extension during continental convergence with application to the tibetan plateau the tibetan plateau is the product of crustal thickening caused by the collision between india and asia and is the largest active example of extensional tectonics in a zone of continental collision throughout most of the tertiary the tectonics of the plateau were dominated by north south shortening a significant proportion of which took place on east west striking thrust faults for the last 5 m y or so the plateau has been thinning by the mechanism of extension on north south trending normal faults previous investigations of the collision have been able to account for the large scale features of the tertiary deformation but have failed to explain the transition in the late tertiary to quaternary strain rate field of the plateau from north south compression to east west extension the transition could in principle be effected by a reduction in the rate of convergence between india and asia or by a uniform reduction in strength of the whole continental lithosphere of asia it could not however be effected by a reduction in strength of the elevated region alone this produces increased compressional strain rates in the weakened zone an alternative explanation for the transition to extension comes from considering the thermal evolution of thickened continental lithosphere the lower part of the lithosphere consists of a thermal boundary layer which when thickened by horizontal shortening is colder and denser than its surroundings convective instability of the thickened thermal boundary layer and its replacement by hot asthenosphere would rapidly raise the surface elevation and gravitational potential energy of the overlying part of the lithosphere the convective instability would happen in a time brief compared with the collision time scale 50 m y in the case of india and asia but would only occur after there had already been substantial thickening of the lithosphere numerical experiments show that for a range of lithospheric parameters the increase in surface height as much as 2 km and of potential energy 5 to 10 1012 n m 1 resulting from convective instability of the lower lithosphere are sufficient for east west extension to replace north south compression as the dominant feature of the stress field of the tibetan plateau

response of daily and annual shell growth patterns of the intertidal bivalve phacosoma japonicum to holocene coastal climate change in japan abstract it is well known that the latitudinal distribution of shallow marine mollusks around the japanese islands has changed episodically during the holocene in response to climate change however few studies have investigated the biotic response of individual species to holocene climate change at daily to annual scales of temporal resolution holocene shells of the venerid phacosoma japonicum recovered from various sites in japan preserve lunar daily and annual growth increments in the outer shell layer and hinge plate as also observed in modern shells sclerochronological analysis of 29 shells with known 14 c ages revealed that the life history traits e g length of the growth period and the mean lunar daily growth rate at a given age have changed markedly over the past 8000 years the intra annual patterns of lunar daily increments and shell oxygen isotope records of the fossil shells can be compared with those of modern specimens from various locations which are constrained by annual patterns of seawater temperature and summer autumn precipitation rates furthermore the temporal trends recorded in the fossil shells show a strong correlation with climate change in mid latitude parts of east asia over the past 8000 years indicating that p japonicum has changed its life history traits in response to holocene terrestrial and coastal climate change

slab detachment during continental collision influence of crustal rheology and interaction with lithospheric delamination abstract collision between continents can lead to the subduction of continental material if the crust remains coupled to the downgoing slab a large buoyancy force is generated this force slows down convergence and promotes slab detachment if the crust resists to subduction it may decouple from the downgoing slab and be subjected to buoyant extrusion we employ two dimensional thermo mechanical modelling to study the importance of crustal rheology on the evolution of subduction collision systems we propose simple quantifications of the mechanical decoupling between lithospheric levels and the potential for buoyant extrusion of the crust the modelling results indicate that a variable crustal rheological structure results in slab detachment delamination or the combination of both mechanisms a strong crust provides coupling at the moho low and remains coherent during subduction low it promotes deep subduction of the crust 180 km and slab detachment exhumation occurs in coherent manners via eduction and thrusting slab detachment triggers the development of topography 4 5 km close to the suture a contrasting style of collision occurs using a weak crustal rheology mechanical decoupling at the moho high promotes the extrusion of the crust high disabling slab detachment ongoing shortening leads to buckling of the crust and development of topography on the lower plate collisions involving rheologically layered crust allow decoupling at mid crustal depths this structure favours both the extrusion of upper crust and the subduction of the lower crust such collisions are successively affected by delamination and slab detachment topography develops together with the buoyant extrusion of crust onto the foreland and is further amplified by slab detachment our results suggest that the occurrence of both delamination apennines and slab detachment himalayas in orogens may indicate differences in the initial crustal structure of subducting continental plates in these regions

size and exhumation rate of ultrahigh pressure terranes linked to orogenic stage abstract a growing set of data indicates a stark contrast between the evolution of two types of ultrahigh pressure uhp terranes large terranes that evolved slowly over 10 30 myr and small terranes that formed and were exhumed on timescales of

lithospheric deformation during the early stages of continental collision numerical experiments and comparison with south island new zealand 1 the nature of lithospheric deformation during continental plate collision still remains unresolved while it has often been proposed that the mantle lithosphere is accommodated by distributed thickening of a viscous root an alternate hypothesis suggests that significant portions of convergent mantle lithosphere essentially undergo underthrusting or subduction to further consider this issue we model the thermochemical evolution of the lithosphere mantle system using arbitrary lagrangian eulerian finite element techniques we incorporate a mix of viscous thermally activated power law creep and plastic frictional coulomb with strain softening rheologies in the numerical experiments to treat disparate composition in the crust and mantle a range of rheological and mechanical parameters is explored to determine controls on the style of lithospheric deformation the models suggest that during the initial stages of plate collision the mantle lithosphere is characterized by plate like behavior and underthrusting subduction of the upper region in conjunction with distributed thickening and rayleigh taylor type viscous instability of the lower portion depending on the material rheology temperature regime and imposed convergence velocity the deforming mantle lithosphere demonstrates various combinations of these end member behavioral modes the modeling results are interpreted in the context of observed lithospheric deformation across south island new zealand a combined style of underthrusting and distributed thickening is consistent with the observed crustal structure in the young collisional orogen as well as seismic imaging of the geometry of the underlying lithosphere

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

the surface tectonics of mantle lithosphere delamination following ocean lithosphere subduction insights from physical scaled analogue experiments many postulated lithospheric removal events occur in regions with an earlier history of subduction but the relationship between the two processes has not been explored in this work we use physical scaled analogue experiments to investigate the evolution from ocean lithosphere subduction to collision and possible delamination of the mantle lithosphere from the crust we test how varying the magnitude of plate convergence alters the behavior of the subduction delamination model our experiments show that a retreating ocean proplate can evolve to continental mantle lithosphere delamination negative surface topography is supported at the delamination hinge and this migrates back with the peeling lithosphere with high plate convergence delamination is suppressed rather the crust and mantle lithosphere split at the collision zone in a form of flake tectonics as oncoming procrust is accreted on top of the retroplate and the promantle lithosphere subducts below localized high topography develops at this zone of crustal accretion and thickening the results suggest that delamination may be a continental continuation of plate retreat and that lithospheric removal is triggered by the transition from one process to another

hp uhp exhumation during slow continental subduction self consistent thermodynamically and thermomechanically coupled model with application to the western alps abstract the exhumation of ultra high pressure uhp crustal material often occurs at the rear of sedimentary accretion wedges e g alps himalaya norway however the mechanisms of deep 100 km burial in slow 10 mm yr 1 and slow at the later stage

a flow law for dislocation creep of quartz aggregates determined with the molten salt cell we have used the molten salt cell to conduct an experimental study on the rheology of a natural quartzite containing 0 15 wt water co axial deformation experiments were conducted at constant piston displacement rates approximating constant strain rates at low strain the strengths of our natural quartzite measured in the molten salt cell are approximately half those measured at the same conditions in solid media because unlike solid confining media molten salt does not contribute to the strength of the sample it reduces the friction on the moving piston and it allows clear identification of the hit point we have limited the experimental conditions to those required for dislocation creep and have used only steady state flow stresses measured during climb accommodated dislocation creep to calculate the flow law parameters two flow laws were determined one for samples containing minor amounts of melt 1 2 and one for melt free samples in both cases the power law stress exponent n is 4 0 0 9 which is greater than that previously reported in flow laws for dislocation creep of quartz aggregates determined in solid media the activation energy q is 137 34 kj mol 1 for samples with melt and 223 56 kj mol 1 for those without within the range of previously determined values for quartz aggregates containing 0 1 wt water the pre exponential term a is 1 1 10 4 2 mpa ns 1 for samples without melt and 1 8 10 8 2 mpa ns 1 for those with melt the lower strengths measured in the molten salt cell indicate that previous piezometer relations for quartz experimentally determined in solid media are not correct extrapolation of the flow law for melt free aggregates to natural strain rates predicts higher strengths than most previous quartz flow laws however accurate extrapolation requires determining the dependence of flow stress on fh2o and or ah

near surface diagnostics of dripping or delaminating lithosphere 1 in various geological regions it has been postulated that the mantle lithosphere has been thinned or completely removed two of the primary removal mechanisms that have been put forward include 1 delamination a wholesale peeling away of a coherent block of the mantle lithosphere and 2 lithospheric dripping viscous rayleigh taylor instability of the mantle lithosphere using computational models we investigate several near surface observables to determine if these may be diagnostic of either often ambiguous removal mechanism surface topography associated with delamination has a broad region of uplift above the lithospheric gap and a localized and mobile zone of subsidence at the delaminating hinge with dripping lithosphere the topographic expression is symmetric and fixed above the downwelling delamination of mantle lithosphere is more efficient than dripping for thermal heating of the crust the onset is more rapid and the elevated temperatures persist longer the resultant crustal p t t paths show modest pressure variations and high temperature increases with large scale delamination or dripping delamination also causes contraction directly above the migrating hinge and distal extension dripping lithosphere induces superimposed contraction and extension above and symmetric about the viscous instability for all the observables if only a portion of the mantle lithosphere is removed by viscous instability delamination inherently removes all of the mantle lithosphere the differences between the two removal mechanisms are even more pronounced with only partial removal of the mantle lithosphere uppermost mantle lithosphere remains well coupled to the crust leading to lower surface temperature variations and broad zones of crustal deformation thickening

coupled crust mantle dynamics and intraplate tectonics two dimensional numerical and three dimensional analogue modeling 1 tectonic deformation of some intraplate regions may be caused by the rayleigh taylor rt instability of dense subcrustal lithosphere mantle lithosphere as it descends into the mantle we report on a series of scaled three dimensional 3 d analogue and 2 d numerical experiments of coupled crust mantle dynamics that study such a process the models investigate the effects of two geometries of rt instability linear versus axisymmetric and use different rheological stratifications of the crust the growth of the rt instability is strongly influenced by crustal rheology while rt growth rates are not greatly influenced by the geometry of the mantle lithosphere instability itself the surface is experiments having a viscous only crust are characterized by significant crustal contraction and thickening above the mantle downwelling and extension and thinning in adjacent regions in experiments with a brittle upper crust the surface deformation is much more subdued however strong mantle flow induced deformation still occurs in the ductile lower crust beneath the quiescent brittle upper crust when the lower crust is relatively strong localized styles of extensional and contractional structure develop in the brittle upper crust owing to the greater degree of coupling between upper crust and the mantle instability in all the experiments a portion of the ductile lower crustal material is entrained within the downgoing rt instability deep into the mantle the interplay between dynamic topography and crustal thickening thinning induced by the underlying mantle flow governs various time dependent phases of subsidence and uplift of the model surface the experiments help to account for a number of first order tectonic behaviors of the lithospheres of earth and other terrestrial planets for example intraplate orogens and basins complex localized tectonic structure in intraplate settings and deep crustal seismic fabrics may arise as the variable crustal response to underlying mantle lithosphere rt instabilities

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

tectonic overpressure and underpressure in lithospheric tectonics and metamorphism the lithostatic pressure concept is most commonly applied on a geological scale for lithospheric processes and related evolution of metamorphic rock complexes here various aspects of non lithostatic overpressure and underpressure phenomena in lithospheric tectonics and metamorphism are reviewed on the basis of recently published literature the main conclusion from this short review is that these phenomena certainly exist in nature on all time and space scales including geological ones they are in particular responsible for some geological processes which are otherwise difficult to explain such as downward water suction into the interior of subducting slabs magnitudes of overpressure and underpressure are strongly variable and may potentially reach up to 100 of the lithostatic pressure and up to a gpa level these magnitudes depend mainly on the rheology of deforming rocks and on the nature of related tectonic process rheological heterogeneity of deforming rock units which is common in nature has a tendency to enhance overpressure and underpressures large overpressure can typically be expected in rheologically strong dry bending rock units in particular in the mantle lithosphere however rheological weakness of rocks and small local deviatoric stresses do not guarantee the absence of large overpressures in these rocks therefore the influence of significant tectonic overpressure and or underpressure cannot be excluded for any metamorphic complex a priori but should be instead tested by exploring realistic thermomechanical models for envisaged tectono metamorphic scenarios many lithospheric rocks subjected to large overpressures and underpressures cannot be studied as they do not exhume to the surface some controversy exists concerning overpressure magnitudes for the ultrahigh pressure uhp rocks and several conflicting hypotheses are proposed which need to be thoroughly tested in the future in this respect the alpine region may offer a unique opportunity for the testing of geological scale overpressures in u hp rocks by combining structural geological and petrological data with realistic lithospheric scale numerical modelling

ablative subduction a two sided alternative to the conventional subduction model subduction is conventionally modelled as a convergence and overlapping of two semirigid plates such a convergence is one sided in that one lithospheric plate descends while the other remains on the surface we suggest an alternative model one based on the dynamics of fluids in this model the viscous lower lithosphere flows downward and the brittle upper lithosphere deforms in passive response this process is potentially double sided for we find that even a buoyant plate can be dragged downward by a dense descending neighbor thus an apparent overriding plate may be worn away by a process of viscous ablation with the rate of ablation a function of plate buoyancy we call this process ablative subduction ablative subduction allows us to simply interpret observations concerning slab profiles interplate seismicity back arc tectonics and complex processes such as double subduction and subduction polarity reversal in performing experiments modelling the evolution of simple fluid slabs we find that slab profile is strongly influenced by ablation in the overriding plate when ablation is weak as when a buoyant continent borders the trench deformable slabs adopt shallow andean style profiles these profiles develop over time from an initially steep shape more vigorous ablation rates as might occur in an ocean ocean convergence yield steeper marianas style profiles thus differences between marianas style and andean style slabs may result from differences in ablation rate and subduction duration the occurrence of ablation might not be easily detected at depth because material from subducting and ablating plates adhere closely as a single slab these slabs show downdip deformations that are consistent with seismic focal mechanisms plate deformations associated with ablative subduction are also consistent with observed patterns of seismicity in particular the relative aseismicity of marianas style plate boundaries is consistent with the ablative model we may thus explain how plates can converge aseismically without requiring that these plates be decoupled ablation should be weakest in andean style subduction although pulses of rapid ablation might lead to episodes of crustal shortening in oceanic regions the manifestation of ablation may depend upon tectonic regime in an extensional area like the marianas ablation may be temporally self limiting and ablative cycles may explain conflicting observations of tectonic erosion and accretion in a compressional regime vigorous ablation might lead to observations of double subduction and subduction reversal in each case the behavior of the lower lithosphere is relatively simple what varies is the rate of ablation and the response of the upper lithosphere

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

tree ring reconstructions of cool season temperature for far southeastern australia 1731 2007 at the global scale reconstructions of cool season temperature over past centuries are relatively rare here we present 277 year reconstructions of cool season july october temperatures for southern australia based on three different data sets a spatial field reconstruction based on highly resolved temperature data from the australian water availability product data reconstructions for the four southeast australian states based on the berkeley earth mean temperature data for each state and reconstructions for individual stations in southeastern australia from the australian bureau of meteorology s australian climate observations reference network surface air temperature data our reconstructions typically capture 25 50 of the variation over the late twentieth century calibration period and are strongest for the southern state of tasmania and the southeastern part of mainland australia all three use tasmanian tree rings sensitive to cool season temperatures and display similar variability in the context of our reconstructions the persistent warming in the observed record since 1950 is unprecedented while the low frequency variability of winter temperatures is generally in step with that in summer december february temperatures high frequency variability is not illustrating the need for seasonal reconstructions to help improve understanding of variability in inter seasonal dynamics and the historical importance of this on the environment the reconstructions covary with changes in the southern annular mode and may be useful for future reconstructions of this phenomenon

exhumation of high pressure rocks driven by slab rollback rocks metamorphosed under high pressure hp and ultra high pressure uhp conditions in subduction zones come back to the surface relatively soon after their burial and at rates comparable to plate boundary velocities in the mediterranean realm their occurrence in several belts related to a single subduction event shows that the burial exhumation cycle is a recurrent transient process using the calabria apennine and aegean belts as examples we show that the exhumation of hp rocks is associated in time and space with the subduction of small continental lithosphere blocks that triggers slab rollback creating the necessary space for the exhumation of the buoyant continental crust that was deeply buried just before the buoyancy force of the subducted crust increases until this crust detaches from the downgoing slab it then exhumes at a rate that depends directly on the velocity of trench retreat to become part of the overriding plate heated from below by the asthenosphere that flows into the opening mantle wedge the exhumed crust weakens and undergoes core complex type extension responsible for a second stage of exhumation at a lower rate the full sequence of events that characterizes this model crust mantle delamination slab rollback and trench retreat hp rock exhumation asthenosphere heating and core complex formation arises entirely from the initial condition imposed by the subduction of a small continental block no specific condition is required regarding the rheology and erosion rate of hp rocks the burial exhumation cycle is transient and can recur every time a small continental block is subducted

mantle lithosphere delamination driving plateau uplift and synconvergent extension in eastern anatolia eastern anatolia is the site of lithospheric thinning plateau uplift heating and synconvergent extension using numerical geodynamic experiments we test the hypothesis that these tectonic anomalies are all related and the consequence of delamination of the mantle lithosphere our findings indicate that delamination during plate convergence results in 2 km high plateau uplift the removal of mantle lithosphere induces distinct regions of contraction and thickening as well as extension and thinning of the crust the latter occurs even within a regime of plate shortening although it is muted with increasing plate convergence detachment of the delaminating slab results in minor surface topographic perturbation but only above the delamination hinge the plateau uplift and pattern of surface contraction and or extension are consistent with a topographic profile at 42 e and geologically interpreted zone of synconvergent extension at eastern anatolia

mechanisms of continental subduction and exhumation of hp and uhp rocks abstract we discuss possible scenarios of continental collision and their relation to mechanisms of exhumation of hp and uhp rocks inferred from thermo mechanical numerical models accounting for thermo rheological complexity of the continental lithosphere due to this complexity mechanisms of continental convergence are versatile and different in many aspects from those that control oceanic subduction elucidating these mechanisms from conventional observations is difficult and requires additional constraints such as those derived from petrological data indeed exhumation of hp uhp rocks is an integral part of convergent processes and burial exhumation dynamics inferred from metamorphic p t t paths provides strong constraints on the collision scenarios metamorphic rocks also play an active role due to their contrasting physical properties rheology density fluid transport capacity numerical thermo mechanical experiments suggest that hp uhp exhumation can only be produced in subduction contexts as well as that long lasting 10 myr continental subduction can only occur in case of cold strong lithospheres t moho t e 50 km and of relatively high convergence rates 3 5 cm yr 1 in this case high density uhp material in the crustal part of subduction interface provides additional pull on the slab and is not always exhumed to the surface in case of slower convergence and or weaker lithosphere t e 1 is generally sufficient to drive continental subduction during the first several myr of convergence but pertinent subduction requires faster convergence rates 3 5 cm yr 1 we suggest that most continental orogenic belts could have started their formation from continental subduction but this process has been generally limited in time

slab breakoff a model for syncollisional magmatism and tectonics in the alps slab breakoff is the buoyancy driven detachment of subducted oceanic lithosphere from the light continental lithosphere that follows it during continental collision in a recent paper davies and von blanckenburg 1994 have assessed the physical conditions leading to breakoff by quantitative thermomechanical modeling and have predicted various consequences in the evolution of mountain belts breakoff will lead to heating of the overriding lithospheric mantle by upwelling asthenosphere melting of its enriched layers and thus to bimodal magmatism breakoff will also lead to thermal weakening of the subducted crustal lithosphere thereby allowing buoyant rise of released crustal slices from mantle depths in this paper we present a test of this model in the tertiary evolution of the european alps in the alps both basaltic and granitoid magmatism occur between 42 and 25 ma following the closure of oceanic basins by subduction and continental collision the granitoids are now well established to result from mixing of basalt with assimilated continental crust to identify the tectonically crucial origin of the partial mantle melts we have compiled all published geochemical and isotopic data of numerous mafic dykes occurring throughout the whole alpine arc their trace element and isotopic composition suggests that they have been formed by low degree melting of the mechanically stable lithospheric mantle we see no evidence for melting of asthenospheric mantle it was thus not decompressed to depths shallower than 50 km once initiated rapid lateral migration of slab breakoff will result in a linear trace of magmatism in locally thermal weakened crust this explains why all alpine magmatic rocks intruded almost synchronously along a strike slip fault the periadriatic lineament a compilation of ages from penninic high pressure rocks subducted to depths of up to 100 km shows that subduction took place at circa 55 40 ma followed by uplift at 40 35 ma from the short time interval between their uplift and the onset of magmatism we infer that both processes have been induced by the breakoff the slab breakoff model fulfills its predictions in the case of the alps and therefore supports the assumptions made in the theoretical model on a geological basis we believe that the characteristic association of magmatic activity with the return of high pressure rocks to the surface allows the identification of this process in the earth s mountain belts

thermobaric structure of a traverse across western dabieshan implications for collision tectonics between the sino korean and yangtze cratons as the best preserved high and ultrahigh pressure hp and uhp metamorphic terrane in the qinling dabieshan sulu orogen western dabieshan is divided into six lithotectonic units along a traverse across the orogen i e from north to south the nanwan balifan huwan xinxian hong an and mulanshan units in this terrane five eclogite bearing zones i v are developed the garnet and clinopyroxene in eclogites from these zones exhibit chemical zoning suggesting that the rims record general peak temperature and pressure thermobarometric study indicates that the peak p t conditions of eclogite are 550 570 c and 21 kbar for zone i 470 500 c and 14 17 kbar for zone ii 620 670 c and 26 29 kbar for zone iii 530 560 c and 20 22 kbar for zone iv and 490 510 c and 19 20 kbar for zone v the symmetrical thermobaric pattern in conjunction with structural and geochronological data demonstrates that the huwan and hong an units belong to the same hp slice overlying the uhp slice this pattern together with the mulanshan lt hp blueschist greenschist belt in the south roughly constitutes a normal metamorphic zonation however clear metamorphic gaps occur between different slices it is inferred that the lt hp hp and uhp slices were broken up from the downgoing slab during subduction and reached different depths along different geothermal gradients the successive subduction of underlying slices leads to a nearly concomitant uplift of overlying slices whereas exhumation of the deepest uhp slice was effected by underthrusting of the lower crust of the yangtze craton

deep subduction and rapid exhumation role of crustal strength and strain weakening in continental subduction and ultrahigh pressure rock exhumation 1 the exhumation of crustal ultra high pressure uhp material depends on temporal and spatial variations in its detachment within the subduction channel this dependence is investigated using numerical models with variable initial crustal strengths representing a range of initial crustal compositions and parameterized strain weakening representing a range of processes that reduce effective crustal viscosity during deformation competition between down channel shear traction favoring subduction and up channel buoyancy favoring exhumation is expressed as the exhumation number e which can vary with time and position along the channel exhumed lower strength crust which resists subduction owing to weak down channel traction records peak conditions 38 kbar given sufficient strain weakening exhumation proceeds at 60 km ma 1 indicating that buoyancy e 1 drives exhumation in these models in all models exhuming uhp material forms a deforming ductile plume with a range of possible structural relationships predicted between exhumed uhp and hp materials

geometry of eclogite facies structural features implications for production and exhumation of ultrahigh pressure and high pressure rocks western gneiss region norway 1 well preserved eclogite facies structural features in metamorphosed gabbro and diorite gneiss in baltica crust formed at a minimum depth of 60 70 km during the scandian orogeny allow direct inferences to be made regarding the geometry of folds kinematics and original structural orientations related to production and exhumation of high pressure rocks folds associated with eclogite facies fabrics are isoclinal to tubular with axes parallel to the trend of a stretching lineation strain estimates and l s or l s fabrics indicate that these structures were formed in a constrictional strain field locally they are well preserved in eclogite facies mylonite zones at least 40 m thick that cut proterozoic gabbro and adjacent gneiss where steeply dipping they show a north side up shear sense across a younger northeast trending shallowly plunging amphibolite facies anticlinorium formed in a constrictional noncoaxial strain field where original structural facing direction can be inferred small circle rotation of the eclogite facies lineation about the anticline axis indicates that the relative motion vector of baltica with respect to overlying imbricated crust and laurentia was oriented 320 in a present day reference frame this result is identical to the orientation of the relative motion vector estimated from paleomagnetic plate reconstructions and consistent with late silurian to early devonian oblique sinistral convergence between baltica and laurentia results of this study indicate that coherent baltica crust at 60 70 km depth experienced subvertical and horizontal shortening while extending toward the foreland during tops southeast thrusting parallel to plate motion high pressure rocks were progressively imbricated and stacked along eclogite facies mylonite zones with the deforming zone separating coherent baltica crust from imbricated basement above the constrictional strains under eclogite facies conditions are interpreted to have resulted from stretching associated with sinking of the cool dense mantle lithosphere

climatic impact of the a d 1783 asama japan eruption was minimal evidence from the gisp2 ice core abstract assessing the climatic impact of the a d 1783 eruption of mt asama japan is complicated by the concur rent eruption of laki iceland estimates of the stratospheric loading of h2so 4 for the a d 1108 eruption of asama derived from the so42 time series in the gisp2 greenland ice core in dicate a loading of about 10 4 tg h2so 4 with a resulting stratospheric optical depth of 0 087 assuming sulfur emis sions from the 1783 eruption were only one third of the 1108 event yields a h2so 4 loading value of 3 5 tg and a strato spheric optical depth of only 0 029 these results suggest minimal climatic effects in the northern hemisphere from the 1783 asama eruption thus any volcanically induced cooling in the mid 1780s is probably due to the laki eruption introduction asama volcano 36 5on 138 9oe honshu japan has erupted many times over the last 1000 years aramaki 1956 1957 1963 but only two of these events have been large enough to definitely warrant a volcanic explosivity index

continental plate collision p t t z conditions and unstable vs stable plate dynamics insights from thermo mechanical modelling there was much debate recently on the mechanisms of continental convergence and related pressure temperature p t conditions both in modeling and petrologic community depending on the mechanisms of convergence subduction collision folding or rt instability one can argue about the possibility of large scale deviations of pressure and temperature in the accretion prism and below it from reference i e lithostatic conditions commonly used for petrologic reconstructions of evolution of exhumed metamorphic rocks these deviations can be caused for example by tectonic overpressure that as suggested in some studies may be responsible for formation of uhp ultra high pressure rocks however overpressure in exhumation zones can be built only in specific collision scenarios associated with plate coupling pure shear folding in this study we analyze conditions that define various mechanisms of convergence and consequently of exhumation these mechanisms can be represented as a superposition of 1 simple shear subduction 2 pure shear collision 3 folding and 4 rayleigh taylor instability we study these scenarios using a thermo mechanical model that accounts for brittle elastic ductile rheology surface processes and metamorphic changes it appears that stable oceanic type subduction may occur in the case of cold lithospheres tmohob550 c and relatively high convergence rates n3 5 cm yr depending on the lower crustal rheology strong or weak either the whole upper and lower crust or only the lower crust can be involved in subduction in case of weak metamorphic rheologies phase changes improve chances for stable subduction pure shear becomes a dominant mechanism when tmohon550 c or convergence rates are lower than 3 cm yr large scale folding is favored in case of tmoho 500 650 c and is more effective in the case of mechanical coupling between crust and mantle e g strong lower crust gravitational rayleigh taylor instabilities overcome other mechanisms for very high values of tmoho n800 c and may lead to development of subvertical cold spots however it is reasonable to assume that in most cases continental collision is initiated at oceanic subduction rate which is rarely slower than 5 cm yr this rate is sufficient to drive continental subduction during the first several myr of collision in this case the subduction channel is characterized by nearly lithostatic pressure conditions large scale zones of tectonic overpressure may be built outside the channel but do not affect the exhumed rocks overpressure may be built inside the channel in the short moment of its closure we suggest that most continental orogenic belts could have started their formation from continental subduction this evokes small tectonic overpressures and thus deep origin of the uhp rocks that may be brought to the surface via the suggested multi level mechanism of exhumation

influence of viscosity pressure dependence on deep lithospheric tectonics during continental collision 1 previous geodynamic models of continental collision show that the behavior of the continental lithosphere is strongly influenced by its rheology we build on previous work by quantitatively investigating with numerical experiments the influence of the pressure dependence of viscosity on the process of tectonic deformation during collision the models demonstrate how the inclusion of viscosity pressure dependence can quite substantially alter the style of continental mantle lithosphere deformation at low activation volumes high convergence rates and low to moderate initial moho temperatures the subduction style of mantle lithosphere deformation is dominant increasing the activation volume of mantle material allows the subduction style of deformation to occur at all convergence rates studied in the experiments at the expense of the subduction drip and ablative drip styles of deformation at low activation volumes high convergence rates and high initial moho temperatures the distributed pure shear style of deformation occurs with these same conditions increasing the activation volume of mantle material produces an ablative subduction style of mantle lithosphere deformation at low activation volumes low convergence rate and moderate to high initial moho temperatures the mantle lithosphere prefers a convective removal style of deformation increasing the activation volume here yields an ablative drip and distributed pure shear styles of deformation the results demonstrate that inclusion of the pressure dependence of viscosity quite often neglected in lithosphere scale geodynamic models can be significant in modulating deformation of the deforming lithosphere

paradigms new and old for ultrahigh pressure tectonism regional ultrahigh pressure uhp metamorphic terranes exhibit a spectrum of lithological structural and petrological characteristics that result from the geodynamic processes that formed and exhumed them at least six geodynamic processes can be envisioned to have carried continental rocks to mantle depths i continental margin subduction ii microcontinent subduction iii sediment subduction iv intracontinental subduction v subduction erosion and vi foundering of a crustal root most of these processes have been investigated through numerical or analog models and most have been invoked for one or more specific occurrences of uhp rocks at least six geodynamic processes can be envisioned to have exhumed uhp continental rocks i eduction ii microplate rotation iii crustal stacking iv slab rollback v channel flow and vi trans mantle diapirs most of these processes have also been investigated through numerical or analog models and all have been invoked to explain the exhumation of at least one uhp terrane more detailed and systematic field investigations are warranted to assess the predictions of numerical models and more sophisticated and realistic numerical models are required to replicate and explain the petrological structural and chronological data obtained from uhp terranes

rifting and subsidence following lithospheric removal in continental back arcs it has been suggested that post orogenic lithospheric removal in continental back arcs promotes extension and surface subsidence however the surface response of this process and its primary difference from classical back arc opening have remained uncertain here the back arc extension process with varying continental mantle lithosphere thickness and thermal heterogeneities is studied by using thermomechanical subduction experiments the experiments illustrate that models with only slab retreat result in minor surface subsidence and extension in the back arc region alternatively there is notable extension due to the slab retreat and a localized high temperature zone in the back arc with uniform lithospheric thickness models with advecting mantle after lithospheric removal in the extending back arc predict rifting stretching factor b 2 and surface subsidence 1 5 km in the center of the basin the results of this work suggest that lithospheric removal may be an important trigger for continental back arc development rather than slab retreat alone causing lithospheric extension and subsidence the findings help explain rift formation and subsidence in the aegean sea west anatolia and possibly other mediterranean back arcs such as the alboran sea and the pannonian basin

tectonics of an ultrahigh pressure metamorphic terrane the dabie shan tongbai shan orogen china ultrahigh pressure metamorphic rocks with coesite and diamond form a tectonic slice over 20 km thick called the eclogite zone within the dabie shan complex in the qinling orogen in central china the orogen separates the sino korean block in the north from the yangtze block in the south the dabie shan complex is a composite terrane made up of eclogite facies and amphibolite facies gneiss slices and represents fragments of the lower continental crust of the yangtze block the dabie shan complex is bounded in the south by a triassic foreland fold thrust belt and in the north by a greenschist facies metaclastic unit the foziling group which probably represents the passive continental apron deposits of the yangtze block farther north is a granulite facies gneiss complex the qinling group which has ultramafic slivers and includes the remnants of an island arc with two bounding suture zones north of the qinling group are early paleozoic active margin deposits of the sino korean block the eclogite zone in the dabie shan complex is sandwiched between amphibolite facies gneiss slices dating by sm nd rb sr and ar ar of two eclogite samples from the eclogite zone gives early to middle triassic ages 236 246 ma the initial end values indicate reworking of a 2 11 and 1 55 ga continental crust a himalayan type tectonic evolution is envisaged for the qinling orogen with the creation of a 100 km thick crustal thrust wedge through continuous underplating during the subduction of the yangtze continental lithosphere exhumation of the ultrahigh pressure metamorphic rocks was chiefly achieved by the southward propagation of the thrust planes thereby isostatically uplifting and eroding the earlier deeply subducted parts of the orogen a total of 680 km of southward thrusting in front of dabie shan is inferred based on the abrupt termination of the tanlu fault normal faulting possibly caused by gravitational collapse probably also had a role in the exhumation process

metamorphic record of catastrophic pressure drops in subduction zones when deeply buried in subduction zones rocks undergo mineral transformations that record the increase of pressure and temperature the fact that high pressure metamorphic parageneses are found at the earth s surface proves that rock burial is followed by exhumation here we use analysis of available data sets from high pressure metamorphic rocks worldwide to show that the peak pressure is proportional to the subsequent decompression occurring during the initial stage of retrogression we propose using a simple mechanical analysis that this linear relationship can be explained by the transition from burial related compression to extension at the onset of exhumation this major switch in orientation and magnitude of principal tectonic stresses leads to a catastrophic pressure drop prior to actual rock ascent therefore peak pressures are not necessarily as commonly believed directly dependent on the maximum burial depth but can also reflect a change of tectonic regime our results which are in agreement with natural data have significant implications for rock rheology subduction zone seismicity and the magnitudes of tectonic pressures sustained by rocks current views of subduction dynamics could be reconsidered in that perspective rocks are altered by high pressure during subduction analysis of exhumed metamorphic rocks suggests that the peak pressures recorded within minerals mark a change in tectonic regime within a subduction zone rather than burial depth

alpine high p low t metamorphism of the afyon zone and implications for the metamorphic evolution of western anatolia turkey abstract carpholite chloritoid pyrophyllite association occurs widely in the triassic metaclastic rocks of the afyon zone in west central turkey fe mg carpholite is associated with rare aragonite pseudomorphs and glaucophane in marbles and metabasites respectively the afyon zone consists stratigraphically of a pan african basement and an overlying mesozoic cover sequence the pan african basement which shows barrovian type amphibolite facies metamorphism comprises garnet mica schists intruded by sodic amphibole bearing metagabbros and leucocratic metagranites it is unconformably overlain by a continuous metasedimentary sequence extending from triassic to early palaeocene this cover sequence begins with metaconglomerates which pass upwards into phyllites fe mg carpholite occurs within this metaclastic sequence as rosette like crystals in metapelites and fibres in quartz segregations the metaclastic rocks are succeeded by metamorphosed platform carbonates grading into latest mesozoic metamorphosed pelagic limestones which in turn progress up to a late mesozoic early tertiary olistostrome this sequence is tectonically overlain by the blueschists of the tavsanli zone fe mg carpholite bearing assemblages imply temperatures of about 350 c and minimum pressures of 6 9 kbar corresponding to burial depths of about 30 km for the mesozoic passive continental margin sediments and the underlying pan african supracrustal metasediments and metaintrusives the metamorphic rocks of the afyon zone are unconformably overlain by upper palaeocene lower eocene sedimentary rocks indicating a paleocene age for the regional hp lt metamorphism this implies continuous younging of hp lt metamorphism in the anatolides related to northward subduction of the anatolide tauride platform beneath the sakarya zone from north to south this involved the tavsanli zone campanian 80 5 ma the afyon zone palaeocene the menderes massif middle eocene and the lycian nappes late cretaceous eocene all of which were probably derived from the frontal part of the anatolide tauride platform

postcollisional lithospheric evolution of the southeast carpathians comparison of geodynamical models and observations seismic evidence and thermal and topographic transients have led to the interpretation of lithospheric removal beneath the southeast carpathians region a series of numerical geodynamic experiments in the context of the tectonic evolution of the region are conducted to test the surface crustal response to lithosphere delamination and slab break off the results show that a delamination type removal plate like migrating instability causes a characteristic pattern of surface uplift subsidence and crustal extension shortening to occur due to the lithospheric deformation and dynamic thermal forcing of the sublithospheric mantle these features migrate with the progressive removal of the underlying lithosphere model results for delamination are comparable with observables related to the geodynamic evolution of the southeast carpathians since 10 ma the mantle structure inferred by seismic tomography migrating patterns of uplift 1 5 km and subsidence 2 km in the region crustal thinning in the carpathian hinterland and thickening at the focsani depression and regional extension in the carpathian corner e g opening of brasov basin correlating with volcanism e g harghita and persani volcanics in the last 3 myr

a comparison of the evolution of diamondiferous quartz rich rocks from the saxonian erzgebirge and the kokchetav massif are so called diamondiferous gneisses magmatic rocks abstract the petrography and chemical composition of minerals of quartz rich diamondiferous rocks from the kokchetav massif especially the zonation of garnet were studied and compared with diamondiferous quartzofeldspathic rocks from the saxonian erzgebirge many compositional and textural features were found to be similar for instance microdiamonds are enclosed systematically in a specific intermediate growth zone of garnet in these rocks on the basis of experimental data a magmatic scenario was constructed to check if the quartz rich diamondiferous rocks are of magmatic origin by this the p t paths derived here for the kokchetav rocks and the textural observations it is concluded that the minerals of the diamondiferous rocks have crystallized from silicate melts these melts originated by anatexis of deeply submerged metasediments erzgebirge at t as high as 1200 c kokchetav massif at 50 100 c lower t and ascended from at least 200 km depth relics of the pre anatectic evolution are still present for instance as garnet cores after ascent and emplacement of the magma in deep portions of thickened continental crust kokchetav massif 45 50 km close to 800 c erzgebirge 55 60 km at 30 50 c lower t considerable quantities of white and or dark micas formed by peritectic reactions from melt for instance garnets could be resorbed at this stage and biotite grew instead after the magmatic stage retrogression took place much stronger in the kokchetav massif this was accompanied by deformation transforming broadly the magmatic texture of quartz rich diamondiferous rocks from the kokchetav massif to a gneissic texture

three centuries of winter temperature change on the southeastern tibetan plateau and its relationship with the atlantic multidecadal oscillation long term high resolution proxy records containing cold season temperature signals are scarce on the southeastern tibetan plateau tp limiting our understanding of regional climate and the potential driving forces in this study we present a nearly three centuries long reconstruction of winter december february mean temperature for the central hengduan mountains southeastern tp the reconstruction is derived from a composite tree ring width chronology of pinus yunnanensis franch from two high elevation sites 3000 m above sea level our reconstruction passes all standard calibration verification schemes and explains nearly 73 of the variance of the original instrumental data however we were constrained to calibrate our full period 1718 2013 reconstruction of december february mean temperature on the calibration period from 1959 to 1992 only due to a decrease in temperature sensitivity of tree ring index exhibited after 1992 spatial correlation analysis shows that our reconstruction represents large scale temperature variations in southwest china and the eastern tp our reconstructed december february mean temperature shows a close association with the atlantic multidecadal oscillation amo over the past three centuries with warm cold periods coinciding with the positive negative phases of the amo this persistent relationship suggests that the amo may have been a key driver of multidecadal winter temperature variations on the southeastern tp

thermomechanical modeling of slab eduction 1 plate eduction is a geodynamic process characterized by normal sense coherent motion of previously subducted continental plate this mechanism may occur after slab detachment has separated the negatively buoyant oceanic plate from the positively buoyant orogenic root eduction may therefore be partly responsible for exhumation of high pressure rocks and late orogenic extension we used two dimensional thermomechanical modeling to investigate the main features of the plate eduction model the results show that eduction can lead to the quasi adiabatic decompression of the subducted crust 2 gpa in a timespan of 5 my large localized extensional strain in the former subduction channel flattening of the slab and a topographic uplift associated with extension of the orogen in order to further investigate the forces involved in the eduction process we ran systematic parametric simulations and compared them to analytic plate velocity estimations these experiments showed that eduction is a plausible mechanism as long as the viscosity of the asthenospheric mantle is lower than 1022 pa s while subduction channel viscosity does not exceed 1021 pa s we suggest that eduction can be a viable geodynamic mechanism and discuss its potential role during the orogenic evolution of the norwegian caledonides

mechanical model for the tectonics of doubly vergent compressional orogens a mechanical model of crustal shortening and deformation driven by the relative convergence of rigid underlying mantle plates explains many features of convergent orogens results based on numerical models and supported by sandbox models show that a coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth 1 block uplift bounded by step up shear zones 2 development of a low taper wedge over the underthrusting mantle plate and 3 development of a low taperwedge overlying the overthrusting mantle plate and verging in the opposite direction when modified by isostasy basal viscous flow surface erosion and denudation and sedimentation the resultant model orogens exhibit a variety of styles with characteristics in common with small rapidly denuded orogens large orogens with plateaus and extensional characteristics and active subduction margins with doubly vergent accretionary wedges and deformed fore arc basins

reconstruction of early spring temperature for central japan from the tree ring widths of hinoki cypress and its verification by other proxy records 1 we describe the results of a dendroclimatic study in central japan using ring width data from living trees of hinoki cypress february april temperatures were reconstructed for the period ad 1719 1961 this reconstruction from which five cooling events were determined for the late little ice age was validated against other climate proxies there is a reasonable agreement with cold periods previously estimated from documentary records in japan and china a notably good agreement is also found for a dendroclimatic reconstruction for korea suggesting that the annual activity of the winter monsoon in the western pacific rim persisted longer in the 19th century than at present records of prolonged famines and unusual climatic phenomena support the reconstruction of the cold periods in the 1780s 1810s and 1830s it is thus revealed that the hinoki chronology has enough potential to reconstruct the climatic variability further into the past

the continental collision zone south island new zealand comparison of geodynamical models and observations the south island zone of oblique continent continent convergence occurs along a 400 km long section of the modern australia pacific plate boundary zone across which about 50 km of shortening has been accommodated since about 10 ma the orogen comprises a central mountain range southern alps flanked on both sides by what are interpreted to be foreland basins two essential features that characterize the orogen are 1 the degree of denudation that accompanied deformation and 2 a fundamental structural asymmetry the architectural asymmetry of the orogen can be explained by plane strain finite element models of continental convergence incorporating mantle subduction comparison of model and orogen polarity implies that pacific plate mantle subducts the models predict two crustal scale dipping shear zones that form above the point where the pacific mantle subducts the localized one more distant from the incoming plate retro step up shear zone corresponds to the alpine fault whereas its conjugate pro step up shear zone corresponds to the distributed strain and thrusting along the eastern margin of the mountain belt parameters that modify the model boundary conditions top surface degree of denudation basal zone subduction load crust mantle velocity discontinuity subduction of lower crust mantle retreat and distributed decrease in mantle velocity and the internal strength of the crust two layer crust with moderate coupling temperature distribution strain weakening are varied in a series of numerical model calculations that establish the combination of material properties and boundary conditions that lead to different cross sectional architectures of the modeled collision zone in turn these are compared with observations about the south island orogen the calculations show how the style and extent of deformation across the whole orogen depend on the rheological properties of the crustal layer and on the balance between its internal strength and the combined effects of the boundary and gravitational stresses north to south along strike differences in the width and two dimensional architecture of the orogen simulated in the experiments by varying the model parameters can be explained by a combination of southward increases in preconvergent crustal thickness geothermal gradient convergence and potentially subduction retreat with the added possibility of a southward decrease in the component of lower crustal subduction

crystallization and very rapid exhumation of the youngest alpine eclogites tauern window eastern alps from rb sr mineral assemblage analysis multimineral rb sr internal isochrons from eclogite facies rocks of the eclogite zone tauern window eastern alps consistently yield an early oligocene age of 31 5 0 7 ma this age has been obtained both for late prograde dehydration related eclogitic veins and for rocks variably deformed and recrystallized under eclogite facies conditions 2 0 2 5 gpa 600 c initial sr isotopic equilibria among all phases indicate absence of significant post eclogitic isotope redistribution processes therefore the ages date eclogite facies assemblage crystallization equilibria also prove that no prolonged pre eclogite facies history is recorded in the rocks instead subduction prograde mineral reactions and eclogitization proceeded rapidly fast exhumation immediately after eclogitization with minimum rates 36 mm a is inferred from a 31 5 0 5 ma internal mineral isochron age of a post eclogitic greenschist facies vein assemblage such rates equal typical subduction rates late eocene to early oligocene subduction of the european continental margin with subsequent rapid exhumation of high pressure nappe complexes has previously been recognized only in the western alps the new data signify synchronous continental collision all along the alpine belt our results demonstrate the unique potential of rb sr assemblage system analysis for precise dating of both eclogite facies and post eclogitic events thus for precisely constraining exhumation rates of deep seated rocks and for straightforward linkage of petrologic evidence with isotopic ages

neotethys closure history of anatolia insights from 40ar 39ar geochronology and p t estimation in high pressure metasedimentary rocks the multiple high pressure hp low temperature lt metamorphic units of western and central anatolia offer a great opportunity to investigate the subduction and continental accretion related evolution of the eastern limb of the long lived aegean subduction system recent reports of the hp lt index mineral fe mg carpholite in three metasedimentary units of the gondwana derived anatolide tauride continental block namely the afyon zone the oren unit and the southern menderes massif suggest a more complicated scenario than the single continental accretion model generally put forward in previous studies this study presents the first isotopic dates white mica 40ar 39ar geochronology and where possible are combined with p t estimates chlorite thermometry phengite barometry multi equilibrium thermobarometry on carpholite bearing rocks from these three hp lt metasedimentary units it is shown that in the afyon zone carpholite bearing assemblages were retrogressed through greenschist facies conditions at c 67 62 ma early retrograde stages in the oren unit are dated to 63 59 ma in the kurudere nebiler unit hp mesozoic cover of the southern menderes massif hp retrograde stages are dated to c 45 ma and post collisional cooling to c 26 ma these new results support that the oren unit represents the westernmost continuation of the afyon zone whereas the kurudere nebiler unit correlates with the cycladic blueschist unit of the aegean domain in western anatolia three successive hp lt metamorphic belts thus formed the northernmost tavsanli zone c 88 82 ma the oren afyon zone between 70 and 65 ma and the kurudere nebiler unit c 52 45 ma the southward younging trend of the hp lt metamorphism from the upper and internal to the deeper and more external structural units as in the aegean domain points to the persistence of subduction in western anatolia between 93 90 and c 35 ma after the accretion of the menderes tauride terrane in eocene times subduction stopped leading to continental collision and associated barrovian type metamorphism because by contrast the aegean subduction did remain active due to slab roll back and trench migration the eastern limb below southwestern anatolia of the hellenic slab was dramatically curved and consequently teared it therefore is suggested that the possibility for subduction to continue after the accretion of buoyant e g continental terranes probably depends much on palaeogeography

a plate tectonics oddity caterpillar walk exhumation of subducted continental crust since plate tectonics began on earth grandiose subduction factories have continually shaped the continents accreting continental blocks and new crust at the convergent plate boundaries an enigmatic product of subduction factories is the high pressure to ultrahigh pressure hp uhp metamorphic crustal rocks regurgitated to earth s surface sometimes from depths as great as 200 km the aegean backarc domain comprises two continental blocks that underwent hp metamorphism during the subduction of the african plate here we use thermomechanical numerical simulations to show that subduction of small continental lithosphere blocks separated by oceanic domains induces variations in the slab buoyancy giving rise to episodic rollback exhumation cycles the single self consistent numerical model successfully reproduces the major structural patterns and pressure temperature time paths of hp rocks across the aegean we suggest that the caterpillar walk of exhuming hp rock units revealed by our simulations is a fundamental mechanism behind hp exhumation globally

evidence for intracontinental thrust related exhumation of the ultra high pressure rocks in china coesite and diamond bearing ultra high pressure metamorphic rocks produced during the triassic continental collision between the sino korean and yangtze plates are offset by about 530 km along the sinistral tanlu fault in eastern china the tanlu fault terminates abruptly south of the ultra high pressure metamorphic terrain in dabie shan and does not extend into the foreland fold belt along the yangtze river this suggests that in this region the mesozoic major sinistral displacement along the tanlu fault was transferred to southward directed thrusting which led to the exhumation of the ultra high pressure metamorphic rocks formed earlier postcollisional himalayan type intracontinental thrusting and concomitant erosion appear to be the main agents in the exhumation of the very high pressure metamorphic rocks in china

a thermomechanical model of exhumation of high pressure hp and ultra high pressure uhp metamorphic rocks in alpine type collision belts using a fully coupled numerical thermomechanical model handling strain localization surface processes and ultra high viscosity contrasts 11 orders of magnitude we test a number of possible mechanisms of high pressure hp low temperature lt high temperature ht exhumation in continental collision zones the model considers two end member cases low or high buoyancy of the downgoing crust the first model case predicts three levels of exhumation in the same collisional context the classical corner flow lp lt low pressure low temperature exhumation in the accretionary prism deeper 70 km hp ht exhumation for the thickened subducting crustal sedimentary wedge and ultra hp ht exhumation from the lower crustal chamber forming at the depth of 100 120 km and separated from the upper one by a narrow crustal channel the width of this channel can oscillate in the process of shortening thus controlling the quantity of the crustal material exchanged between the crustal wedge and the lower crustal chamber although both zones of crustal accumulation and the narrow channel between them resemble a vortex shaped nozzle this nozzle appears to be too soft to produce any significant overpressures from the upper crustal wedge the material is exhumed following the ascending shear flow created by the overriding plate assisted by positive buoyancy of the heated crustal material from the lower crustal chamber the material is transported upward to the upper crustal wedge by a flow induced by the asthenospheric traction and a small scale convective instability forming in the lower crustal chamber due to its heating by the overriding asthenosphere in the second modelled case of high buoyancy the latter mechanisms become dominant resulting in hyper fast exhumation of the crust to the surface accelerated or slowed subduction in case of full or partial crustal decoupling respectively and upper plate extension d 2001 elsevier science b v all rights reserved

exhumation of high pressure metamorphic rocks during crustal extension in the d entrecasteaux region papua new guinea the d entrecasteaux islands of eastern papua new guinea consist of a number of active metamorphic core complexes formed under an extensional tectonic setting related to sea floor spreading in the west woodlark basin the complexes are defined by mountainous domes 2500 m high of fault bounded high grade metamorphic rocks including eclogite facies intruded by 2 4 ma granodiorite plutons garnet clinopyroxene exchange thermometers indicate that the temperature of equilibration of the eclogites was 730 900 c the jadeite component of omphacite indicates minimum pressure of 21 kbar suggesting depths of 70 km the metamorphic rocks have undergone widespread retrogression to amphibolite facies retrogression of the metamorphic basement is associated with shearing and formation of the metamorphic core complexes p t conditions in the early stages of shear zone activity determined using the garnet biotite exchange thermometer and the gasp and grips barometers were 570 730 c and 7 11 kbar a second phase of re equilibration at much lower pressures appears to be related to the widespread intrusion of granodiorite plutons one re equilibrated gneiss indicated maximum temperature of 730 c at estimated pressures of approximately 4 kbar this late high temperature metamorphism is also indicated by reactions involving the production of hercynite and corundum in aluminous gneisses and formation of sillimanite at the expense of kyanite two major episodes of granodiorite intrusion occurred during uplift and exhumation of the core complexes both closely coincide spatially with high temperature metamorphic rocks the onset of deformation in extensional shear zones and subsequent uplift of the metamorphic basement these observations indicate a fundamental link between uplift and granodiorite intrusion during continental extension and the formation of the d entrecasteaux islands metamorphic core complexes

polyphase formation and exhumation of high to ultrahigh pressure rocks in continental subduction zone numerical modeling and application to the sulu ultrahigh pressure terrane in eastern china 1 high to ultrahigh pressure hp uhp metamorphic rocks commonly form and exhume during the early continental collision and many questions related to their origin still remain unresolved we focus our study on explaining the poly metamorphic origins of many hp uhp terranes composed of tectonic units having strongly variable ages peak metamorphic conditions and p t paths these features are especially well characterized for the sulu uhp terrane in eastern china which we have chosen therefore as the reference case we conducted 2 d thermomechanical numerical modeling of continental subduction associated with formation and exhumation of hp uhp rocks our experiments suggest existence of several consequent episodes of u hp rocks exhumation related to the inherently cyclic origin of continental crust subduction detachment exhumation processes three major phases of these processes are identified in our reference model for the sulu uhp terrane 1 first and 2 second exhumation episodes of hp rocks originated in the subduction channel at lithospheric depths and 3 exhumation of uhp rocks originated at asthenospheric depths numerical models also suggest that subducted uhp rocks which are positively buoyant compared to the mantle may detach from the slab forming a flattened plume underplating the overriding lithosphere this sublithospheric plume may exist for several million years being heated to 800 900 c by the surrounding hot mantle at the later stage upward extrusion of hot partially molten rocks from the plume may exhume high temperature ht uhp complexes toward the surface

geodynamic models of mature continental collision evolution of an orogen from lithospheric subduction to continental retreat delamination 1 the behavior of deep lithospheric processes in continental collision are frequently speculated on but poorly understood in this study we build on previous work to explore the styles of continental lithosphere deformation during mature collision where 1800 km of convergence has been accommodated by horizontal shortening we conducted a suite of numerical geodynamic experiments that test the sensitivity of mature continent collision to varying mantle lithosphere density mantle lithosphere yield stress lower crustal strength and to the presence of phase change related density changes in the lower crust i e eclogitization of mafic lower crust the models suggest that the early stages of collision are accommodated by subduction of lower crust and mantle lithosphere along a discrete shear zone beneath the overriding plate following this initial stage of subduction the subducting lower crust and mantle lithosphere can retreat from the collision zone permitting the sub lithospheric mantle to upwell and intrude the overriding plate as a result the lower crust and mantle lithosphere of the overriding plate delaminate from the overlying crust with local isostatic adjustment subduction and delamination driven crustal processes plateau like uplift occurs the sub crustal evolution also causes bands of syn convergent crustal extension to develop in models with a rheologically weaker lower crust surface crustal response to the deep lithosphere dynamics becomes more diffuse as an example the numerical experiments satisfy a number of surface observables of the himalayan tibetan orogen namely 1 the current general mantle lithosphere architecture as defined by seismic analyses 2 the long wavelength plateau uplift of the western tibetan plateau 3 the surface heat flow measurements in india the tethyan himalaya qiangtang and in qaidam basin and 4 the anomalous syn convergent extension in the southern portion of the orogen

exhumation of blueschists along a tethyan suture in northwest turkey a blueschist belt formed during the mid cretaceous 88 ma high pressure metamorphism of a passive continental margin sequence of shale siltstone quartzite and limestone occurs immediately south of the tethyan suture in northwest turkey a transect across the tethyan suture was mapped to constrain the exhumation of blueschist facies assemblages the presence of jadeite glaucophane and lawsonite in the blueschists indicates p t conditions of 20 kbar and 430 c the blueschists are tectonically overlain by a cretaceous oceanic accretionary complex showing an incipient blueschist metamorphism a peridotite slab with subvertical layering lies along a low angle fault contact over the blueschists and the accretionary complex isolated slices of early cretaceous 101 4 ma garnet amphibolites occur at the base of the peridotite the garnet amphibolites which show an incipient blueschist facies overprint have probably formed during the oceanic subduction in the foot wall of the subduction zone and later underplated the hanging wall blueschists and peridotites are abruptly truncated in the north by the steeply dipping suture fault that juxtaposes unmetamorphosed and slightly deformed triassic to jurassic sediments of the northern continent against the peridotite and the accretionary complex tectonic juxtaposition of the lower pressure accretionary complex and peridotites over the higher pressure blueschists implies the removal of a 45 km thick oceanic mantle sequence above the blueschists regional geological and geometric constraints suggest that the exhumation of the blueschists was achieved by a combination of two simultaneously acting mechanisms the first was the detachment of the upper crustal blueschist sequence from its basement in the subduction zone and its incorporation to the hanging wall followed by buoyant ascent the second mechanism is the progressive shallowing of the subducted continental lithosphere resulting in the thinning of the mantle wedge above the subducted slab the blueschists were partly exposed prior to the paleocene continent continent collision post collisional eocene 52 48 ma calc alkaline plutons that were emplaced in the blueschists and the overlying peridotite at 10 km depth are interpreted to have formed by crustal anatexis from advective heating by mantle melts these have probably been generated during the upwelling of the asthenosphere under the shallowing continental lithosphere

plate velocity exhumation of ultrahigh pressure eclogites in the pakistan himalaya u pb ages of zircon and allanite from the coesite bearing ultrahigh pressure uhp units in the kaghan valley northern pakistan demonstrate that peak uhp metamorphism along the northern margin of the indian plate occurred at 46 4 0 1 ma at peak pressure temperature conditions of 27 5 kbar 100 km and 720 770 c much lower pressure retrogressive growth of titanite took place between 46 4 and ca 44 ma indicating that the eclogites were exhumed to 35 km depth at or before 44 ma implying very rapid exhumation rates within the mantle of 30 80 mm yr or more comparable to rapid plate velocities once entrained in the base of the crust the eclogites followed a slower cooling history from 45 ma similar to the amphibolite facies gneisses of the pakistan himalaya

pre collisional high pressure metamorphism and nappe tectonics at active continental margins a numerical simulation the evolution of a subduction channel and orogenic wedge is simulated in 2d for an active continental margin with p t paths being displayed for selected markers in our simulation subduction erosion affects the active margin and a structural pattern develops within a few tens of millions of years with four zones from the trench into the forearc i an accretionary complex of low grade metamorphic sedimentary material ii a wedge of nappes with alternating upper and lower crustal provenance and minor interleaving of oceanic or hydrated mantle material iii a megascale melange composed of high pressure hp and ultra high pressure uhp metamorphic rocks extruded from the subduction channel and iv the upward tilted frontal part of the remaining lid the p t paths and time scales correspond to those typically recorded in orogenic belts the simulation shows that hp uhp metamorphism of continental crust does not necessarily indicate collision but that the material can be derived from the active margin by subduction erosion and extruded from the subduction channel beneath the forearc during ongoing subduction

modelling tectonic styles and ultra high pressure uhp rock exhumation during the transition from oceanic subduction to continental collision the tectonic styles of the transition from oceanic subduction to continental collision and their implications for exhumation of uhp material are tested in a series of numerical thermal mechanical models that address subduction collision dynamics at constant convergence rate reference models examine the sensitivity to continental mantle lithosphere density pro continental margin temperature and rheology and retro continental margin rheology three tectonic styles are observed 1 continental subduction without significant deformation of the over riding continent favoured by relatively dense mantle lithosphere and a strong retro continental margin 2 deformation and subduction of the leading edge of the retro continent favoured by low density mantle lithosphere and strong retro continental crust and 3 backthrusting of the retro continental margin and subsequent double subduction favoured by low density mantle lithosphere and weak retro continental crust continental u hp material is formed in all three tectonic styles although it is only efficiently exhumed in the first case three mechanisms contribute to u hp exhumation their importance depending on the evolving geometry of the subduction zone the strength of the continental margin crust and the density of the subducting mantle lithosphere weaker hotter continental margins decouple in the subduction channel and are subsequently exhumed by buoyancy and or driven circulation of material and or forced expulsion in response to insertion of strong continent into the subduction channel plunger effect stronger colder continental material is subducted without detachment and is then transported laterally underplating the retro continental mantle lithosphere neither surface erosion nor slab break off are required for uhp exhumation

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c

crustal structure a key constraint on the mechanism of ultra high pressure rock exhumation abstract the distribution of ultra high pressure uhp metamorphic rocks demonstrates that burial to 100 km and rapid exhumation 1 cm a 1 of continental crust is a normal part of early 10 ma continental collision currently there is no comprehensive model for this fundamental tectonic process that also satisfactorily explains the upper crustal structures resulting from early collisional uhp rock exhumation characteristic features requiring explanation include structural domes that are cored by uhp nappes associated medium to high pressure nappes displaying a distinct pressure gap overlying lower grade rocks including suture zone ophiolites and coeval foreland directed thrust faults and syn exhumation normal faults we present a geodynamical model involving crustal burial and exhumation in a subduction channel below an accretionary wedge competition between down channel shear traction and up channel buoyancy forces expressed as the exhumation number e controls burial and exhumation leading to rapid up channel flow when e 1 exhuming uhp material forms a nappe stack and structural dome as it penetrates and destabilises the overlying wedge driving thrusting and extension this solution is compelling because it explains both the geology and the petrology of the tso morari and other uhp complexes and because it demonstrates that pulse like buoyant exhumation from deep in the subduction channel creates observed upper crustal structures this places constraints on the exhumation mechanism and provides a test of alternative models other proposed mechanisms such as continuous circulation in a lithospheric scale wedge or overpressured subduction channel predict different types of upper crustal structures and are therefore unsatisfactory explanations for early collisional exhumation of uhp terranes

the role of water in the deformation of dunite specimens of two natural dunites anita bay of 100 m grain size and aheirn of 900 m grain size have been deformed in experiments at temperatures between 1200 and 1400 c and constant strain rates between 10 4 and 10 6 s 1 in a gas medium deformation apparatus at 300 mpa confining pressure prior to these experiments the specimens were given a heat treatment in a controlled oxygen fugacity furnace at 1200 c for 60 hours to dehydrate the trace layer silicate minerals present and drive off adsorbed water the predried specimens are much stronger than specimens tested under wet conditions in an earlier study the predried specimens also do not show the apparent grain size dependence seen under wet conditions infrared absorption measurements on the present specimens and those of the earlier study indicate that for a given rock flow stress can be correlated with the intensity of a residual water related broad absorption the addition of water to a predried specimen results in both a mechanical reweakening and an intensifying of the broadband infrared component the weakening of the dunites under wet conditions is probably strongly influenced by the presence of small amounts of a water related species in the grain boundaries but intragranular weakening may also be contributing it is suggested that the concentration of the water related species in the grain boundaries is governed by its concentration in the small melt fraction present the amounts of water that have been found to be of importance for the strength of these dunites are of the order of 0 01 wt amounts much less than most estimates of the water content of the earth s undepleted upper mantle such water may have an important role in determining upper mantle flow behavior stress strain curves for all experiments are available with entire manuscript on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 008 2 50 payment must accompany order

a mechanism for syn collisional rock exhumation and associated normal faulting results from physical modelling abstract properly scaled physical modelling of the subduction of continental lithosphere is performed with a three layer lithospheric model the model includes a strong upper crust a weak ductile lower crust and a strong mantle part 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 slightly denser than the asthenosphere the results of the modelling are tested on the example of the himalayas in the experiments all lithospheric layers subduct into the mantle to a depth corresponding to 200 300 km in nature until the upper crustal layer fails in front of the subduction zone forming a first major thrust fault the mct in our interpretation of the himalayas underthrusting along this fault increases relief which upon reaching a height corresponding to several kilometres in nature is removed mechanically by a blade the blade is moved horizontally back and forth throughout the experiment this erosional unloading mechanism causes the previously subducted segment of the buoyant upper crust to slide upward over the weak lower crust at a rate comparable to the subduction rate the rise of the crustal slice accelerates displacement along the crustal thrust while producing a normal sense motion and exhumation of the material from great depth up to 40 50 km depending on the model parameters along the upper surface of the slice the rapid uplift of the crust then ceases and the orogenesis occurs further due to continuing underthrusting along the first major thrust at a certain stage the crust fails again producing a new major crustal thrust the mbt in the himalayas

an arbitrary lagrangian eulerian formulation for creeping flows and its application in tectonic models summary this paper presents and analyses numerical techniques developed to investigate viscoplastic stokes flows within a model of lithospheric deformation in particular the techniques are related to a subduction model of compressional orogens the driving mechanism in the model corresponds to the near rigid convergence and subduction of one mantle lithosphere beneath another in plane strain and this boundary condition forces flow in an overlying viscoplastic model crust the numerical techniques use the arbitrary eulerian lagrangian formulation in which flows with free surfaces and large deformation are computed on an evolving eulerian finite element grid that conforms to the material domain a regridding algorithm allows the associated lagrangian motion and fields to be followed and in addition coupled back to the eulerian calculation of the flow mass flux boundary conditions are used so that the effects of erosion and deposition by surface processes and mass loss by subduction can be included in the model calculation the evolving model crustal layer is flexurally compensated using a general elastic beam formulation the applicability of the numerical techniques to problems ranging from accretionary wedges to crustal and lithospheric scale deformation is discussed simple flows a linear viscous subduction model a whirl flow and a quasi convection model are used to show that the mass conservation regridding and surface tracking errors are small the broader applicability of the modelling techniques is reviewed

modeling the behavior of the continental mantle lithosphere during plate convergence in studying orogenic processes the mechanism of viscous rayleigh taylor type removal of gravitationally unstable lithosphere is often invoked to explain the behavior of the mantle lithosphere using numerical models we consider this mechanism and explore alternate styles of deep lithospheric deformation during tectonic convergence the numerical experiments incorporate a mix of viscous and plastic rheologies to model the mechanical evolution of the lithosphere asthenosphere system our results suggest that there are a number of deformational modes of the model mantle lithosphere 1 a dripping or rayleigh taylor type instability 2 an asymmetric underthrusting or subduction 3 symmetric ablative plate consumption 4 slab breakoff the failure and detachment of the strong lithosphere and 5 mixed modes with combinations of these processes the development of the modes is controlled by the rate of convergence associated with the background tectonic regime the density field and the rheology of the mantle lithosphere it is important to determine whether these modes occur in the earth beneath collisional orogens

insights from geodynamical modeling on possible fates of continental mantle lithosphere collision removal and overturn geodynamic modeling demonstrates various modes of behaviour of the tectonically active continental mantle lithosphere at continental collision mantle lithosphere below thickening crust can be accommodated by mixed subduction like consumption and viscous drip like instability depending on the material rheology temperature and convergence velocity late stage slab steepening dual sided and ablative consumption and breakoff can occur as the buoyant crust resists subduction removal of accreted crust by erosion can modify how even the deepest portions of the mantle lithosphere evolves during contraction when gravitational forcing rather than plate shortening dominates mantle lithosphere may be removed through viscous dripping like instability or delamination the removal induces crustal heating modified topography and deformation but distinctive styles of these develop depending on whether mantle lithosphere delaminates or drips with a modified density stratification postulated for the archean r

a matter of divergence tracking recent warming at hemispheric scales using tree ring data 1 no current tree ring tr based reconstruction of extratropical northern hemisphere enh temperatures that extends into the 1990s captures the full range of late 20th century warming observed in the instrumental record over recent decades a divergence between cooler reconstructed and warmer instrumental large scale temperatures is observed we hypothesize that this problem is partly related to the fact that some of the constituent chronologies used for previous reconstructions show divergence against local temperatures in the recent period in this study we compiled tr data and published local regional reconstructions that show no divergence against local temperatures these data have not been included in other large scale temperature reconstructions utilizing this data set we developed a new completely independent reconstruction of enh annual temperatures 1750 2000 this record is not meant to replace existing reconstructions but allows some degree of independent validation of these earlier studies as well as demonstrating that tr data can better model recent warming at large scales when careful selection of constituent chronologies is made at the local scale although the new series tracks the increase in enh annual temperatures over the last few decades better than any existing reconstruction it still slightly under predicts values in the post 1988 period we finally discuss possible reasons why it is so difficult to model post mid 1980s warming provide some possible alternative approaches with regards to the instrumental target and detail several recommendations that should be followed in future large scale reconstruction attempts that may result in more robust temperature estimates