tectonic control on the early cretaceous bentheim sandstone sediments in the schoonebeek oil field the netherlands the reservoir rock of the schoonebeek oil field is formed by the sandstones of the bentheim sandstone member the sedimentology and depositional environment of this sandstone have been extensively studied but the relationship between the geometry of the sandstone and tectonic activity in the schoonebeek area remains poorly understood 355 boreholes and two threedimensional 3d seismic surveys were used to study this relationship an eroded zone in the west of the field and an area where the original depositional thickness is still intact were identified using the ezvalidator software package it can be seen that uplift of a local anticline played an important role in the erosion of the sandstone deposition of the sands of the bentheim sandstone member and the overlying vlieland sandstone and claystone formations occurred on an unstable changing palaeotopography whereby the instability was probably driven by halokinetic movement of the underlying late permian zechstein salt syndepositional tectonic movements affected local thickness variations in the bentheim sandstone member in the west of the field leading to westwards thinning 2018 the author s

late proterozoic high pressure granulite facies meta morphism in the north east ox inlier north west ireland high pressure granulite facies gneisses in the ne ox inlier in nw ireland have undergone extensive caledonian retrogression in the local area of slishwood however reworking was negligible and the gneisses psammites semipelites pelites metabasites and ultramafites preserve evidence of p t changes at high grade which mainly post date pre caledonian polyphase deformation temperatures reached 850 900 c based on garnet clinopyroxene geothermometry and the presence of mesoperthite during and after decompression from earlier eclogite facies conditions inferred from textural evidence of plagioclase release in sieve textured augite subsequent cooling at high pressure is inferred from the unequivocal replacement of sillimanite by kyanite a sm nd mineral isochron gt cpx plag wr of 605 37 ma is taken to date a point on the cooling path and confirms the hitherto suspected pre caledonian age of the high grade metamorphism geochemical and sm nd isotopic data indicate that the protoliths were probably late proterozoic arkosic sediments and tholeiites following metamorphism they apparently came to reside near the base of the crust where they slowly cooled the eventual exhumation of these gneisses is attributed to caledonian crustal imbrication followed by rapid isostatic recovery

connemara its position and role in the grampian orogeny in the irish and british caledonides the early ordovician grampian orogeny was the result of collision between the laurentian rifted margin and an oceanic island arc the connemara terrain in western ireland differs in position and character from all other parts of the exposed dalradian rocks of the grampian orogen in lying south of the collided arc and fore arc and in having north verging fold nappes that developed synchronously with the intrusion of huge volumes of calc alkaline magmas that provided the heat for regional barrovian metamorphism we have tested this hypothesis with a numerical model which demonstrates its admissablity connemara is not a terrane displaced with respect to the remainder of the grampian orogen but was overridden northwards by the arc and its fore arc basin south mayo trough frontal ophiolite complex deer park and accretionary complex killadangan deposition in the south mayo trough occurred below sea level and above the evolving grampian orogen which developed

implications for caledonian plate tectonic models of chemical data from volcanic rocks of the british old red sandstone volcanic rocks associated with the continental sediments known as the old red sandstone outcrop widely in northern britain and form a dominantly calc alkaline suite comparable to those developed on modern continental margins rocks rich in ni and cr are unusually abundant and are thought to have incompatible element concentrations little different from those of primary magmas such rocks show pronounced spatial chemical variation with concentrations of sr ba k light rare earth elements and p 2 o 5 and the ratio la y showing up to a six fold increase in a northwestward traverse across scotland in more evolved rocks a similar increase is shown by sr ba and la y but regional comparisons are made difficult by the varying effects of fractional crystallization the chemistry of all old red sandstone volcanic rocks n of the southern uplands fault is consistent with the former existence of a subduction zone that changed in strike from ene in ireland and southern scotland to n in the north sea this orientation in conjunction with almost e w plate motion may explain the great contrast in end silurian deformation styles between britain and scandinavia old red sandstone volcanic and plutonic rocks s of the southern uplands fault are thought to be younger and are perhaps unrelated to subduction activity

u pb isotopic dating of fluid infiltration and metasomatism during dalradian regional metamorphism in connemara western ireland a metasomatic diopside rock occurs at the top of the dolomitic connemara marble formation of western ireland and contains titanite and k feldspar in addition to around 90 diopside xmg 0 90 0 97 u pb isotopic measurements on this mineral assemblage show that the titanite is both unusually uranium rich and isotopically concordant with the result that a precise u pb age of 478 2 5 ma can be determined the age is identical within error to a less precise rb sr age of diopside k feldspar of 483 6 ma petrological evidence indicates that the assemblage crystallized at c 620 c close to or below the closure temperature of titanite the age thus provides a precise estimate of the time of metamorphism this age is 11 3 ma younger than the 490 ma age for nearby gabbroic plutons which has previously been used to constrain the peak metamorphic age this difference accords well with geological evidence that the gabbros were emplaced prior to the metamorphic peak analysis of minerals with high closure temperature from assemblages whose crystallization is unambiguously associated with a specific episode of fluid infiltration at the peak of metamorphism provides the basis for a new approach to dating metamorphism the success of this approach is demonstrated by the results from connemara

nappe geometry in the western swiss alps abstract detailed geological mapping during the last 20 years in the western swiss alps has shown clearly that most of the lower basement nappes are fold nappes possessing normal and inverted limbs moreover their cores are made of strongly deformed gneisses indicating that important ductile strain took place during the formation of the fold nappes it is therefore probably wrong to imagine deep basement nappes as rigid slices as often actually claimed especially when interpreting seismic profiles true brittle type thrust nappes involving basement rocks only occur in the internal and upper parts of the belt cover nappes on the contrary are in most parts of the alpine belt thrust sheets following more or less the rules of thin skinned tectonics many basement fold nappes lost part of their sedimentary cover during or just before their formation by decollement along ductile horizons the result is that many cover thrust nappes in the external part of the alps are directly related to their original basement fold nappes

the identification of mantle reflections below hungary from deep seismic profiling abstract in the 1970 s the eotvos lorand geophysical institute performed seismic reflection deep sounding experiments with multiple coverage in several parts of the pannonian basin posgay 1975 one of the most interesting results was that arrivals were received in the time range corresponding to the depth of the asthenosphere they were interpreted as reflections posgay et al 1981 conspicuous arrivals from the time range of the asthenosphere 15 20 s were also received along the pannonian geotraverse in 1987 in the majority of sections published in other countries the upper mantle seems to be transparent the present paper discusses investigations and preliminary results which lead to the conclusion that this apparent transparency is due to the frequency range below 10 hz being neglected in both signal generation and recording in most deep continental reflection surveys the seismogeological conditions of the pannonian basin are certainly favourable for reflection studies of the upper mantle the correctness of the assumption that arrivals from the time range of the asthenosphere are primary reflections is subjected to further investigation and conclusions are made concerning the structure of the asthenosphere

emplacement of a fold nappe betic orogen southern spain the aguilon nappe sierra alhamilla southeast spain is a north closing recumbent isoclinal fold with a greatly thinned lower limb this large scale geometry can be demonstrated using small scale structures the fold nappe is detached along the lower limb and lies on a carpet of mylonitic schist mylonite and cataclasite formation of the nappe must have involved northward relative transport a point of current controversy in the betic zone during emplacement the nappe underwent extensive ductile deformation accomplished partly by pressure solution of silica in the highly deformed footwall rocks however quartz was deformed by crystal plastic processes the change in deformational mechanism suggests a downward increase in deviatoric stress which is compatible with a gravitational mechanism of nappe emplacement the core of the low grade nappe is occupied by medium grade schist the abrupt change in grade suggests that the schist contact is an extensional fault that excised part of the original metamorphic zonation and was subsequently refolded in the nappe such relations are common in the betic zone and suggest extension by listric normal faulting a characteristic feature of nappes emplaced by gravity spreading

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

permeability anisotropy and its relations with porous medium structure 1 the complete permeability tensor of 18 porous rock cores was determined by means of x ray tomography monitoring during the displacement of a salty tracer to study the effect of the pore space geometry on the anisotropy of permeability we compared the three dimensional shape of the invasion front with the x ray porosity maps obtained before injection the samples clean and shale bearing sandstones limestones and volcanic rocks belong to a broad variety of granulometry and pore space geometry their porosity ranges from 12 to 57 and their permeability ranges from 1 5 10 14 to 4 10 12 m2 for sandstones the permeability anisotropy is well correlated with the presence of bedding for volcanic rocks it is clearly related to the orientation of vesicles or cracks however for limestones no evident link between the geometry of the porous network and the permeability anisotropy appears probably because of the influence of the nonconnected porosity that does not contribute to the hydraulic transport this systematic work evidences the ability and the limitations of the tracer method to characterize the anisotropy of permeability in the laboratory in a simple and rapid way

the ordovician evolution of the south mayo trough western ireland the south mayo trough a broad synclinorium containing a 10 km sequence of early ordovician turbidites passing up into shallow water sandstones is interpreted as a forearc basin with an ophiolitic basement that formed a backstop to an accretionary prism on the northern edge of a north facing arc the arc collided with the laurentian margin in early llanvirn times to deform the dalradian supergroup rifted margin clastic sequence prior to the late llandovery sinistral transcurrent faulting transposed the connemara dalradian terrane to its present position on the outboard ocean ward side of the south mayo trough the south mayo trough was fed principally by arc and ophiolite derived elastics during the arenig with an increasing metamorphic component during the llanvirn the late ordovician transpressional docking of connemara provided the southerly source for high grade metamorphic clasts in the deny veeny formation

the geology and structural evolution of a caledonian fold and ductile thrust zone kyle of tongue region sutherland northern scotland the middle to upper proterozoic metasedimentary rocks of the northern highland moine in sutherland are disposed in a series of major caledonian c 470 430 ma metamorphic thrust nappes produced by nw wnw directed shortening and reworking of parts of a pre existing precambrian c 1000 ma orogen lewisian inliers occur throughout sutherland and represent tectonically interleaved units of the original basement gneisses upon which the moine cover sequence was deposited in the western moine nappe of the kyle of tongue region the rocks are deformed by two main episodes d1 and d2 which are thought to represent the effects of precambrian and caledonian orogenesis respectively little basement cover interleaving or large scale folding occurred during the d1 event in the moine nappe but the associated metamorphism to ut leust garnet grade produced peak metamorphic mineral assemblages subsequent caledonian reworking d2 produced a foreland propagating system of wnw directed ductile thrusts and related major f2 sheath folds both of which resulted in much of the observed moine lewisian interleaving metamorphism associated with d2 decreases in grade westwards occurring at similar or slightly lower grades than the earlier d1 event this retrogressive overprint is most obvious at the w margin of the moine nappe the d2 folds and ductile thrusts are geometrically and kinematically similar to the caledonian structures recognized within the moine thrust zone to the west the very low 5 ramp angles suggesting that the thrusts must have very shallow dipping trajectories through the crust f2 fold formation is attributed to an early phase of wnw directed overthrusting compression which was then progressively superseded by a regional ductile extensional strain contemporaneous with continued wnw directed thrusting the occurrence of internal extension in the orogenic wedge during active thrust related foreshortening implies that the shear strength of the material close to the basal detatchment s was continuously reduced i e wedge motion became supercritical in this context strain softening processes within the ductile thrust mylonites were probably of major significance

evolution of the lower saxony basin abstract the lower saxony basin is a 300 km long and 65 km wide late jurassic and early cretaceous trough which parallels the northern margin of the rhenish massif this basin became inverted during the senonian and paleocene the lower saxony basin is the most important oil province of germany past production amounts to 200 10 6 t of oil 1 5 10 9 bbl the lower saxony basin is superimposed on the late carboniferous variscan foredeep basin and the frontal structures of the variscan fold belt during the permian to middle jurassic the area formed part of the northwest european basin differential subsidence of the lower saxony basin was initiated during the late jurassic and persisted till albian time this involved reactivation of permo carboniferous fracture systems by divergent wrenching a major aptian deep seated laccolith is evident in the western part of the lower saxony basin during the senonian and paleocene sinistral convergent wrenching induced uplift of the basin floor and folding of its sedimentary fill the margins of the inverted lower saxony basin are thrust over the adjacent stable platform areas the differential subsidence of the lower saxony basin can be related to a period of major crustal extension in the north sea rift basin inversion coincides with important orogenic events in the alpine system and is thought to have been induced by intra plate compressional stresses

major northwest directed caledonian thrusting and folding in precambrian rocks northwest mayo ireland structural mapping of the north west mayo inlier has investigated the kinematics of ductile fault and shear zones within the dalradian and pre dalradian rocks the area is characterized by an intense east to southeast trending elongation direction pre caledonian deformation is preserved as west northwest trending sinistral mylonite zones these are cross cut by grenville pegmatites and by undated mafic dykes subsequent deformation has been influenced by the majorbasement structures during the caledonian deformation the pre existing sinistral structures were reactivated coaxially as steep sinistral shear zones deforming the mafic dykes major northwesterly verging thrusts are widespread in flat lying zones which occur over much of the inlier these are interpreted as d2 in age the s2 fabric is the dominant fabric and contains an east to northeast trending mineral lineation the southeastern part of the inlier is characterized by very large scale wavelengths of kilometres reclined d2 folding this is interpreted as a crustal scale ramp zone in the vicinity of this ramp there is evidence of earlier f1 folds which are possibly the product of ramprelated deformation all of these structures are interpreted as the product of northwest verging thrusting with crustal scale frontal and lateral ramps

a geological and tectonic cross section of the caledonides of western ireland geological and geophysical criteria allow division of the caledonides of western ireland into four principal zones zone 1 in the north contains grenvillian basement with late proterozoic supra crustals zone 2 contains the westport complex the deer park complex and the south mayo trough connemara and the delany dome formation occupy zone 3 zone 4 contains the south connemara group and mid to late ordovician and silurian sediments and volcanic rocks the various tectonic models proposed for these zones are discussed and a minimum tectonic model is presented which involves early ordovician collision of the northern part of zone 2 with the laurentian margin zone 1 followed by subduction flip northern part of zone 4 the site of early collision between zones 1 and 2 subsequently underwent orogenic collapse and strike slip dismemberment leading to the sinistral transpressive emplacement of zone 3 assemblages in zone 4 were accreted to the now south facing arc northwards stress transfer consequent on suturing deformed the thinned crust and overlying silurian sequences in zones 2 and 3

decompression induced growth of albite porphyroblasts fleur de lys supergroup western newfoundland abstract albite porphyroblasts are widely distributed in pelitic and semi pelitic schists of the fleur de lys supergroup western newfoundland textures and mineral assemblages indicate that albite grew during nearly isothermal decompression from p t conditions of about 500 c 9 kbar to conditions of 550 c 6 5 kbar three compositional varieties of albite bearing schists here termed pmaq paragonite muscovite albite quartz mmaq microcline muscovite albite quartz and pmmq paragonite muscovite margarite quartz can be distinguished on the basis of pre porphyroblast mineral assemblages analysis of these assemblages in terms of the composition of the coexisting fluid log a na h versus log a k h suggests that as pressure and temperature changed the stability field of albite expanded at the expense of coexisting matrix phyllosilicates this promoted growth of albite on pre existing or newly formed nuclei late oligoclase in pmaq and pmmq samples is associated with replacement of matrix garnet by plagioclase mica chlorite particularly in strongly sheared samples

a magnetic deep structural geology interpretation of ireland a new 10 nt compilation of magnetic data for ireland and the immediately adjacent sea area shows distinct geophysical provinces of characteristic magnetic signature separated by major magnetic linears which can be related to important geological lines in the far northwest there is a structurally coherent grampian tectonic thermal zone bounded to the southeast by the fair head clew bay line between the fair head clew bay line and the western extension of the southern uplands line which forms the northern margin to the southern uplands of scotland is a segmented zone of relatively small magnetic anomalies and geological bodies of mixed crustal affinity grampian massifs lower palaeozoic sedimentary basins bounded by major shear faults relics of lower palaeozoic oceanic and related crust and possible older microcontinent fragments these separate areas have quite individual structural and metamorphic histories there is good evidence for the grampian massifs having undergone significant lateral movement during lower palaeozoic times and it can be inferred that many of the now adjacent bodies are only docked the original position of each block is now not known in the far southeast of ireland are relics of a cadomian craton which are now tectonically imbricated with lower palaeozoic rocks this is probably the segmented margin of the stable cadomian craton which is inferred beneath wales and south britain between the southern uplands line and the northernmost extent of the relics of the southern craton is a zone of deep seated low amplitude magnetic anomalies and major linears these are about 30 oblique to the southern margin of this vestiges of the iapetus ocean zone but are nearly parallel with the northern margin

the distribution geometry and kinematic significance of caledonian buckle folds in the western moine nappe northwestern scotland within internal regions of orogenic belts systems of ductile thrusts and folds may form which are geometrically and kinematically analogous to those observed in foreland thrust belts the moine nappe of sutherland northwestern scotland developed during wnw directed caledonian displacements under amphibolite facies conditions broad zones of mylonite associated with the ductile ben hope and moine thrusts both contain numerous small scale folds which can be attributed to transient flow perturbations it is possible to isolate and differentiate generations of such folds basedon their association with particular major thrust zones and or their geometry in regions of lower strain between these thrusts there are a series ofkilometre scale buckle folds the axial traces of which define arcuate trends those with hinges orthogonal to transport are northwestvergent whilst those plunging subparallel to the mineral lineation display variable vergence the spatial distribution of these geometries suggests a causal relationship with displacements on specific underlying thrusts we propose that they reflect larger kilometre scale and possibly longer lived flow perturbations the transport parallel folds are persistent features that often centre on earlier structural culminations and are thought to reflect components of differential shear the most prominent set of regional buckle folds root downwards into the ductile moine thrust and openly refold the structurally overlying ben hope thrust and its associated buckle folds thus supporting a foreland propagating sequence of ductile displacements

bentheimer sandstone revisited for experimental purposes abstract bentheimer sandstone outcrop samples are ideal for laboratory studies due to their lateral continuity and block scale homogeneous nature over the years they have been used to investigate reservoir topics ranging from passive and active properties of oil gas water rock interaction and processes to flow and transport this work shows an evaluation of bentheimer sandstone properties and their characteristics to advance understanding on the mineral accessory and the physical and electrical transport properties on the basis of the nature of depositional environments and diagenesis we measure and characterize spatial attributes of the matrix mainly by qualitative analysis laboratory and stereological measurements and statistical 2d 3d reconstructions the main contribution of this paper is the impact of mineral composition on the petrophysical quality and block scale homogeneity of the reservoir with 3d techniques reconstructions of the new rock grain framework pore structures have been created based on measurements a review of literature data and interpretation of variation between the outcrops and flow relevant parameters we conclude that bentheimer sandstone is a rock that shows constant mineralogy and is largely free of paramagnetic impurities in accordance with the results of 3d reconstructions techniques and in line with the depositional settings they show a well sorted grain framework and a pore network that can be used to calculate the permeability and resistivity without conducting any direct laboratory measurements of either parameter it presents a porosity with a range of 0 21 0 27 and permeabilities varying between 0 52 and 3 02 darcy based on our work and literature it is found that high permeability together with similar distribution of pore throats and bodies make the sandstone an attractive and easy obtainable candidate for comparative experimental studies moreover we state by comparing various techniques that the efficiency and resolution accuracy of the applied method must be taken into consideration when planning the measurements

evidence for a major neoproterozoic orogenic unconformity within the dalradian supergroup of nw ireland an intriguing and contradictory scenario has recently developed in the caledonides of the british isles concerning the ages of deposition and deformation of the neoproterozoic dalradian supergroup isotopic evidence although limited suggests that the lower parts of the sequence may have been deposited and undergone precambrian deformation i e pre 600 ma prior to deposition of the upper parts of the sequence i e post 600 ma given these existing constraints it is clear that a major break or breaks would be required in the intervening sequence to maintain a coherent tectonostratigraphy we present evidence for such an unconformity preserved at the base of the easdale subgroup in nw ireland reworked clasts that contain a pre existing tectonometamophic history are identified within a conglomerate that lies along this regionally recognized boundary the underlying sequence also exhibits pre existing deformational fabrics that display erosional truncation at the base of the conglomerates these relationships together with significant erosion and excision of the footwall sequence and extensive thickness and facies variations in the hanging wall units imply that a major tectonic unconformity exists within this succession as the conglomerate lies stratigraphically below precambrian c 600 ma lavas the structural fabrics contained within the clasts and the underlying sequence must also be of precambrian age and totally unrelated to the well established early palaeozoic caledonian orogenic deformation observed in the younger parts of the sequence the dalradian supergroup as defined may actually comprise at least two distinct tectonostratigraphic sequences

reflections from mantle fault zones around the british isles british institutions reflection profiling syndicate birps deep seismic reflection surveys show that the upper mantle around the british isles is reflective exhibiting discrete isolated reflecting zones within an otherwise seismically transparent mantle there are 20 known occurrences of reflectors within the upper mantle across the continental shelf of the british isles many delimited by intersecting or closely spaced profiles most of the mantle reflectors have similar reflection characteristics they dip 10 degree to 30 degree and are thin approximately 0 5 s approximately 2 km and discrete the geologic history of the continental shelf around the british isles an area 1000 x 1000 km is complex and the tectonic settings of these reflectors very each mantel reflector is associated with a major zone of deformation in the crust this association the dips of the reflectors and their high reflectivity all indicate a structural origin the birps mantle reflectors include probable early paleozoic thrusts and mesozoic normal faults in contrast to birps data deep reflection profiles in other areas of the world in a variety of geologic settings have only rarely imaged reflectors within the upper mantle

comparison of outcrop and subsurface sandstone permeability distribution lower cretaceous fall river formation south dakota and wyoming quantification of permeability structure in outcrop reservoir analogs documents the distribution of flow units and barriers important for reservoir management and simulation differences in burial history and diagenesis of the outcrop analog and the subsurface reservoir must be quantified however before outcrop permeability data can be used to model subsurface strata appropriately a case study of the lower cretaceous fall river formation shows that permeability differences between facies are accentuated by diagenesis and permeability variation increases with the extent of burial diagenesis the fall river formation which is exposed in outcrop around the black hills uplift in wyoming and south dakota usa produces oil from fluvial and estuarine valley fill sandstones in the adjacent powder river basin outcrop sandstones were buried to 2 km before being uplifted at the end of the cretaceous whereas reservoir sandstones remained at depths of 2 4 km throughout the tertiary the permeability of outcropping fall river sandstones has a log normal distribution the main controls on permeability in outcrop sandstones are 1 ductile grain content 2 grain size both properties being controlled by the energy of the depositional environment and 3 hematite cement an uplift related diagenetic feature not present in the subsurface hematite was preferentially precipitated along zones of permeability contrast including sequence boundaries complicating the determination of changes in primary depositional permeability at important flow unit boundaries in contrast the permeability of fall river reservoir sandstones at a depth of 4 km has a bimodal distribution and is controlled by ductile grain content grain size and quartz cement volume quartz cementation and compaction due to ductile grain deformation were more extensive in the estuarine facies resulting in a greater permeability reduction during burial diagenesis than was observed in fluvial sandstones abundant stylolites developed in sandstones with clay partings or mud clasts features that are common in estuarine sandstones the stylolites acted as an internal source of silica sandstones near the stylolites typically have low permeability that probably resulted from above average volumes of quartz cement diagenesis increased the coefficient of variation of permeability in all facies in the subsurface this diagenetic overprint should be considered if outcrop permeability data are used to model a subsurface reservoir having a very different burial history

salt and ice crystallisation in porous sandstones salt and ice crystallisation in the pore spaces causes major physical damage to natural building stones the damaging effect of these processes can be traced back to physically induced stress inside of the rock while crystallizing the increasing scientific research done during the past century has shown that there are numerous parameters that have an influence on the weathering resulting from these processes however the working mechanisms of the stress development within the rock and its material dependency are still subject to discussion this article gives an overview of salt and ice weathering additionally laboratory results of various sandstones examined are presented salt crystallisation tests and freeze thaw tests were done to obtain information about how crystallisation weathering depends on material characteristics such as pore space water transportation and mechanical features simultaneous measuring of the length alternating during the salt and ice crystallisation has revealed detailed information on the development of crystal in the pore spaces as well as the development of stress these findings can help to understand the damaging mechanisms

sea ice anomalies observed in the greenland and labrador seas during 1901 1984 and their relation to an interdecadal arctic climate cycle two independent ice data sets from the greenland and labrador seas have been analyzed for the purpose of characterizing interannual and decadal time scale sea ice extent anomalies during this century sea ice concentration data for the 1953 1984 period revealed the presence of a large positive anomaly in the greenland sea during the 1960s which coincided with the great salinity anomaly an upper ocean low salinity water mass that was observed to travel cyclonically around the northern north atlantic during 1968 1982 this ice anomaly as well as several smaller ones propagated into the labrador sea and then across to the labrador and east newfoundland coast over a period of 3 to 5 years a complex empirical orthogonal function analysis of the same data also confirmed this propagation phenomenon an inverse relation between sea ice and salinity anomalies in the greenland labrador sea region was also generally found an analysis of spring and summer ice limit data obtained from danish meteorological institute charts for the period 1901 1956 indicated the presence of heavy ice conditions i e positive ice anomalies in the greenland sea during 1902 1920 and in the late 1940s and generally negative ice anomalies during the 1920s and 1930s only limited evidence of the propagation of greenland sea ice anomalies into the labrador sea was observed however probably because the data were from the ice melt seasons on the other hand several large ice anomalies in the greenland sea occurred 2 3 years after large runoffs in the early 1930s and the late 1940s from northern canada into the western arctic ocean similarly a large runoff into the arctic during 1964 1966 preceded the large greenland sea ice anomaly of the 1960s these facts together with recent evidence of climatic jumps in the northern hemisphere tropospheric circulation suggest the existence of an interdecadal self sustained climate cycle in the arctic in the greenland sea this cycle is characterized by a state of large sea ice extent overlying an upper layer of cool relatively fresh water that does not convectively overturn which alternates every 10 15 years with a state of small sea ice extent and relatively warm saline surface water that frequently overturns

the caledonian strike swing and associated lineaments in nw ireland and adjacent areas sedimentation deformation and igneous intrusion patterns part of the major swing in strike associated with the greenland labrador promontory of the north atlantic caledonides is exposed in considerable detail in the metasedimentary and meta igneous rocks of the neo proterozoic dalradian supergroup in northwest ireland we show that the strike swing controlled dalradian stratigraphy and sedimentation patterns and is therefore at least c 600 ma in age our interpretation of the stratigraphy allows a reconstruction of this promontory in the laurentian continental margin and shows that it separated a ne trending gently inclined and broad continental shelf in the north from an e trending more steeply inclined and narrower shelf to the south once established in the geometry of the dalradian sequence the strike swing and basement promontory had a major influence on caledonian deformation patterns it controlled the nucleation of major folds the geometry of the regionally developed primary cleavages the facing direction of the early folds and it also acted as a major buttress in perturbing and deflecting on a regional scale the orogenic transport vector accurate restoration of the sinistral displacement on a late caledonian fault system shows that the apices of the strike swing at different stratigraphic levels and their associated sedimentary facies changes define a line trending approximately n12oe this line is coincident with again after fault restoration six out of the eight devonian donegal granites the vast majority of the mantle related ultrabasic appinite suite with most of these bodies concentrated at the intersection of this line and the ne trending main donegal granite shear zone and the highest density of the neo proterozoic basic tholeiite suite thes e and other data are used to suggest that the donegal lineament is the expression of a major steeply inclined fault in the basement beneath the dalradian which is intimately related to the morphology of the promontory the fault which might be as old as 1800 ma probably reached down into the lithospheric mantle controlling igneous activity including the ascent emplacement site and perhaps sources of the magmas another complementary strike swing and parallel lineament is described from the dalradian rocks of the adjacent part of scotland it is suggested that these and other major lineaments i e old pre caledonian faults which have either nne ne or ese trends reflect not so much the location of the northern british isles at a 120o triple junction during iapetus rifting but rather the exploitation of these pre existing trends in the pre caledonian atlantic borderlands during iapetus opening

point of zero charge determination in soils and minerals via traditional methods and detection of electroacoustic mobility points of zero charge were determined on two highly weathered surface soils from puerto rico an oxisol and ultisol as well as mineral standard kaolinite and synthetic goethite using three methods 1 potentiometric titration measuring the adsorption of h and oh on amphoteric surfaces in solutions of varying ionic strength i point of zero salt effect 2 direct assessment of surface charges via non specific ion adsorption as a function of ph and i point of zero net charge and 3 electroacoustic mobility of reversible particles as it varies with ph and i isoelectric point the first two methods yielded points of zero charge for kaolinite 2 7 3 2 and synthetic goethite 7 4 8 2 comparable to those reported previously indicating the reliability of these analyses the soil values ranged from 3 9 to 4 4 for the oxisol and 2 3 to 3 7 for the ultisol electroacoustic mobility as measured by the acoustosizer is a parameter that has yet to be thoroughly tested for mineral or soil systems as a viable alternative to pzc assessment the points of zero charge from electroacoustic mobility of kaolinite 3 8 4 1 and synthetic goethite 8 1 8 2 were similar to values obtained by electrophoretic mobility furthermore the values found for the oxisol 3 4 3 5 and ultisol 2 6 2 7 were in the range expected for these soils

the antrim galway line a resolution of the highland border fault enigma of the caledonides of britain and ireland the westward continuation of the highland border fault of scotland hbfz into ireland is problematic it is widely thought to follow a pronounced magnetic and gravity lineament the fair head clew bay line fcl the advantage of this interpretation is that it places all the ordovician ophiolitic complexes and associated sedimentary basins to the south of the fcl which would represent the contact between laurentia and the outboard terranes its main shortcomings are that both the deep structure and timing of strike slip are different on the hbfz and fcl in ireland the fcl is a north dipping feature that can be traced to the moho on birps profiles while the hbfz has no such signature terrane amalgamation in western ireland was completed by the late ordovician while in scotland the midland valley terrane did not finally dock until the early devonian these considerations suggest that in western ireland a branch of the hbfz exists which was active in post ordovician time and must lie south of connemara an examination of irish geological geophysical and image processed magnetic data shows that a profound lineament can be traced from antrim to galway bay the antrim galway line stitching plutons date movement on it as pre 405 ma we propose that the antrim galway line represents the continuation of the scottish hbfz while the fcl is a preserved ordovician splay of the hbfz system whose northdipping geometry is a product of ordovician collapse of the orogen in western ireland

winter atmospheric circulation in the arctic basin and possible relationships to the great salinity anomaly in the northern north atlantic variations in synoptic activity are assessed in terms of potential influences on ice transport and indicators of shifts in the arctic atmospheric circulation related to the great salinity anomaly of the northern north atlantic winter pressure anomalies suggest an increased contribution to the fram strait sea ice flux by multiyear ice from along the northern coasts of greenland and ellesmere island this may be related to a teleconnection involving changes in anticyclonicity over the central arctic ocean and canadian arctic archipelago

impact of recent north atlantic anomalies on surrounding land areas based on dendroclimatic evidence tree rings are useful indicators of interannual to decadal scale climate variability in this paper we show that tree ring data from the northwestern atlantic sector may reflect the great negative salinity gsa and related positive sea ice anomalies of the 1960 s 70 s along with cold surface air temperatures at coastal land stations there were declines in ring widths in the 1970 s in temperature sensitive birch trees in southern greenland white spruce in labrador and white spruce and white pine from newfoundland tree ring and other data from both sides of the north atlantic also indicate a decadal scale warm event with subsequent cooling around the 1820 s 1830 s

the structure and tectonic evolution of the aguilon fold nappe sierra alhamilla betic cordilleras se spain abstract detailed structural work in the sierra alhamilla se spain shows that the aguilon nappe comprising triassic and older metasedimentary rocks is a fold nappe the most prominent set of small scale folds changes from dominantly n vergent in the upper right way up limb to s vergent in the greatly thinned lower limb the nappe closes to the north and must have been emplaced in this direction nappe formation was accompanied by small scale folding and extensive solution transfer producing a pronounced differentiated crenulation cleavage these structures overprint an earlier set of folds and cleavage and are overprinted in turn by late n vergent structures the lower limb of the nappe was thinned and disrupted during continued nappe transport pre triassic schist in the core of the nappe was affected by medium grade metamorphism of probable post triassic age the contact with low grade permo triassic sediments above and below the schist coincides with a distinct change in metamorphic grade this contact may be a post metamorphic extensional fault that is now folded around the nappe which suggests that nappe formation was preceded by extensional faulting this is consistent with gravity spreading as the driving process for nappe emplacement

the structure of the helvetic nappes and its relation to the mechanical stratigraphy abstract this paper discusses structure and variations in structural style due to changes in mechanical stratigraphy of the helvetic nappes based on five cross sections the ratio n of the thickness of mechanically weak to the thickness of adjacent mechanically strong layers is of particular importance a low value of n favours imbricate thrusting and harmonic folding a high value favours detachment and disharmonic folding the transition at around n 0 5 is characterized by polyharmonic folding stratigraphic thickness variations due to local basin subsidence resulted in arcuate fold hinges and important plunges upon basin inversion lateral variations in the overall structure of the helvetic nappes is largely controlled by the initial irregular arrangements of the various small basins one more or less coherent thrust sheet the cretaceous stockwerk of the wildhorn drusberg santis nappe can be traced along the entire fold and thrust belt

effects of stratigraphic heterogeneity on permeability in eolian sandstone sequence page sandstone northern arizona outcrop and core data from the eolian middle jurassic page sandstone of northern arizona show that realistic reservoir characterization can be based on stratigraphic architecture outcrop displays of differential weathering coloration cementation and seepage reflect stratigraphic control of fluid flow within the sandstone measurements of permeability using a field permeameter and conventional analysis yield a five order of magnitude range of values between distinct populations extra erg and or interdune deposits are least permeable 0 67 1 800 md followed by wind ripple strata 900 5 200 md with grain flow strata being the most permeable 3 700 12 000 md extra erg and interdune deposits and some extensive bounding surfaces form significant flow barriers and ten to compartmentalize the reservoir reservoir characteristics of the dune cross strata are a function of the types distributions and orientations of the internal stratification directional flow properties within the strata are most pronounced within wind ripple laminae recognition of the levels of reservoir heterogeneity is sampling scale dependent general models for flow within single sets of cross strata as well as within eolian sequences can be generated based upon relative permeability between stratigraphic units

s c mylonites two types of foliations are commonly developed in mylonites and mylonitic rocks a s surfaces related to the accumulation of finite strain and b c surfaces related to displacement discontinuities or zones of relatively high shear strain there are two types of s c mylonites type i s c mylonites described by berthe et al typically occur in deformed granitoids they involve narrow zones of intense shear strain which cut across mylonitic foliation type ii s c mylonites described here have widespread occurrence in quartz mica rocks involved in zones of intense non coaxial laminar flow the c surfaces are defined by trails of mica fish formed as the result of microscopic displacement discontinuities or zones of very high shear strain the s surfaces are defined by oblique foliations in the adjacent quartz aggregates formed as the result of dynamic recrystallization which periodically resets the finite strain clock these oblique foliations are characterized by grain elongations alignments of segments of the grain boundary enveloping surfaces and by trails of grains with similar c axis orientations examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of s c tectonites and a deviation from the general tradition of purely geometric analysis of foliation and time relationships kinematic indicators such as those discussed here allow the recognition of kilometre scale zones of intense non coaxial laminar flow in crustal rocks and unambiguous determination of the sense of shear

the caledonian evolution of the laurentian margin in western ireland the highland border clew bay fair head baie verte brompton line which probably marks the laurentian margin passes through achill island in western ireland as a zone of ductile transpression within which the clew bay complex and dalradian supergroup are juxtaposed dextral greenschist facies transpression fabrics in the clew bay complex record the northwest directed emplacement of terranes bearing high p low t mineral assemblages against laurentia causing the dalradian continental margin sediments to be thrust westward there are significant differences in timing structural geometry and regional facing between w ireland and the scottish highland border zone

basement cover relationships in the tambo nappe central alps switzerland geometry structure and kinematics in the pennine zone of the central alps the tambo nappe forms a thin crystalline sliver embedded in the mesozoic cover four tertiary alpine deformations are observed the d1 ductile deformation is linked to the progressive eocene stacking of the nappes towards the northwest during d1 basement deformation is governed by imbricate tectonics whereas cover is thin skinned and intensively folded these different structures reflect the original strong rheological contrast between basement and cover during the heterogeneous and ductile d2 deformation the behaviour of the basement and cover became similar the strong vertical d2 shortening associated with a top to the east shear led to the folding of the prior se dipping structures developing se vergent folds with axes close to the e w l2 stretching lineation d2 corresponds to post collisional crustal thinning following d2 crustal thickening the d3 and d4 deformations occurred under retrograde conditions and can be correlated with uplift and late dextral movement on the insubric line respectively

metamorphism of an early palaeozoic continental margin western baie verte peninsula newfoundland metamorphic mineral assemblages and textures from early palaeozoic continental margin rocks in north western newfoundland indicate that different structural levels have contrasting metamorphic histories rocks of the east pond metamorphic suite which represent the older structurally lower level of the margin experienced an early high pressure low temperature stage of metamorphism 10 12 kbar minimum 450 500 c which produced eclogite in mafic dykes and phengite garnet assemblages in pelites this was overprinted by higher temperature lower pressure amphibolite facies metamorphism 700 750 c 7 9 kbar minimum which produced complex symplectic textures in rocks of all compositions rocks of the fleur de lys supergroup which were deposited in the stratigraphically higher levels of the rifted margin reached pressures of 7 8 5 kbar at about 450 c during the early stages of metamorphism overprinted by assemblages which indicate maximum temperatures of 550 600 c at about 6 5 kbar the metamorphic history of both units is interpreted to be the result of thermal relaxation following initial burial of a continental margin by overriding thrust sheets since there is no evidence that maximum pressures or temperatures within the fleur de lys supergroup were ever as high as those reached in the east pond metamorphic suite these rocks may have followed parallel nested p t t paths with the more deeply buried east pond metamorphic suite subjected to greater thermal relaxation effects quantitative modelling of p t t paths is not possible with the present data owing to both large uncertainties in p t estimates and in the time of metamorphism

orogen parallel extension and oblique tectonics the relation between stretching lineations and relative plate motions many collisional orogens exhibit two distinct deformation phases the first and relatively early phase forms within an obliquely subducting continent and is characterized by regionally consistent stretching lineations subparallel to the orogen and to associated isoclinal and sheath folds second phase folds and stretching lineations develop during and after imbrication of the subducting plate into the overriding plate and they become progressively disharmonic with stretching lineations oriented perpendicular to the orogen structural morphology and stretching lineations indicate that a fundamental kinematic transition occurs as material is imbricated from footwall to hanging wall kinematic modeling based on modern oblique convergent boundaries and considerations of buoyancy forces suggest that only the relatively early structures reflect the relative plate motion but that the correspondence is not simple in particular the orientation of stretching lineations is a function of finite strain kinematics of deformation and angle of subduction and convergence and it will consistently underestimate the angle of oblique convergence second phase structures reflect isostatic emergence of the hanging wall and are independent of relative plate convergence examples are drawn from the south central canadian rockies the western alps the variscides of ireland and france and the himalaya

geochemical evolution of an ordovician island arc south mayo ireland early ordovician volcanic rocks exposed in the south mayo region of western ireland document the history of a volcanic arc complex produced following the initiation of south dipping subduction within the iapetus ocean in the late cambrian or early ordovician lavas of the lough nafooey group tremadoc arenig show an eruption history marked by an initial period of tholeiitic basaltic volcanism followed by a shift to increasing silica compositions of calc alkaline affinity subsequent volcanism of the tourmakeady group arenig shows a mixture of rhyolitic and andesitic compositions which are in turn succeeded by tuffs of the rosroe and mweelrea formations of the murrisk group llanvirn these are also rhyolitic and andesitic but are generally more siliceous the overall increase in silica contents may reflect increasing fractional crystallization due to crustal thickening trace element studies of the basaltic members of the lough nafooey group show a strong subduction zone influence while absolute levels of immobile trace and rare earth elements decrease upsection relative depletion of the most incompatible elements decreases over the same time period these trends suggest decreasing degrees of partial melting of a progressively more depleted source mantle upsection the basaltic bohaun volcanic formation tremadoc arenig shows greater depletion in its incompatible trace elements than the lough nafooey group so indicating a more depleted mantle source comparison with modern arc systems suggests that the bohaun volcanic formation was probably generated shortly after the initiation of subduction and was erupted in a more trenchward position than the lough nafooey group i e in the forearc this confirms a south dipping polarity to the subduction zone and suggests an age of initiation during the late cambrian or earliest ordovician

the stereographic analysis of facing abstract facing directions represent an accurate means by which tectonic structure and sedimentary way up may be related a simple method is presented for determining facing directions accurately using a stereonet once determined larger amounts of facing data can be quickly collated and analysed using similar techniques to those employed for other lineations an example is given for some sheath fold structures in the moine rocks of sutherland n scotland

formation of recumbent folds during synorogenic crustal extension austroalpine nappes switzerland in sedimentary rocks of the austroalpine nappes in eastern switzerland several generations of alpine cretaceous tertiary folds can be distinguished by overprinting relations the second generation d 2 probably late cretaceous in age includes spectacular kilometre scale folds refolding the earlier formed nappe pile d 2 folds are interpreted here as resulting from east southeast directed crustal extension rather than as previously assumed from north south crustal shortening for the following reasons 1 d 2 folds are restricted to a predominantly ductile layer underlying the brittlely extended orogenic lid and 2 d 2 folds are associated with structures indicating extension in a direction parallel to the extension direction of the brittle layer before the east southeast extension started the layering of the rocks was already steeply inclined owing to earlier d 1 folding the deformation producing the second phase folds probably had a noncoaxial component with a top to the east southeast shear sense and a component of coaxial deformation leading to shortening in the vertical direction and stretching in an east southeast horizontal direction the folds developed with axes oblique to parallel to the direction of bulk stretching i suggest that folding of initially nonhorizontal layers by ductile crustal extension is an important deformation mechanism in the austroalpine and possibly also in other parts of the alps initially steep orientations of layers resulted from transpressive deformation in the obliquely convergent alpine belt and crustal extension parallel to the chain resulted from gravitational spreading i propose that this process can lead to the often encountered phenomenon of stretching parallel to the axes of folds

a high precision u pb age for the ben vuirich granite implications for the evolution of the scottish dalradian supergroup high precision high accuracy u pb analyses of zircons from the ben vuirich granite in the scottish dalradian succession were performed on carefully picked high integrity abraded fractions of specific morphologies zircon needles which grew during crystallization give an age of 590 2 ma stubby prisms show inheritance with an upper concordia intercept of 1448 7 ma the age of intrusion at 590 ma contrasts strongly with a previous u pb zircon age of 514 6 7 ma determined on bulk fraction unabraded grains which it is argued must have suffered significant pb loss as the ben vuirich granite was emplaced between the regional d2 and d3 deformation events in the dalradian block the new data imply that all dalradian sedimentation was precambrian supposed lower cambrian fauna1 links between the dalradian succession and laurentia are thus discounted and the possibility that the dalradian supergroup has non laurentian affinities must be considered the early d1 d2 nappe forming grampian structures are also precambrian and may be associated with terrane docking along a tectonically active ocean margin later d3 structures and associated peak barrovian metamorphism suggest a subsequent thickening event possibly c 520 ma ago

major southeast directed caledonian thrusting and folding in the dalradian rocks of mid ulster implications for caledonian tectonics and mid crustal shear zones the dominant structure controlling the disposition of dalradian stratigraphy in mid ulster has hitherto been regarded as a southeast facing gently inclined f 1 anticline a gross geometry modelled on and thought to be a possible correlative of the tay nappe in scotland remapping of the supposedly inverted southern limb of this major fold reveals that much of it is in fact the correct way up however a stratigraphie repetition coupled with a reversal in younging does occur in the sperrin mountains much further south than previously realized this hitherto unrecognized upward southeast facing isoclinal sperrin nappe is however a d 2 structure traceable for at least 40 km along strike and responsible for a regional stratigraphie inversion over an area of 300 km 2 following d 2 a major 10 km thick d 3 ductile shear zone resulted in translation towards the east southeast in the south this deformation carried the dalradian over ordovician volcanics of the tyrone igneous complex along the omagh thrust penecontemporaneity of magmatism with deformation clearly demonstrates that d 3 is caledonian arenig llanvirn this deformation correlates with similar southeast directed caledonian thrusting in southern donegal and connemara the apparent absence of dalradian deformation of this age in southwestern scotland may imply that caledonian collision of outboard terranes with the miogeoclinal margin was initiated in ireland and or subsequent strike slip has removed the evidence for deformation of this age from southwestern scotland the d 3 shear zone in the sperrin mountains affects a very large volume of psammitic rocks within this shear zone the strain is not markedly higher than surrounding areas however its existence is demonstrated by the reorientation of mineral lineations over a large area such broad zones of only moderate strain may we believe be typical of translatory tectonics in areas of the mid crust where there is little lithological diversity

reconstruction of past ice conditions in a hudson bay estuary using tree rings descriptions of ice conditions ranging from lake and sea ice to glacial advances and retreats form an important portion of the climatic record ice formation deterioration and changing areal extent are of increasing interest in climatology sea transportation petroleum exploration and the fishing industry with few exceptions quantitative information on ice conditions is generally brief with many records in the us and canada beginning only in the 1950s1 tree ring data from long lived trees can be used as quantitative proxy data for past climatically related parameters the record of first complete freezing of the churchill river estuary on hudson bay is reconstructed here for 298 yr 1680 1977 by means of tree ring analysis

the interpretation of the icelandic sea ice record the representativeness of the icelandic sea ice record one of the longest climate indicators for the north atlantic sector is assessed by statistical analysis correlation of the sea ice index with ice concentration data for the adjacent seas suggests that the index is a reasonable measure of the ice cover of a large area of the northern north atlantic sector associations with surface air temperature and sea level pressure are strong but regional rather than hemispheric in scale the immediate causes of the variable advance of ice onto the icelandic coasts are clear in the analysis of the atmospheric circulation data severe ice conditions are associated with enhanced anticyclonicity in the icelandic region and stronger northerly and westerly winds over the greenland sea it is concluded that the icelandic index is a useful indicator of the state of the atmospheric flow north of iceland and of the character of the east greenland current one of the major avenues of meridional heat exchange in the climate system

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

wetting weakening of tertiary sandstones microscopic mechanism the micromechanism accounting for wetting weakening of tertiary sandstones was studied it was found that intragranular fracture prevails for all dry sandstones however when the sandstone is wet intergranular fracture occurs for type b sandstones therefore one sandstone from type a sandstones ms1 and another from type b tk were selected to further investigate the nature of the matrix it was found that 1 for both sandstones the major mineral components of the matrix are illite and kaolinite except that the ms1 sandstone has more chlorite 2 leaching of matrix induced an increase of porosity and consequently results in leaching softening and 3 among the mineral composition chlorite is easiest to be dissolved and leached out and induces a more significant increase of porosity which in turn results in a more significant leaching softening

ultrasonic properties of sedimentary rocks effect of pressure saturation frequency and microcracks seismic velocities and attenuations are influenced by lithology porosity and permeability as well as the kind and quantity of the pore fluids the microstructure of rocks is another important factor influencing seismic properties this influence can be used to investigate the pressure dependent closure of microcracks for this purpose velocities and attenuations of p and s waves have been determined at ultrasonic frequencies in three different rocks vacuum dry and partially saturated with water as a function of hydrostatic pressure up to 200 mpa a new combined model has been developed with this model an explicit description of the microstructure local fluid flow lf and macrostructure gassmann effect global fluid flow is possible assuming a patchy saturation i e inhomogeneous distribution of the fluid in the pores and modulus reduction both the saturation dependent measurements and the pressure dependent data can be explained the modelling and the phenomenological interpretation of the data gained under increasing hydrostatic pressure yields consistent results concerning the amount of crack closure as well as the range in which crack closure appears in high porosity sandstones bentheimer and obernkirchner sandstone a strong closure of cracks already with a small increase of pressure must be concluded in a low porosity harzer greywacke a significantly lower closure of cracks can be concluded within the pressure range of 0 100 mpa it is shown however that there must be open cracks in all rocks even under elevated hydrostatic pressure resulting in a measurable effect due to lf

does the birchy complex of newfoundland extend into ireland metabasalts from the birchy complex in newfoundland are chemically similar to the kill callow suite of metabasalts in co mayo ireland the latter which are also chemically akin to cambrian amphibolites of the highland border series of scotland lie along the shear zone associated with the fair head clew bay line which has been equated with the baie verte brompton line in newfoundland the birchy complex occupies an analogous tectonic position along the baie verte brompton line and hence we postulate that chemical similarities suggest a correlation between the birchy complex of newfoundland and the kill callow suite of ireland

role of shear in development of the helvetic fold thrust belt of switzerland the geometric features of the helvetic nappes of the alps show intense internal deformation set up predominantly by simple shear during nappe transport the nappes are bounded by thrusts that parallel incompetent layers and ramp upward stratigraphically across competent units in the direction of tectonic transport stratal layering is oblique to nappe boundaries thrust surfaces formed after ductile shear zones developed in underlying pre triassic gneissic basement the broad zone of shearing narrowed down as deformation increased to form zones of intense shear and ultimately to form thrust faults contraction along competent layers obliquely inclined to shear surfaces produced buckle folds the geometry of these folds was modified by further shearing as their normal limbs became aligned in the extension field of the strain ellipse for simple shear producing normal faults

structural and stratigraphic relationships across the continuation of the highland boundary fault in western ireland t he relationship between the dalradian supergroup and the highland border complex in scotland has remained contentious for over a century in western ireland the contact between the dalradian supergroup and the clew bay complex a correlative of the highland border complex is superbly exposed on the island of achill beg on the north mayo coast the unfossiliferous south achillbegsuccessionhasbeentraditionally assignedtotheclewbaycomplex andthisinterpretation is supported by a combination of sm nd model age data heavy mineral analysis and lithostratigraphic correlation tdm ages range from 1 99 2 66 ga mean 2 28 ga n 6 detailed structural mapping shows that both the dalradian and the clew bay complex share the same structural history a d1 high strain event is common to both units and is associated with the development of tectonic slides the d2 event is responsible for the formation of crustal scale nappes in both units beds are consistently downward facing on the s2 foliation later dextral shearing d3 resulted in the tilting of the originally recumbent s facing d2 nappes into this downward facing orientation rb sr and 40 ar 39 arradiometricdatingofmuscoviteconfirmsthatbothunitsweredeformedcontemporaneously as the s2 nappe fabric in each is dated at c 460 ma this middle ordovician age for deformation of the clew bay complex is highly significant not least because published microfossil data suggest a silurian age

u pb zircon ages of basic rocks and gneisses intruded into the dalradian rocks of cashel connemara western ireland new isotopic ages are presented from the syntectonic cashel lough wheelaun basic intrusion and an associated k feldspar granite gneiss of the connemara gneiss complex from lettershinna hill the linear normally discordant distribution of data points from the cashel lough wheelaun metagabbro indicates that the zircons in this intrusion have no significant inherited radiogenic pb they yield an upper concordia intercept age of 477 25 6 ma and a more precise 207 pb 206 pb age of 490 1 ma in contrast zircons from the k feldspar granite gneiss show distinct reverse discordance the non magnetic fractions show a good linear arrangement yielding a lower intercept age of 454 6 14 ma and a very uncertain upper intercept age of 1807 646 570 ma the lower intercept age is younger than the 477 9 ma k ar age of hornblendes in gneisses from lettershinna hill elias et al 1988 and therefore reflects a pb loss episode younger than igneous crystallization the imprecise proterozoic upper intercept age indicates contamination by older inherited and only partially reset zircons

parallel stretching lineations and fold axes oblique to a shear displacement direction a model and observations abstract spatial variations in shear strain rate are expected in ductile shear zones where the variation is a change in shear strain perpendicular to the displacement direction the effect is to rotate the shear slip planes this is a mechanism for giving a rotation of fold axes into parallelism with the slip and extension direction in a rock if such a variation in shear strain affects rocks with a strong planar anisotropy it is possible to produce a fabric with an apparent stretching lineation parallel to fold axes but both significantly oblique to the slip direction a possible example of this is seen in strongly deformed quartz mica schists from syros greece where a stretching lineation is seen parallel to fold hinges over a range of fold axes orientations of at least 40

basalts water or shear zones in the lower continental crust abstract bright sub horizontal laterally extensive multi cyclic layered reflections have now been recorded in the lower continental crust from many areas in contrast this reflection signature is rare in the continental upper crystalline crust and upper mantle and in the oceanic lithosphere with the present state of knowledge it is sensible to seek a single quantitative explanation for all such layering qualitative explanations for lower crustal layering abound and include igneous metamorphic structural and fluid related models measurements of reflection coefficients from the lower crust place strong quantitative constraints on possible models structurally induced anisotropy metamorphic layering or free fluids cannot realistically produce sufficiently bright reflections igneous layering or the juxtaposition of pre existing heterogeneities by shear zones can produce strong reflections results from crustal xenoliths helium isotope ratios palaeo geothermal gradients and chemistry of flood basalts together with numerical modelling of lithospheric extension and asthenospheric melting suggest that underplating and intrusion of the lower crust with mantle derived mafic igneous rocks is common the apparent spatial relationship between lower crustal layering and continental extension and the strength and continuity of the reflections suggest that underplating related to extension and or elevated mantle temperatures is a good explanation for lower crustal layering

relationships between distributed and localized shear in the tectonic evolution of a caledonian fold and thrust zone northwest ireland broad zones of distributed shear operating through mid crustal regions of orogenic belts may incorporate narrow horizons of intense localized deformation culminating in discrete large magnitude displacements the relative importance and relationship between distributed and localized shear are influenced by a variety of factors including lithological variation pre existing structural anisotropy strain rate and migration of fluids rigorous structural analysis of lower amphibolite facies dalradian metasediments in northwestern ireland demonstrates that an early d 1 discrete ductile detachment was subsequently reactivated during distributed non coaxial d 2 deformation operating in a broad zone through the structural pile regional shear was directed towards the southeast and resulted in the generation and translation of kilometre scale isoclinal recumbent sheath folds which close and face towards the transport direction the d 1 detachment is clearly folded around the hinges of these major folds whilst on fold limbs it was reactivated and acted as a local decollement within the zone of distributed shear shear criteria along the detachment indicate a southeast directed translation of the major folds in sympathy with regional shear a broad zone of d 3 translation operating through the nappe pile resulted in coaxial refolding of large scale f 2 folds by the d 3 ballybofey nappe producing a complex fold interference pattern non coaxial d 3 deformation resulted in continued reactivation of local decollements together with the initiation of east southeast directed oblique thrusts and partial dismemberment of d 2 folds detailed structural investigation allows concepts of distributed and localized shear to be evaluated and models of crustal deformation to be assessed

the great salinity anomaly in the northern north atlantic 1968 1982 the widespread freshening of the upper 500 800m layer of the northern north atlantic which this paper describes represents one of the most persistent and extreme variations in global ocean climate yet observed in this century though a range of explanations have been advanced to explain this event including in situ changes in the surface moisture flux this paper describes the great salinity anomaly as largely though not entirely an advective event traceable around the atlantic subpolar gyre for over 14 years from its origins north of iceland in the mid to late 1960s until its return to the greenland sea in 1981 1982 the overall propagation speed around this subpolar gyre is estimated at about 3cm s 1 of the total salt deficit associated with the anomaly as it passed south along the labrador coast in 1971 1973 about 72 109 tonnes a deficit equivalent to about two thirds of this figure 47 109 tonnes ultimately passed through the faroe shetland channel to the barents sea arctic ocean and greenland sea during the mid 1970s possible effects on deep water formation and on the representativeness of historical section data are discussed

caledonian extension in the north irish dalradian implications for the timing and activation of gravity collapse following caledonian crustal thickening and prograde metamorphism of the dalradian rocks on the se laurentian margin in mid ulster down dip gravity controlled extension due to orogenic collapse was concentrated in the inverted limbs of the major ese facing fold nappes structural and metamorphic studies show that this extensional collapse which can be correlated along strike for at least 100 km occurred immediately after the crustal thickening episode such a relationship is compatible with dynamic wedge models for orogenic belts our data also imply that a collapse was triggered by the release of fluids during the prograde metamorphism and b late stage mobilization of the extensional zone created the gold mineralization in the sperrin mountains

chemostratigraphic correlation structure and sedimentary environments in the dalradian of the nw co mayo inlier nw ireland a thorough geochemical survey of the dalradian rocks in the nw mayo inlier has shown that each formation has a characteristic geochemical signature within regional geochemical trends that vary systematically with stratigraphic age this geochemical characterization enables correlation between geographically and tectonically isolated stratigraphic successions in the nw mayo inlier an almost complete succession of late proterozoic grampian appin and argyll group dalradian rocks structurally overlies earlier basement annagh division gneisses in structural blocks bounded by caledonian thrusts extensional low angle faults and steeply dipping faults grampian appin group relationships are better seen here than anywhere else in scotland or ireland but no tayvallich subgroup or southern highland group dalradian rocks are present comparison with chemostratigraphic data from scottish dalradian rocks reveals broadly similar chemical trends with time suggesting that the geological evolution of the dalradian took place in a related depositional environment probably an unbroken continental margin along the entire dalradian outcrop seen in both scotland and ireland

continental crust under the southern porcupine seabight west of ireland abstract two new seismic refraction wide angle reflection profiles demonstrate that the crust beneath the southern porcupine seabight out to water depths in excess of 4000 m is of continental type they also reveal the rifted margin of the porcupine basin on its eastern side crustal thickness under the seabight inclusive of sediments which are up to 6 km thick decreases from 23 km in the east to about 10 km at a sharp continent ocean transition in the west

models of strain variation in nappes and thrust sheets a review abstract strain within nappes and thrust sheets can be modelled using simple boundary conditions an original rectangular prism of material resting on a thrust plane is considered to be subject to various pure and simple shear components operating parallel to the thrust deformation gradient tensors are determined for the models and solutions found for the principal strains a two dimensional model considers heterogeneous simple shear combined with a stretch in the direction of shear simple shear zones arise as a special case where 1 recumbent fold nappes characteristic of gravitational collapse can be modelled with 1 and thrust sheets with layer parallel shortening and folding with differential transport of nappes gives rise to additional wrench type shears solutions for the principal strains in this type of model are found and used to interpret side wall ramps and steep zones parallel to nappe movements by including a stretch in the transport direction these differential transport models produce prolate strains 1 and oblate strains finally a brief consideration of bending strains produced in a sheet passing over a ramp indicates reversals in the shear direction during the strain history these predict possible crenulation of early thrust fabrics at ramps

extensional shearing deformation and kilometer scale a type folds in a cordilleran metamorphic core complex raft river mountains northwestern utah cordilleran metamorphic core complexes are characterized by ductile mylonitic deformation and regional stretching lineations in the raft river mountains a mylonitic zone and a late superimposed detachment fault separate precambrian crystalline and metasedi mentary basement rocks from an upper plate made of allochthonous paleozoic rocks which suffered a brittle extensive deformation we suggest that the mylonitic ductile deformation is the result of tertiary extensional tectonics strain analysis shows that 1 strain results from large scale progressive shearing deformation in a non coaxial regime 2 e w regional stretching lineations represent the direction of shearing and transport 3 shear criteria indicate an east directed movement 4 strain magnitude is increasing eastward 5 kilometer scale folds parallel to the lineations are interpreted as a type folds formed by eastward shearing of a previously northward dipping layering measurement of striated faults in the upper plate gives an e w direction of extension parallel to the lineations thus ductile and brittle deformations are closely related to the same regional scale extensional tectonics a model is proposed to explain the observed geometry and kinematics of the structures the mesozoic to paleocene cordilleran compressional tectonics induced an overthickening of the crust in the hinterland belt at the future emplacement of the complexes important uplift abnormal heating and granite emplacement occurred later during the collapse of the former belt the combined effects of thermal and gravitational instabilities during tertiary extensional tectonics enabled the development of a ductile shear zone localized at the precambrian basement paleozoic cover interface the brittle extension of the upper crust is displaced laterally along this flat shear zone and transformed farther east into the extended regions of the middle and lower crust

terrane amalgamation in the clew bay region west of ireland the caledonides of the west of ireland provide a well exposed and well mapped example of an oblique collision zone the east northeast trending deer park and achill beg fault system is a crustal scale ductile sinistral strike slip duplex of late ordovician age imbricating late precambrian granulite facies lower crustal rocks near eclogite facies supracrustal rocks up to amphibolite facies dalradian metasedimentary rocks and greenschist facies cambro ordovician rocks this fault system is correlated with a pre devonian component of the highland boundary fault system in southern scotland in the clew bay area the high pressure low temperature facies metamorphic rocks in tectonic contact with greenschist facies cambro ordovician rocks are together interpreted as an accretionary prism complex related to northwestward directed subduction both of these are allocthonous terrains with respect to the dalradian terrane to the north north west mayo to the south the cambro ordovician rocks docked with a probable dalradian block containing ultramafic intrusives deer park complex during the late ordovician the deer park complex and south mayo trough linked earlier during the arenig silurian and lower middle devonian redbed successions sit unconformably on the metamorphic rocks deposition and deformation of these cover rocks was controlled by oblique strike slip movements on the leek fault whose strike swings from west northwest to north northeast following earlier basement trends as it is traced eastwards from clew bay the leek fault system may be correlated with the leannan fault of northwest donegal a splay of the great glen fault system of central scotland east of clew bay this sinistral shear generated local dilation on the more northerly trending bend of the leek fault lower and middle old red sandstone redbeds were developed here the west northwest trend of the leek fault in clew bay acted as a compressional bend during these sinistral movements and transpressional southwest directed thrusting developed in silurian rocks post middle old red sandstone pre late tournaisian dextral displacement on the leek fault reversed this pattern with transtension in clew bay allowing intrusion of small carbonated peridotite bodies into silurian rocks and easterly directed thrusting of middle old red sandstone rocks east of the bay on the transpressional north south bend a tectonic model for the region is presented here this model involves a northwestward directed subduction system 150 to 750 km of arenig sinistral strike slip movement and eastwards insertion of the connemara block with formation of the ordovician south mayo trough as a pull apart basin subsequently a further 130 to 650 km eastward displacement of rocks took place south of the deer park fault in later ordovician times the magnitudes of these estimates are directly proportional to an assumed maximum wavelength of 1500 km for promontories on the original laurentian margin and using the current juxtaposition of terranes a minimum wavelength of 300 km is inferred

pore scale imaging of geological carbon dioxide storage under in situ conditions 1 while geological carbon dioxide co2 storage could contribute to reducing global emissions it must be designed such that the co2 cannot escape from the porous rock into which it is injected an important mechanism to immobilize the co2 preventing escape is capillary trapping where co2 is stranded as disconnected pore scale droplets ganglia in the rock surrounded by water we used x ray microtomography to image at a resolution of 6 4 m the pore scale arrangement and distribution of trapped co2 clusters in a limestone we applied high pressures and temperatures typical of a storage formation while maintaining chemical equilibrium between the co2 brine and rock substantial amounts of co2 were trapped with an average saturation of 0 18 the cluster sizes obeyed a power law distribution with an exponent of approximately 2 1 consistent with predictions from percolation theory this work confirms that residual trapping could aid storage security in carbonate aquifers

kinematics of the northern part of the simplon line central alps abstract structural studies were carried out along the northern part of the simplon line in order to constrain the kinematic history of the line during tertiary times the ductile strain accumulated within both basement units and mesozoic cover reveals the upper units sheared towards the west southwest the simplon line can be described as a major shear zone along which the main movement occurred perpendicular to the line this shearing developed during retrograde metamorphic conditions and postdates earlier deformations associated with the high temperature lepontine episode finally it was followed by the formation of the berisal syncline back folding episode which generated some brittle movements towards the south localized at the contact between the lower and upper pennine units i e on the simplon line

caledonian transpressional terrane accretion along the laurentian margin in co mayo ireland the highland border fair head clew bay baie verte brompton suture clew bay suture on achill island western ireland is an e w trending zone of vertical ductile dextral transpression bounding the southern edge of the dalradian terrane the achill dalradian a sequence of coarse grained psammite overlain by conglomerate quartzite schist and limestone disposed in recumbent curvilinear folds represents laurentian continental margin deposits which were translated westward during deformation parallel to the suture trace studies of fabric development and albite porphyroblast growth indicate that e w dextral transpression and west directed recumbent folding were synchronous and are interpreted as separate expressions of the same nw directed terrane impingement event this process of terrane docking was marked by a c 4kbar phase of decompression within the suture four separate terrane slivers are recognized the presence of pyrope garnet and crossite amphibole within two imply the clew bay zone experienced sufficient displacement to exhume rocks from deep lithospheric levels in addition the extensive retrogression and destruction suffered by these remnant high pressure mineral assemblages during the albite generating transpressional phase implies that ductile deformation was enhanced by the introduction of fluid into and along the suture zone during impingement

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

an ordovician ophiolite in county tyrone ireland ophiolites are regarded widely by geologists as remnants of old oceanic crust that has been obducted onto the continental margins during ocean or marginal basin closure and orogeny therefore they provide evidence for the existence of old oceans and oceanic basins and are a key to the extension of plate tectonic modelling into pre mesozoic rock assemblages in the caledonide appalachian orogen ophiolites are known although very few of these are dated well enough to provide accurate constraints on plate tectonic kinematics we report here the discovery of an ophiolitic sequence within the rocks of the tyrone igneous complex in northern ireland u pb dating combined with field evidence indicates that the tyrone ophiolite was formed and then obducted onto the northern margins of the iapetus ocean close to 470 myr bp arenig

sequential porphyroblast growth and crenulation cleavage development during progressive deformation abstract six stages of crenulation cleavage development during a single deformation event d2 can be recognised in both matrix and porphyroblast inclusion trails in the robertson river formation n e australia these stages progress from undeformed to crenulation and differentiated crenulation of s1 through development of differentiated crenulation cleavage s2 and differentiated schistosity s2 to homogeneous foliation s2 these rocks underwent prograde metamorphism during d2 and chloritoid garnet staurolite andalusite and sillimanite isograds have been mapped most rocks are rich in porphyroblasts which commonly contain well defined inclusion trails the geometry of these trails varies from one mineral type to the next depending on the timing of porphyroblast growth relative to the stage of crenulation cleavage development in the schistose matrix the deformation history involved progressive bulk inhomogeneous shortening and the strain is very heterogeneous on all scales partly as a consequence of this hence each of the six stages of crenulation cleavage development formed locally at various times during d2 therefore if a porphyroblast grew late in the deformation it could overgrow all stages of crenulation cleavage development including the very early ones the temperature increased during metamorphism in any one locality and consequently some porphyroblasts overgrew others that were unstable in the new conditions preserving them from destruction hence the precise timing of mineralogical reactions deduced from isograds and mineral chemistry relative to the stages in schistosity development can also be determined in the andalusite zone the sequence of porphyroblasts in decreasing order of age is biotite garnet biotite staurolite andalusite dissolution of garnet occurred during growth of staurolite and especially andalusite and dissolution of staurolite accompanied andalusite growth

new method for measuring compressibility and poroelasticity coefficients in porous and permeable rocks over the last decades a large understanding has been gained on the elastic properties of rocks rocks are however porous materials which properties depend on both response of the bulk material and of the pores because in that case both the applied external pressure and the fluid pressure play a role different poroelasticity coefficients exist while theoretical relations exist measuring precisely those different coefficients remains an experimental challenge accounting for the different experimental complexities a new methodology is designed that allows attaining accurately a large set of compressibility and poroelasticity coefficients in porous and permeable rocks this new method relies on the use of forced confining or pore fluid pressure oscillations in total seven independent coefficients have been measured using three different boundary conditions because the usual theories predict only four independent coefficients this overdetermined set of data can be checked against existing thermodynamic relations measurements have been performed on a bentheim sandstone under water and glycerine saturated conditions for different values of confining and pore fluid pressure consistently with the poroelasticity theory the effect of the fluid bulk modulus is observed under undrained conditions but not under drained ones using thermodynamic relations i the unjacketed quartz and skeleton zimmerman s relation bulk moduli fit ii the drained and undrained properties fit and iii it is directly inferred from the measurements that the pore skeleton compressibility c phi is expected to be constant with pressure and to be exceedingly near the bulk skeleton c s and mineral c m compressibility coefficients plain language summary poroelastic properties of rocks are a major information for a large variety of applications linked to rupture processes in saturated media theories exist to link the properties to measurable parameters yet only little measurements exist to confront those theories this article aims at providing a method for accurately measuring those properties moreover for one particular rock a large amount of compressibility and poroelastic coefficients is measured and used to confront those theories

strike slip terranes and a model for the evolution of the british and irish caledonides evidence is presented that many of the major strike faults in the british and irish caledonides were active as sinistral strike slip zones in the end silurian to pre mid devonian period some such as the highland boundary fault moved in this way at an earlier stage in the ordovician these data allow the caledonian rocks lying between the laurentian miogeocline whose basement is represented by the lewisian moine and possibly the dalradian and the gondwanaland miogeocline midland platform and welsh basin to be re analysed as a group of disorganized terranes which originated to the southwest in north america and southwest europe africa prior to the silurian the highland border terrane and northern belt terrane are interpreted as duplicated pieces of a mid ordovician sequence which was a back are to northwest subduction the midland valley terrane is interpreted as a slice of laurentian foreland onto which ophiolites were obducted in the lower ordovician but which became the basement of a continental margin arc to northwest subduction in the mid ordovician the cockburnland terrane is inferred to be part of the same arc repeated and then broken up and dispersed by continuing strike slip the connemara terrane is regarded as an allochthonous piece of the dalradian miogeocline and the south mayo terrane as a remnant of an early ordovician arc and fore arc which in mid ordovician times became a back arc marginal basin to northwest subduction the lake district wexford terrane is part of an arc to southeast subduction under gondwanaland whose activity climaxed in the mid ordovician the central terrane is interpreted as a silurian overstep assemblage which blankets the junction between laurentian and gondwanaland derived oceanic terranes and therefore iapetus is regarded as an ordovician ocean which closed prior to the silurian the model suggests that at the end of the silurian a clockwise rotating gondwanaland having carried laurentia into collision with baltica broke free and created a major sinistral strike slip zone which disrupted the ordovician palaeogeography in the british isles north american sector of iapetus

nontypical birps on the margin of the northern north sea the shet survey summary striking similarities in the reflectivity of the crust and upper mantle on birps profiles has led to the development of the typical birp a model seismic section for the british continental lithosphere the shet survey collected in the region of the shetland islands and the northern north sea fits the general pattern to a certain extent caledonian structures and devonian or younger basins are imaged in the otherwise acoustically transparent upper crust an unexpected and exciting feature imaged on shet is a short wavelength structure on the moho or abrupt moho offset beneath the strike slip walls boundary fault shet differs markedly from the swat typical birp however by showing a poorly reflective lower crust only a narrow zone 1 s at the base of the crust contains high amplitude reflections the shet survey therefore highlights the wide variation in lower crustal reflectivity within the total birps data set rather than the similarities

a deep seismic reflection transect across the irish caledonides new reflection profiles recorded to 18s c 60km along a 600 km north south transect west of ireland wire lines provide a cross section of the irish caledonides from lewisian basement in the north to the variscan fold and thrust belt in the south and constrain the offshore extrapolations of major caledonide structures we interpret the profiles as showing the great glen fault and associated ne trending strike slip faults as near vertical structures which cut the whole crust and the uppermost mantle beneath the surface trace of the fair head clew bay line we image a north dipping zone which soles into horizontal layers just above the reflection moho the north dipping zone separates south dipping reflections above and to the north from north dipping reflections below and to the south from the fair head clew bay line as far south as the iapetus suture the lower crust contains north dipping packages of reflectors possibly related to acadian accretionary underplating the iapetus suture zone can be identified as a zone more than 50 km wide dipping north at 30 from 5 km depth offshore dingle peninsula to sole into the moho at c 30 km depth west of the shannon estuary in the upper crust the shallow part of the iapetus suture zone appears to be transported to the north by late caledonian retrocharriage or by variscan thrusting the iapetus suture does not penetrate through the present day reflection moho but there are scattered reflectors in the mantle not continuous with crustal reflectors perhaps relics of a subducted slab we recognize lower crustal reflectivity and the reflection moho along almost all the wire profiles regionally the travel time to the reflection moho varies from 9 s c 28 km offshore donegal northwest of ireland to 12 s c 35 km offshore kerry and cork southwest of ireland apparent moho offsets rapid lateral changes in the travel time to the reflection moho north of donegal and north of mayo are probably due to the great glen fault and associated faults the lower crustal reflectivity varies considerably in thickness and prevalent dip direction along the wire profiles in donegal bay this reflectivity is observed at depths less than 5 km c 1 5s whereas west of kerry prominent reflectivity is only observed beneath 6s c 17 km though the age of crustal reflectors cannot in general be determined beneath central ireland from donegal to dingle the lower crustal reflectors mimic in dip direction the structural vergence of the exposed dalradian and early palaeozoic rocks so that the lower crustal reflectivity patterns and probably much of the crust must be caledonian in age

comparison of the anisotropic behaviour of undeformed sandstones under dry and saturated conditions abstract this article presents a systematic analysis of the anisotropic behaviours of the bentheim and rothbach sandstones using ultrasonic p wave velocity electrical conductivity and magnetic susceptibility measurements for each sandstone the data were obtained from three core samples drilled perpendicularly to each other and tested in dry and water saturated conditions for acoustic and magnetic investigations the same statistical analysis was applied in order to present the data on comparable stereoplots surprisingly the bentheim sandstone which appeared homogeneous at macroscopic scale showed a stronger elastic and electrical anisotropy than the rothbach sandstone in which cross laminations were clearly identified as confirmed by a sedimentary magnetic fabric a discussion on the velocity contrasts between dry and saturated samples led us to consider two different origins for the observed anisotropies first by comparing electrical and acoustic properties in the bentheim sandstone we conclude that the nature of the anisotropic behaviour is linked to the anisotropy of pore shape the inclusion model developed by kachanov kachanov m 1993 elastic solids with many cracks and related problems advances in applied mechanics vol 30 academic press boston ma pp 259 445 accounts for our observations if one considers that the pore space is made of parallel flat pores with moderate pore aspect ratio second acoustic electrical and magnetic properties indicate that the observed anisotropy in the rothbach sandstone can be attributed to the matrix and more specifically to cementation we modified the dvorkin and nur geophysics 61 5 1996 1363 model of cemented granular media by introducing a spatially variable contact length and the model suggests that a very small variability of cemented contact length is enough to account for the observed p wave velocity anisotropy we emphasise the fact that combining several kinds of measurements is of great help in capturing the nature of the anisotropic behaviour of porous rocks

short paper stratigraphical correlations adjacent to the highland boundary fault in the west of ireland an ordovician age for at least part of the clew bay supercomplex a dismembered ophiolite together with contiguous deep water sediments and volcanics occurring adjacent to the highland boundary fault in western ireland supplement comparisons with the highland border complex of scotland based on lithological similarities an early phase of terrane development associated with ophiolite emplacement is represented in western ireland whilst the recognition of similar intra terrane magnafacies patterns permit a more complete analysis of the evolution of the highland border terrane