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

highly fractionated i type granites in ne china ii isotopic geochemistry and implications for crustal growth in the phanerozoic ne china is the easternmost part of the central asian orogenic belt caob the area is distinguished by widespread occurrence of phanerozoic granitic rocks in the companion paper part i we established the jurassic ages 184 137 ma for three granitic plutons xinhuatun lamashan and yiershi we also used geochemical data to argue that these rocks are highly fractionated i type granites in this paper we present sr nd o isotope data of the three plutons and 32 additional samples to delineate the nature of their source to determine the proportion of mantle to crustal components in the generation of the voluminous granitoids and to discuss crustal growth in the phanerozoic despite their difference in emplacement age sr nd isotopic analyses reveal that these jurassic granites have common isotopic characteristics they all have low initial 87 sr 86 sr ratios 0 7045f0 0015 positive end t values 1 3 to 2 8 and young sm nd model ages 720 840 ma these characteristics are indicative of juvenile nature for these granites other late paleozoic to mesozoic granites in this region also show the same features sr nd and oxygen isotopic data suggest that the magmatic evolution of the granites can be explained in terms of two stage processes 1 formation of parental magmas by melting of a relatively juvenile crust which is probably a mixed lithology formed by pre existing lower crust intruded or underplated by mantle derived basaltic magma and 2 extensive magmatic differentiation of the parental magmas in a slow cooling environment the widespread distribution of juvenile granitoids in ne china indicates a massive transfer of mantle material to the crust in a post orogenic tectonic setting several recent studies have documented that juvenile granitoids of paleozoic to mesozoic ages are ubiquitous in the central asian orogenic belt hence suggesting a significant growth of the continental crust in the phanerozoic d 2003 elsevier science b v all rights reserved

orogen parallel tectonic transport in the variscan belt of northeastern sardinia italy implications for the exhumation of medium pressure metamorphic rocks a transpressive crustal scale dextral shear zone is documented in the variscan basement of northeastern sardinia it indicates the presence of a shear deformation parallel to the belt overprinting previous d1 structures related to nappe stacking and top to the s and sw thrusting the l2 stretch ing lineation points to an orogen parallel stretching and to a general change in the tectonic transport from d1 to d2 phase d1 developed during initial frontal collision whereas the d2 deformation was characterized by dextral shearing during the increasing curvature of the ibero armorican arc transpressional deformation developed in a regime of decreasing pressure it caused telescoping of the barrovian isograds and the exhumation of the low to medium grade metamorphic rocks in this sector of the variscan belt exhumation is due to continuing compression with an increasing compo nent of horizontal displacement the overall change of the shortening direction in a large sector of an orogenic belt with the occurrence of increasing orogen parallel displacement may be regarded as a general mechanism affecting the exhumation of rocks and preventing the thickened collisional crust from undergoing a generalized gravitational collapse

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

spatial and temporal distribution of the orogenic gold deposits in the late palaeozoic variscides and southern tianshan how orogenic are they abstract a principal uncertainty in models of orogenic ore deposits concerns their ages relative to orogenic processes the yardstick of the relation has resided loosely in the peak of metamorphism age estimates in the variscides and tianshan indicate that most orogenic ore deposits were formed in the course of the late carboniferous to middle triassic with a peak between 305 and 280 ma their locations settings and ages suggest an association with coeval lithosphere scale strike slip deformation which played a role in the destruction of the orogenic system following crustal shortening this destruction tends to invalidate the orogenic association of the orogenic ore deposits observation of a mantle signature in some of these ore deposits and in coeval magmatic rocks suggests that the strike slip belts tapped into sub crustal levels here lithological composition had been modified by subduction during the stage of construction consequently any orogenic element in the ore deposit models is apart from location only an element of inheritance of subduction modified lithosphere features acquired during construction of the orogen these features may however well have been crucial for the formation of the ore deposits in view of the translithospheric extent of the strike slip belts fluids which contributed to the ore deposits may have come from at least the entire in part metasomatized lithosphere column the ore deposits could probably not have been formed without these deep reaching strike slip systems a model is proposed for the geological setting of the orogenic gold deposits in the variscides and the southern tianshan it consists of plutono metamorphic elements in the mantle in the lower and in the middle crust within a transpressional to transtensional tectonic framework of translithospheric faults and associated domes and pull apart basins with shallow marine to terrestrial sediments and interlayered felsic and mafic volcanics the thermal engines of these systems were probably in localized upwellings of the asthenosphere prompted and controlled by the lithosphere scale deformation at the time at an even larger scale oroclinal bending of the old orogenic backbone may have played yet another role in the localization of the ore deposits the thesis of specific localized tectono thermal engines is at odds with the original proposals of a relation between orogenic ore deposits and regional metamorphism it suggests that the late palaeozoic gold antimony mercury ore deposits in the variscides and the southern tianshan are more akin to the intrusion related ore systems

cybertectonic earth and gaia s weak hand sedimentary geology sediment cycling and the earth system tectonics climate and sea level are dominant controls on the nature and distribution of sedimentary environments by inverse logic recognition of influences on sedimentation from the stratigraphic record as a result of perturbations in these variables leads to clarification of past environmental and structural change the succession of sedimentary environments in time and place is determined by a wider combination of variables namely sediment flux biological evolution atmosphere and water body productivity and chemistry the feedbacks and interrelationships between all variables cause the complexity seen in sedimentation processes over geological time the processes of global tectonics that cause widespread mountain belt and continental plateau uplift have produced numerous severe events during earth history these often random workings out of the plate tectonic cycle define the state of cybertectonic earth from cyber after the greek kubernan to steer or govern this has worked within a usually zonally arranged series of climatic belts and within the framework provided by biological evolution at certain times in the geological past a combination of factors arose as a response to continental uplift and have acted to cause certain earth surface conditions and variables notably mean global surface temperature atmospheric p co 2 p o 2 to have varied by very large amounts 2 10 compared with present values these large fluctuations such as those responsible for the neoproterozoic late palaeozoic and late tertiary glaciations lead one to doubt the reality of homeostatic control of surface conditions as proposed in lovelock9s gaia hypothesis the gaian kuberne0te0s steersman had a weak hand on the helm frequently unable to prevent vast areas of the globe experiencing rapid fluctuations in environmental conditions and inimical conditions to life for very long periods during neoproterozoic and phanerozoic times at the same time biogenic and abiogenic processes have proved capable of returning earth to states of mean stability although biogeochemical cycling models seem alarmingly ad hoc and largely untestable as scientific hypotheses in any true geological sense

is there a remnant variscan subducted slab in the mantle beneath the paris basin implications for the late variscan lithospheric delamination process and the paris basin formation abstract the paris basin northern france is a late paleozoic mesozoic intracratonic basin that settled upon the collapsed variscan collisional belt the lithospheric roots of the variscan orogenic system below the paris basin have been investigated using a european scale p wave velocity tomographic model tomography points out the existence of a significant high velocity anomaly in the upper mantle below the western part of the basin at 150 200 km depth the anomaly extends with a nw se trend along the buried northern france trace of the northern variscan suture zone i e the bray segment of the upper carboniferous lizard rhenohercynian lrh suture moreover the high velocity anomaly is spatially correlated with the prominent paris basin magnetic anomaly its downdip extent reaches depths greater than 200 km below the southern margin of the paris basin as suggested in previous tomographic studies below ancient suture zones these data argue for such anomaly being the remnant of a variscan subducted slab that escaped the extensive late orogenic delamination process affecting the lithospheric roots by late carboniferous early permian times and that was preserved stable over 300 ma at the base of the lithosphere on a general geodynamical perspective these results provide a new insight into the long term evolution of subducted lithosphere into the mantle in the case of the western european variscan orogenic belt they suggest that the subduction of the lrh slab below the previously thickened variscan crust and its final detachment from the orogenic root have played an important role in the collapse of the belt inducing thermal erosion and extension of the overriding lithosphere the spatial evolution of late orogenic extension across the belt and of subsequent thermal subsidence in the paris basin is suggested to result from the heterogeneous delamination of the lithospheric roots along strike and from the resultant pattern of asthenospheric rise

multistage anatexis during tectonic evolution from oceanic subduction to continental collision a review of the north qaidam uhp belt nw china abstract the north qaidam ultra high pressure uhp metamorphic belt in the northern tibetan plateau is considered as a typical alpine type uhp metamorphic belt due to the early paleozoic subduction of the qaidam block beneath the qilian block the well preserved paleozoic metatexite migmatite diatexite migmatite felsic sheet dyke and anatectic granite in the north qaidam uhp metamorphic belt provide us with an excellent opportunity to study the generation transport and final fate of crustal melt and geodynamic processes associated with orogenesis based on structural relationships petrology geochronology whole rock geochemistry and sr nd isotope data the early paleozoic anatexis during oceanic subduction to continental collision in the north qaidam uhp metamorphic belt can be further divided into three stages of development 470 ma 446 428 ma and 432 420 ma the first stage granitic leucosomes are rich in k poor in na with low sr y and enrichment of large ion lithophile elements liles which were probably derived from partial melting of ancient felsic gneiss in continental arc circumstances during oceanic subduction the second stage na rich leucosomes and tonalite plutons are characterized by high na sr sr y and la yb and low heavy rare earth elements hrees with positive end t values of 0 1 4 3 and zircon ehf t values of 8 3 10 2 similar to typical tonalite trondhjemite granodiorites ttgs these ttg like magmas were produced through partial melting of newly emplaced gabbroic rocks with arc affinity under high pressure hp granulite facies conditions in a thickened lower crust during continental collision the volumetrically significant migrated plutons evolved from ttg like melt will become segments of continental crust and contribute to crustal growth with partial residual products of hp granulite and or garnet pyroxenite to the mantle by delamination the third stage of anatexis was preserved in both migmatitic uhp gneiss and eclogite in the xitieshan and luliangshan terranes the kfs rich felsic leucosomes inside uhp gneisses in the third stage are characterized by high alkali contents and low mafic component with feot mgo tio2 2 0 2 high la yb and sr y and low y and hrees 3 high al2o3 and low mg values and 4 enriched in liles e g rb ba k sr pb and poor in high field strength elements hfses partial melting of uhp eclogite and felsic gneiss during the initial retrogression stage with pl rich and kfs rich leucosome formation was triggered by dehydration melting involving predominant zoisite and muscovite the anatectic melts from partial melting of deeply subducted uhp gneiss have accumulated and migrated outside the deeply subducted crustal slice in the form of syn collisional plutons the melt evolution from the leucosomes produced at the early exhumation stage to syn collisional granitoids produced at the late exhumation might contribute greatly to the exhumation of the north qaidam uhp metamorphic belt from mantle depths to the lower crustal levels

excess methane in continental hydrothermal emissions is abiogenic thermogenic hydrocarbons entirely deriving from the thermal degradation of organic matter usually exhibit methane to ethane plus propane ratios smaller than 100 we present hydrocarbon distribution data of continental hydrothermal gases whose methane has been independently identified to derive from the abiogenic reduction of co2 we find that excess amounts of methane with respect to thermogenic hydrocarbon distributions are characteristic for the investigated gases a similar pattern is observed for well discharges whose temperatures are too high to support any microbially mediated methanogenesis these findings strongly suggest that abiogenic methane production in continental hydrothermal systems is a more widespread process than previously assumed the maximum contribution of such emissions to the modern atmospheric ch4 budget is estimated at 1

scrubbing process and chemical equilibria controlling the composition of light hydrocarbons in natural gas discharges an example from the geothermal fields of el salvador 1 the compositional features of fluids from both fumarolic discharges and productive geothermal wells of ahuachapan chipilapa berlin chinameca and san vicente geothermal systems el salvador are described and discussed in order to investigate the complex geochemical interactions involving geothermal fluids within the shallowest part of the hydrothermal circulation pathways our results highlight that secondary processes are able to strongly affect and modify the chemical characteristics of geothermal gases once they discharge to the surface as natural manifestations mainly in relation to the chemical physical properties of each gas species the effects of both gas dissolution in shallow aquifers and gas water rock chemical interactions on gas discharge composition make it difficult to get a correct evaluation of the thermodynamic conditions that characterize the geothermal reservoirs by applying the common geoindicators based on the chemical equilibria of the h2o co2 h2 ch4 co system differently the composition of the c1 c2 c3 alkanes and the c3 and c4 alkane alkene pair established within the geothermal reservoirs under the control of chemical reactions remains stable in samples collected from discharging gas vents these results suggest that the relative abundances of hydrocarbons characterized by similar structure and molecular size seem to be mainly regulated by the diffusion velocity of gases through the liquid dominated system therefore the chemical features of the light organic gas fraction of naturally discharging fluids can be successfully utilized for the evaluation of geothermal reservoir temperatures and redox conditions providing useful indications in terms of geothermal exploration and exploitation on this basis the distribution speciation and relative abundances of light hydrocarbons can also be considered highly promising in geochemical monitoring of active volcanic systems

the vaasa migmatitic complex svecofennian orogen finland buildup of a lp ht dome during nuna assembly tectonic evolution of the paleoproterozoic vaasa migmatitic complex vmc in the central part of the svecofennian accretionary orogen is deciphered using aeromagnetic and gravity maps deep seismic

empirical mode skeletonization of deep crustal seismic data theory and applications seismic skeletonization is a pattern recognition technique used to decompose reflection seismic data into events or reflectors and their seismic attributes and to store the results in an event file for an analysis toward seismic interpretation the extraction of instantaneous frequency from the event file for any statistical analysis of the seismic attribute analysis is currently not possible thus precluding the complete parameterization of the reflected seismic wave field in terms of its decomposition products empirical mode decomposition of reflection seismic data has uniquely addressed the question of computing the instantaneous frequency from its decomposition products using the hubert transform although the decomposition products of the seismic skeletonization and the empirical mode decomposition are conceptually different they share a common link to the primitive features of the seismic waveforms in this paper we introduce a new decomposition technique empirical mode skeletonization that combines the features of both seismic skeletonization and empirical mode decomposition by this process it is now possible to subject the reflection seismic data to an analysis that could include a parameterization of the reflected wave field we apply the new technique to a segment of seismic line 1 of the lithoprobe slave northern cordillera lithosphere evolution snorcle transect to extract event oriented instantaneous frequency attributes and also to analyze the decomposition products for any scaling behavior of the reflected wave field

diachronous cooling on both sides of a major strike slip fault in the variscan maures massif south east france as deduced from a detailed 40ar 39ar study abstract a detailed 40ar 39ar geochronological study was performed in the maures massif se france to define the thermal history of this geochronologically poorly constrained segment of the variscan belt a total of 22 plateau ages were obtained on single grains and bulk samples of amphibole muscovite and biotite extracted from 21 metamorphic and magmatic rocks sampled on both sides of the grimaud fault migmatites micaschists gneisses pegmatites and granites displayed muscovite and biotite plateau ages ranging from 317 2 1 0 to 322 9 1 7 ma on the western side and from 300 2 0 6 to 306 0 2 4 ma to the east of the grimaud fault in the western block amphiboles from amphibolites yielded plateau ages of 328 1 2 8 and 329 9 2 1 ma whereas in the eastern block amphiboles from amphibolite and magmatic bodies yielded variable ages ranging from 307 9 1 2 to 317 4 2 4 ma these data clearly demonstrate distinct cooling histories between at least 330 and 300 ma on both sides of the grimaud fault which consequently appears as a major crustal fault during the late variscan orogeny as shown by the concordant for each type of mineral data obtained on biotite muscovite and amphibole from magmatic and metamorphic rocks the two crustal blocks cooled down independently each one homogeneously in the range of temperatures 550 300 c two distinct periods of fast cooling appear at 320 ma to the west and 305 300 ma to the east these data are consistent with those previously obtained on other regions of the european variscan belt

nature of the moho beneath the scottish highlands from a receiver function perspective abstract the geology of scotland documents a protracted geological history from precambrian basement formation through the caledonian orogeny and to the tertiary opening of the atlantic a temporary deployment of 22 closely spaced broadband seismic stations in scotland traverses many of the major terrane boundaries in the region and provides a unique opportunity to place spatial and temporal constraints on variations in crustal structure with teleseismic p wave receiver functions we use the h method to determine variations in bulk crustal parameters crustal thickness h and v p v s ratio mean crustal thickness is 28 km varying from 23 km in the ne highlands and increasing to 30 km near the highland boundary faultin the southern part of the study area mean v p v s values of 1 76 show no significant variation across the study area an abrupt increase in crustal thickness of 4 5 km nw across the moine thrust is not easily linked to tertiary recent tectonic activity instead it appears that scotland s crust has retained features for hundreds of millions of years since it was first formed despite abundant volcanic activity in tertiary times our work favours the view that the moho is a compositional boundary rather than a mineralogical one defined by a reaction in p t space

hydrocarbon provinces in the swiss southern alps a gas geochemistry and basin modelling study abstract in order to evaluate the petroleum potential of southern switzerland south alpine fold and thrust belt gas geochemical investigations including free and adsorbed gases were combined with a basin modelling analysis the palaeotectonic setting and alpine orogenic movements were recognized as main controls in the evolution of the petroleum generation potential three main hydrocarbon provinces are distinguished 1 the arbostora anticline which developed from an early jurassic high has excellent but immature source rocks which probably at no time generated hydrocarbons because of their swallow level in the nappe edifice 2 the stabio anticline south of the arbostora anticline in southernmost switzerland where wet gases are observed at the surface that are likely to have been generated from late cretaceous to present day from triassic source rocks occurring deeper in the nappe pile and 3 the m generoso basin with prevailing petroleum generation in the early jurassic and possible minor generation in marginal areas during cretaceous and tertiary times

mesozoic granitic magmatism in extensional tectonics near the mongolian border in china and its implications for crustal growth abstract the yagan area of the southernmost sino mongolian border is characterized by an extensional structure where a large metamorphic core complex yagan onch hayrhan and voluminous granitoids are exposed new isotopic age data indicate that the granitoids which were previously regarded as paleozoic in age were emplaced in early and late mesozoic times the early mesozoic granitoids have 228 7 ma u pb zircon age and consist of linear mylonitic quartz monzonites and biotite monzogranites their chemical compositions are similar to those of potassic granites and shoshonitic series and show an intraplate and post collisional environment in tectonic discrimination diagrams their fabrics reveal that they experienced syn emplacement extensional deformation all these characteristics suggest that the adjustment thinning and extensional deformation at middle to lower crustal levels might have occurred in the early mesozoic the late mesozoic granitoids have a u pb zircon age of 135 2 ma and are made up of large elliptical granitic plutons they are high k calc alkaline and were forcefully emplaced in the dome extensional setting both the early and late mesozoic granitoids have end t values of 2 3 to 5 in strong contrast with the negative end t values 11 of the precambrian host rocks this suggests that juvenile mantle derived components were involved in the formation of the granitoids the similar situation is omnipresent in central asia this study demonstrates that tectonic extension magmatism and crustal growth are closely related and that post collisional and intraplate magmatism was probably a significant process for continental growth in the phanerozoic

3 d ground penetrating radar applied to fracture imaging in gneiss three dimensional ground penetrating radar georadar techniques suitable for geological engineering applications have been developed and tested initial experiments were conducted on the floor of a quarry in southern switzerland from which ornamental gneissic rock is extracted during a brief two day period constant offset georadar data were recorded over a 650m2 area with a grid cell size of 0 1 m 0 2 m georadar velocities were estimated from the results of expanding spread surveys all georadar data and associated geometry files were recorded automatically in seismic industry formats the experimental georadar data set was processed image enhanced and interpreted using 3 d seismic reflection software operating on a workstation arbitrary vertical sections time slices 3 d images and animated movies in which the observer travels through the entire data volume were constructed from the resultant migrated georadar data semi automatic tracking routines allowed continuous subhorizontal reflectio

noble gas isotopes and the chemical composition of geothermal gases from the eastern part of the buyuk menderes graben turkey abstract we present noble gas isotope and gas composition data from eight production wells of the kizildere geothermal field and from six gas discharging pools from the adjacent tekke hamam geothermal field situated in the eastern segment of the buyuk menderes graben system in western anatolia turkey gas from both locations is composed mainly of co 2 with minor contributions of n 2 h 2 s ch 4 o 2 ar h 2 c 2 and he with higher contents of h 2 s he and c 2 in tekke hamam samples air corrected helium isotope ratios reveal a mantle origin for samples from both locations but 3 he 4 he ratios are lower and more variable at kizildere 0 96 2 06 r a than at tekke hamam 2 36 2 91 r a r a is the atmospheric 3 he 4 he ratio of 1 39 10 6 kizildere gas contains less 4 he 0 7 2 2 ppmv and 3 he 2 5 10 6 ppmv than gas from tekke hamam 2 5 3 5 ppmv and 1 10 5 ppmv respectively co 2 3 he ratios of 2 8 10 11 at kizildere and 0 7 1 10 11 at tekke hamam are higher than average mantle values 2 10 9 within the kizildere geothermal field reservoir temperatures and co 2 3 he and ch 4 c 2 h 6 ratios increase and 3 he 4 he ratios decrease from southwest to northeast interpolation of a linear trajectory between 3 he 4 he and temperature from kizildere delivers temperature of 173 c 184 c for the source reservoir of tekke hamam in good agreement with measured reservoir temperatures from well th 2 at tekke hamam 171 c the low helium content the variable helium isotope composition and the higher co 2 3 he ratios at kizildere can be explained by temperature dependent partial removal of mantle derived helium e g through vapor separation followed by addition of radiogenic helium kizildere samples derive from the remaining water phase after gas separation while tekke hamam samples may represent the gas phase liberated by the former process the pristine 3 he 4 he ratio of helium at kizildere can be calculated as 2 2 r a close to helium isotope ratios observed at tekke hamam partial removal of helium may also account for the higher and t dependent co 2 3 he ratios at kizildere compared to tekke hamam but such process alone cannot explain the overall high co 2 3 he ratios 7 10 10 and calls for contribution of co 2 from interaction between geothermal fluids and carbonate reservoir rock noble gas abundances indicate mixing with cold meteoric water in the reservoir

migmatites granites and orogeny flow modes of partially molten rocks and magmas associated with melt solid segregation in orogenic belts abstract this paper presents a model for the genesis of migmatites and granites during orogenic evolution based on the analysis of several phanerozoic crustal segments and a review of the physical properties of partially molten rocks and magmas this model inventories the modes of bulk flow of partially molten rocks and magmas and the mechanisms of melt solid segregation for each mode partial melting of rocks is associated with a strength decrease of two to three orders of magnitude leading to strain partitioning expressed by channel flow driven either by forces related to plate tectonics vertical channel flow or by the gravity force associated with lateral variations of crustal thicknesses horizontal channel flow although the structural characteristics of migmatites indicate that deformation plays a role in melt migration the emplacement of laccoliths of leucogranites above migmatites attests to the efficiency of the buoyancy force another decrease in apparent strength of about ten orders of magnitude is associated with the loss of continuity of the solid framework marking the transition from partially molten rocks to magmas the breakdown of the solid framework also allows for settling of the solid in suspension increasing the buoyancy of the remaining magma accordingly domes cored by diatexites former heterogeneous magmas and mantled by metatexites former partially molten rocks are interpreted as gravitational instabilities driven by the relative buoyancy of the magma and permitted by the weakness of the partially molten rocks this model provides a first order framework to elucidate the development of a crustal scale horizontal layering during the evolution of orogenic belts that are affected by partial melting in this case the middle crust is dominated by migmatites with domes cored by diatexites and mantled by metatexites that correspond to a partially molten and magmatic zone respectively the granitic dikes and sills and the associated laccoliths of leucogranites correspond to an intrusive zone overlying the partially molten zone the refractory lower crust is potentially in part formed by accumulation of solids segregated from the overlying heterogeneous magmas

radar experiments in isotropic and anisotropic geological formations granite and schists 1 in order to understand various aspects of radar wave propagation a survey of electromagnetic wave behaviour relative to the geological characteristics of the formations prospected was undertaken the sites chosen for the tests were a granite quarry and an underground schist working by investigating an electrically resistive isotropic site and a conductive anisotropic site it was demonstrated that non conventional use of a radar system antennae raised various orientations of the transmitter receiver etc could improve data quality and could allow information other than reflector depth to be collected volume scattering intensity isotropy etc by studying wave propagation velocities we underlined the difficulties encountered in establishing a velocity versus depth law despite recourse to seismic data processing such as nmo corrections the results of field experiments complemented by laboratory measurements of dielectric permittivities clearly showed anisotropy effects in the case of a path that is perpendicular to the schistosity plane an electromagnetic wave propagates more slowly and is more attenuated than a wave parallel to the schistosity plane

moho depths and poisson s ratios of precambrian crust in east africa evidence for similarities in archean and proterozoic crustal structure abstract we investigate the structure of precambrian crust in east africa by using estimates of moho depth and poisson s ratio for 37 new seismic stations in uganda and tanzania and for 32 previously deployed seismic stations in kenya and tanzania the dataset includes estimates of crustal structure distributed between all of the major archean and proterozoic terrains obtained from modeling p wave receiver functions the average crustal thickness for the ubendian belt is 42 km and between 37 and 39 km for all of the terrains the average poisson s ratio for all of the terrains is either 0 25 or 0 26 indicating a felsic to intermediate bulk crustal composition a similar composition and thickness of the crust in east africa for terrains spanning some 4 byr of earth history is different than in many other precambrian regions our results suggest that there may have been few changes over earth s history in the processes that formed the east african crust or else that processes have been at play to homogenize crustal structure such as the flow of lower crustal material in orogenic systems or through the foundering of eclogites into the mantle the finding that crustal structure is similar between the archean and proterozoic terrains indicates that crustal structure through its influence on lithospheric rheology and strength has not had a first order effect on the location of rifting we also find that there is little correlation between moho depth and elevation across the east african plateau indicating that variations in crustal thickness exert few if any influences on topography in east africa this conclusion lends support to many studies arguing that mantle structure and processes provide the primary buoyant support for the plateau

large extensional structures developed during emplacement of a crystalline thrust sheet the mondonedo nappe nw spain the mondonedo nappe is a crystalline thrust sheet characterized by large recumbent folds regional intermediate pressure metamorphism synkinematic intrusion of granitoids during nappe emplacement and an extensional ductile shear zone developed within the nappe during thrusting a large tectonic window permits the study of the footwall unit revealing another extensional shear zone contemporaneous with thrusting and a low pressure metamorphic evolution in contrast to that of the hanging wall unit the two main extensional shear zones produced e w extension parallel to the direction of orogenic shortening and normal to the orogenic structural trend furthermore subordinate n s longitudinal extension was accommodated by normal faults in the footwall and some of these faults were used as lateral ramps in late stages of thrusting the role of the extensional shear zones and faults described is discussed in the context of an evolving orogenic wedge dominated by plate convergence but characterized by large scale rheological heterogeneities within it deep seated viscous flow triggered by heat accumulation seems to account for the horizontal stretching and probable tapering of the orogenic wedge which was induced by gravitational instabilities due to partial melting and underplating by buoyant continental crust

protracted coeval crust and mantle melting during variscan late orogenic evolution u pb dating in the eastern french massif central the late stages of the variscan orogeny are characterized by middle to lower crustal melting and intrusion of voluminous granitoids throughout the belt which makes it akin to hot orogens these processes resulted in the development of large granite migmatite complexes the largest of which being the 305 300 ma old velay dome in the eastern french massif central fmc this area also hosts a wide range of late variscan plutonic rocks that can be subdivided into four groups i cordierite bearing peraluminous granites cpg ii muscovite bearing peraluminous granites mpg iii k feldspar porphyritic calc alkaline granitoids kcg and iv mg k rich monzo diorites and lamprophyres vaugnerites new results of la sf icp ms u pb zircon and monazite dating on 33 samples from all groups indicate that both granites and mafic rocks emplaced together over a long period of 40 million years throughout the carboniferous as shown by intrusion ages between 337 4 1 0 and 298 9 1 8 ma for the granitoids and between 335 7 2 1 and 299 1 1 3 ma for the vaugnerites low zircon saturation temperatures and abundant inherited zircons with predominant late ediacaran to early cambrian ages indicate that the cpg and mpg formed through muscovite or biotite dehydration melting of ortho and paragneisses from the lower gneiss unit the kcg and vaugnerites contain very few inherited zircons if any suggesting higher magma temperatures and consistent with a metasomatized lithospheric mantle source for the vaugnerites the kcg can be explained by interactions between the cpg mpg and the vaugnerites or extensive differentiation of the latter the new dataset provides clear evidence that the eastern fmc was affected by a long lived magmatic episode characterized by coeval melting of both crustal and mantle sources this feature is suggested here to result from a lithospheric scale thermal anomaly triggered by the removal of the lithospheric mantle root the spatial distribution of the dated samples points to a progressive southward delamination of the lithospheric mantle perhaps in response to rollback following continental subduction or to retro delamination owing to the retreat of a south verging subduction zone

interaction between basic and acid magmas during the latest stages of the post collisional variscan evolution clues from the gabbro granite association of ota corsica sardinia batholith abstract the intrusive association of ota post variscan corsica sardinia batholith consists of mafic rocks and granites displaying mingling relations the mafic rocks are olivine gabbros to quartz diorites and the granites are subsolvus and characterised by biotite as the only mafic phase minor amounts of amphibole bearing granodiorites to granites develop along the contacts between mafic rocks and granites the rather high initial e nd 4 5 of the most primitive mafic sample indicates that parental basic melts had an extensive contribution from a depleted mantle source in the mafic rocks the decrease of initial e nd to 2 1 is associated with an increase of th and u concentrations in clinopyroxene thus indicating that the ota basic melts underwent a process of crustal contamination associated biotite granites have low initial e nd 0 4 and high th and u concentrations the chemical evolution of ota basic melts most likely occurred through fractional crystallisation and concomitant contamination with the acid magma that gave rise to associated biotite granites the incompatible element signature of late stage anhedral amphibole from variably evolved mafic rocks is similar thus suggesting that the whole ota mafic sequence recorded the contamination process possibly by percolation within the gabbroic crystal mush of a melt contaminated by the acid magma hybridisation between basic and acid magmas is also documented by contact amphibole bearing granitoids in particular major and trace element zoning of amphibole from granodiorites reveals that such a hybridisation was associated with reactions between crystals from the basic magma amphibole and clinopyroxene and the acid melt the occurrence of chemically homogeneous amphiboles in associated granites suggests that contact hybridisation comprised also a process of gradient induced chemical diffusion

geochronological constraints on post collisional shear zones in the variscides of sardinia italy terra nova 24 42 51 2012 abstract the post collisional tectonics in the inner zone of the variscan belt of corsica sardinia recorded a dextral transpression developed with an orogen parallel tectonic transport recent works documented the presence of nearly parallel dextral and sinistral crustal scale shear zones responsible for the exhumation of the metamorphic complexes to constrain for the first time the timing of both shear zones and the timing of the transpressional shearing zircons and monazites were collected for u th pb ages results indicate that the shear zones have been active at c 320 ma in a transpressional setting widespread in the southern european variscan belt

origin of carbon in fumarolic gas from island arcs abstract we have measured 3 he 4 he ratios 13c values and co 2 3 he ratios of fumaroles and hot spring gases in three volcanic areas in japan the 3 he 4 he ratios 13c values and co 2 3 he ratios vary from 02 05 ratm 9 7 to 2 7 and from 7 74 109 to 1 18 1011 respectively based on these data and those in the literature we present the first quantitative estimates of the upper mantle sediment and limestone including slab components in co2 of volcanic and hydrothermal fluids from subduction zones taking compiled c isotopic data of limestone and organic sediment assuming that the co 2 3 he ratio and 13c value of mid ocean ridge basalt morb are representative of the upper mantle component and that later isotopic and elemental fractionations are minimal it is possible to deconvole each fraction of c on the basis of simple mixing equations up to 20 of the carbon in high temperature volcanic gases in subduction zones is derived from a morb type source while the major fraction of the gases is attributable to co2 produced by decarbonation of subducted marine limestone and slab carbonate

georadar anisotropy in the gotthard gneiss switzerland georadar data recorded in a crystalline gneiss of southern switzerland exhibit two effects that are related to anistropic metamorphic fabric in the first signal coherence appears to be better for waves propagating parallel to the direction of foliation and in the second rms velocities of waves propagating along the foliation are higher than for waves propagating at a high angle to foliation

the timing of w sn rare metals mineral deposit formation in the western variscan chain in their orogenic setting the case of the limousin area massif central france new 40ar 39ar dating performed on rare metal granites and w sn deposits in the northern limousin has provided evidence of two metallogenic episodes an early namurian episode c 325 ma was contemporaneous with the emplacement of the large peraluminous leucogranite bodies which are associated with small w sn deposits but also with some larger deposits at puy les vignes 323 4 0 9 ma and moulin barret 323 7 0 8 ma formed at a shallower level above cryptic granite plutons these new data indicate that the metallogenic potential of the namurian leucogranites might have been underestimated most other w sn deposits in the northern limousin area are attributed to a mid westphalian episode c 310 ma and are contemporaneous with the emplacement of all the rare metal granites both episodes were related to leucogranite emplacement and associated fluid circulations but in two different geodynamic contexts the early namurian episode may be related to syncollisional extension of the variscan belt whereas the mid westphalian one occurs during generalized extension and rapid exhumation of the belt associated with the granulite facies metamorphism of the lower lithosphere probably related to the delamination of the lower lithosphere thus w sn and rare metals ta nb be li deposits are clearly temporally and probably genetically related to leucogranitic magmatism

successful application of ground penetrating radar in the exploration of gem tourmaline pegmatites of southern california application of ground penetrating radar has been successful in delineating gem bearing zones in the himalaya pegmatite mine of the mesa grande district of southern california the high frequency of the electromagnetic signal allows features as small as a few centimetres to be resolved within 1 2 m of the surface of a mine wall careful initial set up consisted of i selection of antennae with sufficiently high central frequencies ii recording with a short time of scan to reduce end of scan noise levels iii choosing appropriate colour schemes to highlight extreme amplitude variations operation during data collection consisted of pre painting marking points on the mine face and air launching the signal to reduce false anomalies caused by rocking of the antenna on the rough surfaces data processing using the hilbert transform provided images of the cavity geometry that were then used by the blasting captain for accurate placement of explosives the instantaneous frequency plot was found to be effective in distinguishing air filled from clay filled pockets and the instantaneous phase plot was helpful in selecting potential targets where the amplitude was less than the maximum range when carefully used in conjunction with good knowledge of the geological conditions the method promises to provide an important tool for mapping internal structures of pegmatites thus assisting future mining activities

syn orogenic extensional and contractional deformation related to granite emplacement in the northern tasman orogenic zone australia abstract the hodgkinson province of the northern tasman orogenic zone comprises uniform low grade metasediments of mid palaeozoic age that host numerous permian granite intrusions three widely developed sequential deformations d2 d4 are recognized on the basis of penetrative fabrics and folds which show clear regionally consistent overprinting relationships d3 and d4 were contemporary with phases of granite emplacement as shown by relationships of s3 and s4 microstructures to porphyroblasts which grew in thermal aureoles adjacent to intrusive contacts granitic dyke relationships with s3 and the deflection of s3 and s4 foliation trajectories around plutons support this view granites of d3 and d4 affiliation have a close spatial relationship and strong geochemical similarities suggesting that they were closely associated in time isotopic dating of granites of d4 association show that this deformational phase represents the hunter bowen orogeny which has been recognized as the terminal permian triassic episode of tectonism for the eastern sector of the tasman orogenic zone d3 fabrics are best developed in zones of enhanced metamorphic grade and are typically subhorizontal or of gentle inclination and associated with recumbent folds when not affected by subsequent deformation d3 is attributed to a phase of tectonism linked to the hunter bowen orogeny but preceding the main episode of compression uplift above tracts of rising magma combined with the thermal weakening of crustal material and the reduction of compressive stress due to plate convergence sponsored widespread ductile extension at a high crustal level the zone of failure was a discontinuous carapace above the magmatic front d4 is attributed to the main episode of the hunter bowen orogenic compression and involved crustal shortening with the imposition of upright structures and the widespread emplacement of granite s3 foliation surfaces have been reoriented in the vicinity of d4 granite bodies which behaved as rigid bodies during d4 compression the preparation of crustal pathways by d3 granite was utilized by granites of the d4 generation with the hodgkinson province displaying an intimate interplay of magmatism stress regime and structural evolution over a time period of perhaps 15 20 ma

a multidisciplinary study of a syntectonic pluton close to a major lithospheric scale fault relationships between the montmarault granitic massif and the sillon houiller fault in the variscan french massif central 1 geochronology mineral fabrics and tectonic implications 1 because of its location along one of the major faults the sillon houiller fault shf of the french massif central fmc the montmarault granitic pluton is well suited to better understand the place of the late orogenic magmatism in variscan orogeny through a methodological approach the close spatial association of the pluton with the lithospheric shf is investigated in order to clarify the relationships between faulting and magmatic processes during the late carboniferous therefore a multidisciplinary study has been carried out on the montmarault massif combining geochronology field and laboratory microstructural observations and anisotropy of magnetic susceptibility ams methods presented in this paper and gravity and aeromagnetic modeling part 2 as well as similar studies from other plutons in the fmc allows us to draw the following conclusions 1 the montmarault pluton dated by the chemical u th pb method on monazite at 321 2 ma was emplaced in a nw se maximum stretching trend which is consistent with the regional extensional tectonic regime 2 the montmarault pluton is rooted in its eastern part along the shf with a laccolite like shape in its western part 3 at circa 320 ma the proto shf acted probably as a normal fault considered as the feeding channel for the magma emplacement and 4 late carboniferous ne sw extensional tectonics reworked the montmarault pluton in a brittle postsolidus stage

from a mountain belt collapse to a sedimentary basin development 2 d thermal model based on inversion of stratigraphic data in the paris basin abstract the thermal mineralogical and mechanical behaviour of the lithosphere determines the genesis and evolution of sedimentary basins subsidence amplitude geometry of sediment bodies sedimentary processes using the large stratigraphic database available for the paris basin we constrain the lithosphere thermal evolution thickness and rigidity as well as the mechanism that gave birth to this wide intracontinental basin the thermal subsidence calculated with a two dimensional thermal model is adjusted to the estimated tectonic subsidence by optimization of three parameters extension profile crustal thickness profile and average mantle heat flow the best fits for the paris basin lithosphere are obtained for a mantle heat flow of 23 mw m 2 and for an elastic thickness lower than 20 km preferred value te 5 km two mechanisms for the origin of the basin subsidence have been tested a pure extensional collapse of the variscan belt and a delamination of the mantle lithosphere at the end of carboniferous it appears that in both cases inverted extension factors are large in the range 1 2 1 85 in the first case and in the range 1 0 1 85 in the second case both cases involve the presence of a thick variscan crustal wedge and a mountain belt elevation of 1500 3500 m located in the paris basin centre the maximum extensional rates are obtained in the centre part of the basin in addition lithospheric delamination at around 300 ma allows for both crustal and mantle melting in agreement with the widespread existence of permian bimodal magmatism in france at the end of extension the basin centre kept a high elevation of about 1000 m before subsiding around 40 my later below sea level

flow of mantle fluids through the ductile lower crust helium isotope trends heat and mass are injected into the shallow crust when mantle fluids are able to flow through the ductile lower crust minimum 3he 4he ratios in surface fluids from the northern basin and range province western north america increase systematically from low crustal values in the east to high mantle values in the west a regional trend that correlates with the rates of active crustal deformation the highest ratios occur where the extension and shear strain rates are greatest the correspondence of helium isotope ratios and active transtensional deformation indicates a deformation enhanced permeability and that mantle fluids can penetrate the ductile lithosphere even in regions where there is no substantial magmatism superimposed on the regional trend are local high 3he 4he anomalies indicating hidden magmatic activity and or deep fluid production with locally enhanced permeability identifying zones with high resource potential particularly for geothermal energy development

rapid extensional unroofing of a granite migmatite dome with relics of high pressure rocks the podolsko complex bohemian massif the podolsko complex bohemian massif is a high grade dome that is exposed along the suprastructure infrastructure boundary of the variscan orogen and records snapshots of its protracted evolution the dome is cored by leucocratic migmatites and anatectic granites that enclose relics of high to ultrahigh pressure rocks and is mantled by biotite migmatites and paragneisses whose degree of anatexis decreases outwards our new u pb zircon ages indicate that the leucocratic migmatites were derived from early ordovician c 480 ma felsic igneous crust the same age is inferred for melting the proto source of the metapelitic migmatites the relics of high to ultrahigh pressure rocks suggest that at least some portions of the complex witnessed an early variscan subduction to mantle depths followed by high temperature anatexis and syntectonic growth of the podolsko dome in the middle crust at c 340 339 ma subsequently the dome exhumation was accommodated by crustal scale extensional detachments similar c 340 ma ages have also been reported from other segments of the variscan belt yet the significance of this tectonothermal event remains uncertain here we conclude that the 340 ma age post dates the high pressure metamorphism the high temperatures required to cause the observed isotopic resetting and new growth of zircon were probably caused by heat input from the underlying mantle and finally the extensional unroofing of the complex requires a minimum throw of about 8 10 km we use this as an argument for significant early carboniferous palaeotopography in the interior of the variscan orogen

from lithospheric thickening and divergent collapse to active continental rifting 1 abstract 32 introduction 33 modelling approach main assumptions and simpli cations 53 1 tectonic thickening 63 2 erosion sedimentation 63 3 thermal relaxation 73 4 gravity driven ow 74 physical model 74 1 temperature pro le 84 2 density pro le and surface elevation 84 3 gravitational force 94 4 strength pro le 104 5 lithospheric geometry through time 105 presentation of the results 115 1 stable thermal boundary layer during convergence 115 2 post convergence removal of the thermal boundary layer 126 discussion and conclusion 137 acknowledgements 158 references 162

precambrian crustal evolution seismic constraints from the canadian shield whether or not plate tectonic processes operated on a younger hotter earth remains ambiguous seismic data from new networks in the hudson bay region of the canadian shield where the precambrian geological record spans more than 2 billion years offer fresh scope to address this problem using receiver function analyses we show that the crust of the rae domain which exhibits ages of paleo to neoarchean 3 9 2 7 ga is likely felsic to intermediate in composition average vp vs 1 73 and seismically transparent with a sharp moho there is little evidence for modern style plate tectonics and based on the simplicity and spatial extent of the felsic crust models favouring vertical tectonic processes such as crustal delamination or plume activity appear better suited to the results data from the hearne domain which exhibits widespread 2 7 ga granite and greenstone geology show a more complex crust with higher vp vs ratios consistent with a greater mafic component the trans hudson orogen tho proposed to be a himalayan scale mountain belt during the paleoproterozoic is thought to have formed during the 1 8 ga collision of the superior and churchill plates results from the quebec baffin island segment of the tho appear to map out the first order shape of the underthrusting superior plate with elevated vp vs ratios likely representing the rifted margin of the superior craton consistently thicker crust is observed beneath central and southern baffin island 43 km coincident with widespread high grade metamorphic surface geology these features can be explained by crustal thickening due to stacking of accreted terranes during continent continent collision analogous to the present day tibetan plateau followed by erosion when reviewed in light of age and compositional constraints from the geological record our seismic observations point towards secular crustal evolution from non plate tectonic during the paleo to mesoarchean evolving towards fully developed modern style plate tectonics during the paleoproterozoic

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

age constraints on late paleozoic evolution of continental crust from electron microprobe dating of monazite in the peloritani mountains southern italy another example of resetting of monazite ages in high grade rocks in situ monazite microprobe dating has been performed for the first time on trondhjemite and amphibolite facies metasediments from the peloritani mountains in order to obtain information about the age of metamorphism and intrusive magmatism within this still poorly known sector of the hercynian belt all samples show single stage monazite growth of hercynian age one migmatite and one biotitic paragneiss yielded monazite ages of 311 4 and 298 6 ma respectively these ages fit with previous age determinations in similar rocks from southern calabria indicating a thermal metamorphic peak at about 300 ma at the same time as widespread granitoid magmatism the older of the two ages might represent a slightly earlier event possibly associated with the emplacement of an adjacent trondhjemite pluton previously dated by shrimp at 314 ma no evidence for pre hercynian events and only a little indication for some monazite crystallization starting from ca 360 ma were obtained from monazite dating of the metasediments suggesting either a single stage metamorphic evolution or a significant resetting of the monazite isotope system during the main hercynian event ca 300 ma rare monazite from a trondhjemite sample yields evidence for a late hercynian age of about 275 ma this age is interpreted as representing a post magmatic stage of metasomatic monazite crystallization which significantly postdates the emplacement of the original magmatic body

the late palaeozoic early mesozoic from the catalan pyrenees spain 60 myr of environmental evolution in the frame of the western peri tethyan palaeogeography the eastern catalan pyrenees form the southernmost segment of the pyrenean axial zone they experienced complex multistage process as the result of both the variscan and the pyrenean orogenic cycles after the late palaeozoic several small extensional sub basins were filled by continental successions as a response to the changing tectonic setting within the frame of the southwestern europe this stratigraphic record perfectly exposed in the studied eastern pyrenean areas preserves the signatures of the late palaeozoic pangea break up during the progressive dismantling of the variscan chain up to the triassic diffused extension despite structural network being severely affected by cenozoic reworking the stratigraphic framework can be unravelled by the detailed sedimentological and stratigraphical analysis of the well exposed sequences in short this paper addresses facies patterns and architectural elements of late carboniferous permian triassic continental sections studied in the catalan pyrenees and aims to use these signatures for correlation not only at a regional scale but also within the broader picture of the southwestern europe our stratigraphical reconstruction clearly demonstrates that the thick studied succession consists of fluvial lacustrine basal units affected by intense volcanic activity and overlaying predominantly fluvial fining upwards units for each unit several lithofacies architectural elements and bounding surfaces of different order have been identified and their lateral and vertical staking characterized deposition and erosion processes are controlled by alternating periods of tectonic activity producing different stages of high and low subsidence the uplift of the sub basin margins and subsequent extended erosive events marked by angular unconformities on the strength of the major rank bounding surfaces analysis the studied sedimentary packages have been grouped into different tectono stratigraphic units and then correlated they have been contextualized within the latest palaeogeographic reconstructions of the studied sector contributing to detail the role of the study area within the frame of the s european palaeogeography

variscan sb au mineralization in central brittany france a new metallogenic model derived from the le semnon district numerous sb au deposits are known in the european variscan belt and are often associated with the latevariscan hydrothermal events linked to postorogenic extension and strike slip faults through the whole belt thecentral part of the armorican massif central brittany hosts several sb au deposits such as the le semnondeposit however little is known about its formation its structural control or hydrothermal history in order toprovide new constraints on the sb au mineralization framework of the variscan armorican massif structural geochronological and chemical analyses have been performed on the le semnon sb au deposit first wehighlight that sb au mineralization consists of fault fill and flat extensional vein systems resulting from smallscalefault valve mechanism controlled by fluid overpressure at shallow depth indeed this deposit is characterizedby the presence of dolerite dykes which acted as the plumbing system for fluids within impermeableslates the low permeability of the slates permitted the establishment of local supralithostatic fluid pressure in apredominantly hydrostatic regime coupled with new micro textural evidences a reappraisal of the parageneticevolution and new physico chemical data 40ar 39ar and u pb analyses show that the le semnon sb aumineralization was emplaced around 360 ma and therefore indicates an early hydrothermal event in thevariscan history of the armorican massif moreover absolute dating of mafic magmatism coupled with hydrothermalalteration associated with the deposition of sb au mineralization strongly suggest that the emplacementof the sb au mineralization was coeval with the emplacement of the host rock doleritic dyke furthermore the high temperature recorded in fluid inclusions and the paragenetic evolution suggests an elevatedthermal anomaly in central brittany thus we propose that the emplacement of the widespread maficmagmatism at ca 360 ma played a major role in the mobilization of fluid flows and the redistribution of metalsin the armorican crust

transpressional tectonics and carboniferous magmatism in the limousin massif central france structural and 40ar 39ar investigations new structural microstructural and 40ar 39ar data from the nw massif central france provide additional constraints on the timing and tectonic setting of late variscan granite magmatism previous studies had emphasized the role of late orogenic extension in the emplacement of granite plutons in the limousin region in contrast the new data set is consistent with syntectonic emplacement of magma in a dextral simple shear active from 350 to 300 ma in a transpressional regime as an alternative hypothesis to late orogenic extension we propose that magmas migrated into tensional bridges between active p shears associated with a lithospheric shear zone comparable to a pop up structure the galician region in the western end of the ibero armorican tectonic arc exhibits major left lateral ductile shear zones which can be interpreted as conjugate structures to the limousin and armorican shear zones citation ge belin a m brunel p monie m faure and n arnaud 2007 transpressional tectonics and carboniferous magmatism in the limousin massif central france structural and 40ar 39ar investigations

geochronological study of zircons from continental crust rocks in the frido unit southern apennines zircon crystals have been separated from gneisses and metagranitoids of the pollino area southern apennines in order to unravel the origin of these crustal slices within the ophiolite bearing frido unit the morphology of the zircon has been investigated by sem and the internal structure was revealed by cathodoluminescence data obtained by u pb dating have been used to deduce the age and significance of the different crystallization stages of zircon connected to the evolutionary stages of the continental crust late paleozoic early mesozoic zircons in gneisses are characterized by inherited cores of magmatic origin bordered by metamorphic rims inherited zircons generally show paleoproterozoic to ordovician ages indicating the provenance of the sedimentary protolith from different sources the exclusive presence of late neoproterozoic zircon cores in leucocratic gneisses may suggest a different magmatic source possibly connected to pan african events late carboniferous early permian ages are found mainly in zircon rims of metamorphic origin these are similar to the emplacement ages of protolith of the metagranites in the middle crust portion late carboniferous early permian metamorphism and magmatism testify the extensional collapse of the hercynian belt recorded in european particularly in the corsica sardinia block and in calabria late permian triassic ages have been detected in zircon rims from gneisses and metagranitoids these younger ages appear related to deformation and emplacement of albite quartz veins in both lithologies and are related to an extensional episode predating the middle triassic to middle jurassic rifting in the tethyan domain followed by middle to late jurassic spreading

precambrian crustal structure in africa and arabia evidence lacking for secular variation we review the thickness and shear wave velocity structure of precambrian crust in africa and arabia where over 90 of the landmass is comprised of archean and proterozoic terranes and examine the data for evidence of secular variation the data come from many published 1d shear wave velocity profiles obtained by jointly inverting receiver functions and surface wave dispersion measurements 35 new 1d shear wave velocity profiles for locations in eastern africa and a new map of crustal thickness for africa and arabia derived from modeling satellite gravity data we find for both archean and proterozoic terranes a similar range of crustal thicknesses 33 45 km similar mean crustal shear wave velocities 3 6 3 7 km s and similar amounts of heterogeneity in lower crustal structure as reflected in the thickness of lowermost crust with shear wave velocities 4 0 km s there is little evidence for secular variation in crustal structure indicating that there may have been few changes over much of earth s history in the processes that form the continental crust post formation tectonic events also may have modified many of the terranes to such an extent that secular trends arising from crustal genesis may be difficult to recognize

early paleozoic orogenic collapse tectonic stability and late paleozoic continental rifting revealed through thermochronology of k feldspars southern norway southern norway was subjected to contractional orogenesis and subsequent extensional collapse during the early to middle paleozoic when baltica collided with laurentia resulting in the caledonian orogen the largely unknown post caledonian paleozoic thermal history of the crust in southern norway has been assessed through application of multiple diffusion domain thermal modeling of k feldspar 40ar 39ar data samples were collected along a 250 km e w traverse from the internides of the orogen along the western coast near bergen through a series of middle to upper crustal nappes to the east the k feldspar thermal models which complement published mica and hornblende 40ar 39ar data are consistent with rapid cooling at the end of the caledonian orogenic cycle when early devonian 400 ma extensional collapse resulted in exhumation through mainly midcrustal conditions following extensional collapse a period of relative tectonic and thermal stability may have prevailed until 300 ma the k feldspar modeling suggests that southern norway entered a new phase of relatively rapid cooling a few c ma in the permo carboniferous and that the increase in cooling rate was diachronous across the traverse the onset of this cooling phase occurs at about the time of the earliest faulting and magmatism in the oslo graben to the southeast the increase in cooling rate may also indirectly date the onset of the first important rift phase and associated sedimentation in the north sea graben system the distribution of permian and younger fission track ages in both southern continental norway and its marginal basin sediments is consistent with the permo carboniferous rapid cooling recorded in the k feldspars the simplest explanation for the late paleozoic increase in cooling rate is that rifting around the margin of southern norway has resulted in a decrease in the base level and an increase in the rate of erosional denudation resulting in essentially uniform exhumation of 2 4 km along the traverse

subsidence of the paris basin and its bearing on the late variscan lithosphere evolution a comparison between plate and chablis models abstract in this study two different models for the lithospheric thermal evolution are used to predict the long term subsidence of the paris basin the plate or chablis models have a constant temperature or constant heat flow at the base of the lithosphere these boundary conditions yield a different thermal relaxation time for a given equilibrium lithospheric thickness the final thermal steady state does not depend on the boundary condition fixed at the base of the lithosphere the thermal evolution of the lithosphere is computed taking into account temperature and pressure dependent thermal characteristics metamorphism in the crust top crustal erosion and phase transition in the mantle spinel garnet the long term tectonic subsidence of the paris basin constrains the mesozoic thermal evolution of the lithosphere and the amplitude of the initial thermal anomaly the chablis model including the effects of spinel garnet phase transition predicts present day lithospheric thickness in better agreement with observation than the plate model the chablis model involves a basal heat transfer of 37 mw m2 similar to the value beneath oceanic plates the long term subsidence of the paris basin obviously results from the decay of a thermal anomaly initiated during late variscan times we first assume that at 300 ma the lithosphere was less than 150 km thick and the thickened crust started returning to its present thickness 35 km by uniform lithospheric extension the subsidence data can be explained by short stephano autunian as well as long stephano triassic lasting extension these hypotheses both implicitly refer to extensional collapse of the variscan belt however the evolution of the variscan belt from 350 to 300 ma may have involved almost no crustal thickening but an anomalously thin lithosphere ca 55 km the corresponding thermal evolution of the lithosphere is computed using the chablis model with an initially high heat transfer of 61 mw m2 at the base of the base of the lithosphere at 280 ma this heat transfer is tuned to 40 mw m2 leading to lithospheric cooling from 280 to 30 ma which can also explain the paris basin subsidence for the short lasting extension hypothesis the late variscan lithospheric evolution predicted with the chablis model yields paleogeography magmatism and retrograde p t t paths in good agreement with those reported from areas around the paris basin conversely the plate model does not allow mantle melting during the end of carboniferous and permian times the long lasting extension hypothesis does not explain the paleogeographic evolution the timing of magmatism and retrograde p t paths showing isothermal decompression the hypothesis with a strong lithospheric delamination cannot be ruled out however it is less plausible than the extensional collapse hypothesis

origin of nitrogen rich natural gases in the california great valley evidence from helium carbon and nitrogen isotope ratios abstract dry natural gases produced from the sacramento and northern san joaquin basins contain up to 87 nitrogen 15n measurements for gases with n2 8 vary from 0 9 to 3 5 methane 13c values vary from 61 to 15 3 he 4 he ratios indicate the presence of mantle derived helium in addition to radiogenic crustal helium and suggest that the gases may originate from mixing between three principal components a hydrocarbon rich gas with 3 he 4 he 0 1r a derived from the microbial and thermogenic alteration of sedimentary organic matter a hydrocarbonrich gas with 3 he 4 he 2 75r a derived from the localized pyrolysis of sediments by magmatic volatiles and a nitrogenrich gas with 3 he 4 he 1r a the nitrogen rich component is characterized by methane 13c less negative than 25 and n 2 ar ratios as high as 22 000 nitrogen concentrations tend to increase towards the basement along the shallow eastern margin of the valley suggesting that the nitrogen rich component is derived from metasedimentary rocks deep within the crust the coast ranges fraciscan complex which was thrust well beneath the great valley during late mesozoic and early cenozoic subduction appears to be the most likely source

sedimentary evolution of the le danois contourite drift systems southern bay of biscay ne atlantic a reconstruction of the atlantic mediterranean water circulation since the pliocene abstract the evolution of the le danois contourite depositional systems cds during the pliocene and quaternary was investigated based on high resolution seismic reflection data from old to young six seismic units u1 u6 bounded by major discontinuities h1 h6 were identified regarding variations of the bottom current circulation four evolution stages of the le danois cds were identified including onset 5 3 to 3 5 3 0 ma initial 3 5 3 0 to 2 5 2 1 ma intermediate 2 5 2 1 to 0 9 0 7 ma and drift growth 0 9 0 7 ma to present day stages the cds associated with the atlantic mediterranean water amw along the middle continental slope initiated at 3 5 3 ma and was widely built after the mid pleistocene transition mpt 0 9 07 ma at a shallower water depth a second cds associated with the eastern north atlantic central water enacw started to develop from the late quaternary 0 47 ma onwards in the amw related drift system the le danois drift was generated both under glacial and interglacial climatic oscilations repeated internal structures in unit 5 that consist of acoustically transparent lower parts moderate amplitude upper parts and high amplitude erosional surfaces at the top are compared with interglacial glacial cycles since the middle pleistocene to the present day these cyclic features suggest coarsening upward sequences of the le danois drift and processes related to enhanced amw during glacial stages the estimated sedimentation rate of the le danois cds reached a maximum during the mpt at least 27 cm ky and then decreased until present day 5 cm ky variations of sedimentary stacking patterns and processes of the le danois cds imply full domination of the intermediate water mass along the central atlantic and southwest european continental slopes from the late pliocene 3 5 3 0 ma onwards

regional and temporal variations in co2 3he 3he 4he and δ13c along the north anatolian fault zone turkey abstract new he and c relative abundance isotope and concentration results from nine geothermal locations situated along an 800 km transect of the north anatolian fault zone nafz turkey that were monitored during the period november 2001 november 2004 are reported the geothermal waters were collected every 3 6 months to study possible links between temporal geochemical variations and seismic activity along the nafz at the nine sample locations the he isotope ratios range from 0 24 to 2 3 r a 13 c values range from 4 5 to 5 8 and co 2 3 he ratios range from 5 10 9 to 5 10 14 the following geochemical observations are noted 1 the highest 3 he 4 he ratios are found near the galatean volcanic region in the central section of the nafz 2 at each of the nine sample locations the 3 he 4 he ratios are generally constant however co 2 3 he ratios and he contents both show one order of magnitude variability and 13 c values show up to 4 variability and 3 at all locations except resadiye 13 c values show positive correlations with co 2 contents the results indicate that at least three processes are necessary to account for the geochemical variations 1 binary mixing between crustal and mantle derived volatiles can explain the general characteristics of 3 he 4 he ratios 13 c values and co 2 3 he ratios at the nine sample locations 2 preferential degassing of he from the geothermal waters is responsible for variations in co 2 3 he values and he contents at each sample location and 3 co 2 dissolution followed by calcite precipitation is responsible for variations in co 2 contents and 13 c values at most locations for each of the geochemical parameters anomalies are defined in the temporal record by values that fall outside two standard deviations of average values at each specific location geochemical anomalies that may be related to seismic activity are recorded on june 28 2004 at yalova where a m 4 2 earthquake occurred 43 days earlier at 15 km distance from the sample location and on april 7 2003 at efteni where a m 4 0 earthquake occurred 44 days later at a distance of 12 km at both locations the sampling periods containing geochemical anomalies were preceded by an increase in m 3 earthquakes occurring within 60 days and less than 40 km distance

nature and composition of the lower continental crust in central spain and the granulite granite linkage inferences from granulitic xenoliths xenolith bearing alkaline ultrabasic dykes were intruded into the of the hercynian basement of the spanish central system in early mesozoic times the suite of lower crustal xenoliths in the dykes divided into three groups felsic peraluminous granulites metapelitic granulites and charnockitic granulites the felsic granulites form 95 of the total volume of the xenoliths whereas the charnockitic and metapelitic granulites are much less abundant 0 01 5 respectively thermobarometric calculations based on mineral paragenesis indicate equilibration conditions around 850 950 c 7 11 kbar thus the xenoliths represent lower continental crustal material superimposed on this high t high p assemblage is a high t low p paragenesis represented mainly by kelyphitic coronas reflecting re equilibration during transport in the clearly restitic mineral assemblages with up to 50 garnet and 37 sillimanite major and trace element modelling supports the idea that the late hercynian peraluminous granites of central spain represent liquids in equilibrium with restitic material of similar composition to the studied lower crustal xenoliths 87sr 86sr and end of the felsic xenoliths calculated at an average hercynian age of 300 ma are in the range 0 706 0 712 and 1 4 to 8 2 respectively these values match the isotopic composition of the outcropping late hercynian granites the sr isotopic composition of the xenoliths is lower than that of the outcropping mid crustal lithologies orthogneisses pelites a major contribution from the lower crust to the source of hercynian granites greatly reduces the necessity of invoking a large mantle contribution in models of granite petrogenesis the felsic nature of the lower continental crust in central spain contrasts with the more mafic lower crustal composition estimated in other european hercynian areas suggesting a non underplated crust in this region of the hercynian orogenic belt

retrogressive fabric development during exhumation of the voltri massif ligurian alps italy arguments for an extensional origin and implications for the alps apennines linkage geological mapping coupled with structural investigations carried out in the voltri massif eastern ligurian alps italy provide new data for the interpretation of the tectonic context controlling main fabric development during exhumation of its high pressure core the voltri massif is here interpreted as a c 30 km long eclogite bearing asymmetric dome formed by the progressive verticalisation of the regional second phase mylonitic foliation developed during retrogressive greenschist metamorphic conditions in this light the exhumation history is driven by a ductile to brittle extensional process operating through low angle top to the w multiple detachment systems a late eocene early oligocene age for this extensional episode is proposed on the basis of structural correlations stratigraphic and radiometric constraints in this scenario the voltri massif is interpreted as an extensional domain developed to accommodate the late eocene early oligocene arching of the western alps northern apennines orogenic system

adjacent releases of mantle helium and soil co2 from active faults observations from the marmara region of the north anatolian fault zone turkey volatile releases in terms of co2 efflux and noble gas of mantle origin are investigated in the marmara region of the seismically active north anatolian fault naf zone a total of 180 co2 efflux measurements 1 to 309 g m 2d 1 were carried out along four transects across the naf spatial variability of surface co2 effluxes was higher at faulted sites relative to comparable background areas the 13c values of soil co2 are 24 0 to 15 6 without correlation with co2 efflux values which suggest that co2 efflux anomaly is caused by fault related biogenic gas flow furthermore 36 gas and water samples from spring sites were investigated for 3he 4he ratios which resulted in a range from 0 26 r ra to 4 22 r ra large mantle helium contributions were observed from several sources along the naf the high co2 efflux sector on the gazikoy saros fault along the naf coincides with the area of high mantle helium flux this shows that fault zones play an important role both in transferring mantle helium to the surface and in causing fault related biogenic gas flow

crustal and upper mantle structure of the western carpathians from celebration 2000 profiles cel01 and cel04 seismic models and geological implications summary celebration 2000 was a large international experiment that focused on the lithospheric structure in central europe it consisted of several wide angle reflection and refraction seismic profiles profiles cel01 and cel04 are located in the transition from the precambrian and palaeozoic platforms to the carpathian orogen the data were modelled using 2 d tomographic and forward modelling techniques the profile cel01 about 900 km long extends from the southwestern margin of the east european craton to the pannonian basin system the depth of moho boundary varies along the profile from 27 33 km under the pannonian basin system area 30 39 under the carpathians and within the tesz to 40 45 km under the east european craton the cel04 profile 630 km long starts in the polish trough pt and reaches the pannonian basin system along the cel04 profile the moho interface shallows from 40 km beneath the pt to about 35 km in the maopolska unit the crustal thickness is 43 km beneath the carpathians and 25 km beneath the pannonian basin system the results obtained are consistent with a commonly accepted view of the carpathians thought to be formed largely due to the interplay between palaeogene to middle miocene subduction of oceanic or suboceanic crust and subsequent collision a feature that we interpret as the subduction related orogenic root is at least partially preserved in both profiles and is manifested by a crustal thicknesses of 39

evidence and timing of crustal extension versus shortening in the early tectonothermal evolution of a proterozoic continental rift sequence at broken hill australia 1 low pressure p high temperature t metamorphism previously attributed to crustal thickening accompanying nappe emplacement in the paleoproterozoic willyama supergroup at broken hill is reinterpreted as a response to crustal extension d1 and bimodal magmatism commencing circa 1700 ma d1 deformation occurred under amphibolite granulite facies conditions producing layer parallel fabrics s1 and metapelitic mineral assemblages in which andalusite dominated over sillimanite crustal extension ceased soon after 1640 ma to be followed by crustal thickening higher pressure metamorphism and northeast vergent recumbent folding d2 attendant on inversion of the original d1 extensional structures d2 recumbent folding has previously gone unrecognized but probably peaked around 1600 ma before further amphibolite facies metamorphism between 1600 and 1590 ma accompanying upright folding and northwest directed thrust faulting d3 in the light of this revised tectonothermal history geodynamic models erected to explain elevated thermal gradients and low p high t granulite facies metamorphism in tectonically thickened crust may not be as relevant to broken hill as once thought above average crustal temperatures and low p high t metamorphism in the willyama supergroup more likely occurred in response to voluminous magmatic intrusion accompanying crustal thinning mantle upwelling and emplacement and unroofing of midcrustal rocks along a major detachment surface a second major structural and or stratigraphic break in the willyama supergroup is indicated by the absence of d1 fabrics from the uppermost part of the sequence

injection of mantle type helium into lake van turkey the clue for quantifying deep water renewal helium neon and tritium concentrations have been measured to study long term vertical mixing and deep water renewal in lake van eastern turkey the largest soda lake on earth helium excesses were found in the water column of lake van although neon concentrations were close to air saturation the excesses of both isotopes are strictly correlated and increase with depth in the bottom water 4he supersaturation is about 20 and the corresponding 3he concentration is 2 5 times the air saturation value the mean excess 3he 4he ratio of 1 1 2 10 5 is slightly higher than the morb ratio found in lake nemrut a neighbouring volcanic crater lake in which a large input of mantle helium was detected mantle helium accounts for the majority of the helium excesses in lake van although part of the 3he excess is attributed to the presence of tritium a one dimensional lake model was used which describes tritium input vertical mixing and gas exchange to reconstruct the evolution of the helium isotopes and the tritium as a conclusion the model based on the measured 3he and 4he profiles shows that 1 the vertical exchange of deep water occurs within 1 2 yr 2 the flux of mantle helium into the lake averaged over the total cross section of lake van is 0 23 0 35 cm3 stp m 2 yr 1 2 3 1011 atoms m 2 s 1 the present estimate of the global mantle helium flux averaged over the total surface area of the earth is 3 9 109 atoms m 2 s 1 the flux into lake van would account for at least 0 04 0 06 of this

the association of continental crust rocks with ophiolites in the northern apennines italy implications for the continent ocean transition in the western tethys abstract the late cretaceous sedimentary melanges from the external liguride units of the northern apennines include large slide blocks of ophiolites and lower and upper continental crust rocks representative of a continent ocean transition between the internal liguride oceanic domain and the thinned continental margin of the adria plate the slide blocks preserve a record of the long lived history of rifting which led to opening of the jurassic western tethys basin the external liguride ophiolites consist of 1 undepleted spinel peridoties partly re equilibrated under plagioclase facies conditions which were interpreted as unroofed subcontinental mantle 2 rare gabbroic rocks mainly troctolite to olivine bearing gabbro probably crystallised from n morb magmas and 3 basalts with n to t morb affinity covered by late callovian early oxfordian radiolarian cherts both gabbroic rocks and basalts locally intrude the mantle peridotites and postdate their re equilibration to plagioclase facies conditions the slide blocks of lower continental crust are composed of gabbro derived mafic granulites and felsic granulites the latter include quartzo feldspathic granulites and rare quartz poor to quartz free charnockitic rocks in both mafic and felsic granulites granulite facies re equilibration was followed by a retrograde metamorphic evolution to amphibolite greenschist and subgreenschist facies conditions retrogression is commonly accompanied by deformations progressively changing from plastic to brittle the upper crustal rocks occurring as slide blocks consist of hercynian granitoids with orogenic affinity mainly biotite bearing granodiorites and peraluminous two mica leucogranites locally the granitoids are intruded by basaltic dykes or capped by basaltic flows and radiolarian cherts the granitoids underwent polyphase brittle deformations under subgreenschist facies conditions which predated the basalt emplacement the tectono metamorphic evolution recorded by the slide blocks of the external liguride units started in the late carboniferous early permian about 290 ma with the emplacement at deep crustal levels of the gabbroic protoliths for the mafic granulites the associated felsic granulites likely represent the remnants of the lower continental crust intruded by the gabbro derived granulites mafic and felsic granulites subsequently underwent tectonic exhumation in permo triassic times as testified by the development of granulite to amphibolite facies ductile shear zones the granulites were finally exhumed to shallow levels probably in association with the subcontinental mantle in late triassic middle jurassic times the latter period was most likely characterized by extensive brittle faulting at shallow crustal levels thus giving rise to extensional allochthons formed by stretched slices of granitoids the western tethys opening is finally testified by the basalt intrusion and effusion in the late jurassic followed by deep sea pelagic sedimentation the external liguride crustal stratigraphy can be regarded as a fossil example of the transitional realm at the continent ocean boundary this reconstruction fits well with the available data on the present day continental margins derived from passive lithosphere stretching

5 late variscan mineralizing systems related to orogenic processes the french massif central abstract the french massif central constitutes an exceptional study area due to the diversity of its metallic deposits its internal position in the variscan belt and the abundance of available geological geophysical and metallogenic data obtained within the geofrance 3d programme the deposits formed towards the end of the orogenic evolution represent the economic products of two distinct mineralizing systems a au sb hydrothermal system and a w sn and rare metals magmatic hydrothermal system which were simultaneously active during a short time span between ca 310 and 300 ma two types of gold deposit can be distinguished on the basis of their depth of emplacement deep seated gold deposits developed under lithostatic to hydrostatic pressure during rapid exhumation and shallow gold deposits emplaced under hydrostatic pressure with no significant uplift deposits of w sn and rare metals were emplaced in the upper crust during final crystallization of specialized magmas after their rapid ascent perhaps enhanced by simultaneous regional uplift the gold bearing systems are associated with a complex network of re activated crustal scale faults initially active during the period between 335 and 315 ma normal motion along the faults coeval with 335 to 315 ma granite migmatite domes played a major role in the 3d distribution of the hydrothermal plumbing system gold and related metals were carried within huge hydrothermal cells which reached ca 100 km by 10 km in area and 30 km in depth in contrast granites rich in magmatophile elements w sn rare metals generated smaller hydrothermal cells 10 km by 10 km in area and

helium isotope variations in turkey relationship to tectonics volcanism and recent seismic activities the distribution of helium isotope ratios in the various tectonic provinces of turkey is examined through a synthesis of previously published data and the results of a recent survey along the north anatolian fault zone nafz following the catastrophic 1999 earthquakes the r ra values where r sample 3he 4he and ra air 3he 4he cover a wide range from 0 05 to 7 87 and the mantle derived helium is clearly identified in most locations the mantle derived component is high 50 of total he in a regions of central and eastern anatolia both of which are associated with historically active volcanoes and b the seismically active west to central segment of nafz while the mantle contribution reaches a maximum at nemrut volcano in eastern anatolia a region of dominantly compressional tectonics and with moderate enthalpy geothermal fields it is relatively low 50 in the western anatolian extensional province even though this region has the highest geothermal potential the average 3he enthalpy ratios estimated for the different provinces suggest lithospheric stretching and rise of the geotherm as the major mechanism of high heat flow yet limited intrusive activity in western anatolia is suggested as the reason for the comparatively low he additions and consequently low 1 7 106 atoms j 3he enthalpy ratios a more extensive magmatic activity appears to be responsible for the greater input of both heat and helium in eastern and central anatolia with intra province 3he enthalpy variations from 3 1 106 atoms j in eastern to 0 25 106 atoms j in central anatolia reflecting the ageing of hydro magmatic systems the relatively high 3he concentrations in low enthalpy waters of northern anatolia 2 8 106 atoms j are particularly significant since there is no evidence of volcanic activity associated with the strike slip motion along the seismically active segment of nafz continuous monitoring of he isotope compositions along nafz should lead to a better understanding of this apparent anomaly as well as the relationship if any between he isotope variations and seismic activities in this region

the velay dome french massif central melt generation and granite emplacement during orogenic evolution this paper is a synthesis of available data on the velay dome that include both small and large scale lithologic and structural mapping strain analysis isotope geochemistry geochronology and pressure temperature estimates the velay dome one of the largest granite migmatite domes of the variscan belt formed during orogenic collapse at around 300 ma its study allows an assessment of the thermal and geodynamic context leading to voluminous crustal anatexis of the variscan orogenic crust a first melting stage developed in connection with south verging thrust zones during the early carboniferous leading to a crustal thickening estimated at 20 km minimum the involvement of fertile lithologies and the intrusion of plutons of deep origin contributed to the development of water saturated melts the volume of biotite granite extracted from melt during this period was limited the second phase of melting corresponded to generalized melting of gneiss achieved by biotite dehydration melting reactions and accompanied by the generation of cordierite bearing granites at this stage crustal scale detachment faults were activeandpartiallyobliteratedtheearlierstructures thenewstructureswereprogressivelytiltedtotheverticalatthemarginofthe velay dome due to the southward and lateral ballooning of the granitic dome the reconstructed p t path indicate that the large volume ofmelt produced was a consequence ofa significant increase in temperature at the onset of biotite dehydration melting at the base of the crust this melting event is coeval with granulite facies metamorphism associated to underplating of mantle derived magmas as suggested by the geochemical signature of late paleozoic lower crustal xenoliths sampled by cenozoic volcanoes and withtheisotopicsignatureofthelategraniticintrusions accordingly itisproposedthatasthenosphericupwellingwasresponsible for the temperature increase favoring melting of hydrous minerals d 2001 published by elsevier science b v

helium carbon relationships in geothermal fluids of western anatolia turkey abstract we investigate the helium carbon and oxygen hydrogen isotopic systematics and co2 3he ratios of 8 water and 6 gas samples collected from 12 geothermal fields in western anatolia turkey 3he 4he ratios of the samples r normalized to the atmospheric 3he 4he ratio ra 1 39 10 6 range from 0 27 to 1 67 and are significantly higher than the crustal production value of 0 05 fluids with relatively high r ra values are generally found in areas of significant heat potential kizildere and tuzla fields co2 3he ratios of the samples ranging from 1 6 109 to 2 3 1014 display significant variation and are mostly higher than values typical of an upper mantle source 2 109 the 13c co2 and 13c ch4 values of all fluids vary from 8 04 to 0 35 and 25 80 to 23 92 vs pdb respectively stable isotope values 18o d of the geothermal waters are conformable with the mediterranean meteoric water line and indicate a meteoric origin the temperatures calculated by gas geothermometry are significantly higher than estimates from chemical geothermometers implying that either equilibrium has not been attained for the isotope exchange reaction or that isotopic equilibration was disturbed due to gas additions en route to the surface evaluation of he co2 abundances indicates that hydrothermal degassing and calcite precipitation controlled probably by adiabatic cooling due to degassing significantly fractionate the elemental ratio co2 3he in geothermal waters such processes do not affect gas phase samples to anywhere near the same extent for the gas samples mixing between mantle and various crustal sources appears to be the main control on the observed he c systematics however crustal inputs dominate the co2 inventory considering that limestone is the main source of carbon 70 to 97 of the total carbon inventory the carbon flux from the crust is found to be at least 20 times that from the mantle as to the he inventory the mantle derived component is found to vary up to 21 of the total he content and is probably transferred to the crust by fluids degassed from deep mantle melts generated in association with the elevated geotherm and adiabatic melting accompanying current extension the range of 3he enthalpy ratios 0 000032 to 0 19 10 12 cm3 stp j of fluids in western anatolia is consistent with the release of both helium and heat from contemporary additions of mantle derived magmas to the crust the deep faults appear to have facilitated the deep circulation of the fluids and the transport of mantle volatiles and heat to the surface

diachronous variscan late orogenic collapse as a response to multiple detachments a view from the internides in france to the foreland in the irish sea