lateral extrusion of lower crust from under high topography in the isostatic limit where there is isostasy the rocks of the lower continental crust are subject to an effective lateral pressure gradient equal to the gradient of topographic load whether compensation is in crustal roots or in the mantle the result is a poiseuille flow planar channel flow in the weak lower crust which removes crust from under mountains and smooths and levels the topography assuming cubic power law creep the flux of crust is proportional to the third power of the topographic gradient to the usual arrhenius term to the tenth power of the absolute temperature of the lower crust and to the negative fifth power of the geothermal gradient the result is that any initial condition with an isolated high tends eventually toward a state where the topography is pancake shaped with a flat central plateau and steeper flanks spreading outward when adjacent pancakes merge most of the relief is eliminated flow may either roughen or smooth the moho depending on whether there are lateral density contrasts in the mantle lithosphere or not although it is difficult to find analytic solutions for this evolution it is easily simulated numerically using various published flow laws for plausible lower crustal rocks lateral extrusion is shown to be insignificant under the oceans marginally significant under shields and platforms important under elevated plains and dominant beneath high plateaus and or hot delaminated regions in particular the basin and range province of the western united states cannot maintain short wavelength 100 km moho relief of more than 1 km for times greater than 10 20 m y at most if the tibetan plateau of china has been delaminated as recently suggested then it may flatten even faster reducing 100 km wavelength topographic features to 200 m relief in no more than 0 03 0 13 m y and leaving only features with wavelengths over 400 km at the present therefore the lower crust of tibet may resemble a hydraulic reservoir as previously suggested because the flatness of the moho is self maintained in these regions information on past tectonics is continually lost and the present moho shape cannot be used to balance cross sections for times in the past the flatness of the moho today is not a constraint on the mechanisms of extension or shortening in these regions nor is it evidence for underplating by intrusions it also follows that very intense and inhomogeneous dilational strains may have occurred locally as in the metamorphic core complexes of the basin and range province

trace elements in titanite a potential tool to constrain polygenetic growth processes and timing abstract the petrological information preserved in the trace element signature of titanite is a valuable complement to in situ u pb geochronology and can be used to refine interpretations on the growth of this mineral we present trace element and u pb isotopic compositions of titanite grains in five amphibolite to granulite facies samples from the east albany fraser orogen of western australia chondrite normalised rare earth element ree abundance patterns discriminate between titanite populations and correlate with backscatter electron bse zonation in two samples titanite ree composition correlates with proximity to garnet and in one sample titanite composition correlates with gneissic compositional banding the dy yb ratio can be used as a geochemical indicator to link titanite growth to garnet growth titanite that crystallised in a garnet bearing assemblage is hree depleted with dy yb 2 whereas titanite that crystallised in a garnet free assemblage typically has flat hree patterns with dy yb by integrating the ree signatures with u pb data the five new titanite u pb ages can be linked to a range of processes magmatic titanite crystallisation metamorphic titanite growth garnet growth and or cooling through the closure temperature for pb diffusion in titanite

high pressure deformation of calcite marble and its transformation to aragonite under non hydrostatic conditions we conducted deformation experiments on carrara marble in the aragonite and calcite stability fields to observe the synkinematic transformation of calcite to aragonite and to identify any relationships between transformation and deformation or sample strength deformation induced microstructures in calcite crystals varied most significantly with temperature ranging from limited slip and twinning at 400 c limited recrystallization at 500 c widespread recrystallization at 600 and 700 c to grain growth at 800 900 c variations in confining pressure from 0 3 to 2 0 gpa have no apparent effect on calcite deformation microstructures aragonite grew in 10 6 10 7 s 1strain rate tests conducted for 18 524 h at confining pressures of 1 7 2 0 gpa and temperatures of 500 600 c as in our previously reported hydrostatic experiments on this same transformation the aragonite nucleated on calcite grain boundaries the extent of transformation varied from a few percent conversion near pistons at 400 c 2 0 gpa and 10 4 s 1 strain rate in a 0 8 h long experiment to 98 transformation in a 21 day test at a strain rate of 10 7 s 7 a temperature of 600 c and a pressure of 2 0 gpa at 500 c porphyroblastic 100 200 m aragonite crystals grew at a rate faster than 8 10 1m s 1 at 600 c the growth of aragonite neoblasts was slower 6 10 1 m s 1 and formed glove and finger cellularprecipitation like textures identical to those observed in hydrostatic experiments the transformation to aragonite is not accompanied by a shear instability or anisotropic aragonite growth consistent with its relatively small volume change and latent heat in comparison with compounds that do display those features

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

the zonal currents and transports at 35 w in the tropical atlantic the total of 13 existing cross equatorial shipboard current profiling sections taken during the woce period between 1990 and 2002 along 35 w are used to determine the mean meridional structure of the zonal top to bottom circulation between the brazilian coast near 5 s and 5 n and to estimate mean transports of the individual identified shallow intermediate and deep current branches one of the results is that on the equator a mean westward equatorial intermediate current below the equatorial undercurrent exists

structural mineralogical and petrophysical effects on deep crustal rocks of fluid limited polymetamorphism western gneiss region norway the proterozoic banded granulite facies complex of bardsholmen western gneiss region norway t 815 845 c was locally transformed to eclogite t 455 510 c p 12 kbar and amphibolite facies rocks t 460 c during the caledonian continental collision the granulite complex consists of mafic two pyroxene granulite and leucocratic orthopyroxene garnet bearing layers alternating on a scale from 1 cm to 10 m the granulite facies rocks change to eclogite facies rocks over centimetre scale distances along well defined fluid infiltration fronts the mafic granulite was transformed to omphacite garnet rich eclogites and the leucocratic rocks were converted to quartz phengite rich assemblages with minor garnet and local omphacite melange like lithologies consisting of mafic lenses of eclogite surrounded by felsic material represent an advanced stage in the process of converting deep crust to eclogite facies during amphibolitization this melange like lithology evolves to a rock where amphibolite lenses and layers are surrounded by granitoid gneiss a lithology typical of the western gneiss region the bardsholmen locality illustrates the profound control exerted by fluids on the timing of metamorphism the structural make up and petrophysical properties such as density and rheology of crustal root zones fluid induced metamorphism will therefore exert control on the attributes of orogenic belts such as topography and moho depth and influence the dynamics of collision zones by controlling the time of orogenic collapse and the buoyancy of the subducted crust we suggest that orogens may develop differently depending on the fluid budget

multistage zircon and titanite growth and inheritance in an archean gneiss complex winnipeg river subprovince ontario abstract the paper presents results from a u pb isotopic investigation of zircon and titanite from three phases of the archean sioux lookout gneiss complex at the margin of the gneissic plutonic winnipeg river subprovince wrs of the superior province the data reveal an intricate data pattern caused by inheritance and multistage crystallization and record the timing of two major orogenic events followed by a protracted post metamorphic evolution a zircon age of 2889 3 ma for a foliated pink granite correlates with that of similar units elsewhere in the superior province and reflects one of the first major episodes of crustal assembly in the pre kenoran evolution of the region tonalite and diorite from the same site yield zircon and titanite ages of 2704 2 and 2705 5 ma which correspond to the timing of major magmatism related to kenoran collision of the wrs with the adjacent wabigoon volcanic domains tonalite and diorite both contain abundant xenocrystic zircon moreover the late kinematic diorite also includes xenocrystic titanite which has a minimum age of 2730 ma and coexists with younger ca 2705 ma titanite and zircon two generations of titanite are also present in both the tonalite and the granite the earliest one reflects the 2704 ma plutonic metamorphic event whereas the second one indicates post metamorphic crystallization at 2650 and 2627 ma caused by superimposed hydrothermal activity and retrogression the data support the concept that the titanite can remain a closed system to pb diffusion at elevated temperatures as long as the crystals escape magmatic or metamorphic recrystallization conversely they also show that titanite can easily be produced by low temperature processes hence generalized interpretation of titanite ages in terms of regional cooling can be inappropriate

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

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

microfabrics of uhp metamorphic granites in the dora maira massif western alps no evidence of deformation at great depth in the dora maira massif western alps essentially undeformed ultrahigh pressure uhp metamorphic granites brossasco granite are embedded in and locally grade into granite gneisses or augengneisses and mylonites in this study the quartz microfabrics of the undeformed granites are compared against the augengneisses and mylonites in a representative number of samples from several locations in the undeformed granites the fine grained quartz aggregates that formed from coesite upon decompression are characterized by a foam structure and random crystallographic orientation in the deformed granites the quartz microstructures and the crystallographic preferred orientation cpo indicate deformation by dislocation creep most of the deformation of the granites if not all must have happened at a late stage during exhumation after transformation of coesite to quartz at greenschist facies conditions in the middle crust the deformed granites provide no evidence of deformation during subduction at u hp metamorphic conditions and in the earlier stages of exhumation the diameter of internally undeformed slices of continental crust subducted to and exhumed from about 100 km can exceed that of the presently exposed brossasco granite i e it can be on the kilometre scale

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

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

protracted thrusting followed by late rapid cooling of the greater himalayan sequence annapurna himalaya central nepal insights from titanite petrochronology the greater himalayan sequence ghs has commonly been treated as a large coherently deforming high grade tectonic package exhumed primarily by simultaneous thrust and normal sense shearing on its bounding structures and erosion along its frontal exposure a new paradigm developed over the past decade suggests that the ghs is not a single high grade lithotectonic unit but consists of in sequence thrust sheets in this study we examine this concept in central nepal by integrating temperature time t t paths based on coupled zr in titanite thermometry and u pb geochronology for upper ghs calcsilicates with traditional thermobarometry textural relationships and field mapping peak zr in titanite temperatures are 760 850 c at 10 14 kbar and u pb ages of titanite range from c 30 to c 15 ma sector zoning of zr and distribution of u pb ages within titanite suggest that diffusion rates of zr and pb are slower than experimentally determined rates and these systems remain unaffected into the lower granulite facies two types of t t paths occur across the chame shear zone csz between c 25 and 17 16 ma hanging wall rocks cool at rates of 1 10 c ma while foot wall rocks heat at rates of 1 10 c ma over the same interval temperatures increase structurally upwards through the hanging wall but by 16 17 ma temperatures converge in contrast temperatures decrease upwards in foot wall rocks at all times while the foot wall is interpreted as an intact structurally upright section the thermometric inversion within the hanging wall suggests thrusting of hotter rocks over colder from c 25 to c 17 16 ma retrograde hydration that is restricted to the hanging wall and a lithological repetition of orthogneiss are consistent with thrust sense shear on the csz the csz is structurally higher than previously identified intra ghs thrusts in central nepal and thrusting duration was 3 6 ma longer than proposed for other intra ghs thrusts in this region cooling rates for both the hanging wall and foot wall of the csz are comparable to or faster than rates for other intra ghs thrust sheets in nepal the overlap in high temperature titanite u pb ages and previously published muscovite 40ar 39ar cooling ages imply cooling rates for the hanging wall of 200 c ma after thrusting causes of rapid cooling include passive exhumation driven by a combination of duplexing in the lesser himalayan sequence and juxtaposition of cooler rocks on top of the ghs by the stds normal sense displacement does not appear to affect t t paths for rocks immediately below the stds prior to 17 16 ma this article is protected by copyright all rights reserved

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

formation of the jinshandian fe skarn ore field in the edong district eastern china constraints from u pb and 40ar 39ar geochronology abstract the jinshandian ore field represents one of the largest concentrations of fe skarn deposits in the edong district of eastern china orebodies of these deposits occur in skarn along the contacts between quartz diorite or quartz monzonite and triassic sedimentary country rocks in this paper we present new u pb geochronological data for magmatic zircon and hydrothermal titanite as well as 40 ar 39 ar ages on hydrothermal phlogopite to determine whether multiple magmatic hydrothermal events associated with formation of the fe skarn deposits in the jinshandian ore field the robust u pb zircon ages demonstrate that the quartz monzonite and quartz diorite were emplaced at 130 ma and 127 ma receptively the u pb and 40 ar 39 ar ages of hydrothermal titanite and phlogopite form the skarn indicate that fe mineralization also formed at ca 131 128 ma but with an additional 118 ma hydrothermal event recorded by phlogopite this geochronology confirms a causative linkage between granitic intrusion and skarn mineralization the data also indicate that at least two hydrothermal events took place in the jinshandian ore field one at 131 128 ma and the other at 118 ma integrating these ages from the ore field with previous geochronological data from the broader edong district two regional magmatic hydrothermal events are recognized in the edong district after 135 ma with the older one at 131 127 ma in excellent agreement with the 131 ma formation age of magnetite apatite deposits in the eastern part of middle lower yangtze river region and the younger one at 122 118 ma post dating emplacement of the ca 126 ma a type granite and syenite in the eastern part of middle lower yangtze river region a detailed comparison between the fe skarn deposits and magnetite apatite deposits in the middle lower yangtze river region suggests that these two deposit types not only share similar alteration assemblages and association with the evaporites within the ore forming systems but also may have formed during the same early cretaceous magmatic hydrothermal event

new evidence for coesite in eclogite and gneisses defining an ultrahigh pressure province in the western gneiss region of norway widespread evidence of ultrahigh pressure metamorphism is reported from the nordfjord and stadtlandet area of the western gneiss region in norway outlining a distinct ultrahigh pressure province coesite has been found as inclusions in garnet omphacite or kyanite in eclogite from 5 new localities and quartz pseudomorphs after coesite occur in eclogite minerals at an additional 12 localities quartz pseudomorphs after coesite within garnet and clinozoisite are reported for the first time from associated gneisses which is evidence for in situ ultrahigh pressure metamorphism at least at a local scale the boundary to an ultrahigh pressure province is defined by a metamorphic break from lower pressure 2 8 gpa within a 10 km wide mixed zone in the nordfjord area this zone may mark the disrupted tectonic boundary to a small ultrahigh pressure unit and casts doubt on the assumption that the western gneiss region behaved as a coherent terrane during burial and exhumation

u pb sphene dating of metamorphism the importance of sphene growth in the contact aureole of the red mountain pluton laramie mountains wyoming the relative importance of thermal diffusion versus new growth or recrystallization on u pb isotopic data from sphene is assessed through a study of amphibolites and granite gneisses within the contact aureole of the red mountain pluton laramie anorthosite complex wyoming samples were collected along a traverse approximately perpendicular to the margin of the intrusion over a distance of 0 13 to 2 65 km from the contact the 207pb 206pb ages of sphene from amphibolite samples range between 1 43 ga the intrusive age of the red mountain pluton to 1 78 ga the age of regional metamorphism the 207pb 206pb ages of sphene in rocks metamorphosed above 700 c are within error of the intrusive age of the pluton and appear to have resulted from diffusional resetting of preexisting sphene and the metamorphic growth of additional sphene at 1 43 ga at greater distance from the contact the 207pb 206pb ages range from 1 45 to 1 72 ga this 300 million year spread in ages is interpreted to result from two periods of sphene growth one produced during regional metamorphism at 1 78 ga and another generation of newly grown or recrystallized sphene that formed during contact metamorphism at 1 43 ga these two age populations may be identified on the basis of petrographic textures the morphologies and color differences of grain separates as well as by the u pb systematics in rocks metamorphosed to temperatures less than 700 c sphene growth was the dominant process controlling the response of the u pb isotope system to contact metamorphism sphene grew well outside the zone of obvious contact metamorphism the u pb sphene ages were reset by diffusion only at high temperatures supporting the experimentally determined closure temperature estimates for the u pb system in sphene of around 650 c cherniak 1993 this study demonstrates that u pb ages of sphene can be used to date metamorphism not only in areas with a simple geologic history such as igneous intrusion or single metamorphic or deformational events but also to date multiple events so long as different generations of sphene can be identified and separated

seismic detection of the untransformed basaltic oceanic crust subducting into the mantle summary subcrustal earthquakes occur in a slab like zone beneath sw japan in association with the subduction of the philippine sea plate they generate a distinct pair of p and s later phases with anomalously low apparent velocities of about 7 and 4 km s 1 which cannot be explained by a laterally homogeneous velocity structure these later phases are observed for particular source receiver geometries the sources at depths shallower than 50 60 km and the receivers beneath which the subcrustal seismic zone comes in contact with the bottom of the continental crust we examined the nature of these later phases using two methods one is to determine two types of apparent velocities an event common apparent velocity and a station common apparent velocity which may be different from one another for a laterally inhomogeneous velocity structure the other is to trace seismic rays numerically for a laterally heterogeneous velocity model an extensive analysis using these two methods leads to a conclusion that the slab like seismic zone constitutes a low velocity channel in the uppermost mantle with p and s velocities comparable to those of the lower continental crust beneath sw japan the waves trapped within the low velocity channel escape from it through its contact with the continental moho and are observed as a distinct pair of later phases the head waves guided by the bottom boundary of the low velocity channel are observed as weak arrivals of initial phases the thickness 10 km and the velocities vp 7 km s 1 vs 4 km s 1 of the subcrustal seismic zone indicates that it is made up of the untransformed basaltic oceanic crust subducted with the underlying lithospheric mantle and that the transformation of basalt to eclogite does not take place in the subducting slab at least down to depths of 50 60 km we present laterally heterogeneous structural models of the crust and the uppermost mantle along two profiles in sw japan the models are characterized by an apparent thickening of the lower continental crust in particular regions due to a direct contact of the subducting oceanic crust to the continental moho such features are qualitatively consistent with the results of explosion seismology and with the bouguer anomaly pattern in the relevant regions the oceanic lithosphere with the untransformed light oceanic crust at its top might still be buoyant after subduction a factor which must be taken into account in considering the driving mechanisms of plate tectonics the slab geometry and the earthquake related tectonic process

polyphase tectono magmatic episodes as revealed by shrimp u pb geochronology and microanalysis of zircon and titanite from the central okcheon belt korea abstract the south central okcheon belt of the southern korean peninsula experienced polyphase granitic magmatism and deformation during the mesozoic this study presents integrated results of ion microprobe u pb geochronology and microtextural and microchemical analyses of zircon and titanite from pre tectonic baegnok and cheongsan and post tectonic boeun and sokrisan plutons with respect to the cheongsan shear zone the baegnok granodiorite sample yielded coherent zircon and titanite 206pb 238u ages of 225 6 1 8 and 223 9 4 4 ma indicating a rapid early cooling of the pluton the cheongsan k feldspar megacryst bearing biotite granite sample yielded a coeval zircon 206pb 238u age of 224 7 1 8 ma on the other hand the titanite grains from this rock are categorized into two age populations corresponding to the late triassic 214 3 7 9 ma and the late cretaceous 96 2 8 2 ma growth the older population is clearly of igneous origin considering partly preserved oscillatory zoning in back scattered electron images therefore the age indicates contrasting early cooling paths of the two pre tectonic plutons the younger population is interpreted to be the product of superimposed metasomatic reactions involving plagioclase ilmenite rutile epidote and calcite the two populations show different patterns in their atomic al fe ratios zircons from the boeun and sokrisan plutons yielded 206pb 238u ages of 172 2 2 1 and 94 2 2 1 ma respectively with the latter being indistinguishable from the younger titanite age of the cheongsan pluton the shrimp u pb age of syn deformational fine grained titanite grains lying on sigmoidal foliations in a mylonite from the cheongsan shear zone has a large error 165 47 ma due to their low u pb ratios possibly affected by post tectonic magmatic activities this study highlights the importance of detailed microanalysis prior to the isotopic dating of samples that have experienced complex tectono magmatic episodes

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

on the valency state of radiogenic lead in zircon and its consequences in zircon u pb systematics extreme robustness up to the temperatures of granulite facies and anatexis contrasts with apparently easy loss of radiogenic pb at low temperatures often without any metamorphic event being in evidence here we propose that this paradoxical behaviour can be understood with the hypothesis that radiogenic pb in zircon is tetravalent we review data and arguments in favour of this hypothesis diffusion profiles calculated for pb sup 2 in a 25 micro m radius zircon xenocryst in a melt at 1000 c combined with the incompatibility of pb sup 2 or for a zircon core inside a younger zircon rim at this temperature show age effects that should have been observed in sims dating further in zircon evaporation as well as in leaching experiments common pb is generally released preferentially to radiogenic pb after removal of less radiogenic pb the evaporation record generally shows pure radiogenic pb during the final evaporation steps the distribution of residual pb in a leached titanite revealed by pixe is similar to that of ti lastly xanes spectra of a 1 ga old titanite predominantly radiogenic pb and an alpine one predominantly common pb are significantly different although the former does not resemble that ofmore pbo sub 2 the arguments why radiogenic pb should be tetravalent are based on analogies with studies relating to the tetravalent state of sup 234 th and the hexavalent state of sup 234 u which show that alpha recoil in silicates generates a strongly oxidizing environment at the site where the recoiling nucleus comes to rest further a zircon grain being small should remain highly oxidizing in its interior by the constant loss of beta particles maintaining the 4 state of radiogenic pb from its effective ion radius similar to that of zr sup 4 and its charge pb sup 4 has to be compatible in the zircon lattice also by analogy with u sup 4 th sup 4 and hf sup 4 its diffusivity should be several orders of magnitude lower than that measured for pb sup 2 in zircon these factors can account for the extreme retentivity of u pb clocks even at high temperatures on the other hand radiogenic pb situated in alpha recoil damaged sites could be leached out by any electrolyte solution that reduces it to the divalent state making it both incompatible and soluble thus discordia can be generated in weathering the curious observation that discordant archaean zircon suites generally define trends to lower intercepts at up to 800 ma instead of zero appears to reflect a c 14 to 28 better average accessibility to leaching of radiogenic sup 207 pb compared to sup 206 pb in accord with the somewhat higher recoil energy of the last branched alpha decays in the sup 235 u chain less

a mineral equilibria study of the hydrothermal alteration in mafic greenschist facies rocks at kalgoorlie western australia the influx of a h2o co2 dominated fluid into actinolite bearing metabasic rocks during greenschist facies metamorphism in the kalgoorlie area of western australia resulted in a zoned alteration halo around inferred fluid conduits that contain gold mineralisation the alteration halo is divided into two outer zones the chlorite zone and the carbonate zone and an inner pyrite zone adjacent to the inferred fluid conduits reaction between the fluid and the protolith resulted in the breakdown of actinolite and the development of chlorite dolomite calcite and siderite in addition rocks in the pyrite zone developed muscovite bearing assemblages as a consequence of the introduction of potassium by the fluid mineral equilibria calculations undertaken using the computer software thermocalc in the model system na2o cao k2o feo mgo al2o3 sio2 h2o co2 show that mineral assemblages in the outer zones of the alteration halo are consistent with equilibrium of the protoliths with a fluid of composition xco2 co2 co2 h2o 0 1 0 25 for temperatures of 315 320 c the inner zone of the alteration halo reflect equilibrium with a fluid of composition xco2 0 25 fluid rock buffering calculations show that the alteration halo is consistent with interaction with a single fluid composition and that the zoned structure of the halo reflects the volume of this fluid with which the rocks reacted this fluid is likely to have also been the one responsible for the gold mineralisation at kalgoorlie

experimental constraints on the depth of olivine metastability in subducting lithosphere the hypothesis that metastable olivine persists in some subducting slabs into the transition zone has wide implications for mantle dynamics and rheology in order to evaluate this possibility we derive new thermo kinetic subduction zone models to predict the extent of olivine metastability within the stability fields of its high pressure polymorphs wadsleyite and ringwoodite our updated models improve on previous work by incorporating experimental kinetic data on realistic mantle compositions mg fe 2sio4 rather than analogue systems furthermore latent heat due to the transformation is fed back into both the kinetics and the thermal model we also consider the effects of transformation stress on growth kinetics and the possibility of an intracrystalline transformation mechanism previously thought to be important only at high shear stresses our models predict significantly smaller wedges of metastable olivine than previous work in the case of tonga for example where high values of lithospheric age 100 140 million years and convergence rate 14 cm per year are most favorable for metastability models considering only grain boundary nucleation and interface controlled growth predict olivine metastability to 600 km depth in contrast to 660 km predicted previously by kirby et al rev geophys 34 1996 261 when intracrystalline transformation is considered the depth of metastability is further reduced by as much as 100 km due to the large increase in the density of nucleation sites inhibition of growth by transformation stress can increase the depth interval over which the transformation takes place but is unlikely to be a dominant factor especially if the intracrystalline mechanism operates these results indicate that the existence of metastable olivine at depths corresponding to those of the deepest earthquakes 680 km requires subduction of old lithosphere 100 million years and a high vertical subduction velocity 15 cm per year such conditions currently may only be achieved in the very northern part of the tonga subduction zone yet earthquakes occur at depths down to 680 km in other subduction zones in which lithospheric age and or subduction velocity are relatively low e g indonesia and the marianas therefore mechanisms other than transformational faulting in metastable olivine must operate to cause the deepest earthquakes

new views on the structure and rheology of the lithosphere over the last 10 years a series of developments have led to a new understanding of what controls the variations in lithosphere strength structure and evolution that produce dramatic contrasts between the geological histories of oceans ancient shields and young orogenic belts those developments involve a wide range of observations from a great diversity of geological geophysical and geochemical disciplines that none the less provide a mutually consistent and coherent overall picture this paper summarizes in one place the essential stages in the evolution of the relevant ideas and observations that have led to this situation

structure and 40ar 39ar thermochronology of an ultrahigh pressure transition in western norway abstract the nordfjord region of western norway hosts an archetypal subducted crustal section underpinned by ultrahigh pressure uhp eclogite overlain by devonian sediments and cored by a crustal scale extensional shear zone structural mapping reveals two distinct displacement zones that played different roles during the formation and exhumation of this section 1 the sandane shear zone is a nw dipping amphibolite facies high strain zone near the base of the eclogite bearing crust that separates allochthonous units from underlying crystalline basement it may have originated during early thrusting but was overprinted by top to the west extensional fabrics at lower crustal depths 2 structurally above this the nordfjord sogn detachment zone is a top to the west amphibolite to greenschist facies detachment shear zone within allochthonous units that defines the upper boundary of the eclogitized crust and was responsible for exhumation through at least mid crustal depths muscovite 40 ar 39 ar ages suggest that amphibolite facies deformation below the nordfjord sogn detachment was mostly finished by c 397 ma whereas muscovite ages from the deeper parts of the uhp domain indicate that it cooled after 390 ma during exhumation through the middle crust west directed stretching was accompanied by north south folding late sinistral transpressional faulting in the middle to upper crust truncated the earlier folds and shear zones supplementary material complete 40 ar 39 ar data and a summary geological map of the nordfjord region are available at http www geolsoc org uk sup18460

petrogenetic grids for metacarbonate rocks pressure temperature phase diagram projection for mixed volatile systems isothermal or isobaric phase diagram sections as a function of fluid composition x f are widely used for interpreting the genetic history of metacarbonate rocks this approach has the disadvantages that 1 the influence of a key metamorphic variable either pressure p or temperature t is obscured 2 the diagrams are inappropriate for systems that are not fluid saturated these problems are avoided by constructing phase diagram projections in which the volatile composition of the system is projected onto a p t coordinate frame i e a petrogenetic grid the univariant curves of such p t projections trace the conditions of the invariant points of isothermal or isobaric phase diagram sections thereby defining the absolute stability of high variance mineral assemblages with and without a coexistent fluid phase petrogenetic grids for metacarbonate rocks are most useful for the study of regional metamorphism and for systems in which fluid composition has not been externally controlled a calculated example of a p t projection for the system cao mgo sio2 h2o co2 suggests that many assemblages e g calcite tale enstatite fluid magnesite tremolite antigorite diopside dolomite and calcite forsterite tremolite in mixed volatile systems have stability fields that make them useful as p t indicators consideration of the principles governing projection topology demonstrates that the univariant curves around a fluid present invariant point cannot be oriented independently with respect to the direction of compositional variation in the fluid phase this has the interesting predictive implication that if the direction of compositional variation along one univariant curve around an invariant point is known then the direction of compositional variation along the remaining curves can be determined solely from topologic constraints the same constraints can be applied to systems containing simple mineral solutions or melts in order to predict compositional variations

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

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

u pb systematics and trace element characteristics in titanite from a high pressure mafic granulite abstract simultaneous acquisition of u pb isotope ratios and trace element abundances across titanite crystals formed in an anatectic high pressure granulite using la icp ms split stream analysis has enabled evaluation of titanite compositional systematics and intracrystalline variability during growth and residence in high temperature melt present environments although the titanite studied here have a comparatively low initial pb pb 0 component pb 0 pb the pb 0 is highly radiogenic relative to model crustal values indicating inheritance from u bearing accessory minerals consumed in the melt titanite forming reactions additionally titanite crystals typically exhibit core rim decreases in pb 0 pb as defined by 204 pb 206 pb calculated 206 pb 0 206 pb t and uncorrected 206 pb 238 u spot date profiles near the margins this is clearly dominated by local u enrichment but in the uniformly low u interiors outwardly decreasing pb 0 pb appears to reflect decreasing pb 0 concentrations during growth the positive correlation among pb 0 and sr concentrations in crystal interiors over length scales of hundreds of micrometers is consistent with each having experienced similarly small degrees of diffusional relaxation given the high crystallization temperatures 800 c and likely slow cooling rates 5 c our data support slow pb diffusivity in titanite even at high temperature conditions as has been proposed in a number of recent studies along the outer 50 100 m u th zr and ree concentrations are variably elevated relative to the crystal interiors with profiles taking one of two forms 1 sharply increasing to highest concentrations inboard from the crystal edge and decreasing again to lower values near the crystal edge or 2 gradually increasing to highest concentrations nearest the crystal edge high contrast bse imaging shows that the former profiles are associated with 1 2 m wide bright bands surrounding polygonal subgrains that tend to be developed where titanite is or formerly was in contact with matrix feldspar i e crystallized melt and are inferred to represent trace element enriched dissolution precipitation reaction fronts the latter profiles are associated with diffuse locally wispy brightness gradients adjacent to resorbed crystal boundaries subgrain boundaries or thicker bright bands formed in contact with matrix hornblende crystals and are interpreted as regions of enhanced element mobility potentially resulting from the development of local micro porosity pathways at some point in the recrystallization process

a linear stratified ocean model of the equatorial undercurrent a linear stratified ocean model is used to study the wind driven response of the equatorial ocean the model is an extension of the lighthill 1969 model that allows the diffusion of heat and momentum into the deeper ocean and so can develop non trivial steady solutions to retain the ability to expand solutions into sums of vertical normal modes mixing coefficients must be inversely proportional to the square of the back ground vaisala frequency the model is also similar to the earlier homogeneous ocean model of stommel i960 he extended ekman dynamics to the equator by allowing his model to generate a barotropic pressure field the present model differs in that the presence of stratification allows the generation of a baroclinic pressure field as well the most important result of this paper is that linear theory can produce a realistic equatorial current structure the model undercurrent has a reasonable width and depth scale there is westward flow both above and below the undercurrent the meridional circulation conforms to the classical picture suggested by cromwell 1953 unlike the stommel solution the response here is less sensitive to variations of parameters ocean boundaries are not necessary for the existence of the undercurrent but are necessary for the existence of the deeper equatorial intermediate current the radiation of equatorially trapped rossby and kelvin waves is essential to the development of a realistic undercurrent because the system supports the existence of these waves low order vertical modes can very nearly adjust to sverdrup balance defined below which in a bounded ocean and for winds without curl is a state of rest as a result higher order vertical modes are much more visible in the total solution this property accounts for the surface trapping and narrow width scale of the equatorial currents the high order modes tend to be in yoshida balance defined below and generate the characteristic meridional circulation pattern associated with equatorial ekman pumping

duration of strike slip movements in large shear zones the red river belt china abstract sixty four size fractions of zircon monazite xenotime and titanite were dated by the u pb method to temporally and spatially constrain left lateral movements in the red river shear zone between the depths of 15 and 20 km the shearing led to the formation of the high grade metamorphic gneisses that form the bulk of the 4200 m high diancang shan and 3100 m high ailao shan mountain ranges the omnipresence of melt layers and pockets in these gneisses shows that temperatures associated with tectonometamorphism lie at and above the solidus of continental crust and deformation patterns are very homogeneous over the entire length of the belt north of the gneisses subvolcanic bodies intrude unmetamorphosed cover rocks in particular eocene red beds along the projected trend of the red river shear zone for a leucogranitic layer in the diancang shan gneisses an age of 24 2 0 2 ma 2 is obtained for monazite and 22 4 0 2 ma is obtained for xenotime whereas zircon formed through two growth stages at 24 23 ma these ages record solidification of the anatectic melts over a period of at least 1 8 0 4 ma during which left lateral shear was continuously active in the ailao shan gneisses zircon and titanite ages are 26 3 0 3 and 26 1 0 2 ma for two monzonitic bodies and 24 1 0 2 and 22 4 0 2 ma for two pegmatitic layers that occur within the monzonitic intrusions these results show that zircon monazite titanite and probably also xenotime are closed systems for u and pb under melting conditions 650 c therefore the ages date mineral formation in the melt rather than cooling below certain closure temperatures a significantly older age of 35 0 0 1 ma is obtained for a monzonitic subvolcanic intrusion north of the diancang shan gneisses because these intrusions follow the trend of the shear zone they most likely represent the surface expression of early activity along that zone indicating that strike slip movement had already occurred in latest eocene time the total set of u pb ages shows that magmatic and metamorphic activity occurred from 35 to 20 ma strengthening the idea that the red river shear zone absorbed most of the intraplate movements required to compensate the opening of the south china sea between 32 and 16 ma the existence of monzonitic syenitic intrusions along the red river zone implies that melting affected source rocks other than the diancang shan and ailao shan gneiss protoliths potential sources to produce such rocks lie in the deep crust possibly including melts from the mantle consequently the red river shear zone must be a deep rooted structure along which melts migrated to the surface moreover the contemporaneity of shear motion and magma generation suggests that melting occurred in direct relation to strike slip activity

geochronology and geochemistry of zircon from the northern western gneiss region insights into the caledonian tectonic history of western norway abstract the western gneiss region wgr of norway is divided by the more trondelag shear zone mtsz into a southern region that contains domains of caledonian ultrahigh pressure uhp metamorphic rocks 2 5 gpa and a northern area of similar caledonian aged rocks that record a maximum pressure reported thus far of 1 5 gpa although both regions contain similar lithologies primarily migmatitic quartzofeldspathic gneiss containing mafic lenses and structural relationship of basement rocks to infolded nappes this difference in maximum pressure implies a difference in tectonic history continental subduction south of the shear zone none to the north and raises questions about the role of the mtsz in the metamorphic history including exhumation of the wgr previous geochronology results indicated a difference in timing of peak metamorphism older in north younger in south in order to better understand the tectonic history of the northern wgr and the mtsz and in particular the late to post caledonian tectonic history u pb zircon geochronology and trace element abundances were obtained using the split stream laser ablation icpms technique from metabasaltic lenses and migmatitic quartzofeldspathic host rocks from the structurally lowest exposed region of the northern wgr roan peninsula basement as well as leucosomes from an intercalated portion of the seve nappe complex and a pegmatite in the mtsz zircon from roan gneiss and metabasite yield metamorphic ages of ca 410 406 ma and zircon from a variety of migmatite samples foliation parallel leucosome to dikes indicate melt crystallization at ca 410 to 405 ma the seve nappe leucosomes yield only early caledonian dates that cluster at ca 437 ma and ca 465 ma suggesting that the allochthons in this region did not experience or record the same scandian tectonic history as the basement rocks zircon from a weakly deformed pegmatite dike within the mtsz crystallized at ca 404 ma indicating that this shear zone was active during the end stages of high grade metamorphism in both the southern and northern wgr domains results of this study show that the northern and southern wgr experienced a coeval scandian metamorphic magmatic and deformation history despite a possible difference in maximum p t conditions

a re assessment of focal depth distributions in southern iran the tien shan and northern india do earthquakes really occur in the continental mantle summary we investigate the depth distribution of earthquakes within the continental lithosphere of southern iran the tien shan and northern india by using synthetic seismograms to analyse p and sh body waveforms in the zagros mountains of southern iran earthquakes are apparently restricted to the upper crust depths of 20 km whereas in the tien shan and northern india they occur throughout the thickness of the continental crust to depths of 40 45 km we find no convincing evidence for earthquakes in the continental mantle of these regions in spite of previous suggestions to the contrary and question whether seismicity in the continental mantle is important in any part of the world in some regions such as iran the aegean tibet and california seismicity is virtually restricted to the upper continental crust whereas in others including parts of east africa the tien shan and northern india the lower crust is also seismically active although usually less so than the upper crust such variations cannot reliably be demonstrated from published catalogue or bulletin locations even from ones in which depth resolution is generally improved in contrast to the oceanic mantle lithosphere in which earthquakes certainly occur the continental mantle lithosphere is we suggest virtually aseismic and may not be significantly stronger than the lower continental crust these variations in continental seismogenic thickness are broadly correlated with variations in effective elastic thickness suggesting that the strength of the continental lithosphere resides in the crust and require some modification to prevalent views of lithosphere rheology

shear generated turbulence in the equatorial pacific produced by small vertical scale flow features we investigate the characteristics of shear generated turbulence in the natural environment by considering data from a number of cruises in the western equatorial pacific in this region the vertical shear of the flow is dominated by flow structures that have a relatively small vertical scale of o 10 m combining data from all cruises we find a strong relationship between the turbulent dissipation rate vertical shear s and buoyancy frequency n examination of at a fixed value of richardson number ri n2 s2 shows that ut2n for a wide range of values of n where ut is an appropriate velocity scale which we assume to be the horizontal velocity scale of the turbulence the implied vertical length scale lv ut n is consistent with theoretical and numerical studies of stratified turbulence such behavior is found for ri 0 4 the vertical diffusion coefficient then scales as v ut2 n at a fixed value of richardson number the amplitude of is found to increase with decreasing ri but only modestly and certainly less dramatically than suggested by some parameterization schemes provided the shear generating the turbulence is resolved our results point to a way to parameterize the unresolved turbulence

inherited pb components in magmatic titanite and their consequence for the interpretation of upb ages abstract the occurrence of inherited radiogenic pb components in 35 ma old magmatic titanite is reported here for the first time the about 810 ma age of the inherited components is identical to that contained in zircon from the same amphibole bearing syenite sample in both minerals the precambrian components were not totally reset during magmatic new growth at 35 ma this age pattern substantiates that the u pb chronometer in titanite is resistant to temperatures in excess of 712 c as is already known for zircon and monazite in consequence u pb ages from crystalline titanites have to be interpreted as the age of mineral formation or re crystallization rather than the time of u pb system closure during cooling of the rock

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

localized granulite and eclogite facies metamorphism at flatraket and krakeneset western gneiss region u pb data and tectonic implications

preserved zr temperatures and u pb ages in high grade metamorphic titanite evidence for a static hot channel in the himalayan orogen abstract titanite grains from 800 c gneisses of the c 6 km thick greater himalayan sequence of central nepal were analyzed for zr in titanite temperatures and u pb ages to investigate the formation and evolution of a former weak mid crustal channel in the himalaya laser ablation icp ms depth profiles and spot analyses of titanite zr and u pb isotopic compositions are inconsistent with diffusional reequilibration on scales greater than 1 2 m and instead data appear to faithfully record both temperature and age titanite records protracted heating from 700 750 c at 37 ma to 775 c at 24 ma and possibly slight cooling to 765 c at 20 ma such temperatures exceed partial melting reactions in associated metapelites and imply profoundly weak rheologies i e a hot channel yet predate initiation of the bounding main central thrust and south tibetan detachment systems by as much as 15 myr evidently thick but weak crustal channels may remain stationary with respect to the convergence direction for 10 myr even during one of earth s biggest continent continent collisions

on the behavior of internal waves in the wakes of storms abstract the rossby adjustment process in the wake of a storm is studied with a view to finding within the context of linear theory how wave dispersion in both the vertical and meridional directions spreads energy which is initially confined entirely to the mixed layer comparisons are made between three cases 1 a periodic storm on the f plane where dispersion is purely vertical 2 a bounded storm on the f plane and 3 a bounded storm centered 2700 km from the equator on an equatorial beta plane particular attention is paid to the initial rate of loss of energy from the mixed layer and some simple formulas which work very well in the cases studied are derived these show that the rate of loss goes up when the mixed layer depth is increased and also that the rate scales as the square of the wavenumber values of the rates are sufficient to provide a major source of energy for internal waves below the mixed layer the often observed tendency for phase lines to propagate upward is found in all case

a view into crustal evolution at mantle depths abstract crustal foundering is an important mechanism in the differentiation and recycling of continental crust nevertheless little is known about the dynamics of the lower crust the temporal scale of foundering and its role in the dynamics of active margins and orogens this particularly applies to active settings where the lower crust is typically still buried and direct access is not possible crustal xenoliths derived from mantle depth in the pamir provide a unique exception to this the rocks are well preserved and comprise a diverse set of lithologies many of which re equilibrated at high pressure conditions before being erupted in their ultrapotassic host lavas in this study we explore the petrological and chronological record of eclogite and felsic granulite xenoliths we utilized accessory minerals zircon monazite and rutile for coupled in situ trace element analysis and u th pb chronology by laser ablation split stream inductively coupled plasma mass spectrometry each integrated analysis was done on single mineral zones and was performed in situ in thin section to maintain textural context and the ability to interpret the data in this framework rutile thermo chronology exclusively reflects eruption 11 17 0 06 ma which demonstrates the reliability of the u pb rutile thermo chronometer and its ability to date magmatic processes conversely zircon and monazite reveal a series of discrete age clusters between 55 11 ma with the youngest being identical to the age of eruption matching age populations between samples despite a lack of overlapping ages for different chronometers within samples exhibit the effectiveness of our multi mineral approach the ree systematics and age data for zircon and monazite and ti in zircon data together track the history of the rocks at a million year resolution the data reveal that the rocks resided at 30 40 km depth along a stable continental geotherm at 720 750 c until 24 20 ma and were subsequently melted densified and buried to 80 90 km depth 20 km deeper than the present day moho at 930 35 c the material descended rapidly accelerating from 0 9 1 7 mm yr 1 to 4 7 5 8 mm yr 1 within 10 12 myr and continued descending after reaching mantle depth at 14 13 ma the data reflect the foundering of differentiated deep crustal fragments 2 9 3 5 g cm 3 into a metasomatized and less dense mantle wedge through our new approach in constraining the burial history of rocks we provided the first time resolved record of this crustal recycling process foundering introduced vestiges of old evolved crust into the mantle wedge over a relatively short period c 10 myr the recycling process could explain the variability in the degree of crustal contamination of mantle derived magmatic rocks in the pamir and neighboring tibet during the cenozoic without requiring a change in plate dynamics or source region

geochemical and u pb age constraints on the occurrence of polygenetic titanites in uhp metagranite in the dabie orogen abstract relict magmatic titanite was identified in the core of a few titanite grains with the overgrown rim of metamorphic titanite in uhp metagranite in the dabie orogen la icpms u pb dating gave neoproterozoic ages for the magmatic titanite but triassic ages for the metamorphic titanite the magmatic and metamorphic titanites are clearly distinct by differences in petrological and geochemical compositions the magmatic titanite occurs as residual cores that show bright bse the presence of allanite and quartz inclusions low contents of cao al2o3 and tio2 but high contents of fe2o3 and mgo in trace elements the magmatic titanite exhibits high ree and hfse contents distinctly negative eu anomalies with flat mree hree patterns and high th u ratios in contrast the metamorphic titanite occurs as rims and grains of homogeneously dark bse that contain inclusions of epidote quartz k feldspar rutile biotite and phengite and have relatively high contents of cao al2o3 and tio2 but low contents of fe2o3 and mgo and relatively low ree and hfse contents slightly negative eu anomalies with hree depletion relative to mree and low th u ratios the zr in titanite thermometry yields 727 to 877 c at 0 5 to 1 0 gpa for the magmatic titanite and 729 to 870 c at 1 5 to 2 0 gpa for the metamorphic titanite the neoproterozoic u pb chronometric system of magmatic titanite survived the triassic continental subduction zone hp uhp metamorphism this suggests a relatively high closure temperature of 800 c for the titanite u pb system the metamorphic titanite is principally a product of retrograde metamorphism during decompression exhumation at the transition from hp eclogite facies to amphibolite facies therefore titanite holds a great potential to geochronological and petrogenetic studies of continental subduction zone metamorphic rocks

inherited zircon and titanite upb systems in an archaean syenite from southwestern australia implications for upb stability of titanite abstract inherited zircon and titanite have been identified in a syenite from the archaean of southwestern australia conventional and shrimp analyses on euhedral zoned zircon and zoned rims on complex grains define a crystallisation age of 2654 5 ma for the syenite in addition shrimp analyses on zircon cores and unzoned subhedral zircons show that zircon has a ca 3250 ma inherited component conventional u pb ages on titanite also fall between ca 3250 ma and ca 2650 ma demonstrating that inherited titanite as well as zircon is present in the syenite the syenite has been affected by regional upper amphibolite facies metamorphism at an estimated temperature of 625 650 c retention of the inherited radiogenic pb in the titanite is evidence that the closure temperature for titanite is greater than 650 c the presence of inherited titanite and zircon also demonstrates a crustal source component for the syenite and indicates it originated by partial assimilation of crustal rocks by a potassic magma at

metastability mechanical strength and the support of mountain belts exhumed high pressure rocks from the caledonian root zone in norway provide analogues for processes occurring today under southern tibet allowing large scale geophysical observations from gravity and earthquakes to be linked with the mechanical properties of metastable rocks under high mountains metastability is essential for the survival of thick mountain roots and hence of high mountains and is in turn controlled by water if water is absent dry granulite formed from earlier melting episodes is both stable and strong and likely to survive in the eclogite stability field of deep root zones for hundreds of million years but if hydrous fluid is introduced the transformation of granulite to eclogite is relatively rapid and accompanied by a dramatic loss of strength in norway this transformation was initiated by water infiltration along fractures formed by earthquakes the same process as marked by deep earthquakes may be occurring today beneath southern tibet

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

polygenetic titanite records the composition of metamorphic fluids during the exhumation of ultrahigh pressure metagranite in the sulu orogen

an automated strategy for calculation of phase diagram sections and retrieval of rock properties as a function of physical conditions we formulate an algorithm for the calculation of stable phase relations of a system with constrained bulk composition as a function of its environmental variables the basis of this algorithm is the approximate representation of the free energy composition surfaces of solution phases by inscribed polyhedra this representation leads to discretization of high variance phase fields into a continuous mesh of smaller polygonal fields within which the composition and physical properties of the phases are uniquely determined the resulting phase diagram sections are useful for understanding the phase relations of complex metamorphic systems and for applications in which it is necessary to establish the variations in rock properties such as density seismic velocities and volatile content through a metamorphic cycle the algorithm has been implemented within a computer program that is general with respect to both the choice of variables and the number of components and phases possible in a system and is independent of the structure of the equations of state used to describe the phases of the system

interpreting titanite u pb dates and zr thermobarometry in high grade rocks empirical constraints on elemental diffusivities of pb al fe zr nb and ce length scales of compositional heterogeneity in titanite from 750 to 1000 c metamorphic rocks from southern madagascar were measured to provide empirical constraints on elemental diffusivities the calculated pb diffusivity is comparable to experimental estimates of sr diffusivity because of this u pb dates from rocks that reached peak temperatures 900 c thus zr in titanite thermobarometry should not be reset by diffusion in all but the smallest grains in the hottest rocks al and nb diffuse at similar rates to zr ce and fe diffuse slower than pb but faster than zr differences in empirical and experimental estimates of elemental diffusivities might be related to the complexity of most natural titanite solid solutions compared to the near end member titanite used in experiments

the current status of titanite rutile thermobarometry in ultrahigh pressure metamorphic rocks the influence of titanite activity models on phase equilibrium calculations abstract titanite an important accessory mineral in ultrahigh pressure uhp rocks commonly deviates significantly from its ideal composition catisio 4 o by the substitution of al and f oh for ti for o this compositional variability of titanite could be used in phase equilibrium calculations if the activity composition relations in al f bearing titanites were sufficiently known experimental investigations by troitzsch and ellis troitzsch u and ellis d j 2002 thermodynamic properties and stability of alf bearing titanite catiosio 4 caalfsio 4 contributions to mineralogy and petrology 142 543 563 and tropper et al tropper p manning c essene e j 2002 the substitution of al and f in titanite at high pressure and temperature experimental constraints on phase relations and solid solution properties journal of petrology 43 1787 1814 derived non ideal mixing models for solid solutions along the join catisio 4 o caalsio 4 f tropper et al troitzsch u and ellis d j 2002 thermodynamic properties and stability of alf bearing titanite catiosio 4 caalfsio 4 contributions to mineralogy and petrology 142 543 563 derived a preliminary regular model in the t range 900 1100 c in which the t dependent interaction parameter w g was negative in contrast troitzsch and ellis troitzsch u and ellis d j 2002 thermodynamic properties and stability of alf bearing titanite catiosio 4 caalfsio 4 contributions to mineralogy and petrology 142 543 563 favored a regular activity model with positive w g the different signs of the interaction parameter strongly influence calculated non ideal titanite activities comparing available simple ideal ionic coupled ionic non coupled ionic activity models with the non ideal models shows that a catisio 4 o calculated with the ideal ionic models is substantially lower at almost all t calculation of two suitable titanite rutile involving equilibria for thermobarometry applied to literature data from rocks from four uhp terranes shows that ideal ionic models yield the best convergence with independently established p estimates although the literature data have to be treated with caution e g retrogression compositional disequilibrium etc the calculations nonetheless indicate that in terms of t estimates high al x al 0 5 titanites yield a large variation of up to 300 c in t and low al x al t estimates this study shows that p and t estimates derived from ideal models and experimentally constrained non ideal models for titanite activity may show large deviations at uhp conditions ranging from 0 05 to 3 0 gpa and up to 300 c therefore the current status of titanite rutile involving thermobarometry allows it only to be applied to al rich x al 0 2 titanites from uhp rocks if independent more robust p t estimates are available until better activity constraints are available it is recommended that the user employ either the ideal coupled ionic model for titanite solid solutions involving mixing on the ti and the o1 site and compare the results to an experimentally derived activity model or adopt a range of different activity models ionic and regular to obtain a range of p t conditions

incomplete high p t metamorphic transitions within the kvamsoy pyroxenite complex west norway a case study of disequilibrium the kvamsoy pyroxenite complex consists of olivine websterite olivine gabbro and leucogabbro norite which have been subjected to regional high p t hpt metamorphism the metamorphism has resulted in a range of disequilibrium textures with the development of coronas and pseudomorphism of the igneous phases reactions between felsic and mafic mineral domains have been controlled by variable and selective diffusion of elements resulting in a variety of local plagioclase breakdown reactions and in significant compositional variations for the product garnet restricted diffusion favours transient reaction stages with garnet spinel corundum zoisite after calcic plagioclase in olivine gabbro and olivine websterite and garnet spinel kyanite quartz sodic plagioclase in leucogabbro norite complete hpt reaction has produced garnet pyroxenite which consists of garnet diopside hornblende zoisite in gabbroic rocks while amphibolitization continued during the cooling and uplift history grt ky pl qtz geobarometry suggests pressures in the range 13 16 kbar for t 750 c comparable with the regional eclogite forming metamorphism

feedback between rifting and diapirism can exhume ultrahigh pressure rocks article i nfo the processes by which crustal rocks are buried to tremendous depths and subsequently exhumed to earth s surface remain controversial rapid exhumation of earth s youngest ultra high pressure uhp rocks in the woodlark basin papua new guinea is occurring within an active rift in contrast to more common scenarios of uhp exhumation during plate convergence we use 2d and 3d thermo mechanical models to demonstrate that uhp exhumation can result from feedback between rifting and the diapiric rise of a previously subducted continental fragment through the lithosphere we infer that this feedback is responsible for the exhumation of the uhp rocks in gneiss domes in the woodlark basin our models successfully reproduce uhp exhumation paths and rates and geological structures within the gneiss domes we show that uhp exhumation by diapirism is mechanically consistent in post collisional rifts our models highlight the complex feedback between diapiric ascent and extension

diapiric exhumation of earth s youngest uhp eclogites in the gneiss domes of the d entrecasteaux islands papua new guinea abstract the woodlark rift in papua new guinea hosts the world s youngest 2 8 ma eclogite facies rocks and extensional deformation has played a key role in exhuming these u hp rocks at rates of 20 mm yr during the eocene papuan arc continent collision australian plate derived continental rocks were subducted to u hp depths there they remained for up to 30 m y until the pliocene when asthenospheric circulation ahead of the west propagating woodlark spreading ridge introduced heat and fluids this caused rocks to break away from the paleosubduction channel recrystallize in the eclogite facies and rise as rayleigh taylor instabilities the diapirs ascended adiabatically undergoing partial melting which lowered their viscosity and increased buoyancy u hp crust ponded near the moho at 2 4 ma thickening the crust to 40 km 11 kb domal uplifts emerged above sea level and these are still underlain by an unusually thick crust 26 km for a rift that has stretched by factor of 3 since 6 ma after ponding they acquired a flat lying foliation during amphibolite facies retrogression vertical shortening accompanied the gravitationally driven outflow of ponded lower crust the weak material was extended parallel to the rift margin thinning ductilely by w k 0 2 and was mechanically decoupled from and orthogonal to plate motion in the rift top e shear fabrics suggest that this flow was westward perhaps driven by isostatic stresses towards a strongly thinned rift corridor ahead of the woodlark spreading ridge at 100 c per m y and were mechanically incorporated into the rift s upper crust final exposure was by normal faulting and minor erosion such exhumation may also apply to other u hp terranes where less evidence for moho ponding is preserved

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

compatible p t conditions for eclogites and surrounding gneisses in the kristiansund area western norway analyses of coexisting garnets clinopyroxenes and plagioclases from eclogites and high pressure granulitic gneisses in the kristiansund area within the west norwegian basal gneiss region are used to establish the p t conditions for the metamorphic peak for these rocks based on the distribution of fe and mg between coexisting garnet and clinopyroxene in both eclogite and granulites equilibrium temperatures are estimated to 750 50 c pressures are derived from the absence of orthopyroxenes in the granulites and from the assemblage clinopyroxene plagioclase quartz present in the gneisses equilibrium pressures are estimated to 18 5 3 0 kilobars and these equilibrium conditions are thus compatible with equilibrium conditions derived for both orthopyroxene free and most orthopyroxene bearing country rock eclogites from adjacent areas

constraints on pb closure temperature in titanite based on rocks from the ungava orogen canada implications for u pb geochronology and p t t path determinations on the basis of multiequilibrium metamorphic pressure temperature p t estimates we have determined that 660 700 c is a reasonable minimum estimate of the closure t of pb diffusion in titanite a temperature significantly higher than previously suggested activity corrected reactions that produce titanite define narrow t stability fields 40 c that correspond closely to metamorphic p t determinations derived for coronitic garnet clinopyroxene hornblende samples growth of titanite occurring between 1789 and 1814 ma in the underthrust archean basement of the paleoproterozoic ungava orogen northern quebec is associated with retrograde metamorphism during underthrusting of the archean basement approx 1 b y after formation of the original granulitic assemblages

exhumation of ultrahigh pressure rocks beneath the hornelen segment of the nordfjord sogn detachment zone western norway the nordfjord sogn detachment zone of western norway represents an archetype for crustal scale normal faults that are typically cited as one of the primary mechanisms responsible for the exhumation of ultrahighpressure uhp terranes in this paper we investigate the role of normal sense shear zones with respect to uhp exhumation using structural geology thermobarometry and geochronology of the hornelen segment of the nordfjord sogn detachment zone the hornelen segment of the zone is a 2 6 km thick top w shear zone primarily developed within amphibolite grade allochthonous rocks that juxtaposes the uhp rocks of the western gneiss complex in its footwall with lower grade allochthons and carboniferous devonian basins in its hanging wall new thermobarometry and sm nd garnet geochronology show that these top w fabrics were initiated at lower crustal depths of 30 40 km between 410 ma and 400 ma structural geology and quartz petrofabrics indicate that top w shear was initially relatively evenly distributed across the shear zone and then overprinted by discrete ductile brittle detachment faults at slower strain rates during progressive deformation and exhumation these results require a three stage model for uhp exhumation in which normal sense shear zones exhumed uhp rocks from the base of the crust along initially broad ductile shear zones that were progressively overprinted by discrete ductile brittle structures

uranium lead zircon and titanite ages from the northern portion of the western gneiss region south central norway abstract the geochronologic history of the northern portion of the western gneiss region south central norway has been investigated by u pb analyses of zircon and titanite from various basement units and one supracrustal gneiss a six point four zircon and two titanite discordia line from a migmatite melt pod and its host gneiss astfjord migmatite gneiss defines upper and lower intercept ages of 1659 1 1 8 ma and 393 6 3 6 ma respectively the upper intercept age is interpreted as a time of tonalite emplacement and migmatite formation in the region a seven point three zircon and four titanite discordia line from the ingdal granite gneiss has similar upper and lower intercept ages 1652 9 1 7 ma and 396 1 4 9 ma respectively and the upper intercept age is interpreted as the time of granite crystallization the lower intercept age from both theastfjord migmatite gneiss and the ingdal granite gneiss is interpreted as the time of regional metamorphic resetting and is evidence for caledonian influence in the region titanite from these two gneiss units as well as from other tonalite granite and supracrustal gneisses throughout the western gneiss region displays a remarkably uniform pattern of discordance titanite discordance is variable from 6 to 100 and in general the degree of discordance is proportional to the caledonian metamorphic grade of nearby supracrustal schists episodic diffusional lead loss from titanite during caledonian metamorphism probably caused the discordance pattern but a combined mechanism of mixing and diffusional lead loss cannot be ruled out in any case the seventeen point titanite and zircon discordia line clearly indicates that 1 the bulk of the granitoid terrane in this portion of the western gneiss region was emplaced migmatized and cooled in a short time interval about 1657 ma ago 2 a second short lived thermal event that exceeded the blocking temperature of titanite occurred about 395 ma ago and 3 titanite and zircon in the region were not isotopically disturbed by comparable geologic events in the period from 1657 to 395 ma or at any time after 395 ma

common lead corrected laser ablation icp ms u pb systematics and geochronology of titanite u pb geochronology using laser ablation icp ms is a fairly recently introduced technique and thus far its development has centred on zircon dating using titanite as an example the application of laser ablation icp ms to u pb dating of common pb bearing accessory phases is presented a simple analytical set up comprising a 213 nm nd yag laser coupled to a quadrupole based icp ms instrument is used it is demonstrated that accurate and precise pb u ratios 2 3 at 2 for 206pb 238u can be obtained on small volumes of analysed material using the zircon geostandard 91500 as an external calibration standard a robust common pb correction necessitating the measurement of the stable 204pb isotope is detailed the isobaric 204hg background interference in icp ms is lowered to a level where it does not dominate the total 204 amu signal using in line gold traps on the carrier gas lines the remaining 204hg is stripped from the 204pb signal mathematically by applying a natural hg isotope ratio common pb correction using this method is demonstrated to work well in analyses with elevated u pb signal intensity where this correction fails an alternative method is required and a novel approach using three dimensional u pb concordia is adopted this approach is shown to be extremely powerful in assessing age precision and common pb composition correction the technique is applied to the well characterised fish canyon tuff demonstrating that accurate 7 8 at 2 compared to reference age and precise 2 4 at 2 on 206pb 238u ratios u pb age data can be obtained from titanite even as young as oligocene

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

are the alps collapsing abstract orogenic collapse is a process that transfers gravitational potential energy from regions of high potential energy to regions of lower potential energy this transfer is classically considered to be accomplished by extension in the orogenic core and by synchronous shortening in foreland regions of the orogen not all extensional features in collisional mountain belts need however reflect orogenic collapse normal faulting thrust faulting and strike slip faulting are all active in different parts of the alps today and reflect complex local responses to ongoing europe adria convergence the western alps is the only area today where extension and shortening radial to orogen trend occur synchronously and where orogenic collapse may be an important process elsewhere in the alps normal faults are oriented at a high angle to orogen trend and were primarily active in oligocene and miocene time most present day activity in the central and eastern alps is on strike slip faults that are accommodati

ductile nappe extrusion in constrictive strain at the origin of transverse segments of the allochthon boundary thrust in the manicouagan imbricate zone central grenville province quebec abstract separating the parautochthonous and allochthonous belts the allochthon boundary thrust abt is a major lithotectonic boundary that generally runs parallel with the length of the grenville province but with some transverse segments in this contribution we investigate one of these segments the manicouagan imbricate zone miz by adding new field based data supplemented by new u pb deformation ages to a compilation of previous research conducted on both sides of the abt in the region our results reveal striking differences between the footwall and hanging wall of the abt in terms of strain and structures and concludes that the transverse orientation of the abt is not related to folding an updated strain analysis allows us to determine that deformation of the miz was dominated by a constrictive type of strain we build on a previously proposed conceptual model to document ductile extrusion over basement ramps driven by tectonic forcing and lateral density contrast in the upper crust at ca 990 ma the insertion of the hot ductile nappe into partially molten metasediments of the surrounding parautochthonous belt was accommodated by constrictive strain within the former and flattening and transverse folding in the latter

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

u pb geochronology of the grenville orogen of ontario and new york constraints on ancient crustal tectonics based on lithological structural and geophysi cal characteristics the proterozoic grenville orogen of southern ontario and new york has been divided into domains that are separated from each other by ductile shear zones in order to constrain the timing of meta morphism u pb ages were determined on metamorphic and igneous sphenes from marbles calc silicate gneisses amphibolites granitoids skarns and pegmatites in addi tion u pb ages were obtained for monazites from meta pelites and for a rutile from an amphibolite these mineral ages constrain the timing of mineral growth the duration of metamorphism and the cooling history of the different domains that make up the southern part of the exposed grenville orogen based on the ages from metamorphic minerals regional and contact metamorphism occurred in the following intervals central granulite terrane