rutile u pb age depth profiling a continuous record of lithospheric thermal evolution abstract understanding of the thermal and geophysical evolution of the lower continental crust is limited by the resolution of conventional thermochronology intracrystalline daughter nuclide distribution profiles preserve a rich and underutilized record of thermal history using laser ablation inductively coupled plasma mass spectrometry we outline here a method to simultaneously acquire 206 pb 238 u age and trace element profiles from u bearing accessory phases inversion of 206 pb 238 u age depth profiles yields thermal history information from an extended temperature range compared to inversion of age versus grain size relationships thermally activated volume diffusion of pb and zr in rutile is sensitive to the thermal evolution of the mid to lower lithosphere we document the ability of laser ablation depth profiling to simultaneously resolve 206 pb 238 u age and zr diffusion profiles in the outer 35 m of lower crustal rutile euhedra from the ivrea zone southern alps with 1 2 m depth resolution inversion of the age profiles reveals a continuous cooling history characterized by initially rapid cooling from 600 c at 180 ma followed by a period of slower cooling from 525 c to 450 c combined with the topology of zr diffusion profiles these data indicate that the ivrea zone underwent a brief thermal pulse in the early jurassic plausibly associated with hyperextension of the adriatic margin inversion of near surface 206 pb 238 u age distributions can be employed to resolve otherwise inaccessible thermal history information from the lower lithosphere

high temperature deformation during continental margin subduction exhumation the ultrahigh pressure western gneiss region of norway abstract a new dataset for the high pressure to ultrahigh pressure western gneiss region allows the definition of distinct structural and petrological domains much of the study area is an e dipping homocline with e plunging lineations that exposes progressively deeper more strongly deformed more eclogite rich structural levels westward although eclogites crop out across the wgr scandian deformation is weak and earlier structures are well preserved in the southeastern half of the study area the scandian reworking increases westward culminating in strong scandian fabrics with only isolated pockets of older structures the dominant scandian deformation was coaxial e w stretching the sinistrally sheared more trondelag fault complex and nordfjord mylonitic shear zone bound these rocks to the north and south there was moderate top e amphibolite facies deformation associated with translation of the allochthons over the basement along its eastern edge and the nordfjord sogn detachment zone underwent strong lower amphibolite facies to greenschist facies top w shearing a northwestward increase in exhumation related melting is indicated by leucosomes with hornblende plagioclase and scandian sphene in the western 2 3 of the study area exhumation related amphibolite facies symplectite formation in quartzofeldspathic gneiss postdated most scandian deformation further deformation was restricted to slip along biotite rich foliation planes and minor local folding that the western gneiss region quartzofeldspathic gneiss exhibits a strong gradient in degree of deformation implies that continental crust in general need not undergo pervasive deformation during subduction

eclogites and eclogites in the western gneiss region norwegian caledonides the western gneiss region wgr marks the outcrop of a composite terrane consisting of variably re worked proterozoic basement and parautochthonous or autochthonous cover units the wgr exhibits a gross structural petrographic and thermobarometric zonation from southeast to northwest reflecting an increasing intensity of scandian late palaeozoic

diffusion closure temperature and age of a mineral with arbitrary extent of diffusion theoretical formulation and applications the commonly used expression of the closure temperature tc of a diffusing species in a mineral as derived by m h dodson contrib mineral petrol 40 1973 259 264 is applicable only to systems which have undergone sufficient diffusion so that even the composition at the center of individual grains is significantly removed from that established at the onset of cooling we have extended dodson s formulation to include cases with arbitrarily small amount of diffusion and applied it to calculate tc and age profiles which would develop in single crystals of different geometries these results permit evaluation of the extent of resetting of mineral age and of ion exchange geothermometers during cooling the measured age profile of a mineral can also be used to constrain its cooling rate these applications have been illustrated for the cases of garnet biotite fe mg exchange geothermometer and the cooling age and closure temperatures of the sm nd and lu hf decay systems in garnet

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

evolution of amphibolite facies structural features and boundary conditions for deformation during exhumation of high and ultrahigh pressure rocks nordoyane western gneiss region norway 1 geologic mapping and structural analysis of baltica basement and overlying thrust nappes have yielded a record of the late exhumation history of high pressure rocks where strain partitioning has preserved evidence for interpreting the evolution of late structural features the earliest of these were extensional detachments juxtaposing eclogite facies rocks against overlying amphibolite facies rocks that show no evidence for eclogite facies metamorphism these early detachments are strongly overprinted and complexly folded and they represent a phase of upper crustal extension that was active during continued convergence at deeper levels younger more localized mylonite zones formed synchronously with tubular sheath isoclinal tight and open folding that shows a progression from wnw to ene trends the earliest mylonite zones interpreted as originally subhorizontal range in strike through a 20 angle from 110 to 90 later steeply dipping mylonite zones formed under lower amphibolite facies conditions strike 75 and locally truncate earlier structures the youngest mylonite zones formed under lowest amphibolite conditions strike 50 and truncate all earlier structures folds developed during this progression show the range in orientation from wnw to ene reflected in the orientations of the mylonite zones that is interpreted to represent progressive evolution during top west shearing these changes in orientation of the late structural features are interpreted to have been caused by changes in boundary conditions related to transtensional deformation during exhumation l s fabrics absence of axial planar foliation and chaotic orientations of axial surfaces of granulite to amphibolite facies folds indicate formation in a constrictional strain field this is also supported by estimates of the finite strain accumulated at 780 c and 45 km and similar observations by previous workers assuming a simple monoclinic deformation for transtension strain estimates and structural measurements indicate apparent transtensional angles of 9 11 that increased to greater than 20 and then decreased to less than 20 these changes in transtensional angle agree with changes in the orientation of the x y plane of strain during exhumation from 45 km to less than 20 km depth and appear to reflect changes in the boundary conditions of deformation this upper crustal transtension is interpreted to have occurred during continued sinistral oblique convergence and provides a mechanism for syncollisional exhumation of hp and uhp metamorphic rocks

u pb zircon geochronology of eclogites from the scandian orogen northern western gneiss region norway 14 20 million years between eclogite crystallization and return to amphibolite facies conditionsthis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh reconstructing tectonic histories involving continental collision subduction and exhumation at plate tectonic rates of 1 cm year requires precise u pb zircon geochronology the western gneiss region has exceptional exposures of high pressure hp and ultra high pressure uhp rocks the strategy adopted here involved sampling eclogite and associated late unstrained pegmatites to acquire the time of eclogite crystallization and subsequent exhumation respectively the oldest eclogite sampled is 415 1 ma from layered probably uhp eclogite at tevik averoya also with a garnet hornblende assemblage at 410 1 ma the flem gabbro eclogite margin with implied uhp conditions is 410 2 ma hornblende eclogite at seth lepsoya never at uhp is 412 2 ma these compare to devonian ages of 401 1 ma for overgrowths on proterozoic baddeleyite in selnes gabbro 402 2 ma for coesite eclogite at hareidlandet 405 400 ma for coesite eclogite at flatraket and 405 2 ma for near uhp eclogite at hjelmeland

segmentation of the caledonian orogenic infrastructure and exhumation of the western gneiss region during transtensional collapse the ultra high pressure western gneiss region of the norwegian caledonides represents an archetypical orogenic infrastructure of a continent continent collision zone to test established exhumation models we synthesize the geochronology and structures of major basement windows and provide new ages from poorly dated areas migmatite u pb zircon samples date melt crystallization at c 405 ma in the oygarden complex expanding the spatial extent of devonian migmatization micas from shear zones in the oygarden and gulen domes yield 40ar 39ar ages mostly between 405 and 398 ma recording the exhumation of metamorphic core complexes on a larger scale the youngest ages of various geochronometers in different segments of the western gneiss region show abrupt breaks 10 30 myr across low angle detachments and sinistral transfer zones which also correspond to metamorphic and structural discontinuities we explain the segmentation of the orogenic infrastructure by partitioned post orogenic transtension due to lateral and vertical rheological contrasts in the orogenic edifice strong cratonic foreland and orogenic wedge v soft infrastructure differential crustal stretching dragged out deep levels of the orogenic crust below low angle detachments and became progressively dominated by sinistral transfer zones collapse obliterated the syn collisional structure of the orogenic root and resulted in the diachronous exhumation of distinct infrastructure segments supplementary material complete analytical results are available at https doi org 10 6084 m9 figshare c 5241710

two stage cooling and exhumation of deeply subducted continents the burial and exhumation of continental crust during collisional orogeny exert a strong controlon the dynamics of mountain belts and plateaus constraining the rates and style of exhumation of dee

the timing of india asia collision onset facts theories controversies abstract the timing of initial collision between india and asia has remained controversial for half a century this paper attempts to review this crucial and hotly debated argument describing first the different methods used to constrain the age of collision and discussing next the rationale results inferences and problems associated with each we conclude that stratigraphy represents the best direct way to unravel collision chronology other methods focusing on the magmatic metamorphic or paleomagnetic record provide additional fundamental constraints but cannot provide a robust direct estimate of collision onset initial collision in the central eastern himalaya is dated directly at the middle paleocene 59 1 ma by the abrupt change in sediment provenance recorded in trench settings the quasi synchronous unconformities documented along both tethyan passive margin of india and active margin of asia from tibet to zanskar ladakh confirm that orogeny was underway at the close of the paleocene 56 ma well before the disappearance of marine seaways in the himalaya during the early middle eocene 50 45 ma sedimentary evolution and provenance changes in marine to fluvio deltaic successions are recorded synchronously within error from the western to the central eastern himalaya failing to provide conclusive evidence for diachronous collision these coherent observations are hard to reconcile with three widely cited hypotheses invoking either paleogene arc continent collision or late cretaceous ophiolite obduction or the protracted existence of a greater india basin which are all not favored after discussing the geological evidence critically point by point a scenario no more complex than the one involving solely the passive continental margin of india and the active continental margin of asia is needed to explain the geological evolution of the nascent himalaya the collision between the tethys himalaya and the transhimalayan arc trench system does represent the collision between india and asia because the yarlung zangbo ophiolite is the forearc basement of the asian active margin its obduction onto india could not have preceded the initial closure of neo tethys ophiolite obduction began when collision began in the middle paleocene

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

petrology and geochronology of rutile rutile tio2 is an important accessory mineral that when present offers a rich source of information about the rock units in which it is incorporated it occurs in a variety of specific microstructural settings contains significant amounts of several trace elements and is one of the classical minerals used for u pb age determination here we focus on information obtainable from rutile in its original textural context we do not present an exhaustive review on detrital rutile in clastic sediments but note that an understanding of the petrochronology of rutile in its source rocks will aid interpretation of data obtained from detrital rutile for further information on the important role of rutile in provenance studies the reader is referred to previous reviews e g zack et al 2004b meinhold 2010 triebold et al 2012 coarse rutile is the only stable tio2 polymorph under all crustal and upper mantle conditions with the exception of certain hydrothermal environments smith et al 2009 as such we will focus on rutile rather than the polymorphs brookite anatase and ultrahigh pressure modifications in this chapter we first review rutile occurrences trace element geochemistry and u pb geochronology individually to illustrate the insights that can be gained from microstructures chemistry and ages then in the spirit of petrochronology we show the interpretational power of combining these approaches using the ivrea zone italy as a case study finally we suggest some areas of future research that would improve petrochronologic research using rutile rutile is a characteristic mineral in moderate to high pressure metapelitic rocks in high pressure metamorphosed mafic rocks and in sedimentary rocks e g force 1980 frost 1991 zack et al 2004b triebold et al 2012 rutile also occurs rarely in magmatic rocks e g anorthosites as well as in some hydrothermal systems coarse grained

nanometre scale faulting in quartz under an atomic force microscope abstract we conducted nano indentation tests of quartz under a load of 294 mn at room temperature and ambient pressure using an atomic force microscope we performed topographic mapping of the indented surface immediately after the nano indentation test and again 46 h later differences in the contour patterns of the two surface topographies reveal that a new fault developed in the quartz specimen while it was secured on the specimen stage of the atomic force microscope between the two mapping times the fault length is 2 m and the maximum displacement on the fault plane is 20 nm when combined with existing displacement length data from natural faults the data suggest that a linear displacement length scaling relationship can be extrapolated to nanometre scale faulting

mirabilite solubility in equilibrium sea ice brines abstract the sea ice microstructure is permeated by brine channels and pockets that contain concentrated seawater derived brine cooling the sea ice results in further formation of pure ice within these pockets as thermal equilibrium is attained resulting in a smaller volume of increasingly concentrated residual brine the coupled changes in temperature and ionic composition result in supersaturation of the brine with respect to mirabilite na2so4 10h2o at temperatures below 6 38 c which consequently precipitates within the sea ice microstructure here mirabilite solubility in natural and synthetic seawater derived brines representative of sea ice at thermal equilibrium has been measured in laboratory experiments between 0 2 and 20 6 c and hence we present a detailed examination of mirabilite dynamics within the sea ice system below 6 38 c mirabilite displays particularly large changes in solubility as the temperature decreases and by 20 6 c its precipitation results in 12 90 and 91 97 reductions in the total dissolved na and so42 concentrations respectively compared to that of conservative seawater concentration such large non conservative changes in brine composition could potentially impact upon the measurement of sea ice brine salinity and ph whilst the altered osmotic conditions may create additional challenges for the sympagic organisms that inhabit the sea ice system at temperatures above 6 38 c mirabilite again displays large changes in solubility that likely aid in impeding its identification in field samples of sea ice our solubility measurements display excellent agreement with that of the frezchem model which was therefore used to supplement our measurements to colder temperatures measured and modelled solubility data were incorporated into a 1d model for the growth of first year arctic sea ice model results ultimately suggest that mirabilite has a near ubiquitous presence in much of the sea ice on earth and illustrate the spatial and temporal evolution of mirabilite within sea ice as it grows throughout an arctic winter reaching maximum concentrations of 2 3 g kg 1

campaign style titanite u pb dating by laser ablation icp implications for crustal flow phase transformations and titanite closure abstract u pb dates of titanite from 150 samples of chiefly quartzofeldspathic gneiss and leucosomes were measured across the western gneiss region of norway to understand deformation and metamorphism of typical crustal rocks during ultrahigh pressure uhp subduction and exhumation titanite is unstable at these high temperatures and pressures and indeed most of the titanite yielded post uhp dates a modest number of titanites sampled across large areas however have pre uhp u pb dates indicating that they survived their excursion to and return from mantle depths metastably this has three important implications 1 titanite grains can remain closed to complete pb loss during regional metamorphism at temperatures as high as 750 c and pressures as high as 3 gpa 2 phase transformations in quartzofeldspathic rocks can be inhibited at the same conditions 3 quartz bearing rocks can remain undeformed even at high temperature and pressure both of the latter were previously recognized the present study simply presents a new method for evaluating both using titanite u pb dates

a systematic evaluation of the zr in rutile thermometer in ultra high temperature uht rocks the zr in rutile geothermometer is potentially a widely applicable tool to estimate peak metamorphic temperatures in rocks from diverse geological settings in order to evaluate its usefulness and reliability to record and preserve high temperatures in granulite facies rocks rutile from uht rocks was investigated to assess different mechanisms of zr re distribution following cooling from high temperature granulite facies paragneisses from the lowermost part of the ivrea zone italy incorporated as thin sheets into the extensive basaltic body of the mafic complex were selected for this study the results show that zr in rutile thermometry if properly applied is well suited to identify and study uht terranes as it preserves a record of temperatures up to 1190 c although the thermometer is susceptible to partial post peak metamorphic resetting by zr diffusion texturally homogeneous rutile grains preserve zr concentrations corresponding to temperatures of prograde rutile growth diverse rutile textures and relationships between some rutile host grains and included or adjacent zr bearing phases bear testimony to varying mechanisms of partial redistribution and resetting of zr in rutile during cooling and link zr in rutile temperatures to different steps of the metamorphic evolution rutile grains that equilibrated their zr concentrations at temperatures above 1070 c i e 1 1 wt zr could not retain all zr in the rutile structure during cooling and exsolved baddeleyite zro2 by subsequent reaction of baddeleyite exsolution lamellae with sio2 zircon needles formed before the system finally closed at 650 700 c without significant net loss of zr from the whole host rutile grain by reintegration of zircon exsolution needles peak metamorphic temperatures of up to 1190 c are derived for the studied rocks which demonstrates the suitability of this solution thermometer to record uht conditions and also confirms the extraordinary geological setting of the lowermost part of the ivrea zone

coesite micro inclusions and the u pb age of zircons from the hareidland eclogite in the western gneiss region of norway abstract we have identified by laser micro raman spectroscopy that inclusions of coesite occur together with other eclogite facies mineral phases within metamorphic zircons separated from the large eclogite body at ulsteinvik dimnoy on hareidland this is the first identification of coesite from this portion of the northwestern western gneiss region wgr and supports continuity of ultrahigh pressure uhp metamorphism between the documented coesite occurrences on stadlandet to the south and the microdiamond and coesite pseudomorph localities on fjortoft in the nordoyene to the north the zircons first analysed by u pb tims in 1973 have been re analysed and have yielded a much more precise age of 401 6 1 6 ma that overlaps with the previously determined age our discovery of coesite and the indication of a close to 402 ma formation age add to a growing number of mid late early devonian ages that signal that the uhp metamorphism in this part of west norway occurred relatively late in the caledonian orogenic cycle these observations should be incorporated in geodynamic models for the exhumation of these rocks and for the metastable preservation of eclogite facies mineralogies

metamorphic rocks seek meaningful cooling rate interpreting 40ar 39ar ages in an exhumed ultra high pressure terrane 40ar 39ar dating of metamorphic white mica is commonly used to elucidate the timing of cooling and or exhumation of metamorphic terranes in theory so long as the dated mineral originally crystallised above its closure temperature the yielded 40ar 39ar ages should not vary significantly within or between samples and should vary only due to grain size 3 ma variation depending on the estimated cooling rate infra red laser single grain fusion and ultra violet laser spot muscovite and or phengite data from mafic eclogite bodies and their host gneisses in the nordfjord region of the western gneiss complex wgc norway show that the ages yielded both within and between white micas from the same mafic eclogite sample vary by up to 284 ma ages yielded within the same pelitic gneiss sample vary by up to 28 ma overall the 40ar 39ar phengite dates from mafic eclogites range from 750 to 440 ma and are significantly older than the previously reported 405 400 ma u pb zircon age constraining peak metamorphism in the nordfjord wgc the felsic host gneiss samples which recrystalised during a post peak exhumation related amphibolite facies overprint yield single grain fusion and laser spot 40ar 39ar white mica dates spanning 420 to 385 ma encompassing previous estimates of the timing of the entire metamorphic cycle these data suggest that ar is heterogeneously distributed both within grains and between grains on the cm to sub mm scale in all samples and imply restricted ar transport and inefficient removal during the metamorphic cycle open system zero grain boundary ar concentration behaviour is an inherent assumption in the commonly applied closure temperature formulation but unless this can be demonstrated linking 40ar 39ar ages to temperature is no longer trivial the high spatial and chronological precision afforded by uv laser ablation mean that intra grain ar concentration distributions may be used to assess open vs closed system behaviour the data presented here suggest that open system behaviour cannot be assumed in metamorphic systems even those in which the pressure temperature and time conditions in theory allow for efficient diffusion during the metamorphic cycle high spatial resolution 40ar 39ar data and robust comparison with numerical models of open system behaviour provide the greatest future potential for unravelling the complexities of metamorphic exhumation timescales

exhuming norwegian ultrahigh pressure rocks overprinting extensional structures and the role of the nordfjord sogn detachment zone 1 the nordfjord sogn detachment zone nsdz is widely cited as one of the primary structures responsible for the exhumation of norwegian ultra high pressure uhp rocks here we review data from the considerable volume of research describing this shear zone and compile a strikeparallel cross section along the nsdz from the solund basin in the south to the soroyane uhp domain in the north this cross section highlights several previously unrecognized patterns revealing a shear zone with top to the west asymmetric fabrics that 1 initiated at amphibolite facies 2 overprints metamorphic breaks and tectonostratigraphic contacts and 3 has a gradational continuum of muscovite cooling ages these patterns constrain the kinematic evolution of the nsdz and suggest a new three step model for the exhumation of norwegian u hp rocks the initial stages of exhumation were characterized by the rise of crustal rocks from u hp depths to the base of the crust by buoyancydriven mechanisms not specified in this paper mantle exhumation was followed by top to thewest normal sense displacement within a broad noncoaxial ductile shear zone near the base of the crust that overprinted tectonostratigraphic contacts formed previously during mantle exhumation in the final stages of crustal exhumation top w brittleductile detachments soled into and partially excised this ductile shear zone dropping the devonian basins into contact with rocks of varying tectonostratigraphic levels this new interpretation of the nsdz is significant as it accounts for the extreme crustal excision observed in western norway using three sequentially overprinting structures active at different stages of uhp rock exhumation citation johnston s m b r hacker and t b andersen 2007 exhuming norwegian ultrahigh pressure rocks overprinting extensional structures and the role of the nordfjord sogn detachment zone tectonics 26 tc5001

microdiamond in high grade metamorphic rocks of the western gneiss region norway three grains of microdiamond were recovered from high grade gneiss exposed in the western gneiss region norway identification and characterization of the diamond grains by raman and infrared spectroscopy indicate the presence of substitutional impurities of h and n primary fluid inclusions in garnet and quartz in the diamond bearing rock demonstrate the evolution of metamorphic volatile fluids from reduced n 2 co 2 compositions during the peak phase of metamorphism to n 2 ch 4 h 2 o bearing compositions during retrograde metamorphism compatible geologic petrologic and fluid composition data imply a metamorphic origin for the microdiamonds if so the metamorphic and fluid conditions recorded by the microdiamonds and gneissic host may be applicable to microdiamond investigations in other high pressure regionally metamorphosed orogens

constraints on the u pb systematics of metamorphic rutile from in situ la icp ms analysis abstract in situ laser ablation icp ms u pb dating of metamorphic rutile from granulite facies metapelitic rocks of the archaean pikwitonei granulite domain manitoba canada provides constraints on pb diffusion and characterizes the closure behavior of rutile the analysis of transects of 35 m spots across 15 rutile grains having a size of 120 to 280 m yielded concordant ages with core ages of ca 2450 ma and core to rim younging towards 2280 ma age profiles indicate that volume diffusion of pb occurs in rutile implying that the ages represent cooling ages to investigate the closure behavior of pb in rutile closure temperature profiles t c x were constructed based on different models combined with experimentally determined diffusion parameters the classical t c x model of dodson 1986 mat sci forum 7 145 154 indicates a rapid decrease of t c in the rims of grains providing unrealistic estimates for the cooling rate when combined with u pb ages a new t c x model was constructed based on the analyzed age profiles that are described by an error function this model shows a more steady decrease in t c throughout the grain from ca 640 c in the core depending on grain size to a rim intercept t c rim of 490 c 7 2 which is interpreted to be the extrapolated theoretical absolute temperature of insignificant pb diffusion in rutile the new model provides a better description of the relation between age and t c for the analyzed grains however both t c x models demonstrate that even in small grains the variations of t c can be significant making it impossible to derive one representative t c for pb in rutile the error function based t c x model allows the determination of cooling rates which show a decrease over time from ca 2 2 to 0 4 c ma agreeing well with previous estimates for the pikwitonei granulite domain this consistency supports the validity of our model and indicates that cooling rates can be estimated from single grains by la icp ms u pb dating of rutile providing constraints on the cooling history of a metamorphic terrane the slow cooling rates imply that exhumation was slow

trace element systematics in granulite facies rutile implications for zr geothermometry and provenance studies metamorphic rutile from granulite facies metapelitic rocks of the archean pikwitonei granulite domain pgd manitoba canada provides constraints on the systematics of trace elements in rutile during high temperature conditions and subsequent slow cooling compositional profiles and maps of the zr concentrations in rutile grains 120 600 m from three metapelitic gneisses were acquired by electron probe micro analysis using a spatial resolution of down to 2 m simultaneously profiles were analysed for nb cr and v which have significantly different diffusion characteristics in rutile the profiles of all elements show relatively homogeneous concentrations within most grains but significant inter grain differences even within a single thin section some rutile grains display a slight concentration decrease from a neighbouring garnet towards the matrix for all measured elements the lack of diffusion profiles for all analysed elements shows that these are highly immobile in rutile and that distributions of these elements are primary and preserve prograde information the nb and cr concentrations overlap with ranges that are ascribed to different provenances indicating that source discrimination based on these elements is not possible in all cases high retentiveness for zr implies that the zr in rutile geothermometer is highly robust to diffusive re equilibration even during very slow cooling 2 c ma 1 from granulite facies conditions most grains have high zr contents 3000 4600 ppm differences between high zr contents suggest that during growth under vapour absent conditions there may not be saturation of zr in rutile even if zircon is present therefore several rutile grains need to be analysed in a sample to obtain a useful minimum peak temperature the highest zr concentrations correspond to 900 c this is significantly higher than previous peak temperature estimates of 820 c based on two feldspar thermometry on a regional scale this implies that part of the pgd was affected by ultra high temperature uht metamorphism it also implies that rutile is able to preserve primary compositions even to uht conditions this study shows that if combined with textural information zr in rutile has the potential to be a very useful tool for estimating rutile crystallization temperatures and peak metamorphic conditions for granulite facies rocks zr in rutile yields more reliable peak metamorphic temperatures than most other exchange geothermometers which tend to partially re equilibrate by diffusion during cooling

protolith ages and exhumation histories of ultra high pressure rocks across the western gneiss region norway the timing of protolith formation ultrahigh pressure uhp subduction and subsequent exhumation for the ultrahigh pressure to high pressure units across the eastern part of the western gneiss region norway were assessed using u pb zircon th pb monazite and 40 ar 39 ar white mica ages u pb zircon ages from eclogites demonstrate that oceanic and continental allochthons were emplaced onto the baltica basement before the entire mass was subducted to ultra high pressure eclogites within the allochthons across the entire western gneiss region are caledonian and show a degree of zircon re crystallization that increases with peak pressure permitting the interpretation that the entire region underwent synchronous subduction 40 ar 39 ar white mica ages of 399 ma indicate that the eastern part of the western gneiss region had been exhumed to shallow crustal levels while uhp metamorphism was ongoing farther west indicating a westward dip to the slab the 40 ar 39 ar white mica ages also show a clear east to west gradient across the entire western gneiss region indicating that the western gneiss region rose diachronously to crustal levels from east to west between 399 and 390 ma

early evolution of the pamir deep crust from lu hf and u pb geochronology and garnet thermometry determining early orogenic processes within the pamir tibet orogen represents a critical step toward constructing a comprehensive model on the tectonic evolution of the region here we investigate the timing and cause of prograde metamorphism of cenozoic metamorphic rocks from the pamir plateau through lu hf geochronology u pb rutile thermochronology and garnet thermometry regional prograde metamorphism and heating to 750 830 c as constrained by thermometry occurred between 37 and 27 ma prograde growth of garnet first occurred in the south pamir and spread to the central pamir during the following 10 m y the early metamorphism is attributed to high mantle heat flow following the ca 45 ma break off of the indian slab south of the pamir our investigation confirms a long lived thermal history of the pamir deep crust before the miocene and provides a causal link between break off enhanced mantle heat flow and prograde heating of the subduction hanging wall

when can muscovite 40ar 39ar dating constrain the timing of metamorphic exhumation abstract metamorphic cooling rates determined using muscovite 40 ar 39 ar ages underpin many tectonic models but are based on several simplifying approximations we demonstrate that a number of these approximations especially those relating to the assignment of a single closure temperature might not hold for certain metamorphic scenarios and particularly for metamorphism during rapid orogenic cycles using numerical diffusion models that include a recently reported significant pressure dependence of ar diffusion in muscovite we systematically interrogate the approximations associated with linking 40 ar 39 ar dates to pressure temperature histories the results of the simulations are presented in a simple graphic form allowing evaluation of the pressure temperature pt regions in which ar diffusion is efficient and hence those in which the muscovite may yield true 40 ar 39 ar cooling ages we suggest that a robust method for determining whether metamorphic muscovite 40 ar 39 ar ages relate to the timing of cooling involves 1 the determination of the pt conditions and relative timing of muscovite crystallisation during the metamorphic cycle 2 the collection of high precision and high spatial resolution 40 ar 39 ar data from muscovite grains and 3 the comparison between analytical data and numerical diffusion models which test different post crystallisation pressure temperature histories

constraints on the early uplift history of the tibetan plateau the surface uplift history of the tibetan plateau and himalaya is among the most interesting topics in geosciences because of its effect on regional and global climate during cenozoic time its influence on monsoon intensity and its reflection of the dynamics of continental plateaus models of plateau growth vary in time from pre india asia collision e g 100 ma ago to gradual uplift after the india asia collision e g 55 ma ago and to more recent abrupt uplift 7 ma ago and vary in space from northward stepwise growth of topography to simultaneous surface uplift across the plateau here we improve that understanding by presenting geologic and geophysical data from north central tibet including magnetostratigraphy sedimentology paleocurrent measurements and 40ar 39ar and fission track studies to show that the central plateau was elevated by 40 ma ago regions south and north of the central plateau gained elevation significantly later during eocene time the northern boundary of the protoplateau was in the region of the tanggula shan elevation gain started in pre eocene time in the lhasa and qiangtang terranes and expanded throughout the neogene toward its present southern and northern margins in the himalaya and qilian shan

stylolites a review abstract stylolites are ubiquitous geo patterns observed in rocks in the upper crust from geological reservoirs in sedimentary rocks to deformation zones in folds faults and shear zones these rough surfaces play a major role in the dissolution of rocks around stressed contacts the transport of dissolved material and the precipitation in surrounding pores consequently they play an active role in the evolution of rock microstructures and rheological properties in the earth s crust they are observed individually or in networks in proximity to fractures and joints and in numerous geological settings this review article deals with their geometrical and compositional characteristics and the factors leading to their genesis the main questions this review focuses on are the following how do they form how can they be used to measure strain and formation stress how do they control fluid flow in the upper crust geometrically stylolites have fractal roughness with fractal geometrical properties exhibiting typically three scaling regimes a self affine scaling with hurst exponent 1 1 0 1 at small scale up to tens or hundreds of microns another one with hurst exponent around 0 5 to 0 6 at intermediate scale up to millimeters or centimeters and in the case of sedimentary stylolites a flat scaling at large scale more complicated anisotropic scaling scaling laws depending of the direction of the profile considered is found in the case of tectonic stylolites we report models based on first principles from physical chemistry and statistical physics including a mechanical component for the free energy associated with stress concentrations and a precise tracking of the influence of grain scale heterogeneities and disorder on the resulting micro structures experimental efforts to reproduce stylolites in the laboratory are also reviewed we show that although micrometer size stylolite teeth are obtained in laboratory experiments teeth deforming numerous grains have not yet been obtained experimentally which is understandable given the very long formation time of such geometries finally the applications of stylolites as strain and stress markers to determine paleostress magnitude are reviewed we show that the scalings in stylolite heights and the crossover scale between these scalings can be used to determine the stress magnitude its scalar value perpendicular to the stylolite surface during the stylolite formation and that the stress anisotropy in the stylolite plane can be determined for the case of tectonic stylolites we also show that the crossover between medium millimetric scales and large pluricentimetric scales in the case of sedimentary stylolites provides a good marker for the total amount of dissolution which is still valid even when the largest teeth start to dissolve which leads to the loss of information since the total deformation is not anymore recorded in a single marker structure we discuss the impact of the stylolites on the evolution of the transport properties of the hosting rock and show that they promote a permeability increase parallel to the stylolites whereas their effect on the permeability transverse to the stylolite can be negligible or may reduce the permeability depending on the development of the stylolite

deep crustal flow within postorogenic metamorphic core complexes insights from the southern western gneiss region of norway 2019 the authors this is an open access article under the terms of the creative commons attribution license which permits use distribution and reproduction in any medium provided the original work is properly cited

underplating in the himalaya tibet collision zone revealed by the hi climb experiment we studied the formation of the himalayan mountain range and the tibetan plateau by investigating their lithospheric structure using an 800 kilometer long densely spaced seismic array we have constructed an image of the crust and upper mantle beneath the himalayas and the southern tibetan plateau the image reveals in a continuous fashion the main himalayan thrust fault as it extends from a shallow depth under nepal to the mid crust under southern tibet indian crust can be traced to 31 n the crust mantle interface beneath tibet is anisotropic indicating shearing during its formation the dipping mantle fabric suggests that the indian mantle is subducting in a diffuse fashion along several evolving subparallel structures

slow exhumation of uhp terranes titanite and rutile ages of the western gneiss region norway abstract u pb ages of titanite and rutile were obtained from the central western gneiss region norway to assess the style and timing of exhumation and cooling of the western gneiss uhp terrane approximately half of the titanite ages are concordant the majority of which yield a limited age range from 393 to 390 ma the other titanite data are discordant and define discordia arrays with upper intercept ages of either 938 ma or 1 6 ga and a lower intercept of 389 ma concordant rutile analyses range from 385 to 392 ma both titanite and rutile ages young wnw toward the core of the orogen and are 4 ma older than 40 ar 39 ar muscovite ages corresponding to a cooling rate of 90 c ma a well defined boundary between concordant and discordant titanite ages in combination with the wnw increasing p t gradient and the similarity between muscovite cooling ages in the east and eclogite ages in the west suggests that the wgr remained coherent throughout its exhumation history and was progressively unroofed from east to west a 390 2 0 8 ma titanite in the soroyane uhp domain indicates that exhumation occurred at a vertical rate of 7 mm yr for 12 ma these rates are slower than estimates from smaller uhp terranes but similar to other large uhp terranes suggesting that there may be fundamental differences in the mechanisms controlling the evolution of large uhp terranes that undergo protracted subduction and exhumation and smaller uhp terranes that undergo rapid subduction and exhumation

pb diffusion in rutile diffusion of pb was measured in natural and synthetic rutile under dry 1 atmosphere conditions using mixtures of pb titanate or pb sulfide and tio2 as the sources of diffusant pb depth profiles were then measured with rutherford backscattering spectrometry rbs over the temperature range 700 1100 c the following arrhenius relation was obtained for the synthetic rutile d 3 9 10 10exp 250 12 kj mol 1 rt m2s 1 results for diffusion in natural and synthetic rutile were quite similar despite significant differences in trace element compositions mean closure temperatures calculated from the diffusion parameters are around 600 c for rutile grains of 100 m size this is about 100 c higher than rutile closure temperature determinations from past field based studies suggesting that rutile is more resistant to pb loss through volume diffusion than previously thought

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

slow subduction of a thick ultrahigh pressure terrane 1 high precision lu hf and sm nd ages reported here cover a large portion of the western gneiss region wgr ultrahigh pressure uhp terrane norway and range from 413 9 3 7 to 397 1 4 8 ma collectively new and existing geochronologic data for eclogites demonstrate that eclogite facies metamorphism occurred over a large area 60 000 km2 for an unexpectedly long period of 20 ma a thermal model using slow subduction 2 4 mm a 1 of a relatively warm and thick slab can reproduce the observed p t t history of the wgr uhp terrane indicating that large uhp terranes with protracted peak or near peak histories probably reflect slow subduction of relatively thick crustal sections

crustal exhumation of the western gneiss region uhp terrane norway 40 ar 39 ar thermochronology and fault slip analysis article i nfo ar 39 ar muscovite thermochronology 40 ar 39 ar k feldspar thermochronology fault slip analysis new 40 ar 39 ar muscovite and k feldspar thermochronology combined with existing data reveal the timing and patterns of late stage exhumation across the western gneiss region u hp terrane muscovite age contours show that exhumation into the mid upper crust progressed westward over a 20 myr period 400 380 ma this exhumation was caused by i e w stretching and eastward tilting north of nordfjord where muscovite ages decrease from the foreland allochthons westward into the uhp domains and ii differential exhumation south of nordfjord where muscovite ages depict a ne sw dome like pattern and the western gneiss region is bounded by overlying units little affected by the scandian metamorphism exhumation of the uhp domains into the mid upper crust by late folding continued through 374 ma the smooth gradient of fairly flat muscovite age spectra demonstrates minimal influence of excess ar which is relatively unusual for a u hp terrane 40 ar 39 ar spectra and modeled cooling histories from k feldspar combined with brittle ductile

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