deformed monazite yields high temperature tectonic ages the deformation of monazite in the polymetamorphic sandmata granulite complex in india has been investigated by electron backscattered diffraction and sensitive high resolution ion microprobe shrimp quantitative microstructural analyses document the development of deformation twins in 100 001 and graphic 1 orientations low angle 10 boundary development associated with dislocation creep and the development of new grains due to dynamic recrystallization these data represent the first quantitative evidence of crystal plastic deformation of natural monazite shrimp u th pb analysis shows that the host monazite preserves discordant ages as old as 1666 28 ma along a trend from ca 1720 ma to ca 1000 ma with increasingly discordant ages recorded in zones of higher lattice distortion domains of recrystallized new grains within the monazite record a tightly clustered concordia age of 970 14 ma this age is interpreted to represent the timing of monazite dynamic recrystallization associated with deformation of the host protolith and is consistent with partial resetting and pb loss from domains deforming by dislocation creep the complex but systematic relationship between microstructure and age data in monazite provide the first direct evidence of pb isotope resetting during deformation the approach illustrates a new methodology for the dating of deformation events in high grade metamorphic rocks which are typically difficult to constrain 1 embed inline graphic 1 gif

the tectonic implications of high pressure metamorphism in the western alps five mineralogical stages are recognized in the metamorphism of alpine ophiolites from one area in the zermatt saas zone constraints on p t values indicate an abrupt change at the metamorphic peak 550 600 c 17 20 kbar from a cold positive slope of 25 c kbar or less to cooling initially without discernible change of pressure correspondence of the prograde path to the p t trend of t in c 150 23 p in kbar shown by a variety of alpine units suggests that they all have p t histories for which an england richardson 1977 relaxation model also england thompson 1984 is inappropriate major tectonism at the peak pressure is indicated by association between minerals kyanite chloritoid lawsonite in eclogite vein growths fluid action and thrust surfaces the proposed metamorphic environment involves thrust repetition subcreting ophiolite to the over riding slab in a thermally mature subduction zone at a depth of approximately 70 km preservation is favoured when ocean closure leads to subduction of sialic crust so providing a combination of buoyant uplift with the continued refrigeration by underthrusting of cold surface materials

new zircon shock phenomena and their use for dating and reconstruction of large impact structures revealed by electron nanobeam ebsd cl eds and isotopic u pb and u th he analysis of the vredefort domethis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh integrated electron nanobeam ebsd cl eds and isotopic measurements u pb u th he of zircon from the collar and centre of the 80 km wide central uplift of the 2020 3 ma vredefort impact structure reveal new shock features in a microstructural progression related to impact basin formation and degree of u pb age resetting 1 planar fractures in 1k0 and 1k2 orientation during initial shock wave compression 2 curviplanar fractures in 1k1 orientation now annealed which host glassy inclusions of partial melt of the host rock 3 microtwin lamellae in an orientation of 65 about 110 attributed to shock wave rarefaction 4 nucleation of impact age crystallites possibly on microtwins during post shock heating by impact melt and 5 crystal plastic deformation linked to crater modification of the core of the central uplift planar fracturing and microtwinning 20 gpa in cold shock zircon in granitoid at a radial distance of 25 km failed to reset zircon age single grain id tims data

empirical constraints on shock features in monazite using shocked zircon inclusions shock deformation microstructures in monazite have been systematically characterized for the first time in grains from the vredefort impact structure in south africa electron backscatter diffraction mapping has identified 12 unique orientations of monazite deformation twins including 7 orientations that have not previously been described in experiments or nature other shock features include planar deformation bands and strain free neoblasts which have been shown to date deformation shock twinned zircon inclusions within the deformed monazite require pressures of 20 gpa thus providing critical empirical constraints on formation conditions confirming a hypervelocity impact origin of the monazite microstructures the vredefort monazite grains described here represent the first case of using shocked mineral inclusions to empirically calibrate shock microstructures formed in the host mineral these results conclusively establish monazite as a recorder of shock deformation and highlight its use in identifying and dating impact structures

incomplete blueschist re crytallization in high grade metamorphics from the sesia lanzo unit vasario sparone subunit western alps a case history of metastability abstract the vasario sparone vs subunit is part of the sesia lanzo unit western alps and represents a thin and discontinuous slice of pre alpine deep continental crust it is tectonically interposed between the eclogitic mica schists and the gneiss minuti subunits metastable relics and a sequence of incomplete transformations controlled by a local equilibrium reaction mechanism are preserved in the vs subunit the vs rocks record a pre alpine high grade of metamorphism t 780 50 c p 6 kbar which is comparable to the metamorphism dated around 500 ma in the ivrea zone during the alpine evolution the vs rocks sustained an early blueschist stage t 300 400 c p 8 10 kbar of probable cretaceous age and a later low t low p stage of tertiary age the blueschist transformations are governed by slow reaction kinetics by the heterogeneous distribution of pervasive deformations and by syn metamorphic fluid phase in particular transient kyanite formed in the early stages of the blueschist re crystallization in response to a process controlled by fluid deficient conditions and by internal buffering of a h 2 o at low values

contrasted metamorphic evolutions in thrusted cover units of the brianconnais zone french alps a model for the conservation of hp lt metamorphic mineral assemblages the evolution of organic matter silicate and fluid phases in cover units of the three structural zones of the vanoise area allows one to distinguish differentp t cooling paths these paths depend on whether the cover has been transported western and northern vanoise or not southern vanoise all units first underwent a common high pressure low temperature hp lt metamorphic stage 300 c 6 kbar of blueschist type fe mg carpholite chloritoid facies the cover units transported on the extermal colder zone coal measure series metamorphosed in albite chlorite facies preserved their hp mineralogy fe mg carpholite lawsonite and organic matter content oils wet gases and kerogen while the unit which remained in contact with its more thermally conductive basement polymetamorphic now shows extensive greenschist facies overprinting breakdown of fe mg carpholite and lawsonite appearance of chlorite pyrophyllite chloritoid and clinozoisite absence of oils and wet gases these two tectonic evolutions might serve as a more general model for the preservation of hp lt mineral assemblages

mechanical twinning of monazite expels radiogenic lead mechanical twins form by the simple shear of the crystal lattice during deformation in order to test the potential of narrow twins in monazite to record the timing of their formation we investigated a ca 1700 ma monazite grain from the sandmata complex rajasthan india deformed at ca 980 ma by electron backscattered diffraction ebsd transmission electron microscopy tem and atom probe tomography apt apt 208 pb 232 th ages indicate that the twin was entirely reset by radiogenic pb loss during its formation at conditions far below the monazite closure temperature the results are consistent with a model where pb is liberated during rupture of rare earth element oxygen ree o bonds in the large ree o 9 polyhedra during twinning liberated pb likely migrated along fast diffusion pathways such as crystal defects the combination of a quantitative microstructural investigation and nanogeochronology provides a new approach for understanding the history of accessory phases

modelling landslide liquefaction mobility bifurcation and the dynamics of the 2014 oso disaster some landslides move slowly or intermittently downslope but others liquefy during the early stages of motion leading to runaway acceleration and high speed runout across low relief terrain mechanisms responsible for this disparate behaviour are represented in a two phase depth integrated landslide dynamics model that melds principles from soil mechanics granular mechanics and fluid mechanics the model assumes that gradually increasing pore water pressure causes slope failure to nucleate at the weakest point on a basal slip surface in a statically balanced mass failure then spreads to adjacent regions as a result of momentum exchange liquefaction is contingent on pore pressure feedback that depends on the initial soil state the importance of this feedback is illustrated by using the model to study the dynamics of a disastrous landslide that occurred near oso washington usa on 22 march 2014 alternative simulations of the event reveal the pronounced effects of a landslide mobility bifurcation t

relative geobarometry from high pressure rocks of quartzofeldspathic composition from the sesia zone western alps italy in the sesia zone of the western alps italy early alpine blueschist to eclogite facies metamorphism of rocks of quartzofeldspathic composition has produced the same high pressure assemblage of quartz na pyroxene na amphibole paragonite phengite zoisite garnet magnetite sphene and fe sulphide the qfs assemblage over an area 150 km2 relative gradients in pressure and temperature over this region are reflected in the variations in mineral chemistries of the individual phases of the quartzofeldspathic assemblage through continuous reactions mineralogical discontinuities do not occur in the qfs assemblage of this region increases in the jadeite content of the pyroxenes xjd 0 48 to xjd 0 93 and in the glaucophane content of the amphiboles xgl 0 89 to xgl 0 96 occur from the southwest to the northeast of the region studied analysis of coexisting garnets and pyroxenes indicate that the compositional variation of amphiboles and pyroxenes is associated with a decrease in the grossular component of the coexisting garnet zoned pyroxenes and garnets together with the regional trends in mineral chemistries suggest that the evolution of the qfs assemblage with increasing pressure may be modelled by pressure sensitive continuous reactions in which amphibole zoisite and the more jadeitic pyroxene constitute the high pressure assemblage chemographic constraints permit the positioning in pressure temperature space of the compositional isopleths of those model continuous reactions involving these phases which meet the textural and chemical criteria observed in the natural assemblages the low dp dt slope 20 bars c of these isopleths causes the continuous reactions to be useful for geobarometric calculations at pressures above the absolute breakdown of albite to jadeite plus quartz in addition the pseudobinary loops for the other continuous reactions which are potentially useful geobarometers and involve either the naalca 1mg 1 exchange or the mgca 1 exchange are calculated comparison of mineral chemistries with the isopleths yields a relative barometric scheme for the localities studied with these barometric observations it is possible to show that the p t path which the sesia body travelled towards the final recorded state was one of increasing pressure other blueschist and eclogite occurrences from syros and sifnos which contain rocks of quartzofeldspathic composition are also examined

thickening history of the western alps a model of evolution for the western segment of the alpine chain is presented available data concerning the structural metamorphic and geochronological evolution in the majorunits involved in the thickening processes are takeninto account io constrain the model the successive stages of the collisional history are correlated with the evolution of thermal structure the african paleomargin was deformed at the onset of collision just after the abduction of the oceanic crust after these initial stages progressive stacking affected more and more external domains of the european plate lastly the deformation of the entire pile is linked io the thermal reequilibration of the considered thickened crust the alpine chain is a good example of a collision zone where the interaction of thermal and mechanical effects can be treated quantitatively

zircon u pb strain chronometry reveals deep impact triggered flow abstract large 100 km meteorite impact cratering events play important roles in surface and biosphere evolution however their potential for widespread ductile modification of the lithosphere has been difficult to assess due partly to our inability to isotopically age correlate deep mineral fabrics with surface records we have integrated benchmark u pb zircon dating methods id tims shrimp with new microstructural techniques ebsd xrd to demonstrate that crystal plastic deformation can cause rapid out diffusion of radiogenic pb and accompanying trace element alteration in crystalline zircon we have used this phenomenon to directly date fabric in archean zircons and xenoliths of the lower crust of south africa at 2023 15 million years coeval with the 2020 3 million year old vredefort cratering event at surface with extent 20 000 km 2 our findings indicate that regional exogenic fabrics similar to high temperature tectonic fabrics exist in ancient crust moreover our results establish that crystal plastic deformation in the lithosphere can now be directly dated and linked to planetary evolution by zircon u pb strain chronometry

assessing the mechanisms of common pb incorporation into titanite abstract common pb the portion of non radiogenic pb within a u bearing mineral needs to be accurately accounted for in order to subtract its effect on u pb isotopic ratios so that meaningful ages can be calculated the propensity to accommodate common pb during crystallization or later is different across the range of u bearing minerals used for geochronology titanite frequently accommodates significant amounts of common pb however the most appropriate method to correct for this requires knowledge on the mechanism and timing of common pb incorporation information that is commonly difficult to extract in this study the spatial and compositional distribution of trace elements including pb in metamorphic titanites from a greenland amphibolite is investigated on the grain to nano scale titanites have an isotopically similar signature for both common and radiogenic pb in all grains but significantly different quantities of the non radiogenic component microstructural and compositional examination of these grains reveals undeformed but high common pb f207 titanites have homogeneous element distributions on the atomic scale suggesting common pb is incorporated into titanite during its growth and not during later processes in contrast deformed titanite comprising low angle boundaries formed by subgrain rotation recrystallization comprise networks of dislocations that are enriched in mg al k and fe smaller cations may migrate due to elastic strain in the vicinity of the dislocation network yet the larger k cations more likely reflect the mobility of externally derived k along the orientation interface the absence of pb enrichment along the boundary indicates that either pb was too large to fit into migrating lattice dislocations or static low angle boundaries and or that there was no external pb available to diffuse along the grain boundary as the common pb composition is distinctly different to regional pb models the metamorphic titanite grew in a homogeneous pb reservoir dominated by the break down of precursor u bearing phases the different quantity of common pb in the titanite grains indicates a mineral driven element partitioning in an isotopically homogeneous metamorphic reservoir consistent with low u low total ree and flat lree signatures in high f207 analyses these results have implications for the selection of appropriate common pb corrections in titanite and other accessory phases

high pressure metamorphism in the nappes of the valle dell orco traverse western alps collisional belt a detailed field and petrological study of rocks from nappes cut by the valle dell orco italian western alps in particular the sesia lanzo composite unit has revealed geological and metamorphic histories which started in pre alpine times and lasted up to the alpine subduction collisional processes during these processes the nappes sustained an early high p low t stage and a later low p greenschist facies stage but followed partly distinctive p t time trajectories this paper discusses the kinematic evolution and the thermal history of the alpine belt from the early subduction underthrust to the later exhumation stage the metamorphic crystallization is often governed by incomplete and or local equilibrium and the pervasive syn metamorphic deformation and the composition of the syn metamorphic fluid phase if present have exerted an effective local control on reaction kinetics

the importance of serpentinite mylonites for subduction and exhumation of oceanic crust abstract in the ultramafic erro tobbio unit voltri massif western alps a set of overprinting structures in serpentinite mylonites is related to alpine subduction to about 80 km depth and to subsequent exhumation antigorite mylonites are cut by en echelon olivine veins which in turn are dissected by multiple sets of shear bands containing olivine and titanian clinohumite the transition from olivine free to olivine bearing structures indicates recrystallization during prograde metamorphism all structures display the same top to the nw kinematics providing evidence for a continuous non coaxial deformation the serpentinite mylonites surround km scale bodies of pre alpine peridotite which show only minor alpine overprint this indicates that during subduction related deformation recrystallization and fluid flow were strongly localized within serpentinite mylonites olivine bearing discontinuous shear planes with top to the se sense of movement crosscut the prograde structures the inversion of shear sense suggests a change in position of the serpentinites relative to the downgoing slab i e from the subducted slab to the upper plate during accretion thus the shear sense inversion marks the change from burial to exhumation of the serpentinites the low density of antigorite serpentinites 2 75 g cm3 causes strong buoyancy thus providing a mechanism for the exhumation of deeply subducted rocks it is suggested that serpentinites may act as carriers for the uprise of eclogite bodies which have higher densities than the peridotitic upper mantle

tectonic implications of lawsonite formation in the sesia zone western alps lawsonite of early alpine age formed in different tectonic sub units of the sesia zone western italian alps in the southern part of the eclogitic micaschists subunit em lawsonite grew with quartz jadeite during the early alpine retrograde path which just post dates the eclogite thermal peak the early retrograde path of the southern em occurred at decreasing temperature and at p t conditions comparable to those of the metamorphic peak in some blueschist facies thrust sheets which separate the em from the southern alps such a post eclogitic cooling path may have been induced by conductive cooling linked to migration of the underthrust surfaces and to stacking with colder nappes like the other sesia subunits or the piemonte metaophiolites stacking and thermal interaction between the different units probably occurred at depth during the early stages of exhumation which followed subduction underthrusting of early mid cretaceous age during the paleogene continental collision between the european and african plates determined further underthrusts of more westerly units these units underwent a high pressure very low temperature recrystallization meso alpine event characterized by the growth of lawsonite at the peak metamorphic conditions consequently propagation of the underthrust surfaces may represent an effective mechanism to prevent the rise of the isotherms and to favor post peak cooling paths in some units of the western alps this effect is more obvious in the structurally upper units which were first brought to deep crustal levels in the early alpine imbricated subduction zone

multiple metamorphic stages within an eclogite facies terrane sesia zone western alps revealed by th u pb petrochronology convergent plate margins typically experience a transition from subduction to collision dynamics as massive continental blocks enter the subduction channel studies of high pressure rocks indicate that tectonic fragments are rapidly exhumed from eclogite facies to midcrustal levels but the details of such dynamics are controversial to understand the dynamics of a subduction channel we report the results of a petrochronological study from the central sesia zone a key element of the internalwestern alps this comprises two polymetamorphic basement complexes eclogitic micaschist complex and gneiss minuti complex and a thin dismembered cover sequence scalaro unit associated with pre alpine metagabbros and metasediments bonze unit structurally controlled samples from three of these units eclogitic micaschist complex and scalaro bonze units yield unequivocal petrological and geochronological evidence of two distinct high pressure stages ages u th pb of growth zones in accessory allanite and zircon combined with inclusion and textural relationships can be tied to the multi stage evolution of single samples two independent tectono metamorphic slices showing a coherent metamorphic evolution during a given time interval have been recognized the fondo slice which includes scalaro and bonze rocks and the druer slice belonging to the eclogitic micaschist complex the new data indicate separate stages of deformation at eclogite facies conditions for each recognized independent kilometer sized tectono metamorphic slice between 85 and 60 ma with evidence of intermittent decompression p 0 5 gpa within only the fondo slice the evolution path of the druer slice indicates a different p t time evolution with prolonged eclogite facies metamorphism between 85 and 75ma our approach combining structural petrological and geochronological techniques yields field based constraints on the duration and rates of dynamics within a subduction channel

crustal reworking and hydration insights from element zoning and oxygen isotopes of garnet in high pressure rocks sesia zone western alps subduction zones represent one of the most critical settings for fluid recycling as a consequence of dehydration of the subducting lithosphere a better understanding of fluid flows within and out of the subducting slab is fundamental to unravel the role of fluids during burial in this study major and trace element geochemistry combined with oxygen isotopes were used to investigate metasediments and eclogites from the sesia zone in order to reconstruct the effect of internal and external fluid pulses in a subducted continental margin garnet shows a variety of textures requiring dissolution precipitation processes in presence of fluids in polycyclic metasediments garnet preserves a partly resorbed core related to pre alpine high temperature low pressure metamorphism and one or multiple rim generations associated with alpine subduction metamorphism in eclogites garnet chemical zoning indicates monocyclic growth with no shift in oxygen isotopes from core to rim in metasediments pre alpine garnet relics show 18o values up to 5 3 higher than the alpine rims while no significant variation is observed among different alpine garnet generations within each sample this suggests that an extensive re equilibration with an externally derived fluid of distinct lower 18o occurred before or in correspondence to the first alpine garnet growth while subsequent influxes of fluid had 18o close to equilibrium the observed shift in garnet 18o is attributed to a possible combination of 1 interaction with sea water derived fluids during pre alpine crustal extension and 2 fluids from dehydration reactions occurring during subduction of previously hydrated rocks such as the serpentinised lithospheric mantle or hydrated portions of the basement

fluid migration above a subducted slab constraints on amount pathways and major element mobility from partially overprinted eclogite facies rocks sesia zone western alps

numerical simulations of an ocean continent convergent system influence of subduction geometry and mantle wedge hydration on crustal recycling the effects of the hydration mechanism on continental crust recycling are analyzed through a 2 d finite element thermomechanical model oceanic slab dehydration and consequent mantle wedge hydration are implemented using a dynamic method hydration is accomplished by lawsonite and serpentine breakdown topography is treated as a free surface subduction rates of 1 3 5 7 5 and 10 cm yr slab angles of 30 45 and 60 and a mantle rheology represented by dry dunite and dry olivine flow laws have been taken into account during successive numerical experiments model predictions pointed out that a direct relationship exists between mantle rheology and the amount of recycled crustal material the larger the viscosity contrast between hydrated and dry mantle the larger the percentage of recycled material into the mantle wedge slab dip variation has a moderate impact on the recycling metamorphic evolution of recycled material is influenced by subduction style tpmax generally representative of eclogite facies conditions is sensitive to changes in slab dip a direct relationship between subduction rate and exhumation rate results for different slab dips this relationship does not depend on the used mantle flow law thermal regimes predicted by different numerical models are compared to pt paths followed by continental crustal slices involved in ancient and recent subduction zones making ablative subduction a suitable precollisional mechanism for burial and exhumation of continental crust

a thermal tectonic model for high pressure metamorphism and deformation in the sesia zone western alps the sesia zone western alps is a slice of austroalpine continental crust that was once metamorphosed at t 500 560 c and p 14 16 kbar at 130 100 ma b p pervasive blueschist eclogite facies assemblages record this event and a later greenschist facies overprint is only well developed in the nw part of the zone on the basis of thermal calculations it is difficult to explain the preservation of high pressure assemblages in a simple model of underthrusting and subsequent uplift erosion an alternative model is proposed in which the sesia zone was emplaced by underthrusting of the austroalpine continental margin prior to subduction of oceanic lithosphere of the piemonte basin subduction of relatively cool oceanic lithosphere below the underthrust sesia zone then continued for a period of time e g 20 ma thus suppressing thermal recovery of this latter unit exhumation of sesia zone rocks began during this subduction episode by uplift on the insubric line erosion and by underplating of oceanic

crystal chemistry of the monazite and xenotime structures monazite and xenotime the re p04 dimorphs are the most ubiquitous rare earth re minerals yet accurate structure studies of the natural phases have not been reported here we report the results of high precision structure studies of both the natural phases and the synthetic re p04 phases for all individual stable rare earth elements monazite is monoclinic p2 n and xenotime is isostructural with zircon space group 14 amd both atomic arrangements are based on 001 chains of intervening phosphate tetrahedra and re polyhedra with a reogpolyhedron in xenotime that accommodates the heavy lanthanides tb lu in the synthetic phases and a re09 polyhedron in monazite that preferentially incorporates the larger light rare earth elements la gd as the structure transforms from xenotime to monazite the crystallographic properties are comparable along the 00i chains with structural adjustments to the different sizes of re atoms occurring principally in 00i there are distinct similarities between the structures that are evident when their atomic arrangements are projected down 001 in that projection the chains exist in 100 planes with two planes per unit cell in monazite the planes are offset by 2 2 a along 010 relative to those in xenotime in order to accommodate the larger light re atoms the shift of the planes in monazite allows the re atom in that phase to bond to an additional 02 atom to complete the re09 polyhedron

uplift and exhumation of high pressure metamorphic terrains the example of the cycladic blueschist belt aegean sea the cycladic blueschists emerged from a depth of more than 50 km during the tertiary these rocks are now covered only by a thin fraction of the synmetamorphic overburden upper plate remnants of the upper plate include low p crystalline rocks and sediments which have been juxtaposed onto the lower plate blueschists in several stages since the early miocene the low angle contacts operated as normal faults which removed the overburden from above the cycladic blueschists detailed investigations on tinos island indicate that the fractured blocks of the upper plate lying above a low angle normal fault did not rotate this shows that the faults initiated as rather gently dipping fractures similar structures were also reported from other blueschist terrains kinematic analysis of such flat lying faults implies that the synmetamorphic overburden should be mostly located several hundred kilometers away from the present exposure of the blueschists considering that the cycladic blueschist belt exposes successively shallower units in the west and that rocks of the upper plate have an anatolian nature we suggest that these blueschists were dragged from beneath turkey we recognize that the blueschist unit has undergone further thrusting in the final stage of the exhumation as have its counterparts elsewhere in the hellenides the emergence of the blueschists in the tertiary was concomitant with continental collision and thrusting in the hellenides the operation of low angle normal faults with large extensional displacements occurred within this context of collision tectonics this extension occurred parallel to the strike of the convergent orogen

detrital high pressure low temperature minerals in a late turonian flysch sequence of the eastern alps western austria implications for early alpine tectonics along the northern margin of the eastern alps structurally intercalated between the main austroalpine thrust and the pennine rhenodanubic flysch nappe a thin tectonically complex zone occurs that comprises a series of tectonic melanges and imbricates of oceanic south pennine and distal continental margin austroalpine origin this zone is the lateral continuation of the ophiolite bearing arosa zone of the swiss alps we refer to it as the walsertal zone in a broken flysch formation of the walsertal zone turbiditic sandstones yield detrital grains of the high p low t metamorphic minerals lawsonite and glaucophane crossite ferroglaucophane the sandstones are dated as late turonian to earliest coniacian about 87 to 89 ma this age falls into the time span of radiometric ages of subduction related high p low t metamorphism along the south pennine austroalpine convergent margin and of coeval cooling ages of medium p medium t metamorphism in the middle austroalpine nappes if the high p low t minerals are related to mid cretaceous high p low t metamorphism as we think the high p metamorphic rocks must have been brought up from about 20 km depth within about 10 m y or even less as isostatic uplift alone would have been too slow alternative tectonic mechanisms such as corner flow must be responsible for rapid uplift and erosion in a fore arc environment

formation mechanisms of planar deformation features in naturally shocked quartz abstract shock waves induce peculiar defects in quartz shock mosaics high pressure polymorphs coesite and stishovite diaplectic glass and planar deformation features pdfs together these features are the indices of shock metamorphism pdfs appear under the optical microscope as straight and narrow defects parallel to the 10 1n rhombohedral planes with n 3 and 2 as most frequent values although n 1 4 and i e basal plane are also found we report in the first part of this paper a detailed investigation by transmission electron microscopy of the fine structure of pdfs in shocked quartz grains from a variety of sites slate islands la malbaie and manson in north america the vredefort complex in south africa the ries crater in germany and the cretaceous tertiary boundary layer at raton basin colorado we distinguish four pdf fine structures 1 bands of dislocations 2 lamellae of mixtures in various proportions of amorphous silica and small crystallites 3 thin brazil twin lamellae 4 short parallel lamellae forming serrated ladder structures the possible formation mechanisms of these pdfs are discussed in the second part of the paper it is suggested that the dislocation bands are probably not original pdf structures but stem from a later recrystallization stage the brazil twins have been produced by relatively large deviatoric stresses which accompanied the shock wave the other pdf structures which consist of partial amorphization in the 10 1 n planes must result from elastic instabilities of the shear modulus of the quartz structure at high pressure in these planes theoretical estimates of the elastic stiffness coefficients indicate that amorphization should start at approximately 10 gpa the growth of the straight and narrow amorphous lamellae would be driven by the front of the shock wave

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

resolution of impact related microstructures in lunar zircon a shock deformation mechanism map abstract the microstructures of lunar zircon grains from breccia samples 72215 73215 73235 and 76295 collected during the apollo 17 mission have been characterized via optical microscopy cathodoluminescence imaging and electron backscatter diffraction mapping these zircon grains preserve deformation microstructures that show a wide range in style and complexity planar deformation features pdfs are documented in lunar zircon for the first time and occur along 001 110 and 112 typically with 0 1 25 m spacing the widest pdfs associated with 112 contain microtwin lamellae with 65 misorientation relationships deformation bands parallel to 100 planes and irregular low angle misorientation axes this geometry is consistent with a dislocation glide system with 010 during dislocation creep nonplanar fractures recrystallized domains with sharp irregular interfaces and localized annealing textures along fractures are also observed no occurrences of reidite were detected shock deformation microstructures in zircon are explained in terms of elastic anisotropy of zircon pdfs form along a limited number of specific hkl planes that are perpendicular to directions of high young s modulus suggesting that pdfs are likely to be planes of longitudinal lattice damage twinned 112 pdfs also contain directions of high shear modulus a conceptual model is proposed for the development of different deformation microstructures during an impact event this shock deformation mechanism map is used to explain the relative timing conditions and complexity relationships between impact related deformation microstructures in zircon

3d reconstruction of fabric and metamorphic domains in a slice of continental crust involved in the alpine subduction system the example of mt mucrone sesia lanzo zone western alps geological mapping multiscale structural analysis and estimations of the degree of fabric evolution and of reaction progress allow the construction of a 3d quantitative model of structural and metamorphic gradients in a portion of continental crust deeply involved in the alpine subduction system and mainly structured under eclogite facies conditions before the continental collision the investigated and modelled rocks outcrop in the surroundings of mt mucrone in the central sesia lanzo zone the geomodeller software allowed a quantitative 3d estimation of domains characterized by homogeneous fabric evolution and metamorphic reaction progress dfe and drp respectively for two of the seven structural and metamorphic imprints detected in this area the d2 eclogitic and d5 greenschist stages which are the most pervasive at km scale such a 3d modelling clarifies mutual relationships between fabric and metamorphic gradients and indicates that i dfe and drp are closely related regardless of the rock type ii the syn deformational thermal regime can influence the degree of metamorphic transformation if dfe remains below the 60 threshold iii the phase transitions can not be properly implemented in quantitative geodynamic modelling without considering the heterogeneity of reaction progress and fabric evolution

nanoscale distribution of pb in monazite revealed by atom probe microscopy abstract the widespread use of monazite lreepo4 in u pb geochronology is underpinned by the assumption that it incorporates negligible amounts of pb during initial growth and that radiogenic pb remains immobile after formation we have investigated the nanoscale distribution of pb in monazite from granulite facies rocks of the sandmata metamorphic complex rajasthan india by atom probe microscopy to further understand the utility of monazite as a geochronometer the studied monazite contains distinct 10 nm clusters enriched in ca and with a bulk composition consistent with them being apatite ca5 po4 3 oh that are also enriched in si and pb relative to the monazite host the 208pb 232th ratios of the clusters ranged from 1 1 0 1 to 1 4 0 2 2 indicating that the clusters hold unsupported pb the 208pb 232th ratios of the whole specimen including clusters and the matrix alone are similar

calcite twin morphology a low temperature deformation geothermometer abstract twinning of the e plane is the dominant crystal plastic deformation mechanism in calcite deformed below about 400 c calcite in a twin domain has a different crystallographic orientation from the host calcite grain so called thin twins appear as thin black lines when viewed parallel to the twin plane at 200 320 magnification under a petrographic microscope thick twins viewed in the same way have a microscopically visible width of twinned material between black lines calcite e twin width and morphology has been correlated with temperature of deformation in naturally deformed coarse grained calcite in this paper we present a compilation and analysis of data from limestones of the frontal alps france and switzerland and the appalachian valley and ridge and plateau provinces eastern united states to document this temperature dependence mean calcite twin width correlates directly with temperature of deformation such that thin twins dominate below 170 c and thick twins dominate above 200 c above 250 c dynamic recrystallization is an important deformation mechanism in calcite mean twin intensity twin planes mm correlates negatively with temperature and a cross plot of twin intensity with twin width can yield information about both strain and temperature of deformation these relationships provide a deformation geothermometer for rocks that might otherwise yield little or no paleotemperature data

structural evolution of the contact between two penninic nappes zermatt saas zone and combin zone western alps and implications for the exhumation mechanism and palaeogeography the boundary zone between two penninic nappes the eclogite facies to ultrahigh pressure zermatt saas zone in the footwall and the blueschist facies combin zone in the hanging wall has been interpreted previously as a major normal fault reflecting synorogenic crustal extension quartz textures of mylonites from this fault were measured using neutron diffraction together with structural field observations the data allow a refined reconstruction of the kinematic evolution of the pennine nappes the main results are 1 the contact is not a normal fault but a major thrust towards northwest which was only later overprinted by southeast directed normal faulting 2 exhumation of the footwall rocks did not occur during crustal extension but during crustal shortening 3 the sesia dent blanche nappe system originated from a continental fragment cervinia in the alpine tethys ocean and the combin zone ophiolites from the ocean basin southeast of cervinia 4 out of sequence thrusting played a major role in the tectonic evolution of the penninic nappes

geochemical and k ar isotopic behaviour of alpine sheet silicates during polyphased deformation abstract several deformational events with several generations of phengites occur in the schistes lustres from the northern cottic alps the phengites belong mainly to 1 1 old forms from a s 1 schistosity 2 2 deformed forms from s 2 cleavage tightening zones 3 3 neoformed forms from second generation stretching zones the si tetrahedral substitutions of these minerals which lie between 3 28 and 3 46 depend on their microstructural site this chemical variation cannot only be related to p t conditions it is also induced by deformation related solution deposition processes the k ar apparent ages of the phengites are widely scattered between 38 and 65 ma the data when plotted in an isochron diagram suggest a two step response at 58 54 ma and 50 49 ma of the phengites to deformation this could happened during the same event which produced the second generation of microstructures