sedimentological effects of a moving terrain to investigate the sedimentologic effects of changing rheology and boundary conditions on lithospheri cflexure due to thrust loading the following simple linear models for flexure have been examined the infinite elastic plate to represent thrusting on the craton the semi infinite plate to represent thrusting near a plate boundary and the infinite viscoelastic plate to consider simple time dependent effects these models make the following sedimentological predictions 1 periodic thrust motion on the infinite and semi infinite elastic plates produces periodic regressions and transgressions on the orogenward and cratonward side of the basin respectively 2 periodic thrust motion on a viscoelastic plate with a small relaxation constant produces out of phrase transgressions and regressions on both sides of the basin and 3 for a given set of flexural parameters the flexure size for these models in order of decreasing amplitude are the semi infinite elastic plate the infinite viscoelastic plate

slow spreading ridge axis tectonics evidence from the lizard complex uk abstract the east coast section of the lizard ophiolite complex cornwall u k preserves a series of interrelated extensional faults and dyke sets which indicate that the formation of this section of obducted oceanic lithosphere resulted from a combination of contemporaneous structural and magmatic events three dolerite dyke sets are evident the first two sets 1 and 2 are cut by and rotated on a series of ridge parallel extensional faults and shear zones later dykes set 3 cross cut the extensional faults these data indicate that both rotation and significant thinning in the region of 40 of this part of the oceanic lithosphere occurred prior to the final period of dyke injection the thinning of the lithosphere was the result of extension along the normal faults all of the dolerite dykes show morb characteristics although the late dykes set 3 are more primitive tending to be aphyric to sparse olivine phyric and show higher cr ni and lower zr values than the typically plagioclase phyric dolerite dykes sets 1 and 2 intruded prior to the ductile shearing event the more primitive dolerites post date the tectonism and indicate the lack of a significant magma chamber following a period of amagmatic extension the data are consistent with observations at present day slow spreading ridge axes and some mafic ultramafic ophiolite sequences and provide insight into the nature of ridge related structures and the interrelationships between tectonism and geochemical evolution of the dykes and record a mechanism listric faulting to account for the presence of peridotites and gabbros commonly dredged at slow spreading centres

discussion on shrimp u pb zircon dating of the exhumation of the lizard peridotite and its emplacement over crustal rocks constraints for tectonic models in this recent contribution nutman et al 2001 provide the first shrimp u pb zircon age data for the lizard peridotite and its immediate envelope and demonstrate the usefulness of ion microprobe geochronology in the unravelling of the complex thermo tectonic history inherent in the emplacement of such assemblages of plausibly oceanic rocks onto continental strata building on the research of cook et al 2000 the authors distinguish clearly between the lizard peridotite proper a mass of c 15 kbar mantle tectonite and the contiguous crousa downs complex of gabbro troctolite and sheeted mafic dykes emplaced into the peridotite following its exhumation and cooling thus although exhibiting many lithologies and structures characteristic of ophiolites the oceanic part of the lizard complex is clearly at least bipartite the studies of cook et al 2000 and nutman et al 2001 rationalize and clarify several problematic relationships in this extensively studied but still ambiguous suite we understand that the editorial handling of nutman et al s paper overlapped with the publication in this journal of our detailed account of the petrogenesis of the kennack gneiss which incorporates an evaluation of the tectonic context of this controversial unit on the basis of an idtims single zircon date sandeman et al 2000 we consider none the less that several aspects of the new contribution require comment these include nutman et al s interpretations of our earlier documentation of the petrology and age relationships of the kennack gneiss sandeman et al 1995 the man of war gneiss sandeman et al 1997 and the porthkerris plagiogranite clark et al 1998 b as well as our overview of the emplacement history of the lizard complex clark et al 1998 a an extended abstract cited but not referenced by nutman et al we employ herein the revised palaeozoic time scale of mckerrow

geochemical characteristics and comparison of the basic rocks of the lizard complex and the basaltic lavas within the hercynian troughs of sw england the s cornish basaltic rocks constitute a separate chemical province relative to the rest of the sw england hercynian volcanic suites in particular they are characterized by a lower incompatible element abundances b generally lower large ion lithophile high field strength element ratios and c being mildly enriched in the light ree within this province some of the basalts have chemical features akin to incompatible element enriched morb tectonically the s cornish province can be divided into a the s cornish melange zone n lizard roseland area and b the mylor sedimentary trough penwith camborne area both of which contain localized basaltic lavas these basalts are compared with the lizard ophiolite and associated hornblende schists which have compositions ranging from depleted to enriched morb the exotic volcanic megaclasts within the melange zone are not chemically related to the ophiolite and as such do not represent the dismembered volcanic portion of the lizard ophiolite if the lizard ophiolite and the hornblende schists represent different segments of a palaeozoic oceanic area lying to the s the ocean floor type hercynian basalts of s cornwall are interpreted as an abortive attempt to produce oceanic crust in a rifted continental margin during the early devonian

structural evolution of intermediate crustal rocks in a strike slip and extensional setting variscan odenwald sw germany differential upward transport of metamorphic complexes and changing deformation mechanisms abstract the variscan basement exposed in the odenwald mountains southwest germany is composed of several allochthonous metamorphic complexes showing low pressure metamorphism with varying pt conditions and of syntectonic plutons i and s type during the late devonian early carboniferous times rapid uplift of these rocks caused rapid cooling before and during the thermal peak of metamorphism the metamorphic rocks experienced penetrative strain d 1 d 2 which was probably generated by crustal shortening and stacking the resulting micro structures infer distributed strain activation of c slip in quartz and postkinematic annealing during and after the thermal peak of metamorphism ne sw trending zones of normal oblique sinistral strike shearing d 3 and nne ssw trending zones of normal shearing d 4 developed these episodes of deformation are characterized by strong localization of strain strain softening was caused by reorientation of the lattice of quartz toward an orientation more favorable for easy glide along a and by a change in deformation mechanisms of feldspar from dislocation creep to grain size sensitive creep normal oblique strike shearing and normal shearing caused juxtaposition of mid crustal slivers of dissimilar pt and structural histories and provided mechanisms for their denudation detachment of rocks occurred along domains in which strain softening was operative further transtension and extension facilitated uplift and emplacement of syntectonic plutons which experienced also d 3 and d 4 strain in the plutonic rocks strain was broadly distributed in the initial hot stages and became increasingly localized as the deformation continued through cooling

composition of modern deep sea sands from arc related basins summary petrographic and chemical examination of modern deep sea sands shows a clear distinction between active and passive plate tectonic settings among active settings sands from forearc basins of island arcs and basins from strike slip continental margins can be distinguished but those from other arc related settings overlap considerably in composition in particular sands from continental margin subduction zones and those from the backarc of island arcs appear to be indistinguishable chemically arc related sands are very close in composition to ancient greywacke sandstones for instance sodium almost always exceeds potassium thus it is not necessary for sodium to be added to sands diagenetically to make greywackes as has been suggested the influence of tectonics on the composition of sandstones has long been a central feature of sandstone petrology the geosynclinal model of global tectonics stimulated much thought on this subject probably the best known ideas are those of krynine e g 1942 subsequently such research declined but the plate model for global tectonics has produced a new wave of interest e g crook 1974 schwab 1975 potter 1978 dickinson suczek 1979 much of this work has relied on data from ancient sediments where diagenetic changes and difficulty in de ciphering the tectonic setting limit the useful ness of the conclusions much more work is needed on recent sediments to establish a base line against which ancient sediments can be compared we have previously reported valloni maynard 1981 the results of our work on the petrography of a set of recent deep sea sands from piston cores in this paper we shall com pare this petrographic information with bulk chemistry and attempt to relate these to tecto nics in arc related settings

strike slip orogens an explanation for the hercynides the nature of oblique plate interactions is not yet well understood the oblique interaction of pacific and n american plates has resulted in large scale migration rotation and accretion of microplates intervening basins are ephemeral individually filled and deformed oblique motion results in zones of both compression and extension the latter may be sufficient to generate new oceanic crust and the former to obduct it both subduction and collision are restricted in time and space the resulting orogen is extraordinarily complex in detail the far more complicated hercynides are the result of dextral interaction between continental plates europe and africa the lateral variability of the orogen the major and long lived strike slip faults the bimodal volcanism and related ore deposits of the european trough the lizard ophiolite and its melanges the nw spanish ophiolites and the iberian pyrite belt are all explained using an oblique collision model in particular the generation deformation and obduction of ophiolites in oblique slip mobile zones is discussed

upper palaeozoic ophiolite generation and obduction in south cornwall the gramscatho flysch sequence of south cornwall including the melanges of the meneage formation and the lizard ophiolite are described in relation to the development of the gramscatho basin the timing and geotectonic evolution of this feature are considered in its regional context within the rhenohercynian zone of northwest europe this zone is interpreted as an intracontinental dextral transform system that developed during the devonian closure of the so called massif central ocean a basin and rise topography is attributed to extension and compression at offsets on major faults e w extension of the gramscatho pull apart basin in middle and upper devonian times is believed to have caused rifting and the development of ocean crust from which the lizard ophiolite was assembled in an oceanic setting at a ridge transform junction the meneage formation was generated as a result of intrabasinal uplift on transcurrent faults a major change of plate motions took place when the massif central ocean finally closed at the end of the devonian and the weak rhenohercynian zone became a site of crustal convergence from which thrust nappes were transported northwards foreland basins were sequentially developed and deformed by thin skinned tectonics as the system advanced northwards during the carboniferous

surface circulation at the tip of the antarctic peninsula from drifters an array of 40 surface drifters drogued at 15 m depth was deployed in february 2007 to the east of the tip of the antarctic peninsula as part of the antarctic drifter experiment links to isobaths and ecosystems adelie project data obtained from these drifters and from a select number of local historical drifters provide the most detailed observations to date of the surface circulation in the northwestern weddell sea the antarctic slope front asf characterized by a 20 cm s 1 current following the 1000 m isobath is the dominant feature east of the peninsula the slope front bifurcates when it encounters the south scotia ridge with the drifters following one of three paths drifters i are carried westward into bransfield strait ii follow the 1000 m isobath to the east along the southern edge of the south scotia ridge or iii become entrained in a large standing eddy over the south scotia ridge drifters are strongly steered by contours of f h coriolis frequency depth as shown by calculations of the first two moments of displacement in both geographic coordinates and coordinates locally aligned with contours of f h an eddy mean decomposition of the drifter velocities indicates that shear in the mean flow makes the dominant contribution to dispersion in the along f h direction but eddy processes are more important in dispersing particles across contours of f h the results of the adelie study suggest that the circulation near the tip of the antarctic peninsula may influence ecosystem dynamics in the southern ocean through antarctic krill transport and the export of nutrients

geochemistry and u pb and 40ar 39ar geochronology of the man of war gneiss lizard complex sw england pre hercynian arc type crust with a sudeten iberian connection the man of war gneiss is a variably deformed meta igneous body that ranges in composition from gabbro to tonalite and comprises part of the basal structural slice of the lizard ophiolite complex sw england u pb dating of zircon demonstrates that the parental magmas crystallized in the earliest ordovician 499t s 3 ma but 40 ar 39 ar step heating analysis of amphiboles suggests that at least the northernmost zone of the gneiss was subjected to an amphibolite grade metamorphic event at c 374 ma probably associated with thrust emplacement of the ophiolite the gneiss exhibits major trace and rare earth element variations in conformity with an origin through partial melting of an amphibolitic source containing hornblende and plagioclase probably with minor garnet and zircon the elemental abundances and variations are characteristic of granitoids formed in supra subduction zone environments a granitoid sill within the adjacent old lizard head series schists previously inferred to be related to the man of war gneiss is trondhjemitic and exhibits higher sio 2 tio 2 and mgo but lower zr v ba and sr contents and therefore is probably not genetically related to the latter the sill may record local melting during ophiolite emplacement the man of war gneiss probably represents a structural inlier of pre hercynian basement incorporated in the hercynian nappes of sw england during nnw directed thrusting we emphasize the geological and geochronological similarities between variably deformed granitoid units of the lizard complex the sudeten massif of southern poland and the allochthonous complexes of northern portugal and nw spain these granitoid rocks may represent dispersed remnants of tremadoc arcs that formed oceanward from the northern margin of gondwana this is further convincing evidence that at this time eastern avalonia was separated from northern gondwana by an ocean basin that may have been connected with the tornquist sea

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

helium radon and hydrothermal circulation associated with the carnmenellis radiothermal granite of southwest england the carnmenellis granite of southwest england has higher than average radioelement concentrations and a heat flow greater than 100 mw m 2 more than twice the united kingdom average measurements were made of 4he concentrations and 222rn activities in surface waters over the granite and its immediate host rocks high values greater than 48 ppb 4he 160 c min 222rn for either or both gases are interpreted as indicating discharge of groundwater from the upwelling limbs of hydrothermal convection cells 4he has a spatial distribution which agrees well with the observed variations of heat flow within the pluton but 222rn is also influenced by lithology and depth of groundwater flow although the distribution of convection cells is partly determined by the presence of nw se faults there is close similarity with theoretical models of groundwater circulation it is concluded that 4he and 222rn surveys provide a simple but effective method for mapping areas of modern hydrothermal activity

a model for the tectonic evolution of south cornwall a stratigraphy for south cornwall is combined with an analysis of the chronology of thrust nappe emplacement the resulting model suggests that northward overthrusting of nappes of flysch and olistostrome ophiolite and continental basement is related to the closure of an oceanic basin in south cornwall involving southward subduction throughout the devonian the subsequent continental collision between the normannian high and part of laurentia occurred in the upper devonian with deformatian continuing until late carboniferous times

patterns of folding within nappes and thrust sheets examples from the variscan of southwest england folding is a common response to deformation within nappes and thrust sheets and a wide variety of attitudes of folds can result two examples of patterns of variably oriented folds from southwest england are analysed in terms of thrust tectonics in northern cornwall the millook nappe contains a downward progression from upright to recumbent folding with a zone of oblique or sheath folds at its base these changes are related to an increasing simple shear component towards the base of the nappe in southwestern cornwall d1 and d2 deformations are related to the thrust emplacement of the lizard complex f1 folds in rocks below and to the north of the lizard complex have variable axial orientations alternating zones of folds trending normal and oblique to the transport direction have been mapped the oblique folds are interpreted as initiating in zones of differential movement within thrust sheets later d2 shear zones cut up section ahead of the advancing lizard complex in which case they result in refolding of f1 folds beneath the lizard complex these d2 shear zones cut down section extending but rarely refolding the d1 structures

siegenian generation of the lizard ophiolite u pb zircon age data for plagiogranite porthkerris cornwall the long controversial age of the lizard ophiolite is resolved by u pb dating of three single magmatic zircons from a plagiogranite dyke near porthkerris point this demonstrates that the axial oceanic crust was undergoing extensional ductile shearing mylonitization amphibolite facies metamorphism and probably anatexis at 397 2 2 ma in conjunction with published k ar 40 ar 39 ar and rb sr data for the complex this early devonian age predicates a protracted interval between intra oceanic magmatism and deformation on the one hand and obduction during late devonian northwestward displacement of the normannian high on the other

indium mineralisation in sw england host parageneses and mineralogical relations abstract the sw england ore region contains significant amounts of indium in in early permian granite related skarn and lode parageneses and to a lesser extent triassic epithermal crosscourse veins ore parageneses that predate granite emplacement devonian and lower carboniferous sedimentary exhalative and vein parageneses are largely devoid of in cadmium cd and gallium ga occur widely in all sulphide bearing parageneses across the region with sphalerite concentrations locally reaching 1 74 wt cd and 1750 ppm ga indium displays a strong affinity to sulphide bearing magmatic hydrothermal parageneses it occurs in silicate sulphide skarns polymetallic sulphide lodes and sulphide bearing portions of greisen bordered sheeted vein systems and quartz tourmaline lodes across the region magnetite silicate skarns and quartz tourmaline lodes that are devoid of sulphide in contrast were comparatively unfavourable for in precipitation the highest in concentrations are found in mineral lodes associated with the carnmenellis and st agnes granites which are the districts that had the highest historical production of sn total in concentrations in these areas locally exceed 430 ppm while concentrations elsewhere are systematically below 200 ppm the principal in hosts are chalcopyrite sphalerite and stannite group minerals with local concentrations within cassiterite and tennantite surprisingly chalcopyrite accounts for the majority of the in budget throughout the region although concentrations are highest in sphalerite and stannite group minerals sphalerite locally contains up to 1 42 wt in chalcopyrite has up to 2200 ppm and stannite group minerals up to 6800 ppm no in was detected in lollingite arsenopyrite rutile haematite magnetite tourmaline biotite chlorite galena bornite chalcocite or pyrrhotite scattered concentrations in pyrite relate to impurities rather than incorporation by solid solution roquesite and possibly dzhalindite or native in formed locally where in bearing chalcopyrite or sphalerite has been replaced by bornite and quartz the in partitioning between sphalerite and chalcopyrite varies broadly between 1 1 and 10 1 sporadic in was included in triassic crosscourse veins as a result of interactions between migrating cacl2 rich basinal brines and earlier formed granite related parageneses the interactions involved at least two distinct components 1 incorporation of clasts of magmatic hydrothermal veins in crosscourse veins during faulting and 2 dissolution and re precipitation of magmatic hydrothermal vein minerals in crosscourse fluids local concentrations reach 140 ppm in the magmatic hydrothermal parageneses in sw england are comparable to the south china tin belt mount pleasant as well as erzgebirge krusne hory magmatic hydrothermal fluids associated with peraluminous granites have developed a variety of skarn greisen lode and veins parageneses by interactions with their host rocks and contained fluids crosscourse epithermal mineralisation occurred as in was transported by cacl2 rich basinal brines in a similar manner as in bearing veins in the west shropshire ore field

palaeozoic evolution of the variscan vosges mountains a geological synthesis of the palaeozoic vosges mountains ne france is presented using existing observations and new data the geodynamic evolution involves 1 early palaeozoic sedimentation and magmatism 2 late devonian subduction triggering back arc spreading 3 early lower carboniferous continental subduction continent continent collision and polyphase deformation and metamorphism of the orogenic root and 4 late lower carboniferous orogenic collapse driven by thermal weakening of the middle crust the evolution is integrated within the framework of the european variscan belt the northern vosges comprise sediments of rhenohercynian affinity separated from tepla barrandian metasediments by a lower carboniferous magmatic arc the latter is correlated with the mid german crystalline rise and is ascribed to the south directed subduction of the rhenohercynian basin the saxothuringian moldanubian suture is thought to be obliterated by the magmatic arc while the lalaye lubine fault is interpreted as the tepla barrandian moldanubian boundary the central vosges are paralleled with the moldanubian domain of the bohemian massif where identical lithologies record the devonian carboniferous se directed subduction of the saxothuringian passive margin below the moldanubian upper plate the southern vosges represent the upper moldanubian crust and are linked to the southern black forest the presence of an oceanic domain to the south of the vosges black forest remains unclear

mantle evolution in the variscides of sw england geochemical and isotopic constraints from mafic rocks abstract the geology of sw england has long been interpreted to reflect variscan collisional processes associated with the closure of the rhenohercynian ocean and the formation of pangea the cornish peninsula is composed largely of early devonian to late carboniferous volcanosedimentary successions that were deposited in pre and syn collisional basins and were subsequently metamorphosed and deformed during the variscan orogeny voluminous early permian granitic magmatism cornubian batholith is broadly coeval with the emplacement of ca 280 295 ma lamprophyric dykes and flows although these lamprophyres are well mapped and documented the processes responsible for their genesis and their relationship with regional variscan tectonic events are less understood pre to syn collisional basalts have intra continental alkalic affinities and have ree profiles consistent with derivation from the spinel garnet lherzolite boundary end values for the basalts range from 0 37 to 5 2 and tdm ages from 595 ma to 705 ma the lamprophyres are extremely enriched in light rare earth elements large iron lithophile elements and are depleted in heavy rare earth elements suggesting a deep garnet lherzolite source that was previously metasomatised they display end values ranging from 1 4 to 1 4 initial sr values of ca 0 706 and tdm ages from 671 ma to 1031 ma suggesting that metasomatism occurred in the neoproterozoic lamprophyres and coeval granite batholiths of similar chemistry to those in cornwall occur in other regions of the variscan orogen including iberia and bohemia by using new geochemical and isotopic data to constrain the evolution of the mantle beneath sw england and the processes associated with the formation of these post collisional rocks we may be able to gain a more complete understanding of mantle processes during the waning stages of supercontinent formation

40ar 39ar phlogopite geochronology of lamprophyre dykes in cornwall uk new age constraints on early permian post collisional magmatism in the rhenohercynian zone sw england the spatial and temporal association of post collisional granites and lamprophyre dykes is a common but enigmatic relationship in many orogenic belts including the variscan orogenic belt of sw england the geology of sw england has long been interpreted to reflect orogenic processes associated with the closure of the rheic ocean and the formation of pangaea the sw england peninsula is composed largely of early devonian to carboniferous volcano sedimentary successions deposited in synrift and subsequent syncollisional basins that underwent deformation and low grade regional metamorphism during the variscan orogeny voluminous early permian granitic magmatism cornubian batholith is considered to be broadly coeval with the emplacement of lamprophyric dykes and lamprophyric and basaltic lava flows largely on the basis of geochronological data from lamprophyric lavas in devon although published geochronological data for cornish lamprophyre dykes are consistent with this interpretation these data are limited largely to imprecise k ar whole rock and biotite analyses hindering the understanding of the processes responsible for their genesis and their relationship to granitic magmatism and regional variscan tectonics 40 ar 39 ar geochronological data for four previously undated lamprophyre dykes from cornwall combined with published data suggest that lamprophyre magmatism occurred between c 293 6 and c 285 4 ma supporting previous inferences that their emplacement was coeval with the cornubian batholith these data provide insights into 1 the relative timing between the lamprophyres and basalts the cornubian batholith and post collisional magmatism elsewhere in the european variscides and 2 the post collisional processes responsible for the generation and emplacement of lamprophyres basalts and granitoids supplementary data complete datasets photomicrographs and photographs of sample locations are available online at http www geolsoc org uk sup18838

the emplacement of peridotites and associated oceanic rocks from the lizard complex southwest england upper mantle peridotites and associated oceanic rocks from the lizard complex south west england preserve evidence for a multistage geological history steeply dipping pre emplacement fabrics record high temperature 900 1100 c shearing and exhumation of the mantle peridotites apparently formed during localized ne sw rifting in a pull apart basin setting c 400 390 ma associated oceanic rocks landewednack amphibolites preserve a pre emplacement prograde brown amphibole bearing metamorphic assemblage and steeply dipping fabric thought to have formed as the newly formed oceanic crust was juxtaposed with newly exhumed hot mantle peridotite during ne sw rifting in both the peridotites and landewednack amphibolites steep pre emplacement structures are cross cut by low angle mylonitic fabrics thought to have formed during the initial phases of emplacement of mantle over crustal rocks in a partially intra oceanic setting c 390 375 ma the fabrics in peridotites and amphibolites exhibit retrograde mineral assemblages c 500 800 c with the amphibolites preserving two superimposed assemblages green amphibole titanite and colourless magnesio hornblende respectively that are thought to record progressive down temperature deforma tion during thrusting emplacement related structures in both the basal peridotites and amphibolites consistently dip at low to moderate angles nw with down dip lineations and kinematic indicators showing consistent top to the nw senses of shear syn emplacement magmatism is recorded by intrusions of foliated kennack gneiss anastomosing serpentine filled faults mark many existing low angle contacts between the peridotites and landewednack amphibolites and appear to represent the final lowest temperature 250 c stages of emplacement c 370 ma this study shows that dynamothermal aureoles in ophiolites may preserve evidence for tectonothermal events that pre date thrust emplacement

an interpretation of the variscan structures in sw england the variscan structures in sw england apart from the high level structures in the culm synclinorium of n devon are recumbent with an overall gentle southward dip of cleavages thrusts and recumbent folds translation was towards the nnw the structures are interpreted on a thin skinned model with a basal decollement dipping gently s related to a s dipping subduction zone s of the lizard the granites are thought to have been generated to the s of their present position and injected northwards above the decollement zone

thrust tectonics of s devon the tectonics of the area from start point to chudleigh sw england are shown to be dominated by northward verging thrust nappes two major thrusts are postulated the dartmouth antiform is considered to be a major thrust structure not a fold structure as previously interpreted the torquay limestones are allochthonous and part of a klippe of a major thrust nappe with a minimum displacement of 13 km northwards a progressive and diachronous sequence of deformation has been established with deformation starting in the s and moving n many of the folds and cleavages are probably related to the strains developed locally in thrust tips and may have no regional correlation more intense and complex deformation is found in the s compared to the n the diachroneity is also reflected in the k ar mineral ages in the s at start point southward verging backfolding is the last significant hercynian deformation phase folding the thrust nappes

evidence for the importance of small faults on block rotation field and earthquake data are presented which show that small faults can be important during block rotation small here means those faults which are at least an order of magnitude smaller than the largest faults at a particular scale of observation the nw se trending dextral faults of sw england have millimetres to kilometres of displacement the largest of these faults such as the sticklepath lustleigh fault do not appear to cause more than a few degrees of rotation from the general e w strike of beds tens of degrees of rotation are often visible however on faults with millimetres of displacement similarly the apparent rotation of beds in the badajoz cordoba shear zone spain increases as the resolution of the faults is increased the power law scaling relationship of earthquake magnitudes in the san andreas fault zone illustrates that small faults can also be important in regions of active block rotation small faults can allow deformation within rotating blocks and can allow high displacement gradients to occur on the block bounding faults this would reduce the need for void creation which is a requirement of rigid block rotation models a tentative model is presented which incorporates the concept that fault behaviour is fractal emphasising the contribution of small faults it is suggested that future studies of block rotation should rigorously test the contribution of small faults and that particular care is needed when comparing palaeomagnetic data with regional scale structures

the tourmaline bearing granite pluton of bodmin cornwall uk magnetic fabric study and regional inference the magnetic fabrics and microstructures of the variscan granite pluton of bodmin cornwall have been studied its low susceptibility magnitude consistent with its lack of magnetite comes from biotite and tourmaline as the magneto crystalline behaviour of tourmaline is inverse compared with that of the phyllosilicates the magnetic fabric was remeasured after heating the specimens to enhance the biotite magnetic signal by the growth of mimetic magnetite the roughly similar magnetic fabrics before and after heating demonstrate that the effect of tourmaline is not important as long as the tourmaline content does not exceed that of the phyllosilicates regionally this study reveals a well defined nnw trend of the magnetic lineations with opposite plunges on each side of a separation line crossing the pluton centre this pattern very similar to that documented in the nearby carnmenellis pluton is ascribed to unroofing during emplacement it agrees with the late to post magmatic nw stretch that characterizes the devonian to carboniferous formations located to the north of this region the thrust and fold structure of which is attributed to southward compression we conclude that syntectonic emplacement may have characterized the whole cornubian batholith at the very end of the variscan orogeny in cornwall

the tibetan side of the india eurasia collision results are reported from the 1980 joint french chinese field expedition in tibet the area covered was from the high himalaya in the south to the region of nagqu 250 km north of yangbajain ophiolites in the zangbo valley represent remnants of the crust of an ocean basin which lay adjacent to the gangdise granodiorite belt in the late mesozoic the ophiolites were thrust to the south onto the cretaceous melange and triassic flysch but the age of this event is unclear in the tertiary sediments of the indian margin and the xigaze basin were folded with steeply dipping cleavage which indicates some north south shortening of the crust however the tertiary volcanic sequence on the lhasa block and further north shows comparatively little folding and thrusting in the quaternary deformation of tibet was characterized by east west extension

the evolution of the hercynides of central sw england towards the end of lower carboniferous times sedimentation accompanied by widespread igneous activity proceeded in a tectonically active unstable and constricted basinal zone visean flysch and olistostromic sequences were generated in front of an advancing deformation front which caused erosion of submarine rises and heralded the beginning of the hercynian orogeny in this region between dartmoor and bodmin moor upper devonian basin and rise facies sediments were thrust northwards over both upper devonian and lower carboniferous northern equivalents which were themselves incorporated onto the allochthon the tectonic deformation history began in the late visean and early namurian with the gravity sliding dl of the bulk of the lower carboniferous northwards to form the greystone nappe subsequently the main thrusting event d2 from late lower namurian times onwards led to the successive overthrusting and emplacement of the heathfield petherwin and tredorn nappes towards the end of the namurian and in the westphalian the intrusion of the granite batholith caused the fold belt to tighten which resulted in the formation of numerous low angle normal faults d3 offsetting earlier thrusts to the n overall the regime is one of very thin skinned thrust and nappe tectonics with the allochthon probably not exceeding 1 km in thickness

correlation of the rhenohercynian variscides recent structural and stratigraphical data from the rhenohercynian variscides of belgium germany and south cornwall indicate tectonic continuity within this zone prior to disruption along a late carboniferous dextral transcurrent fault this extends from the bristol channel to the vosges parallel to similar lineaments of variscan age in europe and north america consequently a tectonic model for south cornwall is extrapolated to europe and a plate tectonic setting for the evolution of the north european variscides by the closure of an oceanic basin is discussed

late carboniferous to triassic reactivation of variscan basement in the western english channel evidence from onshore exposures in south cornwall south cornwall represents the westernmost part of the variscan basement massif in sw england field based investigations into the distribution geometry kinematics and relative chronology of post variscan deformation have been accompanied by preliminary palaeostress analyses these indicate three episodes of basement reactivation 1 stephanian to early permian nnw sse extension 2 early to late permian strike slip deformation and 3 late permian to triassic ene wsw extension these data have been used to test and refine previously published models based upon seismic and well data of late carboniferous to triassic basement reactivation and sedimentary basin development within the western approaches trough we confirm that nnw sse regional extension and reactivation of ene striking variscan thrusts had initiated by the latest carboniferous and that ene wsw regional extension and dip slip reactivation of nnw striking high angle faults occurred during the triassic permian strike slip deformation recognized onshore may have contributed to the development of widespread unconformities within the ovshore basins

the kennack gneiss of the lizard peninsula cornwall sw england commingling and mixing of mafic and felsic magmas accompanying givetian continental incorporation of the lizard ophiolite the kennack gneiss comprises a suite of interlayered mafic and felsic igneous rocks that intruded the 397 ma lizard ophiolite complex cornwall at 376 4 1 7 ma u pb single zircon date and were shortly thereafter c 370 ma metamorphosed to the amphibolite facies weakly deformed examples at the type locality of kennack sands reveal net veining of the mafic by the felsic component dispersed enclaves of the former in the latter and flame like interfingering of the two features indicative of magmatic commingling and mixing the silicic components of the kennack gneiss range from granodiorite to syenogranite and from metaluminous to peraluminous the least silicic rocks being the most peraluminous they were generated through anatexis of a predominantly metasedimentary crustal source having trace element and isotopic compositions comparable to those of the devonian gramscatho group of south cornwall the mafic component of the gneiss includes members group 1 which are weakly enriched in light ree have minor negative ta nb p and ti anomalies in extended trace element plots and display primitive time corrected 143 nd 144 nd and 87 sr 86 sr values and others group 2 which are more strongly enriched in all incompatible trace elements exhibit more prominent negative ta nb p and ti anomalies and have isotopic ratios intermediate between group 1 mafic components and the associated felsic gneiss group 2 represents mixtures of group 1 and granitic melts whereas group 1 rocks are comparable to emorb and were probably generated through partial melting of a weakly enriched mantle source emplacement of two commingled magmas into the base of the lizard ophiolite one representing the products of crustal anatexis implies that the ophiolite was removed from its oceanic setting and was in the process of obduction in the givetian

major crustal reflectors beneath sw england strong wide angle reflection events were detected during a crustal seismic experiment carried out in sw britain in 1979 three crustal reflectors have been identified referred to as rl r2 and r3 rl lies at a depth of about 8 km and appears to be restricted to the interior of the granite batholith underlying sw england the nature of this reflector is unknown r2 is a mid crustal reflector lying at a depth of 10 15 km and interpreted as rising gradually northwards it coincides locally with the base of the granite as predicted by gravity modeling but extends well beyond the northern limit of the granite body it is suggested that r2 may represent a major thrust of late variscan age r3 is the deepest reflector identified and lies at a depth of 27 30 km r3 is interpreted as the base of the crust and its computed depth is in close agreement with the local depth to the moho derived from an earlier seismic experiment in sw britain

faunal evidence for oceanic separations in the palaeozoic of britain the faunal criteria for the recognition of the relative separation of old continents are 1 the contrast between shallow not deep water faunas off opposite shorelines 2 the recognition of deeper water facies surrounding the edges of continents and 3 the recognition of the disposition of ancient climatic belts these criteria are defined and used to assess the relative position of the various parts of what is now britain in the palaeozoic the iapetus ocean is well characterized by faunal differences from the cambrian until the lower ordovician but from caradoc times onwards these differences dwindle until by the latest silurian only the distribution of the ostracods separates the two sides of the ocean on faunal grounds in the early ordovician s british faunas are comparable with those from bohemia france and elsewhere to the s indicating connection with gondwanaland and these faunas differ from those in the baltic area a distribution which we attribute to a true ocean tornquist9s sea between n and s europe tornquist9s sea probably closed in the upper ordovician and from ashgill times onwards the s british and baltic faunas remained similar during the middle and late silurian there is evidence of increasing faunal distinction between britain the baltic area and north america on the one hand and southern europe and gondwanaland on the other indicating an opening rheic ocean which appears to have persisted until late carboniferous times

tectonic environment of the devonian gramscatho basin south cornwall framework mode and geochemical evidence from turbiditic sandstones the portscatho formation within the allochthonous unit of the middle and upper devonian gramscatho group is a thick sequence of deep water sandstones and interbedded slates deposited by southerly derived turbidity currents into the gramscatho basin of south cornwall throughout an approximately 3 5 km thick sequence the portscatho formation is petrographically and chemically coherent except that the upper section shows a higher proportion of metamorphic clasts high but variable cr and low uniform zr abundances complementary framework mode and bulk geochemistry indicate that the sandstones were derived from a dissected continental magmatic arc of predominantly acidic composition similar to average upper continental crust but with an admixture of minor intermediate basic material flysch deposition took place in a fore arc setting the presence of an arc to the south of cornwall during the devonian implies that there was subduction at the margin of the gramscatho basin whose ultimate closure was accommodated by the northward stacking of flysch ophiolite nappes

post magmatic hydrothermal circulation and the origin of base metal mineralization cornwall uk a detailed fluid inclusion study of post variscan quartz carbonate sulphide anhydrite fluorite veins hosted by palaeozoic basement porthleven menheniot cornwall and permo triassic sediments western approaches basin has identified two broad fluid types a high salinity 20 27 wt nacl low temperature 80 150 c brine is present in base metal mineralized veins hosted by palaeozoic rocks onshore and in sulphate bearing veins within the permo triassic offshore succession microthermometric data indicate that saline fluids found in the permo triassic basinal sequences have similar compositions to the base metal mineralizing fluids and confirms that such basins are likely sources for the mineralizing fluids a second hotter 200 c more dilute 0 5 wt fluid has been identified in fault hosted east west trending veins in the porthleven area it is likely that the temperatures of both the dilute and saline fluids have been affected by the local high heat producing cornubian granite batholith there is also limited evidence for fluid mixing of these two fluids which may have been responsible for base metal precipitation

the padstow confrontation north cornwall a reappraisal the gravel caverns section new polzeath north cornwall shows clear evidence of multiphase deformation tight f1 folds with axial planar s1 slaty cleavage are reported for the first time these are deformed by north verging f2 folds with a south dipping spaced cleavage the fold and fabric relationships are inconsistent with the widely accepted model of structural confrontation previously described for the region this model is reassessed

hydrothermal recharge and discharge across 50 km guided by seamounts on a young ridge flank hydrothermal circulation within the sea floor through lithosphere older than one million years myr is responsible for 30 of the energy released from plate cooling and for 70 of the global heat flow anomaly the difference between observed thermal output and that predicted by conductive cooling models 1 2 hydrothermal fluids remove significant amounts of heat from the oceanic lithosphere for plates typically up to about 65 myr old3 4 but in view of the relatively impermeable sediments that cover most ridge flanks5 it has been difficult to explain how these fluids transport heat from the crust to the ocean here we present results of swath mapping heat flow geochemistry and seismic surveys from the young eastern flank of the juan de fuca ridge which show that isolated basement outcrops penetrating through thick sediments guide hydrothermal discharge and recharge between sites separated by more than 50 km our analyses reveal distinct thermal patterns at the sea floor adjacent to recharging and discharging outcrops we find that such a circulation through basement outcrops can be sustained in a setting of pressure differences and crustal properties as reported in independent observations and modelling studies

variscan very low grade metamorphism in southwest england a diastathermal and thrust related origin abstract regional metamorphism in the external variscides of southwest england varied from diagenetic level to greenschist facies there is a fundamental difference in the metamorphic character between the northern and southern regions of the area in the north m1 metamorphism is of a sedimentary burial character associated with high heat flow whilst to the south it is related to tectonic burial during thrust thickening processes with lower geothermal gradients this pattern appears to be related to the character of basin development and its subsequent tectonic evolution the northern region has features that accord with a diastathermal extensional origin for the very low grade metamorphism whilst in the southern region the very low grade metamorphism is linked to thrusting as a consequence of variscan compression the tintagel high strain zone presents an anomaly in this regional pattern where an m2 metamorphic phase is attributed to localized d2 thrust stacking along the southern margin of the culm basin there is no extensive overprint of the regional metamorphic pattern by the contact aureoles surrounding the granite plutons of the region however there is a noticeable coincidence between the areas of regional epizone grade and the extent of the geophysically defined subsurface limit of the granite batholith excluding the north devon area this link is attributed largely to the late stage structural up doming of the higher grade areas over the roof of the batholith

ridge axis deformation and coeval melt migration within layer 3 gabbros evidence from the lizard complex u k ridge parallel extensional shear zones within the layer 3 gabbros of the lizard ophiolite have recorded varied and numerous chemical and mineralogical changes under a variety of p t conditions these changes accompany tectonic exhumation of lower crustal levels by a listric fault mechanism during amagmatic extension at a slow spreading centre deformation within the mid crustal level of layer three resulted in the migration and impregnation of an evolved iron titanium rich silica poor melt along the shear zones this syntectonic infiltration occurred along the lowermost portions of listric faults which rooted in a partially molten zone beneath the ridge axis residual melt was tapped from this underlying zone and tectonically redistributed about deforming gabbros situated immediately above the partially molten zone the occurrence of sub solidus synkinematic assemblages within the shear zones which were generated at various temperatures and water rock ratios is explained in terms of their simultaneous development on listric fault planes in the presence of a hydrothermal fluid phase the sequential overprinting of pre existing assemblages arises from repeated block rotation during amagmatic episodes of extension within an oceanic slow spreading environment

volatile production during contact metamorphism the role of organic matter in pelites elemental analyses of palaeozoic mudrocks from south cornwall indicate the presence of 0 04 0 74 wt carbon primarily in an elemental and or organic form in addition to 0 04 0 10 wt nitrogen as nh 4 in micas and in organic material both elements are depleted in samples from the lithologically homogeneous mylor slate formation next to the tregonning granite an offshoot of the cornubian batholith the depletions are due to an h 2 o co 2 n 2 ch 4 fluid formed during contact metamorphic devolatilization of the pelitic country rocks fluids of this type have been reported from a number of hercynian sn w provinces and may play an important role in mobilizing metals in pelitic rocks during granite related contact metamorphism and hence in the metallogenic evolution of such regions

lower crustal heterogeneity beneath britain from deep seismic reflection data 450 km of deep seismic reflection data have been acquired over the uk landmass a qualitative assessment of lower crustal reflectivity based on the amplitude and continuity of deep crustal events has been made in the cratonic area of central england the lower crust is generally poorly reflective while in the foldbelt terrains of southern eastern and northern england it is more strongly reflective beneath the variscides of southern england an important distinction can be made between poorly reflective foreland type lower crust lying beneath the variscan front thrust and strongly reflective orogenic lower crust farther south a brief review is given of hypotheses for the origin of lower crustal reflectivity and the relationship of the latter to crustal tectonic regime both in the uk and elsewhere it is concluded that the types of lower crustal heterogeneity required to produce the observed reflections are more easily generated in orogenic provinces fold thrust metamorphic belts than in areas which have undergone only low to moderate crustal extension beneath typical sedimentary basins the generation of granites by partial melting within the lower crust and processes of metamorphic differentiation and recrystallization which accompany orogenic activity are likely to play a significant role in the development of such heterogeneities in addition post orogenic processes of magmatic underplating and low viscosity ordering may also have played a part

provenance and depositional environment of rhenohercynian synorogenic greywackes from the giessen nappe germany the giessen nappe of the south eastern rhenish massif can be subdivided into a northern group and southern group of frasnian lower carboniferous greywackes that differ slightly in their sedimentology clast petrography heavy mineral assemblages and bulk geochemistry

the rb sr isochron age of the kennack gneiss and its bearing on the age of the lizard complex cornwall an acid vein from a borehole in the kennack gneiss has given a whole rock rb sr isochron age of 369 12 ma this is interpreted as a metamorphic age recording the time of emplacement of the lizard complex this emplacement in late devonian times was part of an early phase of the hercynian orogeny which also affected the gramscatho 9series9 of s cornwall

short paper the start perranporth line a devonian terrane boundary in the variscan orogen of sw england in the variscides of sw britain the start perranporth line spl trends e w and shows a history of dextral transpression it is proposed that during the devonian the spl marked a major basement fault which formed the northern margin of a continually infilled group of dextral pull apart basins locally floored by oceanic crust in sw britain transtension may have arisen due to dextral motions along a terrane boundary which separated the ors continent from an armorican microplate to the south the faulted continental margin may then have acted as an oblique buttress to the advancing variscan nappes inducing dextral transpression large scale backfolding and forming the spl