a numerical dynamic model for the norwegian danish basin abstract a 2d numerical dynamic model for the late palaeozoic and mesozoic formation and evolution of the norwegian danish basin ndb is presented this basin developed in the eastern part of the northern permian basin within the system of northwest european sedimentary basins the basin forming processes are modelled in a lithospheric plane strain model along a 400 km profile across the basin layered rheology is used with elasto plastic behaviour in the brittle parts and elasto viscous behaviour at higher temperatures and pressures the mechanical equations of equilibrium and the heat equation are solved by the finite element technique the model contains two tectonic events basin initiation in late carboniferous and early permian times is modelled by combined lithospheric heating and lithospheric extension additional lithospheric extension however less pronounced and taking place over a longer period is introduced during the triassic isostatic effects associated with lithospheric extension combined with thermal contraction and sediment loading accounts for up to 6 km of sediments of which 2 3 km are of triassic age the model shows a generally good agreement with observations in terms of basin geometry sediment layer thicknesses stratigraphy crustal thickness and heat flow in the central parts of the basin the model implies a crustal thinning factor of about 1 5 from 35 km to about 23 km close to that observed in seismic data

phenocryst crystallization during ascent of alkali basalt magma at rishiri volcano northern japan abstract phenocrysts in volcanic rocks are commonly used to deduce crystallization processes in magma chambers a fundamental assumption is that the phenocrysts crystallized in the magma chambers at isobaric and nearly equilibrium conditions on the basis of their large sizes however this assumption is not always true as demonstrated here for a porphyritic alkali basalt kutsugata lava from rishiri volcano northern japan all phenocryst phases in the kutsugata lava plagioclase olivine and augite have macroscopically homogeneous distribution of textures showing features characteristic of rapid growth throughout the crystals rarely a core region with distinct composition is present in all phenocryst phases phenocrysts excluding this core are occasionally in direct contact with each other forming crystal aggregates the equilibrium liquidus temperature of plagioclase the dominant phase 35 vol in the kutsugata lava can never exceed the estimated magmatic temperature unless the liquidus temperature increases significantly due to vesiculation of the magma during ascent this suggests that most phenocrysts in the kutsugata lava were formed by decompression of the magma during ascent in a conduit rather than by cooling during residence in a magma reservoir in the magma chamber before eruption probably located at depth of more than 7 km only cores of the phenocrysts were present and the magma was nearly aphyric 4 wt in the magma chamber and liquidus temperatures of phenocryst phases were significantly suppressed large undercooling caused by decompression and degassing of the magma was the driving force for significant crystallization during ascent because of the increase in liquidus temperature due to vapor exsolution low ascent rate of the kutsugata magma which is suggested by pahoehoe lava morphology and no association of pyroclastics gave sufficient time for crystallization furthermore the large degree of superheating of plagioclase in the magma chamber caused plagioclase crystallization with low population density and large crystal size which characterizes the porphyritic nature of the kutsugata lava alkali basalt is likely to satisfy these conditions and similar phenomena are suggested to occur in other volcanic systems

structural development of the fennoscandian border zone offshore denmark abstract the hans 1 saeby 1 and terne 1 wells are located within the danish part of the fennoscandian border zone and provide significant new data pertaining to the evolution of this important tectonic belt all three wells encountered palaeozoic and mesozoic rocks the cambrian to lower silurian deposits can be closely correlated to sections known from bornholm and scania and tectonic activity within the fennoscandian border zone cannot be verified during this time interval the thick upper silurian shales were probably deposited in a rapidly subsiding foreland basin marginal to the speculative caledonian deformation front which lies south of the ringkobing fyn high a major hiatus comprises the devonian and early carboniferous upper carboniferous sediments were deposited before a phase of rifting upper carboniferous intrusive and extrusive volcanic rocks suggest tectonic activities heralding the rifting phase extensive block faulting occurred in the border zone during the rotliegende and more than 650 m of reworked volcanic rocks were deposited in an alluvial fan environment syndepositional erosion created a major unconformity before the deposition of post rift zechstein sediments during the mesozoic the fennoscandian border zone was divided into two zones the sorgenfrei tornquist zone to the west and the skagerrak kattegat platform to the east the sorgenfrei tornquist zone continued to experience tectonic activity in triassic and middle jurassic times the zone has experienced an uplift of 1700 2000 m during late cretaceous to early tertiary inversion tectonics and the late tertiary uplift of fennoscandia

assimilation and fractional crystallization controlled by transport process of crustal melt implications from an alkali basalt dacite suite from rishiri volcano japan mechanisms of fractional crystallization with simultaneous crustal assimilation afc are examined for the kutsugata and tanetomi lavas an alkali basalt dacite suite erupted sequentially from rishiri volcano northern japan the major element variations within the suite can be explained by boundary layer fractionation that is mixing of a magma in the main part of the magma body with a fractionated interstitial melt transported from the mushy boundary layer at the floor systematic variations in sio2 correlate with variations in the pb sr and nd isotopic compositions of the lavas the geochemical variations of the lavas are explained by a constant and relatively low ratio of assimilated mass to crystallized mass r value in the magma chamber in which the kutsugata and tanetomi magmas evolved a strong thermal gradient was present and it is suggested that the marginal part of the reservoir was completely solidified the assimilant was transported by crack flow from the partially fused floor crust to the partially crystallized floor mush zone through fractures in the solidified margin formed mainly by thermal stresses resulting from cooling of the solidified margin and heating of the crust the crustal melt was then mixed with the fractionated interstitial melt in the mushy zone and the mixed melt was further transported by compositional convection to the main magma causing its geochemical evolution to be characteristic of afc the volume flux of the assimilant from the crust to the magma chamber is suggested to have decreased progressively with time proportional to t 1 2 and was about 3 10 2 m year at t 10 years and 1 10 2 m year at t 100 years it has been commonly considered that the heat balance between magmas and the surrounding crust controls the coupling of assimilation and fractional crystallization processes i e absolute value of r however it is inferred from this study that the ratio of assimilated mass to crystallized mass can be controlled by the transport process of the assimilant from the crust to magma chambers

age relations among oslo rift magmatic rocks implications for tectonic and magmatic modelling abstract this paper presents a compilation of 67 published and unpublished radiometric age determinations by the rb sr method on magmatic rocks in the continental oslo rift in southeast norway the data include most major rock types and structural units in the subaerial part of the rift the implications of these data with respect to the tectonomagmatic evolution of the rift and processes in the mantle and crust are discussed the oslo rift was active for about 60 million years the earliest recorded magmatic activity was the intrusion of basaltic and syenitic sills in the pre rift lower paleozoic sediments at about 300 ma extrusion of plateau lavas of basaltic and intermediate composition rhomb porphyries started about 295 ma and lasted to about 275 ma with largest magma volumes and highest extrusion rate from 295 to 285 ma in the vestfold graben segment gs one flow per 250 000 years and 0 30 km3 per 103 years the main faulting activity and graben formation occurred during this period the plateau volcanism was succeeded by a stage of bimodal central volcanoes most of which later underwent caldera collapses ring dykes and central intrusions in these calderas give ages in the range 268 266 ma in the vestfold and 266 243 ma in the akershus gs large composite intrusive complexes were emplaced in the periods 27 268 and 273 241 ma in the vestfold and akershus gs respectively a rate of propagation of the rift from ssw towards the nne has been estimated to 1 2 cm y 1 on the basis of 87 sr 86 sr initial ratios and type of magmatic products we estimate magmas to have been emplaced into the crust only during the first 20 25 ma of the magmatic period some of the magmatism appears to be initiated by movement along precambrian fault zones which were reactivated during the oslo rifting event

elemental fractionation in lavas during post eruptive degassing evidence from trachytic lavas rishiri volcano japan abstract trachytic lavas of rishiri volcano northern japan show a peculiar geochemical variation across lava flow units samples collected systematically in a vertical cross section from a lava flow unit with a thickness of about 20 m are nearly homogeneous in major element compositions however some trace elements including li b and cs are considerably depleted in samples collected from the main part of the flow unit compared to those obtained from the surface of the lava flow clinker layer in particular cs content of the main flow unit is as low as 30 of the clinker layer 11b 10b ratios of samples from the main flow unit are also slightly lower than those of the clinker samples and the isotope compositions positively correlate with boron concentrations these geochemical variations cannot be explained by magmatic processes in magma chambers post eruptive weathering or alteration process rather we infer these systematics resulted from escape of these elements from the lava flow unit during post eruptive degassing vapor phases in which li b and cs dissolved are suggested to have been transported through veins formed in the main flow unit as fractures due to slight shearing along the flow planes after lava emplacement in the tanetomi lava only rocks of the clinker layer preserve original composition of magmas although they are porous and brownish due to extensive oxidization on the other hand rocks of the main flow unit do not retain original magma compositions although they are dense and grayish and seem to be much fresher compared to the clinkers a similar geochemical modification of lavas can occur in other volcanic systems especially for lavas consisting of relatively thick flow units

the oligocene succession in the eastern north sea basin development and depositional systems the oligocene sedimentary succession in the eastern north sea is revised and re interpreted by applying new state of the art reflection seismic data integrated with new bio and sr stratigraphy data from three key wells in the study area the oligocene succession in the eastern north sea is divided into four transgressive regressive t r sequences characterized by non accretional and or aggradational transgressive systems tracts and prograding regressive systems tracts detailed studies of three wells including biostratigraphy and sr analysis constrain the age relationships between the t r sequences internal clinoform geometry indicates that the sediments were sourced from the present southern norwegian mainland to the north of the depositional area the direction of progradation shifted from being se directed in the earliest rupelian early oligocene to s and sw directed during chattian time late oligocene rapid basin subsidence is indicated by the development of non accretionary transgressive systems tracts with subsequent progradation into water depths of hundreds of metres the creation of accommodation space was out of phase relative to eustatic sea level changes and mainly controlled by regional scale differential vertical movements where uplift and exposure of landmasses of the hinterland southern norway occurred concurrently with basin subsidence halokinesis had an intra basinal influence on the main sediment transport direction but probably did not contribute much in creation of accommodation space

paleostress field reconstruction in the oslo region abstract the geodynamic history of a region is archived in its geologic record which in turn may reflect deformation patterns that causally can be related to certain configurations of paleostresses in the oslo region the exposed geological record ranges from precambrian high grade metamorphic rocks through cambro silurian sedimentary rocks to permo carboniferous sedimentary and magmatic rocks the latter being related to the development of the oslo rift system we investigate the kinematics of outcrop scale faults to derive the diversity of paleostress states responsible for the observed strain for this purpose we combine different graphical and numerical approaches to separate heterogeneous fault slip data sets and estimate the associated reduced stress tensors a reduced stress tensor consists of the directions of the three principal stress axes with 1 2 3 and the ratio of principal stress differences r 2 3 1 3 the present study gives evidence for three major regional paleostress fields that affected the oslo region a compressional stress field with a nw se directed maximum compression 1 has been identified as a caledonian imprint the most prominent regional stress field however is tensional characterised by a wnw ese directed minimum compression 3 and related to the permo carboniferous rifting later the area was affected by a wrench regime with a roughly n s directed 1 and a maximum age of permian whereas the absolute timing of this stress field is unclear due to the lack of exposed rocks younger than permian for a large number of estimated stress states none of the principal axes are sub vertical these oblique paleostresses cannot be integrated into a common regional stress field but rather correspond to local effects of intense magmatic activity during the permo carboniferous phase of rifting the present study tends to show that the oslo region remained unaffected by major tectonic activity for much of the mesozoic and cenozoic which is when the central european basin system to the south experienced several phases of intense deformation

textural and thermal history of partial melting in tonalitic wallrock at the margin of a basalt dike wallowa mountains oregon columbia river basalt group dikes invade biotite hornblende tonalite to granodiorite rocks of the wallowa mountains most dikes are strongly quenched against wallrock but rare dike segments have preserved zones of partial melt in adjacent wallrock and provide an opportunity to examine shallow crustal melting at maxwell lake the 4 m thick wallrock partial melt zone contains as much as 47 vol melt glass plus quench crystals around mineral reaction sites and along quartz feldspar boundaries bulk compositional data indicate that melting took place under closed conditions excepting volatiles with progressive melting hornblende biotite and orthoclase were consumed but plagioclase quartz and magnetite persisted in the restite clinopyroxene orthopyroxene plagioclase and fe ti oxides were produced during dehydration melting reactions involving hornblende and biotite reacting phases became more heterogeneous with progressive melting crystallizing phases were relatively homogeneous progressive melting produced an early clear glass followed by light high k and dark high ca brown glass domains overprinted by devitrification melts were metaluminous and granitic to granodioritic thermal modeling of the partial melt zone suggests that melting took place over a period of about 4 years thus rare dikes with melted margins represent long lived portions of the columbia river basalt dike system and may have sustained large flows

the tectonic evolution of the fennoscandian border zone in denmark abstract the fennoscandian border zone is a complex tectonic zone defining the border between the stable fennoscandian baltic shield and the metastable danish basin and polish trough it forms the northwestern part of the tornquist teisseyre lineament one of the major european tectonic sutures intricate tectonic movements took place within the fennoscandian border zone from early paleozoic to recent time it is postulated that its evolution was dominated during the early palaeozoic by tensional tectonics and during the late paleozoic by dextral wrench movements the latter led to the development of curvilinear faults and the subsidence of pull apart basins transtensional stresses governing the triassic to early cretaceous evolution of the fennoscandian border zone induced the differential subsidence of rift basins during the late cretaceous and early cenozoic dextral transpressional stresses induced the partial inversion of mesozoic basins associated with the fennoscandian border zone this is reflected by the uplift of a major anticlinorium over which cretaceous strata became progressively truncated whilst adjacent areas developed into late cretaceous early tertiary depocentres basin inversion diminishes to the northwest along the fennoscandian border zone and disappears in the skagerrak area

seismic amplitude and attribute data from mesozoic strata in the skagerrak danish norwegian north sea indicators for fluid migration and seal bypass systems abstract the skagerrak builds the transition from the exposed palaeozoic basement of south norway to thick mesozoic to cenozoic sediments covering danish jutland it states the southern norwegian trench and comprises from w to e the mesozoic farsund basin the northernmost sector of the sorgenfrei tornquist zone and the palaeozoic skagerrak graben over the past decades fluid escape structures at the southern skagerrak shelf and in the central norwegian trench were investigated and layer parallel fluid migration from mesozoic rocks was suggested however a fluid source or a corresponding migration system of the fluids has not been proposed yet in this study a joint interpretation of seismic imagery and seismic attributes reveals fluid indications and migration systems in the farsund basin and the central skagerrak we propose that source rocks of the lower jurassic fjerritslev formation generated thermogenic fluids in the farsund basin as well as in the fjerritslev trough for the central skagerrak a time structure map of the base of the f iii member of the fjerritslev formation shows that these fluids could migrate upward forced by buoyancy and partially bypassed by faults and seep out in the norwegian trench furthermore we propose that fluid escape structures on the southern shelf were likely caused by seepage of biogenic fluids under bottom current action

an experimental deformation study of partially molten amphibolite application to low melt fraction segregation an amphibolite has been experimentally deformed under subsolidus and partially molten conditions to evaluate the influence of melt on the mechanical behavior of a natural mafic rock and to assess low melt fraction segregation processes experiments were performed under fluid absent conditions at 1 8 gpa between temperatures of 650 c and 1000 c these conditions are similar to those of thickened lower continental crust or intermediate depths in subducting oceanic lithosphere at 850 c melt is granitoid in composition and its viscosity is that of wet granite 103 105 pa s the results can be summarized as follows 1 under subsolidus conditions tests at 650 c and 750 c the amphibolite is macroscopically ductile and deformation is homogeneously distributed throughout the sample 2 at near solidus conditions 800 c 0 5 vol melt fractures 1 10 m in width displace hornblende and plagioclase grains and melt formed in situ is found in some of these cracks the formation of a ductile shear zone in one sample is attributed to the presence of very fine grained reaction products from combined dehydration hydration reactions that involve plagioclase the dehydration reaction products have apparently changed the deformation mechanism and even overrode the melt embrittlement process trapping melt in pockets of lower strain with higher melt fractions 10 15 vol broad melt bearing shear zones form and grains within these zones are brittlely deformed 3 at 20 vol melt 1000 c additional weakening occurs but fractures are not observed and the overall deformation is by viscous flow the results show that at low melt fractions 15 vol fluid absent melting reactions can induce fracture in previously ductile rocks this suggests that the percentage at which melt may escape by fracture is lower than the theoretical critical melt fraction or cmf of 26 40 vol in general estimating the fraction of melt at the onset of segregation cannot be predicted by the cmf melt segregation models have to be adapted to variations in such major factors as pressure temperature type of melting reaction rate of melting and strain rate such models will greatly assist our understanding of continental growth and evolution during orogenesis

microstructurally constrained versus bulk fault gouge k ar dating abstract structural mineralogical and geochronological data were collected from two brittle fault zones fault i and ii deforming the ordovician rolvsnes granodiorite in sw norway microstructurally constrained k ar data from cohesive damage zone samples are compared to bulk k ar data from fault gouges from the adjacent fault cores in fault i fluid ingress along the fault core caused pervasive alteration of the host granodiorite plagioclase and biotite are almost completely altered to kaolinite quartz illite and smectite in fault ii a complex deformation history is recorded by multiple fault rock microdomains including an ultracataclasite with 50 authigenic illite and an illite jarosite bearing pseudotachylyte microdomains hosting authigenic illite jarosite from both studied faults were separated characterized mineralogically and dated by k ar results confirm that each domain contains only one generation of authigenic illite jarosite and provide robust time constraints on fault initiation in the carboniferous 339 8 ma and reactivation in the permian 289 6 ma k ar geochronology of the adjacent fault gouges yields inclined age versus grain size relationships the ages of the coarsest grain size fractions are similar to the age range of the damage zone data whereas the ages of the finer fractions are significantly younger our study provides new and important insights into the dynamics of long lived fault zones isotopic signatures and structural features representative of fault initiation are preserved in the damage zones while they are progressively overprinted due to cumulative strain localization in the fault cores finally our study further clarifies the significance of age plateaus in age versus grain size plots which allows for a robust and geologically meaningful interpretation of k ar data

experimental study on the effects of crustal temperature and composition on assimilation with fractional crystallization at the floor of magma chambers abstract when a hot basaltic magma is emplaced into continental crust or a pre existing silicic magma chamber the processes of assimilation with fractional crystallization afc are likely to control the liquid line of descent of the magma these processes are particularly important at the floor of the magma chamber because evolved light liquids generated by floor melting readily mix with the overlying basaltic magma in order to clarify the effects of temperature and composition of the floor on the afc processes we experimentally investigated simultaneous melting and crystallization of a nh 4 cl h 2 o binary eutectic system in the experiments evolution of temperature and compositional profiles of a hot solution overlying a cold solid mixture of variable initial temperatures and compositions were measured the initial nh 4 cl concentrations of solid and liquid are chosen to be higher than the eutectic composition such that the density change of the experimental material by crystallization and melting is qualitatively the same as that of natural magmas and crusts the results show that a mushy layer forms at the floor due to simultaneous crystallization and partial melting and that the liquid evolves due to mixing with liquids released by crystallization and melting the ratio of melting mass to crystallization mass m c ratio depends on the initial floor temperature and composition as the initial floor temperature decreases the rate of melting largely decreases so that the m c ratio becomes smaller as the initial nh 4 cl concentration of the solid floor decreases the degree of partial melting of the floor increases however it does not necessarily result in an increase in the m c ratio the higher melt fraction of the mushy layer increases permeability within the mushy layer so that vertical exchange between the liquid in the mushy layer and the more concentrated overlying liquid is enhanced this effect promotes crystallization in the mushy layer and decreases the m c ratio it is suggested that the m c ratio during afc processes depends on details of the mixing process in the liquid layer such as spacing and meandering of buoyant plumes

boundary layer fractionation constrained by differential information from the kutsugata lava flow rishiri volcano japan detailed investigation of an erupted magma with limited compositional diversity provides instantaneous information on incremental magma differentiation processes in a magma chamber kutsugata lava a suitable target of such study is a quaternary alkali basalt 51 3 53 2 wt in sio2 from rishiri volcano northern japan despite the narrow range in the whole rock compositional variation chemical and modal compositions of mineral phases crystallized in the magma chamber vary systematically with the whole rock composition in the north lava 51 3 51 9 wt in sio2 the less differentiated portion of the kutsugata lava most crystals which include low ni olivine and plagioclase were derived from the mushy boundary layer the main part of the magma body was principally aphyric 30 vol by such mechanisms as compaction and compositional convection this fractionated melt was mixed with the overlying main magma causing differentiation of the kutsugata magma the average temperature of the extracted melt is 1010 c significantly lower than 1100 c estimated for the main magma a quantitative model of magmatic differentiation which includes thermal and compositional evolution of a mushy boundary layer can successfully reproduce the observed compositional trends of the north lava

a laboratory investigation of assimilation at the top of a basaltic magma chamber the fluid dynamic processes affecting the mechanism and amount of assimilation of the roof of a magma chamber have been studied in a series of analogue laboratory experiments an aqueous solution of na 2 co 3 was first cooled and crystallized at the top of a laboratory tank to form a solid roof layer of prescribed composition the tank was then drained and refilled with warmer solution either na 2 co 3 of the same or different composition or in two experiments kno 3 the metal top in all cases was held below the eutectic temperature for the system na 2 co 3 h 2 o the experiments were continued until substantial crystallization and in some cases complete solidification were achieved starting with a dense solid roof layer of na 2 co 3 10h 2 o and ice on the na 2 co 3 side of the eutectic and a warmer eutectic na 2 co 3 solution below there was rapid convective cooling of the liquid layer accompanied by melting of the roof producing a compositional

early magmatic phase in the oslo rift and its related stress regime abstract the earliest magmatic rocks in the oslo graben consist of a complex of sills and dykes intermediate to felsic syenites termed maenaites and basic camptonites intruded into cambrian and lower ordovician pre rift sediments no volcanism appears to be associated with this intrusive phase the sill intrusions are regional in extent and can be found in all parts of the oslo graben except in the north samples from six maenaite sill occurrences dated by the rb sr method yielded late carboniferous ages ranging from 304 to 294 ma these ages place the sill intrusions in the same evolution phase as the proto rift asker group sediments interpreting the sill complex as a paleostress indicator and applying the tectonic model for the sill forming mechanism the deviatoric stress in the crust at the time of intrusion is found to be compressive at the level of the sills and shallower in contrast the succeeding evolution of the oslo graben was dominated by tensile deviatoric stress the timing of the incipient magmatic event in the oslo graben and its state of deviatoric compressive stress support the concept that the oslo rift developed initially in response to the build up foreland compressional stresses in conjunction with the hercynian orogeny there is no evidence for an early pre rift crustal doming stage

structural analysis of the smeaheia fault block a potential co2 storage site northern horda platform north sea abstract smeaheia a prominent fault block located on the horda platform northern north sea is identified as a potential subsurface co2 storage site we utilise the gn1101 3d seismic survey to generate a high resolution subsurface geomodel to inform the structural style and evolution of the fault block to investigate geological controls on proposed co2 storage and provide a geometric framework as a basis for future analyses two basement involved first order north south trending fault systems the vette fault zone vfz and the oygarden fault complex ofc bound the 15 km wide fault block the vfz bifurcates down section where it is hard linked with two separate basement structures a phenomenon we term as dual rooted apart from activity during the permo triassic rift phase 1 and the late jurassic early cretaceous rift phase 2 we present evidence that rifting in this part of the north sea continued into the late cretaceous with minor reactivation in the palaeocene eocene two segments of the vfz interacted and linked at a relay ramp during rift phase 2 second order thin skinned faults show basement affinity and developed during rift phase 2 in two distinct pulses a population of polygonal faults intersects the overburden and developed during the eocene to middle miocene we have revised the areal extent of two structural closures that define the smeaheia fault block alpha vfz footwall and beta ofc hanging wall which consist of upper jurassic viking group target formations simplified cross fault juxtaposition analysis of the vfz and second order intra block faults are presented and inform pressure communication pathways between the smeaheia and tusse fault block as well as reservoir integrity and compartmentalisation the geomodel further identifies important geological controls on co2 storage in the fault block including a thinning caprock above the alpha structure and identification of hard linkage between deep tectonic faults and shallow polygonal faults

on the origin of the southern permian basin central europe abstract a detailed study of the structural and stratigraphic evolution of the southern permian basin during latest carboniferous to early jurassic times supported by quantitative subsidence analyses and forward basin modelling for 25 wells leads us to modify the conventional model for the rotliegend zechstein development of this basin the late permian early jurassic tectonic subsidence curves are typical for a permian to early triassic extensional stage that is followed by thermal subsidence however a purely extensional model is extremely problematic because active faulting during this time is minor and generally hard to document using inverse techniques to model the subsidence curves we quantitatively show that a significant component of late permian and triassic tectonic subsidence can be explained by thermal relaxation of early permian lithospheric thinning and by delayed infilling of paleo topographic depressions that developed during the early permian in this interpretation stephanian autunian wrenching resulted in thermal destabilisation of the lithosphere deep fracturing of the crust disruption and erosion of its sedimentary cover and regional uplift of the area of the future southern permian basin upon termination of wrench tectonics and associated volcanism towards the end of the autunian the southern permian basin began to subside in response to thermal contraction of the lithosphere the evolving basin was isolated from the world oceans and had subsided possibly up to some 700 m below their level at the beginning of upper rotliegend sedimentation after catastrophic flooding of this paleo topographic depression at the beginning of the zechstein changing sea level sedimentation and subsidence rates remained essentially in balance although the effects of triassic rifting overprinted parts of the southern permian basin its overall subsidence pattern persisted well into the jurassic in contrast to the remainder of the southern permian basin permian and triassic crustal extension contributed significantly to the subsidence of the polish trough

some thermal constraints on crustal assimilation during fractionation of hydrous mantle derived magmas with examples from central alpine batholiths following assimilation at different levels in the crust we test by heat and mass balance the extent of assimilation of crustal country rocks in different local plutonic

permian and mesozoic extensional faulting within the caledonides of central south norway palaeomagnetic data from fault rocks along major faults in the laerdal gjende fault system cutting the caledonian structure in the jotunheimen area of central south norway reveals a multi component remanence pattern sample and site mean directions from the fault rocks obtained by thermal cleaning demonstrate a simple pattern of normal polarity low blocking components and reverse polarity high blocking directions the magnetic signature of the laerdal gjende fault system fault rocks is identical to that observed on breccias on late faults along the west coast of norway based on available palaeomagnetic reference data we assign ages of mid late permian and late jurassic early cretaceous for important phases of faulting and breccia formation along the laerdal gjende fault system in central south norway structural windows partly exposing basement along the axis of the caledonides in southern norway were exhumed by footwall uplift on major faults in the laerdal gjende fault system the consanguinity of fault rock data from the laerdal gjende fault system and fault rocks in western norway and comparison with displacement on the offshore continuation of the laerdal gjende fault system along the hardangerfjorden shear zone indicate that the main tectonic events responsible for the development of the north sea basin also significantly affected the geology of central south norway

analog experiments on melting and contamination at the roof and walls of magma chambers abstract a series of analogue laboratory experiments were carried out to model melting and mixing processes occurring at the roof and sides of a magma chamber when hot dense mantle derived magmas are emplaced in the deep crust the experiments investigated the influence of a viscosity contrast between crustal melts and input magma the presence of crystals in the input magma and roof geometry the crust was modelled by aqueous ice or a cold low melting point wax arranged as a flat roof or in an upside down u the input magma was modelled by a hot dense aqueous solution of nano3 or feso4 which was injected into the cavity between the walls the experiments revealed the potentially important role that side wall processes may have in crustal melting and mantle magma contamination in this environment the walls melted back much faster than the roof and the melts from the opposite walls ponded next to each other under the roof and mixed with each other only in the central region when the input solution was able to crystallise it did so preferentially in the active side wall boundary layers the tiny dense crystals which were dragged up in the melt boundary layer and then dropped into the interior fluid enhanced hybridisation of the melt and contamination of the input fluid and through their influence on the boundary layer structure reduced the melting rate

new constraints on the timing of late carboniferous early permian volcanism in the central north sea the permo carboniferous evolution of the central north sea is characterized by three main geological events 1 the development of the west european carboniferous basin 2 a period of basaltic volcanism during the lower rotliegend latest carboniferous early permian and 3 the development of the northern and southern permian basins in late permian times the timing of the late carboniferous permian basaltic volcanism in the north sea is poorly constrained as is the timing of extensional tectonic activity following main inversion during the westphalian due to the propagation of the variscan deformation front results of high precision ar ar dating on basalt samples taken from a core from exploration well 39 2 4 amerada hess in the uk sector of the central north sea suggests that basaltic volcanism was active in the late carboniferous at c 299 ma the presence of volcanics below the dated horizon suggests that the onset of permo carboniferous volcanism in the central north sea commenced earlier probably at c 310 ma westphalian c this is contemporaneous with other observations of tholeiitic volcanism in other parts of nw europe including the oslo graben the north east german basin southern sweden and scotland interpretations of available seismic data show that main extensional faulting occurred after the volcanic activity but the exact age of the fault activity is difficult to constrain with the data available

the age and tectonic significance of dolerite dykes in western norway coast parallel dykes in sw norway primarily of permo triassic age have been linked regionally to the early tectonic evolution of the norwegian continental shelf we demonstrate from palaeomagnetic data mean declination 206 1 inclination 30 1 a 95 11 8 that dolerite dykes in the coastal sunnfjord region of western norway and immediately west of the devonian basins are also of permian c 250 270 ma age and not lower or pre devonian as previously advocated the sunnfjord dykes appear to be contemporaneous with dykes from sw norway at sotra 262 6 ma and the oldest dykes from sunnhordland 260 280 ma and geochemical data attest to a transition from sub alkaline to alkaline magmatism at the dawn of the mesozoic the sunnfjord dykes are not simple records of e w extension and magma intrusion but instead represent significant mid late permian time markers within a complex zone of fault activation and rejuvenation late mesozoic cenozoic magnetic overprinting mean declination 348 6 inclination 68 9 a 95 12 and metamorphic alteration documented by these dykes are directly dependent upon proximity to major e w brittle faults south of the hornelen devonian basin hence some motion and related fluid activity do post date dyke intrusion the e w high angle normal or oblique slip faults can be regionally traced offshore to the oygarden fault zone onshore these faults truncate the hornelen low angle detachment which in turn cuts folded d evonian strata these observations along with evidence for permian and late jurassic cretaceous extension from the nearby dalsfjord region demonstrate important reactivation of a late to post caledonian detachment and high angle fault system in western norway

petrogenesis of early neogene magmatism in the northern puna implications for magma genesis and crustal processes in the central andean plateau of uolm 2 and mm group rocks assuming the same endnew compositional data and petrogenetic models are presented for member compositions the modelling suggests genesis of the mm pre upper miocene volcanism in the northern puna of argentina magmas at higher pressure than the uolm 2 centres 10 kbar 22 s 24 s two phases of volcanism produced small dome vs 7 kbar which may reflect the influence of crustal thickening in complexes of mainly silicic andesite to low sio2 rhyolite the the plateau region by the mid miocene the felsic dome complexes upper oligocene lower miocene phase uolm 20 17 ma of this study are compositionally similar to the large volume calderaproduced two distinct groups of rocks the uolm 1 group is sourced felsic ignimbrites that dominated volcanism in the region metaluminous and mainly andesitic with isotopic compositions like from 10 to 2 ma and our results suggest that there is no fundamental those of the recent arc sr srt 0 706 ndt 3 the difference in magma genesis between them the differences in the uolm 2 group is more silicic and peraluminous and has isotopic volumes and the mode of eruption reflect changes in the stress and compositions indicating a substantial crustal contribution sr thermal regime with time srt 0 713 ndt 8 the mid miocene phase mm 15 12 ma produced rocks similar in composition to those of the uolm 2 group sr srt 0 710 ndt 7 but with higher incompatible element contents ratios of ba nb and zr

ce isotope ratios of n type morb abstract isotope ratios of ce nd and sr and light rare earth elements la ce nd and sm concentrations were measured for sixteen n type morb samples from dsdp sites 420 421 429 597 b and 597 c east pacific rise 504 b costa rica rift and 395 519 a 522 b 562 and 564 mid atlantic ridge all the samples were confirmed as n type morb based on nd and sr isotope ratios and lree patterns the ce isotope ratios ranged from 0 0225616 ce 1 6 to 0 0225633 ce 1 0 averages of ce and nd of n type morb were 1 4 and 9 5 respectively the average of ce is quite different from previous estimates we propose that ce 1 4 should be used as the typical morb ce isotope ratio in future studies

seismic stratigraphic subdivision of the triassic succession in the central north sea integrating seismic reflection and well data the triassic sedimentary succession in the central north sea has been investigated to establish a broader understanding of the triassic period from the combined interpretation of seismic reflection data and well data the triassic succession has been subdivided into four seismic units where unit boundaries are characterized by regional seismic amplitude anomalies reflecting changes in gross sedimentary facies or rock properties a successful correlation between sedimentary facies interpreted within the well sections and distinct seismic reflection patterns allowed a thorough mapping of the gross palaeoenvironment throughout the triassic the method presented of subdividing a continental sedimentary succession into seismic units should be applicable elsewhere in other basins the main source area during the triassic was scandinavia to the north and sediment transportation was mainly along north south and ne sw trending lineaments which are at present located onshore southern norway and in the asta graben and the varnes graben offshore an uplifted skagerrak graben area acted as source area in the early and early middle triassic with sediment dispersal to the south and sw high relief existed for a longer period in western scandinavia than in eastern scandinavia which supports an asymmetric shape of the scandinavian mountains during the triassic accommodation space in the early triassic was mainly controlled by the relief inherited from a late carboniferous permian rift phase although thermally induced regional subsidence continued in the middle and late triassic creation of local accommodation space was mainly limited to halokinesis including redistribution and withdrawal of salt from the subsurface the upper triassic succession is eroded across the western and central parts of the study area although the upper triassic unit is preserved in synforms adjacent to salt structures in the western part of the study area dry playa conditions prevailed during the early triassic although fluvial systems supplied long transported sandy detritus southeastwards in the late early triassic more sandy detritus was transported into the sedimentary basin in the middle and late triassic concurrently with a gradually wetter climate supplementary material uninterpreted seismic sections are available at www geolsoc org uk sup18726

two stage contamination during crustal assimilation isotopic evidence from volcanic rocks in eastern nevada thousands of cubic kilometers of dominantly intermediate composition metaluminous magma erupted at approximately 35 ma in eastern nevada two stages of crustal contamination are inferred from detailed study of the earliest vocanic rocks fine grained mafic rocks and rocks with mixed textures ranging from basalt to rhyolite were contaminated with a crustal component rich in nd but poor in sr overlying plagioclaserich andesites and dacites have greater sr and 87sr 86sr but less nd and lower nd and are interpreted to have been contaminated by a crustal component with the opposite elemental signature i e poor in nd but rich in sr the first contaminant represents a partial melt of the crust with plagioclase as a residual phase and the second contaminant is the residue of the partial melting event or bulk crust the net effect is bulk crustal assimilation but in two bites the separation of the crustal souree into two elementally distinct contaminants causes divergent trends with respect to the variation of 87sr 86sr versus end that could be misinterpreted to indicate the existence of isotopically distinct crustal reservoirs comparison of the calculated contaminants to melting relationships in pelites is consistent with the two contammants representing melt and residue at about 30 melting the model age of the bulk crust is approximately 2 2 ga consistent with an early proterozoic crustal province inferred by other workers

geochemical evolution of a shallow magma plumbing system during the last 500 years miyakejima volcano japan constraints from 238u 230th 226ra systematics abstract in order to unravel magma processes and the geochemical evolution of shallow plumbing systems beneath active volcanoes we investigated u series disequilibria of rocks erupted over the past 500 years 1469 2000 ad from miyakejima volcano izu arc japan miyakejima volcanic rocks show 238u 230th 226ra disequilibria with excess 238u and 226ra due to the addition of slab derived fluids to the mantle wedge basaltic bombs of the 2000 ad eruption have the lowest 230th 232th ratio compared to older miyakejima eruptives yielding the youngest 238u 230th model age of 2 kyr this reinforces our previous model that fluid release from the slab and subsequent magma generation in the mantle wedge beneath miyakejima occur episodically on a several kyr timescale in the last 500 years miyakejima eruptives show 1 a vertical trend in a 230th 232th 238u 232th diagram and 2 a positive linear correlation in a 226ra 230th 0 1 230th diagram which is also observed in lavas from some of the single eruptions e g 1940 1962 and 1983 ad the variations cannot be produced by simple fractional crystallization in a magma chamber with radioactive decay of 230th and 226ra but it is possibly produced by synchronous generation of melts in the mantle wedge with different upwelling rate or addition of multiple slab derived fluids a much more favorable scenario is that some basaltic magmas were intermittently supplied from deep in the mantle and injected into the crust subsequently modifying the original magma composition and producing variations in 230th 232th and 226ra 230th 0 ratios via assimilation and fractional crystallization afc the assimilant of the afc process would be a volcanic edifice of previous miyakejima magmatism due to the relatively short timescales involved the interaction between the assimilant and recent miyakejima magmatism has not been recorded by the sr nd pb isotopic systems in such cases th isotopes and 226ra 230th ratio are excellent geochemical tracers of magmatic evolution

the oslo rift a review the tectonomagmatic history of the oslo rift may be subdivided into 5 main periods stage 1 300 ma development of a shallow depression stage 2 300 295 ma roughly simultaneous onset of widespread basalt volcanism b1 and vertical movement along nnw sse to n s trending faults stage 3 295 275 ma main rifting period accompanied by volcanism dominated by rhomb porphyry lavas latites stage 4 275 240 ma change of magmatic style from basaltic shield volcanism to central volcanoes of mixed compositions many of which went through caldera collapses intrusion of composite batholiths of intermediate to granitic compositions stage 5 240 ma dike intrusions may have continued into triassic time both the crust and total lithosphere under the rift are thinned relative to the precambrian basement on both sides a 12 km thick high density and high velocity body occupies the lower crust along the rift axis and extends under the eastern and western rift shoulder petrological and geochemical data imply that this massive layer represents dense cumulates and gabbroic rocks formed from mantle derived magmas in deep crustal magma chambers two mantle sources one nearly undepleted age corrected nd of about 1 and sr of 10 to 15 the other mildly depleted nd of about 4 sr of 10 contributed melts to the oslo rift magmatism the two sources are interpreted as different parts of a heterogeneous upper mantle which before the rifting event belonged to the subcontinental lithosphere on their way to the surface the mantle derived magmas were retained in deep crustal magma chambers where they underwent extensive fractional crystallization some also suffered moderate contamination anatectic melts and melts formed by mixing between mantle derived and anatectic melts gave rise to syenitic and granitic intrusions

mechanism for middle to upper crustal contamination evidence from continental margin magmas lavas and feeder dikes of two quaternary volcanic complexes in southwest british columbia provide evidence of an upper crustal syntectic origin for many dacite magmas contamination initially involves solid liquid and liquid liquid interdiffusion between the magma and 1 cm wide partially melted 60 margins of wall rock xenoliths subsequent abrasion of mechanically unstable margins contributes both silicic melt and xenocrysts to the magma hybridization continues by elemental exchange between the magma and crustal liquid where mechanical mixing produces a meshlike distribution of 1 mm rhyolite strands in the lava matrix the magma may assimilate large quantities of crustal material through disaggregation of xenoliths within high level subvolcanic dikes

northward sediment transport from the late carboniferous variscan mountains zircon evidence from the oslo rift norway proto rift late carboniferous fluvial and deltaic sandstones from the asker group are locally preserved in the upper palaeozoic oslo rift high precision u pb ages of single detrital zircon grains from a moscovian late westphalian to early stephanian sandstone and overlying tuffaceous sediments that predated initial basaltic volcanism have been used to study the provenance of the detritus and to evaluate the tectonic significance of the deposits we find late neoproterozoic of avalonian affinity cambro ordovician and early carboniferous detrital zircons requiring that one of the source regions was located along or south of the axis between southern england and northeastern germany and implying sediment transport from the south probably as a consequence of uplift during the asturic compressional phase of the variscan orogeny other age groups reflect typical baltic sources possibly from the ringkobing fyn high detrital zircons and monazite from underlying silurian sandstone wenlock ludlow yield svecofennian to sveconorwegian as well as late ordovician ages but lack late neopoterozoic avalonian ages it is thus unlikely that the bulk of the carboniferous detritus could have been derived by recycling of preexisting silurian sandstones these data imply that the rivers feeding the asker group originated in the variscan mountains and entered the oslo proto rift from the south possibly via the horn graben or along the sorgenfrei tornquist zone

reply to comment on apatite fission track and u th he data from fennoscandia an example of underestimation of fission track annealing in apatite by b w h hendriks and t f redfield abstract in the absence of any remnant sedimentary rocks or other tangible geological evidence apatite fission track aft data from fennoscandia have been interpreted in terms of deep burial beneath a caledonian foreland basin we rejected this interpretation of aft data from central finland in 2005 in favor of one incorporating radiation enhanced annealing over geologically extended periods of time the physics underlying our hypothesis have been discussed extensively during the past two decades by various peer reviewed studies in this reply we contend that in their comment larson et al incorrectly interpret published geological data to support deep burial beneath an aerially extensive caledonian foreland basin we show the avalonia baltica tectonic collision better explains data that intimate slightly elevated paleo temperatures and paleo pressures in southern sweden we demonstrate that most of the geological data cited by larson et al are geographically isolated from the area we discussed in our 2005 contribution and thus completely unrelated to our conclusions we consider the comment by larson et al as unfounded and consequently uphold our original interpretation

skagerrak graben the offshore continuation of the oslo graben abstract new seismic mapping of skagerrak reveals eight structural provinces in particular the skagerrak graben appears to be the continuation of the oslo graben the skagerrak graben is defined by two major late paleozoic faults trending northeast southwest connecting with the faults bounding the oslo graben extensive rotated block faulting is observed within the graben and a thickness of more than 2 s twt of lower paleozoic sediments has been preserved along the boundary faults sediments of probable late paleozoic age have been deposited as erosional products from the fault scarps and a central uplifted area a substantial part of these sediments may have a volcaniclastic origin the skagerrak graben continues to the north as a province of igneous rocks exposed at the sea floor with precambrian basement on either side the skagerrak graben is flanked by small half grabens containing probable early paleozoic sediments indicating that a wider area was involved in the rift process the formation of the skagerrak graben is related to the dextral strike slip movements along the sorgenfrei tornquist zone in late paleozoic time the kimmerian extensional movements along this zone indicate however a decoupling of the skagerrak graben at this time only minor inversion in the southwestern part of the skagerrak graben reflects the major inversion tectonism along the sorgenfrei tornquist zone in late cretaceous early tertiary times

crustal stretching in the scandinavian caledonides as revealed by deep seismic data the normalization of overthickened orogenic crust after a continent continent collision typically involves several different processes including erosion plate divergence gravity driven collapse of the orogenic wedge and viscous flow of the lower crust as a contribution to this discussion we here utilize reprocessed deep seismic data to image the scandinavian part of the major caledonian continent continent collision zone where we identify a double set of extensional shear zones one in the upper to middle crust hardangerfjord shear zone and a major oppositely dipping moho offsetting shear zone in the lower crust and upper mantle the latter shear zone may have a true displacement close to 50 km and appears to offset the moho vertically by 10 km extending downward beyond the 50 km depth limit of the seismic data while the upper structure offsets the basal caledonian thrust the moho shear zone is more difficult to date however they are kinematically consistent and located at the transition zone between thick and thin skinned post collisional tectonics and we suggest that they may represent a crustal pinch or zipper like structure that separated viscously flowing caledonian lower crust from cooler rigid basement representative for the rest of the baltic shield later extension created the north sea rift but the general rift axis and the associated riftrelated thinning developed in a narrower zone that runs oblique to the devonian trend hence the two generations of extension and thinning can be distinguished at the crustal scale

mantle temperature and lithospheric thinning beneath the midcontinent rift system evidence from magmatism and subsidence the tectono magmatic history of the midcontinent rift system can be explained by the presence of a mantle plume bringing elevated temperature mantle beneath the rift system at about 1110 ma huge volumes of extrusive and intrusive igneous rocks were generated as abnormally hot mantle decompressed beneath the lithospheric rift geochemical and isotopic data from the keweenawan volcanics show that the earliest melts were derived from small degree melting of primitive plume mantle coupled with enriched metasomatic melts derived from the continental lithosphere as rifting progressed the main bulk of the volcanics was generated primarily from the plume mantle with the melting starting at depths of about 120 km and extending to as shallow as the base of the stretched lithosphere at 45 km depth elevated mantle temperatures of 1500 1560 c approximately 150 200 c above normal are inferred from the rare earth element concentrations in the volcanic rocks further constraints on the mantle temperature come f

magmatic differentiation examined with a numerical model considering multicomponent thermodynamics and momentum energy and species transport abstract magmatic differentiation processes in a cooling magma body were examined using a numerical model considering multicomponent thermodynamics and momentum energy and species transport the model accounts for melt transport induced by its density variation resulting primarily from solid liquid phase change the equilibrium mineral assemblages their mass fractions and their chemical compositions are determined by multicomponent thermodynamic models and these parameters are linked with the calculations of velocity pressure temperature and species fields at each iteration and time step for simplicity solid phases are assumed to be stationary and only olivine and plagioclase that are the earliest crystallization phases in common basaltic magmas are considered as fractionating phases application of the model to natural magmatic system shows that crystallization occurs selectively along the chilled boundaries soon after the cooling and the magma body is separated into high crystallinity mush zones and mostly crystal free main magma because of the melt exchange between the mush zones and the main magma by compositional convection the main magma evolves progressively in composition and the spatial chemical heterogeneity is grown with time the melt segregated from the sidewall mush zone is accumulated below the roof mush zone forming compositional stratification in the upper part of the main magma on the other hand the fractionated melt from the floor mush zone principally mixes with the overlying ambient magma in the middle and lower parts of the main magma body this study shows that the direct treatment of nonlinear coupling among momentum energy and species transport provides useful information of the thermal and chemical evolution of magma chambers as a function of time and space

seismic images of caledonian lithosphere scale collision structures in the southeastern north sea along mona lisa profile 2 abstract the unexposed suture between baltica and eastern avalonia is imaged by coincident normal incidence reflection and wide angle reflection refraction seismic data of the mona lisa project we present new results of the upper lithospheric seismic structure from the n s striking profile 2 across the caledonian deformation front which represents the crustal collision suture between baltica and eastern avalonia that formed after closure of the tornquist sea during the caledonian orogeny in late ordovician times three different crustal types are identified with great similarities to the nearby profile 1 1 a three layered crust typical of shields to the north 2 a transitional crust suture zone in the central part and 3 a two layered crust of caledonian origin to the south characterized by very low velocities throughout the crust the crustal thickness varies from 38 35 km under the northern margin of the ringkobing fyn high baltica crust to 28 27 km beneath the north german basin in the caledonian crust to the south the suture zone is imaged by s dipping crustal reflections from 1 9 to 10 6 s two way travel time twt over a horizontal distance of 70 km within the transitional crust the reflection suture zone terminates in a 60 km wide reflective lens with velocities of 6 6 6 8 km s in the lowermost crust the reflective lens may be interpreted as a remnant of oceanic or island arc crust that was accreted to the leading edge of baltica during closure of the tornquist sea alternatively the reflective lens may represent an indentor of baltica crust into the avalonian terrane the change in lower crustal reflectivity and the abrupt transition from a three to a two layered crust further south suggest that the trans european fault and the elbe lineament continue into the north sea the upper mantle reflectivity is dominated by bands of strong amplitude s dipping reflections from 13 5 to 21 8 s twt and a weaker band of n dipping reflections from 12 to 16 s twt reversed wide angle refractions and reflections indicate the presence of a s dipping high velocity layer 8 65 8 8 km s in the uppermost mantle the sub moho high velocity layer partially coincides with the strong s dipping normal incidence mantle reflections we propose that the s dipping mantle structure may represent a late caledonian or late carboniferous to early permian mantle shear zone

palaeozoic structural development along the tornquist zone kattegat area denmark abstract the interpretation of newly released commercial 2d reflection seismic data in the kattegat area denmark has provided us with a better understanding of the palaeozoic tectonic processes along the tornquist fault zone a base palaeozoic time structure map a lower palaeozoic twt isopach map a true lower palaeozoic twt isopach map an upper carboniferous lower permian syn rift twt isopach map a top pre zechstein time structure map and a zechstein combined twt isopach and palaeogeography map have been generated the uniform lower palaeozoic sequence thickness in the kattegat both inside and outside the tornquist zone indicates only minor lateral movements if any whereas the extensive upper silurian sequence increasing in thickness to the north indicates a relatively fast regional subsidence the base palaeozoic time structure map and the late palaeozoic syn rift isopach map show a clear late palaeozoic extension in the area the syn rift isopach map in combination with the time equivalent opening of the skagerrak graben at right angles to the tornquist zone in the kattegat indicates that this extensional tectonic event had a dextral slip component measurements on internal extensional faults in the tornquist zone give a minimum right lateral displacement of 10 4 km the footwall blocks were deeply eroded during the early permian rifting and at zechstein times the area became a peneplane the tornquist zone was later exposed to several tectonic phases where dextral slip played a role indicated by the push up and pull down structures caused by restraining and releasing bends of the borglum fault the dextral displacement along the borglum fault since the beginning of the permian is in the order of 5 7 km based on the displacement of a lower palaeozoic local depocentre early permian depocentres and faults which gives a total amount of right lateral displacement since the early palaeozoic in the order of 15 20 km the continuously repeated tectonic episodes along the tornquist zone throughout most of the phanerozoic show that the zone was easily reactivated implying deep seated basement faults the tornquist zone can be seen as a buffer zone between continental blocks whenever changes in the regional stress field are induced

boundary layer crystallization in a basaltic magma chamber evidence from rishiri volcano northern japan the evolution of basaltic magma by mushy boundary layer processes introduction in a magma chamber is documented for an alkali basalt kutsugata the evolution of magma chambers is of major petrologic lava from rishiri volcano northern japan on the basis of zoning interest and has been studied extensively experimental patterns of plagioclase phenocrysts and their spatial distribution in and theoretical studies that have investigated the cooling the lava plagioclase phenocrysts with complex zoning patterns can of magma chambers have demonstrated that a magma be divided into four types type 1 is characterized by an extremely chamber solidifies through formation of a mushy boundan rich core an71 90 which is commonly corroded and filled with ary layer composed of crystals and interstitial melt along sodic plagioclase type 2 has an ab rich core surrounded by a calcic the chamber walls e g brandeis jaupart 1986 turner mantle characteristic of partial dissolution type 3 is characterized by et al 1986 marsh 1988 tait jaupart 1992 layered igneous complexes e g morse 1969 shirley 1987 an ab rich core that is commonly surrounded by a reversely zoned mcbirney 1996 provide useful information on how slightly calcic mantle type 4 has an ab rich core without a calcic mafic magma chambers evolve and consolidate but they mantle the an rich cores of the type 1 plagioclase are too calcic do not offer much insight into the instantaneous state of to have crystallized from a liquid represented by any whole rock a chamber because those intrusions integrate processes composition at the estimated temperatures of the kutsugata magma that occurred over long solidification times mcbirney system the an rich cores are inferred to have been formed in a noyes 1979 tait 1988 volcanic products such as mushy boundary layer along the wall of a magma chamber where lava flows and tephra deposits can sample the inthe magma was relatively cool and rich in water because of significant stantaneous state of a magma chamber there are a few olivine fractionation and possibly because of addition of water studies along this line of approach tait 1988 de silva expelled from the chilled margin and or groundwater from the 1989 nakada et al 1994 these studies focused on surrounding crust the ab rich cores of types 2 4 plagioclase crystal aggregates in volcanic products and concluded phenocrysts are interpreted to have crystallized in the main magma that the crystal formation in a mushy boundary layer body of the reservoir the types 2 and 3 ab rich cores were brought plays a dominant role in magmatic evolution in a magma to the mushy layer by convection the temporal and spatial variations reservoir however these studies were mostly on silicic of zoning patterns of plagioclase phenocrysts in basaltic lava flows magmas and there are significant differences in physical give useful information on the evolution of mafic magma chambers and chemical features between silicic and mafic magma chambers in this paper the magmatic evolution of an alkali basalt kutsugata lava from rishiri volcano northern japan is studied the kutsugata lava is divided into three lava flows on the basis of whole rock chemical

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

the influence of structural inheritance and multiphase extension on rift development the northern north sea the northern north sea rift evolved through multiple rift phases within a highly heterogeneous crystalline basement the geometry and evolution of syn rift depocentres during this multiphase evolution and the mechanisms and extent to which they were influenced by pre existing structural heterogeneities remain elusive particularly at the regional scale using an extensive database of borehole constrained 2d seismic reflection data we examine how the physiography of the northern north sea rift evolved throughout late permian early triassic rp1 and late jurassic early cretaceous rp2 rift phases and assess the influence of basement structures related to the caledonian orogeny and subsequent devonian extension during rp1 the location of major depocentres the stord and east shetland basins was controlled by favorably oriented devonian shear zones rp2 shows a diminished influence from structural heterogeneities activity localises along the viking sogn graben system and the east shetland basin with negligible activity in the stord basin and horda platform the utsira high and the devonian lomre shear zone form the eastern barrier to rift activity during rp2 towards the end of rp2 rift activity migrated northwards as extension related to opening of the proto north atlantic becomes the dominant regional stress as rift activity in the northern north sea decreases through documenting the evolving syn rift depocentres of the northern north sea rift we show how structural heterogeneities and prior rift phases influence regional rift physiography and kinematics controlling the segmentation of depocentres as well as the locations styles and magnitude of fault activity and reactivation during subsequent events

pre permian sedimentary basins in the north sea images from reprocessed and pre stack depth migrated mona lisa data pre permian sedimentary basins in the southeastern north sea have been previously interpreted from potential field data but only poorly imaged on seismic sections due to the presence of salt layers and a thick mesozoic and cenozoic cover furthermore potentially low prospectivity for hydrocarbon resources has resulted in sparse penetration of pre permian sediments in the southeastern north sea we present images of large palaeozoic basins obtained through reprocessing and pre stack depth migration of seismic data from the mona lisa project the depth images show in detail the structure of pre permian basins beneath the danish basin and the ringkobing fyn high providing new insights into upper palaeozoic basin formation in the north sea

absolute dating of brittle fault movements late permian and late jurassic extensional fault breccias in western norway 40ar 39ar geochronological and palaeomagnetic dating methods applied to fault breccias in western norway have isolated two brittle reactivation episodes of the syn post caledonian extensional nordfjord sogn detachment these events of latest permian and latest jurassic early cretaceous ages demonstrate temporal relationships between development of chemical remanent magnetism and partial resetting of ar isotopic systems during distinct breccia forming episodes a third event of carboniferous age was also identified in the breccias with the 40ar 39ar technique and is a relict unroofing signature inherited from the fault wall rocks these brittle faults are significant time markers and become relevant to interpretations of offshore seismic data which attempt to place ages on faults that have undergone multiple reactivation episodes

the structural evolution of the northern viking graben and its bearing upon extensional modes of basin formation recent advances in the understanding of rift basin formation coupled with the increasing public availability of seismic and well data across the northern viking graben 60 61 n have enabled a detailed analysis of its development two rifting episodes can be recognized late permian early triassic and bathonian ryazanian major regional unconformities thought to be primarily tectonic rather than eustatic in origin separate and subdivide the rifting episodes the earlier episode involved extension about a n s axis ensuing triassic mid jurassic thermal subsidence was accommodated on steep faults during the later episode a new ne sw fault trend was superimposed on pre existing patterns major block rotation marking active rifting ceased at the end of the ryazanian during the second post rift episode there was a progressive migration of active faulting towards the basin margins and as a result a widening with time of the area undergoing subsidence asymmetric subsidence of the central part of the basin was hinged at the western margin of the horda platform and accommodated to the nw on major faults within the tampen spur factors for the second rifting episode were calculated both by relating subsidence to extension and by measuring observed extension values calculated by both methods increase consistently towards the basin axis for both rifting and thermal subsidence phases but are greater for the latter phase subsidence patterns are similar for both rifting and thermal subsidence episodes so that there is vertical stacking of relatively thick sequences in the axis of the northern viking graben these factors preclude the application of models involving uniform and non uniform stretching and also preclude oblique extension depth dependent stretching is preferred

evidence for multiple mechanisms of crustal contamination of magma from compositionally zoned plutons and associated ultramafic intrusions of the alaska range

melt segregation in migmatites anatectic stromatic migmatites have a symmetrical layered structure with a low ratio of thickness to length and a periodicity features that have not been explained satisfactorily but which are related to physical processes of melt segregation we evaluate the compaction model for segregation as it applies to migmatites and develop models for melt segregation based upon convection driven by volume change and upon advection down pressure gradients that result from applied differential stress acting on an anisotropic multilayer protolith compaction by gravity driven two phase flow results in asymmetric separation yields calculated segregation times that are slow or geologically unreasonable and yields relative volumes that are not consistent with the abundance of leucosome seen in natural migmatites although calculated segregation times for very wet granite might allow segregation by compaction some driving force in addition to gravity is needed to cause widespread melt segregation in the crust for segregation at low volume fraction of melt we develop a two dimensional two phase matrix and fluid viscous flow cell model length much greater than thickness with phase changes which predicts either expansion or contraction convection depending on whether volume change vr is positive or negative convection leads to melt segregation to form a stromatic structure in a geologically reasonable timescale at moderate volume fraction of melt segregation may occur by filter pressing in compositionally layered rocks in response to applied differential stress melt migration is by porous medium flow driven by differences in mean normal stress between the layers as a consequence of differences in rheology and shear enhanced matrix collapse calculated segregation rates are fast and the model yields adequate volume of leucosome the positive vr for volatile phase absent melting reactions under crustal pressures promotes melt escape unless extensional deformation facilitates melt accumulation if the rate of melt production exceeds the rate of melt escape then the increase in melt pressure may lead to melt enhanced embrittlement and fracture and melt may migrate out of the system in contrast water rich volatile phase present melt producing reactions have a negative vr which promotes melt retention after segregation unless deformation generates melt escape pathways evolution of stromatic structure through anatectic erosion of mesosome by increasing the melt fraction in situ may lead to breakdown of the solid matrix and will lead to instability due to buoyancy and magma may become mobile and entrain residual material restite

stratigraphy of the rotliegend group in the danish part of the northern permian basin north sea a stratigraphic study of the rotliegend offshore denmark has resulted in a coherent lithostratigraphic framework and a better understanding of the mid permian evolution of the eastern northern permian basin the rotliegend group is subdivided into the karl formation composed of syn rift volcanics and volcaniclastic sediments and the younger post rift auk formation dominated by aeolian and fluvial sandstones radiometric age data based on k ar dating of plagioclase from the karl formation volcanics combined with previously published age data suggests that volcanism took place during two separate events 276 281 ma and 261 269 ma both events post date the lower rotliegend volcanism 288 300 ma in the southern permian basin suggesting that the northern permian basin was formed during a later tectonic event the main volcanic event in the northern permian basin was broadly coeval with volcanic activity in the altmark area of ne germany reflecting simultaneous rifting and onset of upper rotliegend 2 sedimentation in the northern and southern permian basins

triassic paleogeography of the greater eastern norwegian danish basin constraints from provenance analysis of the skagerrak formation abstract zircon u pb geochronometry and heavy mineral ccsem analysis are used to determine the provenance of the skagerrak formation which was deposited during most of the triassic period as an up to 3 km thick succession the sampled sandstones are collected in well cores from the norwegian danish basin the sorgenfrei tornquist zone and the skagerrak kattegat platform with the goal of obtaining a geographically and stratigraphically representative dataset usable for investigating provenance and the implications for triassic paleogeography the basement in the source terranes has pronounced differences in zircon ages so it is possible to distinguish which terranes the zircon grains were derived from the zircon ages found in the skagerrak formation are comparable to source terranes in southern norway and southwestern sweden including the telemarkia terrane the idefjorden terrane and the eastern segment of the sveconorwegian orogen the basement and paleozoic sediments of the skagerrak graben and the skagerrak kattegat platform were presumably exposed to erosion during the early to middle triassic as indicated by the abundant rock fragments present in the skagerrak formation south of this elevated area this local input was mixed with vast amounts of material transported from other basement areas in the northeast and their cover the material was transported by rivers that downstream developed into braid plains with braided rivers and aeolian activity the decreasing grain size up through the skagerrak formation is reflecting the gradual overstepping of the basin margins thus sediment supplied by local erosion is absent in the upper part of the formation the heavy mineral assemblages are mineralogically immature due to the arid depositional conditions and the limited temporary storage during transport the assemblages are very uniform in the norwegian danish basin whereas they are characterized by local inputs on the skagerrak kattegat platform a locally deposited basement conglomerate indicates that the basement beneath northern denmark has ages corresponding to the telemarkia terrane in southern norway similarly to the ages of the basement beneath southern denmark

rates of thermal and chemical evolution of magmas in a cooling magma chamber a chronological and theoretical study on basaltic and andesitic lavas from rishiri volcano japan rates of magmatic processes in a cooling magma chamber were investigated for alkali basalt and trachytic andesite lavas erupted sequentially from rishiri volcano northern japan by dating of these lavas using u th radioactive disequilibrium and c dating methods in combination with theoretical analyses we obtained the eruption age of the basaltic lavas to be 29 3 0 6 ka by c dating of charcoals the eruption age of the andesitic lavas was estimated to be 20 2 3 1ka utilizing a whole rock isochron formed by u th fractionation as a result of degassing after lava emplacement because these two lavas represent a series of magmas produced by assimilation and fractional crystallization in the same magma chamber the difference of the ages i e 9 kyr is a timescale of magmatic evolution the thermal and chemical evolution of the rishiri magma chamber was modeled using mass and energy balance constraints as well as quantitative information obtained from petrological and geochemical observations on the lavas using the timescale of 9 kyr the thickness of the magma chamber is estimated to have been about 1 7 km the model calculations show that in the early stage of the evolution the magma cooled at a relatively high rate 40 18c year and the cooling rate decreased with time convective heat flux from the main magma body exceeded 2w m when the magma was basaltic and the intensity diminished exponentially with magmatic evolution volume flux of crustal materials to the magma chamber and rate of convective melt exchange compositional convection between the main magma and mush melt also decreased with time from 0 1m year to 10 3 m year and from 1m year to 10 2 m year respectively as the magmas evolved from basaltic to andesitic compositions although the mechanism of the cooling i e thermal convection and or compositional convection of the main magma could not be constrained uniquely by the model it is suggested that compositional convection was not effective in cooling the main magma and the magma chamber is considered to have been cooled by thermal convection in addition to heat conduction

latest caledonian to present tectonomorphological development of southern norway abstract the regional resultant stress field of the northeastern north atlantic has shifted significantly throughout the phanerozoic in fennoscandian parts of the caledonian orogen mountain building which was characterized by nw se contraction reference to present north was followed by a collapse with transport both parallel and transverse to the mountain chain the late palaeozoic mesozoic saw several stages of e w to nw se extension varying in time and position local episodes of inversion are traceable in some cases particularly in connection with deep seated and long lived zones of weakness the cenozoic has to a larger degree been affected by compression including folding and basin inversion again some of the more pronounced effects of local inversion are related to pre existing fault systems neogene uplift of the western mountainous area in scandinavia can be unravelled by potential field study aft data and reflection seismic sections assuming that the region is close to isostatic equilibrium the uplifted areas must be supported at depth by substantial volumes of low density material within the crust or the mantle close to the crust mantle interface or close to the lithosphere asthenosphere interfaces a series of nw se oriented cross sections representing successive stages of development from the late caledonian to present were prepared and used to analyse the tectonomorphological development as related to the shifting stress configurations three stages are inferred stage 1 devonian late permian denudation of the caledonian mountain chain during the latest stage of closing of the iapetus ocean in early to mid devonian times the caledonian mountain chain reached its maximum elevation of perhaps as much as 8 9 km including the foreland basins to the east the average topographic gradient of the eastern flank may have been in the order of 1 3 the denudation of the caledonian mountains continued into the permian and by the end of the permian the mountain system was more or less obliterated with a gradient of the eastern flank probably continued to fall to less than 0 1 stage 2 triassic late cretaceous tectono thermal uplift of central south norway uplift of central south norway was rejuvenated in triassic times and continued throughout the jurassic and early cretaceous the gradient of the eastern flank increased to a maximum of approximately 0 15 and master structural elements like the laerdal gjende olestol fault complex again played important roles in the development stage 3 tertiary present renewed tectono thermal activity and post glacial rebound a second phase of post caledonian tectono thermal uplift of south central norway started in oligocene time again increasing the gradient of the eastern flank of south norway around 0 2

early paleozoic orogenic collapse tectonic stability and late paleozoic continental rifting revealed through thermochronology of k feldspars southern norway southern norway was subjected to contractional orogenesis and subsequent extensional collapse during the early to middle paleozoic when baltica collided with laurentia resulting in the caledonian orogen the largely unknown post caledonian paleozoic thermal history of the crust in southern norway has been assessed through application of multiple diffusion domain thermal modeling of k feldspar 40ar 39ar data samples were collected along a 250 km e w traverse from the internides of the orogen along the western coast near bergen through a series of middle to upper crustal nappes to the east the k feldspar thermal models which complement published mica and hornblende 40ar 39ar data are consistent with rapid cooling at the end of the caledonian orogenic cycle when early devonian 400 ma extensional collapse resulted in exhumation through mainly midcrustal conditions following extensional collapse a period of relative tectonic and thermal stability may have prevailed until 300 ma the k feldspar modeling suggests that southern norway entered a new phase of relatively rapid cooling a few c ma in the permo carboniferous and that the increase in cooling rate was diachronous across the traverse the onset of this cooling phase occurs at about the time of the earliest faulting and magmatism in the oslo graben to the southeast the increase in cooling rate may also indirectly date the onset of the first important rift phase and associated sedimentation in the north sea graben system the distribution of permian and younger fission track ages in both southern continental norway and its marginal basin sediments is consistent with the permo carboniferous rapid cooling recorded in the k feldspars the simplest explanation for the late paleozoic increase in cooling rate is that rifting around the margin of southern norway has resulted in a decrease in the base level and an increase in the rate of erosional denudation resulting in essentially uniform exhumation of 2 4 km along the traverse

the oslo rift new palaeomagnetic and 40ar 39ar age constraints summary 40ar 39ar whole rock and alkali feldspar ages demonstrate that dioritic to monzonitic dykes from boverbru and lunner belong to the youngest recorded magmatic activity in the oslo rift region southeast norway these dykes represent the terminal phase of rift and magmatic activity in the oslo graben at the dawn of the triassic 246 238 ma the boverbru and lunner dyke ages are statistically concordant however the palaeomagnetic signature of the boverbru dyke is complex and directions from the margins and the interior of the dyke diver in polarity therefore the new early triassic palaeomagnetic pole for baltica eurasia is exclusively based on the less complex lunner dykes and contacts palaeomagnetic pole latitude 52 9 n longitude 164 4 e dp dm 4 5 7 3 the early triassic palaeomagnetic pole mean age 243 5m a 2s is slightly diverent from the upper carboniferous permian 294 274 ma and kiaman aged poles from the oslo rift

upper carboniferous and lower permian tectonostratigraphy on the southern margin of the central north sea hydrocarbon exploration in uk licence blocks 31 26 and 39 2 on the southern margin of the central north sea in 1996 and 1997 proved the presence of a palaeozoic sedimentary succession previously undescribed in the area biostratigraphic data from the sediments and radiometric dating of interbedded basaltic lavas indicate that the succession spans the upper carboniferous westphalian b to lower permian asselian interval this has been informally split into three sedimentary units the lower flora sandstone is of westphalian b c age this clastic interval is overlain by the first of two main lavas which is in turn overlain by the westphalian c d to stephanian upper flora sandstone the flora sandstones are interpreted to be lithostratigraphic equivalents to the schooner formation of the southern north sea the flora sandstones are separated from the grensen formation by an angular unconformity possibly related to variscan asturic inversion the grensen formation is in turn overlain by a second basaltic lava flow both basaltic units are assigned to the inge volcanics formation rotliegend group petrographic data indicate that the grensen formation also forms part of the rotliegend group a major extensional event of permian age post dates all of these strata extensional faults are planed by a regional unconformity saalian unconformity that is overstepped by permian to cretaceous strata

the relative roles of boundary layer fractionation and homogeneous fractionation in cooling basaltic magma chambers abstract in a cooling magma chamber magmatic differentiation can proceed both by fractionation of crystals from the main molten part of the magma body homogeneous fractionation and by mixing of the main magma with fractionated melt derived from low temperature mush zones boundary layer fractionation in this study the relative roles of boundary layer fractionation and homogeneous fractionation in basaltic magma bodies were examined using a thermodynamics based mass balance model model calculations show that boundary layer fractionation cannot be a dominant fractionation mechanism when magma chambers are located at low pressures 2 wt such as arc basalt and the magma chambers are located at depth 100 mpa because the melt derived from mush zones is enriched in alkalis and h 2 o crystallization from the main magma is suppressed by mixing with the mush melt as a consequence of depression of the liquidus temperature therefore homogeneous fractionation is more effectively suppressed in magma chambers in which boundary layer fractionation is more active if magmatic differentiation proceeds primarily by boundary layer fractionation magmas can remain free of crystals for long periods during magmatic evolution

interaction of permo triassic and jurassic extensional fault blocks during the development of the northern north sea the main structural elements resulting from late permian early triassic extension have been resolved across the total width of the northern north sea the northern segment of the basin north of 61 n bounded to the west by the hutton fault and to the east by the norwegian mainland was occupied by four westerly tilted half grabens 25 35 km in width prior to jurassic extension the central segment of the basin 60 20 61 n exhibits a full graben beneath the jurassic horda platform flanked by half grabens tilted towards the basin margins the change in permo triassic fault block configuration defining structural domains is largely constrained by a basement discontinuity which runs diagonally ne sw across the basin maximum jurassic extension is represented by several centres of subsidence aligned nne ssw across a n s oriented and broader permo triassic basin jurassic extension was concentrated within the viking graben proper south of 60 n which in this area is represented by single westerly tilted en echelon half graben segments 20 25 km wide the area affected by jurassic extension becomes progressively wider to the north again governed by the basement discontinuity eventually encompassing the total width of the basin c 200 km north of 61 n the central segment of the northern north sea displays the greatest contrast in permo triassic and jurassic structural styles and basin geometry jurassic stretching and faulting was negligible to the east horda platform precisely the area which had been the site of maximum permo triassic stretching and fault activity it has generally been assumed that jurassic fault configuration along the eastern margin of the central segment of the viking graben to a large extent reflects the reactivation of major permo triassic faults this study demonstrates the interference of permo triassic and jurassic basin geometries in this area due to major faults of the two generations dipping in opposite directions as a consequence the e tilted oseberg fault block of jurassic origin and c 20km in width is underlain by a wider c 35 km and w tilted permo triassic fault block

danish basin subsidence by triassic rifting on a lithosphere cooling background i report here the first quantitative study of the entire post palaeozoic subsidence history in the ensemble of north west european sedimentary basins data obtained from wells suggests that the late permian through mesozoic subsidence of the danish basin is a result of lithosphere wide cooling following an igneous event of late carboniferous to early permian age this cooling subsidence was superposed by major triassic rifting and minor mid jurassic uplift both events apparently not having significant thermal consequences the danish basin did not experience the late jurassic to early cretaceous rifting and subsequent cooling which were so important in the development of the central north sea rifts this result suggests that a reappraisal of north sea tectonics should look for pre jurassic subsidence

a stretching model for the oslo rift abstract data on the tectonic and magmatic activity crustal thickness seismic velocity and gravity have been integrated to constrain a stretching model for the late carboniferous early permian oslo rift the crustal structure and numerical modelling of temperature and melt generation during rifting suggest that the oslo rift in skagerrak developed in response to crustal stretching with a factor between 1 4 and 1 6 however upper crustal extension by faulting accounts for a stretching factor of only 1 1 1 2 thermal expansion of the lithosphere due to an increased asthenospheric potential temperature the intrusion of melted mantle material into the lower crust and probably a lowering of the erosional base by a fall in sea level all contributed to the erosion of 2 4 km of lower palaeozoic pre rift sediments over large areas the lateral extent of the thermal anomaly reached far beyond the oslo and skagerrak grabens

the horn graben and its relationship to the oslo graben and the danish basin abstract the tectonic development of the horn graben the danish basin and the oslo graben are compared with special emphasis on the timing of their initiation the comparison is based on seismic interpretation and is supported by analysis of tectonic subsidence devonian and carboniferous sedimentary strata and rotliegende volcanics constitutes the pre rift succession in the horn graben in the central part of the graben it rests unconformably on precambrian possibly dalslandian basement to the south on caledonian metamorphic basement whereas lower palaeozoics occur to the north the distribution of these pre devonian assemblages suggests initial early devonian uplift of the ringkobing fyn high as the lower palaeozoic sediments are believed to have covered the high originally further uplift appears to have taken place in the early permian a subdivision of the horn graben into a northern and a southern segment are recognized the rifting was apparently heralded only in the southern segment in the rotliegende and the main rifting phase did not start before the triassic subsidence modelling shows that the thermal anomaly created by rifting of the horn graben was less significant than in the danish basin and resulted in much less post rift thermal subsidence the reduced thermal anomaly may account for an earlier consolidation of the crust thereby rendering the area resistant to late cretaceous early tertiary inversion tectonism the oslo graben was tectonically active during late carboniferous to early permian times and essentially became inactive during triassic times when the main tectonism of the horn graben occurred subsidence modelling shows that the danish basin south of the fjerritslev trough was tectonically active mainly during the zechstein whereas the rapid triassic subsidence is modelled to be caused by phase transformations as indicated by the minor amount of basement attached faulting based on the differing subsidence patterns it is concluded that the three basins did not develop simultaneously and that they are not genetically related

quantitative analysis of triassic extension in the northern viking graben it has been recognized for over a decade that large displacement pre jurassic faults are present in the northern viking graben part of the north sea rift these faults define a series of major fault blocks below the more obvious jurassic rift basin we attempt here to quantify the amount of extension associated with this older rift event which is probably of early triassic age quantitative modelling of the triassic rift and the succeeding period of thermal subsidence has been undertaken using a combination of flexural backstripping and flexural cantilever forward modelling these techniques suggest that triassic extension across the horda platform norwegian sector reached c 40 s 1 4 the consequences of this extension were deposition of a thick 3 km triassic upper jurassic syn rift and post rift sequence prior to renewed but minor extension in the late jurassic earliest cretaceous the thickness of the viking group reservoirs in the troll area appears to have been almost entirely controlled by sediment loading during post triassic thermal subsidence jurassic extension on the horda platform was in the uk sector of the viking graben triassic structures are less obvious than those below the horda platform because of jurassic overprinting they are however still present average triassic extension across the east shetland basin was c 15 comparable with the magnitude of jurassic extension in the same area we believe that the tern eider and cormorant fault blocks with proven shallow basement comprised a large eroded horst during the early triassic uplifted in the footwalls of major faults flanking the magnus and statfjord half graben during the triassic the magnus half graben was contiguous with the unst basin now situated in the western footwall of the younger jurassic basin the presence of the unst basin suggests that triassic extension occurred across the area that is now the northern shetland platform continuing into the west shetland area although the more obvious structures in the viking graben are jurassic in age the earlier triassic event was equally as important in controlling the structural and stratigraphic history of the basin

the 23 october 1904 ms 5 4 oslofjord earthquake reanalysis based on macroseismic and instrumental data abstract on 23 october 1904 the area of the oslofjord was struck by an m s 5 4 earthquake that was felt over much of northern europe we collected information available from seismological bulletins of the time and also seismogram copies from a number of seismic stations in europe this allowed for the first time for this earthquake an instrumental epicentral location even if the observation capabilities at that time were poor due to low seismograph amplification and timing problems after a careful selection and weighting of published onset times the reported observations from the seismic stations in uppsala hamburg potsdam gottingen leipzig and tartu could be used for this instrumental location of the event we also performed an inversion of the available macroseismic observations based on the kinematic function kf for the radiation of body waves from a line source because the problem is nonlinear and also bimodal for pure dip slip mechanisms we used a sharing niching genetic algorithm to perform the inversion the new epicenter obtained from the kf intensity inversion is consistent with the new instrumental solution giving a location in the lower crust 25 30 km and close to the eastern coast of the oslofjord near the junction of two major fault zones the kf inversion was moreover able to constrain the fault plane mechanism with an almost vertical rupture plane striking north northeast south southwest 206 212 with a mixed mechanism approximately 64 rake angle the ambiguity of which is resolved by the polarities of the pn onset observed on the horizontal components of the uppsala wiechert seismograph the magnitude of the event has previously been assessed to m s 5 4 based on the size of the felt area observations and a value of m s 5 4 has now been independently confirmed both through the new intensity inversion and through the instrumental data

the middle devonian basins of western norway sedimentary response to large scale transtensional tectonics abstract the devonian basins of western norway were formed during late to post orogenic extension of overthickened caledonian crust the basins are situated in the hanging wall of the extensional nordfjord sogn detachment zone nsdz and display extensional half graben geometries in sections parallel to the local direction of principal extension based on overall facies configurations paleocurrent patterns and intrabasinal structures we infer an anticlockwise rotation of the syndepositional extension direction from nw se in the south solund basin to wsw ene in the north hornelen basin the axes of folds that are roughly parallel to the local extension direction are rotated correspondingly the kvamshesten basin is located between the solund and hornelen basins sedimentological and structural data show evidence of an early southeastwards tilt direction followed by a more eastwards tilt and associated e w flowing paleodrainage correspondingly nw se trending folds and reverse faults are superposed by e w trending ones at low to intermediate stratigraphic levels the variations in apparent tilt direction for the basins together with variations in intrabasinal structure is interpreted to reflect an anticlockwise rotation of the regional syndepositional strain field the above observations and inferences indicate that the devonian basins in western norway formed in a strain field dominated by regional transtension accommodated by extension along the nsdz and sinistral strike slip along orogen parallel shear zones and faults to the north of the basins alternatively nw directed extension preceded the introduction of a sinistral strike slip component the models are in accordance with recent work carried out in the footwall of the nsdz and illustrates the tectono sedimentary response to a complex interplay between extension and strike slip that appears to have been fundamental in the late stage disintegration of the caledonian orogen

palaeomagnetic dating of fault rocks evidence for permian and mesozoic movements and brittle deformation along the extensional dalsfjord fault western norway summary palaeomagnetic and structural studies of the dalsfjord nappe western norway show that the basal ow angle detachment dalsfjord fault is a long lived fault zone and that the most important phase of faulting was of devonian extension probably nucleated on an earlier silurian scandian thrust fault rocks produced during subsequent movements indicate that the dalsfjord fault underwent periods of brittle low angle extensional reactivation during the permian 250 260 ma and upper jurassic lower cretaceous c 150 ma corresponding with stages of major extensional movements on the continental shelf palaeomagnetic studies may be of great importance for dating faults and major movement stages in long lived fault systems the particular importance of the results is that they show that low angle normal faults can operate in a brittle upper crustal regime

internal differentiation of kutsugata lava flow from rishiri volcano japan processes and timescales of segregation structures formation article i nfo internal differentiation processes in a solidifying lava flow were investigated for the kutsugata lava flow from rishiri volcano in northern japan in a representative 6 m thick lava flow that was investigated in detail in this study segregation products darker than the host lavas manifested mainly in the form of pipes vesicle cylinders and layers vesicle sheets occurring around 0 5 2 3 m and 2 0 4 0 m above the base respectively both the cylinders and sheets are significantly richer in incompatible elements such as tio2 and k2o than the host lavas which suggest that these products essentially represent residual melt produced during solidification of the lava flow field observation and the geochemical features of the lavas suggest that the vesicle cylinders grew upward from near the base of the flow by continuous feeding of residual melt from the neighboring host lavas to the heads of the cylinders on the other hand the vesicle sheets were produced in situ in the solidifying lava flow as fracture veins caused by horizontal compression the vesicle cylinders have a remarkably higher mgo content up to 8 wt than the host lava b6 wt whereas the vesicle sheets display mgo depletion as low as 3 5 wt the relatively high mgo content of the vesicle cylinders cannot be explained solely by the mechanical mixing of olivine phenocrysts with the residual melt it is suggested that the vesicle cylinders were produced by the extraction of olivine bearing interstitial melt from an augite plagioclase network in the host lava whereas the vesicle sheets were formed by the migration of the residual melt from a crystal network consisting of plagioclase augite and olivine in the host lava into platy fractures we infer that this selective crystal fractionation for forming the vesicle cylinders resulted from processes in which abundant vesicles rejected from the upward migrating floor solidification front prevented olivine crystals from being incorporated into the crystal network in the host lava the vesicle cylinders are considered to have formed in 1 day after the lava flow came to rest while relatively large vesicle sheets n1 cm thick appeared much later after 9 days the formation of these segregation products was essentially complete within 20 days after the lava emplacement

magmatic flare up at the carboniferous permian boundary in the ne german basin revealed by shrimp zircon ages shrimp ages of 12 volcanic samples indicate intense magmatic activity to have occurred at the carboniferous permian boundary throughout much of the ne german basin rhyolitic crystal rich samples have been taken from quarries in the halle volcanic complex hvc and in the flechtingen ignimbrite and from drill cores of the kotzen mirow friedland and penkun areas ten samples yielded 206pb 238u ages between 302 and 297 ma 3 ma respectively which indicate that the magmatic activity took place concentrated in a relatively short time span throughout much of the ne german basin two hvc samples have ages of 307 and 294 ma this remarkably synchronous magmatic activity occurred during the initial phase of the basin development considering published volume estimates 48 000 km3 extrusion rates during this time period probably were in the order of 0 01 km3 a 1 implying magma production rates of 0 1 km3 a 1 the calc alkaline sio2 rich volcanic rocks originated from magmas that presumably formed during anatexis and subordinate magma mixing in an intra continental transtensional setting the basaltic magma which must have provided the thermal input into the lower crust probably formed during decompressional melting of lithospheric mantle fertilised by previous magmatic processes the dated volcanic rocks occur in three geotectonic provinces namely the a mid german crystalline rise which forms the northern part of the internal variscides the b external variscides and the variscan foreland which is considered as c eastern avalonia many of the old zircons found in the hvc samples reflect the magmatic activity of the mid german crystalline rise 325 400 ma however cadomian 500 650 ma and older gondwanian elements 1674 2373 ma are also present although the flechtingen and kotzen areas are located within the external variscides it seems more likely that the middle to lower crust that experienced anatexis formed part of the eastern avalonia plate overridden by the variscan front thus the old zircons found in flechtingen 350 and 538 ma and kotzen 345 ma but certainly those present in the penkun drill cores 1483 ma represent eastern avalonia tectonic history the anatectic magmas in the friedland area probably formed partly from baltic crust thrusted below eastern avalonia during the caledonian orogeny among the old zircons from the friedland samples in addition to proterozoic ages 1456 ma we found testimony of caledonian 443 ma and of clearly post caledonian 387 ma magmatism