extension during continental convergence with application to the tibetan plateau the tibetan plateau is the product of crustal thickening caused by the collision between india and asia and is the largest active example of extensional tectonics in a zone of continental collision throughout most of the tertiary the tectonics of the plateau were dominated by north south shortening a significant proportion of which took place on east west striking thrust faults for the last 5 m y or so the plateau has been thinning by the mechanism of extension on north south trending normal faults previous investigations of the collision have been able to account for the large scale features of the tertiary deformation but have failed to explain the transition in the late tertiary to quaternary strain rate field of the plateau from north south compression to east west extension the transition could in principle be effected by a reduction in the rate of convergence between india and asia or by a uniform reduction in strength of the whole continental lithosphere of asia it could not however be effected by a reduction in strength of the elevated region alone this produces increased compressional strain rates in the weakened zone an alternative explanation for the transition to extension comes from considering the thermal evolution of thickened continental lithosphere the lower part of the lithosphere consists of a thermal boundary layer which when thickened by horizontal shortening is colder and denser than its surroundings convective instability of the thickened thermal boundary layer and its replacement by hot asthenosphere would rapidly raise the surface elevation and gravitational potential energy of the overlying part of the lithosphere the convective instability would happen in a time brief compared with the collision time scale 50 m y in the case of india and asia but would only occur after there had already been substantial thickening of the lithosphere numerical experiments show that for a range of lithospheric parameters the increase in surface height as much as 2 km and of potential energy 5 to 10 1012 n m 1 resulting from convective instability of the lower lithosphere are sufficient for east west extension to replace north south compression as the dominant feature of the stress field of the tibetan plateau

topographic inversion and changes in the sediment routing systems in the variscan orogenic belt as revealed by detrital zircon and monazite upb geochronology in post collisional continental basins abstract the basal alluvial and fluvial siliciclastic successions within the late carboniferous intracontinental basins of the west central bohemian massif provide an intriguing sedimentary record of changes in paleotopography and paleodrainage systems during and after orogenic collapse we analyze this record using the u pb laser ablation icp ms detrital zircon and monazite geochronology five distinct age groups have been identified across the examined stratigraphic succession i archean paleoproterozoic ii late neoproterozoic iii cambro ordovician iv late devonian and v mid to late carboniferous these age groups indicate mixing of both local and distal extra basinal sources and also point to a two stage post convergent geodynamic evolution of the variscan belt the first stage followed collapse of the central bohemian orogenic plateau at around 346 337 ma and involved rapid cooling at ca 340 320 ma and removal of ca 2 3 km of sedimentary overburden from the top of the plateau no sedimentary record of this erosional event suggests that the river systems transported detritus radially outwards from the inner part of the orogen to its forelands a paleorelief inversion and thus major reorganization of the sediment routing systems were then associated with uplift in the southerly and southwesterly peripheral domains of the orogen at ca 330 310 ma in turn these uplifted domains occupied by rapidly unroofed voluminous granite plutons shed detritus into the bohemian sink this was represented by the former plateau collapsed to relatively low elevations but still with significant internal paleorelief dominated by a system of tens of kilometers long nne ssw trending paleovalleys some of the material was further transported in extensive river systems crossing the former plateau to the northerly variscan foreland basin thus the radial sediment routing system changed into axial unidirectional finally we suggest that these major inversions of paleorelief and changes in the surface processes and paleodrainage systems may have been linked to distinct late orogenic lithospheric mantle delamination events first stage delamination occurred beneath the center of the bohemian massif to cause the plateau collapse denudation and radial sediment transport whereas the second stage delamination occurred along its periphery to cause uplift sourcing the intracontinental basins and unidirectional sediment transport end of deposition and magmatic shutdown at around 305 ma may then define the very cessation of late orogenic vertical movements and the transition to a post orogenic intra plate setting

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

late variscan tectonic evolution by thinning of earlier thickened crust an 40ar39ar study of the montagne noire southern massif central france abstract 40ar 39ar age determinations on samples selected on deformational criteria form the basis of a proposal of a new detailed tectonometamorphic history of the montagne noire the southern part of the massif central france this range is classically divided into a cover and an axial gneissic and migmatitic core which was metamorphosed under high t medium p conditions the basement is structurally overlain by epimetamorphic paleozoic sediments in the north in the south equivalent rocks form a well known pile of recumbent nappes the variscan history of this area comprises early shortening and thickening of the continental crust the climax of this event took place around 320 ma ago as is shown by previous rb sr data and by 40ar 39ar measurements new developments in structural analyses have led to a better understanding of the late evolution of the area in terms of diapiric uplift of the core south southwestwards verging recumbent nappes have been emplaced and were weakly metamorphosed in their lower parts this movement was accompanied and followed by dextral extensional wrenching of the paleozoic cover along the northern and southern margins of the axial zone attendantly ductile shear zones were formed in order to decipher the complex history three structural levels have been dated with the 40ar 39ar method these levels are 1 the migmatitic and gneissic series in the axial core where micas yield ages of c 315 ma 2 mylonites from the northern and southern shear zones where biotites and muscovites yield ages of c 310 ma 3 muscovites and biotites from the epimetamorphic nappe structures yielding ages of c 297 ma the later ages may represent a younger thermal pulse these data are compared and discussed in the light of previous radiometric studies of the area and precise biostratigraphic constraints a detailed geodynamic evolution of the studied segment is proposed

thermotectonic evolution of the barrandian czech republic as revealed by apatite fission track analysis abstract a fission track ft study was carried out on apatite from upper proterozoic to upper carboniferous sedimentary and volcanic rocks in the barrandian czech republic apatite ft ages vary between 324 8 ma lower carboniferous and 196 14 ma lower jurassic mean confined track lengths range from 11 68 1 76 to 13 30 1 41 m the temperature time path of selected samples was simulated by applying the laslett annealing model and independent geological constraints from the barrandian the first stage of increased temperature that is recorded in nearly all samples occurred at the upper devonian lower carboniferous boundary the change in temperature is due to the thickening of the crust by increased sediment load and stacking of sedimentary rocks along thrusts ft data indicate a limitation of the extension of thrust sheets towards the west cooling during the carboniferous is attributed to exhumation of parts of the barrandian deposition of upper carboniferous to lower permian sediments caused a second increase in temperature in lower permian time variation in maximum temperature indicates variable thickness of the sedimentary cover the second cooling during the mesozoic is linked to the long term slow to medium exhumation process of the central variscan belt

extending a thickened crustal bulge toward a new geodynamic evolution model of the paleozoic nw bohemian massif german continental deep drilling site se germany abstract fault bounded tectonic metamorphic complexes assembling the nw bohemian massif around the german continental deep drilling ktb site are seen to be extremely heterogeneous in tectonic and metamorphic histories in current models the different complexes were supposed to reflect a puzzle of small pre devonian microplates and the related collision events supposedly lasted until the carboniferous opposed to these models it will be shown that all the boundaries among the complexes were formed by detachment late in a prolonged overall geodynamic history of a thickened crustal bulge during extensional tectonics and associated thermal events that outlasted the onset of collision in the silurian lower devonian by about 70 80 ma micro structures petrological and geochronological data of individual complexes predominantly preserve the late stages rather than the unbroken record of their tectonometamorphic histories such partial histories strongly different among individual complexes depict diverse snapshots taken at different places in the evolving thickened crustal bulge and at different instants in its overall evolution and do not define different precollisional microplates predominantly p t and deformation episodes after terrane juxtaposition are preserved this article presents an integrated view of the structural geology microscopic fabrics p t data and geochronology of such diverse metamorphic complexes this integrated view provides a new understanding of 1 the tectonic evolution during upper silurian devonian collision of the gondwana derived central european lithosphere with laurussia 2 the postaccretionary events that lasted through the upper carboniferous and 3 the earlier lower ordovician metamorphic and magmatic history which is only locally recorded metamorphic complexes occupying the structurally highest position upper tectonic complexes record devonian and earlier tectonometamorphic and magmatic events after the mid devonian such complexes did not experience any metamorphism the recorded devonian events consist of subduction and exhumation of hp rocks and their exhumation involved thrusting and extensional tectonics upper tectonic complexes comprise fragments of both the plate that was subducted in this period and the overriding plate in the footwall carboniferous extension has brought upper tectonic complexes against metamorphic complexes that essentially record lower and upper carboniferous tectonometamorphic and magmatic events lower tectonic complexes in the lower tectonic complexes such events are 1 consecutive extensional stages that created at least three sets of ductile normal detachment systems intersecting each other and various associated thermal pulses as well as 2 predetachment events that are only recorded as petrologic and structural relics transposed during extension inferred as predetachment events are crustal subduction as well as stacking that outlasted thrusting and exhumation of the upper tectonic complexes in the deeper portion of the thickened crustal bulge represented by the lower tectonic complexes spatial variations of p t t d histories during decompression occurred as a result of continued differential exhumation on the three sets of detachment systems and also from various thermal pulses that have affected different parts of this section during progressive shearing and exhumation to various degrees whereas the lower tectonic complexes of the erzgebirge gneiss dome preserve the record of a lower carboniferous history of hp metamorphism and crustal thickening followed by extension in those of the oberpfalz region ktb site a major upper carboniferous thermal pulse mostly erased the pre upper carboniferous strain fabrics and metamorphic record in the oberpfalz region ongoing extension emplaced mid crustal rocks that pervasively equilibrated in the upper carboniferous at high temperatures and low pressures low p t ratio against rocks not exposed to high temperatures at that time in summary a prolonged postaccretionary history concealed large scale structural relationships linked to crustal subduction and thickening associated with former juxtaposition of gondwana lithosphere to laurussia polyphase crustal extension and strike slip created the variety of different metamorphic tectonic complexes observed in the nw bohemian massif

porphyroclast systems as kinematic indicators porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material the geometry of a porphyroclast and its tails called a porhyroclast system can serve as a valuable indicator of the sense of vorticity porphyroclast systems have been divided into and types on the basis of the geometry of the tails types have wedge shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system types are further subdivided into a a types in which the porphyroclast is isolated in a relatively homogeneous matrix and b types in which the porphyroclast system is associated with a shear band foliation in the matrix types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast consequently embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends a porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non coaxial flow in the adjacent matrix this behaviour has been modelled in shear box experiments passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear the experiments were modified to simulate a change in recrystallization rate r with respect to rate of deformation by decreasing the diameter of the rigid cylinder during deformation at variable rates the ratio r appears to be one of the most important factors in determining which porphyroclast system will develop at high r values flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge shaped a type tails develop at low r values relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast in addition most porphyroclast systems at low shear strains are of a type or lack monoclinic symmetry whereas types are only developed at high shear strain values complex porphyroclast systems characterized by two generations of tails are observed in many of the natural and model shear zones studied and may form due to fluctuating r conditions that allow isolated a and type porphyroclast systems to be used as sense of vorticity indicators are the systems should have a monoclinic shape symmetry matrix grain size should be small with respect to porphyroclast size matrix fabric should be homogeneous deformation history should be simple and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite

ductile normal faulting along the west bohemian shear zone moldanubian tepla barrandian boundary evidence for late variscan extensional collapse in the variscan internides structural and kinematic investigations of the west bohemian shear zone wbs clearly indicate late variscan orogen parallel wsw ene extension within the variscan internides along the wbs the western part of the tepla barrandian tb was downthrown to the east against the adjacent moldanubian according to seismic data the steeply east dipping wbs flattens with depth forming a prominent detachment zone the western part of the tb was tilted along this zone producing the patterns of metamorphic isograds the age of which is probably cadomian cross cutting relationships of wbs mylonites and carboniferous granites as well as the overall cooling ages of hornblende and mica suggest that ductile normal faulting along the wbs was active from about 330 to 310 ma

crustal thinning recorded by the shape of the namurian westphalian leucogranite in the variscan belt of the northwest massif central france in the western part of the variscan belt of the french massif central the gueret massif is an extensional allochthon emplaced to the southeast during namurian westphalian time the internal structure of the coeval leucogranites surrounding the western edge of the gueret massif consists of the plateau d9aigurande brame st sylvestre and st goussaud massifs these plutons are asymmetric diapirs rooted in the marche bussiere madeleine and arrenes faults respectively the plutons indicate diverging extravasation from the gueret massif whatever their shapes the plutons have a northwest trending stretching lineation that is also marked in the granite host rocks by retrograde metamorphic minerals the same extension direction is also inferred from namurian westphalian leucogranites and dikes intruding the gueret massif extension related granitoids present typical structural patterns such as asymmetric shapes extravasation from the collapsing area and persistence of a stretching lineation parallel to the regional extension direction these features may be used to diagnose crustal thinning

deciphering the variscan tectonothermal overprint and deformation partitioning in the cadomian basement of the tepla barrandian unit bohemian massif the tepla barrandian unit tbu has long been considered as a simply bivergent supracrustal median massif above the saxothuringian subduction zone in the variscan orogenic belt this contribution reveals a much more complex style of the variscan tectonometamorphic overprint and resulting architecture of the neoproterozoic basement of the tbu for the first time we describe the crustal scale ne sw trending dextral transpressional krakovec shear zone ksz that intersects the tbu and thrusts its higher grade northwestern portion severely reworked by variscan deformation over a southeastern very low grade portion with well preserved cadomian structures and only brittle variscan deformation the age of movements along the ksz is inferred as late devonian 380 370 ma on the basis of structural microstructural and anisotropy of magnetic susceptibility data from the ksz we propose a new synthetic model for the deformation partitioning in the tepla barrandian upper crust in response to the late devonian to early carboniferous subduction and underthrusting of the saxothuringan lithosphere we conclude that the saxothuringian tepla barrandian convergence was nearly frontal during 380 346 ma and was partitioned into pure shear dominated domains that accommodated orogen perpendicular shortening alternating with orogen parallel high strain domains that accommodated dextral transpression or bilateral extrusion the synconvergent shortening of the tbu was terminated by a rapid gravity driven collapse of the thickened lithosphere at 346 337 ma followed by or partly simultaneous with dextral strike slip along the baltica margin parallel zones driven by the westward movement of gondwana from approximately 345 ma onwards

a study of retrogression in eclogites of the oberpfalz forest north east bavaria west germany and their significance in the tectonic evolution of the bohemian massif

hercynian low pressure high temperature regional metamorphism and subhorizontal foliation development in the canigou massif pyrenees france evidence for crustal extension a detailed study has been made of the structural and metamorphic history of part of the hercynian low pressure high temperature regional metamorphic terrane exposed in the canigou massif france microtextural relations between and compositional zoning within porphyroblast phases in pelitic rocks indicate that the pressure temperature time p t t trajectory followed by the massif during metamorphism involved decompression under prograde and retrograde conditions clockwise p t t trajectory the geometric relations between inclusion trails within these porphyroblasts and the structures in the adjacent matrix indicate that metamorphism was synchronous with the development of the regional subhorizontal foliation s3 the s3 foliation formed in response to noncoaxial bulk strain the absence of a marked hiatus in sedimentation in the pyrenean region during hercynian metamorphism and deformation and the widespread preservation of low grade hercynian metasedimentary rocks suggest that low p high t metamorphism occurred in an extensional tectonic environment and that extension manifested itself in asymmetric top to the northwest shear during d3

evidence for deformation induced k feldspar replacement by myrmekite several examples of deformation induced myrmekite have been found in two amphibolite facies mylonites derived from granitic protoliths namely a muscovite poor s c mylonite and a single foliation muscovite poor mylonitic gneiss back scattered sem and conventional optical microscopy show that in both rock types syntectonic myrmekitic intergrowths of oligoclase and quartz formed on the two sides of k feldspar grains that faced the local inferred incremental shortening direction for the mylonite myrmekite does not occur on the two ends of the grain that faced the incremental stretching direction the replacement of k feldspar by plagioclase and quartz results in a volume decrease and is favoured on high normal stress sites around the grains we suggest that the ambient temperature pressure and chemical activities were such that the replacement reaction was favoured but the addition of extra strain energy along the high pressure sides of the grains localized the reaction at these sites this energy could arise from elastic strain or strain associated with tangled dislocations or twin boundaries the relative roles of stress and strain energy concentrations in driving the replacement reaction are not known but both were probably important

the petrology of retrograded eclogites of the oberpfalz forest northeastern bavaria west germany abstract in the bavarian margin of the moldanubian basement of the bohemian massif retrograded eclogitic metabasites are found within cordierite gneisses associated with garnet free serpentinised peridotites the primary eclogite assemblage gar omph qz rut ky is rarely retained typically garnet pyr 12 37 alm 38 56 gro 14 29 is prograde zoned with thin retrograde rims and a kelyphitic corona of orthopyroxene plagioclase rare or mg hastingsite plagioclase an 60 common primary omphacitic clinopyroxene jd 25 35 is preserved within garnet groundmass omphacite jd 15 25 always showing an early exsolution of plagioclase is usually replaced by a vermicular di plag an 20 symplectite itself overgrown by a diablastic hastingsite plagioclase an 40 aggregate quartz is rimmed by hypersthene with an outer margin of diopside kyanite never preserved in the metabasites is altered anisochemically to a symplectite of hercynite anorthite commonly only anorthite aggregates remain geothermobarometric studies indicate equilibration around 710 c minimum pressure 15 kb for the primary eclogite assemblage the granulite facies overprint occurred at probably slightly higher temperatures but lower pressures with later retrogression into the amphibolite fades as mostly shown by the surrounding gneisses the p t t path preserved within these metabasic rocks is that expected from a region having undergone crustal thickening by subduction and crustal imbrication their present day surface exposure underlain by crust of normal thickness would indicate the presence of a basal thrust complex that has driven under the imbricate crustal pile aiding uplift and erosion a geometry confirmed by recent geophysical studies

elevator tectonics and orogenic collapse of a tibetan style plateau in the european variscides the role of the bohemian shear zone variscan collision of peri gondwanan terranes led to a doubly vergent crustal wedge that was thicker than 55 km in the area of the bohemian massif this crustal thickness resulted in a highly elevated bohemian plateau with a topographic height 3 4 km the bohemian plateau was covered with unmetamorphic paleozoic strata all of which are today well preserved in the tepla barrandian unit because of crustal scale vertical slip along the bohemian shear zone bsz the bsz forms a subvertical ca 500 km long and up to 2 km wide belt of dip slip mylonites which show several 90 deflections in map view tepla barrandian down movements were active under retrograde metamorphic conditions starting with granulite and ceasing with greenschist facies conditions as slip along the bsz was largely vertical and led to a minimum throw of 10 km this type of crustal scale deformation is referred to as elevator tectonics the elevator style movements caused the juxtaposition of the supracrustal tepla barrandian lid the elevator against high grade rocks of the extruding orogenic root the bsz has further governed the foci of mantle derived plutonism new u pb zircon and monazite tims dating of six plutons suggest that emplacement of mantle derived melts along the bsz lasted for at least 20 m y starting with the emplacement of the klatovy granodiorite at 347 4 3 ma and ceasing with the emplacement of the drahotin pluton at 328 1 ma when taking into account the new ages of synkinematic plutons the simultaneous vertical slip along the individual segments of the bsz north west and central bohemian shear zone is bracketed to the period 343 337 ma elevator tectonics was probably controlled by delamination of thickened mantle lithosphere that caused a dramatic thermal turnover and heating up of the orogenic root the overheated lower crust was thermally softened by anatexis and diffusion creep resulting in channel flow vertical extrusion fast uplift and exhumation of the orogenic root

a preliminary electron probe study of microchemical variations in desert varnish in western new south wales australia examination of desert varnish samples from australia by electron microprobe and energy dispersive spectrometry has shown that gross layering of iron and manganese enrichments is variable surface upper varnish may be similar to lower varnish or be relatively enriched in either iron or manganese when viewed in sections cut parallel to the varnished surface microlaminations with high manganese content up to 30 per cent were sometimes found in concentrically zoned patterns around grains dorn et al 1986 have proposed that chemical changes occur with increasing varnish age in this study the possibility of such changes was examined at the within varnish scale by comparing near surface and deeper upper and lower portions of the same varnishes variations in elemental ratios k ca ti at this scale were shown to be largely unsystematic suggesting either that samples were unrepresentative that element inputs to varnish have varied over time that the varnish was too young for differentiation by weathering into an upper and lower layer that varnish cover formed rapidly and at one time or that existing varnish cover represents the coalescence of numerous individual patches of varnish each of which formed over a relatively short period

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

structural model for the saxothuringian moldanubian suture in the variscan basement of the oberpfalz northeastern bavaria f r g interpreted from geophysical data abstract the moldanubian saxothuringian boundary marks a prominent subduction zone in the internal parts of the european variscides in the oberpfalz area deep reflection seismic profiles show a high density of various reflectors mainly in the upper crust indicating a complex polyphase deformation during the collision process seismic structures and related data from surface geology suggest that the moldanubian saxothuringian collision consisted of two main phases the first phase is marked by extensive nw directed overthrusting of moldanubian onto saxothuringian crust along se dipping master decollements some of which can be traced down to the moho during an advanced stage progressive overthrusting led to the development of wedge structures associated with se directed backthrusting and backfolding in the upper structural levels subsequently some of these wedges and their related decollements were folded to form the main ne striking anticlines the first phase of crustal imbrication correlates with a regional low pressure high temperature metamorphism at about 320 ma decreasing in grade northwestward the second phase is characterized by a reorientation of the main direction of crustal shortening from nw se to sw ne which is interpreted as the result of the n directed indentation of the vindelician terrane into the moldanubian this event is recorded by conjugate systems of overthrusting and subsequent strike slip faulting in the oberpfalz and the bavarian forest and the black forest vosges area in the area of the southern german block between these basement outcrops the indentation front is marked by the arc shaped axes of gravimetric and magnetic anomalies and permo carboniferous troughs

late archaean synorogenic basins of the eastern goldfields superterrane yilgarn craton western australia abstract siliciclastic supersequences of the kalgoorlie and gindalbie terranes post date 2660 ma deformation uplift and erosion associated with the amalgamation of arc related terranes but were tectonically buried to greenschist or amphibolite facies during the orogenic shortening that accreted the eastern goldfields superterrane to the yilgarn craton the supersequences are synclinal remnants of late stage basins that were linear zones of subsidence adjacent to north trending strike slip faults fluvial facies merougil navajo jones creek scotty creek and yandal basins are dominated by sheet sandstones that represent the deposits of medial braid plains but proximal channel and bar complexes of cobble and boulder conglomerates are also preserved submarine facies kurrawang basin range from boulder to pebble conglomerates but are dominantly bouma turbidites with rare black shales and record the filling of a proximal canyon that fed a distal submarine fan axial depositional systems are preferentially preserved in all basins but transverse systems are preserved at the base of some fluvial deposits the gross vertical succession in all basins is from proximal up to distal the basins developed during two tectonic stages separated by an intervening period of uplift tilting and erosion with the southern kalgoorlie terrane recording change from a stage i fluvial to a stage ii deep marine basin stage ii fluvial deposits are preserved in the northern kalgoorlie terrane and in the gindalbie terrane sparse palaeocurrents establish sediment dispersal was to the south or southeast implying uplift to the north or northwest controlled palaeoslopes stacking of lithofacies is aggradational and is consistent with limited lateral migration of facies tracts in fault confined basins similar lithofacies and aggradational styles of facies stacking characterise syncollisional sequences of the cenozoic coastal range basin of taiwan and the present day syncollisional basins of southeastern taiwan are good analogues of late stage basins of the kalgoorlie and gindalbie terranes

some late precambrian microfossils from the bohemian massif and their correlation abstract a brief account is given of the main biostratigraphical conclusions resulting from the micropalaeontological study of the bohemian upper proterozoic the relatively rich microfossil material presently known from the barrandian area enables comparison to be made both with middle and upper brioverian microfossils of the west european brioverian complex and also with those from the upper riphean and vendian platform sediments a correlation between the relatively close geographical areas of bohemia lusatia and saxony on the basis of microorganisms is evident biocommunities from siliceous rocks display algal mat assemblages their features are very similar to those of biogenic rocks reported from other regions especially australia and north america in the moldanubian supergroup the cesk krumlov formation in which graphitized phytoclasts with anatomical structures of primitive land plants have recently been found was examined the question of the age of this formation remains open and its study is still in progress the correlation of the sedimentary complex of the east sudeten the presumably proterozoic zab eh formation with the palaeozoic assemblage is possible due to the finds of chitinozoa remains of megascopic algae were recently found in this assemblage along with chilinozoan chambers this association is highly specialized and comprises new taxa of higher thallophytes

structural evidence for the relationship between thrusts extensional faults and granite intrusions in the variscan belt of galicia spain abstract new structural and petrofabric data are presented from the hombreiro pluton a synkinematically emplaced granite located in the northern part of the variscan belt of spain this pluton shows the imprint of two plastic deformations the first is related to the motion of the mondonedo nappe with a top to the east shear sense and the later one to the vivero fault an extensional shear zone with a top to the west motion these deformations initiated at high temperatures close to the granite solidus as shown by the preservation of c slip fabrics in quartz this fact indicates that extensional faulting was activated soon after the thrusting of the mondonedo nappe during which the hombreiro granite was emplaced we propose that the position of the vivero fault was controlled by a crustal instability induced by the intrusion of a number of plutons that delineate the footwall block of this fault

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

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

lateral displacement of crustal units relative to underlying mantle lithosphere example from the bohemian massif abstract we propose a mechanical model of deformation of the entire lithosphere of the bohemian massif bm whose core is formed by an asymmetric block of the tepla barrandian tb unit in between the saxothuringian st and moldanubian md units for the modelling we have re processed p wave travel times recorded during the last two decades at dense networks of seismic stations installed in the bm during several passive seismic experiments we also use previous results of anisotropic studies based on splitting of teleseismic shear waves this allows us to refine estimates of the lithosphere thickness and delimit deep margins of the individual mantle lithosphere domains the domains are rigid enough to preserve pre orogenic olivine fabrics differently oriented in each of the units shapes and dips of the mantle boundaries representing major zones of weakness inherited from the variscan amalgamation of independent microplates indicate that north westward subductions beneath the tb unit dominated tectonic development of the core of the bm two mantle lithosphere domains with different fabric orientations separated by a wsw ene striking shear zone underlie the tb crust the nw domain is the tb mantle lithosphere while the se domain is the md mantle lithosphere thrust under the tb crust lithosphere of the north western tb domain compressed between early variscan subductions of the st continental lithosphere from the northwest and the md continental lithosphere from the southeast was pushed south westward by about 50 km though the crust of the south westerly tb promontory is commonly attributed to the md unit apparently it preserves the tb mantle lithosphere the shifted tb lithosphere provides compelling evidence in support of older views suggesting that the zone erbendorf vohenstrauss zev originally belonged to the tilted western rim of the tb unit during the final phase of the assemblage of the bm the rigid tb lithosphere was disrupted by the southward pushing st lithosphere along the newly formed nw se striking jachymov fault zone jfz this lithosphere scale process most likely changed the tectonic regime released subduction related forces and started the gravity dominated tectonics