the bristol mendip foreland thrust belt the thrusts and associated fold structures of the bristol mendip area are interpreted in terms of a thin skinned piggy back foreland thrust belt of variscan age with dominant northward transport strike normal balanced sections and compatible strike parallel sections have been constructed to illustrate the 3 d geometry of the linked thrust system north south shortening of c 20 km 40 is calculated from deformed and restored strike normal sections a thick and extensive thrust wedge of pre old red sandstone basement in the west of the area is responsible for culminations expressed at the surface as the mendip periclines uplifted culminations permit erosion to lower stratigraphic levels in the west and south of the area strike parallel sections and a hanging wall cut off map indicate that the majority of thrusts in the area are mainly oblique ramps rather than simple frontal and lateral ramps or flats the en echelon pattern of the mendip periclines may be related to the oblique ne climb of two of these thrusts the angle of frontal ramp climb through the dinantian stratigraphy is generally low and this explains the presence of numerous klippen of lower on upper carboniferous rocks whilst suggesting that certain thrust sheets were much more extensive than present outcrops indicate early pre thrusting movement on basement lineaments in the variscan foreland created uplifts angular unconformities and a predominance of n s structures in the north of the area

the stratigraphy and structure of the devonian rocks around liskeard east cornwall with regional implications a formal stratigraphy based on new macrofaunal evidence is proposed for a little known area of slates thin limestones and volcaniclastic rocks of lower devonian to early upper devonian age around liskeard between south devon and north cornwall open shallow marine shelf conditions persisted from the emsian until the onset of basinal sedimentation in the frasnian with the local development of pillow lavas and pillow fragment breccias during eifelian and givetian times regional stratigraphic correlations identify a middle devonian axial ridge which runs from tor bay to bodmin and separates a redefined trevone north cornwall basin from the south cornish basin a moderate to steeply dipping slaty cleavage dl dips southward throughout the area and is associated with early e w major folds inferred to be north or upward facing which plunge gently east a penetrative closely spaced crenulation cleavage d2 is developed in a zone 0 7 km thick which trends e w and dips at 15o to 30o south this fabric previously misidentified as the d1 slaty cleavage is associated with major d2 folds which have near horizontal east west trending axes structures of this age and geometry have not been reported from elsewhere in the region the portwrinkle fault has been traced nw se across the area and passes west of liskeard it shows a large dextral strike slip displacement and may have developed early in the history of the thrust belt

the calculation of bulk strain in oblique and inclined balanced sections abstract the calculation of bulk strain from balanced sections is a technique commonly used in the study of orogenic belts the validity of a balanced section is often enhanced by controls such as seismic and or borehole data the construction of a conventional vertical section perpendicular to orogenic strike i e in the transport direction may not allow control data to be directly incorporated into the section the control data can be projected into the plane of the section or alternatively the section may be constructed in an orientation such that it contains the available control data in the case of a vertical section constructed at an oblique angle to the transport direction the bulk strain value obtained is a minimum and can be corrected to the true value of bulk strain in the perpendicular vertical section by a simple trigonometric calculation if the section is perpendicular to strike but inclined at an angle to the vertical the correction of the bulk strain values is mathematically unique to the section this problem can be overcome by using a digitizing table in conjunction with a simple computer program the general case of a section both oblique to the strike direction and inclined to the vertical can be resolved by removing the effects of the inclination to produce a vertical oblique section from which the bulk strain can be obtained and corrected in realistic geological problems the oblique section will be a more common problem than the inclined section in both cases deviations of 10 and less introduce such small errors in the estimates of bulk strain that their effects can be ignored

stratigraphy of the upper devonian argillite successions in north cornwall some 880 m of grey green and purple argillites are estimated to be present in complexly deformed upper devonian successions which occur in several overlapping coastal sections four lithostratigraphical formations are formally defined in order of decreasing age the marble cliff limestone longcarrow cove slate harbour cove slate and polzeath slate formations improved correlation and biostratigraphical control is available for the two younger formations which are studied in detail the formations range in age from lowest frasnian to mid famennian despite the intense deformation there are no major structural breaks in the successions

strain variation in the henaux basse normandie duplex northern france abstract a completely exposed section through a miniature duplex developed in carboniferous limestones exists in a disused quarry and an adjacent railway cutting in the boulonnais region of northern france the structure is unique in that the propagating tip of the duplex where the shortening dies to zero is exposed a balanced restored section reveals an overall contraction of 38 7 increasing from zero at the duplex tip to over 70 at the distal end the limestone beds above the roof thrust of the duplex have shortened by 24 5 in a complex of foreland and orogen dipping thrusts which are typically developed above decollements

source and partitioning of b and sn in the cornubian batholith of southwest england abstract emplacement of the variscan s type cornubian batholith initiated the formation of the historical world class sn and cu deposits and extensive tourmalinisation of southwest england the hypothesis is tested that the sn and tourmaline mineralisation were wholly formed by magmatically derived hydrothermal fluids with no requirement for a contribution from the stripping of country rocks by circulating hydrothermal fluids from simple geochemical modelling sufficient sn concentrations to match those in the main 90 of granite outcrop biotite granites average of 23 mg kg and to produce the large sn deposits can be derived by partial melting of pre granite southwest england crust at geologically realistic levels of partial melting 30 fractionation 10 to 50 and melt aqueous fluid partitioning assumed to be 6 wt of the mass of the granite batholith in contrast insufficient b 700 mg kg or b concentrations found in many of the granites we conclude that tourmaline in the biotite granites was formed from magmatically derived hydrothermal fluids which emanated as a result of first and possibly second boiling an additional source of b for the hydrothermal fluids may have been the devolatilisation of stoped blocks of country rock mudstones these findings provide fundamental constraints on the evolution of the cornubian batholith which is used as a model for a large number of sn provinces worldwide

the structure and evolution of the wessex basin southern england an example of inversion tectonics abstract the wessex basin of southern england consists of four main sub basins each sub basin is thought to be fundamentally controlled by the normal reactivation of hercynian basement thrust and wrench or transfer faults post hercynian polyphase reactivation of both east west thrust and northwest southeast transfer faults tended to compartmentalise the basement thereby leading to the initiation of discrete permian cretaceous depocentres while rifting was confined to the western portions of the area during the permo triassic easterly migration in the mesozoic allowed the initiation of the weald and channel basins the sub basins represent therefore simple sinistral pull apart basins opening along northwest southeast faults this same fault direction is postulated to have also played a major role in controlling and or modifying the development of european basins in general major pre aptian uplift and erosion is largely attributed to the thermal effects and hence the isostatic uplift of the basin associated with continental rifting of the bay of biscay and the north atlantic margins major inversion and hence basin destruction began in the late cretaceous early tertiary with the uplift of the channel weald and vale of pewsey basins and culminated in the oligocene miocene synchronous with the helvetic phase of alpine deformation wessex basin inversion was facilitated by the thrust reactivation of variscan faults above an intracrustal detachment surface the same surface which allowed upper crustal extension and hence basin initiation in the permo triassic the structural development of the wessex basin both during extension basin formation phase and compression inversion phase was ultimately controlled by the plate motion of africa relative to europe such plate motions successfully predict the observed nw se sinistral motion from the stephanian to the aptian e w sinistral motion during the aptian cenomanian and largely nw se dextral motion from the cenomanian to the present day

evidence of an imbricate crustal thrust belt in the southern british variscides contributions of southwestern approaches traverse swat deep seismic reflection profiling recorded through the english channel and the celtic sea the southwestern approaches traverse swat seismic reflection profiles recorded through the celtic sea and the english channel provide unexpected data concerning notably the deep structure of the irish and south british variscan crust one of the most significant results is the recognition of prominent deep southerly dipping reflectors regarded as large scale foreland directed variscan thrusts which constitute in the southern innermost zones a crustal stacking wedge the resulting overthickened crust may have induced the petrogenesis of the cornubian granitic batholith by an anatectic melting process the roots of this crustal duplex are not imaged on the swat lines because of their subsequent assimilation by the more recent layering of the present day lower crust upward the deep crustal ramps are assumed to flatten out and to join a roof thrust which acts as a major midcrustal decoupling zone not revealed by the swat profiles and which underlies the thick devonian allochthonous high strained units of cornwall northward this shallow northerly verging shear zone is overstepped by the tintagel antithetic back thrusts which are closely related to deeper northward dipping basement faults considered as caledonian features southward the imbricate crustal thrust unit is overridden by the lizard ophiolitic suture which appears as a 8 km thick gently southerly dipping sheared zone crosscuttting the entire crust and rooting deep beneath the present day moho its hanging wall is constituted by the channel cadomian block characterized by an almost seismically featureless upper crust which is not involved in the main variscan thrust stacking northward the imbricate thrust unit is delineated by a moderately southerly dipping ramp which penetrates straight down into the upper crust without any evidence of an intervening flat lying decollement level this frontal ramp emerges along a broadly 110 n trending discontinuous thrust front located above the northern downfaulted margins of the munster ireland and pembroke wales devonian basins the swat deep seismic data relate the overall variscan crustal shortening undergone by the south british domain to an oblique converging process involving a continental shelf pinched between the rigid allochthonous channel block and the northern autochthonous foreland this crustal shortening occurred during a carboniferous intracontinental collisional context subsequent to the closure and the obduction of a small oceanic crust floored basin lizard

the relation between regionally consistent stretching lineations and plate motions abstract regional stretching lineations in the himalayas and southern tibet coincide with the known relative motions of the converging plates this relation is used to infer plate motions from the stretching lineations in the variscan and caledonian belts of europe and in the late proterozoic belts of northeast and east africa in each region high angle convergence of plates produced transverse stretching lineations and high angle collision was followed by or combined with large displacements first by ductile shear and later by brittle fractures parallel to the plate boundaries apparent stretching as a result of volume loss ramsay wood 1973 is common but recognisable as such reorientation by later deformations must be allowed for ramsay 1967 in ductile shear zones simple shearing leads to the progressive approach towards the shear plane of the xy plane of the strain ellipsoid and progressive approach of x the extension axis to the shear line ramsay graham 1970 thus large extensions approximate to the shear line large extensions tend to produce sheath folds whose acutely curved axes approach the stretching direction quinquis et al 1978 also the pre existence of a strong linear fabric predisposes the development of later folds with axes parallel to the earlier linear fabric for all of these reasons the extension axis x rather than the fold axis is of prime significance

strains developed in the hangingwalls of thrusts due to their slip propagation rate a dislocation model reply abstract fault planes propagate radially from a point source and this failure propagation process is very similar to the movement of a dislocation through a crystal lattice an elastic strain represented by an extra half plane is necessarily imparted to the lattice at an edge dislocation and this is equivalent to the ductile bead which accompanies the propagating tip of a thrust fault this ductile bead migrates with the fault tip and imparts a characteristic internal strain to the thrust sheet a two dimensional model is presented to illustrate the inter relationship between fault plane slip fault tip propagation and internal strain in multilayered sequences internal strain is usually represented by asymmetric folds verging in the thrust transport direction a simple technique the displacement distance plot utilizes the fact that displacement dies out towards the fault tip this plot can be used to quantify the relative stretch measured parallel to the fault movement which is dependent on the slip propagation rate and it may be used to define exactly the position of the fault tip on a cross section examples of fold thrust structures from devon england and pembrokeshire wales are used to illustrate the technique

chemical composition of fresh snow from glaciar marinelli tierra del fuego chile a fresh snow sampling campaign was conducted during the late austral summer of 2006 in the accumulation zone of glaciar marinelli located in the cordillera darwin tierra del fuego chile snow samples were analyzed for stable isotopes d 18 o major soluble ions na k ca 2 mg 2 cl no3 so4 2 ms and major and trace elements na mg al fe ca sr cd cs ba la ce pr dy ho er bi u as ti v cr mn the dominance of marine chemistry resembles that in studies from patagonian glaciers snow chemistry was dominantly loaded by marine species cl na and ssso4 2 while contributions of crustal species e g al and fe were very low empirical orthogonal function analysis suggests two possible dust sources one represented by al and fe and the other by la ce and pr enrichment factor calculations suggest the majority of elements are within average upper crustal ratios but major enrichments of bi and cd hundreds of times suggest possible anthropogenic sources linear correlation ofd 18 o and barometric pressure r 0 60 p 0 007 suggests a potential amount effect relationship between depletedd 18 o ratios and stronger storm conditions e g greater precipitation the snow chemistry records from glaciar marinelli are the first measured in tierra del fuego the southernmost glaciated region outside antarctica

the irish variscides a fold belt developed within a major surge zone an arcuate fold belt characterizes the irish variscides south of the dingle dungarvan line the folds are cross cut by numerous faults which tend to be sinistral in the west and dextral in the east clockwise transection of major fold axes by minor folds and by cleavage has been recorded in the east while anticlockwise transections are common in the west these and other features suggest that north south dextral and sinistral shearing occurred in the east and west respectively this is considered to be a consequence of the morphology of the munster basin which allowed the development of a surge zone in the centre of the fold belt the variscan sole thrust is believed to lie at the base of or within the old red sandstone however to the east thinning of the upper palaeozoic cover sequence forces the sole thrust to cut down laterally stratigraphic section into lower palaeozoic basement in the depocentre the sole thrust surged ahead by propagating along bedding surfaces or possibly a devonian carboniferous extensional detachment beneath the basin while to the east and offshore to the west its propagation was inhibited by the absence of easy decollement horizons within the basement consequently the tip line became strongly arcuate and the thrust developed into a surge zone the sole thrust propagated northwards by stick slip processes much of the deformation within the cover sequence occurred during sticking events particularly fold tightening and accommodation faulting

geodynamic model for the palaeozoic crustal consolidation of western and central europe abstract the crystalline basement of western and central europe consists of a mosaic of crustal elements which were consolidated during pre grenvillian the grenvillian dalslandian morarian cadomian caledonian and hercynian orogenic cycles contemporaneous with the caledonian suturing of the precambrian laurentia greenland and fennosarmatian shields a number of gondwana derived cadomian micro cratons were accreted to the southern margin of laurasia following the late caledonian paroxysm the devonian and early carboniferous evolution of europe was dominated by continued northward subduction of the proto tethys plate at an arc trench system parallelling the southern margin of laurasia the accretion of additional gondwana derived continental fragments back arc rifting and a sinistral translation of major proportions between the laurentia greenland and the fennosarmatian sub plates the acadian and bretonian orogenies were of the pacific type the visean collision of gondwana with laurasia marked the onset of the himalayan type variscan orogeny during which collision related compressive stresses overpowered the devonian early carboniferous back arc rift systems and caused the development reactivation of a subduction zones the central armorican and saxothuringian successor basins became folded and destroyed during the latest visean whilst the variscan foredeep became scooped out in part by nappes involving the basement during the late westphalian major crustal shortening during the variscan diastrophism was accompanied by the anatectic remobilisation of subducted crustal and upper mantle material and a widespread syn and late orogenic magmatism the latest carboniferous early permian reorientation of the convergence direction between gondwana and laurasia induced the development of a complex wrench fault system transecting the variscan fold belt and an extensive post orogenic volcanism the hypotheses summarised here require confirmation by further palaeomagnetic and radiometric data and faunal analyses

geological significance of seismic reflections in collision belts geological problems raised by the interpretation of deep seismic reflections in collision belts result from the structural complexity of the crust and the lack of surface control and well documeted references some of these problems are illustrated by examples coming from various tectonic settings in france and adjacent areas and tentative interpretations are proposed

tectonics and plate tectonics model for the variscan belt of europe abstract a plate tectonics model is presented to explain the tectonometamorphic characteristics of the european variscides after the closing of two oceanic domains by two sided subduction 500 420 ma and obduction 420 380 ma collision of the european and african continental plates occurred we propose that the subsequent complex intracontinental deformation 380 290 ma is the result of a double subduction of the continental lithosphere accompanied by crust mantle decollement this mechanism explains the progressive crustal thickening and migration of the deformation through time from the sutures toward the external parts of the variscan belt accounting for this model and for the relationships between the european variscides and the other paleozoic peri atlantic belts caledonides appalachian mauritanides and morocco we infer the relative positions of africa america and europe between the silurian and the permian

west antarctica s sensitivity to natural and human forced climate change over the holocene the location and intensity of the austral westerlies strongly influence southern hemisphere precipitation and heat transport with consequences for human society and ecosystems with future warming global climate models project increased aridity in southern mid latitudes related to continued poleward contraction of the austral westerlies we utilize antarctic ice cores to investigate past and to set the stage for the prediction of future behaviour of the westerlies we show that holocene west antarctic ice core reconstructions of atmospheric circulation sensitively record naturally forced progressive as well as abrupt changes we also show that recent poleward migration of the westerlies coincident with increased emission of greenhouse gases and the antarctic ozone hole has led to unprecedented penetration compared with 100 000 years ago of air masses bringing warmth extra antarctic source dust and anthropogenic pollutants into west antarctica copyright 2012 john wiley sons ltd

tectonics of the bude formation north cornwall the recognition of northerly directed decollement the upper carboniferous bude formation of north cornwall and devon underwent deformation beginning shortly after its deposition which resulted in a complex geometry of folding and thrusting the deformation initially exploited slumped beds as decollement horizons and resulted in a significant northerly translation of thrust sheets the development of chevron folds for which the area is famous commenced at this stage but continued causing folding of thrust planes and cleavages during the deformation the dominant sense of thrust movement switched from a northerly to a southerly direction and this is interpreted as the effect of backthrusting associated with the northwards migration of the deformation front

fold and thrust systems in mass transport deposits abstract improvements in seismic reflection data from gravity driven fold and thrust systems developed in offshore mass transport deposits mtds reveal a number of significant features relating to displacement along thrusts however the data are still limited by the resolution of the seismic method and are unable to provide detail of local fold and thrust processes investigation of exceptional gravity driven contractional structures forming part of mtds in lacustrine deposits of the dead sea basin enables us to present the first detailed outcrop analysis of fold and thrust systems cutting unlithified soft sediments we employ a range of established geometric techniques to our case study including dip isogons fault propagation fold charts and displacement distance diagrams previously developed for investigation of thrusts and folds in lithified rocks fault propagation folds in unlithified sediments display tighter interlimb angles compared to models developed for lithified sequences values of stretch which compares the relative thickness of equivalent hangingwall and footwall sequences measured along the fault plane may be as low as only 0 3 which is significantly less than the minimum 0 5 values reported from thrusts cutting lithified rocks and reflects the extreme variation in stratigraphic thickness around thrust related folds we suggest that the simple shear component of deformation in unlithified sediments may modify the forelimb thickness and interlimb angles to a greater extent than in lithified rocks the average spacing of thrust ramps and the thickness of the thrust sequence display an approximate 5 1 ratio across a range of scales in mtds in general thicker hangingwall and footwall sequences occur with larger thrust displacements although displacement patterns on thrusts cutting unlithified yet cohesive sediments are more variable than those in lithified rocks line length restoration of thrust systems in mtds reveals 42 shortening which reduces to 35 in overlying beds a 23 reduction in shortening by folding and thrusting along individual thrusts suggests that heterogeneous lateral compaction may increase by 10 towards the sediment surface thrust systems cutting unlithified sediments display distinct steps in cumulative displacement distance plots representing increased rates of slip along the floor thrust while displacement distance plots along individual thrusts also reveal horizontal steps relating to lithological variation competent units cut by thrust ramps may display the greatest displacement which then progressively reduces both upward and sometimes downward along the ramp this relationship demonstrates that ramps do not necessarily propagate upwards from the underlying flat as in some traditional models but rather initiate by offset of competent horizons in the hangingwall of the detachment critical taper angles in mtds may be an order of magnitude less than in accretionary complexes or lithified rocks overall thrusts cutting unlithified sediments in mtds display more variable displacement and more pronounced displacement gradients toward fault tips compared to thrusts cutting lithified sequences

superimposed variscan caledonian and proterozoic features inferred from deep seismic profiles recorded between southern ireland southwestern britain and western france abstract from southern ireland to the northern bay of biscay subduction was active from the proterozoic to the late palaeozoic they led to the cadomian caledonian and variscan collisions features characterizing these three orogenies have been crossed by the winch swat wam and bay of biscay deep seismic profiles recorded by the birps and ecors groups in the celtic sea the variscan front is characterized by a s dipping ramp rising directly from the middle crust with much less northward flattening than shown by the front in the northern france and west germany profiles south of the variscan front other s dipping reflections occur above a possible deep detachment the lizard ophiolitic suture marks s dipping events traversing the entire crust and the present moho in the english channel several sets of prominent n dipping reflections occurring in the celtic sea and south irish sea are interpreted as caledonian features running parallel to the caledonian iapetus suture and the pattern of folds and faults recognized in wales and southeastern ireland proterozoic s dipping subduction is recorded by geological data in northern wales and the english channel related crustal structures correspond to deep seismic events cutting across the lower crust and the moho and show the same apparent dip as the variscan features parallelism between dips and or strikes of structural features with different ages suggests a tectonic heritage the crust thus appears to be characterized by n and s dipping features which cut across large parts of it and may be assigned to variscan caledonian or candomian thrusts or shear zones the lower part of the crust shows a band of high amplitude subhorizontal reflections and a flat moho reflection which seem to have overprinted the above mentioned dipping features in a later period the lack of evidence of any shallow detachment in the caledonian and variscan belts together with the presence of dipping crustal thrust like features suggest that the region may have reached the very latest collisional stage the end of both of these orogenies

potential for southern hemisphere climate surprises climate model results suggest that future climate change in antarctica will be accompanied by continued strengthening and poleward contraction of the southern ocean westerly wind belt paleoclimate records suggest past changes in the westerly winds can be abrupt and that healing of the antarctic ozone hole could lead to poleward contraction of the westerlies and increased meridional atmospheric transport of warm air regionally into antarctica an abrupt shift to more meridional circulation could lead to notable changes in moisture availability for extra antarctic regions increased antarctic ice sheet disintegration and more rapid sea level rise copyright 2015 the authors journal of quaternary science published by john wiley sons ltd

the bude formation sw england a three dimensional intra formational variscan imbricate stack abstract the deformation of the upper carboniferous bude formation sw england is re interpreted in terms of a sedimentary multilayer sequence which represents a compromise between mechanically competent and mechanically incompetent behaviour the chevron folding that characterises the sandstone shale multilayer sequence is shown to be only a part of a more complex regional scale polyphase deformational history that developed at high structural levels in the late variscan orogeny the major tectonic features are thrust faults which are responsible for the imbrication of the original sedimentary pile six distinct thrust sheets are recognised in the 5 km coastal section studied northerly directed thrusts pre date chevron folding whilst structurally necessary southerly propagating back thrusts both pre and post date folding where they post date folding they impose a southerly shear onto the rocks leading to modification of existing folds the significance of these thrust structures has previously been ignored due to their tendency to develop parallel to bedding and to be overprinted by flexural slip features during chevron folding a model is presented that views the bude formation as a three dimensional intra formational imbricate stack which is at any single locality an instantaneous representation of an orogenic front propagating into its own foreland basin

zircon la icpms geochronology of the cornubian batholith sw england abstract available u pb age data for the cornubian batholith of sw england is based almost entirely on monazite and xenotime and very little zircon u pb age data has been published as a result no zircon inheritance data is available for the batholith by which the nature of the unexposed basement of the rhenohercynian zone in sw england might be constrained zircon la icpms data for the cornubian batholith provides concordia ages bodmin moor granite 316 4 ma carnmenellis granite 313 3 ma dartmoor granite 310 ma st austell granite 305 5 ma and land s end granite 300 5 ma that are consistently 20 30 ma older than previously published emplacement ages for the batholith and unrealistic in terms of geologic relative age relationships with respect to the country rock this discrepancy is likely as a consequence of minor pre granitic pb inheritance several of the batholith s granite plutons contain a component of late devonian inheritance that may record rift related lower crustal melting or arc related magmatism associated with subduction of the rheic ocean in addition the older granites likely contain mesoproterozoic inheritance although the highly discordant nature of the mesoproterozoic ages precludes their use in assigning an affinity to the rhenohercynian basement in sw england

anisotropic crosshole vsp traveltime tomography through triangular cell model with a normalized jacobian matrix and multistage inversion strategy to overcome the difficulty in forward and inversion problem in complex geological model including undulated topography and irregular subsurface interface we in this paper realize a multistage triangular shortest path method for multi phase seismic ray tracing and combine a nonlinear inversion solver damped minimum norm and constrained least squares problem solved by a conjugated gradient method to simultaneously invert four elastic parameters or four thomsen parameters in anisotropic media using combined direct and reflected arrival times for trading off between different parameter updating we normalize the sensitivity functions traveltimes derivatives with respect to different elastic or thomsen parameter changes to balance four updated elastic or thomsen parameters in simultaneous inversion process due to the magnitude and pattern of partial derivatives more sensitive to different phase slowness angles in addition in order to solve the multi solution problems of qsv wave inversion a strategy of multistage inversion is proposed for testing the efficiency and correctness of proposed inversion method we use the picked traveltimes in synthetic seismograms as the observed traveltimes in the inversion the results show that both inverted images by taking the picked traveltimes in synthetic seismograms as the observed data and by taking predicted traveltimes using ray tracing method as observed data have a similar imaging pictures which verify the efficiency and correctness of the proposed inversion method furthermore the inverted results by normalized kernel functions are better than that of inversion without applying the normalized kernel functions for both qp and qsv data and the results of the multistage inversion are better than that of the simultaneous inversion without multistage inversion applied in qsv wave inversion in addition both elastic and thomsen parameter inversions for the same anisotropic model indicate that there are no big differences between the two inverted results even though the sensitivity kernels related to the thomsen parameters have relatively large values

thin skinned tectonics in the northern rhenish massif germany a network of profiles in the hercynian mountain system geo traverse rhenoherzynikum is needed to determine details of the geological structure and find the reasons for tectonic movements outstanding features within the mountain system such as the large fault zone along the southern boundary of the hunsruck1 and the left flank of the rhinegraben have already been investigated by reflection seismology2 3 we describe here a new reflection refraction experiment carried out in 1978 in the most northern part of the rhenish massif across the stavelot venn massif near aachen fig 1 while layers in the lower crust dip to the nnw a strong upper reflector at 3 4 km depth shows a slight dip to the sse and indicates a prominent fault zone near aachen known in belgium as the faille du midi here a comparable and apparently identical reflector was found some 100 and 175 km to the west of our profile we interpreted this reflector as a prominent and well lubricated thrust fault along which a huge horizontal nappe displacement took place during the last stages of the variscan orogeny

a reappraisal of the facing confrontation in north cornwall fold or thrust dominated tectonics the problem of conflicting fold or thrust solutions to the interpretation of the structure of north cornwall is discussed via new detailed mapping of the coastal sections in addition to facing evidence shear criteria have been extensively applied in order to sustain new interpretations the facing confrontation at polzeath is confirmed but the high strain zone centred upon tintagel is ascribed to later superimposed northwards directed ductile shearing peak metamorphism was synchronous with this event subsequent northwards displacement became confined to discrete thrust planes dismembering the south facing structures which developed during an intermediate phase of backthrusting the major structures are thrust dominated though large recumbent folds were generated during the early ductile phases the facing confrontation is probably allochthonous having been carried northwards on a blind thrust it is suggested that the important rusey fault zone is the roof thrust to a triangle zone in which the lower grade rocks of the culm basin are uplifted over an underlying antiformal stack

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

emplacement of synkinematic plutons in the variscan odenwald germany controlled by transtensional tectonics in the odenwald area sw germany an intermediate level of variscan magmatic arc is exposed between 360 and 335 ma the neunkirchen intrusive complex intruded into an active ne sw trending zone of ductile divergent strike shear after the thermal peak of metamorphism but before the emplacement of the magmatic rocks shear zones developed in the wall rocks the microfabrics of these zones infer plastic deformation of feldspar hornblende and quartz and an increasing degree of postkinematic annealing towards the intrusive complex during emplacement and solidification the magmatic rocks experienced bulk strain the intrusive complex locally crosscut the shear zones of the wall rocks but show magmatic and also mylonitic foliations which parallel these zones a change from plastic to brittle behavior of feldspar and amphibole and a varying degree of postkinematic annealing of quartz suggest that in the intrusive complex strike shear continued during cooling in wall rocks and plutonic rocks kinematic indicators imply sinistral displacement in the wall rocks adjacent to the intrusive complex strike shear was combined with normal displacement this suggests that a component of horizontal extension which was associated with strike shear transtension facilitated ascend and emplacement of the magma transtension occurred at the same time when the schwarzwald vosges area in the south of the odenwald was subjected to nw se compression transtension was probably initiated by lateral escape of crustal fragments during overall convergent plate movements transtension is also supposed to be associated with significant uplift of metamorphic rocks

pan african structures ophiolites and melange in the eastem desert of egypt a traverse at 26 n a detailed structural traverse across the basement rocks of the eastern desert of egypt shows that they consist apart from intrusions of four broadly recumbent tectonic units the lowest of arkosic metasediments of continental shelf facies is exposed in a dome this unit is overlain by an allochthonous ophiolitic melange containing complete and dismembered ophiolitic masses in a matrix of deep oceanic graphitic pelites and turbidites a near horizontal schistose to mylonitic fabric most intense near and below the base of the melange indicates extensive lateral tectonic transport of the ophiolitic melange over the shelf metasediments but the melange originated as an olistostrome before being tectonically transported the melange extends over an area of at least 10 000 km 2 it is locally overlain by calc alkaline volcanics in which deformation is less intense but increases and schistosity flattens downwards indicating some translation over the melange unconformable on the melange and calc alkaline volcanics is a molasse facies series itself also locally strongly deformed late tectonic granites preceded and locally post date the molasse facies sediments still later diapiric peralkaline riebeckite granites locally up domed the recumbent structures the tectonic evolution is related to a late proterozoic subduction zone to the se of the region

upper palaeozoic successions and nappe structures in north cornwall for many years the geology of north cornwall has been interpreted in terms of a south facing overfold model which predicts a major confrontation at padstow with the north facing structures of south cornwall new evidence indicates the existence of contrasting successions separated by flat lying thrusts in upward succession the port isaac tredorn and boscastle nappes are identified the first appears to have been emplaced by gravity sliding in late visean times followed by the tredorn and boscastle nappes in the early namurian strata represented in the inverted limb of the port isaac nappe not previously distinguished are described as the jacket9s point slate formation throughout the nappe pile there is consistent evidence of north facing and northward transport for distances measured in tens of kilometres not more than 3 km of south facing strata may now exist at the padstow confrontation and this feature is unlikely to be of great regional significance the allochthon extends northwards to the rusey fault zone where the autochthon was locally backfolded over the allochthon in late westphalian times this appears to be the only significant confrontation of facing directions in north cornwall

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

the origin of the basse normandie duplex boulonnais france abstract two hercynian duplexes are developed in visean limestones in the basse normandie quarry the lower duplex is completely exposed in a subvertical quarry wall the partially exposed upper duplex lies immediately above the lower duplex the duplexes are both located in the footwall of the hydrequent thrust which emplaced devonian clastic sediments above the visean limestones the lower duplex exposes all the internal thrusts a reference bed of chalky limestone the roof and floor thrusts and the duplex tip the duplex has been graphically restored to its pre deformation geometry by line length and area balancing and its resultant geometry is close to the model of boyer elliott the lower duplex shortened by two different mechanisms an initial phase of layer parallel shortening which produced no cleavage followed by thrust imbrication the average contraction of the front portion of the duplex was 49 natural strain of which 27 is layer parallel shortening and 22 is thrust imbrication however locally the bulk shortening increases from zero at the duplex tip to over 120 in a down dip direction the area balancing provides the most accurate estimates of bulk shortening line length balance calculations give minimum estimates only an area balance on the whole of the lower duplex gives a bulk shortening of 84 an area balance of the upper duplex yields an average contraction of 75 and the total contraction produced by both duplexes is 92

recumbent folds key structural elements in orogenic belts abstract this review has two main parts the first of them presents existing ideas and data related to recumbent folds reviewing aspects such as the physical conditions of the development of these folds the strain inside the folded layers the kinematic mechanisms of their formation the role of gravitational forces the tectonic context of their development and the structures associated with them in the second part the above ideas are discussed and possible mechanisms for the development of these folds are presented it is proposed that initial perturbations of the layers are essential to give rise to the asymmetry of recumbent folds these perturbations may be non planarities of the layering or may be linked to the existence of a core or basement of competent rock that hinders the normal propagation of the deformation this could explain why many large recumbent folds have a root zone deformation with an important component of simple shear is a general condition for the formation of recumbent folds in areas with very low grade metamorphism competent layers often play an active role during the deformation and undergo buckling with the development of an overturned fold limb which can be stretched and thinned to finally produce a pair of recumbent folds separated by a thrust in areas with low or medium metamorphism buckling under a simple shear regime is probably the most important mechanism for producing large folds with gentle or moderately dipping axial surfaces subsequent kinematic amplification by coaxial strain components with vertical maximum shortening is important for the formation of recumbent folds these components involve a sub horizontal stretching that can cause a problem of strain compatibility and give rise to a basal thrust in areas deformed under high p and t conditions recumbent folds can develop by flow perturbations and kinematic amplification of folds this is probably a common mechanism in ductile shear zones

dynamic ray tracing in an anisotropic medium abstract we derive a dynamic ray tracing system for seismic waves in an anisotropic medium it consists of ten equations and can be solved numerically for isotropic media it does not reduce to the drt systems derived by other authors

new la icp ms cryocell and calibration technique for sub millimeter analysis of ice cores ice cores provide a robust reconstruction of past climate however development of timescales by annual layer counting essential to detailed climate reconstruction and interpretation on ice cores collected at low accumulation sites or in regions of compressed ice is problematic due to closely spaced layers ice core analysis by laser ablation inductively coupled plasma mass spectrometry la icp ms provides sub millimeter scale sampling resolution on the order of 100 m in this study and the low detection limits ng l 1 necessary to measure the chemical constituents preserved in ice cores we present a newly developed cryocell that can hold a 1m long section of ice core and an alternative strategy for calibration using ice core samples from central greenland we demonstrate the repeatability of multiple ablation passes highlight the improved sampling resolution verify the calibration technique and identify annual layers in the chemical profile in a deep section of an ice core where annual layers have not previously been identified using chemistry in addition using sections of cores from the swiss italian alps we illustrate the relationship between ca na and fe and particle concentration and conductivity and validate the la icp ms ca profile through a direct comparison with continuous flow analysis results

interpretation of recent southern hemisphere climate change climate variability in the high latitude southern hemisphere sh is dominated by the sh annular mode a large scale pattern of variability characterized by fluctuations in the strength of the circumpolar vortex we present evidence that recent trends in the sh tropospheric circulation can be interpreted as a bias toward the high index polarity of this pattern with stronger westerly flow encircling the polar cap it is argued that the largest and most significant tropospheric trends can be traced to recent trends in the lower stratospheric polar vortex which are due largely to photochemical ozone losses during the summer fall season the trend toward stronger circumpolar flow has contributed substantially to the observed warming over the antarctic peninsula and patagonia and to the cooling over eastern antarctica and the antarctic plateau

geometry and thermal regime of the king george island ice cap antarctica from gpr and gps king george island is the largest of the south shetland islands close to the tip of the antarctic peninsula the annual mean temperature on the island has increased by 1 c during the past three decades and the ice cap that covers the majority of the island is sensitive to climatic change we present data from two field campaigns 1997 and 2007 700 km of global positioning system gps and ground penetrating radar gpr profiles were collected on arctowski icefield and on the adjacent central part the data were analysed to determine the surface and bed topography and the thermal regime of the ice average ice thickness is 250 m and maximum thickness is 420 m the gpr profiles show isochrones throughout the ice cap which depict the uparching of raymond bumps beneath or close to the ice divides a water table from percolation of meltwater in the snowpack shows the firn ice boundary at 35 m depth the firn layer may be temperate due to the release of latent heat in the area below 400 m a s l backscatter by water inclusions is abundant for ice depths below the water table we interpret this as evidence for temperate ice scatter decreases significantly above 400 m ice temperatures below the water table in this part of the ice cap are subject to further field and modelling investigations

vertical seismic profile synthetics by dynamic ray tracing in laterally varying layered anisotropic structures dynamic ray tracing drt is important in evaluating high frequency seismic wave fields in complicated structures two formulations of the drt equations for inhomogeneous anisotropic media are presented one of them is represented by the classical drt system suggested by cerven 15 years ago both systems are specified in cartesian coordinates the drt equations are supplemented with formulae for the transformation of drt across a smoothly curved interface between two inhomogeneous anisotropic media cerven s formulation of the drt is applied to the computation of vertical seismic profile vsp synthetics the results of drt are used for the evaluation of the geometrical spreading and of the coefficients of the paraxial ray approximation for travel times and ray amplitudes in addition the drt is also used in the interval ray tracing procedure a procedure searching for rays starling from the source and terminating in a specified interval on a profile results of numerical modeling of vsp measurements in a three dimensional laterally varying structure consisting of isotropic and anisotropic layers separated by curved interfaces are presented ray diagrams of selected elementary waves time distance curves and multisource three component vsp synthetics generated for two different source locations calculated for the anisotropic model are compared with the results determined for a reference isotropic model the latter is obtained by averaging phase velocities of the anisotropic model this comparison clearly shows the effects of anisotropy and the lateral variation of the model on seismic wave fields the reliability of the ray synthetics is briefly discussed

deformation around a thrust tip in carboniferous limestone at tutt head near swansea south wales abstract the tutt head thrust zone is the tip of a forward propagating lateral ramp which links two levels of detachment within the langland mumbles anticline an early pressure solution accommodated cataclastic phase of deformation is partially overprinted by later plastic deformation involving dislocation glide dislocation climb and grain boundary sliding mechanisms two and three dimensional strain determinations indicate that displacement reduction in the tip zone is primarily accommodated by footwall extension with subsidiary contribution from hangingwall contraction localized oblique movement in the tip zone is explained by the process of tip line pinning geometrical considerations reveal the requirement for diachronous emergence of the tip line from an obstacle to propagation this enables differential displacement about a pinning point and the consequent generation of shear strain coplanar with the thrust plane such shear strains are necessarily localized and limited in magnitude the stress pattern around the thrust tip is compatible with that predicted by the elastic solution to the stress distribution around a mode ii shear crack the observation of a recognizable stress concentration suggests that quite high stress concentrations may accompany thrust tip propagation

is the water still hot sustainability and the thermal springs at bath england the hot springs at bath are the largest natural thermal source in britain sustainable use of the waters for a spa requires maintenance of their temperature and flow rate together with smaller springs at hotwells bristol they form the outflow from a regional thermal aquifer that occurs where the carboniferous limestone is buried at depths 2 7 km in the bristol bath structural basin the aquifer is recharged via limestone outcrops forming the south and west portions of the basin rim current knowledge of the basin s structure is reviewed and important uncertainties identified concerning the hydrogeological role of thrust faults which may cut the limestone at depth a simple numerical model is used to determine the possible influence of thrusts upon groundwater flow within the thermal aquifer comparison of the modelled flow patterns with geochemical data and structure contours eliminates the hypothesis that thrusts completely disrupt the continuity of the aquifer the most successful model is used to simulate the possible impact of dewatering by large quarries at the limestone outcrops north and south of bath substantial reductions in modelled flow at bath result from proposed dewatering in the eastern mendips although the steady state approach adopted has severe limitations in that it does not take account of the incremental staging of actual dewatering nor allow for partial restitution of groundwater levels the geological uncertainties highlighted by the modelling could be addressed by future research into the effect of thrusts on the continuity of the carboniferous limestone more refined modelling to predict the timing of possible impacts of quarry dewatering will require measurements of the storativity of the thermal aquifer

major southward thrusting of the dalradian rocks of connemara western ireland the connemara schists are of dalradian late precambrian cambrian age and since the work of kilburn et al 1 it has been apparentthat they occupy an anomalous geographical position south southeast of the strike continuation of the dalradian rocks of scotland and ireland and south of the inferred position of the highland boundary fault fig 1 also the connemara schists are unusual in having reached a higher metamorphic grade and having had a different metamorphic history from most other parts of the dalradian the model which we propose here for the connemara dalradian that of an allochthonous thrust sheet offers an explanation for these anomalies and highlights hitherto unsuspected similarities between the tectonic histories of the south east margins of the metamorphic caledonides in the british isles and scandinavia

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

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

variscan oceans ages origins and geodynamic implications inferred from geochemical and radiometric data abstract a review of the magmatic associations providing evidence for the existence of extensional regimes during the palaeozoic evolution of variscan europe is presented the available geochronological data allow recognition of two major rifting events one in the early ordovician and the other in the middle late devonian scarce relicts of ophiolitic complexes indicate that both extensional episodes reached a true ocean spreading stage it is suggested that the variscan belt resulted from a complex two stage collision process through successive suturing in the silurian and the early carboniferous of these oceanic domains the superposition of two distinct opening closure cycles involving geographically separate oceanic basins may partly account for the unusual width of the variscan orogen

initial reconnaissance for a south georgia ice core we present the first snow ice chemistry and ice radar results ever collected from south georgia as part of an initial reconnaissance with the ultimate goal of assessing the feasibility of a south georgia ice core to reconstruct past climate in the south atlantic south georgia is well situated to capture a record of past atmospheric chemical composition over the south atlantic and of past variability in the position and intensity of the austral westerlies the question is how well preserved an ice core record can be recovered from a region experiencing accelerated melting the results presented in this paper offer only a preliminary step in determining the feasibility of future deep ice coring on south georgia however this initial reconnaissance does provide some basic information including the chemistry of the atmosphere over south georgia relative to other southern hemisphere ice coring sites the potential for preservation of annual layers in old ice on the island a possible age for deep ice in the region and an estimate of glacier health in the lower elevation regions of the island

seismic attenuation in the continental crust sw of england summary significant variability in crustal attenuation exists off england s sw coast as shown by birps deep seismic reflection lines the presence of the thick cornubian batholith with no sedimentary cover acting as a low attenuating region compared to normal upper crust with about 300 m of sedimentary cover can account for the observed differences in the strength of signal returning from the lower crust application of the spectral amplitude ratio method to determine the effective quality factor q of the lower crust beneath the granite yields a value of 400 200 a modification of the spectral amplitude ratio method to measure relative q s at different locations indicates that the average q of the upper and middle crust containing the granite is twice that elsewhere although the hypothesis that the lower crustal composition differs significantly beneath the granite cannot be ruled out geological and seismic observations suggest that differential attenuation is the main factor causing variation in the observed reflectivity thus thick granites act as low attenuating windows to the lower crust

historically unprecedented global glacier decline in the early 21st century observations show that glaciers around the world are in retreat and losing mass internationally coordinated for over a century glacier monitoring activities provide an unprecedented dataset of glacier observations from ground air and space glacier studies generally select specific parts of these datasets to obtain optimal assessments of the mass balance data relating to the impact that glaciers exercise on global sea level fluctuations or on regional runoff in this study we provide an overview and analysis of the main observational datasets compiled by the world glacier monitoring service wgms the dataset on glacier front variations 42000 since 1600 delivers clear evidence that centennial glacier retreat is a global phenomenon intermittent readvance periods at regional and decadal scale are normally restricted to a subsample of glaciers and have not come close to achieving the maximum positions of the little ice age or holocene glaciological and geodetic observations 5200since 1850 show that the rates of early 21st century mass loss are without precedent on a global scale at least for the time period observed and probably also for recorded history as indicated also in reconstructions from written and illustrated documents this strong imbalance implies that glaciers in many regions will very likely suffer further ice loss even if climate remains stable

late magmatic immiscibility during batholith formation assessment of b isotopes and trace elements in tourmaline from the land s end granite sw england quartz tourmaline orbicules are unevenly distributed in the roof segment of the land s end granite sw england this study shows that the orbicules formed from an immiscible hydrous borosilicate melt produced during the late stages of crystallization and differentiates tourmaline formed by dominantly magmatic and dominantly hydrothermal processes trace elements and boron isotope fractionation can be tracked in tourmaline and create a timeline for crystallization tourmaline from the granite matrix has higher v cr and mg content and is isotopically heavier than the later crystallizing inner orbicule tourmaline overgrowths of blue tourmaline occurring together with quartz showing hydrothermal cathodoluminescence textures crystallized from an aqueous fluid during the very last crystallization and are significantly higher in sr and sn and isotopically heavier tourmaline associated with sn mineralization is also high in sr and sn but has boron isotopic compositions close to that of the magmatic tourmaline and is not formed by the same fluids responsible for the blue overgrowths the ore forming fluids precipitating tourmaline and cassiterite are likely derived from the same magma source as the granite but exsolved deeper in the magma chamber and at a later stage than orbicule formation tourmaline from massive quartz tourmaline rocks is concentrically zoned with major and trace element compositions indicating crystallization from a similar melt as for the orbicules but shows a more evolved signature