the influence of microstructure on seismic wave speed anisotropy in the crust computational analysis of quartz muscovite rocks summary we study the influence of microstructural variables on seismic wave speed anisotropy in crustal rocks the bulk elastic properties and corresponding wave velocities are calculated for synthetic rock samples with varying amounts of muscovite and quartz different muscovite and quartzgrainorientationsandvaryingspatialdistributionsofthemuscovitegrainstoinvestigate the sensitivity of seismic wave speed anisotropy on these characteristics the asymptotic expansionhomogenizationmethodcombinedwithfiniteelementmodellingisusedtocalculate bulk stiffness tensors for representative rock volumes and the wave velocities are obtained from these tensors using the christoffel equation the aim of this paper is to 1 demonstrate how wave speeds computed from the rigorous asymptotic expansion homogenization method compare with those generated using stiffness tensors derived from commonly applied analytic estimates and 2 explore how different microstructural variables influence seismic wave speeds our results show that the muscovite grain orientations have a significant influence on the wave speeds increasing the modal fraction and alignment of muscovite grains leads to greater seismic anisotropy of the rock the p wave speed at an incidence angle of 45 between the

simple shear deformation and quartz crystallographic fabrics a possible natural example from the torridon area of nw scotland ln this paper the microstructures and crystallographic fabrics of a 30 cm wide deformed quartz vein from the torridon area of nw scotland are described the vein which lies along a joint separating two blocks of proterozoic gneiss was deformed by crystal plastic processes when the rigid joint blocks moved in opposite directions parallel to the vein margins mylonitic foliation within this quartzose ductile shear zone locally displays the classic orientation variation expected for heterogeneous simple shear deformation in thin section the vein is a mylonite composed of dynamically recrystallized elongate quartz grains aligned oblique to the mylonitic foliation fabrics within the central part of the vein have been examined by optical and x ray texture goniometry methods together with orientation distribution function odf analysis the a axis fabric consists of a dominant single point maximum orientated within the xz plane at 9 to the lineation x this point maximum occupies a pole position to the corresponding single girdle c axis fabric for individual positions on the c axis fabric skeleton the orientations of potential crystallographic slip systems slip plane and direction relative to shear zone co ordinates have been calculated by odf analysis the calculation of relative resolved shear stresses schmidfactors for the potential slip systems assuming a simple shear kinematic framework reveals an elegantly simple relationship between individual grain c axis orientation and correspond ing orientations of potential slip systems these results are readily interpreted in terms of bulk simple shear deformation

mica deformation fabrics and the seismic properties of the continental crust abstract seismic anisotropy originating within the continental crust is used to determine kinematic flow lines within active mountain belts and is widely attributed to regionally aligned mica however naturally deformed micaceous rocks commonly show composite e g s c fabrics it is necessary therefore to understand how both varying mica content and differing intensities of multiple foliations impact on seismic interpretations in terms of deformation fields an outcrop analogue for granitic mid crustal deformed zones is used here to calibrate the seismic response against both parameters seismic responses are modelled using crystallographic preferred orientations for polymineralic micaceous granitic gneisses measured using electron back scatter diffraction the sample results are generalised by modelling the effects of variations in modal composition and the relative importance of deformation fabrics of variable orientation so called rock and fabric recipes the maximum p and s anisotropy are calculated at 16 6 and 23 9 for single foliation gneisses but for mixed i e s c foliation gneisses these values reduce to 5 8 and 7 5 respectively furthermore mixtures of multiple foliations generate significant variations in the geometry of the seismic anisotropy this effect coupled with the geographical orientation of fabrics in nature can generate substantial variations in the orientation and magnitude of seismic anisotropy especially for shear waves as measured for the continental crust using existing receiver function and teleseismic near vertical incidence methods thus maps of seismic anisotropy varying with depth in deforming continents need not imply necessarily depth varying deformation kinematics and tectonic decoupling

grain boundary contact effects during faulting of quartzite an sem ebsd analysis abstract during low temperature faulting of cambrian quartzite assynt nw scotland stress concentrations develop at grain contacts either at the onset of deformation prior to the establishment of a through going fault plane or within the damage zone remote from the main displacement segment such concentrations contribute to the development of intragranular microfractures cataclastic microstructures and fault rocks this contribution considers the progressive deformation sequence that precedes microfracturing and cataclasis the complexity of this deformation is revealed by scanning electron microscope sem electron backscattered diffraction ebsd dauphine twinning is a widespread feature associated with grain contact stress concentration and forms distinctive ebsd microstructures automatic sem ebsd analysis reveals that whilst initial indentation causes dauphine twinning of many grains continued indentation results in the formation of an arcuate array of subgrains via low temperature plasticity and or microcracking which overprint the dauphine twins these observations are consistent with transmission electron microscopic analysis of quartz crystals used for microhardness indentation tests which reveal that indentation causes an intensely deformed region to develop comprising a high density of microfractures and a submicron scale blocky microstructure that accommodates any plastic deformation deformation mechanisms and associated microstructures develop sequentially with progressive indentation and may provide sites of microfracture nucleation via low temperature ductile fracture the new microstructures assist diffusive mass transfer dmt processes by the formation of a cellular or subgrain array that represents a reduction of several orders of magnitude in apparent grain size and hence in diffusion path length concomitantly associated microfracturing perturbs local thermodynamic equilibrium leading to enhanced dmt crack healing and cementation overgrowths together these processes form the aseismic creep and sealing components of fault zone development

marine incursions and the influence of andean tectonics on the miocene depositional history of northwestern amazonia results of a palynostratigraphic study abstract new palynological and sedimentological data permit the age of neogene sediments in northwestern amazonia to be established more precisely and indicate that major environmental changes occurred in the area during this time based on a study of borehole samples the age of the solimoes formation solimoes basin northwestern brazil is determined as miocene and five pollen zones are distinguished which are correlated with existing zonations for northern south america in addition 30 new sporomorphs are described which belong to the following 12 genera psilamonocolpites retimonocolpites retitricolpites retibrevitricolpites psilatriporites nov gen bombacacidites psilatricolporites retitricolporites rugutricolporites psilastephanoporites nov gen psilastephanocolporoties and heterocolpites a correlation is made between the brazilian wells and some of the studied outcrops in colombian and peruvian amazonia based on palynological marker species the presence of coastal elements such as mangroves zonocostites indicates that during the miocene this area was influenced by marginal marine conditions caused by several marine incursions these incursions may be related to global sea level fluctuations miocene marine phases also are known from sites elsewhere in northern south america which at present like northwestern amazonia are entirely ruled by continental conditions sediment composition shows that during the early miocene the guyana shield was the major source area of sediment input in the basins of northwestern amazonia in the interval from early to middle miocene the andes became the major sediment source area the change in provenance is related to the uplift of the eastern cordillera this event caused a major change in palaeoenvironment and palaeogeography in northwestern amazonia which was characterized by the reverse of a northwestward directed fluvial system into an eastward directed fluvial lacustrine system with an estuarine character

individual orientation measurements in quartz polycrystals advantages and limitations for texture and petrophysical property determinations individual orientation determination of quartz grains by electron channelling in principle gives the complete orientation however in routine analysis the noise level in electron channelling patterns ecps does not permit the determination of handedness of a quartz grain in a polycrystal in practice all quartz grains are arbitrarily indexed as right handed hence dauphine twins can be identified but not brazil twins this practice also means that only the centrosymmetric petrophysical properties can be determined from texture measurements these include most geologically relevant properties e g thermal conductivity thermal expansion and elasticity however other properties e g piezoelectricity which are not centrosymmetric cannot be calculated from such texture measurements some texture forming processes e g dislocation glide can also be considered to be centrosymmetric in quartz whereas others e g grain boundary migration may not be the method of quantitative texture analysis from individual measurements is briefly recalled as an example 382 grains from tongue quartzite are used to illustrate the advantages of texture analysis from ecps the orientation distribution function odf is calculated from ecps and x ray pole figures of the same sample the agreement is found to be good between the two methods proving that ecps can be used for quantitative analysis the methods used in local texture analysis and the definitions of the various misorientation distribution functions modfs are given data collected from a traverse of a quartzo feldspathic shear zone in lewisian gneiss torridon quartzite are used to illustrate local texture analysis examples from a region of shear strain of about one are given of core and mantle subgrains and dauphine twins dispersion trails of the crystallographic axes within a single grain show an apparent rotation about the intermediate structural axis y detailed analysis of the subgrain misorientation axes in specimen and crystallographic co ordinates show an important scatter implying that the subgrains resulted from local incompatibility strains rather than specimen scale kinematics the method of calculation of physical properties from individual orientation measurements is given for second and fourth order tensors using the texture data from tongue quartzite we have calculated thermal conductivity thermal expansion and seismic velocities all these properties are extremely anisotropic in quartz however it is emphasized that the presence of a second phase on grain boundaries e g water graphite may completely alter a physical property e g electric and render the values calculated from texture measurements inappropriate

microstructural and crystal fabric evolution during shear zone formation abstract the microstructures and crystal fabrics associated with the development of an amphibolite facies quartzo feldspathic mylonitic shear zone torridon nw scotland have been investigated using sem electron channelling our results illustrate a variety of microstructures and fabrics which attest to a complex shear zone deformation history microstructural variation is particularly pronounced at low shear strains significant intragranular deformation occurs via a domino faulting style process whilst mechanical incompatibilities between individual grains result in characteristic grain boundary deformation accommodation microstructures a sudden reduction in grain size defines the transition to medium shear strains but many of the boundaries inherited from the original and low shear strain regions can still be recognized and define distinctive bands oriented at low angles to the shear zone margin grains within these bands have somewhat steeper preferred dimensional orientations these domains persist into the high shear strain mylonitic region where they are oriented subparallel to the shear zone margin and consist of sub 20 m grains the microstructures suggest that the principal deformation mechanism was intracrystalline plasticity with contributions from grain size reduction via dynamic recrystallization grain boundary migration and grain boundary sliding crystal fabrics measured from the shear zone vary with position depending on the shear strain involved and are consistent with the operation of several crystal slip systems e g prism basal rhomb and acute rhomb planes in a consistent direction probably parallel to a and or m they also reveal the presence of dauphine twinning and suggest that this may be a significant process in quartz deformation a single crystal fabric evolution path linking the shear zone margin fabric with the mylonitic fabric was not observed rather the mylonitic fabric reflects the instantaneous fabric which developed at a particular location for a particular shear strain and original parental grain orientation the mature shear zone therefore consists of a series of deformed original grains stacked on top of each other in a manner which preserves original grain boundaries and intragranular features which develop during shear zone evolution the stability of some microstructures to higher shear strains the exploitation of others at lower shear strains and a continuously evolving crystal fabric mean that the strain gradient observed across many shear zones is unlikely to be equivalent to a time gradient

first results on the lpo derived seismic properties of iron ores from the quadrilatero ferrifero region southeastern brazil abstract determinations of the lpo derived seismic properties of iron ores were carried out in five samples with contrasting mineralogy hematite magnetite and quartz and deformed in different conditions all the samples are seismically quasi isotropic or weakly anisotropic which reflect i an absent or weak crystallographic preferred orientation cpo in some samples ii the high modal content of magnetite iii the relatively weak anisotropy of elastic stiffness of hematite single crystal such variables induce a low anisotropic seismic behavior even in high strained iron ores with strong preferred orientation of hematite a plane of seismic transversal isotropy parallel to the foliation of the aggregates is developed in hematite magnetite aggregates and in itabirites with strong cpo of hematites in these high strained aggregates some relationships between the crystallographic axes of hematite and the propagation velocities can be observed the magnitudes of p and s wave velocities derived of hematite cpo are lower than the values experimentally determined in iron ores such a difference probably reflects other microstructural variable which were not taken into account in the present contribution e g shape preferred orientation

d rossetti p mann de toledo a m goes new geological framework for western amazonia brazil and implications for biogeography and evolution quaternary research 63 2005 78 89 letter to the editor

provenance of quaternary and modern alluvial deposits of the amazonian floodplain brazil inferred from major and trace elements and pb nd sr isotopes abstract chemical and pb nd and sr isotope data from floodplain sedimentary deposits in the solimoes river northwest brazil constrain the variability of the deposits through the quaternary and help reconstruct the paleogeography of amazonia since the uplift of the andes compared to the modern alluvial deposits in the active channel the quaternary terrace deposits in the solimoes river have a higher chemical index of alteration cia higher sio2 al2o3 ratios higher concentrations of immobile elements higher radiogenic sr isotopic compositions more negative end t 0 values and older nd tdm ages these geochemical characteristics indicate that the quaternary sediment sources were composed by more felsic and or recycled old continental crust derived rock materials than the younger source areas which contributed major arc volcanic materials to the flood plain deposits this shift in the composition of the sediment source areas indicates that a significant reorganization of the architecture of the solimoes river has occurred since the uplift of the andes

seismic anisotropy of the upper mantle 1 factors that affect mineral texture and effective elastic properties 1 flow induced mineral alignment in the earth s mantle affects the nature of seismic wave propagation since measurements of seismic travel time and shear wave splitting are a key means by which the structure of the upper mantle can be imaged it is important to understand the factors that contribute to variability in elastic properties seismic anisotropy associated with lattice preferred orientations of minerals in peridotite is the focus of this study one way to better understand anisotropy in a convecting upper mantle is to simulate texture evolution based on certain assumptions simulations of the development of olivine and orthopyroxene alignment along streamlines of a mantle flow field illustrate how continuously varying strain conditions affect the resulting orientation distribution there are various uncertainties in rock texture simulations and the aim here is to investigate how much model assumptions may influence the results a model of upper mantle flow in the vicinity of an oceanic spreading center is used to illustrate these points first we assess how sensitive results are on assumptions of different polycrystal plasticity models specifically lower bounds and viscoplastic self consistent approaches we also investigate how recrystallization during deformation might affect the texture that develops along a streamline the effects of grain growth and nucleation produce as expected textures that are different from the deformation only case however the basic p wave structure of the predicted anisotropy is similar between models for streamlines in a slow spreading passive flow model as is commonly used for simulating flow near this type of plate boundary shear wave splitting patterns are more complex and differ somewhat more between the models in the off axis region we also compare predictions for texture development of olivine only aggregates to that of mixed composition with 70 olivine and 30 orthopyroxene although we consider only one aspect of such polyphase deformation i e plastic deformation on specified crystal slip systems our results are consistent with field observations of the orientation distributions in ophiolitic peridotites finally we determine how simplifying assumptions about the symmetry of the elastic anisotropy can bias interpretations of seismic travel time and shear wave splitting whereas a hexagonal approximation can lead to underestimates of the degree of anisotropy an orthorhombic approximation is found to closely match the results predicted for a general elastic tensor corresponding to an orientation distribution whose symmetry is as low as monoclinic or triclinic

interpretation of sks waves using samples from the subcontinental lithosphere abstract the seismic properties of five south african kimberlite nodules have been calculated from petrofabric measurements these garnet lherzolite and harzburgite composition nodules 56 80 olivine are known from previous studies to have originated at depths of 120 170 km in the subcontinental lithosphere their in situ seismic properties have been calculated by extrapolating the elastic constants to the appropriate temperature 900 1050 c and pressure 3 0 3 5 gpa conditions the average of the five samples has a maximum s wave anisotropy v s of 3 7 which is sufficiently high to explain previously reported teleseismic sks delay times t in continental shield areas of up to 1 7 s the biggest delay time is observed when the foliation is vertical the lineation horizontal and the fastest s wave polarized parallel to the lineation i e an orientation for transcurrent motion delay times of over 1 s can only be generated by this orientation a strong mantle crust mechanical coupling is suggested for such situations the potential use of seismic anisotropy as an indicator of strain in the lithosphere has been investigated using experimental and simulated fabric data for olivine the most abundant phase 70 in the upper mantle the v p and v s seismic anisotropy increases with fabric strength and finite strain recrystallization tends to reduce fabric strength and seismic anisotropy resulting in saturation values for experimental and simulated fabrics which correspond to approximately 50 60 strain or 8 9 v s for pure olivine aggregates a survey of 25 naturally deformed peridotites of oceanic origin suggests an average maximum s wave anisotropy for the olivine component of 9 or about 8 for a lherzolite or harzburgite rock composition when the orthopyroxene component is taken into account the ophiolite samples are twice as anisotropic as the kimberlite nodules if an average anisotropy value is representative of the lithosphere then the sks delay times represent variations in anisotropic layer thickness for delay times over 1 s as the orientation is constrained to be foliation vertical and lineation horizontal the magnitude of s wave anisotropy is less sensitive than v p to variations in olivine volume fraction in range 50 100 and to deformation or fabric intensity further suggesting that s wave anisotropy is particularly apt for the determination of the anisotropic lithosphere thickness for smaller delay times there is some trade off between structural orientation and thickness

numerical analysis of x ray texture data an implementation in fortran allowing triclinic or axial specimen symmetry and most crystal symmetries abstract the use of complete descriptions of preferred orientation for studies of experimentally and naturally deformed rocks is discussed diagrammatic representations of preferred orientations are defined and conventions for coordinate systems are given methods of extracting information from the orientation distribution function are described a system of fortran programs which enables the orientation distribution function to be calculated for most suitable minerals is described

evidence for regional dauphine twinning in quartz from the santa rosa mylonite zone in southern california a neutron diffraction study preferred orientation in granitic mylonites from the santa rosa mylonite zone in southern california is investigated with time of flight neutron diffraction quartz and biotite display strong preferred orientation whereas feldspar alignment is weak for quartz a c axis maximum in the intermediate fabric direction is consistent with dynamic recrystallization pole figures for positive and negative rhombs such as rz 10 11 and zz 01 11 are distinct over a wide region suggesting that the pattern reflects tectonic deformation and was not

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

petrofabric derived seismic properties of a mylonitic quartz simple shear zone implications for seismic reflection profiling the link between petrofabric lpo and seismic properties of an amphibolite facies quartzofeldspathic shear zone is explored using sem ebsd the shear zone lpo develops by a combination of slip systems and dauphine twinning with a maximum parallel to lineation x and c maximum normal to mylonitic foliation xy the lpo are used to predict elastic parameters from which the three dimensional seismic properties of different shear zone regions are derived results suggest that lpo evolution is reflected in the seismic properties but the precise impact is not simple in general the p wave velocity vp minimum is parallel to the a axis maximum the direction of maximum shear wave splitting avs is parallel to mylonitic foliation and the vp maximum and avs minimum are parallel to the c axis maximum the seismic anisotropy predicted is significant and increases from shear zone wall rock to mature mylonite p wave anisotropy ranges from 11 13 fast and slow shear waves anisotropy range from 6 15 and the magnitude of shear wave splitting ranges from 9 16 nevertheless such anisotropy requires a considerable thickness of rock with this lpo before it becomes seismically visible i e 100 s m for local earthquakes 10 s m for controlled source experiments however reflections and mode conversions provide much better resolution particularly across tectonic boundaries the low symmetry and strong anisotropy due to the lpo suggest that multi azimuth wide angle reflection data may be useful in the determination of the deformation characteristics of deep shear zones a number of factors may produce elastic anisotropy in rocks including variations in the spatial distribution of mineral phases layering grain size and shape fabrics grain boundary properties and the presence of oriented pores or fractures e g kern wenk 1985 mainprice et al 2003 in addition because elastic properties vary with respect to direction in single crystals the majority of rock forming minerals are elastically anisotropic e g christensen 1966 thus deformation processes such as dislocation creep which produce strong crystal lattice preferred orientations lpo in anisotropic minerals may also induce bulk elastic anisotropy it is no surprise therefore that petrophysical properties such as seismic velocities are often highly anisotropic in bulk rock aggregates e g babuska cara 1991 understanding of the impact of microstructural variables on elastic anisotropy has been obtained typically from experimental measurements of elastic properties of minerals or theoretical models that incorporate microstructural variables e g burlini kern 1994 wendt et al 2003 experimental studies have focused primarily on the influence of fractures or lpo on the elastic properties of minerals and rocks in particular the seismic properties can be measured via direct laboratory methods at ambient or elevated temperatures and pressures e g kern 1982 although it is often difficult or impossible to obtain the complete three dimensional property orientation distribution from a single experiment however see pros et al 2003 furthermore experimental investigation of seismic properties usually involves rocks in which all microstructural fabric elements contribute effectively masking any lpo component an alternative approach is to calculate the seismic properties of a rock aggregate from individual crystal orientation measurements incorporating the single crystal elastic constants and the lpo for each mineral weighted according to its modal content e g mainprice humbert 1994 the seismic phase velocities and anisotropies then can be calculated in every direction e g mainprice silver 1993 recently the advent of electron backscattered diffraction ebsd in the scanning electron microscope sem has made it possible to measure lpo in statistically viable numbers from different minerals within a rock aggregate whilst maintaining a oneto one relationship with other microstructural elements e g mainprice et al 1993 lloyd 2000 mauler et al 2000 bascou et al 2001 in this contribution the seismic properties of an amphibolite facies mylonitic quartz shear zone are predicted by averaging the single crystal elastic properties according to the sem ebsd derived lpo in order to study the relationship between microstructure petrofabric and seismic properties this approach illustrates the usefulness of integrating geological studies of rock microstructures and particularly petrofabrics with geophysical studies of seismic properties to obtain a fuller understanding of geodynamic processes

effects of crystallographic anisotropy on fracture development and acoustic emission in quartz 1 transgranular microcracking is fundamental for the initiation and propagation of all fractures in rocks the geometry of these microcracks is primarily controlled by the interaction of the imposed stress field with the mineral elastic properties however the effects of anisotropic elastic properties of minerals on brittle fracture are not well understood this study examines the effects of elastic anisotropy of quartz on the geometry of brittle fracture and related acoustic emissions ae developed during indentation experiments on single crystals at ambient pressure and temperature a hertzian cone crack developed during blunt indentation of a single crystal of flawless brazilian quartz parallel to the c axis shows geometric deviation away from predictions based on the isotropic case consistent with trigonal symmetry the visible cone crack penetration depth varies from 3 to 5 mm and apical angle from 53 to 40 electron backscatter diffraction ebsd mapping of the crack tip shows that fracturing initiates along a 40 m wide process zone comprising damage along overlapping en echelon high index crystallographic planes shown by discrete bands of reduced electron backscatter pattern ebsp quality band contrast coalescence of these surfaces results in a stepped fracture morphology monitoring of ae during indentation reveals that the elastic anisotropy of quartz has a significant effect on ae location and focal mechanisms ninety four ae events were recorded during indentation and show an increasing frequency with increasing load they correspond to the development of subsidiary concentric cracks peripheral to the main cone crack the strong and complex anisotropy in seismic velocity 28 vp 43 vs with trigonal symmetry resulted in inaccurate and high uncertainty in ae locations using geiger location routine with an isotropic velocity model this problem was overcome by using a relative master event location algorithm that only requires a priori knowledge of the velocity structure within the source volume the ae location results correlate reasonably well to the extent of the observed cone crack decomposition of ae source mechanisms of the geiger relocated events shows dominantly end member behavior between tensile and compressive vector dipole events with some double couple dominated events and no purely tensile or compressive events the same events located by the master event algorithm yield greater percentage of vector dipole components and no double couple events indicating that ae source mechanism solutions can depend on ae location accuracy and therefore relocation routine that is utilized calculations show that the crystallographic anisotropy of quartz causes apparent deviation of the moment tensors away from double couple and pure tensile compressive sources consistent with the observations preliminary modeling of calcite anisotropy shows a response distinct from quartz indicating that the effects of anisotropy on interpreting ae are complex and require detailed further study

sub slab mantle flow parallel to the caribbean plate boundaries inferences from sks splitting abstract upper mantle deformation near the margins of the caribbean plate is investigated using observations of shear wave splitting in teleseismic and local shear phases the caribbean plate is almost stationary in the hot spot reference frame and is wedged between the north america south america nazca and cocos plates collisional belts and major shear zones encircle the plate data from seismic stations operated by iris geoscope the venezuelan seismological network and the british geological survey have provided nearly 2000 seismic records for analysis analysis of shear wave splitting in teleseismic core phases e g sks at stations reveals fast shear wave polarisations that are conformal to the plate boundary paralleling the major structural features the magnitude of the splitting is in general quite large 1 2 2 1 s in northern venezuela the magnitude of splitting increases towards the caribbean south american collisional front local shear phases from earthquakes up to 200 km deep beneath northeastern venezuela show very small amounts of splitting 0 1 0 3 s analysis of the depth dependence in the magnitude of the splitting suggests that most of the upper mantle wedge is isotropic and the splitting in the local phases is mostly accrued in the crust and uppermost mantle in ne venezuela fast shear wave polarisations in both the local and teleseismic phases closely parallel the fault systems in the region suggesting that the crust and mantle are coupled in this area stations on the island of montserrat and near bucaramanga columbia also show very small magnitudes in local s wave splitting but large amounts of sks splitting in general the small magnitude of the local s phase splitting suggests that the teleseismic phases accrue considerable splitting beneath the top of the slab we interpret the bulk of the splitting in terms of sub slab flow that is forced around the caribbean plate due to the convergence of the surrounding plates

fracture characterization at valhall application of p wave amplitude variation with offset and azimuth avoa analysis to a 3d ocean bottom data set the delineation and characterization of fracturing is important in the successful exploitation of many hydrocarbon reservoirs such fracturing often occurs in preferentially aligned sets if the fractures are of subseismic scale this may result in seismic anisotropy thus measurements of anisotropy from seismic data may be used to delineate fracture patterns and investigate their properties here fracture induced anisotropy is investigated in the valhall field which lies in the norwegian sector of the north sea this field is a chalk reservoir with good porosity but variable permeability where fractures may significantly impact production e g during waterflooding to investigate the nature of fracturing in this reservoir p wave amplitude variation with offset and azimuth avoa is analyzed in a 3d ocean bottom cable obc data set in general 3d ocean bottom seismic obs acquisition leads to patchy coverage in offset and azimuth and this must be addressed when considering such data to overcome this challenge and others associated with 3d obs acquisition a new method for processing and analysis is presented for example a surface fitting approach which involves analyzing azimuthal variations in avo gradients is used to estimate the orientation and magnitude of the fracture induced anisotropy this approach is also more widely applicable to offset azimuth analysis of other attributes e g traveltimes and any data set where there has been true 3d data acquisition land or marine using this new methodology we derive high resolution maps of p wave anisotropy from the avoa analysis for the top chalk reflection at valhall these anisotropy maps show coherent but laterally varying trends synthetic avoa modeling using effective medium models indicates that if this anisotropy is from aligned fracturing the fractures are likely liquid filled with small aspect ratios and the fracture density must be high furthermore we show that the fracture normal direction is parallel to the direction of most positive avo gradient in other situations the reverse can be true i e the fracture normal direction can be parallel to the direction of the most negative avo gradient effective medium modeling or comparisons with anisotropy estimates from other approaches e g azimuthal variations in velocity must therefore be used to resolve this ambiguity the inferred fracture orientations and anisotropy magnitudes show a degree of correlation with the positions and alignments of larger scale faults which are estimated from 3d coherency analysis overall this work demonstrates that significant insight may be gained into the alignment and character of fracturing and the stress field variations throughout a field using this high resolution avoa method

a novel ebsd based finite element wave propagation model for investigating seismic anisotropy application to finero peridotite ivrea verbano zone northern italy a novel electron backscatter diffraction ebsd based finite element fe wave propagation simulation is presented and applied to investigate seismic anisotropy of peridotite samples the fe model simulates the dynamic propagation of seismic waves along any chosen direction through representative 2d ebsd sections the numerical model allows separation of the effects of crystallographic preferred orientation cpo and shape preferred orientation spo the obtained seismic velocities with respect to specimen orientation are compared with voigt reuss hill estimates and with laboratory measurements the results of these three independent methods testify that cpo is the dominant factor controlling seismic anisotropy fracture fillings and minor minerals like hornblende only influence the seismic anisotropy if their volume proportion is sufficiently large up to 23 the spo influence is minor compared to the other factors the presented fe model is discussed with regard to its potential in simulating seismic wave propagation using ebsd data representing natural rock petrofabrics

ebsd measured lattice preferred orientations and seismic properties of eclogites we investigated the deformation mechanisms and the seismic properties of 10 eclogite samples from different localities alps norway mali and eastern china through the analysis of their microstructures and lattice preferred orientations lpo these samples are representative of various types and intensity of deformation under eclogitic metamorphic conditions omphacite and garnet lpo were determined from electron backscatter diffraction ebsd technique garnet appears to be almost randomly oriented whereas omphacite develops strong lpo characterized by the 001 axes concentrated sub parallel to the lineation and the 010 poles concentrated sub perpendicular to the foliation in order to analyze the deformation mechanisms that produced such omphacite lpo we compare our observations to lpo simulated by viscoplastic self consistent numerical models a good fit to the measured lpo is obtained for models in which the dominant slip systems are 1 2h110i 11 0 001 110 and 001 100 dominant activation of these slip systems is in agreement with tem studies of naturally deformed omphacite seismic properties of eclogite are calculated by combining the measured lpo and the single crystal elastic constants of omphacite and garnet although eclogite seismic anisotropies are very weak less than 3 for both p and s wave they are generally characterized by a maximum p wave velocity sub parallel to the lineation and by a minimum velocity approximately normal to foliation the mean p and s wave velocities are high respectively 8 6 and 4 9 km s the s wave anisotropy pattern displays complex relationships with the structural frame but the fast polarization plane generally tends to be parallel to the foliation calculated reflection coefficients show that an eclogite crust interface is generally a good reflector rc 0 1 whereas an eclogite body embedded in the upper mantle would be hardly detectable

seismic properties of highly strained marbles from the splugenpass central alps abstract seismic reflection profiling across the central alps nfp20 shows strong reflectors some of which correspond to a mesozoic marble zone which underlies the contact between the tambo and suretta nappes by means of experimental velocity measurements at confining pressures up to 300 mpa on rock samples collected in the marble zone we determined the seismic properties of these marbles and calculated their reflection coefficients in order better to constrain the seismic interpretation the main findings are as follows 1 a marbles exhibit v p anisotropy up to 9 with orthorhombic or transverse isotropic tetragonal symmetry due to the crystallographic preferred orientation of calcite 2 b compressional waves shear waves and shear wave splitting are very sensitive to the type of fabric presence or absence of an r axes concentration 3 c interfaces between marbles and amphibolites or orthogneisses could be good reflectors in normal incidence reflection

terrigenous sediment on ceara rise a cenozoic record of south american orogeny and erosion abstract ceara rise located east the amazon river mouth is covered with a thick blanket of pelagic carbonate and hemipelagic terrigenous sediment the terrigenous component has been extracted from 57 bulk sediment samples at ocean drilling program odp sites 925 and 929 on ceara rise to obtain a cenozoic record of riverine discharge from northern south america from the early eocene to early miocene 55 20 ma terrigenous accumulation was dominated by moderate amounts of generally large grained gray to green sediment especially depleted in elements that are enriched in post archaean shale e g cs th yb however pulsed inputs of relatively small grained gray to green terrigenous sediment less depleted in the above elements occurred in the late eocene and oligocene the accumulation of terrigenous sediment decreased significantly until 16 5 ma in the middle miocene 16 5 13 ma terrigenous accumulation was dominated by small amounts of small grained tan sediment notably depleted in na and heavy rare earth elements the accumulation rate of terrigenous sediment increased markedly from the latest miocene 10 ma to the present day a change characterized by deposition of gray green sediment enriched in elements that are enriched in post archaean shale observed changes in terrigenous sediment at ceara rise record tectonism and erosion in northern south america the brazil and guyana shields supplied sediment to the eastern south american margin until the middle miocene 20 16 5 ma when a period of thrusting shortening and uplift changed the source region probably first to highly weathered and proximal phanerozoic sediments by the late miocene 9 ma there was a transcontinental connection between the andes and eastern south america weathering products derived from the andes have increasingly dominated terrigenous deposition at ceara rise since the late miocene and especially since the late pliocene

a determination of the three dimensional seismic properties of anorthosite comparison between values calculated from the petrofabric and direct laboratory measurements bernard eront and david mainprice zaboratoire de tectonophysique i h iversit montpe lier ii montpe lier france nikolas i christensen department of earth and atmospheric sciences purdue unh ersity west lafayette btdiana we have undertaken a coinparative study of the seisnfic properties of an anorthosite the seisnfic properties have been calculated froin the lattice preferred orientation lpo of plagioclase 90 by volume and olivine 10 by voluine and the elastic constants of these minerals laboratory measurements of vp 24 directions and vs 6 directions have been made on the same sample at room temperature and pressures up to 1 0 gpa the twe techniques agree with 0 1 km s for all orientations studied when the voigt averaging method is used hill and reuss averaging methods give calculated velocities significantly lower than the experimental ones a study of the difference between the velocity values given by the two methods reveals a systematically larger departure in orientations normal to the foliation the orientation dependent difference between the two tedmiques can be explained if there are small errors in the elastic constants for single crystal plagioclase used in the initial calculations the axial lpo symmetry of the plagioclase with all the crystallographic b axes parallel to the specimen symmetry axis allowed us to calculate c22 norinal to 010 directly from the laboratory measurement of vp some constraints can be put on c22 c44 c66 c12 and c23 on average our data require 10 modification of the single crystal constants reported by ryzhova 1964

the role of crystallographic fabric in the generation of seismic anisotropy and reflectivity of high strain zones in calcite rocks abstract for a relatively pure calcite ultramylonite grain size 5 m from the ivrea verbano zone of the the n w italian alps the crystallographic preferred orientation was measured by x ray texture goniometry the fabric is characterized by a 3 times uniform axisymmetric c axis maximum normal to the mesoscopic foliation from this and available single crystal elastic properties of calcite the expected variation of acoustic p wave velocity with orientation can be calculated this is compared with high pressure up to 520 mpa at 20 c ultrasonic velocity measurements made on seven different core orientations from the same rock and good agreement obtained between the calculated and measured velocity distribution seismic velocity contrasts range up to 3 3 relative to an isotropically textured protolith sufficient to give rise to detectable seismic reflectivity for normal incidence p waves on a horizontal shear zone crystallographic texture of the type observed maximises the obtainable seismic reflectivity for a c maximum fabric oriented obliquely to the plane of the high strain zone as is observed in some other natural shear zones or for a gently dipping shear zone reflectivity may be either enhanced or weakened thus visibility may be very sensitive to the local dip of a shear zone and the fabric orientation and intensity within it

holocene flooding and species diversity in southwestern amazonia abstract a thick blanket of holocene alluvium lies over southwestern lowland amazonia and may possibly occur throughout much of the amazon basin these deposits resulted from massive seasonal flooding from about 11 000 to about 5000 yr b p that was followed by two cycles of erosion and deposition interpretations based on these geologic data suggest that southwestern lowland amazonia is ecologically an island in a state of supersaturated disequilibrium as a result of colonization from pleistocene refugia on its perimeter and that habitats of highest diversity may be the most recent in origin conservation efforts and studies of amazonian biogeography soils and paleoethnography should be reevaluated in light of the geologic data

climate and tectonic influences on continental erosion of tropical south america 0 13 ma is tectonic uplift or climate change the primary driving force behind continent scale erosion here we examine these two hypotheses for tropical south america by comparing a high resolution record of terrigenous sediment accumulation rates from the western tropical atlantic ocean with continental tectonic reconstructions and both low latitude and high latitude climate indices that span the past 13 m y over the full record terrigenous sedimentation rates most closely follow lowland aridity as recorded by abundance ratios of terrigenous oxide minerals an indication that basin integrated climate is an important control of erosion step like transitions in the flux and composition of terrigenous material near 8 and 4 5 ma however follow dated tectonic events which suggests progressive expansion of the drainage basin into less humid or less vegetated regions that were more prone to erosion on long time scales high latitude climate variability and sea level as recorded by 1 8 o do not closely track terrigenous sedimentation we conclude that regional climate change provides a mechanism for long term erosion of tropical south america tectonics plays a significant but secondary role in this area perhaps by shaping the drainage basin to include regions in which the climate is more favorable to erosion

lattice preferred orientation and seismic anisotropy in sedimentary rocks summary although it is well known that sedimentary rocks can be seismically anisotropic there have been few detailed investigations of the underlying cause of such anisotropy here we investigate anisotropy due to the preferred orientation of minerals or lattice preferred orientation lpo in a suite of sedimentary rocks seismic properties are predicted by averaging single crystal elastic constants of minerals according to their crystal orientation and modal volume fraction in the rock aggregate both electron backscattered diffraction ebsd and x ray texture goniometry xtg are tested as quantitative techniques for measuring the lpo of sedimentary rocks although ebsd has promise for future lpo measurements in polymineralic sedimentary rocks problems currently remain in measuring low symmetry phases e g feldspars and very small clay or mica particles however the lpo of very fine grained phyllosilicates can be measured using xtg and the lpo of low symmetry minerals can be measured using manual ebsd pattern analysis here we use such a hybrid approach to estimate lpo in a suite of sedimentary samples the seismic properties calculated from lpo data show anisotropy values for p waves ranging from 1 5 per cent in sandstones to over 3 5 per cent in a siltstone to 12 per cent in a shale the effect of thin multilayering on long wavelength propagation in a siltstone is predicted by applying backus type averaging the layering does not enhance the anisotropy because of the small differences in density and in elasticity between the two layer types the lpo of phyllosilicates and to a lesser extent dolomite and siderite seem to contribute significantly to the seismic anisotropy of phyllosilicate rich rocks siltstones and shales the weak lpo of quartz in sandstones causes a few per cent anisotropy cumulatively our results suggest that field observations of seismic anisotropy have the potential to be used as an indicator of rock lithology

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

new geological framework for western amazonia brazil and implications for biogeography and evolution abstract although many of the current hypotheses to explain the origin and distribution of the amazon biodiversity has been based directly or indirectly on geological data the reconstruction of the geological history of the amazon region is still inadequate to analyze its relationship with the biodiversity this work has the main goal to characterize the sedimentary successions formed in the brazilian amazon in the neogene quaternary discussing the evolution of the depositional systems through time and analyzing their main controlling mechanisms in order to fill up this gap radar image interpretation sedimentological studies and radiocarbon dating allowed the mapping of plio pleistocene to holocene units along the solimoes amazonas river brazil this integrated work led to the characterization of five sedimentary successions overlying miocene deposits of the solimoes pebas formation which include the following ica formation plio pleistocene deposits q1 37 400 43 700 14c yr b p deposits q2 27 200 14c yr b p deposits q3 6730 2480 14c yr b p and deposits q4 280 130 14c yr b p these deposits occur mostly to the west of manaus forming nw se elongated belts that are progressively younger from sw to ne indicating a subsiding basin with a depocenter that migrated to the ne the reconstruction of the depositional history is consistent with significant changes in the landscapes hence the closure of a large lake system at the end of the miocene gave rise to the development of a plio pleistocene fluvial system this was yet very distinct from the modern drainage with shallow energetic highly migrating braided to anastomosed channels having an overall northeast outlet this fluvial system formed probably under climatic conditions relatively drier than today s during the early pleistocene there was pronounced erosion followed by a renewed depositional phase ca 40 000 14c yr b p with the development of prograding lobes and or crevasse splays associated with a lake system i e fan delta and or fluvial flood plain areas after a period of erosion a fluvial system with eastward draining channels started to develop at around 27 000 14c yr b p the fluvial channels were overflooded in mid holocene time this flooding is attributed to an increased period of humidity with a peak between 5000 and 2500 14c yr b p the data presented herein support that rather than being a monotonous area the amazonia was a place with frequent changes in landscape throughout the neogene quaternary probably as a result of climatic and tectonic factors we hypothesize that these changes in the physical environment stressed the biota resulting in speciation and thus had a great impact on modern biodiversity

an experimental and numerical study of the microstructural parameters contributing to the seismic anisotropy of rocks 1 the elastic properties of rocks are influenced by several microstructural variables including the lattice preferred orientation and grain shape fabric of the mineral phases variations in the spatial distribution of the mineral phases the properties of the grain boundaries and the presence of porosity fractures consequently in principle directional variations in these variables can be inferred from seismic velocity anisotropy observations experimental seismic velocity measurements made on rocks of well characterized microstructure may be used to improve the quality of such interferences however since most rocks are microstructurally complex in order to interpret the measurements fully theoretical analyses which can accommodate all the relevant microstructural variables are required theoretical analyses of the requisite sophistication have only recently been developed we have tested one of these due to ponte castaneda and willis by using it to calculate the elastic properties of an upper mantle harzburgite and by then comparing the results with experimental velocity measurements to determine if the values of those microstructural variables which are difficult to quantify grain boundary properties fracture shape are physically realistic we find that they are and conclude that the ponte castaneda and willis analysis provides a powerful method for a more detailed assessment of the causes of elastic property anistropy in rocks than has previously been possible