strain localization at the margins of strong lithospheric domains insights from analog models the lateral variation of the mechanical properties of continental lithosphere is an important factor controlling the localization of deformation and thus the deformation history and geometry of intraplate mountain belts a series of three layer lithospheric scale analog models with a strong domain sd embedded at various depths are presented to investigate the development of topography and deformation patterns by having lateral heterogeneities within a weak continental lithosphere the experiments performed at a constant velocity and under normal gravity indicate that the presence or absence of the sd controls whether deformation is localized or distributed at a lithospheric scale deformation and topography localize above the edges of the sd while the sd region itself is characterized by minor amounts of surficial deformation and topography the depth of the sd within the ductile crust ductile mantle lithosphere or both controls the pattern of deformation and thus the topography the presence of a sd in the ductile crust or in the mantle results in limited surficial topographic effects but large variations in the moho topography strong moho deflection occurs when the sd is in the ductile crust while the moho remains almost flat when the sd is in the mantle when the sd occupies the ductile lithosphere the sd is tilted these analog experiments provide insights into intraplate strain localization and could in particular explain the topography around the tarim basin a lithospheric scale heterogeneity north of the india asia collision zone

statistical cyclicity of the supercontinent cycle supercontinents like pangea impose a first order control on earth s evolution as they modulate global heat loss sea level climate and biodiversity in a traditional view supercontinents form and break up in a regular perhaps periodic manner in a cycle lasting several 100 myr as reflected in the assembly times of earth s major continental aggregations columbia rodinia and pangea however modern views of the supercontinent cycle propose a more irregular evolution on the basis of an improved understanding of the precambrian geologic record here we use fully dynamic spherical mantle convection models featuring plate like behavior and continental drift to investigate supercontinent formation and breakup we further dismiss the concept of regularity but suggest a statistical cyclicity in which the supercontinent cycle may have a characteristic period imposed by mantle and lithosphere properties but this is hidden in immense fluctuations between different cycles that arise from the chaotic nature of mantle flow

source characteristics of the 2017 mw6 4 moijabana botswana earthquake a rare lower crustal event within an ancient zone of weakness abstract the 2017 moijabana earthquake in central botswana mw6 4 was a large and deep event for a continental interior and occurred in a region with little historical seismicity based on insar measurements of surface deformation spanning the event and teleseismic observations we determine the ruptured fault plane and finite rupture model of the earthquake although this oblique normal faulting earthquake is too deep to uniquely determine the rupture plane geometry from insar alone the best fitting fault plane constrained by the joint inversion of teleseismic waveforms and insar data has a southwest dip and a strike of 126 roughly consistent with the geologically mapped strike of the kaapvaal craton s northern edge our results indicate that the earthquake had a total duration of 10 s characterized by two major asperities the first asperity nucleated in the lower crust and then the rupture propagated up dip the lower crustal asperity shows a much shorter rise time compared with the shallower asperity indicating that contrasts in stress or material properties may have played an important role in the rupture process the earthquake appears to have occurred in the limpopo belt a proterozoic orogenic belt that represents an ancient zone of weakness between the archean zimbabwe and kaapvaal cratons in the present day this zone of weakness may be responding to the stress field imposed by the east african rift system

mantle convection with a brittle lithosphere thoughts on the global tectonic styles of the earth and venus summary plates are an integral part of the convection system in the fluid mantle but plate boundaries are the product of brittle faulting and plate motions are strongly influenced by the existence of such faults the conditions for plate tectonics are studied by considering brittle behaviour using byerlee s law to limit the maximum stress in the lithosphere in a mantle convection model with temperature dependent viscosity when the yield stress is high convection is confined below a thick stagnant lithosphere at low yield stress brittle deformation mobilizes the lithosphere which becomes a part of the overall circulation surface deformation occurs in localized regions close to upwellings and downwellings in the system at intermediate levels of the yield stress there is a cycling between these two states thick lithosphere episodically mobilizes and collapses into the interior before reforming the mobile lid regime resembles convection of a fluid with temperature dependent viscosity and the boundary layer scalings are found to be analogous this regime has a well defined nusselt number rayleigh number relationship which is in good agreement with scaling theory the surface velocity is nearly independent of the yield stress indicating that the plate motion is resisted by viscous stresses in the mantle analysis suggests that mobilization of the earth s lithosphere can occur if the friction coeycient in the lithosphere is less than 0 03 0 13 lower than laboratory values but consistent with seismic field studies on venus the friction coeycient may be high as a result of the dry conditions and brittle mobilization of the lithosphere would then be episodic and catastrophic

the role of history dependent rheology in plate boundary lubrication for generating one sided subduction we have developed a two dimensional dynamical model of asymmetric subduction integrated into the mantle convection without imposed plate velocities in this model we consider that weak oceanic crust behaves as a lubricator on the thrust fault at the plate boundary we introduce a rheological layer that depends on the history of the past fracture to simulate the effect of the oceanic crust the thickness of this layer is set to be as thin as the earth s oceanic crust to treat 1 kilometer scale structure at the plate boundary in the 1000 kilometer scale mantle convection calculation we introduce a new numerical method to solve the hydrodynamic equations using a couple of uniform and nonuniform grids of control volumes using our developed models we have systematically investigated effects of basic rheological parameters that determine the deformation strength of the lithosphere and the oceanic crust on the development of the subducted slab with a focus on the plate motion controlling mechanism in our model the plate subduction is produced when the friction coefficient 0 004 0 008 of the modeled oceanic crust and the maximum strength 400 mpa of the lithosphere are in plausible range inferred from the observations on the plate driving forces and the plate deformation and the rheology experiments in this range of the plate strength yielding induces the plate bending in this case the speed of plate motion is controlled more by viscosity layering of the underlying mantle than by the plate strength to examine the setting of the overriding plate we also consider the two end member cases in which the overriding plate is fixed or freely movable in the case of the freely movable overriding plate the trench motion considerably changes the dip angle of the deep slab especially in the case with a shallow angle plate boundary retrograde slab motion occurs to generate a shallow angle deep slab

trace element degassing and enrichment in the eruptive plume of the 2000 eruption of hekla volcano iceland during its last eruption in february 2000 hekla volcano iceland emitted a sub plinian plume that was condensed and scavenged down to the ground by heavy snowstorms offering the unique opportunity to study the chemistry of the gaseous plume released during highly explosive eruptions in this paper we present results on trace element and minor volatile species sulfates chlorides and fluorides concentrations in snow samples collected shortly after the beginning of the eruption the goal of this study is to better constrain the degassing and mobility of trace elements in gaseous emissions trace element volatility at hekla is quantified by means of enrichment factors ef relative to be well known volatile trace elements e g transition metals heavy metals and metalloids are considerably enriched in the volcanic plume of hekla their abundances are governed by the primary magmatic degassing of sulfate and or halide compounds which are gaseous at magmatic temperature their volatility is however slightly lower than in basaltic systems most likely because of the lower magma temperature and higher magma viscosity at hekla more surprisingly refractory elements e g ree th ba and y are also significantly enriched in the eruptive plume of hekla where their apparent volatility is two orders of magnitude higher than in mafic systems in addition ree patterns normalized to the hekla 2000 lava composition show a significant enrichment of hree over lree suggesting the presence of ree fluorides in the volcanic plume such enrichments in the most refractory elements and ree fractionation are difficult to reconcile with primary degassing processes since ree fluorides are not gaseous at magma temperature ree enrichments at hekla could be attributed to incongruent dissolution of tephra grains at low temperature by f rich volcanic gases and aerosols within the eruptive plume this view is supported by both leaching experiments performed on hekla tephra and thermodynamic considerations on ree mobility in hydrothermal fluids and modeling of glass dissolution in f rich aqueous solutions tephra dissolution may also explain the observed enrichments in other refractory elements e g th y and ba and could contribute to the degassing mass balance of some volatile trace elements provided they are mobile in f rich fluids it thus appears that both primary magmatic degassing and secondary tephra dissolution processes govern the chemistry of eruptive plumes released during explosive eruptions

modeling of craton stability using a viscoelastic rheology archean cratons belong to the most remarkable features of our planet since they represent continental crust that has avoided reworking for several billions of years even more it has become evident from both geophysical and petrological studies that cratons exhibit deep lithospheric keels which equally remained stable ever since the formation of the cratons in the archean dating of inclusions in diamonds from kimberlite pipes gives archean ages suggesting that the archean lithosphere must have been cold soon after its formation in the archean in order to allow for the existence of diamonds and must have stayed in that state ever since yet although strong evidence for the thermal stability of archean cratonic lithosphere for billions of years is provided by diamond dating the long term thermal stability of cratonic keels was questioned on the basis of numerical modeling results we devised a viscoelastic mantle convection model for exploring cratonic stability in the stagnant lid regime our modeling results indicate that within the limitations of the stagnant lid approach the application of a sufficiently high temperature dependent viscosity ratio can provide for thermal craton stability for billions of years the comparison between simulations with viscous and viscoelastic rheology indicates no significant influence of elasticity on craton stability yet a viscoelastic rheology provides a physical transition from viscously to elastically dominated regimes within the keel thus rendering introduction of arbitrary viscosity cutoffs as employed in viscous models unnecessary

pacific offshore record of plinian arc volcanism in central america 2 tephra volumes and erupted masses 1 sediment gravity cores collected from the pacific seafloor offshore central america contain numerous distal ash layers from plinian type eruptions at the central american volcanic arc dating back to more than 200 ka in part 1 of this contribution we have correlated many of those ash layers between cores and with 26 tephras on land the marine ash layers cover areas of up to 106 km2 in the pacific ocean and represent a major fraction 60 90 of the erupted tephra volumes because the pacific coast lies within a few tens of kilometers downwind from the volcanic arc combining our own mapping efforts on land and published mapping results with our marine data yields erupted volumes of all major tephras along the arc that range from 1 to 420 km3 recalculated to erupted magma mass the widespread tephras account for 65 of the total magma output at the arc complementing our tephra data with published volumes of the arc volcanic edifices and volcano ages we calculate the long term average magma eruption rates for each volcano moreover we use incompatible element variations to calculate the cumulate masses that were fractionated during variable degrees of differentiation this yields a minimum estimate of long term average magma production rate at each volcano because intrusives without surface expression and losses by erosion are not accounted for peak magma production rates increase from costa rica to guatemala but there is considerable scatter within each region and large differences even between neighboring volcanoes

isotopic disequilibrium among commingled hybrid magmas evidence for a two stage magma mixing commingling process in the mt perkins pluton arizona this is the publisher s version also available electronically from http www jstor org stable 30070774 seq 1 page scan tab contents

dynamic cause of marginal lithospheric thinning and implications for craton destruction a comparison of the north china superior and yilgarn cratons we present a comparative tectonic analysis of the north china craton ncc which has lost parts of its root with the yilgarn and superior cratons which preserve their roots we compare the geophysical structure and tectonic histories of these cratons to search for reasons why some cratons lose their roots while others retain them based on the comparison and analysis of geological geophysical and geochemical data it is clear that the lithospheric thinning beneath craton margins is a common phenomenon which may be caused by convergence between plates however craton destruction is not always accompanied by lithospheric thinning except for cratons that suffered subduction and collision from multiple sides the western block also known as the ordos block of the ncc yilgarn and superior cratons have not experienced craton destruction the common ground among them is that they are surrounded by weak zones e g mobile belts or orogens that sheltered the cratons from deformation which contribute

the supply and accumulation of silica in the marine environment rivers and submarine hydrothermal emanations supply 6 1 1014g sio2 yr to the marine environment approximately two thirds of the silica supplied to the marine environment can be accounted for in continental margin and deep sea deposits siliceous deep sea sediments located beneath the antarctic polar front convergence account for over a fourth 1 6 1014g sio2 yr of the silica supplied to the oceans deep sea sediment accumulation rates beneath the polar front are highest in the south atlantic with values as large as 53cm kyr during the last 18 000 yr siliceous sediments in the bering sea sea of okhotsk and subarctic north pacific accumulate 0 6 1014g sio2 yr or 10 of the dissolved silica input to the oceans the accumulation of biogenic silica in estuarine deposits removes a maximum of 0 8 1014g sio2 yr although estuarine silica versus salinity plots indicate extensive removal of riverine silica from surface waters the removal rates must be considered as maximum values because of dissolution of siliceous material in estuarine sediments and bottom waters siliceous sediments from continental margin upwelling areas e g gulf of california walvis bay or peru chile coast have the highest biogenic silica accumulation rates in the marine environment 69 g sio2 cm2 kyr despite the rapid accumulation of biogenic silica upwelling areas account for less than 5 of the silica supplied to the marine environment because they are confined laterally to such small areas

eruption dynamics of the 22 23 april 2015 calbuco volcano southern chile analyses of tephra fall deposits abstract after 54 years since its last major eruption in 1961 calbuco volcano ensenada southern chile reawakened with few hours of warning on 22 april 2015 at 18 05 local time the main explosive eruption consisted of two eruption pulses lasting 1 5 and 6 h each one on 22 and 23 april producing stratospheric 15 km height eruption columns the erupted materials correspond to porphyritic basaltic andesite 55 wt of sio2 the tephra fall affected mainly the area northeast of the volcano and the finest ash was deposited over southern chile and patagonia argentina we studied the tephra fall deposits of both pulses in terms of stratigraphy distribution volume emplacement dynamics and eruption source parameters here we show field observations that have been made 5 470 km downwind and distinguish five layers layers a b b1 c and d representing different stages of the eruption evolution eruption onset layer a pulse 1 followed by the first paroxysmal event layer b pulse 1 in some places interbedded by layer b1 tentatively representing the sedimentation of a secondary plume during the end of pulse 1 we recognized a second paroxysm layer c pulse 2 followed by the waning of the eruption layer d pulse 2 the total calculated bulk tephra fall deposit volume is 0 27 0 007 km3 0 11 0 13 km3 dense rock equivalent 38 of which was erupted during the first phase and 62 during the second pulse this eruption was a magnitude 4 45 event vei 4 eruption of subplinian type

crustal structure of a proterozoic craton boundary east albany fraser orogen western australia imaged with passive seismic and gravity anomaly data abstract we use passive seismic and gravity data to characterize the crustal structure and the crust mantle boundary of the east albany fraser orogen in western australia a proterozoic orogen that reworked the southern and southeastern margin of the archean yilgarn craton the crustal thickness pattern retrieved from receiver functions shows a belt of substantially thickened crust that follows the trend of the orogen but narrows to the southwest common conversion point profiles show a clear transition from a wide symmetric moho trough in the northeast to a one sided northwestern moho dip in the southwest where the moho appears to underthrust the craton towards its interior this change appears to coincide with the inferred trace of the ida fault a major terrane boundary within the yilgarn craton forward modelling of gravity anomaly data using the retrieved moho geometry as a geometric constraint shows that a conspicuous elongated gravity low on the northwestern side of the east albany fraser orogen is almost certainly caused by thickened archean crust to obtain a model that resembles the regional gravity pattern the following assumptions are necessary high density rocks occur in the upper crust of the fraser zone at depth inside the moho trough and in parts of the eastern nornalup zone east of the moho trough although our gravity models do not constrain at which crustal level these high density rocks occur active deep seismic surveys suggest that large extents of the east albany fraser orogen s lower crust include a mesoproterozoic magmatic underplate known as the gunnadorrah seismic province the simplest interpretation of the imaged crustal structure is that the gunnadorrah seismic province is underthrust beneath the yilgarn craton likely as a consequence of crustal shortening during accretion further east the imaged geometry overall appears to show a wedge of archean lower crust that was driven between the exhumed fraser zone and the gunnadorrah seismic province effectively splitting the paleo to mesoproterozoic crust of the east albany fraser orogen the vertical splitting of proterozoic crust by a cratonic crustal wedge comparable to what we image in this study may be a process that contributed to forming many craton margins around the world

tectonics and melting in intra continental settings abstract most of the geodynamic theories of deformation as well as metamorphism and melting of continental lithosphere are concentrated on plate boundaries and are dominated by the effects of subduction upon deformation of the margins of continental lithospheric blocks however it is becoming increasingly apparent that suture zones or mobile belts presumably representing fossil subduction zones or other types of pre deformation which occur far from present plate boundaries play a key role in intra continental deformation in such zones the crust is strongly sheared and the mantle lithosphere is metasomatized reworking of such settings reveals a surprisingly large range of instabilities that develop in compressed lithosphere with lateral heterogeneities inherited from previous deformational processes structural complexity arises which is sensitive to lithospheric age and tectonic setting this complexity influences localization of deformation topographic evolution melt generation and melt generation ascent and emplacement in this paper using fully coupled petrological thermomechanical modeling various tectonic responses are correlated with magmatic events in intra continental settings and are compared to observed intra cratonic orogenies and magmatic events

simulating faults and plate boundaries with a transversely isotropic plasticity model abstract in mantle convection simulations dynamically evolving plate boundaries have for the most part been represented using an visco plastic flow law these systems develop fine scale localized weak shear band structures which are reminiscent of faults but it is a significant challenge to resolve the large and the emergent small scale behavior we address this issue of resolution by taking into account the observation that a rock element with embedded planar failure surfaces responds as a non linear transversely isotropic material with a weak orientation defined by the plane of the failure surface this approach partly accounts for the large scale behavior of fine scale systems of shear bands which we are not in a position to resolve explicitly we evaluate the capacity of this continuum approach to model plate boundaries specifically in the context of subduction models where the plate boundary interface has often been represented as a planar discontinuity we show that the inclusion of the transversely isotropic plasticity model for the plate boundary promotes asymmetric subduction from initiation a realistic evolution of the plate boundary interface and associated stresses is crucial to understanding inter plate coupling convergent margin driven topography and earthquakes

volcanic degassing of bromine and iodine experimental fluid melt partitioning data and applications to stratospheric chemistry abstract in order to understand the degassing behavior of heavy halogens in volcanic processes we experimentally studied the distribution of cl br and i between albite melt and hydrous fluids containing 0 01 2 wt of nacl nabr or nai respectively all experiments were carried out at 2 kbar and 900 c in rapid quench cold seal autoclaves with a run duration of 7 days the major element compositions and cl contents of the glassy run products were measured by electron microprobe bromine and iodine were measured by proton induced x ray emission fluid compositions were obtained by mass balance all halogens investigated were found to partition strongly into the fluid phase over the range of concentrations studied the halogen contents in the melt are proportional to the concentrations in the fluid the fluid melt partition coefficients df m are 8 1 0 2 for cl 17 5 0 6 for br and 104 7 for i the logarithm of df m is linearly correlated with the ionic radius of the halogenide ion on the basis of our experimental data we estimate the amount of bromine injected into the stratosphere by major volcanic explosions for the 1991 mount pinatubo eruption we obtain br yields of 11 25 kt as minimum estimates these numbers are comparable to the total annual influx of bromine into the stratosphere from all other natural and anthropogenic sources about 100 kt year since bromine is much more efficient in destroying stratospheric ozone than chlorine it could at least be partially responsible for the massive ozone depletion observed after the 1991 mount pinatubo eruption

the late holocene history of gormire lake ne england and its catchment a multiproxy reconstruction of past human impact sedimentary evidence spanning the last 3500 years from cores taken from the centre of gormire lake cleveland northeast england points to two periods of deforestation and catchment erosion the first mainly late iron age romano british phase began c 200 bc and ended c ad 600 the second began c ad 1200 the time of early monastic activity and continued into the nineteenth century the chronology for the core rests on 137cs 210pb and 14c measurements and on the history of lead accumulation in the sediments the majority of the ams 14c dates are shown to be too old to varying degrees depending on the source of the organic matter in the sediments the record of vegetation change is derived from pollen analysis evidence for changes in sediment composition and source and for catchment erosion comes from magnetic measurements element analyses using xrf and organic biogeochemical analyses there is a high degree of coherence between all these lines of evidence interpretation of the magnetic record

the role of mobile belts for the longevity of deep cratonic lithosphere the crumple zone model many archean cratons are surrounded by proterozoic mobile belts that have experienced episodes of tectonic re activation over their lifetimes this suggests that mobile belt lithosphere may be associated with long lived inherited weakness it is proposed that the proximity of this weakness can increase the longevity of deep archean lithosphere by buffering archean cratons from mantle derived stresses the physical plausibility of this idea is explored through numerical simulations of mantle convection that include continents and allow for material rheologies that model the combined brittle and ductile behavior of the lithosphere within the simulations the longevity of deep cratonic lithosphere does increase if it is buffered by mobile belts that can fail at relatively low stress levels

radiative forcing and climate impact resulting from so2 injections based on a 200 000 year record of plinian eruptions along the central american volcanic arc we present for the first time a self consistent methodology connecting volcanological field data to global climate model estimates for a regional time series of explosive volcanic events using the petrologic method we estimated so2 emissions from 36 detected plinian volcanic eruptions occurring at the central american volcanic arc cava during the past 200 000 years together with simple parametrized relationships collected from past studies we derive estimates of global maximum volcanic aerosol optical depth aod and radiative forcing rf describing the effect of each eruption on radiation reaching the earth s surface in parallel aod and rf time series for selected cava eruptions are simulated with the global aerosol model maecham5 ham which shows a relationship between stratospheric so2 injection and maximum global mean aod that is linear for smaller volcanic eruptions 5 mt so2 and nonlinear for larger ones 5 mt so2 and is qualitatively and quantitatively consistent with the relationship used in the simple parametrized approximation potential climate impacts of the selected cava eruptions are estimated using an earth system model of intermediate complexity by rf time series derived by 1 directly from the global aerosol model and 2 from the simple parametrized approximation assuming a 12 month exponential decay of global aod we find that while the maximum aod and rf values are consistent between the two methods their temporal evolutions are significantly different as a result simulated global maximum temperature anomalies and the duration of the temperature response depend on which rf time series is used varying between 2 and 3 k and 60 and 90 years for the largest eruption of the cava dataset comparing the recurrence time of eruptions based on the cava dataset with the duration of climate impacts based on the model results we conclude that cumulative impacts due to successive eruptions are unlikely the methodology and results presented here can be used to calculate approximate volcanic forcings and potential climate impacts from sulfur emissions sulfate aerosol or aod data for any eruption that injects sulfur into the tropical stratosphere

bromine counts from xrf scanning as an estimate of the marine organic carbon content of sediment cores 1 xrf sediment core scanning technology is increasingly used to quantify sediment composition the overall good correlation between biophilic halogen bromine br and sedimentary total organic carbon toc potentially allows the fast estimation of down core toc profiles by xrf scanning in order to test this approach we present data from the arabian sea and a mediterranean brine basin comparing xrf core scanning br data with discrete sample toc analyses overall br counts and toc show a clear correlation except when stable carbon isotope and c n data indicate intervals characterized by enhanced input of terrestrial organic matter hence solid phase br is exclusively associated with marine organic matter moc and can be used as a direct estimate of this parameter after a calibration is established high pore water br in the brine core steepens the br toc correlation but after salt correction shows a nearly identical gradient to that of the arabian sea core

layered convection induced by phase transitions we report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection two dimensional numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the extended boussinesq approximation i e considering the effects of adiabatic gradient latent heat and frictional heating in the energy equation we find that the critical value of the negative clapeyron slope which must be surpassed in order to induce layered convection decreases in magnitude with increasing rayleigh number ra in the range 104 ra 2 106 near the critical clapeyron slope vacillations between double and single layer convection or strongly leaking double layer convection are possible the breakdown into layers is influenced very little by the latent heat release but depends solely on the phase boundary deflection caused by lateral temperature differences the value of the critical clapeyron slope also seems little affected by the width of the transition zone or by its depth a possible superplastic rheology within the transition zone would tend to favor layered convection scaling the model results to the 670 km discontinuity in the earth s mantle as a possible endothermic phase boundary we estimate the critical clapeyron slope to be in the range of 4 to 8 mpa k 40 to 80 bar k the possibility that the spinel perovskite periclase transition is within this range appears to be remote but certainly cannot be neglected

computational approaches to studying non linear dynamics of the crust and mantle we outline a mathematical formulation for mantle convection which can deal with the viscoelastic plastic rheology of the cool parts of the lithosphere this formulation is then analyzed to expose the numerical challenges inherent in the equations and a suitable solution strategy is outlined with this strategy in place we discuss a parallel implementation explaining how we maintain computational efficiency in a tiered and modular software environment we show two examples from geodynamics which demonstrate the capability of the numerical method and its implementation c 2007 elsevier b v all rights reserved

geochemical and mineralogical characterization of sediments from lake futalaufquen 42 8 s andean patagonia to evaluate their potential as paleoclimatic proxies lake sediments are key archives for paleoenvironmental investigation as they provide continuous records of the depositional history of the lake and its watershed lake futalaufquen 42 8 s is an oligotrophic waterbody located in los alerces national park in the andes of northern patagonia south america a sedimentary sequence covering 1600 years was recovered to analyze the potential for paleoenvironmental reconstructions of the last millennia integration of different geochemical and mineralogical parameters and comparison with climatic reconstructions from other patagonian records give clues for the identification of a warm period around ad 800 1000 associated with the medieval climatic anomaly the high frequency of tephra layers beginning in the mid sixteenth century precludes identification of the little ice age recorded in northern patagonia as a cold period from the fourteenth to the eighteenth century furthermore the parameters analysed do not provide evidence of late twentieth century global warming however zn deposition a long distance atmospheric transport process of anthropogenic origin was identified during the last century

viscoelastic mantle convection and lithospheric stresses summary accurate predictions of stress distribution in the lithosphere are of major importance for approaching more realistic numerical models of the mantle lithosphere system since stress fields in the lithosphere computed in convection models differ substantially between viscous and viscoelastic rheologies it is essential to employ a viscoelastic rheology when accurate stresses are to be predicted in mantle convection models involving the lithosphere this difference in stress distribution and magnitude has important implications for accurate modelling of stress dependent processes like power law creep shear heating and plasticity a further requirement for computation of accurate stress fields is to ensure numerically divergence free solutions in the boussinesq approximation we present the technical background required for implementation of numerically incompressible solutions and for implementation of a maxwell viscoelastic rheology in the frame of the finite element method fem we employ the jaumann invariant stress derivative in our implementation and demonstrate that the choice of the invariant stress derivative is irrelevant for geodynamic simulations we discuss potential numerical advantages of a viscoelastic rheology when large viscosity variations are applied in thermal convection models due to the physical transition from effectively viscous to elastic behaviour in a viscoelastic model the introduction of viscosity cut offs generally applied in viscous models can be avoided

self consistent generation of tectonic plates in three dimensional mantle convection abstract despite the fundamental importance of plates in the earth s mantle convection plates have not generally been included in numerical convection models or analog laboratory experiments mainly because the physical properties which lead to plate tectonic behavior are not well understood strongly temperature dependent viscosity results in an immobile rigid lid so that plates where included at all in 3 d models have always been imposed by hand an important challenge is thus to develop a physically reasonable material description which allows plates to develop self consistently this paper focuses on the role of ductile shear localization in two dimensional geometry it is well established that strain rate softening non newtonian rheologies e g power law visco plastic cause weak zones and strain rate localization above up and down wellings resulting in a rudimentary approximation of plates three dimensional geometry however is fundamentally different due to the presence of transform plate boundaries with associated toroidal motion since power law and visco plastic rheologies do not have the property of producing shear localization it is not surprising that they do not produce good plate like behavior in three dimensional calculations here it is argued that a strain rate weakening rheology previously shown to produce plate like behavior in a two dimensional sheet representing the lithosphere is a reasonable generic description of various weakening processes observed in nature one and two dimensional models are used to show how this leads to shear localization and the formation of faults this rheology is then applied to the high viscosity lithosphere of 3 d mantle convection calculations and the velocity pressure viscosity solution for the entire 3 d domain lid and underlying mantle is solved self consistently it is found that the lithosphere divides into a number of very high viscosity plates separated by narrow sharply defined weak zones with a viscosity many orders of magnitude less than the plate interiors broad weak zones with dominant convergent divergent motion above up and down wellings are interconnected by a network of narrow weak zones with dominant strike slip motion passive spreading centers are formed in internally heated cases while the resulting plates are not fully realistic these results show that self consistent plate generation is a realizable goal in three dimensional mantle convection and provide a promising avenue for future research

mantle convection modeling with viscoelastic brittle lithosphere numerical methodology and plate tectonic modeling the earth s tectonic plates are strong viscoelastic shells which make up the outermost part of a thermally convecting predominantly viscous layer brittle failure of the lithosphere occurs when stresses are high in order to build a realistic simulation of the planet s evolution the complete viscoelastic brittle convection system needs to be considered a particle in cell finite element method is demonstrated which can simulate very large deformation viscoelasticity with a strain dependent yield stress this is applied to a plate deformation problem numerical accuracy is demonstrated relative to analytic benchmarks and the characteristics of the method are discussed

modification of the lithospheric stress field by lateral variations in plate mantle coupling 1 the presence of deeply penetrating continental roots may locally increase the magnitude of basal shear tractions by up to a factor of 4 compared to a layered viscosity structure here we examine how these increases in mantle lithosphere coupling influence stress patterns in the overlying elastic lithosphere by coupling a mantle flow model to a model for the elastic lithosphere we show that the amplification of mantle tractions beneath cratons increases elastic stress magnitudes by at most a factor of only 1 5 in a pattern not correlated to local basal traction changes this disconnect is explained by the transmission of elastic stresses across large distances which makes them sensitive to regionally averaged changes in basal tractions but not local variations our results highlight the importance of regional variations in lithospheric strength which could allow stress patterns to more closely match regional changes in basal shear

bromine as indicator of source of lacustrine sedimentary organic matter in paleolimnological studies bromine br to organic matter om concentration ratio is studied in lake sediment sequences to provide information on environmental changes modifying om production the sequences studied were extracted from shallow lakes morenito el trebol escondido and portezuelo and deep lakes futalaufquen moreno and traful north patagonia andean range lake morenito a former lake moreno bay until its closure in ad 1960 showed a decrease in br om ratios from 1 38 to 0 74 after lake closure associated with an increase of primary autochthonous productivity attributable to the development of submerged and emerging macrophytes sedimentary sequences from lakes escondido portezuelo and el trebol with large participation of macrophytes in primary productivity and from lakes moreno futalaufquen and traful with little development of littoral macrophytes showed br om ratios consistent with the lake morenito pattern consistently the morphometric parameters mean depth and shoreline development correlate with br om ratios therefore br om ratios can be associated with the composition of primary autochthonous productivity with values of about 0 7 associated to significant macrophyte contributions and higher values associated with more pelagic contributions accordingly br om variations along a sedimentary sequence can be associated with modifications on the composition of the primary autochthonous productivity of the water body providing information on environmental changes

temporal evolution of sediment supply in lago puyehue southern chile during the last 600 yr and its climatic significance short term climate changes in southern chile are investigated by a multi proxy analysis of a 53 cm long sedimentary sequence selected among eight short cores retrieved in lago puyehue chile 40 s this core contains a 600 yr long undisturbed record of paleo precipitation changes two measurement methods for sediment density organic matter and biogenic silica contents are compared and the most appropriate techniques are selected together with aluminium and titanium concentrations grain size and geochemical properties of the organic matter these proxies are used to demonstrate paleo precipitation changes around 40 s increase of terrigenous particle supply between a d 1490 and a d 1700 suggests a humid period contemporaneously y 13 c data show increasing lake productivity in response to the high nutrient supply the a d 1700 1900 interval is characterized by a decreasing terrigenous supply and increasing y 13 c values interpreted as a drying period the magnetic susceptibility signal reflecting the terrigenous biogenic ratio demonstrates that similar variations occur in all the undisturbed sedimentary environments of lago puyehue the a d 1490 1700 wet period is associated with the onset of the european little ice age lia andinterpreted asits local signature this worksupports the fact that the liawasaglobal event not only restricted to thenorthern hemisphere d 2005 university of washington all rights reserved

the pinto shear zone a laramide synconvergent extensional shear zone in the mojave desert region of the southwestern united states the pinto shear zone is one of several late cretaceous shear zones within the eastern fringe of the mesozoic magmatic arc of the southwest cordilleran orogen that developed synchronous with continued plate convergence and backarc shortening we demonstrate an extensional origin for the shear zone by describing the shear zone geometry and kinematics hanging wall deformation style progressive changes in deformation temperature and differences in hanging wall and footwall thermal histories deformation is constrained between w74 and 68 ma by 40 ar 39 ar thermochronology of the exhumed footwall including multi diffusion domain modeling of k feldspar we discount the interpretations applied in other areas of the mojave desert region that widespread late cretaceous cooling results from refrigeration due to subduction of a shallowly dipping laramide slab or to erosional denudation and suggest alternatively that post intrusion cooling and exhumation by extensional structures are recorded widespread crustal melting and magmatism followed by extension and cooling in the late cretaceous are most consistent with production of a low viscosity lower crust during anatexis and or delamination of mantle lithosphere at the onset of laramide shallow subduction q 2005 published by elsevier ltd

fallout and distribution of volcanic ash over argentina following the may 2008 explosive eruption of chaiten chile 1 the major explosive eruption of chaiten volcano chile in may 2008 provided a rare opportunity to track the long range dispersal and deposition of fine volcanic ash the eruption followed 10 000 years of quiescence was the largest explosive eruption globally since hudson chile in 1991 and was the first explosive rhyolitic eruption since novarupta alaska in 1912 field examination of distal ashfall indicates that 1 6 1011 kg of ash dense rock equivalent volume of 0 07 km3 was deposited over 2 105 km2 of argentina during the first week of eruption the minimum eruption magnitude estimated from the mass of the tephra deposit is 4 2 several discrete ashfall units are identifiable from their distribution and grain size characteristics with more energetic phases showing a bimodal size distribution and evidence of cloud aggregation processes ash chemistry was uniform throughout the early stages of eruption and is consistent with magma storage prior to eruption at depths of 3 6 km deposition of ash over a continental region allowed the tracking of eruption development and demonstrates the potential complexity of tephra dispersal from a single eruption which in this case comprised several phases over a week long period of intense activity

radially anisotropic 3 d shear wave structure of the australian lithosphere and asthenosphere from multi mode surface waves abstract a new radially anisotropic shear wave speed model for the australasian region is constructed from multi mode phase dispersion of love and rayleigh waves an automated waveform fitting technique based on a global optimization with the neighbourhood algorithm allows the exploitation of large numbers of three component broad band seismograms to extract path specific dispersion curves covering the entire continent a 3 d shear wave model is constructed including radial anisotropy from a set of multi mode phase speed maps for both love and rayleigh waves these maps are derived from an iterative inversion scheme incorporating the effects of ray path bending due to lateral heterogeneity as well as the finite frequency of the surface waves for each mode the new s wave speed model exhibits major tectonic features of this region that are in good agreement with earlier shear wave models derived primarily from rayleigh waves the lateral variations of depth and thickness of the lithosphere asthenosphere transition lat are estimated from the isotropic voigt average s wave speed model and its vertical gradient which reveals correlations between the lateral variations of the lat and radial anisotropy the thickness of the lat is very large beneath the archean cratons in western australia whereas that in south australia is thinner the radial anisotropy model shows faster sh wave speed than sv beneath eastern australia and the coral sea at the lithospheric depth the faster sh anomaly in the lithosphere is also seen in the suture zone between the three cratonic blocks of australia one of the most conspicuous features of fast sh anisotropy is found in the asthenosphere beneath the central australia suggesting anisotropy induced by shear flow in the asthenosphere beneath the fast drifting australian continent

bromine and chlorine emissions from plinian eruptions along the central american volcanic arc from source to atmosphere highlights data set of br and cl emissions from 29 large cava eruptions vei 5 melt inclusions are strongly enriched in chlorine and bromine compared to their respective matrix glasses fluid partitioning is 4 to 68 times more efficient for br than for cl subducted calcareous sediments are the major control on the arc magmatic bromine contents average cava eruption would add 368 eesc of recent annual loading to stratosphere abstract large explosive volcanic eruptions inject gases aerosols and fine ashes into the stratosphere potentially influencing climate and atmosphere composition on a global scale although the potential climate effect of chlorine cl and bromine br injections into the stratosphere is known the global mass fluxes are poorly constrained in this study we focus on the magmatic degassing systematics and budgets of br and cl and on constraining the major sources of br in a subduction setting we therefore present a regional time series of br and cl emissions from 29 highly explosive eruptions throughout the central american volcanic arc cava covering the last 200 ka and a range of magmatic compositions and eruption magnitudes we have measured br and cl in matrix glasses and melt inclusions using synchrotron radiation micro x ray fluorescence spectrometry sr micro xrf and electron microprobe respectively melt inclusions of the cava tephras generally have higher br 0 9 to 17 9 ppm and cl 770 to 3800 ppm contents than the matrix glasses 0 39 to 1 5 ppm br 600 to 2800 ppm cl moreover the difference between maximum and minimum concentrations observed in melt inclusions of a given sample ranges between 9 and 90 of the maximum observed concentration for br and between 2 and 40 for cl such intra sample variations arise from variable pre eruptive degassing of these halogens into a magmatic fluid phase the relative loss of br from the melt is 4 to 68 times higher than that of cl the masses of br 2 1100 kt and cl 0 1 to 800 mt emitted by the eruptions generate instantaneous additions to the stratosphere potentially amounting to 6 5600 of the present day stratospheric annual global loading of equivalent effective stratospheric chlorine as the size of the stratospheric impact is primarily a function of eruption magnitude we use magnitude frequency relationships to estimate that eruptions adding 10 to resident eesc loading would occur every 40 years while every 200 years an eruption would double the eesc loading comparing the variations in br and cl concentrations and particularly minimum cl br ratios in melt inclusions with geochemical trace element proxies e g u la ba th and lead isotope compositions which change along the arc in response to changing subduction conditions we suggest that subducted calcareous sediment is a major source of magmatic br but also infer an important role of fluids expelled from serpentinized subducted mantle extrapolation of cava volcanic br emissions to the global subduction system thus needs to consider variations in the nature of subducted lithologies

geodynamic evolution of the southern abitibi and pontiac terranes evidence from geochemistry of granitoid magma series 2700 2630 ma four distinct granitic series developed during the evolution of the archean abitibi southern volcanic zone svz 1 a minor synvolcanic tonalite trondhjemite granodiorite ttg series was emplaced at 2700 ma which has extremely low k2o 1 5 wt rb 50 ppm and sr 200 ppm higher nb y and sc flat rare earth element ree patterns and negative eu anomalies 2 a voluminous tonalite granodiorite granite quartz monzonite tggm series developed syntectonically at 2695 2685 ma and displays low k2o 1 3 wt rb 10 50 ppm ba 1000 ppm and u enhanced sr depletion of ta nb and ti and strongly fractionated ree patterns la ybn 49 21 3 a late tectonic quartz syenite quartz monzonite granite smg series was emplaced from 2685 to 2675 ma and is grossly similar to the tggm but has lower cao k2o na2o and greater concentrations of rb ba th and u 4 a late tectonic to posttectonic alkali feldspar syenite alkali feldspar quartz syenite ss series was emplaced

origin of silicic magmas along the central american volcanic front genetic relationship to mafic melts abstract silicic pyroclastic flows and related deposits are abundant along the central american volcanic front these silicic magmas erupted through both the non continental chorotega block to the southeast and the paleozoic continental chortis block to the northwest the along arc variations of the silicic deposits with respect to diagnostic trace element ratios ba la u th ce pb oxygen isotopes nd and sr isotope ratios mimic the along arc variation in the basaltic and andesitic lavas this variation in the lavas has been interpreted to indicate relative contributions from the slab and asthenosphere to the basaltic magmas carr m j feigenson m d bennett e a 1990 incompatible element and isotopic evidence for tectonic control of source mixing and melt extraction along the central american arc contributions to mineralogy and petrology 105 369 380 patino l c carr m j and feigenson m d 2000 local and regional variations in central american arc lavas controlled by variations in subducted sediment input contributions to mineralogy and petrology 138 3 265 283 with respect to along arc trends in basaltic lavas the largest contribution of slab fluids is in nicaragua and the smallest input from the slab is in central costa rica similar trends are observed in the silicic pyroclastic deposits data from melting experiments of primitive basalts and basaltic andesites demonstrate that it is difficult to produce high k 2 o na 2 o silicic magmas by fractional crystallization or partial melting of low k 2 o na 2 o sources however fractional crystallization or partial melting of medium to high k basalts can produce these silicic magmas we interpret that the high silica magmas associated central america volcanic front are partial melts of penecontemporaneous mantle derived evolved magmas that have ponded and crystallized in the mid crust or are melts extracted from these nearly completely crystallized magmas

structural criteria for identifying granitic cumulates abstractcumulates are igneous rocks that reflect relative concentration of crystals and or loss of melt and that therefore did not crystallize entirely from a magma of their current whole rock composition however this definition does not help to identify cumulates and so the problem is to determine whether the current whole rock composition reflects cumulate processes which is where structural criteria can be useful identification of cumulates in granitoids can be difficult especially in the absence of major whole rock compositional differences between cumulate and host structural and structural mineralogical criteria diagnostic of or at least consistent with granitic cumulates include 1 locally high concentrations of particular minerals relative to concentrations in the bulk of the intrusion 2 abundant euhedral crystals in contact at least with regard to cores that touched before any overgrowths and consequent molding of minerals about each other occurred 3 abundant antecrysts or mantle

postglacial vegetation climate and fire history along the east side of the andes lat 41 42 5 s argentina abstract the history of the low elevation forest and forest steppe ecotone on the east side of the andes is revealed in pollen and charcoal records obtained from mid latitude lakes prior to 15 000 cal yr bp the vegetation was characterized by steppe vegetation with isolated stands of nothofagus the climate was generally dry and the sparse vegetation apparently lacked sufficient fuels to burn extensively after 15 000 cal yr bp a mixture of nothofagus forest and shrubland steppe developed fire activity increased between 13 250 and 11 400 cal yr bp contemporaneous with a regionally defined cold dry period huelmo mascardi cold reversal the early holocene period was characterized by an open nothofagus forest shrubland mosaic and fire frequency was high in dry sites and low in wet sites the data suggest a sharp decrease in moisture eastward from the andes a shift to a surface fire regime occurred at 7500 cal yr bp at the wet site and at 4400 cal yr bp at the dry site preceding the expansion of austrocedrus by 1000 1500 yr the spread of austrocedrus is explained by a shift towards a cooler and wetter climate in the middle and late holocene the change to a surface fire regime is consistent with increased interannual climate variability and the onset or strengthening of enso the present day mixed forest dominated by nothofagus and austrocedrus was established in the last few millennia

thermal evolution of the upper yangtze craton secular cooling and short lived thermal perturbations abstract the upper yangtze craton uyc has stayed in a long term stabilization state its thermal history can be portioned into two parts secular evolutionary trend and short lived thermal perturbations the former is simulated by a two dimensional forward transient thermal model and the latter is discussed by reviewing previous studies the numerical modeling indicates the uyc tended to be cooling as it adjusted to the asynchronous rates of change in the internal radiogenic heat production and in the convecting mantle temperature below the cratonic root the average cooling rate of the uyc has been 58 c ga since the end of archean faster than that of the convecting mantle the surface heat flow presents a decreasing trend from 82 mw m2 at 2 5 ga to 53 mw m2 at present day which constructs the long term evolutionary path the thermal perturbations derived from the occasional tectono thermal events such as mantle plume activity and regional lithosphere extension serves as paroxysmal factors the thermal effects of the emeishan mantle plume were very strong yielding to the maximum heat flow anomaly of 100 mw m2 at the late permian in contrast the influences of regional lithosphere extension during both the early paleozoic period and the early permian middle triassic period were rather weak resulting in a surface heat flow increase of

tephrochronology of recent events in the andean range northern patagonia spatial distribution and provenance of lacustrine ash layers in the nahuel huapi national park tephrochronology is a powerful tool for dating sedimentary sequences especially in patagonia where a large number of active volcanoes have produced frequent historical eruptions short lacustrine sedimentary sequences were extracted from the lakes moreno oeste and ton cek nahuel huapi national park seventeen volcanic ash layers were identified in both cores 210 pb and 137 cs were used for dating techniques and historical volcanic records were employed for correlation white pumice and glass shards from the tephras were characterised by measuring major and trace element contents by instrumental neutron activation analysis two volcanic sources the cordon caulle and the calbuco volcanoes were recognised as the prevailing systems that impacted the area in the past 800 1000 a the calbuco volcano was mainly responsible for the nine tephras identified in the lake ton cek sequence four of these nine tephras also contained material from the cordon caulle complex and could be interpreted as composite tephras or possible reworked ones the calbuco volcano showed predominant influences in the lake moreno oeste sequence six of the eight tephras identified in such a sequence were from the calbuco volcano and three of these six contained material from the cordon caulle complex the other two were from the cordon caulle complex with contributions from the calbuco volcano in one of them these findings show that the high frequency of volcanic events in the study region demands an accurate characterisation of the products generated by each source as well as an evaluation of their spatial distribution to obtain a consistent framework for dating recent environmental changes copyright 2010 john wiley sons ltd supporting information can be found in the online version of this article

lithospheric thickness and mechanical strength of the indian shield abstract we determined the relative mechanical strength of the indian subcontinent by a robust coherence method based on multitaper spectral analysis on overlapping windows of equal size representing the elastic properties of the lithosphere by a transitional coherence wavelength we bypass the need to input rheological variables such as young s modulus or poisson s ratio which have been shown to vary on other continents we compare our measurements in terms of age and find that regions underlain by proterozoic basement are on average stronger than regions of archaean age this is in stark contrast to the common assumption that mechanical strength should increase progressively with time this suggests they are indeed mechanically distinct and that the transitional coherence wavelength adequately captures this distinction between regions of relative strength difference conversions of these values to absolute effective elastic thickness show a greater overlap which is due to the many strong assumptions required in obtaining such absolute values and argues against their usage as a robust and well constrained property of the elastic lithosphere

a critical assessment of neoarchean plume only geodynamics evidence from the superior province abstract criticisms of archean plate tectonic models rarely offer detailed descriptions of alternative paradigms the catalytic delamination driven model for the coupled genesis of archean crust and sub continental lithospheric mantle described by bedard 2006 is an exception in that a relatively comprehensive but essentially 2 d model is described although this novel model is frequently cited as an example of the diversity in geodynamic thought for the archean it is rarely assessed with reference to events in a specific craton such as the superior province or a particular greenstone belt such as the abitibi when this type of assessment is undertaken it becomes clear that the apparent simplicity of the model one of its supposed virtues constitutes a critical weakness although bedard 2006 developed the catalytic model to overcome the perceived melt productivity deficit of archean plate tectonic models the catalytic process at the heart of the model is likely to be inefficient and provides insignificant improvements to crustal generation rates associated with subduction and non catalytic plume tectonics the model implies a near global craton in the mesoarchean to neoarchean but fails to address that fate of this craton during and after crustal rigidification seismic reflectors observed beneath archean cratons are characterized as failed delamination features but their global abundance suggests that syn to post tectonic ttg suites could not have been generated as a result of crustal restite delamination the brief descriptions of suggested mechanisms for the generation of minor rock types such as sanukitoids mask the complexity inherent in the overall model when considered in terms of the evolution of the superior province as a whole it is evident that many of these suggestions are not only implausible they are inconsistent with the timing and distribution of geological criteria used to mark the onset of crustal rigidification

strong lateral variations of lithospheric mantle beneath cratons example from the baltic shield abstract understanding mechanisms for creation and evolution of precambrian continental lithosphere requires to go beyond the large scale seismic imaging in which shields often appear as laterally homogeneous with a thick and fast lithosphere we here present new results from a seismic experiment polenet lapnet in the northern part of the baltic shield where we identify very high seismic velocities vs 4 7 km s in the upper part of the mantle lithosphere and a velocity decrease of 0 2 km s at approximately 150 km depth we interpret this velocity decrease as refertilisation of the lower part of the lithosphere this result is in contrast to the lithospheric structure immediately south of the study area where the seismic velocities within the lithosphere are fast down to 250 km depth as well as to that of southern norway where there is no indication of very high velocities in the lithospheric mantle vs of 4 4 km s while the relatively low velocities beneath southern norway can tentatively be attributed to the opening of the atlantic ocean the velocity decrease beneath northern finland is not easily explained with present knowledge of surface tectonics our results show that shield areas may be laterally heterogeneous even over relatively short distances such variability may in many cases be related to lithosphere erosion and or refertilisation at the edge of cratons which may therefore be particularly interesting targets for seismic imaging

modeling shear zones in geological and planetary sciences solid and fluid thermal mechanical approaches abstract shear zones are the most ubiquitous features observed in planetary surfaces they appear as a jagged network of faults at the observable brittle surface of planets and in geological exposures of deeper rocks they turn into smoothly braided networks of localized shear displacement leaving centimeter wide bands of mylonitized reduced grain sizes behind the overall size of the entire shear network rarely exceeds kilometer scale at depth although mylonitic shear zones are only visible to the observer when uplifted and exposed at the surface they govern the mechanical behavior of the strongest part of the lithosphere below 10 15 km depth mylonitic shear zones dissect plates thus allowing plate tectonics to develop on the earth we review the basic multiscale physics underlying mylonitic ductile shear zone nucleation growth and longevity and show that grain size reduction is a symptomatic cause but not necessarily the main reason for localization we also discuss a framework for analytic and numerical modeling including the effects of thermal mechanical couplings thermal elasticity the influence of water and void volatile feedback the physics of ductile shear zones relies on feedback processes that turn a macroscopically homogenously deforming body into a heterogeneously slipping solid medium positive feedback can amplify strength heterogeneities by cascading through different scales we define basic intrinsic length scales of strength heterogeneity such as those associated with plasticity grain size fluid inclusion and thermal diffusion length scale for an understanding ductile shear zones we need to consider the energetics of deformation shear heating introduces a jerky flow phenomenon potentially accompanied by ductile earthquakes additional focusing due to grain size reduction only operates for a narrow parameter range of cooling rates for the long time scale deformational energy stored inside the shear zone through plastic dilation or crystallographic and shape preferred orientation consumes only a maximum of 10 of energy dissipated in the shear zones but creates structural anisotropy shear zones become long living features with a long term memory a special role is attributed to the presence of water in nominally anhydrous minerals we show that water directly affects the mechanical equation of state and has the potential to synchronize viscous and plastic flow processes at geological time scale we have shown that fully coupled finite element calculations using mechanical data from the laboratory can reproduce the basic mode of deformation of an entire mylonitic shear zone the next step of modeling lies in benchmarking basic feedback mechanism in field studies and zooming into the braided network of shear zone structure without losing the large scale constraint numerical methods capable of fulfilling the goal are emerging these are adaptive wavelet techniques and hybrid particle finite element codes which can be run over a computational grid across the net

magmatic evolution of the puyehue cordon caulle volcanic complex 40 s southern andean volcanic zone from shield to unusual rhyolitic fissure volcanism abstract magmas erupted from quaternary volcanoes of southern andes between 37 and 46 s latitude are mainly basaltic to andesitic however pccvc 40 s shows a singular magmatic evolution due to the abnormal evacuation of rhyolites especially in the last 100 ka in addition pccvc is the result of juxtaposing products from the nw trending alignment of cordillera nevada caldera cordon caulle fissure volcano and the puyehue stratocone using 40 ar 39 ar and 14 c geochronology it can be established that they evolved since ca 500 ka as coeval but separated vents with a first stage of shield volcanism followed by repeated collapses that formed an internal nw elongated graben from ca 100 ka volcanic activity occurred in both a fissure system cordon caulle and a central volcano puyehue holocene explosive eruptions mainly in the puyehue crater accompanied the dome growing along a nw trending fissure system last historical eruptions were in 1921 and 1960 when nw fissures of cordon caulle fed rhyodacitic lava flows in 1960 the fissure eruption was triggered by a remote mw 9 5 thrust earthquake cordillera nevada caldera presents a reduced compositional range 52 63 sio 2 and geochemical features of low pressure magma mixing and assimilation instead cordon caulle and puyehue volcanoes have a wide silica range 48 71 sio 2 and an outstanding affinity which can be modelled with initial high pressure fractional crystallization moderate magma mixing and subsequent low pressure fractional crystallization from a common parental source the exceptional magmatic evolution and eruptive style of pccvc in southern andes could be related with the physics of the plumbing system which in turn can be controlled by external factors as the structure of the continental crust and the ongoing stress regime

seismic and mechanical anisotropy and the past and present deformation of the australian lithosphere abstract we interpret the three dimensional seismic wave speed structure of the australian upper mantle by comparing its azimuthal anisotropy to estimates of past and present lithospheric deformation we infer the fossil strain field from the orientation of gravity anomalies relative to topography bypassing the need to extrapolate crustal measures and derive the current direction of mantle deformation from present day plate motion our observations provide the depth resolution necessary to distinguish fossil from contemporaneous deformation the distribution of azimuthal seismic anisotropy is determined from multi mode surface wave propagation mechanical anisotropy or the directional variation of isostatic compensation is a proxy for the fossil strain field and is derived from a spectral coherence analysis of digital gravity and topography data in two wavelength bands the joint interpretation of seismic and tectonic data resolves a rheological transition in the australian upper mantle at depths shallower than 150 200 km strong seismic anisotropy forms complex patterns in this regime the seismic fast axes are at large angles to the directions of principal shortening defining a mechanically coupled crust mantle lid deformed by orogenic processes dominated by transpression here seismic anisotropy may be considered frozen which suggests that past deformation has left a coherent imprint on much of the lithospheric depth profile the azimuthal seismic anisotropy below 200 km is weaker and preferentially aligned with the direction of the rapid motion of the indo australian plate the alignment of the fast axes with the direction of present day absolute plate motion is indicative of deformation by simple shear of a dry olivine mantle motion expressed in the hot spot reference frame matches the seismic observations better than the no net rotation reference frame thus seismic anisotropy supports the notion that the hot spot reference frame is the most physically reasonable independently from plate motion models seismic anisotropy can be used to derive a best fitting direction of overall mantle shear

age dependent seismic thickness and mechanical strength of the australian lithosphere 1 we present constraints on the regional variations of the seismic and mechanical thickness of the australian lithosphere we infer the seismic thickness from a waveform tomographic model of s wave speed and as a proxy for the elastic thickness we use the wavelength at which the coherence of surface topography and bouguer gravity drops below half of its long wavelength maximum our results show that on scales 1000 km the relationship between the age of the crust and the thickness of the lithosphere is more complicated than longer wavelength or global averages suggest recent geochemical and geodynamical evidence for small scale secular variations of the composition and stability of continental cratons further illustrates the complexity of the age dependence of seismo mechanical lithospheric properties on regional scales

the significance of microgranitoid enclave shapes and orientations abstract enclaves are often incorrectly used to measure magmatic strains in plutons we emphasize that microgranitoid enclaves are not like other ellipsoidal markers used to determine strain for the following reasons 1 adjacent enclaves may form at different times and different places and initially have non spherical shapes with axial ratios up to 2 7 2 the final shapes and orientations of enclaves are a complex function of a initial shape and temperature of enclaves b subtle changes in composition melt percents volatiles grain sizes and thus temporally variable viscosity contrasts between the enclave and magma c a competition between strain and interfacial energies and d deformation path which may include internal strain and rigid rotations caused by magma flow during ascent convection expansion chamber boundary processes and tectonism 3 enclaves spend much of their time in magma as relatively rigid objects and thus rigidly rotate and potentially break apart rather than strain at matrix strain rates 4 in some instances enclaves do not record or track finite strain because of the above final enclave populations are heterogeneous and the use of single enclaves or enclave populations as strain markers violates many assumptions needed to complete strain analyses on the other hand a comparison of the preserved characteristics of igneous layering mineral fabrics and carefully evaluated enclave fabrics including internal mineral alignment in enclaves may provide qualitative data on the changing magnitude and kinematics of magmatic strains

generation and driving forces of plate like motion and asymmetric subduction in dynamical models of an integrated mantle lithosphere system abstract the dynamical effects of an asymmetric subduction structure on the generation of plate like motion were investigated using two dimensional numerical models of the integrated lithosphere mantle system to dynamically generate the plate boundary we introduce a history dependent rheology in which the yield strength is determined by past fractures only the buoyancy due to the internal density contrast consistently drives convective flow including the motion of the viscous lithosphere without imposed boundary conditions we first investigate the effects of plate yield strength friction at the plate boundary and plate age on the emergence of plate like motion with asymmetric subduction plate like motion is generated when maximum plate strength is as high as that estimated by experimental rheology studies the reason for this is that asymmetric subduction requires a plate bending force much less than that for symmetric subduction because the plate gently bends when one sided subduction occurs in contrast the strength of the plate boundary has to be very small for emergence of subduction as several previous studies on the numerical convection and subduction modeling have pointed out development of the subducted slab is also controlled by the age of the plate in the early stages of subduction older plates increase their velocities faster because of their larger negative buoyancy after the slab develops the plate stiffness that is both the yield strength and the plate thickness control plate velocity so that an older plate subducts more sluggishly we next explore effects of viscosity layering in the underlying mantle focusing on the mechanism in which the asthenosphere promotes plate motion the low viscosity under the lithosphere enhances a mantle drag force that drives the plate not only concentrating the convective flow beneath the plate but also enlarging its aspect ratio we also examine longevity of the plate like motion using the convection models with asymmetric subduction the asymmetrically structured subduction process continues stably at high yield strength at which episodic resurfacing or the stagnant lid mode occurs in the previous convection simulation studies the asymmetric subduction structure therefore has key roles in generating plate like motion as well as reducing the strength at the plate boundary