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

numerical modelling of spontaneous slab breakoff dynamics during continental collision slab detachment or breakoff is directly associated with phenomena like morphological orogenesis occurrenceofearthquakesandmagmatism atdepththedetachmentprocessisslowand characterized by viscous rheolgy whereas closer to the surface the process is relatively fast and plastic using a 2d mantle model 1500 km deep and 4000 km wide we investigated with finite difference and marker in cell numerical techniques the impact of slab age convergence rate and phase transitions on the viscous mode of slab detachment in contrast to previous studies exploring simplifiedbreakoffmodelsinwhichtheblockageresponsibleforinducingbreakoffiskinematically prescribed we constructed a fully dynamic coupled petrological thermomechanical model of viscousslabbreakoff inthismodel forcedsubductionofa700 km longoceanicplatewasfollowed bycollisionoftwocontinentalplatesandspontaneousslabblockingresultingfromthebuoyancyof thecontinentalcrustonceithadbeensubductedtoadepthof100 124 km typically fivephasesof model development can be distinguished a oceanic slab subduction and bending b continental collision initiation followed by the spontaneous slab blocking thermal relaxation and unbending in experiments with old oceanic plates in this phase slab roll back occurs c slab stretching and necking d slab breakoff and accelerated sinking and e post breakoff relaxation our experiments confirm a correlation between slab age and the time of spontaneous viscous breakoff as previously identified in simplified breakoff models the results also demonstrate a non linear dependence of the duration of the breakoff event on slab age a positive correlation being characteristic of young 50 ma slabs while for older slabs the correlation is negative the increasing duration of the breakoff with slab age in young slabs is attributed to the slab thermal thickness which increases both the slab thermal relaxation time and duration of the neckingprocess inolderslabsthistendencyiscounteractedbynegativeslabbuoyancy whichgen eratehigherstressesthatfacilitateslabneckingandbreakoff apredictionfromourbreakoffmodels isthattheolivine wadsleyitetransitionplaysanimportantroleinlocalizingviscousslabbreakoffat depths of 410 510 km due to the buoyancy effects of the transition

generation of the early cenozoic adakitic volcanism by partial melting of mafic lower crust eastern turkey implications for crustal thickening to delamination abstract early cenozoic 48 50 ma adakitic volcanic rocks from the eastern pontides ne turkey consist of calc alkaline and high k calc alkaline andesite and dacite with sio2 contents ranging from 56 01 to 65 44 wt this is the first time that early eocene volcanism and adakites have been reported from the region the rocks are composed of plagioclase amphibole quartz and mg rich biotite they have high and low mg values ranging from 55 to 62 and 13 to 42 respectively high mg rocks have higher ni and co contents than low mg samples the rocks exhibit enrichments in large ion lithophile elements including the light rare earth elements depletions in nb ta and ti and have high la yb and sr y ratios their relative high isr 0 70474 0 70640 and low end 50 ma values 2 3 to 0 8 are inconsistent with an origin as partial melts of a subducted oceanic slab combined major and trace element and sr nd isotope data suggest that the adakitic magmas are related to the unique tectonic setting of this region where a transition from a collision to an extension stage has created thickening and delamination of the pontide mafic lower crust at 50 ma the high mg adakitic magmas resulted from partial melting of the delaminated eclogitic mafic lower crust that sank into the relatively hot subcrustal mantle and its subsequent interaction with the mantle peridotite during upward transport leaving garnet as the residual phase elevates the mgo content and mg of the magmas whereas low mg magmas formed by the melting of newly exposed lower crustal rocks caused by asthenospheric upwelling which supplies heat flux to the lower crust the data also suggest that the mafic lower continental crust beneath the region was thickened between the late cretaceous and the late paleocene and delaminated during late paleocene to early eocene time which coincides with the initial stage of crustal thinning caused by crustal extensional events in the eastern pontides and rules out the possibility of an extensional regime before 50 ma in the region during the late mesozoic to early cenozoic

delamination and detachment of a lithospheric root abstract orogens can be formed by shortening of the lithosphere on time scales much shorter than that of thermal diffusion for time scales of a few ma the major part of the uplift of an orogen which may occur during the transition from crustal shortening to extension may be due to delamination of the mantle lithosphere rather than to convective thinning the negative buoyancy forces of a lithospheric root may cause lateral shortening while the lithospheric root delaminates peels off from the crust detaches and is replaced by hot upwelling asthenosphere this provides a large input of heat into the crust geological observations show that the late orogenic phase may be characterized by volcanic activity high temperature low pressure metamorphism extension high heat flow and a rapid within a few ma increase of the topography we present dynamically and thermally consistent numerical models of the evolution of an orogenic root mantle rheology is based on laboratory data of olivine while models with the rheology of olivine do not allow for delamination due to its very high viscosity the incorporation of a quasi brittle rheology is necessary for modelling delamination of the mantle lithosphere successful delamination models are always associated with a rapid change in topography we examined the parameter range of our models and found different regimes of behaviour of the orogenic root of the mantle lithosphere no delamination delamination delamination and full detachment for a fixed viscosity of the upper crust of 1023 pa s delamination occurs for a range of viscosities of 1023 pa s to 1020 pa s for the lower crust depending on the thickness of the lithospheric root but full detachment of the delaminated lithospheric slab only occurs if the viscosity of the lower crust is greater than approximately 1021 pa s hot asthenosphere is welling up into the gap opening during delamination further implications of our models are lithospheric roots or unsupported slabs of at least 100 170 km depth extent are needed to provide sufficient negative buoyancy to allow delamination and detachment in all delamination models significant amounts of lower crustal material are subducted into the mantle in applying our models to selected orogens we argue that mantle delamination may have played an important role in the variscan and the himalayan orogenies

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

geodynamics layer cake or plum pudding whether convection in the earth s mantle extends through its entire depth or if the mantle is layered has long been debated recent research suggests that spatially and temporally intermittent or partial layering is the most likely solution

convective instability of thickening mantle lithosphere mantle lithosphere being colder and therefore denser than the underlying mantle is prone to convective instability that can be induced by horizontal shortening numerical experiments on a cold layer with imposed horizontal shortening are carried out to examine the relative importance of mechanical thickening thermal diffusion and gravitational instability in deforming the layer this analysis is then used to develop a method for determining which of these styles dominates for a layer thickening at a given rate if viscosity is non newtonian the imposition of shortening decreases the lithospheric strength which causes perturbations to the lithosphere s temperature structure to grow exponentially with time once these perturbations become sufficiently large they then grow super exponentially with time eventually removing the lithospheric base because lithospheric viscosity is highly temperature dependent at most only the lower 30 per cent of the lithosphere participates in the downwelling associated with this initial super exponential growth event after this event however a downwelling develops that removes material advected into the region of downwelling by horizontal shortening the magnitude of this persistent downwelling depends on the rate and duration of shortening if the total amount of shortening does not exceed 50 per cent doubling of crustal thickness then this downwelling extends to a depth three to four times the thickness of undeformed lithosphere and forms a sheet significantly thinner than the width of the region undergoing shortening once shortening stops this downwelling is no longer replenished by the shortening process and should then detach due to its inherent gravitational instability the hottest 60 per cent of the mantle lithosphere could be removed in such an event which would be followed by an influx of hot buoyant asthenosphere that causes rapid surface uplift because more cold material is removed after the cessation of shortening than by the initial gravitational instability the former has a potentially greater influence on surface uplift the tibetan interior is thought to have been shortened by about 50 per cent in 30myr and afterwards at 8ma experienced a period of rapid uplift that may have resulted from the removal of a large downwelling finger of cold lithosphere generated by shortening

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

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

diamond from the dabie shan metamorphic rocks and its implication for tectonic setting diamond occurs in ultrahigh pressure metamorphic rocks from dabie shan anhui province eastern china diamond bearing rocks include eclogite gamet pyroxenite and jadeitite diamond occurs in a mineral assemblage with coesite and jadeite the diamonds and diamondiferous rocks of dabie shan are interpreted to be the products of ultrahigh pressure metamorphism in the underthrust basement of the yangtze continental plate during the early mesozoic at greater than 4 0 gigapascals and 900 c this interpretation is based on the distribution of rock units the stability field of diamond and isotopic data indicating a crustal origin for the rocks most diamonds occur as euhedral inclusions in garnets and are 10 to 60 micrometers across although some are up to 700 micrometers across

geodynamic evolution of the lithosphere and upper mantle beneath the alboran region of the western mediterranean constraints from travel time tomography an edited version of this paper was published by the american geophysical union copyright 2000 agu see also http www agu org pubs crossref 2000 2000jb900024 shtml http atlas geo cornell edu morocco publications calvert2000 htm

robust characteristics method for modelling multiphase visco elasto plastic thermo mechanical problems abstract we have extended our previous 2d method gerya t v yuen d a 2003 characteristics based marker in cell method with conservative finite differences schemes for modeling geological flows with strongly variable transport properties phys earth planet interiors 140 295 320 which is a combination of conservative finite differences with marker in cell techniques to include the effects of visco elasto plastic rheology self gravitation and a self consistently derived evolving curvilinear planetary surface this code is called i2elvis and can solve a new class of computationally challenging problems in geodynamics such as shear localization with large strains crustal intrusion emplacement of magmas bending of realistic visco elasto plastic plates and core formation by vigorous shell tectonics activities related to a global rayleigh taylor instability of a metal layer formed around silicate rich lower density primordial core during planetary accretion we discuss in detail the computational strategy required the rheological constraints to be satisfied at each time step and spatial location we show analytical benchmarks and examples drawn from comparing between numerical and analogue experiments in structural geology subducting slab bending with a visco elasto plastic rheology and equilibrium spherical configurations from self gravitation we have also tested possibilities of future applications by addressing 3d geometries based on multigrid method and including inertial effects in the momentum equation with tracers in order to simulate meteoritic impact events and eventually earthquake instabilities

coupled and decoupled regimes of continental collision numerical modeling useful geodynamic distinction of continental collision zones can be based on the degree of rheological coupling of colliding plates coupled active collision zones which can be either retreating or advancing are characterized by a thick crustal wedge and compressive stresses i e himalaya and western alps while decoupled end members which are always retreating are defined by a thin crustal wedge and bi modal distribution of stresses i e compressional in the foreland and extensional in the inner part of the orogen northern apennines in order to understand physical controls defining these different geodynamic regimes we conducted a 2d numerical study based on finite differences and marker in cell techniques in our experiments we systematically varied several major parameters responsible for the degree of rheological coupling between plates during collision such as convergence rate crustal rheology and effective velocity of upward propagation of aqueous fluids and melts in the mantle wedge low convergence rates and fluids melts propagation velocities favor continuous coupling and convergence between the plates coupled collision zones are characterized by continuous accretion of the weak upper continental crust resulting in the development of a thick and broad crustal wedge by hot temperature in the inner parts of the orogen due to radiogenic heating of the thickened crust by compressive orogenic stresses and appearance of a double seismogenic brittle layer involving upper crust and sub moho mantle in contrast high convergence rates and fluid melt percolation velocities produce efficient weakening of the mantle wedge and of the subduction channel triggering complete decoupling of two plates mantle wedging into the crustal wedge and retreating style of collision the evolution of fully decoupled collision zones are characterized by the disruption of the accretionary wedge formation of an extensional basin in the inner part of the orogen and delamination of the weak portion of the continental crust that is first thrusted toward the foreland and subsequently dissected by extensional tectonics transition from coupled to decoupled regime occurs always at the early stages of continental collision indicating that insertion of rheologically weak crustal material in the subduction channel is critical for the subsequent evolution of the collision zone we found good correlations of our numerical results with some of the major collisional orogens in particular the decoupled retreating collision regime reproduces what is observed in the northern apennines

multiple zircon growth during fast exhumation of diamondiferous deeply subducted continental crust kokchetav massif kazakhstan diamondiferous rocks from the kokchetav massif kazakhstan represent deeply subducted continental crust in order to constrain the age of ultra high pressure uhp metamorphism and subsequent retrogression during exhumation zircons from diamondiferous gneisses and metacarbonates have been investigated by a combined petrological and isotopic study four different zircon domains were distinguished on the basis of transmitted light microscopy cathodoluminescence trace element contents and mineral inclusions mineral inclusions and trace element characteristics of the zircon domains permit us to relate zircon growth to metamorphic conditions domain 1 consists of rounded cores and lacks evidence of uhp metamorphism domain 2 contains diamond coesite omphacite and titanian phengite inclusions providing evidence that it formed at uhp metamorphic conditions p 43 kbar t 950 c domain 3 is characterised by low pressure mineral inclusions such as garnet biotite and plagioclase which are common minerals in the granulite facies overprint of the gneisses p 10 kbar t 800 c this multi stage zircon growth during cooling and exhumation of the diamondiferous rocks can be best explained by zircon growth from zr saturated partial melts present in the gneisses domain 4 forms idiomorphic overgrowths and the rare earth element pattern indicates that it formed without coexisting garnet most probably at amphibolite facies conditions p 5 kbar t 600 c the metamorphic zircon domains were dated by shrimp ion microprobe and yielded ages of 527 5 528 8 and 526 5 ma for domains 2 3 and 4 respectively these indistinguishable ages provide evidence for a fast exhumation beyond the resolution of shrimp dating the mean age of all zircons formed between uhp metamorphic conditions and granulite facies metamorphism is 528 3 ma indicating that decompression took place in less than 6 ma hence the deeply subducted continental crust was exhumed from mantle depth to the base of the crust at rates higher than 1 8 cm year we propose a two stage exhumation model to explain the obtained p t t path fast exhumation on top of the subducted slab from depth 140 to 35 km was driven by buoyancy and facilitated by the presence of partial melts a period of near isobaric cooling was followed by a second decompression event probably related to extension in a late stage of continental collision

parental magmas of the nain plutonic suite anorthosites and mafic cumulates a trace element modelling approach equilibrium melt trace element contents are calculated from proterozoic nain plutonic suite nps mafic and anorthositic cumulates and from plagioclase and orthopyroxene megacrysts assumed trapped melt fractions tmf 20 generally eliminate all minor phases in most mafic cumulate rocks reducing them to mixtures of feldspar pyroxene and olivine which would represent the high temperature cumulus assemblage in anorthosites tmf 15 generally reduce the mode to a feldspar only assemblage all model melts have trace element profiles enriched in highly incompatible elements relative to normal mid ocean ridge basalt nmorb commonly with negative nb and th anomalies most mafic cumulates yield similar profiles with constant incompatible element ratios and can be linked through fractional crystallization high k la subtypes probably represent crust contaminated facies mafic cumulates are inferred to belong to a tholeiitic differentiation series variably contaminated by upper and lower crustal components and probably related to coeval tholeiitic basaltic dyke swarms and lavas in labrador model melts from anorthosites and megacrysts have normalized trace element profiles with steeper slopes than those calculated from mafic cumulates indicating that mafic cumulates and anorthosites did not crystallize from the same melts orthopyroxene megacrysts yield model melts that are more enriched than typical anorthositic model melts precluding an origin from parental melts jotunites have lower k rb ba y yb and higher la ce than model residues from fractionation of anorthositic model melts suggesting they are not cosanguineous with them but provide reasonable fits to evolved mafic cumulate model melts incompatible element profiles of anorthositic model melts closely resemble those of crustal melts such as tonalites with steep y yb lu segments that suggest residual garnet in the source inversion models yield protoliths similar to depleted lower crustal granulite xenoliths with aluminous compositions suggesting that the incompatible trace element budget of the anorthosites are derived from remobilization of the lower crust the similarity of the highly incompatible trace elements and lile between anorthositic and mafic cumulate model melts suggests that the basalts parental to the mafic cumulates locally assimilated considerable quantities of the same crust that yielded the anorthosites the reaction between underplating basalt and aluminous lower crust would have forced crystallization of abundant plagioclase and remobilization of these hybrid plagioclase rich mushes then produced the anorthosite massifs

migmatites granites and orogeny flow modes of partially molten rocks and magmas associated with melt solid segregation in orogenic belts abstract this paper presents a model for the genesis of migmatites and granites during orogenic evolution based on the analysis of several phanerozoic crustal segments and a review of the physical properties of partially molten rocks and magmas this model inventories the modes of bulk flow of partially molten rocks and magmas and the mechanisms of melt solid segregation for each mode partial melting of rocks is associated with a strength decrease of two to three orders of magnitude leading to strain partitioning expressed by channel flow driven either by forces related to plate tectonics vertical channel flow or by the gravity force associated with lateral variations of crustal thicknesses horizontal channel flow although the structural characteristics of migmatites indicate that deformation plays a role in melt migration the emplacement of laccoliths of leucogranites above migmatites attests to the efficiency of the buoyancy force another decrease in apparent strength of about ten orders of magnitude is associated with the loss of continuity of the solid framework marking the transition from partially molten rocks to magmas the breakdown of the solid framework also allows for settling of the solid in suspension increasing the buoyancy of the remaining magma accordingly domes cored by diatexites former heterogeneous magmas and mantled by metatexites former partially molten rocks are interpreted as gravitational instabilities driven by the relative buoyancy of the magma and permitted by the weakness of the partially molten rocks this model provides a first order framework to elucidate the development of a crustal scale horizontal layering during the evolution of orogenic belts that are affected by partial melting in this case the middle crust is dominated by migmatites with domes cored by diatexites and mantled by metatexites that correspond to a partially molten and magmatic zone respectively the granitic dikes and sills and the associated laccoliths of leucogranites correspond to an intrusive zone overlying the partially molten zone the refractory lower crust is potentially in part formed by accumulation of solids segregated from the overlying heterogeneous magmas

climatic and environmental changes at southeastern coast of lake biwa over past 3000 years inferred from borehole temperature data abstract in order to infer past climatic change in central japan we measured temperatures in a borehole at the karasuma site on the southeastern coast of lake biwa and reconstructed sediment surface temperature history during the last 3000 years the reconstructed temperature history shows apparent medieval warm period little ice age and contemporary temperature warming however the large amplitude of the temperature changes up to 4 5 k cannot be explained by past climatic change only suggesting that there was some other cause of the larger amplitude temperature changes the onsets of temperature decrease in the late 12th century a d and temperature increase in the mid 17th century a d appear to coincide with occurrences of two destructive earthquakes 1185 and 1662 a d that caused water level changes of lake biwa it suggests that the reconstructed sediment surface temperature history reflects the environmental change due to tectonically induced water level changes of the lake if the annual mean of the ground surface temperature was higher than that of the bottom water temperature in a shallow part of the lake which is consistent with the present day data the large amplitude of the sediment surface temperature change may be attributed to a combined effect of past climatic and environmental changes thus we suggest that the borehole temperature at the karasuma site preserves information not only on past climate changes but also on environmental changes due to tectonically induced water level changes

asymmetric delamination and convective removal numerical modeling comparison with evolutionary models for the alboran sea region convective removal and mantle delamination are geodynamical mechanisms proposed to explain the presence of extension in the alboran sea within a regional context of compression using a new thermomechanical algorithm we present here a quantitative evaluation and comparison of conceptual models based on these geodynamical mechanisms in contrast to the in situ convective removal process the laterally propagating delamination mechanism is shown here to be consistent with first order features of the alboran sea such as the thinning thickening distribution intermediate depth seismicity and upper mantle structure imaged by seismic tomography the lower crust is predicted to reach depths of 100 150 km in some areas due to mechanically driven viscous drag of the downwelling mantle

subduction initiation by thermal chemical plumes numerical studies abstract prior suggestions for the initiation of subduction have included sediment loading compression and plate reconfiguration as potential triggers here we investigate the possibility of subduction initiation by the interaction of the lithosphere with a buoyant mantle plume numerical testing of this hypothesis has been conducted in 2d with the i2vis thermo mechanical code accounting for phase transitions and a viscoplastic model of a thin oceanic lithosphere hit by a partially molten thermal chemical or purely thermal plume we demonstrate that a mantle plume can break the lithosphere and initiate self sustaining subduction provided the plume causes a critical local weakening of the lithospheric material above it the intensity of the required weakening depends on the plume volume plume buoyancy and the thickness of the lithosphere and is the highest for the least buoyant purely thermal plumes another necessary condition is the presence of high pressure fluids at the slab upper interface reducing the effective friction coefficient there to very low values based on our results we suggest that sheet like instabilities of the archean mantle convection could have initiated subduction on earth where ocean was already present in less stable tectonic settings provided that mantle plumes sheets at that time were rich in water and melt which could drastically reduce the effective friction coefficient in the lithosphere above the plume our numerical models are also in good agreement with suggested concepts for corona formation on venus

continental delamination insights from laboratory models 1 one of the major issues of the evolution of continental lithospheres is the detachment of the lithospheric mantle that may occur under certain conditions and its impact on the surface in order to investigate the dynamics of continental delamination we performed a parametric study using physically scaled laboratory models the adopted setup is composed of a three layers visco elastic body analog for upper crust lower crust lithospheric mantle locally thickened thinned to simulate a density anomaly lithospheric root and an adjacent weak zone lying on a low viscosity material simulating the asthenosphere the results emphasize the interplay between mantle flow deformation surface topography and plate motion during a three phases process 1 a slow initiation phase controlled by coupling and bending associated with contraction and dynamic subsidence 2 lateral propagation of the delamination alongside with extension and a complex topographic signal controlled by coupling and buoyancy while poloidal mantle flow develops around the tip of the delaminating lithospheric mantle and 3 a late phase characterized by a counterflow that triggers retroward motion of the whole model a semiquantitative study allows us to determine empirically two parameters 1 an initiation parameter that constrains the propensity of the delamination to occur and correlates with the duration of the first stage 2 a buoyancy parameter characterizing the delamination velocity during late stages and therefore its propensity to cease finally we point out similarities and differences with the sierra nevada california usa in terms of topography deformation and timing of delamination

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

tectonic delamination and subcrustal imbrication of the precambrian lithosphere in northwestern canada mapped by lithoprobe seismic reflection profiling of the precambrian lithosphere in northwestern canada has produced images of delamination structures tectonic wedges at crustal scale to about 30 km depth and for the first time at lithospheric scale to about 100 km depth and has further delineated subcrustal structures that were likely caused by imbrication of subducted lithosphere coupled with observations of similar structures in thrust and fold belts delamination and lower plate imbrication are persistent features at many scales

continental subduction and exhumation of high pressure rocks insights from thermo mechanical laboratory modelling abstract thermo mechanical physical modelling of continental subduction is performed to investigate the exhumation of deeply subducted continental crust the model consists of two lithospheric plates made of new temperature sensitive analogue materials the lithosphere is underlain by liquid asthenosphere the continental lithosphere contains three layers the weak sedimentary layer the crust made of a stronger material and of a still stronger lithospheric mantle the whole model is subjected to a constant vertical thermal gradient causing the strength reduction with depth in each lithospheric layer subduction is driven by both push force and pull force during subduction the subducting lithosphere is heating and the strength of its layers reduces the weakening continental crust reaches maximal depth of about 120 km and cannot subduct deeper because its frontal part starts to flow up the subducted crust undergoes complex deformation including indicated upward ductile flow of the most deeply subducted portions and localised failure of the subducted upper crust at about 50 km depth this failure results in the formation of the first crustal slice which rises up between the plates under the buoyancy force this process is accompanied by the delamination of the crustal and mantle layers of the subducting lithosphere the delamination front propagates upwards into the interplate zone resulting in the formation of two other crustal slices that also rise up between the plates average equivalent exhumation rate of the crustal material during delamination is about 1 cm year the crust asthenosphere boundary near the interplate zone is uplifted the subducted mantle layer then breaks off removing the pull force and thereby stopping the delamination and increasing horizontal compression of the lithosphere the latter produces shortening of the formed orogen and the growth of relief the modelling reveals an interesting burial exhumation evolution of the sedimentary cover during initial stages of continental subduction the sediments of the continental margin are dragged to the overriding plate base and are partially accreted at the deep part of the interplate zone at 60 70 km depth these sediments remain there until the beginning of delamination during which the pressure between the subducted crust and the overriding plate increases this results in squeezing the underplated sediments out part of them is extruded upwards along the interplate zone to about 30 km depth at an equivalent rate of 5 10 cm year

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

geochemistry and age of ultrahigh pressure metamorphic rocks from the kokchetav massif northern kazakhstan isotopic and geochemical data of the zerenda series metamorphic rocks from the kokchetav massif are reported some of these rocks contain microdiamond inclusions in garnets and other indicators of ultrahigh pressure metamorphism p 40 kbar t 900 1000 c the diamond bearing rocks exhibit distinctive geochemical characteristics compared to typical crustal rocks the ree patterns range from lree depleted to slightly lree enriched chondrite normalized la yb n 0 1 5 4 with a negative eu anomaly they are depleted in incompatible elements e g sr ba u th with respect to the upper crust in contrast non diamondiferous rocks of the zerenda series exhibit normal crustal geochemistry all rocks of the zerenda series have very radiogenic lead isotopes the measured values 238u 204pb compared with those calculated for the interval between crust formation and ultrahigh pressure uhp metamorphism suggest a decrease by factors of up to 200 during the uhp metamorphism the sm nd mineral isochrons from the diamond bearing rocks and other rock types of the zerenda series give a middle cambrian 524 535 ma age of metamorphism the nd model ages show that crust formation occurred about 2 3 ga ago significant fractionation of sm and nd and loss of incompatible elements may be due to partial melting of the protoliths the ar ar age determinations of secondary biotite and muscovite from the diamond bearing rocks yield an age of 517 5 ma this cooling age requires a short time interval between uhp metamorphism and uplift to a crustal level ultrahigh pressure metamorphism might be a significant source of pb for the mantle we propose that the radiogenic pb of the oceanic array is the contamination traces of numerous uhp events beside the geological aspect we demonstrate a method of dating a high grade metamorphic terrain using nd isotopes we compare whole rock isochrons and mineral isochrons and in this way get some insight into the behaviour of the sm nd system during very high grade metamorphic events

evidence for the chaotic origin of northern annular mode variability 1 exponential spectra are found to characterize variability of the northern annular mode nam for periods less than 36 days this corresponds to the observed rounding of the autocorrelation function at lags of a few days the characteristic persistence timescales during winter and summer is found to be 5 days for these high frequencies beyond periods of 36 days the characteristic decorrelation timescale is 20 days during winter and 6 days in summer we conclude that the nam cannot be described by autoregressive models for high frequencies the spectra are more consistent with low order chaos we also propose that the nam exhibits regime behaviour however the nature of this has yet to be identified

thermomechanical modelling of slab detachment abstract slab detachment or breakoff is appreciated as an important geological process as shown by recent tomographic imaging using a 2 d upper mantle model 660 km deep and 2000 km wide we have investigated with a 2 d finite difference and marker in cell numerical technique the multi resolutional character of the thermomechanical phenomena related to this complex geological process our experiments show that this process can be initiated in form of slab necking by a prolonged 8 30 my period of slab weakening due to thermal diffusion 60 c my due to strong positive thermal feedback from shear heating detached slab fragments sink rapidly with a tendency for coherent rotation the influence of temperature and pressure dependent thermal conductivity on the process of thermal weakening of the slab is quite significant this supports the idea that the breakoff process is triggered by thermal diffusion on a time scale linearly dependent on heat conductivity rapid topographic changes and increasing volcanic activities due to the melting of subducted oceanic crust are possible scenarios of this vigorously driven geodynamic process

from lithospheric thickening and divergent collapse to active continental rifting 1 abstract 32 introduction 33 modelling approach main assumptions and simpli cations 53 1 tectonic thickening 63 2 erosion sedimentation 63 3 thermal relaxation 73 4 gravity driven ow 74 physical model 74 1 temperature pro le 84 2 density pro le and surface elevation 84 3 gravitational force 94 4 strength pro le 104 5 lithospheric geometry through time 105 presentation of the results 115 1 stable thermal boundary layer during convergence 115 2 post convergence removal of the thermal boundary layer 126 discussion and conclusion 137 acknowledgements 158 references 162

delamination wedge structure beneath the hidaka collision zone central hokkaido japan inferred from seismic reflection profiling in the hidaka collision zone of hokkaido northern japan the kuril island arc collides with the northeast japan arc in order to better understand the collision process new high resolution information about lithospheric structure was obtained by means of a series of seismic reflection surveys these seismic profiles reveal distinct zones of seismic lamination in the lower crust of the kuril arc the upper portion of the lower crust is characterized by numerous east dipping reflectors in contrast west dipping reflectors dominate the lower part of the lower crust from this reflector configuration the lower crust of the kuril arc is interpreted to be delaminated by the collision the geometry of delamination is consistent with other geophysical data as well as the peak metamorphic grades exposed in the hidaka mountains as earthquakes indicate that delamination here is ongoing the hidaka collision zone represents an active model for continental growth by arc accretion with delamination of lower arc crust

slab behaviour and its surface expression new insights from gravity modelling in the se carpathians abstract we use lithosphere scale gravity models to calculate gravity anomalies resulting from oceanic subduction continental collision slab steepening delamination and break off local isostasy was assumed for determining vertical movements caused by mass changes related to these tectonic processes our results show that subduction is accompanied by basin subsidence on the upper plate caused by the heavy lithospheric root of the subducting slab the basin evolution goes parallel with the slab evolution and shows considerable modifications when the processes at depth change slab steepening delamination break off characteristic gravity anomaly curves were acquired for the different tectonic scenarios these curves together with other data e g basin evolution on the upper and the lower plate were used for the reconstruction of the tectonic evolution of the se carpathians which includes tertiary subduction and collision followed by slab steepening and delamination

re os isotopic evidence for a lower crustal origin of massif type anorthosites massif type anorthosites are large igneous complexes of proterozoic age they are almost monomineralic representing vast accumulations of plagioclase with subordinate pyroxene or olivine and fe ti oxides the 930 myr old rogaland anorthosite province in southwest norway1 represents one of the youngest known expressions of such magmatism the source of the magma and geodynamic setting of massif type anorthosites remain long standing controversies in precambrian geology with no consensus existing as to the nature of the parental magmas or whether these magmas primarily originate in the earth s mantle or crust at present massif type anorthosites are believed to have crystallized from either crustally contaminated mantle derived melts that have fractionated olivine and pyroxenes at depth2 or primary aluminous gabbroic to jotunitic melts derived from the lower continental crust3 here we report rhenium and osmium isotopic data from the rogaland anorthosite province that strongly support a lower crustal source for the parental magmas there is no evidence of significantly older crust in southwest scandinavia and models invoking crustal contamination of mantle derived magmas fail to account for the isotopic data from the rogaland province initial osmium and neodymium isotopic values testify to the melting of mafic source rocks in the lower crust with an age of 1 400 1 550 myr

heat flow in the alboran sea western mediterranean abstract the results of the first regional heat flow survey carried out in the alboran basin are presented the survey consists of 98 heat flow measurements obtained using a violin type probe 697 nautic miles of gravity profiles 1446 nautic miles of bathymetric survey and 22 gravity cores a remarkable difference in heat flow patterns exists between the western wab and eastern eab parts of the alboran basin the average heat flow in the wab is 69 6 mw m 2 with a generally increasing trend towards the centre and to the east in contrast the heat flow pattern in the eab shows an average value of 124 8 mw m 2 and it is maintained rather constant for the overall area superimposed on this general pattern there are some local thermal anomalies associated with hydrothermal activity which have been detected in the central wab up to 123 mw m 2 in the south alboran basin sab up to 153 mw m 2 and in the djibouti bank db up to 254 mw m 2 after corrections for thermal refraction sedimentation and cooling of volcanic bodies the resulting heat flow distribution in the wab is smoother but still shows the increasing trend towards the centre and to the east in the eab the application of these corrections did not lead to any noticeable changes a 1 d approach that combines heat flow data crustal structure and elevation shows a dramatic decrease in lithospheric thickness from the wab 50 90 km to the eab 38 40 km likewise the resulting crustal thickness is around 14 16 km in the central part of the wab increasing towards the borders of the basin whereas in the eab the crustal thickness varies between 12 5 and 14 5 km in its western part and between 10 and 11 5 km in its eastern part

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

mantle flow and dynamic topography associated with slab window opening insights from laboratory models abstract we present dynamically self consistent mantle scale laboratory models that have been conducted to improve our understanding of the influence of slab window opening on subduction dynamics mantle flow and associated dynamic topography over geological time scales the adopted setup consists of a two layer linearly viscous system simulating the subduction of a fixed plate of silicone lithosphere under negative buoyancy in a viscous layer of glucose syrup mantle our experimental setting is also characterized by a constant width rectangular window located at the center of a laterally confined slab modeling the case of the interaction of a trench parallel spreading ridge with a wide subduction zone we found that the opening of a slab window does not produce consistent changes of the geometry and the kinematics of the slab on the contrary slab induced mantle circulation quantified both in the vertical and horizontal sections using the feature tracking image analysis technique is strongly modified in particular rollback subduction and the opening of the slab window generate a complex mantle circulation pattern characterized by the presence of poloidal and toroidal components with the importance of each evolving according to kinematic stages mantle coming from the oceanic domain floods through the slab window indenting the supra slab mantle zone and producing its deformation without any mixing between mantle portions the opening of the slab window and the upwelling of sub slab mantle produce a regional scale non isostatic topographic uplift of the overriding plate that would correspond to values ranging between ca 1 and 5 km in nature assuming that our modeling results can be representative of the natural behavior of subduction zones we compared them to the tectonics and volcanism of the patagonian subduction zone we found that the anomalous backarc volcanism that has been developing since the middle miocene could result from the lateral flow of sub slab mantle and that part of the patagonian uplift could be dynamically supported

closing the gap between regional and global travel time tomography recent global travel time tomography studies by zhou 1996 and van der hilst et al 1997 have been performed with cell parameterizations of the order of those frequently used in regional tomography studies i e with cell sizes of 1 2 these new global models constitute a considerable improvement over previous results that were obtained with rather coarse parameterizations 5 cells the inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved this study aims at resolving lateral heterogeneity on scales as small as 0 6 in the upper mantle and 1 2 3 in the lower mantle this allows for the adequate mapping of expected small scale structures induced by for example lithosphere subduction deep mantle upwellings and mid ocean ridges there are three major contributions that allow for this advancement first we employ an irregular grid of nonoverlapping cells adapted to the heterogeneous sampling of the earth s mantle by seismic waves spakman and bijwaard 1998 second we exploit the global data set of engdahl et al 1998 which is a reprocessed version of the global data set of the international seismological centre their reprocessing included hypocenter redetermination and phase reidentification finally we combine all data used p pp and pwp phases into nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal to noise ratio is improved in the approximate solution of the huge inverse problem we obtain a variance reduction of 57 1 synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells 0 6 or 1 2 in the shallow parts of subduction zones decreasing to approximately 2 3 resolution in well sampled regions in the lower mantle vertical resolution can be worse up to several hundreds of kilometers in subduction zones with rays predominantly pointing along dip important features of the solution are as follows 100 200 km thick high velocity slabs beneath all major subduction zones sometimes flattening in the transition zone and sometimes directly penetrating into the lower mantle large high velocity anomalies in the lower mantle that have been attributed to subduction of the tethys ocean and the farallon plate and low velocity anomalies continuing across the 660 km discontinuity to hotspots at the surface under iceland east africa the canary islands yellowstone and the society islands our findings corroborate that the 660 km boundary may resist but not prevent present day large scale mass transfer from upper to lower mantle or vice versa this observation confirms the results of previous global mantle studies that employed coarser parameterizations

thermal mechanical effects of low temperature plasticity the peierls mechanism on the deformation of a viscoelastic shear zone we studied for the first time the effects of low temperature plasticity on the formation of shear zones a thermal mechanical model has been developed for describing the shear deformation of maxwell viscoelastic material with a rheology close to dry olivine we employed a one dimensional model with a half width of l deforming under a constant velocity u at the boundary and the spatially averaged strain rate u l was set to o 10 14 s 1 in addition to diffusion and power law creep we included deformation by low temperature plasticity called the peierls mechanism which is significant at low temperatures and has a strong exponential dependence on the stress when a sufficient magnitude of heat is generated by the rapid conversion from elastically stored energy into viscous dissipation thermal instability takes place and the deformation localizes in a narrow region by comparing the condition for thermal instability we found that the low temperature plasticity inhibits the development of thermal instability in shear zones in case of constant strain rate the peierls mechanism enhances deformation at a significantly lower stress compared to the rheology with solely diffusion creep and power law creep the enhanced deformation by low temperature plasticity produces lower amount of dissipative heating and thus stabilizes the shear zone comparing the stability between constant strain rate and constant stress boundary conditions we found that the peierls mechanism exerts an opposite destabilizing effect in the case of constant stress for dry olivine rheology and realistic magnitude of the strain rate the effect of low temperature plasticity is significant for temperatures between around 800 k and 1000 k this finding suggests that the low temperature plasticity may be crucial in determining the thermal mechanical stability in the shallow portion of slabs

diamond and coesite discovered in saxony type granulite solution to the variscan garnet peridotite enigma diamond and coesite were discovered in situ as inclusions in garnet kyanite and zircon in high pressure granulites from northern bohemian massif these continental crustal rocks were therefore subducted to depths of c 140 km which also explains their common association with mantle garnet bearing peridotites models involving crustal thickening for these high pressure granulites need to be significantly modified whole variscan belt with numerous hp granulite occurrences can represent a large ultrahigh pressure terrain

influence of continental roots and asthenosphere on plate mantle coupling the shear tractions that mantle flow exerts on the base of earth s lithosphere contribute to plate driving forces and lithospheric stresses we investigate the sensitivity of these tractions to sub lithospheric viscosity variations by comparing shear tractions computed from a mantle flow model featuring laterally varying lithosphere and asthenosphere viscosity with those from a model with layered viscosity lateral viscosity variations generally do not change the direction of shear tractions but deeply penetrating continental roots increase traction magnitudes by a factor of 2 5 compared to 100 km thick lithosphere a low viscosity asthenosphere decreases traction magnitudes by a smaller amount and is important only if 100 km thick increased plate mantle coupling beneath thick continental lithosphere may increase plate driving forces surface deformation and mantle derived lithospheric stresses in these regions by contrast a low viscosity asthenosphere does not decouple the lithosphere from mantle flow highlighting the geological importance of mantle tractions on the lithosphere

deep and near surface consequences of root removal by asymmetric continental delamination abstract the occurrence of continental delamination has been proposed for a number of areas characterized by highly variable geodynamic settings in this study we present results of numerical simulations considering different initial setups representative for geodynamic scenarios where delamination could potentially develop to mimic a post collisional orogenic scenario we have designed an initial state characterized by the presence of an area of orogenic lithosphere with both crustal and lithospheric roots in a second setup we have considered a lithospheric root representative of a remnant slab with a flat overlying crust we focus on predicted evolution of surface and near surface observables namely the crustal structure surface heat flow and isostatic and dynamic topography evolution our results show that a high density orogenic lower crust likely related to the presence of eclogite significantly accelerates the sinking of the lithospheric mantle the pattern of local isostatic elevation is characterized by laterally migrating surface uplift subsidence this pattern is shown to be little sensitive to lower crust density variations in contrast predicted dynamic topography is more sensitive to these changes and shows surface subsidence adjacent to the delaminating lithospheric mantle for the model with a high density lower crust and surface uplift above the slab for a model with a less dense lower crust the reason for uplift in this second model is that the effect of the positive buoyancy of the thickened crust overwhelms the effect of negative buoyancy of the slowly sinking lithospheric mantle we infer from our modeling that there is not a specific characteristic pattern of topography changes associated with delamination but it depends on the interplay between highly variable factors as slab sinking velocity asthenospheric upwelling and changes in crustal thickness

steady propagation of delamination events delamination of the lithospheric thermal boundary from overlying continental crust propagates laterally from the line of initiation accelerating as the sinking slab of detached lithosphere grows longer this propagation has been numerically modeled with steady state equations in a moving reference frame by matching an interior finite element solution to flexible boundary conditions which represent the mechanical and thermal response of the surroundings the form of the solution depends on the shear coupling of intruding asthenosphere to the top of the sinking slab across a thin layer of crustal material without coupling the tip of the intrusion cools and stiffens to form a wedge dividing the crust cold mode with coupling the intrusion is forced to convect and remains ductile hot mode the cold mode can propagate at all velocities the hot mode has a lower limiting velocity of 1 2 cm year but offers less resistance at higher speeds resistance to delamination includes a constant term from the buoyant crustal downwarp plus a velocity proportional term representing viscous deformation however the proportionality constant of the latter term is only weakly dependent on crust and lithosphere viscosities matching this resistance to loading lines of 100 to 800 km slabs sinking in a mantle of 1022 p velocities of 0 3 8 0 cm year are obtained changes in viscosity affect this rate but cold mode delamination is unstoppable except at continental margins or by failure in the sinking slab the surface expression of delamination is a leading outer rise followed by a submarine trough with a large negative free air anomaly which finally evolves into a 1 km plateau if crustal viscosity and velocity are both low however there is a montonic crustal uplift with no trough thus the present lack of linear supracontinental oceans does not preclude delamination at up to 4 cm year driven by slabs up to 400 km in length

delamination in collisional orogens during continental collision the weight of the downgoing slab may lead to extensional shearing in the descending plate permitting separation of the downdip dense oceanic lithosphere from the adjacent lighter continental lithospher separation may occur by means of a lithosphere penetrating normal sense shear zone by means of pure shear necking of the lithosphere or by some combination of these mechanisms during separation the continental crust attached to the lower plate may undergo an episode of extensional tectonics the proximity of asthenosphere to crust may result in syntectonic regional metamorphism and plutonism if the continents continue to converge following delamination a compressive tectonic regime will be reestablished in the crust the subsequent cooling of asthenosphere at the base of the crust may lead to the establishment of a continuous flat moho the likely imbrication of crustal and mantle lithologies during delamination and during the subsequent compressive tectonic regime as well as underplating associated with syntectonic plutonism may account for the seismic complexities observed at moho depths beneath some mountain belts

conditions for a crustal block to be sheared off from the subducted continental lithosphere what is an essential factor to cause features associated with collision 1 crustal blocks are detached from the subducting lithosphere at various depths i call such phenomena shearing off and investigate the mechanical conditions for shearing off to occur in the continental lithosphere the buoyancy of the crustal block does not work selectively as a driving force until it has been sheared off and i suppose that shearing off occurs when the shear traction at the thrust whose maximum value is calculated from the strength along the thrust overcomes the strength of the boundary faults of the block and determine the depth of the leading edge of the subducted crustal block when shearing off occurs the temperatures of the slab and at its surface are calculated by the steady state formula of molnar and england 1990 as a function of the surface heat flow dip angle and convergence velocity of the subducted continental plate as the shear stress and temperature at the ductile part of the thrust zone are consistent with the flow law the shear stress at the brittle part is a strong function of the pore fluid pressure ratio i show that shearing off of an upper crustal block is possible only for a small value of such as 0 4 when the surface heat flow value is 70 mw m2 i suggest that the essential factor to cause shallow offscraping of crustal blocks in collision zones is not a buoyancy of the continental crust but a low value of which would result from a little amount of dehydration from the subducted continental plate

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

convective thinning of the lithosphere a mechanism for the initiation of continental rifting thinning of the lithosphere by heat carried convectively to its base is a possible mechanism for the initiation of continental rifting the lithosphere asthenosphere boundary marks the transition from a quasi rigid behavior of the mantle on geological time scales to a viscous one the boundary lies at or close to the solidus of the mantle material and its position is a function of temperature and pressure additional heat carried to the boundary will cause it to rise and the lithosphere to thin a one dimensional model is used to study the upward movement of the boundary from an initial equilibrium position as a consequence of an increase in the heat flux from the convecting mantle the ratio of the amount of thinning to the initial thickness is directly proportional to the increase in heat flux from the asthenosphere and inversely proportional to the difference between the final subcrustal geothermal gradient and the slope of the transition temperature profile the rate of thinning is determined by the initial and final equilibrium thicknesses the thermal diffusivity the transition temperature profile and the plume temperature profile it depends only slightly on any latent heat associated with the lithosphere asthenosphere transition during a large fraction of the time between the initial and final equilibrium states the lithosphere thins at a rate which is inversely proportional to the square root of time t even 300 km thick lithospheres may be thinned to 50 km or crustal thicknesses in tens of million years by the heat advected by mantle plumes this heat contributes to an enhanced surface heat flow because of the constant level of radioactive heat production in the crust the ratio of the final to the initial surface heat flow increases much less than the ratio of the final to the initial heat flow from the asthenosphere for large increases in asthenospheric heat flow the lithosphere is almost thinned to the crust before any significant change of surface heat flow occurs uplift due to thermal expansion upon lithospheric thinning is of the order of 102 103 m a rayleigh taylor instability acting on the thinning lithosphere may further increase thinning rates and cause magmatic intrusions of the lithosphere and surface volcanism

linking the alps and apennines subduction systems new constraints revealed by high resolution teleseismic tomography abstract we report a new model of the upper mantle structure beneath italy obtained by means of p wave teleseismic tomography besides the recent and remarkable development of the italian seismic network a high model resolution has been achieved improving the inversion method upon the ach method used in previous investigations and picking high quality arrival times with the multi channel cross correlation technique the finer details of our vp model yield new insights into the heterogeneous structure of the adria continental lithosphere involved in the collision between the africa and europe plates a wide low vp anomaly located in the northern adria mantle facing the alpine high vp slab supports the idea that the adria lithosphere has been hydrated and thinned during the alpine subduction we argue that this mantle softening may have played a key role in favoring the subsequent delamination of the adria lithosphere in the northern apennines we hypothesize that delamination of continental lithosphere previously thinned in a back arc setting may be considered a key process to favor subduction polarity reversal and recycling of continental material into the mantle circulation conversely in the central southern apennines the velocity structure is consistent with the existence of a deeper oceanic slab that flattens at the base of the upper mantle in agreement with the widely accepted geodynamic evolution of the central mediterranean by slab retreat and back arc spreading the oceanic slab is discontinuously detached from the surface plate suggesting a different structure of the adria lithosphere which resists subduction instead of favoring delamination

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

lithospheric strength in compression initiation of subduction flake tectonics foreland migration of thrusting and an origin of displaced terranes compressive strength profiles through continental and oceanic lithosphere show under all likely conditions continental lithosphere to be significantly weaker than oceanic manifestations of this include subduction initiation within continental lithosphere north africa eastern japan and the existence of continental large scale strike slip faults these latter are associated with the production of displaced exotic terranes once failure has begun and crustal shortening occurs through thrusting reduction of the geotherm causes an increase in total compressive strength and burial of the strength maximum in the thickened section this result leads to a mechanistic view of flake tectonics such that the modified strength profile of the overriding thickened plate enables it to act as a chisel and strip the upper crust from an encroaching continent the increase in total strength leads to a horizontal strength gradient such that the most recently thrust shortened section is stronger than the as yet und

rift induced delamination of mantle lithosphere and crustal uplift a new mechanism for explaining rwenzori mountains extreme elevation with heights of 4 5 km the topography of rwenzori mountains a large horst of old crustal rocks located inside a young passive rift system poses the question why are the rwenzori mountains so high the cenozoic western rift branch of the east african rift system is situated within the late proterozoic mobile belts between the archean tanzania craton and congo craton the special geological setting of the massif at a rift node encircled by the ends of the northern western rift segments of lake albert and lake edward suggests that the mechanism responsible for the high elevation of the rwenzoris is related to the rifting process our hypothesis is based on the propagation of the rift tips surrounding the stiff old lithosphere at rwenzori region thereby triggering the delamination of the cold and dense mantle lithosphere ml root by reducing viscosity and strength of the undermost lower crust as a result this unloading induces fast isostatic pop up of the less dense crustal rwenzori block we term this rid rift induced delamination of mantle lithosphere the physical consistency of the rid hypothesis is tested numerically viscous flow of 2d models is approximated by a finite difference method with markers in an eulerian formulation the equations of conservation of mass momentum and energy are solved for a multi component system based on laboratory data of appropriate rock samples a temperature pressure and stress dependent rheology is assumed assuming a simple starting model with a locally heated ml the ml block between the weakened zones becomes unstable and sinks into the asthenosphere while the overlying continental crust rises up thus rid seems to be a viable mechanism to explain geodynamically the extreme uplift important conditions are a thermal anomaly within the ml a ductile lower crust with visco plastic rheology allowing significant strength reduction and lateral density variations the special situation of a two sided rifting or offset rift segments to decouple the ml laterally from the surrounding continental lithosphere seems to be most decisive further support for the rid mechanism may come from additional crustal thickness and an extensive stress field some parameters such as the excess temperature and yield stress are very sensitive small changes determine whether delamination takes place or not

numerical simulations of the mantle lithosphere delamination 1 sudden uplift extension and increased igneous activity are often explained by rapid mechanical thinning of the lithospheric mantle two main thinning mechanisms have been proposed convective removal of a thickened lithospheric root and delamination of the mantle lithosphere along the moho in the latter case the whole mantle lithosphere peels away from the crust by the propagation of a localized shear zone and sinks into the mantle to study this mechanism we perform two dimensional 2 d numerical simulations of convection using a viscoplastic rheology with an effective viscosity depending strongly on temperature depth composition crust mantle and stress the simulations develop in four steps 1 we first obtain classical sublithospheric convection for a long time period 300 myr yielding a slightly heterogeneous lithospheric temperature structure 2 at some time in some simulations a strong thinning of the mantle occurs progressively in a small area 100 km wide this process puts the asthenosphere in direct contact with the lower crust 3 large pieces of mantle lithosphere then quickly sink into the mantle by the horizontal propagation of a detachment level away from the asthenospheric conduit or by progressive erosion on the flanks of the delaminated area 4 delamination pauses or stops when the lithospheric mantle part detaches or when small scale convection on the flanks of the delaminated area is counterbalanced by heat diffusion we determine the parameters crustal thicknesses activation energies and friction coefficients leading to delamination initiation step 2 we find that delamination initiates where the moho temperature is the highest as soon as the crust and mantle viscosities are sufficiently low delamination should occur on earth when the moho temperature exceeds 800 c this condition can be reached by thermal relaxation in a thickened crust in orogenic setting or by corner flow lithospheric erosion in the overriding lithosphere of subduction zones