to 2 0 wt fe in the glass shards lack biotite have colorless platy bubble wall and bubble junction glass shards dacitic ashes are light gray to grayish brown have more than 0 55 wt fe and ca and have a complex mixture of colorless to light brown shards including pumiceous fibrous chunky and bubble wall and bubble junction types w type rhyolitic ashes possibly formed from magmas that were cooler 695 to 830 c and thus more viscous than the hotter 870 to 1000 c magma progenitors of g type ashes based on fe ti oxide mineral compositions primary microphenocrysts in the ash beds comprise assemblages of the following quartz sanidine plagioclase biotite amphibole clinopyroxene orthopyroxene fayalite zircon apatite allanite chevkinite sphene magnetite and ilmentite fayalite and chevkinite only have been found in g type biotite and sphene only have been found in w type rhyolitic ashes are highly evolved and contain 74 79 wt sio2 dacitic ashes vary from 67 77 wt sio2 al ranges from 10 5 to 14 5 wt al2o3 mg ranges from 0 06 to 1 0 wt mgo and fe as fe3o4 ranges from 0 50 to 4 1 wt dacitic ash beds have the most 0 80 3 8 wt as fe3o4 and rhyolitic ashes have the least 0 50 1 9 wt as fe3o4 ca varies from an unusually low amount 0 25 wt cao in a few rhyolitic ashes to as much as 2 2 in some dacitic ashes rhyolitic glasses show larger eu deficiencies compared to dacitic glasses on rare earth element diagrams w type rhyolitic glasses have moderately large la to sm ratios and have la contents from about 30 to 100 times chondrite values g type rhyolitic glasses also have moderately large la to sm ratios and contain somewhat more la than do the w type rhyolitic ashes generally from 100 to 300 times chondrites amounts

a perovskitic lower mantle inferred from high pressure high temperature sound velocity data determination of the shear wave velocities for silicate perovskite and ferropericlase under the pressure and temperature conditions of the deep lower mantle indicates that perovskite constitutes much more of the lower mantle than predicted by the conventional mantle model and is consistent with the chondritic earth model earth s upper mantle is apparently depleted of silicon when compared with meteorites which are thought to represent the material from which the earth formed this missing silicon problem has provoked intense debate it suggests that the deficit might be balanced by silicon in the core or that the upper mantle is not representative of the entire mantle and that the lower mantle is enriched in silicon murakami et al provide evidence to support the latter case they made laboratory measurements of sound wave transmission through silicate perovskite and ferropericlase minerals at pressures and temperatures matching those of the lower mantle and compared the resulting shear wave values with seismic data that sample the lower mantle they conclude that the mineralogical model that provides the best fit is one in which perovskite constitutes greater than 93 of the lower mantle this suggests that the lower mantle is enriched in silicon in comparison with the upper mantle consistent with the chondritic earth model and that there is limited mass transport between the upper and lower mantle the determination of the chemical composition of earth s lower mantle is a long standing challenge in earth science accurate knowledge of sound velocities in the lower mantle minerals under relevant high pressure high temperature conditions is essential in constraining the mineralogy and chemical composition using seismological observations1 but previous acoustic measurements were limited to a range of low pressures and temperatures here we determine the shear wave velocities for silicate perovskite and ferropericlase under the pressure and temperature conditions of the deep lower mantle using brillouin scattering spectroscopy2 the mineralogical model that provides the best fit to a global seismic velocity profile1 indicates that perovskite constitutes more than 93 per cent by volume of the lower mantle which is a much higher proportion than that predicted by the conventional peridotitic mantle model it suggests that the lower mantle is enriched in silicon relative to the upper mantle which is consistent with the chondritic earth model such chemical stratification implies layered mantle convection with limited mass transport between the upper and the lower mantle

elasticity of mgo to 130 gpa implications for lower mantle mineralogy abstract the aggregate shear wave velocities of mgo periclase have been determined throughout earth s lower mantle pressure regime approaching 130 gpa using brillouin spectroscopy in conjunction with synchrotron x ray diffraction technique in a diamond anvil cell apparatus we found that the extrapolations of the high pressure shear wave velocities and shear moduli to ambient pressure are highly consistent with earlier studies however the measurements over a wide pressure range revealed that the pressure derivative of the shear modulus d g d p g 0 of mgo is 1 92 2 which is distinctly lower than that of previous lower pressure experiments compared with the previous results on mg fe o ferropericlase there is no clear correlation between iron content and g 0 we calculate that the shear wave velocity profile of lower mantle along the adiabatic geotherm applied by the lower g 0 value of periclase can remarkably well reproduce the global seismological 1 d velocity profile model with uniform composition model the best fitting result indicates the possibility of a lower mantle mineralogy with 92 vol silicate perovskite phase implying that the bulk composition of lower mantle is likely not to be pyrolitic but more chondritic the present acoustic measurements performed over the large pressure range have thus led us to a better understanding of compositional model of the earth s lower mantle

paleoenvironments of the earliest stone toolmakers gona ethiopia fluvio lacustrine sediments of the hadar and busidima formations along the northern awash river ethiopia archive almost three million years 3 4 to 0 6 ma of human evolution including the earliest documented record of stone toolmaking at 2 5 2 6 ma this paper brings together sedimentologic and isotopic evidence for the paleoenvironmental context of early hominids from both formations but with particular emphasis on the setting for the early toolmakers the pre 2 92 ma record hadar formation is characterized by low gradient fl uvial paludal and lacustrine deposition in an undissected topography most analogous to reaches of the modern middle awash river near gewane the gona area experienced repeated deep dissection and aggradation by the awash river starting between 2 92 and ca 2 7 ma and continuing through the top of the record at 0 6 ma busidima formation each aggradational succession is 10 20 m in thickness and fi nes upward from wellrounded conglomerates at the base to capping paleosols at the top during this period the ancestral awash represented by these fi ning upward sequences was dominantly meandering and fl owed northeast as it does today smaller channels tributary to the axial awash system are also extensively exposed in the busidima formation compared to the axial system conglomerates the tributary channels transported fi ner less mature volcanic clasts mixed with abundant carbonate nodules reworked from adjacent badlands stone artifacts oldowan 2 6 2 0 ma at the oldest archaeological sites are only associated with the axial awash system in the bedded silts or capping paleosols of the fi ning upward sequences the implements were made from rounded cobbles from the channels but manufacture and use of the tools was always away from the channel bars on the nearby sandy banks and silt dominated fl oodplains archaeological sites higher in the record acheulian 1 7 ma occur in similar axial river contexts as well as along tributary channels further removed from artifact raw material sources mature paleosols in the hadar and busidima formations are mostly pale to darkbrown vertisols typifi ed by abundant clay slickensides pseudo anticlinal and vertical fracturing and carbonate nodules such calcic vertisols are common in the region today demonstrating that the paleoclimate over the past 3 4 m y has been semi arid and strongly seasonal carbon isotopic results from pedogenic carbonates in the vertisols allow reconstruction of the proportion of c 3 plants trees and shrubs to c 4 plants grasses through time the 13

a revised position for the rotated falkland islands microplate the early stages of transform margin formation are associated with crustal fragmentation and block rotation the restricted size of the resultant microcontinental blocks precludes palaeogeographical reconstructions and reliable estimations of the amount of rotation they can undergo an example considered here is the falkland plateau this is located adjacent to the agulhas falkland fracture zone and its westernmost province is the falkland islands microcontinent the position of the plateau and the islands prior to gondwana break up remains contentious this study integrates seismic reflection and gravity data to propose a revised position of the falkland islands microcontinent constrained by 1 the presence of a mega decollement controlling the gondwanide orogen described north of the falkland islands and underneath south africa and the outeniqua basin and 2 the similar architecture of fault networks mapped north of the islands and in the northernmost outeniqua basin this revised position requires a re evaluation of the timing and rate of rotation of the falkland islands microcontinent and affects the expected crustal architecture adjacent to the islands our model yields rotation rates between 5 5 and 8 ma 1 and two potential times for rotation and predicts more unstretched crust beneath the basin east of the falkland islands than previous models

elasticity of mg 0 83 fe 0 17 o ferropericlase at high pressure ultrasonic measurements in conjunction with x radiation techniques abstract the elasticity of ferropericlase with a potential mantle composition of mg 0 83 fe 0 17 o is determined using ultrasonic interferometry in conjunction with in situ x radiation techniques x ray diffraction and x radiography in a dia type cubic anvil high pressure apparatus to pressures of 9 gpa nacl pressure scale at room temperature in this study we demonstrate that it is possible to directly monitor the specimen length using an x ray image technique and show that these lengths are consistent with those derived from x ray diffraction data when no plastic deformation of the specimen occurs during the experiment by combining the ultrasonic and x ray diffraction data the adiabatic elastic bulk k s and shear g moduli and specimen volume can be measured simultaneously this enables pressure scale free measurements of the equation of state of the specimen using a parameterization such as the birch murnaghan equation of state the elastic moduli determined for mg 0 83 fe 0 17 o are k s0 165 5 12 gpa g 0 112 4 4 gpa and their pressure derivatives are k s0 4 17 20 and g 0 1 89 6 if these results are compared with those for mgo they demonstrate that k s0 and k s0 are insensitive to the addition of 17 mol feo but g 0 and g 0 are reduced by 14 and 24 respectively we calculate that the p and s wave velocities of a perovskite plus ferropericlase phase assemblage with a pyrolite composition at the top of the lower mantle 660 km depth are lowered by 0 8 and 2 3 respectively when compared with those calculated using the elastic properties of end member mgo consequently the magnitudes of the calculated wave velocity jumps across the 660 km discontinuity are reduced by about 11 for p wave and 20 for s wave if this discontinuity is considered as a phase transformation boundary only ringwoodite perovskite ferropericlase

sound velocity measurements of casio 3 perovskite to 133 gpa and implications for lowermost mantle seismic anomalies abstract we report the measurements of aggregate shear velocity v s of casio 3 perovskite capv at high pressure p between 32 and 133 gpa and room temperature t on the basis of brillouin spectroscopy the sample had a tetragonal perovskite structure throughout the experiments the measured p v s data show the shear modulus and its pressure derivative at ambient condition to be g 0 115 8 gpa and g 1 20 respectively the zero pressure shear velocity is determined to be v s0 5 23 km s in good agreement with the previous estimate inferred from the ultrasonic measurements on ca si ti o 3 perovskite at 1 bar our experimental results are broadly consistent with the earlier calculations on tetragonal capv but exhibit lower velocity at equivalent pressure such tetragonal capv is present in cold subducting slabs and possibly in wide areas of the lowermost mantle while primitive mantle includes certain amount of capv a depleted peridotite former harzburgite layer in subducted oceanic lithosphere is deficient in capv and enriched in ferropericlase in the lower mantle such harzburgite exhibits 0 9 faster v s and 0 7 slower bulk sound velocity v at the lowermost mantle p t conditions if capv is present in the tetragonal form in the surrounding mantle the observed fast v s and slow v anomalies in the d layer underneath the circum pacific region might be attributed in large part in the presence of subducted harzburgitic materials

first principles computation of mantle materials in crystalline and amorphous phases abstract first principles methods based on density functional theory are used extensively in the investigation of the behavior and properties of mantle materials over broad ranges of pressure temperature and composition that are relevant a review of computational results reported during the last couple of decades shows that essentially all properties including structure phase transition equation of state thermodynamics elasticity alloying conductivity defects interfaces diffusivity viscosity and melting have been calculated from first principles using mgo the second most abundant oxide of earth s mantle as a primary example and considering many other mantle materials in their crystalline and amorphous phases we have found that most properties are strongly pressure dependent sometimes varying non monotonically and anomalously with the effects of temperature being systematically suppressed with compression the overall agreement with the available experimental data is excellent it is remarkable that the early calculated results such as shear wave velocities of two key phases mgo and mgsio 3 perovskite were subsequently reproduced by experimentation covering almost the entire mantle pressure regime as covered in some detail the defect formation and migration enthalpies of key mantle materials increase with pressure the predicted trend is that partial mgo schottky defects are energetically most favorable in mg silicates but their formation enthalpies are high so the diffusion in the mantle is likely to be in the extrinsic regime preliminary results on mgo and forsterite hint that the grain boundaries can accommodate point defects including impurities and enhance diffusion rates at all pressures the structures are highly distorted in the close vicinity of the defects and at the interface with excess space recent simulations of mgo sio 2 binary and other silicate melts have found that the melt self diffusion and viscosity vary by several orders of magnitudes with pressure temperature and composition the predicted high compressibility and complex dynamical behavior can be associated with structural changes involving non bridging oxygen oxygen tri clusters si o pentahedra etc occurring on compression we envision future prospect for massively parallel distributed computing of unprecedented magnitude and scope in the study of relevant materials for earth super earth and other planets a renewed computational theme perhaps should be the first principles simulations of large systems with long runs that are necessary to explore realistic natural compositions polycrystalline phases multi component melts crystal melt interfaces trace element partitioning etc

faunal age of the usno shungura b and hadar formations ethiopia recent work by brown1 correlating the sidi hakoma or sidiha koma2 tuff at hadar with tuffs b and u 10 of the shungura and usno formations respectively reverses a recent finding3 indicating an age of the hadar formation fauna of 2 9 to greater than 3 6 myr bp the difference in these determinations is 0 4 myr a significant period of evolutionary time we compare the faunas from shungura unit b10 usno and hadar and discuss the implications for interpretation of the early hominid fossil record

interpretations of the magnetostratigraphy of the hadar hominid site ethiopia as part of an investigation to determine the age of the hominidbearing strata of the hadar formation of ethiopia we have determined the magnetostratigraphy of two separate sections of the strata two major zones of reversed polarity occur within the predominantly normally magnetized sequence one at the top and the other in the middle there are difficulties with the absolute age determinations of volcanic units within the strata which result in uncertainties in correlating the hadar magnetostratigraphy with the standard magnetic reversal time scale the age of the upper part of the section is established by the bkt 2 tuff at 2 8 myr refs 1 4 the k ar dating of the kadada moumou basalt which lies in the middle of the sequence yields variable results ranging from 3 1 to 3 6 myr refs 1 4 regional geochemical5 and bio stratigraphical6 studies suggest the younger age for the kadada moumou basalt we show here that this is also the age that results in the most consistent interpretation of the observed magneto stratigraphy of the formation which is that the hadar formation was deposited during the gauss normal epoch and that the two reversals represent the kaena and mammoth events within that epoch

the world s oldest stone artefacts from gona ethiopia their implications for understanding stone technology and patterns of human evolution between 2 6 1 5 million years ago the systematic archaeological and geological survey and excavations at gona between 1992 1994 led to the discovery of well flaked stone artefacts which are currently the oldest known from anywhere in the world more than 3000 surface and excavated artefacts were recovered at 15 localities documented east and west of the kada gona river based on radioisotopic dating 40 ar 39 ar and magnetostratigraphy the artefacts are dated between 2 6 2 5 million years ago ma eg10 and eg12 from east gona are the most informative with the highest density providing the best opportunity for characterizing the oldest assemblages and for understanding the stone working capability of the earliest tool makers slightly younger artefact occurrences dated to 2 4 2 3 ma are known from hadar and omo in ethiopia and from lokalalei in kenya cut marked bones dated to 2 5 ma from bouri in ethiopia are now providing important clues on the function of these artefacts in addition australopithecus garhi known from contemporary deposits at bouri may be the best candidate responsible for the oldest artefacts surprisingly the makers of the gona artefacts had a sophisticated understanding of stone fracture mechanics and control similar to what is observed for oldowan assemblages dated between 2 0 1 5 ma this observation was corroborated by the recent archaeological discoveries made at lokalalei because of the similarities seen in the techniques of artefact manufacture during the late pliocene early pleistocene it is argued here that the stone assemblages dated between 2 6 1 5 ma group into the oldowan industry the similarity and simplicity of the artefacts from this time interval suggests a technological stasis in the oldowan 2000 academic press

age of the salmon springs glaciation in washington a tephra bed within the salmon springs drift at its type locality near sumner washington has a zircon fission track age of 0 84 0 21 m y and the immediately overlying silts are reversely magnetized the same distinctive tephra at auburn washington has a glass fission track age of 0 66 0 04 m y and a zircon age of 0 87 0 27 m y the bracketing silts are likewise reversely magnetized therefore the salmon springs drift as defined at its type section is of middle quaternary age and is much older than the currently accepted age of about 70 000 yr

magnetostratigraphy of the koobi fora formation lake turkana kenya the koobi fora formation a pliocene and pleistocene sequence of sedimentary deposits northeast of lake turkana has yielded numerous fossils and stone artifacts of early hominids stratigraphic correlation of the hominid bearing deposits throughout the turkana region was established primarily by the chemistry and isotopic ages of volcanic tuffs and complemented by magnetostratigraphic studies we have reinterpreted previously published magnetostratigraphy from the upper part of the koobi fora formation because the original stratigraphy and dating of tuffs have been revised in our reinterpretation we include previously unpublished data from the uppermost part of the formation the upper magnetozones correlate with parts of the brunhes normal polarity chron and matuyama reversed polarity chron about 0 6 0 85 ma and are separated from the magnetozones of the upper part of the matuyama 2 0 1 25 ma by a disconformity the olduvai normal polarity subchron is represented within the matuyama but the lower part of the matuyama 2 4 2 0 ma is missing due to an erosional disconformity we have also determined magnetozones in the lower part of the koobi fora formation which had not been sampled for paleomagnetism during the earlier studies our time calibration of the magnetozones is made possible by isotopic dating of several tuffs and by chemical correlation of koobi fora tuffs with dated tuffs in the shungura formation of southern ethiopia the tephra correlations are corroborated by the excellent concordance between the magnetostratigraphies of the koobi fora and shungura formations the lower part of the koobi fora spans the interval from about 4 ma to 2 4 ma within the gilbert reversed polarity and gauss normal polarity chrons rock magnetic studies indicate that detrital magnetite carries most of the stable remanence although hematite contributes to the remanence as indicated by thermal demagnetization the hematite which presumably formed by postdepositional oxidation of original iron oxide grains carries chemical remanent magnetization crm that diminishes the sharpness of polarity zone boundaries the crm accumulated continuously within the uppermost 10 m of sediment below the land surface so that the crm reinforces the depositional remanent magnetization within thick magnetozones but obscures magnetozones having durations of roughly less than 70 000 years in sections where the sedimentation rate was approximately 15 cm 1000 years

fossil elephantoids awash paleolake basins and the afar triple junction ethiopia abstract the most diverse collection of fossil elephantoids from a single area are contained in the 1 km thick hominid bearing awash group these deposits range from late miocene to holocene in age and are found in the awash valley of the afar depression ethiopia uniquely the afar elephantoids inhabited a series of internal lake basins splayed out across an evolving subaerial triple junction created by the separation of the african east african and arabian plates the spatial and temporal distribution of elephantoids in these basins demonstrates that these animals were progressively drawn into the central afar with the divergence of the three plates as such the elephantoids moved from the higher margins of the east african and african plates to the depressed lowlands of the triple junction where subsiding and migrating lake basins served as ideal habitats for large herbivores with high water requirements this pattern of migration serves as a model for the migration of animals into intercontinental areas and for their dispersal across plate boundaries

new data from hadar ethiopia support orbitally tuned time scale to 3 3 ma support for the astronomically calibrated geomagnetic polarity time scale is provided by new 40ar 39ar dating and magnetostratigraphic data from the hadar formation ethiopia three tuffs and a basalt flow at hadar span the mammoth subchron of the gauss chron and range in age from 3 18 0 01 to 3 40 0 03 ma interpolation of sediment accumulation rates in two magnetostratigraphic sections yields an estimated age range of 3 30 0 02 to 3 21 0 01 ma for the mammoth subchron closely coinciding with orbitally based predictions of 3 33 to 3 22 ma rather than with previous estimates of 3 15 to 3 05 ma

tephra sources and correlations in ethiopia application of elemental and neodymium isotope data abstract combined elemental and neodymium nd isotope data for proximal and distal pyroclastic deposits from the ethiopian volcanic province illustrate the utility of these parameters for establishing potential source regions for distal tephra and for correlation of widely separated distal deposits in the ethiopian volcanic province nd isotopic compositions and to some extent elemental characteristics of volcanic products from distinct geographic tectonic sub provinces allow first order indentification of geochemical domains these domains are used to target potential source regions for the numerous distal tephra deposits exposed within the fossiliferous sedimentary sequences of the afar and omo turkana basins pliocene distal deposits from the hadar and middle awash regions of afar ethiopia and the omo turkana region of southern ethiopia northern kenya are examined combined elemental and nd isotope data are consistent with derivation of these tephra from sources within or adjacent to the central sector of the main ethiopian rift or within the central to northern afar the data presented in this paper also point to the need for a better understanding of the physical and chemical characteristics of the eruptions giving rise to many of the distal deposits until information regarding the range of chemical and isotopic variability in these magmatic systems is available it is difficult to link a specific set of geochemical characteristics to one particular eruption or vent