formation and maintenance of shear zones the processes that initiate and maintain prograde ductile shear zones are not well understood we have performed shear experiments 1 to 3 on a fine grained 100 150 m gneiss 59 quartz 28 plagioclase 13 aligned but not interconnected biotite to determine the evolution of deformation mechanisms that produce dramatic strain weakening and localization at conditions 1 5 gpa 800 c dot gamma 2 10 5 s where pure quartz aggregates deform homogeneously by dislocation creep initial yield occurs where stress concentrations at the tips of weak biotite grains produce semibrittle deformation in intervening quartz or plagioclase allowing local biotite interconnection by slip on 001 and initiating strain weakening after yield the interconnected biotites kink and thus strengthen but the highly strained parts of grains react to a fine grained mixed phase assemblage which deforms by grain size sensitive creep allowing further strain weakening and localization at 3 3 the strain and strain rate in the 100 m thick shear zone are 100 times that of the enclosing host rock similar to the localization observed in natural shear zones thus the processes that initiate strain localization are not necessarily the same as those that preserve the weak shear zone and once formed a shear zone may be permanently weakened

pre caledonian granulite and gabbro enclaves in the western gneiss region norway indications of incomplete transition at high pressure the western gneiss region of norway is a continental terrane that experienced caledonian high pressure and ultrahigh pressure metamorphism most rocks in this terrane show either peak caledonian eclogite facies assemblages or are highly strained and equilibrated under late caledonian amphibolite facies conditions however three kilometre size rock bodies flatraket ulvesund and krakenes in outer nordfjord preserve pre caledonian igneous and granulite facies assemblages and structures where these assemblages are preserved the rocks are consistently unaffected by caledonian deformation the three bodies experienced high pressure conditions 20 23 kbar but show only very localized about 5 eclogitization in felsic and mafic rocks commonly related to shear zones the preservation of pre caledonian felsic and mafic igneous and granulite facies assemblages in these bodies therefore indicates widespread 95 metastability at pressures higher than other metastable domains in norway late caledonian amphibolite facies retrogression was limited the degree of reaction is related to the protolith composition and the interaction of fluid and deformation during the orogenic cycle whereby metastability is associated with a lack of deformation and lack of fluids either as a catalyst or as a component in hydration reactions the three bodies appear to have been far less reactive than the external gneisses in this region even though they followed a similar pressure temperature evolution the extent of metastable behaviour has implications for the protolith of the western gneiss region for the density evolution of high pressure terranes and hence for the geodynamic evolution of mountain belts

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

the brittle plastic transition in experimentally deformed quartz aggregates deformation experiments have been conducted to provide constraints on the processes responsible for the brittle plastic transition in quartz aggregates a correlation between mechanical behavior and distinctive microstructural characteristics indicates that the brittle plastic transition in nonporous quartzite involves at least three transitions in deformation mechanism that occur with increasing temperature and or pressure first there is a transition from cataclastic faulting to semibrittle faulting microstructural observations indicate that this transition occurs due to the activation of dislocations in addition faulting is more stable in the semibrittle faulting regime due to the blunting of the stresses at the advancing fault tip by dislocation glide second there is a transition from semibrittle faulting to semibrittle flow this transition corresponds to a change from localized to distributed deformation microstructural observations indicate that microcracks nucleate in response to stress concentrations at dislocation pileups in the semibrittle flow regime we conclude that the transition to semibrittle flow occurs when the stress intensity at crack tips is insufficient to allow propagation across grain boundaries third there is a transition from semibrittle flow to dislocation creep microstructural observations suggest that this transition occurs as a result of an increase in grain boundary mobility with increasing temperature in addition microstructural observations indicate that a transition from dominantly mode i axial to mode ii shear microcracking occurs with an increase in confining pressure from 0 4 to 0 8 gpa regardless of temperature the differential stresses supported by the experimentally deformed samples are higher than those expected under geologic conditions however a comparison of the experimentally produced microstructures to those reported from natural fault zones suggests that similar processes are operative in the laboratory and in the earth the results of this study provide further evidence to indicate that the brittle plastic transition in the continental crust occurs over a relatively wide range of conditions

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

softening trigerred by eclogitization the first step toward exhumation during continental subduction direct observation of peak pressure deformation in exhumed subduction channels is difficult because little evidence of this deformation survives later syn exhumation deformation most ultrahigh pressure parageneses are found in continental derived metamorphic rocks making continental subduction the best context to observe peak pressure deformation whereas many studies have enlightened the main driving parameters of exhumation such as buoyancy forces low viscosity in the subduction channel overburden removal by erosion and normal faulting a basic question is seldom considered why is a tectonic unit disconnected from the descending lithosphere and why does it start its way towards the surface this event seminal to exhumation processes must involve some deformation and decoupling of the exhumed slice from the descending slab at peak pressure conditions or close to it our field observations in the bergen arc show that caledonian eclogitization and later amphibolitization of a granulitic terrane was achieved with a consistent component of simple shear compatible with the sense of the caledonian subduction thus the sequence of deformation preserved in the bergen arc documents the decoupling of subducted crustal material from the descending slab at the onset of exhumation this observation suggests that deformation in the subduction channel is largely controlled by kinematic boundary conditions i e underthrusting of the subducting slab in this context of simple shear metamorphic reactions assisted by fracturating fluid infiltration and ductile deformation lower the resistance of rocks and allow the localisation of shear zones and the decoupling of buoyant tectonic units from the subducting slab these tectonic units can then be incorporated into the channel circulation and start their upward travel

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

the role of recrystallisation on the deformation behaviour of calcite rocks large strain torsion experiments on carrara marble abstract the rheological microstructural and textural evolution of carrara marble was studied during experimental deformation in torsion up to very large shear strains 0 4 50 experiments were performed at constant angular displacement rates corresponding to shear strain rates of 6 10 5 3 10 3 s 1 a confining pressure of 300 mpa and temperatures of 500 600 and 727 c microstructures and crystallographic preferred orientations cpo were analysed using light microscopy and electron backscatter diffraction ebsd at all conditions deformation occurred dominantly by dislocation creep even up to very large shear strains after peak stresses 2 recrystallisation mostly by subgrain rotation caused weakening 5 20 grain size reduction and a change in the cpo a monoclinic cpo that formed at small strains at all temperatures evolved into two different large strain cpos at different temperatures at large strains even after complete recrystallisation the cpo continued to strengthen and a secondary foliation was formed at a steep angle to the shear zone boundary this secondary foliation was continuously reset by both subgrain rotation and grain boundary migration recrystallisation and eventually overprinted the primary foliation misidentifying the primary and secondary foliations could result in misinterpretation of the applied strain field more generally information about the amount of strain is lacking for microstructures and cpos of highly deformed ultramylonites

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

pervasive eclogitization due to brittle deformation and rehydration of subducted basement effects on continental recycling the buoyancy of continental crust opposes its subduction to mantle depths except where mineral reactions substantially increase rock density sluggish kinetics limit such densification especially in dry rocks unless deformation and hydrous fluids intervene here we document how hydrous fluids in the subduction channel invaded lower crustal granulites at 50 60 km depth through a dense network of probably seismically induced fractures we combine analyses of textures and mineral composition with thermodynamic modeling to reconstruct repeated stages of interaction with pulses of high pressure hp fluid at 650 670 c rehydrating the initially dry rocks to micaschists sims oxygen isotopic data of quartz indicate fluids of crustal composition hp growth rims in allanite and zircon show uniform u th pb ages of 65 ma and indicate that hydration occurred during subduction at eclogite facies conditions based on this case study in the sesia zone western italian alps we conclude that continental crust and in particular deep basement fragments during subduction can behave as substantial fluid sinks not sources density modeling indicates a bifurcation in continental recycling chiefly mafic crust once it is eclogitized to 60 are prone to end up in a subduction graveyard such as is tomographically evident beneath the alps at 550 km depth by contrast dominantly felsic hp fragments and mafic granulites remain positively buoyant and tend be incorporated into an orogen and be exhumed with it felsic and intermediate lithotypes remain positively buoyant even where deformation and fluid percolation allowed them to equilibrate at hp

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

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

lithospheric pressure depth relationship in compressive regions of thickened crust we investigate the pressure distribution with depth in regions undergoing horizontal shortening and experiencing crustal thickening both analytically and numerically our results show that in a convergent tectonic setting pressure can be considerably higher than lithostatic the pressure resulting from the weight of the overburden increases in pressure with respect to lithostatic conditions result from both the contribution of horizontal stresses and the flexural vertical loads the latter generated by the deflection of the upper crust and of the mantle because of the presence of topographic relief and a root respectively the contribution of horizontal stresses is particularly relevant to the upper crust and uppermost mantle where rocks are thought to deform brittlely in these domains pressure gradients twice lithostatic can be achieved the contribution of horizontal stresses is less important in the ductile domains as diverential stresses are progressively relaxed nevertheless the evects are still noteworthy especially close to the brittle ductile transition flexural vertical loads generated by the deflection of the upper crust and lithospheric mantle are relevant for rocks of the weaker lower crust as a result of the combination of the two mechanisms the pressure gradient varies vertically through the lithosphere ranging from negative inverted gradients to gradients up to several times the lithostatic gradient the pressure values range from one to two times the lithostatic values 1rgz to 2rgz

changes in dip of subducted slabs at depth petrological and geochronological evidence from hp uhp rocks tianshan nw china article high resolution seismic imaging has previously revealed sudden changes in the dip of subducted oceanic plates this kinking feature is crucial because in many subduction zones it is thought to coincide with the disappearance of the low velocity layers associated with subducted oceanic crust in this study we present petrological evidence for this phenomenon derived from oceanic blueschist and eclogite facies rocks from the chinese tianshan the investigated samples span a large range of peak metamorphic conditions with temperatures between 330 and 580 c at 1 5 to 2 3 gpa such variable peak metamorphic conditions together with the intimate interlayering of high and ultrahigh pressure rocks have also been reported from other tianshan localities these observations suggest that the rocks were derived from varying depths within the subduction zone and then juxtaposed during exhumation in the subduction channel multi point lu hf isochrons from four high pressure rocks yield consistent garnet growth ages of 313 12 315 8 2 9 313 9 4 8 and 315 2 1 6 ma these inconjunctionwiththe 311 maclusterof published 40

the western alps from the jurassic to oligocene spatio temporal constraints and evolutionary reconstructions abstract despite extensive research in the last 150 years the regional tectonic reconstruction of the western alps has remained controversial the curved orogenic belt consists of several ribbon like continental terranes sesia austroalpine internal crystalline massifs brianconnais which are separated by two or more ophiolitic sutures piemonte valais antrona lanzo canavese high pressure hp metamorphism of each terrane occurred during distinct orogenic episodes at 65 ma in the sesia austroalpine at 45 ma in the piemonte zone and at 35 ma in the internal crystalline massifs it is suggested that these events reflect individual accretionary episodes which together with kinematic indicators and the speed and direction of plate motions provide constraints for the discussed reconstruction model the model involves a prolonged orogenic history that took place during relative convergence of europe and adria here considered as a promontory of the african plate the first accretionary event involved the sesia austroalpine terrane final closure of the piemonte ocean occurred during the eocene 45 ma and involved ultra high pressure uhp metamorphism of the piemonte oceanic crust incorporation of the brianconnais terrane in the accretionary wedge occurred thereafter possibly during or after subduction of the valais ocean in the late eocene 45 35 ma this subduction was terminated at ca 35 ma when the internal crystalline massifs i e the assumed internal parts of the brianconnais terrane were buried into great depths and underwent hp and uhp metamorphism

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

high temperature geochronology constraints on the tectonic history and architecture of the ultrahigh pressure dabie sulu orogen 1 new u pb zircon and th pb monazite ages are presented from the giant sulu ultrahigh pressure uhp terrane combined with sm nd ages rb sr ages inclusion relationships and geologic relationships they help define the timing of peak recrystallization the timing of subsequent amphibolite facies metamorphism and the architecture of the dabie sulu suture zone between the collided sino korean and yangtze cratons the data indicate a 15 myr record of uhp recrystallization the first clearly documented for a giant uhp terrane this requires that continental subduction in the dabie sulu orogen involved multiple uhp tectonic or recrystallization events a 244 236 ma precursor uhp event seen only in the dabie shan was followed by a second 230 220 ma main uhp event which was itself terminated by a 220 205 ma amphibolite facies overprint older eclogite facies events seen in the qinling segment of this orogenic belt raise the possibility that these rocks have undergone u hp metamorphism three or four times but at present there is no geochronological evidence in the dabie sulu area to support this the subduction of the lower yangtze plate did not proceed in a simple fashion the ages of inherited zircon cores demonstrate that a ribbon continent of yangtze affinity escaped subduction and became wedged against the sino korean plate hanging wall

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

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

kinematic linkage between internal zone extension and shortening in more external units in the nw alps in the alps a major extensional shear zone unroofed eclogites in its footwall from 30 kbar to c 10 kbar implying of the order of 60 km of relative vertical movement in the period 45 36 ma the horizontal component of divergence was of the order of 100 km we use the foreland basin sedimentary record to prove that the extension was contemporaneous with thrusting in units structurally beneath the eclogites the brianconnais and or external zones this thrust related convergence during that time period was of the order of 44 56 km and therefore of the same order of magnitude as internal zone divergence extension may have been driven by internal density contrasts buoyant continental crust underneath the mafic eclogites or dense subducted slab offset from the orogen can induce extension any dynamic model must account for the comparable magnitudes of extension and shortening in adjacent parts of the orogen relative plate motions in the period 45 36 ma played a subsidiary role

thermo mechanical modeling of the obduction process based on the oman ophiolite case obduction emplaces regional scale fragments of oceanic lithosphere ophiolites over continental lithosphere margins of much lower density for this reason the mechanisms responsible for obduction remain enigmatic in the framework of plate tectonics we present two dimensional 2d thermo mechanical models of obduction and investigate possible dynamics and physical controls of this process model geometry and boundary conditions are based on available geological and geochronological data and numerical modeling results are validated against petrological and structural observations of the oman semail ophiolite our model reproduces the stages of oceanic subduction initiation away from the arabian margin the emplacement of the oman ophiolite on top of it and the domal exhumation of the metamorphosed margin through the ophiolitic nappe a systematic study indicates that 350 400 km of bulk shortening provides the best fit for both the maximum pressure temperature conditions of the metamorphosed margin 1 5 2 5 gpa 450 600 c and the dimension of the ophiolitic nappe 170 km width our results confirm that a thermal anomaly located close to the arabian margin 100 km is needed to initiate obduction we further suggest that a strong continental basement rheology is a prerequisite for ophiolite emplacement

rheological and geodynamic controls on the mechanisms of subduction and hp uhp exhumation of crustal rocks during continental collision insights from numerical models while subduction of crustal rocks is increasingly accepted as a common scenario inherent to convergent processes involving continental plates and micro continents its occurrence in each particular context as well as its specific mechanisms and conditions is still debated the presence of ultra high pressure uhp terranes is often interpreted as a strong evidence for continental subduction subduction of continental crust since the latter is seen as the most viable mechanism of their burial to uhp depths yet if one admits nearly lithostatic pressure conditions in the subduction interface or channel the presumed links of continental subduction to exhumation of high and ultra high pressure hp uhp units also remain a subject of controversy despite the fact that recent physically consistent thermo mechanical numerical models of convergent processes suggest that subduction can create specific mechanisms for uhp exhumation we hence review and explore possible scenarios of subduction of continental crust and their relation to exhumation of hp and uhp rocks as inferred from last generation of thermo mechanical numerical models accounting for thermo rheological complexity and structural diversity of the continental lithosphere the inferences from these models are matched with the petrology data in particular with p t t paths allowing for better understanding of subtleties of both subduction and burial exhumation mechanisms numerical models suggest that exhumation and continental subduction are widespread but usually transient processes that last for less than 5 10 myr while long lasting 10 15 myr subduction can take place only in rare cases of fast convergence of cold strong lithospheres e g india the models also show that tectonic heritage can play a special role in subduction exhumation processes in particular when thicker continental terrains are embedded in subducting oceanic plate exhumation of uhp terranes results in the formation of versatile metamorphic belts and domes and in series of slab roll back and exhumation events with remarkably different p t t records

garnet omphacite phengite thermobarometry of eclogites from the coesite bearing unit of the southern dora maira massif western alps abstract using relevant geothermobarometric methods p t data were collected for the reconstruction of the metamorphic evolution of 34 eclogite samples taken from small lenses and boudins within the ultrahigh pressure uhp metamorphic coesite bearing brossasco isasca unit biu of the dora maira massif the mineral phases used clinopyroxene garnet phengite or growth zones thereof were identified as being coexistent for different stages of metamorphism on the basis of careful petrographic studies of several published geothermobarometers the garnet clinopyroxene thermometer of powell powell r 1985 regression diagnostics and robust regression in geothermometer geobarometer calibration the garnet clinopyroxene geothermometer revisited j metamorph geol 3 pp 231 243 combined with the garnet clinopyroxene phengite barometer after waters and martin waters d martin h n 1993 the garnet clinopyroxene phengite barometer terra abstr 5 pp 410 411 was chosen here because it provided the most reliable results nevertheless the scatter of p t data points for the prograde stage i peak metamorphic stage ii and retrograde stage iii development of the eclogites is still considerable among the many possible reasons for this inconsistency discussed a partial lack of equilibration of some of the eclogites during their metamorphic history should be taken into account despite the data scatter an average p t path could be estimated which includes the following coordinates for stage i 15 kbar 500 c 25 kbar 570 c 32 kbar 650 c for stage ii 36 kbar 720 c and for stage iii 24 kbar 680 c and 14 kbar 620 c this is in fair agreement with p t paths derived earlier for other rock types of the biu on the basis of other geothermobarometers

crustal flow modes in large hot orogens abstract crustal scale channel flow numerical models support recent interpretations of himalayan tibetan tectonics proposing that gravitationally driven channel flows of low viscosity melt weakened middle crust can explain both outward growth of the tibetan plateau and ductile extrusion of the greater himalayan sequence we broaden the numerical model investigation to explore three flow modes homogeneous channel flow involving laterally homogeneous crust heterogeneous channel flow involving laterally heterogeneous lower crust that is expelled and incorporated into the mid crustal channel flow and the hot fold nappes style of flow in which mid lower crust is forcibly expelled outward over a lower crustal indentor to create fold nappes that are inserted into the mid crust the three flow modes are members of a continuum in which the homogeneous mode is driven by gravitational forces but requires very weak channel material the hot fold nappe mode is driven tectonically by for example collision with a strong crustal indentor and can occur in crust that is subcritical for homogeneous flows the heterogeneous mode combines tectonic and gravitationally driven flows preliminary results also demonstrate the existence and behaviour of mid crustal channels during advancing and retreating dynamical mantle lithosphere subduction an orogen temperature magnitude t m diagram is proposed and the positions of orogens in t m space that may exhibit the flow modes are described together with the characteristic positions of a range of other orogen types

ultrarapid exhumation of ultrahigh pressure diamond bearing metasedimentary rocks of the kokchetav massif kazakhstan the diamond bearing ultrahigh pressure kokchetav massif recrystallized at eclogite facies conditions deep in the mantle at 180 km depth at 535f3 ma and yet new 40 ar 39 ar ages suggest that it may have been exhumed to crustal depths as indicated by closure of mica to ar loss by f529 ma these data indicate a possible exhumation rate of tens of kilometers per million years i e a rate that is comparable to rates of horizontal plate motion and subduction d 2003 elsevier b v all rights reserved

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

incorporating metamorphism in geodynamic models the mass conservation problem summary geodynamic models incorporating metamorphic phase transformations almost invariably assume the validity of the boussinesq approximation that violates conservation of mass in such models metamorphic density changes take place without volumetric effects we assess the impact of the boussinesq approximation by comparing models of orogeny accompanied by lower crustal eclogitization both with and without the approximation our results demonstrate that the approximation may cause errors approaching 100 per cent in characteristic measures of orogenic shape mass conservation errors in boussinesq models amplify with model time mass conservative models of metamorphism are therefore essential to understand long term tectonic evolution and to assess the importance of the different geodynamic processes

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

recycling of crustal materials through study of ultrahigh pressure minerals in collisional orogens ophiolites and mantle xenoliths a review abstract newly recognized occurrences of ultrahigh pressure uh p minerals including diamonds in ultrahigh temperature uh t felsic granulites of orogenic belts in chromitites associated with ophiolitic complexes and in mantle xenoliths suggest the recycling of crustal materials through deep subduction mantle upwelling and return to the earth s surface this circulation process is supported by crust derived mineral inclusions in deep seated zircons chromites and diamonds from collision type orogens from eclogitic xenoliths in kimberlites and from chromitities of several alpine himalayan and polar ural ophiolites some of these minerals contain low atomic number elements typified by crustal isotopic signatures ophiolite type diamonds in placer deposits and as inclusions in chromitites together with numerous highly reduced minerals and alloys appear to have formed near the mantle transition zone in addition to ringwoodite and inferred stishovite a number of nanometric minerals have been identified as inclusions employing state of the art analytical tools reconstitution of now exsolved precursor uh p phases and recognition of subtle decompression microstructures produced during exhumation reflect earlier uh p conditions for example tibetan chromites containing exsolution lamellae of coesite diopside suggest that the original chromitites formed at p 9 10 gpa at depths of 250 300 km the precursor phase most likely had a ca ferrite or a ca titanite structure both are polymorphs of chromite and at 2000 c would have formed at minimum pressures of p 12 5 or 20 gpa respectively some podiform chromitites and host peridotites contain rare minerals of undoubted crustal origin including zircon feldspars garnet kyanite andalusite quartz and rutile the zircons possess much older u pb ages than the time of ophiolite formation these uh p mineral bearing chromitite hosts evidently had a deep seated evolution prior to extensional mantle upwelling and partial melting at shallow depths to form the overlying ophiolite complexes these new findings together with stable isotopic and inclusion characteristics of diamonds provide compelling evidence for profound underflow of both oceanic and continental lithosphere recycling of surface organic carbon into the lower mantle and ascent to the earth s surface through mantle upwelling intensified study of uh p granulite facies lower crustal basement and ophiolitic chromitites should allow a better understanding of the geodynamics of subduction and crustal cycling

what drives orogeny in the andes the andes the world9s second highest orogenic belt were generated by the cenozoic tectonic shortening of the south american plate margin overriding the subducting nazca plate we use a coupled thermomechanical numerical modeling technique to identify factors controlling the intensity of the tectonic shortening from the modeling we infer that the most important factor was accelerated westward drift of the south american plate changes in the subduction rate were less important other important factors are crustal structure of the overriding plate and shear coupling at the plates9 interface the model with a thick 40 45 km at 30 ma south american crust and relatively high friction coefficient 0 05 at the nazca south american interface generates 300 km of tectonic shortening during 30 35 m y and replicates the crustal structure and evolution of the high central andes the model with an initially thinner

consequences of progressive eclogitization on crustal exhumation a mechanical study summary the very low water content of the granulitic unit of holsnoy in the bergen arcs norway caused its partial metastable preservation throughout caledonian burial end exhumation leading to the observed mixture of completely eclogitized and uneclogitized rocks the eclogitization of the granulite facies protolith led to a density increase 10 but also to a large rheological weakening that resulted in the localization of all deep ductile deformation in the eclogite fraction we address the consequences on subduction dynamics of the concomitant evolution of large scale density and rheology during progressive eclogitization of crust by comparing the behaviour of buried crust with various properties in the channel flow model large scale buoyancy and density are integrated in a single adimensional parameter the exhumation number which describes the capacity of the crust to exhume for a crust whose eclogitization causes a very large viscosity decrease burial resistant end member goes through a maximum in the partially eclogitized zone that is there is a given proportion of eclogite for which the crust is strongly weakened but still significantly lighter than surrounding mantle for this partially eclogitized crust with max the maximum possible downward flow is very low this zone acts as a bottleneck and all incoming crust in excess is forced backwards and starts exhuming this return flow zone is bounded downwards in the partially eclogitized zone by max while its upper limit propagates upwards at a constant rate the curve eclogite fraction controls the maximum proportion of eclogite of the crust that can be exhumed we also demonstrate that large exhumation rates reached by certain uhp units are incompatible with any stationary flow regime whatever the nature of subducted crust the additional presence of a weak layer on top of the channel favours burial for narrow channels and exhumation for large ones but does not qualitatively change the dynamics of the subduction channel this study shows that deep crustal circulation in subduction zones and exhumation from large depths are controlled by the evolution of large scale crustal properties with progressive eclogitization

to conserve or not to conserve mass in numerical models mass conservation is a fundamental physical law yet computer models aiming to simulate earth evolution commonly fail to respect it indeed most geodynamic models implement phase transitions of rocks such as metamorphism solid solid phase change or melting solid liquid phase change following a simplifying assumption dating back to 1897 which is conserving volume rather than mass the underlying problem is present at different scales illustrated here by three examples metamorphism in the continental crust phase changes in the mantle transition zone and melt crystallization during columnar jointing these illustrate that phase changes may become a driving force of a system s deformation and point to important differences with respect to simplified models developing and applying mass conserving approaches in future modelling tools are therefore not an option but a necessity

texture development of polycrystalline anhydrite experimentally deformed in torsion a polycrystalline aggregate of anhydrite was deformed in torsion to a maximum shear strain of 8 1 at 700 c and a maximum shear strain rate of 5 10 3 s 1 the crystallographic preferred orientation cpo or texture was investigated as a function of shear strain shear strain rate in a radial profile from the centre to the edge of the sample a deformation texture developed at shear strains of 1 5 2 corresponding to shear strain rates of 1 to 1 3 10 3 s 1 and reached a stable position relative to the kinematic frame at a shear strain of 3 7 2 3 10 3 s 1 further shear strain only led to a small increase in texture strength but no change in the orientation relative to the kinematic frame the cpo is very similar to naturally observed textures and can be explained by the activity of the 001 and 012 slip systems although independent mechanical data indicate that a change of mechanism from dislocation to diffusion controlled creep occurred at a shear strain of approximately 1 5 the texture does not weaken but rather increases in strength with higher shear strains

buoyancy driven rapid exhumation of ultrahigh pressure metamorphosed continental crust abstract preservation of ultrahigh pressure uhp minerals formed at depths of 90 125 km require unusual conditions our subduction model involves underflow of a salient 250 150 km wide 90 125 km long of continental crust embedded in cold largely oceanic crust capped lithosphere loss of leading portions of the high density oceanic lithosphere by slab break off as increasing volumes of microcontinental material enter the subduction zone buoyancy driven return toward midcrustal levels of a thin 2 15 km thick low density slice finally uplift backfolding normal faulting and exposure of the uhp terrane sustained over 20 million years rapid 5 mm year exhumation of the thin aspect ratio uhp sialic sheet caught between cooler hanging wall plate and refrigerating downgoing lithosphere allows withdrawal of heat along both its upper and lower surfaces the intracratonal position of most uhp complexes reflects consumption of an intervening ocean basin and introduction of a sialic promontory into the subduction zone uhp metamorphic terranes consist chiefly of transformed yet relatively low density continental crust compared with displaced mantle material otherwise such complexes could not return to shallow depths relatively rare metabasaltic metagabbroic and metacherty lithologies retain traces of phases characteristic of uhp conditions because they are massive virtually impervious to fluids and nearly anhydrous in contrast h2o rich quartzofeldspathic gneissose schistose more permeable metasedimentary and metagranitic units have backreacted thoroughly so coesite and other uhp silicates are exceedingly rare because of the initial presence of biogenic carbon and its especially sluggish transformation rate uhp paragneisses contain the most abundantly preserved crustal diamonds

eclogites of the snowbird tectonic zone petrological and u pb geochronological evidence for paleoproterozoic high pressure metamorphism in the western canadian shield eclogite occurs within the southern domain of the east athabasca mylonite triangle in northern saskatchewan situated at the boundary between the archean rae and hearne provinces of the western canadian shield the east athabasca mylonite triangle is a fundamental exposure of the 3 000 km long snowbird tectonic zone the eclogite occurs in association with a variety of lower crustal high pressure granulites that record a complex metamorphic history from 2 6 to 1 9 ga temperatures of the eclogite facies metamorphism are constrained by garnet clinopyroxene exchange thermometry at 920 1 000 c minimum pressure conditions are recorded by the jadeite quartz albite geobarometer at 1 8 2 0 gpa a near isothermal decompression path to granulite facies conditions is inferred from retrograde reaction textures involving the formation of granulite facies assemblages such as orthopyroxene plagioclase and pargasite plagioclase u pb idtims zircon geochronology of the eclogite yields a weighted mean 207pb 206pb date of 1 904 0 0 3 ma which we interpret as the time of peak eclogite facies metamorphism shrimp in situ analyses of metamorphic zircons included within omphacitic clinopyroxene support this interpretation with a weighted mean 207pb 206pb date of 1 905 19 ma inclusion suites of high pressure phases and the petrographic setting of zircon are a direct link between zircon growth and eclogite facies metamorphism zircon from one eclogite sample has older cores that are 2 54 ga which is a minimum age for the emplacement or earliest metamorphism of the gabbroic protolith u pb rutile data indicate slow cooling at 1 c ma below 500 c from 1 88 to 1 85 ga the formation and exhumation of the eclogites at ca 1 9 ga has important implications for the tectonic significance of the snowbird tectonic zone during the paleoproterozoic the eclogites described here are consistent with transport of continental crust to mantle depths during the paleoproterozoic followed by rapid buoyancy driven exhumation to normal lower crustal depths

shear senses and viscous dissipation of layered ductile simple shear zones velocity profiles and shear heat profiles for inclined layered newtonian simple shear zones are considered reverse fault like simple shear of the boundaries and upward net pressure gradient act together in such shear zones as the velocity of the boundary increases the point of highest velocity shifts from the lower layer of less viscosity into the upper layer the shear heat profile shows a temperature peak inside the lower layer for a more viscous upper layer the point of highest velocity is located inside the upper layer and shifts towards the upper boundary of the shear zone the shear heat profile shows a maximum temperature within the upper layer depending on the flow parameters of the two layers the slip rate of the boundary and the dip and thickness of the shear zone a shear sense in reverse to the relative movement of the shear zone boundaries may develop these models can decipher thermo kinematics of layered shear zones in plate scale hot orogens

nonlithostatic pressure during sediment subduction and the development and exhumation of high pressure metamorphic rocks a subduction shear zone can be modeled as a long narrow channel with the flow of subducted metasedimentary rocks in the channel driven by two sets of forces the downward shearing force exerted by the subducting slab and the gradient in the hydraulic potential which combines the effect of both pressure and buoyancy if the channel walls are effectively rigid very slight narrowing or broadening of the channel convergence angles 2 gpa in the channel at only 40 km depth the model is consistent with a horizontal balance of forces across the plates and with a reasonable value for the thickness of subducted sediment 650 m the practical limit for overpressures attainable in subduction zones is determined by the strength and permeability of the channel walls at 40 km depth the channel is effectively confined on both sides by cold lithospheric mantle which should be strong enough to support a significant tectonic overpressure episodic failure of the upper plate to produce great earthquakes at 30 40 km focal depth could vent overpressured fluid from the channel allowing a cyclical buildup and release of both rock and fluid pressure topography on the subducting plate e g seamounts and thinned continental crust may lead to an anvil like jamming of the channel and local high overpressures tectonic erosion by topography on the lower plate of slivers from overlying continental crust and the compression of these slivers between the topography and the narrowing channel walls could produce high overpressures in continental rocks a decrease in the convergence rate or cessation of subduction with a consequent general warming within the channel and associated viscosity decrease promotes exhumation by buoyant reverse flow the most rapid reverse flow occurs in the region of previously greatest overpressure since the exhumation distance is shorter than for a simple lithostatic pressure distribution and any increase in temperature is coupled with a strong increase in the rate of exhumation preservation of high pressure assemblages at the surface in fossil subduction zones is promoted for such a model

quantifying different modes of the late cenozoic shortening in the central andes since the late miocene the two main segments of the central andean high plateau altiplano and puna demonstrate different styles and magnitudes of tectonic shortening through numerical simulation of thermomechanical processes we show that different shortening modes pure and simple shear accompanied by thin or thick skinned tectonics might be controlled by strength of the foreland uppermost crust and by temperature of the foreland lithosphere mechanical weakening and failure of the thick paleozoic sediments overlying the cold lithosphere in the altiplano foreland at 13 9 ma explains the transition from pure to simple shear shortening accompanied by broad thin skinned thrusting started before the major uplift of the plateau however the high strength of the uppermost crust combined with a relatively warm lithosphere results in the thick skinned shortening typical for the foreland of the puna failure of paleozoic sediments in the altiplano foreland significantly reduces the force required to shorten the lithosphere which may be the reason for the increased bulk shortening rate in the late miocene

liquid co2 breakout and the formation of recent small gullies on mars we show that the action of a co2 suspended flow could have produced the recent small gullies on mars and hence that liquid water is not required the model involves the build up of a liquid co2 aquifer behind and below a dry ice barrier dam cap rock in the pore spaces a few meters into the rock from the cliff face and on order of a hundred meters below the top of the cliff brink surface seasonal or obliquity cycle seasonal heating causes pinching out of the dry ice barrier and rapid release of the liquid co2 erosion of the gullies occurs as the rapid vaporization of the liquid co2 with entrainment of rock and clathrate hydrate ice produces a density flow analogous to a terrestrial nue ardente

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

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

rapid exhumation of the zermatt saas ophiolite deduced from high precision smnd and rbsr geochronology sm nd isotope data from garnets in ultrahigh pressure coesite bearing eclogite from the western alps were obtained to determine the age of peak metamorphism and exhumation rates of deeply buried oceanic crust in the zermatt saas ophiolite complex we report an exceptionally well constrained sm nd isochron age of 40 62 6 ma from the lago di cignana eclogite difficulties in dating alpine eclogites using this method were overcome by using an hf acid leaching procedure on garnet to remove mineral inclusions the zermatt saas rocks reached greenschist facies conditions around 38 2m a on the basis of rb sr whole rock phengite isochrons these data combined with existing estimates of the pressure and temperature conditions of the eclogite give an estimate for the initial exhumation rate of 10 to 26 km m y depending on the inferred pressure for the recrystallization of phengite followed by slow exhumation of 0 3 km m y from 34 to 14 ma the initial exhumation rates at lago di cignana are among the highest yet determined for high pressure terranes and are well constrained because the age has been determined directly on the high pressure assemblage the rapid exhumation rates presented here place critical constraints on collisional tectonic models 1999 elsevier science b v all rights reserved

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

thinning of continental backarc lithosphere by flow induced gravitational instability abstract many continental backarcs have thin 60 km lithosphere for 100s of km behind the volcanic arc even where there has been no significant extension one mechanism to produce thin backarc lithosphere is through gravitational thinning of normal thickness lithosphere by subduction related mantle flow the stability of backarc mantle lithosphere is examined using thermal mechanical models of subduction of an oceanic plate beneath continental lithosphere with an initial thickness of 120 km and a thermal structure similar to average phanerozoic continental lithosphere subduction induced mantle flow shears the base of the backarc lithosphere producing lateral density perturbations owing to the non newtonian lithosphere rheology shearing also reduces the effective viscosity of the lowermost lithosphere enabling the density perturbations to become gravitationally unstable the perturbations develop into rapidly growing downwellings which result in removal of lower backarc lithosphere on timescales of 5 10 ma conductive heating and shearing of the remaining lithosphere results in a second more muted phase of gravitational instability and thinning lower lithosphere instability is enhanced by higher subduction rates weaker intrinsic rheology higher compositional density and hotter initial thermal structure the numerical model results are in good agreement with a buoyancy stability analysis which includes the vertical gradients in temperature and strain rate through the lithosphere as both rheology and density depend on lithosphere composition significant thinning may be restricted to continental mantle lithosphere that is fertile and contains a small amount of water to produce a final thickness comparable to that observed without lithospheric contraction or extension it is necessary to have a weak lithosphere rheology an initial thermal structure hotter than average phanerozoic continental geotherms or a combination of the two the region of thin lithosphere at the northern cascadia backarc coincides with terranes that were accreted to the north american craton the known fertile composition of the backarc lithosphere may allow it to be thinned while the drier more refractory craton lithosphere is resistant to thinning

continental subduction and a mechanism for exhumation of high pressure metamorphic rocks new modelling and field data from oman abstract the physical model presented reveals two principal regimes of continental subduction a highly compressional hc regime and a low compressional lc regime characterised by high and low pressure between the overriding and subducting plates respectively the pressure is inversely proportional to the pull force which depends on the difference between average density of the subducting lithosphere and density of the mantle the subducting continental crust reaches a maximum depth which is proportional to the strength of the crust inversely proportional to the interplate pressure and is ca 200 km on average the crust then fails forming a major crustal thrust the location of failure depends on the regime of subduction on the interplate pressure in the hc regime failure occurs near the front of the subduction collision zone whereas the crust fails at greater depth under the base of the overriding plate in the lc regime the failure is followed by buoyancy driven uplift of the subducted crustal slice while the lithospheric mantle keeps subducting the uplift causes a normal sense displacement formation of a normal fault along the upper surface of the crustal slice for the lc regime failure of the crust near the base of the overriding plate a rapid spontaneous crustal uplift intrusion into the interplate zone brings the deeply subducted crust to shallow depths under the pressure of this crust the frontal part of the overriding plate undergoes local extension and then fails forming a tectonic window the rising material the high pressure rocks is exhumed within this window in the hc regime uplift of the subducted crust after its failure in front of the overriding plate is possible only with erosion of the relief which provides an unloading effect allowing the subducted crustal slice to rise up the exhumation depth is generally smaller in this regime but the volume of the exhumed material is larger the hc subduction regime has been shown earlier to match the himalayan situation the lc regime fits the situation in the oman mountains considered in this paper

precollisional multistage exhumation of subducted continental crust the sesia zone western alps 1 the sesia zone within the tertiary arc of the western alps is a relic of the subducted part of the adriatic continental margin along the se border of the tethyan ocean the sesia zone comprises three basement nappes which individuated during late cretaceous 65 80 ma subduction to different depths at high pressure hp blueschist eclogite facies conditions peak pressures of 1 0 1 2 1 0 1 5 and 1 5 2 0 gpa the thrusts bounding these nappes developed where the crust was previously thinned during jurassic rifting crustal scale shear zones partly overprinted these early thrusts and exhumed coherent slices of crust containing hp rocks initial exhumation of the internal part of the accreted margin involved thrusting d1 and transpressional shearing d2 along a subvertical e w trending mylonitic shear zone under retrograde blueschist to greenschist facies conditions this exhumation was nearly isothermal to a depth of about 25 km where the basement nappes were juxtaposed subsequent exhumation of these nappes to a common depth of about 15 20 km occurred in the footwall of a greenschist facies top se extensional shear zone d3 preserved in some of the highest mountain peaks of the sesia zone new rb sr mineral ages constrain d2 to have occurred at about 60 65 ma and d3 at about 45 55 ma thus top se extensional exhumation was broadly coeval with eocene se directed subduction of the liguro piemont oceanic lithosphere beneath the adriatic margin slow cooling and erosional denudation of the sesia zone from 45 to 30 ma occurred in the hanging wall of the gressoney extensional shear zone d4 which itself contributed to the exhumation of eocene hp and ultra hp oceanic rocks in its footwall by 30 ma hp rocks of the sesia zone were intruded by shallow granitic plutons which were eroded and redeposited within volcanoclastic sediments oligo miocene insubric backfolding and thrusting d5 only exhumed northeastern parts of the sesia zone where hp metamorphism is absent or was overprinted by tertiary temperature dominated metamorphism most exhumation of continental hp rocks in the sesia zone therefore preceded tertiary alpine collision and coincided with late cretaceous to early tertiary subduction of the adriatic and tethyan lithosphere the transition from d2 to d3 in the sesia zone is interpreted to mark a change from high stress oblique se directed subduction and accretion of the distal adriatic continental margin to nw retreating low stress subduction of the liguro piemont oceanic lithosphere

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

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

intraplate tectonics feedback between radioactive thermal weakening and crustal deformation driven by mantle lithosphere instabilities abstract two dimensional viscous plastic numerical experiments show that weak radioactive crust may be susceptible to significant tectonic activity by the effect of rayleigh taylor type instability of the sub crustal lithosphere below a continental interior in particular crust having a localized or regional enrichment of radioactive elements can respond to an underlying mantle lithosphere downwelling by initial subsidence then thickening and uplift owing to the positive feedback among thickening heating and weakening such models may later undergo orogenic deflation as spontaneous crustal extension thinning and surface subsidence occur while mantle downwelling continues strong homogeneous crust subsides dynamically above mantle downwellings and undergoes essentially no internal deformation a uniform distribution of radioactive elements through the crust or a concentration at depth is much more effective in causing thermal weakening of the crust and subsequent intraplate deformation than with radioactive material concentrated nearer the surface for wide regions of high heat production in the crust the length scale of the intracrustal tectonic zone is controlled by the width of the mantle downwelling in contrast with narrow heat production regions which control their own length scale in general the numerical experiments demonstrate that the presence of radioactive elements may make the crust vulnerable to intraplate tectonic deformation and represent a primary control on the way the strength of the crust evolves during sub crustal forcing the results may explain first order features of intraplate tectonics including the subsidence mechanism for intracontinental basins orogenic crustal contraction thickening and surface uplift and subsequent extension and crustal deflation the processes have particular implications for regions of high radioactive heat production and juvenile crust which would more likely have a higher concentration of radioactive elements in the lower crust

symmetric and asymmetric lithospheric extension relative effects of frictional plastic and viscous strain softening 1 strain dependent rheologies may play a critical role in the deformation of the lithosphere and particularly in the development of focused shear zones we investigate the effects of strain softening on lithospheric extension using plane strain thermomechanical finite element model experiments parametric softening is specified as a linear decrease of the effective internal angle of friction the effective viscosity or both in the model rheologies the sensitivity of deformation to the choice of softening parameters is investigated in cases where the crust is either strongly or weakly coupled to the mantle lithosphere results are classified according to the symmetry s or asymmetry a of the deformation of the upper and lower lithosphere during rifting strain softening is required for rifting asymmetry but is not always sufficient a range of model tectonic styles occurs including pure and simple shear modes with focused shear zones mode selection is mostly determined by the feedback between two primary controls the dominant rheology and the parametric strain softening mechanisms listed above softening of the dominant rheology promotes asymmetric extension of that part of the lithosphere controlled by the dominant rheology model results are consistent with the proposed primary controls and the factors that contribute to these controls in particular decreasing and increasing the rifting velocity can change the mode by changing the dominant rheology asymmetry is strongest in coupled models which include a decrease in the internal angle of friction and have low rifting velocities

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

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

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

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

diffusion obstacles and opportunities in petrochronology many of the approaches in petrochronology are rooted in the assumption of equilibrium diffusion is an expression of disequilibrium the movement of mass in response to chemical potential gradients and isotopes in response to isotopic gradients it is extremely important that we be aware of how the effects of diffusion can place obstacles across our path towards petrochronologic enlightenment conversely the effects of diffusion also provide opportunities for understanding rates processes and conditions experienced by rocks the enormity of the field does not permit us to provide a comprehensive review of either the mathematics of diffusion or quantitative data that have been obtained relevant to the interpretation of diffusive processes in rocks and minerals many resources cover these topics including rimg volume 72 diffusion in minerals and melts zhang and cherniak 2010 and several textbooks crank 1975 glicksman 2000 particularly relevant to the discussion of petrochronology are summaries of the theory and controls on diffusion brady and cherniak 2010 zhang 2010 as well as diffusion rates in feldspar cherniak 2010a accessory minerals cherniak 2010b garnet ganguly 2010 mica pyroxene and amphibole cherniak and dimanov 2010 and melts zhang and ni 2010 zhang et al 2010 rather than duplicate that material our goal is to explore the obstacles and opportunities presented by the effects of diffusion as they inform the rates of petrologic processes to achieve this goal we emphasize key principles and illustrative examples quantitative interpretation of the effects of diffusion assumes predictability of numerous factors that may affect chemical or isotopic transport including temperature initial and boundary conditions water and oxygen fugacities activities of other components multiple mechanisms of diffusion and crystal chemistry coupling of the substitution of elements into different crystallographic sites additionally the extraction of meaningful ages durations of events and temperatures requires

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

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred