an experimental study of the relationship between joint spacing and layer thickness abstract two methods for measuring joint spacing are described and compared the area method is a constant for a given outcrop area and is not affected by joint distribution within that area in contrast the line method depends on the location of the linear traverse two kinds of joint sets are distinguished on bedding surfaces 1 a poorly developed set represents the early stages of development when typical joint lengths are less than typical spacing 2 a well developed set represents later stages when lengths are much greater than spacing the area method is applicable to both poorly and well developed sets whereas the line method produces inconsistent results for poorly developed joint sets surface textures on many joints indicate point fracture origins and propagation parallel to bedding whereas most numerical model studies of spacing assume linear origins and propagation perpendicular to bedding the laboratory experiments described in this paper do not suffer from these restrictions an important concept confirmed during these experiments is fracture saturation when the applied strain reaches a certain value fracture spacing stops evolving and remains nearly constant these fracture sets are well developed spacing at saturation is a function of layer thickness but is independent of strain whereas spacing before saturation varies strongly with applied strain thus plotting spacing vs thickness and comparing the slopes of lines fit to such data for poorly developed joint sets in different layers is unlikely to be a diagnostic test for differences in material properties on the other hand spacing may be a sensitive indicator of strain for layers with poorly developed joint sets assessing fracture saturation is a lirst order consideration when gathering spacing data

transportable seismic array tomography in southeast australia illuminating the transition from proterozoic to phanerozoic lithosphere abstract the phanerozoic tasmanides of eastern australia is comprised of a series of orogenic belts that developed along the east margin of gondwana following the breakup of the supercontinent rodinia and subsequent formation of the pacific ocean the tectonic complexities of this region have been well studied but most work has been confined to evidence collected from the near surface where extensive mesozoic and cenozoic basin cover masks large tracts of palaeozoic basement we apply teleseismic tomography to distant earthquake data recorded by wombat the largest transportable seismic array experiment in the southern hemisphere to image p wavespeed variations in the mantle lithosphere beneath the southern portion of the tasmanides in detail in order to seamlessly suture together the teleseismic datasets from each of the 14 sub arrays of wombat we use p wavespeeds from the ausrem model to construct a laterally heterogeneous starting model that captures the long wavelength structural variations that would otherwise be lost through the use of relative arrival time residuals synthetic resolution tests indicate good horizontal resolution of 50 km within most of the array between depths of 50 350 km a key feature of the 3 d p wave model is a pronounced easterly high velocity salient in the mantle lithosphere beneath the northern limit of the new england orocline which may indicate the presence of underpinning proterozoic lithosphere that was instrumental in its formation another pronounced high velocity anomaly underlies the curnamona province a large crustal block with a strong archean provenance which is clearly separated from the gawler craton to the west at upper mantle depths by a low velocity zone beneath the adelaide fold belt we also estimate the location of the eastern boundary of precambrian australia at depth and show that it extends eastward further than previously thought

localized basal freezing within george vi ice shelf antarctica hobbs pool is an area of thin ice shelf situated within george vi ice shelf antar ctica thicker ice shelf surro unding hobbs pool isolates the upper 155 m of the water column from water lying at the same depth else where under the ice shelf summer melt water lakes drain through crevasses at hobbs pool forming a 155 m thick layer of low s alinity water close to its freezing point colder and more saline water in the lower part of this layer leads to in situ freezing of fresher water lying above it below 155 m depth the water tempera ture and salinity are linearly related by basal melting which is observed elsewhere under the ice shelf the surface ice shows areas of defo rmation and deposits of subglacial rock debris which may result from upward particle paths in the area the raising of subgl acial rock debris on to the ice surface may provide a mechanism for the transp ort of erratics across the ice shelf to alexa nder island from the base of palmer land glaciers

sr isotopic heterogeneity in primitive basaltic rocks southeastern australia correlation with mantle metasomatism rb sr data for seven basaltic provinces k ar ages 50 0 ma in southeastern australia imply isotopic heterogeneities in the mantle sources the total range of 87sr 86sr is 0 7031 0 7054 effects of crustal contamination are negligible since the rocks analyzed represent primary or primitive magma compositions the inferred scales of heterogeneity range from 1 km for small intraprovince variations to in the order of 100 km for the larger differences between provinces correlation of regional high 87sr 86sr in basaltic rocks with the presence of amphibole bearing upper mantle xenoliths suggests that the degree of metasomatic activity in the underlying mantle is a major control on the rb sr and 87sr 86sr values of mantle source volumes and partial melts derived from these xenolith data also indicate that both pervasive metasomatism and the presence of crystallized melts or cumulates as veins and dykes in mantle wall rock are possible mechanisms for metasomatic additions mantle isochrons can be constructed both within some provinces and between provinces however episodic metasomatism in the mantle source regions with correlated enrichment in rb sr and 87sr 86sr can produce artificial isochrons which may have no relevance to mantle differentiation events

penetration of mid crustal low velocity zone across the kunlun fault in the ne tibetan plateau revealed by ambient noise tomography abstract the ne tibetan plateau composed of the mesozoic accretions of lhasa qiangtang and songpan ganze terranes are bounded by the east kunlun qaidam block in the north with the boundary delineated by the kunlun fault the ne tibetan plateau is at a nascent stage of plateau growth resulting from the collision between the indian and eurasian plate starting 50 million years ago and is one of the best areas to study the growth mechanism of the tibetan plateau in this study we process continuous ambient noise data collected from 280 stations during 2007 and 2010 and generate rayleigh wave phase velocity maps at 10 60 s periods with a lateral resolution of 30 50 km for most of the study region by adopting a bayesian monte carlo method we then construct a 3 d vsv model of the crust using the rayleigh wave dispersion maps our 3 d model reveals that strong lvzs exist in the middle crust across the ne tibetan plateau and the lateral distribution of lvzs exhibit significant west east variations along the kunlun fault in the west of 98 e lvzs are confined to regions of the kunlun fault and the eastern kunlun ranges but absent beneath the qaidam basin while in the east of 98 e lvzs extend and penetrate northward into the east kunlun and qinling orogens over 100 km across the kunlun fault the strong contrast of the lvzs distribution along the kunlun fault may be related to the distinct neighboring tectonic units in the north a strong crust of the qaidam basin in the west blocking the penetration of lvzs but a weak crust in the qinling orogens facilitating the extrusion of lvzs the distribution of lvzs in the ne tibetan plateau is consistent with the crustal channel flow model which predicts a branch of north eastward mid crustal channel flow our 3d model clearly delineates the north extent of the mid crustal lvzs probably reflecting the status of channel flow in the ne tibetan plateau

magnetotelluric evidence for serpentinisation in a cambrian subduction zone beneath the delamerian orogen southeast australia abstract the delamerian orogen in southeast australia represents a proterozoic continental rift margin overprinted by convergent margin andean style subduction in the cambrian a detailed 150 km east west magnetotelluric transect was collected across the orogen to investigate the electrical resistivity structure the magnetotelluric transect follows an existing full crustal reflection seismic transect of which interpretations support a westward dipping cambrian subduction model as derived from field mapping and geochemistry a 2d inversion of the data from the 68 station broadband magnetotelluric transect imaged a heterogeneous crust with lateral changes as large as 10 000 m occurring over 15 km the crust within the western glenelg zone is resistive in contrast to the eastern glenelg zone and the grampians stavely zone above the paleo subduction zone which host three conductive pathways the main low resistivity regions 1 10 m reside at mid lower crustal depths 10 30 km extending up to the surface with a higher resistivity 300 m but still much less than surrounding resistivity mantle 1000 m crust 10 000 m fluids released from the upper mantle during the cambrian west dipping subduction are interpreted to have moved up crustal faults to create the observed low resistivity pathways by serpentinisation and magnetite creation in mafic ultramafic rocks the electrical conductivity of hand samples of serpentinised mafic ultramafic rocks in the region was found to be much greater than most other rock types present in addition to adding insight into the crustal structure the magnetotelluric data also supports geological surface mapping as the major lawloit and yarramyljup faults that bound different geological domains also mark domains of different electrical structure

deflection of dykes into sills at discontinuities and magma chamber formation abstract many rift zones contain composite volcanoes central volcanoes most of which are supplied with magma from shallow crustal magma chambers volcanotectonic studies of eroded rift zones such as in iceland as well as geophysical studies indicate that many shallow magma chambers evolve from sills here i provide field description of sills and extinct magma chambers in the quaternary and tertiary palaeorift zones of iceland focusing on the felsic pluton of slaufrudalur southeast iceland as a fine example of a magma chamber developed through the injection of sills in this paper i review analyse and develop further two related mechanisms for the deflection of dykes into sills and thus the potential initiation of a magma chamber namely a cook gordon debounding delamination and b stress barriers and propose a third mechanism c favourable material toughness ratios due to elastic mismatch difference in young s moduli or stiffnesses of layers in contact in the cook gordon mechanism a weak contact opens up as a result of dyke induced tensile stress this mechanism is likely to operate primarily at shallow depths as is supported by field observations and numerical models a stress barrier is a layer where the local stress is unfavourable to a particular type of rock fracture here a dyke field observations and numerical models show that on meeting a stress barrier a dyke either changes into a sill or becomes arrested the material toughness mechanism indicates that when the upper layer at a contact has the same or less stiffness than the lower layer hosting the dyke there is little tendency for the dyke to become deflected into the contact however when the upper layer is stiffer such as when a stiff basaltic lava flow is on the top of a soft pyroclastic layer the dyke tends to become deflected into the contact to form a sill field results suggest that all these mechanisms may cause dyke deflection into sills and may operate together particularly where the rock consists of alternating soft and stiff layers since the quaternary lava pile in iceland has many more soft hyaloclastite layers than the tertiary lava pile these mechanisms were probably very efficient in generating sills and by implication shallow magma chambers during the quaternary this may be one reason why in comparison with the tertiary lava pile the quaternary lava pile contains so many sills and extinct plutons and active shallow magma chambers

the moho in australia and new zealand abstract australia and new zealand share in part a history in the gondwana supercontinent australia has a long and complex tectonic history with the last major accretion in the early paleozoic whereas new zealand is still undergoing major plate boundary processes the australian continent is relatively well covered with both active and passive seismic techniques multiple sources of information are therefore available for building a model of moho depth results from on shore and off shore refraction experiments are supplemented by receiver functions from a large number of portable stations and the recently augmented set of permanent stations moho picks from more than 10 500 km of full crustal reflection profiles provide valuable additional constraints the composite data set provides good sampling of much of australia though coverage remains low in some remote desert areas the various datasets provide multiple estimates of the depth to moho in many regions and the consistency between the different techniques is high some of the thinnest crust lies beneath the archean craton in the pilbara and in the neighbourhood of the simpson desert thick crust is encountered beneath parts of the proterozoic in central australia and beneath the paleozoic lachlan fold belt in southeastern australia there are a number of zones of sharp contrast in depth to moho notably in the southern part of central australia despite most of the continental material around new zealand being submerged moho data for this region is mainly onshore concentrating on the australia pacific plate boundary two major wide angle reflection transects provide the bulk of the active source data with just a few traditional reflection profiles offshore the plate boundary provides an abundance of local earthquakes for tomographic imaging and this data is supplemented with receiver functions from both portable and permanent networks onshore the combined coverage is as dense as that of australia although it could be argued that a higher spatial resolution is required to capture the nature of the moho of tectonically active new zealand three regions of thickened crust can be identified one beneath the southern alps another beneath fiordland and below the wanganui basin between the north and south island thin crust is identified west of the volcanic arc with extensive underplating below the back arc region

absence of a regional surface thermal high in the baikal rift new insights from detailed contouring of heat flow anomalies heat flow in active tectonic zones as the baikal rift is a crucial parameter for evaluating deep anomalous structures and lithosphere evolution based on the interpretation of the existing datasets the baikal rift has been characterized in the past by either high heat flow or moderately elevated heat flow or even lacking a surface heat flow anomaly we made an attempt to better constrain the geothermal picture by a detailed offshore contouring survey of known anomalies and to estimate the importance of observed heat flow anomalies within the regional surface heat output a total of about 200 new and close spaced heat flow measurements were obtained in several selected study areas in the north baikal basin with an outrigged and a violin bow designed thermoprobe of 2 3 m length both the sediment temperature and thermal conductivity were measured the new data show at all investigated sites that the large heat flow highs are limited to local heat flow anomalies the maximum measured heat flow reaches values of 300 35000 mw m2 but the extent of the anomalies is not larger than 2 to 4 km in diameter aside of these local anomalies heat flow variations are restricted to near background values of 50 70 mw m2 except in the uplifted academician zone the extent of the local anomalies excludes a conductive source and therefore heat transport by fluids must be considered in a conceptual model where all bottom floor heat flow anomalies are the result of upflowing fluids along a conduit an extra heat output of 20 mw including advection is estimated for all known anomalies in the north baikal basin relative to a basal heat flow of 55 65 mw m2 these estimations suggest an extra heat output in the northern lake baikal of only 5 corresponding to a regional heat flow increase of 3 mw m2 the source of this heat can be fully attributed to a regional heat redistribution by topographically driven ground water flow thus the surface heat flow is not expected to bear a signal of deeper lithospheric thermal anomalies that can be separated from heat flow typical for orogenically altered crust 40 70 mw m2 the new insights on the geothermal signature in the baikal rift once more show that continental rifting is not by default characterized by high heat flow

ice dammed lateral lake and epishelf lake insights into holocene dynamics of marguerite trough ice stream and george vi ice shelf alexander island antarctic peninsula abstract we present new data regarding the past dynamics of marguerite trough ice stream george vi ice shelf and valley glaciers from ablation point massif on alexander island antarctic peninsula this ice free oasis preserves a geological record of ice stream lateral moraines ice dammed lakes ice shelf moraines and valley glacier moraines which we dated using cosmogenic nuclide ages we provide one of the first detailed sediment landform assemblage descriptions of epishelf lake shorelines marguerite trough ice stream imprinted lateral moraines against eastern alexander island at 120 m at ablation point massif during deglaciation lateral lakes formed in the ablation and moutonnee valleys dammed against the ice stream in george vi sound exposure ages from boulders on these shorelines yielded ages of 13 9 to 9 7 ka following recession of the ice stream george vi ice shelf formed in george vi sound an epishelf lake formed at 15 20 m asl in ablation and moutonnee valleys dated from 9 4 to 4 6 ka suggesting that the lake was stable and persistent for some 5000 years lake level lowering occurred after this with the lake level at 12 m at 3 1 0 4 ka and at 5 m asl today a readvance of the valley glaciers on alexander island at 4 4 0 7 ka is recorded by valley glacier moraines overlying epishelf lake sediments we speculate that the glacier readvance which occurred during a period of warmth may have been caused by a dynamic response of the glaciers to a lowering in surface elevation of george vi ice shelf

crustal structure of the carpathian pannonian region from ambient noise tomography we use ambient noise tomography to investigate the crust and uppermost mantle structure beneath the carpathian pannonian region of central europe over 7500 rayleigh wave empirical green s functions are derived from interstation cross correlations of vertical component ambient seismic noise recordings 2005 2011 using a temporary network of 54 stations deployed during the south carpathian project 2009 2011 56 temporary stations deployed in the carpathian basins project 2005 2007 and 100 permanent and regional broad band stations rayleigh wave group velocity dispersion curves 4 40 s are determined using the multiple filter analysis technique group velocity maps are computed on a grid of 0 2 0 2 from a non linear 2 d tomographic inversion using the subspace method we then inverted the group velocity maps for the 3 d shear wave velocity structure of the crust and uppermost mantle beneath the region our shear wave velocity model provides a uniquely complete and relatively high resolution view of the crustal structure in the carpathian pannonian region which in general is validated by comparison with previous studies using other methods to probe the crustal structure at shallow depths 30 km are relatively fast presumably related to shallowing of the moho consequent on the extensional history of the pannonian region

dynamics of continental accretion three dimensional dynamic computer models show how accretionary tectonic margins evolve from the initial plate collision state through a period of plate margin instability and then re establish a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region subduction zones where one plate dives under another become congested when they try to accommodate buoyant exotic crust louis moresi et al describe new numerical models of continental accretion that follow the entire process from the initial collision state through a period of plate margin instability to the re establishment of a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region subduction zones become congested when they try to consume buoyant exotic crust the accretionary mountain belts orogens that form at these convergent plate margins have been the principal sites of lateral continental growth through earth s history modern examples of accretionary margins are the north american cordilleras and southwest pacific subduction zones the geologic record contains abundant accretionary orogens such as the tasmanides1 along the eastern margin of the supercontinent gondwana and the altaides which formed on the southern margin of laurasia2 in modern and ancient examples of long lived accretionary orogens the overriding plate is subjected to episodes of crustal extension and back arc basin development often related to subduction rollback3 and transient episodes of orogenesis and crustal shortening4 5 6 7 coincident with accretion of exotic crust here we present three dimensional dynamic models that show how accretionary margins evolve from the initial collision through a period of plate margin instability to re establishment of a stable convergent margin the models illustrate how significant curvature of the orogenic system develops as well as the mechanism for tectonic escape of the back arc region the complexity of the morphology and the evolution of the system are caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction we find geological and geophysical evidence for this process in the tasmanides of eastern australia and infer that this is a recurrent and global phenomenon

impurity sources of f cl no3 and so4 2 in greenland and antarctic precipitation five polar impurity sources are proposed anthropogenic no3 and so42 in recent greenland but not antarctic snow marine aerosols the dominant cl source volcanic eruptions which produce detectable but variable peaks in so42 f and or cl concentrations biogenic sulfurous gases from soil and coastal waters the dominant source for so42 in nonvolcanic strata an unidentified source of background nitrate which is more active in the northern hemisphere and which displays a summer concentration maximum solar activity and supernova events do not modulate no3 concentrations in greenland snow

crustal structure of the east gondwana margin in southeast australia revealed by transdimensional ambient seismic noise tomography 1 ambient seismic noise data from the ongoing wombat transportable seismic array in southeast australia the largest deployment of its kind in the southern hemisphere are used to produce a high resolution 3 d shear wave velocity model of the region we apply a two stage transdimensional hierarchical bayesian inversion approach to recover phase velocity maps at periods of 1 20 s and then invert phase velocity dispersion for 3 d shear wave velocity structure to the base of the crust data uncertainty is propagated through the sequence of inversions ensuring that model complexity is justified by the quality and quantity of the measurements the pattern of 3 d velocity variations helps elucidate the geometry and position of key crustal features such as the torrens hinge zone associated with the transition from paleozoic eastern australia to precambrian central and western australia that formed along the proto pacific margin of east gondwana

non darcian fluid flow during the alleghenian orogeny the late paleozoic remagnetization of rocks in north america may in part related to the expulsion of 22 500 km3 of metamorphic fluids from beneath the blue ridge and piedmont decollement fluid inclusions from veins and cements associated with this remagnetization event in the valley and ridge give temperatures 50 150 c higher than other indicators of peak temperatures one explanation for the high temperatures is the passage of hot metamorphic fluids along 1 10 cm wide open fractures which extended laterally 30 100 km using the above thermal constraints and a model for fluid flow in an open fracture flow is found to be turbulent and to have mean velocities of 1 10 m s the duration of the flow ranged from approximately a day to a few years these fluid velocities are seven orders of magnitude faster than the maximum velocities predicted by darcian fluid flow models and the duration of flow is four to eight orders of magnitude shorter than those predicted by darcian models

dielectric behaviour of firn and ice from the antarctic peninsula antarctica les mesures dielectriques de 14 echantillons de glace et neve preleves en antarctique pour des temperatures de 2 a 70 c et des frequences de 10 hz a 100 khz ont ete comparees a celles de la glace prelevee au groenland

electrical resistivity sounding of the east antarctic ice sheet electrical resistivity soundings using a schlumberger array have been carried out at dome c east antarctica 74 39 s 124 10 e elevation 3400 m to sound the entire 3500 m depth of the ice sheet changes in density and temperature are largely separated in the ice at dome c so activation energies for both firn and ice could be determined we find an activation energy of 0 25 ev in both solid ice and firn between 15 c and 58 c a common value of the activation energy points to a single transport regime in which the charge carriers and conduction paths are the same in firn and ice to evaluate the variation of resistivity with density we have considered five dielectric mixture models that fit the available data on the high frequency dielectric constant of firn only looyenga s equation fits the field data for dc resistivity in the upper 900 m of the ice sheet where impurity concentrations are known from core samples we find no correlation between resistivities and the concentrations of salts or acids instead we find it likely that resistivities are correlated with the crystal size hence with the holocene wisconsin boundary in the ice column a pronounced increase in resistivity to a value comparable with that in temperate glacier ice occurs deep within the ice sheet we attribute this to a large increase in the size and irregularity of the ice crystals which destroys the continuity of the impurity shells surrounding the ice crystals that we believe supply the conduction paths shabtaie and bentley 1994a high resistivity does not imply removal of the impurities from the system moderate concentrations of impurities can be accommodated by locating them in disconnected domains

fluid flow in sedimentary basins abstract the pore waters in sedimentary basins are ultimately derived from sea water meteoric water mineral bound water or from the underlying basement geochemically these waters initially have different signatures however the pore water compositions are easily altered by reactions with minerals or by mixing with other fluids meteoric water is driven downwards into sedimentary basins by the gravitational potential which approximately corresponds to the elevation of the ground water table above the sea or lake level it is difficult to model such flow however because the permeability on a large scale is almost impossible to represent realistically the continuity of confined sandstone and limestone aquifers plays an important role in determining the flux of meteoric water received by rocks in different parts of the basin but this is again difficult to predict pore water flow driven by compaction typically has velocities several orders of magnitude lower than what is commonly found in meteoric water flow regimes the average rate of upwards flow is lower than the rate of subsidence and the difference is the rate of incorporation of sea water in the topmost layer at the sea floor the salinity of formation water provides important constraints on fluid flow in sedimentary basins in the north sea basin and the gulf coast basin these types of data indicate that vertical mixing by compaction driven flow and convection is limited compaction driven flow obeys darcy s law but there are complex interdependencies between pressure permeability and compaction given the low compressibility of water the flow which can result from pressure release alone is very limited except on a local scale along permeable faults and fractures the main flow of pore water during burial is driven by compaction which is a slow process even when overpressure is released abruptly with a resulting increase in the net effective stress in the case of high permeability faults the flow into the fault from low permeability sedimentary rocks at depth may be rate limiting the displacement of pore water in adjacent sediments near the top of the faults may also slow down fluid flow on fractures that do not reach the sea floor modelling of fluid flow in sedimentary basins should be based on permeability distributions which are mostly determined by primary facies diagenesis and the tectonic history of the basin extensional tectonics produce a predominantly vertical fracture network which may serve as conduits for pore water flowing deep into the crust upward flow of hot fluids in basement fractures must have a high velocity in order to produce large thermal anomalies concentrations of dissolved components precipitate at the surface and form deposits of quartz and ore minerals if basement fractures are overlain by a thick sequence of soft sediments the rate of flow on the fractures will be reduced because of the low permeability in the sedimentary cover and conductive heat transport will dominate flow of water at high enough velocities to bring hot water to the surface and produce hot springs is most likely to occur in fractured basements rocks or well cemented sedimentary rocks concentrated precipitation of dissolved components will occur near the surface where the rate of cooling is highest also sediment hosted ores precipitated from flow of hot water on fractures are therefore likely to form before accumulation of thick overlying sedimentary sequences

australia s moho a test of the usefulness of gravity modelling for the determination of moho depth abstract in general seismic methods provide a reliable way to image the crust mantle interface which is marked by a rapid increase in seismic velocity the moho however the coverage provided by seismic networks is necessarily limited due to access difficulties and the cost and labour involved in collecting data gravity data provide an alternative way to model the depth to the moho and provide more consistent and broader coverage we discuss the usefulness of gravity data to model moho depth and the advantages and disadvantages of several gravity modelling methods as an example a model of australia s moho is generated through seismically constrained gravity inversion including an estimate of modelling uncertainty the inversion results demonstrate that gravity inversion is generally useful but that its usefulness is subject to the following limitations 1 gravity inversion cannot spontaneously generate thick high density crust nor thin low density crust and unless constrained will not generate a correct moho where such crust exists 2 major errors in the definition of the a priori density structure in particular features that are fixed during inversion will influence the moho results 3 applying a broad range of inversion parameters is necessary to characterise uncertainty model variability maps for australia show that the average error is less than 5 km there is a general relationship with seismic coverage but the areas of highest uncertainty are not necessarily those with the lowest seismic estimate density comparison with previous seismic and seismic gravity models of australia s moho indicates that low seismic data density limits usefulness due to higher uncertainty in the gravity inversion high seismic data density also limits usefulness because moho depth is largely known and there is little scope for change the usefulness of gravity inversion is maximum under conditions where seismic coverage is moderately dense but estimates are well distributed

the yellowstone magmatic system from the mantle plume to the upper crust the yellowstone supervolcano is one of the largest active continental silicic volcanic fields in the world an understanding of its properties is key to enhancing our knowledge of volcanic mechanisms and corresponding risk using a joint local and teleseismic earthquake p wave seismic inversion we revealed a basaltic lower crustal magma body that provides a magmatic link between the yellowstone mantle plume and the previously imaged upper crustal magma reservoir this lower crustal magma body has a volume of 46 000 cubic kilometers 4 5 times that of the upper crustal magma reservoir and contains a melt fraction of 2 these estimates are critical to understanding the evolution of bimodal basaltic rhyolitic volcanism explaining the magnitude of co2 discharge and constraining dynamic models of the magmatic system for volcanic hazard assessment

a ground based radar for measuring vertical strain rates and time varying basal melt rates in ice sheets and shelves the apres autonomous phase sensitive radio echo sounder instrument is a robust lightweight and relatively inexpensive radar that has been designed to allow long term unattended monitoring of ice shelf and ice sheet thinning we describe the instrument and demonstrate its capabilities and limitations by presenting results from three trial campaigns conducted in different antarctic settings two campaigns were ice sheet based pine island glacier and dome c and one was conducted on the ross ice shelf the ice shelf site demonstrates the ability of the instrument to collect a time series of basal melt rates the two grounded ice applications show the potential to recover profiles of vertical strain rate and also demonstrate some of the limitations of the present system

three dimensional s wave velocity model of the bohemian massif from bayesian ambient noise tomography abstract we perform two step surface wave tomography of phase velocity dispersion curves obtained by ambient noise cross correlations in the bohemian massif in the first step the inter station dispersion curves were inverted for each period ranging between 4 and 20 s separately into phase velocity maps using 2d adjoint method in the second step we perform bayesian inversion of the set of the phase velocity maps into an s wave velocity model to sample the posterior probability density function the parallel tempering algorithm is employed providing over 1 million models from the model samples not only mean model but also its uncertainty is determined to appraise the reliable features the model is correlated with known main geologic structures of the bohemian massif the uppermost low velocity anomalies are in agreement with thick sedimentary basins in deeper parts 4 20 km the s wave velocity anomalies correspond in general to main tectonic domains of the bohemian massif the exception is a stable low velocity body in the middle of the high velocity moldanubian domain and high velocity body resembling a promontory of the moldanubian into the tepla barrandian domain the most pronounced high velocity anomaly is located beneath the eger rift that is a part of a tertiary rift system across europe

linking mainland australia and tasmania using ambient seismic noise tomography implications for the tectonic evolution of the east gondwana margin for nearly half a century a number of conflicting tectonic models have been postulated to explain the enigmatic geological relationship between tasmania and victoria with a view to unifying our understanding of the evolution of the eastern margin of gondwana in australia in this study ambient noise data from an array of 24 broadband seismometers is used to produce a high resolution 3 d crustal shear wave velocity model of bass strait the key to understanding the missing link we apply a novel transdimensional and hierarchical bayesian inversion approach to construct group velocity maps in the period range of 2 30 s and subsequently invert group velocity dispersion for 3 d shear wave velocity structure this allows us to image for the first time the entire crust beneath bass strait in high detail and elucidate the geometry and position of key crustal features with corroboration from complementary datasets the three sedimentary basins related to the failed rifting event associated with the australia antarctica breakup in particular bass basin clearly emerge from the tomographic solution model a key feature of the 3 d shear wavespeed model is a distinct mid lower crustal nw se high velocity zone which extends from northwestern tasmania to south central victoria confirming a proterozoic geological connection we also image three north south high velocity belts that appear to span bass strait with some interruption from velocity variations possibly related to more recent tectonic events these belts are consistent with recent gravity and magnetic maps and may indicate the presence of an exotic precambrian terrane the selwyn block the model also images the crustal velocity structure of the southern stawell and bendigo zones and their internal large scale multi layer characters a legacy of their early paleozoic intra oceanic origins another high velocity anomaly imaged in the mid lower crust is an east west lineament beneath the northern part of bass strait which may be an intrusive feature associated with the failed rift

partial melt in the upper middle crust of the northwest himalaya revealed by rayleigh wave dispersion article i nfo seismic shear wave velocities are sensitive to the partial melts that should be present in the himalayan orogen if low viscosity channel flow is active at the present day we analyzed regional earthquakes in the western himalaya and tibet recorded on 16 broadband seismometers deployed across the nw indian himalaya from the indian platform to the karakoram range we used a multiple filter technique to calculate the group velocity dispersion of fundamental mode rayleigh waves and then inverted the dispersion records to obtain separate one dimensional shear wave velocity models for five geologic provinces the tibetan plateau ladakh arc complex indus tsangpo suture zone tethyan himalaya and himalayan thrust belt our velocity models show a low velocity layer lvl with 7 17 velocity reduction centered at 30 km depth and apparently continuous from the tethyan himalaya to the tibetan plateau this lvl shows good spatial correspondence with observations of low resistivity from magnetotelluric studies along the same profile of the possible explanations for low velocity and low resistivity in the mid crust only the presence of melts or aqueous fluids or both satisfactorily explains both sets of observations elevated heat flow observed in the nw himalaya implies that if aqueous fluids are present in the mid crust then the mid crust is well above its solidus comparison of our results with laboratory measurements and theoretical models suggests 3 7 melt is present in a channel in the upper middle crust of the nw himalaya at the present day and the physical conditions to enable active channel flow may be present

spatial analysis of an intra plate basaltic volcanic field in a compressional tectonic setting south eastern australia abstract the newer volcanics province nvp is a pliocene to recent intra plate basaltic volcanic field bvf that has formed in a compressive tectonic setting v

a synoptic view of the distribution and connectivity of the mid crustal low velocity zone beneath tibet 1 based on 1 2 years of continuous observations of seismic ambient noise data obtained at more than 600 stations in and around tibet rayleigh wave phase velocity maps are constructed from 10 s to 60 s period a 3 d vsv model of the crust and uppermost mantle is derived from these maps the 3 d model exhibits significant apparently inter connected low shear velocity features across most of the tibetan middle crust at depths between 20 and 40 km these low velocity zones lvzs do not conform to surface faults and significantly are most prominent near the periphery of tibet the observations support the internal deformation model in which strain is dispersed in the deeper crust into broad ductile shear zones rather than being localized horizontally near the edges of rigid blocks the prominent lvzs are coincident with strong mid crustal radial anisotropy in western and central tibet and probably result at least partially from anisotropic minerals aligned by deformation which mitigates the need to invoke partial melt to explain the observations irrespective of their cause in partial melt or mineral alignment mid crustal lvzs reflect deformation and their amplification near the periphery of tibet provides new information about the mode of deformation across tibet

regional teleseismic tomography of the western lachlan orogen and the newer volcanic province southeast australia summary from 1998 may to september a portable array of 40 short period digital seismograph stations was operated in western victoria southeast australia across the western end of the midpaleozoic lachlan foldbelt and the newer volcanic province consisting of four parallel almost w e oriented receiver lines the array covered an area of about 270 150 km 2 the major aim of the lf98 lachlan foldbelt survey 1998 project is to map lateral variations in p wave speeds vp in the crust and upper mantle using teleseismic arrival time tomography primarily in order to investigate whether the major surface structural zones are associated with seismic velocity signatures at depth little a priori information from seismic profiling is available we invert 4067 relative arrival time residuals for a minimum structure vp model in the upper few hundred km using non linear iteration and 3 d ray tracing the most prominent negative anomaly 3 8 per cent in vp is found at a depth of about 45 km underneath the eastern part of the newer volcanic province it correlates spatially with the highest density of pliocene and pleistocene eruption centres northwest of melbourne and is therefore interpreted as a hotspot related high temperature anomaly causing reduced mantle velocities the related coherent volume of significantly lower than average velocities extends down to depths greater than 100 km in the east and extends west underneath the newer volcanic province a strong velocity contrast with average velocities 2 per cent greater in the west is found down to about 100 km across the moyston fault zone which forms the major structural boundary between the early paleozoic delamerian orogen in the west and the lachlan orogen in the east this result suggests that the moyston fault zone should be seen as a major lithospheric boundary in the south this boundary is also expressed by a distinct discontinuity in sr isotopic ratios of xenoliths the so called mortlake discontinuity and a change in the geochemistry of plutons of similar age however if the east to west velocity contrast originally existed in this southern zone it is now overprinted by the thermally reduced mantle velocities beneath the newer volcanic province

complex continental growth along the proto pacific margin of east gondwana we use ambient noise recordings from the largest transportable seismic array in the southern hemisphere to image azimuthal variations in rayleigh wave phase anisotropy in the crust beneath southeast australia this region incorporates a transition from the precambrian shield region of australia in the west to younger phanerozoic terranes in the east which are thought to have been formed by subduction accretion processes our results which span the shallow to lower crust show a strong and consistent pattern of anisotropy that is oriented north south approximately parallel to the former margin of east gondwana however significant deviations from this trend persist through the period range 2 5 s to textgreater10 s one of the most notable deviations occurs along the edge of cratonic australia where the curnamona province forms a salient into the younger accretionary terrane here the fast axis of anisotropy follows the boundary almost exactly and is virtually coincident with magnetic lineations extracted from aeromagnetic data to the east of this boundary beneath the lachlan orogen a region masked by the cenozoic murray basin the fast axis of anisotropy becomes strongly curved and traces out a semicircular pattern with a radius of 200 250 km farther east the fast axis of anisotropy returns to a dominantly north south orientation these new findings provide strong observational support to recent geodynamic modeling results that demonstrate how large scale oroclinal structures can become embedded in accretionary mountain belts 2014 geological society of america

seismic evidence of on going sublithosphere upper mantle convection for intra plate volcanism in northeast china abstract a 3 d crustal and upper mantle s wave velocity model of ne china is constructed by inversion of phase velocity dispersion curves at 6 140 s periods from ambient noise tomography and two plane surface wave tomography the seismic data used in this study are collected from 120 china earthquake administration cea permanent stations and 127 portable stations of necessarray we observe strong low s wave velocity beneath the changbaishan volcano in the upper mantle to at least 200 km depth which is interpreted as a mantle upwelling beneath the changbaishan volcano that is consistent with the body wave tomographic image the songliao basin is dominated by a high velocity extending to at least 200 km depth built upon the observed velocity anomalies we propose a sub lithosphere mantle convection model for ne china in which the upwelling of upper mantle materials from the mantle transition zone to the changbaishan volcano could induce a local sub lithosphere convection in the upper mantle and the strong high velocity of the upper mantle beneath the songliao basin corresponds to the downwelling limb of this convection cell the downwelling beneath the songliao basin could also induce secondary local convection in the asthenosphere to the west leading to local asthenospheric upwelling beneath the abaga and halaha volcanoes in the xing an mongolia orogenic belt

circulation and melting beneath george vi ice shelf antarctica introduction george vi ice shelf sandwiched between the western coast of palmer land and the eastern coast of alexander island is the largest and most studied of the west antarctic peninsula ice shelves it covers an area of approximately 25 000 km2 and is underlain by circumpolar deep water cdw with temperatures in excess of 1oc giving rise to rapid basal melting bishop and walton 1981 lennon et al 1982 the maximum ice thickness of about 500 m occurs about 70 km from the southern ice front where a ridge of thick ice extends across george vi sound near 70ow see figure 1 effectively dividing the upper water column into northern and southern regions the northern ice front which faces marguerite bay appears to be near the geographical limit of ice shelf viability and has undergone a gradual retreat in recent decades lucchitta and rosanova 1998 a timeframe over which much of the nearby wordie ice shelf disintegrated doake and vaughan 1991 there is extensive surface melting over the northern parts of the ice shelf and much of the ice column near the northern ice front appears to be temperate paren and cooper 1986 conditions in the south where the ice front faces into ronne entrance are colder and the ice front position appears to be steady the vast majority of the flow into the ice shelf comes from palmer land but basal melting is sufficient to remove most of this so that the ice is derived almost exclusively from local accumulation by the time it reaches the ice fronts potter et al 1984 there is some evidence at the margins of the ice shelf to suggest that it may have disappeared completely during the early holocene before reforming sugden and clapperton 1981 bentley et al 2005

new insight into cainozoic sedimentary basins and palaeozoic suture zones in southeast australia from ambient noise surface wave tomography 1 detailed images of rayleigh wave group velocity are derived from ambient seismic noise recorded by wombat a large rolling seismic array project in southeast australia group velocity maps sensitive to crustal structure exhibit low velocity anomalies in the presence of sedimentary basins and recent hot spot volcanism and high velocities in regions of out cropping metamorphic and igneous rocks distinct and well constrained patches of low velocity within the murray basin provide new insight into the spatial extent and possible composition of pre tertiary infra basins in a broader tectonic context our results show little evidence for the palaeozoic building blocks of the southeast australian continent that have been inferred from geological mapping and potential field data this may mean that apparent changes in basement terrane near the surface are not associated with major changes in composition at depth

crustal structure of the newer volcanics province se australia from ambient noise tomography abstract intraplate volcanism is a widespread phenomenon and is generally regarded to be independent of plate tectonics the newer volcanics province nvp of se australia represents the most recent and arguably still active intraplate volcanism on the australian continent and has been postulated to originate from the combined effect of localized shear flow and edge driven convection in this study we adopt ambient noise tomography and monte carlo inversion methods to construct the first local scale 3d crustal vs model of the nvp region with a resolution of 35 km the model displays distinct crustal velocity features near the eastern and western margins of the nvp which may point to the existence of a lithosphere scale plumbing system for the migration of melt associated with the boundary between the delamerian orogen and the lachlan orogen which underlies the nvp in particular exceptionally high velocities are observed in the middle crust of the delamerian orogen and are best explained by buried magmatic arcs this interpretation is consistent with a subduction accretion origin for the delamerian orogen trans lithospheric faults might have developed during the accretion processes providing possible pathways for the migration of nvp magmas to the crust and surface our 3d model also images small localized velocity reductions in the lower crust at the region where the two distinct lithospheric units meet the low velocity zone is spatially correlated with the top of a prominent lithosphere scale low resistivity zone 10 30 m which we interpret to represent intruded magmatic sills with small proportions of melt remaining

teleseismic p wave tomogram of the yellowstone plume 1 inversion of a new data set of teleseismic p wave travel times from three passcal seismic deployments around the yellowstone hotspot reveals a 100 km diameter upper mantle plume that extends from the yellowstone volcanic caldera to 500 km depth and dips 20 to the northwest a monotonic decrease in the velocity perturbation of the plume from 3 2 at 100 km to 0 9 at 450 km is consistent with a uniform thermal anomaly of 180 c where the plume crosses the 410 km discontinuity previous research shows a depression in the 410 km discontinuity consistent with a warm plume fee and dueker 2004 additionally a region of high velocities extends to 250 km beneath the wind river basin in nw wyoming that may represent a convective downwelling of the lithosphere

intraplate volcanism off south greenland caused by glacial rebound plate tectonic mechanisms explain most of the dominant types of volcanism intraplate volcanism has been attributed to hot spots associated with mantle plumes though alternative explanations have been proposed for settings not fed by plumes we report young intraplate volcanism in an amagmatic margin south of greenland in a 40 ma old plate an elongated structure with a length of 110 km and a height of 2500 m above basement extends at an oblique angle to the mid atlantic ridge and the fracture zones we propose that this structure is related to reactivation of a rudimentary fracture zone by edge driven convection across minna fracture zone and partial melting caused by glacial rebound during the periodic melting of ice on greenland in the last 3 ma

decompression melting driving intraplate volcanism in australia evidence from magnetotelluric sounding free to read a long period magnetotelluric mt survey with 39 sites covering an area of 270 by 150 km has identified melt within the thinned lithosphere of pleistocene holocene newer volcanics province nvp in southeast australia which has been variously attributed to mantle plume activity or edge driven mantle convection two dimensional inversions from the mt array imaged a low resistivity anomaly 10 30 m beneath the nvp at 40 80 km depth which is consistent with the presence of 1 5 4 partial melt in the lithosphere but inconsistent with elevated iron content metasomatism products or a hot spot the conductive zone is located within thin juvenile oceanic mantle lithosphere which was accreted onto thicker proterozoic continental mantle lithosphere we propose that the nvp owes its origin to decompression melting within the asthenosphere promoted by lithospheric thickness variations in conjunction with rapid shear where asthenospheric material is drawn by shear flow at a step at the base of the lithosphere

automated detection extraction and measurement of regional surface waves our goal is to develop and test an effective method to detect identify extract and quantify surface wave signals for weak events observed at regional stations we describe an automated surface wave detector and extractor designed to work on weak surface wave signals across eurasia at intermediate periods 8 s 40 s the method is based on phase matched filters defined by the rayleigh wave group travel time predictions from the broadband group velocity maps presented by ritzwoller and levshin 1998 and ritzwoller et al 1998 and proceeds in three steps signal compression signal extraction or cleaning and measurement first the dispersed surface wave signals are compressed in time by applying an anti dispersion or phase matched filter defined from the group velocity maps we refer to this as the compressed signal second the surface wave is then extracted by filtering noise temporally isolated from the time compressed signal this filtered signal is then redispersed by applying the inverse of the phase matched filter finally we adaptively estimate spectral amplitude as well as group and phase velocity on the filtered signal the method is naturally used as a detector by allowing origin time to slide along the time axis we describe preliminary results of the application of this method to a set of nuclear explosions and earthquakes that occurred on or near the chinese lop nor test site from 1992 through 1996 and one explosion on the indian rajasthan test site that occurred in may of 1998

a large magmatic sill complex beneath the toba caldera an understanding of the formation of large magmatic reservoirs is a key issue for the evaluation of possible strong volcanic eruptions in the future we estimated the size and level of maturity of one of the largest volcanic reservoirs based on radial seismic anisotropy we used ambient noise seismic tomography below the toba caldera in northern sumatra to observe the anisotropy that we interpret as the expression of a fine scale layering caused by the presence of many partially molten sills in the crust below 7 kilometers this result demonstrates that the magmatic reservoirs of present non eroded supervolcanoes can be formed as large sill complexes and supports the concept of the long term incremental evolution of magma bodies that lead to the largest volcanic eruptions

the flow of hot brines in cracks and the formation of ore deposits we examined the thermal evolution of solutions transported vertically by turbulent flow in fractures and its implications for mississippi valley type ore deposition the main parameter governing the flow is fracture width which determines the flow rate volume and thermal evolution of the transported fluid we assumed local equilibrium and traced the chemical evolution of the transported solutions the mineral deposits associated with the cooling of these solutions and the limits of fracture clogging the volume of fluid expelled and the fracture width determine the location of the deposit and hence the deposit type i e carbonate hosted or vein we show that a network of thin cracks can indeed transport the volume of brines needed for the genesis of large mississippi valley type ore deposits such as the southeast missouri lead district

distribution of the crustal low velocity zones beneath the central and northeastern tibetan plateau insights from joint analysis of receiver functions and surface wave dispersion observations abstract a crustal and uppermost mantle shear wave velocity model beneath the central and northeastern tibetan plateau is obtained from the joint analysis of rayleigh wave phase dispersions and teleseismic p wave receiver functions at 188 broadband seismic stations deployed in this region this newly presented model reveals wide spread low velocity zones in the middle crust and these zones are mainly confined to the region south of the kunlun fault the velocities in the low velocity zones are extremely low

character and kinematics of faults within the turbidite dominated lachlan orogen implications for tectonic evolution of eastern australia reply fault zones within turbidite dominated orogenic systems typified by the lachlan orogen of eastern australia are characterised by higher than average strain and intense mica fabrics transposition foliation and isoclinal folds poly deformation with overprinting crenulation cleavages and steeply to moderately plunging meso and micro folds they have a different character compared to the brittle ductile fault zones of classic foreland fold and thrust belts such as the appalachians and the canadian rocky mountains multiple cleavages and transposition layering record a progressive shear related deformation history an intense mica fabric evolves initially during shortening of the overlying sedimentary wedge but is progressively modified during rotation and emplacement to higher structural levels along the steep parts of inferred listric faults the deformed wedge outside the fault zones generally undergoes one phase of deformation shown by a weak to moderately developed slaty cleavage which is parallel to the axial surface of upright subhorizontally plunging chevron folds other faults within the turbidites of the lachlan orogen include the steep zones of ductile strike slip deformation that bound a centrally located high t low p metamorphic complex characterised by s c mylonites these ductile shear zones indicate a southward passage of the metamorphic complex as a crustal wedge with emplacement to higher structural levels along a leading edge ductile thrust fault ar ar dating constrains the timing of regional deformation to be mostly late ordovician through silurian across the lachlan orogen faults in the low grade turbidite sequences record the kinematic evolution of the developing lachlan orogen and indicate progressive deformation associated with simultaneous eastward propagating and migrating deformation fronts in both the western and eastern parts of the fold belt these deformation fronts are related to accretionary style deformation at the leading edges of overriding plates in an inferred southwest pacific type subduction setting from the late ordovician to the mid devonian along the former gondwana margin the fault zones effectively accommodate and preserve movements within the structurally thickening migrating and prograding accretionary wedge

source of the lachlan fold belt flysch linked to convective removal of the lithospheric mantle and rapid exhumation of the delamerian ross fold belt during the cambrian ordovician the paleo pacific margin of gondwana underwent a rapid transition from a site of convergent deformation delamerian ross orogeny to one of sedimentation during which the thick turbiditic sequences of the adjacent basal lachlan fold belt were deposited laser probe 40 ar 39 ar data show that the synkinematic 523 486 ma mid crustal i and s type granites and metamorphic country rocks of the delamerian fold belt were rapidly cooled at 490 485 ma coincident with the intrusion of a suite of high level postkinematic a type granites and gabbros ca 497 481 ma 40 ar 39 ar data on detrital muscovites from the basal sections of the lachlan fold belt provide information on exhumation rather than cooling but yield an age pattern 507 480 ma identical to that of the delamerian fold belt demonstrating that this was the source of the sediments the combined data require that some 15 km of exhumation occurred very rapidly 5 15 mm yr 1 coincident as implied by overlapping ages with the cessation of convergent deformation and partial melting to form the postkinematic magmas this scenario implies a causative link which we infer to have been convective removal of lithospheric mantle following orogenic thickening the model is analogous to the tertiary uplift of the india asia orogen which provided the sediment source for the bengal fan our results suggest that similar processes were important in the evolution of mountain belts at least as far back as the early phanerozoic

isotopic fronts in hydrothermally mineralized carbonate rocks advective dispersive fluid flow through permeable and porous rock causes systematic alteration of the infiltrated rock o and sr isotopes can be used as tracers to monitor exchange processes between mineralizing hydrothermal solutions and carbonate host rocks fluid infiltration into rock of initially uniform isotopic composition leads to characteristic changes in 18o and 87sr in the rock that depend on infiltration distance fluid velocity diffusivity and reaction kinetics for fast fluid solid equilibrium during advective flow the shape of such alteration profiles can be described by the dimensionless peclet number p e which expresses the fluid flux infiltration distance and diffusivity in the case of mineralization from low to moderate temperature solutions isotope exchange is not instantaneous but kinetically controlled and a similar parameter the damkohler number n d expressing fluid flux isotope exchange rate and infiltration distance becomes more useful n d and p e values have been estimated for various examples of carbonatehosted hydrothermal mineralization in the eastern alps austria and the benue trough nigeria by modelling o and sr isotope data from ore or gangue minerals and host rock the values obtained enable a rough differentiation between pervasive and channelled fluid flow as the prevailing transport mechanism to be made results indicate that both highly channelled fluid flow through fractures without significant interaction with the wall rock and pervasive fluid flow with infiltration distances into the percolated host rock of the order of 100 m occur near mineral deposits in the latter case the extent of o and sr isotopic alteration of the host rock can be used as an effective exploration tool

teleseismic tomography of the upper mantle beneath the southern lachlan orogen australia with the goal of better understanding the deep structure and tectonic setting of the lachlan orogen 50 short period seismic stations were deployed across the southern end of the great dividing range in victoria southeast australia between 2005 and 2006 to record distant earthquakes a total of 7452 relative pwave arrival time residuals from 169 teleseismic events have been extracted from the continuous records using an adaptive stacking technique which exploits the coherency of global phases across the array these residuals are mapped as three dimensional perturbations in p wavespeed in the upper mantle beneath the array using a recently developed iterative non linear tomographic procedure which combines a grid based eikonal solver and a subspace inversion technique the capability of the new scheme to include interface geometry is utilised in order to investigate the effects of a priori moho topography on the resolution of upper mantle structure the resultant images show a pattern of p wavespeed anomalies that lacks a predominant orientation and therefore does not favour a purely w e subduction accretion model for the formation of the lachlan orogen one of the main features of the three dimensional model is a zone of elevated wavespeed beneath the northern end of the array which extends to a depth of approximately 150 km and contrasts with significantly lower wavespeeds to the south this anomaly which does not appear to be an artifact of relative arrival time residual contributions from the adjoining mountainous terrane may reflect the presence of a substantial piece of proterozoic lithosphere incorporated within the phanerozoic subduction accretion setting of the lachlan orogen another key feature of the solution model is a zone of relatively low velocity beneath the newer volcanic province northwest of melbourne which extends from the crust to a depth of approximately 200 km this is likely to represent the signature of elevated temperatures associated with a diffuse mantle source for the quaternary volcanism in victoria 2008 elsevier b v all rights reserved

modeling the growth and decay of the antarctic peninsula ice sheet a model of the growth and decay of the antarctic peninsula ice sheet during the last glacial interglacial cycle is used to identify the main controls on ice sheet behavior using as input glaciological assumptions derived by w f budd and i n smith 1982 annals of glaciology 3 4249 bedrock topography isostatic compensation and mass balance relationships the model is driven by sea level change over the last 40 000 yr in association with assumed changes in the rate of melting beneath ice shelves an ice sheet dome over 3 5 km thick grows on the offshore shelf and straits west of the antarctic peninsula and reaches a maximum at 18 000 yr b p collapse begins at 14 000 yr b p but becomes rapid and continuous after 10 000 yr b p the present stable ice cover is achieved at 6500 yr b p ice growth and decay are characterized by thresholds which separate periods of steady state from periods of rapid transition the thresholds usually relate to topography tests show that ice sheet behavior is most sensitive to sea level change basal marine melting and accumulation and is less sensitive to isostasy spatial variation in accumulation calving rates and ice flow parameterization tests of the model against field evidence show good agreement in places as well as discrepancies which require further work o

ice ocean interaction on ronne ice shelf antarctica detailed glaciological studies have been completed at 28 sites lying on an approximate flow line extending 760 km across ronne ice shelf parameters measured at each location include ice velocity thickness principal strain rates surface elevation temperature and accumulation rate the data have been used in a steady state model to derive the basal mass flux and the temperature profile with depth at each site from the principles of mass and energy conservation these calculations indicate basal melting in excess of 1 m yr 1 over the first 100 km of the flow line downstream of the grounding line where the ice shelf is between 1200 m and 1600 m thick the maximum melt rates in this region occur near the inland margin and exceed 4 m yr 1 melting continues at a lesser rate over the next 200 km before freezing commences freezing then dominates up to the final 100 km before the ice front resulting in the accumulation of a layer of basal sea ice up to 50 m thick this is rapidly removed as melt rates increase to over 6 m yr 1 at the ice front the cumulative effect of this pattern of basal accumulation and ablation is the wastage by melting of 85 of the mass discharged across the grounding line before it reaches the ice front supporting evidence for a layer of saline ice underlying the ice shelf is provided by the strength of basal radar reflections observed during radio echo sounding of ice thickness in the region where mass balance calculations suggest an accretion of basal sea ice reflection coefficients are consistently low ranging from 6 db to 38 db most of these weak reflections are believed to originate from the true base of the ice shelf the additional energy loss being the result of increased attenuation of the radar signal within the saline layer the derived pattern of basal melting and freezing is consistent with a simple model of sub ice shelf oceanic circulation involving a deep thermohaline convection cell dense saline water which is formed over the continental shelf during winter when the sea surface freezes drains into the deepest parts of the sub ice shelf cavity at the inland margin where this water mass comes into contact with the ice shelf its temperature is 1 c above the local pressure freezing point melting of ice results producing a buoyant outflow of cold relatively fresh water along the ice shelf base basal freezing occurs towards the ice front where the ascending water becomes supercooled this circulation has important implications for the production of antarctic bottom water and for the response of the ice shelf to driving stresses through the temperature dependent viscosity of ice

glaciological and oceanographic calculations of the mass balance and oxygen isotope ratio of a melting ice shelf on a etudie l alimentation du george vi ice shelf en relation avec l accumulation le velage et la fonte on releve la relation temperature taux d isotope de l oxygene ces donnees sont couplees a des donnees oceanologiques sur les eaux chaudes et son interaction avec le shelf

distribution of recent volcanism and the morphology of seamounts and ridges in the glimpse study area implications for the lithospheric cracking hypothesis for the origin of intraplate non hot spot volcanic chains 1 lithospheric cracking by remotely applied stresses or thermoelastic stresses has been suggested to be the mechanism responsible for the formation of intraplate volcanic ridges in the pacific that clearly do not form above fixed hot spots as part of the gravity lineations intraplate melting petrology and seismic expedition glimpse project designed to investigate the origin of these features we have mapped two volcanic chains that are actively forming to the west of the east pacific rise using multibeam echo sounding and side scan sonar side scan sonar reveals the distribution of rough seafloor corresponding to recent unsedimented lava flows in the hotu matua volcanic complex recent flows and volcanic edifices are distributed over a region 450 km long and up to 65 km wide with an apparent irregular age progression from older flows in the west to younger in the east the 550 km long southern cross seamount sojourn ridge brown ridge chain appears to have been recently active only at its eastern end near the east pacific rise a third region of recent flows is found 120 km north of southern cross seamount in seafloor approximately 9 myr old no indication of lithospheric extension in the form of faulting or graben formation paralleling the trend of the volcanic chains is found in the vicinity of recent flows or anywhere else in the study area thermoelastic cracking could be a factor in the formation of a few small very narrow volcanic ridges but most of the volcanic activity is broadly distributed in wide swaths with no indication of formation along narrow cracks the sojourn and brown chains appear to begin as distributed zones of small seamounts that later develop into segmented ridges perhaps under the influence of membrane stresses from self loading we suggest that the linear volcanic chains are created by moving melting anomalies in the asthenosphere and that lithospheric cracking plays at most a secondary role

seismic structure of the southeast australian lithosphere from surface and body wave tomography abstract teleseismic arrival time residuals from the wombat transportable seismic array experiment are inverted to construct a high resolution 3 d p wave velocity model of the upper mantle beneath southeast australia in order to address one of the principal limitations of teleseismic tomography that long wavelength structure is filtered out when data from multiple arrays operating at different times are used an initial model with lower spatial resolution derived from surface tomography is constructed to preserve the broad scale features that would otherwise be lost although the absolute velocities of the final model are not strongly constrained due to the assumption of radial vp vs ratios and differences in regularisation the relative variations appear robust across all scales these reveal a wealth of features that can be related to the geology and tectonic history of the region the most significant being 1 an easterly dipping velocity transition zone which involves a higher velocity delamerian orogen extending beneath the western subprovince of a lower velocity lachlan orogen 2 a distinct region of low velocity in the upper mantle north of melbourne which can be associated with recent quaternary hot spot volcanism 3 a gradual east southeast decrease in velocity towards the coast which is consistent with lithospheric stretching and thinning near a passive margin and 4 a zone of high velocity north of adelaide that may correspond to the presence of the palaeoproterozoic curnamona province at depth these results have important implications for the palaeozoic evolution of the east margin of gondwana the subsequent break up of australia and antarctica and opening of the tasman sea

interactions of 3d mantle flow and continental lithosphere near passive margins abstract we investigate the time evolution of 3d numerical models of convection in the upper mantle which incorporate both plate motions and thick continental lithosphere the resultant flow in the upper mantle is driven by a combination of bottom heated convection and applied shear velocity boundary conditions that represents plate motion both the plate velocity and continental lithosphere topography are varied in a way to assess the general influence of 3d geometry as well as a more specific tectonic analogue of the australian plate transient thermal events offshore of the trailing passive margin are observed and include plume migration boundary layer instability growth at the passive margin and variations in surface heat flux the geometry and plate velocity both play a significant role in controlling the magnitude and duration of these transient features in particular there are large differences between the different models in the oceanic region downstream of the trailing edge of the continent at near stationary plate speeds cold linear downwelling sheets propagate away from the 3d edge of the continent with regions offshore of the continents central axis localising hot cylindrical upwelling plumes at very fast plate speeds the shear flow is dominated by the plate motions this causes regions neighbouring the trailing edge of the continent to produce broad hot upwellings and the cold linear sheets to migrate around the continent at moderate 2 cm yr plate speeds oceanic lithosphere neighbouring the passive margin along the trailing edge of the continent is buffered by cold downwelling instabilities sinking along the edges of the continental lithosphere such neighbouring regions are subjected to larger heat flux than for regions distant to the passive margin yet also record smaller and less variable vertical surface velocities these dynamics have implications for volcanism and surface topography for which 3d aspects play a significant role

layered mafic sill complex beneath the eastern snake river plain evidence from cyclic geochemical variations in basalt the eastern snake river plain in southern idaho western united states is characterized by 1 2 km of pleistocene to late pliocene basalt overlying rhyolite caldera complexes cyclic variations in the chemical composition of basalts from 1136 m of scientific drill core show that the parent magmas of these lavas evolved by crystal fractionation at shallow to intermediate crustal depths punctuated by episodic recharge with more primitive compositions and assimilation of adjacent wall rock we have identified 10 upward fractionation cycles and four reversed cycles assimilation of sialic crust was limited and mainly affects the oldest basalts which directly overlie rhyolites we infer that the crystal fractionation and or recharge cycles took place in a series of sill like intrusions at intermediate crustal depths that now form a layered mafic intrusion that underlies the eastern snake river plain at depth this layered sill complex is represented by the 10 km thick basaltic sill that has been imaged seismically at 12 22 km depth the association of this mid crustal sill complex with geochemical fractionation cycles in basalt supports the concept that exposed layered mafic intrusions may be linked to overlying basalt provinces that have since been removed by erosion

tomography from 26 years of seismicity revealing that the spatial extent of the yellowstone crustal magma reservoir extends well beyond the yellowstone caldera the yellowstone volcanic field has experienced three of earth s most explosive volcanic eruptions in the last 2 1 ma the most recent eruption occurred 0 64 ma forming the 60 km long yellowstone caldera we have compiled earthquake data from the yellowstone seismic network from 1984 to 2011 and tomographically imaged the three dimensional p wave velocity vp structure of the yellowstone volcanic system the resulting model reveals a large low vp body interpreted to be the crustal magma reservoir that has fueled yellowstone s youthful volcanism our imaged magma body is 90 km long 5 17 km deep and 2 5 times larger than previously imaged the magma body extends 15 km ne of the caldera and correlates with the location of the largest negative gravity anomaly a 80 mgal gravity low this new seismic image provides important constraints on the dynamics of the yellowstone magma system and its potential for future volcanic eruptions and earthquakes

numerical modeling of the late weichselian svalbard barents sea ice sheet abstract previous reconstructions of the ice cover of the svalbard barents sea region during the late weichselian have ranged from small ice masses on svalbard to complete inundation of the barents shelf region by an ice sheet several kilometers thick we have used a time dependent finite difference numerical model to undertake a new glaciological reconstruction for the svalbard barents sea ice sheet over the last 30 000 yr the numerical model requires environmental forcing functions in the form of air temperature and precipitation and their behavior with respect to altitude together with sea level change and an iceberg calving relation ice buildup on svalbard is calculated to have begun 28 000 yr ago and maximum dimensions were reached by 20 000 yr ago covering svalbard and the northwestern barents sea with a center of mass 1 3 km thick around eastern svalbard decay was complete by about 10 000 yr ago the margin of the modeled ice sheet at its maximum is in good agreement with observed sea floor morphological features but there are discrepancies in timing between the modeled ice sheet decay and i a dated meltwater spike in fram strait and ii the observed rebound curves for svalbard an inverse approach was used to predict ice sheet decay and it was found that increasing the rate of iceberg calving within the model produces a deglaciation some 2000 yr earlier which is compatible with these two independent datasets the reconstruction is also compatible with geological evidence on the isostatic response of bjornoya close to the southern limit of the ice sheet and seismically observed deposits interpreted to be ice proximal facies located in the northwestern barents sea our time dependent model reconstructions of the svalbard barents sea ice sheet indicate ice cover over only the northwestern barents sea during the late weichselian but this does not preclude the presence of ice derived from fennoscandia and the kara sea region elsewhere in the barents sea

crustal structure in the junction of qinling orogen yangtze craton and tibetan plateau implications for the formation of the dabashan orocline and the growth of tibetan plateau

patterns of intraplate volcanism controlled by asthenospheric shear volcanism observed far from plate boundaries in the interior of oceanic and continental plates may result from flow in the underlying mantle comparison between a numerical model of mantle flow and the spatial distribution of intraplate volcanism indicates that rapid shear motion in the mantle may drive melting that causes intraplate eruptions most of earth s volcanism occurs at plate boundaries in association with subduction or rifting a few high volume volcanic fields are observed both at plate boundaries and within plates fed by plumes upwelling from the deep mantle1 the remaining volcanism is observed away from plate boundaries it is typically basaltic effusive and low volume occurring within continental interiors2 3 4 5 6 7 or creating seamounts on the ocean floor8 9 10 11 this intraplate volcanism has been attributed to various localized processes12 such as cracking of the lithosphere8 13 14 small scale convection in the mantle beneath the lithosphere15 16 17 or shear induced melting of low viscosity pockets of asthenospheric mantle that have become embedded along the base of the lithosphere18 here we compare the locations of observed intraplate volcanism with global patterns of mantle flow from a numerical model we find a correlation between recent continental and oceanic intraplate volcanism and areas of the asthenosphere that are experiencing rapid shear due to mantle convection we detect particularly high correlations in the interior of the continents of western north america eastern australia southern europe and antarctica as well as west of the east pacific rise in the pacific ocean we conclude that intraplate volcanism associated with mantle convection is best explained by melting caused by shear flow within the asthenosphere whereas other localized processes are less important

stacking autocorrelograms to map moho depth with high spatial resolution in southeastern australia current estimates of moho depth in southeastern australia are based on sparse sampling the results are augmented with 180 new moho estimates constructed from spatial stacks of crustal p wave reflectivity derived from autocorrelograms at over 750 stations the spatial stacks of reflectivity are constructed using a gaussian with half width 0 5 picks of the base of crustal reflectivity are made with the aid of the previous moho model based on the sparser data and knowledge of the variation in the character of the crust mantle boundary across the region good ties can be made to previous results from deep reflection profiling the new information fills in many holes in coverage and provides a moho map with closer ties to geological provinces the procedure exploits the continuous records at the stations and just the vertical components and so can be applied to older data for which receiver function techniques cannot be used

electrical resistivity of ice from the antarctic peninsula antarctica l objectif etait de determiner le comportement electrique de la glace dans une zone ou existe une fonte estivale substantielle ainsi que des regimes thermiques contrastes des mesures electriques effectuees en trois sites le long d une ligne de courant sur le george vi ice shelf montrent que a la resistivite de la glace en profondeur est semblable a celle des autres shelfs antarctiques b la resistivite de la glace a la surface de la glace du shelf qui est soumise a la percolation et au regel de l eau de fusion est semblable a celle de la glace en profondeur et donc la glace a un caractere polaire

changbaishan volcanism in northeast china linked to subduction induced mantle upwelling the changbaishan volcanic complex in china cannot be easily explained as the consequence of a mantle plume seismic images from the region identify buoyant mantle material that may have been entrained and dragged downwards by the subducting pacific plate but is now escaping upwards through a gap in the plate and producing the intraplate volcanism

a 10 000 yr b p extensive ice shelf over viscount melville sound arctic canada abstract late wisconsinan age glacial landforms and deposits indicate that an ice shelf of at least 60 000 km 2 flowed northwestward into viscount melville sound probably from the m clintock dome of the laurentide ice sheet the ice shelf overlapped coastal areas and laid winter harbour till up to 125 m above present sea level on the southern coast of melville island to 135 m on byam martin island to possibly 90 m on the northeast tip of banks island and to 150 m on the north coast of victoria island the contemporary sea level was 50 to 100 m higher than present it now rises eastward a maximum age of 10 340 150 yr b p for the till and thus the ice shelf advance is provided by shells in marine sediments which underlie it whereas a minimum age of 9880 150 yr b p is provided by overlying shells that postdate the ice advance the major advance of shelf ice into viscount melville sound may be the result of the rapid disintegration of the m clintock dome while the climate ameliorated in the western arctic

seismic image and origin of the changbai intraplate volcano in east asia role of big mantle wedge above the stagnant pacific slab abstract we determined p wave tomography of the crust and upper mantle under the changbai volcanic area in northeast asia by using 289 318 arrival time data from 4802 local earthquakes and 9599 teleseismic events a prominent low velocity anomaly is clearly imaged in the crust and upper mantle down to about 410 km depth beneath the changbai volcano a broad high velocity anomaly is revealed in the mantle transition zone and deep earthquakes occurred actively there suggesting that the subducting pacific slab is stagnating in the mantle transition zone under east asia our results suggest that the formation of the changbai and other intraplate volcanoes in northeast asia is related to the upwelling of hot and wet asthenospheric materials in the big mantle wedge above the stagnant pacific slab hence the active intraplate volcanoes in northeast asia are not hotspots related to deep mantle plumes but are caused by plate tectonic processes in the upper mantle

crust and upper mantle structure beneath southeast australia from ambient noise and teleseismic tomography abstract in the last decade the lithospheric structure beneath southeast australia has been intensively studied using passive seismic data from wombat the largest transportable seismic array in the southern hemisphere the two primary imaging methods that have been applied are ambient noise tomography for the crust and teleseismic tomography for the upper mantle despite these recent studies no attempt has yet been made to provide an integrated view of the crust mantle system here we perform teleseismic tomography using wombat data that includes a detailed crustal model from ambient noise tomography in the starting model a moho surface from the australian seismological reference earth model ausrem is also included this has the dual benefit of accounting for the unresolved crustal component of the teleseismic arrival time residuals and producing a model that reveals a high level of detail in both the crust and upper mantle our new integrated p wave model contains a number of noteworthy features including i low velocity anomalies in the lower crust and high velocity anomalies in the lithospheric mantle beneath the gawler craton and curnamona province which are of paleoproterozoic archean origin ii a marked velocity transition in the crust and lithospheric mantle near the moyston fault which we interpret as the boundary between the lachlan and delamerian orogens iii a rapid eastward decrease in upper mantle velocity 200 km inboard of the east coast of australia which is consistent with a marked thinning of the lithosphere iv an increase in upper mantle velocity north of the gawler craton and curnamona province which points to the presence of thicker lithosphere associated with the precambrian shield region of the australian continent v cenozoic intraplate basaltic volcanic centres distributed exclusively above the zone of thinner lithosphere inboard of the east coast with the exception of low volume leucitite volcanics

introduction to special section on physical processes in geodynamics on may 16 and 17 1992 a meeting was held at cornell university as part of activities to honor donald l turcotte on the occasion of his 60th birthday which actually occurred on april 22 1992 the papers in the following special section are a result of that meeting which included invited presentations and posters representing the diverse areas of geology and geophysics to which donald turcotte has contributed three of the papers deal with aspects of mantle convection a c fowler addresses the problem of initiation of subduction by extending boundary layer theory to a viscoplastic rheology turcotte and oxburgh 1967 pioneered such studies by showing how boundary layer theory could be used to describe the dependence of geophysical observables such as heat flux on distance from a midocean ridge or age of the oceanic crust d w sparks et al use a model of three dimensional convection beneath a segmented ridge to explore how buoyancy effects and melt production beneath a spreading center influence ridge parallel variations in crustal thickness and gravity magma migration and the dynamics of midocean ridges have been major focuses of donald turcotte s research in papers dating back to 1978 e g turcotte and ahern 1978 ahern and turcotte 1979 turcotte 1982a kenyon and turcotte 1987 g a glatzmaier and g schubert calculate models of three dimensional two layer mantle convection in a spherical shell and compare with whole layer spherical shell three dimensional convection models these models represent the end member states of a spectrum of possible modes of mantle convection turcotte and oxburgh 1972 oxburgh and turcotte 1978 turcotte 1979a silver et al 1988

modelling ice sheet sensitivity to late weichselian environments in the svalbard barents sea region ice proximal sedimentological features from the northwestern barents sea suggest that this region was covered by a grounded ice sheet during the late weichselian however there is debate as to whether these sediments were deposited by the ice sheet at its maximum or a retreating ice sheet that had covered the whole barents sea to examine the likelihood of total glaciation of the late weichselian barents sea a numerical ice sheet model was run using a range of environmental conditions total glaciation of the barents sea originating solely from svalbard and the northwestern barents sea was not predicted even under extreme environmental conditions therefore if the barents sea was completely covered by a grounded late weichselian ice sheet then a mechanism not accounted for within the glaciological model by which grounded ice could have formed rapidly within the central barents sea may have been active during the last glaciation such mechanisms include i grounded ice migration from nearby ice sheets in scandinavia and the central barents sea ii the processes of sea ice induced ice shelf thickening and iii isostatic uplift of the central barents sea floor

mantle instability beneath the sierra nevada mountains in california and death valley extension the southern sierra nevada mountain range rapidly uplifted at 3 5 ma simultaneously with a pulse of basaltic volcanism xenoliths recovered from volcanics indicate that the range lost a dense crustal root after the miocene the vertical motions and removal of the root have been linked to a fast seismic velocity anomaly that extends 200 km into the mantle but is offset to the west of the range with visco elasto plastic thermo mechanical numerical models we have tested the influence of crustal strength on the kinematics of removal and on the amount of associated uplift we find that delamination of the dense root is the most likely mechanism for gravitational instability to occur the model satisfies the plio quaternary vertical motions the shift of the mantle anomaly to the west of the range and intense miocene extension to the east based on those results we propose the existence of a dynamic link between the sierra nevada mantle instability and death valley rifting

the role of acids in electrical conduction through ice 1 electrical conduction through meteoric polar ice is controlled by soluble impurities that originate mostly from sea salt biomass burning and volcanic eruptions the strongest conductivity response is to acids yet the mechanism causing this response has been unclear here we elucidate conduction mechanisms in ice using broadband dielectric spectroscopy of meteoric polar ice cores we find that conduction through polycrystalline polar ice is consistent with jaccard theory for migration of charged protonic point defects through single ice crystals except that bulk dc conduction is impeded by grain boundaries neither our observations nor modeling using archie s law support the hypothesis that grain boundary networks of unfrozen acids cause significant electrolytic conduction common electrical logs of ice cores by electrical conductivity measurement ecm or dielectric profiling dep and the attenuation of radio waves in ice sheets thus respond to protonic point defects only this response implies that joint interpretation of electrical and chemical logs can determine impurity partitioning between the lattice and grain boundaries or inclusions for example in the greenland ice core project grip ice core from central greenland on average more than half of the available lattice soluble impurities h cl nh4 create defects understanding this partitioning could help further resolve the nature of past changes in atmospheric chemistry

p wave tomography and origin of the changbai intraplate volcano in northeast asia abstract we present the first seismic image of the upper mantle beneath the active intraplate changbai volcano in northeast asia determined by teleseismic travel time tomography the data are measured at a new seismic network consisting of 19 portable stations and 3 permanent stations our results show a columnar low velocity anomaly extending to 400 km depth with a p wave velocity reduction of up to 3 high velocity anomalies are visible in the mantle transition zone and deep focus earthquakes occur at depths of 500 600 km under the region suggesting that the subducting pacific slab is stagnant in the transition zone as imaged clearly by global tomography these results suggest that the intraplate changbai volcano is not a hotspot like hawaii but a kind of back arc volcano related to the deep subduction and stagnancy of the pacific slab under northeast asia

isotopic evidence for rapid continental growth in an extensional accretionary orogen the tasmanides eastern australia the trace element signature of earth s continental crust resembles that of arc lavas but the continents may have formed during ancient igneous pulses that are hard to reconcile with supra subduction zone magmatism we explore the role of coupled arc back arc accretionary processes in crust generation by considering the tectonic context of whole rock nd isotope and zircon hf o isotope data from igneous rocks of the australian tasmanides 515 230 ma which is thought to have evolved by the repeated opening and closure of sediment filled back arc basins behind a long lived subduction zone the significance of this process for continental crust formation has yet to be evaluated from an isotopic perspective granitic rocks in this area define striking secular end ehf 18o trends that correlate with the pattern of deformational events and register changes in magma source during tectonic activity these trends reveal that juvenile magmatic input was enhanced during extensional back arc rifting episodes that immediately followed crustal thickening suggesting a relationship between slab rollback and continental growth interaction between juvenile magma and sedimentary units deposited during a preceding back arc rifting cycle was integral to the formation of stable continental material this highlights the importance of back arc environments for both the generation and differentiation of continental crust the juvenile component within the tasmanide igneous rocks increased from the cambrian to the triassic consistent with a diminished input from cratonderived metasedimentary material as the subduction zone migrated outboard of the gondwana margin subduction zone retreat formed large tracts of new crust in eastern australia at comparable rates to crust generation at modern island arcs providing a mechanism for rapid continental growth at convergent margins using isotopic tracers to link tectonic evolution and crust generation in modern and ancient orogens can lead to a better understanding of the geodynamic controls on planetary differentiation