anisotropy of compressional wave velocities complex electrical resistivity and magnetic susceptibility of mylonites from the deeper crust and their relation to the rock fabric abstract laboratory measurements of compressional wave velocities v p complex electrical resistivity and magnetic susceptibility have been carried out on a mylonitic clinopyroxene amphibolite mca and an epidote muscovite quartz ultramylonite mqu from the ivrea zone both mylonites exhibit significant anisotropies that are related to the rock fabric at low pressure of about 50 mpa the anisotropy of v p for the mca and mqu is 17 and 34 respectively it is due to oriented microcracks intragranular transgranular grain boundary cracks and cleavage cracks in addition to preferred orientation of anisotropic rock forming minerals at high pressure 600 mpa where most of the cracks are closed a residual anisotropy of about 10 and 12 respectively is observed that is mainly controlled by preferred orientations of mineral constituents the anisotropies of the complex electrical resistivity measured with an electrolyte tap water at 0 1 mpa are 40 and 36 for the mca and mqu whereas at 80 mpa these values increase to 44 for both samples in contrast measured with an electrolyte of 0 1 m nacl solution the resistivity is lower and gives anisotropies of 71 mca and 63 mqu at 0 1 mpa the anisotropies of the magnetic susceptibility ams are about 27 mca and 14 mqu for both mylonites the axes of the ams ellipsoid coincide with the macroscopic fabric elements where the main axis of magnetic susceptibility k max lies approximately parallel to the lineation an intercorrelation of the different physical properties in terms of rock composition and anisotropic rock fabric is difficult nevertheless the laboratory determination of anisotropic in situ physical properties provide more objective data for the modelling of in situ crustal conditions

folding within seconds hypervelocity impacts of cosmic projectiles larger than 200 m diameter are capable of forming complex craters on earth at these craters shock loading shock damage and excavation flow are followed by a gravity driven collapse of the deep transient cavity such impact structures are characterized by a central uplift a flat crater floor and a terraced crater rim collapse induced deformation features like folds and brittle fault zones have many similarities to tectonic structures typical deformation patterns of complex terrestrial impact craters of 5 15 km diameter are compiled and analyzed with respect to their kinematic development unlike their tectonic counterparts deformation structures are always the result of non plane strain deformation and are formed in a single event that takes place in seconds to minutes to understand the high strain rate processes the microstructure of an impact induced fold of the crooked creek impact crater 7 km diameter missouri united states is investigated in detail a period of 20 30 s at the most is determined for the collapse phase of this crater the gross plastic deformation behavior of the fold is achieved by localized brittle deformation along millimeter to centimeter spaced fault zones forming a network of veins shock damage has fractured 40 of grain boundaries the onset of collapse and associated deformation started in rocks with a reduced cohesion and is friction controlled

hydrogen isotopes in dinosterol from the chesapeake bay estuary abstract the hydrogen isotope ratio of the dinoflagellate sterol dinosterol 4 23 24 trimethyl 5 cholest 22e en 3 ol was measured in suspended particles and surface sediments from the chesapeake bay estuary in order to evaluate the influence of salinity on hydrogen isotope fractionation d h fractionation was found to decrease by 0 99 0 23 per unit increase in salinity over the salinity range 10 29 psu a similar decrease to that observed in a variety of lipids from hypersaline ponds on christmas island kiribati we hypothesize that the hydrogen isotopic response to salinity may result from diminished exchange of water between algal cells and their environment lower growth rates and or increased production of osmolytes at high salinities regardless of the mechanism the consistent sign and magnitude of dinosterol d response to changing salinity should permit qualitative to semi quantitative reconstructions of past salinities from sedimentary dinosterol d values

anisotropy of vp and vs in an amphibolite of the deeper crust and its relationship to the mineralogical microstructural and textural characteristics of the rock abstract laboratory seismic measurements of compressional v p and shear wave v s velocities have been carried out on a deformed amphibolite from the ivrea zone at pressures and temperatures up to 600 mpa and 600 c respectively amphibolite is considered to be an important rock of the deeper crust from crystallographic orientations of the component minerals hornblende plagioclase and diopside and the respective elastic constants we calculated the average elastic constants voigt average for the rock specimen compressional and shear wave velocities in three orthogonal directions normal and parallel to foliation measured at high confining pressure 600 mpa are in good agreement with the calculated velocity surface this suggests that seismic wave velocity anisotropy is controlled by preferred orientations of minerals in rocks texture at high confining pressure where the effect of microcracks on seismic wave velocities is eliminated in the amphibolite sample the velocity anisotropy is dominated by the texture of hornblende because of the strong preferred orientation and large volume fraction of this mineral species a large s wave splitting is documented in the directions parallel to the foliation particularly in the lineation which strongly argues against the microcrack origin of s wave splitting at least at conditions of high effective pressure

elastic anisotropy of tambo gneiss from promontogno switzerland a comparison of crystal orientation and microstructure based modelling and experimental measurements author s vasin rn kern h lokajicek t svitek t lehmann e mannes dc chaouche m wenk hr abstract the authors 2017 felsic and mafic gneisses constitute large proportions of the upper and lower continental crust gneisses often display high anisotropy of elastic properties associated with preferred orientations of sheet silicates here we study the elastic anisotropy of a sample of tambo gneiss from promontogno in the central alps we apply optical microscopy time of flight neutron diffraction neutron and x ray tomography to quantify mineral composition and microstructures and use them to construct self consistent models of elastic properties they are compared to results of ultrasonic measurements on a cube sample in a multi anvil apparatus and on a spherical sample in an apparatus that can measure velocities in multiple directions both methods provide similar results it is shown that models of microstructure derived elastic properties provide a good match with ultrasonic experiment results at pressures above 100 mpa at a pressure of 0 1 mpa the correspondence between the model and the experiment is worse this may be caused by an oversimplification of the model with respect to microfractures or uncertainties in the experimental determination of s wave velocities and elastic tensor inversion the study provides a basis to determine anisotropic elastic properties of rocks either by ultrasonic experiments or quantitative models based on microstructures this information can then be used for interpretation of seismic data of the crust

shear wave anisotropy of laminated lower crust beneath urach sw germany a comparison with xenoliths and with exposed lower crustal sections in order to investigate the laminated lower crust beneath urach sw germany we analysed 1 the anisotropy and reflectivity of seismic shear waves and 2 the composition of xenoliths samples we conclude that the directional dependence of shear waves of the lower crust is effectively transversely isotropic with a maximum velocity difference of 3 to 6 both seismic signature and petrological information are compared with reference profiles deduced from the exposed lower crustal sections of ivrea and calabria italy the comparison based on synthetic seismograms shows that the observed combination of shear wave anisotropy and reflectivity pattern corresponds to alternating layers containing a high amount of metapelites such as found in the calabria profile and in the val strona subsection of ivrea the main portion of the whole rock anisotropy is caused by the lattice preferred orientation of anisotropic minerals so called intrinsic anisotropy the effect of alternating rock layering so called layering anisotropy contributes only a minor portion to the whole rock anisotropy layered mafic intrusions such as found in the val sesia subsection of ivrea are nearly isotropic both seismic anisotropy and xenoliths indicate that mafic layers do not play a major role for the laminated lower crust at urach they may be restricted to a thin layer at the crust mantle boundary

crustal anisotropy in northeastern tibetan plateau inferred from receiver functions rock textures caused by metamorphic fluids and lower crust flow abstract the crust of tibetan plateau may have formed via shortening thickening or large scale underthrusting and subsequently modified via lower crust channel flows and volatile mediated regional metamorphism the amplitude and distribution of crustal anisotropy record the history of continental deformation offering clues to its formation and later modification in this study we first investigate the back azimuth dependence of ps converted phases using multitaper receiver functions rfs we analyze teleseismic data for 35 temporary broadband stations in the ascent experiment located in northeastern tibet we stack receiver functions after a moving window moveout correction major features of rfs include 1 ps arrivals at 8 10 s on the radial components suggesting a 70 90 km crustal thickness in the study area 2 two lobed back azimuth variation for intra crustal ps phases in the upper crust

the kinematics of movements along the insubric line and the emplacement of the ivrea zone schmid sm zingg a and handy m 1987 the kinematics of movements along the insubric line and the emplacement of the ivrea zone in h j zwart m martens i van der molen c w passchier c spiers and r l m vissers editors tectonic and structural processes on a macro meso and micro scale tecta a hys 135 47 66 the insubric line west of locamo is characterized by a 1 km thick greenschist facies mylonite belt east of locamo these mylonites are overprinted by a discrete brittle fault the mylonites are derived from basement units of the central alps sesia zone and the southern alps ivrea zone as well as from the permo mesozoic cover of the southern alps canavese sense of shear criteria indicate that the mylonites accomm ated backt s g followed by dextral strike slip motion mylonitization during backtbrusting was synchronous with backfolding of the central alpine nappes under higher metamorphic conditions a horizontal temperature gradient resulted from the rapid juxtaposition of the warm central alpine block against the cold southern alpine block mylonites formed during the later dextral strike slip event are related to large transcurrent displacements in the central alps deduced from regional kinematic considerations thus both mylo t ation events are contempor eous with deformation to the north and south of the insubric line insubric phase extensively modifying the pre insubric crustal configuration of the alps the insubric phase post dates the bergell intrusion 30 m y the emplacement of the geophysical ivrea body is a combined effect of vertical uplift due to e w directed crustal thinning during the early jurassic and underplating by continental crust associated with late cretaceous compression a deep crustal normal fault pogallo line subsequently rotated during tertiary alpine orogenesis separates deeper parts of the southern alpine crust ivrea zone from intermediate crustal levels strona ceneri zone the rigid ivrea body localir ed large strains within the insubric mylonite belt and is responsible for the present curvature of the insubric line in uetion

compound specific d h ratios of the marine lakes of palau as proxies for west pacific warm pool hydrologic variability abstract we tested the use of hydrogen isotopic ratios d of lipids in marine lake sediments from the micronesian republic of palau against the instrumental record of the last century to assess their capacity to record past hydrological changes of the western pacific warm pool d values of the algal lipid biomarker dinosterol d dino and the more generic palmitic acid d pa were found to be sensitive indicators of the intensity of regional precipitation as recorded by the southern oscillation index soi the observed sensitivity is caused by the combined effect of 1 the amount effect in tropical precipitation 2 dilution of the isotopically heavy saline surface waters with light precipitation 3 a salinity effect on the biosynthetic d h fractionation between lipid and lake water both lake water d d lake and d dino could be expressed as a quadratic function of either precipitation or lake water salinity d dino values were used to reconstruct past hydrological changes of the region long term variations in the strength and sign of the el nino southern oscillation enso since the little ice age lia 1450 1850 a d and during the early holocene 7 9 kyr bp appeared to dominate decadal variability and indicate very dry conditions during the lia early holocene d dino values were on average 10 higher than those of recent centuries which we interpret as a result of millennial scale hydrologic and water mass changes on a global level the similar 35 range of d changes during the early holocene and last several centuries imply a similar range of decadal centennial hydrologic variability during those two climate regimes our results indicate that a correlation exists between solar irradiance levels and tropical pacific climate

seismic anisotropy and mantle creep in young orogens summary seismic anisotropy provides evidence for the physical state and tectonic evolution of the lithosphere we discuss the origin of anisotropy at various depths and relate it to tectonic stress geotherms and rheology the anisotropy of the uppermost mantle is controlled by the orthorhombic mineral olivine and may result from ductile deformation dynamic recrystallization or annealing anisotropy beneath young orogens has been measured for the seismic phase pn that propagates in the uppermost mantle this anisotropy is interpreted as being caused by deformation during the most recent thermotectonic event and thus provides information on the process of mountain building whereas tectonic stress and many structural features in the upper crust are usually orientated perpendicular to the structural axis of mountain belts pn anisotropy is aligned parallel to the structural axis we interpret this to indicate mountainparallel ductile i e creeping deformation in the uppermost mantle that is a consequence of mountain perpendicular compressive stresses the preferred orientation of the fast axes of some anisotropic minerals such as olivine is known to be in the creep direction a consequence of the anisotropy of strength and viscosity of orientated minerals in order to explain the anisotropy of the mantle beneath young orogens we extend the concept of crustal escape or extrusion tectonics to the uppermost mantle we present rheological model calculations to support this hypothesis mountain perpendicular horizontal stress determined in the upper crust and mountain parallel seismic anisotropy in the uppermost mantle require a zone of ductile decoupling in the middle or lower crust of young mountain belts examples for stress and mountain parallel pn anisotropy are given for tibet the alpine chains and young mountain ranges in the americas finally we suggest a simple model for initiating mountain parallel creep

seismic reflection imaging of mineral systems three case histories mineral deposits can be described in terms of their mineral systems i e fluid source migration pathway and trap source regions are difficult to recognize in seismic images many orebodies lie on or adjacent to major fault systems suggesting that the faults acted as fluid migration pathways through the crust large faults often have broad internal zones of deformation fabric which is anisotropic this coupled with the metasomatic effects of fluids moving along faults while they are active can make the faults seismically reflective for example major gold deposits in the archaean eastern goldfields province of western australia lie in the hanging wall block of regional scale faults that differ from other nearby faults by being highly reflective and penetrating to greater depths in the lower crust coupled thermal mechanical and fluid flow modeling supports the theory that these faults were fluid migration pathways from the lower to the upper crust strong reflections are also recorded from two

velocity anisotropy and shear wave splitting in rocks from the mylonite belt along the insubric line ivrea zone italy velocity anisotropy and shear wave splitting has been measured up to 600 mpa on seventeen mylonitic samples from the mylonite belt along the insubric line the rocks which range from felsic to mafic in composition have experienced large ductile strain seismic anisotropy a and shear wave splitting delta vs is significantly pressure dependent velocity anisotropies of p and s waves range from 9 to 27 vp and 10 to 25 vs at 10 mpa and from 4 to 15 and 6 to 17 at 600 mpa respectively oriented microcracks in addition to preferred orientation of anisotropic rock forming minerals contribute largely to velocity anisotropy and shear wave splitting in the low pressure range at high pressure 600 mpa where most of the cracks are closed velocity anisotropy and shear wave splitting is dominated by preferred mineral orientation texture the experimental data at 600 mpa are in fairly good agreement with velocity calculations voigt averages based on volume percentages of major minerals the stiffness tensor of respective rock forming minerals and the measured crystallographic orientation texture related reflection coefficients rc at the protolith mylonite interfaces normal incidence no density change are 0 04 vp and 0 03 vs for extremely anisotropic mylonites a approximate 12 calculations of the variation of reflection coefficients due to velocity anisotropy of p waves in mylonites of different lithologies literature data included give 90 less than 0 1 66 less than 0 05 and 44 less than 0 03 these results suggest that the directional variation of wave velocities in anisotropic mylonites alone does not provide the impedance contrast necessary to give rise for strong seismic reflections

magnetic susceptibility of the rock matrix related to magnetic fabric studies non ferromagnetic minerals constitute what is called the rock matrix whose susceptibility k1 is directly accessible using only high magnetic fields measurements on minerals and a wide range of rock types show that k1 is mainly due to paramagnetism and hardly exceeds 10 3 si with an anisotropy degree p less than 1 35 different methods to estimate the role of the matrix component in low field susceptibility k and its anisotropy include petrological and chemical analysis evolution of p vs k low temperature studies and comparison of k with remanent magnetizations

compositional variation and the origin of deep crustal reflections abstract identifying the sources of crustal reflections is essential for deriving more geological information from deep crustal reflection profiles theoretical and model studies help place constraints on the role of compositional variation in producing deep crustal reflections analysis of laboratory derived velocities and densities of rock types typical for the continental crust indicates that reflection coefficients are generally small but significantly 17 of the possible reflection coefficients have magnitudes between 0 1 and 0 2 comparison between reflection coefficients derived from well logs and reflections observed in associated seismic profiles indicates that constructive interference associated with geological layering is at least as important as the magnitude of the reflection coefficients for producing detectable reflections constructive interference can increase reflection amplitude by two to three times but is limited to a relatively narrow range of layer thickness for a typical 10 40 hz seismic wavelet and typical crustal velocities of about 6 km s constructive interference occurs for layer thickness ranging between about 35 and 80 m layers thinner than 35 m interfere destructively if reflections result from compositional variation seismic models of hypothetical and observed geologic relations provide analogs for interpreting complex reflection patterns observed in deep crustal reflection profiles such models show reflection patterns similar to those observed in the reflection profiles the models indicate that the reflections could originate in the complexly deformed and intruded terranes that are common in the crystalline crust and it may not be necessary to appeal to unobserved phenomena such as special lamellae or fluid filled fractures to explain the reflections

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

mt and reflection an essential combination summary at many localities in the world there have been coincident comprehensive electromagnetic em studies and seismic reflection profiles conducted unfortunately over many more regions the seismic reflection images are interpreted without the constraints afforded by electrical conductivity information this paper is an attempt to convince the reader that a collocated magnetotelluric mt study should in almost every case be made wherever a seismic reflection survey is undertaken examples are shown from six studies in which the em results aided the geological tectonic interpretations of the seismic sections also difficulties with the mt technique are discussed and the interpretations of conducting zones within the lower crust are examined finally a generalised model is proposed for the continental crust that may account for both the reflectivity and conductivity of the zone at the top of the lower crust

high temperature and pressure seismic properties of a lower crustal prograde shear zone from the kohistan arc pakistan anastomosing shear zones exposed in the lower crust of the kohistan palaeoisland arc in pakistan have parageneses indicating increasing pressure during deformation therefore they represent a rare example of strain localization during crustal thickening we investigated the seismic properties of a sheared gabbro with constant bulk chemistry across one of these shear zones the compressional wave velocity was measured at confining pressures of up to 0 5 gpa and temperatures of up to 600 8c the density the average vp compressional p wave velocity and the acoustic impedance at room temperature increase from the undeformed protolith through the strain gradient to the intensely sheared mylonite the seismic anisotropy is largest for the strain gradient the vp dependence of the velocities on temperature is much higher in the mylonite than in the protolith the acoustic impedance contrast between protolith and mylonite is high enough to generate seismic reflections at low temperatures but not at high temperatures this suggests that the seismic reflectivity may also depend on the temperature consequently ductile shear zones can be detected in reflection profiles of crusts with low geothermal gradients and may be transparent in crusts with high geothermal gradients

impact of the messinian salinity crisis on black sea hydrology insights from hydrogen isotopes analysis on biomarkers the messinian salinity crisis 5 96 5 33 ma ago was a dramatic oceanographic event when evaporites kilometers thick precipitated in a desiccating mediterranean basin trapping more than 5 of the world s oceanic salt hydrological changes in the adjacent black sea and water exchange with the mediterranean region are crucial but poorly understood factors influencing messinian evaporite formation here we present compound specific hydrogen isotope d data from messinian black sea sedimentary rocks that show a rapid change to heavy waters at 5 8 ma when major glaciations occurred at the same time highly depleted d values of long chain n alkanes derived from plant waxes indicate that fresh river transported water originated from colder northern latitudes the d values of alkenones biosynthesized by haptophyte algae show an unprecedented increase of 60 within 100 kyr the corresponding rapid change to 110 for d of the black sea waters seem unrealistic being heavier than anywhere in the present day oceans regardless of the applied relation between the d values of the alkenones and d of the waters where they were produced the 60 enrichment in the d values of alkenones indicates strongly enhanced evaporitic conditions still the relative distribution of the alkenones implies in situ growth and reproduction of haptophyte algae requiring sustained marine conditions in the black sea up to 5 6 ma this indicates that mediterranean black sea connectivity persisted during the first msc phase when gypsum precipitated in the mediterranean basin when the black sea became isolated at the peak of the msc 5 6 ma it had a strongly negative hydrological budget and rapidly desiccated due to excess evaporation

shear wave velocities anisotropy and splitting in high grade mylonites shear wave velocities have been measured for 10 high grade granulite and upper amphibolite facies mylonites from the snowbird tectonic zone canada at various pressures up to 600 mpa in order to explore the kinematic significance of shear wave splitting in the lower crust at 600 mpa vs anisotropy ranges from 11 in amphibolitic mylonites to 2 in granulite facies pyroxene bearing and tonalitic mylonites hornblende bearing granitic mylonites display intermediate vs anisotropy 6 7 vs anisotropy is lower than vp anisotropy in hornblende rich mylonites whereas vs anisotropy is higher than vp anisotropy in feldspar rich mafic tonalitic granitic and diatexitic mylonites shear wave splitting is pronounced in rocks displaying strong s wave anisotropy in most of the high grade mylonites studied the polarization direction of the first arrival vs1 corresponds to the extension lineation x when propagation is parallel to y perpendicular to lineation and parallel to foliation or to the y direction when propagation is parallel to x for all other propagation directions the polarization direction of vs1 is oblique to the x y and z directions therefore it may be difficult to infer the structural implications of shear wave splitting in lower continental crustal shear zones if one does not know 1 the orientation of the propagation and fast vibration directions with respect to in situ lineation and foliation and 2 the lattice preferred orientation of in situ rock forming minerals

natural deformation fabrics of plagioclase implications for slip systems and seismic anisotropy abstract the optical and tem microstructures and fabrics of twelve naturally deformed plagioclase containing rocks of compositions from an0 an80 have been studied with a view to determining the deformation mechanisms the optical indicatrix fabrics have been recorded in all samples with respect to the foliation and lineation in general and form a girdle in the foliation plane with a high concentration of normal to the foliation in some cases the orientation of 010 and 001 cleavages or albite and pericline twins has allowed the construction of pole figures in all cases 010 has been shown to be the dominant slip plane with either 001 or 100 the dominant slip direction tem shows that the dominant slip system is 010 001 with secondary 010 100 the seismic p wave anisotropy has been calculated for the first time from the lpo data of plagioclase the anisotropy coefficient is high at around 15 the minimum p velocity is normal to the lineation in the foliation plane and the maximum is sub parallel to the foliation normal the combined information on deformation kinematics and seismic anisotropy is valuable for interpretations of deep crustal seismic sections

hydrologic cycling over antarctica during the middle miocene warming global warmth 20 15 million years ago allowed vegetation to grow on formerly ice covered areas of antarctica leaf wax and pollen data show that this growth was supported by increased hydrologic activity over the antarctic coast derived from a local moisture source

complex resistivity measurements of confined rock complex resistivity was measured for samples of westerly granite and berea sandstone saturated with 0 01 m kclaq a four electrode technique was employed to make measurements over a frequency range of 10 3 to 106 hz and a confining pressure range of 0 2 to 200 mpa when the electrical response was analyzed in terms of equivalent conductivity and relative permittivity k both of these quantities varied more with confining pressure for the granite samples than they did for the sandstone in addition both rock types exhibited large low frequency dispersion of permittivity while previous studies have measured the response of near dc conductivity to changes in pressure measurements of pressure dependence of the full complex resistivity provide greater insight into earthquake related phenomena as well as induced polarization measurements and other remote sensing techniques

hydrogen isotopic compositions of long chain alkenones record freshwater flooding of the eastern mediterranean at the onset of sapropel deposition sapropels are organic rich sediment layers that were intermittently deposited in the mediterranean sea especially in its eastern basin during the last 10myr the associated anoxic events that gave rise to sapropel formation resulted indirectly from the impact of african monsoon maxima on the basin s hydrography sharp shifts in oxygen isotopes 18o to values more depleted in the heavy isotope 18o in carbonates from surface dwelling planktonic foraminifera slightly preceding sapropel deposition suggest that the mediterranean was flooded by large amounts of freshwater leading to the development of a low salinity of the surface water and a strong density stratification of the water column however the degree of freshwater flooding and concomitant drop in sea surface salinity sss remain elusive recent work has shown that the hydrogen isotope d values of long chain alkenones produced by haptophyte algae depend mainly on the d of the water and on salinity and may therefore offer a new tool for salinity reconstructions our analysis of the d of alkenones from last interglacial sapropel s5 from the aegean sea shows a large decrease in d of 25 at the onset of sapropel formation suggesting a drop in sss of 6 from 39 to 33 although the absolute sss estimates should be interpreted with care as they are subject to relatively large uncertainties the estimated sss values appear quite reasonable as they for example yield sss before sapropel deposition similar to that of the present day aegean sea to reduce uncertainties in sss estimates the d salinity relationship has to be better constrained with cultures and also tested in field studies however our results do illustrate the promise of a combined use of d of alkenones u37k of alkenones and 18o of surface dwelling planktonic foraminifera for sss reconstructions

the effect of plagioclase textures on velocity anisotropy and shear wave splitting at deeper crustal levels abstract average elastic data for lower crustal rocks have been calculated on the basis of crystallographic plagioclase orientations from greenschist to granulite facies rocks and the respective single crystal elastic constants the anisotropy of the p wave velocity caused by different plagioclase textures is between 2 and 8 whereas the highest differences in the two perpendicular polarized shear wave velocities vary between 0 08 and 0 18 km s these relatively low values compared with those of the single crystal result from generally weak preferred crystallographic plagioclase orientations as well as from the opposing influence of the preferred orientation of different crystallographic directions model rocks of identical composition and density but with different types of plagioclase textures show poisson s ratios in the range of 0 16 0 23 the impedance contrasts between these rocks are generally low only the contrast between strong plagioclase texture and random orientation would produce reflection coefficients large enough to result in seismic reflection thus the influence of plagioclase textures upon the seismic anisotropy of rocks with strongly oriented minerals such as amphiboles and pyroxenes should be fairly low

elastic properties fabric and seismic anisotropy of amphibolites and their contribution to the lower crust reflectivity in this paper the elastic properties of amphibolites from the serie dei laghi and the ivrea zone southern alps n italy were investigated as a function of their mineralogical composition microfabric and density three orthogonal cores were cut parallel and normal to foliation and lineation from those bulk and grain density were measured and the interconnected porosity was calculated bulk density varies from 2 75 to 3 07 g cm 3 and calculated porosity ranges from 0 02 to 0 88 the same cores were also used to measure seismic velocity of ultrasonic waves at room temperature and at increasing confining pressure up to 300 mpa at high pressure the matrix properties are separated from the crack induced properties p wave velocity varies with respect to the direction of propagation the slowest direction is always normal to foliation and the fastest parallel to the mineral lineation the mineral lineation is typically defined by the elongation of amphibole crystals in which the fastest vp direction is parallel to the c axis that is also the elongation axis the vp ranges between 6 76 and 7 54 km s in the direction parallel to lineation and between 6 32 and 7 06 km s in the direction normal to foliation this defines a vp anisotropy of up to 14 whose shape varies from orthorhombic to axially symmetric either prolate or flattened it was observed that both vp and vp anisotropy increase with the amount of amphibole and decrease with the amount of plagioclase moreover the c axis distribution of amphiboles is responsible of the vp anisotropy intensity and shape in agreement with observations from previous studies the seismic properties calculated with the approach of mainprice 1990 using the fabric data the elastic constants and the modal composition gave results in good agreement with the measurements exposed rocks in the ivrea and serie dei laghi zones show that amphibolites are interlayered with metapelites on a scale from 1 to 100 m because of the very large acoustic impedance contrast 20 34 1 75 for amphibolites 17 16 0 4 for metapelites they represent a very reflective portion in the middle lower crust 2007 elsevier b v all rights reserved

2h 1h fractionation in lipids of the mangrove bruguiera gymnorhiza increases with salinity in marine lakes of palau abstract hydrogen isotope ratios 2h 1h or d h of sedimentary mangrove lipid biomarkers can be exploited as a quantitative proxy of past salinity and water isotopes this approach is based on the observation that apparent 2h 1h fractionation between surface water and mangrove leaf lipids increases with surface water salinity in order to better understand the mechanisms responsible for this empirical relationship we analyzed the isotopic composition of surface water xylem water leaf water and leaf lipids from bruguiera gymnorhiza mangroves growing around eleven marine lakes and a lagoon on the rock islands of palau spanning a salinity range of 5 32 parts per thousand ppt net fractionation increased with increasing salinity for both nc31 alkane 0 7 0 1 ppt 1 and for the pentacyclic triterpenoid lupeol 0 5 0 2 ppt 1 these trends could not be attributed to changes in biosynthetic fractionation with salinity but seem more likely to be due to increasing disequilibrium between xylem water and water vapor as salinity increases in palau s humid climate this most likely causes leaf water to become less 2h enriched relative to surface water and to xylem water as salinity increases this supposition is supported not only by measurements of leaf water 2h enrichment but also by the correlation r2 0 66 between leaf water isotopes and those of rain water which are assumed to be in equilibrium with water vapor isotopes and by the dependence of leaf water isotopes on water vapor isotopes in a peclet modified craig gordon model these results should inform the application of sedimentary mangrove lipid hydrogen isotope ratios to infer past hydroclimatic changes

petrofabric p wave anisotropy and seismic reflectivity of high grade tectonites abstract ji s salisbury m h and hanmer s 1993 petrofabric p wave anisotropy and seismic reflectivity of high grade tectonites tectonophysics 222 195 226 in order to understand seismic anisotropy and its influence on the reflectivity of lower crustal fault zones we have undertaken experimental measurements of p wave velocities up to 600 mpa for 20 granulite and upper amphibolite facies mylonites from the snowbird tectonic zone canada the rocks whose composition ranges from ultramafic to felsic have experienced extremely large ductile strain it is found that the amphibolitic mylonites exhibit significant vp anisotropy 6 13 at 600 mpa whereas the pyroxene bearing granulite facies mylonites and the quartz or feldspathic tonalitic granitic and diatexitic mylonites are quasi isotropic in order to constrain interpretation of the measured vp properties microstructural analyses have been systematically performed in the samples studied it is shown that the interaction between feldspar and quartz or pyroxene and the absence of amphibole cause anisotropy in the granulite facies mylonites to be low the amphibolitic mylonites are strongly anisotropic because of a large volume fraction of hornblende with strong lpo 001 lineation and 100 foliation the anisotropy pattern in the amphibolitic mylonites is consistently orthorhombic vp x vp y vp z where x is parallel to stretching lineation y is normal to the lineation in the foliation plane and z is normal to the foliation plane but the anisotropy pattern in the mica rich mylonites is transversely isotropic vp x vp y vp z if sufficiently large volumes of the crust display such patterns this difference may be an important indicator for kinematic analysis in the middle to lower crust using modern seismic techniques the study demonstrates that while seismic reflectivity is strongly lithology controlled fabric induced anisotropy can both enhance and decrease seismic reflectivity hence the discontinuity of seismic reflections along ductile shear zones in the lower crust is not necessarily indicative of the discontinuity of the shear zones but may indicate changes in composition or metamorphic grade

measurements of wave velocity and electrical conductivity of an amphibolite from southwestern margin of the tarim basin at pressures to 1 0 gpa and temperatures to 700 c comparison with field observations summary in situ measurements of elastic wave velocities and electrical conductivities in the three structural directions normal to foliation z perpendicular to lineation in foliation y and parallel to lineation x for an amphibolite collected from southwestern margin of the tarim basin northwest china were carried out in the laboratory the elastic wave velocity was measured with thecombinedtransmission reflectionmethodatpressuresupto1 0gpa atroomtemperature and temperatures up to 700 c at 1 0 gpa and the electrical conductivity was measured with the impedance spectroscopy from 250 to 700 c at 1 0 gpa the experimentally determined data included compressional vp and shear wave velocities vs velocity anisotropy av intrinsic pressure and temperature derivatives of vp and vs electrical conductivity electrical conductivity anisotropy a and the parameters of the arrhenius relationship elastic wave velocities increase in the structural directions z y x with vp of 6 63 6 78 and 6 95 km s 1 and vs of 3 75 3 82 and 3 96 km s 1 for z y and x respectively at pressure of 1 0 gpa elastic wave velocities increase linearly with pressure at room temperature and pressures between 0 25 and 1 0 gpa and decrease linearly with increasing temperature at 1 0 gpa the pressure coefficients of the sample are in the range of 0 1883 0 2308 km s 1 gpa 1 for vp and 0 1149 0 1678 km s 1 gpa 1 for vs the temperature coefficients are in the range of 2 09 2 35 10 4 km s 1 gpa 1 for vp and 1 28 1 68 10 4 km s 1 gpa 1 for vs t he electrical conductivity increases with increasing temperature consistent with the arrhenius relationship activation energies for the three structural directions of the amphibolite are in the range of 0 71 0 75 ev the amphibolite shows velocity anisotropy 4 15 4 86 per cent for vp and 5 29 5 84 per cent for vs at 0 25 1 0 gpa and electrical conductivity anisotropy 11 1 25 2 per cent based on the regional crust model and geothermal gradient velocity and electrical conductivity depth profiles were calculated for the sample these profiles were then compared with those derived from seismic reflection refraction data and from electromagnetic data our results showed that the amphibolite sample has vp and vs in agreement with those of the middle and lower crust obtained from seismic reflection refraction data and in accord with that of the lower crust deduced from electromagnetic data the lower crust of the electromagnetic crust model is roughly equivalent to the middle and lower crust layers of the seismic crust model therefore it is suggest that the amphibolite may be one of the constituents of the present middle and lower crust in the tarim basin

southward movement of the pacific intertropical convergence zone ad 1400 1850 tropical rainfall patterns control the subsistence lifestyle of more than one billion people seasonal changes in these rainfall patterns are associated with changes in the position of the intertropical convergence zone which is characterized by deep convection causing heavy rainfall near 10 n in boreal summer and 3 n in boreal winter dynamic controls on the position of the intertropical convergence zone are debated but palaeoclimatic evidence from continental asia africa and the americas suggests that it has shifted substantially during the past millennium reaching its southernmost position some time during the little ice age ad1400 1850 however without records from the meteorological core of the intertropical convergence zone in the pacific ocean quantitative constraints on its position are lacking here we report microbiological molecular and hydrogen isotopic evidence from lake sediments in the northern line islands galapagos and palau indicating that the pacific intertropical convergence zone was south of its modern position for most of the past millennium by as much as 500km during the little ice age a colder northern hemisphere at that time possibly resulting from lower solar irradiance may have driven the intertropical convergence zone south we conclude that small changes in earth s radiation budget may profoundly affect tropical rainfall

elastic wave velocities in rocks from a lower crustal section in southern calabria italy compressional and shear wave velocities vp and vs have been measured on 14 rock samples recovered from a high grade metamorphic terrane in the southern part of the calabrian massif which is considered to represent a continuous section of a former deep continental crust the experiments have been carried out in a cubic anvil apparatus at constant confining pressures up to 600 mpa and temperatures ranging from 20 to 700 c using the ultrasonic transmission technique at 600 mpa the compressional and shear wave velocities range from 6 33 km s 1 to 7 74 km s 1 and from 3 54 km s 1 to 4 30 km s 1 respectively variations in velocity are strongly related to variations in mineralogy both vp and vs increase with increasing amphibole pyroxene garnet and sillimanite content the range of velocities measured in the granulite facies rocks under conditions of lower crustal depth is similar to the range of seismic velocities for the crust beneath calabria importantly the metapelite rocks with intermediate silica and high alumina contents have in situ compressional and shear wave velocities which are equivalent to the mafic rocks or even significantly higher the detected high velocity layer at a depth between 15 and 20 km can be explained by a sequence of garnet cordierite sillimanite gneisses for which actual velocities between 6 6 and 7 6 km s 1 have been determined experimentally

the nature of seismic reflections from deep crustal fault zones deep seismic reflection profiles have recorded reflections from ductile shear zones within the crystalline basement perhaps the best example is the consortium for continental reflection profiling cocorp wind river line in wyoming which shows the wind river thrust fault to be a strong reflector from the surface to depths of about 30 km to identify the physical properties responsible for the seismic reflections from fault zones we report measurements of compressional wave velocity and velocity anisotropy in mylonites recovered from exhumed ductile shear zones these rocks are characterized by extensive ductile deformation of plastic minerals brittle deformation of the more rigid minerals grain size reduction and development of a strong fabric average velocity and density were not found to change significantly and systematically between the mylonites and the adjacent undeformed rock seismic anisotropy of 7 or greater is present in several mylonites dependent upon their composition and fabric using our data on mylonite properties and models for the crustal structure near the wind river line we compute true amplitude synthetic reflection seismograms and compare them to true amplitude plots of the cocorp wind river reflection data when anisotropy and the finely laminated structure of the fault zone are considered our modeling indicates fairly strong reflections from mylonite zones in most cases we also show that elevated pore pressure in shear zones may produce strong reflections however a permeability of the order of 10 16 darcy is required to maintain sufficient pore pressure to produce a velocity anomaly in a fault zone which has long been inactive

p wave velocity across a noncoaxial ductile shear zone and its associated strain gradient consequences for upper crustal reflectivity in order to simulate a normal incidence reflection profile across a noncoaxial ductile shear zone we determined p wave velocities of samples cut parallel and normal to mylonite foliation along a closely spaced profile 27 cm long through a transition zone and its associated strain gradient the ductile shear zone developed within an aplitic leucogranite was sampled from a kilometer wide ductile transcurrent fault in the northern french massif central strain analysis indicates that the sample experienced heterogeneous and progressive simple shear deformation shear strain systematically increases from zero in the undeformed protolith to 30 in the mylonite the transition zone thickness t is about 30 cm and the mylonite thickness m is about 10 cm the amount of quartz and mica increases relative to feldspar toward the mylonite indicating that a mineralogical composition change accompanied mylonitization mica and quartz developed a strong crystallographic preferred orientation cpo in the least strained domain seismic anisotropy is low and mean vp is 6 km s at 600 mpa anisotropy increases up to 10 and vp decreases up to 5 35 km s for propagation normal to the mylonite foliation through the transition zone this systematic velocity change correlates with the increasing through the transition zone and can be directly related to the cpo of mica and the increase in volume percent mica within the mylonite zone these results indicate that velocity and anisotropy gradients may in some cases be associated with ductile shear zones and that mylonite boundaries may not represent first order discontinuities the reflectivity of a ductile shear zone depends on the thickness of the transition zone relative to the seismic wavelength and on the t m ratio synthetic seismograms show that for a given seismic wavelength the reflectivity decreases when the transition zone thickness increases and when the ratio t m increases we show that layers with second order boundaries velocity gradients in transition zones are only seismically detectable within a narrow thickness range extrapolation to thicker shear zones is based on the assumption that the strain gradient thickness relative to shear zone thickness is to a first approximation scale independent in granitic domains ductile shear zones with similar geometrical and petrophysical features to the example studied here will be detected on deep seismic profiles only if their width is between 20 and 400 m development of ductile shear zones with strain gradients of the appropriate thickness to enhance reflectivity is favored under low temperature conditions in the granitic upper crust indeed low temperature strain gradient may explain the high seismic reflectivity of the upper crust in the scandinavian caledonides whereas high temperature strain gradient may explain in part the relative transparency of the european variscan upper crust

elastic properties of anisotropic synthetic calcite muscovite aggregates 1 a set of synthetic aggregates containing mixtures of calcite and muscovite were compacted with varying uniaxial loads ranging from 20 mpa up to 400 mpa their elastic properties have been measured using compressional and shear waves in confining hydrostatic pressures up to 475 mpa measured seismic velocities are shown to depend on 1 the ratio of calcite to muscovite 2 the uniaxial load used during sample manufacturing and 3 the porosity amount prior to velocity measurements the matrix framework may also affect the seismic velocities but this effect is not easily quantifiable in general measured seismic velocities decrease with increasing muscovite content and porosity elastic properties have been calculated based on texture measurements which were obtained by neutron diffraction the calculated velocities are based on the calcite and muscovite single crystal elastic tensors their orientation distribution functions and their modal fractions a large discrepancy is apparent between calculated and measured velocities where the former always overestimate the actual velocities shear waves display less of a difference between calculated and measured values than do compressional waves indicating that waves propagating by shearing are less affected by sample porosity and matrix framework a better agreement between measured and calculated seismic velocities arises when the initial porosity is taken into account using a differential effective medium model seismic anisotropy is evident from both measured and calculated velocities and is low for samples containing pure calcite but becomes prominent as the muscovite concentration increases the intensity of anisotropy further depends on the uniaxial load used during sample compaction and the initial porosity

importance of pre impact crustal structure for the asymmetry of the chicxulub impact crater the final geometry of an impact crater can be significantly influenced by the geological and geomorphological structure of the impact site therefore it may not on its own provide accurate information regarding the direction and angle of impact

the effect of oriented microcracks on seismic velocities in an ultramylonite the influence of oriented microcracks on the directional dependence of p and s wave velocities has been investigated in an epidote muscovite ultramylonite optical and sem studies revealed that the fabric of the mylonite is characterized by oriented intragranular and grain boundary cracks comparison of orientation patterns of cracks with calculated elastic properties voigt average of a model rock isotropic matrix ultramylonite crack geometry and laboratory seismic data indicates that wave propagation is particularly affected by oriented grain boundary cracks microcracks are most effective at low pressure and largely closed above about 200 300 mpa the residual part of velocity anisotropy observed above this crack closing pressure is mainly due to preferred mineral orientation in particular of mica maximum concentration of poles to 001 mica planes corresponds to the maximum of poles to oriented grain boundary cracks so that the effects of texture and oriented cracks on wave velocities line up giving rise to extreme values of velocity anisotropy and shear wave splitting at low pressure conditions

origin of reflections from the brevard fault zone the brevard fault zone is a belt of mylonitic metamorphic rocks that extends northeast over 700 km from alabama into virginia seismic reflection studies show that the fault zone produces excellent reflections which dip gently to the southeast and sole at depth the origin of these reflections has been investigated by measuring compressional wave velocities to 800 mpa confining pressure in 127 samples obtained from drill core which continuously sampled the fault zone to a depth of over 300 m densities are reported as well as chemical analyses at selected depths anisotropy at elevated pressures ranges within the fault zone from less than 5 to over 26 similarly the fault zone rocks show significant chemical variability with sio2 ranging from 58 0 to 70 4 by weight synthetic seismograms generated from the velocity and density measurements show that the reflections originate within the brevard fault zone from the complex interaction of compositional variations and seismic anisotropy resulting from ductile strain essential to this study has been the continuous coring of a major section of the brevard fault zone which has provided the necessary stratigraphic control as well as unweathered samples for laboratory studies

seismic and magnetic susceptibility anisotropy of middle lower continental crust insights for their potential relationship from a study of intrusive rocks from the serre massif calabria southern italy abstract we investigated the relationships between fabric seismic and magnetic anisotropy on lithotypes representative of a continental crust exposed in the serre massif southern italy in particular from five granitoids and one metagabbro cubes were cut according to the fabric elements and seismic properties up to 400 mpa confining pressure were measured with a triaxial multi anvil apparatus we also calculated the elastic properties based on the mineral content and composition in granitoids measured average compressional wave velocity vp of the fracture free aggregate at 400 mpa is 6 2 km s whereas average shear wave velocity vs is 3 6 km s with poisson s ratio ranging from 0 240 to 0 257 related to the modal proportions of quartz in metagabbro average vp and vs at 400 mpa are 6 9 km s and 3 7 km s respectively results showed that intrinsic velocity distribution after microcracks closure depends on progressive alignment of anisotropic minerals such as biotite amphibole and pyroxene with maxima velocities localized within the foliation plane mean magnetic susceptibility k m of the granitic rocks is 6 si units indicating that paramagnetic minerals such as biotite and amphibole control the intensity of magnetic anisotropy comparison of seismic and magnetic anisotropies highlighted the different role of constituting minerals over the petrophysical properties moreover a positive correlation between seismic and magnetic anisotropy has been recognized indicating that biotite and amphibole contribute to the petrophysical and textural anisotropy in the middle crust conversely in metagabbro the anisotropy of magnetic susceptibility ams is controlled by magnetite and pyrrhotite although these form

seismic reflectivity of mylonite zones in the crust deep fault zones are characterized by mylonitic rocks with strong preferred orientation of constituent minerals and retrograde mineral assemblages sonic velocities are consequently lower for propagation directions normal to mylonite foliation than in surrounding unmylonitized rocks synthetic seismograms computed for low velocity near horizontal thick mylonite zones of complex laminated geometry show multicyclic reflections with amplitudes up to twice those generated by a single interface seismic characteristics of mylonite zones are sufficient to produce reflections in crystalline rocks provided noise levels are relatively low

seismic anisotropy in mylonites an example from the mannin thrust zone southwest connemara ireland abstract mylonites associated with the mannin thrust zone of southwesternmost connemara formed when the high grade metamorphic rocks typical of most of the connemara massif were thrust to the southeast over low metamorphic grade low greenschist facies acid volcanics and volcaniclastic sediments while being metamorphosed in the epidote amphibolite facies triaxial and biaxial ultrasonic velocity measurements of mylonite specimens from a 240 m borehole have established that there is significant seismic anisotropy up to about 11 when comparing velocities perpendicular and parallel to the foliation this would ultimately lead to a reflection coefficient of about 0 02 when comparing the mean isotropic seismic velocity with that perpendicular to the foliation the finely striped discontinunous mineral lithons that define mylonitic foliation but which form no real and continuous surfaces could interact with seismic energy to produce reflections that do not relate to lithological contacts within the rocks but to a tectonically induced orientated acoustic impedance however the results support the work of others in suggesting that on its own the fabric would not produce the high amplitude reflections observed on deep seismic lines and other mechanisms need to be investigated

the conductivity of the crust and mantle beneath the kapuskasing uplift electrical anisotropy in the upper mantle summary kapuskasing uplift has been interpreted as an oblique cross section of up to 25km of crust and thus provides the opportunity to examine the properties of exposed mid to lower crustal material a magnetotelluric mt survey mapped a remarkably uniform upper crust and provided no evidence for upper crustal conductive zones that could be related to the often observed increase in mid and lower crustal conductivity the only shallow conductive anomaly is related to the ivanhoe lake cataclastic zone but its electrical signature does not appear to extend more than one kilometer in depth a regional apparent resistivity curve was determined from mt data and confirmed by subsequent controlled source electromagnetic surveys regional curves are essential for proper structural interpretation but are often difficult to determine because of electric field distortions the data show a decrease in resistivity at depths below 15 km which is typical of continental crust in many areas of the world there is a clear difference in orthogonal apparent resistivity and phase curves at periods greater than 10s which is most pronounced in a n65 e and n25 w coordinate frame two different models are proposed to explain the data the first consists of a conducting 2 d slab of approximately 50 km width in the lower crust and striking n65 e the strike direction is consistent with a number of regional structural trends but there is no other direct supporting evidence for it the second model invokes either micro or macro electrical anisotropy in the upper mantle there is considerable evidence for seismic anisotropy in the region the electrical anisotropy could be the result of preferential conduction along the c axis of olivine crystals hydrogen dissolved in the olivine lattice with preferential conduction along the a axis or possible alignment of dykes joints and faults in the upper mantle if the anisotropy is related to strain induced orientations of crystals or fabric it will provide evidence for the motions of the mantle associated with plate tectonics

reflection characteristics of a mylonite zone based on compressional wave velocities of rock samples summary a combined field and laboratory study of the seismic reflection response and seismic properties of the mylonite zone capping the kettle dome metamorphic core complex provides important constraints on reflectivity of mylonites because of its structural and petrologic characteristics this mylonite zone can be regarded as a model for mid to deep crustal mylonite zones cdp reflection profiling revealed that the mylonite zone is reflective laboratory measurements of compressional wave velocities v at high confining pressures in samples collected directly up dip from the cdp reflection line revealed a wide variation of v in common mid crustal lithologies mafic and carbonate rock mean v values range from 6 67 to 7 31 km s at 600 mpa and quartzite and quartzo feldspathic values range from 6 16 to 6 52 km s seismic anisotropy at low pressures can be related to open microcracks and preferred mineral orientation whereas the latter effect controls anisotropy at high pressures the laboratory data provide the basis for a seismic model which indicates that the kettle dome mylonite zone can generate reflections from zones where high and low velocity lithologies are interlayered these reflections correlate with those observed on the cdp data thus demonstrating that the reflectivity of the mylonite zone is a direct result of its lithologic diversity and layered nature in this case it is not necessary to explain the reflections by the effects of anisotropy retrogression high pore pressure or constructive interference

coincident seismic reflection refraction studies of the continental lithosphere a global review nearly 50 coincident seismic reflection refraction studies to depths of at least the moho provide an improved understanding of the continental lithosphere some conclusions include the following 1 a transparent upper crust a common observation on vertical reflection profiles cannot generally be correlated with velocity gradients or low velocity zones rather a commonly transparent upper crust may be explained by short wavelength steeply dipping features in the brittle upper crust and to a lesser degree by signal contamination from source generated noise 2 the reflective lower crust in extensional terranes appears to be characterized by a high average seismic velocity 6 6 7 3 km s and to consist of laminated high and low velocity layers with typical thicknesses of 100 200 m 3 landward dipping reflectors observed in the middle to lower crusts of convergent zones have been identified as paired high and low velocity slabs which represent oceanic crust and mantle accreted via underplating to the continental margin 4 the crust mantle boundary may differ sufficiently when imaged with vertical incidence and wide angle data to justify the retention for the present of the concept of separate reflection and refraction mohos while there is good evidence that these features are coincident within measurement uncertainties in most regions recently recorded data from the basin and range admit the possibility for noncoincidence in that area 5 upper mantle reflections which cannot be migrated into the lower crust remain rare despite isolated unequivocal examples thus the upper mantle appears to be relatively homogeneous at seismic reflection wavelengths and to lack the laminations inferred for the lower crust the wide angle method will likely provide the most reliable information on the velocity structure and physical state of this portion of the lithosphere for some years to come 6 there appear to be clear and consistent basic differences between convergent and extensional terranes which have been identified from coincident experiments these differences may be sufficiently universal to infer the tectonic history of poorly exposed terranes 7 no truly three dimensional coincident experiment i e including three dimensional migration has been conducted but some three dimensional data have been collected using both methods measurements of attenuation poisson s ratio and anisotropy within the crust using coincident data sets remain frontiers

seismic velocity anisotropy in mica rich rocks an inclusion model summary we calculated seismic wave velocity anisotropy caused by the preferred orientation of mica minerals by using the differential effective medium method dem spheroidal biotite crystals with their c axes coinciding with the symmetry axis of the spheroid are embedded in an isotropic matrix up to a volume ratio of 30 per cent all crystals are aligned with their c axes parallel to the symmetry axis of the effective homogeneous medium which shows transverse isotropy the effect of crystal shape on anisotropy was studied by changing the aspect ratio the ratio between the minor axis and the major axis of the spheroid from 0 01 flat spheroid to 1 sphere the s velocity anisotropy becomes large as the crystal shape becomes flat whereas the p velocity anisotropy shows only small changes with changes of the crystal shape in particular biotite generates large s wave anisotropy and the anisotropy becomes stronger as the aspect ratio of the biotite crystal becomes smaller when the volume ratio of the mica mineral is large the p wave phase velocity surface shows considerable deviation from the ellipse and the sv wave phase velocity surface forms a large bulge and crosses the sh wave phase velocity surface singularity in the plane including the symmetry axis these results show an interesting contrast when compared with the effect of crack or pore shape on seismic velocity anisotropy crack or pore shape affects the p velocity more than the s velocity we also calculated thomsen s anisotropic parameters e and as functions of the crystal aspect ratio and the mica volume ratio

a model of velocity structure beneath calabria southern italy based on laboratory data abstract laboratory velocity measurements of representative rock samples from across a nearly complete crustal section from the serre mountains of southern calabria have been carried out as an aid to interpreting in situ seismic velocity data the section is composed of interlayered acidic mafic and pelitic rocks metamorphosed to granulite and amphibolite facies conditions in addition a suite of post tectonic granitoids intrudes the section the experimental data include the measurement of compressional wave vp and shear wave vs velocities at confining pressure in conjunction with structural and stratigraphic field data the experimentally determined data are used to construct a seismic velocity model for the crust beneath calabria in the upper crustal levels vp and vs vary in different manners below the uppermost amphibolite facies gneisses a weak velocity inversion of vp is associated with the post tectonic granites at the granite granodiorite and the tonalite dioritic gneiss boundary vp increases in steps of about 200 and 300 m s respectively in contrast vs remains constant or even decreases slightly through this same sequence of rocks this contrasting behaviour is primarily related to the quartz feldspar ratio in the rocks the most pronounced velocity jump occurs at the top of the granulite facies metapelite unit here both vp and vs increase markedly due to mineralogic changes that accompany increasing metamorphic grade and the greater abundance of mafic and garnet bearing restites of partially melted pelite the mafic rocks of the lowermost crustal layer have p and s wave velocities that are significantly lower than those in the overlying metapelites the variation of poisson s ratio with depth suggests that this parameter which is a combination of vp and vs is more sensitive to lithology than the p and s wave velocities alone in rock units of upper and mid crustal levels with average p wave velocities

reflections from a mylonitized zone in central sweden dipping reflections imaged on a deep seismic reflection profile acquired within the baltic shield can be indirectly linked to strain induced acoustic impedance contrasts within a regional brittle ductile shear zone the singo shear zone measurements made at various scales and frequencies estimate rock velocity within the shear zone and correlate well with the ductile component of strain previously determined by structural analysis velocity measurements include some made on cored field samples at confining pressures of 0 4 gpa in a laboratory 10 mhz sensor frequencies and some made in situ using ultrasonic 54 khz sensors on exposures within the shear zone laboratory measurements show a 10 change in seismic anisotropy and a contrast of 0 5 km s 1 in bulk rock velocity between granitoid protolith and highly strained ultramylonite the in situ velocity measurements demonstrate that the velocity variations persist over considerable volumes including zones where strain and associated volume loss of silica remain elevated without mylonitization synthetic seismograms generated using a one dimensional velocity model of the shear zone show that velocity variations observed in the field were large enough in magnitude and of sufficient scale to produce reflections observable on the deep seismic reflection profile collected using 6 25 hz frequencies ray trace two dimensional forward modeling of a high density wide angle reflection section demonstrates that deep dipping reflections could result from strain induced acoustic impedance contrasts within the singo or similar nearby shear zones

seismic anisotropy in the morcles nappe shear zone implications for seismic imaging of crustal scale shear zones microstructures and textures of calcite mylonites from the morcles nappe large scale shearzone in southwestern switzerland develop principally as a function of 1 extrinsic physical parameters incl

electrical conductivity temperatures and fluids in the lower crust we have compiled both laboratory and worldwide field data on electrical conductivity to help understand the physical implications of deep crustal electrical profiles regional heat flow was used to assign temperatures to each layer in regional electrical conductivity models we avoided those data where purely conductive heat flow suggested temperatures more than about 1000 sup 0 c substantially higher than solidus temperatures and outside the range of validity of heat flow models the resulting plots of log conductivity sigma versus 1 t demonstrate that even low conductivity layers lcl have conductivities several orders of magnitude higher than dry laboratory samples and that the data can be represented by straight line fits in addition tectonically active regions show systematically higher conductivities than do shield areas because volatiles are usually lost in laboratory measurements and their absence is a principal difference between laboratory and field conditions these materials probably account for the relatively higher conductivities of rocks in situ in the crust free water in amounts of 0 01 0 1 in fracture porosity could explain crustal conductivities other possibilities are graphite hydrated minerals in rare instances and sulfur in combination with other volatiles as most of the temperatures are less than 700 sup 0 c partial melting seems likely onlymore in regions of highest heat flow where the conductive temperature profiles are inappropriate another result is that at a given temperature crustal high conductivity layes hcl are more conductive by another order of magnitude and show more scatter than do lcl s less

fabric related velocity anisotropy and shear wave splitting in rocks from the santa rosa mylonite zone california the directional dependence of p and s wave velocities have been measured at pressures up to 600 mpa and temperatures up to 700 c in rocks from the santa rosa mylonite zone southern california during tectonism these were progressively deformed from granodiorite protolith to mylonite and ultimately phyllonite the mineralogical and chemical composition of protolith and mylonite is nearly identical thus these rocks provide excellent material for documenting the effect of microstructural and textural changes on rock anisotropy velocity anisotropy increases significantly with the degree of deformation whereas average velocities and densities do not change at low pressure 50 mpa the velocity anisotropy ranges from 1 7 in granodiorite up to 19 in phyllonite and is due to both oriented microfractures and crystallographic preferred orientation at high pressure 600 mpa the residual anisotropy up to 12 is mainly due to preferred mineral orientation in particular of biotite significant shear wave splitting is measured parallel to the foliation plane and shows a good correlation with the biotite texture these observations confirm that oriented microcracks and preferred orientation of minerals should be taken into account in the interpretation of seismic reflection and refraction data in terranes with deformed rocks

seismic velocity and anisotropy in mylonites and the reflectivity of deep crystal fault zones compressional wave velocity anisotropy has been measured to 1 kb confining pressure in several mylonites in one cataclasite and in their protoliths no significant difference in the velocity or density was found between the protolith and the tectonically altered rock at 1 kb strong seismic anisotropy greater than 10 was present in only two of nine mylonites measured the anisotropy was due to their aphanitic texture high content of layered silicates and pervasive foliation most mylonitic rocks studied were not strongly anisotropic despite a well developed macroscopic foliation where anisotropy was present velocities usually were slowest perpendicular to the foliation and fastest parallel to both foliation and lineation these preliminary results suggest that ductile fault zones in the crust are not strong seismic reflectors unless mylonites in the fault zone are aphanitic and contain large amounts of foliated phyllosilicates it is also possible that anomalous environmental conditions such as high pore pressure in the fault zone in situ may explain observed seismic reflectivity

complete seismic properties obtained from microcrack fabrics and textures in an amphibolite from the ivrea zone western alps italy abstract laboratory seismic measurements of compressional wave velocities vp and shear wave splitting vs have been carried out on a clinopyroxene amphibolite from the ivrea zone western alps italy at pressures up to 600 mpa from the texture of the major minerals hornblende plagioclase and pyroxene and the respective elastic constants the elastic properties voigt average and seismic anisotropies were calculated at high confining pressure where the effect of microcracks is eliminated vp and shear wave velocities vs vss are in good agreement with the calculated velocities reflecting texture induced anisotropies microcracks are most effective at low confining pressure but are largely closed at about 200 300 mpa microscopic studies revealed that the crack fabric is characterized by oriented transgranular and cleavage cracks the bulk anisotropy at low pressure conditions was computed by superposing the texture induced anisotropy with the crack induced anisotropy these calculated values are consistent with the measured values indicating that anisotropy of vp and shear wave splitting is affected by both texture and oriented microcracks based on detailed fabric analyses the presented method allows to describe the complete elastic and seismic properties of crystalline rocks providing an ideal data basis for three dimensional seismic modelling to investigate the influence of anisotropic media on the reflectivity

preferred lattice orientations of plagioclase from amphibolite and greenschist facies rocks near the insubric line western alps abstract preferred lattice orientations of plagioclase measured by the universal stage from deformed amphibolite and greenschist facies rocks near the insubric line western alps reflect the type and orientation of the strain system either 001 or 010 are oriented parallel or subparallel to the schistosity and form more or less complete girdles around the stretching direction by the preferred orientation patterns dominant 010 001 slip is indicated and additionally slip on 001 parallel to the a direction possibly at somewhat higher temperatures and or at a higher an content of the plagioclase it is suggested that obliquities of the symmetry planes of the orientation patterns against the principle axes of strain are related to the sense of shearing

flin flon belt seismic anisotropy elastic symmetry heterogeneity and shear wave splitting laboratory measurements of compressional and shear wave velocities and shear wave splitting have been carried out on a set of upper greenschist lower amphibolite facies of metasediments and metavolcanics and plutonic rocks from two ductile shear zones in the flin flon belt ffb of the trans hudson orogen tho selected metamorphic rocks vary in composition from felsic to mafic test sites with outcrops of sheared metamorphic rocks were correlated with a series of inclined seismic reflectors possibly extending from the midcrust and intersecting a well mapped shear zone at the surface determination of the lithological and physical properties of highly deformed metamorphic rocks is essential for proper interpretation of the nature of observed seismic reflectors to investigate the anisotropic properties of the rocks compressional velocity was measured at confining pressure up to 300 mpa in three mutually orthogonal directions aligned with respect to visible textural features in addition on selected

the influence of quartz textures on the seismic anisotropy in lower crustal granulites analyses that are based on the spherical harmonic representation of the orientation distribution function odf which can describe the three dimensional nature of textures can be used to evaluate the effective elastic response of quartz in polycrystalline aggregates and to account for the dependence of crystal and sample symmetries the aggregate elastic constants are determined using the voigt method by averaging overall orientations weighted by the odf the application of the results is given for three different commonly observed quartz textures the directional dependence of the seismic wave velocities vp and shear wave splitting that are calculated by odf using the texture pole figures can also be shown using the respective single crystal velocities the experimental seismic data at 400 mpa for a muscovite bearing quartzite where most of the cracks are closed are in good agreement with the calculated velocity anisotropies voigt average that are based on volume percentages of major minerals the stiffness tensors of rock forming minerals and the measured crystallographic orientation the combination of the calculated elastic constants from different quartz textures together with plagioclase muscovite orthoclase garnet and sillimanite explain the texture related directional dependence of vp vs and vss in lower crustal granulites the results suggest that the directional variation of wave velocities of quartz rich granulites do not provide the necessary impedance contrast that gives rise to strong seismic reflections

northern hemisphere controls on tropical southeast african climate during the past 60 000 years the processes that control climate in the tropics are poorly understood we applied compound specific hydrogen isotopes d and the tex86 tetraether index of 86 carbon atoms temperature proxy to sediment cores from lake tanganyika to independently reconstruct precipitation and temperature variations during the past 60 000 years tanganyika temperatures follow northern hemisphere insolation and indicate that warming in tropical southeast africa during the last glacial termination began to increase 3000 years before atmospheric carbon dioxide concentrations d data show that this region experienced abrupt changes in hydrology coeval with orbital and millennial scale events recorded in northern hemisphere monsoonal climate records this implies that precipitation in tropical southeast africa is more strongly controlled by changes in indian ocean sea surface temperatures and the winter indian monsoon than by migration of the intertropical convergence zone

delayed hydrological response to greenland cooling at the onset of the younger dryas in western europe during the younger dryas cold event about 12 800 years ago environmental change in western europe seems to occur 170 years after cooling over greenland lake sediment analyses confirm this delay and suggest european hydrological and vegetation change occurred only after the build up of sea ice in the north atlantic pushed the westerly wind system south

rupture creep lamellae and crocodiles happenings in the continental crust from the observation of certain seismic reflectivity patterns in phanerozoic continental crusts a relationship is suggested between 1 rupture and seismicity generally restricted to the upper crust 2 crocodile tectonics a kind of vertical indentation observed by diverging plane reflectors preferably in the middle crust 3 creep and the formation of reflecting lamellae in the lower crust based on these observations and on viscosity depth estimates a conceptual model for the above mentioned structures is presented which is also compatible with observations in older crusts boundary conditions are given for rupture as well as for lamellae forming creep

laboratory seismic measurements an aid in the interpretation of seismic field data this contribution reports data and evidence from laboratory measurements of compressional and shear wave velocities and mineralogical microstructural and chemical analyses in crustal rocks that may provide an aid to understanding and interpreting geophysical data it is shown that the correlation of laboratory seismic data with stratigraphic field data can provide the necessary data base for constructing a seismic model of the earth s crust in a particular geological area

development of shape and lattice preferred orientations application to the seismic anisotropy of the lower crust abstract we review the physical basis of the development of fabrics in plastic and viscous flow and illustrate the typical fabrics formed by these processes in the main rock forming silicates of the lower crust feldspar quartz pyroxene and amphibole the orientation process in plastic deformation where a single slip is dominant is recalled and the role of the constraint of neighbouring grains is emphasized the fabric development of anisometric crystals in viscous flow is discussed as a function of the main controlling parameters shear strain aspect ratio and interference between crystals the same sense of fabric asymmetry is introduced by plastic and viscous flow between the flow plane and the shape preferred orientation and hence coherent kinematic analysis can be undertaken in both modes of flow in order to assess the role of such fabrics in the seismic laminations of the lower continental crust we have calculated the seismic p wave properties of typical fabrics for hypothetical monomineralic and polymineralic rocks the calculations show that the strongest anisotropies develop in monomineralic rock with values between 5 and 16 compared with 5 and 8 for typical rock compositions the strongest anisotropies for layered monomineralic rocks generated by fabrics is only 6 compared to the 14 suggested by model studies of the observed seismic laminations we suggest that other effects such as compositional layering and or constructive interference of seismic waves are responsible for augmenting the apparent anisotropy

taraxerol and rhizophora pollen as proxies for tracking past mangrove ecosystems angola basin and cape basin southeast atlantic surface sediments and sediment cores show that maxima in the abundance of taraxerol relative to other land derived lipids covary with maxima in the relative abundance of pollen from the mangrove tree genus rhizophora and that in the surface sediments offshore maxima in the relative abundance of taraxerol occur at latitudes with abundant coastal mangrove forests together with the observation that rhizophora mangle and rhizophora racemosa leaves are extraordinarily rich in taraxerol this strongly indicates that taraxerol can be used as a lipid biomarker for mangrove input to the se atlantic the proxy environment relations for taraxerol and rhizophora pollen down core show that increased taraxerol and rhizophora pollen abundances occur during transgressions and periods with a humid climate these environmental changes modify the coastal erosion and sedimentation patterns enhancing the extent of the mangrove ecosystem and or the transport of mangrove organic matter offshore analyses of mid pleistocene sediments show that interruption of the pattern of taraxerol maxima during precession minima occurs almost only during periods of low obliquity this demonstrates the complex environmental response of the interaction between precession related humidity cycles and obliquity related sea level changes on mangrove input

the relationship between open dry microcracks and p wave velocity of crystalline rocks application to in situ stress estimates on ktb drill cores after stress relaxation experiments were performed to quantitatively confirm the relationship between open dry microcracks and p wave velocity on crystalline rocks equations derived by hudson 1980 first order corrections only describe the experimentally obtained relationship microcrack analysis consists of chemical contact imagery to make the cracks visible image analysis and stereologic data processing to obtain three dimensional crack characteristics from data obtained in three orthogonal plane images pressure dependent measured p wave velocities are used to estimate intrinsic p wave velocities velocities characteristic for the uncracked rock by extrapolating the linear part of the pressure velocity curve from high pressure to low pressure therefore the impact of open microcracks on p wave velocity is given by normalized p wave velocities which are the ratio between measured and intrinsic velocities at a hydrostatic pressure of 1 mpa three dimensional crack characteristics and normalized p wave velocities are obtained in three orthogonal directions velocities in all other directions were estimated by interpolation assuming an orientation distribution that can be described by a triaxial ellipsoid in this way it is possible to study the relationship between open dry cracks and p wave velocity experimentally and to compare the results with the theoretically predicted relationship using one single model in a case study pressure dependent measured p wave velocities have been used to infer the distribution of open microcracks in 84 drill cores from the continental deep drilling project of the federal republic of germany kontinentales tiefbohrprogramm der bundesrepublik deutschland or ktb covering the depth range between 621 8080 m the cores were studied after stress relaxation 1 7 years after drilling it was not possible to obtain information about the in situ stress field owing to the anisotropic nature of the rock fabric

amphibole and lower crustal seismic properties abstract measurements of seismic anisotropy frequently offer insights of the deformation kinematics in situ within the earth in the continental crust seismic anisotropy is commonly ascribed to regionally aligned lattice preferred orientation lpo of mica crystals however determinations of composition suggest that the deep continental crust contains rather little mica in this contribution the role of amphibole a far more plausible constituent especially for deep continental crust of basic composition is assessed a field analogue approach is adopted where the lpo are measured correlated with the strain state of the rocks and used to calculate the seismic properties the seismic properties may be up scaled through the specimen using the modal proportions of the constituent mineral phases to examine the role of composition in controlling the intensity of seismic anisotropy seismic properties are computed using an array of modal compositions so called rock recipes the case history comes from the central block of the lewisian of nw scotland where a regional basic dyke swarm the scourie dykes is locally deformed within amphibolite facies shear zones conditions that are representative of the deep crust shear strain profiles across a 100 m wide shear zone using deflected banding in the host gneisses yield values of 0

spatial gradients in plant leaf wax d h across a coastal salt marsh in southern california coastal salt marsh ecosystems contain strong environmental gradients that are anticipated to influence the d h ratios recorded in the leaf waxes of salt tolerant plants we characterized the molecular and hydrogen isotopic composition of alkanes in plant and sediment samples as well as the d h ratios of environmental and plant waters across an elevation and inundation gradient in a southern californian coastal salt marsh we sampled the dominant salt marsh plant species salicornia virginica arthrocnemum subterminale and jamuea carnosa all succulents as well as monanthochloe littoralis and limonium californicum nonsucculents plant xylem water hydrogen isotopic compositions indicate a shift in source waters from meteoric influences at upland sites dd value 20 to seawater dominated values 0 at lowland areas we found leaf water d enrichment relative to xylem water ranging from mean dd values of 54 upland to 28 lowland interpreted as a reduction of transpiration with increasing inundation time this has the effect of increasing the net fractionation between source water and leaf wax product across the environmental gradient from mean values of 101 upland to 134 lowland with an attenuated signal recorded in the dd values of plant leaf wax n alkanes 122 to 136 these results constrain the hydrogen isotopic composition of salt marsh organic matter that may contribute to marine carbon budgets of the santa barbara basin and further indicate the potential for plant leaf waxes to resolve paleoenvironmental change including sea level change in sediment cores from salt marsh ecosystems

structural geology the buried record of chicxulub impact craters often have asymmetric shapes which have been used to infer the direction and angle of impact but pre existing structural or topographic heterogeneities also play an important role in crater asymmetry