high precision microsampling and preparation of zoned garnet porphyroblasts for sm nd geochronology abstract zoned garnet porphyroblasts contain one of the longest continuous records of tectonic processes with growth histories spanning many millions of years well known challenges exist in the physical preparation purification and geochronologic analysis of zoned garnets which have limited the progress and application of this potentially powerful geochronologic tool here we present details of an integrated methodology specifically for the acquisition of high resolution sm nd geochronologic records from zoned garnets that overcome these challenges we develop and demonstrate our methods on large test garnets from a shear zone in the tauern window in western austria chemically contoured microdrilling permits physical sampling of many discrete age annuli each as narrow as 100 m within a single garnet refined hno 3 hf hclo 4 partial dissolution methods cleanse coarse crushed 75 150 m size garnet annuli of detrimental inclusions yielding high 147 sm 144 nd ratios 0 9 to 4 6 and consequently accurate and precise ages experiments on powdered garnet 147 sm 144 nd 143 nd 144 nd with 10 20 ppm 2 rsd precision good enough for 147 sm 144 nd 1 0 in total this integrated sampling and analytical protocol permits the acquisition of 3 11 discrete high precision

garnets recording deep crustal earthquakes syn eclogite facies pseudotachylytes of the bergen arcs western norway were formed during co seismic faulting at a depth of 60 km during caledonian continent collision concurrent with fluid induced partial eclogitization of precambrian anorthositic granulites the pseudotachylytes contain the high pressure assemblage omphacite garnet and kyanite and formed in the metastable granulites at the border of eclogitized areas rock volumes corresponding to outcrops of 100 m2 in area were shattered by pseudotachylytes associated with ultramylonites cataclasites and shear zones the granulite facies garnets unzoned and inclusion free away from the pseudotachylyte veins display planar and curved fractures brecciation and melting with decreasing distance from the veins sieve textured garnet developed by the disintegration of granulite facies garnet crystallization of eclogite facies mineral inclusions and welding together of garnet fragments fractured granulite facies garnets trend to more almandine rich compositions with variable grossular contents in the pseudotachylyte veins a second generation of minute 10 200 m euhedral or dendritic garnets has a higher femg ratio than the granulite facies garnet and a wide range in grossular content grs11 grs89 suggesting rapid growth and cooling fracturing of garnet and its preferential disappearance in the pseudotachylyte show that garnet has a relatively low fracture toughness seismic faulting caused enhanced reaction rates during ongoing fluid induced eclogitization of the deep crust by increasing the surface to volume ratio of garnet and its dislocation density the sudden volume reduction associated with eclogitization of metastable crust may have nucleated further faulting and pseudotachylyte formation

accelerated hydration of the earth s deep crust induced by stress perturbations the metamorphic cycle associated with the formation of mountain belts produces a lower crust containing little or no free fluid1 2 the introduction of external fluids to dry and impermeable volumes of the earth s crust is thus a prerequisite for the retrogressive metamorphism later observed in such regimes such metamorphism can cause significant changes in the crust s physical properties including its density rheology and elastic properties3 4 on a large scale the introduction of fluids requires the presence of high permeability channels such as faults or fractures which are the result of external tectonic stresses but extensive interaction between externally derived fluids and the fractured rock requires efficient mass transport away from the initial fractures into the rock itself and this transport often occurs over distances much longer than expected from grain boundary diffusion here we present both field observations and a simple network model that demonstrate how the transport of fluids into initially dry rock can be accelerated by perturbations in the local stress field caused by reactions with fluids we also show that the morphology of reaction fronts separating dry from wet rocks depends on the anisotropy of the external stress field

the effect of fluid and deformation on zoning and inclusion patterns in poly metamorphic garnets within the bergen arcs of w norway caledonian eclogite facies assemblages t 650 c p 15 kbar have formed from grenvillian granulites t 800 900 c p 10 kbar along shear zones and fluid pathways garnets in the granulites grti pyr56 40 alm45 25gro19 14 are unzoned or display a weak ca 1 wt feo over 1000 m zoning the eclogite facies rocks contain garnets inherited from their granulite facies protoliths these relict garnets have certain areas with compositions identical to the garnets in their nearby granulite but can be crosscut by bands of a more almrich composition grtii pyr31 41alm40 47gro17 21 formed during the eclogite facies event these bands orientated preferentially parallel or perpendicular to the eclogite foliation may contain mineral filled veins or trails of eclogite facies minerals omphacite amphibole white mica kyanite quartz and dolomite steep compositional gradients up to 9 wt feo over 40 m separate the two generations of garnets indicating limited volume diffusion the bands are interpreted as fluid rich channels where element mobility must have been infinitely greater than it was for the temperature controlled volume diffusion at mineral interfaces in the granulites the re equilibration of granulite facies garnets during the eclogite facies event must therefore be a function of fracture density deformation and fluid availability the results cast doubts on modern petrological and geochronological methods that assume pure temperature controlled chemical re equilibration of garnets

high pressure metamorphism of the lanzo peridotite and its oceanic cover and some consequences for the sesia lanzo zone northwestern italian alps abstract this paper presents new data on the alpine high pressure evolution of the lanzo peridotite massif and its metasedimentary cover western alps field relations in the northwestern lanzo massif show that the serpentinized part is covered by a metasedimentary sequence including local occurrences of metaophicarbonate metabasalt calcschist metaquartzite and gneiss remnants of granulite facies mafic gneiss associated with metaophicarbonate are found within the serpentinite major element garnet zonation profiles in metaquartzite and mafic gneiss were used to explore the contrasting origin of metasediments bell shaped mn zoning in garnet from metaquartzite indicates a one stage alpine metamorphic evolution while inverse zoning in garnet of the mafic gneiss shows that the protolith equilibrated under high grade metamorphic conditions in pre alpine times similar to kinzigites within the neighbouring sesia zone phase equilibria in metaquartzite and mafic gneiss point to 500 50 c at pressures of 0 9 to 1 3 gpa for the alpine metamorphic equilibration that contrasts with the 550 620 c and pressures in excess of 2 0 gpa obtained for kyanite talc chloritoid and talc chloritoid garnet domains in a metagabbro dike from the internal part of the lanzo massif these different p t trajectories can be explained by a much stronger retrograde overprint at the basement cover contact than in the internal part of the lanzo massif

synchronous peak barrovian metamorphism driven by syn orogenic magmatism and fluid flow in southern connecticut usa recent work in barrovian metamorphic terranes has found that rocks experience peak metamorphic temperatures across several grades at similar times this result is inconsistent with most geodynamic models of crustal over thickening and conductive heating wherein rocks which reach different metamorphic grades generally reach peak temperatures at different times instead the presence of additional sources of heat and or focusing mechanisms for heat transport such as magmatic intrusions and or advection by metamorphic fluids may have contributed to the contemporaneous development of several different metamorphic zones here we test the hypothesis of temporally focussed heating for the wepawaug schist a barrovian terrane in connecticut usa using sm nd ages of prograde garnet growth and u pb zircon crystallization ages of associated igneous rocks peak temperature in the biotite garnet zone was dated via sm nd on garnet at 378 9 1 6 ma 2r whereas peak temperature in the highest grade staurolite kyanite zone was dated via sm nd on garnet rims at 379 9 6 8 ma 2r these garnet ages suggest that peak metamorphism was pene contemporaneous within error across these metamorphic grades ion microprobe u pb ages for zircon from igneous rocks hosted by the metapelites also indicate a period of syn metamorphic peak igneous activity at 380 6 4 7 ma 2r indistinguishable from the peak ages recorded by garnet a 388 6 2 1 ma 2r garnet core age from the staurolite kyanite zone indicates an earlier episode of growth coincident with ages from texturally early zircon and a previously published monazite age along the prograde regional metamorphic t t path the timing of peak metamorphism and igneous activity as well as the occurrence of extensive syn metamorphic quartz vein systems and pegmatites best supports the hypothesis that advective heating driven by magmas and fluids focussed major mineral growth into two distinct episodes the first at c 389 ma and the second corresponding to the regionally synchronous peak metamorphism at c 380 ma

precise eclogitization ages deduced from rb sr mineral systematics the maksyutov complex southern urals russia abstract the maksyutov complex southern urals russia is a well preserved example of subduction related high pressure metamorphism one of its two litho tectonic units consists of rocks that experienced eclogite facies conditions published 40ar 39ar data on phengite u pb data on rutile and sm nd mineral data define a cluster of ages around 370 to 380 ma nevertheless no consensus exists as to the detailed interpretation of data and the exact age of eclogitization we present new high precision internal mineral rb sr isochrons for eclogite facies metabasites felsic eclogites and eclogite facies quartz veins nine isochrons mainly controlled by omphacite and white mica phases give concordant ages with an average value of 375 2 ma 2 microtextural features such as prograde growth zoning in eclogite facies phases suggest that the assemblages dated formed at a stage of prograde metamorphism sr isotopic equilibria among eclogite facies phases and among eclogite facies fluid veins and the host rocks indicate that our ages reflect crystallization ages related to the prograde metamorphic probably fluid mediated eclogitization reactions this interpretation is reinforced by data from fluid precipitated quartzitic eclogites whose modal composition together with intergrowth relationships conclusively imply closed system behavior after crystallization the possible occurrence of a pre 375 ma event of ultra high pressure metamorphism uhpm in the maksyutov complex is disproved by isotope systematics microtextures and mineral zoning patterns

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

processes leading to eclogitization densification of subducted and tectonically buried crust 1 the transformation of subducted and tectonically buried crustal rocks to denser eclogite plays a fundamental role in the dynamics of mountain building and crustal recycling however a complex of partially eclogitized granulites on the island of holsnoy in the western part of the norwegian caledonides reveals that such densification depends in part on the antecedent history of rock masses and that large bodies of untransformed rock can persist metastably even after long residence times in the lower crust the rocks on holsnoy suggest that conversion to eclogite was delayed until lower or subcrustal seismic events allowed aqueous fluids to enter the previously dry and unusually strong granulite complex metamorphism then occurred in a spatially heterogeneous manner with several distinct physical processes involving feedbacks between deformation fluid infiltration and chemical reactions operating simultaneously in different parts of the rock mass field relations show that the introduction of fluids and conversion to eclogite significantly weakened the rocks a reservoir flux systems model is used to represent the various processes contributing to eclogitization of the granulites which operated at vastly different rates over disparate intervals of time the model suggests that the metamorphic process may have been brief 1 myr and ultimately self limiting arrested by the change in rheology associated with the conversion of brittle granulite to ductile eclogite

geochronology of fluid induced eclogite and amphibolite facies metamorphic reactions in a subduction collision system bergen arcs norway rb sr multimineral isochron data for metamorphic veins allow to date separate increments of the mineral reaction history of polymetamorphic terranes granulite facies rocks of the lindas nappe bergen arcs norway were subducted and exhumed during the caledonian orogeny the rocks show petrographic evidence for two distinct events of local fluid infiltration and vein formation along fractures and shear zones the first occurred at eclogite facies 15 21 kbar 650 750 c and a later one at amphibolite facies conditions 8 10 kbar 600 c the presence of fluids enabled local metamorphic equilibration only near fluid pathways in fluid absent domains preexisting assemblages were metastably preserved this resulted in a heterogeneity of metamorphic signatures on meter to m scales well preserved granulite facies rocks preserve their proterozoic rb sr mineral ages as does the u pb system of zircon in most lithologies six rb sr multimineral isochron ages for eclogite facies veins and their immediate wallrocks date the fluid induced eclogitization at 429 9 3 5 ma 2 weighted average mswd 0 39 an eclogite facies vein has yielded metamorphic zircon with concordant u pb ages of 429 3 ma identical to the u pb age of 427 4 0 9 ma for zircon xenocrysts in an amphibolite facies vein seven rb sr mineral isochron ages date amphibolite facies fluid infiltration at 414 2 2 8 ma mswd 1 5 an age value testifying to residence of the rocks in the deep orogenic crust at temperatures 600 c for nearly 15 ma the new data show that rb sr mineral isochron ages effectively date fluid induced re crystallization events rather than stages of cooling the direct link between isotopic ages and distinct petrographic equilibrium assemblages aids to constrain the evolution of rocks in the p t reaction time space which is essential for understanding exhumation histories and the internal dynamics of orogens in general

geophysics hot fluids and cold crusts conventional wisdom says that changes to crustal rocks pushed down deep when continents collide develop over millions of years but it seems that some metamorphism may be caused by tectonic events lasting only a decade

geophysics hot fluids or rock in eclogite metamorphism reply arising from a camacho j k w lee b j hensen j braun nature 435 1191 1196 2005 camacho et al reply the mechanisms by which mafic rocks become converted to denser eclogite in the lower crust and mantle are fundamental to our understanding of subduction mountain building and the long term geochemical evolution of earth based on larger than expected gradients in argon isotopes camacho et al 1 propose a new explanation co seismic injection of hot 700 c aqueous fluids into much colder 400 c crust for the localized nature of eclogite metamorphism during caledonian crustal thickening as recorded in the rocks of holsnoy in the bergen arcs western norway we have studied these unusual rocks2 3 4 which were thoroughly dehydrated under granulite facies conditions during a neoproterozoic event about 945 million years 945 myr ago we also concluded that fracture hosted fluids were essential as catalysts and components in the conversion to eclogite about 425 myr ago5 however we are sceptical of the assertion by camacho et al that eclogite temperatures were reached only in the vicinity of fluid filled fractures determining whether these rocks were strong enough to fracture at depths of 50 km because they were cold or because they were very dry is crucial to understanding the mechanics of the lower crust in mountain belts including for example the causes of seismicity in the indian plate beneath the modern himalayas6

rare earth element distribution in 400 c hot hydrothermal fluids from 5 s mar the role of anhydrite in controlling highly variable distribution patterns abstract two submarine hydrothermal vent fields at 5 s mid atlantic ridge mar turtle pits and comfortless cove emanate vapor phase fluids at conditions close to the critical point of seawater 407 c 298 bars in this study the concentration and distribution of rare earth element ree and yttrium y has been investigated independent of the major element composition the fluids display a strong temporal variability of their ree y concentrations and relative distributions at different time scales of minutes to years chondrite normalized distributions range from common fluid patterns with light ree enrichment relative to the heavy ree accompanied by positive eu anomalies type i to strongly ree y enriched patterns with a concave downward distribution with a maximum enrichment of sm and weakly positive or even negative eu anomalies type ii the larger the sum of ree the smaller cecn ybcn and eu eu we also observed a strong variability in fluid flow and changing fluid temperatures correlating with the compositional variability as evident by the positive correlation of total ree ca and sr concentrations in turtle pits and comfortless cove fluids precipitation dissolution of hydrothermal anhydrite controls the variability in ree concentrations and distributions in these fluids and the transformation of one fluid type to the other the variable distribution of ree can be explained by the accumulation of particulate anhydrite with concave downward ree distribution and negative eu anomaly into a fluid with common ree distribution type i followed by the modification of the ree fluid signature due to dissolution of incorporated anhydrite a second model in which the type ii fluids represent a primary ree reaction zone fluid pattern which is variably modified by precipitation of anhydrite can also explain the observed correlations of total ree fractionation of lree hree and size of eu anomaly as well as ca sr the emanation of such a fluid may be favored in a young hydrothermal system in its high activity phase with short migration paths and limited exchange with secondary minerals however this model is not as well constrained as the other and requires further investigations the strongly variable ree fluid signature is restricted to the very hot actively phase separating hydrothermal systems turtle pits and comfortless cove at 5 s and has not been observed at the neighboring red lion vent field which continuously emanates 350 c hot fluid and displays a stable ree distribution type i

structural development and petrofabrics of eclogite facies shear zones bergen arcs western norway implications for deep crustal deformational processes caledonian eclogite facies shear zones developed from grenvillian garnet granulite facies anorthosites and gabbros in the bergen arcs of western norway allow direct investigation of the relations between macroscopic structures and crystallographic preferred orientation cpo in lower continental crust field relations on the island of holsnoy show that the eclogites formed locally from granulite facies rocks by progressive development of 1 eclogite adjacent to fractures 2 eclogite in discrete shear zones 2 m thick 3 eclogite breccia consisting of 80 well foliated eclogite that wraps around rotated granulite blocks and 4 anastomosing subparallel eclogite facies shear zones 30 100 m thick continuous over distances 1 km within the granulite terrane these shear zones deformed under eclogite facies conditions at an estimated temperature of 670 50 c and a minimum pressure of 1460 mpa which corresponds to depths of 55 km in the continental crust detailed investigation of the major shear zones shows the development of a strong foliation defined by the shape preferred orientation of omphacite and by alternating segregations of omphacite garnet rich and kyanite zoisite rich layers a consistent lineation throughout the shear zones is defined by elongate aggregates of garnet and omphacite the cpo of omphacite determined from five axis universal stage measurements shows a strong b axis maximum normal to foliation and a c axis girdle within the foliation plane with weak maxima parallel to the lineation direction these patterns are consistent with deformation of omphacite by slip parallel to 001 and suggest glide along 010 the lineation and cpo data reveal a consistent sense of shear zone movement although the displacement was small localized faulting of high grade rocks accompanied by fluid infiltration can be an important mode of failure in the lower continental crust field relations show that granulite facies rocks can exist in a metastable state under eclogite facies conditions and imply that the lower crust can host differing metamorphic facies at the same depth deformation of granulite and partial conversion to eclogite such as is exposed on holsnoy island may be an orogenic scale process in the lowermost crust of collisional orogens

the high pressure iberian czech belt in the variscan orogen extrusion into the upper gondwanan plate a siluro devonian high pressure hp belt the iberian czech or ic belt extends from iberia cordoba coimbra shear zone through armorica and the massif central to the bohemian massif iberian czech ic belt and includes arc and periarc rocks morb and supra subduction ophiolites and passive margin sequences it has generally been interpreted to be either a a suture of the galician south brittany massif central and moldanubian ocean b a nappe rooted to the north in the rheic ocean or c the result of post collisional strike slip shuffling of the gondwanan margin during which a slice of the rheic ocean was inserted into the gondwanan margin we agree with a rheic ocean origin but propose that the ic belt represents a pre collisional part of the gondwanan continental oceanic margin that was removed by subduction erosion underwent hp metamorphism and was then extruded into the overlying gondwanan plate this model explains i the lack of paleolatitudinal and faunal differences across the ic belt ii the juxtaposition of active margin tectonics within synchronous passive margin sequences and iii the pre collisional age of most of the hp metamorphism the complete range of hp rocks in the ic belt from blueschist to hot eclogites indicates that previous correlations with b and a type subduction respectively cannot be sustained

retrogression of eclogite facies shear zones by short lived fluid infiltration during the caledonian orogeny lofoten islands norway norwegian eclogite facies shear zones in the lofoten islands served as major pathways for short lived pulses of both 40 ar rich and 40 ar poor hot fluids during eclogitization and retrogression events related to the caledonian orogeny the open system for ar affected most of the minerals leading to old 40 ar 39 ar ages particularly in trioctahedral micas 40 ar concentration in the fluids appears to decrease during the amphibolite facies retrogression and ca rich amphiboles yield 40 ar 39 ar ages of c 415 ma scandian event late and post caledonian 40 ar 39 ar muscovite and k feldspar ages and rb sr biotite ages coincide with multiple extensional events fluid infiltration and thermal activity during the final exhumation of the crustal rocks potentially reflecting major tectonic episodes such as rifting of pangaea and seafloor spreading between europe and north america

temporal and tectonic evolution of the granulite eclogite association from the bergen arcs western norway the u pb and sm nd dating of deep crustal rocks from the bergen arcs system helps resolve enigmatic aspects of the tectonic evolution of the caledonian orogen in western norway and yields insights into the arrested stages of eclogite development within the granulites of the area the u pb dating of zircon from one of the eclogite facies shear zones yields an upper intercept age of 945 5 ma all errors two standard deviations 2 which is similar to other zircon ages from the granulite facies protolith the age is interpreted to represent the time of late proterozoic sveconorwegian granulite metamorphism the u pb ages of sphene and epidote show that the eclogites formed early in the evolution of the caledonian orogen pre scandian phase at about 460 ma an eclogite facies quartz vein yields a sm nd whole rock garnet isochron of 440 12 ma that may reflect the onset of cooling immediately after peak eclogite facies conditions although the sm nd systematics reveal some isotopic disequilibrium within the sample in tandem with previous 40ar39ar age determinations from an adjacent eclogite of 450 ma for hornblende and 430 ma for muscovite these data indicate that 30 ma elapsed between formation of the eclogites and the initial stages of cooling and exhumation to at least mid crustal levels this corresponds to minimum cooling rates of 14 c m y the timing relations suggest that the formation and exhumation of these eclogites from the overlying caledonian nappe wedge in western norway are related to an early phase of crustal subduction during or somewhat before the major phase of continent continent collision the short period of time between the formation of the eclogites and the initial stages of exhumation and rapid cooling is consistent with the only partial and localized transformation of the granulite to eclogite isolated occurrences of eclogite within the granulite the formation of eclogite along metasomatic fronts and the formation of hydrous eclogite facies minerals within the dry granulite all point to the importance of fluids in the transformation and re equilibration of the granulite to eclogite together field and isotopic data demonstrate that both the localized and limited access of fluids and the rapid cycling of continental crust through the deepest portions of the orogen to upper crustal levels resulted in the preservation of the arrested stages of eclogite formation and survival of the granulites metastably through eclogite facies conditions

subduction of continental margins and the uplift of high pressure metamorphic rocks abstract the mechanism by which high pressure metamorphosed continental material is emplaced at high structural levels is a major unsolved problem of collisional orogenesis we suggest that the emplacement results from partial subduction of the continental margin which because of its high flexural rigidity produces a rapid change in the trajectory of the descending slab we assume a two fold increase in effective elastic thickness of the lithosphere as the continental margin approaches the subduction zone and calculate the flexural profile of a thin plate for progressive downward migration of the zone of increased rigidity we assess the effect of changes in the flexural profile on the overlying accretionary prism and mantle wedge as the continent approaches by estimating the extra stresses that are imposed on the wedge due to the bending moment exerted by the continental part of the plate the wedges overlying the subduction zones and the subducting slab itself experience substantial extra compressional stress at depths of around 100 km and extensional stress at shallower depths as the continental margin passes through the zone of maximum curvature the magnitudes of such extra stresses are probably adequate to effect significant deformation of the wedge and or the descending plate and are experienced in a time interval of less than 5 m y for typical subduction rates the spatial variation of yield stresses in the region of the wedge and descending slab indicates that much of this deformation may be taken up in the crustal part of the descending slab which is the weakest region in the deeper parts of the subduction zone this may result in rapid upward migration of the crust of the partially subducted continental margin against the flow of subduction high pressure metamorphosed terranes emplaced by the mechanism envisaged in this paper would be bounded by thrust faults below and normal faults above movement on the faults would have been coeval and would have resulted in rapid unroofing of the high pressure terranes synchronous with arrival of the continental margin at the subduction zone and therefore relatively early in the history of a collisional orogen

mechanics of normal fault networks subject to slip weakening friction summary we seek to understand how the stress interactions and the slip weakening process combine within a non coplanar normal fault network to allow a slip instability to develop and shape the final slip distribution on the system in a first part we perform a non linear spectral analysis to investigate the conditions of stability and the process of slip initiation in an antiplane non coplanar fault system subject to a slip dependent friction law that numerical model allows determining the zones that are able to slip within a fault network as well as the location of the stress singularities the resulting slip profiles on the faults show only a few different shapes some of them with long linear sections this leads to formulate a general classification of slip profiles that can be used to infer the degree of fault interaction within any non coplanar system in a second part of work we use our modelling to try reproducing the cumulative slip profiles measured on three real normal interacting faults forming a large scale en echelon system for that we assume that cumulative slip profiles can be compared to the first static modal solution of our conceptual model we succeed reproducing the profiles quite well using a variable weakening along the faults overall the weakening rate decreases in the direction of propagation of the fault system yet modelling the slip along the propagating isolated termination segment of the system requires an unlikely distribution of weakening this suggests that factors not considered in our analysis may contribute to slip profile shaping on isolated propagating faults

simultaneous calculation of metamorphic fluid fluxes reaction rates and fluid rock interaction timescales using a novel inverse modeling framework abstract to study metamorphic carbonation at greenschist facies conditions in the sw scottish highlands a novel inverse modeling framework which combines solutions of the transport equation with a global optimization method of differential evolution was developed using this framework we calculated simultaneously time integrated and time averaged metamorphic fluid fluxes of 83 4 35 4 m 3 m 2 and 10 10 1 0 5 m 3 m 2 s 1 respectively a time averaged reaction rate constant of 10 12 7 10 2 m s 1 and comparable timescales for fluid flow and fluid driven reaction of 10 4 3 0 5 yr and 10 2 7 5 2 yr respectively these parameters were calculated using an empirical estimate of the coefficient of molecular diffusion and a calculated value for metamorphic porosity our estimates are 1 consistent with single pass flow of fluid released by metamorphic devolatilization 2 within the range where heat is transported by conduction and matter is transported by advection 3 in agreement with an emerging consensus that metamorphic events are relatively short lived and 4 supportive of applying laboratory based estimates of kinetic parameters to metamorphic systems based on a sensitivity analysis we show that 1 selecting the diffusion coefficient rather than fluid velocity reaction rate or flow duration as an input parameter yields more robust estimates of metamorphic fluid flow parameters and 2 ignoring reaction dependent porosity and reaction rates can result in an order of magnitude uncertainty in best fit flow parameters evaluated from concentration profiles finally similarity between our calculated time averaged metamorphic fluid fluxes which were obtained numerically and those which were obtained analytically confirms the validity of using the quasi stationary state assumption to quantify metamorphic fluid flow parameters

a tem study of disequilibrium plagioclase breakdown at high pressure the role of infiltrating fluid high pressure metamorphism 600 c 20 kbar of the allalin gabbro western alps resulted in the breakdown of plagioclase an63 to fine grained zoisite jadeite kyanite and quartz in rare cases this reaction failed to reach completion the resulting textures of partial reaction have been studied by transmission and analytical electron microscopy in localised regions of a plagioclase crystal where the extent of reaction is 10 only zoisite developed and the orientation relationship left 100 right text zo left 101 right text p1 1 and left 012 right text zo left 010 right text p1 1 is frequently present in regions where 10 50 of plagioclase has transformed the reaction plagioclase h2o zoisite kyanite quartz nasica 1al 1 pl has occurred the systematic orientation relationship between plagioclase and zoisite is absent at 50 transformation complete breakdown of plagioclase occurred in localized micron scale domains by the reaction plagioclase h2o zoisite jadeite kyanite quartz and the reaction products are variably orientated with respect to each other incomplete reaction together with the concentration of reaction products around cracks in original plagioclase grains suggests that extent of reaction was controlled primarily by the availability of h2o the textural observations are interpreted in terms of two possible disequilibrium reaction models 1 plagioclase persists metastably with its original igneous composition to a pressure 17 kbar at 600 c reaction to the equilibrium assemblage then develops adjacent to cracks in response to the presence of aqueous fluid at intracrystalline sites only partial reaction occurs because jadeite and sometimes kyanite and quartz fail to nucleate for kinetic reasons 2 localized regions of a plagioclase crystal partially equilibrate at several stages during the increase of pressure 9 17 kbar at 600 c possibly due to discrete episodes of fluid infiltration in both these models the extent of reaction may be limited by nasi caal interdiffusion in plagioclase

tectonic mode switches and the nature of orogenesis abstract the birth and death of many mountain belts occurs in lithosphere that over rides major subduction zones here the tectonic mode shortening versus extension can abruptly switch even during continuous and otherwise smooth convergence if the hinge line of the foundering slab rapidly retreats i e rolls back the foundering slab creates a gravitational potential well into which the orogen collapses this motion coupled with stress guides can pull the orogen apart a slowing of roll back or of hinge retreat means that the subduction flexure may subsequently begin to be pushed back or be pushed over by the advancing orogen the consequence of such changes in relative motion is that orogenic belts are affected by abrupt tectonic mode switches the change from push to pull leads to a sudden change from horizontal extension to horizontal shortening potentially throughout the entire mass of the orogenic lithosphere that over rides the subducting slab the sequencing of these tectonic mode switches affects the thermal evolution of the orogen and thus fundamentally determines the nature of orogenesis this insight led to us to our quite different views as to how orogens work it is evident that orogens affected by abrupt push pull mode switches are characterized by high pressure metamorphism whereas orogens affected by abrupt pull push mode switches are characterized by high temperature metamorphism magmatism and anatexis

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

metastability of granulites and processes of eclogitisation in the uhp region of western norway the flatraket complex a granulite facies low strain enclave within the western gneiss region provides an excellent example of metastability of plagioclase bearing assemblages under eclogite facies conditions coesite eclogites are found 28 kbar unprecedented metastability of plagioclase and quartz is implied alternatively a tectonic break exists between the flatraket complex and uhp eclogites supporting the concept of a tectonic boundary to the uhp zone of the western gneiss region the distribution of eclogite and amphibolite facies metamorphic overprints demonstrates that the reactivity of the crust during deep burial and exhumation is strongly controlled by fluid availability and is a function of the protolith

creep of garnet in eclogite mechanisms and implications abstract the rheological properties of subducting rocks represent a fundamental parameter in the dynamics of subduction zones making robust predictions about these properties and the general strength evolution of subducting plates is hampered by a relatively restricted understanding of the mechanisms by which rocks deform at high and ultrahigh pressure this uncertainty is relates to the discrepancy between experimental and field based observations of eclogite deformation and in particular the role of garnet to further investigate this important aspect we performed a textural and micro structural investigation applying optical microscopy element mapping and electron backscatter diffraction on deformed garnet polycrystals from an eclogite mylonite the results were compared to those from a study of undeformed polycrystals that formed from a supercooled frictional melt at hp conditions the mylonites polycrystals are flattened parallel to the main high pressure foliation in the rock and individual grains were shortened by an average 15 although dislocation creep is commonly presumed to dominate garnet straining in eclogites no record of this mechanism was found instead the garnet grains have dissolution surfaces indicative of deformation by intergranular pressure solution the observations provide compelling evidence for the role of fluids and syn tectonic porosity in the weakening of garnet a supposedly rigid eclogite component such weakening represents a crucial step in the fundamental feedback loop between fluid ingress metamorphism and near instantaneous competence loss in rocks undergoing deep subduction

evidence for a caledonian amphibolite to eclogite facies pressure gradient in the middle allochthon lindas nappe sw norway the proterozoic anorthosite mangerite charnockite complex dominating the lindas nappe in the scandinavian caledonides was locally eclogitized in the southwestern part of the nappe during the caledonian orogeny whereas only amphibolite facies assemblages are recorded in the rest of the nappe sveconorwegian granulites of anorthositic to jotunitic composition in the northernmost eclogite free exposures of the nappe exhibit large garnet phenoblasts ca 900 c that are fractured and partly replaced by a caledonian symplectitic amphibolite facies assemblage ca 515 c metamorphic zircon attributed to this garnet breakdown is dated by id tims u pb at 430 3 ma suggesting that the amphibolite facies overprint was coeval with the formation of eclogite 30 km further south probably implying that the section across the nappe represents a caledonian pressure gradient the rocks also preserve a complex sveconorwegian history including an age of 969 6 ma which we interpret as dating magmatic emplacement of jotunitic anorthositic portions of the complex 936 12 ma reflecting the granulite facies metamorphism and 908 16 ma representing a late generation of zircon best explained as having formed by metasomatic processes caledonian shearing severely deformed zircon grains in an amphibolite facies shear zone resetting their u pb systems and forming new ones hereby also demonstrating a case of resetting and recrystallization of low u zircon our data gained from diverse lithologies illustrate several processes involved in making and resetting zircon as well as indicate the contemporaneous evolution and similar origin of the lindas nappe and the jotun nappe complex

textural evolution of plagioclase feldspar across a shear zone implications for deformation mechanism and rock strength caledonian amphibolite facies shear zones developed in granulitefacies anorthosites and anorthositic gabbros of the bergen arcs western norway allow a detailed study of the relationships between fluid infiltration mineral reactions the evolution of microstructure and deformation mechanisms a sequence of rocks from the relatively pristine granulites into a shear zone has been studied by optical microscopy electron microprobe microanalysis empa scanning electron microscopy sem electron backscatter diffraction ebsd and transmission electron microscopy tem focusing on the progressive development of microstructure in the plagioclase feldspars leading up to their deformation in the shear zone at the outcrop scale fluid infiltration into the granulites is marked by a distinct colour change in the plagioclase from lilac brown to white this is associated with the breakdown of the intermediate composition plagioclase an50 in the granulite to a complex intergrowth of na rich and ca rich domains ebsd analysis shows that this intergrowth retains the crystallographic orientation of the parent feldspar but that the ca rich domains contain low angle grain boundaries within the shear zone this complex intergrowth coarsens by grain boundary migration annihilating these grain boundaries but retaining the na rich and ca rich zoning pattern analysis of nearestneighbour misorientations of feldspar grains in the shear zone demonstrates that local crystallographic preferred orientation cpo is inherited from the parent granulite grain orientations random pair misorientation angle distributions show that there is no cpo in the shear zone as a whole nor is there significant shape preferred orientation spo in single grains these observations are interpreted in terms of fluid induced weakening and deformation by dissolution precipitation pressure solution creep

bimodal behavior of extended continental lithosphere modeling insight and application to thermal history of migmatitic core complexes abstract extensional migmatitic core complexes micc expose partially molten crustal rocks exhumed during post thickening extension low heat flow in active collisional mountains suggests that there is i either input from extra crustal heat sources or ii under assessment of the pre extensional collisional heat content to form high grade metamorphic core complexes we evaluate these two hypotheses by varying thermal gradients and crustal thicknesses of two dimensional thermo mechanical numerical models simulating the extension of a stratified lithosphere for moho temperatures 35 and 50 km extension leads to a rifting mode dictated by the upwelling of the asthenosphere in that case no micc is formed for moho temperatures 700 c and or crustal thicknesses 60 km extension develops micc while maintaining a flat moho similar to wide extensional systems such as the aegean and the basin and range however when crustal strain softening is switched on it produces for both mentioned conditions miccs with similar crustal geometries testifying that partial melting in miccs can result from both extra crustal syn extensive heat asthenospheric advection and collisional heat typically miccs induced by asthenospheric heat show the sequence lower crustal migmatization doming and crustal ultra high temperature uht conditions and mantle melting in a short temporal range from 5 to 10 model ma after asthenospheric upwelling the convective migmatitic dome is shifted horizontally with respect to the asthenospheric culmination and dome migmatites experience isobaric heating to peak conditions in contrast collisional heat induced miccs exhibit pre extensional migmatization and subsequent doming at 3 4 model ma in this case the asthenosphere generating lower crustal uht conditions and mantle melts upwells lately and is not symptomatic for micc formed in short extensional pulses

fine scale isotopic heterogeneities and fluids in the deep crust a40ar 39ar laser ablation and tem study of muscovites from a granulite eclogite transition zone spectacular exposures on holsnoy island western norway reveal that eclogites formed in situ from adjacent anhydrous granulites as a result of the localized infiltration of fluids stepwise and laser40ar 39ar experiments on muscovite from the eclogites have been used in conjunction with electron microprobe and transmission electron microscopy tem analysis to examine the behavior of argon under high pressure metamorphism and subsequent cooling muscovites with 0 10 0 17na na k yield40ar 39ar plateau and integrated ages in the range 450 465 ma 20 35 myr older than40ar 39ar muscovite plateau ages from adjacent eclogite samples laser40ar 39ar analyses on 75 m spots across two single muscovite grains from these samples are approximately uniform at 444 2 1 9 21 spots and 443 3 1 7 ma 26 spots however incremental heating of high sodic 0 18 0 32 na na k muscovite failed to yield a plateau laser40ar 39ar analyses of 100 areas from two single grains from this sample reveal complex two dimensional patterns with variable apparent ages in the range of 445 625 ma electron microprobe measurements obtained from one of the grains reveal that the paragonite content is also highly variable both parallel and perpendicular to 001 all the tem bright field images obtained from this mica yield information consistent with a single homogeneous phyllosilicate phase yet several of the selected area electron diffraction saed analyses document locally intergrown muscovite and sodic muscovite possibly in an exsolution relationship on a unit cell scale analytical electron microscopy aem analyses obtained over a 5 100 nm scale on the same sample reveal great range and heterogeneity inna k including pure muscovite although no separate paragonite domains were found in contrast muscovite grains with consistent laser spot fusion and plateau ages show little heterogeneity inna k at the microprobe level these data provide evidence of variable fluid mediated incorporation of excess40ar under high pressure eclogite facies conditions at the time of the development of the sodic muscovite compositions the data indicate that fine scale heterogeneities in argon isotopic compositions were generated during eclogite facies equilibration and were preserved as the rocks were cycled back to the surface despite the influx of free fluids in the rocks at high pressure the heterogeneities are best reconciled with local buffering and limited mobility of the fluids at least during the final stages of equilibration

trace element signature and u pb geochronology of eclogite facies zircon bergen arcs caledonides of w norway secondary ion mass spectrometry sims u pb and trace element data are reported for zircon to address the controversial geochronology of eclogite facies metamorphism in the lindas nappe bergen arcs caledonides of w norway caledonian eclogite facies overprint in the nappe was controlled by fracturing and introduction of fluid in the proterozoic sveconorwegian granulite facies meta anorthosite norite protolith zircon grains in one massive eclogite display a core rim structure sveconorwegian cores have trace element signatures identical with those of zircon in the granulite protolith i e 0 31 th u 0 89 heavy rare earth element hree enrichment and negative eu anomaly weakly zoned to euhedral oscillatory zoned caledonian rims are characterized by th u 0 13 low lree content minimum normalized abundance for pr or nd variable enrichment in hree and no eu anomaly a decrease of ree towards the outermost rim especially hree is documented this signature reflects co precipitation of zircon with garnet and clinozoisite in a feldspar absent assemblage and consequently links zircon to the eclogite facies overprint the rims provide a mean 206pb 238u crystallization age of 423 4 ma this age reflects eclogite forming reactions and fluid rock interaction this age indicates that eclogite facies overprint in the lindas nappe took place at the onset of the scandian silurian collision between laurentia and baltica

intracratonic strike slip partitioned transpression and the formation and exhumation of eclogite facies rocks an example from the musgrave block central australia mountain belts developed in an intracratonic setting during the neoproterozoic in the center of australia mesoproterozoic granulite and amphibolite facies gneisses of the musgrave block were overprinted during the petermann orogeny 550 ma with zones of high strain concentrated along broadly east west trending shear zones these zones formed under eclogite facies t 650 c and p 12 kbars to greenschist facies conditions and may form part of a strike slip related crustal scale flower type structure granulite facies gneisses preserve ages older than 700 ma suggesting that these rocks did not experience temperatures greater than 350 c at 550 ma for these rocks not to have been thermally equilibrated to temperatures of 350 c at a depth of 12 kbars requires either an extraordinarily low geothermal gradient 9 c km 1 or if the geothermal gradient was more typical 20 c km 1 then transport to that depth and subsequent exhumation occurring in a short time interval 40 myr

evidence for shear heating musgrave block central australia the phenomenon of shear heating is generally difficult to recognise from petrologic evidence alone establishing that shear zones attain higher temperatures than the surrounding country rocks requires independent evidence for temperature gradients in the musgrave block central australia there is a clear spatial association between shear zones and interpreted elevated temperatures eclogite facies shear zones that formed at 550 ma record temperatures of 650 700 c outside the high pressure shear zones minerals with low closure temperatures such as biotite 450 c in the 40ar 39ar and rb sr systems preserve ages 800 ma suggesting that these rocks did not experience temperatures greater than about 450 c at 550 ma for any extended period thus the shear zones record temperatures that are 200 c higher than the surrounding country rocks simple calculations show that the combination of relatively high shear stresses 100 mpa and high strain rates 10 11 s 1 for short durations 1 ma can account for the observed apparent temperature variations the evidence indicates that shear heating is the dominant mechanism for localised temperature increases in the shear zones while the country rock remained at relatively lower temperatures

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

short lived orogenic cycles and the eclogitization of cold crust by spasmodic hot fluids collision tectonics and the associated transformation of continental crust to high pressure rocks eclogites are generally well understood processes but important contradictions remain between tectonothermal models and petrological isotopic data obtained from such rocks here we use 40ar 39ar data coupled with a thermal model to constrain the time integrated duration of an orogenic cycle the burial and exhumation of a particular segment of the crust to be less than 13 myr we also determine the total duration of associated metamorphic events to be 20 kyr and of individual heat pulses experienced by the rocks to be as short as 10 years such short timescales are indicative of rapid tectonic processes associated with catastrophic deformation events earthquakes such events triggered transient heat advection by hot fluid along deformation shear zones which cut relatively cool and dry subducted crust in contrast to current thermal models that assume thermal equilibrium and invoke high ambient temperatures in the thickened crust our non steady state cold crust model satisfactorily explains several otherwise contradictory geological observations the duration of the geological processes that shape the earth generates vigorous debate mountain building where crust is consumed continuously by burial and recycled as buried remnants rise to the surface is one process where new dating methods can resolve the issue high precision dating of rock remnants left by continental collision in the caledonides of southern norway shows that the entire cycle can happen quickly in about 13 million years in addition the rapid transport of hot fluid zones through the cold crust may explain many puzzling geological observations

the role of fluids in lower crustal earthquakes near continental rifts earthquakes in the lower continental crust of the earth are rare as the lower crust is thought to be too hot to allow brittle failure so it is surprising that they occasionally occur where continental crust is rifting apart a study of three dimensional crustal structure and seismicity in the taupo volcanic zone an active continental rift in new zealand has now provided insight into how these earthquakes arise the crustal earthquakes form a continuous band along the rift and often occur in swarms suggesting fluid movement in critically loaded fault zones seismic velocities within the band are consistent with the presence of fluids and the linking of upper and lower crust seismicity to crustal structure point to a common explanation for all of the seismicity probably the weakening of faults on the periphery of an otherwise dry crust by hot fluids a report on three dimensional tomographic imaging of seismic velocities and attenuation in the taupo volcanic zone an active continental rift where earthquakes are known to occur in the lower crust finds that crustal earthquakes form a continuous band along the rift and often occur in swarms suggesting fluid movement in critically loaded fault zones the occurrence of earthquakes in the lower crust near continental rifts has long been puzzling as the lower crust is generally thought to be too hot for brittle failure to occur1 2 such anomalous events have usually been explained in terms of the lower crust being cooler than normal3 4 but if the lower crust is indeed cold enough to produce earthquakes then the uppermost mantle beneath it should also be cold enough2 and yet uppermost mantle earthquakes are not observed5 numerous lower crustal earthquakes occur near the southwestern termination of the taupo volcanic zone tvz an active continental rift in new zealand6 here we present three dimensional tomographic imaging of seismic velocities and seismic attenuation in this region using data from a dense seismograph deployment7 we find that crustal earthquakes accurately relocated with our three dimensional seismic velocity model form a continuous band along the rift deepening from mostly less than 10 km in the central tvz to depths of 30 40 km in the lower crust 30 km southwest of the termination of the volcanic zone these earthquakes often occur in swarms suggesting fluid movement in critically loaded fault zones8 seismic velocities within the band are also consistent with the presence of fluids and the deepening seismicity parallels the boundary between high seismic attenuation interpreted as partial melt within the central tvz and low seismic attenuation in the crust to the southwest this linking of upper and lower crustal seismicity and crustal structure allows us to propose a common explanation for all the seismicity involving the weakening of faults on the periphery of an otherwise dry mafic crust by hot fluids including those exsolved from underlying melt such fluids may generally be an important driver of lower crustal seismicity near continental rifts

pseudotachylytes generated during seismic faulting and eclogitization of the deep crust pseudotachylytes are typically interpreted to have formed by frictional melting during coseismic faulting within the upper to middle crust pseudotachylytes in the bergen arcs of western norway contain microlites including omphacite garnet plagioclase and quartz this eclogite facies assemblage is stable at temperatures of about 800 c and pressures of 18 to 19 kilobars corresponding to depths of 60 kilometers or more the pseudotachylytes are exposed in grenvillian granulites that locally underwent fluid induced eclogitization and corresponding volume reduction of approximately 10 percent during the caledonian continental collision the pseudotachylytes may have formed as a result of the rapid relaxation of stresses caused by the eclogitization process