slip system domains 2 kinematic aspects of fabric development in polycrystalline aggregates abstract in the first paper of the present series cobbold and gapais 1986 we develop theoretical models of plane strain domainal deformation by slip along inextensible and indestructible fibres for such extreme mechanical conditions we show that deformation is largely controlled by kinematic factors we use our models to explain domainal patterns of natural slip systems including bedding and foliation planes faults ductile shear zones and lattice planes the present paper focusses on kinematic aspects of fabric development in polycrystalline aggregates a three dimensional kinematic model is developed it reveals that common quartz fabrics may develop domainally by reorientation and selection of inextensible slip directions a axes or slip planes with initially random orientation stable orientations of slip directions are close to both surfaces of no finite extension and orientations of large amount of shear we explain 1 1 common patterns of quartz a and c axis fabrics e g small circles and crossed girdles 2 2 characteristic differences between fabric patterns according to the shape of the finite strain ellipsoid 3 3 development of asymmetric fabrics during non coaxial deformation histories this last process involves selective destruction or replacement of certain orientation domains so that a preferred orientation of slip lines develops close to the bulk shear direction rotation recrystallization is shown to be an efficient natural selection mechanism the models also account for many microstructural features found in oriented polycrystalline rocks especially 1 1 alternating band like or kink like domains of twinned preferred orientations 2 2 individual grains or clusters of grains with retort like shapes and extensive recrystallisation at boundaries 3 3 small scale shear bands formed by strain partitioning and shear localization

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

the role of hercynian granites in the deformation and metamorphism of brioverian and palaeozoic rocks of central brittany the late proterozoic rocks of central brittany represented essentially by upper brioverian sediments are considered to be a late to post tectonic molasse to the cadomian chain the poly orogenic character cadomian and hercynian of the deformation structures affecting these rocks is noted from the baie de douarnenez to the rennes region however the main syn metamorphic deformation event is clearly of hercynian age furthermore we show that this event and the associated metamorphism are directly related to the emplacement of the hercynian leucogranites the syntectonic nature of the leucogranites with respect to the deformation of their envelope is therefore affirmed and the concept of a brioverian basement in the mechanical sense of the term and contrasted with a palaeozoic cover must be abandoned

deep seismic imaging of the cadomian thrust wedge of northern brittany abstract the armor seismic profile ar1 ar2 crosscuts the major units of the cadomian domain 600 540 ma of northern brittany and extends the offshore swat 10 profile birps ecors program we thus have a combined 95 km long nnw sse section for describing the regional scale structure of the cadomian crust in the area following reprocessing and migration of the southern end of the swat 10 profile so that it could be compared with the ar1 ar2 armor profile a structural interpretation of the composite profile was carried out taking into account gravity modelling and magnetotelluric soundings the swat 10 armor profile reveals the existence of steeply dipping thrust faults in the upper crust representing a thrust wedge with a gently northwest dipping sole this cadomian thrust system continues below the unmetamorphosed brioverian and palaeozoic sediments of central brittany and beyond the southern end of the profile in the central armorican domain it supports previous interpretations of dominant oblique type thrusting during cadomian tectonics proposed on the basis of geological field evidence and confirms the existence of a major northwest dipping thrust fault that brings the volcanic arc formations of the saint brieuc unit onto the metasediments of the saint malo unit the profile also emphasises the regional extent of a thrust wedge at the scale of the whole cadomian domain of northern brittany

multiple crust reworking in the french armorican variscan belt implication for the genesis of uranium fertile leucogranites muscovite peraluminous granites mpgs form by partial melting of the continental crust and can be related to metalliferous deposits such as tin tungsten and uranium u metal enrichment in mpgs commonly results from fractional crystallization but the metal contents of the source play a major role for their fertility between ca 320 and 300 ma late carboniferous the french armorican variscan belt was intruded by numerous u fertile mpgs that contain inherited zircon grains with a wide range of ages from archean to carboniferous u pb and hf isotopic data of zircon grains from brioverian to carboniferous sediments cambrian to early carboniferous granitoids and late carboniferous mpgs indicate that the crust of the armorican massif is made up by detritus mainly derived from the west african craton 3500 1600 ma tdm 3 8 2 3 ga grenvillian belt 1200 900 ma tdm 2 7 1 2 ga and avalonian cadomian belt 800 550 ma tdm 2 5 0 8 ga and that the crust was affected by magmatic events at 510 470 ma tdm 1 6 0 6 ga 410 330 ma tdm 1 6 1 ga and 320 300 ma furthermore they reveal that the late carboniferous mpgs were mainly formed by partial melting of brioverian sediments with cambro ordovician and devonian carboniferous granitoids which are all genetically linked with each other and characterized by th u 4 the new data suggest that the u fertile mpgs result from multiple reworking of u rich brioverian sediments deposited ca 550 ma ago on the northern margin of gondwana and partially molten during several paleozoic events causing a successive increase in u content in the middle upper crust

u pb la icp ms dating of apatite in mafic rocks evidence for a major magmatic event at the devonian carboniferous boundary in the armorican massif france apatite is a ubiquitous accessory mineral found in most magmatic rocks and is often the only u bearing mineral available to date mafic rocks because primary zircon and or baddeleyite are not present in this paper u pb la icp ms dating of apatite was applied to seven different dike and sill samples of dolerite from the variscan belt of brittany armorican massif western france these dolerites which are characterized by a within plate tholeiite geochemical signature are organized in several dense swarms across the belt their geochemical compositions are homogeneous although they intrude a large geographical area subdivided into several domains each characterized by different tectonic metamorphic settings their emplacement ages were so far poorly constrained due to the difficulty to date these mafic rocks using either the 40ar 39ar or the u pb methods on classical minerals like mica plagioclase or zircon although the closure temperature of apatite is lower than the emplacement temperature of the magma physical models show that the time needed to solidify and cool these mafic dikes and sills below the apatite closure temperature is basically of the order of 100 years or less consequently the u pb dates obtained on apatite can be interpreted as the emplacement ages for these mafic intrusions our results demonstrate that in all cases the apatite grains do carry enough radiogenic pb to be dated by in situ u pb analyses and yield a 207pb corrected mean age of 363 4 5 8 ma these results reveal the existence of a major and short lived magmatic event in the variscan belt of brittany during the devonian carboniferous transition a feature further highlighted by field evidence beyond the geological implications of these results u pb la icp ms dating of apatite appears to represent an ideal tool to date small size mafic intrusions

u pb geochronology and hf nd isotope compositions of the oldest neoproterozoic crust within the cadomian orogen new evidence for a unique juvenile terrane new u pb dates combined with nd and hf isotopic data from rocks within the port morvan area of the baie de st brieuc region of brittany identify a unique portion of the neoproterozoic cadomia terrane two gneisses near port morvan yielded u pb dates of 754 6 0 8 ma and 746 0 0 9 ma ages that are more than 130 myr older than the oldest units formed during the main phase of early cadomian magmatism two trondhjemite boulders from the monogenetic facies of the cesson conglomerate yielded identical ages of 665 2 0 5 ma and 665 5 0 7 ma and a cobble from the polygenetic facies yields a 207pb 206pb date of 637 2 ma individual detrital zircons from a sandstone associated with the cesson conglomerates yield concordant u pb dates ranging from 650 3 ma to 624 1 0 6 ma initial nd values for the rocks in this region range from 5 0 to 6 6 indicative of a substantial input from depleted mantle initial hf values determined on zircons from these neoproterozoic rocks including the detrital zircons range from 6 7 to 14 5 consistent with the nd isotopic results maximum initial hf values for two 2 ga icartian gneisses considered basement to cadomia average 8 4 and 8 7 in contrast to the results of the port morvan rocks 616 608 ma syn tectonic intrusions from normandy and the british channel islands all have negative initial nd values 10 4 to 8 3 consistent with significant contamination by ancient crust such as the 2 ga gneisses the oldest arc related magmas should have interacted most extensively with cadomian basement buffering younger mantle derived magmas that were generated in subsequent magmatic episodes the rocks within the port morvan region are thus inconsistent as examples of the earliest cadomian intrusions as they show no evidence of interaction with 2 ga basement instead the older ages and mantle like isotopic composition of these rocks suggest they are part of an independent terrane that formed prior to and independently from the cadomian arc possible terrane scale structural boundaries have recently been identified including the newly recognized port morvan thrust fault and the nw dipping main cadomian thrust

integrated 3d geophysical and geological modelling of the hercynian suture zone in the champtoceaux area south brittany france this paper combines geological knowledge and geophysical imagery at the crustal scale to model the 3d geometry of a segment of the hercynian suture zone of western europe in the champtoceaux area brittany france the champtoceaux complex consists of a stack of metamorphic nappes of gneisses and micaschists with eclogite bearing units the exhumation of the complex during early carboniferous times was accompanied by deformation during regional dextral strike slip associated with a major hercynian shear zone the south armorican shear zone sasz dextral shearing produced a km scale antiformal structure with a steeply dipping axial plane and a steeply eastward plunging axis armor 2 deep seismic profile shows that the regional structure was cut by a set of faults with northward thrusting components based on the seismic constraint direct 2d crustal scale modelling was performed throughout the champtoceaux fold on seven radial gravity profiles also using geological data and density measurements from field and drill hole samples the 3d integration of the cross sections the digitised geological map and the structural information foliation dips insure the geometrical and topological consistency of all sources of data the 2d information is interpolated to the whole 3d space using a geostatistical analysis finally the 3d gravity contribution of the resulting model is computed taking into account densities for each modelled geological body and compared to the bouguer anomaly the final 3d model is thus compatible with the seismic and gravity data as well as with geological data main geological results derived from the modelling are i the overall 3d geometry of the south dipping thrust system interpreted on the seismic profile emphasises northward thrusting and folding of the champtoceaux complex which was coeval with strike slip along the south armorican shear zone ii the gravity modelling suggests the presence of a relatively dense body below the champtoceaux complex that could be interpreted as a result of relative uplift of midcrustal material during thrusting along the e w trending wrench thrust system iii the northern limb of the champtoceaux anticline is a relatively shallow feature and iv vigneux synkinematic granitic body is a laccolith sheared and rooted along the southern branch of the sasz and spreads away from the strike slip zone within weak country rocks d 2004 elsevier b v all rights reserved

variable variscan thermal rejuvenation in the st malo region cadomian orogen france evidence from 40ar 39ar mineral ages abstract the st malo region in north west france contains migmatites and anatectic granites derived by partial melting of metasedimentary protoliths during cadomian orogenesis at c 540 ma previously reported rb sr model ages for muscovite and biotite range from c 550 to c 300 ma and suggest variable resetting of mineral isotopic systems these rocks display microscopic evidence for variably intense cadomian intracrystalline plastic strain but record no obvious evidence of penetrative palaeozoic regional deformation 40ar 39ar mineral ages have been determined to evaluate better the extent timing and significance of palaeozoic overprinting eleven muscovite concentrates and one whole rock phyllite have been prepared from various units exposed in the st malo and adjacent mancellian regions in the mancellian region muscovite from two facies of the bonnemain granite complex record 40ar 39ar plateau ages of c 527 and 521 ma an internally discordant 40ar 39ar release spectrum characterizes muscovite from protomylonitic granite within the cadomian alexain deux evailles ize granite complex and probably records the effects of variscan displacement along the north armorican shear zone muscovite concentrates from anatectic granite and from cadomian mylonites along ductile shear zones within the north western sector of the st malo region exhibit internally discordant 40ar 39ar release spectra which suggest variable and partial late palaeozoic rejuvenation by contrast muscovite concentrates from samples of variably mylonitic brioverian metasedimentary rocks exposed within the south eastern sector of the st malo region display internally concordant apparent age spectra which define plateaux of 326 320 ma a whole rock phyllite sample from brioverian metasedimentary rocks exposed along the eastern boundary of the st malo region displays an internally discordant argon release pattern which is interpreted to reflect the effects of a partial late palaeozoic thermal overprint muscovite from the plelan granite part of the variscan plelan bobital granite complex yields a 40ar 39ar plateau age of c 307 ma the 40ar 39ar results indicate that cadomian rocks of the st malo region have undergone a widespread and variable palaeozoic carboniferous rejuvenation of intracrystalline argon systems which apparently did not affect the mancellian region this rejuvenation was not accompanied by penetrative regional deformation and was probably of a static thermal hydrothermal origin the heat source for rejuvenation was probably either the result of heating during variscan extension or advection from variscan granites which are argued to underlie the st malo region

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

the aeromagnetic map of the french cadomian belt abstract in order to obtain an image of the subsurface of the armorican massif two detailed aeromagnetic surveys were combined to obtain an aeromagnetic map at 350 m altitude with an interpolated grid of 250 m 250 m the map shows good correlation between the magnetic anomalies and known geological structures and also distinguishes between long and short wavelength anomalies the long wavelength anomalies are associated with the overall subsurface arcuate structure of the cadomian belt the short wavelength anomalies are commonly associated with the lanvollon acid volcanic rocks and syntectonic diorites with one set of short wavelength anomalies in the east of the saint malo region being associated with dykes that are exposed onshore and continue offshore

kinematics of pre miocene ductile deformation in the santa catalina core complex and adjacent regions this paper describes structures within mylonites and metamorphic rocks from the santa catalina and tortolita suizo metamorphic core complexes southeastern arizona united states in these areas mylonitic rocks constitute an easterly low dipping slab more than 4 km thick mapping of principal strain directions and analysis of shear sense have been carried out through the whole pile of mylonites it is shown that although the stretching lineation appears to have a uniform mean sw ne trend three successive shearing events are involved the first shearing event is toward the west and is accompanied in part by the emplacement of the leatherwood quartz diorite dated at 64 ma the second shearing event is toward the ne and is accompanied by the emplacement of the wilderness granitic suite dated at 49 ma the third shearing event is localized and directed toward the sw the top to west shearing and top to ne shearing are possibly related to laramide thrusting the last ductile shearing is related to post eocene extension

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

mathematical removal of regional ductile strains in central brittany evidence for wrench tectonics abstract hercynian deformation in central brittany is responsible for folding of paleozoic and earlier sediments with concurrent development of cleavage essentially e w and vertical and stretching lineation essentially horizontal in the past the regional motion was inferred to be a n s shortening more recent studies emphasize e w transcurrent motions associated with major crustal faults values and orientations of ductile strain at 29 localities in central brittany show that the regional deformation is basically a horizontal plane strain with local three dimensional perturbations we first remove the perturbations rendering the problem two dimensional then a selected domain is divided into an array of finite elements parallel to strain trajectories using strain values extant at element centroids each element is unstrained independently of its neighbours resulting gaps and overlaps are minimized using a least squares technique which allows elements to translate and rotate rigidly thus we obtain the configuration of the domain in the undeformed state to within an unknown but constant rigid rotation the domain has undergone a heterogeneous dextral simple shear with vertical shearing plane striking n125 and horizontal shear direction a band of low strain running through guer is flanked by two zones of more intense strain the banding is approximately parallel to a major transcurrent fault in the area the south armorican shear zone

the impacts of ski slope development on stream channel morphology in the white river national forest colorado usa article i nfo the combined influence of tree clearing road construction snowmaking and machine grading can cause increased flow and sediment loads along streams in or adjacent to commercial ski resorts these changes to stream channels can increase bank failures bed material size pool scour and in extreme cases channel incision we used field data from the white river national forest in colorado which includes several major ski resorts to test the hypothesis that ski slope development causes a significant difference in bank stability undercut banks fine sediment wood load pool residual depth and particle size d84 between the ski area project streams and reference streams we further hypothesize that the changes in a stream are mitigated by the density and type of vegetation growing along the banks a significant difference is defined as a project stream that is outside the range of variability of the reference streams to test these hypotheses we surveyed channel conditions channel dimensions and vegetation along 47 stream reaches 200 300 m in length twenty four of these streams are within ski areas project streams either adjacent to or downstream from ski slopes twenty three reference streams with very little to no development in their basins are used to define reference conditions of bank stability bank undercutting bank height wood load pool residual depth sediment size and vegetation structure a combination of statistical techniques including principal components analysis and classification and regression tree analysis was used to assess the controls on stream channel morphology and to analyze the differences between project and reference streams project streams that are significantly different than reference streams have a combination of a higher percentage of fine sediment smaller pool residual depth and higher percentage of unstable banks the impacted project streams have bed material derived from granitic rocks and a lower density of understory vegetation these results show the importance of considering vegetative and geologic influences on channel form and processes when assessing impacts of land use change roads and machine grading have the most significant impact on the streams causing bed fining and pool filling these data and results will help in revising a forest management plan to provide guidelines for planning and development of ski areas on public lands

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 influence of riparian vegetation on stream width eastern pennsylvania usa we surveyed adjacent reaches with differing riparian vegetation to explain why channels with forested banks are wider than channels with nonforested banks cross sections and geomorphic mapping demonstrate that erosion occurs at cutbanks in curving reaches while deposition is localized on active floodplains on the insides of bends our data indicate that rates of deposition and lateral migration are both higher in nonforested reaches than in forested reaches two dimensionless parameters and e explain our observations represents the influence of grassy vegetation on rates of active floodplain deposition it is 5 times higher in nonforested reaches than in forested reaches e is proportional to rates of cutbank migration it is 3 times higher in nonforested reaches than in forested reaches differences in width between forested and nonforested reaches are proportional to e in forested reaches channels are wide with banks that are difficult to erode dense tree roots create a low value of e and the channel migrates slowly e is high however because is very low shade from trees inhibits the growth of grass on active floodplains in nonforested reaches channels are narrow with banks that are easy to erode e is high and the channel migrates rapidly e is low however due to a very large value of grass grows readily on nonforested convex bank floodplains thus differences in width between forested and nonforested reaches are related to a balance between rates of cutbank erosion and rates of deposition on active floodplains implying that equilibrium widths develop to equalize rates of cutbank erosion and vegetation mediated rates of deposition on active flood plains these results suggest that accurate models of width adjustment should consider the combined effects of bank erodibility and floodplain depositional processes rather than focusing on these processes in isolation from one another

characteristics of log and clast bed steps in step pool streams of northwestern montana usa the role of log and clast steps was studied in 53 step pool reaches in 32 streams in northwestern montana in each reach 20 consecutive steps were sampled drainage areas ranged from 0 5 to 11 5 km2 mean step spacing was found to be proportional to channel gradient width of the bankfull channel and drainage area mean step height was proportional to the width of the channel no significant difference exists between clast and log steps with regard to step spacing and step height this suggested either that 1 woody debris is mobile in even the smallest channels in the study area or 2 channel flows arrange clast steps around immobile woody debris calculations of theoretical entrainment and transport of sediment suggest that the coarse clasts forming steps are capable of being entrained at bankfull discharge in most channels the presence of immobile log steps composed of living roots in the upstream reaches an increase in the percentage of log steps with gradient and a decrease in this percentage with the width of the bankfull channel and drainage area imply that woody debris is mobile primarily in the downstream reaches calculations of flow resistance suggest that most steps serve to maximize flow resistance these results support the second hypothesis that the height and spacing of clast steps are adjusted around immobile log steps to maintain consistent step characteristics

application of organic geochemistry to sequence stratigraphic analysis four corners platform area new mexico u s a abstract the sequence stratigraphic model of sedimentation postulates the existence of a time condensed section deposited at very low sedimentation rates occurring in the uppermost transgressive systems tract and the lowermost highstand systems tract this condensed interval has of often been equated with zones of increased hydrocarbon source potential and invoked as a petroleum source rock often in the absence of supporting geochemical data in this study isotopic molecular lithostratographic and paleontological data are presented for 51 outcrop samples from a cenomanian turonian section at red wash new mexico u s a interpretations are made within a sequence stratigraphic framework ramp model sedimentation rates are estimated in order to define the location of the codensed intervalm for comparison with the zone of best source potential approximately 80 m of section were sampled the interval of maximum flooding and westernmost progression of the cretaceous seaway shoreline as defined by paleontological data is stratigraphically immediately below the occurrence of discontinuities in total organic carbon content hydrogen index and fluoramorphinite concentration in the kerogen the overall transgressive event is accompanied by increases in relative concentrations of 24 n propylcholestane total steranes and rearranged steranes these changes are consistent with a simultaneous decrease in the ratio of terrigenous organic matter to marine algal input as westward flooding proceeded the simultaneous occurrence of discontinuities in kerogen related parameters and continuous trends in bitumen related parameters probably implies mobilization of soluble organic matter across the flooding event boundary sedimentation rate estimates for six intervals in the red wash section show that the best oil source rock potential occurs immediately above the condensed interval and maximum flooding surface within the highstand systems tract previous studies of cretaceous sections in other portions of the western interior basin have located the best source rock potential below the condensed zone we conclude that the equivalence between source rock occurence and condensed sections as applied through the sequence stratigraphic model requires modification before the model may be used to predict the location of oil source rocks in a ramp setting in this respect it is critical that the data in sequence stratigraphic studies be used to define the model and not allow the model to dictate the interpretation of the data

vein arrays as kinematic indicators in kinked anisotropic materials abstract vein arrays associated with natural kink bands have been even recently used by some workers as kinematic markers to distinguish between the migration and the rotation models of kink band mechanics in an outcrop of schistose meta agglomerate conjugate kinks and synthetic sinistral shears are associated with a component of sinistral slip along the foliation discordant scalariform vein arrays form in association with the shears but not the kinks tension gash elements of the arrays form by combined layer parallel slip and layer parallel extension along the rotated schistosity they are only kinematically significant at the scale of the length of the rotated segment anisotropy applying this analysis to kink bands elsewhere it is concluded that discordant vein arrays are not diagnostic of the rotational kink band model or of high resolved shear stress on kink boundaries

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

aureole deformation by flattening around a diapir during in situ ballooning the cannibal creek granite the cannibal creek granite a foliated pluton within a strongly deformed aureole shows several diagnostic features of a diapir the intensity of the aureole deformation diminished away from the contact the aureole foliation is continuous with that of the granite and forms closed elliptical trend lines and the contact metamorphic minerals are synkinematic with regard to the aureole foliation the pluton deformed its aureole in three stages involving initially brittle then ductile and finally brittle deformation styles initial intrusion of a low strength magma stoped kinked and flexed aside its overburden crystallization of the magma prevented further rise and it ballooned and deformed its aureole evidently entirely by ductile bulk shortening perpendicular to the contact this the major stage of aureole deformation records no evidence of vertical movement of the pluton or of progressive simple shear along its margin during this stage the magma may have been theologically a high strength bing

metallogenesis of the french part of the variscan orogen part ii time space relationships between u au and snw ore deposition and geodynamic events mineralogical and upb data abstract mineralogical geochemical structural and isotopic u pb studies were carried out on representative uranium and gold deposits from the french part of the variscan orogen results are compared with complementary data from the literature in order to propose a reconstruction of the metallogenesis in the variscan terranes with a particular emphasis on u deposits the data show that the early metal concentrations or pre enrichments in the terranes are affected by successive metallogenic stages which caused significant metal transport and deposition thus the significant geodynamic events which affected the variscan terranes were responsible for multistage metal deposition or alteration of the early concentrations and for the superimposed features of the metal deposits u pb geochronological results complemented by a compilation of geochronological data on u and pb zn ba f deposits show that three major stages of u deposition took place in western europe in the periods 325 300 ma 290 260 ma and 190 170 ma in addition small uranium mineralization events have been dated at 425 ma 90 110 ma and as tertiary about 40 ma the permo stephanian stage is associated with late brittle deformation associated with major shear zone movements and constitutes the major stage of u concentration the jurassic phase has a prominent part in the hydrothermal remobilization of uranium and in f ba pb zn deposition throughout western europe especially in the eastern part of the massif central other stages correspond to subordinate uranium migration within variscan terranes during earlier or later geodynamic phases the silurian metamorphism in vendee and the mesozoic to tertiary tectonic movements related to the pyrenean or alpine orogenies in the south eastern part of the massif central and in the alps data available on au and sn w as sb deposits have been considered for comparison with the u metallogenesis stages related to the deposition of such metals display significant similarities with those related to uranium deposition they occurred mostly during visean to permian events related to the intrusion of the late visean sn w and late carboniferous sn w li f granitoids and to the late brittle deformation stages which affected the hercynian basement in the vicinity of major shear zones au as sb however the significant discrepancies between the physical chemical conditions of solubilization transport and deposition of the metal led to distinct deposit location independently of the metal source location within the terranes

lithologic tomography an application to geophysical data from the cadomian belt of northern brittany france abstract a probabilistic description of subsurface lithologic structures can be established by inverting multidisciplinary geophysical data constrained by geological and geostatistical priors the methodology is based on the joint modelling of several media properties and on a statistical description of the relationships between them the information provided by the geophysical data and the geological and geostatistical priors is represented by probability density functions pdf that are combined into a posterior pdf composed by 1 a prior pdf in the space of the primary lithologic model parameters 2 a pdf of the secondary physical model parameters conditional to the primary model parameters and 3 a joint likelihood function that is the product of the independent likelihood functions for each observed geophysical field applying a markov chain sampling method enables a large sample of joint models to be generated from the posterior pdf the true configuration of the media is then determined from the representation of models pulled from the chain and the elaboration of statistics from the large sample of posterior joint models the method was used to invert gravity and magnetic data jointly characterising the mass density field the magnetic susceptibility field and the lithotype field along two 2 d sections of the geological units in the cadomian belt of northern brittany besides generating 10 6 joint models consistent with the observations and priors some features of the joint models and the statistical tomographic images provided additional insights to the geologic configuration of the area for example the main cadomian thrust shows an irregular geometry that could have resulted from the belt emplacement and or from variscan tectonism and the hercynian granitic intrusion shows a deep subsurface continuation the cosimulation of magnetic susceptibility and mass density inside each lithologic region was performed according to a multivariate gaussian model or a mixed multivariate gaussian functions model that was developed specially to describe multimodal distributed properties

environment and structural controls on the intrusion of the giant rare metal greenbushes pegmatite western australia the greenbushes pegmatite district has been the main center of production of alluvial tin in western australia since the beginning of this century and more recently for the production of tin tantalum and lithium from one of the largest rare metal pegmatite deposits in the world the intrusive rocks in the greenbushes pegmatite district are concentrated along the ancient crustal scale donnybrook bridgetown shear zone analogous to the present san andreas fault system and are characterized by steeply dipping planar mylonitic fabrics with horizontal stretching lineations asymmetric folds asymmetric pressure shadows and shear bands suggesting sinistral strike slip movement the greenbushes pegmatite occurs in a higher temperature and higher pressure metamorphic terraine than would be expected for pegmatites containing rare element mineralization the pegmatite contains the same shear fabrics as its host rocks and has evidence for syntectonic crystallization of minerals such as tourmaline fantalite garnet and cassiterite it is proposed that intrusion of the greenbushes pegmatite magma was controlled by the donnybrook bridgetown shear zone any melts or fluids present during movement along the shear zone would have been channeled into it leading to intrusion of the pegmatite the fluid pressure in the pegmatite magma may then have increased causing further failure zones of structural weakness and further intrusion of pegmatite generally parallel to the mylonitic fabric in the shear zone according to present models and classifications the greenbushes pegmatite group should be unmineralized however it is apparent from this study that giant rare metal pegmatites can occur in higher grade metamorphic terranes and that these types of pegmatites need not have obvious parental granitoids they may contain a variety of mineralization and are likely at least in the archean to be associated with tectonism along crustal scale fault systems

tree roots as a dominant agent of streambed habitat profile pattern and grade control abstract tree roots are a ubiquitous biotic agent in numerous fluvial settings but their role in controlling streambed habitat and profile patterns is understudied although their importance in resisting bank erosion is more widely reported tree roots also have the potential to span the channel bed particularly in narrower first to third order streams this enables tree roots to drive profile patterns and maintain important habitat elements such as steps that induce deep pools in both perennial and ephemeral systems in a study across two humid temperate ecoregions tree roots were associated with more abundant and larger steps than large wood lw forming 91 of the steps in the high gravel load setting and 40 of the steps in the steeper coarser low sediment supply setting when combined with lw a majority of the steps in both settings were formed by biotic agents 97 and 55 respectively furthermore 70 of the deepest pools across all 40 study reaches were associated with biotic controls with root steps being the most dominant driver 45 of the deepest pools in all study reaches including the maximum pool depths measured in both ecoregions 0 57 m and 0 38 m respectively many questions need further exploration for example what tree sizes and species are most likely to form root steps i e trunk diameters 40 cm and american sycamore platanus occidentalis in this study however even this somewhat limited study has clear implications to land use planning and stormwater management for example on land developments draining to root step dominated streams maintaining the integrity of the riparian zone would seem to be as important as ensuring hydrologic mimicry if channel integrity is to be preserved the study also has insights to the stream restoration industry where conventional practice typically involves large tree clearing operations to facilitate stream regrading

low pressure metamorphism in the sierra albarrana area variscan belt iberian massif a low pressure metamorphic zonation ranging from biotite to migorogenic belts including the variscan belt of southwest matite zones occurs in the sierra albarrana area variscan belt of europe de yoreo et al 1991 presented a review of some southwestern iberian peninsula in uppermost precambrian to lower possible tectonic settings of low pressure metamorphism palaeozoic metasedimentary rocks the principal deformation in those workers considered low pressure metamorphism this area is related to a major ductile shear zone whose central part to be that in which peak temperature is above 500 c is localized immediately to the southwest of the sierra albarrana and pressure does not exceed that of the aluminium quartzites the metamorphism is synchronous with respect to this silicate als triple point the following four tectonic deformation the metamorphic zones are symmetrically distributed situations were envisaged for the low pressure metawith respect to the sierra albarrana quartzites pressure temperature morphism de yoreo et al 1991 1 magmatic arcs p t conditions are 3 5 4 kbar and range from 400 c 2 regions of crustal extension 3 continent continent biotite zone to 500 c staurolite garnet zone up to 650 700 c collision zones where thermal relaxation occurs after migmatite zone we have not detected pressure variations along crustal thickening 4 regions with significant fluxes of the different metamorphic zones relic kyanite is observed in the aqueous fluids especially those located above subduction form of inclusions in andalusite within veins in the lower grade part zones of the staurolite andalusite zone the low pressure metamorphism of this paper is concerned with the description of a highthe sierra albarrana area arises from a two stage history including temperature low pressure facies series terrane the sierra moderate crustal thickening followed by subsequent localization of albarrana area variscan belt southwestern iberian masdeformation in a transcurrent shear zone during peak p t conditions sif figs 1 and 2 in this area a metamorphic zonation channelized fluid flow within the major ductile shear zone may from the biotite zone up to the migmatite zone developed have contributed to the heat budget of the low pressure metamorphism at low pressure garrote 1976 gonzalez del tanago peinado 1990 as will be shown here the metamorphic zonation is not related to post thickening thermal relaxation but rather exhibits a roughly symmetric pattern

magnetic modelling in the french cadomian belt northern armorican massif abstract the french cadomian belt northern armorican massif constitutes an excellent area for studying panafrican orogenic processes in europe for which geophysical methods are useful for obtaining a realistic interpretation of the geological structures at depth magnetic modelling of the area to the west of saint brieuc bay was carried out to determine the geometry of the cadomian geological units modelling was computed along profiles extracted from the regional aeromagnetic map the shape of the magnetic bodies was determined with the help of interpretative geological sections and the total magnetization of the magnetic bodies was determined from a data base of about 350 rock sample measurements the measured susceptibility ranges from 0 15 10 3 si to 221 5 10 3 si the induced magnetization im 40 a m the natural remanent magnetization nrm is weak and shows varied directions it is therefore negligible at the formation scale and only the magnetic susceptibility was taken into account for modelling a computed mean im was attributed to each magnetic body in the model measurements of the magnetic properties and thin section observations of field samples show that magnetite is the main carrier of the magnetization the strong susceptibilities are consistent with ferromagnetic behaviour of multidomain grains whereas the weak susceptibilities accord with dominantly paramagnetic behaviour sample measurements show that the saint quay intrusion the acid part of the lanvollon formation and one of the three lenses of squiffiec metagabbro are the most highly magnetic formations magnetic modelling made it possible to estimate the long wavelength fold shape of the binic formation the basin within this formation reaches about 2 0 2 5 km depth the modelling also indicates that the saint quay intrusion cuts the binic basin at its centre with vertical contacts modelling shows that the intrusion is apparently composed of two imbricated magnetic bodies in its eastern part and of homogeneous magnetic bodies in its western part which is consistent with the geological observations the lanvollon formation exhibits an heterogeneous magnetic behaviour consistent with its lithology of intercalated acid and basic metavolcanic bodies magnetic modelling indicates a difference in thickness of the acid part of the lanvollon formation between the profiles of the binic basin area and those of the squiffiec plouha area this we interpret as reflecting an initial variation in the thickness of the acid volcanic bodies the binic and the squiffiec plouha profiles show that the southern boundary of the basic part of the lanvollon formation is probably vertical or steeply dipping to the north

multi scale strainfield analysis using geostatistics investigating the rheological behavior of the hot variscan crust of the pyrenees axial zone the present study provides a quantitative way to analyze and interpolate strainfields from ductile fabric field measurements using geostatistics based on variogram analysis and kriging interpolations the developed methodology allows computing automatic cleavage and lineation trajectory maps at different scales from a same field structural dataset for the example provided by the hot variscan crust of the pyrenees the analysis of cleavage stretching lineations orientation and kinematic data document pure shear conditions recorded during convergence strain was vertically partitioned between an upper crust submitted to homogeneous thickening responding to n s orogen normal shortening and a hot and partially molten mid lower crust affected by horizontal longitudinal orogen parallel flow the emplacement of plutons and gneiss domes generated heterogeneities in the upper crust and induced strain localization into steep anastomosed transpressional shear zones flanking those structures the results show that geostatistics applied to structural geology are powerful tools for retrieving tectonic regimes of large orogenic segments by identifying and quantifying perturbations in the strainfields induced by protracted and potentially complex tectonic histories the variogram analysis also gives first order clues to evaluate whether the considered structural domains underwent single continuous ductile or polyphased brittle ductile deformation

influence of bank vegetation on channel morphology in rural and urban watersheds stream bank vegetation significantly influences the morphology of streams in the piedmont region of the united states we surveyed the morphology of 26 paired stream reaches in southeastern pennsylvania northern maryland and delaware one member of each pair has a forested riparian zone whereas the other has a riparian zone composed pri marily of grass the paired reaches are nearly contiguous so all significant channel forming variables except riparian vegetation are held constant the extent of urban development of the watersheds upstream of the paired reaches also varies considerably allowing us to determine the combined influence of riparian vegetation and urbanization on channel morphology statistical analyses indicate that 1 channels with forested riparian zones are wider than channels with nonforested riparian zones 2 channels in urbanized watersheds are wider than channels in nonurbanized watersheds and 3 the effect of riparian vegetation is independent of the level of urbanization

antimony deposits in the variscan armorican belt a link with mafic intrusives the variscan armorican belt was the world leading producer of antimony at the beginning of the twentieth century however geological controls on the deposits remain unconstrained here we illustrate the field setting of mineralisations and perform a statistical analysis using geographical information system software geophysical and geological data the analysis shows that sb deposits are spatially correlated with high density and magnetic zones this may reflect the presence of mafic ultramafic bodies at depth a suggestion that is further supported by numerous occurrences of doleritic dykes throughout the region future antimony prospecting should focus on the high density and high magnetic zones which comprise about 18 of the armorican massif links between sb deposits and mafic intrusions are recognised elsewhere in the west european variscan belt emphasising that these may be key for antimony prospecting in deformation belts

ductile extensional shear zones in the lower crust of a passive margin we describe and interpret an unpublished set of ion geophysical seismic reflection profile showing strong organized seismic reflectors at the base of the continental crust of the uruguayan volcanic rifted margin we distinguish two main groups of reflectors in the lowermost continental crust a first group at depths ranging from 32 km below the continent to 16 km in the continent ocean transition comprises reflectors continuous over tens of kilometers peculiarly visible near the mantle crust boundary a second group of reflectors dipping toward the ese oceanward is widely distributed in the lower crust these reflectors are slightly curved and tend to merge and become sub parallel with the first group of reflectors together they draw the pattern of thick shallow dipping top to the continent shear zones affecting the lower continental crust such sense of shear is also consistent with the continentward dip of the normal faults that control the deposition of the thick syn tectonic volcanic formations sdr a major portion of the continental crust behaved in a ductile manner and recorded a component of top to the continent penetrative simple shear during rifting indicative of a lateral movement between the upper crust and the mantle

strain magnetic fabric and paleomagnetism of the deformed red beds of the pont rean formation brittany france a joint study of strain anisotropy of magnetic susceptibility ams and paleomagnetism has been conducted in the deformed red beds of the ordovician pont rean formation brittany france analysis of results from 13 sampling sites shows that homogeneous strain estimated from the shape of elliptical reduction spots in each site controls the development of a magnetic fabric during the hercynian folding of this formation the good correlation between strain and ams tensors is interpreted as mainly arising from a strain induced progressive rotation of planar hematite that tends to align within cleavage plane this rotation of magnetic carriers in turn induces a rotation of net measurable pretectonic remanent magnetization toward cleavage because of the high angle between the initially subvertical ordovician remanent magnetization and the regionally subhorizontal shortening direction this deviation of the pretectonic magnetization is achieved without any scattering but with a probable clustering effect upon within site initial dispersions for the same reason the overall distribution of mean site pretectonic magnetization vectors is clustered at the in situ deformed state this results in an anomalous behavior of the classical fold test that could erroneously lead to assignment of a syntectonic to post tectonic age to the magnetizations finally the strain removal technique based upon the passive behavior hypothesis is applied to the deviated vectors it allows recovery of the initial direction of these pretectonic ordovician magnetizations d 235 i 75 95 6 5

synkinematic mafic ultramafic sheeted intrusions emplacement mechanism and strain restoration of the permian huangshan ni cu ore belt eastern tianshan nw china abstract whilst petrology geochemistry and metal content of small mafic ultramafic ni cu bearing complexes have been extensively studied their structural controls and emplacement mechanisms are still poorly documented this study addresses the last two points with the huangshan ni cu ore belt chinese eastern tianshan as a case study the huangshan intrusions are early permian a period when the tianshan orogenic belt recorded major right lateral wrench tectonics characterized by crustal scale shear zones detailed mapping petro structural analysis and strain rate calculation within and around the intrusions allow us to establish that the huangshan ni cu bearing mafic ultramafic complexes are not layered intrusions instead they emplaced by injection of several mafic ultramafic magma batches within kilometre scale tension gashes generated by permian dextral shearing and should be considered as synkinematic sheeted intrusions finite strain analysis across the huangshan kangguer shear zone provides rather high shear strain rates 4 5 considering the location and alignment of the ni cu bearing mafic ultramafic bodies along regional shear zones throughout eastern tianshan it appears that wrench tectonics most likely controlled and focussed the intrusion of parent magmas as a consequence rifting related to post orogenic extension is not required to account for permian magmatic features of the tianshan belt finally the huangshan ni cu bearing mafic ultramafic intrusions are neither parts of an ophiolitic suture nor of a dunite cored alaskan type ore deposit as postulated in some previous studies in the light of these new results we believe that structural controls and emplacement mechanisms of many ni cu sulphides deposits hosted by small intrusions particularly funnel shaped ones should be re evaluated from a structural and geophysical point of view

identification of ordovician block tilting in the hercynian fold belt of central brittany france field evidence and computer models abstract the significance of the brioverian lower ordovician unconformity in central brittany has been discussed by many authors a new interpretation in terms of extensional tectonics is presented based on structural sedimentological and stratigraphical data however because sedimentary formations have been strongly folded by hercynian deformation extensional structures are obscured and difficult to recognize to identify the normal faults the geological map is compared with computer models of the folded tilted blocks it is shown that major normal faults had a mean n70 trend dipping toward the se with an almost regular spacing of 7 8 km

on the role of diapirism in the segregation ascent and final emplacement of granitoid magmas only bodies of magma with a high crystal content and partially molten crustal country rocks can ascend as diapirs once such an envelope is pierced the diapiric ascent of the pluton is arrested by the high viscosity of a solid aureole deformation by shortening of the carapace of these bodies may lead to the expulsion of a magma with a relatively low crystal content which may then continue ascent via fractures and dykes the details of the mechanisms of granitoid magma segregation are still unknown but it appears that many magmas hegin their ascent through the crust as mushes with at least 50 melt and that such magmas are rheologically able to ascend through a thickness of crust this ascent mechanism explains the dearth of structures attributable to the ascent of granitoids in contrast to the abundance of structures that developed during their final emplacement when a magma becomes too crystalline melt 25 to continue its ascent via dykes it is immobilised at approximately this stage a hydrous magma may become saturated with water and release fluids into the aureole making it particularly susceptible to deformation magma that continues to arrive at this level is also immobilised and the pluton grows as a ballooning diapir these characteristically deform themselves and their aureoles by bulk shortening magmas that are able to ascend to shallow depths largely by virtue of lower water contents and higher initial temperatures tend to become finally accommodated by such brittle processes as stoping and cauldron subsidence high level intrusions lend to be tabular are also fed by dykes or conduits and assemble in tabular batholiths

bed coarsening riffle shortening and channel enlargement in urbanizing watersheds northern kentucky usa abstract stream systems naturally respond to watershed land use dynamics particularly in urban developments with unmanaged impervious areas such urban provoked alterations to channel morphology cause water quality impairments have adverse effects on aquatic biota and pose risks to adjacent public infrastructure over the past four years we have collected detailed hydrogeomorphic data at 40 unique stream locations throughout northern kentucky with at least two rounds of annually repeated surveys at 70 of the sites and three rounds of surveys at 50 of the sites analysis of this time series data encompassed measured rates of instability across three distinct dimensions including 1 channel cross sections 2 longitudinal profiles and 3 bed material particle composition regression analyses between geomorphic change and 2011 watershed imperviousness indicated stream cross sections in urban suburban watersheds tend to be getting larger their overall shape is both deepening and widening additionally stream riffle lengths are shrinking and their pools are becoming both longer and deeper and finally their bed material composition is coarsening particularly in streams in the early stages of watershed development by documenting fluvial geomorphologic dynamics in such detail this study highlights the process by which unmitigated urbanization homogenizes stream habitat and degrades aquatic ecosystems this improved process based understanding of the urban induced channel response sequence has clear implications to both stormwater management and stream ecosystem restoration particularly in stream systems where headcut migration is a primary driver of channel instability

where along a river s length will vegetation most effectively stabilise stream banks riparian vegetation has different impacts on stream processes depending upon its position in a catchment native riparian vegetation is increasingly becoming the favoured stream management tool but managers need to locate revegetation schemes where they will most effectively achieve ecological geomorphological or other project goals using the latrobe river in se australia as an example this paper illustrates a structured decision making approach for assessing the role of vegetation in stream bank erosion at different points throughout a catchment three bank erosion process groups are identified subaerial preparation fluvial entrainment and mass failure although these processes act on banks throughout the catchment there exists spatial zoning in the dominance of each process group over the others bank erosion in upper reaches is dominated by subaerial preparation in mid basin reaches by fluvial entrainment and in the lower reaches by mass failure we find that in upper reaches windthrown trees are responsible for most bank sediment transfer to the flow where direct fluvial entrainment of bank material is the dominant erosion process flow resistance due to vegetation becomes crucial in reaches where bank slumping is the dominant erosion process increased bank shear strength due to root reinforcement is the major role of vegetation in stabilising banks other effects such as tree surcharge and altered bank hydrology appear to exert only minor influences on the slumping process considering the above variables we are able to define a critical zone in which revegetation will be most effective in reducing bank erosion on the latrobe river this zone occurs in that portion of the river where it first leaves the mountain front and meanders across a broad floodplain this reach occupies the second quarter of the river s length this information combined with other scale analyses e g ecological hydrological will assist river managers to plan physically based riparian revegetation strategies

striding athabasca mylonite zone implications for the archean and early proterozoic tectonics of the western canadian shield study of the northern saskatchewan district of mackenzie segment of the snowbird tectonic zone suggests that fragments of relatively stiff mid archean crust possibly arc related have controlled the localization shape and complex kinematics of the multistage striding athabasca mylonite zone during the archean as well as the geometry of the early proterozoic rifted margin of the western churchill continent by the late archean the striding athabasca mylonite zone was located in the interior of the western churchill continent well removed from the contemporaneous plate margins except for the alberta segment the snowbird tectonic zone was not the site of an early proterozoic plate margin we suggest that the geometry of the archean early proterozoic boundary in the western canadian shield represents a jagged continental margin composed of a pair of reentrants defined by rifted and transform segments these segments were inherited from early proterozoic breakup and controlled by the archean structure

characterisation of the seismological pattern in a slowly deforming intraplate region central and western france abstract we analyzed the seismicity of central and western france using historical data a compilation of all recorded earthquakes from 1962 to 2002 4574 events relocated and all published focal mechanisms 119 focal solutions the aim is to understand what are the causes of earthquakes and stress accumulation in a slowly deforming intraplate region the distribution of earthquakes and focal mechanisms is first correlated with recognized faults geological structures and tomographic images then in order to better understand the distribution of hypocenters and seek deeper crustal sources for stress accumulation euler solutions are computed from the available bouguer anomaly data the analysis of the obtained pattern for heat flow values provides a better understanding of the concentration of seismicity in some particular zones two different behaviors of this slowly deforming intraplate region are evidenced one is linked to the presence of a hot spot under the massif central the other to reactivation of the hercynian structural heritage our results highlight that several possible sources for earthquake clustering can be invoked in central western france

tectonic subsidence flexure and global changes of sea level tectonic models for the evolution of passive continental margins predict that following rifting sediments should progressively onlap basement at the edge of a margin as the lithosphere cools and increases its flexural rigidity with age the pattern of modelled onlap is strikingly similar to that used by vail and colleagues to estimate sea level rise through geological time this similarity suggests that major portions of stratigraphical sequences at margins may have a tectonic rather than eustatic control the patterns of onlap used by vail and colleagues may be widespread however because several widely separated passive margins rifted at similar times but they are unlikely to be worldwide

regional vs contact metamorphism of garnet metapelites in the vicinity of late variscan granites central armorican domain brittany france late variscan granites intruded brioverian upper proterozoic and lower paleozoic pelitic sequences to the north of the south armorican shear zone in the vicinity of the granites brioverian garnet micaschists contain pre syn s2 assemblages with garnet staurolite and post s2 assemblages with staurolite andalusite andalusite appeared pre syn and post s2 in garnet free micaschists the garnets in the brioverian micaschists are zoned with increasing mg and decreasing mn and ca from core to inner rim only poor garnet zonations occur in paleozoic hornfelses of enclaves in the rostrenen granite the results of a microstructurally controlled application of garnet biotite geothermometers and garnet plagioclase geobarometers are similar to p t trends obtained by the gibbs method of garnet zonation modelling in the system ncfmnmash the p t paths of a pre syn s2 regional metamorphism are clockwise between 500 550 c 8 kbar and 700 c 5 kbar followed by cooling decompression they contrast with isobaric contact metamorphism between 500 and 700 c at 2 5 3 kbar in paleozoic hornfelses this points to a two stage variscan metamorphism with a pre granitic pressure dominated event in the brioverian micaschists followed by late variscan contact metamorphism and suggests the existence of a pre granitic tectonic boundary between the micaschists and overlying low grade sequences

examining the physical meaning of the bank erosion coefficient used in meander migration modeling widely used models of meander evolution relate migration rate to vertically averaged near bank velocity through the use of a coefficient of bank erosion e in applications to floodplain management problems e is typically determined through calibration to historical planform changes and thus its physical meaning remains unclear this study attempts to clarify the extent to which e depends on measurable physical characteristics of the channel boundary materials using data from the sacramento river california usa bend average values of e were calculated from measured long term migration rates and computed near bank velocities in the field unvegetated bank material resistance to fluvial shear k was measured for four cohesive and noncohesive bank types using a jet test device at a small set of bends for which both e and k were obtained we discovered that variability in k explains much of the variability in e the form of this relationship suggests that when modeling long term meander migration of large rivers e depends largely on bank material properties this finding opens up the possibility that e may be estimated directly from field data enabling prediction of meander migration rates for systems where historical data are unavailable or controlling conditions have changed another implication is that vegetation plays a limited role in affecting long term meander migration rates of large rivers like the sacramento river these hypotheses require further testing with data sets from other large rivers

relationship between dip and magma flow in the saint malo dolerite dyke swarm brittany france abstract a low elevation aeromagnetic survey has been recently carried out in northern brittany france revealing magnetic anomalies over both the onshore and offshore areas the shaded aeromagnetic map displays a striped pattern related to a dolerite dyke swarm dated at ca 330 ma the dyke swarm is slightly fanning at its northern and southern ends and is affected by some major n60 sinistral transcurrent faults after sliding the different segments of the dyke swarm along the actually observed offsets about 20 km one can see the dyke swarm as it was originally trending a study of the anisotropy of the magnetic susceptibility ams has also been conducted over 250 specimens uniformly distributed on 13 sites in section the k max plunge of the ams is broadly the same as the dip of the dykes measured on the field on this section the centre of the fanning pattern is superimposed both with the centre of a magma chamber calculated after gravity and magnetic modelling and with the axis of symmetry of the dyke swarm it can also be observed that the k max plunge which is usually vertical above the magma chamber is oblique with respect to the rims of the dykes in the shallow dipping dykes this may be interpreted as a result of the crystallization of an oblique flow also it seems that the magnetic susceptibility increases when the magma flow is steeper beyond a k max inclination of 45 the bulk susceptibility tends to increase in a linear way

3d volumetric modelling of cadomian terranes northern brittany france an automatic method using voronoi diagrams abstract an automatic method based on the use of voronoi diagrams is proposed for the 3d volumetric reconstruction of geological objects it enables volumes to be constructed starting from the combined use of geological maps and cross sections defined in multiple directions and can also take into account incomplete information on geological interfaces and faults the method is suitable for global modelling of a set of geological objects it gives a consistent 3d volumic partition of space according to geological information contained in the maps and the cross sections the method is described and applied to modelling the main domains of the cadomian collisional orogenic belt of panafrican age in northern brittany france thanks to the contrasting densities of the main units involved in the collisional process the gravimetric effect of the volumic model can be calculated its comparison with the observed anomaly allows us to discuss the validity of the geological model although some improvements appear to be necessary it is concluded that the method can make geological modelling more efficient through providing the possibility of rapidly testing many hypotheses

vail s sea level curves aren t going away academic researchers are beginning to gather support for exxon s new stratigraphic organizing scheme as its interpretation becomes more sophisticated

giant quartz vein formation and high elevation meteoric fluid infiltration into the south armorican shear zone geological fluid inclusion and stable isotope evidence giant quartz veins associated with the south armorican shear zone record important fluid circulation during the hercynian period regional scale mapping of veins allows two groups of veins to be identified on the basis of their geometric relationship with the south armorican shear zone veins in the first group are parallel to the shear zone whereas those in the second group developed in a direction oblique to it the former probably record infiltration of fluids along permeable pathways in highly deformed zones the latter may represent crustal scale tension gashes in the regional context most quartz veins have 18 o values between 10 and 16 indicating a mid crustal origin for the fluids microthermometry on fluid inclusions from euhedral quartz indicates that late fluids were mostly aqueous with very low salinity 0 1 7 wt eq and with homogenization temperatures ranging between 150 and 270 c together with very low 18 o values of some euhedral quartz down to 2 these features argue for a surface origin corresponding 18 o fluid values estimated near 11 are probably related to the high palaeo elevation of meteoric precipitation scarce but significant h 2 o co 2 fluid inclusions in euhedral quartz indicate a metamorphic contribution these were probably sourced from the exhumed metamorphic basement in the southern part of the region

ductile thrusting recorded by the garnet isograd from blueschist facies metapelites of the ile de groix armorican massif france

wood distribution along streams draining old growth floodplain forests in congaree national park south carolina usa abstract we measured wood volume and other characteristics along a series of instream and floodplain transects in and around congaree national park south carolina seven transects were surveyed at most sites one within the bankfull channel and three on each bank at 10 50 and 100 m lateral distances respectively from the channel along each transect we measured piece dimensions location along transect orientation and decay category we also characterized channel width drainage area vegetation type average forest basal area and inundation category for each transect forest transects 100 m from a channel were also surveyed instream wood loads for unmanaged channels draining old growth forests were 102 178 m 3 ha which is lower than most instream wood loads documented for old growth temperate forests we attribute the lower wood loads to the combined effects of high transport capacity and rapid decay floodplain wood loads are lowest close to the channel but increase with distance reaching comparable values up to 172 m 3 ha at distances 100 m from a channel channel geometry does not appear to exert the significant influence on instream wood loads that has been documented for more laterally constrained channels channel spanning jams are uncommon and appear to be transient features

geomorphic changes upstream of beaver dams in bridge creek an incised stream channel in the interior columbia river basin eastern oregon channel incision is a widespread phenomenon throughout the dry interior columbia river basin and other semi arid regions of the world which degrades stream habitat by fundamentally altering natural ecological geomorphological and hydrological processes we examined the extent of localized aggradation behind beaver dams on an incised stream in the interior columbia river basin to assess the potential for using beaver castor canadensis dams to restore such channels and the effect of the aggradation on riparian habitat we estimated aggradation rates behind 13 beaver dams between 1 and 6 years old on bridge creek a tributary to the john day river in eastern oregon vertical aggradation rates are initially rapid as high as 0 47 m yr 1 as the entrenched channel fills then level off to 0 075 m yr 1 by year six as the sediment begins accumulating on adjacent terraces we found that a 0 5 m elevation contour above the stream channel approximately coincided with the extent of new riparian vegetation establishment therefore we compared the area surrounding reaches upstream of beaver dams that were within 0 5 m elevation of the stream channel with adjacent reaches where no dams existed we found that there was five times more area within 0 5 m elevation of the channel upstream of beaver dams presumably because sediment accumulation had aggraded the channel our results suggest that restoration strategies that encourage the recolonization of streams by beaver can rapidly expand riparian habitat along incised streams copyright 2007 john wiley sons ltd

uranium metallogenesis of the peraluminous leucogranite from the pontivy rostrenen magmatic complex french armorican variscan belt the result of long term oxidized hydrothermal alteration during strike slip deformation in the french armorican variscan belt most of the economically significant hydrothermal u deposits are spatially associated with peraluminous leucogranites emplaced along the south armorican shear zone sasz a dextral lithospheric scale wrench fault that recorded ductile deformation from ca 315 to 300 ma in the pontivy rostrenen complex a composite intrusion the u mineralization is spatially associated with brittle structures related to deformation along the sasz in contrast to monzogranite and quartz monzodiorite 3 3 the leucogranite samples are characterized by highly variable u contents 3 to 27 ppm and th u ratios 0 1 to 5 suggesting that the crystallization of magmatic uranium oxide in the more evolved facies was followed by uranium oxide leaching during hydrothermal alteration and or surface weathering u pb dating of uranium oxides from the deposits reveals that they mostly formed between ca 300 and 270 ma in monzogranite and quartz monzodiorite apatite grains display magmatic textures and provide u pb ages of ca 315 ma reflecting the time of emplacement of the intrusions in contrast apatite grains from the leucogranite display textural geochemical and geochronological evidences for interaction with u rich oxidized hydrothermal fluids contemporaneously with u mineralizing events from 300 to 270 ma infiltration of surface derived oxidized fluids leached magmatic uranium oxide from fertile leucogranite and formed u deposits this phenomenon was sustained by brittle deformation and by the persistence of thermal anomalies associated with u rich granitic bodies

cadomian tectonics in northern brittany a contribution of 3 d crustal scale modelling abstract both available and newly acquired geological and geophysical data processed in the framework of the armor project have been used to construct a 3 d geometric and geological model of the cadomian domain of northern brittany the major results of the data processing are presented and discussed relative to a composite profile transecting the studied domain and constructed from the surface geology reflection seismics magnetotelluric soundings and gravimetric data they are then extrapolated at regional scale through a series of cross sections compiled from the surface geology and gravimetric data and calibrated onto the composite profile the 3 d crustal scale models were computed according to two procedures the first using a strim matra datavision modeler defines various interfaces through correlation between adjacent cross sections and is well adapted to the 3 d representation of geological structures the second based on voronoi diagrams leads to a volumetric model of the geological units and is a powerful tool for calculating 3 d geophysical responses of the model when the physical properties of the rock units are known the tectonic implications of the computed 3 d crustal scale geometry of the cadomian domain are then discussed in terms of volcanic arc thrusting with special emphasis being placed on the obliquity of the shortening direction by reference to the thrust fault trend finally the data are used to demonstrate the reactivation of some cadomian faults during the deposition of ordovician sandstones and following the emplacement of the paleozoic dolerite dyke swarm of northern brittany

precise u pb zircon ages from alderney channel islands growing evidence for discrete neoproterozoic magmatic episodes in northern cadomia the northernmost exposures of rocks formed during the late neoproterozoic cadomian orogeny in the channel islands northern france region occur on alderney the island mainly comprises foliated quartz diorite once considered to be 2 ga pre cadomian basement and an undeformed basic to intermediate plutonic complex a precise age of 610 2 ma based on u pb analyses of single and small groups of zircons for the foliated fort tourgis quartz diorite demonstrates that the oldest rocks were emplaced and deformed during a cadomian magmatic event the age is virtually identical to ages from similar foliated syntectonic quartz diorite bodies on the islands of guernsey and sark and at la hague north normandy indicating that this magmatic and deformational event was regional in extent discordant zircon xenocrysts define an upper intercept age of c 2 ga indicating the presence of palaeoproterozoic basement at depth single zircons from the undeformed bibette head granodiorite give a precise u pb age of 572 1 ma this age is closely similar to that for the emplacement of the northern igneous complex of guernsey the emerging data indicate that cadomian magmatism in the northern channel islands region was not a protracted continuum but occurred during two distinct short lived events separated by c 30 40 my

refraction wide angle reflection investigation of the cadomian crust between northern brittany and the channel islands abstract a seismic refraction experiment has been carried out off the coast of northern brittany nw france to investigate the deep structure of the north armorican block the seismic source was an array of airguns and the shots were recorded simultaneously both by ocean bottom seismometers and land stations we present 2d velocity models computed from two crossing seismic refraction profiles labelled p1 and p2 and each about 135 km long oriented ne sw and nnw sse i e parallel and perpendicular respectively to the regional basement structures wide angle reflection profiles were also performed to extend the 2d structures to a 3d crustal interpretation observed and calculated p wave traveltimes were matched by forward modelling prior to their inversion in order to produce two crustal velocity models the model which best fit the data consists of 1 high basement velocities at the top of the crust except in an area with shallow seismic velocity which can be as low as 3 0 km s at the surface and possibly related to the existence of shallow paleozoic sedimentary basins 2 the mean crustal velocity ranges from 5 5 to 6 8 km s at a depth of 15 km depth 3 lower crustal velocities range from 7 0 to 7 5 km s at a depth of 35 38 km and then from 7 5 to 8 1 km s at the crust mantle boundary located between 35 and 38 km depth comparisons with the crustal section imaged by the coincident seismic vertical reflection profile swat 10 show a good correlation between velocity gradients and lower crustal reflectivity when compared to surrounding basement areas the north armorican cadomian crust displays a slightly thicker crust and also a thicker reflective lower crust which can be related to the relative preservation of this upper proterozoic block from later paleozoic and mesozoic tectonic episodes the crustal structure of the north armorican block thus appears to be an intercalated feature between the surrounding paleozoic areas and older shield or platform domains

widespread miocene deep sea hiatuses coincidence with periods of global cooling high resolution biostratigraphic analyses of miocene deep sea cores reveal eight intervals of widespread hiatuses in the world ocean at 23 0 22 5 20 0 18 0 16 0 15 0 13 5 12 5 12 0 11 0 10 0 9 0 7 5 6 2 and 5 2 4 7 m y ago in complete sections these hiatuses correspond to intervals of cool faunal and floral assemblages rapid enrichment of 18 o and sea level regressions these factors suggest that miocene deep sea hiatuses result from an increased intensity of circulation and corrosiveness of bottom currents during periods of increased polar refrigeration

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

genesis of the two main types of peraluminous granitoids two mica monzogranites to leucogranites and biotite rich cordierite bearing tonalites to monzogranites form two distinct groups of peraluminous granitoids they can be distinguished by mineral and rock associations and by the variation of their peraluminosity during differentiation except for the rare muscovite bearing granitoids produced by extreme fractionation or local contamination of metaluminous magmas the majority of peraluminous granitoids are produced by partial melting of crustal rocks production of either muscovite bearing granites or biotite rich cordierite bearing granitoids does not depend only on the nature of the sources but is also controlled by the physical parameters of partial melting and consequently by the way anatexis of a thickened crust is enhanced biotite rich cordierite bearing granitoids are generated where mantle derived magmas are injected into or have underplated crustal rocks two mica granites are generated where thickened crust is affected by major crustal shears or thrusts correlations between differentiation and peraluminosity indicate the dominant role of either restite unmixing or fractional crystallization in the production and evolution of the various types of peraluminous granitoids

graben inversion in nature and experiments the inversion of crustal scale basement grabens is studied here through laboratory experiments on small scale models and available oil industry seismic lines from the southern north sea two basic configurations are considered first both the basement and the sedimentary cover are brittle and inversion does not involve any potential decollement between them second the basement and sedimentary cover are separated by a weak ductile layer e g salt which can allow decollement of the cover during both extension and later compression and inversion the second configuration is more complicated and can lead to a large variety of geological structures laboratory experiments were carried out on brittle ductile models built with sand to represent brittle layers basement and sedimentary cover and silicone putty to simulate the decollement layer between basement and cover a mechanically based classification of inversion structures is proposed the effects of some crucial parameters are investigated including obliquity between the direction of shortening and normal faults as well as strength profiles and the presence or absence of salt diapirs the experimental investigation leads to the following conclusions 1 the inversion of the graben by reactivation of normal faults implies that the angle between the direction of compression and the graben is less than 45 2 if there is a superficial decollement e g basement cover interface inversion initiates low dipping thrust faults in the cover localized at graben borders 3 salt diapirs or salt walls localized along the graben borders in the cover are preferential sites for the development of thrust faults and 4 when the cover is decoupled from the basement by a decollement layer inversion induced deformation in the cover which is partitioned between thrust faults along the graben borders and strike slip faults within the graben trending oblique to the graben borders experimental results are compared with field examples in particular from the southern north sea

geochemical differences between the cadomian granites of mancellia and the st malo migmatites armorican massif france data sets used to suggest geochemical similarity between the st malo migmatites and mancellian granites brown d lemos 1991 are reinterpreted the samples have complex distributions with outliers three non parametric tests show significant statistical differences between the samples in a1 2 o 3 cao k 2 o cr ni sr y and zr if outliers are culled from the samples the differences become more extreme the differences are also apparent on both principal component and multidimensional scaling plots geotectonic models based on a similar petrogenesis for the two rock units d lemos et al 1992 must be able to account for these differences

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

dynamic single thread channels maintained by the interaction of flow and vegetation most rivers on earth today flow as a single channel in some cases with occasional islands and follow a more or less sinuous course however single thread channels have proven difficult to reproduce and study experimentally experimental self formed channels tend to widen and subdivide leading to a braided pattern cohesive sediment has been the main mechanism studied for stabilizing banks and producing a single thread channel we show how laboratory experiments using vegetation to stabilize banks can organize the flow and convert the planform morphology from braided to single thread our experimental strategy a repeated cycle of short periods of high water discharge alternating with longer periods of low discharge accompanied by plant seeding and growth leads to the evolution of a dynamic self maintaining single thread channel with well defined banks and floodplain by eliminating weak flow paths the vegetation corrals the water into a single dominant channel until the reduction in total wetted width leads to a new self organized state in which the flow removes vegetated area as fast as it is produced the new channel is deeper and has a broader distribution of depths than the braided one with channel size adjusted to carry almost all the flood flow the resulting system maintains a dynamic steady state via similar mechanisms to those that operate in meandering channels in the field specifically erosion at the outside of bends bend growth and bar development our methodology provides a basis for experimental development of self sustaining high amplitude meanders and has applications for river management and basic research purposes

is there a close spatial relationship between faults and plutons abstract using spatial relationships between individual plutons and faults to support genetic correlations between faulting and magmatism is meaningless since even random magmatic or tectonic processes will result in some plutons adjacent to faults our initial analyses of populations of faults and carboniferous plutons in the armorican massif france and faults and alleghanian plutons in the southern appalachians usa indicate that plutons have broad distributions with respect to faults but with a tendency for plutons to occur away from faults maxima of integrated pluton areas occur at 1 4 appalachians and 1 2 armorican of the distance between the average fault spacing in these orogens although there is a great need for statistical evaluations of relationships between populations of igneous bodies and structures in a wide variety of settings and crustal depths our initial studies suggest that faults do not preferentially channel magma during ascent or emplacement and that these are relatively unfocused processes within orogenic belts

petrogenesis of hercynian leucogranites from the southern armorican massif contribution of ree and isotopic sr nd pb and o geochemical data to the study of source rock characteristics and ages abstract ree geochemistry and isotopic pb sr nd o compositions have been studied in up to nine separate leucogranite massifs from three structurally adjacent zones in south brittany ages decrease from 340 ma in the north to 300 ma in the south whereas initial 87 sr 86 sr ratios increase from 0 706 to 0 710 0 717 ree abundances are relatively low la n 20 80 chondrites with a strong enrichment of lree relative to hree la n lu n 13 4 26 6 pb isotope ratios are homogeneous for a given massif but heterogeneous among massifs and plot very close to a normal terrestrial lead growth curve at 300 ma ago 18 o 16 o ratios are high and relatively uniform at 10 7 to 12 8 mean 11 9 u pb studies on zircon indicate the presence of an inherited radiogenic pb component and that the sediment dominated source region had been metamorphosed during the siluro devonian ligerian negativee nd t values imply that the source region had been enriched in lree prior to the melting episode based on the combined data large scale partial melting of crustal material dominantly of sedimentary origin is proposed the model age of the source rocks is not very old 600 700 to 1200 ma the pb and sr isotopic variations observed among different massifs is probably the result of age differences in the source materials of the different leucogranites

the floodplain large wood cycle hypothesis a mechanism for the physical and biotic structuring of temperate forested alluvial valleys in the north pacific coastal ecoregion abstract a floodplain large wood cycle is hypothesized as a mechanism for generating landforms and influencing river dynamics in ways that structure and maintain riparian and aquatic ecosystems of forested alluvial river valleys of the pacific coastal temperate rainforest of north america in the cycle pieces of wood large enough to resist fluvial transport and remain in river channels initiate and stabilize wood jams which in turn create alluvial patches and protect them from erosion these stable patches provide sites for trees to mature over hundreds of years in river valleys where the average cycle of floodplain turnover is much briefer thus providing a future source of large wood and reinforcing the cycle different tree species can function in the floodplain large wood cycle in different ecological regions in different river valleys within regions and within individual river valleys in which forest composition changes through time the cycle promotes a physically complex biodiverse and self reinforcing state conversely loss of large trees from the system drives landforms and ecosystems toward an alternate stable state of diminished biogeomorphic complexity reestablishing large trees is thus necessary to restore such rivers although interactions and mechanisms may differ between biomes and in larger or smaller rivers available evidence suggests that large riparian trees may have similarly fundamental roles in the physical and biotic structuring of river valleys elsewhere in the temperate zone