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

flow laws for rocks containing two non linear viscous phases a phenomenological approach abstract mylonitic rocks and rock analogue materials reveal two basic types of structure 1 a load bearing framework lbf of strong phase contains isolated pockets of weak phase 2 interconnected layers of weak phase iwl separate boudins and clasts of strong phase aggregates with the lbf microstructure are characterized by nearly uniform strain rate stress is concentrated in the load bearing framework in aggregates with an iwl microstructure strain rate and sometimes also stress are higher in the interconnected weak phase than in the boudins and clasts of strong phase the degree of stress and strain partitioning depends strongly on the viscous strength contrast and on the relative amounts of the constituent mineral phases based on these observations the rheology of two phase rock is modelled with separate functions for lbf and iwl microstructures a new flow law is derived for rock with iwl structure in which two phases undergo dislocation creep the flow law expresses composite creep strength in terms of temperature bulk strain rate and the volume proportions and creep parameters of the minerals in the rock strain rate and stress are averaged in the constituent phases and slip along phase boundaries maintains strain compatibility within the aggregate composite strengths predicted with the iwl flow law fall well within the uniform stress and uniform strain rate bounds and are generally consistent with the viscous strengths of experimentally deformed bimineralic aggregates a hypothesis of viscous strain energy minimization is used to determine the relative stability of the lbf and iwl microstructures during steady state creep the iwl microstructure is predicted to be stable over a broad range of two phase compositions and mineral strength contrasts whereas the lbf microstructure is stable only in rocks with low volume proportions of weak phase and low to moderate mineral strength contrasts the iwl flow law indicates that rheological stratification in the lithosphere depends strongly on rock composition especially in rocks with low volume proportions of a weak phase and high mineral strength contrasts

constraining the relative strengths of high grade metamorphic rocks using foliation refraction angles an example from the northern new england appalachians foliation refraction angles are used to estimate effective viscosity contrasts between metapelitic and metapsammitic layers in amphibolite facies metaturbidites in the presidential range of the white mountains in eastern new hampshire an early formed foliation developed during km scale nappe folding consistently displays larger bedding foliation angles in metapelitic units than metapsammitic units suggesting that the metapelitic units had higher effective viscosities during this deformation we collected bedding foliation angle measurements from a combination of outcrops and locally derived boulders to test the hypothesis that foliation refraction angles could be used to estimate the effective viscosity ratio between different rock types at high metamorphic grades our results show that the metapelitic layers were between 2 and 3 times more viscous than the metaspsammitic layers which we attribute to the presence of large up to 15 cm long abundant up to 30 vol effectively rigid andalusite porphyroblasts in the metapelitic layers we present a methodology for using foliation refraction angles to determine the effective viscosity ratios of different rock types address the practical limitations we encountered and suggest this method is an easy way of estimating the strength contrast between different rock types finally we hypothesize that metamorphism resulting in porphyroblast growth can strengthen large crustal volumes during orogenesis

rheological transitions in progressive melting of rock and their geological constraints from the fuhu metatexite diatexite profile in guangdong province se china abstract integration of the published experimental data of partially melted amphibolite orthogneiss granite and aplite reveals the existence of three rapid strength drops of the rock assemblage at melt fraction around 7 21 and 41 respectively the first and the last drops are equivalent to the well known melt connectivity transition mct and solid to liquid transition slt the drop at melt fraction around 21 is newly identified and termed as the framework melting transition fmt the fmt is defined as the transition from eutectic quartz feldspars melting to refractory biotite melting the three rheological transitions are well constrained by the geological data from the fuhu profile in guangdong province se china this profile is a 15 m high cliff rocks exposed on the profile are zoned as mottled stripped narrow banded and wide banded migmatites and diatexitic granite from the top of the profile downwards the leucosome proportions of the zones measured on the outcrops combined with the experimental data of the partially melted rocks have significant implications for understanding the role of rheological transitions during crustal melting the mct which corresponds to the boundary between the mottled and the narrow banded migmatites suggests the segregation of melt within the partially melted system along with the compaction of matrix thus the mct is deemed to be the transition of the partially melted system from matrix supported melt stored in pore space to framework supported melt stored in foliation bedding space of protolith the fmt corresponding to the boundary between the narrow and the wide banded migmatites denotes the inception of melting of the solid framework mechanically the fmt can be regarded as the transition of the framework from compaction to fusion and geochemically the transition from eutectic melting of quartz feldspars to that of refractory biotite the increasing melt due to framework melting after the fmt is basically limited within the solid framework until the melt fraction exceeding the slt the slt is geologically constrained by the in situ metatexite diatexite transition that is also termed as magma interface mi the slt mi represents the upper limit of a crustal magma layer because of the downward increasing temperature and melt fraction there should be no consecutive layer of felsic rock existed in the space between the slt mi and the mafic lower crust except the isolated rock blocks within the magma layer

deformation of partially molten granite a review and comparison of experimental and natural case studies experimental and natural investigations of partially molten granite are compared and reviewed experiments suggest that deformation of partially molten granite with low viscosity melt 104 pa s exhibits a rheological critical melt percentage rcmp in case of high viscosity melts 108 pa s however the relationship between melt fraction and log viscosity of the partially molten granite may be linear considerations about viscosity rheological thresholds and segregation of natural melts suggest that low viscosity melt experiments simulate natural conditions more realistically therefore an rcmp is to be expected under natural conditions both diffusion creep and dislocation creep may occur under natural conditions whereas cataclastic flow is only observed under experimental conditions a melt induced transition from dislocation creep to diffusion creep occurs under experimental and natural conditions melt topology is controlled by the magnitude of differential stress under experimental conditions if differential stress is higher than 100 150 mpa melt pockets are elongate and oriented at a low angle to the maximum compressive stress in contrast in nature melt pockets tend to be oriented subparallel to the foliation plane i e presumably at a high angle to the maximum compressive stress

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

strain partitioning during partial melting and crystallizing felsic magmas abstract the rheology of partly molten felsic systems changes dramatically during the onset of melting and through crystallization solid to liquid and liquid to solid transitions are not rheologically identical contradicting the concept of a unique rheological critical melt percentage we use four thresholds separating the successive stages according to the melt fraction for the melting transition 1 the liquid percolation threshold connects the melt pockets at about 8 of melt thus allowing local magma displacement at the metric scale and 2 for greater melt percentages 20 25 a melt escape threshold corresponds to the onset of transport over large distances of the melt and part of the residual solid phase leading ultimately to granitic bodies for the solidification transition 3 the first formation of a rigid framework of crystals which can accumulate stress although the melt can still flow corresponds to the rigid percolation threshold 55 solids and 4 the system becomes totally locked at about 72 75 solid which corresponds to the particle locking threshold the present paper focuses on strain partitioning induced by the coexistence of two contrasting rheological phases between these thresholds on a variety of scales non coaxial deformation is preferentially accommodated by the lower viscosity melt which increases the vorticity in the melt phase and reinforces asymmetries in the flow pattern strain partitioning also increases the scale length of the weakest phase during melting and magma ascent whereas strain partitioning decreases the scale length during crystallizing strain partitioning also affects the migration of melt by raising the particle locking threshold and lowering the melt escape threshold for three dimensional deformations such as transpression the direction of melt segregation e g horizontal may differ from the direction of melt migration e g vertical

an experimental deformation study of partially molten amphibolite application to low melt fraction segregation an amphibolite has been experimentally deformed under subsolidus and partially molten conditions to evaluate the influence of melt on the mechanical behavior of a natural mafic rock and to assess low melt fraction segregation processes experiments were performed under fluid absent conditions at 1 8 gpa between temperatures of 650 c and 1000 c these conditions are similar to those of thickened lower continental crust or intermediate depths in subducting oceanic lithosphere at 850 c melt is granitoid in composition and its viscosity is that of wet granite 103 105 pa s the results can be summarized as follows 1 under subsolidus conditions tests at 650 c and 750 c the amphibolite is macroscopically ductile and deformation is homogeneously distributed throughout the sample 2 at near solidus conditions 800 c 0 5 vol melt fractures 1 10 m in width displace hornblende and plagioclase grains and melt formed in situ is found in some of these cracks the formation of a ductile shear zone in one sample is attributed to the presence of very fine grained reaction products from combined dehydration hydration reactions that involve plagioclase the dehydration reaction products have apparently changed the deformation mechanism and even overrode the melt embrittlement process trapping melt in pockets of lower strain with higher melt fractions 10 15 vol broad melt bearing shear zones form and grains within these zones are brittlely deformed 3 at 20 vol melt 1000 c additional weakening occurs but fractures are not observed and the overall deformation is by viscous flow the results show that at low melt fractions 15 vol fluid absent melting reactions can induce fracture in previously ductile rocks this suggests that the percentage at which melt may escape by fracture is lower than the theoretical critical melt fraction or cmf of 26 40 vol in general estimating the fraction of melt at the onset of segregation cannot be predicted by the cmf melt segregation models have to be adapted to variations in such major factors as pressure temperature type of melting reaction rate of melting and strain rate such models will greatly assist our understanding of continental growth and evolution during orogenesis

melt segregation in the continental crust before melt will segregate from its restite and source there must be both a driving force and space into which the melt can move the continental crust is anisotropic and in most cases is subject to deviatoric stresses during melting under these conditions melt segregation begins when an interconnected network of melt forms and the source becomes permeable melt is driven by pressure gradients from its restite toward dilatant sites set up in the heterogeneously deforming source the source is continually drained of melt and so need never contain more melt than that required to maintain permeability 26 where the solid matrix breaks down and diapirism can occur

diurnal variation and horizontal extent of the low level jet over the northern gulf of california abstract this note describes aspects of the diurnal variation and geographical location of the summertime low level jet over the gulf of california based on wind soundings made at two sites near the northern end of the gulf during a two week summer field program in 1995 the low level wind observations from puerto penasco a site along the northern shore of the gulf showed a strong diurnal cycle decreasing in amplitude to near 0 by the 1800 m altitude a weaker but deeper diurnal cycle was observed at mexicali an inland site some 200 km west northwest of puerto penasco the strongest winds were observed at the coastal site the observations suggest that the strongest southerly winds in the northern gulf lie closer to the eastern shore of the gulf

shear criteria in granite and migmatite deformed in the magmatic and solid states microstructural criteria for the determination of the sense of shear in rocks homogeneously deformed in the partially melted state are similar to those which apply to solid state deformation sense of shear determination is either direct deduced from the sense of rotation of markers or indirect involving the obliquity between the shear and foliation planes or between the successive foliations imprinted at different stages of progressive deformation this study is a by product of the detailed structural and microstructural investigation of a high grade metamorphic rock pile variscan vosges massif france which underwent subhorizontal shearing during partial melting and further solidification depending on the rock chemistry on the position in the pile and the relative timing of progressive deformation layered migmatites and homogeneous granites were variously deformed in the partially melted and solid states the sense of shear obtained from these rock types using the criteria presented here consistently gives a top to sw direction

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

rheology of crystal bearing silicate melts an experimental study at high viscosities the viscosity of partially crystallized mg3al2si3o12 melts has been measured under uniaxial compression in the interval 1010 1013 poise as a function of the volume fraction of crystals these inclusions are well rounded spherulites of aluminous enstatite having the same composition as the melt and whose growth rate is negligible at the temperature of the measurements the viscosity increases by less than 1 order of magnitude for crystal fractions of 40 vol and remains newtonian up to the maximum stress exerted namely 1 kbar the einstein roscoe equation 0 1 m n provides very good fits to the measurements only if either the m or n parameter is allowed to depend on temperature for modeling of magmatic processes however the widely recommended constant values m 0 6 and n 2 5 should be adequate the rheology changes abruptly when the clustered spherulites begin to oppose shear deformation at a crystal fraction of about 40 vol the viscosity becomes non newtonian with yield strengths of a few tens of bars at temperatures at which the viscosity of the melt is higher than 1010 poise as long as the crystal fraction remains lower than 70 vol the deformation proceeds in an irregular manner with a nonuniform distribution of crystals and melt the deformation becomes again regular at low stresses with lower melt fractions but samples undergo extensive fracturation along the direction of uniaxial stress similar rheology changes have been observed during the isothermal crystallization of li2si2o5 melts which produces small ellipsoidal inclusions these results suggest that the influence of solid suspensions on the rheology of magmas is primarily determined by the crystal fraction even though additional measurements would be useful to determine the possible influence of other factors such as the size distribution or the shape of the inclusions

manaslu leucogranite a collision signature of the himalaya a model for its genesis and emplacement the manaslu pluton is one of 10 leucogranites that formed in the overthrusted higher himalaya after the indo eurasian collision field and analytical data indicate that the underlying migmatites of the tibetan slab may be where the leucogranitic melts were generated the himalayan crustal thrusting of a hot slab over a rather cold volcano sedimentary pile le fort 1975a provides the necessary release of fluids these fluids cross the main central thrust mct induce the partial anatexis of the overheated tibetan slab and produce a leucogranitic magma the emplacement of the magma at first is located along the main disharmonic plane above the mct between the infrastructure and the superstructure there it generates a convective hydrothermal system extending very far laterally according to the stratification of permeabilities progressive emplacement of the granite proceeds as the convected fluids including the fluids released by the saturated magma dissolve the mainly calcareous host rocks of tibetan sedimentaries a frozen image of this caving out progression is given by the extensive network of granitic dikes outside the pluton the two quite independant fluid cycles of the generation and of the emplacement were triggered or guided by the tectonics due to collision this two fold model dominated by fluid activity may be of importance for other leucogranites and granites

a model for the rheology of particle bearing suspensions and partially molten rocks an edited version of this paper was published by agu copyright 2009 american geophysical union

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

crustal anatexis and melt migrations in the higo metamorphic terrane west central kyushu kumamoto japan abstract this paper deals with the anatectic migmatites and leucogranites developed in a high t low p metamorphic terrane kyushu japan the migmatites in this terrane are divided into metatexites and diatexites and they occur at and above the garnet cordierite grade layers lenses and pods of leucogranite a few centimeters up to 6 m in thickness are wide spread in the metamorphic terrane and are most abundant not in the highest grade zone but in intermediate grade zones in which migmatites do not develop the migmatite leucosomes are typically depleted in orthoclase component while many leucogranites contain abundant orthoclase it is suggested that the leucogranites are crystal cumulates precipitated from peraluminous melts that had been generated and ascended from deeper levels in the crust and that the zone of metatectic migmatites represents both the source region and passage region for the anatectic melts it is thus clear that the anatectic melts had segregated and ascended on the order of 5 km in the crust of this region diatexites on the other hand represent solid melt mixtures at more advanced stages of partial melting dominated by biotite breakdown implications of the observations made in the higo terrane are discussed in reference to an evolutionary scheme for crustal anatexis and granite magma genesis

importance of melt fraction and source rock composition in crustal genesis the example of two granitic suites of northern portugal in the montalegre area northern portugal two varieties of syn to late tectonic hercynian aluminous granitoids are distinguished 1 1 muscovite rich two mica granites forming many successively emplaced bodies with little chemical variation the chemical characteristics of these units suggest partial melting of various crustal rocks under distinct p t and water saturation conditions there has been no homogenization between the different melt batches the rocks contain very few restitic minerals segregation processes may have operated either in the source rock or in an early magmatic stage 2 2 a cordierite biotite and muscovite bearing granitic body its mineralogical composition is extremely heterogeneous and it contains many xenoliths the genesis of this granite results essentially from partial melting of peraluminous orthogneisses the melt fraction was higher than the rheological critical melt percentage rcmp during anatexis the rigid framework of the source rock broke down leading to migration of both melt and residual minerals towards upper crustal levels the heterogeneity of the resulting granite is mostly related to an incomplete separation of residual crystals and melt the existence of biotite and cordierite rich samples is the result of accumulation of residual minerals during the magma emplacement this study emphasizes the role of the source rock composition which controls the degree of partial melting at given p t and water saturation conditions during crustal melting the strong geochemical differences between the two kinds of granitoids are related to the near absence of residual minerals in the muscovite rich granites and to the presence of residual minerals in the cordierite bearing granite it is suggested that the muscovite rich granites are mostly low melt fraction rcmp the presence of many residual crystals alkali feldspars and plagioclases as well as dark minerals in the cordierite granite implies a high viscosity of the melts and therefore water undersaturated melting conditions

imaging a low viscosity zone beneath the kutcharo caldera eastern hokkaido japan using geodetic data abstract geodetically observed volcano deformation has been analysed in terms of either elastic or viscoelastic crustal rheology for which the presence of magma has usually been considered as a deformation source from a rheological viewpoint however a magma and the peripheral rocks it influences are presumably low viscosity materials which allows detection of the presence of magma as a zone that has rheologically less strength in this study using a 3 dimensional finite element model that introduces a rectangular low viscosity zone lvz into relatively high viscosity crust the response of the linear maxwell viscoelastic crust to the emplacement of a spheroidal sill is adopted for interferometric synthetic aperture radar insar data of the kutcharo caldera eastern hokkaido japan to image a spatial crustal viscosity variation the observed temporal displacement at the surface point where the line of sight los displacement is largest in the deformation field requires the lvz to have a spatially uniform viscosity of 4 5 10 17 pa s the lvz is also required to be present immediately below a 5 km elastic layer down to mid crustal depths or deeper with a horizontal extent of at least 10 km models with acceptable minimum sizes of lvz are found to satisfy the overall temporal displacement field with root mean square misfits of less than 2 cm the geodetic data reasonably accept the presence of an lvz beneath the kutcharo caldera that is consistent with the magnetotelluric image of the accumulated magma providing the mechanical aspect of the low resistivity domain the lvz model behaviour suggests an important implication that the accumulation of magma in the middle to lower crust is detectable using geodetic data after magma emplacement into the upper crust where the deformation rate represents the degree of magmatic influence on the crustal viscosity

melt segregation in migmatites anatectic stromatic migmatites have a symmetrical layered structure with a low ratio of thickness to length and a periodicity features that have not been explained satisfactorily but which are related to physical processes of melt segregation we evaluate the compaction model for segregation as it applies to migmatites and develop models for melt segregation based upon convection driven by volume change and upon advection down pressure gradients that result from applied differential stress acting on an anisotropic multilayer protolith compaction by gravity driven two phase flow results in asymmetric separation yields calculated segregation times that are slow or geologically unreasonable and yields relative volumes that are not consistent with the abundance of leucosome seen in natural migmatites although calculated segregation times for very wet granite might allow segregation by compaction some driving force in addition to gravity is needed to cause widespread melt segregation in the crust for segregation at low volume fraction of melt we develop a two dimensional two phase matrix and fluid viscous flow cell model length much greater than thickness with phase changes which predicts either expansion or contraction convection depending on whether volume change vr is positive or negative convection leads to melt segregation to form a stromatic structure in a geologically reasonable timescale at moderate volume fraction of melt segregation may occur by filter pressing in compositionally layered rocks in response to applied differential stress melt migration is by porous medium flow driven by differences in mean normal stress between the layers as a consequence of differences in rheology and shear enhanced matrix collapse calculated segregation rates are fast and the model yields adequate volume of leucosome the positive vr for volatile phase absent melting reactions under crustal pressures promotes melt escape unless extensional deformation facilitates melt accumulation if the rate of melt production exceeds the rate of melt escape then the increase in melt pressure may lead to melt enhanced embrittlement and fracture and melt may migrate out of the system in contrast water rich volatile phase present melt producing reactions have a negative vr which promotes melt retention after segregation unless deformation generates melt escape pathways evolution of stromatic structure through anatectic erosion of mesosome by increasing the melt fraction in situ may lead to breakdown of the solid matrix and will lead to instability due to buoyancy and magma may become mobile and entrain residual material restite

some comments on the rheologically critical melt percentage abstract the concept of rheologically critical melt percentage rcmp originally proposed by arzi tectonophysics 44 1978 173 184 for partially molten granitic rocks is re examined it is shown that there is no experimental support to show the presence of rcmp the published experimental data suggest that the effective viscosity of partially molten granitic rocks is reduced rapidly and continuously with increasing melt fraction it is also shown that the experimental data may be modeled by means of the upper bound behavior the voigt bound of two phase material by assuming a melt localization which implies that there is no partitioning of strain between the solid and the melt

controls on low pressure anatexis low pressure anatexis whereby rocks melt in place after passing through the andalusite stability field develops under more restricted conditions than does low pressure metamorphism our thermal modelling and review of published work indicate that the following mechanisms operating alone may induce anatexis in typical pelitic rocks without inducing wholesale melting in the lower crust i magmatic advection by pervasive flow ii crustal scale detachment faulting and iii the presence of a high heat producing layer of these only magmatic advection by pervasive flow and crustal scale detachment faulting have been shown quantitatively to provide sufficient heat to cause widespread melting combinations of the above mechanisms with pluton scale magmatic advection shear heating removal of the lithospheric mantle or with each other provide additional means of developing suitable high temperatures at shallow crustal levels to generate low pressure anatexis

observations of infrasound and subsonic disturbances related to severe weather summary during the past 10 years the geoacoustics group of noaa s wave propagation laboratory studied travelling low frequency pressure variations related to thunderstorms and severe weather two general categories of waves were associated with severe weather conditions subsonic pressure disturbances and infrasonic waves with acoustic velocities the low frequency pressure variations were measured at the earth s surface using microphone arrays located at times thousands of kilometres from the severe weather disturbance the radiated infra sound was related to thunderstorms penetrating the tropopause and spectral analyses were performed on several signals possible practical applications to storm warning and classification are discussed for both infrasound and subsonic pressure disturbances past measurements of these signals are reviewed

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

experimental deformation of partially melted granitic aggregates abstract the effects of varying amounts of partial melt on the deformation of granitic aggregates have been tested experimentally at conditions 900 c 1500 mpa 10 4 to 10 6 s where melt free samples deform by dislocation creep with microstructures approximately equivalent to those of upper greenschist facies experiments were performed on samples of various grain sizes including an aplite 150 m and sintered aggregates of quartz albitemicrocline 10 50 and 2 10 m water was added to the samples to obtain various amounts of melt 1 15 in the aplite 1 5 in the sintered aggregates optical and tem observations of the melt distribution in hydrostatically annealed samples show that the melt in the sintered aggregates is homogeneously distributed along an interconnected network of triple junction channels while the melt in the aplites is inhomogeneously distributed the effect of partial melt on deformation depends an melt amount and distribution grain size and strain rate for samples deformed with 1 melt all grain sizes exhibit microstructures indicative of dislocation creep for samples deformed with 3 5 melt the 150 m and 10 50 m grain size samples also exhibit dislocation creep microstructures but the 2 10 m grain size samples exhibit abundant tem scale evidence of dissolution precipitation and little evidence of dislocation activity suggesting a switch in deformation mechanism to predominantly melt enhanced diffusion creep at natural strain rates melt enhanced diffusion creep would predominate at larger grain sizes although probably not for most coarse grained granites the effects of melt percentage and strain rate have been studied for the 150 m aplites for samples with 5 and 10 melt deformation at 10 6 s squeezes excess melt out of the central compressed region allowing predominantly dislocation creep conversely deformation at 10 5 s produces considerable cataclasis presumably because the excess melt cannot flow laterally fast enough and a high pore fluid pressure results for samples with 15 melt deformation at both strain rates produces cataclasis presumably because the inhomogeneous melt distribution resulted in regions of decoupled grains which would produce high stress concentrations at point contacts at natural strain rates there should be little or no cataclasis if an equilibrium melt texture exists and if the melt can flow as fast as the imposed strain rate however if the melt is confined and cannot migrate a high pore fluid pressure should promote brittle deformation