tropical pacific atlantic ocean interactions at multi decadal time scales analysis of sea surface temperature sst observations suggests a pan oceanic interaction between the tropical pacific and the atlantic ocean at multi decadal time scales such that periods of anomalously high ssts in the tropical pacific are followed by a basin wide sst dipole in the atlantic ocean with a time delay of a few decades the sst anomaly structure in the atlantic ocean is reminscent of variations in the north atlantic thermohaline circulation the two ocean basins are linked through an atmospheric bridge involving anomalous fresh water fluxes based on the observational findings the atlantic thermohaline circulation may strengthen during the next decades in response to the strong decades long increase in tropical pacific sst which will have strong impacts on the climates of north america and europe through changes in the north atlantic oscillation nao

east african lake evidence for pliocene millennial scale climate variability late cenozoic climate history in africa was punctuated by episodes of variability characterized by the appearance and disappearance of large freshwater lakes within the east african rift valley in the baringo bogoria basin a well dated sequence of diatomites and fluviolacustrine sediments documents the precessionally forced cycling of an extensive lake system between 2 70 ma and 2 55 ma one diatomite unit was studied using the oxygen isotope composition of diatom silica combined with x ray fluorescence spectrometry and taxonomic assemblage changes to explore the nature of climate variability during this interval data reveal a rapid onset and gradual decline of deepwater lake conditions which exhibit millennial scale cyclicity of 1400 1700 yr similar to late quaternary dansgaard oeschger events these cycles are thought to reflect enhanced precipitation coincident with increased monsoonal strength suggesting the existence of a teleconnection between the high latitudes and east africa during this period such climatic variability could have affected faunal and floral evolution at the time

origin of the oslo graben in relation to the hercynian alleghenian orogeny and lithospheric rifting in the north atlantic abstract the contemporaneous extrusion of basalts in the oslo graben and intrusion of dolentes in northern britain and southern sweden at 295 ma calls for a common explanation two hypotheses are investigated 1 1 the graben and dykes resulted from extensional stresses associated with progressive lithosphere separation to the northwest of europe in the late carboniferous 2 2 the dykes and therefore the graben were related in some way to oblique collision of plates in the hercynian orogeny which was developing to the south the first hypothesis is mechanically more satisfying and makes a number of testable predictions it states that in late carboniferous times the lithosphere separated in two places along the orogenic grain to the west of britain and norway the two embryonic oceanic rifts were divided by thick cold lithosphere with an archaean crust to the north of scotland extensional stresses were focussed in this region fanning out in an arcuate zone to the south and east causing failure where the lithosphere was relatively thin in norway the strain was restricted to a zone previously thinned and weakened in the early palaeozoic i e the oslo graben in britain the caledonoid grain is oblique to the expected direction of extensional strain and a dyke swarm trending e w and about 300 km wide was formed the first hypothesis predicts that the dykes should die out to the west but continue along an arc and widen to the east northeast under the western north sea interpretation of aeromagnetic maps shows that the dykes behave as predicted by hypothesis one but that their trends and extent are at variance with the expectations of hypothesis two the apparent contradiction of rifting to the northwest of europe occurring at the same time as compression and oblique collision in the heart of europe is resolved in principle by two plate tectonic reconstructions of pangaea drawn for late carboniferous and early permian times the oslo graben was conceived in mid carboniferous times and born towards the end of the carboniferous as the thin lithosphere failed and allowed the extrusion of basalt magma it was then abandoned as the two collinear oceanic rifts to the west and northwest joined up but the graben carried on a life of its own as the mafic magma cushion caught in the density trap beneath the crust continued to differentiate and alkali magma pods rose buoyantly to freeze in the upper crust or extrude on the graben floor

development of basins in the inner moray firth and the north sea by crustal extension and dextral displacement of the great glen fault abstract reflection seismic data provide evidence that mesozoic dextral movements along the great glen fault line have had an important influence on the development of the inner moray firth basin geophysical evidence further indicates that deep structure beneath the inner basin is dissimilar to that beneath the outer part and viking and central grabens in the north sea tectonic development of the inner basin can nevertheless be fitted into a pattern of north sea extensional movements which led to the formation of the graben system with which the major north sea hydrocarbon resources are associated

lower tertiary sand development in viking graben north sea a regional study of lower tertiary sands in the viking graben northern north sea involving heavy mineral analysis geophysical log interpretation and palynologic dating has led to the successful differentiation of sand bodies with two distinct source areas being recognized these both lie on the orkney shetland platform one being the metamorphic basement in the north the other the preexisting sediments to the south probably of late jurassic age correlation of sand bodies by these means has led to a more complete breakdown of the depositional history of the area four phases of basinal subsidence and source area uplift have been recognized in this way and these are correlated with similar events in the central north sea the changes in subsidence pattern and source area uplift are related to events in the north atlantic prior to crustal separation between greenland and rockall although provenance is the fundamental control on heavy mineral distribution intrastratal solution also plays a major role solution is largely a function of burial depth and it is estimated that up to 50 of the assemblage may have been lost by this process over a large part of the area

plate tectonics and the evolution of the british isles this paper is an essay on the rationalism and difficulties of the geological corollaries of plate tectonics with illustrations particularly from the geological history of the british isles there is special emphasis on the mechanics of continental lithosphere in extension when basins are formed and in convergence whereby continental lithosphere thinned in extension is shortened and restacked to form orogenic belts an upper lithosphere stress concentrating elastic lid suffers extremely complex inhomogeneous deformations above a simpler viscoelastic lower lithosphere whose deformation drives strain and displacement in the elastic lid the very nature of plate tectonics as relative motion among a multi plate mosaic of plates provides a rational semi quantitative framework for large scale geological complexity in space and time the caledonian and hercynian history of the british isles may be explained semi quantitatively in terms of plate boundary evolution but it is not possible to define unequivocal solutions the rationalism of plate tectonics is exhibited on a smaller more direct field scale by many of the complexities of ophiolite complexes such as ballantrae scotland that can be explained as the effects of the evolution of ridge ridge transforms and by diachronous deformation and sedimentation in subduction accretion prisms such as the southern uplands deformational and sedimentary patterns in nw europe from triassic times to the present day resulted from a complex interaction of extensional events related to diachronous atlantic opening and compressional events related to sequential alpine shortening phases those widespread events are typical of intracontinental plate boundary systems and create great difficulties for the geologist wishing to decipher relative plate motion from geological sequences because huge areas much larger than the british isles must be analysed and synthesized before much sense can be extracted further difficulties arise from the palinspastic and preservation problems of convergent boundaries orogenic shortening telescopes and partially subducts facies belts most orogenic belts were assembled as a collage of displaced and rotated terrain elements that were subsequently flattened elongated and straightened to an apparent but deceiving simplicity therefore while plate tectonics provides a rational geological framework against which for example british basin evolution can be tested and explained we cannot yet use the geological effects of plate boundary evolution to reconstruct relative plate motion quantitatively

origin of ultramafic inclusions and megacrysts in a monchiquite dyke at streap inverness shire scotland abstract spinel lherzolite nodules composed of olivine fo 88 7 89 2 clinepyroxene 6 5 al 2 o 3 and al rich spinel and websterite nodules as well as megacrysts of clinopyroxene orthopyroxene and magnetite occur in a monchiquite at streap scotland petrographic data are given and microprobe analyses of coexisting phases in six spinel lherzolite nodules and one websterite nodule are reported along with analyses of both types of pyroxene megacrysts the spinel lherzolites show internal chemical homogeneity and their mineral chemistries suggest equilibrium conditions of 1100 1200 c and 14 23 kb the websterite nodules are on the basis of mineral chemistry and petrography considered to be crustal material the megacrysts constitute a separate group differing in composition from analogous phases in associated lherzolites and websterites as well as from monchiquite phenocryst phases and show systematic chemical variations corresponding to low pressure crystal fractionation processes

structural evolution of the witch ground graben the witch ground graben is a nw se trending extensional basin in the central part of the northern north sea lying between the moray firth and the viking graben ne sw extension occurred from triassic to cretaceous times extension took place on listric normal faults along the sw margin with downthrow to the ne giving rise to the asymmetry of the basin one of the principal phases of fault controlled subsidence occurred in the lower cretaceous from the valanginian to albian compilation of biostratigraphic data from this interval shows the development of an increasing number of stratigraphic breaks across the basin from sw to ne these breaks increasing in time span also in this direction this is a result of rotation of the subsiding fault block on the listric extensional faults the rotation is recorded in dipmeter data and occurred in several distinct episodes the triassic the volgian and the valanginian to aptian in reconstructing a cross section of the basin the interpreted fault curvature is constrained by the observed rotation of bedding surfaces and the amount of extension across the listric faults is constrained by the observed basin subsidence a depth to decollement for the faults of 4 km below sea level and an extension across the basin of 40 are estimated

reply to discussion on multiple post caledonian exhumation episodes across nw scotland revealed by apatite fission track analysis journal vol 167 675 694 we thank john hudson for his welcome comments on the nature and extent of the now eroded mesozoic sedimentary cover across parts of the present day scottish landmass that is indicated by the apatite fission track analysis afta results presented by holford et al 2010 as hudson points out low temperature thermochronological techniques including afta can provide unique insights into the post palaeozoic geological evolution of basement terranes that lack significant phanerozoic cover as witnessed by studies including green et al 2000 2001 belton et al 2004 weber et al 2005 as we demonstrated holford et al 2010 these techniques become even more powerful when applied to basement terranes where some sedimentary cover is preserved whereas low temperature thermochronological methods can provide information on only the cooling histories of basement rocks often reflecting exhumation the age of a sedimentary unit however thin it may be overlying eroded basement defines a time when the basement had been exhumed to the surface and was being buried thus integration of low temperature thermochronological data from basement rocks and overlying sedimentary units can define more complex thermal histories encompassing both heating and cooling reflecting episodic burial and exhumation application of this approach to basement and cover samples from a number of regions where phanerozoic sedimentary rocks are sparsely preserved including nw scotland has revealed that some continental regions that have long been thought to

deep structure of the scottish caledonides revealed by the moist reflection profile the moine and outer isles seismic traverse moist is a deep crustal reflection profile shot at sea off the north coast of scotland in 1981 spectacular reflections are observed from the moho and from thrust zones within the caledonian fold belt and foreland deep reflection profiling of the continental crust of the united states under the direction of the cocorp group has amply demonstrated the value of this technique when applied to suitable geological problems see for example refs 1 2 a similar group the british institutes reflection profiling syndicate birps has now been set up in the uk to organize such projects funds are provided by the natural environment research council nerc through the core group based at cambridge university however a preliminary experiment of which moist is the result was organized through the institute of geological sciences igs

northern scotland as an atlantic north sea divide president9s anniversary address 1984 from the end of the caledonian orogeny until the present northern scotland has acted as a structural high separating the north sea and atlantic basins or their forerunners the geological history of this positive massif is discussed in relation to that of the adjacent cratonic basins with the objective of determining the extent to which it adapted to regional tectonic developments and the manner in which its responses were modified by the pre existing basement structure from devonian to early palaeocene times the massif was generally low lying and its behaviour with respect to vertical movements was essentially neutral the land surface carved in the crystalline basement during the devonian period was preserved with relatively small modifications and little metamorphic detritus was supplied to the adjacent basins tectonic events of major importance in the basins such as the formation of the north sea and hebridean rift systems produced little change of conditions in the intervening massif it is inferred that the underlying keel of lithospheric mantle was protected from the effects of lithospheric stretching the uneventful history of the massif was interrupted by two phases of mafic igneous activity in late carboniferous and palaeocene times which record responses to regional plate motions the formation of the palaeocene igneous province was followed by a unique phase of uplift as a result of which the ancient sub devonian land surface was elevated and dissected and abundant crystalline detritus was supplied to the north sea the style of these events and the anomalous character of the continental margin off nw britain may have been determined by the refractory nature of the precambrian basement which inhibited stretching prior to sea floor spreading

low temperature effects of the skye tertiary intrusions on mesozoic sediments in the sea of hebrides basin fission track analysis of samples from the sea of hebrides basin and surrounding regions demonstrates two very distinct age groups around 50 ma and 300 ma regardless of the stratigraphic age apatite fission track results from sediments within 8 km of the tertiary igneous complex on skye were totally reset by temperatures 110 c during intrusion of the centres but now yield ages younger than the granites zircon fission track results from the granites also demonstrate ages significantly younger than their hosts mean track lengths in apatites of 13 5 m and reduced apatite and zircon ages suggest that temperatures remained elevated throughout the 6 ma during which intrusive activity occurred but were hotter within the granite bodies than the surrounding sediments prior to intrusion of the complex temperatures in the basin sediments presently at outcrop were unlikely to have been higher than 50 c and beyond the effects of the tertiary intrusions age and track length distributions from northern skye the isle of lewis and the west coast of scotland illustrate that the area is unlikely to have been buried beneath more than 2 km of sediment at any one time since the end of the devonian north of the highland boundary fault tertiary uplift and erosion in scotland is considered to have been between 1 1 5 km this is consistent with a regional pattern of tertiary erosion identified across the british isles but considerably less than that recognised in northern england the geological society 1992

the geometrical evolution of normal fault systems abstract the purpose of this paper is to examine the kinematic behaviour of normal fault systems and see what general conditions govern their geometrical evolution we pay particular attention to seismological and surface data from regions of present day active normal faulting as the instantaneous three dimensional geometry at the time of fault movement is better known in active regions than in areas where the faults are now static most normal faults are concave upward or listric this shape can be produced by geometric constraints either because the faults reactivate curved thrusts or because they must be curved to accommodate rotations another effect which will produce curved faults is the variation of rheology with depth brittle failure at shallow depths produces less fault rotation than does distributed creep in the lower part of the crust an important geometric feature of normal faulting is the uplift of the footwall the amount of such uplift is related not only to the elastic properties of the lithosphere but also to the throw and dip of the fault a striking feature of active normal faults is that they occur in groups in which all the faults dip in the same direction this behaviour arises because the faults cannot intersect if they do one must cease to be active the rotation which such fault systems produce reduces the dip of the faults until a new steeply dipping fault is formed once a new fault cuts pre existing faults the earlier faults become locked and a new set of faults must propagate rapidly across the whole region involved many of these geometric constraints also apply to thrust faulting

magma and sediment i emplacement mechanism of late carboniferous tholeiite sills in northern britain president s anniversary address 1981 like other great tholeiite sheets of the world the midland valley sill and whin sill were emplaced in thick accumulations of sediments which at the time of intrusion were not much older nor had been orogenically deformed however the carboniferous sediments of northern britain had been deposited under the influence of continuous differential subsidence so that they were already disposed in syn sedimentary basins and swells when intrusion began by drawing palinspastic contours of the sills relative to a late westphalian marine band approximating to the surface at the time of emplacement it is established that the dolerites mainly follow bedding planes dipping at angles of as much as 5 down to the bottoms of the basins isopachs of the sills indicate a further relationship for the intrusions are thickest at the bottoms of the basins these facts are not explained by any intrusion mechanism previously postulated though there seems to be broad agreement as to the processes which have given rise to thinning and en echelon lensing at leading edges as well as to step and stair transgression of sediment bedding it is proposed that emplacement must have been controlled in part by gravitational flow down dip from feeder dykes which extended to between 0 5 and 1 0 km below the contemporary surface thereafter hydrostatic equilibrium was attained first by accumulation at the bottom of the basins and then by further advance up dip under pressure of head the sheets are so similar in aspect to flood basalts that it must be assumed that the sediments offered only low frictional resistance to splitting and flow

birps deep seismic reflection studies of the british caledonides the western isles north channel winch traverse fig 1 an extension of the successful moist profile1 was recorded in 1982 at sea along the west coast of britain for birps british institutions reflection profiling syndicate by the geophysical company of norway geco the purpose of the winch traverse was to study crustal structure of the caledonian foreland to cross the caledonian orogen and to establish the three dimensional geometry of mantle reflectors originally seen on moist we describe here the data first emphasizing british caledonian structures and then discussing features of the deep crust of wider significance the data are of very good quality and contain clear moho and upper mantle reflections the lower crust is surprisingly reflective deeply penetrative thrusts are observed and there is firm evidence that several of the mesozoic basins round the shores of the united kingdom were formed by reactivation of older features

upper palaeozoic basins of the british isles caledonide inheritance versus hercynian plate margin processes hypotheses to explain the origin and location of upper palaeozoic sedimentary basins are presented group a basins were those old red sandstone basins whose location and subsidence were controlled by caledonide features including active and remnant fore arc basins midland valley and major transcurrent movements orcadian basin newer granite plutonic magmas in the southern uplands subduction complex post dated subduction and resulted from post collision cratonization processes as yet little understood group b basins were those whose initial positions in space were controlled by palaeo plate factors in particular the location of granitoid plutons but whose subsidence was controlled by active hercynian neoplate processes some group a basins were regenerated by this process midland valley a new hypothesis is presented to account for the origin of a carboniferous rift to sag trend similar to that proven in modern continental margin basins such as the north sea the hypothesis is an adaptation of mckenzie9s lithospheric stretching model some of whose major features are well shown in the carboniferous history of the north and central british isles group b processes were controlled by ligurian bretonic plate consumption in the s brittany massif central vosges areas group c basins in the rheno hercynian zone also owe their location and origin to back arc rifting and continental crustal oceanization processes caused by liguro bretonic subduction

comments and reply on paleomagnetic evidence for a large 2 000 km sinistral offset along the great glen fault during carboniferous time

the thrust and shear zones of the moine thrust zone and the nw scottish caledonides the caledonian thrust belts of nw scotland exhibit many features typical of foreland fold and thrust belts the major structures propagated westwards towards the foreland the thin carapace of cambro ordovician sediments being accreted in imbricate and duplex zones beneath the major thrust sheets on a broad scale the rules of thrust geometry are obeyed but many local anomalies are found particularly in the assynt region in assynt several large fault bounded gravity spreading structures surge zones have been identified extensional faults have been traced into strike slip and then contractional faults there are zones of extensional and contractional imbricate faults and extensional and contractional ductile flow similar structures have been found along the length of the moine thrust belt suggesting that gravity spreading was an important mechanism during the later stages of movement within the main caledonian fold belt there are several major tectonic slides which are recognized by a local increase in intensity of deformation the majority are nw verging and were originally gently dipping in the highlands se of the great glen there are early major se verging recumbent folds associated with considerable crustal thickening reflected in high pressure 12kbar metamorphism many of the major slides may have formed by spreading away from this thickened crust the caledonian deformation and metamorphism occurred later in the moines and moine thrust zone than in the dalradian and grampian moines to the se of the great glen this may reflect different levels of deformation alternatively it may argue for a large strike slip displacement on the great glen fault juxtaposing crustal blocks which had experienced different ages of caledonian deformation and metamorphism the late movement on the moine thrust involved spreading from an area of thickened caledonian crust formed either by the stair step trajectory of the basal thrust to the moines and or by thickening due to intrusion of large masses of late caledonian granites

moist and the continuity of crustal reflector geometry along the caledonian appalachian orogen new aspects of the deep structure of the scottish caledonides are revealed by the moine and outer isles deep seismic reflection traverse moist the caledonian foreland is underlain by an easterly dipping strongly reflecting surface cutting through the moho and traceable to more than 45 km depth thrusts within the foreland basement and caledonian orogen have been reactivated as normal faults bounding half grabens filled with sediments of late palaeozoic to mesozoic age the moine thrust which carries rocks of the orogen over lewisian foreland dips at 20 25 to the e on moist and is either a the westernmost of a series of easterly dipping reflections thrusts which flatten or terminate at 17 20 km depth or b a thrust lying further e which structurally overlies these easterly dipping reflectors by comparison with easterly dipping reflections on cocorp lines in the northern and southern appalachians the latter interpretation is preferred implying that many of the easterly dipping reflections on moist correspond to off shelf metasedimentary rocks with slivers of basement stacked and imbricated against the lewisian basement edge regional seismic refraction and conductivity data from northern scotland suggest that regions of the caledonian orogen have lower mid crustal velocity gradients and higher conductivities than the foreland which might be explained by this interpretation of the easterly dipping reflections despite similarities in crustal reflector geometries there are important differences between the moist and cocorp lines including 1 a remarkably continuous relatively horizontal moho seen on moist data at about 25 km depth and 2 the sedimentary basins offshore from northern scotland which have formed by reactivation under crustal extension of the easterly dipping thrusts furthermore it is probable that the easterly dipping reflectors in these areas of the moist and cocorp lines were formed at different times in the early palaeozoic

basin structure and tertiary magmatism on skye nw scotland the emplacement of igneous material into upper crustal rocks of sedimentary basins is likely to be strongly controlled by the geometry of the pre existing basin structures these controls are investigated using examples from the tertiary igneous complexes of skye part of the sea of hebrides basin of nw scotland the basin consists of an array of half graben related to se dipping normal faults these pre volcanic mesozoic structures are traced near the igneous complexes using geological relationships preserved unconformably beneath the widespread basaltic lava fields the unconformity represents a period of cretaceous uplift and denudation of the basin and its flanks entirely pre dating the tertiary volcanism of nw scotland this unconformity seals stratigraphically the major basin faults preserving field relationships that permit the tracing of these faults in the country rocks to the tertiary intrusions the major camasunary fault is separated from the raasay fault via a series of minor graben linked by a series of steep nw se trending faults that transfered mesozoic displacements between the principal fault strands a broad range of igneous material of various compositions was intruded into part of the mesozoic sea of hebrides basins and their flanks during palaeocene times different emplacement styles and different structural controls are found the major gabbroic centres do not appear to be controlled by upper crustal structures having been emplaced into the footwalls of major faults however minor synmagmatic displacements on the basin faults may have been sufficient to generate dilatational sites in these footwall positions thereby facilitating emplacement in contrast the granitic melts have been emplaced as sheets and domed into the sediments and overlying lava pile reactivating segments of the basin fault network doming occurred from an array of sills the stratigraphic levels of which can be reconstructed using structural relationships preserved in the roofs and walls of the intrusions the sill levels and their transgressive forms are strongly related to inferred mesozoic basin structures the major fold structures of tertiary age in southern skye are interpreted as accommodating granitic emplacement rather than crustal shortening the nw se mesozoic transfer fault trend appears to have strongly influenced the segmentation of the granite domes these interpretations are illustrated using field relationships mapped in the vicinity of the coire uaigneich granophyre it is concluded that although the higher parts of the basin faults were reactivated to facilitate the doming of granitic intrusions the deeper levels of the mesozoic faults show no evidence of substantial reactivation

sedimentological constraints on the late silurian history of the highland boundary fault scotland implications for midland valley basin development the relationship between movement on the highland boundary fault and deposition of the lower old red sandstone in the midland valley basin of scotland is controversial most models favour mid silurian to early devonian sinistral movement on the highland boundary fault and development of a transtensional midland valley basin to constrain highland boundary fault movement during the late silurian we examine the basal lower old red sandstone alluvial succession exposed adjacent to the highland boundary fault a lack of synsedimentary fault movement indicators coupled with an increase in stratal thickness across the fault indicates that the highland boundary fault was not active during lower old red sandstone sedimentation a transtensional basin model cannot be sustained

potassium and argon loss patterns in weathered micas implications for detrital mineral studies with particular reference to the triassic palaeogeography of the british isles abstract potassium argon ages are reported for 16 muscovite and 14 biotite samples separated from triassic sandstones of the manx furness carlisle solway firth and lough neagh arran basins disturbance of the k ar isotopic system in these minerals during weathering erosion transport and lithification is discussed and loss of potassium shown to be as important a parameter as argon loss published data from tropical weathering profiles are used to establish the pattern of loss for these two elements during the formation of clay minerals from biotite the data being displayed in a potassium argon loss diagram the loss curve so produced is then used to interpret our new biotite data and previously published age data for detrital biotite from the cheshire basin the limit of interdigitation of northerly and southerly derived materials in the irish sea basins is shown to lie north of the fylde district of lancashire age data from detrital muscovite are used to establish a potassum argon loss curve for muscovite the features of the two curves are markedly similar little effect on the apparent k ar age is observed until about 20 loss of potassium occurs at this point massive argon loss takes place with little loss of potassium due to either potassium stabilisation or structural expansion the suitability of micas as k ar chronometers may be a consequence of the correlated potassium argon loss rather than intrinsic high argon retentivity if both mica curves are applied to detrial micas of the namurian millstone grit series a source area of age of ca 900 ma is indicated for both muscovite and biotite indicating a probable pre cambrian age source area probably in scandinavia