relative array analysis of upper mantle lateral velocity variations in southern california spatial averaging of short period teleseismic p wave arrivals across scarlet constrains upper mantle lateral velocity variations beneath southern california we compare dt d back azimuth and averaged arrival time estimates determined from the entire network for 154 events to the same parameters derived from small subsets of scarlet the resulting relative array diagrams and plots of net subarray delays computed for 171 overlapping subarrays exhibit large anomalies that vary smoothly as a function of subarray position implying substantial lateral heterogeneity under the network patterns of mislocation vectors for more than 100 subarrays permit identification of an east west striking high velocity antiform beneath the transverse ranges further constraints on both the lateral configuration and depth of the anomaly are obtained through thin lens analysis of the net delay data where all the time perturbations are assumed to originate at a single depth projection of 2798 net delays requires a lens deeper than 100 km a representative lens at 150 km depth features low velocities beneath the salton trough as well as a concentrated high velocity zone between 116 3 118 3 w and 33 7 34 8 n consistent with the relative array diagrams and the pkp delay contours of hadley and kanamori 1977 therefore the bulk of the high velocity material observed by hadley and kanamori may in fact lie at or deeper than 150 km rather than 40 100 km this possibility prompts renewed speculation regarding the relationship of this body to the plate tectonic history of the region

sedimentological and palaeoenvironmental study from waregi hill in the hiwegi formation early miocene on rusinga island lake victoria kenya

new oligocene early miocene microflora from the southwestern turkana basin palaeoenvironmental implications in the northern kenya rift we report five oligocene early miocene pollen assemblages from the loperot 1 exploration well drilled in the semi desert lokichar basin latitude 02 21 46 15 n longitude 35 52 23 47 e ground elevation 615 m above msl near lake turkana in northern kenya they represent the second oldest plant microfossils so far recovered from east africa and add significantly to the paleogene neogene tropical african fossil plant record the loperot pollen indicate a mosaic environment of semi deciduous forest and humid woodland whose floristic composition presents strong affinities with the vegetation occurring today in the guinea congolia zambezia phytogeographical transition zone with a rainfall more than 1000 mm year and a well defined dry season the weak representation of poaceae and herbaceous taxa characteristic of grassland dry bushland or savanna and the abundance of shade tolerant plants such as ferns all point to a vegetation composed to a variety of communities with closed forest formations predominant the lack of typical temperate mountains elements mainly podocarpus and juniperus today widespread on the east african highlands indicates that the geography of the region was different from that of today the plateaux or uplands adjacent to the lokichar basin were probably still not high enough during this period of early rifting in east africa to support the temperate coniferous forests characteristic of the plio pleistocene

spatio temporal trends in normal fault segmentation recorded by low temperature thermochronology livingstone fault scarp malawi rift east african rift system the evolution of through going normal fault arrays from initial nucleation to growth and subsequent interaction and mechanical linkage is well documented in many extensional provinces over time these processes lead to predictable spatial and temporal variations in the amount and rate of displacement accumulated along strike of individual fault segments which should be manifested in the patterns of footwall exhumation here we investigate the along strike and vertical distribution of low temperature apatite u th he ahe cooling ages along the bounding fault system the livingstone fault of the karonga basin of the northern malawi rift the fault evolution and linkage from rift initiation to the present day has been previously constrained through investigations of the hanging wall basin fill the new cooling ages from the footwall of the livingstone fault can be related to the adjacent depocentre evolution and across a relay zone between two palaeo fault segments our data are complimented by published apatite fission track aft data and reveal significant variation in rock cooling history along strike the centre of the footwall yields younger cooling ages than the former tips of earlier fault segments that are now linked this suggests that low temperature thermochronology can detect fault interactions along strike that these former segment boundaries are preserved within exhumed footwall rocks is a function of the relatively recent linkage of the system our study highlights that changes in ahe and potentially aft ages associated with the along strike displacement profile can occur over relatively short horizontal distances of a few kilometres this is fundamentally important in the assessment of the vertical cooling history of footwalls in extensional systems temporal differences in the rate of tectonically driven exhumation at a given location along fault strike may be of greater importance in controlling changes in rates of vertical exhumation than commonly invoked climatic fluctuations

pn velocity anisotropy in southern california we analyze p n propagation as a function of azimuth across a 28 station 150 km aperture subarray of the scarlet network centered near the central transverse ranges california we selected signals from 81 earthquakes and explosions with epicentral distances ranging from 150 to 400 km covering all azimuths except a 40 gap from the southwest and a lesser gap from the northeast direction for each source the apparent velocity of p n was determined using a one norm measure of misfit the apparent p n velocity does not show any systematic variation with epicentral distance but exhibits a strong azimuthal dependence our preferred interpretation calls for a slightly dipping 2 to n40w planar moho with 3 to 4 per cent anisotropy of subcrustal material transverse isotropy with a nearly horizontal symmetry axis is sufficient to explain the data the direction of sagittal symmetry is n50w the isotropic velocity of p n is 7 8 km sec in contrast a higher 8 1 km sec p n velocity is found in the mojave block with no indication of anisotropy these observations are consistent with a subcrustal model of the pacific north america plate boundary where ductile flow is characterized by simple shear in a vertical plane with strike parallel to the direction of relative plate motion

tectonic uplift and eastern africa aridification the history of eastern african hominids has been linked to a progressive increase of open grassland during the past 8 million years this trend was explained by global climatic processes which do not account for the massive uplift of eastern african topography that occurred during this period atmosphere and biosphere simulations quantify the role played by these tectonic events the reduced topographic barrier before 8 million years ago permitted a zonal circulation with associated moisture transport and strong precipitation our results suggest that the uplift itself led to a drastic reorganization of atmospheric circulation engendering the strong aridification and paleoenvironmental changes suggested by the data

accelerated contribution of the paleo congo river to global seawater 87 sr 86 sr change following eocene oligocene collapse of the african surface

digital processing of regional network data abstract much more than hypocenter location can be done with digital time histories obtained from regional seismic networks examples are given of how these data can be routinely processed to provide readily accessible intermediate results for subsequent studies among which are velocity inversion using teleseismic p residuals composite focal mechanism studies and spectral analysis the processing must be designed to be as routine and as complete as possible only with these two objectives achieved can the seismic networks be as productive as they should be

the lamu basin deepwater fold and thrust belt an example of a margin scale gravity driven thrust belt along the continental passive margin of east africa in recent decades advances in seismic processing and acquisition of new data sets have revealed the presence of many deepwater fold and thrust belts dw ftbs often developing along continental passive margins these kinds of tectonic features have been intensively studied due to their substantial interest this work presents a regional scale study of the poorly explored lamu basin dw ftb a margin scale gravity driven system extending for more than 450 km along the continental passive margin of kenya and southern somalia east africa a 2 d seismic data set was analyzed consisting of both recently acquired high quality data and old reprocessed seismic profiles for the first detailed structural and stratigraphic interpretation of this dw ftb the system originated over an early to mid cretaceous shale detachment due to a mainly gravity spreading mechanism analysis of synkinematic strata indicates that the dw ftb was active from the late cretaceous to the early miocene but almost all of the deformation occurred before the late paleocene the fold and thrust system displays a marked n s variation in width the northern portion being more than 150 km wide and the southern portion only a few dozen kilometers wide this along strike variation is thought to be related to the complex tectonosedimentary evolution of the continental margin at the somalia kenya boundary also reflected in the present day bathymetry locally a series of volcanic edifices stopped the basinward propagation of the dw ftb a landward change in the dominant structural style from asymmetric imbricate thrust sheets to pseudo symmetric detachment folds is generally observed related to the landward thickening of the detached shales

three dimensional velocity variations and their relation to the structure and tectonic evolution of the new madrid seismic zone ninety five teleseisms and 270 local earthquakes are used to determine three dimensional velocity models of the crust and upper mantle beneath the new madrid seismic zone p wave travel time residuals from teleseisms yield a three dimensional model of velocity perturbations in the crust and upper mantle to a depth of 156 km travel time residuals of local earthquakes lead to an upper crustal model which is consistent with the model for that region obtained from teleseismic data and provide details which can be related to geological structure the local earthquake data also permit the resolution of tradeoffs between the upper and lower crust which may result from the analysis of teleseismic data only lateral variations in velocity occur both as broad continuous or nearly continuous bands and as small scale localized features a band of anomalous velocities correlates in position and orientation with a basement rift system which has been delineated from gravity and magnetic data this band occurs as a low velocity zone through the upper crust and uppermost mantle and may be offset beneath the branch of the new madrid seismic zone having the most intense earthquake activity after trade offs with upper crustal velocities are removed the lower crust exhibits somewhat higher than average velocities in the rift region several localized regions of high velocity in the crust inferred from teleseismic data correlate with the sites of intrusions found from gravity and magnetic data a number of localized regions of both high and low velocities occur throughout the upper mantle with no clear cut relation to near surface features

locations and spectral properties of earthquakes accompanying an eruption of mount st helens a joint hypocenter determination routine has been developed to accurately determine the locations of earthquakes occurring under mount st helens the method has a special application to mount st helens where most of the earthquakes are suspected to occur at shallow depth in a rather limited volume under the volcano differences in elevation among stations are allowed for in this routine the location procedure determines a best half space velocity and one station correction for each station simultaneously with earthquake hypocenters arrival time data from an array of portable digital seismic event recorders are supplemented with data from stations of the permanent university of washington seismic network operating at the volcano earthquakes are found to occur along a line trending south east of the dome inside the crater the shallowest events occur near the dome with hypocentral depth increasing with increasing distance from the dome station corrections can be interpreted to imply the existence of a high velocity body beneath the crater which extends at least between depths of 5 and 2 1 km beneath the crater floor spectra are calculated from earthquake waveforms recorded at three stations the spectra show very little evidence for local site or path effects on frequency content of waveforms stress drops of earthquakes are calculated from measurements of corner frequency and low frequency spectral amplitude stress drops of earthquakes vary between 5 and 5 bars there is no evidence for a limiting corner frequency for small earthquakes as has been found for earthquakes occurring in mammoth lakes california data fit well a linear trend between log seismic moment and log source radius fit to data for earthquakes ranging in size from 1014 to 1027 dyn cm suggesting that there is no limiting source radius observed for earthquakes in this range of size

northeast african vegetation change over 12 m y intense debate surrounds the evolution of grasses using the c 4 hatch slack photosynthesis pathway and the emergence of african grasslands often assumed to be one and the same here we bring new insights with the combination of plant leaf wax carbon isotopic composition 13 c wax and pollen data from marine sediments of the gulf of aden northeast africa which show that c 4 biomass increases were not necessarily associated with regional grassland expansion we find broadly opposing trends toward more enriched 13 c wax values and decreased grass pollen proportions between 12 and 1 4 ma this apparently contradictory evidence can be reconciled if a greater proportion of the late miocene northeast african landscape were covered by c 3 grasses than previously thought such that c 4 grasses and shrubs replaced a c 3 ecosystem including trees and productive grasslands in addition 13 c wax and pollen both indicate that true rainforests were unlikely to have been extensive in northeast africa at any time in the last 12 m y although seasonally dry forests were a significant component of the regional landscape since the late miocene here we extend regionally integrative marine archives of terrestrial vegetation back to 12 ma and we evaluate them in the context of an updated compilation of pedogenic carbonate 13 c values from east african rift strata we identify two distinct phases of increasing c 4 biomass between 11 and 9 ma with a reversal by 4 3 ma and then a re expansion between 4 3 and 1 4 ma surprisingly neither was associated with grassland expansion

stratigraphy depositional history and environments of deposition of cretaceous through tertiary strata in the lamu basin southeast kenya and implications for reservoirs for hydrocarbon exploration abstract the lamu basin which is characterized by an extensional tectonic style is the failed arm of a tri radial rift system and possesses tectonic and stratigraphic elements in the post rift series that are analogous to those of the west coast of africa development of the southern part of the basin as a passive margin is closely related to considerations of the pre drift position of madagascar and formation of the indian ocean basin during mesozoic time cretaceous and tertiary strata in the basin comprise an eastward thickening gross succession of sediments on which eustatic sea level fluctuations and a sequence of unconformities related to pulses of transgressive and regressive depositional trends are superimposed recognition of these trends has provided the basis for classification of the strata into megasequences representing distinct provinces with regard to time stratigraphy sedimentation tectonics depositional environments and hydrocarbon potential megasequence i karoo group and jurassic includes strata of the permian through jurassic the discussion of which is beyond the scope of this paper megasequence ii sabaki group includes strata of the cretaceous and early paleocene deposited in tide influenced shelf and marine settings these are in ascending order the ewaso sands the walu shale the hagarso limestone the freretown limestone and the kofia sands megasequence iii tana group includes strata of the eocene through oligocene which were deposited in fluvial deltaic and restricted shelf settings these are in ascending order the kipini formation the pate limestone the linderina limestone the dodori limestone and the barren beds formation megasequence iv coastal group includes strata of the miocene through pliocene which were deposited in restricted shelf middle to outer shelf deep marine and fluvial settings these are in ascending order the baratumu formation the lamu reefs the simba shales and the marafa formation determination and description of these lithostratigraphic units their distribution and palaeohistory have been imperative for predicting reservoirs for hydrocarbon exploration

source parameters of seismic events at heinrich robert mine ruhr basin federal republic of germany evidence for nondouble couple events abstract a three component digital seismic station is in operation at heinrich robert coal mine in the ruhr basin at a depth of 890 m a cluster of seismic events occurred at the beginning of 1987 at a distance of about 5 km from the station and continued in 1988 source parameters of 46 tremors selected from the cluster with local magnitude from 1 2 to 2 3 are studied the ratio of p wave to s wave corner frequency is variable implying its dependence on the angle of observation and in turn the variability of focal mechanism with time the scaling relations indicate that characteristic tremors with a fault length of about 200 m are possibly generated in the area but the evidence is not conclusive the seismic energy for a given seismic moment can vary by a factor of 20 and the ratio of s wave to p wave energy ranges from 1 5 to 30 stress release estimates show low values not exceeding a few bars typical for tremors induced by mining a comparison of the apparent stress with the static stress drop divides the selected tremors into high and low apparent stress events the latter events are characterized by high p wave energy and almost constant and large ratio of p wave to s wave corner frequency which differs from the lower ratios of other events implying similar focal mechanism these high p wave energy tremors are the most likely candidates for nondouble couple events

a new species of africanacetus odontoceti ziphiidae found on the deep ocean floor off the coast of brazil abstract a fossil skull of a beaked whale was newly collected by a manned submersible from the sao paulo ridge of the atlantic floor about 2900 m deep off the coast of brazil it was found in the middle of the manganese nodules on the seafloor the whale fossil can be dated sometime between the middle miocene and the early pliocene based on the timing of the onset of the tectonic movements of the sao paulo ridge whence the specimen came and the precipitation of the manganese layers which entirely encrusted the specimen this is the first instance of a fossil beaked whale skull retrieved from such great depths this skull belongs to a new species of africanacetus which may be a genus with a broad distribution in southern oceans in neogene times expands the geographic distribution of the genus as the northernmost record and adds the taxonomic and morphological diversity within the genus

from source to sink in central gondwana exhumation of the precambrian basement rocks of tanzania and sediment accumulation in the adjacent congo basin apatite fission track aft and u th he ahe thermochronometry data are reported and used to unravel the exhumation history of crystalline basement rocks from the elevated 1000 m a s l but low relief tanzanian craton coeval episodes of sedimentation documented within adjacent paleozoic to mesozoic basins of southern tanzania and the congo basin of the democratic republic of congo drc indicate that most of the cooling in the basement rocks in tanzania was linked to erosion basement samples were from an exploration borehole located within the craton and up to 2200 m below surface surface samples were also analysed aft dates range between 317 33 ma and 188 44 ma alpha ft corrected ahe dates are between 433 24 ma and 154 20 ma modelling of the data reveals two important periods of cooling within the craton one during the carboniferous triassic 340 220 ma and a later less well constrained episode during the late cretaceous the later exhumation is well detected proximal to the east african rift 70 ma thermal histories combined with the estimated geothermal gradient of 9 c km constrained by the aft and ahe data from the craton and a mean surface temperature of 20 c indicate removal of up to 9 2 km of overburden since the end paleozoic the correlation of erosion of the craton and sedimentation and subsidence within the congo basin in the paleozoic may indicate regional flexural geodynamics of the lithosphere due to lithosphere buckling induced by far field compressional tectonic processes and thereafter through deep mantle upwelling and epeirogeny tectonic processes

multi mode analysis of rayleigh type lg part 2 application to southern california and the northwestern sierra nevada the uc diagram technique described in the companion paper part 1 is applied to nine sets of lg phases recorded through the cedar system in southern california and two sets of lg phases recorded along the northwestern margin of the sierra nevada a clear image of the signal is obtained in time frequency wavenumber space and we discuss in particular observations at 2 5 sec period for events 200 to 300 km outside the profiles from the gross features of uc diagrams we conclude that a representation of lg as a single coherent multi mode wave train is oversimplified in the case of southern california but is more appropriate for the sierra block in southern california peaks observed at group velocities smaller than 3 2 km sec are not predicted by realistic crustal models of the area and are probably due to lateral heterogeneities effects such as mode conversion and multipathing on the other hand for group velocities between 3 2 and 3 6 km sec peaks observed in either area can generally be interpreted in terms of overtones excited at the source and propagating through spatially averaged structures although care must be taken to monitor the stability of the algorithm on actual short period records

kinematic evolution of a regional scale gravity driven deepwater fold and thrust belt the lamu basin case history east africa the deepwater fold and thrust belts dwftbs are geological structures recently explored thanks to advances in offshore seismic imaging by oil industry in this study we present a kinematic analysi

modelling vegetation change during late cenozoic uplift of the east african plateaus abstract the present day vegetation in the tropics is mainly characterized by forests worldwide except in tropical east africa where forests only occur as patches at the coast and in the uplands these forest patches result from the peculiar aridity that is linked to the uplift of the region during the late cenozoic the late cenozoic vegetation history of east africa is of particular interest as it has set the scene for the contemporary events in mammal and hominin evolution in this study we investigate the conditions under which these forest patches could have been connected and a previous continuous forest belt could have extended and fragmented we apply a dynamic vegetation model with a set of climatic scenarios in which we systematically alter the present day environmental conditions such that they would be more favourable for a continuous forest belt in tropical east africa we consider varying environmental factors namely temperature precipitation and atmospheric co 2 concentrations our results show that all of these variables play a significant role in supporting the forest biomes and a continuous forest belt could have occurred under certain combinations of these settings with our current knowledge of the palaeoenvironmental history of east africa it is likely that the region hosted these conditions during the late cenozoic recent improvements on environmental hypotheses of hominin evolution highlight the role of periods of short and extreme climate variability during the late cenozoic specific to east africa in driving evolution our results elucidate how the forest biomes of east africa can appear and disappear under fluctuating environmental conditions and demonstrate how this climate variability might be recognized on the biosphere level

simultaneous inversion for hypocenters and lateral velocity variations an iterative solution with a layered model an iterative inversion technique has been developed that uses the direct p and s wave arrival times from local earthquakes to compute simultaneously a three dimensional velocity structure and relocated hypocenters crustal structure is modeled by subdividing flat layers into rectangular blocks an interpolation function is used to smoothly vary velocities between blocks allowing ray trace calculations of travel times in a three dimensional medium tests using synthetic data from known models show that solutions are reasonably independent of block size and spatial distribution but are sensitive to the choice of layer thicknesses application of the technique to observed earthquake data from north central utah shows the following 1 lateral velocity variations in the crust as large as 7 occur over 30 km distances 2 earthquake epicenters computed with the three dimensional velocity structure were shifted an average of 3 0 km from locations determined assuming homogeneous flat layered models and 3 the laterally varying velocity structure correlates with anomalous variations in the local gravity and aeromagnetic fields suggesting that the new velocity information can be valuable in acquiring a better understanding of crustal structure

wallrock alteration at the western system of the csa mine cobar new south wales australia abstract petrological and geochemical characteristics of the sequence from chloritic siltstone through siliceous siltstone to elvan which occurs approaching ore on the hanging wall of the western system in the csa mine cobar indicate its formation by extensive dilational silicification together with minor addition of s and k very close to the ore the manner whereby more chloritic members of this sequence developed from background muscovite rich siltstones remote from mineralization is less clear elvan is not an exhalative chert the distribution of wallrock alteration products and characteristics of the ores do not support suggestions that cobar orebodies are remobilized stratiform deposits alteration preceded epigenetic ore emplacement in the early stages of a regional metamorphic episode which took place soon after early devonian turbidite sedimentation without associated igneous activity mineralization is attributed to diagenetic dewatering or metamorphic dehydration of the sediment pile

a 17 my old whale constrains onset of uplift and climate change in east africa timing and magnitude of surface uplift are key to understanding the impact of crustal deformation and topographic growth on atmospheric circulation environmental conditions and surface processes uplift of the east african plateau is linked to mantle processes but paleoaltimetry data are too scarce to constrain plateau evolution and subsequent vertical motions associated with rifting here we assess the paleotopographic implications of a beaked whale fossil ziphiidae from the turkana region of kenya found 740 km inland from the present day coastline of the indian ocean at an elevation of 620 m the specimen is 17 my old and represents the oldest derived beaked whale known consistent with molecular estimates of the emergence of modern strap toothed whales mesoplodon the whale traveled from the indian ocean inland along an eastward directed drainage system controlled by the cretaceous anza graben and was stranded slightly above sea level surface uplift from near sea level coincides with paleoclimatic change from a humid environment to highly variable and much drier conditions which altered biotic communities and drove evolution in east africa including that of primates

isotopic dating of basin inversion the palaeozoic cobar basin lachlan orogen australia abstract the age of inversion of the early devonian cobar basin is poorly defined by field geological data which have to be extrapolated from the coeval shelf west of the basin in order to better constrain the age of deformation a whole rock isotopic dating program was undertaken and yields the following results rb sr isotopic dates predate the depositional age giving a regional isochron of 442 4 ma three k ar dates from zone 1 are mutually indistinguishable 402 8 402 8 405 8 ma suggesting a mid to late early devonian age of cleavage formation four whole rock 40ar 39ar spectra from the high strain zone 1 give intermediate temperature plateaux ranging from ca 389 ma to 404 ma in the lower strain zone 2 total gas ages from three samples range from ca 401 ma to ca 423 ma an approximate intermediate temperature plateau for one sample indicates an age ca 405 ma the other sample yields a good plateau of ca 437 ma these data are interpreted as reflecting a low grade cleavage forming inversion event which took place at ca 395 400 ma isotopic data from zone 1 suggest near complete rejuvenation of detrital phases during the 395 400 ma event in contrast the older zone 2 rb sr ages and some of the 40ar 39ar ages reflect at least in part basement provenance ages preserved within detrital micas and feldspars which did not undergo complete rejuvenation because of lower deformational strain and or temperature of metamorphism the intense 395 400 ma late early devonian inversion event in zones 1 and 2 of the former cobar basin largely bypassed the southwestern part of the basin and the adjacent shelf to the west these only underwent minor block faulting and uplift at this time but were more strongly deformed along with the overlying fluviatile mulga downs group in the carboniferous

hydrocarbon potential of the meso cenozoic turkana depression northern kenya ii source rocks quality maturation depositional environments and structural control abstract source rocks identified by drilling loperot 1 well in the lokichar basin of the northern kenya rift were evaluated for source potential using rockeval pyrolysis and palynofacies analysis results indicate moderate to good oil source potential from algal prone source rocks the original sediments accumulated in a large freshwater lake similar to some modern rift lakes in the loperot 1 well the section with the highest source potential lies within the lokhone shale member up to 17 total organic carbon toc average 2 4 toc from well log calculation the other potential source interval is the deeper loperot shale member which is highly mature to post mature r 0 1 1 average 1 2 toc from well log calculation the poorer quality of the lower shale interval while partly due to maturity is thought to be largely a result of the basin evolution the loperot shale member was restricted to the lokichar basin while the lokhone shale member was deposited during a time when displacement on the main boundary fault system was greater and adjacent basins had been established consequently lacustrine conditions appear to have extended into adjacent basins thus allowing the accumulation of relatively higher concentrations of algal material in sediment starved parts of the basin