the collapse of the sella zerbino gravity dam abstract when a severe flood wave completely filled the ortiglieto reservoir on august 13 1935 the 14 m high sella zerbino secondary dam failed catastrophically causing 100 casualties both of the dams sella zerbino zerbino saddle and bric zerbino zerbino peak fig 1 were overtopped but only the sella zerbino failed whereas the main barrage did not suffer any damage the lawsuit that followed this tragic event ended with a full acquittal of the dam s designers since the plaintiff experts succeeded in demonstrating that the collapse was due to an extreme rainfall storm of unpredictable intensity the case was then officially closed and still today the failure of the sella zerbino dam is attributed to the unpredictable hydrological event recently natale and petaccia 2013 re examined the case assessing the capacity of the flood spillways which equipped the bric zerbino dam this paper thoroughly reviews the mechanics of the collapse of the sella zerbino dam focusing on the stability of the structure the water pressure underneath the dam and the poor quality of the foundation rock is believed to have played a major role in the sequence of events that ended in the collapse of the barrage

unpairing metamorphic belts p t paths and a tectonic model for the ryoke belt southwest japan southwest japan is divided into outer and inner zones by the median tectonic line mtl a major transcurrent fault the outer zone is composed of the sambagawa high pressure intermediate or high p t type metamorphism chichibu and shimanto belts in the inner zone the ryoke belt andalusite sillimanite or low p t type metamorphism was developed mainly within a jurassic accretionary complex this spatial relationship between high p t type and low p t type metamorphic belts led miyashiro to the idea that metamorphic belts were developed as paired systems textural relationships and petrogenetically significant mineral assemblages in pelites from the ryoke belt imply peak p t conditions of 5 kbar and up to 850 c in migmatitic garnet cordierite rocks from the highest grade metamorphic zone it is likely that the thermal anomaly responsible for metamorphism of the ryoke belt was related to a segment of the farallon izanagi ridge as it subducted under the eastern margin of the asian continent during the cretaceous the sequence of mineral assemblages developed in pelites implies a metamorphic field gradient with shallow dp dt slope inferred to have been generated by a nested set of hairpin like clockwise p t paths these p t paths are characterized by limited prograde thickening minor decompression at peak t and near isobaric cooling features that may be typical of p t paths in low p t type metamorphic belts caused by ridge subduction a ridge subduction model for the ryoke belt implies that juxtaposition of the high p t metamorphic rocks of the sambagawa belt against it was a result of terrane amalgamation belt parallel ductile stretching recorded as syn metamorphic predominantly constrictional strain in both ryoke and sambagawa belt rocks and substantial sinistral displacement on the mtl are consistent with left lateral oblique convergence diachroneity in fast cooling of the ryoke belt is implied by extant thermochronological data and is inferred to relate to progressive sw to ne docking of the sambagawa belt thus an alternative interpretation of paired metamorphic belts in japan is that they represent laterally contemporaneous terranes rather than outboard and inboard components of a trench arc paired system amalgamation of laterally contemporaneous terranes during large translations of forearcs along continental margins may explain other examples of paired metamorphic belts in the geological record

modelling granite migration by mesoscale pervasive flow abstract mesoscale pervasive magma migration leads to granite injection complexes common in hot crustal terranes pervasive migration is limited by magma freezing when intruding cold country rock here we explore numerically the feedback mechanism between magma intrusion and heating of the country rock which allows younger intrusive batches to reach increasingly shallower cooler levels this process relies on the higher solidus temperature of a rock compared to that of its melt once melt is segregated we define the free ride layer as the region above the melt source where magma may freely migrate because rock temperature is above melt solidus the top of the free ride layer which corresponds to the melt solidus t s isotherm is at the limiting depth z s after magma passes through the free ride layer the magma front is always at the limiting depth we modeled the thickening and heating of the crust above the source as melt at its liquidus t l intrudes it pervasively from below we found that a magma quickly warms crust below z s to about t l forming a step in temperature at z s b the front z s moves up through the crust as more magma is intruded c as magma is emplaced at the front a mingled layer of about half magma half crust forms below it so that the total rise of the front corresponds approximately to half of the thickness of magma added to the free ride layer d the rate of rise of the front depends on the temperature difference between crust and t l and slows down as the magma front rises e for most reasonable intrusion rates and volumes the crust above z s feels little influence of the intrusion because the diffusion time scale is much smaller than the rise rate of the front in summary pervasive migration is an efficient way of heating the lower to middle crust and can result in an injection complex several kilometers thick consisting of about half magma and half original crust

pervasive granitoid magma transfer through the lower middle crust during non coaxial compressional deformation granitic magmas migrated through early proterozoic middle lower crust at mt hay central australia via a diverse network of narrow structurally controlled channelways during a period of progressive w sw directed thrusting d1a d1d they utilized existing folds boudins and shear zones or created new channels by magmatic fracture either parallel to layering or rarely in irregular arrays the magmas rose obliquely parallel to the plunging 50 60 regional elongation direction which was defined by coaxial folds boudin necks and a strong mineral elongation lineation megacrystic charnockitic magmas migrated through metre scale conduits during d1a d1b but leucosomes were generally restricted to smaller centimetre scale structures that existed throughout the entire deformation history thus d1a d1b leucosomes were potential feeders of in situ partial melts to the adjacent larger conduits of charnockite magma thereby providing a pervasive interconnected network that allowed efficient migration of all magma types during the early stages of thrusting the upper middle crust of the anmatjira reynolds range area contains abundant megacrystic granitoid sheets that are of similar age and geochemistry to those at mt hay they are considered to have formed as syntectonic intrusions emplaced during w sw directed thrusting as at mt hay suggesting that granitic magmas formed near the base of the continental crust passed through the mid lower crustal level 25 30 km exposed at mt hay and accumulated in batholithic proportions at shallower crustal levels 12 20 km such as the anmatjira reynolds range area the observations imply that granitoid magmas in the deep crust are capable of pervasive migration through the crust during major compressive noncoaxial shear deformation localization of magmas by sequentially developed narrow compressive structures suggests that dilatancy followed successive foliation forming events a situation that can occur during steady state deformation if the effective confining pressures are low which would be a result of high and possibly variable rates of magma influx the inferred rapid melt segregation and migration during deformation suggest that large chambers do not form until magma reaches neutral buoyancy in the middle to upper continental crust

triassic 40ar 39ar ages from the sakaigawa unit kii peninsula japan implications for possible merger of the central asian orogenic belt with large scale tectonic systems of the east asian margin the 218 4 0 4 228 8 0 9 and 231 9 0 7 ma 40ar 39ar laser probe pseudo plateau ages 2 49 63 39ar release of very low grade meta pelitic whole rocks from the sakaigawa unit date high p t metamorphism we argue that this event occurred in a subduction accretion complex not along the east asian continental margin but on the pacific side of the proto japan superterrane proto japan was a permian magmatic arc presently dispersed in the japanese islands which also contained older subduction accretion complexes the arc system was fringing but not yet part of the eurasian continent the middle to late triassic high p t tectono metamorphic event was partly coeval with proto japan s collision with proto eurasia along the southward extension of the central asian orogenic belt causing the main metamorphism in the hida oki terrane it is possible that this system continued via the cathaysia block china to indochina the late permian to middle triassic indosinian event might stem from docking of pacific derived terranes with southeast asia s continental margin the concept of the proto japan superterrane implies that the qinling dabie sulu suture zone joined the central asian orogenic belt to the east of the north china craton and did not continue to japan as commonly assumed

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

low p high t metamorphism and the role of heat transport by melt migration in the higo metamorphic complex kyushu japan this paper characterizes the metamorphic thermal structure of the higo metamorphic complex hmc and presents the results of a numerical simulation of a geotherm with melt migration and solidification reconstruction of the geological and metamorphic structure shows that the hmc initially had a simple thermal structure where metamorphic temperatures and pressures increased towards apparent lower structural levels subsequently this initial thermal structure has been collapsed by e w and nne ssw trending high angle faults pressure and temperature conditions using the analysis of mineral assemblages and thermobarometry define a metamorphic field p t array that may be divided into two segments the array at apparent higher structural levels has a low dp dt slope whereas that at apparent lower structural levels has a high dp dt slope this composite array cannot be explained by heat conduction in subsolidus rocks alone migmatite is exposed pervasively at apparent lower structural levels but large syn metamorphic plutons are absent at the levels exposed in the hmc transport and solidification of melt within migmatite is a potential mechanism to generate the composite array thermal modelling of a geotherm with melt migration and solidification shows that the composite thermal structure may be formed by a change of the dominant heat transfer from an advective regime to a conduction regime with decreasing depth the model also predicts that strata beneath the crossing point will consist of high grade solid metamorphic rocks and solidified melt products such as migmatite this prediction is consistent with the observation that migmatite was associated with the very high dp dt slope the melt migration model is able to generate the very high dp dt segment due to the high rate of heat transfer by advection

valid garnet biotite gb geothermometry and garnet aluminum silicate plagioclase quartz gasp geobarometry in metapelitic rocks abstract at present there are many calibrations of both the garnet biotite gb thermometer and the garnet aluminum silicate plagioclase quartz gasp barometer that may confuse geologists in choosing a reliable thermometer and or barometer to test the accuracy of the gb thermometers we have applied the various gb thermometers to reproduce the experimental data and data from natural metapelitic rocks of various prograde sequences inverted metamorphic zones and thermal contact aureoles we have concluded that the four gb thermometers perchuk l l lavrent eva i v 1983 experimental investigation of exchange equilibria in the system cordierite garnet biotite in saxena s k ed kinetics and equilibrium in mineral reactions springer verlag new york berlin heidelberg pp 199 239 kleemann u reinhardt j 1994 garnet biotite thermometry revised the effect of alvi and ti in biotite european journal of mineralogy 6 925 941 holdaway m j 2000 application of new experimental and garnet margules data to the garnet biotite geothermometer american mineralogist 85 881 892 model 6av kaneko y miyano t 2004 recalibration of mutually consistent garnet biotite and garnet cordierite geothermometers lithos 73 255 269 model b are the most valid and reliable of this kind of thermometer more specifically we prefer the holdaway holdaway m j 2000 application of new experimental and garnet margules data to the garnet biotite geothermometer american mineralogist 85 881 892 and the kleemann and reinhardt kleemann u reinhardt j 1994 garnet biotite thermometry revised the effect of alvi and ti in biotite european journal of mineralogy 6 925 941 calibrations due to their small errors in reproducing the experimental temperatures and good accuracy in successfully discerning the systematic temperature changes of the different sequences in addition after applying the gasp barometer to 335 natural metapelitic samples containing one kind of aluminosilicate and 33 natural metapelitic samples containing two kinds of aluminosilicate as well as to rocks in limited geographic areas and thermal contact aureoles we propose that the calibrations of holdaway holdaway m j 2001 recalibration of the gasp geobarometer in light of recent garnet and plagioclase activity models and versions of the garnet biotite geothermometer american mineralogist 86 1117 1129 and newton and haselton newton r c haselton h t 1981 thermodynamics of the garnet plagioclase al2sio5 quartz geobarometer in newton r c navrotsky a wood b j eds thermodynamics of minerals and melts new york springer verlag 131 147 based on kleemann and reinhardt s kleemann u reinhardt j 1994 garnet biotite thermometry revised the effect of alvi and ti in biotite european journal of mineralogy 6 925 941 thermometer are the most valid gasp barometers other gb thermometers and gasp barometers are not recommended

geothermometry geobarometry and the al2sio5 triple point at mt moosilauke new hampshire the distribution of aluminum silicate minerals in pelitic schists constrains pressure and temperature conditions during the acadian orogeny to have been approximately those of the al 2 sio 5 invariant point thus providing an excellent opportunity to evaluate the internal consistency of a variety of published geothermometers and geobarometers with the al 2 sio 5 invariant point of holdaway 1971 four fluid independent equilibria yield p t estimates consistent with holdaway 1971 two fluid dependent equilibria yield internally consistent but unrealistically high temperatures our data are consistent with the conclusions that 1 only pyrope grossular mixing in the quaternary garnet system is significantly non ideal and 2 the activity coefficient for the anorthite component in plagioclase is approximately 2 at triple point p t conditions modified journal abstract

hypotheses for the origin of the hypanis fan shaped deposit at the edge of the chryse escarpment mars is it a delta abstract we investigated the origin of the fan shaped deposit at the end of hypanis valles that has previously been proposed as an exomars mars 2020 and human mission candidate landing site and found evidence that the landform is an ancient delta previous work suggests that the deposit originated from a time of fluvial activity both distinct from and prior to catastrophic outflow and crater counting placed the deposit s age at 3 6 ga we found over 30 thin sedimentary strata in the proposed delta wall and from our slope analysis conclude that the fluvial sequence is consistent with a lowering retreating shoreline we measured nearly horizontal bedding dip angles ranging from 0 to 2 over long stretches of cliff and bench exposures seen in hirise images and hirise stereo dtms from themis night ir images we determined that the fan shaped deposit has a low thermal inertia 150 240 jm 2k 1s 1 2 and the surrounding darker toned units correspond to thermal inertia values as high as 270 390 jm 2k 1s 1 2 we interpret these findings to indicate that the fan shaped deposit consists mostly of silt sized and possibly finer grains and that the extremely low grade and large lateral extent of these beds implies that the depositional environment was calm and relatively long lived our assessment of the geomorphology and composition seems to indicate that the alluvial fan and mudflow hypotheses are less compatible from our stratigraphic mapping we interpret the order of events which shaped the region after the chryse impact sediment filled the basin a confined lake or sea formed allowing a large delta to be deposited near its shoreline the water level receded to the north darker sedimentary volcanic units covered the region and capped the light toned deposit as hydro volcanic eruptions shaped the interior of lederberg crater freeze thaw cycles and desiccation induced local fracturing and finally wrinkle ridges associated with rounded cones warped the landscape following trends in degraded crater rims and existing tectonic features the ancient deltaic deposit we observe today was largely untouched by subsequent catastrophic outflows and its surface has been only moderately reshaped by over 3 billion years of aeolian erosion

electrical conductivity measurement of gneiss under mid to lower crustal p t conditions abstract we conducted electrical conductivity measurements perpendicular and parallel to mineral foliation in dry gneiss at up to 1000 k and a constant pressure of 1 gpa the analyzed gneisses were obtained from the higo metamorphic belt kyushu japan as the metamorphic conditions of these rocks have been well determined by previous studies we were able to select samples that were representative of the middle to lower crust prior to the conductivity measurements the samples were maintained at the maximum temperature for a long period until the electrical conductivity had stabilized our experiment results reveal linear and reproducible conductivity data between temperatures of 600 and 1000 k conductivity measured perpendicular and parallel to foliation differ by an order of magnitude over the same temperature window a plausible explanation for this discrepancy in conductivity is the contrasting configuration of minerals in the two sample orientations as observed by backscattered electron image bei and electron probe microanalysis epma we evaluated the conductivity and computed activation energy for each of the samples and compared the results with those of previous studies our results are consistent with the conductivity values reported for other types of rocks we also compared the experiment results with data derived from electromagnetic em soundings electrical conductivity measurements undertaken perpendicular to foliation can account for the subsurface conductivity structure beneath central kyushu japan

phase equilibria and the pressure temperature path of the highest grade ryoke metamorphic rocks in the yanai district sw japan the garnet cordierite zone the highest grade zone of the ryoke metamorphic rocks in the yanai district sw japan is defined by the coexistence of garnet and cordierite in pelitic rocks three assemblages in this zone are studied in detail i e spinel cordierite biotite garnet cordierite biotite and garnet biotite all of which contain quartz k feldspar and plagioclase the mg fe mg in the coexisting minerals decreases in the following order cordierite biotite garnet and spinel two facts described below are inconsistent with the paragenetic relation in the k2ofeomgoal2o3sio2h2o kfmash system in terms of an isophysical variation first garnet and biotite in the last assemblage have mg fe mg higher than those in the second second the first two assemblages are described by the reaction

large scale melt depletion in granulite terranes an example from the archean ashuanipi subprovince of quebec this study uses field petrographic and geochemical methods to estimate how much granitic melt was formed and extracted from a granulite facies terrane and to determine what the grain and outcrop scale melt flow paths were during the melt segregation process the ashuanipi subprovince located in the north eastern superior province of quebec is a large 90 000 km2 metasedimentary terrane in which 85 of the metasediments are of metagreywacke composition that was metamorphosed at mid crustal conditions 820 900 c and 6 7 kbar in a late archean dextral transpressive orogen decrease in modal biotite and quartz as orthopyroxene and plagioclase contents increase together with preserved former melt textures indicate that anatexis was by the biotite dehydration reaction biotite quartz plagioclase melt orthopyroxene oxides using melt orthopyroxene ratios for this reaction derived from experimental studies the modal orthopyroxene contents indicate that the metagreywacke rocks underwent an average of 31 vol partial melting the metagreywackes are enriched in mgo cao and feot and depleted in sio2 k2o rb cs and u have lower rb sr higher rb cs and th u ratios and positive eu anomalies compared to their likely protolith these compositions are modelled by the extraction of between 20 and 40 wt granitic melt from typical archean low grade metagreywackes a simple mass balance indicates that about 640 000 km3 of granitic melt was extracted from the depleted granulites the distribution of relict melt at thin section and outcrop scales indicates that in layers without leucosomes melt extraction occurred by a pervasive grain boundary porous flow from the site of melting across the layers and into bedding planes between adjacent layers in other rocks pervasive grain boundary flow of melt occurred along the layers for a few to tens of centimetres followed by channelled flow of melt in a network of short interconnected and structurally controlled conduits visible as the net like array of leucosomes in some outcrops the leucosomes contain very little residual material 5 biotite orthopyroxene indicating that the melt fraction was well separated from the residuum left in situ as melt depleted granulite only 1 3 vol percentage melt remained in the melt depleted granulites hence the extraction of melt generated by biotite dehydration melting in these granulites was virtually complete under conditions of natural melting and strain rates in a contractional orogen

high radiogenic heat producing granites and metamorphism an example from the western mount isa inlier australia the origins of metamorphism 600 c and 3 4 kbar in the western mount isa inlier australia remain controversial for a number of reasons 1 no synmetamorphic intrusive bodies can be recognized 2 high temperatures appear to be sustained for periods 100 m y and 3 metamorphism follows an extended phase of thermal subsidence we show that the burial of granite batholiths enriched in radiogenic elements beneath the thick insulating sedimentary succession of the mount isa group deposited in response to repeated rift sag cycles was capable of generating steep upper crustal thermal gradients immediately prior to the isan orogeny these gradients are appropriate to peak metamorphic conditions such that the ensuing isan orogeny required no significant additional heat input this result is significant in that it may provide a mechanism for understanding the origins of high temperature metamorphism in other terranes where the involvement of transient heating is not obvious

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

development of migmatites and the role of viscous segregation in high t metamorphic complexes example from the ryoke metamorphic complex mikawa plateau central japan abstract the ryoke metamorphic complex of the mikawa plateau central japan contains well preserved geological and thermal structures formed during cretaceous high temperature high t metamorphism these structures can be explained by a pervasive melt migration model however the estimated flux of melt is much higher than that expected for compaction flow of felsic melt an additional tectonic force and or melt segregation to network of veins or layers is therefore required this paper presents the results of numerical modeling of the structural evolution of immiscible viscous fluids under simple shear deformation and discusses the role of viscous segregation in enabling pervasive melt flow in hot crust the results of numerical modeling of simple shear deformation of immiscible viscous fluids reveal that low viscosity fluids segregate into lenses and layers this segregation is a type of viscous segregation that takes place to minimize the dissipation of energy in the system the rate of energy dissipation is proportional to viscosity therefore this process is expected to occur universally in immiscible viscous fluids with large viscosity contrasts and is an important process in focusing melt in partially melted rocks the structures produced by the numerical simulation are similar to those of migmatites the simulation results suggest that melts distributed randomly in partially melted rock will segregate into lenses or layers with progressive simple shear deformation during deformation enhanced segregation a melt network will form dynamically if this process occurs at small to large scales in the hot crust melts will be effectively transported upward due to buoyancy forces it is possible that pervasive migmatites at low structural levels within high t metamorphic complexes may represent fossil melt pathways formed by crustal scale viscous segregation

contrasting ages between isotopic chronometers in granulites monazite dating and metamorphism in the higo complex japan abstract the higo complex of west central kyushu western japan is a 25 km long body of metasedimentary and metabasic lithologies that increase in metamorphic grade from schist in the north to migmatitic granulite in the south where granitoids are emplaced along the southern margin the timing of granulite metamorphism has been extensively investigated and debated previously published sm nd mineral isochrons for garnet bearing metapelite yielded ca 220 280 ma ages suggesting high grade equilibration older than the lower grade schist to the north which yielded ca 180 ma k ar muscovite ages ion and electron microprobe analyses on zircon have yielded detrital grains with rim ages of ca 250 ma and ca 110 ma electron microprobe ages from monazite and xenotime are consistently 110 130 ma two models have been proposed 1 high grade metamorphism and tectonism at ca 115 ma with older ages attributed to inheritance and 2 high grade metamorphism at ca 250 ma with resetting of isotopic systems by contact metamorphism at ca 105 ma during the intrusion of granodiorite these models are evaluated through petrographic investigation and electron microprobe th u total pb dating of monazite in metapelitic migmatites and associated lithologies in situ investigation of monazite reveals growth and dissolution features associated with prograde and retrograde stages of progressive metamorphism and deformation monazite th u pb isochrons from metapelite diatexite and late deformational felsic dykes consistently yield ca 110 120 ma ages earlier and later stages of monazite growth cannot be temporally resolved the preservation of petrogenetic relationships coupled with the low diffusion rate of pb at

1923 gleno dam break case study and numerical modeling on the morning of december 1 1923 the gleno dam located in the central italian alps suddenly collapsed a few days after the first complete reservoir filling nearly 4 5 1 0 6 m 3 of water was released the consequent inundation caused significant destruction along the downstream valley and a death toll of at least 356 lives this failure is the only historical case of dam break caused by structural deficiencies that has occurred in italy as a result it has deeply influenced the evolution of italian regulations regarding dam design and hydraulic risk evaluation however in spite of its relevance this event has never been characterized from a hydraulic standpoint this paper reports the main information obtained from the analysis of a vast amount of historical documents regarding the gleno dam break to set up a case study useful for validating dam break models in mountain settings moreover it presents the main results of one dimensional 1d modeling of the dam break wave propagation accomplished with a first order finite volume numerical scheme recently proposed in the literature for field applications the overall effectiveness and reliability of the model are evaluated for this case characterized by very irregular topography finally the practical relevance of several choices that the numerical reconstruction of this kind of event demands is tested doi 10 1061 asce hy 1943 7900 0000327 2011 american society of civil engineers ce database subject headings dam failures shallow water numerical models hydraulic models case studies italy author keywords dam failure 1d dam break modeling shallow water finite volume numerical model hydraulic hazard case study

thermal modelling in low pressure high temperature metamorphic belts abstract low pressure high temperature metamorphic belts occur throughout the world they display a characteristic metamorphic style which has remained unchanged with time they are found in disparate tectonic settings including magmatic arcs regions of extension regions of thickened crust in continent continent collision zones and accretionary wedges they are commonly characterized by an abundance of granitoid plutons pressures of metamorphism are primarily less than or equal to that of the aluminosilicate triple point approximately 4 kbar while temperatures are in the range 500 750 c with rare exceptions inferred ptt paths are nearly isobaric and recrystallization was rapid and prograde metamorphic thermal gradients are commonly in the range 60 150 c km these conditions necessitate the presence of a large component of advective heat transfer and or anomalously high basal heat flow during metamorphism the large magmatic fluxes associated with the formation of magmatic arcs provide an adequate heat source for low pressure metamorphism lpm lithospheric extension is likely to produce lpm only if the degree of thinning is large 3 and there is additional heat from magmas the generation of lpm by crustal thickening requires either a large component of advective heating by magmas anomalously high basal heat flow or pre thickening lithospheric extension lpm in this environment must necessarily be accompanied by plutonic activity advection of heat by aqueous fluids may provide a significant contribution to the thermal budget but acting along is unlikely to produce lpm regional lpm induced by advective sources rarely occurs along a geotherm which increases monotonically with depth or synchronously across the region at a given depth it occurs over time as a series of overlapping temporally separate and spatially localized thermal events

thermal modelling in shallow subduction an application to low p t metamorphism of the cretaceous shimanto accretionary complex japan this paper presents the results of numerical modelling to investigate the regional occurrence of prehnite bearing metamorphic rocks at shallow levels in subduction zones the modelling assumes a simple geometrical configuration in which the thermal structure in a prism is controlled by boundary conditions at the top and base of the prism it is expected that the predominant metamorphic facies in a prism will change with decreasing age of the descending slab the results of thermal modelling show that the facies boundary between pumpellyite actinolite and prehnite actinolite facies including prehnite pumpellyite facies overlaps with an array of p t conditions in the prism when the age of a descending slab is younger than 10 myr this implies that the change of the predominant metamorphic facies from pumpellyite actinolite to prehnite actinolite facies will switch drastically the critical age of the switch depends on subduction parameters in particular the critical age of the descending slab is 5 myr in the case of no shear heating with a subduction rate of v 75 200 mm y 1 and subduction angle of 5 15 for shear heating constant shear stress 30 mpa with a subduction rate of v 75 mm y 1 and subduction angle of 10 the critical age is 7 myr to test this switching behaviour in the development of prehnite actinolite facies in the prism petrologic data from the cretaceous shimanto accretionary complex csac in kyushu japan were compiled the regional occurrence and mineral assemblages of prehnite bearing metamorphic rocks suggest that the most of csac was metamorphosed under prehnite actinolite facies this conclusion is consistent with subduction of a young hot slab as has been proposed based on other geological observations this suggests that the regional extent of the prehnite actinolite facies metamorphic rocks may be a unique evidence for the subduction of a young hot slab

mesoscale pervasive felsic magma migration alternatives to dyking abstract this paper reviews the literature on dyking as a mechanism of felsic magma extraction from a source and transport to shallower crustal levels and review the recent literature suggesting a range of alternative mechanisms of magma migration in hot crustal zones which produce mesoscale pervasive granite sheet intrusions recent papers have strongly favoured dyking as the main mechanism controlling magma migration however the initiation of dykes from a felsic magma source is fraught with difficulties even when magma is immediately available for transportation as in magma chambers within a partially molten source magma may reside in a range of structures with a wide range of shapes sizes and degrees of connectivity whereas the growth of individual dykes within a partially molten zone and the self propagation of large dykes into subsolidus crust have both been studied in some detail little attention has been given to the crucial intermediate step of the growth of a dyke network capable of producing wide crustal scale dykes the rarity of granite dyke swarms suggests that if dyking is the preferred mechanism of magma transport felsic magma sources produce only few major transporting dykes during their lifetime alternatively dyking is not an important mechanism the parameters controlling the volume of the catchment drained by one such dyke as well as other basic geometrical parameters controlling the structure of the dyke network within the source are unknown the ability of dyking to drain a partially molten source depends crucially on these variables and particularly on the horizontal permeability of the source the slow velocity of viscous felsic magmas traveling in rock pores implies that magma drained during dyking is mostly that previously extracted from the pores and resident in irregular magma bodies or dyke networks the observation that large volumes of buoyant magma are commonly present in migmatite zones and that dyking in these zones plays a secondary role suggest that dyking is inefficient and is able to extract only a fraction of the total melt available in the source in support of this conclusion recent detailed studies of exhumed hot crustal zones have revealed a range of alternative migration mechanisms characterized by mesoscale pervasive magma flow outcrop scale as opposed to porous flow pervasive migration gives rise to magma sheets preferentially emplaced parallel to high permeability zones such as foliation or bedding planes apart from local dyking three alternative mechanisms have been proposed to account for pervasive migration of magmas namely tectonic pumping magma wedging into low viscosity rocks and volatile driven intrusion because of their unfocused pervasive character these mechanisms are restricted to hot crust where magmas are not exposed to rapid freezing a model is proposed whereby heat advected with the magma pushes crustal geotherms upwards allowing pervasive magma migration to shallower depths

petrogenesis of metatexite and diatexite migmatites determined using zircon u pb age trace element and hf isotope data higo metamorphic terrane central kyushu japan metatexite and diatexite migmatites are widely distributed within the upper amphibolite and granulite facies zones of the higo low p high t metamorphic terrane here we report data from an outcrop in the highest grade part of the granulite facies zone in which diatexite occurs as a 3 m thick layer between 2 m thick layers of stromatic structured metatexite within pelitic gneiss the migmatites and gneiss contain the same peak mineral assemblage of biotite plagioclase quartz garnet k feldspar with retrograde chlorite muscovite and some accessory minerals of ilmenite rutile titanite apatite zircon monazite pyrite zinc sulphide calcite calculated metamorphic p t conditions are 800 900 c and 9 12 kbar zircon in the diatexite forms elongate euhedral crystals with oscillatory zoning but no core rim structure zircon from the gneiss and metatexite forms euhedral subhedral grains comprising inherited cores overgrown by thin rims the overgrowth rims in the metatexite have lower th u ratios than zircon in the diatexite and yield a 206pb 238u age of 116 0 1 6 ma which is older than the 110 1 0 6 ma 206pb 238u age derived from zircon in the diatexite zircon from the diatexite has variable ree contents with convex upward patterns and flat normalized hree whereas the overgrowth rims in the metatexite and gneiss have steep hree enriched patterns however both types have similar positive ce and negative eu anomalies 176hf 177hf ratios in the overgrowth rims from the metatexite are more variable and generally lower than values from zircon in the diatexite based on u pb ages trace element and hf isotope data the zircon rims in the metatexite are interpreted to have crystallized from a locally derived melt following partial dissolution of inherited protolith zircon during anatexis whereas the zircon in the diatexite is interpreted to have crystallized from a melt that included an externally derived component by integrating zircon and petrographic data for the migmatites and pelitic gneiss the metatexite migmatite is interpreted to have formed by in situ partial melting in which the melt did not migrate from the source whereas the diatexite migmatite included an externally derived juvenile component the cretaceous high temperature metamorphism of the higo metamorphic terrane is interpreted to reflect emplacement of mantle derived basalts under a volcanic arc along the eastern margin of the eurasian continent and advection of heat via hybrid silicic melts from the lower crust post peak crystallization of anatectic melts in a high t region at mid crustal depths occurred in the interval c 116 110 ma as indicated by the difference in zircon ages from the metatexite and diatexite migmatites

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

sapphirine bearing granulites and related high temperature metamorphic rocks from the higo metamorphic terrane west central kyushu japan the higo metamorphic unit in west central kyushu island southwest japan is an imbricated crustal section in which a sequence of units with increasing metamorphic grade from high northern part to low southern part structural levels is exposed the basal part of the metamorphic sequence representing an original depth of 23 24 km consists mainly of garnet cordierite biotite gneiss garnet orthopyroxene gneiss orthopyroxene bearing amphibolite and orthopyroxene bearing s type tonalite these metamorphic rocks underwent high amphibolite facies up to granulite facies metamorphism with peak p t conditions of 720 mpa 870 c in addition sapphirine bearing granulites and related high temperature metamorphic rocks also occur as tectonic blocks in a metamorphosed peridotite intrusion the sapphirine bearing granulites and their related high temperature metamorphic rocks can be subdivided into five types of mineral assemblages reflecting their bulk chemical compositions as follows 1 sapphirine corundum spinel cordierite 2 corundum spinel cordierite 3 garnet corundum spinel cordierite 4 garnet spinel gedrite corundum and 5 orthopyroxene spinel gedrite these metamorphic rocks are characterized by unusually high al2o3 and low sio2 contents which could represent a restitic nature remaining after partial melting of pelitic granulite under the ultra high temperature contact metamorphism at the peak metamorphic event of the higo metamorphic unit the metamorphic conditions are estimated to be about 800 mpa and above 950 c which took place at about 250 ma as a result of the thermal effect of the regional gabbroic rock intrusions

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

high t low p metamorphism in the palaeoproterozoic halls creek orogen northern australia the middle crustal response to a mantle related transient thermal pulse high t low p metamorphic rocks of the palaeoproterozoic central halls creek orogen in northern australia are characterised by low radiogenic heat production high upper crustal thermal gradients locally exceeding 40 c km 1 sustained for over 30 myr and a large number of layered mafic ultramafic intrusions with mantle related geochemical signatures in order to account for this combination of geological and thermal characteristics we model the middle crustal response to a transient mantle related heat pulse resulting from a temporary reduction in the thickness of the mantle lithosphere this mechanism has the potential to raise mid crustal temperatures by 150 400 c within 10 20 myr following initiation of the mantle temperature anomaly via conductive dissipation through the crust the magnitude and timing of maximum temperatures attained depend strongly on the proximity duration and lateral extent of the thermal anomaly in the mantle lithosphere and decrease sharply in response to anomalies that are seated deeper than 50 60 km maintained for 5 myr in duration and or have half widths 100 km maximum temperatures are also intimately linked to the thermal properties of the model crust primarily due to their influence on the steady state background thermal gradient the amplitudes of temperature increases in the crust are principally a function of depth and are broadly independent of crustal thermal parameters mid crustal felsic and mafic plutonism is a predictable consequence of perturbed thermal regimes in the mantle and the lowermost crust and the advection of voluminous magmas has the potential to raise temperatures in the middle crust very quickly although pluton related thermal signatures significantly dissipate within 10 myr even for very large high temperature intrusive bodies the interaction of pluton and mantle related thermal effects has the potential to maintain host rock temperatures in excess of 400 450 c for up to 30 myr in some parts of the mid crust the numerical models presented here support the notion that transient mantle related heat sources have the capacity to contribute significantly to the thermal budget of metamorphism in high t low p metamorphic belts especially in those characterised by low surface heat flow very high peak metamorphic geothermal gradients and abundant mafic intrusions

contact metamorphism of a cretaceous accretionary prism by the 14 ma okueyama granite a single post kinematic pluton in central kyushu japan svd analysis of metamorphic reactions and thermal release petrologic analysis is used to determine the intensity and extent of the thermal effects of a postkinematic granite on its surrounding rocks to understand the thermal evolution of an accretionary prism in central kyushu japan the relatively young 14 ma okueyama granite represents a single magma chamber which solidified as a zoned pluton and is therefore suitable for the study of the thermal effects by a single heat source the lower shimanto group country rock an accretionary prism of cretaceous age has undergone contact metamorphism to form an orthopyroxene zone cordierite zone and biotite zone in order of decreasing metamorphic grade singular value decomposition svd analysis of composition matrices of relevant mineral assemblages results in the following isograd reactions batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document 0 475bt crd 0 538opx 0 179bt opx 0 665kfs 0 075crd 0 067ilm for the orthopyroxene isograd and end document batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document 0 612bt bt 0 218ms bt 0 063ilm 0 292kfs crd 0 686bt crd 0 137crd crd for the cordierite isograd end document peak metamorphic temperatures for each zone were determined using ternary feldspar and garnetbiotite thermometers yielding a peak thermal gradient of 0 071 c m this thermal gradient yields isograd temperature conditions of 740 c for the orthopyroxene isograd and 550 c for the cordierite isograd respectively

thermal modeling of the chugach metamorphic complex heating from hot young subducting oceanic lithosphere and from a subducting spreading center has been proposed to explain the metamorphism and associated plutonism observed in former accretionary prism sedimentary rocks in the gulf of alaska we evaluated the thermal effects on the overlying accretionary wedge by heat conduction from a hot young subducting slab through numerical modeling the temperature limiting parameter in the modeling is the age of the subducted crust the rate of convergence angle of subduction amount of radiogenic heating and values of thermal conductivity are of secondary importance in determining temperatures in the accretionary prism a possible former setting for the subduction of young oceanic crust is recorded in the valdez metasedimentary rocks in the chugach metamorphic complex of the chugach terrane in southern alaska petrologic data indicate that rocks now at the surface were at 10 km depth when they were metamorphosed under greenschist facies conditions 350 400 c and subsequently intruded by felsic plutons using parameter values appropriate for chugach terrane rocks our results indicate that for accretionary prisms less than 20 km thick oceanic lithosphere aged 1 0 ma or less must be subducted in order to obtain temperatures greater than 300 c at 10 km depth within the prism we also modeled the transient heating of the accretionary prism due to the cessation of subduction of young oceanic lithosphere recent seismic refraction surveys from the trans alaska crustal transect have been interpreted to show underplated sedimentary rocks and relict oceanic crust beneath the chugach mountains transient heating due to cessation of subduction or underplating of hot oceanic crust does not appreciably relax the requirement of subducting oceanic crust aged 1 0 ma or less in order to attain temperatures greater than 30o c at 10 km depth however at depths greater than 25 km it may provide the heat necessary to generate hot fluids and felsic magma which can augment the temperatures at shallow depths by advection of heat as well as produce the postmetamorphism or synmetamorphism emplacement of the anatectically derived plutons observed in the chugach mountains our modeling requires subduction of such young crust that subduction of a spreading ridge was probable although underplating of extremely young oceanic crust would also have provided sufficient heat to cause the formation of the chugach metamorphic complex

high geothermal gradient metamorphism during thermal subsidence the burial of a basement sequence enriched in heat producing elements during thermal subsidence following rifting produces two concomitant changes in the thermal structure of the crust firstly the burial of the enriched layer produces high geothermal gradients in the overlying sedimentary succession with the high gradients propagating down into but not through the enriched basement sequence secondly the lithospheric thickening that drives thermal subsidence reduces the heat flowing into the deeper crust from the mantle because the process of thermal subsidence promotes burial it naturally increases the depth extent of the high geothermal gradients in the upper crust potentially inducing significant temperature increases in the mid upper crust during burial the lowering of the thermal gradients in the deep crust accompanying burial severely limits the temperature changes affecting the moho potentially allowing moho cooling while the mid upper crust heats these effects can promote high geothermal gradient 40 c km metamorphism in the mid upper crust without inducing significant melting in the lower crust providing the basement heat production contributes 70 mw m 2 to the surface heat flow and that the horizontal length scale for the basement heat production anomaly is 50 km these conditions appear to be met in several australian intermediate to high temperature low pressure metamorphic terranes where the thermal causes of metamorphism have hitherto remained enigmatic one of these terrains the mt painter province in the northern flinders ranges south australia is used to illustrate some of the attributes of the model

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press