new global marine gravity model from cryosat 2 and jason 1 reveals buried tectonic structure gravity models are powerful tools for mapping tectonic structures especially in the deep ocean basins where the topography remains unmapped by ships or is buried by thick sediment we combined new radar altimeter measurements from satellites cryosat 2 and jason 1 with existing data to construct a global marine gravity model that is two times more accurate than previous models we found an extinct spreading ridge in the gulf of mexico a major propagating rift in the south atlantic ocean abyssal hill fabric on slow spreading ridges and thousands of previously uncharted seamounts these discoveries allow us to understand regional tectonic processes and highlight the importance of satellite derived gravity models as one of the primary tools for the investigation of remote ocean basins

effect of partial melting on rock deformation experimental and natural evidences on rocks of granitic compositions abstract deformation tests at constant strain rate 10 5 s 1 at 210 250 mpa confining pressure and at 560 900 c are performed on an aplite with varying water contents the mechanical behaviour is correlated with the microstructure after a stage of plastification depending on the temperature and on the water content the rock strength falls dramatically under such confining pressures before melting and before straining the pores of the wet rocks burst as the thermal expansion of water develops a local high pore pressure which causes cracking and fracturing in our opinion this pore bursting will increase the effects of thermal cracking due to the differential expansion of the minerals the rocks exhibit a uniform crack network with increasing temperature melting occurs in the cataclased rock the crack development and melting depend on the initial water content the loss of cohesion obtained either by cracking or by cracking and melting provokes a dramatic weakening axial shortening induces cracking parallel to 1 and melt transfer into extensional regions deformation is concentrated in dilatant shear bands at the highest temperature 900 c a melt film network is rapidly established and induces a very low flow stress melt effects in naturally deformed gneisses suggest that biotite breakdown furnishes a silicate fluid which induces hydraulic fracture and melt dispersion

on the rheology of partially molten synthetic granite abstract in an attempt to obviate some of the difficulties inherent in using natural rocks to study experimentally the rheology of partially molten granitic rocks we report some preliminary experiments using hot pressed synthetic aggregates with quartz as the solid phase and a quartz albite glass to form the melt phase undrained constant displacement rate and stress relaxation experiments were performed at 1100 c and 300 mpa total confining pressure using nominally 16 and 28 wt anhydrous melt and at 980 c with 12 wt of water saturated melt strain rate varies non linearly with stress corresponding to a stress exponent of 3 6 for the anhydrous melt tests and 1 9 with the wet melt strain rate is enhanced about 160 fold through doubling the melt fraction from 16 wt intracrystalline plasticity appears to contribute insignificantly to the observed deformation and the preservation of relatively equant grains implies a granular flow process microcracking occurs during isostatic hot pressing and also contributes to subsequent non isostatic deformation at high stresses and strain rates models of flow of partially molten rock that assume diffusive transfer accommodation of granular flow predict linear viscosity but only at lower strain rates than accessed experimentally it is suggested that the non linear granular flow observed in these experiments involved the episodic unsticking of sintered contacts between grains by a rate dependent cracking and or dislocation accommodation process

global marine gravity from retracked geosat and ers 1 altimetry ridge segmentation versus spreading rate 1 three approaches are used to reduce the error in the satellite derived marine gravity anomalies first we have retracked the raw waveforms from the ers 1 and geosat gm missions resulting in improvements in range precision of 40 and 27 respectively second we have used the recently published egm2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean third we have used a biharmonic spline interpolation method to construct residual vertical deflection grids comparisons between shipboard gravity and the global gravity grid show errors ranging from 2 0 mgal in the gulf of mexico to 4 0 mgal in areas with rugged seafloor topography the largest errors of up to 20 mgal occur on the crests of narrow large seamounts the global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate for rates less than about 60 mm a the typical ridge segment is 50 80 km long while it increases dramatically at higher rates 100 1000 km this transition spreading rate of 60 mm a also marks the transition from axial valley to axial high we speculate that a single mechanism controls both transitions candidates include both lithospheric and asthenospheric processes

toward 1 mgal accuracy in global marine gravity from cryosat 2 envisat and jason 1 more than 60 of the earth s land and shallow marine areas are covered by 2 km of sediments and sedimentary rocks with the thickest accumulations on rifted continental margins figure 1 free air marine gravity anomalies derived from geosat and ers 1 satellite altimetry fairhead et al 2001 sandwell and smith 2009 andersen et al 2009 outline most of these major basins with remarkable precision moreover gravity and bathymetry data derived from altimetry are used to identify current and paleo submarine canyons faults and local recent uplifts these geomorphic features provide clues to where to look for large deposits of sediments while current altimeter data delineate large offshore basins and major structures they do not resolve some of the smaller geomorphic features and basins yale et al 1998 fairhead et al 2001 improved accuracy and resolution is desirable to facilitate comparisons between continental margins as an exploration tool and to permit extrapolation of known structu

experimental deformation of partially molten westerly granite under fluid absent conditions with implications for the extraction of granitic magmas the mechanical behavior of partially molten westerly granite was investigated in the temperature range 800 1100 c 250 mpa confining pressure by means of constant strain rate creep and stress relaxation tests the only water in the samples came from the breakdown of hydrous phases biotite minor chlorite and muscovite and alteration products of feldspars thus the amount of melt was controlled by the test temperature and ranged from 3 at 800 c to 50 at 1100 c over that temperature range strength decreased from 500 mpa to less than 1 mpa and a preliminary constitutive flow law for the partially molten rock was obtained to allow extrapolation to low strain rates the comparative viscosity of the melt alone was estimated at 950 and 1000 c from the distance it could be made to penetrate into a porous sand under a known pressure gradient under all conditions deformation of the matrix of solid grains was by brittle fracture only samples containing up to 10 vol melt failed with the formation of a shear fault zone at higher melt fractions melt filled pores collapsed by shear enhanced compaction squeezing the melt into axial cracks above 40 vol melt unfractured solid grains were carried about passively in the flowing liquid there was no evidence of a rheologically critical melt percentage in this system by analogy with the uniaxial compaction of water saturated soils a simple model is erected to describe a two stage process for the extraction of granitic melts from their protoliths with the aid of nonhydrostatic stress shear enhanced compaction is inferred to drive melt into a network of melt filled veins whereupon porous flow through the high permeability vein network allows rapid drainage of melt to higher crustal levels

direct evolution of w rich brines from crystallizing melt within the mariktikan granite pluton west transbaikalia uneconomic tungsten mineralization associated with the mariktikan granite pluton of the transbaikalian igneous province eastern siberia is confined to a marginal part of the pluton referred to as the andreyevsky body this is composed of rocks similar to those of the main pluton but is likely to be an autonomous although kindred body on the basis of cross cutting relationships between rock varieties and their textural features the crystallization history is subdivided into three stages of unequal duration melt and fluid inclusions related to each of them have been studied using microthermometric and micro analytical procedures including a technique for atomic emission spectroscopy of individual fluid inclusions opened by a laser microprobe during the main crystallization stage 1045 1012 c more than 70 of the parental magma is believed to have crystallized at the level studied resulting in the formation of a crystalline framework rigid enough for the appearance of fractures within which some portion of the intercrystalline melt accumulated giving rise to thin aplite veins almost complete crystallization of intercrystalline and fracture hosted melt occurred during the late stage 1012 990 c whereas during the final stage only small bunches of schlieric sometimes miarolitic pegmatite were formed from a few pockets of residual melt 990 917 c in spite of the low water content of the melt about 1 wt fluid separation took place from the onset of crystallization during the main crystallization stage the exsolved fluid divided into two immiscible phases co2 rich gas and salt rich liquid brine however it was homogeneous thereafter major ore components of the brine mn fe w displayed different behaviour as crystallization progressed w concentration was below 0 1 wt at the main stage attained 1 wt during the late stage and increased to 1 8 wt at the final stage comparison of these data with calculations carried out using estimated parameters for the parental magma enables us to infer that obtained concentration values are reasonable for the special case studied here however they are unlikely to be attainable if the magma is h2o enriched and lacks an anomalously high w content in the andreyevsky body of the mariktikan pluton the following features of the parental magma made possible the generation of w rich solutions 1 high liquidus temperature 1045 c and elevated cl content c 0 15 wt that resulted in enlargement of the fluid melt distribution coefficient c 10 2 low water content c 1 and elevated w content c 0 001 wt that provided a relatively high w h2o ratio within the system at the same time because of the low water content of the magma the total mass of w bearing solutions has proved to be insufficient for the production of large scale mineralization a reduced w content of the great bulk of the exsolved brine may have had an unfavourable effect as well

inversion of combined gravity and bathymetry data for crustal structure a prescription for downward continuation abstract we propose a new and improved recipe for inverting for crustal thickness variations from combined gravity and bathymetry data the new ingredients with respect to previous studies are the posing of this problem as an inverse problem and the use of a downward continuation filter which can be tailored to find the crustal thickness solution which satisfies a minimum slope or maximal smoothness criteria or a weighted combination of these desired solution features we explore several suggestions for determining the optimum smoothing parameters for a given crustal thickness inversion then we compare this new method to that used previously in marine gravity studies we find it to be an improved recipe for dealing with gravity data with observation errors of 3 5 mgal but not by much this recipe will be most useful in extracting more geological information from marine gravity data where apparent observation errors are 1 mgal e g from future global positioning satellite navigated studies

viscosity and thickness of the sub lithospheric low viscosity zone constraints from geoid and depth over oceanic swells abstract the medium wavelength geoid to depth anomalies ratio gdr at oceanic hotspot swells has been found to increase from 0 5 m km to 5 m km according to the age of the lithosphere they occur on in order to interpret this trend the geoid and topography anomalies associated with mantle convective plumes crossing a sublithospheric low viscosity zone lvz have been derived from numerical models and a systematic investigation of the gdr dependence on the viscosity and depth extent of the lvz on the thickness and thermal structure of the lithosphere and on the rayleigh number has been conducted it is shown that for viscosity drops across the base of the lvz greater than one order of magnitude the gdr is strongly dependent on the depth of shallow interfaces such as the lithosphere athenosphere boundary and on the lvz s thickness consequently the empirical trend can be accounted for by the thickening of the lithosphere with age provided it occurs at the expense of a lvz whose base is at a fixed depth around 200 km in such a frame no significant variation with age of the lvz s viscosity is required by the gdr data best fit with the empirical trend is found for a lvz about 50 times less viscous than the underlying mantle the mantle flow starts to fluctuate when the local rayleigh number of the low viscosity layer exceeds the rayleigh number of the underlying mantle the fluctuations are initiated in the upper boundary layer in the diverging part of the plume at a distance of a few hundreds of kilometers from the main ascending current for viscosity contrasts in the range of 40 60 deduced from the present study the conditions for the development of these small scale instabilities are realized only where the lithosphere has not yet grown significantly downwards ages

the possible role of fluids for the structuring of the continental crust abstract some well known seismic reflectivity patterns of continental crusts characterize specific tectonic terranes the role of fluids in enhancing or destroying such structures is studied seismicity is strongly dependent on the different pore pressure of fluids in intraplate settings in strike slip or extensional areas hydrostatic pore pressure seems to be approached in accordance with standard rheological models and standard seismicity depth distributions in suture zones or zones of developing loads possibly also along zones of near horizontal normal faults extremely high pore pressure develops which approaches lithostatic values they are accompanied by a number of weakening processes with enhanced shear and reduced viscosity these values correlate with low seismic velocities v p in accordance with an empirical v p relationship

models of the bouguer gravity and geologic structure at yucca flat nevada the bouguer gravity anomaly at yucca flat nevada has been modeled by two different techniques the cordell henderson and parker oldenburg methods the three dimensional model has incorporated known density and structural information where possible these models predict the structural relief on the cenozoic paleozoic contact to within 150 m or about 15 percent of the actual depth the three dimensional parker oldenburg method has been found to be efficient in an application involving a large 9000 sample data base numerical stability was ensured by the application of a consistent regularization a low pass filter tuned to suppress the noise dominated portion of the data spectrum of the downward continuation operator the use of a single regularizing filter for the entire model is not completely satisfactory due to the oversmoothing of shallow regions of the basin the model is useful in the delineation of the geologic history of the area structural features in the model support the hypothesis that regional stress fields rotated significantly during the tertiary major structural elements of the basin are well defined on the cenozoic paleozoic interface the principal basin bounding fault is the large throw carpetbag fault on the west this fault was most active during the earliest phases of subsidence the yucca fault is seen to be a much smaller feature in the model presented here the basin is rotated down to the west with normal hinge faults on the eastern margin

the downward continuation of magsat crustal anomaly field over southeast asia a general method for the local downward continuation at ground level of geopotentials measured by low orbiting satellites at different altitudes is proposed such a continuation is the inverse solution of the external dirichlet problem the stability of the solution with respect to the altitude and the distribution of data points is discussed principal component analysis is shown to perform well and leads to a stable solution the method proposed has the advantage of being directly applicable to measured data points which are not evenly spaced and at different altitude as an illustration it is applied to magsat data over southeast asia two independent maps of the vertical component of the anomaly field at a constant altitude near ground level are derived for dawn descending orbits and dusk ascending orbits data separately the two maps reveal similar anomalies and confirm the correlation of the magsat derived anomaly field with the mosaic structure of the continental crust in southeast asia stable continental blocks are associated with positive anomalies and intervening collision belts correspond to well defined negative anomalies a very prominent low is observed above the himalaya and extends over the tibet plateau confirming an anomalous crustal and thermal structure of the plateau

the effects of porphyroblast growth on the effective viscosity of metapelitic rocks implications for the strength of the middle crust numerical models are used to examine the effects of porphyroblast growth on the rheology of compositionally layered rocks metapelites and metapsammites and by extension the middle crust during prograde metamorphism as porphyroblast abundance increases during prograde metamorphism metapelitic layers will strengthen relative to porphyroblast free metapelitic units and potentially relative to quartzofeldspathic metapsammitic units as metapelitic layers become stronger the integrated strength of compositionally layered successions increases potentially causing large volumes of mid crustal rock to strengthen altering the strain rate distribution in the middle crust and affecting the geodynamic evolution of an orogenic belt the growth of effectively rigid porphyroblasts creates strength heterogeneities in the layer undergoing porphyroblast growth which leads to complex strain rate distributions within the layer at the orogen scale the strengthening of large crustal volumes on the order of thousands of cubic kilometres changes the strain rate distribution which may change exhumation rates of high grade metamorphic rocks the geothermal structure and the topography of the orogen the presence of a strong zone in the middle crust causes strain rate partitioning around the zone suppressed uplift rates within and above the zone and leads to the development of a basin on the surface

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

pseudotachylite from the main zone of the hidaka metamorphic belt hokkaido northern japan pseudotachylite veins have been found in the mylonite zone of the hidaka metamorphic belt hokkaido northern japan they are associated with faults with wnw ese to ene wsw or ne sw trends which make a conjugate set cutting foliations of the host mylonitic rocks with high obliquity the mylonitic rocks comprise greenschist facies to prehnite pumpellyite facies mineral assemblages the mode of occurrence of the pseudotachylite veins indicates that they were generated on surfaces of the faults and were intruded as injection veins along microfractures in the host rocks during brittle deformation in near surface environments an analysis of the deformational and metamorphic history of the hidaka main zone suggests that the ambient rock temperature was 200 300 c immediately before the formation of the hidaka pseudotachylite three textural types of veins are distinguished cryptocrystalline microcrystalline and glassy the cryptocrystalline or glassy type often occupies the marginal zones of the microcrystalline type veins the microcrystalline type is largely made up of quench microlites of orthopyroxene clinopyroxene biotite plagioclase and opaque minerals with small amounts of amphibole microlites the interstices of these microlites are occupied by glassy and or cryptocrystalline materials the presence of microlites and glasses in the pseudotachylite veins suggests that the pseudotachylites are the products of rapid cooling of silicate melts at depths of less than 5 km the bulk chemical composition of the pseudotachylite veins is characterized by low sio2 and a high water content and is very close to that of the host mylonitic rocks this indicates that the pseudotachylite was formed by virtual total melting of the host rocks with sufficient hydrous mineral phases local chemical variation in the glassy parts of the pseudotachylite veins may be due to either crystallization of quench microlites or the disequilibrium nature of melting of mineral fragments and incomplete mixing of the melts pyroxene microlites show a crystallization trend from hypersthene through pigeonite to subcalcic augite with unusually high al contents the presence of pigeonite and high al pyroxene microlites of hornblende and biotite microlites and rare plagioclase microlites may indicate the high temperature and high water content of the melt which formed the pseudotachylite veins the melt temperatures were estimated to be up to 1100 c using a two pyroxene geothermometer using published data relating water solubilities in high temperature andesitic magmas to pressure a depth estimate of about 4 km is inferred for the hidaka pseudotachylites evidence derived from pseudotachylites in the hidaka metamorphic belt supports the conclusion that pseudotachylite is formed by frictional melting along fault surfaces at shallow depths from rocks containing hydrous minerals

a zoned low p high t complex at the level of anatexis structural and plutonic patterns in metasediments of the archean yellowknife supergroup near bathurst inlet n w t canada abstract within metagreywackes and schists of the archean yellowknife supergroup the development of an anatectic stratigraphy during the late stages of regional deformation has resulted in the formation of a complex with a metamorphicplutonic structural zoning similar to that occurring in phanerozoic low p high t terrains from bottom to top the stratigraphy consists of 1 small folded migmatite grading with decreasing partial melting to 2 sillimanite schist strongly strained by the emplacement of two mica granites 3 andalusite cordierite schist the site of large folds related to the emplacement of the complex each zone is characterized by a different structural style resulting from the interplay between metamorphic grade and deformation for instance in the sillimanite zone the structure was influenced by the extraction of syntectonic melt and its emplacement into fractures at low effective pressure this zone was also the ductile envelope into which sills and plutons of two mica granites derived from the lower migmatites migrated the time taken for evolution of the complex has been constrained by u pb zircon and monazite dating of a member of the early tonalite diorite suite and members of the two mica granite suite this shows the evolution of the complex was rapid ca 10 ma but possibly polyphase the formation of a distinctively styled complex accompanied by syn tectonic anatexis and plutonism may be typical of lower crustal levels of the yellowknife supergroup and other low p high t terrains

melt segregation in the continental crust distribution and movement of melt in anatectic rocks the grain and outcrop scale distribution of melt has been mapped in anatectic rocks from regional and contact metamorphic environments and used to infer melt movement paths at the grain scale anatectic melt is pervasively distributed in the grain boundaries and in small pools consequently most melt is located parallel to the principal fabric in the rock typically a foliation short branched arrays of linked melt bearing grain boundaries connect melt depleted parts of the matrix to diffuse zones of melt accumulation protoleucosomes where magmatic flow and alignment of euhedral crystals grown from the melt developed the distribution of melt leucosome and residual rocks normally melanocratic in outcrop provides different but complementary information the residual rocks show where the melt came from and the leucosomes preserve some of the channels through which the melt moved or sites where it pooled different stages of the melt segregation process are recorded in the leucosome melanosome arrays regions where melting and segregation had just begun when crystallization occurred are characterized by short arrays of thin branching leucosomes with little melanosome a more advanced stage of melting and segregation is marked by the development of residual rocks around extensive branched leucosome arrays generally oriented along the foliation or melting layer places where melting had stopped or slowed down before crystallization began are marked by a high ratio of melanosome to leucosome because most of the melt has drained away very few leucosomes remain to mark the melt escape path this is common in melt depleted granulite terranes many migmatites contain abundant leucosomes oriented parallel to the foliation mostly these represent places where foliation planes dilated and melt drained from the matrix via the branched grain boundary and larger branched melt channel leucosome arrays collected melt collected in the foliation planes was partially or fully expelled later when discordant leucosomes formed leucosomes or veins oriented at high angles to the foliation layering formed last and commonly lack melanocratic borders hence they were not involved in draining the matrix of the melting layer discordant leucosomes represent the channels through which melt flowed out of the melting layer

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

transition from dislocation creep to melt enhanced diffusion creep in fine grained granitic aggregates abstract sintered aggregates composed of quartz albite and microcline of two different grain sizes 2 10 and 10 50 m each with two different melt contents

metamorphism transient mid crustal rheology strain localization and the exhumation of high grade metamorphic rocks 1 we present a series of three dimensional numerical models investigating the effects of metamorphic strengthening and weakening on the geodynamic evolution of convergent orogens that are constrained by observations from an exposed mid crustal section in the new england appalachians the natural mid crustal section records evidence for spatially and temporally variable mid crustal strength as a function of metamorphic grade during prograde polymetamorphism our models address changes in strain rate partitioning and topographic uplift as a function of strengthening weakening in the middle crust as well as the resultant changes in deformation kinematics and potential exhumation patterns of high grade metamorphic rock results suggest that strengthening leads to strain rate partitioning around the zone and suppressed topographic uplift rates whereas weakening leads to strain rate partitioning into the zone and enhanced topographic uplift rates deformation kinematics recorded in the orogen are also affected by strengthening weakening with complete reversals in shear sense occurring as a function of strengthening weakening without changes in plate boundary kinematics

on the rheologically critical melt fraction abstract with increasing melt fraction the strength of partially molten granite decreases from a value characteristic of solid competent rock to a value nearly equal to that of the melt previously published mechanical and microstructural data indicate that deformation in partially molten rocks often involves brittle processes thus the pressure in the melt is expected to be important in determining strength when the volume changes during deformation strength and fluid flow will be coupled by such parameters as permeability k storage capacity per unit volume or storativity s melt compressibility f grain size d fluid viscosity and strain rate experiments on brittle rock at low temperatures show that the strain rate at which the internal fluid pressure can be maintained constant is approximately proportional to k s an evaluation of published experimental data suggests that this relation also holds for partially molten granites indicating that the strength of these rocks depends on their transport properties since the permeability is related to below a critical melt fraction rcmf the melt pressure will change if deformation is not iso volumetric by assuming a power law relationship between k and we estimate that rcmf f d m 1 n 1 where m and n relate permeability to grain size and respectively

pervasive melt migration from migmatites to leucogranite in the shuswap metamorphic core complex canada control of regional deformation abstract the shuswap metamorphic core complex exhumed in the hinterland of the canadian cordillera displays a 15 km structural section from migmatites to leucogranite it comprises a lower unit dominated by anatectic migmatites exposed in the core of domes at higher structural level the transition from a middle amphibolite facies unit to an upper crustal unit is marked by a low angle detachment associated with the emplacement of laccoliths of leucogranite the amphibolite facies middle unit is permeated by a network of granitic veins feeding the laccoliths and structurally connected to the migmatites of the lower unit this paper describes the geometric textural and structural characteristics and the distribution of the granitic fraction at the outcrop scale in the amphibolite facies unit separating the migmatites from the leucogranite this analysis constrains the mechanisms of melt migration within a plurikilometer scale section across high grade rocks of the middle crust in fertile lithologies metapelites felsic amphibolites the granitic fraction in part generated in situ forms a diffuse network of concordant veins feeding discordant veins and structural sites such as the foliation shear zones and boudin necks discordant granitic veins are oriented perpendicular and to a lesser extent parallel to the mineral and stretching lineation in refractory lithologies the granitic fraction is dominantly intrusive and displays sharp contacts with the host rock however distribution of the granitic fraction shows the same characteristics as for fertile lithologies the melt fraction migrated through a network of fractures taking advantage of mechanical weaknesses such as the foliation plane or forming dikes oriented dominantly perpendicular to the regional stretching and mineral lineations these geometric and textural characteristics suggest that granite migration in these rocks is achieved dominantly by viscous flow of the melt solid through the solid matrix driven by its buoyancy and controlled by mechanical anisotropy of the rock and local dilation created by heterogeneous deformation the relationship between the regional fabric and the distribution of the granitic fraction indicates that regional deformation incremental and finite played a major role in providing pathways for melt migration the formation of laccoliths of leucogranite at higher structural levels suggests that upward melt migration led to accumulation of granitic magmas these observations are consistent with a model of pervasive melt migration through a network of interconnected concordant and discordant granitic veins feeding laccolithic plutons extracted from the dominantly solid matrix during vertical thinning and compaction at a regional scale the presence of both transposed and intact granitic networks indicate an overlap between deformation and granite migration as a continuum or in pulses the methodology and terminology proposed in this paper provide some insights on the behavior of the granitic fraction in the middle crust this approach should be applicable to other high grade terrains exhumed in the cores of orogenic belts

constraints on melting and magma production in the crust major intrusions of granitic rocks are found in several tectonic settings and in all cases crustal melts may contribute to the volumes of granitic magma high grade metamorphism and partial melting of the crust take place predominantly under fluid absent conditions we present a model for calculating the amounts of melt that may be formed by fluid absent breakdown of micas and amphiboles in common crustal rock types pelitic quartzofeldspathic intermediate and mafic melt proportions depend mainly on the kind of source rock the pressure at which melting takes place the temperature and the hydrous mineral h2o content of the source as a consequence of the pressure dependence of water solubility in silicate melts any given source rock will produce more melt by a given fluid absent reaction at lower pressure at a given pressure higher temperature reactions can produce more melt from a given source rock based on a survey of the compositions of common rock types we show that the amounts of melt can vary from 50vol thus crustal rocks vary widely in their fertility as magma sources depending on the types and amounts of hydrous phases they contain in general muscovite breakdown in pelites will yield only small quantities of melt and lead to migmatite formation biotite breakdown in pelites occurs at higher temperature and because most high grade pelites below granulite grade are biotite rich can yield up to about 50 vol of granitoid melt rocks of intermediate composition and hornblende rich mafic rocks are potentially highly fertile magma sources also provided that the high temperatures necessary for biotite and hornblende breakdown are realized pyroxene rich mafic rocks and quartzofeldspathic rocks are much less potentially fertile data suggest that mechanisms exist for the efficient segregation of melt and restite in systems with 30and probably 20vol melt the pressure temperature history of a region can greatly influence crustal source fertility through its control over the occurrence of subsolidus dehydration and melting equilibria

transformation plasticity and the effect of temperature on the mechanical behaviour of hawkesbury sandstone at atmospheric pressure abstract uniaxial compressive strength ucs testing of hawkesbury sandstone was carried out at various temperatures between 25 and 950 c to explore the mechanical response of the sandstone to significant changes in temperature as expected for enhanced geothermal energy systems nuclear waste disposal and underground coal gasification the ucs testing results demonstrate a mechanical dependence on temperature whereby the compressive strength and elastic modulus for the sandstone increases with increasing temperature for temperatures less than c 500 c and decreases with increasing temperature for temperatures greater than c 500 c x ray diffraction analyses performed on material from the failed 25 and 950 c specimens highlights a distinct difference in mineralogy between the two specimens that has been related to mineralogical changes in the sandstone cement with heating progressive dehydroxylisation of kaolinite in the sandstone cement at temperatures beyond 500 c appears to have enabled transformation plasticity explaining the weakening and softening of the sandstone that was observed with increasing temperature beyond 500 c transformation plasticity is mineralogy dependent and thus its influence on mechanical behaviour of rock will vary with bulk mineralogy and the relative distribution of mineral species comparison of the results from the ucs testing to those obtained from similar experimental work carried out on different sandstone units highlights variability in the response of rock to heating the study provides a word of caution regarding the need for accurate understanding of the influence of temperature on the mechanical behaviour of the specific rock unit considered for a given elevated temperature engineering application such understanding requires consideration of the geological history of the rock in addition to its physical properties and mineralogy

partial melt topology in statically and dynamically recrystallized granite microstructures of the murray granite pluton central ontario canada show evidence of both static and dynamic crystallization subsequent to partial melting backscattered electron analyses reveal interstitial k feldspar and plagioclase at triple junctions of strain free isometric quartz grains the geometry of the quartz feldspar boundaries mimics the original topology of the quartz melt contacts during crystallization this conclusion is suggested by the occurrence of both rounded and planar faces of quartz grains and by low 27 dihedral angles of quartz quartz feldspar boundaries similar to dihedral angles in experimentally crystallized quartz quartz silicic melt systems in contrast feldspar seams in deformed granites have high axial ratios are usually elongated perpendicular to the foliation plane and are located preferentially along individual grain boundaries quartz grains are dynamically recrystallized and occasionally transected by feldspar seams indicating that fracturing occurred in the presence of melt during crystal plastic deformation of quartz the subparallel orientation in quartz grains of intragranular feldspar bearing fractures and interstitial feldspar seams suggests that these features originated as intragranular and intergranular fractures respectively partial melt topology was therefore controlled by intergranular and occasionally by intragranular fracturing

hydrothermal events in the valle de tena spanish western pyrenees as evidenced by fluid inclusions and trace element distribution from fluorite deposits abstract this paper provides further evidences from fluorite and calcite geochemistry and from fluid inclusion studies for the origin of the valle de tena fluorite deposits the valle de tena spanish western pyrenees contains two kinds of fluorite mineralizations 1 vein type deposits located in the vicinity of the panticosa granite and enclosed by devonian carbonate facies and 2 carbonate hosted mineralizations on highly silicified lower carboniferous limestones based on geological evidences the latter deposits can be referred to as permo triassic microthermometrical measurements on two phase water rich inclusions with high liquid to vapor ratios yield homogenization temperatures ranging from 100 to 200 c for the two deposit types salinities range from 4 0 to 11 6 eq wt nacl for carbonate hosted deposits and from 12 5 to 21 3 eq wt nacl for vein mineralizations ree contents are variable and highest abundances are found in the fluorite from lanuza and tebarray green flurite these later fluorite mineralizations have primary ree patterns with positive eu anomalies which possibly reflect the alteration of feldspar the ree distribution patterns of fluorites and calcites from the carbonate hosted mineralizations are different from those occurring as veins the decrease of lree in the former resulted in roof shaped ree patterns the negative eu anomalies as well as the trace element distribution allow us to deduce that the fluid from which these later ca minerals were formed was probably equilibrated with the sedimentary country rocks the fluid inclusion geochemical geologic and preliminary stable isotope data support two temporally distinct hydrothermal systems accounting for the fluorite formation and suggest a sedimentary basin source for the ore fluids high salinity fluids basement brines are involved in the formation of the vein type deposits whereas the subsequent carbonate hosted mineralizations were formed by mobilization of the fluorite veins by formation waters the most likely fluorite deposition mechanism is cooling with minor contribution of fluid rock interaction in such context magmatic activity is considered as heat source the above mineralization events possibly span a time from permian to triassic which is in close agreement with the timing of hydrothermal processes documented throughout the hercynian europe

satellite gravity anomalies and their correlation with the major tectonic features in the south china sea abstract the south china sea scs is a region of interaction among three major plates the pacific indo australian and eurasian the collision of the indian subcontinent with the eurasian plate in the northwest back arc spreading at the center and subduction beneath the philippine plate along manila trench in the east and the collision along palawan trough in the south have produced complex tectonic features within and along the scs this investigation examines the satellite derived gravity anomalies of the scs and compares them with major tectonic features of the area a map of bouguer gravity anomaly is derived in conjunction with available seafloor topography to investigate the crustal structure the residual isostatic gravity anomaly is calculated assuming that the cenozoic sedimentary load is isostatically compensated the features in the gravity anomalies in general correlate remarkably well with the major geological features including offsets in the seafloor spreading segments major faults basins seamounts and other manifestations of magmatism and volcanism on the seafloor they also correlate with the presumed location of continental oceanic crust boundary the region underlain by oceanic crust in the central part of the scs is characterized by a large positive bouguer gravity anomaly 220 330 mgal as well as large free air and residual isostatic anomalies there are however important differences among spreading segments for example in terms of free air gravity anomaly the southwest section of mid ocean has an approximately 50 km wide belt of gravity low superimposed on a broad high of 45 mgal running nw se whereas there are no similar features in other spreading segments there are indications that gravity anomalies may represent lateral variation in upper crustal density structure for instance free air and isostatic anomalies show large positive anomalies in the east of the namconson basin which coincide with areas of dense volcanic material known from seismic surveys the red river fault system are clearly identified in the satellite gravity anomalies including three major faults songchay fault in the southwest songlo fault in the northeast and central fault in the center of the basin they are elongated in nw se direction between 20 30 n and 17 n and reach to vietnam scarp fault around 16 30 n it is also defined that the crustal density in the south side of the central basin is denser than that in the north side of the central basin

fast inversion of magnetic field maps of unidirectional planar geological magnetization 1 scanning magnetic microscopes are being increasingly utilized in paleomagnetic studies of geological samples these instruments typically map a single component of the sample s magnetic field at close proximity with submillimeter horizontal spatial resolution however in most applications an image of the magnetization distribution within the sample is desired rather than its external magnetic field this requires carefully solving an ill posed inverse problem to obtain solutions that are nearly free of artifacts and consistent with both natural and laboratory magnetization processes we present a new fast inversion technique based on classic methods developed for the fourier domain that retrieves planar unidirectional magnetization distributions from magnetic field maps whereas our approach considers the subtle peculiarities of scanning magnetic microscopy which otherwise can complicate this technique much of the formalism and algorithms described in this work can also be directly applied to province scale magnetic field data from aeromagnetic surveys and may be used as an initial step in the modeling of magnetic sources with complex three dimensional geometries we discuss sources of inaccuracy observed in practical implementations of the technique and present strategies to improve the quality of inversions numerous examples of inversion of both synthetic and experimental data demonstrate the performance of the technique under different conditions in particular we retrieve magnetization distributions of a hawaiian basalt and compare it to inversions calculated in a previous work we conclude by showing a reconstructed magnetization for the eucrite meteorite alha81001 that displays in high resolution the spatial distribution of high coercivity grains within the sample

how temperature dependent elasticity alters host rock magmatic reservoir models a case study on the effects of ice cap unloading on shallow volcanic systems abstract in geodynamic numerical models of volcanic systems the volcanic basement hosting the magmatic reservoir is often assumed to exhibit constant elastic parameters with a sharp transition from the host rocks to the magmatic reservoir we assess this assumption by deriving an empirical relation between elastic parameters and temperature for icelandic basalts by conducting a set of triaxial compression experiments between 200 c and 1000 c results show a significant decrease of young s modulus from 38 gpa to less than 4 7 gpa at around 1000 c based on these laboratory data we develop a 2d axisymmetric finite element model including temperature dependent elastic properties of the volcanic basement as a case study we use the snaefellsjokull volcanic system western iceland to evaluate pressure differences in the volcanic edifice and basement due to glacial unloading of the volcano first we calculate the temperature field throughout the model and assign elastic properties accordingly then we assess unloading driven pressure differences in the magma chamber at various depths in models with and without temperature dependent elastic parameters with constant elastic parameters and a sharp transition between basement and magma chamber we obtain results comparable to other studies however pressure changes due to surface unloading become smaller when using more realistic temperature dependent elastic properties we ascribe this subdued effect to a transition zone around the magma chamber which is still solid rock but with relatively low young s modulus due to high temperatures we discuss our findings in the light of volcanic processes in proximity to the magma chamber such as roof collapse dyke injection or deep hydrothermal circulation our results aim at quantifying the effects of glacial unloading on magma chamber dynamics and volcanic activity

ages and magnetic structures of the south china sea constrained by deep tow magnetic surveys and iodp expedition 349 combined analyses of deep tow magnetic anomalies and international ocean discovery program expedition 349 cores show that initial seafloor spreading started around 33 ma in the northeastern south china sea scs but varied slightly by 1 2 myr along the northern continent ocean boundary cob a southward ridge jump of approximate to 20 km occurred around 23 6 ma in the east subbasin this timing also slightly varied along the ridge and was coeval to the onset of seafloor spreading in the southwest subbasin which propagated for about 400 km southwestward from approximate to 23 6 to approximate to 21 5 ma the terminal age of seafloor spreading is approximate to 15 ma in the east subbasin and approximate to 16 ma in the southwest subbasin the full spreading rate in the east subbasin varied largely from approximate to 20 to approximate to 80 km myr but mostly decreased with time except for the period between approximate to 26 0 ma and the ridge jump approximate to 23 6 ma within which the rate was the fastest at approximate to 70 km myr on average the spreading rates are not correlated in most cases to magnetic anomaly amplitudes that reflect basement magnetization contrasts shipboard magnetic measurements reveal at least one magnetic reversal in the top 100 m of basaltic layers in addition to large vertical intensity variations these complexities are caused by late stage lava flows that are magnetized in a different polarity from the primary basaltic layer emplaced during the main phase of crustal accretion deep tow magnetic modeling also reveals this smearing in basement magnetizations by incorporating a contamination coefficient of 0 5 which partly alleviates the problem of assuming a magnetic blocking model of constant thickness and uniform magnetization the primary contribution to magnetic anomalies of the scs is not in the top 100 m of the igneous basement

flow of synthetic wet partially molten granite under undrained conditions an experimental study 1 major problems in experimental studies of the flow of partially molten granitic rocks are 1 maintaining control of the grain size of the matrix of solid grains 2 controlling the melt fraction and 3 controlling the melt viscosity closely linked to water content to overcome these problems we used a synthetic granitoid comprising a solid matrix of quartz grains 50 m grain size mixed with an albite quartz melt prepared from oxides with added water 2 5 wt dissolved in the melt phase viscosity 9 104 pa s experiments were performed undrained with melt fractions of 0 1 0 2 0 25 and 0 3 constant strain rate creep and stress relaxation experiments were carried out in a gas medium apparatus at 1273 and 1173 k mostly at 300 mpa confining pressure up to 15 shortening strain strain rates ranged between 10 4 to less than 10 7 s 1 and fully ductile mechanical behavior was observed melt fraction has a more profound effect on strength than water content temperature or total confining pressure low strain rate 10 6 s 1 data were fitted to an empirical flow law of the form d dt a exp b m exp h rt n with the parameters loga 1 39 m 3 n 1 8 b 192 h 220 kjmol 1 where d dt is strain rate s 1 is flow stress mpa is melt fraction and t is temperature k strength is expected to fall rapidly above about 0 4 melt fraction to levels characteristic of flow of the melt containing suspended crystallites extrapolation to geological strain rates using this flow law shows that in nature migmatites bearing granitic melt will be extremely weak much weaker than silicate rocks deforming by intracrystalline plasticity microstructural study shows grains remain equant at all strains with no discernable formation of a grain flattening fabric nor a large strain contribution from microfracturing thus bulk intracrystalline plasticity is of minor importance and intergranular sliding is implied the nonlinear flow observed is tentatively attributed to a combination of sintering of grain contacts and sliding failure of such contacts with a substantial contribution from diffusive transfer processes

deformational behaviour of migmatites and problems of structural analysis in migmatite terrains an understanding of the response of migmatites to deformation is crucial to an interpretation of their structures and in anatectic and intrusive migmatite terrains due consideration must be given to the modification of deformation processes imposed by melts in partially molten systems containing more than 30 melt the classical theories of fold formation are inapplicable the folds produced are of the viscous fold type their geometry is unpredictable and may mimic structures due to refolding this is particularly true at very high degrees of melting where flow of melt carrying rafts of solids begins failure to distinguish viscous folds from the products of refolding will lead to errors in interpreting the structural and metamorphic history of an area and to consequent misinterpretation of the true structural relations between geometrically complex high grade and more regularly deformed low grade areas

downward continuation of airborne gravity data by means of the change of boundary approach within the modelling of gravity data a common practice is the upward downward continuation of the signal i e the process of continuing the gravitational signal in the vertical direction away or closer to the sources respectively the gravity field being a potential field satisfies the laplace s equation outside the masses and this means that it allows to unambiguously perform this analytical continuation only in a source free domain the analytical continuation problem has been solved both in the space and spectral domains by exploiting different algorithms as well known the downward continuation operator differently from the upward one is an unstable operator due to its spectral characteristics similar to those of a high pass filter and several regularization methods have been proposed in order to stabilize it in this work an iterative procedure to downward upward continue the gravity field observations acquired at different altitudes is proposed this methodology is based on the change of boundary principle and it has been expressively thought for aerogravimetric observations for geophysical exploration purposes within this field of application usually several simplifications can be applied basically due to the specific characteristics of the airborne surveys which are usually flown at almost constant altitude as close as possible to the terrain for instance these characteristics as shown in the present work allow to perform the downward continuation without the need of any regularization the goodness of the proposed methodology has been evaluated by means of a numerical test on real data acquired in the south of australia the test shows that it is possible to move the aerogravimetric data acquired along tracks with a maximum height difference of about 250 m with accuracies of the order of 10 3 mgal