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

antigorite induced seismic anisotropy and implications for deformation in subduction zones and the tibetan plateau the present study which is a follow up of the journal of geophysical research paper by ji et al 2013a provides a new calibration for both seismic and fabric properties of antigorite serpentinites comparisons of the laboratory velocities of antigorite serpentinites measured at high pressures with crystallographic preferred orientation data measured using electron backscatter diffraction techniques demonstrate that seismic anisotropy in high t serpentinite which is essentially controlled by the antigorite c axis fabric is independent on the operating slip system but strongly dependent on the regime and magnitude of finite strain experienced by the rock extrapolation of the experimental data with both pressure and temperature suggests that vp anisotropy decreases but shear wave splitting vs and vp vs increase with increasing pressure in either cold or hot subduction zones for a cold steeply subducting slab antigorite is most likely deformed by nearly coaxial flattening or trench parallel movements forming trench parallel seismic anisotropy for a hot shallowly subducting slab however antigorite is most likely deformed by simple shear or transpression trench normal seismic anisotropy can be observed when the subducting dip angle is smaller than 30 the geophysical characteristics of the tibetan plateau such as strong heterogeneity in vp vs and attenuation shear wave splitting and electric conductivity may be explained by the presence of strongly deformed serpentinites in lithospheric shear zones reactivated along former suture zones between amalgamated blocks hydrated zones of subducting lithospheric mantle and the crust mantle boundary if the temperature is below 700 c in the region of interest

structure and seismic response of extended continental crust stochastic analysis of the strona ceneri and ivrea zones italy a two dimensional stochastic seismic model of the crustal cross section exposed in the strona ceneri and ivrea zones northern italy was derived from geologic maps and petrophysical data with techniques used in microfabric analysis the model has a layered self affine structure which agrees with the intrinsic scale independence of geologic phenomena synthetic seismograms from this model replicate many pertinent features observed in deep seismic reflection data and allow insight into crustal structure 1 reflectivity depends on the lithological populations present and their distribution within a crustal unit 2 transparent zones within the reflective lower crust can arise from metamorphic reactions 3 the transition from upper to lower crust may be a change in small scale seismic fabric rather than a first order seismic discontinuity

composition and tectonic evolution of the chinese continental crust constrained by poisson s ratio article i nfo we have measured both p and s wave velocities vp and vs and poisson s ratios of 60 typical ultrahigh pressure uhp metamorphic rock samples from the chinese continental scientific drilling ccsd main and pre pilot holes and surface outcrops in the sulu dabie orogenic belt at hydrostatic confining pressures up to 850 mpa the experimental results together with those compiled in handbook of seismic properties of minerals rocks and ores ji s c wang q xia b 2002 handbook of seismic properties of minerals rocks and ores polytechnic international press montreal 630 pp reveal that except for monomineralic rocks such as quartzite serpentinite anorthosite limestone and marble the rest of the rock types have poisson s ratios falling along an upward convex curve determined from the correlations between elastic moduli and density poisson s ratios increase with density as the lithology changes from granite felsic gneiss and schist through diorite syenite intermediate gneiss and metasediment to gabbro diabase amphibolite and mafic gneiss and then decrease as the rocks become ultramafic in composition eclogite has a higher density but a lower poisson s ratio than peridotite the results were applied to constrain the crustal composition and tectonic evolution of the chinese continental crust based on crustal thickness h and poisson s ratio from 248 broadband seismic stations measured using teleseismic receiver function techniques the north china yangtze south china and northeast china blocks and songpan ganzi terrane are dominated by low b0 26 and moderate 0 26 b0 28 values n70 suggesting the dominance of felsic composition in the crust the lhasa terrane qiangtang terrane and indochina block are characterized by high proportions 33 42 of measurements with very high values 0 30 and h is found for the south china block northeast block lhasa block qiangtang terrane and indochina block indicating either tectonic thickening of the felsic upper and middle crust by folding and thrusting or the removal of mafic layers from the lower crust into the upper mantle by delamination

quartz microstructures and c axis preferred orientations in high grade gneisses and mylonites around the morin anorthosite grenville province quartz in deformed rocks from two large high grade shear zones around the morin anorthosite morin terrane grenville province displays distinctive microstructures as well as c axis preferred orientations in the west dipping morin shear zone east of the morin anorthosite four distinct quartz microstructures types 1 4 are identified based on deformation features and grain size the c axis orientations are characterized by a single maximum near the stretching lineation and two maxima in type 1 microstructure and by an asymmetrical single girdle in type 2 microstructure quartz c axes show crossed girdle pattern in type 3 microstructure both quartz microstructures and c axis preferred orientations suggest that crystal plastic slip and dynamic recrystallization are the dominant deformation mechanisms the asymmetry of c axis orientations with respect to the mylonitic foliation as well as the substructures developed in quartz indicates a dextral sense of shear in the morin shear zone type 4 micr

the origin of reflections beneath the blue ridge piedmont allochthon a view through the grandfather mountain window a perplexing variety of crustal models have been proposed for the region beneath the southern appalachian overthrust determining the true nature of crustal composition and structure in this region carries important scientific and economic implications to evaluate the numerous models compressional wave velocities and densities were measured on 30 rock samples collected within the grandfather mountain window in the blue ridge of north carolina the window breaches high grade crystalline rocks exposing grenville gneisses late proterozoic metasedimentary and metavolcanic rock and cambrian quartzite phyllite and dolostone that are likely to underlie the regional blue ridge piedmont thrust measured velocities range widely from 4 21 km s 1 for cambrian phyllite to 7 47 km s 1 for tectonized cambrian shady dolomite at 200 mpa velocity anisotropy is as great as 47 in the phyllite the samples exhibit a broad range in densities as well from 2500 kg m 3 in fractured quartzite to 3000 kg m 3 in metabasalts synthetic reflection seismograms were generated using these physical properties to consider the origin of subthrust reflection events in addition to the much publicized interpretation of paleozoic sedimentary rocks residing beneath the crystalline sheet modeling reveals that the reflections could also be explained by metamorphosed paleozoic strata by the regional late proterozoic metavolcanic and metaclastic rift sequence and by pervasive shear zones i e mylonites within compositionally homogeneous gneisses we interpret field data to strongly favor mylonitic gneisses as the subthrust reflectors still surface data remain sufficiently ambiguous that the question will not be definitively settled without continental drilling

seismic evidence of mylonite reflectivity and deep structure in the kettle dome metamorphic core complex washington the first direct test of the seismic reflectivity of mylonites is provided by a seismic reflection profile over a mylonite zone exposed in the kettle dome metamorphic core complex northeastern washington the mylonite zone developed at mid crustal levels and has undergone a minimum of retrogression and cataclasis during uplift downdip projection of the exposed mylonite zone into the seismic profile indicates that the compositionally layered mylonite zone is reflective and traceable to at least 5 km depth as a complex zone of multicyclic reflections the seismic data also show that the crust beneath the kettle dome is characterized by abundant strongly reflective moderately dipping layers down to at least 18 km these layers can be interpreted in terms of compressional or extensional orogenic events or a combination of both models for the structure and genesis of metamorphic core complexes must take these data into account

u pb zircon dates of morin anorthosite suite rocks grenville province quebec u pb zircon ages of samples of anorthosite pyroxene monzodiorite jotunite and pyroxene quartz monzonite quartz mangerite of the morin anorthosite complex grenville province quebec are 1155 plus minus 3 1146 plus minus 4 and 1135 plus minus 3 ma respectively these dates are very similar to available dates for equivalent units of the lac st jean and adirondack anorthosite suite occurrences and slightly predate estimates of 1075 1100 ma for peak metamorphic conditions during the grenville orogeny in this region sup 87 sr sup 86 sr initial ratios of 0 7048 0 7051 for the three morin units sampled permit a comagmatic origin if the different emplacement or cooling times can be reconciled the mangerite sampled cannot have been formed by fusion of the upper crust but may have been derived from a relatively juvenile crust at depth all three units have interacted with continental crust given the likely depleted nature of the mantle in this region

seismic evidence for late proterozoic orogenic structures below the phanerozoic sedimentary cover in the kattegat area sw scandinavia 1 commercial seismic reflection profiles from sw scandinavia image sveconorwegian orogenic structures in the late precambrian basement beneath the thin phanerozoic cover we interpret 1 pronounced west to wnw dipping reflections as sveconorwegian localized ductile shear which overprint 2 weaker east dipping reflections interpreted as gothian or early sveconorwegian compressional structures folded and laminar seismic fabrics may represent f and c type crystalline thrust sheets emplaced during the gothian and sveconorwegian events some of the dipping reflections are observed to sole into 3 an interval of subhorizontal reflections at 2 5 4 5 s two way travel time which we interpret as a fossil ductile to brittle transition that was unroofed during the gothian orogeny 4 nw se striking thrusts cutting the sveconorwegian structures may indicate a shift in the late sveconorwegian stress field they may represent an early stage in the subsequent phanerozoic basin evolution in the danish area

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

the lithoprobe abitibi grenville transect two billion years of crust formation and recycling in the precambrian shield of canada we summarize the results of lithoprobe studies in the neoarchean southeastern superior province and the mesoproterozoic grenville province in the southeastern precambrian shield of canada through two composite cross sections based on seismic reflection data which define dramatically different styles of crust formation and tectonic accretion in the neoarchean and mesoproterozoic in the neoarchean the structures at the surface are steep with discontinuous and flatter structures at depth much of the crust appears to be juvenile and the predominant process of crustal growth is inferred to have been subduction accretion of primitive crust in a prograding arc system in the mesoproterozoic surface structures are shallow and the seismic character of the crust is continuous over the entire cross section archean parautochthonous rocks and reworked archean crust comprise a very significant proportion of the preserved crust in the mesoproterozoic and provided the backstop to the grenvillian orogeny result

correlation between seismic reflectivity and rheology in extended lithosphere southern tuscany inner northern apennines italy abstract in southern tuscany inner zone of the northern apennines the lithosphere has been affected by extensional deformation since the middle miocene and is characterized by high heat flow 120 mw m 2 both local shallow and regional deep seismic reflection profiles clearly show the occurrence of a sharp reflector k horizon within the upper crust which is probably located at the top of a shear zone acting as the decollement horizon for the latest pliocene quaternary normal faults its depth varies between 3 and 6 km in geothermal areas and 9 and 12 km elsewhere borehole control in the geothermal areas shows that it occurs at a temperature of approximately 450 c the hypothesis that the k horizon represents the top of the brittle ductile transition agrees with two lines of evidence a it approximately follows the same isotherm over the whole region and b its depth coincides with rheological predictions of the brittle ductile transition for the range of high pore fluid pressures and strain rates estimated for southern tuscany it appears therefore that at least under specific conditions very high pore fluid pressures strain concentration the brittle ductile transition may have a seismic signature

p wave anisotropy of mylonitic and infrastructural rocks from a cordilleran core complex the ruby east humboldt range nevada abstract in order to evaluate the possibility that compressional wave anisotropy might characterize the upper to middle crust of the north american cordillera we examined the nature and degree of seismic anisotropy of rock samples collected from the mylonite zone and underlying infrastructure of the ruby east humboldt metamorphic core complex in northeastern nevada seismic anisotropy in quartzofeldspathic rocks from all structural levels is low and generally related to the volume of mica relative to feldspar upper amphibolite facies infrastructure quartzite has a stronger and different quartz crystallographic preferred orientation cpo and thus higher seismic anisotropy than the lower grade mylonitic quartzites a feature related to the higher temperature deformation conditions that characterized the infrastructure schistose mylonite zone rocks as a group have the highest anisotropy we infer that this is in large part due to mica cpo although the anisotropy patterns suggest that quartz cpo may be a factor anisotropy is higher in infrastructural quartzites calc silicates and marbles than in mylonite zone equivalents seismic anisotropy data for other cordilleran core complex samples suggest that these patterns are common characteristics of cordilleran core complex rocks

origin of deep crystal reflections seismic profiling across high grade metamorphic terranes in canada abstract in an attempt to better understand the origin of deep crustal reflections lithoprobe has sponsored or co sponsored seismic reflection surveys across tracts of high grade metamorphic rock in the archean superior craton the proterozoic grenville orogen and the phanerozoic cordilleran orogen common to these three diverse terranes are near surface zones of prominent seismic reflectivity that are typically associated with velocity discontinuities at highly strained contacts between gneissic rocks of varying lithology at some locations the reflective layering resulted from transposition and rearrangement of previously layered rocks stratified assemblages sills etc whereas in other regions it was generated by extreme attenuation stretching and ductile flow of weakly layered or irregularly organized rocks it seems likely that compositionally layered gneissic rock is a common source of reflections in the deep crust with reflections originating at lithological boundaries and zones of mylonite

seismic properties of rocks from an exposure of extended continental crust new laboratory measurements from the ivrea zone abstract compressional wave velocities for new samples from the ivrea verbano zone ivz and strona ceneri zone scz have been measured to 600 mpa in order to better understand the petrologic controls on velocity and to provide an expanded base for seismological models of the crust as represented by these terrains these data coupled with previous measurements show that preferred orientation of mica controls both the velocity hysteresis with pressure and seismic anisotropy of scz and ivz schists and gneisses ivz paragneisses form the basis of a linear velocity density relationship for metagraywackes of various metamorphic grade this relationship falls close to that expected for mafic rocks and overlaps velocity density data for ivz mafic gneisses one dimensional near vertical reflection models based on these data and recent structural reconstructions of the terrain show that lower crustal reflectivity can be related to high reflection coefficients associated with interlayered sequences of upper amphibolite granulite facies paragneiss mafic gneiss and ultramafic rocks transparent upper crustal levels in the model result from the overall similarity of velocities for schists gneisses and granites from the structurally and lithologically complex scz a mean lower crustal velocity of 7 1 km s is calculated for the part of the ivz composed of interlayered mafic gneiss and paragneiss and for another area dominated by a mafic layered igneous complex the velocity density analysis coupled with this observation emphasize the ambiguity inherent in interpretation of crustal composition from p wave velocity data in the absence of other constraints

the velocity structure of the britt domain southwestern grenville province from laboratory and refraction experiments the britt domain in the southwestern grenville province of the canadian shield is believed to be an exposure of high grade upper amphibolite facies mid crustal rocks of predominantly granitic and granodioritic composition a 270 km refraction line was conducted by lithoprobe across the britt domain and the grenville front tectonic zone to the north in order to determine the deep velocity structure under the region this data set demonstrates a uniform velocity structure to a depth of 15 km in the central britt domain with near surface p and s wave velocities of 6 15 and 3 55 km s respectively and linear vertical gradients at depth of 0 02 and 0 01 s 1 respectively the data also show that the grenville front tectonic zone is strongly anisotropic at shallow depths to determine the acoustic properties of rocks from the britt domain 80 velocity samples representing different lithologies in this area were measured at confining pressure up to 600 mpa these studies show that p wave velocities at 600 m

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

reflectivity and seismic properties of the deep continental crust the recovery of drill core from a section of upper amphibolite facies terrane located in the inner piedmont of south carolina provides a rare opportunity to study in detail the seismic character of rocks which were once part of the deep continental crust seismic reflection profiles over the inner piedmont in the vicinity of the drill hole reveal subhorizontal high amplitude reflections the origin of these reflections has been investigated by measuring compressional wave velocities as a function of confining pressure and densities in 150 samples obtained from 305 m of drill core synthetic seismograms generated from the velocity and density measurements demonstrate that reflections originate from subhorizontal metamotphic layering probably originating from lower crustal ductile flow fine tuning of the metamotphic layering which is temperature and pressure related can produce bright spots in the deep crust anisotropy resulting from preferred mineral orientation is another important propeny of this sectim velocities are low for vertical wave propagatim whereas horizontal velocities are relatively high horizontal velocities show mly a slight variatim with azimuth which is unlikely to be detected using standard refractim techniques reflectivity decreases with increasing anisotropy critical to this study has been the continuous coring of a major section of high grade metamotphic terrane which has provided the necessary stratigraphic and lithologic cmtrol for the generation of accurate synthetic seismograms

post permian evolution of the central north sea a numerical model abstract a dynamic thermomechanical model is presented for the post permian evolution of the central north sea including the central graben permo carboniferous and jurassic thermal events as well as triassic and jurassic extensional phases and late cretaceous shortening are considered the equations of force balance for a continuum and the transient heat equation are solved using the finite element method the lithosphere responds to applied forces by elastic deformation at low deviatoric stress levels fracture at low temperature and pressure conditions and nonlinear viscous flow at high temperature and pressure a first order rheological strain softening is incorporated to simulate a transition from strong to soft material behaviour such as a change from dislocation creep to diffusion creep seismic refraction and reflection data indicate crustal thinning below the central graben by a factor of 2 while the triassic rifting appears to have been associated with a more distributed deformation pattern the upper jurassic rifting is more localised pure shear extensional models have been advocated to explain this however dipping seismic reflectors in the upper mantle indicate that simple shear could be important the strain softening of the present model allows for modelling of strain localisation during the upper jurassic rifting phase and detailed modelling of moho structures the present model accounts for the main structural and stratigraphic observations in the central north sea furthermore the results indicate that in the part of the central north sea investigated in this paper a pure shear regime in triassic changed into a simple shear dominated regime with mantle shear zones developing during late jurassic extension

laboratory measurements of seismic p wave velocities on rocks from the betic chain southern iberian peninsula abstract ultrasonic laboratory measurements of seismic compressional velocities vp carried out on rock samples encompasing a wide range of metamorphic grades and lithologies from the betic chain are presented measurements were performed at a high confining pressure on a total of 12 representative samples of the alboran domain internal betics p wave mean velocities referred to pressure conditions of 280 10 mpa range from 5 82 to 7 13 km s 1 the maximum values corresponding to high amphibolite facies metapelites garnet gneiss under these pressure conditions vp anisotropies range from 1 2 to 16 1 with the lowest values being associated with granitic rocks and the highest values associated with metapelites from the upper structural units p wave velocities were calculated for prevailing p t conditions at depths of 10 20 30 and 40 km we discuss the significance of the analyzed rock types in terms of metamorphic grade and derived vp at different crustal levels p wave velocities and densities measured on low to middle and middle to high grade metamorphic rocks are compatible with values obtained from previous seismic and gravity studies for the upper and middle crust respectively at lower crustal levels the integration of our measurements with other geophysical data suggests the presence of partial melting of high grade metamorphic rocks the estimation of the impedance contrast from the measured physical properties suggests that the reflective character of the mid lower crust is associated with shear mylonitic zones and or fine scale lithological layering enhanced by partial melting

magnitude and symmetry of seismic anisotropy in mica and amphibole bearing metamorphic rocks and implications for tectonic interpretation of seismic data from the southeast tibetan plateau we calibrated the magnitude and symmetry of seismic anisotropy for 132 mica or amphibole bearing metamorphic rocks to constrain their departures from transverse isotropy ti which is usually assumed in the interpretation of seismic data the average bulk vp anisotropy at 600 mpa for the chlorite schists mica schists phyllites sillimanite mica schists and amphibole schists examined is 12 0 12 8 12 8 17 0 and 12 9 respectively most of the schists show vp anisotropy in the foliation plane which averages 2 4 for phyllites 3 3 for mica schists 4 1 for chlorite schists 6 8 for sillimanite mica schists and 5 2 for amphibole schists this departure from ti is due to the presence of amphibole sillimanite and quartz amphibole and sillimanite develop strong crystallographic preferred orientations with the fast c axes parallel to the lineation forming orthorhombic anisotropy with vp x vp y vp z effects of quartz are complicated depending on its volume fraction and prevailing slip system most of the mica or amphibole bearing schists and mylonites are approximately transversely isotropic in terms of s wave velocities and splitting although their p wave properties may display orthorhombic symmetry the results provide insight for the interpretation of seismic data from the southeast tibetan plateau the n s to nw se polarized crustal anisotropy in the sibumasu and indochina blocks is caused by subvertically foliated mica and amphibole bearing rocks deformed by predominantly compressional folding and subordinate strike slip shear these blocks have been rotated clockwise 70 90 around the east himalayan syntaxis without finite eastward or southeastward extrusion in responding to progressive indentation of india into asia

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

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

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

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

ductility of garnet as an indicator of extremely high temperature deformation discoidal shaped garnets alm63 prp33 grs3 sps1 occur in the foliation plane of lineated quartzofeldspathic mylonites of the morin shear zone located along the eastern boundary of the morin anorthosite complex grenville province quebec the flattened shape of garnets records a component of coaxial strain x y z different from the dominant rotational shear strain x y z in this mylonite tem observations indicate dislocation slip and recovery to be the main mechanism for the deformation of garnet extrapolation of available experimentally determined flow laws to natural strain rates shows an inversion of flow strength between quartz feldspar and garnet garnet much stronger than quartz and feldspar at temperatures below 700 c becomes weaker than quartz and feldspar above 900 c the coaxial ductile strain recorded by the garnets could be produced by a local uncharacterized deformation under extremely high temperature conditions at the roof of the rising anorthositic diapir 1155 ma subsequent lower temperature rotational deformation is related to strike slip movement along the morin shear zone 1020 ma

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

racoro continental boundary layer cloud investigations 3 separation of parameterization biases in single column model cam5 simulations of shallow cumulus climatically important low level clouds are commonly misrepresented in climate models the fast physics system testbed and research faster project has constructed case studies from the atmospheric radiation measurement climate research facility s southern great plain site during the racoro aircraft campaign to facilitate research on model representation of boundary layer clouds this paper focuses on using the single column community atmosphere model version 5 scam5 simulations of a multi day continental shallow cumulus case to identify specific parameterization causes of low cloud biases consistent model biases among the simulations driven by a set of alternative forcings suggest that uncertainty in the forcing plays only a relatively minor role in depth analysis reveals that the model s shallow cumulus convection scheme tends to significantly under produce clouds during the times when shallow cumuli exist in the observations while the deep convective and stratiform cloud schemes significantly over produce low level clouds throughout the day the links between model biases and the underlying assumptions of the shallow cumulus scheme are further diagnosed with the aid of large eddy simulations and aircraft measurements and by suppressing the triggering of the deep convection scheme it is found that the weak boundary layer turbulence simulated is directly responsible for the weak cumulus activity and the simulated boundary layer stratiform clouds increased vertical and temporal resolutions are shown to lead to stronger boundary layer turbulence and reduction of low cloud biases

the reflectivity of magmatic underplating using the layered mafic intrusion analog abstract the reflection characteristics of layered mafic intrusions provide an analog for characterizing the reflectivity of magmatic underplates one dimensional seismic models of a variety of mafic intrusions with different parent magmas demonstrate that cumulate layering may typically be strongly reflective with reflection coefficients as high as 0 18 a seismic profile across the bjerkreim intrusion in southern norway shows that even a moderately reflective cumulate sequence with reflection coefficients on the order of 0 05 produces detectable reflections two dimensional models demonstrate that the subhorizontal cumulate layering in typical mid to upper crustal intrusions is conducive to seismic imaging but more complicated geometries such as concentric layering created during synmagmatic deformation complicate the seismic image reflection coefficients predicted for granulite facies mineral assemblages are similar to those predicted for the igneous assemblages reflection coefficients predicted for cumulates intruded or re equilibrated at eclogite facies conditions may be either decreased or enhanced with respect to the igneous assemblages depending on the abundance of plagioclase in the cumulate layers and the pressure results suggest that magmatic underplating should not be invoked as an explanation for non reflective crust except in cases where eclogitization has occurred

seismic reflectivity of a finely layered granulite facies ductile shear zone in the southern grenville province quebec abstract lithoprobe reflection profile 54 across the granulite facies morin shear zone msz in the grenville tectonic province quebec canada reveals a west dipping zone of strong reflections the origin of these reflections has been investigated by detailed field mapping laboratory measurements of p wave velocities and densities of representative rock samples from the main lithological units and forward synthetic modeling petrofabric and microstructural analyses were performed in order to interpret the measured seismic properties synthetic seismograms generated from the velocity and density data demonstrate that reflections originate from lithologic thin layers and thin layer clusters likely resulted from intensive tectonic transposition and metamorphic differentiation lattice preferred orientation lpo induced anisotropy of granulite facies mylonites except for those containing abundant sillimanite biotite or amphibole is very low and hence unlikely to be the primary cause for reflections confining pressure does not significantly affect the reflectivity of the shear zone because the reflection coefficients are not sensitive to pressure up to at least 600 mpa since composite reflections of thin layer clusters can cause high reflectivity the strong reflectivity from some deep crustal sections with low refraction velocities may be interpreted to be due to the presence of thin layer mafic clusters within dominantly felsic rocks

plagioclase preferred orientation by tof neutron diffraction and sem ebsd abstract the lattice preferred orientation lpo of an anorthosite composed of andesine sampled from a highly deformed anorthositic mylonite grenville province quebec was measured by tof neutron diffraction and sem ebsd the quantitative texture analysis of neutron data was accomplished by using the rietveld texture analysis with the wimv algorithm implemented in the program package materials analysis using diffraction maud the texture calculations of the ebsd data were performed by using the program beartex analyses from neutron and electron diffraction data gave similar results if ebsd data are smoothed to account for grain statistics the principal pole figures show 010 roughly parallel to the rock foliation 001 poles exhibiting a low angle 25 to the pole to foliation and 100 poles close to the y direction perpendicular to the lineation and foliation pole the 100 crystallographic direction shows a maximum in the lineation direction 010 directions concentrate near the foliation pole the geological deformation conditions and the constructed pole figure patterns indicate that the preferred orientation could be attributed to intracrystalline slip dominantly on 010 with 100 as slip direction elastic properties calculated by averaging document weak anisotropy that has implications for the seismic structure of the lower crust

seismic reflection response of folded structures and implications for the interpretation of deep seismic reflection profiles seismic reflection characteristics of several common folded structures have been investigated by forward synthetic modeling in order to interpret correctly the seismic data from ancient and modern orogenic belts the studied folds include single order open folds close folds and asymmetric inclined folds as well as multi order folds the results show that complex folded structures and particularly those in deep crust cannot be accurately imaged by conventional 2d reflection profiling it is possible that widely observed subhorizontal discontinuous reflections in the continental crust of compressional folded belts do not necessarily correspond to subhorizontal compositional lamination multi order folded structures may equally enable the formation of such reflections furthermore trains of asymmetric inclined folds produce straight discontinuous dipping reflections that are potentially misinterpreted as thrusting faults or shear zones thus structural geologists must be extremely cautious when trying to convert directly seismic reflection sections to geological profiles for the continental crust

new tectonic divisions of the grenville province southeast canadian shield on the basis of geological geophysical and geochronological data the grenville province has been divided into three first order longitudinal belts the parautochthonous belt pb allochthonous polycyclic belt apb and allochthonous monocyclic belt amb these are set apart by three first order tectonic boundaries the grenville front gf allochthon boundary thrust abt and monocyclic belt boundary zone mbbz the belts are subdivided into terranes based on internal lithological character the gf separates the archean to proterozoic foreland northwest of the orogen from reworked equivalents to the southeast continuous at the scale of the orogen its main characteristic is that of a crustal scale contraction fault the pb although less clearly identified along the length of the orogen in most places represents upgraded and tectonically reworked rocks of the adjacent foreland the boundary between the pb and the apb to the southeast the abt is most clearly delineated in the eastern half of the province it is the locus of major crustal delamination along which high grade mostly middle proterozoic polycyclic terranes were tectonically transported northwest toward and onto the pb the amb comprises two separate areas underlain by the wakeham supergroup and what is currently known as the grenville supergroup respectively its basal contact the mbbz is a decollement zone of variable kinematic significance between older polycyclic rocks and tectonically overlying monocyclic rocks this first order zonation implies a tectonic polarity to the grenville province superimposed on which are second order features evident from contrasting tectonic styles and radiometric ages these characteristics are consistent with a diachronous or oblique collisional model for the grenville orogen

kinematics of horizontal simple shear zones of concentric arcs taylor couette flow with incompressible newtonian rheology we present preliminary kinematic analyses of taylor couette flow we consider deformation of a newtonian incompressible ductile material inside rotating horizontal listric concentric circular boundaries the velocity profile is curved indicating non uniform shear strain but leads to the same shear sense each material point on progressive shear keeps increasing shear strain linearly with time a curve of no movement the neutral curve may exist inside the shear zone irregular geometries of initially regular markers and their individual non matching strain paths indicate inhomogeneous deformaion in such taylor couette flow

sense of shear in high temperature movement zones from the fabric asymmetry of plagioclase feldspars abstract a new method for determining the sense of shear in plagioclase bearing tectonites from the 010 orientation of plagioclase feldspar is presented the method is based on the asymmetry of the 010 plane with respect to the structural frame foliation and lineation and the dominant activity of the 010 slip plane in the high temperature plasticity of plagioclase feldspar using examples from the zabargad gneisses red sea the method is applied to plagioclases of an25 an45 and compared with other methods of shear sense determination quartz c axis fabrics and microstructural criteria

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

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

pressure dependence and anisotropy of p wave velocities in ultrahigh pressure metamorphic rocks from the dabie sulu orogenic belt china implications for seismic properties of subducted slabs and origin of mantle reflections the compressional wave velocities vp pressure derivatives vpv and anisotropy a of three types of eclogites and country rocks from the dabie sulu ultrahigh pressure uhp metamorphic belt china have been measured under confining pressures up to 800 mpa type 1 eclogites which are coarse grained and subjected to almost no retrograde metamorphism experienced recovery accommodated dislocation creep at peak metamorphic conditions in the diamond stability field type 2 eclogites are fine grained reworked type 1 materials that experienced recrystallizationaccommodated dislocation creep under quartz coesite boundary conditions during the early stage of exhumation type 3 eclogites are retrogressed samples that were overprinted by significant amphibolite facies metamorphism during a late stage of exhumation within the crust type 1 eclogites are richer in al2o3 and mgo but poorer in sio2 and na2o than type 2 and type 3 eclogites anisotropy of type 1 and type 2 eclogites is generally low b4 because volumetrically important garnet is elastically quasi isotropic while type 3 eclogites can exhibit high anisotropy n10 due to the presence of strongly anisotropic retrograde minerals such as amphibole plagioclase and mica the transition of the pressure dependence of velocity from the poroelastic to elastic regimes occurs at a critical pressure pc which depends mainly on the density and distribution of microcracks and in turn on the exhumation history of rocks the vp pressure relationship can be expressed by vp a lnp 2 blnp c pvpc and vp v0 dp pzpc where p is the confining

ductile extensional shear zones in the lower crust of a passive margin we describe and interpret an unpublished set of ion geophysical seismic reflection profile showing strong organized seismic reflectors at the base of the continental crust of the uruguayan volcanic rifted margin we distinguish two main groups of reflectors in the lowermost continental crust a first group at depths ranging from 32 km below the continent to 16 km in the continent ocean transition comprises reflectors continuous over tens of kilometers peculiarly visible near the mantle crust boundary a second group of reflectors dipping toward the ese oceanward is widely distributed in the lower crust these reflectors are slightly curved and tend to merge and become sub parallel with the first group of reflectors together they draw the pattern of thick shallow dipping top to the continent shear zones affecting the lower continental crust such sense of shear is also consistent with the continentward dip of the normal faults that control the deposition of the thick syn tectonic volcanic formations sdr a major portion of the continental crust behaved in a ductile manner and recorded a component of top to the continent penetrative simple shear during rifting indicative of a lateral movement between the upper crust and the mantle

large scale distinctions between mjo and non mjo convective initiation over the tropical indian ocean abstractin this study the authors seek large scale signals that may distinguish mjo from non mjo convective events before they start over the indian ocean three such signals were found low level easterly anomalies extend from the surface to the midtroposphere and move from the western to eastern indian ocean surface pressure anomalies exhibit a zonal structure of wavenumber 1 with an equatorial low pressure surge penetrating eastward from africa through the indian ocean and reaching the maritime continent negative temperature anomalies in the middle to upper troposphere start over the indian ocean and move eastward all of them emerge 20 days before convective initiation of the mjo and move eastward at speeds close to that of the mjo without any direct connection to mjo convection they are not obviously related to the extratropics in any discernible way or any preceding mjo events they are absent in non mjo convective events these signals provide useful information for forecasting mjo initiation o

crustal scale shortening and extension across the grenville province of western quebec a deep seismic reflection survey shot in 1993 crosses almost the full width of the grenville province in western quebec the seismic transect provides a very clear image of the crust mantle boundary and the most precise definition to date of the various grenvillian terranes the crust is around 44 km thick beneath the grenville front but thins rapidly to 36 km some 60 km to the southeast it is also notable that the greatest crustal thickness of 50 km occurs at the southeast end of the transect far from the inferred location of the main grenvillian collision the grenville front zone in which nw directed thrusting at about 1 ga was followed by se directed extension is defined by discontinuous se dipping reflections which extend down to the moho to the southeast the overlying migmatitic archean parautochthon almost half of the transect is characterised instead by nw dipping reflectors extending into the lower crust these reflectors are in turn truncated to the southeast by a 12 km thick zone of intense se dipping reflections the baskatong crustal ramp the base of the allochthonous terranes allochthon boundary thrust is likely located at this ramp which flattens out at around 30 km depth into the base of the relatively transparent intermediate crust a highly reflective upper crustal deck corresponding to rocks of the mont laurier terrane was thrust over the baskatong crustal ramp and is represented further to the northwest by the klippelike cabonga allochthon the synformal transparent morin anorthosite charnockite complex belongs to the same upper crustal level ramp anticlines and the overriding basal thrust of the upper allochthons demonstrate the nw directed propagation of tectonic transport during the grenvillian orogeny involving deformation and displacement along the baskatong ramp with a relay into the grenville front zone postaccretional extension appears to have been primarily accommodated along the latter two crustal discontinuities thinning the crust immediately south of the grenville front and affecting the crust up to 350 km away from the front

basalts water or shear zones in the lower continental crust abstract bright sub horizontal laterally extensive multi cyclic layered reflections have now been recorded in the lower continental crust from many areas in contrast this reflection signature is rare in the continental upper crystalline crust and upper mantle and in the oceanic lithosphere with the present state of knowledge it is sensible to seek a single quantitative explanation for all such layering qualitative explanations for lower crustal layering abound and include igneous metamorphic structural and fluid related models measurements of reflection coefficients from the lower crust place strong quantitative constraints on possible models structurally induced anisotropy metamorphic layering or free fluids cannot realistically produce sufficiently bright reflections igneous layering or the juxtaposition of pre existing heterogeneities by shear zones can produce strong reflections results from crustal xenoliths helium isotope ratios palaeo geothermal gradients and chemistry of flood basalts together with numerical modelling of lithospheric extension and asthenospheric melting suggest that underplating and intrusion of the lower crust with mantle derived mafic igneous rocks is common the apparent spatial relationship between lower crustal layering and continental extension and the strength and continuity of the reflections suggest that underplating related to extension and or elevated mantle temperatures is a good explanation for lower crustal layering

on the measurement of plagioclase lattice preferred orientations abstract there are two major difficulties in measuring plagioclase lattice preferred orientations lpos by u stage the first which is caused by dependence of the lpo determination on anorthite content cannot be overcome using the u stage method at present the second difficulty which is caused by the dependence on orientation of thin sections can be overcome by measuring all twinned grains in any two orthogonal sections compared with previous methods published in the literature the present method using two rather than three orthogonal sections is more statistically meaningful and time saving in practice this method was used for measuring lpos of plagioclase an 50 from a typical anorthosite mylonite deformed under the granulite facies conditions in the morin ductile shear zone within the canadian grenville tectonic province

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

acoustic reflections from complex strata rock strata may contain fine scale structures which are too small to be identified directly but which can give rise to anomalous seismic reflection amplitudes the variance caused by such anomalies severely hampers the use of reflection amplitudes for layer identification and techniques are required which can identify or correct for the effects of such fine structure we have done calculations which explore the consequences of two general structure types wavy interfaces and very thin layers in each case a formalism is set up which enables us to identify systematically all of their effects computer modeling of realistic formations verified our calculations we suggest that the strongly wavelength dependent response of such layers and their characteristic response waveforms may provide convenient tools for identifying fine scale features in strata

a new calibration of seismic velocities anisotropy fabrics and elastic moduli of amphibole rich rocks 1 a large portion of the middle to lower crust beneath the continents and oceanic island arcs consists of amphibolites dominated by hornblende and plagioclase we have measured p and s wave velocities vp and vs and anisotropy of 17 amphibole rich rock samples containing 34 80 vol amphibole at hydrostatic pressures p up to 650 mpa combined petrophysical and geochemical analyses provide a new calibration for mean density average major element contents mean vp p and vs p coefficients intrinsic vp and vs anisotropy poisson s ratios the logarithmic ratio rs p and elastic moduli of amphibole rich rocks the vp values decrease with increasing sio2 and na2o k2o contents but increase with increasing mgo and cao contents the maximum 0 38 0 40 km s and minimum s wave birefringence values occur generally in the propagation direction parallel to y and normal to foliation respectively amphibole plays a critical role in the formation of seismic anisotropy whereas the presence of plagioclase quartz pyroxene and garnet diminishes the anisotropy induced by amphibole crystallographic preferred orientations cpos the cpo variations cause different anisotropy patterns illustrated in the flinn diagram of vp x vp y vp y vp z plots the results make it possible to distinguish in terms of seismic properties the amphibolites from other categories of lithology such as granite granodiorite diorite gabbro diabase felsic gneiss mafic gneiss eclogite and peridotite within the earth s crust hence amphibole aligned by dislocation creep anisotropic growth or rigid body rotation is the most important contributor to the seismic anisotropy of the deep crust beneath the continents and oceanic island arcs which contains rather little phyllosilicates such as mica or chlorite

p wave velocities anisotropy and hysteresis in ultrahigh pressure metamorphic rocks as a function of confining pressure 1 the chinese continental drilling project ccsd has drilled to a depth of 5100 m at maobei n34 40 e118 67 donghai county jiangsu province in the eastern segment of the dabie sulu ultrahigh pressure uhp metamorphic terrane the borehole which penetrated through all of the high velocity layers and seismic reflectors observed within the uppermost crust on seismic refraction and reflection profiles reveals the main lithologies to be coesite bearing felsic gneisses metabasic rocks i e amphibolite retrogressed and non retrogressed eclogites and ultramafic rocks i e garnet peridotite and serpentinite p wave velocities anisotropy and hysteresis of 31 typical ccsd core samples and 35 representative surface samples collected from the sulu uhp belt were measured at hydrostatic confining pressures up to 800 mpa the velocity pressure curves can be well described by a four parameter exponential equation derived from theory v p v0 dp b0 exp kp where v0 is the projected velocity at zero pressure if pores cracks were absent d is the intrinsic pressure derivative of velocity in the linear elastic regime b0 is the initial velocity drop caused by the presence of pores cracks at zero pressure and k is the decay constant of the velocity drop in the nonlinear poro elastic regime the seismic hysteresis is caused by irreversible changes in grain contact increases in microcrack aspect ratios and reduction of void space during the pressurization depressurization cycle the statistical properties of p wave velocities in the uhp rocks provide an important set of basic information for the interpretation of field seismic data from the root zones of continental convergent orogenic belts and modern and ancient subduction zones

localized rheological weakening by grain size reduction during lithospheric extension abstract grain size reduction may be a possible mechanism for the origin of localized deformation in the ductile regime i investigated the effects of grain size reduction due to dynamic recrystallization cataclasis and syntectonic metamorphic reaction on the stress envelope in the lithospheric mantle during extension by using a simple one dimensional model in this model the lithosphere extends uniformly with a constant strain rate and a fall in rock strength appears as a decrease in stress because grain size distribution at the onset of extension is unknown i regarded the steady state grain size due to dynamic recrystallization as the initial size then i evaluated the maximum effects of grain size reduction by dynamic recrystallization during extension and consequently examined the effects of grain size reduction by cataclasis and metamorphic reaction under conditions when dynamic recrystallization occurs significantly i find that it is difficult to bring about localized rheological weakening by grain size reduction owing to dynamic recrystallization in contrast grain size reduction by cataclasis can cause localized weakening during extension there is a wide ranging rate of grain size reduction by means of cataclasis that causes localized weakening just below the moho i specified the reaction from spinel lherzolite to plagioclase lherzolite that plays a role in grain size reduction by syntectonic metamorphism the reaction occurs at depths less than 35 km which is independent of the initial thermal state of the lithosphere localized rheological weakening can occur if the following conditions are satisfied 1 grain size before the reaction is greater than 0 7 mm under dry conditions and greater than 0 5 mm under wet conditions and it decreases down to those values by the reaction 2 grain size decreases down to less than initial grain size when the dominant deformation mechanism is gss creep at the onset of extension it is also noted that dry conditions are more favourable for localized weakening

preferred orientation of anorthite deformed experimentally in newtonian creep abstract synthetic anorthite aggregates were deformed in a paterson gas deformation apparatus at confining pressures up to 400 mpa in torsion and axial compression at temperatures between 950 c and 1200 c samples deformed in torsion under newtonian creep display development of texture or crystallographic preferred orientation as documented with synchrotron x ray diffraction measurements complex diffraction patterns were deconvoluted with the rietveld method to obtain quantitative texture information torsion samples deformed up to shear strains of 4 and samples deformed in compression at higher stresses to total strains of 0 3 develop clear textures texture and shape preferred orientation spo of torsion samples display a monoclinic pattern with an asymmetry inclined against the sense of shear consistent with polycrystal plasticity simulations that assume the deformation is accomplished by dislocation glide these results show that a material deforming in linear viscous creep can develop a strong texture in striking contrast to the paradigm that the presence of a texture precludes low stress newtonian behavior our observations show that the presence or absence of crystallographic preferred orientation is not sufficient to uniquely infer the dominant rheological mechanical regime as sometimes applied for interpretation of seismic anisotropy in the earth

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

the calculated seismic properties of quartz mylonites with typical fabrics relationship to kinematics and temperature summary quartz mylonites produced by intense ductile deformation in shear zones are often invoked as seismic reflectors in the lower continental crust the seismic properties vp vsl v av and polarization planes of v and v have been calculated for five quartz mylonites that display typical lattice preferred orientations lpos all specimens display considerable seismic anisotropy with v anisotropy coefficients of between 8 and 12 per cent there are good correlations between the lpo of the a axes and the minimum v direction and the c axes and the maximum v direction the other seismic properties are not reliable indicators of lpo as the a axis is the major flow direction in quartz a correlation between the ductile flow direction and the v minimum can be made with increasing temperature and pressure alpha quartz transforms to beta quartz in the beta field quartzites are seismically transverse isotropic furthermore there is a considerable increase in v from 6 km s l in the alpha field to nearly 7 km s in the beta field the transition is instantaneous and hence will result in a sharp seismic discontinuity with a reflection coefficient of 0 08 between pure aggregates of alpha and beta phases however under typical conditions of the continental crust the seismological importance of this phase transition will be limited by the presence of secondary minerals and partial melting

seismic reflectivity of the whipple mountain shear zone in southern california the whipple mountains shear zone of southeastern california comprises part of the whipple mountains metamorphic core complex of the north american cordillera the 3 9 km thick shear zone displays excellent exposures of ductilely deformed mid tertiary mylonitic gneisses as well as kinematically related but younger cataclasites which underlie a major low angle normal fault the whipple detachment fault prominent crustal reflections beginning at 3 4 s were recorded on calcrust seismic profiles southwest of the whipple mountains in order to interpret these seismic reflections and to understand why detachment faults are imaged on some profiles but not on others we collected oriented rock specimens from all major structural units in the whipple mountains and determined p wave velocities for these samples parallel to three principal fabric directions under laboratory induced confining pressures up to 500 mpa using a geologic section through the mylonitic gneisses from a previous investigation we then constructed a detailed acoustic impedance section for the whipple mountain shear zone based upon laboratory results this section was in turn used to compose two dimensional synthetic reflection seismograms for comparison with the calcrust records significant impedance contrasts are enhanced by the opposing fabric orientations between the interlayered but structurally isolated relict domains of nonmylonitized rocks and the surrounding mylonitic gneisses the finite thickness and lateral extent of these rock units cause constructive interference in seismic modeling to create the seismic fabric of strong laterally discontinuous but subparallel reflections as seen in the calcrust profiles southwest of the whipple mountains chloritic breccias beneath the whipple detachment fault wdf have much lower seismic impedance than that of rocks occurring both structurally above and below however the significant thinning of the chloritic breccia zone to the southwest of the whipple mountains may have caused the poor image of the wdf on the calcrust profiles the absence of a detachment reflection is thus not necessarily indicative of the absence of a subsurface fault zone

3 d seismic velocities calculated from lattice preferred orientation and reflectivity of a lower crustal section examples of the val sesia section ivrea zone northern italy summary to quantify the seismic properties of lower crustal rocks and to better constrain the origin of the lower crustal seismic reflectivity we determined the complete 3 d seismic properties of a lower crustal section eight representative samples of the main lithologic and structural units outcropping in the val sesia ivrea zone were studied in detail the seismic velocities were calculated using the single crystal stiffness coefficients and the lattice preferred orientation lpo of each mineral in all samples the 21 stiffness coefficients characterizing the elastic behaviour of each rock are determined mafic and ultramafic rocks such as pyroxenite and pyroxenebearing gabbros display complex shear wave properties these rocks are weakly birefringent maximum 0 1 km s and it is difficult to find consistent relationships between the seismic properties and the rock structure on the other hand seismic properties of deformed felsic rocks are essentially controlled by mica they display strong s wave birefringence 0 3 km s and relatively high vp anisotropy 7 6 per cent amphibole also strongly influences the rock birefringence patterns for both kind of rocks the foliation is highly birefringent and the fast polarized shear wave is systematically oriented parallel to the foliation we show that the number of mineral phases in the rock strongly controls the anisotropy the seismic anisotropy has a complex role in the p wave reflectivity compared to the isotropic case anisotropy enhances the reflection coefficient for about 60 per cent of the possible lithological interfaces for 40 per cent of the interfaces the reflection coefficient is much lower when one considers the medium as anisotropic

recrystallization and fabric development in plagioclase the microstructures and fabrics of naturally deformed intermediate composition plagioclase feldspars from the amphibolite granulitic gneisses from the lower crust have been studied special interest in the effects of dynamic recrystallization and deformation fabrics has been taken the plagioclase feldspars an40 50 have undergone extensive ductile deformation by intracrystalline slip and twinning they show strong crystallographic preferred orientation which was produced by dominant 010 001 slip dynamic recrystallization is indicated by fine grained new grains neoblasts interspersed with deformed relicts porphyroclasts the neoblasts have formed preferentially along grain boundaries kink band boundaries kbbs intracrystalline shear zones and around solid inclusions such as garnet and pyroxene optical microstructural evidence suggests that dynamic recrystallization occurred primarily by grain boundary migration with subsidiary subgrain rotation which appears to have occurred only at low st

paleoproterozoic subduction in northwestern canada from near vertical and wide angle seismic reflection data near vertical incidence and refraction wide angle reflection seismic data recorded as part of lithoprobe studies in the paleoproterozoic archean domains of canada s northwest territories show remarkable reflections from within the upper mantle a parallel pair of reflectors imaged by the near vertical data can be traced from moho levels 33 km down to 70 km depth in a previous study the reflectors were interpreted as the top and bottom of an 1 8 ga subducted oceanic crust beneath the hottah terrane further inboard where the seismic line changes its direction from east west to nearly north south another pair of reflectors extends subhorizontally for about 100 km at 70 km depth before dipping downward the subhorizontal reflectors were not correlated with the dipping slab instead they were interpreted as a separate feature however they roughly coincide with a horizontal interface modeled from wide angle data by an earlier study considering the crooked line acquisition geometry we re exami

numerical modelling of strain localisation during extension of the continental lithosphere dipping reflectors in the upper mantle are observed on reflection seismic data and in some cases have been interpreted to represent mantle shear zones it is not clear however how these shear zones develop and under what circumstances previous attempts to model strain localisation by strain softening in the ductile regime have failed to show the development of mantle shear zones we present here results from a numerical model of lithospheric extension the model uses an elasto visco plastic rheology dependent on temperature pressure deviatoric stress and composition strain softening is incorporated into the mechanical model via a simple ad hoc strain viscosity relationship the results demonstrate that strain localisation can dominate lithospheric deformation in an extensional regime we illustrate the effects of varying the parameters which define the strain viscosity relationship on the geometry of lithospheric scale shear zones and the rift basin we also investigate the effects on shear zone development of introducing a variety of pre extension weak zones in the crust and or mantle our results suggest that if dipping reflectors observed on deep seismic data around the world are evidence of the existence of mantle shear zones there is a need to review our conceptual models of strain strength evolution during the development of continental rifts

modeling studies of interference reflections in thin layered media bounded by coal seams high resolution seismic data collected over a major u s coal basin indicated potential complex problems associated with interference reflections these problems differed from those normally encountered in the exploration of oil and gas because of differences in the geologic boundary conditions modeling studies were conducted to investigate the effects of overlapping primary reflections and the composite reflection that result from stacking individual wavelets a modified empirical formula of lindseth s linear relationship between acoustic impedance and velocity is used to extrapolate velocity information from density logs to provide appropriate geophysical properties for modeling the synthetic seismograms generated from density and synthetic sonic logs correlated well with the processed seismic data a 150 hz ricker wavelet is used to convolve with the computer models and the models showed that certain anomalous composite reflections result from the superposition of overlapping primary reflections de

some results of cocorp seismic reflection profiling in the grenville age adirondack mountains new york state cocorp deep seismic reflection profiling in the adirondack mountains of northern new york state has revealed a prominent zone of layered reflectors in the lower crust of the east central adirondacks the strong layered reflectors here termed the tahawus complex occur between 18 and 26 km depth beneath the sparsely reflective granulite grade surface terrane which has been uplifted from depths greater than 20 km the tahawus complex apparently represents layered rocks of some type in the lower crust of the adirondacks possibilities include gneissic layering cumulate igneous layering a layered sill complex and underthrust sedimentary strata the tahawus complex may be spatially coincident with a previously detected high conductivity zone in the lower crust suggesting that either unusual mineralogies or interstitial electrolytes are present in the tahawus complex in contrast to layered reflections discovered in the lower crust of the east central adirondacks and southeast of the adirondacks cro

low velocity layers in the northwestern margin of the south china sea evidence from receiver functions of ocean bottom seismometer data abstract the receiver function method was applied to teleseismic recordings of an active source ocean bottom seismometer array in the northwestern margin of the south china sea forward modeling of the receiver functions was used to derive velocity interfaces beneath the stations two main groups of low velocity layers were imaged the first group in the reef area with a thickness of 7 10 km exhibits a seaward uptrend of the buried depth but poor continuity the second group with a thickness of 3 4 km is more continuous along the zhongsha trough the buried depth decreases asymmetrically from the flanks to the rift center a high shear wave velocity layer of 4 0 km s is imaged at the crust bottom beneath the zhongsha trough with thickness of 3 km the average vp vs in the cenozoic sediment is 2 0 3 0 and was affected by high fluid activity induced by intensive faults the vp vs of the crust varies between 1 77 and 1 88 suggesting an intermediate to basic composition under the continental extension together these results suggest a two stage evolution model of the continental crust in the first stage the crust deformed in a ductile manner and thus created considerable shearing structures with high anisotropy that caused the obvious decrease of the shear wave velocity in the second stage the shearing structures connected asymmetrically driven by detachment faults in the upper crust and upwelling of the lower crust the zhongsha trough then formed probably with magmatic underplating due to lithospheric extension or rising of hot mantle materials

fabric asymmetry and shear sense in movement zones asymmetry between lattice preferred orientation and shape fabrics is commonly observed in microstructural studies of many deformed mineral aggregates e g ice carbonate rocks peridotites and quartzites theoretical predictions are limited by the nature of the models available and more can be gained by discussion of the mechanisms which give rise to such asymmetry in specific natural and experimental situations