the tectonothermal evolution of the cadomian belt of northern brittany france a neoproterozoic volcanic arc abstract the neoproterozoic cadomian belt northern brittany france results from the accretion of crustal blocks that record distinctive p t deformation paths in addition abundant gabbro dioritic to granitic intrusions during and after deformation influence the thermal pattern and are used as time markers as a whole the deformation history of the cadomian belt is characterised by a migration in time of the active zones which become younger towards the southeast relics of eocadomian rocks reveal that arc development took place at about 660 640 ma abundant calc alkaline magmatism intruding strongly deformed palaeoproterozoic orthogneisses characterise the northernmost unit of the cadomian belt i e the tregor unit which cooled before 600 ma the lanvollon basin a short lived intra arc rift containing abundant mafic and felsic volcanics followed by greywackes opened at about 600 580 ma the subsequent tectonothermal history of the cadomian belt is divided into two major episodes the first episode is related to the shortening of the intra arc rift and its southwestward thrusting over the guingamp unit this led to crustal thickening inducing partial melting in the guingamp unit exhumation of the latter rocks is accommodated by late extensional shearing along the boundary between the saint brieuc and guingamp units cooling of the saint brieuc unit is dated at about 570 560 ma the second episode is dominated by sinistral wrenching mainly localised in the eastern part of the cadomian belt at the boundary between the domnonean i e the arc domain and mancellian possibly a fore arc basin domains several shear zones developed during this episode some of them being associated with minor granitic intrusions dated around 550 ma the sinistral shear located southeast of the arc domain is reworking or crosscutting earlier high temperature fabrics within the saint brieuc unit in addition the vertical component of this shearing episode contributes significantly to the exhumation of the saint malo migmatites near the proterozoic cambrian boundary cordierite biotite granitoids intruded at shallow depths the fougeres unit their emplacement outlasting deformation of the sedimentary sequences minor stocks of muscovite leucogranites of cambrian age are possible equivalents of the abundant volcanism in an adjacent nw se trending rift this marks a major change in the kinematic framework with predominance of extension finally minor reactivation of the cadomian structures occurred during the carboniferous associated with the variscan orogeny

cadomian terranes wrench faulting and thrusting in the central europe variscides geophysical and geological evidence sixty five per cent of the paleozoic basement of western and central europe is hidden by a sedimen tary cover and or sea this work aims to remove that blanket to detect new structures which could used to build a more comprehensive model of the variscan or ogeny it is based on the interpretation of various forms of data a published gravity maps corrected for the ef fects of the crust mantle boundary topography and light sedimentary basins b aeromagnetic maps c measurements of densities and d induced and rema nent magnetizations on rocks from paleozoic outcrops of the upper rhenish area from the northern b ohem ian massif to the eastern paris basin the saxothurin gian is characterized by a 500 km long belt of gravity highs the most important being the kraichgau high most of the corresponding heavy bodies are buried un der a post early visdan cover they are interpreted as relics of late proterozoic terranes overlain by an early to middle paleozoic sequence equivalent to the bo hemian t errane in the bohemian massif the most probable continuation of these dense bohemian ter ranes toward the west is the southern channel north ern brittany cadomian terrane the gravity lows are correlated with variscan granites and pre and early variscan metagranites gravity and magnetic maps demonstrate large scale displacement in devonian early carboniferous times along the parallel and equidistant nw se striking vistula elbe bavarian bray and south armorican dextral wrench faults in the vosges schwarzwald and central massif the faults continue with the east west striking lalaye lubine ba den baden and marche faults and with south vergent thrusts the bavarian faults shift the kraichgau terrane by 150 km relative to the bohemian terrane whereas the offset of the north ern brittany cadomian relative to the northern vosges kraichgau terranes is estimated at 400km along the bray fault sinistral wrench faults are the ne sw striking sillon houiller rheingraben rodl vitis and diendorf faults the southern vosges schwarzwald devonian dinanti an basin is interpreted as a pull apart basin at the south easterly extremity of the bray fault the bohemian and kraichgau body form allochthonous terranes which were thrust over the saxothuringian crust thrusting to the north west was accompanied by back thrusting and led to the forma tion of pop up structures contemporaneous dextral and sinistral wrench faulting resulted in transpressive strain during collision the zonal structure of the varis cides in the sense of kossmat 1927 is relevant only to the rhenohercynian foreland belt kossmat 1927 al ready spoke of a moldanubian region because it dis plays no real zonal structure the saxothuringian zone was formed by terrane accretion their apparent zonal structure is not a pre collisional feature but only the result of accretion and collision

transition from island arc to passive setting on the continental margin of gondwana u pb zircon dating of neoproterozoic metaconglomerates from the se margin of the tepla barrandian unit bohemian massif abstract sedimentological petrological and geochemical data combined with shrimp and la icp ms u pb ages of pebbles of granitic rocks and detrital zircons from neoproterozoic vletice metaconglomerates of the se margin of the tepla barrandian unit were used to characterize the regional geotectonic setting at the proterozoic paleozoic boundary the metaconglomerates form several metres thick intercalations in metasediments of the svrchnice formation roof pendants of central bohemian plutonic complex which is considered to have been deposited in neoproterozoic cambrian times a sedimentological study indicates that the metaconglomerates are products of debris flows deposited in submarine canyons or proximal parts of submarine fans geochemistry of metaconglomerate matrix and petrography of pebbles revealed an obvious difference from the western and central part of the tepla barrandian unit proterozoic where pebbles eroded from uplifted parts of the basin and island arc sources are most common the abundance of quartz quartzite granite and crystalline limestone in pebbles together with the more mature composition of host metagreywackes indicate the presence of relatively evolved continental crust in the source area for the vletice metaconglomerates and shallowing of the neoproterozoic basin to the se in the present geographic co ordinate system the neoproterozoic to lowermost cambrian age of the svrchnice formation suggested by its comparison with similar rocks in the central part of the tepla barrandian unit was confirmed by u pb dating ages of detrital zircons from the metaconglomerate matrix cluster around two values archaean to paleoproterozoic u pb ages between ca 2580 and 1780 ma and neoproterozoic to early cambrian ages between 660 and 520 ma with a significant peaks at 600 and 544 ma suggesting local volcanic activity extending shortly beyond the proterozoic paleozoic boundary zircons separated from two trondhjemite pebbles from the metaconglomerate yielded a mean u pb age of 610 17 ma arguing for active arc magmatism during the latest neoproterozoic the overall age distribution of detrital zircons and the absence of grenvillian zircon ages favour a location of the tepla barrandian unit close to the west african craton at the neoproterozoic paleozoic boundary the rapid onset of a transtensional geotectonic regime and maturing of sediments is evidenced by the overlying cambrian shallow water clastics alternating with predominantly felsic calc alkaline to alkaline volcanics during the cambrian oblique plate convergence the active continental margin was transformed into belts of horsts and narrow pull apart or small rift basins after the ordovician break off of armorican terrane assemblage blocks from gondwana the extensional regime in the tepla barrandian unit continued until the beginning of variscan orogeny in the middle devonian

sm nd isotope characteristics of late cadomian granite magmatism in northern france and the channel islands sm nd isotopic studies of granites within the late precambrian cadomian orogenic belt of the north armorican massif northwestern france and channel islands reveal differences between arc related granite magmatism in outboard terranes and intracrustal granite magmatism in inboard terranes late cadomian c 570 ma arc related granitoids exhibit a range of e nd 2 to 6 and nd model ages t dm 1 0 1 3 ga reflecting variable contamination between late precambrian mantle derived magmas and ancient c 2 0 ga continental crust the contamination did not involve exposed granitic icartian basement to anygreat degree a more likely contaminant being unexposed lower crust of intermediate to acidic granulitic composition or early cadomian plutons which were themselves contaminated by lower crust voluminous granites of the mancellian region c 550 540 ma share common isotopic characteristics e nd 4 to 7 t dm 1 5 1 7 ga with migmatites and anatectic granites produced by partial melting of metasedimentary sequences within the st malo region consistent with a common source

the cadomian orogeny in the north armorican massif a brief review the north armorican massif nam of nw france is the type locality of the late precambrian cadomian orogenic belt the nam largely escaped reworking during the upper palaeozoic variscan cycle with the result that earlier geological relationships are commonly preserved intact a large part of the cadomian belt of the nam comprises variably deformed and metamorphosed supracrustal rocks of the brioverian succession barrois 1895 1896 1908 graindor 1957 cogne 1962 le corre 1977 rabu et al 1982 1983 chantraine et al 1982 the cadomian orogeny which takes its name from cadomus caen in normandy was defined by bertrand 1921 as the late precambrian orogeny which in the nam resulted in the folding and uplift of the brioverian succession prior to the deposition of lower palaeozoic red bed sequences events which have been ascribed to the cadomian orogeny span the period c 700 425 ma during which time the brioverian was heterogeneously deformed and metamorphosed and intruded by calc alkaline igneous complexes the evolution of the belt has been interpreted in terms of the gradual cratonization of calc alkaline arc complexes and syn orogenic sediments in an andean type continental margin brown et al 1990 dupret et al 1990 this contribution reviews briefly the geology of the cadomian belt of the nam fig 1 and outlines some of the more controversial debates concerning the distribution and status of precadomian basement the stratigraphy of the brioverian succession the nature of cadomian magmatism the kinematic significance of major structures and the geotectonic setting of the component parts of the belt

emplacement of lobate sheet flows with hyaloclastites onto soft sediment the erquy neoproterozoic lava pile armorican massif france abstract the neoproterozoic volcano sedimentary succession of the heussaye headland erquy belongs to the cadomian block located in the north east of the armorican massif france the volcanism dated at approximately 608 7 ma zircon u pb data occurred in a back arc basin this site exhibits a noteworthy verticalized pile of moderately metamorphosed greenschist facies basaltic andesite flows of arc back arc tholeiite affinity alternating with metasedimentary layers sandstones and siltstones a large range of volcanic facies characteristic of an underwater soft substrate environment can be continuously studied for over than 200 m parallel to the flow edges they include pillow lavas lava lobes fluidal peperites and sediment matrix hyaloclastites the presence of parallel lava lobes in a sector where one lava flow is capped by a variably thick hyaloclastite carapace together with the occurrence of asymmetric turbiditic folds in interbedded metasedimentary layers suggests the existence of an easterly facing gentle paleoslope the sudden thinning of this hyaloclastite carapace in two spots located approximately 60 m from each other is attributed to the synvolcanic activity of two conjugate normal paleofaults this hypothesis is supported by the observation of a hyaloclastite accumulation in one of the two spots close to a strike slip fault which may have been extensional at the time of the volcanism water entered the lava flow along the faults resulting in an interstitial network of hyaloclastites by quench fragmentation in the upper part of the hanging wall block all these observations are consistent with a model of lava emplacement along a subaqueous slope resulting from synmagmatic extensional faulting in a marginal basin context

brioverian volcanism and cadomian tectonics baie de st brieuc brittany stages in the evolution of a late precambrian ensialic basin

short paper cadomian terrane tectonics and magmatism in the armorican massif the north armorican composite terrane nw france is a collage of displaced terranes which result from the amalgamation of cadomian continental arcs and marginal basin complexes by sinistral transpression along a continental margin above a subduction zone early cadomian arc activity occurred at c 700 650 ma but terrane accretion did not occur until c 540 ma and post tectonic magmatism persisted well into the palaeozoic cadomian events thus span a considerably greater period than previously supposed

tectonothermal chronology of early cadomian arc development in guernsey and sark channel islands the cadomian orogen exposed in nw france and the channel islands comprises a collage of variably displaced terranes which record the late proterozoic evolution of a continental margin within a complex subduction zone on the islands of guernsey and sark calc alkaline quartz diorite stocks were emplaced during early stages of the cadomian orogeny zircons from two stocks of deformed quartz diorite display evidence of inheritance in larger and more magnetic fractions upper intercept ages for these discordant concentrates are c 1300 ma sm nd whole rock model ages t dm suggest older ages of c 2000 ma conversely the smallest and least magnetic zircons exhibit a distinct lead loss pattern suggesting that the 207 pb 206 pb dates of c 700 ma correspond to the time of crystallization of the stocks and provide a lower age constraint for initiation of subduction related magmatism in the cadomian orogen hornblende concentrates from deformed quartz diorite stocks exposed on guernsey and sark display discordant 40 ar 39 ar age spectra five samples record well defined isotope correlation ages ranging between 596 ma and 606 ma a hornblende concentrate from host orthogneiss sark yields a plateau age of 608 ma the hornblende ages are interpreted as dating post metamorphic cooling following late proterozoic deformation and metamorphism within the arc system both magmatic and cooling ages from this early cadomian arc contrast with those of younger initial tectonothermal activity c 570 ma and c 540 ma in other cadomian terrane elements and suggest palinspastic separation until very late stages of the orogeny the polyphase nature of this late proterozoic activity is similar to that of related orogens which probably occupied similar tectonic settings along the margin of gondwana e g sw iberia west africa

the cadomian granites of mancellia northeast armorican massif of france relationship to the st malo migmatite belt petrogenesis and tectonic setting abstract the north armorican shear zone nasz divides the cadomian belt of the armorican massif of france into two major terranes the north armorican composite terrane nact and the central armorican terrane cat the nact is a pastiche of displaced blocks and terranes st brieuc terrane st malo terrane and mancellian terrane which results from the amalgamation of cadomian continental arc and basin complexes by sinistral transpression along a continental margin above a southerly dipping subduction zone in the nact the neoproterozoic brioverian succession was deformed and metamorphosed during the cadomian orogeny and intruded by syn to post tectonic plutonic complexes the st malo migmatite belt is a syn tectonic suite of anatectic migmatites derived through partial melting of the brioverian succession using the t test on whole rock geochemical data there is no evidence to suggest that a sample of st malo diatexites comes from a population with a different mean than some samples of the brioverian succession sandstones resulting homogeneous diatexites anatectic granites of the core were emplaced syn kinematically into cadomian strike slip shear zones thus the age of anatexis of 540 ma is also the date of strike slip shearing and provides a minimum age for the sinistral transpression which amalgamated the terranes of the nact the mancellian granites to the southeast of the st malo migmatites were emplaced at 540 ma and are overlain unconformably by cambrian sediments overall geochemistry and a variety of specific geochemical parameters show that the mancellian granites are similar to the diatexites and anatectic granites of the st malo migmatite belt using the t test on whole rock geochemical data there is no evidence to suggest that the sample of mancellian granites comes from a population with a different mean than the sample of st malo diatexites additionally both suites of rocks are peraluminous they have similar distributions of rb sr and ba they have k rb ratios around 225 and they plot largely within the vag field using trace element discrimination diagrams for the interpretation of tectonic setting samples of the brioverian succession sandstones the st malo migmatites and the mancellian granites all exhibit similar primordial mantle normalized element patterns preliminary results of nd and sr isotope studies reveal a similar restricted range of values for nd between 4 0 and 7 3 and sr between 3 and 42 and depleted model ages in the range 1 7 to 1 5 ga tdmur for both mancellian granites and st malo migmatites isotopic features which set these rock types apart from other cadomian granites within the nact the st malo migmatite belt and the mancellian granites are coeval at 540 ma and both developed by anatexis at intermediate crustal depths of the brioverian succession within an inverted but only moderately overthickened behindarc basin in comparison with the st malo migmatites the mancellian granites represent farther travelled products of intracrustal melting but their geochemistry still preserves the characteristic signature of the source

geochemical study of the brioverian late proterozoic volcanic rocks in the northern armorican massif france implications for geodynamic evolution during the cadomian

evidence for multiple high t metamorphism and regional migmatization within the cadomian belt of northern brittany france the cadomian orogeny is characterized by two major tec tonometamorphic events one that pre dates 570 ma and another at c 540 ma until recently the majority of migmatites occurring within the cadomian belt of northern brittany were interpreted as having been formed during the second event the discovery of the jouguet migmatites which have been hornfelsed by the st brieuc diorite emplaced at 533 l2 ma suggests that a migmatitic event occurred before 540 ma these data demand a reinterpretation of the structural sketch of the cadomian orogeny

geochemistry and geotectonic interpretation of the penthievre crystalline massif northern brittany france abstract in the armorican massif of brittany and normandy northern france and the channel islands u k an episode of crust formation spanning the period from 700 ma to the early palaeozoic can be recognized and is attributed to the cadomian orogenic cycle during the early part of this orogenic cycle the components of the penthievre crystalline massif were emplaced to produce the basement upon which local brioverian volcanism and sedimentation took place three separate episodes of crustal accretion have been recognised in the penthievre crystalline massif the first is represented by the yffiniac group which is dominantly of basic to intermediate composition with a minor acidic component this suite which was metamorphosed at amphibolite facies shows geochemical characteristics of island arc tholeiitic to continental calc alkaline igneous activity ree profiles are flat to lree enriched cen ybn in the range 1 18 4 89 incompatible element spidergrams show relative enrichment of for example th u lree and lil elements and depletion of ta nb and ti ta la and nb la ratios are low the second suite is represented by the port morvan group a linear belt of leucocratic orthogneisses containing net veined rafts of deformed yffiniac group rocks the port morvan group gneisses are trondhjemitic to quartz dioritic in composition with na2o k2o up to 32 9 the third assemblage comprises the jospinet group and the fort de la latte complex these are part of a compositionally expanded mantle derived plutonic suite ranging from ultrabasic to aplitic in composition flat chondritic ree profiles characterise gabbros whereas lree enriched profiles characterise calc alkaline and alkali calcic granitoid plutons there is evidence in penthievre for protracted destructive plate margin activity and associated plutonism of andean type the penthievre crystalline massif probably represents the roots of juxtaposed island arc and continental margin terranes produced in three crust forming episodes during the cadomian orogenic cycle brioverian sedimentation and volcanism took place in ensialic basins on rifted crust the closure of these brioverian basins was accompanied by the emplacement of calc alkaline plutons into the crystalline basement

precadomian relicts in the armorican massif their age and role in the evolution of the western and central european cadomian hercynian belt abstract well dated precambrian is mostly developed in the north of the armorican massif in which area voluminous cadomian magmatism is dated at between 650 and 550 ma much older relicts occur at cap de la hague in guernsey and in the tregor and are also found in the northern continental margin of the iberian peninsula in all these occurrences whole rock systems have been opened so that the true ages cannot be determined by the rb sr whole rock isochron method four u pb zircon ages are between 1 8 and 2 ga in guernsey icart orthogneisses in tregor port beni trebeurden morguignen orthogneisses there is no evidence from strontium isotopes that these isolated and scattered relicts have a wide extension or that such ancient continental crust played an important role in magma genesis from 650 ma to 270 ma ago on the contrary the evolution of initial 87sr 86sr ratios with time shows that the observed mid and west european continental crust is probably not older than 700 ma the increase of the initial 87sr 86sr ratios of the magmas with time suggests that after its formation in cadomian times this segment of continental crust evolved virtually as a closed system and hercynian magmatism arose principally from re melting of relatively young sialic components

behaviour of rb sr whole rock and u pb zircon systems during partial melting as shown in migmatitic gneisses from the st malo massif ne brittany france the age of regional metamorphism and partial melting in the late proterozoic succession of the st malo area has been established at around 540 ma whole rock homogenization of the sr isotope systems has been achieved only in the anatectic granites whereas the associated metatexitic and diatexitic gneisses remain highly heterogeneous at lower grade than the sillimanite isograd metasedimentary gneisses contain detrital zircons which record the regional metamorphic event as a u pb lower intercept age 536 14 ma in the sillimanite zone u pb zircon lower intercept ages are around 500 10 ma and suggest a recent loss of radiogenic pb from the observed overgrowths zircons from the anatectic granites do not yield u pb intercept ages due to the presence of inherited radiogenic pb but the finer fractions define a discordant array whose upper intercept on concordia yields 555 13 ma an age significantly older than the partial melting event indicated by the lower grade zircon ages the lower intercept defined by the fine grained anatectic zircons yields a date in the future negative ordinate value in their regional context the st malo migmatites can be considered as resulting from late stage crustal melting in the cadomian orogenic belt following genesis of calc alkaline magma at an active margin around 600 ma ago an age of 540 ma for partial melting in the st malo migmatites places the end of the cadomian orogeny near the precambrian cambrian boundary

incipient strain partitioning in a slate belt evidence from the early variscan monts d arree slate belt brittany france abstract partitioning of strain is a fundamental process during mountain building it commonly causes a compartmentalisation of a bulk regional strain into deformational domains with contrasting strain characteristics and largely oriented parallel to the orogenic grain the monts d arree slate belt brittany france offers an opportunity to study strain partitioning in a slate belt deformed in an overall transpressional regime the slate belt consists of highly deformed low grade siliciclastic metasediments of upper silurian to lower devonian age the deformation occurred during an early variscan nappe stacking event bretonian phase an extensive structural analysis has demonstrated that the slate belt reflects the initial stages of strain partitioning the slate belt primarily reflects coaxial contraction dominated deformation it resulted in nw verging folding and a pervasive cleavage development giving rise to a pronounced mechanical anisotropy during the later stages of deformation incipient strain partitioning lead to the development of punctuated strain heterogeneities consistently reflecting dextral belt parallel strike slip strain these structures are not organised in networks or domains incipient strain partitioning in the monts d arree slate belt did not reach the stage of compartmentalisation by the formation of an interlinked discontinuity network or wrench dominated deformational domains

the structural evolution of the english channel area the structural evolution of the english channel area is controlled by structure and particularly by the pre existing cadomian and variscan crustal discontinuities which have been reactivated repeatedly in post variscan times they controlled the crustal subsidence that produced basin development in the mesozoic prior to the sea floor spreading in the north atlantic region they were then reactivated during the cenozoic compression and basin inversion the english channel development is ascribed to mid tertiary differential uplift oligocene to miocene during late tertiary to quaternary times the channel displays characteristics of a tectonically controlled fluvial basin periodically invaded by the sea at the lithospheric scale the channel can be considered as an active intraplate area influenced by the nw se alpine push the nw se atlantic ridge push and glacial rebound stresses copyright 2003 john wiley sons ltd

u pb geochronological constraints on the tectonothermal evolution of the paleoproterozoic basement of cadomia la hague nw france abstract the la hague region of nw france provides some of the best exposures of paleoproterozoic basement in the northernmost segment of the cadomian orogen one of several neoproterozoic arc fragments in the circum atlantic region the basement rocks which include orthogneiss metasediments and metabsites contain a comparable series of structures with a common metamorphic history thus the basement units are regarded to have undergone the same broad tectonothermal evolution the metasedimentary succession includes migmatites that preserve relict kyanite overprinted by sillimanite associated with the development of garnet bearing leucosome biotite dehydration at a metamorphic peak of 700 800 c and 8 9 kbar is considered responsible for the anatexis evident in both the metasediments and orthogneiss units a u pb zircon crystallization age of 2061 2 7 ma for the culeron orthogneiss a member of the metamorphic suite confirms the paleoproterozoic protolith age idioblastic rims to 2 06 ga zircons from the culeron orthogneiss yield shrimp u pb dates of c 618 ma providing the first geochronological constraints on the likely age of high grade metamorphism and anatexis of the paleoproterozoic basement in cadomia foliated c 615 610 ma intrusions that intrude the metamorphic units record magmatic to solid state fabrics that parallel metamorphic fabrics within the basement consistent with deformation and metamorphism being late neoproterozoic in age

late precambrian thrust and wrench zones in northern brittany france abstract brioverian units of northern brittany armorican massif france were deformed and metamorphosed during a late precambrian tectonic episode the cadomian orogeny kinematic study strain and displacement in the st cast st brieuc area shows that this tectonic episode was characterized by the obduction of a back arc basin over a continental margin around 580 590 ma ago strain pattern analysis reveals a constant interaction between ssw directed imbricated thrust zones and ductile sinistral wrench zones trending n 50 e the arcuate pattern of cadomian structures in this area results from such an interaction thickening as a result of the thrusts induced migmatization and the development of a high temperature belt around 540 ma in the st malo guingamp area

gravity modellings of the cadomian active margin of northern brittany abstract the geometrical relationships existing between the saint brieuc saint malo and guingamp units which form the cadomian active margin of northern brittany have been determined by gravity analyses and 2 5d modelling correlated with other geophysical magnetic seismic etc and geological observations automatic structural analyses were performed before modelling to identify lateral gravity discontinuities averaged density measurements were used for 2 5d gravity modelling along six profiles across the arcuate geological and gravity shape of the cadomian basement this study allowed the third dimension to be obtained through correlating each profile with its neighbours we propose that orthogneiss formations equivalent to the port morvan orthogneiss constitute the saint brieuc upper crustal unit below the lanvollon formation 2 5d gravity modelling outlines downward geological differences in the western and eastern parts of the ploufragan complex as previously indicated by surface mapping these results stress the necessity for distinguishing between the guingamp and saint malo units

the conductivity structure across the trans european suture zone from magnetotelluric and magnetovariational data modeling abstract to achieve an electrical conductivity model of the trans european suture zone tesz magnetotelluric measurements in the period range of 10 20 000 s along two almost parallel profiles with a total of 65 sites starting from the north german polish basin crossing the tesz and ending on the east european craton were conducted magnetovariational responses were combined with other magnetotelluric transfer functions because for large periods they are free of near surface galvanic distortions and thus lead to more reliable resolution of deep structures following the results of strike and dimensionality analysis input data for two dimensional inversion were created by rotating all transfer functions to the common strike direction models derived from different combinations of data were compared and several sensitivity analyses were carried out the results show a highly conductive layer of sediments saline aquifer and the tesz as a broad region of enhanced conductivity it was also clarified that the lower crust and upper mantle of the east european craton eec are more resistive than the paleozoic platform pp underlying the northeast german basin and this resistivity extends to the greater depths

analytical solutions for analysing pumping tests in a sub vertical and anisotropic fault zone draining shallow aquifers summary we present new analytical solutions for examining the influence during a pumping test in a well of an infinite linear and anisotropic strip aquifer that drains shallow aquifers of different diffusivity and thickness the whole system is confined and the aquifer geometry can be represented by a t an aquifer geometry resembling a sub vertical fault or a sub vertical vein cross cutting shallower aquifers the proposed solutions are based upon an unconventional application of well image theory without limitation of the diffusivity contrast between the three domains solutions for drawdown were developed for the three domains i e the strip aquifer and the two shallow compartments and flow signatures are discussed in detail and compared to numerical modelling the proposed solutions are not shown to be exact solutions to the appropriate partial differential equation but very good and useful approximations the solutions were applied to a 63 day pumping test in a steep fault zone in crystalline aquifer rock of brittany france after that the flow contributions of the fault zone and of the shallow aquifers deduced from groundwater dating were compared to analytical solutions the solutions and theoretical type curve examples can help in understanding flow processes from tests conducted in settings that are similar to such a conceptual model

neoproterozoic shear zone tectonics within the icartian basement of guernsey and sark channel islands the channel islands of guernsey and sark are amongst the few localities within the neoproterozoic cadomian orogenic belt where palaeoproterozoic basement is exposed basement units collectively referred to as icartian comprise orthogneisses e g the c 2000 ma icart granite gneiss metasediments and amphibolites on guernsey the protolith to the icart granite gneiss was intruded into metasediments already deformed during a d 1 deformation event both were variably deformed during a d 2 event within an approximately north south trending steeply dipping oblique dextral shear zone on sark metasediments and amphibolites carry a d 1 fabric not necessarily correlatable with that on guernsey which was deformed during d 2 to form recumbent gently inclined folds and tectonic fabrics which locally carry top to the south kinematic indicators early cadomian quartz diorites which intruded the icartian basement carry variably developed magmatic and moderate temperature 400 550 c ls solid state fabrics which are similarly oriented to d 2 basement structures this indicates that the quartz diorites were emplaced and deformed during a protracted d 2 event published 40 ar 39 ar amphibole cooling ages of c 600 ma from guernsey and sark quartz diorites and a host icartian amphibolite are consistent with reworking of basement units during the cadomian orogeny the contrasting kinematic pattern during cadomian deformation implies that a flat lying shear zone on sark may have accommodated oblique dextral strike slip movement on guernsey with an overall southerly direction of tectonic transport deformation most likely occurred in the hanging wall of a major south dipping neoproterozoic subduction zone

chronology of cadomian tectonothermal activity in the baie de saint brieuc north brittany france evidence from 40ar 39ar mineral ages the late precambrian cadomian orogen exposed in the north armorican massif northwest france is a collage of displaced terranes that in part developed during amalgamation of continental arc and marginal basin complexes 40ar 39ar mineral ages reported here place new constraints on the timing of cadomian tectonothermal activity in the southern part of the st brieuc terrane in the baie de saint brieuc area brioverian supracrustal units were deformed metamorphosed and intruded by calc alkaline plutonic complexes metamorphic hornblende from a metabasic amphibolite sheet within brioverian rocks records an isotope correlation age of 568 4 2 6 ma interpreted to date postmetamorphic cooling through appropriate argon closure temperatures similar isotope correlation cooling ages are recorded by metamorphic hornblende within both an amphibolite sheet intrusive into the penthievre complex 567 5 1 2 ma and the la croix gibat amphibolite 574 8 2 1 ma igneous hornblende from the late tectonic to pos

provenance of quartz rich metamorphic tectonite pebbles from the black flysch w pyrenees n spain an ebsd and detrital zircon la icp ms study abstract metasandstone and quartz tectonite conglomerate pebbles from the albian black flysch encapsulate evidence of pre mesozoic igneous and metamorphic events quartz deformation microstructures and crystallographic fabrics characterize plastic flow under sub greenschists 250 300 c to high grade metamorphic conditions above 550 600 c detrital zircon grains from the pebbles provide concordant 206pb 238u radiometric ages ranging from 473 26 ma to 2261 59 ma with four major populations clustered in the intervals 1 96 1 80 ga paleoproterozoic 1 11 0 93 ga stenian tonian 765 571 ma neoproterozoic and 500 475 ma cambro ordovician the youngest zircons and the probability distribution of the studied population permit us to establish an early ordovician maximum depositional age for tectonite pebble protoliths and relate them with the armorican quartzite detrital zircon signature we identify the currently submerged landes massif of the armorican platform as the most likely candidate to being the source of black flysch metamorphic conglomerate pebbles

preferred mineral orientation of a chloritoid bearing slate in relation to its magnetic fabric 2014 elsevier ltd a regional analysis of the anisotropy of the magnetic susceptibility on low grade metamorphic chloritoid bearing slates of the paleozoic in central armorica brittany france revealed very high values for the degree of anisotropy up to 1 43 nonetheless high field torque magnetometry indicates that the magnetic fabric is dominantly paramagnetic chloritoid s intrinsic degree of anisotropy of 1 47 0 06 suggests that chloritoid bearing slates can have a high degree of anisotropy without the need of invoking a significant contribution of strongly anisotropic ferromagnetic s l minerals to validate this assumption we performed a texture analysis on a representative sample of the chloritoid bearing slates using hard x ray synchrotron diffraction the preferred orientation patterns of both muscovite and chloritoid are extremely strong 38 6m r d for muscovite 20 9m r d for chloritoid and display roughly axial symmetry about the minimum magnetic susceptibility axis indeed suggesting that chloritoid may have a profound impact on the magnetic fabric of chloritoid bearing rocks however modeling the anisotropy of magnetic susceptibility by averaging single crystal properties indicates that the cpo of chloritoid only partially explains the slate s anisotropy

inherited arc signature in ediacaran and early cambrian basins of the ossa morena zone iberian massif portugal paleogeographic link with european and north african cadomian correlatives abstract geochemical data from clastic rocks of the ossa morena zone iberian massif show that the main source for the ediacaran and the early cambrian sediments was a recycled cadomian magmatic arc along the northern gondwana margin the geodynamic scenario for this segment of the avalonian cadomian active margin is considered in terms of three main stages 1 the 570 540 ma evolution of an active continental margin evolving oblique collision with accretion of oceanic crust a continental magmatic arc and the development of related marginal basins 2 the ediacaran early cambrian transition 540 520 ma coeval with important orogenic magmatism and the formation of transtensional basins with detritus derived from remnants of the magmatic arc and 3 gondwana fragmentation with the formation of early cambrian 520 510 ma shallow water platforms in transtensional grabens accompanied by rift related magmatism these processes are comparable to similar cadomian successions in other regions of gondwanan europe and northwest africa ediacaran and early cambrian basins preserved in the ossa morena zone portugal and spain the north armorican cadomian belt france the saxo thuringian zone germany the western meseta and the western high atlas morocco share a similar geotectonic evolution probably situated in the same paleogeographic west african peri gondwanan region of the avalonian cadomian active margin

helium trapping in apatite damage insights from u th sm he dating of different granitoid lithologies apatite u th sm he ahe thermochronometry is widely used to constrain thermal histories and rates of tectonic exhumation and erosion processes however data interpretation is often challenging especially when the thermal history includes extended residence time in the he partial retention zone heprz with highly dispersed dates revealing the complexity of diffusion processes in natural systems this study investigates chemical and physical factors that may have impacted he diffusion in apatite over long timescales in a context of protracted residence in the heprz nine samples from the ploumanac h pluton and north tregor armorican massif france were collected in granitoids differing in petrography and chemisty this area was chosen because these samples underwent a similar thermal history since 300 ma we report new u th sm he dates along with apatite fission track aft data as well as lithological and chemical characterization the results show dispersed u th sm he dates ranging from 87 7 to 291 23 ma whereas central aft dates vary from 142 6 to 199 9 ma current predictive models for he diffusion and fission track annealing in apatite could not reproduce the two datasets together however this apparent discrepancy gives insight into the parameters influencing he diffusion at geological timescales the data confirm that radiation damage enhances he trapping as the ahe dates are positively correlated to effective uranium eu concentration the he age dispersion for constant eu content cannot be explained just by variations in grain size or chemical composition to explore the potential influence of recoil damage trapping behavior and annealing kinetics on ahe dates we tested a new diffusion model from gerin et al 2017 given the expected model of the thermal history provided by aft inversion we investigated the influence of the trapping energy on ahe dates the ahe date variations can be explained only if the trapping energy evolves from one crystal to another increasing with the amount of damage for a given trapping energy minor variations in the recoil damage annealing rate can consistently explain most of the remaining dispersion of the ahe dates

comparison of 40ar 39ar conventional and laser dating of biotites from the north tregor batholith bulk samples and single grains of biotites from the north tregor batholite armorican massif france were analysed by the 40ar39ar method apparently pure single biotite grains analysed by the step heating procedure with the laser probe displayed plateau ages or slightly disturbed age spectra one strongly chloritized single biotite grain gave a very disturbed age spectrum with younger ages the bulk samples of biotites of the same rocks displayed disturbed age spectra characterized by younger low temperature ages and a hump at intermediate temperature steps these disturbances are explained by 39ar recoil from the biotite into the chlorite which is clearly evidenced by hrtem observations and not by excess 40ar the more concordant results obtained on single biotites are probably due to a more careful selection of the minerals in most cases we obtain good concordance of the ages 600 610 ma measured on the apparently pure single biotite grains and the high temperature ages of both the bulk samples of amphiboles and biotites from the same rocks

timing of cadomian deformation and magmatism within la hague nw france the la hague peninsula normandy nw france contains the most northerly components of the late neoproterozoic cadomian orogeny u pb dates for single grain and small multiple grain zircon separates from carefully selected plutons provide geochronological constraints on the timing of deformation and magmatism within the region the syntectonic jardeheu quartz diorite and omonville granodiorite were emplaced and contemporaneously deformed during the development of a greenschist facies fabric pervasive across the peninsula u pb zircon dates of 610 1 ma for the jardeheu quartz diorite and 608 2 ma for the omonville granodiorite are similar to a previously reported sensitive high resolution ion microprobe u pb date for metamorphism of the basement and indicate that a regionally significant period of metamorphism and magmatism occurred between 620 and 608 ma an emplacement age of 573 9 1 3 0 9 ma for the syntectonic thiebot granodiorite places limits on the timing of deformation along the thiebot shear zone zircons from the undeformed st martin monzonite give a precise u pb age of 581 3 1 1 ma the new dates point to reactivation of magmatism and localized greenschist facies deformation between 582 and 573 ma

late devonian early carboniferous contraction dominated deformation in central armorica monts d arree brittany france and its relationship with the closure of the rheic ocean abstract in the monts d arree western brittany france a high strain slate belt is well exposed the slate belt is located in the central armorica terrane a low grade middle to upper crustal domain in the armorican massif composed of a cadomian basement and its neoproterozoic and palaeozoic metasedimentary cover sequence the slate belt consists of highly deformed siliciclastic rocks of the pridolian to lochkovian plougastel formation a structural analysis has demonstrated that the slate belt primarily reflects a coaxial contraction dominated deformation history resulting from a top to the nw shearing on top of a weakly dipping decollement it resulted in nw verging folding and a pervasive cleavage development giving rise to a pronounced subvertical mechanical anisotropy only during the later stages of deformation history incipient strain partitioning on this anisotropy lead to the development of punctuated strain heterogeneities consistently reflecting dextral belt parallel strike slip strain the deformation largely occurred prior to the emplacement of the early carboniferous granitoid intrusions in central armorica and can therefore be correlated with the late devonian early carboniferous bretonian tectonometamorphic event the kinematics inferred in the monts d arree moreover are consistent with the top to the nw thrusting and nappe stacking inferred in the leon terrane situated to the northwest of the central armorican terrane these new insights allow linkage of the early variscan contraction dominated deformation in central armorica to the closure of the rheic ocean and the continental collision of the leon microcontinental block with armorica our study also reveals the importance of the bretonian tectonometamorphic event in the entire north western parts of central armorica the main cleavage forming deformation was diachronous and followed by subordinate wrench related deformation

the cadomian active margin north armorican massif france a segment of the north atlantic panafrican belt abstract within the european variscan belt the armorican massif northwestern france contains an isolated fragment of the north atlantic panafrican belt the cadomian block mostly unaffected by variscan deformation this block is composed of neoproterozoic rocks ranging in age from 620 to 540 ma brioverian locally overlying a composite basement of icartian 2000 ma or pentevrian 750 ma age the cadomian tectonics the last magmatic manifestations of which are dated at around 540 ma were sealed by a cover of early cambrian rocks in normandy and early ordovician rocks in brittany five units are distinguished in this cadomian block tregor saint brieuc guingamp saint malo and fougeres although the units are bound by major faults the overall coherence of their different protolithic characteristics suggests that they can be fitted into a simple geodynamic model the tregor unit represented by the north tregor batholith of crustal affinity and its icartian host rock probably reflects an early cadomian evolution the saint brieuc unit essentially made up of magmatic rocks of juvenile affinity includes two assemblages juxtaposed by the cadomian tectonics 1 a volcanosedimentary succession formed in a typical island arc setting reflecting subduction around 610 ma and 2 a volcanic plutonic complex emplaced around 590 ma in an extensional active continental margin of pentevrian age the guingamp unit predominantly migmatitic includes a tectonic slice of deep origin the extrusion of which accompanied the exhumation of the migmatites it may represent the trace of a possible suture the saint malo unit is composed of brioverian detrital rocks the significance of which is uncertain passive continental margin or accretionary prism it is structured around a late migmatitic dome 540 ma the fougeres unit corresponds to the outer domain of the cadomian orogeny it was only weakly deformed by cadomian tectonics and is intruded by the large mancellian batholith of crustal origin dated at 540 ma together the structural units are interpreted in the framework of an evolving active margin juxtaposing continental margin marginal basin and volcanic arc before being shortened during the cadomian convergent tectonics

estimating drilling direction for optimum production in a fractured reservoir production from many naturally fractured reservoirs is only marginally profitable thus determining the drilling direction for maximum production in such reservoirs is very important a simple analysis is presented for determining the optimum drilling direction in a naturally fractured reservoir two dimensional scanline analysis is used to find the maximum fracture interception rate then is generalized to find the maximum fracture flow interception rate finally the analysis is further generalized to the three dimensional case in this analysis fractures are presumed to occur in sets in fracture sets fractures are assumed to have a common orientation uniform closest spacing s and a representative flow aperture e which govern the capacity of each set to contribute to flow into the well bore the optimum drilling direction is controlled by the aperture spacing and orientation of fracture sets with respect to the well bore the optimum direction favors the direction of closest spacing for the fracture set having the largest ratio of flow aperture to spacing e3 s i e the fracture set that has the greatest capacity for conducting fluid to the well bore to introduce the method of analysis focus is given to finding the optimum direction to drill a horizontal well through two intersecting sets of vertical fractures a nomograph for this solution is provided the analysis is extended to a general case where any number of fracture sets may exist each with its own representative flow aperture and orientation a degenerate form of the general case pertains to sets of vertical fractures penetrated by an inclined hole this degenerate case can be reduced to a two dimensional form by projecting the well bore onto the horizontal plane the optimum drilling direction can then be found by the two dimensional solution outlined in this paper but the resulting fracture interception rate is reduced as a function of the well bore inclination

late proterozoic zircon ages from a basic ultrabasic complex a possible cadomian orogenic complex in the hercynian belt of western europe the basic ultrabasic complex of belle isie en terre northwestern part of the armorican massif france has been dated at 602 4 1 m y by the u pb zircon method the calc alkaline trend of the metabasic rocks and the geologic context suggest that this ophiolite like complex is related to a back arc orogenic setting similar to the geodynamic environment that has been proposed for pan african calc alkaline ophiolite like complexes of late proterozoic age around the west african craton our results are compatible with the closing in late proterozoic time of a channel ocean north of the basement and formed during the cadomian orogeny rather than with possible caledonian or hercynian subduction