anisotropy of schists contribution of crustal anisotropy to active source seismic experiments and shear wave splitting observations we have made sets of five independent compressional and shear wave velocity measurements which with density allow us to completely characterize the transverse isotropy of samples from five metamorphic belts the haast schist terrane south island new zealand poultney slate chugach phyllite coldfoot schist and pelona schist united states these velocity measurements include compressional wave velocities for propagation parallel perpendicular and at 45 to the symmetry axis shear wave velocity for propagation and particle motion perpendicular to the symmetry axis and shear wave velocity for propagation parallel to the symmetry axis velocity measurements were made up to pressures of 1 gpa 35 km depth where microcracks are closed and anisotropy is due to preferred mineral orientation our samples exhibit compressional wave anisotropy of 9 20 as well as significant shear wave splitting metamorphic terranes that are anisotropic to ultrasonic waves may also be anisotropic at the scale of active and passive seismic experiments our data suggest that a significant thickness 10 20 km of appropriately oriented steeply dipping foliation schist in the crust could contribute as much as 45 of observed shear wave splitting our data set can also be used to model the effects of crustal anisotropy for active source seismic experiments in order to determine if the anisotropy of the terrane is significant and needs to be taken into account during processing and modeling of the data

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

quartz fabrics in the alpine fault mylonites influence of pre existing preferred orientations on fabric development during progressive uplift strong quartz crystallographic preferred orientations cpos were developed during dislocation creep in the mylonite zone exposed in the hanging wall of the dextral reverse alpine fault zone new zealand the cpos have a consistent asymmetry indicating a high ratio of simple to pure shear strain with a shear sense of dextral up to the nw consistent with the mesoscopic shear sense indicators and with slip on the active fault there is a transition from y maxima and asymmetric single girdles in mylonites and ultramylonites within 300 m of the present fault trace to cross girdle fabrics in the protomylonites further from the fault the strong y maxima or single girdle cpos are ascribed to high ductile shear strains under amphibolite facies conditions while the cross girdle patterns found in the protomylonites are interpreted to represent deformation under lower temperature conditions however the observed fabric transition cannot logically be attributed to variations in temperature during the last increment of deformation the highly oriented y maximum fabrics formed at high temperature contain very few grains suitably oriented for basal slip so that the slip systems activated during subsequent shear at lower temperatures during exhumation were prism or rhomb further from the fault where shear strains under high temperature conditions were lower weaker fabrics developed under these conditions were modified at higher levels in the crust into crossed girdle patterns this interpretation implies that intense localisation of shear strain along the fault zone within the lower crust must have occurred in order for the high temperature fabrics to become sufficiently intense to be preserved this in turn implies that deep seated localised shear was taking place early during the evolution of the current oblique slip plate boundary fault the results of this study also indicate that the current model that operation of certain slip systems in quartz is mostly a function of temperature is incorrect future work should consider the effects of deformation history total strain and the presence of other mineral phases

development of crystallographic preferred orientation and microstructure during plastic deformation of natural coarse grained quartz veins 1 the microstructure and crystallographic preferred orientation cpo of quartz were quantified in 17 samples of natural monomineralic tabular veins the veins opened and were deformed up to shear strain 15 in a small temperature window about 25 c above 500 c as established by ti in quartz thermometry the veins filled a set of fractures within the adamello tonalite southern alps italy and localized homogeneous simple shear during postmagmatic cooling the local square millimeter scale and bulk square centimeter cpo were investigated by computer integrated polarization microscopy cip and x ray texture goniometry weakly deformed veins wdv 1 consist of millimeter to centimeter sized crystals with a strong cpo showing a c axis girdle slightly inclined mostly with the shear sense to the foliation xy plane and a strong maximum close to the lineation x moderately deformed veins mdv 2 3 consist of elongated nonrecrystallized ribbon grains and most have a cpo showing a strong y maximum of c axes some with weak extension into a yz girdle strongly deformed veins sdv 4 to 15 are pervasively to completely recrystallized to fine 34 40 m grain size aggregates with a strong cpo similar to that of mdv the slip systems during plastic deformation were dominantly prism a with subordinate rhomb and basal a slip recrystallization occurred rather abruptly for 3 4 in contrast to dislocation creep experiments in quartz and other minerals a steady state recrystallized fabric is achieved at early stages of deformation 4 as there is no evidence with increasing strain of strengthening of the cpo of rotation of the fabric skeleton or of change in grain size wdv represent weakly deformed relicts of veins with an initial cpo believed to have developed during crystal growth but unsuitably oriented for prism a slip during subsequent shear mdv and sdv appear to derive from veins different from wdv where the vein crystals grew with orientation favorable for prism a slip the relationship between the initial growth cpo and the kinematic framework suggests that veins opened at a temperature close to that at which there is a switch between the activity of prism c and prism a slip with the temperature of growth causing growth of crystals well oriented for slip the initial cpo of veins from which quartz mylonites are commonly derived plays a critical role in the fabric evolution the strong growth and strain induced cpos of these sheared veins inhibited significant reworking during lower temperature stages of pluton cooling when basal a slip would have been dominant

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

experimental studies of the role of water in the plasticity of quartzites experiments on several quartzites at 300 mpa confirming pressure show them to behave as if near the brittle ductile transition at 900 sup 0 c when oven dry and in a more ductile fashion at 1000 sup 0 c when extra water is added in both cases the stress strain curves are substantially higher than would be expected on the basis of the strength of hydrolytically weakened synthetic crystals containing less water than the rocks as measured by the broad infrared absorption at low temperature thus would appear that relative to the single crystals the effective concentration of the water related species that is active in mechanical weakening within the grains is only a small fraction of the water present the presence of glass in the grain boundaries of wet deformed specimens suggest that grain boundary weakening may also be significant in these experiments questions of equilibrium and of geological extrapolation are discussed

infrared microspectroscopy analysis of water distribution in deformed and metamorphosed rocks infrared microspectroscopy has been applied to thin sections of various deformed and metamorphosed rocks in order to investigate water content of quartz in these rocks the broad ir band absorbance around 3400 cm 1 probably due to fluid inclusion molecular water h2o was used to calculate water contents deformed granitic rocks from the yanazawa kamimura area near the median tectonic line mtl showed an increase of water content in quartz from about 300 ppm to 2500 ppm toward the mtl with increasing degree of deformation metacherts from sambagawa metamorphic rocks asemigawa route showed a systematic decrease of water content in quartz from about 1000 ppm to 200 ppm with increasing metamorphic degree from chlorite garnet albite biotite to oligoclase biotite zones an archaean metachert from the napier complex granulite facies has only 40 ppm water comparing this with an inuyama unmetamorphosed chert sample having water contents of 3500 to 7000 ppm a starting point of metamorphism the systematic decrease of water in quartz with increasing metamorphic grade may extend from unmetamorphosed cherts to the highest grade metacherts water contents in high pressure metamorphic rocks in japan are mostly of the order of 500 ppm 300 700 ppm except for those from kurosegawa with a water content as high as 1700 ppm these results represent an exploratory analysis of intragranular water contents in quartz in various deformed and metamorphosed rocks further micro ft ir studies of the distribution of water in crustal rocks will provide a quantitative basis for examining the geochemical cycle of water in the earth s crust

tectono thermal evolution magma emplacement and shear zone development in the caruaru area borborema province ne brazil abstract the neoproterozoic borborema province bp in northeastern brazil is characterized by a regional flat lying foliation and by abundant magmatic rocks commonly spatially associated with large transcurrent shear zones combined field micropetrographic thermobarometric and magmatic fabric studies carried out on plutons country rocks and shear zones in the eastern domain of the province the caruaru area reveal that 1 an early episode of regional deformation with a top to the ne displacement was followed by the development of conjugate strike slip shear zones 2 similar low pressure 650 c metamorphic conditions occurred during the two events 3 plutons intruded the flat lying foliation 4 magma emplacement slightly predated strain localization in transcurrent shear zones but 5 plutons underwent strike slip deformation before complete crystallization therefore regional deformation pluton emplacement and shear zone development were successive events occurring over a relatively short time span 40ar 39ar laser dating of amphibole 584 ma and biotite 545 553 ma single grains supports slow cooling 5 c ma of country rocks through the argon closure temperatures of these minerals complex sometimes discordant amphibole ages bracketed between 552 and 575 ma for shear zones may be attributed to episodic activity and or dike swarm intrusions into them which disturbed the ar system biotite ages for the shear zones 533 545 ma also imply a relatively low cooling rate these data show that the massive injection of magmas in the continental crust of the bp during the brasiliano pan african orogeny produced a long lived thermal anomaly of regional extent the 40ar 39ar ages in the caruaru area are older than in other segments of the bp with similar geological characteristics this indicates a tectono thermal history for the bp that is more complex than previously recognized with either diachronous deformation magmatism and associated low p high t metamorphism or contrasting cooling of the different domains possibly due to differential vertical movements

deformation regime variations in an arcuate transpressional orogen ribeira belt se brazil imaged by anisotropy of magnetic susceptibility in granulites the ribeira belt of southeastern brazil displays an arcuate shape with a structural trend that varies from wns in the northern domain to ene wsw in the southern domain this curvature is accompanied by a transition from contraction dominated to transcurrent dominated tectonics the transition in deformation regime is accommodated in the central domain of the belt where granulitic rocks dominate and mineral stretching lineations are commonly concealed by metamorphic recrystallization we present anisotropy of magnetic susceptibility ams data from 664 samples from 62 sites in high temperature gneisses granulites and migmatites of the transitional central domain of the belt with the aim of investigating 1 how well ams allows one to map the mineral stretching lineation and foliations in domains displaying a complex kinematic framework and 2 to investigate the kinematic pattern at the transition between the thrusting dominated and a wrenchfaulting dominated orogenic segments the mean magnetic susceptibility is 7 54 10 k3 si the degree of anisotropy varies from 1 32 up to 4 31 with an average value of 1 53 the shape parameter t is generally o0 meaning that the ams ellipsoid is dominantly oblate magnetic lineations and foliations form a consistent pattern correlated with the modification of the structural characteristics observed along the ribeira belt in the southern wrenchfault dominated domain the magnetic lineation is subhorizontal parallel to the trend of the steeply dipping magnetic foliation this correlation with the fabric observed in mylonites suggests that the magnetic fabric is a valid proxy of the tectonic fabric in granulites results from the northern domain show that it comprises two sub domains both displaying a wns trending magnetic foliation eastward over a broad area the magnetic foliation is consistently steeply dipping and bears a shallowly to moderately plunging magnetic lineation westward the dip of the foliation decreases progressively and the lineation rotates from nearly parallel to nearly orthogonal to the trend of the foliation suggesting a progressive transition from transcurrent to contractional tectonics at the junction between the northern and southern domains the magnetic fabric is composite the magnetic foliation displays a constant nne ssw orientation but the magnetic lineation spreads within the foliation plane from parallel to normal to the foliation direction the ams pattern mapped in the central ribeira belt supports a scenario in which the transcurrent and contractional deformation regimes are coeval with a transition between the two regimes accommodated progressively over a large transpressional area where both strain regimes were active q 2005 elsevier ltd all rights reserved

rheology and strength of the lithosphere abstract mechanical properties of lithosphere are of primary importance for interpretation of deformation at all spatial and time scales from local scale to large scale geodynamics and from seismic time scale to billions of years depending on loading conditions and time scale lithosphere exhibits elastic brittle plastic or viscous ductile properties as can be inferred from rock mechanics data a large part of the long term lithospheric strength is supported in the ductile or ductile elastic regime while it also maintains important brittle strength yet at short seismic time scale s the entire lithosphere responds in elastic brittle elastic regime even though rock mechanics experiments provide important insights into the rheological properties of the lithosphere their conditions e g time scales strain rates temperature and loading conditions are too far from those of real earth therefore these data cannot be reliably extended to geological time and spatial scales strain rates 10 17 to 10 13 s 1 without additional parameterization or validation based on geological time scale observations of large scale deformation for the oceanic lithosphere the goetze and evan s brittle elastic ductile yield strength envelopes yses were validated by geodynamic scale observations such as the observations of plate flexure however oceanic lithosphere behavior in subduction zones and passive continental margins is strongly conditioned by the properties of the continental counterpart whose rheology is less well understood for continents and continental margins the uncertainties of available data sources are greater due to the complex structure and history of continental plates for example in a common continental rheology model dubbed jelly sandwich the strength mainly resides in crust and mantle while in some alternative models the mantle is weak and the strength is limited to the upper crust we address the problems related to lithosphere rheology and mechanics by first reviewing the rock mechanics data te flexure and ts earthquake data and long term observations such as folding and subsidence data and then by examining the physical plausibility of various rheological models for the latter we review the results of thermo mechanical numerical experiments aimed at testing the possible tectonic implications of different rheology models in particular it appears that irrespective of the actual crustal strength the models implying weak mantle are unable to explain either the persistence of mountain ranges for long periods of time or the integrity of the subducting slabs although there is certainly no single rheology model for continents the jelly sandwich is a useful first order model with which to parameterize the long term strength of the lithosphere it is concluded that dry olivine rheology laws seem to represent well the long term behavior of mantle lithosphere in oceans margins and continents as to the continent and margin crust rheology analysis of the results of thermo mechanical models and of te data based on the most robust variants of flexural models suggests that continental plates with te 30 50 smaller than their theoretical mechanical thickness hm i e te 20 60 km should be characterized by a weak lower or intermediate crustal rheology enabling mechanical decoupling between crust and mantle older plates such as cratons are strong due to crust mantle coupling and specific properties of the cratonic mantle lithosphere

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

electron microprobe monazite dating of ca 1 71 1 63 ga and ca 1 45 1 38 ga deformation in the homestake shear zone colorado origin and early evolution of a persistent intracontinental tectonic zone the homestake shear zone one of the principal precambrian structures within the colorado mineral belt has a history of tectonism that extends from the proterozoic to the tertiary new field mapping microstructural analysis and electron microprobe u th pb monazite dates define the style kinematics and timing of multistage proterozoic ductile deformation early subhorizontal shearing d1 progressed to northwest southeast crustal shortening d2 during a protracted ca 1710 1630 ma orogeny involving high temperature metamorphism partial melting and emplacement of the ca 1 7 ga cross creek batholith the subvertical northeast trend of the shear zone was established by d2 shortening a major episode of simple shear displacement occurred ca 1 4 ga during emplacement of the st kevin batholith complex kinematics are recorded by 1380 ma southeast side down dextral mylonite zones d3 and post 1380 ma southeast side up dextral ultramylonite pseudotachylyte zones d4 that were both localized in d2 foliation domains the succession of ductile tectonites in the homestake records 300 m y of episodic deformation at progressively shallower crustal levels as colorado evolved from an active accretionary margin 1 8 1 7 ga through crustal assembly 1 7 1 6 ga to a tectonically active continental interior post 1 45 ga the homestake shear zone constitutes a classic example of ductile shear zone reactivation and the development of persistent tectonic zones in the continental lithosphere

elasticity of single crystal orthoferrosilite the elastic moduli of a synthetic single crystal of pure fesio3 orthoferrosilite have been measured at 1 bar and 20 c by brillouin spectroscopy the moduli are in megabars c11 1 98 c22 1 36 c33 1 75 c44 0 59 c55 0 58 c66 0 49 c12 0 84 c13 0 72 c23 0 55 a comparison of the pyroxene crystal structure with elasticity data shows that distortion of the tetrahedral chains parallel to c is related to c33 straight or extended chains increase c33 and highly kinked chains weaken the structure however the ferrosilite data show that beyond a certain degree of distortion the tetrahedral chains are essentially passive previous models of pyroxene elasticity have indicated that c11 should be most sensitive to composition this is supported by the current study and is reflected in the elasticity data via a positive linear correlation of c11 with the size of the m1 octahedron as measured by the average m1 0 distance the average longitudinal modulus c 11 c11 c22 c33 3 varies linearly with composition in the system orthoenstatite orthoferrosilite whereas the average shear moduli c 44 and c 12 are constant for fe contents of at least 20 by comparison data for the solid solution forsterite fayalite are consistent with a linear variation of modulus versus composition between the end members for all three average moduli we suggest that this difference between orthopyroxene and olivine is due to a strong preference of fe2 for the m2 octahedral site in pyroxene the isotropic shear modulus is 30 lower for ferrosilite than enstatite despite a similar contrast in end member values of the olivine data are consistent with a linear variation of between fo100 and fa100 at high temperatures fe and mg in orthopyroxene will be randomly distributed and a linear variation of with molar content of fe is expected a comparison of laboratory elasticity data with observed mantle properties shows that vp vs and of the upper mantle are satisfied by pure olivine however the data strongly suggest that without the coexistence of a garnet phase no more than 10 orthopyroxene may be present in a model mantle assemblage ca clinopyroxene increases the upper limit on mantle garnet content but does not greatly affect constraints on the quantity of orthopyroxene in the upper mantle

seismic anisotropy and shear wave splitting in lower crustal and upper mantle rocks from the ivrea zone experimental and calculated data to quantify the relationships between anisotropy s wave splitting and tectonics we determined the seismic properties of lower crustal and upper mantle rocks outcropping in the ivrea zone northern italy we obtained p and s wave seismic velocities by laboratory direct velocity measurements and or by calculations based on the modal compositions of the rocks the lattice preferred orientations lpos and the single crystal stiffness coefficients measured p and s wave velocities 6 0 7 5 km s 1 and 3 6 4 2 km s 1 are typical of the lower crust the p wave anisotropy is in the range 0 10 shear wave birefringence is in the range 0 0 0 6 km s 1 with typical values between 0 0 and 0 2 km s 1 in many cases the birefringence is clearly related to fabric elements foliation lineation mafic rocks such as anorthosite or pyroxene bearing gabbros exhibit low p wave anisotropies 5 and low shear wave birefringences less than 0 1 km s 1 in contrast the seismic properties of felsic rocks such as biotite bearing gneisses and mafic rocks such as amphibolites display high vp anisotropy 10 and strong birefringence 0 3 km s 1 biotite and amphibole preferred orientations clearly control seismic anisotropy and particularly shear wave splitting in these rocks maximum splitting is observed in directions parallel to the foliation with the fast split shear wave polarized parallel to the foliation plane to have an overview of the seismic properties of this lower crustal section at a broader scale we calculated from our data the anisotropic seismic properties of several hypothetical samples that are perhaps more representative of the regional anisotropy than each sample individually for instance the average lower crustal sample displays an anisotropy of 5 5 for p waves and a birefringence around 0 14 km s 1 for s waves propagating parallel to the foliation we observe little splitting for waves propagating at high angle to the foliation

albite crystallographic preferred orientation and grain misorientation distribution in a low grade mylonite implications for granular flow abstract crystallographic orientation data from pure albite domains in a low grade sheared metagabbro from the combin zone of the western italian alps were measured by electron backscatter diffraction crystallographic preferred orientations cpos in four high strain domains are non random and have a triclinic symmetry the clusters of 100 010 and 001 show an angular relationship that corresponds to that of the albite crystal lattice however the orientations of axis clusters to the kinematic axes vary from domain to domain cpos from a low strain domain also show clustered axes with triclinic symmetry but with more intense clustering than those in the high strain domains grain misorientation distributions are presented both for the low strain domain and one of the high strain domains in the high strain domain the distribution of misorientation angles between neighbouring grains displays a peak at about 70 the equivalent distribution in the low strain domain has a peak at 30 for both domains the misorientation axis distributions between neighbouring and non neighbouring grains are random except for some of axes with 160 180 misorientation that exhibit a slight concentration around 010 the diversity of cpos among the domains suggests that these cpos could not be produced by dislocation creep they are likely to have been inherited from plagioclase parents as a result of host control on the nucleation of the new albite grains these cpos do not contain any direct information about the deformation kinematics we interpret that deformation of these domains occurred by granular flow crystallographic axis dispersion due to grain boundary sliding gbs caused weakening of cpos modification of misorientation angle distributions and randomisation of misorientation axis distributions the fact that a cpo can survive gbs even after a high strain indicates that cpo is not always a sensitive indicator of deformation mechanisms misorientation distribution may provide a complementary and possibly a more sensitive indicator of deformation mechanisms

comments on the interpretation of deformation textures in rocks abstract in rocks that undergo ductile deformation preferred orientation develops as a result of intracrystalline slip and mechanical twinning the orientation distribution is a consequence of the microscopic mechanisms and of the strain path it can be used to get some insight into the deformation history however it is never unique the interpretation relies largely on polycrystal plasticity theory the concepts of stress equilibrium and strain compatibility which are two extreme assumptions made to model deformation are discussed new approaches such as the viscoplastic self consistent theory are a compromise and may be applicable to mineral systems which display a high degree of plastic anisotropy important extensions allow for heterogeneous deformation in the polycrystal from grain to grain and even within grains in correspondence with microstructural observations all these theories defy the popular notion which is becoming entrenched in the geological literature that the microscopic slip plane normal aligns with the axis of maximum principal compressive stress and that in simple shear the crystallographic slip plane rotates into the macroscopic shear plane and the slip direction into the macroscopic shear direction an orientation referred to by geologists as easy glide it is emphasized that future work on texture development of rocks should be based on rigorous physics rather than ingenious intuition in accordance with an old recommendation of walter schmidt

faults shear zones in the earth s mantle abstract geodetic data support a short term continental deformation localized in faults bounding lithospheric blocks whether major faults observed at the surface affect the lithospheric mantle and if so how strain is distributed are major issues for understanding the mechanical behavior of lithospheric plates a variety of evidence from direct observations of deformed peridotites in orogenic massifs ophiolites and mantle xenoliths to seismic reflectors and seismic anisotropy beneath major fault zones consistently supports prolongation of major faults into the lithospheric mantle this review highlights that many aspects of the lithospheric mantle deformation remain however poorly understood coupling between deformation in frictional faults in the uppermost crust and localized shearing in the ductile crust and mantle is required to explain the post seismic deformation but mantle viscosities deduced from geodetic data and extrapolated from laboratory experiments are only reconciled if temperatures in the shallow lithospheric mantle are high 800 c at the moho seismic anisotropy especially shear wave splitting provides strong evidence for coherent deformation over domains several tens of km wide in the lithospheric mantle beneath major transcurrent faults yet it cannot detect narrow strain localization zones or shallowly dipping faults seismic profiling images shallow dipping seismic reflectors in the lithospheric mantle interpreted as the continuation of normal or less frequently inverse faults in the mantle however the nature of these reflectors is poorly understood there is a large discrepancy between the scale at which deformation processes are studied a few kilometers at most due to the limited size of continuous mantle exposures at the earth s surface and the scale inferred for strain localization in the mantle from geophysical observations tens of kilometers combining data on deformation microstructures and crystal preferred orientations in naturally deformed peridotites and seismologic observations allows nevertheless discussing strain localization processes that may play a role on the development of mantle shear zones

development of lattice preferred orientation in clinoamphiboles deformed under low pressure metamorphic conditions a sem ebsd study of metabasites from the aracena metamorphic belt sw spain the mechanical properties of the lower continental crust are intimately related to the rheology of calcic clinoamphibole naturally deformed clinoamphibole typically shows brittle behaviour under a wide range of deformation conditions whereas crystal plasticity is rarely observed in this study the microfabric of clinoamphiboles from metabasites deformed under low pressure medium to high temperature metamorphic conditions is presented amphiboles from all samples have developed lpo that can be attributed to different deformation mechanisms depending on deformation temperature fluid content structure and phase strength contrasts rigid body rotation of amphibole prisms within a weaker plagioclase matrix corresponds to metabasites deformed under medium deformation temperatures that show a weak layer structure and a high phase strength contrast between plagioclase and clinoamphibole lower temperatures and the presence of fluids mean that one of the studied rocks was affected also by dissolution precipitation creep and cataclastic flow in the final sample dislocation creep was accommodated by recovery and subgrain rotation dynamic recrystallization in monominerallic hornblende layers at higher temperatures which could be representative of the rheology of amphiboles in a lower continental crust at high temperature conditions

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

pyroxene orientation within the upper mantle petrofabric analyses of orthopyroxene and olivine for a suite of rocks from the bay of islands ophiolite newfoundland show systematic relationships between the minerals9 fabrics the plane of the mohorovicic discontinuity determined from the contact of the gabbro and ultramafic sections and the spreading direction inferred from the attitude of the sheeted dikes the orthopyroxene c crystallographic axes maxima which are aligned subparallel to the spreading direction coincide with olivine a axes maxima the orthopyroxene b axes and olivine c axes also lie approximately within the plane of the mohorovicic discontinuity but parallel the ridge whereas the orthopyroxene a axes maxima and olivine b axes maxima are approximately perpendicular to the mohorovicic discontinuity using olivine and orthopyroxene elastic constants and their pressure derivatives velocities have been calculated from composite petrofabric data and contoured for varying proportions of olivine and pyroxene the results of these calculations show that within a vertical direction compressional wave velocities change only slightly with increasing pyroxene content whereas parallel to the mohorovicic discontinuity velocities decrease with increasing pyroxene content anisotropy decreases from 4 for a rock with 100 olivine to 2 6 for a rock with 50 olivine and 50 orthopyroxene also within the plane of mohorovicic discontinuity the maximum and minumum velocities occur at approximately the same azimuth for differing percentages of orthopyroxene a comparison of the calculated anisotropy and the observed seismic anisotropy off the coast of california shows a striking agreement from this comparison it is estimated that the percentage of orthopyroxene within the upper mantle of this region varies between 0 and 30

a quantitative evaluation of the contribution of crustal rocks to the shear wave splitting of teleseismic sks waves abstract the seismic elastic properties of the crust have been modelled using hypothetical polycrystals with typical crustal compositions and commonly measured petrofabrics this modelling allows us to better evaluate and quantify the crustal contribution to splitting of teleseismic sks waves mafic rocks such as anorthosite or pyroxene bearing gabbros have complex s waves properties i e with small shear wave anisotropies

deformation and recrystallization of plagioclase along a temperature gradient an example from the bergell tonalite abstract syn to post mylonitic tilting of the bergell tonalite allows investigation of the deformation and recrystallization of plagioclase grains along a temperature gradient from the lower to the upper amphibolite facies at the lowest temperatures recrystallization occurs by nucleation and growth of new grains having a different composition from the old grains in contrast at the highest temperatures recrystallization occurs by subgrain rotation associated with grain boundary migration the temperature increase is inferred to induce this transition in two ways first at higher temperatures the anorthite content of the new grains is higher and thus the comositional difference between old and new grains is lower hence the chemical driving potential is small compared with recrystallization at lower temperatures second higher temperatures facilitate climb of dislocations and thus subgrain formation as well as grain boundary mobility the temperature increase is also associated with a transition in the dominant deformation mechanism intracrystalline plasticity dominates deformation at lower temperatures whereas at higher temperatures deformation occurs primarily by diffusion accommodated grain boundary sliding as indicated by the weakening of the indicatrix preferred orientation and the formation of a mixed plagioclase biotite matrix with increasing deformation therefore the strength of the mid to lower crust may be overestimated by dislocation creep flow laws for plagioclase lower crust rheology for polyphase rocks is better approximated by constitutive relationships involving diffusion accommodated grain boundary sliding

an 40ar 39ar investigation of the mertz glacier area george v land antarctica implications for the ross orogen east antarctic craton relationship and gondwana reconstructions george v land antarctica includes the boundary between late archean paleoproterozoic metamorphic terrains of the east antarctic craton and the intrusive and metasedimentary rocks of the early paleozoic ross delamerian orogen this therefore represents a key region for understanding the tectono metamorphic evolution of the east antarctic craton and the ross orogen and for defining their structural relationship in east antarctica with potential implications for gondwana reconstructions in the east antarctic craton the outcrops closest to the ross orogenic belt form the mertz shear zone a prominent ductile shear zone up to 5 km wide its deformation fabric includes a series of progressive overprinting shear structures developed under different metamorphic conditions from an early medium p granulite facies metamorphism through amphibolite facies to late greenschistfacies conditions 40 ar 39 ar laserprobe data on biotite in mylonitic rocks from the mertz shear zone indicate that the minimum age for ductile deformation under greenschist facies conditions is 1502 9 ma and reveal no evidence of reactivation processes linked to the ross orogeny 40 ar 39 ar laserprobe data on amphibole although plagued by excess argon suggest the presence of a 1 7 ga old phase of regional scale retrogression under amphibolite facies conditions results support the correlation between the east antarctic craton in the mertz glacier area and the sleaford complex of the gawler craton in southern australia and suggest that the mertz shear zone may be considered a correlative of the kalinjala shear zone an erratic immature metasandstone collected east of ninnis glacier 180 km east of the mertz glacier and petrographically similar to metasedimentary rocks enclosed as xenoliths in cambro ordovician granites cropping out along the western side of ninnis glacier yielded detrital white mica 40 ar 39 ar ages from 530 to 640 ma and a minimum age of 518 5 ma this pattern compares remarkably well with those previously obtained for the kanmantoo group from the adelaide rift complex of southern australia thereby suggesting that the segment of the ross orogen exposed east of the mertz glacier may represent a continuation of the eastern part of the delamerian orogen 2006 elsevier b v all rights reserved

magma assisted strain localization in an orogen parallel transcurrent shear zone of southern brazil in a lithospheric scale orogen parallel transcurrent shear zone of the pan african dom feliciano belt of southern brazil two successive generations of magmas an early calc alkaline and a late peraluminous have been emplaced during deformation microstructures show that these granitoids experienced a progressive deformation from magmatic to solid state under decreasing temperature conditions magmatic deformation is indicated by the coexistence of aligned k feldspar plagioclase micas and or tourmaline with undeformed quartz submagmatic deformation is characterized by strain features such as fractures lattice bending or replacement reactions affecting only the early crystallized phases high temperature solid state deformation is characterized by extensive grain boundary migration in quartz myrmekitic k feldspar replacement and dynamic recrystallization of both k feldspar and plagioclase decreasing temperature during solid state deformation is inferred from changes in quartz crystallographic fabrics decrease in grain size of recrystallized feldspars and lower ti amount in recrystallized biotites final low temperature deformation is characterized by feldspar replacement by micas the geochemical evolution of the synkinematic magmatism from calc alkaline metaluminous granodiorites with intermediate 87sr 86sr initial ratio to peraluminous granites with very high 87sr 86sr initial ratio suggests an early lower crustal source or a mixed mantle crustal source followed by a middle to upper crustal source for the melts shearing in lithospheric faults may induce partial melting in the lower crust by shear heating in the upper mantle but whatever the process initiating partial melting lithospheric transcurrent shear zones may collect melt at different depths because they enhance the vertical permeability of the crust these zones may then act as heat conductors by advection promoting an upward propagation of partial melting in the crust synkinematic granitoids localize most if not all deformation in the studied shear zone the regional continuity and the pervasive character of the magmatic fabric in the various synkinematic granitic bodies consistently displaying similar plane and direction of flow argue for accommodation of large amounts of orogen parallel movement by viscous deformation of these magmas moreover activation of high temperature deformation mechanisms probably allowed a much easier deformation of the hot synkinematic granites than of the colder country rock and consequently contributed significantly to the localization of deformation finally the small extent of the low temperature deformation suggests that the strike slip deformation ended approximately synchronously with the final cooling of the peraluminous granites the evolution of the deformation reflects the strong influence of synkinematic magma emplacement and subsequent cooling on the thermomechanical evolution of the shear zone magma intrusion in an orogen scale transcurrent shear zone deeply modifies the rheological behavior of the continental crust it triggers an efficient thermomechanical softening localized within the fault that may subsist long enough for large displacements to be accommodated therefore the close association of deformation and synkinematic magmatism probably represents an important factor controlling the mechanical response of continental plates in collisional environments

high temperature plastic deformation of quartz plagioclase multilayers by layer normal compression it is a common practice in rheological modeling to take the extrapolated compressive flow strengths of quartzite and plagioclase rock as the bulk flow strengths of the upper and lower continental crusts respectively such a practice implies that the bulk flow strength of polyphase rocks is identical to that of the pure weak phase to test this assumption we performed deformation experiments on synthetic layered and particulate quartz anorthite 50 50 composites and aggregates of the end members in a paterson gas medium apparatus at a confining pressure of 300 mpa a constant strain rate of 10 5 s 1 and temperatures from 1273 to 1473 k under these conditions anorthite deforms by recrystallization accommodated dislocation creep while quartz is semibrittle electron backscatter diffraction measurements show a strong lattice preferred orientation lpo of anorthite developed in deformed specimens and the lpo pattern is interpreted as a result of dislocation slip on the 010 100 system our experimental results show that the bulk flow strength of layered composites increases with decreasing thickness of the layers thinlayered composites are significantly stronger than pure anorthite aggregates and a homogeneous quartz anorthite mixture i e particulate composites with the same modal composition but weaker than pure quartz aggregates our experimental results together with the theoretical overview presented previously in the materials science literature demonstrate that 1 compressive flow strength of a polyphase rock cannot be represented by that of the weak phase and 2 thin layered rocks compressed normal to the layering are rheologically stronger than homogeneous isotropic mixtures under the same deformation conditions thus weak phase based rheology will lead to an underestimation of the bulk flow strength of the continental crust in which polyphase rocks dominate

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

effect of temperature on the b to c type olivine fabric transition and implication for flow pattern in subduction zones abstract the effect of temperature on the b to c type fabric transition in olivine was investigated by large strain shear deformation experiments a new series of deformation experiments was conducted at 1273 1373 k 2 0 gpa and strain rates ranging from 3 8 10 4 to 3 2 10 5 s 1 under water saturated conditions and combined with previous results at 1473 k the influence of temperature on deformation fabrics was investigated two types of olivine fabrics were identified one in which the olivine 0 0 1 axis is subparallel to the shear direction and the 0 1 0 plane is parallel to the shear plane b type and another one in which the 0 0 1 axis is parallel to the shear direction and the 1 0 0 plane is parallel to the shear plane c type the b type olivine fabric occurs at higher stress than the c type but the stress magnitude at which the b to c type fabric transition occurs decreases significantly with decreasing temperature the temperature dependence of the transition stress is parameterized using a range of exponential flow law that is constrained by the experimental study when the experimental results are extrapolated to lower temperatures the fabric transition between b and c type is predicted to occur at 1 mpa and 1000 k this prediction is consistent with the olivine fabrics in naturally deformed peridotites found in plate convergent regions the present study indicates that when some amounts of water 200 ppm h si are present the b type fabric will dominate in low temperature regions of the upper mantle whereas the c type or e type fabric will dominate at high temperatures the complex seismic anisotropy in subduction zones can be explained by changes in the dominant type of olivine fabrics caused by spatial variations in physical and chemical variables such as temperature and water content rather than by complexities in mantle wedge flow pattern

seismic velocities and anisotropy of the lower continental crust a review seismic anisotropy is often neglected in seismic studies of the earth s crust since anisotropy is a common property of many typically deep crustal rocks its potential contribution to solving questions of the deep crust is evaluated the anisotropic seismic velocities obtained from laboratory measurements can be verified by computations based on the elastic constants and on numerical data pertaining to the texture of rock forming minerals for typical lower crustal rocks the influence of layering is significantly less important than the influence of rock texture surprisingly most natural lower crustal rocks show a hexagonal type of anisotropy maximum anisotropy is observed for rocks with a high content of aligned mica it seems possible to distinguish between layered intrusives and metasediments on the basis of in situ measurements of anisotropy which can thus be used to validate different scenarios of crustal evolution

archean magmatism and deformation were not products of plate tectonics abstract the granite and greenstone terrains that dominate upper crust formed from about 3 6 to about 2 6 ga and record magmatic and tectonic processes very different from those of a younger time they indicate heat loss by the archean earth primarily by voluminous magmatism from a mantle much hotter than that of the present plate tectonic processes were not then operating the distinctive array of petrologic structural and stratigraphic features that characterize phanerozoic convergent plate systems ophiolites magmatic arcs accretionary wedges fore arc basins etc have no viable analogues in archean terrains purported archean plate tectonic indicators consist merely of rock types that even superficially resemble actual phanerozoic indicators only when considered in isolation from their association and structure archean ultramafic and mafic volcanic rocks neither resemble ophiolitic rocks in petrology nor occur in ophiolite type successions they often depositionally overlie felsic basement rocks and often overlie and are intercalated with sedimentary and felsic volcanic rocks and they require a mantle about 200 c hotter than now archean graywackes are coherent strata derived from nearby volcanic rocks late in the histories of their regions and they lack the setting and disruption that characterize modern accretionary wedges the lithologic structural and stratigraphic assemblages that typify proterozoic and phanerozoic rifted and reassembled margins also lack archean analogues and no evidence has been found for archean rifting rotation and reassembly of continental plates conversely characteristic archean assemblages lack modern equivalents in any tectonic setting ultramafic lavas erupted at liquidus temperatures are voluminous granite and greenstone terrains have no modern analogues greenstone belts are typically anastomosing networks of upright synforms formed by crowding aside by and sinking between large rising diapiric elliptical composite batholiths the batholiths include both the products of new crustal melts and variably remobilized mid crustal gneisses the greenstone belts are defined by late deformation of regionally semiconcordant volcanic and sedimentary successions and are not relics of successively formed volcanic chains little deformation generally preceded the diapirism and metamorphism was primarily of contact type the regionally uniform areal density and accordant crustal level of the diapiric batholiths their contacts primarily against the oldest strata of the synforms their general age 10 20 million years younger than most of the flanking stratiform rocks and considerations of high archean radiogenic heat productivity all fit the explanation that the batholiths were mobilized by partial melting of hydrous lower crust by radiogenic heating diapirism was accompanied by modest regional orthogonal shortening and extension of the hot upper crust producing the orientations of the batholiths rise of the batholiths greatly increased the petrologic fractionation of the crust and the concentration of radionuclides high in it resulting in cooling of the deeper crust and subjacent mantle and thus cratonization the upper crust containing the granite and greenstone aggregates was decoupled from the gneissic middle crust which underwent flattening and extension subparallel to the elongation of the shallow batholiths this deep deformation may have been driven by flow of dense restites toward delamination loci from which they sank into the mantle the early earth was probably wholly molten the surface of the earth like that of the moon must have been wholly recycled by impacts before 3 9 ga and heavily modified by them until 3 8 ga zircons as old as 4 2 ga have been found as clastic grains in much younger archean quartzites and polycyclic migmatites last partly melted and reconstituted under hydrous conditions only after 3 6 ga contain relict zircons as old as 4 0 ga the lithologies of the early earth protoliths in which these zircons formed have not been established but impact melts and breccias must be represented and magma ocean fractionates may be the nature of the transition in tectonic style into the granite and greenstone mode is unknown plate tectonic rifting and convergence were operating by about 2 0 ga and were in an essentially modern mode by about 0 8 ga the nature of the transition from the granite and greenstone mode at about 2 6 ga to plate mode by about 2 0 ga has yet to be defined the change may have been facilitated by the increasing content of water and carbon dioxide in the mantle as dense but hydrated delaminated archean crust sank into it

ultrahigh temperature deformation microstructures in felsic granulites of the napier complex antarctica abstract detailed electron microscope and microstructural analysis of two ultrahigh temperature felsic granulites from tonagh island napier complex antarctica show deformation microstructures produced at 1000 c at 8 10 kbar high temperature orthopyroxene al 7 wt and 11 wt exhibits crystallographic preferred orientation cpo and frequent subgrain boundaries which point to dislocation creep as the dominating deformation mechanism within opx two different main slip systems are observed in opx bands with exclusively opx grains containing subgrain boundaries with traces parallel to 010 and a strong coupling of low angle misorientations 2 5 5 with rotation axes parallel to 010 the dominating slip system is 100 001 isolated opx grains and grain clusters of 2 5 grains embedded in a qtz fsp matrix show an additional slip system of 010 001 the latter slip system is harder to activate we suggest that differences in the activation of these slip systems is a result of higher differential stresses imposed onto the isolated opx grains and grain clusters in contrast to opx large qtz grains up to 200 m show random crystallographic orientation this together with their elongate and cuspate shape and the lack of systematic in the rotation axes associated with the subgrain boundaries is consistent with diffusion creep as the primary deformation mechanism in quartz our first time detailed microstructural observations of ultrahigh temperature and medium to high pressure granulites and their interpretation in terms of active deformation mechanisms give some insight into the type of rheology that can be expect at lower crustal conditions if qtz is the mineral phase governing the rock rheology newtonian flow behaviour is expected and only low differential stress can be supported however if the stress supporting mineral phase is opx the flow law resulting from dislocation creep will govern the rheology of the rock unit hence an exponential relationship between stress and strain rate is to be expected

strain softening and microstructural evolution of anorthite aggregates and quartz anorthite layered composites deformed in torsion torsion experiments of anorthite an aggregates and layered composites with equal volume fractions of quartz qtz and an were performed in a gas medium apparatus at a confining pressure of 400 mpa temperatures from 1373 to 1473 k and twist rates from 1 0 x 10 4 to 3 0 x 10 4 rad s dense specimens were fabricated from an glass and qtz crystalline powder using hot isostatic pressing hip techniques both an aggregates and qtz an layered composites show a continuous strain weakening from a peak stress at gamma 0 2 0 3 to gamma 3 2 and steady state flow has not reached under the experimental conditions the weakening is even more pronounced in the layered composites than the monolithic aggregates suggesting channeling or localization of flow into the weak material between strong layers the sheared an specimens developed pervasively c s c structures which are similar to those observed in natural ductile shear zones tem and electron backscattering diffraction ebsd fabric analyses suggest that grain boundary migration recrystallization accommodated dislocation creep with 010 100 as the dominant slip system was operating in the an the strain softening may be due to the development of crystallographic preferred orientation cpo the operation of dynamic recrystallization and the formation of extremely fine grained recrystallized material in the narrow c shear bands c 2004 elsevier b v all rights reserved

shear zone controlled magma emplacement or magma assisted nucleation of shear zones insights from northeast brazil abstract in the borborema province of northeast brazil neoproterozoic granitoids and large scale transcurrent shear zones are spatially associated suggesting a genetic link between magma bodies and shear zones in some cases magma emplacement was clearly favored by shear zone activity but for several plutons this model is not satisfactory in these plutons pre full crystallization strike slip deformation evidenced by parallelism of magmatic foliations and lineations with the solid state mylonitic fabric and by a transition from magmatic to solid state flow is restricted to the vicinity of the shear zones evidence of shear zone activity prior to magma emplacement is lacking and the magmatic foliation away from the shear zones is in most cases shallowly dipping and concordant with the slightly older gently dipping regional gneissic foliation field and anisotropy of magnetic susceptibility mapping together with petrographic and geochemical studies performed in one of the magmatic complexes of the borborema province have revealed a structure and a magmatic fabric incompatible with the shear zone controlled emplacement model away from the shear zones this complex has retained a stratification inherited from the mixing of crystal poor magmas of contrasting composition and a magmatic fabric characterized by low to moderate dip magmatic foliations bearing a nw trending lineation which contrast with vertical foliations bearing ne or e w trending stretching lineations in the shear zones and indicates that crystallization started prior to shear zone development based on evidence that magma emplacement predated strike slip shearing and on information about the transition from magmatic to solid state deformation observed in the studied plutons we suggest that incompletely solidified plutons within the crust represent rheological heterogeneities that may induce strain localization and favor shear zone nucleation we propose that in the studied cases deformation first concentrated within the plutons where shear zones nucleated and grew subsequently as the solid phase component increased the crystal mush began to behave as a solid and the shear zones propagated into the country rocks

the role of strain localization in the segregation and ascent of anatectic melts namaqualand south africa granulite facies gneisses of the late proterozoic okiep copper district of the namaqua province in south africa preserve evidence of a range of anatectic melt features that reflect the initial stages of segregation and ascent of crustally derived magmas during high grade metamorphism these melt bodies include both in situ and sharply transgressive subvertical pipe like bodies that show vertical dimensions of several hundreds of metres and horizontal dimensions of tens to hundreds of metres migmatite bodies are spatially closely linked with narrow upright zones characterized by intense high strain fabrics locally referred to as steep structures they display a progressive textural development from diktyonitic textures in in situ bodies via schollen and raft textures to largely homogeneous intrusive granites that have migrated vertically for distances of several hundreds of metres or more the intimate association of the migmatite bodies with the steep structures reflects strain induced melt segregation facilitated by increased permeabilities in these ductile deformation zones and migration of melts into sites of strain incompatibility at the intersections between the regional subhorizontal gneissosity and the superimposed high strain zones subsequent melt migration was focused along the network of subvertical structural anisotropies provided by the steep structures melt migration was controlled by a combination of buoyancy shear enhanced melt compaction during ongoing deformation melt compaction due to the settling of wall rock fragments from higher stratigraphiclevels and subordinate brittle fracturing the unusual geometry of the steep structures and the intensely heterogeneous nature of the strain and the absence of similar strain features and voluminous melt bodies elsewhere in the granulite facies terrane suggest a positive feed back mechanism between melt generation and strain localization in steep structures the structural development of the migmatite bodies illustrates that the efficiency of melt segregation and migration in a mid crustal segment is dependent not only on the fertility of its lithologies but also on its deformational style

structures and fabrics in a crustal scale shear zone betic cordillera se spain abstract a broad zone of dominantly ductile high strain deformation lies beneath the aguilon nappe in the sierra alhamilla southern spain it forms part of a crustal scale movement zone traceable through much of the betic cordillera which separates the higher betic nappes from the underlying nevado filabride complex the zone is characterized in outcrop by a distinctive platy foliation and a strong nne trending stretching lineation microstructural characteristics include quartz ribbons mica fish augen of feldspar and other minerals in a matrix of dynamically recrystallized quartz and extensional crenulation cleavages narrow bands of ultramylonite and cataclasite occur within and on the margins of the movement zone deformation occurred under lower greenschist facies conditions and was accompanied by retrogression of earlier higher grade mineral assemblages structures in the movement zone developed in a temporal sequence beginning with isoclinal folding and transposition of older foliations this was followed by the formation of extensional crenulation cleavages and the progressive localization of strain into the ultramylonite bands mylonitic foliation in these bands is deformed by syn mylonite folds restricted to the bands all these structures were then deformed by s to se vergent small scale folds restricted to the movement zone as a whole cataclasis associated with alteration is localized along the ultramylonite bands and indicates a transition to late stage brittle deformation the lower boundary of the movement zone is gradational strain decreases recrystallized grainsize and the degree of recrystallization of quartz increases and pressure solution becomes the dominant deformation mechanism in mica schist asymmetric quartz fabrics in the movement zone indicate a nne sense of shear but variations in the degree of asymmetry suggest that flow was partitioned with the ultramylonite bands taking up much of the shear strain and the intervening rocks deforming more slowly and with a lower degree of non coaxiality diffuse fabrics in the fine grained ultramylonite bands may indicate a switch to a grainsize sensitive deformation mechanism and an overall downward increase in the opening angle of crossed girdle fabrics may reflect increased water activity at depth

fabrics produced mimetically during static metamorphism in retrogressed eclogites from the zermatt saas zone western italian alps abstract lattice preferred orientations lpos are commonly interpreted to form by dislocation creep consequently they are used to infer deformation at the metamorphic grade at which the minerals were stable especially if those minerals show a shape fabric here we show that lpos can occur through mimicry of a pre existing lpo so they formed statically not during deformation omphacite and glaucophane lpos occur in eclogite facies rocks from the zermatt saas unit of the northwest italian alps barroisite grew during greenschist facies retrogression and has an lpo controlled significantly by the eclogite facies omphacite and glaucophane lpos rather than directly by deformation using spatially resolved lattice orientation data from the three key minerals collected using electron backscatter diffraction we deploy a new technique of interphase misorientation distribution analysis to prove this barroisite lpo develops by mimicry of omphacite via a particular lattice orientation relationship and by direct topotactic and epitactic replacement of glaucophane lpo in turn influenced anisotropic grain growth resulting in a barroisite grain shape fabric thus regional retrogression during exhumation of the zermatt saas high pressure rocks was in large part static rather than dynamic as previously interpreted in general the possibility of mimetic fabrics forming during metamorphic reactions must be borne in mind when interpreting direct structural observations and seismic anisotropy data in terms of deformation in both crust and mantle

the influence of grain boundary fluids on the microstructure of quartz feldspar mylonites quartz rich domains in pre alpine water deficient amphibolite facies 510 580 c 250 450 mpa pegmatite mylonites from mont mary mm western alps preserve a fine dynamically recrystallized grain size without significant annealing despite the high synkinematic temperatures and subsequent static greenschist facies alpine overprint the microstructure is dramatically different from more typical water rich amphibolite facies mylonites such as from the simplon fault zone in the central alps where the recrystallized grain size is on the millimetre scale the difference reflects the dominant strain induced recrystallization mechanism 1 progressive subgrain rotation and grain boundary bulging for the dry mm examples and 2 fast grain boundary migration for the wet simplon examples the grain boundary microstructure imaged with sem is also very different with most grain boundaries in the dry mm samples lacking porosity whereas grain boundaries in the wet samples are decorated by a multitude of pores quartz grain boundaries from both wet and dry samples are locally coated by thin 100 s of nanometres possibly amorphous silica films despite the differences in microstructure the crystallographic preferred orientations cpos of quartz rich domains from both areas are very similar water deficient conditions hinder grain boundary mobility and thereby modify the dominant recrystallization mechanism s but apparently have little influence on the intracrystalline slip systems as reflected in the cpos strong c axis y maxima in mm mylonites both k feldspar and plagioclase an33 38 dynamically recrystallize consistent with the inferred metamorphic conditions under water deficient conditions mid to lower crustal rocks can deform heterogeneously under transitional ductile brittle conditions at high differential stress for mm ca 300 500 mpa as estimated for dry mohr coulomb failure and preserve this high stress microstructure because of the low mobility of dry grain boundaries

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

superimposed neoarchaean and paleoproterozoic tectonics in the terre adelie craton east antarctica evidence from th u pb ages on monazite and 40ar 39ar ages in order to understand the tectonic behaviour of a stabilized neoarchaean continental crust during subsequent tectonic activity we investigated the compositemetamorphic basement along the terre adelie and george vth land coastline also known as the terre adelie craton east antarctica two domains are recognized 1 a neoarchaean basement composed of granulite rocks to the east and overlain to thewest by amphibolites and 2 two paleoproterozoic detrital basins overly theneoarchaean crust and extend further west newgeochronological data fromthe terre adelie craton define a tectonic evolution with threemajor peaks activity th u pb electron probe analyses of monazite from the neoarchaean granulites constrain the main structuration event at ca 2 45 ga in agreement with zircon ages from throughout the neoarchaean domain local resetting together with low temperature recrystallization of monazites occurred at ca 1 7ga along hydrated anastomozing metre scale shear zones new 40ar 39ar ages obtained by stepwise heating techniques on amphibole biotite and muscovite from both the neoarchaean basement and the paleoproterozoic basins illustrate the differential evolutions of basement and its sedimentary cover during the major 1 7ga transpressive event a final event at ca 1 55 1 50 ga is only recognized close to the mertz shear zone 145 e bounding the terre adelie craton to the east the new data allow us to propose detailed geological pictures of the terre adelie craton geodynamic evolution from the neoarchaean to the mesoproterozoic

the deformation of plagioclase investigated using electron backscatter diffraction crystallographic preferred orientation data abstract extensive plagioclase grain orientation datasets have been collected using the rapid and accurate electron backscatter diffraction sem technique the crystallographic preferred orientation cpo of a porphyroclastic rich plagioclase layer and an equigranular plagioclase layer in an amphibolite facies shear zone from harris scotland were investigated in order to study their cpos and deformation mechanisms the layers have random cpos together with other microstructures and misorientation distributions these suggest dynamic recrystallisation followed by dominant grain boundary diffusion creep this is surprising given the coarse 100 250 m recrystallised grain size of the layers plagioclase within tectonites of these grain sizes in the amphibolite facies are generally inferred to deform by dislocation creep from microstructures and observed strong cpos deformation mechanism maps show that this style of deformation is possible at very slow strain rates

strain partitioning in ductile shear zones an example from a lower pennine nappe of switzerland abstract the rocks of the maggia nappe a lower pennine nappe of the swiss alps were deformed in at least three phases during alpine orogeny hercynian granites in the core of the nappe developed ductile shear zones during the early phases of deformation but between shear zones they remained almost undeformed both heterogeneous simple shear and heterogeneous volume change played an important role in the formation of the ductile shear zones partitioning of different strain components indicates a volume loss of nearly 80 90 towards the centres of some of the shear zones such a large amount of volume loss is also corroborated by strain analysis of deformed xenoliths in other shear zones of the same region a comparative study of theoretical strain paths and the natural results indicates constant shear strain rate but changing dilation rate during deformation

plagioclase preferred orientation in layered mylonites evaluation of flow laws for the lower crust thesis s m joint program in marine geology and geophysics massachusetts institute of technology dept of earth atmospheric and planetary sciences and the woods hole oceanographic institution 2008

magnetic fabric study across the ailao shan red river shear zone the anisotropy of magnetic susceptibility ams of 351 specimens from 51 sites across the ailao shan red river shear zone asrr was measured to determine its magnetic fabric rocks range westward from core schistose gneiss through lowgrade schist to triassic sediment magnetic ellipticity analysis shows that 41 of 51 sites have an oblate compressional fabric and the other 10 sites have a prolate fabric p j value drops by 22 4 in the low grade schist and by 27 4 in the triassic sediment on average with respect to the gneiss suggesting a rapid decrease of deformational intensity the directions of principal susceptibilities are closely related to the deformation of the ailao shan red river shear zone the susceptibility plane always coincides with the schistosity or cleavage plane most of the maximum susceptibility axes trend nw se in the shear zone the maximum susceptibility axes k max are parallel to the lineation within the foliation plane with increasing distance from the shear zone there is a trend that they become parallel to the down dip of reverse faults or cleavage this indicates changes in deformation mode inside and outside the shear zone within the shear zone horizontal movement is dominant outside shortening prevails the overall minimum magnetic axes align ne sw with subhorizontal to low dip angles suggesting that the dominant shortening is ne sw directed caution should be exercised when ams is used to determine shear sense in strong shear zones because the angle between the minimum susceptibility axis k min and pole of foliation is small and also because the attitude of foliation varies from place to place they result in unreliable or even wrong shear sense another important result is the axial ratio of magnetic susceptibility ellipsoid along the study section with these data it is possible to establish an axial ratio relationship between the finite strain ellipsoid and magnetic susceptibility ellipsoid for quantitative calculation of offset c 2002 elsevier science b v all rights reserved

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

granulite microfabrics and deformation mechanisms in southern madagascar optical microstructures and crystallographic preferred orientations were studied in naturally deformed granulite to high amphibolite facies quartzo feldspathic rocks in southern madagascar the microstructures of coarse grained granulite suggest that feldspar and quartz accommodated deformation by both dislocation and diffusion creep in the absence of melt the extreme ductility of feldspar in dynamically recrystallized granulite is explained by activity of dislocation creep in conjunction with stress controlled intracrystalline diffusion in the studied rocks the considerable volume of quartz is not interconnected even at high strain the lack of its interconnectivity in coarse grained granulites and in amphibolite facies granoblastic platy quartz rocks is explained by an extreme stability of the load bearing framework structure at high temperatures in dynamically recrystallized granulite the feldspar viscosity decreases so that quartz becomes enveloped by a weak feldspar matrix which inhibits its coalescence and interconnectivity we predict an important decrease in strength of quartzo feldspathic granulites due to activity of diffusional creep and convergence of viscosity of recrystallized feldspar and quartz

the initiation of strain localisation in plagioclase rich rocks insights from detailed microstructural analyses abstract in order to shed light on the cause for onset of strain localisation in plagioclase rich rocks we have performed detailed microstructural analyses on a sheared anorthosite leucogabbro using optical microscopy electron backscatter diffraction ebsd and chemical analyses the analysed sample is from an archaean unit sw greenland deformed at lower to mid crustal conditions t 675 700 c and moderate pressure the initial deformation occurred dominantly by dislocation creep and the grain size was reduced primarily by subgrain rotation recrystallisation recrystallised plagioclase grains average size 80 m are dominantly found in i clusters ii lenses and iii continuous bands subparallel to shear zone boundaries recrystallised grains in clusters and lenses display inherited crystallographic orientations their bulk crystallographic preferred orientation cpo is random however crystallographic characteristics show that parent and daughter grains have the same misorientation axes and possibly the same active slip systems recrystallised grains in continuous bands show a cpo with a single dominant active slip system 001 aligned with the structural xyz framework for these parent and daughter grains misorientation axes are random and the dominant slip system is different grain rotations of recrystallised grains are traceable back to the orientation of the adjacent porphyroclast we infer that the cause for strain localisation is recrystallisation and development of a cpo in continuous recrystallised bands microstructures in combination with misorientation and slip system analyses indicate a possible change from dislocation creep in clusters and lenses to dislocation accommodated grain boundary sliding disgbs in continuous bands this inferred shift in dominant deformation mechanism would lower the strength of the shear zone

details of colliding thunderstorm outflows as observed by doppler lidar abstract three cases of colliding outflow boundaries are examined using data collected from the noaa doppler lidar and a meteorological tower during the summer of 1986 near boulder colorado the data are unique because the lidar and the 300 m tower were colocated providing measurements of both kinematic and thermodynamic properties lidar data reveal small scale vortex roll instabilities within the leading edge of the outflow observations of the post collision interactions showed that the warmer of the two outflows was deflected upward by the colder outflow to heights of 2 km in all cases this forced mechanical lifting was sufficient to produce convection a simple model of two colliding density currents also suggests that deeper outflows are more efficient in initiating convection

evolution of quartz microstructure and c axis crystallographic preferred orientation within ductilely deformed granitoids arolla unit western alps abstract we have studied quartz microstructures and the c axis crystallographic preferred orientations cpos in four granitoid samples representative of increasing ductile shear deformation from a weakly deformed granitoid stage 1 to a mylonitic granitoid stage 4 the quartz c axis cpo measured in the mylonitic granitoid has been compared with the one observed in a fully recrystallized quartz mylonite from the same area all the samples belong to the austroalpine arolla unit western alps and were deformed at greenschist facies conditions the quartz c axis cpo was analyzed using a u stage and the optical orientation imaging technique the magmatic plagioclase forming more than 50 of the volume of the granitoid is extensively replaced by a mica rich aggregate even in weakly deformed samples of stage 1 these aggregates flow to form an interconnected weak matrix with increasing deformation wrapping relatively less strained quartz grains that undergo dominantly coaxial strain recrystallization of quartz ranges from less than 1 in the weakly deformed granitoid to up to 85 in the mylonitic granitoid with average grain strain of 41 and 64 respectively with increasing strain and recrystallization quartz grains in the granitoids show a sequence of transient microstructures and cpos crystal plastic deformation is initially accomplished by dislocation glide with limited recovery and at 50 grain strain it results in a cpo consistent with dominantly basal a slip at 60 grain strain recrystallization is preferentially localized along shear bands which appear to develop along former intragranular cracks and the recrystallized grains develop a strong c axis cpo with maxima orthogonal to the shear band boundaries and independent of the host grain orientation within the granitoid mylonite at an average quartz grain strain of 64 recrystallization is extensive and the c axis cpo of new grains displays maxima overlapping the host c axis orientation and therefore unrelated to the bulk sense of shear the host controlled cpo is inferred to reflect pervasive recrystallization by progressive subgrain rotation the switch from shear band control to host control on c axis cpo occurred between 40 and 70 of recrystallization in the quartz mylonite the quartz c axis cpo develops an asymmetric single girdle consistent with the bulk sense of shear and the synkinematic greenschist facies conditions this study indicates that the cpo evolution of quartz may significantly differ in cases of polymineralic vs monomineralic rocks under the same deformation conditions if quartz in the polymineralic rock behaves as a strong phase

structural setting of the neoarchean terrains in the commonwealth bay area 143 145 e terre adelie craton east antarctica lithological and structural maps from the east commonwealth bay unit ecb terre adelie and georges v land antarctica are presented with a summary of the main structural and metamorphic data obtained in the area the ecb unit is structured during the late archean lower proterozoic at 2 4 2 5 ga with i granulite metamorphism at 9 6 kbar and 850 50 c of lower mid crust and amphibolite metamorphism p 5 kbar t 750 c of the upper crustal levels ii the lower crustal granulites are uplifted and suffered local partial melting and then retrogression into the amphibolite facies at 550 50 c 5 kbar granulites are extruded in the core of a crustal scale anticlinal fold granulites being retrogressed only on the rims of the anticline crustal scale folding along with other structural features results from intense ne sw shortening that prevailed during the late archean orogenic cycle strike slip and extensive motions are only minor in that process iii top to the east thrusting and nappe piling has at least locally occurred under lower amphibolite to greenschist facies conditions finally it seems that iv the paleoproterozoic 1 7 ga structural imprint may have only affected the rims of the archean units the tectonic context observed in the 1 7 ga cape hunter phyllites features mainly e w shortening component and vertical extrusion the eastern mertz and western port martin parts of the archean block were reactivated by localized dextral shearing

grain size reduction mechanisms and rheological consequences in high temperature gabbro mylonites of hidaka japan abstract the study of microstructures and crystallographic fabrics in a granulite facies shear zone of the hidaka metamorphic belt showed that the strong shearing localized within the mylonite resulted in the asymmetrical elongation of the inherited orthopyroxene porphyroclasts and the generation of fine grained plagioclase and orthopyroxene layers as asymmetric tails of orthopyroxene porphyroclasts the orthopyroxene porphyroclasts and the coarse plagioclase matrix surrounding them have a strong crystallographic preferred orientation acquired through deformation by dislocation creep in contrast the small orthopyroxene and plagioclase grains located in the tails have equant shapes and random fabric that are interpreted as the result of deformation by grain boundary sliding the small orthopyroxene grains are generated on the sheared rims of the orthopyroxene porphyroclasts by subgrain rotation inheriting the orientation of the porphyroclasts before deforming by grain boundary sliding gbs and losing this fabric additional mechanism of grain size reduction is the disruption of orthopyroxene porphyroclasts by synthetic shear zones localized on clinopyroxene exsolutions the switch in deformation mechanism from dislocation creep to gbs associated with the grain size reduction yielded estimates of deviatoric stress one order smaller than lithostatic pressure besides such rheological evolution attests of the mechanical softening during deformation which contributed to the localization of the strain within the mylonite

a mesoscale eddy driving spatial and temporal heterogeneity in the productivity of the euphotic zone of the northeast atlantic in this paper we show how different water masses from a similar geographic region provide an explanation for perturbations in the signal of declining productivity at the porcupine abyssal plain pap study site in the northeast atlantic furthermore we show that the passage of these different water masses is affected by the filamentary instabilities of a cyclonic eddy just southwest of the pap site we describe a high resolution spatial hydrographic survey conducted with a towed instrument package complemented by biogeochemical sampling maximum rates of primary production of 110 mmol c m 2 d 1 seen at the centre of the survey area were associated with the passage of an eddy filament and were enhanced 3 fold relative to far field conditions 36 mmol c m 2 d 1 the rotation and stirring influence of the eddy resulted in the sequential passage of 3 distinct water masses past the observation point this understanding of the lateral stirring around the site enabled us to explain the sharp changes observed in daily primary production rates and other biogeochemical parameters the spatial survey also revealed a fluorescence maxima associated with the cyclonic eddy that was laterally displaced northwards away from the core an observation supportive of recent modelling studies

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

microstructures deformation mechanisms and seismic properties of a palaeoproterozoic shear zone the mertz shear zone east antarctica abstract the mertz shear zone msz is a lithospheric scale structure that recorded mid crustal deformation during the 1 7 ga orogeny we performed a microstructural and crystallographic preferred orientation cpo study of samples from both mylonites and tectonic boudins that constitute relics of the terre adelie craton tac the deformation is highly accommodated in the msz by anastomosed shear bands which become more scattered elsewhere in the tac most of the msz amphibolite facies mylonites display similar cpo thermal conditions intensity of deformation and dominant shear strain preserved granulite facies boudins show both coaxial and non coaxial strains related to the previous 2 45 ga event this former deformation is more penetrative and less localized and shows a deformation gradient later affected by a major phase of recrystallization during retrogression at 2 42 ga both msz samples and granulite facies tectonic boudins present microstructures that reflect a variety of deformation mechanisms associated with the rock creep that induce contrasted cpo of minerals quartz feldspar biotite amphibole and orthopyroxene in particular we highlight the development of an uncommon cpo in orthopyroxene from weakly deformed samples characterized by 010 planes oriented parallel to the foliation plane 001 axes parallel to the stretching lineation and clustering of 100 axes near the y structural direction lastly we computed the seismic properties of the amphibolite and granulite facies rocks in the msz area in order to evaluate the contribution of the deformed intermediate and lower continental crust to the seismic anisotropy recorded above the msz our results reveal that i the low content of amphibole and biotite in the rock formations of the tac and ii the interactions between the cpo of the different mineralogical phases generate a seismically isotropic crust thus the seismic anisotropy recorded by the seismic stations of the tac including the msz must be due to mantle rather than crustal structures

mylonitic deformation of gabbro in the lower crust a case study from the pankenushi gabbro in the hidaka metamorphic belt of central hokkaido japan abstract the mylonitization of the pankenushi gabbro in the hidaka metamorphic belt of central hokkaido japan occurred along its western margin at 600 mpa and 660 700 c through dynamic recrystallization of plagioclase and a retrograde reaction from granulite facies to amphibolite facies orthopyroxene clinopyroxene plagioclase h2o hornblende quartz the reaction produced a fine grained 100 m polymineralic aggregate composed of orthopyroxene clinopyroxene quartz hornblende biotite and ilmenite into which strain is localized the dynamic recrystallization of plagioclase occurred by grain boundary migration and produced a monomineralic aggregate of grains whose crystallographic orientations are mostly unrelated to those of porphyroclasts the monomineralic plagioclase aggregates and the fine grained polymineralic aggregates are interlayered and define the mylonitic foliation while the latter is also mixed into the former by grain boundary sliding to form a rather homogeneous polymineralic matrix in ultramylonites however in both mylonite and ultramylonite plagioclase aggregates form a stress supporting framework and therefore controlled the rock rheology crystal plastic deformation of pyroxenes and plagioclase with dominant 100 001 and 001 1 2 1 1 0 slip systems respectively produced distinct shape and crystallographic preferred orientations of pyroxene porphyroclasts and dynamically recrystallized plagioclase grains in both mylonite and ultramylonite euhedral to subhedral growth of hornblende in pyroxene porphyroclast tails during the reaction and its subsequent rigid rotation in the fine grained polymineralic aggregate or matrix produced clear shape and crystallographic preferred orientations of hornblende grains in both mylonite and ultramylonite in contrast the dominant grain boundary sliding of pyroxene and quartz grains in the fine grained polymineralic aggregate of the mylonite resulted in their very weak shape and crystallographic preferred orientations in the fine grained polymineralic matrix of the ultramylonite however pyroxene and quartz grains became scattered and isolated in the plagioclase aggregate so that they were crystal plastically deformed leading to stronger shape and crystallographic preferred orientations than those seen in the mylonite

development of shape and lattice preferred orientations of amphibole grains during initial cataclastic deformation and subsequent deformation by dissolution precipitation creep in amphibolites from the ryoke metamorphic belt sw japan amphibolites from the ryoke metamorphic belt sw japan were deformed initially by cataclasis and subsequently by dissolution precipitation creep initial cataclastic deformation produced a rather weak shape preferred orientation spo of brown amphibole grains with small aspect ratios as well as a poorly developed amphibole lattice preferred orientation lpo with na a 100 axes scattered subnormal to the foliation and ng or c 001 axes scattered around the lineation during later deformation by dissolution precipitation creep preferential dissolution at grain boundaries subparallel to the foliation and simultaneous compaction normal to the foliation have likely produced a distinct spo of elongate brown amphibole grains subparallel to the foliation as well as their lpo such that their ng or c axes are scattered around the lineation while na a and nb b 010 are spread along a girdle normal to the lineation also during this deformation green amphibole precipitated as isolated grains or in pressure shadow regions around brown amphibole grains nucleation and anisotropic growth of isolated green amphibole grains according to the orientations of the principal stress directions produced an lpo of these grains such that their na a are oriented normal to foliation nb b within the foliation normal to the lineation and ng or c axes are parallel to the lineation in addition there is an associated spo growth of green amphibole in pressure shadow regions around brown amphibole grains occurs either syntaxially or anisotropically according to the orientations of the principal stress directions q 2003 elsevier ltd all rights reserved

evaluating quartz crystallographic preferred orientations and the role of deformation partitioning using ebsd and fabric analyser techniques quartz crystallographic preferred orientations cpo from three distinct orthogneisses using both the electron back scatter diffraction ebsd and fabric analyser fa techniques reveal a clear trend from basal and rhomb slip for high p t conditions 670 20 degrees c 9 kbar rhomb and basal slip for medium p t 590 15 degrees c 6 kbar and a dominance of prism slip for lower p t conditions 570 degrees c 4 5 kbar the textural variations are interpreted in terms of a temperature field gradient and microscale strain partitioning controlled by a weak feldspar matrix that can locally invert the expected slip system sequences locally quartz cpus are different within one thin section and in comparison to bulk orientation measurements both ebsd and the fabric analyser techniques are ideal to determine such textural heterogeneities while the ebsd is a powerful technique to determine the full cpo measurements from similar locations inside several quartz grains from the orthogneisses and an annealed and undeformed quartzite show that the fa is an accurate tool to determine cpus of c axis orientations in uniaxial materials in a cpo focussed study the fa is a cheap alternative to ebsd as the analysis of whole thin section can be accomplished in a very short time with minimal sample preparation with the fa it is possible to evaluate the cpus of several samples quickly with an accuracy that allows identification of the main slip systems and their homogeneity

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

a change in the geodynamics of continental growth 3 billion years ago models for the growth of continental crust rely on knowing the balance between the generation of new crust and the reworking of old crust throughout earth s history the oxygen isotopic composition of zircons for which uranium lead and hafnium isotopic data provide age constraints is a key archive of crustal reworking we identified systematic variations in hafnium and oxygen isotopes in zircons of different ages that reveal the relative proportions of reworked crust and of new crust through time growth of continental crust appears to have been a continuous process albeit at variable rates a marked decrease in the rate of crustal growth at 3 billion years ago may be linked to the onset of subduction driven plate tectonics

seismic anisotropy of the archean crust in the minnesota river valley superior province the minnesota river valley mrv subprovince is a well exposed example of late archean lithosphere its high grade gneisses display a subhorizontal layering most likely extending down to the crust mantle boundary the strong linear fabric of the gneisses results from high temperature plastic flow during collage related contraction seismic anisotropies measured up to 1 gpa in the laboratory and seismic anisotropies calculated through forward modeling indicate vp 5 6 and vs 3 the mrv crust exhibits a strong macroscopic layering and foliation and relatively strong seismic anisotropies at the hand specimen scale yet the horizontal attitude of these structures precludes any substantial contribution of the mrv crust to shear wave splitting for vertically propagating shear waves such as sks the origin of the regionally low seismic anisotropy must lie in the upper mantle a horizontally layered mantle underneath the united states interior could provide an explanation for the observed low sws

crystallographic preferred orientation in albite samples deformed experimentally by dislocation and solution precipitation creep abstract the crystallographic preferred orientations of a series of experimentally deformed fine grained albite aggregates were measured by synchrotron source x ray diffraction most samples were deformed and extensively recrystallized by low temperature recrystallization accommodated dislocation creep in axial compression as well as simple shear these samples developed weak but distinct crystallographic preferred orientations consistent with intracrystalline slip on 001 the sheared samples have a marked asymmetry of the maxima with respect to the shear zone boundaries one sample was axially compressed by solution precipitation creep it developed a somewhat different but equally strong preferred orientation perhaps reflecting crystallographic anisotropy in rates of dissolution and growth

dynamic recrystallization of quartz correlation between natural and experimental conditions quartz veins in the eastern tonale mylonite zone italian alps were deformed in strike slip shear due to the synkinematic emplacement of the adamello pluton a temperature gradient between 280 c and 700 c was effected across this fault zone the resulting dynamic recrystallization microstructures are characteristic of bulging recrystallization subgrain rotation recrystallization and grain boundary migration recrystallization the transitions in recrystallization mechanisms are marked by discrete changes of grain size dependence on temperature differential stresses are calculated from the recrystallized grain size data using paleopiezometric relationships deformation temperatures are obtained from metamorphic reactions in the deformed host rock flow stresses and deformation temperatures are used to determine the strain rate of the tonale mylonites through integration with several published flow laws yielding an average rate of approximately 10 14s 1 to 10 12s 1 the deformation conditions of the natural fault rocks are compared and correlated with three experimental dislocation creep regimes of quartz of hirth tullis linking the microstructures of the naturally and experimentally deformed quartz rocks a recrystallization mechanism map is presented this map permits the derivation of temperature and strain rate for mylonitic fault rocks once the recrystallization mechanism is known

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

initiation and propagation of shear zones in a heterogeneous continental lithosphere continental plates represent rheologically heterogeneous media in which complex finite strain fields may develop due to interaction of plate tectonic processes with intraplate heterogeneities such a deformation pattern is displayed by the borborema shear zone system in northeastern brazil it involves continental scale curvilinear e w trending right lateral transcurrent shear zones that branch off from a major ne trending right lateral strike slip deformation zone formed at the northern termination of the sao francisco craton and that finally terminate in transpressive metasedimentary belts we suggest that this strain field results from the compressional deformation of a highly heterogeneous continental lithosphere composed of a stiff domain craton and rheologically weaker domains basins the effect of a low viscosity domain on the deformation of a continental plate and in particular the perturbation induced by this domain on the development of a shear zone formed at the termination of a stiff block were investigated using numerical modeling the low strength domain induces an enhanced strain localization and the geometry of the shear zone is significantly modified it is either split forming a branched shear zone system in which one branch maintains its original orientation while the other rotates toward the low viscosity domain or completely rotated toward the weak block the perturbation of the finite strain field depends on the ability of the weak domain to accommodate deformation which is controlled by its initial viscosity contrast relative to the surrounding lithosphere its orientation relative to the convergence direction and its distance from the shear zone initiated at the termination of the stiff block the interaction between imposed boundary conditions tectonic forces and plate geometry and the internal structure of the plate may give rise to highly heterogeneous strain fields as exhibited by the borborema shear zone system in which intraplate rheological heterogeneities induce the development of branched or sinuous shear zones a heterogeneous continental plate subjected to a normal convergence may therefore display significant lateral variations in strain intensity with shear zones bordering nearly undeformed blocks and in deformation regimes and vertical strains that would result in contrasting metamorphic and uplift histories

amphibole fabric formation during diffusion creep and the rheology of shear zones we conducted experiments to examine the effect of amphibole forming hydration reactions on mineral fabric development and the strength of mafic rocks both hydrostatic and general shear deformation experiments were conducted on powdered basalt with added water at lower continental crust conditions 800 c 1 gpa amphibole that formed under hydrostatic conditions exhibits a random lattice preferred orientation lpo in contrast amphibole formed during deformation exhibits both a strong shape preferred orientation spo and lpo with a 001 maximum aligned subparallel to the shear direction plagioclase in both hydrostatic and deformed samples shows a very weak to random lpo at low effective strain rates 10 5 s 1 to 10 6 s 1 the stress exponent is 1 1 5 consistent with deformation accommodated by diffusion creep the correlation of the spo and lpo coupled with the rheological evidence for diffusion creep indicates that the amphibole fabric results from oriented grain growth and rigid grain rotation during deformation the experimentally produced fabrics are strikingly similar to those observed in amphibolite grade natural shear zones supporting interpretations that such rocks deform by diffusion creep in addition the rheology of the fine grained experimental amphibolite is comparable to that predicted using flow laws for wet anorthite thus both our experiments and field analyses indicate that wet plagioclase rheology provides a good constraint on the strength of hydrated lower crust

dislocation interactions in experimentally deformed biotite abstract biotite single crystals shortened experimentally in orientations that maximize critical resolved shear stresses on 001 in directions 110 010 and 310 have been examined by transmission electron microscopy in all samples dislocations are confined to the basal plane with burgers vectors of 1 2 110 and 100 consistent with previous determinations of mica slip systems occasional partial dislocations that bound stacking faults form extended dislocations of these same systems typically dislocations are concentrated in complex arrays in local regions of the samples isolated or widely spaced dislocations are the exception within these arrays a variety of interactions between the dislocations are suggested by the geometrical arrangements of dislocations that cross over one another on parallel 001 planes sufficiently close together for elastic interaction and dislocations on the same 001 plane in networks and parallel arrays crossing dislocations of opposite sign on separate 001 planes form stable dipole segments where they overlap attractive and repulsive interactions between overlapping dislocations of like sign are evident as well dislocation networks occur in all samples and may exhibit a variety of complexly distorted geometries the spacing of dislocations in parallel arrays do not match models for standard dislocation pile ups consistent with observations showing these arrays involve dislocation multipoles the observed dislocation arrays and interactions resemble those reported for stage i easy glide deformation of fcc and hcp metals basal shear strengths are probably largely controlled by the development of multipoles and dislocations that pile up behind them

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