earthquake locations using single station deep borehole recordings implications for microseismicity on the san andreas fault in southern california we have located and estimated source parameters of 109 earthquakes 0 to 5 m using seismograms recorded at 2 5 km depth in the cajon pass borehole southern california the borehole is about 4 km from the san andreas fault at the boundary between the locked almost aseismic mojave and san bernardino segments where the san jacinto fault approaches the san andreas this area is of interest both on account of its tectonic complexity and its high potential seismic hazard the clear relatively unattenuated downhole recordings are rotated to determine the incoming azimuth of the p wave and the delay time between the p and s arrivals is used to estimate the hypocentral distance the difference between the hypocenters located this way and the southern california seismic network scsn locations of the same earthquakes increases with hypocentral distance from the borehole of the earthquakes within 20 km of the borehole 95 are within 1 2 km of the scsn epicenters and all are within 5 km of the scsn depths all but three of the 58 earthquakes located here which the scsn did not record were within 20 km of the borehole and so the location errors are similar to those of scsn a and b quality events most of the small earthquakes occurred within the relatively active san jacinto fault zone but at least eight are located close to or within the san andreas fault zone the stress drops for these events range from 0 1 to 18 mpa these earthquakes appear similar to the other events in this study suggesting that the san andreas fault is similar to other faults at the scale of these small earthquakes the variation in stress drop and slip orientation of these small earthquakes suggests that larger scale heterogeneity continues to these small scales also the location of these small earthquakes in major established fault zones implies that earthquake source dimensions are not geometrically controlled by fault zone width

stable oxygen and carbon isotopes in late glacial holocene freshwater carbonates from belarus and their palaeoclimatic implications abstract four sets of 18 o and 13 c curves were derived from late glacial to mid holocene spring and lacustrine carbonates from belarus stratigraphic division was carried out by palynological studies and 14 c dating it was revealed that variations of isotopic compositions and the dynamics of terrestrial vegetation were affected to some extent by common climatic forcing birch expanded during relatively warm and or arid phases higher 18 o and or 13 c fir spread during relatively cold and or humid phases lower 18 o and or 13 c and grasses were abundant during relatively cold lower 18 o or arid phases higher 13 c a comparison of four pairs of 18 o and 13 c curves allowed the recognition of several palaeoclimatic events at a regional scale two main cold events interrupted by a warmer interval occurred in the younger dryas a short term episode of active permafrost decay at the end of the younger dryas and in the very beginning of the preboreal caused an abrupt rise of lake levels followed by their rapid fall a shift toward a lower precipitation evaporation balance and lower lake levels took place roughly in the middle of the boreal a trend of decreasing temperatures throughout most of the boreal culminated in a short lived very cold phase at 8100 14 c year bp or 9050 cal 14 c year bp according to the proposed age model in the atlantic 1 warmer phases were coincident with the phases of the lower precipitation evaporation balance an episode of significant short term warming occurred in the last third of the atlantic 1 correlation of the 18 o curve derived from authigenic calcites precipitated between the beginning of the preboreal 2 and the end of atlantic 1 in the ptich river valley with that from the gisp2 greenland ice core allowed good matching of the trends and spikes i e of the air temperature changes under assumption that the ages of the ptich pollen based time model are 600 1000 years too old

sedimentology stratigraphy and paleoclimate at the late miocene coffee ranch fossil site in the texas panhandle abstract the late miocene coffee ranch fossil assemblage contains some of the earliest evidence of c4 herbivory in north america however little is known regarding the environmental setting associated with the fauna and a general lack of detailed paleoclimate data exists for the late miocene from the north american continental interior in this study the sedimentary environments stratigraphy geochronology and paleoclimate are interpreted for a series of outcrops of the late miocene ogallala formation in the texas panhandle that includes the coffee ranch locality updated magnetostratigraphy coupled with previously published geochronology indicates that all exposed strata were deposited over a 277 kyr interval within chron c3an 2n from 6 42 6 70 ma depositional environments include fluvial channels floodplain ponds floodplain paleosols eolian paleosols riverine tufa and reworked volcanic ash trunk and tributary fluvial channels are differentiated using channel dimensions sedimentary structures and bounding surface architecture and indicate seasonally variable discharge paleosols from nine pedofacies are described and preserve a spectrum of weakly developed entisols and weakly to moderately mature inceptisols and vertisols constitutive mass balance calculations reveal that mature paleosols formed along distinct pedogenic pathways calcic vertisols accumulated pedogenic carbonate and exhibited either net volumetric dilation or collapse as a result of mineral weathering in contrast non calcic vertisols show patterns of decalcification and variable degrees of volumetric dilation and collapse the presence of the paleosols described in this study requires paleoprecipitation between of 900 1150 mm yr approximately double modern values and a westward shift of the continental udic ustic soil moisture boundary by 400 km during the late miocene this finding suggests that climates in the southern midcontinent were not sufficiently arid to select for c4 photosynthetic pathways over c3 photosynthesis in the late miocene

what does the oxygen isotope composition of rodent teeth record oxygen isotope compositions of tooth phosphate 18op were measured in 107 samples defined on the basis of teeth obtained from 375 specimens of extant rodents these rodents were sampled from pellets collected in europe from 38 n portugal to 65 n finland with most samples coming from sites located in france and spain large oxygen isotopic variability in 18op is observed both at the intra and inter species scale within pellets from a given location this isotopic variability is partly explained by heterochrony in tooth formation related to the short time of mineralization for all rodent species as well as the duration of mineralization that is species dependent consequently tooth phosphate of rodents records a short seasonal interval in the oxygen isotope compositions of meteoric waters 18omw in addition inter species isotopic variability observed in the same pellets suggests behavioural differences implying distinct isotopic compositions for species living in the same location at the scale of europe a robust linear oxygen isotope fractionation equation was determined for muroidea between the midrange 18op values and 18omw values 18op 1 21 0 20 18omw 24 76 2 70 with r2 0 79 n 9 p 0 0001

paleoecology of aphelops and teleoceras rhinocerotidae through an interval of changing climate and vegetation in the neogene of the great plains central united states abstract the rhinocerotidae was one of most widespread and abundant mammalian groups in north america during the oligocene and miocene but were extinct by the early pliocene potential drivers of extinction include the expansion of c4 grasslands cooling climate and increased seasonality here we examine potential changes in the relative abundance of two common genera aphelops and teleoceras from the middle miocene to the pliocene of the great plains we also use stable isotopes in tooth enamel to test whether great plains rhinoceroses were able to incorporate c4 vegetation into their diet after the expansion of c4 grasslands relative abundance of rhinoceroses fluctuates during the middle to late miocene but then rapidly declines at the end of the miocene around 6 5 ma coinciding with the expansion of c4 grasslands on the great plains stable carbon isotopes indicate that both aphelops and teleoceras were predominantly c3 feeders for most of their history during c4 expansion some individuals were able to adapt to modest c4 consumption but were not as successful as contemporaneous horses our results suggest that the extinction of rhinoceroses was associated with environmental stress related to diminishing food resources following the expansion of c4 vegetation possibly coupled with a shorter growing season as climate cooled in addition tooth enamel 18o values in rhinoceroses do not support the interpretation that teleoceras a hippo ecomorph had a semi aquatic lifestyle tooth enamel 18o values reflect a strong latitudinal temperature gradient suggesting that rhinoceroses effectively track 18o values in surface waters and are potentially useful for paleoclimatic reconstructions

integrating stalagmite vertebrate and pollen sequences to investigate holocene vegetation and climate change in the southern midwestern united states abstract speleothem carbon and oxygen isotopic records from onondaga cave south central missouri and beckham creek cave north central arkansas are compared with the cupola pond and oldfield swamp pollen series from southeastern missouri and the rodgers shelter and modoc shelter vertebrate biostratigraphic sequences from central missouri and southwestern illinois similar and roughly contemporaneous shifts between deciduous forest and steppe indicators throughout the holocene are revealed in each database these independent proxies record steppe conditions between approximately 9000 and 1500 cal yr b p a shift toward lighter speleothem carbon may reflect a change from warm and dry to cool and dry conditions between 4500 and 3000 yr b p the sensitive response of speleothem 13c to changes in vegetation emphasizes their importance as paleoclimate records in an area containing few other millenial scale climate proxies

oxygen and hydrogen isotopes in the hydrologic cycle changes of the isotopic composition of water within the water cycle provide a recognizable signature relating such water to the different phases of the cycle the isotope fractionations that accompany the evaporation from the ocean and other surface waters and the reverse process of rain formation account for the most notable changes as a result meteoric waters are depleted in the heavy isotopic species of h and o relative to ocean waters whereas waters in evaporative systems such as lakes plants and soilwaters are relatively enriched during the passage through the aquifers the isotope composition of water is essentially a conservative property at ambient temperatures but at elevated temperatures interaction with the rock matrix may perturb the isotope composition these changes of the isotope composition in atmospheric waters surface water soil and groundwaters as well as in the biosphere are applied in the characterization of hydrological system as well as indicators of paleo climatological conditions in proxy materials in climatic archives such as ice lake sediments or organic materials

stress orientation inferred from shear wave splitting in basement rock at cajon pass s waves generated by mode conversion of p waves from surface compressional sources and recorded by oriented geophones at depths between 1300 and 1820 meters in the cajon pass scientific drillhole arrive at higher apparent velocities when polarized along the direction n70 e than when polarized in the orthogonal direction the observed polarization dependent velocities are consistent with seismic anisotropy due to in situ fractures or cracks that are preferen tially aligned n70 e subnormal to the san andreas fault this result is consistent with the maximum principal horizontal stress direction at cajon pass also being subnormal to the san andreas fault

the origin and spread of grass dominated ecosystems in the late tertiary of north america preliminary results concerning the evolution of hypsodonty abstract sediment samples were collected in northwestern nebraska from the following lithologic units 1 the upper oligocene monroe creek formation the lower miocene harrison formation and upper harrison beds of the arikaree group and 2 the lower miocene runningwater formation and the dawes clay member box butte formation of the ogallala group the samples were processed using heavy liquid extraction and analyzed for siliceous plant microfossils phytoliths all samples yielded well preserved and diverse assemblages dominated by grass phytoliths but included phytoliths produced by woody and herbaceous dicotyledons palms and sedges the abundance of woody dicotyledons and palms decreased up section the grass assemblages consisted mainly of festucoid c3 phytolith morphotypes an index assessing the amount of tree cover d p the ratio of dicotyledon phytoliths to grass phytoliths was employed to interpret the data according to this analysis there was open c3 dominated grassland in the study area between 25 and 17 ma at least 7 ma before supposed adaptations to grasslands in ungulates hypsodonty originate

the range of effects of azimuthal isotropy and eda anisotropy in sedimentary basins summary combinations of bedding or lithology induced azimuthal isotropy with an axis of symmetry perpendicular to the bedding plane and crack induced extensivedilatancy anisotropy eda with a horizontal axis of symmetry are believed to be common in sedimentary basins and cause the widely observed phenomenon of shear wave splitting combinations of two such transversely isotropic forms of anisotropy with orthogonal axes of cylindrical symmetry lead to orthorhombic symmetry this has two major effects 1 the polarizations of the faster split shear waves may no longer be parallel to the strike of the cracks or fractures even for near vertical propagation and 2 such orthorhombic symmetry systems necessarily have a number of directions called shear wave point singularities where shear waves display disturbed or anomalous behaviour again possibly in near vertical directions unless these effects are correctly identified they could be interpreted mistakenly for the effects of structural irregularities or discontinuities in contrast recognition of the 3 d geometry of this behaviour places comparatively tight constraints on possible combinations of anisotropy in the rockmass in order to give some understanding of the geometry of these phenomena this paper presents 3 d patterns of the behaviour of shear wave splitting that have been computed for a range of combinations of crack and bedding induced anisotropy

earthquake prediction a new physical basis summary subcritical crack growth in the laboratory occurs slowly but progressively in solids subjected to low stresses at low strain rates the cracks tend to grow parallel to the maximum compressive stress so that when this stress is aligned over a large enough region the cracks will also be aligned and possess effective seismic anisotropy it is suggested that such subcritical crack growth produces extensive dilatancy anisotropy eda throughout earthquake preparation zones this process is a possible driving mechanism for earthquake precursors observed at substantial distances from impending focal zones and provides in the shear wave splitting which has been observed in several seismic regions a possible technique for monitoring the build up of stress before earthquakes

near surface attenuation and site effects from comparison of surface and deep borehole recordings abstract near surface attenuation and site effects are investigated using the seismograms of 17 local earthquakes recorded at depths of 0 0 3 1 5 2 5 and 2 9 km in the cajon pass borehole southern california the borehole penetrates 500 m of miocene sandstone and then crystalline granitic basement rock previous estimates of site response have been limited to shallower holes where the surface reflection interferes with the upgoing direct wave and the deepest sensor is not below the severe near surface effects in bedrock spectral ratios of the direct p and s waves for each earthquake between the different recording levels are well modeled with frequency independent q and amplification at the borehole q p 27 8 and q s 21 7 in the upper 2 9 km increasing from q p 26 and q s 15 in the upper 300 m to q p 133 and q s 47 between 1 5 and 3 km one event was also recorded at a surface granite site less than 1 5 km from the wellhead comparison of the 2 9 km recording with that at this granite site gives q p 50 and q s 23 the similarity of these values with those of previous studies at a wide range of sites suggests that q is very low in the near surface independent of rock type near surface amplification appears considerably more site dependent at the wellhead the amplifications at 1 hz of the direct p and s waves are 12 7 and 13 7 respectively these values include the free surface effect and are in reasonable agreement with the impedance contrast from the borehole logs at the granite site amplification of both p and s waves is less than four direct wave amplification at the wellhead is therefore at least three times that of the granite site the spectra of the direct and coda waves of the one earthquake recorded at both the wellhead and granite sites are compared with the corresponding 2 9 km recording coda wave amplification is in good agreement with the direct wave at the rock site but at the borehole the coda wave amplification factor overestimates the direct wave amplification by a factor of 3

strain accumulation in southern california 1973 1984 repeated surveys of selected lines from five trilateration networks along the san andreas fault in southern california have been used to deduce the 1973 1984 strain accumulation records at five localities the secular rate of engineering shear strain accumulation is about 0 3 rad yr with the plane of maximum shear parallel to the local strike of the san andreas fault the secular rate of accumulation of areal dilatation is negligible the data were examined to detect evidence for fluctuations in the rate of strain accumulation for this examination 19 lines were removed from the data set four because they exhibited an obvious coseismic offset and 15 others because they contained at least one very anomalous measurement that could reasonably be attributed to a survey blunder the incidence of such blunders appears to be one in every 75 measurements the remaining data consist of 104 lines with an average of 10 measurements each although the strain accumulation plots for the five networks may exhibit marginally significant temporal fluctuations we are not convinced that those fluctuations are greater than could be attributed to survey error in particular we are unable to demonstrate that the 1973 1979 southern california strain anomaly reported by savage and others is real given the uncertainty in the random and systematic errors in measurement the strain measurements in southern california are marginally consistent with linear in time strain accumulation the strain accumulation plots for the salton network clearly established that unlike the deformation reported after the 1940 imperial valley earthquake no acceleration in the shear strain rate has yet been observed following the 1979 imperial valley earthquake

geologic setting and lithologic column of the cajon pass deep drillhole the cajon pass deep drillhole penetrates a late tertiary basin developed on crystalline basement in the sw mojave desert 4 km from the san andreas fault cores continuous cuttings and geophysical logs from phase i show great diversity in rock types structure and alteration the hole encountered cajon formation arkoses granodiorite tonalite unusual megacrystic granite and augen gneiss granitic and pelitic gneisses with quartzite quartzofeldspathic orthogneiss cut by sheets of hornblende gabbro and interlayered mafic and quartzofeldspathic orthogneisses with rare calcsilicate intervals foliation and compositional layering have low dips throughout the column and layered gneisses contain 10 cm scale recumbent folds faults and alteration zones bound several rock units with low apparent dips basement cores are typically cut by steep fractures 1 mm wide that contain zeolites calcite or chlorite epidote fractures and faults decrease in abundance with depth

diet reconstruction for an extinct deer cervidae cetartiodactyla from the quaternary of south america abstract among the extinct cervids of the pleistocene in south america morenelaphus has the most abundant fossil record and the broadest geographic distribution however the paleoecology of morenelaphus is poorly known especially its dietary patterns thus this study aims to recognize the feeding habits of this extinct cervid through analysis of microwear the microwear analysis indicated a mixed feeder diet for morenelaphus both high pit values and frequency of individuals with low number of fine scratches indicated the consumption of grasses possibly including grit siliciclastic sediment ingested during feeding also our results suggest that morenelaphus possibly went extinct at the pleistocene holocene transition as a result of climate environmental changes and or a physiological nutritional crisis

strain accumulation in southern california 1973 1980 frequent surveys of seven trilateration networks in southern california over the interval 1973 1980 suggest that a regional increment in strain may have occurred in 1978 1979 prior to 1978 and after late 1979 the strain accumulation has been predominantly a uniaxial north south compression this secular trend was interrupted sometime in 1978 1979 by an increment in both north south and east west extension in five of the seven networks the onset of this change appears to have occurred first in the networks farthest south the changes occurred without any unusual seismicity within the networks but the overall seismicity in southern california was unusually low prior to and has been unusually high since the occurrence the average principal strain rates for the seven networks in the 1973 1980 interval are 0 17 mu strain yr north south contraction and 0 08 mu strain yr east west extension although the observed increment in strain could be related to unidentifed systematic error in the measuring system a careful review of the measurements and comparison with three other measuring systems reveal no appreciable cumulative systematic error

grip deuterium excess reveals rapid and orbital scale changes in greenland moisture origin the northern hemisphere hydrological cycle is a key factor coupling ice sheets ocean circulation and polar amplification of climate change here we present a northern hemisphere deuterium excess profile covering one climatic cycle constructed with the use of 18 o and d greenland ice core project grip records past changes in greenland source and site temperatures are quantified with precipitation seasonality taken into account the imprint of obliquity is evidenced in the site to source temperature gradient at orbital scale at the millennial time scale grip source temperature changes reflect southward shifts of the geographical locations of moisture sources during cold events and these rapid shifts are associated with large scale changes in atmospheric circulation

modelling the compliance of crustal rock ii response to temporal changes before earthquakes summary there have been several claims that seismic shear waves respond to changes in stress before earthquakes the companion paper develops a stress sensitive model ape for the behaviour of low porosity low permeability crystalline rocks containing pervasive distributions of fluid filled intergranular microcracks and this paper uses ape to model the behaviour before earthquakes modelling with ape shows that the microgeometry and statistics of distributions of such fluid filled microcracks respond almost immediately to changes in stress and that the behaviour can be monitored by analysing seismic shear wave splitting the physical reasons for the coupling between shear wave splitting and differential stress are discussed in this paper we extend the model by using percolation theory to show that large crack densities are limited at the grain scale level by the percolation threshold at which interacting crack clusters lead to pronounced increases in rock matrix permeability in the simplest formulation the modelling is dimensionless and almost entirely constrained without free parameters nevertheless ape modelling of the evolution of fluid saturated rocks under stress reproduces the observed fracture criticality and the narrow range of shear wave azimuthal anisotropy in crustal rocks it also reproduces the behaviour of temporal variations in shear wave splitting observed before and after the 1986 m 6 north palm springs earthquake southern california and several other smaller earthquakes the agreement of ape modelling with a wide range of observations confirms that fluid saturated crystalline rocks are stress sensitive and respond to changes in stress by critical fluid rock interactions at the microscale level this means that the effects of changes in stress and other parameters can be numerically modelled and monitored by appropriate observations of seismic shear waves

active faulting near the cajon pass well southern california implications for the stress orientation near the san andreas fault the dosecc cajon pass well is located 4 km ne of the san andreas fault in a region that includes a diversity of secondary structures active faults close to the well are subparallel to the right lateral san andreas fault but exhibit left lateral and normal slip the stress orientation measured in the well is not consistent with right lateral slip on the san andreas fault or the stresses inferred from focal mechanisms of earthquakes within 10 km of the fault but may drive the e striking slightly normal left lateral cleghorn fault the closest active fault to the well if the in situ stress is everywhere consistent with the highly variable styles of active secondary faulting like the cleghorn the orientation of stresses along the san andreas fault may be quite variable

suggestions for a consistent terminology for seismic anisotropy seismic anisotropy is an unfamiliar concept to many geophysicists and the use of misleading and ambiguous terminology has made it more difficult to understand i suggest here a consistent terminology in which simple expressions have specific meanings similar to their colloquial meanings it is hoped that use of such language will help to make the increasing number of papers reporting seismic anisotropy more readily comprehensible to the non specialist this not a manual of anisotropy and it is not intended for theoreticians it is a list of terms which may make anisotropy a little easier to understand for those more familiar with wave propagation in isotropic solids

paleodietary reconstruction of fossil horses from the eocene through pleistocene of north america paleodietary trends for north american horses from hyracotherium early eocene through equus late pleistocene were examined using dental microwear analysis over 1200 fossil specimens were analyzed for enamel microwear and results were compared to an extensive microwear database of extant ungulates this study tests the hypothesis that eocene horses browsed on low abrasion foods whereas oligocene and later horses had more abrasive diets concomitant with the spread of more open habitats and examines the correlation between the acquisition of hypsodonty and the beginning of grazing microwear results indicate that early to middle eocene species engaged mostly in fruit browsing in contrast late eocene to oligocene taxa have microwear scratch numbers typical of grazing and mixed feeding ungulates but with very fine scratch textures which are unusual in modern c4 grazers and indicate consumption of relatively low abrasion grasses an increase in overall dietary abrasion is evident beginning in the early miocene but members of the anchitherinae sensu stricto engaged in leaf browsing while other forms committed to grazing or alternating between grass and leaves anchitherinae ss continue to rely on leaves until they became extinct in the late miocene while members of the equinae were either grazing or mixed feeding through the pleistocene dietary abrasion increased further beginning in the late miocene and continued to increase through the pleistocene most pleistocene horses were not grazing but rather alternating between grass and leaves or even browsing increased pitting of dental enamel and coarser scratch textures in plio pleistocene horses reinforces the idea that increased abrasion may be due to grit encroachment on food items and or a shift toward coarser grasses than due to a steady increase in the amount of grass consumed

fossil horses and carbon isotopes new evidence for cenozoic dietary habitat and ecosystem changes in north america abstract the transformation from low crowned to high crowned horse teeth during the miocene has traditionally been interpreted as an adaptive response to the spread of savanna grasslands during the middle miocene by about 15 m y ago carbon isotope data from 50 north american horse teeth spanning eocene to pleistocene age indicate that savanna grasslands presumably dominated by c 4 grasses as they are today first became widespread much later than do horses with high crowned teeth prior to 7 m y horses had a c 3 based diet and after 7 m y horses started eating c 4 grasses this change in diet occurred when the major drop in the diversity of horses occurs during the late miocene the change in vegetation reflected in horses diet may be related to a significant reduction in atmospheric co 2 level toward the end of the miocene which provided the c 4 grasses with an adaptive advantage and led to their expansion at the expense of c 3 plants

shear wave splitting in cross hole surveys modeling shear wave splitting diagnostic of some form of effective seismic anisotropy is observed along almost all near vertical raypaths through the crust the splitting is caused by propagation through distributions of stress aligned vertical parallel fluid filled cracks microcracks and preferentially oriented pore space that exist in most crustal rocks shear waves have severe interactions with the free surface and may be seriously disturbed by the surface and by near surface layers in principle cross hole surveys chss should be free of much of the near surface interference and could be used for investigating shear waves at higher frequencies and greater resolution along shorter raypaths than is possible with reflection surveys and vsps synthetic seismograms are examined to estimate the effects of vertical cracks on the behavior of shear waves in chs experiments the azimuth of the chs section relative to the strike of the cracks is crucial to the amount of information about seismic anisotropy that can

multiply polarized shear wave vsps from the cajon pass drillhole a 3 component vsp data set was acquired in the cajon pass drillhole with a multiply polarized shear wave source and a conventional p wave source producing a 9 component data set the vsp data show effects on the full wavefield from the sediment granite interface mode converted waves are identified with significant scattering of s wave energy from the leading p wavefront hodographs are used to display s wave particle motions in two dimensions shear wave splitting is observed in the hodographs the shear wave particle motion is used to infer anisotropy orientation with respect to the local strike of the san andreas fault stress induced fracturing is assumed to be a possible cause of the shear wave splitting a variation of particle motion orientation with depth is observed implying a variation in anisotropy orientation as a function of depth

a decade of shear wave splitting in the earth s crust what does it mean what use can we make of it and what should we do next it is 10 years since shear wave splitting throught to be diagnostic of some form of seismic anisotropy was first positively identified in the earth s crust from the beginning it was argued that the splitting was probably associated with the presence of stress aligned cracks inclusions in the crust and that this would provide the opportunity for monitoring the in situ geometry of cracks and stress in a variety of different circumstances and in a variety of different applications

shear wave anisotropy and the stress field from borehole recordings at 2 5 km depth at cajon pass summary 53 local earthquakes recorded at 2 5 km depth in the cajon pass scientific borehole are analysed for shear wave splitting the time delays between the split shear waves can be positively identified for 32 of the events modelling these observations of polarizations and time delays using genetic algorithms suggests that the anisotropic structure near cajon pass has orthorhombic symmetry the polarization of the shear waves and the inferred strike of the stress aligned fluid filled intergranular microcracks and pores suggests that the maximum horizontal compressional stress direction is approximately n13 w this is consistent with previous results from earthquake source mechanisms and the right lateral strike slip motion on the nearby san andreas fault but not with stresses measured within the uppermost 3 km of the borehole this study suggests that the san andreas fault is driven by deeper tectonic stresses and the present understanding of a weak and frictionless san andreas fault may need to be modified the active secondary faulting and folding close to the fault are probably driven by the relatively shallow stress as measured in the 3 5 km deep borehole

tertiary history of c4 biomass in the great plains usa we use the carbon isotope composition of paleosols to reconstruct the history of c4 biomass on the great plains from ca 23 to 1 ma the proportion of c4 biomass was uniform and moderate 12 34 throughout the miocene increased between 6 4 and 4 0 ma and reached modern levels by 2 5 ma ecological changes in great plains ungulates preceded the increase in c4 biomass the contrasts in the paleosol and ungulate records may indicate initial development of c3 grasslands after the middle miocene or a greater role for ecological interactions within communities in structuring ungulate faunas contrasts in paleosol records from different continents point to regional rather than global controls on the evolution of c4 grasslands

the evolution of pco2 ice volume and climate during the middle miocene abstract the middle miocene climatic optimum 17 15 ma mco is a period of global warmth and relatively high co2 and is thought to be associated with a significant retreat of the antarctic ice sheet ais we present here a new planktic foraminiferal 11b record from 16 6 to 11 8 ma from two deep ocean sites currently in equilibrium with the atmosphere with respect to co2 these new data demonstrate that the evolution of global climate during the middle miocene as reflected by changes in the cyrosphere was well correlated to variations in the concentration of atmospheric co2 what is more within our sampling resolution 1 sample per 300 kyr there is no evidence of hysteresis in the response of ice volume to co2 forcing during the middle miocene contrary to what is understood about the antarctic ice sheet from ice sheet modelling studies in agreement with previous data we show that absolute levels of co2 during the mco were relatively modest 350 400 ppm and levels either side of the mco are similar or lower than the pre industrial 200 260 ppm these new data imply the presence of either a very dynamic ais at relatively low co2 during the middle miocene or the advance and retreat of significant northern hemisphere ice recent drilling on the antarctic margin and shore based studies indicate significant retreat and advance beyond the modern limits of the ais did occur during the middle miocene but the complete loss of the ais was unlikely consequently it seems that ice volume and climate variations during the middle miocene probably involved a more dynamic ais than the modern but also some component of land based ice in the northern hemisphere

role of fluids in faulting inferred from stress field signatures the stress orientation signature of weak faults containing high pressure fluids has been observed for segments of the san andreas fault system in southern california the inferred lithostatic fluid pressures extend into the surrounding relatively intact rock in a zone scaling with the width of the interseismic strain accumulation repeated strain related fracturing and crack sealing may have created low permeability barriers that seal fluids into the network of currently active fractures

a new stable isotope record of neogene paleoenvironments and mammalian paleoecologies in the western great plains during the expansion of c4 grasslands abstract stable isotopes in horse teeth from the north american great plains show that equids began to incorporate c4 plants in their diets about 6 5 million years ago as c4 grasslands expanded however the ecological response of many other ungulates to this expansion is poorly documented we use stable carbon isotopes in mammalian tooth enamel to test whether other ungulates adapted by incorporating c4 vegetation in their diets the openness of habitats before the expansion of c4 grasslands is uncertain with plant phytolith studies suggesting a patchy environment with open habitat grasses and forest and pollen and macrofloral studies suggesting more forested habitats to address this problem we use a model that predicts carbon isotope values for tooth enamel for a variety of environments based on values in modern plants carbon and oxygen isotopes were analyzed in medium to large herbivores from three late miocene and three pliocene local faunas comprising a total of 13 and 7 genera respectively and 59 and 42 individuals results indicate that before the expansion of c4 grasslands taxa with high crowned teeth were consuming predominantly c3 vegetation in contrast by the late pliocene most taxa studied were consuming a component of c4 vegetation and only the peccary platygonus sp had a pure c3 diet c4 consumption increased in the late pliocene 3 0 ma big springs local fauna probably in response to increased c4 biomass most landscapes in the late miocene of nebraska were open such as woodland savanna or c3 grassland although low carbon values from the pratt slide local fauna suggest a denser presumably forested area this general pattern suggests an expansion of open habitats no later than 12 ma early clarendonian through the miocene pliocene transition there was an overall shift to lower 18o enamel values which parallels the long term decrease in global mean annual temperature inferred from the marine record our results indicate that major changes in the diets of medium and large herbivores broadly corresponded with increased c4 biomass and cooling climate from the latest miocene to the late pliocene

in situ stress orientation near the san andreas fault preliminary results to 2 1 km depth from the cajon pass scientific drillhole stress induced wellbore breakouts were analyzed in the cajon pass scientific drillhole 4 3km ne of the san andreas fault to determine the orientation of the principal horizontal stresses using borehole televiewer digitized data in the depth interval 1750 2115 m the mean orientation of the maximum horizontal stress is 073 the scatter in the orientation of breakouts is as much as 37 and variations occur with wavelengths of few hundred meters in this interval reflecting either true changes in stress orientation with depth or the variability of rock anisotropy all the measured orientations however result in left lateral shear stress resolved onto planes parallel to the san andreas fault contrary to its post miocene right lateral slip of 300 km the stress orientation is consistent however with reported left lateral displacement across the cleghorn fault 3 km northeast of and possibly subparallel to the san andreas near cajon pass since the pleistocene the cajon pass stress orientation may reflect the current state of stress across the san andreas fault in which case it should change prior to right lateral slip on this fault or a shallow local perturbation to the san andreas stress field in which case it should rotate couterclockwise with depth determination of the stress orientations at greater depths in the cajon pass drillhole will better sample the near field of the san andreas fault and help resolve this question

source parameters of small earthquakes recorded at 2 5 km depth cajon pass southern california implications for earthquake scaling a 2 5 km deep triaxial seismometer at cajon pass in southern california has recorded several hundred earthquakes ml4 0 occurring within the san andreas fault system at 2 5 km seismic background noise is below amplifier sensitivity and the 2 250 hz spectral range of recorded seismic motion is wider and higher than that of most natural event catalogs compared with downhole recorded motion seismic amplitudes at the surface are amplified below 10 hz and severely attenuated above 30 hz we estimate that qs is at least 1000 for wave motion at 2 5 km and below and qp is over 2000 the range of source dimensions in the downhole recorded catalog is 10 m to 70 m ml 2 0 mo 108 nm to ml 2 7 mo 1013 nm the plot of log source radius vs log moment has a straight line trend compatible with earthquake scaling at constant stress drop inferred stress drops are scattered between 1 and 500 bars there is no evidence in the catalog for the proposed minimum source dimension at 100 m when the cajon pass borehole catalog containing some of the smallest recorded natural earthquakes is combined with 800 larger events from previous studies the moment radius trend suggests that natural earthquakes are self similar over a magnitude range m 2 to 8 we suggest that inferences of minimum source dimension are more likely due to bias in bandlimited individual catalogs than to properties of the seismic crust

regional scale variability in the spread of grasslands in the late miocene abstract a new paleovegetation reconstruction based on both phytolith and paleosol carbon isotopic data is presented for the middle late miocene 11 2 9 5 ma of southwestern montana both sources of data indicate an open habitat c3 grassland ecosystem with a small c4 component

palaeotemperature reconstruction during the last glacial from δ18o of earthworm calcite granules from nussloch loess sequence germany abstract the nussloch loess palaeosol sequence rhine valley germany is considered to be one of the most complete records of the last glacial period in western europe due to its very high sedimentation rate and its good chronological control this sequence is therefore a good framework in which to develop new proxies for palaeoenvironmental reconstructions in this study we explore for the first time the potential of earthworm calcite granules as a new bio indicator and climatic proxy of absolute air and soil temperature in the context of last glacial loess these granules are composed of rhomboedric calcite crystals organized in a radial crystalline structure as these granules are individually generated by earthworms at a relative fast rate they are expected to record intra annual variations in the available sources of oxygen percolating waters of meteoric origin we extracted thirty earthworm calcite granules from 11 of 5 cm layers thick from tundra gley and brown soil horizons previously dated at 45 to 23 ka oxygen isotope ratios were measured on each individual granule the 18o of calcite granules and interlinked transfer functions between water cycle air and soil temperatures allowed us to estimate air temperatures ranging from 10 to 12 4 c which most likely reflect the warm periods of the year when earthworms were the most active

effects of point singularities on shear wave propagation in sedimentary basins summary in most directions of propagation in anisotropic solids seismic shear waves split in regular and predictable ways that in principle can be directly related to the degree of anisotropy and the anisotropic symmetry of the rockmass in all anisotropic solids however there are directions of propagation known as shear wave singularities where the split shear waves have the same phase velocities for directions of propagation near the commonest type of singularity the point singularity the relationship between the phase and group velocities may undergo rapid variations for small changes in direction this results in shear waves along rays propagating at the grozq velocity behaving anomalously with irregular polarizations and amplitude changes as if they were propagating near cusps although the degree of anisotropy may be too small to cause conventional cusps on the group velocity wave surfaces the effects of propagation near such point singularities have been identified in sedimentary basins where they are features of the well established phenomenon of azimuthal isotropy transverse isotropy with a vertical axis of symmetry caused by horizontal lithology or by fine layering ptl anisotropy combining with the more recently recognized azimuthal anisotropy caused by distributions of near parallel near vertical fluid filled inclusions eda anisotropy this paper demonstrates these irregular effects by calculating synthetic shear waves in directions near a point singularity in a material simulating a possible sedimentary basin such anomalies may be important in exploration seismology as point singularities can occur along nearly vertical ray paths in sedimentary basins if not identified correctly the effects of such point singularities could be mistakenly attributed to structural irregularities and if correctly identified the directions of such singularities can place tight constraints on possible combinations of ptl and eda anisotropy in sedimentary basins

cenozoic expansion of grasslands and climatic cooling abstract mountains ocean currents forests and swamps have played an important role in regulating global climate for hundreds of millions of years but the truly novel event of the cenozoic was the evolution and expansion of grasslands with their uniquely coevolved grasses and grazers neogene expansion of the climatic and geographic range of grasslands at the expense of woodlands is now revealed by recent studies of paleosols fossils and their stable isotopic compositions grasslands and their soils can be considered sinks for atmospheric co2 ch4 and water vapor and their cenozoic evolution a contribution to long term global climatic cooling grassland soils are richer in organic matter than are woodland and desert soils of comparable climates and when eroded their crumb clods form sediment unusually rich in organic matter grasslands also promote export of bicarbonate and nutrient cations to lakes and to the oceans where they stimulate productivity and c burial this increased productivity and

stress orientation profile to 3 5 km depth near the san andreas fault at cajon pass california a detailed profile of the orientation of the apparent maximum horizontal compressive stress was constructed over the depth interval 1 75 3 46 km in the cajon pass drill hole located 4 2 km from the san andreas fault in southern california the profile is based on the analysis of stress induced well bore breakouts and consists of 32 616 orientation determinations at a basic sampling interval of 4 cm the azimuth of the apparent maximum horizontal compressive stress around the borehole is 057 s d 19 depth dependent variations in breakout azimuths from a few degrees to as much as 100 occur over depth intervals of several centimeters to hundreds of meters in the borehole with wavelengths showing a self similar distribution over a range of depth intervals the variations in breakout orientation at depth seem to reflect stress fluctuations associated with active faults penetrated by the drill hole assuming linear elastic behavior perturbations in stress magnitude and orientation were computed for predrilling slip on these faults fault stress drops limited to the ambient shear stress i e up to total stress drop along with occasional local extreme stress drops and geometrical complications can explain the variations in breakout orientations in the drill hole the penetrative stress inhomogeneity may suggest that the average orientation of borehole breakouts in cajon pass which indicates left lateral shear on planes parallel to the san andreas fault may not be representative of the state of stress near the fault at depth in this region rather the stress orientation profile indicates the superposition of numerous local stress perturbations on an average stress field which is characterized by an absence of appreciable right lateral shear on planes parallel to the san andreas combining these observations with the highly variable stress state in shallow boreholes in the western mojave desert it is suggested that the heterogeneous stress field in this region representing a spectrum of seismic events may extend over substantial distances across and along the san andreas fault zone if the average measured breakout orientation in cajon pass is representative of the state of stress near the san andreas in this region then substantial left lateral shear stress is resolved on planes parallel to the fault in contradiction to the sense of its long term motion we use an elastic dislocation model to compute the net stress change near the cajon pass drill site since just prior to the large 1812 earthquake taking into account coseismic slip in major earthquakes and aseismic accumulation of slip on the san andreas and san jacinto faults the model suggests that the net change in fault parallel shear stress during the current earthquake cycle in the cajon pass area is nearly negligible the difference between the measured and computed results requires that additional left lateral shear stress be superimposed on a generally fault normal maximum horizontal compressive stress the results suggest that during the great fort tejon earthquake of 1857 left lateral shear stress parallel to the san andreas fault in the region of cajon pass may have played a role in terminating the southeastward extent of the rupture propagation

seismic anisotropy in the shallow crust of the loma prieta segment of the san andreas fault system we study seismic anisotropy beneath the santa cruz mountains using digital recordings of s waves from 60 aftershocks of the 1989 loma prieta earthquake m 7 0 recorded at 18 local stations three component particle motion covariance matrix decomposition combined with a cross correlation method is used to estimate the polarization direction of the fast shear wave arrival and the delay time between the split shear waves results from this procedure are compared with those obtained through visual inspection of horizontal particle motions inclusion of the vertical component in the particle motion analysis allows the recognition and elimination of s to p converted phases from our study uncertainties in the polarization of the fast shear wave and in the delay time between arrival of the fast and slow waves are estimated to be about 20 and 10 ms respectively about half of the s wave records examined show clear evidence of shear wave splitting two persistent polarization directions of fast shear wave arrivals are observed regardless of the focal mechanism and hypocenter location of the individual events 52 of the polarizations are parallel to the strike of the san andreas fault nw se and 24 are parallel to the probable direction of local maximum horizontal compressive stress ne sw the fault parallel fast polarization direction dominates at 10 stations and may result from mineral or fracture alignment caused by shearing along the plate boundary the fast polarization direction perpendicular to the local axes of minimum horizontal compressive stress dominates at one station farthest from the san andreas fault and is consistent with the hypothesis of extensive dilatancy anisotropy eda where parallel alignment of fluid filled fractures produces the anisotropy the predominance of fault parallel fast shear wave polarizations indicates that the use of shear wave splitting results to deduce the orientation of local axes of principal stress may not always yield accurate results the splitting time between fast and slow shear waves varies from 10 to 125 ms and shows no systematic relationship with either hypocentral distance or focal depth this suggests that the seismic anisotropy is no deeper than 2 km the depth of the shallowest earthquakes analyzed for an anisotropic zone 2 km thick the average velocity anisotropy is about 5 in our study region

using phytolith assemblages to reconstruct the origin and spread of grass dominated habitats in the great plains of north america during the late eocene to early miocene abstract this paper outlines a general approach for analyzing siliceous microfossils from plants phytoliths in geologic sediments and uses this method to reconstruct the evolution of grasslands in the central great plains during the late tertiary phytolith assemblage analysis is widely employed for reconstructing vegetation types in pleistocene holocene paleoecology and archaeology but no standardized methodology is currently available published phytolith studies are often system specific and tailored after specific modern analogues as a result they vary in extraction technique classification scheme and method of inference for ancient phytolith assemblages whose modern analogues are not well understood a more general analytic approach is necessary this analytic approach includes studying all size fractions 2 250 mm using a classification scheme based on the available literature and a comprehensive modern phytolith reference collection and comparing relative frequencies through time of all diagnostic phytoliths to interpret changes in habitat structure this general method was applied to late eocene to early miocene sediment samples from northwestern nebraska to deduce vegetation history and the results were compared to those obtained using a more specific technique stromberg paleogeogr paleoclimatol paleoecol 177 2002 59 in both studies phytoliths were extracted by means of heavy liquid separation from the following lithostratigraphic units 1 the late eocene chadron formation and the early oligocene brule formation white river group 2 the late oligocene early miocene monroe creek formation the early miocene harrison formation and anderson ranch formation arikaree group and 3 the early miocene runningwater formation and box butte formation ogallala group the two techniques general vs specific produced results that differ markedly data from the general approach indicate the presence of relatively closed habitats in late eocene to early oligocene typified by the presence of phytoliths from bambusoid grasses woody and herbaceous dicotyledons and palms the introduction and spread of modern open habitat grasses such as pooids arundinoids and panicoids occurred between the late oligocene and early miocene this resulted in a savanna or woodland type habitat by the early miocene in contrast analysis using the specific method suggested the presence of open habitats in northwestern nebraska from the late eocene onward the difference in interpretation based on the two approaches is principally due to inclusion of phytoliths 50 mm and the use of an expanded reference collection in the current study general analytic approach these results point to the importance of methodological choices in phytolith analysis and emphasize the usefulness of phytoliths as paleoecological indicators in the fossil record

environmental change in the great plains an isotopic record from fossil horses abstract carbon and oxygen isotope ratios of fossil horse tooth enamel from nebraska and texas show evidence for late neogene environmental changes in the great plains the earliest unambiguous c4 dietary signal among texas equids coincides with the age of the classic late hemphillian age coffee ranch fauna which we suggest is 6 6 ma based on volcanic ash correlations c4 vegetation was present in the diets of a small fraction of late hemphillian equids in nebraska and was thereafter ordinary in the diets of both nebraska and texas equids there is no unequivocal evidence for abundant c4 vegetation in the diets of pre late hemphillian equids and we suggest that the ensuing dietary change reflects c4 biomass expansion in the latest miocene carbon isotope ratios of post hemphillian horses in nebraska can be divided into two statistically distinct populations on the basis of whether tortoise remains co occur with horse remains indicating that the two proxies carbon isotopes and presence absence of tort

pedogenic carbonate stable isotope record of environmental change during the neogene in the southern great plains southwest kansas usa carbon isotopes and the evolution of c4 dominated grasslands fossiliferous strata in the meade basin southwest kansas preserve numerous superposed mammalian faunas and calcareous paleosols that range in age from the clarendonian north american land mammal age nalma 12 0 9 0 ma early late miocene to the early irvingtonian nalma ca 2 5 ca 1 0 ma early pleistocene faunas from these sections document the evolution of the small mammal community of the modern grassland ecosystem of the region and the stable isotope composition of paleosol carbonates provides a means by which the environmental context of the evolution of the modern ecosystem may be documented we used the stable carbon isotope composition 13 c relative to vienna peedee belemnite vpdb of 194 pedogenic carbonates from 19 measured sections to reconstruct the history of c 4 grass abundance in the meade basin paleosol carbonate 13 c values reflect the proportion of c 3 trees shrubs cool climate grasses and c 4 warm climate grasses plants that grew in an ancient soil and provide a means with which to reconstruct past mammalian habitats paleosol carbonate 13 c values record a three phase increase in the abundance of c 4 biomass during the neogene in the meade basin late miocene sections have mean 13 c values of 7 6 0 90 clarendonian and 6 5 0 31 hemphillian nalma 9 0 4 9 ma consistent with 17 and 26 c 4 biomass respectively miocene 13 c values from meade are statistically identical to published 13 c values for miocene paleosol carbonates elsewhere in the southern great plains supporting the widespread presence of 20 c 4 biomass on average in the region throughout the miocene the abundance of c 4 biomass increased between the end of the hemphillian section and the beginning of the early blancan nalma 5 0 3 0 ma early and middle blancan 3 0 2 5 ma carbonates have statistically identical 13 c values 4 9 0 90 and 5 0 1 10 respectively suggesting a stable ecosystem during the early pliocene although high 13 c variability in densely sampled intervals suggests a high degree of landscape scale variation in c 4 abundance the final phase geochronologically controlled by two well characterized ashes huckleberry ridge 2 10 ma cerro toledo b 1 47 1 23 ma and magnetostratigraphy is a trend to higher 13 c values from the late blancan to early irvingtonian ca 2 5 ca 1 0 ma from 4 at the base of the section to 1 at the top corresponding to an increase from almost 50 to 65 c 4 biomass the abundance of c 4 biomass first reaches modern levels for the region 78 10 9 around the level of the cerro toledo b ash indicating that a modern like grassland ecosystem first appeared in the region ca 1 3 ma although 13 c values do not remain consistently high through the rest of the section

the cajon pass scientific drilling experiment overview of phase 1 the cajon pass scientific drilling project is a broad interdisciplinary experiment involving over two dozen principal scientists phase i of drilling coring and downhole experimentation began dec 8 1986 and ended april 2 1987 with the hole at a depth of 2115 m 82 m of core recovered and a wide range of downhole experiments successfully completed in this paper we briefly outline the scientific motivation for the project and provide an overview of the scientific program we also indicate some of the varied research areas in the earth sciences where data obtained in this experiment will provide unique and important insight into active in situ processes regional geologic structure and rock and fluid composition and origin

investigating equid mobility in miocene florida usa using strontium isotope ratios abstract despite extensive research on the evolutionary history and ecology of horses surprisingly little is known about the daily and seasonal movements i e mobility of extinct species we used strontium isotope ratios 87sr 86sr in tooth enamel to estimate mobility patterns of equids from two miocene fossil sites in northern florida usa thomas farm ca 18 ma and love bone bed ca 9 5 9 ma the species included in our sample have a range of body sizes 45 252 kg and reconstructed diets gomphotheres and tapirs from love bone bed were also included to represent browsers based on modern species behavior we predicted small bodied and browsing taxa living in closed habitats had limited mobility and 87sr 86sr similar to local northern florida limestone we anticipated larger bodied and grazing taxa living in open environments had increased mobility with higher and more variable 87sr 86sr reflecting larger movements across differing geologies contrary to expectations the majority of taxa at both sites have higher 87sr 86sr than expected for local eocene bedrock and instead are similar to contemporary late oligocene to mid miocene seawater the most plausible explanation for these findings is that most individuals foraged close to the paleo coastline where vegetation would have been influenced by marine derived strontium via sea spray or precipitation tapirs have considerably higher and more variable 87sr 86sr than horses or gomphotheres at love bone bed which may reflect foraging on aquatic vegetation in the river channel that is preserved at the site our results suggest that horses were relatively sedentary in florida even during their peak radiation

pedogenic carbonate stable isotope record of environmental change during the neogene in the southern great plains southwest kansas usa oxygen isotopes and paleoclimate during the evolution of c4 dominated grasslands neogene strata in the meade basin southwest kansas preserve numerous superposed calcareous paleosols in sections that range in age from the clarendonian north american land mammal age nalma 12 0 9 0 ma early late miocene to the early irvingtonian nalma ca 2 5 ca 1 ma early pleistocene the carbon isotope compositions 13 c relative to vienna peedee belemnite vpdb of pedogenic carbonates from these sections record the protracted regional increase in the abundance of grasses using the c 4 photosynthetic pathway over this interval with the first appearance of a modern like grassland ecosystem having 70 c 4 biomass around 1 3 ma we use the stable oxygen isotope composition 18 o relative to vienna standard mean ocean water vsmow of 194 paleosol carbonates from 19 measured sections to reconstruct the climatic conditions in the meade basin during the rise of c 4 grasses to ecological dominance in the region pedogenic carbonate 18 o values are sensitive to soil temperature and the 18 o of soil water and hence they are a paleoclimate proxy carbonate 13 c values do not exhibit consistent trends in relation to 18 o values indicating no consistent relationship between short term climatic conditions in terms of temperature or aridity and the abundance of c 4 biomass mean carbonate 18 o values within biostratigraphic intervals decrease from the clarendonian 25 3 0 72 to the early and middle blancan 21 8 0 87 and 22 1 0 69 respectively and they also exhibit a decreasing trend in the late blancan early irvingtonian from 25 to 21 the increase in 18 o values between the end of the middle blancan and the beginning of the late blancan early irvingtonian sections could reflect the onset of northern hemisphere glaciation and a change in the isotope composition of the hydrosphere due to increased ice volume the long term trend in 18 o values suggests that c 4 biomass increased coincident with some combination of decreasing temperature increasing proportion of winter precipitation recharge of soil water and or increasing soil moisture thus the ecological dominance of c 4 grasses in the region today does not seem to be linked to warmer temperatures or increased aridity

ungulate diets reveal patterns of grassland evolution in north america abstract equids from the late miocene hemphillian north american land mammal age of texas evolved c4 dominated diets earlier than their nebraska counterparts leading researchers to hypothesize a northward expansion of c4 grasses between 8 and 6 ma however competing hypotheses from multiple paleoecological proxies support one of three hypotheses that northward c4 expansion occurred at the expense of c3 woodlands c3 grasslands or was not occurring during the hemphillian we test these alternative hypotheses by comparing the ecosystems of hemphillian texas and nebraska 6 3 7 5 ma using hoofed mammal dietary reconstruction hypsodonty and digestive strategy and by incorporating published tooth enamel isotopes paleosol isotopes and phytolith assemblage data we also use hypsodonty mesowear and microwear in a detailed comparison of two localities that are roughly contemporary cambridge nebraska at 7 ma and coffee ranch texas at 6 6 ma to rule out time averaging as an explanatory variable through analogy with modern african communities we reconstruct texas and nebraska ecosystems as bushland and woodland respectively using the proportions of hypsodont taxa we estimate mean annual precipitation values of 1217 mm year for nebraska and 1368 mm year for texas using rarefaction we also do not find differences in the richness of brachydont hypsodont and hindgut fermenting taxa we therefore conclude that both ecosystems were largely similar in hoofed mammal faunal structure during the late miocene when compared to published paleosol and phytolith data which show no differences in c3 c4 biomass among latitudes our results allow us to reject hypotheses of northward c4 expansion during the late miocene we therefore suggest that the equid enamel isotopes may have been biased by two potential factors the primary reliance on a single locality of appropriate age and possible behavioural lag in c4 feeding among nebraska equids these results suggest that the formation of a modern c3 c4 latitudinal gradient occurred later than is previously suggested by equid enamel carbon isotopes

spatial distribution of shear wave anisotropy in the crust of the southern hyogo region by borehole observations summary in order to estimate the detailed orientation and density of cracks around an active fault area we investigate the spatial distribution of shear wave anisotropy in the crust of the southern hyogo region with high quality waveform data recorded by a borehole network operated by the geological survey of japan we introduce the vertically aligned crack model of hudson 1980 1981 to explain its anisotropic structure at stations more than 10 km away from the earthquake fault zone i e ikeda and inagawa crack orientation is nearly consistent with the direction of the regional maximum stress in this area wnw ese cracks are distributed at depths below 5 km with a nearly constant crack density e of 0 015 at ikeda and below 0 km with e 0 009 at inagawa these upper limit depths of crack distribution are roughly comparable with those of microearthquakes there in contrast cracks are aligned nearly parallel to the strike of the earthquake fault ne sw at stations at both ends of the fault system i e takarazuka and ikuha and their intensities of anisotropy are three or four times larger than those at ikeda and inagawa at takarazuka in particular cracks are distributed at a depth shallower than 4 5 km with a large crack density e of 0 06 this result suggests that major stress release by the faulting of the main shock dominates the present stress field at both ends of the fault system in the middle of the fault system i e hirabayashi cracks are aligned parallel to its regional maximum stress and the intensity of anisotropy is twice as large as those at ikeda and inagawa this result contrasts with that of tadokoro et al 1999 who determined the crack orientation at a station close to hirabayashi to be parallel to the fault direction for events in a period earlier than this study by about a year suggesting that the stress field in this area may have changed over the timescale of a year around inagawa the distribution of anisotropy seems to have strong azimuthal variations this result indicates small scale of the order of several kilometres variations of anisotropic structure in the upper crust probably due to strong heterogeneities in the fluid and or constitutive materials

focal mechanisms and the state of stress on the san andreas fault in southern california focal mechanisms have been determined from p wave first motion polarities for 138 small to moderate 2 6 m 4 3 earthquakes that occurred within 10 km of the surface trace of the san andreas fault in southern california between 1978 and 1985 on the basis of these mechanisms the southern san andreas fault has been divided into five segments with different stress regimes earthquakes in the fort tejon segment show oblique reverse sup on east west and northwest striking faults the mojave segment has earthquakes with oblique reverse and right lateral strikesup motion on northwest strikes the san bernardino segment has normal faulting earthquakes on north south striking planes while the banning segment has reverse strike sup and normal faulting events all occurring in the same area the earthquakes in the indio segment show strike slip and oblique normal faulting on northwest to north south striking planes these focal mechanism data have been inverted to determine how the stresses acting on the san andreas fault in southern california vary with position along strike of the fault one of the principal stresses is vertical in all of the regions the vertical stress is the minimum principal stress in fort tejon and mojave the intermediate principal stress in banning and indio and the maximum principal stress in san bernardino the orientations of the horizontal principal stresses also vary between the regions the trend of the maximum horizontal stress rotates over 35 from n15 w at fort tejon to n20 at indio except for the san bernardino segment the trend of the maximum horizontal stress is at a constant angle of about 65 to the local strike of the san andreas fault implying a weak fault the largest change in the present stress state occurs at the end of the rupture zone of the 1857 fort tejon earthquake it appears that the 1857 rupture ended when it propagated into an area of low stress amplitude possibly caused by the 15 angle between the strikes of the san jacinto and san andreas faults the strong correlation between present state of stress and segmentation in previous earthquakes suggests that the stress state is important in controlling rupture propagation

carbon and oxygen isotopic variability in neogene paleosol carbonates constraints on the evolution of the c4 grasslands of the great plains usa abstract understanding the development of grassland ecosystems is critical for understanding the evolution of their associated vertebrate faunas we focus on the central and southern portions of the great plains which is the largest continuous grassland in north america and has a well documented mammalian fossil record modern biomass in this region is dominated by c4 grasses but paleontological evidence indicates the region was forested prior to the middle miocene implying the region was formerly dominated by c3 plants we examine long and short term variations in the abundance of c4 grasses and climatic conditions in the central and southern great plains using the stable carbon and oxygen isotope composition of paleosol carbonates from 28 sections of 11 lithostratigraphic units in the region the carbon isotope composition 13c of paleosol carbonate reflects the proportion of c3 and c4 plants that grew in an ancient soil the long term pattern of 13c values indicates that the percentage of c4 grasses in the great plains was moderate ca 20 throughout most of the miocene increased from 6 4 to 4 0 ma and reached modern levels by 2 5 ma based on the range in 13c values within sections and the mean 13c values for different carbonate morphologies that reflect different intervals of carbonate accumulation the abundance of c4 biomass did not vary substantially on short time scales the variability in c4 biomass on short time scales does not correlate with age of section latitude longitude or the mean percentage of c4 biomass the oxygen isotope composition 18o of soil carbonate is controlled by soil temperature and the 18o value of soil water great plains paleosol 18o values vary considerably in relation to time and latitude removing the variation due to latitude with least squares linear regression reveals a temporal pattern that is consistent with the global pattern of neogene climate change quantitative interpretation of the 18o record is difficult without independent control on temperature or the oxygen isotope composition of soil water both of which have probably varied with long term climate change comparisons of measured 18o values for each section with predicted values based on modern temperature data and estimated modern meteoric water 18o values suggest that 1 most miocene sections have 18o values consistent with warmer temperatures and more positively shifted soil water 18o values than today 2 three miocene sections in western nebraska probably had soil water compositions that were more negative than similar aged localities in the region and 3 the plio pleistocene sections have 18o values consistent with cooler temperatures and more negative soil water 18o values than today the paleosol 13c values do not correlate strongly with the pattern of climate change recorded by the 18o values suggesting that long term change in mean temperature is not the main control on the abundance of 13c biomass in the great plains we propose two testable hypotheses to account for our data and other published data that bear on the evolution of the great plains ecosystem

holocene changes in atmospheric circulation recorded in the oxygen isotope stratigraphy of lacustrine carbonates from northern sweden the oxygen isotope composition of local precipitation delta 18 o p is reconstructed from carbonate lake sediment components in a sediment core covering the last 10000 calendar years from lake tibetanus a small hydrologically open groundwater fed take in the abisko area northern sweden comparison of the delta 18 o p history with a pollen based palaeotemperature record from the same core clearly reveals pronounced deviations from the normally expected temporal delta 18 o p temperature relation so called dansgaard relation that may be a function of changing oceanicity the transition from relatively moist maritime conditions in the early holocene to a much drier climate after 6500 cal bp is reflected by major changes in forest extent and composition as recorded by pollen and plant macrofossil data at the time of maximum influence of westerly air mass circulation high zonal index c 9500 cal bp brought about by high summer insolation and enhanced meridional pressure gradients delta 18 o p at lake tibetanus was about 2 higher than would be predicted by the modem isotope temperature relation the occurrence of long term changes in delta 18 o p temperature relations which are more sensitive measures of palaeoclimate than either delta 18 o p or temperature alone needs to be taken into account when extracting palaeoclimatic information from continental oxygen isotope records less

late cenozoic tectonics of the northwestern san bernardino mountains southern california the late cenozoic structural and stratigraphic history of the northwestern san bernardino mountains supports two distinct episodes of uplift in late miocene to earliest pliocene and quaternary time that we hypothesize are related to movements on low angle structures beneath the range in this paper we document the nature distribution and timing of late cenozoic deformation and deposition in the northwestern san bernardino mountains and we illustrate the neotectonic evolution of the area in a series of interpretive paleotectonic block diagrams in the first episode of deformation late miocene to earliest pliocene motion on the south southwest directed squaw peak thrust system disrupted drainage in pre existing miocene nonmarine basins and uplifted the western third of the present range to form the ancestral san bernardino mountains crystalline rocks of the san bernardino mountains were thrust southward across the present site of the san andreas fault between 9 5 and 4 1 ma at a time when the san gabriel fault was the active strand of the san andreas transform system we speculate that the liebre mountain crystalline block at the northern margin of the ridge basin may be the missing upper plate of the squaw peak thrust now offset along the san andreas fault the second episode of deformation began with uplift of the northern plateau of the modern san bernardino mountains on north directed range front thrusts in early pleistocene time between 2 0 and 1 5 ma synchronous uplift of the northern plateau recorded in early pleistocene fanglomerates on the northwestern margin of the range is interpreted to be the result of movement of a relatively coherent crustal block northward up a south dipping detachment ramp beneath the central range in middle pleistocene time activity on the northern range front began to wane and the locus of uplift shifted to a narrow zone of arching and northward tilting adjacent to the san andreas fault which subsequently migrated rapidly northwestward along the san andreas fault from the western san bernardino mountains into the northeastern san gabriel mountains we attribute this pattern of deformation to the passage of a bulge or strike slip ramp attached to the southwest side of the san andreas fault at depth

crustal stress field in southern california and its implications for fault mechanics we present a new high spatial resolution image of stress orientation in southern california based on the inversion of earthquake focal mechanisms we use this image to study the mechanics of faulting in the plate boundary region the stress field contains significant spatial heterogeneity which in some cases appears to be a result of the complexity of faulting and in other cases appears to be a cause temporal changes in the stress field are also observed primarily related to major earthquakes the observed 15 10 rotation of the stress axes due to the 1992 m7 3 landers mainshock implies that the deviatoric stress magnitude in the crust is low of the order of 10 mpa this suggests that active faults in southern california are weak the maximum principal stress axis near the san andreas fault is often at 50 to the fault strike indicating that the shear stress on the fault is comparable to the deviatoric stress the san andreas in southern california may therefore be a weak fault in a low strength crust

tsunami sedimentary deposits of crete records climate during the minoan warming period 3350 yr bp earthquakes or explosive eruptions generate tsunami which are at the origin of thick and chaotic coastal sediments these commonly fossiliferous deposits are formed instantaneously at the historic

niche modeling reveals lack of broad scale habitat partitioning in extinct horses of north america abstract the classic traits of modern horses equidae large body size high crowned teeth hypsodonty and a single toe monodactyly are often considered adaptations to grassland environments however extinct horses that varied in these three traits overlapped geographically for millions of years during the miocene and pliocene it has been hypothesized that co occurring horse species partitioned habitats with large bodied hypsodont and monodactyl equids dominating open grasslands while equids with different combinations of traits lived in more wooded areas we tested for the presence of broad scale habitat partitioning by compiling a large database of north american horse fossil occurrences with data on trait state body size hypsodonty index and toe number and paleoenvironment derived from paleovegetation records null modeling of niche overlap in each of the north american land mammal ages of the miocene and pliocene revealed that taxonomic and trait based groups show no differences in habitat occupancy cluster dendrograms visualizing niche overlap showed that some ecological guilds shared derived traits but derived traits were not associated exclusively with grassland habitats trait values are not predicted by the proportion of grassland habitats in time bins further the three traits show no co evolution across the equid tree when corrected for phylogenetic relatedness together these results suggest that the evolution of large body size hypsodonty and monodactyly in equids was not due to a shared selective regime in response to expanding grassland habitats instead these traits may have evolved separately likely due to a variety of small scale selective pressures acting across the variety of habitats present in the miocene and pliocene

mollusca stable isotope record of a core from lake frassino northern italy hydrological and climatic changes during the last 14 ka a core retrieved from lake frassino northern italy provided evidence of palaeohydrological change in this area during the last 14 ka lithological malacological and in particular stable isotope composition of freshwater shells allowed the delineation of the main phases of lake evolution between the lateglacial and holocene lateglacial conditions were drier than the holocene although a wetter period was inferred before c 14 ka according to the oxygen isotope composition of freshwater shells the holocene showed a clear bipartition the first part which lasted from 9100 to 7000 yr bp was drier and was followed by a rapid increase in humidity at 7000 6800 yr bp between 6800 and 5000 yr bp there were wetter and more stable conditions as indicated by smaller 18 o oscillations of pisidium and valvata shells from c 5000 to 2600 yr bp the record is characterized by larger fluctuations which may indicate that short term and particularly pronounced alternation of wet and dry periods occurred

hydraulic fracturing in situ stress measurements to 2 1 km depth at cajon pass california stress measurements to 2 1 km reveal stress changes with depth that cannot be explained by an elastic response to uniform crustal strain the data at about 1 km depth suggest that the stress is limited by the frictional strength of rock and is perturbed at greater depths by faults which intersect the borehole the stress data indicate that there is little or no right lateral shear stress acting on planes parallel to the san andreas fault

inversion for seismic anisotropy using genetic algorithms1 a general inversion scheme based on a genetic algorithm is developed to invert seismic observations for anisotropic parameters the technique is applied to the inversion of shear wave observations from two azimuthal vsp data sets from the conoco test site in oklahoma horizontal polarizations and time delays are inverted for hexagonal and orthorhombic symmetries the model solutions are consistent with previous studies using trial and error matching of full waveform synthetics the shear wave splitting observations suggest the presence of a shear wave line singularity and are consistent with a dipping fracture system which is known to exist at the test site application of the inversion scheme prior to full waveform modelling demonstrates that a considerable saving in time is possible whilst retaining the same degree of accuracy

what are the climate controls on δd in precipitation in the zongo valley bolivia implications for the illimani ice core interpretation abstract controversy has surrounded the interpretation of the water isotopic composition d or 18o in tropical and subtropical ice cores in south america although recent modeling studies using agcm have provided useful constraints at interannual time scales no direct calibration based on modern observations has been achieved in the context of the recent ice core drilling at nevado illimani 16 39 s 67 47 w in bolivia we examine the climatic controls on the modern isotopic composition of precipitation in the zongo valley located on the northeast side of the cordillera real at about 55 km from nevado illimani monthly precipitation samples were collected from september 1999 to august 2004 at various altitudes along this valley first we examine the local and regional controls on the common d signal measured along this valley we show that 1 local temperature has definitely no control on d variations and 2 local rainout is a poor factor to explain d variations we thus seek regional controls upstream the valley potentially affecting air masses distillation based on backtrajectory calculations and using satellite data trmm precipitation noaa olr and direct observations of precipitation iaea gnip we show that moisture transport history and the degree of rainout upstream are more important factors explaining seasonal d variations analysis of a 92 yr simulation from the echam 4 model t30 version implemented with water stable isotopes confirms our observations at seasonal time scale and emphasize the role of air masses distillation upstream as a prominent factor controlling interannual d variations lastly we focus on the isotopic depletion along the valley when air masses are lifted up our results suggest that if the temperature gradient between the base and the top of the andes was higher by a few degrees during the last glacial maximum lgm less than 10 of the glacial to interglacial isotopic variation recorded in the illimani ice core could be accounted for by this temperature change it implies that the rest of the variation would originate from wetter conditions along air masses trajectory during lgm

the fracture criticality of crustal rocks summary the shear wave splitting observed along almost all shear wave ray paths in the earth s crust is interpreted as the effects of stress aligned fluid filled cracks microcracks and preferentially oriented pore space once away from the free surface where open joints and fractures may lead to strong anisotropy of 10 per cent or greater intact ostensibly unfractured crustal rock exhibits a limited range of shear wave splitting from about 1 5 to 4 5 per cent differential shear wave velocity anisotropy interpreting this velocity anisotropy as normalized crack densities a factor of less than two in crack radius covers the range from the minimum 1 5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near surface rocks this narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4 5 and 10 per cent marking the boundary between essentially intact and heavily fractured rock when the level of fracture criticality is exceeded cracking is so severe that there is a breakdown in shear strength the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability the range of normalized crack dimensions below fracture criticality is so small in intact rock that any modification to the crack geometry by even minor changes of conditions or minor deformation particularly in the presence of high pore fluid pressures may change rock from being essentially intact below fracture criticality to heavily fractured above fracture criticality this recognition of the essential compliance of most crustal rocks and its effect on shear wave splitting has implications for monitoring changes in any conditions affecting the rock mass these include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery and in monitoring changes before and after earthquakes amongst others