carbon mass balance isotopic tracers of biogenic methane and the role of acetate in coal beds powder river basin usa abstract a multi isotope c h o investigation of biogenic gas 13c ch4 13c co2 d ch4 and coal bed waters d 18o 13c dic 13c acetate was conducted along a 25 km basinward sampling transect in the powder river basin prb in wyoming and montana usa to address inconsistencies among isotope fingerprinting techniques and to apply mass balance isotope modeling to biogenic coalbed methane cbm results include the first published compound specific 13c acetate values from a natural biogenic cbm system providing new constraints on acetate s role in biogenic ch4 generation coal bed water chemistry is anoxic and dominated by na and hco3 with high alkalinity concentrations 14 8 33 0 meq l the deep basin interior environment contains sulfate free na hco3 dominated waters exhibiting signs of methanogenesis ch4 and co2 generally become 13c enriched from the shallow basin edge to the deep basin interior 13c ch4 78 2 to 56 2 and 13c co2 24 7 to 4 7 also compound specific 13c acetate is more positive in the basin interior compared to the basin edge setting range 34 3 to 15 1 using a steady state mass balance isotope model the observed variations in 13c ch4 and 13c co2 can be explained in terms of the favorability of methanogenesis fch4 relative to heterotrophic bacterially mediated non methanogenic pathways such as bacterial sulfate reduction which compete with methanogenesis for substrates at the basin interior fch4 exceeds 0 4 approaching its inferred limiting value hypothesized from the oxidation state of low molecular weight compounds detected in prb formation waters therefore methanogenesis is likely a dominant biogeochemical pathway consuming substrates in the deeper coal beds the basinward shift toward 13c enriched acetate broadly indicates linkage between acetate ch4 and co2 and likely records acetate synthesis effects modified by methanogenic fractionation these results suggest that acetate plays a role in methanogenesis in prb coal beds not necessarily a dominant role while other c isotope and microbial evidence suggests that hydrogenotrophic methanogenesis is also a significant methane producing pathway the results of this study broadly suggest multiple viable methanogenic pathways and c cycling that affects acetate a more complex view of coal bed geochemistry than implied by traditional c and h isotope fingerprinting of accumulated products ch4 co2 and coexisting water

geodynamic evolution of the sw europe variscides 1 the early evolution of sw europe variscides started by opening of the rheic ocean at 500 ma splitting avalonia from armorica iberia subduction on the se side of rheic generated the paleotethys back arc basin 430 390 ma splitting armorica from iberia with development of porto tomar ferreira do alentejo ptfa dextral transform defining the boundary between continental armorica and finisterra microplate to the w obduction of paleotethys was followed by armorica iberia collision and emplacement of nw iberian allochthonous units at 390 370 ma whereas toward the west of ptfa there was antithetic ophiolite obduction beja acebuches and rheic ophiolites plus finisterra continental slices on top of ossa morena zone with simultaneous development of eclogites and orogenic magmatism under a flake double wedge tectonic regime continued convergence 370 ma proceeded by intracontinental deformation with progressive tightening of the ibero armorican arc through dextral transpression on the cantabrian indentor from iberia to armorica the proposed model is discussed at the light of the driving mechanism of soft plate tectonics

application of carbon isotopes to detect seepage out of coalbed natural gas produced water impoundments abstract coalbed natural gas cbng production from coal bed aquifers requires large volumes of produced water to be pumped from the subsurface the produced water ranges from high quality that meets state and federal drinking water standards to low quality due to increased salinity and or sodicity the powder river basin of northeastern wyoming is a major coalbed natural gas producing region where water quality generally decreases moving from the southeastern portion of the basin towards the center most produced water in wyoming is disposed into impoundments and other surface drainages where it may infiltrate into shallow groundwater groundwater degradation caused by infiltration of cbng produced water holding impoundments into arid soluble salt rich soils is an issue of immense importance because groundwater is a major source for stock water irrigation and drinking water for many small communities in these areas this study examines the potential of using stable c isotope signatures of dissolved inorganic c 13 c dic to track the fate of cbng produced water after it is discharged into the impoundments other geochemical proxies like the major cations and major anions were used in conjunction with field water quality measurements to understand the geochemical differences between cbng produced waters and ambient waters in the study area samples were collected from the cbng discharge outfalls produced water holding impoundments and monitoring wells from different parts of the powder river basin and analyzed for 13 c dic the cbng produced waters from outfalls and impoundments have positive 13 c dic values that fall within the range of 12 to 22 distinct from the ambient regional surface and groundwaters with 13 c dic values ranging from 10 to 14 the results from the study demonstrate that these contrasting 13 c dic signatures can be used to trace seepage out of cbng produced water impoundments into shallow groundwaters

p t t evolution of ultrahigh temperature granulites from the saxon granulite massif germany part ii geochronology the granulites of the saxon granulite massif equilibrated at high pressure and ultrahigh temperature and were exhumed in large part under near isothermal decompression this raises the question of whether p t t data on the peak metamorphism may still be retrieved with confidence felsic and mafic granulites with geochronologically useful major and accessory phases have provided a basis to relate p t estimates with isotopic ages presented in a companion paper the assemblage garnet clinopyroxene in mafic granulite records peak temperatures of 1010 1060 c consistent with minimum estimates of around 967 c and 22 3 kbar obtained from the assemblage garnet kyanite ternary feldspar quartz in felsic granulite multiple partial overprint of these assemblages reflects a clockwise p t evolution garnet and kyanite in the felsic granulite were successively overgrown by plagioclase spinel plagioclase sapphirine plagioclase and biotite plagioclase most of this overprinting occurred within the stability field of sillimanite garnet clinopyroxene in the mafic granulite were replaced by clinopyroxene amphibole plagioclase magnetite the high p t conditions and the absence of thermal relaxation features in these granulites require a short lived metamorphism with rapid exhumation the ages of peak metamorphism 342 ma and shallow level granitoid intrusions 333 ma constrain the time span for the exhumation of the saxon granulites to about 9 ma

correlation of allochthonous terranes and major tectonostratigraphic domains between nw iberia and the bohemian massif european variscan belt the nw iberian allochthon and the tepla barrandian and moldanubian zones represent the internal parts of the variscan belt in their respective domains a correlation based on the lithological association protolith ages metamorphic evolution detrital zircon age spectra and tectonic setting is attempted between the nw iberian massif and se bohemian massif in order to check whether they could have formed part of the same allochthonous stack the galicia tras os montes zone of the iberian massif and the internal zones of the bohemian massif include from bottom to top a parautochthon and lower allochthon representing the outer edge of the northern gondwana margin an oceanic middle allochthon with cambro ordovician and early devonian ophiolites and an upper allochthon interpreted as a peri gondwanan terrane early variscan subduction related high pressure metamorphism characterizes many of the allochthonous units with ages younging from the structurally upper to the lower units from 400 385 ma to 370 360 ma respectively high and ultrahigh pressure metamorphism occurred also in the saxothuringian autochthon at 360 340 ma but not in the nw iberian autochthon the different behavior of the autochthon in the iberian and bohemian massifs accounts for their distinct evolutions from 360 ma onward we conclude that the upper allochthon was a unique peri gondwanan terrane whereas the middle allochthon represents units of the same peri gondwanan ocean opened at the cambro ordovician boundary and having recorded localized renewed activity in the silurian early devonian no other oceans separated the lower allochthon parautochthon and autochthon

variscan tectonics of central europe postaccretionary intraplate deformation of weak continental lithosphere the area occupied by the variscan belt of central europe forms part of gondwana derived microplates essentially avalonia and armorica that according to paleomagnetic data juxtaposed baltica and laurentia from the upper ordovician to the lower devonian the actual structure of this area is a puzzle of rather small fault bounded crustal blocks different in p t t histories which was created by later tectonic processes at around 360 to 320 ma according to a common view and by contrast to paleomagnetic data the structural record of fault bounded blocks is interpreted in terms of mid devonian to lower carboniferous amalgamation of narrow continental plates represented by the fault bounded blocks themselves however this is not supported in this article instead it will be shown that fault systems and fault bounded blocks were created by intraplate deformation of a weak domain of gondwana derived continental lithosphere significantly after its accretion deformation was partitioned into newly formed deep seated strike slip and kinematically connected reverse or normal detachment systems large translation magnitudes caused juxtaposition of crustal areas of variable thickness until they were emplaced along the fault systems the various categories of fault bounded complexes record different tectonic events category 1 metamorphic complexes represent deep seated parts of thickened crust some time before unroofing on the detachment systems between about 340 360 and 320 ma however the record of subduction and extreme crustal thickening during earlier stages of the collision history was erased to a high degree and is only preserved by mineral and structural relics strike slip and detachment systems propagated across and dismembered tectonic boundaries formed during these early events category 2 metamorphic complexes and the barrandian basin recording predominant pre late devonian metamorphic or stratigraphic events represent the brittle upper crustal parts during continuing metamorphism in the category 1 complexes weakly or unmetamorphosed lower paleozoic category 3 basins represent areas of thinned continental crust and record subsidence before and to some degree during activity of detachment systems the complex overall kinematic pattern of block bounding strike slip and detachment systems indicates compression and extension acting simultaneously this probably reflects the effects of continuing oblique plate convergence rotation and wsw extrusion of fault bounded blocks in addition to heating and extensional flow of previously thickened crustal areas

core complex geometries and regional scale flow in the lower crust a simple flexural model that treats the lithosphere as a thin elastic plate overlying a fluid asthenosphere produces an excellent fit to the domal detachment fault geometries observed in typical core complexes of the basin and range province however this mode of isostatic compensation requires an unacceptably large reduction in local topographic elevation 3 5 km above core complex structures because topographic lows of this magnitude are not observed above core complexes today it requires mountain ranges 3 5 km higher than the surrounding areas to have existed at the site of each future core complex and requires these mountain ranges to have had large crustal roots that were subsequently uplifted to form a flat moho as the core complexes developed an alternate mode of isostatic compensation whereby compensation occurs primarily by regional scale flow of material within the lower crust beneath an elastic midcrust provides an equally good fit to the observed detachment fault geometries while requiring little reduction in local topographic elevation above developing core complex structures and providing an obvious means of maintaining a flat moho beneath these zones of upper crustal doming if this interpretation is correct then the effective elastic plate thickness below the basin and range province probably cannot exceed a maximum of about 4 km with a best guess estimate of about 0 5 to 1 km computation of the temperature structure associated with crustal flow during doming shows that an increase in temperature at the moho of about 100 c follows doming by several million years perhaps explaining the observed coincidence of core complexes with crustally derived igneous rocks subsequent cooling of the mid to lower crust beneath the core complex could result in freezing of the initially ductile lower crust within the core of the dome effectively locking the domal shape into the crust even after the detachment surface is cut by younger faults

age geodynamic setting and mantle enrichment processes of a k rich intrusion from the meissen massif northern bohemian massif and implications for related occurrences from the mid european hercynian the plutonic complex of the meissen massif northern margin of the bohemian massif comprises dioritic to mainly monzonitic and granitic rocks the diorite to monzonite intrusions show major and trace element patterns typical for shoshonitic series the chemical signatures of less crustally contaminated diorites are similar to arc related shoshonitic rocks derived from continental lithospheric mantle clm sources previously enriched by subduction of altered oceanic crust laser step heating 40ar 39ar analyses on actinolitic to edenitic amphiboles from geographically different occurrences of the monzonitic intrusion yielded concordant plateau ages as well as total gas ages ranging from 329 1 1 4 to 330 4 1 4 ma and from 330 4 2 1 to 330 6 1 8 ma respectively these cooling ages are indistinguishable from sensitive highresolution ion microprobe shrimp 238u 206pb intrusion ages measured on magmatic zircon rims from the monzonite nasdala et al submitted this shows that the monzonite intrusion is probably not related temporally to active subduction because it postdates eclogites of the adjacent saxonian erzgebirge by approximately 20 ma the shoshonitic magmas intruded during strike slip tectonism along the elbe valley zone the enrichment of their mantle sources may be of upper devonian lower carboniferous age or older intrusions of shoshonitic to ultra potassic k rich rocks during the upper visean namurian are widespread in the moldanubian zone based on similar ages and structural relationships a similar post collisional setting to the meissen shoshonitic rocks can be demonstrated most of these occurrences cut high grade nappe units which were subducted during the upper devonian lower carboniferous in contrast to the meissen massif at least the ultra potassic members of the central and the south bohemian batholiths were derived from clm sources enriched by fluids or melts released from subducted oceanic crust and by greater portions of crustal material despite the similar post collisional geodynamic setting of the k rich intrusions different enrichment processes generated mid european hercynian clm sources with heterogeneous major and trace element and isotopic signatures

topography of the variscan orogen in europe failed not collapsed the variscan orogenic collage consists of three subduction collision systems rheno hercynian saxo thuringian and massif central moldanubian devonian to early carboniferous marine strata are widespread not only in the individual foreland fold and thrust belts but also in post tectonic basins within these foreland belts and on the cadomian crust of peri gondwanan microcontinental fragments which represent the upper plates of the subduction collision zones these marine basins preclude high elevations in the respective areas and also in their neighbourhood widespread late carboniferous intra montane basins with their coal bearing sequences are likewise incompatible with high and dry plateaus while narrow belts with high elevations remain possible along active margins within the orogen comparison of the variscides with the himalaya tibetan plateau is unfounded plausible reasons for the scarcity of high variscan relief include subduction of oceanic and even continental crust subduction erosion orogen parallel extension and most important lithospheric thinning accompanied by high heat flow and magmatism in many areas timing and areal array of magmatism and ht metamorphism are not compatible with a model of tectonic thickening and subsequent gravitational collapse it is suggested instead that lithospheric thinning and heating are due to mantle activities caused by the tethys rift the lower and middle crust were thermally softened and rendered unfit for stacking and isostatic uplift in terms of topography the variscides represent a failed orogen the ht regime also explains the abundance of granitoids and ht lp metamorphic rocks typical of the variscides melting in the ht regime extracted mafic components from variscan and cadomian crust as well as from cadomian metasomatized lithospheric mantle thus mimicking subduction related magmatism the onset of the ht regime at c 340 ma may also have triggered the final ascent of hp uhp felsic metamorphic rocks

the palaeozoic variscan oceans revisited abstract geological evidence supported by biogeographical data and in accord with palaeomagnetic constraints indicates that one ocean models for the variscides should be discarded and confirms instead the existence of three gondwana derived microcontinents which were involved in the variscan collision avalonia north armorica franconia and thuringia subdivided by a failed vesser rift and south armorica central iberia armorica bohemia all divided by small oceans in addition parts of south eastern europe including adria and apulia are combined here under the new name of palaeo adria which was also peri gondwanan in the early palaeozoic oceanic separations were formed by the break up of the northern gondwana margin from the late cambrian onwards most of the oceans or seaways remained narrow but much like the alpine cenozoic oceans gave birth to orogenic belts with hp uhp metamorphism and extensive allochthons the saxo thuringian ocean between north and south armorica and the galicia moldanubian ocean between south armorica and palaeo adria only the rheic ocean between avalonia and peri gondwana was wide enough to be unambiguously recorded by biogeography and palaeomagnetism and its north western arm closed before or during the emsian in europe ridge subduction under the northernmost part of armorica in the emsian created the narrow and short lived rheno hercynian ocean it is that ocean and not the rheic whose opening and closure controlled the evolution of the rheno hercynian foldbelt in south west iberia south west england germany and moravia czech republic devonian magmatism and sedimentation set within belts of early variscan deformation and metamorphism are probably strike slip related the first arrival of flysch on the forelands and or the age of deformation of foreland sequences constrains the sequential closure of the variscan seaways galicia moldanubian in the givetian saxo thuringian in the early famennian rheno hercynian in the tournaisian additional mid to late devonian and partly early carboniferous magmatism and extension in the rheno hercynian saxo thuringian and galicia moldanubian basins overlapped with variscan geodynamics as strictly defined the early carboniferous episode was the start of episodic anorogenic heating which lasted until the permian and probably relates to tethys rifting

water rock interaction and methanogenesis in formation water in the southeast huaibei coalfield china abstract the hydrogeochemical characteristics of aquifers in coal bearing strata can provide an important foundation for understanding the evolution of the aquifers and recognizing the formation process and occurrence of biogenic gas by analyzing the hydrogeochemical characteristics of water produced from the cenozoic the limestone and the coal strata aquifers and the coalbed gas wells this study focused on water rock interactions and methanogenesis in the formation water of the suzhou mining area huaibei coalfield china tectonically this is one of the most complex coalfields in china the dissolution of evaporites silicates and carbonates controlled the chemical composition of the formation water the cenozoic aquifers have a relatively stable water quality type and balanced ionic composition but more complex water rock interactions occur in the coal strata and the limestone aquifers extensive methanogenic activity and methanogenesis could be recognized by the meteoric water recharge positive 13 c signatures for dissolved inorganic carbon dic 15 4 26 2 lower sulfate concentrations

mid crustal flow during tertiary extension in the ruby mountains core complex nevada structural analysis and geochronologic data indicate a nearly orthogonal late eocene oligocene flow pattern in migmatitic infrastructure immediately beneath the kilometer thick extensional mylonitic shear zone of the ruby mountains metamorphic core complex nevada new u pb radiometric dating indicates that the development of a northward trending lineation in the infrastructure is partly coeval with the development of a pervasive west northwest trending lineation in the mylonitic shear zone u pb monazite data from the leucogranite orthogneiss of thorpe creek indicate a crystallization age of ca 36 39 ma zircon fractions from a biotite monzogranite dike yield an age of ca 29 ma the three dated samples from these units exhibit a penetrative approximately north south trending elongation lineation this lineation is commonly defined by oriented bundles of sillimanite and or elongated aggregates of quartz and feldspar indicating a synmetamorphic and syndeformational origin the elongation lineation can be interpreted as a slip line in the flow plane of the migmatitic nonmylonitic infrastructural core of the northern ruby mountains a portion of this midcrustal flow is coeval with the well documented west northwest sense of slip in the structurally overlying kilometer thick mid tertiary mylonitic shear zone lineations in the mylonitic zone are orthogonal to those in the deeper infrastructure suggesting fundamental plastic decoupling between structural levels in this core complex furthermore the infrastructure is characterized by overlapping oppositely verging fold nappes which are rooted to the east and west one of the nappes may be synkinematic with the intrusion of the late eocene orthogneiss of thorpe creek in addition the penetrative elongation lineation in the infrastructure is subparallel to hinge lines of parasitic folds developed synchronous with the fold nappes suggesting a kinematically related evolution the area is evaluated in terms of a whole crust extension model magmatic underplating in the lower crust stimulated the production of late eocene early oligocene granitic magmas which invaded metasedimentary and mesozoic granitic rocks of the middle crust the midcrustal rocks weakened by the magmatic heat influx acted as a low viscosity compensating material decoupled from an extending upper crust the fold nappes and lineation trends suggest large scale flow of the weakened crust into the study area the inflow pattern in the migmatitic infrastructure can be interpreted as a manifestation of midcrustal migration into an area beneath a domain of highly extended upper crustal rocks at present the inferred eocene early oligocene phase of upper crust extension remains unknown but available data on relative and geochronologic timing are not inconsistent with our model of return flow into an area already undergoing large scale upper crustal extension

formation burial and exhumation of basement nappes at crustal scale a geometric model based on the western swiss italian alps abstract information from geological and reflection seismic data from the western swiss italian alps and from numerical models is used to build a geometrical model that can explain some of the major tectonic metamorphic features of alpine type basement nappes the model gives a geometrical and mechanical explanation for the initiation burial and subsequent uplift and partial exhumation of basement nappes at a crustal scale three main tectonic stages during convergence are distinguished and each correlated with the formation of specific nappe structures the first two stages are single vergent nw and correspond to the subduction of continental margin crust and the formation and uplift of high pressure rocks simple shear flow and superimposed wedge shaped pure shear flow is proposed for the creation and intrusion of high pressure nappes of the monte rosa type the third stage is characterized by a doubly vergent style with both pro and retro movements the former created nw vergent nappes as seen in the external alpine massifs and the latter caused backfolding and thrusting typical of the internal alps this third stage corresponds to the neoalpine movements in the western swiss italian alps and is accompanied by a generalized uplift mountain building and molasse sedimentation

timescales of exhumation and cooling inferred by kinetic modeling an example using a lamellar garnet pyroxenite from the variscan granulitgebirge germany we present a numerical modeling approach to infer timescales of the exhumation and cooling history recorded in the chemical composition of minerals in a garnet pyroxenite from the granulitgebirge saxony germany the studied sample contains remarkable exsolution textures from former megacrysts that produced up to millimeter wide alternating lamellae of garnet grt and clinopyroxene cpx compositional profiles of major and minor elements measured with the electron microprobe perpendicular to the grt cpx interfaces reveal systematic zoning patterns for fe mg al si cr ti in clinopyroxene and ca fe mg mn in garnet respectively in addition to simple thermal modeling that is used to constrain the conditions of emplacement of the granulitgebirge massif at shallow crustal levels we combine thermodynamic data with a numerical finite difference scheme that simulates growth and simultaneous diffusive exchange between garnet and clinopyroxene along a virtual cooling path the latter model assumes local equilibrium at the interface diffusive fluxes are constrained by mass balance it is shown that zoning patterns such as fe mg exchange between garnet and clinopyroxene can be used to extract cooling rates and thus timescales of exhumation while the profiles for the minor elements are provisionally related to the growth history of the lamellae furthermore zoning profiles in the lamellae can only be reproduced with ultrahigh cooling rates similar to contact metamorphic conditions this in turn suggests that the massif was emplaced at temperatures above 900 c in agreement with the observed spatial extent of a contact aureole within low grade metasedimentary rocks surrounding the granulite massif as predicted by thermal modeling exhumation of the massif without cooling below 900 c requires an exhumation rate of several centimeters per year thus we propose an almost isothermal exhumation period of 1 ma followed by isobaric cooling from 900 to 600 c within less than 10 ka

structure of the saxonian granulites geological and geophysical constraints on the exhumation of high pressure high temperature rocks in the mid european variscan belt

white mica 40ar 39ar ages of erzgebirge metamorphic rocks simulating the chronological results by a model of variscan crustal imbrication

quantitative analysis of detrital modes statistically rigorous confidence regions in ternary diagrams and their use in sedimentary petrology abstract detrital modes are the basic type of quantitative data used in sedimentary petrology they represent estimates of the petrographic framework composition of sand or sandstone obtained by point counting of thin sections at present geological inferences from analysis of these numerical data are semi quantitative only because many of the data processing methods employed in sedimentary petrology lack a firm theoretical basis a review of existing methods reveals a number of fundamental statistical problems associated with the use of univariate statistics and the construction of so called hexagonal fields of compositional variation it is shown that these problems can be overcome by using multivariate methods that honour the non negativity and unit sum constraints on compositional data and incorporate the covariance structure spread in compositional data can be modelled on two levels 1 by regarding the grain as unit of observation and the total point count as a sample from a multinomial distribution 2 by regarding the total count as unit of observation and a set of counts as a sample from an additive logistic normal distribution numerical methods are presented for constructing statistically rigorous confidence regions in ternary diagrams by inversion of significance tests for these two distributions statistical analyses of the framework composition and heavy mineral assemblages of modern beach and river sands illustrate their use the case histories are followed by a brief overview of popular multivariate methods for reconstructing sediment provenance

computer simulations of california tectonics confirm very low strength of major faults recent improvements in the technique of modeling fault networks with thin plate finite elements yield full convergence of the solutions without any compromise in the representation of the frictional rheology of the upper crust or the transition to dislocation creep in the lower crust we apply these techniques to model the california region incorporating all faults with estimated slip rate over 1 mm yr as well as variations in elevation heat flow and crustal thickness velocity boundary conditions on the model sides are based on the nuvel 1 plate model and an approximation of deformation in the great basin the frictional and dislocation creep rheologic constants of the crust are calibrated to reproduce the observed variations in the maximum depth of seismicity which occurs at model temperatures of 350 410 c this leaves two free parameters the time averaged coefficient of friction on faults and the apparent activation energy for creep in the lower crust these parameters are systematically varied in three sets of 10 81 and 64 experiments respectively the predictions of each model are tested against three published data sets a set of 79 geologic limits on average slip rate on faults a set of 221 principal stress directions and a set of 841 secular rates of geodetic baselines both trilateration and vlbi very long baseline interferometry the patterns of model scores indicate that the time averaged friction coefficient of major faults is only 0 17 0 25 or only 20 30 of the value 0 85 that is assumed for the friction in the intervening blocks the laser trilateration data taken alone would indicate a preference for a high stress model but this model is one with very little fault slip and seismicity which can be rejected on other grounds in the final three parameter set of 64 models we test whether the anomalous weakness of faults is uniform or proportional to net slip with slip dependent weakening there is a general reduction in prediction errors which supports the hypothesis the geologic data and geodetic data agree in implying that fault weakness that is friction of 0 17 is general with an additional 30 slip dependent weakening of the san andreas to friction of 0 12 or 14 of normal the stress data however are fit best if all weakness is slip dependent effectively only the san andreas is weak thus the hypothesis that all weakening is slip dependent should not be rejected the best models predict all available data with rms root mean square mismatch of as little as 3 mm yr 6 of the relative plate velocity so their predictions may be useful for seismic hazard estimation at least along faults where no data are available this study extends the finding of very low friction from subduction zones where it was previously documented to a dominantly strike slip system in which underthrusting of wet sediment is not widespread return of previously subducted water from the mantle and its chemical or physical binding in fine grained gouge are possible explanations for the very low friction

multi tracer and hydrogeophysical investigation of the hydraulic connectivity between coal seam gas formations shallow groundwater and stream network in a faulted sedimentary basin abstract characterization of geological fault networks in sedimentary basins targeted for coal seam gas development typically use deep penetrating seismic data such methods can reveal the presence of numerous fault zones at depths greater than 100 m but not necessarily show if these extend to the surface or provide a significant degree of hydraulic connectivity between deep and shallower geological formations our combined multi tracer and hydrogeophysical study used transient electromagnetic surveys to image the subsurface in a sedimentary coal seam gas basin gloucester basin nsw australia and indicated that deep faults greater than 200 m from seismic analysis may extend to within metres of the ground surface analysis of environmental tracers in surface water and groundwater samples near fault traces showed the presence of hotspots of deeper groundwater from the coal seams discharging into the stream network via the alluvial aquifer the isotopic composition of methane was extremely variable in groundwater 13c ch4 from 81 6 to 29 2 v pdb and surface water 13c ch4 from 56 0 to 19 9 v pdb suggesting several sources of methane being recycled in the connected alluvial aquifer and stream network a 4he hotspot 1 67 10 6 cm3 stp g 1 in the alluvial aquifer close to a fault line showed high methane 463 g l 1 with a thermogenic signature 13c ch4 of 62 4 and 2h ch4 of 174 7 the gloucester basin which has experienced multiple episodes of complex restructuring displays high dilation tendency of fractures close to fault damage zones with fracture orientations parallel to the orientation of maximum horizontal stress as a result there is increased permeability for wells close to the fault damage zone and with increasing depth these findings resulted in a robust hydrogeological conceptual model of a faulted basin which can be used in a monitoring and modelling framework for improved evaluation of risk associated with unconventional gas developments

exotic crustal components at the northern margin of the bohemian massif implications from uthpb and hf isotopes of zircon from the saxonian granulite massif abstract the saxonian granulite massif is located at the northern margin of the saxo thuringian zone of the peri gondwana bohemian massif eight felsic and mafic granulites were studied with respect to their geochemistry and u pb zircon geochronology the felsic granulites are interpreted to be derived from continental crust of possible granitoid composition an origin from depleted mantle sources with iat to morb composition can be assumed for the mafic granulites the peak of metamorphism is thought to be timed at about 340 ma while several earlier metamorphic events are supposed to have occurred at about 355 360 370 375 405 and 450 ma they reveal a complex and polyphased geologic evolution of the saxonian granulite massif protolith emplacement likely took place at c 450 and 494 ma hf isotopic data suggest mesoproterozoic crustal ages at least for parts of the massif as these crustal ages are exotic for the bohemian massif their origin has to be searched elsewhere potential source areas could be amazonia and baltica of which the latter is the one preferred furthermore a composite architecture with at least two components the felsic granulites with mesoproterozoic crustal model ages and the mafic granulites of potential island arc origin is hypothesised their amalgamation to the recent appearance of the saxonian granulite massif is likely bracketed between 375 and 340 ma

effect of metamorphic reaction history on the u pb dating of titanite drill core samples of garnet clinopyroxene granulite at tirschheim and a reference sample at waldheim saxon granulite massif germany endured the same p t conditions but developed variable mineral assemblages due to differences in bulk chemistry reaction progress deformation and retrogression titanite formed during peak metamorphic conditions of 22 24 kbar and 1020 1050 c dating titanite from the various samples should yield the same age for all the observed age variation which exceeds the duration of the entire metamorphic cycle originates from the contrasting preservation of isotopic inheritance during peak metamorphism and from post peak re equilibration 1 pb inheritance observed in some peak metamorphic titanite demonstrates that geochronologically relevant elements are redistributed among remaining reactants and reaction products during prograde metamorphism and that the sequence of metamorphic reactions does not result in isotopic homogenization instead metamorphic minerals inherit the radiogenic signatures of the precursor minerals and may in extreme cases approach the age of the precursor mineral 2 titanite that formed at peak metamorphic conditions is characterized by high a1 contents and x f 0 8 1 texturally comparable titanite that re equilibrated during cooling reduced al contents and x f yields too young u pb ages the age of such re equilibrated titanite does not correspond to the age of the event indicated by the texture

exhumation and metamorphism of an ultrahigh grade terrane geochronometric investigations of the sudete mountains bohemia poland and czech republic the sudete mountains ne bohemian massif czech republic and poland preserve abundant eclogitic and granulitic centimetre to decimetre scale boudins enveloped in a predominantly migmatitic matrix published geochronometry and thermobarometry from the uhp and uht rocks broadly constrain the crystallization and initial stage 1 exhumation history for these units however the timing of stage 2 metamorphism and associated unroofing is less well constrained new in situ ion microprobe th pb monazite results together with complementary u pb zircon and electron microprobe analyser total pb monazite results on 11 amphibolite facies gneissic to migmatitic samples place important temporal constraints on the second stage of uhp and uht metamorphism exhumation the orlica snieznik dome records uhp metamorphism occurring at 375 ma and subsequent exhumation to midcrustal levels in supra barrovian conditions at c 345 330 ma in contrast the western gory sowie block preserves evidence of hp granulite conditions at c 400 ma and exhumation to mid crustal levels at 380 370 ma revealing a c 30 million years difference in exhumation events between the neighbouring terranes the eastern gory sowie block preserves ages similar to the orlica sneiznik dome suggesting that different preserved metamorphic cooling histories are juxtaposed across the sudetic marginal fault the bounding niemcza shear zone yields preliminary th pb dates that range from 380 8 ma to 283 2 ma preserving a protracted metamorphic record that spans the exhumation history of the region the distinct collapsed geochronologies of both terranes probably reflect rapid vertical transport of low viscosity crust under supra barrovian conditions near the mid crustal high strength lid during oblique transpressional convergence

compositional evolution and substitutions in disseminated and nodular tourmaline from leucocratic granites examples from the bohemian massif czech republic two distinct textural types of tourmaline were distinguished in leucocratic granites of the bohemian massif moldanubicum saxothuringicum i commonly euhedral disseminated tourmaline dt crystallized in relatively early stage of the granite consolidation and ii typically interstitial nodular tourmaline nt formed during the stage transitional from late solidus to early subsolidus crystallization very similar chemical composition and similar fractionation trends in both dt and nt tourmaline types indicate crystallization in a quasi closed system from early solidus to early subsolidus stage of granite consolidation the substitutions in tourmaline from nt granites in the moldanubicum are more similar to the substitutions in tourmaline from li poor granitic pegmatites in the same region relative to tourmaline from dt granites plotting of emp analyses of tourmaline in diverse diagrams strongly suggests using combination of ternary diagrams al fe mg and na ca xsite vacancy and simple plots involving single cations elements such as na al f na fe mg rather then more complicated diagrams e g r1 r2 r3 to characterize chemical composition and to derive substitution mechanisms in ca poor tourmaline from granitic rocks the complex diagrams e g r1 r2 versus r3 do not enable to investigate compositional evolution in the xsite and wsite properly and oversimplifies the real substitutions we recommend using specific diagrams for specific tourmaline compositions e g carich lirich

intracrustal subduction and gravity currents in the deep crust sm nd ar ar and thermobarometric constraints from the skagit gneiss complex washington isotopic and thermobarometric pressure temperature p t data on a gneissic quartz diorite combined with previous u pb ages and p t data on wall rock metapelites define a precise p t time series for a portion of the skagit gneiss complex a deep crustal crystalline complex within a zone of intracontinental collision concordant u pb and sm nd ages on zircon indicate crystallization at 68 ma which preceded metamorphism of nearby pelitic wall rocks garnet core assemblages and the pluton plagioclase hornblende coronas on garnet at 800 900 mpa 700 800 c following nearly isothermal decompression and sillimanite cordierite metamorphism of wall rocks garnet rims plus matrix at 300 500 mpa 650 700 c a sm nd mineral isochron including two garnet separates records an initial phase of cooling of the complex at 60 ma slow cooling 10 c m y extends from approximately 60 ma to 50 ma hornblende and biotite ar ar ages of 47 ma and 45 ma respectively define a second unroofing event cooling rates during this period were approximately 100 c m y late cretaceous deep burial followed by two distinct unroofing episodes is also observed in some cordilleran metamorphic core complexes and may suggest a common origin we propose that regional shortening in the cordilleran interior is accommodated by a process of intracrustal subduction i e the upper crust is forced downward into a crustal asthenosphere resulting in a cold buoyant root heating and weakening of the root cause the formation of gravity currents effective viscosity being in the range of 10 16 10 19 pa s spreading outward below nonfluid upper crust spreading of the gravity current which does not necessarily contribute to crustal thinning is driven by the small density contrast between the root and its mid crustal surroundings hence unroofing is neither a response to overthick crust nor controlled by the orientation of principal stress axes in the upper crust and upper mantle the root then cools slowly until final unroofing to near surface levels which may occur via either extension or shortening with consequent erosion

further evidence for an early carboniferous 340 ma age of high grade metamorphism in the saxonian granulite complex u pb and pb pb zircon ages for metamorphic zircons from granulites in the saxonian granulite complex are reported using the shrimp ion microprobe conventional multigrain and single gain techniques and the evaporation method this is complemented by a pb pb evaporation age for a post granulite granite emplaced into the schist mantle around the granulites during uplift of the complex we also demonstrate that zircon ages are not reset during high grade metamorphism as commonly argued but have a very high closure temperature and usually preserve the isotopic composition reflecting the time of their formation multifaceted zircons from four granulite samples that probably grew close to the peak of high grade metamorphism yielded identical u pb and pb pb ages of 340 ma which support previously published data and unambiguously show that the granulites formed during a lower carboniferous event and not in the early palaeozoic or precambrian as previously suggested older cores in some of the metamorphic zircons reveal early palaeozoic components at 470 485 ma that we interpret as ages reflecting magmatic crystallization of the granulite precursors one sample suggests an inherited component as old as 1700 ma the post granulite granite has a pb pb evaporation age of 333 1 1 0 ma and the short time interval between granulite metamorphism and granite intrusion implies that uplift crustal extension and cooling of the granulite complex occurred rapidly after peak metamorphic conditions

the effect of lower crustal flow on continental extension and passive margin formation the great variety of styles of continental extension may reflect different crustal thickness and thermal states of continental lithosphere during the initiation of rifting to investigate how these and other factors affect rifting and the development of passive continental margins we develop a simplified model of lithospheric extension we consider the evolution of extensional deformation for a three layer model lithosphere bounded laterally by much stronger lithosphere the cold part of the crust and mantle are treated as thin brittle plastic layers the lower crust is approximated as a thin viscous channel each brittle plastic layer can extend in only one location determined by the strength of the layer shear of the lower crust and buoyancy forces related to both crustal thickness variations and thermally induced density differences the lower crust flows in response to crustal thickness variations and is sheared when the loci of extension for the two brittle plastic layers are horizontally offset a situation we term shear decoupling as in previous studies we see three distinct patterns or modes of extensional deformation that occur under different sets of model parameters the core complex mode the wide rift mode and the narrow rift mode shear decoupling occurs only in cases with a crustal rheology at the weak end of the spectrum of laboratory estimated values we are aware of no observations that require that the upper crust and upper mantle strain at laterally displaced positions we show that for large magnitudes of extension there can be transitions between modes as inferred for some highly extended continental areas predicted patterns of crustal thickness and heat flow for some models are similar to observations at several rifted continental margins including very wide and asymmetric margins

fold cleavage relationships as indicator for late variscan sinistral transpression at the rheno hercynian saxo thuringian boundary zone central european variscides abstract the boundary between the rheno hercynian and the saxo thuringian zones of the european variscides is characterized by a ne sw striking late orogenic fold and thrust belt affecting the intervening rheic suture classical models used the first order strike of this zone as an indicator for perpendicular plate convergence i e nw se we present structural data from both sides of the suture focusing on fold cleavage relationships the statistical analysis reveals an orientation maximum of the youngest cleavage that deviates from the strike of the fold and thrust belt by c 22 the presence of clockwise transection of the folds by the cleavage up to 16 indicates pervasive sinistral transpression three types of fold cleavage relationships are observed ne trending folds i with or ii without a transecting cleavage and iii non transected ene trending folds we explain the occurrence of different fold cleavage types by strain partitioning due to nnw convergence obliquely to pre existent ne trending mechanical anisotropies in terms of plate tectonics we propose that the classical boundary of the rheno hercynian and the saxo thuringian zone represents an initial transform plate boundary that was finally affected by sinistral transpression

palaeozoic terrane amalgamation in central europe a ree and sm nd isotope study of the pre variscan basement ne bohemian massif

the saxonian granulite massif new aspects from geochronological studies geochronological and geochemical analyses were carried out to determine the age of protolith formation and the timing of eclogite and granulite metamorphism of the saxonian granulite massif sgm the age of the ultramafic protoliths of the garnet pyroxenites from the rim of the sgm has been determined to be late riphean 828 ma which together with geochemical and isotope data implies the extraction of the melt from an enriched mantle reservoir with an initial nd value of 3 4 the minimum emplacement age of the mafic to felsic granulites has been dated at 470 ma the source material of these granulites reflects an enriched mantle reservoir with an nd value of 2 6 470 ma

carbonate budget of a cold water coral mound challenger mound iodp exp 307 abstract cold water coral ecosystems represent a worldwide distributed carbonate factory which is so far not considered in global carbonate budget estimations the herein presented study evaluates the carbonate budget of a complete cold water coral mound sequence from challenger mound belgica mound province porcupine seabight sw offshore ireland drilled during iodp expedition 307 reaching back to the late pliocene versus background sediment derived calcite dominated carbonate cores from adjacent drift deposits were examined for comparison studies challenger mound which developed in a fast growth phase unit m1 2 7 to 1 6 ma and a slower growth phase or mound decline unit m2 0 8 ma until today separated by a so called mound crisis displays increased carbonate accumulation rates for both phases 17 3 g cm 2 ka for unit m1 and 5 7 g cm 2 ka for unit m2 compared to adjacent drift deposits but only about 33 to 40 wt of the carbonate is derived from corals bulk sedimentation rates of on and off mound deposits suggest a decline of challenger mound since the onset of drift deposition in the area about 1 2 ma ago yet total carbonate accumulation rates as well as hemipelagic background sediment derived carbonate accumulation rates from challenger mound unit m2 still exceed the carbonate accumulation rates of the adjacent drift deposits the selective enrichment of background sediment derived carbonate by a factor of 2 0 in a cwc mound is herein observed for the first time this might be caused by the different hydrodynamic behaviour and slower settlement of marine fluff rich in calcareous detritus compared to siliciclastic sediments and its effective baffling by the coral framework and or the active catch of the calcareous plankton by the corals the carbonate accumulation rates of challenger mound are about 4 to 12 of the carbonate accumulation rates of tropical shallow water reefs but exceeds the carbonate accumulation rates of continental slopes by a factor of 3 9 to 11 8 the carbonate production for the entire belgica mound province is estimated with 3 2 10 3 t a for unit m1 and 1 0 10 3 t a for unit m2

time of formation and peak of variscan hp ht metamorphism of quartz feldspar rocks in the central erzgebirge saxony germany the variscan erzgebirge represents an antiform with a core of gneisses and mica schists surrounded by a phyllitic mantle the gneiss eclogite unit geu in the central part is a composite tectonometamorphic assemblage characterized by a hp ht imprint and comprises migmatitic para and orthogneisses ht mylonites hp granulites eclogites and garnet peridotites it is tectonically sandwiched between two major units with distinctly lower pt histories the geu experienced a characteristic kinked retrograde pt path after hp ht equilibration with 1 strong near isothermal decompression at high temperatures 2 extensive re equilibration at medium pressures followed 3 by rapid cooling during continued uplift we dated zircons pb pb evaporation from granitoid orthogneisses and metapelites of the geu the orthogneisses contain euhedral long prismatic zircons of igneous origin that provided protolith ages between 470 and 524 ma metapelites retain well preserved granulite facies mineral assemblages and contain spherical multifaceted metamorphic zircons that grew near the peak of hp ht metamorphism inclusions of prograde hp phengite 15 kbar and rutile are included in one such zircon metamorphic zircons of three samples from different localities yielded identical 207pb 206pb ages of 340 5 0 7 ma 341 2 0 5 ma and 341 6 0 5 ma respectively consideration of these zircon ages with published 39ar 40ar white mica ages suggests fast cooling and uplift rates in excess of 50 c ma and 4 km ma this is typical for large scale extensional tectonic unroofing of the ultra deep part of a fossil thickened variscan continental crust 60 km during continuing continental collision and orogenic collapse

accretion and exhumation at a variscan active margin recorded in the saxothuringian flysch the saxothuringian flysch basin on the north flank of the central european variscides was fed and eventually overthrust by the northwestern active margin of the tepla barrandian terrane clast spectra mineral composition and isotopic ages of detrital mica and zircon have been analyzed in order to constrain accretion and exhumation of rocks in the orogenic wedge the earliest clastic sediments preserved are of early famennian age ca 370 ma they are exposed immediately to the nw of the suture and belong to the par autochthon of the foreland besides ultramafic ophiolite material these rocks contain clasts derived from early paleozoic continental slope sediments originally deposited at the nw margin of the saxothuringian basin these findings together with the paleogeographic position of the famennian clastics debris on the northwestern passive margin indicate that the saxothuringian narrow ocean had been closed by that time microprobe analyses of detrital hornblendes suggest derivation from the randamphibolit unit now present in the middle part of the saxothuringian allochthon munchberg nappes detrital zircons of metamorphic rocks formed a little earlier ca 380 ma indicate rapid recycling at the tectonic front the middle part of the flysch sequence ca early to middle visean both in the par autochthon and in the allochthon contains abundant clasts of paleozoic rocks derived from the northwestern slope and rise together with debris of cadomian basement 500 ma granitoids and 380 ma early variscan crystalline rocks all of these source rocks were still available in the youngest part of the flysch c middle to late visean but some clasts record in addition accretion of the northwestern shelf our findings permit deduction of minimum rates of tectonic shortening well in excess of 10 30 mm per year and rates of exhumation of ca 3 mm a and possibly more

plutonism and the origin of metamorphic core complexes the formation of metamorphic core complexes may be triggered by plutonic activity during episodes of continental extension pulses of ductile deformation have taken place during short lived thermal events initiated by the heat input from intruded plutons sills or dikes such intrusions may be the underlying cause for differential uplift of the footwall during tectonic denudation of metamorphic core complexes fast cooling inferred from 40 ar 39 ar apparent ages may have taken place after periods of magma arrival and need not be the result of rapid erosional or tectonic denudation heterogeneity of 40 ar 39 ar apparent ages can be explained by rapid cooling of deforming mylonites formed at shallow crustal levels in the thermal aureoles of intruded sills or sill swarms under such circumstances apparently isothermal decompression paths may actually link pressure temperature points set during periods of transient mineral growth during and after periods of igneous activity

evolutionary model for the taiwan collision based on physical modelling abstract 2 d and 3 d physical modelling of lithospheric convergence in the luzon taiwan ryukyu region is performed with properly scaled laboratory models the lithospheric model consists of two parts continental the asian plate ap and oceanic the philippine sea plate psp the oceanic lithosphere has one layer while the continental lithosphere includes both mantle and crustal layers the continental margin is covered by sediments a low viscosity asthenosphere underlies the lithosphere the opposing luzon and ryukyu subduction zones are initiated by inclined cuts made within the psp the subduction collision is driven by a piston pre collisional intraoceanic subduction along the luzon and ryukyu boundaries results in the formation of a transform zone between them with two tear faults at the ends the psp undergoes strong compression along this zone subduction of the chinese margin under the luzon boundary further increases the compression compressive stresses reach the yield limit of the psp in the arc area which is a weak zone in the experiments the plate fails at the western side of the arc along an eastward dipping fault the longitudinal valley fault underthrusting of the frontal wedge of the psp along this fault results in the closure of the fore arc basin and is then blocked the psp fails at the opposite side of the luzon arc along the westward dipping fault the failure releases lithospheric compression in this region and results in the initiation of southward propagating subduction of the psp under northeastern taiwan the incipient subduction zone becomes part of the southeastward retreating ryukyu subduction zone which allows the okinawa back arc rift to propagate into taiwan the taiwan collision thus includes the following succession of major processes over time or from south to north 1 an e w shortening of the psp in the luzon arc 2 a failure of this plate at the western side of the arc and the formation of the eastward dipping longitudinal valley fault the transient plate boundary 3 a closure of the fore arc basin and a rapid uplift of the orogen 4 a failure of the psp at the eastern side of the luzon arc partly overthrusting the orogen and the initiation of westward wn ward subduction of the psp 5 and finally back arc rifting in the rear of this incipient subduction zone i e in northern taiwan all these processes commence with some delay with respect to the preceding ones and propagate southwards

geochemical signature of formation waters associated with coalbed methane formation waters associated with coalbed methane have a common chemical character that can be an exploration tool regardless of formation lithology or age effectively devoid of sulfate calcium and magnesium the waters contain primarily sodium and bicarbonate and where influenced by water of marine association also contain chloride the distinct geochemical signature evolves through the processes of biochemical reduction of sulfate enrichment of bicarbonate and precipitation of calcium and magnesium cation exchange with clays may also deplete the dissolved calcium and magnesium but is not prerequisite low sulfate bicarbonate ratios characterize these waters and are also common but less pronounced with occurrences of conventional oil and gas waters rich in sulfate calcium and magnesium occur in many coalbed aquifers but are not found in association with methane users of total dissolved solids data should ensure that the values reflect adjustments of bicarbonate concentrations to simulate evaporation residues results that erroneously sum the entire bicarbonate content can be far too high in these bicarbonate rich waters thereby exacerbating the issues of disposal evaluations of prospects and choices of exploration targets can be enhanced by an added focus on the geochemical signature that should be expected in association with methane knowledge of the geochemical signature may also be useful in the commonly protracted testing of wells the appearance of high sulfate concentrations in water analyses can justify early curtailment of test pumping and can prompt the siting of subsequent drill holes farther from areas of recharge

an early miocene age for a high temperature event in gneisses from zabargad island red sea egypt mantle diapirism in the last decade much attention has been focused on the tectonothermal and geochemical evolution of rocks outcropping on zabargad island red sea egypt this island though of limited size 4 km has an almost unique geodynamic setting 50 kmwest of the red sea axis and reveals a close association of mafic ultramafic rocks with a metamorphic gneiss complex understanding the geodynamic evolution of the island was expected to elucidate the processes operating in a young rift setting environment and crust mantle interface tectonics since the first work of bonatti et al 1981 however genetic models have been limited by the the scant data on the timing of juxtaposition between the peridotites and the gneisses as well as the nature of the mafic ultramafic rocks one interpretation considers the peridotites as an asthenospheric mantle diapir intruding a pan african continental crust during the early stages of the red sea opening e g nicolas et al 1987 the fewattempts todate the time of intrusion of the peridotites are consistent with this view nicolas et al 1985 for example inferred a k ar age of 23 7 myr for amphiboles extracted fromanamphibolite this value concurs with the more precise ar ar age 18 7 1 3 myr given by hornblende extracted from a pegmatoidal pocket of a coarse grained hornblendite in harzburgite trieloff et al 1997 both ages are identical within errors of a 18 4 1 0myr zircon age obtained by oberli et al 1987 although there is a good agreement between the three methods oberli s zircon value has been presented only in an abstract thus making it difficult to properly evaluate k ar and ar ar ages have also been questioned for decoupling of parent and daughter nuclides villa 1990 or for their meaning in terms of a geological event uplift hydrothermalism or emplacement age due to their known susceptibility to low grade events an alternative interpretation considers the peridotites and the gneisses to be panafrican brueckner et al 1988 1995 and that juxtaposition of both rock types occurred shortly after differentiation from a common depleted mantle source 700 ma this is apparently supported by the so called slap errorchron resulting from alignment of whole rock peridotite samples and cpx separates in the sm nd isochron diagram brueckner et al 1988 this model however has been challenged in two recent papers on the grounds of fluid inclusion boullier et al 1997 and noble gases and argon chronological studies trieloff et al 1997 in both models gneisses are seen to represent remnants of the pan african continental deep crust left behind during opening of the red sea rift pan african ages for the metamorphic gneiss complex are well established by sm nd and rb srmineral isochrons lancelot and bosch 1991 brueckner et al 1995 as well as by a zircon pb pb evaporation date brueckner et al 1995 from a felsic gneiss located a few hundred metres from the contact zone between the central peridotite body and the gneisses these ages however provide no constraints on the timing of juxtaposition of gneisses and peridotites in order to solve this fundamental problem and because of the known resistance to alteration of the mineral zircon we focused on the u pb dating of single zircon grains extracted from a felsic gneiss located at the contact zone with the central peridotite body

the saxonian granulites a metamorphic core complex the brachyanticlinal structure of the saxonian granulite complex was formed or at least overprinted by large scale crustal extension which is documented in an extreme reduction of the metamorphic profile and a pressure temperature path with nearly isothermal decompression the granulite is situated within a south west northeast trending belt characterized by basaltic volcanism and an intrabasinal sediment producing rise both active in frasnian times these features are taken to reflect at least part of the extension which has exhumed the high grade rocks however cambro ordovician rifting possibly initiated the process the frasnian palaeogeography and geophysical data suggest that the axis of the granulite dome extends at least 150 km towards the south west the overall setting strongly resembles the metamorphic core complexes of the western usa but the exhumation of rocks preserving evidence of metamorphic pressures of 10 12 kbar requires much higher extensional strain possibly due to the longer period of stretching early carboniferous compression and minor post collisional extension late carboniferous to permian have not significantly modified the earlier geometry u pb and sm nd data ranging between 380 and 340 ma are interpreted to record retrogression during uplift rather than peak metamorphism

a statistical approach to estimate the 3d size distribution of spheres from 2d size distributions size distribution of rigidly embedded spheres in a groundmass is usually determined from measurements of the radii of the two dimensional 2d circular cross sections of the spheres in random flat planes of a sample such as in thin sections or polished slabs several methods have been devised to find a simple factor to convert the mean of such 2d size distributions to the actual 3d mean size of the spheres without a consensus we derive an entirely theoretical solution based on well established probability laws and not constrained by limitations of absolute size which indicates that the ratio of the means of measured 2d and estimated 3d grain size distribution should be 4 785 actual 2d size distribution of the radii of submicron sized pure fe 0 globules in lunar agglutinitic glass determined from backscattered electron images is tested to fit the gamma size distribution model better than the log normal model numerical analysis of 2d size distributions of fe 0 globules in 9 lunar soils shows that the average mean of 2d 3d ratio is 0 84 which is very close to the theoretical value these results converge with the ratio 0 8 that hughes 1978 determined for millimeter sized chondrules from empirical measurements we recommend that a factor of 1 273 reciprocal of 0 785 be used to convert the determined 2d mean size radius or diameter of a population of spheres to estimate their actual 3d size

the building of the central swiss alps an experimental approach abstract a kinematic model explaining the building of the central swiss alps is proposed from a review of various kind of data including structural geophysical petrological and geochronological data the eo alpine event is taken into account in order to determine the thermo mechanical evolution from the cretaceous to the late tertiary the stacking process of crustal wedges which took place during cretaceous time has led to the formation of a thickened crust with a very low geothermal gradient high pressure low temperature metamorphism this period was followed by a lull in tectonic activity during which thermal relaxation took place the major tertiary deformation is thought to result from horizontal compression of a thickened crust having a geothermal gradient higher than the average horizontal surface forces are due to the adriatic sub plate acting as a rigid indenter the model explaining the present day structure of the swiss alps emphasizes vertical extrusion and horizontal intrusion of the deepest highly ductile basement units several scale model experiments have been carried out in order to test this basic kinematic idea they display quite similar structures to those identified for the swiss alps striking features of the swiss alps such as their metamorphic and uplift history are discussed and compared with the timing of the tectonic evolution we conclude that the general process of vertical extrusion and horizontal intrusion can possibly explain the overall structure of the central swiss alps external crystalline massifs and the penninic zone

initiation of asymmetric extension in continental lithosphere govers r and wortel m r 1993 initiation of asymmetric extension in continental lithosphere in m j r wortel u hansen and r sabadini editors relationships between mantle processes and geological processes at or near the earth s surface tectonophysics 223 x 96 the physical conditions are investigated under which the lithospheric scale style of extension is pure shear or simple shear we focus on the initial stages of continental extension to monitor how symmetric or asymmetric modes of extension evolve from specific tectonic conditions continental collision magma intrusions and interaction between the lithosphere and the underlying mantle are investigated as sources for extension we use a finite element method to model the thermo mechanical evolution of continental lithosphere experimental flow laws are used to model the elastic hutle power law creep or diffusion creep rheology of lithospheric rocks our results indicate that if in plane forces change from compressive to tensile immediately after a rapid mountain building phase initiation of a lithosphere scale detachment fault is possible we find a strong dependence of the extensional style on the distribution with depth of residual stresses from the collision phase this result is consistent with observations of gravitational collapse in regions like the aegean and the basin and range province where detachment faults have exhumed lower crustal rocks the predicted dip direction of the fault also agrees with observations in these areas intrusion of magma into continental lithosphere which is subject io in plane tensile forces will cause localization of pure shear deformation the style of deformation resulting from mantle plumes impinging to the base of the lithosphere is symmetric delamination of lithospheric mantle may initiate detachment faults if delamination occurs at the end of a collision phase when in plane forces change sign from compressive to tenstle this result also strongly depends on the assumed residual stress distribution if delamination occurs during the mountam huilding phase the style of deformation will be pure shear another interesting outcome from our modeling is that dramatic strain weakening as a result of a deformation mechanism change from dislocation creep to diffusion creep reduces the tendency to strain localization a large volume of the published work on continental extension has been concentrated on quantifying the stretching factor p of the pure shear model of mckenzie 1978a in some basins it was found that the kinematics of deformation could be more accurately described by depth dependent stretching factors royden and keen 19801 which introduced asymmetry near basin edges in the simple shear model of wemicke

post variscan evolution of the lithosphere in the rhine graben area constraints from subsidence modelling in the area of the cenozoic rhine rift system crustal and lithospheric thicknesses range between 24 and 35 km and 60 and 120 km respectively this rift system transects the deeply truncated variscan orogen and superimposed permo carboniferous wrench induced troughs and late permian and mesozoic thermal sag basins at the time of its westphlian consolidation the variscan orogen was probably characterized by 45 60 km deep crustal roots that were associated with its rheno hercynian saxo thuringian saxo thuringian bohemian and bohemian moldanubian sutures all of which are transected by the cenozoic rhine rift system during the stephanian early permian wrench induced disruption of the variscan orogen subducted lithospheric slabs were detached causing upwelling of hot mantle material during the resulting thermal surge partial delamination and or thermal thinning of the continental mantle lithosphere induced regional uplift at the same time the variscan orogenic roots were destroyed and crustal thicknesses reduced to 28 35 km in response to the combined effects of mantle derived melts interacting with the lower crust regional erosional unroofing of the crust and on a more local scale by its mechanical stretching towards the end of the early permian the potential temperature of the asthenosphere returned to ambient levels with this regional long term thermal subsidence of the lithosphere commenced controlling the development of a new system of late permian and mesozoic thermal sag basins however the evolution of these basins was repeatedly overprinted by minor short term subsidence accelerations that reflect the build up of far field stresses related to rifting in the tethyan and atlantic domains comparison of observed and modelled subsidence curves suggests that in the area of the rhine rift system the lithosphere had equilibrated with the asthenosphere at the end of the cretaceous at depths of 100 120 km before it became thermally destabilized again by cenozoic rifting and plume related magmatism modelled subsidence curves indicate that by the end of early permian times the thermal thickness of the remnant mantle lithosphere ranged between 10 and 50 km in areas that were later incorporated into mesozoic thermal sag basins this corresponds to mid permian thermal lithosphere thicknesses of 40 80 km the geological society of london 2004

transformation of a magmatic arc and an orogenic root during oblique collision and it s consequences for the evolution of the european variscides mid german crystalline rise the architecture of the european variscides has been subdivided by kossmat 1927 into paleogeographically coherent units which are presently interpreted as former plate fragments the mid german crystalline rise mgcr separates two fragments rhenohercynian and saxothuringian belts at the site of an inferred plate boundary and reequilibrated orogenic root the commonly favoured model interprets the mgcr as the magmatic arc on saxothuringian crust above a south dipping subduction zone in upper devonian and carboniferous times data from the mgcr the kinematic evolution of the mid european variscides and first order volume balancing suggest a reinterpretation of this unit which challenges classical views on the mgcr as well as on the subdivision of variscan architecture the mgcr is composed of two rock groups with different tectonic identity a lower carboniferous low pressure high temperature magmatic arc association on lower paleozoic basement rests tectonically on a stack of medium pressuremedium temperature rocks of inferred rhenohercynian origin the latter were tectonically accreted to the base of the overriding plate by tectonic underplating the entire process was controlled by oblique convergence this led to regional partitioning of the plate kinematic vector into contractional domains lower rhenohercynian plate and back arc area of the upper saxothuringian plate bulk heterogeneous plate margin parallel extensional domains mgcr and plate margin parallel wrench domains mgcr boundaries during this process material was continually transferred from the lower plate to the upper plate uplifted and exhumed by net crustal extension the concomitant removal of parts of the former arc and the entire orogenic root necessitates a reappraisal of variscan architecture and evolution

new 40ar 39ar ages and geochemistry of late carboniferous early permian lamprophyres and related volcanic rocks in the saxothuringian zone of the variscan orogen germany

from cadomian subduction to early palaeozoic rifting the evolution of saxo thuringia at the margin of gondwana in the light of single zircon geochronology and basin development central european variscides germany

lower crustal rejuvenation and growth during post thickening collapse insights from a crustal cross section through a variscan metamorphic core complex the variscan lower crust of europe acquired its main features seismic layering intrusion of mantle derived magmas granulite grade metamorphism when the upper crust underwent post thickening extension through gravitational collapse we suggest that post thickening collapse is a geodynamic process that contributes to the growth and differentiation of continental lithosphere in collapsed mountain belts

zircon dating of north bohemian granulites czech republic further evidence for the lower carboniferous high pressure event in the bohemian massif u pb zircon and rutile multigrain ages and 207pb 206pb zircon evaporation ages are reported from high pressure felsic and metapelitic granulites from northern bohemia czech republic the granulites in contrast to those from other occurrences in the bohemian massif do not show evidence of successive ht mplp overprints multigrain size fractions of nearly spherical multifaceted metamorphic zircons from three samples are slightly discordant and yield a u pb concordia intercept age of 348 10 ma whereas single zircon evaporation of two samples resulted in 207pb 206pb ages of 339 1 5 and 339 1 4 ma respectively a rutile fraction from one sample has a u pb concordia intercept age of 346 14 ma all ages are identical within error and a mean age of 342 5 ma was adopted to reflect the peak of hp metamorphism because rutile has a lower closing temperature for the u pb isotopic system than zircon the results and the p t data imply rapid uplift and cooling after peak metamorphism the above age is identical to ages for high grade metamorphism reported from the southern bohemian massif and the granulite massif in saxony it can be speculated that all these granulites were part of the same lower crustal unit in early carboniferous being separated later due to crustal stacking and subsequent late variscan orogenic collapse

long term landscape evolution of the northeastern margin of the bohemian massif apatite fission track data from the erzgebirge germany to study the relative and absolute timing of post variscan cooling and denudation processes in the erzgebirge of the mid european variscides eight samples for apatite fission track aft analysis were collected from a 1 300 m drill core the fission track data reveal two stages of accelerated cooling through the apatite partial annealing zone apaz i e 110 10 60 c in the late jurassic late cretaceous and in the late cenozoic respectively late jurassic late cretaceous cooling corresponding to denudation of 1 5 5 9 km has been related to wrench tectonics along the elbe zone during triassic jurassic pangea breakup late cenozoic exhumation of 2 1 5 6 km and the increase of the geothermal gradient from 17 5 c km 1 oligocene miocene to 25 27 c km 1 recent is likely connected to the formation of the eger graben starting from the oligocene as a result of the late alpine orogenic phases

comment on beronaguirre puelles abalos gil ibarguchi garcia de madinabeitia short duration regional metamorphic event recorded in a variscan subduction channel malpica tui eclogites nw iberia international journal of earth sciences 2019 108 2037 2046 beranoaguirre et al int j earth sci 108 2037 2046 2019 have added a valuable piece of information to the carboniferous tectono thermal record of europe by documenting a short term pulse of heating in high pressure metamorphic rocks of the nw iberian allochthons c 350 ma malpica tui unit they correctly state that the short duration of the thermal event is untypical of orogenic subduction collision processes but still envisage short time heating within the subduction channel the present comment is intended to point out that short term thermal events of similar age are widespread in europe and are part of a polyphase metamorphic history controlled by episodic heating from the mantle probably over the plume generation zone tuzo

compositional variations in deep sea gravity flow deposits a case study from the voirons flysch voirons wagital complex chablais prealps france abstract grain size control is frequently invoked to explain the compositional variation of detrital sediments in fluvial sediments a similar possibility is here investigated in the fossil deep sea gravity flow deposits from the voirons flysch voirons wagital complex chablais prealps about 270 samples of diverse depositional settings were analysed using the framework composition including cement and porosity mutti facies and grain size measurements six lithofacies have been identified from cluster analysis they correspond to channel l1 to l3 and lobe l4 and l5 depositional settings and to reworked turbidites l6 our results emphasize the close relationship between petrography and grain size which alters the detrital signal from extrabasinal sources the abundant polycrystalline component of proximal deposits is progressively removed while skeletal grains and additional intrabasinal grains become more abundant in distal deposits alongside porosity is reduced by the simultaneous increase of cement which constrains subsequent diagenetic modifications hence proximal depositional settings keep most of the inherited signal from extrabasinal sources whereas distal deposits will be homogenised when plotted in provenance analysis diagrams the successive depositional settings show distinct fields possibly corresponding to different tectonic settings of the dickinson model this compositional variation thus creates a bias in provenance interpretation resulting from both grain size control and flow mechanisms finally our results emphasize that petrographic data must be handled with care in provenance analyses especially when comparing various deep sea gravity flow systems with different grain size ranges

comment on dorr and zulauf elevator tectonics and orogenic collapse of a tibetan style plateau in the european variscides the role of the bohemian shear zone int j earth sci geol rundsch 2010 99 299 325 the present comment disproves the tectonic model of a late devonian early carboniferous tibetan style collisional plateau in the tepla barrandean tb part of the bohemian massif which later collapsed by thermal weakening of the underlying crust contrary to this model the tb neither reveals major crustal thickening nor uplift and erosion and eastern continuations of the tb were during the relevant time span areas of open marine sedimentation late devonian early carboniferous marine sediments widespread also in the armorican and central massifs of france testify to low topography in central parts of the variscan orogen notional traces of a permo carboniferous ice cap on the french massif central do not support the plateau model because they are questionable and much younger than the inferred plateau stage of the tb the relative uplift of high grade metamorphic rocks to the nw and the se of the tb is not due to sinking of an elevated tb but instead to the hydraulic and buoyant expulsion of hp material from the saxo thuringian and moldanubian subduction channels the rise of lower grade ht rocks along the southwestern margin of the bohemian massif was effected by late carboniferous transpression the high temperature and the resulting low viscosity of the rising materials were probably not caused by variscan mantle delamination but relate to lithospheric thinning and heating at the tip of the westward propagating tethys rift

smnd and upb dating of eclogites and granulites from the oberpfalz ne bavaria germany abstract the formation of eclogites and felsic granulites of the moldanubian basement of the oberpfalz area of winklarn ne bavaria has been dated at 424 ma using the sm nd method on minerals garnet zircon rutile and whole rock only estimates on the age of the tholeiitic protoliths of the eclogite boudins are possible from the scattered u pb zircon data and from sm nd systematics both methods suggest protolith ages around 1 ga and indicate together with the major trace and rare earth element data extraction of the tholeiitic melts from a suboceanic mantle with an initial nd of 7 5 based on the geochronological data of three typical moldanubian cordierite bearing paragneisses which surround the winklarn area in distances of 30 60 km tectonic contacts to the low pressure country rocks of the eclogites are probable as these paragneisses yielded ordovician ages for their amphibolite facies metamorphism and post pan african ages for the deposition of their sedimentary precursors low pressure high temperature metamorphism at 323 ma fully reset monazites in the granulites this widespread and synchronous carboniferous overprinting of the moldanubian basement is probably also responsible for the variable opening of u pb zircon rb sr whole rock and sm nd mineral systems a terrane or microcontinent model still appears most suitable to explain the ordovician silurian devonian and carboniferous high pressure subduction events detected all over the european variscan belt in this model oceanic basins between gondwana derived microcontinents were successively closed since the ordovician causing differently old subduction zone related high pressure metamorphisms

k ar geochronology of different tectonic units at the northwestern margin of the bohemian massif abstract the polymetamorphic moldanubian mo of the northeasthern margin of the bohemian massif has been thrust to the north onto the mainly paleozoic sedimentary saxothuringian of the fichtelgebirge fg these two units have undergone polyphase deformation and the last regional event to affect both units was a low pressure metamorphism in which temperatures decreased towards the north in contrast the nappe units of the erbendorf vohenstrauss zone zev and the erbendorf greenschist zone egz which partly cover the border of the moldanubian and the saxothuringian and the munchberg nappe pile mm which lies on the saxothuringian were in parts subjected to a late medium pressure metamorphic event the zev the egz the mo and the fg are intruded by late carboniferous granites conventional k ar analyses mainly of hornblendes and muscovites from the autochthonous fg and mo the units beneath the nappes have yielded exclusively carboniferous dates the oldest dates point to a regional cooling of the rocks which outcrop at the present day surface at about 330 320 ma i e at the early late carboniferous boundary the late carboniferous cooling history was largely governed by the thermal influence of the post kinematic granites 320 295 ma especially in the fg and the northern mo the high grade metamorphic rocks in the western part of the zev and in the upper three nappes of the mm mostly yield dates around 380 ma i e early devonian the results show a relatively wide scatter moreover biotites frequently appear to be older than the coexisting muscovites both observations indicate that the rocks underwent a later thermal influence whether some groups of older dates e g 400 ma are due to excess argon or to inherited argon is still open to discussion slightly scattered muscovite dates around 366 ma were obtained for the prasinite phyllite series one of the lower nappes of the mm a single hornblende from the egz gave the same age these two nappes have therefore probably been affected by a late devonian thermal and or tectonic event the muscovite dates obtained from the paleozoic bavarian lithofacies the lowermost nappe of the mm and the hornblende dates from the eastern part of the zev are indistinguishable from those of the autochthonous units fg and mo

generation accumulation and resource potential of biogenic gas biogenic gas is generated at low temperatures by decomposition of organic matter by anaerobic microorganisms more than 20 of the world s discovered gas reserves are of biogenic origin a higher percentage of gases of predominantly biogenic origin will be discovered in the future biogenic gas is an important target for exploration because it occurs in geologically predictable circumstances and in areally widespread large quantities at shallow depths in rapidly accumulating marine sediments a succession of microbial ecosystems leads to the generation of biogenic gas after oxygen is consumed by aerobic respiration sulfate reduction becomes the dominant form of respiration methane generation and accumulation become dominant only after sulfate in sediment pore water is depleted the most important mechanism of methane generation in marine sediments is the reduction of co2 by hydrogen electrons produced by the anaerobic oxidation of organic matter co2 is the product of either metabolic decarboxylation or chemical decarboxylation at slightly higher temperatures the factors that control the level of methane production after sediment burial are anoxic environment sulfate deficient environment low temperatu e availability of organic matter and sufficient space the timing of these factors is such that most biogenic gas is generated prior to burial depths of 1 000 m in marine sediments most of the biogenic gas formed can be retained in solution in the interstitial pore waters because of higher methane solubility at the higher hydrostatic pressures due to the weight of the overlying water column under certain conditions of high pressures and or low temperatures biogenic methane combines with water to form gas hydrates biogenic gas usually can be distinguished from thermogenic gas by chemical and isotopic analyses the hydrocarbon fraction of biogenic gas consists predominantly of methane the presence of as much as 2 of heavier hydrocarbons can be attributed to admixture of minor thermogenic gas due to low temperature degradation of organic matter the amounts of hydrocarbon components other than methane generally are proportional to temperature age and organic matter content of the sediments biogenic methane is enriched in the light isotope 12c dgr13c1 lighter than 55 ppt owing to kinetic isotope fractionation by methanogens the variations in isotopic composition of biogenic methane are controlled primarily by dgr13c of the original co2 substrate which reflects the net isotopic effect of both addition and removal of co2 the methane isotopic composition also can be affected by mixing of isotopically heavier thermogenic gas the possible complicating factors require that geologic chemical and isotopic evidence be considered in attempts to interpret the origin of gas accumulations accumulations of biogenic gas have been discovered in canada germany italy japan trinidad the united states and ussr in cretaceous and younger rocks at less than 3 350 m of burial and in marine and nonmarine rocks other gas accumulations of biogenic origin have undoubtedly been discovered however data that permit their recognition are not available

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

subsurface granites in the franconian basin as the source of enhanced geothermal gradients a key study from gravity and thermal modeling of the bayreuth granite the franconian basin in ne bavaria is a region of gravity low between the bohemian massif in the east and the kraichgau terrane in the west borehole measurements have identified the northern part of the franconian basin as a regional geothermal anomaly and new heat flow calculations give values of 100 mw m2 distinct negative bouguer anomalies observed in this basin are modeled as granitic intrusions in the saxothuringian basement that underlies the permo mesozoic units the interpretation of gravity gradients in combination with the filtering of gravity data gives the possible depth constraints of the intrusive bodies the resulting depths were cross checked using microstructural studies of quartz veins in the basement rocks at 1341 m depth the quartz shows structures typical for low temperature plasticity and we infer a deformation temperature of ca 300 c this indicates a considerable pre permian uplift of at least 7 km for parts of the saxothuringian basement and supports depth estimates from gravity data the heat supply of granitic intrusions by radiogenic decay is modeled considering several scenarios for the geological conditions recovered by the obernsees borehole model for conductive heat transfer the 1390 m deep drillhole is in a marginal position to the most pronounced negative bouguer anomaly it could be shown that the saxothuringian basement including heat producing granites heat production rates 4 6 w m3 covered by the insulating sedimentary rocks 1 35 km of permian to lower jurassic units can account for the enhanced geothermal gradient 38 c km that were measured in the borehole

transitional blueschist epidote amphibolite facies metamorphism in the frankenberg massif germany and geotectonic implications bimodal metavolcanic rocks granitic gneisses and metasediments are associated in the frankenberg massif germany these rocks are faulted against underlying very low grade palaeozoic sequences and adjacent metamorphic complexes of the variscan basement the granitic gneisses record an rb sr whole rock isochron age of 461 20 ma that is taken as at least a minimum protolith age the bimodal meta igneous suites are interpreted to have formed during rifting of the gondwana continental margin in the cambro ordovician the various metamorphic units have all experienced a common p t history the peak pressure stage is constrained to around 490 520 c and 10 14 kbar 10 12 kbar being most realistic the metamorphism proceeded along a clockwise p t path towards conditions of around 580 610 c and 7 8 5 kbar at the thermal peak followed by a final low pressure overprint which spanned amphibolite facies to prehnite actinolite facies temperatures owing to a secondary rb sr whole rock isochron age of 381 24 ma interpreted to date the retrograde stage the whole metamorphic cycle in the frankenberg massif is ascribed to the late silurian early devonian high pressure event widely recorded in the european variscides the antiformal complexes bordering the frankenberg massif underwent a well documented early carboniferous metamorphism suggesting that the frankenberg massif constitutes a klippe which was overthrust towards the end of this second metamorphic cycle

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

model of syn convergent extrusion of orogenic lower crust in the core of the variscan belt implications for exhumation of high pressure rocks in large hot orogens reflection seismic section field structural analysis and gravimetric modelling of orogenic lower crust in the core of a carboniferous orogenic root reveal details of the polyphase process of exhumation subvertical amphibolite facies fabrics strike parallel to former plate margins that collided in the nw the fabrics are developed in both mid crustal and lower crustal high pressure granulite units as a result of intensive nw se intraroot horizontal shortening driven probably by the west directed collision in granulites the steep fabrics originated as a result of extrusion of orogenic lower crust in a 20 km wide vertical ascent channel from lower crustal depths at 350 340 ma the large granulite bodies preserve older granulite facies fabrics documenting a two stage evolution during the exhumation process surface exposures of granulites coincide with the absence of subhorizontal seismic reflectors at depth suggesting preservation of the 20 km wide subvertical tabular structure reaching moho depths horizontal seismic reflectors surrounding the vertical channel structure corroborate a dominant flat migmatitic fabric developed in all tectonic units this structural pattern is interpreted in terms of subhorizontal spreading of partially molten orogenic lower crust in mid crustal levels 765 c and 0 76 gpa at 342 337 ma large massifs of extruded and progressively dismembered felsic granulites disturbed mid crustal fabrics in the surrounding horizontally flowing partially molten crust the horizontal mid crustal flow resulted in collapse of the supra crustal tepla barrandian unit interpreted as the orogenic lid along a large scale crustal detachment above the extruded lower crustal dome the presence of felsic granulites at the bottom of the orogenic root is considered to be a key factor controlling the exhumation of orogenic lower crust in large hot orogens