crustal scale mass transfer geotherm and topography at convergent plate boundaries end member cases for the fate of the crust at convergent lithospheric plate boundaries accommodated by subduction are considered combining i plate kinematics slab retreat vs slab advance and ii mechanical coupling decoupling within the subducting slab slab advance is accommodated by thickening of the overriding lithospheric mantle whereas slab retreat is associated with thinning crust mantle mechanical coupling results in continental subduction whereas decoupling leads to continental accretion the thermal and topographic evolutions of the associated orogenic belts are functions of the crust mantle thickness ratio resulting from the combined effects of crust mantle coupling decoupling and slab advance retreat crustal accretion associated with slab advance results in crustal thickening and contributes to an increase in radioactive heat production and buoyancy which is impeded by concomitant thickening of the conductive and dense lithospheric mantle in contrast crustal accretion combined with slab retreat results in thickening simultaneous with thinning of the lithospheric mantle this peculiar situation is the most favourable for generating a high temperature and buoyant orogenic belt

high frequency earthquake swarm associated with the may 1991 dome extrusion at unzen volcano japan abstract the 1990 1995 eruption of unzen volcano in southwestern japan was characterized by the extrusion of a dacite lava dome and frequent pyroclastic flows during the dome growth associated with the dome emergence on may 20 1991 an intense swarm of high frequency hf microearthquakes occurred just beneath the crater at very shallow depths we used data from fg3 a seismic station located 500 m ssw of the crater to identify 29 401 hf earthquakes between may 11 and 31 1991 the rate of hf earthquakes increased starting on may 12 and peaked on may 17 the high seismicity continued until may 26 then dropped sharply coinciding with a marked decline in the swelling on the southern upper flank of the volcano the seismicity increased and decreased repeatedly within a 1 to 2 h period which was correlated with tilt cycles observed 680 m west of the crater in such a way that the seismicity increased during uplifting on the craterward side defining an earthquake group as a series of earthquakes with waveforms that are similar or vary only slightly over time we identified 10 such groups each containing more than 300 events the largest group comprised 3 214 events over 18 days seismicity rates of eight groups including the largest increased and decreased repeatedly correlated with the tilt cycles as the waveform data from station fg3 were considerably clipped for relatively large events we analyzed data from two additional stations kra and cja located 3 5 and 8 7 km from the crater respectively and identified a total of five new groups the activity of two groups recorded at kra was correlated with the tilt cycles but those of three groups recorded at cja did not always show such a correlation hypocenters for the groups recorded at kra were distributed to the east side of the conduit and a focal mechanism suggested that the events in this area occurred due to compressional stress produced by the inflation of the conduit the groups recorded at cja occurred to the north side of the dike trending westward from the conduit the focal mechanisms have p axes roughly trending to the dike which can be explained by the compressional stress generated by the increasing thickness of the dike

magma ascent beneath unzen volcano sw japan deduced from the electrical resistivity structure the resistivity structure of unzen volcano has been revealed by extensive magnetotelluric surveys since the first eruption on november 17 1990 this structure comprises a highly resistive surface layer a low resistive second layer at several hundred meters depth interpreted as a water saturated layer a resistive third layer and a low resistive fourth layer at 10 km depth possibly related to the deep magmatic activity the structure has influenced the volcanic activity of unzen this activity was characterized by a series of dramatic changes in eruption type a minor phreatic eruption on november 17 1990 phreatic eruptions after february 12 1991 preceded by several weeks of volcanic tremor phreatomagmatic eruptions after april 9 and dome effusion beginning may 19 1991 this paper presents a hypothesis in which the top of the magma column rose about 20 m day reached the base of the water saturated layer at the end of january 1991 and approached the upper boundary of this layer on april 9 thus the temporal change of eruption type and associated phenomena are systematically explained by an interaction between magma and groundwater contained in the saturated layer

water fluxed crustal melting produces cordilleran batholiths most granites and related calc alkaline silicic volcanic rocks from the united states and new zealand cordillera are saturated with zircon between 65 and 70 wt sio2 for this silica interval zircon saturation temperatures t zr are universally lower 900 c the values contrast with t zr from alkaline rocks from the cenozoic u s cordillera which are typically 800 c for 65 70 wt sio2 case studies of titanium in zircon thermometry from the u s cordillera also suggest that alkaline magma injections into granitic magma chambers are hot but calc alkaline magma injections are usually cooler a model is presented suggesting that silicic cordilleran magmas form in magmatic arcs where hydrous basaltic magmas solidify in the arc root producing mafic underplates that exsolve aqueous fluids which transfer to the crust and promote water fluxed partial melting at ambient pressure temperature 750 800 c at 8 kbar conditions subsequent rock buffered melting reactions modulate the water content of arc magmas the granitic partial melts are water undersaturated rise adiabatically as increments but stall in the middle to upper crust building cool and hydrous crystal rich magma chambers batholiths however injections of hotter magmas are required to drive volcanic eruption in the backarc granitic magma chambers are intermittently recharged with hotter drier alkaline magmas which are produced mostly by decompression melting during lithospheric extension not hydrous fluxing this highlights the control of subduction dynamics on water content and consequently magmatic temperatures in silicic magma systems

igneous garnet and amphibole fractionation in the roots of island arcs experimental constraints on andesitic liquids to evaluate the role of garnet and amphibole fractionation at conditions relevant for the crystallization of magmas in the roots of island arcs a series of experiments were performed on a synthetic andesite at conditions ranging from 0 8 to 1 2 gpa 800 1 000 c and variable h2o contents at water undersaturated conditions and fo2 established around qfm garnet has a wide stability field at 1 2 gpa garnet amphibole are the high temperature liquidus phases followed by plagioclase at lower temperature clinopyroxene reaches its maximal stability at h2o contents 9 wt at 950 c and is replaced by amphibole at lower temperature the slopes of the plagioclase in boundaries are moderately negative in text t text x text h 2 text o space at 0 8 gpa garnet is stable at magmatic h2o contents exceeding 8 wt and is replaced by spinel at decreasing dissolved h2o the liquids formed by crystallization evolve through continuous silica increase from andesite to dacite and rhyolite for the 1 2 gpa series but show substantial enrichment in feo mgo for the 0 8 gpa series related to the contrasting roles of garnet and amphibole in fractionating fe mg in derivative liquids our experiments indicate that the stability of igneous garnet increases with increasing dissolved h2o in silicate liquids and is thus likely to affect trace element compositions of h2o rich derivative arc volcanic rocks by fractionation garnet controlled trace element ratios cannot be used as a proxy for slab melting or dehydration melting in the deep arc garnet fractionation either in the deep crust via formation of garnet gabbros or in the upper mantle via formation of garnet pyroxenites remains an important alternative despite the rare occurrence of magmatic garnet in volcanic rocks

bounded influence magnetotelluric response function estimation author posting blackwell 2004 this article is posted here by permission of blackwell for personal use not for redistribution the definitive version was published in geophysical journal international 157 2004 988 1006 doi 10 1111 j 1365 246x 2004 02203 x

inversion for parameters of tensile earthquakes the tensile source model generalizes the shear source model by assuming that the slip vector can be arbitrarily oriented with respect to the fault and is not constrained to lie within the fault plane the proposed inversion for the parameters of tensile sources is based on the evaluation of the isotropic iso compensated linear vector dipole clvd and double couple dc components in seismic moment tensors the most significant parameters inverted are the ratio at the fault denoted as the parameter and the inclination of the slip vector from the fault the parameter is significant for discriminating noisy moment tensors of shear earthquakes from those of tensile earthquakes the inclination can be accurately determined from the dc component in the moment tensor because the dc component rapidly decreases with increasing for example the inclination of 20 causes dc being 50 60 only the inversion is applied to earthquakes which occurred in january 1997 in west bohemia czech republic it is shown that some of these earthquakes display tensile faulting the parameter is 0 1 the inclination of the slip from the fault attains values of up to 20 this inclination is a result of tensile traction and reduced shear traction along the fault and high fluid pressure in the region

seismic pulses suggesting an implosive source at the 1989 ito oki submarine eruption central japan four unusual seismic pulses of magnitude about 2 were observed at the 1989 ito oki submarine eruption on july 13 1989 their seismograms show two distinct characteristics ineffective shear wave excitation and downward motions of p onsets for all azimuths source mechanisms are studied by using data obtained on the local seismic network hypocenters were estimated to be in a very shallow part of the upper crust beneath a new crater found after the eruption the downward first motion and ineffective shear wave excitation are difficult to explain simultaneously by a double couple type source mechanism suggesting an implosive type source

volcano tectonic seismicity at unzen volcano japan 1985 1999 abstract the eruption of unzen volcano in southwestern japan began on november 17 1990 and an extrusion of a dacite lava dome lasted from may 20 1991 until early february 1995 the seismic network operated by the shimabara earthquake and volcano observatory of kyushu university sevo located about 4000 volcano tectonic earthquakes during the period from 1985 to 1999 the seismicity was greatest in the period between one year before the eruption and the start of the lava dome extrusion the hypocenters were mostly distributed within a 12 20 km area on the western side of the volcano an inclined seismic aseismic boundary shallows eastward to the summit probably reflecting the thermal structure beneath the volcano within 4 km west from the summit however the seismicity is very low it was suggested that the high temperature ductile and low q bodies exist there we obtained focal mechanism solutions for about 90 earthquakes among them orientations of p axes for the earthquakes occurring on the upper side of the inclined seismic aseismic boundary suggest a pressure source located below this boundary although most of the focal mechanisms have t axes nearly horizontal in the n s direction consistent with the n s extensional tectonic stress several earthquakes whose focal mechanisms are inconsistent with the tectonic stress were observed within a horizontal distance of 5 km from the summit this is because the pressurization of volcanic gas or magma beneath the summit produced a strong local stress in the summit surroundings

s type granitic magmas petrogenetic issues models and evidence abstract despite a perception that it represents a perverse divergence it is perfectly possible to believe in the existence of s and i type granites and the implications for the nature of their protoliths and to disbelieve in the applicability of the restite unmixing model for chemical variation in granitic magmas white and chappell erected the s i classification with impeccable validity the isotopic evidence demands contrasting source reservoirs for s and i type granitic magmas however the major advance was not the classification but the recognition that highly contrasting parental materials must be involved in the genesis of granitic magmas the restite unmixing model is commonly seen as a companion to the s i classification but it is really a separate issue this model implies that the compositions of granites image those of their source rocks in a simple way however there are other equally valid models that can explain the data and none of them represents a unique solution the most cogent explanation for the high grade metasedimentary enclaves in most s type granites is that they represent mid crustal xenoliths restitic enclaves are either rare or absent inherited zircons in s type rocks are certainly restitic however the occurrence of a substantial restitic zircon population does not imply an equally substantial restitic component in the rest of the rock zircon and zirconium behaviours are controlled by disequilibrium and kinetics and zr contents of granitic rocks can rarely be used to infer magma temperatures since the dominant ages among inherited zircons in lachlan fold belt lfb s type granites are ordovician and proterozoic it seems likely that crust of this age but geochemically different from the exposed rocks not only underlies much of the lfb but also forms a component in the granite magma sources the evidence is overwhelming that the dark microgranular enclaves that occur in both s and i type granites are igneous in origin they represent globules of quenched more mafic magma mingled and modified by exchange with the host granitic magma however magma mixing does not appear to be a significant process affecting the chemical evolution of the host magmas likewise the multicomponent mixing models erected for some granitic rock suites are mathematically nonunique and in some cases violate constraints from isotopic studies s and i type magmas commonly retain their distinct identities this suggests limited source mixing limited magma mixing and limited wall rock assimilation though intermediate types certainly exist they are probably relatively minor in volume crystal fractionation probably plays the major role in the differentiation of very many granitic magmas including most s types especially those emplaced at high crustal levels or in the volcanic environment minor mechanisms include magma mixing wall rock assimilation and restite unmixing isotopic variations within plutons and in granite suites could be caused by source heterogeneities magma mixing assimilation and even by isotopic disequilibrium however source heterogeneity coupled with the inefficiency of magma mixing is probably the major cause of observed heterogeneity normal geothermal gradients are seldom sufficient to provide the necessary heat for partial melting of the crust and crustal thickening likewise fails to provide sufficient heat generally the mantle must be the major heat source this might be provided through mantle upwelling and crustal thinning and possibly through the intra and underplating of mafic magmas upper crustal extension seems to have been common in regions undergoing granitic magmatism migmatites probably provide poor analogues of granite source regions because they are mostly formed by fluid present reactions granitic magmas are mostly formed by fluid absent processes where we do see rare evidence for arrested fluid absent partial melting the melt fraction is invariably concentrated into small shear zones veinlets and small dykes thus it seems likely that dyking is important in transporting granitic magma on a variety of scales and at many crustal levels however one major missing link in the chain is the mechanism by which melt fractions in small scale segregations occurring over a wide area can be gathered and focused to efficiently feed much wider spaced major magma conduits answers may lie in the geometry of the melting zones and in the tendency of younger propagating fractures to curve toward and merge with older ones self organization almost certainly plays a role

the four plutonic belts of the transhimalaya himalaya a chemical mineralogical isotopic and chronological synthesis along a tibet nepal section

a dual tube model for gas dynamics in fractured nanoporous shale formations gas flow through fractured nanoporous shale formations is complicated by a hierarchy of structural features ranging from nanopores to microseismic and hydraulic fractures and by several transport mechanisms that differ from the standard viscous flow used in reservoir modelling in small pores self diffusion becomes more important than advection also slippage effects and knudsen diffusion might become relevant at low densities we derive a nonlinear effective diffusion coefficient that describes the main transport mechanisms in shale gas production in dimensionless form this coefficient depends only on a geometric factor or dimensionless permeability and on the kinetic model that describes the gas to simplify the description of the complex structure of fractured shales we observe that the production rate is controlled by the flow from the shale matrix which has the smallest diffusivity into the fracture network which is assumed to produce instantaneously therefore we propose to model the flow in the shale matrix and estimate the production rate with a simple bundle of dual tubes model bodtm in which each tube is characterized by two diameters one for transport and the other for storage the solution of a single tube is approximately self similar at early time but not at late time when the gas flux decays exponentially owing to the finite length of the tube to construct a bodtm a reliable estimate of the joint statistics of the matrix porosity parameters is required this can be either inferred from core measurements or postulated on the basis of some a priori assumptions when information from laboratory and field measurements is scarce by comparison with field production data from the barnett shale gas field we demonstrate that bodtm can be calibrated to estimate structural parameters of the shale formation and to predict the cumulative production of shale gas our framework has enough flexibility to construct models of increasing complexity that can be employed in the presence of a complex dataset or when more information is available

partial convective overturn of archaean crust in the east pilbara craton western australia driving mechanisms and tectonic implications strain varies systematically from weakly developed outward dipping s tectonites in the 3320 3310 10 ma mount edgar batholith to intensely deformed subvertical l tectonites in greenstones of the warrawoona syncline a consistent greenstone down batholith up sense of shear is recorded in batholithic domal margins and adjacent high grade supracrustal rims lineations converge to a central zone of subvertical extension zone of sinking along the synclinal axis at domal margins early kinematic granitoid sheets and intrusive diatexites are subconcordant to a well developed dome parallel schistosity but late to post kinematic intrusives are discordant high level plutons all granitoids are the same age within analytical error these syn doming features conform with structural tests for diapirs and differ from those expected during metamorphic core complex formation and cross folding diapirism is part of a larger process involving partial convective overturn of the crust based on strain patterns and kinematic criteria we argue that deformation was initiated by sinking of greenstones which rapidly subsided the domes then rose passively as accommodation structures ongoing doming d4 partitioned strain to the southern rim of the batholith and uplifted a wedge of the warrawoona syncline keel we consider that crustal overturn occurred in response to mantle plume activity eruption of a 5 10 km thick 3325 ma mafic ultramafic greenstone pile onto an older granite greenstone terrain created a negatively buoyant crust but convective overturn occurred some 5 20 ma later triggered by widespread anatexis thermal softening convective overturn may have been a common archaean process occurring in response to a hotter mantle and represents an end member deformation mechanism that includes thrust accretion of lithotectonic assemblages in other archaean terranes such as the superior province of canada

eruption dynamics at mount st helens imaged from broadband seismic waveforms interaction of the shallow magmatic and hydrothermal systems 1 the current eruption at mount st helens is characterized by dome building and shallow repetitive long period lp earthquakes waveform cross correlation reveals remarkable similarity for a majority of the earthquakes over periods of several weeks stacked spectra of these events display multiple peaks between 0 5 and 2 hz that are common to most stations lower amplitude very long period vlp events commonly accompany the lp events we model the source mechanisms of lp and vlp events in the 0 5 4 s and 8 40 s bands respectively using data recorded in july 2005 with a 19 station temporary broadband network the source mechanism of the lp events includes 1 a volumetric component modeled as resonance of a gently nnw dipping steam filled crack located directly beneath the actively extruding part of the new dome and within 100 m of the crater floor and 2 a vertical single force attributed to movement of the overlying dome the vlp source which also includes volumetric and single force components is 250 m deeper and nnw of the lp source at the sw edge of the 1980s lava dome the volumetric component points to the compression and expansion of a shallow magma filled sill which is subparallel to the hydrothermal crack imaged at the lp source coupled with a smaller component of expansion and compression of a dike the single force components are due to mass advection in the magma conduit the location geometry and timing of the sources suggest the vlp and lp events are caused by perturbations of a common crack system

the sources of energy for crustal melting and the geochemistry of heat producing elements crustal melting to produce granite magmas requires a tremendous amount of energy in principle there are three main mechanisms of heating that can extensively melt a fertile crust radiogenic heat caused by the decay of 40k 230th 235u and 238u increased subcrustal heat flux caused by the upwelling of deeper therefore hotter mantle materials advection of heat caused by the emplacement and crystallization of hot mantle magmas two dimensional finite elements modeling reveals that a fertile crust thickened to 65 70 km would produce copious granite magmatism after 30 40 m y if its average heat production is a 1 2 w m 3 but it would scarcely melt if a 0 65 w m 3 increasing the subcrustal heat flow from the normal value of qm 0 025 w m 2 to qm 0 04 w m 2 may also lead to extensive crustal melting especially if the crust does not thin to less than 30 35 km very high qm 0 06 w m 2 affecting the continental crust is unlikely but the combination of moderately high qm 0 04 w m 2 and a thick fertile crust with a 1 2 w m 3 such as often happens in the volcanic and back arc areas of subduction zones is ideal to produce copious granite magmatism lastly the emplacement of hot mantle magmas in a fertile crust can produce crustal melts in just a few thousand years but the volume of these is equal to or less than the volume of the intruding magma a clue for understanding the relative importance of each of these three mechanisms comes from the radiogenic heat production of granite rocks calculated from the concentration of 40k 230th 235u and 238u at the time of their formation this parameter estimated on more than 3400 granites samples of different ages and provenance reveals a strongly asymmetric distribution peaking around 2 4 w m 3 a value much higher than the average continental crust about 1 1 2 w m 3 and certainly much higher than the average lower continental crust about 0 4 0 8 w m 3 only those granite rock types that are clearly connected with mantle heat sources such as the archean ttg post archean subduction related trondhjemites and recent adakites have a heat production equal to or smaller than the lower continental crust since the bulk melt solid partition coefficient of the heat producing elements hpe k th and u is k 1 the elevated hpe contents of granites indicates that most of them have been derived from hpe rich sources we conclude that radiogenic heating is often essential and always advantageous for generating large volumes of granite magmas and that granite magmatism is the main cause of the accumulation of hpe in the upper crust

a review of the relationships between granitoid types their origins and their geodynamic environments abstract granitoids are divided into several types according to their mineral assemblages their field and petrographical features and their chemical and isotopic characteristics this typology complements most of the recent classifications because it is not based solely on chemical and isotopic criteria but also on the field petrographical and mineralogical criteria it thus has the advantage of distinguishing the various granitoid types in the field in most cases the proposed classification shares many similarities with the twenty most used genetic classifications of granitoids both types of peraluminous granitoids are of crustal origin the tholeiitic alkaline and peralkaline granitoids are of mantle origin and both types of calc alkaline granitoids are of mixed origin and involve both crustal and mantle materials each granitoid type is generated and emplaced in a very specific tectonic setting each stage of the wilson cycle is characterised by typical associations of granitoids well typed and precisely dated granitoids can then complement structural approaches and indicate on the geodynamic environment with reference to some case studies the use of granitoids as tracers of the geodynamic evolution is also proposed and discussed

neo tethyan evolution in southeastern extension of tibet constraints from early paleocene to early eocene granitic rocks with associated enclaves in tengchong block abstract the granite is one of most widely distribution rocks in crust and its petrogenesis is crucial to advance our knowledge of thermal condition in continental crust the long time tectonic evolution of crust in the active continental margin could be recorded by multi stages of granitic assemblage in order to reveal the subduction of neo tethys and initial collision of indian asian continents in eastern tibetan plateau we selected the early paleocene to early eocene granitic rocks with associated enclaves in tengchong block southeastern extension of gangdese magmatic belt to clarify their different derivation regimes using geochemical and isotopic compositions mineral compositional variation the zircon data show that early paleocene granites formed at 64 ma and early eocene granitic rocks formed at 54 ma combing with the field contact relationship the similar and enriched sr nd pb and zircon hf isotopic compositions indicate that the two stages of granitic rocks have a common and ancient lower crustal source the enclaves in early paleocene granites could represent crystallization of coeval and cognate magmas at margin of magmatic conduit but the enclaves in early eocene granitic rocks are products of magma mixing between mafic and felsic magma according to rock assemblages and geochemical signatures of granitic rocks during early paleocene to early eocene in tengchong block we propose that early paleocene granitic rocks are derived from partial melting of lower crustal rocks induced by long lived heat accumulation with no or minimal basalt flux during the neo tethyan subduction whereas early eocene granitic rocks are derived from melting of crustal materials resulted from high mafic magma flux during indian asian continental collision the initial collision occurred at ca 55 ma

clockwise p t paths for crustal melting and h2o recycling in granite source regions and migmatite terrains abstract clockwise pressure temperature p t paths of metamorphism pass through two kinds of melting reactions with increasing temperature the first generates h 2 o saturated partial melts at the h 2 o saturated solidus only if excess h 2 o is present usually this accounts for less than 3 melt the second generates h 2 o undersaturated melts through dehydration melting reactions dmrs of micas or other hydrous minerals and occurs at temperatures higher than the h 2 o saturated solidus if h 2 o undersaturated melts remain in situ and the source regions are not exhumed then the crystallisation reactions encountered along the near reverse p t path may produce the same mineralogy as in the source rock in regions of exhumation along the decompression segments of clockwise p t paths two other types of crystallisation reactions occur the better known type occurs at the recrossing of the pelite h 2 o saturated solidus at shallower depth this can occur at mid crustal depths or much shallower at pressures lower than the appropriate invariant point which relates all of these reactions and will result in granitic pegmatites at the other type remaining mica can react with hydrous melt to generate anhydrous granulite with free h 2 o fluid such suprasolidus decompression dehydration reactions sddrs result in freezing of pelitic melts at mid crustal depth to k feldspar quartz assemblages and release of h 2 o which can induce h 2 o saturated melting in adjacent fertile lithologies lacking h 2 o e g psammite older granite distinct mineral proportions and textures will result from each reaction the actual reactions occurring are related through specific invariant points whose locations are known in the p t space the displacement of these invariant points due to additional components in the solid melt and fluid phases is reasonably well known the flat lying sddr very low d p d t will extend to much higher pressures if a low ah 2 o fluid is present sddr involving amphibole or epidote will occur in mafic to intermediate compositions over a large depth range depending on fe mg ca na etc of the rock compositions

joint seismological magnetotelluric investigation of shallow and implosive non dc and dc earthquakes beneath the gravitationally unstable heisei shinzan lava dome unzen volcano japan abstract shallow implosive non double couple ndc earthquakes have been detected beneath unzen volcano japan by a routinely operated seismometer network here we analyze the waveforms from 14 temporary seismic stations around mt fugen fugendake and heisei shinzan lava dome which formed during the 1990 1995 eruptions to further investigate this shallow seismicity we also estimate the subsurface electrical resistivity structure using broadband magnetotelluric observations and investigate its relationship to the shallow seismicity we find that 1 implosive ndc earthquakes occur at the craters that were active during the 1990 1991 eruptions which are currently buried by the approximately 200 m thick heisei shinzan lava dome 2 the detected ndc earthquakes generally possess downward p wave arrivals although upward p wave arrivals are observed at a few seismic stations 3 the tensile axes of the ndc earthquakes show various slip directions 4 repeated ndc earthquakes are in some cases detected within 20 s of the initial event and 5 a high resistivity zone exists beneath the ndc earthquakes we conclude that the implosive ndc earthquakes were generated by the collapse of small randomly oriented vapor filled voids at the two buried craters with active volcanic conduits still present beneath these craters the shallow dc earthquakes consist of two clusters that are located 2 3 km below the ndc earthquakes the focal mechanisms of the shallower cluster approximately 2 km depth indicate various fault mechanism whereas those of the deeper cluster approximately 3 km depth indicate strike slip faulting the top of the shallower cluster corresponds to the base of the conductive zone which suggests that the dc earthquakes in the shallower cluster are generated in a zone of high pore pressure that is capped by a hydrothermally altered clay layer

generation of felsic crust in the archean a geodynamic modeling perspective abstract as a consequence of secular cooling of the earth there is generally no modern analog to assist in understanding the tectonic style that may have operated in the archean higher mantle temperatures and higher radiogenic heat production in the archean earth would have impacted the thickness and composition of the crust for this reason well constrained numerical modeling based on the fragmentary evidence preserved in the geological record is the most appropriate tool to evaluate hypotheses of archean crust formation the main lithology of archean gray gneiss complexes is the sodic tonalite trondhjemite granodiorite ttg suite melting of hydrated basalt at garnet amphibolite granulite or eclogite facies conditions is considered to be the dominant process that generated the archean ttgs taking into account geochemical signatures of possible mantle contributions to some ttgs models proposed for the formation of archean crust include subduction melting at the bottom of thickened continental crust and fractional crystallization of mantle derived melts under water saturated conditions we evaluated these hypotheses using a 2d coupled petrological thermomechanical tectono magmatic numerical model with initial conditions appropriate to the eoarchean mesoarchean based on the result of our experiments we identify three tectonic processes by which intermediate to felsic melts may be generated from hydrated primitive basaltic crust 1 delamination and dripping of the lower mafic crust into the mantle 2 local thickening of the crust and 3 small scale crustal overturns in the context of a stagnant deformable lid tectono magmatic geodynamic regime that is terminated by short lived subduction we identify two distinct types of continental crust the first type is a pristine granite greenstone like crust with dome and keel geometry formed over delaminating upwelling mantle which is mostly subjected to vertical tectonics processes by contrast the second type is a reworked accreted crust comprising strongly deformed granite greenstone and subduction related sequences and subjected to both strong horizontal shortening and vertical tectonics processes thus our study has identified a possible spatial and temporal transition from pristine granite greenstone like crust with dome and keel geometry to reworked accreted crust forming more felsic gneiss terranes in the archean we suggest that the contemporaneity of the proposed mechanisms can explain the variety and complexity of the archean geological record

water fluxed melting of the continental crust a review abstract water fluxed melting also known as fluid or water present melting is a fundamental process in the differentiation of continents but its importance has been underestimated in the past 20 years during which research efforts focused mostly on dehydration melting reactions involving hydrate phases in the absence of a separate aqueous phase the presence of a free aqueous phase in anatectic terranes influences all major physical and chemical aspects of the melting process from melt volumes viscosity and ability to segregate from rock pores to melt chemical and isotopic composition a review of the literature shows that melting due to the fluxing of aqueous fluids is a widespread process that can take place in diverse tectonic environments active tectono magmatic processes create conditions for the release of aqueous fluids and deformation driven transient high permeability channels capable of fluxing high temperature regions of the crust where they trigger voluminous melting water fluxed melting can be either congruent in regions at the water saturated solidus or incongruent at suprasolidus p t conditions incongruent melting reactions can give rise to peritectic hornblende or to nominally anhydrous minerals such as garnet sillimanite or orthopyroxene in this case the presence of an aqueous phase is indicated by a mismatch between the large melt fraction generated and the much smaller fractions predicted in its absence the relatively small volumes of aqueous fluids compared to that of rocks imply that melting reactions are generally rock buffered fluids tend to move upwards and down temperature however there are cases in which pressure gradients drive fluids up temperature potentially fluxing suprasolidus terranes crustal regions at conditions equivalent to the water saturated solidus represent a natural impediment to the up temperature migration of aqueous fluids because they are consumed in melting reactions in this case continued migration into supra solidus terranes take place through the migration of water rich melts thus melts become the transport agent of water into supra solidus terranes and responsible for water fluxed melting other processes such as the relatively rapid fluid migration through fractures also allow regional aqueous fluids to by pass the water saturated solidus fluid trap and trigger melting above solidus conditions when aqueous fluids or hydrous melts flux rocks at supra solidus conditions they equilibrate with the surroundings through further melting decreasing water activity and giving rise to undersaturated melts it is in these conditions that hornblende or anhydrous peritectic phases are stabilized unlike dehydration melting the melt fraction generated in this case is not limited by the water contained in hydrous minerals but by the volume of water added to the system unlike melting at the water saturated solidus these melts are capable of rising without freezing and do give rise to upper crustal granitic bodies

strengths and limitations of zircon lu hf and o isotopes in modelling crustal growth abstract the robust nature of the mineral zircon combined with our analytical ability to readily acquire in situ uranium lead u pb lutetium hafnium lu hf and oxygen o isotopic data has resulted in a rapid rise in the use of zircon isotopic datasets for studying both the generation of continental crust and its growth through earth history in such studies there has been a strong focus on developing methods to determine the timing and or proportion of juvenile magmatic addition to the continental crust one widespread approach to determine the timing of crustal growth has been the construction or fitting of reworking arrays to regional hf isotopic datasets simple stochastic models are presented which highlight that in many cases apparent reworking arrays are much more likely to represent a process of on going dilution and refertilisation of ancient crust consistent with hot zone models of granitoid generation and the need to refertilise lower crustal reservoirs to maintain magmatism a new compilation of magmatic rock zircon lu hf and o isotope data is used to demonstrate that the use of mantle like o isotope data as a screening tool for meaningful hf model ages is also unlikely to be reliable with independently constrained data indicating that as few as 14 of hf model ages provide a meaningful indicator of the timing of crustal growth the limitations of hf model ages are discussed with regard to existing approaches for continental growth and we demonstrate that popular inverse modelling approaches suffer from a bias created by both the use of model ages and numerical artefacts in an effort to address some of the limitations within existing models we develop stochastic models based on joint calibration of multiple datasets which allow for more unique solutions

the compositional variation of i type granites constraints from geochemical analyses and phase equilibrium calculations for granites from the qinling orogen central china abstract there is a large compositional variation of i type granites although partial melting and fractional crystallization are the two primary mechanisms for the variation it is intriguing to know how they jointly work this issue is addressed by a combined study of geochemistry and phase equilibrium modelling for a triassic pluton from the qinling orogen the pluton consists of high k calc alkaline i type granites geochemical results suggest that the granites are likely sourced from triassic mafic intermediate igneous rocks compared with experimental melts it is inferred that the natural granites are not the crystallized product from pure magmatic melts this is confirmed by phase equilibrium calculations that have utilized the triassic mafic intermediate igneous rocks as the source yielding modelled melts that contain too low feot mgo and mg to match the target granites the addition of restitic mineral assemblages to melts can relieve the discrepancy but the modelled magmas have much higher cao than the granites replacing equilibrium melting by disequilibrium melting in terms of the plagioclase behavior can lead to a satisfactory match furthermore by taking the average of the target granites as a parental magma the modelled crystalline products can well reproduce the variation trends in the high k calc alkaline i type granites from the qinling orogen in this regard the crustal anatexis would produce the parental magma at first which experienced fractional crystallization to form the granitic pluton with the compositional variation as observed therefore these two processes would jointly lead to the compositional variation of i type granites in nature

the composite archaean grey gneisses petrological significance and evidence for a non unique tectonic setting for archaean crustal growth abstract the geodynamic context of formation of the archaean continental crust is a matter of debate the crust is largely made of grey gneiss complexes a composite rock assemblage dominated by granitoids that are generally regarded as belonging to the ttg tonalite trondhjemite granodiorite series using a large database of published ttg and grey gneiss compositions it is possible to show that the granitoids forming grey gneiss complexes actually belong to at least four main geochemical groups i a potassic component made of granodiorites and formed by melting of existing crustal lithologies and ii three sodic groups ttg proper that comprise low medium and high pressure groups the geochemistry of the low pressure group is consistent with derivation from a plagioclase and garnet amphibolite the medium pressure group was formed in equilibrium with a garnet rich plagioclase poor amphibolite whereas the high pressure group derived from a rutile bearing eclogite as the temperature of melting of metamafic rocks is largely independent from pressure this corresponds to melting along a range of contrasting geothermal gradients in turn reflecting a range of tectonic sites for the formation of the archaean continental crust

implosive earthquakes at the active accretionary plate boundary in northern iceland non double couple earthquake focal mechanisms imply a mode of seismic failure other than simple shear and movements perpendicular to the fault plane reports of such events are rare and it is controversial whether such failure can occur at depth in the earth following reports of such earthquakes from the hengill and reykjanes volcanic systems on the accretionary plate boundary in iceland1 3 a seismological survey was conducted in the krafla system iceland to explore whether such earthquakes occur elsewhere along the boundary here we report the observation of a mixed suite of focal mechanisms at krafla including shear events explosive tensile crack events such as were observed at hengill and reykjanes and implosive events these last represent cavity collapse and constitute an entirely new class of earthquake the krafla system had undergone a crustal spreading episode4 10 during the decade before the observations were made and our results indicate that immediately after such an episode the accretionary plate boundary is characterized by a heterogeneous stress regime with shear extensional and compressional sources in close juxtaposition

origin of peraluminous granites and granodiorites iberian massif spain an experimental test of granite petrogenesis the discrimination between potential source materials involved in the genesis of iberian granites and granodiorites as well as the role of mantle crust inter actions are examined using constraints imposed by melting experiments melting assimilation experiments and sr nd isotope systematics the sr nd isotope re lationships indicate the existence of diaerent genetic trends in which juvenile mantle materials are involved by diaerent mechanisms 1 a source trend traced by a particular evolution of the pre hercynian basement and indicating mantle participation at the time of sedimen tation 2 a set of magmatic trends traced by gabbro tonalite enclave granodiorite associations implying the incorporation of new mantle material at the time of granite generation these relationships strongly support a pure crustal origin for the peraluminous leucogranites derived from partial melting of crustal protoliths and a hybrid origin for the peraluminous granodiorites these granodiorites are the most abundant granitic rocks of the central iberian zone ciz of the iberian massif implying that processes of hybridisation by assimilation and or magma mixing played an important role in granitoid production during the hercynian orogeny these hypotheses have been tested by means of melting and assimilation experiments melting experiments in the range 800 900 c and at pressures of 3 6 10 and 15 kbar indicate that 1 several potential source materials such as bt ms gneisses and metagreywackes are suitable for the production of peraluminous leucogranite melts 2 the melt compositions are always leucogranitic re gardless of pressure 3 pressure exerts a strong influ ence on the fertility of the source experiments at 3 kbar produce more than 20 vol of melt compared with less than 5 vol of melt produced at 10 and 15 kbar and at the same temperature the melting assimilation experi ments carried out at 1000 c and 4 7 and 10 kbar and using a proportion of 50 gabbro and 50 gneiss give high melt proportions more than 50 vol and noritic residues these melts have the composition of le ucogranodiorites and overlap with part of the compo sitional range of peraluminous granodiorites of the iberian massif the generation of more mafic granodiorites may be explained by the incorporation of some residual orthopyroxene to the granodiorite mag mas the low solubility of fe mg prevents the gen eration of granodiorite melts with more than 3 wt of mgo feo at all crustal pressures the large volumes of peraluminous hybrid granodiorites produced by assimilation of crustal rocks by mantle magmas imply that an important episode of crustal growth took place during the late palaeozoic hercynian orogeny in the iberian massif

seismicity associated with the 1991 1995 dome growth at unzen volcano japan abstract from may 1991 until february 1995 seismicity in the crater area of unzen volcano southwest japan intensified in conjunction with the growth of a dacite lava dome we used data from seismic stations located near the crater to identify approximately 580 000 summit earthquakes with maximum amplitudes equal to or greater than 1 10 3 cm s the temporal characteristics of the seismicity level were different for exogenous and endogenous periods of dome growth periods of solely exogenous growth were accompanied by several days or weeks of increased seismicity and levels of seismicity were notably reduced between successive seismically active periods in contrast levels of seismicity were generally high during periods when the dome grew endogenously with repeated cycles of increasing and decreasing seismicity of one to two months duration we classified the waveforms of summit earthquakes into high frequency hf medium frequency mf and low frequency lf types on the basis of spectral analysis dominant waveform types varied significantly over time hf was dominant in may 1991 lf from june 1991 until august 1993 mf during september and october 1993 and hf and mf from november 1993 hf and mf events are mainly distributed at depths of 500 1100 m above sea level asl just below the lava dome while lf events are widely distributed at depths from 500 m asl to the interior of the dome itself to efficiently detect earthquake families we conducted a cross correlation analysis of waveforms using the events one by one as reference events we calculated peak correlation coefficients between each reference event and events that occurred within 24 h either side of the reference event the results show that many earthquake families of all waveform types occurred throughout the growth period of the dome the durations of most families were less than two weeks and were not related to the intensity of seismic activity the incidence rate of events within each family reached a peak in the middle of the respective activity period the results also suggest that the waveforms in several analyzed families gradually evolved over time during periods when hf events intensified several families appeared contemporaneously in contrast during periods when lf events intensified the occurrence pattern of earthquake families was relatively simple a new family became active only once the activity of the previous family had declined or ceased completely a possible source mechanism for families of hf events is stick slip within the stiff rocks surrounding the conduit however several different types of source mechanisms should be considered for families of lf events

resistivity structure of unzen volcano derived from time domain electromagnetic tdem survey abstract time domain electromagnetic tdem surveys were carried out around unzen volcano shimabara peninsula south west japan in 2001 and 2002 in the eastern part of the peninsula the surveys were a completion of the previously interpreted tdem data collected from surveys in 1995 in the western part of the peninsula interpretation of the recorded transient magnetic fields resulted in a general feature of the resistivity structure which are composed of three main layers a resistive surface layer a conductive layer below 10 m from about sea level to 2 5 km below the surface and a third resistive layer the conductive layer is considered to be a complex of water saturated and altered layers which spreads widely beneath shimabara peninsula the spatial conductance distribution shows a w e trending pattern of high conductance from tachibana bay to the summit of unzen fugen dake this evidence indicates that volcanic gas was supplied to shallow layer in the western part of the peninsula which is in accordance with the path of magma migration derived from seismic and deformation studies another high conductance zone was also found near mayu yama in the eastern part of shimabara peninsula indicating magma degassing process due to the possibility of the presence of a magma system beneath it

selective heat producing element enrichment in a crustal segment of the mid european variscan chain abstract heat production has been calculated from u th and k content measurements on selected rock from the armorican massif western france geographical geochemical and age criteria have been introduced during the selection in order to obtain a representative sampling of the outcropping formations this procedure allows the delineation of a high heat production zone with values nearly twice the average upper crustal heat production ones the high heat production zone laterally ends through a high gradient belt indicating an upper crustal origin this zone cross cuts the geological structures and encompasses various granitoid plutons of different geochemical typologies and ages however among the granites belonging to the same geochemical type only those plutons located in this zone are enriched in radioelements those located outside with the exception of a few massifs present normal radiogenic element concentrations the enrichment affects all intruded granitoids or orthogneisses irrespective of their age when geographical features are restored prior to the late carboniferous shear deformation then the high heat production zone no longer appears as fortuitous it shapes a region intruded by numerous granitoids and structured during the early proterozoic we therefore propose that the high heat production zone reflects a previous selective enrichment of this crustal segment related to the late cambrian plutons

generation of tonalitic and dioritic magmas by coupled partial melting of gabbroic and metasedimentary rocks within the deep crust of the famatinian magmatic arc argentina the source regions of dioritic and tonalitic magmas have been identified in a deep crustal section of the famatinian arc sierras pampeanas of western argentina the source zones of intermediate igneous rocks are located at the transition between a gabbro dominated mafic unit and a tonalite dominated intermediate unit in the upper levels of the mafic unit mafic magmas intruded into metasedimentary wall rocks crystallized mainly as amphibole gabbronorite and caused the partial melting of the surrounding metasediments in turn the leucogranitic melts sourced from the metasedimentary rocks intruded into the newly crystallized but still hot mafic layers and catalysed the process of partial melting of the gabbroic plutonic rocks the gabbroic rocks became mafic migmatites comprising amphibole rich pyroxene bearing mesosomes and leucotonalitic veins significantly most of the mafic migmatites have isotopic compositions sr sr t 50 7063 and end t 1 4 0 94 to 2 24 similar to those of the gabbroic rocks and distinct from those of their complementary leucotonalitic veins sr sr t 1 4 0 7075 0 7126 and end t 5 2 65 providing evidence for the idea that melting of the mafic rocks was triggered by the intrusion of leucogranitic anatectic melts sr sr t 1 4 0 715 and end t 1 4 6 21 mass balance calculations show that the model reaction plagioclase amphibole leucogranitic melt leucotonalitic melt clinopyroxene orthopyroxene can better explain the partial melting of the gabbroic rocks based on field observations we argue that the coalescence of leucotonalitic veins in the mafic migmatites led to breakdown of the solid matrix to form melt dominated leucotonalitic pools however the leucotonalitic veins that crystallized before leaving behind the mafic migmatitic rock are chemically elemental and isotopic more evolved than the dioritic and tonalitic rocks we envisage that once detached from their source region the leucotonalitic magmas were able to react commingle and mix with entrained fragments of both mafic and metasedimentary rocks this process gave rise to melts that became tonalitic and dioritic magmas this study concludes that the generation of intermediate magmas is a multistage process with three critical steps 1 influx and emplacement of hydrous mafic magmas into a deep crust containing metasedimentary country rocks 2 physically and chemically coupled melting of mafic and metasedimentary rocks leading to the formation of a leucotonalitic vein and dyke system that coalesces to form leucotonalitic or tonalitic magma bodies 3 retrogression of the leucotonalitic magmas by partially assimilating entrained fragments of their mafic and metasedimentary precursors the dimensions of the

hot orogens tectonic switching and creation of continental crust many granulite terrains were too hot to have formed during continental collision rather along with many high grade metamorphic terrains that typify continental crust most formed in accretionary orogens during tectonic switching when prolonged lithospheric extension was interrupted by intermittent transient contraction based on modern and ancient examples tectonic switching occurs when slab retreat induces upper plate extension causing arc splitting formation of microcontinent slivers and backarc basins then intermittent arrival of buoyant oceanic plateaus induces transient flat subduction or slab flip and crustal thickening during slab retreat basaltic magmas produced from decompressed asthenosphere advect into the extending orogen causing granulite facies metamorphism and granite generation but subsequent thickening during flat subduction cools the region thickening is focused in the thermally softened backarc region and if sediment filled a hot short lived 10 m y narrow 50 100 km orogenic belt forms such thickening is often misleadingly ascribed to arc or microcontinent collision once slab retreat mode is reestablished lithospheric extension recommences and a new arc backarc system forms generally outboard arrival of another plateau will reverse the procedure and another short lived hot orogen will form within the orogenic system cycles of tectonic switching efficiently produce continental crust

pressure sources beneath unzen volcano inferred from leveling and gps data abstract unzen volcano erupted from 1990 to 1995 and the resultant volcanic deformations were measured by gps leveling tilt meter and edm the largest vertical displacement observed was subsidence of 8 cm in the western part of shimabara peninsula numerous magma chamber models were proposed based on the geodesic data obtained however these models could not explain the volcanic deformations observed around chijiwa bay after the eruption had finished because previous studies only used data collected during the eruption we therefore re examined the magma chamber model using leveling and gps data and included measurements collected both during and after the eruption on leveling we applied the mogi model with a height correction applied to the measuring points there are four parameters for each source longitude latitude depth and volume change each source parameter was estimated using a grid search scheme we also used aic akaike information criterion to fix the number of pressure sources beneath unzen to explain the vertical and horizontal deformations during 1986 2004 the results show that there are four pressure sources under unzen volcano and that magma ascended obliquely at an angle of about 45 from beneath chijiwa bay to the crater created by this eruption after the eruption had stopped magma inflows into deep sources were identified based on our leveling survey conducted after 1996 we compare our source model with several seismic data sets before the unzen eruption an earthquake swarm occurred from 1989 to 1990 in which pressure sources were detected just below the earthquake hypocenters other seismic refraction studies support our estimates of the location of these pressure sources using volume change values we inferred that magma supply had continued into this deep source at the rate of 2 0 2 5 m 3 yr from at least 1984 until after eruption stopped

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