collapse of the thickened variscan crust in the french massif central mont pilat extensional shear zone and st etienne late carboniferous basin abstract a large scale extensional shear zone is described from the mont pilat area in the northeastern part of the french massif central this shear zone is about 2 km thick and is characterized by well developed n dipping foliation and stretching lineations deformation occurred under conditions of retrograde metamorphism from high temperature conditions to greenschist facies it is superimposed on a composite foliation related to the earlier variscan 380 350 ma compressional events associated with southward emplacement of nappes and hp mt metamorphism regional n s lineations and shear criteria observed at a regional scale indicate a northward displacement of the hanging wall during extension large strains locally recorded in thick mylonitic zones suggest that displacement was substantial some tens of kilometres late superimposed extensional brittle deformation and pseudotachylites show that northward displacement on normal faults continued at a shallower depth a large granitic pluton granite du velay intruded late in the the variscan orogeny was associated with extension some small leucogranitic syntectonic melts were emplaced within the shear zone and are strongly deformed geochronological rb sr data on these leucogranite bodies give an age of 320 ma and 40 ar 39 ar dating of synkinematic biotites from gneissose rocks in the shear zone yields a plateau age of 313 ma these data show that extensional ductile deformation and magmatism were still active during the latest late carboniferous thus extension occurred immediately after the well known late palaeozoic collision type compressional tectonics which had significantly thickened the crust we propose that the extensional tectonism described here is due to the gravitational collapse of the initially thick variscan crust the stephanian intracontinental sedimentary basin of st etienne was formed along the hanging wall of the normal shear zone during the late stages of this post orogenic crustal extension

deformation and metamorphism associated with crustal rifting the permian to liassic evolution of the lake lugano lake como area southern alps abstract in connection with movements between europe and adria the southern alps underwent extension leading to the formation of the middle jurassic south alpine passive continental margin as a result of mainly s vergent alpine thrusting and folding a substantially preserved mesozoic crustal section ranging from the surface to a paleo depth of ca 15 km is exposed in the lugano lake como area tectonic processes associated with permo mesozoic extension can thus be investigated at different crustal levels rifting began with a thermal anomaly during which rocks at middle crustal levels were sheared at ca 650 750 c deformation was distributed on a several km wide zone and late kinematic pegmatites were emplaced at this stage temperatures then decreased and beginning in the norian extension was accommodated by a major e dipping normal fault the lugano val grande normal fault the fault which can be followed down to a paleo depth of ca 12 km was listric and flattened to 20 at ca 7 9 km depth in the upper 8 10 km deformation was only brittle whereas at deeper crustal levels greenschist mylonites were formed with continued normal faulting the colder hanging wall cooled the fault zone and greenschist mylonites were progressively replaced by lower temperature ultramylonites and cataclasites the thickness of the fault zone varies from some tens of meters in the upper 5 6 km to several hundred meters at deeper levels deformation was thus discrete at a crustal scale

increase in background stratospheric aerosol observed with lidar at mauna loa observatory and boulder colorado article no l15808 the stratospheric aerosol layer has been monitored with lidars at mauna loa observatory in hawaii and boulder in colorado since 1975 and 2000 respectively following the pinatubo volcanic eruption in june 1991 the global stratosphere has not been perturbed by a major volcanic eruption providing an unprecedented opportunity to study the background aerosol since about 2000 an increase of 4 7 per year in the aerosol backscatter in the altitude range 20 30 km has been detected at both mauna loa and boulder this increase is superimposed on a seasonal cycle with a winter maximum that is modulated by the quasi biennial oscillation qbo in tropical winds of the three major causes for a stratospheric aerosol increase volcanic emissions to the stratosphere increased tropical upwelling and an increase in anthropogenic sulfur gas emissions in the troposphere it appears that a large increase in coal burning since 2002 mainly in china is the likely source of sulfur dioxide that ultimately ends up as the sulfate aerosol responsible for the increased backscatter from the stratospheric aerosol layer the results are consistent with 0 6 0 8 of tropospheric sulfur entering the stratosphere

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

zr bearing minerals as indicators for the polymetamorphic evolution of the eastern lower austroalpine nappes stubenberg granite contact aureole styria eastern alps austria abstract contact metamorphism during emplacement of the permian stubenberg granite has led to the formation of the assemblage forsterite calcite titanian clinohumite phlogopite chlorite in the adjacent marbles during intrusion of the granite veins rich in ti zr ree and actinides u th formed these veins show a distinct mineralogical zoning sequence with four zones going from the center of the vein to the margin these zones include the assemblages 1 geikielite baddeleyite zirconolite apatite calcite chlorite magnetite pyrrhotite assemblage 2 calcite chlorite 3 forsterite titanian clinohumite chlorite calcite phlogopite and 4 calcite forsterite baddeleyite is always replaced by zirconolite possibly via the model reaction baddeleyite 2 geikielite 3 calcite co 2 zirconolite 2 dolomite zirconolite zirc i shows a strong internal oscillatory zoning and distinct overgrowths zirc ii which have a different chemical composition the chemical variation between the cores zirc i and the rims zirc ii can be explained by using the substitutions me 5 me 2 ti 4 me 3 and ree 3 me 5 me 2 ca 2 2ti 4 in contrast to zirconolite from metacarbonates associated with contact aureoles these analyses show elevated nb contents of up to 4 5 wt nb 2 o 5 and unusually high w contents of 1 2 wt wo 3 a strong eo alpine metamorphic overprint led locally to the formation of the assemblage chlorite dolomite calcite ilmenite zirconolite ii geikielite fe sulfides late zircon grew locally presumably as the last zr bearing mineral in the carbonates which formed during permian contact metamorphism the zr mineral sequence baddeleyite zirconolite zircon implies an increasing a sio 2 and f co 2 during the growth of these minerals electron microprobe dating of zirconolite zirc i yields a weighted average age of 263 16 ma and indicates that the hfse and ree rich assemblages formed during the permian emplacement of the stubenberg granite as a result of the subsequent high p eo alpine metamorphic overprint 111 15 ma hfse and ree were locally re mobilized leading to dissolution of zirc i and reprecipitation of the ree and nb rich overgrowths of zirc ii

from granulites to eclogites in the sesia zone italian western alps a record of the opening and closure of the piedmont ocean the sesia zone italian western alps offers one of the best preserved examples of pre alpine basement reactivated under eclogite facies conditions during the alpine orogenesis a detailed mineralogical study of eclogitized acid and basic granulites and related amphibolites is presented in these rare weak to undeformed rocks microstructural investigations allow three main metamorphic stages to be distinguished a a medium to low p granulite stage giving rise to the development of orthopyroxene garnet plagioclase brown amphibole ilmenite biotite in basic granulites and garnet k feldspar plagioclase cordierite sillimanite biotite ilmenite in acid granulites b a post granulite re equilibration associated with the development of shear zones producing discrete amphibolitization of the basic granulites and widespread development of biotite sillimanite cordierite spinel in the acid rocks c an eo alpine eclogite stage giving rise to the crystallization of high p and low t assemblages in an effort to quantify this evolution independent well calibrated thermobarometers were applied to basic and acid rocks for the granulite event p t estimates are 7 9 kbar and 700 800 c and for subsequent retrograde evolution p t was 4 5 kbar and 600 c for the eo alpine eclogite metamorphism pressure and temperature conditions were 14 16 kbar and 550 c the inferred p t path is consistent with an uplift of continental crust produced by crustal thinning prior to the subduction of the continental rocks in the light of the available geochronological constraints we propose to relate the pre alpine granulite and post granulite retrograde evolution to the permo jurassic extensional regime the complex granulite eclogite transition is thus regarded as a record of the opening and of the closure of the piedmont ocean

metamorphic evolution and oxygen isotope geochemistry of rift precursor amphibolites from hole 1067a odp leg 173 off west iberian galicia bank rifted margin abstract the paper documents permian igneous rocks in the vicinity of an ocean continent transition in the west iberian abyssal plain odp leg 173 hole 1067a was drilled through a sequence of foliated to massive amphibolites of gabbroic protolithic origin the 270 3 ma to 246 5 ma rock sequence experienced several metamorphic events during the interval of time spanning from 138 ma up to 55 ma metamorphic parageneses record two major thermobarometric events for which the temperature and pressure conditions are 670 40 c and 0 7 0 1 gpa and 550 50 c and 0 5 0 1 gpa according to radiometric dating these metamorphic events might have occurred around 138 136 ma and 113 110 ma respectively the low cl content and the preserved igneous 18 o of amphibole imply seawater absent 580 c isothermal plagioclase amphibole equilibrium related to the development of the strong foliation of the amphibolite during the second metamorphic event late isotopic equilibration involved lower temperature seawater derived fluid and is younger than 110 ma the gabbroic intrusion is believed to be a precursor for initiation of continental rifting melting of such metamorphosed rocks in a rift setting would generate magmatic rocks with complex oxygen isotopic signatures

permian deposits of the southern alps as product of initial alpidic taphrogenesis sedimentological and morphotectonic investigations of permian deposits between the val camonica and the karawanken mountains indicate the development of a complex rift system which initiated the alpidic sedimentation phase

influence of deformation partitioning and metamorphic re equilibration on p t path reconstruction in the pre alpine basement of central southern alps northern italy the dominant foliation s2 in the metapelites of the southalpine basement near the western side of the tertiary adamello intrusive stock is a variscan greenschist facies planar fabric slightly reworked during thick skin alpine tectonics s2 is defined by muscovite and chlorite and was formed by decrenulation of pre existing foliations which are confined to metre size less deformed domains and defined by biotite and white mica the pre s2 fabric is composite d1a d1b and defined by contrasting amphibolite facies metamorphic assemblages in diverent residual sites cld bt i grt ms i pl qtz and st bt ii grt ms ii pl qtz assemblages mark d1a and d1b fabrics respectively these developed during successive steps of a single temperature prograde polyphase event rather than during separate tectonometamorphic imprints avecting diverent tectonic units later coupled during a d2 greenschist facies stage thermobarometric estimates of assemblages formed during d1a d1b and d2 show a transition from t 480 540 c during d1a to t 570 660 c during d1b corresponding to a slight pressure increase from 0 75 0 95 gpa to 0 85 1 15 gpa d2 greenschist retrogression corresponds to a pressure and temperature decrease t 400 550 c and p 0 3 0 4 gpa this p t deformation time path is inferred to be the result of uplift from a depth of c 35 km after palaeozoic subduction and continental collision it is consistent with models postulated for other metamorphic units of the variscan belt in europe this is the first documented example in the southern alps of temperature prograde metamorphism before palaeozoic collision

magma pulses in the central variscan belt episodic melt generation and emplacement during lithospheric thinning magmatism in the central part of the variscan orogenic belt in europe has occurred in several distinct episodes each episode spanned a few million years and was characterized by a distinct geochemical signature and geodynamic setting i between 340 and 330 ma visean high k diorites and granitoids within a zone extending parallel the orogen axis possibly tracing a palaeosuture ii at 310 307 ma westphalian calc alkaline granitoids in the intra alpine domain iii at 304 295 ma stephanian subalkaline granitoids in intra alpine and pennine domains and finally iv at 280 270 ma permian granitoids in pennine austroalpine and south alpine domains the generation and emplacement of granitoid melts in the variscan crust are suggested to be short term pulses that are controlled by extensional processes in the lithosphere such as thermal erosion and relaxation of the lithospheric mantle and decompressional melting

tectonics and plate tectonics model for the variscan belt of europe abstract a plate tectonics model is presented to explain the tectonometamorphic characteristics of the european variscides after the closing of two oceanic domains by two sided subduction 500 420 ma and obduction 420 380 ma collision of the european and african continental plates occurred we propose that the subsequent complex intracontinental deformation 380 290 ma is the result of a double subduction of the continental lithosphere accompanied by crust mantle decollement this mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the variscan belt accounting for this model and for the relationships between the european variscides and the other paleozoic peri atlantic belts caledonides appalachian mauritanides and morocco we infer the relative positions of africa america and europe between the silurian and the permian

from permo triassic lithospheric thinning to jurassic rifting at the adriatic margin petrological and geochronological record in valtournenche western italian alps abstract slices of polycyclic metasediments marbles and meta cherts are tectonically amalgamated with the polydeformed basement of the dent blanche tectonic system along a major alpine shear zone in the western alps becca di sale area valtournenche valley a combination of techniques structural analysis at various scales metamorphic petrology geochronology and trace element geochemistry was applied to determine the age and composition of accessory phases titanite allanite and zircon and their relation to major minerals the results are used to reconstruct the polyphase structural and metamorphic histories comprising both pre alpine and alpine cycles the pre alpine evolution is associated with low pressure high temperature metamorphism related to permo triassic lithospheric thinning in meta cherts microtextural relations indicate coeval growth of allanite and garnet during this stage at 300 ma textures of zircon also indicate crystallisation at ht conditions ages scatter from 263 to 294 ma with a major cluster of data at 276 ma in impure marble u pb analyses of titanite domains with variable al and f contents yield apparent 206pb 238u dates range from permian to jurassic chemical and isotopic data suggest that titanite formed at permian times and was then affected by extension related fluid circulation during the triassic and jurassic which redistributed major elements al and f and partially opened the u pb system the alpine cycle lead to early blueschist facies assemblages which were partly overprinted under greenschist facies conditions the strong alpine compressional overprint disrupted the pre alpine structural imprint and or reactivated earlier structures the pre alpine metamorphic record preserved in these slices of metasediments reflects the onset of the permo triassic lithospheric extension to jurassic rifting

late orogenic extension in mountain belts insights from the basin and range and the late paleozoic variscan belt from two examples of orogenic domains some general mechanisms significant of late orogenic tectonic processes in mountain belts are characterized the basin and range province and the variscan belt in the french massif central have both suffered important compressional orogenic crustal thickening and the results of late orogenic processes can be observed in the field both areas are covered by deep seismic profiling providing constraints on the geometry of a crust which has been restored to a normal thickness late orogenic features from the two domains are compared at different scales and their tectonic significance for extension mechanisms is discussed at the scale of the orogenic domains the most prominent tectonic features are the metamorphic core complexes mcc which expose deformed rocks from the middle crust generally affected by high temperature low to medium pressure metamorphism partial melting and widespread granite emplacement in these mcc large scale extensional shear zones present an intense mylonitic deformation characterized by low dipping foliations and pervasive stretching lineations they show a complete evolution from early deep seated ductile deformation generally achieved under high temperature low to medium pressure metamorphism to a late shallow brittle stage characterized by cataclastic deformation the late detachment stage generally controls the development of asymmetrical extensional sedimentary basins filled by continental deposits two main geometries of mcc are defined that are characterized by differing geometry and kinematics of low angle shear zones in the first case two low angle shear zones with opposite vergence develop along the flanks of a roughly symmetrical mcc often one system is dominant over the other the second geometry characterizes asymmetrical mcc bounded by a single normal shear zone which is upwarped during uplift and doming of the core caused by tectonic denudation detailed strain analysis performed in several extensional shear zones shows that the deformation regime is heterogeneous and results from general noncoaxial flow deformation along the shear zones evolves progressively from slight homogeneous pure shear strain to intense heterogeneous noncoaxial shear strain strain distribution within the lower crust is less well constrained by field observation however analogies between cocorp and ecors deep seismic reflection profiles give important constraints on crustal structure wide zones of highly subhorizontally layered lower crust and a flat high amplitude reflection moho characterize both evolved orogenic domains suggesting that major deformations and flow occur within the lower crust during extension a kinematic model involving heterogeneous crustal deformation and regional scale flow fits relatively well with late orogenic structures observed in continental domains a weak hot upper mantle allows large scale flow of lower crust material from zones of deep ductile extension to uplifted domains of upper crustal denudation heterogeneous strain is accommodated by low angle extensional shear zones from localized zones of extension in the brittle crust to ductile lower crust combined pure and simple shear occurs along localized shear zones whereas at the scale of the whole lithosphere deformation nearly corresponds to a vertical pure shear such deformation processes which affect a thick and hot crust seem to be common in both compared domains suggesting that late orogenic extensional processes are slightly dependent of the type of contractional tectonics thus as much in the andean type west american cordilleran belt as in the collision type variscan belt late orogenic processes produced similar extensional features

two subduction events in a polycyclic basement alpine and pre alpine high pressure metamorphism in the suretta nappe swiss eastern alps this study is essentially based on coupling macrostructures microstructures and metamorphic petrology in polymetamorphic mafic rocks from the swiss eastern alps suretta nappe penninic domain petrographic criteria are used in conjunction with structural analysis and microprobe work to define crystallization deformation relationships and to establish a relative but precise sequence of tectono metamorphic events a first eclogite facies overprint and related exhumation occurred before emplacement of late palaeozoic intrusives during the alpine cycle the suretta nappe was part of the thinned european continental margin the tertiary burial due to subduction and collision is responsible for d1 ductile thrusting and blueschist facies metamorphism late deformation phases related to exhumation are responsible for the development of extensional structures under greenschist facies conditions quantitative metamorphic petrology based on gibbs free energy minimization domino by de capitani gives a constraint on the p t conditions during the polymetamorphic and polycyclic evolution the first high p metamorphic event related to pre alpine structures occurred at c 700 c and at least 2 0 gpa these conditions are compatible with pre alpine high p re equilibration already described in several alpine units the alpine high p metamorphism occurred under blueschist facies conditions at c 400 450 c and 1 0 gpa similar high p low t conditions have already been described in the mesozoic and permian rock types the two high p events are clearly related to two different geothermal regimes and geodynamic environments

increase in background stratospheric aerosol observed with lidar at mauna loa observatory and boulder colorado 1 the stratospheric aerosol layer has been monitored with lidars at mauna loa observatory in hawaii and boulder in colorado since 1975 and 2000 respectively following the pinatubo volcanic eruption in june 1991 the global stratosphere has not been perturbed by a major volcanic eruption providing an unprecedented opportunity to study the background aerosol since about 2000 an increase of 4 7 per year in the aerosol backscatter in the altitude range 20 30 km has been detected at both mauna loa and boulder this increase is superimposed on a seasonal cycle with a winter maximum that is modulated by the quasi biennial oscillation qbo in tropical winds of the three major causes for a stratospheric aerosol increase volcanic emissions to the stratosphere increased tropical upwelling and an increase in anthropogenic sulfur gas emissions in the troposphere it appears that a large increase in coal burning since 2002 mainly in china is the likely source of sulfur dioxide that ultimately ends up as the sulfate aerosol responsible for the increased backscatter from the stratospheric aerosol layer the results are consistent with 0 6 0 8 of tropospheric sulfur entering the stratosphere

permian triassic high thermal regime in the alps result of late variscan collapse or continental rifting validation by numerical modeling 1 the permian triassic igneous activity associated with regional scale deformation developed under high temperature low pressure ht lp metamorphic conditions and widely recorded in the pre alpine crust of the european alps can result from late orogenic collapse of a collisional belt or from lithospheric thinning leading to a continental rifting process in order to reduce this ambiguity we use a two dimensional finite element model to give new insights on the sequence of mechanisms operating during active ocean continent convergence followed by continental collision and pure gravitational evolution and on the regional geodynamic interpretation of the paleozoic mesozoic evolution of the alpine area the modeling predictions are compared with the pt climax conditions of variscan and permian triassic metamorphism affecting the continental crust of the helvetic to southalpine domains the good agreement between model predictions and natural data realized for the early to neovariscan evolution indicates that during paleozoic the pre alpine crust of the alps was part of an active ocean continent convergence margin and an intracontinental suture zone furthermore modeling results support the interpretation envisaging a permian triassic lithospheric extension as responsible for the ht metamorphism and related intense igneous activity this evolution was precursor of the mesozoic oceanization during which the tectonic units coupled and accreted during the variscan subduction and collision were separated to form the two passive european and adriatic continental margins

pre mesozoic alpine basements their place in the european paleozoic framework prior to their alpine overprinting most of the pre mesozoic basement areas in alpine orogenic structures shared a complex evolution starting with neoproterozoic sediments that are thought to have received detrital input from both west and east gondwanan cratonic sources a subsequent neoproterozoic cambrian active margin setting at the gondwana margin was followed by a cambrian ordovician rifting period including an ordovician cordillera like active margin setting during the late ordovician and silurian periods the future alpine domains recorded crustal extension along the gondwana margin announcing the future opening of the paleotethys oceanic domain most areas then underwent variscan orogenic events including continental subduction and collisions with avalonian type basement areas along laurussia and the juxtaposition and the duplication of terrane assemblages during strike slip accompanied by contemporaneous crustal shortening and the subduction of paleotethys under laurussia thereafter the final pangea assemblage underwent triassic and jurassic extension followed by tertiary shortening and leading to the buildup of the alpine mountain chain recent plate tectonic reconstructions place the alpine domains in their supposed initial cambrian ordovician positions in the eastern part of the gondwana margin where a stronger interference with the chinese blocks is proposed at least from the ordovician onward for the visean time of the variscan continental collision the distinction of the former tectonic lower plate situation is traceable but becomes blurred through the subsequent oblique subduction of paleotethys under laurussia accompanied by large scale strike slip since the pennsylvanian this global collisional scenario has been replaced by subsequent and ongoing shortening and strike slip under rising geothermal conditions and all of this occurred before all these puzzle elements underwent the complex alpine reorganization

contrasting thermomechanical evolutions in the southalpine metamorphic basement of the orobic alps central alps italy in the southern alps a progressive metamorphic zonation with an increase in the geothermal gradient from ne to sw has been widely proposed however recent investigations have shown that the greenschist metamorphic imprint of the low grade zone corresponds to a metamorphic retrogression following amphibolite facies conditions on the other hand in the medium grade zone a later low pressure high temperature lpht metamorphic event has also been proposed in an attempt to resolve these different interpretations new petrological and partly new structural data have been obtained for two sectors of the orobic alps traditionally attributed to different metamorphic zones thermo barometric determinations supported by microstructural analysis indicate the following different pressure retrograde paths in each sector 1 in the val vedello basement wb rocks a first metamorphic imprint characterized by p 7 9 kbar and t 570 610 c was followed by a greenschist retrogression p 4 kbar and t 500 c 2 in the lario basement lb rocks the first detectable metamorphic stage characterized by mineral assemblages indicating p 7 9 kbar and t 550 630 c was followed by a lpht event synkinematic with f2 extensional deformation a greenschist retrogression marks the final uplift of these rocks reinterpretation of the available geochronological data indicates a diachronism for the two thermo mechanical evolutions in the light of these data we interpret the retrograde p t t path of the vvb rocks as a pre permian post thickening uplift and the retrograde p t t evolution of the lb rocks as a permo mesozoic uplift related to the extensional tectonic regime of the tethyan rifting

tectonics of the dolomites southern alps northern italy abstract in post variscan times the dolomites underwent a number of tectonic events which may be summarized as follows permian and triassic rifting phases broke the area into n s trending basins with different degrees of subsidence a middle triassic transpressive event then deformed the region along a n70 e axis generating flower structures within the basement volcano tectonic domal uplift and subsequent caldera formation occurred at the same time as the late ladinian magmatism early jurassic rifting also controlled the subsidence which increased eastward this long period of deformation was followed by a pre neogene late cretaceous palaeogene e w ene wsw compression which generated a w vergent belt possibly equivalent to the folded foreland of the dinaric chain a 70 km e w section of the dolomites indicates shortening of at least 10 km during the neogene the dolomites as far north as the insubric lineament were the innermost part of a s vergent thrust belt the basement of the dolomites was thrust southwards along the valsugana line onto the sedimentary cover of the venetian prealps for at least 10 km this caused a regional uplift of 3 5 km the valsugana line and its backthrusts on the northern side of the central dolomites generated a 60 km wide pop up in the form of a synclinorium within which the sedimentary cover adapted itself mainly by flexural slip often forming triangle zones the shortening linked to this folding is about 5 km with neogene thrusts faulting and folding pre existing thrust planes on the north eastern side of the dolomites neogene deformation is apparently more strictly controlled by the transpressive effects of the insubric lineament and shortening of the sedimentary cover may be greater than in the central dolomites minor deformation linked to the giudicarie belt is present in the western dolomites the present structure of the dolomites is thus the result of a number of tectonic events of different significance and different strike only a 3 dimensional restoration can unravel the true structure of the dolomites

a variscan pressure temperature time path for the n e mont blanc massif the northeastern portion of the mont blanc massif in western switzerland is predominantly comprised of the granitic rocks of the mont blanc intrusive suite and the mont blanc basement gneisses within these metamorphic rocks are a variety of sub economic fe skarns the mineral assemblages and fluid inclusions from these rocks have been used to derive age pressure temperature and fluid composition constraints for two variscan events metamorphic hornblendes within the assemblages from the basement amphibolites and iron skarns have been dated using 40ar 39ar and indicate that these metamorphic events have a minimum age of approximately 334 ma garnet hornblende plagioclase thermobarometry and stable isotope data obtained from the basement amphibolites are consistent with metamorphic temperatures in the range 515 to 580 c and pressures ranging from 5 to 8 kbar garnet hornblende magnetite thermobarometry and fluid inclusion studies indicate that the iron skarns formed at slightly lower temperatures ranging from 400 to 500 c in the presence of saline fluids at formational pressures similar to those experienced by the basement amphibolites late paleozoic minimum uplift rates and geothermal gradients calculated using these data and the presence of ladinien ichnofossils are on the order of 0 32 mm year and 20 c km respectively these uplift rates and geothermal gradients differ from those obtained from the neighbouring aiguilles rouges massif and indicate that these two massifs experienced different metamorphic conditions during the carboniferous and permian periods during the early to late carboniferous period the relative depths of the two massifs were reversed with the aiguilles rouges being initially unroofed at a much greater rate than the mont blanc but experiencing relatively slower uplift rates near the termination of the variscan orogeny

the experimental deformation of dunite deformation experiments have been carried out on two dunites anita bay of 100 m grain size and aheim of 900 m grain size at strain rates from 10 3 to 10 6 s 1 and temperatures from 1000 c to 1300 c in a gas medium deformation apparatus at 300 mpa confining pressure most of the tests were under wet conditions defined by the presence of small amounts of water from hydrous minerals initially present constant strain rate and relaxation experiments covering ranges of flow stress down to about 70 mpa and 7 mpa respectively show that there is a change in flow law in going below about 100 mpa differential stress and that the coarser grained rock is stronger than the finer grained one power law parameters above the transition are n 4 48 0 31 and q 498 38 kj mol 1 for aheim dunite and n 3 35 0 17 and q 444 24 kj mol 1 for anita bay dunite while below the transition relaxation tests on anita bay dunite give n 2 44 0 18 and q 386 27 kj mol 1 it is concluded that there is a weakening effect of water that this effect is mainly in the grain boundaries and that grain boundary sliding is probably a significant deformation mechanism at the lower stresses under wet conditions

sequences of mesostructures related to the development of alpine thrusts in the eastern pyrenees abstract the mesostructures formed during the development of an antiformal stack in the eastern pyrenees are studied the following structural evolution is proposed a pressure solution cleavage develops by layer parallel shortening at initial stages of the thrusting the cleavage is present in all units but mainly in the marls and marly limestones in the siliceous layers a weak cleavage and small scale thrust structures develop the cleavage and the bedding are deformed during the thrusting different structures form in various places of the antiformal stack where the bedding is flat lying southward directed layer parallel shears predominate in the hangingwall ramps kink bands disjunctive stylolitic cleavage and layer parallel shear opposite to the sense of thrust movement develop the described structures constitute a kinematical model for mesostructures developed during the evolution and ramping of thrusts

permian metamorphic event in the alps in addition to the well known variscan and alpine orogenic cycles a permian thermal event occurred in the european alps the geodynamic background of this event is not well understood here we suggest that the event is a reflection of lithospheric thinning accompanied by magmatic underplating causing partial melting of the lower crust and low pressure high temperature metamorphism the event was terminated by early triassic opening of the meliata ocean and was followed by sag stage subsidence during slow lithospheric cooling this last stage of the evolution allowed the onset of the mesozoic marine evolution with the orogen wide formation of triassic carbonate platforms well known from the dolomites and the northern calcareous alps the combined evidence for magmatism metamorphism and late subsidence identifies the permian event of the alps as an excellent example of underplating events suggested as a cause for low pressure high temperature terrains worldwide

multistage accretion and exhumation of the continental crust ivrea crustal section italy and switzerland the ivrea crustal section exposes in map view all levels of the southern alpine continental crust from ultramafic mafic and felsic granulite facies rocks of the deep crust ivrea verbano zone through medium grade basement rocks strona ceneri zone and val colla zone to unmetamorphosed permo mesozoic sediments the oldest part of the crustal section is preserved in the medium grade basement units which are interpreted to be the overprinted remains of an ordovician 440 480 ma magmatic arc or forearc complex during variscan subduction this arc was technically underplated by a carboniferous accretion subduction complex 320 355 ma containing metasediments and slivers of rheic oceanic crust presently found in the ivrea verbano zone during the late stages of variscan convergence 290 320 ma lithospheric delamination triggered magmatic underplating and lead to polyphase deformation under amphibolite to granulite facies conditions this was broadly coeval with extensional exhumation and erosion of the variscan overprinted ordovician crust presently exposed in the strona ceneri and val colla zones post variscan transtensional tectonics 270 290 ma were associated with renewed magmatic underplating mylonitic shearing and incipient exhumation of the lower crust in the ivrea verbano zone this coincided with the formation of elongate basins filled with volcanoclastic sediments in the upper crust early mesozoic tethyan rifting of the southern alpine crust 180 230 ma reduced crustal thickness to 10 km or less in the lower crust most of this thinning was accommodated by granulite to retrograde greenschist facies mylonitic shearing the lower crust was exhumed along a large noncoaxial mylonitic shear zone that was linked to asymmetrical rift basins in the upper crust the composite structure resulting from this complex evolution is probably typical of thinned late variscan continental crust on the passive margins of western europe alpine faulting and folding 20 50 ma fragmented the crustal section the originally deepest levels of the crustal section in the ivrea verbano zone as well as some segments of the basement cover contact were steepened whereas other parts of the crustal section particularly the strona ceneri zone underwent only minor to moderate alpine rotation

gondwana derived microcontinents the constituents of the variscan and alpine collisional orogens abstract the european variscan and alpine mountain chains are collisional orogens and are built up of pre variscan building blocks which in most cases originated at the gondwana margin such pre variscan elements were part of a pre ordovician archipelago like continental ribbon in the former eastern prolongation of avalonia and their present day distribution resulted from juxtaposition through variscan and or alpine tectonic evolution the well known nomenclatures applied to these mountain chains are the mirror of variscan resp alpine organization it is the aim of this paper to present a terminology taking into account their pre variscan evolution at the gondwana margin they may contain relics of volcanic islands with pieces of cadomian crust relics of volcanic arc settings and accretionary wedges which were separated from gondwana by initial stages of rheic ocean opening after a short lived ordovician orogenic event and amalgamation of these elements at the gondwanan margin the still continuing gondwana directed subduction triggered the formation of ordovician al rich granitoids and the latest ordovician opening of palaeo tethys an example from the alps external massifs illustrates the gradual reworking of gondwana derived pre variscan elements during the variscan and alpine tertiary orogenic cycles

the evolution of the polymetamorphic basement in the central alps unravelled by precise u pb zircon dating the u pb age determinations of zircon and rutile from the aar massif reveal a complex evolution of the central alpine basement the oldest components are found in zircons of metasediments which bear cores of archean age the u pb age of discordant prismatic zircons of the same rocks ranges between 580 and 680 ma an age that is typical for pan african metamorphism the zircons are interpreted as pan african detritus with archean inheritance the provenance region of the pan african zircons is assumed to be a terrane of gondwana affinity i e the w african craton or the pentevrian microplate the caledonian metamorphism left a pervasive structural imprint in amphibolite facies on the rocks of the aar massif it is dated at 456 2 and 445 ma by zircons of a layered migmatitic gneiss and a migmatitic leucosome respectively both occurring in the northernmost zones of the massif hercynian metamorphism never exceeded greenschist facies conditions and is recorded by zircon in a garnet amphibolite and by rutile in a meta psammite that yield an age of 330 ma both zircon and rutile are considered to be products of retrograde mineral reactions and therefore do not date the peak conditions of hercynian metamorphism the gastern granite at the western end of the aar massif is a contaminated granite that intruded at 303 4 ma contemporaneously with the wide spread late hercynian post collisional i type magmatism the study demonstrates the potential of isotope dilution u pb dating of single grains and microfractions in deciphering complex evolutionary histories of polymetamorphic terrains

isotopic evidence from the ivrea zone for a hybrid lower crust formed by magmatic underplating the large mafic complex overlying mantle peridotites in the ivrea zone of northern italy originated by underplating of mantle derived magma onto the base of continental crust and may be a unique outcrop of typical mafic lower crust sampled elsewhere only by mafic xenoliths neodymium and strontium isotope data show that the magmas assimilated large and roughly uniform proportions of crustal material the resulting hybrid magmas have the same isotopic signature as and may represent the lower crustal roots of upper crustal hercynian granitoid intrusions

magmatic underplating extension and crustal reequilibration insights from a cross section through the ivrea zone and strona ceneri zone northern italy the thermal impact of magmatic underplating at various crustal levels is studied along a traverse through the ivrea verbano zone and strona ceneri zone in northern italy geochronological and petrologic data are compared to a two dimensional thermal kinematic model field data and numerical simulation show the strong disturbance of the temperature field in the lower and intermediate crust in relation to magmatic underplating leading to granulite to amphibolite facies metamorphism as well as reequilibration of mineral chemical and isotopic systems magmatic underplating leaves a crust with an apparently heterogeneous tectonometamorphic evolution as information on the earlier history is preserved only at upper crustal levels

deep oxygen penetration drives nitrification in intertidal beach sands we discovered a hotspot of elevated nitrate concentration median 431 m in shallow beach pore water that extended across the entire length of two barrier islands in the southeastern united states of america we investigated this feature by surveying groundwater geochemistry measuring fluctuations in in situ dissolved oxygen do concentrations modeling groundwater flow and quantifying nitrification rates nitrification of groundwater ammonium was the only possible nitrate source with a measured potential rate of 0 84 mol m 2 h 1 however the observed nitrate concentrations were far in excess of the predicted maximum achievable by aerobic nitrification assuming a 2 1 ratio of o n and around 200 m do in air saturated seawater groundwater do concentrations within the hotspot 65 cm depth were consistently 20 50 m the nitrate hotspot was located at the top of the water table beneath dry undersaturated sand that allowed the penetration of air and the dissolution of excess oxygen into the pore fluids the total dissolved nitrogen concentration of the hotspot was higher than anywhere else on the island indicating nitrogen accumulation within the hotspot most likely via ammonium adsorption vertical dispersion was the dominant pathway for nitrate loss from the hotspot this nitrate was consumed in underlying anoxic sand coupling microbial pathways of nitrogen oxidation and reduction and removing bioavailable nitrogen from the beach ecosystem

sm nd isotope systematics of high ree accessory minerals and major phases id tims la icp ms and epma data constrain multiple permian triassic pegmatite emplacement in the koralpe eastern alps abstract detailed sm nd mineral id tims empa and la icp ms analysis of meta pegmatite minerals from the koralpe stubalpe area austria involving single crystal separation step wise dissolution and acid leaching techniques yielded i information about the partitioning of ree between accessory and major mineral phases in meta pegmatites and ii age constraints on the long lasting permian triassic event in the austroalpine basement units of the eastern alps internal sm nd mineral isochrons performed on porphyroclast domains from the three localities rostockbach kuppergrund and wildbachgraben including garnet xenotime apatite monazite and feldspar indicate multiple emplacement of pegmatoid melts between 273 2 and 258 3 ma other localities yielded younger ages indicating that magmatic garnet crystallisation persisted from 251 7 to c 230 ma ages calculated between high ree accessories monazite apatite xenotime and the low ree major phases feldspar and garnet are largely consistent zircon however plots clearly off the internal regression line h 2 so 4 leaching experiments on meta pegmatite garnet rich in micron sized xenotime and apatite inclusions locally affected by dynamic recrystallisation yielded information about the behaviour of the sm nd system during strong tectono metamorphic overprinting the data suggest multiple injections of pegmatite melts at distinctly different times during protracted crustal thinning and heating in the permian triassic ky paramorphs after andalusite and micro inclusions of kyanite staurolite and sillimanite in magmatic garnet indicate crystallisation of the garnet at moderately high but variable pressure conditions during subsequent eclogite facies overprinting peak pt c 700 c 2 2 gpa and intense deformation in cretaceous time the magmatic chemical and isotopic signatures of porphyroclasts were largely preserved except for localized within grain micro shear zones and outermost rim domains whereas part of the feldspar rich matrix recrystallised during pervasive mylonitisation along the eclogite to amphibolite facies grade exhumation path

the mafic ultramafic complex near finero ivrea verbano zone ii geochronology and isotope geochemistry whole rock nd and sr isotopic compositions of the mafic ultramafic complex near finero demonstrate that the magma was derived from a depleted perhaps morb type mantle reservoir the sm nd data for the amphibole peridotite unit can be interpreted as an isochron with an apparent age of 533 20 ma which is consistent with a 207pb 206pb evaporation age of 549 12 ma of a single zircon grain from the internal gabbro unit however the interpretation of these apparent ages remains open to question we therefore retain the alternative hypotheses that the intrusion occurred either about 533 or 270 ma ago the latter being the most likely age of emplacement of the much larger magma body near balmuccia val sesia the implication of the older emplacement age if correct would be that the igneous complex may be related to the numerous amphibolite units which are intercalated with the metapelites of the overlying kinzigite formation and together with them may constitute an accretionary complex in this case the mafic ultramafic complex itself might also be part of such an accretionary complex as has been proposed for the balmuccia peridotite internal sm nd isochrons involving grt cpx plag and amph from the internal gabbro unit yield concordant ages of 231 23 226 7 223 10 214 17 and 203 13 ma these results confirm published evidence for a separate regional heating event about 215 15 ma ago initial end 533 values average 6 3 0 4 for six samples of the amphibole peridotite unit and 6 0 1 2 for ten samples of the external gabbro unit 87sr 86sr ratios require little or no age correction and range from 0 7026 to 0 7047 with two outliers at 0 7053 and 0 7071 strong correlations between 87sr 86sr and k2o and weaker correlations between initial end and k2o imply a comparatively minor 10 contamination of the external gabbro magma by crustal material and a later alteration by a crustal or seawater derived fluid these results contrast sharply with the isotopic composition negative end and high 87sr 86sr values of the associated mantle rocks the phlogopite peridotite unit which has been pervasively metasomatized by crustal fluids this type of metasomatism and its isotopic signature are never seen in the magmatic complex this evidence rules out any direct genetic relationship between the igneous complex and the mantle peridotite the crust mantle interaction is the opposite of that seen at balmuccia where the mantle peridotite is essentially pristine and the magmatic body has been extensively contaminated by assimilation of crustal rocks

implications of extinction due to meteoritic smoke in the upper stratosphere 1 recent optical observations of aerosols in the upper stratosphere and mesosphere show significant amounts of extinction at altitudes above about 40 km where the stratospheric sulfate aerosol layer ends recent modeling of this region reveals that meteoritic smoke settling from the mesosphere and its interaction with the upper part of the sulfate aerosol layer is the origin of the observed extinction extinction in this region has major implications for the interpretation and analysis of several kinds of aerosol data satellite and lidar we compare observations from the sage ii satellite and from noaa s lidar located at mauna loa hawaii to extinction profiles derived from the whole atmosphere community climate model waccm coupled with the community aerosol and radiation model for atmospheres carma our results show that a major source of extinction exists in the region above about 30 km that must be addressed by all remote sensing instruments that have traditionally used the stratosphere above about 30 km as an aerosol free region to estimate the molecular component of their total extinction it is also shown that meteoritic smoke not only contributes to but also becomes the dominant source of aerosol extinction above 35 km and poleward of 30 degrees in latitude as well as above 40 km in the tropics after addressing the concerns described here current and past observations of this region could be reanalyzed to further our understanding of meteoritic dust in the upper stratosphere

origin of eclogites from the austroalpine otztal basement tirol austria geochemistry and smnd vs rbsr isotope systematics geochemical and combined sm nd vs rb sr isotope data from the otztal metabasite suite point to production of gabbros and basalts in an extensional regime in late cambrian times the source of these magmas and cumulates had a chemical and isotopic composition similar to morb the time of intrusion is constrained by sm nd mineral isochrons clinopyroxeneplagioclase of relictic gabbro occurrences within the eclogite amphibolite series in the time span between 530 and 521 ma subsequently the basic eclogite precursors suffered three metamorphic stages the imprint of the caledonian event is unclear during the hercynian cycle the basic magmatic rocks were transformed into eclogites at surprisingly high p t conditions of 27 kbar 730 c the time of this subduction event is given by mineral isochrons garnet clinopyroxene zoisite whole rock as 370 340 ma with a most probable time span for the pressure peak between 360 and 350 ma during subsequent exhumation and pressure release the eclogites were partly transformed into amphibolites alpine overprint during a greenschist facies grade metamorphism caused further and partly intense retrograde alterations in the metabasic suite

the lac cornu retrograded eclogites aiguilles rouges massif western alps france evidence of crustal origin and metasomatic alteration abstract metabasic rocks interbedded in amphibolite facies supracrustal gneisses outcrop around lac carnu in the aiguilles rouges massif western alps the cores of the thickest boudinaged lenses are made up of eclogitic amphibolites grading outwards into amphibolites the common assemblage is unzoned garnet symplectitic clinopyroxebe hornblende plagioclase in a slightly amphibolitized sample minimum p and t conditions of equilibrium between garnet and omphacite inclusions are 780 c and 11 kb a polymetamorphic pre alpine evolution of the massif is thus demonstrated eclogitic amphibolites show variations in major element composition similar to the skaergaard evolution in the al2o3 feo fe2o3 mgo triangle variation diagrams in which zr is taken as differentiation index also indicate magmatic trends for mn ti p ni co y v and cr ca k na sr rb and si were mobile during the evolution of the rocks the igneous trend can be described by a quantitative model of fractional crystallization in which a noritic assemblage separated in the initial stage this provides evidence of crustal p t conditions of differentiation for the original materoal and in situ evolution for the eclogites a second group of amphibolites shows banded structure and transitional terms with the surrounding gneisses a volcano sedimentary origin is suggested though the process cannot be modelled the rocks may result from complex interactions of magnetic metasomatic and sedimentary processes it is concluded that lac cornu metabasites were originally continental tholeiites though several geochemical criteria tend to indicate an oceanic origin this casts some doubt on the validity of these criteria when applied to metamorphic rocks

the metamorphic evolution of garnet cordierite sillimanite gneisses of the gruf complex eastern pennine alps metapelitic gneisses occuring as lenses and bands within the migmatites of the gruf complex in the eastern pennine alps contain various combinations of the minerals quartz biotite cordierite garnet sillimanite plagioclase k feldspar spinel orthopyroxene anthophyllite and muscovite the most common rock type is represented by a darkschistosebiotiterichcordierite garnet sillimanite gneiss a consistent pressure temperature range of 3 4 kb and 600 650 c has been calculated for the last metamorphic equilibration from six geological thermobarometers

the emplacement of variscan ht metamorphic rocks linked to the interaction between gondwana and laurussia structural constraints in ne sardinia italy terra nova 22 369 377 2010 abstract the emplacement of variscan ht metamorphic rocks in sardinia italy occurred in a transpressive regime related to the oblique collision between gondwana and laurussia during the early visean c 340 ma bashkirian c 320 ma the restored directions of the main structural features in ne sardinia allow us to infer a ne sw oriented direction of shortening during this time as has been established for the french massif central and the maures tanneron massif which could lead to the exhumation of ht rocks by telescoping processes field evidence and finite strain analysis emphasize that the exhumation of ht metamorphic rocks could be driven by a restraining bend that developed in ne sardinia along dextral transpressive shear zones the tectonic model proposed in this paper allows us to conclude that crustal melt production usually attributed to extensional regimes can be related to compressional transpressional regimes

magmatic plumbing of a large permian caldera exposed to a depth of 25 km we report evidence for a 13 km diameter permian caldera associated with a renowned section through the continental crust comprising the ivrea verbano zone and serie dei laghi of northwest italy correlation of ages of volcanic and middle to deep crustal plutonic rocks suggests that they constitute an unprecedented exposure of a subcaldera magmatic plumbing system to a depth of 25 km and points to a cause and effect link between intrusion of mantle derived basalt in the deep crust and large scale silicic volcanism

deep crustal extensional faulting in the ivrea zone of northern italy abstract the ivrea zone in northern italy consists of amphibolite and granulite facies metasedimentary metabasic and ultrabasic rocks and is thought to represent a section through the deep continental crust detailed mapping of the high grade part of this zone has revealed a series of high temperature shear zones which after the effects of later low temperature faulting and folding have been removed appears to accommodate extension of the sequence the shear zones formed at temperatures similar to those at which thermal equilibration occurred in the host rocks and locally show evidence for development under prograde conditions these observations fit well with thermal effects calculated from a simple heat flow model for extensional faulting in this model the effects of shear heating and displacement of cool hanging wall rocks against hotter footwall rocks are calculated these observations indicate that faulting as a mode of failure may be important during extension of the lower continental crust especially when basic rocks are the dominant components of the rock pile a restored section through the extended lower crust as observed in the ivrea zone is presented and the extent to which the features observed are able to explain seismic reflectivity of the lower crust is discussed

eclogites of paleozoic or early alpine age in the basement of the penninic siviez mischabel nappe wallis switzerland outcrops of eclogites and retroeclogites occur in the normal limb of the siviez mischabel fold nappe penninic alps wallis switzerland the ecologites are contained in the gneisses of the ergischhorn and barneuza units turtmanntal and val de moiry where they usually associate with banded amphibolites high pressure mineral assemblages in the eclogites pyrope rich almandine and omphacite are overprinted by later amphibolite and greenschist facies metamorphic minerals according to coleman s classification the eclogites belong to the b type with pt values around 650 c for a minimum pressure of 15 kb geochemical investigations suggest a tholeiitic basalt affinity some problems still remain to solve these are 1 the geodynamic contexts and 2 the age of the eclogite facies overprint whether alpine or pre alpine in the absence of radiometric ages the following arguments favour a pre alpine lower paleozoic or older age of the high pressure metamorphism 1 1 the presence of polymetamorphic paragenesis the eclogitic mineral assemblages being the oldest 2 2 the b type of the eclogites which are associated tectonically with polymetamorphic gneisses 3 3 the presence in the same tectonic units of permian gabbros which show no signs of eclogite facies overprint 4 4 the presence of autochthonous sedimentary cover rocks that have been dated triassic to eocene this makes it difficult to assume an eo alpine cretaceous subduction of the underlying basement 5 5 the existence of striking similarity with other pre alpine eclogites related to a lower paleozoic 500 460 ma high pressure event well documented in some alpine and extra alpine basement units it could have associated with a caledonian subduction of oceanic crust

the south western branch of the variscan belt evidence from morocco abstract this work is based on the compilation and re evaluation of the most significant data either personal or from the literature concerning the moroccan variscides the latter constitute the only moderately disturbed or even undisturbed part of the south western branch of the variscan belt facing directly nw gondwana they include two orogenic segments namely the northern mauritanides and the meseta domain exposed in the saharan and atlas meseta regions respectively and a foreland belt cropping out essentially in the anti atlas the eastward thrust units of saharan morocco oulad dlim mostly originate from the west african craton wac border in an area of thin palaeozoic sedimentation thin skinned fold thrust foreland arcs develop progressively northward zemmour at the expense of the increasingly thick palaeozoic series whereas thick skinned deformation characterizes the inverted proximal paleomargin in the anti atlas domain as suggested by the meseta and anti atlas stratigraphic similarities the meseta domain corresponds to a collage of moderately displaced thinned crustal blocks from the distal gondwana paleomargin variscan deformation is dominated by nw verging thrusts and metamorphism developed in the thickened tectonic prism in relation with crustal anatexis at depth the meseta anti atlas boundary is a major ene trending transpressional dextral fault referred to as the south meseta fault smf discussing the correlations between the variscan segments of morocco and sw iberia allows us to suggest that a latitudinal transform zone similar to the smf separated these segments during the late palaeozoic subduction of the rheic ocean crust would have been directed se ward along both the iberian and moroccan meseta and nw ward south of the smf i e along the wac

reviewing pre transalp dss models abstract the interpretation of dss deep seismic soundings profiles in central and eastern alps is recalled in the paper and the models of the lower crust and moho proposed several years ago are compared to the results of the transalp seismic reflection profile this evaluation highlights a good agreement as far as the geometry of the deep crustal structure is concerned therefore the reliability of the interpretative models previously exclusively based on dss profiles becomes improved the deep structure beneath the whole alpine range is examined reconsidering the map of the moho boundary and the structural model already proposed for the central eastern sector five main interpretative transects are put side by side starting from the western alps and moving eastwards to the swiss lombardian central alps european geotraverse to the cross section from southern bavaria to the euganei hills to the transalp profile and finally to the easternmost profile available so far southern bavaria trieste the comparison outlines lateral variations of the deep crustal structure as well as a sharp contrast between the adria and the european lower crust and moho the transition from the adria plate to the dinaric domain remains up to now undefined

evolution of large silicic magma systems new u pb zircon data on the nw permian athesian volcanic group southern alps italy abstract during the permian the southern alps were affected by prominent igneous activity that produced voluminous basic to acidic volcanic and plutonic rocks the magmatism originated during a period in which postorogenic extension and transtensional faulting controlled magma ascent and emplacement within this scenario the athesian volcanic group ag with lithologies ranging from basaltic andesites to rhyolites constitutes a major part of the permian magmatism in the central and eastern southern alps we present an up to date stratigraphic reconstruction of the northwestern sector of the ag volcanic sequence combining detailed field mapping and new geochronological data single zircon u pb age data suggest that the volcanism in the ag lasted ca 10 m yr between 285 and 274 ma with a deposition rate that increased throughout the eruptive cycle the processes of magma deposition and basin formation were strongly influenced by tectonic activity over the entire history of the ag at least two major

south variscan terrane accretion sardinian constraints on the intra alpine variscides structural data are presented that show that the sardinian variscides form the boundary of the variscan intra alpine terrane thrust onto a gondwana foreland this model replaces the currently held hypothesis that the south armorican suture zone continues into the sardinian variscides the orogenic activity propagated toward the foreland in two distinct waves 1 thrusting burial and barrovian type metamorphism that reached the internal orogen in late devonian times and lasted in the external orogen until early carboniferous times transpressional strain partitioning is suggested by frontal thrusting in the external orogen and lateral displacement in the internal orogen 2 normal faulting exhumation and local buchan type metamorphism were superimposed on the internal orogen probably since early carboniferous times and reached the external orogen contemporaneously with the late carboniferous emplacement of voluminous granitic intrusions extensional doming and lateral displacement indicate a transtensional regime the sense of orogen parallel displacement during both stages was left lateral on inward dipping movement planes and right lateral on outward dipping movement planes the resulting left lateral displacement between the intra alpine terrane and the gondwana foreland is due to the bending of the variscan belt during which the sardinian segment underwent a bookshelf type movement within an overall right lateral displacement regime

emplacement at granulite facies conditions of the sesia lanzo metagabbros an early record of permian rifting the evolution of the pre alpine corio and monastero metagabbros points to strong chemical and mineralogical similarities with that of other permian gabbro bodies of the alps which are concentrated in the southalpine and austroalpine domains the structural and metamorphic pre alpine evolution of these gabbros records a re equilibration following the emplacement in the deep crust p 0 6 0 9 gpa and t 850 70 c exhumation through amphibolite facies conditions p 0 5 0 35 gpa and t 570 670 c followed by a greenschist facies imprint 0 25 p 0 35 gpa and t 550 c this retrograde p t evolution suggests that the exhumation occurred in a high thermal gradient regime such as that induced by upwelling of an asthenospheric plume during continental rifting this would be consistent with the crustal thinning known to have occurred in both the southalpine and austroalpine domains during permian times the gabbros and their country acid granulites are spatially associated with the serpentinised subcontinental mantle of the lanzo massif this lithologic association and the metamorphic evolution is similar to that of the fedoz gabbro austroalpine domain of the central alps and completely different from that observed in passive margins where no remnants of the lower crust occur and the upper granitic crust directly overlies the serpentinized lherzolites the location of permian gabbro bodies in the austroalpine and southalpine domains and their absence in the helvetic domain is evidence for asymmetric rifting

u pb zircon ages for post variscan volcanism in the ligurian alps northern italy abstract ages of permian volcanic rocks from the ligurian brianconnais domain western italian alps have been determined by laser ablation inductively coupled plasma mass spectrometry u pb dating of zircon three major volcanic units yielded zircons that were dated calc alkaline rhyolites 285 6 2 6 ma andesites with inherited cores yielded ages c 476 ma and older and voluminous rhyodacites rhyolites 272 7 2 2 ma following an amagmatic sediment starved time gap of c 14 ma alkaline volcanic activity is recorded at the top of the sequence by k alkaline rhyolites dated at 258 5 2 8 ma the ligurian segment of the southern variscan belt records transtensional and then extensional tectonics associated with the volcanic activity the switch from calc alkaline to alkaline activity corresponds to the transition from a post orogenic to an anorogenic setting in the southern variscides it may represent progressive and increasing delamination of the continental lithosphere accompanied by partial melting of the lithospheric mantle supplementary material technical and data acquisition parameters u th pb isotope analyses and calculated ages of zircons from samples and standard and trace element compositions of selected zircons are available at http www geolsoc org uk sup18322

thermal evolution rate of exhumation and tectonic significance of metamorphic rocks from the floor of the alboran extensional basin western mediterranean high grade metamorphic rocks drilled at ocean drilling program site 976 in the alboran sea show a pt path characterized by decompression from about 1050 mpa 40 km depth to 350 mpa 13 km depth accompanied by an increase in temperature from about 550 50 c to 675 25 c the ar ar dating on muscovite and apatite fission track analysis indicate that the final stage of exhumation and cooling occurred very rapidly in the interval 20 5 18 ma which coincides with the initiation of sedimentation in the alboran sea basin the alboran sea formed by miocene extension on the site of a late cretaceous to paleogene contractional orogen and extension coincided with thrusting in the peripheral parts of the betic rif arc which surrounds the basin on three sides thermal modeling of the pt path was carried out with the aim of constraining geodynamic models for the formation of the basin variables considered in the modeling included 1 the thickness and thermal gradient of the postorogenic lithosphere 2 the radiogenic heat production in the thickened crust 3 the time gap pause between the end of contractional tectonics and the start of extension 4 removal of lithospheric mantle below 125 75 or 62 5 km and 5 the rate of extension the only combinations of variables that produce modeled pt paths with the observed characteristics involve high radiogenic heat production combined with a significant postcontractional pause to produce high temperatures in rocks initially at 40 km depth removal of lithosphere below 62 5 km to produce further heating during decompression extension by a factor of 3 in 6 m y to delay the attainment of the maximum temperature until the rocks reached shallow depths and final exhumation and cooling in about 3 3 m y to satisfy radiometric and petrological constraints this gives a maximum of about 9 m y for exhumation from 40 km depth to the surface lithospheric stretching in response to plate boundary forces such as trench rollback without removal of lithosphere cannot explain the late onset of heating and the high temperatures reached by these rocks removal of lithosphere at depths significantly greater than 62 5 km cannot explain the combination of high temperatures reached by these rocks and the shallow depth at which they attained the maximum temperature only a combination of significant postcollisional radiogenic heating then wholesale removal of lithospheric mantle below the orogenic crust followed by rapid stretching can explain the observed pt path these results appear to support some form of lithospheric delamination as the primary cause for the formation of the alboran sea basin

amphibole evolution in variscan eclogite amphibolites from the savona crystalline massif western ligurian alps italy controls on the decompressional p t t path abstract the variscan metamorphic evolution of eclogite amphibolites from the savona crystalline massif is marked by the occurence of calcic amphiboles microtextural relations together with coupled electron microprobe and x ray investigations on amphiboles pointed out a complex decompressional evolution a prograde stage of the evolution in the eclogite facies temperature conditions estimated as ca 620 c is recorded by compositional zoning and inclusions in garnet the eclogitic event pressure conditions 12kbar produced assemblages of garnet omphacite ca amphibole zoisite quartz and rutile composition and zoning of matrix eclogitic amphiboles reveal that the subsequent evolution in the eclogite facies contemplates a late re equilibration under lower conditions of temperature ca 540 c and pressure the eclogitic amphiboles are pargasitic to edenitic hornblendes and are marked by rather high na contents at the m4 site up to 0 5 apfu the forming of coarse grained diopside plagioclase symplectites and of amphibole plagioclase coronas represent the early results of the destabilization of omphacites and garnets in the amphibolite facies pressure conditions of this stage have been estimated of ca 10 kbar while temperature conditions are poorly constrained however the increase in edenite naalsi 1 at the outermost rims of the matrix eclogitic amphiboles and the very high values of edenite and tschermack al 2 mg 1 si 1 components in the coronitic amphiboles subsilicic pargasites suggest that temperature increase accompanied the early decompression to the amphibolite facies subsequent retrogression in the low grade amphibolite facies resulted in the complete breakdown of omphacite producing finer grained symplectites this stage is also recorded by the amphibole growth magnesio hornblendes to edenitic hornblendes with negligible na amounts at the m4 site at the expense of the clinopyroxene symplectites and as outward rims around the coronitic amphiboles frequently amphibolites are completely recrystallized and do not retain any relic of high pressure assemblages temperature conditions for this late amphibolitic event have been estimated of ca 510 c while pressure conditions are inferred in the range 4 7 kbar such a complicated decompressional evolution is indicative of complex uplift tectonics after a subduction event

the late variscan ht lp metamorphic event in nw and central iberia relationships to crustal thickening extension orocline development and crustal evolution

variscan oceans ages origins and geodynamic implications inferred from geochemical and radiometric data abstract a review of the magmatic associations providing evidence for the existence of extensional regimes during the palaeozoic evolution of variscan europe is presented the available geochronological data allow recognition of two major rifting events one in the early ordovician and the other in the middle late devonian scarce relicts of ophiolitic complexes indicate that both extensional episodes reached a true ocean spreading stage it is suggested that the variscan belt resulted from a complex two stage collision process through successive suturing in the silurian and the early carboniferous of these oceanic domains the superposition of two distinct opening closure cycles involving geographically separate oceanic basins may partly account for the unusual width of the variscan orogen

exhumation controlled by transcurrent tectonics the argentera mercantour massif sw alps new apatite fission track aft and u th he ages from the argentera mercantour external crystalline massif of the sw alps document an exhumation history along transcurrent dextral faults significant aft age differences across the strikes of the main faults have been obtained from the ne 12 9 ma to the sw 5 2 ma and are linked to vertical uplifts along these right lateral transpressional structures such segmentation is not observed in ahe ages ahe ages have preserved younger ages 4 5 ma on the eastern hangingwall side of the high durance extensional system such an exhumation history is interpreted to result from a transition of transpressional to transtensional regimes at 8 5 ma during continuing adria europe convergence these data show that recent extensional deformation in the sw alps can be related to the development of a transtensional domain in the ubaye embrunais depression linked to ongoing strike slip deformation along the argentera mercantour

preservation of permo triassic low pressure assemblages in the cretaceous high pressure metamorphic saualpe crystalline basement eastern alps austria metapelites and intercalated metapegmatites of the saualpe crystalline basement which forms part of the austroalpine nappe complex in the eastern alps display a polyphase tectonometamorphic history here we focus on the evolution that these rocks underwent prior to cretaceous eo alpine high pressure metamorphism and related penetrative deformation geothermobarometry on coarse grained porphyroclastic parageneses garnet biotite muscovite plagioclase sillimanite quartz which occur as relics in kyanite garnet two mica gneiss yielded 600 c 0 4 gpa results from a corundum bearing lithology suggest that higher temperatures may have been reached in very restricted areas the matrix of these rocks displays intense recrystallization during a pressure dominated metamorphic overprint microstructures and mineral chemistry indicate that this low pressure metamorphism was the first significant metamorphic imprint in these rocks mineral relics in all metapelitic rock types reflect low pressure conditions for this interkinematic crystallization phase the distribution macroscopic and microscopic observations and the mineralogical composition of intercalated metapegmatites point to regionally elevated temperature conditions during their emplacement therefore pegmatite formation is correlated with mineral formation in metapelites sm nd dating of magmatic garnet from the pegmatite gneiss yielded 249 3 ma which is interpreted to represent the age of pegmatite emplacement and low pressure metamorphism in the metapelites since the pegmatites are overprinted by mylonitisation and high pressure metamorphism this permo triassic age also sets an upper age limit to the eclogite facies metamorphic event which affected considerable parts of the saualpe crystalline basement

lidar measurements of stratospheric aerosol over mauna loa observatory dual wavelength aerosol lidar backscatter measurements at mauna loa observatory are used to monitor and characterize the 15 30 km stratospheric aerosol layer the decay of aerosol loading following the el chichon mexico 17 n and pinatubo philippine islands 15 n volcanic eruptions of 1982 and 1991 respectively depends on the phase of the quasibiennial oscillation qbo in tropical stratospheric winds averaged over a 3 year period these effects are removed and an exponential decay with a characteristic e 1 decay time of about 1 year is observed for both eruptions by the end of 1996 about 5 years after the pinatubo eruption stratospheric aerosol levels at mauna loa had decayed to pre eruption levels approximately matching the lowest aerosol levels seen here in the past 17 years about 6 10 5 sr 1 at 694 nm integrated between 15 8 and 33 km however this background stratospheric aerosol level at mauna loa may depend on the qbo being slightly lower during the westerly phase analyses of aerosol backscatter backscatter wavelength dependence and trajectories provide evidence for a minor injection of aerosol from the rabaul eruption in papua new guinea 4 s in september of 1994

eo alpine eclogitisation of permian morb type gabbros in the koralpe eastern alps austria new geochronological geochemical and petrological data new petrological geochemical and isotopic data sm nd rb sr from meta gabbros eclogites and metapelites from the southern koralpe are presented pl cpx gabbros mostly olivine free that have preserved their magmatic minerals to varying extent occur as relics within eclogites transformation of gabbro into eclogite is documented in different stages ranging from macro to microscale based on textural characteristics the eclogites are classified into four groups types 1 4 typical eclogite parageneses are grt omp zo ky qtz amp phe rut the metapelitic eclogite host rocks are composed of qtz ms bt pl grt ky p t estimates for both metabasites and metapelites range at 1 8 2 0 gpa 600 650 c sm nd mineral isochrons pl cpx for the gabbroic eclogite precursors give ages of between 247 16 and 255 9 ma their initial nd and sr isotopic compositions are clearly within the range of modern depleted mantle values minerals from type 1 metagabbroic eclogites are in clear nd isotopic disequilibrium pervasively recrystallized type 3 eclogites yield grt wr or grt omp zo amp wr ages of 97 5 101 7 and 109 8 ma the metapelitic eclogite host rocks mica schists yield nd dm depleted mantle model ages of 1 44 1 63 ga grt wr isochron ages from these rocks are 86 1 87 4 and 89 3 ma the new data further support the permian early mesozoic rifting event for this part of the austroalpine basement which is related to initial oceanization at the western end of the alpine tethys during alpine plate convergence pre alpine continental basement and oceanic crustal fragments where subducted to depths of c 60 km the most likely time span for the high pressure peak is c 100 10 ma rapid exhumation in the upper cretaceous is inferred from mineral cooling ages between 80 and 60 ma