exhumation processes during post collisional stage in the variscan belt revealed by detailed 40ar 39ar study tanneron massif se france detailed 40ar 39ar geochronology on single grains of muscovite was performed in the variscan tanneron massif se france to determine the precise timing of the post collisional exhumation processes thirty two plateau ages obtained on metamorphic and magmatic rocks sampled along an east west transect through the massif vary from 302 2 to 321 2 ma and reveal a heterogeneous exhumation of the lower crust that lasted about 20 ma during late carboniferous in the eastern part of the massif the closure of the k ar isotopic system is at 311 315 ma whereas in the middle part of the massif it closes earlier at 317 321 ma these cooling paths are likely to be the result of differential exhumation processes of distinct crustal blocks controlled by a major ductile fault the la moure fault that separates both domains in the western part of the massif the ages decrease from 318 to 303 ma approaching the rouet granite which provides the youngest age at 303 6 1 2 ma this age distribution can be explained by the occurrence of a thermal structure spatially associated to the magmatic complex these ages argue in favour of a cooling of the magmatic body at around 15 ma after the country rocks in the western tanneron the emplacement of the rouet granite in the core of an antiform is responsible for recrystallization and post isotopic closure disturbances of the k ar chronometer in the muscovite from the host rocks these new 40ar 39ar ages clearly outline that at least two different processes may contribute to the exhumation of the lower crust in the later stage of collision during the first stage between 320 and 310 ma the differential motion of tectonic blocks limited by ductile shear zones controls the post collisional exhumation this event could be related to orogen parallel shearing associated with crustal scale strike slip faults and regional folding the final exhumation stages at around 300 ma take place within the tectonic doming associated to magmatic intrusions in the core of antiformal structures local ductile to brittle normal faulting is coeval to upper carboniferous intracontinental basins opening

the tertiary structural and thermal evolution of the central alps compressional and extensional structures in an orogenic belt the western alpine arc has been created during the cretaceous and the tertiary orogenies the interference patterns of the tertiary structures suggest their formation during continental collision of the european and the adriatic plates with an accompanying anticlockwise rotation of the adriatic indenter extensional structures are mainly related to ductile deformation by simple shear these structures developed at a deep tectonic level in granitic crustal rocks at depths in excess of 10 km in the early palaeogene period of the tertiary orogeny the main tertiary nappe emplacement resulted from a nw thrusting of the austroalpine penninic and helvetic nappes heating of the deep zone of the upper cretaceous and tertiary nappe stack by geothermal heat flow is responsible for the tertiary regional metamorphism reaching amphibolite facies conditions in the lepontine gneiss dome geothermal gradient 25 c km the tertiary thrusting occurred mainly during prograde metamorphic conditions with creation of a penetrative nw se oriented stretching lineation xi finite extension parallel to the direction of simple shear earliest cooling after the culmination of the tertiary metamorphism some 38 ma ago is recorded by the cooling curves of the monte rosa and mischabel nappes to the west and the suretta nappe to the east of the lepontine gneiss dome the onset of dextral transpression with a strong extension parallel to the mountain belt and the oldest s vergent backfolding took place some 35 to 30 ma ago during retrograde amphibolite facies conditions and before the intrusion of the oligocene dikes north of the periadriatic line the main updoming of the lepontine gneiss dome started some 32 30 ma ago with the intrusion of the bergell tonalites and granodiorites concomitant with s vergent backfolding and backthrusting and dextral strike slip movements along the tonale and canavese lines argand s insubric phase subsequently the center of main updoming migrated slowly to the west reaching the simplon region some 20 ma ago this was contemporaneous with the westward migration of the adriatic indenter between 20 ma and the present the western aar massif toce culmination was the center of strong uplift the youngest s vergent backfolds the glishorn anticline and the berisal syncline fold the 12 ma rb sr biotite isochron and are cut by the 11 ma old rhone simplon line the discrete rhone simplon line represents a late retrograde manifestation in the preexisting ductile simplon shear zone this fault zone is still active today the oligocene neogene dextral transpression and extension in the simplon area were concurrent with thrusting to the northwest of the helvetic nappes the prealpes 35 15 ma and with the jura thin skinned thrust 11 3 ma it was also contemporaneous with thrusting to the south of the bergamasc 35 5 ma and milan thrusts 16 5 ma the oligocene neogene dextral transpression in the central alps is of the same age as the thrusting to the west and southwest in the external french part and to the east in the internal italian part of the alpine arc of the western alps

proterozoic contraction extension tectonics of the southern sao francisco region minas gerais brazil the southern sao francisco region comprises the southern portion of the sao francisco craton which is underlain by archean basement and its fringing proterozoic orogenic belts included in this region are three geologic provinces the quadrilatero ferrifero qf the sao francisco basin and the cordilheira do espinhaco we synthesize preliminary results of ongoing structural analysis reconnaissance field studies and published reports to provide a concise review of the geology of the region and to propose a working tectonic model for its proterozoic evolution interpretation of superimposed folds faults and foliations leads to the conclusion that the southern sao francisco region records the effects of four principal tectonic events that occurred subsequent to deposition of the lower proterozoic minas supergroup the first event resulted in formation of a northwest verging fold thrust belt the second event led to development of high angle reverse faults open folds and basement uplift the third event is manifested in the qf by the formation of normal faults and by the intrusion of mafic dikes these extensional structures may be associated with formation of the sedimentary basins east and south of the sao francisco craton the final event resulted in formation of a west verging fold thrust belt that affected the qf the cordilheira espinhaco and the sao francisco basin the events that we describe can be tentatively associated with regionally recognized orogenic periods trans amazonian uruacuano brasiliano

granite ascent and emplacement during contractional deformation in convergent orogens based on a case study in the central maine belt of west central maine u s a it is proposed that crustal scale shear zone systems provide an eaective focussing mechanism for transfer of granite melt through the crust in convergent orogens during contractional deformation flow of melt in crustal materials at depths below the brittle plastic transition is coupled with plastic deformation of these materials the flow is driven by pressure gradients generated by buoyancy forces and tectonic stresses within the oblique reverse central maine belt shear zone system stromatic migmatite and concordant to weakly discordant irregular granite sheets occur in zones of higher strain which suggests percolative flow of melt to form the migmatite leuco somes and viscous flow of melt channelized in sheet like bodies possibly along fractures cyclic fluctuations of melt pressure may cause instantaneous changes in the eaective permeability of the flow network if self propa gating melt filled tensile and or dilatant shear fractures are produced due to melt enhanced embrittlement in homogeneous migmatite and schlieric granite occur in zones of lower strain which suggests migration of partially molten material through these zones en masse by granular flow and channelized flow of melt carrying entrained residue founded on the central maine belt case study we develop a model of melt extraction and ascent using the driving forces stress conditions and crustal rheologies in convergent especially transpressive orogens ascent of melt becomes inhibited with decreasing depth as the solidus is approached for intermedi ate a h2o muscovite dehydration melting the water saturated solidus occurs between 400 and 200 mpa near the brittle plastic transition during high t low p metamorphism where the balance of forces favors sub horizontal fracture propagation emplacement of melt may be accommodated by ductile flow and or stoping of wall rock and inflation may be accommodated by lifting of the roof at shallower crustal levels and or sink ing of the pluton floor the resultant plutons have sub horizontal tabular geometries with floors that slope down to the ascent conduits although these plutons may have locally discordant relations with country rock structures when viewed at the crustal scale granite ascent and emplacement in convergent orogens are syn tec tonic processes 1998 elsevier science ltd all rights reserved

timing and duration of partial melting and magmatism in the variscan montagne noire gneiss dome french massif central unravelling the detailed pressure temperature time deformation p t t d evolution of magmatic and metamorphic rocks provides essential insights into the timing and duration of partial melting and related plutonism during crustal flow and migmatitic dome formation the montagne noire axial zone mnaz is a migmatitic dome located within the variscan orogen in the southern french massif central the timing of the main thermal event that was responsible for intense partial melting is still highly debated in this study we present new laser ablation inductively coupled plasma mass spectrometry la icp ms age data on micaschists migmatites and granites that clarify the p t t d evolution of the mnaz structurally controlled samples were collected in order to constrain the timing of metamorphism migmatization and plutonism regarding the main structural pattern d1 d2 and d3 d1 and d2 correspond to nappe stacking and dextral transpression respectively d3 is related to vertical shortening and coaxial thinning with a preferential ne sw to e w directed stretching la icp ms analyses on the syntectonic angles soulie and martys granites yielded u th pb monazite ages of 305 1 5 306 1 9 and 314 2 ma respectively five migmatitic rocks sampled in the eastern and central espinouse area yielded in situ ages ranging between 312 2 and 301 2 ma along the dome envelope two garnet staurolite bearing micaschists near saint pons de thomieres village gave in situ u th pb ages of 312 1 2 1 and 309 0 3 1 ma a fine grained gneiss with a d3 fabrics in the eastern dome envelope yield a 208pb 232th mean age at 305 7 3 9 ma all ages obtained in this study for the micaschists migmatites and granites range between 315 and 301 ma we interpret this time span as the record of the high thermal event responsible for intense crustal partial melting within the lower and middle crust the onset of partial melting occurred at ca 315 ma that marked the beginning of transpressional deformation d2 based on structural and petrological studies our new u th pb results suggest that 1 partial melting may have started at ca 315 ma and lasted 15 10 myr and 2 d2 et d3 developed between 315 and 300 ma and were synchronous d1 deformation ended at 315 ma the onset and duration of d1 related to nappe stacking and crustal thickening is still uncertain

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

the mechanism of ascent and emplacement of granite magma during transpression a syntectonic granite paradigm we propose a model for syntectonic ascent and emplacement of granite magma based on structural relations in part of the northern appalachians in the study area in western maine strain was distributed heterogeneously during devonian acadian transpression metasedimentary rocks migmatites at high grades record two contrasting types of finite strain in zones that alternate across strike rocks in both types of zones have a penetrative moderately to steeply ne plunging mineral elongation lineation defined by bladed muscovite fibrolite sillimanite at high grades in straight belts of enhanced deformation rocks have s l fabrics that record apparent flattening to plane strain apparent flattening zones afzs but rocks between these belts have l s fabrics that record apparent constriction apparent constriction zones aczs at metamorphic grades above the contemporary solidus rocks in afzs developed stromatic structure in migmatite which suggests that percolative flow of melt occurred along the evolving flattening fabric stromatic migmatites are intruded by concordant to weakly discordant m scale composite sheet like bodies of granite to suggest magma transport in planar conduits through the afz rocks inhomogeneous migmatite is found in the intervening aczs which suggests migration of partially molten material through these zones en masse probably by melt assisted granular flow inhomogeneous migmatites are intruded by irregular m scale bodies of granite that vary from elongate to sub circular in plan view and seem cylindrical in three dimensions these bodies apparently plunge to the northeast parallel to the regional mineral elongation lineation to suggest magma transport in pipe like conduits through the acz rocks we postulate that the form of magma ascent conduits was deformation controlled and was governed by the contemporaneous strain partitioning magma ascent in planar and pipe like conduits through migmatites is possible because oblique translation during contraction displaces isotherms upward in the orogenic crust to form a thermal antiform within this hot corridor it is the difference in temperature between melt producing reactions in the anatectic zone and the wet solidus for granite melt that enables magma to migrate pervasively up through the orogenic crust without congealing heat advected with the migrating melt promotes amplification of the thermal antiform in a feedback relation that extends the zone of plastic deformation and pervasive melt migration to shallower levels in the crust at the wet solidus we suggest melt flows obliquely toward axial culminations in the thermal antiform which are sites of melt accumulation and perturbations from which magma may escape to form plutons batches of melt that escape from these perturbations may be trapped by tectonic structures higher in the crust or ascent may become inhibited with decreasing depth by thermal arrest and solidification if the rate of arrival of subsequent melt batches exceeds the rate of crystallization at the site of pluton construction melt pressure ultimately may lead to sub horizontal magma fracture or viscous flow of wall rocks may allow lateral spreading the resultant plutons have sub horizontal tabular geometries with floors that slope down to the ascent conduit as the thermal antiform decays the height of sub solidus crust that separates the deepest part of the pluton from the anatectic zone increases consequently pluton inflation declines and solidification leads to infilling of the magma feeder channel to form a root zone that passes downward into migmatite which may explain the difficulty of determining precisely the floor to these deeper segments of large plutons using gravimetry

experimental deformation of partially molten westerly granite under fluid absent conditions with implications for the extraction of granitic magmas the mechanical behavior of partially molten westerly granite was investigated in the temperature range 800 1100 c 250 mpa confining pressure by means of constant strain rate creep and stress relaxation tests the only water in the samples came from the breakdown of hydrous phases biotite minor chlorite and muscovite and alteration products of feldspars thus the amount of melt was controlled by the test temperature and ranged from 3 at 800 c to 50 at 1100 c over that temperature range strength decreased from 500 mpa to less than 1 mpa and a preliminary constitutive flow law for the partially molten rock was obtained to allow extrapolation to low strain rates the comparative viscosity of the melt alone was estimated at 950 and 1000 c from the distance it could be made to penetrate into a porous sand under a known pressure gradient under all conditions deformation of the matrix of solid grains was by brittle fracture only samples containing up to 10 vol melt failed with the formation of a shear fault zone at higher melt fractions melt filled pores collapsed by shear enhanced compaction squeezing the melt into axial cracks above 40 vol melt unfractured solid grains were carried about passively in the flowing liquid there was no evidence of a rheologically critical melt percentage in this system by analogy with the uniaxial compaction of water saturated soils a simple model is erected to describe a two stage process for the extraction of granitic melts from their protoliths with the aid of nonhydrostatic stress shear enhanced compaction is inferred to drive melt into a network of melt filled veins whereupon porous flow through the high permeability vein network allows rapid drainage of melt to higher crustal levels

an experimental deformation study of partially molten amphibolite application to low melt fraction segregation an amphibolite has been experimentally deformed under subsolidus and partially molten conditions to evaluate the influence of melt on the mechanical behavior of a natural mafic rock and to assess low melt fraction segregation processes experiments were performed under fluid absent conditions at 1 8 gpa between temperatures of 650 c and 1000 c these conditions are similar to those of thickened lower continental crust or intermediate depths in subducting oceanic lithosphere at 850 c melt is granitoid in composition and its viscosity is that of wet granite 103 105 pa s the results can be summarized as follows 1 under subsolidus conditions tests at 650 c and 750 c the amphibolite is macroscopically ductile and deformation is homogeneously distributed throughout the sample 2 at near solidus conditions 800 c 0 5 vol melt fractures 1 10 m in width displace hornblende and plagioclase grains and melt formed in situ is found in some of these cracks the formation of a ductile shear zone in one sample is attributed to the presence of very fine grained reaction products from combined dehydration hydration reactions that involve plagioclase the dehydration reaction products have apparently changed the deformation mechanism and even overrode the melt embrittlement process trapping melt in pockets of lower strain with higher melt fractions 10 15 vol broad melt bearing shear zones form and grains within these zones are brittlely deformed 3 at 20 vol melt 1000 c additional weakening occurs but fractures are not observed and the overall deformation is by viscous flow the results show that at low melt fractions 15 vol fluid absent melting reactions can induce fracture in previously ductile rocks this suggests that the percentage at which melt may escape by fracture is lower than the theoretical critical melt fraction or cmf of 26 40 vol in general estimating the fraction of melt at the onset of segregation cannot be predicted by the cmf melt segregation models have to be adapted to variations in such major factors as pressure temperature type of melting reaction rate of melting and strain rate such models will greatly assist our understanding of continental growth and evolution during orogenesis

strain patterns in archaean dome and basin structures the dharwar craton karnataka south india abstract models simulating the role of gravity in the earth s crust are the main source of criteria used for the field identification of diapirs after a short review of the distribution of finite strain in experimental diapir models we describe an example where differential erosion of a tilted craton has provided 3d geometrical information on patterns in a diapiric system at various depths from greenschist to granulitic facies conditions within the archaean continental crust of the dharwar craton south india diapirs and anti diapirs sagducted greenstones are observed at different levels structures and strain are consistent with all the field identification criteria inferred from experimental simulation but some peculiar structural features are also observed which are not predicted by the models these features such as strain trajectory triple junctions result from i the progressive deformation of diapiric domes or sagducted basins ii interferences between diapiric structures body forces and iii the superimposition of diapiric strain fields on regional horizontal shortening boundary forces

maximum and minimum water contents of granitic melts implications for chemical and physical properties of ascending magmas new results obtained by the investigation of liquidus and solidus phase relations in the system qz ab or and by the determination of h2o solubility in haplogranitic melts are used to discuss the evolution of granitic quartzo feldspathic magmas during their ascent in the crust particular attention is given to the role of h2o and its effect on physical properties of ascending magmas the evolution of melt fraction h2o content of the melt and viscosity of melt and magma during ascent are discussed for several examples adiabatic ascent ascent with cooling ascent with crystal fractionation it is shown that decompression melting processes may be very important significant increase of the melt proportion especially at low pressure in magmas ascending along adiabats or with slow cooling rates the comparison of the discussed examples shows that melts formed at low t are significantly less viscous than at high t although the degree of melting may remain very low in the case of dehydration melting at low temperature the low viscosity of the generated melts may enhance the extraction of these melts from the source area to produce leucogranites

mobilization and redistribution of rees and thorium in a syenitic lateritic profile a mass balance study abstract ree th geochemistry and mineralogy have been studied in a lateritic profile derived from a syenite at akongo in sw cameroon ree and th mass balance calculations for the host rock minerals show that at least 70 of the lree s and 50 of the hrees are contained in allanite apatite titanite and epidote and at least 50 of the th is controlled by the same accessory minerals which represent about 2 wt of the unaltered syenite these accessory phases are destroyed during the first stages of weathering causing most of the rees and th to be rapidly released into the soil comparison of the variation in the zr ti and th content as a function of the apparent density of the different zones of the saprolite shows that th is the least mobile element the presence of secondary thorianite tho2 the etched surface on zircon grains and the presence of ti in secondary cerianite support this geochemical interpretation the concentration of thorium was therefore chosen as invariant relative to the concentration of the other elements especially the rees in mass balance calculations most of the rees are leached in the iron rich upper horizons loose nodular horizon iron crust and top of mottled clay horizon where the groundwater table moves saprolite and bottom of the mottled clay horizon the rees are fractionated and redistributed there is a juxtaposition of leached and accumulation zones with precipitation of lree aluminous hydrated phosphates this study supports the existence of two different cycles for the redistribution of elements in the soil 1 1 as dissolved ions in the saprolite horizon and 2 2 as individual particles in the upper part of the profile

granitic magma transport by fracture propagation granitic magmas commonly ascend tens of kilometres from their source terranes to upper crustal emplacement levels or to the earth s surface apart from its obvious bearing on the interpretation of the geology and geochemistry of felsic igneous rocks the magma ascent mechanism critically affects any modelling of the metamorphic evolution of the upper lithosphere as well as its rheology we propose that in general granitic magmas ascend via propagating fractures as dykes in extremely short time periods long distance diapiric transport of granitoid magmas through crustal sections is not viable on thermal or mechanical grounds and there is an apparent total absence of field evidence for diapiric rise of such magmas even in supposed type localities neither the shapes nor internal or external characteristics of granitic plutons necessarily reveal anything about the transport of their precursor magmas these are purely arrival phenomena dictated by local structure kinematics and stress states based on existing numerical treatments of the problem we show that granitic magmas are apparently sufficiently inviscid to travel through fractures to high crustal levels without suffering thermal death by freezing for example a 2000 km3 granitic batholith can be inflated by a 1 km 3m 20 km deep dyke system in less than 900 years the model proposed has numerous far reaching petrological and rheological consequences some of which are outlined

isograds and mineral assemblages in the eastern axial zone montagne noire france implications for temperature gradients and p t history detailed petrography across a metapelitic sequence in the eastern axial zone of the montagne noire france is the basis for a sequence of isograds marking the first appearance of biotite cordierite staurolite andalusite and sillimanite the juxtaposition of low grade biotite free rocks against medium grade rocks at the gently dipping biotite cordierite isograd is attributed to tectonic telescoping of the metamorphic sequence study of mineral assemblages with respect to an afm reaction sequence indicates the staurolite isograd is related to changes in rock composition and complex assemblages in the sillimanite zone may be the result of unstable persistence of minerals formed when metamorphic grade was lower these assemblages are interpreted to contain a record of part of the p t history during which pressure decreased as temperature increased p t profiles show that temperature gradients of 200 300 c km suggested by previous workers are not required to explain the isograd pattern gradients of 37

radiation effects in zircon the widespread distribution of zircon in the continental crust its tendency to concentrate trace elements particularly lanthanides and actinides its use in age dating and its resistance to chemical and physical degradation have made zircon the most important accessory mineral in geologic studies because zircon is highly refractory it also has important industrial applications including its use as a lining material in high temperature furnaces however during the past decade zircon has also been proposed for advanced technology applications such as a durable material for the immobilization of plutonium or when modified by ion beam irradiation as an optic waveguide material in all of these applications the change in properties as a function of increasing radiation dose is of critical importance in this chapter we summarize the state of knowledge on the radiation damage accumulation process in zircon

inverted metamorphic zonation and variscan thrust tectonics in the rouergue area massif central france p t t record from mineral to regional scale

two stage partial melting during the variscan extensional tectonics montagne noire france one of the striking features that characterise the late stages of the variscan orogeny is the development of gneiss and migmatite domes as well a s extensional late carboniferous and permian sedimentary basins it remains a matter of debate whether the formation of domes was related to the well documented late orogenic extension or to the contractional tectonics that preceded migmatization and magmatism are expected to predate extension if the domes are compression related regional anticlines but they must both precede and be contemporaneous with extension if they are extensional core complexes in the montagne noire area s outhern french massif central where migmatization magmatism and the deformation framework are well documented the age of the extensional event was unequivocally constrained to 300 0 ma therefore dating migmatization in this area is a key point for discriminating between the two hypotheses and understanding the late palaeozoic evolution of this part of the variscan belt for this purpose a migmatite and an associated anatectic granite from the montagne noire dome were dated by la icp ms u th pb on zircon and monazite and laser probe 40 ar 39 ar k ar on muscovite although zircon did not record any variscan age unequivocally related to compression 380 330ma two age groups were identified from the monazite crystals a first event at ca 319 ma u th pb on monazite is interpreted as a first stage of migmatization and as the emplacement age of the granite respectively a second event at ca 298 295 ma recorded by monazite u th pb and by the muscovite 40 ar 39 ar system in the migmatite and in th e granite could be interpreted as a fluid induced event probably related to a second melting event identified through the syn extensional emplacement of the nearby montalet leucogranite ca 295 ma ago the age s of these two events post date the variscan compression and agree with an overall extensional context for the development of the montagne noire dome shaped massif comparison of these results with published chemical epma dating of monazite from the same rocks demonstrates that the type of statistical treatment applied to epma data is crucial in order to resolve different monazite age populations

origin of a large scale fold nappe in the montagne noire variscan belt france abstract detailed structural analysis of a large recumbent fold in low grade palaeozoic sediments of the montagne noire and the previously established tectonometamorphic history of its crystalline substratum indicate a two stage development history first relatively homogeneous crustal thickening produced folds with steep axial planes associated with a regional cleavage s1 subsequently these structures became rotated and amplified in a subhorizontal flow with pure shear and simple shear components related to low angle thrusting the development of an associated horizontal crenulation cleavage s2 with a component of vertical shortening of s1 plus evidence for a synchronous decompression of autochthonous units allows this flow regime to be placed in the context of a thrust wedge undergoing gravitational spreading above a continuously contracting footwall decoupling of an extending thrust wedge from a contracting footwall by a basal thrust with low shear resistance explains bulk shortening uplift and exhumation of mid to lower crustal autochthonous basement during thrusting and recumbent folding at higher levels structural relationships in the southern montagne noire demonstrate that traditional tectonic units recognized in this area the so called nappes are in fact unrelated to the recumbent folding as inferred previously these nappes postdate not only the recumbent folding but also a later phase of refolding d3 associated with the formation of a gneiss dome the traditional nappe division can be simplified to a hanging wall and footwall domain of a major decollement that formed during a second syn collisional collapse d4 associated with the formation of intermontane stephanian basins the late timing of the nappes and simplified tectonic structure of the southern montagne noire imply a thinner d2 recumbent fold structure with only one major anticline syncline pair and is in better agreement with the low metamorphic grade of the area

palaeoproterozoic intraplating exposed by resultant gravity overturn near kiruna northern sweden abstract the tectonic history of the kiruna area is reexamined on the basis of four newly available digitised geophysical maps a literature survey and some first hand field observations a basement complex of archaean gneisses was unconformably overlain by a palaeoproterozoic sequence of metabasalt andesite and keratophyric porphyries with interstratified tuffs and sediments the cover was then detached from its basement by an orogeny with a northerly tectonic grain a pile of ductile nappes separated by mylonites in the east pushed westward an imbricate fold thrust belt that may have decoupled over an evaporite unit of regional extent conglomerates and quartzites accumulated in a foredeep migrating in front of a wedge of emergent thrusts the allochthonous cover then developed a vertical north trending foliation in high t low p conditions as it foundered among buoyant diapirs of remobilised granitoid basement the near constant lateral spacing of the centres of these diapirs their mushroom shapes and the absence of contemporaneous volcanic rocks suggest that these diapirs rose in the solid state the region continued to distort episodically even after diapir emplacement ductile shear zones evolved through vein systems to faults as the region cooled and was uplifted before the cambrian the youngest fault movements clearly displace 9000 years old glacial deposits post or late orogenic plutons in the kiruna area appear to be contemporaneous with syn svecofennian plutons further south we therefore suggest reinstating the label svecokarelian for the orogeny in the kiruna area heat advected by melts from the mantle is invoked to account for the archaean basement risking in two convective stages the first as porphyritic lavas the second as solid state diapirs melts rose rapidly and erupted before the svecokarelian orogeny while the slower diapirs ended it we attribute the current southwestern dip of the diapir zone to extension of svecokarelian crust before the svecofennian magmatic arc was built above it

sinking of the dharwar basin south india implications for archaean tectonics abstract archaean tectonic regimes can be constrained by structural analysis of greenstone belts and adjacent granite gneiss masses the late archaean bababudan greenstone belt dharwar craton south india contains a low grade volcanosedimentary basin sequence the dharwar supergroup which unconformably overlies a previously eroded basement made up mainly of granite and gneiss fabric trajectories kinematic indicators and superimposed deformation patterns show that greenstones have undergone a radial converging downward flow with respect to underlying basement rocks as well as strike slip shearing this is attributed to the progressive sagduction gravity sinking of the greenstone belt into its basement along a decollement horizon and to late transcurrent shearing we propose a deformation model for the bababudan belt that may explain common features of many greenstone belts such as decollements and superimposed upright fold patterns which are attributed to the progressive sinking of the supracrustals into their basement the results of our study and other field investigations in the crust of the dharwar craton as well as structural evidence from other cratons zimbabwe and pilbara raise the possibility that a unique continental tectonic regime existed during the archaean controlled by the emplacement of dense volcanic traps mantle underplating and thermal blanketing of the continental crust this tectonic regime differs from that observed within the modern lithosphere as it is characterized by the pervasive development of gravity i e rayleigh taylor instabilities between the volcanic traps and the rest of the continental crust it allowed heat and material transfer through the crust and has to be seen as an efficient recycling mechanism for the young continental crust during periods of mantle underplating the diapiric phenomenon is independent of the boundary forces acting at the limits of the protocontinents and is associated in time and space with large scale accretion events of the continental crust perhaps in hot spot environments

internal structures and dating of complex zircons from meissen massif monzonites saxony abstract u pb zircon ages were determined for accessory zircons from two monzonites from the meissen massif northern bohemian massif germany by shrimp ion microprobe bse and cl imaging revealed that the monzonite zircons showing oscillatory growth zoning often contain inherited cores raman microprobe measurements showed that zoned rims u 277 1426 ppm th 144 910 ppm are slightly disordered to moderately metamict whereas cores have a wide spectrum of structural states from well crystallized to highly metamict the radionuclide u th content of cores varies from a few hundred to more than 20 000 ppm shrimp ion probe measurements on zoned rims interpreted as zircon growth during crystallization of the monzonite magma gave concordant carboniferous 206 pb 238 u ages of 326 6 ma monzonite from leuben and 330 5 ma monzonite from heidenschanze 95 confidence respectively these ages are consistent with recently published 40 ar 39 ar ages of 329 1 2 8 to 330 4 2 8 ma 2 on amphiboles from the same rocks in contrast shrimp measurements on cores range from slightly discordant proterozoic relic ages in low u and th cores to highly discordant data in the most metamict cores previously obtained 207 pb 206 pb ages single zircon evaporation technique in the range 340 16 to 357 16 ma 1 are probably mixed ages between magmatic and inherited zircon components the u pb ages of the monzonite zircons postdate the upper devonian lower carboniferous subduction and collision events in the area by about 20 ma due to their shoshonitic chemical signature which is typical of rocks from destructive plate boundary settings the meissen massif monzonites might be interpreted as originating during these 350 ma events and the earlier 207 pb 206 pb ages appeared to confirm this interpretation however both the 40 ar 39 ar ages and our current results show that only the enrichment of the lithospheric mantle source but not the monzonite intrusion itself was directly related to these events the monzonite intrusion itself took place 330 ma ago in the framework of strike slip tectonics in the elbe valley zone widely occurring zircon cores are indicative of crustal contamination of the magma causing considerable modification of chemical signatures trace element content of the monzonites

crustal thickening and deformation sequence in the footwall to the suture of the variscan belt of northwest spain abstract the sequence of formation and propagation of structures as well as the evolution of crustal thickening in the variscan belt of northwest spain are proposed in this paper the foreland of this mountain belt shows thin skinned tectonics a polyphase deformation with associated regional metamorphism is recognized in the hinterland zones two important culminations the narcea and lugo antiforms are present in an e w cross section through the footwall to the suture of this orogen such culminations are derived from the stacking of several thrust systems generated during two successive periods the sequence of thrust emplacement helps to clarify the formation of different types of structures at the same time in different places in the orogen

accretionary history and crustal evolution of the variscan belt in western europe abstract the variscan belt of western europe is part of a large intra paleozoic belt extending on both sides of the atlantic from the ouachitas in the us and the mauritanides in west africa to the bohemian massif in czechoslovakia and poland the belt was constructed between 500 and 250 ma as a result of convergence and collision between two main continents laurentia baltica and africa after the closure of various oceanic basins iapetus rheic and galicia massif central now recorded as discrete in part cryptic sutures which form the roots of large crystalline nappes in which are found dismembered remnants of oceanic crust and mantle the accretionary history of the variscan belt in western europe is related to the progressive closure of probably two oceanic basins the rheic in the north and another the galicia massif central in the south by respectively southward and northward intraoceanic subduction followed by obduction and intracontinental lithospheric subduction the result is a broad 700 800 km fan like divergent belt which must be considered as a classical obduction collision orogen with a long intracontinental history of about 100 150 ma its arcuate shape is probably due to the northwestward impingement of a promontory of the african continent into laurentia baltica

mantle underplating granite tectonics and metamorphic p t t paths arrival of mantle derived magma at the base of the continental crust leads to melting major crustal reworking and overturn these processes can form both counterclockwise and high t clockwise pressure temperature time p t t paths in the same tectonic episode regions with low p high t counterclockwise p t t paths form where the upper crust moves down to compensate for the upward ascent of granite high t shear zones separate these regions from ascending granite plutons and their migmatitic envelopes where mediumto high pressure rocks record rapid uplift along a clockwise p t t path the increase in pressure in counterclockwise p t t paths does not require tectonic thickening of the crust but results from ensialic mantle heat initiated overturn of the continental crust nor does the attenuation of deep crustal rocks denote extension instead it results from lateral migration of low density melts in the source region of the ascending granites as the crustal overturn is gravity driven to create isostatic equilibrium the high t metamorphic paths end in isobaric cooling the mantle derived magma may have its source in a mantle plume impinging beneath continental crust or in a back arc setting may result from slab pull back and replacement by hot asthenosphere

erratum to metamictisation of natural zircon accumulation versus thermal annealing of radioactivity induced damage annealing of the radiation damage generated in decay events is a common phenomenon in natural zircons we have studied relationships between radiation damage age and radionuclide content of natural zircons on a micro scale the degree of metamictisation was estimated by means of confocal laser raman spectroscopic measurements raman band broadening in well ordered to moderately metamict zircons that have not experienced significant healing of the radiation damage since the time of their growth shows a nearly linear dependence on the fluence this dependence is proposed as a calibration line for examining unknown zircons in order to consider complete or incomplete damage storage which may contribute to the understanding of the low t thermal history of their host rocks seven examples for the application are presented radiation damage may anneal heterogeneously with preferred re ordering in more damaged micro areas results suggest that single point defects do not anneal continuously over geologic periods of time but are in contrast a comparably stable type of radiation damage in contrast to recrystallisation processes structural recovery due to simple thermal annealing is not necessarily connected with loss of radiogenic pb the most prominent example for this are concordant zircons from sri lanka although they are excellent standards for u pb dating sri lankan zircons should not be used as standards for structural radiation damage the experimental work is completed by monte carlo simulations of decay events in zircon

u pb zircon evidence for a pan african granulite facies metamorphism in the central african republic a new interpretation of the high grade series of the northern border of the congo craton abstract the results of u pb zircon and sm nd whole rock analyses are reported on three meta igneous granulitic samples from the central african republic basement formerly ascribed to an archaean complex u pb data point to late proterozoic ages 833 66 ma 652 19 ma and 639 3 ma and sm nd model ages in the 1700 2200 ma range preclude an exclusive derivation from an archaean source whatever their precise geologic significance either first crystallization of zircons at the time of igneous emplacement or complete resetting during metamorphic recrystallization the u pb data clearly rule out an archaean age for the high grade event in contrast with previous opinion indeed the granulite facies metamorphism cannot be older than late proterozoic this is in good agreement with the recent recognition of a major collision type orogen oubanguides belt of pan african age in this area it is proposed that two main generations of granulitic rocks occur at the northern margin of the congo craton an archaean one in the foreland of the oubanguides belt e g ntem complex cameroon and a pan african one in the internal zones of this orogen

on the origin of early proterozoic gneiss domes and metamorphic nodes northern michigan biotite 40 r 39 r cooling ages from medium pressure 500 600 mpa rocks in the watersmeet district northern michigan suggest significant cooling uplift and concomitant deformation during gneiss dome formation at 1755 ma well after the close of the 1870 1830 ma penokean orogeny however an 1822 ma hornblende plateau date indicates that the isograds surrounding the dome are penokean in age we attribute gneiss dome formation and doming of penokean aged isograds to an episode of orogenic collapse superimposed on an earlier history of crustal shortening this contrasts with the compressional origin for gneiss domes preserved in the low pressure 200 300 mpa republic district the different origins may reflect the fact that collapse was localized along the overthickened region of the orogenic belt in contrast to the watersmeet area hornblende and biotite 40ar 39ar ages obtained from the republic area are 1720 1680 ma given the relatively shallow depth of this region it is unlikely that temperatures re

opposite vergence of nappes and crustal extension in the french italian western alps in the internal pennine zone of the french italian western alps dora maira monviso rocciavre massifs and queyras region the structural trends define an apparent dome geometry that is truncated to the east by the po basin this geometry is classically interpreted as a backthrusting system the kinematic history of the area has been investigated using structural metamorphic and geochronologic data on the basis of which two main deformation episodes have been distinguished that occurred under differing metamorphic conditions 1 the first deformation occurred under greenschist facies metamorphism it caused a regional scale foliation and an ew trending lineation that lie parallel to ductile deformation zones and thrust contacts sense of shear indicators associated with the lineation indicate contradictory top to the east and top to the west senses of shear from these it is deduced that coeval east and west vergent thrust faulting accommodated major displacements between nappes phengite ar ar data yield late eocene early oligocene ages for the greenschist facies metamorphism 2 the second deformation caused the widespread development of high angle shear bands and normal faults on opposite sides of the dora maira massif resulting in the apparent dome geometry of the massif a steep 50 80 network of west dipping normal faults is particularly well developed on the west side of the dora maira massif differences of 2 3 kbar and 100 150 c between juxtaposed monviso rocciavre and queyras nappes implies 8 km of vertical displacement along the monviso upper tectonic contact these normal faults formed in a more brittle regime only a post early oligocene age can be estimated from microstructural studies for these final movements these deformations postdate hp lt metamorphism and resulted in the juxtaposition of nappes of distinctly different origin age grade and p t t history a kinematic model is presented in which opposite vergence of nappes followed by large scale crustal extension occurred in response to uplift during the convergence between the european and south alpine plates this model may account for rapid uplift rapid unroofing and cooling of high pressure low temperature rocks thereby explaining the absence of amphibolite facies metamorphism overprint in this part of the alps that may have led to the preservation of coesite bearing rocks in the southern part of the dora maira massif

characteristics and mode of emplacement of gneiss domes and plutonic domes in central eastern pyrenees abstract gneiss domes and plutonic granitoid domes make up almost 50 of the pre hercynian terrains in the central and eastern pyrenees from a structural study of the shape and internal structure of the domes and of their relationships with the enclosing rocks it can be shown that both types of domes were emplaced diapirically during the major regional deformation phase and the peak of regional metamorphism the study also shows that the internal structure the overall shape and general behaviour relative to the host rocks are similar for plutonic domes and for gneiss domes this appears to be in good agreement with h ramberg s 1967 gravity deformation and the earth s crust academic press london 1970 model studies in relation to intrusion of plutonic bodies in mechanisms of igneous intrusion edited by newall g rast n geol j spec issue 2 261 286 model studies showing that dome or mushroom like structures similar to those observed develop when there is a small viscosity ratio between the rising body and its enclosing medium this implies a high crystal content for the granitoid magma this crystal content has been estimated by i calculating the viscosity and density in natural conditions from petrological data for the magma considered as a suspension using the model and program of j p carron et al 1978 bull soc geol fr 20 739 744 ii using the recent results of experimental deformation of partially melted granites of i van der molen m s paterson 1979 contr miner petrol 70 299 318 and ii comparing the preceding results with the data obtained by deformation experiments on rocks similar to those enclosing the domes the minimum crystal content for the development of a dome like structure has been thus estimated to about 70 i e a value very close to that estimated by van der molen paterson 1979 to be the critical value separating the granular framework flow from suspension like behaviour the effect of small variations in the viscosity of the rising body are then simulated by centrifuge experiments these small variations appear to exert a strong control on the shape and rate of rise of the domes they are thought to be sufficient to account for the variations in shape and structure and the level of emplacement of the different types of gneissic and plutonic domes finally more complex experiments with models built in order to simulate as closely as possible the natural structural evolution of the region as deduced from petrological and structural data are reported their implications for the regional interpretation of the relationships between gneissic and plutonic domes is then discussed

tectonics and seismic sequence stratigraphy a two day meeting convened by graham williams university of keele and angela dobb mobil north sea ltd was held at the geological society in london on 6 and 7 february 1991 the aim of the meeting was to address the problems of the interaction between eustacy sediment supply rate and tectonics in seismically imaged basin sequences the meeting discussed the importance of tectonics in generating recognizable sequence boundaries on both a regional and local scale it was felt that eustacy as the dominant factor in controlling basin stratigraphy has been overplayed in the past there were 190 participants at the conference representing the oil industry and academia some 21 oral presentations were given covering theoretical aspects of sequence stratigraphy numerical modelling and field analogues case histories from europe north america africa and the uk were presented basin scale numerical modelling in the simplest case stratigraphy in extensional basins and passive margins may be subdivided into pre syn and post rift sequences representing stratigraphic fill related to a single rifting event a widely recognized unconformity or megasequence boundary between syn and post rift sequences is referred to as the breakup unconformity numerical modelling on a basin scale in extensional regimes has been carried out by m j newall and n j kusznir shell research liverpool who concentrated on the post rift subsidence history and palaeobathymetric evolution of snorre magnus and other areas of the northern north sea basin subsidence resulting from thermal effects compaction and sediment loading has been numerically modelled using flexural

p t t deformation paths recorded by kinzigites during diapirism in the western variscan belt golfe du morbihan southern brittany france the high grade metamorphic rocks of southern brittany underwent a complex tectonic evolution under various p t conditions high p high t related to stacking of nappes during palaeozoic continentcontinent collision the east to west thrusting observed in the whole belt is strongly perturbed by vertical movements attributed to the ascent of anatectic granites in the high t area the field reconstruction of subvertical closed elliptical structures in gneisses and migmatites associated with the subhorizontal doubly radial pattern of stretching lineation in the mica schists suggests the existence of an elliptical diapiric body buried at depth beneath the present erosion level deformation is associated with a complex p t evolution partly recorded in aluminous gneisses kinzigites e g morbihanites a chronology of successive episodes of mineral growth at different compositions is established by detailed studies of the mineral microstructure relationships in x z sections using the deformation partitioning concept low and high strain zones several thermometric and barometric calibrations are applied to mineral pairs either in contact or not in contact but in equivalent microstructiiral positions with respect to the deformation history this methodology provides a continuous microstructural control of p t variations through time and leads to three p t t d paths constructed from numerous successive p t estimations path 1 is a clockwise retrograde path preserved in low strain zones which records general exhumation movements after crustal thickening paths 2 and 3 are clockwise prograde retrograde paths from high strain zones they are interpreted and discussed in the light of models of crustal anatexis and upward movement of magma diapirism deformation and p t effects induced by diapirism can be distinguished from the general deformation metamorphic history of a belt and would seem to be produced during a late stage of its history the present microstructural petrological approach to defining successive mineral equilibria in relation to progressive deformation steps provides a far more accurate evaluation of the metamorphic evolution than is possible by standard thermobarometry

pan african structural evolution of paleoproterozoic basement gneiss and cu co mineralized shear zones in the domes region of the lufilian belt mwombezhi dome zambia abstract in the domes region of the lufillian belt world class stratiform cu co ni u pb zn deposits are closely associated with tectonized pre existing basement unconformities viz the pre katanga basement decollement kd in the kd footwall on the mwombezhi dome the lumwana mine exploits mineralization within

mid crustal magmatic sheets in the cascades mountains washington implications for magma ascent abstract diking diapirism ascent along faults and ascent during heterogeneous ductile flow have all been championed as the most important means of magma ascent in the crust we suggest that these mechanisms are end members in a complex spectrum of ascent processes in an attempt to evaluate which combination of ascent processes formed sheet like bodies in the mid crustal 20 25 km entiat pluton washington we examined the tip regions of these sheets the sheets have length width ratios ranging from 6 to 75 with increasing ratios strongly correlated to decreasing sheet tip radii from 850 to 100 m and decreasing ratios of tip diameter sheet width from 0 66 to 0 33 thus these bodies have geometries falling between those of dikes and those associated with elliptical diapirs the sheets are not associated with faults or fracture zones extending from their tips instead sheet walls are oriented parallel to the axial planes of upright syn emplacement folds in sheets with high length width ratios magmatic foliations in sheets are folded or parallel to axial planes of host rock folds with decreasing length width ratios margin parallel foliations in both sheet and host rock are increasingly common our studies indicate that the sheets are emplaced at high angles to 1 not 3 as proposed in elastic dike models and are always associated with complex viscoelastic flow of host rock these observations rule out elastic dike and fault models and instead favor diapiric rise of magma sheets during viscoelastic behavior of host rock

eocene early miocene foreland basin development and the history of himalayan thrusting western and central nepal sedimentologic petrographic and u pb detrital zircon ages from middle eocene through early miocene sedimentary rocks in the lesser himalayan zone of western and central nepal indicate that a peripheral foreland basin system had developed in the eastern himalayan collision zone by middle eocene time the shallow marine eocene bhainskati formation accumulated in a back bulge depozone between a southward migrating forebulge and the indian craton migration of the forebulge through this region during eocene oligocene time produced a regional unconformity that spans 15 20 myr by early miocene time the forebulge unconformity was onlapped by the distal fringes of the southward migrating foredeep depozone represented by fluvial deposits of the dumri formation continued southward migration of the foredeep during the neogene accommodated the fluvial siwalik group light mineral provenance data and u pb detrital zircon ages suggest that the bhainskati was derived partly from tethyan sedimentary rocks of the tibetan himalayan zone during initial growth of the himalayan fold thrust belt the dumri was derived from metasedimentary and crystalline rocks of the greater himalayan zone during emplacement of the main central thrust and contemporaneous tectonic unroofing by normal faulting along the south tibetan detachment system the lesser himalayan crystalline thrust sheets were emplaced soon after deposition of the dumri formation 15 10 ma paleocurrent and lithofacies data from the dumri formation indicate deposition by west southwestward flowing rivers that drained into the indus portion of the himalayan foreland basin system during the early miocene thick channel sandstones in the lower dumri may represent the early miocene counterpart of the modern ganges river eastward diversion of the ganges drainage system to near its present location had occurred by 15 ma as the high standing aravalli range on the northern indian shield approached the front of the fold thrust belt assuming reasonable values for the flexural rigidity of indian lithosphere the time span of the forebulge unconformity yields a velocity of 14 33 mm yr for the southward migration of the fold thrust belt relative to india this range of values is consistent with neogene and present day estimates and suggests that only one third to one half of india eurasia convergence has been accommodated by shortening in the himalayan fold thrust belt since the onset of collision

age of metamorphism and deformation in the montagne noire dome french massif central tapping into the memory of fine grained gneisses using monazite u th pb geochronology recent work has revealed that the montagne noire dome located in the foreland of the variscan belt french massif central contains a record of nearly coeval late variscan eclogitization and migmatization given these new results it is important to understand the chronology of events that produced high grade metamorphism and exhumation using u th pb dating of monazite this study confirms ages of 315 300 ma for high t metamorphism in the augen gneiss that makes up a large fraction of the montagne noire dome and documents for the first time 295 ma monazite ages in compositionally varied fine grained gneisses that form folded continuous layers within the core of the dome the fine grained gneiss layers are intensely sheared and are interpreted to have localized late high t deformation in the core of the dome these sheared fine grained gneisses form a network of shear zones that were kinematically linked to the extensional and strike slip deformation zones that exhumed the montagne noire dome in a pull apart s l domain continued deformation recrystallization and fluid flow within these shear zones likely drove rejuvenation of monazite for 5 million years after much of the melt had crystallized in the montagne noire dome

reappraisal of variscan tectonics in the southern french massif central abstract the comprehensive kinematic analysis of the variscan fold nappes and associated wildflysch in the southeastern montagne noire shows that the nappes were thrust northeastwardly along the cevennes fault as a result of strike slip dominated sinistral transpression between 340 and 323 ma macro and map scale recumbent folding and nappe stacking were achieved by ene directed strike parallel thrust shearing the recumbent folds evolved into sheath folds by interplay of continued strike parallel thrust shearing and strike normal shortening manifested by north verging folds that grew during wildflysch sedimentation these results call for a tectonic reassessment of the southern side of the variscides the foreland basin was decoupled from the hinterland along orogen scale sinistral fault s trending at a high angle to the orogenic trend which interacted with a variety of hinterland ward thrust systems the shift from sinistral transpression to dextral transtension along the cevennes fault after 317 ma accompanied the closure of the iberian armorican syntax by accommodating northeastward escape and gravitational collapse of the northern limb of the syntax these results illustrate the impact of the shift from plate convergence controlled to escape controlled kinematics on shear sense reversal and lower crust exhumation along long lived strike slip fault systems

variscan crustal thickening extension and late overstacking during the namurian westphalian in the western montagne noire france abstract study of the structural relationships between major lithotectonic units that is the axial zone x schists and southern thrust sheets on the southwestern slopes of the montagne noire in southern france has cast new light on the main processes and timing for this segment of the variscan mountain belt the contact between x schists and the gneissic axial zone is a major shear zone early movements to the southwest along this contact appear to have been contemporary with the emplacement of nappes on the south slope subsequent anatexis of the axial zone was synchronous with regional dextral strike slip shearing and granitization at the end of anatexis sealed the transcurrent structures extension commenced during the final stage of granite crystallization finally crustal shortening with north northeast vergence completed the deformation and controlled the emplacement of gold mineralization we propose a new model for the geodynamic evolution of the montagne noire during namurian westphalian times crustal thickening took place around 330 325 ma and was followed by uplift anatexis and dextral transcurrent shearing between 325 and 319 ma the subsequent unroofing of the thickened domain continued between 319 and 315 ma during a stage of doming and extensional detachment this crustal thinning was succeeded in the westphalian early stephanian 315 300 ma by compressive tectonism during which the lithological stacking of the southern thrust sheets was achieved

on uplift and exhumation during convergence a simple kinematic model is used to illustrate the vertical motions of rocks during convergent deformation and erosion at the surface it is shown that rocks may move upward or downward in the crust depending on the relative rates of erosion and thickening and depending on their initial depth in the crust exhumation during thickening can only occur if rapid denudation accompanies the thickening process during homogeneous thickening with erosion that is elevation dependent the initial depth from which rocks can be exhumed is only determined by the density distribution in the column and is independent of erosion or thickening rates the maximum initial depth from which rocks can be exhumed is of the order of 30 km or less there is therefore no conflict in the observation of synchronous exhumation and convergent deformation if the peak pressure of rocks is equivalent to no more than 30 km of burial it is also shown that uplift defined as vertical motion of the surface with respect to a reference level for example the geoid and exhumation defined as vertical motion of rocks with respect to the surface may follow different patterns in time and that the difference between the two evolutions may be a useful indicator of the exhumation process the model serves to emphasize the important differences between uplift and exhumation which are often not distinguished in the literature

collapse of a hercynian tibetan plateau into a late palaeozoic european basin and range province the large width the importance of crustal shortening and the likelihood of thick crust in the late palaeozoic hercynian or variscan belt of southern europe suggest a tectonic evolution analogous to the evolution of the tibetan plateau and its neighbouring himalaya1 and of the altiplano and other high plateaux of the andes here we extend dewey and burke s analogy1 of the hercynian chain with tibet to include the subsequent stage of pervasive normal faulting and crustal extension and thinning which characterize the active tectonics of tibet parts of the high andes and the basin and range province of western north america it appears that normal faulting and rifting also characterize the late palaeozoic tectonic activity of the hercynian chain

behavior of sm and nd in a lateritic soil profile a study of lateritic soils and samples of ground and river waters was carried out in the nsimi zoetele a tropical watershed in the southern cameroon the nd isotopic compositions and concentrations of nd and sm were determined it was found that the nd isotopic composition of the river waters was much more radiogenic than the parent rocks and that the nd in the waters is not homogeneous but is carried by different dissolved and complexed components that are not isotopically homogenized the soil profile shows a regular increase in e nd going from the parent rock e nd 36 to e nd 18 near the top of the profile the nd transported in the river is thus not representative of the parent rock but reflects the results of differential weathering of constituent minerals and the redeposition of ree in phosphates and a significant contribution of radiogenic nd from dust the concentration of nd in the river water is far above that found in temperate climate rivers and thus this type of tropical river may play a dominant role in the marine nd and ree budget it is suggested that the correlation of ree with doc is related to doc fixing some dissolved ree but that the ree in solution is governed by other mechanisms no major shifts were found in sm nd however a regular progression from the parent rock through the lateritic profile was found the upper laterite profile shows large almost uniform depletions in all ree below tb and enrichment above complementary behavior was found in the lower part of the section the concentration of nd relative to the immobile elements zr and ti in the laterite is depleted by a factor of 10 th nd and sm are enriched in the lowest zone sampled and must reflect redeposition of ree from the upper part of the weathering section and is associated with phosphate formation it is concluded that the soil evolution involves both differential dissolution of primary phases from the parent rock significant to major input of ree from atmospheric dust from other regions and the formation of diagenetic phases particularly phosphates

fluids p t paths and the fates of anatectic melts in the earth s crust this paper presents a model for the longevity and physical behaviour of anatectic melts in the earth s crust it is assumed that a thermal anomaly has caused partial fusion of biotite plagioclase quartz rocks to produce a granitic melt and restitic solids the focus is on the potential chemical and physical outcomes as a function of the various p t fluid conditions rocks undergoing fluid present melting will sustain an increase in bulk density and will become more gravitationally stable diapiric up welling of such material should not occur in fluid absent melting the bulk density will decrease and foster gravitational instability stresses in the surrounding rocks could however lead to fracture and rapid evacuation of the melt with excess fluid and without melt segregation isobaric cooling will cause immediate freezing if t rises above the biotite quartz melting reaction freezing will still occur at the original reaction t diapiric up welling should not occur biotite bearing diatexites might be formed by large degrees of fusion but magma ascent is unlikely in isothermal decompression immediate freezing will ensue and the migmatite should contain orthopyroxene rather than biotite with excess fluid and melt segregation cooling will induce freezing at the fluid saturated granite solidus magma ascent could however occur even for xflh2o 1 in isothermal decompression melt will persist to the granite solidus for the imposed xflh2o limited magma ascent could occur all granites should contain primary igneous biotite for fluid deficient rocks without melt segregation freezing will occur at the t of the original melting if the reaction is overstepped solidification will occur at the original xflh2o magma formation is unlikely migmatites should contain either pyroxene retrogressed to biotite or biotite alone on decompression immediate freezing is likely if t remains at the biotite quartz reaction for higher t melt will survive decompression and freeze at the fluid saturated solidus for some increased xflh2o magma ascent is possible and the melt will freeze very close to the wet granite solidus in fluid deficient rocks with melt segregation cooling and crystallization will cause fluid saturation in situ segregated melt will freeze at the original xflh2o melt ascent will however cause exsolution of a co2 rich fluid with fluid escape melt will freeze close to the wet granite solidus an increase in melt proportion may accompany magma ascent or source up welling any low p granite intrusion should be charnockitic for fluid absent rocks without melt segregation freezing will occur on cooling to the t of the original reaction only transitory migmatites could be formed but melt will survive decompression almost to the h2o saturated granite solidus the rocks will reveal former melt presence only in the existence of veins of coarse retrograde biotite with isothermal decompression further melting may occur forming an orthopyroxene bearing granulite facies migmatite in fluid absent situations with melt segregation magma could be efficiently transported to higher crustal levels if the melt proportion were insufficient for magma formation granulite facies migmatites would result ascending melt would progressively dissolve any entrained restite the granites produced should be charnockitic to aid in interpretation of the modelling results we suggest that it may be slightly more common for the highest grade metamorphic terranes to exhibit isobaric cooling paths that fluid present anatexis is normally limited to the shallower parts of the crust and that fluid absent reactions are more likely at greater depths we conclude that the best models for natural fluid present anatexis are those dealing with essentially pure h2o fluid and that fluid deficient conditions will dominate over fluid excess we also believe that it is common for melts to be physically segregated on scales larger than the diffusion paths of many elements

mineralogy and stable isotope geochemistry of hydrothermally altered oceanic rocks mineralogical and isotopic variations observed in altered glassy and crystalline rocksfrom the east pacific rise and the mid atlantic ridge provide information about the temperatures of alteration and seawater rock ratios for various hydrothermal regimes within the oceanic crust a systematic increase in alteration temperature is evident for the glassy rocksin the sequence 1 nontronite and celadonite vesicle fillings 35 c 2 saponite rich pillow breccias 130 170 c 3 calcite rich greenstone breccias and epidote rich greenstone 200 350 c theseresults include the highest temperatures thus far reported for saponite formation the seawater dominated hydrothermal alteration process that formed the saponite rich pillow breccias is characterized by high water rock ratios 0 1 low to moderate temperatures a seawater origin of most of the carbon in vein calcites 13 c 0 and the predominance of fe rich saponite and calcite as secondary phases greenstones chlorite quartz epidote and greenstone breccias chlorite quartz albite calcite are altered in a rock dominated system with lower water rock ratios 50 1 to 13 c 4 the crystalline rocks diabase gabbro and metagabbro are affected to varying degrees by pervasive high temperature seawater interactions that commence soon after solidification producing varying proportions of fine grained secondary minerals including talc smectite chlorite vermiculite actinolite and sodicplagioclase hydrothermal solutions derived from alteration of the crystalline rocks are of the appropriate temperature and isotopic composition to alter the overlying glassy rocks to the observed mineralogies as well as being the source of metal rich deposits associated with the oceanic spreading centers

internal structures of zircons from archaean granites from the darling range batholith implications for zircon stability and the interpretation of zircon u pb ages internal structures in zircons from granitoids from the late archaean darling range batholith show secondary features revealed by hf etching which record reconstitution of the zircons and modification of the distribution of trace elements during post crystallisation cooling of the granitoid zircons from the granites commonly contain unzoned to weakly zoned cores surrounded by rims showing oscillatory zoning which has been modified by recrystallisation the most striking feature is the development of high trace element concentration areas found in zircons from a number of granites these structures range from enhanced trace element concentrations in primary zones to a single accumulation of most trace elements in one band about half way between the outer edge and the centre of the zircon in any zircon the extent of the concentration of trace elements towards the formation of a single trace element band appears to be inversely related to the fading and broadening of primary oscillatory zones in the outer rim this suggests that the trace element bands formed by migration of trace elements from the outer primary zones to new concentration sites on an inner set of primary zones this explanation is supported by the formation of multiple curved trace element bands that transgress primary zoning and the determination of younger shrimp ages on depleted zircon outer rims compared to remnant primary oscillatory zoned areas of the zircon and unzoned centres also observed in some granite zircons is a finely convoluted zoning which overprints oscillatory zoning in parts of a zoned zircon and in rare cases occurs throughout the zircon this structure is explained in terms of secondary migration and reconcentration of trace elements in curved bands all structures can be transgressed by generally rounded lobes and patches of low u weakly nebulously zoned zircon this is interpreted as a late stage interaction between the zircon and fluids formed during cooling and crystallisation of the granitoid resulting in recrystallisation of affected parts of the zircon with accompanying loss of trace elements from the zircon

pervasive melt migration from migmatites to leucogranite in the shuswap metamorphic core complex canada control of regional deformation abstract the shuswap metamorphic core complex exhumed in the hinterland of the canadian cordillera displays a 15 km structural section from migmatites to leucogranite it comprises a lower unit dominated by anatectic migmatites exposed in the core of domes at higher structural level the transition from a middle amphibolite facies unit to an upper crustal unit is marked by a low angle detachment associated with the emplacement of laccoliths of leucogranite the amphibolite facies middle unit is permeated by a network of granitic veins feeding the laccoliths and structurally connected to the migmatites of the lower unit this paper describes the geometric textural and structural characteristics and the distribution of the granitic fraction at the outcrop scale in the amphibolite facies unit separating the migmatites from the leucogranite this analysis constrains the mechanisms of melt migration within a plurikilometer scale section across high grade rocks of the middle crust in fertile lithologies metapelites felsic amphibolites the granitic fraction in part generated in situ forms a diffuse network of concordant veins feeding discordant veins and structural sites such as the foliation shear zones and boudin necks discordant granitic veins are oriented perpendicular and to a lesser extent parallel to the mineral and stretching lineation in refractory lithologies the granitic fraction is dominantly intrusive and displays sharp contacts with the host rock however distribution of the granitic fraction shows the same characteristics as for fertile lithologies the melt fraction migrated through a network of fractures taking advantage of mechanical weaknesses such as the foliation plane or forming dikes oriented dominantly perpendicular to the regional stretching and mineral lineations these geometric and textural characteristics suggest that granite migration in these rocks is achieved dominantly by viscous flow of the melt solid through the solid matrix driven by its buoyancy and controlled by mechanical anisotropy of the rock and local dilation created by heterogeneous deformation the relationship between the regional fabric and the distribution of the granitic fraction indicates that regional deformation incremental and finite played a major role in providing pathways for melt migration the formation of laccoliths of leucogranite at higher structural levels suggests that upward melt migration led to accumulation of granitic magmas these observations are consistent with a model of pervasive melt migration through a network of interconnected concordant and discordant granitic veins feeding laccolithic plutons extracted from the dominantly solid matrix during vertical thinning and compaction at a regional scale the presence of both transposed and intact granitic networks indicate an overlap between deformation and granite migration as a continuum or in pulses the methodology and terminology proposed in this paper provide some insights on the behavior of the granitic fraction in the middle crust this approach should be applicable to other high grade terrains exhumed in the cores of orogenic belts

cenozoic and mesozoic structure of the eastern basin and range province utah from cocorp seismic reflection data cocorp seismic reflection data collected from the eastern basin and range in west central utah provide information on cenozoic extensional tectonics mesozoic thrusting and their interrelationships those data show a series of remarkably continuous low angle reflectors that extend more than 120 km perpendicular to strike and can be traced as deep as 15 20 km over that distance none of these events are significantly cut by any high angle normal faults a major detachment beneath the sevier desert can be traced from a surface zone of normal faulting to a depth of 12 15 km with a regional apparent westward dip of 12 tentative correlation of upper and lower plate events suggests 30 60 km of extensional displacement on this detachment whether this structure is a reactivated mesozoic thrust is uncertain west steepening splays off the end of the detachment reach depths of 20 km and may represent a major mesozoic ramp or zones of distributed ductile shearing during extension some events are interpreted to be mesozoic thrusts of which at least one beneath the house range has been reactivated during the cenozoic the snake range decollement dips gently east and has a sense of cenozoic displacement opposite to that of other cenozoic detachments farther east deep events are most numerous beneath the east side of the sevier desert where they occur to depths of 30 km at the top of or perhaps partly within the anomalously low velocity upper mantle

metamorphic core complex dynamics and structural development field evidences from the liaodong peninsula china east asia metamorphic core complexes mcc constitute remarkable features within wide rifts based on analogue and numerical modelling mcc dynamics and structural development are mainly controlled by first geothermal gradient second the compositional layering and after the strain rate and partial melting in the late mesozoic continental extension occurred in east asia leading to the development of mcc magmatism and extensional sedimentary basins based on an integrated study i e structural and finite strain analysis petrofabrics anisotropy of magnetic susceptibility ams and u pb on zircon dating this paper aims at constraining the tectonic evolution and deformation mechanisms in the south liaodong peninsula ne china the gudaoling massif is identified as a migmatitic mcc developed from upper jurassic ca 157 154 ma to early cretaceous ca 128 113 ma intrusion of the yinmawanshan synkinematic pluton early cretaceous to the south of the dome marks out the final stages of shearing along the gudaoling detachment zone and of exhumation of the mcc into the upper crust the gudaoling mcc and the yinmawanshan pluton stay in line with the coeval south liaodong mcc to the south all making a 140x30km wide extensional band formed during a regional e w to nw se crustal stretching the area shows a bi phased development with a slow and a fast stage which correspond to 1 crustal necking and 2 dome amplification exhumation stages according to published thermo mechanical modelling results finally the gudaoling mcc lower unit almost exclusively displays migmatites and anatectic granitoids of late jurassic and early cretaceous respectively occurrence of partial melting during earlier stages of extension seems controlling the initiation of mcc as a soft anomaly within lower crust in east asia a regional scale thermal event during jurassic cretaceous times may have significantly reduced the bulk lithosphere strength and locally induced partial melting emphasizing pervasive deformations and development of a wide rift system

hydrothermalism and diapirism in the archean gravitational instability constraints abstract in archean granite greenstone terrains greenstone belts are interspersed between granitic plutons to form dome and basin structures although the hypothesis is widely debated gravitational instabilities have been invoked to explain such structures which are typical of archean cratons we used a numerical approach based on a rayleigh taylor model to examine the possibility that these structures result from gravitational processes giving a special emphasis to the alteration of the mafic ultramafic volcanic rocks we investigated a the influence of contemporaneous hydrothermal activity upon density and viscosity of mafic ultramafic volcanics and b the perturbation of thermal conditions in the granitoid basement on its rheology variables included 1 varying proportions of mafic ultramafic volcanics and their thickness relative to a granitoid basement 2 density and viscosity differences between fresh and altered rocks 3 kinetics of alteration and 4 thermal regime and viscosity of the basement we adopted a multi layer model with variable viscosity in which the upper layers represent an ultramafic mafic volcanic pile and the underlying layers a granitic basement we show that the high viscosities of fresh volcanic rocks and underlying basement granitoids normally prevent the development of gravity driven deformation the conditions required to produce the observed structures are realized only when a a high proportion of the volcanic rocks are altered to a low viscosity assemblage and b the underlying granitic basement is moderately heated or migmatized

experimental evidence for mobility of zr and other trace elements in soils a soxhlet extraction was carried out over a period of 27 d on a column comprising 3 cm of quartz overlain by 4 cm of soil from the b horizon and then 1 cm of soil from the a horizon of a granitic podzol major and trace elements were leached from the column and accumulated in a reservoir at the base of the column total loss of elements from the soil over the course of the experiment ranged from 0 002 to 1 wt with major elements and the light and heavy rare earth elements ree showing the largest percentage losses zirconium 0 002 and then al 0 008 showed the lowest percentage loss the light ree were leached out of the soil preferentially to the mid ree all elements showed accumulation by a factor of 2 to 11 in the quartz layers at the base of the column particularly in the upper first 1 cm of the quartz major elements were leached from the column at a rate of 0 02 to 0 59 mol h 1 whereas zr nd sm gd dy rb and sr were leached at the rate of 0 5 to 30 10 6 mol h 1 concentrations of other ree in the reservoir increased over the duration of the experiment but they were poorly correlated with time so leaching rates were not calculated normalization of the major element leaching rates to take into account the constant flushing of water through the column the average annual rainfall in the allt a mharcaidh catchment in scotland from where the soil was sampled and the cross sectional area of the soil in the column together with the temperature of the soil in the column 70 c compared with the average annual temperature of the allt a mharcaidh catchment 5 7 c gave major element release rates from the soil of 0 002 to 0 97 meq m 2 yr 1 depending on the choice of ea the dissolution activation energy which are generally less than those measured in the field of 0 1 to 40 9 meq m 2 yr 1 calculations showed that despite the redistribution and loss of zr from the column assumptions of zr mobility would have had a negligible effect on calculated element release rates of na ca fe and mg however significant underestimates of the release of k 5 ti 57 al 5 and si 10 as well as some trace elements e g nd 23 rb 54 sr 24 would have occurred concentrations of ca and sr leached from the column correlated well rsq 0 93 p 0 01 supporting the idea of the use of sr release as a proxy for ca release in weathering rate calculations the release rates and percentage loss of ree from the soil varied between elements indicating that ree distribution patterns of rocks and soils may not be preserved in drainage waters

impact of erosion and decollements on large scale faulting and folding in orogenic wedges analogue models and case studies deformation mechanisms long term kinematics and evolution of fold and thrust belts subjected to erosion are studied through 2d analogue experiments involving large convergence first order parameters tested include 1 decollements and or plastic layers interbedded at different locations within analogue materials and 2 synconvergence surface erosion weak layers depending on their location in the model favour deformation partitioning characterized by the simultaneous development of underplating domains in the inner part of the wedge basal accretion and frontal accretion where the wedge grows forward interaction between tectonics and surface processes influences this behaviour development of antiformal thrust stacks controlled by underplating shows small and large scale cyclicity thin plastic layers induce folding processes which are studied at wedge scale recumbent and overturned folds with large inverted limbs develop in a shear induced asymmetric deformation regime via progressive unrolling of synclinal hinges surface erosion and underplating at depth induce further rotation passive tilting and horizontalization of fold limbs model results give insights to discuss the mechanisms responsible for the large scale structures i e antiformal nappe stacks klippen and kilometre scale recumbent fold nappes encountered in several mountain belts such as the montagne noire french massif central the galicia variscan belt spain and the northern apennines italy supplementary material raw data of the experiments are available at www geolsoc org uk sup18658

paleomagnetic evidence for rotation during stephano permian extension in southern massif central france we present results of a preliminary paleomagnetic study devised as a test of a tectonic model proposed by van den driessche and brun 1 to interpret the main structural features of the montagne noire south massif central france this model suggests that crustal extension during stephanian to permian times is responsible for a major fiat lying detachment the inferred geometrical relationships between fault azimuth and extension direction is befieved to induce a counter clockwise rotation of the paleozoic cover during its northward motion in order to test this model we have sampled autunian lower permian grey and red sandstone beds within two basins opened during the extension the lod ve and the st affrique basins the paleomagnetic study of six sites three in each basin using thermal demagnetization procedures reveals the presence of 3 magnetization components a first one a which is unblocked below 400 o c is interpreted as a recent overprint due to weathering of the sandstones the b component is unblocked between 400 o and 575 o c this b component is assumed to be primary although the gentle dip of the beds does not allow a fold test it fits the lower permian field direction in the lod ve basin in situ d m 194 i m 6 o k 50 tilt corrected d m 194 i m 10 o k 53 in contrast it shows a systematic difference in declination in the st affrique basin in situ d m 173 i m 2 5 o k 101 tilt corrected d m 172 lm 6 o k 37 this is interpreted as a post autunian counter clockwise rotation of at least 20 o of the st affrique basin thus supporting one of the main implications of the tectonic model finally red sediments from two sites of the lod ve basin yield a third component c with unblocking temperatures up to 670 c and carried by hematite its direction is also consistent with the permian field direction showing that hematite crystallisation occurred early in the diagenesis history of the sediments

core complex model proposed for gneiss dome development during collapse of the paleoproterozoic penokean orogen minnesota gneiss domes preserved throughout the paleoproterozoic 1870 1820 ma penokean orogenic belt are commonly viewed as collisional features however 1700 ma mica 40ar 39ar ages from archean rocks of the mcgrath gneiss dome minnesota are in marked contrast with 1750 1760 ma mica ages obtained everywhere outside the dome suggesting the dome may have formed well after compression had ceased we hypothesize that the bimodal age pattern is a result of crustal excision between archean basement and paleoproterozoic cover during extensional collapse of the overthickened orogen removal of structural section and progressive unroofing may have led to the development of a domed basement cover shear zone in this respect the mcgrath gneiss dome would represent one of the oldest analogues of a metamorphic core complex postorogenic collapse of the penokean orogen was apparently long lived with significant pulses occurring at 1755 ma and at 1700 ma the sudden and widespread crustal fusion at 1770 1760 ma that preceded the proposed collapse suggests that rapid subcrustal lithospheric thinning may have triggered collapse as has been proposed recently for several phanerozoic collapsed orogens

syn extension rotations in the permian st affrique basin massif central france paleomagnetic constraints abstract we present the results of a paleomagnetic study of the permian sedimentary basin of st affrique southern massif central france this study was devised to test for tectonic rotations in the montagne noire area during the stephano permian extension in the hecynian chain and to complete a preliminary study 1 in which we suggested an counterclockwise rotation of st affrique basin with respect to the neighbouring lodeve basin this rotation was attributed to the domino like rotation of the basement of st affrique i e the paleozoic monts de lacaune imbricate units as expected from the tectonic interpretation of the montagne noire area made by van den driessche and brun 2 3 however the rotation of the basin during permian should induce declination differences between the basal and the upper part of the stratigraphic sequence we therefore conducted vertical sampling of the basin throughout the permian sequence two sites of stephanian age gave no results but twelve sites from the lower to the upper permian have been sampled in the st affrique basin and did provide results paleomagnetic analysis using thermal procedures confirms our previous analysis and allowed us to demonstrate three magnetization components the low temperature a component is unblocked by 300 c and conforms to the present day dipole field direction an intermediate temperature component b has also been isolated with unblocking temperatures of 300 to 550 600 c and this component was possibly carried by magnetite between 600 and 680 c a third component c may be suspected at some sites but it has not been isolated with confidence at a site showing a tilted block geometry produced by syn sedimentary normal faulting site p7 it is shown that the b component is an early magnetization acquired during faulting of the succession and we are therefore able to give it a stratigraphic age the mean directions of the b component which are computed at the stratigraphic stage level show the following counterclockwise deviations from the reference magnetic field direction for stable europe 21 4 5 for the upper autunian 10 7 for the saxonian 10 9 for the lower thuringian and 4 6 and 3 5 4 5 for two successive stages of middle upper thuringian age we thus consider that 1 the comparison of the lowest units of st affrique and lodeve basins 1 and 2 the evolution in paleomagnetic declination during the permian within the st affrique basin itself provide evidence in space and time of a rotation of the basin during its formation we attribute this rotation to the domino like rotation of its basement the monts de lacaune imbricate units during the stephano permian extension in the hercynian chain 2 3

a dynamic model of the structural evolution of the hercynian pyrenees abstract the structural history of the hercynian pyrenees is considered with respect to the relationships between the metamorphism and the successive deformations the characteristics and mode of emplacement of the gneissic and anatectic cores of the metamorphic domes the migmatization and the intrusions of plutonic and basic and ultrabasic rocks the metamorphism can be shown to have progressed upwards with time attaining the presently exposed levels just before during and a short time after the major penetrative regional deformation the intrusion of major plutonic anatectic and gneissic massifs which has determined the metamorphic domes in country rocks was roughly synchronous with the progression of the metamorphism but seems to have continued later in the uppermost levels the basic and ultrabasic rocks uplifted by the intrusion of the anatectic cores of the metamorphic domes have been deformed and foliated by the major regional penetrative deformation thus showing that their intrusion predated this deformation in the lowermost levels these results together with the presently available sedimentological data lead us to propose a dynamic model of the tectonic evolution of the hercynian pyrenees the first step was a crustal extension and thinning attaining its maximum during frasnian time marked in the upper levels by a horst and graben controlled sedimentation and in the lower crust by intrusions of basic magmas and the onset of the very low pressure metamorphism and anatexis characteristic of the hercynian pyrenees the second step was a crustal shortening marked by large scale crustal thrusting followed by widespread regional penetrative deformation the flysch troughs formed in front of the progressing and emerging thrusts in lower levels the thrusts are believed to have controlled the progressive ascent of the plutonic and anatectic magmas which continued their uplift and emplacement during major penetrative regional deformation in the actual mesozone and probably after it in the uppermost levels

the montagne noire migmatitic dome emplacement french massif central new insights from petrofabric and ams studies in the southern french massif central the montagne noire axial zone is a ne sw elongated granite migmatite dome emplaced within visean south verging recumbent folds and intruded by syn to late migmatization granitoids the tectonic setting of this dome is still disputed thus several models have been proposed in order to better understand the emplacement mechanism of this dome petrofabric and anisotropy of magnetic susceptibility ams studies have been carried out in the granites and migmatites that form the dome core magmatic texture and to a lesser extent weak solid state texture are dominant as a paramagnetic mineral biotite is the main carrier of the magnetic susceptibility on the basis of 135 ams sites the magnetic fabrics appear as independent of the lithology but related to the dome architecture coupling our results with previous structural and geochronological studies allows us to propose a new emplacement model between 340 325 ma the palaeozoic series underwent a compressional deformation represented by nappes and recumbent folds involving the thermal event leading to partial melting until 325 310 ma the dome emplacement was assisted by diapiric processes an extensional event took place at 300 ma after the emplacement of the late to post migmatitic granitic plutons in the northeast side of the dome a brittle normal dextral faulting controlled the opening of the graissessac coal basin

gneiss domes and orogeny many gneiss domes record positive feedback between decompression and partial melting of orogenic middle crust exhumed orogens are riddled with gneiss domes cored by migmatites that underwent dehydration melting during decompression the decreasing buoyancy associated with increasing melt fraction drives further decompression at near isothermal conditions as the partially molten crust rises diapirically this combination of processes may explain the generation and retention of large volumes of crustally derived melt recorded in many deep seated migmatite terranes and inferred for active orogens in exhumed orogens the signature of the rapid ascent of partially molten crust is a gneiss dome cored by migmatite granite the large volume of material involved in the vertical transfer of partially molten crust indicates that the formation of gneiss domes is an efficient mechanism for heat advection during orogenesis

the eclogite facies rocks of the montagne noire france eclogite facies rocks are very rare in the montagne noire the local typical metamorphism being a low pressure type with extensive development of migmatites the cabardes and le jounie eclogites garnet omphacite quartz rutile 700 800 c 9 2 kbar derive from tholeiitic basalts interbedded in paragneisses very close to the orthogneissic basement both are strongly retromorphosed and re equilibrated in the conditions of the amphibolite facies metamorphism of the surrounding gneisses the eclogite facies garnet ultramafite of the airette pod derives from a ferromagnesian calc alkalic cumulate its reactional mineral associations show a complex polyphase evolution a primary spinel websterite cpx opx pleonaste chromite is reset in the eclogite facies where garnet develops as coronas on the spinel a further retromorphic evolution leads to a paragenesis hornblende tremolite picotite olivine fo70 phlogopite and finally to the paragenesis chlorite amphibole those rocks may be interpreted either as tectonically emplaced slabs or as the only remnants of a medium to high pressure regional metamorphism but it must be pointed out that all these rocks occur only in the southern part of the axial zone of the montagne noire i e in the root zone of the large penninic nappes which characterize this part of the hercynian belt

chemical weathering in the drainage basin of a tropical watershed nsimi zoetele site cameroon comparison between organic poor and organic rich waters abstract this study deals with the weathering processes operating at the scale of a small catchment nsimi zoetele cameroon and is focused on the role of organic colloids on mineral weathering and transport of elements in natural waters samples of river spring and groundwaters from nsimi zoetele were filtered through membranes of decreasing pore size 0 22 m 0 025 m or 300 000 da 5000 da to separate colloidal fractions from the truly dissolved one major and trace elements and dissolved organic carbon doc were analysed in each fraction two kinds of waters can be distinguished in the catchment clear and coloured waters clear waters exhibit low concentrations of major and trace elements and doc elements are carried in these solutions in a true dissolved form except al and rare earth elements rees by contrast the higher abundances of al fe and trace elements in coloured waters are controlled by the colloidal fraction thermodynamic equilibrium calculations show that clear waters are in equilibrium with kaolinite and iron oxi hydroxide which are major minerals in the weathered soil for coloured waters the aqueous speciation of ca mg cu fe al la and th was calculated taking into account the complexes with humic acids speciation calculations for cu fe al la th show a strong complexation with humic acids in good agreement with the results of the filtration experiments by contrast although filtration experiments show a strong control of major cations by organic matter for example 75 for ca speciation calculations reveal that their complexes with humic ligands do not exceed a few percent of total dissolved elements this discrepancy is explained as an artefact induced by the organic colloids and occurring during the filtration procedure finally both filtration experiments and speciation calculations show that organic matter plays an important role in natural doc rich waters organic acids increase significantly the dissolution rates of silicates and oxi hydroxides and thus the amounts of solutes and of complexed elements leaving the catchment

neogene kinematics of the central and western alps evidence from fission track dating zircon and apatite fission track analysis has established the continuation of a zone of rapid change in mineral ages extending from the simplon fault zone in the simplon alps through the rhone valley and along the southeastern side of the mont blanc and belledonne massifs apatite fission track ages to the northwest of this zone generally range from 1 4 to 6 ma whereas those to the southeast range from 6 to 13 ma corresponding values for zircon are 11 13 and 16 26 ma respectively the zone corresponds to the position of the frontal pennine thrust in the mont blanc belledonne region this structure a prominent feature on recent deep seismic profiles may have been reactivated in the neogene with an important normal fault component this is suggested by apatite and zircon fission track ages in the footwall that are younger than those in the hanging wall and by the observed elimination of the lower pennine or simplon nappes along this fault zone in the rhone valley mont blanc belledonne region resulting in the juxtaposition of north penninic and helvetic ultra helvetic nappes at the current erosion level the distribution of fission track ages reflects this movement and the related neogene exhumation of the mont blanc and belledonne external massifs which has continued until less than 1 4 m y ago i e it is almost certainly still active and has tilted the overlying nappe sequence

tectono metamorphic evolution of the neyriz metamorphic complex quri kor e sefid area sanandaj sirjan zone sw iran abstract the neyriz region includes outcrops of metamorphic rocks that are thrust over the neotethyan ophiolites these rocks are affected by a major deformational event the result of which includes a shearing polyphase foliation present in gneissic core domes overprinted by a crenulation cleavage these fundamental structures developed contemporaneously with a medium pressure metamorphism which is characterized by the syn kinematic crystallization of kyanite and the beginning of anatexis followed by the development of retrometamorphic mineral parageneses the major deformation phase in the area occurred during the early cimmerian orogeny in the late triassic following the orogeny the gneiss domes started to rise into the upper levels of the crust from the geodynamic point of view after the mid permian the studied area was situated at southern passive margin of the iranian plate the central iranian microcontinent at that time was separated by the neotethys ocean from the gondwanian supercontinent after the late triassic the region became an active margin associated with an accretionary prism the margin was finally involved in an orogenic wedge after the closure of the neotethyan oceanic basin in the late mesozoic closure of the basin resulted in a major thrusting of the metamorphic rocks of the southern iranian margin over the neotethyan ophiolites

dome emplacement and formation of kilometre scale synclines in a granite greenstone terrain quadrilatero ferrifero southeastern brazil the quadrilatero ferrifero is a portion of the brazilian precambrian shield containing a granite greenstone terrain characterized by granite gneiss domes encircled by kilometre scale synclines integration of porphyroblast matrix microtextures c axis fabrics analysis and strain analysis with outcrop scale structural relationships indicates that the main phase of domes and synclines evolution occurred during the westerly directed 0 8 0 6 ga pan african brasiliano regional shortening this tectonic event has produced the single pervasive foliation observed in the folded supracrustal sequences of the quadrilatero ferrifero dip slip ductile shear zones developed at the dome supracrustal interface are synchronous with dome evolution during the pan african brasiliano deformation as indicated by contemporaneous growth of aureole porphyroblasts in the shear planes these shear zones facilitated steeply directed downward flow of country rock material around the granite gneiss domes the marked density contrast between granite gneiss material and the iron rich supracrustals provided a gravitationally favourable environment for downward wallrock flow variations in shear zone kinematics away from the domes combined with kilometer scale viscous drag contributed to produce kilometre scale synclines in the supracrustal rocks constrained between upward moving domes

timing of formation and exhumation of the montagne noire double dome french massif central new u pb monazite zircon and xenotime ages date the late carboniferous crystallization of the anatectic vialais granite in the montagne noire axial zone and the high temperature deformation and metamorphism of the augen gneiss ordovician granite protolith from which the granite was likely derived the u pb monazite ages obtained from the augen gneiss 308 3 ma late kinematic vialais granite 303 4 ma and post kinematic leucogranite 298 2 ma bracket the high temperature deformation and metamorphism at 310 300 ma clearly postdating regional contraction and nappe emplacement 320 ma the planar linear and locally linear fabrics in the augen gneiss outline a regional scale double dome structure caroux and espinouse sub domes containing smaller km scale upright folds compared to the caroux sub dome the larger migmatitic espinouse sub dome contains abundant leucosome leucogranite bodies and late kinematic intrusions vialais granite the vialais granite displays a weak magmatic foliation that defines the main layering of the espinouse migmatite ellipsoidal quartz sillimanite nodules concentrated above the roof of the granite recorded an increment of strain vertical shortening e ne elongation during granite emplacement and crystallization consistent with the extension event that exhumed the domes these new geochronological and structural data suggest that the montagne noire double dome formed in a pull apart structure within a dextral strike slip system upright folding in the sub domes has been traditionally assigned to a regional contraction event alternatively we propose that local contraction is associated with the convergence of low viscosity crust beneath the upper crust pull apart dynamic models of extension of hot crust indicate that contraction at depth is generated by flow of low viscosity orogenic crust converging and rising to fill the gap created by upper crust extension this interpretation solves the long standing problem of apparent coeval contraction and extension in the montagne noire double dome and establishes migmatite dome emplacement in strike slip corridors as a style of late variscan tectonics

dome and keel provinces formed during paleoproterozoic orogenic collapse core complexes diapirs or neither examples from the quadrilatero ferrifero and the penokean orogen paleoproterozoic dome and keel provinces in which troughs of deformed and metamorphosed paleoproterozoic supracrustal rocks surround domes of archean basement continue to puzzle geologists in current literature some authors refer to the domes as diapirs implying basement flowed vertically upward and others consider them to be cordilleran type metamorphic core complexes implying that the contact between basement and cover is an upwarped detachment geochronological studies suggest that dome emplacement occurred during extensional collapse of contractional orogens seemingly supporting the core complex analogy however structural analyses in the quadrilatero ferrifero brazil and the penokean orogen michigan demonstrate that the domes resemble diapirs in shape and in terms of surface kinematics the domes differ from diapirs however in that they did not flow penetratively within but were emplaced by movement along steeply dipping shear zones we suggest that contrasts between dome and keel and core complex provinces reflect contrasts between paleoproterozoic and phanerozoic crustal structure specifically during the paleoproterozoic when crust was warmer and supracrustal assemblages denser core complex like detachment faulting and associated plutonism resulted in juxtaposition of hot basement beneath a denser tectonically thickened supracrustal layer the resulting viscosity contrast and density inversion triggered vertical rise of diapir shaped basement domes by slip on steep shear zones

paleocene eocene migmatite crystallization extension and exhumation in the hinterland of the northern cordillera okanogan dome washington usa the okanogan gneiss dome washington is located in the hinterland of the north american cordillera and is part of a chain of metamorphic core complexes containing gneiss and migmatite domes exhumed during eocene extension of thickened crust u pb sensitive high resolution ion microprobe shrimp analyses of zircon monazite and titanite and 40 ar39 ar analyses of biotite from migmatites exposed in the footwall of the okanogan detachment coupled with a detailed structural analysis document the timing and duration of migmatite crystallization and indicate coeval crystallization extensional deformation and exhumation of the dome okanogan migmatites are folded and deformed and preserve successive generations of leucosomes generated by synkinematic anatexis analyses of migmatite samples from a highmelt fraction subdome near stowe mountain suggest that the okanogan dome records a history of migmatite crystallization spanning at least 12 m y as indicated by 206 pb

dissolution of radiation damaged zircon in lateritic soils zircon crystals from lateritic soils at nsimi cameroon were investigated using electron microprobe analysis empa transmission electron microscopy tem scanning electron microscopy sem and raman spectroscopy to determine the extent of radiation damage from alpha decay events the soils belong to a small watershed developed on granitic rocks of the congo craton 2 9 ga interactions with fluids are evidenced by significant cao up to 1 5 wt al2o3 up to 2 9 wt and fe2o3 up to 2 9 wt concentrations in uo2 rich regions 0 05 to 1 wt of the zircon regional heating up to 500 c related to the pan african orogeny about 0 6 ga ago has lead to the recrystallization of the radiation damaged grains and the formation of a nanoporous microstructure the correlation observed between the presence of dissolution features and the actual damage state of zircon shows that zircon dissolution occurs under tropical weathering conditions and with preferential dissolution of the highly radiation damaged regions congruent dissolution of zircon and the limited mobility of zr are supported by the absence of zirconium oxide precipitates in the fractures of weathered grains of zircon

present weathering rates in a humid tropical watershed nsimi south cameroon the study of biogeochemical and hydrological cycles in small experimental watersheds on silicate rocks common for the temperate zone has not yet been widely applied to the tropics especially humid areas this paper presents an updated database for a six year period for the small experimental watershed of the mengong brook in the humid tropics nsimi south cameroon this watershed is developed on precambrian granitoids north congo shield and consists of two convexo concave lateritic hills surrounding a large flat swamp covered by hydromorphic soils rich in upward organic matter mineralogical and geochemical investigations were carried out in the protolith the saprolite the hillside lateritic soils and the swamp hydromorphic soils biomass chemical analyses were done for the representative species of the swamp vegetation the groundwater was analysed from the parent rock saprolite weathering front to the upper fringe in the hillside and swamp system the chemistry of the wet atmospheric and throughfall deposits and the mengong waters was monitored in the nsimi watershed the carbon transfer occurs primarily in an organic form and essentially as colloids produced by the slow biodegradation of the swamp organic matter these organic colloids contribute significantly to the mobilization and transfer of fe al zr ti and th in the uppermost first meter of the swamp regolith when the organic colloid content is low i e in the hillside groundwater th and zr concentrations are extremely low 3 pmol l icp ms detection limits strongly insoluble secondary thorianite tho 2 and primary zircon zrsio 4 crystals control their mobilization respectively this finding thus justifies the potential use of both these elements as inert elements for isoelement mass balance calculations pertaining to the hillside regolith chloride can not be used as a conservative tracer of hydrological processes and chemical weathering in this watershed biogenic recycling significantly influences the low cl input fluxes sodium is a good tracer of chemical weathering in the watershed the sodium solute flux corrected from cyclic salt input was used to assess the chemical weathering rate even though low 2 8 mm kyr the chemical weathering rate predominates over the mechanical weathering rate 1 9 mm kyr compared to the rio icacos watershed the most studied tropical site the chemical weathering fluxes of silica and sodium in the mengong are 16 and 40 times lower respectively this is not only related to the protective role of the regolith thick in both cases but also to differences in the hydrological functioning this is to be taken into account in the calculations of the carbon cycle balance for large surfaces like that of the tropical forest ecosystems on a stable shield at the global level

structures indicative of helicoidal flow in a migmatitic diapir bacao complex southeastern brazil abstract the migmatitic bacao complex southeastern brazil shows foliation and lineation patterns that depict an internal flow structure with helicoidal geometry microstructures show that a rheological transition exists from a predominantly magmatic flow represented by isotropic undeformed granitic textures to solid state flow mylonites towards the diapir margins nebulites and contorted migmatites occur in the diapir core whereas the peripheral domains show different kinds of banded migmatites gneisses and high temperature mylonites along the margins low temperature mica rich mylonites and phyllonites occur two major flow domains were recognized the central cell and the peripheral cell each cell is characterized by one or more systems of concentric foliations subcells the mineral lineation produced by the morphological alignment of biotite flakes on the foliation planes defines helicoidal geometries in all subcells this lineation was interpreted as being parallel to the local flow vector the foliation and lineation patterns suggest that the principal ascent trunk occurred along a medium plunge spiral in the central cell a dip flow regime the constant injection of molten material in the central cell pushed the material previously emplaced into the peripheral cell where flow upwards continued through a shallower plunge spiral a strike flow regime

thrust related diapiric and extensional doming in a frontal orogenic wedge example of the montagne noire southern french hercynian belt abstract the montagne noire which is situated at the toe of the orogenic wedge of the french massif central south european variscides appears to be a well suited area for studying the origin and evolution of middle to upper crustal domes adjacent to foreland basins the data reported in the present paper show that the montagne noire dome is a particular type of basement involved frontal culmination in an orogenic wedge and foreland basin system this frontal culmination is characterized by a syn contractional ht decompression recorded by clockwise ptt paths and widespread strata overturning in thrust and fold structures which controlled the sedimentation in the adjacent foreland basin these unusual characteristics are interpreted to be a result of the succession of thrusting diapirism and extensional collapse antiformal stacking of syn metamorphic thrust sheets controlled the first stages of the foreland basin development diapirism was essentially responsible for the ht decompression and widespread strata overturning extensional doming was a result of late to post metamorphic collapse acting on the pre existing high amplitude dome diapirism and associated isothermal decompression metamorphism which constitute the essential difference between the montagne noire and ordinary frontal ridges in orogenic wedges were probably enhanced by a local partial melting of the upper to middle crust it is suggested that the occurrence of these phenomena in front of an orogenic wedge was related to local over thickening due to the superposition of an upper crustal antiformal stack on top of a lower crustal ramp anticline

tectonic evolution of the montagne noire and a possible orogenic model for syncollisional exhumation of deep rocks variscan belt france structural analysis in the montagne noire including a study of inclusion trail patterns in andalusite garnet and staurolite porphyroblasts reveals a succession of five foliations in alternately steep and flat lying orientations this is inferred to reflect multiple gravitational collapse phases that intervened during continuous plate convergence initial crustal thickening by foliation development and folding d1 was responsible for eclogite facies metamorphism in the gneissic basement subsequent thrusting and recumbent folding d2 generated a subhorizontal crenulation cleavage during namuro westphalian times while previous steep structures became rotated petrological and microstructural evidence indicate that thrust nappes were emplaced by a gravitational spreading mechanism while undergoing internal thinning an important amount of basement exhumation took place synchronously these observations form the basis for a new orogenic model in which thermal reequilibration of a previously thickened orogen induces gravitational instability in the upper crust collapse of the upper crust causes it to spreadout horizontally over continuously converging relatively cool plates thereby creating low angle thrust nappes the upper crustal thinning facilitates simultaneous extrusion of lower crust in vertical root zones at the orogen center which on reaching the higher part of the orogen can be incorporated in the nappes and displaced horizontally following such a mechanism of nappe tectonics renewed horizontal shortening d3 produced the montagne noire gneissic dome by upright folding of nappe related foliations with orogen parallel stretching previous crustal thinning had led to an increased heat flux between basement and cover so that during d3 cover sequences were still rising in temerature while basement rocks had already commenced their retrograde path during d2 peak metamorphic conditions in the cover triggered a second gravitational collapse during namurian times d4 producing a 2 km wide subhorizontal shear zone in which orogen parallel stretching continued at the surface deformation was accommodated by decollements normal faulting and basin formation gneissic rocks below the shear zone were not affected by the collapse because a stronger rheology and horizontal shortening was probably continuous here retrograde cooling of the cover restored gravitational equilibrium and caused its renewed upright folding during late variscan times d5 permian postorogenic extension d6 oriented at a high angle to the previous orogen parallel stretching completed exhumation of the gneiss dome the dome structure of the montagne noire metamorphic core complex was acquired during compressional tectonics and later modified flattened by synorogenic and postorogenic extensional events a similiar polyphase origin appears not to be excluded for other metamorphic core complexes with poorly constrained preextension histories

improved u th total pb dating of zircons by electron microprobe using a simple new background modeling procedure and ca as a chemical criterion of fluid induced u th pb discordance in zircon abstract we present a simple new background modeling method particularly for the determination of trace concentrations of pb u and th to date zircons chemically by electron microprobe this method reduces the acquisition time considerably and at the same time increases the precision of a single pb u and th analysis depending on the analytical conditions theoretical detection limits down to 45 ppm 99 confidence level for u th and pb could be obtained the final precision on age for a single measurement was between 8 and 35 l at pb concentrations between 100 and 200 ppm in proterozoic zircons precise u th and pb analyses however do not necessarily facilitate a straightforward chemical age calculation u th total pb data of zircons from four granites could not be objectively interpreted because of a severe disturbance of the u th pb system however it is shown that chemical ages derived only from zircon areas containing less than 0 2 wt cao agree very well with published conventional radiometric ages of these rocks we propose a model which explains the ca gain and pb loss of zircons which have not undergone any metamorphism after emplacement to be the result of a hydration of radiation damaged zircon domains through low temperature aqueous solutions it is suggested that the ca content can be used independently from u th and pb analyses as a chemical criterion to discriminate between altered zircon domains having suffered a low temperature fluid induced pb loss and non altered zircon domains for which there is a high probability that they will give concordant u th pb ages 2000 elsevier science b v all rights reserved

flow of partially molten crust and the internal dynamics of a migmatite dome naxos greece 1 migmatite domes are common in metamorphic core complexes dome migmatites deform in the partially molten or magmatic state and commonly record complex form surfaces folds and fabrics while units mantling the dome display a simpler geometry typically formed by transposition during crustal extension we use field observations and magnetic fabrics in the naxos dome greece to quantify the complex flow of anatectic crust beneath an extensional detachment system the internal structure of the naxos dome is characterized by second order domes subdomes pinched synforms and curved lineation trajectories which suggest that buoyancy driven flow participated in dome evolution subdomes broadly occur within two compartments that are separated by a steep n s oriented high strain zone this pattern has been recognized in domes formed by polydiapirism and in models of isostasy dominated flow the preferred model involves a combination of buoyancy and isostasy driven processes the naxos dome may have been generated by regional n s extension that triggered convergent flow of partially molten crust at depth and the upwelling of anatectic migmatites within the dome this pattern is complicated by gravitational instabilities and or overturning of the high melt fraction crust leading to the growth of subdomes as the migmatites within the naxos dome reached a higher structural level they were affected by regional top to the nne kinematics of the detachment system dome formation therefore occurred by a combination of coeval and coupled processes upper crustal extension deep crust contraction during convergent flow of anatectic crust diapirism and or density driven crustal convection forming subdomes and north directed detachment kinematics

deep seismic structure and thermo mechanical modelling of continental collision zones abstract a series of 2d thermo mechanical models are presented which quantify crustal thickening and the resulting temperature perturbations associated with thrusting in an attempt to constrain the existence of temperatures responsible for the generation of anatectic granites in a compressional environment deformation is assumed to occur in the crust by a simple thrusting mechanism the moho acting as a detachment surface as has been observed on deep seismic reflection data while the more ductile sub crustal lithosphere is assumed to behave elastically and deform in a pure shear manner investigated are the relative effects of the rate of crustal shortening the distribution of radioactive elements within the crust the reduced heat flow from the mantle isostasy and erosion our results show that previous oversimplified 1d models significantly overestimate the elevated temperatures in lower crustal regions caused by radiogenic heating and that this mechanism will only have a significant effect in regions of relatively thin lithosphere or where there are unusually high rates of radiogenic heating in the crust generation of synthetic p t t data for locations deep down in the lower crust enables us to quantify the temperatures produced following a period of compressional deformation providing insight into the time scales involved for the generation of crustal melt granites within an orogenic belt

the tectono metamorphic evolution of the very low grade hangingwall constrains two stage gneiss dome formation in the montagne noire southern france the montagne noire in the southernmost french massif central is made of an ene elongated gneiss dome flanked by palaeozoic sedimentary rocks the tectonic evolution of the gneiss dome has generated controversy for more than half a century as a result a multitude of models have been proposed that invoke various tectonic regimes and exhumation mechanisms most of these models are based on data from the gneiss dome itself here new constraints on the dome evolution are provided based on a combination of very low grade petrology k ar geochronology field mapping and structural analysis of the palaeozoic western mont peyroux and faugeres units which constitute part of the southern hangingwall of the dome it is shown that southward directed variscan nappe thrusting d1 and a related medium p metamorphism m1 are only preserved in the area furthest away from the gneiss dome the regionally dominant pervasive tectono metamorphic event d2 m2 largely transposes d1 structures comprises a higher metamorphic thermal gradient than m1 transition low p and medium p metamorphic facies series and affected the rocks between c 309 and 300 ma post dating d1 m1 by more than 20 ma d2 related fabrics are refolded by d3 which in its turn is followed by dextral normal shearing along the basal shear zone of both units at c 297 ma in the western mont peyroux and faugeres units d2 m2 is largely synchronous with shearing along the southern dome margin between c 311 and 303 ma facilitating the emplacement of the gneiss dome into the upper crust d2 m2 also overlaps in time with granitic magmatism and migmatization in the zone axiale between c 314 and 306 ma and a related low p high t metamorphism at c 308 ma the shearing that accompanied the exhumation of the dome therefore was synchronous with a peak in temperature expressed by migmatization and intrusion of melts within the dome and also with the peak of metamorphism in the hangingwall both the intensity of d2 fabrics and the m2 metamorphic grade within the hangingwall decrease away from the gneiss dome with grades ranging from the anchizone epizone boundary to the diagenetic zone the related zonation of the pre d3 metamorphic field gradients paralleled the dome these observations indicate that d2 m2 is controlled by the exhumation of the zone axiale and suggest a coherent kinematic between the different crustal levels at some time during d2 m2 based on integration of these findings with regional geological constraints a two stage exhumation of the gneiss dome is proposed during a first stage between c 316 and 300 ma dome emplacement into the upper crust was controlled by dextral shear zones arranged in a pull apart like geometry the second stage from 300 ma onwards was characterized by northeast to northward extension with exhumation accommodated by north dipping detachments and hangingwall basin formation along the northeastern dome margin

viscous collision in channel explains double domes in metamorphic core complexes in hot orogens gneiss domes are a response to upper crustal stretching and lower crustal flow two dimensional thermal mechanical modeling shows that localization of extension in the upper crust triggers in the deep crust oppositely verging horizontal flows that converge beneath the extended region upon viscous collision both flowing regions rotate upward to form two upright domes of foliation double domes separated by a steep median high strain zone in such systems horizontal shortening in the infrastructure develops in an overall extensional setting dome material follows a complex depth dependent strain history from shearing in the deep crustal channel to contraction upon viscous collision in the median high strain zone to extension upon advection into the shallow crust this depth dependent strain history is likely a general feature of dome evolution and is arguably well preserved in double domes such as the montagne noire france and naxos greece gneiss domes