the influence of melting and melt drainage on crustal rheology during orogenesis partial melting significantly weakens crustal rocks by introducing a low viscosity liquid phase however near concomitant melt drainage can remove this weak phase potentially reversing the rheological effects such that the strength of a specific lithology depends on when the prograde pressure temperature path intersects a melting reaction how much melt is produced and how long this melt is retained before it is lost phase equilibria and mixed rheology modeling of typical metapelite and metagreywacke compositions indicate that these rocks undergo continuous but pulsed melt production during prograde metamorphism depending on whether melt removal is continuous or episodic and assuming geological strain rates the lithologies can retain a very low strength less than 1 mpa or transiently strengthen to 5 mpa following melt loss lithologies undergoing episodic melt loss can therefore cycle between being relatively weak and relatively strong components within a composite crustal section melt production retention and weakening in the middle to lower crust as a whole is more sustained during heating and melt production consistent with geodynamic inferences of weak melt bearing lower crust however the long term consequence of melting and melt loss is a 50 400 increase in the strength of residual lithologies the strengthening is more pronounced in metapelite than metagreywacke and is achieved through a combination of dehydration and the removal of the weak mica framework coupled to increased proportions of strong feldspars and garnet despite prolonged weakness melting and melt loss therefore ultimately result in a dry and elastic lower crust

relative strength of mafic and felsic rocks during amphibolite facies metamorphism and deformation field observations of mafic dykes intruded into felsic rocks from the lewisian complex nw scotland suggest that the mafic rocks are weaker than the felsic ones contrary to experimental results in order to resolve this conflict samples were studied to constrain the deformation mechanisms active under amphibolite facies conditions crystallographic preferred orientation cpo in plagioclase is used to infer deformation by dislocation creep on the 112 11 0 11 2 110 001 11 0 and 001 11 0 slip systems with increasing strain this cpo became weaker due to grain boundary sliding that accompanied diffusion creep in rocks where grain size reduction of plagioclase occurred by chemically dominated recrystallisation there is no cpo suggesting deformation was wholly accommodated by diffusion creep and grain boundary sliding in metamorphosed dykes plagioclase grains have orientations that are not consistent with dislocation creep deformation amphibole has a cpo consistent with either dislocation creep on 100 or deformation by diffusion creep with anisotropic dissolution and precipitation rates it is inferred that metamorphism of the dykes lead to the production of fine grained amphibole and plagioclase both of which deform by diffusion creep but that anisotropic dissolution and precipitation in the amphibole produced a cpo the mafic dykes are weaker than the felsic gneisses because grain size reduction is more extreme in the dykes even though both may be deforming primarily by grain size sensitive mechanisms this work highlights the variation in processes active in polyphase rocks and how these can lead to variations in cpo of the same mineral among different rock types

reactions involving hydration of orthopyroxene in anorthosite gabbro abstract hydration of orthopyroxene en 60 in contact with plagioclase an 36 leads to oronas of the type orthopyroxene cummingtonite edenite quartz edenitic hornblende the compositions of the minerals involved indicate that kyanite needles in the adjacent plagioclases form as part of the reaction orthopyroxene en 60 plagioclase an 36 h 2 o cummingtonite edenite edenitic hornblende plagioclase 36 kyanite quartz the reaction proceedings consumption first of the orthopyroxene and later of the cummingtonite to produce a corona structure consisting of a core of intergrown edenite and quartz surrounded by edenitic hornblende corona evolution of this type is controlled by diffusion

possible orogeny parallel lower crustal flow and thickening in the central andes the relatively low elevation and thick crust in the altiplano in comparison to the higher elevation but thinner crust in the puna plateau together with geophysical data suggests that isostatic equillibrium is achieved by cooler and denser lithospheric mantle in the altiplano excess density in the altiplano mantle could create differential horizontal stress in the order of 25 mpa between both lithospheric columns numerical models accounting for pressure and temperature dependent rheology show that such stress can induce horizontal ductile flow in the lower crust from the puna towards the altiplano with a minimum viscosity of 1019 pa s this flow reaches 1 cm year displacing more than 50 km of material within 5 ma if the lower crust viscosity is smaller the amount of orogeny parallel lower crustal flow can be even greater such a mechanism of channel flow may explain that different amounts of crustal material have been accommodated by shortening in the altiplano and in the puna because of the strength of the elastic brittle upper crust this channel flow does not necessitate large amounts of surface deformation except vertical uplift making it difficult to detect from the geology

processes controlling vertical coupling and decoupling between the upper and lower crust of orogens results from fiordland new zealand the pre cenozoic configuration of western new zealand allows determination of the effects of magmatism and a changing lower crustal rheology on the evolution of a cretaceous orogen from upper to lower crustal levels 10 50 km beginning at 126 ma a composite batholith dominated by diorite was emplaced into the lower crust during emplacement deformation was partitioned into zones weakened by magma and heat leading to the development of two layer parallel shear zones at the upper and lower contacts of the batholith transient vertical decoupling of the crust above and below the batholith occurred from 126 ma until 120 ma as magma was emplaced into and moved through a weak thick lower crust by 116 ma however much of the batholith had crystallized and the lowermost crust had cooled from 750 c t 850 c to t 650 700 c cooling was aided by the juxtaposition of pre existing crust against hot new crust and by the efficient extraction of partial melts out of the lower crust cooling together with dehydration of the lower crust and mafic compositions led to the development of a strong dry lower crustal root by 116 ma a strong lower crust resulted in high degrees of vertical coupling between the upper and lower crust during contraction from 116 to 105 ma even as magma continued to be emplaced into the mid upper crust a narrow focused orogenic style in the upper crust at this time reflected a highly viscous lower crust through which compressional stresses were transferred vertically the results imply that changes in plate boundary dynamics rather than the thermal weakening of thick lower crust during convergence controlled the onset of regional extension at 108 105 ma

microstructural evidence for magma confluence and reusage of magma pathways implications for magma hybridization karakoram shear zone in nw india in the karakoram shear zone ladakh nw india miocene leucogranitic dykes form an extensive varied and complex network linking an anatectic terrane exposed in the pangong range with leucogranites of the karakoram batholith mineral paragenesis of the heterogeneous anatectic source rocks suggests melting has resulted from water influx into rocks at upper amphibolite facies conditions and microstructures suggest anatexis was contemporaneous with shearing the network is characterized by continuous and interconnected dykes with only rare cross cutting relationships forming swarms and chaotic injection complexes where magmatic rocks cover up to 50 of the outcrop area despite this volume of magma the system did not lose continuity suggesting that it did not flow en masse and that the magma network was not all liquid simultaneously leucogranites in this network including leucosomes in migmatites carry an isotopic signature intermediate between the two main anatectic rocks in the source suggesting efficient homogenization of the magmatic products here we describe a number of microscopic features of these magmatic rocks which suggests that several pulses of magma used the same pathways giving rise to textural and chemical disequilibrium features these include i narrow tortuous corridors of fine grained minerals cutting across or lining the boundaries of larger grains interpreted to be remnants of magma filled cracks cutting across a pre existing magmatic rock ii corrosion of early formed grains at the contact with fine grained material iii compositional zoning of early formed plagioclase and k feldspar grains and quartz overgrowths documented by cathodoluminescence imaging iv incipient development of rapakivi and anti rapakivi textures and iv different crystallographic preferred orientation of early formed quartz and fine grained quartz mapping of the fine grained corridors interpreted to represent late melt channels reveal an interlinked network broadly following the s c fabric defined by pre existing magmatic grains we conclude that early formed dykes provided a pathway exploited intermittently or continuously by new magma batches new influxes of magma opened narrow channels and migrated through a microscopic network following predominantly grain boundaries along an s c fabric related to syn magmatic shearing a mixed isotopic signature resulted not from the mixing of magmas but from the micro scale interaction between new magma batches and previously crystallized magmatic rocks through local equilibration

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

dynamic recrystallization processes in plagioclase porphyroclasts abstract recrystallization processes in plagioclase porpyroclasts were studied in samples from an anorthositic gabbro of the jotun complex south norwegian caledonides the deformation took place at a temperature of approximately 700 c and at pressures below 900 mpa two populations of plagioclase porphyroclasts can be distinguished based on microstructural observations the crystallographic orientation and grain shapes for the two populations are interpreted as soft and hard orientations of the dominant slip system 010 001 with respect to the kinematic reference system the orientations of porphyroclasts and recrystallized grains were measured with a u stage microscope analysis of misorientations between porphyroclasts and adjacent recrystallized grains yields different results for the two types of porphyroclasts in type 1 porphyroclasts soft orientation the misorientation is inferred to be caused by subgrain rotation due to the activation of the 010 001 slip system resulting in a host control orientation relationship whereas in type 2 porphyroclasts hard orientation no dominant misorientation relationship could be identified the slip systems of the dislocations inferred from transmission electron microscopy tem analysis correspond well with the results of the u stage misorientation analysis in type 2 porphyroclasts hard orientation micro fracturing was observed in the tem whereas microfractures are absent in type 1 porphyroclasts soft orientation micro fracturing is likely to be the mechanism for nucleation of new grains in type 2 porphyroclasts resulting in a no host control orientation relationship between porphyroclasts and recrystallized grains

two stage rifting of zealandia australia antarctica evidence from 40ar 39ar thermochronometry of the sisters shear zone stewart island new zealand the sisters shear zone is a newly discovered late cretaceous detachment fault system exposed for 40 km along the southeast coast of stewart island southernmost new zealand footwall rocks consist of variably deformed ca 310 and 105 ma granites that range from undeformed to protomylonite mylonite and ultramylonite the hanging wall includes non marine conglomerate and brittley deformed granite k feldspar thermochronometry of the footwall indicates moderately rapid cooling 20 30 c m y due to tectonic denudation over the interval ca 89 82 ma return to slow cooling at 82 ma coincides with the age of the oldest seafloor adjacent to the campbell plateau and reflects the mechanical transition from continental extension to lithospheric rupture and formation of the pacific antarctic ridge our findings support a two stage rift model for continental breakup of this part of the gondwana margin stage one ca 101 88 ma is the northward propagation of continental extension and the tasman ridge as recorded in mylonite dredged from the ross sea and the paparoa core complex stage two ca 89 82 ma is extension between the campbell plateau and west antarctica leading to formation of the pacific antarctic ridge

migmatites granites and orogeny flow modes of partially molten rocks and magmas associated with melt solid segregation in orogenic belts abstract this paper presents a model for the genesis of migmatites and granites during orogenic evolution based on the analysis of several phanerozoic crustal segments and a review of the physical properties of partially molten rocks and magmas this model inventories the modes of bulk flow of partially molten rocks and magmas and the mechanisms of melt solid segregation for each mode partial melting of rocks is associated with a strength decrease of two to three orders of magnitude leading to strain partitioning expressed by channel flow driven either by forces related to plate tectonics vertical channel flow or by the gravity force associated with lateral variations of crustal thicknesses horizontal channel flow although the structural characteristics of migmatites indicate that deformation plays a role in melt migration the emplacement of laccoliths of leucogranites above migmatites attests to the efficiency of the buoyancy force another decrease in apparent strength of about ten orders of magnitude is associated with the loss of continuity of the solid framework marking the transition from partially molten rocks to magmas the breakdown of the solid framework also allows for settling of the solid in suspension increasing the buoyancy of the remaining magma accordingly domes cored by diatexites former heterogeneous magmas and mantled by metatexites former partially molten rocks are interpreted as gravitational instabilities driven by the relative buoyancy of the magma and permitted by the weakness of the partially molten rocks this model provides a first order framework to elucidate the development of a crustal scale horizontal layering during the evolution of orogenic belts that are affected by partial melting in this case the middle crust is dominated by migmatites with domes cored by diatexites and mantled by metatexites that correspond to a partially molten and magmatic zone respectively the granitic dikes and sills and the associated laccoliths of leucogranites correspond to an intrusive zone overlying the partially molten zone the refractory lower crust is potentially in part formed by accumulation of solids segregated from the overlying heterogeneous magmas

three stage evolution of lower crustal gneiss domes at breaksea entrance fiordland new zealand 1 new structural analyses reveal the presence of several kilometer scale gneiss domes and many subdomes within an exhumed section of cretaceous lower crust in sw fiordland the domes are defined by complex nearly concentric trajectories of foliations and steep high strain zones in migmatitic granulite and eclogite facies orthogneiss structural patterns combined with evidence of deformation that was synchronous with partial melting and high grade metamorphism steep mineral lineation orientations sense of shear indicators and near isothermal decompression suggest that buoyancy driven diapiric flow of hot partially molten crust aided initial dome formation mafic intermediate magmatism appears to have enhanced this process superimposed on all high grade structures are upper amphibolite facies shear zones that record a change in the style and conditions of lower crustal flow lineation orientations sense of shear evidence of cooling with decompression and other patterns suggest that the retrograde shear zones record bulk horizontal flow and crustal thinning driven by regional tectonic forces a preferred sequence begins with the initial emplacement of mafic intermediate magma into the lower crust at depths up to 70 km d1 diapirism and initial dome formation occurred during a period of waning arc magmatism and episodic partial melting and high grade metamorphism d2 following melt crystallization dominantly horizontal flow of a cooler strain hardened lower crust occurred during late cretaceous continental extension d3 this example highlights the interplay between mechanisms of horizontal and vertical flow at the root of a continental arc and illustrates their roles in the formation of gneiss domes

quartz grain size reduction in a granitoid rock and the transition from dislocation to diffusion creep abstract in the gran paradiso metagranodiorite western alps small scale lower amphibolite facies shear zones record the transition from a mylonite composed of polycrystalline mineral aggregates to a homogeneous ultramylonite with a grain scale phase mixture polycrystalline quartz aggregates in the mylonite deform by dislocation creep developing a crystallographic preferred orientation cpo and a monoclinic surface orientation distribution function odf the polymineralic matrix of the mylonite and the ultramylonite deform by diffusion creep in the ultramylonite the quartz cpo is randomized and the surface odf becomes orthorhombic the transition from mylonite to ultramylonite is accompanied by a grain size decrease and a disintegration of quartz aggregates concomitant with the precipitation of k feldspar biotite between quartz grains in quartz reduction from the dynamically recrystallized grain size in the aggregates 110 m to the size of the dispersed grains in the ultramylonite 25 m occurs through the following processes k feldspar precipitates at opening sites along grain boundaries strain incompatibility pinning the grain size in quartz aggregates coalescence of k feldspar leads to enhanced grain boundary sliding and disintegration of the quartz aggregates solution precipitation reduces the size of the dispersed grains to less than subgrain size 45 m

lateral variations of crustal seismic attenuation along the indepth profiles in tibet from lg q inversion 1 fourier spectra of crustal guided lg waves are collected to study seismic attenuation or q along the indepth profiles the 1 hz lg q q0 values are measured between many pairs of two stations and input to a back projection algorithm to obtain laterally varying q0 models along the indepth iii profile in central tibet the q0 model consists of low and nearly constant values of 90 these low values are consistent with abnormally high temperature as well as partial melts in the crust along the indepth ii profile in southern tibet the q0 values increase southward they are extremely low 60 over the northernmost segment but increase by a factor of 2 over a distance of 100 km between the indus yalong suture and the kangmar dome farther south and into the high himalayas q0 values are higher than 300 regional rayleigh waves observed along the northern indepth ii profile are used to infer a low velocity low q layer at midcrustal depths between 15 and 30 km in southern tibet the aqueous fluid trapped in the upper crust and a midcrust partial melting zone associated with the underthrusting of the indian lithosphere are the likely causes of the high lg attenuation in southern tibet low q rather then a systematic lg conversion along a dipping moho is the main cause of the previously observed lg blockage over paths crossing the southern tibetan boundary

on the pseudomorphing of melt filled pores during the crystallization of migmatites pseudomorphs of melt filled pores recognized by their generally cuspate shape are used as diagnostic for the former presence of partial melt they are commonly observed in migmatites from the mid to deep crust although they occur in the smaller pores in migmatites from shallower levels 1 2 kbar the pseudomorphing of meltfilled pores is controlled by the kinetics of nucleation and is a consequence of the greater supersaturation required for nucleation in a small pore compared with a larger one we examine three migmatites in detail a contact metamorphosed cherty band from an iron formation an archaean regional granulite from an accretionary prism and an amphibolite facies sample from the roots of an archaean mountain chain the greater undercooling required for nucleation in progressively smaller pores is recorded by the composition of plagioclase pseudomorphs a study of dihedral angles at the corners of pseudomorphed pores demonstrates that melt solid textural equilibrium was probably attained only in the contact aureole the regional granulite preserves an almost unmodified reaction controlled melt distribution with little evidence for either melt solid textural equilibration or solid solid re equilibration whereas the reactioncontrolled melt distribution in the regional amphibolite facies example has been modified by a partial approach to solid solid textural equilibrium it is not clear whether the differences in dihedral angle population are due to differences in uplift and exhumation rates or due to the presence of h2o on grain boundaries

the interplay and effects of deformation and crystallized melt on the rheology of the lower continental crust fiordland new zealand abstract microstructural electron backscatter diffraction ebsd and misorientation analyses of a migmatitic granulite facies orthogneiss from the exhumed lower crust of a cretaceous continental arc in fiordland new zealand show how deformation was accommodated during and after episodes of melt infiltration and high grade metamorphism microstructures in garnet omphacite plagioclase and k feldspar suggest that an early stage of deformation was achieved by dislocation creep of omphacite and plagioclase with subsequent deformation becoming partitioned into plagioclase continued deformation after melt infiltration resulted in strain localization in the leucosome of the migmatite where a change of plagioclase deformation mechanism promoted the onset of grain boundary sliding most likely accommodated by diffusion creep in fine recrystallized plagioclase grains our results suggest three distinctive transitions in the rheology of the lower crust of this continental arc where initial weakening was primarily achieved by deformation of both omphacite and plagioclase subsequent strain localization in plagioclase of the leucosome indicates that the zones of former melt are weaker than the restite and that changes in deformation mechanisms within plagioclase and an evolution of its strength primarily control the rheology of the lower crust during and after episodes of melting and magma addition

omphacite fabric variation in the cabo ortegal eclogite nw spain relationships with strain symmetry during high pressure deformation petrostructural study of cabo ortegal eclogite tectonites provides an insight into the relationships between omphacite lattice preferred orientation and the fundamental deformation parameters in particular strain symmetry microstructural evidence is consistent with a tectonic origin of eclogite fabrics by coeval pyroxene plastic deformation and recrystallization under eclogite facies metamorphic conditions omphacite shape fabrics and garnet spatial distributions are anisotropic and representative of the fields of either apparent flattening or constriction omphacite lattice preferred orientations are dominated by 010 axis maxima normal to the foliation s type or flattening fabrics by 001 axis maxima parallel to the lineation l type or constriction or correspond to other fabric patterns described in the literature omphacite crystallographic fabrics are consistently asymmetric with respect to the structural framework defined by foliation and lineation this suggests that non coaxial deformation components either simple or general shear accompanied eclogite fabric development in addition to any flattening or constriction strain paths and permit shear sense determination these petrostructural features were acquired in the course of eo hercynian ductile deformation probably during deep subduction under 1 5 1 7 gpa confining pressure temperatures above 600 700 c low differential stress 15 20 mpa and slow strain rate 10 12 10 15s 1

dislocation generation slip systems and dynamic recrystallization in experimentally deformed plagioclase single crystals abstract three samples of gem quality plagioclase crystals of an60 were experimentally deformed at 900 c 1 gpa confining pressure and strain rates of 7 5 8 7 10 7 s 1 the starting material is effectively dislocation free so that all observed defects were introduced during the experiments two samples were shortened normal to one of the principal slip planes 010 corresponding to a hard orientation and one sample was deformed with a schmid factor of 0 45 for the principal slip system 001 010 corresponding to a soft orientation several slip systems were activated in the soft sample dislocations of the 001 010 and 110 001 system are about equally abundant whereas 110 111 and 101 in 131 to 242 are less common in the soft sample plastic deformation is pervasive and deformation bands are abundant in the hard samples the plastic deformation is concentrated in rims along the sample boundaries deformation bands and shear fractures are common twinning occurs in close association with fracturing and the processes are clearly interrelated glissile dislocations of all observed slip systems are associated with fractures and deformation bands indicating that deformation bands and fractures are important sites of dislocation generation grain boundaries of tiny defect free grains in healed fracture zones have migrated subsequent to fracturing these grains represent former fragments of the fracture process and may act as nuclei for new grains during dynamic recrystallization nucleation via small fragments can explain a non host controlled orientation of recrystallized grains in plagioclase and possibly in other silicate materials which have been plastically deformed near the semi brittle to plastic transition

the inception and growth of leucosomes microstructure at the start of melt segregation in migmatites the beginning stages of melt segregation and the formation of leucosomes are rarely preserved in migmatites most arrays of leucosomes record a more advanced stage where flow dominates over segregation however the early stages in the formation of leucosomes and the segregation of melt are preserved in a partially melted meta argillite from the metatexite zone 800 c of the contact aureole around the duluth complex minnesota the rock contains 2 4 modal leucosome in a matrix consisting of 40 5 in situ neosome and 57 1 cordierite plagioclase framework the domainal microstructure in the matrix is a pre anatectic feature resulting from the bulk composition terminal chlorite reactions produced a large volume of cordierite which with plagioclase formed a framework that enclosed patches of biotite quartz plagioclase k feldspar upon melting these fertile domains became patches of in situ neosome plagioclase in the neosome is less sodic than in the leucosome hence segregation of melt occurred during crystallization not melting segregation was delayed because the cordierite plagioclase framework was strong enough to resist dilatation and compaction until after crystallization started the leucosomes are small i e they are microleucosomes and display a systematic progression in morphology as length and aspect ratio increase from 1 to 19 mm and from 2 5 to 30 respectively small equant micropores form first and in places these coalesce into small 1 mm aspect ratio 2 5 isolated blunt ended elliptical microleucosomes in the next stage micropores develop ahead of and at 45 to the left and right of the blunt tip of a microleucosome one of these develops into an elliptical leucosome and an en echelon array of either a left or right stepping elliptical microleucosome forms each elliptical microleucosome in the en echelon arrays is separated by a bridge of matrix next microleucosomes of greater length 4 mm and aspect ratio 5 form when the bridges of cordierite plagioclase matrix rupture and the elliptical microleucosomes link together to form a zigzag shaped microleucosome finally still longer microleucosomes with greater aspect ratios 30 are formed by the joining of zigzag arrays such a progression is characteristic of the way ductile fractures grow the segregation of melt was driven by the pressure gradient between the dilatant fracture and an adjacent in situ neosome which drew melt to the growing fracture thereby creating a microleucosome the microleucosomes are filled arrays of ductile fractures melt was contiguous only between microleucosomes and adjacent patches of in situ neosome the length scale of segregation was 5 mm the size of a typical patch of in situ neosome and restricted by the surrounding impermeable cordierite plagioclase framework the melt in the microleucosome was the most fractionated and the last to crystallize all microleucosomes contain entrained minerals as a consequence of their mechanism of growth rupture of the bridges resulted in the entrainment of pre anatectic phases however microleucosomes that cross patches of in situ neosome are also contaminated with peritectic phases that were transported with the melt

effect of random sea surface and bottom roughness on propagation in shallow water the potential difficulties associated with modeling acoustic propagation in shallow water environments are well documented larger scale deterministic features combine with random fluctuations in the water column sediment sea surface and water sediment interface to produce an extremely complicated propagation regime the extent to which each of these factors needs to be included in realistic propagation modeling remains to be quantified toward this end results from a series of detailed simulations generated using the parabolic equation method are presented beginning with a deterministic downward refracting sound speed profile and a known sloping bottom realizations of the random features are sequentially added to the simulation wind driven surface gravity waves and power law bottom roughness are considered the individual and cumulative effects of these scattering mechanisms on the acoustic wavefront are quantified successive interactions with the random interfaces are studied a modal decomposit

intragranular dynamic recrystallization in naturally deformed calcite marble diffusion accommodated grain boundary sliding as a result of subgrain rotation recrystallization abstract intragranular microshear zones within a greenschist facies calcite marble were studied to try to constrain better the processes of dynamic recrystallization as well as the deformation processes that occur within newly recrystallized grains intragranular recrystallized grains within large twinned calcite porphyroclasts can be related to the host from which they have recrystallized and are the focus of an electron backscatter diffraction study lattice distortions low angle boundaries and some high angle boundaries 15 in the microshears within a porphyroclast have the same misorientation axes suggesting that deformation occurred by climb accommodated dislocation creep involving subgrain rotation recrystallization changes in the ratio of host and twin domain as the deformation zone is entered show that twin boundary migration also occurred recrystallized grains have similar sizes 10 60 m to subgrains suggesting that they formed by subgrain rotation however within the intragranular microshear zones the misorientations between recrystallized grains and porphyroclasts are considerably larger than 15 and misorientation axes are randomly oriented moreover recrystallized grain orientations average around the porphyroclast orientation we suggest that the recrystallized grains once formed are able to deform partly by diffusion accommodated grain boundary sliding which is consistent with predictions made from lab flow laws

rheology of synthetic anorthite diopside aggregates implications for ductile shear zones 1 we investigated the high temperature creep strength of fine grained anorthite diopside rocks at temperatures ranging from 1323 k to 1523 k and at 300 mpa confining pressure in a paterson type gas medium deformation apparatus flow stress varied between 20 and 450 mpa resulting in strain rates between 6 1 10 7 s 1 and 7 5 10 4 s 1 pure diopside and anorthite samples were hot pressed from crushed natural single crystals and glass powders respectively two phase samples were produced by hot isostatic pressing of mechanically mixed powders of anorthite glass with 25 50 and 75 vol diopside particles arithmetic mean grain size of the anorthite matrix is dan 3 5 m three different ranges of diopside particle size were used ddi 25 m 35 m and 45 m water content of as is samples was about 0 05 0 02 wt h2o and predried samples contain about 0 004 0 001 wt h2o at experimental conditions as is samples are assumed to be water saturated water content of predried samples is about 3 times less than that of starting diopside single crystals the specimens contain about 1 vol glass located at fluid inclusions and some multiple grain junctions two grain boundaries examined by high resolution transmission electron microscopy did not show amorphous layers to a resolution of 1 nm at experimental conditions pure diopside aggregates are about 2 3 orders of magnitude stronger than pure anorthite samples for as is and predried specimens respectively in general strength of the two phase aggregates increases with increasing diopside content but remains between isostress and isostrain rate bounds aggregate strengths predicted from continuum mechanics models are in good agreement with the experimental data for dilute diopside particle mixtures and high volume fractions when diopside particles form a load bearing framework at low stresses 100 200 mpa the stress exponent is n 1 suggesting diffusion controlled creep at higher stresses mechanical data and microstructures suggest that samples deformed in the transition region between diffusion controlled creep and dislocation creep for pure anorthite and diopside aggregates deforming in dislocation creep we estimated stress exponents of n 3 and n 5 5 respectively for the two phase aggregates n is between n 3 and n 5 depending on diopside content at low stresses deformation microstructures indicate load transfer from a weak anorthite matrix to stronger diopside particles creep activation energies for pure diopside and anorthite mixtures range from 286 kj mol 1 for wet anorthite deformed at low stresses to 691 kj mol 1 for dry diopside deformed at high stresses activation energies of two phase mixtures are between or close to those of the end members as is samples have significantly lower activation energies than predried samples

the effect of pre tectonic reaction and annealing extent on behaviour during subsequent deformation insights from paired shear zones in the lower crust of fiordland new zealand granulite facies pargasite orthogneiss is partially to completely reacted to garnet granulite either side of narrow 20 mm felsic dykes in fiordland new zealand forming 10 80 mm wide garnet reaction zones the metamorphic reaction changed the abundance of minerals and their shape and grain size distribution the extent of reaction and annealing temperature related coarsening and nucleation is greatest close to the dykes whereas further away the reaction is incomplete as a consequence grain size and the abundance of garnet decreases away from the felsic dykes over a few centimetres the aspect ratios of clusters of s1 pyroxene and pargasite in the orthogneiss which are variably reacted to post s1 garnet decrease from high in the host to near equidimensional close to the dyke post reaction deformation localized in the fine grained partially reacted areas this produced a pattern of paired shear zones located at the outer parts of the garnet reaction zone our study shows that grain size sensitive deformation occurs where the grain size is sufficiently reduced by metamorphic reaction the weakening of the rock due to the change in grain size distribution outweighs the addition of nominally stronger garnet to the assemblage

mechanisms of mass and heat transport during barrovian metamorphism a discussion based on field evidence from the central alps switzerland northern italy 1 tectonic and metamorphic data for the central alps switzerland italy are used to discuss this classic example of a barrovian metamorphic terrain notably the evolution of its thermal structure in space and time available p t t data indicate variable contributions of advective and conductive heat transport during collision and subsequent cooling and exhumation some areas experienced a prolonged period of partial melting while other areas at the same time show but moderate heating the barrow type metamorphic field gradient observed in the final orogen is the result of two distinct tectonic processes with their related advective and conductive heat transport processes the two tectonic processes are 1 accretion of material within a subduction channel related to decompression and emplacement of high pressure units in the middle crust and 2 wedging and related nappe formation in the continental lower plate the second process postdates the first one wedging and underthrusting of continental lower plate material produces heat input into lower crustal levels and this process is responsible for predominantly conductive heat transport in the overlying units the interacting processes lead to different maximum temperatures at different times producing the final barrovian metamorphic field gradient the south experienced rapid cooling whereas the north shows moderate cooling rates this discrepancy principally reflects differences in the temperature distribution in the deeper crust prior to cooling differences in the local thermal gradient that prevailed before the cooling also determined the relationships between cooling rate and exhumation rate in the different areas citation berger a s m schmid m engi r bousquet and m wiederkehr 2011 mechanisms of mass and heat transport during barrovian metamorphism a discussion based on field evidence from the central alps switzerland northern italy tectonics 30 tc1007 doi 10 1029 2009tc002622

western fiordland orthogneiss early cretaceous arc magmatism and granulite facies metamorphism new zealand u pb isotopic analyses of zircons from a distinctive suite of previously undated granulite facies metaplutonic rocks here termed the western fiordland orthogneiss wfo in fiordland southwest new zealand indicate synkinematic magmatic emplacement between 120 and 130 ma ago these rocks were previously interpreted as possibly being of precambrian age initial pb and sr ratios are consistent with arc subduction related magmagenesis with little or no involvement of ancient continental crust subsequent high pressure 12 kb metamorphism of the wfo may reflect a major collision event involving crustal thickening by overthrusting of a 15 km thick sequence metamorphism ceased 116 ma ago based on206pb 238u ages of zircon from a retrogressed granulite u pb isotopic analysis of apatite along with previously published rb sr mineral ages indicate that final uplift and cooling to 300 400 c was largely completed by 90 ma the average uplift rate during this period is inferred to have been in excess of 1 mm yr unmetamorphosed gabbronorites of the darran complex in eastern fiordland inferred by some investigators to be the granulite protolith yield concordant u pb zircon ages of 137 1 ma u pb ages of apatite and previously published k ar mineral ages indicate that these rocks experienced a rapid and simple cooling history lasting only a few million years the high grade wfo and unmetamorphosed darran complex are now separated by a profound structural break however the ages and similarities in initial pb and sr isotopic ratios suggest that both suites are products of the same early cretaceous cycle of subduction related magmatism the timing of early cretaceous magmatism and metamorphism collision and resultant crustal thickening and subsequent great uplift and erosion in fiordland has important implications for terrane accretion and the evolution of relative plate motions along the new zealand segment of the gondwana margin

mode of crustal extension determined by rheological layering the results of numerical modelling show that the mechanical stratification of the crust provides the fundamental control on fault spacing and ultimately the mode of extension macroscopic pre existing structures and weaknesses are often thought to govern the behaviour of continental crust under extension and many prior studies have focussed on the effect of heterogeneities in triggering faulting and exhumation of lower crustal material the role played by such features is in fact subordinate to that exerted by the rheological contrast from upper to lower crust in our numerical model the temperature gradient dictates the transition from a strong brittle upper crust to a weaker ductile lower crust we see two distinct extension modes that depend on this vertical rheological contrast the distributed faulting mode and the metamorphic core complex mode the ratio of the integrated strength of the upper to lower crust is an indicator of the resulting mode of extension when this strength ratio is small i e the lower crust is relatively strong the result is distributed densely spaced faulting with limited slip on each fault and no exposure of lower crustal rocks an example could be faulting in the north sea a large strength ratio hence a weak lower crust that flows easily leads to stretching being strongly localised onto relatively few normal fault zones each fault accommodates large displacements eventually dissecting the upper crust and resulting in exhumation of the lower crust this is representative of the metamorphic core complexes of the western u s a and the aegean the actual critical strength ratio for the transition between modes will depend upon secondary factors such as the relative thickness of the lower crust with respect to the upper crust and the degree of fault weakening d 2005 elsevier b v all rights reserved

formation of elliptical garnet in a metapelitic enclave by melt assisted dissolution and reprecipitation metapelitic residual enclaves in the neogene volcanic province of se spain are residues left after melt extraction glass quenched melt of granitic composition occurs as inclusions in most minerals and as intergranular pockets the most common enclave types show one stage of garnet growth that is interpreted to have occurred at the same time as glass production some of these show a well developed foliation outlined by fibrolite biotite graphite and glass which wraps around elongate garnet crystals that have aspect ratios up to 10 1 based on microstructures and chemistry the garnet within these rocks shows clear core and mantle structure the core has an average composition of alm76 prp08 sps14 grs03 and contains primary inclusions of biotite and melt trapped during garnet growth a thin c 100 lm irregular mantle overgrows the garnet core enclosing oriented fibrolite inclusions in strain caps and biotite in strain shadows in places the overgrowths form skeletal elongated arms which extend parallel to the foliation the garnet mantle contains less mn and higher xmg but both core and mantle display flat mn profiles the contact being a sharp break ternary feldspar and grt bt thermometry yield temperatures in the range 800 900 c with no systematic differences among the different microstructural domains of elliptical garnet based on the observed intracrystalline microstructures the high amount of melt extraction in the rock by flattening component strain and the chemical zoning of garnet the formation of elliptical garnet is modelled by a multistage sequence this involves pressure solution and reprecipitation of the core followed by post kinematic partly mimetic growth of the garnet mantle

plastic deformation and development of clinopyroxene lattice preferred orientations in eclogites abstract we use an anisotropic viscoplastic self consistent vpsc model to simulate the development of omphacite lattice preferred orientations lpos in response to deformation by dislocation glide in these simulations we consider slip systems identified either in naturally deformed omphacite or in experimentally deformed diopside simulated lpos reproduce very well the characteristic omphacite lpo pattern in naturally deformed eclogites a strong concentration of 001 axes sub parallel to the lineation and of 010 poles sub perpendicular to the foliation these models reconcile the interpretation of omphacite lpos in eclogites and tem observations of naturally deformed omphacite since they show that omphacite lpos in naturally deformed eclogites may develop by dislocation glide on 1 2 110 110 001 110 and 001 100 we also investigate the effect of the strain regime on the omphacite lpo development the simulations show that changes in deformation regime lead to second order variations in omphacite lpo patterns similar to those observed in eclogites such as an asymmetry of the lpo relative to the structural frame a stronger concentration of the 001 axes relative to the 010 pole concentration or a dispersion of 001 axes in the foliation plane this suggests that omphacite lpo patterns may carry information on the deformation regime simple shear transtension active during the high pressure events

dynamics and structural development of metamorphic core complexes the development of metamorphic core complexes mccs in a thickened continental lithosphere is studied using fully coupled thermomechanical numerical code accounting for elastic brittle ductile properties of constituent rocks for mccs to develop the middle lower crust and the sub moho mantle are required to be weak enough to flow laterally so as to simultaneously feed the exhuming dome and enable the moho to keep a flat geometry the conditions are satisfied with initial moho temperatures of 800 c or higher with crustal thicknesses of 45 km or greater and with initial effective viscosities lower than 1020 pa s and 1022 pa s in the lower crust and the underlying mantle respectively a compositional mainly density anomaly with the properties of granite is placed centrally in the crust to localize strain at the onset of deformation during a first stage of upper crust necking the deformation pattern is relatively symmetrical and dominated by graben formation in the upper crust when the first ductile layers reach the surface a second stage of dome amplification and widening occurs dome amplification is accommodated by horizontal flow in the ductile crust giving an early symmetrical pattern of conjugate shear zones with no obvious detachment zone the system then rapidly becomes asymmetric with the localization of a detachment zone along one dome limb further accommodating dome widening thus the exhumation process of a metamorphic dome results in the progressive development of a detachment zone depending on initial moho temperature the detachment zone can migrate in space or die out and be replaced by a new one with an opposite dip

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

the role of second phases for controlling microstructural evolution in polymineralic rocks a review abstract we present a state of the art review of the microstructural evolution in rocks under static and deformational conditions first the general concepts and processes are introduced using monomineralic aggregates then they are expanded into the more complex context of polymineralic rocks with a dominant matrix phase the first part of this contribution delivers information on sample strategies to quantify polymineralic microfabrics based on comparisons between microfabrics of monomineralic and polymineralic rocks we use the common knowledge collected over the past decades for monomineralic systems and discuss the differences to polymineralic ones in terms of microstructures modal compositions spatial distribution of phases crystallographic preferred orientations and associated processes the article puts particular emphasis on the effect of coupled grain growth mass transfer processes and deformation mechanisms we speculate on the effect of mineral reactions during the evolution of microstructures and rheology in polymineralic aggregates at different metamorphic conditions at the end of the article we demonstrate the great potential of grain size evolution maps as microstructural tool to unravel the geological history of polymineralic rocks that evolved under a variety of geodynamic situations

mode coupling and the environmental sensitivity of shallow water propagation loss predictions variations of seabed parameters particularly the acoustic compressional velocity relative to that of the water column have a very strong effect on the predicted magnitude of the propagated acoustic field in many cases the sensitivity of the predicted losses to the assumed seabed parameters is sufficiently severe to render the predictions meaningless in this article coupled mode theory is employed to study how the presence of lateral seabed inhomogeneities affects this sensitivity the dependence of predicted propagation losses on sediment sound speed is first examined for horizontally stratified sediments then a rough layered structure of clay silt interbedded with sand is assumed with the introduction of rough sub bottom layering coupling occurs between modes so that the conversion of energy into progressively higher order modes which attenuate rapidly becomes an important loss mechanism the extent to which acoustic energy penetrates the seabed and interacts with the sub bottom inhomogeneiti

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

geoacoustic parameters in a stratified sea bottom from shallow water acoustic propagation due to the difficulty of direct measurement there is a need to develop inverse techniques for remote sensing bottom geoacoustic parameters in the lowan mode measurements are extended to extract acoustic attenuation and speed in a horizontally stratified bottom in shallow water as a function of frequency and depth the computational and experimental results show that for a limited frequency band we can find an equivalent depth profile of sea bottom acoustic attenuation with a linear frequency dependence that simulates the effect of nonlinear frequency dependence without depth structure on some field characteristics such as the attenuation rate of individual mode the frequency response of long range sound propagation and the amplitude ratio of mode 2 to mode 1 however the resultant equivalent negative gradient for the sea bottom attenuation is too strong to be accepted in light of available data the conclusion is that nonlinear frequency dependence of the acoustic attenuation in the upper sedimentary layer is required to explain many aspects of shallow water sound propagation

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

cation ordering in omphacite and effect on deformation mechanism and lattice preferred orientation lpo we present microstructural data and lattice preferred orientations lpos of omphacites from a suite of eclogites from the adula cima lunga nappe central alps our work shows a surprisingly strong correlation between the measured lpo and the ordering state of cations in omphacite estimates of deformation temperature from metamorphic petrology together with measured omphacite compositions and lpos determine the field ordering state on the omphacite phase diagram into which each sample falls lpos dominated by l type and s type signatures are developed in samples that fall in the p2 n field ordered structure and c2 c field disordered structure respectively dislocations with b 1 2 110 or b 001 are observed in the transmission electron microscope tem in all samples the former change from a perfect dislocation in the c2 c structure to a partial in p2 n any movement of a partial dislocation requires the formation or growth of a stacking fault furthermore in order to pass an obstacle a partial dislocation has to constrict to a unit dislocation the energy to form a constriction is high in omphacite due to the large separation width thus the activity of the b 1 2 110 dislocation is hindered in the p2 n structure relative to the c2 c structure which change the balance between the two and might give rise to the different lpos

metamorphic core complexes windows into the mechanics and rheology of the crust metamorphic core complexes are products of normal fault displacements sufficient to exhume rocks from below the brittle ductile transition these faults detachments may initiate within the brittle crust at steep angles but they sole into the ductile middle crust and during displacement rotate to gentler dips due to hanging wall extension the exhumed footwall commonly adopts an arched or domed geometry owing to flexural isostatic readjustment and may be overlain by strongly extended upper crustal rocks that slipped on gently dipping low friction shallow segments of the detachment metamorphic rocks exhumed beneath the detachment record progressively increasing flow stress strain localization and strain rate with decreasing temperature providing a window into physical conditions and deformational processes in the mid crust the metamorphic and deformational history of the footwall rocks may reflect tectonic processes that predate formation of the detachment fault in addition to those accompanying exhumation these processes may include diapiric emplacement of gneiss domes or exhumation in a subduction channel and may not be directly related to formation of the core complex factors favouring core complex formation are high gravitational potential energy of the extending crust weak rheology and a change in the tectonic boundary conditions such as a cessation or slowing of plate convergence

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

large scale deformation in a locked collisional boundary interplay between subsidence and uplift intraplate stress and inherited lithospheric structure in the late stage of the se carpathians evolution 1 the interplay between slab dynamics and intraplate stresses in postcollisional times creates large near surface deformation particularly in highly bent orogens with significant lateral variations in mechanical properties this deformation is expressed through abnormal foredeep geometries and contrasting patterns of vertical movements intraplate folding is often the controlling mechanism particularly when the orogenic belt is locked the study of these tectonic processes in the se carpathians indicates a generalized subsidence period during latest miocene pliocene times driven by the slab pull and an intraplate folding due to an overall quaternary inversion the latter accommodates 5 km ese ward movement of this area with respect to the neighboring units which creates complicated three dimensional deformation patterns potentially driven at a larger scale by the interaction between the adriatic indentor and the entire carpathians system the lithospheric anisotropy inherited from the subduction times concentrates strain and induces large scale deformation far away from the active plate margins this anisotropy is dynamic because of deep mantle processes related to the subducted slab during postcollisional times such as thermal reequilibration or increase in slab dip

flow of partially molten crust and the internal dynamics of a migmatite dome naxos greece 1 migmatite domes are common in metamorphic core complexes dome migmatites deform in the partially molten or magmatic state and commonly record complex form surfaces folds and fabrics while units mantling the dome display a simpler geometry typically formed by transposition during crustal extension we use field observations and magnetic fabrics in the naxos dome greece to quantify the complex flow of anatectic crust beneath an extensional detachment system the internal structure of the naxos dome is characterized by second order domes subdomes pinched synforms and curved lineation trajectories which suggest that buoyancy driven flow participated in dome evolution subdomes broadly occur within two compartments that are separated by a steep n s oriented high strain zone this pattern has been recognized in domes formed by polydiapirism and in models of isostasy dominated flow the preferred model involves a combination of buoyancy and isostasy driven processes the naxos dome may have been generated by regional n s extension that triggered convergent flow of partially molten crust at depth and the upwelling of anatectic migmatites within the dome this pattern is complicated by gravitational instabilities and or overturning of the high melt fraction crust leading to the growth of subdomes as the migmatites within the naxos dome reached a higher structural level they were affected by regional top to the nne kinematics of the detachment system dome formation therefore occurred by a combination of coeval and coupled processes upper crustal extension deep crust contraction during convergent flow of anatectic crust diapirism and or density driven crustal convection forming subdomes and north directed detachment kinematics

nanogranite and glassy inclusions the anatectic melt in migmatites and granulites using as a case study a granulite from the kerala khondalite belt india we show that a former anatectic melt can be preserved as tiny 25 m droplets within refractory minerals in this case garnet the melt is either fully crystallized as a qtz ab kfs bt cryptocrystalline aggregate nanogranite or completely glassy in inclusions 15 m both nanogranite and glassy inclusions have a peraluminous ultrapotassic granitic composition that in this case does not correspond to a minimum melt and points to high melting temperatures in agreement with the ultrahigh temperature origin of the rock this discovery indicates that peritectic minerals growing during incongruent melting reactions act as hosts for inclusions of anatectic melt and that in the general case of slow cooling of the crust these inclusions will occur as nanogranite exceptionally in the smallest inclusions glass may be present due to inhibition of crystallization our results extend the frontiers of petrological and geochemical research in crustal melting as the composition of natural anatectic melts can be directly analyzed rather than assumed

the influence of phase and grain size distribution on the dynamics of strain localization in polymineralic rocks abstract deformation microstructures of a quartzo feldspathic pegmatite deformed at mid crustal levels allow the study of the dynamics of strain localization in polymineralic rocks strain localization results from i difference in grain sizes between phases both original and obtained during fluid present reactions and ii initial compositional banding due to original difference in grain size stress concentrates in the initially finer grained phases resulting in their intense grain size reduction via subgrain rotation recrystallization sgr when the grain size is sufficiently reduced through either deformation or interphase coupled dissolution precipitation replacement of the coarse grained feldspar aggregates start to deform by dominantly diffusion accommodated grain boundary sliding gbs phase mixing inhibits grain growth and sustains a grain size allowing gbs consequently discontinuous microscale shear zones form locally within initially coarse grained areas at the same time difference in strain rate between feldspar rich and quartz rich domains needs to be accommodated at domain boundaries this results in the formation of continuous mesoscale shear zones deformed by gbs once these are formed deformation in the coarse grained parts is arrested and strain is mainly accommodated in the mesoscale shear zones resulting in superplastic behaviour consistent with diffusion creep

transformation of two pyroxene hornblende granulite to garnet granulite involving simultaneous melting and fracturing of the lower crust fiordland new zealand granulite facies gabbroic and dioritic gneisses in the pembroke valley milford sound new zealand are cut by vertical and planar garnet reaction zones in rectilinear patterns in gabbroic gneiss narrow dykes of anorthositic leucosome are surrounded by fine grained garnet granulite that replaced the host two pyroxene hornblende granulite at conditions of 750 uc and 14 kbar major and trace element whole rock geochemical data indicate that recrystallization was mostly isochemical the anorthositic veins cut contacts between gabbroic gneiss and dioritic gneiss but change in morphology at the contacts from the anorthositic vein surrounded by a garnet granulite reaction zone in the gabbroic gneiss to zones with a septum of coarse grained garnet surrounded by anorthositic leucosome in the dioritic gneiss the dioritic gneiss also contains isolated garnet grains enclosed by leucosome and short planar trains of garnet grains linked by leucosome partial melting of the dioritic gneiss mostly controlled by hornblende breakdown at water undersaturated conditions is inferred to have generated the leucosomes the form of the leucosomes is consistent with melt segregation and transport aided by fracture propagation limited retrogression suggests considerable melt escape dyking and melt escape from the dioritic gneiss are inferred to have propagated fractures into the gabbroic gneiss the migrating melt scavenged water from the surrounding gabbroic gneiss and induced the limited replacement by garnet granulite

effect of random hydrodynamic inhomogeneities on low frequency sound propagation loss in shallow water low frequency 100 500 hz sound propagation loss on the us atlantic continental shelf and in the barents sea in the presence of stochastic surface waves and for the us atlantic shelf also in the presence of internal waves is studied for the range of up to 150 km by means of numerical simulations surface waves simulations are carried out with pierson neumann spectrum internal wave field realizations are modeled according to frequency spectrum of thermocline displacements that were recorded in shallow water 06 experiment qualitative difference between sound propagation loss behavior on the us atlantic shelf and in the barents sea is demonstrated for summertime conditions even without random inhomogeneities it is shown that whereas internal waves on average have a weak effect on propagation loss surface waves result in its considerable increase in both areas under wintertime conditions with a wind speed of more than 9 m s the authors wish to acknowledge and thank dr j f lynch for discussing this work the work was supported by the russian foundation for basic research project 08 02 00283 and by the civilian research and development foundation project no rec 010

syndeformational recrystallization dynamic or compositionally induced dynamic recrystallization in the strict sense of the term is the reconstitution of crystalline material without a change in chemical composition driven by strain energy in the form of dislocations driving potentials additional to internal strain energy may contribute to the recrystallization of naturally deformed minerals which form solid solutions such as feldspar amphiboles and pyroxenes if they change their composition during recrystallization to estimate the relative importance of these driving potentials the chemical composition of porphyroclasts and recrystallized grains of plagioclase clinopyroxene and hornblende have been investigated in samples from a high grade shear zone of the ivrea zone italy the plagioclases show two different recrystallization microstructures bulging recrystallization at grain boundaries and discrete zones of recrystallized grains across porphyroclasts probably involving fracturing deformation took place under amphibolite facies conditions on a retrograde p t path porphyroclast and recrystallized compositions from bulging recrystallization microstructures differ only in their or content and yield a g between mean host grain and mean recrystallized grain composition at fixed p t conditions of approximately 5 joules 10 4 m3 extreme compositional variations yield approximately 60 j 10 4 m3 the increase of free energy due to dislocations calculated for common glide systems in plagioclase are on the order of 100 joules 10 4 m3 for high values of dislocation densities of 1014 m 2 thus the effect of chemically induced driving energies on grain boundary velocity appears small for mean compositions but may be as great as that of deformational energies for larger chemical differences in the other type of microstructure porphyroclasts and recrystallized grains in discrete zones differ in their anorthite content the maximum g induced by the compositional disequilibrium is on the order of 100 j 10 4 m3 this maximum value is of the same magnitude as the g derived from high dislocation densities of 1014 m 2 the resulting combined g is approximately twice as high as for deformational g alone and heterogeneous nucleation may become a feasible recrystallization mechanism which is evident from the microstructures the recrystallization mechanism depends on the nature of the driving potential grain boundary migration gbm and heterogeneous nucleation can release gibbs free energy induced by compositional disequilibrium whereas this is not likely for subgrain rotation therefore only gbm and heterogeneous nucleation may link metamorphism and deformation so that syndeformational recrystallization may represent a transitional process ranging from dynamic recrystallization to metamorphic reaction

the misorientation index development of a new method for calculating the strength of lattice preferred orientation abstract using orientation data from experimentally deformed olivine we explore some practical problems with the j index a commonly applied measure of fabric strength we show that the j index is highly dependent on several factors including the number of discrete data in the orientation distribution function odf and arbitrary numerical parameters specified for its calculation because of this non uniqueness we conclude that the j index is difficult to interpret and should only be applied with caution as an alternative to the j index we propose a new measure of fabric strength that is based on the distribution of uncorrelated misorientation angles this m index is shown to be insensitive to the parameters specified for its calculation for typical deformed olivine samples we show that 150 discrete data are adequate to quantify fabric strength using the m index technique the m index correlates well with seismic anisotropy particularly for materials of the same fabric type therefore we conclude that the m index technique is well suited for the quantification of fabric strength and the comparison of like materials

geochronology and geochemistry of high pressure granulites of the arthur river complex fiordland new zealand cretaceous magmatism and metamorphism on the palaeo pacific margin the arthur river complex is a suite of gabbroic to dioritic orthogneisses in northern fiordland new zealand the arthur river complex separates rocks of the median tectonic zone a mesozoic island arc complex from palaeozoic rocks of the palaeo pacific gondwana margin and is itself intruded by the western fiordland orthogneiss new shrimp u pb single zircon data are presented for magmatic metamorphic and deformation events in the arthur river complex and adjacent rocks from northern fiordland the arthur river complex orthogneisses and dykes are dominated by magmatic zircon dated at 136 129 ma a dioritic orthogneiss that occurs along the eastern margin of the complex is dated at 154 4 3 6 ma and predates adjacent plutons of the median tectonic zone rims on zircon cores from this sample record a thermal event at c 120 ma attributed to the emplacement of the western fiordland orthogneiss migmatitic palaeozoic orthogneiss from the arthur river complex 346 6 ma is interpreted as deformed wall rock very fine rims 5 20 lm also indicate a metamorphic age of c 120 110 ma a post tectonic pegmatite 81 8 1 8 ma may be related to phases of crustal extension associated with the opening of the tasman sea the arthur river complex is interpreted as a batholith emplaced at mid crustal levels and then buried to deep crustal levels due to convergence of the median tectonic zone arc and the continental margin

deformation processes in eclogitic rocks evidence for the rheological delamination of the oceanic crust in deeper levels of subduction zones the deformation mechanisms and associated growth processes operating in omphacite during foliation development and veining have been studied in a km scale eclogitic ductile shear zone from the monviso meta ophiolitic complex italian western alps dislocation creep due to multiple slip and or climb is the dominant deformation mechanism whereas 100 and 001 twinning fracturing and intergranular fluid phase diffusional creep are subordinate active slip systems include 110 001 12 110 110 100 001 and 12 010 100 fracturing is interpreted in terms of brittle instabilities arising from strain and pore fluid pressure partitioning between adjacent omphacite domains the transition from plastic to brittle behaviour of rocks is cyclic in nature as the internally generated fluids can be released and trapped repetitively during progressive deformation the significance of the eclogitic shear zone is discussed in terms of a mechanically weak zone that developed during burial or at least prior to the peak of temperature at a minimum depth of 40 km in a subduction zone and which subsequently acted as a zone of detachment during a late stage of the alpine orogeny a rheological model for oceanic crust delamination in deeper levels of subduction zones is presented

ores and orogenesis circum pacific tectonics geologic evolution and ore deposits jon e spencer and spencer r titley editors pp 618 2008 arizona geological society digest 22 isbn 10 978 1 891924 10 1 price us 75 00 this publication was born out of an arizona geological society meeting with the same title held in october 2007 and consists of 45 papers representing a fraction of the material presented at the meeting the compilation is dedicated to william r bill dickinson emeritus professor of geology at the university of arizona who was one of the first geologists to recognize and promote the inter relationship between tectonics and ore deposit formation the editors proclaim the volume to be a measure of the current level of cross pollination between and advancement within the fields of tectonics and economic geology as the title suggests the papers within the volume cover a broad range of tectonics and ore deposits studies from the global to deposit scale thus this 618 page long book is divided into six sections with a large scale pacific margins orogenesis and metallogeny section an exploration mining and modeling section and four other sections each of which contains papers on one of the four quadrants of the circum pacific orogenic belts even within the individual sections as outlined below the range of topics is exceptionally broad five papers make up the first section which opens with a paper by dickinson that outlines basic observations of the current configuration of circum pacific orogenic belts and sets the stage for reconstructing older dissected belts based on these observations kerrich et al present a summary paper of metallogenic provinces of north america through super continent assembly and breakup and sengor et al propose potential for strata bound

viscous collision in channel explains double domes in metamorphic core complexes in hot orogens gneiss domes are a response to upper crustal stretching and lower crustal flow two dimensional thermal mechanical modeling shows that localization of extension in the upper crust triggers in the deep crust oppositely verging horizontal flows that converge beneath the extended region upon viscous collision both flowing regions rotate upward to form two upright domes of foliation double domes separated by a steep median high strain zone in such systems horizontal shortening in the infrastructure develops in an overall extensional setting dome material follows a complex depth dependent strain history from shearing in the deep crustal channel to contraction upon viscous collision in the median high strain zone to extension upon advection into the shallow crust this depth dependent strain history is likely a general feature of dome evolution and is arguably well preserved in double domes such as the montagne noire france and naxos greece gneiss domes