subduction collision and exhumation in the ultrahigh pressure qinling dabie orogen c abstract high pressure metamorphism and ophiolite emplacement songshugou ophiolite attended suturing of the yangtze craton to rodinia during the c l o ga grenvillian oro geny the qinling microcontinent then rifted from the yangtze craton at c 750ma the erlangping intraoceanic arc formed in the early ordovician was emplaced onto the qinling microcontinent in the ordovician silurian and then both units were accreted to the sino

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

numerical modelling of melting processes and induced diapirism in the lower crust summary the processes of partial melting and magmatic diapirism within the lower crust are evaluated using a numerical underplating model fully molten basalt t 1200 c is emplaced at the moho beneath a solid granite t 750oc in order that a melt front grows into the granite if diapirism does not occur this melt front in the granite reaches a minimal depth in the crust before like in the molten basalt crystallization takes place the density contrast between the partially molten granite layer and the overlying solid granite can lead to a rayleigh taylor instability rti which results in diapiric rise of the partially molten granite assuming a binary eutectic system for both the granite and the underplating basalt and a temperature and stress dependent rheology for the granite we numerically solve the governing equations and find a that diapirism occurs only within a certain but possibly realistic range of parameters and b that if diapirs occur they do not rise to levels shallower than 15 or perhaps 12km the growth rate depends on the degree of melting and the thickness of the partially molten layer as well as the viscosity of the solid and the partially molten granite from a comparison of the growth rate with the velocity of a stefan front it is possible to predict whether a melt front will become unstable and result in diapiric ascent or whether a partially molten layer is created which remains at depth we carry out such a comparison using our thermodynamically and thermomechanically consistent model of melting and diapirism

tectonic evolution of the european variscan belt constrained by palaeomagnetic structural and anisotropy of magnetic susceptibility data from the northern vosges magmatic arc eastern france geochronology structural and anisotropy of magnetic susceptibility data from the northern vosges batholith which belongs to the northern vosges mid german crystalline rise arc show contrasting emplacement modes of southern granodiorites and northern granites the ene wsw trending fabrics of granodiorites 346 334 ma are parallel to the metamorphic cleavage affecting the host rocks developed during regional compression the nnw sse trending fabrics of younger granitoids c 330 ma reveal an extensional emplacement mode associated with a normal shear zone separating the two magmatic suites palaeomagnetism shows that the switch from a compressive to an extensional regime coincides with a regional counterclockwise rotation the 330 325 ma extension is further supported by palaeomagnetic and seismic data indicating southeastward tilt of the whole batholith finally the system is rotated clockwise without any structural overprint data from the early carboniferous northern vosges magmatic arc and remote bohemian massif suggest that this evolution is valid for the whole eastern branch of the european variscan belt for which the following model is proposed 1 late devonian early carboniferous east west shortening of the variscides above the rhenohercynian subduction zone 2 axial nnw sse shortening of the assembled variscan orogen associated with 3 internal sinistral rotation of inherited rhenohercynian transform faults and shortening of intervening blocks 4 east west extension and dextral external rotation of blocks between dextrally reactivated transforms supplementary materials analytical procedures for geochronology magnetism anisotropy of magnetic susceptibility and palaeomagnetism are available at www geolsoc org uk sup18635

2d thermomechanical modelling of continent arc continent collision abstract arc continent collision is a key process of continental growth through accretion of newly grown magmatic arc crust to older continental margin we present 2d petrological thermo mechanical models of arc continent collision and investigate geodynamic regimes of this process the model includes spontaneous slab bending dehydration of subducted crust aqueous fluid transport partial melting of the crustal and mantle rocks and magmatic crustal growth stemming from melt extraction processes results point to two end member types of subsequent arc continent collisional orogens i orogens with remnants of accretion prism detached fragments of the overriding plate and magmatic rocks formed from molten subducted sediments and ii orogens mainly consisting of the closed back arc basin suture detached fragments of the overriding plate with leftovers of the accretion prism and quasi insignificant amount of sediment derived magmatic rocks transitional orogens between these two endmembers include both the suture of the collapsed back arc basin and variable amounts of magmatic production the orogenic variability mainly reflects the age of the subducting oceanic plate older therefore colder and denser oceanic plates trigger subduction retreat which in turn triggers necking of the overriding plate and opening of a backarc basin in which new oceanic lithosphere is formed from voluminous decompression melting of the rising hot asthenosphere in this case subducted sediments are not heated enough to melt and generate magmatic plumes on the other hand young and less dense slabs do not retreat which hampers opening of a backarc basin in the overriding plate while subducted sediments may reach their melting temperature and develop trans lithospheric plumes we have also investigated the influences of convergence rate and volcanic plutonic rocks ratio in newly forming lithosphere the predicted gross scale orogenic structures find similarities with some natural orogens in particular with deeply eroded orogens such as the variscides in the bohemian massif

esr and mass spectrometric uranium series dating studies of a mammoth tooth from stanton harcourt oxfordshire england abstract the age of the channel deposits at stanton harcourt oxfordshire england has been a topic of debate with important implications for british pleistocene stratigraphy recent excavations led by k scott reveal ample evidence for ancient environmental conditions characteristic of an interglacial however the question remains on the assignment of its age at present it is thought to represent an interglacial corresponding to either marine oi stage 7 or 5e in an attempt to constrain the chronology of the site and to assess the techniques reliability we have made electron spin resonance esr measurements on enamel and mass spectrometric u series measurements on both enamel and dentine from a mammoth tooth buried in the channel deposits at stanton harcourt four dentine samples gave u series dates between 65 4 0 4 and 146 5 1 0 ka and two enamel samples between these dentine layers were dated to 53 3 0 2 and 61 1 0 6 ka the corresponding esr age estimates for the enamel samples are 59 6 and 62 4 ka early u uptake eu and 95 11 and 98 7 ka linear u uptake lu the recent u uptake ru dates are 245 38 and 238 31 ka but in light of the u series data we would not expect these to represent realistic age estimates similar esr results were obtained from two other adjacent enamel samples the effect of the large size of the mammoth tooth on the external gamma dose and the internal gamma contribution from the high u content of the dentine are considered while the recent uptake esr dates appear to coincide with oi stage 7 all the early and linear uptake esr and mass spectrometric u series dates are younger than the expected age estimation based on recent geological interpretation and amino acid racemisation measurements 200 ka and optical dating studies 200 450 ka possible causes of the unexpected dating results are discussed we conclude that our mass spectrometric u series and eu and lu esr measurements represent minimum age estimates for the tooth analysed our results seem to suggest that the tooth and hence the channel deposits are at least 147 ka in age i e predating the last interglacial

exhumation mechanisms of melt bearing ultrahigh pressure crustal rocks during collision of spontaneously moving plates a series of 2d petrological thermomechanical numerical experiments was conducted to i characterize the variability of exhumation mechanisms of ultrahigh pressure metamorphic uhpm rocks during collision of spontaneously moving plates and ii study the possible geodynamic effects of melting at ultrahigh pressure conditions for the exhumation of high temperature ultrahigh pressure metamorphic ht uhpm rocks to this end the models include fluid and melt induced weakening of rocks five distinct modes of exhumation of u hpm rocks associated with changes in several parameters in the models of plate collision and continent subduction are identified as follows vertical crustal extrusion large scale crustal stacking shallow crustal delamination trans lithospheric diapirism and channel flow the variation in exhumation mechanisms for u hpm rocks in numerical models of collision driven by spontaneously moving plates contrasts with the domination of the channel flow mode of exhumation in a majority of the published results from numerical models of collision that used a prescribed plate convergence velocity and or did not include fluid and melt induced weakening of rocks this difference in the range of exhumation mechanisms suggests that the prescribed convergence velocity condition and the neglect of fluid and melt related weakening effects in the earlier models may inhibit development of several important collisional processes found in our experiments such as slab breakoff vertical crustal extrusion large scale stacking shallow crustal delamination and relamination and eduction of the continental plate consequently the significance of channel flow for the exhumation of uhpm rocks may have been overstated based on the results of the earlier numerical experiments in addition the results from this study extend over a larger proportion of the high temperature range of p t conditions documented from uhpm rocks including those retrieved from ht uhpm rocks than the results of experiments from previous numerical models in particular the highest peak metamorphic temperatures up to 1000 degrees c are recorded in the case of the vertical crustal extrusion model in which subducted continental crust is subjected to a period of prolonged heating by asthenospheric mantle abutting the continental side of the vertically hanging slab nonetheless some extreme temperature conditions which have been suggested for the kokchetav and bohemian massifs perhaps up to 1100 1200 degrees c are still to be achieved in experiments using numerical models

a method for coupled esr u series dating of teeth showing post depositional u loss abstract an additional method of coupled esr u series dating is developed for teeth showing post depositional u loss the fundamental parameter for the dose rate calculation is the present day 230th activity because of the geochemical immobility of thorium uranium and thorium concentrations are measured by thermal ionization mass spectrometry tims due to the unknown uranium history an average saturation uptake of uranium is assumed leading to an average development of u series activities therewith an internal dose rate and the age t can be calculated on condition that the modelled 230th activity at time t corresponds with the measured value using this new method teeth found in archaeological sites in france gramat bramefond could be dated even though they show u loss after u uptake two teeth from gramat could be dated to 128 3 8 6 and 130 5 10 ka two teeth from bramefond have ages of 104 4 8 4 and 115 1 10 2 ka both sites can be ascribed to oxygen isotope stage 5

comparison of soil freezing curve and soil water curve data for windsor sandy loam unfrozen water content as a function of temperature was measured in the laboratory using pulsed nuclear magnetic resonance pnmr for a windsor sandy loam soil the pnmr data were related to previously measured soil moisture retention data through the modified clausius clapeyron equation with suitable adjustment for surface tension the transformed measured unfrozen water content data and the previously measured soil moisture retention data were expressed by a brooks and corey type of equation with the required set of regression parameters determined it was found that a single set of parameters were sufficient to correctly express the behavior of these data when suitable constraints were imposed on the unfrozen water content data additional insight into the traditional form of expressing unfrozen water content data is presented in terms of air or ice entry pressure

numerical simulations of subduction zones effect of slab dehydration on the mantle wedge dynamics abstract in oceanic subduction zones dehydration of slab s minerals may favor asthenospheric flow in the mantle wedge by decreasing rocks strength this should enhance the upper plate base reheating and markedly alter its thermal structure to quantify this phenomenon we model slab subduction within a viscous mantle dehydration hydration processes and the rock strength dependence on water content we use accurate phase diagrams for a h2o satured mantle peridotite and a gabbroic crust to determine at each time step the amount of water released or absorbed by each unit of rock transition phases are supposed to be not metastable water is released from the oceanic crust and from the altered peridotite portion of the slab dehydration of the subducting lithosphere occurs first at 60 75 km depth when the crust is eclogitized and second deeper around 105 km when serpentine and chlorite in serpentinite layer below the crust become unstable for high convergence rates because of cold p t t paths in the slab serpentine can be transformed into the hydrated phase a and water is recycled by the slab until great depth however in all investigated cases the released water is sufficient to hydrate by dissolution the whole mantle wedge until 217 55 km away from the trench and as it goes up to form hydrated minerals in the overlying lithosphere over a significant volume the convergence rate slightly shifts dehydration fronts location and consequently widens or reduces the hydrated mantle wedge note that for the dynamic corner flow modelled here with a non newtonian rheology the slab surface is significantly warmer than for an isoviscous analytical corner flow model yielding plausible crust melting we assume a strength reduction associated to hydration larger for rocks containing nominally hydrous minerals than for rocks with only dissolved water the rock strength thus becomes quite uniform at the base of the hydrated upper plate and in the wedge this results in cooler temperatures along the slab top but in an enhanced corner flow effect on the upper plate thermal structure for large hydration strength reductions a strong thermal erosion of the overlapping lithosphere develops in less than 15 myr due to convective destabilization this weakens drastically the upper plate at a distance from 110 to 220 km away from the trench the geometry of the eroded region could correspond to the low velocity zone observed below the arc region

subduction and eduction of continental crust major mechanisms during continent continent collision and orogenic extensional collapse a model based on the south norwegian caledonides during continental collision in the middle silurian the thickness of the lithosphere under the caledonides of s norway was doubled by subduction of the western margin of baltica including the western gneiss region under laurentia crustal rocks of the baltic plate reached sub moho depths of near 100 km or more as inferred from the presence of coesite in eclogites isostatic calculations indicate an average elevation of the mountain chain of about 3 km at this stage the subducted lithosphere experienced vertical constrictional strains as a result of slab pull by its heavy and cold root eduction of the deeply buried crustal material was initiated by decoupling of the thermal boundary layer in the subducted lithosphere isostatic rebound resulted in very rapid uplift 1 2 mm yr 1 and the deep crust was exhumed mainly by tectonic extensional stripping over a period of 30 40 myr the eduction was probably related to a rolling hinge footwall uplift mechanism and the early high pressure coaxial fabrics were overprinted by extensional simple shear as the deep crust reached middle and upper crustal levels the model explains the present day normal crustal thickness under the exhumed deep rocks without necessarily invoking large scale lateral flow of material in the lower crust or igneous underplating

redox zonation of elements at an oxic post oxic boundary in deep sea sediments abstract deep sea sediments generally provide limited evidence of redox control on the post depositional redistributions of elements this is because only a fraction of any element is reactive and observed elemental distributions are the result of a number of competing processes which are simultaneously active so that redox effects are not dominant a detailed investigation is reported of a core which is unusual in showing unequivocal evidence of direct redox control operating in a homogeneous environment that is relatively free from competing processes a progressive oxidation front has migrated slowly downwards into a uniform turbidite over a long period ca 250 ky the reactive fractions of several redox sensitive elements now show a remarkable degree of organisation around the redox boundary of this front with the different elements exhibiting individual responses solid phase enrichments are not coincident but are arranged in a clear succession of concentration peaks along the redox gradient from oxic to postoxic conditions the succession is mn i fe se and pt cu and pb v zn and sb and u the movement directions of most elements can be inferred from the peak shapes against the homogeneous turbidite background composition the system seems to have been essentially isolated within the sediment for most elements so that no loss or gain can be ascribed to redox processes exceptions are u and v which have been augmented by uptake from ocean bottom waters into the sediments by an efficient redox trap mechanism

u series dating of bone using the diffusion adsorption model abstract u series dating of bone has suffered problems of reliability since its inception because bone remains an open system with respect to uranium commonly applied a priori assumptions of u uptake such as early uptake or linear uptake are inadequate because they have no physical or chemical bases no means of demonstrating which model is suitable for a particular bone and no intrinsic tests of reliability despite this and numerous examples of anomalous u series dates such assumptions are still routinely applied we address this problem using the diffusion adsorption d a model of u uptake millard and hedges 1996 which incorporates a physicochemical description of u uptake using this model we show how the u uptake of a bone responds to geochemical changes in the burial environment which can lead to phenomena such as the removal of u from bones leaching or u uptake late in their burial history recent uptake and we show how the overall uptake history is reflected in distributions profiles of u and u series isotopes across a bone section we present measurements of u concentration profiles and 230 th 234 u profiles on archeological bone from a number of different sites and burial environments and compare the results to profiles predicted by the d a model bones that have undergone complex uptake histories which include u leaching or recent uptake are identified on the basis of these profiles and rejected as unsuitable for dating for bones that appear to have undergone uptake under constant geochemical conditions the d a model is applied to calculate u series dates with much improved reliability

dynamics and implications of slab detachment due to ridge trench collision 1 the approach of a buoyant spreading ridge to a subduction zone may lead to detachment of a subducted slab previous work has called upon the detachment process as an explanation for observed ridge abandonment and slab window related magmatism e g in baja california but such a scenario has not been tested using fully dynamic numerical models we use dynamic two dimensional models including a non newtonian rheology to study the approach of a trench parallel spreading ridge to a subduction zone we find that before the ridge approaches within 100 km of the trench detachment of the subducted slab occurs due to the combined effects of increased buoyancy and reduced strength of the lithosphere with ridge proximity in models exploring effects of subducted slab length distance of the ridge from the trench shear zone strength and lithospheric yield strength we find the following 1 a decrease in subduction velocity as the ridge approaches the trench 2 a shrinking surface plate that maintains a uniform plate like subduction velocity 3 ridge abandonment distances 100 275 km from the trench and 4 slab gap distances 195 285 km from the trench these results are consistent with observations in baja california where detachment of the cocos slab may explain abandonment of observed segments of the east pacific rise 50 200 km outboard of the trench and the presence of nonarc related magmatism located 100 250 km inboard of the trench ridge subduction does not occur within any of these two dimensional cases of ridge trench collision indicating that 3 d geometric effects may be responsible for ridge subduction

holocene climate variability in europe evidence from δ18o textural and extension rate variations in three speleothems time series o isotope profiles for three u th dated stalagmites have revealed that for much of the holocene a site on the atlantic seaboard sw ireland exhibits first order 18o trends that are almost exactly out of phase with coupled 18o curves from two southern european sites se france and nw italy in the irish stalagmite cc3 from crag cave sw ireland low 18o at 10 000 cal yr bp reflects cool conditions by the early to mid holocene 9000 6000 cal yr bp 18o had increased reflecting the onset of warmer conditions on the atlantic seaboard this shift to higher 18o was accompanied by a marked increase in the stalagmite extension rate reinforcing our interpretation that this was a period of relative warmth except for an episode of increased extension rate about 5500 yr ago 18o in the crag stalagmite exhibits a gradual decrease accompanied by declining extension rates between 7800 and 3500 cal yr bp interpreted as a cooling trend there is evidence for increases in both 18o and stalagmite extension rate in the period from 3500 cal yr bp to the present day suggesting a return to warmer conditions on the atlantic seaboard in the stalagmite from nw italy er76 grotta di ernesto trentino province the early holocene c 9200 7800 cal yr bp is characterised by high 18o probably indicative of warm and or dry conditions exceptionally low 18o from 7800 to 6900 cal yr bp at this site reflects a well defined wet phase cerin wet phase in the last three millennia this stalagmite exhibits a shift to lower 18o interpreted as some combination of cooler and or wetter conditions unlike the irish stalagmite the italian sample does not show a correlation between 18o and extension rate instead its extension rate correlates roughly with 13c presumably reflecting a climate driven vegetation change in the early holocene 18o in the french stalagmite cl26 grotte de clamouse herault province se france was low relative to its holocene average for much of the period since c 3500 cal yr bp this stalagmite exhibits higher 18o than in the early holocene suggesting warmer conditions like the irish stalagmite the french sample exhibits a well defined correlation between 18o and extension rate had drip water availability been the dominant control on 18o at this semi arid site then higher 18o would have been accompanied by lower not higher extension rates this suggests strongly that temperature rather than rainfall amount was the dominant control at this site while conclusions regarding the patterns of climate variability on a continent scale must remain tentative because of the limited number of stalagmites studied we argue that early holocene warm conditions on the atlantic seaboard irish site coincided with relatively cool conditions at the clamouse site by c 3500 yr ago the pattern appears to have been reversed

formation and exhumation of ultra high pressure rocks during continental collision role of detachment in the subduction channel 1 uhp rocks commonly form and exhume during the transition from oceanic subduction to continental collision their exhumation in subduction channels depends on the balance between down channel shear traction and up channel buoyancy thermal mechanical upper mantle scale numerical models are used to investigate how variations in material properties of the subducting continental margin affect this balance changes in shear traction leading to crustal decoupling detachment are investigated by varying the onset of strain weakening thermal parameters and convergence velocity variations in buoyancy force are investigated by modifying subducted material density and volume the model results are interpreted in terms of the exhumation number e which expresses the role of the pressure gradient channel thickness effective viscosity and subduction velocity peak metamorphic conditions exhumation velocity and timing of exhumation are temporally and spatially variable and are sensitive to the evolution of e the models reproduce natural ptt constraints and indicate that neither slab breakoff nor surface erosion is required for uhp exhumation

tectonic erosion and crustal relamination during the india asian continental collision insights from eocene magmatism in the southeastern gangdese belt abstract understanding the processes of tectonic erosion and crustal relamination during continental collision has important implications for the growth and differentiation of the continental crust the discrepancy in isotopic compositions between the pre and syn collision magmatic rocks from the gangdese belt in south tibet provides an opportunity for studying these processes during the india asian collision the nyingchi granites and confluence hornblende gabbros from the eastern gangdese belt have zircon u pb ages of ca 50 ma the nyingchi granites have high sr y and dy yb n ratios indicating that the magma was generated under eclogite facies conditions their sr nd pb hf isotopic compositions require significant incorporation of ancient supracrustal materials from the gangdese belt and the indian continent the confluence hornblende gabbros display arc like trace element patterns but have enriched sr nd pb hf isotopic compositions compared with those from the jurassic cretaceous arc magmatic rocks indicating significant input of ancient components into their mantle sources the occurrence of the cenozoic felsic metamorphic rocks in the lower crust of the gangdese belt allows us to propose that the nyingchi high sr y granites were derived from partial melting of relaminated crustal materials which were removed from the gangdese belt by tectonic erosion and the subducted indian continent the confluence gabbros were sourced from lithospheric mantle which was metasomatized by inputs from relaminated crustal materials derived from the gangdese belt and the subducted indian continent the estimated tectonic erosion rate is 150 188 km3 km my indicating significant crustal loss occurred during continental collision our study demonstrates that tectonic erosion and crustal relamination play an important role in the refinement of the composition of continental crust during continental collision

further studies for uranium series dating of fossil bone abstract in an earlier paper the authors suggested that bones from well sealed depositional matrices might be deteable by the u series method this paper presents the results of further case studies on bones from archaeological sites and also in vitro laboratory studies of the u bone system these suggest that the u series method may only be of limited use for bones and that in most sites samples are exposed to an unknown number of wetting events over their period of deposition the laboratory experiments indicate a more complex process than the simple diffusion of u through the bone cross section

mechanisms for the generation of plate tectonics by two phase grain damage and pinning abstract weakening and shear localization in the lithosphere are essential ingredients for understanding how and whether plate tectonics is generated from mantle convection on terrestrial planets we present a new theoretical model for the mechanism of lithospheric weakening and shear localization and hence plate generation through damage grain evolution and zener pinning in two phase polycrystalline lithospheric rocks grain size evolves through the competition of coarsening which drives grain growth with damage which drives grain reduction however in a two phase medium the interface between phases induces zener pinning which impedes grain growth and facilitates damage the size of the pinning surfaces is given by the roughness of the interface and damage to the interface causes smaller pinning surfaces which in turn drive down the grain size forcing the rheology into the grain size dependent diffusion creep regime this process allows damage and rheological weakening to co exist which is normally considered impossible in single phase assemblages moreover pinning greatly inhibits grain growth and shear zone healing which is much faster in single phase materials hence the resulting shear localization is rapid less than 1 myr but the healing time for a dormant weak zone is very slow greater than 100 myrs these effects therefore permit rapidly forming and long lived plate boundaries the model therefore provides a key ingredient and predictive theory for the generation of plate tectonics on earth and other planets

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

synconvergent ductile flow in variable strength continental crust numerical models with application to the western grenville orogen 1 we present results from numerical models for a convergent orogen with laterally variable lower crustal strength representing a simplified orogenic system in which a strong craton flanked by progressively weaker terranes collides with another continent with progressive convergence crustal thickening and thermal relaxation lower crust becomes decoupled from upper and middle crust forming a ductile orogenic infrastructure beneath a stronger superstructure collision with strong external crust results in uplift and expulsion of ductile nappes from the orogenic core creating allochthonous terranes overlying a lower crustal indentor the extent of transport and exhumation of lower crustal nappes over the indentor reflects the amount of convergence and the erosion rate the western grenville orogen displays across strike variations in age tectonic history and protolith association suggesting a systematic variation in precollision crustal strength the laurentian craton margin and accreted terranes were variably reworked at synorogenic depths of 25 35 km during the ottawan orogeny deformation propagated from younger monocyclic rocks in the southeast into older polycyclic rocks flanking the craton on the northwest a comparison between numerical model results and crustal scale cross sections from the georgian bay and montreal val d or transects shows close correspondence between crustal structure and model geometry this indicates that the models produce geologically realistic results and provides a context for interpreting the tectonic evolution of the western grenville orogen contrasts between the results of homogeneous channel flow models and the present ductile nappe models suggest that the effects of different styles of ductile flow can be distinguished in the geologic record

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

relamination of mafic subducting crust throughout earth s history earth has likely cooled by several hundred degrees over its history which has probably affected subduction dynamics and associated magmatism today the process of compositional buoyancy driven upwelling and subsequent underplating of subducted materials commonly referred to as relamination is thought to play a role in the formation of continental crust given that archean continental crust formation is best explained by the involvement of mafic material we investigate the feasibility of mafic crust relamination under a wide range of conditions applicable to modern and early earth subduction zones to assess if such a process might have been viable in an early earth setting our numerical parametric study illustrates that the hotter thicker crust conditions of the early earth favour the upward relamination of mafic subducting crust the amount of relaminating subducting crust is observed to vary significantly with subduction convergence rate having the strongest control on the volume of relaminated material indeed removal of the entire mafic crust from the subducting slab is possible for slow subduction 2 cm yr under archean conditions we also observe great variability in the depth at which this separation occurs 80 120 km with events corresponding to shallower detachment being more voluminous and that relaminating material has to remain metastably buoyant until this separation depth which is supported by geological geophysical and geodynamical observations furthermore this relamination behaviour is commonly episodic with a typical repeat time of approximately 10 myrs similar to timescales of episodicity observed in the archean rock record we demonstrate that this relamination process can result in the heating of considerable quantities of mafic material to temperatures in excess of 900 c which is then emplaced below the over riding lithosphere as such our results have implications for archean subduction zone magmatism for continental crust formation in the early earth and provide a novel explanation for the secular evolution of continental crust

advanced strain and mass transfer analysis in crustal scale oroclinal buckling and detachment folding analogue models abstract the piv particle image velocimetry method became a standard tool for the calculation of displacement fields in physical geodynamic models for understanding the deformation dynamics of geodynamic models in our study we implemented several post processing algorithms on the derived displacement field and calculated the velocity and strain rate components such as the divergence of the velocity field vorticity and shear strain rate in the model of oroclinal buckling we focused on strain analysis of the upper crust and correlated the shear strain rate vorticity and divergence anomalies with visual deformation patterns in the upper crust the divergence of velocity fields in these models correspond to the pop up and pop down belts oriented along the axial trace of the oroclinal bends high shear strain rate domains correlate with horizontal isovolumic shear zones alongside these belts while vorticity shows rotational trend of fold axial traces of the pop up and pop down belts around orocline inflection in another series of models we simulated the development of melt cored crustal scale detachment folds and employed the same set of parameters to investigate the ductile deformation visible in side view of the model domain we developed a method that allows tracing the divergence in subcells locked on target subdomains we tracked and quantified melt flow between the melt source area at the bottom of the model and progressively developing folds this mass transfer analysis revealed polyphase fold evolution where initial fold perturbations quickly amplify as the melt accumulates in the triangular hinge sector below and between the rotating fold limbs while the early amplification leads to decompression driving the melt into the hinge zone area between the limbs the fold lock up stage and continued attenuation of the vertical limbs is associated with melt expulsion from the fold interlimb domain back into the source layer where it can be transferred laterally to the foreland

a comparison of numerical surface topography calculations in geodynamic modelling an evaluation of the sticky air method summary calculating surface topography in geodynamic models is a common numerical problem besides other approaches the so called sticky air approach has gained interest as a free surface proxy at the top boundary the often used free slip condition is thereby vertically extended by introducing a low density low viscosityfluid layer this allows the air crust interface to behave in a similar manner to a true free surface we present here a theoretical analysis that provides the physical conditions under which the sticky air approach is a valid approximation of a true free surface two cases are evaluated that characterize the evolution of topography on different timescales 1 isostatic relaxation of a cosine perturbation and 2 topography changes above a rising plume we quantitatively compare topographies calculated by six different numerical codes using finite difference and finite element techniques using three different topography calculation methods i direct calculation of topography from normal stress ii body fitting methods allowing for meshing the topography and iii lagrangian tracking of the topography on an eulerian grid it is found that the sticky air approach works well as long as the term st ch hst l 3 is sufficiently small where st and hst are the viscosity and thickness of the sticky air layer and ch and l are the characteristic viscosity and length scale of the model respectively spurious lateral fluctuations of topography as observed in some marker based sticky air approaches may effectively be damped by an anisotropic distribution of markers with a higher number of markers per element in the vertical than in the horizontal direction

dated paleontological cave sites of central europe from late middle pleistocene to early upper pleistocene ois 5 to ois 8 abstract caves are terrestrial depositories that preserve a large variety of organic and inorganic remains these may contain important quaternary climatic and ecological information most of the faunal remains however cannot be linked to any interglacial or glacial period exclusively reliable dating of such remains is therefore required experience has however shown that esr dating of speleothems or 230 th u dating of bones are of disputable value only tims 230 th u dating of speleothems appears to yield reliable ages dating the bottom and top of speleothem layers permit assigning pleistocene faunal remains to the ois chronology if the deposition of the speleothems and the faunal remains are clearly correlated care must be taken to consider the depositional situation of each site before interpreting any age dates in this paper we present an overview of all numerically dated paleontological cave sites in central europe between ois 5 and ois 8 a total of 25 strata were dated from 13 sites most of them deposited during ois 5 the rest belonging to ois 6 and 7 numerically dated paleontological sites older than ois 8 are not known

transient hot channels perpetrating and regurgitating ultrahigh pressure high temperature crust mantle associations in collision belts abstract two dimensional numerical modeling of early continental collision associated with subduction of the lithospheric mantle shows that the formation and exhumation of coesite and diamond bearing rocks metamorphosed at 700 to 900 c in the presence of dense supercritical silicate fluids and melts may be explained by a transient hot channel effect anomalously high temperature is caused by intense viscous and radiogenic heating in the channel composed of deeply subducted radiogenic upper crustal rocks especially sediments of passive margin origin and mantle rocks heating is associated with partial melting of crustal rocks caused by pervasive flow of aqueous fluids relieved by rapid dehydration deserpentinization of the overriding mantle lithosphere that has been hydrated during earlier subduction stages the channel penetrates the plate interface to the bottom of the lithosphere of the overriding plate 150 200 km and is characterized by metamorphic temperatures reaching 700 to 900 c low effective viscosity of rocks subsequent to increased temperature partial melting and fluid infiltration permits profound mixing of hydrated mantle and crustal rocks the hot channel exists only during early collision but rapidly produces large amounts of ultrahigh pressure high temperature rocks further collision closes the channel by squeezing rheologically weak partially molten buoyant rocks between the strong lithospheric mantles of the two colliding plates assemblages of complicated p t paths with repetitive loops characterize exhumation of ultrahigh pressure rocks in the convoluted flow pattern of the hot channel

exhuming norwegian ultrahigh pressure rocks overprinting extensional structures and the role of the nordfjord sogn detachment zone 1 the nordfjord sogn detachment zone nsdz is widely cited as one of the primary structures responsible for the exhumation of norwegian ultra high pressure uhp rocks here we review data from the considerable volume of research describing this shear zone and compile a strikeparallel cross section along the nsdz from the solund basin in the south to the soroyane uhp domain in the north this cross section highlights several previously unrecognized patterns revealing a shear zone with top to the west asymmetric fabrics that 1 initiated at amphibolite facies 2 overprints metamorphic breaks and tectonostratigraphic contacts and 3 has a gradational continuum of muscovite cooling ages these patterns constrain the kinematic evolution of the nsdz and suggest a new three step model for the exhumation of norwegian u hp rocks the initial stages of exhumation were characterized by the rise of crustal rocks from u hp depths to the base of the crust by buoyancydriven mechanisms not specified in this paper mantle exhumation was followed by top to thewest normal sense displacement within a broad noncoaxial ductile shear zone near the base of the crust that overprinted tectonostratigraphic contacts formed previously during mantle exhumation in the final stages of crustal exhumation top w brittleductile detachments soled into and partially excised this ductile shear zone dropping the devonian basins into contact with rocks of varying tectonostratigraphic levels this new interpretation of the nsdz is significant as it accounts for the extreme crustal excision observed in western norway using three sequentially overprinting structures active at different stages of uhp rock exhumation citation johnston s m b r hacker and t b andersen 2007 exhuming norwegian ultrahigh pressure rocks overprinting extensional structures and the role of the nordfjord sogn detachment zone tectonics 26 tc5001

the giant shakhdara migmatitic gneiss dome pamir india asia collision zone 2 timing of dome formation 1 cenozoic gneiss domes exposing middle lower crustal rocks cover 30 of the surface exposure of the pamir western india asia collision zone they allow an unparalleled view into the deep crust of the asian plate we use titanite monazite and zircon u th pb mica rb sr and 40ar 39ar zircon and apatite fission track and zircon u th he ages to constrain the exhumation history of the 350 90 km shakhdara alichur dome southwestern pamir doming started at 21 20 ma along the gunt top to n normal shear zone of the northern shakhdara dome the bulk of the exhumation occurred by nnw ward extrusion of the footwall of the crustal scale south pamir normal shear zone along the southern shakhdara dome boundary footwall extrusion was active from 18 15 ma to 2 ma at 10 mm yr slip and with vertical exhumation rates of 1 3 mm yr it resulted in up to 90 km n s extension coeval with n s convergence between india and asia erosion rates were 0 3 0 5 mm yr within the domes and 0 1 0 3 mm yr in the horst separating the shakhdara and alichur domes and in the southeastern pamir plateau rates were highest along the dome axis in the southern part of the shakhdara dome incision along the major drainages was up to 1 0 mm yr thermal modeling suggests geothermal gradients as high as 60 c km along the trace of the south pamir shear zone and their strong n s variation across the dome the gradients relaxed to 40 45 c km since the end of doming

delamination in collisional orogens thermomechanical modeling 1 modes of mantle delamination in collision zones are discussed in the light of 2d numerical modeling freely evolving thermomechanical models take into account phase changes in the mantle and melting in the crust distinct modes are identified in which orogens undergo mantle delamination concurrently or after collision delamination propagates along the moho of the subducted plate together with the retreating trench provided that slab pull is sufficient and that the meta stability of the crust lithospheric mantle interface is overcome topography is an instantaneous response to delamination and migrates with the focused and localized separation between crust and lithospheric mantle delamination front early exhumation of high pressure rocks is followed by exhumation of high temperature partially molten rocks convective stabilization of delamination outlasts slab break offs and impedes renewed build up of mechanically strong lithospheric mantle by static cooling parameter studies investigate the sensitivity of delamination to the age of the subducted oceanic plate the collision rate the mantle rheology and explore the effects of melting and sedimentation mantle flow patterns that form around delamination and subduction are sensitive to and often limited by the semi permeable 670 km spinel perovskite transition demonstrating that phase changes are essential factors delamination may account for the formation of complex boundary zones between continents such as between africa and europe in the mediterranean where transient compression and extension the lack of mantle lithosphere and the exhumation of high grade metamorphic core complexes are features also obtained in our modeling results

esr dating of micoquian industry and neanderthal remains at kůlna cave czech republic abstract electron spin resonance esr and mass spectrometric u series dating has been carried out on teeth recovered from k lna cave during the excavations of 1961 76 linear and early uptake lu and eu age estimates are similar due to low internal u content of the teeth in general esr ages decrease upward in the section but teeth from stone rich lumpy layers give anomalously young ages due to uneven dose rate distribution elephant molars also give anomalously low ages apparently due to self absorption of gamma dose disregarding anomalous data earliest appearance of micoquian lithics and last occurrence of dicerorhinus kirchbergensis in layer 9b is dated to 69 8 ka which places them near the oxygen isotope stage ois 5a 4 boundary neanderthal remains and the latest occurrence of micoquian lithics in level 7a are dated to 50 5 ka in good agreement with an uncalibrated radiocarbon date for this level of about 45 ka the interstadial character of the fauna of this level also agrees with its date which places it in ois 3 we also report that three stadial interstadial cycles recognized in the lithological faunal and palynological record of levels 9b through 7a at kolna occurred during the timespan from about 70 to 50 ka

patterns of diagenesis in bone i the effects of site environments abstract we have measured a suite of diagenetic parameters for several populations of archaeological bones buried in a number of northwest european sites since the last ice age these are the structural damage due to microbes changes in bone micro and macro porosity protein content a measure of the crystallinity of hydroxyapatite as reflected in the phosphate infra red spectrum peak splitting and a measure of the carbonate content as given by the ratio of carbonate to phosphate infra red absorption peaks the results provide a database that clearly characterises the patterns which are often site dependent of diagenetic change in buried bone our main conclusions are that individual site hydrology appears to have a strong influence on the outcome of bone preservation and that porosity is the most effective single diagenetic parameter which both determines and reflects the preservation of bone in the burial environment

new permafrost is forming around shrinking arctic lakes but will it last widespread lake shrinkage in cold regions has been linked to climate warming and permafrost thaw permafrost aggradation however has been observed within the margins of recently receded lakes in seeming contradiction of climate warming here permafrost aggradation dynamics are examined at twelvemile lake a retreating lake in interior alaska observations reveal patches of recently formed permafrost within the dried lake margin colocated with discrete bands of willow shrub we test ecological succession which alters shading infiltration and heat transport as the driver of aggradation using numerical simulation of variably saturated groundwater flow and heat transport with phase change i e freeze thaw simulations support permafrost development under current climatic conditions but only when net effects of vegetation on soil conditions are incorporated thus pointing to the role of ecological succession furthermore model results indicate that permafrost aggradation is transitory with further climate warming as new permafrost thaws within seven decades

probing the depths of the india asia collision u th pb monazite chronology of granulites from nw bhutan 1 rocks metamorphosed to high temperatures and or high pressures are rare across the himalayan orogen where peak metamorphic conditions recorded in the exposed metamorphic core the greater himalayan sequence ghs are generally at middle to upper amphibolite facies however mafic garnet clinopyroxene assemblages exposed at the highest structural levels in bhutan eastern himalaya preserve patchy textural evidence for early eclogite facies conditions overprinted by granulite facies conditions monazite hosted within the leucosome of neighboring granulite facies orthopyroxene bearing felsic gneiss yields la mc icp ms u th pb ages of 13 9 0 3 ma monazite associated with sillimanite grade metamorphism in granulite hosting migmatitic gneisses yields u th pb rim ages between 15 4 0 8 ma and 13 4 0 5 ma monazite associated with sillimanite grade metamorphism in gneiss at structurally lower levels yields u pb rim ages of 21 17 ma these data are consistent with miocene exhumation of ghs material from a variety of crustal depths at different times along the himalayan orogen we propose that these granulitized eclogites represent lower crustal material exhumed by tectonic forcing over an incoming indian crustal ramp and that they formed in a different tectonic regime to the ultrahigh pressure eclogites in the western himalaya their formation and exhumation in the miocene therefore do not require diachroneity in the timing of the initial india asia collision

uranium thorium protactinium dating systematics abstract with precise 234u 230th and 231pa data available 230th and 231pa ages can now be tested rigorously for concordancy if the material is not concordant the isotopic characteristics of this material may be examined in some detail here models similar to those used to describe the u pb system are evaluated for use in u th pa studies for the case in which initial 230th and 231pa concentrations are effectively zero the systematics of concordia plots in relation to models of variation in 234u episodic u loss or gain continuous u loss or gain and continuous 234u 230th and 231pa gain or loss are considered for the case in which initial u concentration is significant for example in many carbonate deposits we also examine linear u uptake models for the case in which initial u concentration is effectively zero for example in teeth and bones such models should prove useful in interpreting data from materials that have behaved as open systems in particular these models may help constrain the nature of diagenetic processes and in some situations it may be possible to determine or constrain true ages with materials that have behaved as open systems

evolutionary model of the himalaya tibet system geopoem based on new modelling geological and geophysical data a two dimensional thermo mechanical laboratory modelling of continental subduction was performed the subducting continental lithosphere includes a strong brittle upper crust a weak ductile lower crust and a strong upper mantle the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is denser than the asthenosphere the lithospheric layers are composed of material whose strength is sensitive to and inversely proportional to temperature throughout the experiment the model surface was maintained under relatively low temperature and the model base at higher temperature the subduction rate satisfied the peclet criterion modelling confirms that the continental crust can be deeply subducted and shows that slab break off delamination and tectonic underplating are fundamental events with drastic consequences on the subsequent evolution of the convergent system combining these results with previous purely mechanical modelling we elaborate a new evolutionary model for the himalaya tibet convergent system the principal successive stages are 1 subduction of the indian continental lithosphere to 200 250 km depth following subduction of the tethys oceanic lithosphere 2 failure and rapid buoyancy driven uplift of the subducted continental crust from ca 100 km depth to some depth that varies along the mountain belt 20 30 km on average 3 break off of the indian subducted lithospheric mantle with the attached oceanic lithosphere 4 subduction underplating of the indian lithosphere under asia over a few to several hundred kilometers 5 delamination roll back and break off of the indian lithospheric mantle 6 failure of the indian crust in front of the mountain belt formation of the main central thrust and underthrusting of the next portion of indian lithosphere beneath tibet for a few hundred kilometers at the beginning of stage 6 the crustal slice corresponding to the crystalline himalayas undergoes erosion activated uplift and exhumation fl 2000 elsevier science b v all rights reserved

continuous deformation of the tibetan plateau from global positioning system data global positioning system velocities from 553 control points within the tibetan plateau and on its margins show that the present day tectonics in the plateau is best described as deformation of a continuous medium at least when averaged over distances of 100 km deformation occurs throughout the plateau interior by ese wnw extension and slightly slower nne ssw shortening relative to eurasia material within the plateau interior moves roughly eastward with speeds that increase toward the east and then flows southward around the eastern end of the himalaya crustal thickening on the northeast ern and eastern margins of the plateau occurs over a zone 400 km wide and cannot be the result of elastic strain on a single major thrust fault shortening there accommodates much of india s penetration into eurasia a description in terms of movements of rigid blocks with elastic strain associated with slip on faults between them cannot match the velocity field

delamination vs break off the fate of continental collision 1 the fate of a convergent continental margin is investigated we perform a set of 2d numerical models to study how and why continental collision can evolve in different scenarios since the rheology of continental lithosphere has a major control on the dynamics of subduction we explore a range of different lithosphere and lower crust viscosity values to understand their sensitivity on the possible scenarios we find that with a rheologically layered crust both delamination and break off are feasible we identify three modes 1 slab detachment in which the lithospheric mantle and the crust are strongly coupled subduction slows down and the slab eventually breaks 2 delamination of the lithospheric mantle that separates from the crust and continue to subduct and 3 an intermediate mode where the lithospheric mantle and the crust remain partially coupled resulting in an initial stage of delamination followed by the slow down and cessation of subduction

missing oligocene crust of the izu bonin arc consumed or rejuvenated during collision the 50 myr old izu bonin mariana ibm arc consists mostly of oligocene middle and lower crust that underlies the upper crust these units are in turn covered by quaternary volcanic rocks seismic imaging forearc geology ocean drilling program drilling and magnetic anomalies suggest that most ibm arc crust was created in eocene oligocene times however remnants of this old crust have never been found at the northern end of the arc where it is colliding with the honshu arc izu collision zone two batholiths in this collision zone thetanzawa tonalites and the kofu granitic complex were emplaced during the miocene 4 17 ma major elements zr y rare earth element ratios and normalized abundance patterns and sr nd isotopic data indicate that these plutonic bodies are compositionally similar to the oligocene ibm volcanic rocks and that they are dissimilar to the miocene pliocene and quaternary ibm lavas and volcaniclastic rocks we suggest that the miocene plutonic rocks in the izu collision zone were derived from partially melted oligocene middle crust a model is proposed in which ibm arc middle crust in the collision zone was partially melted during the collision and then intruded into the overlying upper crust of the honshu and ibm arcs this resulted in the complete loss of chronological information related to their original source

geochemistry and geochronology of mississippian volcanic rocks from sw mongolia implications for terrane subdivision and magmatic arc activity in the trans altai zone abstract baaran and baytag terranes form important part of the trans altai zone of the central asian orogenic belt in the sw mongolia the trans altai zone represents an oceanic domain built by ordovician to devonian ophiolites covered by devonian carboniferous oceanic sediments this oceanic assemblage was intruded by a spectrum of carboniferous volcanic rocks ranging from basalts to rhyolites 325 6 1 4 ma to 351 0 5 9 ma la icp ms u pb dating on zircon that display similar whole rock trace element and sr nd isotopic signatures regardless their silica contents and the terrane they belong to they are normal k calc alkaline rocks whose nmorb normalized multielement patterns show an arc like enrichment in lile large ion lithophile elements and deep negative tnt ta nb ti anomalies based on the relatively high nb yb and th yb ratios the arc was probably formed on immature continental rather than typical oceanic crust low 87sr 86sr i 0 7043 0 7047 high e nd 350 5 2 to 5 8 and the observed in situ hf isotopic variation in dated zircons e hf t 7 4 to 13 are all attributed to a partial melting of primitive and youthful neoproterozoic ordovician arc type lower continental crust metamorphosed intermediate acid igneous rocks and or arc derived greywackes augmented with and most likely facilitated by contemporaneous arc related basaltic intrusions a distinct possibility remains the previously formulated model assuming that the carboniferous magmatism in gobi altai and trans altai zones sampled the juvenile neoproterozoic to cambrian arc crust relaminated under the ordovician devonian oceanic crust in any case the studied arc association together with lithological sequence of ophiolites can be closely correlated with the dulate arc in eastern junggar it is proposed that the two arc sequences form a single belt c 500 km long that represents one of the main known arc structures in the central asian orogenic belt

uranium series dating uranium decays through a number of radioactive daughter isotopes some of which have half lives comparable to the time scale of prehistoric archaeology the growth of these isotopes in naturally occurring materials at archaeological sites can be used to determine the age of sites the growth of 230th from its parent 234u can be used over a time range from a few hundred to half a million years calcite precipitated from running or dripping water in springs and caves as well as marls and soil deposited calcretes may be spatio temporally associated with archaeological materials they can be dated by 230th 234u and 234u 238u measurements with a precision of 5 10 of the age by alpha counting or 1 by mass spectrometry bones teeth mollusk and egg shells are also datable but present problems due to migration of parent u in and out of the samples during their burial history

shear deformation and eclogite formation within granulite facies anorthosites of the bergen arcs western norway the anorthosites of the bergen arcs contain two distinct metamorphic mineral assemblages which are stable under different p t conditions a granulite facies assemblage which equilibrated at t 800 900 c p 10 kbar is defined by the minerals plagioclase al rich diopside cpx i garnet gnt i scapolite orthopyroxene dark brown hornblende this assemblage is linked by corona structures to a primary magmatic mineralogy where coexistence of olivine and plagioclase limits the maximum crystallization pressure of the anorthosites to 8 kbar at 1200 c the anorthosites are transected by a series of fine grained shear zones in which the granulite facies assemblage is replaced by an eclogite mineralogy defined by omphacite cpx ii jd40 50 garnet kyanite epidote clinozoisite phengite quartz amphibole with plagioclase sometimes remaining in the marginal parts mineral compositions indicate equilibration conditions for the eclogite paragenesis of t 700 800 c p 16 19 kbar a series of textural and mineralogical changes which can be observed as the strain increases into the shear zones demonstrates that the eclogite mineralogy is formed at the expense of the granulite facies minerals during the early stages of shear zone development the eclogite mineralogy defines a strong mylonitic fabric which again is truncated by centimetres thick secondary shear zones containing clasts of both granulite and eclogite minerals these latter fine grained ultramylonites may be connected to uplift of this deep seated complex the eclogite shear zones are highly oblique to the granulite structures indicating that the eclogite shear zones formed under a new stress regime rb sr dating shows that the granulite facies assemblage formed in sveconorwegian time or earlier while minerals from shear zones define ages around 400 ma indicating a caledonian formation of the eclogites this work demonstrates the critical role of deformation in triggering metamorphic reactions whether the shear stress also was important in stabilizing the eclogite assemblages is not clear from the available data

single subduction zone for the generation of devonian ophiolites and high p metamorphic belts of the variscan orogen nw iberia within the variscan orogen early devonian and late devonian high p belts separated by mid devonian ophiolites can be interpreted as having formed in a single subduction zone early devonian convergence nucleated a laurussia dipping subduction zone from an inherited lithospheric neck peri gondwanan cambrian back arc slab retreat induced upper plate extension mantle incursion and lower plate thermal softening favouring slab detachment within the lower plate and diapiric exhumation of deep seated rocks through the overlying mantle up to relaminate the upper plate upper plate extension produced mid devonian suprasubduction ocean floor spreading devonian ophiolites while further convergence resulted in plate coupling and intraoceanic ophiolite imbrication accretion of the remaining cambrian ocean heralded late devonian subduction of inner sections of gondwana across the same subduction zone and the underthrusting of mainland gondwana culmination of nw iberian allochthonous pile oblique convergence favoured lateral plate sliding and explained the different lateral positions along gondwana of terranes separated by palaeozoic ophiolites

chronology of the saxothuringian subduction in the west sudetes bohemian massif czech republic and poland isotopic dating of monazite and garnet from high pressure metamorphic rocks exposed in the northern part of the saxothuringian palaeo suture in the bohemian massif revealed a diachronous metamorphism of various rock types that are now closely associated within allochthonous units representing the devonian carboniferous subduction accretionary complex mafic blueschists of the middle unit yielded a lu hf garnet age of 363 9 1 3 ma the blueschists occur within high pressure garnet free phyllites monazite extracted from this rock type yielded a u pb isotope dilution thermal ionization mass spectrometry age of 336 5 0 5 ma garnet bearing micaschist of the lower unit contains monazite with a u pb secondary ion mass spectrometry age of 341 3 ma consistent with lu hf garnet whole rock ages of 344 5 1 3 and 342 7 ma obtained from the same rock type existing tectonic models of the bohemian massif and particularly of its northern part assume that the period of oceanic subduction was terminated at c 380 375 ma by the arrival of an attenuated saxothuringian continental crust which was partly subducted and partly relaminated underneath the overriding tepla barrandian domain however our data as well as data from mafic high pressure rocks in the southern part of the saxothuringian domain suggest that the initial collision was probably caused by the arrival of a smaller crustal block present within the saxothuringian oceanic domain after its subduction and relamination the oceanic subduction was re established and terminated by continental subduction and later collision of the saxothuringian passive margin at c 345 335 ma

low temperature high stress deformation of olivine under water saturated conditions abstract low temperature high stress rheology of olivine was investigated under water saturated conditions by large strain shear deformation experiments using the griggs type apparatus samples at t 1273 1373 k and p 2 gpa under water saturated conditions deformed under a significantly lower applied stress than expected from the power law relation and the strain rate increases more rapidly with stress these observations suggest that an exponential flow law operates under the present experimental conditions in which the stress dependence of activation enthalpy is characterized by the peierls stress the peierls stress is determined between 1 6 and 2 9 gpa depending on the choice of the form of stress dependence of the activation enthalpy these values are significantly smaller than the peierls stress under dry conditions 9 1 gpa we suggest that water hydrogen incorporated in the olivine lattices decreases the peierls stress and hence the rate of deformation in the exponential creep is significantly enhanced by the presence of water the reduction of the peierls stress by water results in the highly anisotropic weakening effect of water that causes the fabric transitions in olivine the transition between the power law and the exponential creep occurs at relatively low stress 100 mpa at a geologically relevant strain rate of 10 15 s 1 when temperature is lower than 1273 k and under water rich conditions hence the exponential creep might be dominated in olivine rich rocks at low temperature and high stress under water rich conditions and such regions could be widespread in the subduction zone upper mantle where the cold lithospheric plates sink into the mantle

bones and groundwater towards the modelling of diagenetic processes this paper develops a theory for describing those diagenetic changes in bone which involve its interaction with groundwater three main processes are considered as examples of such changes namely the uptake of uranium the dissolution of bone and the increase of crystallinity of the bone mineral carbonate hydroxyapatite or dahllite here simple models of the chemistry involved are postulated although how bone interacts with water on amolecularscale is not clearly known in order to demonstrate the theory with explicit mechanisms and values greater emphasis is given to uranium uptake since the model used is comparatively detailed being based on the authors previous work the basic assumption is that the rate limiting process in diagenetic change is the movement of solutes to from or within the bone therefore the main thrust of the paper is to show how the physical structure of the bone itself together with the hydrology of the burial site interact to determine how water and its solutes move into within and from a bone during burial this interaction can be of three kinds defined by the site hydrology these are termed here diffusion hydraulic flow and recharge all three types may operate together and their relative importance depends on the extent to which the pore structure of a bone has been altered by diagenesis as well as the type of chemical change taking place it is shown that diffusion is usually the most common and important process but that it is possible to predict the hydrological regimes in which other mechanisms dominate it is shown how knowledge of site hydrology mainly the specification of soil structure and moisture variation the physical state of the bone and the chemistry of the diagenetic process can provide estimates for the rate and manner of the particular diagenetic process such estimates agree with observation suggesting this approach to be on the right lines qualitative predictions also result from the theory the main value of this approach is to identify those situations where particular diagenetic changes are simplest e g sites where the hydrology gives rise to a single and quantifiable hydraulic process so that they may be decisively tested against the quantitative predictions of the theory

measurements and relationships of diagenetic alteration of bone from three archaeological sites four diagenetic parameters have been chosen to represent the state of diagenesis of bone buried on archaeological sites they are histological preservation protein content crystallinity and porosity how these parameters are measured is described and results from populations of bones from three different sites are presented the results show the extent and variation in the degree of change both within a site and between sites in particular the correlations between diagenetic parameters are examined which give clues about the processes which cause alteration the value of porosity determinations both at the intercrystalline level and at coarser levels in revealing the degree of diagenetic change in bone and in underlying the dynamics of the interaction between buried bone and the surrounding water is stressed the data also indicate but are too restricted to prove the following microbiological attack is generally complete within less than 500 years substantial levels of protein may remain in bone after maximal micromorphological alteration loss of protein appears to be independent of other diagenetic change the correlated lose of microporosity with increase of crystallinity suggests these changes may arise from the dissolution perhaps with subsequent recrystallization of the smallest hydroxyapatite crystallites

anatomy of a diffuse cryptic suture zone an example from the bohemian massif european variscides the fate of the lower plate during continental collision can be examined in deeply eroded orogens such as the late paleozoic variscan belt in continental europe in particular the bohemian massif at its eastern extremity preserves well the evolution of an andean type orogen involved in continental collision this process included relamination of subducted light felsic material rich in radioactive elements underneath a dense mafic lower crust of the upper plate this led to gravity driven overturns and overprinting of the original suture by a broad zone of mixed upper and lower plate materials in the studied example this zone of interaction repeatedly reappears within the orogen forming a so called diffuse cryptic suture zone

diapiric exhumation of earth s youngest uhp eclogites in the gneiss domes of the d entrecasteaux islands papua new guinea abstract the woodlark rift in papua new guinea hosts the world s youngest 2 8 ma eclogite facies rocks and extensional deformation has played a key role in exhuming these u hp rocks at rates of 20 mm yr during the eocene papuan arc continent collision australian plate derived continental rocks were subducted to u hp depths there they remained for up to 30 m y until the pliocene when asthenospheric circulation ahead of the west propagating woodlark spreading ridge introduced heat and fluids this caused rocks to break away from the paleosubduction channel recrystallize in the eclogite facies and rise as rayleigh taylor instabilities the diapirs ascended adiabatically undergoing partial melting which lowered their viscosity and increased buoyancy u hp crust ponded near the moho at 2 4 ma thickening the crust to 40 km 11 kb domal uplifts emerged above sea level and these are still underlain by an unusually thick crust 26 km for a rift that has stretched by factor of 3 since 6 ma after ponding they acquired a flat lying foliation during amphibolite facies retrogression vertical shortening accompanied the gravitationally driven outflow of ponded lower crust the weak material was extended parallel to the rift margin thinning ductilely by w k 0 2 and was mechanically decoupled from and orthogonal to plate motion in the rift top e shear fabrics suggest that this flow was westward perhaps driven by isostatic stresses towards a strongly thinned rift corridor ahead of the woodlark spreading ridge at 100 c per m y and were mechanically incorporated into the rift s upper crust final exposure was by normal faulting and minor erosion such exhumation may also apply to other u hp terranes where less evidence for moho ponding is preserved

in situ u series dating by laser ablation multi collector icpms new prospects for quaternary geochronology the capabilities and potential applications of in situ dating of quaternary materials using laser ablation mc icpms are explored 234 u 238 u and 230 th 234 u can be measured with precision sufficient for dating at a spatial resolution of 100mm or better in samples that contain as a little as 1 ppm uranium moreover u and th concentrations and u series isotope ratios can be continuously profiled to determine changes in age that occur with sample growth e g in speleothems these capabilities additionally permit the dating of bones teeth and possibly molluscs which are subject to post mortem open system behaviour of useries isotopes and can be employed to elucidate processes of u series migration during weathering and diagenesis a drawback of laser ablation mc icpms is that it cannot in general provide u series age estimates with the high precision and accuracy of conventional tims or solution mc icpms methods however sample preparation is straightforward the amount of sample consumed negligible and it can be used to rapidly characterise or screen and select samples from which more precise and accurate dates can be obtained using conventional methods given further instrumental developments and the establishment of suitable matrix matched standards for carbonates and other materials we foresee that laser ablation mc icpms will play an increasingly important role in quaternary dating research r 2005 published by elsevier ltd

the variscan belt in central europe main structures geodynamic implications open questions abstract the variscan belt of europe originated from the confrontation of northern europe and gondwana with intervening pre variscan blocks cadomian and older armorica tepla barrandean moravo silesian though a strike slip component cannot be excluded geological evidence suggests a subduction collision process newly discovered features include basaltic rocks with morb affinities high pressure metamorphic rocks trench deposits and large scale allochthonism which also affects the pre variscan basement and has often led to an inversion of the metamorphic isogrades large scale thrusting is evident at all crustal levels and is also observable by geophysical methods the structure of the variscan crust is best described as horizontal tectonic layering late ordovician through carboniferous convergence closed one rhenohercynian and one saxo thuringian basin to the north and at least one mediterranean basin to the south of the pre variscan blocks subduction occurred from the north as well as from the south down under the dorsal core region the exact timing and the driving mechanism of this bilateral activity remains a problem likewise uncertain are the exact limination of the pre variscan blocks and the existence and width of oceanic areas in the saxothuringian zone and in the south facing part of the moldanubian zone there is good evidence of early palaeozoic rifting and of late ordovician through carboniferous plate convergence palaeomagnetic and palaeoclimatic constraints however suggest only a limited extent of oceanic basins in these areas while they appear to allow a wider ocean at the rhenohercynian saxothuringian boundary which is relatively unobtrusive from the geological point of view research during the past decade has produced a wealth of information on the variscan fold belt in europe the resulting geodynamic models however are extremely diverging evidently it is not the time yet for definitive statements the present paper intends to review the state of knowledge to deduce such basic items of a geodynamic model as appear reasonably well documented and to point out the most urgent open questions stress will be laid on central europe which offers a complete cross section through the northern flank of the variscan belt and where we have carried out most of our studies during the last years

a preliminary report on the stanton harcourt channel deposits oxfordshire england geological context vertebrate remains and palaeolithic stone artefacts a summary is given of the geological faunal and archaeological information obtained during excavations in the stanton harcourt channel deposits from 1990 to 1995 the channel deposits underlie the cold climate stanton harcourt gravel member of the summertown radley terrace formation the channel sediments are attributed to oxygen isotope stage 7 when the thames was undergoing down dip migration and eroding the weymouth member of the oxford clay upper jurassic the contemporary jurassic corallian escarpment being near to stanton harcourt at that time abundant large vertebrate remains have been recovered mainly from the base of the channel deposits where a cobble and boulder bed rests on thin silt or sand horizons or in scour hollows in the clay bedrock smaller bones occur throughout the deposits which are mainly poorly sorted gravels but especially at erosive horizons several palaeolithic artefacts have been found in the same contexts many of the bones and some of the artefacts appear not to have been transported far although the artefacts cannot be linked directly with the bones a study of them adds to our knowledge of the middle pleistocene human settlement of the upper thames valley it is of interest that mammoth is abundant as part of the interglacial faunal assemblage and the significance of this is discussed the environment clearly included substantial areas of open grassland although there was also some forest in the vicinity evidence appears to be accumulating for important faunal and floral differences between particular interglacial events during the british middle and late pleistocene

model of syn convergent extrusion of orogenic lower crust in the core of the variscan belt implications for exhumation of high pressure rocks in large hot orogens reflection seismic section field structural analysis and gravimetric modelling of orogenic lower crust in the core of a carboniferous orogenic root reveal details of the polyphase process of exhumation subvertical amphibolite facies fabrics strike parallel to former plate margins that collided in the nw the fabrics are developed in both mid crustal and lower crustal high pressure granulite units as a result of intensive nw se intraroot horizontal shortening driven probably by the west directed collision in granulites the steep fabrics originated as a result of extrusion of orogenic lower crust in a 20 km wide vertical ascent channel from lower crustal depths at 350 340 ma the large granulite bodies preserve older granulite facies fabrics documenting a two stage evolution during the exhumation process surface exposures of granulites coincide with the absence of subhorizontal seismic reflectors at depth suggesting preservation of the 20 km wide subvertical tabular structure reaching moho depths horizontal seismic reflectors surrounding the vertical channel structure corroborate a dominant flat migmatitic fabric developed in all tectonic units this structural pattern is interpreted in terms of subhorizontal spreading of partially molten orogenic lower crust in mid crustal levels 765 c and 0 76 gpa at 342 337 ma large massifs of extruded and progressively dismembered felsic granulites disturbed mid crustal fabrics in the surrounding horizontally flowing partially molten crust the horizontal mid crustal flow resulted in collapse of the supra crustal tepla barrandian unit interpreted as the orogenic lid along a large scale crustal detachment above the extruded lower crustal dome the presence of felsic granulites at the bottom of the orogenic root is considered to be a key factor controlling the exhumation of orogenic lower crust in large hot orogens

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

styles of post subduction collisional orogeny influence of convergence velocity crustal rheology and radiogenic heat production abstract a number of models of continental collision already exist but the role of foregoing oceanic plate subduction or variable coupling between plates still remains to be explored in addition heat generation by radioactive decay may be quite variable for example our geochemical data from low to high grade metamorphic rocks of the lesser and higher himalayan sequences reveal measured heat production values on the order 4 5 w m 3 which is considerably higher than normal inferred upper crustal values 3 therefore we performed 2d numerical experiments of oceanic continental subduction followed by continental collision for varying continental lower crustal strength convergence velocity and radiogenic heating rate oceanic subduction produces thinning and upwarping of the overriding lithospheric wedge such as is observed in andean type collisional settings cascadia and central andes after collision two main orogenic styles develop depending on the values of key parameters either a slow convergence rates weak shear heating and consequently strong rheological coupling between plates and or a weak lower crust generate a two sided thick and narrow symmetrical collisional zone or b high convergence rates strong shear heating and therefore weak rheological coupling between plates and a strong lower crust produce a one sided thin and broad asymmetrical collisional zone in this case a vertical lithospheric body divides two upper crustal wedges that develop distinct deformation patterns in both cases enhanced radiogenic heating favors the formation of midcrustal partially melted channels that propagate toward the surface and the pro foreland partial melting triggers lateral extrusion of metamorphic rocks from different depths 15 23 kbar 700 c on top of coherent lp lt units the geometry of metamorphic isograds inverted shapes of p t paths at different grades and exhumation rates are similar to those observed in parts of the himalayan chain indicating that the models capture the essence of real systems the model results demonstrate that internal heat production rate is a crucial parameter in determining the timing of slab breakoff polarity reversal and exhumation of the inverted metamorphic sequence with respect to the timing of partial melting late stages of some numerical experiments showed interesting geodynamical evolution of the collisional zone with the appearance of polarity reversal roll over of both plates and roll back of the subducting slab irrespective of these details hundreds of km of continental lithosphere are recycled into the mantle in all models supporting the idea that continental crustal consumption is indeed a common and considerable process during collision this affects inferences about extents of convergence and or shortening during orogenesis moreover crustal recycling into the deep mantle during collisional orogeny needs to be considered in models of crustal growth and evolution

strain partitioning and metamorphism in a deformable orogenic wedge application to the alpine belt abstract various processes have been proposed in the literature to explain the exhumation and preservation of blueschist and eclogitic facies rocks among these processes corner flow circulation within an accretionary wedge is frequently advocated in the alpine collisional belt geological petrological and chronological evidences indicate that high pressure low temperature metamorphic rocks have been generated in the subduction zones which existed before the collisional event indeed the internal part of the alps have been compared to the inner zone of modern accretionary prisms in order to discuss the corner flow circulation model and its applications to the internal part of the alps taking into account the metamorphic and structural data we have numerically explored in a deformable orogenic wedge 1 the possible generation of hp lt metamorphism in this context and the ptt paths related to the exhumation process 2 the duration of the exhumation and 3 the strain partitioning and the related geometries imposed by this circulation the velocity field is computed from mass conservation law no attempt has been made to include variable viscosity we show that 1 hp lt metamorphism can be achieved in the computed conditions ptt paths differ according to the velocity of subduction 2 the exhumation process lasts less than 15 ma and generates high strains which erase the initial deformation phases and 3 metamorphic and structural data from the internal part of the alps are compatible with the numerical constraints

the fate of subducted continental margins two stage exhumation of the high pressure to ultrahigh pressure western gneiss region norway thermobarometry suggests that ultrahigh pressure uhp to high pressure hp rocks across the western gneiss region ponded at the moho following as much as 100 km of exhumation through the mantle and before exhumation to the upper crust eclogite across the c 22 000 km 2 study area records minimum pressures of c 8 18 kbar and temperatures of c 650 780 c one orthopyroxene eclogite yields an uhp of c 28 5 kbar and evidence of former coesite has been found c 50 km farther east than previously known despite this widespread evidence of uhp to hp thermobarometry of metapelite and garnet amphibolite samples reveals a surprisingly uniform supra barrovian amphibolite facies overprint at c 11 kbar and c 650 750 c across the entire area chemical zoning analysis suggests that garnet in these samples grew during heating and decompression presumably during the amphibolite facies event these data indicate that the norwegian uhp hp province was exhumed from mantle depths of c 150 km to lower crustal depths where it stalled and underwent a profound high temperature overprint the ubiquity of late stage supra barrovian metamorphic overprints suggests that large scale collisional uhp terranes routinely stall at the continental moho where diminishing body forces are exceeded by boundary forces significant portions of the middle or lower crust worldwide may be formed from uhp terranes that were arrested at the moho and never underwent their final stage of exhumation

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain

magnetic susceptibility applied as an age depth climate relative dating technique using sediments from scladina cave a late pleistocene cave site in belgium abstract here we demonstrate that magnetic susceptibility ms data from scladina cave belgium provide a time depth climate relationship that is correlated to the marine oxygen isotopic record and thus yields a high resolution relative dating method for sediments recovered from many archaeological sites this methodology will help resolve one of the major problems facing archaeologists namely the difficulty of acquiring absolute dates with reasonable precision for the period from 40 000 to 400 000 years or so the problem is that dating techniques applicable to most materials within this age range are subject to significant errors relative dating techniques such as magnetic secular variation or stable isotope methods offer the potential to improve this precision but both methods suffer from problems that make broad application to many sites impossible however for most archaeological cave sites ms measurements of cave sediments offers the potential for intra site correlation and paleoclimate estimation this is possible in protected cave environments because the ms of cave sediments results from climate processes active outside caves which cause variations in magnetic properties of the sediments that ultimately accumulate inside caves once deposited these materials are often preserved and their stratigraphy provides a time depth climate signal that can be identified therefore ms data can be used as an independent methodology alongside conventional methods such as sedimentology and palynology for relative age dates and correlation within and between sites by tracing evidence of paleoclimatic change this correlation has been used to infer an age of 90 000 7000 years for neanderthal skeletal remains recovered from scladina cave an important middle paleolithic archaeological site in belgium

high precision constraints on timing of alpine warm periods during the middle to late pleistocene using speleothem growth periods a long standing problem in the reconstruction of warm climate periods during the late quaternary is the difficulty of determining absolute ages for undisturbed terrestrial climate archives speleothems offer an important source of information about the timing and duration of middle and late pleistocene warm phases a flowstone from the high elevation spannagel cave zillertal alps austria provides a new absolutely dated climate record which includes the most prominent warm phases between 260 and 50 ka before present new data from the penultimate interglacial improve our knowledge about the timing and progression of warm periods in the alps corresponding to marine isotope stages 7 and 8 speleothem growth phases were identified from 261 to 249 from 236 to 229 and from 200 to 192 ka an early onset of the last interglacial is corroborated by a growth phase commencing at 136 7f2 8 ka and the youngest section of the flowstone provides further evidence for an alpine climate during time intervals corresponding to marine isotope stages 5a and 3 sufficiently warm to allow speleothems to grow the high resolution stable isotope profile taken along the growth axis of the flowstone provides information about the hydrological conditions during the growth phases d 2005 elsevier b v all rights reserved

effects of mantle and subduction interface rheologies on slab stagnation and trench rollback abstract trench rollback has been a widely discussed phenomenon in recent years and multiple studies have concentrated on various parameters that may influence trench migration and related aspects of slab deformation in the upper mantle here we concentrate on the effects of rheological description yield stress lower mantle viscosity viscosity of crust in controlling the rollback and associated stagnation of slabs in the transition zone 410 660 km depth we perform numerical simulations of slab evolution in a 2d cartesian model with strongly nonlinear rheology combining diffusion creep dislocation creep and a power law stress limiter we demonstrate that trench retreat develops in most models considered regardless of the subducting plate age or prescribed strength rollback then mostly produces slabs that are horizontally deflected at the 660 km phase boundary and remain subhorizontal at the bottom of the transition zone slab morphologies are in agreement with stagnant horizontally deflected structures reported in the transition zone by seismic tomography furthermore if the strength of the slab is limited to less than 0 5 gpa the slab experiences a significant amount of horizontal buckling the amplitude of the rollback velocity is sensitive to several model parameters as one might expect it increases with the age of the subducting plate thus reflecting its increasingly negative buoyancy on the other hand rollback velocity decreases if we increase the viscosity of the crust and thereby strengthen the coupling between the subducting and overriding plates high friction on the contact between the subducting and overriding plates may even result in slabs penetrating into the lower mantle after a period of temporary stagnation also reducing the additional negative buoyancy imparted by the 410 km exothermic phase transition suppresses trench rollback interpretation of the controls on slab rollback and stagnation may be rather complex in strongly nonlinear rheological models where for example buoyancy effects may be counteracted by associated yield stress weakening

mafic injection as a trigger for felsic magmatism a numerical study 1 the origin of crustal scale silicic magmatism remains a matter of debate and notable uncertainty exists concerning the physical mechanisms that drive ascent and emplacement of felsic magmas in upper crustal regions a 2 d numerical model demonstrates that injection of mantle derived mafic magma into a partially molten hot zone in the lower crust can drive felsic magma ascent and intrusion into upper crustal levels the injection of mafic magma induces overpressure in the reservoir which increases crustal stresses and triggers development of brittle plastic shear zones and can drive significant surface uplift the emerging topography causes a nonuniform overpressure distribution in the reservoir and can trigger felsic magma ascent along crustal shear zones based on systematic numerical experiments we investigate the influence of crustal strength and injection rate the initial upper crustal strength controls the degree of crustal faulting and surface uplift and therefore whether felsic magma ascent can be initiated or not the final upper crustal strength influences the depth and final style of felsic intrusion the injection rate of mafic magma determines the time scale of overpressure growth and surface uplift stage in contrast the duration of the subsequent felsic ascent and intrusion emplacement stages remains nearly constant our results imply that mafic underplating and intrusion into the lower crust may not only be a prime control for the generation of felsic magmas in the lower crust but may also be an important physical driving mechanism for felsic magma ascent and intrusion into upper crustal levels

dynamics of double subduction numerical modeling abstract double subduction is a geodynamic process in which two plates following each other are synchronously subducted double subductions are known for both modern izu bonin marianas and ryukyu arcs and ancient west himalaya collision zone plate tectonics however our knowledge about this process is limited to conceptual schemes and some restricted analogue experiments in order to fill this gap we performed 2d numerical experiments using a coupled petrological thermomechanical approach based on finite differences and marker in cell techniques combined with thermodynamic database for the mantle we investigated the influence of convergence rate intermediate plate length activation volume of the mantle dislocation creep and age of the lithosphere based on these experiments we conclude that a subduction rates at two zones running in parallel differ and vary in time even when the total convergence rate remains constant supremacy of either subduction zone depends on physical parameters such as i relative rates of the plates ii slab ages and iii length of the middle plate b subduction dynamics of the double subduction system involves several processes unknown in simple subduction systems such as i eduction i e un subduction ii subduction re initiation iii subduction flip triggered by shallow slab breakoff and iv turn over of detached slabs to up side down attitudes c simulated tomographic structures related to slab propagation account for both penetration and non penetration of the 660 km discontinuity non penetration is favored by i low convergence rate ii faster relative movement of the overriding plate iii young age of the subducting slab and iv up side down turn over of detached slab

building the pamir tibetan plateau crustal stacking extensional collapse and lateral extrusion in the central pamir 1 geometry and kinematics asian deep crust exposed in the pamir permits determination of the amount sequence and interaction of shortening extension and lateral extrusion over 30 km of crustal section during the india asia collision in the central pamir gneiss domes and their hanging walls record paleogene tripling of the 7 10 km thick phanerozoic upper crustal strata total crustal thickness may have amounted to 90 km two thrust sheets comprising cambro ordovician respectively carboniferous to paleogene strata straddle the domes amphibolite facies metamorphic rocks within the domes equivalent to lower grade rocks outside the domes form fold nappes with dome scale wavelengths e w stretching occurred contemporaneously with top to n imbrication and folding at 22 12 ma bivergent top to n and top to s normal sense shear zones exhumed the crystalline rocks most of the extension occurred along the northern dome margins shortening resumed at 12 ma with opposite sense thrusting and folding focused along the dome margins throughout the building of the central and south pamir dominant n s shortening interacted with e w extension along mostly dextral shear fault zones in the neogene shear is concentrated along a dextral wrench corridor south of the domes we interpret the paleogene shortening to record thickening and northward growth of the pamir tibetan plateau and short lived miocene crustal extension as gravitational adjustment i e collapse of the thickened asian crust to indian slab breakoff synconvergent paleogene lateral extrusion thickened the afghan hindu kush crust west of the india asia collision and the miocene recent dextral shear and e w extension have accommodated collapse of the pamir plateau into the tajik depression

precise dating of dansgaard oeschger climate oscillations in western europe from stalagmite data the signature of dansgaard oeschger events millennial scale abrupt climate oscillations during the last glacial period is well established in ice cores and marine records but the effects of such events in continental settings are not as clear and their absolute chronology is uncertain beyond the limit of 14 c dating and annual layer counting for marine records and ice cores respectively here we present carbon and oxygen isotope records from a stalagmite collected in southwest france which have been precisely dated using 234u 230th ratios we find rapid climate oscillations coincident with the established dansgaard oeschger events between 83 000 and 32 000 years ago in both isotope records the oxygen isotope signature is similar to a record from soreq cave israel and deep sea records indicating the large spatial scale of the climate oscillations the signal in the carbon isotopes gives evidence of drastic and rapid vegetation changes in western europe an important site in human cultural evolution we also find evidence for a long phase of extremely cold climate in southwest france between 61 2 0 6 and 67 4 0 9 kyr ago

successful application of uranium series dating of fossil bone abstract a novel approach to uranium series disequilibrium dating is presented and used successfully to date bones from several european archaeological and palaeontological sites a method is described based on a proposed mechanism of uranium uptake and fixation derived from present understanding of the biochemistry of fossilizing bones and the geochemistry of uranium stability of uranium in fossil bone is predicted under particular conditions of local depositional environment and the importance of the inhomogeneous distribution of uranium through bone to uranium series dating is explained sample bones from several different sites have been examined taking into account the three factors of uptake mechanism uranium distribution and sedimentary environment and have yielded ages in good agreement with those derived by alternative dating methods or from chronologically fixed stratigraphical positions

ductile rebound of an orogenic root case study and numerical model of gravity tectonics in the western gneiss complex caledonides southern norway the main caledonian deformation in the western gneiss complex southern norway has been interpreted as resulting from the gravity driven ductile rebound of the orogenic root detailed field data from the sognefjord profile a 60 km long continuously exposed shore section through the complex are compared with the results of a numerical scale model of root rebound dimensioned according to reconstructions of the overall tectonic evolution the strain pattern in the model can be closely tied to the previously recognized structural regimes from east to west r1 little deformed precambrian basement stiff orogenic lid basal part of upper crust r2 heterogeneously sheared basement simple shear domain at upper crust lower crust boundary r3 completely overprinted basement with eclogitic remnants pure shear regime in the lower crust of the rebounded root subsequent to root rebound the profile remained practically unaffected by later tectonic events except for tilting uplift erosion providing a unique example of deep seated gravity tectonics