kinematics fabrics and geochronology analysis in the medog shear zone eastern himalayan syntaxis abstract the uplift history and tectonic evolution of the eastern himalayan syntaxis ehs have been extensively studied in the last several decades the medog shear zone the eastern boundary of the ehs has preserved considerable significant information on the structural and tectonic evolution of the ehs in this study we report kinematics fabrics and geochronology data of the medog shear zone in the ehs analyses of the crystallographic preferred orientation cpo of quartz ebsd analysis demonstrated that there are three major slip systems 1 basal slip 2 prism c slip and 3 prism slip these slip systems are consistent with microstructures of low temperature shearing medium temperature shearing and high temperature shearing respectively zircons from the two gneiss samples possess inherited magmatic cores and metamorphic overgrown rims yielding a metamorphic age of 29 4 28 6 ma it is suggested that the dextral shearing along the medog shear zone was not earlier than the early oligocene the 40ar 39ar analysis indicates that the medog shear zone experienced three thermo tectonic events from the late oligocene to the pliocene e g 23 4 ma 16 9 12 6 ma and 5 3 ma we correlate the oligocene metamorphic event and the late oligocene to the pliocene multi stage thermo tectonic events that resulted from subduction of the northeast corner of the india plate

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

on the structure of grain boundaries in tectonites abstract grain boundaries in several tectonites have been examined using high voltage and scanning transmission electron microscopy on a gross scale grain boundaries are observed to differ from the host crystal in exhibiting faster rates of preferential etching during ion beam thinning and faster rates of ionization damage during exposure to the electron beam grain boundary features can be grouped according to decreasing size as follows grain junction voids that can form linked networks of tubules arrays of isolated voids and inclusions along the grain boundary surface and a narrow grain boundary film of distorted crystal structure the presence of inclusions in voids and the propensity for electron damage along the boundary suggests that the former were initially fluid filled and that the latter may be hydrated initial microchemical analyses using stem indicate that no compositional difference exists between the grain boundaries and host crystals the overall complex grain boundary structure in tectonites suggests that the effective grain boundary width for intercrystalline diffusion is much greater than that proposed based on metals and that diffusion path lengths along grain boundaries are much less than the theoretical maximum the net effect is to greatly enhance deformation mechanisms dependent on intercrystalline mass transport

accelerating thinning of kenai peninsula glaciers alaska 1 temperate mountain glaciers are thinning at high rates and significantly contributing to sea level rise due to these glaciers remote locations remote sensing and digital elevation models dems are an effective way to calculate their thinning rates and contribution to sea level rise comparisons of 1950s united states geological survey usgs and 2000 shuttle radar topographic mission srtm dems with dems produced from lidar profiles of glacier center lines indicate thinning rates from the mid 1990s to 1999 0 72 0 13 m y 1 accelerated by a factor of 1 5 as compared with 1950 to mid 1990s 0 47 0 01 m y 1 for glaciers on the harding icefield overall comparison of usgs and srtm dems indicate the harding icefield and grewingk yalik glacier complex alaska are thinning 0 61 0 12 m y 1 from 1950 to 1999

modelling the anisotropic seismic properties of partially molten rocks found at mid ocean ridges abstract the problem of modelling the seismic properties of mid ocean ridge rocks of the axial magma chamber amc and the low velocity triangle lvt immediately below it has been addressed using two samples from oman ophiolite as examples the specimens are a layered gabbro from the lower oceanic crustal sequence and a harzburgite from the upper most mantle section at the palaeo ridge axis the seismic properties have been simulated at a temperature of 1200 c and pressure of 200 mpa so that the basalt melt is above its solidus various effective medium methods are discussed in the perspective of modelling rocks that have a strong background elastic anisotropy due to crystal preferred orientation cpo and an introduced anisotropy due to oriented melt filled inclusions calculations using various methods show a wide range of predicted melt fractions for a given seismic velocity the self consistent scheme scs and differential effective medium dem are compared in some detail a tensorial model was developed using a poro elastic method of gassman at low seismic frequency and a standard dem with isolated basalt inclusions at high frequency at low frequency the basalt pore fluid is considered to be everywhere connected and the pressure uniform whereas at high frequency basalt inclusions are isolated assuming the medium to be a standard linear solid the low and high frequency velocities were used to calculate the anisotropy of attenuation calculations with spherical basalt inclusions show that the seismic velocities decrease and attenuation increase with increasing melt fraction the symmetry of the background anisotropy due to cpo is preserved but gradually reduced with increasing melt fraction for the seismic velocities observed for the axial magma chamber amc with v p of 3 5 to 4 7 km s the model predicts 50 70 melt for the low velocity triangle lvt below the amc with v p of 4 8 5 6 km s the model predicts 35 50 melt predictions from the observed attenuations q 1 at the amc of 0 05 0 02 and in the lvt of 0 02 0 01 are around 60 and 45 melt respectively seismic anisotropy has been modelled with ellipsoidal basalt pancake shaped inclusions with their circular sections in the xy foliation plane only a 2 3 of ellipsoidal basalt inclusions are required to over print the anisotropy of the background medium there is a rapid decrease in v p and increase of q 1 for propagation normal to the foliation z with increasing axial ratio of the inclusions with increasing axial ratio the model predicts decreasing amounts of melt for given velocity v p in the z direction for the lvt only 15 25 melt is predicted for a 10 1 inclusion instead of the 35 50 of the spherical inclusion

crystallographic preferred orientation magnetic and seismic anisotropy in rocks from the finero peridotite ivrea verbano zone northern italy interplay of anisotropy contributions from different minerals abstract mineral alignment can provide valuable information on a rock s geological history many structural tectonic or geodynamic studies rely on determining the preferred orientation of rock forming minerals either by direct texture determination or using anisotropy measurements as proxies robust interpretations of anisotropy require detailed understanding of its relationship with texture this study investigates the texture magnetic and seismic anisotropies of ultramafic rocks from the ivrea verbano zone ivz northern italy with a special focus on the interplay between anisotropy contributions carried by different minerals texture was obtained from electron backscatter diffraction magnetic anisotropy from low and high field anisotropy of susceptibility measurements and seismic anisotropy from p and s wave velocities along the three principal directions of the macroscopic fabric using 22 mm diameter cores texture based models of magnetic anisotropy agree well with corresponding measurements larger variability is observed for seismic anisotropy possible explanations are 1 velocities were measured in three directions insufficient to determine the full elasticity tensor and 2 seismic anisotropy depends on grain boundaries pores and cracks in addition to crystallographic preferred orientation the contributions to magnetic and seismic anisotropy of each constituent mineral can interfere positively leading to a larger overall anisotropy or negatively resulting in a weaker anisotropy seismic anisotropy is mostly controlled by olivine whereas magnetic anisotropy can be dominated by olivine pyroxene or hornblende this study illustrates that although the relationship between texture and anisotropy can be complex it is possible to quantitatively predict magnetic and to a lesser degree seismic anisotropy from texture data even though both magnetic and seismic anisotropies are common proxies for texture they are not coaxial and no simple relationship between the degree of magnetic and seismic anisotropy was found these results underline the importance of understanding different minerals contributions to anisotropy in upper mantle rocks

heterogeneous deformation and quartz crystallographic fabric transitions natural examples from the moine thrust zone at the stack of glencoul northern assynt quartz crystallographic fabric transitions in well exposed mylonites immediately beneath the moine thrust at the stack of glencoul nw scotland have been investigated by optical microscopy x ray texture goniometry and orientation distribution function analysis a progressive change is observed from asymmetrical kinked single girdle c axis fabrics at 0 5 cm beneath the moine thrust through asymmetrical type i cross girdle fabrics to symmetrical type i cross girdle fabrics at 30 cm beneath the thrust this c axis fabric transition is accompanied by a transition from asymmetrical single a axis maximum fabrics 0 5 cm beneath the thrust through asymmetrical two maxima fabrics to essentially symmetrical two maxima a axis fabrics odf analysis of these s l and l s tectonites indicates that c axis positions on the leading edge of the fabric skeleton are related by a common a direction oriented within the xz plane at a moderate angle to the lineation x in contrast c axis positions on the peripheral trailing edge are related by a positive r rhomb pole oriented close to z a directions lying within this common rhomb plane progressively change through 180 in orientation traced around the c axis fabric skeleton such contrasting single crystal rhomb a preferred orientations on the leading and trailing edges of the fabric skeleton are interpreted as indicating localized grain scale plane strain and flattening deformation respectively they result in tectonites with essentially symmetrical c and a axis fabrics which display strongly asymmetrical positive r and negative z rhomb pole figures the observed transition in quartz c and a axis fabrics is interpreted as indicating an increasing importance of non coaxial plane strain deformation as the moine thrust is approached even immediately 1 cm beneath the thrust however flow has still significantly departed from bulk simple shear and involved an important heterogeneous component of contemporaneous flattening deformation

northwest africa 1500 plagioclase bearing monomict ureilite or ungrouped achondrite northwest africa nwa 1500 is an ultramafic meteorite dominated by coarse 100 500 m olivine 95 96 augite 2 3 and chromite 0 6 1 6 in an equilibrated texture plagioclase 0 7 1 8 occurs as poikilitic grains up to 3 mm in vein like areas that have concentrations of augite and minor orthopyroxene other phases are cl apatite metal sulfide and graphite olivine ranges from fo 65 73 with a strong peak at fo 68 69 most grains are reverse zoned and also have 10 30 m reduction rims in terms of its dominant mineralogy and texture nwa 1500 resembles the majority of monomict ureilites however it is more ferroan than known ureilites fo 75 and other mineral compositional parameters are out of the ureilite range as well furthermore neither apatite nor plagioclase have ever been observed and chromite is rare in monomict ureilites nevertheless this meteorite may be petrologically related to the rare augite bearing ureilites and represent a previously unsampled part of the ureilite parent body upb the mn mg ratio of its olivine and textural features of its pyroxenes are consistent with this interpretation however its petrogenesis differs from that of known augite bearing ureilites in that 1 it formed under more oxidized conditions 2 plagioclase appeared before orthopyroxene in its crystallization sequence and 3 it equilibrated to significantly lower temperatures 800 1000 c from two pyroxene and olivine chromite thermometry formation under more oxidized conditions and the appearance of plagioclase before orthopyroxene could be explained if it formed at a greater depth on the upb than previously sampled however its significantly different thermal history compared to ureilites may more plausibly be explained if it formed on a different parent body this conclusion is consistent with its oxygen isotopic composition which suggests that it is an ungrouped achondrite nevertheless the parent body of nwa 1500 may have been compositionally and petrologically similar to the upb and may have had a similar differentiation history

experimental evidence of fast transport of trace elements in planetary basaltic crusts by high temperature metamorphism incompatible elements ies such as k p ti and rare earth elements rees provide important constraints on the geochemistry and chronology of basaltic meteorites most of which experienced complicated post crystallization histories these elements are immobile under subsolidus conditions because of their slow diffusion rates in planetary basalt minerals such as pyroxene and plagioclase thus ies are considered to preserve in most cases reliable records of formation processes even in ancient rocks that have undergone moderate thermal processing however observations of natural planetary samples suggest that melting of ie accessory carrier phases enhances the mobilization of such elements here we show that ies are rapidly transported by near solidus partial melting of highly ie enriched minor phases including ca phosphate and ti rich phases these partial melts occur as interconnected veins along cracks and fractures and as thin films on surfaces of pore spaces indicating that the melt mobilization is driven by surface tension the melt transport provides the necessary condition for melt migration consistent with the presence of the depleted basaltic eucrites also reaction between major minerals pyroxene and plagioclase and partial melts may cause disturbance and resetting of some isotopic systems these results have important implications for a range of geochemical investigations in particular the elemental fractionation resulting from such partial melting may result in improper age determinations

variability of the rebound hardness as a proxy for detecting the levels of continuity and isotropy in archaeological quartz abstract the equotip hardness tester is applied to different types of quartz and other lithic raw materials in order to determine if variations in the rebound hardness measured by this device may serve as a proxy for detecting the levels of continuity and isotropy of toolstones continuity the amount of cracks and other internal defects and isotropy the measure of the directional properties of a raw material are known to have important effects on the level of consistency and predictability of fractures which in turn is traditionally considered a main factor for defining the flaking quality of a given raw material the results demonstrate the existence of differences among the raw materials analysed in both the average value and the variability of the rebound hardness while cryptocrystalline samples show the most homogeneous results quartz and metamorphic rocks display a comparatively higher variance in the specific case of quartz such variability seems to be mainly caused by the presence of internal planes and discontinuities while anisotropy although also active had played a comparatively modest role the results achieved by the equotip hardness tester suggest that this device could be successfully used for a basic or preliminary estimating of some of the physical and mechanical properties of stone raw materials therefore it is a potential index test for complementing other more accurate laboratory methods

thermal diffusivity of olivine single crystals and polycrystalline aggregates at ambient conditions a comparison 1 heat transfer is a key process for the mantle dynamics however analysis of experimental data on thermal transport properties of upper mantle materials highlights a large scatter of absolute values of thermal diffusivity in particular conduction of heat in single crystals is systematically higher than in polycrystalline samples here we present new thermal diffusivity measurements on san carlos olivine single crystals and mantle rocks at 300k measured components of the olivine thermal diffusivity tensor along the 100 010 and 001 directions are 2 73 1 70 and 2 49 mm 2 s 1 respectively measurements of thermal diffusivity of peridotites as a function of structural orientation show absolute values and anisotropy in good agreement with those predicted by petrophysical models based on the crystal preferred orientation of olivine and the above thermal diffusivity tensor this suggests that the upper mantle thermal diffusivity is up to 50 higher than indicated by previous measurements on mantle rocks

different frequencies and triggers of canyon filling and flushing events in nazare canyon offshore portugal submarine canyons are one of the most important pathways for sediment transport into ocean basins for this reason understanding canyon architecture and sedimentary processes has importance for sediment budgets carbon cycling and geohazard assessment despite increasing knowledge of turbidity current triggers the down canyon variability in turbidity current frequency within most canyon systems is not well constrained new ams radiocarbon chronologies from canyon sediment cores illustrate significant variability in turbidity current frequency within nazare canyon through time generalised linear models and cox proportional hazards models indicate a strong influence of global sea level on the frequency of turbidity currents that fill the canyon radiocarbon ages from basin sediment cores indicate that larger canyon flushing turbidity currents reaching the iberian abyssal plain have a significantly longer average recurrence interval than turbidity currents that fill the canyon the recurrence intervals of these canyon flushing turbidity currents also appear to be unaffected by long term changes in global sea level furthermore canyon flushing and canyon filling have very different statistical distributions of recurrence intervals this indicates that the factors triggering and thus controlling the frequency of canyon flushing and canyon filling events are very different canyon filling appears to be predominantly triggered by sediment instability during sea level lowstand and by storm and nepheloid transport during the present day highstand canyon flushing exhibits time independent behaviour this indicates that a temporally random process signal shredding or summation of non random processes that cannot be discerned from a random signal are triggering canyon flushing events

petrology and geochemistry of the unbrecciated achondrite northwest africa 1240 nwa 1240 an hed parent body impact melt nwa 1240 is an unusual eucrite recently recovered in morocco as a single stone of 98 g it is an unbrecciated greenish brown rock nearly devoid of fusion crust it displays porphyritic texture consisting of skeletal hollow low ca pyroxene phenocrysts set in a variolitic fan spherulitic mesostasis of fine elongate pyroxene and plagioclase crystals minor phases are skeletal chromite iron silica troilite ilmenite and minute amounts of phosphate and fayalite pyroxenes are unequilibrated and show one of the widest ranges of composition so far described for a eucrite from en 76 0 wo 1 9 fs 22 1 to compositions nearly devoid of mg unusual ferrosilite and fe augite symplectites and possibly pyroxferroite plagioclase crystals contain significant amounts of fe and mg which are possibly controlled by the ca mg fe2 si3o8 plagioclase component

mechanical properties of quartz at the α β phase transition implications for tectonic and seismic anomalies 1 the anelastic response of single crystal and polycrystalline quartz and quartz rich sandstones to applied stress at low frequencies has been investigated by dynamic mechanical analysis and force torsion pendulum measurements the dynamic modulus was measured on heating and cooling through the phase transition under shear and bending stress applied at low frequency 1 hz single crystal quartz shows highly anisotropic properties in both the real and imaginary part of the modulus polycrystalline novaculite quartz displays the phase transition temperatures tc around 8 c higher than seen in single crystal quartz due to the pinning stress effect of grain boundaries the properties of the sandstone vary with heating and cooling cycles as the cementation and microfracturing of the grain structure changes at high temperature the phase transition in quartz is ferrobielastic and as such energy differences between domains in the transformation microstructure arise under conditions of applied shear stress pre transition softening and an increase in anelastic loss above tc are attributed to ferrobielastic switching of incipient domain microstructures that arise in the silica microstructure the influence of temperature on this microstructure is to induce increased switching of domains as a response to thermal stress inherent in the anisotropic thermal expansion of the polycrystalline structure these results indicate that ferrobielastic switching in quartz is an important mechanism in controlling changes in mechanical properties as such we anticipate that it will induce measurable anomalies in seismic signatures of quartz rich portions of the earth s crust in regions of high thermal flux the transition may also be responsible for observed seismic velocity reductions and tectonic weakening in western north america and in certain parts of tibetan plateau

crustal differentiation in the early solar system clues from the unique achondrite northwest africa 7325 nwa 7325 the unique achondrite nwa 7325 is an unusual olivine gabbro composed chiefly of calcic plagioclase an85 93 diopsidic pyroxene en50 1 54 0 wo44 8 49 3 fs0 6 1 3 and forsteritic olivine fo97 it is al and mg rich and fe and na poor and displays very low concentrations of incompatible trace elements much below 0 3 x ci abundances for many of them it is also characterized by huge eu and sr anomalies eu eu 65 srn cen 240 although the o isotopic composition of nwa 7325 and some ureilites those with olivine cores in the range fo75 to fo88 are similar a genetic relationship between them is unlikely due to the fe poor composition of nwa 7325 it is almost certainly derived from a distinct planetesimal not previously sampled by other achondrites the low na al ga al zn al ratios as well as the low k rb and cs shown by nwa 7325 suggest a volatile depleted parent body this unique gabbro is demonstrably a cumulate but the composition of its parental melt cannot be precisely assessed however the liquid from which nwa 7325 crystallized would have been very poor in incompatible trace elements yb in the range of 0 25 to 1 5 x ci abundance with a very large positive eu anomaly such a melt cannot be the product of the early magmatic activity on a small parent body instead we propose that the parental melt to nwa 7325 formed as a consequence of the total melting of an ancient gabbroic lithology possibly upon impact in agreement with the systematics of 26al 26mg based on recent dating the crustal material that was parental to nwa 7325 must have been older than 4562 8 ma and formed possibly 4566 ma ago if this scenario is correct nwa 7325 provides evidence of one of the earliest crusts on a differentiated body so far studied

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

polystage deformation of the gaoligong metamorphic zone structures 40ar 39ar mica ages and tectonic implications abstract the gaoligong metamorphic zone is located southeast of the eastern himalayan syntaxis in western yunnan china the zone is characterized by four stages of deformation d1 d4 d1 structures record early compressive deformation during the indosinian orogeny which formed tight to isoclinal f1 folds of bedding with a penetrative s1 foliation developed parallel to fold axial planes mid crustal horizontal shearing during d2 resulted in overprinting of d1 structures d1 and d2 structures are associated with granulite facies metamorphism d3 doming resulted in late crustal thickening and the development of a regional nw se trending f3 antiform synchronous with or slightly subsequent to d3 deformation the zone experienced d4 ductile strike slip shearing resulting in its exhumation to shallow crustal levels and retrograde metamorphism granitic d4 mylonites predominantly yield 40 ar 39 ar mica ages of 15 16 ma indicating that d4 dextral strike slip shearing occurred in the miocene weakly deformed leucogranite and protomylonite yield 40 ar 39 ar ages of 10 11 ma suggesting that ductile strike slip shearing continued to the late miocene the new 40 ar 39 ar data indicate that escape related deformation along the gaoligong strike slip shear zone occurred in the miocene in association with recent geophysical studies and on the basis of the structural crystal preferred orientation cpo and geochronological data presented in this paper we suggest that the gaoligong metamorphic zone formed in response to intracontinental transpression in the southeast of tibet characterized as intense deformation and metamorphism at middle upper crustal levels

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

local atomic structure of fontainebleau sandstone evidence for an amorphous phase 1 rocks are traditionally viewed as crystallite aggregates in general the treatment of the composition and mechanical properties never includes solid non crystalline components such glass like materials are difficult to detect in small quantities by standard techniques e g thin section polarimetry x ray diffraction but if they exist at critical locations i e grain contacts they could affect the behavior of rocks considerably neutron scattering measurements on a solid sample of fontainebleau sandstone have shown clear evidence for the presence of an unexpected glass like component atomic pair distribution function pdf analysis shows significant local structural deviations from pure quartz these deviations appear as an excess of 5 10 of nearest neighbor nn si o and o o bonds which is consistent with a 5 10 volume fraction of vitreous silica these measurements may provide significant information about the still unexplained causes for the peculiar mechanics and dynamics of sedimentary rocks

experimental investigation of fe mg distribution between olivine and clinopyroxene implications for mixing properties of fe mg in clinopyroxene and garnet clinopyroxene thermometry

an illustration of the advantages of a complete texture analysis described by the orientation distribution function odf using quartz pole figure data abstract the preferred orientation of eight crystal directions of quartz was measured with an x ray texture goniometer for selected quartz bearing tectonites with a clear pattern of c axis preferred orientation the advantages of data analysis by means of calculations of the orientation distribution function and other forms of data presentation derived from the odf are demonstrated

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

experimental evidence of transformation plasticity in silicates minimum of creep strength in quartz abstract mechanical weakening due to solid state transformation of mineral phases has long been proposed to be a significant mechanism for localization of deformation in the earth s lithosphere and the mantle transition zone however experimental observations confirming such a proposition are lacking here we present a novel approach to prove the existence of a minimum in the creep strength of quartz at the transition by observing the deformation of fluid inclusions in a quartz crystal using a hydrothermal diamond anvil cell pressure differences required for permanent deformation of the quartz around fluid inclusions were significantly lower at the phase transition than in either the stability fields of or quartz these results indicate that transformation plasticity of silicates can indeed cause a considerable localized reduction in the strength of the earth s crust and mantle

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

3d seismic study of a ductile shear zone from laboratory and petrofabric data saint barthelemy massif northern pyrenees france in order to constrain interpretations of seismic reflection records more effectively the seismic properties of a middle crustal section exposed in the saint barthelemy massif have been determined the massif transected by a 200 m thick shear zone has been systematically sampled for density measurements and modal analysis has been performed in order to define the spatial variations of physical properties seismic velocities vp v5 shear wave birefringence have been measured on five representative samples to 600 mpa and 600 c simultaneously in the three structural directions x y and z for two samples the expep mental data have been compared with calculated values based on petrofabric analyses the lattice preferred orientation lpo is determined using universal stage electron channelling microscopy and neutron diffraction goniometry using the experimental and calculated velocity data we establish a lateral homogeneous anisotropie model

preferred orientation of phyllosilicates in phyllonites and ultramylonites abstract x ray texture goniometry operated in the transmission mode has been applied to fine grained phyllosilicate rich sc mylonites from the santa rosa mylonite zone california the whipple mountains core complex california and the austroalpine nappes of the eastern alps austria basal planes 001 phyllosilicate pole figures depart in two respects from orthorhombic symmetry a the maximum pole density deviates from the pole to the mylonitic foliation and lies along a great circle which is normal to the foliation and contains the mylonitic lineation and b the contours of equal pole density are more widely spaced on one side of the 001 maximum than on the other measured along the mylonitic lineation optical microscopy reveals s and c surfaces and sets of shear bands sb characteristic of non coaxial deformation the pole figures are used for two purposes a the first is strain estimation after the march model by imposing orthorhombic symmetry the resulting march strains are more or less the same in all californian samples this is interpreted as evidence for a steady state foliation b the second is shear sense determination it is demonstrated that consistently asymmetric preferred orientations reflect structures which are typical of non coaxial deformation it is concluded that x ray analysis of phyllosilicate rich mylonites can in addition to providing an estimate of total strain give information about deformation regime and shear sense furthermore the study documents the heterogeneous nature of deformation on macroscopic and microscopic scales and the importance of non coaxial deformation in accommodating large strains

effect of crystal preferred orientations on the thermal diffusivity of quartz polycrystalline aggregates at high temperature article i nfo thermal diffusivity of synthetic quartz single crystals and natural polycrystalline aggregates were measured using a modified angstrom method at ambient pressure and up to 800 c in order to investigate the effect of crystal preferred orientation on thermal diffusivityof quartz rich rocks the thermal diffusivitytensorof quartz was determined by measuring the thermal diffusivity of an oriented synthetic quartz single crystal along the 100 and 001 directions a naturally deformed fine grained quartzite mylonite was used to compare the single crystalproperties with aggregate properties using thecrystal preferred orientations cpo measuredby the electron back scattered diffraction ebsd method and the measured thermal diffusivity tensor of quartz the three dimensional aggregate thermal diffusivity was modelled using volume averaging models in order to test the validity of these models that are only based on the properties of the constituent minerals the thermal diffusivity was then measured on oriented cores as a function of temperature along three orthogonal directions parallel to the x lineation y z pole to foliation structural frame both modelled and direct measurement of thermal diffusivity show an anisotropy of thermal diffusivity heat transport being more effective parallel to y direction and less effective parallel to the lineation x direction thermal diffusivities measured in a given direction on the natural rock are lower by less than 10 than the modelled thermal diffusivity in the same direction showing that rock imperfections have a weak effect on heat transport in addition the modelled thermal diffusivity anisotropy based on the characterization of crystal orientations and thermal diffusivity tensor shows a good agreement with measurements in both and stability field this validation of the petrophysical simulation allows for evaluating the 3d thermal diffusivity of quartz rich rocks in different crustal conditions the 3d thermal diffusivity of five naturally deformed quartzites displaying quartz fabrics expected in different crustal pressure and temperature ranges were simulated the resulting thermal diffusivity anisotropies of quartzites range between 40 and 10 depending on temperature crystallographic fabric patterns and fabric strength under crustal conditions quartzites deformed in the granulite facies display a high thermal diffusivity in the xz foliation plane whereas quartzites deformed in greenschist facies have highest thermal diffusivity in the z direction normal to the foliation plane

microscopic evidence of stishovite generated in low pressure shock experiments on porous sandstone constraints on its genesis it has been almost exactly half a century since the first synthesis of stishovite in shock experiments on quartz was reported but its formation conditions during shock is still under debate here we present direct transmission electron microscopic observation of stishovite within material recovered from high explosive shock experiments on porous sandstone shocked at 7 5 and 12 5 gpa our observations allow for new conclusions on the genesis of stishovite in a close to nature environment the formation of stishovite in short time shock experiments proves that its crystallization is ultrafast 1 s crystals were found only embedded in amorphous veins indicating homogeneous nucleation crystallization from melt rather than from glass can be concluded from the observation of roundish defect free crystals up to 150 nm in diameter embedded in nondensified glass the formation of stishovite at 7 5 gpa is in accordance with the phase diagram of silica if rapid undercooling is present that becomes only possible by the existence of small hot spots in an otherwise cold material which is supported by transient heat calculation the absence of coesite at 7 5 gpa suggests kinetic hindrance of its crystallization from melt and thus smaller critical cooling rates compared to stishovite where critical cooling rates are estimated to be as large as 1011 k s 1 while the amorphous veins containing stishovite represent unambiguously hot spots no associated pressure amplification could be verified within these veins the rapid liquidus crystallization of stishovite only in hot spots generated in porous material is an alternative formation mechanism to the widely accepted theory of solid solid transition from quartz to stishovite and might represent the more general mechanism occurring in nature for low shock pressure events