gravity above subduction zones and forces controlling plate motions 1 short and intermediate wavelength gravity and geoid anomalies are used to provide constraints on the mechanical structure of subduction zones and on the forces involved this study is based on 2d cartesian dynamically self consistent models with newtonian or power law rheologies we show that both strong decoupling of the two convergent plates shear stresses of the order of 107 pa and weakened bending lithosphere are necessary to reproduce the observed geoid and gravity data good fits are found for relatively low failure stresses 30 50 mpa for all models providing reasonable predictions of gravity only a small fraction of the downgoing slab weight is transmitted to the surface plates about 10 of the energy is dissipated in the contact zone between the two plates 10 to 20 in the bending region and more than 70 in the sublithospheric mantle the basal tractions on the order of 1 4 1012 n m induce a net motion of the plates with the subducting lithosphere moving faster than predicted by the no net motion principle a marked positive geoid anomaly is predicted above subduction zones at intermediate wavelength 2000 4000 km in the case of pure whole mantle convection such large geoid highs are not observed introducing partial layering i e mass anomalies hampering mantle flow through the transition zone is necessary to reconcile model predictions and observations for these wavelengths

a slab in depth three dimensional geometry and evolution of the indo australian plate geoscientists use plate tectonics to explain many aspects of both continental evolution and evolution of the planet as a whole the subduction of material at convergent plate boundaries forms a fundamental component to the theory of plate tectonics plates continents subduction zones and spreading centers all exhibit motion and geometric evolution so to try and resolve the past geometries of the planet geologists have utilized plate tectonic reconstructions here we present a three dimensional image of the subducted indo australian plate below southeast asia and show that the geometry of the subducted slab at depth is intimately related to the geometric evolution of se asia over the past 50 ma including the collision of india with the asian continent we show how the once semicontinuous subducting indo australian plate has been segmented during collision between india australian and the subduction margin to the north thus we have found that the geometry of the subducted plate should form a key component to the interpretation of the evolution of earth s surface as complexities and evolution of the subducted plate are manifest in the evolution of the overriding plate

controls on back arc basin formation the relationship between subduction and back arc spreading has been well known since the early days of plate tectonics however the reasons why back arc basins are associated with some subduction systems but not all has remained elusive we examine the kinematic controls on subduction and back arc basins for both the present day and cenozoic to differentiate between the major competing hypotheses for back arc basin formation and to explain their temporal and spatial distribution our new data set of subduction and back arc basin parameters uses a new set of paleo oceanic age grids muller et al 2005 associated with a moving atlantic indian ocean hot spot reference frame o neill et al 2005 the plate model includes detailed reconstructed spreading histories of back arc basins based on marine geophysical and satellite gravity data our combined rotation and oceanic paleo age model provides the age distribution of subducting lithosphere through space and time convergence rates and the absolute motion of the downgoing and overriding plates we find that back arc basins develop when the age of subducting normal oceanic lithosphere is greater than 55 million years additionally we establish an age dip relationship showing that the intermediate dip angle of the subducting slab is always greater than 30 with back arc spreading our results suggest that back arc basin formation is always preceded by an absolute motion of the overriding plate away from the subduction hinge thereby creating accommodation space between the overriding and subducting plates once back arc extension is established it continues regardless of overriding plate motion indicating back arc spreading is not a simple consequence of overriding plate behaviour the landward migration of the overriding plate as a precursor to back arc extension may indicate that extension on the overriding plate is influenced by the oceanward lateral flow of the mantle however once back arc extension is established rollback of the subduction hinge appears to be the primary force responsible for the continued creation of accommodation space our analysis indicates the driving mechanism for back arc extension is a combination of surface kinematics properties of the downgoing slab the effect of lateral mantle flow on the slab and mantle wedge dynamics

constraints on the lateral strength of slabs from three dimensional dynamic flow models if the viscosity of subducted lithosphere is described purely by temperature and pressure it should remain considerably more viscous than the surrounding material as it descends through the upper mantle many dynamic models of the long wavelength geoid associated with slabs require an increase in viscosity from the upper to the lower mantle but assume the perturbation to the lateral viscosity structure arising from the slabs themselves can be ignored previous studies indicate that strong and localized viscosity variations should have a dramatic influence on the geoid we present 3d finite element models of the regional geoid of the western pacific subduction zones slab buoyancies and viscosities are defined using the distribution of seismicity the geoid is very sensitive to the lateral strength of the slab very viscous slabs penetrating a low viscosity mantle generate significant 10 50 m long wavelength geoid lows opposite to the geoid high which is observed over slabs to obtain a geoid high comparable to that observed the lower mantle viscosity must be 60 200 times greater than the upper mantle viscosity and the slab must be in contact with the lower mantle these strict requirements suggest that slabs have been weakened and cannot act as stress guides from the deep mantle to the surface

the role of h 2 o in rapid emplacement and crystallization of granite pegmatites resolving the paradox of large crystals in highly undercooled melts granite pegmatite sheets in the continental crust are characterized by very large crystals there has been a shift in viewing pegmatites as products of very slow cooling of granite melts to viewing them as products of crystal growth in undercooled liquids with this shift there has been a renewed debate about the role of h2o in the petrogenesis of pegmatites based on data on nucleation of minerals and new viscosity models for hydrous granite melts it is argued that h2o is the essential component in the petrogenesis of granite pegmatites h2o is key to reducing the viscosity of granite melts which enhances their transport within the crust it also dramatically reduces the glass transition temperature which permits crystallization of melts at hundreds of degrees below the thermodynamic solidus which has been demonstrated by fluid inclusion studies and other geothermometers published experimental data show that because h2o drastically reduces the nucleation rates of silicate minerals the minerals may not be able to nucleate until melt is substantially undercooled in a rapidly cooling intrusion nucleation starts at its highly undercooled margins followed by inward crystal growth towards its slower cooling hotter core delay in nucleation may be caused by competition for crystallization by several minerals in the near eutectic melts and by the very different structures of minerals and the highly hydrated melts once a mineral nucleates however it may grow rapidly to a size that is determined by the distance between the site of nucleation and the point in the magma at which the temperature is approximately that of the mineral s liquidus assuming components necessary for mineral growth are available along the growth path granite pegmatites are apparently able to retain h2o during most of their crystallization histories within the confinement of their wall rocks pegmatitic texture is a consequence of delayed nucleation and rapid growth at large undercooling both of which are facilitated by high h2o li b f and p contents in granite pegmatite melts without retention of h2o the conditions for pegmatitic textural growth may be difficult to achieve loss of h2o due to decompression and venting leads to microcrystalline texture and potentially glass during rapid cooling as seen in rhyolites in contrast slow cooling within a large magma chamber promotes continuous exsolution of h2o from crystallizing magma growth of equant crystals and final solidification at the thermodynamic solidus these are the characteristics of normal granites that distinguish them from pegmatites

glaciodynamically upthrusted bands of englacially transported debris in the pleistocene of central poland two kinds of buried structures are described from dzirzenin north east of warsaw where they occur within a glaciofluvial landform 1 narrow till ridges showing vertically oriented structures excavated from stratified gravel and sands and 2 a narrow vertical zone of massive gravelly sandy material involving vertically oriented lens like layers composed of massive sand with pebbles or of diamicton the gravelly sandy zone is also closely surrounded by stratified glaciofluvial sediments in spite of their vertical position and internal deformation the till ridges and gravelly sandy zone show non tectonic contacts with the surrounding stratified undisturbed sediments the glaciofluvial sediments that occur immediately next to the structures under discussion are characterized by the occurrence of comparatively coarse material and interbeddings of diamicton which wedge out away from these structures the gravelly sandy zone separates different kinds of water laid deposits the buried structures are interpreted as former debris laden bands thrust upwards within the frontal part of the ice sheet and then transformed into still frozen debris ridges projecting over the already dead ice further melting of the decaying ice resulted in abundant glaciofluvial sedimentation and the debris ridges also supplied material for the deposition of the neighbouring stratified deposits one of the ridges separated different glaciofluvial environments the glaciofluvial sediments completely buried the ice cemented ridges which were finally transformed by a melting out process into the till ridges and the gravelly sandy zone the former are interpreted as having been transformed from upturned debris laden bands with a high concentration of debris or from the bands composed of frozen up sediment slabs the gravelly sandy zone is interpreted as having most probably been deposited from upturned bands characterized by a lesser concentration of debris

fluid inclusion examination of the transition from magmatic to hydrothermal conditions in pegmatites from san diego county california thermometric properties and compositions of fluid inclusions in quartz are used to constrain the roles that fluid soluble elements principally li b cl and f have in controlling the transition from magmatic to hydrothermal mineral paragenesis in pegmatites and to ultimately understand why some pegmatites in the san diego county pegmatite district contain abundant gem quality li bearing minerals in pockets whereas others do not in this district lithium cesium tantalum type pegmatites occur in the mesozoic peninsular ranges batholith emplacement of the dikes occurred at low pressures 200 300 mpa that resulted in the formation of large miarolitic cavities pockets some of which contain gem quality li bearing minerals two pegmatite suites were studied the gem bearing himalaya and the more barren la posta the inclusions measured in this study further underscore highly undercooled crystallization of pegmatites pressure corrected homogenization temperatures t h of 400 to 515 and 70 to 425 c were obtained for primary inclusions in the intermediate zone and the core respectively of a la posta dike primary inclusions in the intermediate zone and the massive quartz core of the himalaya pegmatite have t h ranges of 350 to 420 and 150 to 300 c respectively the high portion of the latter temperature range is interpreted to represent the conditions that existed during the initial crystallization of minerals that line pegmatite pockets the most important cations in fluid inclusions in both pegmatites are na b3 and li lithium concentrations are much higher in inclusions in the himalaya pegmatite up to 51 at of all cations within the massive quartz in the core zone in the la posta pegmatite few primary inclusions contain appreciable li the b content of inclusions in both pegmatites is high up to 65 at of cations the dominant anions in the inclusions are cl f and formula 1 the data suggest that hydrothermal fluids that collected in pockets were acidic and promoted the growth of tourmaline and other minerals that are stable in acidic solutions in both pegmatites na and b dominate secondary inclusions these inclusions reveal fluids stripped of li and k by crystallization of lepidolite within fractures of primary minerals throughout the pegmatites and sometimes as an alteration product in pockets the lowering of alkali h ratios in the fluid stabilized clays including kaolinite that line the walls of pockets coeval crystallization of terminated quartz crystals with clays is consistent with its precipitation from the fluids 1 embed mml math 1 gif

the deforming bed debris rich basal ice continuum and its implications for the formation of glacial landforms flutes and sediments melt out till abstract there are two potentially important shear zones at the base of the glacier the deforming layer and or the debris rich basal ice layer these two layers have a significant effect on glacier motion and glacial erosional depositional and sedimentary processes however it is shown that most of the processes operating within these two layers are very similar this includes compression at the ice margin longitudinal extension and simple shear upglacier similar styles of shear zone and associated fabric development and similar incorporation transport and depositional processes in the past many criterion have been developed to identify distinct deposition from the deforming layer and the debris rich basal ice layer however if the processes within these layers are similar then it makes the distinct identification of these two separate layers in the geological record very difficult however it is also shown that the main differences between the two layers include rates of sediment movement processes and preservation potential the later is very important as it appears that the two layers are interconnected and that material is melted out from the debris rich basal ice to form a significant component of the deforming layer in this way the final depositional imprint is from the deforming layer and there is little likelihood of the debris rich basal ice layer being preserved undeformed this is demonstrated from modern glacial environments pleistocene flute studies and studies of pleistocene melt out tills from the u k u s a and germany

a petrologic assessment of internal zonation in granitic pegmatites abstract cameron et al 1949 devised the nomenclature and delineated the patterns of internal zonation within granitic pegmatites that are in use today zonation in pegmatites is manifested both in mineralogy and in fabric mineral habits and rock texture although internal zonation is a conspicuous and distinctive attribute of pegmatites there has been no thorough effort to explain that mineralogical and textural evolution in relation to the zoning sequence presented by cameron et al 1949 or in terms of the comprehensive petrogenesis of pegmatite bodies pressure temperature and whole rock composition this overview of internal zonation within granitic pegmatites consists of four principal parts 1 a historic review of the subject 2 a summary of the current understanding of the pegmatite forming environment 3 the processes that determine mineralogical and textural zonation in pegmatites and 4 the applications of those processes to each of the major zones of pegmatites based on the concepts presented in london 2008 the fundamental determinates of the internal evolution of pegmatite zones are 1 the bulk composition of melt 2 the magnitude of liquidus undercooling prior to the onset of crystallization 3 subsolidus isothermal fractional crystallization by which eutectic or minimum melts fractionate by sequential non eutectic crystallization 4 constitutional zone refining via the creation of a boundary layer liquid chemically distinct from but continuous with the bulk melt at the crystallization front and 5 far field chemical diffusion the long range and coordinated diffusion of ions particularly of alkalis and alkaline earths through melt

granitic magma transport by fracture propagation granitic magmas commonly ascend tens of kilometres from their source terranes to upper crustal emplacement levels or to the earth s surface apart from its obvious bearing on the interpretation of the geology and geochemistry of felsic igneous rocks the magma ascent mechanism critically affects any modelling of the metamorphic evolution of the upper lithosphere as well as its rheology we propose that in general granitic magmas ascend via propagating fractures as dykes in extremely short time periods long distance diapiric transport of granitoid magmas through crustal sections is not viable on thermal or mechanical grounds and there is an apparent total absence of field evidence for diapiric rise of such magmas even in supposed type localities neither the shapes nor internal or external characteristics of granitic plutons necessarily reveal anything about the transport of their precursor magmas these are purely arrival phenomena dictated by local structure kinematics and stress states based on existing numerical treatments of the problem we show that granitic magmas are apparently sufficiently inviscid to travel through fractures to high crustal levels without suffering thermal death by freezing for example a 2000 km3 granitic batholith can be inflated by a 1 km 3m 20 km deep dyke system in less than 900 years the model proposed has numerous far reaching petrological and rheological consequences some of which are outlined

solubility and stability of beryl in granitic melts beryllium contents of granitic melts at saturation in beryl or chrysoberyl phenakite or quartz have been determined by forward and reverse direction experiments at 650 850 degrees c and 200 mpa using natural beryl or the compositionally equivalent mixture of phenakite alumina quartz added to metaluminous haplogranite peraluminous haplogranite and macusanite a peraluminous li b f and p enriched rhyolite obsidian from macusani peru chrysoberyl coexists with beryl or quartz in moderately to strongly peraluminous systems asi approximately or 1 05 the assemblage phenakite beryl chrysoberyl is stable in silica undersaturated bulk composition the beo content of the melts which ranged between 0 03 0 40 wt varies principally with temperature the solubility of beryl falls with decreasing t and with the activity product of beryl alpha beo 3 alpha al2o3 alpha sio2 6 beryllia contents are lowest in strongly peraluminous and quartz saturated haplogranite melts however the highest beo content measured in glass products occurs for the strongly peraluminous macusanite at 850 degrees c 4016 ppm we infer that the greater solubility of beryl in macusanite melt results from melt speciation reactions involving li f b and p for all bulk compositions studied the beo content of melt projects to a narrow range of similar values at low t near the solidus of haplogranite thus metaluminous to peraluminous granitic magmas with and without common volatile and fluxing components will if cooled to subsolidus temperatures have similarly low beo requirements for beryl saturation the small beo content of melt near the solidus of haplogranite results in a minor 10 degrees c depression of the freezing point compared to be free haplogranite melt the mean melt composition at the beryl saturated granite minimum shifts slightly toward quartz beryl is common in peraluminous granitic rocks in part because lower beo contents are required to saturate these melts in beryl and possibly because these melts acquire higher beo contents by mica melting reactions at their sources beryl is also a common constituent of border facies assemblages in granitic pegmatite dikes regardless of their asi values this early crystallization of beryl results from the low beo content required to saturate any granitic melt in beryl at the low t environments in which pegmatite dikes solidify

40ar 39ar geochronological evidence for multiple postmetamorphic hydrothermal events focused along faults in the southern abitibi greenstone belt thermal histories in the abitibi and pontiac subprovinces vary for three general domains 1 zones of impermeable rock with cooling rates of 8 c ma 2 zones of impermeable rock that are spatially associated with monzogranite plutons with apparent cooling rates of 1 2 cima and 3 permeable high strain zones with apparent cooling rates of 2594 ma from the volcanogenic massive sulphide related sericite alteration with the coarser mica yielding the younger date based on radiometric dating impermeable rocks were affected weakly by postmetamorphic hydrothermal events minerals in zones of higher permeability were reset during the intrusion of monzogranite plutons highly permeable zones such as the larder lake cadillac and porcupine destor faults were overprinted repeatedly this produces a drawn out series of dates from minerals with varying closure temperatures radiometric ages from fault zones or ore deposits cannot be used to interpret the cooling history and evolution of the abitibi belt as a whole

estimation of crystallization pressure of granite intrusions abstract a numerical method is presented to estimate the crystallization pressure of granite intrusions based on two polynomial equations obtained by an analysis of the existing haplogranite ternary phase diagram and associated dataset the results indicate that the pressure is correlated respectively with normative quartz qtz content and with normative albite ab plus orthoclase or contents of granitic rocks as follows 1 p 0 2426 qtz 3 26 392 qtz 2 980 74 qtz 12563 2 p 0 2426 ab or 3 46 397 ab or 2 2981 3 ab or 64224 r 2 0 9943 where p is pressure in mpa and r denotes correlation coefficient it is noted that the procedure of normalizing the sum of cipw norm quartz albite orthoclase contents to 100 is required before using eqs 1 and 2 the difference in pressure calculations between these two equations is 16 mpa for the range of normative quartz contents from 15 to 40 wt an example of how to use these equations to estimate the crystallization pressure of a granite intrusion is also provided to show the validity and convenience of this method the uncertainty of such pressure estimation is not well known although it must fall into the uncertainty range of the existing experimental work on pressure constraints the simplicity of this empirical method is appreciable although its applicability to natural granitoids needs further test more experimental work is required to constrain the effects of components such as cao feo mgo f cl co2 on the granite phase equilibria these equations however can be used for estimating crystallization pressures of water saturated and quartz oversaturated granitic systems

the nature origin and physicochemical controls of hydrothermal mo bi mineralization in the cadillac deposit quebec canada mo bi mineralization occurs in subvertical and subhorizontal quartz muscovite k feldspar veins surrounded by early albitic and later k feldspathic alteration halos in monzogranite of the archean preissac pluton abitibi region quebec canada molybdenite is intergrown with muscovite in the veins or associated with k feldspar in the alteration halos mineralized veins contain five main types of fluid inclusions aqueous liquid and liquid vapor inclusions aqueous carbonic liquid liquid vapor inclusions carbonic liquid and vapor inclusions halite bearing aqueous liquid and liquid vapor inclusions trapped mineral bearing aqueous liquid and liquid vapor inclusions the carbonic solid in frozen carbonic and aqueous carbonic inclusions melts in most cases at 56 7 0 1 c indicating that the carbonic fluid consists largely of co2 all aqueous inclusion types and the aqueous phase in carbonic inclusions have low initial melting temperatures 70 c requiring the presence of salts other than nacl leachate analyses show that the bulk fluid contains variable proportions of na k ca cl and traces of mg and li the following solids were identified in the fluid inclusions by sem eds analysis halite calcite muscovite millerite nis barite and antarcticite cacl2 6h2o all are interpreted to be trapped phases except halite which is a daughter mineral and antarcticite which formed during sample preparation freezing aqueous inclusions homogenize to liquid at temperatures between 75 c and 400 c the mode is 375 c aqueous carbonic inclusions homogenize to liquid or vapor between 210 c and 400 c halite bearing aqueous inclusions homogenize by halite dissolution at approximately 170 c aqueous inclusions containing trapped solids exhibit liquid vapor homogenization at temperatures similar to those of halite bearing aqueous inclusions temperatures of vein formation based on oxygen isotopic fractionation between quartz and muscovite range from 342 c to 584 c the corresponding oxygen isotope composition of the aqueous fluid in equilibrium with these minerals ranges from 1 2 to 5 5 per mil with a mean of 3 9 per mil suggesting that the liquid had a significant meteoric component isochores for aqueous fluid inclusions intersect the modal isotopic isotherm of 425 c at pressures between 590 and 1900 bar a model is proposed in which molybdenite was deposited owing to decreasing temperature and or pressure from co2 bearing moderate to high salinity fluids of mixed magmatic meteoric origin that were in equilibrium with k feldspar and muscovite these fluids resulted from the degassing of a monzogranitic magma and evolved through interaction with volcanic komatiitic and sedimentary country rocks

cooling rates and crystallization dynamics of shallow level pegmatite aplite dikes san diego county california pegmatites of the pala and mesa grande pegmatite districts san diego county california are typically thin sheet like composite pegmatite aplite dikes aplitic portions of many dikes display pronounced mineralogical layering referred to as line rock characterized by fine grained garnet rich bands alternating with albite and quartz rich bands thermal modeling was performed for four dikes in san diego county including the 1 m thick himalaya dike the 2 m thick mission dike the 8 m thick george ashley dike and the 25 m thick stewart dike calculations were based on conductive cooling equations accounting for latent heat of crystallization a melt emplacement temperature of 650 c into 150 c fractured gabbroic country rock at a depth of 5 km and an estimated 3 wt initial h 2o content in the melt cooling to 550 c at the center of each dike occurred in 9 years for the stewart dike 340 days for the george ashley dike 22 days for the mission dike and 5 days for the himalaya dike based on these calculations growth rates for large pegmatitic minerals such as the 10 cm long himalaya hanging wall tourmaline crystals may have been on the order of 10 5 cm s crystal size distribution csd studies of garnet from layered aplites suggest growth rates of about 10 6 cm s these results indicate that the dikes cooled and crystallized rapidly with variable nucleation rates but high overall crystal growth rates initial high nucleation rates coincident with emplacement and strong undercooling can account for the millimeter size aplite grains lower nucleation rates coupled with high growth rates can explain the decimeter size minerals in the hanging walls cores and miarolitic cavities of the pegmatites the presence of tourmaline and or lepidolite throughout these dikes suggests that although the melts were initially h 2o undersaturated high melt concentrations of incompatible or fluxing components such as b f and li h 2o aided in the development of large pegmatitic crystals that grew rapidly in the short times suggested by the conductive cooling models

drake passage and cenozoic climate an open and shut case drake passage opening has often been viewed as a single discrete event possibly associated with abrupt changes in global circulation and climate at or near the eocene oligocene boundary a new plate tectonic model based on recent reinterpretations of the opening history of basins in the scotia sea suggests that an effective ocean gateway may have developed even earlier during the middle eocene this is consistent with a growing body of evidence from sediment core proxy data for eocene changes in southern ocean circulation and biological productivity the period between earliest opening after 50 ma and the latest eocene was characterized by the evolution of various current pathways across the subsiding continental shelves and intervening deep basins this shallow opening may have caused important changes in southern ocean circulation contributing to eocene cooling and the growth of antarctic ice sheets

revisiting complete miscibility between silicate melts and hydrous fluids and the extreme enrichment of some elements in the supercritical state consequences for the formation of pegmatites and ore deposits abstract using water and major and trace element data obtained from melt inclusions primarily in pegmatite quartz we have shown in this and previous papers that melt melt fluid immiscibility is widespread and an important process during the generation of granitic pegmatites furthermore we have shown that the formation of pegmatites actually begins in the supercritical melt fluid stage in this study we extend previous work and in particular explore the behavior of 10 different elements be f p s cl as rb sn cs and ta in the supercritical range from preliminary studies we suggest that other elements such as b na k zn nb and sb will behave similarly near the solvus crest of a generalized pseudo binary melt water system this provides important evidence that pegmatite forming processes have already begun at high temperatures in the range of 850 750 c and that may require a re think on the partitioning behavior of metals in late stage residual melts exsolved from granitic magmas the existence of this parental supercritical silicate melt fluid although derived from granitic magmas imposes significant constraints of its own so drawing conclusions about pegmatite forming processes from data gained from slightly modified granite compositions becomes highly problematic we propose that the extremely high concentration of some elements over a small but well defined water concentration range symmetrically distributed around the critical point of the pseudo binary solvus must be derived from the unusual properties of the near critical or supercritical phases these unusual properties include extremely low density viscosity and surface tension and high diffusivity reactivity and mobility these are exactly the qualities that make such supercritical fluids an excellent medium for the formations of pegmatites by reactions with the existing matrix melt crystal mush and for the extreme enrichment of some ore forming elements highly soluble in alkali and water rich supercritical fluids a very important point in mineral crystallization is the non uniform local enrichment of rare and economically significant elements at specific locations in pegmatites granites and other rocks given the properties of the supercritical phase identified from melt inclusions the process of ostwald ripening can operate very effectively the driving force for ostwald ripening arises because the concentration of solute in the vicinity of small crystals or clusters is greater than and in the vicinity of larger crystals or clusters less than the average supersaturation the solute in the supercritical fluid therefore flows from the small crystals to the larger crystals and permits the growth of very large crystals at the expense of smaller ones this is favored by the extraordinary properties of supercritical fluids given the low surface tension the supercritical fluid can move also through grain boundaries and small channels or dislocation pipes aiding incompatible element transport additionally their consequences for the formation of some important granite related ore deposits are briefly outlined

influence of the subducting plate velocity on the geometry of the slab and migration of the subduction hinge abstract geological observations indicate that along two active continental margins east asia and mediterranean major phases of overriding plate extension resulting from subduction hinge retreat occurred synchronously with a reduction in subducting plate velocity in this paper results of fluid dynamical experiments are presented to test the influence of the velocity of the subducting plate on the hinge migration velocity and on the geometry of the slab results show that hinge retreat decreases with increasing subducting plate velocity in addition phases of hinge retreat alternate with phases of hinge advance for relatively high subducting plate velocities due to interaction of the slab with the bottom of the box simulating the upper lower mantle discontinuity such slab kinematics could explain the episodic behaviour of back arc opening observed in convergent settings the geometry of the slab and the kinematics of subduction are significantly affected by the velocity of the subducting plate three subduction modes with accompanying slab geometry can be recognized a relatively low subducting plate velocity is accompanied by relatively fast hinge retreat with backward sinking of the slab and a backward draping slab geometry with increasing subducting plate velocity hinge migration is relatively small resulting in subvertical sinking of the slab and a folded slab piling geometry for a very high velocity the hinge migrates forward resulting in forward oriented subduction vectors and a forward draping slab geometry

dynamics of plate bending at the trench and slab plate coupling 1 the bending strength of subducting lithosphere plays a critical role in the earth s plate tectonics and mantle convection modulating the amount of slab pull transmitted to the surface and setting the boundary conditions under which plates move and deform however it is the subject of a lively debate how much of the potential energy of the downgoing plate is consumed in bending the plate and how the lithospheric strength is defined during this process we model the subduction of a viscoelastic lithosphere driven solely by the downgoing plate s buoyancy freely sinking in a passive mantle represented by drag forces to investigate the dynamics of bending 1 we vary the density and the viscosity profile within the plate from isoviscous where strength is distributed to strongly layered where strength is concentrated in a thin core and 2 we map the stress strain and dissipation along the downgoing plate the effective plate strength during bending is not a simple function of average plate viscosity but is affected by rheological layering and plate thinning earth like layered plates allow for the transmission of large fractions of slab pull 75 80 through the bend and yield a net slab pull of fspnet 1 to 6 1012 n m 1 which varies with the buoyancy of plates in all models only a minor portion of the energy is dissipated in the bending surprisingly bending dissipation hardly varies with lithospheric viscosity because in our dynamic system the plates minimize overall dissipation rate by adjusting their bending curvature as a result bending dissipation b is mainly controlled by the bending moment work rate exerted by slab pull we propose a new analytical formulation that includes a viscosity dependent bending radius which allows for assessment of the relative bending dissipation in the earth s subduction zones using parameters from a recent global compilation this yields estimates of b tot 20 these results suggest that plates on earth weakly resist bending yet are able to propagate a large amount of slab pull

geobarometry differential block movements and crustal structure of the southwestern abitibi greenstone belt canada the southern volcanic zone of the archean abitibi greenstone belt comprises several fault bounded tectonic blocks with distinct volcanic sedimentary sequences pressure temperature time p t t histories granite compositions and metallogenic provinces pressure distributions have been mapped out from an extensive new database derived from al in hornblende and other geobarometers in the composite crustal structure of the southern volcanic zone determined from the pressure time data the lacorne block and pontiac subprovince represent deeper crustal levels overlain by rocks exposed at the surface in the round lake or lac abitibi type blocks which are in turn capped by those exposed at the surface in a blake river rouyn type block exposure of the deeper crustal levels and juxtaposition of these distinct tectonic blocks are interpreted to have occurred by differential block uplift caused by strike slip faulting which was related to oblique collisional tectonics during late tectonic development of the superior province in late archean time

the physical and chemical evolution of fluids in rare element granitic pegmatites associated with the lacorne pluton quebec canada abstract co magmatic rare element be li nb ta granitic pegmatites associated with the lacorne monzogranite pluton of the preissac lacorne batholith quebec range from a least evolved beryl type in the pluton through a transitional beryl spodumene type to a most evolved spodumene type in mafic country rocks they possess internal lithologic fabrics and primary fluid inclusions that provide evidence for saturation of the magma with aqueous fluids immediately after the formation of aplite in the beryl pegmatite and pene contemporaneously with formation of the beryl spodumene and spodumene pegmatites the early fluids are represented by aqueous liquid vapor type 1a and aqueous liquid vapor solid type 2 inclusions in beryl from beryl and beryl spodumene pegmatites and are estimated to have been trapped between 500 and 550 c and at 3 5 kb on the basis of microthermometric measurements analyses of the compositions of the precipitates from decrepitated inclusions and the nature of the solids trapped these fluids are interpreted to have been nacl dominated 16 wt nacl eq and to have contained appreciable fe and dissolved co2 in spodumene pegmatite the equivalent fluid which was trapped at 450 to 500 c contained significant mn li and cs in place of fe consistent with corresponding trends in the chemical evolution of the host rock volatile rich mainly secondary aqueous fluid inclusions type 1b trapped in beryl are interpreted to reflect temporary sharp drops in pressure later in the crystallization history of the beryl bearing pegmatites due to local fluid overpressures that led to brecciation and consequent pressure release these inclusions have high salinity are cacl2 rich and are considered to represent magmatic hydrothermal fluids that acquired their calcium as a result of their interaction with the host lacorne pluton incursion of cacl2 rich fluids into the spodumene pegmatites is interpreted to have occurred earlier in their crystallization history i e upon emplacement of the pegmatites as shown by the fact that a high proportion of primary inclusions in spodumene in these pegmatites contains considerable calcium and calcium contents are generally higher in inclusions in spodumene than in beryl the inclusions in the spodumene pegmatites are thought to have acquired their calcium through interaction of the fluids with the amphibolites metabasalts that host these intrusions the crystallization of the pegmatites terminated with the entrapment of low salinity aqueous carbonic fluids type 3 inclusions in quartz 6 10 wt nacl eq which most likely formed as a result of the unmixing of the magmatic fluid into separate aqueous and carbonic fluids probably at temperatures between 250 and 350 c the results of this study show that aqueous fluid saturation can take place at an early stage of pegmatite emplacement and that magma evolution is matched by changes in the solute chemistry of the exsolving aqueous fluid they also show that evolution to a more volatile rich composition h2o f li may have contributed to the increased mobility of later magma batches thereby helping to explain the regional zonation of the pegmatites from beryl type within the pluton to spodumene type in the adjacent amphibolite

evolution of subduction zone curvature and its dependence on the trench velocity and the slab to upper mantle viscosity ratio 1 three dimensional laboratory models of upper mantle subduction are presented investigating the effect of the trench velocity vt and the slab to upper mantle viscosity ratio sp um on trench curvature and slab curvature one set of experiments varies sp um from 66 to 1375 another set of experiments modifies vt through applying different velocities at the trailing plate the results show that the radius of trench curvature rtc progressively decreases with continuous trench retreat due to quasi toroidal mantle return flow from the subslab region around the lateral slab edges and toward the mantle wedge region but starts to increase when trench retreat changes to trench advance due to quasi toroidal return flow in the opposite direction furthermore rtc increases with increasing sp um i e progressively stronger slabs are progressively less curved following a cubic root function the force required to curve the slab and trench varies with progressive subduction but is only 0 1 2 5 of the total negative buoyancy force of the slab while the viscous dissipation rate due to progressive slab curvature is only 0 01 0 60 of the potential energy release rate comparison of trench curvature in the models scaled slab width w 750 km with that of the scotia subduction zone w 800 km indicates that for the scotia subduction zone the effective sp um is of the order 1 2 102 finally the elastic sphere indentation model for arc curvature is revisited demonstrating that four main predictions are not met by observations implying that the model should be rejected

the role of magmatic and hydrothermal processes in the evolution of be bearing pegmatites evidence from beryl and its breakdown products beryl and euclase crystals from the mina do santino and the jacu pegmatites in the borborema pegmatite province in northeastern brazil contain several generations of melt and fluid inclusions which allow interpretation of p t x conditions responsible for beryl crystallization and for alteration of a primary pegmatitic mineral assemblage to a mixture of hydrothermal minerals euclase bertrandite kaolinite and quartz primary melt and fluid inclusions hosted by beryl were trapped simultaneously however their homogenization temperatures are significantly higher 870 900 c than the values previously reported for pegmatitic systems 712 c and should be treated with caution an isobaric drop of temperature resulted in the exsolution of a fluid a low salinity co2 enriched phase and a saline water rich phase were trapped in pseudosecondary inclusions in beryl at a pressure of 2 1 2 7 kbar and temperature of 390 480 c cooling of the country rocks below 400 c caused a ductile to brittle transition and allowed infiltration of cold groundwater which further decreased the temperature in the system to 190 240 c at the same time the pressure dropped from a lithostatic 2 1 2 7 kbar to a hydrostatic value 0 57 0 73 kbar consequently minerals deposited under magmatic conditions feldspars and beryl became unstable and a newly formed hydrothermal mineral paragenesis euclase bertrandite kaolinite and quartz overprinted the earlier one the hydrothermal fluids responsible for the alteration differ from the earlier exsolved fluids in having a lower salinity lower homogenization temperature the absence of co2 and the presence of ch4

asymmetric deformation in the backarc region of the kuril arc northwest pacific new insights from analogue modeling 1 the eocene to middle late miocene tectonic evolution of the kuril arc and backarc region has been simulated with analogue experiments the experiments simulate asymmetric deformation in the overriding plate due to anticlockwise rollback of the subducting pacific plate the results show the formation of a n s to ne sw oriented dextral shear zone near the far edge of the retreating boundary analogous to the sakhalin hokkaido dextral shear zone contemporaneously normal faults and grabens form striking parallel to the retreating boundary near the far edge but striking more oblique near the hinge point and away from the retreating boundary this is similar to extensional structures observed in the kuril basin and the sea of okhotsk furthermore the model shows that the amount of extension progressively decreases away from the retreating boundary this appears to have also happened in the kuril okhotsk region as evidenced by crustal thickness variation in the region finally the model results show that extension is increasingly accommodated by the region close to the retreating boundary with progressive deformation this can account for the eocene early miocene extension in the sea of okhotsk followed by miocene spreading in the kuril basin

the geochemical differentiation of s type pegmatites constraints from major trace element and li b isotopic composition of muscovite and tourmaline pegmatites often exhibit regional as well as internal zonation in this study we use major and trace element and li b isotopic composition of muscovite and tourmaline from internally zoned and un zoned s type pegmatites and their host granites to characterize the geochemical and isotopic fractionation associated with their formation the internally zoned pegmatites comprise three distinct textural zones namely wall intermediate and core muscovite and tourmaline occur in all the zones of these pegmatites the trace element concentrations ratios of muscovite from the three zones form well defined differentiation trends marked by enrichment of incompatible elements such as rb cs sr b zn nb ta p and the depletion of ni co v sc ti ba from the wall zone through the intermediate zone to the core zone this is suggestive of a strong role of fractional crystallization in producing the compositional diversity in the internally zoned pegmatites alkali element ratios such k rb and k cs in muscovite exhibit near exponential decline from the wall to the core zone which is suggestive of rayleigh type fractional crystallization fractional crystallization modelling reveals that the formation of the wall zone requires 69 crystallization the intermediate zone 85 95 and the core zone ca 99 crystallization leading to extreme enrichment of rb and cs and other incompatible elements muscovites and tourmaline from the un zoned pegmatites display similar compositional trends as the internally zoned ones but with a significant compositional gap between the host granite and the pegmatite lithium isotopic composition of muscovites and b isotopic composition of tourmalines become progressively lighter from wall to core zone of the internally zoned pegmatite and from the granite to the pegmatite for the un zoned pegmatites this is suggestive of an important role of vapour exsolution in the formation of the pegmatites taken together the geochemical and isotopic trends in the pegmatites can be explained by rayleigh fractional crystallization operating in tandem with vapour exsolution

subduction zones observations and geodynamic models abstract this review of subduction and geodynamic models is organized around three central questions 1 why is subduction asymmetric 2 are subducted slabs strong or weak 3 how do subducted slabs interact with phase transformations changes in mantle rheology and possibly chemical boundaries in the mantle based on laboratory measurements of the temperature dependence of olivine one would conclude that the core of a subducting slab is at least 10 000 times more viscous than ambient mantle however there are a number of complementary but independent observations that suggest that slabs are much weaker than this slabs undergo significant deformation in the upper mantle and may thicken to twice their original width by the time they reach the base of the transition zone the lack of a clear correlation between the observed dip angle of deep slabs and plate velocity rate of trench migration and slab age in modern subduction zones is consistent with hypothesis that subduction is a time dependent phenomenon both tank and numerical convection experiments with plates conclude that subduction is not a steady phenomenon but that slabs bend thicken stretch and change dip through time this is at odds with the assumptions used in steady state slab thermal models where slab deformation is not considered

crystallization conditions and evolution of magmatic fluids in the harney peak granite and associated pegmatites black hills south dakota evidence from fluid inclusions abstract a microthermometric study of inclusions in granites and pegmatites in the proterozoic harney peak granite system identified four types of inclusions type 1 inclusions are mixtures of co2 and h2o and have low salinities on average 3 5 wt nacleq type 2 inclusions are aqueous solutions of variable salinities from 0 to 40 wt nacleq type 3 inclusions are carbonic dominated by co2 with no detectable water and type 4 inclusions consist of 20 to 100 solids with the remaining volume occupied by a co2 h2o fluid many inclusions have a secondary character however a primary character can be unambiguously established in several occurrences of the type 1 inclusions these inclusions were trapped above the solidus and represent the exsolved magmatic fluid the secondary populations of types 1 2 and 3 probably formed as a result of reequilibration and unmixing of the type 1 fluid that progressively changed composition and density with decreasing temperature and pressure and was finally trapped along healed microfractures under subsolidus conditions type 4 inclusions are primary and are interpreted to be trapped fluid bearing complex silicate melts that subsequently solidified or underwent other posttrapping changes it is demonstrated that primary type 1 fluid inclusions that coexist with crystallized melt inclusions in the complex li bearing tin mountain pegmatite were trapped along the two fluid phase boundary in the system co2 h2o nacleq consequently the temperature and pressure conditions of trapping are identical to the bulk homogenization conditions on average 340 c and 2 7 kbar these conditions indicate that this li cs rb p and b rich pegmatite crystallized at some of the lowest known temperatures for a silicate melt in the crust an internally consistent empirical solvus surface in p t xco2 coordinates was generated for the pseudobinary co2 h2o 4 3 wt nacleq pegmatite fluid system distribution coefficients for the major species co2 h2o nacl and ch4 between the immiscible co2 rich and h2o rich fluid phases as a function of pressure and temperature were extracted from data for the two cogenetic fluid inclusions types

exploring the effect of lithium on pegmatitic textures an experimental study the effect of lithium li on the development of pegmatitic textures was evaluated experimentally at h 2 o saturated and h 2 o undersaturated conditions at 500 mpa pressure and temperatures ranging from 400 to 800 c the addition of 3700 ppm li to a starting material of common granitic composition lake county obsidian reduces the crystallization and melting temperatures by approximately 50 70 and 200 c respectively and promotes the formation of quartz feldspar graphic and granophyric intergrowths the presence of li in the hydrous granitic system lowers significantly the degree of undercooling needed for the development of graphic and granophyric intergrowths to 85 110 c in comparison with the li free systems where approximately 200 c of undercooling is necessary the feldspar crystals developed in li bearing samples can incorporate from 150 to 250 ppm li assume skeletal and spherulitic morphologies and exhibit one order of magnitude faster growth rates than crystals in li free samples the results of this study attest to the effectiveness of li as a fluxing agent and highlight the crucial role it plays in the development of pegmatitic textures leading to important implications for the conditions of formation of li bearing granitic pegmatites

fluid evolution in the strange lake granitic pluton canada implications for hfse mobilisation abstract strange lake is a mid proterozoic peralkaline granite pluton quebec labrador canada that underwent extreme enrichment in high field strength elements hfse including the rare earth elements ree the hfse mineralisation is confined to highly altered pegmatites and the most altered parts of the granites implying a genetic association between hydrothermal fluids and hfse enrichment this study uses analyses of fluid inclusions to investigate the hydrothermal evolution of the strange lake pluton and the role of hydrothermal processes in concentrating the hfse to potentially exploitable levels five groups of inclusions were distinguished from earliest to latest these groups are primary aqueous inclusions 25 wt nacl eq associated with melt inclusions group 1 primary aqueous inclusions 13 23 wt nacl eq associated with ch 4 inclusions groups 2a and b primary aqueous inclusions 9 wt nacl eq associated with co 2 inclusions group 3 primary aqueous inclusions 9 wt nacl eq which contain no carbonic component group 4 and finally secondary aqueous inclusions 19 wt nacl eq including inclusions that outline mineral pseudomorphs also with no detectable carbonic component group 5 most of the inclusions except those in group 5 have conspicuous implosion haloes evident as numerous tiny voids this indicates that the inclusions re equilibrated most likely during isobaric cooling fluid evolution commenced with the exsolution of a saline aqueous liquid 25 wt nacl eq and an immiscible ch 4 h 2 gas from the pegmatitic melt at temperatures of 450 500 c and a pressure of 1100 bars during isobaric cooling the gas component of the fluid was gradually oxidised evolving from being ch 4 dominant to a ch 4 fluid with a significant proportion of higher order hydrocarbons due to oxidative coupling of methane induced by the consumption of o 2 through the alteration of arfvedsonite to aegirine 325 360 c and finally to a co 2 dominated fluid at 300 c the apparent salinity of the aqueous fluid decreased from 25 to 4 5 wt nacl eq due to fluid rock interaction the latter also caused precipitation of nahcolite as a result of the reaction of newly formed co 2 with sodium from decomposing minerals formation of pseudomorphs after primary na zirconosilicates and na titanosilicates and replacement of primary ree silicates by bastnasite ce owing to this interaction the carbonic component of the fluid was consumed which together with the consumption of h 2 o to form al k and fe phyllosilicates contributed to an increase in the fluid salinity up to 19 wt nacl eq light rare earth elements lree were remobilised over 10s to 100 s of metres by the high temperature high salinity fluid preserved as group 1 and 2a inclusions whereas heavy rare earth elements hree were remobilised on a much smaller scale by a late low temperature high salinity fluid trapped as inclusion group 5 fluid preserved as inclusion group 3 may have been responsible for remobilisation of zr and ti

bending mechanics and mode selection in free subduction a thin sheet analysis summary to elucidate the dynamics of free buoyancy driven subduction of oceanic lithosphere i study a model in which a 2 d sheet of viscous fluid with thickness h and viscosity 1 subducts in an infinitely deep ambient fluid with viscosity 1 numerical solutions for the sheet s evolution are obtained using the boundary element method bem starting from an initial configuration comprising a short protoslab attached to a longer horizontal plate that is free to move laterally beneath an impermeable traction free surface interpretation of the solutions using thin viscous sheet theory shows that the fundamental length scale controlling the subduction is the bending length lb defined at each instant as the length of the portion of the sheet s midsurface where the rate of change of curvature is significant geophysically speaking lb is the sum of the lengths of the slab and of the region seaward of the trench where flexural bulging occurs the bending length in turn enters into the definition of the sheet s dimensionless stiffness s h lb 3 which controls whether the sinking speed of the slab is determined by the viscosity of the sheet itself s 1 or by that of the ambient fluid s 1 motivated by laboratory observations of different modes of subduction retreating versus advancing trench folding versus no folding etc in fluid layers with finite depth i calculate numerically the dip d of the slab s leading end as a function of and the normalized depth d h to which it has penetrated the contours of the function d d h strongly resemble the intermode boundaries in the laboratory based regime diagram of schellart supporting the hypothesis that the mode of subduction observed at long times in experiments is controlled by the dip of the slab s leading end when it reaches the bottom of the layer in particular the bem solutions explain why trenches advance in the laboratory only when lies in an intermediate range and why they retreat when is either smaller or larger than this application of the bem model to wu et al s compilation of the minimum curvature radii of subducted slabs suggests 140 510 for the earth this is too small to permit the laboratory type trench advancing mode in agreement with the lack of tomographic evidence for slabs that are bent over backwards

geology and geochronology of the super large bailongshan li rb be rare metal pegmatite deposit west kunlun orogenic belt nw china abstract the bailongshan pegmatite deposit is a newly discovered super large li rb be ta nb rare metal deposit in the west kunlun orogenic belt northwest china fifty two rare metal orebodies 50 1230 m long by 1 5 157 m wide have been discovered by detailed field surveying making up a metallogenic field that is larger than 8250 400 m the deposit is estimated to have ore reserves of more than 5 06 million tonnes t li2o 160 020 t beo 316 200 t rb2o 40 060 t nb2o5 and 10 750 t ta2o5 pegmatite dikes are zoned around a granodiorite batholith ranging from barren through orebodies rich in be nb ta rb and li with increasing distance from the batholith similar to the classical model of a zoned li ce ta lct type pegmatite la icp ms analyses of coltan and zircon yield 206pb 238u ages of 208 1 ma and 212 3 ma respectively representing the emplacement ages of the pegmatites and granodiorite these data indicate a temporal link between the emplacement of the granodiorite batholith and rare metal mineralization in the pegmatites and that the bailongshan rare metal pegmatites are temporospatially related to the granodiorite batholith the bailongshan pegmatites and other rare metal pegmatite deposits in the west kunlun orogenic belt likely make up a 600 km long li metallogenic belt and further exploration for rare metal pegmatites should be focused on the area adjacent to late triassic granitoid plutons in the belt this belt likely extends to the east kunlun and west sichuan rare metal belts forming the 2000 km long late triassic west kunlun songpan garze rare metal metallogenic belt

mantle flow in the south sandwich subduction environment from source side shear wave splitting 1 the occurrence of seismic anisotropy in the earth s upper mantle is a global phenomenon related to subcrustal deformation and flow processes the shear wave splitting analysis method has led to a global set of anisotropy maps mainly derived from receiver side analysis remote places with few seismometers deployed remain unexamined source side splitting analysis allows mapping of mantle fabrics in these regions here we investigate seismic anisotropy in the south sandwich islands subduction environment core reflected scs waves recorded at the neumayer seismograph network are corrected for well constrained receiver anisotropy and then analysed for source anisotropy sub slab mantle minerals are aligned horizontally almost parallel to the trench indicating a westward flow around the subducting slab this is consistent with a model of horizontal mantle flow due to slab rollback that was previously inferred from marine and geochemical studies in the back arc region

late wisconsinan sub glacial clastic intrusive sheets along lake erie bluffs at bradtville ontario canada abstract numerous clastic intrusive sheets a few decimetres to more than 16 m long 1 120 cm thick and extending one to more than 25 m laterally occur along a 350 m long section of the late wisconsinan catfish creek drift in the lake erie bluffs at bradtville southwestern ontario most of them are downglacier dipping dikes the largest one terminating in the underlying middle wisconsinan tyrconnell formation most dikes strike nne ssw at right angles to the local direction of glacier movement during the deposition of catfish creek drift the tops of some of them are truncated or displaced downglacier by shearing the main concentration of clastic intrusive sheets is on the upglacier side of a glaciotectonically folded anticline of tyrconnell formation clays and silts underlying the catfish creek drift the host sediments are catfish creek till gravel sand and silt and tyrconnell formation silt and clay most intrusive sheets particularly the small to medium ones consist of massive to crudely laminated sand and silt intruded from below by a dewatering process the largest dike reflects in its composition mainly the adjoining or higher lying host sediment materials and its main part was formed by downward infilling or by gravity flows into an open fracture the large dike is flanked by small laminated silty sand sheets and several small apophyses some of them injected downward and sideways others upward by dewatering the clastic intrusive sheets were formed under a moving glacier the erie lobe probably both at the beginning and towards the end of the deposition of catfish creek till their location and position was predetermined by glaciotectonically induced listric planar structures and zones of weakness mainly tension fractures that strike transverse to glacier movement and dip downglacier and also by confinement of pore water in a permeable sediment wedge between the less pervious tyrconnell formation and massive catfish creek basal till

influence of trench width on subduction hinge retreat rates in 3 d models of slab rollback subduction of tectonic plates limited in lateral extent and with a free trailing tail i e free subduction is modeled in a three dimensional 3 d geometry the models use a nonlinear viscoplastic rheology for the subducting plate and exhibit a wide range of behaviors depending on such plate characteristics as strength width and thickness we investigate the time evolution of this progressive rollback subduction measure the accompanying return flow in the upper mantle and quantify the plate kinematics due to the 3 d geometry flow is allowed to accompany slab rollback around the lateral edges of the slab the toroidal component as opposed to 2 d geometry where material is forced to flow underneath the slab tip the poloidal component a simple force balance is provided which relates the speed of backward trench migration to the resistive forces of generating flow and weakening the plate our results indicate most of the gravitational energy of the system i e the negative buoyancy of the subducting slab is converted into a toroidal flow 69 a much smaller amount goes into weakening the plate 18 and the remaining amount goes into driving flow parallel to displacement of the slab 13 for the trench widths w we investigate 1500 km a maximum trench retreat rate occurs for trenches 600 km wide which is attributed to the interaction between a plate of finite width and the induced flow which has a lengthscale in the horizontal direction these numerical results quantitatively agree with comparable 3 d laboratory experiments using analogue models with a purely viscous plate material schellart 2004a 2004b including correlations between increasing retreat rate with increasing plate thickness trench width for maximum retreat rate 500 km and estimated amount of slab buoyancy used to drive rollback induced flow 70 several implications for plate tectonics on earth result from these models such as rollback subduction providing a physical mechanism for ephemeral slab graveyards situated above the more viscous lower mantle and endothermic phase transition prior to a flushing event into the lower mantle mantle avalanche

fluid inclusion microthermometry in be ta li sn bearing pegmatites from the borborema province northeast brazil abstract fluid inclusions in be ta li sn bearing pegmatites have been reported in the literature as hypersaline aqueous brines or low salinity aqueous or aqueous carbonic fluids the evolution paths of volatile fluids in equilibrium with the different crystallizing phases of rare element pegmatites do not seem to be unique the aim of this work is to contribute to this discussion on the basis of fluid inclusion information from the borborema pegmatite province bpp in northeast brazil some well known heterogeneous be ta li sn mineralized pegmatites of this province were systematically sampled along cross sections with the purpose of studying the fluid inclusions in the different pegmatite zones i muscovite rich contact zone ii homogeneous or graphic pegmatite iii blocky feldspar zone and iv quartz core the observed fluid inclusions mainly in quartz were almost always secondary or pseudosecondary grouped along healed fractures with random orientation even some trails paralleling quartz grain limits were not undoubtedly primary almost all these fluid inclusions were filled with low salinity aqueous or aqueous carbonic fluids primary inclusions were recognized only in some well formed crystals from druses with erratic distribution in the pegmatites microthermometric data from these samples confirm that the various crystallizing phases of the pegmatites were in equilibrium with low salinity aqueous or aqueous carbonic fluids with insignificant n 2 or ch 4 contents with frequent accidentally trapped solids the various types of primary inclusions found in these crystals compare well with the types of secondary inclusions of the samples from the systematic cross sections the relative age of the crystals hosting primary fluid inclusions the relative age of primary inclusion types in successive crystal growth zones the relative age of secondary fluid inclusion trails and the corresponding microthermometric data allowed to establish the most probable evolution of the fluid characteristics during pegmatite crystallization an early low salinity aqueous carbonic fluid with co 2 contents of 20 mol was present during the crystallization of early minerals such as garnet and tourmaline of the border zone and graphic quartz beryl manganotantalite and blue tourmaline of zones ii and iii the end of zone iii crystallization occurred in the presence of a low salinity aqueous fluid with very low co 2 contents as indicated by inclusions in the smoky core of quartz crystals at the limit between zones iii and iv later aqueous fluids barren in co 2 and with salinities increasing from 5 to 20 coexisted with the formation of the pegmatite nucleus and replacement bodies as indicated by inclusions in the border of these crystals and in euclase the estimated trapping conditions of 3 5 kbar and 580 c for the earliest inclusion types agree with the regional metamorphic conditions and with the known liquidus conditions of rare element bearing pegmatite crystallization the trapping conditions of inclusions in euclase and coexisting quartz crystal rims in the pegmatitic core at 3 5 kbar and 400 c are slightly below the conditions admitted for the solidus of rare element pegmatites and indicate an isobaric cooling during pegmatite crystallization

spatial and temporal evolution of the subducting pacific plate structure along the western pacific margin 1 tomographic images of the subducting pacific plate beneath the izu bonin mariana arc illustrate a progression of geometries from shallow dipping to vertical from north to south along the arc recent advances in technology and inversion techniques have improved resolution of slab structure beneath the western pacific island arcs but reasons for the variation in geometry and morphology are still poorly understood by comparing high resolution tomographic images of the western pacific and updated paleogeographic reconstructions we are able to link the spatiotemporal evolution of the subducting pacific plate back to the mid miocene we have reconstructed tectonic motions along the kurile japan izu bonin mariana arc system and the ryukyu arc to provide an independent additional interpretation of the subducting pacific plate using the most current plate motion data we then investigate the plausibility of our model and three other proposed models based on the interpreted slab structure from tomographic images the new reconstruction agrees with the basic characteristics of former trench retreat models but illustrates the importance of the collision of the ogasawara plateau with the trench in the mid miocene and its subsequent effect on the slab structure at depth and the impact of other aseismic ridge collisions along the plate boundary the combination of evidence in changed physical properties imaged with tomography and the current interpreted slab morphology can be analyzed with the past plate motions to understand subduction zone processes

eruptions from zoned magma chambers summary an appreciation of the positions within a magma chamber from which magma will be erupted can be gained from an understanding of the pattern of magma removal upwardly converging streamlines followed by a fluid escaping from a flat topped reservoir into a cylindrical conduit cause magma from widely separated horizontal and vertical locations in a pre eruption chamber to be simultaneously erupted the volume erupted v is related to the maximum depth tapped r by the equation v 2 9 r 3 large ash flow sheets 100 km 3 may therefore sample magma up to 5 km below the chamber roof analysis of the flow field suggests that about one half of a given sample of erupted magma represents material from the deepest part of the chamber being tapped at the time of eruption many petrographic features of chemically zoned ignimbrites can best be explained by the operation of such a flow field during eruption where a series of sub spherical sampling shells intersect the horizontal layers in the magma chamber to produce heterogeneous ejecta mixed magmas erupted late in some zoned ash flow sequences can originate by tapping across the boundary between silicic and underplating basaltic magmas which coexist in the source chamber rather than by convective mixing prior to eruption flow banding in rhyolites can result from the same fluid dynamic processes

fossil remnants of up turned debris bands in pleistocene glacial deposits of poland fossil remnants of up turned debris rich bands of glacial ice are recorded from three localities in the polish lowlands in each case the fossil debris bands occur in proximal parts of hills built of glacitectonically disturbed sediments the characteristics of these remnants and of the associated deposits and structures are described and they are compared to present day examples from spitsbergen

formation and ascent of granitic magmas new results obtained by the investigation of liquidus and solidus phase relationships in the haplogranite system qz ab or are used to discuss the evolution of magmas during their ascent in the crust it is assumed that the magmas are formed at 720 c 820 c 920 c and at a depth corresponding to a pressure of 8 kbar the starting composition of the magma is taken as 50 melt plus 50 quartz and feldspars in case of a closed system no heat exchange and no transfer of elements the melt fraction of magmas the water activity and the viscosity increase with decreasing pressure the temperature slightly decreases at 700 c the viscosity is approximatively 2 orders of magnitude lower than at 900 c this is related to the higher amount of water in the h2o undersaturated melt at low t it is also shown that dehydration melting is only realistic at high t 900 c at lower temperatures water has to be added from outside to obtain an intrusive magma with approximatively 50 melt

sm nd geochemistry and u pb geochronology of the preissac and lamotte leucogranites abitibi subprovince the lacorne block in the southern volcanic zone of the abitibi subprovince is composed of interleaved metavolcanic and metasedimentary rocks that are intruded by syn to posttectonic diorites granodiorites and granites these rocks form the lacorne lamotte and preissac plutons which can be divided into an early suite of dioritic granodioritic rocks and a later suite of s type leucocratic granites with an estimated age of 2640 ma this study presents sm nd data and u pb monazite and titanite ages for the late leucocratic granites of the preissac and lamotte plutons a biotite muscovite monzogranitic phase of the lamotte pluton is dated at 2647 2 ma but similar phases of the preissac pluton are dated at 2681 2660 ma these ages extend the period of leucogranitic plutonism for this area to 40 ma and suggest that the age of collision of the abitibi and the pontiac subprovinces occurred before 2685 ma the end values for the leucogranites range from 1 to 3 and suggest an origin largely through melti

complex slab subduction beneath northern sumatra 1 new data provided by the 2004 2005 sumatra andaman great earthquake sequences allow us to image with improved detail the p wave velocity structure beneath sumatra and adjacent regions below northern sumatra we find that the slab is folded at depth exhibiting geometry similar to that of the volcanic arc and the trench at the surface we speculate that this fold plays a major role in the segmentation of the sumatra megathrust and may impede rupture propagation in the region north of sumatra significant slab material in the mantle transition zone is imaged for the first time and we infer the presence of a major tear between the upper mantle and transition zone there

magma driven hydraulic fracturing and infiltration of fluids into the damaged host rock an example from dronning maud land antarctica abstract excellent outcrops in dronning maud land antarctica provide unique insight into the mode and extent of fluid infiltration into metamorphic and plutonic rocks in the middle crust the fluids are liberated from pegmatitic veins and give rise to alteration halos in the alteration halos the conspicuous change in colour is correlated with 1 hydration mineral reactions and 2 high density of microcracks in quartz and feldspar exceeding that observed in the unaltered host rock by an order of magnitude the field relations indicate that the veins originated as melt driven hydraulic fractures sealed by pegmatite and aplite crystallising from volatile rich melts with the alteration halo being the wake of the process zone formed at the tip of the propagating fractures it is proposed that 1 the size of the alteration zone and the width of the vein are correlated resulting in higher values of both these quantities for cracks propagating at higher velocities and consequently higher crack propagation toughnesses 2 the damage zone is characterised by a transient state of high permeability which was short lived due to rapid healing and sealing of microcracks 3 the infiltration and retrogression of the high grade rocks can be considered as a quasi instantaneous process on geologic time scales with a duration of hours to weeks

genesis of hummocky moraines by thrusting in glacier ice evidence from svalbard and britain irregular mounds of glacial debris commonly referred to as 9hummocky moraine9 until recently were linked to ice stagnation during rapid climatic amelioration however recent work in scotland has demonstrated that some hummocky moraine dating from the younger dryas event c 10 000 years bp was the product of deposition at active ice margins observations at modern high arctic glacier margins in svalbard 76 80 n indicate that some moraines of this type form under a dynamic glaciological regime mainly by thrusting in active polythermal ice undergoing strong longitudinal compression this situation is particularly common where there is a transition from warm based ice in the interior of a glacier to cold based ice at the margins or snout from their morphological similarity rectilinear slopes facing up glacier and irregular downglacier faces and apparent stacking of slabs of sediment of subglacial derivation it is concluded that some assemblages of british moraines are also the product of thrusting this process appears to be most dominant in polythermal glaciers rather than in temperate glaciers on this basis it is tentatively suggested that the younger dryas in britain was characterized by a climatic regime similar to that in svalbard today mean annual temperature 5 c 400 1000 mm precipitation and that the associated glaciers may have been of polythermal character

40ar 39ar age constraints on the thermal history of the archean abitibi greenstone belt and the pontiac subprovince implications for terrane collision differential uplift and overprinting of gold deposits 40ar 39ar mineral age spectra of granitic and metamorphic rocks in conjunction with existing conventional zircon geochronology indicate that at least two major late archean thermal events affected tectonic blocks of the abitibi southern volcanic zone svz and the juxtaposed pontiac subprovince the earlier thermal activity 2690 2670 ma was accompanied by the intrusion of voluminous syntectonic plutons and caused low pressure greenschist facies metamorphism in the svz and intermediate pressure metamorphism in the pontiac subprovince the second thermal event 2660 2630 ma was coeval with the emplacement of syncollisional s type garnet muscovite granites in the pontiac subprovince and the higher grade lacorne block of the abitibi svz and reset the k ar systems in preexisting rocks magmatic amphibole from the syntectonic round lake batholith 2695 ma u pb zircon age of the abitibi svz has a slightly disturbed ar release spectrum with an upper plateau age of 2669 6 ma signifying that the low gra

alkali metal and rare earth element evolution of rock forming minerals from the gatumba area pegmatites rwanda quantitative assessment of crystal melt fractionation in the regional zonation of pegmatite groups abstract this study presents a general model for the evaluation of rayleigh fractional crystallisation as the principal differentiation mechanism in the formation of regionally zoned common and rare element pegmatites the magmatic evolution of these systems from a granitic source is reconstructed by means of alkali element and rare earth element ree analyses of rock forming minerals feldspars micas and tourmaline which represent a whole sequence of regional pegmatite zonation the gatumba pegmatite field rwanda central africa is chosen as case study area because of its well developed regional zonation sequence the pegmatites are spatially and temporally related to peraluminous g4 granites 986 10 ma the regional zonation is developed around a g4 granite and the proximal pegmatites grade outwardly into biotite two mica and muscovite pegmatites rare element nb ta sn pegmatites occur most distal from the granite alkali metal fractionation trends in pegmatitic k feldspar rb 350 6000 ppm cs 2 160 ppm and muscovite rb 670 7000 ppm cs 10 150 ppm define a single and continuous trend which is modelled by rayleigh fractional crystallisation starting from a parental granite composition g4 composition k 3 1 wt rb 222 ppm cs 11 ppm the fractionation model shows moreover that the pegmatites adjacent to the parental pluton are the least fractionated and the distal pegmatites are the most fractionated biotite pegmatites form from 0 to 69 crystallisation two mica pegmatites from 69 to 92 and muscovite pegmatites from 92 crystallisation onwards the extreme rb and cs enrichment in rare element pegmatites requires at least 98 fractionation of the initial g4 granite composition mathematical derivation of the k rb versus cs relationship in k feldspars confirms rayleigh fractional crystallisation as the main differentiation process in the development of regional pegmatite zonation moreover 1 it demonstrates the continuity of the fractionation process from biotite pegmatites to rare element pegmatite and indicates a genetic link among them and 2 it allows a general evaluation of pegmatite fields in terms of system parameters i e the initial element concentration in the granitic melt and partition coefficients ree patterns of the rock forming minerals feldspars muscovite biotite and schorl show a distinct transition along the regional zonation they evolve from sloping fractionated to more flat patterns starting from the biotite up to the muscovite pegmatites minerals from the rare element pegmatites feldspars muscovite and elbaite show again more fractionated heavy ree depleted patterns the observed evolution in the ree patterns corresponds to early crystallisation of light ree enriched monazite and a late crystallisation of mainly cogenetic heavy ree enriched phases such as apatite columbite group minerals and beryl modelling of rayleigh fractionation starting from an initial parental granite g4 composition lan ybn 10 and ree 83 shows that this evolution in mineral ree pattern is the result of fractional crystallisation of the pegmatitic melt and precipitation of ree incorporating minerals such as monazite and apatite consequently trace element modelling indicates that rayleigh fractional crystallisation governs the mineralogical and geochemical evolution from a granite source to common and eventually rare element pegmatites this mechanism shows that granitic pegmatites are extremely fractionated residual melts which are genetically and directly connected to a main granite body

experimental phase equilibria in the system lialsio4 sio2 h2o a petrogenetic grid for lithiumx rich pegmatites phase relations for the bulk composition lia1sis01z system lia1si04 sioz hzo have been investigated experimentally over the range 340 950 c and 0 5 6 0 kbar p hzo stability relations among eucryptite spodumene petalite b spodumene tetragonal and virgilite hexagonal have been determined for quartz saturated subsolidus conditions the result is a quantitative p t phase diagram that is applicable to quartz saturated lithium aluminosilicate assemblages in pegmatites and some volcanic rocks thermochemical data derived from these experimental results are internally consistent and in close agreement with values obtained from calorimetry under the quartz saturated conditions that prevail in pegmatites stability relations among the lithium aluminosilicates are a function of p and t and are largely independent of the nature and proportions of other phases in the chemically complex pegmatite system thus the lithium aluminosilicate phase diagram constitutes a petrogenetic grid from which p t conditions of emplacement and crystallization can be ascertained for many lithium alumino silicate bearing pegmatites

a mechanism for preferential h 2 o leakage from fluid inclusions in quartz based on tem observations preferential leakage of h2o from fluid inclusions containing multiple gas components has been suspected in natural metamorphic rocks and has been demonstrated experimentally for synthetic h2o co2 rich inclusions in natural quartz knowledge of the physical and chemical characteristics of the leakage mechanism which may be very complex increases the value of natural fluid inclusions to metamorphic geology it is proposed that crystal defects play a major role in nondecrepitative preferential h2o leakage through quartz and remain effective during metamorphism inclusions with either an internal overpressure or underpressure produce strain in the adjacent quartz crystal via the nucleation of many dislocations and planar defects like dauphine twin boundaries these defects allow preferential loss of h2o from h2o co2 rich inclusions at supercritical conditions the transport capacity of this leakage mechanism is enhanced by nucleation of small bubbles on defect structures the nucleation of these bubbles seems to be a recovery process in strained crystals solubility gradients of quartz in water in a crystal with internally underpressurized inclusions may result in optical visible implosion halos in a three dimensional spatial arrangement caused by the growth of small bubbles at the expense of the larger original fluid inclusion natural fluid inclusions from naxos greece are always associated with numerous interlinked dislocations these dislocations may have been produced by plastic derormation or by crystal growth related processes e g crack healing the presence of small bubbles on these dislocations indicates that a similar leakage mechanism for h2o must have occurred in these rocks

melting behavior of fluid inclusions in laboratory grown halite crystals in the systems naclh2o naclkclh2o naclmgcl2h2o and naclcacl2h2o abstract the chemical composition of aqueous fluid inclusions in crystals of halite can be accurately determined from observed melting behaviors of ice hydrohalite and sylvite some fluid inclusion melting behaviors observed in laboratory grown halite crystals systems nacl h 2 o nacl kcl h 2 o nacl mgcl 2 h 2 o and nacl cacl 2 h 2 o differ from predicted stable equilibrium relations in the nacl h 2 o and nacl kcl h 2 o systems observed first melt temperatures are up to 15 c below the equilibrium eutectic temperatures of 21 2 and 22 9 c respectively the final melting temperature of ice in the presence of hydrohalite and the final melting temperature of hydrohalite are reproducible and match predicted melting temperatures the limit of detection of sylvite daughter crystals in the nacl kcl h 2 o system is approximately 5 wt 1 molal kcl final melting temperatures of sylvite match published equilibrium data to within 0 3 c in the nacl kcl h 2 o system at halite saturation m kcl can be determined from the final sylvite dissolution temperature or from the final melting temperature of hydrohalite fluid inclusions in the nacl mgcl 2 h 2 o and nacl cacl 2 h 2 o systems that form stable salt hydrates mgcl 2 12h 2 o and cacl 2 6h 2 o during freezing first melt within 3 c of predicted eutectic temperatures 37 and 52 c however fluid inclusions with mgcl 2 or cacl 2 may also start melting at temperatures as low as 80 c such low first melt temperatures indicate the presence of metastable salt hydrates presumably mgcl 2 8h 2 o mgcl 2 6h 2 o or cacl 2 4h 2 o the formation of metastable phases during freezing of fluid inclusions can lead to misinterpretation of the chemical composition of fluid inclusions in natural samples this is especially true for fluid inclusions with first melt temperatures below 37 c which may be erroneously interpreted as being rich in cacl 2 the final melting of ice in the presence of hydrohalite may vary by more than 15 c in fluid inclusions of different size but identical bulk composition and occurs at lower temperatures than predicted in fluid inclusions from the nacl mgcl 2 h 2 o and nacl cacl 2 h 2 o systems however the final melting temperature of ice in inclusions which fail to nucleate hydrohalite and the final melting temperature of hydrohalite are reproducible to within 0 1 c and can be used to determine mgcl 2 and cacl 2 molalities

rapid crystallization of the animikie red ace pegmatite florence county northeastern wisconsin inclusion microthermometry and conductive cooling modeling we evaluated the crystallization regime of a zoned pegmatite dike and the degree of magma undercooling at the onset of crystallization by analyzing coeval fluid and melt inclusion assemblages the liquidus temperature of the pegmatite magma was 720 c based on re melting of crystallized melt inclusions in heating experiments the magma crystallized sequentially starting with a thin border zone which formed in less than one day at an average temperature of 480 c based on primary fluid inclusions meaning 240 c undercooling the primary inclusions from the outer zones were postdated by secondary inclusions trapped between 580 and 720 c representing fluid exsolved from hotter still crystallizing inner pegmatite units the huge temperature contrast between the pegmatite s inner and outer zones was simulated by conductive heat numerical modeling a 2 5 m wide dike emplaced in 220 c rocks cools entirely to 400 c in less than 50 days unidirectional and skeletal textures also indicate rapid disequilibrium crystallization

ice flow directions of the last scandinavian ice sheet in central poland abstract the article presents the results of studies on the ice flow directions of the last scandinavian ice sheet sis in central poland at the southern periphery of the sis and particularly within the plock lobe the ice flow pattern was derived mainly from direct indices such as till fabric vergence of glaciotectonic compression structures orientation of small scale subglacial deformations formed beneath the flowing ice orientation of pavement stone longitudinal axes stone lee ends and clast surface striae the study is supplemented with a reconstruction of ice movement directions based on orientation analysis of both concave tunnel valley and convex eskers glacial morpholineaments linear glacial landforms till age was also determined using luminescence osl datings from sand deposits these were collected directly at numerous sites from under and above the last sis till unit combined with previously published data the results of the present investigation of the last ice sheet advance during the late weichselian ca 23 18 ka indicate that there were two different ice flow phases and that the maximum extent was asynchronous the western part of the said area experienced two advances during the leszno brandenburg and poznan frankfurt phases while during the earlier one the ice masses came from the nw presumably along the eastern limb of the odranian b2 after punkari 1997 ice stream and then from the n along the vistulian b3 stream the last sis occupied the eastern sector of the area only once during the younger poznan phase building the plock lobe the lobe shaped ice margin with a fan like flow pattern within the lobe may be indicative of a palaeo ice stream terminal zone

lithium isotope fractionation during fluid exsolution implications for li mineralization of the bailongshan pegmatites in the west kunlun nw tibet abstract granitic pegmatites are considered significant host rocks for the rare metal deposits however the enrichment mechanisms for the rare metal elements e g li be nb and ta remain debated here we report the whole rock and mineral e g muscovite and spodumene major trace elements and li isotopic compositions of the bailongshan granitic pegmatites in the west kunlun nw tibet to address this issue the bailongshan pegmatites hosting a super large li rb polymetallic deposit are composed of li poor and li rich pegmatites the li poor pegmatites display high 7li values in the whole rock 2 30 4 94 and muscovite 2 22 7 55 in contrast the li rich pegmatites show low 7li values in the whole rock 1 89 to 0 35 muscovite 3 39 to 4 49 and spodumene 2 83 to 1 87 these results together with the variations of the elements suggest that the li poor and li rich pegmatites were produced by the melt fluid separation during the late stage of granitic magma evolution this means that the li poor pegmatites were formed in a h2o poor silicate rich melt system while the li rich pegmatites were generated in a h2o rich silicate poor melt supercritical fluid system the melt fluid separation can lead to significant li isotope fractionation with 7li enriched in the strongly bonded residual silicate melt and 6li preferring to weaker hydrated bonds in the fluid alternatively the li rich pegmatites enriched in 6li could have been induced by preferential accumulation of the faster diffusing 6li in the supercritical or near critical fluid the li rich pegmatites experienced extensive crystal fluid interaction in a relatively closed magmatic hydrothermal system which resulted in the leaching and reprecipitation of ore forming elements but produced restricted li isotope fractionation the exsolved supercritical fluid is more enriched in alkali fluxing components li rb cs p and f by the continuous interaction with the residual silicate melt therefore the supercritical fluid exsolution plays an important role in the li isotope fractionation and rare metal mineralization

seismological evidence for a slab detachment in the tonga subduction zone abstract the tonga wadati benioff zone is characterized by a large seismicity gap beneath the lau basin that raises the question of the slab continuity between the shallow to intermediate part 60 300 km and the deep part 400 700 km to address this problem we investigated the wadati benioff zone geometry and stress regime through a detailed analysis of the spatial distribution of moment tensors and variation of the stress tensor using the global seismicity engdahl e van der hilst r buland r 1998 global teleseismic earthquake relocation with improved travel times and procedures for depth determination bull seism soc am 88 722 743 and the centroid moment tensor solutions cmt catalogs dziewonski a m chou t woodhouse j h 1981 determination of earthquake source parameters from waveform data for studies of global and regional seismicity j geophys res 86 2825 2852 the stress tensors were computed using the gephart s program gephart j w forsyth d 1984 an improved method for determining the regional stress tensor using earthquake focal mechanism data application to the san fernando earthquake sequence j geophys res 89 9305 9320 the stress inversion results indicate that between 21 s and 27 s and depths down to 700 km the slab is under homogeneous down dip compressional stress regime while north of 21 s we found strong variations of the stress orientations between the intermediate and deep portions of the slab we also show that between 14 and 19 s the stresses at intermediate depth 60 300 km can be resolved into two slab parallel domains a thin upper part of the slab that is under downdip compression and the lower part that is under downdip extension this pattern of two zones with opposite mechanical behavior is characteristic of a subducted plate with a free lower limit that does not interact with the 670 km depth boundary these results together with the large seismicity gap within the slab argue for a slab detachment

kinematics and flow patterns in deep mantle and upper mantle subduction models influence of the mantle depth and slab to mantle viscosity ratio 1 three dimensional fluid dynamic laboratory simulations are presented that investigate the subduction process in two mantle models an upper mantle model and a deep mantle model and for various subducting plate mantle viscosity ratios sp m 59 1375 the models investigate the mantle flow field geometrical evolution of the slab sinking kinematics and relative contributions of subducting plate motion and trench migration to the total rate of subduction all models show that the subducting plate is always moving trenchward resulting from slab pull furthermore all deep mantle models show trench retreat as do upper mantle models in the initial stage of subduction before slab tip transition zone interaction upper mantle models with a low sp m 66 217 continue to show trench retreat after interaction upper mantle models with a high sp m 378 709 show a period of trench advance after interaction followed by trench retreat upper mantle models with a very high sp m 1375 show continued trench advance after interaction the difference in trench migration behavior and associated slab geometries is attributed to both sp m and the mantle depth to plate thickness ratio tm tsp which both affect the slab bending radius to mantle thickness ratio rb tm four subduction regimes can be defined regime i with rb tm 0 3 trench retreat slab draping and a concave trench regime ii with 0 3 rb tm 0 5 episodic trench migration slab folding and a concave trench regime iii with rb tm 0 5 trench advance slab rollover geometries and minor trench curvature and regime iv with rb tm 0 8 trench retreat slab draping and a rectilinear trench in all models slab parallel downdip motion induces poloidal mantle flow structures in addition trench retreat and rollback motion of the slab induce quasi toroidal return flow around the lateral slab edges toward the mantle wedge rollback induced poloidal flow around the slab tip is not observed in any of the experiments finally comparison between the slab geometries observed in the upper mantle models and slab geometries observed in nature imply that the effective viscosity ratio between slab and ambient upper mantle in nature is less than 103 and of the order 1 7 102 with a best estimate of 1 3 102

till and moraine emplacement in a deforming bed surge an example from a marine environment the glacier sefstrombreen in spitsbergen surged across an arm of the sea between qsr 1882 and 1886 and rode up onto the island coraholmen marine and terrestrial geological observations and archive records show that the glacier advanced on a deforming carpet of marine mud which was eroded from its original location transported and smeared over the sea bed and coraholmen as a deformation till the glacier emplaced about 210sm 0 2 km 3 of drift in the terminal 2 km of its advance in a maximum of 14 years leaving a thickness of up to 20 m on coraholmen which was doubled in size as a result during the surge subglacial muds were characterised by high water pressures low effective pressures and low frictional resistance to glacier movement original sedimentary inhomogenities permit fold structures to be identified but repeated refolding and progressive remoulding produce mixing and homogenisation of deformation tills the surge was probably shortlived and as the heavily crevassed glacier stagnated underlying water saturated muds were intruded into crevasses and then extruded on the glacier surface reticulate crevasse intrusion ridges on coraholmen and the sea floor reflect the orientation of surge generated crevasses water and sediment was also extruded beyond the glacier at its maximum extent to form extensive flows producing till tongues both on coraholmen and the sea floor extending over 1 3 km from the glacier it is argued that subglacial deformation of pre existing sediment will almost invariably be associated with glaciation of marine areas and that this process will not only produce deformation tills through remoulding of pre existing sediments but will also play a fundamental role in glacier dynamics criteria which perrait glacial tills produced by such events from marine and glaciomarine muds are discussed copyright 1996 elsevier science ltd