proterozoic contraction extension tectonics of the southern sao francisco region minas gerais brazil the southern sao francisco region comprises the southern portion of the sao francisco craton which is underlain by archean basement and its fringing proterozoic orogenic belts included in this region are three geologic provinces the quadrilatero ferrifero qf the sao francisco basin and the cordilheira do espinhaco we synthesize preliminary results of ongoing structural analysis reconnaissance field studies and published reports to provide a concise review of the geology of the region and to propose a working tectonic model for its proterozoic evolution interpretation of superimposed folds faults and foliations leads to the conclusion that the southern sao francisco region records the effects of four principal tectonic events that occurred subsequent to deposition of the lower proterozoic minas supergroup the first event resulted in formation of a northwest verging fold thrust belt the second event led to development of high angle reverse faults open folds and basement uplift the third event is manifested in the qf by the formation of normal faults and by the intrusion of mafic dikes these extensional structures may be associated with formation of the sedimentary basins east and south of the sao francisco craton the final event resulted in formation of a west verging fold thrust belt that affected the qf the cordilheira espinhaco and the sao francisco basin the events that we describe can be tentatively associated with regionally recognized orogenic periods trans amazonian uruacuano brasiliano

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

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

deformation microstructures and magnetite texture development in synthetic shear zones abstract we present observations of deformation features in magnetite from synthetic magnetite bearing silicate aggregates deformed between 1000 c and 1200 c in transpressional shear experiments with strains of up to 300 anisotropy of magnetic susceptibility and shape preferred orientation spo analysis were combined with electron backscatter diffraction ebsd to characterize the magnetite deformation fabrics and intragrain microstructures crystallographic preferred orientation cpo in magnetite is very weak in all deformed samples and does not vary as a function of either temperature or shear strain magnetic anisotropy and spo increase strongly with both strain and deformation temperature and indicate that strain partitioning between magnetite and the plagioclase matrix decreases at higher temperatures ebsd orientation mapping of individual magnetite particles revealed substantial dispersions in intragrain orientation analogous to undulose extinction after deformation at 1000 and 1100 c indicating that dislocation creep processes were active in magnetite despite the lack of a well developed cpo geometrical analysis of crystallographic orientation dispersions from grain map data indicates that low angle grain boundary formation in magnetite could have been accommodated by slip on 110 or 100 planes but no evidence for dominant slip on the expected 111 planes was found evidence for activation of multiple slip systems was seen in some magnetite grains and could be partially responsible for the lack of cpo in magnetite these results suggest that at least in polyphase rocks crystallographic textures in magnetite may be inherently weak or slow to develop and cpo alone is not an adequate indicator of magnetite deformation mechanisms these results may aid in the interpretation of deformation textures in other spinel structured phases such as chromite and ringwoodite

39ar recoil artifacts in chloritized biotite biotite separates from the metamorphosed granitic rocks of eastern taiwan yield disturbed 40ar39ar age spectra their disturbed profiles become more pronounced with increasing degree of chloritization and are attributed to mixed release of argon from sub micron scale biotite and chlorite intercalations 39ar internal recoil best explains the discordance of the age spectra chlorite is randomly intercalated with biotite at scales ranging from 10 a to several tens of microns both 40ar and k were evidently lost during the chloritization reaction during the neutron irradiation 39ar atoms recoil into the chlorite from the adjacent biotite intercalations the recoil and chloritization combine to produce a variable 40ar39ar spatial distribution in the intercalated biotite chlorite during 40ar39ar step heating analyses argon in biotite is released in two pulses centered at 650 c and 950 1050 c which represent two stages of dehydroxylation of the biotite chlorite out gasses in two pulses centered at 550 c and 700 900 c during dehydroxylation and then decomposes this differential thermal release of argon from each mineral phase with variable 40ar39ar ratios yields discordant incremental heating age spectra

microstructures textures and deformation mechanisms in hematite abstract deformation and recrystallization of hematite in iron formations and high grade ore bodies of the caue formation from the quadrilatero ferrifero district minas gerais brazil resulted in several types of fabrics depending on the tectonic and metamorphic imprint the fabrics vary from randomly oriented granoblastic to strongly oriented lepidoblastic mylonitic with local development of cataclastic features complete pole figures of the crystallographic planes 003 basal plane 110 prism and 104 rhombohedron were measured on several samples from different areas by means of neutron diffraction revealing a typical crystallographic preferred orientation for the different fabric types the configuration and intensity of the maxima are directly related to the magnitude of the deformation and the type of response of the ore the analysis of the textures and microstructures of the iron ores and comparison with experimental deformation data lead us to the conclusion that three main mechanisms are active basal slip diffusion processes and anisotropic grain growth post tectonic recrystallization and secondary grain growth did not affect the textures developed during deformation

new u pb zircon ages and the duration and division of devonian time newly determined u pb zircon ages of volcanic ashes closely tied to biostratigraphic zones are used to revise the devonian time scale they are 1 417 6 1 0 ma for an ash within the conodont zone of icriodus woschmidti i w hesperius early lochkovian 2 408 3 1 9 ma for an ash of early emsian age correlated with the conodont zones of po dehiscens lower po inversus 3 391 4 1 8 ma for an ash within the po c costatus zone and probably within the upper half of the zone eifelian and 4 381 1 1 3 ma for an ash within the range of the frasnian conodont palmatolepis punctata pa punctata zone to upper pa hassi zone u pb zircon ages for two rhyolites bracketing a palyniferous bed of the pusillites lepidophyta spore zone are dated at 363 8 2 2 ma and 363 4 1 8 ma respectively suggesting an age of363 ma for a level within the late famennian pa g expansa zone these data together with other published zircon ages suggest that the base and top of the devonian lie close to 418 ma and 362 ma respectively thus lengthening the period by20 over current estimates we suggest that the duration of the middle devonian eifelian and givetian is rather brief perhaps no longer than 11 5 myr 394 ma 382 5 ma and that the emsian and famennian are the longest stages in the period with estimated durations of15 5 myr and 14 5 myr respectively 1998 elsevier science b v all rights reserved

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

laboratory measurements of elastic anisotropy parameters for the exposed crustal rocks from the hidaka metamorphic belt central hokkaido japan summary to study crustal rock seismic anisotropy and its effect on seismic wave propagation we measure the seismic velocity anisotropy of two amphibolites one biotite gneiss and one biotite schist from the hidaka metamorphic belt in central hokkaido japan under confining pressures up to 150 mpa the rock microstructures show foliation and lineation characterized by lattice preferred orientation lpo of hornblende or biotite p and two s wave velocities are measured along the direction perpendicular to the foliation plane and two directions in the foliation plane perpendicular and parallel to the lineation we assume orthorhombic symmetry based on the rock microstructures and obtain tsvankin s anisotropic parameters an extension of thomsen s parameters for orthorhombic symmetry p and s wave phase velocity surfaces are calculated from anisotropy parameters and compared with the measured velocities along particular directions and with the velocity contour maps calculated from the voigt averages of singlecrystal elastic constants based on the orientation of measured lpo data qualitatively the measured velocity anisotropy agrees with the velocity contour calculated from the lpo data although large quantitative differences exist between them all anisotropy patterns can be approximated as transverse isotropy or its modification appearing as orthorhombic symmetry biotite schist containing 30 per cent volume ratio biotite shows strong s wave anisotropy and the phase velocity surfaces of p waves show a large deviation from ellipticity in the plane perpendicular to the foliation and parallel to the lineation in the same plane s waves show a singularity due to a large bulge of the sv velocity surface

compressibility of porous rocks four compressibilities are defined for porous rock relating changes in the bulk and pore volumes to changes in the pore and confining pressures using a micromechanical theory based on classical linear elasticity three relations are found between these compressibilities two of these relations are verified experimentally for berea and bandera sandstone bounds are derived for these compressibilities involving only the porosity and the elastic moduli of the rock forming minerals for the strains to be unique functions of the stresses the compressibilities must be functions only of the difference between the confining and pore pressures this dependence is verified for berea bandera and boise sandstone while the strains cannot be expressed as functions of an effective stress regardless of how it is defined the elastic volumetric behavior of porous rocks can be studied through tests conducted at zero pore pressure

transformation of magnetite to hematite and its influence on the dissolution of iron oxide minerals deformed rocks of the itabira iron formation itabirites in brazil show microstructural evidence of pressure solution of quartz and iron oxides it appears that magnetite was dissolved and hematite precipitated the dissolution of magnetite seems to be related to its transformation to hematite by oxidation of fe2 to fe3 the transformation of magnetite to hematite occurs along 111 planes and results in the development of hematite domains along 111 that are parallel to the foliation the difference in volume created by the transformation of magnetite to hematite and the shear stress acting on the interphase boundaries allow fluids to migrate along these planes the dissolution of magnetite involves the hydrolyzation of the fe2 o bonds at interphase boundaries of high normal stress the high fugacity of oxygen in the fluid phase promotes the reaction of fe2 in solution with oxygen fe2 ions oxidize to fe3 and precipitate as hematite platelets with their longest axes oriented parallel to the direction of maximum stretching the transformation of magnetite to hematite during deformation plays an important role in the fabric evolution of the iron formation rocks the transformation along 111 creates planes of weakness that facilitate fracturing the fracturing plus the dissolution result in a reduction of magnetite grain size and the oriented precipitation results in layers of hematite platelets these processes produce a new fabric characterized by a penetrative foliation and lineation

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

contrasting provenance signals in riphean and vendian sandstones in the sw urals russia constraints for a change from passive to active continental margin conditions in the neoproterozoic abstract two contrasting provenance areas for neoproterozoic sandstones can be distinguished in the southwestern urals by light and heavy mineral analyses as well as by mineral chemistry these reflect a dramatic change of geotectonic conditions at the eastern border of the baltica protocontinent at around 610 620 ma detritus from riphean and lower vendian sandstones representing about 1 ga of continuous sedimentation is characterised by a continental platform provenance reflecting its derivation from the proterozoic basement of the east european platform source rocks include mainly minerals and few lithic clasts from granitoids and high grade metamorphic rocks as well as reworked clastic sediments and silicic volcanic rocks the provenance signal changes abruptly with upper vendian detritus representing a recycled orogenic provenance this includes mineral and lithic clasts of low grade siliciclastic metasediments and mylonites containing phengites with a high pressure signature as well as clasts of bimodal volcanic rocks and reworked siliciclastic sediments the composition of phengites contrasts strongly with those of detrital white mica in the riphean rocks also the composition of tourmalines derived from a metapsammopelitic source indicates a mainly al poor metasediment provenance whereas riphean tourmalines were derived from mainly al rich metapsammopelites zircon morphology tourmaline zoning and a reduced heavy mineral spectrum provide evidence for polycyclic sedimentation during riphean and vendian upper vendian sedimentary rocks were deposited in a foreland basin derived from a proximal uplift to the east provenance characteristics of the upper vendian detritus are consistent with areas affected by a high pressure low temperature metamorphism during a pre uralian orogenic event most likely from the metamorphic complex of beloretsk which was emplaced and exhumed during the upper vendian the change of geotectonic conditions in the upper vendian reflects a change from a passive continental margin in the south urals throughout the entire riphean since at least 1350 ma to a convergent continental margin within a presumed transpressional setting

an olivine fabric database an overview of upper mantle fabrics and seismic anisotropy we present a unique database of 110 olivine petrofabrics and their calculated seismic properties the samples come from a variety of the upper mantle geodynamic environments ophiolites subduction zones and kimberlites with a wide range of micro structures a phenomenological relationship is established between p and s wave seismic anisotropy and the degree of crystal alignment fabric strength seismic anisotropy increases rapidly at low fabric strength before reaching a near saturation level of 15 to 20 for p waves and 10 to 15 for s waves despite a large variation in the symmetry of fabric patterns the average seismic properties of the different fabric micro structural and geodynamic settings have similar anisotropies in both magnitude and symmetry hence it would seem possible to determine some measure of fabric strength from seismic anisotropy if the dimensions of the anisotropic region are known but not geodynamic environment or details of the petrofabric pattern a simple pattern of seismic anisotropy characterises the average sample of the database which has the following features the polarisation plane of the fastest s wave is parallel or sub parallel to the foliation plane the maximum shear wave splitting is parallel to the y structural direction in foliation plane and normal to the lineation the maximum of the p wave velocity is parallel to the high concentration of 100 axes which is sub parallel to the lineation the 100 orientation distribution has the greatest influence on the p wave seismic anisotropy the 100 and 001 orientation distributions have the greatest influence on the symmetry of s wave anisotropy although the magnitude of anisotropy is influenced by the distribution of all three principal axes although the database only contains olivine petrofabrics this statistical study clearly shows that seismic anisotropy can be used to deduce the orientation of the structural frame in the upper mantle 1998 elsevier science b v all rights reserved

transamazonian orogeny in the southern sao francisco craton region minas gerais brazil evidence for paleoproterozoic collision and collapse in the quadrilatero ferrifero abstract the paleoproterozoic transamazonian orogeny yielded a series of discrete orogens in brazil the present field study in the quadrilatero ferrifero qf indicates that the southern sao francisco craton region of the brazilian highlands preserves a portion of one of these orogens two sets of transamazonian structures occur in this region the first consists of northwest verging folds and thrusts affecting supracrustal sequences it is suggested that this set formed in a fold thrust belt setting shortly after 2 125 ga during the closure of a passive margin basin that had initiated along the margins of a preexisting continental mass at ca 2 5 ga the second set consists of structures defining the prominent dome and keel architecture of the qf this set a consequence of the emplacement of basement domes against supracrustal rocks at 2 095 ga may reflect the consequences of orogenic collapse narrow conglomerate filled intermontane basins may have formed coevally with dome emplacement formation of an ocean basin east of the present day sao francisco craton eventually occurred in late mesoproterozoic in effect the transamazonian orogen of the qf represents the collision and collapse stages of a paleoproterozoic wilson cycle the contractional phase of the transamazonian orogeny probably represents accretion of an offshore arc to the eastern and southeastern margin of the present day sao francisco craton region the arc and an associated suture may be traced in the brasiliano pan african orogen east of the sao francisco craton northwards into the northeastern lobe of the sao francisco craton clearly initial assembly of crustal blocks to form a larger continent involving south america occurred during the paleoproterozoic 2 1 ga post transamazonian rifting of this continent created the basins which were later inverted during the brasiliano assembly of gondwana

microstructures crystallographic fabric development and deformation mechanisms in natural hematite aggregates deformed under varied metamorphic conditions abstract naturally deformed hematite aggregates from 15 different iron ore mines located in quadrilatero ferrifero region brazil were analyzed in order to verify the influence of increasing temperature and deformation intensity on their microstructural and textural aspects as well as the deformation mechanisms associated with the metamorphic conditions the electron backscattered diffraction ebsd technique was applied in order to get qualitative and quantitative data concerning with microstructural parameters crystallographic preferred orientation cpo and misorientation between hematite grains the microstructures of these aggregates vary from randomly oriented hematite grains with approximately equant grains to strongly oriented and elongated grains following the increase in deformational and metamorphic polarity toward east in the region in the low deformation domain western region the deformation mechanisms are typically microfracturing and dissolution precipitation creep for magnetite rich aggregates in the high strain domain eastern region the deformation is accommodated by a combination of basal intracrystalline slip c and grain boundary sliding with rotation around hematite c axis no evidences for recrystallization processes in these aggregates can be supported by our results probably due to the superposition of subsequent processes

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

structure and evolution of the magnitogorsk forearc basin identifying upper crustal processes during arc continent collision in the southern urals the southern urals of russia contain a well preserved example of a paleozoic arc continent collision in which the intraoceanic magnitogorsk volcanic arc and its forearc basin sediments accreted to the east european craton during the devonian the magnitogorsk arc records the evolution from incipient intraoceanic subduction to a mature arc and by comparing its surface geological features with those in active arc continent collision settings it is possible to identify upper crustal processes that were active in the southern urals the arc edifice can be divided into western and eastern volcanic fronts that were active during different stages of arc evolution and for which two distinct phases of forearc basin development can be recognized the late lower to middle devonian aktau formation represents a remnant of the intraoceanic to collisional forearc basin to the irendyk volcanic front whereas the middle devonian to lower carboniferous ulutau koltubanian and zilair formations were deposited in a suture forearc basin to the east magnitogorsk volcanic front it was not until the late devonian that these two basins were joined structural mapping combined with reflection seismic profiling shows these basins to be affected by open nonlinear volcanic basement cored synsedimentary folds the karamalytash anticline appears to have the geometry of a growth fold that formed during deposition of sediments in the suture forearc basin the forearc region is affected by minor thrusting that involves the volcanic basement although it is not clear if these thrusts reactivate preexisting trench parallel faults synsedimentary deformation slumping and olistostrome development were common throughout the suture forearc basin history but were especially widespread during the late devonian when the full thickness of the continental crust is interpreted as having arrived at the subduction zone

breakdown of feldspar volume gain and lateral mass transfer during mylonitization of granitoid in a low metamorphic grade shear zone abstract in a low metamorphic grade shear zone from quadrilatero ferrifero southeastern brazil granitoid rocks have been transformed into phyllonites and mylonites via activation of crystal plastic processes in quartz and fluid assisted reaction softening in feldspars quartz deformed by basal a and prism a slip with concurrent subgrain rotation recrystallization breakdown of matrix plagioclase an 22 35 via mica producing softening reactions required cationic exchanges between ca na and k forming replacement perthites in the k feldspar megacrysts which occupy around 60 of the original rock volume subsequent breakdown of the perthitic plagioclase an 12 15 led to disruption of the megacrysts enabling strain accommodation that localized the mylonite and phyllonite zones domainal volume and mass balance calculations based on major and minor elements indicate only small positive changes in mass and volume around 15 in the whole system however accentuated differences ranging from mass and volume losses of 25 and 35 at the margins to gains of 85 and 95 in the centre respectively exist between the individual longitudinal subzones occupied by the different tectonite types this scenario indicates that deformation proceeded as a nearly isochemical process although with lateral mobility of major elements principally si al and k within the shear zone these results are different from those previously reported for other granitic mylonites most of them indicating volume losses in compressional tectonic settings it is suggested that the extensional tectonic framework attributed to the moeda bonfim shear zone where mass volume losses are in principle not required enhanced the access of fluid and consequent transport of components into the shear zone principally fe si and k producing the small gains of mass and volume observed

seismic anisotropy in trinidad a new tool for lithology prediction a great challenge facing explorationists is identification of potential intervals of reservoir quality sand in areas with little well control this is especially acute in deep water where reservoir presence can be elusive and well costs are considerable substantial evidence suggests the occurrence of transversely isotropic ti anisotropy in the subsurface associated primarily with shales while intervals of clastic material show greatly reduced and or near zero ti anisotropy although this has been demonstrated conclusively on core samples the need exists to translate these concepts into a geologic framework on a basinwide and prospect scale this article presents a method for estimating interval anisotropy from nonhyperbolic moveout on common midpoint gathers and relating these values to subsurface geology alkhalifah and tsvankin 1995 demonstrated that for ti media with a vertical symmetry axis vti media only two parameters are sufficient for performing all time related processing such as nmo correction including nonhyperbolic moveout correction if necessary dmo correction and prestack and poststack time migration one of these parameters is dimensionless its departure from zero implies anisotropy the other parameter vnmo is the short spread nmo velocity for horizontal reflectors these two parameters can be obtained solely from surface p wave data using estimates of stacking velocity for reflections from interfaces having two distinct dips alkhalifah and tsvankin 1995 vti media induce nonhyperbolic moveout of reflections that depends on vnmo and at large offsets alkhalifah 1997 showed that these parameters can be obtained by analyzing the moveout at far offsets the direct output from such an inversion or velocity analysis is the effective values of vnmo and that include the combined average influence of the overburden these effective values are converted to interval values using layer stripping techniques using data from offshore africa alkhalifah et al 1996 showed that anisotropic

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

the metamorphic complex of beloretzk sw urals russia â a terrane with a polyphase meso to neoproterozoic thermo dynamic evolution abstract an integrated geological study of the tectono metamorphic evolution of the metamorphic complex of beloretzk mcb which is part of the eastern bashkirian mega anticlinorium bma sw urals russia shows that the main lithological units are neoproterozoic riphean and vendian age siliciclastic to carbonate successions granitic syenitic and mafic intrusions together with subaerial equivalents comprise the neo and mesoproterozoic magmatic rocks the metamorphic grade ranges from diagenetic and very low grade in the western bma to high grade in the mcb the n s trending zuratkul fault marks the change in metamorphic grade and structural evolution between the central and eastern bma structural data pb pb single zircon ages 40 ar 39 ar cooling ages and the provenance signature of riphean and vendian siliciclastic rocks in the western bma give evidence of mesoproterozoic grenvillian rifting deformation and eclogite facies metamorphism in the mcb and a neoproterozoic cadomian orogenic event in the sw urals three pre ordovician deformation phases can be identified in the mcb the first sse vergent isoclinal folding phase d 1 is younger than the intrusion of mafic dykes pb pb single zircon 1350 ma and older than the eclogite facies metamorphism high p low t eclogite facies metamorphism is bracketed by d 1 and the intrusion of the achmerovo granite pb pb single zircon 970 ma an extensional sinistral top down to nw directed shearing d 2 is correlated with the first exhumation of the mcb e vergent folding and thrusting d 3 occurred at retrograde greenschist facies metamorphic conditions the tremolite 40 ar 39 ar cooling age 718 5 ma of amphibolitic eclogite and muscovite 40 ar 39 ar cooling ages about 550 ma of mica schists indicate that a maximum temperature of 500 50 c was not reached during the neoproterozoic orogeny the style and timing of the neoproterozoic orogeny show similarities to the cadomian aged timan range nw of the polar urals geochronological and thermochronological data together with the abrupt change in structural style and metamorphism east of the zuratkul fault suggest that the mcb is exotic with respect to the se margin of the east european platform thus the mcb is named the beloretzk terrane recognition of the beloretzk terrane and the neoproterozoic orogeny at the eastern margin of baltica has important implications for neoproterozoic plate reconstruction and suggests that the eastern margin of baltica might have lain close to the avalonian cadomian belt

the pb pb age of the minas supergroup carbonate rocks quadrilatero ferrifero brazil abstract pb pb isochron ages determined on carbonate rocks from the minas supergroup quadrilatero ferrifero qf show that the deposition of the gandarela formation the intermediate unit of the minas supergroup took place at 2420 19 ma the entire sedimentary sequence and underlying basement rocks have been deformed at a regional scale and data from greenschist facies carbonate rocks from the piracicaba group indicate that the pb isotopes were reset at about 2100 ma this age is in agreement with whole rock rb sr and u pb titanite ages determined by many authors for metamorphism of the surrounding archean granite gneiss terrane

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

the tectonic evolution of the quadrilatero ferrifero minas gerais brazil abstract the structural framework of the quadrilatero ferrifero is the result of two main deformational events the first is an extensional event of palaeoproterozoic age between 2 100 and 1700 ma the transamazonian event which resulted in the formation of archaean granite gneiss domes and the nucleation of regional synclines in the overlying rio das velhas and minas supergroups strata such regional synclines are continuous with each other and are bordered by structural granite gneiss highs at the contact between these units ductile brittle to ductile extensional shear zones are developed representing the dislocation surfaces of archaean blocks this tectonism is probably related to the evolution of a metamorphic core complex formed in the hinterland to the west of a transamazonian collision zone which structured the paramirim craton the second event is compressive and associated with the closure of the pan african brasiliano proto ocean 650 500 ma situated to the east of quadrilatero ferrffero qf during this event a west verging fold and thrust belt ftb developed causing inversion amplification translation and rotation of the basinal synclines the ftb affected mainly the eastern portion of qf and obliterated many of the tectonic features of the extensional event

seismic anisotropy and global geodynamics for many years seismic anisotropy was often neglected mostly because of the inherent heavy mathematical and computational tools needed to describe and model its effects on seismic waves the usual basic knowledge about propagation in isotropic media cannot easily apply to anisotropic media where new phenomena come up such as birefringence or shear wave splitting or difference between directions of propagation of phase velocity and of group velocity consequently geophysicists often claimed that it was a second order effect and considered the earth as isotropic this hypothesis was assumed to be a good approximation because of the random orientation of crystals in most parts of the earth and of the random sampling of anisotropic regions by seismic rays this assumption furthermore made easier the description of wave propagation as well as the parameterization of media in inverse problems an isotropic elastic medium can be described by two independent elastic parameters and lame parameters but the simplest anisotropic medium transverse isotropy with a vertical symmetry axis requires 5 independent parameters love 1927 anderson 1961 to date seismic observations have been explained in terms of isotropic and often thermal lateral heterogeneities ignoring manifestations of anisotropy however since the 1960s it was recognized that most parts of the earth are not only laterally heterogeneous but also anisotropic and that seismic anisotropy provides a simple explanation of different observational data seismic anisotropy contrary to isotropy is reflecting some inherent

physical properties and seismic imaging of massive sulfides laboratory studies show that the acoustic impedances of massive sulfides can be predicted from the physical properties vp density and modal abundances of common sulfide minerals using simple mixing relations most sulfides have significantly higher impedances than silicate rocks implying that seismic reflection techniques can be used directly for base metals exploration provided the deposits meet the geometric constraints required for detection to test this concept a series of 1 2 and 3 d seismic experiments were conducted to image known ore bodies in central and eastern canada in one recent test conducted at the halfmile lake copper nickel deposit in the bathurst camp laboratory measurements on representative samples of ore and country rock demonstrated that the ores should make strong reflectors at the site while velocity and density logging confirmed that these reflectors should persist at formation scales these predictions have been confirmed by the detection of strong reflections from

petrofabrics and seismic properties of garnet peridotite from the uhp sulu terrane china implications for olivine deformation mechanism in a cold and dry subducting continental slab abstract lattice preferred orientations lpo of olivine diopside enstatite and garnet from the zhimafang garnet peridotite body in the sulu ultrahigh pressure uhp metamorphic terrane china were measured using the electron backscatter diffraction ebsd technique the peridotite was captured from a mantle wedge immediately adjacent the subducted yangtze slab and then experienced the uhp metamorphism at 750 950 c and 4 7 gpa the olivine lpo is characterized by the 001 axis close to the stretching lineation and the 100 plane subparallel to the foliation indicating the prevailing of 100 001 slip enstatite lpo displays the dominance of 100 001 slip diopside developed complex lpo patterns that are difficult to explain using a single slip system of 100 001 garnet is almost randomly oriented due to its low volume fractions cubic symmetry and the presence of numerous slip systems calculated seismic properties of the peridotite yield a maximum p wave velocity normal to the foliation and a minimum along the foliation with anisotropy up to 8 in strongly sheared samples the s wave velocity pattern is complex but the fast polarization plane generally normal to the foliation the inferred shear sense from the olivine lpo is top to se in contrary to exhumation induced top to nw thrusting recorded in the quartz lpo implying that the olivine lpo formed at early uhp metamorphic conditions the olivine crystals have relatively low water contents 141 475 h 106 si indicating a fluid deficient environment for the lpo formation the present study suggests that a combination of low temperature and uhp plays a much more important role than the water content to promote the activation of 100 001 slip in olivine

crystallographic fabric development along a folded polycrystalline hematite abstract detailed analyses of microstructures and crystallographic preferred orientations cpos of hematite rocks were conducted in samples of polycrystalline hematite around a tight fold in the quadrilatero ferrifero region southeastern brazil grain size is dominantly very fine in all samples 15 30 m with a slight increase toward the hinge grain aspect ratio increases substantially toward the hinge from an average of 2 in the limb to 9 in the hinge distribution patterns of 0001 axes and poles of planes 10 1 0 11 2 0 and 10 1 4 suggest that intracrystalline slip operate on the basal plane along the direction the distribution of the poles of prismatic planes parallel to the foliation of reference frame indicate that the all the symmetric slip directions of hematite crystals were equally efficient during activation of basal intracrystalline slip increasing aspect ratio is accompanied by cpo intensification toward the hinge where intensification was aided during deformation by bulk rotation of the hematitic plates and possibly by some grain boundary sliding such cpo intensification with the increase of aspect ratio toward the hinge is used to infer that the fold developed by flexural flow

seismic anisotropy of metapelites from the ivrea verbano zone and serie dei laghi northern italy abstract laboratory measurements of compressional and shear wave velocities up to 600 mpa confining pressure were carried out on amphibolite to granulite facies metapelites from the serie dei laghi and ivrea verbano zone northern italy the metapelite samples show moderate to high compressional wave anisotropy and significant shear wave splitting for propagation directions parallel to foliation serie dei laghi samples are generally transversely isotropic for compressional wave propagation a pattern that based on calculated velocities is the consequence of strong preferred orientation of micas ivrea verbano samples generally have an orthorhombic anisotropy pattern for compressional waves that is caused by the preferred orientation of both mica and sillimanite shear wave anisotropy and shear wave splitting patterns differ significantly for the two fabric types intense static recrystallization can obliterate seismic anisotropy but partial static recrystallization does not appreciably reduce anisotropy we conclude that seismic anisotropy may occur in mid to lower continental crust where it is predominantly composed of metapelitic rocks and if there is a consistent regional fabric this seismic anisotropy may be diminished if there is no consistent regional fabric or if postmetamorphic intrusions have caused static recrystallization

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

laser 40ar 39ar ages of single detrital white mica grains related to the exhumation of neoproterozoic and late devonian high pressure rocks in the southern urals russia single grains of detrital white mica from two different synorogenic sediments in the southern urals were analysed using the in situ ultraviolet laser ablation ar ar dating technique to discriminate between age signatures associated with a high pressure signal phengites from those related to muscovite only two disparately aged sandstone formations of neoproterozoic upper vendian and upper devonian famennian age were formed by the erosion of high relief source areas with contemporaneously exhumed high pressure rocks a bimodal distribution of ages and chemical compositions can be detected in the two detrital populations there is no age overlap between the two populations reflecting completely different source areas containing high pressure rocks of different ages within the upper vendian sandstones detrital white mica from a 571 609 ma age group is phengitic in composition si 3 3 3 41 per formula unit while an older 645 732 ma age group is comprised of muscovite composition grains only the first group is compatible with the time of late exhumation and emplacement of a source area containing high pressure rocks the neoproterozoic beloretzk terrane the older age range is compatible with a long history of cooling and the allochthonous nature of this terrane detrital white mica from the famennian sandstones zilair formation comprises one age group 342 421 ma containing phengite si 3 21 3 39 per formula unit and muscovite and a second group 446 496 ma containing muscovite only while the derivation of the second group cannot be correlated with any as yet known regional data the first age group indicates the earliest arrival of high pressure rocks at the surface along the suture zone after late devonian arc continent collision

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

microscale data to macroscale processes a review of microcharacterization applied to mineral systems microanalysis can provide rapid quantitative characterization of mineral systems that complements the field and core scale observations traditionally made in ore deposits we review recent innovations in microanalytical procedures and their application to studies of ore deposits case studies are presented examining how microanalysis can provide constraints on macroscopic processes within mineral systems synchrotron x ray fluorescence shows centimetre scale chemical variations associated with proximity to mineralization in samples from sunrise dam gold mine western australia pseudomorphs of igneous plagioclase and chemically driven recrystallization interpreted from electron backscatter diffraction suggest that the system was dominated by fluid driven brecciation with very little shearing both the fluid chemistry and fluid pressure evolved during a protracted sequence of vein formation and alteration accompanying gold mineralization a second case study of sulphide mineralogy at the mt keith nickel sulphide deposit western australia demonstrates how x ray computed tomography combined with trace element mapping can constrain the chemistry and dynamics of magmatic systems large scale interaction between silicate and sulphide melts shown by homogenous palladium enrichment in pentlandite leads to a large proportion of globular ores with a high nickel content increasing use of microanalysis in ore deposit geology is resulting in the constant reassessment of established models for ore genesis though a combination of micro and macroscale datasets

episodic rapid uplift in the himalaya revealed by 40ar 39ar analysis of detrital k feldspar and muscovite bengal fan detrital k feldspar and muscovite samples from ocean drilling program leg 116 cores have been dated by the sup 40 ar sup 39 ar technique and have depositional ages from 0 to 18 ma from 4 to 13 individual k feldspars and 1 to 12 individual muscovites have been dated from 7 stratigraphic levels in every level at least one k feldspar and one muscovite yielded a minimum age identical within uncertainty to the age of deposition these results indicate that a significant portion of the material in the bengal fan is first cycle detritus derived from the himalaya therefore the substantial amount of sediment deposited in the distal fan in early to middle miocene time can be ascribed to a significant pulse of uplift and erosion in the collision zone at this time moreover these data indicate that throughout the neogene some part of the himalayan orogen was undergoing rapid erosion 1 to 10 mm yr this erosion must have been less than or equal to uplift relative to sea level the lack of granulite facies rocks in the eastern himalaya and tibetan plateau suggests to us that very rapid uplift must have been distributed in brief pulses over different parts of the mountain belt these datamore are incompatible with tectonic models in which the himalaya and tibetan plateau are uplifted either uniformly over the past 40 m y or mostly within the past 2 to 5 m y less