sequence stratigraphy of the palaeozoic welsh basin the timing and extent of unconformities in the welsh basin are investigated using rock preservation curves derived from outcrop stratigraphic logs four basin wide unconformities occur focussed in late precambrian late tremadoc pusgillian early ashgill and mid devonian times these bound three megasequences equating with newly defined lithostratigraphic units the dyfed gwynedd and powys supergroups less extensive unconformities bound 18 component sequences the majority of the sequence boundaries reflect a component of tectonic or volcanotectonic activity rather than a pure eustatic sea level change the megasequence boundaries are attributed to late precambrian to early cambrian onset of rifting to form a passive margin tremadoc onset of subduction with intra arc then back arc extension late caradoc end to subduction and late early devonian collisional deformation the megasequences and controlling events can be tentatively matched with other basins on the avalonian margin more generally this study shows that sequence analysis is feasible in onshore basins lacking well and seismic data and that the global eustatic interpretation of sequence stratigraphy is only partially applicable to active margin basins

mylonitic mafic granulite in fault megabreccia at clarke head nova scotia a sample of avalonian lower crust the clarke head fault megabreccia was produced within a major late palaeozoic transcurrent structure the minas fault system which separates the displaced meguma terrane of southern nova scotia from rocks more typical of the avalon superterrane a large clast of anomalously high grade metabasite embedded in the clay matrix of the fault megabreccia displays a fresh granulite facies mineralogy 2 pyroxene garnet plagioclase and mylonitic to ultramylonitic textures induced by anhydrous shearing deep in the roots of the fault zone whole rock geochemistry reveals the granulite protolith to have been a continental within plate mafic magma transitional between theoleiitic and alkaline the original geochemical signature has survived strong dynamic recrystallization at granulite grade well preserved ree abundances testify to a lack of metasomatic fractionation during high grade shearing under water absent conditions sm nd data indicate that the basic granulite has a t dm age of c 1 ga isotopic comparisons with adjacent areas reveal similar t dm ages both north and south of the minas fault system the high grade clast may be typical of the lower crust in nova scotia and is thought to offer a rare window into the deep crust of the avalon superterrane in north america

discussion on dating the late precambrian volcanicity of england and wales whilst new data constraining the evolution of the avalonian superterrane in southern britain are most welcome the conclusion that compston et al 2002 draw from this data raise some points that merit comment firstly the conclusion that the new age data for the arfon group enables a redefining of the sarn complex terrane affinity is questioned in particular the statement it also now seems unlikely that the sarn complex should be compared with the padarn area as the parwyd gneiss has yielded a rb sr date of 542 17 ma beckinsale et al 1984 this is not now regarded as the age of the amphibolite facies metamorphism horak et al 1996 but must indicate a considerable retrogression event in the gneissose basement in late precambrian cambrian time does not appear to have any relevance to any of the data presented the parwyd gneiss is represented by a single exposure of retrogressed garnet amphibolite and felsic gneiss which lies entirely within the ll n shear zone although previously included within the sarn complex recent work has defined this as a separate unit of uncertain affinity e g gibbons horak 1990 gibbons mccarroll 1993 in contrast to the gneisses the sarn complex is a poorly exposed heterogeneous plutonic complex of which the most northwestern margin has been involved in ductile and brittle deformation the rb sr isotopic data age for the parwyd gneisses has no bearing on the affinity or age of the sarn complex and furthermore the published u pb age of 615 2 ma for a gabbro not granite as stated by compston et al 2002 from the complex horak

menai strait fault system an early caledonian terrane boundary in north wales a steep fault system runs parallel to the menai strait which separates anglesey from the mainland of north wales the menai strait fault system is interpreted as a major terrane boundary along which terrane docking took place during the late precambrian or earliest cambrian the evidence is based on 1 marked geologic dissimilarities exhibited by the pre late early cambrian basement to these terranes and 2 the existence of early major ductile shear zones within the fault system rocks as varied as blueschists low grade melange and shallow level granitic complexes were juxtaposed along these steep shear zones prior to the eruption of ash flow tuffs and deposition of upper lower cambrian marine sediments in the welsh basin since the amalgamation of the anglesey area with the avalonian terrane then a part of gondwana frequent phanerozoic reactivation of the resulting composite terrane boundary has been restricted to mostly dip slip fault movements

high pressure elastic anisotropy of mgsio3 perovskite and geophysical implications using plane wave pseudopotential method within the local density approximation lda we calculate single crystal elastic constants cij of orthorhombic mgsio3 perovskite generally accepted to be the major component of the lower mantle as a function of pressure up to 150 gpa our results are in excellent agreement with experimental data at zero pressure and compare favorably with other pseudopotential predictions over the pressure regime studied here we use our elastic constants to calculate anisotropy of seismic wave velocities as a function of pressure depth mgsio3 perovskite is shown to be highly anisotropic in all portions of the lower mantle and the nature of anisotropy changes significantly with depth the absence of significant seismic anisotropy in most of the lower mantle suggests that mgsio3 perovskite assumes nearly random orientation in most of this region anisotropy at the topmost lower mantle suggested by some studies can be attributed to the preferred orientation of perovskite however anisotropy in the d layer is difficult to be attributed to preferred orientation of perovskite some other mechanisms including the presence of the aligned melt pockets and or lattice preferred orientation of magnesiowustite are needed

is a cation ordering transition of the mg fe olivine phase in the mantle responsible for the shallow mantle seismic discontinuity beneath the indian craton 1 we use first principles molecular dynamics simulations to study the behavior of cation ordering in the non equivalent octahedral sites of mg fe olivine solid solutions our theoretical calculations confirm the previous experimental finding that mg2 and fe2 can invert their octahedral site occupancy at a critical temperature assuming that the site preference of fe changes discontinuously between two states in which it is completely restricted to either m1 or m2 sites we have calculated the transition temperature tt between the two extreme states under ambient pressure tt is calculated to be 520 c that agrees fairly with the experimental finding in which however the ordering state changed discontinuously over a much smaller range of the site occupancy of fe tt is found to be pressure sensitive showing an increase by 30 to 100 c per unit gpa depending upon the iron content using the indian continental geotherm we estimate a depth of around 75 km corresponding to the calculated transition pressure and temperature of cation ordering which matches well with the depth for the hales discontinuity marked by a jump of shear wave velocity by 4 for olivine solid solutions with 12 5 iron the ordering transition increases vs from 4 5 to 4 7 km s both the inferences viz depth of discontinuity and magnitude of velocity increase find support from the modeling of teleseismic earthquake waveforms recorded over broadband seismographs on the dharwar craton this leads us to infer that the cation ordering transition in ferromagnesian olivine might be a potential factor for the hales discontinuity

palaeomagnetic analysis of fault gouge and dating fault movement anglesey north wales the palaeomagnetic properties and magnetic fabric of clay rich fault gouge in a prominent fault at porth y pistyll on the northern coast of anglesey north wales have been investigated in an attempt to constrain the age of last movement on this fault the texture and mineralogy of the gouge has been examined by scanning electron microscopy the porth y pistyll fault is one of a set of e w trending faults that cut metasedimentary rocks of the late precambrian early cambrian monian supergroup on the island of anglesey a high coercivity component of magnetization with a reverse polarity has been identified in the gouge this is believed to be a chemical remanent magnetization resulting from fluid movement and oxidation which post dated the last major movement on the fault this strongly suggests that movement has not occurred during the present normal polarity chron thereby establishing a minimum age of 0 73 ma for the last movement on the fault a well defined planar magnetic fabric is present in the gouge and this corresponds in geometric orientation to a planar tectonic fabric observed from sem studies these fabric data provide additional information on the kinematics of motion on the fault

displaced terranes in britain and ireland a meeting entitled displaced terranes in britain and ireland was held at burlington house on 2 november 1988 this was the third terranes meeting held in britain since the inception of international geological correlation project no 233 terranes in the circum atlantic palaeozoic orogens the others having been held at cardiff in 1986 and glasgow in 1987 igcp 233 grew out of igcp 27 the caledonide orogen many of the developments promoted by which are recorded within two special publications of the geological society harris et al 1979 harris fettes1988 over the last few years research on displaced terranes in the british caledonides has accelerated with new data now challenging what have become traditional applications of plate tectonic theory to the british caledonides the reports of work in progress on caledonian terranes and it was thought appropriate to take advantage of the rapid publication facilities afforded by the short papers scheme because of the four page limit on short papers much introductory material has had to be omitted so that we provide here an introduction to the concept of displaced terranes and its applications to british palaeozoic geology displaced terranes the essential value of the terrane concept lies in its recognition of the likelihood of large scale lateral fault movements occurring along active plate margins the concept of terrane tectonics is therefore no more than a fuller appreciation of the kinematic implications of plate tectonic theory the impact of this recognition upon our perception of geotectonic processes in general and the

geotectonic framework of the blueschist unit on anglesey lleyn uk and its role in the development of a neoproterozoic accretionary orogen abstract a 560 550 ma 5 km 25 km blueschist unit extends ne sw on the island of anglesey wales uk and continues to the southwest for more than 70 km along the northern coast of the lleyn peninsula it has the shape of a shallow dipping slab or sheet up to a few kilometres thick that was exhumed from the subduction zone and emplaced into the accretionary complex to the northwest today the top boundary of the slab is commonly a thrust that was originally a low angle normal fault at its top in the subduction zone above it is an accretionary complex and a unit of high grade gneisses the bottom boundary is today a thrust that was originally a thrust below which is a unit of low grade or unmetamorphosed rocks belonging to an olistostrome type accretionary complex this thrust was originally at the bottom of the slab when it was exhumed in the subduction zone the sandwiched assemblage was created by the tectonic insertion of the blueschist between the other units folding particularly during late doming of the tectonic sandwich rotated the top normal fault boundary into a thrust later high angle ne sw trending normal faults displaced the tectonic stratigraphy in horst and graben a 200 km long and 200 km wide 680 450 ma calc alkaline volcano plutonic belt extends southeastwards from the blueschist belt via northern wales to central england suggesting that the subduction of oceanic lithosphere was to the southeast and this is consistent with the extrusion of the thin high pressure slab into the accretionary complex to the northwest this took place on the western accretionary margin of avalonia finally we outline the key steps involved in unravelling the structure of a pacific type accretionary complex

spin states and hyperfine interactions of iron incorporated in mgsio3 post perovskite abstract using density functional theory hubbard u dft u calculations we investigate the spin states and nuclear hyperfine interactions of iron incorporated in magnesium silicate mgsio 3 post perovskite ppv a major mineral phase in the earth s d layer where the pressure ranges from 120 to 135 gpa in this pressure range ferrous iron fe 2 substituting for magnesium at the dodecahedral a site remains in the high spin hs state intermediate spin is and low spin ls states are highly unfavorable as to ferric iron fe 3 which substitutes magnesium at the a site and silicon at the octahedral b site to form mg fe si fe o 3 ppv we find the combination of hs fe 3 at the a site and ls fe 3 at the b site the most favorable neither a site nor b site fe 3 undergoes a spin state crossover in the d pressure range the computed iron quadrupole splittings are consistent with those observed in mossbauer spectra the effects of fe 2 and fe 3 on the equation of state of ppv are found nearly identical expanding the unit cell volume while barely affecting the bulk modulus

interpretation of the regional gravity and magnetic surveys of wales using shaded relief and euler deconvolution techniques regional gravity and aeromagnetic data of wales have been processed using a variety of techniques image processing has greatly assisted qualitative interpretation whilst automated procedures have provided additional quantitative information the shaded relief images emphasize gradients in the potential fields and are useful for displaying strong linear features the euler deconvolution method produces plotted solution maps which define the position of the source of the gravity and magnetic anomalies euler solution maps of wales and the adjacent continental shelf are presented for the first time these maps are interpreted in relation to the known geology with special emphasis on the lower palaeozoic welsh basin it is proposed that the euler solutions define a network of fault bounded blocks within the precambrian basement

palaeogene dyke swarm nw wales evidence for cenozoic sinistral fault movement the berw and aber dinlle faults represent the brittle manifestation of a major lineament the menai strait fault system which is known to have been active intermittently since late precambrian times documentation of historical seismicity in this area provides information on the most recent activity along this structure although there is a paucity of field evidence to support conclusively cenozoic or later movements this work documents data from three separate aeromagnetic datasets combined with field relationships which reveal a well defined cenozoic dyke swarm traversing anglesey and snowdonia showing clear sinistral offset up to 1 5 km along the berw fault the most northwesterly component of the menai strait fault system these data provide conclusive evidence of cenozoic

reconstruction of ocean plate stratigraphy in the gwna group nw wales implications for the subduction accretion process of a latest proterozoic trench forearc abstract the gwna group in anglesey island and lleyn peninsula wales consists of a latest proterozoic volcano sedimentary trench melange which has a complicated accretionary structure and is poorly constrained by isotopic ages the melange contains oceanic trench rocks including pillow basalts cherts mudstones and sandstones which have not previously been interpreted as ocean plate stratigraphy ops we reconstructed imbricated ops at 5 localities in the coastal lleyn peninsula in order to constrain the depositional u pb age of the upper clastic sediments detrital zircons separated from 9 clastic sediments were analyzed with a nu attom single collector inductively coupled plasma mass spectrometer the ages indicate that there are two gwna groups maximum depositional ages of 1 at 608 601 ma and 2 at 564 539 ma that were deposited between the late neoproterozoic and the middle cambrian contemporaneously with dated calc alkaline arc magmatism and regional metamorphism in the anglesey lleyn complex the age spectra of the detrital zircons show a prominent peak at ca 650 600 ma and several proterozoic and archean ages to account for the older ages we integrated our new isotopic data with published radiometric and fossil ages and conclude that the clastic sediments at the top of the ops were deposited in a trench on the western active margin of avalonia when it was close to the amazonian craton and that the gwna group ops began to be incorporated into an accretionary wedge in an active subduction zone in the latest proterozoic

fundamentals of elasticity of mg1 x fex 2sio4 olivine we study the influence of iron on the elasticity of mg1 x fex 2sio4 olivine 0 x 0 125 a major constituent of the earth s upper mantle we calculate static elastic properties by first principles for this solid solution and investigate the effect of atomic arrangement an artifact of supercell calculations on all single crystal and poly crystalline elastic moduli from calculated wave propagation velocities we find the heterogeneity ratios of shear to compressional wave velocity and bulk sound to shear wave velocity their values are though limited to composition considerations marginally consistent with seismic tomography

elastic constants and anisotropy of forsterite at high pressure we have determined from first principles the athermal elastic constant tensor of mg2sio4 forsterite with the plane wave pseudopotential method over a wide range of pressure 0 100 gpa that encompasses the full range over which forsterite has been observed experimentally the computed elastic constants are in excellent agreement with experimental data up to the maximum pressure of the experiments 16 gpa we calculate the single crystal elastic anisotropy from the elastic constants we find that the anisotropy is strong azimuthal p and s wave anisotropy 25 and 20 respectively polarization anisotropy 15 in agreement with experiment and that it depends weakly on pressure over the range 0 25 gpa in contrast to the behavior of other silicates and oxides

aeromagnetic data in the uk a study of the information content of baseline and modern surveys across anglesey north wales a number of modern multiparameter high resolution airborne geophysical surveys termed hires have been conducted over the past decade across onshore uk these were undertaken in part as a response to the limited resolution of the existing uk national baseline magnetic survey data set acquired in the late 1950s and early 1960s modern magnetic survey data obtained with higher precision and reduced line spacing and elevation provide an improved data set however the distinctions between the two available resources existing and new are rarely quantified in this contribution we demonstrate and quantify the improvements that can be anticipated using the new data the information content of the data sets is examined using a series of modern processing and modelling procedures that provide a full assessment of their resolution capabilities the framework for the study involves two components the first relates to the definition of the shallow magnetic structure in relation to an ongoing 1 10 k and 1 50 k geological map revision the second component relates to the performance of the datasets in defining maps of magnetic basement and assisting with larger scale geological and structural interpretation one of the smaller hires survey areas the island of anglesey ynys mon off the coast of nw wales is used to provide a series of comparative studies the geological setting here is both complex and debated and cultural interference is prevalent in the low altitude modern survey data it is demonstrated that successful processing and interpretation can be carried out on data that have not been systematically corrected decultured for non geological perturbations across the survey area a wide number of near surface magnetic features are evident and are dominated by a reversely magnetized palaeogene dyke swarm that extends offshore the average depth to the upper surfaces of the dykes is found to be 44 m the existing baseline data are necessarily limited in resolving features 3 km to upper surface assessments of magnetic basement this further assessment is successfully provided by the larger scale baseline data which locates and defines a mid crustal magnetic basement feature centred beneath the snowdon massif and illustrates that basement of similar characteristic extends beneath much of anglesey

terrane assemblage of the leinster massif se ireland during the lower palaeozoic compilation of irish geological survey mapping is used to construct a synthesis of the tectonic evolution of the leinster massif during the lower palaeozoic the massif is interpreted as a series of ne sw trending tectonostratigraphic terranes of diverse origin and provenance brought together along major sinistral transcurrent faults with minor discrepancies the age of these terranes decreases from se to nw the precambrian basement rosslare terrane is succeeded by the early cambrian continental margin cullenstown cahore terrane in the waterford wicklow terrane late cambrian early ordovician accreted ocean floor materials are overlain by a late ordovician volcanic arc the early to mid ordovician accreted ocean floor dublin terrane to the nw is overthrust by the early cambrian continental margin bray terrane and is followed to the nw by the mainly silurian midlands terrane these terranes were juxtaposed after substantial sinistral movement along transcurrent faults during the silurian consolidation of the massif also took place from se to nw stages in the consolidation are marked by the intrusion of the saltees 437 ma carrigmore 415 ma and leinster 404 ma stitching plutons fault movement ceased before the deposition of late devonian and carboniferous sediments on a linking unconformity across the massif

a discussion of monian supergroup stratigraphy in northwest wales new borehole data and trench exposures in northern anglesey show the gwna group melange resting directly upon disrupted sedimentary rocks of the new harbour group field evidence from anglesey and the ll n peninsula shows that units previously given formal lithostratigraphic status in the higer part of the monian sopergroup occur as clasts within the gwna group melange me wbdivision of the monian supergroup is therefore simplified to comprise only three groups the south stack lowest new har bour and gwna groups highest the disruption of the upper part of the new harbour group is interpreted to have been efferied during emplacement of the gwna group melange

pressure sensitivity of olivine slip systems and seismic anisotropy of earth s upper mantle the mineral olivine dominates the composition of the earth s upper mantle and hence controls its mechanical behaviour and seismic anisotropy experiments at high temperature and moderate pressure and extensive data on naturally deformed mantle rocks have led to the conclusion that olivine at upper mantle conditions deforms essentially by dislocation creep with dominant 100 slip the resulting crystal preferred orientation has been used extensively to explain the strong seismic anisotropy observed down to 250 km depth the rapid decrease of anisotropy below this depth has been interpreted as marking the transition from dislocation to diffusion creep in the upper mantle but new high pressure experiments suggest that dislocation creep also dominates in the lower part of the upper mantle but with a different slip direction here we show that this high pressure dislocation creep produces crystal preferred orientations resulting in extremely low seismic anisotropy consistent with seismological observations below 250 km depth these results raise new questions about the mechanical state of the lower part of the upper mantle and its coupling with layers both above and below

basement fractures in north wales their recognition and control on caledonian deformation a detailed survey of the patterns of regional deformation in north wales has revealed the presence of several discrete linear zones of anomalous deformation these zones are distinguished by atypical finite strain values intense tight to isoclinal folding non axial planar cleavages and high metamorphic gradients a history of prolonged tectonic activity along these zones is indicated by their influence on sedimentation and volcanism it is proposed that these zones reflect the deformation of the lower palaeozoic cover above pre existing basement fractures these basement fractures form a framework which divides north wales into a series of blocks and sub blocks the end caledonian deformation is explained by a transpression model dominated by se directed shear with minor components of strike slip localized in these zones the application of this model to other parts of the welsh basin is briefly considered

brillouin scattering and x ray diffraction of san carlos olivine direct pressure determination to 32 gpa the single crystal elastic moduli of san carlos olivine mg 0 9fe0 1 2sio4 were determined at seven pressures between 2 5 and 32 5 gpa by brillouin spectroscopy in a diamond anvil cell the unit cell volume was also determined at each pressure and ambient pressure by single crystal x ray diffraction the shear elastic moduli exhibit non linear pressure dependencies in particular a non linear dependence is required for the elastic modulus c55 above 10 gpa aggregate bulk and shear moduli were determined from hashin shtrikman averages of the individual constants pressures were determined by fitting the bulk modulus reuss bound and volume data to a third order eulerian finite strain equation and integrating the results are compared to independent pressure measurements made using the ruby fluorescence scale the ruby measurements yield pressures that are in good agreement with the brillouin results on average the ruby fluorescence measurements produce pressures that are larger than the brillouin determinations by 1 6 1998 elsevier science b v all rights reserved

chitinozoan reflectance in the lower palaeozoic of the welsh basin vitrinite woody matter and spores are generally uncommon or absent in pre devonian sediments to determine organic maturity in the lower palaeozoic alternative methods such as chitinozoan reflectance must be employed chitinozoa are marine vase shaped organic walled microfossils of unknown affinity the reflectance of incident light from polished chitinozoa walls can provide reliable and accurate maturity data and due to their relative facies independency a high degree of resolution to regional maturity studies calibration of the chitinozoan reflectance scale with vitrinite reflectance enables palaeotemperature estimates to be made from pre devonian sediments the range of chitinozoan reflectance for the welsh basin is from under 1 to 8 this corresponds to estimated palaeotemperatures of under 100 c to over 300 c the regional spatial distribution of organic maturation recorded by chitinozoan reflectance in the welsh basin is attributed to palaeobasin form the increase in reflectance with age in the rocks of the myddfai steep belt and associated areas allows an estimation of a palaeogeothermal gradient in the silurian sequences around llandovery the gradient is calculated to be in the region of 50 c km 1 similar gradients are derived from other studies clay minerals metabasite mineral assemblages conodont colours the results from myddfai steep belt suggest the thermal peak occurred during the acadian event chitinozoan reflectivity demonstrates that with respect to hydrocarbon generation mature conditions occur on the palaeoplatform where the sedimentary sequences are relatively thin overmature conditions are prevalent in the palaeobasin where there is a thick sedimentary pile which has been subsequently deformed

high pressure elastic properties of major materials of earth s mantle from first principles the elasticity of materials is important for our understanding of processes ranging from brittle failure to flexure to the propagation of elastic waves seismologically revealed structure of the earth s mantle including the radial one dimensional profile lateral heterogeneity and anisotropy are determined largely by the elasticity of the materials that make up this region despite its importance to geophysics our knowledge of the elasticity of potentially relevant mineral phases at conditions typical of the earth s mantle is still limited measuring the elastic constants at elevated pressure temperature conditions in the laboratory remains a major challenge over the past several years another approach has been developed based on first principles quantum mechanical theory first principles calculations provide the ideal complement to the laboratory approach because they require no input from experiment that is there are no free parameters in the theory such calculations have true predictive power and can supply critical information including that which is difficult to measure experimentally a review of high pressure theoretical studies of major mantle phases shows a wide diversity of elastic behavior among important tetrahedrally and octahedrally coordinated mg and ca silicates and mg ca al and si oxides this is particularly apparent in the acoustic anisotropy which is essential for understanding the relationship between seismically observed anisotropy and mantle flow the acoustic anisotropy of the phases studied varies from zero to more than 50 and is found to depend on pressure strongly and in some cases nonmonotonically for example the anisotropy in mgo decreases with pressure up to 15 gpa before increasing upon further compression reaching 50 at a pressure of 130 gpa compression also has a strong effect on the elasticity through pressure induced phase transitions in several systems for example the transition from stishovite to cacl2 structure in silica is accompanied by a discontinuous change in the shear s wave velocity that is so large 60 that it may be observable seismologically unifying patterns emerge as well eulerian finite strain theory is found to provide a good description of the pressure dependence of the elastic constants for most phases this is in contrast to an evaluation of birch s law which shows that this systematic accounts only roughly for the effect of pressure composition and structure on the longitudinal p wave velocity the growing body of theoretical work now allows a detailed comparison with seismological observations the athermal elastic wave velocities of most important mantle phases are found to be higher than the seismic wave velocities of the mantle by amounts that are consistent with the anticipated effects of temperature and iron content on the p and s wave velocities of the phases studied an examination of future directions focuses on strategies for extending first principles studies to more challenging but geophysically relevant situations such as solid solutions high temperature conditions and mineral composites

sedimentology structural evolution and tectonic setting of the late precambrian longmyndian supergroup of the welsh borderland uk abstract the longmyndian supergroup is an approximately 6500 m thick sedimentary sequence of late precambrian age major unconformities within this sequence are lacking the longmyndian supergroup can be divided into four facies associations 1 turbidite 2 subaqueous delta 3 alluvial floodplain and 4 braided alluvial these are organized into a single broadly upwards coarsening progradational sequence with the turbidite association at the base and the braided alluvial association towards the top the sediments are interpreted to have been deposited in a forearc or possibly interarc basin which lay to the north of a contemporaneous magmatic arc of which the now adjacent uriconian volcanic complex is considered to have been a part the longmyndian supergroup has been deformed into a tight nne ssw trending syncline fault patterns and strain markers within the longmyndian indicate that it was deformed during sinistral transpression the adjacent pontesford linley and church stretton fault systems are considered to be major strike slip fault systems which were initiated at about the same time as the deformation of the longmyndian supergroup strike slip movements along these fault systems resulted in the juxtaposition of the uriconian volcanic complex with the longmyndian supergroup late precambrian early cambrian deformation is indicated by some stratigraphic and radiometric age dating evidence since there is evidence that the fault systems have suffered strike slip displacements during the palaeozoic it is difficult to be certain of the magnitude and style of the late precambrian early cambrian deformation however the recognition of late precambrian early cambrian sinistral transpressional deformation in northwest wales suggests that the precambrian of the welsh borderland was similarly deformed during the late precambrian early cambrian

is the welsh borderland fault system a terrane boundary the welsh borderland fault system forms one of the most prominent tectonic lines in southern britain and along with the menai straits and malverns fault systems is a candidate for a terrane boundry a review of stratigraphic linkage across the system reveals significant contrasts in pre ashgill sequences these contrasts occur particularly across its north west edge the pontesford and tywi lineaments rather than across the church stretton lineament the ashgill event produced strike slip dominated structure and an unconformity that expands from the welsh basin towards the platform an important linkage sequence was produced by the subsequent late ashgill to llandovery marine transgression across the fault system silurian facies changes are marked but gradational and no more than expected across a basin margin from stable to thinned crust devonian acadian sinistral transpression and later phanerozoic movements reactivated the fault system but did not engineer major terrane amalgamation across it this reactivation has obscured the early history of the fault system the remaining evidence admits though does not prove major ashgill or earlier transcurrent movements broader considerations suggest that the welsh borderland fault system originated either as a cambro ordovician extensional system on the south eastern margin of the welsh basin or during late precambrian early cambrian transcurrent movements occurring within avalonia inboard from the menai strait line

avalonian magmatism and terrane linkage new isotopic data from the precambrian of north wales new u pb isotope data from the precambrian sarn igneous complex of north wales establishes that calc alkaline plutonic rocks of the same age occur on either side of the menai strait fault system zircon fractions from the sarn igneous complex gabbro give a near concordant u pb age of 615 2 ma which is interpreted as the first accurate age of igneous crystallization and supersedes previous dating attempts using the rb sr method the new u pb age is the same within error as recent zircon dates obtained from the arfon group ignimbrites along strike from the sarn igneous complex and from the coedana complex granite on anglesey these data emphasize the importance of what appears to have been the major phase of calc alkaline magmatism at 630 600 ma recognizable along the avalonian arc in britain newfoundland and elsewhere despite the similar ages and tectonic setting of the coedana and sarn plutonic rocks however they remain petrogenetically and geochemically distinct and are not directly linked as part of the same igneous complex instead the coedana complex granite is interpreted as having been generated in the avalonian arc during a widespread magmatic event that is also recorded in britain not only by the sarn igneous complex and arfon group but by volcanic and plutonic rocks in central england investigation of granite clasts in the gwna group melange monian supergroup on anglesey failed to produce zircons adequate for dating purposes but revealed rb sr whole rock geochemical and petrographic characteristics similar to the nearby coedana complex and suggest linkage between fault bounded 9monian9 terranes on anglesey in the light of these new data rocks on either side of the menai strait terrane boundary are interpreted as belonging to the same arc system that was dismembered dispersed and laterally duplicated by transcurrent faulting after late precambrian magmatism

laboratory evaluation of k0 for overconsolidated clays most of the laboratory testing methods available for the evaluation of in situ horizontal stresses are applicable to normally consolidated or lightly overconsolidated clays this paper describes a new laboratory method for the determination of in situ horizontal stresses of heavily overconsolidated clays using a stress path triaxial apparatus the proposed method is based on the concept that if the radial stress exceeds the in situ horizontal stress while maintaining the axial stress constant and equal to the in situ vertical effective stress only then will the sample experience significant axial strain the results obtained for undisturbed samples of an overconsolidated clay crust are found to be in agreement with some available methods for verification of the applicability of the proposed method k0 was determined for artificially prepared samples that had been subjected to known stress paths simulating field stress history key words k0 overconsolidation in situ stress in situ test clay crust

the east irish sea and adjacent basins new faults or old the east irish basin is characterized by three sets of faults striking n s ne sw and e w the menai strait fault system bounds the basin to the south and shows pre caledonian to cenozoic movement this span of activity contrasts with similarly oriented faults to the north of the basin which did not exist before the early carboniferous n s faults have a similarly diverse history some may have influenced silurian sedimentation and controlled the disposition of ordovician volcanic centres in snowdonia a contrast with n s striking faults in the north of the area which originated as steep strike slip faults during the late silurian e w faults control dinantian and permo triassic thickness variations although unlike the other fault trends pre carboniferous precursors are not apparent some of the basin bounding faults developed as wholly new faults during permo triassic extension

reappraising the neoproterozoic east avalonian terranes of southern great britain abstract inliers of late neoproterozoic basement rocks exposed in southern great britain described in summary herein have been previously attributed to a series of terranes bounded by major crustal lineaments that form a dissected remnant of a single subduction complex together these terranes have been termed east avalonia and interpreted as correlative with west avalonia of the northern appalachian orogen a review of age and nd and o isotopic data shows an essentially ensialic complex with two broad overlapping divisions 1 the monian composite terrane of nw wales and fenland terrane of eastern england that are characterised by felsic intrusions at around 615 ma and have more evolved end t 7 5 to 2 5 older t dm 1868 ma to 1269 ma and generally more elevated 18 o up to 13 7 and 2 a more diverse assemblage represented by the cymru wrekin and charnwood terranes including more substantive volcano sedimentary successions and intrusions ranging in age between around 710 ma and 566 ma in which 615 ma magmatic rocks are not represented less evolved end t 2 5 to 3 5 younger t dm 1501 994 ma and lower 18 o 6 5 to 10 1 comparison with other neoproterozoic basement units throughout the northern appalachian orogen shows that both divisions from southern great britain have closer isotopic affinity with igneous components of ganderia which generally have evolved compositions interpreted to reflect an ensialic setting and the input of gondwanan cratonic source components they are mostly dissimilar from magmatic rocks of west avalonia which typically have less evolved compositions interpreted to reflect the contribution of juvenile mesoproterozoic source components